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Article

Highly Efficient Production of DMF from Biomass-Derived HMF on Recyclable Ni-Fe/TiO2 Catalysts

by
Martyna Przydacz
,
Marcin Jędrzejczyk
,
Jacek Rogowski
,
Małgorzata Szynkowska-Jóźwik
and
Agnieszka M. Ruppert
*
Institute of General and Ecological Chemistry, Faculty of Chemistry, Lodz University of Technology, ul. Zeromskiego 116, 90-924 Lodz, Poland
*
Author to whom correspondence should be addressed.
Energies 2020, 13(18), 4660; https://doi.org/10.3390/en13184660
Submission received: 6 August 2020 / Revised: 31 August 2020 / Accepted: 2 September 2020 / Published: 8 September 2020
(This article belongs to the Section D1: Advanced Energy Materials)
Browse Figures
Versions Notes

Abstract

:
5-Hydroxymethylfurfural, which can be derived from lignocellulosic biomass, can be transformed via the hydrodeoxygenation process to potential biofuels, such as 2,5 dimethylfuran or other chemicals of industrial importance. Non-noble metal catalysts constitute a robust and cheap solution for this process. In this work, the modification of the Ni/TiO2 catalyst by the addition of iron and support modification was evaluated. It was shown that bimetallic Ni-Fe catalysts are more selective and stable than their monometallic counterparts. This improvement in properties depends on the Ni:Fe ratio, but the support plays an equally important role—namely the high surface area anatase titania support improves the metal dispersion, resulting in a higher catalytic activity, and the formation of NiFe alloy facilitates the C=O bond cleavage. Such catalysts are active and stable and can be easily separated from the reaction mixture thanks to their magnetic properties.

1. Introduction

Lignocellulose biomass is a sustainable, abundant, and cost-efficient source of a wide spectrum of chemicals with applications ranging from pharmaceuticals to fuels [1]. The acidic hydrolysis of cellulose leads to multiple platform molecules, including 5-(hydroxymethyl)furfural (HMF) [2]. HMF hydrodeoxygenation (HDO) is an attractive transformation which provides access to potential biofuels, such as 2,5-dimethylfuran (DMF) and other multiple chemicals [3]. Indeed, DMF possesses higher energy density, boiling point and octane number, as well as lower volatility, than the bioethanol benchmark and is immiscible with water, which all make its application more attractive [1,4,5].
The HMF hydrodeoxygenation reaction is presented in Scheme 1. First, competitive hydrodeoxygenation and hydrogenation of HMF leads to the formation of 5-methylfurfural (5-MF) and 2,5-bishydroxymethylfuran (BHMF), according to paths A and B, respectively. Then, 5-methylfurfuryl alcohol (5-MFA) is formed either by the hydrodehydration of BHMF, or by 5-MF hydrogenation. Then, 5-MFA can further be hydrogenated to 5-methyltetrahydrofurfuryl alcohol (MTHFA), or undergo hydrodehydration for forming DMF, whose subsequent hydrogenation leads to 2,5-dimethyltetrahydrofuran (DMTHF). Additionally, the BHMTHF hydrodeoxygenation with MTHFA as intermediate, is a parallel way for obtaining DMTHF [6]. Taking into account that, in the HMF hydrodeoxygenation reaction, various products can be obtained, the key challenge is to conduct this process selectively [7,8].
To address this issue, noble metals, such as Au, Ru, Pt, Ru, and Pd, have often been used, whereas non-noble metal catalysts have received less attention [1]. Non-noble metals constitute, however, an attractive solution in terms of their abundance [4,9], good stability [10] and low price [11]. Among them, Ni can be an ideal candidate to replace precious metals due to its high ability for hydrogenation. Monometallic Ni, however, can cause undesired reactions, such as decarbonylation and ring hydrogenation [12], and often shows limited stability [13].
Therefore, the activity of the Ni-based catalysts should be strictly controlled, which can be achieved by doping Ni with another metal. Solutions involving other non-noble metals are particularly interesting. The group of Shimazu showed that the formation of a metal alloy between Ni and Sn can strongly influence the selectivity. Ni–Sn-based alloy catalysts can selectively perform the hydrogenation of C=O rather than C=C, and, therefore, in the hydrogenation of unsaturated carbonyl compounds a high yield of unsaturated alcohols was obtained [14]. Seemala et al. showed that the preparation of a bimetallic catalyst by the addition of Cu to Ni/TiO2 allowed the highly efficient production of DMF. This high activity was associated with the metal segregation that occurred on the support surface, which was selectively enriched with Cu, whereas Ni stayed close to the support surface. This core-shell-like structure allows for reaching high catalytic performance [11].
Very recently, Yu et al. in their pioneering work synthesized bimetallic robust Ni-Fe carbon nanotube-supported (CNT) catalysts for the HMF hydrodeoxygenation. Monometallic Ni/CNT catalyst showed poor selectivity, giving DMF (46.3%) as the main product and significant amounts of by-products. Monometallic Fe/CNT merely induced HMF conversion (3.2%), whereas a physical mixture of Ni/CNT and Fe/CNT strongly resembled the behavior of the monometallic Ni catalyst [4].
Different ratios of Ni:Fe were investigated by keeping the Fe amount constant and increasing the Ni loading. Such a proof-of-principle study showed the great potential of this system taking into account both the increased selectivity and stability. The increased performance was associated with the Fe-Ni alloy formation [4]. On the other hand, the role of the support has also been considered to be crucial in the HMF hydrogenation. High isoelectric point supports, such as γ-Al2O3, promote the HMF ring hydrogenation towards BHMTHF, [15] whereas supports showing Brønsted acidity, such as SiO2, promote the formation of polyols and polymers by ring opening [16]. Moreover, titania was shown to be the crucial support for the selectivity improvement. The key-role played by the titania support in the segregation of metal atoms on the surface of the catalyst was reported, as it induces strong Ni-TiO2 interactions and enriches the surface with Cu in the case of Ni-Cu bimetallic catalysts. As a result, the Ni-core and Cu-shell-like system was formed, which was not observed in the case of Al2O3 support [11]. In our previous work, we also showed that, by the choice of titania support for Ni catalysts, we can strongly modify the selectivity and conversion of HMF hydrogenation [7]. The support plays a key role not only in the selectivity and activity improvement but also stability [17], particularly taking into account the possible deactivation pathway of the catalysts used for DMF synthesis that is usually associated with the carbon deposit formation. The organic species can adsorb on the surface and further polymerize and, therefore, the support choice should also allow for effective carbon removal by temperature treatment, which excludes carbon-based supports.
Therefore, in our work we investigated titania based Ni-Fe catalysts for HMF hydrodeoxygenation. We evaluated both the impact of the support (titania with different rutile to anatase ratio) and the presence of Ni-Fe interaction on the catalyst performance. The application of high-surface area and low-acidity anatase as a support of the Ni-Fe catalyst suppressed the formation of over-hydrogenated products from DMF. Tuning the Fe:Ni ratio allowed us to obtain a high selectivity towards DMF and provided a robust and recyclable catalyst. Characterization with a series of physico-chemical methods allowed us to deeply understand the relationship between the catalyst structure and activity.

2. Materials and Methods

5-Hydroxymethylfurfural (98%), Ni(NO3)2 6 H2O (98%), Fe(NO3)3·9 H2O (99%) and 1,4-dioxane (99%) were supplied by Fluorochem Ltd. (Hadfield, United Kingdom), Chempur (Piekary Śląskie, Poland), Sigma-Aldrich (Poznań, Poland), and Merck (Darmstadt, Germany), respectively.

2.1. TiO2 Supports and Catalysts

Several commercially available TiO2 supports displaying different anatase/rutile crystallographic phase contents and specific surface areas, were used. Anatase–rutile mixed phase Aeroxide© TiO2 P25 and P90 materials with a rutile content of 20% and 10%, respectively [7], were supplied by Evonik-Degussa, Germany. Cristal-ACTIV PC500 was supplied by Millenium-Crystal (France), and consisted of pure anatase TiO2. PC500, P90 and P25 TiO2 supports a displayed specific surface area of 336, 90 and 25 m2/g, respectively.
Monometallic catalysts preparation. Monometallic Ni and Fe catalysts (Ni/P25 and Fe/P25) were prepared by the wet impregnation of the TiO2 (P25) support, using Ni(NO3)2·6 H2O or Fe(NO3)3·9 H2O as precursor salts, respectively, and water as a solvent. The catalyst was dried at 120 °C for 2 h, calcined in the air flow at 500 °C for 5 h and reduced in hydrogen flow at 500 °C for 1 h.
Bimetallic catalyst preparation. Bimetallic Ni-Fe catalysts were prepared by the wet co-impregnation method using different types of TiO2 as a support (P25, P90, PC500), Ni(NO3)2·6 H2O and Fe(NO3)3·9 H2O as precursor salts and water as a solvent. The catalysts were dried at 120 °C for 2 h, calcined in the air flow at 500 °C for 5 h and reduced in hydrogen flow at 500 °C for 1 h. Bimetallic catalysts were denoted as NiFe(x:y) where x and y represent the weight content of Ni and Fe metals, respectively, relative to the TiO2 support.
All catalysts (mono and bimetallic) contain 10 wt % of metal loading.

2.2. Characterization Techniques

Temperature-programmed reduction (TPR) was carried out on AMI1 system (Altamira Instr., Pittsburgh, PA, U.S.A.) using a thermal conductivity detector, for studying the catalyst reducibility. Before measurement, the catalysts were calcined at 300 °C (heating rate of 10 °C/min) for 0.5 h in a 2/98 vol. %/vol.% O2/Ar flow with a space velocity W/F = 1.11 × 10−5 g/h cm3. TPR profiles were recorded in the 35–800 °C range, with a 7 °C/min heating rate, using a mixture of 5/95 vol. %/vol.% H2/Ar at the same space velocity.
Temperature-Programmed Desorption (TPD) of NH3 was performed for studying the catalyst acidity. The NH3-TPD experiments were implemented in a home-made quartz-based flow micro-reactor. Before all experiments, the surface of the catalyst was cleaned under hydrogen flow for 30 min at 500 °C. The catalyst was then cooled down to 100 °C and NH3 was adsorbed on the surface of the catalyst for 15 min at 100 °C. Prior to measurement, physically adsorbed NH3 was removed from the surface of the catalyst by He flow cleaning for 15 min before cooling down the sample to ambient temperature. The NH3-TPD experiment was performed from room temperature to 500 °C with a 25 °C/min heating ramp.
Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) measurements were carried out in an ION-TOF GmbH instrument (TOF-SIMS IV) equipped with a 25 kV pulsed Bi3+ primary ion gun in static mode. To obtain a flat sample surface (so that better resolved mass spectra can be achieved), powdery samples were tableted before being fixed at the sample holder using double-sided adhesive tape. The analyzed area of the sample surface was 500 × 500 μm. Three spectra from different surfaces were analyzed for each sample. During the analysis, charge neutralization was obtained by using a pulsed low-energy electron flood gun. The number of counts of selected ions from the mass spectra was normalized on the basis of the total ion count for allowing semi-quantitative analysis.
X-ray diffraction (XRD) measurements were performed on a PANalyticalX’Pert Pro MPD diffractometer, using a Cu long-fine focus XRD tube working at 30 mA and 40 kV as an X-ray source. Data were recorded in the 2θ mode with a 0.0167° step (5–90°). Crystalline phases were identified by referring to the ICDD PDF-2 database (version 2004).
The FTIR spectra of adsorbed CO were registered on a Nicolet-6700 spectrometer composed of a diffuse reflection chamber and an MCT-detector. Prior to CO exposure, the catalysts were reduced for 60 min using a H2(5%)/Ar flow at an appropriate temperature, and further cooled to ambient temperature in an Ar flow for removing the hydrogen adsorbed before recording the background spectra. CO adsorption was performed under a pressure of 5 bar of CO(5%)/Ar for 30 min. Spectra were recorded with a resolution of 4 cm−1 by performing 64 scans.

2.3. Catalytic Tests

The catalysts were tested in the hydrogenation of 5-hydoxymethylfurfural (5-HMF). The activity test was performed in a 60 mL stainless-steel batch autoclave (Premex Solutions GmBH, Lyss, Switzerland). The reaction was performed with 1 g of 5-HMF, 0.15 g of the catalyst and 30 mL of 1,4-dioxane. The reactor was flushed twice under hydrogen flow prior to pressurizing at room temperature to 30 bar of hydrogen. The autoclave was subsequently heated to 220 °C and this temperature was maintained for 1 to 3 h. The reaction was carried out under strong mechanical agitation (at 650 rpm). After reaction, the reactor was cooled down to room temperature, and the reaction mixture was subsequently centrifuged for 5 min at 3500 rpm and then the liquid sample and catalyst were separated, and liquid was analyzed with a GC instrument (Agilent 7820 A) using a FID detector and a high polarity Wax column Agilent J&W CP-Wax 52 CB. Analytical conditions were an injection volume of 1.0 μL, an inlet temperature of 300 °C, a detector temperature of 300 °C, a split-flow of 163.69 mL/min, a column flow of 1.63 mL/min (Nitrogen). Initially maintained at 5 °C for 5 min, the column temperature was increased to 200 °C with a heating ramp of 12 °C/min, and finally maintained for 20 min.
As far as the recycling procedure was concerned, the post-reaction mixture was centrifuged at 3500 rpm for 5 min, before the liquid sample and the catalyst were separated. The catalyst was then dried at 80 °C for 5 h, calcined in the air flow at 500 °C for 5 h and reduced in hydrogen flow at 500 °C for 1 h. The catalyst prepared in this way was reused in the reaction.

3. Results

3.1. Catalytic Activity

Monometallic 10% Ni/P25 catalyst was used as a reference benchmark and tested in the HMF hydrodeoxygenation (Table 1). It exhibited 69% yield towards DMF, although a significant amount of undesired over-hydrogenated product (22% of DMTHF) was also observed. Therefore, in order to increase the selectivity, two different approaches were undertaken, firstly doping with Fe. Secondly, the effect of the support was tested.
The addition of 2–3 wt.% of iron to the Ni/TiO2 catalyst resulted in a decrease in DMF and DMTHF yields, which may suggest a decrease in reaction rate. Additionally, in both cases a significant yield of BHMTHF, which is a further BHMF ring-hydrogenation product, was also observed. Other unidentified products were also found, which might result from the unwanted polymerization-dehydration and the furan ring-opening reactions that can be co-catalyzed by the presence of acidic sites in the used catalyst [17,18]. A more significant addition of iron (4–5 wt.%) to the Ni/TiO2 catalyst limited the amount of the overhydrogenated product and allowed an increase in the DMF yield. The latter was also improved by the increase in Fe addition, reaching the maximum value for the Ni:Fe(5:5) catalyst (DMF yield of 75%). This remains in contrast to the literature for Ni-Fe supported on carbon nanotubes, where the most effective catalytic systems possessed a higher Ni:Fe ratio and are probably caused by the support properties [4].
The support effect was evaluated for the catalysts with those metal ratios that showed the best performance (Table 2). Based on this, NiFe(5:5) catalysts supported on rutile-containing titanias (P25 and P90) and on pure anatase PC500, were tested in 2 h HMF hydrogenation. All catalysts provided full conversion of the substrate. However, in the presence of rutile-containing catalysts (P25 and P90), a significant quantity of unidentified products was observed (about 10 %). The NiFe(5:5)/P90 catalyst gave the lowest DMF yield (40%). Moreover, the occurrence of other products (BHMF, BHMTHF, MTHFA, DMTHF) indicates that the catalyst supported on P90 is non-selective in this reaction. In contrast, the NiFe(5:5)/PC500 catalyst was highly selective, giving 90% DMF yield and a negligible amount of unidentified products. Prolongation of the reaction time to 3 h further improved the DMF selectivity to 97% and did not affect the formation of over-hydrogenated products.
Reusability of the bimetallic catalyst NiFe(5:5)/PC500 was also investigated, and the results are shown in Figure 1a. Those tests showed that the HMF conversion stays intact throughout all the cycles (100%). The DMF yield was nearly the same, with only a very slight decrease from 97% to 92% after the fifth observed cycle. This confirms the high stability of bimetallic catalysts, particularly in comparison with the monometallic Ni reference system.
As a reference, the monometallic Ni/PC500 catalyst was examined. Although a higher DMF yield was reached (87%) in comparison to the benchmark Ni/P25 (69%), the catalyst was, however, not stable (Figure 1b). A slight loss in DMF yield was already visible in the second cycle (77%), and in the third it dropped more strongly (44%)—after the fifth cycle the drop, it was already significant, as only 18% yield of DMF was reached, and conversion decreased to 72%. Such a behavior has already been observed in the literature and can be caused by carbon deposit formation or/and Ni leaching [17]. In order to understand the activity behavior, a series of physicochemical properties was investigated.

3.2. Catalyst Characterization

3.2.1. Temperature-Programmed Reduction Analysis

Reducibility of the catalysts was investigated by performing temperature-programmed reduction (TPR) analysis, shown in Figure 2. The reduction profile for the monometallic 10% Ni/TiO2 (P25) catalyst displayed a broad peak ranging from 300 to 500 °C with the maximum of hydrogen consumption located at 460 °C. This peak can be assigned to NiOx strongly interacting with titania [19,20,21]. The visible shoulder of the peak may indicate the presence of large crystallites [22].
In the case of Fe catalysts, mainly the two-step reduction of iron oxides ((I) hematite to magnetite and (II) magnetite to metallic iron) is reported in the literature, according to the equation: (I) Fe2O3 → Fe3O4 (II) Fe3O4 → Fe [4,23,24]. In the TPR profiles of the studied materials, the reduction in iron oxides on the surface of monometallic Fe/TiO2 catalysts occurs in a broad temperature range (200–650 °C) and it is difficult to distinguish clear maxima, but this broad effect might reflect the two reduction steps of iron oxide. The addition of Ni to a Fe-based catalyst in a low amount (2–3%) resulted in TPR profiles similar to that of monometallic Fe/P25, which means that the effects of the iron oxide reduction are dominant. The only difference is the slightly more visible low-temperature maximum, which can result from the reduction of NiOx amorphous small crystallites that do not interact with Fe and are therefore reduced more easily [25,26,27]. This effect can be also related to the facilitated reduction of FeOx interacting with Ni [28].
Higher loading of Ni (4–6 wt.%) in the bimetallic catalyst resulted in the occurrence of a distinct peak with maximum around 400–450 °C. This may be attributed to the reduction of NiOx crystallites interacting with Fe species [4]. A shift in this peak towards higher temperatures is visible for samples possessing higher Ni loading (7–8%). In this case, the reduction peak resembles the one for monometallic Ni [29], suggesting that the NiO phase can be dominant. The Ni-Fe interaction cannot be excluded but is probably less significant.
The TPR profiles of bimetallic NiFe(5:5) catalysts supported on different types of titania are shown in Figure 3. The effect of the support is strongly pronounced. The reduction effect for NiFe(5:5)/P90 is shifted towards higher temperatures (460 and 555 °C) in comparison to the benchmark NiFe(5:5)/P25. This can be related to the strong interaction of metallic species with the support and the formation of Ni-Fe alloy [24,30,31]. For NiFe(5:5)/PC500, although the reduction temperature ranges are similar, a more distinct peak is observed with a clear maximum at 390 °C, which suggests a more uniform type of species that are reduced in the same temperature range.

3.2.2. X-ray Diffraction Measurements

Figure 4 shows the X-ray diffraction patterns of the synthesized catalysts supported on TiO2. Main rutile and anatase diffraction peaks were observed for all samples at 2θ = 27.4, 36.1, 41.2, 54.3, 62.7° and 25.3, 37.8, 48.1, 55.1°, respectively (not shown). For the monometallic catalysts (Ni/P25 and Fe/P25) the main diffraction peaks of metallic Fe and Ni are present at 2θ = 44.6 and 44.4° (JCPDS 00-001-1262 and JCPDS 04-0850). Interestingly, the reflexes of metallic iron are visible only in the case of monometallic catalyst, which might suggest that in the bimetallic systems the crystallites are smaller (<5 nm) or amorphous.
Additionally, in the case of all bimetallic catalysts, the peak in the region of 44–45° was gradually shifted towards smaller angles with the increasing Fe content. This can suggest the formation of the Ni-Fe alloy. The structure of the Fe-Ni binary alloys strongly depends on the Fe and Ni content [32].
The formed solid solution can lead to form Ni-rich Ni-Fe solid solution alloy with fcc structure Fe-Ni (FCC) or Fe-rich Fe-Ni bcc structure (BCC) depending on the Fe:Ni proportions [33].

3.2.3. Acidity

The acidity of TiO2 (P25) as well as mono- and bimetallic catalysts supported on TiO2 (P25) was measured using ammonia (NH3) as a molecular probe and the results are shown in Table 3. The monometallic Ni catalyst (Entry 10) showed higher acidity than its Fe analogue (Entry 1). The acidity of catalysts is higher than that of the support itself. The bimetallic catalysts possess the acidity in a similar range as their monometallic counterparts, which, however, depends on the ratio of metals. It is the highest for the Ni:Fe ratio ranging from 4:6 to 7:3. This is in accordance with the literature, where it has been reported that the addition of Fe can provide a higher number of acid sites [34]. Those changes are related to the alloy formation, which changes the interaction of metals with the support [35,36].
For the catalysts supported on different titanias, the acidity changes are more visible. The lowest acidity was observed for the catalyst supported on pure anatase—i.e., NiFe (5:5)/PC500. In contrast, the acidity of the catalyst supported on rutile-containing titania—i.e., NiFe(5:5)/P90—was a few times higher. As we have recently showed for monometallic titania-supported catalysts, the higher acidity of the rutile-containing catalysts in comparison to that of their anatase-supported counterparts can be related to the higher availability of the Lewis acid centers on the rutile surface [7,37].

3.2.4. FTIR Analysis

FTIR spectra of adsorbed CO were recorded, as the FTIR tests using CO as a probe molecule allows us to identify the metal centers present in the catalyst (both the kind and the number of active sites), which are shown in Figure 5. No CO adsorption bands were observed for the Fe catalyst, which suggests that the adsorption visible on bimetallic catalysts occurs only on Ni species.
In general, the bands in the 2100–2000 cm−1 range correspond to linear CO adsorption, while the bands below 2000 cm−1 are characteristic for the bridged form of CO adsorbed on the metal [38,39].
In the case of monometallic nickel catalyst, bands at 2056, 2025 and 1980 cm−1 are visible. The bands at 2056 and 2025 cm−1 are assigned, respectively, to physically and linearly adsorbed CO on moderately dispersed metal [40,41]. In contrast, the band at 1980 cm−1 corresponds to bridged CO adsorption on poorly dispersed nickel, which suggests that the crystallites are non-uniform and rather large.
In the spectra of Ni-Fe/PC500 bimetallic catalysts, a band at 2084 cm−1, related to CO adsorption, centered on well-dispersed nickel, is also visible. The 1980 cm−1 band does not appear, which additionally confirms a high metal dispersion.
Comparing the band intensities in the spectra of mono- and bimetallic catalysts supported on P25 titania, it can be stated that, generally, the presence of iron limits the adsorption of carbon monoxide on nickel. This can result from strong interactions between both metals. However, NiFe/PC500 has much higher CO adsorption in comparison with its bimetallic counterpart supported on P25 titania. Its adsorption is at the same level as for the monometallic Ni/P25 (the same intensity of the band at 2056 cm−1 is observed). This indicates that the NiFe/PC500 and Ni/P25 possess a similar number of CO adsorption centers.

3.2.5. ToF-SIMS

Figure 6 presents the intensities of selected ions identified by the ToF-SIMS analysis of the surface of the bimetallic NiFe/PC500 catalyst (which shows the best catalytic performance). The results for the NiFe/P25 catalyst are shown for comparison. The intensities of Ni and Fe ions are much stronger in the case of the NiFe(5:5)/PC500 catalyst. It can be explained by smaller Ni and Fe particle sizes [42,43]. Interestingly, the Fe/Ni intensity ratio is higher in the case of NiFe(5:5)/PC500 catalyst, which suggests that Fe is more dispersed there in comparison its P25 supported counterpart, or is just more accessible on the catalyst surface.
Additionally, in both samples Ni-Fe+ ions are observed, and their relative intensity is associated with the Fe and Ni ions intensity. They can be related to the formation of Ni-Fe alloy or the strong interaction between those two metals [42,43]. The calculated ratio of respective metal ions to the Ni-Fe ratio in the catalyst suggests that the Ni-Fe interaction depends on the proportion of both metals in the catalyst.
For both catalysts, FeO+ ions are also observed, which can suggest that iron is partially in the oxide form. On the other hand, NiO+ ions can be observed only in a negligible amount, on the verge of sensitivity of the device.

4. Discussion

Several parameters play an important role in designing active, selective and stable catalysts for the DMF synthesis. The key issues are related to the type of active phase and support properties.
Monometallic Ni, despite being active, leads to undesired reactions, such as ring hydrogenation and formation of DMTHF. Additionally, it shows a very limited stability, which is the key aspect for heterogeneous catalysis [44]. Its modification with iron slightly boosts the DMF yield, which can be attributed to the presence of NiFe alloy.
Sitthisa et al. showed, with the use of density functional theory (DFT) calculations, that Fe species are beneficial to the adsorption of the furan ring, owing to their oxophilic nature [45]. This, in consequence, allows a stronger interaction between the furfural ring and the NiFe alloy in comparison with the respective interaction between furfural and pure Ni. Additionally, the C=O bond cleavage was facilitated on the NiFe alloy, which accounted for a higher DMF yield.
The NiFe alloy formation is, however, not the sole factor increasing the product yield. The structure of the NiFe alloy is related to metal proportion; therefore, different solid solution alloys possessing fcc crystallographic structure Fe-Ni (FCC) or bcc Fe-Ni (BCC) structure can be formed [32]. It is difficult to establish a clear relationship between the alloy composition—its formation and reaction performance. We proved that the Ni:Fe ratio close to 1:1 is optimal for reaching the highest product yield, whereas Putro et al. in their work showed that the excess of Ni is the most beneficial for the C-O cleavage in the case of Ni:Fe(2:1)/CNT catalysts [46]. In the work of Chen et al., the optimum Ni:Fe ratio for the C=O hydrogenation was 1:2 in the active carbon-supported catalysts [23]. It is therefore clear that other factors responsible for the catalytic performance cannot be omitted, including the different availability of acid sites, metal dispersion and metal-support interaction. These features can be strongly modified by the support influence. Modification of the catalyst possessing the optimal Ni:Fe composition by high-surface area titania can improve its performance. The high product yield obtained over the NiFe/PC500 catalyst results from its increased number of active centers (FTIR, ToF-SIMS), probably due to the high metal dispersion in comparison with a bimetallic catalyst supported on standard (P25) titania material. The effect of improved metal dispersion, thanks to the Fe addition, remains in accordance with the literature [47]. We also noticed that increased dispersion of Ni for monometallic Ni/TiO2 catalysts allows us to tune the selectivity and influence the reaction pathway. Large Ni particles can favor the reaction path via BHMF and, depending on the number and strength of the acid sites, the reaction might go towards BHMTHF or via 5-MF to DMF [7]. It was also shown that the preferred adsorption reported for monometallic Ni possessing large crystallites occurs most probably via ɳ2 (C=O), which brings the aromatic ring close to the surface, allowing, consequently, its reduction and facilitating, therefore, the DMTHF production in a significant quantity via the reduction of the furanic ring [7,11]. Moreover, high catalytic performance is only partially beneficial if a catalyst is not stable, as in the case of heterogeneous catalysts the stability is considered a key aspect.
Here we showed that catalysts with a balanced Ni:Fe ratio and optimum modification of the support are not only active and selective, but also stable during several catalytic cycles. Last but not least, thanks to their magnetic properties, the TiO2 PC500 supported Ni:Fe catalysts can be easily removed from the reaction solution, as visualized in Figure 7.

5. Conclusions

We showed that the modification of Ni/TiO2 catalysts by the addition of iron and support adjustment can significantly improve the product yield and catalyst stability in the HMF hydrodeoxygenation. Monometallic Ni leads to undesired reactions, such as the ring hydrogenation and the formation of DMTHF. Its stability is also very limited. Modification of the monometallic catalyst with iron slightly improves the DMF yield, thanks to the NiFe alloy formation which facilitates the C=O bond cleavage. It was also shown that the Ni:Fe ratio, close to 1:1, is optimal for reaching the highest product yield.
Moreover, we showed that a further yield improvement can be reached thanks to the titania modification. High surface area titania allows us to obtain better metal dispersion and lower acidity. Those features allow us to tune the reaction selectivity even more, while retaining excellent stability. In consequence, the DMF yield reaching 97% can be achieved.

Author Contributions

Conceptualization, A.M.R.; supervision, A.M.R.; funding acquisition, A.M.R.; project administration, A.M.R.; writing—original draft preparation, M.P. and A.M.R.; methodology, A.M.R.; investigations, M.P., M.J. and J.R.; resources, M.S.-J. All authors have read and agreed to the published version of the manuscript.

Funding

The authors acknowledge gratefully the National Center of Science (NCN), Krakow, Poland, for financially supporting the through a SONATA BIS grant (2016/22/E/ST4/00550).

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Chen, S.; Wojcieszak, R.; Dumeignil, F.; Marceau, E.; Royer, S. How catalysts and experimental conditions determine the selective hydroconversion of furfural and 5-hydroxymethylfurfural. Chem. Rev. 2018, 118, 11023–11117. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  2. Menegazzo, F.; Ghedini, E.; Signoretto, M. 5-hydroxymethylfurfural (HMF) production from real biomasses. Molecules 2018, 23, 2201. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  3. Van Putten, R.; Van Der Waal, J.C.; De Jong, E.; Rasrendra, C.B.; Heeres, H.J.; De Vries, J.G. Hydroxymethylfurfural, a versatile platform chemical made from renewable resources. Chem. Rev. 2013, 113, 1499–1597. [Google Scholar] [CrossRef] [PubMed]
  4. Yu, L.; He, L.; Chen, J.; Zheng, J.; Ye, L.; Lin, H.; Yuan, Y. Robust and recyclable nonprecious bimetallic nanoparticles on carbon nanotubes for the hydrogenation and hydrogenolysis of 5-hydroxymethylfurfural. ChemCatChem 2015, 7, 1701–1707. [Google Scholar] [CrossRef]
  5. Hu, L.; Lin, L.; Liu, S. Chemoselective hydrogenation of biomass-derived 5-hydroxymethylfurfural into the liquid biofuel 2,5-dimethylfuran. Ind. Eng. Chem. Res. 2014, 53, 9969–9978. [Google Scholar] [CrossRef]
  6. Nishimura, S.; Ebitani, K. Catalytic conversions of biomass-derived furaldehydes toward biofuels. Green Chem. Process. Synth. 2017, 51–68. [Google Scholar] [CrossRef] [Green Version]
  7. Przydacz, M.; Jędrzejczyk, M.; Brzezińska, M.; Rogowski, J.; Keller, N.; Ruppert, A.M. Solvothermal hydrodeoxygenation of hydroxymethylfurfural derived from biomass towards added value chemicals on Ni/TiO2 catalysts. J. Supercrit. Fluids 2020, 163, 104827. [Google Scholar] [CrossRef]
  8. Wang, J.; Wei, Q.; Ma, Q.; Guo, Z.; Qin, F.; Ismagilov, Z.R.; Shen, W. Constructing Co@N-doped graphene shell catalyst via Mott-Schottky effect for selective hydrogenation of 5-hydroxylmethylfurfural. Appl. Catal. B Environ. 2020, 263, 118339. [Google Scholar] [CrossRef]
  9. Wang, X.; Liang, X.; Li, J.; Li, Q. Catalytic hydrogenolysis of biomass-derived 5-hydroxymethylfurfural to biofuel 2, 5-dimethylfuran. Appl. Catal. A Gen. 2019, 576, 85–95. [Google Scholar] [CrossRef]
  10. Kohli, K.; Prajapati, R.; Sharma, B.K. Bio-based chemicals from renewable biomass for integrated biorefineries. Energies 2019, 12, 233. [Google Scholar] [CrossRef] [Green Version]
  11. Seemala, B.; Cai, C.M.; Wyman, C.E.; Christopher, P. Support induced control of surface composition in Cu-Ni/TiO2 catalysts enables high yield co-conversion of HMF and furfural to methylated furans. ACS Catal. 2017, 7, 4070–4082. [Google Scholar] [CrossRef] [Green Version]
  12. Hu, S.; Xue, M.; Chen, H.; Shen, J. The effect of surface acidic and basic properties on the hydrogenation of aromatic rings over the supported nickel catalysts. Chem. Eng. J. 2010, 162, 371–379. [Google Scholar] [CrossRef]
  13. Sitthisa, S.; Resasco, D.E. Hydrodeoxygenation of furfural over supported metal catalysts: A comparative study of Cu, Pd and Ni. Catal. Lett. 2011, 141, 784–791. [Google Scholar] [CrossRef]
  14. Rodiansono; Khairi, S.; Hara, T.; Ichikuni, N.; Shimazu, S. Highly efficient and selective hydrogenation of unsaturated carbonyl compounds using Ni-Sn alloy catalysts. Catal. Sci. Technol. 2012, 2, 2139–2145. [Google Scholar] [CrossRef]
  15. Alamillo, R.; Tucker, M.; Chia, M.; Pagán-Torres, Y.; Dumesic, J. The selective hydrogenation of biomass-derived 5-hydroxymethylfurfural using heterogeneous catalysts. Green Chem. 2012, 14, 1413–1419. [Google Scholar] [CrossRef]
  16. Davda, R.R.; Shabaker, J.W.; Huber, G.W.; Cortright, R.D.; Dumesic, J.A. Aqueous-phase reforming of ethylene glycol on silica-supported metal catalysts. Appl. Catal. B Environ. 2003, 43, 13–26. [Google Scholar] [CrossRef]
  17. Gyngazova, M.S.; Negahdar, L.; Blumenthal, L.C.; Palkovits, R. Experimental and kinetic analysis of the liquid phase hydrodeoxygenation of 5-hydroxymethylfurfural to 2,5-dimethylfuran over carbon-supported nickel catalysts. Chem. Eng. Sci. 2017, 173, 455–464. [Google Scholar] [CrossRef]
  18. Sun, K.; Shao, Y.; Li, Q.; Liu, Q.; Wu, W.; Wang, Y.; Hu, S.; Xiang, J.; Liu, Q.; Hu, X. Cu-based catalysts for hydrogenation of 5-hydroxymethylfurfural: Understanding of the coordination between copper and alkali/alkaline earth additives. Mol. Catal. 2019, 474, 110407. [Google Scholar] [CrossRef]
  19. Xiong, J.; Chen, J.; Zhang, J. Liquid-phase hydrogenation of o-chloronitrobenzene over supported nickel catalysts. Catal. Commun. 2007, 8, 345–350. [Google Scholar] [CrossRef]
  20. Huo, W.; Zhang, C.; Yuan, H.; Jia, M.; Ning, C.; Tang, Y. Vapor-phase selective hydrogenation of maleic anhydride to succinic anhydride over Ni/TiO2 catalysts. J. Ind. Eng. Chem. 2014, 20, 4140–4145. [Google Scholar] [CrossRef]
  21. Ho, S.W.; Chu, C.Y.; Chen, S.G. Effect of thermal treatment on the nickel state and CO hydrogenation activity of titania-supported nickel catalysts. J. Catal. 1998, 178, 34–48. [Google Scholar] [CrossRef]
  22. Raj, K.J.A.; Prakash, M.G.; Mahalakshmy, R.; Elangovan, T.; Viswanathan, B. Selective hydrogenation of acetophenone over nickel supported on titania. Catal. Sci. Technol. 2012, 2, 1429–1436. [Google Scholar] [CrossRef]
  23. Cheng, C.; Shen, D.; Xiao, R.; Wu, C. Methanation of syngas (H2/CO) over the different Ni-based catalysts. Fuel 2017, 189, 419–427. [Google Scholar] [CrossRef]
  24. Hammedi, T.; Triki, M.; Ksibi, Z.; Ghorbel, A.; Medina, F. Catalytic wet hydrogen peroxide oxidation of p-hydroxybenzoic acid over Fe/TiO2 and 0.5Ru–3Fe/TiO2. J. Sol-Gel Sci. Technol. 2015, 76, 679–685. [Google Scholar] [CrossRef]
  25. Valinejad-Moghaddam, S.; Rezaei, M.; Meshkani, F.; Daroughegi, R. Carbon dioxide methanation over Ni-M/Al2O3 (M: Fe, CO, Zr, La and Cu) catalysts synthesized using the one-pot sol-gel synthesis method. Int. J. Hydrogen Energy 2018, 43, 16522–16533. [Google Scholar] [CrossRef]
  26. Wu, H.; Zou, M.; Guo, L.; Ma, F.; Mo, W.; Yu, Y.; Mian, I.; Liu, J.; Yin, S.; Tsubaki, N. Effects of calcination temperatures on the structure-activity relationship of Ni-La/Al2O3 catalysts for syngas methanation. RSC Adv. 2020, 10, 3166–3174. [Google Scholar] [CrossRef] [Green Version]
  27. Ali, S.; Khader, M.M.; Almarri, M.J.; Abdelmoneim, A.G. Ni-based nano-catalysts for the dry reforming of methane. Catal. Today 2020, 343, 26–37. [Google Scholar] [CrossRef]
  28. Song, Y.; Lin, W.; Guo, X.; Dong, L.; Mu, X.; Tian, H.; Wang, L. Aromatization and isomerization of methylcyclohexane over Ni catalysts supported on different supports. Green Energy Environ. 2019, 4, 75–82. [Google Scholar] [CrossRef]
  29. Mebrahtu, C.; Krebs, F.; Perathoner, S.; Abate, S.; Centi, G.; Palkovits, R. Hydrotalcite based Ni-Fe/(Mg, Al)O:X catalysts for CO2 methanation-tailoring Fe content for improved CO dissociation, basicity, and particle size. Catal. Sci. Technol. 2018, 8, 1016–1027. [Google Scholar] [CrossRef]
  30. Sreekanth, P.M.; Smirniotis, P.G. Selective reduction of NO with CO over titania supported transition metal oxide catalysts. Catal. Lett. 2008, 122, 37–42. [Google Scholar] [CrossRef]
  31. Lin, Y.; Zhu, Y.; Pan, X.; Bao, X. Modulating the methanation activity of Ni by the crystal phase of TiO2. Catal. Sci. Technol. 2017, 7, 2813–2818. [Google Scholar] [CrossRef]
  32. Ebrahimi, F.; Li, H.Q. Structure and properties of electrodeposited nanocrystalline FCC Ni-Fe alloys. Rev. Adv. Mater. Sci. 2003, 5, 134–138. [Google Scholar]
  33. Yousefi, E.; Sharafi, S.; Irannejad, A. The structural, magnetic, and tribological properties of nanocrystalline Fe-Ni permalloy and Fe-Ni-TiO2 composite coatings produced by pulse electro co-deposition. J. Alloys Compd. 2018, 753, 308–319. [Google Scholar] [CrossRef]
  34. Jia, P.; Lan, X.; Li, X.; Wang, T. Highly selective hydrogenation of furfural to cyclopentanone over a NiFe bimetallic catalyst in a methanol/water solution with a solvent effect. ACS Sustain. Chem. Eng. 2019, 7, 15221–15229. [Google Scholar] [CrossRef]
  35. Cherifi, O.; Bettahar, M.M.; Auroux, A. Microcalorimetric study of the acidity and basicity of Ni/SiO2 catalysts modified by metallic additives Fe, Co, Zr and Ce. Thermochim. Acta 1997, 306, 131–134. [Google Scholar] [CrossRef]
  36. Tu, M.; Shen, J.; Chen, Y. Preparation, characterization and microcalorimetric studies of nickel-iron hydrotalcites and their decompositions. Thermochim. Acta 1997, 302, 117–124. [Google Scholar] [CrossRef]
  37. Scaranto, J.; Giorgianni, S. A quantum-mechanical study of CO adsorbed on TiO2: A comparison of the Lewis acidity of the rutile (1 1 0) and the anatase (1 0 1) surfaces. J. Mol. Struct. 2008, 858, 72–76. [Google Scholar] [CrossRef]
  38. Zhu, X.; Zhang, Y.P.; Liu, C.J. CO adsorbed infrared spectroscopy study of Ni/Al2O3 catalyst for CO2 reforming of methane. Catal. Lett. 2007, 118, 306–312. [Google Scholar] [CrossRef]
  39. Zhu, X.; Huo, P.; Zhang, Y.-P.; Cheng, D.-G.; Liu, C.-J. Structure and reactivity of plasma treated Ni/Al2O3 catalyst for CO2 reforming of methane. Appl. Catal. B Environ. 2008, 81, 132–140. [Google Scholar]
  40. Mihaylov, M.; Hadjiivanov, K.; Knözinger, H. Formation of Ni(CO)4 during the interaction between CO and silica-supported nickel catalyst: An FTIR spectroscopic study. Catal. Lett. 2001, 76, 59–63. [Google Scholar] [CrossRef]
  41. Bartholomew, C.H.; Pannell, R.B. The stoichiometry of hydrogen and carbon monoxide chemisorption on alumina-and silica-supported nickel. J. Catal. 1980, 65, 390–401. [Google Scholar] [CrossRef]
  42. Ruppert, A.M.; Jędrzejczyk, M.; Potrzebowska, N.; Kaźmierczak, K.; Brzezińska, M.; Sneka-Płatek, O.; Sautet, P.; Keller, N.; Michel, C.; Grams, J. Supported gold-nickel nano-alloy as a highly efficient catalyst in levulinic acid hydrogenation with formic acid as an internal hydrogen source. Catal. Sci. Technol. 2018, 8, 4318–4331. [Google Scholar] [CrossRef] [Green Version]
  43. Soszka, E.; Reijneveld, H.M.; Jędrzejczyk, M.; Rzeźnicka, I.; Grams, J.; Ruppert, A.M. Chlorine influence on palladium doped nickel catalysts in levulinic acid hydrogenation with formic acid as hydrogen source. ACS Sustain. Chem. Eng. 2018, 6, 14607–14613. [Google Scholar] [CrossRef]
  44. Ye, R.P.; Gong, W.; Sun, Z.; Sheng, Q.; Shi, X.; Wang, T.; Yao, Y.; Razink, J.J.; Lin, L.; Zhou, Z.; et al. Enhanced stability of Ni/SiO2 catalyst for CO2 methanation: Derived from nickel phyllosilicate with strong metal-support interactions. Energy 2019, 188, 116059. [Google Scholar] [CrossRef]
  45. Sitthisa, S.; An, W.; Resasco, D.E. Selective conversion of furfural to methylfuran over silica-supported NiFe bimetallic catalysts. J. Catal. 2011, 284, 90–101. [Google Scholar] [CrossRef]
  46. Putro, W.S.; Kojima, T.; Hara, T.; Ichikuni, N.; Shimazu, S. Selective hydrogenation of unsaturated carbonyls by Ni-Fe-based alloy catalysts. Catal. Sci. Technol. 2017, 7, 3637–3646. [Google Scholar] [CrossRef]
  47. Li, C.; Xu, G.; Zhai, Y.; Liu, X.; Ma, Y.; Zhang, Y. Hydrogenation of biomass-derived ethyl levulinate into gamma-valerolactone by activated carbon supported bimetallic Ni and Fe catalysts. Fuel 2017, 203, 23–31. [Google Scholar] [CrossRef]
Energies 13 04660 sch001 550
Scheme 1. HMF hydrodeoxygenation.
Scheme 1. HMF hydrodeoxygenation.
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Figure 1. Reusability tests of (a) NiFe(5:5)/PC500, (b) Ni/PC500.
Figure 1. Reusability tests of (a) NiFe(5:5)/PC500, (b) Ni/PC500.
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Figure 2. H2-TPR profiles of titania (P25)-supported mono- and bimetallic catalysts.
Figure 2. H2-TPR profiles of titania (P25)-supported mono- and bimetallic catalysts.
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Figure 3. H2-TPR profiles of Ni:Fe(5:5) catalysts supported on different types of titania.
Figure 3. H2-TPR profiles of Ni:Fe(5:5) catalysts supported on different types of titania.
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Figure 4. XRD analysis of the titania (P25 and PC500)-supported mono- and bimetallic catalysts.
Figure 4. XRD analysis of the titania (P25 and PC500)-supported mono- and bimetallic catalysts.
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Figure 5. FTIR analysis of mono- and bimetallic catalysts supported on P25 and bimetallic catalyst supported on PC500.
Figure 5. FTIR analysis of mono- and bimetallic catalysts supported on P25 and bimetallic catalyst supported on PC500.
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Figure 6. ToF-SIMS results collected from the surface of titania (P25 and PC500) supported bimetallic catalysts; (a) intensity of Fe+, Ni+ and NiTiO+ ions (b) selected ion intensity ratios.
Figure 6. ToF-SIMS results collected from the surface of titania (P25 and PC500) supported bimetallic catalysts; (a) intensity of Fe+, Ni+ and NiTiO+ ions (b) selected ion intensity ratios.
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Figure 7. Visualization of the easy-to-process magnetic recovery of the PC500 supported NiFe(5:5) catalyst from the dioxane-based post-reaction mixture: (a) catalyst suspension in the post-reaction mixture; (b) magnetic recovery of the catalyst from the post-reaction mixture.
Figure 7. Visualization of the easy-to-process magnetic recovery of the PC500 supported NiFe(5:5) catalyst from the dioxane-based post-reaction mixture: (a) catalyst suspension in the post-reaction mixture; (b) magnetic recovery of the catalyst from the post-reaction mixture.
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Table
Table 1. Influence of the Ni:Fe ratio on the catalytic activity and the product yields in the HMF hydrodeoxygenation.
Table 1. Influence of the Ni:Fe ratio on the catalytic activity and the product yields in the HMF hydrodeoxygenation.
CatalystHMF Conversion (%)Product Yield (%)
5-MFBHMFBHMTHF5-MFADMFMTHFADMTHFOTHERS
Ni/P251000060693220
NiFe(8:2)/P2510000288431425
NiFe(7:3)/P2510000160578109
NiFe(6:4)/P2510000100705411
NiFe(5:5)/P251000070754212
NiFe(4:6)/P2510001101659311
Fe/P251664100005
Reaction conditions: 220 °C; 1 h; 0.15 g of catalyst; 1 g of HMF; 30 mL of dioxane; 30 bar of hydrogen.
Table
Table 2. The influence of the type of titania in Fe-Ni/TiO2 catalysts on their activity and product yields in the hydrodeoxygenation of HMF.
Table 2. The influence of the type of titania in Fe-Ni/TiO2 catalysts on their activity and product yields in the hydrodeoxygenation of HMF.
CatalystHMF Conversion (%)Product Yield (%)
5-MFBHMFBHMTHF5-MFADMFMTHFADMTHFOTHERS
NiFe(5:5)/P251000040746610
NiFe(5:5)/P901000223040141011
NiFe(5:5)/PC500100100390006
NiFe(5:5)/PC500(1)100000097005
Reaction conditions: 220 °C; 2 h; 0.15 g of catalysts; 1 g of HMF; 30 mL of dioxane; 30 bar of hydrogen. (1) Reaction time was extended to 3 h.
Table
Table 3. Acidity of the mono- and bimetallic catalysts supported on P25 and PC500.
Table 3. Acidity of the mono- and bimetallic catalysts supported on P25 and PC500.
EntryCatalystAcidity (μmol/m2)
1Fe(10)/P258.7
2NiFe(1:9)/P258.3
3NiFe(2:8)/P258.1
4NiFe(3:7)/P257.6
5NiFe(4:6)/P259.8
6NiFe(5:5)/P259.7
7NiFe(6:4)/P2511.1
8NiFe(7:3)/P2510.9
9NiFe(8:2)/P259.2
10Ni(10)/P259.6
11P257.5
12NiFe (5:5)/P906.0
13NiFe (5:5)/PC5001.8

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Przydacz, M.; Jędrzejczyk, M.; Rogowski, J.; Szynkowska-Jóźwik, M.; Ruppert, A.M. Highly Efficient Production of DMF from Biomass-Derived HMF on Recyclable Ni-Fe/TiO2 Catalysts. Energies 2020, 13, 4660. https://doi.org/10.3390/en13184660

AMA Style

Przydacz M, Jędrzejczyk M, Rogowski J, Szynkowska-Jóźwik M, Ruppert AM. Highly Efficient Production of DMF from Biomass-Derived HMF on Recyclable Ni-Fe/TiO2 Catalysts. Energies. 2020; 13(18):4660. https://doi.org/10.3390/en13184660

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Przydacz, Martyna, Marcin Jędrzejczyk, Jacek Rogowski, Małgorzata Szynkowska-Jóźwik, and Agnieszka M. Ruppert. 2020. "Highly Efficient Production of DMF from Biomass-Derived HMF on Recyclable Ni-Fe/TiO2 Catalysts" Energies 13, no. 18: 4660. https://doi.org/10.3390/en13184660

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