Zeolite-assisted acid hydrolysis of cellulose: Optimization of reaction conditions and chemical pretreatments for enhancing HMF yields

Abstract

This study explores the hydrolysis of microcrystalline cellulose under soft conditions using a combination of both homogeneous (HCl) and heterogeneous catalysts (β-zeolite). The impact of temperature and HCl concentration is studied, revealing that the most favorable results (highest HMF yield) are achieved at 140ºC with 0.02 %w/w of HCl (19% of selectivity, 23.5% of conversion). Nevertheless, the inherent recalcitrance of cellulose limits the conversion and the HMF concentration. Three chemical pretreatments (HCl, H2O2, HNO3) are considered, assessing the effect of concentration. Except when using 69% of HNO3, the bulk structure of cellulose remains largely unaffected (low effect on conversions). However, the surface of cellulose undergoes chemical alteration due to the acidic and predominantly oxidizing pretreatment, leading to the detection of C5 compounds. These modifications have a positive influence on the reaction, significantly enhancing the selectivity towards HMF up to 50% (34% of conversion).