Repulsive and attractive interactions between Bronsted sites and hydrocarbon species with partial carbocationic character in restricted spaces: comparison of IR results and ab initio calculations

A novel type of repulsive interaction between anchored oligomeric chains with partial carbocationic character (formed inside the channels of the zeolite upon proton catalyzed oligomerization of propene) and unreacted Bransted groups in H-mordenite is described and discussed. This anomalous interaction causes the stretching frequency of the unreacted Bronsted groups to shift upwards (instead of downward as usually observed). This finding is compared with those obtained for the interaction of Bronsted groups with propene oligomers similarly formed in the channels of H-ZSM-5, H-beta and H-Y and with those caused by the interaction of Bronsted groups with neutral molecules like n-heptane. In order to estimate both the value and the sign of the OH stretching frequency shift of the acidic site caused by the interaction with positively charged -(CH2)(n)CH3 chains, ab initio density functional calculations on a model compound have been performed to simulate the interaction of acidic centers with net and partially positively charged species.