Developing catalysts for efficient biomass upgradation is essential for establishing a sustainable chemical industry. Selective glucose isomerization to fructose is an important step in upgrading cellulosic biomass (plant matter) to high valued products. Fructose can be used in the form of high-fructose corn syrup or it can be upgraded to platform chemicals such as 5-(hydroxymethyl)furfural. While industrially, the isomerization is catalyzed using immobilized enzymes, recent reports demonstrate that simple homogeneous tertiary amines can selectively convert glucose to fructose.1
Herein, we present the synthesis, characterization, and catalytic performance of silica supported tertiary amine catalysts for the selective (~75%) isomerization of glucose to fructose. Systematic investigations reveal that various catalyst design parameters including the amine loading, amine structure,2
and support pore characteristics affect the catalytic performance significantly. Observations indicate an inverse relation between the catalytic performance and amine-surface silanol interactions. To limit these interactions, previously reported silanol-capping methods using capping agents like hexamethyldisilazane are found to be ineffective as they increase catalyst hydrophobicity and induce mass transfer limitations. However, shortening the length of the amine tether from a propyl to a methyl group spatially limits the possibility of amine-silanol interactions and results in a four-fold increase in the fructose yield. As the immobilization procedure involves consumption of surface silanols, synthesizing catalysts with high amine loadings probabilistically limits amine-silanol interactions. This results in a four-fold increase in fructose yield as well. These insights are then employed for designing the optimal catalytic material.
(1) Liu, C.; Carraher, J. M.; Swedberg, J. L.; Herndon, C. R.; Fleitman, C. N.; Tessonnier, J. P. ACS Catal. 2014, 4 (12), 4295â4298.
(2) Deshpande, N.; Pattanaik, L.; Whitaker, M. R.; Yang, C. T.; Lin, L. C.; Brunelli, N. A. J. Catal. 2017, 353, 205â210.