Probing the formation of soluble humins in catalytic dehydration of fructose to 5-hydroxymethylfurfural over HZSM-5 catalyst

Abstract

Understanding the formation paths of humins and corresponding influencing factors in fructose dehydration catalyzed by a solid acid would be helpful for the rational design of heterogeneous catalysts for efficient synthesis of 5-hydroxymethylfurfural (HMF). In this work, the fructose-to-HMF dehydration has been studied over HZMS-5 with various Si/Al ratios to get insights into the evolution pathways of soluble humins (i.e., oligomeric products). It was found that three main representative paths were involved in the formation of soluble humins under the catalysis of HZMS-5 solid acids: 1) oligomer I formed through dehydrated condensation of fructose and/or HMF, 2) oligomer II formed through degradative (-FA) condensation of fructose and/or HMF, and 3) oligomer III formed from degradative (–HCHO) condensation of fructose and/or HMF. We demonstrated that the Brønsted acids with weak (i.e., Alpair) and medium-strong (i.e., Alpair) strengths on HZSM-5 along with those dissolved from Lewis acidic sites (i.e., Alsingle and Alq3) both contributed to the formation of oligomer I, the medium-strong Brønsted acids and strong Lewis acids on HZSM-5 were responsible for the formation of oligomer II, and the Brønsted acids with high acid density promoted the formation of oligomer III. HZSM-5 with a low density ratio between medium-strong and weak Brønsted acids (Dmedium-strong/Dweak = 0.74) along with a low density of strong Lewis acids was found to be effective for fructose-to-HMF dehydration by inhibiting humin formations to the maximum extent. This work highlighted the rational design of heterogeneous catalysts with moderate acid density and acid strength to comprehensively avoid the formation of undesired humins in HMF biorefinery.