J. Chern. SOC., Faraday Trans, I, 1989, 85(4), 837-841 OH Groups in Boralites Jerzy Datka and Z. Piwowarska Faculty of Chemistry, Jagiellonian University, Karasia 3, 30-060 Cracow, Poland Four kinds of OH groups (3740, 3720, 3680 and 3460 cm-') are present in H-boralites. The 3740 and 3720 cm-' OH groups were previously assigned to silanol, Si-OH, and acidic B..-HO-Si groups. The results obtained in the present study suggest that both 3680 and 3460 cm-I OH groups were formed by the hydrolysis of B-0-Si bond upon ionic exchange in an acidic NH,C1 solution. The broad 3460 cm-I band was attributed to O-..H-O-Si groups forming the hydrogen bond, and the 3680 cm-l band to B-OH groups in B-O..-H-O-Si fragments. The OH groups in these fragments are unstable and prone to dehydroxylation. H I Isomorphically substituted zeolites have recently been extensively studied because of their interesting chemical properties and potential industrial applications [for a review see ref.(l)]. Boron-substituted zeolites, boralites, which owing to the small size of the boron atom show properties not observed in Al-substituted zeolites, draw the greatest attention. The boron atom is three-coordinated in dehydrated boralites and becomes four-coordinated upon the adsorption of electron-donor molecules. The strength of the acid sites in boralites is much lower than in ~eolites,~-~ even though the electronegativity of boron is higher than that of aluminium (2.93 and 2.22, respectively). It can be explained by a weak interaction between the OH groups and the small boron atom in the B-..HO-Si units.Our previous i.r. study5 showed that only 3740 cm-l silanol Si-OH groups are present in Na-boralite, but three new OH bands (at 3460, 3680 and 3720 crn-l) appear upon Na+/NH,+ ion exchange and decomposition of the NH; ions. The 3720 cm-l OH groups were found to be weak Bronsted acid sites and were assigned to B..-HO-Si groups. The origin of the 3460 and 3680 cm-' OH groups is not known. The present study was undertaken in order to elucidate the nature and to study the properties of both the 3460 and 3680 cm-l OH groups in H-boralites. Experimental The samples of boralites synthesized at the Department of Chemical Technology of the Jagiellonian University were highly crystalline, as shown by X-ray analysis. Magic-angle spinning n.m.r. spectra showed that the boron atoms were in lattice positions.6 The NH,-boralite was obtained from the Na-form by ionic exchange in NH,Cl solution at room temperature.The compositions of the Na- and NH,-boralites were studies the boralites were pressed into thin wafers (4-7 mg cm-2) and activated in situ in an i.r. cell. The cell used in this study was similar to that described in ref. (7). The boralite wafer was heated in dynamic vacuum at various temperatures (580-780 K) for 1 h. The temperature was measured by a thermocouple situated near the wafer. The spectra were recorded using a Specord 75 infrared spectrometer (Zeiss Jena) working on-line with a KSR-4100 minicomputer. ~ ~ 1 . 0 ~ 0 . 1 t ~ ~ ~ , ~ 1 . 1 ~ ~ ~ ~ 2 ~ 9 4 , 1 and respectively.For infrared 837838 OH Groups in Boralites Results and Discussion In order to study the thermal stability of OH groups in boralite wafers Na- and NH,-boralites were activated in situ in the i.r. cell for 1 h at temperatures increasing in a stepwise manner between 580 and 780 K, and the spectra were recorded 320 K after each activation step. The spectra are presented in fig. 1 and 2. Na-boralite activated at 580 K and above shows an Si-OH band at 3740 cm-l; its intensity does not depend on the activation temperature (fig. 1). The spectrum of H- boralite (fig. 2) shows the 3740 cm-' OH band as well as three others, viz. 3460,3680 and 3720 cm-l. The 3460 cm-l band is broad and typical of hydrogen-bonded OH groups. The intensities of both the 3460 and 3680cm-l bands decrease as the activation temperature increases.The 3460 and 3680 cm-' bands could be assigned either to structural OH groups, in which case their decrease at higher temperatures would be due to dehydroxylation, or else to physisorbed water which is desorbed upon heating. The latter assignment would be confirmed by the frequencies and shapes of the bands being analogous to those of water sorbed in However, the characteristic deformation vibration [6(H20)] is missing from the spectra of H-boralites. The observed 1640 cm-l band is independent of the activation temperature and thus is due to a skeletal vibration of boralite. In order to test the 'water hypothesis', water was sorbed into boralite activated at 770 K. This resulted in the appearance of bands at 1630, 3410 and 3670 cm-l (fig.3). However, all these bands disappeared upon heating in vacuo at 380 K (a temperature lower than the activation temperatures previously applied), indicating that the 3460 and 3680 cm-' bands observed in the spectra of boralites activated above 380 K cannot be due to physisorbed water. Thus structural OH groups must be responsible for the 3460 and 3680 cm-' vibrations. The observed dehydroxylation begins at a relatively low temperature of 590 K. As the bands at 3460 and 3680 cm-' were absent in the Na-form of boralite (fig. l), it was of interest to see if they were formed during Na+/NH,+ exchange or during NHZ decomposition. Unfortunately the direct recording of the spectrum following water desorption and before the decomposition of NH; was impossible, because both processes occur simultaneously.An indirect method had to be applied, and thus the number of 3720cm-' OH groups in H-boralite was compared with the amount of NH,+ ions in its NH, precursor. Our previous study5 showed that of the OH groups under discussion, only the 3720 cm-l ones can protonate pyridine, and their concentration could be determined by measuring the concentration of pyridinium ions (PyH+) thus formed. The concentration of PyH+ ions can be calculated from the intensity of the PyH+ band at 1545 cm-l and the extinction coefficient of this band. The following experimental procedure has been applied. Small portions of pyridine (ca. 0.2 pmol) were introduced at 443 K into the cell containing an activated wafer of H-boralite, and the i.r.spectrum was recorded at the same temperature after each adsorption step. The intensity of the 1545 cm-l pyridinium ion band increased with the amount of pyridine, but after the neutralization of all Bronsted-acid sites it attained a constant level, This maximum intensity was used to calculate the concentration of the Bronsted-acid sites (3720 cm-l OH groups) using the extinction coefficient of the 1545 cm-l band (0.058 cm2 pmol-l) determined in a previous study.12 The details of this method of i.r. studies of acid properties of zeolites were described in previous papers. 12-14 The concentration of Bronsted-acid sites in H-boralites activated at 590 and 780 K were found to be the same (0.81 per u.c.) and comparable, within experimental error, to that of NH: ions before activation (1 .O per u.c.), indicating that all the protons liberated upon decomposition of the NH; ions were used for the formation of the 3720 cm-l groups; thus the 3460 and 3680 cm-l OH groups originate from a different source.J .Datka and Z . Piwowarska 839 1 K K K K K r n I I I I 3 I I - l 1300 1500 1700 3200 m 3600 3800 wavenumber/cm-' Fig. 1. Infrared spectra of Na-boralite activated at various temperatures. 1 1 I t o 1 2300 1500 I 17b 3200 34m 3600 I 3dm wavenumber/m-' Fig. 2. Infrared spectra of NH,-boralite activated at various temperatures.840 OH Groups in Boralites 0.11 dm 1500 ' 1700 $00 3ioo ' 3600 ' 3eb wavenumber/cm-' Fig. 3. Water sorption in H-boralite: (a) NH,-boralite activated at 770 K, (b) water adsorption at room temperature and (c) desorption at 380 K.The following scheme is proposed to explain the formation of the 3460 and 3680cm-l OH groups by boralite hydrolysis during ion exchange in acidic NH,C1 solution : Na+(NH:)HzO Na+(NHt)HzO H O---H-O\ I / O 0 \ /O\ /O\ / O H20 0 \ 0 / \oo/ \ 00 / \o H+ o/ \oo/ \o o/ \* /'\ / B- Si Si B- Si - Si One B-0-Si bond is broken in the first hydrolysis step. If the neighbouring B-0-Si bonds were also hydrolysed, this would result in boron extraction from the lattice. This was indeed observed with b~ralites.~$ l5 Similarly, aluminium is extracted from zeolites in acid solutions. The extent to which this process occurs depends strongly on the boron content in b0ra1ites.l~ The broad band at 3460 cm-l can be attributed to Si-OH groups forming a hydrogen bond with a neighbouring oxygen atom, O-.-H-O-Si, and the 3680cm-l band to B-OH groups whose oxygen is engaged in hydrogen-bond formation.These assignments are supported by the results of Ghiotti et all6 and of Morrow et al.,17 who observed a 3540 cm-l band in Si-O...H-O-Si dimers analogous to our B-O---H-O-Si species. Winde et a1.l' assigned an observed 3707 cm-l band to isolated B-OH groups. The involvement of the oxygen atom in the hydrogen bonding results in a weakening of the 0-H bond,13 in agreement with the Gutmann rule1' and in the shift of the B-OH band to lower frequencies.J. Datka and Z . Piwowarska 84 1 H I The B-O--.H-O-Si fragments seem to be unstable and prone to dehydroxylation. The data presented in fig. 2 show that this is truly the case.Both the 3460 and 3680 cm-l bands decrease noticeably with the activation temperature. Terminal Si-OH (3740 cm-l) and acidic (3720 cm-l) OH groups are much more stable. The intensity ratio of the 3720 and 3680 cm-l OH bands depends strongly on the activation temperature. At lower activation temperatures the 3680 cm-' band is the strongest and at higher activation temperatures the 3720 cm-l band becomes dominant. This explains why some authors3 observed only the 3695 cm-' (B-OH) band, while othersz0 only the 3725 cm-' (B.-.HO-Si) band, depending on the activation conditions. Conclusions Four kinds of OH groups (3740, 3720, 3680 and 3460 cm-') are present in H-boralite. The 3740 and 3720 cm-' hydroxyls were previously attributed to silanol Si-OH and acidic B..SHO-Si groups, re~pectively.~ The results obtained in the present study suggest that both 3680 and 3460 cm-l OH groups were formed by the hydrolysis of the B-0-Si bond upon ion exchange in acidic NH,Cl solution. The broad 3460 cm-' barid is assigned to O-..H-O-Si groups forming the hydrogen H I bond, and the 3680 cm-I band to B-OH groups in B-O.-.H-O-Si fragments. H I The OH groups in B-O--.H-O-Si fragments are unstable and prone to the dehydroxylation. We thank Dr A. Cichocki from Jagiellonian University for the samples of boralites and Dr M. Tencer from Lehigh University for helpful discussions. References 1 M. Tielen, M. Gailen and P. A. Jacobs, Proc. Int. Symp. Zeolite Catalysis, Siofok, 1985, p. 1 2 K. F. M. G. J. Scholle, A. P. M. Kentgens, W. S.Veeman, P. Frenken and G. M. P. van der Velden, 3 G. Coudurier and J. C. Vedrine, Pure Appl. Chem., 1986, 58, 1389. 4 P. Ratnasamy, S. G. Hedge and A. J. Chandwalkar, J . Catal., 1986, 102, 467. 5 J. Datka and Z. Piwowarska, J . Chem. Soc., Faraday Trans. I , 1989, 85, 47. 6 A. Cichocki and J. Datka, to be published. 7 A. Bielanski and J. Datka, Bull. Acad. Pol. Sci., Ser. Sci. Chim., 1972, 20, 81. 8 G. J. Frohnsdorf and G. L. Kington, Proc. R. SOC. London, Ser. A, 1958, 247, 469. 9 H. A. Szymanski, D. N. Stamires and G. N. Lynch, J. Opt. SOC. Am., 1960, 50, 1223. J. Phys. Chem., 1984, 88, 5. 10 J. W. Ward, J. Phys. Chem., 1968, 72, 421 1. 11 A. Bielanski and J. Datka, Bull. Acad. Pol. Sci., Ser. Sci. Chim., 1970, 18, 173. 12 J. Datka and E. Tuznik, J. Catal., 1986, 102, 43. 13 A. Bielanski and J. Datka, Bull. Acad. Polon. Sci., Ser. Sci. Chim., 1974, 22, 341. 14 J. Datka, J . Chem. Soc., Faraday Trans. I , 1980, 76, 2437. 15 A. Cichocki, to be published. 16 G. Ghiotti, E. Garrone, C. Morterra and F. Boccuzzi, J . Phys. Chem., 1979, 83, 2863. 17 B. A. Morrow and A. Cody, J . Phys. Chem., 1975, 79, 761. 18 H. Winde, P. Fink and A. Kohler, Z . Chem. 1977, 17, 41. 19 V. Gutmann, The Donor-Acceptor Approach in Molecular Interactions (Plenum Press, New York, 20 C. T. W. Chu and C. D. Chang, J . Phys. Chem., 1985, 89, 1569. 1978). Paper 8/01295D; Received 29th March, 1988