Exploring the effect of acid modulators on MIL-101 (Cr) metal–organic framework catalysed olefin-aldehyde condensation: a sustainable approach for the selective synthesis of nopol

Abstract

The development of efficient and sustainable strategies that evade the utilization of petroleum reserves is highly challenging yet inevitable today. In this regard, the conversion of pine tree-derived β-pinene to highly recognized nopol is particularly attractive owing to its widespread applications. Herein, we describe an approach that enables the selective synthesis of nopol based on the extraordinary activity of MIL-101(Cr). This remarkable activity of MIL-101(Cr) is attributed to its high specific surface area (SSA), accessible active sites in the mesopore architecture and unsaturated Cr³⁺ Lewis acid sites. We have established a good correlation between the superior catalytic performance and textural properties of the materials, which can be tuned by using different mineralizing agents. To realize the unprecedented catalytic activity, the influence of reaction parameters, solvent properties, and mineralizing agents has been investigated systematically. MIL-101(AA) (AA-acetic acid) showed the highest catalytic activity, which is superior to most of the reported materials for this transformation to date. The results of catalyst recycling and hot filtration experiments have emphasized that the catalyst is resistant towards leaching of active sites and retained its original catalytic activity beyond four recycles. An Eley–Rideal based model was used to study the reaction kinetics and establish a plausible reaction mechanism. The activation energy of the reaction was found to be 102 kJ mol⁻¹ and the enthalpy of formaldehyde adsorption was −86.88 kJ mol⁻¹. This approach opens up new avenues for the valorization of biomass-based molecules into useful chemicals.