Applied Catalysis A: General 377 (2010) 1–8

Alkylation of isobutane/1-butene on methyl-modified Nafion/SBA-15 materials

Wei Shen a,b, Yi Gu a, Hualong Xu a, David Dube b, Serge Kaliaguine b,*a Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials and Laboratory of Advanced Materials,

Fudan University, Shanghai 200433, PR Chinab Department of Chemical Engineering, Laval University, Quebec City, QC, Canada G1K 7P4

A R T I C L E I N F O

Article history:

Received 3 September 2009

Received in revised form 30 November 2009

Accepted 5 December 2009

Available online 7 February 2010

Keywords:

Methyl modification

Silylation

Ethoxytrimethylsilane

Nafion

SBA-15

Alkylation of isobutane/1-butene

A B S T R A C T

Hydrophobicity modification of the intrinsic polarity of the surface of SBA-15 mesoporous by

ethoxytrimethylsilane was used in this work to make hybrid organic–inorganic mesoporous matrix. This

matrix was functionalized with perfluorosulfonic acidic Nafion resin by a post-synthetic impregnation

method. Characterized by N2-physisorption, XRD, and transmission electron micrographs (TEM), all the

materials synthesized were highly ordered. Elemental analysis, 29Si MAS NMR, thermal gravimetric

analysis (TGA), energy-dispersive X-ray (EDX) and potentiometric titration showed that trimethylsilane

is grafted on the surface by capping the OHs and the Nafion resin was incorporated, revealing a strong

solid acid with hydrophobic surface. The alkylation of isobutane/1-butene was thereafter evaluated on

each material under specified conditions. Compared with the polar surface of conventional SBA-15 and

commercial Nafion silica nanocomposite SAC-13, methyl-modified surface of SBA-15 material (denoted

as Me-SBA-15) is a much better solid acid catalyst for isobutane/1-butene alkylation.

� 2009 Elsevier B.V. All rights reserved.

Contents lists available at ScienceDirect

Applied Catalysis A: General

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

1. Introduction

Alkylation of isobutane with light (C3–C5) olefins is animportant refining operation which produces mixtures ofbranched alkanes. The formed product, designated as alkylate, isthe highest quality hydrocarbons for the gasoline pool because ofits high octane number, low Reid vapor pressure, very low sulfurcontent and freedom from aromatics or olefins [1]. The currentalkylation technology makes use of strong liquid acids (HF orH2SO4). Both processes, while producing high quality, environ-mentally benign gasoline components, suffer from a number ofdrawbacks.

HF is a corrosive and highly toxic liquid with a boiling pointclose to room temperature. Tests indicated that liquid HF can formaerosol clouds containing lethal levels of HF, which drift downwindat ground level for several kilometers. Two incidents occurred in1986 and 1987 [2]. Sulfuric acid is also a dangerous chemical, butthe dangers are relatively localized. The major disadvantage ofsulfuric acid is its unusually high catalyst consumption, which canbe as high as 70–100 kg of acid/tonnes of alkylates [3]. Further-more, the spent acid containing conjunct polymers forms a toxicsludge that must be sent to a recovery facility. The transport ofspent and fresh acid to and from the sulfuric acid regeneration

* Corresponding author. Tel.: +1 418 656 2708; fax: +1 418 656 3810.

E-mail address: Serge.Kaliaguine@gch.ulaval.ca (S. Kaliaguine).

0926-860X/$ – see front matter � 2009 Elsevier B.V. All rights reserved.

doi:10.1016/j.apcata.2009.12.012

plant has given rise to some concern and increased the pressure onrefiners to establish sulfuric acid regeneration plants near thealkylation unit. Indeed, the cost of the recovered acid has beenestimated to be two to three times that of sulfuric acid available onthe market [4]. About one-third of the operating cost of the H2SO4

process can be attributed to acid consumption [5].The obvious environmental risks of an unintentional release

upon separation and disposal of these catalysts is a strongmotivation for the petroleum industry to develop a heterogeneouscatalyst. A variety of strong acidic solids has been tested asalkylation catalysts, including sulfated zirconia and relatedmaterials [6,7], heteropolyacids [8,9], acid resins [10,11], chlori-nated alumina [12], and acidic zeolites [13–22]. None of them metany commercial success because of their unacceptably rapiddeactivation. During zeolite-catalyzed isobutane/butene alkyl-ation, which almost exclusively produces isoalkane products, ahighly unsaturated and highly branched polymer is formed. Thepolymer is strongly adsorbed on the acid sites and completely fillsthe pores at the end of the reaction [23]. The alkylation reactionproceeds mainly via addition of n-butene to an isobutyl carbeniumion. The resulting octyl carbenium ion is removed (after possibleisomerization) from the active site by hydride transfer fromisobutane leading to trimethylpentanes as ideal products and anisobutyl ion, which perpetuates the reaction. A high ratio of therate of hydride transfer vs. the rate of oligomerization is crucial forgood catalytic performance. Strong acid sites are necessary toeffectively catalyze hydride transfer [24]. Hydrophobic surface

W. Shen et al. / Applied Catalysis A: General 377 (2010) 1–82

which allows a higher paraffin steady concentration in the poresystem is important to hydride transfer. The hydrophilicity of thezeolite pores enhances the adsorption of polarizable molecules[25]. The preferential adsorption of alkene on zeolite surface leadsto high rate of oligomerization. Moreover, the small pore size ofzeolite limits the diffusion of reactants to active sites and productsout of the pore system [26]. Thus, a catalyst with large pore size,strong acid sites and hydrophobic surface is desired.

Perfluorosulfonic acid Nafion resin has been used as catalysts ina wide range of organic reactions. The presence of electron-withdrawing fluorine atoms in the structure significantly increasesthe acid strength of the terminal sulfonic-acid groups, whichbecomes comparable to that of pure sulfuric acid [27]. However,Nafion presents low surface areas. It performed poorly inisobutene/n-butene alkylation [10,28]. Supporting Nafion on highsurface area carriers provided perfluorosulfonic acid materialswith higher surface areas and higher density of available acid sites,which leads to the enhancement of their catalytic performance[29]. Martinez et al. reported that Nafion/SBA-15 made byimpregnation is a very good solid acid catalyst for acylation ofanisole [30]. Wang and Guin reported that impregnation is morebeneficial than the sol–gel technique to make Nafion/silica catalystfor etherification of olefins [31]. Perfluorosulfonic acid graftedmesoporous materials synthesized by post-synthetic graftingstrategies [28,32] or direct one-step synthesis [33] are catalystswith similar advantages. However, the limited availability ofsuitable fluorinated silanes hinders the applicability of this kind ofmaterials for large scale production. Comparatively, impregnationof Nafion resin in mesoporous silica such as SBA-15 is a practicalway to produce catalyst with higher surface area, large pore sizeand high strength acid sites. In addition, the hydrophobic/hydrophilic balance of the catalyst can be tuned by capping thesurface –OHs with alkyl trimethoxysilane. This is beneficial toincrease the isoparaffin/olefin ratio in the alkylation reactionconditions.

In this work, ethoxytrimethylsilane was used to cap the surface–OHs of SBA-15 to tune its hydrophobicity. Nafion was impreg-nated on hydrophobicity modified SBA-15. The activity ofhydrophobicity-modified Nafion/SBA-15 (denoted as Nafion/Me-SBA-15) materials was evaluated and compared with Nafion/SBA-15 and SAC-13 on the alkylation of isobutene/1-butene. Theinfluence of the hydrophobic nature of support, reaction tempera-ture, alkane/alkene ratio and 1-butene space velocity were studied.

2. Experimental

2.1. Catalyst preparation

SBA-15 mesoporous silica sample was synthesized by the acid-catalyzed hydrolysis and condensation of tetraethyl orthosilicate(TEOS, Aldrich) using the triblock copolymer Pluronic P123(EO20PO70EO20, BASF) as the structure-directing agent, accordingto the procedure adopted by Zhao et al. [34]. In a typical synthesis,4 g of P123 was dissolved in 125 g of 2 M HCl solution understirring at 40 8C. This was followed by adding 9.12 g of TEOS intothe solution as silicon source. After being stirred vigorously for24 h at 40 8C, the resulting gel was transferred to a Teflon-linedautoclave and heated at 100 8C for an additional 48 h. After coolingto ambient temperature, the solid in the autoclave was recoveredby filtering, washing and drying at 80 8C. Finally, the solids werecalcined at 550 8C for 5 h to remove the organic surfactant.

Silylation of the surface –OHs of SBA-15 was carried out asfollows: 2.6 g SBA-15 was pre-dried under vacuum at 200 8C for12 h before adding 3.5 g ethoxytrimethylsilane (Aldrich) and 30 mlof dry toluene under Argon. The mixture was refluxed at 100 8C foranother 12 h. Then, the hydrophobicity-modified SBA-15 material

was filtered and washed by toluene and anhydrous ethanol in turn.At last, the solid was dried at 80 8C overnight [35].

The supported Nafion catalysts were prepared by impregnatingNafion (5 wt.% Nafion in water–alcohol solution, Dupont) on theabove prepared SBA-15 (pure SBA-15 or hydrophobicity-modifiedSBA-15), stirring at 60 8C and atmospheric pressure for 6 h. Thesolid was firstly dried at room temperature for 12 h in staticconditions, and then the water and alcohols were evaporatedthoroughly under vacuum at 60 8C for an additional 12 h. Theresultant materials were denoted as Nafion(X)/SBA-15 orNafion(X)/Me-SBA-15, where X indicates the theoretical wt.% ofNafion loading (in this work, two Nafion loading were studied, i.e.X = 15 or 30). Me-SBA-15 indicates that the surface –OHs of SBA-15were silanized by ethoxytrimethylsilane. For comparison, Nafionresin/silica composite, SAC 13, with resin content of ca. 13 wt.%obtained from Aldrich was also studied.

2.2. Catalysts characterization

Nitrogen adsorption–desorption isotherms at 77 K wereperformed using a Micromeritics TRISTAR 3000 apparatus. Thesamples were degassed at 120 8C and high vacuum prior to themeasurements. BET model was used to estimate the surface areasof the materials. BJH model was performed to calculate themesopore size using adsorption branches of isotherms, and thepore diameter was estimated out from the peak position of BJHpore size distribution.

Powder XRD spectra were recorded using a Bruker D4 X-raydiffractometer with nickel-filtered Cu Ka radiation (l = 1.5418 A).The tube voltage was 40 kV, while the current was 40 mA.Diffraction patterns were recorded with scan step of 0.028 for 2theta between 0.58 and 58

Transmission electron micrographs (TEM) were obtained usinga JEOL 2011 microscope operated at 200 kV. Scanning electronmicroscope (SEM) and energy-dispersive X-ray (EDX) microana-lyses were obtained using a Philip-XL30 apparatus operated at20 kV.

Solid-state 29Si MAS NMR spectra were recorded at roomtemperature on a Bruker ASX 300 spectrometer at a frequency of59.6 MHz and at 8 kHz spinning rate. Tetramethylsilane (TMS) wasused as external reference for 29Si MAS NMR analyses.

The ion exchange capacities (corresponding to acid siteconcentration) of the Nafion-modified SBA-15 materials weredetermined using aqueous solutions of NaCl and titratedpotentiometrically by NaOH [36]. In a typical experiment, 0.1 gof solid was added to 20 g of 2 M NaCl solution, and vigorouslystirred at room temperature overnight. The resulting suspensionwas filtered and washed thoroughly with a total amount of 80 ml2 M NaCl to retain the hydrogen ions in the solution, and thereaftertitrated potentiometrically by a drop wise addition of a 0.1 MNaOH.

Thermal gravimetric analysis (TGA) was conducted on aPerkinElmer TGA 7 from ambient temperature to 800 8C at aheating rate of 10 8C/min in air.

Sulfur and total organic content were determined by elementalanalysis using a Vario EL III apparatus (CHNS model).

2.3. Reaction procedure

Liquid phase alkylation of isobutane/1-butene experimentswere carried out in an automated stainless steel fixed bedcontinuous reactor. In each run, 0.35 g of catalyst pellets withdiameters of 0.5–0.8 mm, obtained by compressing the powderinto tablets, crushing, and sieving, were loaded in a reactor tubewith internal diameter of 8 mm. The catalyst bed was fixedbetween two plugs of quartz wool and the remaining empty

Fig. 1. (a) Nitrogen adsorption–desorption isotherms at 77 K and (b) X-ray diffraction patterns for Nafion-modified mesostructured materials.

W. Shen et al. / Applied Catalysis A: General 377 (2010) 1–8 3

volume filled with quartz beads. The catalysts were pretreated in

situ at 110 8C for 10 h in a flow of N2 of 15 ml/min. The reactor wascooled to reaction temperature and a pressure of 3.2–3.4 MPa wasestablished with nitrogen before feeding the reactants. Thefeedstock consisting of an isobutane/1-butene mixture with amolar ratio of 20/1 and 40/1 was delivered to the reactor using apiston-type pump, and it goes through the catalyst bed located inthe middle zone of the reactor. The product stream coming out ofthe reactor is then collected and stored using a 16-loop samplingvalve. In this way samples at intervals of 1 min can be taken duringthe run and analyzed automatically once the experiment has beenfinished. The reaction products were analyzed using a GC (ThermoTrace GC Ultra) equipped with a 100m capillary column (CP SILPONA CB) and a FID detector. The individual C4–C8 hydrocarbonsidentified by means of available reference standards and GC–MSanalysis.

3. Results and discussion

3.1. Catalyst characterization

Hybrid Nafion/SBA-15 materials combine the merits of Nafionresin and mesoporous substrates as a whole in these dedicatedmaterials. Nafion resin, a perfluorosulfonic acid, has an aciditycomparable to that of concentrated sulfuric acid, providingeffective active sites in alkylation reactions. Mesoporous SBA-15 support has uniform pore size, together with large surface andpore void, ensuring more interacting surface of reactants andmeanwhile has a potential for loading more well-dispersed Nafionresin as active sites. Silylation of the surface –OHs intomethylsilane further contributed to the function of the materials.Hydrophobicity-modified surface of SBA-15 is expected to besuitable for alkylation reaction by changing the surface concen-tration ratios of isobutane/1-butene which affects the reactionremarkably.

Fig. 1a and b presents nitrogen adsorption–desorption iso-therms and XRD patterns of SBA-15, Me-SBA-15 and their Nafion-modified samples. All the materials displayed type IV isothermswith H1-type adsorption–desorption hysteresis loop. The narrowH1-type hysteresis loop of SBA-15 is maintained after the graftingof trimethylsilane (OH capping), indicating that the structure is notaffected by silylation. It becomes broader slightly after impregna-tion of 15 wt.% of Nafion. That means the Nafion resin thatpenetrated into the porous framework seems to be homogeneouslydeposited along the cylindrical mesoporous channels of materialswhen the Nafion loading is lower than 15 wt.%. With increasingNafion loading, the hysteresis loop in the N2 isotherm becomesbroader, suggesting Nafion resin places within the mesoporousstructure in forms of polymeric aggregates [37]. XRD patterns ofNafion/SBA-15 samples show the characteristic (1 0 0), (1 1 0) and(2 0 0) diffractions clearly, indicating the presence of mesostruc-ture with hexagonal p6mm symmetry. The secondary diffractionpeaks of materials modified by silylation and impregnationdecrease in intensity, demonstrating the deterioration of themesopore order. However, the sharp peaks of (1 0 0) of thematerials synthesized show that mesoporous textures can bemaintained in all cases.

Textural parameters deduced from nitrogen isotherms and XRDpatterns of the synthesized materials are summed in Table 1. Me-SBA-15 has textural parameters similar to those of SBA-15,indicating little influence of the silylation treatment on mesopor-ous textures. Impregnation of Nafion induces reduction of BETsurface area, pore diameter and pore volume. BJH analysis of theadsorption branches of the isotherms reveals that the mesoporediameter decreased from ca. 7.6 nm to 6.6 nm, while the wallthickness increased systematically from 2.8 nm to 4.5 nm with theincorporation of 30 wt.% Nafion loadings. The decrease of porediameter and increase of wall thickness confirm the deposition ofNafion resin on the pore walls. Though the pore diameter decreasesafter OH capping and Nafion loading, the mean pore size remains

Table 1Textural properties for Nafion-modified mesostructured materials.

Sample SBET (m2/g) DPorea (nm) VPore

a (cm3/g) d100b (nm) Wall thicknessc (nm)

SBA-15 702 7.5 0.96 9.3 2.9

Nafion(15)/SBA-15 474 7.0 0.65 9.6 4.1

Nafion(30)/SBA-15 404 6.6 0.40 9.6 4.5

Me-SBA-15 727 7.6 1.05 9.0 2.8

Nafion(30)/Me-SBA-15 363 6.6 0.45 9.5 4.4

a Total pore size and pore volume of Nafion-modified SBA-15 materials were calculated from BJH adsorption branch, and the pore diameter was deduced from the peak

position of BJH pore size distribution.b d(1 0 0) spacing, measured from small-angle XRD.c Pore wall thickness calculated as ao: pore diameter with ao = 2d(1 0 0)/H3.

W. Shen et al. / Applied Catalysis A: General 377 (2010) 1–84

beyond 6.6 nm for all the materials, which makes these acidcatalysts potential applicants in catalytic reactions involving bulkymolecules. The impregnation of Nafion affects the narrow pore sizedistribution of the purely siliceous SBA-15 support, leading tobroader distributions centered at smaller pore sizes (Fig. 2). Thatsuggests Nafion resin may be placed within the mesopores aspolymeric aggregates especially at higher loading.

29Si MAS NMR spectrum of Nafion(30)/Me-SBA-15 is presentedin Fig. 3. Four peaks at �110, �101, �90, and �64 are attributableto Q4 [Si(OSi)4], Q3 [OHSi(OSi)3], Q2 [(OH)2Si(OSi)2] and T3

[RSi(OSi)3] resonances respectively [38,39]. The result indicatesthat ethoxytrimethylsilane is grafted on the surface of the SBA-15catalyst. The surface ratios of the peak components associated tothese silica species are recognized to be Q4:Q3:Q2:T3 = 57:23:5:15.While there is no occurrence of T3 [RSi(OSi)3] resonances forNafion(30)/SBA-15 sample and its Q4:Q3:Q2 is 49:43:8. The

Fig. 2. Pore size distributions of the Nafion-modified SBA-15 materials.

increase in T3 and Q4 corresponds to the disappearance of someof the surface OH’s upon capping. Result of elemental analysis(Table 2) indicates that 0.6 mmol/gcat of methyl groups was graftedon SBA-15.

Fig. 4a shows a typical SEM image of Nafion(30)/Me-SBA-15material which consists of irregular particles. TEM images (Fig. 3band c) of this sample show the hexagonal array of uniformchannels with the typical honeycomb appearance of SBA-15materials with an estimated pore diameter around 6.6 nm which isin agreement with the pore size obtained by nitrogen adsorption–desorption isotherms.

Table 2 shows several parameters related to the chemicalproperties including the acidity and methyl incorporation of thehybrid materials. TG analyses have been used to evaluate theexperimental Nafion loading of each sample. The weight lossbetween 300 8C and 550 8C was assigned to the thermo decompo-sition of the Nafion resin[30]. The incorporation yield ranges from84% to 88%. It demonstrated that the Nafion loading (between15 wt.% and 30 wt.%) and polarity of SBA-15 did not make greateffect on incorporation yield. Results of H+ concentrationsdetermined by drop-wise potentiometric titration and the sulfurcontent determined by elemental analysis are consistent with TGresult. The comparison between S and [H+] contents in the varioussolids suggests that Nafion is protonated and essentially all acidsites can exchange sodium ions from NaOH solution. S/Si atomicratios given by EDX also indicate the trend of Nafion incorporationand accessibilities of acid sites. All these results evidence that theactive sites provided by Nafion resin are highly accessible,revealing the effective role of the mesostructured silica support.

Methyl capping of surface –OHs, though not providing theactive sites as the Nafion resin, plays an important role in tuningthe physical property of the surface of the catalyst. It changes therelative affinity of the catalyst for the sorption of less-polarizedreactant, i.e. isobutane, which is beneficial to increasing the surfaceisoparaffin/olefin ratio in the alkylation reaction conditions.

Fig. 3. 29Si MAS NMR spectrum of Nafion(30)/Me-SBA-15.

Table 2Methyl incorporation and acidic-related properties of Nafion modified mesostructured materials.

Sample Nafion incorporation Acidic Properties Methyl content

(mmol methyl/gcat)a

wt.%b Yieldc (mmol S/gcat)c (mmol H+/gcat)

d S/Si (molar ratio)e

Nafion(15)/SBA-15 12.9 86 0.124 0.130 0.007 –

Nafion(30)/SBA-15 26.3 88 0.260 0.270 0.017 –

Nafion(30)/Me-SBA-15 25.2 84 0.248 0.258 0.016 0.6� 0.1

a Carbon content evaluated by elemental analysis.b Percentage of weight loss from 300 to 550 8C. Value corrected by deducing the weight decrease by the dehydration of the corresponding mesoporous silica prepared

without Nafion, i.e. siliceous SBA-15 or Me-SBA-15 [30].c Sulfur content evaluated by elemental analysis.d Obtained by cationic-exchange in 2 M NaCl, and titrated potentiometrically with 0.1 M NaOH after filtration and thorough wash.e S/Si atomic ratio given by EDX, average ratio of 20 random ordered regions per sample.

Fig. 4. (a) SEM image and TEM images of Nafion(30)/Me-SBA-15 viewed (b) in the [1 0 0] direction and (c) in the [1 1 0] direction.

W. Shen et al. / Applied Catalysis A: General 377 (2010) 1–8 5

Furthermore, methyl groups improve the catalytic performancewith an efficient removal from the catalyst surface of the waterintroduced by the feedstock or moisture that are difficult toremove by thermo heating at low temperature due to thelimitation of the thermostability of the Nafion resin. In thecatalysis result shown below, the benefit of the methyl groups onthe overall conversion of alkylation will be discussed.

3.2. Catalytic performance

Nafion/silica composites, such as commercially available SAC-13, are reported to be good catalysts for isobutane/n-butenealkylation [40]. Nafion/SBA-15 with similar H+/gcat is a better solidacid catalyst for acylation [30]. In this part of the work, Nafionimpregnated SBA-15 and Me-SBA-15 catalysts were synthesizedand compared with SAC-13 in the alkylation of isobutane/1-butenereaction. The influence of reaction temperature, isobutane/1-butene ratio, alkene space velocity was studied. Isobutane/1-

butene alkylation on acid catalysts yields trimethylpentanes(TMPs) as the primary products. A set of parallel and consecutivereaction steps can occur, giving mixtures of hydrocarbons rangingfrom C5 to C9+. C8 consists of trimethylpentanes (TMPs),dimethylhexanes (DMHs) and C8 olefins (C8

55). The TMPs+ arecommonly considered to be formed by the reaction of t-butylcarbenium ion with butene, they can undergo hydride transferfrom isobutane to form the TMPs. The tert-butyl cation isregenerated and the chain sequence can continue. The DMHsand C8

55 are believed to be formed by dimerization of olefin [10] orcracking of heavier compounds [41] depending on the hydridetransfer activity. The product distribution is governed by therelative rates of alkene addition, isomerization, and hydridetransfer. Hydride transfer/alkene addition determines the selec-tivity to single and multiple alkylation. A high ratio of hydridetransfer vs alkene addition retards the buildup of long hydrocarbonchains, leads to high ratio of TMPs in products and long catalyticlifetime.

Fig. 5. Catalytic performance of Nafion-based catalysts. Reaction conditions:

T = 100 8C; olefin WHSV = 2 h�1; isobutane/1-butene (I/O) molar ration = 40.

Table 3Textural and catalytic properities of catalysts.

Sample Textural properties Catalytic

properties

SBET

(m2/g)

DPorea

(nm)

VPorea

(cm3/g)

mmol

S/gcatb

mmol

H+/gcatc

Nafion(30)/Me-SBA-15 363 6.6 0.45 0.248 0.258

Nafion(30)/SBA-15 404 6.6 0.40 0.260 0.270

Nafion(15)/SBA-15 474 7.0 0.65 0.124 0.130

SAC-13 200 0.43 0.147 0.133

a Total pore size and pore volume were calculated by BJH method.b Sulfur content evaluated by elemental analysis.c Obtained by cationic-exchange with NaCl and titration with NaOH.

W. Shen et al. / Applied Catalysis A: General 377 (2010) 1–86

Alkylation is catalyzed by sites with strong acidity, whiledouble bond isomerization is catalyzed by very weak acid sites [6].Even a fully deactivated zeolite retains some activity forisomerizing butenes. Thus the conversion is calculated based ondisappearance of butenes.

Three Nafion/SBA-15 materials were tested and compared withSAC-13 at same reaction conditions. C5, C6, C7, TMPs, DMHs, C8

55

and C9+ are obtained. TMPs are the primary products. Fig. 5 showsthe catalytic performance of each material in terms of butenesconversion, TMPs yield and C5–C7 selectivity with time on stream,whereas their relevant textural and catalytic properties aresummarized in Table 3.

The results shown indicate that SAC-13 gives a higher activitythan Nafion(15)/SBA-15 which has similar H+ concentration andlarger surface area. It is inverse to the results got by Martinez et al.in acylation reaction [30]. This can be explained by the different

acid strength these two reactions required. Alkylation needs verystrong acid sites to catalyze. The interaction between the sulfonicgroups of Nafion and silanol groups of the silica leads to a decreasein acidity. Corma and co-workers reported that Nafion/silicacomposites with the same polymer content but a larger surfacearea showed a considerably lower activity in isobutane/2-butenealkylation [10]. Nafion(15)/SBA-15 has a higher number of silanolgroups resulting in more interaction between the active phase andthe silica support. In this way, a charge transfer may occur from theprotons of the sulfonic groups to the silanol groups of the silica,decreasing the acid strength of the resin. Palinko et al. studied theinteraction between–SO2OH groups of Nafion and the silanols ofthe silica using physical characterization techniques [42]. Theyfound the interaction leads to a decrease in acidity due to thelevelling effect of the hydrating environment. Thus, the greater thenumber of silanol groups, the weaker the global acidity of theactive phase. Consequently, the catalytic conversion of alkylationwill be lower. This was also confirmed by the selectivity of C5–C7,which are the result of cracking. Cracking is catalyzed only by thestrongest acid sites [6]. SAC-13 shows higher initial crackingactivity than Nafion(15)/SBA-15. This can be attributed to theincrease in acid strength.

With the increase in Nafion loading (from 15 wt.% to 30 wt.%),Nafion resin is present within the mesoporous structure under theform of polymeric aggregates. The negative effect of the interac-tions between Nafion and silanol groups diminishes. The numberand strength of acid sites which catalyze isobutane/1-butenealkylation increase. The initial butenes conversion was increasedfrom 54.2% to 92.3% when the Nafion loading of Nafion/SBA-15 waschanged from 15 wt.% to 30 wt.%. High Nafion content favors theformation of TMPs and C5–C7 products. The initial yield of TMPs ofNafion(30)/SBA-15 is not as high as expected because the strongperfluorosulfonic acid sites show high initial cracking activity.With time on stream, deactivation of the catalysts, decrease inselectivity to C5–C7 and a higher content of octenes and C9+ can beobserved. The strongest acid sites are the first ones to be poisoned.

The activity of Nafion(30)/SBA-15 is obviously improved afterits surface OHs were capped by ethoxytrimethylsilane. Nafion(30)/Me-SBA-15 gives higher initial butenes conversion (98.6% vs.92.3%) and TMPs yields (63.5% vs. 51.7%). At same time, theselectivity of C5–C7 of two catalysts is almost same. That means theOH capping does not seem to increase activity through increasingthe acid strength. Catalyst characterization indicated there are noessential structure changes induced by the capping procedure. Thiscapping is intended to enhance the pore surface hydrophobicitywhich is reflected by 29Si MAS NMR data indicating a diminution ofthe surface silanol density. The higher activity obtained withNafion(30)/Me-SBA-15 in comparison to Nafion(30)/SBA-15 can berelated to a higher isobutane concentration in the hydrophobicenvironment of the capped catalyst comparatively to thehydrophilic silica support. As mentioned above, for isobutane/1-butene alkylation reaction, the octyl carbenium ion is removed

Table 4Influence of I/O ratio on the initial (TOS = 1 min) butenes conversion and products

distribution on Nafion(30)/SBA-15 and Nafion(30)/Me-SBA-15a.

Nafion(30)/SBA-

15 catalyst

Nafion(30)/

Me-SBA-15

catalyst

40b 20b 40b 20b

Butenes conversion (wt.%) 92.3 49.8 98.6 98.2

Distribution of C5+ (wt.%)

C5–C7 41.5 30.8 42.7 36.7

C8 44.2 52.8 48.2 55.2

C9+ 14.3 16.4 9.1 8.1

Distribution of C8 (wt.%)

TMP 62.1 42.1 65.5 50.2

DMH 30.7 40.9 33.4 45.8

C855 7.2 17.0 1.1 4.0

a Other experimental conditions: T = 100 8C, olefin WHSV = 2 h�1.b I/O ratio.

W. Shen et al. / Applied Catalysis A: General 377 (2010) 1–8 7

from the active site by hydride transfer from isobutane leading totrimethylpentanes as ideal products and an isobutyl ion, whichperpetuates the reaction. The high ratio of the rate of hydridetransfer vs. the rate of oligomerization is crucial for a good catalyticperformance. High ratio of isobutane/1-butene is necessary forhydride transfer. The electrostatic field at the surface of a solid acidcatalyst enhances the adsorption of alkenes. The preferentialadsorption of alkene leads to high rate of oligomerization, whicheither blocks the pores or poisons the active sites. To enhancesurface carbophilicity by capping the silanols of SBA-15 increasesthe I/O ratio in the reaction conditions and thus improves thecatalytic performance.

The influence of reaction parameters on the catalytic perfor-mance of Nafion/Me-SBA-15 was studied. The results are shown inFig. 6. It can be seen that with same 1-butene space velocity andisobutane/1-butene ratio, the conversion of butenes and yield ofTMPs increase dramatically with temperature. The initial conver-sion of butenes increases from 71.4% to 98.6% when the reactiontemperature increases from 70 8C to 100 8C (WHSV = 2 h�1, I/O = 40). Moreover, this increase in conversion is accompanied byan increase in yield to TMPs. The fraction of the C5–C7 products isfavoured at higher temperature while C9

+ increases at lowertemperature (not shown).

Fig. 6. Variation of the conversion of butenes (a) and yield of TMPs (b) with time on

stream over Nafion(30)/Me-SBA-15 under varying experimental conditions: 100 8C,

WHSV = 1.4 h�1, I/O = 40 (^); 100 8C, WHSV = 2 h�1, I/O = 40 (~); 70 8C,

WHSV = 2 h�1, I/O = 40 (^); 100 8C, WHSV = 2 h�1, I/O = 20 (4).

The influence of WHSV is also shown in Fig. 6. When WHSV of1-butene decreases from 2 h�1 to 1.4 h�1 (I/O = 40, T = 100 8C), thedecay of catalyst is much slower and the initial conversion ofbutenes increases slightly (from 98.6 wt.% to 100 wt.%). As theWHSV of 1-butene increases, the contact time decreases, resultingin an increase of the C8

55 and C9+ fractions, which not only

decreases the selectivity of TMPs but also fastens the catalystdecay.

The isoparaffin/olefin ratio is a very important parameter inisobutane/n-butene alkylation. Lower I/O ratio leads to higher rateof oligomerization. It can be seen in Fig. 6 that I/O shows obviouseffect on catalytic performance. The conversion of butenesdecreases much faster when the I/O ratio decreases from 40 to20 (WHSV = 2 h�1, T = 100 8C). High I/O ratio obviously favours theyield of TMPs. However, high I/O ratio will leads to high cost ofseparating products from the excess isobutane, which limits the I/O ratio that can be used industrially.

The I/O ratio determines the concentration of isobutane inthe reactor and thereby the rate of hydride transfers. The I/Oratio also sets the product concentration, which affects the ratesof the product degradation reactions [23]. The local concentra-tion of 1-butene in the pore system is strongly affected by thepolarity of surface. Capping the OHs enhances the hydrophobicnature of the SBA-15 surface, which allows higher concentra-tions of isobutane, thereby mitigating the effect of a decrease inI/O ratio. The effect of I/O ratio on initial catalyst activity withand without OH capping is compared in Table 4. As the I/O ratiodecreases from 40 to 20, the apparent concentration of 1-buteneis doubled, the activity of Nafion(30)/Me-SBA-15 decreasesslightly while that of Nafion(30)/SBA-15 decreases dramatically.The initial conversion Nafion(30)/Me-SBA-15 decreases from92.3 wt.% to 49.8 wt.%. The result confirms that OH cappingprovides hydrophobic enviroment and allows comparativelyhigher local paraffin concentration even at low I/O ratio. Thisenhanced activity of Nafion(30)/Me-SBA-15 at lower I/O ratiohas potential commercial interest by reducing the cost ofseparation.

The main reason for deactivation of Nafion based solid acidcatalysts is due to the formation of heavy products that arestrongly adsorbed over the active sites [30,40]. No leaching ofNafion resin was detected by elemental and TG analysis on testedcatalysts. Both Nafion/SBA-15 series and SAC-13 catalysts wereregenerated with washing by acetone and further treatment withHNO3 [30]. The catalytic performance of fresh and regeneratedcatalysts is illustrated in Fig. 7. About 85% of the initial catalyticactivity was achieved.

Fig. 7. Initial catalytic activity of fresh and regenerated Nafion-containing catalysts.

Reaction conditions: T = 100 8C; olefin WHSV = 2 h�1; I/O = 40.

W. Shen et al. / Applied Catalysis A: General 377 (2010) 1–88

4. Conclusions

Mesostructured SBA-15 hydrophobically modified by OHcapping and functionalized with Nafion resin by means ofimpregnation was described. The resulting solid acid catalystspreserve the mesostructure of SBA-15 and show high surface area(ca. 400 m2/g) and narrow pore size distribution centred in themesoscale range (ca. 7 nm). Capping of surface OH diminishes thesurface silanol density and provides a hydrophobic environmentfor the isobutane/1-butene reaction.

Hydrophobically modified Nafion(30)/Me-SBA-15 shows excel-lent activity and efficiency in the production of isooctanecompared with Nafion/SBA-15 and SAC-13. The catalytic perfor-mance of Nafion/SBA-15 is obviously improved after its surfaceOHs were capped. To enhance the surface carbophilicity increasesthe local I/O ratio in the reaction conditions and results in a higherratio of hydride transfer/oligomerization rate and good catalyticperformance. The catalytic activity of hydrophobic Nafion/Me-SBA-15 is much superior to that of hydrophilic Nafion/SBA-15especially at low I/O ratio. No Nafion leaching being detectedevidences a high stability of the impregnated resin. The usedcatalysts can be regenerated by washing with actone.

Acknowledgement

This work was supported by NSERC of Canada and Science& Technology Commission of Shanghai Municipality(08DZ2270500).

References

[1] A. Corma, A. Martinez, Catal. Rev. Sci. Eng. 35 (1993) 483–570.[2] L.F. Albright, in: I.T. Howath (Ed.), Encyclopedia of Catalysis, vol. 1, John Wiley

and Sons, New York, 2003, pp. 226–281.[3] J. Weitkamp, Y. Traa, in: G. Ertl, H. Knoezinger, J. Weitkamp (Eds.), Handbook of

Heterogeneous Catalysis, vol. 4, VCH, Weinheim, 1997, pp. 2039–2069.[4] E. Furimsky, Catal. Today 30 (1996) 223–286.[5] L.F. Albright, Chemtech (1998) 46–53.[6] A. Corma, A. Martinez, C. Martinez, J. Catal. 149 (1994) 52–60.[7] A.S. Chellappa, R.C. Miller, W.J. Thomson, Appl. Catal. A 209 (2001) 359–374.[8] T. Blasco, A. Corma, A. Martinez, P. Martinez-Escolano, J. Catal. 177 (1998) 306–

313.[9] V. Sarsani, Y. Wang, B. Subramaniam, Ind. Eng. Chem. Res. 44 (2005) 6491–6495.

[10] P. Botella, A. Corma, J.M. Lopez-Nieto, J. Catal. 185 (1999) 371–377.[11] C.J. Lyon, V.S.R. Sarsani, B. Subramaniam, Ind. Eng. Chem. Res. 43 (2004) 4809–

4814.[12] S.I. Hommeltoft, Appl. Catal. A 221 (2001) 421–428.[13] A.W. Chester, Y.F. Chu, Zeolite 6 (1986) 195–200.[14] A. Corma, A. Martinez, C. Martinez, Catal. Lett. 28 (1994) 187–201.[15] M. Stocker, H. Mostad, T. Rorvik, Catal. Lett. 28 (1994) 203–209.[16] F. Cardona, N.S. Gnep, M. Guisnet, G. Szabo, P. Nascimento, Appl. Catal. A 128

(1995) 243–257.[17] A. Corma, A. Martinez, P.A. Arroyo, J.L.F. Monteiro, E.F. Sousa-Aguiar, Appl. Catal. A

142 (1996) 139–150.[18] D.E. Sherwood, R.J. Taylor, Appl. Catal. A 155 (1997) 195–215.[19] G.S. Nivarthy, Y. He, K. Seshan, J.A. Lercher, J. Catal. 176 (1998) 192–203.[20] R. Loenders, P.A. Jacobs, J.A. Martens, J. Catal. 176 (1998) 545–551.[21] A. Feller, A. Guzman, I. Zuazo, J.A. Lercher, J. Catal. 224 (2004) 80–93.[22] M. Mukhergee, J. Nehlsen, Hydrocarbon Process 86 (2007) 110–114.[23] A. Feller, I. Zuazo, A. Guzman, J. Barth, J.A. Lercher, J. Catal. 216 (2003) 313–

323.[24] A. Corma, A. Martinez, C. Martinez, J. Catal. 146 (1994) 185–192.[25] S. Ramachandran, T.G. Lenz, W.M. Skiff, A.K. Rappe, J. Phys. Chem. 100 (1996)

5898–5907.[26] K.P. de Jong, C.M.A.M. Mesters, D.G.R. Peferoen, P.T.M. van Brugge, C. de Groot,

Chem. Eng. Sci. 51 (1996) 2053–2060.[27] M.A. Harmer, W.E. Farneth, Q. Sun, Adv. Mater. 10 (1998) 1255–1257.[28] W. Shen, D. Dube, S. Kaliaguine, Catal. Commun. 10 (2008) 291–294.[29] A. Heidekum, M.A. Harmer, W.F. Holderich, J. Catal. 176 (1998) 260–263.[30] F. Martinez, G. Morales, A. Martin, R. van Grieken, Appl. Catal. A 347 (2008) 169–

178.[31] S. Wang, J.A. Guin, Energy Fuels 15 (2001) 666–670.[32] M. Alvaro, A. Corma, D. Das, V. Fornes, H. Garcia, Chem. Commun. (2004) 956–957.[33] D.J. Macquarrie, S.J. Tavener, M.A. Harmer, Chem. Commun. (2005) 2363–

2365.[34] D. Zhao, J. Feng, Q. Huo, N. Melosh, G.H. Fredrickson, B.F. Chmelka, G.D. Stucky,

Science 279 (1998) 548–552.[35] S. Parambadath, M. Chidambaram, A.P. Singh, Catal. Today 97 (2004) 233–240.[36] D. Margolese, J.A. Molero, S.C. Christiansen, B.F. Chmelka, G.D. Stucky, Chem.

Mater. 12 (2000) 2448–2459.[37] M. Choi, F. Kleitz, D. Liu, H.Y. Lee, W.S. Ahn, R. Ryoo, J. Am. Chem. Soc. 127 (2005)

1924–1932.[38] G. Morales, G. Athens, B.F. Chmelka, R. Van Grieken, J.A. Melero, J. Catal. 254

(2008) 205–217.[39] S. Hamoudi, S. Royer, S. Kaliaguine, Microporous Mesoporous Mater. 71 (2004)

17–25.[40] P. Kumar, W. Vermeiren, J.P. Dath, W.H. Hoelderich, Energy Fuels 20 (2006) 481–

487.[41] L. Lee, P. Harriott, Ind. Eng. Chem. Process Des. Dev. 16 (1977) 282–287.[42] I. Palinko, B. Torok, G.S.K. Prakash, G.A. Olah, Appl. Catal. A Gen. 174 (1998) 147–

153.