effect of the acid properties on the diffusion of c 7 hydrocarbons in ul-zsm-5 materials

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Microporous and Mesoporous Materials 92 (2006) 117–128

Effect of the acid properties on the diffusion of C7 hydrocarbonsin UL-ZSM-5 materials

Hoang Vinh-Thang a, Qinglin Huang b, Adrian Ungureanu a, Mladen Eic b,Do Trong-On a, Serge Kaliaguine a,*

a Department of Chemical Engineering, Universite Laval, Ste-Foy, Que., Canada G1K 7P4b Department of Chemical Engineering, University of New Brunswick, P.O. Box 4400, Fredericton, NB, Canada E3B 5A3

Received 8 July 2005; received in revised form 22 August 2005; accepted 26 August 2005Available online 20 February 2006

Abstract

In this study, a series of UL-MFI materials with different Si/Al ratios has been synthesized following a solid-state crystallization start-ing from amorphous wormhole-like mesostructured aluminosilicates. The acid properties of UL-MFI materials are varied by changingthe content of aluminum as observed by FTIR spectra of adsorbed pyridine.

The effect of the acid properties on the diffusion of C7 hydrocarbons n-heptane and toluene in UL-MFI materials was further studiedusing the ZLC technique. As expected, the interaction with the acid sites causes a decrease in the diffusivities of both sorbates. The valuesof the activation energy of diffusion for both C7 hydrocarbons in H-form UL-MFI materials are higher than those in Na-forms. Theeffect of the acid properties on the decrease of these values is more pronounced at high Si/Al ratio.� 2005 Published by Elsevier Inc.

Keywords: UL-ZSM-5; Bronsted/Lewis-acid; Diffusion of n-heptane and toluene; Micro/mesoporosity; ZLC technique

1. Introduction

In our previous papers [1,2], we reported the diffusionof linear and aromatic C7 hydrocarbons in steam-stablemesostructured zeolitic UL-ZSM-5 materials using thezero length column (ZLC) method. UL-ZSM-5 is a dualmicro/mesostructured pore UL-zeolite materials, whichhas been developed by our group since 1999 [3,4]. Materialswith a micro/mesostructure are of particular interestbecause they combine the advantages of both microporouscrystalline zeolites such as strong acidity and good thermal/hydrothermal stability as well as those of mesoporousmaterials, i.e., highly ordered large pores and high specificsurface areas [5–8]. In these UL-ZSM-5 materials, the dif-fusion of hydrocarbons was found to be controlled by a

1387-1811/$ - see front matter � 2005 Published by Elsevier Inc.

doi:10.1016/j.micromeso.2005.08.030

* Corresponding author. Tel.: +1 418 656 2708; fax: +1 418 656 3810.E-mail addresses: kaliagui@gch.ulaval.ca, Serge.Kaliaguine@gch.

ulaval.ca (S. Kaliaguine).

combination of mesopore diffusion, which is due to a sur-face slip in the main pores, and surface/micropore diffusionin the intrawalls of the bi-porous structure. The activationenergies of diffusion for smaller critical diameter n-heptaneare higher than for toluene which has however a larger crit-ical diameter [2].

Recently, the acid properties of these UL-ZSM-5 mate-rials have been thoroughly investigated using several tech-niques such as FTIR of adsorbed pyridine, heat flowmicrocalorimetry of adsorbed ammonia, TPD of ammoniaand FTIR of adsorbed DTBPy (2,6-diterbutylpyridine) [9–11]. The results indicated that overcoming the low acidstrength of the amorphous mesostructured precursors,UL-ZSM-5 materials show medium-strong acid sites com-parable to those of ZSM-5 zeolites.

Up to now, several studies related to the influence ofacid sites on diffusivities of porous materials have beenreported [12–15]. For example, the diffusion of benzene inZSM-5 was found to be twice slower than that in silicalite

118 H. Vinh-Thang et al. / Microporous and Mesoporous Materials 92 (2006) 117–128

using the constant volume method [12]. These results are ingood agreement with those reported for the same system byShen and Rees [13] using the frequency–response method.The intracrystalline diffusivities of benzene, toluene andpara-xylene in MFI zeolites with different amounts of acidsites reported by Masuda et al. [14] using the constant vol-ume method were found to decrease significantly withincreasing density of acid sites in the zeolite. The influenceof the acid sites on the diffusion of n-hexane within MFIzeolite was also studied by Koriabkina et al. [15] usingthe positron emission profiling (PEP) technique.

Although most promising strategies for improving theacidity of micro/mesostructured materials have been pro-posed [5–8,16–30], in particular with regard to catalyticapplications [5,7], no influence of acid sites on the transportprocess of hydrocarbons within these materials has beenreported yet.

Thus, the main objective of this work is to clarify theinfluence of acid sites on the diffusion of C7 hydrocarbonswithin UL-MFI structures with different Si/Al ratios. First,the structural characteristics as well as acid properties ofthese materials were measured using a variety of analyticaland spectroscopic techniques, including X-ray diffraction(XRD), N2 adsorption/desorption, scanning electron micro-scope (SEM), atomic adsorption spectrometer (AAS) andFTIR spectra of adsorbed pyridine. Then, by comparingthe transport properties obtained for two C7 hydrocarbonsin these UL-MFI materials using the zero length column(ZLC) method, the influence of acid properties on thediffusivities of these materials was examined.

2. Experimental

2.1. Synthesis

UL-MFI (Silicalite and ZSM-5 types) materials wereprepared according to the synthesis method described byTrong-On and Kaliaguine [3,4]. This procedure involves atemplated solid-state secondary crystallization of nano-zeolites starting from the wormhole-like mesostructuredaluminosilicates prepared by hydrolysis of chlorides in eth-anol reported by Stucky and coworkers [31,32] as precur-sors. In the first step, four wormhole-like mesostructuredprecursors having Si/Al ratio from infinity (aluminum-free)to 20, designated as Si-Meso, Al-Meso-x (where x is theatomic Si/Al ratio, x = 100; 50 and 20), were synthesizedusing SiCl4 and AlCl3 as silicon and aluminum sources,respectively and poly(alkylene oxide) triblock copolymer(Pluronic P123, BASF) in ethanol (EtOH) as surfactant.In the second step, the surfactant containing precursorswere impregnated with 10% aqueous solution of tetrapro-pylammonium hydroxide (TPAOH) followed by dryingfor several days. The molar composition was: 0.017 P123,21.7 EtOH, SiCl4, 1/x AlCl3, 0.1 TPAOH, 2.0 H2O. Thesolid-state crystallization was performed at 120 �C for dif-ferent lengths of time in a Teflon-lined autoclave after theaddition of a small quantity of water not contacting the

sample. The final partially crystalline products were driedin air at 80 �C and calcined at 550 �C for 6 h to removeorganics. A microporous ZSM-5 sample with the atomicSi/Al ratio of 50 was also synthesized according to thereported method [33] and denoted as ZSM-5-50. Theobtained zeolite was crystallized in a Teflon-lined stainlesssteel autoclave at 175 �C for 5 days under autogeneouspressure.

Further, four UL-MFI samples with different Si/Alratios and crystallinities in the range of 40–60%, were pre-pared and designated as UL-Silicalite (aluminum-freeform) and UL-ZSM-5-x respectively. Subsequently, 1.0 gof the calcined UL-Silicalite or UL-ZSM-5-x was treatedwith 100 ml of 0.1 N solution of NaCl or NH4Cl(pH = 7) at room temperature for 24 h in three times, fol-lowed by washing with deionized water, drying at 80 �Covernight and heating at 550 �C for 6 h. The end solidsare designated as UL-Silicalite-Na+, UL-Silicalite-H+,UL-ZSM-5-x-Na+ and UL-ZSM-5-x-H+, respectively.

2.2. Structural characterization

Porosity and surface area studies were carried out usingan Omnisorp-100 sorptometer with nitrogen as the sorbateat liquid nitrogen temperature (77 K). All the calcined sam-ples were degassed at 200 �C and 10�5 mmHg for at least4 h before adsorption measurements. The specific surfaceareas (SBET) of the samples were calculated from adsorp-tion isotherm data using the Brunauer–Emmett–Teller(BET) method [34]. The micropore volume was also calcu-lated from the nitrogen adsorption branch using as-plotmethod [35]. On the other hand, the pore diameter andmesopore volume were obtained from the nitrogen desorp-tion branch using the standard Barrett–Joyner–Halenda(BJH) method [36].

Powder XRD patterns were performed on a PhilipsX-ray diffractometer using nickel-filtered CuKa radiation,0.025� step size and 1 s step time. The average size of indi-vidual particles of samples was calculated from the SEMimages obtained using a JEOL JSM-840 scanning electronmicroscope operated at 15 kV. Elemental analysis of thesolid product was carried out using a Perkin–Elmer1100B AAS.

The acidity was studied using the pyridine adsorption/desorption experiments. Prior to pyridine adsorption, thesample wafers were evacuated overnight at 400 �C underhigh vacuum followed by pyridine adsorption at room tem-perature. Subsequently, the wafers were evacuated at vari-ous temperatures. The FTIR spectra were recorded using aBiorad FTS-60 spectrometer on sample wafers.

2.3. Diffusion measurements

Diffusion tests of C7 hydrocarbons on UL-MFI sampleswere carried out on a ZLC system. The ZLC set-up wassimilar to the one used by Jiang and Eic [37]. Prior to mea-surement, a small amount of sample powder (1–2 mg) was

Relative pressure, P/Po

0.0 0.2 0.4 0.6 0.8 1.0

Ad

sorb

ed v

olu

me

(cm

3 .g

-1/S

TP

)

(b)

(c)

(d)

(a)

Mesopore diameter (nm)0 2 4 6 8 10 12

Po

resi

zed

istr

ibu

tio

n,d

V/d

D(c

m3 .g

-1.n

m-1

)

0.01

(d)

(a)

(c)

(b)

250

Fig. 1. Nitrogen adsorption/desorption isotherms at 77 K for the worm-hole-like mesostructured precursors with different atomic silicon toaluminium ratio in gel: (a) Si-Meso, (b) Al-Meso-100, (c) Al-Meso-50and (d) Al-Meso-20 (inset: BJH pore diameter distribution).

H. Vinh-Thang et al. / Microporous and Mesoporous Materials 92 (2006) 117–128 119

placed between two sintered metal disks and activated at270 �C overnight in order to remove water present in theair exposed sample. Helium of 99.95% purity was used asthe inert carrier gas. n-Heptane (99% grade, Aldrich) or tol-uene (99% grade, Aldrich) was kept in a bubbler and itsvapor carried by a small flow of helium before beingdiluted in the main He stream. The sorbate concentrationswere adjusted and maintained at a low level, i.e., 0–0.01 Torr absolute pressure range or 0–0.0002 on the rela-tive pressure scale, corresponding to the linear region of theadsorption isotherm (Henry’s Law region). This was con-firmed by adsorption isotherms independently measuredusing a CAHN digital microbalance as described in ourprevious study [2]. The sample powder was initially equili-brated with sorbate diluted in a helium flow and thendesorption was performed by purging with helium at a flowrate high enough to maintain a very low sorbate concentra-tion at the external surface of the particles, thus minimizingexternal mass and heat transfer resistances as required bythe ZLC theory. Essentially the same diffusivity resultswere extracted from the initial runs using the purge flowrates in the range of 80–140 cm3 (STP)/min, thus confirm-ing kinetically controlled process. A purge flow rate of100 cm3 (STP)/min helium was chosen as a standard flowrate for all experiments in this study. Detailed analysis ofZLC curves has been described elsewhere [38].

3. Results and discussion

3.1. Synthesis and characterization

As mentioned above, in the first step of the synthesis ofUL-MFI materials, four wormhole-like mesostructuredprecursors with different Si/Al ratios were prepared. Inorder to characterize the textural properties of these pre-cursors, the nitrogen adsorption/desorption isothermsand the pore size distributions are plotted in Fig. 1. In allcases, the isotherms are of type IV and exhibit hysteresisloops of H2 type at relative pressure in range of 0.4–0.9according to the IUPAC classification, typical of materials

Table 1Textural properties of different samples

Sample Si/Al ratio Crystallizationtime (day)

SBET

(m2/g)SBJH

a

(m2/g)M(In gel In productd

Si-Meso 1 1 – 835 815 1Al-Meso-100 100 100.2 – 800 780 1Al-Meso-50 50 50.8 – 745 725 1Al-Meso-20 20 20.3 – 680 630 1UL-Silicalite 1 1 1.5 375 225 1UL-ZSM-5-100 100 100.5 2.0 440 230 1UL-ZSM-5-50 50 51.3 4.0 470 245 1UL-ZSM-5-20 20 21.2 10.0 395 215 1

a Determined by the standard BJH method.b Calculated from as-plots.c Calculated from XRD data.d Determined by AAS.

with wormhole-like mesopore structures [31,32]. The tex-tural properties of the four parent materials are summa-rized in Table 1. Their mesopore diameter increases withSi/Al ratio from 5.4 nm (Si/Al = 20) to 7.8 nm (alumi-num-free, or infinite Si/Al ratio). However, the pore sizedistribution becomes more narrow when the amount ofaluminum in the initial gel decreases. As shown in

esopore volumea

cm3/g)Mesoporediametera (nm)

Microporevolumeb (cm3/g)

Crystallinityc

(%)

.724 7.8 – –

.626 6.9 – –

.586 6.2 – –

.424 5.4 0.008 –

.186 24.5 0.098 52.0

.235 30.0 0.127 58.0

.275 32.5 0.133 60.0

.204 27.0 0.110 40.0

(b)

(c)

(d)

(a)

Mesopore diameter (nm)0 10 20 30 40 50

Po

resi

zed

istr

ibu

tio

n,d

V/d

D(c

m3 .g

-1.n

m-1

) 0.00

5 (d)

(a)(c)

(b)

250

Ad

sorb

ed v

olu

me

(cm

3 .g

-1/S

TP

)

120 H. Vinh-Thang et al. / Microporous and Mesoporous Materials 92 (2006) 117–128

Table 1, with the increase of the aluminum content, thespecific surface area (SBET) and mesopore volume decreasefrom Si-Meso to Al-Meso-20 sample. Note the absence ofmicropores in all parent precursors, which is in agreementwith the reports of a wormhole-like mesostructured mate-rial prepared using SiCl4 as silica source [31,32].

Fig. 2 presents the XRD patterns for the four UL-MFIsamples as well as Al-Meso-50 and ZSM-5-50 as references.As shown in Fig. 2, UL-MFI samples (Fig. 2b–e) displaydistinct broad diffraction peaks in the 8–10 and 20–25� 2hranges which is consistent with the ZSM-5-50 (Fig. 2f),while Al-Meso-50 did not show crystalline features(Fig. 2a). By considering ZSM-5-50 as being 100% crystal-line, the crystallinity of these samples was also establishedas reported in Table 1. All four UL-MFI samples havecrystallinity higher than 40%. The crystallinity increasesin the order UL-ZSM-5-20 < UL-Silicalite < UL-ZSM-5-100 < UL-ZSM-5-50.

The nitrogen adsorption/desorption isotherms of thesefour selected samples are shown in Fig. 3 and the physico-chemical properties obtained from N2 adsorption/desorp-tion isotherms are also summarized in Table 1. As can beseen from Fig. 3, all UL-MFI samples show a typical typeIV isotherm with a H1 hysteresis loop in the partial pres-sure (P/P0) range 0.7–0.95 and steep rise at low relative

2θ, Degree5 10 15 20 25 30

Rel

ativ

e In

ten

sity

, cts

/s

(f)

(e)

(d)

(c)

(b)

(a)

2000

Fig. 2. XRD patterns (a) Al-Meso-50, (b) UL-Silicalite, (c) UL-ZSM-5-100, (d) UL-ZSM-5-50, (e) UL-ZSM-5-20 and (f) ZSM-5-50, shifted by0, 1000, 2400, 3600, 5100 and 6800 cts/s, respectively.

Relative pressure, P/Po

0.0 0.2 0.4 0.6 0.8 1.0

Fig. 3. Nitrogen adsorption/desorption isotherms at 77 K for (a) UL-Silicalite, (b) UL-ZSM-5-100, (c) UL-ZSM-5-50 and (d) UL-ZSM-5-20materials (inset: BJH pore diameter distribution).

pressure (P/P0 < 0.3). This result indicates that both micro-and mesopores are present in UL-MFI materials. The BJHanalysis shows (Fig. 3) that all samples still exhibit a rela-tively narrow pore size distribution with mean mesoporediameter varying from 24.5 to 27.0 nm. From Table 1,due to the solid state crystallization, an increase in micro-pore volume and a decrease in specific surface area betweenparent mesoporous precursor and UL-MFI products wereobserved. This suggests that the changes in textural proper-ties are consistent with the gradual transformation of theamorphous walls of these parent materials into micropo-rous ZSM-5 zeolitic nano-crystals [4].

The changes in structural properties of the fourUL-MFI samples after treatment with diluted aqueoussolution of NaCl or NH4Cl were further investigated bynitrogen adsorption/desorption and XRD technique. In arecent study of our group on UL-ZSM-5 materials withSi/Al ratio of 50 [10], we found that the treatment withaqueous solution of NH4OH (pH = 10) is followed by aslight decrease in SBET as well as micropore volume. Thisis attributed to the partial blocking of the micropores withsome amorphous phase likely resulting from the decompo-sition of the amorphous walls left from the parent precur-sors. In order to minimize the effect of basic treatmentcondition on the structural decomposition, two neutral

H. Vinh-Thang et al. / Microporous and Mesoporous Materials 92 (2006) 117–128 121

aqueous solutions of NaCl and NH4Cl were used. After theion-exchange followed by the calcination at 550 �C, Na-form and H-form UL-MFI samples were obtained. Com-bining nitrogen isotherms and XRD results (not shown),the physico-chemical properties of Na-form and H-formUL-MFI samples are summarized in Table 2. As can beseen from Table 2, Na-form samples show higher SBET aswell as mesopore diameters compared to the H-form. Thelatter species exhibit however slightly higher micropore vol-umes as compared to the former ones. In accordance withthe results reported in Table 1, the micropore volume is inthe following order: UL-Silicalite-Na+ < UL-ZSM-5-20-Na+ < UL-ZSM-5-100-Na+ < UL-ZSM-5-50-Na+ and UL-Silicalite-H+ < UL-ZSM-5-20-H+ < UL-ZSM-5-100-H+ <UL-ZSM-5-50-H+.

Table 2Textural properties of sodium and ammonia treated forms of the UL-MFI sa

Sample Si/Al ratio Crystallizationtime (day)

SBET

(m2/g)In gel In productd

(a) UL-Silicalite-Na+ 1 1 1.5 320(b) UL-ZSM-5-100-Na+ 100 101.8 2.0 400(c) UL-ZSM-5-50-Na+ 50 51.7 4.0 450(d) UL-ZSM-5-20-Na+ 20 22.5 10.0 350(e) UL-Silicalite-H+ 1 1 1.5 315(f) UL-ZSM-5-100-H+ 100 102.4 2.0 380(g) UL-ZSM-5-50-H+ 50 52.5 4.0 435(h) UL-ZSM-5-20-H+ 20 23.1 10.0 340

a Determined by the standard BJH method.b Calculated from as-plots.c Calculated from XRD data.d Determined by AAS.

Wavenumber/cm-1

14001450150015501600

Ab

sorb

ance

/a.u

.0.

1

(c)

(d)

(b)

(a)

A

1549

1446

Fig. 4. FTIR spectra of adsorbed pyridine in the 1400–1600 cm�1 range fotreatment at (a) 150 �C, (b) 200 �C, (c) 300 �C and (d) 400 �C.

3.2. Acidity

In order to evaluate the strength and type of acid sites inthe semi-crystalline walls of Na-form and H-form UL-MFIsamples (note that the materials designated as UL-ZSM-5-x-H+ having been calcined at 550 �C are actually underthe H-form), FTIR of adsorbed pyridine experiments werecarried out. Fig. 4 shows the FTIR spectra of pyridineadsorbed on the UL-ZSM-5-50-Na+ (Fig. 4A) as well asUL-ZSM-5-50-H+ (Fig. 4B) after desorption at differenttemperatures in the range of 1600–1400 cm�1. FTIR bandsdue to pyridine adsorbed on Bronsted acid sites (1549 cm�1)and pyridine adsorbed in Lewis acid sites (1455 cm�1) canbe observed as well as a band at 1490–1500 cm�1 that canbe assigned to pyridine associated with both Bronsted and

mples

SBJHa

(m2/g)Mesoporevolumea

(cm3/g)

Mesoporediametera

(nm)

Microporevolumeb

(cm3/g)

Crystallinityc

(%)

215 0.854 22.5 0.063 55.0225 1.122 26.0 0.105 60.0230 1.231 28.0 0.121 62.0210 0.918 24.0 0.074 43.0195 0.842 21.5 0.068 57.0195 1.075 23.5 0.112 62.0200 1.178 26.5 0.128 63.0195 0.872 22.5 0.081 45.0

Wavenumber/cm-1

14001450150015501600

Ab

sorb

ance

/a.u

.0.

1

(a)

(b)

(c)

(d)

B

1549

1455

r (A) UL-ZSM-5-50-Na+ and (B) UL-ZSM-5-50-H+ following thermal

122 H. Vinh-Thang et al. / Microporous and Mesoporous Materials 92 (2006) 117–128

Lewis acid sites [2,4,29,39]. As can be seen from Fig. 4, theintensity of the bands corresponding to Bronsted acid sitesas well as those corresponding to Lewis acid sites decreaseswith increasing desorption temperature. It can be observedthat the intensity of the band corresponding to Bronstedacid sites is higher for UL-ZSM-5-50 treated with dilutedaqueous NH4Cl solution than for the one treated withdiluted aqueous NaCl solution. This observation clearlyindicates that the density of Bronsted acid sites inUL-ZSM-5-50-H+ is higher than in UL-ZSM-5-50-Na+.However, when the desorption temperature increases to400 �C, no band at 1549 cm�1 was obtained while the bandsat 1455 cm�1 corresponding to Lewis acid sites are still pres-ent. Fig. 5 also indicates a shift in the Lewis-bound pyridinebands from 1455 cm�1 to lower wave numbers at about1446 cm�1. This is attributed to the weaker interaction ofthe Na+ cation. It is known that, for a given coordination,the strength of interaction of cations which also function asLewis acid sites increases with increase in the formal charge/radius ratio of the cation [40,41].

Fig. 5 shows the FTIR spectra after pyridine desorptionat 200 �C of UL-ZSM-5-50-Na+ and UL-ZSM-5-50-H+ aswell as Al-Meso-50 and ZSM-5-50 for comparison. Theseresults clearly indicate that the density of Bronsted acidsites in UL-ZSM-5-50-H+ (Fig. 5d) as well as as-preparedUL-ZSM-5-50 (Fig. 5c) is higher than in Al-Meso-50, whileUL-ZSM-5-50-Na+ (Fig. 5b) has the lowest value. How-ever, all samples have lower densities of Bronsted acid sitesthan purely crystalline ZSM-5-50 (Fig. 5e). This trend is in

Wavenumber/cm-1

14001450150015501600

Ab

sorb

ance

/a.u

.0.

1

(e)

(d)

(c)

(b)

(a)

1455

1446

1549

Fig. 5. FTIR spectra of adsorbed pyridine in the 1400–1600 cm�1 rangefor (a) Al-Meso-50, (b) UL-ZSM-5-50-Na+, (c) UL-ZSM-5-50, (d) UL-ZSM-5-50-H+ and (e) ZSM-5-50 following thermal treatment at 200 �C.

line with our earlier studies of the acid properties ofUL-ZSM-5 (Si/Al ratio of 50) using several methods, i.e.,TPD-NH3, heat flow microcalorimetry of adsorbed ammo-nia and FTIR of adsorbed DTBPy (2,6-diterbutylpyridine)[9–11]. In these studies, the overall content of Bronsted acidsites in UL-ZSM-5 (medium-strong) materials was foundto be intermediate between those of Al-Meso (medium)and ZSM-5 (strong). On the other hand, the intensity ofthe bands corresponding to Lewis acid sites (1446–1455cm�1) decreases in the following order: Al-Meso-50 >UL-ZSM-5-50 > UL-ZSM-5-50-Na+ > UL-ZSM-5-50-H+ > ZSM-5-50. The decrease in Lewis acidity of Na- andH-form samples in comparison to parent UL-ZSM-5-50has been attributed to the elimination of the hexa-coordi-nated extra-framework aluminum, which may exist inthe semi-crystalline walls of UL-MFI materials after treat-ment with diluted aqueous solution of NaCl or NH4Cl[10,42].

Fig. 6 shows the FTIR spectra after pyridine desorptionat 200 �C of the four UL-MFI samples after treatment withdiluted aqueous solution of NaCl (Fig. 6a–d), together withthose after treatment with diluted aqueous solution ofNH4Cl (Fig. 6e–h). For both Na- and H-form UL-MFIseries, the intensity of the bands corresponding to Bronstedacid sites (1549 cm�1), as well as those assigned to Lewisacid sites (1444–1455 cm�1) increases with decreasingSi/Al ratios. In the case of aluminum-free UL-Silicalites(Na- and H-form), these bands seem to disappear similarto pure silica (Fig. 6a and e). Moreover, both Na- andH-form UL-Silicalites showed no peak corresponding toBronsted acid sites at 1549 cm�1. A careful investigationof the band position corresponding to the Lewis acid sitesat 1444–1455 cm�1 showed that this band position wasslightly shifted toward lower wave numbers for the fourNa-form UL-MFI samples (Fig. 6A), which is in accor-dance with increasing degree of sodium ion-exchange. Onthe contrary, a shift toward higher wave numbers wasfound for the four H-form UL-MFI samples (Fig. 6B),which is associated with increasing aluminum content.Thus, it is also noteworthy that the number of both Bron-sted and Lewis acid sites in Na- and H-forms of UL-MFIseries varies in the order of increasing Al content.

It should also be noted that for both Na- and H-formUL-MFI samples, the band at 1490–1500 cm�1 is split intotwo bands (Figs. 4–6). As mentioned in our previous paper[2], the band at the higher-field wave number (1496 cm�1) isassociated with some Lewis acidity. On the other hand, theband at the lower-field wave number (1491 cm�1) is associ-ated with both Bronsted and Lewis acid sites. As a conse-quence, there are at least two kinds of Lewis acid sites inthe semi-crystalline mesoporous UL-zeolites and only thestrongest one yields the wave number of 1496 cm�1. Itcan be observed from Fig. 4Ac that the band at1496 cm�1 is the only one present in the spectrum of thesodium form of UL-ZSM-5-50 after desorption at200 �C, where the Lewis acid peak is located at1446 cm�1. In that respect the 1496 cm�1 corresponds to

Wavenumber/cm-1

14001450150015501600

Ab

sorb

ance

/a.u

.0.

1

(c)

(d)

(b)

(a)

1549

1444

1450

A

Wavenumber/cm-1

14001450150015501600A

bso

rban

ce/a

.u.

0.1

(h)

(g)

(f)

(e)

1549

1455

1450

B

Fig. 6. FTIR spectra of adsorbed pyridine in the 1400–1600 cm�1 range for (A) Na-form and (B) H-form materials following thermal treatment at 200 �C:(a) UL-Silicalite-Na+, (b) UL-ZSM-5-100-Na+, (c) UL-ZSM-5-50-Na+, (d) UL-ZSM-5-20-Na+, (e) UL-Silicalite-H+, (f) UL-ZSM-5-100-H+, (g)UL-ZSM-5-50-H+ and (h) UL-ZSM-5-20-H+.

Time (s)

0 200 400 600 800

C/C

o

0.001

0.01

0.1

1

80oC

70oC

90oC

(A)

Time (s)

0 200 400 600 800

C/C

o

0.001

0.01

0.1

1

40oC

30oC

50 C

(B)

Fig. 7. Experimental (symbols) and theoretical (solid lines) ZLC curvesfor (A) n-heptane and (B) toluene at different temperatures in UL-ZSM-5-20-H+ sample.

H. Vinh-Thang et al. / Microporous and Mesoporous Materials 92 (2006) 117–128 123

the sodium ion in this case. This is confirmed by the seriesof spectra illustrated in Fig. 6A which shows that the1496 cm�1 grows in intensity as the Na content is raised.It is interesting to note however that a very minor peakat 1496 cm�1 is also present in the Na+ and NHþ4 treatedUL-Silicalites. This may indicate that the Lewis acid sites,corresponding to low coordination silicon yielding the1450 cm�1 peak, also contributes to the 1496 cm�1 peak.

From Fig. 4B it can be concluded that since the peak at1496 cm�1 increases in intensity as the desorption temper-ature is raised, the corresponding Lewis acid site is thestrongest one in this sample. However at a desorption tem-perature of 400 �C (Fig. 4Bd) the Lewis acid site still hold-ing the pyridine molecule is yielding a peak 1455 cm�1.Thus, it appears that the 1496 cm�1 peak is associated withat least three different kinds of Lewis acid sites. On theother hand, the weakest one yielding the band at1491 cm�1 is attributed to another kind of Lewis acid sites,which is associated with the five-coordinated aluminum inthe extra-framework aluminate phase. Evidence for suchtype of five-coordinated aluminum has been inferred from27Al MAS NMR studies of water or acetylacetonateadsorbed in Al-MCM-41 surface [43].

In Al-Meso-50 which is entirely amorphous (spectrum(a) in Fig. 5) the Lewis acid sites have a peak at1455 cm�1 also associated with a high 1496 cm�1 compo-nent. In the completely crystalline ZSM-5-50 sample (spec-trum e in Fig. 5) the Lewis acid site with a band at1455 cm�1 is however yielding a peak at 1491 cm�1 (alsoassociated with the Bronsted acid site). Thus it seems thatthe 1455–1491 cm�1 doublet allows to us distinguish the

sodium ion in a crystalline aluminosilicate from the one inan amorphous solid whose doublet is at 1455–1496 cm�1.

124 H. Vinh-Thang et al. / Microporous and Mesoporous Materials 92 (2006) 117–128

3.3. Diffusion results

In order to investigate the effect of the acid properties onthe diffusivity of UL-MFI materials, the eight UL-MFIsamples with different acid strengths were further studiedby the ZLC method. Representative experimental and the-

Fig. 8. SEM micrographs of (a) UL-Silicalite-Na+, (b) UL-ZSM-5-100-Na+

(f) UL-ZSM-5-100-H+, (g) UL-ZSM-5-50-H+ and (h) UL-ZSM-5-20-H+ sam

oretical ZLC curves at different temperatures for n-heptaneand toluene in UL-MFI materials, for example, in UL-ZSM-5-20-H+, are illustrated in Fig. 7. All the samplesshow good agreement between experimental data and thetheoretical fittings [44] yielding theoretical estimates forDeff/R2 (Tables 4 and 5). In order to obtain the effective

, (c) UL-ZSM-5-50-Na+, (d) UL-ZSM-5-20-Na+, (e) UL-Silicalite-H+,ples.

H. Vinh-Thang et al. / Microporous and Mesoporous Materials 92 (2006) 117–128 125

diffusivity Deff, the UL-MFI particle R radii were estimatedfrom their SEM images. Fig. 8 shows the SEM images ofthe eight UL-MFI samples. The average size of individualparticles determined from SEM images is listed in Table 3.As can be seen from Tables 4 and 5, all L values obtainedfrom the theoretical fittings are significantly higher than 5which is considered a point of transition from an equilib-rium-controlled to the diffusion-controlled regime [37].

Furthermore, the effective diffusivity values of n-heptanein UL-MFI samples measured in the temperature range of60–100 �C were found to be in the order 10�13–10�12 m2/s.The same order of magnitude was obtained for toluene, butfor the measurements carried out over the lower tempera-ture range of 20–60 �C (Table 5). These results clearly indi-cate that the transport process of linear C7 hydrocarbons inUL-MFI materials is slower than that of aromatics (seeFig. 9). In both case the diffusivities are some five ordersof magnitude higher than the values we measured for a

Table 3Average size of individual particles calculated from SEM images

Sample Atomic Si/Alratio in gel

Particlesize (lm)

(a) UL-Silicalite-Na+ 1 150.0(b) UL-ZSM-5-100-Na+ 100 120.0(c) UL-ZSM-5-50-Na+ 50 128.0(d) UL-ZSM-5-20-Na+ 20 132.0(e) UL-Silicalite-H+ 1 136.0(f) UL-ZSM-5-100-H+ 100 102.0(g) UL-ZSM-5-50-H+ 50 110.0(h) UL-ZSM-5-20-H+ 20 124.0

Table 4Diffusivity data of n-heptane in different samples

Sample Si/Al T (�C) L

(a) UL-Silicalite-Na+ 1 60 44.070 33.080 24.4

(b) UL-ZSM-5-100-Na+ 100 70 34.080 25.090 18.0

(c) UL-ZSM-5-50-Na+ 50 80 24.690 17.4

100 15.0(d) UL-ZSM-5-20-Na+ 20 70 38.0

80 28.490 19.6

(e) UL-Silicalite-H+ 1 60 47.070 35.080 27.0

(f) UL-ZSM-5-100-H+ 100 80 21.690 15.2

100 12.4(g) UL-ZSM-5-50-H+ 50 80 33.0

90 22.0100 16.0

(h) UL-ZSM-5-20-H+ 20 70 44.080 29.490 20.6

ZSM-5 sample of Si/Al = 50 (see Ref. [2] for a completedescription of this sample). Similar observation was madefor the diffusion of C7 hydrocarbons in UL-ZSM-5 materi-als synthesized at different crystallization times, and earlierreported by our group [2]. From such data, the variationsof the effective diffusivity for n-heptane and toluene withmicropore volume at corresponding temperatures wereplotted in Fig. 10. The effective diffusivities for the two sor-bates decrease with increasing micropore volume in thesesamples. These results strongly suggest that micropores inthe semi-crystalline walls of UL-MFI materials play a sig-nificant role in the overall diffusion process through itsbi-porous structure. This is consistent with our earlier studyregarding the diffusion of n-heptane, cumene and mesityl-ene in SBA-15 materials, as well as of n-heptane and tolu-ene in UL-ZSM-5 materials of varied crystallinity, andwith Si/Al ratio of 50 [2,44,45]. The extracted effective acti-vation energies of n-heptane and toluene are also summa-rized in Tables 4 and 5. Similar observation was madefor the diffusion of C7 hydrocarbons in UL-ZSM-5 materi-als synthesized at different crystallization times as wasreported by our group [2]. The values of activation energyfor n-heptane are twice higher than those of toluene,although n-heptane has a smaller kinetic diameter than tol-uene. Those values for n-heptane are relatively close to thelimiting heat of adsorption for that sorbate in a UL-ZSM-5sample with Si/Al ratio of 50, which was found to be 64 kJ/mol [2]. However, they are significantly higher compared todiffusion of saturated hydrocarbons, e.g., C-5 to C-10 car-bon range in silicalite/H-ZSM-5 zeolites, i.e., 17–24 kJ/mol[46]. Contrary to this, toluene effective activation energies

Deff/R2 (s�1) Deff (m2 s�1) E (kJ/mol)

0.12 · 10�3 6.92 · 10�13 45.20.20 · 10�3 1.11 · 10�12

0.31 · 10�3 1.75 · 10�12

0.13 · 10�3 4.66 · 10�13 51.60.21 · 10�3 7.73 · 10�13

0.35 · 10�3 1.26 · 10�12

0.18 · 10�3 7.46 · 10�13 55.10.31 · 10�3 1.26 · 10�12

0.50 · 10�3 2.04 · 10�12

0.14 · 10�3 6.10 · 10�13 58.90.25 · 10�3 1.09 · 10�12

0.44 · 10�3 1.90 · 10�12

0.11 · 10�3 5.29 · 10�13 47.50.19 · 10�3 8.62 · 10�13

0.30 · 10�3 1.40 · 10�12

0.17 · 10�3 4.54 · 10�13 62.50.31 · 10�3 8.10 · 10�13

0.55 · 10�3 1.42 · 10�12

0.14 · 10�3 4.34 · 10�13 66.30.27 · 10�3 8.06 · 10�13

0.48 · 10�3 1.46 · 10�12

0.11 · 10�3 4.05 · 10�13 70.00.21 · 10�3 8.05 · 10�13

0.41 · 10�3 1.57 · 10�12

Table 5Diffusivity data of toluene in different samples

Sample Si/Al T (�C) L Deff/R2 (s�1) Deff (m2 s�1) E (kJ/mol)

(a) UL-Silicalite-Na+ 1 20 22.0 0.23 · 10�3 1.29 · 10�12 20.030 19.0 0.30 · 10�3 1.68 · 10�12

40 18.6 0.39 · 10�3 2.18 · 10�12

(b) UL-ZSM-5-100-Na+ 100 20 25.0 0.16 · 10�3 5.70 · 10�13 23.730 21.0 0.22 · 10�3 7.87 · 10�13

40 19.0 0.29 · 10�3 1.06 · 10�12

(c) UL-ZSM-5-50-Na+ 50 40 19.6 0.20 · 10�3 8.08 · 10�13 25.050 18.8 0.26 · 10�3 1.07 · 10�12

60 18.8 0.35 · 10�3 1.44 · 10�12

(d) UL-ZSM-5-20-Na+ 20 30 32.0 0.17 · 10�3 7.47 · 10�13 30.640 28.0 0.25 · 10�3 1.11 · 10�12

50 24.0 0.36 · 10�3 1.58 · 10�12

(e) UL-Silicalite-H+ 1 20 30.0 0.21 · 10�3 9.54 · 10�13 22.330 24.3 0.28 · 10�3 1.29 · 10�12

40 20.4 0.37 · 10�3 1.71 · 10�12

(f) UL-ZSM-5-100-H+ 100 30 23.0 0.16 · 10�3 4.07 · 10�13 30.340 19.4 0.23 · 10�3 5.98 · 10�13

50 18.0 0.33 · 10�3 8.56 · 10�13

(g) UL-ZSM-5-50-H+ 50 40 23.2 0.13 · 10�3 3.79 · 10�13 36.150 20.8 0.19 · 10�3 5.78 · 10�13

60 20.4 0.29 · 10�3 8.73 · 10�13

(h) UL-ZSM-5-20-H+ 20 30 39.0 0.14 · 10�3 5.29 · 10�13 37.540 33.6 0.22 · 10�3 8.46 · 10�13

50 29.4 0.35 · 10�3 1.33 · 10�12

1/T [1/K]0.0024 0.0026 0.0028 0.0030

lnD

eff [

ln(m

2 /s)]

-40-38

-28

-26

UL-MFI

ZSM-5

(A)

1/T [1/K]0.0026 0.0028 0.0030 0.0032 0.0034

lnD

eff [

ln(m

2 /s)]

-40-38

-28

-26

UL-MFI

ZSM-5

(B)

Fig. 9. Arrhenius plot showing the temperature dependence of theeffective diffusivity (Deff) for (A) n-heptane and (B) toluene: (d)UL-Silicalite-Na+, (m) UL-ZSM-5-100-Na+, (j) UL-ZSM-5-50-Na+,(r) UL-ZSM-5-20-Na+, (s) UL-Silicalite-H+, (n) UL-ZSM-5-100-H+,(h) UL-ZSM-5-50-H+, (�) UL-ZSM-5-20-H+ and ( ) ZSM-5 samples.

126 H. Vinh-Thang et al. / Microporous and Mesoporous Materials 92 (2006) 117–128

are comparable with activation energies for benzene, ethyl-benzene and o-xylene diffusion in silicalite/H-ZSM-5 zeolites,i.e., 25–34 kJ/mol [46]. The above observed phenomenaclearly suggest that the transport behavior of both sorbatesin UL-MFI samples can also be interpreted by the theoret-ical model considering the combination of mesopore diffu-sion due to a surface slip in the main pores with anactivated surface/micropore diffusion in the intrawalls ofthe bi-porous structure, which has been discussed in detailin our recent study [2]. According to the experimentalresults and the proposed model, it can be concluded thatthe transport of n-heptane in UL-MFI materials is mainlydominated by mesopore diffusion in the main channelstructure, while that of toluene is dominated by intrawalldiffusion process.

Fig. 10 shows that for both sorbates, Na-forms ofUL-MFI samples have higher effective diffusivities thantheir corresponding H-forms, which is consistent with thedata reported in the literature [12–15]. However, theseresults also show an opposite trend in comparison withthe diffusivities of various hydrocarbons, i.e., hexadecane,cumene, 1,3,5-trimethylbenzene, 1,3,5-triethylbenzene and1,3,5-tri-isopropylcyclohexane in NaY and HY zeolites[47]. The diffusion rates for these sorbates in HY werefound to be generally faster than in NaY. This has beenattributed to the cavities of HY being more open than inNaY, due to the removal of the Na cations upon ion-exchange. In our case, a plausible explanation for suchbehavior could be attributed to the nature of Na cationsin the new structures. In the case of NaY zeolites, Na cat-ions exist in two forms, i.e., the Na valence cations neededto balance the net negative charge of zeolites created after

Micropore volume (cm3/g) Micropore volume (cm3/g)

0.06 0.08 0.10 0.12 0.14

Def

f (m

2 .s-1

x1013

)

0

5

10

15

20

25

(a)

(b)(d)

(h)

(f)(g)

(c)

(e)

n-Heptane80oC

0.06 0.08 0.10 0.12 0.14

Def

f (m

2 .s-1

x1013

)

0

5

10

15

20

25

(a)

(b)(d)

(h) (f)

(g)

(c)

(e)

Toluene40oC

Fig. 10. Variation of the effective diffusivity for n-heptane at 80 �C and toluene at 40 �C with micropore volume in UL-ZSM-5 samples (see Fig. 8 forsample identification).

H. Vinh-Thang et al. / Microporous and Mesoporous Materials 92 (2006) 117–128 127

the substitution of a tetravalent Si cation by a trivalent Alcation, and the extra-framework Na cations. The latterones are responsible for blocking the channels of zeolites,which are formed by using up of an excess amount ofsodium in the process of the zeolite formation from natris-ilicate and natrialuminate as silicon and aluminium sourcesrespectively, according to the typical synthesis procedure[48]. Subsequently, in the preparation of HY zeolite from

Al/Si molar percentage (%)

0 2 4 5

Act

ivat

ion

en

erg

y (k

J/m

ol)

10

20

30

40

50

60

70

80

(a)

(b )

(d)

(h)

(f)(g)

(c)(e)

n-Heptane

(e)

(f)

(g)(h)

(a)(b)

(c)

(d)

Toluene

1 3

Fig. 11. Effect of the Al/Si molar percentage on the activation energy ofthe diffusion of n-heptane and toluene in UL-ZSM-5 samples (see Fig. 8for sample identification).

NaY, not only Na+ counter ions, but also the extra-frame-work Na has to be removed. In the case of our Na-formUL-MFI materials, only Na valence cations were presentin the ion exchanged material. Thus, the faster diffusivityof Na-form of UL-MFI samples could be attributed totheir lower acidity as mentioned above.

The values of effective activation energy for n-heptaneand toluene in Na-form UL-MFI samples are relativelylower compared to the H-forms. Fig. 11 illustrates thedependence of the activation energy for the two sorbatesin the eight UL-MFI samples as a function of aluminumcontent. It is easily noticeable from the figure, that as theAl/Si molar percentage decreases the range of activationenergy (E) becomes more limited, thus again indicatingthat these differences are associated with the interactionsof the sorbates with the Bronsted acid sites.

4. Conclusions

In this paper, a series of UL-MFI materials with dif-ferent Si/Al ratios was synthesized and characterizedby several techniques, including XRD, N2 adsorption/desorption, AAS and SEM. The acid properties were inves-tigated from FTIR spectra of adsorbed pyridine and variedas a function of atomic Si/Al ratio.

The diffusion measurements of n-heptane and toluene ineight UL-MFI materials with different densities of acidsites were carried out using the ZLC method. Effective dif-fusivity values for both sorbates in Na-form of UL-MFIsamples are higher than in corresponding H-forms, andshow a decrease when analyzed with regard to the increas-ing micropore volume, which suggests the significant con-tribution stemming from sorbate–acid site interactions.Moreover, activation energy for n-heptane in these

128 H. Vinh-Thang et al. / Microporous and Mesoporous Materials 92 (2006) 117–128

UL-MFI materials was found to be twice that of toluene.The range of these values is strongly dependent on theatomic Si/Al ratio. Similarly, as has been observed in ourrecent study [2], mesopore diffusion in main channelsseemed to be rate controlling for n-heptane diffusion, whilethe intrawall diffusion was a dominant mechanism for tol-uene transport.

The effective diffusivities for both sorbates in UL-MFImaterials are much higher than in conventional MFI zeo-lites [2]. Combined with the higher acidity and higherhydrothermal stability, as compared to amorphous meso-structured materials, this suggests that UL-MFI materialshave a potential as catalysts for applications in petroleumrefining and fine chemical synthesis.

Acknowledgments

This project was supported by the Natural Science andEngineering Research Council of Canada (NSERC)through the Industrial Chair in Nanoporous Materials atLaval University.

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