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Materials Chemistry and Physics 127 (2011) 278–284

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Materials Chemistry and Physics

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

reparation of organic-functionalized mesoporous ZSM-5 zeolites by consecutiveesilication and silanization

haron Mitchell a, Adriana Bonillab, Javier Pérez-Ramíreza,∗

Institute for Chemical and Bioengineering, Department of Chemistry and Applied Biosciences, ETH Zurich, HCI E 125, Wolfgang-Pauli-Strasse 10, CH-8093 Zurich, SwitzerlandInstitute of Chemical Research of Catalonia (ICIQ), Av. Països Catalans 16, 43007 Tarragona, Spain

r t i c l e i n f o

rticle history:eceived 24 September 2010eceived in revised form 5 January 2011ccepted 3 February 2011

eywords:esoporous zeolites

SM-5

a b s t r a c t

Zeolites with various levels of micro- and mesoporosity are prepared by alkaline treatment of ZSM-5 atdifferent temperatures. Terminal silanol groups generated at the mesopore surface during desilication arereacted with aminopropyltriethoxysilane or mercaptopropyltrimethoxysilane leading to the obtainmentof hierarchical zeolites with organic-functionalized hybrid mesopores. Grafting of 3-aminopropyl and 3-thiopropyl moieties is directly evidenced by elemental analysis, nitrogen adsorption, thermogravimetry,and infrared spectroscopy, and the relative extents of surface functionalization of the purely microp-orous and of the mesoporous ZSM-5 zeolites are compared. Based on these results the mechanism oforganosilane attachment is thought to be equivalent in both cases. Organic-functionalized mesopores

ost-synthesis treatment

esilicationilanizationurface functionalizationnzymatic catalysis

exhibit good hydrothermal stability. The adsorption properties of the ZSM-5 samples both prior and postsilanization are assessed for the uptake of lipase enzyme. Mesoporous zeolites show increased uptakesthan the parent zeolite in all cases. This demonstrates that the mesoporosity enhances the adsorption oflarge guest species that cannot be accommodated within the micropores of their conventional counter-parts. Immobilization on surface functionalized zeolites greatly improves the retention of lipase activity

olysi

when applied in the hydr

. Introduction

Hierarchical porous zeolites possessing auxiliary mesoporosityere conceptually sought after to overcome diffusion limitations

ssociated with purely microporous materials, leading to theirnhanced effectiveness in catalytic applications [1,2]. Of the exist-ng routes for the preparation of mesoporous zeolites [2], alkalinereatment, in which intracrystalline mesoporosity is introduced byesilication of the zeolite framework, is attractive due to its rela-ive simplicity and controllability. Zeolites with differing levels of

icro- and mesoporosity may be obtained depending on (i) the zeo-ite studied (i.e. framework structure and composition) [3], (ii) thereatment conditions (e.g. base concentration, temperature, treat-

ent length) [4], and (iii) the presence of pore directing agentsframework aluminum or external) [5,6].

From the perspective of efficient utilization, besides the micro-ores, the mesopores themselves offer a potential functional

pace within the zeolite crystal. Accessible to larger molecu-ar species [7], the composition of the mesoporous surface maylso be modified [8], which can be exploited to introduce dis-inctive reactivity in addition to the Brønsted acidity inherent

∗ Corresponding author. Fax: +41 44 633 1405.E-mail address: jpr@chem.ethz.ch (J. Pérez-Ramírez).

254-0584/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.matchemphys.2011.02.003

s of p-nitrophenylbutyrate.© 2011 Elsevier B.V. All rights reserved.

to the micropores. The internal surface of mesopores formed bydesilication is decorated with reactive terminal silanol groups[9], which could be functionalized by common methods. Cross-coupling with organosilane molecules, referred to as silanization, isone of the post-synthetic techniques by which this may be achieved[10–16], and a library of differing functionalities are availableto permit the specific engineering of surface properties. In otherclasses of mesoporous materials, such as ordered mesoporous sil-ica (e.g. MCM-41, SBA-15), reaction with organosilanes has widelyextended the range of potential applications, permitting controlledvariation of pore diameter [10], tethering of active species withinthe mesopores [17], and modification of their adsorption properties[12,18].

Due to diffusion constraints, size exclusion effects, and thehigh reactivity of terminal silanol groups, grafting of the organosi-lane molecules is typically confined to the outer surface and poremouths of conventional zeolite crystals and internal functionaliza-tion of the micropores is limited [19,20]. In this context, reactionwith organosilanes has been used to modify the properties of theexternal crystal surface, for example to passivate the activity of

terminal silanol groups, or for the preparation of colloidal zeolitedispersions [13]. A different approach has been the use of organosi-lanes to cap the entrances to micropores, and hence trap speciespreviously introduced within the zeolite crystal, in a ‘ship in thebottle’ type strategy [16]. Organosilane-modified zeolites have also

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S. Mitchell et al. / Materials Chem

een used as intermediates for the introduction of more complexunctionalities [14].

The idea of functionalizing hierarchical porous zeolites byilanization has only recently been addressed; Ryoo et al. [21]eported the preparation of a 3-aminopropyl functionalized meso-orous zeolites, obtained by the surfactant templating route, andang et al. [22], reported the preparation of a 3-thiopropyl func-ionalized mesoporous MCM-22 zeolite obtained by desilication.n related work, the silanization of a defected and mesopore-ontaining pure silica beta zeolite, prepared by mild NaOHreatment of a purely microporous zirconia-coated parent [23], andhe functionalization of mesoporous zeolite Y prepared by steam-ng [24] have also been reported. As the silanization of hierarchicaleolites was not the primary focus of any of the above studies,he ‘success’ of functionalization was judged more empirically,ased on further application, and detailed characterization of therganic-functionalized zeolites was not reported. Methods such asilanization [16] form part of an extensive toolbox of approachesvailable to modify the properties of conventional zeolites. Thus,heir applicability to hierarchical zeolites is of significant interest.

Herein, we prepare organic-functionalized mesoporous zeolitessing a combined two-step approach involving desilication anduccessive silanization. A ZSM-5 with a Si/Al ratio of 40 is selecteds the starting zeolite because (i) the alkaline treatment of the MFIramework is well understood and (ii) this particular Si/Al ratio isnown to give optimal mesoporosity development for conventionalesilication in NaOH solution [25]. Hierarchical ZSM-5 zeolitesith different degrees of mesoporosity are prepared by alkaline

reatment at different temperatures. The influence of mesoporosityn organic-functionalization is investigated by comparison of theilanization of mesoporous and conventional ZSM-5 with amino-ropyltriethoxysilane (APTES) or mercaptopropyltrimethoxysilaneMPTMS). These agents are commonly applied for the function-lization of conventional zeolites [12,13]. Several techniques aresed to characterize the zeolites before and after surface function-lization, including a detailed analysis of the variation in texturalroperties and composition. The (hydro)thermal stability of therganic-modified zeolites are also investigated. The influence ofesilication and surface functionalization on the adsorption prop-rties is assessed by comparison of the uptake of lipase enzymey purely microporous and combined meso-/microporous zeolitesefore and after surface functionalization. The potential advantagesf surface functionalization are also shown by preliminary compar-son of the influence on lipase retention on catalytic application ofhe immobilized enzyme.

. Experimental

.1. Desilication

Commercial NH4-ZSM-5 zeolite with a nominal Si/Al of 40 (CBV 8014) was pur-hased from Zeolyst International. The as-received powder was calcined in staticir at 550 ◦C for 5 h using a heating ramp of 5 ◦C min−1, leading to sample P (par-nt). Hierarchical porous ZSM-5 zeolites, with different degree of mesoporosity,ere prepared by desilication following previously reported methodologies [25].riefly, alkaline treatment of the calcined zeolite (1 g) was undertaken with aqueousodium hydroxide (0.2 M, 30 cm3) for 30 min at 45, 55, or 65 ◦C. The alkaline-treatedroducts (denoted H1, H2, or H3, respectively) were isolated by filtration, washedith distilled water until the emitted filtrate was pH neutral, and dried at 80 ◦C.

he hierarchical zeolites were converted to the protonic form by three consecu-ive exchanges in 0.1 M NH4NO3 solution (298 K, 6 h) and calcined using the samerocedure as for the parent zeolite.

.2. Silanization

Prior to attempted functionalization, the zeolites in their protonic form wereeated in a static oven at 350 ◦C for 4 h, in order to remove the majority ofhysisorbed water present. Reactions with aminopropyltriethoxysilane or mercap-opropyltrimethoxysilane were undertaken in n-hexane for 24 h at 25 ◦C. The zeolite0.25 g) was dispersed in a stirred solution of n-hexane, to which a fixed amount of

nd Physics 127 (2011) 278–284 279

organosilane (0.001 mol) was added dropwise. The 3-aminopropyl (coded N) and3-thiopropyl (coded S) functionalized zeolites were filtered, washed with n-hexanefollowed by distilled water, and dried at 80 ◦C. The hydrothermal stability of theorganic-modified zeolites was assessed by treatment in an autoclave with distilledwater (1 wt.% solid) at 110 ◦C for 24 h.

2.3. Characterization

X-ray diffraction (XRD) was undertaken using a Siemens D5000 diffractome-ter with Bragg-Brentano geometry and Ni filtered Cu K˛ radiation (� = 0.1541 nm).Data were recorded in the range of 5–50◦ 2� with an angular step size of 0.05◦

and a counting time of 8 s per step. Elemental analysis was undertaken by theMicro-laboratory at the Laboratory for Organic Chemistry, ETH Zurich. The con-tent of carbon, nitrogen, hydrogen, and sulfur of each sample was determinedusing a LECO model CHNS-932 elemental analyzer, which has a relative error ofless than 0.1%. Transmission electron microscopy (TEM) imaging was performedwith a Phillips CM12 instrument operated at 100 kV. The sample was dispersedfrom ethanol onto a laced carbon-coated copper grid, which was subsequentlydried at room temperature. Nitrogen isotherms were measured in a QuantachromeQuadrasorb-SI gas adsorption analyzer at −196 ◦C. Samples were degassed in vac-uum at 180 ◦C for 24 h prior to measurement. The total pore volume was derivedfrom the amount of nitrogen adsorbed at p/p0 = 0.98, the t-plot method [26] wasused to discriminate between micro- and mesoporosity, the BET method [27] wasapplied to determine the total surface area (for comparative purposes), and theBJH method [28] applied to the adsorption branch of the isotherm was used toestimate the mesopore size distribution. Fourier transform infrared spectroscopywas carried out in a Thermo Nicolet 5700 spectrometer (Thermo Scientific) usinga SpectraTech Collector II diffuse reflectance (DRIFT) accessory equipped witha high-temperature chamber, ZnSe windows, and a mercury cadmium telluride(MCT) detector. Spectra were recorded at 180 ◦C, after flushing with nitrogenfor 2 h at this temperature prior to measurement, using KBr (Aldrich, IR spec-troscopy grade) treated equivalently as the background. The range 650–4000 cm−1

was covered by co-addition of 32 scans at a nominal resolution of 4 cm−1. Ther-mogravimetry was measured in a Mettler Toledo TGA/SDTA851e microbalance.Analyses were performed in air ramping the temperature from 30 to 1000 ◦C at5 ◦C min−1.

2.4. Lipase adsorption and hydrolytic activity

The accessibility of the functionalized mesopores in the hierarchical zeolites wasassessed by the adsorption of lipase (triacylglycerol ester hydrolase, EC no. 3.1.1.3),purchased from Anamo Enzyme Inc. The as-supplied powdered lipase enzyme wasdissolved (20 mg cm−3) in aqueous sodium phosphate buffer (0.05 M, pH 7) and anyinsoluble content was removed by filtration. The zeolite (0.1 g) was then introducedand an orbital shaker was used to stir the support-enzyme containing solutions for2 h. The lipase-containing zeolites were collected by filtration and dried at roomtemperature.

The biocatalytic activity of the supported lipase enzyme was assessed spec-troscopically, as previously reported, for the hydrolysis of p-nitrophenylbutyrate[29]. Assays were undertaken in standard cells with a reaction volume of 10 cm3 at25 ◦C using orbital stirring (200 rpm). Cells were charged with a known amount ofsupported enzyme. Aqueous phase reactions were undertaken in 4 cm3 of sodiumphosphate buffer solution (0.05 M, pH 6) with 0.75 cm3 of isopropanol and the reac-tion was initiated by addition of 0.25 cm3 of p-nitrophenylbutyrate (pNPB) substratesolution. After reaction for 30 min, the solid catalyst was filtered. The absorbancewas measured in the 250–500 nm range using a Shimadzu UV-2401PC spectropho-tometer with 10 mm optical path cells. Standard solutions of known quantities ofthe pNPB substrate, and of the p-nitrophenol (pNP) hydrolysis product were pre-pared for calibration of the results of activity screening. The degree of conversionachieved was estimated by comparison of the relative intensities of the absorbanceof the pNPE substrate and pNP product. The retention of catalytic activity on repeatcatalytic cycling was evaluated by comparison of the relative conversion observed inthe third catalytic run, X3, as a percentage of that achieved during the first catalyticcycle.

3. Results and discussion

Hierarchical zeolites with differing levels of mesoporosity wereprepared by alkaline treatment of ZSM-5 at different tempera-tures. The influence of the auxiliary mesoporosity, introduced byalkaline treatment, on the extent of achievable silanization wasinvestigated by comparing the variation in textural properties and

composition following reaction of mesoporous and conventionalzeolites with organosilanes. The exact changes depend both on theextent of surface coverage, and on the mode of interaction of theorganosilane with the zeolite surface (e.g. the degree of organosi-lane hydrolysis and the extent of reaction with the surface versus

280 S. Mitchell et al. / Materials Chemistry a

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ig. 1. Summary of possible modes of attachment of the organosilane leading toonolayer or multilayer coverage (X = N/S). The variation in the ratio of C/X with

he extent of organosilane hydrolysis (for R = –CH2CH3(–CH3)), and the thickness (d)f the organosilane layer are also shown.

elf-condensation). Some of the possible modes of interaction areummarized in Fig. 1.

.1. Textural properties

The modification of porosity, on alkaline treatment of the parentnd on surface functionalization of conventional and mesoporouseolites, was evidenced by nitrogen adsorption and transmission

lectron microscopy. Table 1 summarizes the textural proper-ies of the zeolites before and after functionalization with APTESr MPTMS. The nitrogen adsorption isotherms of P, H3, H3–N,nd H3–S zeolites and the corresponding mesopore size distri-utions are compared in Fig. 2. The P zeolite exhibits a type I

able 1nalysis of the textural properties and organic composition of the zeolites.

Zeolite Vporea [cm3 g−1] Vmicro

b [cm3 g−1] Smesob [m2 g−1] SB

P 0.26 0.16 72 4H1 0.31 0.12 91 3H2 0.50 0.12 140 4H3 0.55 0.11 165 4P–N 0.19 0.11 50 3P–N (HT)f 0.19 0.08 48 2H1–N 0.29 0.10 89 3H2–N 0.38 0.07 131 2H3–N 0.48 0.06 164 3H3–N (HT) 0.47 0.05 125 2P–S 0.22 0.09 80 3P–S (HT) 0.20 0.08 69 2H2–S 0.38 0.07 139 3H3–S 0.41 0.08 146 3H3–S (HT) 0.52 0.05 180 4

a Measured as Vads at p/p0 = 0.98.b t-plot method.c BET method.d Weight percentage of C or X = S or N dependent on the organosilane studied, determie Surface density of organosilane groups calculated based on values of Smeso of the unfuf HT: after hydrothermal treatment.

nd Physics 127 (2011) 278–284

isotherm with a high uptake at low relative pressures, consis-tent with the presence of micropores, and a much lower uptakeat high relative pressures. Correspondingly, no evidence of meso-porosity is observed in the pore size distribution. The introductionof mesoporosity upon alkaline treatment is evidenced by changesin the form or the nitrogen adsorption isotherms of the hier-archical porous zeolites obtained, which exhibit a combinationof type I and IV behavior. A broad peak is visible in the poresize distribution, centered around 12 nm, which is consistentwith other equivalently prepared mesoporous ZSM-5 zeolites [25].Increasing the temperature of alkaline treatment led to greatermesoporosity development as previously reported [27]. The totalpore volume (Vpore) increased from 0.26 to 0.55 cm3 g−1 and themesopore surface area (Smeso) increased from 72 to 165 m2 g−1 fol-lowing the trend H3 > H2 > H1 > P. The micropore volume (Vmicro)is similar for all mesoporous zeolites (Vmicro = 0.11–0.12 cm3 g−1),showing the expected decrease with respect to the parent zeolite(Vmicro = 0.16 cm3 g−1).

Silanization results in a reduction in porosity with respect tothe unfunctionalized counterparts. Despite the reduced nitrogenuptake, functionalization does not appear to significantly impactthe form of the isotherm. In all cases the functionalized mesoporouszeolites retain a higher Vpore and Smeso than those of the unfunc-tionalized ZSM-5 parent. Furthermore, the relationship in Vpore

and Smeso, with respect to the extent of mesoporosity developedduring alkaline treatment, remains comparable to that observedprior to functionalization with H3–N/S > H2–N/S > H1–N/S > P–N/S.Grafting of the organosilane leads to a similar reduction in Vmicroin all cases and is thought to be a consequence of capping of themicropores [14], resulting in their effective blockage, rather thandue to their internal functionalization. The decrease in the averagepore diameter (7 nm), seen by a shift in the mesopore size distribu-tion, is of particularly interest as it confirms functionalization of themesopore walls formed during alkaline treatment (Fig. 2, right).

Observation of N-functionalized P and H3 zeolites by TEM sup-ports the findings of N2 adsorption and XRD (Section 3.2). TEMmicrographs of P–N and H3–N zeolites are compared in Fig. 3.As no significant differences were observed upon silanization ofthe zeolite crystals (i.e. the TEM micrographs of H3, H3–N, and

H3–S appear identical), these images were selected as representa-tive examples. Intra-crystalline mesoporosity was not detected inthe functionalized parent ZSM-5 (P–N), which shows low transmis-sion of the electron beam. In contrast, the auxiliary mesoporosity

ETc [m2 g−1] Organicd [wt.%] C/Xd [–] DX

d,e (×1018) [mol m−2]

33 – – –92 – – –23 – – –36 – – –12 5.3 5.0 4.852 4.6 3.6 4.925 4.7 7.9 2.496 5.6 4.1 2.818 5.9 3.6 2.755 6.1 3.4 3.204 8.0 3.6 6.953 7.2 3.4 6.113 6.1 5.3 2.227 6.2 5.3 2.114 6.2 4.3 2.4

ned by elemental analysis.nctionalized zeolites.

S. Mitchell et al. / Materials Chemistry and Physics 127 (2011) 278–284 281

rrespo

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Fig. 2. Comparison of (a) the N2 adsorption isotherms and (b) the co

eveloped by desilication is clearly observable in the functional-zed hierarchical zeolite (H3–N). Well-distributed mesopores, ofizes comparable with the estimated N2-mesopore size distribu-ion (Fig. 2), remain visible following reaction with organosilaneonfirming that mesoporosity is preserved on functionalization.

Fig. 3. TEM images of the functionalized parent (P–N) and hierarchical (H3

nding BJH pore size distributions of P, H3, H3–N, and H3–S zeolites.

3.2. Structure and composition

No noticeable changes were observed by XRD following alkalinetreatment of P or on functionalization of the P/H zeolites, indicatingthat the treatments proceeded with minimal variation in the crys-

–N) zeolites obtained on reaction with aminopropyltriethoxysilane.

282 S. Mitchell et al. / Materials Chemistry and Physics 127 (2011) 278–284

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ciated with silanols interacting by hydrogen bonding), and of thesmall band at 3691 cm−1 (associated with the presence of frame-work imperfections e.g. silanol nests), as previously reported [25].This is consistent with the occurrence of rapid base hydrolysis atlattice imperfections such as hydroxyl nest sites during desilica-

Table 2Assignment of the infrared bands in the samples (Fig. 5).

Band Wavelength[cm−1]

Description Assignment

a 3741 Sharp, intense �(O–H) stretch terminal silanolb 3691 Sharp, weak �(O–H) stretch vicinal silanolc 3654 Sharp, weak �(O–H) stretch vicinal silanold 3600 Sharp, intense �(O–H) stretch Brønsted acid sitee 3470 Broad, intense �(O–H) stretch hydrogen bondedf 3360 Sharp, weak �(N–H) asymmetric stretch

ig. 4. XRD patterns confirming the retention of structural integrity and absencef formation of crystalline impurities during preparation and silanization of theierarchical zeolite.

allinity of the MFI framework and that no crystalline impuritiesere formed (Fig. 4). The appearance of clear lattice fringes in theigh resolution TEM images (Fig. 3) provided further evidence forhe retention of zeolite crystallinity throughout.

Elemental analysis of C, N, and S permitted estimation ofhe organic content (Organic [wt.%]), the degree of organosilaneydrolysis, and the approximate surface density of organosilaneroups present post zeolite functionalization (Table 1). A slightlyigher organic content is observed on reaction with MPTMS thanith APTES. As the density of surface hydroxyl groups and amount

f water present was approximately equivalent in both cases,his is thought to be a result of the faster hydrolysis kinetics of

ethoxy–compared with ethoxy–substituents [30]. Consequently,eaction with MPTMS appears to be less controllable than withPTES leading to greater prevalence of non-ideal functionalization

Fig. 1). The C/X ratio (where X = S or N) provides an indicationf the extent of hydrolysis of the alkoxy groups on silanization.

C/X ratio of 3 is indicative of complete organosilane hydrol-sis (top scheme), while a value greater than 3 suggests theresence of unhydrolyzed alkoxy groups (bottom scheme). Foreaction with APTES, the C/X ratio is observed to decrease withncreasing mesoporosity, as expected due to the increased avail-bility of surface hydroxyl groups. This confirms the expectedncrease in grafted 3-aminopropyl groups for mesoporous zeo-ites, despite the similarity in total organic loadings observedor P and H zeolites, due to increased organosilane hydrolysis.s the kinetics of APTES hydrolysis is thought to be controlledy condensation with the surface, with little self-polymerization,

n the presence of fewer available surface silanol groups thethoxy-groups remain unhydrolyzed, leading to increased organicontent.

Assuming ideal monolayer coverage and selective function-lization of the mesoporous surface area, the surface densityf organosilane groups, DX [molecules m−2], achieved on reac-ion of organosilane can be estimated. The maximum theoreticalurface density for monolayer coverage of 3-aminopropyl organosi-ane groups on a silica surface is thought to be approximately× 1018 molecules m−2 [10]. A value exceeding this is suggestivef the occurrence of non-ideal surface coverage due to self-

olymerization of the organosilane. High values of DX are observedn functionalization of the P zeolite, with S-functionalized zeoliteshowing higher values than that expected for monolayer cov-rage, providing further evidence for self-polymerization in thisase. The slightly lower surface densities observed for the meso-

Fig. 5. Infrared spectra of the parent, hierarchical, and organic-functionalized hier-archical zeolites in the OH-stretching region. Assignment of the bands a–k is givenin Table 2. The results clearly show the generation and consumption of terminalsilanol groups on desilication and silanization, respectively.

porous zeolites might be expected given the additional presence ofaluminum-containing sites in zeolites, which are known to be moreprevalent at the mesopore/external surface following desilication[8], and are known not to contribute as readily as terminal silanolgroups to condensation with organosilane [14].

3.3. Role of surface silanol groups

Infrared spectroscopy was used to study the identity and role ofsurface silanol groups formed upon alkaline treatment and reactedupon silanization reactions. Fig. 5 compares the silanol stretch-ing region of the DRIFT spectra of P, H3, H3–N, and H3–S zeolites.Table 2 collects the assignments of the absorption bands. Alkalinetreatment of P results in an increase in the relative intensity of theband at 3740 cm−1 (associated with terminal silanol groups), anda decrease in the intensity of the broad band at 3490 cm−1 (asso-

g 3300 Sharp, weak �(N–H) symmetric stretchh 2970 Sharp, intense �(C–H) stretch (CH3)i 2933 Sharp, intense �(C–H) stretch (CH2 asymmetric)j 2870 Sharp, intense �(C–H) stretch (CH3/CH2 symmetric)k 2576 Sharp, weak �(S–H) stretch

S. Mitchell et al. / Materials Chemistry and Physics 127 (2011) 278–284 283

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ion and to the formation of isolated silanol groups on the newlyeveloped mesopore surface.

Silanization leads to disappearance of the band at 3740 cm−1,onfirming complete reaction of all terminal silanol groups. Addi-ional bands due to alkyl (2850–3000 cm−1) and thiol (2580 cm−1)r amine (3360/3300 cm−1) groups confirm the organic modifica-ion of the zeolite. No variation in intensity of the band at 3600 cm−1

s observed indicating that the Brønsted sites, present within theeolite micropores, do not partake in the reaction.

.4. Thermal stability

Thermogravimetry (TG) was used to assess the thermal stability,rganic composition and degree of hydration of the zeolites stud-ed. Fig. 6 (left) shows the TG profiles of P, H3, H3–N, and H3–Seolites, which clearly show the variation in thermal propertiesesulting from alkaline treatment of the microporous parent andn subsequent surface functionalization of the mesoporous zeolitebtained. A relatively large initial weight loss, corresponding to theemoval of physisorbed water, is observed below 200 ◦C in all cases.bove 200 ◦C the functionalized H3–N and H3–S samples undergo a

urther distinct weight loss, which is attributed to thermal decom-osition of the organic moieties grafted to the surface followingeaction with organosilane. The relative weight losses from 30 to00 ◦C and from 200 to 800 ◦C are compared in Fig. 6 (right).

Interestingly, for the unfunctionalized zeolites, a direct cor-elation is observed between the weight loss associated withhysisorbed water and the variation in textural properties on alka-

ine treatment. Comparison of the P and H3 zeolites indicateseight losses of 3.7 and 7.2 wt.%, with corresponding Vpore = 0.26

nd 0.55 cm3 g−1, respectively. Such a relationship is not observedor the functionalized samples, providing evidence for a variationn the degree of hydration following organic-modification of thentracrystalline mesopore surface.

For the functionalized samples, decomposition of the organicomponent occurs at a similar temperature, irrespective ofhe identity of the grafted organosilane (3-aminopropyl or 3-

hiopropyl) studied. In agreement with the results of elementalnalysis (Table 1), the magnitudes of this weight loss measuredor H3–N and H3–S appear almost identical providing independentonfirmation of the extent of organosilane incorporation. The widthf the decomposition step may be related to the mode of organosi-

of the parent, hierarchical, and organic-functionalized hierarchical zeolites.

lane attachment. A gradual weight loss is indicative of non-idealsurface coverage, with the presence of less stable linkages such asunhydrolyzed alkoxy groups or organosilane multilayers. For H3–Nthe decomposition step observed is sharper than that of H3–S. Thissuggests a more uniform attachment or organosilane to the H3–Nsurface, with fewer unhydrolyzed alkoxy substituents remainingfollowing silanization. This is in agreement with the lower C/X valuedetermined for H3–N by compositional analysis.

3.5. Hydrothermal stability

The hydrothermal stability of the grafted organosilaneswas examined by hydrothermal treatment of the N- and S-functionalized zeolites in an autoclave at 110 ◦C for 24 h. Thetextural and compositional properties of the zeolites before andafter hydrothermal treatment (denoted HT) are compared inTable 1. Based on compositional analysis, no significant reductionin the estimated organosilane surface density, DX, was observedfor any of the functionalized P or H zeolites. Hydrothermal treat-ment did, however, lead to a reduction in the organic content andin the C/X ratio. This is thought to be a consequence of the removalof any unhydrolyzed alkoxy groups remaining from surface func-tionalization. These results demonstrate the stability of the graftedorganosilane moieties with respect to hydrolysis. This is also evi-denced by analysis of the textural properties, which show littlevariation following hydrothermal treatment.

3.6. Adsorption properties

The organic-functionalized zeolites prepared by consecutivedesilication and silanization possess mesopores capable of host-ing large guest species which are unable to fit in the micropores ofconventional zeolites. Lipase enzyme was chosen as a model guestspecies in order to investigate the influence of desilication andsilanization on the zeolite adsorption properties. The biocatalyticactivity and stability of the supported enzyme was also assessed.Table 3 compares the uptake of lipase AK by the P and H3 zeo-

lites, before and after N- and S-functionalization. As expected,mesoporous zeolites show significantly increased enzyme load-ings, E, consistent with the resulting increase in Smeso. Althoughsilanization results in a decrease in the lipase uptake with respectto the unfunctionalized counterparts (consistent with the reduc-

284 S. Mitchell et al. / Materials Chemistry a

Table 3Comparison of the lipase uptake (E) by unfunctionalized (U) and functionalized(N/S) zeolites in conventional (P) and mesoporous (H3) forms. The relative conver-sion retained during the third catalytic run (X3), on application for the enzymatichydrolysis of p-nitrophenylbutyrate, is also shown.

Zeolite Ea [mg g−1] X3b [%]

P H3 H3

U 82 265 10N 64 100 85

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S 76 224 95

a Estimated by elemental analysis.b Percentage of activity retention upon three catalytic runs.

ion in total pore volume), organic-functionalized mesoporouseolites (H–N/S) retain higher lipase uptakes than functionalizedonventional zeolites (P–N/S). This confirms that the functionalizedesopores remain accessible to large guest species. Comparison of

he percentage activity retention after three consecutive catalyticuns, X3 [%], for the hydrolysis of p-nitrophenylbutyrate, providesvidence for the improved retention of lipase enzyme by the surfaceunctionalized zeolites. While the activity of the unfunctionalized3 zeolite dropped to only 10% of its initial activity, after threeonsecutive runs, the percentage of activity retension of the func-ionalized H3–N and H3–S remained above 85%. This result furtherevices the potential of silanized mesoporous zeolites in practicalpplications.

. Conclusions

Hierarchical zeolites possessing intracrystalline organic-odified mesopores of tunable composition and textural

roperties were obtained by controlled alkaline treatment andubsequent reaction with organosilane. Silanization of hierarchicalSM-5 led to complete reaction of terminal silanol groups, presentt both the mesopore and the external surface. This demonstrateshat the mechanism of functionalization is similar to that of purely

icroporous zeolites, occurring by cross condensation of terminalilanol groups with organosilane, despite possible differences inhe surface composition and/or the distribution and accessibility ofhese groups. Grafting of 3-aminopropyl and 3-thiopropyl groupsithin the mesopores introduced into ZSM-5 by desilicationas further evidenced by a reduction of the average mesoporeiameter, and of the mesoporous surface area, demonstrating theirccessibility to organosilanes. The grafted groups exhibited goodtability against hydrolysis on hydrothermal treatment. Organic-unctionalized hierarchical zeolites retained higher mesoporous

urface areas than their functionalized microporous counterparts.he mesopores were accessible to large guest species as demon-trated by the higher uptakes observed for the adsorption of lipasenzyme. Silanization was found to be beneficial in improvingnzyme retention on application in catalysis. The ability to apply

[[[

[

nd Physics 127 (2011) 278–284

the toolbox of methods currently available to modify the proper-ties of purely microporous zeolites for the specific engineering ofmesopores in hierarchical zeolites prepared by desilication greatlyextends the range of potential applications.

Acknowledgements

ETH Zurich is acknowledged for financial support. TEM imag-ing was undertaken with the support of the Electron MicroscopyCentre of the Swiss Institute of Technology (EMEZ). Dr. F.Gramm is thanked for assistance and support with microscopytraining.

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