(683d) Selective Methanol Oxidation to Formaldehyde in a Chemical Looping System: A Promising Alternative

Formaldehyde, a critically important platform chemical, is produced from partial oxidation of methanol. The major industrial formaldehyde manufacture processes, silver catalyst and formox, face drawbacks like catalyst deactivation and operational safety hazards. Chemical looping (CL) is a reaction scheme that splits a redox reaction into two sub-reactions, facilitated by a solid reaction intermediate called the oxygen carrier (OC). It enables the spatio-temporal separation of the reaction's reduction and oxidation segments and can produce chemicals like formaldehyde via the selective partial oxidation route. During reduction, the OC donates its lattice oxygen to methanol, generating formaldehyde and steam. The reduced OC subsequently replenishes its lattice oxygen by reacting with air during oxidation. The CL scheme provides an additional degree of freedom for product optimization and avoids direct contact of air and methanol, rendering a safer operation. To ensure a high product yield and stable long-term performance of a CL system, the optimal design of its OC is crucial.

This work demonstrates a novel CL route for the partial oxidation of methanol to formaldehyde. Vanadium Phosphorous Oxide (VPO), a known catalyst capable of C-H bond activation in low-carbon alkanes, is employed as the OC. The prepared β-VPO₅ is dispersed on silica support (VPO-Si), enabling high active site dispersion. VPO-Si OC exhibited stable redox performance over 10 redox cycles. The results were supported by several solid characterization techniques including Temperature-programmed reduction (TPR), X-Ray diffraction (XRD), Scanning electron microscopy (SEM) with Electron dispersive X-ray spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS). Fixed-bed studies to validate the process at larger operational scale, displayed high methanol conversion of ~85% with an exceptional formaldehyde selectivity of ~45%. Density functional theory simulations are performed to elucidate the reaction pathway and the role of surface oxygen vacancies. This study facilitates the development of safer and more robust industrial formaldehyde production.