Chemical Engineering Journal 179 (2012) 285 294

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Chemical Engineering Journal

j ourna l ho mepage: www.elsev ier .c

Direct f poxygen

Van-Huy unla Department ob Institute of En

a r t i c l

Article history:Received 10 AReceived in reAccepted 2 No

Keywords:EpoxidationPropylene oxidPhotocatalysisMolecular oxyMCM-41TiO2

opylee envd at iO2 aic epopress

and malyticwith

ssuretreamg the

2011 Elsevier B.V. All rights reserved.

1. Introduction

As the dicals demanspecicallytant in theas key inteinstant, prointermediatindustry sinwas approxin 2009. Altby 5.6% in 2annual growrecovers [4]

Indeed, studies per

CorresponE-mail add

epoxidation by using molecular oxygen under high temperature

1385-8947/$ doi:10.1016/j.evelopment of third world countries continues, chem-d worldwide reaches an unprecedented level. More

, the epoxidation of hydrocarbons becomes more impor- industrial production of chemicals, which are usedrmediates for polymer and organic syntheses. Forpylene oxide (PO), the second most important chemicale, has become increasingly important to the chemicalce 1950s [13]. In 1991, the global PO consumptionimately 3.9 million tons and reached to 6.0 million tonshough the global PO consumption has been decreased009 due to economic crisis, it is predicted that averageth of over 7% will be seen from 2009 to 2014 as demand.

there has been great experimental and theoreticalformed recently focusing on gas-phase propylene

ding author. Tel.: +886 2 23631994; fax: +886 2 23623040.ress: cswu@ntu.edu.tw (J.C.S. Wu).

and/or high pressure as shown in Eq. (1).

CH3 CH CH2 + O2 CH 3 CH CH 2

Othermocatalyst

high temp., high pressure12

(1)

In 2004, Jin et al. reported that PO selectivity of 60.3% wasachieved over the 20%Ag4%MoO3/ZrO2 catalyst under 400 C,0.1 MPa and GHSV = 7500 h1 [5]. In 2008, Suo et al. obtainedPO selectivity of 17.9% with 0.9% propylene conversion at theinitial 10 min of reaction over Au/SiO2 catalyst at 325 C, andGHSV = 6000 h1 [6]. The other approaches on exploring the selec-tive catalytic epoxidation of propylene via the mixture of hydrogenand oxygen are shown in Eq. (2).

CH3 CH CH + H2 2 + O2 CH3 CH CH + H 2 2O

Othermocatalyst

high temp., high pressure (2)

In 2005, Cumaranatunge and Delgass enhanced Au content inAu/TS-1 and achieved 10% conversion, 76% selectivity by using agas mixture at 473 K [7]. In 2007, the study from Oyama and co-workers achieved a conversion of 1.4%, a PO selectivity of 99%,and a H2 efciency of 17% under 423 K and 0.1 MPa by usinggold supported on a Ti-containing silicate mesoporous material[8]. The approaches developed for the thermocatalysts by using

see front matter 2011 Elsevier B.V. All rights reserved.cej.2011.11.003gas-phase photocatalytic epoxidation o by photocatalysts

Nguyena, Hsiang-Yu Chana, Jeffrey C.S. Wua,, Hsf Chemical Engineering, National Taiwan University, Taipei 10617, Taiwanvironmental Engineering, National Chiao Tung University, Hsin Chu 300, Taiwan

e i n f o

ugust 2011vised form 28 October 2011vember 2011

e

gen

a b s t r a c t

The photocatalytic epoxidation of prduce propylene oxide (PO) from thachieve high PO selectivity and yielTiO2, V-Ti/MCM-41, V2O5/SiO2, Au/Tphoto-epoxidation. The photocatalytC3H6:O2:N2 = 1:1:18 at atmospheric hyde, acetone, acetaldehyde, ethanolfavorable photocatalysts for photocatformation rate of 114 mol g cat1 h1

after 4 h at 50 C and atmospheric pretivity of products was very stable on sknowledge of species presented durinom/ locate /ce j

ropylene with molecular

ing Baib

ne by molecular oxygen is one of the best methods to pro-ironmental viewpoints. The key is using photo-energy tomild conditions. A series of photocatalysts including SiO2,nd TS-1, were used to evaluate their performance in thexidation of propylene was carried out in the gas mixture of

ure. In addition to PO, other products, such as propionalde-ethanol, were detected. The results indicated that the most

epoxidation were silicates supported group. The highest PO selectivity of 47% was obtained over V-Ti/MCM-41 on stream

under UVA irradiation of 0.2 mW/cm2. In addition, the selec-. A possible reaction mechanism was proposed based on the

photocatalytic reaction.

286 V.-H. Nguyen et al. / Chemical Engineering Journal 179 (2012) 285 294

hydrogenoxygen mixture or only molecular oxygen are very cleanand have shown good activity performance. However, the extraenergy lost in the reaction, low hydrogen efciency and safety con-sideration that requires H2O2 production facilities to be restrictedin size have

It is wellsource andneer work engineeringcatalysis, hafew researclysts and thYoshida et metal oxideTiOx/SiO2 wyield and 1Xe lamp irrproposed Vwhich has dand a PO sewith UV-C lronmental require furtidation proprocess (20dation procliquid-phasepoxidationronmentallconditions still needs t

It is expwith the aidapproach fothe main stdirect gas-pis to explorvia an idealenergy to acEq. (3).

CH3 CH

The objephotocatalySiO2, TiO2, conditions. for photocayields of POcatalytic epof species p

2. Experim

2.1. Prepara

Titaniumcial P25 (Dwas carriedHAuCl4 solpH 5.5 by uwas irradia

stirring. The photocatalyst was ltered and washed before dryingin an oven at 393 K and calcined in a ow of air at 773 K for 5 h.

Amorphous SiO2 was prepared from tetraethyl orthosilicate(TEOS, Aldrich, 98%) by using the solgel method. The pH of a mix-

ntairoxim

for 2n a r.niumlizatnd tsenc

Flukaurathe ra5 (0

ed by cheK forere csopod bimetng te.01Tisis ped int of

van titane waf 2 M0.5 e sowas ae wae wan at sultinand te w

arac

lighectiv00). Bectrotrum

on Hiffra

strucred bline photohe chof vaecics, ASdiedotro

andon b2G Crepa made these approaches only a temporary solution. known that photo-energy is the Earths ultimate energy

friendly to environment. Following the excellent pio-of Fujishima and Honda in 1972 [9], scientic and

interests in TiO2, particularly in the eld of photo-ve grown signicantly [1016]. However, there are onlyhes on photo-epoxidation in the presence of photocata-eir performances are not advanced yet [1723]. In 2000,al. investigated more than 50 types of silica-supporteds for their photo-oxidation activities and found thatas the most effective photocatalyst attaining 4.7% in PO9.2% in PO selectivity under batch reaction with 200 Wadiation and at 318 5 K [18]. In 2006, Amano et al.2O5 (0.1%)/SiO2 as the best photocatalyst until now,emonstrated a PO formation rate of 85 mol g cat1 h1

lectivity of 37% under continuous reaction at 303 K andight of 240 < < 440 nm [19]. From economic and envi-viewpoints, most of the current studies on epoxidationher improvements. Although there were two new epox-cesses developed recently, a cumenehydroperoxide03 in Japan) and a hydrogen peroxide (H2O2) epoxi-ess (2008 in Belgium), they still based on multi-stagee reactions and required hydrogen. The photocatalytic

of propylene is a promising process because it is envi-y friendly, and the process can be carried out under mildin the absence of hydrogen; however, its performanceo be improved.ected that direct epoxidation process to produce PO

of light energy over photocatalysts will be a promisingr the production of chemicals in the future. Therefore,rategy being considered in this study is to develop ahase one-pass reaction under mild conditions. Our aime the selective photocatalytic epoxidation of propylene

oxidant, i.e., molecular oxygen. The key is using photo-hieve high PO selectivity at mild conditions, as show in

CH2 + O2 CH3 CH CH2

Ophotocatalyst

mild conditions12

(3)

ctive of this study is to evaluate the direct gas-phasetic epoxidation on a series of photocatalysts such asV-Ti/MCM-41, V2O5/SiO2, Au/TiO2 and TS-1 at mildThe optimal reaction conditions were also studiedtalytic epoxidation by comparing the selectivities and

for different photocatalysts. The mechanism of photo-oxidation was also proposed based on the knowledgeresent during the photocatalytic reaction.

ental

tion of photocatalysts

dioxide (TiO2) powder used in this study was commer-egussa). The photo-deposition of Au particles on TiO2

out by the method suggested in the literature [24].ution and TiO2 were mixed together and adjusted tosing 0.1 M Na2CO3 before illumination. The solution

ted with a 200 W mercuryarc lamp for 120 min with

ture coto appstirredcined ipowde

Titacrystal98%) athe prewater,monol[25]. T

V2OreportStreamat 353 ples w

MethesizecetyltrdirectiSiO2:0synthedissolvamounand/orfor themixtur20 ml opH to 1ring, thwater mixturmixturan ovethe rewater templa

2.2. Ch

Theby reCary 1ning el230 insformedX-ray dtallinemeasucrystalX-ray mine tstatus The spmetricwas stuSynchrK-edgemode DE-20were pning ethanol, TEOS, and deionized water was adjustedately 2 by HCl (0.1 M) solution and the mixture was

h. After that, it was dried in an oven at 393 K and cal-ow of air at 973 K for 5 h to yield the amorphous SiO2

silicalite-1 (TS-1) was prepared by the hydrothermalion of gel from tetraethylorthosilicate (TEOS, Aldrich,itanium (IV) tetrabutoxide (TBOT, Alfa Aesar, 98%) ine of tetrapropylammonium hydroxide (TPAOH, 20% ina) with a small amount of polyoxyethylene sorbitane (Tween 20), which was reported by Khomane et al.tio of Si to Ti for TS-1 was 19:1 in this study..5 wt%)/SiO2 was prepared using the same method as

Amano and Tanaka [26]. First, silicon(IV) oxide (99.5%;mical) was stirred with an aqueous solution of NH4VO3

2 h, followed by evaporation to dryness. Next, the sam-alcined in dry air at 773 K for 5 h.rous V-Ti/MCM-41 molecular sieves were syn-y simple hydrothermal treatment method usinghylammonium bromide (CTAB) as the structure-mplate. The molar composition of the gel mixture wasO2:0.01V2O5:0.2CTAB:0.89H2SO4:120H2O. In a typicalrocedure, 21.2 g of sodium metasilicate monohydrate

100 ml DI water was combined with the appropriate metal precursors like titanium oxysulfate hydrateadyl sulfate hydrate (dissolved in 20 ml of 2 M H2SO4)ium and vanadium sources, respectively. The resultings stirred vigorously for 30 min. Then, approximately

H2SO4 was added to the above mixture to adjust thewith constant stirring to form a uniform gel. After stir-lution containing 7.28 g of CTAB dissolved in 25 ml of DIdded slowly into the above mixture and the combineds stirred for three additional hours. The resulting gels transferred into a Teon coated autoclave and kept in145 C for 36 h. After cooling to the room temperature,g solid was recovered by ltration, washed with DIdried in an oven at 110 C for 8 h. Finally the organicas removed by calcination at 550 C for 10 h.

terization

t absorption of photocatalysts was fully characterizede diffusive UVvis spectroscopy (UVvis DRS, VarianaSO4 was used as a standard reection reference. Scan-n microscopy (SEM) was carried out on Nova Nano SEMent. Transmission electron microscopy (TEM) was per-itachi H-7100 electron microscope instrument. Powderction (XRD, Xray-M03XHF, Ultima IV) was used for crys-ture and calculation of photocatalyst sizes. All peak datay XRD analysis were checked and assigned to known

phases by comparing with those of JCPDS database. Theelectron spectroscopy (XPS) was carried out to deter-emical composition of the elements and the chemicalrious species on the Thermo Theta Probe instrument.

surface area of photocatalyst was measured by Micro-AP 2000. The atomic structure of V-Ti/MCM-41 and TS-1

by X-ray absorption spectroscopy (XAS) at the Nationaln Radiation Research Center in Hsin Chu, Taiwan. For Ti

V K-edge spectra, they were analyzed in uorescenceeam-line 16A. The analysis was performed using ARSlosed Cycle Cryostat with Lytle detector. The samplesred as powder form, and dispersed on Kapton tape. The

V.-H. Nguyen et al. / Chemical Engineering Journal 179 (2012) 285 294 287

Fig. 1. The scheme of powder packed-bed reactor.

energy step used was 0.2 eV, and the counting time per step was2 s around the absorption edge.

2.3. Direct gas-phase photocatalytic epoxidation of propylene

All photocatalytic epoxidation experiments were conductedcontinuously with the reactant gas mixture of C3H6:O2:N2 = 1:1:18(controlled by rotameters) at a gas hourly space velocity (GHSV) of6000 h1 under atmospheric pressure. The photocatalyst (0.02 g)was spread in a powder packed-bed reactor (0.55 cm3) with aquartz window on top for transmission of light as shown inFig. 1. The UV-A (wavelength 320500 nm) from Exfo S1500 (USA)

Fig. 2. XRD paAu/TiO2; V2O5

Fig. 3. Diffuse reectance UVvis spectroscopy of different photocatalysts (a) SiO2,(b) V-Ti/MCM-41, (c) TS-1, (d) P25, (e) V2O5(0.5 wt%)/SiO2, and (f) Au(3 wt%)/TiO2.

equipped with 200 W mercuryarc lamp was used as the irradi-ation source and guided to reactor by an optical ber. The lightintensity was measured at the window of the reactor by GOLDILUXRadiometertor was placwas requiregas chromaame ioniz(TCD). A 6-wthe two colto separatehyde (PA), aand ethanoarate O2 anpropylene cselectivity o

C3H6 consu

C3H6 conve

=

rattterns of different photocatalysts (a) V-Ti-/MCM-41; TS-1 and (b) P25;/SiO2; SiO2./Photometer (UV-A Probe/UV-C Probe). The photoreac-ed on the hot plate in case high-temperature conditiond. The feed gases were analyzed periodically using atograph (Young Lin, YL6100 GC) equipped with both aation detector (FID) and a thermal conductivity detectoray valve was used to switch the ow of the sample into

umns of the GC. A molecular sieve-5A column was used propylene and oxygenates, including PO, propionalde-cetone (AC), acetaldehyde (AA) and alcohol (methanoll). The Porapak-N on the other hand was used to sep-d CO2. The following (Eqs. (47)) are the denitions ofonversion, consumption and adsorption rates, and thef products for the reaction.

mption rate = C3H6feed rate C3H6out rate (4)

rsion rate

e of all products covered to C3 products (5)Fig. 4. SEM image of titanium silicalite (TS-1).

288 V.-H. Nguyen et al. / Chemical Engineering Journal 179 (2012) 285 294

Fig. 5. TEM im

C3H6 adsorp

= C3H6 co

Product sele

= moles o

3. Results

3.1. Photoc

The X-rshown in Fof V-Ti/MCMpore struct24.3 and 4shown in thtive peaks from monoc[28]. In Fig. sive XRD peP25. Au is acteristic Xand 77.55

structure.ages of photocatalysts: (a) Au(3 wt%)/TiO2; and (b) V-Ti/MCM-41.

tion rate

nsumption rate C3H6 conversion rate (6)

ctivity

f the formation one C3 productmoles of all C3 products

100% (7)

and discussions

atalyst characterization

ay diffraction patterns of various photocatalysts areig. 2. As shown in Fig. 2(a), the main diffraction peaks

-41 at 2 = 1.9 and 3.9 conrm the family of meso-ure. The diffraction peaks of TS-1, at 2 = 23.2, 23.8,5, are consistent with the MFI topology structures ase inset of Fig. 2(a) [27]. The presence of single diffrac-at 2 = 24.3 in the XRD pattern indicates a changelinic symmetry (S-1) to orthorhombic symmetry (TS-1)2(b), the photocatalysts containing TiO2 showed inten-aks for anatase phase and rutile phase as expected inhighly dispersed on the TiO2 which exhibits a char-RD pattern with ne structures at 2 = 44.38, 64.56

. No peak is shown for SiO2 due to their amorphous

Fig. 6. XPS spV-Ti/MCM-41

Fig. 3 shoferent phot200380 nminset of Figwidely usedTi4+ in a tecenters. Moindicates thTS-1 [7]. Foobserved atof anatase ectra of: (a) Si 2p: TS-1, V-Ti/MCM-41, V2O5/SiO2; (b) Ti 2p: TS-1, and (c) V 2p: V2O5/SiO2, V-Ti/MCM-41.

ws the UVvis diffusive reectance spectroscopy of dif-ocatalysts. There is a slight absorption edge at around

for amorphous SiO2 and V-Ti/MCM-41 as shown in the. 3(a)(b). The spectrum of TS-1 as shown in Fig. 3(c) is

to conrm the electronic transitions between O2 andtrahedral environment of isolated framework of Ti(IV)reover, the absence of absorption band at 320400 nmat no anatase or extra framework of Ti is observed inr TiO2 (P25) shown in Fig. 3(d), the absorption edge is

around 400 nm, which is between the absorption edge(387 nm) and rutile (418 nm). For V2O5 (0.5 wt%)/SiO2

V.-H. Nguyen et al. / Chemical Engineering Journal 179 (2012) 285 294 289

shown in Ffrom O2 toto be a tetris shown ilight absorpabsorption surface plas

Fig. 4 shoamount of Fig. 5(a) anTi/MCM-41as shown inMCM-41 ca

Fig. 6(a)104.1 eV. TFig. 7. Summary of the Ti K-edge characterization of (a) V-Ti/MCM-41 and (b) TS-1 by

ig. 3(e), a ligandmetal-charge-transfer (LMCT) band V5+ emerges at around 200550 nm, which is assignedahedral VO4 monomer [19]. The spectrum of Au/TiO2n Fig. 3(f). The Au on TiO2 signicantly affects thetion as compared with bare TiO2, and shows a secondband in the visible-light region at 600 nm, revealingmonic resonance [29].ws the SEM micrograph of TS-1, indicating that a small

TiO2 aggregated on the SiO2 surface in some regions.d (b) shows the TEM images of Au (3 wt%)/TiO2 and V-, respectively. The size of Au particles is around 10 nm

Fig. 5(a). With obvious contrast, the nano-structure ofn be observed in Fig. 5(b).

shows the binding energies of Si 2p at 103.2 eV andhe peak of TS-1 at 104.1 eV belongs to silica oxide

compound.V2O5/SiO2oxides but hFig. 6(b) shconrmingon V-Ti/MCTi state of VFig. 6(c) shTi/MCM-41The intensitloading of V

The Ti Kand TS-1 arcan be obta(XANES). Th X-ray near edge absorption spectroscopy (XANES).

There is no distinguished shift for V-Ti/MCM-41 andat 103.2 eV, which indicates that silicates are mainlyex-uorosilicate and other anion are also included [30].ows the binding energies of Ti 2p for TS-1 at 457.5 eV,

the presence of Ti4+ state. Due to the little amount of TiM-41, its peak of Ti 2p could not be observed clearly. The-Ti/MCM-41 was conrmed by XAS as shown below.

ows the binding energies of V 2p3/2 and 2p1/2 for V- and V2O5/SiO2 at 517.3 eV and 520.9 eV, respectively.ies of peaks were closed to background due to the small2O5 (0.5 wt%).-edge X-ray absorption spectroscopy of V-Ti/MCM-41e shown in Fig. 7. Generally, two kinds of informationined from the X-ray absorption near edge spectroscopye shift of absorption edge indicates the oxidation state

290 V.-H. Nguyen et al. / Chemical Engineering Journal 179 (2012) 285 294

Table 1Photocatalytic epoxidation of propylene.a

Entry Catalysts BET surfacearea(m2 g1)b

Temp.(C)

Lightintensity(mW/cm2)

C3H6 PO formationrate (mol g1 h1)

Selectivity (%)c

Adsorptionrate (molg1 h1)

Conv. rate(molg1 h1)

AA ROH PO PA AC CO2

1 P25 50.0 50 0.2 3444.8 619.5 7.6 ND ND 0.4 17.6 74.42 Au (3%)/TiO2 37.9 50 0.1 2900.9 455.4 4.13 33.7 ND 1.0 5.4 21.7 38.23 SiO2 560.1 50 0.1 2113.6 4.5 0.21 77.3 ND 10.6 12.1 ND ND4 V2O5 (0.5%)/SiO2 50 0.2 1459.9 26.1 49.9 ND ND 50.1 ND ND

5 TS-1 368.9 25 0.2 59.6 25.4 1.31 31.6 ND 24.1 ND ND 44.350 0.2 3886.7 50.7 19.69 29.0 0.9 40.1 22.9 7.1 trace50 0.1 521.9 55.7 24.08 15.9 ND 44.7 32.3 7.0 trace70 0.1 n.s. 59.7 17.91 38.3 ND 31.0 24.6 6.1 trace50 0.05 n.s. 16.9 5.02 38.5 ND 33.2 16.6 12.0 trace

6 V-Ti/MCM-41 790 50 0.2 11972.5 243.5 114.18 21.3 ND 46.8 26.2 5.7 ND25 0.2 n.s. 163.1 54.52 41.4 2.3 32.0 17.8 6.5 ND75 0.2 2013.6 223.2 87.92 22.0 1.7 40.2 26.9 9.2 ND120 0.2 13624.9 207.5 81.96 11.8 ND 39.6 45.1 3.5 ND50 0.4 9323.8 283.0 92.80 24.3 1.4 33.2 22.6 18.5 ND50 0.1 12915.6 121.8 48.35 21.1 ND 40.1 29.8 9.0 ND

ND: not detected by GC.a Reaction conditions: photocatalyst 0.02 g; feed gas C3H6:O2:N2 = 1:1:18 vol% at a gas hourly space velocity (GHSV) of 6000 h1. The data is the mean value obtained on

stream after 4 h.b BET surface area was measured by Micromeritics, ASAP 2000.c PO: propylene oxide; PA: propionaldehyde; AC: acetone; AA: acetaldehyde; ROH: ethanol and methanol.

of elementcoordinatiothe averageshould be Tand (b), rescoordinatioshown in Fthat of TS-1is close to gdistorted. Tto propylen

irect

le 1 sene wckedpylenvity rch a

nol wramand . The pre-edge peak shows the information of localn structure [31]. Compared with the Ti(III) and Ti(IV),

oxidation state of titanium in V-Ti/MCM-41 and TS-1i4+ from the shifts of absorption edges shown in Fig. 7(a)pectively. From the pre-edge peaks of Fig. 7(a), the localn structure of Ti4+ is suggested to be tetrahedral. Asig. 7(b), the pre-edge peak of TS-1 fresh is higher than

reacted. The local coordination structure of TS-1 freshenuine tetrahedral while that of TS-1 reacted is slightlyhis suggests that TS-1 is distorted due to Ti connectinge or other oxygenates during reaction.

3.2. D

Tabpropylous paof proselectiucts, sumethamost d(TiO2Fig. 8. The effective light of intensity and temperature for photocatalytic epoxidation ogas-phase photocatalytic epoxidation of propylene

ummarizes the results of photocatalytic epoxidation ofith oxygen over a series of photocatalysts in a continu-

-bed reactor at mild conditions. The conversion ratese range from 4.5 to 619.5 mol g cat1 h1 while POanges from 1% to 46.8%. In addition to PO, other prod-s propionaldehyde, acetone, acetaldehyde, ethanol andere detected and their selectivities were calculated. The

atic differences between two supported photocatalystsSiO2) are in the production of side products and thever TS-1: (a) PO formation rate and (b) PO selectivity.

V.-H. Nguyen et al. / Chemical Engineering Journal 179 (2012) 285 294 291

Fig. 9. The time-dependent behavior of selectivity for the formation of PO (), PA (), AC (), ROH () and MeCHO () on stream over TS-1 under different light intensityconditions: (a) 0.1 mW/cm2 and (b) 0.2 mW/cm2.

Fig. 10. The effects of light intensity and temperature on the photocatalytic epoxidation over V-Ti/MCM-41: (a) PO formation rate (b) PO selectivity and (c) C3H6 conversionrate.

292 V.-H. Nguyen et al. / Chemical Engineering Journal 179 (2012) 285 294

Slec

tivity

(%)

0

10

20

30

40

50MeCHOPO PA AC

(a)_V- Ti/MCM -41_0.1mW_50 oC

Time on strea m (min)0 50 10 0 15 0 20 0 25 0

50

60MeCHOPO PA AC

(b)_V -Ti/MCM -41_0.2mW_50 oCSl

ectiv

ity (%

)

0

10

20

30

40MeCHOPO PA AC

(d)_V -Ti/MCM -41_0.2mW_120 oC

Fig. 11. The t0.1 mW/cm2, 5

conversion total oxidatC3H6 conve

The adsoduring phothe averagereaction. Thand chemicface area anV-Ti/MCM4material.

Commerity. The C3HCO2 in the pis very impoAu particle of C3H6 to CCO2 is prodlene, but alproducts. HTiO2.

The UV avery low C3Table 1. HoThe main r560.1 m2 g cin order tocatalyst andTime on strea m (min)0 50 10 0 15 0 20 0 25 0

Slec

tivity

(%)

0

10

20

30

40

MeCHOPO PA AC ROH

(c)_V- Ti/MCM -41_0.4mW_50 oC

Slec

tivity

(%)

0

10

20

30

40

50Time on strea m (min)0 50 10 0 15 0 20 0 25 0 0

ime-dependent behavior of selectivity for the formation of PO (), PA (), AC (), ROH0 C; (b) 0.2 mW/cm2, 50 C; (c) 0.4 mW/cm2, 50 C and (d) 0.2 mW/cm2, 120 C.

rate of C3H6. TiO2 support can convert more C3H6 toion while SiO2 support leads to low oxidation at lowerrsion.rption of propylene was observed for all photocatalyststoreaction after switching on the UV light. Table 1 lists

adsorption rates based on the carbon balance in 4 he propylene adsorption is the combination of physicalal adsorptions on the supports, which depends on sur-d TiO2 content. The amount of propylene adsorbed on1 was substantial because of the mesoporous nature of

cial TiO2 (entry 1) shows high photo-oxidation activ-6 consumption rate is 619.5 mol g1 h1 with mostroducts, and no PO is observed. The size of Au particlesrtant on epoxidation. This study also conrms that the

with size higher than 5.0 nm prefers complete oxidationO2 as shown in entry 2 [32]. A considerable amount of

uced not only from the direct photo-oxidation of propy-so from the successive photo-oxidation of oxygenatedence, only a few PO can be generated over Au/TiO2 and

bsorption of amorphous SiO2 is very low (Fig. 3), thusH6 conversion rate was obtained as listed in entry 3 ofwever, the selectivity to form PO still can reach 10.6%.eason may be the high specic surface area of SiO2,at1. Therefore, SiO2 can harvest more photo-energy

produce sufcient electronhole pairs on the photo- carry out partial oxidation successfully. The dispersed

vanadium absorption Table 1, POC3H6 convephous SiO2

The moand V-Ti/Mentry 5 and

0.05 C3H

6 con

v. &

PO

form

atio

n ra

te (

mol

.g-1

.h-1

)

0

50

100

150

200

250

300

Fig. 12. The lifor V-Ti/MCMTime on strea m (min) 50 10 0 15 0 20 0 25 0

() and MeCHO () over V-Ti/MCM-41 at different conditions: (a)

oxide on silica had signicantly improved the lightat around 200550 nm (Fig. 3). As listed in entry 4 of

still cannot be produced over V2O5/SiO2 although thersion rate increased 5 times as compared with amor-.st favorable photocatalytic epoxidation was on TS-1CM-41 under various reaction conditions as shown in

6 of Table 1, respectively. These two photocatalysts

Light i ntensit y (mW/cm2)0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45

C3H

6 ads

orpt

ion

rate

(m

ol.g

-1.h

-1)

9000

10000

11000

12000

13000

14000

Conv. ra te PO ra te Adsorp tion ra te

ght effect on PO formation, C3H6 conversion and C3H6 adsorption rate-41.

V.-H. Nguyen et al. / Chemical Engineering Journal 179 (2012) 285 294 293

C

C

CH3H

O*O

C

C

CH3H*

O

C

C

CH3

O

ucts o

prepared frble for the poptimal con114.18 moV-Ti/MCM-

The phoferent lightoptimum cowhich can Further invtivity are shachieved uFig. 9(a). Thof the Ti spfore, stableepoxidationlight intenssupplied, malyst. Thus mresulting in

V-Ti/MCepoxidationthe light anfor V-Ti/MCimpacts on condition inin Fig. 10(a)than TS-1 (Fenhances thever, oversuand formati

Comparetime-depenepoxidationble selectiv0.10.4 mWties at the is that V-TiTS-1, hencePO is still thslight increat 50 C (Figtially increathat of PO r

Fig. 12 tion, C3H6 cThe adsorpto its porouwith increathe C3H6 c

er, fu, the m P

opos

intrible f speativeataly

ataly

-phan as

e spes. He

by sho

+ h+

he epts. Foct wducts

CH C

C

CH3H

H H

O*

H H

C

C

CH3H

H H

O*

Fig. 13. The formation mechanism of PO and oxygenate prod

om SiO2 supported materials are all active and sta-roduction of PO, despite having signicantly differentditions. For instance, the highest PO formation rate ofl g cat1 h1 with selectivity of 46.8% was observed for41at 50 C under light intensity of 0.2 mW/cm2.tocatalytic epoxidation over TS-1 was studied at dif-

intensities and temperatures as shown in Fig. 8. Thenditions of 0.10.15 mW/cm2 and 4050 C were foundachieve the highest PO formation rate and selectivity.estigations on time-dependent behavior of PO selec-own in Fig. 9. The stable selectivities of products are

nder the light intensity of 0.1 mW/cm2 as shown ine XANES (Fig. 7(b)) revealed that the oxidation statesecies are the same before and after reaction. There-

active sites can be maintained during photocatalytic. However, the selectivities are unstable with higherity as shown in Fig. 9(b). When higher light intensity isore elections and holes are generated on the photocat-ore active intermediate oxygenates may be generated

unstable selectivities.M-41 exhibited excellent selectivity for the photo-

of propylene with molecular oxygen. Fig. 10 showsd temperature effects on photocatalytic epoxidationM-41. Light intensity and temperature have signicantphotocatalytic epoxidation and there exists an optimum

the range of 0.20.3 mW/cm2 and 4060 C as revealed. V-Ti/MCM-41 has much lower UVvis absorption edgeig. 3). Hence, increase in light intensity can signicantlye C3H6 conversion rate as shown in Fig. 10(c). How-pply of light would eventually reduce the selectivityon rate of PO as shown in Fig. 10(a) and (b).

to TS-1, V-Ti/MCM-41 also shows a quite similardent behavior in PO selectivity for the photocatalytic. However, as shown in Fig. 11, V-Ti/MCM-41 holds sta-ities on stream within a wide range of light intensity,/cm2, while TS-1 can only maintain stable selectivi-light irradiation of 0.1 mW/cm2. The possible reason/MCM-41 has the lower UVvis light absorption than

HowevHenceoptimu

3.3. Pr

TheA possedge oof negphotoc

photoc

Gaselectro

O2(g) +The

procesformedhole as

O2(ads)

In tproduccan reaall pro

CH3 more photo-energy is required than TS-1. Althoughe dominant product with increasing light intensity, a

ase in acetone and alcohol concentration is observed. 11(a)(c)). Increasing temperature to 120 C substan-ses the amount of propionaldehyde produced whileemains at the same level (Fig. 11(d)).shows the inuence of light intensity on PO forma-onversion and C3H6 adsorption rate over V-Ti/MCM-41.tion rate of C3H6 is signicant on V-Ti/MCM-41 dues structure. The adsorption rate decreases noticeablyse in light intensity. Increasing light intensity enhancesonversion rate thus also increases PO formation rate.

Carter ananism of proxypropenyangle of H-aand the reaate intermeabstractionintermediapionaldehywe suggestthe pore reHH

CC

CH3

H

H

H

*

HH

O

CC

CH3

HH

H

ver V-Ti/MCM-41 photocatalyst.

rther increasing light intensity inhibits PO selectivity.PO formation rate starts to decline after reaching anO formation rate at 0.2 mW/cm2.

ed mechanism of photocatalytic epoxidation

insic epoxidation mechanism is still not quite clear.reaction mechanism is proposed based on the knowl-cies present during the photocatalytic reaction. A pair

electron (e) and positive hole (h+) is formed in thest with light irradiation as in Eq. (8).

sthve + h+ (8)

se O2 is adsorbed and becomes O2 when react with anshown in Eq. (9).

O2(ads) (9)

cies O2(ads) is stable and not directly active in thisnce, the active species, surface atomic oxyradicals, arethe reaction of the preceding species with the positivewn in Eq. (10).

2O(ads) (10)

oxide reaction, there are several pathways to create ther example, the above surface atomic oxyradicals O(ads)

ith propylene to yield 3 kinds of products (8090% for) as listed in Eq. (11).

O* CH2 +

CH3 CH CH 2

O

CH3 CH 2 CH

O

CH3 C CH 3

O

(11)d Goddard [33] proposed the selective oxidation mech-opylene to epoxide via H-atom abstraction by formingl intermediate. The product selectivities depend on thetom abstraction from the propylene attacked by O(ads)

ction conditions. Fig. 13 displays the mechanism to cre-diate species of PO and another product via H-atom

in a limited space, e.g. inside pore. The oxypropenylte of PO is favorable inside the pore while that of pro-de cannot be formed due to space limitation. Therefore,ed that the selectivity of PO could be increased due tostriction of MCM-41.

294 V.-H. Nguyen et al. / Chemical Engineering Journal 179 (2012) 285 294

4. Conclusions

A direct photo epoxidation process to produce PO from propy-lene by O2 with the aid of light energy over photocatalysts is apromising technology for the production of chemicals in the nearfuture. Compared to other thermal processes of propylene epox-idation, no hydrogen is required in the photocatalytic process.Furthermore, sunlight may be applied in the photo-epoxidationin the future. In this study, under mild conditions, Ti-V/MCM-41and TS-1 promote the photocatalytic epoxidation of propylene withmolecular oxygen at steady state in a ow reactor system. The high-est PO formation rate and PO selectivity achieved on stream after 4 hover Ti-V/MCM-41 is 114 mol g cat1 h1 and 47%, respectively,at 50 C and atmospheric pressure. In summary, V-Ti/MCM-41 isthe most favorable photocatalyst for the photocatalytic epoxidationin this study, and the selectivity of PO could be further enhancedpossibly by solving the pore restriction of reaction intermediate.

Acknowledgment

We gratefully acknowledge the National Science Council ofTaiwan for nancial supporting this research under contractnumbers,NSC 99-2923-E-002-002-MY2 and NSC 99-2221-E-009-037-MY3.

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Direct gas-phase photocatalytic epoxidation of propylene with molecular oxygen by photocatalysts1 Introduction2 Experimental2.1 Preparation of photocatalysts2.2 Characterization2.3 Direct gas-phase photocatalytic epoxidation of propylene

3 Results and discussions3.1 Photocatalyst characterization3.2 Direct gas-phase photocatalytic epoxidation of propylene3.3 Proposed mechanism of photocatalytic epoxidation

4 ConclusionsAcknowledgmentReferences