Product selectivity and mass transport in levulinic acid transfer hydrogenation by monolithic MIL-100, MIL-88B and ZIF-8@Pd MOFs

The diffusion processes between adsorbent and adsorbate naturally play a significant role in the efficiency and selectivity of the heterogenous catalytic process. This paper considers the importance of diffusion processes in the transfer hydrogenation reaction of levulinic acid to gamma-valerolactone by MIL-88B, MIL-100, and ZIF8@Pd monolithic catalysts. Over a period of five catalytic cycles, it was shown that the Fe-based catalysts can achieve similar conversions to the ZIF-8 supported Pd, with the only current limitation being the lower aqueous stability of these MOFs. Diffusion constants were calculated using the ZLC method, with micropore diffusion limitation found for ZIF-8 and MIL-100 monolithic frameworks at 2.7 x 10(-8) and 4.6 x 10(-8) cm(2) s(-1) respectively. This diffusion limitation was also confirmed by IR spectroscopy with an increasing concentration of C-H bands on the MOF substrate post-reaction. Mass transfer coefficients, also calculated by ZLC, revealed increased mass transport for the hydrophobic ZIF-8 framework, which perhaps aids in the gamma-valerolactone selectivity over side products that are produced in the absence of catalytic material, as seen for MIL-88B and MIL-100 after multiple uses.

Automated summary

This paper examines the significance of diffusion processes in the transfer hydrogenation reaction of levulinic acid to gamma-valerolactone using MIL-88B, MIL-100, and ZIF8@Pd monolithic catalysts. The Fe-based catalysts showed similar conversions to the ZIF-8 supported Pd, but with lower aqueous stability. By calculating diffusion constants and mass transfer coefficients, it was found that ZIF-8 had better mass transport performance compared to MIL-100.