The Reaction of Isopropylbenzene on y-Irradiated Silica Gels

Abstract

Dealkylation of isopropylbenzene was studicd 011 three y -irradiated silica gels. Little or 110 visible coloration occurs in these solids on irradiation at 37° to doses of 4 X 1021 ev g-1. The yield of benzene initially increases linearly with dose to the solid but approaches a limiting valué at large doses; for one of the solids, limiting yields were 2.6 and 15.5 in units of 1017 molecules (g of solid) -1 at 37 and —196°, respectively. Initial slopes correspond to G0(C6H6) = 0.067 at 37° and fTo(C6H6) = 4 at —196°; the latter valué suggests a trapping efficiency for free charge carriers of near 100%. Room-temperature annealing of solid irradiated to saturation at —196° gives a limiting yield identical with that attained in a 37° irradiation. Therefore, the 37° limiting yield must correspond to essentially complete population of certain solid defects. Room-temperature irradiation of solid irradiated to saturation at —196° reduces the benzene yield at a considerably greater rate (than simple annealing) to the 37° limit; (?(loss in benzene yield) « 4. Thus, the— 196° limiting yield appears to correspond to a steady-state population of certain defects that are not populated appreciably at 37°. A remarkably good correlation exists between the behavior of benzene yields on irradiated silica gel and the reported behavior of certain trapped-electron color centers associated with specific vacancies in a silica matrix. The over-all results suggest that limiting yields, particularly at —196°, are not governed by aluminum content. Irradiation of isopropylbenzene adsorbed on silica gel also was studied.Results are similar to those obtained previously with a silica-alumina. The results suggest that dealkylation of adsorbed isopropylbenzene occurs under irradiation, (j(CgH6) ~2, by direct capture of free charge carriers produced in the solid with an efficier.cy approximating that for their capture by silica matrix defects at —196°.