Temperature-dependent Synthesis Of Micro-and Meso-Porous Silica Employing Poly(N-isopropylacrylamide) As Structure-directing Agent And Its Application

The synthesis and catalytic application of porous silica is investigated in this work. The main contents of this work are summarized as follows:synthesis of thermo-responsive polymer of PNIPAM and PNIPAM-co-PAA, temperature dependent synthesis of porous silica employing thermo-responsive polymer as structure-directing agent (SDA), and simultaneous immobilization of Pd catalyst during synthesis of mesoporous silica employing SDA of the thermo-responsive and chelate copolymer of PNIPAM-co-PAA to afford Pd/SiO2catalyst.The polymer of PNIPAM is synthesized by atom transfer radical polymerization, while PNIPAM-co-PAA is synthesized by free radical polymerization. The polymer is characterized by GPC,1H NMR and UV-vis.Temperature dependent synthesis of micro-and meso-porous silica employing the thermo-responsive homopolymer poly(N-isopropylacrylamide)(PNIPAM) or the random copolymer poly(N-isopropylacrylamide-co-acrylic acid) as structure-directing agent (SDA) and Na2SiO3as silica source is proposed. The thermo-responsive character of the SDA provides the advantages including (1) temperature dependent synthesis of microporous silica, hierarchically micro-mesoporous silica, and mesoporous silica just by changing the aging temperature below or above the low critical solution temperature of the thermo-responsive SDA, and (2) elimination of the thermo-responsive SDA from silica matrix by water extraction. The synthesis mechanism is discussed, and the effect of the aging temperature and the weight radio of SDA/Na2SiO3on the synthesis of micro-and meso-porous silica are studied. Microporous silica, hierarchically micro-mesoporous silica and mesoporous silica with the surface area at3.5～9.0×102m2/g and the pore volume at0.30～1.13cm3/g and the average pore size ranging from1.1to9.0nm are synthesized. The strategy affords a new and environmentally benign way to fabricate porous silica materials, and is believed to bridge the gap between the synthesis of microporous and mesoporous silica materials. Simultaneous immobilization of Pd catalyst during synthesis of mesoporous silica employing the structure directing agent of the thermo-responsive and chelate copolymer of poly(N-isopropylacrylamide-co-acrylic acid) to afford Pd/SiO2catalyst is studied. Thanks to the structure directing agent of the thermo-responsive and chelate copolymer, immobilization of the Pd catalyst and synthesis of the catalyst support of mesoporous silica are combined into a one-pot process. Following the proposed method, the Pd/SiO2catalysts, in which the size of the immobilized Pd nanoparticles is below4.2nm and the Pd weight content ranges from0.5%to10%, are fabricated. It is supposed that the Pd nanoparticles are encapsulated within the mesopores of the in-situ synthesized mesoporous silica. The synthesized Pd/SiO2catalyst is tested by three different model reactions including hydrogenation of nitrobenzene, hydrogenation of cinnamyl alcohol, and Suzuki coupling reaction, and a robust and reusable Pd/SiO2catalyst is demonstrated.