Nuclear Magnetic Resonance Studies of Hydroxyl Groups in Decationated Zeolites Y

Transcription

1 Nuclear Magnetic Resonance Studies of Hydroxyl Groups in Decationated Zeolites Y D. FREUDE, D. MULLER, A~D H. SCHMIEDEL Sektion Physik der Karl-Marx-Universitgt, ~01 Leipzig, Linn$stra~e 5, DDR Received July 20, 1970; accepted January 6, 1971 A detailed study by stationary nmr techniques has been made on the influence of decationation and of the pretreatment temperature on line width, second moment, line shape, and intensity of proton resonance signals caused by strueturm OH-groups in a series of decationated zeolites type Y. Line shape analysis shows that the hydroxyl groups mainly exist in pairs with an inner proton-proton distance of about 3.7 ~. A study of the temperature dependence of the proton line width points to two sorts of hydroxyls of different correlation times attributed to different thermal stabilities. INTRODUCTION Zeolites are porous, crystalline aluminosilicates of great technical interest. The decationated forms of zeolites are characterized by a special catalytical activity attributed to their structural hydroxyl groups. These structural OH-groups are created after the exchangeable cations are replaced by NH4 -ions and the temperature is rinsed to 300 C, when the ammonium ions decompose into NH~ and protons, which enter the latrice to form OH-groups (1). In the past few years numerous studies have been made dealing with these structural hydroxyl groups. Most of the authors used infrared spectroscopy (e.g., references 1-3); there are also X-ray studies of single crystals (4), and studies using nmr techniques (5). Since in references 1 and 2 the results of the recent studies are summarized and discussed, we refer to these papers. In the present work the line shapes of proton resonance spectra of structural hydroxyl groups are studied as a function of the degree of deeationation, the pretreatment temperature, and the measuring temperature. The line shape permits statements about the arrangement of the OH-groups in the zeolite. From the temperature dependence of the line width conclusions about the mobility of Journal of Colloid and Interface Science, ol. 36, No. 3, July the OH-groups can be drawn. In addition, from the integral of the line shape the number of hydroxyl groups in the zeolite can be determined directly. EXPERIMENTAL In this work a NaY-zeolite with a SiA1- ratio of 2.6 was used; 25 %, 50 %, and 75 % of the Na~-ions were exchanged by NH4 n- ions (see, for example, reference 2). The samples were pretreated at 300 C (or above) in vacuo at less than 10 -~ torr for 10 hours. After this no ammonium ions could be detected with nmr techniques and infrared spectroscopy. The nmr spectra were measured using a wide-line spectrometer of the bridge type (KRB 35, Akademiewerkst~tten Berlin) at 21 MHz. To avoid saturation only weak rf fields (0.1 to 0.3 rag) could be used since the longitudinal relaxation times of the hydroxyls are of the order of 10 sec in our case. Owing to the low signal-to-noise ratio for each curve several spectra with a sweep time of about 40 rain per spectrum had to be accumulated. The accuracy depends on the signal-tonoise ratio. The signal-to-noise ratio is defined as the ratio of the maximum magnitude of the signal and the spread of the zero line

2 NUCLEAR MAGNETIC RESONANCE STUDIES 321 out of resonance. A detailed study with spectra of different signal-to-noise ratios showed their influence on the accuracy of the derived quantities 3H and M2. The errors resulted from the spread of 20 values for M2 and ~H, respectively, calculated on a computer using the experimental spectra. In evaluation of single signals with a signalto-noise ratio of 25, an error of 4-10 % must be taken into account for the line width and one of 4-50 % for M2. Only in the ease of a signal-to-noise ratio of 100 WIU the error in 32 be less than 4-10 %. By accumulating several spectra of hydroxyls the signal-to-noise ratio could be improved so far that the error of all derived quantities is less than 10 %. The line shapes were measured with the use of an amplitude of the magnetic field modulation of 1~ ~H. Proton densities were determined in the usual way by comparing the integral of the nmr signal of the hydroxyls with those of an aqueous solution of manganese. As the line shape did not vary in the temperature range from liquid nitrogen to 100 C those measurements were carried out at room temperature, but special measurements were used for higher temperatures up to 275 C. RESULTS In Table I the number of protons per cubooetahedron determined in the abovementioned way are shown. Since the measured signal is caused by protons of OH-groups only, this quantity is equal to the number of 0H-groups. The accuracy of the results is about 4-10%. The number of cubooctahedra per gram zeolite used for the calculation of these values obtained under the assumption of an ideal zeolite lattice (see, e.g., reference 4) with a SiA1 ratio of 2.6. The cation deficiency corresponds to the theoretical number of hydroxyl groups without dehydroxylation. Since a weak dehydroxylation starts even below 300 C, the experimental OH concentrations at 300 C are in a good agreement with the cation deficiencies. The weak decrease in the OH concentration between 400 C and 500 C is remarkable. In the temperature range between C and 100 C the line widths do not depend on the measuring temperature. In Fig. 1 the line widths of the samples NaY De 75 pretreated at 300 C and 400 C, respectively, are plotted logarithmically versus the reciprocal temperature. According to Abragam [6] the second moment M2 and therefore the line width, too, do not depend on the correlation time rc, if V~ 3~2 re >> t, where v denotes the gyromagnetic ratio. However, if vx~2"r~ << 1, the line width is proportional to rc. This means that the line width has the same dependence on temperature as r~, which can be assumed in a certain temperature range as rc = r0" exp (ERT). Here E denotes the activation energy and R is the gas constant. If both the temperature-dependent and temperature-independent parts of the curve are extrapolated an intersection results. For this temperature (120 C) we have v~~ re = 1, and ~ can be calculated to be 4.0 X 10-5 see from the experimental value of M2 ; rc is the correlation time of the magnetic energy of interaction. For structural hydroxyl groups r~ corresponds to the time in which a proton jumps to another oxygen atom of the lattice. The activation energy of 4.8 kealmole determined from Fig. 1 is in agreement with the value given by Uytterhoeven et al. (7) corresponding to a process of migration of the hydroxyl protons. TABLE I RESULTS OF NMR STUDIES OF HYDROXYL GROUPS IN DECATIONATED ZEOLITES TYPE Y Samples Cation deficiency per cubooctohedron Pretreatment temperature 300 C 400 C 500 C 600 C NaY De NaY De NaY De Hydroxyls per eubo oetahedron Journal of Colloid and Interface Science, Vol. 36, No. 3, July 197l

3 322 FREUDE, MULLER AND SCHMIEDEL C ~ K 3 5~.0 1 Fro. 1. Temperature dependence of the line width ~H of deeationated zeolites Y. --@--@--: sample NaYDe 75 pre~reated at 300 C; --m --m-: sample NaYDe 75 pre~reated at 00 C. For zeolites pretreated at temperatures above 300 C the decrease of the line width starts at higher temperatures only. The measurements have been carried out up to temperatures of 275 C and no significant decrease of the line width occurs. Therefore for these samples the only statement that can be made is that rc is greater than 4 N 10 -~ sec at 275 C. A detailed study of the mobility of the hydroxyls will be the subject of a further work. The second moments M: determined from the line shape can be compared with second moments calculated from configurations of the protons given by the oxygen skeleton of the zeolite by Van Vleek's equation (see, e.g., reference 6). Unfortunately there are many configurations which have second moments in agreement with the experimental results, and it is impossible to determine the kind of oxygen atoms (e.g., O~ or O~ (4)) to which the protons are attached. Nevertheless the second moment allows a rough estimation of the proton@roton distances of different hydroxyls. Assuming regular geometrical arrangements with a constant proton-proton distaonee this distance can be calculated to 2.72 A in the case of proton pairs, and to 3.66 A in the case of six neighbors. This we get for all our samples. It means that the proton-proton distance is between 2.72 and 3.66 A. Three Journal of Colloid and Interface Science, VoL 36, No. 3, July 1971 I 1 and more spin arrangements in the close neighborhood are not likely because of the resulting high electrostatical energy. The experimental results can be described on the assumption that arrangements of pairs of OH groups predominate. An analysis of the line shape of the hydroxyls (8) gives more detailed informations in comparison with those obtained from the second moment. The assumption that the spins are distributed at random in space leads to a line shape which does not agree with our experimental results obtained by stationary nmr techniques (8). A satisfactory line shape analysis can be carried out, however, assuming spirt pairs (i.e., hydroxyl pairs) for which the distances between different pairs are distributed at random. The corresponding line shape f@) is given by (8) with g(e, r~) = I f(co) = ~ pk(r~)g(~, rk), [1] k f0 7r do sin t~. "'i 1+ [ o -- co ~k3 3~'2~ ( cos~#)12 T22 + sin # 1+ I o~- ~oo + 3~2h 4-~- ~ ( c s~t~); 2T2 Here 0 denotes the angle between the vector describing the relative positions of the nuclei and the magnetic field, rk is the distance between the nuclei of one pair, pk is the relative number of two spin pairs with a distance r~, and T~ is the inverse line width of the single line of a doublet. T2 (in the quantity microsee) can be calculated using the experimental spin density n of the pairs according to the statistical theory o 3 1 (microsec_l) = 2.88n(A- ) T2 [2] [3] for protons. With several experimentally determined spin densities the line shape functions g( o, r~) were calculated for discrete rk

4 NUCLEAR MAGNETIC RESONANCE STUDIES 323 (in steps of 0.3 A from 2.0 to 5.1 A). By the method of least squares the pk(rk) fitting the experimental curves could be determined in the limit of the accuracy of the measurements and the following results were obtained. The OH groups mainly exist in pairs corresponding to two different oinner distances of 3.7 A and about 5.1 A, respectively. Whereas in the ease of a 25 % decationated sample there are 87 % of the pairs with a distance of 3.7 A and 13 % with a distance of o about 5.1 A, in the ease of a 75 % decationated sample pretreated at the same temperature there are 94 % and 6 % of the pairs with distances of 3.7 A- and 5.1 A, respectively. Since the line shape functions were calculated for pairs with discrete inner protonproton distancesoonly, a distribution of the radii around 3.7 A cannot be excluded. How- ever, the width of such a distribution must o be less than 0.3 A. Since for great rk the functions g (co, rk) do not differ very much, the "pairs" attributed to 5.1 A only should be considered as single hydroxyl groups separated about 5 A or more from one another. DISCUSSION To find out the possible arrangements of hydroxyl groups we built up a model of a cubooetahedron from the distances and angles given by Olson et al. (4). According to Uytterhoeven et al. (1) we supposed a tetrahedrical charge distribution of the oxygen and assumed that the proton is attached to one of the two free orbitals not being used to form the Si--0--A1 lattice. Considering only the apparent distances determined from the second moment, there had to exist paired hydroxyl groups mainly with two OH groups on one silicon atom. Such a conclusion also had to be drawn from inner pair distances of about 2.3 A for protons on deeationated zeolites Y determined by Gvakhariya et al. (5). The line shape analysis, however, shown that 3.7 A is the most frequent inner pair distance. This difference results from averaging the distances according to rij -~ with the use of the second moment for an estimation of the mean distance. In this procedure the result can be influenced strongly by a small amount of spin pairs having short distances. In our model this distance can be realized in different ways. For instance, if 4-5 protons are distributed at random on O1 and O8 sites (4) in a cubooctahedron (SiA1 ratio 2.6) and if there are no two directly neighbored A104- tetrahedra (according to Loewenstein) the resulting mean distance of about 3.7 is understandable. The hydroxyls maiitly are neighboring. In the case of four OH groups per cubooctahedron there are on the average just two hydroxyls on the silicon atoms of a hexagonal prism. If there is only one OH group per cubooetahedron in the average (see Table I) a line shape analysis aolso yields a mean distanee of about 8.7 A. Arbitrary model arrangements, however, yield greater distances on the average. In this case it is not possible to speak about a random distribution of the protons on all the possible sites. A mean distance of 3.7 A can be realized only if there are either two or no OH groups on the silicon atoms of the hexagonal prism linking two eubooctahedra. In Table I it can be seen that the thermal stability of the hydroxyls does not depend on the degree of decationation essentially. The reason for this behavior may be the proeedure of pretreatment: The samples were heated very slowly and at the pretreatment temperature they were evacuated for 10 hours. The samples of different degrees of deeationation, which were pretreated at 400 C, showed the highest stability in relation to dehydroxylation caused by an increase of the temperature. Independent on the degree of deeationation as shown in Table I about half of the hydroxyls are removed below 400 C whereas the remaining OH groups show a higher stability up to 500 C. The thermal correlation times of the hydroxyl protons in 300 C and 400 C samples (cf. Fig. 1) point to different mobilities of the OH protons. Whereas the 75 % decationated sample pretreated at 300 C shows a correlation time of ~ sec at 275 C, the sample pretreated at 400 C shows a re of greater than 4 X 10-5 sec at the same temperature. So it must be concluded that the Journal of Colloid and Interface Science, Vol. 36, No. 3, July 1971

5 324 FREUDE, MiJLLER AND SCHMIEDEL mean lifetime of a proton on an oxygen atom remaining after pretreatment at 400 C is more than 6 times longer than in the ease of the 300 C sample. This corresponds to the thermal stability of the OH groups. The proton signal caused by terminal hydroxyls (using a nondeeationated NaYzeolite) was negligible. OH groups which could arise as a result of hydrolysis of Si- O-A1 bonds during the ammonium exchange cannot be distinguished from those introduced by deammoniation, but their amount should be negligible. Residual water molecules could not be observed in our deeationated zeolites, and we question the results of Gvakhariya et al. (5). Their separation of the nmr spectra into contributions from residual water molecules and hydroxyl groups is not warranted, in our opinion, because of magnetic dipole interaction between water and hydroxyl protons and because of insufficient accuracy in the nmr spectrometer (8). Note added in proof: Our recent measurements by instationary nmr techniques showed that the assumption made in this work neglecting the magnetic dipole dipole interaction between protons and aluminum is not correct. This changes the values obtained by hne shape analysis and second moment. The results of proton intensities and thermal stabilities are not influenced. ACKNOWLEDGMENTS We are indebted to Prof. H. Pfeifer for his helpful discussions, and we thank Mr. W. Eisoldt, Mr. B. Richter, and Mr. J. Zacharias, who carried out some of the measurements. REFERENCES 1. UYTTERHOEYEN J. B., JACOBS, P., MAKAY, M., AND SCtIOONHEYDT, R., J. Phys. Chem. 72, 1768 (1968). 2. WA~D, J. W., J. Phys. Chem. "ls, 2086 (1969). 3. WHITE, J. L., JELLI, A. N., ANDRE, J. M., ANn FRIPIAT, J. J., Trans. Faraday Soc. 63, 461 (1967). 4. OLSON, D. H., AND DEMPSEY, E., J. Catal. IS, 221 (1969). 5. GVAKHARIYA, V. G., KVILIVIDZE, V. I., KISE~ LEV, V. F., PYLOVA, M. B., AND ZIZISHVILI, G. V., Dokl. Akad. Naulo SSSR 188, 379 (1969). 6. ABnAGAM, A., "The Principles of Nuclear Magnetism." Clarendon Press, Oxford, UYTTE~HOEVEN, J. B., SCHOONItEYDT, T~., AND FaIPIAT, J. a., International Symposium on l~eaction Mechanisms of Inorganic Solids, Chem. Soc. (London) Abstr. 5-2 (1966). 8. FREUDE, D., Mi~ILLER, D., ANn SCHMIEDEL, I-I., "On the Line Shape Problem of Hydroxyl Groups on Solid Surfaces," Surface Sci., 25, 289 (1971). Journal of Colloid and Interface Science, Vol. 36, No. 3, July 1971