INFRARED STUDIES OF 1-HEXENE ADSORBED ONTO Cr3+-EXCHANGED MONTMORILLONITE

-When 1-hexene was adsorbed by Cr3+-montmorillonite at room temperature, all evidence of C=C(str) vibrations was lost. Protonation of the alkene occurred, and the secondary carbocation formed was bound at a site on the primary coordination sphere of the interlayer cation. Some of the hydrogen atoms of these primary-sphere water molecules were involved in strong hydrogen bonds to the silicate sheets, whereas others did not form such bonds, but were free and directed into the interlayer space. These latter hydrogen atoms were labile and protonated the alkene molecules. Key Words--Adsorption, Alkene, Catalysis, Chromium, Hexene, Infrared spectroscopy, Water. I N T R O D U C T I O N The reactions of alkenes and dialkenes over montmorillonite clays have received considerable attention in recent years (Adams et al., 1978, 1979, 1981 a, 1982a, 1982b, 1983a, 1983b; Bylina et aL, 1980; Pinnavaia et al., 1979; Pinnavaia and Raythatha, 1983). In many of the clay-catalyzed reactions studied, the proposed mechanisms have involved stages where proton addition to form carbonium ions is considered to be the principal step (Adams et al., 1981b, 1982b, 1982c, 1983a, 1983b; Ballantine et al., 1981a, 1981b). There are, however, some reactions where this mechanistic approach is not satisfactory (Adams et al., 1978, 1979, 1981 a, 1982a; Pinnavaia, 1977) and others where the site of reaction is an adsorbed (intercalated) active species such as a rhodium phosphine complex (Pinnavaia et al., 1979; Pinnavaia and Raythatha, 1983). It is therefore of interest to study the interaction of alkenes with certain cation-exchanged clays in an attempt to clarify the reaction mechanisms involved. The chosen adsorbate was 1 -hexene, whose reactions over montmorillonites have been studied extensively (see, e.g., Adams et al., 1982a; Pinnavaia and Raythatha, 1983). Over M 2+or M3+-exchanged montmorillonites the principal product below 100~ is di2,2'-hexyl ether, which cannot be synthesized in significant yields by any other route (Adams et al., 1979). This reaction is unusual in that all other acid-catalyzed reactions which occur over these catalysts at low temperatures proceed via tertiary or allylic carbocation intermediates (Adams et al., 1983b), whereas a reaction mechanism written assuming that the clay acts merely as a solid source of protons would involve secondary carbocation production on protonation of the alkene (Figure 1). A further problem exists in that the later steps in such a mechanism imply the production of a secondary alcohol intermediate, yet doping the reaction with this alcohol slows the reaction (Adams Copyright 9 1985, The Clay Minerals Society et aL, 1982a). This effect, however, could be purely a result of miscibility or phase-transfer problems. To avoid these difficulties Adams et al. (1981 a) proposed a reaction mechanism that involves direct coordination of the double bonds of the alkene molecules to the interlayer cation. Although this second proposal has many attractive features, including an explanation of the shape selectivity of the reaction, it has some faults, the main one being that many ion-exchanged clays catalyze the reaction, even those with interlayer cations which normally do not bond to alkenes. Inasmuch as hydrocarbons have strong infrared (IR) absorption bands in the regions around 3000 and 1500 cm -1, the water of the clay investigated here was exchanged for deuterium oxide. In this way the v(O-H str.) at 3500-2900 cm -1 was exchanged for v(O-D str.) at 2600-2300 cm -1. Thus, a 'window' was opened in which the adsorbate v(C-H str.) bands could be observed. In addition the 6(H-O-H) band was lost at 1620 cm I thus avoiding interference with the alkenes v(C=C) band. Comparisons of the spectra of the adsorbed and free species were then made to establish the nature of the adsorbed state.