Modification of silicalite-1 with ammonium compounds aimed at preparation of acidic catalyst for acetalization of glycerol with acetone

Post-synthesis modification of silicalite-1 (MFI) with solutions of ammonium compounds (NH4F, NH4Cl, NH4NO3, NH4OH) without any additional alkaline agent followed by further thermal treatment appeared a new, facile, and cheap method to generate the acidic sites in starting material. The number and strength of resulting acid sites depend on the nature of anion in the applied ammonium salt. Used procedure resulted in partial removal of external silanol groups and in the formation of acidic internal isolated as well as hydrogen bonded OH groups (e.g. silanol nests). The novel acidic sites show a high catalytic activity for acetalization of glycerol with acetone and high selectivity towards 2,2-dimethy1-1,3-dioxolane-4-methanol (solketal). The modified silicalite-1 indicates also high stability with time on stream. The proposed reaction mechanism considers a combined contribution of both Lewis and very weak Bronsted acid sites.