alkali-treatment technique — new method for modification of structural and acid-catalytic...

Applied Catalysis A: General 219 (2001) 33–43

Alkali-treatment technique — new method formodification of structural and acid-catalytic

properties of ZSM-5 zeolites

Masaru Ogura, Shin-ya Shinomiya, Junko Tateno, Yasuto Nara,Mikihiro Nomura, Eiichi Kikuchi, Masahiko Matsukata∗

Department of Applied Chemistry, Waseda University, 3-4-1 Okubo, Shinjuku, Tokyo 169-8555, Japan

Received 15 January 2001; received in revised form 7 May 2001; accepted 8 May 2001

Abstract

ZSM-5 zeolite having a SiO2/Al2O3 molar ratio of 39.4 was treated in a NaOH alkali solution and the changes in structuraland acidic properties were investigated. A siliceous species was selectively dissolved from the framework of zeolite, althougha lower amount of Al was also eluted. In this procedure, mesopores with a uniform size were formed on the zeolite, whilethe microporous structure remained. The acidic property was changed very little quantitatively or qualitatively, even thoughthe catalytic activity for cracking of cumene was enhanced by the alkali-treatment. This can be explained by the facts thatadsorptive and diffusive properties of cumene through micropores of the ZSM-5 are increased by the creation of mesopores.© 2001 Elsevier Science B.V. All rights reserved.

Keywords: Alkali-treatment; ZSM-5; Mesopore; Acidity

1. Introduction

Zeolites have been encountering a growinginterest because of their applications for catalystsin industry, and for this reason there are currentlymassive efforts in trying to synthesize new kinds ofzeolite and zeolite-like materials [1]. Nowadays, aquite large number of materials, synthetic methodsand modifying methods has been devoted to opti-mize catalytic processes [2–4]. Active sites in thesenewly synthesized materials are acid/base or redoxcenters, which are associated with the presence ofprotons and extraframework metal cations not only

∗ Corresponding author. Tel.: +81-3-5286-3850;fax: +81-3-5286-3850.E-mail address: [email protected] (M. Matsukata).

alkaline and alkaline earth cations, but also transi-tion metal cations. Zeolites with metal cations thatisomorphously substitute framework atoms are alsointeresting new catalysts [5–7]. Such a large varietyof sites, whose activities very often depend on thelocal structure of zeolites [8], is characterized by useof various instruments, and are evaluated by variouskinds of catalytic reactions [9].

Among such reactions, cracking of hydrocarbonshas been widely recognized to be important since itis related to conversion of petroleum in transporta-tion fuels [10]. It is generally accepted that crackingactivity should be attributed to Brønsted acid sites ofzeolite, and that the reaction can be described by thecarbenium ion mechanism [11]. Zeolite Y has beenextensively used for refining oil in fluidized catalyticcracking processes. In order to control the catalytic

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34 M. Ogura et al. / Applied Catalysis A: General 219 (2001) 33–43

performance of zeolite Y, one often needs to im-prove the hydrothermal stability. From this viewpoint,dealumination treatment has widely been attempted.Zeolite Y can be dealuminated in an acidic mediumor steam at high temperatures to form ultra-stable Yzeolite (USY) [12]. Many researchers have found thatdealumination leads to changes in the zeolite on amicro scale, resulting in higher conversions and bet-ter hydrothermal stability [13]. The relation betweenstructural change and catalytic improvement upondealumination are summarized as in the followingthree types. Firstly, destruction of some portion ofthe zeolite framework occurs. Several reports haverevealed the formation of mesopores by using electronmicroscopy [14], and such formation is one of thereasonable explanations for a higher catalytic activity,because cracking is often limited by diffusion insidethe micropore of zeolite [15]. The mesoporous struc-ture shows an advantage for the diffusion of reactants:the external surface area of zeolite grain becomeslarger and reactants become more easily accessible tothe catalytically active acid sites. Secondly, the Al ionsremoved from the framework exist as extraframeworkAl species. As evidenced by NMR, octahedrally coor-dinated Al exists on dealuminated USY zeolite. Themost acceptable explanation for the enhanced activityis the acid strength model [16]. This model assumesthat, as a result of dealumination, strong Brønsted acidsites are generated which work as catalytically moreactive sites, and that the extraframework Al is nec-essary to form the strengthened Brønsted acid sites.Finally, Lewis acid sites and hydroxyl groups gener-ated on the dealuminated zeolite are claimed to causethe enhancement in catalytic activity [17]. Numerousconfusing proposals prevent us from concluding whatoccurs essentially on the dealuminated zeolites. More-over, recently, Williams et al. proposed a new possiblemodel for acidic catalysis [13]. They suggested that theacidity of zeolite is not related to the cracking activity,and they emphasized a change in reaction mode dur-ing cracking inside the catalyst bed. This phenomenoncan uniquely explain the activities of zeolite [18].

The acid treatment of zeolites is well known as amethod to change the SiO2/Al2O3 ratio in the zeoliteframework, and thus, the properties connected with theratio [19]. Less information has been available aboutthe base treatment of zeolite. In contrast to the acidtreatment which preferentially removes framework Al

atoms, the base treatment by use of alkali solutions wasfound to preferentially remove framework siliceousspecies [20]. Previous studies on the alkali-treatmentof zeolites such as ZSM-5 [21–24] or silicalite [23,24],by use of a 0.8 M sodium carbonate or 1–5 M sodiumhydroxide solution, showed that selective removalof the siliceous species from the zeolite frameworkoccurs without changes in the zeolitic structure. Thedissolved siliceous species is, however, easy to pre-cipitate onto the surface of ZSM-5 crystals, forminga layer of amorphous silica [23,24]. This may causepore blocking, and this must create a disadvantage inutilizing such alkali-treated ZSM-5 as a catalyst.

In our previous report on the alkali-treatment ofZSM-5 zeolite using a relatively low concentration(0.2 M) of sodium hydroxide aqueous solution, it wasshown that mesopores uniform in size were createdwithout any change in microporous structures of thezeolite [25]. The objective of this paper is to inves-tigate the mechanism of mesopore formation and theeffects of alkali-treatment on the structural and cat-alytic properties of ZSM-5, using low concentrationsof NaOH solution as treatment media. The possibilityof this treatment as a method for improving catalysiswill be discussed.

2. Experimental

2.1. Alkali-treatment of ZSM-5 zeoliteusing NaOH solutions

ZSM-5 zeolite used in this study, with a SiO2/Al2O3molar ratio of 39.4, was supplied by Tosoh Corp. in anNH4 form, designated as “as-received” in this work,which had smooth morphological features. Fig. 1shows SEM image of the zeolite.

Alkali-treatment of the ZSM-5 zeolite was per-formed with aqueous solutions of 0.05, 0.1, and 0.2 MNaOH. A flask made of polyethylene was used to avoiddissolution of any compounds by the treatment usinghigh alkaline solutions. First, 300 ml of the aqueoussolution was heated to 338 or 353 K in the flask witha reflux condenser using a water bath. Then, 4 g ofZSM-5 was put into the heated solution, and the solu-tion was kept at that temperature while stirring. Aftera period of 5, 30, 90, 120, or 300 min, the slurry wascooled down immediately using an ice bath and filtered

M. Ogura et al. / Applied Catalysis A: General 219 (2001) 33–43 35

Fig. 1. A SEM image of as-received ZSM-5.

using a 0.8 �m pore size filter made of cellulose-mixedester and a funnel made of polyethylene. The filtratewas analyzed to determine the concentrations of Si andAl dissolved during the alkali-treatment. The filteredcake was dried in an air oven at 383 K overnight.

The alkali-treated ZSM-5, abbreviated as ZSM-5-AT,was obtained in a Na-exchanged form, so that itwas ion-exchanged into H form for investigating itsacidity and catalytic performance. After drying, 5 gof ZSM-5-AT was put into a 1 M NH4NO3 solutionand then stirred at 353 K for 2 h, followed by fil-tering and rinsing with distilled water to remove allNa. This procedure was repeated five times to obtainNH4-ZSM-5-AT. After drying at 383 K overnight, theNH4-ZSM-5-AT was calcined in an electric furnaceto form H-ZSM-5-AT.

2.2. Physicochemical properties ofalkali-treated ZSM-5 zeolite

Concentrations of Si and Al were determined bymeans of inductively coupled plasma atomic emis-sion spectroscopy. A field-emission scanning electronmicroscope (S-4500S, Hitachi Corp.) and a transmis-sion electron microscope (JEM-2010, JEOL) wereused for the investigation of particle morphology ofZSM-5-AT. X-ray diffraction patterns were measuredusing RINT 2000 (Rigaku Instrument Corp.) withCu K� radiation for confirming the structure andcrystallinity of ZSM-5.

Total surface area, micropore and mesopore volu-mes, and external surface area of ZSM-5 and ZSM-5-AT were determined by use of the N2 adsorptionmethod at 77 K with Belsorp 28SA (BEL Japan, Inc.).The total surface area was calculated according to theBET isothermal equation, and the micropore volumeand external surface area were evaluated by the t-plotmethod [26]. The mesopore volume was determinedfrom a pore size distribution curve by the DH method[27].

2.3. Acidic and catalytic properties ofalkali-treated ZSM-5 zeolite

Temperature-programmed desorption (TPD) spec-tra of ammonia were measured for the identificationsof acidity. The sample of 50 mg was degassed at773 K in 20% oxygen stream for 30 min, evacuatedand exposed to NH3 at 373 K. Desorption of NH3 wasmonitored at a temperature ramp of 10–1073 K min−1

using a TPD-AT-1 (BEL Japan, Inc.) connected to amass spectroscopic instrument. Infrared spectra wereobtained with a JASCO IR610 system; the sample waspressed into a small wafer of 20 mg and 20 mm ∅,and activated after having been mounted in a specialIR cell with CaF2 windows allowing in situ thermaltreatments under controlled atmospheres. The samplewas degassed at 773 K below 10 Torr, followed byadsorption of 30 Torr pyridine at 323 K.

Al coordination states on the zeolite samples wereconfirmed by 27Al-MASNMR, recorded on a JEOLGSX-400, at the frequency of 104 MHz, the pulsewidth of 4.2 �s, the recycle time of 5 s, and the scan-ning times of 200. Chemical shifts were referenced toan external standard of [Al(H2O)6]3+.


Cumene cracking was conducted in a pulse-injec-table flow reactor connected to an on-line TCD gaschromatograph with a Shimadzu GC-6A integrator.After pretreatment at 573 K for 1 h to dehydrate, thetemperature of the catalyst bed was kept at 523 K,and 1 ml (7.2 �mol) of cumene was injected into aHe carrier flow (30 cm3 min−1). The same reactor wasused for evaluation of adsorptive/diffusive propertiesof ZSM-5 and ZSM-5-AT for benzene in the tem-perature range of 503–543 K. The injected amount ofbenzene was 1 �l (11.2 mmol).

3. Results

The concentrations of Si and Al dissolved fromZSM-5 during alkali-treatment were measured, andtypical results using 0.05 M NaOH solution are givenin Fig. 2. Mainly Si was eluted into the alkaline so-lution, and the amount of Si in the filtrate increasedwith increasing treatment period. The concentra-tion of Al in the filtrate was much less than thatof Si; it first increased, reached a maximum after120 min, and decreased by further treatment. Thechange in the SiO2/Al2O3 molar ratio in ZSM-5 af-ter alkali-treatment is summarized in Table 1. TheSiO2/Al2O3 ratio remained almost unchanged during120 min of the alkali-treatment. The amount of Sieluted in 300 min corresponds to 2.8% of Si in theframework of the as-received ZSM-5. Therefore, itcan be concluded that the alkali-treatment leads toselective extraction of Si, and does not change theframework composition significantly.

On the other hand, there is found a differencein physicochemical properties between as-receivedZSM-5 and ZSM-5-AT. Fig. 3 shows the N2 adsorp-tion isotherms on as-received and ZSM-5 treated

Table 1SiO2/Al2O3 molar ratio in the alkali-treated ZSM-5a

Alkali-treatment period (min) Si (mg g−1 zeolite) Al (mg g−1 zeolite) Si/2 Al ratio

0 446 26.2 32.85 428 25.9 31.8

10 420 25.1 32.130 420 26.5 30.4

120 427 27.1 30.3300 430 28.6 28.9

a Conditions for alkali-treatment were the same as those in Fig. 2.

Fig. 2. Concentrations of Si (a) and Al (b) in the filtrate ofalkali-treatment of ZSM-5. Treatment was carried out in a 0.05 MNaOH solution at 338 K.

by a 0.2 M NaOH solution. The initial slope of theisotherm corresponds to adsorption in the microstruc-ture of zeolite and it was not much changed by thealkali-treatment. A hysteresis loop clearly appeared onZSM-5-AT, suggesting that ink-bottle type mesopores


Fig. 3. Nitrogen isotherms for as-received and alkali-treated ZSM-5. Treatment was carried out in a 0.2 M NaOH solution at 353 K. Adotted line corresponds to the desorption branch of N2 adsorption measurement.

are formed on the zeolite. Fig. 4 illustrates the poresize distributions calculated by the DH equation.When we extended the period of alkali-treatment, thediameter of the mesopores slightly increased and asharp distribution around 4 nm was observed after300 min of alkali-treatment. Table 2 shows the BETsurface area (SBET), micropore volume (Vmicro), meso-

Fig. 4. Pore size distributions of as-received and alkali-treated ZSM-5: (�) as-received; (�) AT30; (�) AT120; (�) AT300. The datawere calculated according to the DH method using the desorption branch of N2 isotherms in Fig. 3.

pore volume (Vmeso), and external surface area (Sext)of ZSM-5 zeolites treated with various concentrationsof NaOH solution. As reported in [23,24], the silicaspecies dissolved from zeolite are easy to precipitateonto the surface of ZSM-5 crystals, forming the layerof amorphous silica, and their presence may causepore blocking. A lower concentration of alkaline


Table 2Summary of physicochernical properties of ZSM-5 as-received and alkali-treated

Alkali concentration (mol l−1)

As-received 0.05 0.1 0.2

5a 10a 30a 120a 300a 120a 300a 30a 120a 300a

BET surtace area, SBET (m2 g−1) 303 211 239 263 279 234 287 292 321 310.8 320.2External surface area, Sext (m2 g−1) 6.63 5.87 6.44 9.37 18.6 27.2 41.5 57.6 60.1 57.5 115.4Micropore volume, Vmicro (mm3 g−1) 184 123 140 163 168 149 140 140 170 177 133Mesopore volume, Vmeso (mm3 g−1) 15.4 13 22.3 42.1 59.4 59.2 29.2 47.4 177 200 279

a Values represent alkali dissolution period (min).

solution and a shorter period of treatment caused areduction in the amount of N2 adsorbed. The valuesof Vmicro and SBET were almost unchanged from theiroriginal values, while those of Vmeso and Sext increasedby alkali-treatment. Further increases in the values ofVmeso and Sext were observed for longer periods ofalkali-treatment. The crystallinity of ZSM-5, however,was confirmed to be lowered by treatment at 353 Kusing a solution with NaOH concentration higher than1 M.

A quite dramatic change in morphology of ZSM-5appeared upon alkali-treatment. Typical SEM imagesof ZSM-5-AT are shown in Fig. 5. As is obvious froma comparison between Fig. 5a and Fig. 1, some cracksand faults appear on the surface of ZSM-5 particlesafter alkali-treatment for 120 min in a 0.1 M NaOHsolution. The cracks on the as-received ZSM-5 parti-cle became much deeper after treatment for 300 minin the alkaline solution. Furthermore, the change inmorphology of ZSM-5 in a 0.2 M NaOH solution for300 min (Fig. 5c) was extremely significant comparedwith the morphology of the as-received. The zeolitegrain was partly collapsed by alkali-treatment undersuch severe conditions. The edges of particles lookmelted to such an extent that the original shape ofas-received ZSM-5 can hardly be recognized.

To find internal changes in particle morphology, thealkali-treated ZSM-5 was elucidated by TEM obser-vation, as shown in Fig. 6. Mesopores through the par-ticle were clearly viewed on the sample, and the edgepart seems to be deformed. The mesopores observedwere similarly traversed inside the particle to those ondealuminated zeolite Y [14].

Even though ZSM-5 did not retain its original mor-phology after alkali-treatment, XRD patterns showed

that the crystal structure was maintained. The resultsare shown in Fig. 7a. The diffraction peaks assignedto MFI did not change in intensity and the lat-tice parameters were the same after alkali-treatmentup to 300 min. It is noted that the alkali-treatmentsharpened the diffraction peaks. Fig. 7b shows thediffraction patterns at around 23◦ (Cu K�). Thepeaks at 23.07 and 23.25◦, respectively assigned to[0 5 1] and [−5, 0, 1] of MFI, were distinctly split-ted by alkali-treatment, indicating that poor crys-tallinity parts of ZSM-5 are selectively dissolved.Judging from the results of chemical analysis, SEMand XRD, we can conclude that the alkali-treatmentresults in the structural change of ZSM-5 on a mi-crometer scale, but not at an atomic or a molecularlevel.

In order to evaluate the acidity of H-ZSM-5-ATcompared with the as-received ZSM-5 zeolite, theacidic properties of H-form zeolites were investigated.When a 0.05 or 0.1 M NaOH solution was utilized asthe treatment medium, almost no change in aciditywas observed after treatment. Fig. 8 shows the spectraof NH3-TPD for the samples treated by a 0.2 M NaOHsolution. The peak intensities slightly decreased withprolonged alkali-treatment, and the peaks becamebroadened. As a result, the amounts of acid sites cal-culated from the desorption at the higher temperature,were 0.704 mmol g−1 zeolite for as-received ZSM-5,0.589, 0.525, and 0.496 mmol g−1 for H-ZSM-5-ATfor 30, 120, and 300 min, respectively.

Typical results of infrared spectroscopic measure-ments for pyridine chemisorption are illustrated inFig. 9. The band at 1545 cm−1 assigned to Brønstedacid sites did not change, while the relative intensityof band due to Lewis acid sites at 1450 cm−1 slightly


Fig. 5. SEM images of alkali-treated ZSM-5. Treatment was carriedout in a 0.1 M NaOH solution at 338 K for 120 min (a), 300 min(b), and in a 0.2 M solution at 353 K for 300 min (c).

increased by alkali-treatment. Little dealuminationoccurred during the alkali-treatment, and only tetra-hedrally coordinated Al was confirmed by use of27Al-MASNMR.

Fig. 6. A TEM image of alkali-treated ZSM-5. Treatment wascarried out in a 0.05 M NaOH solution at 338 K for 120 min.

H-ZSM-5-AT and as-received H-ZSM-5 were eval-uated as acid-catalysts by cumene cracking. Theresults are summarized in Table 3. For 20 pulses ofcumene injection, the catalytic activity of every cata-lyst declined very little. It is obvious that cumene con-version into benzene is increased by alkali-treatment.H-ZSM-5-AT30 showed almost twice the conversionof as-received ZSM-5 on a basis of the catalytic per-formance per a Brønsted-acidic Al atom. From theseresults, it can be deduced that alkali-treatment ofzeolite is effective to increase the catalytic activity.

To investigate the diffusion properties of H-ZSM-5and H-ZSM-5-AT, benzene was pulsed on these zeo-lites under the similar conditions to those utilized inthe cumene cracking test. Fig. 10 shows half-widthsof the chromatogram when 1 �l of benzene waspulsed onto the 20 mg of the zeolites in the tempera-ture range of 483–543 K. Benzene did not react underthe conditions used; therefore, all of the injectedbenzene came through the zeolite bed and the areaof each chromatogram was almost unchanged, whilebenzene was eluted at a different rate depending onadsorption onto or diffusion through the zeolites. Thehalf-width of the chromatogram could be consideredas a sorptive–diffusive parameter in the zeolite pores.As shown in Fig. 10, alkali-treatment was found to


Fig. 7. XRD patterns for as-received and alkali-treated ZSM-5. Treatment was carried out in a 0.2 M NaOH solution at 353 K for 300 min.

Fig. 8. Temperature-programmed desorption of NH3 from as-received and alkali-treated H-ZSM-5. Treatment was carried out in a 0.2 MNaOH solution at 353 K.

result in the increase of the half-width of benzenechromatogram, whereas the slope against the bedtemperature was almost constant.

4. Discussion

4.1. What is the alkali-treatment of zeolite?

The results of chemical analyses (Fig. 2 andTable 1) clearly show that the alkali-treatment is amethod to extract a siliceous species from the frame-work of ZSM-5. Alkali-treatment is another way ofmodifying zeolite, becoming an alternative for acidtreatment, which is often utilized in order to extractAl to obtain higher SiO2/Al2O3 molar ratios in the

Table 3Catalytic activities of alkali-treated ZSM-5 for cumene cracking

Catalyst % Conversion

mmol cumene g−1

catalysismol cumene/mol Al

H-ZSM-5as-received

22 0.32 0.49

H-AT30 33 0.47 0.86H-ATI20 24 0.35 0.71H-AT300 35 0.50 0.78

framework. During alkali-treatment, the structuralunit is partly extracted from ZSM-5 particles. In thisstudy, for example, 2.6 Si atoms per unit cell of MFIwere extracted per min (2.6 Si (u.c.)−1 min−1) dur-ing the initial period of alkali-treatment until 10 min,


Fig. 9. Infrared spectra of pyridine adsorbed on as-receivedH-ZSM-5 and H-ZSM-5-AT300. The sample was exposed to30 Torr pyridine after calcination at 773 K, followed by evacuationand cooling down to 323 K.

while 0.1 Al (u.c.)−1 min−1. The SiO2/Al2O3 ratio inthe extracted part is 52 min−1. Mesopores are thenformed with no collapse in crystal structure of theas-received ZSM-5, resulting in the enlargement ofexternal surface area of the zeolite.

From TEM observation, one can see that the meso-pores created by dissolving the portion of ZSM-5 withpoor crystallinity might show the existence of facetsby twinning ZSM-5 crystallites. A part of a zeoliteparticle, weak against alkali-treatment, would become

Fig. 10. Half-widths of chromatogram of benzene through ZSM-5:(�) as-received; (�) AT30; (�) AT120; (�) AT300.

a defect in the zeolite framework. The amounts ofsiliceous species eluted into the alkaline solution dur-ing alkali-treatment were almost the same as thoseestimated from the value of mesopore volumes in-creased during the treatment [25]. These results ledus to conclude that mesopores are created betweenthe ZSM-5 crystallites.

4.2. Effect of alkali-treatment on acidicproperties of ZSM-5 zeolite

As previously mentioned, dealumination in minorto desilicification occurred during alkali-treatment,so that the amount of acid sites is little changed inaccordance with the results of NH3-TPD (Fig. 8).On the other hand, the desorption profiles of NH3from alkali-treated samples are slightly different: thedesorption peak of NH3 from H-ZSM-5-AT seems toshow tailing. However, infrared spectroscopic studiesusing pyridine adsorption (Fig. 9) reveal that acidicproperties of H-ZSM-5-AT are maintained as the orig-inal state of as-received ZSM-5. 27Al-NMR spectraindicate that extraframework aluminum, which hasoften been reported to cause strengthened acidity ofzeolite [13], never exists on H-ZSM-5-AT.

It can be deduced from the results of catalytic ac-tivity tests and benzene adsorption that the number ofadsorption sites for reactant is increased on the zeoliteby alkali-treatment. From the value of the half-widthof the benzene desorption chromatogram shown inFig. 10, the diffusion coefficient of benzene (D) wascalculated according to the Einstein equation:

τ = L2

6D

where τ is the delayed time for the elution of benzenegiven by the half-width of the chromatogram, and L theaverage half-length of a zeolite particle. This calcula-tion was based on an assumption that the diffusion ofbenzene through ZSM-5 particles is rate-determining.Fig. 11 shows the values of D through ZSM-5 par-ticles either as-received or alkali-treated. From thefigure, the apparent activation energy Ea for benzeneadsorption–diffusion was calculated; these values aresummarized in Table 4 along with the D-values at523 K where cumene cracking was carried out. Thevalues of D are quite similar to those reported fordiffusion of benzene through zeolite pores in [28,29];


Fig. 11. Arrhenius plots of the apparent diffusivity for benzenethrough ZSM-5: (�) as-received; (�) AT30; (�) AT120; (�)AT300.

Table 4Apparent difflisivity and activation energy for benzene observedon alkali-treated ZSM-5 samples

Zeolite As-received AT30 AT120 AT300

D at 523 K(×10−14 m2 s−1)

2.68 1.38 1.49 1.48

Ea (kJ mol−1) 18.4 11.5 12.8 11.3

they are in the order of 10−13 or 10−14 m2 s−1 at about500 K. Therefore, it is reasonable to say that benzene iseluted through zeolite pores. The D on H-ZSM-5-ATwas smaller than that on H-ZSM-5. This might be ex-plained by the proposal [29,30], in which acid sites af-fect benzene sorption, resulting in a lower D-value onMFI having a lower SiO2/Al2O3 ratio, which relatesto adsorption. The values of Ea, on the other hand,are smaller than reported values, such as 25 kJ mol−1

[28] or ca. 45 kJ mol−1 [29]. It is noted that the valuesof Ea are largely changed by alkali-treatment. Thisindicates that alkali-treatment leads to a larger con-tribution for the adsorption–diffusion process throughmicropores. The investigations by benzene injec-tion would show the diffusivity of zeolite throughmicropores, which is affected by the coexistingmesopores.

Apparent dependencies can be seen among theamount of mesopores formed in the samples, the half-widths of chromatogram of benzene, and also thecatalytic activities, indicating that the mesopores en-hance the diffusivity of reactant and catalytic activity,

as has been illustrated in [31]. Therefore, we can con-clude that alkali-treatment increases mesopores andthe external surface of zeolite, increases the numberof adsorption sites located near the pore mouth on theexternal surface of zeolite, and as a result, promotesthe diffusion of reactant through zeolite micropores.

5. Conclusions

ZSM-5 zeolite was treated in NaOH solutions andthe changes in structural and acidic properties wereinvestigated. A siliceous species was selectively dis-solved from the framework of zeolite, although Al wasalso eluted to a less extent. The siliceous species wasdissolved from relatively weaker parts of zeolite suchas growing faces, resulting in better crystallinity of theremaining zeolite. A morphological change of ZSM-5particles by the alkali-treatment could be observed andmany cracks and faults were formed on the outer sur-face of zeolite grains. Mesopores with a uniform sizewere formed on zeolite particles, although the micro-porous structure remained under the conditions usedin this work. The acidity was hardly changed, eventhough the catalytic activity for cumene cracking wasenhanced by the treatment. Alkali-treatment led to anincrease in the number of adsorption sites and also inthe diffusivity of benzene through zeolite micropores.The enhancement in catalytic performance can beexplained due to the fact that the adsorptive–diffusiveproperty of ZSM-5 is improved by alkali-treatment.

Acknowledgements

The authors would show great appreciation to Dr.Yukichi Sasaki, Japan Fine Ceramics Center (JFCC),who has collaborated with the studies on the TEMobservation.

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alkali-treatment technique — new method for modification of structural and acid-catalytic...

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