Change in benzene adsorption with acido- basicity of (Cs,Na)X zeolites studied by i.r. spectroscopy A. de Mallmann and D. Barthomeuf Laboratoire de Rhactivite’ de Su ace et Structure, U.A. 1106 CNRS, Universite’ Paris VI, 4, ! PlaceJussieu, 75252 Paris Ce ex 05, France (Receaved 23 October 1987; accepted 26 November 1987) The existence of two benzene-adsorbed species has been shown by i.r. spectroscopy on (Cs,Na)X zeolites with various cesium loadings. They have been assigned to benzene molecules interacting either with cations (C species), within the supercage, or with the oxygen of the 12-R windows (W species). Frequencies of the i.r. bands of the adsorbed species are shifted compared to liquid benzene; the shifts are particularly important for the C-H out-of-plane combination bands (5 + 17) and (IO + 17). The change in the intensity of those bands with the benzene loading allowed the concentration on each site to be determined. As the cesium content of the (Cs,Na)X zeolites increases, the amount and the stability of W species compared to C species is increased, both at low and high loadings. This is related to the increase of the framework oxygen charge and to the simultaneous decrease of the cation acidity as the cesium content of the zeolite increases. In samples with high cesium content, new C-C bands at 1466 and 3015 cm-’ originate from the strong CH-oxygen interaction with O4 on both sides of the benzene plane. Keywords: Faujasite; CBHB adsorption; i.r. spectroscopy; acido-basicity INTRODUCTION The interaction with faujasites of benzene used as a probe molecule has been studied using several means of investigation in order to characterize the active adsorption sites for benzene and other aromatics. Electronic and vibrational perturbations of the adsorbed benzene molecule have been evidenced by U.V., ‘*’ i.r.,2-6 and Raman’ spectroscopies. The mobil- ity of the benzene molecules was studied using n.m.r. s-‘z In order to explain the strong affinity of faujasites for benzene, calculations have been per- formed on the formation of a rc complex in the supercage between the cations and the aromatic ring. l3 The existence of such interactions have also been shown in gaseous phase.14-16 In fact, n.m.r.,” recent neutron diffraction,“*‘s and i.r.6 studies showed that strongly adsorbed benzene can interact either with cations or with oxygen atoms of the 12-rings within the faujasite structure. In a previous study on Na and Rb faujasites,6 it has been shown that the preferred adsorption on cations or on oxygen atoms is governed by the balance between the cation acidity and the oxygen basicity. The purpose of this paper is to study, by means of i.r. spectroscopy, the influence of the increase of the basicity in (Cs,Na)X faujasites on the mode of benzene adsorption, as the cesium content of the solid is raised. 0 1988 Butterworth Publishers 292 ZEOLITES, 1988, Vol8, July EXPERIMENTAL Materials Seven zeolites have been studied with different cesium, rubidium, and sodium contents. They have been obtained by one or several exchanges of commercial NaX zeolite (Linde SK 20) with aqueous solutions containing different concentrations of cesium or rubidium chloride, except for the CsdoNaX sample that was prepared by an exchange with a cesium-hydroxide solution. The samples were then dried at 80°C in an oven for 18 h. The samples are labeled according to their cesium or rubidium con- tent, expressed as the number of cesium or rubidium ions per u.c.: NaX, Cst.+NaX, Csp3NaX, Cs3sNaX, CsaaNaX, Css’NaX, and Rbs7NaX. The sum [Na+ + Cs+ [or Rb+]). equals the number of A104 tetra- hedras. The chemical composition of these seven zeolites is given in Table 1 for the dehydrated forms. The crystallinity of all the samples is very good. 1.r. studies The study was carried out on self-supported wafers (15 mg for 18 mm diameter). The pretreatment was conducted as in (ReJ 6) by heating the sample under 200 Torr of oxygen and then treating it under vacuum (10m5Torr) at 450°C for 14 h. Spectra were