Diethylenetriamine[propyl(silyl)]-Functionalized (DT) Mesoporous Silicas as CO2 Adsorbents

Mesoporous silica substrates were functionalized with N-[3-(trimethoxysilyl)propyl]diethylenetriamine to form diethylenetriamine[propyl(silyl)]- (DT-) functionalized hybrid products suitable for CO2 adsorption. The materials prepared were characterized by N2 adsorption/desorption at 77 K, C and N elemental analysis, helium pycnometry, X-ray diffraction (XRD), CO2 adsorption, and thermal decomposition and were compared to analogous aminopropylsilyl- (AP-) and ethylenediamine[propyl(silyl)]- (ED-) functionalized materials. The extent of surface functionalization varied with substrate morphology. CO2 adsorption capacities and heats of adsorption were determined via combined thermogravimetric analysis and differential thermal analysis (TGA/DTA). Functionalization of the substrates was found to enhance their CO2 adsorption capacities at 20 °C under anhydrous conditions. Higher temperature led to reduced adsorption capacities but higher heats of adsorption (H ads) of CO2, thought to be due to the reduced role of weak physisorption sites. When CO2 was supplied in a moist gas stream, the adsorption capacity was reduced, but the value of H ads(CO2) was essentially unchanged. The thermal stabilities of one substrate and its AP-, ED-, and DT-functionalized products both in N2 and in mildly oxygenated N2 were also characterized by combined TGA/DTA. These materials were found to be stable up to 170 °C in both atmospheres and, furthermore, had no particular affinity for either N2 or O2 over this temperature range. Oxidative decomposition data from TGA at higher temperature were found useful for estimating the N content of these materials.