Direct aldol and nitroaldol condensation in an aminosilane-grafted Si/Zr/Ti composite hollow fiber as a heterogeneous catalyst and continuous-flow reactor

• APS-grafted Si-Zr-Ti/Torlon composite hollow fibers were used for cooperative catalysts.
• Amine-silanol cooperativity is tuned by adjusting the ZrO2, TiO2, and SiO2 nanoparticles.
• Aldol and nitroaldol condensation are very sensitive to amine-silanol cooperativity.
• High primary aldol and nitroalcohol yields were obtained in a continuous-flow system.

A proof-of-concept study for a new type of continuous-flow reactor based on porous aminosilane-grafted Si-Zr-Ti/Torlon composite hollow fibers was considered for aldol and nitroaldol condensations of aromatic aldehydes with acetone and nitromethane. These novel hollow fiber reactors consist of bifunctional groups in fiber wall that make them bifunctional catalysts for cooperative interactions (i.e., acid–base catalysis). In this study, the effects of reactant flow rate, reaction time and temperature, and electron-donating and electron-withdrawing groups of para-substituted benzaldehyde derivatives on the catalytic activity of aldol and nitroaldol condensation reactions were systematically investigated. The yield of products was found to be dependent on the cooperative interactions of acid–base pairs, para-substituted benzaldehyde, and reaction conditions such as temperature and contact time. Moreover, relatively high yields of ca. 100% were obtained at higher flow rates for primary aldol and nitroalcohol products, while benzylideneacetone and nitrostyrene yields were higher at longer contact times. The obtained results indicate that the aminosilane-grafted Si-Zr-Ti/Torlon composite hollow fibers provide a new platform as flow reactors for heterogeneously catalyzed reactions that may facilitate the ultimate scale-up of practical fiber catalysts for the synthesis of complex organic compounds.

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