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Impact of Al and Ti ions on the dispersion and performance

Impact of Al and Ti ions on the dispersion and performance

Impact of Al and Ti ions on the dispersion and performance

Impact of Al and Ti ions on the dispersion and performance of supported

NiMo(W)/SBA-15catalysts in the HDS and HYD reactions

A.Olivas,T.A.Zepeda*

Centro de Nanociencias y Nanotecnolog?′a-UNAM,Km.107Carretera Tijuana-Ensenada,CP.22800,Ensenada,B.C.Me′xico,Mexico

1.Introduction

Today the environmental regulations on the sulfur and

aromatics content of diesel fuels are leading re?ners to develop

novel catalysts for the production of ultra-low sulfur diesel.In

consideration of this for2009the sulfur contents from diesel

fractions need to be lower than10ppm and the cetane number

higher than51[1–3].The molybdenum-based hydroprocessing

catalysts promoted by Co and Ni are usually supported on g-Al2O3

[4].Unfortunately these catalysts in the hydroprocessing of the

heavy petroleum fractions,especially in the Maya-Mexico oil type

complex fractions,are not effective enough to satisfy the new

environmental legislation.The need to produce clean engine fuel

combined with the prospect of processing poor-quality feedstock

petroleum,has stimulated the interest in the synthesis of novel

effective catalysts for deep hydroprocessing of diesel and gas–oil

[5,6].

Current strategies for the design of novel HDS catalysts often

include variations in the preparation method,active phase

formulation and the variation or modi?cation of the support[7].

Many new materials have been chosen as support for the

preparation of Mo(W)active component and Co or Ni promoters

[7].For this reason,the use of mesoporous silicate materials such

as HMS,MCM-41and SBA-15,which have large pore diameters

and higher surface areas than the Al2O3[7–9],have attracted

widespread attention as new potential materials for the prepara-

tion of novel supported hydrotreating catalysts[7–10].Several

reports have been made to develop high performance hydrotreat-

ing catalysts supported on heteroatoms-containing(Ti,Al,Zr,etc.)

mesoporous silicate materials[11–13].In particular,the Al and Ti-

containing SBA-15material has thicker pore walls and acceptable

hydrothermal stability[12].This is an important characteristic for

the preparation of hydrotreating catalysts.Important works have

been reported on the potential of the SBA-15material by the

preparation of hydroprocessing catalysts.Vradman et al.[14]

reported the excellent performance of NiW/SBA-15catalyst in the

deep hydrodesulfurization(HDS)of petroleum feedstock’s.Murali

Dhar et al.[15]studied the hydrodesulfurization of thiophene(TP)

over Mo,CoMo and NiMo catalysts supported on SBA-15material.

The authors reported higher activities of the all supported-SBA-15-

catalysts compared with a traditional alumina-supported catalyst,

and the superior activity was observed in the CoMo catalysts.

Sampieri et al.[16]studied the effect of Mo loading supported on

SBA-15and MCM-41materials.The authors observed superior

performance on the overall HDS of dibenzothiophene(DBT)in the

SBA-15supported catalysts in comparison with MCM-41sup-

ported ones.Nava et al.[12]studied the effect of Ti-loading CoMo/

SBA-15catalysts on the overall activity in the HDS of DBT.The

authors reported that the Ti-containing catalysts were more active

than the Ti-free counterpart and traditional alumina-supported

catalyst.Recently Murali Dhar et al.reported the effect of Al[17]

and Zr[18]loading on the HDS of TP and hydrogenation(HYD)of

Catalysis Today143(2009)120–125

A R T I C L E I N F O

Article history:

Available online21October2008

Keywords:

Hydroprocessing catalysts

NiMo and NiW catalysts

Hydrodesulfurization

Hydrogenation

SBA-15material

Al and Ti effect

Acidity effect on the dispersion

A B S T R A C T

The effect of the aluminum and titanium presence on the catalytic activity of the NiMo/SBA-15and NiW/

SBA-15catalysts was evaluated in the hydrodesulfurization of dibenzothiophene and biphenyl

hydrogenation.The samples were characterized by SP MAS29Si NMR,S BET,XRD,chemisorption of H2

and TPD-NH3.The Ti-containing samples showed higher catalytic performance in both hydroprocessing

reactions,in comparison with the Al-containing samples.The presence of Ti ions enhanced the dispersion

of the supported active components,and the dispersion of the active species was related to the acidity of

the supports.Finally,the W based catalysts showed higher activity in the hydroprocessing reactions than

the traditional Mo based catalysts.

?2008Elsevier B.V.All rights reserved.

*Corresponding author.Tel.:+526461744602;fax:+526461744603.

E-mail address:trino@icp.csic.es(T.A.Zepeda).

Contents lists available at ScienceDirect

Catalysis Today

j o u r n a l h o m e p a g e:w w w.e l s e v i e r.c o m/l o ca t e/ca t t o d

0920-5861/$–see front matter?2008Elsevier B.V.All rights reserved.

doi:10.1016/j.cattod.2008.09.001

cyclohexene(CH)over of Mo,CoMo and NiMo catalysts supported on SBA-15material.The Zr-loading catalyst manifested superior activity in HDS of TP,while the Al-loading catalyst was more active in the HYD of CH,and these samples were more active than traditional alumina-supported catalyst.Superior activity in both HDS and HYD reactions was observed over NiMo catalysts,in comparison with Mo and CoMo catalysts.Probably the use of heteroatoms-containing SBA-15materials could be a candidate for newer more active and effective supported hydrotreating catalysts than the typical industrial supported Co(Ni)Mo/g-Al2O3.In this work we report the effect of Al and Ti-load on the catalytic activity of NiMo/SBA-15and NiW/SBA-15catalysts in the HYD of biphenyl (BP)and the HDS of DBT.The samples were characterized by SP MAS29Si NMR,S BET,XRD,chemisorption of H2and TPD-NH3.

2.Experimental

2.1.Synthesis of supports and catalysts

The SBA-15,Al-SBA-15and Ti-SBA-15supports(with an atomic ratio of Si/Al(Ti)=40)were synthesized by following a similar procedure to that described by Zhao et al.[19]and Yue et al.[20]. The incorporation of Al and Ti ions into the mesostructure SBA-15 was carried out by the direct-synthesis method using aluminum isopropoxide(98%,Aldrich)and titanium butoxide(97%,Aldrich) as the Al and Ti source following the published procedure[20].The solid product obtained was?ltered,dried at room temperature for 24h and then at393K for3h,and?nally calcined in air at773K for 6h with a heating rate of18C/min.

The catalysts were prepared by simultaneous impregnation method using the pore?lling method source following the published procedure[9].The samples were prepared with the same atomic loading of the Mo and W elements,taking into account the difference of the atomic weight between these atoms. The loading of the Mo(W)and Ni species were determinate by ICP and they are reported in Table1.

2.2.Characterization methods

The samples were characterized by solid-state single pulse(SP) mass atomic spectra(MAS)nuclear magnetic resonance(29Si NMR) measurements,the textural properties of the supports and catalysts,the X-ray patterns,the metal dispersion and acidity of the pure supports.The experimental details are reported else-where[21–22].

2.3.Catalytic activity measurements

Before of the reaction,the catalysts were activated by sul?dation performed in a U-shaped glass?ow reactor.First,the sample was?ushed in a nitrogen?ow,gradually increasing the temperature to423K and then switching to the sul?dation mixture(H2/H2S10vol.%H2S)at a?ow of60mL minà1and increasing the temperature to673K,reaching this temperature within2.5h and maintaining this temperature for2h.

The catalytic activity in the reaction of HYD of BP and HDS of DBT were measured in a batch Parr reactor.The experimental details are reported elsewhere[13].

3.Results and discussion

The29Si NMR spectra of the calcined SBA-15,Al-SBA-15and Ti-SBA-15supports are shown in Fig.1.All spectra showed three peaks observed close toà92,à100and atà110ppm.The peak center atà92ppm corresponds to the resonance associated with Q2species and the peaks atà100andà110were assigned to the resonances corresponds to the Q3and Q4species,respectively[23]. The Q4,Q3and Q2resonances are assigned to Si(OSi)4,Si(OSi)3OH and Si(OSi)2(OH)2species,respectively[23,24].In previous report [22]it was compared the29Si NMR spectra of Ti ions containing varying the Ti content into HMS framework(the samples

Impact of Al and Ti ions on the dispersion and performance

were Fig.1.29Si-Nuclear magnetic resonance spectra of the pure supports.

Table1

Metal composition and Textural properties of the supports and catalysts and H2-chemisorption data of the catalysts.

Samples Mo a

(mmol g supportà1)W a

(mmol g supportà1)

Ni a

(mmol g supportà1)

S BET

(m2/g)

APD

(nm)

APV

(cm3/g)

H2uptake b

(m mol gà1)

SBA-15–––1167 5.6 1.91–Al-SBA-15–––1083 5.5 1.87–Ti-SBA-15–––971 5.4 1.74–NiMo/SBA-150.98–0.53598 5.2 1.1869.3 NiMo/Al-SBA-15 1.02–0.54698 5.3 1.2978.2 NiMo/Ti-SBA-150.99–0.54672 5.3 1.1184.3 NiW/SBA-15–0.970.53515 4.0 1.3275.3 NiW/Al-SBA-15– 1.020.52621 4.1 1.0987.1 NiW/Ti-SBA-15–0.980.53587 4.70.9792.2

a Determined by ICP.

b Chemisorption capacity as determined by pulse method of H

2chemisorption in the oxidized catalysts.

A.Olivas,T.A.Zepeda/Catalysis Today143(2009)120–125121

prepared with a Si/Ti molar ratio equal to that of our samples).A dramatic increment in the amount of Q4,Q3and Q2species was observed in all Ti-containing samples.This observation was attributed to the formation of a cross-linked framework by the presence of Ti ions.Therefore,we could expect in our samples higher values of Q4,Q3and Q2in the Al and Ti-containing samples than those for the SBA-15sample,and indeed this happened.This broadening could be attributed to the effect of the heteroatom sites on the chemical environment of the adjacent Si atoms.Indeed,the amount of Q4,Q3and Q2species are incremented strongly in the presence of Al and Ti ions,but the increment of the Q2resonance intensity was higher than the Q3value,and the increment of the intensity of both,Q2and Q3,was higher than the increment of the Q4resonance http://www.wendangku.net/doc/c03427db9b6648d7c0c746a6.htmlparing the29Si NMR spectra of the pure SBA-15sample with the Al and Ti-containing SBA-15samples, we can note that the increment on the intensities of Q4,Q3and Q2 resonances is higher in the Ti-containing sample than those of the Al-SBA-15sample.In agreement with previous report[22],we could take the Q4/(Q2+Q3)intensity ratio to have an idea on the incorporation of Al and Ti ions into the silica framework.As it was expected,this ratio is lower in the Al and Ti-containing samples in comparison with the pure SBA-15sample,but the lowest ratio was observed in the Ti-containing sample.Then we could conclude that the Al and Ti ions were incorporated into of SBA-15material due to the modi?cation in the chemical environment of the adjacent Si. Also,a higher effect of Ti ion incorporation was observed as compared to the effect of Al ion incorporation.

The nitrogen adsorption isotherms of the pure supports and catalysts(data not shown here)were of type IV isotherms,according to the IUPAC classi?cation[25].All samples showed an appreciable type H1hysteresis loops indicating the presence of textural mesopores and cylindrical pores[26].The textural properties,such as surface area(S BET),average pore diameter(APD)and average pore volume(APV)are summarized in Table1.In this table,we can note that the pure SBA-15support shows the highest speci?c area (1167m2gà1).The speci?c area of the supports follows the order: SBA-15>Al-SBA-15>Ti-SBA-15.This means that the introduction of Al or Ti ions into the SBA-15material modi?es the speci?c area. Also,both APD and APV decreases in the presence of heteroatoms-containing,the last order is also valid for these two parameters.

The S BET speci?c area of the supports decreased upon incorpora-tion of the NiMo or NiW phases(Table1).Independently of the Al and Ti content,we can note higher surface area values in the NiMo samples in comparison with the NiW samples.The S BET values of supported NiMo catalysts follow the order:NiMo/SBA-15

The X-ray patterns recorded in the range5–608in2u of the supported NiMo and NiW catalysts are shown in Fig.2.The wide-angle X-ray patterns of all the samples exhibited a broad line between158and358,which was attributed to the siliceous amorphous phase[11–13].All NiMo samples show an intense re?ection peak at22.782u overlapping with the broad diffraction peaks of the amorphous part of the support,which is characteristic at crystalline of MoO3phase(JCPDS card1-076-1003).An additional intense re?ection peak at25.682u was observed in the NiMo/SBA-15sample and this re?ection is attributed to the presence of the b-NiMoO4phase(JCPDS card21-0868).One can note that the intensities of the re?ections observed in the NiMo/SBA-15sample are higher in comparison with both,Al and Ti-containing NiMo samples.The absence of the re?ection characteristic to the b-NiMoO4phase in both,the Al and Ti-containing NiMo samples, could indicate that the presence of Ti and Al ions inhibit the formation of this phase,and it is dif?cult to sul?de under the conditions employed in this work.Contrary to the NiMo samples, the NiW samples showed a small re?ection peak at23.582u associated with the presence of WO3phase.Newly,we can note that the free-heteroatoms sample(NiW/SBA-15)exhibits the more intense re?ection peak,in comparison with the Al and Ti-containing samples.Also,in general we can note that the intensities of the re?ections are higher in the NiMo samples than that in the NiW samples.We could conclude that the presence of Al and Ti ions enhances the dispersion of the supported species,and superior dispersion of the active components could be in the NiW samples,in comparison with the NiMo samples.This was corroborated by H2-chemisorption data.The H2-chemisorption data of the catalysts are shown in Table1.We can note superior H2-chemisorption in

Impact of Al and Ti ions on the dispersion and performance

the Fig.2.X-ray patterns correspond to the supported NiMo catalysts(a)and supported NiW catalysts(b).

A.Olivas,T.A.Zepeda/Catalysis Today143(2009)120–125

122

NiW based samples in comparison with the NiMo samples,in agreement with DRX results.Also,independently of the phase type (NiMo or NiW),we can note lower H 2-chemisorption in the free-heteroatoms samples (NiMo/SBA-15and NiW/SBA-15)and higher H 2-chemisorption in the Ti-containing samples (NiMo/Ti-SBA-15and NiW/Ti-SBA-15).Our results could indicate that the presence of Ti ions into of SBA-15sample enhances the dispersion of the supported transition metals.These results are opposed to our previous study on the dispersion of transition metals supported on Al and Ti-containing HMS system [13].This could be related to the structure type of the substrate (HMS and SBA-15)and the formation the different surface Al and Ti species on the different substrates.With our results we cannot explain this point,but more experi-ments are necessary to clarify this question.

The TPD-NH 3pro?les of the pure supports are shown in Fig.3.The concentrations after ?tting of the experimental curves to sum of Gaussian lines of the weak (T <500K),medium (500K 700K)strength acid sites,expressed as mmole of NH 3desorbed are summarized in Table 2.The total acidity of the samples was calculated as the sum of the weak,medium and strong acid sites.One can note that the SBA-15support shows weak and medium acid sites with the main contribution of weak type.The Al and Ti-containing SBA-15supports show the presence of weak,medium and strong acid sites,with the main contribution of medium type.We can note that the Ti-containing sample has higher acidity medium type,while acidity strong type is similar to the Al-containing sample,but both medium and strong acidity sites contribute on the total acidity.The higher total acidity was observed in the Ti-containing sample,in comparison with the other samples.In agreement with Del Arco et al.[27],the medium and strong acidity could be associated with the presence of Br?nsted acidity sites.Then

in agreement with the last author and with our previous report [21],the presence of Al and Ti ions into of SBA-15material enhances the population of Br?nsted acidity sites.We found a tendency between the supports acidity and the dispersion of the supported species;this tendency is shown in Fig.4.Then,it is possible to conclude that the dispersion of the transition metals in SBA-15system depends strongly on the substrate acidity,and speci?cally the dispersion of the supported metals could be involved with the medium and strong acidity sites,this in agreement with Del Arco et al.[27].

We tested the samples in both reactions,HYD of BP and HDS of DBT in batch reactor.The HYD and HDS reactions were carried out under the same reaction conditions.The rate constants normalized by surface area for these two reactions are shown in Table 3.The product distributions to 50%of conversion by the HYD of BP and HDS of DBT are shown in Figs.5and 6,respectively.The reaction products by the HYD of BP were dicyclohexyl (DCH)and cyclohexylbencene (CHB),while the

Impact of Al and Ti ions on the dispersion and performance

tetra-hydro-dibenzothio-Fig.3.TPD-NH 3pro?les and the Gaussian deconvolution lines of the calcined samples.

Table 2

TPD-NH 3acidity of the pure supports.Sample

Strength of acid sites a (mmol NH 3g cat à1)Weak <500K

Medium 500–700K Strong >700K Total SBA-150.250.07–0.32Al-SBA-150.100.180.110.39Ti-SBA-15

0.05

0.30

0.10

0.45

a

Calculated from amount of desorbed NH 3by TPD

Impact of Al and Ti ions on the dispersion and performance

pro?les.

Fig.4.Correlation between the acidity of the pure supports with the dispersion of the supported species.

Table 3

Rate constants for the HYD a of BP and HDS a of DBT.Samples

k (HYD)?109

[mol DBT s

à1

m à2

]

k (HDS)?109

[mol DBT s à1m à2]NiMo/SBA-157.8 2.2NiMo/Al-SBA-158.4 3.3NiMo/Ti-SBA-159.2 4.1NiW/SBA-158.1 3.8NiW/Al-SBA-1510.6 5.7NiW/Ti-SBA-1513.87.7NiW/g -Al 2O 3

20.4

9.2

a

In batch reactor carried out for 8h at 598K under hydrogen pressure of 5.5MPa.

A.Olivas,T.A.Zepeda /Catalysis Today 143(2009)120–125123

phene (THDBT),biphenyl,DCH and CHB were the reactions products by the HDS of DBT.We could take the DCH/CHB and (THDBT +DCH +CHB)/BP ratios as measurements of the hydro-genation ability for the HYD and HDS,respectively.The NiW samples showed higher hydrogenation ability in both,HYD and HDS reactions,in comparison with the NiMo samples,but the predominant reaction pathway for the HDS of DBT,in all samples,was via direct desulfurization.Also,independently of the reaction and the active phase (NiMo or NiW),the Ti-containing samples showed superior hydrogenation ability,while the heteroatoms-free samples showed the lower hydrogenation ability.Then,we could conclude that the Ti and Al ions enhance the hydrogenation ability of the samples,but the higher hydrogenation performance is in the presence of Ti ions.Independently of the reaction (HDS or HYD),the hydrogenation ability of the samples could be associated with the acidity properties [27,28].An interesting correlation between the total acidity of the samples with the hydrogenation ability in both HYD and HDS reactions is shown in Fig.7(a)and (b),respectively.Analyzing these ?gures we could assume that the ring saturation ability could be related to the acidity of the samples.The W loaded samples showed higher saturation ability of the ring molecules [27].This observation is consistent that the active W sites are characterized for having a strong acid,and this was observed with our measures of the acidity [27,28].Also in agreement with our previous reports [29]the acidity of the samples could be related to the ability by the heterolitic hydrogen dissociation,involving the hydrogen dissociated in the ring saturation of the samples [29].Then,we could conclude that the acidity of the samples has a strong effect on the hydrogenation ability of the samples.

Finally,we found that the catalytic performance,in both reactions,is related to the dispersion of the active components.Fig.8shows a correlation between the dispersion of the supported metals (for H 2-chemisorption data)with the catalytic activity in both,HYD and HDS reactions.Then,in agreement with our results and with our previous reports [11,13,22],we could conclude that the dispersion of the active components has a strong effect on the response of the catalysts in the tested reactions.

Analyzing the rate constants shown in Table 3,again we can note that,independently of the reaction,the NiW samples showed higher catalytic performance,in comparison with the NiMo samples.Newly,the Ti-containing samples showed superior catalytic activity in comparison with the other samples.In general,the hydrogenation activity was approximately 2.6times more active than the hydrodesulfurization activity,this could be related to the formation of H 2S during the HDS reaction,and the presence of this gas could be inhibiting the active sites [21].For comparison,in Table 3,we included the reactions constant (normalized

Impact of Al and Ti ions on the dispersion and performance

by

Fig.5.Products distribution by the HYD of BP to 50%of

Impact of Al and Ti ions on the dispersion and performance

conversion.

Fig.6.Products distribution by the HDS of DBT to 50%of

Impact of Al and Ti ions on the dispersion and performance

conversion.

Fig.7.Correlation between the acidity of the samples with the catalytic hydrogenation ability in both,HYD of BP (a)and HDS of DBT (b)reactions.

A.Olivas,T.A.Zepeda /Catalysis Today 143(2009)120–125

124

surface area)for the HYD and HDS reactions over the reference NiW/g -Al 2O 3sample,test under same reaction http://www.wendangku.net/doc/c03427db9b6648d7c0c746a6.htmlparing our reaction rate constants normalized by surface area with those of the reference sample,we could note superior performance for the reference sample.Nevertheless,is valid to assume that we could optimize the load of the active components to that of our samples having between 2.5and 3times more surface area than that of the reference one,and obtain supported NiMo or NiW catalysts on the mesoporous SBA-15material with superior catalytic performance than the reference one.4.Conclusions

The introduction of Ti ions into the framework of the SBA-15material results in complete incorporation in comparison with the Al ions.Incorporation of Al and Ti ions leads to signi?cant changes in the catalytic properties of the catalysts in both,HYD and HDS reactions enhance the hydrogenation ability of the samples,and this hydrogenation ability was related to the acidity of the samples.Superior performance and hydrogenation ability were observed on the Ti and W content samples.Finally,the catalytic performance of the samples was related to the dispersion of the supported metals,and the dispersion of the supported metals was related to the acidity properties of the supports.

Acknowledgements

T.A.Zepeda is grateful to CNyN-UNAM Me

′xico for ?nancial support.The authors are grateful to Eric Flores and Eloisa Aparicio for the valuable technical assistance.References

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Fig.8.Correlation between the dispersion of the supported metals (H 2-chemisorption data)with the catalytic activity in both,HYD of BP and HDS of DBT reactions.

A.Olivas,T.A.Zepeda /Catalysis Today 143(2009)120–125125