(770e) Atomic Level Design of Bifunctional Titanium-Amine Materials for Cooperative Catalysis

Moving beyond carbon capture and sequestration, there is an increasing call for materials suitable for carbon capture and conversion. Many of the reactions that would be interesting in this context could potentially be carried out if suitably cooperative acid-base catalysts could be designed and synthesized. Acid-base bifunctional catalysis is a common motif in enzymes and some types of supported catalysts, and has been applied to the conversion of CO₂ into industrially desirable compounds like solvents (cyclic carbonates) or fuels (via photoreduction). Generally, 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. 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 poison each other through ion pairing; e.g. forming ‘frustrated’ pairs..

We report here on the synthesis of solid materials comprising Lewis acidic, dispersed Ti centers and propylamine groups supported in close proximity on a silica surface. Materials are characterized by ¹³C CP/MAS NMR, X-ray absorption near-edge spectroscopy (XANES), non-aqueous potentiometric titrations, TGA, and elemental analysis. The typical aminopropyltriethoxysilane (APTES) precursor is used as a control; here amines are generated by grafting a carbamate-containing 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 Ti site remains similar to the starting material (and thus catalytically active) in the presence of the amine liberated from the carbamate, but is deactivated after typical grafting of APTES. This talk will discuss several probe reactions used to assess the functionality of the acid site, the base site, and cooperative beneficial interactions that occur between the two. 

CO₂ adsorption at low pressures is used as a probe of the base site accessibility in the presence of Ti. Properly normalized CO₂ adsorption isotherms show that 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.

The Knoevenagel condensation of ethyl cyanoacetate with benzaldehyde are also presented as a probe of cooperativity. The Knoevenagel condensation is an oft-cited base-catalyzed reaction that is thought to proceed through and enamine intermediate.  However, it has also been suggested in the literature that in the presence of Bronsted acid sites, the reaction proceeds with enhanced rates and through an imine intermediate.  Our materials allow us to probe the effect of adjacent surface Lewis acids. Initial results suggest that the presence of Ti does have an effect on the rates of reaction that is strongly dependent upon the catalyst’s amine to Ti ratio. Additional results will be presented from other reactions, time permitting.