The challenges of increasing atmospheric CO2 and increasing demand for new sources of liquid transportation fuels create an opportunity to utilize excess CO2 as a resource to meet fuel needs. However, the conversion of CO2 into useable fuels is an enormously complex problem. Reverse water-gas shift chemically reduces CO2 with H2 over metal catalysts, but this creates no new fuel value. Thus, solar photoreduction of CO2 is being investigated. Catalytic materials for this process require an adsorption/capture function and the electron transfer and reducing center. A challenge in combining these two functions is that the former often occurs on basic sites, and the latter at Lewis acid sites, which may not be mutually compatible.1 We report here on the synthesis of a heterogeneous catalyst consisting of a silica surface supporting both well-defined isolated Lewis acid Ti centers and amine groups in close proximity. The Ti site is protected with a bulky organic ligand either during synthesis alone or also during operation. The undercoordinated site is characterized by diffuse reflectance UV-visible spectroscopy and XANES. A carbamate precursor to an amine site is grafted to these Lewis acidic surfaces such that the acid and base sites do not annihilate each other. The carbamate also acts as a template for CO2 adsorption upon thermal deprotection. The Ti4+ site is monitored with diffuse reflectance US-Vis and is shown to remain in tact in the presence of the amine liberated from the carbamate, in contract to direct grafting of an amine. A fundamental investigation into the adsorption of CO2 onto these acid-base materials of different surface densities was undertaken and is complemented by additional analyses of supported materials by 13C CP-MAS NMR and TGA/MS. Preliminary results of acid-base catalysis over these materials are also presented.
1Srivastava, R.; Srinivas, D.; Ratnasamy, P., Sites for CO2 activation over amine-functionalized mesoporous Ti(Al)-SBA-15 catalysts. Micropor. Mesopor. Mat. 2006, 90, 314-326.