Assessment of acid strength in sodium-exchanged resin catalysts: consequences on glycerol etherification with isobutene in batch and flow reactors

Abstract

Consequences of a decrease in the number of acid sites by the cation exchange on the acid strength of the resin catalysts were assessed using ammonia as a probe molecule. IR spectra illustrated that v(N-H) bands on NH3-saturated Amberlyst 15 (TM) shift linearly to higher frequencies with an increase in the degree of sodium exchange associated with an increase in the Bronsted acid strength. These results were further confirmed by density functional theory calculations illustrating that the deprotonation energy of a sulfonated styrene group decreases upon Na+-exchange on its neighboring counterparts. Consequences of these changes in acid strength were investigated on glycerol etherification with isobutene. Batch reactor measurements at high conversions illustrated that selectivity to desired glycerol ethers increases as that of isobutene dimerization is suppressed with an increase in acid strength. The effects of acid strength on the complex reaction network were further investigated using a once-through flow reactor specifically focusing on low conversions. These measurements showed that mono-tert-butyl glycerol ether and di-isobutene were the primary products on pristine Amberlyst 15 (TM), while di-and tri-tert-butyl glycerol ethers also become a primary product on sodium-exchanged counterpart catalysts.