J . Am. Chem. SOC.1990, 112, 7144-7147
7144
indicate that the motional states of species A and B do not depend appreciably on Si/AI, at least in the range 34 I %/AI I 120. The strength and number of acid sites in a zeolite depend on the concentration of tetrahedral aluminum atoms2' and their locations in the pore structure.22 For a highly dealuminated zeolite ZSM-5 such as sample 490, aluminum sites will generally be limited in number and methoxyl groups bound to silanol groups will have local structural and dynamic environments substantially different from that of a similar group adsorbed in H-ZSM-5 with a lower Si/AI ratio, in which case interactions with nearby aluminum centers is possible. The temperature dependence of the relaxation of species C (Figure 5 ) is like that of a spin relaxing through dipole-dipole coupling when W ~
