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Insights into deactivation mechanism of Cu–ZnO catalyst in hydrogenolysis of glycerol to 1,2-propanediol
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JIEC-2782; No. of Pages 6 Journal of Industrial and Engineering Chemistry xxx (2016) xxx–xxx
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Insights into deactivation mechanism of Cu–ZnO catalyst in hydrogenolysis of glycerol to 1,2-propanediol Yan Du a, Cancan Wang b, Hong Jiang b, Changlin Chen b, Rizhi Chen b,* a b
College of Environment, Nanjing Tech University, Nanjing 210009, PR China State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing Tech University, Nanjing 210009, PR China
A R T I C L E I N F O
Article history: Received 16 November 2015 Received in revised form 23 December 2015 Accepted 4 January 2016 Available online xxx Keywords: Submerged catalysis/membrane filtration system Glycerol hydrogenolysis 1,2-Propanediol Cu–ZnO catalyst Catalyst deactivation
A B S T R A C T
The deactivation mechanism of Cu–ZnO catalyst was investigated in detail. During the glycerol hydrogenolysis cycles, the morphology of Cu–ZnO catalyst was firstly changed from spherical nanoparticles to lamellar structure and then to rod-like structure, and the uniform surface composition was deteriorated. The obvious aggregation of Cu and ZnO crystallite sites led to the decrease in specific surface area and pore volume. Furthermore, the Cu content on the catalyst surface significantly decreased with the increase of the number of reaction cycle. These findings provide in-depth insights on the deactivation of Cu–ZnO catalyst and the obvious decrease in glycerol conversion. ß 2016 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Introduction Biomass, as the only natural renewable carbon source, can be applied for sustainable production of liquid fuels and chemicals, which effectively alleviates the world’s dependence on fossil fuels [1–4]. As one typical instance of biomass utilization, biodiesel is usually produced by transesterification of triglycerides in vegetable oils or animal fats [5,6]. However, as the by-product, large amount of glycerol is generated and takes approximately 10 wt% of the total products in the process of biodiesel production. The chemical conversion of glycerol to polymers or fine chemicals enables the substitute of the polyol feedstocks produced from petrochemical industry. 1,2-Propanediol is widely used as the monomer for the synthesis of unsaturated polyester resins, and it is of great use as the solvent in paints, liquid detergents, cosmetics and functional fluids. Following the petroleum-based route, 1,2propanediol is produced by the hydrolysis of propylene oxide that is converted through selective oxidation of propylene. Alternatively, the hydrogenolysis of glycerol into 1,2-propanediol is environmentally friendly and value-added, which receives more attention from academia and industry [7,8].
* Corresponding author. Tel.: +86 25 83172286. E-mail address: [email protected] (R. Chen).
Heterogeneous catalysts with Ru, Ag, Pt, Pd, Ni, Cu or their alloys supported on oxides have been intensively probed into in attempts to achieve high performance of glycerol hydrogenolysis [9–18]. In spite of the high catalytic activity, the noble metal catalysts are not preferred in glycerol hydrogenolysis because of their high cost and unfavorable selective cleavage between C–C and C–O bonds [9–12]. Meanwhile, Ni-based catalysts for glycerol hydrogenolysis have a high cleavage selectivity of C–C bonds instead of C–O bonds, leading to the lower 1,2-propanediol selectivity. Recently, the indepth evaluations have reached that Cu active sites in Cu-based catalysts are superior to those in noble metal and Ni-based catalysts due to their essential ability in selective cleavage of C–O bonds rather than C–C bonds in the glycerol molecule, contributing to 1,2-propanediol formation, which makes it easier for the posttreatment process of glycerol hydrogenolysis including the separation of target product from the reaction system [14–18]. Therefore, much attention has been paid to the study of Cu-based catalysts especially Cu–ZnO on the hydrogenolysis of glycerol [15–18]. Up to date, the studies have been focused on the development of catalyst preparation methods and conditions optimization [10,16]. However, little effort was given to the exploration of Cu–ZnO deactivation mechanism, although Cu–ZnO deactivation had already been observed in hydrogenolysis of glycerol to 1,2propanediol [15,16]. The glycerol conversion and 1,2-propanediol
http://dx.doi.org/10.1016/j.jiec.2016.01.002 1226-086X/ß 2016 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
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selectivity were strongly related to the Cu and ZnO particle sizes, i.e., the smaller the Cu and ZnO particle sizes, the higher the glycerol conversion and 1,2-propanediol selectivity. Wang et al. [15] adopted Cu–ZnO catalysts prepared by a co-precipitation method for the hydrogenolysis of glycerol to 1,2-propanediol. It was found that the glycerol conversion and 1,2-propanediol selectivity decreased with the increase of the initial pH value, which was mainly due to the aggregation of Cu and ZnO particles at higher pH. Bienholz et al. [16] prepared a CuO–ZnO catalyst by an oxalate gel method for the glycerol hydrogenolysis, and the glycerol conversion of 55% and 1,2-propanediol selectivity of 86% were obtained with the use of 1,2-butanediol as a solvent. Different from 1,2-butanediol, the presence of water as a solvent made the size of Cu crystallites of CuO–ZnO catalyst grow drastically, resulting in a drop in active surface area and a loss of catalytic activity. In our previous work [18], a submerged catalysis/ membrane filtration system was developed for the semi-continuous hydrogenolysis of glycerol to 1,2-propanediol over Cu–ZnO catalyst, in which the in-situ separation and reuse of catalyst was achieved. It was found that the glycerol conversion decreased obviously during eight reaction cycles due to the aggregation of Cu and ZnO crystallite sites. Some explanations have been proposed for the deactivation of Cu–ZnO catalysts. However, the deep understanding of deactivation mechanism still remains a real challenge. In this work, in order to seek out the reasons for the drop of glycerol conversion and further to explore the deactivation mechanism of Cu–ZnO catalyst, a series of characterization techniques including XRD, FESEM, HRTEM, BET, TPD, N2O chemisorption, XPS and ICP were applied for the research of micromorphology and physicochemical properties of the Cu–ZnO catalysts.
normal boiling point (77 K), prior to which the sample was evacuated at 393 K for 4 h. Temperature-programmed desorption of ammonia (NH3-TPD) was used to determine the acidities of the catalysts by a BELCAT-A equipment connected to a thermal conductivity detector (TCD). H2-TPD was carried out with the same procedure as NH3-TPD. In order to determine the Cu surface area and its dispersion in the recovered catalysts, N2O chemisorption was carried out by means of the BELCAT-B equipment with a TCD detector. In a typical experiment, the pre-reduction of CuO in the catalysts to Cu0 was performed with 10% H2/Ar mixture at 573 K for 1 h. The sample was cooled down in Ar, and then Cu0 exposed to an atmosphere of 5% N2O/He was oxidized to Cu2O at 353 K for 30 min. Finally, temperature-programmed reduction (TPR) was carried out to reduce Cu2O on the catalyst surface to Cu0. A TCD detector was used to measure the amount of H2-uptake. During the after-treatments of recovered Cu–ZnO catalyst, parts of Cu on the surface might be oxidized to CuO. Therefore, it was necessary to perform the pre-reduction of recovered Cu–ZnO catalyst in the NH3-TPD and N2O chemisorption analyses. The X-ray photoelectron spectroscopy (XPS) measurements were performed on a Thermo ESCALAB 250 spectrophotometer with a monochromatic Al Ka (1486.6 eV) radiation source. The C1s peak (284.8 eV) was used for the calibration of binding energy. The catalyst composition was characterized by inductively coupled plasma optical emission spectrometry (ICP-AES, Optima 2000 DV). Glycerol hydrogenolysis
The Cu–ZnO catalyst was prepared by a co-precipitation method according to our previous work [18]. First, the aqueous solutions of Cu(NO3)23H2O and Zn(NO3)26H2O were mixed and the precursor was yielded with the Cu2+/Zn2+ molar ratio of 1:1.4. Second, the as-prepared precursor and the Na2CO3 solution (2 mol/ L) were transferred to a flask by peristaltic pumps, and the reaction was performed at 343 K with a constant pH value of 8.0 for 1 h. After the reaction, the precipitate was collected by vacuum suction filtration and washed using deionized water to completely remove sodium ions. Third, the rinsed precipitate was treated at 383 K overnight and then calcined in an oven at 723 K for 3 h. Finally, the as-prepared Cu–ZnO catalyst was ground into the fine enough particles for the hydrogenolysis of glycerol. During the catalyst preparation, all chemicals were purchased from commercial sources and used without further treatments.
The hydrogenolysis of glycerol to 1,2-propanediol over Cu–ZnO catalyst, depicted in Scheme 1 [17], was carried out in a submerged catalysis/membrane filtration system, in which a ceramic membrane tube was introduced to the reaction system for the in situ separation of Cu–ZnO catalyst from the suspension slurry [18]. A semi-continuous glycerol hydrogenolysis reaction process was performed to investigate the feasibility of the submerged catalysis/membrane filtration system for the glycerol hydrogenolysis. In the initial reaction cycle, 9 g of Cu–ZnO catalyst and 20 g of glycerol dissolved in 1 L of deionized water were loaded into the reactor, and the reaction was carried out for 8 h under the optimal reaction conditions: stirring rate of 400 rpm, hydrogen pressure of 5 MPa, temperature of 220 8C. In each cycle, the membrane filtration operation was carried out under the optimal filtration conditions: stirring rate of 0 rpm, transmembrane pressure of 0.2 MPa, temperature of 30 8C. The Cu–ZnO catalyst particles were retained in the reactor by the membrane, and then the same volume of fresh reaction solution with the permeation mixture was added into the reactor for next cycle. The glycerol conversion was defined as the ratio of sum of C-based mol of all products to the C-based mol of initially added glycerol, while the 1,2-propanediol selectivity was calculated on the basis of the ratio of C-based mol of the 1,2-propanediol to sum of C-based mol of all products [17,18].
Catalyst characterization
Results and discussion
The X-ray powder diffraction (XRD) patterns were recorded on a Rigaku MiniFlex600 diffractometer using Cu Ka radiation at 40 kV and 15 mA with the 2u range of 20–808. The morphology of Cu–ZnO catalyst was examined by field emission scanning electron microscope (FESEM, Hitachi S-4800II, Japan), and the element distribution was determined by energy dispersive X-ray spectrum (EDX, HORIBA). To verify the distribution and morphology of CuO, Cu and ZnO particles, high resolution transmission electron microscope (HRTEM, JEM 2100) was adopted for the fresh and recovered catalysts. The N2 adsorption–desorption isotherms were measured by using an ASAP 2020 analyzer (Micromeritics) at its
Catalytic stability of Cu–ZnO catalyst
Experimental Preparation of Cu–ZnO catalyst
In order to investigate the catalytic stability of Cu–ZnO catalyst, a number of catalytic reaction cycles were carried out in a submerged catalysis/membrane filtration system [18]. The influence of the
Dehydration HO
OH OH
ZnO
O
Hydrogenation OH
Cu
HO
Scheme 1. The two-step reaction mechanism of glycerol hydrogenolysis.
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OH
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Table 1 Specific surface area and pore volume of Cu–ZnO catalysts.
1.0 0.9 Glycerol conversion 1,2-Propanediol selectivity
0.8 0.7 0.6 0.5 0.4 0
1
2
3
4
5
6
7
8
9
Number of catalytic reaction cycle Fig. 1. Change of glycerol hydrogenolysis performance with the number of catalytic reaction cycle.
number of catalytic reaction cycle on the glycerol conversion and 1,2-propanediol selectivity is shown in Fig. 1. Obviously, the selectivity of 1,2-propanediol remained stable while the glycerol conversion sharply decreased by 56.7% through eight cycles, which indicated the obvious deactivation of Cu–ZnO catalyst. As discussed in our previous work [17], the selectivity of 1,2-propanediol was mainly related to the solvent in the hydrogenolysis of glycerol over Cu–ZnO catalyst. During all the catalytic reaction cycles, water was served as the solvent, thus the 1,2-propanediol selectivity could remain stable. Deactivation mechanism of Cu–ZnO catalyst
Fresh
After the first cycle
After the eighth cycle
Specific surface area (m2/gcat) Pore volume (cm3/gcat)
22.8 0.17
11.3 0.047
7.6 0.022
Fig. 2 gives the nitrogen adsorption-desorption isotherms of different Cu–ZnO catalysts, i.e., the fresh one, the one after the first cycle and the one after the eighth cycle. All the samples exhibited Langmuir type IV isotherms. The overlapped adsorption-desorption curves were found at the low pressure. At the high pressure, obvious hysteresis loops appeared especially for the fresh Cu–ZnO catalyst, which was the characteristic of mesoporous materials [19,20]. The existence of mesoporous might be caused by the aggregation of crystals as shown in the following FESEM analyses. However, compared to the fresh catalyst, the recovered catalysts had drastic drop of the N2 adsorption amount, especially when the eighth cycle was finished. This indicated that the severe aggregation of the Cu–ZnO catalyst crystals took place during the catalytic reaction cycles. Table 1 lists the specific surface area and pore volume of the fresh and recovered Cu–ZnO catalysts. Compared to the fresh catalyst, the recovered catalysts had smaller values of specific surface area and pore volume, and the specific surface area and pore volume decreased gradually with the increase of the number of reaction cycle. These results suggested that the Cu–ZnO crystals tended to aggregate during the glycerol hydrogenolysis cycles, leading to a reduction of specific surface area and active sites, then to a loss of catalytic activity and glycerol conversion [21]. The acidities of recovered Cu–ZnO catalysts were detected by NH3-TPD according to the literatures [22–24], as shown in Fig. 3. For both samples, two NH3 desorption peaks located in the range of 260–450 and 500–650 8C, respectively, were observed, indicating that the two Cu–ZnO catalysts all had a strong and a medium acid sites [23,24]. Furthermore, the amount of the medium acid site was much larger than that of the strong acid site. By calculating the peak area, the total amount of acidity (16.85 mmol/g) of the Cu–ZnO catalyst after the eighth cycle was much less than that (43.61 mmol/g) of the one after the first cycle, which demonstrated that the total amount of acidity decreased gradually with the increase of the number of reaction cycle. This was in consistent with the XRD results [17]. According to the reaction mechanism of glycerol hydrogenolysis over Cu–ZnO catalyst as presented in Scheme 1, glycerol is dehydrated to acetol on ZnO acidic sites followed by the hydrogenation on Cu active
Intensity (a.u.)
Quantity adsorbed (cm3/g STP)
To explore the deactivation mechanism of Cu–ZnO catalyst, the fresh and recovered catalysts were characterized in detail by XRD, N2 adsorption-desorption, NH3-TPD, H2-TPD, FESEM, EDS, HRTEM, N2O chemisorption, XPS and ICP. In our previous work [18], XRD was adopted to analyze the microscopic crystalline structure such as the Cu and ZnO crystallites. It was found that CuO phase in the fresh catalyst could be completely converted to metallic Cu during the reaction, which was necessary for the formation of 1,2-propanediol [17]. The Cu and ZnO crystallites aggregated during the continuous reaction, and continually grew up with the number of catalytic reaction cycle, leading to the decrease in catalytic active sites and glycerol conversion.
Cu–ZnO catalyst
(a)
(a)
(b) (c) 0.0
(b) 0.2
0.4
0.6
0.8
1.0
P/P0 Fig. 2. N2 adsorption–desorption isotherms of Cu–ZnO catalysts: (a) fresh; (b) after the first cycle; (c) after the eighth cycle.
100
200
300
400
500
600
700
800
Temperature Fig. 3. NH3-TPD diagrams of Cu–ZnO catalysts: (a) after the first cycle; (b) after the eighth cycle.
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Table 2 EDS analysis of Cu–ZnO catalysts.
Intensity (a.u.)
Elements (mol.%)
Cu Zn Cu/Zn molar ratio
(a)
Fresh
After the first cycle
After the eighth cycle
1
2
3
4
5
6
0.170 0.273 0.623
0.174 0.275 0.633
0.060 0.201 0.299
0.061 0.184 0.331
0.095 0.405 0.235
0.273 0.414 0.659
(b)
100
200
300
400
500
600
700
800
900
Temperature Fig. 4. H2-TPD diagrams of Cu–ZnO catalysts: (a) after the first cycle; (b) after the eighth cycle.
sites [15,25]. Therefore, by increasing the number of reaction cycle, the total amount of ZnO acidic sites decreased, leading to a drop of glycerol conversion. The adsorption amount of hydrogen is related to the hydrogen activation and spillover capacity, and the greater the amount of hydrogen adsorption, the better the catalytic performance [26]. Fig. 4 shows the H2-TPD curves of the recovered Cu–ZnO catalysts involving after the first cycle and the eighth cycle. For each sample, only one peak in 300–800 8C was observed, but the amount of hydrogen desorption of the Cu–ZnO catalyst after the eighth cycle was less than that after the first cycle. The results indicated that the hydrogen desorption capacity of Cu–ZnO catalyst decreased with the increase of the number of reaction cycle, contributing to the reduction of glycerol hydrogenation ability. Fig. 5 shows the FESEM images and EDS analysis images of Cu– ZnO catalysts. Compared to the fresh catalyst, the morphologies of
the recovered catalysts were remarkably changed. The fresh catalyst was mainly composed of nanoparticles, and larger spherical particles were formed by piling up the nanoparticles. In the recovered catalysts, besides the particles, the lamellar structure was observed after the first cycle and the rod-like structure was formed after the eighth cycle. To explore the reasons for the change of Cu–ZnO morphology, EDS was further performed to analyze the compositions of the fresh and recovered catalysts. Six regions labeled by 1–6 in Fig. 5 were selected for EDS analysis. Table 2 lists the metal element content including Cu and Zn on the surface of Cu–ZnO catalysts in different regions. The results showed that the Cu/Zn molar ratio in regions 1 and 2 was almost the same, indicating the uniform distribution of Cu and Zn in the spherical particles of the fresh Cu–ZnO catalyst. On the surfaces of the lamellar structure, the Cu/Zn molar ratio was close to that on the spherical particles, as indicated in regions 3 and 4. Hence, there was no obvious difference in the elements composition between the lamellar structure and spherical particles. Differently, for the Cu–ZnO catalyst after the eighth cycle, the Zn content on the surface of rod-like structure in region 5 was much larger than that of spherical particles in region 6. It could be concluded that the rodlike structure mainly consisted of the ZnO crystallites. The structure and morphology of Cu–ZnO catalysts have been characterized in more detail by HRTEM, as shown in Fig. 6. The fresh catalyst was composed of nearly uniform spherical particles
Fig. 5. FESEM images of Cu–ZnO catalysts: (a and d) fresh; (b and e) after the first cycle; (c and f) after the eighth cycles. EDS analysis images of Cu–ZnO catalysts: (g) fresh; (h) after the first cycle; (i) after the eighth cycle.
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Fig. 6. TEM images and electron diffraction patterns of Cu–ZnO catalysts: (a–c) fresh; (d–f) after the first cycle; (g–i) after the eighth cycle.
with a diameter of approximately 25 nm, and the aggregation of spherical particles was observed (Fig. 6(a)). The lattice fringes of 0.230 and 0.261 nm were found on the surface of spherical particle, corresponding to CuO (2 0 0) and ZnO (0 0 0 2) planes, respectively. And the electron diffraction pattern depicted in Fig. 6(c) indicated that the fresh catalyst was a mixture of CuO and ZnO crystal structure [27,28], in agreement with the SEM and EDS analyses. After the first cycle, the Cu–ZnO catalyst mainly presented the lamellar structure where the Cu nanoparticles with an average diameter of 60 nm were dispersed, and a small amount of ZnO rodlike structure was also found (Fig. 6(d)). The lattice fringes of 0.210 and 0.261 nm were attributed to Cu (1 1 1) and ZnO (0 0 0 2) planes, respectively, indicating that the lamellar structure consisted of the Cu and ZnO [29]. As shown in Fig. 6(f), the structure of regular hexagon should be indexed as ZnO (0 0 0 2) plane of hexagonal wurtzite structure, while the diffraction rings were ascribed to Cu (1 1 1) plane [28,30]. After the eighth cycle, the morphology of Cu–ZnO catalyst was changed into the micro-sized rod-like structure (Fig. 6(g)). The lattice spacing of 0.261 nm of ZnO nanorods corresponded to ZnO (0 0 0 2) plane where the Cu nanoparticles were dispersed with an average diameter of 70 nm. As given in Fig. 6(i), the diffraction pattern of ZnO nanorods presented the single crystallinity structure [31,32]. By SEM and TEM analyses, during the glycerol hydrogenolysis cycles, the structure and morphology of Cu–ZnO catalyst were changed significantly. The morphology was shifted from spherical nanoparticles to lamellar structure and then to rod-like structure. The composition was changed from even to uneven, and both Cu and ZnO sizes increased, especially for the ZnO. These changes took into account the loss of catalytic activity and glycerol conversion. The Cu dispersion is regarded as one of the key factors for the evaluation of Cu-based catalysts [33]. Table 3 lists the dispersion Table 3 Cu dispersion and specific surface area measured by N2O chemisorption. Cu–ZnO catalyst
Dispersion (%)
Specific surface area (m2/gcat)
After the first cycle After the eighth cycle
0.83 0.59
1.71 1.22
and specific surface area of Cu calculated from N2O chemisorption. Compared to the recovered catalyst after the first cycle, both the dispersion and specific surface area of Cu decreased for the recovered catalyst after the eighth cycle. It could be explained that the aggregation of Cu crystals generated with the increase of the number of reaction cycle, leading to the reduction of Cu active surface area, and then to the loss of catalytic activity and glycerol conversion. The results were in good accordance with the XRD analyses, and further confirmed the causes of catalyst deactivation. Table 4 lists the contents of Cu, Zn and O elements on the surface of Cu–ZnO catalysts according to the XPS characterization. The results showed that the content of Cu element on the surface of the recovered Cu–ZnO catalysts decreased along with the increase of Zn content in comparison with the fresh catalyst, and the changes became more evident with the increase of the number of reaction cycle. The phenomena might be due to the change of catalyst microstructure, i.e., parts of Cu nanoparticles were covered by ZnO crystals during the catalytic cycles. During the glycerol hydrogenolysis over Cu–ZnO, glycerol is dehydrated to acetol on ZnO acidic sites followed by the hydrogenation on Cu active sites [15,25]. Thus, the decrease of Cu content on the Cu–ZnO surface would account for the deactivation of Cu–ZnO catalyst in the hydrogenolysis of glycerol. In order to determine whether the leaching of Cu–ZnO catalyst was responsible for the decrease of glycerol conversion, the permeate through the membrane was analyzed by ICP, and the contents of Cu and Zn metal elements in the Cu–ZnO catalysts were also measured by ICP. The ICP analyses showed that no obvious Cu and Zn elements were found in the permeate, indicating that the Cu–ZnO catalyst could be completely retained by the ceramic Table 4 Atomic concentration of different elements for Cu–ZnO catalysts. Cu–ZnO catalyst
Atomic concentration (%) Cu2p
Zn2p
O1s
Fresh After the first cycle After the eighth cycle
13.03 3.95 3.06
33.86 34.21 41.65
53.11 61.84 55.29
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6 Table 5 Element analysis of Cu–ZnO catalysts.
Enterprises in Jiangsu Province (BC2015008) of China are gratefully acknowledged.
Cu–ZnO catalyst
Cu (wt.%)
Zn (wt.%)
Cu/Zn (%)
Fresh After the first cycle After the eighth cycle
39.41 33.26 37.37
60.59 54.05 62.63
65.04 61.54 59.67
membrane and the decrease of glycerol conversion was not caused by the leaching of Cu–ZnO catalyst. Table 5 gives the contents of Cu and Zn metal elements in the fresh and recovered Cu–ZnO catalysts. It turned out that the total composition of Cu–ZnO catalyst almost kept stable through eight reaction cycles. These results confirmed that the decrease of Cu atomic concentration on the surface of the recovered catalyst as presented by XPS analyses was not attributed to the leaching of Cu during the reaction. Conclusions In this work, the physicochemical properties of the fresh and recovered Cu–ZnO catalysts were characterized to explore the deactivation mechanism of the Cu–ZnO catalyst in the hydrogenolysis of glycerol to 1,2-propanediol. The drop of catalytic activity of Cu–ZnO and glycerol conversion were mainly caused by the obvious changes of the structure and morphology of Cu–ZnO catalyst during the catalytic cycles: the aggregation of Cu and ZnO crystallite sites, the nonuniform distribution of Cu and Zn, and the decrease of Cu content on the catalyst surface. The present study suggested that the Cu and ZnO crystals, especially for the ZnO, tended to aggregate during the hydrogenolysis of glycerol, and ZnO might be not a feasible catalytic component, and thus more suitable components should be selected. Acknowledgments Financial supports from the Jiangsu Natural Science Foundation for Distinguished Young Scholars (BK20150044), the Natural Science Foundation of Jiangsu Province (BK20130920), the National Natural Science Foundation of China (91534110, 21306081), he Foundation from State Key Laboratory of Materials-Oriented Chemical Engineering (ZK201402, ZK201407), and the Technology Innovation Foundation for Science and Technology
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Insights into deactivation mechanism of Cu–ZnO catalyst in hydrogenolysis of glycerol to 1,2-propanediol Yan Du a, Cancan Wang b, Hong Jiang b, Changlin Chen b, Rizhi Chen b,* a b
College of Environment, Nanjing Tech University, Nanjing 210009, PR China State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing Tech University, Nanjing 210009, PR China
A R T I C L E I N F O
Article history: Received 16 November 2015 Received in revised form 23 December 2015 Accepted 4 January 2016 Available online xxx Keywords: Submerged catalysis/membrane filtration system Glycerol hydrogenolysis 1,2-Propanediol Cu–ZnO catalyst Catalyst deactivation
A B S T R A C T
The deactivation mechanism of Cu–ZnO catalyst was investigated in detail. During the glycerol hydrogenolysis cycles, the morphology of Cu–ZnO catalyst was firstly changed from spherical nanoparticles to lamellar structure and then to rod-like structure, and the uniform surface composition was deteriorated. The obvious aggregation of Cu and ZnO crystallite sites led to the decrease in specific surface area and pore volume. Furthermore, the Cu content on the catalyst surface significantly decreased with the increase of the number of reaction cycle. These findings provide in-depth insights on the deactivation of Cu–ZnO catalyst and the obvious decrease in glycerol conversion. ß 2016 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Introduction Biomass, as the only natural renewable carbon source, can be applied for sustainable production of liquid fuels and chemicals, which effectively alleviates the world’s dependence on fossil fuels [1–4]. As one typical instance of biomass utilization, biodiesel is usually produced by transesterification of triglycerides in vegetable oils or animal fats [5,6]. However, as the by-product, large amount of glycerol is generated and takes approximately 10 wt% of the total products in the process of biodiesel production. The chemical conversion of glycerol to polymers or fine chemicals enables the substitute of the polyol feedstocks produced from petrochemical industry. 1,2-Propanediol is widely used as the monomer for the synthesis of unsaturated polyester resins, and it is of great use as the solvent in paints, liquid detergents, cosmetics and functional fluids. Following the petroleum-based route, 1,2propanediol is produced by the hydrolysis of propylene oxide that is converted through selective oxidation of propylene. Alternatively, the hydrogenolysis of glycerol into 1,2-propanediol is environmentally friendly and value-added, which receives more attention from academia and industry [7,8].
* Corresponding author. Tel.: +86 25 83172286. E-mail address: [email protected] (R. Chen).
Heterogeneous catalysts with Ru, Ag, Pt, Pd, Ni, Cu or their alloys supported on oxides have been intensively probed into in attempts to achieve high performance of glycerol hydrogenolysis [9–18]. In spite of the high catalytic activity, the noble metal catalysts are not preferred in glycerol hydrogenolysis because of their high cost and unfavorable selective cleavage between C–C and C–O bonds [9–12]. Meanwhile, Ni-based catalysts for glycerol hydrogenolysis have a high cleavage selectivity of C–C bonds instead of C–O bonds, leading to the lower 1,2-propanediol selectivity. Recently, the indepth evaluations have reached that Cu active sites in Cu-based catalysts are superior to those in noble metal and Ni-based catalysts due to their essential ability in selective cleavage of C–O bonds rather than C–C bonds in the glycerol molecule, contributing to 1,2-propanediol formation, which makes it easier for the posttreatment process of glycerol hydrogenolysis including the separation of target product from the reaction system [14–18]. Therefore, much attention has been paid to the study of Cu-based catalysts especially Cu–ZnO on the hydrogenolysis of glycerol [15–18]. Up to date, the studies have been focused on the development of catalyst preparation methods and conditions optimization [10,16]. However, little effort was given to the exploration of Cu–ZnO deactivation mechanism, although Cu–ZnO deactivation had already been observed in hydrogenolysis of glycerol to 1,2propanediol [15,16]. The glycerol conversion and 1,2-propanediol
http://dx.doi.org/10.1016/j.jiec.2016.01.002 1226-086X/ß 2016 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
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selectivity were strongly related to the Cu and ZnO particle sizes, i.e., the smaller the Cu and ZnO particle sizes, the higher the glycerol conversion and 1,2-propanediol selectivity. Wang et al. [15] adopted Cu–ZnO catalysts prepared by a co-precipitation method for the hydrogenolysis of glycerol to 1,2-propanediol. It was found that the glycerol conversion and 1,2-propanediol selectivity decreased with the increase of the initial pH value, which was mainly due to the aggregation of Cu and ZnO particles at higher pH. Bienholz et al. [16] prepared a CuO–ZnO catalyst by an oxalate gel method for the glycerol hydrogenolysis, and the glycerol conversion of 55% and 1,2-propanediol selectivity of 86% were obtained with the use of 1,2-butanediol as a solvent. Different from 1,2-butanediol, the presence of water as a solvent made the size of Cu crystallites of CuO–ZnO catalyst grow drastically, resulting in a drop in active surface area and a loss of catalytic activity. In our previous work [18], a submerged catalysis/ membrane filtration system was developed for the semi-continuous hydrogenolysis of glycerol to 1,2-propanediol over Cu–ZnO catalyst, in which the in-situ separation and reuse of catalyst was achieved. It was found that the glycerol conversion decreased obviously during eight reaction cycles due to the aggregation of Cu and ZnO crystallite sites. Some explanations have been proposed for the deactivation of Cu–ZnO catalysts. However, the deep understanding of deactivation mechanism still remains a real challenge. In this work, in order to seek out the reasons for the drop of glycerol conversion and further to explore the deactivation mechanism of Cu–ZnO catalyst, a series of characterization techniques including XRD, FESEM, HRTEM, BET, TPD, N2O chemisorption, XPS and ICP were applied for the research of micromorphology and physicochemical properties of the Cu–ZnO catalysts.
normal boiling point (77 K), prior to which the sample was evacuated at 393 K for 4 h. Temperature-programmed desorption of ammonia (NH3-TPD) was used to determine the acidities of the catalysts by a BELCAT-A equipment connected to a thermal conductivity detector (TCD). H2-TPD was carried out with the same procedure as NH3-TPD. In order to determine the Cu surface area and its dispersion in the recovered catalysts, N2O chemisorption was carried out by means of the BELCAT-B equipment with a TCD detector. In a typical experiment, the pre-reduction of CuO in the catalysts to Cu0 was performed with 10% H2/Ar mixture at 573 K for 1 h. The sample was cooled down in Ar, and then Cu0 exposed to an atmosphere of 5% N2O/He was oxidized to Cu2O at 353 K for 30 min. Finally, temperature-programmed reduction (TPR) was carried out to reduce Cu2O on the catalyst surface to Cu0. A TCD detector was used to measure the amount of H2-uptake. During the after-treatments of recovered Cu–ZnO catalyst, parts of Cu on the surface might be oxidized to CuO. Therefore, it was necessary to perform the pre-reduction of recovered Cu–ZnO catalyst in the NH3-TPD and N2O chemisorption analyses. The X-ray photoelectron spectroscopy (XPS) measurements were performed on a Thermo ESCALAB 250 spectrophotometer with a monochromatic Al Ka (1486.6 eV) radiation source. The C1s peak (284.8 eV) was used for the calibration of binding energy. The catalyst composition was characterized by inductively coupled plasma optical emission spectrometry (ICP-AES, Optima 2000 DV). Glycerol hydrogenolysis
The Cu–ZnO catalyst was prepared by a co-precipitation method according to our previous work [18]. First, the aqueous solutions of Cu(NO3)23H2O and Zn(NO3)26H2O were mixed and the precursor was yielded with the Cu2+/Zn2+ molar ratio of 1:1.4. Second, the as-prepared precursor and the Na2CO3 solution (2 mol/ L) were transferred to a flask by peristaltic pumps, and the reaction was performed at 343 K with a constant pH value of 8.0 for 1 h. After the reaction, the precipitate was collected by vacuum suction filtration and washed using deionized water to completely remove sodium ions. Third, the rinsed precipitate was treated at 383 K overnight and then calcined in an oven at 723 K for 3 h. Finally, the as-prepared Cu–ZnO catalyst was ground into the fine enough particles for the hydrogenolysis of glycerol. During the catalyst preparation, all chemicals were purchased from commercial sources and used without further treatments.
The hydrogenolysis of glycerol to 1,2-propanediol over Cu–ZnO catalyst, depicted in Scheme 1 [17], was carried out in a submerged catalysis/membrane filtration system, in which a ceramic membrane tube was introduced to the reaction system for the in situ separation of Cu–ZnO catalyst from the suspension slurry [18]. A semi-continuous glycerol hydrogenolysis reaction process was performed to investigate the feasibility of the submerged catalysis/membrane filtration system for the glycerol hydrogenolysis. In the initial reaction cycle, 9 g of Cu–ZnO catalyst and 20 g of glycerol dissolved in 1 L of deionized water were loaded into the reactor, and the reaction was carried out for 8 h under the optimal reaction conditions: stirring rate of 400 rpm, hydrogen pressure of 5 MPa, temperature of 220 8C. In each cycle, the membrane filtration operation was carried out under the optimal filtration conditions: stirring rate of 0 rpm, transmembrane pressure of 0.2 MPa, temperature of 30 8C. The Cu–ZnO catalyst particles were retained in the reactor by the membrane, and then the same volume of fresh reaction solution with the permeation mixture was added into the reactor for next cycle. The glycerol conversion was defined as the ratio of sum of C-based mol of all products to the C-based mol of initially added glycerol, while the 1,2-propanediol selectivity was calculated on the basis of the ratio of C-based mol of the 1,2-propanediol to sum of C-based mol of all products [17,18].
Catalyst characterization
Results and discussion
The X-ray powder diffraction (XRD) patterns were recorded on a Rigaku MiniFlex600 diffractometer using Cu Ka radiation at 40 kV and 15 mA with the 2u range of 20–808. The morphology of Cu–ZnO catalyst was examined by field emission scanning electron microscope (FESEM, Hitachi S-4800II, Japan), and the element distribution was determined by energy dispersive X-ray spectrum (EDX, HORIBA). To verify the distribution and morphology of CuO, Cu and ZnO particles, high resolution transmission electron microscope (HRTEM, JEM 2100) was adopted for the fresh and recovered catalysts. The N2 adsorption–desorption isotherms were measured by using an ASAP 2020 analyzer (Micromeritics) at its
Catalytic stability of Cu–ZnO catalyst
Experimental Preparation of Cu–ZnO catalyst
In order to investigate the catalytic stability of Cu–ZnO catalyst, a number of catalytic reaction cycles were carried out in a submerged catalysis/membrane filtration system [18]. The influence of the
Dehydration HO
OH OH
ZnO
O
Hydrogenation OH
Cu
HO
Scheme 1. The two-step reaction mechanism of glycerol hydrogenolysis.
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Table 1 Specific surface area and pore volume of Cu–ZnO catalysts.
1.0 0.9 Glycerol conversion 1,2-Propanediol selectivity
0.8 0.7 0.6 0.5 0.4 0
1
2
3
4
5
6
7
8
9
Number of catalytic reaction cycle Fig. 1. Change of glycerol hydrogenolysis performance with the number of catalytic reaction cycle.
number of catalytic reaction cycle on the glycerol conversion and 1,2-propanediol selectivity is shown in Fig. 1. Obviously, the selectivity of 1,2-propanediol remained stable while the glycerol conversion sharply decreased by 56.7% through eight cycles, which indicated the obvious deactivation of Cu–ZnO catalyst. As discussed in our previous work [17], the selectivity of 1,2-propanediol was mainly related to the solvent in the hydrogenolysis of glycerol over Cu–ZnO catalyst. During all the catalytic reaction cycles, water was served as the solvent, thus the 1,2-propanediol selectivity could remain stable. Deactivation mechanism of Cu–ZnO catalyst
Fresh
After the first cycle
After the eighth cycle
Specific surface area (m2/gcat) Pore volume (cm3/gcat)
22.8 0.17
11.3 0.047
7.6 0.022
Fig. 2 gives the nitrogen adsorption-desorption isotherms of different Cu–ZnO catalysts, i.e., the fresh one, the one after the first cycle and the one after the eighth cycle. All the samples exhibited Langmuir type IV isotherms. The overlapped adsorption-desorption curves were found at the low pressure. At the high pressure, obvious hysteresis loops appeared especially for the fresh Cu–ZnO catalyst, which was the characteristic of mesoporous materials [19,20]. The existence of mesoporous might be caused by the aggregation of crystals as shown in the following FESEM analyses. However, compared to the fresh catalyst, the recovered catalysts had drastic drop of the N2 adsorption amount, especially when the eighth cycle was finished. This indicated that the severe aggregation of the Cu–ZnO catalyst crystals took place during the catalytic reaction cycles. Table 1 lists the specific surface area and pore volume of the fresh and recovered Cu–ZnO catalysts. Compared to the fresh catalyst, the recovered catalysts had smaller values of specific surface area and pore volume, and the specific surface area and pore volume decreased gradually with the increase of the number of reaction cycle. These results suggested that the Cu–ZnO crystals tended to aggregate during the glycerol hydrogenolysis cycles, leading to a reduction of specific surface area and active sites, then to a loss of catalytic activity and glycerol conversion [21]. The acidities of recovered Cu–ZnO catalysts were detected by NH3-TPD according to the literatures [22–24], as shown in Fig. 3. For both samples, two NH3 desorption peaks located in the range of 260–450 and 500–650 8C, respectively, were observed, indicating that the two Cu–ZnO catalysts all had a strong and a medium acid sites [23,24]. Furthermore, the amount of the medium acid site was much larger than that of the strong acid site. By calculating the peak area, the total amount of acidity (16.85 mmol/g) of the Cu–ZnO catalyst after the eighth cycle was much less than that (43.61 mmol/g) of the one after the first cycle, which demonstrated that the total amount of acidity decreased gradually with the increase of the number of reaction cycle. This was in consistent with the XRD results [17]. According to the reaction mechanism of glycerol hydrogenolysis over Cu–ZnO catalyst as presented in Scheme 1, glycerol is dehydrated to acetol on ZnO acidic sites followed by the hydrogenation on Cu active
Intensity (a.u.)
Quantity adsorbed (cm3/g STP)
To explore the deactivation mechanism of Cu–ZnO catalyst, the fresh and recovered catalysts were characterized in detail by XRD, N2 adsorption-desorption, NH3-TPD, H2-TPD, FESEM, EDS, HRTEM, N2O chemisorption, XPS and ICP. In our previous work [18], XRD was adopted to analyze the microscopic crystalline structure such as the Cu and ZnO crystallites. It was found that CuO phase in the fresh catalyst could be completely converted to metallic Cu during the reaction, which was necessary for the formation of 1,2-propanediol [17]. The Cu and ZnO crystallites aggregated during the continuous reaction, and continually grew up with the number of catalytic reaction cycle, leading to the decrease in catalytic active sites and glycerol conversion.
Cu–ZnO catalyst
(a)
(a)
(b) (c) 0.0
(b) 0.2
0.4
0.6
0.8
1.0
P/P0 Fig. 2. N2 adsorption–desorption isotherms of Cu–ZnO catalysts: (a) fresh; (b) after the first cycle; (c) after the eighth cycle.
100
200
300
400
500
600
700
800
Temperature Fig. 3. NH3-TPD diagrams of Cu–ZnO catalysts: (a) after the first cycle; (b) after the eighth cycle.
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Table 2 EDS analysis of Cu–ZnO catalysts.
Intensity (a.u.)
Elements (mol.%)
Cu Zn Cu/Zn molar ratio
(a)
Fresh
After the first cycle
After the eighth cycle
1
2
3
4
5
6
0.170 0.273 0.623
0.174 0.275 0.633
0.060 0.201 0.299
0.061 0.184 0.331
0.095 0.405 0.235
0.273 0.414 0.659
(b)
100
200
300
400
500
600
700
800
900
Temperature Fig. 4. H2-TPD diagrams of Cu–ZnO catalysts: (a) after the first cycle; (b) after the eighth cycle.
sites [15,25]. Therefore, by increasing the number of reaction cycle, the total amount of ZnO acidic sites decreased, leading to a drop of glycerol conversion. The adsorption amount of hydrogen is related to the hydrogen activation and spillover capacity, and the greater the amount of hydrogen adsorption, the better the catalytic performance [26]. Fig. 4 shows the H2-TPD curves of the recovered Cu–ZnO catalysts involving after the first cycle and the eighth cycle. For each sample, only one peak in 300–800 8C was observed, but the amount of hydrogen desorption of the Cu–ZnO catalyst after the eighth cycle was less than that after the first cycle. The results indicated that the hydrogen desorption capacity of Cu–ZnO catalyst decreased with the increase of the number of reaction cycle, contributing to the reduction of glycerol hydrogenation ability. Fig. 5 shows the FESEM images and EDS analysis images of Cu– ZnO catalysts. Compared to the fresh catalyst, the morphologies of
the recovered catalysts were remarkably changed. The fresh catalyst was mainly composed of nanoparticles, and larger spherical particles were formed by piling up the nanoparticles. In the recovered catalysts, besides the particles, the lamellar structure was observed after the first cycle and the rod-like structure was formed after the eighth cycle. To explore the reasons for the change of Cu–ZnO morphology, EDS was further performed to analyze the compositions of the fresh and recovered catalysts. Six regions labeled by 1–6 in Fig. 5 were selected for EDS analysis. Table 2 lists the metal element content including Cu and Zn on the surface of Cu–ZnO catalysts in different regions. The results showed that the Cu/Zn molar ratio in regions 1 and 2 was almost the same, indicating the uniform distribution of Cu and Zn in the spherical particles of the fresh Cu–ZnO catalyst. On the surfaces of the lamellar structure, the Cu/Zn molar ratio was close to that on the spherical particles, as indicated in regions 3 and 4. Hence, there was no obvious difference in the elements composition between the lamellar structure and spherical particles. Differently, for the Cu–ZnO catalyst after the eighth cycle, the Zn content on the surface of rod-like structure in region 5 was much larger than that of spherical particles in region 6. It could be concluded that the rodlike structure mainly consisted of the ZnO crystallites. The structure and morphology of Cu–ZnO catalysts have been characterized in more detail by HRTEM, as shown in Fig. 6. The fresh catalyst was composed of nearly uniform spherical particles
Fig. 5. FESEM images of Cu–ZnO catalysts: (a and d) fresh; (b and e) after the first cycle; (c and f) after the eighth cycles. EDS analysis images of Cu–ZnO catalysts: (g) fresh; (h) after the first cycle; (i) after the eighth cycle.
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Fig. 6. TEM images and electron diffraction patterns of Cu–ZnO catalysts: (a–c) fresh; (d–f) after the first cycle; (g–i) after the eighth cycle.
with a diameter of approximately 25 nm, and the aggregation of spherical particles was observed (Fig. 6(a)). The lattice fringes of 0.230 and 0.261 nm were found on the surface of spherical particle, corresponding to CuO (2 0 0) and ZnO (0 0 0 2) planes, respectively. And the electron diffraction pattern depicted in Fig. 6(c) indicated that the fresh catalyst was a mixture of CuO and ZnO crystal structure [27,28], in agreement with the SEM and EDS analyses. After the first cycle, the Cu–ZnO catalyst mainly presented the lamellar structure where the Cu nanoparticles with an average diameter of 60 nm were dispersed, and a small amount of ZnO rodlike structure was also found (Fig. 6(d)). The lattice fringes of 0.210 and 0.261 nm were attributed to Cu (1 1 1) and ZnO (0 0 0 2) planes, respectively, indicating that the lamellar structure consisted of the Cu and ZnO [29]. As shown in Fig. 6(f), the structure of regular hexagon should be indexed as ZnO (0 0 0 2) plane of hexagonal wurtzite structure, while the diffraction rings were ascribed to Cu (1 1 1) plane [28,30]. After the eighth cycle, the morphology of Cu–ZnO catalyst was changed into the micro-sized rod-like structure (Fig. 6(g)). The lattice spacing of 0.261 nm of ZnO nanorods corresponded to ZnO (0 0 0 2) plane where the Cu nanoparticles were dispersed with an average diameter of 70 nm. As given in Fig. 6(i), the diffraction pattern of ZnO nanorods presented the single crystallinity structure [31,32]. By SEM and TEM analyses, during the glycerol hydrogenolysis cycles, the structure and morphology of Cu–ZnO catalyst were changed significantly. The morphology was shifted from spherical nanoparticles to lamellar structure and then to rod-like structure. The composition was changed from even to uneven, and both Cu and ZnO sizes increased, especially for the ZnO. These changes took into account the loss of catalytic activity and glycerol conversion. The Cu dispersion is regarded as one of the key factors for the evaluation of Cu-based catalysts [33]. Table 3 lists the dispersion Table 3 Cu dispersion and specific surface area measured by N2O chemisorption. Cu–ZnO catalyst
Dispersion (%)
Specific surface area (m2/gcat)
After the first cycle After the eighth cycle
0.83 0.59
1.71 1.22
and specific surface area of Cu calculated from N2O chemisorption. Compared to the recovered catalyst after the first cycle, both the dispersion and specific surface area of Cu decreased for the recovered catalyst after the eighth cycle. It could be explained that the aggregation of Cu crystals generated with the increase of the number of reaction cycle, leading to the reduction of Cu active surface area, and then to the loss of catalytic activity and glycerol conversion. The results were in good accordance with the XRD analyses, and further confirmed the causes of catalyst deactivation. Table 4 lists the contents of Cu, Zn and O elements on the surface of Cu–ZnO catalysts according to the XPS characterization. The results showed that the content of Cu element on the surface of the recovered Cu–ZnO catalysts decreased along with the increase of Zn content in comparison with the fresh catalyst, and the changes became more evident with the increase of the number of reaction cycle. The phenomena might be due to the change of catalyst microstructure, i.e., parts of Cu nanoparticles were covered by ZnO crystals during the catalytic cycles. During the glycerol hydrogenolysis over Cu–ZnO, glycerol is dehydrated to acetol on ZnO acidic sites followed by the hydrogenation on Cu active sites [15,25]. Thus, the decrease of Cu content on the Cu–ZnO surface would account for the deactivation of Cu–ZnO catalyst in the hydrogenolysis of glycerol. In order to determine whether the leaching of Cu–ZnO catalyst was responsible for the decrease of glycerol conversion, the permeate through the membrane was analyzed by ICP, and the contents of Cu and Zn metal elements in the Cu–ZnO catalysts were also measured by ICP. The ICP analyses showed that no obvious Cu and Zn elements were found in the permeate, indicating that the Cu–ZnO catalyst could be completely retained by the ceramic Table 4 Atomic concentration of different elements for Cu–ZnO catalysts. Cu–ZnO catalyst
Atomic concentration (%) Cu2p
Zn2p
O1s
Fresh After the first cycle After the eighth cycle
13.03 3.95 3.06
33.86 34.21 41.65
53.11 61.84 55.29
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6 Table 5 Element analysis of Cu–ZnO catalysts.
Enterprises in Jiangsu Province (BC2015008) of China are gratefully acknowledged.
Cu–ZnO catalyst
Cu (wt.%)
Zn (wt.%)
Cu/Zn (%)
Fresh After the first cycle After the eighth cycle
39.41 33.26 37.37
60.59 54.05 62.63
65.04 61.54 59.67
membrane and the decrease of glycerol conversion was not caused by the leaching of Cu–ZnO catalyst. Table 5 gives the contents of Cu and Zn metal elements in the fresh and recovered Cu–ZnO catalysts. It turned out that the total composition of Cu–ZnO catalyst almost kept stable through eight reaction cycles. These results confirmed that the decrease of Cu atomic concentration on the surface of the recovered catalyst as presented by XPS analyses was not attributed to the leaching of Cu during the reaction. Conclusions In this work, the physicochemical properties of the fresh and recovered Cu–ZnO catalysts were characterized to explore the deactivation mechanism of the Cu–ZnO catalyst in the hydrogenolysis of glycerol to 1,2-propanediol. The drop of catalytic activity of Cu–ZnO and glycerol conversion were mainly caused by the obvious changes of the structure and morphology of Cu–ZnO catalyst during the catalytic cycles: the aggregation of Cu and ZnO crystallite sites, the nonuniform distribution of Cu and Zn, and the decrease of Cu content on the catalyst surface. The present study suggested that the Cu and ZnO crystals, especially for the ZnO, tended to aggregate during the hydrogenolysis of glycerol, and ZnO might be not a feasible catalytic component, and thus more suitable components should be selected. Acknowledgments Financial supports from the Jiangsu Natural Science Foundation for Distinguished Young Scholars (BK20150044), the Natural Science Foundation of Jiangsu Province (BK20130920), the National Natural Science Foundation of China (91534110, 21306081), he Foundation from State Key Laboratory of Materials-Oriented Chemical Engineering (ZK201402, ZK201407), and the Technology Innovation Foundation for Science and Technology
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