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

Authors: 
Young, P., Northwestern University
Notestein, J. M., Northwestern University


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.

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