Triazole-based covalent gels assembled from small molecules with superior stability for supported catalysis in a monolithic microfluidic reactor

Abstract

A novel catalysis-assisted gelation strategy is exploited to assemble a catalogue of covalent gels with superior stability from small molecules via strong covalent bonding. Triazole-based covalent gels with hierarchical porous structure and large surface area are fabricated based on copper(I)-catalyzed azide–alkyne cycloaddition (CuAAC) polymerization between 1,4-diazidobenzene (A1) and tetrakis(4-ethynylphenyl)methane (B1) /2,2',7,7'-tetraethynyl-9,9'-spirobifuorene (B2) (denoted as triazole-A1B1 and -A1B2 gels). The BET specific surface areas of the triazole aerogels are up to 368 m2 g-1. The triazole gels show superior chemical stability, offer the hierarchical porous structure and rich triazole coordination sites for metal nanoparticles immobilization, and can therefore serve as catalyst supports. A range of metal nanoparticles are readily supported in the gels after wet impregnation and reduction. Among them, Ru nanoparticles are anchored within the gel matrix with small nanoparticle size of 2.95 nm. A microfluidic reactor is fabricated by loading Ru@triazole-A1B2 gel monolith in fused-silica capillary. The triazole-A1B2 gel and Ru@A1B2 gel monoliths have high permeability of 2.27 × 10-11 m2 and 1.59 × 10-11 m2, respectively. The Ru@triazole-A1B2 microfluidic reactor achieves high catalytic activity and selectivity, and ultrafast flow synthesis is developed for the reduction of nitrobenzene derivatives to the corresponding anilines with maximum conversion of 91.8 % and 87.3 % selectivity within 25 s. The reactor shows superior stability and the conversion of nitrobenzene remains above 92 % during 60 h of continuous-flow catalytic reaction.