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.