Hydrogenolysis of Bio-Glycerol over In Situ Generated Nanosized Cu-ZnO Catalysts

Abstract

Due to the growth of biodiesel production, utilization of the glycerol formed as a by-product is still of considerable importance. This study is devoted to a novel approach for glycerol hydrogenolysis with use of in situ generated Cu-ZnO catalysts. The main product formed is 1,2-propanediol, with the by-products being lactic acid and ethylene glycol. The Cu-ZnO catalysts are characterized by AAS, XRD, XPS, SEM, TEM, EDX, BET, and chemisorption N₂O. The proportion of ZnO turns out to have a significant effect on the activity and selectivity of the catalyst formed. Increasing the ZnO content enables one to obtain more dispersed, active, selective, and agglomeration-resistant catalysts. The transition from monometallic Cu catalysts to Cu-ZnO with a ZnO content of 65 wt% allows one to increase selectivity from 74 to 86%, TOF from 0.136 to 0.511 s⁻¹, and S_Cu from 1.9 to 7.1 m²/g-Cu. The morphology of the synthesized Cu-ZnO catalysts resembles the structure of oxide/metal inverse catalysts.

1. Introduction

Nowadays, FAME-based biodiesel fuel is of great significance. Its production is growing every year and is expected to reach USD 72.29 billion by 2030 [1]. Along with this increase in biodiesel production, the problem of the utilization of glycerol, mostly produced during the transesterification of vegetable oil in biodiesel manufacturing, is still relevant. Glycerol as a versatile chemical compound is widely used in the food, medical, cosmetic, and polymer industries. The production of other chemicals with high added value from glycerol is of the considerable interest [2,3]. One of these products is propylene glycol (1,2-propanediol), which is currently obtained mainly from fossil-derived propylene oxide. Propylene glycol is used as precursor in the synthesis of unsaturated polyester resins, as a solvent and plasticizer, humectant, and a base for technical functional fluids, water-based heat carriers, and deicing reagents for aviation needs [4]. In 2012, a plant obtaining bio-propylene glycol from bio-glycerol was successfully launched by Oleon and BASF [5]. The proposed technology turned out to be quite successful, enabling them to reduce CO₂ emissions by at least 60% compared to the technology that produces PG from fossil-derived propylene oxide. Ten years later, in Poland, together with ORLEN Południe, BASF, and Air Liquide Engineering & Construction, another new plant opened [6]. Despite the successes achieved, research aimed at obtaining catalysts with improved characteristics of activity, selectivity, and stability, as well as the optimization of process conditions, is continuing.

In general, the hydrogenolysis of the glycerol molecule can occur with scission of C-O as well as of C-C bonds. The cleavage of C-O bonds is preferred since it enables one to obtain the more valuable 1,2-propanediol and 1,3-propanediol, while C-C bond scission leads to less desirable ethylene glycol and C₁ molecules. The most common catalysts for hydrogenation and hydrogenolysis processes are based on precious metals (Rh [7,8,9], Ru [10,11], Pt [12,13,14], Pd [15,16,17], and some transition metals (Co [11,18,19], Ni [20,21], and Cu [15,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36]). Precious-metal-based catalysts tend to be highly active in the hydrogenolysis of different oxygen-bearing compounds. However, these catalysts suffer from relatively low selectivity to C-O bond cleavage (for precious metal catalysts): the hydrogenolysis process runs with scission of both C-O and C-C bonds.

Copper-based catalysts have a number of benefits, such as low cost, relatively low toxicity, and high selectivity for the hydrogenation of C–O bonds. The main drawback is their low activity compared to the platinum group metals. One of the ways to solve the problem of low activity is the promotion of copper by other elements. Examples of the promotion of copper by oxides of chromium, aluminum [30,37,38], zinc [25,29,30,33,34,35], and zirconium [39] are well known. Moreover, there are examples of copper promotion by boron [22], nickel [20,21], and magnesium [31]. Another way to increase the activity of copper catalysts is the synthesis of finer and more dispersed metallic particles, as the activity is proportional to the metal surface area [36].

Among the vast number of methods to produce nanosized particles, the polyol method to produce metal nanoparticles deserves special attention [40,41]. The polyol method is a way to obtain finely dispersed metals from their oxides, hydroxides, and salts in liquid polyhydric alcohol. The main functions of the latter are to solve the metal precursor and to prevent the formed nanoparticles from coagulating. This method offers several advantages, including the opportunity to produce nanoparticles with tailored sizes and shapes, cheapness, and ease of use in industry.

Previously, unsupported copper catalysts were successfully applied in the selective hydrogenation of furfural [42]. At the same time, in situ formation of Cu₂O was observed in the reactions of azide-alkyne cycloaddition to afford 1,4-triazoles, using a CuI/oleylamine catalytic system in glycerol, where the long-chain amine stabilized the in situ formed Cu₂O [43]. Taking into account the fact that glycerol, in addition to being the medium for the catalyst formation, could be a substrate for the hydrogenolysis reaction, there was a hypothesis that the copper particles formed from inorganic precursors could catalyze the hydrogenolysis reaction of glycerol in the hydrogen environment. Previously, the concept was successfully tested in the synthesis of monometallic dispersed copper particles in situ, which were active in glycerol hydrogenolysis [26]. As the monometallic generated copper catalysts in situ were found to lose their activity quite quickly due to the coagulation and sintering, in this study, the system was modified by the addition of a zinc oxide component, generated in situ from a water-soluble precursor salt along with copper particles. To explore the modified concept, the characterization of the in situ generated Cu-ZnO catalysts by several analytic techniques (N₂ adsorption, chemisorption N₂O, TEM, EDX, SEM, XRD, XPS, and AAS) was performed, followed by a description of their catalytic performance in the reaction of glycerol hydrogenolysis.

Cu-ZnO catalysts are well known in the industry for methanol stream reforming [44], methanol synthesis from synthesis gas [45,46,47], and water–gas shift reaction [48]. Nevertheless, Cu-ZnO catalysts are extremely promising in glycerol hydrogenolysis [25,29,30,33,35]. Nowadays, most of the Cu-ZnO catalysts for the hydrogenolysis of glycerol are synthesized by the co-precipitation method [25,33,35]. The approach of obtaining catalysts in situ has significant benefits. Firstly, the additional procedure to synthesize a heterogeneous catalyst is omitted. Secondly, in situ synthesis in polyol medium allows us to obtain dispersed particles and to carry out the exothermic reaction of the glycerol hydrogenolysis using slurry technologies. Thirdly, it permits one to produce catalysts with uncommon morphology, and therefore the catalysts can have unique properties.

2. Results and Discussion

2.1. Effect of Zinc Precursor on Glycerol Hydrogenolysis

In our earlier study on monometallic copper catalysts synthesized in situ, we found that the addition of alkali had a significant effect on glycerol (Gly) hydrogenolysis. In its absence, the conversion was low, 3.1%, and the optimum number of KOH addition was 2.3 eq. It should be mentioned that 2 eq. were consumed to form the catalyst. The remaining 0.3 eq. participate in hydrogenolysis, since OH⁻ catalyzes the dehydration step [26].

In the present research, as well as in previous research, without the potassium hydroxide precursor, X_Gly was low, 0.8%; therefore, the effect of zinc precursor on glycerol hydrogenolysis was studied with an alkali addition equal to 3 eq. to the total amount of precursors. The molar ratio of Cu²⁺ and Zn²⁺ varied, while the total mole number of copper and zinc precursors to the substrate was constant and amounted to 1% (Figure 1). The main product of the glycerol hydrogenolysis was propylene glycol (PG), and the by-products were lactic acid (lactate-ion) (LA) and ethylene glycol (EG), with small traces (up to 0.1%) of glycerol monoacetate found. The formation of these products was confirmed by GC-MS. It should be pointed out that without the copper precursor, hydrogenolysis did not occur—X_Gly did not exceed 0.6%. Increase in the mole fraction of copper acetate from 0 to 25% raised X_Gly and Y_PG sharply, and the X_Gly extremum was observed at 25% of the copper precursor equal to 20.2%. A further increase in the mole fraction of copper to 100% led to a smooth slowdown in X_Gly and Y_PG to 5.7 and 4.2%, respectively. Previously, Zhou et al. also found that increasing the zinc content from 27 to 57 mol% in the Cu-ZnO-Al₂O₃ catalyst prepared by co-precipitation method allowed one to increase the Gly conversion from 33.2 to 69.7% [30].

In addition to the fact that the maximum of X_Gly was observed for a point with an initial molar copper content of 25%, the highest selectivity toward PG, equal to 86%, was also achieved.

The mechanism of the glycerol hydrogenolysis with use of in situ generated catalysts should be considered (Scheme 1). In an alkaline medium [49], the mechanism of the Gly hydrogenolysis is likely to proceed through the stage of Gly dehydrogenation with the formation of the intermediate product, glyceraldehyde, and its subsequent dehydration into the second intermediate, pyruvaldehyde (methylglyoxal). Next, methylglyoxal is hydrogenated to PG by 2 eq. of hydrogen, and at the same time, methylglyoxal can be transformed into lactate ion in the presence of the base by intramolecular Cannizzaro reaction [32,50]. The dehydrogenation/hydrogenation stage is catalyzed by metallic sites, while the dehydration step is catalyzed by OH⁻. Thus, the addition of potassium hydroxide to the initial mixture of precursors has a cocatalytic effect on the X_Gly, and changing the amount of KOH added can be a way to control the reaction selectivity. For example, Moreira et al. demonstrated that the lactic acid yield and selectivity could be increased by increasing the NaOH content [50].

Since the optimal ratios of precursors (Cu:(Cu + Zn) = 25%) permitting us to obtain the most active catalysts with the highest selectivity towards PG were established above, the influence of the hydrogen pressure and the reaction time on the Gly conversion and the product yields was studied (Figure 2). Dasari et al. [15] observed an increase in the Gly conversion with increasing hydrogen pressure, as well as, in our case, with an increase in hydrogen pressure from 1.0 MPa to 5.0 MPa, during which the Gly conversion was raised from 13.5 to 32.9% and the PG selectivity from 64 to 90% (Figure 2a). The lack of hydrogen led to the excessive formation of by-product—lactic acid (lactate ion). In the case of 1.0 MPa, the selectivity for lactate ion was 30%, and in the case of a slight excess of hydrogen and a pressure of 5.0 MPa, 6%. Moreover, the lack of hydrogen was not desirable because in an alkaline medium, polyglycerols might theoretically be formed under heating above 220 °C [51]. Hence, after conducting catalytic tests, it can be stated that the apparent order with respect to hydrogen was non-zero.

Using monometallic copper catalysts obtained in situ, a sharp reaction slowdown was observed over time due to the sintering of the catalyst [26]. Studying the influence of time on the Gly conversion and the product yields with the catalytic system modified by zinc precursor, it is worth emphasizing that 2 h later, the Gly conversion amounted to 14.6% (Figure 2b). With an increase in the time of the catalytic experiment, the conversion growth continued: 16 h later, the Gly conversion amounted to 47.9%; 24 h later, the Gly conversion and the PG yield amounted to 65.3 and 59.5%, respectively; and 48 h later, it amounted to 81.0 and 76.5%, respectively. Therefore, there was no significant slowdown in the rate of Gly hydrogenolysis over time.

The reproducibility of catalytic experiments using catalysts obtained in situ is demonstrated in Table S1 (for 15 experiments with the same synthesis conditions). The results obtained (s = 2.6%) showed that the error of in situ catalysis was within the error inherent in heterogeneous catalysis.

2.2. Composition of Cu-ZnO Catalysts Synthesized In Situ

2.2.1. XRF Analysis

Since the catalyst was formed during the hydrogenolysis, it was crucial to describe the main reactions occurring during the formation of the catalyst in situ. Glycerol and potassium hydroxide were consequently added to copper and zinc salts pre-dissolved in water. The reactions that took place can be characterized by the following equations (Equations (1)–(8)):

(CH₃COO)₂Cu + 2KOH → Cu(OH)₂↓ + 2CH₃COOK

(1)

Cu(OH)₂ + 2C₃H₈O₃ ↔ [Cu(C₃H₇O₃)₂] + 2H₂O

(2)

It is possible that the formation of copper glycerate complex occurred in parallel with the formation of potassium tetrahydroxocuprate (II) complex.

Cu(OH)₂ + 2KOH ↔ K₂[Cu(OH)₄]

(3)

The zinc precursor underwent the following changes:

(CH₃COO)₂Zn + 2KOH → Zn(OH)₂↓ + 2CH₃COOK

(4)

then

Zn(OH)₂ + 2KOH ↔ K₂[Zn(OH)₄]

(5)

Furthermore, in a hydrogen environment, copper from the complex was reduced to Cu⁰ (E⁰_Cu2+/Cu0 = +0.345 V).

K₂[Cu(OH)₄] + H₂ → Cu↓ + 2KOH + 2H₂O

(6)

An alternative route of Cu²⁺ reduction by glycerol with the formation of glyceric acid salt was possible:

2K₂[Cu(OH)₄] + C₃H₈O₃ → 2Cu↓ + 3KOH +C₃H₅O₄K + 4H₂O

(7)

As for the zinc complex, the reductive potential of hydrogen was not enough to transform it into a metallic one (E⁰_Zn2+/Zn0 = –0.762 V). Tetrahydroxozincate complex decomposed into potassium hydroxide, water, and ZnO at 220 °C [52]. The last one partially dissolved once again in alkali medium.

K₂[Zn(OH)₄] ↔ ZnO + 2KOH + H₂O

(8)

Thus, the oxide form of zinc is in equilibrium with the complex form in the described catalytic system. The equilibrium can be influenced by temperature, as well as changes in glycerol concentration, since the equilibrium will also depend on the concentration of water.

Considering all the processes described above, it was significant to establish the conversion degree of the precursors to characterize the solid catalytic phase formed. The reaction mixtures after catalytic tests with different initial contents of precursor salts, varying for copper acetate from 0 to 100 mol%, were thoroughly centrifuged, with the supernatants examined by XRF (Table S2). The initial total molar content of precursor salts and alkali relative to glycerol was fixed, while the molar ratio of the salt precursors varied. Copper acetate was completely converted into a solid phase in all cases. After copper reduction, KOH was released, partially dissolving ZnO (Equations (6)–(8); Table S2, Entry 1–4). Without the initial copper precursor (Table S2, Entry 6), with 3 eq. KOH, after consumption of 2 eq. in the formation of zinc hydroxide, the following complex can be formed:

Zn(OH)₂ + KOH ↔ K[Zn(OH)₃]

(9)

The complex decomposed at temperature

K[Zn(OH)₃] ↔ ZnO + KOH + H₂O

(10)

Without an additional source of KOH, the zinc oxide did not dissolve; thus, the conversion of the zinc acetate was 100% (Table S2, Entry 6). Therefore, the dissolved copper and zinc precursors turned into a catalytic phase during the glycerol hydrogenolysis.

2.2.2. XRD Analysis

Taking XRF data of supernatants into account, the precipitates were studied by XRD (Figure 3,

Table 1. Zn/Cu ratio in the bulk and on the surface, crystallite size, total (S_BET) and metal surface area (S_Cu), and copper dispersion (D_Cu).

Catalyst	AAS	XPS	XRD		BET	Chemisorption N2O
	Atomic Zn/Cu Ratio (%)	Atomic Zn/Cu Ratio (%)	Cu (nm)	ZnO (nm)	SBET (m2/g)	SCu (m2/g-Cu)	DCu (%)
Zn-100	Only Zn	– *	–	16	35.4	– *	– *
CuZn-82	3.7	23.4	28	20	15.9	12.2	1.89
CuZn-65	1.5	8.7	27	30	13.8	7.1	1.10
CuZn-10	0.09	1.5	36	42	5.6	2.6	0.40
CuZn-2	0.02	0.4	33	25	2.7	2.2	0.33
Cu-100	Only Cu	– *	35	–	1.3	1.9	0.29

* The analysis was not carried out for the sample.

). The catalysts investigated were highly crystalline without amorphous components. In accordance with the X-ray diffraction pattern obtained, the phase composition of the formed catalyst Cu-100 was a crystalline metallic copper Cu⁰ with a typical face-centered cubic lattice (PDF 04-0836). The catalysts CuZn-82, CuZn-65, CuZn-10, and CuZn-2 were composed of both crystalline Cu⁰ and ZnO with a wurtzite-type hexagonal structure (PDF 65-3411). The sizes of crystallites were estimated by FWHM of the patterns, corresponding to the (111), (200), (220), (311), and (222) planes in Cu⁰ and corresponding to the (100), (002), (101), (102), (110), and (103) planes in ZnO according to the Hall–Williamson method. The smallest average crystallite sizes were observed for sample CuZn-82, with the lowest copper content equal to 28 nm for the Cu⁰ phase and 20 nm for the ZnO phase. The phase composition of the catalysts indicated that the copper acetate was reduced to Cu⁰, and the absence of Zn(OH)₂ demonstrated that the zinc precursor was thermally decomposed to ZnO during the glycerol hydrogenolysis.

2.3. Effect of Promotion by Zinc Oxide on Morphology and Structure of Cu-ZnO Catalysts Generated In Situ

To understand the effect of zinc oxide on the catalytic properties of copper catalysts, the sorption properties of catalysts with different ZnO content were considered (Table 1). In this paper, the zinc oxide content of the catalysts was equated to the degree of promotion. The degree of promotion of Cu-ZnO catalysts synthesized in situ was determined using AAS. The total surface area (S_BET) changed from 2.7 to 15.9 m²/g, with an increase in ZnO content from 0 to 82 wt%.

With an increase in the degree of promotion, changes in the metal surface and dispersion of catalysts were observed, as determined by the N₂O chemisorption method. Considering CuZn-2 and Cu-100 catalysts with Cu weight concentration 97.9 and 100.0%, respectively, the zinc oxide promotion was only 2.1 wt%, and a significant increase in copper metal surface was not observed. At the same time, when the promotion degree of Cu by ZnO was 10 wt%, it allowed one to increase S_Cu by 37%, from 1.9 to 2.6 m²/g-Cu. Comparing catalysts CuZn-10 and CuZn-65, with a rise in the degree of promotion by 6.5 times from 10 to 65 wt%, D_Cu increased by 2.8 times from 0.40 to 1.10%. As for catalysts CuZn-65 and CuZn-82, with a further increase in the degree promotion from 65 to 82 wt%, D_Cu raised by 1.7 times from 1.10 to 1.89%. Therefore, increasing the promotion degree from 0 to 82 mass % increased the total surface area, the metallic surface area per gram of copper, and the dispersion of copper particles.

The correlation between copper dispersity and zinc oxide content was also previously noted in research [33,35,47]. Wang et al. found that with an increase in the mole fraction of ZnO in the Cu-ZnO catalyst prepared by co-precipitation method for glycerol hydrogenolysis from 33.3 to 71.4 mol%, S_Cu changed from 34.2 to 49.5 m²/g-Cu [33]. For industrial catalysts of methanol synthesis, Behrens et al. found an increase in S_Cu by 4.3 times (from 6 to 26 m²/g) after doping copper with ZnO in the amount of 30 mol% [47].

In addition to the obtained data on the sorption of catalysts, changes in dispersity can be traced using SEM micrographs (Figure 4). In contrast to the catalyst containing only monometallic Cu⁰ (Cu-100), on the surface of which monolithic areas were observed, for the catalyst promoted with 10% wt ZnO (CuZn-10), such areas were absent. Although, in this case, the surface of this catalyst consisted of ingrown agglomerates. With a further increase in the degree doping to 65 wt%, instead of agglomerates, a highly dispersed stabilized system was already observed, and doping to 82 wt% led to a continued increase in dispersion. In addition, a growth of needle-shaped particles was noted, which, as shown by EDX (SEM-EDX), mostly consisted of ZnO (Figure S1).

The effect of the degree of promotion by ZnO up to 10 wt% on the morphology and the structure of the catalysts with use of TEM was considered (Figure 5). ZnO was not found in the CuZn-2 catalyst, using the EDX technique (however, as shown by XPS, ZnO was presented in a thin nm layer). Nevertheless, the morphology of the CuZn-2 sample differed from the catalyst containing only Cu⁰. The Cu-100 catalyst was represented by a single agglomerate about 5 µm in length and about 2.5 µm in width. The CuZn-2 catalyst consisted of interconnected agglomerates about 2 µm in size with cavities between them. In the case of the CuZn-10 catalyst, the cavities were already filled with ZnO, while the agglomerates themselves were slightly smaller, down to 2 µm in each dimension. It is possible that even a small-scale promotion by ZnO prevented a consolidation of agglomerates and could slow down the degradation and sintering of the catalytic phase observed in the case of monometallic copper catalysts [26].

The effect of increasing the degree of promotion by zinc oxide to 65 and 82 wt% on the morphology of Cu-ZnO catalysts is shown in Figure 6 and Figure 7, respectively. For the CuZn-65 catalyst, EDX showed that copper particles, round in shape and with a size distribution from 50 to 250 nm, were surrounded by less dense zinc oxide particles, ranging in size from 25 to 100 nm with a predominance of 50 nm particles. As catalytic tests demonstrated, using a catalyst with this composition and morphology, there was no significant slowdown in the glycerol hydrogenolysis reaction over time (Figure 2b), because CuZn-65 was resistant to agglomeration and degradation during the 48 h catalytic experiment. For the CuZn-82 catalyst, the average size of copper particles ranged from 35 to 190 nm, with the particles being spherical in shape, surrounded by zinc oxide particles ranging in size from 25 to 150 nm, of either round or oval shape. Therefore, it can be said that the promotion by a large amount of zinc oxide (more than 50 wt%) allowed us to obtain more dispersed Cu-ZnO catalysts, which was confirmed by TEM and chemisorption N₂O.

Attention should be paid to the arrangement of zinc oxide in Cu-ZnO catalysts. For obtained XPS patterns, Cu 2p^1/2, Cu 2p^3/2, Zn 2p^1/2, and Zn 2p^3/2 peak areas were calculated. Atomic Zn/Cu concentrations were determined taking into account the relative sensitivity factor. The XPS results indicated that the formation of the zinc oxide phase occurred mainly on the surface of the copper particles formed, since the concentration of zinc atoms on the surface significantly exceeded its concentration in the bulk (Table 1). For instance, the Zn/Cu atomic ratio for the catalyst CuZn-2 in bulk was 0.02% according to AAS, and the Zn/Cu atomic ratio on the surface according to XPS was 0.4%; for catalyst CuZn-10, this was 0.09 in bulk and 1.5% on the surface. For the CuZn-65 catalyst, the Zn/Cu atomic ratio was 1.5 in the bulk and 8.7% on the surface, and for the CuZn-82 catalyst, 3.7 in the bulk and 23.4% on the surface. Therefore, considering the reactions forming Cu-ZnO catalysts (Equations (1)–(10)), it was concluded that the reaction rate of copper particle reduction from solution was faster than the thermal decomposition of the potassium tetrahydroxozincate complex into zinc oxide, and as a result, copper particles were coated with zinc oxide.

2.4. The Activity of Cu-ZnO Catalysts Synthesized In Situ in Glycerol Hydrogenolysis

For the synthesized in situ series of catalysts with different contents of Cu and ZnO, the TOF values were calculated for 4 h of glycerol hydrogenolysis. The values of TOF per active center (surface Cu atoms) for a line of catalysts obtained in situ, some industrial catalysts, and catalysts prepared by co-precipitation, as well as one by oxalate gel method, are shown in

Table 2. TOF value calculated for Cu-ZnO and Cu-ZnO-Al₂O₃ catalysts.

Catalyst Preparation Method	Catalyst	Molar Ratio Cu:Zn:Al	TOF (10−3 s−1)	Conditions	Reference
in situ	CuZn-82	1:3.7	288	T = 220 °C, p(H2) = 3.0 MPa, τ = 4 h, 5.05 g Gly, 30–35 mg catalyst, batch reactor	Current research
	CuZn-65	2:3	511
	CuZn-10	12:1	404
	CuZn-2	60:1	221
	Cu-100	Only Cu	136
commercial copper catalysts	1	3:2	34	T = 200 °C, p(H2) = 5.0 MPa, τ = 7 h, 177 g Gly, 3 g catalyst, batch reactor	[36]
	2	5:5:2	18
	3	6:4:1	12
co-precipitation	4	2:5	1.7	T = 200 °C, p(H2) = 6.0 MPa, τ = 6 h, 10 g Gly, catalyst weight not specified, batch reactor	[33]
	5	1:1	2.6
	6	2:1	1.2
	7	Only Cu	0.4
	8	2:3	19	T = 220 °C, p(H2) = 5.0 MPa, τ = 8 h, 20 g Gly, 9 g catalyst, batch reactor	[53]
	9	2:3	9	T = 200 °C, p(H2) = 5.0 MPa, τ = 7 h, 140 mL Gly, 3 g catalyst, batch reactor	[54]
oxalate gel method	10	2:3	13

For catalysts 1–3 and 8–10, the TOF values were calculated from the corresponding reference.

. The lowest values were observed for catalysts prepared by co-precipitation (catalysts 4–7), and the maximum for catalyst 8 and 9, with Cu:Zn molar ratio = 2:3 equal to 19 and 9 × 10⁻³ s⁻¹, respectively. Commercial catalysts (catalyst 1–3) exhibited TOF values an order of magnitude higher compared to catalysts prepared by co-precipitation method (catalysts 4–7), and the maximum value of 34 × 10⁻³ s⁻¹ was achieved for the catalyst consisting of CuO and ZnO with molar ratio Cu:Zn = 3:2 (catalyst 1). For catalyst prepared by the oxalate gel method, the value was at the same magnitude as for the commercial catalysts, equal to 19 × 10⁻³ s⁻¹.

The TOF values even for monometallic catalysts prepared in situ (Cu-100) were an order of magnitude higher—136 × 10⁻³ s⁻¹. Maximum TOF values equal to 511 × 10⁻³ s⁻¹ were examined for CuZn-65 with molar ratio Cu:Zn = 2:3. Compared to catalysts prepared ex situ, the method of in situ catalyst synthesis allows us to obtain catalysts that are at least 4 times more active (Cu-100 compared to catalyst 1). The main cause for very high catalyst activity is the special structure and morphology of catalysts formed in situ (Figure 3, Figure 4, Figure 5 and Figure 6), which differs from catalysts prepared by other methods. For the catalysts generated in situ, TOF value varied, depending on the fraction of zinc oxide in the catalyst. With an increase in copper promotion by zinc oxide from 0 to 65 wt%, catalyst activity raised by 3.8 times, from 136 to 511 × 10⁻³ s⁻¹.

The first possible cause of the increased activity for Cu-ZnO catalysts in comparison with monometallic copper catalysts obtained in situ was the significant influence of zinc oxide, which, being a Lewis acid, facilitates one of the reaction stages—dehydration. This cause was put forward as the main one by Wang et al. and Balaraju et al. [25,29]. However, in addition to Cu-ZnO, strong base OH⁻ was in our catalytic system, in the presence of which zinc oxide could not exhibit acidic properties. Therefore, the first cause was considered unlikely in our reaction conditions.

The second possible cause was that the activity of the catalyst was raised with the increase in Cu dispersion. ZnO particles played a role as spacers between copper particles to prevent them from sintering. Previously, Bienholz et al. established a correlation between TON and S_Cu. It turned out that the TON value was directly proportional to S_Cu [36] in glycerol hydrogenolysis. The same dependence was constructed for the catalysts Cu-100, CuZn-2, CuZn-10, CuZn-65, and CuZn-82 (Figure S2), which also proved to have a linear growth pattern for Cu-100, CuZn-2, CuZn-10, and CuZn-65 catalysts.

The third possible cause was the synergistic interactions of Cu and ZnO affecting the dehydrogenation stage of glycerol hydrogenolysis. Strong metal-oxide interactions between Cu and ZnO influenced the electron transfer and consequently the d-electron density of Cu particles. Microstrain in the forming catalysts had a significant effect. Microstrain caused by a lattice defect can be contained in Cu particles larger than 5 nm, and the most common forms are twins and packing defects. Metal-oxide interactions between Cu and ZnO increase the number of microstrains in the catalyst. As a result, the activity of Cu-ZnO catalysts was related to the amount of microstrain. This phenomenon was described by Kasatkin et al. [37] in the methanol synthesis reaction, and Wang et al. [25] in the reaction of glycerol hydrogenolysis. The latter authors found a linear relationship between the percentage of microstrain and the activity of the catalyst, with the maximum amount of microstrain observed for a catalyst with a Cu:Zn molar ratio of 1:1. For supported Cu/ZnO catalysts, the phenomenon of increasing activity was attributed to strong metal support interactions (SMSI) [44,45,55,56]. Due to SMSI, ZnO atoms migrated and copper nanoparticles were covered with graphite-like ZnO_x layers, and as a result, the catalyst activity increased [44,55]. For instance, the exposure of commercial Cu/ZnO/Al₂O₃ catalyst in H₂/H₂O/CH₃OH/N₂ mixture at 300 °C under atmospheric pressure accelerated the migration of ZnO_x particles onto the Cu surface, which eventually led to a threefold increase in the long-term stability and a twofold increase in activity in the methanol stream reforming reaction [44].

In terms of their morphology, Cu-ZnO catalysts generated in situ were closer to inverse ZnO/Cu catalysts: larger and denser Cu particles were covered by several zinc oxide particles. The coating of copper with thin zinc oxide particles, demonstrated by XPS (Table 1) and TEM (Figure 6 and Figure 7), allowed us to consider the in situ synthesized catalysts as inverse ZnO/Cu catalysts, which, as shown by Palomino et al. [57], could be even more active than supported Cu/ZnO—the reaction rate of the methanol synthesis was 2 times (for ZnO/Cu(111)) and 3.75 times (for ZnO/Cu(100)) higher than for supported Cu/ZnO ones. The large number of electrons in metallic support could modify the spatial distribution in semiconductor-zinc oxide in a boundary layer of 10–100 nm in width, giving such inverse catalysts new catalytic properties [56].

2.5. Reusability Tests for Cu-ZnO Catalyst Generated In Situ in Glycerol Hydrogenolysis

In order to estimate catalytic stability for the most active Cu–ZnO catalyst (CuZn-65), five catalytic reaction cycles were carried out (Figure S3). After the first cycle, the catalyst was separated by centrifugation; then, 5.05 g of fresh glycerol, 0.27 g H₂O, and 0.01 g KOH were added for a new catalytic test. For cycle 2, Gly conversion decreased from 18.9 to 11.6% with an increase in selectivity towards PG from 84 to 90%. For cycle 3, Gly conversion continued to drop and was only 4.0%. To establish the reasons for the decrease in activity, the catalyst after the fifth cycle (CuZn-c5) was examined using TEM, EDX, and XPS. The first cause supposed that the drop in dispersion was due to the coagulation of copper particles. Nonetheless, according to EDX and TEM (Figures S4 and S5), there were no significant changes in the size and shape of Cu-ZnO after the fifth cycle. However, amorphous regions were found on the surface of the CuZn-c5 catalyst (Figure S6), which were not previously observed for the CuZn-85 catalyst after the first cycle in TEM micrographs. It was estimated, with the use of EDX technique, that the amorphous areas were most likely carbon deposits (Figure S7). To assess the change in surface composition, the atomic concentrations of copper, zinc, carbon, and oxygen on the catalyst surface (Table S3, Figures S8 and S9) were calculated for the catalysts after the first cycle (CuZn-65) and after the fifth cycle (CuZn-c5). The oxygen content on the surface, estimated by XPS, remained virtually unchanged (for cycle 1—29.2; for cycle 5—30.0 at. %), whereas the carbon content increased by 16.5% from 45.9 to 55.0 at. %.

It should be emphasized that if we evaluate the catalyst activity after the third cycle with the assumption that the D_Cu has not changed, we obtain a TOF value equal to 101 × 10⁻³ s⁻¹, which is higher than the values for catalysts 1–10 from Table 2.

To summarize, the decrease in the activity of Cu-ZnO catalyst is associated with its structural changes. According to TEM, one of the possible changes is the formation of amorphous structures on the catalyst surface. However, given the small mass of the catalyst, it cannot be ruled out that the loss of activity is not related to an elementary mass loss of the catalyst from cycle to cycle. In the future, we will clarify the issue of catalyst deactivation. If necessary, we plan to find methods for reactivating the deactivated Cu-ZnO catalyst and, possibly, modifying the catalyst composition to improve its long-term stability, maintaining the high activity and selectivity to PG.

3. Materials and Methods

3.1. Materials

Copper (II) acetate monohydrate (98%, Komponent-Reaktiv, Moscow, Russia), zinc acetate dihydrate (≥98%, Reakhim, Moscow, Russia), potassium hydroxide (98%, Chimmed, Moscow, Russia), glycerol (≥99.3%, Komponent-Reaktiv, Moscow, Russia), and distilled water were used to carry out the hydrogenolysis of glycerol and form the catalyst in situ. For calibration by GC-FID method, 1,2-propanediol (>99%, Carl Roth, Karlsruhe, Germany), ethylene glycol (≥99.5%, Komponent-Reaktiv, Moscow, Russia), D-lactic acid (95%, abcr, Karlsruhe, Germany), and the internal standard 1,2-Butanediol (98%, abcr, Karlsruhe, Germany) were used after derivatization into the corresponding TMS esters. Trimethylsilylating reagent TMS-HT (hexamethyldisilazane and trimethylchlorosilane in anhydrous pyridine; abcr, Karlsruhe, Germany) was used for derivatization. For qualitative determination by GC-MS analysis, the samples were diluted with methylene chloride (>98%, Chimmed, Moscow, Russia). The in situ formed catalysts were separated from liquid products by centrifugation, washed twice with distilled water and then twice with isopropyl alcohol (>99%, Komponent-Reaktiv, Moscow, Russia), dried at room temperature in dry Ar (≥98%) flow, and stored under an inert Ar atmosphere prior to the characterization analyses. Hydrogen (grade A in accordance with GOST 3022-80, MGPZ, Moscow, Russia) was used for hydrogenolysis.

3.2. Catalytic Tests

The catalytic tests were carried out in a stainless steel batch reactor equipped with a pressure gauge, thermocouple, polytetrafluoroethylene (PTFE) liner, and magnetic stirrer. The internal volume of the batch reactor was 50 cm³. A water-soluble copper and zinc precursor was dissolved in 1.56 g H₂O; then, 5.05 g of glycerol and 0.092 g KOH were added, and the mixture of precursors and substrate was loaded into a batch reactor. After the loading, the reactor was purged twice with 3.0 MPa of hydrogen and then filled with an operating hydrogen pressure. The closed reactor was placed in an electric furnace, then stirring was turned on. This moment was taken as the beginning of the reaction. The set temperature of 220 °C inside the reactor was achieved within 30–35 min.

The stirrer speed was set to 700 rpm (the absence of diffusion limitations was demonstrated by individual tests). At the end of the experiment, the reactor was promptly chilled in air, then the pressure was gently released. Each catalytic experiment was performed at least three times. The carbon balance in the catalytic tests was 100 ± 5%. The gaseous products were found to be negligible and, as a result, were not included in further calculations.

3.3. Analysis of Liquid Products

X-ray fluorescence method of analysis (XRF). To estimate the completeness of the conversion of copper and zinc precursors into the solid phase, the supernatants of the liquid products of the hydrogenolysis reaction were analyzed by X-ray fluorescence analysis on a Thermo ARL Perform’x Sequential XFR instrument (Thermo Fisher Scientific, Zürich, Switzerland). Quantitative analysis for copper and zinc in water–glycerol mixtures was carried out using the external standard method.

Gas chromatography with mass spectrometric detector (GC-MS). The qualitative composition of liquid products of the glycerol hydrogenolysis was studied by GC-MS on a ThermoFocus DSQ II instrument (Fisher Scientific, Waltham, MA, USA) with a Varian VF-5 ms capillary column (30 m × 0.25 mm × 0.25 μm). The carrier gas was helium. Capillary column temperature programming mode was 40–300 °C, heating rate was 15 °C/min, holding time was 10 min. The sample ionization method was electron ionization. Operating mode of the mass spectrometric detector: ionization energy was 70 eV, source temperature was 230 °C, and scanning was in the range of 10–800 Da at a speed of 2 scans/s, with unit resolution over the entire mass range. For the GC-MS analysis, 10 μL of the sample was diluted with 1 mL CH₂Cl₂. The GC-MS analysis did not reveal the formation of 1,3-propanediol and hydroxyacetone.

Gas chromatography with flame ionization detector (GC-FID). Liquid products of the hydrogenolysis after preliminary derivatization were analyzed on a Crystallux-4000M chromatograph (Meta-chrome, Yoshkar-Ola, Russia) equipped with a flame ionization detector and an Optima-1 capillary column (25 m × 0.32 mm × 0.35 μm). The carrier gas was helium. Temperature programming mode: the temperature of 70 °C was maintained for 1 min; from 70 to 100 °C, heating rate was 3 °C/min; the temperature of 100 °C was maintained for 1 min; from 100 to 230 °C, heating rate was 30 °C/min; the temperature of 230 °C was maintained for 1 min.

Silylation protocol. To obtain more volatile derivatives, liquid hydrogenolysis products with added 1,2-butanediol were converted using the commercially available trimethylsilylation reagent into TMS esters, and then analyzed by GC-FID on an Optima-1 column. For derivatization, 5 μL of the sample was mixed with 500 μL of the derivatizing agent. The resulting mixture was kept for 1 h at 70 °C.

Glycerol conversion (X), product yields (Y), selectivity (S), and activity (TOF) were calculated using the following equations:

$$\mathrm{X}\left(\%\right)={\displaystyle \frac{\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{e} \mathrm{o}\mathrm{f} \mathrm{c}\mathrm{o}\mathrm{n}\mathrm{s}\mathrm{u}\mathrm{m}\mathrm{e}\mathrm{d} \mathrm{s}\mathrm{u}\mathrm{b}\mathrm{s}\mathrm{t}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e}}{\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l} \mathrm{m}\mathrm{o}\mathrm{l}\mathrm{e} \mathrm{o}\mathrm{f} \mathrm{s}\mathrm{u}\mathrm{b}\mathrm{s}\mathrm{t}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e}}}\times 100\%$$

$$\begin{gathered} \mathrm{Y}\left(\%\right)={\displaystyle \frac{\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{e} \mathrm{o}\mathrm{f} \mathrm{f}\mathrm{o}\mathrm{r}\mathrm{m}\mathrm{e}\mathrm{d} \mathrm{p}\mathrm{r}\mathrm{o}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{t}}{\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l} \mathrm{m}\mathrm{o}\mathrm{l}\mathrm{e} \mathrm{o}\mathrm{f} \mathrm{s}\mathrm{u}\mathrm{b}\mathrm{s}\mathrm{t}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e}}}\times 100\% \\ \mathrm{S}(\%)={\displaystyle \frac{\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{e} \mathrm{o}\mathrm{f} \mathrm{f}\mathrm{o}\mathrm{r}\mathrm{m}\mathrm{e}\mathrm{d} \mathrm{p}\mathrm{r}\mathrm{o}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{t}}{\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{e} \mathrm{o}\mathrm{f} \mathrm{c}\mathrm{o}\mathrm{n}\mathrm{s}\mathrm{u}\mathrm{m}\mathrm{e}\mathrm{d} \mathrm{s}\mathrm{u}\mathrm{b}\mathrm{s}\mathrm{t}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e}}}\times 100\% \end{gathered}$$

$$\mathrm{T}\mathrm{O}\mathrm{F}={\displaystyle \frac{\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{e} \mathrm{o}\mathrm{f} \mathrm{c}\mathrm{o}\mathrm{n}\mathrm{s}\mathrm{u}\mathrm{m}\mathrm{e}\mathrm{d} \mathrm{s}\mathrm{u}\mathrm{b}\mathrm{s}\mathrm{t}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e}}{\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{e} \mathrm{o}\mathrm{f} \mathrm{s}\mathrm{u}\mathrm{r}\mathrm{f}\mathrm{a}\mathrm{c}\mathrm{e} \mathrm{C}\mathrm{u} \mathrm{a}\mathrm{t}\mathrm{o}\mathrm{m}\mathrm{s}\times \mathrm{t}\mathrm{i}\mathrm{m}\mathrm{e}}}={\displaystyle \frac{\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{e} \mathrm{o}\mathrm{f} \mathrm{c}\mathrm{o}\mathrm{n}\mathrm{s}\mathrm{u}\mathrm{m}\mathrm{e}\mathrm{d} \mathrm{s}\mathrm{u}\mathrm{b}\mathrm{s}\mathrm{t}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e}}{\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{e} \mathrm{o}\mathrm{f} \mathrm{C}\mathrm{u}\times \mathrm{D}(\%)\times \mathrm{t}\mathrm{i}\mathrm{m}\mathrm{e}}}\times 100\%$$

3.4. Characterization of Cu-ZnO Catalysts

In situ technique for the preparation of Cu-ZnO catalysts.

Catalysts (

Table 3. Names of catalysts, composition, and conditions of their synthesis in situ.

Catalyst	Molar Ratio Cu/(Cu + Zn) in Precursor Mixture (%)	Composition (wt%)
		Cu	ZnO
Zn-100	only Zn2+	–	100
CuZn-82	12.5	17.6	82.4
CuZn-65	25	34.9	65.1
CuZn-10	50	90.0	10.0
CuZn-2	75	97.9	2.1
Cu-100	100	100	–

) were prepared in situ from copper (II) acetate monohydrate and zinc acetate dihydrate in 1.56 g of water, 5.05 g of glycerol, and 0.092 g of KOH at p(H₂) = 3.0 MPa, 220 °C, τ = 4 h.

Chemical analysis (AAS). The content of copper and zinc in the samples was determined using the method of atomic adsorption spectroscopy on an AAnalyst 400 instrument (Perkin Elmer, Waltham, MA, USA) with a flame atomizer. The sample preparation procedure included mineralization with concentrated nitric acid until complete dissolution.

X-ray phase analysis (XRD). The phase composition of the samples was studied by X-ray diffraction on a Rotaflex D/Max-RC diffractometer (Rigaku, Tokyo, Japan) with a rotating copper anode and a secondary graphite monochromator (CuK_α radiation wavelength 0.1542 nm) in Bragg–Brentano geometry in continuous θ–2θ scanning mode in the angular range 2θ = 25 ÷ 100°. Scanning speed was 2°/min, and scanning step was 0.04°. The experimental diffraction patterns were processed in the MDI Jade 6.5 program; the phase composition was identified using the ICDD PDF-2 diffraction database.

X-ray photoelectron spectroscopy (XPS). The elemental composition of the catalyst surface was investigated by XPS with a PHI 5500 ESCA X-ray photoelectron spectrometer (Physical Electronics, Chanhassen, MN, USA). Nonmonochromatic AlK_a radiation (hn = 1486.6 eV) with a power of 300 W was used for the excitation of photoemission. The photoelectron peaks were calibrated based on the C^1s line of carbon with a binding energy of 284.5 eV. The obtained spectra were processed with the use of CasaXPS software (version 2.3.26).

Transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The structure and morphology of the catalyst samples were studied by a Tecnai Osiris transmission electron microscope (FEI Company, Hillsboro, OR, USA) equipped with a field emission electron gun operated at 200 kV and an attachment for energy dispersive X-ray (EDX) spectroscopy analysis.

Electron microscopic images were obtained using a Tabletop Microscope TM3030Plus (Hitachi, Tokyo, Japan) equipped with a Bruker Silicon Drift Detector (SDD) for EDX.

N₂O Chemisorption. Metal surface area of catalyst was obtained using the chemisorption analyzer Chemosorb (Neosib, Novosibirsk, Russia). The sample was reduced in a flow of H₂ (30 mL/min) at 250 °C for 2 h, and then cooled to 50 °C and oxidized to Cu₂O in a flow of 1%N₂O/He (30 mL/min) for 1 h until a stable TCD signal was obtained. To determine the amount of Cu₂O, H₂-TPR was carried out in a flow of 9.7% H₂/Ar (30 mL/min), while heated at 10 °C/min to 300 °C. Water was removed before gas entered the detector, using a trap at −80 °C. Copper surface area per gram of copper (S_Cu) and dispersity (D) were calculated using the following equations:

$${\mathrm{S}}_{\mathrm{C}\mathrm{u}}=2\times {\displaystyle \frac{{\mathrm{V}}_{\mathrm{H}2}}{{\mathrm{V}}_{\mathrm{m}}}}\times {\mathrm{N}}_{\mathrm{a}}\times {\displaystyle \frac{{\mathrm{a}}_{\mathrm{C}\mathrm{u}}}{{\mathrm{m}}_{\mathrm{c}\mathrm{a}\mathrm{t}}\times {\mathsf{\omega }}_{\mathrm{C}\mathrm{u}}}}$$

$$\mathrm{D}(\%)={\displaystyle \frac{2\times \frac{{\mathrm{V}}_{\mathrm{H}2}}{{\mathrm{V}}_{\mathrm{m}}}}{{\mathrm{m}}_{\mathrm{c}\mathrm{a}\mathrm{t}}\times {\mathsf{\omega }}_{\mathrm{C}\mathrm{u}}/{\mathrm{M}}_{\mathrm{C}\mathrm{u}}}}\times 100\%$$

where ${\mathrm{V}}_{\mathrm{H}2}$—the volume of adsorbed hydrogen (L), ${\mathrm{V}}_{\mathrm{m}}$—molar volume (22.414 L/mol), ${\mathrm{N}}_{\mathrm{a}}$—Avogadro constant (6.022 × 10²³), M_Cu—molar weight of Cu (63.55 g/mol) ${\mathrm{a}}_{\mathrm{C}\mathrm{u}}$—the area of Cu atom (6.85 Ų or 6.85 × 10⁻²⁰ m²), ${\mathrm{m}}_{\mathrm{c}\mathrm{a}\mathrm{t}}$—weight of catalyst (g), and ${\mathsf{\omega }}_{\mathrm{C}\mathrm{u}}$—Cu weight fraction.

Specific surface area by the BET method. The study of the surface characteristics of the samples was carried out using a Belsorp mini X device (MicrotracBEL Corp., Tokyo, Japan). The preliminary preparation of samples included thermal vacuuming at 200 °C and a pressure of 10 Pa for 8 h. The total specific surface area was estimated in accordance with the BET method in the relative pressure range p/p₀ = 0.05–0.35.

4. Conclusions

The synthesis of 1,2-propanediol from renewable glycerol is of considerable significance due to rapidly increasing FAME-based biodiesel fuel. In this paper, a novel approach for glycerol hydrogenolysis with use of Cu-ZnO catalysts generated in situ was proposed. This approach has a number of advantages: it allows us to synthesize the catalyst directly in the reaction medium and to obtain highly active and selective catalysts (TOF = 0.511 s⁻¹ and S_PG = 86%). The effect of the precursor composition (molar ratio of zinc and copper acetates) and hydrogen pressure on the Gly conversion and product yields were investigated. For the most active catalyst with Cu:Zn (molar ratio = 3:2), X_Gly = 81.0% and Y_PG = 76.5% were obtained in the 48 h catalytic experiment. In situ generated catalysts were characterized by AAS, XRD, XPS, SEM, TEM, EDX, BET, and chemisorption N₂O. An increase in the ZnO proportion in the catalyst from 0 to 82 wt% increased S_BET from 1.3 to 15.9 m²/g, S_Cu from 1.9 to 12.2 m²/g-Cu, and D_Cu from 0.29 to 1.89%. The changes in morphology and increase in copper dispersion were also observed by SEM and TEM.

Compared to monometallic copper catalysts generated in situ, modification with zinc oxide equal to 65 wt% enabled one to produce more active (TOF increase from 0.136 to 0.511 s⁻¹), selective (S_PG increase from 74 to 86%), and agglomeration-resistant Cu-ZnO catalysts. Zinc oxide served as a spacer between the copper particles, preventing them from sintering. An almost 4-fold increase in activity was achieved due to two factors—the production of more dispersed catalysts (D_Cu increased by 3.8 times from 0.29 to 1.10%) and the synergistic effect of copper and zinc oxide. According to XPS, TEM, and EDX, in terms of their morphology, in situ synthesized Cu-ZnO catalysts were similar to oxide/metal inverse catalysts—larger copper particles (50–250 nm in size) were coated with smaller, thin zinc oxide particles (25–100 nm in size).