alkaline treatment on commercially available aluminum rich mordenite

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Applied Catalysis A: General 382 (2010) 65–72

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Applied Catalysis A: General

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lkaline treatment on commercially available aluminum rich mordenite

dri N.C. van laak, Robert W. Gosselink, Sophia L. Sagala, Johan D. Meeldijk, Petra E. de Jongh,rijn P. de Jong ∗

norganic Chemistry and Catalysis, Debye Institute for Nanomaterials Science, Utrecht University, NL-3584 CA Utrecht, Netherlands

r t i c l e i n f o

rticle history:eceived 11 February 2010eceived in revised form 9 April 2010ccepted 9 April 2010vailable online 24 April 2010

eywords:

a b s t r a c t

Several commercially available samples consisting of agglomerated small mordenite crystallites with lowSi/Al ratios (5.7–10 at/at) have been treated in aqueous NaOH solution. It was found that the porosity canbe enhanced when the sodium hydroxide solution is sufficiently concentrated. Treatment in 1 M NaOHfor 15 min resulted in inter-crystalline porosity and the mesopore volume was increased from 0.01 to0.21 cm3 g−1 together with an increased external surface area from 36 to 85 m2 g−1. The micropore volumeand crystallinity were preserved after the treatment. Both H-MOR and Na-MOR mordenite agglomerates

ordeniteesoporosityesilicationlkylationatalysis

have been successfully treated: the Na-MOR requires a longer contact time to obtain similar porosity.By carefully choosing the alkaline concentration and contact time, intra-crystalline mesoporosity canbe obtained for mordenite with Si/Al ratios as low as 10. Catalytic tests with proton mordenite showedthat alkaline treatment leads to more than one order of magnitude of activity gain in the liquid-phasealkylation of benzene with propene to form cumene, while selectivity is preserved. These results demon-strate that alkaline treatment also on high-aluminum content mordenites is an effective tool to enhance

its ca
accessibility and thereby
. Introduction

Zeolites are microporous crystalline materials that present anmportant class of catalysts because of their specific properties suchs high Brønsted acidity, thermal stability, facile regeneration andhape selectivity [1]. There are over 191 different types of zeo-ite framework structures [2], but only a limited number is usedn industrial applications. Mordenite, with an ideal composition ofa8Al8Si40O96·nH2O, is one of the industrially important zeolitesnd has been used for hydroisomerization, alkylation and dewax-ng processes [3–5]. The micropore system of mordenite consist ofwo pore channels: an elliptical 12 membered ring (MR) channel6.7 Å × 7.0 Å) which runs parallel to the c-axis, and an 8 MR sideocket (3.4 Å × 4.8 Å) [6] that runs in the b-axis direction. Morden-

te is considered a one-dimensional zeolite due to inaccessibility ofhe 8 MR side pocket for all but the smallest molecules [7].

The shape selectivity is a result of the micropores that are of sim-lar size as the reacting molecules and the intended products. As theatalytic sites are located within these micropores, the formation
f high molecular weight side products is suppressed resulting inigh selectivity. The drawback of this size similarity is slow intra-rystalline diffusion, due to strong interaction of the molecules withhe micropore walls [8]. A second disadvantage of the micropore
∗ Corresponding author. Tel.: +31 302537400; fax: +31 30 251 1027.E-mail address: [email protected] (K.P. de Jong).

926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.apcata.2010.04.023

talytic performance.© 2010 Elsevier B.V. All rights reserved.

structure, especially with one-dimensional zeolites such as mor-denite, is that they are prone to pore blocking [9]. A reduction ofthe diffusion pathlength, however, may alleviate these problemsand increase the activity and stability.

A first approach is the reduction of the crystal size by alteringthe synthesis routes [10,11]. This has been proven successful overthe years for numerous zeolites. Mordenite is challenging in thisrespect because the range of synthesis conditions are very narrow.Industrial zeolite producers such as Zeolyst and Tosoh have beenable to synthesize small crystallites ∼100 nm but those are mostoften agglomerated into larger particles of 1–5 �m.

A second approach to decrease the diffusion pathlength is byintroducing mesoporosity [12,13]. This can be realized during syn-thesis or by using post-synthesis treatments such as steaming andacid leaching [7,14–19]. For the last two methods the introductionof mesoporosity is closely linked with the Si/Al ratio of the zeolite.As a high silicon to aluminum ratio often also has a positive effecton the catalytic performance of zeolites these methods have beenthoroughly investigated in the past decades. One of the industrialapplications of acid treatment was described by employees of Dow[20,21]. They reported that by deep dealumination they obtainedmordenite that provided shape selective, active and stable catalysis
of the trans-alkylation of mononuclear aromatics. This was the firstzeolite to be commercialized in a cumene synthesis process, as anadd-on trans-alkylation process.
Another post-synthesis treatment to introduce mesoporesinvolves alkaline solutions and was studied in the late 1960s and

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970s [22–26] and has recently seen a revival. This method iseferred to as desilication, base leaching or alkaline treatment27–31]. In a typical experiment the zeolite is treated with a lowoncentration of NaOH in water at 60–80 ◦C for a short time.uring contact with the solution, silicon is extracted from the

ramework resulting in mesoporous zeolites. Results have beenublished for various zeolites such as MFI [29,31,32], BEA [33] andOR [22,23,34,35]. There are however some general requirements

eported for successful alkaline treatment. Firstly, the zeolite mustave preferably a Si/Al ratio (at/at) between 25 and 50 [36]. At loweralues there is a limited effect of alkaline treatment due to the rela-ively high-aluminum content, while at higher values the structuralntegrity of the zeolite crystal is lost. Secondly for mordenite onlyarge zeolite crystals, in the micrometers range, have been suc-essfully treated using this method. Groen et al. [34] reported theeneration of intra-crystalline mesoporosity for mordenite crystalsf 4 and 12 �m in diameter having Si/Al ratios of 20 and 30, respec-ively. A commercially available mordenite, Zeolyst CBV 20A, wasound to be hardly susceptible for desilication. van Bokhoven ando-workers [35] reported no mesoporosity for a 13 �m crystal withi/Al ratio of 15 upon desilication, only after boosting the Si/Al ratiof the parent mordenite to 25.

In this study we use commercially available mordenites, whichost often have low Si/Al ratios and consist of small crystal-

ites that are assembled in agglomerates. The effect of alkalinereatment to improve the porosity of these systems and theiratalytic properties for the production of cumene from propenend benzene will be discussed.

. Experimental

.1. Sample preparation

Three samples of mordenite were supplied by BP Amoco, BASFnd Zeolyst named LZM-5, BASF and CBV 21A with Si/Al ratios of 5.7,.5 and 10, respectively. LZM-5 and BASF mordenite were provided

n sodium form, Zeolyst CBV 21A was provided in ammonia form.As received LZM-5 mordenite, referred to as Na-MOR-parent,

as ion-exchanged under stirring in an aqueous 1 M ammoniumitrate solution at 353 K for 24 h, followed by filtering and washing.er gram of mordenite 12 ml of ammonia nitrate solution was used7]. This procedure was repeated twice to ensure complete removalf sodium ions. The sample was then converted from the ammoniaorm to the proton form by calcination at 723 K for 3 h at a heatingate of 1 K min−1; the obtained sample will be referred to as H-OR-parent.Two gram of H-MOR-parent was stirred in pre-heated 100 ml

odium hydroxide solution (0.2, 0.5 or 1.0 M) at 343 K for 15r 30 min. The solid sample was subsequently filtered andashed with demineralised water until the pH of the filtrateas neutral. After drying at 333 K the resulting solid was named-MOR-at-[NaOH]-time. One sample, H-MOR-at-1.0-15, was re-

on-exchanged/calcined into the H-MOR as described above andill be referred to as H-MOR-at. Na-MOR-parent was likewise

reated with NaOH solutions and referred to as Na-MOR-at-NaOH]-time. The following sample code H-MOR-at-1.0-15 thanranslates into: alkaline treatment on the parent H-MOR for 15 minn 1 M NaOH that has sodium as a compensating cation.

As received BASF and Zeolyst CBV 21A mordenite were referredo as BASF-parent and CBV 21A-parent. After alkaline treatmentf the parent an identical naming scheme was used as describedbove.

.2. Structural characterization

Powder X-ray diffraction (XRD) patterns were obtained usingBruker-Axs D8 series 2 with a Co�1,2 source (� = 0.179 nm).

is A: General 382 (2010) 65–72

The porosity of the samples was studied using N2-physisorptionisotherms, which were recorded with a Micromeritics Tristar 3000at 77 K. Prior to the physisorption measurements, the mordenitesamples were dried overnight at 573 K in flowing nitrogen. The t-plot method [37] was applied to obtain the micro pore volume andexternal surface area. Pore size distributions were obtained fromthe adsorption branch using the BJH-method [38]. The mesoporevolume was calculated by integrating these plots from 2 to 50 nm.

Morphology and crystal sizes were determined with a Tecnai FEIXL 30SFEG Scanning Electron Microscope (SEM) at 15 kV and witha Tecnai 20 Transmission Electron Microscope operated at 200 kV.Electron tomography (ET) grids, to which 10 nm gold fiducial mark-ers were added to aid alignment on a polymer coated copper grid.Mordenite samples were suspended in ethanol and dispersed for30 min in an ultrasonic bath and deposited on the TEM grids usinga thin film. ET was performed in bright field TEM by acquiring tiltangles from −67◦ to +67◦. Reconstruction of the tomogram wasperformed in IMOD [39].

27Al MAS NMR experiments were performed at room tempera-ture on a 500 MHz Varian Inova Spectrometer using a 4 mm probe.In the experiments, a single pulse of 1 �s, a relaxation delay of 2 s,and a spinning rate of ca. 13 kHz were used. The 27Al chemical shiftwas referenced to 1 M Al(NO3)3 in H2O.

2.3. Adsorption and catalytic measurements

Cumene uptake was measured with a tapered element oscil-lating microbalance (TEOM) (Rupprecht & Pataschnick TEOM 1500PMA). Prior to the measurements the catalyst was pelletized at apressure of 1 ton/cm2 for 25 s and subsequent crushed and sieved,resulting in particles between 150 and 425 �m. Approximately50 mg of catalyst powder was then transferred into a 100 �l sam-ple container of the TEOM and held in place between two layersof quartz wool. The sample was dried for 4 h at 623 K in nitrogenflow (grade 5.0). Cumene (Acros 99.9% pure) uptake was measuredwith using nitrogen as a carrier gas at a total pressure of 1.3 bar at623 K. Liquid cumene was injected into the system using an ISCO260D syringe pump with 3 �l min−1 and was evaporated in flow-ing nitrogen resulting in a cumene partial pressure of 9 mbar. Fora detailed description of the TEOM we refer to Chen et al. [40] andZhu et al. [41]

The catalytic tests were carried out in a stirred autoclave at400 rpm. Benzene and propylene were tapped from industrialstreams (99% plus) supplied by Dow Terneuzen, The Netherlands.For the alkylation approximately 1 g of catalyst powder (sievefraction 425–800 �m) and 270 g of benzene were loaded into anitrogen purged 1 l autoclave. After raising the temperature to150 ◦C approximately 38 g of propylene was fed to the reactorresulting in a benzene to propylene molar ratio ∼4. The reactorwas then pressurized to ∼40 bar by feeding nitrogen. During reac-tion samples were withdrawn from the reactor and analyzed by GC.Similar experiments have been described by Bellussi et al. [42].

3. Results and discussion

Fig. 1 presents SEM images of LZM-5, BASF and Zeolyst CBV21A mordenite samples. All three samples consist of sub-micronsized crystals that have agglomerated into larger particles. LZM-5 mordenite was chosen as a model system to study mesopore
development upon alkaline treatment because of its overall uni-form morphology. LZM-5 consists of coffin-shaped crystallites withtypical dimensions of ∼100 nm × 40 nm and a Si/Al atomic ratio of5.7. These crystallites are mostly organized in a parallel fashion intoparticles of about 0.5–2 �m.

A.N.C. van laak et al. / Applied Catalysis A: General 382 (2010) 65–72 67

Fig. 1. SEM image of (A) LZM-5, (B) BASF and (C) Zeolyst CBV 21A.

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ig. 2. (A) N2 adsorption (solid symbols) and desorption (open symbols) isotherm; at-0.5-30.

.1. H-MOR-parent (LZM-5)

The H-MOR-parent sample was treated in 0.2 M sodium hydrox-de for 30 min, similar to what is described in earlier studies forntroducing mesopores in large mordenite crystals [34]. This treat-

ig. 3. (A) N2 adsorption (solid symbols) and desorption (open symbols) isotherm; and (Bt-1.0-15.

) BJH pore size distribution): (�) H-MOR-parent, (�) H-MOR-at-0.2-30, (�) H-MOR-

ment did not result in significant changes in the porosity of themordenite most likely due to the Si/Al ratio which is well belowthe Si/Al ratio of 25–50 that is reported to be necessary to introducemesoporosity [36,43]. Increasing the sodium hydroxide solution to0.5 M resulted in a slight increase in mesoporosity as illustrated in

) BJH pore size distribution): (�) H-MOR-parent, (�) H-MOR-at-1.0-30, (�) H-MOR-

68 A.N.C. van laak et al. / Applied Catalysis A: General 382 (2010) 65–72

Table 1Textural properties of parent and alkaline treated LZM-5 H-mordenite.

Sample Vmicroa (cm3 g−1) Aext

a (m2 g−1) Vmesob (cm3 g-1) Vtotal

c (cm3 g−1) Si/Ald (at/at) Cumene uptake (wt.%)

Na-MOR-parent 0.15 35 0.02 0.21 5.5H-MOR-Parent 0.17 36 0.01 0.20 5.5 4.1H-MOR-at-0.2-30 0.15 35 0.04 0.24 –H-MOR-at-0.5-30 0.15 55 0.11 0.30 –H-MOR-at-1.0-30 0.12 54 0.09 0.35 –H-MOR-at-1.0-15 0.15 67 0.16 0.44 5.0filtrate 6.1H-MOR-at 0.17 85 0.21 0.53 5.0 3.6

bchamre1ipfttsHoprSslitibiett

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a t-plot method.b BJH-method (adsorption branch).c @ p/po = 0.995.d ICP-AES.

y the results of N2-physisorption (Fig. 2). Surprisingly, a NaOH con-entration of 1.0 M brought about a sharp increase in adsorption atigh relative pressures, indicating the formation of large mesoporesnd small macropores (Fig. 3). Simultaneously, a substantial part oficropore volume was lost, indicating that the applied conditions

esulted in the degradation of the framework. However, when thexperiment in 1 M NaOH was repeated with a contact time of only5 min (to obtain the H-MOR-at-1.0-15 sample), a similar increase

n mesoporosity was observed, while the micropore volume wasreserved (Fig. 3). Table 1 summarizes the N2-physisorption resultsor the different samples. In general the external surface area andotal pore volume both increase with increasing alkaline concentra-ion and contact times, with the exception of the H-MOR-at-1.0-30ample. In Picture 3B the pore size distribution is shown for the-MOR-at-1.0-15 sample and an average pore size of 35 nm wasbserved. These pores are too large to be intra-crystalline meso-ores, as the crystallites have a dimension of 40 nm × 100 nm. Theesulting mesopores are therefore most probably inter-crystalline.i/Al ratio ratios were determined for the H-MOR-at-1.0-15 andhowed a decrease from 5.7 to 5.0. This suggests that upon alka-ine treatment predominantly silicon was dissolved. The Si/Al ration the filtrate after alkaline solution was 6.1, showing that duringhis treatment both silicon and aluminum are being dissolved andn rather large amounts. For this particular sample the dry weightsefore and after the alkaline experiment was determined, which

ndicated that 35–40% of the mordenite was dissolved. Similarxperiments were performed by Cundy et al. [44,45], who showedhat large 1 �m sized ZSM-5 crystals can also be subjected to con-rollable dissolution.

The H-MOR-at-1.0-15 sample was chosen for further studies. Itas exchanged with ammonium nitrate to remove sodium ions fol-

owed by calcination to obtain the proton form that is referred to as-MOR-at. When H-MOR-parent and H-MOR-at are compared webserve that micropore volume as determined with N2 is similar

Fig. 5. TEM images of LZM-5 mordenite: (A

Fig. 4. XRD patterns of LZM-5 mordenite (A) H-MOR-Parent and (B) H-MOR-at.

for both samples. The uptake of benzene however, is ∼15% lowerfor the H-MOR-at indicating that as a result of alkaline treatmentsome micropores have become partially blocked. This could be dueto the precipitation of amorphous species during alkaline treat-ment or due to the formation of extra-framework aluminum uponcalcination. From 27Al MAS NMR (Fig. 9) we did not observe a signif-icant change in framework to extra-framework aluminum speciesupon calcination, which indicates that the former reason is mostlikely the cause of the partial blockade. Fig. 4 depicts the XRD pat-terns of both samples, showing that there is no significant changein crystallinity after alkaline treatment. If the crystallinity is cal-
culated according to O’Donovan et al. [46] the crystallinity is 100%and 102% for H-MOR-parent and H-MOR-at, respectively.
Fig. 5 shows TEM images of the H-MOR-parent sample andthe H-MOR-at sample. After alkaline treatment the density of theagglomerates has decreased significantly and individual crystallites

) H-MOR-parent and (B) H-MOR-at.


graph

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Fig. 6. Reconstructed thin slices obtained from electron tomo

an be observed throughout the whole sample, while the parentample is much more compact. To study the changes in porosity inore detail we applied electron tomography. Fig. 6A and B is thin

lices obtained from the center of the reconstructed tomograms. Forhe H-MOR-parent this illustrates the tight packing of crystallites,eading to dense particles. Between the crystallites small cavitiesre visible that are most probably inkbottle type mesopores origi-ating from imperfect stacking. However, for the treated sample aery open structure is observed. An average mesopore size of about0 nm is observed from tomography, thereby supporting the resultsbtained with N2-physisorption.

.2. Na-MOR-parent (LZM-5)

Direct alkaline treatment on the Na-MOR-parent would be veryttractive, because in this case the additional exchange and calcina-ion to obtain H-MOR could be avoided. Identical conditions werepplied on the Na-MOR-parent as on the H-MOR-parent, i.e., 1 MaOH solution. However, less mesoporosity was introduced than

or the H-MOR-parent. A series of measurements was performed tonvestigate the mesoporosity development in time (Table 2). After5 min of alkaline treatment the mesopores are still very small andith SEM (Fig. 7A) a similar morphology is observed, although it

ppears that the crystallites have become more round compared to
he parent (Fig. 1A). After 45 min of treatment the small mesoporesave expanded into large mesopores and small macropore thatesult in open particles that are close to falling apart (Fig. 7B). After20 min of treatments the particles are no longer observed (Fig. 7C),nd a uniform structure with large macropores is observed.
able 2extural properties of parent and alkaline treated LZM-5 Na-mordenite.

Sample Vmicroa (STP) (cm3 g-1) Aext

a (STP) (m2 g-1) Vmesob (

Na-MOR-parent 0.15 35 0.03Na-MOR-at-1.0-5 0.15 45 0.08Na-MOR-at-1.0-10 0.15 48 0.08Na-MOR-at-1.0-15 0.16 52 0.11Na-MOR-at-1.0-23 0.15 65 0.14Na-MOR-at-1.0-30 0.15 79 0.17Na-MOR-at-1.0-45 0.15 82 0.18Na-MOR-at-1.0-60 0.13 101 0.13Na-MOR-at-1.0-90 0.14 84 0.12Na-MOR-at-1.0-120 0.13 99 0.14


y of LZM-5 mordenite (A) H-MOR-parent and (B) H-MOR-at.

We observed a slight decrease in Si/Al ratios upon alkaline treat-ment (Table 2, right column), and after 120 min the Si/Al ratio waslower than 5, suggesting the presence of extra-framework alu-minum. Simultaneously we observe a decrease in Na/Al ratio thatis combined with a decreasing micropore volume. This leads to theconclusion that some extra-framework aluminum has exchangedwith sodium or is being deposited on the outer surface. Both mightlead to blocking of the micropores.

From the N2-physisorption data we can conclude that Na-MOR-at-1.0-30 is very similar to the H-MOR-at-1.0-15 (Fig. 8) in terms ofporosity. Hence sodium mordenite can be used as a starting mate-rial for alkaline treatment, but requires a longer treatment time dueto higher stability. An additional advantage is that longer treatmenttimes provide more control, which might be necessary for largescale treatment. A possible explanation for the higher stability ofsodium mordenite towards alkaline treatment can be the result ofits larger cations, effectively shielding the framework of the mor-denite from the alkaline treatment. A second possibility is thatalthough the bulk Si/Al ratios are similar some extra-frameworkaluminum was formed in H-MOR upon calcination [47,48] leadingto structural defects and thus higher susceptibility towards alkalinetreatment. In Fig. 9A 27Al MAS NMR is shown for the Na-MOR-parent. Only one peak is visible at 54 ppm chemical shift, whichrepresents tetrahedrally coordinated aluminum [49], and can beascribed to framework aluminum. In the H-MOR-parent (Fig. 9B)
three types of Al coordinations were observed; tetrahedrally (peakat 54 ppm) coordinated aluminum (AlIV), octahedrally (peak at−1 ppm) coordinated aluminum (AlVI) and a shoulder at 41 ppmwhich is suggested to represent penta-coordinated aluminum [50](AlV). The AlVI is probably extra-framework, which could explain
STP) (cm3 g-1) Vtotalc (STP) (cm3 g-1) Si/Ald (at/at) Na/Ald ratio

0.18 5.5 0.960.260.280.27 5.3 0.900.370.49 5.3 0.880.500.530.660.53 4.9 0.66

70 A.N.C. van laak et al. / Applied Catalysis A: General 382 (2010) 65–72

t-1.0-15; (B) Na-MOR-at-1.0-45 and (C) Na-MOR-at-1.0-120.

attsosaa(

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Fig. 7. SEM images of LZM-5 mordenite: (A) Na-MOR-a

higher susceptibility towards alkaline treatment. After alkalinereatment (Fig. 9C) AlVI is no longer observed and the intensity ofhe shoulder at 41 ppm is greatly diminished. The absence of AlVI

uggests that the extra-framework aluminum species are dissolvedr re-inserted into the lattice [51,52]. After ion exchange and con-ecutive calcination AlVI is present (Fig. 9D) in similar AlIV/AlVI ratios for H-MOR-parent. No changes in the aluminum coordinationre observed upon direct alkaline treatment on Na-MOR-parentFig. 9E).

.3. BASF and Zeolyst CBV 21A mordenites

The acquired insight for alkaline treatment on Na LZM-5 (Si/Alatio 5.5) was applied to two other commercial mordenite samplesrom BASF (Si/Al ratio 7.5) and Zeolyst (Si/Al ratio 10). BASF-parent

ordenite (sodium form) was treated similar as the parent sodiumZM-5 sample, although the contact time was slightly shorter15 min), in view of the higher Si/Al ratio of the BASF morden-te. This resulted in an increased external surface area from 32o 88 m2 g−1, enhanced total porosity and the preservation of the

icropore volume (Table 3), similar to what was observed for theZM-5 mordenite. The pore size distribution (Fig. S2) after alka-ine treatment shows that a large variety of pores are present, bothmaller and larger compared the LZM-5 mordenite. This is mostikely a direct influence of the morphology of the material which,

able 3extural properties of BASF (Na-MOR) and Zeolyst CBV 21A (NH4-MOR).

Sample Vmicroa (STP) (cm3 g−1) Aext

a (STP) (m2 g−1)

BASF-parent 0.17 36BASF-at-1.0-15 0.17 80CBV-21A-parent 0.17 45CBV-21A-at-1.0-10 0.17 88CBV-21A-at-0.5-20 0.16 106


Fig. 8. BJH pore size distribution derived from adsorption branch of LZM-5 morden-ite: (�) Na-MOR-parent; (�) H-MOR-at-1.0-15 and (�) Na-MOR-at-1.0-30.

compared to the LZM-5 sample, is more diverse in terms of particleand crystallite size.

For the NH4-Zeolyst CBV 21A material an even shorter contact
time of 10 min was applied, because the starting Si/Al ratio of 10is considerable higher than for LZM-5 mordenite, resulting in afaster desilication process. Upon alkaline treatment we observed anincrease in external surface area (45–88 m2 g−1), while the micro-
Vmesob (STP) (cm3 g−1) Vtotal

c (STP) (cm3 g−1) Si/Ald (at/at)

0.03 0.26 8.50.09 0.36 –0.04 0.25 100.03 0.37 –0.10 0.36 –


Fp

p1(dprpab((u2Nmr

3

cdtpHliAltcais

Fig. 11. Benzene alkylation with propylene on LZM-5 mordenite: (�) H-MOR-parentand (�) H-MOR-at.

Table 4Benzene alkylation with propylene; influence of alkaline treatment on LZM-5 H-mordenite.

H-MOR-parent H-MOR-at

ActivityInitial rate (mol g−1 s−1) 2.9 E−5 5.5 E−4Initial rate (normalized) 1 19

Propene selectivitya (mol %)Cumene 82.6 85.7

ig. 9. 27Al MAS NMR spectra of LZM-5 samples: (A) Na-MOR-parent; (B) H-MOR-arent; (C) H-MOR-at-1.0-15, (D) H-MOR-at and (E) Na-MOR-at-1.0-15.

ore volume was preserved. Additionally an average pore size of00 nm was observed, thus resulting in a low mesopore volumeTable 3). From these results it can be concluded that at these con-itions fast dissolution of the sample occurs. To slow down thisrocess a 20 min experiment with 0.5 M NaOH was performed. Thisesulted in an external surface area of 106 m2 g−1 and a meso-ore volume of 0.10 ml g−1 (Table 3). When these conditions arepplied not only large mesopores and small macropores are presentut also a considerable amount of mesopores in the 3-8 nm rangeFig. S3). TEM images of CBV 21A-parent and of CBV-21A-at-0.5-20Fig. 10) show that small intra-crystalline mesopores are formedpon alkaline treatment. Groen et al. [34] reported that the Zeolyst0A sample with similar Si/Al ratio was not reactive towards 0.2 MaOH for 30 min. We can conclude that the porosity of the 21Aordenite can be improved using a 0.5 M NaOH for 20 min, which

esults in intra- and inter-crystalline mesoporosity.

.4. Liquid-phase benzene alkylation with propylene

The alkylation of benzene with propylene to form cumene washosen as a model reaction to evaluate the effect of the intro-uced mesoporosity in LZM-5 on the activity and selectivity. Athe start of the reaction an excess of benzene was used (benzene toropylene molar ratio ∼4), and ∼1 g of catalyst (H-MOR-parent or-MOR-at) was added. As can be seen in Fig. 11 and Table 4 alka-

ine treatment results in a catalytic activity, as measured by thenitial rate constant, of more than an order of magnitude higher.fter 4 h of reaction 60% of the propene has reacted with the alka-

ine treated sample, while only 10% of propene has reacted using
he parent mordenite as catalyst. After 4 h the selectivity towardsumene is slightly higher for the parent mordenite. However, if wessume that di-substituted benzenes can also be transformed backnto cumene via a trans-alkylation reaction the selectivity of bothamples effectively was about 99%.
Fig. 10. TEM images of (A) Zeolyst CBV 21A

di-Isopropylbenzene 16.1 13.3

a After 4 h of reaction.

The comparable selectivity gives an indication that no strongLewis acid sites have formed on the surface as a result of alkalinetreatment [53], otherwise it is expected that propene oligomer-ization would have taken place. It is unlikely that the increase inactivity is related to an increase in acid site density, as based onthe increase in Si/Al ratio only a slight increase in acid site densitywould be expected, as was confirmed by ammonia TPD (see Fig. S1).Furthermore, also for the heavily dealuminated mordenite reportedpreviously by employees of Dow [21,54] an increase in activity wasobserved, showing that the acid site density or strength is not thelimiting factor in this process. Rather the observed increase in activ-ity can be ascribed to a decrease in average diffusion distances,and related to two parameters that were changed as result of alka-line treatment. First, inter-crystalline mesopores were generated toincrease the accessibility of the mordenite agglomerates, thereby
preventing possible mass transfer limitations between the parti-cles. Second, the mordenite crystallites were slightly decreased insize resulting in a shorter intraparticle diffusion pathlength.
and (B) Zeolyst CBV 21A-at-0.5-20.

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. Conclusion

Commercially available, aluminum rich mordenites comprisedf small (100–200 nm) agglomerated crystallites, were subjectedo alkaline treatment. Alkaline treatment in a NaOH solution of 1 Mor 15 min at 343 K on proton mordenite resulted in the forma-ion of inter-crystalline porosity, while the microporosity as wells the crystallinity were preserved. It was also possible to applylkaline treatment directly on the sodium parent mordenite. How-ver, with an identical alkaline treatment, the treatment time hado be prolonged by a factor 2 to obtain similar porosity as com-ared to the proton mordenite. Using sodium mordenite has thedvantage that the whole process involves less steps and as theeaction is slower it is more easily controlled, especially relevantor scale-up. Different commercial samples of mordenite have beenuccessfully treated, although slightly different conditions had toe applied as a result of slight differences in Si/Al ratios and mor-hology of the parent mordenite. With careful adjustments of timend alkaline concentration, it was possible to obtain both inter-rystalline and intra-crystalline porosity for mordenite samplesith Si/Al ratio as low as 10. Upon alkaline treatment of morden-

te with a Si/Al ratio of 5.7 an increase in catalytic activity by morehan one order of magnitude was observed without losing selec-ivity. Hence alkaline treatment of aluminum rich mordenite is anffective post-synthesis treatment to enhance porosity and therebyatalytic activity.

cknowledgements

We would like to thank Dr. J.T. Miller (BP Amoco, USA), Dr. K.R.ayense (BASF, Utrecht, The Netherlands) and Dr. F. Winter (Shell,msterdam, The Netherlands) for supplying mordenite samples,r. M.J. Janssen (ExxonMobil, Belgium) for 27Al MAS NMR mea-

urements and M.S.U. Samson, Dr. M.J.M. van der Aalst, and Dr. G.R.eima (Dow Terneuzen, The Netherlands) for catalytic tests and

CTS-ASPECT for financial support.

ppendix A. Supplementary data

Supplementary data associated with this article can be found, inhe online version, at doi:10.1016/j.apcata.2010.04.023.

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