Thiol group functionalization of mesoporous SiO2 SBA-15 using supercritical CO2

Chemical modification of mesoporous SiO2 SBA-15 with thiol groups was performed using mercapto-propyltrimethoxysilane (MPTMS) dissolved in supercritical CO2 (scCO(2)). Thiol groups serve as adsorbents for the selective removal of contaminant metal cations and in catalysis. Functionalization was carried out in scCO(2) at temperatures ranging from 40 to 150 degrees C and pressures from 15.0 to 29.0 MPa. For comparison purposes, the reaction was also performed in toluene at 80 and 110 degrees C. As opposed to toluene, scCO(2) is considered a green solvent. Grafting of the thiol groups was confirmed by FTIR spectroscopy, thermogravimetric analysis (TGA) and elemental analysis. Grafting density and surface coverage of the materials modified using scCO(2) increased with temperature, CO2 density, time and stirring and varied from 1.3 to 4.4 mmol g(-1) and from 1.3 to 4.0 molecules nm(-2), respectively. On the other hand, surface area and pore size decreased as grafting density increased. At temperatures of 80 degrees C or higher, the pore size remained constant, suggesting the formation of a compact monolayer. Modification at higher temperatures led to larger grafting densities but very low surface areas. Assuming total hydrolysis and condensation of the precursor, the optimum grafting density and surface coverage of 2.3 mmol g(-1) and 2:4 molecules nm(-2), respectively, were obtained in scCO(2) at 80 degrees C and 25.0 MPa for 4 h. Grafting densities of the samples prepared in toluene were by far much lower than those obtained using scCO(2) at lower temperatures and shorter times, which demonstrates the advantages of CO2 as a green functionalization medium. 2017 Elsevier Inc. All rights reserved.