Catalysis Communications 9 (2008) 1949–1954

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Catalysis Communications

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Immobilized PVA-stabilized gold nanoparticles on silica show an unusualselectivity in the hydrogenation of cinnamaldehyde

Hui Shi, Na Xu, Dan Zhao, Bo-Qing Xu *

Innovative Catalysis Program, Key Lab of Organic Optoelectronics and Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing 100084, China

a r t i c l e i n f o a b s t r a c t

Article history:Received 15 December 2007Received in revised form 20 February 2008Accepted 5 March 2008Available online 20 March 2008

Keywords:Catalysis by goldSelective hydrogenationCinnamaldehydeHydrocinnamaldehydeImmobilized Au colloids

1566-7367/$ - see front matter � 2008 Elsevier B.V. Adoi:10.1016/j.catcom.2008.03.025

* Corresponding author. Tel./fax: +86 10 6279 2122E-mail address: bqxu@mail.tsinghua.edu.cn (B.-Q.

This work reports for the first time an ‘‘unusual” property of immobilized PVA-stabilized gold nanopar-ticles (AuNPs) on SiO2 for highly selective hydrogenation at the C@C bond of cinnamaldehyde (CAL) at150–180 �C, uncovering a catalytic route for selective hydrocinnamaldehyde (HCAL) production fromCAL. When the same colloidal gold nanoparticles are immobilized on ZrO2, they effect competitive hydro-genations at both C@C and C@O bonds of the CAL molecules under identical reaction conditions.

� 2008 Elsevier B.V. All rights reserved.

1. Introduction

The application of metal nanoparticles (MeNPs) in catalysis is atrendy frontier of research in recent years [1–4]. Among many syn-thetic strategies proposed for the preparation of MeNPs, one of themost commonly used synthetic methods is their generation insolution by reduction of suitable metal precursors in the presenceof a polymer stabilizer able to interact with the metal surface andthus prevent agglomeration [5]. Polyvinyl alcohol (PVA) and poly-vinyl pyrrolidone (PVP) are two types of the frequently used poly-mer stabilizers in the syntheses of MeNPs [6], which could bepromising for versatile catalytic applications. For instance, PVA-stabilized AuNPs have been widely investigated in the selectiveoxidation of alcohols and sugars [7–10]. And, the use of PVP-stabi-lized colloidal Au has very recently been extended to catalyzinghydrogenation reactions [11].

The direct use of colloidal Au catalyst, however, could be unfa-vorable in practical applications, e.g., separation and reuse of thecatalyst after reaction would be problematic. It has been wellknown that supported gold nanoparticles can offer interestingselectivity for a number of reactions including hydrogenation[12]. In the hydrogenation of a,b-unsaturated aldehydes or ke-tones, the supported gold catalysts were shown to be in favor ofthe hydrogenation at the C@O bond [11,13–25]. An important reac-tion in the hydrogenation of a,b-unsaturated aldehydes is theselective semi-hydrogenation of cinnamaldehyde (CAL) since its

ll rights reserved.

.Xu).

products, cinnamyl alcohol (COL) from the hydrogenation at C@Oand hydrocinnamaldehyde (HCAL) from the hydrogenation at theconjugated C@C bond, are both important intermediates in themanufacture of perfumes and pharmaceuticals [26–28]. Recently,HCAL was even considered as an important intermediate for thesynthesis of pharmaceuticals for the treatment of HIV [28]. In thepresent work, we investigate the synthesis and immobilization ofPVA-stabilized AuNPs on SiO2 and ZrO2, and their catalytic selectiv-ity in the liquid-phase hydrogenation of cinnamaldehyde (CAL). Incontrast to earlier documentations that COL was the favorableproduct over supported gold catalysts [21,22], we show for the firsttime that the immobilized PVA-stabilized AuNPs on SiO2 are highlyselective for the production of HCAL in the hydrogenation of CAL.

2. Experimental

2.1. Preparation and immobilization of AuNPs

Three samples of narrowly sized colloidal Au particles withdiameters of 10.0 ± 1.2, 5.0 ± 1.5 and 3.0 ± 0.6 nm, named asAu(10), Au(5) and Au(3), respectively, were synthesized by reduc-tion of AuCl�4 ions in aqueous solutions containing varying amountof PVA. In particular, the Au(10) sample was prepared by using tri-sodium citrate for the reductant as in the work of Enustun andTurkvich [29]. The other two Au samples having different PVA con-tents were prepared following the method of Porta et al. [8], with afew modifications detailed in Ref. [30].

Immobilization of the AuNPs was conducted by adding a desir-able amount of SiO2 (Degussa, AEROSIL 300, SBET = 90 m2 g�1) in

1950 H. Shi et al. / Catalysis Communications 9 (2008) 1949–1954

the colloidal solution of AuNPs, followed by a careful adjustment ofthe solution acidity to pH = 0.5 with 1 M HNO3. When ZrO2 (pre-pared as in Ref. [31], SBET = 100 m2 g�1) was used as the supportingmaterial, the final solution acidity was adjusted to pH = 3.0. Thewhole process was accompanied by vigorous stirring. The solidswere then separated, extensively washed by deionized water andair-dried at 110 �C for 2 h to produce the Au/SiO2 or Au/ZrO2

samples.

2.2. Characterizations

The immobilization or adsorption of Au colloids on SiO2 andZrO2 from the colloidal solution of Au was confirmed to be totallyquantitative by ICP-AES analysis on a Leeman Prodigy spectrome-ter of Au both in the solid catalysts and the filtrates separated fromthe catalysts. BET measurements of the solid materials were car-ried out with nitrogen adsorption at 77 K on a Micromeritics ASAP2010C instrument; pretreatments of the solid materials were doneby degassing at 110 �C for 2 h. The morphology, size (diameter) andsize distribution of Au colloids in the solution and immobilized onthe supporting materials were characterized by analytic transmis-sion electron microscopy (TEM) using a JEOL JEM-2010 system,equipped with an Oxford INCA analyzer, operating at 200 kV. X-Ray photoelectron spectroscopy (XPS) measurements were carriedout on a PHI 5300 ESCA1610 SAM instrument equipped with MgKa radiation.

2.3. Catalytic hydrogenation and product analysis

The hydrogenation of CAL was performed in a 25 ml stainlesssteel autoclave under magnetic stirring. Typically, the reactorwas loaded with 50 mg of an immobilized catalyst, 0.5 ml CALand 4.0 ml toluene (solvent); the CAL/Au molar ratio was 1600.The autoclave was repeatedly (six times) flushed with H2, beforeit was pressurized to the desired H2 pressure (1.0 MPa) and thenplaced in a heating jacket preheated to a designated reaction tem-perature. Zero reaction time was taken as soon as the autoclave

Fig. 1. Representative TEM micrographs of un-immobilized (A1, A2, and A3), and immobiSiO2; B2: 1% Au(5)/SiO2; B3: 1% Au(10)/SiO2.

was placed in the heating jacket. SiO2 (and ZrO2) powders werepreviously proofed under identical reaction conditions to be totallyinactive for the hydrogenation of CAL. Also, hydrogenation of thesolvent toluene was not observed over all of the catalysts investi-gated in this work. The reacted liquid, containing products andunreacted CAL, was separated from the solid catalyst by filtrationand analyzed with a gas chromatograph (HP4890). n-Nonane wasused as an internal standard to calibrate the reaction products.

3. Results and discussion

In this work, the synthesis of AuNPs with different sizes wasrealized via chemical reduction of AuCl�4 in aqueous solutions bytuning the PVA/Au (wt/wt) ratio from 1 to 0. The complete reduc-tion of AuIII to form zero-valent AuNPs was visually witnessed by acolor change of the solution from yellow to wine-red/blood-red,which was also supported by UV–vis measurements (e.g., Ref.[30]). Shown in the upper row (A1–A3) of Fig. 1 are representativeTEM images of the AuNPs from the as-prepared colloidal solutions,which assume approximately spherical shape with size distribu-tion in the range of 3.0 ± 0.6, 5.0 ± 1.5 and 10.0 ± 1.2 nm, respec-tively; their corresponding PVA/Au (wt/wt) ratio in the synthesissolution was 1, 0.5 and 0, respectively. The color and UV–vis spec-tra of the solution containing the PVA-stabilized Au colloids (A1and A2 in Fig. 1) remained unchanged over a long period of time(at least 20 days!), indicating that the as-prepared AuNPs werewell stabilized against agglomeration during storage owing to aneffective stabilization by PVA [32]. Nevertheless, a post-processof immobilization would still be necessary since the morphologyand activity of Au colloids might not be preserved for long duringreactions, as was observed in glucose oxidation [33]. After theimmobilization including subsequent washing and drying treat-ments, the dimensions of AuNPs in Au(3)/SiO2 and Au(5)/SiO2 re-mained essentially the same as their corresponding Au(3) andAu(5) colloids. However, the average size of PVA-free AuNPs inAu(10)/SiO2 grew to ca. 16 nm (B1–B3 in Fig. 1), which was signif-icantly larger than the as-prepared Au(10) colloids in solution.

lized AuNPs on SiO2 (B1, B2, and B3). A1: Au(3); A2: Au(5); A3: Au(10). B1: 1% Au(3)/

H. Shi et al. / Catalysis Communications 9 (2008) 1949–1954 1951

Washing cycles performed after the immobilization of AuNPson the support materials aimed at removing any free PVA notbound to AuNPs or the support surface. Evidence for the presenceof surface-bound PVA residues was given in Fig. 2A, which showsthe XPS signals of C1s and Si2p. The measured C1s/Si2p ratio atthe surface of Au(3)/SiO2 was remarkably higher than that of thecorresponding ‘‘clean” SiO2 support. In a blank experiment, the‘‘clean” silica support was treated with an Au-free aqueous PVAsolution (10 mg-PVA ml�1) by immersing the support material inthe solution (pH 3.0). After washing extensively with deionizedwater and drying at 110 �C, no PVA residues were detected on thisreference SiO2 by means of thermogravimetric analysis (TG/DTA),indicating no chemical interaction between PVA and the silica sur-face. Therefore, the surface-bound PVA residues detected by XPS

76 81 86 91 96Binding Energy (eV)

Inte

nsity

(a.u

.)

Au4f7/2 Au4f5/2

90 100 280 290 300

C1s

Si2p

Inte

nsity

(a.u

.)

Binding Energy (eV)

A

B

I

II

Fig. 2. (A) XPS spectra of C1s and Si2p for the silica support (I) and PVA-stabilized1% Au(3)/SiO2 catalyst (II); (B) XPS signals of Au4f for the PVA-stabilized 1% Au(3)/SiO2 catalyst.

Table 1Hydrogenation of cinnamaldehyde over PVA-stabilized gold nanoparticles immobilized on

PVA-stabilizO + H2

toluene, 18

Entry Support Au loading (%) Surface area (m2/g) AuNP size

1 SiO2 1.0 87 3.0 (3.0)2c SiO2 3.0 83 3.0 (3.0)3 SiO2 1.0 85 5.0 (4.9)4 SiO2 1.0 78 10.0 (16.35 ZrO2 1.0 86 3.0 (–d)6 ZrO2

e 0.76 90 8.0

a Unless otherwise specified, the reaction was carried out in 4.0 ml toluene using 0.5b Average gold particle size determined by TEM. The values outside and inside the pac Twenty-five milligram catalyst was used in this case, other reaction parameters beid Not determined.e This catalyst was used in previous work [36,37] with the denotation of 0.76% Au/Zr

(Fig. 2A) were actually associated with the immobilized AuNPs,but not the SiO2 support, which is not surprising since PVA wasthe stabilizer of AuNPs. However, it remains unclear whether thestabilizer PVA was just to maintain a separation of the Au particlesagainst agglomeration or it could additionally effect significantsurface modification on its stabilized AuNPs. In the Pt-PVA system,it was shown earlier that the core-level binding energy of Pt wasbasically not affected by the interaction between PVA and PtNPs[34]. Similarly, we hardly scented any sign of obvious surface mod-ification of PVA on AuNPs in terms of the XPS BEs of Au4f7/2 andAu4f5/2 (Fig. 2B) in the Au/SiO2 samples [30,35].

The immobilized AuNPs on SiO2 and ZrO2 were used to catalyzethe liquid-phase hydrogenation of CAL at 180 �C, the results aresummarized in Table 1. Under the same reaction conditions (notoptimized), the Au(3)/SiO2 catalyst effected the highest CAL con-version and thus exhibited the highest activity for the hydrogena-tion reaction. In order to make a good estimation of thehydrogenation activity by TOF of the immobilized AuNPs, the reac-tion time over Au/SiO2 catalysts was shortened, when necessary, toless than an hour to limit the CAL conversion to be lower than 10%.We obtained TOF rates based on reacted CAL and gold dispersion(or exposed percentage) of the AuNPs [36,37] to be 600 h�1 over1.0% Au(3)/SiO2, 430 h�1 over 1.0% Au(5)/SiO2, and 360 h�1 over1.0% Au(10)/SiO2 catalysts. Therefore, the very small (3.0 nm)PVA-stabilized AuNPs in 1.0% Au(3)/SiO2 were unique in showingthe highest activity in terms of the TOF rate. When the loading ofAu(3) particles was increased to 3.0% (Table 1, entry 2), it exhibiteda TOF (620 h�1) for the consumption of CAL at CAL conversion of ca.20%. Thus, the loading level or concentration of AuNPs on the sup-port surface does not significantly affect the activity of the Au(3)/SiO2 catalyst in terms of the TOF rate.

An outstanding feature of the immobilized AuNPs on SiO2 forthe hydrogenation of CAL is their high selectivity for the formationof hydrocinnamaldehyde (HCAL), which is the product of hydroge-nation at the C@C bond. The potentially competitive hydrogenationat the C@O bond, with cinnamyl alcohol (COL) being the product,was always undetectable as long as the reaction temperaturewas controlled to no higher than 180 �C, (see also Table 2).Au(3)/SiO2 catalyst containing the smallest AuNPs showed alsothe highest selectivity for HCAL. In comparison, Au(10)/SiO2 cata-lyst carrying the biggest AuNPs produced a considerable selectivity(12%) for the formation of hydrocinnamyl alcohol (HCOL), theproduct of a total hydrogenation at the conjugated C@C and C@Obonds. And, accordingly, the HCAL selectivity over this Au(10)/SiO2 catalyst was ca. 10% lower than that over the Au(3)/SiO2 cat-alyst; HCAL and HCOL add up to more than 99% of total products

silica and zirconia at 180 �Ca

ed AuNPs O

0 °C, 4 h

(nm)b Conversion (%) Selectivity (%)

HCAL HCOL COL Others

58.9 96.2 3.6 0.0 0.283.0 95.3 4.5 0.0 0.220.3 90.8 8.9 0.0 0.3

) 9.5 87.1 12.5 0.0 0.415.0 80.1 7.6 12.0 0.38.2 57.6 11.8 29.5 1.1

ml CAL and 50 mg catalyst (molar CAL/Au = 1600) under 1.0 MPa H2 for 4 h.rentheses are the sizes measured before and after the immobilization step.ng unchanged.

O2 – 673.

Table 2Effect of reaction temperature on the catalytic performance of 1.0% Au(3)/SiO2

catalyst in the hydrogenation of cinnamaldehydea

Entry T (�C) Time (h) Conversion (%) Selectivity (%)

HCAL HCOL COL Othersb

1 120 4 12.3 57.6 41.5 0.0 0.92 150 4 30.2 92.8 6.9 0.0 0.33 180 4 58.9 96.2 3.6 0.0 0.24 180 6 79.5 94.3 5.5 0.0 0.25 180 10 100.0 48.7 50.2 0.0 1.16 210 4 69.6 66.6 7.7 25.4 0.3

a Refer to the footnote of Table 1 for reaction conditions other than reaction time.b Two main by-products were identified as the dehydration products of HCOL, 1-

propenyl benzene and 2-propenyl benzene, whose area percentages in GC analysisof the products accounted for 90% of the total area of all detected by-products. Theselectivity to all by-products was calculated according to the formula: selectiv-ityothers (%) = 100% � (SelectivityHCAL + SelectivityHCOL + SelectivityCOL).

1952 H. Shi et al. / Catalysis Communications 9 (2008) 1949–1954

over the present Au/SiO2 catalysts, the by-products including 1-and 2-propyl benzenes and any of the unidentified were less than1%. While the conversion levels of CAL over the three Au/SiO2 cat-alysts were remarkably different in Table 1, a relationship betweenthe size increment of AuNPs and the decrease in HCAL productioncould still be straightforward since the chemo-selectivity of thepresent hydrogenation reaction hardly varied with the conversionof CAL, when it was less than 100% (see also the data in Tables 2and 3).

Size effect of AuNPs in the activity and selectivity issues of Au-catalyzed hydrogenation of a,b-unsaturated aldehydes is still un-der debate since the property of support materials would affectremarkably the performance of Au catalysts [15,16,20–22]. SiO2-supported Au catalysts had been prepared via diversified methodsdifferent from ours in the present work, and they were used to cat-alyze the hydrogenation of other a,b-unsaturated aldehydes suchas acrolein and crotonaldehyde [13–19]. Though the Au particlesize was varied in a wide range from 150 nm [13] to 3.9 nm [18]and 5.3 nm [19], these SiO2-supported Au catalysts often showeda high selectivity (e.g., up to 98% in [13]) toward the hydrogenationat the C@C bonds to form the semi-hydrogenated aldehyde prod-ucts. On ignorance of the different molecular structure of CAL fromacrolein and crotonaldehyde, these earlier observations could beconsidered similar to our observed high selectivity of HCAL inthe hydrogenation of CAL. However, the C@C bond in a CAL mole-

Table 3Reusability of 1.0% Au(3)/SiO2 catalyst for selective hydrogenation of cinnamaldehydeat 180 �Ca

Runb Usec Reaction time(min)

Conversion(%)

Selectivity (%)

HCAL HCOL COL Othersd

1 1-a 170 96.7 96.5 3.3 0.0 0.21-b 190 100.0 86.9 12.6 0.0 0.51-c 220 100.0 80.4 18.9 0.0 0.71-d 220e 100.0 79.7 19.6 0.0 0.7

2 2-a 170 95.6 87.2 13.3 0.0 0.52-b 220 100.0 74.2 24.9 0.0 0.9

3 3-a 170 87.2 93.1 6.5 0.0 0.43-b 220 93.4 85.3 14.1 0.0 0.6

a The catalyst load was 100 mg (molar CAL/Au = 800), other conditions exceptreaction time were the same as those specified in the footnote of Table 1.

b The reacted catalyst was recovered from the reaction mixtures by separationwith centrifugation and then washing with anhydrous ethanol (finally air-dried at110 �C). Gold loss during the reaction and catalyst recovery was hardly detected byICP-AES analysis of the recovered catalyst.

c 1-* symbolizes the use of fresh catalyst; 2-, and 3-* symbolize the first andsecond reuses of the used catalyst, respectively.

d See Footnote b in Table 2.e A repeat of Run 1-c.

cule conjugates not only with the terminal C@O bond, but alsowith the benzene ring of the phenyl group, its reactivity towardhydrogen can be intrinsically much lower than those of the C@Cbonds in acrolein and crotonaldehyde molecules [38], which wouldalso highlight the significance of the present finding that theimmobilized PVA-stabilized AuNPs on SiO2 are highly selectivefor HCAL production in the hydrogenation of CAL.

Based on the identified product molecules, a reaction network isproposed to account for the hydrogenation reactions of CAL overour Au/SiO2 catalyst (Scheme 1). Irrespective of average size ofthe immobilized AuNPs, no formation of COL was detected overour Au/SiO2 catalysts. This seems rather unusual since COL was re-ported to be the major product of CAL hydrogenation over nano-sized Au particles supported on interacting oxides like Fe2O3 [22]and Al2O3 [21]. Such a difference in product selectivity could hinta significant effect of metal-support interaction on the hydrogena-tion catalysis by gold, if difference in catalyst preparation could notinvalidate the comparison because none of these referenced COL-selective Au catalysts was prepared by immobilization of AuNPsfrom preformed AuNPs in solution. The catalytic performance ofAuNPs on SiO2, a far less interacting support than Fe2O3, Al2O3

and etc., could be more informative for understanding the intrinsiccatalysis by AuNPs.

The effect of interacting support on the hydrogenation catalysisof Au was dealt with by comparing the reaction data over 1.0%Au(3)/SiO2 and 1.0% Au(3)/ZrO2, which were obtained with similarpreparations. In comparison with the 1.0% Au(3)/SiO2 catalyst, the1.0% Au(3)/ZrO2 catalyst (Table 1, entry 5) effected a much lowerCAL conversion but a considerable selectivity (12%) for the forma-tion of COL, and its selectivity for HCAL formation was significantlylower. Such differences in both activity and selectivity of the sim-ilarly immobilized PVA-stabilized AuNPs on ZrO2 and SiO2 cer-tainly demonstrated a modification of an interacting oxidesupport on the catalysis of PVA-stabilized AuNPs. Moreover, an-other reference 0.76% Au/ZrO2 catalyst, prepared by the well-known deposition–precipitation method in the absence of any sta-bilizer [36,37], showed even higher selectivity for the formation ofCOL (ca. 30%); its selectivity for HCAL was even much lower (Table1, entry 6), indicating that presence of the PVA stabilizer was ben-eficial to the selective formation of HCAL. In the absence of anysupport material, ‘‘free” PVP-stabilized AuNPs in solution showedalso high selectivity for the formation of allylic alcohols in thehydrogenation of a,b-unsaturated aldehydes or ketones [11],although CAL was not involved in the work. A clarification of therole of the PVA or PVP stabilizer in the hydrogenation catalysisby AuNPs remains to be addressed in the future work.

CAL

+H2

CinnamaldehydeHCAL

Hydrocinnamyl alcohol HCOL

1-propenyl benzeneBy-product

2-propenyl benzene By-product

+H2

-H2O

O O OH

Hydrocinnamaldehyde

-H2O

Rearra

ngem

ent

+H2

OH

Cinnamyl alcohol COL

Scheme 1. Proposed reaction pathways based on identified products in the hydr-ogenation of cinnamaldehyde over Au/SiO2 at 150–180 �C.

H. Shi et al. / Catalysis Communications 9 (2008) 1949–1954 1953

Selectivity in the hydrogenation of a,b-unsaturated aldehydesor ketones was often found sensitive to the reaction temperatureas well as the reactant conversion [39–42]. The 1.0% Au(3)/SiO2

catalyst was further used to investigate the effect of reaction tem-perature in the range of 120–210 �C; the results are shown in Table2. As it would be expected, the temperature increase up to 180 �Cresulted in a steady increment of the CAL conversion. However, theselectivity for HCAL was surprisingly low (57.6%) at 120 �C. Instead,the formation of HCOL, the product from two consecutive hydroge-nation reaction pathways of the reactant CAL (Scheme 1), appearedto be favored at this low temperature. When the reactant CAL be-came completely converted by extending the reaction for a longtime at 180 �C (Table 2, entry 5), the selectivity and yield for HCOLincreased remarkably due to the occurrence of a further hydroge-nation of HCAL. In support of this observation, the selectivity andyield of HCOL continued to increase with the reaction time afterthe reactant CAL was completely consumed in the experimentssummarized in Table 3.

COL, the product of selective CAL hydrogenation at the C@Obond, was not detected unless the reaction was conducted at210 �C (Table 2, entry 6). The requirement of such a high temper-ature for COL formation may be explained by a significantly higherenergy barrier in the activation of the C@O bond since the dissoci-ation energy of the C@O bond (174 kcal/mol) is much higher thanthat of the C@C bond (141 kcal/mol) [43]. Preferred hydrogenationat the C@O bond of citral (3,7-dimethyl-2,6-octadienal) using high-er reaction temperatures was previously observed over Pt/SiO2 cat-alysts [39,40].

It is noticeable that not only the selectivity for HCOL, but also itsproduction rate from HCAL decreased with increasing the reactiontemperature up to 180 �C in Table 2. With reference to Singh andVannice, who proposed a simple Langmuir–Hinshelwood kineticsand a weak adsorption of hydrogen (hydrogen adsorption on Auis known to be weak [12]) in their study of citral hydrogenationover Pt/SiO2 catalyst [39,40], the product formation rate of thehydrogenation reaction (HCAL from CAL, and HCOL from HCAL)would be mainly determined by the reaction rate constant andthe adsorption equilibrium constant of the reactant molecules.The higher selectivity of HCOL at lower temperatures (Table 2)should indicate a much higher adsorption constant of HCAL rela-tive to the CAL adsorption since the rate constant would becomegreater at higher temperature.

Table 3 shows the reusability of the 1.0% Au(3)/SiO2 catalyst forthe hydrogenation of CAL at 180 �C; in these reaction tests the cat-alyst load in the reactor was increased to 100 mg (CAL/Au = 800).The first use of this catalyst offered an excellent selectivity (ca.96%) for HCAL (Table 3, Run 1-a) at CAL conversion as high as96%. This observation indicates a good resistance of the primaryHCAL product against a further hydrogenation to form HCOL, aslong as the reactant CAL is not completely consumed. Recallingthat the reactor underwent a heating-up from room temperature,and that the further hydrogenation of the primary product HCALto form the secondary product HCOL occurred more easily at lowertemperatures (Table 2), it is almost certain that the formation ofHCOL under incomplete CAL consumption was mainly due to thesecondary hydrogenation of HCAL occurred during the tempera-ture increase via heat exchange; formation of HCOL from HCALat 180 �C was significant only after a complete conversion of thereactant CAL (Table 3). Judged by the CAL conversion, the recov-ered 1.0% Au(3)/SiO2 catalyst (compare the data of Runs 1-a with2-a, and 1-c with 2-b in Table 3) showed almost the same activityas the fresh catalyst in its first reuse, though the selectivity forHCOL increased considerably at the expense of HCAL with increas-ing the recovery and reuse cycles. We consider two explanationsfor the enhanced formation of HCOL in the catalyst reuse: (1) agrowth of AuNPs would lead to easier formation of HCOL (Table

1); (2) a partial removal of the PVA stabilizer could happen duringthe catalyst recovery, by washing with anhydrous ethanol, whichwould result in more unprotected Au surface to effect the furtherreaction of HCAL. However, these two factors may be interrelatedand could also be responsible for the activity decline with in-creased reuses. TEM characterization of the fresh and reacted1.0% Au(3)/SiO2 catalyst witnessed that the average size of AuNPsgrew from 3.0 nm in the unreacted fresh state to 4.1 nm in itsrecovered state, i.e., after the catalyst had been reacted in the reac-tor for ca. 4 h at 180 �C. Nevertheless, the recovered catalyst stillshowed no activity for the formation of COL at 180 �C, and thusmaintained the outstanding feature of the immobilized PVA-stabi-lized AuNPs for the hydrogenation at the C@C bond of CAL.

4. Conclusions

Our data have shown that the immobilization on SiO2 of PVA-stabilized AuNPs (especially those with smaller sizes, e.g., ca.3 nm) from colloidal Au solutions can lead to highly selective Au/SiO2 catalysts for HCAL production at 180 �C from CAL hydrogena-tion at the C@C bond. This kind of chemo-selectivity of Au/SiO2 cat-alyst seems unique since any competitive hydrogenation at theconjugated C@O bond was completely avoided. For comparison,we immobilized the same AuNPs on ZrO2 but the resultant Au/ZrO2 catalyst effected competitive hydrogenation at both C@Cand C@O bonds of the reactant CAL molecules. This unique selec-tivity of the immobilized PVA-stabilized AuNPs has been so far be-yond the references for the hydrogenation of CAL and could enrichthe knowledge of hydrogenation catalysis by gold.

Acknowledgements

The authors acknowledge the financial support of this work byNSF (Grant No. 20573062) and National Basic Research Project ofChina (Grant No. 2003CB615804).

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