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Research ArticleReceived: 31 March 2010 Revised: 17 May 2010 Accepted: 17 May 2010 Published online in Wiley Online Library: 18 June 2010

(wileyonlinelibrary.com) DOI 10.1002/jctb.2456

Hydrogenolysis of glycerol to 1,2-propanediolcatalyzed by Cu-H4SiW12O40/Al2O3 in liquidphaseShun-Li Hao,a,b Wei-Cai Peng,a Ning Zhao,a Fu-Kui Xiao,a Wei Weia∗and Yu-Han Suna,c∗

Abstract

BACKGROUND: Crude glycerol will increase to over 400 million L year−1, and the market is likely to become saturated due to thelimited demand for glycerol. The main aim of this work is to develop a novel process for the sustainable conversion of glycerolto 1,2-propanediol (l,2-PD).

RESULTS: Cu-H4SiW12O40/Al2O3 catalysts with different H4SiW12O40 (STA) loadings were prepared for the hydrogenolysis ofglycerol to produce l,2-PD in liquid phase. At 513 K, 6 MPa and LHSV of 0.9 h−1 in 10% (w/w) glycerol aqueous solutions, thecatalyst with 5% (w/w) STA showed the best performance with 90.1% of glycerol conversion and 89.7% selectivity to l,2-PD.More important, both the initial glycerol conversion and l,2-PD selectivity were maintained over 250 h.

CONCLUSION: l,2-PD can be continuously produced with high yields via the liquid phase hydrogenolysis of glycerol overCu-H4SiW12O40/Al2O3. Furthermore, the characterization indicated that catalyst acidity could be greatly modified by STA,which promoted Cu reducibility. It was also found that hydrogenolysis could be favored by a bi-functional catalyst with theappropriate amount of both acid and metal sites.c© 2010 Society of Chemical Industry

Keywords: glycerol; hydrogenolysis; 1; 2-propanediol; Cu–H4SiW12O40/Al2O3

INTRODUCTION1,2-propanediol, a three-carbon diol with a stereogenic siteat the central carbon atom, is a non-toxic and importantmedium-value commodity chemical, which is widely used forpolyesters and alkyd resins, pharmaceuticals, cosmetics, brakefluid and humectants, and green alternatives to an ethyleneglycol based toxic deicing/antifreeze agent.1 – 5 Nowadays, l,2-PDis produced industrially from petroleum-derived5 chemicals viathe chlorohydrin or hydroperoxide processes.1,2 These are nowrestricted by the rapid increase in oil price and the dwindlingpetroleum resource, therefore the production of l,2-PD fromrenewable resources is highly desirable.

However, with the annual world production of biodieselexpected to increase to over four billion liters by the end ofthis decade, the crude glycerol by-product of the process willincrease to over 400 million liters per year.6 The glycerol marketis likely to be saturated because of limited utilization of glycerolat present. However, glycerol is regarded as one of the buildingblocks in the bio-refinery feedstock, and much attention has beenpaid recently to the conversion of glycerol to high value-addedproducts, such as glycols and acrolein.7,8 The hydrogenolysis ofglycerol has been reported in several patents9 – 12 and academicliterature,13 – 31 mainly focused on the development of catalysts,such as CuO/ZnO,2,19,25 Cu–Cr,3 Raney Ni,3,27 sulfided Ru,11,23

Ru/C,2,15,17,18 or Rh/C,2,18 Pt/C,18 Rh/SiO2+ion-exchange resin,15,17

Cu–Pt and Cu–Ru26 bimetallic catalysts etc. Among them, the

noble metal based catalysts appear more active than Cu basedcatalysts for the hydrogenolysis reaction, but the selectivity tol,2-PD is relatively lower,2,20 – 23,27 – 29 which may be due to anexcessive C–C bond cleavage.11,20,21 Copper-containing catalystshave exhibited excellent activity for C–O bond breakage priorto C–C bond breakage.20,24 However, most of these studieswere performed in a stainless autoclave with discontinuousoperation.30,31 This can lead to drawbacks with regard to productseparation, the essential addition of organic solvents and thedeactivation of catalysts. In this study, the production of 1,2-PD has been carried out from glycerol aqueous solution withhigh selectivity over Cu–H4SiW12O40/Al2O3 catalysts; the catalystexhibited high activity and selectivity with remarkable stability forthe hydrogenolysis of glycerol.

∗ Correspondence to: Wei Wei and Yu-Han Sun, State Key Laboratory of Coalconversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan030001, Shanxi, P.R. China. E-mail: weiwei@sxicc.ac.cn; yhsun@sxicc.ac.cn

a State Key Laboratory of Coal conversion, Institute of Coal Chemistry, ChineseAcademy of Sciences, Taiyuan 030001, Shanxi, P.R. China

b Graduate School of Chinese Academy of Sciences, Beijing 100049, P.R. China

c Low Carbon Energy Conversion Center, Shanghai Advanced Research Institute,Chinese Academy of Sciences, Shanghai 201203, P.R. China

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EXPERIMENTALCatalyst preparationCu-based catalysts were prepared by incipient wetness impreg-nation which has been described previously.32 Typically, Al2O3

(20–40 mesh) was impregnated with the desired amount of STAaqueous solution for 24 h at room temperature, then dried at 393 Kovernight and calcined at 623 K for 5 h in air. The STA/Al2O3 sam-ples obtained were further impregnated with an aqueous solutionof Cu(NO3)2· 3H2O at room temperature for 24 h to obtain 5%(w/w) Cu loading. After drying at 393 K, the samples were calcinedin air at 623 K for 5 h. The catalysts are labelled 5Cu-xSTA/Al2O3,where x represents the nominal weight loading of STA.

CharacterizationBET surface areas, pore volumes, and average pore diameters weredetermined by N2 adsorption–desorption method at 77 K, mea-sured using a Micromeritics Tristar 3000 (Particle & Surface SciencesPty. Limited, American) instrument. The samples were degassedunder vacuum at 473 K for 12 h before the measurement. The aver-age pore diameters were calculated according to the BJH method.X-ray powder diffraction (XRD) patterns were recorded on a RigakuMiniflex (M/s. Rigaku Corporation, Japan) X-ray diffractometer us-ing Ni filtered Cu Kα radiation (k = 1.5406 Å) with a scan speedof 2 min−1 and a scan range of 5–90◦ at 30 kV and 15 mA. Tem-perature programmed desorption of NH3 (NH3-TPD) analysis wasconducted using an in-house constructed apparatus. The catalystsample (200 mg) was first purged in Ar for 2 h at 473 K, then cooledto room temperature and NH3 adsorption was carried out by pass-ing a 30% NH3/Ar mixture for 1 h. Any residual NH3 was removed ina flow of dry Ar at the same temperature, the temperature was thenraised to 873 K at a heating rate of 10 K min−1 and the desorptionof NH3 was recorded by Balzers Omnistar mass spectrometry (Balz-ers Inc., Switzerland) (m/z = 16 was used to avoid the influence ofwater). Temperature programmed reduction (H2-TPR) analysis wasperformed on a conventional apparatus with a thermal conduc-tivity detector (TCD). 50 mg samples were loaded in a quartz tubeand purged with Ar at 423 K for 60 min, and then cooled to 333 K.Reduction was carried out from 333 K up to 1073 K (10 K min−1) ina 10% H2/Ar mixture with a flow rate of 30 mL min−1.

Catalytic evaluationThe hydrogenolysis of glycerol in liquid phase was performed ina fixed-bed stainless steel reactor (length 800 mm and i.d. 20 mm)with a back pressure valve to control the reaction pressure. Theglycerol aqueous solution was introduced by a high-pressurepump into the reactor. Prior to reaction, the catalysts were reducedin hydrogen with a flow rate of 40 mL min−1 at 573 K for 5 h.The products were analyzed by HPLC (Schimadu LC-2010Avp,Kromasil-C18, 5 µ, 4.6 mm × 250 mm stainless steel column).Conversion of glycerol and selectivity of products were defined asfollows:

Conversion of glycerol (%)

= Sum of C mol of all liquid products

Added glycerol before reaction× 100%

Selectivity (%)

= C mol of each liquid product

Sum of C mol of all liquid products× 100%

RESULTS AND DISCUSSIONStructure and textureThe textural properties of a range of catalysts are presented inTable 1. Both surface area and pore volume of Al2O3-supportedCu-STA decreased with an increase in STA loading, indicating thatsome of the pores in Al2O3 were plugged by STA. Figure 1 showsthe XRD patterns of Al2O3 impregnated with 5% (w/w) Cu at differ-ent STA loadings. All of them had similar XRD patterns, except thosepeaks at 32.35◦, 37.63◦, 39.62◦, 45.52◦, 60.27◦ and 67.01◦ due to thepresence of γ -Al2O3. For all the catalysts, however, no diffractionlines were associated with STA, even at STA content up to 30%(w/w), indicating no formation of any mixed oxide phases. This wasconsistent with another report.33 Thus, Cu and STA appeared tobe completely X-ray amorphous on the Al2O3 surface. This couldbe due to the strong interaction between STA and Al2O3,33 whichpossibly leads to high dispersion of STA on the surface.

Surface acidityNH3-TPD was carried out to determine the surface acidity of 5Cu-xSTA/Al2O3 catalysts (see Fig. 2). The 5Cu/Al2O3 catalyst showeda desorption peak at 410 K, which can be assigned to the surfaceacidity of Al2O3. It was also observed that the acidity of 5Cu/Al2O3

catalyst could be tuned by the addition of STA, and all the5Cu-xSTA/Al2O3 showed a new desorption peak at 560 K, whichmay be related to the strong acid nature of STA.34 The areaof the desorption peak increased significantly with increasing

Table 1. BET of catalysts

CatalystsBET surface

area (m2 g−1)Average porediameter (nm)

Pore volume(cm3 g−1)

0Cu-0STA/Al2O3 293 10.2 0.75

5Cu-0STA/Al2O3 287 10.3 0.74

5Cu-3STA/Al2O3 276 10.0 0.65

5Cu-5STA/Al2O3 262 9.9 0.62

5Cu-15STA/Al2O3 226 10.5 0.57

5Cu-30STA/Al2O3 202 10.2 0.52

Figure 1. XRD patterns of 5Cu-xSTA/Al2O3 (a: x=0; b: x=3; c: x=5; d: x=15;e: x=30).

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Figure 2. NH3-TPD profiles of 5Cu-xSTA/Al2O3 (a: x=0; b: x=3; c: x=5; d:x=15; e: x=30).

Figure 3. TPR profiles of 5Cu-xSTA/Al2O3 (a: x=0; b: x=3; c: x=5; d: x=15;e: x=30).

STA loading, suggesting that more acid sites were formed. Theinteraction between support and STA had a positive effect on theacidity and improved the proportion of weak and medium acidsites, thus the current increase in acid sites could not be onlyrelated to the increasing loading of STA, but also to its interactionwith Al2O3.

ReducibilityFor supported metal catalysts, the metal–support interaction caninfluence performance. H2-TPR was carried out to measure thereducibility of the 5Cu-xSTA/Al2O3 catalysts and the effect ofSTA on CuO–Al2O3 interaction (see Fig. 3). Two reduction peakscentered at 500 K and 525 K were observed for the 5Cu/Al2O3

catalyst, which can be attributed to the reduction of Cu2+ to Cu+and Cu+ to Cu0, respectively.35,36 The higher temperature neededfor the reduction may result from CuO–Al2O3 interaction.36 Uponaddition of STA, the intensity of the high temperature peakgradually decreased, and disappeared at a STA loading up to 15%

Table 2. Effect of STA loading on the catalytic performance

Selectivity (%) a

Catalysts Conversion (%) 1,2-PD 1-PO EG EtOH MeOH

5Cu-0STA/Al2O3 60.6 85.2 1.0 8.8 0.4 1.8

5Cu-3STA/Al2O3 77.6 87.8 1.3 5.8 0.4 1.7

5Cu-5STA/Al2O3 90.1 89.7 1.5 4.2 0.5 2.0

5Cu-15STA/Al2O3 41.9 91.6 1.2 1.6 0.5 1.5

5Cu-30STA/Al2O3 8.6 92.1 1.0 1.0 0.4 1.5

0Cu-5STA/Al2O3 3.7 – – – – –

Reaction conditions: 10% (w/w) glycerol aqueous solution; H2 pressure:6 MPa; catalyst weight: 4 g; reaction temperature: 513 K; LHSV: 0.9 h−1.a 1,2-PD = 1,2-propanediol; 1-PO = 1-propanol; EG = ethylene glycol,EtOH=ethanol; MeOH = methanol.

(w/w), and meanwhile, the intensity of the low temperature peakincreased dramatically with increasing STA loading. This clearlydemonstrates that the reducibility of Cu species was promotedby the addition of STA due to the weakening of the CuO–Al2O3

interaction. In this case, the one reduction peak around 500 K maybe ascribed to the reduction of CuO to metallic Cu.31 It should benoted that all the catalysts are readily reduced before the reactionunder the conditions used in this work.

Catalytic performanceThe catalytic performance of the 5Cu-xSTA/Al2O3 catalysts inglycerol hydrogenolysis is shown in Table 2. It can be observed thatthe presence of STA improved the conversion of glycerol withina 5% (w/w) loading, beyond which the conversion decreasedrapidly. According to the results of NH3-TPD, more acid sites inthe catalysts at high STA content may be responsible for thedecrease of glycerol conversion. As the acid sites can result incoking,8,37 catalyst deactivation may result. It is also worth notingthat glycerol conversion was as low as 3.7% over 5STA/Al2O3 (seeTable 2) compared with the high conversion over 5Cu-5STA/Al2O3,indicating that Cu played a key role in glycerol conversion, which isconsistent with previous reports.19,38 It is believed that the catalytichydrogenolysis of glycerol requires a bi-functional catalyst system,in which glycerol dehydration to dehydrated intermediates occursand then the hydrogenation takes place simultaneously.3,15,17,19

In the present 5Cu-xSTA/Al2O3 catalysts, STA and Cu serve as twokinds of active sites responsible for high activity and selectivity.

Since the 5Cu-5STA/Al2O3 catalyst showed a better perfor-mance, it was used for further investigation of the effect ofreaction conditions on performance. Table 3 gives the influence oftemperature on glycerol conversion and 1,2-PD selectivity. As thetemperature was increased from 453 K to 543 K, glycerol conver-sion increased from 2.5% to 97.4%, and the selectivity of 1,2-PDdecreased from 97.3% to 37.7%. However, the selectivity of sideproducts such as ethylene glycol and 1-propanol increased dra-matically. So, it can be concluded that the higher temperatureresults in more side reactions. A proper temperature was thenneeded for conversion of the dehydrated intermediate to l,2-PD,and at a relatively higher temperature, further hydrogenolysisof l,2-PD into unwanted by-products is favored. Moreover, Cu-containing catalysts show poor C–C bond cleavage activity butefficient activity for C–O bond hydro-dehydrogenation, which iscritical for the hydrogenolysis of glycerol.

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Table 3. Effect of temperature on the catalytic performance

Selectivity (%)a

Reactiontemperature (K) Conversion (%) 1,2-PD 1-PO EG EtOH MeOH

453 2.5 97.3 – – – –

483 31.2 95.4 1.6 – 1.3 –

513 90.1 89.7 1.5 4.2 0.5 2.0

543 97.4 37.7 49.5 5.4 1.8 2.3

Reaction conditions: 10% (w/w) glycerol aqueous solution; H2 pressure:6 MPa; catalyst weight: 4 g; LHSV: 0.9 h−1.a 1,2-PD = 1,2-propanediol; 1-PO = 1-propanol; EG = ethylene glycol,EtOH=ethanol; MeOH = methanol.

Table 4. Effect of liquid hourly space velocity on the catalyticperformance

Selectivity (%)a

Space velocity(h−1) Conversion (%) 1,2-PD 1-PO EG EtOH MeOH

1.8 81.2 95.1 1.1 – 0.3 –

0.9 90.1 89.7 1.5 4.2 0.5 2.0

0.45 95.7 70.5 15.1 6.8 2.8 2.1

Reaction conditions: 10% (w/w) glycerol aqueous solution; H2 pressure:6 MPa; catalyst weight: 4 g; reaction temperature: 513 K.a 1,2-PD = 1,2-propanediol; 1-PO = 1-propanol; EG = ethylene glycol,EtOH=ethanol; MeOH = methanol.

The consecutive conversion of the dehydrated intermediate toother products via l,2-PD was also observed when the reaction wasperformed under different liquid hourly space velocities (LHSVs),as shown in Table 4. A longer residence time gives rise to lower l,2-PD selectivity due to its further conversion and degradation. Thisdemonstrated that the glycerol conversion decreased with theincrease of LHSV, and in the present work, however, a reasonableglycerol conversion and l,2-PD selectivity was observed even atLHSV of 1.8 h−1, which was the best result in the open literature.30,31

The long-term test was carried out at 513 K, 6 MPa and LHSV of0.9 h−1, and as shown in Fig. 4, both glycerol conversion and l,2-PDselectivity were maintained at the initial levels over 250 h with nocatalyst deactivation.

CONCLUSIONSThe introduction of STA into Cu/Al2O3 catalysts led to both highactivity and selectivity towards the synthesis of 1,2-PD from thehydrogenolysis of glycerol. Catalyst characterization indicated thatthe acidity of catalyst can be tuned by the addition of STA, andmore acid sites were formed with increasing STA loading. Thereexisted a synergistic effect between Cu and STA species on thecatalysts, and the existence of STA changed the reduction behaviorof copper oxides. As a result, the activity of glycerol hydrogenolysisdepended not only on the acid sites but also the metal sites. Thereaction can proceed via a two-step catalyzed pathway, i.e. theacid sites provided by STA/Al2O3 catalyzed the dehydration ofglycerol followed by subsequent hydrogenation to 1,2-PD overmetallic Cu.

Figure 4. Long-term stability of 5Cu-5STA/Al2O3 catalyst.

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