Journal of Catalysis 290 (2012) 202–209

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Journal of Catalysis

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Boosted selectivity toward high glycerol tertiary butyl ethers bymicrowave-assisted sulfonic acid-functionalization of SBA-15 and beta zeolite

María Dolores González a, Yolanda Cesteros a,⇑, Jordi Llorca b, Pilar Salagre a

a Departament de Química Física i Inorgànica, Universitat Rovira i Virgili, C/ Marcel�lí Domingo s/n, 43007 Tarragona, Spainb Institute of Energy Technologies and Centre for Research in Nanoengineering, Universitat Politècnica de Catalunya, Avda. Diagonal 647, ed. ETSEIB, 08028 Barcelona, Spain

a r t i c l e i n f o a b s t r a c t

Article history:Received 17 February 2012Revised 16 March 2012Accepted 20 March 2012Available online 20 April 2012

Keywords:Beta zeoliteSBA-15Sulfonic acid-functionalizationMicrowavesh-GTBEGlycerol etherificationDealumination

0021-9517/$ - see front matter � 2012 Elsevier Inc. Ahttp://dx.doi.org/10.1016/j.jcat.2012.03.019

⇑ Corresponding author. Fax: +34 977559563.E-mail address: yolanda.cesteros@urv.cat (Y. Ceste

We report for the first time the post-synthesis sulfonic acid-functionalization of commercial beta zeolitein one step, by conventional heating as well as with microwaves, and the introduction of higher amountsof sulfonic acid groups in SBA-15 with microwaves. In beta zeolite, dealumination occurred due to theacidic medium used during sulfonation that favored the generation of new silanols groups, which in turnreacted with the organosulfonating agent resulting in the incorporation of the sulfonic acid groups.Microwave-assisted sulfonated catalysts showed higher selectivity toward di- and triethers of glycerol(83–91%) than those sulfonated by conventional heating and much more than a commercial acid macro-porous resin type Amberlyst (35%) after 4 h of reaction. It is remarkable the high selectivity to the bulkiertriether achieved with functionalized beta samples (32–36%) demonstrating that microporosity is not abarrier for the formation of triether when having appropriate amount and strength of Brønsted acidity.

� 2012 Elsevier Inc. All rights reserved.

1. Introduction

During biodiesel production, by transesterification of vegetableoils with methanol, glycerol (glycerine or 1,2,3-propanotriol) isformed as by-product in high amounts (10 wt% of the total prod-uct). The price of glycerol is falling as fast as biodiesel plants arebeing built. Research is currently starting to find new outlets toconvert the surplus of glycerol into high-added value products thatimprove the economy of the whole process [1,2].

One challenging option is the catalytic etherification of glycerolwith isobutene to obtain di- and tri-tertiary butyl ethers of glyc-erol, the so-called high ethers (h-GTBE), which can be used as oxy-genated fuels replacing the highly toxic to the environment methyltertiary butyl ether (MTBE). Besides, when h-GTBE was incorpo-rated in standard 30–40% aromatic-containing diesel fuel, emis-sions of particulate matter, hydrocarbons, CO and unregulatedaldehydes in the exhaust gases decreased significantly [3]. Regard-ing heterogeneous catalysis, the best activity results were achievedfor this reaction with acid ion-exchange resins of Amberlyst type(Amberlyst 15 and 35) [4–7] and with silicas functionalized withorganosulfonic acid groups introduced by conventional heating[8,9]. When H-Beta and H-Y zeolites were tested for this reaction,H-Beta showed less conversion but higher selectivity to glycerol

ll rights reserved.

ros).

diethers than Amberlyst 15. However, the formation of the trietherwas not observed for this zeolite [7]. This was attributed to sterichindrance effects due to its microporosity.

The organic tailoring of the surface of materials has receivedgreat attention in the last decade in terms of their application inthe fields of catalysis, sensing and adsorption. Specifically, theincorporation of organosulfonic acid groups in mesoporous ormicroporous materials can generate effective solid acid catalystswith enhanced catalytic properties. Functionalization of mesopor-ous silicas with organosulfonic acid groups can be performed bypost-synthesis grafting, in which the material is prepared and thenfunctionalized [10,11], and co-condensation, where the functional-ized silane is included in the mesoporous molecular sieve synthesissol–gel mixture so that it is incorporated in the structure as themolecular sieve forms [12,13]. 3-Mercaptopropyltrimethoxysilane(MPTMS) and phenethyltrimetoxysilane (PETMS), which require asubsequent treatment to led to the sulfonic acid groups, and 2-(4-chlorosulfonylphenyl)ethyltrimethoxysilane (CSPTMS) are themost usual compounds used to introduce the sulfonic acid groupsin mesoporous materials. In the case of zeolites, it is difficult tointroduce bulky organic species due to their microporosity. Thereare only few studies about the preparation of zeolites containingsulfonic acid groups within their pores by direct synthesis throughthe incorporation of phenethyl groups (PETMS) covalently tetheredto silicon atoms and subsequent sulfonation of the phenyl rings[14,15]. However, there are no studies about the incorporation of

M.D. González et al. / Journal of Catalysis 290 (2012) 202–209 203

sulfonic acid groups in zeolites by post-synthesis due to the lack ofreactant silanol groups („SiAOH), which are required to reactwith the sulfonating agents [16].

The use of microwaves for the synthesis or modification ofmaterials is becoming an important tool to reduce the synthesistime (energy saving) and modify the samples properties [17–20].Nevertheless, there are no studies about the use of microwavesfor the sulfonation of zeolites or mesoporous silicas.

The aim of this work was the post-synthesis sulfonic acid-functionalization of commercial beta zeolite in one-step, by con-ventional heating as well as with microwaves, and the introductionof sulfonic acid groups during functionalization of SBA-15 withmicrowaves. Samples were widely characterized by XRD, N2

physisorption, TGA, XPS, XRF, FTIR and potentiometric titrationtechniques. Sulfonated SBA-15 and sulfonated beta materials weretested as catalysts for the acid-catalyzed etherification of glycerolwith isobutene to obtain selectively h-GTBE.

2. Experimental

2.1. Preparation of the catalysts

SBA-15 sulfonated by conventional heating (SBA-15-CS) wassynthesized with a molar composition of 1.2 SiO2/0.2 chlorosulfo-nylphenylethyltrimethoxysilane (CSPTMS, Gelest)/6.5 HCl/180H2O, following the method reported elsewhere [21]. The sulfona-tion step was made at 40 �C for 24 h by traditional refluxing.SBA-15 sulfonated with microwaves (SBA-15-MwS) was preparedat the same conditions than SBA-15-CS, but the sulfonation stepwas made at 40 �C for 2 h by refluxing with microwaves (MilestoneETHOS-TOUCH CONTROL). The reaction mixtures were transferredto a Teflon-lined autoclave and heated at 100 �C in a conventionaloven for 24 h. The resulting products were filtered, washed repeat-edly with a large amount of water and dried at 80 �C in an ovenovernight. The surfactant template was removed by extractionwith ethanol under reflux for 24 h and later calcination in air at200 �C for 24 h. Lastly, one SBA-15 was synthesized following thesame procedure as for preparing sulfonated SBA-15 samples butomitting the sulfonation step (SBA-15).

Two gram of commercial Na-Beta (Zeochem, Si/Al = 10) wastreated with 1.4 g of CSPTMS in 150 mL of 2 M HCl at 40 �C for2 h by conventional refluxing (Beta-CS) or by refluxing with micro-waves (Beta-MwS). Samples were filtered, washed extensivelywith deionized water and dried at 80 �C overnight. Amberlyst-15was supplied by Aldrich (39 m2/g, pore size of 103 Å, acidity of4.7 meq H+/g).

2.2. Characterization methods

X-ray diffraction (XRD) patterns were obtained with a SiemensD5000 diffractometer. The patterns of beta zeolite samples wererecorded over a range of 2h angles from 5� to 40�. Crystallinephases were identified using the Joint Committee on Powder Dif-fraction Standards (JCPDS) files (48-0074 corresponds to beta).The integrated intensity of the signal at 2H = 22.4� was used toevaluate the crystallinity of beta samples. For small-angle powderXRD patterns of SBA-15 samples, the scanning range was set from0.5� to 10� with a step of 0.02�.

Textural characterization of the solids was performed by N2

(rN2 = 0.162 nm2) adsorption–desorption at 77 K using a Quadra-sorb SI surface analyzer. Before measurements, all samples wereoutgassed at 573 K for 6 h. The BET-specific surface areas were cal-culated using adsorption data in the relative pressure range 0 < P/P0 < 0.1. Micropore and external surface areas were obtained by t-plot analysis of the adsorption data in the 3.5 � t � 5 Å t range by

adopting the de Boer reference isotherm equation, whereas porevolumes and pore size distributions were determined by theBarrett–Joyner–Halenda (BJH) method.

Themogravimetric analyses (TGA) were performed with a TAinstruments equipment from 50 �C to 800 �C at 10 �C/min underairflow.

XP spectra (XPS) were collected at a pressure below 5 � 10�7 Pawith a SPECS system using a Al anode XR50 X-ray source (150 W)and a 9-channel Phoibos 150 MCD detector with pass energy of25 eV at 0.1 eV steps. Quantification of surface elements was car-ried out using Shirley baselines and Gaussian–Lorentzian (1:1) lineshapes. Binding energy values were referred to the C1s adventi-tious signal.

Elemental analyses of the beta samples were obtained with aPhilips PW-2400 sequential XRF analyzer with Phillips Super Qsoftware. All measures were made in triplicate.

Infrared spectra were recorded on a Bruker-Equinox-55 FTIRspectrometer with a MCT detector using a DRIFT cell connectedto a temperature controller. Samples were dehydrated at 623 Kfor 2 h under nitrogen. The spectra were then acquired at this tem-perature by accumulating 64 scans at 4 cm�1 resolution in therange of 400–4000 cm�1.

Acid capacity was measured through the determination of cat-ion-exchange capacities using aqueous sodium chloride (2 M) solu-tions as cationic-exchange agent. Released protons were thenpotentiometrically titrated [22].

2.3. Catalytic activity

Etherification of glycerol with isobutene (glycerol/isobutenemolar ratio of 0.25) was carried out in a stainless steel stirred auto-clave (150 mL) equipped with temperature controller and a pres-sure gauge. Liquid phase pressurized isobutene was injected intothe reactor, previously charged with glycerol and catalyst (0.5 g),using nitrogen at 10 bar as pushing agent. The temperature wasthen raised to 75 �C and the pressure increased accordingly follow-ing the liquid–vapor equilibrium. Stirring was fixed for all experi-ments at 1200 rpm to avoid external diffusional limitations.Catalytic experiments were made at 4 and 24 h. Reaction productswere analyzed by gas chromatography using a SupraWax-280 col-umn and a FID detector. Glycerol conversion and selectivity toMTBG were determined from calibration lines obtained from com-mercial products. For glycerol diethers (DTBG) and triether (TTBG),which were not available commercially, we isolated them from thereaction products by column chromatography and identified by 13Cand 1H NMR (see Supplementary data) for proper quantification[23]. Turnover frequency values (TOF) were calculated as molesof glycerol converted per mole of active species (H+) per hour.

3. Results and discussion

3.1. Characterization of the catalysts

X-ray diffraction patterns (XRD) of the sulfonated SBA-15 sam-ples (Fig. 1) showed three peaks corresponding to the 100, 110 and200 reflections of the ordered 2D hexagonal (P6mm) mesostruc-ture. After sulfonation of beta, we observed the maintenance ofthe zeolite structure although some decrease in crystallinity wasdetected for the sulfonated samples (Fig. 2), especially for that sul-fonated with conventional heating (Table 1).

Fig. 3 shows the N2 adsorption–desorption isotherms of the sul-fonated samples. N2 adsorption isotherms were Type I for the zeo-lite samples, attributed to microporous materials, and Type IV forthe SBA-15 samples, associated with mesoporous materials,according to the Brunauer, Emmett and Teller classification.

a

b

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Lin

(co

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)

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Fig. 1. XRD patterns of SBA-15-CS (a) and SBA-15-CMwS (b).

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nts)

0100200300400500600700800900

10001100120013001400150016001700

2-Theta - Scale5 10 20 30

a b

c

Fig. 2. XRD patterns of beta (a), Beta-CS (b) and Beta-MwS (c).

204 M.D. González et al. / Journal of Catalysis 290 (2012) 202–209

Sulfonated SBA-15 and sulfonated beta samples showed lower sur-face areas and lower total pore volumes than their correspondingstarting material counterparts (Table 1). This could be related tothe incorporation of the organosulfonic acid groups.

In order to confirm and quantify the introduction of the sulfonicacid groups, X-ray photoelectron spectroscopy (XPS) and thermo-gravimetric analysis (TGA) techniques were used. XPS is a usefultechnique for determining the type of sulfur species and measuringquantitatively the sulfonic acid groups near the surface region[13,24], whereas the weight loss observed between 360 and660 �C in the TGA of organosulfonated samples has been relatedin the literature to the loss of organosulfonic acid groups [12,22],allowing us to calculate the mmol organic sulfonic acid groups/gsample (Table 1).

The S 2p XP spectra of sulfonated SBA-15 samples only showedone peak at ca. 168–169 eV with higher intensity for SBA-15-MwS(Fig. 4a). This peak has been associated with sulfate (S6+) speciesdue to sulfonic (ASO3H) acid groups [13,24]. Additionally, TGA ofthe sulfonated SBA-15 samples (Fig. 5a and b) showed a clearweight loss between 360 and 660 �C, indicated between bars on

the figures, confirming functionalization. Therefore, the use ofmicrowaves for the sulfonation of SBA-15 allowed us a faster intro-duction of higher amounts of sulfonic acid groups in the mesopor-ous silica, according to the higher S/Si atomic ratio, obtained fromXPS, higher sulfur content due to sulfonic acid groups [12,22]determined by TGA, lower surface area and higher acidity observedfor SBA-15-MwS (Table 1).

Regarding beta samples, XPS confirmed sulfonation of betawhen using conventional heating as well as microwaves duringthe sulfonation step since only one S 2p peak, corresponding to sul-fonic acid groups, was observed with similar intensity for bothsamples (Fig. 4b). This is the first time that the post-synthesis sul-fonic acid-functionalization of a zeolite is reported. The S/Si atomicratio, the sulfur content and the acid capacity values were verysimilar for both samples (Table 1). However, Beta-CS showed ahigher decrease in BET, micropore and external surface area thanBeta-MwS with respect to the starting Beta sample (Table 1). Thiscan be explained by the higher loss of crystallinity suffered by theconventionally sulfonated sample, as observed by XRD (Fig. 2 andTable 1). This effect of losing surface area when losing zeolite

Table 1Characterization of the samples.

Samples Crystallinitya

(%)BET area(m2/g)

Micropore area(m2/g)

External surfacearea (m2/g)

Pore volume(cm3/g)

S/Si surfaceratiob

Sulfurcontentc

Acid capacity(meq H+/g)d

Si/Al surfaceratiob

Si/Al bulkratioe

SBA-15 – 1082 – – 1.68 – – –- – –SBA-15-CS – 640 – – 0.81 0.03 0.83 0.35 – –SBA-15-MwS – 575 – – 0.74 0.07 1.01 0.75 – –Beta 100 584 462 122 0.23 – – – 9 10Beta-CS 27 330 243 87 0.14 0.06 0.74 0.75 53 55Beta-MwS 45 505 394 111 0.18 0.06 0.70 0.72 69 71

a Calculated from XRD patterns.b Determined from XPS.c (mmol organic sulfonic acid group/g sample) calculated from TGA.d Obtained by potentiometric titration.e Determined from XRF.

0

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300

0.0 0.2 0.4 0.6 0.8 1.0

Ads

orbe

d vo

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e (c

m3ST

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Relative pressure (p/po)

0

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600

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orbe

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e (c

m3ST

P/g)

Relative pressure (p/po)

a b

dc

Fig. 3. Nitrogen adsorption–desorption isotherms of SBA-15-CS (a), SBA-15-MwS (b), Beta-CS (c) and Beta-MwS (d).

M.D. González et al. / Journal of Catalysis 290 (2012) 202–209 205

crystallinity has been previously reported for other beta-modifiedsamples [18,25].

Taking into account that the main difficulty to introduce bulkyorganic species in a zeolite is the lack of reactant silanol groups(„SiAOH) [16], an important key to understand these good betasulfonation results is the beta dealumination occurring under theacidic conditions used during sulfonation. The Si/Al atomic surfaceratio and the Si/Al bulk ratio of Beta-CS and Beta-MwS, calculatedfrom XPS and XRF respectively, confirmed dealumination. It is wellknown that zeolite beta is easier to dealuminate than ZSM-5 ormordenite [17,18,26]. This has been related to the flexibility of

the zeolite framework, and the accessibility of the aluminumatoms depending on the pores arrangement and sizes [26]. In aprevious study, we also conclude that the use of microwaves dur-ing dealumination in acidic medium led to higher dealuminationthan conventional heating for the same treatment conditions[18]. We believe that zeolite dealumination during sulfonationfavors the generation of new silanol groups than can react withthe organosulfonating agent resulting in the incorporation of thesulfonic acid groups (Scheme 1). In order to confirm this proposedmechanism, we compared the FTIR spectra of commercial beta, apartially dealuminated beta sample, which was obtained by

Beta-CS

Beta-MwS

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174 172 170 168 166 164 162 160

174 172 170 168 166 164 162 160

Binding Energy (eV)

Binding Energy (eV)

CPS

C

PS

SBA-15-CS

SBA-15-MwS

(a)

(b)

Fig. 4. (a) XP spectra of samples SBA-15-CS (conventional heating) and SBA-15-MwS (microwaves) in the S 2p core level region; (b) XP spectra of samples Beta-CS(conventional heating) and Beta-MwS (microwaves) in the S 2p core level region.

206 M.D. González et al. / Journal of Catalysis 290 (2012) 202–209

treatment of commercial beta in HCl 2 M for 15 min, and the sulfo-nated beta sample Beta-CS (Fig. 6). As we can observe, there was aclear increase in the intensity of the silanol band (around3745 cm�1) for the partially dealuminated sample, whereas aftersulfonation, the silanol band decreased as a consequence of thereaction of the silanol groups, formed during dealumination, withthe organosulfonating agent. Although the zeolite sulfonated withmicrowaves suffered higher dealumination than when sulfonatedby conventional heating, similar amounts of organosulfonic acidgroups were incorporated for both samples (Table 1). This couldbe related to the generation of some silanols in Beta-MwS whichwere not accessible to the sulfonating agent.

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ght (

%)

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gth

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-CH2CH2PhSO3H

-CH2CH2PhSO3H

a

c

Fig. 5. Thermogravimetric weight loss curves of SBA-15-C

3.2. Catalytic activity

Table 2 shows the catalytic activity of SBA-15 and beta samplescompared with that of a commercial Amberlyst-type catalyst forthe etherification of glycerol with isobutene. The reaction productsobtained were mono-tert-butyl glycerol ether (MTBG), di-tert-bu-tyl glycerol ether (DTBG) and tri-tert-butyl glycerol ether (TTBG).Besides, diisobutylene (DIB) was detected in low amounts for allthe sulfonated samples.

Pure silica (SBA-15) and commercial beta zeolite showed thelowest conversion values due to their low amount of acid centers.However, functionalized catalysts were very active due to the pres-ence of the sulfonic acid groups. Sulfonated SBA-15 and sulfonatedbeta catalysts showed higher conversion and higher TOF valuesthan the commercial Amberlyst-type catalyst at the same reactionconditions (Table 2). This can be explained by the higher surfacearea of the SBA-15 and beta zeolite catalysts that should favor abetter distribution of the acid centers and, therefore, the accessibil-ity of the reagents to the acid sites. SBA-15 sulfonated by conven-tional heating (SBA-15-CS) showed higher TOF than the rest ofsulfonated samples (SBA-15-Mw-S, Beta-Mw-S and Beta-CS),which exhibited similar TOF values. Taking into account that thiscatalyst has the lowest acid capacity (Table 1), this can be relatedagain to its higher surface area and pore volume (Table 1), whichfacilitate the accessibility of the reagents to the acid sites.

With respect to the selectivity results, after 4 h of reaction, cat-alyst Beta showed higher selectivity toward the diethers thanAmberlyst-15 and SBA-15, in this order (Table 2). This agrees withthe results reported by Klepácová et al. for one H-Beta catalystwith respect to the Amberlyst-15 [7]. For our Na-Beta, this behav-ior can be attributed to the presence of free Brønsted protons, asobserved by 1H NMR for commercial Na-Beta zeolites [18,27]. Ina previous study, we reported that the same commercial Beta usedin this study led high yield to b-phenylacetaldehyde for the isom-erization of styrene oxide, catalyzed by Brønsted acid sites [18].The detection of glycerol triether in very low amounts for catalystBeta, in contrast to the absence reported by Klepácová et al. [7] forone H-Beta catalyst, suggests that microporosity is not a barrier for

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-CH2CH2PhSO3H

-CH2CH2PhSO3H

b

d

S (a), SBA-15-MwS (b), Beta-CS (c) and Beta-MwS (d).

Scheme 1. Mechanism of sulfonic acid-functionalization of zeolite beta.

0,5

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cm- 1

Abs

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b

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Fig. 6. FTIR spectra of commercial beta (a), partially dealuminated beta (b) andsample Beta-MwS (c).

M.D. González et al. / Journal of Catalysis 290 (2012) 202–209 207

the formation of the bulkier glycerol triether, since the reaction cantake place with the appropriate amount and strength of Brønstedacid centers, as later confirmed for sulfonated beta catalysts.

Sulfonated SBA-15 and sulfonated beta catalysts showed higherselectivity to h-GTBE than the Amberlyst catalyst at the same reac-tion conditions. It is important to note that Amberlyst-15 producesdiisobutylenes (DIB), the non-desired byproducts which can beproblematic when formulated with fuels, in higher amounts thanthe rest of catalysts (Table 2). These acid-based resins seem to beable to further dimerize the remaining isobutylene to producethe corresponding DIB. This is in agreement with the results previ-ously reported by other authors [5,7,8].

Interestingly, the samples sulfonated with microwaves showeda remarkably higher selectivity to h-GTBE than those functional-ized by conventional heating, especially after 4 h of reaction. ForSBA-15 catalysts, this can be mainly related to the higher amountof Brønsted acid sites incorporated in the sample sulfonated with

microwaves (Table 1). However, for Beta catalysts, which had sim-ilar amount of acid centers (Table 1), the higher selectivity to h-GTBE of catalyst Beta-MwS could be attributed to a higher accessi-bility of its acid sites, since this catalyst showed slight higher porevolume (Table 1), slight higher mesoporosity (Fig. 7) and higherexternal surface area after sulfonation than the beta catalyst ob-tained by sulfonation with conventional heating (Table 1).

The effect of microwaves on sulfonic acid-functionalization, andtherefore on the catalytic activity results, when compared withconventional heating, can be mainly related to the most homoge-neous heating achieved with microwaves [28,29], which favorscrystallinity of the synthesized samples [20,28]. In addition, formodified beta samples, microwaves in acidic medium favored dea-lumination and therefore the formation of slight higher mesopo-rosity, as previously reported [18].

All sulfonated catalysts maintained the sulfur content afterreaction, as determined by TGA. This confirms that there was noleaching of the sulfonic acid groups during reaction. However, fromN2 adsorption–desorption results, we observed a decrease in sur-face area after reaction for all of them, especially after longer reac-tion time. In order to evaluate the nature of the products remainingin the catalytic pores after reaction, several used catalysts, after fil-tration, were submitted to extraction by refluxing with 50 mL ofethanol for 1 h. After rotary evaporation of the solvent, the result-ing solution was analyzed by gas chromatography. All chromato-grams showed the presence of several peaks corresponding toglycerol and products of reaction (mainly monoethers, diethersand triether in less proportion). Additionally, from 4 h to 24 h ofreaction, we observed a considerably decrease in TOF (Table 2).This confirms that some deactivation occurred for all catalysts (in-cluded Amberlyst-15). From all these results, we can conclude thatthe deactivation process can be explained by the presence of

Table 2Catalytic results for the glycerol etherification with isobutene.

Catalysts Reaction time (h) Conversion (%) TOFa �10 (h�1) Selectivity to MTBGb (%) Selectivity to h-GTBEc (%) DIBd (wt%)

Amberlyst-15 4 73 0.76 65 35 (3) 33.024 99 0.17 23 77 (19) 36.2

SBA-15 4 41 – 87 13 (0) 3.324 62 – 86 14 (2) 10.0

SBA-15-CS 4 98 7.60 39 61 (5) 0.724 99 1.28 15 85 (28) 2.2

SBA-15-MwS 4 99 3.58 9 91 (36) 1.724 100 0.60 9 91 (39) 6.8

Beta 4 44 – 32 68 (1) 5.324 49 – 39 61 (2) 6.1

Beta-CS 4 93 3.37 49 51(5) 2.324 100 0.60 12 88 (32) 10.4

Beta-MwS 4 100 3.77 17 83 (15) 2.024 100 0.63 9 91 (36) 9.4

a TOF: turnover frequency.b MTBG: glycerol monoethers.c h-GTBE: glycerol diethers + glycerol triether. In parenthesis, selectivity to glycerol triether (%).d DIB: diisobutylene.

Fig. 7. BJH pore size distribution of sulfonated zeolites.

208 M.D. González et al. / Journal of Catalysis 290 (2012) 202–209

reagents and reaction products in the pores. This decreases theaccessibility of the reagents to the acid sites.

The selectivity to the triether obtained for Beta-MwS after 24 hof reaction (36%) is the best result achieved for this reaction up tonow using a zeolite as catalyst. Additionally, this result was com-parable to that obtained with SBA-15-MwS and much higher thanthat obtained with the Amberlyst catalyst, which is a macroporousacid ion-exchange resin.

4. Conclusions

Commercial beta zeolite was successfully sulfonated in onestep, for the first time, by a post-synthesis simple method usingmicrowaves as well as conventional heating. The zeolite dealumi-nation occurring due to the acidic medium used during acid-functionalization appears to be a key factor in the generation ofnew silanol groups that can react with the organosulfonatingagent. Although the two samples had similar acidity, the samplesulfonated with microwaves presented higher surface area (micro-pore and external), slight higher pore volume, slight higher meso-porosity and a less decrease in zeolite crystallinity than theconventional sulfonated sample. Additionally, SBA-15 was sulfo-nated with microwaves during its synthesis, for the first time,achieving a faster incorporation of higher amount of sulfonic acidgroups than by conventional heating.

Acid-functionalized samples resulted in outstanding activityand selectivity to h-GTBE catalysts for the etherification of glycerol

with isobutene. It is important to note the higher selectivity toh-GTBE achieved for the microwaved-sulfonated samples after4 h of reaction (83–91%). This has been related to the higheramounts of Brønsted acid sites incorporated in SBA-15 sulfonatedunder microwaves whereas for beta acid-functionalized withmicrowaves, this could be associated with a slight higher accessi-bility of its Brønsted acid sites. The excellent catalytic resultachieved with functionalized beta samples revealed the greatimportance of acidity in front of porosity for this etherificationreaction.

Acknowledgments

The authors are grateful for the financial support of the Ministe-rio de Ciencia e Innovación and FEDER funds (CTQ2008-04433/PPQand CTQ2009-12520). M.D.G. acknowledges FPU Grant AP2007-03789. J.L. is grateful to ICREA Academia program. We thankMs. Irene Marín for her help in the performance and interpretationof 13C and 1H NMR spectra.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.jcat.2012.03.019.

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