**Contents**


## **About the Editors**

#### **Juan Cecilia**

Juan Antonio Cecilia has a degree in Chemistry (2005) and is a Doctor in Inorganic Chemistry (2011) and an Associated Professor (2020) for the University of Malaga (Spain).

His main research line is the synthesis of adsorbents and catalysts for environmental applications, such as hydrotreating reactions, dehydration reactions, hydrogenating reactions, oxidation reactions, the valorization of biomass in valuable products, H2 purification processes, CO2 capture and the storage or the adsorption of biomolecules, dyes, pesticides or olefins between them. He has published 135 articles in scientific journals with a high impact factor (JCR) in the fields of materials, catalysis and adsorption (h-index: 32; citations: 3410). He is a reviewer for 60 journals and he has been an editor and guest editor of Special Issues.

#### **Carmen Pilar Jim´enez G ´omez**

Carmen Pilar Jimenez G ´ omez has a degree in Chemical Engineering (2014) and is a Doctor of ´ Inorganic Chemistry (2018) for the University of Malaga (Spain). She has carried out several studies at the Queen Mary University of London as well as the Universities of Ancona and Bologna (Italy).

Her main research line is the design synthesis of catalysts for the valorization of biomass of sugar, mainly regarding the synthesis of chemicals coming from furfural through hydrogenation and oxidations reactions.

She has published 26 articles in scientific journals with a high impact factor (JCR) in the fields of materials, catalysis and adsorption (h-index: 11; citations: 528). She is a reviewer for 10 journals and she has been the editor and guest editor of two Special Issues.

## **Preface to "Catalytic Applications of Clay Minerals and Hydrotalcites"**

Clay minerals are inexpensive and available materials with a wide range of applications (adsorbent, ion exchanger, support, catalyst, paper coating, ceramic, and pharmaceutical applications, among others). Clay minerals can be easily modified through acid/basic treatments, the insertion of bulky ions or pillars into the interlayer spacing, and acid treatment, improving their physicochemical properties.

Considering their low cost and high availability, clay minerals display a relatively high specific surface area in such a way that they have a great potential to be used as catalytic supports, since they can disperse expensive active phases as noble metals on the porous structures of their surfaces. In addition, the low cost of these supports allows their implementation on an industrial scale more easily than other supports, which are only feasible at the laboratory scale. Hydrotalcites (considered as anionic or basic clays) are also inexpensive materials with a great potential to be used as catalysts, since their textural properties could also be modified easily through the insertion of anions in their interlayer spacing. In the same way, these hydrotalcites, formed by layered double hydroxides, can lead to their respective mixed oxides after thermal treatment. These mixed oxides are considered basic catalysts with a high surface area, so they can also be used as catalytic supports.

> **Juan Cecilia and Carmen Pilar Jim´enez G ´omez** *Editors*

### *Editorial* **Catalytic Applications of Clay Minerals and Hydrotalcites**

**Juan Antonio Cecilia \* and Carmen Pilar Jiménez-Gómez**

Departamento de Química, Inorgánica, Cristalografía y Mineralogía, Facultad de Ciencias, Campus de Teatinos, Universidad de Málaga, 29071 Málaga, Spain; carmenpjg@uma.es

**\*** Correspondence: jacecilia@uma.es

#### **1. Introduction**

Clay minerals are the most abundant minerals on the surface of Earth. These minerals are well-known from the paleolithic period where the primitive person used clay minerals to produce ceramics or pottery. The number of uses and applications has been increasing as life has evolved. Nowadays, clay minerals are employed in a wide range of applications. Between them, their use is highlighted in the ceramic field as potteries, refractories, and porcelain, in rubber industry as fillers, in paper industry as fillers or coatings in the pharmaceutical field. Other interesting applications are as adsorbent and filter, drilling fluids of deodorizing agent, catalyst, catalytic support among others. In an elemental classification, the clay minerals can be grouped as cationic clays, which are very common in nature, and anionic clays, which are rare in nature although the cost for their synthesis is relatively low [1,2]. Considering the structural and morphological composition of cationic and anionic clays, this editorial is focused on the use of these clay minerals for catalytic applications, highlighting the possible modifications of its structure to improve its catalytic behavior.

#### **2. Cationic Clays**

Among clay minerals, phyllosilicates are the minerals that have received the most attention due to their chemical composition and textural properties. Phyllosilicates are lamellar silicates which are composed by tetrahedral sheets [MO4] <sup>4</sup>−, where M can be Si 4+ , Al 3+ or Fe 3+ , and octahedral sheets which are connected through sharing edges. The partial substitution of Si 4+ by Al 3+ in the tetrahedral sheets and the partial substitution of Al 3+ or Fe 3+ by Mg 2+ in the octahedral sheets generate a deficiency of positive charge, which is counterbalanced by the incorporation of alkaline or alkaline-earth cations in the interlayer spacing between adjacent sheets [3]. These phyllosilicates display both Lewis and Brönsted acid sites on the edge of these sheets. Thus, Brönsted acid sites are attributed to the external hydroxyl groups while Lewis acid sites are ascribed to the partial substitution of Si 4+ by Al 3+ . The acidity on the surface decreases according to water adsorbed on the clay increases [4]. Besides acid sites, the presence of Fe 3+ ions in the edges of the sheets can also provide electron-accepting or oxidizing sites [5]. In the same way, the redox behavior of the clay minerals can be modified by the substitution of the alkaline or alkaline-earth cations located in the interlayer spacing by other cations such as Ag + , Cu 2+ or Fe 3+ [6]. However, in many cases, the amount of available active sites is very limited. It has been reported in the literature that the interlayer spacing is between 7Å and 14Å in such a way that most of the molecules cannot access all active sites, so the only active centers available are those located on the outer surface [7].

It has been reported in the literature that the specific surface area determined by the BET equation is ranged between 10–150 m2g −1 [8]. In most cases, N<sup>2</sup> molecules cannot access the interlayer spacing, so the specific surface area is mainly ascribed to the interparticular voids [7]. In any case, the surface area is remarkable so that these phyllosilicates can also be used as catalytic supports to disperse a wide variety of active phases.

**Citation:** Cecilia, J.A.; Jiménez-Gómez, C.P. Catalytic Applications of Clay Minerals and Hydrotalcites. *Catalysts* **2021**, *11*, 68. https://doi.org/ 10.3390/catal11010068

Received: 21 December 2020 Accepted: 3 January 2021 Published: 6 January 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

In this sense, the acid treatment of the phyllosilicates can increase the surface area because of a partial solution of the octahedral sheet [9]. This partial digestion is more pronounced in the case of Mg-rich phyllosilicates as saponites or sepiolite [10]. The acid treatment also produces a higher amount of acid sites due to the generation of partially coordinated Al3+-species, forming Brönsted acid sites as well as the cationic exchange of the alkaline or alkaline-earth cations by H<sup>+</sup> -species in the interlayer spacing [11].

The ability to change ions located in the interlaminar spacing by other bulkier ions provide some phyllosilicates as smectites great potential in the field of catalysis as pillared structures may be formed with a specific surface area much higher than the starting materials. In this sense, the substitution of Na<sup>+</sup> by oligomeric (hidr)oxy aluminum cations provokes an increase of the interlayer spacing. In the next step, a thermal treatment favors the condensation between the silanol groups of the smectites sheets and the -OH groups of the oligomeric cations, leading to pillared clays (PILCs) [12]. Besides Al2O<sup>3</sup> pillars, a wide variety of pillars have been synthesized from their respective polyoxocations. Between them, it can be highlighted the formation of Zr, Cr, Ti, Fe, Al/Ce, Al/Fe or Al/Co. The formation of these pillars can provide Lewis or Brönsted acid sites [13]. In addition, the increase of the surface area can allow for the use of these pillared clays as support, leading to bi-functional catalysts in many cases [13].

The synthesis of porous clay heterostructures (PCHs) is another alternative to obtain materials with high specific surface area [14]. The synthesis of PCHs consists in the substitution of the cations of the interlayer space by a bulky organic cation, which enhances the interlayer spacing. Then, silicon precursor in the form of alkoxide is polymerized around the organic cation, forming a silica structure between adjacent layers. Finally, the organic matter is removed under thermal treatment [15]. Through this synthetic strategy, it is possible to modulate the pore diameter according to the specific conditions of the catalytic reactions [15]. From this methodology, it is also possible to incorporate some heteroatoms in the pillars such as Al, Zr or Ti [16,17]. In these structures, Brönsted acid sites are observed in the sheets of smectite while Lewis acid sites are ascribed to the existence of Si-O-M bridges in the pillared structure. In addition, the increase of the specific surface area, modulated pore diameter and narrow pore diameter distribution allows its use of catalytic support [17].

Future perspectives for cationic clays could be based on the increase of the availability of sheets through a controlled delamination, the combination of organic–inorganic interactions, the deposition of nanoparticles, and the design of hierarchical structures, among others.

#### **3. Anionic Clays**

The number of natural anionic clays is fewer than that observed for natural cationic clays. However, these anionic clays also named hydrotalcites or layered double hydroxides can be easily and quickly synthesized in the laboratory [1]. This family of clays have also attracted the scientific community due to its wide range of applications in many fields such as medicine, biochemistry, electrochemistry, photochemistry, and polymers as additives. In addition, this material can also be used as an ion exchanger, an adsorbent, and a catalytic support or catalyst [18].

The most studied and best-known anionic clay are those composed by Mg2+ and Al3+ species. Its elemental base is composed of octahedrons, which are shared through their edges in such a way that each octahedron consists of one Mg2+ in the center of the octahedron and six OH<sup>−</sup> on their vertexes. The partial substitution of Mg2+ by Al3+ causes a charge deficiency on the brucite sheet, which must be counterbalanced by the inclusion on anions, mainly CO<sup>3</sup> <sup>2</sup><sup>−</sup> in the interlayer spacing [18].

The structure of the anionic clays is not limited to Mg2+ and Al3+ species. Thus, Mg2+ cations can be partially or totally replaced by Cu2+, Zn2+ or Ni2+, while Al3+ cations can be partially or totally replaced by Cr3+ or Fe3+ [18]. Traditionally, anionic clays have been synthesized by co-precipitation method, although alternative methods such as mechanochemical, tribochemistry, microwave or sonication methods have been reported to obtain anionic clays with different crystallinity in the last years.

From a catalytic point view, anionic clays display poor catalytic behavior as a consequence to its basicity is very low, which is probably due to H2O blocking the basic sites [19]. However, a thermal treatment causes dehydroxilation in the brucite sheets as well as the removal of CO<sup>3</sup> <sup>2</sup><sup>−</sup> located in the interlayer spacing causes a collapse of its structure leading to their respective mixed oxides. The properties of the obtained oxides depend on M2+/M3+ molar ratio and the chemical composition. Thus, the presence of MgO can provide basic sites while Al2O<sup>3</sup> can provide acid sites. On the other hand, other oxides such as CuO or NiO are reducible in their respective metallic species. Considering the great variety of M2+ and M3+ species, it is possible to synthesize many polyfunctional catalysts from anionic clays [18].

The thermal treatment leads to the formation of small particles. This fact generates high porosity ascribed to the voids between particles, achieving a surface area between 100–300 m2g −1 in such a way that the obtained oxide can also be employed as catalytic support.

The lamellar structure of the anionic clays can also form pillared structures due to the substitution of the CO<sup>3</sup> <sup>2</sup><sup>−</sup> by a bulkier anion such as isopolyanions, heteropolyanions or ferro/ferricyanides. However, the formation of these pillars are not easy, as the anionic exchange capacity is more complex than that observed in cationic clays since the hydrolysis between the -OH groups and the anion can also collapse the layered structure or the low stability of the anionic clays [19]. Considering these disadvantages, the pillared anionic clays may be prepared by different methods such as the exchange of inorganic/organic anions, and the structure reconstruction or direct coprecipitation. In any case, the design of pillared anionic clays is less common than cationic clays [19].

The future perspectives related to the use of anionic clays in the catalysis field are focused on the design of an efficient method to incorporate active metal anions in the interlayer spacing with high regioselectivity. Another parameter that can be developed is the controlled exfoliation of the sheets to obtain materials with high surface area. The incorporation of metal complexes or metal nanoparticles to form functionalized hydrotalcites and to carry out one-pot catalytic reactions is another important point to be addressed.

**Funding:** The authors thanks to RTI2018-099668-BC22 and RTI2018-094918-B-C44 of Ministerio de Ciencia, Innovación y Universidades, and project UMA18-FEDERJA-126 of Junta de Andalucía and FEDER funds for the financial support.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not Applicable.

**Data Availability Statement:** Data sharing not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


## *Communication* **An Efficient Catalyst Prepared from Residual Kaolin for the Esterification of Distillate from the Deodorization of Palm Oil**

**Alex de Nazaré de Oliveira 1,2,3, \*, Irlon Maciel Ferreira 3 , David Esteban Quintero Jimenez 3 , Fernando Batista Neves 3 , Linéia Soares da Silva 4 , Ana Alice Farias da Costa 1,2 , Erika Tallyta Leite Lima 1,2 , Luíza Helena de Oliveira Pires 5 , Carlos Emmerson Ferreira da Costa 1,2 , Geraldo Narciso da Rocha Filho 1,2 and Luís Adriano Santos do Nascimento 1,2, \***


**Abstract:** The distillate from the deodorization of palm oil (DDPO) is an agro-industrial residue, approximately 84% of which consists of free fatty acids (FFAs), which can be used for the production of fatty acid ethyl esters (FAEE). A catalyst (10HPMo/AlSiM) obtained from a waste material, Amazon *flint* kaolin, was applied in the esterification of the DDPO, reaching a conversion index of 94%, capable of maintaining satisfactory activity (>75%) after four consecutive cycles. *Flint* kaolin is therefore proven to be an efficient option in the search for new heterogeneous low-cost catalysts obtained from industrial by-products, contributing to the reduction of environmental impact and adding value to widely available wastes that would otherwise be discarded directly into the environment. Based on the catalytic results, esterification of DDPO using 10HPMo/AlSiM can be a cheaper alternative for the production of sustainable fuels.

**Keywords:** kaolin; mesoporous; heterogeneous catalyst; esterification; waste valorization

#### **1. Introduction**

Since the discovery of the family of mesoporous molecular sieves known as M41S in 1992 [1,2], MCM–41 (*Mobil Composition of Matter No. 41*) has been the most widely studied, due to its widerange of potential applications either as a catalyst [3] or catalytic support [4]. This is due to its combination of elevated surface areas and a well-defined pore size, which can be controlled and stabilized, while it is also easy to obtain [5,6]. The preparation of MCM–41 commonly involves sodium or ammonium hydroxide, hexadecyltrimethylammonium bromide (CTABr) as a driver and silica gel or tetraethylorthosilicate (TEOS) as a source of silica [2,6–8]. However, the use of TEOS for obtaining MCM–41 has toxic effect, and the materials involved in the process are expensive [7,9–11]. For economic and environmental reasons, efforts to find new sources of inorganic silicates containing high levels of silica at a low cost are increasing [12–14].

To circumvent such challenges, the use of alternative sources of silica, such as kaolin, to replace commercial options, applied in the synthesis of mesoporous materials, reduces the toxicity of the process and reduces costs, as they represent a natural raw material,

**Citation:** de Nazaré de Oliveira, A.; Ferreira, I.M.; Jimenez, D.E.Q.; Neves, F.B.; Soares da Silva, L.; Farias da Costa, A.A.; Lima, E.T.L.; de Oliveira Pires, L.H.; Ferreira da Costa, C.E.; Narciso da Rocha Filho, G.; et al. An Efficient Catalyst Prepared from Residual Kaolin for the Esterification of Distillate from the Deodorization of Palm Oil. *Catalysts* **2021**, *11*, 604. https://doi.org/10.3390/catal11050604

Academic Editor: Juan Antonio Cecilia

Received: 15 April 2021 Accepted: 3 May 2021 Published: 7 May 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

that is both inexpensive and widely available [4,15–18]. The preparation of mesoporous aluminosilicates from natural clay minerals has been investigated, due to the high silicon and aluminum content used in the synthesis of these mesoporous materials [15,19,20].

In the Amazon, the production of kaolin generates significant amounts of tailings, among which *flint* kaolin, FK, is deposited in the mine after the exploitation of soft kaolin, generating major environmental impacts, as it mainly comprises clay mineral kaolin, and contains high levels of silicon, aluminum, titanium and iron [21–23]. These low-quality kaolinitic tailings have been used as a raw material in various chemical processes, including as a catalyst [18,22,24–26], catalytic support [4,27], in zeolite synthesis [21,28,29] and as mesopore [15–17].

New heterogeneous catalysts and catalytic supports have been developed for application in the esterification of FFAs, including mesoporous silica [4,15–17,30], clay minerals [4,18,22,24–26,31] and several catalytic supports with heteropoly acids (HPAs) [4,15,30–35]. The HPAs are notable due to their high Brønsted acidity and redox properties [36,37], which favor the protonation of the carbonyl of FFAs during alcoholic esterification [4,15,30,31,34]. Supporting an HPA in a suitable matrix is known to control the strength and distribution of its active sites [36–38], and it can be applied in a wide range of heterogeneous reactions, including the esterification of low-cost raw materials with high FFA levels for the clean and efficient synthesis of mono alkyl esters (biodiesel), and with water as a by-product [4,30,31,33,34,39–44].

Several previous studies have investigated the possibility of using low-cost raw materials [4,16,26,31,45–48] which can potentially be applied in the synthesis of monoalkyl esters [4,30,31,33,34,39–44]. Notable among these is a by-product from the refining of vegetable oil, a distillate from the deodorization of palm oil (DDPO), around 84% of which is made up of FFAs [4,16,26,31]. The use of these FFAs to generate coproducts with a higher added value is a viable option for the mitigation of possible environmental impacts due to the disposal of such waste [49]. In this case, heterogeneous acid catalysts can esterify the FFAs into esters [4,16,26,31,50,51], and offer advantages such as separation and recovery at the end of the process, as well as subsequent reuse capacity, while avoiding the generation of secondary products and toxic effluents [4,30,31,34,40,43,52].

The use of 12-molybdophosphoric acid (H3PMo12O40, HPMo) functionalized on silica (alternative source, FK) with a well-ordered hexagonal arrangement as an efficient acid catalyst for the esterification of eugenol, was recently reported by our group [15]. This study focused on the synthesis, characterization and application of a catalytic material as a contribution to Green Chemistry, since this catalyst comes from residues which are abundant in the Amazon region [15]. Therefore, the present work aimed to apply this mesoporous material, with an ordered hexagonal phase, in addition to describing a purpose for FK, to produce a low cost catalyst, anchored with HPMo in its mesopores [15], in the DDPO ethanolic esterification reaction. The raw materials used were chosen due to their low-cost and wide availability as waste in the Amazon region. Ethanol was used as it is produced from renewable sources, unlike methanol, which is generally derived from fossil sources [4,31,53]. The reaction between ethanol and DDPO (as a source of FFAs) was chosen as both reagents have greener properties and are widely available for the production of renewable FAEE.

#### **2. Results and Discussion**

#### *2.1. Influence of Time on the Esterification Reaction of DDPO*

The duration of the reaction process strongly influences the consumption of reagents and the formation of products of interest. As expected, for the esterification reaction, the initial conversion of 62% obtained 30 min after the start of the reaction gradually increased over time (Figure 1a). The reaction conversion percentage was higher in the 30 to 120 min intervals, with conversions varying from 62 to 94%, respectively. However, from 120 to 150 min, the conversion remained practically unchanged. The results obtained herein after 2 h of reaction were better than those obtained by Pires et al. [4] using 12-tungstophosforic acid supported on metakaolin waste (25%HPW/MK700) to catalyze DDPO, where after 2 h an 83% conversion to FAEE was obtained. Although Aranda et al. [54] achieved a 90% conversion of DDPO in 1 h of reaction using H2SO<sup>4</sup> as a catalyst, the disadvantages of the homogeneous sulfuric acid catalyst are well known, such as its non-reusability and corrosion of equipment characteristics. The results achieved in this study still provided a better reaction time (≤2 h) than other previously published studies [26,34,52,53,55,56].

**Figure 1.** Influence of reaction parameters on esterification: (**a**) reaction time, (**b**) reaction temperature, (**c**) molar ratio DDPO: EtOH, (**d**) Catalyst amount. Reaction conditions: 1:30 molar ratio (DDPO: EtOH), reaction temperature 140 ◦C, reaction time 2 h and 5% catalyst.

#### *2.2. Influence of Temperature on DDPO Esterification Reaction*

The effect of the reaction temperature was investigated, and the results are shown in Figure 1b. The reaction was studied by varying the temperature from 120 to 150 ◦C without altering the other experimental conditions. An 80% increase in conversion was observed at 120 ◦C. At higher temperatures, the reaction attained equilibrium. with a conversion of 94% at 140 ◦C. These results show that higher temperatures increase the kinetic energy of the molecules, which accelerates the reaction, facilitating the mass transfer between the reactants and the catalyst surface [57]. Therefore, the ideal reaction temperature was determined to be 140 ◦C, lower than that required for other catalysts (150 to 210 ◦C) in similar reactions [4,26,31,33,34,44,52,53,56,58].

#### *2.3. Influence of the Molar Ratio of DDPO and Ethanol in the Esterification Reaction*

Theoretically, the esterification reaction occurs at a stoichiometric ratio of 1 mol of FFA to 1 mol of alcohol, generating 1 mol of ester and 1 mol of water. As the esterification reaction is reversible (hydrolysis), an excess of alcohol is required to drive the reaction towards ester formation and avoid hydrolysis [4,22,24,26,31,51].

To study the influence of the substrate molar ratio, the reaction was carried out by varying the DDPO: EtOH molar ratio from 1:10 to 1:40. According to Figure 1c, the conversion of DDPO increased with the molar ratio, reaching a maximum value of 94% with the DDPO: EtOH molar ratio of 1:30. As the molar ratio increased, however, an appreciable increase in conversion was not observed, which can be attributed to the occurrence of the reverse hydrolysis reaction of part of the esters produced in the presence of excess ethanol [4,30,31]. Thus, the 1:30 molar ratio was determined to be ideal for the esterification reaction, achieving maximum DDPO conversion. It should be stated that lower proportions, such as 1:10 and 1:20, were also used, attaining 80 and 85% conversion, respectively, showing that the 10HPMo/AlSiM catalyst maintained a strong catalytic performance. This can reduce the production costs of the process on a nonlaboratory scale, as less ethanol would be needed to promote the reaction. Even so, studies have shown that a high molar ratio of 1:30, 1:40 or 1:90 is necessary to shift the reaction balance to a direct basis, in order to achieve maximum conversion [27,31,33,34,37,38,40].

#### *2.4. Influence of Amount of Catalyst on DDPO Esterification*

Control experiments with AlSiM and HPMo were also carried out under optimized conditions. At first, the esterification reaction of DDPO with EtOH without a catalyst exhibited a conversion close to 20%, while the esterification reaction in the presence of AlSiM did not reach 23%. Therefore, AlSiM alone is unable to promote a high conversion during the esterification of DDPO, indicating that the catalytic activity is mainly due to the HPMo present in AlSiM, which makes 10HPMo/AlSiM highly active. The same reaction was carried out with the active amount of HPMo (64 mg) which converted 90% of DDPO in homogeneous esterification.

Figure 1d shows the variation in the conversion of DDPO based on the amount of catalyst during ethanolic esterification. The effect of the amount of the 10HPMo/AlSiM catalyst was investigated by the variation in the dosage, which varied from 1 to 6% by mass, based on DDPO mass. It was observed that the DDPO conversion increased from 41 to 94% as the catalyst dosage increased from 1 to 5%. A 4% load can be used without drastically reducing the conversion of DDPO (86%), which shows the efficiency of the catalyst. An additional increase in the amount of catalyst resulted in not significant changes in the conversion of DDPO, indicating that 5% is the catalyst dosage that ensures the appropriate number of active acid sites for the esterification of DDPO.

#### *2.5. Comparison with Data from Literature*

A comparison with other types of heterogeneous and homogeneous catalysts used in the esterification of FFAs described in literature was performed (Table 1). The results obtained in this study indicate that the catalyst has a strong potential for use in the esterification reaction of DDPO, a low-quality organic matter (see Table 1). The 94% observed value for the conversion to esters during the esterification of DDPO with EtOH using the 10HPMo/AlSiM catalyst is close to the values of 62% [59], 90% [54] and 99.9% [45] achieved by sulfuric acid as a catalyst, a substance which has well-known disadvantages. Similar results were obtained using solid acid catalysts such as CsHPW/MCM (92%) [60], MF9S4 (<93%) [26], BLMW (<94%) [31], with even better results than those reported for MP-S-16 (15) (<82%) [30], 25% HPW/MK700 (83%) [4] and CrWO<sup>2</sup> and CrWTiO<sup>2</sup> (<86%) [52,56]. However, the use of chromium is not environmentally friendly in comparison with the aforementioned HPMo material. H3PMo/Al2O<sup>3</sup> (97%) [33], AM41–2H–O (98%) [16], HPMo/Nb2O<sup>5</sup> (99.9%) [34] were better than the results achieved in the present study. Although all of the catalysts mentioned are reasonably efficient in the esterification

reactions of FFA residues, with high conversion to esters, in most cases, higher temperatures (≥140 ◦C) and longer reaction times (≥2 h) than those of the present study were necessary. In addition, the cost of the preparation of our material modified with HPMo was low, since it used kaolinitic residue as a precursor to aluminosilicate, and we still believe that it is more environmentally friendly.

**Table 1.** Comparison of the catalytic activity of 10HPMo/AlSiM with other catalysts for the esterification of different free fatty acids.


<sup>a</sup> Molar ratio; <sup>b</sup> Temperature; <sup>c</sup> Time; <sup>d</sup> Conversion of FFA, Methanol = MeOH.

Therefore, the reaction of esterification of DDPO with EtOH is accelerated by HPMo as an active species (Brønsted acid sites) in the structure of AlSiM, making the catalyst reasonably effective. The results obtained using 10HPMo/AlSiM as a catalyst for the esterification of DDPO with EtOH are largely satisfactory, as the results are similar or superior to those presented in literature using different reagents and catalysts (Table 1).

Finally, it must be noted that the costs of production of the catalyst from Amazonian FK are encouraging. The use of this waste as a raw material for a low-cost silica source for the production of catalytic supports proved to be quite feasible [15–17]. While in the reaction EtOH was also used as a solvent and DDPO by-product as FFAs, all are raw materials that have greener properties for sustainable FAEEs production [31]. Thus, the use of industrial by-products, such as FK and DDPO, in the synthesis of new mesoporous material and FAEEs makes production sustainable, reduces the environmental liabilities caused by its disposal and adds economic value to waste.

#### *2.6. Catalyst Deactivation and Recyclability*

Recovery, stability and reuse are important aspects of heterogeneous catalysts [30,34,37]. The reuse of 10HPMo/AlSiM in the esterification reaction was analyzed for four successive catalytic cycles under the optimal reaction conditions established in this study. The results in relation to the repeated use of the catalyst are shown in Table 2.

Table 2 also shows the results obtained for DDPO conversion using the recycled catalyst. A gradual reduction in DDPO conversion, from 94 to 74.6% in the fourth cycle, was observed, results which are still superior to those of the autocatalysis (20%), indicating that the material can be reused for several cycles and retain the potential to protonate FFAs. The TOF values followed the same trend. This reduction in catalytic activity can be attributed to factors such as loss of mass or even the blockage or destruction of pores by remaining impurities, even after the purification stage (filtration, washing and drying), in addition to the leaching of the active species of the support shown in Table 2 (as no amounts of HPMo were added to the catalyst) [15,30,31,34]. The leaching of the active species (Mo) from the reused catalyst was monitored by the XRF and UV−vis techniques [4,15,27,31,61].


**Table 2.** Properties of acidity, leaching of active species and catalyst conversion during recycling.

<sup>a</sup> Dry mass of the catalyst recovered after each reuse cycle for 2 h at 140 ◦C (the reagents were always recalculated to maintain the same conditions: DDPO: EtOH = 1: 30 and 5% catalyst); b surface acidity calculated by titration; <sup>c</sup> XRF of the catalysts after tests; <sup>d</sup> mass of HPMo anchored in AlSiM measured by UV−vis; <sup>e</sup> mass of leached HPMo detected by UV−vis; <sup>f</sup> percentage of leaching in the reaction medium; g conversion of DDPO; <sup>h</sup> TOF (turnover frequency) = ((Conversion% × moles of DDPO fed) / (No. of mol of Mo species) × (reaction time)); No. of moles of Mo species = (mass of HPMo / 1825.25) × (95.95 / 1825.25) [15,30,31], according to the results of the UV−vis analysis; R: refers to the reused catalyst.

> The results of XRF analysis of the reused catalysts showed that the percentage content of MoO<sup>3</sup> in the new and reused catalysts was practically the same, until the third reuse cycle (see Supplementary Materials). Analysis of the reaction mixtures by UV−vis confirmed there was leaching of active species (Mo) of close to 3% in all the reuse cycles. This can be attributed to the loss of active sites during friction in the reactor during the reaction, and also during the recovery process [15,30,31,34]. These observations ensure that HPMo leaching from AlSiM support is within acceptable limits (3%) [15,27,31]. The acidity values of the reused catalyst measured by titration revealed a decline after each run, in comparison with the new catalyst, which was consistent with the declining values of the DDPO conversion (Table 2).

> To verify the integrity of the catalyst after successive esterification reactions, it was analyzed by the XRD, DRS and FTIR techniques after a fourth reuse cycle (Figure 2a–c). Figure 2a shows the XRD of the 10HPMo/AlSiM and 10HPMo/AlSiM R4 catalysts. There was a clear reduction in peak intensity corresponding to reflection (100), followed by the absence of reflection (110, 200 and 210), a strong indication that the hexagonal mesostructure had become disordered, [4,15,62], but was still preserved [4,62]. This was expected, since the reactions were conducted under aggressive mechanical agitation and temperature conditions [15,30,37,62].

**Figure 2.** (**a**) Comparison of XRD standards, (**b**) DRS spectra, (**c**) FTIR spectra of the new catalyst and after the fourth reuse cycle. R: refers to the reused catalyst.

In the DRS absorption spectrum of 10HPMo/AlSiM–R4 shown in Figure 2b, broad and less intense bands appeared at 220 and 324 nm, indicating the presence of HPMo and the stability of the catalyst after the four reaction cycles [15,62–65]. Analyzing the FTIR spectra of 10HPMo/AlSiM–R4 in Figure 2c, there was a marked presence of bands in the 800 to 1100 cm−<sup>1</sup> range, characteristic of HPMo with a Keggin structure, which was maintained after the fourth catalytic cycle. Some new prominent adsorption bands, such as at 2932 and 2852 cm−<sup>1</sup> , were attributed to the symmetrical stretching of the CH<sup>3</sup> bonds, while a very discrete band, at 1465 cm−<sup>1</sup> , was attributed to the asymmetric CH<sup>3</sup> deformation. The adsorption of organic molecules in the reused catalyst can be clearly seen, and may involve impurities such as triglycerides and unsaponifiable matter present in around 16% of the DDPO [4,16,26,31]. The adsorption of these molecules on the catalyst surface contributed to the reduction of catalytic activity [31,66,67]. This is in line with studies that demonstrated that catalysts with PAHs or other anchored solid acids could be recycled and have been found to have effective recycling capacity [4,15,30,34,38].

Finally, through the use of FRX, UV–vis, DRX, DRS and FTIR techniques, it was observed that active species were still present in the material, and it was concluded that the hexagonal structure of the mesopore was preserved after the recyclability tests. From previously published works [15,19,20] and the results achieved here with 10HPMo/AlSiM, it is possible to predict the use of AlSiM, synthesized at low cost, as a catalytic support to be applied in other types of organic transformation reactions, operating in a predominantly heterogeneous phase.

#### **3. Experimental Section**

#### *3.1. Materials*

All chemical reagents and solvents were analytical grade and used without further purification. DDPO is a residue (viscosity at 60 ◦C = 12.296 mm<sup>2</sup> s −1 ; density at 60 ◦C = 0.862 g mL−<sup>1</sup> ; water content < 0.5%; oxidative stability > 150 h; acidity index = 177.15 mg KOH g−<sup>1</sup> ) consists of 84.0-wt% free fatty acid (FFA) (42% palmitic, 41% oleic, 10% linoleic, 5% stearic, 2% lauric and 1.5% myristic), 12-wt% triglycerides, diglycerides and monoglycerides and 4-wt% unsaponifiable matter [4,16,26,31], kindly donated by Companhia Refinadora da Amazônia, Agropalma S/A (Brazil). *Flint* kaolin of the Capim River Region (Pará, Brazil) was used as Si and Al source and was kindly supplied by the Institute of Geology-UFPA, ethanol (EtOH, 98%, synthetic grade, Nuclear, São Paulo, SP–Brazil), hydrochloric acid (HCl, 37%, Fmaia, Belo Horizonte, MG–Brazil) and sodium hydroxide (NaOH, VETEC, Rio de Janeiro, RJ–Brazil). The preparation and characterization of the 10HPMo/AlSiM were described in a previous work [15].

#### *3.2. Characterization of Fresh and Reused Catalyst*

The chemical compositions of the samples were obtained with Shimadzu EDX-700 energy dispersive X-ray spectrometer (EDX; EDX-700, SHIMADZU, Kyoto, Japan), with a rhodium X-ray source tube (40 kV, SHIMADZU, Kyoto, Japan). For each analysis, approximately 500 mg (powder) of each sample was deposited in a lower sample holder made of polyethylene film in order to determine the MoO<sup>3</sup> content present in the fresh and reused catalyst.

X-ray diffraction analysis were performed on a Bruker D8 Advance diffractometer (Bruker D8Advance; Bruker Corp, Billerica, MA, USA), using powder method, at a 1 ◦ < 2θ > 10◦ interval. Cu Kα (λ = 1.5406 Å, 40 kV e 40 mA) radiation was used.

Diffuse ultraviolet−visible reflectance spectroscopy (DRS) readings were recorded, in the range of 200–500 nm, on a Shuimadzu UV−vis model ISR-2600 Plus spectrophotometer (EDX; EDX-700, SHIMADZU, Kyoto, Japan).

Fourier transform infrared spectroscopy (FTIR) spectra were obtained from a spectrophotometerof Shimadzu (Kyoto, Japan) model IRP Prestige-21A with a resolution of 32 and 100 scans and analyzed by Thermo Electron Corporation, IR 100 model with a resolution of 4 and 32 scans. For the analysis of all materials KBr pellets were used and the spectra were obtained in the region 4000–400 cm−<sup>1</sup> .

The leaching of the HPMo catalysts was performed in UV−vis equipment of Thermoscientific (Waltham, MA, USA), model Evolution array UV−vis spectrophotometer, with 200 to 600 nm scan and 30 scan resolution. The liquids were placed in a quartz tube. The quantification of the HPMo leached in the reaction medium was made using the analytical curve constructed from the postreaction solution (1 to 5 mg L−<sup>1</sup> of HPMo), which was diluted with 0.1 mol L−<sup>1</sup> HCl to avoid any hydrolysis anion [PMo12O40] <sup>3</sup>−, with an absorbance equation (y = 0.0999 + 0.0025) of λmax = 310 nm and an excellent correlation coefficient (R<sup>2</sup> = 0.9999) based on our previous studies [15,31].

#### *3.3. Catalytic Activity*

Prior to the experiments, the catalysts were activated at 130 ◦C for 2 h. Tests of the catalysts were conducted in one run on a PARR 4871 multi-reactor (Parr Instrument Company, Moline, IL, USA). In a typical experiment, the DDPO was mixed with alcohol at a molar ratio of 1:10, 1:20, 1:30 and 1:40 (DDPO: EtOH) and 1, 2, 3, 4, 5 and 6% *w/w* of the solid acid catalyst (as compared to the mass of DDPO). The reaction mixture was stirred (500 rpm) and heated from room temperature to 120, 130, 140 and 150 ◦C. Once the desired temperature was reached, the system was maintained for 30, 60, 90, 120 and 150 min, considered to be the kinetic contact time. At the end of the reaction, the catalyst was separated from the reaction medium by vacuum filtration, and the excess methanol and water produced were removed by evaporation at 120 ◦C. The percentage conversion of DDPO to the corresponding ester was estimated by acid measurement of the product by titration with 0.1 mol L−<sup>1</sup> hydroxide, according to the methods described in the literature. [4,16,26,31]. Reaction parameters such as time, molar ratio, temperature, and catalyst loading were optimized and evaluated. In addition, the recyclability of the catalyst was assessed in the DDPO esterification reaction under the same conditions listed above. At the end of each reaction cycle, the catalyst was filtered under vacuum, washed with 50 mL of ethanol to remove residues and dried at 150 ◦C for 12 h. The catalyst was reused in four reaction cycles.

#### **4. Conclusions**

Experimental investigations revealed that the 10HPMo/AlSiM catalyst exhibited excellent catalytic activity during the esterification reaction of DDPO with ethanol, reaching a conversion of 94%. The XRF, XRD, DRS and FTIR characterization results for this material confirmed the preservation of HPMo after the fourth cycle of reuse in the reaction, where it exhibited a conversion rate of over 75%. The use of two industrial by-products—*Flint* kaolin for the synthesis of mesoporous material and DDPO for the production of ethyl esters—enables sustainable production, reducing possible environmental impacts arising from their provision, in addition to adding economic value to such residues. The results obtained were comparable with previously published results on the use of this reaction and the availability of FK makes this material a promising alternative to those already used. Thus, the results obtained in the present study encourage the search for varied applications for both the 10HPMo/AlSiM catalyst and AlSiM support in oxidation reactions and heavy metal removal, among others.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/article/10 .3390/catal11050604/s1, Elemental analysis by XRF of Table 2.

**Author Contributions:** Conceptualization, L.A.S.d.N.; methodology, C.E.F.d.C. and G.N.d.R.F.; formal analysis, A.d.N.d.O., L.H.d.O.P., F.B.N., D.E.Q.J., L.S.d.S., A.A.F.d.C., and E.T.L.L.; investigation, A.d.N.d.O., E.T.L.L. and L.H.d.O.P.; resources, I.M.F., D.E.Q.J. and C.E.F.d.C.; data curation, L.A.S.d.N.; writing—original draft preparation, A.d.N.d.O., L.H.d.O.P., I.M.F., and E.T.L.L.; writing—review and editing, A.d.N.d.O., L.H.d.O.P., F.B.N., D.E.Q.J., L.S.d.S., A.A.F.d.C. and L.A.S.d.N.;visualization, G.N.d.R.F. and L.A.S.d.N.; supervision, C.E.F.d.C., G.N.d.R.F. and L.A.S.d.N.; project administration, L.A.S.d.N.; funding acquisition, G.N.d.R.F. and L.A.S.d.N. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Banco da Amazônia grant number 2018/212 and CNPQ, grant number 432221/2018-2.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The authors would like to thank the laboratories that supported this work: Laboratory of Research and Analysis of Fuels (LAPAC/UFPA), Laboratory of Catalysis and Oil Chemistry (LCO/UFPA) and LABNANO-AMAZON/UFPA. CAPES/UNIFAP, BIORG/UNIFAP and PROPESP/UFPA for the financial support.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


## *Article* **MgF2-Modified Hydrotalcite-Derived Composites Supported Pt-In Catalysts for Isobutane Direct Dehydrogenation**

**Zhen Song, Jiameng Wang, Fanji Liu, Xiqing Zhang, Énio Matusse and Lihong Zhang \***

Department of Catalysis Science and Technology and Tianjin Key Laboratory of Applied Catalysis Science & Technology, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300350, China; songzhens12345@163.com (Z.S.); wangjiameng0216@163.com (J.W.); lfj19961018@163.com (F.L.); zhangxiqing1997@163.com (X.Z.); enio\_matusse@outlook.com (É.M.)

**\*** Correspondence: zlh\_224@tju.edu.cn; Tel.: +86-150-2225-5828

**Abstract:** Here, a simple method was developed to prepare an MgF<sup>2</sup> -modified hydrotalcite-derived composite, which was used as support for the Pt-In catalyst for isobutane direct dehydrogenation. The catalysts, composites, and their precursors were characterized by numerous characterization techniques. The results provided evidence for the MgF<sup>2</sup> promoter effect on the physical–chemical properties and dehydrogenation performance of the supported Pt-In catalysts. The catalyst with MgF<sup>2</sup> shows exceptional isobutene selectivity that can be stabilized at 95%, and the conversion increases from 50% to 58% during the reaction process. Moreover, the existence of MgF<sup>2</sup> plays an important role in the resistance to coke formation and Pt sintering by improving the Pt dispersion, inhibiting the reduction of the In 3+ species, and adjusting the acidity of the catalyst.

**Keywords:** isobutane dehydrogenation; MgF<sup>2</sup> promoter; hydrotalcite-derived composites; supported Pt-In catalysts

#### **1. Introduction**

In recent years, the sharp increase in the global demand for olefins is driven by the rapid growth in the demand for downstream products in the world [1]. Additionally, isobutene, as a raw material of butyl rubber [2], polyisobutene, and other downstream products, has attracted a lot of attention. At present, the direct dehydrogenation of isobutane represents an environmentally friendly and cost-effective preparation method [3,4].

It is well known that Pt is the most effective active metal for dehydrogenation of light alkanes, but it is easy to sinter and has relatively poor stability [5,6]. Some metallic promoters, such as Sn [7–9], In [10–13], Cu [14], Zn [15,16], Ga [17,18], K [19,20], or Ge [21,22], are usually used to enhance the interaction with Pt from the electronic and geometric aspects so as to resist coke deposition, suppress Pt sintering, and improve the catalytic performance. In addition, the non-metallic promoters, involving element B [23], F [24,25], Cl [26], and P [27], are usually applied to adjust the acid sites and promote the dispersion of active sites on the surface of catalysts. In general, these promoters can not only modify the surrounding environment of the Pt active sites of catalyst, but also adjust some properties of the supports.

The support materials can also influence the catalytic performance, and a lot of studies have been conducted on the support materials, such as Al2O<sup>3</sup> [9,28,29], MgO [30], SiO<sup>2</sup> [27,31], ZrO<sup>2</sup> [15,23,32], and spinel ZnAl2O<sup>4</sup> [7,33,34], for isobutane dehydrogenation catalysts. Now, the focus has been switched to calcined hydrotalcite or hydrotalcitelike (HT) composites, which have been used in direct dehydrogenation of propane and have good performance compared to other supports [10–12]. Calcined hydrotalcite or hydrotalcite-like (HT) materials are the typical composite metal oxides [35–37]. These have suitable surface acidic characteristics and high specific surface area, which is conducive to

**Citation:** Song, Z.; Wang, J.; Liu, F.; Zhang, X.; Matusse, É.; Zhang, L. MgF2-Modified Hydrotalcite-Derived Composites Supported Pt-In Catalysts for Isobutane Direct Dehydrogenation. *Catalysts* **2021**, *11*, 478. https://doi.org/10.3390/ catal11040478

Academic Editors: Juan Antonio Cecilia and Carmen Pilar Jiménez Gómez

Received: 16 March 2021 Accepted: 6 April 2021 Published: 8 April 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

the adsorption of alkanes and the desorption of alkenes and enhances Pt particle dispersion. Among recent studies, some have reported that PtIn catalysts with calcined MgAl hydrotalcite-like as supports compared to spinel as supports exhibited high activity and better selectivity in propane dehydrogenation reaction processes [12]; others reported that Pt-based catalysts substituting Al with In cation on calcined hydrotalcite-like supports also displayed excellent performance of alkanes dehydrogenation [38]. In particular, as far as we know, no report discusses the catalytic performance of Pt-In catalysts supported on MgF2-modified calcined hydrotalcite-like carriers in isobutane direct dehydrogenation.

In our work, we successfully synthesized the MgF2-modified HT-derived composite supported Pt-In catalyst, which exhibited great catalytic performance. The synthesis process includes hydrothermal, alkali-etching, calcination, and impregnation of Pt and In precursors, together with calcination and reduction pretreatment. To discuss the relationships of the isobutane dehydrogenation performance of catalysts with the physicochemical properties, numerous characterization techniques were employed for the as-prepared and spent catalysts.

#### **2. Results and Discussion**

#### *2.1. Characterization of Composite Supports and Catalysts* 2.1.1. The X-Ray Diffraction (XRD)

Figure 1A,B shows the X-ray diffraction (XRD) patterns of the support composites and corresponding supported Pt-In catalysts with calcining and reducing treatment. The characteristic peaks of HT phase (JCPDS file No. 51-1525) are observed in the composites (Figure 1A). Obviously, the HT phase is the only crystalline phase for the reference HT composite. At the same time, an additional MgF<sup>2</sup> phase (JCPDS file No. 41-1443) can be detected in the HT-MgF<sup>2</sup> composite, followed by the decrease in HT diffraction peak intensity. After calcination and reduction (see Figure 1B), the diffraction peaks of the periclase MgO phase (JCPDS file No. 45-0946) appear. However, the diffraction peaks of the Pt and In species cannot be found. This arises from their small particle size and/or low concentration below XRD detection limit, indicating that Pt and In particles are well dispersed on the supports.

**Figure 1.** X-ray diffraction (XRD) patterns of (**A**) the support composites and (**B**) the calcined and reduced catalysts.

#### 2.1.2. N2-Adsorption–Desorption Isotherms

The textural properties of the catalysts were characterized by a low-temperature N<sup>2</sup> adsorption–desorption technique, and the results are depicted in Figure 2A,B. As shown in Figure 2A, the isotherms exhibit the type IV curves with the H2 hysteresis loops indicating the characteristics of the hierarchical mesoporous structure. The corresponding pore size distributions are broad and mainly concentrated in the range of 3–30 nm, further confirming the hierarchical mesoporous feature. Compared with the textural properties of PtInHTC, PtInHTC-MgF<sup>2</sup> exhibits an increase in SBET of 216 m<sup>2</sup> ·g −1 , D<sup>p</sup> of 4.9 and 12.3 nm and Vp. Additionally, the SBET of PtInHTC-MgF<sup>2</sup> is higher than that of the previous

dehydrogenation catalysts [13,38,39]. This means that the pore channel of PtInHTC-MgF<sup>2</sup> can provide more surface and space for adsorption and reaction of isobutane.

−

**Figure 2.** (**A**) Low temperature N<sup>2</sup> adsorption–desorption isotherms, (**B**) pore size distributions curves, and textural data of the calcined catalysts, involving BET special surface area (SBET), the most probable pore size determined by the BJH method (Dp), and total pore volume (Vp).

2.1.3. The Scanning Electron Microscopy (SEM) and the Transmission Electron Microscopy (TEM)

The morphologies of calcined catalysts are described by SEM images in Figure 3. Overall, the typical mesoporous morphology can be found for these composites. The calcined sample PtInHTC without MgF<sup>2</sup> mainly presents the large block mass particles [11]. The catalyst PtInHTC-MgF<sup>2</sup> shows that the abundant well-defined triangular pore channels are constructed by intersecting nanosheets. This means that the MgF<sup>2</sup> plays a key role in tuning the morphology and pore structure of catalysts. The main reason is that the presence of F- anions can activate the substrates to liberate more metal ions for nucleation and growth to obtain interconnected nanosheets in the synthesis process [40]. Figure 3C–F gives TEM images and particle size distribution (PSD) of the reduced catalysts. Their PSD are narrow, and the Pt (111) plan from Pt particles can be found on the reduced catalysts according to the lattice spacing of 0.226 nm, although there is no peak of metal Pt in the XRD phase (Figure 1B). These indicate the metal particles are well dispersed on these catalysts. It is important to point out that the average particle size decreases from 1.3 nm of PtInHTR to 1.2 nm of PtInHTR-MgF2, with a simultaneous narrowing of PSD. This can be attributed to the additional dispersion effect of MgF<sup>2</sup> on active metals. The small size of active metals is more favorable for the dehydrogenation reaction because the small active metals are less active for cracking reaction and deep dehydrogenation [41].

**Figure 3.** SEM images of PtInHTC (**A**) and PtInHTC-MgF (**B**), TEM micrographs of catal **Figure 3.** SEM images of PtInHTC (**A**) and PtInHTC-MgF<sup>2</sup> (**B**), TEM micrographs of catalysts PtInHTR (**C**,**D**), andPtInHTR-MgF<sup>2</sup> (**E**,**F**).

#### 2.1.4. The Temperature-Programmed Reduction (H2-TPR)

The H2-TPR results in Figure 4 show the reducibility of catalysts PtInHTC and PtInHTC-MgF2. It can be clearly seen that the catalyst PtInHTC exhibits a wide reduction peak with the maximum value at 466 ◦C (peak I) and shoulder peak at 560 ◦C (peak II), while the three relative separated peaks are mainly at 460 ◦C (peak I), 550◦C (peak II), and 634 ◦C (peak III) for catalyst PtInHTC-MgF2. According to the previous literature [12,42,43], peak I is attributed to the reduction of PtO2, and peak II can be related to the co-reduction of the Pt and In species. The formation of the peak III may be due to the removal of a small amount of surface hydroxyl. In addition, it can be found that the lower reduction temperature of peak I and peak II can be obtained for the catalyst PtInHTC-MgF2. This indicates that the formation of MgF<sup>2</sup> can reduce the reduction temperature of the Pt species to a certain extent. In other words, the weaker interaction between the Pt species and

supports can be achieved when the MgF<sup>2</sup> species modified the supports. Additionally, it can be seen that the Pt species can be reduced before 600 ◦C for two catalysts.

**Figure 4.** H<sup>2</sup> -TPR profiles of the catalysts.

2.1.5. X-Ray Photoelectron Spectroscopy (XPS)

The surface elemental compositions and chemical states of In, Mg, and F elements on the reduced catalysts were analyzed using X-ray photoelectron spectroscopy (XPS), and the XPS spectra of whole survey, In 3d, Mg 1s, and F 1s regions are shown in Figure 5, with a summary of the binding energy (BE) and ratio of In3+/In<sup>0</sup> for the samples in Table 1.


**Table 1.** XPS results of the In 3d regions for the reduced catalysts.

<sup>a</sup> Calculated from the corresponding fitting peak area.

In Figure 5A, it can be seen that the F element is exactly detectable in PtInHTR-MgF2, compared with the sample PtInHTR. According to the results of XRD above, this further demonstrates the existence of MgF<sup>2</sup> on PtInHTR-MgF2. To explore the metal–support interaction in depth, the XPS spectra are mainly focused on the In 3d regions instead of the Pt 4f regions owing to the overlapping of the Pt 4f and Al 2p region peaks [44]. As shown in Figure 5B, the broad In 3d peak in the range of 440–460 eV can be deconvoluted into four peaks, which refer to two In species on the surface of PtInHTR-MgF2. The low BE value is attributed to the zero-valent In (In<sup>0</sup> ), and the high BE is ascribed to the oxidation state of the surface In species (In3+). As listed in Table 1, the ratio of In3+/In<sup>0</sup> of PtInHTR-MgF<sup>2</sup> is higher than that of PtInHTR, indicating that the presence of MgF<sup>2</sup> can inhibit the reduction of In3+ ions on the surface to avoid the formation of a PtIn alloy. Compared with PtInHTR and PtInHTR-MgF2, the same BE values for the different In species indicate that there is no electron transfer between the In species and MgF2. Accordingly, it can be deduced that the smaller amount of In<sup>0</sup> species should be due to the coverage of MgF<sup>2</sup> resulting in the difficult reduction of In3+ species. Usually, it is proposed that the In3+ species are favorable to dehydrogenation reaction, in view of the blockage of the active Pt sites by the In<sup>0</sup> species [10,13,45].

Then, Figure 5C illustrates the Mg 1s XPS spectra of the samples, and it can be observed that Mg species present in two chemical sates. The peaks appearing at BE of 1304.1 eV and 1305.2 eV can be attributed to MgO and MgF<sup>2</sup> species in the reduced catalysts, respectively [46]. Moreover, according to the deconvolution of the spectra of F 1s (in Figure 5D), we can see two relevant fitted peaks, representing two different coordination states of the F species. The peak of F 1s at 686.0 eV comes from the saturated MgF2, and the peak with BE of 685.4 eV is attributed to F bound to under-coordinated Mg, namely, four- and five-fold coordinated, which is responsible for the Lewis acid sites [47]. Usually, the small MgF<sup>2</sup> particles are deemed to be the reason of the formation of the under-coordinated Mg and even weak acid sites [47]. However, the weak acid sites are favorable for coking-resistance in the dehydrogenation reaction. Therefore, it is reasonable to conclude that the formation of MgF2, especially the under-coordinated Mg species in MgF2, significantly affects the acidity and stability of catalysts and facilitates the resistance to coking and sintering.

#### *2.2. Catalytic Dehydrogenation Performance of Catalysts*

Figure 6 depicts the isobutane conversion, isobutene yield, and selectivity of isobutene and by-product methane over the reduced catalysts in the isobutane dehydrogenation reaction for 9 h. As can be seen from Figure 6A, the catalysts PtInHTR and PtInHTR-MgF<sup>2</sup> exhibit a rapid loss in conversion during the first 30 min and then attain a period of stable conversion throughout the dehydrogenation test. In detail, the catalyst PtInHTR gives the lowest conversion, while the conversion of PtInHTR-MgF<sup>2</sup> increases from 50% to 58% within 9 h. That is to say, the formation of MgF<sup>2</sup> really enhances the activity of the catalysts. It can be assigned to the special pore channels and surface features. From Figure 6B, it can be found that the isobutene selectivity of the catalyst PtInHTR-MgF<sup>2</sup> can be stabilized at 95% during the reaction process. Correspondingly, the catalyst PtInHTR exhibits declining isobutene selectivity. In addition, the selectivity of by-product methane is less than 5% and much lower than the corresponding isobutene selectivity. In particular, for catalyst PtInHTR-MgF<sup>2</sup> the by-product methane is almost completely inhibited during the reaction. This indicates that the MgF2-modification can inhibit the cracking reaction and improve the selectivity and stability of catalysts. Additionally, it is clear that the isobutene yield of PtInHTR-MgF<sup>2</sup> is no less than 55% and is much higher than that of PtInHTR. The excellent catalytic dehydrogenation performance is closely related to the properties of the active species, promoters, and supports. The small size of Pt particles [48], stable In2O<sup>3</sup> state [10,45], and suitable acidic properties of the supports [49] on catalysts can greatly improve the activity and selectivity of the catalyst. According to the TEM and XPS analysis above, PtInHTR-MgF<sup>2</sup> has a small active metal particle size, a stable chemical state of the In3+ species, and abundant weak acid sites, which are responsible for resistance to coking and sintering. Therefore, PtInHTR-MgF<sup>2</sup> exhibited high activity and stable selectivity.

**Figure 6.** (**A**) Isobutane conversion and isobutene selectivity, (**B**) isobutene yield and by-product methane selectivity as functions of time. (Reaction conditions: 600 ◦C, 1 atm, H<sup>2</sup> :iC4H<sup>10</sup> = 1:1 (molar ratio), WHSV (iC4H10) = 3 h−<sup>1</sup> , mcat = 0.5 g).

−

In view of the superior dehydrogenation performance of PtInHTR-MgF2, the detailed information compared with previously reported catalysts is collected in Table 2. In terms of conversion and selectivity, it demonstrates that the investigation of the catalyst PtInHTR-MgF<sup>2</sup> is meaningful.


**Table 2.** Comparison of catalytic performance of various catalysts in isobutane dehydrogenation <sup>a</sup> .

<sup>a</sup> From the considered articles, only the best catalytic performance is indexed. <sup>b</sup> Two data are recorded from the initial and the end stage, respectively.

#### *2.3. Characterization of the Spent Catalysts*

#### 2.3.1. Thermogravimetric Analysis (TG-DTA) and the X-ray Diffraction (XRD)

According to the TG curves in Figure 7A, the total mass losses of PtInHTU and PtInHTU-MgF<sup>2</sup> are 60% and 17%, respectively. As expected, the coke deposition can be suppressed by forming MgF<sup>2</sup> micro-crystals. The positive anti-coking ability is mainly related to the small active metals particles and weak acid sites supplied by MgF<sup>2</sup> nanoparticles over PtInHTR-MgF2. From the differential thermal analysis(DTA) peaks of the spent PtInHTU, it can be determined that there are two successive coke combustion regions, representing two different coke deposits. The small DTA peak at the low temperature range is assigned to the amorphous coke, while the big peak at a high temperature of 570 ◦C corresponds to the formation of serious graphitized coke [53]. Interestingly, only a small DTA peak, resulting from the combustion of amorphous coke, can be detected

for PtInHTU-MgF2. This suggests that it is more difficult for the active metal sites on PtInHTU-MgF<sup>2</sup> to be fully covered by the coke deposits and easier to be regenerated than those on PtInHTU. Additionally, from the XRD patterns shown in Figure 7B, the diffraction peaks of carbon at 2θ of 26◦ can be detected for the PtInHTU catalyst, but it is not detected on the PtInHTU-MgF<sup>2</sup> catalyst. This explains that there is a large amount of carbon on the θ θ

**Figure 7.** Thermogravimetric Analysis (TG-DTA) curves (**A**) and XRD patterns (**B**) of the spent catalysts.

#### 2.3.2. SEM and TEM

The formation of coke deposits also can be confirmed by the SEM and TEM images of the spent catalysts (see Figure 8). Firstly, typical flake mesoporous materials can be kept for each spent catalyst, suggesting that there is no significant texture change for these catalysts after reaction. Additionally, more graphibtized coke can be seen on the surface of PtInHTU. As expected, only the granular amorphous coke deposits can be seen on PtInHTU-MgF2, which is consistent with the TG results. By analyzing the particle size distribution of the spent catalysts, it can be found that the average diameters of PtInHTU and PtInHTU-MgF<sup>2</sup> has a slight increase from 1.3 to 2.7 nm and 1.2 to 2.1 nm, respectively. This demonstrates that the anti-sintering ability can be enhanced by introducing the MgF<sup>2</sup> species.

**Figure 8.** SEM images of spent catalysts: (**A**) PtInHTU, (**B**) PtInHTU-MgF<sup>2</sup> ; TEM micrographs of spent catalysts (**C**,**D**) PtInHTU, (**E**) PtInHTU-MgF<sup>2</sup> .

#### **3. Materials and Methods**

#### *3.1. Materials Used*

Mg(NO3)2·6H2O (Analytical grade chemicals, Fuchen Chemical Regents Factory, Tianjin, China), Al(NO3)3·9H2O (Analytical grade chemicals, Fuchen Chemical Regents Factory, Tianjin, China), urea (Analytical grade chemicals, Fuchen Chemical Regents Factory, Tianjin, China), SiO<sup>2</sup> (>98%, TANSAIL Advanced Materials Co. Ltd., Nanjing, China), KF (Analytical grade chemicals, Aladdin Industrial Corporation, Shanghai, China), H2PtCl6·6H2O (Analytical grade chemicals, Mascot Chemical Co. Ltd., Tianjin, China), In(NO3)3·xH2O (Analytical grade chemicals, Aladdin Industrial Corporation, Shanghai, China).

#### *3.2. Synthesis of Composites and Precursors*

The HT-MgF<sup>2</sup> precursors were prepared by using the hydrothermal and alkali-etching method. Firstly, 0.2 g SiO2, 0.02 mol KF, 2.31 g Mg(NO3)2·6H2O, 1.69 g Al(NO3)3·9H2O, and 2.7 g urea were dissolved into 65 mL deionized water and stirred vigorously for 30 min. Then, the mixed solution was poured in a 100 mL Teflon autoclave and maintained at 100 ◦C for 20 h. The as-prepared product was filtered, washed with deionized water to neutrality, and dried in air at 100 ◦C overnight. Finally, 1 g the dried sample was put into 50 mL NaOH solution (1 mol·L −1 ) and stirred for 10 h. The resulting suspension was washed with deionized water to pH = 7, and the solid product was dried overnight at 100 ◦C. The obtained precursor was labeled as HT-MgF2.

The HT samples were prepared under same conditions, except without adding 0.02 mol KF into the initial solution. The corresponding precursor was named as HT.

#### *3.3. Synthesis of Catalysts*

The calcined products were acquired by calcining at 600 ◦C for 4 h with a heating rate of 2 ◦C·min−<sup>1</sup> .The corresponding PtInHTC-MgF<sup>2</sup> catalyst was obtained via the stepwise incipient wetness impregnation method. Firstly, the In-based precursor was obtained by impregnating calcined HT-MgF<sup>2</sup> with In(NO3)3·xH2O aqueous solution at room temperature for 6 h and dried at 120 ◦C for 12 h. After that, the solid was calcined at 550 ◦C for 4 h. At the same time, the same procedure as In impregnation was conducted to introduce the Pt species using H2PtCl6·H2O as a precursor, except for an impregnation time of 2 h. The loading amount of Pt and In was 0.5 wt% and 1.4 wt%, respectively. After drying and calcination, the resulting solids were defined as PtInHTC-MgF2. PtInHTC was prepared in a same manner.

The calcined catalysts were reduced by 5 vol% H2/N<sup>2</sup> at a flow rate of 30 mL·min−<sup>1</sup> and 600 ◦C for 2 h with a heating rate of 5 ◦C·min−<sup>1</sup> to obtain the corresponding reduced catalysts, which were labeled as PtInHTR and PtInHTR-MgF2.

After the reaction of isobutane dehydrogenation to isobutene, the spent catalysts were marked as PtInHTU and PtInHTU-MgF2.

#### *3.4. Precursors, Composites and Catalysts Characterization*

The XRD patterns of samples were collected on a Bruker D8-Focus X-ray diffractometer (Germany) equipped with a Cu Kα radiation (λ = 0.15418 nm).

Low-temperature N<sup>2</sup> adsorption¬–desorption tests were carried on a TriStar 3000 micromeritics apparatus (Micromeritics, Norcross, GA, USA).

The scanning electron microscopy (SEM) images were obtained using a MAIA3 TESAN.

The transmission electron microscopy (TEM) morphologies were observed on a JEM-2100F field-emission transmission electron microscope.

The temperature-programmed reduction (H2-TPR) was carried out by automatic multi-purpose adsorption apparatus (tp 5080 XQINSTRUMENT CO., Tianjin, China).

The X-ray photoelectron spectra (XPS) of catalysts were tested on a Thermo ESCALAB 250Xi (US) using Al Kα radiation.

Thermogravimetric analysis (TG-DTA) was carried out on a DTG-50/50H (PerkinElmer, Waltham, MA, USA).

#### *3.5. Catalytic Dehydrogenation Performance Test*

The isobutane dehydrogenation to isobutene reactions were performed in a fixed-bed continuous-flow reactor at 600 ◦C under atmospheric pressure. The calcined catalyst (0.5 g, 40–60 mesh) was placed into the reactor and reduced at 600 ◦C for 2 h with a heating rate of 5 ◦C·min−<sup>1</sup> in 5 vol% H2/N2. After reduction, the isobutane and hydrogen (the molar ratio of iC4H10:H<sup>2</sup> = 1:1) were introduced into the reactor, in which the weight hourly space velocity (WHSV) of isobutane was 3 h−<sup>1</sup> .The reactions were performed at 600 ◦C, and an online gas chromatograph (GC) equipped with a flame ionization detector (Al2O<sup>3</sup> packed column) was employed to analyze the gaseous products.

#### **4. Conclusions**

In summary, the MgF2-modified hydrotalcite-derived composites supported Pt-In catalyst PtInHTR-MgF<sup>2</sup> can be synthesized by a combination of the hydrothermal method, alkali-etching, and impregnation strategy. The formation of MgF<sup>2</sup> can not only construct the special texture and morphology of catalyst, but also disperse the active metals, inhibit the reduction of the In3+ species, and adjust the acidity of the catalyst. These features can improve the activity and selectivity of isobutane direct dehydrogenation and make the catalyst obtain a high durability and excellent resistance to coking and sintering.

**Author Contributions:** Conceptualization, Z.S. and J.W.; methodology and experiments, Z.S. and J.W.; Statistics and data validation, Z.S. and É.M.; writing—original draft preparation, Z.S.; writing review and editing, F.L. and X.Z.; research ideas, analyzed data, revised the manuscript and funding acquisition, L.Z. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Natural Science Foundation of China (21776214).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Data is contained within the article.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


## *Article* **Easily Recycled CuMgFe Catalysts Derived from Layered Double Hydroxides for Hydrogenolysis of Glycerol**

**Xiaopeng Yu 1,\*, Fubao Zhang 2 , Yi Wang <sup>1</sup> and Dejun Cheng 1**


**Abstract:** A series of CuMgFe catalysts with different (Cu + Mg)/Fe molar ratios derived from hydrotalcites were prepared by coprecipitation for the hydrogenolysis of glycerol to 1,2-propanediol (1,2-PDO). X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectra (XPS), vibrating sample magnetometer (VSM), hydrogen temperature-programmed reduction (H<sup>2</sup> -TPR), CO<sup>2</sup> -TPD, and H<sup>2</sup> -TPD (temperature-programmed desorption of CO<sup>2</sup> and H<sup>2</sup> ) were used to investigate the physicochemical properties of the catalysts. The CuMgFe-layered double oxides (CuMgFe-4LDO) catalyst with (Cu + Mg)/Fe molar ratio of 4 exhibited superior activity and stability. The high glycerol conversion and 1,2-propanediol selectivity over CuMgFe-4LDO catalyst were attributed to its strong basicity, excellent H<sup>2</sup> activation ability, and an increase in the surface Cu content. The CuMgFe catalysts could be easily recycled with the assistance of an external magnetic field due to their magnetism.

**Keywords:** CuMgFe; layered double hydroxides; hydrogenolysis of glycerol; 1,2-propanediol; recycled

#### **1. Introduction**

Biodiesel is considered as a possible new pattern of renewable energy. Large-scale biodiesel production has brought about a surplus by-product of glycerol. Undoubtedly, the conversion of excess glycerol into higher-value chemicals can increase the economic value of the biodiesel industry. Different processes such as oxidation, dehydration, and hydrogenolysis have been proposed for the conversion of glycerol [1–5]. One of the attractive ways is hydrogenolysis to 1,2-propanediols (1,2-PDOs) because 1,2-PDO is widely used as a monomer for antifreeze agent, polyester resins, paints additive, liquid detergent, food, etc. Some results have been reported in selective catalytic hydrogenolysis of glycerol to 1,2-PDO [6–24].

Noble metals such as Rh, Ru and Pt are extensively used in the hydrogenolysis of glycerol owing to their high reactivity [9–15]. Nevertheless, these catalysts usually facilitate excessive C–C cleavage, resulting in a poor selectivity to 1,2-PDO. Cu-based catalysts exhibit high selectivity to 1,2-PDO in the hydrogenolysis of glycerol due to poor activity for C–C bond cleavage and high efficiency for C–O bond hydro-dehydrogenation. Cu–Cr [16], Cu/ZnO [17,18], Cu/Al2O<sup>3</sup> [17,19], Cu/SiO<sup>2</sup> [20,21], Cu/MgO [22,23] catalysts have been reported by several groups. It has been demonstrated that the activity of Cu-based catalysts for hydrogenolysis of glycerol depends strongly on the dispersion and/or the surface area of exposed Cu [8,20,21]. Additionally, the acidity/basicity of Cu-based catalysts also plays an important role in the hydrogenolysis reaction of glycerol [22,24].

Layered double hydroxides (LDHs), also known as hydrotalcite-like compounds, are a class of anionic clay materials that allow the uniform mixing of different bivalent and trivalent cations. Thermal decomposition of LDHs leads to the formation of mixed oxides with small crystal size, basicity, high dispersion, and large specific surface area.

**Citation:** Yu, X.; Zhang, F.; Wang, Y.; Cheng, D. Easily Recycled CuMgFe Catalysts Derived from Layered Double Hydroxides for Hydrogenolysis of Glycerol. *Catalysts* **2021**, *11*, 232. https://doi.org/ 10.3390/catal11020232

Academic Editors: Juan Antonio Cecilia and Carmen Pilar Jiménez Gómez Received: 20 January 2021 Accepted: 7 February 2021 Published: 9 February 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

It has been reported that Cu-based catalysts derived from hydrotalcites are of basicity and dispersed copper particles [8,24–27], which could improve the catalytic performance in the hydrogenolysis of glycerol. Geng et al. [28] disclosed that the Cu–Ca–Al catalyst derived from hydrotalcite is more active and selective for the formation of propanediols. Nonetheless, these catalysts suffer from serious difficulties in the recovery and reuse from the perspective of ecological and economical sustainability. Magnetic catalysts can be easily separated by an external magnetic field, reducing the consumption of auxiliary substances, saving energy and time in separation, and bringing significant economic and environmental benefits. However, to the best of our knowledge, there is no detailed understanding of magnetic Cu-based catalysts derived from hydrotalcites in the hydrogenolysis of glycerol.

The aim of the present work is to develop easily recycled Cu-containing catalysts in the hydrogenolysis of glycerol. Magnetic CuMgFe mixed-oxide catalysts derived from hydrotalcites were prepared by coprecipitation, and the effect of (Cu + Mg)/Fe molar ratio on the activity of the catalysts was discussed in detail.

#### **2. Results and Discussion**

#### *2.1. Characterization Results*

#### 2.1.1. Structure and Morphology of CuMgFe-xLDH and CuMgFe-xLDO

The XRD patterns of CuMgFe-xLDH are shown in Figure 1a. CuMgFe-2LDH, CuMgFe-3LDH, CuMgFe-4LDH, and CuMgFe-5LDH exhibited the characteristic reflections of hydrotalcite. Compared with CuMgFe-2LDH, the sharp diffraction peaks of CuMgFe-3LDH evidenced a better crystallization of the phase of hydrotalcite. Oppositely, CuMgFe-4LDH and CuMgFe-5LDH exhibited a lower crystallization with the increase of (Cu + Mg)/Fe molar ratio. Increasing (Cu + Mg)/Fe molar ratio might lead to the structural distortion and the orderliness decline of hydrotalcite.

**Figure 1.** XRD patterns of (**a**) CuMgFe-xLDH and (**b**) CuMgFe-xLDO.

The XRD patterns of CuMgFe-xLDO catalysts with different (Cu + Mg)/Fe molar ratios are shown in Figure 1b. After calcination at 600 ◦C, all LDHs were transformed to mixed oxides and spinel phase. For all the CuMgFe-xLDO samples, the diffraction peaks at around 30.2◦ , 35.5◦ , 57.1◦ , and 62.7◦ corresponded to the (220), (311), (511), and (440) planes of CuFe2O<sup>4</sup> spinel phase (JCPDS card no. 77-0010), respectively. Meanwhile, the CuO phase (at around 35.5◦ (002) and 58.3◦ (202)) also might be present, but its diffraction peaks were partially overlapped with those of CuFe2O4. Consequently, it was difficult to distinguish in the XRD results. The diffraction peaks at 30.3◦ and 43.3◦ could correspond to the (220) and (400) planes of the Fe2O<sup>3</sup> phase (JCPDS card no. 39-1346), respectively. Additionally, the diffraction peaks at 42.9◦ and 62.3◦ were also observed, which could be associated with the (200) and (220) planes of the MgO phase (JCPDS card no. 04-0829).

The morphology of CuMgFe-3LDH and CuMgFe-4LDH revealed by SEM is shown in Figure 2. CuMgFe-3LDH displayed a layered structure of solid lamellar (Figure 2A), whereas some solid lamellar structures accumulated obviously in CuMgFe-4LDH (Figure 2B). Figure 2C,D shows the images of CuMgFe-3LDH and CuMgFe-4LDH after calcination, which maintained the plate-like morphology of the original precursor. High resolution transmission electron microscope (HRTEM)was also used to reveal the structure of CuMgFe-4LDO sample. A typical HRTEM image of CuMgFe-4LDO showed two identified reflection patterns with interplanar distances of 0.253 nm and 0.205 nm (Figure 2E), corresponding to the (002) plane of CuO phase and (220) plane of CuFe2O<sup>4</sup> phase [29,30], respectively, which was in line with the XRD results.

**Figure 2.** SEM images of the (**A**) CuMgFe-3LDH, (**B**) CuMgFe-4LDH, (**C**) CuMgFe-3LDO, and (**D**) CuMgFe-4LDO. HRTEM image of (**E**) CuMgFe-4LDO.

#### 2.1.2. Magnetic Behavior of CuMgFe-xLDO Catalysts

The magnetic behavior of the catalysts was analyzed using VSM. All catalysts showed narrow S-shape type loops in Figure 3, indicating that all catalysts were ferromagnetic. Magnetic saturation (Ms), remanence (Mr), and coercivity (Hc) calculated from magnetically recorded data are listed in Table 1. The lower saturation magnetization and coercivity values might be due to the presence of CuO and MgO in the catalysts. The ratio of the remanence to the saturation magnetization (Mr/Ms) decreased in the following order: CuMgFe-5LDO > CuMgFe-3LDO > CuMgFe-2LDO > CuMgFe-4LDO, which was related to the inter- and intragrain exchange interactions, sub-lattice magnetization, magnetic anisotropy, and morphology of the tested sample [31]. The lower ratio corroborated its significant superparamagnetic behavior [32].

**Figure 3.** Magnetic hysteresis curves for the CuMgFe-xLDO catalysts.

**Table 1.** The magnetic properties of CuMgFe-xLDO catalysts.


#### 2.1.3. H2-TPR of CuMgFe-xLDO Catalysts

The reducibility of catalysts was investigated by hydrogen temperature-programmed reduction (H2-TPR). As shown in Figure 4, the first shoulder peak (at 205 ◦C, 209 ◦C, 191 ◦C, and 210 ◦C) in the low-temperature range for all the samples was ascribed to the reduction of CuO to Cu. The subsequent peak (at 235 ◦C, 242 ◦C, 225 ◦C, and 246 ◦C) was attributed to the reduction of CuFe2O<sup>4</sup> to metallic Cu and Fe2O<sup>3</sup> [33,34]. The third peak in the range of 300–550 ◦C might correspond to the transformation of Fe2O<sup>3</sup> to Fe3O<sup>4</sup> [35]. The peak in the high-temperature range of 550–900 ◦C for all the samples could be attributed to the continuous reduction of Fe3O<sup>4</sup> to metallic Fe via FeO [33,36]. It should be noted that the reduction temperature of CuO in the CuMgFe-xLDO samples shifted to lower temperature compared with that of pristine CuO (around 290 ◦C) [33], and the reduction peak of CuFe2O<sup>4</sup> in the CuMgFe-xLDO samples decreased significantly in comparison with that of pristine CuFe2O<sup>4</sup> (around 340 ◦C) [33]. The findings indicated that the strong synergistic effect between copper and iron occurred in the CuMgFe-xLDO samples. Furthermore, the reduction peaks of CuO and CuFe2O<sup>4</sup> in the CuMgFe-4LDO catalyst were lower than those of other catalysts, suggesting that the stronger interaction between copper and iron might occur in the CuMgFe-4LDO catalyst. Cu species could be well dispersed on the CuMgFe-4LDO catalyst surface.

#### 2.1.4. XPS Analysis of CuMgFe-xLDO Catalysts

Cu 2p and Fe 2p spectra of CuMgFe-xLDO catalysts are shown in Figure 5a,b. A Cu 2p3/2 main peak at 933.5–933.9 eV was accompanied by a satellite peak at 942.2–942.8 eV, which was related to CuFe2O4. Additionally, all catalysts exhibited a Cu 2p3/2 peak at 931.8–931.9 eV together with a satellite peak at 940.0–940.2 eV, which could be associated with the presence of CuO. A Cu 2p1/2 main peak at 952.2–952.8 eV along with a satellite peak at 961.6–962.0 eV was also observed, in accordance with those of Cu2+ [37,38]. All Fe 2p spectra in Figure 5b showed two main peaks at 710.9–711.1 eV and 724.2–724.5 eV, which belonged to Fe 2p3/2 and Fe 2p1/2, respectively. Two accompanying satellite peaks at the binding energies of 718.5–718.8 eV and 732.3–732.8 eV were characteristics of Fe3+ cations [38–40]. Mg 1s spectra of CuMgFe-xLDO catalysts are also presented in Figure 5c. All catalysts showed a peak at 1303.5–1304.1 eV, which could be due to the presence of MgO [41].

**Figure 5.** XPS spectra of (**a**) Cu 2p, (**b**) Fe 2p, and (**c**) Mg 1s of the CuMgFe-xLDO catalysts.

The surface element composition of CuMgFe-xLDO catalysts is summarized in Table 2. The surface Cu content and surface Cu/(Mg + Fe) atomic ratio of the CuMgFe-4LDO catalyst were slightly higher than those of the other catalysts, suggesting that Cu species could be well dispersed on the CuMgFe-4LDO catalyst. This was in line with the H2- TPR results.

−

−


**Table 2.** The surface element composition and the base sites amounts of CuMgFe-xLDO catalysts.

<sup>a</sup> The values were calculated from XPS; <sup>b</sup> W: T < 200 ◦C; M: 200 ◦C < T < 400 ◦C; S: T > 400 ◦C.

#### 2.1.5. Basicity of Reduced Catalysts

The base properties of the CuMgFe-LDO catalysts were evaluated by the adsorption of CO<sup>2</sup> on the basic sites. The CO2-TPD profiles of CuMgFe-LDO catalysts are presented in Figure 6. There were two obvious desorption domains occurring at 50–400 ◦C and 400–700 ◦C for all the catalysts. The stronger the basic sites of a catalyst were, the higher the desorption temperature of CO<sup>2</sup> was [42]. The low-temperature peaks below 200 ◦C could be attributed to the desorption of CO<sup>2</sup> on weak basic sites, while the desorption peaks in the range of 200–400 ◦C could be ascribed to the desorption of CO<sup>2</sup> on medium basic sites. The peaks at 549–599 ◦C were associated with the desorption of CO<sup>2</sup> on strong basic sites. The weak and medium basic sites corresponded to surface OH<sup>−</sup> and Lewis acid-base pairings, respectively, and the strong basic sites were related to the contribution of low-coordination surface O2<sup>−</sup> [43–45]. Notably, the desorption temperature on strong basic sites of the CuMgFe-4LDO catalyst was significantly higher than those of the other catalysts. It has been reported that dehydrogenation of glycerol leads to glyceraldehyde and it subsequently dehydrates to hydroxyacrolein on basic sites of the catalyst [46]. The strongest basic sites on the CuMgFe-4LDO catalyst might enhance its catalytic performance. As shown in Table 2, the total amount of basic sites decreased in the following order: CuMgFe-4LDO > CuMgFe-3LDO > CuMgFe-5LDO > CuMgFe-2LDO, indicating that the total amount of basic sites of the catalyst increased with the moderate increase of (Cu + Mg)/Fe molar ratio. Conversely, excessive (Cu + Mg)/Fe molar ratio decreased significantly the contribution of medium and strong basic sites, and thereby reducing the total amount of basic sites of the CuMgFe-5LDO catalyst. This could be associated with the morphology, crystal plane, and crystal size of MgO in the CuMgFe-5LDO catalyst. Li et al. found that MgO (100) surface primarily ended with alternant Mg2+/O2<sup>−</sup> ions providing medium basic sites, while MgO (111) surface primarily ended with O2<sup>−</sup> ions providing strong basic sites [47]. It was reported by Marianou et al. that the basicity of MgOs was strongly affected by the morphology, texture, and chemical composition of the materials [48]. Samples with smaller crystal size and higher surface area exhibited a higher total number of basic sites [48]. − −

**Figure 6.** CO<sup>2</sup> -TPD patterns of the reduced CuMgFe-xLDO catalysts.

α

α

β γ

#### 2.1.6. H2-TPD of Reduced Catalysts

H2-TPD profiles of the CuMgFe-xLDO catalysts are shown in Figure 7. In the range of 50–700 ◦C, the desorption of H<sup>2</sup> might be assigned to three different H species. The desorption peak at low temperature (50–350 ◦C, Hα) might be attributed to hydrogen desorption from Cu sites. The desorption peak at high temperature (350–700 ◦C, Hβ, Hγ) could correspond to the hydrogen desorption from two different Fe sites. Apparently, the CuMgFe-4LDO catalyst exhibited the highest H<sup>α</sup> desorption peak temperature, indicating that a stronger metal-hydrogen interaction occurred on the surface of the CuMgFe-4LDO catalyst. This could be due to the high Cu dispersion in the CuMgFe-4LDO catalyst, which afforded more unsaturated coordination centers for the hydrogen adsorption [33]. It has been reported that enhanced H<sup>2</sup> activation ability could improve the activity for glycerol hydrogenolysis [24]. α β γ α

− −

**Figure 7.** H<sup>2</sup> -TPD profiles of the CuMgFe-xLDO catalysts.

#### *2.2. Hydrogenolysis of Glycerol*

2.2.1. Effect of (Cu + Mg)/Fe Molar Ratio on Catalytic Performance of Reduced CuMgFe-xLDO Catalysts

The conversions of glycerol over CuMgFe-xLDO catalysts for the hydrogenolysis of glycerol are summarized in Table 3. With the increase in (Cu + Mg)/Fe molar ratios from 2 to 4, the conversion of glycerol increased from 38.0% to 47.8%. On the contrary, the conversion of glycerol over the CuMgFe-5LDO catalyst decreased. The CuMgFe-4LDO catalyst exhibited the highest glycerol conversion and 1,2-PDO selectivity among all the catalysts.

.


**Table 3.** Hydrogenolysis of glycerol on reduced CuMgFe-xLDO catalysts <sup>a</sup>

<sup>a</sup> Reaction conditions: 8.0 g 75% glycerol solution, 2.0 MPa H2, 180 ◦C, 10 h, 0.60 g reduced catalyst. <sup>b</sup> Ethylene glycol, methanol, ethanol, and 1-propanol.

According to the reaction mechanism from glycerol to 1,2-PDO proposed by Montassier [49], first, dehydrogenation of glycerol on copper would form glyceraldehyde in equilibrium with its enolic tautomer. Then, a nucleophilic reaction of water or adsorbed OH species led to a dehydroxylation reaction. Subsequently, 1,2-PDO was formed by hydrogenation of the intermediate unsaturated aldehyde (2-hydroxy acrolein). Therefore, it can be concluded that the hydrogenolysis of glycerol to 1,2-PDO needs both metal sites for activation of hydrogen and base sites for dehydration. The higher hydrogenolysis activity

over CuMgFe-4LDO catalyst might be due to the following factors. Primarily, the increase in the surface Cu content might be a factor in improving the hydrogenolysis activity of glycerol (Table 2). Furthermore, compared with the other catalysts, the stronger basic sites and the higher amount of basicity (Figure 6 and Table 2) on the CuMgFe-4LDO catalyst favored dehydration reaction of glyceraldehyde and its enolic tautomer. Consequently, the intermediate unsaturated aldehyde (2-hydroxy acrolein) was formed. Finally, on the basis of the H2-TPD results (Figure 7), enhancing H<sup>2</sup> activation on Cu metal sites accelerated hydrogenation of intermediate unsaturated aldehyde (2-hydroxy acrolein), thereby improving the selectivity to 1,2-PDO.

#### 2.2.2. Hydrogenolysis of Glycerol on Reduced CuMgFe-4LDO Catalyst at Different Temperatures

The activity of CuMgFe-4LDO catalyst for hydrogenolysis of glycerol at different temperatures is summarized in Table 4. The conversion of glycerol increased significantly from 47.8% (at 180 ◦C) to 75.3% (at 200 ◦C), suggesting that glycerol hydrogenolysis accelerated with increasing reaction temperature. Nevertheless, the selectivity to 1,2-PDO declined slightly from 97.5% (at 180 ◦C) to 96.5% (at 200 ◦C), indicating that no obvious cleavage of C–C bonds over CuMgFe-4LDO catalyst occurred even at higher temperatures.

**Table 4.** Hydrogenolysis of glycerol on reduced CuMgFe-4LDO catalyst at different temperatures <sup>a</sup> .


<sup>a</sup> Reaction conditions: 8.0 g 75% glycerol solution, 2.0 MPa H2, 10 h, 0.60 g reduced catalyst. <sup>b</sup> Ethylene glycol, methanol, ethanol, and 1-propanol.

#### 2.2.3. Recycled Usage of Reduced CuMgFe-4LDO Catalyst

The recycling procedure of the CuMgFe-4LDO catalyst was performed for examining the stability of the catalyst. The spent catalysts were separated by an external magnetic field. Due to the magnetism for CuMgFe-4LDO catalysts, the catalysts could be easily recycled, as shown in Figure 8. The activity of recycled CuMgFe-4LDO catalyst is summarized in Table 5. The conversion of glycerol decreased slightly from 47.8% (of the fresh catalyst) to 46.9% (in the second recycle), and then it remained stable (in the third and the fourth recycles). After five times of recycling, the conversion of glycerol over the CuMgFe-4LDO catalyst decreased by 3.5%. No apparent weight loss of catalysts was observed after five times recycles.

**Figure 8.** Images of magnetic characteristics and magnetic separation for reduced CuMgFe-4LDO catalyst.


**Table 5.** Hydrogenolysis of glycerol and composition on recycled CuMgFe-4LDO <sup>a</sup> .

<sup>a</sup> Reaction conditions: 8.0 g 75% glycerol solution, 2.0 MPa H2, 180 ◦C, 10 h, 0.60 g catalyst. <sup>b</sup> Ethylene glycol, methanol, ethanol, and 1-propanol. <sup>c</sup> the values were determined by induced coupled plasma-optical emission spectroscopy (ICP–OES).

To explore the reasons for activity loss, the compositions of the reduced catalyst and five-times-recycled catalyst were determined by induced coupled plasma-optical emission spectroscopy (ICP–OES). No obvious leaching Cu was observed after five recycles (Table 5). Meanwhile, to confirm further no Cu leaching, the catalyst was filtered off after 5 h reaction (halfway through the reaction) in the fifth recycle. At this moment, glycerol conversion and the selectivity to 1,2-propanediol were 28.5% and 97.0%, respectively. Then, the filtrate was transferred to a 100 mL stainless steel autoclave. Moreover, hydrogenolysis of glycerol was still performed under no catalyst conditions. Once more, after 5 h reaction, glycerol conversion and the selectivity to 1,2-propanediol did not further increase, indicating that no Cu leached into the filtrate. Furthermore, the XRD results of the reduced catalyst and five-times-recycled catalyst were also analyzed (Figure S1), Cu metal sizes were calculated from the diffraction peaks (220) according to the Scherrer equation. Cu metal size increased from 9.2 nm to 10.8 nm after five recycles, suggesting that the decreased activity of CuMgFe-4LDO catalyst could be due to slight sintering of copper metal.

#### **3. Materials and Methods**

#### *3.1. Preparation of CuMgFe-Mixed Oxides Catalysts*

Four CuMgFe-LDH precursors with different (Cu2+ + Mg2+)/Fe3+ molar ratios were prepared by coprecipitation. The A solution was a 0.2 M aqueous solution containing the nitrates of Cu2+, Mg2+, and Fe3+. The B solution was an aqueous solution of NaOH and Na2CO<sup>3</sup> with a concentration of 0.25 M and 0.8 M, respectively. Solution A and B were added drop-wise into the deionized water with vigorous stirring. During the coprecipitation, the slurry was kept at pH 10.0 ± 0.1 by adjusting dropping rates. The resulting suspension was aged at 60 ◦C for 18 h. The final precipitate was filtered, washed, and dried at 110 ◦C for 12 h. Layered double hydroxides were gained, and they are donated as CuMgFe-*x*LDH (*x* = 2, 3, 4 or 5 according to the (Cu2+ + Mg2+)/Fe3+ molar ratios of 2, 3, 4, or 5). Subsequently, the hydrotalcites were calcined at 600 ◦C for 5 h in air, and the products are designated as CuMgFe-*x*LDO (*x* = 2, 3, 4, or 5). The nominal Cu content is 9 wt% in the CuMgFe- *x*LDO samples.

#### *3.2. Characterization of Precursors and Catalysts*

The X-ray diffraction patterns were detected on a Philips X'pert-PRO diffractometer using Cu Kα radiation (45 kV, 50 mA) (PANalytical, Etten-Leur, Nethelands).

The morphology of the CuFeMg-LDH was investigated using a Quanta 400 FEG scanning electron microscope (FEI, Hillsboro, Oregon, OR, USA) with an accelerating voltage of 20 kV.

Transmission electron microscopy (TEM) images were obtained using a Tecnai G<sup>2</sup> F20 transmission electron microscope (FEI, Hillsboro, Oregon, OR, USA). The samples were ultrasonically dispersed in ethanol.

The reducibility of the catalysts was studied by hydrogen temperature-programmed reduction (H2-TPR) using a fixed-bed reactor. The catalysts were reduced under a 10% H2/Ar mixed gas (30 mL/min) from 50 ◦C to 900 ◦C at a rate of 10 ◦C/min. The hydrogen consumption was analyzed on-line using an SC-200 gas chromatograph (Chuanyi, Chongqing, China) equipped with a thermal conductivity detector (TCD).

The X-ray photoelectron spectra (XPS) were obtained using an XSAM800 spectrometer (Kratos, Manchester, UK) with an Al Ka (hm = 1486.6 eV) X-ray source, and the binding energies were corrected using C (1 s) at 284.6 eV.

The basicity and the H2-activation ability of the reduced catalysts were determined by temperature-programmed desorption of CO<sup>2</sup> and H<sup>2</sup> (CO2-TPD and H2-TPD). The catalysts were reduced at 400 ◦C in 10% H2/Ar for 2 h and then cooled to 50 ◦C in a He flow. Subsequently, CO<sup>2</sup> was fed into the reactor for 0.5 h. Then the catalysts were purged at 50 ◦C with He for 3 h. Finally, the samples were heated linearly to 750 ◦C at a rate of 10 ◦C/min in a He flow. While the desorbed CO<sup>2</sup> was recorded continuously by a TCD detector. H2-TPD was carried out by the same procedure. Only CO<sup>2</sup> was supplanted by the 10% H2/Ar.

The magnetization was characterized by a superconducting quantum interference SQUID magnetometer (Quantum Design, San Diego, CA, USA) with a maximum field of 20 kOe at room temperature. The saturation magnetization (Ms), coercive force (Hc), and residual magnetization (Mr) were measured.

Chemical composition was analyzed by using an induced coupled plasma-optical emission spectroscopy (ICP–OES) analyzer (Spectro Arcos, SPECTRO Analytical Instruments GmbH, Kleve, Germany).

#### *3.3. Catalytic Experiments*

Hydrogenolysis of glycerol was performed in a 100 mL stainless steel autoclave with a mechanical stirrer and an electric temperature controller, operated under H<sup>2</sup> pressure of 2.0 MPa. Prior to reaction, the catalysts were reduced by 10% H2/Ar stream at 400 ◦C for 2 h in a fixed-bed flowed reactor. A total of 8.0 g aqueous solution of 75 wt% glycerol and 10 wt% (based on glycerol) of the catalysts were charged into the autoclave. The liquid products were analyzed by using a Scion 456C GC gas chromatograph (Techcomp, Shanghai, China) equipped with a flame ionization detector (PEG-20M column: 30 m × 0.25 mm × 0.5 µm). The gas products were analyzed by using a Scion 456C GC gas chromatograph equipped with a thermal conductivity detector (TDX-01 column: 3 m × 3 mm).

#### **4. Conclusions**

The CuMgFe-*x*LDO catalysts derived from different (Cu + Mg)/Fe metal ratios were prepared by coprecipitation. The activity of the CuMgFe-4LDO catalyst was higher than those of other CuMgFe-*x*LDO catalysts, and the conversion of glycerol and the selectivity to 1,2-PDO reached 47.8% and 97.5% at 180 ◦C, respectively. The superior catalytic performance of CuMgFe-4LDO was associated with its strong basicity, excellent H<sup>2</sup> activation ability, and an increase in the surface Cu content. The CuMgFe-4LDO catalyst also exhibited good stability. Furthermore, the CuMgFe-*x*LDO catalysts could be easily recycled with the assistance of an external magnetic field due to their magnetism.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/2073-434 4/11/2/232/s1, Figure S1: XRD patterns of the reduced and spent CuMgFe-4LDO catalysts.

**Author Contributions:** Conceptualization, X.Y.; methodology, X.Y. and F.Z.; software, X.Y.; validation, X.Y.; formal analysis, X.Y.; investigation, X.Y. and F.Z.; resources, X.Y. and Y.W.; data curation, X.Y.; writing—original draft preparation, X.Y.; writing—review and editing; X.Y., F.Z., and Y.W.; supervision, X.Y.; project administration, Y.W. and D.C.; funding acquisition, X.Y. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Natural Science Foundation of China (Grant No. 21706168).

**Data Availability Statement:** Data presented in this study are available on request from the corresponding author.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


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