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Ag/Al2O3 for glycerol hydrogenolysis to 1,2-propanediol: activity,selectivity and deactivation

Jinxia Zhou,*a Jing Zhang,a Xinwen Guo,b Jingbo Maoa and Shuguang Zhang*a,b

Received 28th July 2011, Accepted 5th October 2011DOI: 10.1039/c1gc15918f

A series of g-Al2O3 supported silver catalysts (Ag/Al2O3) prepared with various Ag loadings andcalcination temperatures were used to convert glycerol to 1,2-propanediol. A catalyst with 2 mmolAg per gram Al2O3 and calcined at 400–500 ◦C presented the highest activity (glycerol conversion46 mol%) and selectivity (96 mol%) at 220 ◦C, glycerol/Ag (molar ratio) = 100/2, 1.5 MPa initialH2 pressure and 10 h. Optimal prereduction, elevated reaction temperature and hydrogen pressurepromote the activity, but the selectivity deteriorates at higher reaction temperatures. Excessivewater is detrimental to the performance. Catalyst deactivation was observed, mainly due to Agsintering under reducing environment. The spent catalyst could be calcined to fully recover theactivity.

1. Introduction

As the worldwide production and consumption of biodieselgrows rapidly, it has been predicted that glycerol, as the majorbyproduct of the biodiesel manufacturing processes, will bereadily available with low cost.1,2 This has triggered intensiveresearch aiming to convert glycerol to value-added chemicals.3–5

Among these studies, the catalytic hydrogenolysis of glycerolto propanediols, which are widely used versatile specialitychemicals, is quite attractive. However, the production of 1,3-propanediol (1,3-PD) with homogeneous or heterogeneouscatalysts is very challenging. It suffers from low selectivityand so far is not competitive with the fermentation routethat usually presents yield above 70%.6–8 On the contrary, thecatalytic conversion of glycerol to 1,2-propanediol (1,2-PD)seems viable. Two categories of catalysts, supported noble metalsand transition metal oxides, have been reported in the literaturefor this reaction. Table 1 summarizes a few representative onesand their catalytic performance.

Among several supported metal catalysts (metal: Rh, Ru, Pt,Pd; support: active carbon, SiO2, Al2O3), Furikado et al.9 foundthat Rh/SiO2 was the most active and selective one, with aglycerol conversion of 19.6% and a propanediol selectivity of39.8%. Feng et al.10 found that a TiO2 supported Ru catalystexhibited a markedly high activity, 90.1% glycerol conversion,but that the 1,2-PD selectivity was only 20.6%. Considering

aCollege of Environmental and Chemical Engineering, Dalian University,Dalian, 116622, China. E-mail: jxzhou@163.com, dluzhang@126.com;Fax: 86-411-87402449; Tel: 86-411-87403214bState Key Laboratory of Fine Chemicals, Dalian University ofTechnology, Dalian, 116012, China

the hypothesized two-step mechanism (dehydration and hydro-genation), many researchers have opted to use bifunctionalcatalysts, i.e. supported noble metal catalysts combined withvarious acidic materials.11–14 The combination of a Ru/C andan Amberlyst resin presented a glycerol conversion of 79.3%and a propanediol selectivity of 74.7%.11 The use of Ru/C withother acidic materials, such as niobia, 12-tungstophosphoricacid (TPA) supported on zirconia, the cesium salt of TPA andthe cesium salt of TPA supported on zirconia, was studied byBalaraju et al.13 They found a linear correlation between con-version and acidity on the catalysts. Alhanash et al.14 prepareda bifunctional catalyst by loading Ru onto a heteropolyacidsalt Cs2.5H0.5[PW12O40], which achieved 96% selectivity to 1,2-PD at 21% glycerol conversion. As for supported bimetallicnoble metal catalysts, Ma et al.15 discovered a promoting effectof Re on the catalytic performances of Ru/Al2O3, Ru/C, andRu/ZrO2, both on the conversion of glycerol and the selectivityto propanediols.

Most transition metal oxide catalysts used for the hydrogenol-ysis of glycerol contain copper. Chaminand et al.16 showed thatnearly 100% selectivity to 1,2-PD was achieved on a CuO/ZnOcatalyst, but the activity of the catalyst was so low that ittook 90 h to reach 19% glycerol conversion. Wang and Liu17

obtained a selectivity of 83.6% to 1,2-PD at 22.5% glycerolconversion. Dasari et al.18 reported that a commercial copperchromite catalyst (pre-reduced at 300 ◦C) converted glycerol to1,2-PD with a selectivity of 85.0% at 54.8% conversion at 200 ◦Cand 1.4 MPa initial H2 pressure, after 24 h. Considering thegood performance of copper chromite catalysts under relativelymild reaction conditions as well as the vulnerability of noblemetal catalysts to impurities in glycerol, the process usingcopper chromite is considered to be the most promising one

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Table 1 Summary of a few typical glycerol hydrogenolysis catalysts

CatalystRef. ConditionsConversion(mol%)

1,2-PD Selectivity(mol%)

Rh/SiO29 120 ◦C, 8.0 MPa initial H2 pressure, 10 h, 20 ml of 2 wt% glycerol aqueous

solution, 150 mg metal catalyst.19.6 39.8

Ru/TiO210 180 ◦C, 5 MPa H2, 12 h, 5 ml of 20 wt% glycerol aqueous solution, 102 mg

catalyst.90.1 20.6

Ru/C+Amberlyst11 120 ◦C, 8.0 MPa H2, 10 h, 20 ml of 2 wt% glycerol aqueous solution, 150 mg Rucatalyst + 300 mg Amberlyst.

79.3 74.9

Ru/Cs2.5H0.5[PW12O40]14 180 ◦C, 5 bar H2, 10 h, 5 ml of 20 wt% glycerol aqueous solution, 0.2 g catalyst. 21 96Ru/C + Re2(CO)10

15 160 ◦C, 8 MPa H2, 8 h, 10 ml of 40 wt% glycerol aqueous solution, 50 mgsupported catalyst.

59.4 56.6

CuO/ZnO16 180 ◦C, 80 bar H2, 90 h, 15 g glycerol in 65 ml H2O, 0.08 mol Cu (50%). 19 100Cu–ZnO17 200 ◦C, 4.2 MPa H2, 12 h, 15 g glycerol in 65 ml H2O, 7.5 mmol Cu (Cu/Zn

atomic ratio of 1).22.5 83.6

Copper-chromite18 200 ◦C, 200 psi, 24 h, 80% glycerol solution, 5 wt% of catalyst. 54.8 85.06RANEY R© Ni19 210 ◦C, 10 atm H2, 20 h, 8.0 g pure glycerol, 2.0 g RANEY R© Ni. 91 48Cu/Al2O3

22 200 ◦C, 1.5 MPa H2, 10 h, 50 wt% glycerol aqueous solution, Cu/glycerol molarratio 3 : 100.

35 94

CuAg/Al2O323 200 ◦C, 1.5 MPa H2, 10 h, 50 wt% glycerol aqueous solution, (Cu+Ag)/glycerol

molar ratio 3 : 100.27 96

30% w/w Ag-OMS-224 200 ◦C, 50 atm H2, 8 h, 10 g of glycerol, 40 g of 2-propanol, 0.5 g catalyst. 59.9 88.9

for commercialization.3,18 Another catalyst that may be used atrelatively low hydrogen pressure19 or even without hydrogen20 isRANEY R© Ni.

In general, the disadvantage of supported noble metal cat-alysts is the low selectivity towards 1,2-PD. The Cu-basedcatalysts exhibited superior performances in terms of theselectivity towards 1,2-PD, whereas their activities are usuallylow. The Cu on these catalysts is initially in an oxidic state.In order to achieve a decent activity, the catalysts often needto be reduced in situ under reaction conditions with very highH2 pressure17 or to be pre-reduced to generate Cu species thatare catalytically active under mild reaction conditions.21 In ourprevious work, we reported that a selectivity to 1,2-PD of about97% with a glycerol conversion of about 50% was achievedon a Cu/Al2O3 catalyst, which also needed to be pre-reducedin H2.22 We also found that the introduction of Ag to theCu/Al2O3 catalyst could eliminate the pre-reduction step.23 Thepresent work focuses on the investigation of the catalytic activity,selectivity and deactivation of Ag/Al2O3. A molecular sievesupported Ag catalyst was recently reported in an online paper24

and a comparison with our catalyst is presented later.

2. Experimental

2.1 Catalyst preparation

Our Ag/Al2O3 catalysts were synthesized using an incipientwetness impregnation method with aqueous solutions of AgNO3

of various concentrations and g-Al2O3 from Shandong Filialeof China Aluminium Co., Ltd., China. After impregnation, thecatalysts were dried at 110 ◦C for 12 h and calcined in air for 3 hat 400 ◦C, except in the study of calcination temperature. Unlessspecifically stated, these catalysts were tested or characterizeddirectly after the calcination without additional treatment. Allcatalysts were in powder form with particle size less than0.32 mm in diameter. The catalysts prepared were designated asAg/Al2O3(X), in which X represents the amount of Ag in mmolloaded on 1 g of Al2O3.

2.2 Catalyst characterization

X-Ray Diffraction (XRD) patterns of the catalysts wererecorded at room temperature on an X-ray diffractometer(D/max-2400) with a graphite monochromator attachment,utilizing Ni-filtered Cu-Ka radiation (40 kV, 100 mA) with ascanning speed (2q) of 1◦ per minute.

Nitrogen adsorption–desorption experiments for pore sizedistribution, pore volume, and BET surface area measurementswere conducted on an ASAP2020 instrument (Micromeritics).All samples were pretreated at 350 ◦C under vacuum before themeasurements.

Temperature Programmed Reduction (TPR) studies of thecatalysts were carried out in a 10% H2/Ar gas mixture at a flowrate of 50 ml min-1 with a temperature ramp of 10 ◦C min-1.Before TPR tests the catalysts were dried in argon at 300 ◦Cfor 2 h. Hydrogen consumption was monitored using a thermalconductivity detector (TCD).

Characterizations with transmission electron microscopy(TEM) were carried out on a JEOL JEM-2011TEM. To preparesamples for TEM, Ag/Al2O3 samples were ground, dispersed inethanol, and deposited onto a copper grid.

Thermogravimetric analysis (TGA) results were acquired ona TGA/SDTA851e instrument (Mettler Toledo). Samples wereheated in a flow of 5% H2/N2 gas mixture (20 ml min-1) fromroom temperature to 700 ◦C with a ramp rate of 10 ◦C min-1.

An X-ray fluorescence (XRF) spectrometer (Thermo Scien-tific ARL QUANT’X EDXRF Analyzer) was used to analyzethe Ag content in the spent catalyst.

2.3 Catalytic performance test

The hydrogenolysis of glycerol was carried out in a high through-put batch reactor system consisting of ten independent stainlesssteel autoclaves (165 ml each) with mechanical stirring. In mostcases, an aqueous solution of glycerol (50 wt% concentration)prepared with pure glycerol (>99%, China National MedicinesCorporation Ltd., China) and deionized water was used as

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feed. In a typical run, 65 g of the glycerol solution and aspecified quantity of the catalyst were loaded into the reactor.The reactor was purged five times with H2 (99.99%, DalianF.T.Z Gredit Chemical Technology Development Co., Ltd.)and pressurized with H2 to 1.5 MPa at room temperature.With stirring at 400 RPM, the mixture of the glycerol andthe catalyst was heated to 220 ◦C and maintained for 10 h.The stirring speed was selected to eliminate the influence ofexternal mass transfer and to avoid creating splash inside thereactor, which would make sampling and temperature controlvery difficult. Hydrogen was fed on demand so as to keep thetotal reaction pressure at 3.6 MPa during the 10 h period. Afterthe reaction, the gas phase products were collected in a gasbag and the liquid phase products were separated from thecatalyst by filtration. These products were analyzed using a gaschromatograph (GC HP5890) equipped with a flame ionizationdetector. The GC column used was a PEG2W capillary column(30 m ¥ 0.32 mm ¥ 0.5 mm) manufactured by Dalian Instituteof Chemical Physics, Chinese Academy of Sciences. Solutionsof n-butanol with known amounts of internal standards wereprepared and used for quantification of various glycerol-derivedcompounds in the products. 1,2-PD was the main product withcertain amount of ethylene glycol (EG). There was no 1,3-PDdetected. Other byproducts in very small amount, such as 1-propanol and methane, were also identified and are listed under“Others” in the tables below. Repeated runs showed that datavariation was in the range of ±5% (relative value). The conversionof glycerol and the selectivity, which have been defined in ourprevious publication,23 were used to evaluate the performanceof each catalyst. Usually, only a trace amount of product wasdetected in the gas phase. The overall carbon balance in theproduct was >98%.

3. Results and discussion

3.1 Study of catalyst preparation

Effect of Ag loading. Reaction results on Ag/Al2O3 catalystswith different Ag loadings (0.5–4.0 mmol g-1) are displayed inTable 2. The glycerol conversion rose with increasing Ag amountand reached a maximum (46 mol%) at 2.0 mmol metal pergram of Al2O3. However, there is no linear relation betweenthe conversion and the Ag loading. The amount of glycerolconverted on each mmol of Ag per hour (last column in Table1) indicates that the efficiency of the Ag usage was lower athigher Ag loading. This may be explained by a decrease ofAg dispersion at elevated loading. As shown in Fig. 1, nosignal assigned to Ag or Ag2O crystallite was detected in the

Fig. 1 XRD patterns of Ag/Al2O3 catalysts with different Ag loadings.

XRD patterns of Ag/Al2O3(1.0) and Ag/Al2O3(2.0), probablydue to their small crystallite size, i.e. good dispersion. In thepattern of Ag/Al2O3(4.0), the peaks at 2q ª 38.1, 44.3, and64.5◦ clearly show the existence of larger metallic Ag crystallites.When the loading was between 2.0 and 3.0 mmol g-1, thecatalytic performance showed little change. Further increaseof Ag loading to 4.0 mmol g-1 resulted in a slightly lowerglycerol conversion. Fewer surface Ag active sites due to poordispersion could be the main reason. The selectivity toward 1,2-PD remained constantly high, irrespective of the metal loadingin the range studied.

Fig. 2 shows the N2 adsorption results of Ag/Al2O3. A steadydecline of surface area and pore volume was observed with theincrease of Ag loading. Therefore, the low accessibility of activeAg sites due to pore blockage at high metal loading could alsocontribute to the low conversion of Ag/Al2O3(4.0).

Fig. 2 Surface areas and pore volumes of Ag/Al2O3 catalysts withdifferent Ag loadings.

Table 2 Effect of Ag loading on the catalytic performance of Ag/Al2O3

Selectivity (mol%)

Catalyst Ag : G (mol mol-1) Conversion (mol%) 1,2-PD EG Others Conv./Ag (mol (mol h)-1)

Al2O3 0 : 100 0 — — — —Ag/Al2O3(0.5) 0.5 : 100 20 94 3 3 4.0Ag/Al2O3(1.0) 1 : 100 29 96 2 2 2.9Ag/Al2O3(2.0) 2 : 100 46 96 2 2 2.3Ag/Al2O3(3.0) 3 : 100 45 97 2 1 1.5Ag/Al2O3(4.0) 4 : 100 41 96 2 2 1.0

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Since Ag/Al2O3(2.0) presented the best catalytic performance,it was used in the following studies of calcination temperatureand reaction conditions.

Effect of calcination temperature. A significant effect ofcalcination temperature on the catalytic activity was observed.The trend in Fig. 3 shows a volcano shape. The conversion ofglycerol was 32 mol% on the sample calcined at 200 ◦C. It passeda maximum (47 mol%) at 500 ◦C and dropped to 39 mol% at700 ◦C. Product distribution was not sensitive to the calcinationtemperature. 1,2-PD selectivity was maintained at 96 mol%.

Fig. 3 The catalytic performance of AgAl2O3(2.0) calcined at differenttemperatures (Ag : G (molar ratio) = 2 : 100).

In the process of catalyst preparation, Ag existed as AgNO3

on the support before calcination. Our TG analysis (notshown here) of a AgNO3 impregnated g-Al2O3 (110 ◦C driedovernight) showed that the sample’s weight did not stabilizeuntil the heating temperature reached about 400 ◦C. Belowthis temperature, we believe that AgNO3 did not decomposecompletely, which was the case for the samples calcined at200 and 300 ◦C. The X-ray photoelectron spectroscopy (XPS)analysis in our previous work23 showed that metallic Ag andAg2O existed on the 400 ◦C calcined sample. The molar ratio ofAg(0) and Ag(I) is about 1. The XRD patterns of these samplesin Fig. 4 did not reveal distinctive differences except that the oneof the 700 ◦C calcined sample presented a small peak at 2q ª38.1◦ (metallic Ag). The surface area and pore volume of this

Fig. 4 XRD patterns of Ag/Al2O3(2.0) calcinated at differenttemperatures.

sample are 160 m2 g-1 and 0.25 ml g-1, respectively, while thecorresponding values of the 400 ◦C calcined sample are 223 m2

g-1 and 0.27 ml g-1. Since the phase transition temperature forg-Al2O3 (about 1000 ◦C) is much higher than 700 ◦C, the supportwe used should be stable under the calcination conditions. Theloss of surface area is therefore attributed to the blockage ofpores due to Ag sintering caused by the calcination at 700 ◦C.

The easiness of reduction of the silver species on Ag/Al2O3

varies, as shown in Fig. 5. The TPR profile of the 200 ◦C calcinedsample had a hydrogen consumption peak centered at 150 ◦C,which should be the decomposition/reduction temperature ofAgNO3 in hydrogen. The sample calcined at 300 ◦C may stillhave some AgNO3, which did not decompose completely. ItsTPR profile displayed a shoulder peak around 110 ◦C besidesa major peak at about 65 ◦C, which was also shown in theprofiles of the three samples calcined at higher temperatures.These two peaks are related to the reductions of AgNO3 andAg2O, respectively. The TPR profile of the 700 ◦C calcinedsample is broad. This is because the sintering indicated by theXRD characterization above resulted in relatively large Ag2Ocrystallites, which should be easy to reduce. This is responsiblefor the left side of the broadened peak. Moreover, studiesconducted by Yoon and coworkers using XPS and UV-visspectroscopy concluded that high calcination temperature wasbeneficial to the transformation of Ag from the ionic state tothe metallic state.25 It means that there will be less ionic Ag onAg/Al2O3 samples calcined at higher temperatures. On the otherhand, we believe that a small amount of silver aluminate, whichis more difficult to reduce than Ag2O due to its ionic properties,was formed on the sample and contributed to the right sideof the broadened peak. According to the study using X-rayAbsorption Near Edge Structure by Iglesias-Juez et al.26 andto the XRD results from Nakatsuji et al.27 and from She and

Fig. 5 TPR profiles of Ag/Al2O3(2.0) calcined at differenttemperatures.

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Table 3 The effect of reaction temperature on the catalytic perfor-mance of Ag/Al2O3

Selectivity (mol%)

Reaction T (◦C) Conversion (mol%) 1,2-PD EG Others

180 17 91 4 5200 21 95 3 2220 46 96 2 2240 66 76 6 18

Catalyst loading: Ag : G (mol mol-1) = 2 : 100.

Flytzani-Stephanopoulos,28 certain types of silver aluminate(AgAlO2) can be formed when alumina supported silver cat-alysts are calcined in the temperature range 500–800 ◦C.

According to the TPR results above, it is likely that all Agspecies would be reduced to metallic Ag under the reactionconditions so as to catalyze the hydrogenolysis. However, thereshould be an induction period whose length depends on theease of reduction and affects the glycerol conversion. Thecatalysts calcined between 400 and 500 ◦C may have the easiestreducible Ag species, the shortest induction period, and thereforepresented the highest glycerol conversion.

3.2 Study of reaction conditions

Effects of reaction temperature, pressure and glycerol concen-tration. Ag/Al2O3(2.0) calcined at 400 ◦C was used to studiedthe performance at different reaction conditions. Data in Table3 presents the effect of reaction temperature. When the reactiontemperature rose from 180 to 240 ◦C, the glycerol conversionincreased from 17 mol% to 66 mol%. The 1,2-PD selectivitywas above 90 mol% and relatively stable before the temperaturepassed 220 ◦C. It dropped to 76 mol% at 240 ◦C, mainly due tothe formation of more 1-propanol and other small compounds.We attempted to carry out the hydrogenolysis at a lower reactionpressure, i.e. 2.6 MPa, but the glycerol conversion dropped to21 mol%, less than half of that at 3.6 MPa (46 mol%). Thisis not a surprise since hydrogen is one of the reactants and itsconcentration in the liquid phase should be a key factor for thecatalytic activity. The pressure had no obvious effect on productdistribution and 1,2-PD selectivity remained above 96 mol%.

The positive effect of increasing glycerol concentration onconversion shown in Table 4 can be looked at from severalaspects. As a solvent, water has a diluting effect, which isdetrimental to the conversion of the reactant, glycerol. Wateris also a byproduct from the hydrogenolysis. Excessive amountsof water will tend to shift the reaction equilibrium to the reactantside. It may also damage the physical structure of the catalyst, asshown later. When the water content was below 50 wt%, it wasnot as influential, probably because the glycerol on the catalystsurface became concentrated enough and its content was not arate controlling factor any more.

Effects of reaction time and catalyst usage. High glycerolconversion could be achieved with extended reaction timeor increased catalyst usage, as shown in Fig. 6 and Fig. 7,respectively. After 20 h at 220 ◦C with Ag : G = 2 : 100, 56 mol% ofglycerol was converted. While keeping other conditions the samebut increasing the catalyst usage to Ag : G = 7 : 100, a glycerol

Table 4 The effect of glycerol concentration on glycerol hydrogenolysis

Selectivity (mol%)Glycerol concentration(wt%)

Conversion(mol%) 1,2-PD EG Others

20 17 91 4 550 46 95 2 380 51 94 3 3

100 53 94 3 3

Catalyst loading: Ag : G (mol/mol) = 2 : 100; reaction temperature220 ◦C.

Fig. 6 The effect of reaction time: Ag/Al2O3(2.0), Ag : G = 2 : 100.

Fig. 7 The effect of catalyst loading: Ag/Al2O3(2.0).

conversion of about 75 mol% was achieved. A combination oflong reaction time and high catalyst usage, i.e. 20 h and Ag : G =7 : 100, resulted in 90 mol% glycerol conversion. The 1,2-PDselectivity remained about 95 mol% in these tests, unlike the caseof using high temperature (240 ◦C) to achieve high conversion.However, it was noticed during the reaction time study that theglycerol conversion increased only 10 mol% when the reactiontime was extended from 10 h to 20 h. This can be a sign ofcatalyst deactivation during prolonged use.

3.3 Study of catalyst deactivation

The deactivation of Ag/Al2O3(2.0) was confirmed by theexperimental results in Table 5. After a typical run at 220 ◦Cand 1.5 MPa initial H2 pressure for 10 h, the catalyst was

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Table 5 Studies of the catalyst deactivation of Ag/Al2O3(2.0)

Selectivity (mol%)

Entry CatalystSurface Area(m2 g-1)

Conversion(mol%) 1,2-PD EG Others

1 Fresh 223 46 96 2 22 Spent and washed 25 93 3 43 Spent, washed and calcined at 400 ◦C in air 220 45 97 2 14 Pretreated with H2 at 200 ◦C for 10 h 52 96 2 25 Pretreated with H2 at 500 ◦C for 10 h 45 97 2 16 Pretreated with H2 at 600 ◦C for 10 h 35 97 2 17 Pretreated with H2 at 700 ◦C for 10 h 15 96 2 28 Pretreated with N2 at 200 ◦C for 10 h 44 96 2 29 Pretreated with 100 wt% glycerol under 1.0 MPa N2 at 200 ◦C for 10 h 26 94 2 410 Pretreated with 50 wt% glycerol under 1.0 MPa N2 at 200 ◦C for 10 h 27 93 3 411 Pretreated with water under 1.0 MPa N2 at 200 ◦C for 10 h 56 7 89 5 6

Fig. 8 TEM images of Ag/Al2O3(2.0): (a) fresh, (b) spent and washed, and (c) spent, washed and calcined at 400 ◦C.

separated from the liquid product by filtration, washed with100 ml deionized water and loaded into the reactor with freshglycerol feed for another test under the same conditions. Thewashed catalyst (Entry 2) achieved 25 mol% conversion, about54% of that on the fresh catalyst (Entry 1). In Entry 3, a spentcatalyst was calcined at 400 ◦C for 3 h in air before being testedagain with fresh feed. This time a glycerol conversion of 45 mol%was obtained, which can be considered to be the same as the oneon the fresh catalyst considering experimental variation. Theselectivity remained high on all the catalysts.

There are four possible deactivation mechanisms here: sup-port structure destruction, Ag leaching, Ag sintering, andcoking. Comparison of the surface areas and pore volumes(Table 5) of the fresh catalyst and the spent and calcined onerevealed that the structure was stable. The Ag content in thespent catalyst obtained from XRF analysis was 1.95 mmolg-1, very close to the value in the fresh sample. In addition,the calcination recovered essentially all the activity. All theseindicate that the first two mechanisms did not play a role inthe deactivation. On the other hand, TEM characterization ofthe fresh and the spent samples (Fig. 8(a), (b)) showed thatsintering of Ag species did happen. The Ag particle size on thefresh sample is about 10 nm, while it grew to about 30 nm afteruse. Comparison of the XRD profiles in Fig. 9 concurs with theTEM finding. The three pronounced peaks at 2q ª 38.1, 44.3,and 64.5◦ are the characteristic signals from large metallic Agcrystallites. The XRD characterization also showed that these

signals disappeared after calcination and the XRD profile ofthe spent, washed and calcined sample is very similar to that ofthe fresh catalyst. This is because the calcination redispersedthe sintered Ag and therefore rejuvenated the catalyst, asevidenced by the TEM image in Fig. 8(c). The redispersion ofmetals like Pt or Ni supported on g-Al2O3, when heated in anoxidative environment in the temperature range 300–500 ◦C,is well known.29–33 Of course, the calcination would burn offcarbonaceous species, i.e. coke, formed during the reaction. Thiswould also regenerate the deactivated catalyst if coking was oneof the sources of deactivation. The TG analysis result of the spentand washed catalyst was shown in Fig. 10. The total weight lossis about 20 wt%, among which about 8 wt% is in the range 200–600 ◦C. In order to eliminate the effect of water and glycerol, asimilar analysis (not shown) was carried out on a Ag/Al2O3(2.0)sample that was soaked with 50 wt% glycerol solution at roomtemperature, washed with 100 ml water and dried at 110 ◦C.The weight loss of this sample between 200 and 600 ◦C (residualwater and glycerol) is about 3 wt%. This serves as a baseline.Therefore, the net weight loss of the spent and washed sample inthe temperature range is about 5 wt%. Considering the catalystusage, we estimate that the total amount of coke formed duringthe reaction was no more than 0.7 wt% of the glycerol in thefeed.

In order to pinpoint the individual contribution from thelast two deactivation mechanisms, i.e. Ag sintering and coking,four more tests were carried out. In Entry 4–7 of Table 5, the

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Fig. 9 XRD patterns of fresh, spent and pretreated Ag/Al2O3 catalysts.

Fig. 10 TG analysis of the spent and washed Ag/Al2O3 catalysts.

fresh catalyst was reduced in hydrogen at different temperatures.The goal was to introduce sintering without coking. The XRDcharacterizations in Fig. 9 indicate that sintering occurredeven at 200 ◦C and became more pronounced at elevatedtemperatures. It is interesting to see that the sample prereducedat 200 ◦C was more active than the fresh catalyst. The glycerolconversion on it was 52 mol%. The improved activity couldbe from the elimination of the induction (prereduction) periodmentioned before in the discussion of the calcination effect. Thispositive effect was completely offset by the negative effect ofsintering on the sample reduced at 500 ◦C, which showed asimilar activity as the fresh catalyst (Entry 1). When the effect

of sintering became dominant, the glycerol conversion droppedto 35 mol% and 15 mol% with 600 ◦C and 700 ◦C reduction,respectively.

To better quantify the correlation between deactivation andsintering, we used the intensity of the XRD signal of Ag at 2theta 38.1◦ as a measurement of sintering and plotted it againstthe glycerol conversion on the corresponding catalysts (Fig.11). The smooth curve indicates a clear trend of deactivationwith the degree of sintering. More interestingly, the data pointfor the spent and washed sample fits the trend very well.This demonstrates that the deactivation under the real reactionconditions was predominantly due to Ag sintering. The 50 wt%glycerol pretreated sample is more like the spent and washedone except that it was not exposed to any hydrogen duringthe pretreatment. Because of the lack of the benefit fromprereduction, its data point was off the curve slightly. Althoughthe 100 wt% glycerol pretreated sample had less sintering, it wasnot more active than the spent and washed one or the 50 wt%glycerol pretreated one. Its data point is away from the curve.We believe that coking, which was more severe in the absence ofwater, contributed to the deviation in this case.

Fig. 11 The correlation between sintering and glycerol conversion.

Qu and co-workers studied the sintering of Ag on SiO2 andfound that H2 treatment at low temperatures (100–300 ◦C)dispersed silver particles, but prolonged treatment in H2 attemperature above 300 ◦C induced aggregated particles.34 Thetemperatures observed in our study, 200 ◦C for slight sintering onthe sample reduced in H2 and 220 ◦C for the pronounced one onthe spent and washed catalyst, are relatively low. The differencecould be from the duration of the treatment and the nature ofthe supports. Under the reaction conditions, the sintering is alsorelated to the system environment. Unlike the one treated in H2,the catalyst treated in N2 at 200 ◦C for 10 h showed little sinteringand its activity (Entry 8 in Table 5) was very similar to the freshcatalyst. Entry 9, 10 and 11 in Table 5 and the correspondingXRD patterns in Fig. 9 illustrated the effect of glycerol and water.The treatment with pure glycerol resulted in some sintering andloss of activity. Replacement of half of the glycerol with water(50 wt% glycerol solution) enhanced the sintering. The treatmentwith water solely (Entry 11) destroyed the catalyst structure, asindicated by the low surface area (56 m2 g-1 vs. 223 m2 g-1 ofthe fresh one) and the activity. This result concurred with the

162 | Green Chem., 2012, 14, 156–163 This journal is © The Royal Society of Chemistry 2012

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finding from the study of the effect of glycerol concentration.On the other hand, it also manifested that 50 wt% glycerol inthe system was enough to prevent the water from damagingthe catalyst structure by staying in the liquid phase and formeda layer on the solid catalyst under the reaction conditions. Insummary, hydrogen, glycerol, water, temperature and durationall contributed to the sintering observed here.

At the time this paper was being prepared, Yadav et al. pre-sented a similar catalytic system in their publication.24 The Ag-OMS-2 catalysts (Ag incorporated into octahedral molecularsieve) they synthesized seem to have higher activity for glycerolhydrogenolysis to 1,2-PD. Their best catalyst has 30 wt% Ag,which was probably highly dispersed in the catalyst frameworkdue to the co-precipitation method used. The high Ag loadingand dispersion are believed to be beneficial to the activity. Thestability could be good because metal sintering would be difficultif Ag is in the framework. Another factor contributing to thehigher activity is the use of 2-propanol as solvent. The negativeeffect of water in the system has been clearly shown above.These authors observed activity loss (33–73%) too from thestudy of catalyst reusability in a batch reactor. The decreaseof activity seems smaller in the second reuse. Their investigationusing a continuous fixed bed reactor at 200 ◦C revealed that theglycerol conversion and 1,2-PD selectivity stabilized at about30% and 70%, respectively, after 10 h on stream, although thecorresponding numbers at 4 h were 65% and 90%, respectively.At 220 ◦C, the conversion (about 60%) seemed to be constantthroughout the 32 h run, but the selectivity was only about 70%.We did not study either multiple reuses or continuous testing,but it is reasonable to expect that the sintering/deactivation willslow down with time while the selectivity in our work remainsabove 95 mol% even at 220 ◦C.

4. Conclusions

The preparation of Ag/Al2O3 (Ag loading, calcination tem-perature) and reaction conditions have been optimized for theconversion of glycerol to 1,2-PD. 2 mmol Ag per gram of Al2O3

is the desired loading and the suitable calcination is between400 and 500 ◦C. About 46 mol% conversion and 96 mol% 1,2-PD selectivity were achieved at 220 ◦C, glycerol/Ag (molarratio) = 100/2, 1.5 MPa initial H2 pressure and 10 h. Theperformance is comparable to other Cu-containing catalysts,but Ag/Al2O3 requires neither prereduction nor high hydrogenpressure. Unlike most supported noble metal catalysts, 1,2-PDselectivity on Ag/Al2O3 is much higher. Catalyst deactivationwas observed and investigated. It is concluded that sinteringdue to the combined contribution of hydrogen, water, glycerol,temperature and reaction duration was responsible for theactivity loss. The deactivation because of coking was minimalunder the reaction conditions. The deactivated catalyst could befully regenerated with a calcination in air, which redispersed thesintered Ag particles and may also burn off the coke formed.

Acknowledgements

The authors are grateful to the State Key Laboratory of FineChemicals at Dalian University of Technology for support-

ing the catalyst characterization in this work under GrantKF0703. We also thank the Department of Education ofLiaoning Province, China for their financial support underGrant L2010037.

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