Tailoring the mesopore structure of HZSM-5to control product distribution in the conversion of propanal
Xinli Zhu,Lance L.Lobban,Richard G.Mallinson,Daniel E.Resasco *
Center for Biomass Refining,School of Chemical,Biological,and Materials Engineering,The University of Oklahoma,Norman,OK 73019,USA
a r t i c l e i n f o Article history:
Received 23September 2009Revised 14December 2009Accepted 2February 2010
Available online 6March 2010Keywords:Propanal HZSM-5Desilication
Biomass conversion Bio-oil upgrading
a b s t r a c t
Conversion of propanal to gasoline-range molecules was investigated over a series of HZSM-5catalysts with controlled mesoporosity generated by desilication.Characterization of the structure of the solid by powder X-ray diffraction (XRD),scanning electronic microscopy (SEM),ammonia and isopr
opylamine temperature programmed desorption (TPD),and n -butane diffusivity measurements confirmed the development of various degrees of mesoporosity in the zeolites.This structural modification seems to have little influence on Brønsted acid density.The catalyst stability was improved upon desilication due to an increase in coke tolerance.The product distribution of the propanal conversion was found to vary with the severity of the desilication.Increasing the extent of desilication gradually reduced the aro-matization and cracking reactions,due to a reduction in the fraction of micropores and in the diffusion path length.Mildly desilicated samples were found to exhibit the best stability on stream and inhibited coke formation.
Ó2010Elsevier Inc.All rights reserved.
1.Introduction
Conversion of lignocellulosic biomass into liquid hydrocarbon fuels provides a CO 2neutral energy production route,which poten-tially can reduce the dependency on fossil fuels [1–3].In the ther-mochemical route (e.g.fast pyrolysis),the molecular structure of biomass is broken down into smaller fragments that subsequently undergo further conversion in the vapor and liquid phases con-densing into a complex product termed bio-oil.Some of the con-stituents of this product are larger than the desi
rable fuel range,others are shorter,but all of them contain significant amounts of oxygen.The chemically unstable,highly viscous,corrosive,and low-heating value liquid product includes acids,aldehydes,ke-tones,phenolic compounds,sugars,and dehydrosugars [4,5].Deoxygenation of the larger oxygenated molecules (guaiacols,vanillins,cresols,catechol,etc.)is being extensively investigated [6–10];and hydrogenation,hydrogenolysis,and decarbonylation are potential reaction pathways to improve the quality of these heavy molecules.By contrast,the short oxygenates (e.g.,alde-hydes,acids,ketones)need to be condensed into larger molecules to become useful fuel components.Under hydrotreating conditions for refining the complete bio-oil,short oxygenates are converted to light hydrocarbons of low value while consuming substantial hydrogen.Alternative strategies that avoid discarding short oxy-genates should be considered since they constitute a significant
fraction of the product [4,5].Due to the highly complex nature of the bio-oil,understanding the reaction pathways for each kind of compound conversion is highly desirable for catalyst and process screening.Therefore,the study of model compounds is the first step in simplifying the complexity of the problem [11–14].While our next studies will include more complex mixtures,in this work,propionaldehyde (propanal)has been selected as a model com-pound to investigate the conversion of short aldehydes into gaso-line-range molecules.
The study of propanal conversion is also relevant to the utiliza-tion of glycerol,a major by-product of bio-diesel production.Glyc-erol is readily converted into acrolein by dehydration [15,16].Subsequent hydrogenation produces propanal [17].Thus,conver-sion of propanal may also represent a potential approach for the conversion of bio-diesel by-products to gasoline-range fuels.
reactions to the online managePrevious studies on the conversion of small oxygenates (meth-anol,ethanol,etc.)to hydrocarbons (alkene/alkane,aromatics)over zeolites have addressed propanal conversion briefly.It has been re-ported that propanal can yield aromatics in higher selectivity than acetone and much higher than other C 3oxygenates (alcohol,acid,ester)[18–20].However,it was also found that propanal causes a rapid catalyst deactivation [19].
Zeolites are widely used in hydrocarbon conversion due to their high density of strong acid sites and their well-defined micropo-rous channel structure that enable shape selective reactions inside the pore channels.However,transport of both reactants and prod-ucts in and out of the micropores may be limited by diffusion.Con-ventional mesoporous materials such as MCM-41and SBA-15have superior diffusion properties but lower thermal/hydrothermal
0021-9517/$-see front matter Ó2010Elsevier Inc.All rights reserved.doi:10.1016/j.jcat.2010.02.004
*Corresponding author.Fax:+14053255813.
E-mail addresses:mallinson@ou.edu (R.G.Mallinson),resasco@ou.edu (D.E.Resasco).
Journal of Catalysis 271(2010)
88–98
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Journal of Catalysis
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t
stability and weaker acidity.To improve the performance of zeo-lites by enhancing transport,a post-synthesis method called desi-lication has been used[21–25].This method selectively removes silica from the zeolite crystals,generating mesopores.The resul-tant material has a hierarchical pore structure with pores of vary-ing dimensions and with a shortened diffusion path length for reactants and products[26,27],as well as improved accessibility for large molecules[28–30].Desilicated zeolites have been investi-gated in several reactions,including cumene cracking,methanol to propylene,methanol to gasoline,hydroxylation of benzene to phe-nol,methane aromatization,and hexene conversion[29–35].In most cases,improved activity,stability,and selectivity have been reported.In general,the observed improvement has been ascribed to enhanced diffusion due to the generation of mesopore channels. In this work,four zeolite samples with varying degree of desilica-tion have been characterized for changes in texture,structure, acidity,and diffusivity,and then used in the conversion of propanal with the objective of investigating the effects of mesopore genera-tion on the co
nversion and selectivity toward gasoline-range prod-ucts.H-ZSM-5has been chosen as the basis for this study due to its well-known activity for the conversion of short oxygenates to aromatics.
2.Experimental
2.1.Zeolite synthesis and desilication
The parent zeolite was synthesized hydrothermally using so-dium aluminate(Aldrich)dissolved in deionized water as the Al source to which tetrapropylammonium hydroxide(Fluka,20%) TPAOH)wasfirst added under stirring as the structure directing agent,silica gel(Ludox,40%)was then added dropwise while stir-ring,as the Si source.The resultant gel composition was 150SiO2:1.0Al2O3:8TPAOH:1600H2O(Si/Al=75).After stirring at 700rpm for10h at room temperature,the gel was transferred to a Teflon-lined autoclave,where the zeolite crystallized at180°C over5days with stirring at60rpm.The solid product was recov-ered byfiltration,washed,dried at110°C,andfinally calcined in air at550°C for6h to remove the template.
Four desilicated samples(DS1,DS2,DS3,and DS4)were pre-pared by varying the basicity of the alkaline solution,treatment temperature,and time,resulting in different extents of silica re-moval.To achieve the desired level of desilication,the parent sam-ple(P)was treated in the following desilication b
aths,keeping in all cases a ratio of30mL solution per gram of zeolite and using 350rpm stirring rate.DS1:0.45M Na2CO3(Aldrich)solution for 30h at75°C;DS2:0.2M NaOH(Aldrich)solution at65°C for 30min;DS3:at80°C for30min,and DS4:at80°C for4h.After each treatment,the desilication was stopped by quenching the sample in an ice-water bath.The resultant samples were thenfil-tered,washed,and dried at110°C overnight.After this step,the samples were further suspended in deionized water at80°C for 2h to remove any amorphous silica and alkaline metals remaining in the solid.Finally,the samples were againfiltered,washed,and dried at100°C overnight.
The H-form of the zeolite was obtained by repeating three times the ion exchange of the Na-form of the zeolite with1M NH4NO3 (Aldrich)solution(10mL/g)at80°C for10h.After each exchange, the samples werefiltered,washed,and dried,andfinally calcined inflowing air at550°C for4h.
2.2.Catalyst characterization
Powder X-ray diffraction(XRD)patterns of the zeolite samples were recorded using a Bruker D8Discover diffractometer,with a Cu K a radiation source(k=1.54056Å).High resolution scanning electronic microscopy(SEM)observations were performed on gold-coated samples in a Jeol JSM-880electron microscope equipped with X-ray elemental analyzer.Nitrogen adsorption measurements[3
6]were performed in an Autosorb-1analyzer (Quantachrome)at liquid nitrogen temperature after outgassing the samples under vacuum at300°C for5h.The micropore volume (V micro)and micropore surface area(S micro)were derived by the t-plot method[37]using the adsorption data of0.2<p/p0<0.6. The mesopore size distribution was obtained from the BJH method [38]applied to adsorption branch[39]data for p/p0>0.35.Finally, the total pore volume was determined at p/p0=0.99.
The acid properties of the various zeolites were characterized by temperature programmed desorption of ammonia(NH3-TPD) and isopropylamine(IPA-TPD),using d.quartz reactor. Before each experiment,the zeolite sample(100mg for NH3-TPD and50mg for IPA-TPD)was pretreated for0.5h inflowing He (30mL/min)at600°C to eliminate any adsorbed water.Then,the temperature was reduced to100°C,and the sample was exposed to NH3(2%NH3/He,30mL/min,30min)or to IPA(4l L/pulse,10 pulses,3min/pulse).After exposure to the respective adsorbate, Heflowed for0.5h to remove weakly adsorbed NH3or IPA.To start the TPD,the temperature was increased to650°C at a heating rate of10°C/min.The evolution of desorbed species was continuously monitored by a Cirrus mass spectrometer(MKS)recording the fol-lowing signals m/z=17and16(NH3),18(H2O),44(IPA),and41 (propylene).The density of acid sites was quantified by calibrating the MS signals using the average of105-mL-pulses of2%NH3/He.
The changes in diffusivity upon desilication were evaluated by sending an n-butane pulse using the same system used for TPD. The zeolite samples(100mg,40–60mesh)were pretreated in flowing He(30mL/min)at400°C for0.5h to remove water.Then, the temperature was reduced to90°C,and a5mL n-butane pulse (10%n-butane/He)was sent underflowing He(30mL/min).The butane concentration was monitored by following the m/z=43sig-nal in a mass spectrometer(MS)and compared to pulses over non-porous blanks.
The amounts of coke deposits were quantified by Temperature Programmed Oxidation(TPO)by passing a2%O2/He stream over a20mg spent catalyst sample,using a linear heating rate of 10°C/min.The signals of H2O(m/z=18),CO2(m/z=44),and CO (m/z=28)were continuously monitored by MS.Quantification was calibrated on the basis of the signals from100l L CO2and CO pulses inflowing He.
2.3.Catalytic measurements
The catalytic performance of the different samples was exam-ined in a quartz reactor(d.)at atmospheric pressure. The catalyst sample(10–400mg,40–60mesh)was packed in the reactor between two layers of quartz wool.The thermocouple was affixed to the external wall of the reactor close to the
catalyst bed.The temperature of the catalyst bed was increased to400°C using a rate of10°C/min and held at400°C for0.5h inflowing H2(35mL/min)before reaction.Liquid propanal(from Aldrich) was fed using a syringe pump(kd scientific)equipped with a nee-dle at a rate of0.12mL/min.The liquid was completely vaporized in the line before entering the reactor.All lines were kept at300°C to avoid condensation of reactant or products.The products were analyzed online using a gas chromatograph(GC6890,Agilent) equipped with aflame ionization detector(FID)and a60m Inno-wax capillary column.The effluent was trapped in methanol using an ice-water bath and analyzed using a QP2010s GC–MS(Shima-dzu)with an Innowax column.Quantification of products was done by combination of GC–MS analysis and injection of known amounts of standard compounds.The space time(W/F)is defined
X.Zhu et al./Journal of Catalysis271(2010)88–9889
as the ratio of catalyst mass(g)to propanal massflow rate(g/h) with a carrier gasflow rate of30mL/min.The range of W/F used in this study was0.1–4.0h.The propanal conversion and product yield were calculated based on carbon atoms.
3.Results
3.1.Catalyst characterization of desilicated samples
3.1.1.Porous structure
As shown in Table1,different levels of desilication were ob-tained by treating the ZSM-5zeolite with various solutions that ranged from a weakly basic solution(Na2CO3)to a strongly basic solution(NaOH)that is very effective in removing silica species from the zeolite crystal even at low temperatures and short times. As a result,the weight loss gradually increased from DS1(22%)to DS4(52%)as the severity of the treatment increased.It has been suggested that negatively charged AlOÀ
4
protects against OHÀat-tack,whereas the Si A O A Si bond is more easily attacked by OHÀ[21–25].In agreement with previous studies,the calculated Si/Al ratios that result from assuming that only silica is removed are consistent with the elemental analysis data obtained in the SEM by EDX.
Fig.1shows that even though a significant amount of silica was removed from the zeolite,the XRD patterns show that the MFI structure is preserved.However,a close examination of the characteristic pe
aks of MFI structure in the2h range of23–25°(see the inset in Fig.1),revealed a slight shift in the peak posi-tions to lower angles for the desilicated samples.This shift may be interpreted as a slight expansion of the unit cell of the zeolites, which could be ascribed to the selective removal of Si.This result is in agreement with those of Ohayon et al.[40],who reported that the micropores were slightly enlarged by the desilication-stabilization process.It must be noted that in the most heavily desilicated sample(DS4),the diffraction peaks appear to shift back to higher angles.This reversed structural modification could be explained by a previously reported‘healing effect’[22,33] undergone by the zeolite after prolonged treatment that allows some of the dissolved Si species to re-insert back into the zeolite framework.
The SEM micrographs of the parent sample(P)show well-crys-tallized particles of$6l m(Fig.2a).The particles are twinned crys-tals(Fig.2b),and the well-defined surfaces are rather smooth with few defects(Fig.2c).Desilication seems to break up some of the particles into smaller fragments(Fig.2d).The surface becomes rougher even for those particles that preserve their original shape and size(Fig.2e).The higher-magnification image(Fig.2f)shows evidence of the presence of etched channels on the surface of the particle,indicating the development of mesoporosity.
Fig.3a and b shows the N2adsorption–desorption isotherms and the results of the BJH analysis for bot
h the parent and desilicat-ed HZSM-5samples.The increased adsorption in the range p/p0> 0.5and the appearance of hysteresis loops in the desorption branch at p/p0of$0.42of the desilicated samples indicate the develop-ment of mesopores.From the analysis of the data,it can be inferred that the treatment in weak base(Na2CO3)for prolonged times (DS1)results in a relatively wide distribution of pore sizes,cen-tered at12.0nm but with a rather small overall pore volume.By contrast,the treatment with the strong base(NaOH)at low tem-peratures(sample DS2)results in a relatively narrower pore size distribution centered at7.3nm and a larger overall pore volume. Increasing the severity of the NaOH treatment by using higher temperatures and longer times(DS3and DS4),results in a gradual increase in both pore size and overall pore volume,with a peak center at10.7and13.9nm,respectively.The results from the N2 adsorption are summarized in Table1.They confirm that the frac-tion of mesoporosity significantly increases at the expense of the microporosity with increasing desilication severity.The results of the effect of temperature and time of alkaline treatment are in good agreement with those reported by Groen et al.[25,39,41].It is noted that the V micro is slightly increased for DS1,possibly as a result of the prolonged treatment time with the weakly basic Na2CO3.This long treatment may lead not only to the removal of Si but also to re-incorporation of some of the dissolved species into the zeolite structure,further creating microporosity.
3.1.2.Acidity
The effects of desilication on the acidity of the HZSM-5zeolites were studied by TPD of adsorbed NH3and IPA,as shown in Figs.4 and5,respectively.For sample P,two distinct desorption peaks are observed at180°C and385°C in the NH3-TPD.These are usually ascribed to NH3desorption from weak and strong acid sites,
Table1
Weight loss,final Si/Al ratio,specific area(S BET),and pore volume of parent(P)and desilicated(DS)zeolite samples.
Sample W loss(wt.%)Si/Al a Si/Al b S BET(m2/g)V total(cm3/g)V micro(cm3/g)V meso c(cm3/g)S micro(m2/g)S meso c(m2/g)V meso/V micro
P––763920.2170.1610.056359330.35 DS12259624140.2970.1710.126369490.74 DS22854524290.3520.1510.20133594  1.33 DS34839424530.4330.1380.296310143  2.14 DS45236374410.4920.1380.354311130  2.56
a Estimated from weight loss(W
loss
)assuming that only silica was removed.
b Estimated by SEM elemental analysis of several particles.
c V
meso =V totalÀV micro,S meso=S BETÀS micro.
90X.Zhu et al./Journal of Catalysis271(2010)88–98
respectively.With increasing desilication severity from DS1to DS4,it is observed that the peak ascribed to strong acid sites gradually loses its intensity and shifts to lower temperatures.These changes are accompanied by a gradual increase in the intensity at interme-diate temperatures ($250°C),but little change in the peak as-cribed to weak acid sites.These changes either indicate that desilication converts some of the strong Brønsted acid sites into sites of weaker acidity or modifies the accessibility of these sites due to the partial removal of silica.
The advantage of the IPA-TPD method is that it can be used to selectively quantify Brønsted acid sites that catalyze the conver-sion of IPA into propylene and NH 3[42–45].For the undissociated IPA (Fig.5a),upon desilication,the intensity of the peak at $190°C becomes smaller than that for the parent sample;at the same time,a small peak at higher temperatures ($250°C)gradually develops
with increasing desilication severity.The former peak could be ascribed to desorption of the undissociated amine from weak acid sites of Si A OH.The reduction in intensity is probably due to the removal of internal Si A OH sites (silanol surface defects).This explanation is in good agreement with previous IR measure-ments that showed that internal Si A OH sites were removed upo
n desilication [25,33,46].A new peak develops at higher tempera-tures with increasing desilication severity.Desorption from these sites occurs at relatively high desorption temperatures,but they are unable to catalyze the IPA decomposition [42–45].Infrared measurements of pyridine adsorption have shown an increase in the density of Lewis acid sites due to dealumination upon severe desilication [29,31,46].Accordingly,this desorption peak may be associated with the presence of those sites.In all samples,the Brønsted sites produced propylene desorption peaks centered at 351°C (Fig.5b),indicating that the density (and possibly
strength)
Fig.2.SEM micrographs of parent zeolite sample P (a–c)and desilicated sample DS2(d–f).
X.Zhu et al./Journal of Catalysis 271(2010)88–9891
of Brønsted acid sites is not significantly changed despite the high desilication severity.The NH3peaks appear slightly later than those of propylene(Fig.5c)due to re-adsorption/desorption,as previously indicated[42–45].The NH3-TPD and IPA-TPD results are summarized in Table2.The Brønsted acid density estimated from the TPD measurements is in good agreement with the nomi-nal density of HZS
M-5corresponding to a Si/Al ratio , 0.22mmol/g).Both techniques show that the total acid density and Brønsted acid density remain largely unchanged upon desilica-tion.A slight increase for mild desilication followed by a slight de-crease with increasing desilication severity seems to be detected by both techniques,but the changes are very small.
3.1.3.Diffusivity
Fig.S1(in Supplementary content)shows the evolution profiles resulting from sending an n-butane pulse through the reactor with and without a zeolite bed.The presence of the zeolite significantly changes the shape and width of the observed peak as a result of the combination of adsorption and diffusion through the zeolite bed.It is observed that,upon mild desilication,the elution peak initially shifts to longer retention time and becomes wider(compare P and DS1),but it shifts back to shorter times and becomes narrower as the degree of desilication increases(compare DS2and DS4).The apparent diffusivity derived from applying the dispersion model for deviations from plugflow to the data[47]are summarized in Table3.Details of thefitting method are included in the Supple-mentary content.It is seen that the apparent diffusivityfirst de-creases slightly(DS1)but then increases gradually(DS4). Increased diffusivities upon desilication have been previously re-ported[26,29].But,at the same time,decreases in diffusivity have also been reported[30].While increased diffusivities can be ex-pected from enhanced m
esoporosity,decreases have been ascribed to enhanced Al concentration in the desilicated zeolite that affects adsorption and may lower diffusivity.
However,the adsorption effects may not be as determinant of the overall zeolite performance as the changes in diffusion path length.For example,Gobin et al.[48]have shown that in MFI crys-tals(3–5l m),the rate-determining step of the overall transport is intracrystalline diffusion.The situation seems to be the same in the present study,in which relatively large crystallite sizes have been used and very minor changes in acid density have been observed after desilication.In fact,at the larger extents of desilication(sam-ples DS2to DS4)significant increases in apparent diffusivity are seen,and they correspond well with the increase in mesoporous volume accompanied by a shortening in the diffusion path length.
Table2
Total acid density(A total)and Brønsted acid density(A B).
Sample P DS1DS2DS3DS4
A total(mmol/g)0.260.290.290.270.27
A B(mmol/g)0.200.230.240.220.22 A total was derived from NH3-TPD,and A
B was derived from and IPA-TPD.Table3
Apparent diffusivity(D)for n-butane.
Sample P DS1DS2DS3DS4 D(Âl0À10m2/s)  5.46  4.93  5.617.939.44
92X.Zhu et al./Journal of Catalysis271(2010)88–98
The slight decrease in apparent diffusivity for the mildly desilicat-ed DS1sample might be ascribed to the increase in microporous volume combined with the minor increase in accessibility of adsor-bate to the micropore structure.
3.2.Catalytic activity
3.2.1.Effect of time on stream
Fig.6shows the evolution of propanal conversion with time on stream.It can be seen that after a few hours under identical reac-tion conditions,the propanal conversion obtained on the desilicat-ed samples is significantly higher than on the original sample P. The order of conversion after a few hours on stream is DS1%DS2>DS3%DS4>P.To distinguish the effects of catalyst stability from level of activity,sample P was run at a higher W/F (0.625h,dotted line in Fig.6)so for thefirst couple of hours the le-vel of conversion was similar to that of DS1.However,after that, the conversion dropped much more rapidly than over DS1.There-fore,it appears that the desilicated samples exhibit not only a high-er activity,but also a higher stability.
Fig.7shows the variation of product yields as a function of con-version for varying time on stream at constant W/version decreasing as the catalyst deactivates.A significant change in prod-uct distribution upon desilication is clearly evident.While the yield of aromatics greatly increased with conversion for all samples,an important difference is observed for the production of C4-9al-kane/alkene(mainly C5+)products compared to C3.While the light hydrocarbon fractions(C1-2and C3)follow a similar trend to that of aromatics,that is,increasing with propanal conversion,the C4-9 alkane/alkene fraction exhibits a maximum at intermediate prop-anal conversions.Products in the C4-9fraction are converted to C1-3and aromatics as propanal conversion increases.With increasing desilication severity,the yields of C1-2,C3,and aromat-ics decrease while the yield of C4-9alkane/alkene increases.Table4 shows the product distribution for all samples at a propanal con-version of$90%.It is evident that desilication causes a decrease in the fraction of C1-2hydrocarbons,toluene,and p-xylene,but an increase in C4-9alkane/alkene and C10+aromatics.A noticeable difference in the product distribution observed in this study com-pared to studies conducted at lower temperatures is the lack of products containing oxygen.Even at the lowest conversion levels, no oxygenates other than unconverted propanal were observed in significant amounts.
3.2.2.Effect of varying space time(W/F)
The product distribution as a function of W/F was compared for the parent sample(P),the mildly desilicated sample(DS1),and the highly desilicated sample(DS4).Fig.8shows the change in prop-anal conversion with increasing W/F,which is higher for DS1and DS4than for P over the entire W/F range.As discussed later,the higher activity of DS1may be associated with rapid transport of propanal into the micropore channels.Fig.9shows the variation
Table4
Product distribution of propanal conversion over parent and desilicated HZSM-5with
a propanal conversion of90%.
Product yield(mol.%)P DS1DS2DS3DS4DS4-Si a
Non-aromatics
C1-2  6.8  4.8  4.0  3.7  2.6  2.8
C322.022.021.020.218.622.1
C4-921.823.030.228.934.934.4
Aromatics
Benzene0.40.6  1.00.9  1.00.9
Toluene8.68.7  6.3  6.3  5.3  5.7
C8aromatics
Ethylbenzene  1.8  2.0  1.4  1.4  1.2  1.3
m-Xylene12.810.3  6.87.0  5.5  5.8
p-Xylene  1.0  2.0  2.8  2.3  2.5  3.0
o-Xylene0.30.80.80.70.70.8
C9aromatics
Propyl-benzene0.80.80.80.80.80.7
Methyl-ethyl-benzene7.67.87.27.47.07.3
Trimethyl-benzene  1.6  2.7  3.3  2.2  2.1  2.4
C10+aromatics  5.3  6.5  5.87.17.7  2.2
P
aromatics40.242.236.236.133.830.3
a DS4surface was deposited with SiO
2
to passivate the strong acid sites on the
surface.Reaction conditions:T=400°C,W/F=0.5h.
X.Zhu et al./Journal of Catalysis271(2010)88–9893

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