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Microporous and Mesoporous Materials 123 (2009) 129–139

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Microporous and Mesoporous Materials

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Synthesis, structure and acid characteristics of partiallycrystalline silicalite-1 based materials

Yin Fong Yeong, Ahmad Zuhairi Abdullah, Abdul Latif Ahmad, Subhash Bhatia *

School of Chemical Engineering, Engineering Campus, Universiti Sains Malaysia, Seri Ampangan, Nibong Tebal, 14300 Seberang Perai Selatan, Pulau Pinang, Malaysia

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

Article history:Received 5 February 2009Received in revised form 14 March 2009Accepted 30 March 2009Available online 5 April 2009

Keywords:SynthesisCharacterizationPartially crystalline silicalite-1Phenethyltrimethoxysilane (PE)Acidity

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* Corresponding author. Tel.: +60 604 5996409; faxE-mail address: [email protected] (Subhash B

A series of partially crystalline silicalite-1 based materials were synthesized by varying the molar ratio oforganosilane source, phenethyltrimethoxysilane (PE) to tetraethylorthosilicate (TEOS) in the range of0.05–0.50, using one step co-condensation hydrothermal synthesis method. The phenethyl group wassubsequently sulfonated to arenesulfonic acid group following strong acid treatment. The resulting mate-rials were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmissionelectron microscopy (TEM), nitrogen adsorption and desorption and elemental analysis. The structureof these materials was determined by Fourier transform infrared spectroscopy (FTIR), 29Si and 13C solidstate NMR. The % crystallinity of the partially crystalline silicalite-1 as determined from XRD was inthe range of 33–73%. The average crystallite size decreased with the increase of PE concentration inthe synthesis mixture. The thermogravimetric analysis shows that the structures were thermally stableup to 550 �C after elimination of the structure directing agents (SDAs) by calcination at 420 �C. The acidcapacities of these materials ranged from 2.52 to 6.63 mmol H+/g.

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1. Introduction

Zeolites are crystalline microporous aluminosilicates materialswith well-defined micropore structure, good thermal and struc-tural stability and resistance to relatively extreme chemical envi-ronment [1,2]. The use of zeolites as acid catalysts for industrialprocesses, particularly in petroleum refining and petrochemicals,has been widely reported in the literature [3,4]. The most impor-tant applications are found in the field of cracking, hydrocracking,isomerization, alkylation and reforming reactions [5,6]. In the uti-lization of zeolitic catalysts, the reaction activity and product selec-tivity depend strongly on the number, strength and nature of theacid sites present, crystal/particle size and morphology, as wellas the shape and size of the micropores which can induce differentshape-selectivity effects on the product distribution [7]. These par-ticular properties are obtained by varying Si/Al ratio, crystallitesize and morphology, or modifications of extra-framework by cat-ion exchange, pore blockage and elimination of external sites, iso-morphous substitution and functionalization with organic group[4,8–11].

Among all the zeolites, MFI-type (ZSM-5 and silicalite-1) hasbeen extensively studied for industrial processes because of itsmedium pore-size dimension (0.54 � 0.56 nm). A significant re-search effort has been devoted to the synthesis of MFI zeolite with

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smaller crystal size due to its advantages [12]. In the early 1980s,Jacobs et al. [13] reported the synthesis of partial crystallineZSM-5 zeolite which contained small crystallites of less than8 nm in size within an amorphous matrix, using shorter hydrother-mal synthesis times. Nicolaides et al. [3,4,14] reported the synthe-sis of partially crystalline ZSM-5 based materials (NAS materials)using lower synthesis temperatures, ranging from 25 to 140 �C.ZSM-5 based materials with XRD crystallinity level as low as 2%exhibited superior catalytic performance (higher selectivities andyields) in the skeletal isomerization of linear butenes to iso-butene,due to the decreased of zeolite pore lengths presented in these lowcrystalline materials [4]. These materials with XRD crystallinitieslower than 30%, partially crystalline samples possessing 30–70%crystallinity and highly crystalline materials with >70% XRD crys-tallinity, were tested for their catalytic performance in n-hexanecracking activity. They reported that the number of strongBronsted acid sites and n-hexane cracking activity were found tobe disproportionately low for the samples with XRD relativecrystallinities <30%, and both become significantly higher only atcrystallinity levels higher than 30% [14].

In reviewing the reports on the improved catalytic performanceby using smaller crystal size and partially crystalline ZSM-5, it hasdrawn interest to synthesize partially crystalline silicalite-1 basedmaterials. These materials could produce smaller crystal size andextra-framework (amorphous species) with active acid sites. Silica-lite-1 is an aluminum-free analogue of ZSM-5 (Si/Al =1) which iscatalytically inactive in its pure form. In defining the acidity of the

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Table 1Parent and partially crystalline silicalite-1 based samples prepared in this study.

Sample Sample after sulfonation TEOS, 5(1 � x) PE, 5x

Silicalite-1 (SIL-1,as reference material)

– 1.00 0.00

SIL–PE5 SIL–PE5–SO3H 0.95 0.05SIL–PE10 SIL–PE10–SO3H 0.90 0.10SIL–PE15 SIL–PE15–SO3H 0.85 0.15SIL–PE20 SIL–PE20–SO3H 0.80 0.20SIL–PE30 SIL–PE30–SO3H 0.70 0.30SIL–PE50 SIL–PE50–SO3H 0.50 0.50

130 Y.F. Yeong et al. / Microporous and Mesoporous Materials 123 (2009) 129–139

partially crystalline silicalite-1 based materials, the amount andtype of extra-framework or amorphous species presence isimportant.

In order to create acid sites in the partially crystalline silicalite-1 based materials, the organic-functional group should be intro-duced [15,16]. Jones et al. [17–19] reported that the organic groupis difficult to introduce into the zeolite micropore due to the largesize of organic species, which resulted in a disruption of the crystalstructure and multiphase, and thus, crystalline/amorphous mix-ture is produced. It is expected that the transformation of the or-ganic group attached on the amorphous materials intoorganosulfonic acid group will result in a useful acid materialswhich could be explored as catalyst for industrial reaction [20].The main advantage of these materials is the absence of blockageof silicalite-1 by acid sites from the extra-framework species thatlocated on the outer surface of the crystals.

In the present study, phenethyltrimethoxysilane (PE) as an org-anosilance source has been utilized in the synthesis of partiallycrystalline silicalite-1 based materials. The effect of the organosi-lanes concentration present in the initial synthesis mixtures onthe formation of partially crystalline silicalite-1 based materialsis systematically studied. The organic group is subsequently trans-formed into arenesulfonic acid via sulfonation. The crystalline andextra-framework/amorphous phases of the samples were charac-terized for their % crystallinity, crystal morphology, composition,structure, thermal stability and surface area characteristic by anumber of physical and chemical techniques. The acid capacitiesof the samples were obtained by acid–base titration. The acidicproperties of these materials are correlated with the compositionof the crystalline and amorphous phases present in the samples.

2. Experimental

2.1. Samples preparation

The synthesis of partially crystalline silicalite-1 based materialsand parent silicalite-1 (siliceous ZSM-5) sample are carried out fol-lowing the method reported by Lai et al. [21] and co-condensationmethod [10,22–24].

A series of samples were synthesized using tetraethylorthosili-cate (TEOS) as inorganic silica source and phenethyltrimethoxysi-lane (PE) as organosilica source. The synthesis solution wasprepared by adding tetrapropylammonium hydroxide (TPAOH,1 M, Merck) in a polypropylene bottle containing deionized (DDI)water. TEOS (>98%, Merck) was added drop wise and the solutionwas stirred gently. An appropriate amount of PE (>97%, Fluka)was then added slowly to complete the reaction mixture. The finalmolar composition of the synthesis solution is presented as:

5ð1� xÞ TEOS : TPAOH : 1000 DDIH2O : 5xðPEÞ

where x is molar composition and x = 0.05, 0.1, 0.15 0.2, 0.3 and 0.5in the present study and for the synthesis of silicalite-1, x = 0. Thereaction mixture was stirred vigorously for 1 day at room tempera-ture. After vigorous stirring, the synthesis solution was transferredinto Teflon-lined reaction pressure vessel. The reaction vessel wassealed and heated to 175 �C for 1 day. After hydrothermal synthesis,the vessel was taken out and quenched to room temperature. Thesamples were collected and washed by repeated centrifugationand decanting until the pH of the seed suspension became 8. Thesamples were then dried at 100 �C overnight and further calcinedat 420 �C for 15 h with heating and cooling ramping rate of 0.5 �C/min.

In the present work, the partially crystalline silicalite-1 basedmaterials were synthesized by increasing the PE concentration in

the synthesis mixture while keeping the synthesis temperatureand duration constant at 175 �C and 1 day, different from themethod reported by Nicolaides [4].

2.2. Post-synthesis modification

The phenethyl group present in partially crystalline silicalite-1based materials was sulfonated to arenesulfonic acid group follow-ing the method reported by Holmberg et al. [25]. The calcined sam-ples were dispersed in 96 wt.% concentrated sulfuric acid andtreated at 80 �C for 24 h under stirring. After 24 h, the suspensionwas filtered, washed extensively with DDI water and dried forovernight at 100 �C. Finally, the prepared samples were stored ina desiccator before characterization.

The sample synthesized with a ratio of 0.95 TEOS/0.05 PE iscoded as SIL–PE5 and SIL–PE5–SO3H after sulfonation. Six sampleswith different PE composition were prepared and presented in Ta-ble 1. For comparison purpose, parent silicalite-1 (SIL-1) samplewas also prepared.

2.3. Characterization

2.3.1. X-ray diffraction (XRD)The crystallinity of the prepared samples was obtained from

XRD analysis. The analysis was done using X-ray diffractometerSiemen (D5000) with Cu Ka radiation (k = 1.5406 Å) operated at40 kV and 30 mA. Data were collected stepwise over 5� 6 2h 640� angular region. The crystallinity of samples was determinedbased on the ratio of the major peak intensities of the samples(at 2h � 7.7�, 8.8� and 23�), relative to those of highly crystallinereference material [4]. The crystallinity is defined as:

% XRD crystallinity ¼ sum of peak int ensities of the samplessum of peak int ensities of the reference

� 100

ð1Þ

A highly crystalline silicalite-1 sample was used as a referencematerial.

2.3.2. Scanning electron microscope (SEM)Surface morphology of the samples was studied using scanning

electron microscope (Zeiss Supra 35VP) equipped with W-Tung-sten filament, operated at 3.00 kV.

2.3.3. Transmission electron microscope (TEM)TEM micrographs were obtained on a CM 12 Philips electron

microscope equipped with an image analyzer (Model Soft ImagingSystem, SIS 3.0), operating at 80 kV. The samples were prepared byevaporating one drop of powdered sample-EtOH suspension (aftersonication) onto a carbon coated film supported on a 3 mmdiameter, 400 mesh copper grid.

2θ, degree

(a)

(b)

(c)

(d)

(e)

(f)

(g)

Inte

nsity

(a.

u.)

5 10 15 20 25 30 35 40

(051) (020) (101) (501)

(303) (151)

Fig. 1. XRD pattern for (a) SIL-1, (b) SIL–PE5–SO3H, (c) SIL–PE10–SO3H, (d) SIL–PE15–SO3H, (e) SIL–PE20–SO3H, (f) SIL–PE30–SO3H, and (g) SIL–PE50–SO3H.

Table 2X-ray results for the samples.

Sample % Crystallinity Average crystallite sizea, D, nm

SIL-1 100 54.3SIL–PE5–SO3H 73 + Amorphous 39.1SIL–PE10–SO3H 60 + Amorphous 37.7SIL–PE15–SO3H 47 + Amorphous 30.5SIL–PE20–SO3H 33 + Amorphous 28.7SIL–PE30–SO3H Amorphous n.dSIL–PE50–SO3H Amorphous n.d

a Crystallite size obtained from Scherrer equation, t = 0.9 k/bcosh measured atd051.

Y.F. Yeong et al. / Microporous and Mesoporous Materials 123 (2009) 129–139 131

2.3.4. 29Si MAS NMR and 13C CP-MAS solid state NMRThe 29Si and 13C CP NMR spectra were recorded at room tem-

perature under magic angle spinning (MAS) on a Bruker AV400spectrometer. The spectra were recorded with a 4 mm probe at asample spinning rate of 7 kHz and quoted relative to tetramethyl-silane (TMS). The single pulse 29Si was acquired using pulses of4 ls and a recycle delay of 240 s. The 13C cross-polarization wasmeasured with a recycle delay of 5 s.

2.3.5. Fourier transform infrared spectroscopy (FT-IR)The degree of chemical interaction of the organic and inorganic

phases and the nature of the acid sites were determined using FT-IR technique. The IR spectra were recorded using a Perkin–ElmerFT-IR (Model 2000) in the range of 400–4000 cm�1 using KBrmethod. In order to determine the natural acid sites, prior to KBrmethod, the sample was first exposed to excess pyridine for 1 h;after degassing at 200 �C, followed by desorption of physically ad-sorbed of pyridine at 150 �C under vacuum.

2.3.6. Elemental analysisThe chemical analysis of the samples was carried out using a

Perkin–Elmer CHNS/O analyzer where the sample was combustedin an oxygen-rich environment at 975 �C and analyzed for carbon,hydrogen and sulfur content.

2.3.7. Acid capacity determination by titration techniqueThe acid capacity of the samples was determined by titration

technique [10,26]. The acid exchange capacity of the sample wasmeasured, using aqueous solutions of sodium chloride (NaCl,2 M), tetramethylammonium chloride (TMAC, 0.05 M) and tetrabu-tylammonium chloride (TBAC, 0.05 M) as ion-exchange agents.After treated 0.1 g sample at 200 �C for one day, the sample wasadded to 20 mL of the aqueous solution containing the correspond-ing salt. The resulting suspension was equilibrated for 1 day and ti-trated by dropwise addition of 0.01 M NaOH aqueous solutionusing phenolphthalein as an indicator. The acid exchange capacityis expressed as mmol H+/g of sample.

2.3.8. Thermal gravimetric analysis (TGA)The thermogravimetric analysis was conducted under nitrogen

gas with a TGA/SDTA analyzer (Metler Teledo 851E) to study thethermal stability of the synthesized samples. The synthesizedand calcined samples were subjected at the heating rate of 10 �C/min, from room temperature until 900 �C.

2.3.9. Nitrogen adsorption–desorption measurementThe pore characteristic (pore volume, pore-size and surface

area) of the samples were measured by nitrogen adsorption usinga Micromeritics ASAP 2000 instrument. Samples of �0.05 g wereoutgassed overnight at 105 �C under vacuum prior to the analysis.

3. Results and discussion

3.1. Crystallinity and crystal morphology by XRD, SEM and TEM

3.1.1. X-ray diffraction (XRD)Fig. 1 shows the powder XRD patterns of the partially crystal-

line silicalite-1 based materials synthesized at various PE concen-tration. Parent silicalite-1 is highly crystalline and its XRDpattern shows the presence of the major peaks matching well withthe peaks reported for silicalite-1 [21]. Samples SIL–PE5–SO3H toSIL–PE20–SO3H exhibited diffraction patterns similar to that ofparent silicalite-1 but at a relatively low intensity. This shows thatsilicalite-1 structure was retained in the materials. The degree ofcrystallinity was indicated by the peak intensity. It can be seen

from Fig. 1 and Table 2 that the intensity of the samples graduallydecreased with increasing PE concentration in the synthesis mix-ture, from 73% for sample SIL–PE5–SO3H to 33% for sample SIL–PE20–SO3H. The XRD patterns for SIL–PE30–SO3H and SIL–PE50–SO3H displayed a broad features (Fig. 1f and g) in the range2h = 20–30�, which could be indicative of an amorphous phase.This observation shows that crystalline materials cannot be formedwhen the PE loading in the synthesis mixture reached 30% of thetotal silica source. The XRD results were in agreement with the re-sults reported by Jones et al. [17–19]. The higher loading of orga-nosilane resulted in the disruption of the crystal structure due tothe difficulties of incorporation of longer organic fragment intothe crystalline zeolitic framework, and thus, resulting in the amor-phous phases.

For all the partially crystalline samples (SIL–PE5–SO3H to SIL–PE20–SO3H) obtained, the peaks were less intense and broadercompared to parent silicalite-1, suggesting that the partially crys-talline silicalite-1 based samples present in smaller crystalline do-mains [27,28]. The average crystallite size of the samples ispresented in Table 2. It was calculated from the Scherrer equationbased on the half-width of diffraction lines assigned to (0 5 1). Theaverage crystallite size for parent silicalite-1 was 54 nm which wascomparable to the standard silicalite-1 reported in the literature(�60 nm) [29]. The crystallite size of the partially crystalline silica-lite-1 based samples decreased, from 39 nm for sample SIL–PE5–SO3H to 29 nm for sample SIL–PE20–SO3H, with the reduction incrystallinity.

3.1.2. Scanning electron microscopy (SEM)The SEM images for the parent silicalite-1 and partially crystal-

line silicalite-1 based samples are shown in Fig. 2. Silicalite-1


exhibited a complete crystalline phase and coffin like crystal shape,where the morphology was similar to the conventional MFI-typezeolites (Fig. 2a) [30]. By loading PE into the synthesis mixture,both of the partial crystalline and amorphous phases were ob-served, but the coffin shape of silicalite-1 crystal was retained(Fig. 2b–e). The amorphous phase increased with the increase ofPE loading, until a highly amorphous phase was produced with30 and 50 mol% PE loading, respectively. Since the crystal sizeswere reported as the length of the prismatic crystals [31], the sizeof the crystal reduced from 19 lm for silicalite-1 to 10, 6, 4.7, and

Fig. 2. SEM micrographs for (a) SIL-1, (b) SIL–PE5–SO3H, (c) SIL–PE10–SO3H, (d) SI

4.6 lm for SIL–PE5–SO3H, SIL–PE10–SO3H, SIL–PE15–SO3H andSIL–PE20–SO3H, respectively. The reduction in the crystal’s sizeand the presence of amorphous phases explain the drop in thecrystallinity of the samples; and vice verse, where the SEM resultswere consistent with the XRD results.

3.1.3. Transmission electron microscope (TEM)Fig. 3 shows the TEM micrograph of the parent silicalite-1 and

partially crystalline silicalite-1 based samples. The parent silica-lite-1 was formed by spherical shape crystallites with dimension

L–PE15–SO3H, (e) SIL–PE20–SO3H, (f) SIL–PE30–SO3H, and (g) SIL–PE50–SO3H.


of about 40–60 nm (Fig. 3a). From TEM micrographs, a consider-able reduction in the crystallite size was observed for the partiallycrystalline silicalite-1 based samples when higher PE compositionwas added into the synthesis mixture. It can be seen that the sam-ples SIL–PE5–SO3H and SIL–PE10–SO3H also consist of small well-formed crystallite of a nearly spherical shape, with an average sizeranged from 20–40 nm. A similar image can be observed for theSIL–PE15–SO3H and SIL–PE20–SO3H sample, except that the sizesare slightly smaller, in the range of 15–30 and 10–30 nm, respec-tively. They are also more separated from each other as comparedto the relatively more aggregated morphology of the SIL–PE5–SO3H

Fig. 3. Transmission electron microscopy (TEM) images of (a) SIL-1, (b) SIL–PE5–SO3H, (c)SIL–PE50–SO3H.

and SIL–PE10–SO3H samples. The crystallite size obtained fromTEM agreed well with the value determined from XRD data (indi-cated in Table 2), and comparable with the crystallite size of NASmaterials reported by Triantafyllidis et al. [7]. The reduction inthe crystallite size was probably due to the formation of a largenumber of nuclei in the first steps that grow very slowly [32],which reduced the relative crystallinity of the partially crystallinesilicalite-1 based samples. However, the presences of these smallparticles are not small enough in order to generate interparticleordered mesoporosity [7,33]. The TEM micrograph for sampleSIL–PE30–SO3H and SIL–PE50–SO3H are shown in Fig. 3f and g,

SIL–PE10–SO3H, (d) SIL–PE15–SO3H, (e) SIL–PE20–SO3H, (f) SIL–PE30–SO3H, and (g)

Table 3Description of the various structural units in the partially crystalline silicalite-1 basedsamples [10,34,35].

T speciesT1 T2 T3

R

H

H

O

O

Si SiO

H

Si

R

O

O

Si SiO

Si

Si

R

O

O

Si SiO

Q speciesQ2 Q3 Q4

H

Si

OH

O

O

Si SiO

Si

Si

OH

O

O

Si SiO

Si

Si

OSi

O

O

Si SiO

Arenesulfonic acid group

SO3HCH2CH2

III

I

II II

II II

VIV

R = �CH2, Tm = RSi(OSi)m(OH)3�m, Qn = Si(OSi)n(OH)4�n.


respectively. The micrographs show that the irregular grains wereobtained, different from the spherical particles formed by otherpartially crystalline silicalite-1 based samples with lower PE load-ing (Fig. 3b–e). The difference in particles shape could be explainedby the presence of the amorphous phase for both of these samplesas indicated by XRD pattern (Fig. 1) and SEM images (Fig. 2).

3.2. Structural studies and incorporation of organic-functional groupas indicated by, 29Si MAS, 13C CP-MAS solid state NMR and FT-IR

3.2.1. 29Si MAS and 13C CP-MAS solid state NMRWang et al. [9,10] reported that 13C and 29Si solid state NMR

spectroscopies are useful for providing chemical informationregarding the condensation of organosiloxane. 29Si MAS NMR spec-trum for SIL–PE10–SO3H sample is shown in Fig. 4a and thedescription of the various structural units of silicon atoms for thesample is presented in Table 3. Three peaks corresponding for Q2

(�92 ppm), Q3 (�103 ppm) and Q4 (�112 ppm) and three weakerpeaks assigned for T1 (�48 ppm), T2 (�56 ppm) and T3

(�65 ppm) were found in the SIL–PE10–SO3H sample. The appear-ance of the Tm peaks indicates that some of the linkages betweenthe silicon atom containing the organic group and adjacent siliconatoms were hydrolyzed to produce silanol groups during the acidtreatment process [25]. Fig. 4a shows that peak T3 was predomi-nant over T2 implying that the organosiloxane (PE) are effectivelycondensed as a part of the sample structure, probably grafted onamorphous phases [10]. The relative area ratio of Q4/(Q3 + Q2) of2.83 shows that very high condensation of TEOS under the synthe-sis condition [9,10]. The 13C CP-MAS NMR spectrum of the SIL–PE10–SO3H sample is shown in Fig. 4b. Three peaks at 128, 141and 150 ppm assigned to carbons on the aromatic ring, CII, CI andCIII, respectively. The corresponding carbon atoms on the methy-lene group, CIV and CV are shown at peaks 13 and 29 ppm, respec-

Fig. 4. (a) 29Si MAS, and (b) 13C CP-MAS solid state NMR spectra of sample SIL–PE10–SO3H.

tively [10,34]. These peaks are characteristic of the arenesulfonicacid group grafted on the amorphous structure, which further con-firms that phenethyl group is co-condensed in the sample struc-ture and sulfonated to acid group [10].

3.2.2. Fourier transform infrared spectroscopyFT-IR spectroscopy was used to identify the structure vibration

and surface hydroxyl groups of the samples [28]. The FT-IR spec-trum of the parent silicalite-1 and pyridine adsorbed partially crys-talline silicalite-1 based samples are shown in Fig. 5. The analyzedIR absorption peaks of the samples and the corresponding refer-ences are listed in Table 4. Parent silicalite-1 shows a strong peakat 550 cm�1, assigned to MFI-structured zeolite [29,36]. It can beseen that with increasing of PE loading in the synthesis mixture,the partially crystalline silicalite-1 based samples show the reduc-tion in peak intensity at 550 cm�1, or even disappears for samplesSIL–PE30–SO3H and SIL–PE50–SO3H. This observation was mainlydue to the reduction of crystallinity of the sample and the forma-tion of amorphous phases for 30 and 50 mol% PE loading. The esti-mated % relative crystallinity based on these IR peak intensities areshown in Table 5. It was found that the value obtained was compa-rable with the% relative crystallinity estimated from XRD analysis(Table 2).

All the samples show the vibration band at 470, 800, and1100 cm�1, corresponding to the typical Si–O–Si bending, Si–O–Sisymmetric stretching and Si–O–Si asymmetric stretching withthe condensed silica network, respectively. An absorption peak ataround 960 cm�1 was attributed to stretching vibration of Si–OHgroup. In comparison to the spectrum of the parent silicalite-1,the partially crystalline silicalite-1 based samples show additionalpeaks at 700, 1250, 1490, 1542, 1638 cm�1 and a broad band cen-tered at 3430 cm�1. Two weak absorption peaks at 700 and1490 cm�1 were assigned to the out-of-plane bending of the paradi-substituted aromatic ring as well as the bending vibration ofthe sulfonic acid group and C–H in aromatic rings, respectively

Fig. 5. FT-IR spectrum for parent silicalite-1 and partially crystalline silicalite-1 based samples.

Table 4Characteristic bands (cm�1) in FT-IR spectrum obtained in the partially crystallinesilicalite-1 based samples.

Peak positions (cm�1) Assignments Ref.

470 Si–O–Si bending vibration [37,39]550 Double ring vibration in the

MFI-structured zeolite[29,36]

700 Oop ring bend [37]

800 Si–O–Si symmetric stretching [37,39,41]970 Si–OH stretching [37]1100 Si–O–Si asymmetric stretching [37,39]1250 Si–C [10,36,42,43]

1490 Ring [37]

1542 Strong Bronsted acid site [40]1638 Strong Bronsted acid site [40]3430 OH [37]

Table 5Percentage relative FT-IR crystallinity based on the intensity peak at 550 cm�1.

Sample % Relative FT-IR crystallinity

SIL-1 100SIL–PE5–SO3H 80SIL–PE10–SO3H 67SIL–PE15–SO3H 55SIL–PE20–SO3H 36SIL–PE30–SO3H AmorphousSIL–PE50–SO3H Amorphous

Table 6C and S content in the synthesis mixture and obtained partially crystalline silicalite-1based samples.

Samples Synthesismixture (wt.%)a

Elementalanalysis (wt.%)b

Elementalanalysis (wt.%)c

C S C S C S

SIL-1 0 0 0 0 0 0SIL–PE5–SO3H 2.29 0 1.52 0 1.14 0.84SIL–PE10–SO3H 4.50 0 1.97 0 1.64 0.89SIL–PE15–SO3H 6.87 0 2.31 0 1.72 1.63SIL–PE20–SO3H 8.97 0 2.87 0 1.83 3.38

a Relative to the weight of the C and S used in the synthesis mixture.b Relative to the weight of the C and S in the as-synthesized samples.c Relative to the weight of the C and S in the sulfonated samples.


[10,37]. These results suggested the condensation of PE group intothe sample structure, and the aromatic ring was successfully sulfo-nated to arenesulphonic acid group, which was consistent with 13CCP-MAS solid state NMR spectra. These peaks intensities increasewith increasing the PE loading in the synthesis mixture confirmingthat the higher amount of PE group was incorporated into thestructure. The absorbance at 1250 cm�1 assigned to the vibrationof Si–C indicates that the Si–C bond did not break after calcinationand strong acid treatment, thus, confirming the presence of organic

group. The peaks corresponding to the S'O stretching vibrationsof sulfonic acid are normally observed in the range of 1000–1200 cm�1. However, these peaks cannot be resolved due to theiroverlap with the absorbance of the Si–O–Si asymmetric stretch inthe 1000–1130 cm�1 range and that of the Si–C stretch in the1200–1250 cm�1 range [38]. The intense band at 3430 cm�1 wasassigned to –OH group asymmetric stretching, indicates waterand hydroxyl functional group residual in the partially crystallinesilicalite-1 based samples.

Based on these results, the FT-IR spectra confirmed that both or-ganic and inorganic structural units were present in the partiallycrystalline silicalite-1 based samples. The existence of broad bandat 3430 cm�1 implied the hydrophilic characteristic [39] of the par-tially crystalline silicalite-1 based samples compared to the hydro-phobic characteristic of parent silicalite-1 sample. The nature ofthe acid sites present in the samples was also confirmed by FT-IRspectra. The typical peaks of 1638 and 1542 cm�1 indicated thatstrong Bronsted acid sites are present in partially crystalline silica-lite-1 based samples [40] and the silicalite-1 sample did not showpresence of acid sites.

3.3. Chemical properties by elemental analysis and acid–base titration

3.3.1. Elemental analysisThe carbon (C) and sulfur (S) content in the parent silicalite-1

and partially crystalline silicalite-1 based samples were further

SIL-PE5 -SO3H

Sample C (synthesis mixture)

SIL-PE10 -SO3H

SIL-PE15 -SO3H

SIL-PE20 -SO3H

30

40

50

60

70

% X

RD

crystallinity

wt%

0

2

4

6

8

10

C (as-synthesized)C (sulfonated) S (as-synthesized)

S (sulfonated)S (synthesis mixture)

80

Fig. 6. Relationship between the samples crystallinity with the C and S content.

Table 8Acid capacity of other types of acid materials reported in the literature.

Samples Acid capacity (mmol H+/g) Ref.

NAS-150 0.53a [7]SBA-15–SO3H-10 0.82b [26]SiMNP–FSO3H 0.78b [44]SiO2–phSO3H-10 1.43b [10]SBA–phSO3H-10 1.12b [10]SBA–PrSO3H-10 1.04b [10]

a Determined from TPD method.b NaCl as an ion-exchange agent.


characterized by elemental analysis. The results are summarized inTable 6. Although the increase of PE loading in the synthesis mix-ture increased the amount of C into the partially crystalline silica-lite-1 based samples, this amount was relatively low as comparedto the carbon content loaded in the synthesis mixture. This was

Table 7Acid capacity of the partially crystalline silicalite-1 based samples obtained bytitration technique.

Samples S content (mmol/g)from EA

Acid capacity (mmol H+/g)

NaCl TMAC TBAC

SIL-1 0 0.00 0.00 0.00SIL–PE5–SO3H 2.65 2.52 1.18 0.95SIL–PE10–SO3H 2.80 2.72 1.48 1.44SIL–PE15–SO3H 4.10 4.03 1.92 1.57SIL–PE20–SO3H 7.50 6.63 2.07 1.92

Temperature ( oC)

0 200 400 600 800

Wei

ght (

%)

Wei

ght (

%)

88

105

100

95

90

85

80

750 200 400 600 800

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

90

92

94

96

98

100

102

Der

ivat

ive

Wei

ght (

%/o C

)D

eriv

ativ

e W

eigh

t (%

/o C)

-0.007

-0.006

-0.005

-0.004

-0.003

-0.002

-0.001

0.000

Temperature ( oC)

o

water SDA

200oC

400oC

a

c

Fig. 7. TGA curves (a) as-synthesized SIL-1, (b) calcined SIL-1, (c) as-syn

mainly due to the difficulties of incorporation of large organicgroup into the sample structure. After the calcination and sulfona-tion process, sulfur was present in the sample confirming the pres-ence of the arenesulfonic acid group. However, there was loss ofsmall amount of carbon. This observation is in agreement withthe FT-IR spectra, where the Si–C bond was present after calcina-tion and strong acid treatment. Moreover, the FT-IR spectrum forpartially crystalline silicalite-1 based samples show reduction at700 and 1490 cm�1 adsorption peaks (indicate aromatic ring, Table4), compared to these peaks obtained for the as-synthesized sam-ples (not shown).

The relationship between the samples crystallinity with the Cand S content is shown in Fig. 6. Based on this figure, the samplescrystallinity reduced gradually with the increase of C content in thesamples. Reasonably, the amount of S presence in the sample in-creased with the increase of C content in the sample.

3.3.2. Acidity measurement by titration techniqueThe accessibility of sulfonic acid in the partially crystalline sili-

calite-1 based samples was determined by acid–base titrationusing different ion-exchange agents and the results are presentedin Table 7. The amount of sulfonic acid sites increased from 2.52to 6.63 mmol H+/g (Table 7) when NaCl was used as the ion-ex-change agent. The acid capacity value is close to those obtained

0 200 400 600 800

0 200 400 600 800

Der

ivat

ive

Wei

ght (

%/o C

)D

eriv

ativ

e W

eigh

t (%

/o C)

Temperature ( oC)

Temperature ( oC)

Wei

ght (

%)

95

96

97

98

99

100

101

-0.004

-0.003

-0.002

-0.001

0.000

Wei

ght (

%)

90

92

94

96

98

100

102

-0.014

-0.012

-0.010

-0.008

-0.006

-0.004

-0.002

0.000PE

550oC

2.9 wt%

b

d

thesized SIL-PE10, and (d) sulfonated SIL–PE10 or SIL–PE10–SO3H.


from EA analysis, confirming the almost-complete sulfonation ofphenethyl group. When larger size cations (TMA+ and TBA+) wereused as an ion-exchange agent, the acid capacities of the samplesdecreased. These results suggest that most of the acid sites graftedon the amorphous phases are confined within internal structureenvironment, as well as the external surface of the sample.

It is noticeable that the acid capacities of the samples preparedin the present study were found to be higher than the acid capacityof other types of acid functionalized materials, which determinedby titration technique (mainly mesoporous type), as well as par-tially crystalline ZSM-5 zeolite based materials as reported in theliterature (Table 8).

Table 9Weight loss analyses obtained from TGA.

Samples % Weight loss of waterin as-synthesized samples

% Weight loss at 500–600 �Cin sulfonated samples

SIL-1 3.4 0SIL–PE5 1.6 2.5SIL–PE10 1.3 2.9SIL–PE15 1.1 3.7SIL–PE20 1.0 4.2

0

20

40

60

80

100

120

140

0 0.2 0.4

0 0.2 0.4 0.6 0.8 1

0 0.2 0.4 0.6 0.8 1

Relative Pre

0

100

200

300

400

500

600

Relative Pressure, P/Po

Volu

me,

cc/

g

Volu

me,

cc/

g

Volu

me,

cc/

g

0

50

100

150

200

250

300

350

400


Silicalit

SIL-PE-5-SO3H

SIL-PE15-SO3H

Fig. 8. Nitrogen adsorption–desorpt

3.4. Thermal stability and surface area characterization

3.4.1. Thermal gravimetric analysisThe thermal stability of all of the samples was determined by

thermal gravimetric analysis under nitrogen atmosphere for theas-synthesized and sulfonated samples. The TGA curves for SIL-1and SIL–PE10–SO3H samples are shown in Fig. 7. The TGA curvesfor other partially crystalline silicalite-1 based samples displayedthe same curve trend as SIL–PE10–SO3H sample (as-synthesizedand sulfonated), and thus, are not shown. However, the weight lossanalysis for all of the samples is presented in Table 9.

The first and second weight loss occurred at 200 and 350–450 �C (Fig. 7a and c) for all of the as-synthesized samples wasdue to surface dehydration and decomposition of SDA molecules,respectively. It can be observed that weight loss of partially crys-talline silicalite-1 based samples due to the water desorptionwas lower than that of parent silicalite-1. This indicates that as-synthesized, partially crystalline silicalite-1 based samples weremore hydrophobic than the as-synthesized parent silicalite-1.The as-synthesized partially crystalline silicalite-1 based samplesbecome more hydrophobic with increasing PE loading in the syn-thesis mixture and their corresponding weight loss due to thewater desorption is reported in Table 9. An additional weight losswas found in all as-synthesized partially crystalline silicalite-1based samples, in temperature range of 500–600 �C. The weight

0.6 0.8 1

0 0.2 0.4 0.6 0.8 1

0 0.2 0.4 0.6 0.8 1

ssure, P/Po

Volu

me,

cc/

gVo

lum

e, c

c/g

050100150200250300350400450


0

50

100

150

200

250


N2 desorption ♦ N2 adsorption

e-1

SIL-PE10-SO3H

SIL-PE20-SO3H

ion isotherms for the samples.


loss was due to the removal of the incorporated PE group, sincethere was no significant weight loss in the parent silicalite-1 at500–600 �C. The corresponding DTG profile displayed three peakscentered at 200, 400, and 550 �C, respectively for partially crystal-line silicalite-1 based samples compared to two peaks centered at200 and 400 �C for parent silicalite-1.

TGA curves for calcined silicalite-1 and SIL–PE10–SO3H samplesare shown in Fig. 7b and d, respectively. It can be seen that thewater and SDA were completely eliminated from the as-synthe-sized samples through calcination and after strong acid treatment.A weight loss was observed at 550 �C for all the partially crystallinesilicalite-1 based samples, indicating that the arenesulfonic acidgroup was thermally stable up to 550 �C. Table 9 shows the totalweight loss of the acid group from the samples, estimated as 2.5,2.9, 3.7, and 4.2 wt.% respectively, which was in agreement withthe C and S contents as determined in the elemental analysis (Sec-tion 3.3.1).

3.4.2. Nitrogen physisorptionThe nitrogen adsorption–desorption isotherms for parent silica-

lite-1 and partially crystalline silicalite-1 based samples are shownin Fig. 8. The isotherm for silicalite-1 has a long and horizontal pla-teau showing typical Type I adsorption isotherm with an H4 hys-teresis loop as defined by IUPAC [4,42]. Type I isotherm is well-defined for micropore adsorbent, particularly for silicalite-1. Nev-ertheless, H4 hysteresis loop is usually associated with thin slit-like inter-crystalline pores where the pores are mainly in themicropore range [9,45,46].

However, the isotherms for all of the partially crystalline silica-lite-1 based samples show a different trend compared to parent sil-icalite-1 isotherm. A sharp inflection was observed in P/Po rangingfrom 0.8 to 1 and overall nitrogen adsorption volume was in-creased for SIL–PE5–SO3H and reduced consistently when the PEloading in the synthesis mixture increased. The isotherms exhib-ited a notable shift of the hysteresis position toward high relativepressures (P/Po = 0.8–1.0) suggesting that the sample itself under-went a change in the structure until it produced H3 hysteresis loop.This observation indicates that the shape of the pores had changedfrom thin slit pore to slit-shaped pores, as shown by the presenceof H3 hysteresis loop [45]. The isotherms for partially crystallinesilicalite-1 based samples did not show any limiting adsorptionat high relative pressure, showing that the samples are typical ofopen surface materials with large meso/micropores (20–30 nmdiameter from BJH analysis), as well as high external surface areasthat allow the formation of multiple adsorbate layers as the P/Po

increases [7].The basic physicochemical and textural properties of the par-

tially crystalline silicalite-1 based samples are shown in Table 10.The specific surface area (SBET) was obtained by analyzing nitrogenadsorption data at 77 K in a relative vapor pressure ranging from0.01 to 0.3. The total pore volume (Vtot) was estimated based onthe volume adsorbed at a relative pressure of 0.99 and the micro-

Table 10Surface area and porosity characteristics of parent silicalite-1 and partially crystallinesilicalite-1 based samples.

Sample SBET, m2 g�1 Total porevolumea,Vtot cm3 g�1

Microporevolumeb,Vmic cm3 g�1

Average porediameter,dp nm

SIL-1 329 0.18 0.08 1.5SIL–PE5–SO3H 326 0.66 0.04 7.9SIL–PE10–SO3H 322 0.41 0.04 6.7SIL–PE15–SO3H 250 0.31 0.03 4.8SIL–PE20–SO3H 244 0.26 0.03 4.1

a At P/Po � 0.99.b t-Plot method.

pore volume (Vmic) was determined by t-plot method. The BET sur-face area of the parent silicalite-1 was 329 m2/g with the total porevolume of 0.18 cm3/g and average pore diameter of 1.5 nm. Thesedata are comparable and consistent with the data reported in theliterature for silicalite-1 structure [47,48]. With the increase ofarenesulfonic acid in the partially crystalline silicalite-1 basedsamples, the SBET decreased gradually from 326 to 244 m2/g, Thisobservation was consistent with the literature [9,49], where pres-ence of organic group in the sample structure resulted in the dropof surface area, which was due to the occupation of large organicgroup in the pore channel.

The total pore volume and the average pore diameter for thepartially crystalline silicalite-1 based samples were higher thanthe parent silicalite-1, as shown in Table 10. The presence of arene-sulfonic acid group increased the total pore volume to 0.66 cm3/gfor SIL–PE5–SO3H and reduced gradually to 0.26 cm3/g for SIL–PE20–SO3H, the average pore diameter of SIL–PE5–SO3H increasedto 7.9 nm for but the average pore diameter of SIL–PE20–SO3Hdropped to 4.1 nm. The change in the pore diameter for the par-tially crystalline silicalite-1 based samples was due to the incorpo-ration of the PE group into the pore structure during co-condensation resulted to the larger pore or pore openings [50], asindicated also in the isotherms (Fig. 8). The further increase of PEgroup gradually reduced the total pore volume and average porediameter in the partially crystalline silicalite-1 based samples,which was due to the amorphous phases and partial crystallineof the structure [46]. Table 10 shows a significant reduction inthe micropore volume for the partially crystalline silicalite-1 basedsamples with the increase of arenesulfonic acid group. The rela-tively low micropore volume was due to the pore opening by theacid group and the presence of the amorphous materials [48].These changes were consistent with the reduction in the crystalsize, as shown in the SEM pictures [27].

4. Conclusion

Partially crystalline silicalite-1 based materials was successfullysynthesized by co-condensation of inorganic silica (TEOS) and or-ganic silica sources (PE) under hydrothermal synthesis, followedby sulfonation of aromatic rings which grafted on amorphousphases, to arenesulfonic acid group. The partially crystalline silica-lite-1 based materials were studied by a variety of chemical andphysical characterization techniques. XRD results show that thecrystallinity of the partially crystalline silicalite-1 based samplesreduced gradually, until highly amorphous phase was formed at30 mol%, indicated that the crystalline materials can only beformed from an organosilane source not more than 20 mol%. Ascharacterized by XRD, SEM and TEM, the surface morphology of sil-icalite-1 was significantly modified, leading to a smaller crystalpresent in the amorphous phases. The combined interpretation ofthe characterization data from 29Si MAS, 13C CP-MAS solid stateNMR, FT-IR and elemental analysis support the presence of the acidgroup in the amorphous phases. Nitrogen adsorption–desorptionanalysis shows that, the presence of acid group in the amorphousstructure causing the reduction in the surface area, and microporevolume. These partially crystalline silicalite-1 based materialsexhibited high acid capacity with a good thermal stability, up to550 �C. The present research shows a potential way to create tun-able zeolite materials with new properties for better separationsand catalytic activities, as well as for other applications.

Acknowledgments

The financial support provided by Ministry of Science, Technol-ogy and Environment under e-Science Fund Grant (Account No:


6013319), Universiti Sains Malaysia under Short Term Grant (Ac-count No: 6035188), Ministry of Higher Education under FRGS (Ac-count No: 6070021) and Research University Grant (Account No:811043) are duly acknowledged.

References

[1] G. Bonilla, D. Vlachos, M. Tsapatsis, Micropor. Mesopor. Mater. 42 (2001) 191.[2] J. Hedlund, J. Sterte, M. Anthonis, A. Bons, B. Carstensen, N. Corcoran, D. Cox, H.

Deckman, W. Gijnst, P. Moor, F. Lai, J. Mchenry, W. Mortier, J. Reinoso, J. Peters,Micropor. Mesopor. Mater. 52 (2002) 179.

[3] C.P. Nicolaides, N.P. Sincadu, M.S. Scurrell, Catal. Today 71 (2002) 429.[4] C.P. Nicolaides, Appl. Catal. A: Gen. 185 (1999) 211.[5] N.Y. Chen, T.F. Degnan, Chem. Eng. Prog. 84 (1988) 32.[6] J. Dwyer, A. Dyer, Chem. Ind. 2 (1984) 237.[7] K.S. Triantafyllidis, L. Nalbandian, P.N. Trikalitis, A.K. Ladavos, T.

Mavromoustakos, C.P. Nicolaides, Micropor. Mesopor. Mater. 75 (2004) 89.[8] A.S.M. Chong, X.S. Zhaoa, A.T. Kustedjo, S.Z. Qiao, Micropor. Mesopor. Mater. 72

(2004) 33.[9] X. Wang, J.C.C. Chan, Y.H. Tseng, S. Cheng, Micropor. Mesopor. Mater. 95 (2006)

58.[10] X. Wang, S. Cheng, J.C.C. Chan, J.C.H. Chao, Micropor. Mesopor. Mater. 96

(2006) 321.[11] W. Zhang, X. Bao, X. Guo, X. Wang, Catal. Lett. 60 (1999) 89.[12] R. Van Grieken, J.L. Sotelo, J.M. Menendez, J.A. Melero, Micropor. Mesopor.

Mater. 39 (2000) 135.[13] P.A. Jacobs, E.G. Derouane, J. Weitkamp, J. Chem. Soc. Chem. Commun. (1981)

591.[14] C.P. Nicolaides, H.H. Kung, N.P. Makgobaa, N.P. Sincadu, M.S. Scurrell, Appl.

Catal. A: Gen. 223 (2002) 29.[15] Y. Shin, S.Z. Thomas, E.F. Glen, L.Q. Wang, L. Jun, Micropor. Mesopor. Mater. 37

(2000) 49.[16] L.M. Yang, Y.J. Wang, G.S. Luo, Y.Y. Dai, Micropor. Mesopor. Mater. 84 (2005)

275.[17] C.W. Jones, K. Tsuji, M.E. Davis, Micropor. Mesopor. Mater. 33 (1999) 223.[18] C.W. Jones, M. Tsapatsis, O. Tatsuya, M.E. Davis, Micropor. Mesopor. Mater. 42

(2001) 21.[19] K. Tsuji, C.W. Jones, M.E. Davis, Micropor. Mesopor. Mater. 29 (1999) 339.[20] B.L. Su, M. Roussel, K. Vause, X.Y. Yang, F. Gilles, L. Shi, E. Leonova, M. Eden, X.

Zou, Micropor. Mesopor. Mater. 105 (2007) 49.[21] Z. Lai, M. Tsapatsis, J.P. Nicolich, Adv. Funct. Mater. 14 (2004) 716.[22] C.W. Jones, Science 300 (2003) 439–440.[23] J. Guo, A.J. Han, H. Yu, J.P. Dong, H. He, Y.C. Long, Micropor. Mesopor. Mater. 94

(2006) 166–172.

[24] I.K. Mbaraka, D.R. Radu, S.Y. Lin, B.H. Shanks, J. Catal. 219 (2003) 329.[25] B.A. Holmberg, S.J. Hwang, M.E. Davis, Y. Yan, Micropor. Mesopor. Mater. 80

(2005) 347.[26] Y. Zheng, X. Su, X. Zhang, W. Wei, Y. Sun, Stud. Surf. Sci. Catal. 156 (2005)

205.[27] D.P. Serrano, J. Aguado, J.M. Rodríguez, A. Peral, From zeolites to porous MOF

materials, in: The 40th Anniversary of International Zeolite Conference, 2007,pp. 282–288.

[28] S.C. Larsen, J. Phys. Chem. C 111 (2007) 18464.[29] S. Tanaka, C. Yuan, Y. Miyake, Micropor. Mesopor. Mater. 113 (2008) 418.[30] K. Yamamoto, T. Tatsumi, Chem. Mater. 20 (2008) 972.[31] C.H. Cheng, T.H. Bae, B.A. McCool, R.R. Chance, S. Nair, C.W. Jones, J. Phys.

Chem. C 112 (2008) 3543.[32] U. Diaz, A. Jose, V. Moya, A. Corma, Micropor. Mesopor. Mater. 93 (2006)

180.[33] D.P. Serrano, J. Aguado, J.M. Escola, J.M. Rodriguez, A. Peral, Chem. Mater. 18

(2006) 2462.[34] M. Rat, M.H. Zahedi-Niaki, S. Kaliaguine, Trong-On Do, porous MOF

materials, in: The 40th Anniversary of International Zeolite Conference,2007, p. 1811.

[35] Y.H. Han, A. Taylor, M.D. Mantle, K.M. Knowles, J. Non-Cryst. Solids 353 (2007)313.

[36] Y. Wang, Y. Tang, A. Dong, X. Wang, N. Ren, Z. Gao, J. Mater. Chem. 12 (2002)1812.

[37] Z. Olejniczak, M. Łeczka, K. Cholewa-Kowalska, K. Wojtach, M. Rokita, W.Mozgawa, J. Mol. Struct. 744 (2005) 465.

[38] X. Wang, S. Cheng, J.C.C. Chan, J. Phys. Chem. C 111 (2007) 2156.[39] X. Zhang, Y. Wu, S. He, D. Yang, Surf. Coat. Technol. 201 (2007) 6051.[40] Y.S. Ooi, S. Bhatia, Micropor. Mesopor. Mater. 102 (2007) 310.[41] B. Samuneva, P. Djambaski, E. Kashchieva, G. Chernev, L. Kabaivanova, E.

Emanuilova, I.M.M. Salvado, M.H.V. Fernandes, A. Wu, J. Non-Cryst. Solids 354(2008) 733.

[42] Y. Luo, P. Yang, J. Lin, Micropor. Mesopor. Mater. 111 (2008) 194.[43] R. Xing, Y. Liu, Y. Wang, L. Chen, H. Wu, Y. Jiang, M. He, P. Wu, Micropor.

Mesopor. Mater. 105 (2007) 41.[44] C.S. Gill, B.A. Price, C.W. Jones, J. Catal. 251 (2007) 145.[45] F. Rouquerol, J. Rouquerol, K. Sing, Adsorption by Powders and Porous Solids

Principles, Methodology and Applications, Elsevier, Amsterdam, 1999.[46] R.M.A. Roque-Malherbe, Adsorption and Diffusion in Nanoporous Materials,

CRC Press, Boca Raton, 2007.[47] D.P. Serrano, G. Calleja, J.A. Botas, F.J. Gutierrez, Sep. Purif. Technol. 54 (2007)

1.[48] Y.H. Chou, C.S. Cundy, A.A. Garforth, V.L. Zholobenko, Micropor. Mesopor.

Mater. 89 (2006) 78.[49] S. Shylesh, S.P. Mirajkar, A.P. Singh, J. Mol. Catal. A: Chem. 212 (2004) 219.[50] I.K. Mbaraka, B.H. Shanks, J. Catal. 244 (2006) 78.

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