(337cq) Novel Chemical Looping Scheme for Selective Methanol Oxidation to Formaldehyde

Formaldehyde (HCHO), an essential organic chemical, is primarily produced from the partial oxidation of methanol (CH₃OH) using air. The two major industrial processes for formaldehyde manufacture, i.e., silver catalyst process and formox process, face drawbacks like operational safety hazards and catalyst deactivation. A chemical looping (CL) scheme splits a redox reaction into two sub-reactions assisted by a metal oxide carrier. The CL scheme can be employed to produce formaldehyde by selective oxidation of methanol. In the first step, reduction, methanol reacts with the lattice oxygen of the oxygen carrier to produce formaldehyde and steam. In the second step, the oxygen-depleted oxygen carrier reacts with air to replenish its lattice oxygen. The reaction scheme for the process is as follows:

Reduction: MO_x + CH₃OH --> MO_x-1 + HCHO + H₂O

Oxidation: MO_x-1 + 1/2 O₂ --> MO_x

Overall: CH₃OH + 1/2 O₂ --> HCHO + H₂O

where 'M' represents a metal.

The Chemical looping (CL) scheme enables the temporal separation of a redox reaction's reduction and oxidation segments by utilizing lattice oxygen from a metal oxide called the oxygen carrier (OC). The chemical looping scheme can not only provide an additional degree of freedom for product optimization but also avoids direct contact with air and feed (methanol), thus rendering a safer operation.

This work proposes and demonstrates a novel chemical looping route for methanol partial oxidation to formaldehyde. We utilize a specialized metal oxide, Vanadium Phosphorous Oxide (VPO), a well-known catalyst for activating the C-H bond of low-carbon alkanes as the oxygen carrier. For enhancing surface properties, active site dispersion, and long-term redox stability, silica (SiO₂) is used as support. The Thermogravimetric analyzer (TGA) studies showcased that the silica-supported VPO (VPO-Si) maintained a stable redox performance for 10 consecutive redox cycles. X-Ray diffraction (XRD) and Scanning electron microscopy combined with Electron dispersive X-ray spectroscopy studies confirmed the OC's stability, recyclability and phase integrity over extended redox cycles. X-ray photoelectron spectroscopy (XPS) studies were conducted to investigate the evolution of OC's surface activity characteristics with redox cycles. Further, fixed-bed studies were conducted to evaluate methanol conversion and formaldehyde selectivity. Notably, VPO-Si showcased a high methanol conversion of ~85% with an exceptional formaldehyde selectivity of ~45% in the operating temperature range of 380-400°C. Moreover, the OC exhibited carbon deposition resistance, evidenced by Transmission electron microscopy (TEM) imaging of the post-reaction fixed bed sample. Furthermore, ab initio atomistic simulations and density functional theory calculations were carried out to elucidate the reaction pathway and determine the role of surface oxygen vacancies in the reaction. The findings from this study contribute towards developing a safer and more robust process for industrial formaldehyde production.