Ultrafiltration Pd-immobilized catalytic membrane microreactors continuously reduce nitrophenol: A study of catalytic activity and simultaneous separation

Catalytic membrane microreactors (CMMRs) are an exciting new technology that combine catalysis with membranes by seeding the high internal surface area of membranes with catalysts. This enables the continuous purification and production of organic compounds with high catalytic activity, while maintaining nanocatalyst size and stability. This study reports a simple two-step batch reaction approach for synthesizing stable Pd-immobilized catalytic membranes to transform 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) in a flow-through CMMR system. The product (4-AP) is an essential intermediate for the polymer and solvent chemical industry. Pd-immobilized membranes exhibited strong catalytic activity for 4-NP reduction in the presence of NaBH4, with 4-fold higher reduction (79.7%) and 2-fold higher reduction rate (1.08 mol m−2h−1) as compared to control membranes without catalysts. The catalytic mechanisms were elucidated, such that the catalytic reduction processes were conducted via a sequential hydrogenation reaction where Pd nanocatalysts facilitated H* transfer from BH4- to 4-NP. Ultimately, the 4-AP product was gradually desorbed from the catalytic sites, achieving continuous reduction with high reduction efficiency. Furthermore, by combing ultrafiltration and catalysis, Pd-immobilized membranes showed >99% of 4-NP conversion and >90% of 1 MDa PEO rejection, demonstrating a great potential to be applied in high-efficient wastewater treatment processes.