Cleavage of dibenzyl ether in the presence of zinc halides

Abstract

The large amount of water liberated during coal liquefaction in the presence of ZnCl 2 is believed to derive from the cleavage of benzylic ethers. The details of this process have been studied using dibenzyl ether as a model structure and ZnCl 2, ZnBr 2, and ZnI 2 as catalysts. The influence of H 2O and HX on the progress of the reaction was also examined. In the absence of HX addition, ZnX 2 activates the ether by forming a Lewis acid-base complex with it. Water interferes with the formation of this complex by competing for ZnX 2. In the presence of HX, an adduct is formed, HX · ZnX 2, that is much more active catalytically than ZnX 2. The presence of water does not influence the activity of the Brønsted acid form of the catalyst. Activation of dibenzyl ether is achieved by protonation of the ether. The relative activity of the zinc halides depends on which of the two forms is present. If anhydrous ZnX 2 is the catalyst, the activity decreases in the order ZnCl 2 > ZnBr 2 > ZnI 2. On the other hand, if HX · ZnX 2 is the prevalent form, the activity decreases in the order HI · ZnI 2 > HBr · ZnBr 2 > HCl · ZnCl 2. It is observed that during the reaction of dibenzyl ether in the presence of ZnX 2, a small amount of HX is liberated, which then combines with ZnX 2 to form HX · ZnX 2. Because the Brønsted acid form of the catalyst is more active than the Lewis form, the apparent activity of the catalyst increases after a period of induction during which HX is generated.