J . Chern. Soc., Faraday Trans. I, 1985,81, 1161-1 166

Catalytic Properties of H Mordenite Modified with Fluorine

BY KURT A. BECKER

Fritz-Haber-Institut der Max-Planck-Gesellschaft, Abteilung Grenzflachenreaktionen, Faradayweg 4-6, 1000 West Berlin 33, Dahlem

AND STANSILAW KOWALAK*

Faculty of Chemistry, A. Mickiewicz University, Grunwaldzka 6, 60-780 Poznan, Poland

Received 18th June, 1984

H mordenite has been modified using ammonium fluoride solutions and gaseous CHF,. During treatment, some of the acidic OH groups were substituted with fluorine and the acid strength of the remaining hydroxyls was enhanced because of the inductive effect of the fluorine. This was reflected in the higher catalytic activity of the samples for cumene cracking and the higher values of the heats of adsorption for ammonia.

Fluorine, the most electro-negative element, is often used as a component or modifier of cata1ysts.l For example, the properties of fluorinated alumina have been

Aneke et ~ 1 . ~ 9 * have studied a catalyst consisting of H-Y zeolite, /I-AlF, and Cu, and have found that the presence of fluoride resulted in an increase in the selectivity for toluene disproportionation, Lok et aL99 lo modified zeolites using F, gas, Araya and Dyerll have described the treatment of zeolites with molten fluorides and Sariev and c o w o r k e r ~ ~ ~ - ~ ~ have reported the modification of zeolite catalysts with diluted hydrofluoric acid, which usually increased the catalytic activity. Information concerning zeolite catalysts modified with fluorine can also be found in the patent literature.

In our recent investigati~nsl~~ l6 we have dealt with the fluorinated aluminium- containing faujasite. The modified zeolite was more active than the parent A13+ form and sometimes more active than the H form. It is clear, however, that this system is not just a mixture of H zeolite and AlF, in the intracrystalline space and the presence of other Al-F species must also be considered.

We believe that on the fluorination of H zeolites we would be able to obtain a less complicated system, in which OH groups would be substituted by fluorine. We have chosen to investigate H mordenite first because of its higher stability; work on H faujasite is in progress.

EXPERIMENTAL Commercial H mordenite (Norton, lot 2) was used as the starting material. Its chemical

composition (wt %) was as follows: SiO,, 86.14; Al,O,, 13.45; Na,O, 0.41; Si/A1, 5.43. The theoretical formula of the unit cell was H8(A102)8(SiOz)40 .mH20. A 0.1 mol dm-, aqueous solution of NH,F was used for the fluorination, which took place at room temperature in Teflon beakers. During this process the pH of the solution increased, as in the experiment of Breck and Skeels: e.g. after fluorination of sample HMF-2, the final pH of the solution was 5.3, whereas the pH of the suspension of H mordenite in water was 4.5. The samples were separated

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1162 CATALYTIC PROPERTIES OF F-MODIFIED H MORDENITE

Table 1. Fluorination procedures and concentration of fluorine in mordenite samples

fluorine content (% 1

sample fluorination conditions a b

- HMF-0 - -

HMF-1 0.43 8.1 HMF-2 0.56 10.1

HMF-3 0.76 14.4

HMF-4 0.80 15.1 HMF-5 1.20 22.7 HMF-G 0.25 4.7

5.0 g of zeolite, 100 cm3 of 0.1 mol dm-, NH,F solution, 20 h 5.0 g of zeolite, 100 cm3 of 0.1 mol dm-3 NH,F solution, 20 h,

5.0 g of zeolite, 500 cm3 of 0.1 mol dm-, NH,F solution, 100 cm3

10 g of zeolite, 500 cm3 of 0.3 mol dmP3 NH,F solution, 20 h 10 g of zeolite, 200 cm3 of 1.2 mol dm-, NH,F solution, 20 h 10 g of H mordenite calcined at 450 "C for 4 h in helium stream (100 cm3 min-l), then CHF, (10 cm3 min-l) added to the gas stream for 20 min, followed by heating in the helium stream for 20 min and finally cooling in the helium stream

then 100 cm3 of the same solution for 20 h

of NH,F solution changed every 12 h

a Wt % of fluorine introduced to the mordenite sample; % of OH groups in zeolite substituted with fluorine.

from the solution by centrifugation, dried without prior washing and finally calcined in air at 450 "C for 12 h. Residual NHt cations in the zeolite before calcination were detected by i.r. spectroscopy. The NH; band did not vanish after washing the sample, which means that some cation exchange occurs during fluorination. The method of gaseous fluorination, similar to that used by McVicker et for alumina reactions, was also used. The parameters for the fluorination process and the compositions of the modified samples are summarized in table 1.

The amount of fluorine introduced to the mordenite was estimated using an ion-selective electrode. Fluoride anions were removed from the zeolite phase by alkaline hydrolysis with NaOH solution.

The presence of aluminium in the solution after the fluorine treatment showed that some aluminium is removed from the mordenite framework. The crystallinity of the modified samples was examined by X-ray diffraction and compared with that of the original H mordenite. The catalytic activity of the modified samples was tested using the cumene cracking reaction. The pulse microreactor was attached directly to a gas chromatograph. The samples (0.01 g) were activated at 450 "C for 0.5 h before being tested. The cumene cracking reaction was carried out at 350 "C in a helium stream (100 cm3 min-l) and 0.5 mm3 pulses were used. Usually only dealkylation products were found in the mixture, but some propene deficiency was seen. The results of the catalytic tests are shown in fig. 1 and 2. Cumene cracking is a well known test for the presence of Brarnsted acid sites. The acid sites were investigated by microcalorimetric measurements of the differential heats of ammonia adsorption using a Calvet twin micro- calorimeter. The method is based on the assumption that the acid strength of active sites is reflected in the amount of the heat liberated by adsorption of a base on this site. The sample (ca. 1 g) was activated at 450 "C under vacuum Torr) for 12 h before measurements were made. Ammonia was then added stepwise in small increments to the sample. The amount of ammonia adsorbed was estimated for the calculation of the differential heats of adsorption. The heats of adsorption for the fluorinated sample are compared with those determined for the initial H mordenite in fig. 3.

1.r. measurements were performed with a Perkin-Elmer model 180 spectrometer. The zeolite sample was compacted into an i.r.-transparent wafer and activated in a conventional vacuum cell at a pressure of Torr at 450 "C for 6 h. Pyridine vapour was adsorbed at room

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K. A. BECKER AND S. KOWALAK 1163

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Fig. 1.

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 fluorine content (wt %)

Fig. 2.

Fig. 1. Cumene conversion for selected samples as a function of pulse number: ., HMF-1;

Fig. 2. Correlation between fluorine content in the catalyst sample and cumene conversion.

v, HMF-G; 0, HMF-0; +, HMF-5.

temperature and then desorbed under vacuum at 250 "C for 3 h. 1.r. spectra in the region of the stretching vibrations of OH groups show some decrease in OH band intensity after introducing fluorine. More significant changes occurred in the intensities of the bands originating from the pyridinium ion (1540 cm-l), formed as a result of the interaction of pyridine with acidic hydroxyl groups, and from pyridine coordinatively bound to Lewis-acid centres (1450 cm-l). Results obtained from the i.r. spectroscopic measurements of the samples after chemisorption of pyridine are given in table 2, where the ratio of intensities of the bands at 1540 and 1450 cm-l is taken as a measure of the contribution of Br~rnsted- and Lewis-acid sites, respectively, to catalytic acidity. Such an approach is based on the fact that the ratio of the extinction coefficients of these bands is ca. 1.6

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1164 CATALYTIC PROPERTIES OF F-MODIFIED H MORDENITE

170 - I - z 1 6 0 -

I? 150-

24 1 a

0 .-

1 4 0 - 8 -0

L

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0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 amount of ammonia adsorbed, nid /mmol g-'

Fig. 3. Comparison of the differential heats of ammonia adsorption on H mordenite (--- and 0, HMF-0) and fluorine-modified H mordenite (- and 0, HMF-1).

Table 2. Effect of fluorination of the Brsnsted-acid to Lewis-acid ratio

ratio of the

intensities of bands at 1540

sample and 1450 cm-l

HMF-0 5.3 HMF-G 4.1 HMF- 1 2.7 HMF-4 2.0 HMF-5 1.6

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K . A. BECKER AND S. KOWALAK 1165

RESULTS AND DISCUSSION

Both gaseous (using CHF,) and solution (with NH,F) methods of fluorination yield the fluorine modification without noticeably changing the crystallinity of the zeolite. Even concentrated NH,F solutions do not damage the mordenite structure. X-ray diffraction patterns were almost the same for the modified samples and the original H mordenite, with only small differences in intensity. Gas treatment was carried out at temperatures < 450 "C in order to avoid dehydroxylation.

Fluorination experiments using NH,F were similar to the experiments of Breck and Skeels,17 who used K F solution to substitute OH groups in zeolite. They observed an increase in pH during this treatment and we have found the same tendency in our experiments. Breck and Skeels did not find any fluorine in the zeolite phase and suggested that it may be present as AlF, in the supernatant solution after fluoride treatment. However, we did find fluorine in the zeolite phase.

Breck and Skeels discussed two possible schemes for the fluoride treatment:

zeolite OH + F-(aq) + zeolite F + OH-(aq) (1)

zeolite Al(OH)& + 3KF (aq) --+ AlF,(aq) + xK+zeolite + (3 - x)KOH(aq) (2)

and have suggested that reaction (2) actually occurs. However, the presence of fluorine in our zeolite samples suggests reaction (1). The catalytic activity of fluorine-treated samples increases very sharply; the highest activity was found for samples containing up to 0.4 wt % fluorine, which would mean that ca. 8% of the OH groups in the mordenite are substituted by fluorine. A further increase of fluorine content results in a decline of activity because of the decrease in the concentration of Brsnsted-acid sites. The sample richest in fluorine (> 15% of hydroxyl groups substituted by fluorine) was less active than the parent H mordenite.

We suggest that the enhanced activity results from the inductive effect of fluorine on the acidity of adjacent hydroxyl groups. Introduction of fluorine reduces the absolute number of OH groups but increases the acid strength of those remaining. It is very likely that fluorine is localized preferentially in the channel sites and that after filling these sites it occupies less accessible positions. The reduced activity of the sample richest in fluorine can result from the deficiency of acid OH groups in the channel positions which are accessible to the cumene molecules (although the total number of OH groups is still high). The results of the microcalorimetric measurements corroborate the results of the catalytic tests. The 'heats of adsorption of ammonia at low surface coverage, particularly for the first adsorption step, are significantly higher for .;.he fluorinated sample (ca. 20 J g-l). This is clear evidence of the creation of stronger acid centres. The spectra confirm our conclusions concerning the substitution of hydroxyl groups with fluorine, since the intensity of the OH bands diminishes with increasing fluorine content. The i.r. spectra of samples covered with chemisorbed pyridine species show a decrease in the Brsnsted-acid to Lewis-acid ratio with increasing fluorine content in mordenite, which is further evidence of the substitution of OH groups (Brsnsted sites) with fluorine resulting in the formation of new Lewis sites (AlF, or other Al-F species).

The nature of the fluorine inductive effect is probably the same as it is in fluorinated alumina. The protonic acidity of the alumina hydroxyl groups is very weak, but after partial substitution with fluorine quite strong Brsnsted acidity was l9 In the case of H mordenite, protonic centres already exist, but their strength is increased by fluorine.

Because some of the hydroxyl groups in mordenite can be combined with A1

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1166 CATALYTIC PROPERTIES OF F-MODIFIED H MORDENITE

cationsz0 it is very likely that after the reaction with fluorine-containing compounds, AlF, or other Al-F species can remain inside the zeolite structure. Therefore the contribution of the hydroxo-aluminium cations to the catalytic activity should be increased by the influence of fluorine. In recent investigations we have studied the A1 form of faujasite modified with fluorine.l5?l6 The catalytic activity of the A1 form was increased after fluorination. If the supposition of Breck and Skeels17 concerning the role of the hydroxo-aluminium cations in the catalytic activity is correct, then the activity of the A1 form should be comparable to that of the H form. In our experiments the activity of the A1Y form is similar to that of the hydrogen form after fluorination. Thus the strength of the hydroxyl groups combined with A1 cations is lower than that of framework OH groups, and we conclude that fluorine is a very effective agent for the modification of zeolitic catalysts for acid-catalysed reactions. reactions .

Substitution of the framework OH groups by fluorine is probably the main reaction during the fluorination of H mordenite. However, other reactions, such as cation exchange, dealumination and reactions of fluorine with Al-bearing cations, should also be taken into consideration.

We thank Mrs U. Kockeritz and Mr W. Kollmitt for their assistance with the experiments and Dr J. Klinowski, University of Cambridge for his critical reading and helpful discussion of the manuscript.

V. R. Choudhary, Znd. Eng. Chem. Prod. Res. Dev., 1977, 16, 12. T. V. Antipina, 0. V. Bulgakov and A. V. Uvarov, in Proc. 4th Znt. Congr. Catal. (Akademiai Kiad6, Budapest, 1971), vol. 2, p. 306. H. J. Reitsma and C. Boelhouwer, J. Catal., 1974, 33, 39. R. Covini, V. Fattore and N. Giordano, J. Catal., 1967, 9, 315. V. C. F. Holm and A. Clark, J. Catal., 1967, 8, 286. F. P. J. M. Kerkhof, J. C. Oudejeans, J. A. Moulijn and E. R. A. Matulewicz, J. Colloid Interface Sci., 1980, 77, 120. L. E. Aneke, L. A. Gerritsen, P. J. van der Berg and W. A. de Jong, J. Catal., 1979, 59, 26. L. E. Aneke, L. A. Gerritsen, J. Eilers and R. Trian, J. Catal., 1979, 59, 37. B. M. Lok and T. P. J. Izod, Zeolites, 1982, 2, 66.

218, 41. A. Araya and A. Dyer, Zeolites, 1981, 1, 35.

Sofia, 1979), p. 391.

lo B. M. Lok, F. P. Gortsema, C. A. Messina, H. Rastelli and T. P. J. Izod, ACS Symp. Ser., 1983,

l2 I. T. Sariev and V. Penchev, in Proc. 4th Znt. Conf. Hetero. Catal. (Bulgarian Academy of Sciences,

l 3 V. Penchev, I. T. Sariev and M. Zhelazkova, Kinet. Katal., 1981, 22, 732. l4 M. D. Zhelazkova, I. T. Sariev and S. A. Koralska, in Proc. 5th Int. Symp. Hetero. Catal. (Bulgarian

l5 S. Kowalak, React. Kinet. Catal. Lett., in press. l6 K. A. Becker and S. Kowalak, to be published. l7 D. W. Breck and G. W. Skeels, in Proc. 6th Int. Congr. Catal. (The Chemical Society, London, 1977),

Academy of Sciences, Sofia, 1983), p. 467.

vol. 2, p. 645. G. B. McVicker, C. J. Kim and J. J. Eggert, J. Catal., 1983, 80, 315. S. Kowalak, Acta Chim. Acad. Sci. Hung., 1981, 107, 27.

2o R. M. Barrer and J. Klinowski, J. Chem. SOC., Faraday Trans. 1 , 1975,71, 690.

(PAPER 4/1037)

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