J. Chem. SOC., Faraday Trans. f, 1989, 85(1), 47-53 The Properties of Boralites studied by Infrared Spectroscopy Jerzy Datka* and Zofia Piwowarska Faculty of Chemistry, Jagiellonian University, Cracow, Poland Because of its small dimensions boron is three-coordinated in boralites and is situated in one face of a tetrahedral hole. The i.r. band of the B-0 asymmetric stretch in BO, units is split into two maxima (at 1380 and 1405 cm-'), thus suggesting that two kinds of boron sites exist in boralites. Boron becomes four-coordinated following adsorption of the electron- donor molecules NH,, H,O and pyridine, but not of benzene. Three kinds of OH groups (at 3680, 3720 and 3460cm-') are formed following the substitution of Na' ions by protons. The 3720cm-' OH groups act as Brernsted-acid sites in the reaction with pyridine.The concentration of Brsnsted sites determined by pyridine sorption is comparable with theoretical values. The low strength of the Bronsted-acid sites (illustrated by pyridine thermodesorption experiments) can be explained by the large separation and weak interaction between boron and the OH groups in the O,B***OH-SiO, units. The electrostatic field of the Lewis-acid sites in boralites is stronger than that in zeolites. There are numerous methods whereby the acidic and catalytic properties of zeolites may be modified. One is the isomorphic substitution of A1 or Si atoms by other elements such as B, Be, Ga, Cr, Fe, V, Ti, Ce, Zr or P. Isomorphically substituted zeolites have been extensively studied,'-14 although as yet only boron-substituted pentasil zeolites have any industrial applications [in the Assoreni (conversion of methyl butyl ether into methanol and isobutene) and Amoco (xylene isomerization and ethylbenzene conversion) processes].Apart from these industrial applications, boralites are also interesting from the chemical view point; there are two features which are characteristic of boralites and do not occur in zeolites: the presence of three-coordinated boron and the low acid strength of the OH groups. T atoms (A1 or Si) in the zeolite lattice are four-coordinated, but boron (being much smaller) is located near three oxygen atoms in a trigonal environment within a tetragonal hole.6. l1 The fourfold coordination of boron is observed only after the adsorption of H,O, CH,OH and NH,.Another interesting feature of boralites is that the acid strength of their OH groups is much lower than that in zeolites, despite the fact that the electronegativity of boron is higher than that of aluminium (2.93 and 2.22 in the Sanderson scale).6'11T12 The aim of our work was to study by i.r. spectroscopy the properties of boron atoms and OH groups in boralites. Na-Boralite, H-boralite and zeolite H-ZSM-5, all containing the same amount of boron and aluminium, were studied. Experimental The boralite samples were synthesized by Dr A. Cichocki (Department of Chemical Technology, Jagiellonian University). Zeolite ZSM-5 (Ultrazet) was synthesized at the Institute of Industrial Chemistry (Warsaw). X-Ray analysis has shown all the samples to be highly crystalline, while an m.a.s.n.m.r.study has shown all the boron atoms to be situated in the boralite 1atti~e.l~ The NH, form of boralite was obtained by ion exchange 4748 I.R. Study of Boralites Table 1. Compositions of the boralites and ZSM-5 zeolite, and the concentration and strength of the Brmsted-acid sites sample composition Brmsted-acid sites per unit cell Na-boralite Na1.0H0.1~B02~1.1~si02~94,9 0.27 - NH,-boralite (NH4)1.0(B02)1.0(si0'2)95 0.67 0.16 NH4-ZSM-5 (NH4,H)0.,Na0.2(A102)1.0(si02)95 0.73 0.65 with an NH,CI solution at room temperature. The compositions of the Na- and NH,- boralites, as well as that of zeolite NH,-ZSM-5, are presented in table 1. Prior to the i.r. experiments, wafers of the boralites and the zeolite were activated at 773 K in situ in the i.r. cell for 1 h.The i.r. spectra were recorded using a Specord 75 i.r. spectrometer working on-line with a KSR 4100 minocomputer. Results BO, Vibrations Spectra of the Na- and H-forms of boralite are presented in fig. 1. A doublet at 1380 and 1405 cm-l is present in the spectra of the activated boralites. Sorption of H,O, NH, or pyridine results in the disappearance of both the 1380 and 1405 cm-' BO, bands [fig. I A and B, spectra (bHd). These effects are reversible. Evacuation at higher temperatures (373-673 K) removes the sorbed molecules and the doublet at 1380 and 1405 cm-' reappears. Sorption of benzene does not influence the BO, vibrations [spectrum (e)]. OH Groups Spectra of OH groups in the Na- and H-forms of boralite, as well as spectra of OH groups in H-ZSM-5, are presented in fig.2A. The OH band at 3740 cm-l, characteristic of silanol Si-OH groups, is present in the spectrum of Na-boralite [fig. 2A, spectrum (a)]. The frequency and the intensity (expressed per 1 g of boralite) of the 3740 cm-' OH band in Na-boralite (3.65 g-l) are practically the same as in the Na-ZSM-5 zeolite (3.35 g-'). The spectrum of H-boralite (obtained by decomposition of the NH,-form) is presented in fig. 2A, spectrum (b). Introduction of protons into the boralite results in the appearance of new OH bands at 3680 and 3720 cm-' as well as a weak and broad band at 3460 cm-'. The band due to Si-OH silanol groups is seen as a shoulder at ca. 3740cm-'. The spectrum of H-ZSM-5 zeolite [fig. 2A, spectrum (c)] contains two distinct OH bands : 3738 cm-' (Si-OH) and 3609 cm-' (Si-OH-AI).Pyridine Adsorption Sorption of pyridine in H-boralite (fig. 2 B) results in a strong decrease (or disappearance) of the 3720 cm-' OH band and in a small diminution of the 3680 cm-' OH band. The 3740 cm-' Si-OH band remains unchanged. The sorption of pyridine results in the formation of pyridinium ions HPy+ (1 545 cm-'), pyridine complexes bonded to Na+ ions PyNa+ (1440 cm-l) as well as pyridine complexes bonded to Lewis-acid sites PyL (1460 cm-'). The same complexes are formed after pyridine sorption in H-ZSM-5. The frequency of the PyL complex formed in H-boralite (1460 cm-') is higher than that in H-ZSM-5 (1450 cm-l).J. Datka and Z . Piwowarska 49 rjoo ubo 1500 l600 wavenumber/cm-' Fig.1. Spectrum of Na-boralite (A) and H-boralite (B): (a) activated boralite, and after sorption of (6) H,O, (c) NH,, ( d ) pyridine and (e) benzene. f ' (b) (4 I 7 l I I l 3400 3600 3800 ' 3bO 3600 38a3 I400 1600 wavenumber/cm- ' w avenumtxr/cm- w avenumtxr/cm-' Fig. 2. (A) OH groups in (a) Na-boralite, (6) H-boralite and (c) H-ZSM-5. (B) OH groups in H-boralite : (a) the activated boralite and (b) after pyridine sorpton. (C) The spectrum of pyridine sorbed in (a) H-boralite and (6) H-ZSM-5.50 I.R. Study of Boralites In order to determine the concentration of Brsnsted-acid sites in boralites and in zeolite, small portions of pyridine were sorbed at 443 K up to a constant intensity of 1545 cm-l PyH+. The concentration of PyH+ (and therefore of Brransted-acid sites reacting with pyridine) was calculated from the maximum intensity of the 1545 cm-l band and from the extinction coefficient of this band determined in a previous study1fi (0.058 cm pmol-2).The calculated values are presented in table 1. The concentrations of Brsnsted-acid sites in Na-boralite, H-boralite and H-ZSM-5 are comparable with the theoretical concentrations of protons calculated from the difference between the contents of B(A1) and Na. In order to study the strength of the Brransted-acid sites, experiments with thermodesorption of pyridine were made. Pyridine was desorbed at 770 K, and the ratio A770/A0 was determined. ( A , and A7,,, are the intensities of the PyH+ 1545 cm-' band before and after desorption.) The values of A770/A0, giving information about the fraction of Brsnsted-acid sites still holding pyridine after desorption at 770 K, are taken as a measure of the acid strength of these sites and are presented in table 1.The strength of the Brsnsted-acid sites in the H-boralites is lower than that in H-ZSM-5. Discussion BO, Vibrations The boron atom is smaller than aluminium (atomic radii 0.82 and 1.18 A, respectively) and occurs preferably in trigonal coordination. In boralites boron is situated between three oxygens at one face of a tetrahedral hole in a planar, or nearly planar, configuration. This conclusion is based on the results of i.r. studies of Coudurier and Vedrine,ll who observed a strong band corresponding to a B-0 vibration in BO, units in dehydrated boralites, and also on the results of Scholle and Veemaq6 who detected a m.a.s.n.m.r.signal for three-coordinated boron. (The quadrupole parameters of this signal were similar to those in planar BO,.) The results obtained in our study are in good agreement with these earlier interpretations. A strong i.r. band of the asymmetric stretching BO, vibration is present in the spectrum of dehydrated Na-boralite and H-boralite [fig. 1 A, spectrum (a)]. This band is split into two maxima at 1380 and 1405 cm-l, thus suggesting that two kinds of BO, units with different B-0 force constants exist in boralites. The configuration of the boron atom in the boralite lattice is presented schematically as (1). The boron atom I 0' B I O \ 0 is situated between three oxygens : the distance to the fourth oxygen is greater, and the interaction is much weaker.The splitting of the BO, band suggests that two different sites exist for the boron atom in boralites. In one such site the distance to this fourth oxygen may be shorter than in the other, and thus the interaction with this oxygen may be stronger. Because the boron atom is shifted closer to this oxygen, its interactions with the three oxygens in the BO, unit are weaker and the B-0 stretching force constant in BO, is lower. Note that 12 crystallographically different 'T' sites have been found in the ZSM-5- type stru~ture.'~ M.a.s.n.m.r. studies1* have shown that the number of different T sites is even higher. The sorption of H20, NH, and pyridine in both Na- and H-boralites results in a reversible decrease in the intensity of the BO, doublet (1380 and 1405 cm-l) (fig.1 A and B). These effects were also observed by Coudurier and Vedrinell following theJ . Datka and Z . Piwowarska 51 adsorption of H20, CH,OH and NH,. Scholle and Veeman' reported the disappearance of the BO, m.a.s.n.m.r. signal and the appearance of a BO, signal upon the adsorption of H20 on H-boralite. The i.r. band of BO, vibrations (at 1150 cm-l) was also observed in our studies of hydrated b0ra1ites.l~ The results obtained in our study (and also the earlier results) prove that boron increases its coordination number (from three to four) after sorption of electron donors such as H,O, NH,, pyridine or CH,OH. An interaction with benzene (a n-electron donor) does not change the coordination number of boron (fig.1). Two possible interpretations of these effects can be considered. (1) The sorption of electron-donor molecules removes a proton or Na+ ion from the framework. Protons form PyH+ or NH; ions, and they can be also dissolved in H20 clusters.6 Na+ ions also form complexes with NH, or pyridine molecules. The abstraction of a proton or Na+ ion from the framework generates four-coordinated boron according to the scheme: O,B...OH(Na)-SiO, - 0,B-0-SO,. The interaction of OH groups with benzene (hydrogen bonding) does not remove the proton from the framework: boron remains three coordinated (fig. 1). (2) The second interpretation (which seems to be less probable) assumes a direct interaction between the boron atom and free electrons of the oxygen atom (in H,O) or nitrogen atom (in NH, or pyridine).This interpretation assumes that the geometry of the tetrahedral hole in which such a boron atom is situated makes such a close contact possible. -H+(Na+) OH Groups An OH maximum at 3740cm-' is present in the spectrum of Na-boralite [fig. 2A, spectrum (a)], thus indicating that silanol Si-OH groups exist in boralites. The frequency and intensity of the Si-OH band in boralite (3740 cm-', A / m = 3.65 g-') approach those in ZSM-5 zeolite (3738 cm-l, A / m = 3.35 g-'), thus suggesting that the nature and properties of the silanol Si-OH groups are the same in both cases. It has been foundlg that silanol Si-OH groups in ZSM-5 zeolites are formed during the composition of TPA ions, and they are situated inside the zeolitic chanels. It may be taken that the same is true of silanol Si-OH groups in boralites.The intoduction of protons into the boralites (by the decomposition of NH; ions) results in the appearance of new OH bands at 3680 and 3720 cm-' as well as a broad, weak band at 3460 cm-l. This last band can be attributed to a hydrogen bond between two adjacent OH groups.20*21 The bands at 3680 and 3720cm-l represent free OH groups. Note that only one OH band (apart from silanol Si-OH) was reported in previous studies of boralites. Chu and Chang' reported an OH band at 3725 cm-l, and Coudurier and Vedrinell reported a band at 3695 cm-l. Both the 3680 and 3720 cm-l OH bands were found in our study. It seems that both kinds of OH groups can be formed in H-boralites, but (depending on the sample composition and on the pretreatment conditions) one becomes dominating and is observed in the spectrum.The frequencies of the 3680 and 3720 cm-' OH bands are markedly higher than the OH frequencies in suggesting that the acid strength of these OH groups is lower. Acidic properties of both kinds of OH groups (3680 and 3720 cm-') in H-boralites were studied by pyridine sorption. The molecules of pyridine react with the 3720 cm-l OH groups, forming PyH+ ions, thus indicating that these OH groups are Brarnsted-acid sites. A decrease of the 3680 cm-l band is also observed after pyridine sorption (fig. 2B); however, it is difficult to decide if this decrease is due to proton transfer or only to a physical interaction. The 3720 cm-l OH groups are the main source of protons in H-52 I.R.Study of Boralites boralites. The numbers of Brransted-acid sites determined by pyridine sorption (presented in table 1) in Na-boralite, H-boralite and zeolite H-ZSM-5 are comparable with the theoretical numbers of protons calculated as the difference between the contents of B (or Al) and Na. A proton deficit in H-boralite and H-ZSM-5 may be due to dehydroxylation, which may occur during the pretreatment (Lewis-acid sites were found in both cases, Experiments involving the desorption of pyridine (table 1) show that the Brarnsted- acid sites in H-boralites are much weaker than those in H-ZSM-5. The same conclusion can also be drawn when comparing the OH vibration frequencies. Low-strength acid sites in boralites were also evidenced by earlier t.p.da6P ''9 l2 and m.a.s.n.m.r.6 studies. This effect is unexpected, since the electronegativity of boron is higher than that of aluminium (2.93 and 2.22, respectively), and (according to the collective model) an increase in average electronegativity is followed by an increase in acid-site strength.The following explanation of the low strength exhibited by the acid sites in boralites can be proposed. Because of its small size, the boron atom is situated between three oxygens in one face of a tetrahedral hole (1). The distance to the fourth oxygen (forming the OH group) is longer and the interaction B--*OH- is much weaker than that in zeolites. Such an OH group behaves more like a silanol Si-OH group than like Al-OH-Si (1). The vibrational frequency vCC(N) of a pyridine molecule bonded to an electron-acceptor site depends on the nature of the site.In a series of cationic forms of zeolite Y Ward23y24 observed an increase in this frequency with the electrostatic field of the cation (an increase of the cation charge and a decrease of the radius). In H-boralite the frequency v ~ ~ ( ~ ) of pyridine bonded to Lewis-acid sites (1460 cm-') is higher than that in zeolite H-ZSM-5 (1450 cm-'), thus indicating that the electrostatic field of Lewis-acid sites in boralite is stronger than in the zeolite. This may be due to the higher electronegativity and and smaller size of boron in comparison with aluminium. fig. 2C). We thank Dr A. Cichocki of the Jagiellonian University for donating samples of boralites. References 1 M. Taramasso, G.Perego and B. Notari, Proc. 5th Znt. Conf. Zeolites, Naples, 1980 (Hayden, London, 2 R. M. Barrer, Hydrothermal Chemistry of Zeolites (Academic Press, London, 1982), p. 251. 3 N. A. 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