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Catalytic applications and FTIR investigation of zeolite SSZ-33 after isomorphous substitution
Microporous and Mesoporous Materials 194 (2014) 174–182
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Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Catalytic applications and FTIR investigation of zeolite SSZ-33 after isomorphous substitution Dana Vitvarová ⇑, Lenka Kurfirˇtová, Martin Kubu˚, Nadeˇzˇda Zˇilková ´ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, v.v.i., Dolejškova 3, CZ-182 23 Prague 8, Czech Republic J. Heyrovsky
a r t i c l e
i n f o
Article history: Received 8 January 2014 Received in revised form 1 April 2014 Accepted 2 April 2014 Available online 13 April 2014 Keywords: SSZ-33 Acidity Toluene alkylation Toluene disproportionation
a b s t r a c t Zeolite SSZ-33 (topology: CON, borosilicate as-synthesized) contains access to the internal void spaces via 10- and 12-ring pores. Borosilicate SSZ-33 was transformed by isomorphous substitution to Al, Ga and Fe form. The acid sites of zeolite samples were investigated using infrared spectroscopy. The different probes (d3-acetonitrile, pivalonitrile, pyridine and 2,6-di-tert-butyl-pyridine) were used in this study. Brønsted acidity confirms the isomorphous substitution of B by Al, Ga and Fe in the SSZ-33 framework. It was found that the strength of Brønsted acid sites of zeolites under study decreased in the series: [Al]-SSZ-33 > [Ga]-SSZ-33 > [Fe]-SSZ-33. Upon adsorption of 2,6-di-tert-butyl-pyridine a part of Brønsted acid sites was not accessible (51% [Al]-SSZ-33, 76% [Ga]-SSZ-33 and 78% [Fe]-SSZ-33). The catalytic performance was studied in toluene disproportionation and toluene alkylation with isopropyl alcohol. Toluene conversions decreased in the order: [Al]-SSZ-33 > [Ga]-SSZ-33 > [Fe]-SSZ-33 in all tested reactions. Selectivities to sum of xylene isomers in toluene disproportionation was as follows: [Al]-SSZ-33 > [Ga]-SSZ-33 > [Fe]-SSZ-33. Zeolite [Al]-SSZ-33 and [Fe]-SSZ-33 exhibited the highest ratio xylene/benzene 0.75. In the case of toluene alkylation, the highest selectivities to cymenes were achieved over [Fe]-SSZ-33 and [Ga]-SSZ-33 at 200 °C. In the case of [Fe]-SSZ-33 almost no n-propyltoluene was found in the reaction mixture. Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction Zeolites are the most important heterogeneous catalysts used in a high number of large scale chemical technologies (oil refinery, petrochemistry) as well as for production of fine chemicals and in environmental catalysis [1–3]. Their structures together with the number of Brønsted and Lewis acid sites, their strength, distributions and their locations, have a significant influence on the activity and seletivity of the particular reaction [4–6]. The acid properties of zeolites and mesoporous molecular sieves can be modified by isomorphous substitution of tetrahedral sites by heteroatoms such as Al [7], Ti [8], Ga [9], Fe [10], V [11] and Cr [12]. The differences in acid strength and catalytic properties resulting from framework substitution offer the potential to design zeolites for new applications [13]. Several approaches can be employed to determine zeolite acidity and accessibility of active sites. Theoretical methods, both Hartee-Fock and density functional theory have been used to study acidity of zeolites and interaction of probe molecules [13–21]. FTIR spectroscopy is probably the most frequently used experimental ⇑ Corresponding author. Tel.: +420 26605 3055; fax: +420 28658 2307. E-mail address: [email protected] (D. Vitvarová). http://dx.doi.org/10.1016/j.micromeso.2014.04.007 1387-1811/Ó 2014 Elsevier Inc. All rights reserved.
technique, particularly in combination with different probe molecules [22,23]. Zecchina et al. [24] proposed nitriles as the probes in the IR studies of solid surfaces. Especially deuterated acetonitrile is largely employed to characterize the active sites in zeolitic materials [25]. Investigation of the external surface and access to the cavities of zeolites can be performed using also nitriles with branched side chains [26–29]. New zeolite structures are expected to improve shape selectivity. Zeolites with connected channels of 10- and 12-rings offer an interesting pore arrangement for catalysis [30]. SSZ-33 (topology: CON, borosilicate as-synthesized) contains a multidimensional pore system formed by intersecting 10- and 12-ring channels of 0.64 0.70 nm, 0.59 0.70 nm (12-ring parallel to [0 0 1] and [1 0 0]), respectively, and 0.45 0.51 nm (parallel to [0 1 0]) providing access to the crystal interior through both pore sizes [31,32]. Borosilicates can be easily transformed into the aluminosilicate form by a low temperature hydrothermal treatment [33]. Gil et al. studied acidic properties and active sites accessibility of [Al]-SSZ33 by FTIR spectroscopy [34,35]. FTIR results were correlated with 27 Al and 1H MAS NMR. SSZ-33 zeolite was found to possess bridging Si–OH–Al groups of virtually uniform and high acid strength. It was found that almost all bridging Si–OH–Al groups in SSZ-33 are located in the 12-rings.
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Reactions of alkyl and dialkyl aromatic hydrocarbons represent an important group of reactions used partly in petrochemical industry as well as model reactions testing shape selectivity properties of zeolites [36]. Novel zeolite SSZ-33 was successfully tested in many catalytic reactions such as toluene and xylene disproportionation, benzene, toluene and p-xylene alkylation. Moreover, SSZ-33 was studied as a catalyst for synthesis of fine chemicals in acylation of ferrocene. In p-xylene alkylation with isopropyl alcohol at 60 min TOS (time on stream) SSZ-33, Beta and SSZ-35 reached p-xylene conversions about 70% [36]. Zeolite SSZ-33 represented intermediate behavior as compared with ZSM-5 and Beta in benzene alkylation with isopropyl alcohol (benzene conversions: ZSM-5 43.0% > SSZ33 39.6% > Beta/Ge 38.8%, benzene to isopropyl alcohol molar ratio 4, 280 °C) [37]. m-Xylene reactions (transalkylation, isomerization and disporportionation) over aluminosilicate SSZ-33 with different aluminum populations was tested by Jones et al. [33] and Adair et al. [38]. It was observed that the intracrystalline pore space of SSZ-33 acts as an ensemble of cages connected by 10- and 12-ring rather than intersecting 10-ring and 12-ring channels [33]. Xylene isomerization/disproportionation and toluene alkylation over SSZ-33 was also tested by Llopis et al. [30]. Toluene conversions in alkylation with isopropyl alcohol decreased in the order: Nu-87 > Beta > SSZ-33 > ZSM-5. The objective of this contribution is characterization of acid sites in SSZ-33 zeolites after its post-synthesis modifications. In particular, type and concentration of active sites, their strength and accessibility were studied by the interaction with d3-acetonitrile, pivalonitrile, pyridine and 2,6-di-tert-butyl-pyridine using FTIR spectroscopy. Differences in catalytic behavior of Al, Fe and Ga form of SSZ-33 zeolite were tested in toluene disproportionation and toluene alkylation with isopropyl alcohol.
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2.2. FTIR spectroscopic experiment The FTIR spectra were recorded with a Nicolet 6700 equipped with AEM module instrument with a resolution 4 cm 1 using pressed disks of pure zeolite powders, activate by outgassing at 450 °C into the IR cell under vacuum. Pyridine, 2,6-di-tert-butyl-pyridine, d3-acetonitrile and pivalonitrile adsorbants were purchased from Sigma–Aldrich. The adsorption procedure of d3-acetonitrile and pivalonitrile involves contact of the activated sample disk with vapors for 20 min at equilibrium pressure and room temperature. Desorption of probe molecules was performed in steps at room temperature or increasing temperatures (100–350 °C). The Pyridine adsorption proceeded at 150 °C for 20 min at partial pressure 3 Torr, followed by 20 min evacuation at 150 °C. The concentration and the type of acid sites were determined by adsorption of pyridine and d3-acetonitrile as probe molecules. To obtain quantitative analysis the molar absorption coefficients for d3-acetonitrile adsorbed on Brønsted acid sites (m(C„N)-B at 2297 cm 1, e(B) = 2.05 ± 0.1 cm lmol 1) and strong and weak Lewis acid sites (m(C„N)-L1 at 2325 cm 1 m(CN)-L2 2310 cm 1, e(L) = 3.6 ± 0.2 cm lmol 1) were used [40]. The concentrations of Brønsted and Lewis acid sites were also calculated from integral intensities of individual bands characteristic of pyridine on Brønsted acid sites at 1545 cm 1 and band of pyridine on Lewis acid site at 1455 cm 1 and molar absorption coefficients of e(B) = 1.67 ± 0.1 cm lmol 1 and e(L) = 2.22 ± 0.1 cm lmol 1, respectively [40]. The adsorption of 2,6-di-tert-butyl-pyridine was carried out at 150 °C and at equilibrium probe vapor pressure for 15 min. Desorption proceeded at the same temperature for 1 h. Extinction coefficient for pyridine was used for quantitative evaluation of the concentration of Brønsted acid sites. 2.3. Catalytic test
2. Experimental 2.1. Catalysts SSZ-33 zeolite in boron form was synthesized at Chevron Research Company. Isomorphous substitution of [B]-SSZ-33 zeolite to Al, Fe and Ga form was carried out by the following procedure. The boron-containing SSZ-33 was converted by a one-step reflux in 1 M solution of corresponding nitrate (pH in the range of 1–2) providing hydrolysis of boron and reinsertion of the respective element into the framework. Procedure was performed at room temperature for 100 h. Finally, molecular sieves were recovered by filtration, washed out with 0.1 M HCl, then with distilled water, dried and calcined using the temperature program: 1 °C/min to 120 °C, maintained at 120 °C for 2 h then heated (1 °C/min) to 540 °C, kept for 5 h, and further heated to 595 °C (1 °C/min) and maintained for another 5 h [39]. The crystallinity of zeolites was determined by X-ray powder diffraction with Bruker D8 X-ray powder diffractometer equipped with a graphite monochromator and position-sensitive detector using CuKa radiation in the Bragg–Brentano geometry. The size and shape of zeolite crystals were determined by scanning electron microscopy using an instrument Jeol, JSM-5500LV. For the determination of the chemical composition of synthesized and modified zeolites, X-ray fluorescence analysis with a Philips PW 1404 spectrometer equipped with an analytical program UniQuant was used. The samples were mixed with dentacryl as a binder and pressed on the surface of cellulose pellets. Adsorption isotherms of nitrogen at 196 °C were determined using GEMINI static volumetric instrument. Before adsorption zeolites were degassed under helium at 350 °C for 16 h.
The alkylation of toluene with isopropyl alcohol was performed in a down-flow glass microreactor with a fixed bed of catalyst under atmospheric pressure (powder was pressed and sieved, and the fraction between 0.70 and 0.50 mm was used for reaction). Reactant stream was prepared by passing the carrier gas through thermostated saturators and the composition was controlled by gas chromatography. The experiments were carried with toluene/ isopropyl alcohol molar ratio 9.6 and weight hour space velocity (WHSV) based on toluene equal to 10. The disproportionation of toluene was carried out in the same apparatus under atmospheric pressure at 500 °C with WHSV 1.9 h 1. Before experiment, zeolite catalysts were activated at 500 °C in nitrogen stream for 1 h. The reaction products were analyzed using an ‘‘on-line’’ gas chromatograph (Agilent 6890 Plus) with flame ionization detector and a high-resolution capillary column (Innowax for the disproportionation and DB-5 for the alkylation). The first analysis was performed after 15 min of timeon-stream (TOS) and the other followed in approx. 55 min interval. 3. Results and discussion 3.1. Characterization X-ray powder patterns of SSZ-33 zeolites used in this study exhibited a high crystallinity and phase purity (Fig. 1). All diffraction patterns are similar with respect to the position and intensity of diffraction lines. No evidence of structure damage or the presence of an additional phase was found. The SEM images of SSZ-33 zeolite are depicted in Fig. 2. They indicate rather homogeneous distribution of crystal sizes and
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D. Vitvarová et al. / Microporous and Mesoporous Materials 194 (2014) 174–182 Table 1 Textural properties of SSZ-33 zeolites. Zeolite
BET (m2/g)
Vmic (cm3/g)
Vtot (cm3/g)
Si/M
[Al]-SSZ-33 [Fe]-SSZ-33 [Ga]-SSZ-33
573 550 462
0.254 0.242 0.201
0.398 0.389 0.347
24 27 20
Fig. 1. XRD patterns of zeolites used: [B]-SSZ-33, [Ga]-SSZ-33, [Fe]-SSZ-33 and [Al]SSZ-33.
shapes of SSZ-33 zeolites exhibiting crystals 0.8 0.4 lm with a few larger agglomerates. Textural properties of zeolites under investigation were determined from adsorption isotherms of nitrogen. The t-plot method was applied to estimate the volume of micropores (Table 1). The N2 adsorption–desorption isotherms of SSZ-33 zeolites (Fig. 3) are similar in the shape and the the BET surface areas are in the range 460–570 m2/g (Table 1). The micropore volume (Vmic) is in the range of 0.20–0.25 cm3/g confirming a high quality of the SSZ-33 zeolites studied when compared with the literature [31,41]. Decrease in BET surface area for [Ga]-SSZ-33 is in agreement with the data reported in literature [42]. Molar ratios of silicon to corresponding metal were determined by X-ray fluorescence analysis (Table 1). 3.2. FTIR investigations Pyridine is very sensitive probe molecule for the quantification of both Brønsted and Lewis acid sites, as both the frequency of bands appearing at 1545 cm 1 (PyH+) and 1455 cm 1 (PyL) are independent of the type of zeolite, its composition or morphology. This remains valid even for zeotypes containing heteroatoms other than aluminum [43]. The region of OH stretching vibrations of zeolites under study is shown in Fig. 4. The band occurring at ca. 3745 cm 1 is assigned to the terminal silanol SiOH groups. The frequencies of the stretching vibration bands of the bridged OH groups increase in the order Al (3610 cm 1) < Ga (3620 cm 1) < Fe (3630 cm 1). Frequencies of bridging OH groups of SSZ-33 zeolites agree with the data reported for isomorphously substituted ZSM-5 [44,45]. The presence of the band of bridging OH groups evidence the isomorphous substitution of the boron atoms into framework of the SSZ-33 structure. The
Fig. 3. N2 adsorption (s) and desorption () isotherms of SSZ-33 zeolites. d[Al]SSZ-33, j[Fe]-SSZ-33, N[Ga]-SSZ-33.
position of this band depends on the nature of the trivalent element, and shifts to higher wavenumbers in the row Al < Ga < Fe, suggest the decrease in the acid strength in the same order. It is important to note that the 1380 cm 1 band observed in the case of parent B-SSZ-33 zeolite attributed to tricoordinated framework boron diminished completely after zeolites modification [46]. Berndt et al. studied zeolite ZSM-5 with isomorphously substituted B, Ga, Fe, In and Al by infrared spectroscopy and bands in the region 3651–3672 cm 1 attributed to the non-framework species [44]. Chao et al. reported that the nonframework Ga species provide a band at 3685 cm 1 which was also observed in the IR spectrum of GaNH4[Si, Al]-Beta [47]. In our case, these bands were not observed in case of [Ga]-SSZ-33 and [Al]-SSZ-33. However, the band 3672 cm 1 of a very low intensity is present in the [Fe]SSZ-33 spectra (Fig. 4). It is well known that ferrisilicate zeolites are less stable than the aluminosilicates due to the size of Fe3+ ion building a framework. The size of ions depends on the valency and coordination number. Fe(III) atoms located in framework positions are unstable and show a tendency towards formation of extraframework oxide-like species in zeolites due to geometrical reasons [48,49]. A broad band at around 3500 cm 1 originated from species, whose hydrogen atom is involved in weak H-bondings with nearer hydroxyls for all three samples. This band is
Fig. 2. SEM images of [Ga]-SSZ-33 (left), [Fe]-SSZ-33 (middle) and [Al]-SSZ-33 (right) zeolites.
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Fig. 4. Pyridine adsorption in SSZ-33 zeolites, (A) IR spectra of hydroxyl vibration region and (B) spectra of pyridine region after its adsorption followed by desorption at 150 °C. For (A) red line: spectra of activated samples and, black line: spectra after 20 min desorption at 150 °C. For B) black line: spectra after 20 min desorption at 150 °C and, blue line: spectra after 20 min desorption at 450 °C. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
typical for SiOH groups located on lattice defects [50,51]. The vibration band of the pyridinium ion at 1545 cm 1 evidenced that all substituted SSZ-33 samples contain Si–OH–M groups. The concentrations of Lewis and Brønsted acid sites at different desorption temperatures are depicted in Table 2. The band assigned to pyridine adsorption on Lewis acid sites is located at 1455 cm 1. The Al- and Fe-containing samples exhibit an additional band at 1462 cm 1 which is well visible after desorption at temperature 450 °C indicating pyridine chemisorptions at Lewis sites of higher acid strength [35] (Fig. 4). Data in Table 2 denotes that with increasing desorption temperature concentration of Lewis and Brønsted acid sites interacting with pyridine decreases for all zeolites under study. The concentration of strong Brønsted acid sites was determined from the ratio of pyridine interacting with acid sites after desorption at 150 °C and 450 °C. The part of strong Brønsted acid sites decreased in the order: Al 38.1% > Ga 14.3% > Fe 11.1%. Brønsted acid sites on [Ga]-SSZ-33 exhibit a lower acid strength in comparison with the [Al]-SSZ-33 sites probably due to the lower electronegativity of the larger softer Ga cation as compared with the value of the smaller harder Al cation [52]. The observed decrease in strong Brønsted acid sites concentration of tested zeolites is in agreement with existing experimental and theoretical results [13–16]. Langenaeker and Geerlings [14] studied the influence of isomorphous substitution of Al (by B and Ga) and Si (by Ge) on the catalytic activity of zeolite system using reactivity indexes based on density functional theory. The authors concluded that the acidities of the zeolite-type model systems are dependent on several characteristics including the hardness (softness), the electronegativity of the substituting atoms and the charge on the hydrogen atom of the bridging hydroxyls. The OH-bond length and the ionicity of the OH bond were found to be suitable descriptors of the acidity, whereas the dipole moment
derivative with respect to the OH-bond length appeared to fail as a reactivity index. Deka et al. [13] also found that acidity of zeolites decrease in the order Al(OH)Si > Ga(OH)Si > B(OH)Si. The authors used quantum chemical methods to study the influence of chemical composition on acidity of zeolite clusters. The same conclusions reported Stave et al. [15] in study of isomorphously substituted ZSM-5 zeolites by density functional theory. [Ga]-SSZ-33 contains the highest concentration of Lewis sites form all tested samples. Chandwadkar et al. reported higher concentration of Lewis acid sites than Brønsted acid sites on [Ga]-Mordenite and it attributed to the relatively less thermally stable bridging OH groups leading to dehydroxylation and formation of Lewis acid sites [53]. After outgassing at 450 °C still more than half of Lewis sites bonding pyridine remained: 56.5% [Fe]-SSZ-33, 55.0% [Ga]-SSZ-33 and 55.6% [Al]-SSZ-33 indicating the steady strength of Lewis acid sites among tested samples. Adsorption of 2,6-Di-tert-butyl-pyridine (DTBpy) on the zeolite SSZ-33 showed that accessibility of the Brønsted acid sites is 49.0% for [Al]-SSZ-33, 23.8% for [Ga]-SSZ-33, and 22.2% for [Fe]-SSZ-33, respectively. The FTIR spectra of DTBpy adsorbed on zeolite SSZ-33 samples are depicted in Fig. 5. DTBpy does not penetrate to the pores of 10-ring zeolites like ZSM-5. On the other hand, it was observed that DTBpy can penetrate to the 12-ring channel system of zeolite Beta [54]. Based on the spectra where we clearly see the band around 1530 cm 1 attributed to the interaction of the Brønsted sites with DTBpy we conclude that the accessible Brønsted acid sites are mainly located on the external surface or in the pore mouth of 12-member ring channels. Nitriles are relatively weak bases and coordinate via the nitrogen of the nitrile group to the acid sites. Because of its small size, d3-acetonitrile (CD3CN) is frequently successfully applied for the
Table 2 Concentration of Lewis (Lc) and Brønsted (Bc) acid sites obtained by FTIR spectroscopy (pyridine adsorption). Desorption temperature (°C)
150 250 350 450
[Al]-SSZ-33
[Fe]-SSZ-33
[Ga]-SSZ-33
cLc mmol/g
cBc mmol/g
cLc mmol/g
cBc mmol/g
cLc mmol/g
cBc mmol/g
0.20 0.15 0.12 0.11
0.21 0.21 0.17 0.08
0.19 0.18 0.16 0.11
0.18 0.18 0.18 0.02
0.27 0.22 0.19 0.15
0.21 0.20 0.16 0.03
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Fig. 5. DTBpy adsorption in SSZ-33, (A) IR spectra of hydroxyl stretching region and (B) spectra of DTBpy region after its adsorption followed by desorption at 150 °C. For (A) red line: spectra of activated samples and black line: spectra after 1 h desorption at 150 °C. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
characterization of a large number of oxides [55]. The spectra of SSZ-33 zeolites obtained after adsorption of CD3CN are shown in Fig. 6. The adsorption of CD3CN on [Al]-SSZ-33 and [Ga]-SSZ-33 gives rise the formation of bands at 2323, 2298 cm 1. The band at 2323 cm 1 can be correlated to the adsorption on Lewis acid sites and band at 2298 cm 1 can be assigned to the adsorption on bridging OH groups [56]. Adsorption of CD3CN on [Fe]-SSZ-33 (Fig. 6) leads to the appearance of a intense band at around 2300 cm 1. The m(CN) stretching frequency for CD3CN adsorption on Brønsted sites associated with framework Fe sites is 2298 cm 1. The new band around 2300 cm 1 is attributed to the Lewis acid sites resulting from nonframework Fe and is virtually indistinguishable from the Brønsted sites associated with framework Fe [57]. The concentration of Lewis and Brønsted acid sites obtained from CD3CN adsorption correlates with concentrations from pyridine for [Ga] and [Al]-SSZ-33. Unfortunately, the concentrations were not possible to calculate for [Fe]-SSZ-33 (Table 3).
Table 3 Concentration of Lewis (Lc) and Brønsted (Bc) acid sites obtained by FTIR spectroscopy (CD3CN adsorption).
[Al]-SSZ-33 [Ga]-SSZ-33
cLc mmol/g
cBc mmol/g
0.19 0.33
0.22 0.20
Pivalonitrile (PN) critical diameter is around 0.62 nm [51,58]. It was proven that PN do not penetrate the zeolite ZSM-5 cavity [27] and therefore it is suitable as a probe for SSZ-33 zeolite. The IR spectra recorded after adsorption of pivalonitrile in the CN stretching region contains four components at around 2294, 2270, 2249 and 2238 cm-1 (Fig. 7). The bands 2249 and 2238 cm 1 can be assigned to the two H-bonding complexes formed with terminal silanol groups absorbing at 3745 and 3732 cm 1 [59]. The component at 3732 cm 1 can be associated with terminal silanol groups located on the internal surface of the zeolite in defect sites
Fig. 6. CD3CN adsorption in SSZ-33, (A) IR spectra of hydroxyl vibration region and (B) spectra of CD3CN region after its adsorption followed by desorption at room temperature. For (A) red line: spectra of activated samples and, black line: spectra after 20 min desorption at room temperature. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 7. Pivalonitrile adsorption in SSZ-33, (A) IR spectra of hydroxyl vibration region and (B) spectra of CN stretching region region. For (A) black line: spectra of activated samples, red line: spectra after desorption at room temperature and blue line: spectra after 20 min desorption at 100 °C. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
[28,60]. Bevilacqua studied adsorption of PN on Mordenite and ascribed the band at 2272 cm 1 to interaction of PN with bridging OH groups and the band at 2295 cm 1 to interaction with Lewis acid sites [58]. The saturation of the SSZ-33 samples with PN caused a full perturbation of silanol groups with mutual formation of a band at 3400 cm 1 (Fig. 7). This band is assigned to the OH stretching of hydrogen-bonded complexes of the silanol groups with the PN nitrogen lone pair. Adsorption of PN on [Al]-SSZ-33 yielded all peaks discussed above. However, upon PN adsorption on [Fe]-SSZ-33 only three components are visible in the spectra: a broad band with a maxima at around 2274 cm 1 and bands at 2249 and 2238 cm 1. Unfortunately, features attributed to PN interaction with Brønsted and Lewis acid sites on [Fe]-SSZ-33 were indistinguishable. The same situation was observed in CD3CN adsorption on [Fe]-SSZ-33, vide supra. Hadjiivanov investigated Fe-containing zeolite Beta by FTIR spectroscopy of adsorbed CO and NO. Adsorption of CO and NO led to the formation of species with unusual frequency due to the high electrophilicity of iron atoms [61]. Finally, the spectra recorded after PN adsorption on [Ga]-SSZ-33 exhibit the intense band with the maximum at 2290 cm 1 and the shoulder at 2270 cm 1 probably caused by a higher concentration of Lewis acid sites. Outgassing at 100 °C restores the band near 3745 cm 1 and the component at 3400 cm 1 disappeared but the band corresponding to bridging OH groups is not restored at all. These observations suggest that the acid sites are located in 12-ring channels and therefore can be well accessible in catalytic reactions. Gil et al. used PN to differentiate between the centers located inside 10-ring and 12-ring channels in zeolite SSZ-33 and reported the same conclusion [35]. According to Zˇilková et al. [36] the intesity of the band at 1374 cm 1 (CH3 umbrella bending of pivalonitrile, not shown in Figure) was taken as the analytical band, presenting the amount of pivalonitrile still kept in zeolite after desorption at 200 °C. The ratio of the band areas at 1374 cm 1 after desorption at 200 °C and after desorption at room temperature decreased in the order: 0.29 [Al]-SSZ-33 > 0.25 [Ga]-SSZ-33 > 0.14 [Fe]-SSZ-33. It supports the results from pyridine and DTBpy adsorption showing that the acid strength and accessibility decreased in the order Al > Ga > Fe. The FTIR study proved that isomorphous substitution of boron was successful. In the case of [Fe]-SSZ-33 zeolite the band 3672 cm 1 (Fig. 4) of a very low intensity attributed to the
nonframework atoms was visible in the spectra. Adsorption of pyridine on SSZ-33 zeolites proved that the concentration of Brønsted acid sites is the same for [Al]-SSZ-33 and [Ga]-SSZ-33 and slightly lower for [Fe]-SSZ-33. The sample [Ga]-SSZ-33 contains the highest concentration of Lewis acid sites. The concentration of strong Brønsted acid sites still present after outgassing at 450 °C decreased in the order: Al > Ga > Fe. The strength of Lewis acid sites was almost the same for all tested samples. DTBpy adsorption showed that accessibility of Brønsted acid sites is 23.8% for [Ga]-SSZ-33, 49.0% for [Al]-SSZ-33 and 22.2% for [Fe]-SSZ-33. Adsorption of CD3CN showed that active sites are well accessible for all zeolites under study. It was found that zeolite [Fe]-SSZ-33 exhibits shift of characteristic adsorption bands of probe molecules probably due to the high electrophilicity of iron ions. 3.3. Catalytic test The catalytic behavior of isomorphous substituted zeolites SSZ-33 were tested in toluene disproportionation and toluene alkylation with isopropyl alcohol. 3.3.1. Toluene disproportionation The disproportionation of toluene to benzene and xylenes occurs via a bimolecular reaction mechanism. Disproportionation of the two toluene molecules yields an equimolar mixture of xylene isomers and benzene. Depending on the reaction conditions, it may be accompanied by a parallel dealkylation of toluene to benzene, and also by secondary transformations of xylenes. The toluene conversions decreased in the order: [Al]-SSZ-33 54.0% > [Fe]-SSZ-33 25.3% > [Ga]-SSZ-33 11.3% (60 min TOS, Fig. 8). Higher toluene conversion can be expected over zeolite possessing more acidic sites [36]. Indeed, the highest toluene conversion was achieved over zeolite [Al]-SSZ-33 containing Brønsted acid sites with the strongest acid sites among tested zeolites. [Fe]-SSZ-33 exhibits almost the same concentration of Brønsted acid sites as [Al]-SSZ-33 but with lower acid strength. In spite of the highest concentration of active sites (sum of Lewis and Brønsted acid sites) on [Ga]-SSZ-33 the toluene conversion was the lowest. In addition, we can observe significant decrease in toluene conversion over [Ga]-SSZ-33 between 15 and 60 min contact time probably due to deactivation. The deactivation of the catalyst
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Fig. 8. Disproportionation of toluene, WHSV 1.9 [Fe]-SSZ-33 and [Ga]-SSZ-33, WHSV 30 [Al]-SSZ-33, 500 °C.
is probably caused by the formation of coke deposit promoting by high density of acid sites. Decrease in toluene conversion was accompanied by an increase in the selectivity to xylenes. Selectivities to sum of xylene isomers after 60 min TOS were as follows [Fe]-SSZ-33 40.2% > [Al]SSZ-33 30.2% – [Ga]-SSZ-33 30.2%. On the other hand, the highest selectivity to p-xylene was achieved over [Ga]-SSZ-33 with the highest concentration of Lewis acid sites. Cˇejka and Wichterlová suggested a direct participation of Lewis sites in toluene disproportionation [62]. Selectivity to p-xylene achieved over [Ga]-SSZ-33 increased considerably with an increasing TOS probably due to the formation of coke deposits retarding the isomerization of pxylene. The highest ratio xylene/benzene equal to 0.73 was achieved over [Al]-SSZ-33 and [Fe]-SSZ-33 zeolites. In the case of [Ga]-SSZ-33 the xylene/benzene ratio reached value around 0.6 180 min TOS. It is expected that coke deposits decrease the free volume in the channels and thus limit the consecutive reactions. Xylenes/benzene ratio over [Fe]-SSZ-33 was quite stable with TOS. 3.3.2. Toluene alkylation with isopropyl alcohol The alkylation of toluene with isopropyl alcohol belongs to the group of electrophilic substitution reactions. It is considered that alkylation reactions catalyzed by zeolites proceed via a carbenium ion-type mechanism. The toluene conversions and the product distributions in the alkylation of toluene with isopropyl alcohol at 200 °C are depicted in Fig. 9. The toluene/isopropyl alcohol molar ratio used in the feed was 9.6 indicative of the theoretical toluene conversion around 10.4% for pure alkylation reaction and complete conversion of isopropyl alcohol. Toluene conversions follow the order: [Al]-SSZ-33 10.7% > [Ga]-SSZ-33 10.1% > [Fe]-SSZ-33 7.6% (60 min TOS). It can be observed that toluene conversions correspond to a decrease in Brønsted acid sites strength. Moreover, the data are consistent with accessibility of acid centers obtained by DTBpy adsorption. Finally, it can be stated that the concentration of Lewis acid sites does not play a key role in this transformation because [Ga]-SSZ-33 exhibited the almost the same toluene conversion although contains the highest concentration of Lewis sites. Toluene conversions over all tested catalysts were quite stable with TOS. This could be assigned to stability and diffusion of the formed products in the channel structure of the zeolites [63]. In our case, pivalonitrile desorption at increasing temperature steps revealed that pivalonitrile desorbs from [Fe]-SSZ-33 the most
easily. Besides, the low concentration of acid sites could enhance the rate of diffusion of both reactants and products [64]. Toluene alkylation leads mainly to a mixture of iso-propyltoluenes (cymenes) whereas desired product is p-cymene, that is need in production of fungicides, pesticides or flavors. Consecutive reaction also proceed yielding n-propyltoluenes [65]. The highest selectivity to cymenes (93.8%, 60 min TOS) as well as the selectivity to p-cymene was achieved over [Fe]-SSZ-33 (around 40%, thermodynamic value [66]). It was reported by Wichterlová et al. that decreasing the diameter of the channels or the windows in the sequence zeolite Y > Mordenite > ZSM-12 > (Al, Fe)ZSM-5 led to a significant increase in the selectivity towards p-cymene [64]. Selectivity to cymenes can be lowered by a secondary transalkylation with toluene forming undesired n-propyltoluene [67]. It was already proved that the formation of n-propyltoluenes is strongly dependent on the dimensions and architecture of the channels of the molecular sieves [30,66]. Cˇejka et al. reported that the lower acid strength and number of acid sites suppress competitive reactions and, therefore, cymenes are predominantly formed with a high selectivity which is in agreement with our results [65]. It was proposed that large pore zeolites with 12-membered rings possess a sufficient reaction space decreasing the probability of the formation of bimolecular complex leading to n-propyltoluene [30,68]. Iso/n-propyltoluene ratios reached relatively high values (100 for [Ga]-SSZ-33 and 81 for [Al]-SSZ-33, 180 min TOS). In the case of [Fe]-SSZ-33 zeolite almost only iso-propyltoluenes were found in the reaction products. The ratio iso/n-propyltoluene was 931 after 15 min of TOS. The ratio iso/n-propyltoluene slightly decreased with TOS. It could be caused the lowest concentration of Brønsted acid sites among tested samples. The experiments carried out at 250 °C confirmed the trends observed already at 200 °C (Fig. 10). The highest toluene conversion was obtained over [Al]-SSZ-33 achieved 12.7% 15 min TOS then decreased continuously in 120 min TOS and after that remained quite stable. Besides alkylation reaction toluene disproportionation could occur The Toluene conversion over [Ga]-SSZ-33 was quite stable with TOS keeping the value around 9%. In the case of [Fe]-SSZ-33 conversion of toluene increased in the first 60 min and then remained stable until 120 min TOS and finally slightly decreased probably caused by deactivation of the catalyst. The decrease in the toluene conversion was accompanied by increase in selectivity to cymenes. It can be stated that selectivity to
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Fig. 9. Alkylation of toluene with isopropyl alcohol at 200 °C, T/P = 9.6.
Fig. 10. Alkylation of toluene with isopropyl alcohol at 250 °C, T/P = 9.6.
cymenes is suppressed by competitive reactions at low TOS. Although toluene conversions over SSZ-33 samples were quite similar after 120 min TOS the selectivity to cymenes differs considerably. Selectivities to cymenes decreased in the order: [Fe]-SSZ-33 93.3% > [Ga]-SSZ-33 60.5% > [Al]-SSZ-33 50.3% (120 min TOS). In addition, the highest selectivity to p-cymene was again obtained over [Fe]-SSZ-33 containing slightly lower concentration of Brønsted acid sites with the lowest acid strength. Selectivity to cymenes around 90% and high iso-/n-propyltoluene ratio (the range: 140–259) denotes that side reactions of toluene leading to other dialkylbenzenes are the most probably suppressed due to sufficient reaction space in the channel system and optimum concentration of Brønsted acid sites. 4. Conclusions Borosilicate SSZ-33 was transformed by isomorphous substitution to Al, Ga and Fe form. Modified zeolites were investigated by
FTIR spectroscopy. Different probe molecules: pyridine 2,6-di-tertbutylpyridine, d3-acetonitrile and pivalonitrile were used as probe molecules. The catalytic performance was tested in the toluene disproportionation and toluene alkylation with isopropyl alcohol. Direct evidence for the successful isomorphous substitution of the boron atoms into the framework position was the presence of the band corresponding to bridged OH groups in FTIR spectra. In the case of [Fe]-SSZ-33 nonframework iron species were observed. It was found by adsorption of pyridine and pivalonitrile that the acid strength of Brønsted acid sites of zeolites under study decreased in the order: [Al]-SSZ-33 > [Ga]-SSZ-33 > [Fe]-SSZ-33. Lewis acid sites exhibited almost the same acid strength. Upon adsorption of DTBpy only a part of Brønsted sites was reached (49.0% [Al]-SSZ-33, 23.8% [Ga]-SSZ-33 and 22.2% [Fe]-SSZ-33) indicating that these sites are located on the external surface and in the pore mouth of 12-ring channels. It was found out that all acid centers are accessible for d3-acetonitrile and even for pivalonitrile. It indicates that acid centers are located in 12-ring channels. In
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addition, it was observed that adsorption of nitriles on [Fe]-SSZ-33 leads to the shift of characteristic bands probably due to the high electrophilicity of iron atom. SSZ-33 zeolites were tested in toluene disproportionation. Toluene conversions decreased in the order: [Al]-SSZ-33 > [Ga]-SSZ-33 > [Fe]-SSZ-33 (15 min TOS). The acid strength of Brønsted sites decreased in the same order (Al > Ga > Fe). Selectivity to sum of xylene isomers was as follows: [Al]-SSZ-33 > [Fe]-SSZ-33 > [Ga]-SSZ-33. However, the highest selectivity to p-xylene was obtained over [Ga]-SSZ-33 containing the highest concentration of Lewis sites. In addition, the highest ratio xylene/ benzene (0.75) was exhibited over [Al]-SSZ-33 and [Fe]-SSZ-33. In the case of toluene alkylation with isopropyl alcohol, toluene conversions were as follows: [Al]-SSZ-33 > [Ga]-SSZ-33 > [Fe]-SSZ-33 at 200 °C as well as at 250 °C. Although the toluene conversion over zeolite [Fe]-SSZ-33 was the lowest the selectivity to cymenes and in particular p-cymene was the highest. It was proven that channel system of SSZ-33 zeolite does not support formation of n-propyltoluene whereas in the case of [Fe]-SSZ-33 n-propyltoluene was found in negligible amount. Acknowledgements The authors thank support of the Czech Science Foundation P106/11/0819. We also thank Stacey Zones from the Chevron Research and Technology Company for donation of zeolite SSZ-33. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25]
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Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Catalytic applications and FTIR investigation of zeolite SSZ-33 after isomorphous substitution Dana Vitvarová ⇑, Lenka Kurfirˇtová, Martin Kubu˚, Nadeˇzˇda Zˇilková ´ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, v.v.i., Dolejškova 3, CZ-182 23 Prague 8, Czech Republic J. Heyrovsky
a r t i c l e
i n f o
Article history: Received 8 January 2014 Received in revised form 1 April 2014 Accepted 2 April 2014 Available online 13 April 2014 Keywords: SSZ-33 Acidity Toluene alkylation Toluene disproportionation
a b s t r a c t Zeolite SSZ-33 (topology: CON, borosilicate as-synthesized) contains access to the internal void spaces via 10- and 12-ring pores. Borosilicate SSZ-33 was transformed by isomorphous substitution to Al, Ga and Fe form. The acid sites of zeolite samples were investigated using infrared spectroscopy. The different probes (d3-acetonitrile, pivalonitrile, pyridine and 2,6-di-tert-butyl-pyridine) were used in this study. Brønsted acidity confirms the isomorphous substitution of B by Al, Ga and Fe in the SSZ-33 framework. It was found that the strength of Brønsted acid sites of zeolites under study decreased in the series: [Al]-SSZ-33 > [Ga]-SSZ-33 > [Fe]-SSZ-33. Upon adsorption of 2,6-di-tert-butyl-pyridine a part of Brønsted acid sites was not accessible (51% [Al]-SSZ-33, 76% [Ga]-SSZ-33 and 78% [Fe]-SSZ-33). The catalytic performance was studied in toluene disproportionation and toluene alkylation with isopropyl alcohol. Toluene conversions decreased in the order: [Al]-SSZ-33 > [Ga]-SSZ-33 > [Fe]-SSZ-33 in all tested reactions. Selectivities to sum of xylene isomers in toluene disproportionation was as follows: [Al]-SSZ-33 > [Ga]-SSZ-33 > [Fe]-SSZ-33. Zeolite [Al]-SSZ-33 and [Fe]-SSZ-33 exhibited the highest ratio xylene/benzene 0.75. In the case of toluene alkylation, the highest selectivities to cymenes were achieved over [Fe]-SSZ-33 and [Ga]-SSZ-33 at 200 °C. In the case of [Fe]-SSZ-33 almost no n-propyltoluene was found in the reaction mixture. Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction Zeolites are the most important heterogeneous catalysts used in a high number of large scale chemical technologies (oil refinery, petrochemistry) as well as for production of fine chemicals and in environmental catalysis [1–3]. Their structures together with the number of Brønsted and Lewis acid sites, their strength, distributions and their locations, have a significant influence on the activity and seletivity of the particular reaction [4–6]. The acid properties of zeolites and mesoporous molecular sieves can be modified by isomorphous substitution of tetrahedral sites by heteroatoms such as Al [7], Ti [8], Ga [9], Fe [10], V [11] and Cr [12]. The differences in acid strength and catalytic properties resulting from framework substitution offer the potential to design zeolites for new applications [13]. Several approaches can be employed to determine zeolite acidity and accessibility of active sites. Theoretical methods, both Hartee-Fock and density functional theory have been used to study acidity of zeolites and interaction of probe molecules [13–21]. FTIR spectroscopy is probably the most frequently used experimental ⇑ Corresponding author. Tel.: +420 26605 3055; fax: +420 28658 2307. E-mail address: [email protected] (D. Vitvarová). http://dx.doi.org/10.1016/j.micromeso.2014.04.007 1387-1811/Ó 2014 Elsevier Inc. All rights reserved.
technique, particularly in combination with different probe molecules [22,23]. Zecchina et al. [24] proposed nitriles as the probes in the IR studies of solid surfaces. Especially deuterated acetonitrile is largely employed to characterize the active sites in zeolitic materials [25]. Investigation of the external surface and access to the cavities of zeolites can be performed using also nitriles with branched side chains [26–29]. New zeolite structures are expected to improve shape selectivity. Zeolites with connected channels of 10- and 12-rings offer an interesting pore arrangement for catalysis [30]. SSZ-33 (topology: CON, borosilicate as-synthesized) contains a multidimensional pore system formed by intersecting 10- and 12-ring channels of 0.64 0.70 nm, 0.59 0.70 nm (12-ring parallel to [0 0 1] and [1 0 0]), respectively, and 0.45 0.51 nm (parallel to [0 1 0]) providing access to the crystal interior through both pore sizes [31,32]. Borosilicates can be easily transformed into the aluminosilicate form by a low temperature hydrothermal treatment [33]. Gil et al. studied acidic properties and active sites accessibility of [Al]-SSZ33 by FTIR spectroscopy [34,35]. FTIR results were correlated with 27 Al and 1H MAS NMR. SSZ-33 zeolite was found to possess bridging Si–OH–Al groups of virtually uniform and high acid strength. It was found that almost all bridging Si–OH–Al groups in SSZ-33 are located in the 12-rings.
D. Vitvarová et al. / Microporous and Mesoporous Materials 194 (2014) 174–182
Reactions of alkyl and dialkyl aromatic hydrocarbons represent an important group of reactions used partly in petrochemical industry as well as model reactions testing shape selectivity properties of zeolites [36]. Novel zeolite SSZ-33 was successfully tested in many catalytic reactions such as toluene and xylene disproportionation, benzene, toluene and p-xylene alkylation. Moreover, SSZ-33 was studied as a catalyst for synthesis of fine chemicals in acylation of ferrocene. In p-xylene alkylation with isopropyl alcohol at 60 min TOS (time on stream) SSZ-33, Beta and SSZ-35 reached p-xylene conversions about 70% [36]. Zeolite SSZ-33 represented intermediate behavior as compared with ZSM-5 and Beta in benzene alkylation with isopropyl alcohol (benzene conversions: ZSM-5 43.0% > SSZ33 39.6% > Beta/Ge 38.8%, benzene to isopropyl alcohol molar ratio 4, 280 °C) [37]. m-Xylene reactions (transalkylation, isomerization and disporportionation) over aluminosilicate SSZ-33 with different aluminum populations was tested by Jones et al. [33] and Adair et al. [38]. It was observed that the intracrystalline pore space of SSZ-33 acts as an ensemble of cages connected by 10- and 12-ring rather than intersecting 10-ring and 12-ring channels [33]. Xylene isomerization/disproportionation and toluene alkylation over SSZ-33 was also tested by Llopis et al. [30]. Toluene conversions in alkylation with isopropyl alcohol decreased in the order: Nu-87 > Beta > SSZ-33 > ZSM-5. The objective of this contribution is characterization of acid sites in SSZ-33 zeolites after its post-synthesis modifications. In particular, type and concentration of active sites, their strength and accessibility were studied by the interaction with d3-acetonitrile, pivalonitrile, pyridine and 2,6-di-tert-butyl-pyridine using FTIR spectroscopy. Differences in catalytic behavior of Al, Fe and Ga form of SSZ-33 zeolite were tested in toluene disproportionation and toluene alkylation with isopropyl alcohol.
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2.2. FTIR spectroscopic experiment The FTIR spectra were recorded with a Nicolet 6700 equipped with AEM module instrument with a resolution 4 cm 1 using pressed disks of pure zeolite powders, activate by outgassing at 450 °C into the IR cell under vacuum. Pyridine, 2,6-di-tert-butyl-pyridine, d3-acetonitrile and pivalonitrile adsorbants were purchased from Sigma–Aldrich. The adsorption procedure of d3-acetonitrile and pivalonitrile involves contact of the activated sample disk with vapors for 20 min at equilibrium pressure and room temperature. Desorption of probe molecules was performed in steps at room temperature or increasing temperatures (100–350 °C). The Pyridine adsorption proceeded at 150 °C for 20 min at partial pressure 3 Torr, followed by 20 min evacuation at 150 °C. The concentration and the type of acid sites were determined by adsorption of pyridine and d3-acetonitrile as probe molecules. To obtain quantitative analysis the molar absorption coefficients for d3-acetonitrile adsorbed on Brønsted acid sites (m(C„N)-B at 2297 cm 1, e(B) = 2.05 ± 0.1 cm lmol 1) and strong and weak Lewis acid sites (m(C„N)-L1 at 2325 cm 1 m(CN)-L2 2310 cm 1, e(L) = 3.6 ± 0.2 cm lmol 1) were used [40]. The concentrations of Brønsted and Lewis acid sites were also calculated from integral intensities of individual bands characteristic of pyridine on Brønsted acid sites at 1545 cm 1 and band of pyridine on Lewis acid site at 1455 cm 1 and molar absorption coefficients of e(B) = 1.67 ± 0.1 cm lmol 1 and e(L) = 2.22 ± 0.1 cm lmol 1, respectively [40]. The adsorption of 2,6-di-tert-butyl-pyridine was carried out at 150 °C and at equilibrium probe vapor pressure for 15 min. Desorption proceeded at the same temperature for 1 h. Extinction coefficient for pyridine was used for quantitative evaluation of the concentration of Brønsted acid sites. 2.3. Catalytic test
2. Experimental 2.1. Catalysts SSZ-33 zeolite in boron form was synthesized at Chevron Research Company. Isomorphous substitution of [B]-SSZ-33 zeolite to Al, Fe and Ga form was carried out by the following procedure. The boron-containing SSZ-33 was converted by a one-step reflux in 1 M solution of corresponding nitrate (pH in the range of 1–2) providing hydrolysis of boron and reinsertion of the respective element into the framework. Procedure was performed at room temperature for 100 h. Finally, molecular sieves were recovered by filtration, washed out with 0.1 M HCl, then with distilled water, dried and calcined using the temperature program: 1 °C/min to 120 °C, maintained at 120 °C for 2 h then heated (1 °C/min) to 540 °C, kept for 5 h, and further heated to 595 °C (1 °C/min) and maintained for another 5 h [39]. The crystallinity of zeolites was determined by X-ray powder diffraction with Bruker D8 X-ray powder diffractometer equipped with a graphite monochromator and position-sensitive detector using CuKa radiation in the Bragg–Brentano geometry. The size and shape of zeolite crystals were determined by scanning electron microscopy using an instrument Jeol, JSM-5500LV. For the determination of the chemical composition of synthesized and modified zeolites, X-ray fluorescence analysis with a Philips PW 1404 spectrometer equipped with an analytical program UniQuant was used. The samples were mixed with dentacryl as a binder and pressed on the surface of cellulose pellets. Adsorption isotherms of nitrogen at 196 °C were determined using GEMINI static volumetric instrument. Before adsorption zeolites were degassed under helium at 350 °C for 16 h.
The alkylation of toluene with isopropyl alcohol was performed in a down-flow glass microreactor with a fixed bed of catalyst under atmospheric pressure (powder was pressed and sieved, and the fraction between 0.70 and 0.50 mm was used for reaction). Reactant stream was prepared by passing the carrier gas through thermostated saturators and the composition was controlled by gas chromatography. The experiments were carried with toluene/ isopropyl alcohol molar ratio 9.6 and weight hour space velocity (WHSV) based on toluene equal to 10. The disproportionation of toluene was carried out in the same apparatus under atmospheric pressure at 500 °C with WHSV 1.9 h 1. Before experiment, zeolite catalysts were activated at 500 °C in nitrogen stream for 1 h. The reaction products were analyzed using an ‘‘on-line’’ gas chromatograph (Agilent 6890 Plus) with flame ionization detector and a high-resolution capillary column (Innowax for the disproportionation and DB-5 for the alkylation). The first analysis was performed after 15 min of timeon-stream (TOS) and the other followed in approx. 55 min interval. 3. Results and discussion 3.1. Characterization X-ray powder patterns of SSZ-33 zeolites used in this study exhibited a high crystallinity and phase purity (Fig. 1). All diffraction patterns are similar with respect to the position and intensity of diffraction lines. No evidence of structure damage or the presence of an additional phase was found. The SEM images of SSZ-33 zeolite are depicted in Fig. 2. They indicate rather homogeneous distribution of crystal sizes and
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D. Vitvarová et al. / Microporous and Mesoporous Materials 194 (2014) 174–182 Table 1 Textural properties of SSZ-33 zeolites. Zeolite
BET (m2/g)
Vmic (cm3/g)
Vtot (cm3/g)
Si/M
[Al]-SSZ-33 [Fe]-SSZ-33 [Ga]-SSZ-33
573 550 462
0.254 0.242 0.201
0.398 0.389 0.347
24 27 20
Fig. 1. XRD patterns of zeolites used: [B]-SSZ-33, [Ga]-SSZ-33, [Fe]-SSZ-33 and [Al]SSZ-33.
shapes of SSZ-33 zeolites exhibiting crystals 0.8 0.4 lm with a few larger agglomerates. Textural properties of zeolites under investigation were determined from adsorption isotherms of nitrogen. The t-plot method was applied to estimate the volume of micropores (Table 1). The N2 adsorption–desorption isotherms of SSZ-33 zeolites (Fig. 3) are similar in the shape and the the BET surface areas are in the range 460–570 m2/g (Table 1). The micropore volume (Vmic) is in the range of 0.20–0.25 cm3/g confirming a high quality of the SSZ-33 zeolites studied when compared with the literature [31,41]. Decrease in BET surface area for [Ga]-SSZ-33 is in agreement with the data reported in literature [42]. Molar ratios of silicon to corresponding metal were determined by X-ray fluorescence analysis (Table 1). 3.2. FTIR investigations Pyridine is very sensitive probe molecule for the quantification of both Brønsted and Lewis acid sites, as both the frequency of bands appearing at 1545 cm 1 (PyH+) and 1455 cm 1 (PyL) are independent of the type of zeolite, its composition or morphology. This remains valid even for zeotypes containing heteroatoms other than aluminum [43]. The region of OH stretching vibrations of zeolites under study is shown in Fig. 4. The band occurring at ca. 3745 cm 1 is assigned to the terminal silanol SiOH groups. The frequencies of the stretching vibration bands of the bridged OH groups increase in the order Al (3610 cm 1) < Ga (3620 cm 1) < Fe (3630 cm 1). Frequencies of bridging OH groups of SSZ-33 zeolites agree with the data reported for isomorphously substituted ZSM-5 [44,45]. The presence of the band of bridging OH groups evidence the isomorphous substitution of the boron atoms into framework of the SSZ-33 structure. The
Fig. 3. N2 adsorption (s) and desorption () isotherms of SSZ-33 zeolites. d[Al]SSZ-33, j[Fe]-SSZ-33, N[Ga]-SSZ-33.
position of this band depends on the nature of the trivalent element, and shifts to higher wavenumbers in the row Al < Ga < Fe, suggest the decrease in the acid strength in the same order. It is important to note that the 1380 cm 1 band observed in the case of parent B-SSZ-33 zeolite attributed to tricoordinated framework boron diminished completely after zeolites modification [46]. Berndt et al. studied zeolite ZSM-5 with isomorphously substituted B, Ga, Fe, In and Al by infrared spectroscopy and bands in the region 3651–3672 cm 1 attributed to the non-framework species [44]. Chao et al. reported that the nonframework Ga species provide a band at 3685 cm 1 which was also observed in the IR spectrum of GaNH4[Si, Al]-Beta [47]. In our case, these bands were not observed in case of [Ga]-SSZ-33 and [Al]-SSZ-33. However, the band 3672 cm 1 of a very low intensity is present in the [Fe]SSZ-33 spectra (Fig. 4). It is well known that ferrisilicate zeolites are less stable than the aluminosilicates due to the size of Fe3+ ion building a framework. The size of ions depends on the valency and coordination number. Fe(III) atoms located in framework positions are unstable and show a tendency towards formation of extraframework oxide-like species in zeolites due to geometrical reasons [48,49]. A broad band at around 3500 cm 1 originated from species, whose hydrogen atom is involved in weak H-bondings with nearer hydroxyls for all three samples. This band is
Fig. 2. SEM images of [Ga]-SSZ-33 (left), [Fe]-SSZ-33 (middle) and [Al]-SSZ-33 (right) zeolites.
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Fig. 4. Pyridine adsorption in SSZ-33 zeolites, (A) IR spectra of hydroxyl vibration region and (B) spectra of pyridine region after its adsorption followed by desorption at 150 °C. For (A) red line: spectra of activated samples and, black line: spectra after 20 min desorption at 150 °C. For B) black line: spectra after 20 min desorption at 150 °C and, blue line: spectra after 20 min desorption at 450 °C. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
typical for SiOH groups located on lattice defects [50,51]. The vibration band of the pyridinium ion at 1545 cm 1 evidenced that all substituted SSZ-33 samples contain Si–OH–M groups. The concentrations of Lewis and Brønsted acid sites at different desorption temperatures are depicted in Table 2. The band assigned to pyridine adsorption on Lewis acid sites is located at 1455 cm 1. The Al- and Fe-containing samples exhibit an additional band at 1462 cm 1 which is well visible after desorption at temperature 450 °C indicating pyridine chemisorptions at Lewis sites of higher acid strength [35] (Fig. 4). Data in Table 2 denotes that with increasing desorption temperature concentration of Lewis and Brønsted acid sites interacting with pyridine decreases for all zeolites under study. The concentration of strong Brønsted acid sites was determined from the ratio of pyridine interacting with acid sites after desorption at 150 °C and 450 °C. The part of strong Brønsted acid sites decreased in the order: Al 38.1% > Ga 14.3% > Fe 11.1%. Brønsted acid sites on [Ga]-SSZ-33 exhibit a lower acid strength in comparison with the [Al]-SSZ-33 sites probably due to the lower electronegativity of the larger softer Ga cation as compared with the value of the smaller harder Al cation [52]. The observed decrease in strong Brønsted acid sites concentration of tested zeolites is in agreement with existing experimental and theoretical results [13–16]. Langenaeker and Geerlings [14] studied the influence of isomorphous substitution of Al (by B and Ga) and Si (by Ge) on the catalytic activity of zeolite system using reactivity indexes based on density functional theory. The authors concluded that the acidities of the zeolite-type model systems are dependent on several characteristics including the hardness (softness), the electronegativity of the substituting atoms and the charge on the hydrogen atom of the bridging hydroxyls. The OH-bond length and the ionicity of the OH bond were found to be suitable descriptors of the acidity, whereas the dipole moment
derivative with respect to the OH-bond length appeared to fail as a reactivity index. Deka et al. [13] also found that acidity of zeolites decrease in the order Al(OH)Si > Ga(OH)Si > B(OH)Si. The authors used quantum chemical methods to study the influence of chemical composition on acidity of zeolite clusters. The same conclusions reported Stave et al. [15] in study of isomorphously substituted ZSM-5 zeolites by density functional theory. [Ga]-SSZ-33 contains the highest concentration of Lewis sites form all tested samples. Chandwadkar et al. reported higher concentration of Lewis acid sites than Brønsted acid sites on [Ga]-Mordenite and it attributed to the relatively less thermally stable bridging OH groups leading to dehydroxylation and formation of Lewis acid sites [53]. After outgassing at 450 °C still more than half of Lewis sites bonding pyridine remained: 56.5% [Fe]-SSZ-33, 55.0% [Ga]-SSZ-33 and 55.6% [Al]-SSZ-33 indicating the steady strength of Lewis acid sites among tested samples. Adsorption of 2,6-Di-tert-butyl-pyridine (DTBpy) on the zeolite SSZ-33 showed that accessibility of the Brønsted acid sites is 49.0% for [Al]-SSZ-33, 23.8% for [Ga]-SSZ-33, and 22.2% for [Fe]-SSZ-33, respectively. The FTIR spectra of DTBpy adsorbed on zeolite SSZ-33 samples are depicted in Fig. 5. DTBpy does not penetrate to the pores of 10-ring zeolites like ZSM-5. On the other hand, it was observed that DTBpy can penetrate to the 12-ring channel system of zeolite Beta [54]. Based on the spectra where we clearly see the band around 1530 cm 1 attributed to the interaction of the Brønsted sites with DTBpy we conclude that the accessible Brønsted acid sites are mainly located on the external surface or in the pore mouth of 12-member ring channels. Nitriles are relatively weak bases and coordinate via the nitrogen of the nitrile group to the acid sites. Because of its small size, d3-acetonitrile (CD3CN) is frequently successfully applied for the
Table 2 Concentration of Lewis (Lc) and Brønsted (Bc) acid sites obtained by FTIR spectroscopy (pyridine adsorption). Desorption temperature (°C)
150 250 350 450
[Al]-SSZ-33
[Fe]-SSZ-33
[Ga]-SSZ-33
cLc mmol/g
cBc mmol/g
cLc mmol/g
cBc mmol/g
cLc mmol/g
cBc mmol/g
0.20 0.15 0.12 0.11
0.21 0.21 0.17 0.08
0.19 0.18 0.16 0.11
0.18 0.18 0.18 0.02
0.27 0.22 0.19 0.15
0.21 0.20 0.16 0.03
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Fig. 5. DTBpy adsorption in SSZ-33, (A) IR spectra of hydroxyl stretching region and (B) spectra of DTBpy region after its adsorption followed by desorption at 150 °C. For (A) red line: spectra of activated samples and black line: spectra after 1 h desorption at 150 °C. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
characterization of a large number of oxides [55]. The spectra of SSZ-33 zeolites obtained after adsorption of CD3CN are shown in Fig. 6. The adsorption of CD3CN on [Al]-SSZ-33 and [Ga]-SSZ-33 gives rise the formation of bands at 2323, 2298 cm 1. The band at 2323 cm 1 can be correlated to the adsorption on Lewis acid sites and band at 2298 cm 1 can be assigned to the adsorption on bridging OH groups [56]. Adsorption of CD3CN on [Fe]-SSZ-33 (Fig. 6) leads to the appearance of a intense band at around 2300 cm 1. The m(CN) stretching frequency for CD3CN adsorption on Brønsted sites associated with framework Fe sites is 2298 cm 1. The new band around 2300 cm 1 is attributed to the Lewis acid sites resulting from nonframework Fe and is virtually indistinguishable from the Brønsted sites associated with framework Fe [57]. The concentration of Lewis and Brønsted acid sites obtained from CD3CN adsorption correlates with concentrations from pyridine for [Ga] and [Al]-SSZ-33. Unfortunately, the concentrations were not possible to calculate for [Fe]-SSZ-33 (Table 3).
Table 3 Concentration of Lewis (Lc) and Brønsted (Bc) acid sites obtained by FTIR spectroscopy (CD3CN adsorption).
[Al]-SSZ-33 [Ga]-SSZ-33
cLc mmol/g
cBc mmol/g
0.19 0.33
0.22 0.20
Pivalonitrile (PN) critical diameter is around 0.62 nm [51,58]. It was proven that PN do not penetrate the zeolite ZSM-5 cavity [27] and therefore it is suitable as a probe for SSZ-33 zeolite. The IR spectra recorded after adsorption of pivalonitrile in the CN stretching region contains four components at around 2294, 2270, 2249 and 2238 cm-1 (Fig. 7). The bands 2249 and 2238 cm 1 can be assigned to the two H-bonding complexes formed with terminal silanol groups absorbing at 3745 and 3732 cm 1 [59]. The component at 3732 cm 1 can be associated with terminal silanol groups located on the internal surface of the zeolite in defect sites
Fig. 6. CD3CN adsorption in SSZ-33, (A) IR spectra of hydroxyl vibration region and (B) spectra of CD3CN region after its adsorption followed by desorption at room temperature. For (A) red line: spectra of activated samples and, black line: spectra after 20 min desorption at room temperature. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 7. Pivalonitrile adsorption in SSZ-33, (A) IR spectra of hydroxyl vibration region and (B) spectra of CN stretching region region. For (A) black line: spectra of activated samples, red line: spectra after desorption at room temperature and blue line: spectra after 20 min desorption at 100 °C. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
[28,60]. Bevilacqua studied adsorption of PN on Mordenite and ascribed the band at 2272 cm 1 to interaction of PN with bridging OH groups and the band at 2295 cm 1 to interaction with Lewis acid sites [58]. The saturation of the SSZ-33 samples with PN caused a full perturbation of silanol groups with mutual formation of a band at 3400 cm 1 (Fig. 7). This band is assigned to the OH stretching of hydrogen-bonded complexes of the silanol groups with the PN nitrogen lone pair. Adsorption of PN on [Al]-SSZ-33 yielded all peaks discussed above. However, upon PN adsorption on [Fe]-SSZ-33 only three components are visible in the spectra: a broad band with a maxima at around 2274 cm 1 and bands at 2249 and 2238 cm 1. Unfortunately, features attributed to PN interaction with Brønsted and Lewis acid sites on [Fe]-SSZ-33 were indistinguishable. The same situation was observed in CD3CN adsorption on [Fe]-SSZ-33, vide supra. Hadjiivanov investigated Fe-containing zeolite Beta by FTIR spectroscopy of adsorbed CO and NO. Adsorption of CO and NO led to the formation of species with unusual frequency due to the high electrophilicity of iron atoms [61]. Finally, the spectra recorded after PN adsorption on [Ga]-SSZ-33 exhibit the intense band with the maximum at 2290 cm 1 and the shoulder at 2270 cm 1 probably caused by a higher concentration of Lewis acid sites. Outgassing at 100 °C restores the band near 3745 cm 1 and the component at 3400 cm 1 disappeared but the band corresponding to bridging OH groups is not restored at all. These observations suggest that the acid sites are located in 12-ring channels and therefore can be well accessible in catalytic reactions. Gil et al. used PN to differentiate between the centers located inside 10-ring and 12-ring channels in zeolite SSZ-33 and reported the same conclusion [35]. According to Zˇilková et al. [36] the intesity of the band at 1374 cm 1 (CH3 umbrella bending of pivalonitrile, not shown in Figure) was taken as the analytical band, presenting the amount of pivalonitrile still kept in zeolite after desorption at 200 °C. The ratio of the band areas at 1374 cm 1 after desorption at 200 °C and after desorption at room temperature decreased in the order: 0.29 [Al]-SSZ-33 > 0.25 [Ga]-SSZ-33 > 0.14 [Fe]-SSZ-33. It supports the results from pyridine and DTBpy adsorption showing that the acid strength and accessibility decreased in the order Al > Ga > Fe. The FTIR study proved that isomorphous substitution of boron was successful. In the case of [Fe]-SSZ-33 zeolite the band 3672 cm 1 (Fig. 4) of a very low intensity attributed to the
nonframework atoms was visible in the spectra. Adsorption of pyridine on SSZ-33 zeolites proved that the concentration of Brønsted acid sites is the same for [Al]-SSZ-33 and [Ga]-SSZ-33 and slightly lower for [Fe]-SSZ-33. The sample [Ga]-SSZ-33 contains the highest concentration of Lewis acid sites. The concentration of strong Brønsted acid sites still present after outgassing at 450 °C decreased in the order: Al > Ga > Fe. The strength of Lewis acid sites was almost the same for all tested samples. DTBpy adsorption showed that accessibility of Brønsted acid sites is 23.8% for [Ga]-SSZ-33, 49.0% for [Al]-SSZ-33 and 22.2% for [Fe]-SSZ-33. Adsorption of CD3CN showed that active sites are well accessible for all zeolites under study. It was found that zeolite [Fe]-SSZ-33 exhibits shift of characteristic adsorption bands of probe molecules probably due to the high electrophilicity of iron ions. 3.3. Catalytic test The catalytic behavior of isomorphous substituted zeolites SSZ-33 were tested in toluene disproportionation and toluene alkylation with isopropyl alcohol. 3.3.1. Toluene disproportionation The disproportionation of toluene to benzene and xylenes occurs via a bimolecular reaction mechanism. Disproportionation of the two toluene molecules yields an equimolar mixture of xylene isomers and benzene. Depending on the reaction conditions, it may be accompanied by a parallel dealkylation of toluene to benzene, and also by secondary transformations of xylenes. The toluene conversions decreased in the order: [Al]-SSZ-33 54.0% > [Fe]-SSZ-33 25.3% > [Ga]-SSZ-33 11.3% (60 min TOS, Fig. 8). Higher toluene conversion can be expected over zeolite possessing more acidic sites [36]. Indeed, the highest toluene conversion was achieved over zeolite [Al]-SSZ-33 containing Brønsted acid sites with the strongest acid sites among tested zeolites. [Fe]-SSZ-33 exhibits almost the same concentration of Brønsted acid sites as [Al]-SSZ-33 but with lower acid strength. In spite of the highest concentration of active sites (sum of Lewis and Brønsted acid sites) on [Ga]-SSZ-33 the toluene conversion was the lowest. In addition, we can observe significant decrease in toluene conversion over [Ga]-SSZ-33 between 15 and 60 min contact time probably due to deactivation. The deactivation of the catalyst
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Fig. 8. Disproportionation of toluene, WHSV 1.9 [Fe]-SSZ-33 and [Ga]-SSZ-33, WHSV 30 [Al]-SSZ-33, 500 °C.
is probably caused by the formation of coke deposit promoting by high density of acid sites. Decrease in toluene conversion was accompanied by an increase in the selectivity to xylenes. Selectivities to sum of xylene isomers after 60 min TOS were as follows [Fe]-SSZ-33 40.2% > [Al]SSZ-33 30.2% – [Ga]-SSZ-33 30.2%. On the other hand, the highest selectivity to p-xylene was achieved over [Ga]-SSZ-33 with the highest concentration of Lewis acid sites. Cˇejka and Wichterlová suggested a direct participation of Lewis sites in toluene disproportionation [62]. Selectivity to p-xylene achieved over [Ga]-SSZ-33 increased considerably with an increasing TOS probably due to the formation of coke deposits retarding the isomerization of pxylene. The highest ratio xylene/benzene equal to 0.73 was achieved over [Al]-SSZ-33 and [Fe]-SSZ-33 zeolites. In the case of [Ga]-SSZ-33 the xylene/benzene ratio reached value around 0.6 180 min TOS. It is expected that coke deposits decrease the free volume in the channels and thus limit the consecutive reactions. Xylenes/benzene ratio over [Fe]-SSZ-33 was quite stable with TOS. 3.3.2. Toluene alkylation with isopropyl alcohol The alkylation of toluene with isopropyl alcohol belongs to the group of electrophilic substitution reactions. It is considered that alkylation reactions catalyzed by zeolites proceed via a carbenium ion-type mechanism. The toluene conversions and the product distributions in the alkylation of toluene with isopropyl alcohol at 200 °C are depicted in Fig. 9. The toluene/isopropyl alcohol molar ratio used in the feed was 9.6 indicative of the theoretical toluene conversion around 10.4% for pure alkylation reaction and complete conversion of isopropyl alcohol. Toluene conversions follow the order: [Al]-SSZ-33 10.7% > [Ga]-SSZ-33 10.1% > [Fe]-SSZ-33 7.6% (60 min TOS). It can be observed that toluene conversions correspond to a decrease in Brønsted acid sites strength. Moreover, the data are consistent with accessibility of acid centers obtained by DTBpy adsorption. Finally, it can be stated that the concentration of Lewis acid sites does not play a key role in this transformation because [Ga]-SSZ-33 exhibited the almost the same toluene conversion although contains the highest concentration of Lewis sites. Toluene conversions over all tested catalysts were quite stable with TOS. This could be assigned to stability and diffusion of the formed products in the channel structure of the zeolites [63]. In our case, pivalonitrile desorption at increasing temperature steps revealed that pivalonitrile desorbs from [Fe]-SSZ-33 the most
easily. Besides, the low concentration of acid sites could enhance the rate of diffusion of both reactants and products [64]. Toluene alkylation leads mainly to a mixture of iso-propyltoluenes (cymenes) whereas desired product is p-cymene, that is need in production of fungicides, pesticides or flavors. Consecutive reaction also proceed yielding n-propyltoluenes [65]. The highest selectivity to cymenes (93.8%, 60 min TOS) as well as the selectivity to p-cymene was achieved over [Fe]-SSZ-33 (around 40%, thermodynamic value [66]). It was reported by Wichterlová et al. that decreasing the diameter of the channels or the windows in the sequence zeolite Y > Mordenite > ZSM-12 > (Al, Fe)ZSM-5 led to a significant increase in the selectivity towards p-cymene [64]. Selectivity to cymenes can be lowered by a secondary transalkylation with toluene forming undesired n-propyltoluene [67]. It was already proved that the formation of n-propyltoluenes is strongly dependent on the dimensions and architecture of the channels of the molecular sieves [30,66]. Cˇejka et al. reported that the lower acid strength and number of acid sites suppress competitive reactions and, therefore, cymenes are predominantly formed with a high selectivity which is in agreement with our results [65]. It was proposed that large pore zeolites with 12-membered rings possess a sufficient reaction space decreasing the probability of the formation of bimolecular complex leading to n-propyltoluene [30,68]. Iso/n-propyltoluene ratios reached relatively high values (100 for [Ga]-SSZ-33 and 81 for [Al]-SSZ-33, 180 min TOS). In the case of [Fe]-SSZ-33 zeolite almost only iso-propyltoluenes were found in the reaction products. The ratio iso/n-propyltoluene was 931 after 15 min of TOS. The ratio iso/n-propyltoluene slightly decreased with TOS. It could be caused the lowest concentration of Brønsted acid sites among tested samples. The experiments carried out at 250 °C confirmed the trends observed already at 200 °C (Fig. 10). The highest toluene conversion was obtained over [Al]-SSZ-33 achieved 12.7% 15 min TOS then decreased continuously in 120 min TOS and after that remained quite stable. Besides alkylation reaction toluene disproportionation could occur The Toluene conversion over [Ga]-SSZ-33 was quite stable with TOS keeping the value around 9%. In the case of [Fe]-SSZ-33 conversion of toluene increased in the first 60 min and then remained stable until 120 min TOS and finally slightly decreased probably caused by deactivation of the catalyst. The decrease in the toluene conversion was accompanied by increase in selectivity to cymenes. It can be stated that selectivity to
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Fig. 9. Alkylation of toluene with isopropyl alcohol at 200 °C, T/P = 9.6.
Fig. 10. Alkylation of toluene with isopropyl alcohol at 250 °C, T/P = 9.6.
cymenes is suppressed by competitive reactions at low TOS. Although toluene conversions over SSZ-33 samples were quite similar after 120 min TOS the selectivity to cymenes differs considerably. Selectivities to cymenes decreased in the order: [Fe]-SSZ-33 93.3% > [Ga]-SSZ-33 60.5% > [Al]-SSZ-33 50.3% (120 min TOS). In addition, the highest selectivity to p-cymene was again obtained over [Fe]-SSZ-33 containing slightly lower concentration of Brønsted acid sites with the lowest acid strength. Selectivity to cymenes around 90% and high iso-/n-propyltoluene ratio (the range: 140–259) denotes that side reactions of toluene leading to other dialkylbenzenes are the most probably suppressed due to sufficient reaction space in the channel system and optimum concentration of Brønsted acid sites. 4. Conclusions Borosilicate SSZ-33 was transformed by isomorphous substitution to Al, Ga and Fe form. Modified zeolites were investigated by
FTIR spectroscopy. Different probe molecules: pyridine 2,6-di-tertbutylpyridine, d3-acetonitrile and pivalonitrile were used as probe molecules. The catalytic performance was tested in the toluene disproportionation and toluene alkylation with isopropyl alcohol. Direct evidence for the successful isomorphous substitution of the boron atoms into the framework position was the presence of the band corresponding to bridged OH groups in FTIR spectra. In the case of [Fe]-SSZ-33 nonframework iron species were observed. It was found by adsorption of pyridine and pivalonitrile that the acid strength of Brønsted acid sites of zeolites under study decreased in the order: [Al]-SSZ-33 > [Ga]-SSZ-33 > [Fe]-SSZ-33. Lewis acid sites exhibited almost the same acid strength. Upon adsorption of DTBpy only a part of Brønsted sites was reached (49.0% [Al]-SSZ-33, 23.8% [Ga]-SSZ-33 and 22.2% [Fe]-SSZ-33) indicating that these sites are located on the external surface and in the pore mouth of 12-ring channels. It was found out that all acid centers are accessible for d3-acetonitrile and even for pivalonitrile. It indicates that acid centers are located in 12-ring channels. In
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addition, it was observed that adsorption of nitriles on [Fe]-SSZ-33 leads to the shift of characteristic bands probably due to the high electrophilicity of iron atom. SSZ-33 zeolites were tested in toluene disproportionation. Toluene conversions decreased in the order: [Al]-SSZ-33 > [Ga]-SSZ-33 > [Fe]-SSZ-33 (15 min TOS). The acid strength of Brønsted sites decreased in the same order (Al > Ga > Fe). Selectivity to sum of xylene isomers was as follows: [Al]-SSZ-33 > [Fe]-SSZ-33 > [Ga]-SSZ-33. However, the highest selectivity to p-xylene was obtained over [Ga]-SSZ-33 containing the highest concentration of Lewis sites. In addition, the highest ratio xylene/ benzene (0.75) was exhibited over [Al]-SSZ-33 and [Fe]-SSZ-33. In the case of toluene alkylation with isopropyl alcohol, toluene conversions were as follows: [Al]-SSZ-33 > [Ga]-SSZ-33 > [Fe]-SSZ-33 at 200 °C as well as at 250 °C. Although the toluene conversion over zeolite [Fe]-SSZ-33 was the lowest the selectivity to cymenes and in particular p-cymene was the highest. It was proven that channel system of SSZ-33 zeolite does not support formation of n-propyltoluene whereas in the case of [Fe]-SSZ-33 n-propyltoluene was found in negligible amount. Acknowledgements The authors thank support of the Czech Science Foundation P106/11/0819. We also thank Stacey Zones from the Chevron Research and Technology Company for donation of zeolite SSZ-33. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25]
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