Sustainable Protocol for the Reduction of Nitroarenes by Heterogeneous Au@SBA‐15 with NaBH₄ under Flow Conditions

Abstract

A gold‐incorporated SBA‐15 catalyst was prepared by a solvent‐free ball‐milling approach. The catalyst showed high reactivity and selectivity in the reduction of a variety of nitroarenes to anilines operating in absolute EtOH with NaBH4 as reducing agent. The catalyst was reused in batch conditions over five consecutive runs without any losses of activity or selectivity. Considering the high chemical stability and reusability of the catalytic system, a continuous‐flow protocol was also investigated and defined to minimize the generation of waste and optimize the continuous reuse of the catalyst. Benefits of flow conditions were proven by turnover numbers that increased from 47.5 to 1902 and also by the minimization of both leaching (9.5 vs. 1 ppm) and E‐factor values (8 vs. 23 in batch).

Challenges and Directions for Green Chemical Engineering—Role of Nanoscale Materials

“…Due to their easy separation from the reaction mixture, heterogeneous systems based on metals, 17,18,19 acid 20 or base 21 catalysts have been developed using both organic and inorganic supports. Among the heterogeneous catalytic systems, nanomaterials and metal nanoparticles-based catalysts have been widely studied in the last ten years thanks to their high surface area.…”

“…Meeting this challenge, to reduce attrition and provide thermal and mechanical stability of heterogeneous supported catalysts, flow technology is proving an effective tool. 17,19,20,24,25 In addition, the use of a flow technology has multiple advantages including safety, temperature control, mixing of reagents in two phase systems, easy scale-up and space economy. The use of flow reactors reduces exposure to toxic intermediates or dangerous reagents or additives, allows operation at high temperature and pressure in a safer manner, and guarantees a cleaner reaction mixture minimizing metal leaching of the catalysts with subsequent minimization of purification waste.…”

“…The use of flow reactors reduces exposure to toxic intermediates or dangerous reagents or additives, allows operation at high temperature and pressure in a safer manner, and guarantees a cleaner reaction mixture minimizing metal leaching of the catalysts with subsequent minimization of purification waste. 17,19,24,25 Moreover, by increasing the number of combined flow reactors (the "numbering-up" approach) it is possible to scale-up processes without any of the common problems found in batch scale-up scenarios. Heterogeneous catalytic systems, based on nanomaterials with high surface energy, are ideal partners for flow technologies which can effectively be useful for the definition of greener processes.…”

Green Synthesis of Pd Nanoparticles for Sustainable and Environmentally Benign Processes

“…The use of solvent-free conditions or alternative green solvents to classical organic solvents such as water, solvent derived from lignocellulosic biomass valorization (carbonates, esters, amides, alcohols) or from industrial waste (PolarClean) in combination with continuous flow technologies and an activation with microwave or ultrasound radiation, contributes to the achievement of low-cost and environmentally sustainable processes [16][17][18][19][20][21][22][23][24][25][26].…”

Silver Nanoparticles Supported on Mesoporous Triazine Carbon Material: A Versatile Catalyst for Reduction of Nitroaromatic Compounds

“…Although, considerable progress has been demonstrated for the preparation of the effective catalyst, but finding the new ones, which has good catalytic performance, great intrinsic activity as well as achieving high standard industrial protocol in mild reaction condition is still of great interest. Among them, heterogeneous catalysts containing noble metal catalysts (Pd, Pt, Rh, Ru, Au, and Ag) have received great attention in diverse researches because of excellent catalytic activity in different organic transformations as well as the reduction of nitro compounds …”