Transformation of 1-butene over AlSBA-15 synthesized at different conditions of pH and hydrolysis

This article appeared in a journal published by Elsevier. The attachedcopy is furnished to the author for internal non-commercial researchand education use, including for instruction at the authors institution

and sharing with colleagues.

Other uses, including reproduction and distribution, or selling orlicensing copies, or posting to personal, institutional or third party

websites are prohibited.

In most cases authors are permitted to post their version of thearticle (e.g. in Word or Tex form) to their personal website orinstitutional repository. Authors requiring further information

regarding Elsevier’s archiving and manuscript policies areencouraged to visit:

http://www.elsevier.com/copyright

http://www.elsevier.com/copyright

Author's personal copy

Applied Catalysis A: General 380 (2010) 118–124

Contents lists available at ScienceDirect

Applied Catalysis A: General

journa l homepage: www.e lsev ier .com/ locate /apcata

Transformation of 1-butene over AlSBA-15 synthesized at different conditions ofpH and hydrolysis

V. Sazoa,∗, C.M. Lópeza, G. Gonzalezb, A. Arreguia, M.E. Gomesb, C. Urbinac

a Escuela de Quimica, Fac. Ciencias, Centro de Catálisis, Petróleo y Petroquímica, Universidad Central de Venezuela, Caracas 1020-A, Venezuelab Lab. Ciencia e Ing. Materiales, Dpto. Ing. Instituto Venezolano de Investigaciones Cientificas, Caracas 1020-A, Venezuelac Centro de Microscopía Electrónica, Fac. Ciencias, Universidad Central de Venezuela, Caracas 1020-A, Venezuela

a r t i c l e i n f o

Article history:Received 14 October 2009Received in revised form 15 March 2010Accepted 19 March 2010Available online 25 March 2010

Keywords:AlSBA-151-ButeneSkeletal isomerizationSynthesisHydrolysis

a b s t r a c t

In the present work a detailed study of the effect of pH, in the range of 0–4.5, over the mesostruc-ture of SBA-15 and catalytic transformation of 1-butene has been carried out. Also, the effect of silicasource hydrolysis on the Al incorporation at different pH was studied varying the order of addition ofan aluminium salt, before and after hydrolysis. The samples were characterized by SEM, TEM, ICP andN2 adsorption. The materials showed a highly ordered hexagonal mesostructure for all the pH rangestudied. A decrease in particle size with increase of pH was observed. Al incorporation was favoured aspH increased; the highest incorporation was obtained at pH 4.5. An important effect of hydrolysis on Alincorporation was observed depending of metal incorporation. Hydrolysis had also an effect on the dis-tribution and strength of the acid sites. The materials with the highest skeletal isomerization selectivitywere those synthesized at pH 4.5.

© 2010 Elsevier B.V. All rights reserved.

1. Introduction

The synthesis, functional modification and catalytic activity ofSBA-15 silica mesoporous materials have been matter of interest inthe last years [1–8]. Stucky and co-workers [1,2] synthesized by thefirst time SBA-15 using amphiphilic triblock copolymers templatesunder acidic conditions to direct the organization of polymerizingsilica species. This group also prepared these materials using syn-theses with and without small amounts fluoride ion over a widerange of pH conditions (pH 0–9) [3,4]. They concluded that hexag-onal mesoporous materials are obtained up to pH < 4 with andwithout fluoride. For materials prepared at pH > 4 without F−, dis-ordered products were obtained however the ordered hexagonalstructure persisted up to pH 9 using F−. The successful incorpo-ration of Al in SBA-15 has not been extensively reported. Camposet al. [9] showed that only a very minor portion of the aluminiumspecies present in the reaction were incorporated into the silicaframework, predominantly in the form of extra framework speciesfor the samples prepared with the addition of fluoride. Li et al. [10]employed a two-step approach (sol–gel reaction at low pH) fol-lowed by crystallization at high pH for the synthesis of AlSBA-15and reported that all aluminium species were incorporated intothe silica framework for the samples prepared with the addition of

∗ Corresponding author. Tel.: +58 212 6051266; fax: +58 212 6934977.E-mail addresses: [email protected], [email protected] (V. Sazo).

fluoride. Kao et al. [11] reported high Al incorporation using a post-synthesis alumination method. Other authors [12,13] synthesizedAlSBA-15 by adjusting the synthesis gel pH below the isoelectricpoint of silica (pH ∼ 2). The solid obtained was active to the acid-catalyzed isopropylation of m-cresol and cumene cracking, despitethe low Al incorporation due to the high solubility of aluminiumsource (aluminium isopropoxide) in the acid medium. The effect ofpH and hydrolysis on the aluminium incorporation has been verylittle reported. In the present work we have studied the effect ofpH on the mesostructure of SBA-15 and the effect of hydrolysis onthe Al incorporation by direct synthesis. The structural Al incorpo-ration and dispersion into the mesoporous silica was evaluated bycatalytic test of 1-butene conversion. The materials were character-ized by scanning electron microscopy (SEM), transmission electronmicroscopy (TEM), N2 adsorption at 77 K and elemental chemicalanalysis by ICP-OES.

2. Experimental

2.1. Synthesis

Triblock copolymer (EO20PO70EO20) (Pluronic P123, MW = 5800,BASF) was used as template. Tetraethoxysilane (TEOS, Aldrich) andAl(NO3)3·9H2O from Fischer were used as silica and aluminiumsources, respectively. The SBA-15 materials were synthesized fol-lowing the Zhao et al. [1,2] procedure, but modifying the pHsynthesis gel to obtain the AlSBA-15 materials. The EO20PO70EO20

0926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved.doi:10.1016/j.apcata.2010.03.035


V. Sazo et al. / Applied Catalysis A: General 380 (2010) 118–124 119

was dissolved in de-ionized water under continuous agitation for12 h at room temperature; acid solution of HCl (12 M) was addeduntil pH zero was reached. Then, TEOS was added under continu-ous agitation. Al incorporation was performed by adding a certainamount of Al(NO3)3·9H2O dissolved in water to the synthesis gelbefore TEOS hydrolysis (BH) or after TEOS hydrolysis (AH), to obtaina Si/Al ratio = 30. The variation of pH (0–4.5) was carried out 1 hafter the silica hydrolysis began with NH4OH solution. Hydrother-mal synthesis was performed under continuous stirring for 48 h at363 K in a closed vessel immerses in a thermostatic bath. The solidproducts obtained were filtered, thoroughly washed with distilledwater and dried at 333 K for 12 h. Calcinations were carried out at793 K for 6 h under an air flux with a heating rate of 1 K/min toremove the template.

2.2. Characterization

The calcined solids were characterized by N2 adsorption–desorption isotherms measured at 77 K on a Micromeritic ASAP2010, previously degassing of the samples was performed at 593 Kfor 4 h. The specific surface areas (SSA) of the samples were cal-culated using the BET method, the mesopore size distribution wasobtained using the BJH method. Samples were analyzed by scan-ning electron microscopy (SEM) using a Philips XL30 and a JEOLEPMA LX, both attached with EDS spectroscopy and by transmis-sion electron microscopy (TEM) in a Philips CM-120 operating at100 kV and in a Tecnai F20 operating at 200 kV. Elemental chem-ical analysis was obtained by inductively coupled plasma-opticalemission spectroscopy (ICP-OES).

2.3. Catalytic activities

The catalytic reaction of 1-butene was performed in a con-tinuous fixed-bed reactor operated at atmospheric pressure. Thecatalysts (0.5 g) were pretreated in situ at 773 K in a N2 flow(60 ml (STP)/min) for 2 h, and the reaction was performed at 743 Kusing a N2 flow of 15 ml/min and 1-butene (99% matheson) flow of4 ml/min. The reaction products were analyzed by on line gas chro-matography, using a Hewlett-Packard 5890 chromatograph with aflame ionization detector. A fused silica KCl/Al2O3 column was usedfor separation purposes.

The total conversion (X) of 1-butene was calculated accordingto Eq. (1)

Xi ={

(˙Ai) − A1-butene

˙Ai

}× 100 (1)

were Ai is the corrected chromatographic area with correspondingresponse factor for a particular compound. For a reaction product,or group of products, the selectivity is defined by

Si ={

Ai

(˙Ai) − A1-butene

}× 100 (2)

The following selectivity were determined: Sisobutene (skeletalisomerization), S2-butene (double-bond isomerization), S<C4 (forma-tion of C1–C3 products), S>C4 (formation of products with 5 or morecarbon atoms) and SC4◦ (butane and isobutene).

The skeletal isomerization efficiency (SIE) is defined by Eq. (3)

SIE ={

Aisobutene

(Aisobutene + ACis2−but. + ATrans2-but.)x0.467

}× 100 (3)

The 0.467 term corresponds to the isobutene equilibrium frac-tion at 743 K [14]. SIE was used to evaluate the thermodynamiccontrol on the production of isobutene at the reaction temperature.

3. Results and discussion

3.1. Characterization

Fig. 1 shows the remarkable change of siliceous SBA-15 parti-cle morphology by SEM for the different pH used. The formation ofmacropores and the reduction of particle size as pH increases can beobserved. Fig. 2 shows TEM images for pH 0, 1.5 and 4.5, preserva-tion of the long range hexagonal ordered mesopores is maintainedwith the change in pH, and the significant reduction in particle sizefor high pH is evident. There are several parameters that influencethe hydrolysis and condensation reactions: activity of metal alkox-ide, water/alkoxide ratio, pH, temperature and nature of the solvent[15]. The microstructure and surface chemistry of materials are verysensitive to variation of these parameters.

Figs. 3 and 4 show the N2 adsorption isotherms and BJH poresize distribution of the materials synthesized at the different pHwithout Al and the effect of hydrolysis on Al incorporation at thedifferent pH, respectively. The pore size distribution calculated

Fig. 1. SEM images of SBA-15 synthesized at different pH. (A) pH 0, (B) pH 1.5, (C) pH 3, (D) pH 4.5


120 V. Sazo et al. / Applied Catalysis A: General 380 (2010) 118–124

Fig. 2. TEM images of SBA-15. (A) pH 0, (B) pH 1.5, (C) pH 3, (D) pH 4.5

by BJH model for each material is presented on the right side.All isotherms are type IV, and exhibit hysteresis loops with welldefined adsorption and desorption branches. The pore size distri-butions (BJH) show a decrease in pore size as pH increases (Fig. 3).Some differences in the shape of the hysteresis loop and in poresize distribution for each pH condition are observed. This varia-tion is attributed mainly to changes in the pore morphology andpore size distribution. At pH zero the isotherm shows a H1-typehysteresis loop [16] indicative of open cylindrical mesopore witha narrow pore size distribution. This is consistent with the typicalwell ordered cylindrical channel forming a hexagonal arrangementin SBA-15. As pH increases, the shape of the loop changes to H2or H3 [16], suggesting a more random distribution of pores andprobably an interconnected pore system.

The hysteresis loops show a step-wise desorption isotherm, sug-gesting the presence of energetically different sites, consistent witha wider pore size distribution. The adsorption branches changeswith increase in pH from a very sharp adsorption at pH 0, locatedat relative pressures of 0.75 to a soft increase adsorption branchshifted to lower relative pressures around 0.6–0.65. At pH 0 a nar-row pore size distribution with a mean value of 9.6 nm from theadsorption branch of the nitrogen isotherms using the BJH model.This siliceous material has a BET surface area of 737 m2/g and a

pore volume of 1.135 cm3/g (Table 1). These values are higher thanthose reported by Zhao et al. [2]. As pH increased a bimodal poredistribution is observed.

As shown in Table 1, the Dp(BJH) calculated by desorptionbranches are different at that obtained by adsorption branches, apossible cause for this difference, would can be the sinuosity ofthe cylindrical pores of SBA-15 and AlSBA-15 materials [17]. Theeffect of Al incorporation is shown in Fig. 4. The N2 desorptioncurves shows two inflections, the first desorption step is assignedto desorption of nitrogen from the open mesopores, the seconddesorption step at P/P0 ≈ 0.4 can be attributed to narrowing of themesoporous channels [18]. The Al species could be located on thepores surface or into the mesostructures, causing a decrease or anincrease in the pore size, respectively. As the pH increased to 1.5a non-uniform hysteresis loop is observed similar to a H2 type,characteristic of solids consisting of particles formed by nearlycylindrical channels of non-uniform size or shape [19]. A widepore size distribution is observed, BET surface area is 776 m2/gand a pore volume of 0.776 this indicates that pH is affecting themesostructure. These two values of pH (0 and 1.5) are below thesilica isoelectric point. At pH 3 and 4.5, above the silica isoelec-tric point a wider pore size distribution is obtained with a meanmesopore size of 5.5 and 5.6 nm (by BJH).

Fig. 3. SBA-15 synthesized at different pH: (A) N2 adsorption isotherms for SBA-15 and (B) BJH pore size distribution. (©) pH 0, (�) pH 1.5, (�) pH 3, (♦) pH 4.5.



Fig. 4. Hydrolysis effect on Al incorporation in SBA-15 at different pH: (A) N2 adsorption isotherms; (B) BJH pore size distribution for before hydrolysis (BH) and afterhydrolysis (AH) conditions. (©) pH 0, (�) pH 1.5, (�) pH 3, (♦) pH 4.5.

The Al addition before and after hydrolysis (Fig. 4) has aneffect on the textural properties, at high pressures the adsorp-tion isotherm shows a sharp increase. The incorporation of Al forthese pH narrows and flattens up the desorption branch, becomingparallel to the adsorption branch similar to a H4 type, suggestingthe presence of solids consisting of aggregates of particles form-ing slit shape pores with uniform shape. As pH is increased thecondensation rate increases while the hydrolysis rate decreases[15]. However in the range of pH below 4, hydrolysis of TEOS isstill high compared to its condensation rate. At low pH, proto-

nated PEO chains are associated to cationic silica species throughweak electrostatic interactions mediated by negatively chargedchloride ions. Below the isoelectric point of silica, cationic silicaspecies will be present as precursors, and the assembly is expectedto proceed through an intermediate form of (S0H+)(X−I+). Theresults suggest that suitable oligomeric silica species, formed fromfully hydrolyzed monomeric silica species, are needed for interac-tion with the structure directing agent block copolymer to obtainordered mesostructure. This is due to competition between the con-densation of partially hydrolyzed silica species and the hydrolysis

Table 1Textural properties of SBA-15 and AlSBA-15 at different synthesis conditions.

Sample pH SSAa (m2/g) Vpb (cm3/g) V�p (cm3/g) %V�p

c Dpd (nm) Dp(BJH-Ads)e (nm) Dp(BJH-Des)

f (nm)

SBA-150

737 1.135 0.115 10 6.2 9.6 8.3AlSBA-15(BH) 938 1.151 0.180 16 4.9 8.3 6.3AlSBA-15(AH) 1012 1.184 0.174 15 4.7 6.6 5.6

SBA-151.5

776 0.776 0.131 17 4.0 5.5 4.6AlSBA-15(BH) 873 0.925 0.144 16 4.2 8.1 6.0AlSBA-15(AH) 942 0.991 0.163 16 4.2 8.5 6.3

SBA-153

763 0.716 0.141 20 3.8 5.5 4.4AlSBA-15(BH) 571 0.685 0.091 13 4.8 8.4 6.3AlSBA-15(AH) 827 0.785 0.171 22 3.8 6.3 5.0

SBA-154.5

809 0.956 0.083 9 4.7 5.6 4.5AlSBA-15(BH) 747 0.870 0.116 13 4.7 8.1 6.3AlSBA-15(AH) 814 1.136 0.087 8 5.6 9.6 7.3

a SSA = area (BET).b Vp = total pore volume.c %V�p = [V�p(t-plot)/Vp] × 100.d Dp = mean pore diameter (BET).e Dp(BJH-Ads) = adsorption pore diameter (BJH).f Dp(BJH-Des) = desorption pore diameter (BJH).



Fig. 5. TEM images of AlSBA-15(AH) pH 4.5. Clusters image and electron diffractionpatron.

of alkoxysilane moieties from incomplete hydrolysis due to relativerapid condensation compared to the rate of hydrolysis [15]. In thisenvironment the Al species can interact with the protonated PEOthrough weak electrostatic interactions and the interaction willchange the final location of the Al species if the process takes placebefore or after hydrolysis. Table 1 shows the variation of the texturalproperties and pore size, for the all the solids synthesized at the dif-ferent pH and with Al incorporation by direct synthesis before andafter hydrolysis of the silica source. The percentage contributionof micropores to total pore volume (Vp) it was increased with pHvalue and origins a diminution of mean pore diameter (Dp) for pH1.5–4.5 respect SBA-15 pH 0. AlSBA-15(AH) at pH 4.5 sample wasan exception, the microporosity has affected by presence of Al con-glomerated as shows by TEM image (Fig. 5). The mesopore diameter(Dp(BJH)) was increased to AlSBA-15 compared to SBA-15 for pH1.5–4.5, it may suggest a notable influence by aluminium species onmicro- and mesoporosity of AlSBA-15 on own synthesis conditions.

The incorporation of Al is strongly affected by the pH condition,as pH increases the Al content also increases. Table 2 shows the Alcontent for each pH, measured by ICP and EDS analysis.

The amount of Al detected before and after hydrolysis by ICPis very similar in all the materials, except for pH 3, where the Alincorporation is higher when added before hydrolysis. The resultshows experimental Si/Al ratios similar to Vinu et al. [12]. The val-ues obtained for EDS are higher than for ICP, this is due that ICPtakes the value of the total volume, while EDS was taken as theaverage value of the analysis in 10-crystals.

3.2. Catalytic activities

The catalytic activity for all the materials synthesized atpH 0 to 4.5, with Al incorporated by direct synthesis addedbefore (BH) or after (AH) hydrolysis, was tested in the 1-

Table 2Si/Al ratio determined by ICP and EDS.

Hydrolysis condition pH Si/Al (ICP) Si/Al (EDS)

Before 0 685 –After 0 371 –Before 1.5 332 48After 1.5 302 57Before 3 127 53After 3 245 82Before 4.5 99 26After 4.5 98 45

butene isomerization reaction as an indirect measurement of thedistribution and type of Brönsted acid sites. The 1-butene iso-merization is an acid-catalyzed reaction, the double-bond shiftand the skeletal rearrangement requires different type acidssites. Monomolecular isomerization mechanism was found to bemore common [20–22], 1-butene isomerizes to 2-butene iso-mers more rapidly than to isobutene (skeletal isomerization).Very strong acid sites lead to undesired side reactions. Bimolec-ular reactions such as dimerization–oligomerization, cracking,hydride transfer and coking can occur. Very weakly acidic sitesare not active to skeletal isomerization [20]. Moderated acid siteswill be more selective towards skeletal isomerization. The acidstrength required for these reactions decreases in the order: crack-ing ∼ oligomerization > skeletal isomerization � double-bond iso-merization [23]. Under evaluated reaction conditions the n-butenescan be dimerized to octenes. The C8 units can be cracked toshort chain olefins and the transformation of alkenes into alka-nes by hydrogen transfer is not discarded. When catalysts havehigh density of acid sites, the C8 units can react to produce C12units, oligomers and aromatics, which are the precursors of coke[22]. Over AEL catalysts (SAPO-11) a close parallelism betweenthe selectivity towards the skeletal isomerization and the numberof (moderate + strong) Brönsted acid sites measured by pyridineabsorption was observed [24,25], the crystal topology of thesesolids minimizes side reactions. Whereas over AlSBA-15 preparedby post-synthesis alumination [26] which showed mild acidity byNH3-TPD the cracking products selectivity reached 13%.

A blank experiment was carried out without catalyst in thereactor at same operation conditions that one with catalyst. Totalconversion (X) was 3.5%, it suggest that the contribution fromhomogeneous reactions is not appreciable under these reactionconditions. Fig. 6 shows the conversion (%) as a function of timeon stream (TOS) for all catalysts, it can be seen that both materi-als (BH and AH) exhibit similar activity. There was no deactivationover TOS investigated. The catalytic activity is related directly to pHmore than hydrolysis (Fig. 6), the highest is shown for the materialssynthesized with pH 4.5 (≈90%) consistent with lower experimen-tal Si/Al ratio. At pH zero, the most important conversion was 9%for AlSBA-15 (AH), the activity for BH material it was close to blankand the low conversion can be attributed at low Al quantity. It iswell known that the Brönsted acidity in zeolites is associated withOH groups in the vicinity of aluminium atoms in tetrahedral coor-dination [27–29]. As in the zeolites, the incorporation of acid sitesin mesoporous molecular sieves is carried out by substitution ofatoms of silicon and aluminium during mesophase formation. At

Fig. 6. 1-Butene conversion over AlSBA-15.



Fig. 7. Skeletal isomerization efficiency (SIE) as function of TOS at different pH andhydrolysis condition.

the respect, several authors [12,30–32] have shown that the Brön-sted acidity in mesoporous molecular sieves, such as AlMCM-41and AlSBA-15 is associated to aluminium atoms in tetrahedral coor-dination. Taking in consideration the catalytic activity and the Si/Alratio obtained by chemical analysis, it can be deduced that a certainamount of tetrahedral Al species in all catalysts has been locatedin accessible sites for the reaction to take place. At pH 1.5 (BH andAH) the Si/Al ratio is similar to pH 0 (AH), nevertheless the catalyticactivities are strongly increased at pH 1.5. It can be assumed thatat lower pH most of Al species locates into the internal structure ofthe material walls, leaving only a small amount in accessible sites.

When Al was incorporate after hydrolysis at pH 3, the conversionis lower than before hydrolysis, which can be attributed to the leastamount of Al in the solid. The AlSBA-15 TEM images showed that forthe samples synthesized at pH 4.5, when Al was added after hydrol-ysis, this was incorporated into the SBA-15 mesostructure formingsmall clusters. If these clusters are located in accessible sites on thesurface of solids, they will be important in the catalytic activity ofthe material. The hydrolysis condition at different pH determinesthe Al distribution and location of tetrahedral aluminium in thesematerials.

As observed, the least active catalysts for isomerization of 1-butene were AlSBA-15 pH 0 (BH and AH). The increase in pH at1.5 also increases the total conversion to both materials, besides topromote the formation of Brönsted acid sites responsible at the for-mation of skeletal isomers. The product distributions over AlSBA-15at pH 1.5 are mainly unimolecular reactions (double-bond andskeletal isomerization), the SIE suggesting a good dispersion of themetal and a moderated to strong Brönsted acidity. AlSBA-15 pH 1.5(BH) is more selective to skeletal isomerization reaction than AH,which increase the SIE almost 10% above. For pH 3, a difference inselectivity is observed between the BH and AH materials. The BHshowed the higher selectivity for bimolecular side reactions, thatwhich indicates stronger acids sites and less Al dispersion than AH.These results are according to Auroux and co-workers [31]; theyshowed that an AlSBA-15 sample Si/Al ≈ 250 obtained by direct syn-thesis exhibit a Brönsted/Lewis ratio of 60/40 and few weak siteson its surface.

The material synthesized at pH 4.5 (BH and AH) showed thehighest conversion (≈90%) of all, the selectivity is oriented to iso-merization and cracking products (<C4). The skeletal isomerizationefficiency (SIE) reached 98% after 0.30 h (Fig. 7), thus, the productionof isobutene is thermodynamically controlled over this catalyticsystem. Due to the high activity and selectivity to isobutene shown

Fig. 8. Products distribution for AlSBA-15 at different pH. Solid lines (—) for AH condition and dash lines (- - -) for BH condition. (⊕) 2-Butenes, (�) isobutene, (ϑ) <C4, (×)>4, (�) C4◦ .



Fig. 9. Bimolecular index as function of TOS.

for these materials, it can be inferred that there is a higher numberof Brönsted acid sites of medium to strong acidity [24,25].

The product distribution for cracking (<C4) and >C4 to bothmesoporous materials synthesized at pH 4.5 is quite similar (Fig. 8).S>C4 is <10% because the dimerization on n-butenes is thermo-dynamically disfavoured at reaction temperature (773 K) on theother hand increases cracking products (>20%(BH) and >15%(AH)).SC4◦ increased by the pH condition; a strong acidity of the materialsynthesized, without any space restriction leads to an extensiveH-transfer [20] even at low experimental Si/Al ratios. Although onshort TOS the 1-butene conversion over AlSBA-15 pH 3 and 4.5 arenot very different, the products distribution clearly indicate that Allocalization in the mesostructure by increased pH and load Al inAlSBA-15 modified the catalysts selectivity.

The cracking products are: methane, ethane, ethylene, propaneand propylene, and the >C4 products are: n-pentenes and pentane.According the reaction scheme proposed by Rutenbeck et al. [22],the ratio C3= + C5=/iso-C4= (C3=: propylene, C5=: pentene and iso-C4=: isobutene) can be considered as bimolecular index (BI). Fig. 9shows the bimolecular index vs. TOS for catalysts at pH 1.5–4.5. Thetendency of curves at higher pH values indicates that BI decreaseon TOS because of a stronger acid sites deactivation; at lower pHvalues, a drop in the bimolecular index value is observed, indi-cating a decrease in the density of the active sites that leads toan increasing in the unimolecular reactions. Bimolecular reactionsincreased with synthesis pH. At initial TOS although the catalystsdo not have deposited coke, the bimolecular index were higher forall BH materials, an opposite behaviour was observed at pH 4.5. Asa distribution of products from the bimolecular reaction predomi-nate suggesting a higher number of sites where Al is conglomerated,as evidenced by TEM for AlSBA-15 pH 4.5(AH).

4. Conclusions

pH variation in the synthesis gel affects the textural proper-ties. Increase in pH increases micro-porosity and superficial area,and decreases particle size of silica SBA-15 materials. The hexag-onal order of the channels is preserved as pH increased from 0 to4.5. For AlSBA-15 (BH and AH) materials synthesized at pH > 0, theadsorption pore diameter (BJH) was increased with respect to sil-ica SBA-15. These materials showed a good activity in 1-butenecatalytic test.

The incorporation of Al before or after TEOS hydrolysis affectsthe distribution and location of the metal. The highest selectivityto skeletal isomerization of 1-butene is shown for materials syn-thesized at pH 4.5 (products distribution close at thermodynamicequilibrium), indicating the highest number of Brönsted sites ofmedium to strong acidity. Al incorporation before hydrolysis (BH)shows a higher accessibility and higher proportion of Brönstedstrong sites which lead to cracking and oligomerization reactions.

Acknowledgments

The authors would like to express their gratitude to CDCH-UCVfor financial support, though PG-03-00-5738-2007 and PG-03-12541-2006.

References

[1] D. Zhao, Q.S. Huo, J. Feng, Q. Huo, N. Melosh, G.H. Fredrickson, B.F. Chmelka, G.Stucky, Science 279 (5350) (1998) 548–552.

[2] D.Y. Zhao, Q.S. Huo, J. Feng, B.F. Chmelka, G.D. Stucky, J. Am. Chem. Soc. 120(1998) 6024–6036.

[3] J.M. Kim, Y.-J. Han, B.F. Chmelka, G.D. Stucky, Chem. Commun. (2000)2437–2438.

[4] P. Schmidt-Winkel, P. Yang, D.I. Margolese, B.F. Chmelka, G.D. Stucky, Adv.Mater. 11 (1999) 303–307.

[5] N.T.S. Phan, C.W. Jones, J. Mol. Catal. A: Chem. 253 (1/2) (2006) 123–131.[6] X.Q. Wang, H.L. Ge, H.X. Jin, Y.J. Cui, Microporous Mesoporous Mater. 86 (1–3)

(2005) 335–340.[7] Y. Kanda, T. Aizawa, T. Kobayashi, Y. Uemichi, S. Namba, M. Sugioka, Appl. Catal.

B: Environ. 77 (1/2) (2007) 117–124.[8] J. Yang, K. Hidajat, S. Kawi, Mater. Lett. 62 (8/9) (2008) 1441–1443.[9] A.A. Campos, L. Martins, L.L. de Oliveira, C. Ramos da Silva, M. Wallau, E.A.

Urquieta-González, Catal. Today 107–108 (2005) 759–767.[10] Y. Li, W. Zhang, L. Zhang, Q. Yang, Z. Wei, Z. Feng, C. Li, J. Phys. Chem. B 108 (28)

(2004) 9739–9744.[11] H.-M. Kao, C.-C. Ting, S.-W. Chao, J. Mol. Catal. A: Chem. 235 (2005) 200–208.[12] A. Vinu, G.S. Kumar, K. Ariga, V. Murugesan, J. Mol. Catal. A: Chem. 235 (2005)

57–66.[13] G.M. Kumaran, S. Garg, H. Soni, M. Kumar, J.K. Gupta, L.D. Sharma, K.S. Rama,

G.M. Dhar, Microporous Mesoporous Mater. 114 (2008) 103–109.[14] H.L. Zubowa, M. Richter, U. Roost, B. Parlitz, R. Fricke, Catal. Lett. 19 (1993)

67–79.[15] C.J. Brinker, G.W. Scherrer, Sol–Gel Science, Academic Press, London, 1990, pp.

97–234.[16] K.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, J. Rouquerol, T.

Siemieniewska, Pure Appl. Chem. 57 (1985) 603–619.[17] J.M. Esparza, M.L. Ojeda, A. Campero, G. Hernández, C. Felipe, M. Asomoza, S.

Cordero, I. Kornhauser, F. Rojas, J. Mol. Catal. A: Chem. 228 (2005) 97–110.[18] V. Meynen, P. Cool, E.F. Vansant, Microporous Mesoporous Mater. 104 (2007)

26–38.[19] G. Leofanti, M. Padovan, G. Tozzola, B. Venturelli, Catal. Today 41 (1998)

207–219.[20] J. Houzvicka, R. Klik, L. Kubelkova, V. Ponec, Appl. Catal. A: Gen. 150 (1997)

101–114.[21] M.A. Asensi, A. Corma, A. Martínez, J. Catal. 158 (1996) 561–569.[22] D. Rutenbeck, H. Papp, D. Freude, W. Schwieger, Appl. Catal. A: Gen. 206 (2001)

57–66.[23] M. Höchtl, A. Jentys, H. Vinek, Appl. Catal. A: Gen. 207 (2001) 397–405.[24] D. Escalante, L. Giraldo, M. Pinto, C. Pfaf, V. Sazo, M. Matjushin, B. Méndez,

C.M. López, F.J. Machado, J. Goldwasser, M.M. Ramírez de Agudelo, J. Catal. 169(1997) 176–187.

[25] C.M. López, F.J. Machado, B. Méndez, M. Pinto, V. Sazo, J. Goldwasser, M.M.Ramírez de Agudelo, Top. Catal. 10 (2000) 65–71.

[26] M. Gómez-Cazalilla, J.M. Mérida-Robles, A. Gurbani, E. Rodríguez-Castellón, A.Jiménez-López, J. Solid State Chem. 180 (2007) 1130–1140.

[27] J.W. Ward, in: R.F. Gould (Ed.), Molecular Sieve Zeolite-I, American ChemicalSociety, Washington, DC, 1971, pp. 380–404.

[28] D.W. Breck, Zeolite Molecular Sieves. Structure, Chemistry, and Use, John Wiley& Sons Inc., New York, 1974.

[29] G.L. Woolery, G.H. Kuehl, H.C. Timken, A.W. Chester, J.C. Vartuli, Zeolites 19(1997) 288–296.

[30] M. Trombetta, G. Busca, M. Lenarda, L. Storaro, M. Pavan, Appl. Catal. A: Gen.182 (1999) 225–235.

[31] B. Dragoi, E. Dumitriu, C. Guimon, A. Auroux, Microporous Mesoporous Mater.121 (2009) 7–17.

[32] S. Zeng, J. Blanchard, M. Breysse, Y. Shi, X. Shu, H. Nie, D. Li, Microporous Meso-porous Mater. 85 (2005) 297–304.

Transformation of 1-butene over AlSBA-15 synthesized at different conditions of pH and hydrolysis

Documents

Transcript of Transformation of 1-butene over AlSBA-15 synthesized at different conditions of pH and hydrolysis