Multi-catalytic active site biochar-based catalysts for glucose isomerized to fructose: Experiments and density functional theory study

Abstract

This work provides an innovative method for preparing different isomerization catalysts by impregnating different proportions of MgCl₂ and AlCl₃ and combining different K compounds on cellulose-derived biochar, followed by pyrolysis. Results show MgO and Al(OH)₃ existing in ₄Mg-₁Al-C catalyst can obtain better catalytic effect on glucose isomerization than the singe of Al presenting in ₀Mg-₁Al-C catalyst. Moreover, the synergism effects of the multi-catalytic active sites such as β-_, γ-Al(OH)₃, KCl, MgO, and K₄H₂(CO₃)₃ in Mg-Al-KHCO₃-C catalyst can further lead to an increase in glucose isomerization, compared to the ₄Mg-₁Al-C catalyst. The X-ray diffraction results present that the value of O/Al in Mg-Al-KHCO₃-C catalyst is as high as 13.38, which provides many unsaturated acidic catalysis sites and benefits the glucose isomerization. Simultaneously, the TPD results reveal that the main active sites (MgO, Al(OH)₃, and K₄H₂(CO₃)₃) in Mg-Al-KHCO₃-C catalyst can provide weakly acidic and basic sites and avoid strongly acidic and basic sites to excessively attack the glucose. Based on the DFT analysis, the results indicate that the MgO has a great effect on the ring-opening reaction to form acyclic glucose, while Al(OH)³⁺ has a great effect on promoting acyclic glucose hydrogen transfer isomerized to form fructose. Compared to other carbon-based metal catalysts, the prepared Mg-Al-KHCO₃-C has excellent catalytic performance, which gives a higher fructose yield (38.7%) and selectivity (87.72%) and glucose conversion (44.12%) at 100 °C in 30 min. In this study, we develop a highly efficient Mg-Al-K-biochar catalyst for glucose isomerization and provide an efficient method for cellulose valorization.