(688a) CO2 Chemisorption On Isolated Amines Grafted On SiO2 and Ti-SiO2

Moving
beyond carbon capture and sequestration, there is an increasing call for
materials suitable for carbon capture and conversion. Materials for carbon
capture and conversion have slightly different requirements than those only
designed for carbon capture, namely that they need both acid and base sites. Acid-base
bifunctional catalysis is a common motif in enzymes and some types of supported
catalysts, and may be applicable to the conversion of CO₂ into
industrially desirable compounds like solvents (cyclic carbonates) or fuels
(via photoreduction). At their most fundamental level, catalytic materials for these
processes require a nucleophilic site to chemisorb and activate CO₂,
which must be proximate to an acidic center capable of activating the
co-reactant or transferring electrons. The challenge of creating such catalysts
lies in controlling materials syntheses so that the two competing functions are
in close proximity, but not so close that they annihilate each other.

We
report here on the synthesis of solid materials comprising Lewis acidic,
dispersed Ti centers and propylamine groups both supported in close proximity
on a silica surface. Materials are characterized by ¹³C CP/MAS NMR,
non-aqueous potentiometric titrations, TGA, and elemental analysis. Rather than
use the typical aminopropyltriethoxysilane (APTES) precursor, amines are
generated by grafting a carbamate precursor onto the Lewis acidic surfaces;
these do not interact with the acid sites as synthesized, but expose a primary
amine upon mild thermal treatment. DRUV-vis shows that the Ti⁴⁺ site
remains uncoordinated (and thus catalytically active) in the presence of the amine
liberated from the carbamate, but is deactivated after typical grafting of
APTES. The bulk of this talk will focus on the behavior of these materials in
the chemisorption of CO₂ at 1 kPa partial pressure and below, as a
function of amine precursor, amine surface density, and surface density of the
Lewis acid centers.

Properly
normalized CO₂ adsorption isotherms show saturation behaviour in
low-pressure CO₂ uptake. This uptake increases dramatically as the
grafted carbamate is converted to a free amine, but at all apparent surface
densities, primary amines synthesized via the carbamate route adsorb much less
CO₂ (maximum of ~0.05 CO₂/amine) vs. those derived from
APTES (maximum of ~0.35 CO₂/amine). This is consistent with much
better dispersion of amines by the carbamate route, which, while detrimental to
the total CO₂ uptake, is needed for acid-base catalysis to give
access to the underlying surface. The threshold for high CO₂ uptake
resulting from cooperative amine-amine interactions occurs at ~0.6 amines/nm²
via APTES, but no cooperativity is apparent up to 0.9 amines/nm² for
the carbamate derived materials, suggesting that APTES materials have their true
local surface density underestimated by as much as 150%.

The
presence of Ti on the silica surface generally diminishes the ability for the
amines to uptake CO₂. The presence of Ti strongly diminishes the
capacity of the APTES materials. For example, uptake falls from 0.13 to 0.01 CO₂/amine
at 1 kPa when 180 μmol/g Ti is added. In contrast, CO₂ uptake
is much less affected when the carbamate route is used to graft the amines; uptake
starts at a lower level but falls only from 0.045 to 0.030 CO₂/amine
when 180 μmol/g Ti is added. The uptake is always above that of the
APTES-derived material, even when a large excess of Ti is present (600
μmol/g Ti). Overall, these results are consistent with our hypothesis that
the presence of the carbamate group during synthesis prevents undesired
interactions between the Ti and the amine.  The precursor dictates that, upon
deprotection, the amines are tethered to the surface in such a way that this
inability of the Ti and N to interact is maintained, but each of the sites
retains their ability to perform independent chemistry.