Datka, J.

Tuznik, E.

English

Datka J.

Tuznik E.

University of Szeged

THE STRENGTH OF OH GROUPS IN NaHZSM-5 ZEOLITES STUDIED BY IB SPECTROSCOPY J. DATE», B* Tuinik Faculty of Chemistry, Jagiellonian University, 30-060 Cracow, Karaaia 3, Poland ABSTRACT The acid strength of OH groups in NaHZSU»^ zeolites increases with the cation exchange degree. It has been evidenced by the results of the studies of thermodesorption of pyridine and of the studies of extinction coefficient of OH groups. This effect is rather unexpected because in highly siliceous zeolites like ZSM-5, the concentration of Na+and Hfis very Ion, the distances between them are long and influence of the exchange Na+ - H+ on the properties of OH groups should be neglegible. Two explanation of this disagreement are proposed, INTRODUCTION Acidity is a very important parameter determining the catslytio properties of zeolites. There are numerous methods of modification of acidity of zeolites, such as the modification of cation exchange degree, change of Si/Al molar ratio, partial dehydroxylation, partial poisoning of acid sites etc. The effects of these modifications on the acid and catalytic properties of zeolites are reviewed in the Jacobs' monograph The present paper concerns the modification of the acid pro-perties of NaHZSifr-5 zeolites by the variation of cation exchange degree. In NaHY zeolites the increase of cation exchange degree increased the number of OH groups, their acid strength and also their catalytic activity (for example ref. 1-5)» In ZS14-5 zeolites, the situation is more complicated. T.p.d» studies of NH^ desorption showed that <6 H<jes values increase with cation exchange degree Similar aituation was also observed in mordenites £7] in which the temperature of t»p.d. peak of NH^ de-sorption does also increase with the cation exchange degree« All these results suggest that in ZSM-5 zeolites and also in mordenites the strength of acid sites inoreaaes with the exchange degree (similarly, as ia 7 zeolites). Contrary I d HaHI zeolites, in ZSM-5 the catalytic activity do oot show the same treads as the acidity. Ia n-hexane cracking [6], o-xylene isomerization [8,9], and in dimethylether conversion [9] the rate constant increase linearly with the number of strong acid sites, but no correlation nas found b etweeo the rate of o-hexane cracking and of NHj desorptioo [6]. Such lack of correlation between the strength of acid sites and their catalytic activity was interpreted assigning the existence of 8 threshold level for the acid strength [6^. Above this level the acid strength does not influence any more the catalytic activity of zeolites. The aim of the present study nas to follow the dependence the acid strength of OH groups in NaHZS№-5 zeolites on the cation exchangt degree» In a previous study [10,11], it has been found that two kinds of BrOnsted acid sites exist in NaHZSM-5 zeolites : 3609 cm"1 OH grouj being strong sites and another BrBnsted sites of lower acidity* The concentrations of both kinds of scid sites were determined. In the present study the efforts were made to obtain the information »•1 concerning the properties of 3609 cm OH groups only. The acid strength of these OH groups was characterized by the i.r. spectro-scopic studies of pyridine thermodesorptioo and by the comparison of extinction coefficients of OH bands. Two series of NbHZSM-5 zeolites were studied. One of them was obtained by the ion exchange of parent № form in the HC1 solution (H.2 series) and another ere obtained by the exchange of Na form in HH^KOj solution (NZ series). EXPBRDfB OTA L KbHZSM-5 zeolites were obtained from a parent sodium form ("Utrazet") sample prepared at the Institute of Industrial Chemistry (Warsaw), the composition of which was : U b ^ ^ H q (Si/Al ratio a 47, and Na/Al = 0.85). Two series of hydrogen forms NaHZSM zeolites were prepared : one by the exchange of Na+ into NH^ ions in IJĤ NOj solution and another one by the exchange in HC1 solutions. Various exchsnge degrees were obtained by using various concentrations of NH^HOj and HC1 solutions. The highest concentrations of these solutions were 1 m and 0.5 m respectively. After the ionic exchange, the zeolites were washed and dried at 390 E. In order to calculate the exchange degrees, the zeolites were dissolved in HP solution and sodium content determined by atomic absorption. The zeolites obtained by the HC1 treatment were denoted as : HZ-41, HZ-81, HZ-88 (exchange degree 41, 81, 88 % rasp.), and the zeolites obtained by the NH^NOj treatment and further decom-position of NH^ ions aa : NZ-43, NZ-83 (exchange degrees 43 and b3 resp.). For the i.r. spectroscopic studies the zeolites were pressed 2 into thin wafers (3-10 mg/cm ). The wafers inserted into i.r. cell were pretreated in vacuo at 450°C for 1 h. At these conditions complete water desorption and decomposition of NH^ ions took place without any appreciable dehydroxylation. Pyridine (POCh Gliwice, analytical grade wss dried over KOH. The i.r. spectra were recorded with a SPBCOBD 75 IR spectrometer. The accumulation and subtraction of spectra were done with a KSR 4100 minicomputer working on-line with the spectrometer. RESUI/TS Adsorption of pyridine on NaHZSM-5 zeolites. Two distinct OH bands are present in the i.r. spectrum of NaHZS№-5 zeolites : at —1 3609 and 3738 cm • Adsorption of pyridine results in the formation of PyH+ ions (1545 cm"1), PyL and PyNa+ complexes (1450 and 1444 cm bands). First portions of pyridine sorbed in zeolite react with 3609 cm""1 OH groups forming PyH+ ions. The intensity of 3609 cm""1 OH —1 + band decreases linearly and the intensity of 15^5 cm PyH bands in-creases linearly with the amount of pyridine sorbed. After the con-sumption of all 3609 cm OH groups the next introduced pyridine molecules react with weaker Br8nsted acid sites forming additionally PyH+ ions and 154-5 cm'"'1 PyH+ bsnd continues to increase. The desorption of pyridine removes the molecules bonded with + —1 weak sites at first, PyH bsnd diminishes, but 3609 cm OH band is still absent. The pyridine molecules neutralizing 3609 cm ' OH groups are removed at higher temperatures. The amount of PyH+ ions decomposed by the desorption before the 3609 cm"1 OH band reappeared was taken afi corresponding to the amount of weak £r(5nsted acid sites, and the remainder of PyH+ ions as corresponding to the amount of 3609 cm OH groups. The concentration of both kinds of BrOnsted acid sites was determined in NaHZSM-5 zeolites of various exchange degrees £ 10,11.], i.r. studies of thermodesorption of pyridine. In order to stud; the acid strength of 3609 cm OH groups the small portions of pyridine were sorbed at 440 K in the zeolite up to the disappearance of this OH band (the excess of pyridine was avoided). The spectrum was then recorded and next the cell wss connected to the liquid nitrogen trap and heated to higher temperatures (in the range 620 -820 E). After each dasorption step the cell was cooled down to 440 K —1 + and the spectrum recorded. The intensities of 1545 cm FyH band were measured after the desorptions of two temperatures : at the te№ perature at which 5609 cm OH band begins to reappear (AQ) and after the desorption at 790 K (A790)• The ratio ̂ 790^0 r0Pr8seafcs ttuB fraction of 3609 cm"1 OH groups which still hold pyridine after the desorption at 790 K. These values which were taken as the measure of the strength of 3609 cm OH groups are presented at the Table. In both series of zeolites HZ and NZ the acid strength of 3609 cm measured by the pyridine thermodesorption increase with the cation exchange degree. The calculation of extinction coefficient of 3609 cm""1 OH band In order to calculate the values of integrated extinction coefficient (IEC) at 3609 cm OH b8nd, the values of the integrated intensities of these bsnds were measured in the spectra of freshly activated zeolites. The concentrations of these OH groups were determined by the pyridine adsorption and desorption (as described above). The values of IEC of 3609 cm OH bands calculated in such a way are presented in the Table. In both series of zeolites they increase with the exchange degree of zeolites. Table —1 AygQ/AQ values and integrated extinction coefficients of 3609 cm OH groups Zeolite x A790/Ao integr. ext. coeff. cm mol HZ - 43 0*22 1.8 NZ - 83 0.72 2.V HZ - 41 0.17 2.3 HZ - 81 0.40 2.8 HZ - 88 0.45 2.'/ x the values of cation exchange degrees are given in the symbols of zeolites. DISCUSSION —1 The information concerning the ecid strength of 3609 cm OH groups nas obtained in the studies of pyridine thermodesorption and by the comparison of the values of extinction coefficients. The>; results of the studies of pyridine thermodesorption (pre-—1 sented at the Table) show that the acid strength of 3609 cm OH groups increases with the cation exchange degree of NaHZSM-5 zeolites« This statement is based on the increase of ̂ go^o values, which re-presents the fraction of 3609 cm"*1 OH groups nhich still hold pyridine after the desorption at 790 K« The integrated extinction coefficient (IBC) of 3609 cm""1 OH band doea also increase nith the exchange degree, which can be interpreted as the increase of the acid strength of these OH groups. The values of ISC of OH i«r« bends can be successfully used as the meaaure of the strength of OH groups. Several authors [3,4, 1 2 - 1 5 J formulated the coo« elusions on the strength of OH groups by the comparison of the values of extinction coefficients and the results obteined in such a way were consistent with the result obtained by othera methods« Recently the quantuifr-chemical calculations [1б] showed that in the clusters HjllOHBtHj where the H atoms were successively substituted by fluorine, the values of dipole moment derivative ( i n c r e a s e with the positive change on hydrogen in OH groups and also with, the ionicity of 0 - H bond. The results of the studies of both thermodesorption of pyridine and extinction coefficients lead to the same conclusion that acid _ л strength of 3609 cm OH groups increase with the cation exchange degree. Such effect was already observed in NaHY zeolites £2-4 J end was interpreted as a consequence of an introduction of more electrophilic protons instead of less electrophilic Na + ions. It results in an in-crease of the polarization in neighbouring OH groups. The same pheno-menon can be also explained in the terms of collective model of acidity of zeolites £l7]« Hydrogen is more electronegetive than sodium (electronegativity 3*55 and 0.70 in the Senderson scale) ; the in-crease of cation exchange degree increases the average electronega-tivity of zeolite and the poaitive change on hydrogen in OH groups. In highly-siliceous zeolites as ZS№-5 the same trends but of muoh lower extent are expected. In ZSM-5 (Si/11 = 47) the average concentration of 11 is much lower than in X (Si/11 > 2.5). The coo-centra tion of Nfe* and of protons is also much lower, the distences 1 nn between them much longer, 8nd therefore the exchange Na+ - H+ should have neglegible affect on the properties of OH groups« The same con-clusion can be drown if considering the collective model of acidity of zeolites. In ZSM-5, the exchange of all Na + to protons results in much lower variation of average electronegativity (ASa„ = 0.033) B Y than in Y zeolite ( 4S0„ = 0.54). sv The results obtained at the present study are not in agreement with such previsions. The increase of the strength of OH groups in NaHZSM-5 zeolites is quite remarkable. Two explanations of this dis-agreement can be proposed at present : 1. in highly-siliceous zeolites like ZSM-5 the OH groups are stroa ger acid sites than in Y. The 0-H bond is more polarized and is mora sensitive to even very weak electrostatic influences« 2. in ZSM-5 zeolites the ¿1 distribution is not homogeneous, there are domaines of lower and of higher ¿1 concentration. In the domaines of higher A1 concentration the inductivs effects are stronger and the exchange Na+ - H+ influences the strength of neighbouring OH groups. It should be noticed that several autho: reported non-homogeneous ¿1 distribution inside the crystals [18,19] and among the ZSM-5 crystals of various shapes REFERENCES I. Jacobs P.A., "Carbohiogenic Activity of Zeolites''. Elsevier Sci Publ. Co., Amsterdam, 1977. 2« Barthomeuf D., Beaumont R., J« Catal., ¿0, 288 (1973). 3. BielaAski A., Datka J., J. Catal., 383 (1975). 4. Datka J., J.C.S. Faraday I, £6, 2437, (1980). 5. Blelanski A., Datka J., Drelinkiewicz A., Malecka A., Acta Physii and Chemica Szegediensis 24, 89, (1978). 6. Post J.G., van Hooff J.H.C., Zeolites, 4, 9, (1984). 7. Hidalgo C.V., Itoh H., Hattori M.N«, Morakaai Y., J. Catal., 85, 362, (1984). 8. Nayak V.S., Choudhary V.S., J. Catal., 81, 26, (1983). 9. Guisnet M., Cormeraia F.X., Chen Y.S., Perot G., Praund E., Zeolites 4, 108, (1984). 10» Datka J., Tuinik E., to be published. II. Datka J., Wiss. Zeitschrift FEU Jena, in press. 12. Jacobs P., Ballmoos R.V., J. Phys. Cham,, 86, 3050, (1982). 13. Datka J., J. Chem. Soc. Faraday I, ¿6, 705, (1980). 14. Datka J., J. Chem. Soc. Faradey I, 21» 51H, (1981). 15. Jacobs P«A., Catal. Eev. Sci. Eng., 24, 3, (1982). 16. Datks J., Geerlings P., Mortier W.J., Jacobs P.A., to be published. 17. Mortier W.J., J. Catal., 1 3 8 , (1978). 18» Lyman C.E., Betterdge P.W., Morao E.F., "Intrazeolite Chemistry". ACS Symposium Sériés 218 G.D. Stucky, F.G» Dnyer Eds., Washington 1983. 19. Auroux A., Dexpert H., Leclercq C., Vedrine J., Appl. Catel., 6, 95, (1983). V9Q 

The strength of oh groups in NaHZSM-5 zeolites studied by ir spectroscopy

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The strength of oh groups in NaHZSM-5 zeolites studied by ir spectroscopy

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