J. CHEM. SOC. FARADAY TRANS., 1994, 90(9), 1351-1354 Adsorption of Benzene on the Acidic and Basic Sites of KH B Zeolite studied by in situ IR Spectroscopy? Jian-ping Shen,* Jun Ma, Tie Sun and Da-zhen Jiang Department of Chemistry, Jilin University, Changchun , 130023,China En-ze Min Research Institute of Petroleum Processing, Beijing , 100083,China In situ IR studies of benzene adsorbed on several KH B zeolites, where the extent of potassium exchange, K/AI (YO),varied from 0 to 88.32%, have been undertaken under different conditions. The bands of adsorbed benzene on KH B zeolites in the range 2050-1700 cm-' were assigned. KH fl zeolite exhibits basicity only when K/ Al d 88.32%, where its basicity is stronger and its acidity weaker. The order of stability of the various adsorbed benzene species on the samples is as follows: 0-(benzene) > H+(benzene) > K+(benzene) > benzene./?Zeolite has been demonstrated to be useful as a catalyst for K and 0.2 Pa for 40 min, and then cooled. Benzene vapour acid-catalysed reactions such as hydroisomerization and was then admitted at room temperature. IR spectra were hydrocracking'V2 and as a sorbent in the separation of recorded with Nicolet FTIR-5DX spectrometer at different aromatics3 since it was first ~ynthesized.~ Its structure, which times, pressures and temperatures. The spectrum of the comprises channels with a three-dimensional 12-membered adsorbed phase was obtained after subtraction of the zeolite ring, has been p~blished.~ However, the acid-base properties contribution.of /? zeolite and its catalytic behaviour have received little attention. In our previous work6 we reported the effects of exchange conditions on the degree of exchange of H /3 zeolite Results and the isomerization reactions of a-@-)methylnaphthalene The main observation is that the frequencies (2050-1700 over a series of KH B zeolites. The IR spectra of adsorbed benzene on some zeolites are cm-') of the overtone and combination bands of C-H It is of interest to elucidate the adsorp- out-of-plane vibrations are shifted to higher wavenumbers already kn~wn.~-'~ tion mode of hydrocarbons, since it may explain the behav- compared with those of liquid benzene. iour and reactivity of the molecules in the pores.To date Fig. 1 shows the IR spectra in the 2050-1700 cm-' region some workers have at one extreme studied the interaction of of benzene adsorbed on KH j3 -3 zeolite at different adsorp- benzene with parent zeolites like Y,' X,12 /?13 and at the other tion times. Compared with the pure liquid [Fig. l(g)] the extreme with highly exchanged NaHY. However, few C-H out-of-plane vibrations of benzene on the sample are attempts have been made to detail the changes that occur in disturbed by the adsorption time. The bands at 1756, 1816, the spectra of adsorbed benzene molecules as the proton 1840 and 1960 cm- are observed at an adsorption time of 1 content in the zeolite increases and to study these zeolites min. With increasing time the intensities of the 1816 and 1960 cm -' bands decrease rapidly, and simultaneously shoulders using the method of in situ IR spectroscopy.In this paper, the are observed at 1840 and 1973 cm-l. From comparison with adsorpton of benzene on the acidic and basic sites of KH B zeolites was studied by in situ IR spectroscopy on the basis of [Fig. lcf)], it can be seen that adsorption of benzene reaches our previous work. Experimental The starting material is a B zeolite of composition, for 64 T atoms per unit cell,' H3.50Nao.olA13~5,Si60~~~~~~~.The preparation of KH B zeolite sample with different degrees of exchange was conducted as in ref. 6. The chemical composi- tions of four samples are given in Table 1. The in situ IR studies were carried out on self-supporting wafers.The wafers placed in an IR cell were evacuated at 673 Table 1 Chemical composition of the samples sample chemical composition K/Ala (%) HB H3.50Na0.01A13.51si60.490128 0 KH B -H1.82K1.88Na0.01A12.52si80.480128 53.40 KH B -2 H1.05K2.45A13.52si60.480128 69.60 KH B -3 H0.41K3.10A13.51Si60.490128 88.32 2050 1875 1700 a Molar ratio. wavenumber/cm-' Fig. 1 IR spectra of benzene adsorbed on KH /? -3 zeolite at 300 Part 2 of 'Properties of Alkali-metal Ion Exchange of H p K after (a) 1 min, (b) 5 min, (c) 15 min, (430 min, (e) 1h, 012 h, (Q) Zeolite'. liquid equilibrium after 1 h. Note that the spectrum of adsorption equilibrium (1 h) is very different from that of initial adsorp- tion: the intensities of the bands at 1960 and 1816 cm-' are lower, the band at 1840 cm-' is shifted to lower wavenum- bers (1838 cm-') and is sharper, and four new bands at 1854, 1876,1973 and 2004 cm-' are formed.From Fig. 2 it can be seen that all of the samples exhibit the bands at 1960 and 1816 cm-' observed in the IR spec- trum of liquid benzene. Similar IR spectra are obtained when the K/A1 of KH /3 zeolite <88.32%, except for a small band shift at 1838 cm-'. With decreasing K/Al of KH #? zeolite the band at 1877 cm-' increases in intensity and is shifted to higher wavenumbers. The IR spectra of adsorbed benzene on KH #? -3 zeolite under different pressures are shown in Fig. 3. With decreasing pressure the intensities of the bands at 1960 and 1816 cm-' decrease rapidly and the peaks at 1973, 1877, 1853 and 1840 cm-' become sharper.Comparison of Fig. 3(u) and (c) reveals the selective desorption of benzene from KH /3 zeolite. IR spectra of the samples with different K/Al at the same pressure are presented in Fig. 4. The band at 1877 cm-' cuco 2 2050 1875 1700 wavenumber/crn-' Fig. 2 IR spectra at equilibrium of benzene on the samples at 300 (d)H /9-1,BKH-2,(~) /!? -3,(6) KH /9KH(a)K: J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 2050 1875 1700 wavenumber/cm-' Fig. 4 IR spectra of benzene adsorbed on the samples at 3.2 Pa and 300K: (a)KH B -3,(b) KH /3 -2, (c) KH fi -1, (d)H B increases in intensity with decreasing potassium content of ICH /3 zeolite and is shifted to higher frequencies [1888 cm-in Fig.4(d)]. The band at 1853 cm-', however, disappears when K/Al < 88.32%. The band at 1844 cm-', not observed in the spectrum of benzene adsorbed on H /3 zeolite, increases in intensity with increasing potassium content and is shifted to lower wavenumbers. Simultaneously, the band at 1984 cm-' is also shifted to lower wavenumbers. Fig. 5 shows for KH /I-3 the progressive change in the spectra as the desorption temperature increases. The inten- sities of all of the bands, except that at 1853 cm-', decrease with increasing temperature. The adsorbed benzene is com- pletely desorbed from the sample at 453 K. The behaviour of adsorbed benzene is reported in Fig. 6 for 3.2 Pa and T < 323 K. The bands at 2004 and 1853 cm-' are observed only for KH -3 zeolite.The band at 1840 cm-' totally disappears on H #?.On the other hand, the band at 1877 cm-' increases with decreasing potassium content. The maximum intensity of this band is observed for H #? zeolite. I 1 v I I 2050 1875 1700 2050 1875 1700 wavenum berlcm- ' wavenumber/cm-' Fig. 3 IR spectra of adsorption of benzene on KH /? -3 zeolite at Fig. 5 IR spectra of benzene adsorbed on KH B -3 zeolite at dif- 300 K and different pressures: (a)1.0 x lo5 Pa, (6) 13.3 Pa, (c) 3.2 Pa ferent temperatures: (a) 300 K, (6) 323 K, (c)423 K, (d)453 K J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Discussion In liquid benzene bands in the range 3050-1400 cm-' have been a~signed'~ to C-H stretching and a combination of C-C bendings in the range 3050-3000 cm-', to overtone and combination bands of the C-H out-of-plane deforma- tion vibration at 1960 and 1816 cm-' and to ring deforma- tion at 1479 cm-' .The 1960 and 1816 cm-' bands are assigned to v5 + v17 and vl0 + ~17," respectively. Upon adsorption of benzene on alkali-metal zeolite^,^*^*'^ a shift in frequencies and a split of this pair of bands usually occur. The bands resulting from the split of the starting pair are assigned to the low-frequency (LF) and high-frequency (HF) pair. The LF pair of bands can be assigned to the interaction of benzene with cations through the n-electron cloud and the HF pair of bands would result from the interaction with framework oxygen. Note that the extent of each type of inter-action depends on the acidity of the cations and on the oxygen basicity.Fig. 1-6 reveal some similarities in the behaviour of benzene adsorbed on the zeolites. The LF and HF pairs consist of the bands at 1840-1973 and 1853-2004 cm-', respectively. Similar trends are observed in the adsorption of benzene on the alkali-metal, Na /I and Cs B zeolites,I3 which lends further credence to an adsorption mechanism that per- turbs the 7c bonds of the aromatic ring while simultaneously locking the ring into a fixed configuration which does not favour out-of-plane vibrations. The frequencies of these bands are affected by the microenvironment, e.g. the presence of cations and framework oxygen. The intensity of the LF pair at 1840 cm-' increases with increasing content of pot- assium; this band, however, is not observed for the adsorp- tion of benzene on H /I zeolite, and can be assigned to the interaction with potassium cations through the nelectron a 2050 1875 1700 wavenumber/cm -' Fig.6 IR spectra of benzene adsorbed on the samples at 323 K and 3.2 Pa:(a) KH f? -3,(b)KH /3 -2,(c) KH f? -l,(d) H f? cloud. The intensity of the other LF pair at 1980 cm-' which reflects of the effects of the alkali metals in the parent zeolites on ben~ene,~.~ does not change obviously when K/A1 of KH /I zeolite is < 88.32%. Its intensity, however, decreases rapidly for the sample with K/Al = 88.32%. This may result from the interaction of benzene with K+ and H+ cations. It is pro- posed that the HF pair of bands (2004 and 1853 an-') results from interaction between the framework oxygen of the 12-membered ring and benzene on KH 8-3 zeolite.This pair of bands, on the other hand, cannot be observed when K/Al is <88.32%. The basicity of the conjugate acid-base pairs in the zeolites suggests that it is associated with framework o~ygen.'~.'~In other words, KH /I shows some basicity when K/A1 is high. Note that in our experiment a new band at 1877 cm-' (higher than that of 1840 cm-') is observed in a series of KH /3 zeolites. With increasing proton content the intensity of this band increases and its frequency is shifted to higher values. Upon adsorption, the greater the interaction of the adsorbed molecules with the zeolite surface, the greater will be the alternation in the force constant of the C-H bond and fluctuating dipole; as a consequence, the bands associ- ated with these vibrations should appear at higher fre-quencies and with greater intensities.Compared with K', the electrostatic field of H+ is very strong and consequently the interaction of H+ with benzene is stronger than that of K+. Moreover, with increasing proton content of the zeolite, the strength of the acid sites on the zeolite surface increases, and the interaction of H+ with benzene becomes strong. There- fore, we suggest that the band at 1877 cm-' should be assigned to the interaction of benzene with a proton through the n-electron cloud. The assignment of these bands is given in Table 2.Table 3 gives the ratios of integrated absorbances of the adsorbed benzene species on the samples. The results for 0--benzene (0-b) and H+-benzene (H'b) show that the basicity of KH /I -3 is stronger and its acidity weaker. This result is in good agreement with that of NH,(CO,) temperature-programmed desorption.6 Dzwigaj et a1.I3 pointed out that the basic strength of Na /I, Cs /I samples is comparable to that of NaY and rather higher than that of ZSM-5. The basicity is stronger than one would predict. The absorbance ratio of IH+b/ZK+bincreases with increasing Table 2 Assignment of bands at 2050-1700 cm-' species of wavenumber liquid assignment interaction /cm - assignment K+ba 1840 '10 + '17 1816 vl0 +v17 0-b 1853 '10 + '17 H+b 1877 v10 + v17 ?/(C-H) 1960 v5 + vl, (K+,H+)b 0-b 1980 2004 '5 + '17 v5 + '17 'b, benzene.Table 3 Ratios of integrated absorbances for four samples -0.80 1.70 -1.08 0.73 0 2.47 0.10 0" 0.72 1.22 0.17 -1.41 0.06 0 4.16 0.04 ob 3.59 1.57 0.10 -3.22 0 0 12.81 0 (Y 300 K, 1.0 x lo5 Pa. 300 K, 13.3 Pa. 323 K, 3.20 Pa. desorption temperature, decreasing pressure and increasing H+ content. It is proposed that the benzene interacting with potassium is preferentially desorbed, which may result from the fact that the electrostatic field intensity of H is stronger + than that of K+. An increase in Ib/IK+b is observed at lower pressures and temperatures. 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