Immobilization of dodecatungstophosphoric acid on dealuminated zeolite Y: a physicochemical study

Applied Catalysis A: General 194 –195 (2000) 137–146

Immobilization of dodecatungstophosphoric acid on dealuminated zeolite Y: a physicochemical study K. Pamin a , A. Kubacka a , Z. Olejniczak b , J. Habera,1 , B. Sulikowski a,∗ a

Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, Niezapominajek 1, PL 30-239 Kraków, Poland b Institute of Nuclear Physics, Radzikowskiego 152, PL 31-342 Kraków, Poland Accepted 6 August 1999

Abstract 12-Tungstophosphoric acid (PW12 ) has been supported on dealuminated zeolite Y. The catalysts were studied by XRD, DTA/DTG, IR, argon adsorption and MAS NMR spectroscopy. The structure of dealuminated faujasite is retained upon contact with a very strong PW12 acid, which becomes thermally stabilized. Two types of Keggin anions exist at the surface: those strongly interacting with the OH groups of the support, prevailing at low coverages, and those weakly interacting with the zeolite, prevailing at high coverages. Transformations of m-xylene proceed along three pathways: isomerization, disproportionation and dealkylation. Pure zeolite shows high selectivity to isomerization. Deposition of PW12 acid dramatically increases the selectivity of disproportionation to toluene and trimethylbenzenes, which rises with the growing acid loading. ©2000 Elsevier Science B.V. All rights reserved. Keywords: Dealuminated faujasite; Dodecatungstophosphoric acid; Isomerization; Disproportionation; Xylene isomers

1. Introduction Heteropoly acids with the Keggin structure are well known as oxidation and acid catalysts, in which the redox and acid–base properties may be tailored by choosing various heteroatoms, various atoms of the addenda or substituting the addenda with different ions [1,2]. The major drawback of the heteropoly acids as catalysts is their relatively low thermal stability, resulting in a more or less rapid degradation of the Keggin unit under catalytic conditions, accompanied by paral∗ Corresponding author. Tel.: +48-12-425-2841; fax: +48-12-425-1923. E-mail addresses: [email protected] (J. Haber), [email protected] (B. Sulikowski). 1 Co-corresponding author.

lel deterioration of the catalytic activity or selectivity. Therefore, many attempts were undertaken to stabilize them by supporting on various carriers. The support which has been extensively studied is silica [3–7] because its use permits the specific surface area and mechanical strength of catalysts to be easily modified. It has been found that deposition of a heteropoly acid at the surface of silica may modify both its primary and secondary structure, what may result in the increase of the thermal stability of the heteropoly acid and in modification of its catalytic properties. It seemed that even more advantages could be gained by using zeolites as a support. The heteropoly acid could be then either deposited at the surface of the zeolite, or encapsulated in its pores, which should impart novel catalytic properties and enable also to introduce the shape selectivity. We report here the results of the studies of the

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properties of dodecatungstophosphoric acid supported or encapsulated on/in zeolite Y and the catalytic behaviour of such samples in the transformations of m-xylene.

Fourier trasform spectrometer equipped with the TGS detector. Typically, 128 scans were accumulated per spectrum with 2 cm−1 resolution using the KBr pellet technique.

2. Experimental

2.2. NMR spectroscopy

Parent zeolite NaY with Si/Al = 2.47 and Na/Al = 1.09, highly crystalline according to X-ray studies, was dealuminated with ethylenediaminotetraacetic acid (H4 EDTA) at 93 ± 2◦ C. H4 EDTA was added to a suspension of the zeolite in water, which was continuously stirred. The rate of H4 EDTA addition, essential to minimize amorphization of the zeolite, was kept low at 0.01 g of H4 EDTA per 1 g of zeolite per 1 h. The suspension was heated and stirred and for 4 h after all the acid had been added. The zeolite was then washed with hot distilled water, dried, and ion-exchanged with 10 wt.% NH4 Cl solution to yield a sample labelled NH4 , Na-Y0 . Chemical analysis of the dealuminated sample was performed by atomic absorption spectroscopy (AAS) giving Si/Al = 4.40. Pure dodecatungstophosphoric acid (H3 PW12 O40 acid, PW12 for brevity) has been synthesized from sodium phosphate and sodium tungstate and purified according to a known procedure [8]. The dealuminated zeolite matrix was impregnated with known amounts of PW12 acid, using water and ethyl ether solutions. After evaporation of a solvent the samples were checked with XRD for zeolite integrity. Encapsulation of PW12 in the zeolitic matrix (dealuminated by SiCl4 treatment, Si/Al = 41) was carried out by an in situ synthesis of this compound [9]. The dealuminated zeolite was added to a solution of sodium phosphate in water and the whole mixture was stirred for 2 h at ambient temperature. Then a solution containing the known amount of sodium tungstate was added dropwise to the suspension. After 1.5 h of stirring, a stoichiometric amount of HCl was added dropwise. The suspension was stirred further for 4 h. Finally, the zeolite was separated from liquid and washed very intensively with hot doubly distilled water.

The NMR spectra were obtained at room temperature on a home-made 300 MHz pulse spectrometer. A home-made MAS probe was spun at 2–6 kHz. For 29 Si spectra, 90◦ pulse (5 ␮s) and 60 s delay were used to acquire about 200–300 transients of 2k complex data. 27 Al spectra were acquired with 18◦ (2 ␮s) pulse and 1 s delay, and 31 P spectra with 90◦ (2 ␮s) and 60 s delay. An exponential line broadening of 20 and 100 Hz was applied to 29 Si and 27 Al spectra, respectively.

2.1. IR spectroscopy The IR absorption spectra in the zeolite framework vibration region were obtained with a Nicolet 800

2.3. X-ray diffraction Powder X-ray diffraction patterns were acquired on a Siemens D5005 automatic diffractometer using Cu K␣ radiation (55 kV, 30 mA) selected by a graphite monochromator in the diffracted beam. Silicon powder was used as an internal standard if necessary. 2.4. Catalytic tests Zeolite crystals were pressed into binder-free wafers, crushed and sieved to 30–40 mesh. Typically, a 50 mg sample was diluted with 1 ml inactive quartz grains (30–40 mesh) and packed into a stainless steel down-flow microreactor (10 mm i.d.) placed in a tubular furnace. Additional amounts of quartz (2 ml) were placed on top of the catalyst bed. Before the test a catalyst was heated to 350◦ C at a rate of 100◦ C/h and activated in a helium flow of 30 ml/min for 2 h. Measurements of the m-xylene (99%, Aldrich) transformation were performed in the pulse mode, in order to avoid excess coke formation which can obscure the intrinsic activity and selectivity of the catalysts. (Test for coking carried out for 3% PW/Y0 -imp. sample at 350–450◦ C has shown that 100 injections of m-xylene, made within 3 days, has not led to observable decrease of the catalyst activity.) The products were analyzed with a Perkin Elmer gas chromatograph equipped with a FID detector and a 3 m column

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packed with 8% Bentone-34, 6% didecyl phthalate and 1% silicon oil A on Chromosorb W (60–80 mesh). The following catalysts were tested in this reaction: (i) pure dealuminated zeolites H,Na-Y0 and Y0 (SiCl4 ); (ii) a series of 1, 3 and 10% PW12 impregnated on zeolite H,Na-Y0 and (iii) a sample prepared by in situ synthesis of PW12 in supercages of the dealuminated zeolite Y0 (SiCl4 )-e. [9]. For brevity, the samples containing heteropoly acid impregnated on zeolite Y0 were denoted as %PW/Y0 -imp., while the sample containing encapsulated PW12 in H,Na-Y0 was labelled as PW/Y0 (SiCl4 )-e. The amount of heteropoly acid was related to volume of the zeolite channel system; this volume was taken as 100%. In Table 1 a list of the samples studied is given.

3. Results and discussion Heteropoly oxometalates with the Keggin structure have very strong acid sites of Brønsted and Lewis character. Recent estimation of acid sites distribution of free heteropoly acids H3 PW12 O40 , H4 SiW12 O40 and H3 PMo12 O40 clearly shows that 12-tungstophosphoric acid is stronger than the two other solids, and is characterized by the presence of very strong acid sites (pKa = −16.3, 0.57 meq H+ /g) [10]. When one is trying to prepare supported heteropoly acids, first of all the stability of the support should be considered. A standard zeolite Y with Si/Al = 2.5 is not resistant against strong mineral acids. For example, treatment of faujasite with the HCl solution at pH < 2.3 yielded unavoidably partially crystalline or amorphous samples, which were not thermally stable [11]. We have therefore prepared a zeolitic matrix

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Fig. 1. X-ray diffraction patterns of: (a) H,Na-Y0 , (b) H3 PW12 O40 , (c) 1% PW/Y0 -imp., (d) 3% PW/Y0 -imp., and (e) 10% PW/Y0 -imp.

by the chemical treatment of a zeolite suspension in water with ethylenediaminotetraacetic acid. During this procedure not only aluminium is removed from the framework, but vacancies left behind are filled with the silicon [12]. In this way the structure of faujasite is retained, a Si to Al ratio in the framework is enhanced, and as a consequence the thermal stability of the solid is increased [13]. The dealuminated sample was then impregnated with water or ethyl ether solutions of PW12 . We found that all the samples prepared from the water solution, independently of the conditions used, were unstable. A collapse of the zeolite crystals was evidenced by X-ray studies. Recognising this fact, ethyl ether has been used as a solvent for all further preparation of the supported samples. In Fig. 1 XRD patterns of the zeolitic matrix (a), pure PW12 (b) and three samples with different amounts of 12-tungstophophoric acid are shown

Table 1 Sorption properties of zeolite Y dealuminated by H4 EDTA or SiCl4 treatment and impregnated or encapsulated samples Sample

wt.% of PW12 a

H,Na-Y0 1% PW/Y0 -imp.b 3% PW/Y0 -imp. 10% PW/Y0 -imp. Y0 (SiCl4 ) PW/Y0 (SiCl4 )-ec

0 1.8 5.3 15.7 0 n.d.

(Si/Al)F in dry zeolite H,Na-Y0 4.24

41

BET (m2 /g) 669 664 676 560 223 299

Calculated as wt.% of anhydrous heteropoly acid in the dehydrated zeolite (27.7% weight loss on heating from 20 to 730◦ C). Impregnated. c Encapsulated. a

b

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(c–e). As seen, trace (a) is typical for a siliceous faujasite. Note that the lines are narrow. Also, the baseline is not forming any broad hump, typical for a partly amorphous material. The sample is therefore highly crystalline. This is confirmed further by a very good sorption properties (Table 1). PW12 is characterized by its ‘fingerprint’ XRD pattern (Fig. 1b). All three impregnated samples reveal only a pattern of the zeolite matrix. For the 10% PW/Y0 sample (Fig. 1e) not even the weakest signal ascribable to PW12 could be found. This is in a line with observations made for heteropoly acids supported on SiO2 and MCM-41 where upon adsorption, the X-ray pattern of PW12 disappeared [14]. On the other hand, the zeolite lines are essentially the same as found in the pure matrix (Fig. 1). The intensities recall those of H-Y0 . All the impregnated catalysts exhibit excellent sorption properties (Table 1). As the dealuminated zeolite contains a secondary pore system, with pores ranging from 12 to 50 Å, and with prevailing radius of 15 Å [15], the Keggin units having diameter of about 12 Å will be easily adsorbed in them. From X-ray studies the first important conclusion can be drawn: the dealuminated zeolite retains the structure of faujasite upon contact with a very strong heteropoly acid PW12 . 3.1. Thermogravimetry The DTA measurements for some chosen samples were performed in air (Fig. 2). The samples were heated from ambient to 700◦ C. Pure heteropolyacid H3 PW12 O40 shows two endothermic effects at ca. 115 and 245◦ C, which can be assigned to the removal of crystallisation water and the water molecules hydrating protons, respectively (Fig. 2a). The exothermic effect at ca. 610◦ C results from the decomposition of the Keggin anion and crystallization of WO3 and P2 O5 (Fig. 2a). The DTA trace of zeolite H-Y0 gives a very broad endothermic peak at ca. 180◦ C (Fig. 2b). This effect is due to the removal of physically adsorbed water from the faujasite channel system. There are two endothermic effects in the 3% HPW/Y0 -imp. sample (Fig. 2c). The first one at ca. 100◦ C is very sharp and corresponds to the removal of the heteropoly acid crystallization water. The presence of the second effect at ca. 160◦ C is due to the removal of physically adsorbed water from the zeolite. It is noteworthy that no exothermic effect appears

Fig. 2. DTA curves for samples: (a) H3 PW12 O40 , (b) H,Na-Y0 and (c) 3% PW/Y0 -imp.

for this sample at higher temperatures because, on decomposition of Keggin anions molecularly dispersed at the surface of the zeolite, the crystallization of P2 O5 or WO3 , which would be responsible for the exothermic effect, is hindered. 3.2. IR spectroscopy Four major bands are seen in the IR spectrum of 12-tungstophosphoric acid, at 1080, 982, 892 and 789 cm−1 (Fig. 3a). The lines at 1080 and 982 correspond to stretching vibrations of P–O and W=O groupings, while the two other are due to W–O–W vibrations. In Fig. 3b the IR spectrum of the dealuminated zeolite is depicted, with two strong absorption bands at 1042 and 1149 cm−1 . Assignment of all the other vibration frequencies for the as-prepared and calcined at 650◦ C samples is given in Table 2.

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technique. The IR spectrum of the sample containing Keggin units encapsulated in the supercage of zeolite Y0 was shown elsewhere [9], Fig. 3c. 3.3. NMR spectroscopy

Fig. 3. FTIR Spectra of: (a) pure 12-tungstophosphoric acid, (b) H,Na-Y0 , (c) 10% PW/Y0 -imp., and (d) 10% PW/Y0 -imp., 650◦ C.

Upon impregnation of PW12 (Fig. 3c) a new weak band at 890 cm−1 appears in the spectrum, which is assigned to W–O–W vibrations of the heteropoly acid. Unfortunately, the other bands of PW12 are hidden under the strong zeolite signals. Besides, the amount of PW12 acid is very low, around the detection limit of the infrared technique. After calcination at 650◦ C the band at 896 cm−1 is still seen (Fig. 3d), albeit its intensity is lower and the band is broader, pointing to the stability of supported Keggin units. This finding confirms therefore the results obtained by DTA/TG

In parent zeolite NaY all five possible environments of a silicon atom can be discerned as separate peaks (a spectrum not shown). The 29 Si MAS NMR spectrum of the zeolite dealuminated by H4 EDTA acid is shown in Fig. 4. As the dealumination of NaY proceeds, the intensity of signals corresponding to Si(0Al) and Si(1Al) groupings increases at the expense of remaining ones. Using a known formula [12] the Si/Al ratio in the framework was calculated as 4.24. A weak signal at −112.7 ppm is assigned to the amorphous highly siliceous part of the sample formed during the H4 EDTA treatment. A 27 Al MAS NMR spectrum reveals (Fig. 5) that the zeolitic matrix contains predominantly tetrahedral framework aluminium (AlFT ). The other signal at −0.9 ppm is due to small amounts of non-framework octahedral aluminium (AlNO ). The sample contains 98.0% of AlFT and 2.0% of AlNO . The 31 P MAS NMR spectrum of PW12 acid shows a single sharp signal at −15.35 ppm and FWHM = 0.1 ppm (Fig. 6a), in agreement with other studies [14,16]. Upon supporting PW12 by the impregnation method, two separate lines of 31 P are observed (Fig. 6b–d). The lines at −15.0 corresponds to the intact Keggin units, weakly interacting with the support, which manifests itself by 0.3 ppm down-field shift of the peak center. The second line at −13.3 ppm may correspond either to a chemically modified Keggin anion, for example a species [14] like P2 W18 or

Table 2 Infrared frequencies corresponding to framework vibrations of parent zeolite H,Na-Y0 and the sample after impregnation, before and after calcination at 650◦ C. The frequencies in the column PW12 relate to the W–O–W vibration of the heteropoly acid Sample

Vibration frequencies (cm−1 ) Asymmetric stretch

PW12

Symmetric stretch

D6R

T–O bend

H,Na-Y0 20◦ C 650◦ C

1149 1170

1042 1054

– –

805 813

726 747

587 588

513 514

456 456

10%PW/Y0 -imp. 1155 20◦ C 650◦ C 1173

1042 1052

890 896

802 819

729 742

587 593

513 515

456 456

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zeolite Y0 with encapsulated acid were tested in transformation of m-xylene. A reference reaction with pure zeolitic matrix H, Na-Y0 was also carried out. Transformation of m-xylene encompasses two main reactions proceeding on acid centres, isomerization and transalkylation (disproportionation) of m-xylene. The isomerization of m-xylene is an industrially important process, it is also a test reaction for shape-selective catalysts. Isomerization is a typical reversible reaction. Starting from any of the three xylene isomers, the system proceeds towards the equilibrium (e.g. at 427◦ C 24.4% p-xylene, 23.4% o-xylene and 52.2% m-xylene [18]). Isomerization may occur according to a unimolecular (by consecutive 1,2-methyl shift) or a bimolecular mechanism. The second reaction catalyzed by acidic centers is transalkylation of two xylene molecules to toluene and three trimethylbenzenes. Bimolecular disproportionation of alkyl aromatic molecules is mostly governed by the intrinsic properties of zeolite and proceed much more easily in large pore zeolites such as zeolites Y, L and beta, where bulky transition state complexes can be accommodated, than in zeolites with narrow pores, Fig. 4. Solid-state 29 Si MAS NMR spectrum of the dealuminated sample H,Na-Y0 (spinning rate 6 kHz ).

P2 W21 [14], or PW12 interacting with the support OH groups [17]. Both lines are significantly broader, having FWHH = 0.9 ppm. This can be explained by the distribution of isotropic chemical shifts of 31 P due to various electronic environments of PW12 adsorbed in the mesopores of the zeolite. In particular, different sizes of mesopores, ranging from 12 to 50 Å [15], as well as the gradient of the Al concentration from the surface to the interior of the zeolite [12] may contribute to the heterogeneity of the adsorption sites, leading to the distribution of isotropic chemical shifts. The relative intensities of the two signals change from 54 : 46 to 63 : 37 for 1 and 10% samples, respectively. With the further increase of the amount of PW12 up to 33% (not shown), the relative intensity of the line at −15.0 ppm consistently increases to 80% and becomes dominating. 3.4. Catalytic properties Catalytic properties of zeolite Y0 impregnated with different amounts of 12-tungstophosphoric acid and

Fig. 5. Solid-state 27 Al. MAS NMR spectrum of the dealuminated sample H,Na-Y0 (spinning rate 6 kHz).

K. Pamin et al. / Applied Catalysis A: General 194 –195 (2000) 137–146

Fig. 6. Solid-state 31 P MAS NMR spectra of the samples: (a) PW12 , (b) 1% PW/Y0 -imp., (c) 3% PW/Y0 -imp., and (d) 10% PW/Y0 -imp. (spinning rate 2 kHz).

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such as ZSM-5, ZSM-35 or NU-10. This reaction requires the presence of stronger acid sites. In addition to isomerization and transalkylation, a third pathway involves dealkylation of alkylaromatic hydrocarbons. Thus, dealkylation of the xylene molecule yields toluene, which may be dealkylated further to benzene. The selectivity of m-xylene transformation along the three pathways: therefore reflects the intrinsic properties of the catalyst, particularly the strength and concentration of acid sites. In Fig. 7 the total conversion of m-xylene observed at 350◦ C is plotted as a function of the PW12 loading at the surface of zeolite Y0 . It may be seen that impregnation of the zeolite with 1% of PW12 increases the conversion of m-xylene by a factor of 2, but on further increasing the surface coverage the rise of activity is smaller so that at 10% of PW12 on the surface of the zeolite the conversion of m-xylene reaches the value of about 60 %. Fig. 8 shows the temperature dependence of m-xylene conversion, plotted in ln x versus 1/T coordinates. Small value of the activation energy observed in the case of pure PW12 at higher temperatures and in the case of samples impregnated with PW12 seems to indicate that diffusion may be an important factor in limiting the rate of the reaction. The selectivities of m-xylene transformation by the isomerization, disproportionation and dealkylation pathways are plotted in Fig. 9 as a function of the surface loading for the reaction carried out at

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Fig. 7. Overall m-xylene conversion on the dealuminated zeolite matrix H,Na-Y0 and impregnated samples.

380◦ C. Zeolite H,Na-Y0 is the most active catalyst for isomerization of m-xylene, the selectivity being higher than 80% at 40% conversion, and simultaneously has the lowest selectivity of m-xylene disproportiona-

tion. Note that the zeolitic matrix can not dealkylate m-xylene molecules. Deposition of PW12 results in dramatic increase of the selectivity to disproportionation products, which for sample containing 20%

Fig. 8. Overall conversion of m-xylene on: (a) dealuminated zeolite Y0 , (b) 1% PW/Y0 -imp., (c) 3% PW/Y0 -imp., and (d) 10% PW/Y0 -imp.

Fig. 9. Selectivity of m-xylene conversion along three pathways: (a) dealkylation, (b) isomerization, and (c) disproportionation.

K. Pamin et al. / Applied Catalysis A: General 194 –195 (2000) 137–146

Fig. 10. Selectivity of the m-xylene transformation towards oand p-xylene on: 10% PW/Y’-imp., (b) 3% PW/Y’-imp., (c) 1% PW/Y’-imp., and (d) the dealuminated zeolite H, NA-Y’.

of PW12 attains the value of more than 60%. Some dealkylation activity also appears as the result of the ability of surface Keggin anions to cleave the C–C bonds. Simultaneously the selectivity of isomerization drops to less than 30%. Figs. 10 and 11 illustrate the temperature dependence of selectivities to isomerization and disproportionation products, respectively. It may be seen that for all samples the selectivity of disproportionation increases with temperature, whereas that of isomerization decreases. The two reactions are parallel ones, and the energy of activation is higher for m-xylene disproportionation. As a consequence, isomerization dominates at lower temperatures, while disproportionation at higher temperatures, as indeed found experimentally. Zeolite H,Na-Y0 is the most selective for isomerization (Fig. 10d). The ratio of p-xylene to o-xylene isomer in the product is close to the value corresponding to thermodynamic equilibrium [18] which indicates that no shape-selectivity effect is observed. Upon loading with the PW12 acid, the selectivity of m-xylene isomerization steadily decreases. The opposite trend

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Fig. 11. Selectivity of the m-xylene transformation towards toluene + trimethylbenzenes. The labels as in Fig. 8.

is seen for disproportionation (Fig. 11). The pure matrix exhibits no disproportionation selectivity. Upon loading with PW12 a pronounced disproportionation selectivity was generated for 1, 5 and 10% catalysts (Fig. 11b–d). For completeness, in Fig. 12 we have shown the isomerization and disproportionation selectivities for the catalyst PW/Y0 (SiCl4 )-e. containing the Keggin units encapsulated in faujasite supercages [9]. As before, the isomerization selectivity of the dealuminated zeolite is the highest, 70–80% (Fig. 12c), and drops to around 50% upon incorporation of the Keggin units into the catalyst (Curve b).

4. Conclusions Discussion of the results enables the following conclusions to be drawn. • The dealuminated zeolite upon contact with a very strong heteropolyacid H3 PW12 O40 retains the structure of faujasite.

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hydrocarbons proceeds on the acid sites with higher strength [19]. Obviously, the more acid is supported in the zeolite mesopore system, the higher concentration of acid sites per unit area of the support is obtained. Such a situation favours transformation of m-xylene molecules by a bimolecular pathway required for the disproportionation process. As the diameter of the mesopores is large enough for the xylene molecules to move freely, no shape selectivity effect appears, as indicated by the observed ratio of p-xylene to o-xylene isomers, close to that corresponding to the thermodynamic equilibrium. References

Fig. 12. Selectivity of m-xylene disproportionation (D) and isomerization (I) on Y0 (SiCl4 ) (c) and PW/Y0 (SiCl4 )-e. (a, b).

• The Keggin anions molecularly dispersed at the zeolite surface and in its mesopores become thermally stabilized and remain stable even after calcination at 650◦ C in contrast to pure H3 PW12 O40 which decomposes on heating at 610◦ C. • Two types of Keggin anions exist at the surface: those interacting with the OH groups of the support and hence chemically modified, prevailing at low coverages, and those weakly interacting with the support and practically unmodified, prevailing at high coverages. • Pure zeolite is an active catalyst for isomerization of m-xylene, but practically no disproportionation to toluene and trimethylbenzenes is observed. Deposition of H3 PW12 O40 results in a dramatic increase of selectivity to disproportionation, which rises with growing surface coverage. The isomerization path is simultaneously suppressed. This is due to the fact that disproportionation reaction of alkylaromatic

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