Highly Efficient Selective Hydrogenation of Cinnamaldehyde to

Article pubs.acs.org/JPCC

Highly Efficient Selective Hydrogenation of Cinnamaldehyde to Cinnamyl Alcohol over Gold Supported on Zinc Oxide Materials Hangning Chen,† David A. Cullen,‡ and J. Z. Larese*,† †

University of Tennessee, Knoxville, Tennessee 37996, United States Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee 37831, United States

‡

S Supporting Information *

ABSTRACT: Au/ZnO catalysts have been used for liquid-phase selective hydrogenation of cinnamaldehyde to cinnamyl alcohol and compared with Au/Fe2O3 catalysts. To investigate the influence of the support on the hydrogenation activity and selectivity, three different Au/ZnO catalysts were synthesized, including Au/rod-tetrapod ZnO, Au/porous ZnO, and Au/ZnO-CP prepared using a coprecipitation method. The influence of calcination temperature was also systematically investigated in this study. The characterization of Au/ZnO catalysts was performed using ICP, N2 adsorption/desorption isotherms, X-ray diffraction, scanning transmission electron microscopy, and X-ray photoelectron spectroscopy. Among all the supported Au catalysts prepared in this study, Au/ZnO-CP exhibits both the highest hydrogenation activity and selectivity. Using a 1.5% Au/ZnO-CP catalyst, 100% selectivity could be achieved with 94.9% conversion. We find that the Au particle (size and shape), the ZnO support (size and surface texture) and the interaction between Au and ZnO are three important parameters for achieving a highly efficient Au/ZnO catalyst.

1. INTRODUCTION Conventional wisdom suggests that a noble metal such as gold is inert and unusable as a catalyst. However, such thinking changed dramatically in the 1970s when Bond1,2 first used supported Au catalysts for the hydrogenation of olefins. In the 1980s, Haruta3−5 developed coprecipitation and deposition− precipitation methods for synthesizing highly active supported Au catalysts, and demonstrated that supported Au catalysts have surprisingly high activity for CO oxidation when the Au particles are smaller than 5 nm. More recently, supported Au catalysts have been employed in catalysis reactions,6 including hydrogenation,7,8 oxidation,9,10 and water gas shift11−14 reaction. The selective hydrogenation of α,β-unsaturated aldehydes is one of the most challenging topics in heterogeneous catalysis,15 because the formation of the CC bond is preferentially hydrogenated over the CO bond for both thermodynamic and kinetic reasons. Thus, when using a Pt group metal as a catalyst, the saturated aldehydes usually dominate in the early production period although some reports suggest that unsaturated alcohols can be a significant product on some Pt-supported catalysts.16−18 However, Au catalysts, because of their unique properties that favor adsorption of the CO group and limited the chemisorption of hydrogen, have outstanding selectivity in α,β-unsaturated aldehydes and ketone hydrogenation reactions. Hutchings and Bailie19,20 were the first to apply Au/ZrO2 and Au/ZnO catalysts in gas-phase selective hydrogenation of crotonaldehyde. They found that up to 81% selectivity could be achieved at conversions of 5−10%. In addition, they proposed that the origin of high selectivity is © XXXX American Chemical Society

associated with large Au particles (about 10−20 nm in diameter). By covering the surface of Au nanoparticles with indium, Claus and co-workers21 suggested that for Au/ZnO catalyst, the Au edges and corners are the active sites for selective hydrogenation of acrolein. Au/Fe2O3 catalysts were used in liquid-phase selective hydrogenation of benzalacetone by Milone and co-workers22,23 Using a 4.2% Au/Fe2O3 catalyst (prepared by deposition-precipitation method), they determined that a 65.8% selectivity to an unsaturated alcohol can be obtained when the conversion reaches 70.2%.22 Recently, Au/ Al2O3,24,25 Au/TiO2,26,27 Au/SiO2,19,26,28 Au/CeO2,29−32 Au/ CNTs,33 and Au/MgxAlO34,35 catalysts were also investigated for selective hydrogenation. Although the nature of the active site is still under debate, there is general agreement on the requirements for preparing highly active supported Au catalysts. First, Au catalysts supported on reducible oxides, such as Fe2O3, CeO2, TiO2, and Co3O4, have higher activity than on inert supports.14 Second, the synthesis method itself also affects the Au catalyst activity. For example, Haruta found that the Au/ TiO2 catalyst prepared using an impregnation method exhibited lower activity than those prepared using a deposition− precipitation method, primarily due to the formation of large spherical Au particles instead of small hemispherical Au particles.14 Goodman and co-workers demonstrated that the oxidation activity of Au/TiO2 catalyst decreases significantly Received: August 11, 2015 Revised: November 25, 2015

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DOI: 10.1021/acs.jpcc.5b07823 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C when Au particles are larger than 2.5 nm.36 Third, Au catalysts are very susceptible to poisoning. As Kung37,38 demonstrated the residual Cl− (from HAuCl4) will cause an agglomeration of Au particles and poison the active sites; hence steps to remove such the contamination should be extensive in order to achieve a highly active Au catalyst. The objective of the present study is to augment current knowledge of how the texture of the support and the calcination process affect the efficiency of a supported Au catalyst. Three Au supported catalysts are synthesized in this study: Au/tetrapod ZnO (Au/TZnO), Au/porous ZnO (Au/PZnO), and Au/ZnO by coprecipitation method (Au/ZnO-CP). The tetrapod ZnO used in this experiment are micrometer in size (as bulk ZnO), with defect-deficient (almost free) surfaces; the porous ZnO is nanometer in size (as nanosized ZnO), with porous structure and a defect-rich surface; while Au/ZnO-CP has the highest dispersion of Au particles and nanosized ZnO support. Because Au/Fe2O3 catalysts synthesized using coprecipitation methods (Au/Fe2O3−CP) are regarded as among the most active supported Au catalysts, we have prepared and included the behavior of these materials for comparison in the evaluation of the Au/ZnO catalysts described here.

extensive washing with hot distilled H2O, and overnight drying at 50 °C. Au/ZnO-CP catalysts at 1.1% and 1.5% were also prepared in this manner by adjusting the amount of HAuCl4. 2.1.4. Au/Fe2O3-CP Catalysts. Au/Fe2O3 catalysts at 1.4%, 1.6%, and 1.9% were also prepared using the coprecipitation method. The synthesis procedure is essentially the same as that used for the Au/ZnO-CP catalysts except Fe(NO3)3 was used instead of Zn(OAc)2, and a solution of Na2CO3 was used to adjust the pH. 2.2. Characterization. To quantify the Au content of our samples, ICP-OES41 data were obtained using a PerkinElmer Optima 2100 DV instrument. A 10 mg sample of Au catalyst was mixed with 50 mL of 30% aqueous solution of aqua regia for 30 min in an ultrasonic bath to dissolve Au particles into an aqueous ionic solution prior to measurement. N2 adsorption/ desorption isotherms were performed using a custom built apparatus designed and assembled by our group.42,43 This adsorption apparatus is equipped with a turbo pump (base pressure of ∼10−7 Torr) to evacuate the sample cell and ensure that the system does not contain a measurable amount of undesired gas or water vapor before starting the adsorption experiment. The sample cell is cooled to 77 K with liquid N2. Brunauer, Emmett and Teller (BET) surface areas were determined using the adsorption branches in the relative pressure range (p/po) between 0.05 and 0.20. XRD patterns were collected on a Phillips powder diffractometer at 20 kV and 15 mA using a Cu target with Ni filter and an INEL CPS120 proportional detector. High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) images were recorded by a JEOL 2200FS aberration corrected STEM operating at 200 kV with a spatial resolution below 0.8 Å. The particle size distributions were calculated using 200−500 random particles. X-ray photoelectron spectroscopy (XPS) were performed on a Thermo Scientific K-Alpha spectrometer with 30 μm spatial resolution. All the Au/ZnO catalysts were prereduced at hydrogenation reaction conditions unless otherwise specified. 2.3. Catalysis Reaction. The selective hydrogenation of cinnamaldehyde (CLAD) (Figure 1 shows the reaction pathway) was performed in a 100 mL high-pressure reactor at 110 °C and 2 MPa of H2. Before starting the hydrogenation reaction, 100 mg of catalyst (dried overnight at 50 °C) was calcined at different temperatures, and maintained at that temperature for 2 h. The resultant catalyst was then prereduced 2 h in 15 mL of isopropyl alcohol under reaction conditions.

2. EXPERIMENTAL SECTION 2.1. Materials. 2.1.1. Tetrapod ZnO. Tetrapod-rod ZnO (TZnO) is produced using a patented vapor transport method which produces gram-quantities of high purity material.39 The reaction conditions such as heat, argon flow, and oxygen flow can be varied to change the morphology of ZnO that is produced. 2.1.2. Porous ZnO. Porous ZnO (PZnO) is prepared by a hard-templating method.40 Typically, 0.5 g of CMK-3 carbon were added to 20 mL of THF solution of Zn(NO3)2 (1.5 mol L−1) and stirred at room temperature for 4 h. After filtration, the impregnated CMK-3 was first dried at room temperature, then heated in air at 573 K to convert the zinc nitrate to zinc oxide. This procedure was repeated twice before a final heating to 773 K where the material was held for 4 h to burn off the carbon residue; the resulting material is porous ZnO. 2.1.3. Au/ZnO Catalysts. Au/TZnO catalysts were prepared by both impregnation (IMP) and deposition−precipitation (DP) methods. To prepare 2.2% Au/TZnO-IMP catalyst, typically, 0.8 g of TZnO was added into 80 mL of an aqueous solution containing 40 mg of HAuCl4 at 80 °C. After stirring and aging for 2 h, respectively, the yellow powder was collected by filtration and washed extensively with distilled water. The resultant powder was dried overnight at 50 °C. The synthesis of 1.1% Au/TZnO-DP catalyst followed the same procedure and used the same quantity of HAuCl4 and TZnO as the 2.2% Au/ TZnO-IMP catalyst, except the pH of the HAuCl4 solution was adjusted to ∼7 by adding 0.1 M Na2CO3 aqueous solution. 2.1.3a. Au/PZnO Catalysts. Au/PZnO-IMP catalysts were prepared at 1.1% and 2.2% following the same procedure as the synthesis of the Au/TZnO-IMP catalyst. 2.1.3b. Au/ZnO-CP Catalysts. Au/ZnO-CP catalysts were prepared following a procedure similar to the one described earlier by Hutchings and co-workers.20 Typically, to synthesize 0.7% Au/ZnO-CP catalyst, 100 mL H2O containing 2.2 g of Zn(OAc)2·6H2O and 40 mg of HAuCl4 was heated to 80 °C. A 24mL sample of NaHCO3 (1 mol/L) was slowly added into the solution with vigorous stirring. After the mixture was stirred for 2 h, the pH gradually reached 8−9. The precipitation was then aged for 2 h prior to vacuum filtration, which was followed by

Figure 1. Reaction pathway of selective hydrogenation of cinnamaldehyde. B


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The Journal of Physical Chemistry C After cooling to room temperature, 0.025 mL CLAD was introduced into the reaction solution for Au/TZnO and Au/ PZnO catalysts, while for Au/ZnO-CP and Au/Fe2O3−CP catalysts, 0.1 mL of CLAD was added. The selectivity and the conversion rate were monitored using an HP 6890 GC−MS system (equipped with an HP-5 ms column).

3. RESULTS 3.1. Characterization. 3.1.1. XRD Measurements. The structure of TZnO and PZnO is typical wurtzite ZnO, as is shown in Figure S1 (Supporting Information), the PZnO (Figure S1b) displays a much wider peak, which indicates the ZnO particle size is smaller than TZnO. The structure of both TZnO and PZnO remains the same even when calcined as high as 300 °C. However, for Au/ZnO-CP catalyst, the crystal structure changes in concert with an increase in the calcination temperature. Figure 2a shows the XRD patterns of Au/ZnOCP catalyst calcined at different temperatures without prereduction. Catalysts calcined below 160 °C exhibit similar diffraction patterns, corresponding to the hydrozincite structure. When calcined at 180 °C, changes in the diffraction pattern are apparent signaling the onset of the structural transformation from hydrozincite-to-ZnO. By increasing the calcination temperature to 200 °C, the diffraction peak at ∼12.5° (hydrozincite structure) completely disappears; establishing that the structural transformation of the support from hydrozincite-to-ZnO is complete. Interestingly, hydrogenation (under reaction condition: 110 °C, 2 MPa H2) of Au/ZnO-CP catalyst aids the transformation of hydrozincite-to-ZnO. Figure 2b, Au/ZnO-CP-180 °C clearly shows that after hydrogenation for 2 h under reaction conditions, the structure is transformed to the ZnO phase. For Au/ZnO-CP-160 °C catalyst, the hydrozincite structure almost transforms to ZnO after hydrogenation for 8 h (Figure 2c). No metallic Au signal (2θ = 38.2, 44.4°) was detected in all Au catalysts, which means that Au particles are highly dispersed or the loading of Au is too small to be detected by XRD. 3.1.2. STEM Analysis. In the previous study, Haruta5 demonstrated that the calcination process could change the interface between Au and support, as well as the Au particle size and shape. Recently, Carlsson et al.44 deposited Au on different supports, such as TiO2, Al2O3, and MgAl2O4, and found that the support also affects the size and shape of Au particles. According to their selective hydrogenation results (see section 3.2), three representative calcination temperatures (50, 180, and 300 °C) were chosen to investigate the changes of Au particles shape, size, and interaction between Au and support for Au/ZnO catalysts. The morphology and size of three types of Au/ZnO catalysts are presented in Figure S2 (see Supporting Information). TZnO particles are micrometer in size, with four highly crystalline legs (up to 0.4 μm in length). PZnO prepared by a hard-templating method has much the same morphology as CMK-3, but the pores are not uniform. Magnified images show PZnO is composed from ZnO nanoparticles (smaller than 20 nm) that are connected to each other. Au-ZnO-CP catalyst is a porous network with individual ZnO cylinders (smaller than ∼5 nm in width). The N2 adsorption/desorption isotherm indicates that the BET surface areas of 2.2% Au/TZnO-IMP, 1.1% Au/PZnO-IMP, and 0.7% Au/ZnO-CP are 6, 57.5, and 60.2 m2/g, respectively, for catalysts dried at 50 °C. Figure 3 shows the STEM images of 2.2% Au/TZnO-IMP catalyst and 1.1% Au/TZnO-DP catalyst. Au/TZnO-IMP-180 °C (Figure 3b) catalyst (2.2 ± 0.8

Figure 2. XRD patterns of 0.7% Au/ZnO-CP catalysts calcined at (a) different temperatures, (b) calcined at 180 °C before and after prereduction and (c) calcined at 160 °C before and after prereduction at different times.

nm) has similar Au particle size as compared to Au/TZnOIMP-50 °C (Figure 3a) catalyst (2.2 ± 0.9 nm). It has been reported that the melting point of Au nanoparticles (smaller than 2 nm) is lowered to 300 °C due to the quantum-size effect. Increasing calcination temperature to 300 °C (Figure 3c) causes the aggregation of Au nanoparticles, leading the particle size to grow to 2.9 nm with narrow distribution (±0.2 nm). Interestingly, Au particles are spherical when Au/TZnO catalyst is dried at 50 °C, but hemispherical at 180 °C. This indicates that Au particles have difficulty wetting the TZnO surface when dried at 50 °C. Increasing the calcination temperature helps drive the wetting process (see supplementary Figures S3 and S4). However, a competition between wetting and aggregation exists and for the Au/TZnO catalyst, the aggregation process dominates when calcination is performed at 300 °C. Thus, C


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Figure 4. HAADF-STEM images of 1.1% Au/MZnO-IMP catalysts calcined at (a) 50 °C, (b) 180 °C, and (c) 300 °C and their corresponding particle size distributions.

IMP catalyst has smaller Au particles than the corresponding Au/TZnO-IMP catalyst calcined at the same temperature. When Au/PZnO is dried at 50 °C, 1.1 ± 0.5 nm mound-shaped Au nanoparticles can be obtained (Figure 4a). Meanwhile, the Au(111) phase has the same orientation as ZnO(002) phase. These results show that Au nanoparticles can be easily wetted on a defect-rich PZnO surface even at 50 °C. Increasing the calcination temperature increases the Au particle size. For example, the Au particle size increases to 1.5 ± 0.9 nm when Au/PZnO-IMP was calcined at 180 °C (Figure 4b). Calcination at 300 °C (Figure 4c) does not cause significant aggregation of Au nanoparticles. The Au particle size (1.6 ± 0.7 nm) is about the size of Au/PZnO-IMP calcined at 180 °C. However, the Au particles form hemispheres and spheres rather than rafts and mounds (calcined below 180 °C). STEM images of 0.7% Au/ZnO-CP catalysts calcined at different temperatures are shown in Figure 5. Obviously, Au/ZnO-CP catalyst has the highest Au dispersion and the smallest Au particles among three types of Au/ZnO catalysts. The Au nanoparticles in these samples are mainly Au atoms and atom clusters. By measuring at least two hundred random Au nanoparticles, we conclude that Au/ZnO-CP-180 °C (Figure 5b) has the smallest particle size and narrowest size distribution. We propose that the calcination process can assist Au nanoparticle wetting on the ZnO surface; thus, the Au/ZnO-CP catalyst calcined at 180 °C has a smaller Au particle size than that calcined at 50 °C (Figure 5a). Similar to Au/TZnO and Au/PZnO catalysts, as

Figure 3. HAADF-STEM images of 2.2% Au/TZnO-IMP calcined at (a) 50 °C, (b) 180 °C, and (c) 300 °C, and (d) 1.1% Au/TZnO−DP calcined at 180 °C along with their corresponding particle size distributions.

spherical Au particles dominate at the Au/TZnO-IMP-300 °C catalyst. It should be pointed out that the direction of Au(111) relative to ZnO(002) is random at 50 °C, however when heated to 180 °C, most Au(111) orients along the same direction as ZnO(002) (supplementary Figures S3 and S4). This behavior suggests that the calcination process helps bind the Au nanoparticles to the TZnO surface. The DP method, developed by Haruta4,5 is an efficient way to decrease the Au particle size. Figure 3d shows the STEM image of 1.1% Au/TZnO−DP-180 °C catalyst. In fact, it is clear that the size of Au particles on the Au/TZnO−DP-180 °C catalyst is significantly smaller than those on the Au/TZnO-IMP-180 °C catalyst. The average particle size is about 1.6 nm in diameter and the shape is more mound-like rather than hemispherical. STEM images of 1.1% Au/PZnO-IMP catalysts calcined at different temperatures are shown in Figure 4. The Au/PZnOD


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Figure 6. Au0 XPS data of three types of Au/ZnO catalysts calcined at 50, 180, and 300 °C.

area, and transformation of Au nanoparticles with increasing calcination temperature on TZnO is simpler than on the other ZnO support. Figure 7 clearly demonstrates that both

Figure 5. STEM images of 0.7% Au/ZnO-CP catalysts calcined at (a) 50 °C, (b) 180 °C, and (c) 300 °C and their corresponding particle size distributions.

shown in Figure 5c), the Au/ZnO-CP catalyst calcined at 300 °C will cause the aggregation of Au particles, and more spherical Au particles are formed at this temperature. 3.1.3. XPS. As both the calcination and hydrogenation affect the Au oxidation state, all Au/ZnO catalysts were characterized using XPS after being calcined and then prereduced at reaction conditions. The XPS spectrum of the Au 4f7/2 core level shows three peaks in all Au/ZnO catalysts (see supplementary Figures S3, S4, and S5), which correspond to Au0, Au1+, and Au3+.45 Figure 6 shows the summary of Au0 percentage in three different Au/ZnO catalysts. Au/TZnO catalysts have the highest concentration (82−85%) of Au0 among three types of catalysts. Interestingly, when calcined at 50, 180, or 300 °C, the Au0 concentration remains at the same level for Au/TZnO or Au/PZnO, but increases steadily for Au/ZnO-CP. Recall the XRD pattern indicated that when Au/ZnO-CP is dried at 50 °C, the support is hydrozincite, and prereduced Au/ZnO-CP180 °C dried for 2 h results in the support being transformed to ZnO. These results indicate that Au nanoparticles are more easily reduced on the ZnO surface than on the hydrozincite surface. Moreover, Au nanoparticles on ZnO with a defectdeficient surface (TZnO) are easier to reduce than those on a defect-rich surface (PZnO). 3.2. Selective Hydrogenation. 3.2.1. Au/TZnO Catalysts. The selective hydrogenation of CLAD was first carried out over 2.2% Au/TZnO-IMP catalyst. On the basis of the characterization results, TZnO, well-crystallized ZnO, has low surface

Figure 7. Selective hydrogenation of CLAD over 2.2% Au/TZnO-IMP catalysts: the dependence of catalyst efficiency on calcination temperature. Reaction conditions: 0.1 g of catalyst (before calcination), 0.025 mL of CLAD, 110 °C, 2 MPa H2, 2 h.

hydrogenation activity and selectivity are highly dependent on calcination temperature. Au/TZnO-IMP-50 °C catalyst has the lowest hydrogenation activity with no unsaturated alcohol produced with a 2 h hydrogenation reaction. Increasing the calcination temperature significantly enhances the catalyst efficiency; however, the catalyst calcined up to 200 °C starts to lose efficiency. Thus, the optimal calcination temperature is 180 °C. Over this catalyst, 43.5% selectivity could be achieved at a conversion of 33.8%. The efficiency of Au/TZnO could be further improved by using the DP method to prepare the catalyst. Despite a reduction in the Au loading by 50%, due to smaller Au particles, 74.5% selectivity and 62.5% conversion can be obtained after hydrogenation for 2 h over 1.1% Au/ TZnO−DP-180 °C catalyst (Figure 8). These hydrogenation results indicate that Au particle shape and size are important parameters to consider in producing highly efficient Au/ZnO catalysts. 3.2.2. Au/PZnO Catalysts. Compared to Au/TZnO catalysts, the Au/PZnO catalyst prepared simply by the E


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dried at 50 °C, which has very small Au particles (1.1 nm) wetted on the PZnO surface, has a very high hydrogenation activity, 88.1% conversion and 74.3% selectivity could be achieved within 30 min under the same reaction conditions as Au/TZnO catalysts. This is remarkable, as 2.2% Au/TZnOIMP catalyst dried at 50 °C has very limited hydrogenation activity and selectivity. The reaction rate is increased almost six times even compared to that of 1.1% Au/TZnO-DP-180 °C catalyst. This result indicates that not only the Au particle size and shape, but also the support size will affect the Au/ZnO catalyst hydrogenation efficiency. On one hand, the smaller Au particles could be easily obtained on a defect-rich surface. On the other hand, the interaction between Au and ZnO is significantly enhanced when using nanosized ZnO instead of bulk ZnO. This finding is in agreement with Corma et al.,46 who recently claimed that Au supported on CeO2 nanoparticles has significantly higher activity than Au supported on conventional CeO2 in the oxidation of alcohols. The highest selectivity of 84.1% can be obtained at a conversion of 49.8% when 1.1% Au/PZnO-IMP catalyst is calcined at 180 °C. While increasing the calcination temperature up to 240 °C, both selectivity and activity decline simultaneously. Adjusting the Au loading or the amount of CLAD affects the hydrogenation rate; however, the selectivity to unsaturated alcohol remains at the same level. For example, as is shown in the Supporting Information, 96.2% conversion was achieved within 25 min when using 2.2% Au/PZnO-IMP-180 °C catalyst, while the selectivity (84.5%) remain the same as 1.1% Au/PZnO-IMP180 °C. Another example is that 78.6% conversion was obtained after 3 h of hydrogenation over 2.2% Au/PZnO-IMP180 °C catalyst when the amount of CLAD is increased from 0.025 to 0.1 mL, and, more importantly, the selectivity remained at about 85%. 3.2.3. Au/ZnO-CP Catalysts. Figure 10 shows the hydrogenation results over 0.7% Au/ZnO-CP catalysts calcined at various temperatures. The hydrogenation efficiency of Au/ ZnO-CP catalyst largely depends on calcination temperature. Au/ZnO-CP-50 °C catalyst is not active in the hydrogenation of CLAD. Less than 5% conversion was achieved after 3 h hydrogenation reaction. Even when calcined at 160 °C, for 3 h, the catalyst shows very low activity, and no selectivity to unsaturated alcohol was observed. However, both activity and selectivity increase if the reaction time is prolonged to 6 h. The XRD pattern shows that at this period of time, the hydrozincite support gradually transforms to ZnO structure. Similar to Au/

Figure 8. Selective hydrogenation of CLAD over 1.1% Au/TZnO-DP180 °C catalyst. Reaction conditions: 0.1 g of catalyst (before calcined at 180 °C), 0.025 mL of CLAD, 110 °C, 2 MPa H2.

impregnation method has much higher hydrogenation activity and selectivity. As is shown in Figure 9, 1.1% Au/PZnO-IMP

Figure 9. Selective hydrogenation of CLAD over 1.1% Au/PZnO-IMP catalysts: The dependence of catalyst efficiency on calcination temperature. Reaction conditions: 0.1 g of catalyst (before calcination), 0.025 mL of CLAD, 110 °C, 2 MPa H2, 0.5 h.

Figure 10. Selective hydrogenation of CLAD over 0.7% Au/ZnO-CP catalysts: The dependence of conversion (a) and selectivity (b) on calcination temperature. Reaction conditions: 0.1 g of catalyst (before calcination), 0.1 mL of CLAD, 110 °C, 2 MPa H2. F


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Figure 11. Selective hydrogenation of CLAD over Au/ZnO-CP catalysts: The dependence of conversion (a) and selectivity (b) on Au content. Reaction conditions: 0.1 g of catalyst (before calcination at 180 °C), 0.1 mL of CLAD, 110 °C, 2 MPa H2.

Figure 12. Selective hydrogenation of CLAD over 1.4% Au/Fe2O3-CP catalysts: The dependence of conversion (a) and selectivity (b) on calcination temperature. Reaction conditions: 0.1 g of catalyst (before calcination), 0.1 mL of CLAD, 110 °C, 2 MPa H2.

not the key ingredient for achieving high catalysis efficiency, at least on Au/ZnO catalyst. First of all, Au/ZnO-CP catalyst dried at 50 °C, which has the highest concentration of cationic Au, is less effective than that calcined at 180 °C. Second, after prereduction of Au/TZnO or Au/PZnO catalyst, the ratio of metallic Au and cationic Au is the same for three different calcination temperatures. If cationic Au plays one of the most important roles, the selectivity and activity should show similar results whether catalysts are calcined at 50, 180, or 300 °C. Thus, we conclude that for Au/ZnO catalysts, cationic Au is less important than the following parameters: (1) Au size and shape, (2) the support size and surface texture, and (3) the interaction between Au and support. 3.2.4. Au/Fe2O3-CP Catalysts. To more fully explore whether the optimal calcination temperature we found for Au/ZnO-CP is applicable to another supported Au catalyst, as noted above, we chose Au/Fe2O3 as a candidate for a variable calcination temperature study. Hydrogenation of α,β-unsaturated aldehydes over Au supported iron oxide catalysts has been studied by Deng,49 Milone,23,50,51 and Volpe.52 Deng demonstrated that a higher selectivity to the unsaturated alcohol can be obtained when using iron oxide supported Au catalyst dried at 50 °C instead of calcined at 400 °C.49 Milone also claimed that Au supported on FeOOH has higher activity and selectivity to unsaturated alcohol than Au supported on αFe2O3 catalyst.51 Our selective hydrogenation results of 1.4% Au/Fe2O3-CP catalysts calcined at various temperatures are shown in Figure 12. In contrast to Au/ZnO-CP catalysts, the

TZnO, both the highest conversion and selectivity can be achieved when Au/ZnO calcined at 180 °C, where hydrozincite transforms to ZnO after 2 h prereduction under hydrogenation reaction conditions. These results indicate that ZnO is a better support than hydrozincite, and it acts as a promotor for the hydrogenation reaction. Upon calcination to 300 °C, the selectivity declines to zero and the conversion decreases significantly. The dependence of conversion and selectivity on Au content over Au/ZnO-CP catalyst was also investigated in our study. As illustrated in Figure 11, increasing the Au loading on Au/ZnOCP catalyst will enhance the selectivity and activity. When using 1.5% Au/ZnO-CP-180 °C catalyst, 100% selectivity could be achieved at a conversion of 94.9%. Saturated alcohol will start to increase if the reaction is prolonged, thus decreasing the selectivity. Compared with Au/PZnO-IMP, the Au/ZnO-CP catalyst has even higher selectivity and activity. We propose that the high efficiency of Au/ZnO-CP for selective hydrogenation of CLAD is due to the smaller, more highly dispersed Au particles and smaller ZnO particles. Whether cationic Au serves as an active site for oxidation or hydrogenation has been debated for a long time. By combining EXAFS, in situ XANES, XPS, and Mössbauer effect spectroscopy, Hutchings and co-workers stated that cationic Au plays a crucial role in CO oxidation over Auα-Fe2O3 catalyst.47 Similarly, Xu et al. claimed that isolated Au3+ ions on ZrO2 is the active site for the selective hydrogenation of 1,3-butadiene to butanes.48 However, our findings prove that cationic Au is G


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The Journal of Physical Chemistry C Table 1. Summary of Catalytic Results for the Hydrogenation of Cinnamaldehydea selectivity % entry 1 2 3 4 5 6 7 8 9

catalyst 2.2% 1.1% 1.1% 1.1% 2.2% 0.7% 1.5% 1.4% 1.9%

Au/TZnO-IMP-180 °Cc Au/TZnO−DP-180 °Cc Au/PZnO-IMP-50 °Cc Au/PZnO-IMP-180 °Cc Au/PZnO-IMP-180 °Cc Au/ZnO-CP-180 °Cd Au/ZnO-CP-180 °Cd Au/Fe2O3−CP-50 °Cd Au/Fe2O3−CP-50 °Cd

Treact., min

TOF %b

conversion

USOL

SAL

SOL

120 120 30 30 25 180 40 90 40

3 10.9 62.6 35.4 41 70.9 148.1 70.3 118.7

33.8 61.5 88.1 49.8 96.2 95.2 94.9 94.6 96.3

43.5 75.4 74.3 84.1 84.5 63.9 100 89.1 89

56.5 14.3 3.7 15.9 0.6 7.9 0 0.9 0

0 10.3 22 0 14.9 28.2 0 10 11

Reaction conditions: All reactions are carried out at 110 °C, 2 MPa H2 and 0.1 g catalyst (before calcination). bTOF was measured as mol substrate per mol Au per hour. c0.025 mL CLAD was used. d0.1 mL CLAD was added. a

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Au/Fe2O3-CP catalyst appears less sensitive to variation in calcination temperature. The activity is about the same when calcined below 180 °C, then gradually decreases with increasing calcinations temperature to 300 °C. The selectivity decreases from 91.5% (calcined at 160 °C) to 79.2% (calcined at 300 °C). When Au/Fe2O3-CP catalyst is calcined at 400 °C, however, the selectivity and activity simultaneously decline to 23.3% and 12.9%, respectively. Increasing the Au loading can enhance the activity, as is shown in the Supporting Information, but the selectivity to unsaturated alcohol remains ∼89% when conversion reaches up to 95%. This finding demonstrates that the calcination temperature of 180 °C is particularly influential for Au/ZnO catalyst, at which temperature the shape and size of the Au nanoparticles and the interaction between Au and ZnO are optimized to achieve the highest selectivity. Moreover, as is shown in Table 1, the 1.5% Au/ZnO-CP catalyst has a higher efficiency for selective hydrogenation of CLAD than the Au/Fe2O3-CP catalyst.

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to express our thanks to Paige Landry, Ben Estes, Harry Meyers, and Nathaniel Bass for useful discussions, comments, and assistance. We would especially like to acknowledge the efforts of Nicholas Strange who provided substantial assistance in improving the graphics in the revised manuscript. This research has benefitted from the technical expertise and instrumentation access made possible by a user project supported by ORNL’s Center for Nanophase Materials Sciences (CNMS), which is a DOE Office of Science User Facility. JZL received partial support for the initial stages of this work from the DOE BES.

4. CONCLUSION In summary, we systematically investigated three types of Au/ ZnO catalysts and a Au/Fe2O3 catalyst for selective hydrogenation of CLAD. The XRD, STEM, XPS, and hydrogenation results reveal that instead of cationic Au, Au nanoparticles (small and mound shape), ZnO support (nanosize and defectrich surface), and interaction between Au and ZnO are three important parameters to achieve a highly efficient Au/ZnO catalyst. There is a variation in the optimal calcination temperature for different supported Au catalysts. For Au/ ZnO, calcination at 180 °C, optimizes the three parameters (mentioned above) and leads to high selectivity. In this study, Au/ZnO-CP catalyst calcined at 180 °C shows the highest efficiency for selective hydrogenation of CLAD; 100% selectivity was obtained at a conversion of 94.9% over 1.5% Au/ZnO-CP-180 °C catalyst.

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AUTHOR INFORMATION

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b07823. X-ray diffraction, photoemission spectra (XPS) with Au oxidation state and relative abundances and bright field scanning transmission electron micrographs (BF-STEM), conversion and selectivity plots of selective of CLAD hydrogenation for various Au-ZnO nanoparticle compositions (PDF) H


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The Journal of Physical Chemistry C (52) Lenz, J.; Campo, B. C.; Alvarez, M.; Volpe, M. A. Liquid Phase Hydrogenation of α,β-Unsaturated Aldehydes Over Gold Supported on Iron Oxides. J. Catal. 2009, 267, 50−56.

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Highly Efficient Selective Hydrogenation of Cinnamaldehyde to

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