(483d) Controlled Synthesis of Bifunctional Acid/Base Catalysts for CO2 Capture and Reaction

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.