Boosting the efficiency of supported undecamolybdophosphate catalyst by tailoring acid-redox properties for direct oxidative cross-esterification of aldehydes.

Highly efficient, heterogeneous catalyst PMo 11 /ZSM-5-H with enhanced activity was developed for direct oxidative esterification of aldehyde with alcohols under environmentally benign reaction conditions. [Display omitted] • Supported undecamolybdophosphate catalyst with tailored acidity and redox properties. • Direct oxidative esterification of aldehydes with environmentally benign oxidants. • > 95 % selectivity for Methyl benzoate with >15,000 TON in the presence of O 2. • Superior performance due to the synergistic effect of modulated acid-redox sites. • Heterogeneous and recyclable catalyst. Aiming to develop a novel heterogeneous catalyst for single-step oxidative cross-esterification of aldehydes to ester with enhanced activity and selectivity under benign conditions, here we report the synthesis of defect-regulated undecamolybdophosphate supported on ZSM-5. The enhancement in acidity was achieved by exchanging the available Na+ with protons using an ion-exchange protocol. The structural and morphological features of synthesized catalysts (20 % PMo 11 /ZSM-5-H) were confirmed by in-depth characterization such as acidity measurement, FT-IR, TGA, surface area measurement SEM, TEM, and PXRD. The synthesized catalysts were used for direct oxidative esterification reaction with environmentally benign oxidants (O 2 and H 2 O 2). The effect of various reaction parameters such as %loading of PMo 11 , amount of catalyst, reaction time, and reaction temperature and volume of methanol on activity and selectivity was studied. The comparative studies of supported analogous phosphomolybdates with the present catalyst showed that the enhancement in acidity and modulation in redox properties of 20 % PMo 11 /ZSM-5-H was found propitious. The novelty lies in obtaining excellent selectivity of the ester (>95 %) with an exceptionally high turnover number (>15323) with an environmentally benign oxidant O 2. The scope and limitations of the present catalytic system were also explored by employing different aldehydes and alcohols under optimized conditions. The stability and recycling studies showed that the synthesized catalyst is truly heterogeneous and can be recycled at a minimum of six cycles. From the comparative data of the reported catalysts as well as control experiments, it was concluded that the synergistic effects of enhanced acidities and tailored redox sites played vital roles in the superior performance of the present catalytic system. Based on the results, a plausible reaction mechanism was also proposed. [ABSTRACT FROM AUTHOR]