Sophia E. Hayes

Elucidation of Surface Species through in Situ FTIR Spectroscopy of Carbon Dioxide Adsorption on Amine-Grafted SBA-15

The nature of the surface species formed through the adsorption of CO2 on amine-grafted mesoporous silica is investigated through in situ FTIR spectroscopy with the aid of 15 N dynamic nuclear polarization (DNP) and 13 C NMR spectroscopy. Primary, secondary, and tertiary amines are functionalized onto a mesoporous SBA-15 silica. Both isotopically labeled 13 CO2 and natural-abundance CO2 are used for accurate FTIR peak assignments, which are compared with assignments reported previously. The results support the formation of monomeric and dimeric carbamic acid species on secondary amines that are stabilized differently to the monocarbamic acid species on primary amines. Furthermore, the results from isotopically labelled 13 CO2 experiments suggest the existence of two carbamate species on primary amines, whereas only one species is observed predominantly on secondary amines. The analysis of the IR peak intensities and frequencies indicate that the second carbamate species on primary amines is probably more asymmetric in nature and forms in a relatively smaller amount. Only the formation of bicarbonate ions at a low concentration is observed on tertiary amines; therefore, physisorbed water on the surface plays a role in the hydrolysis of CO2 even if water is not added intentionally and dry gases are used. This suggests that a small amount of bicarbonate ions could be expected to form on primary and secondary amines, which are more hydrophilic than tertiary amines, and these low concentration species are difficult to observe on such samples.

Single Crystal to Single Crystal Topochemical Photoreactions: Measuring the Degree of Disorder in the [2 + 2] Photodimerization of trans-Cinnamic Acid Using Single-Crystal 13C NMR Spectroscopy

A single crystal of α-trans-cinnamic acid was synthesized with a (13)C-label at the β-carbon position and photoreacted to yield the [2+2] cycloaddition product, α-truxillic acid. (13)C{(1)H} cross-polarization (CP) single-crystal NMR experiments were performed on the unreacted and sequentially photoreacted samples for different goniometer orientations, and the spectra were simulated using the SIMMOL and SIMPSON software packages. Atomic coordinates from single-crystal X-ray diffraction data were used as inputs in the simulations, which allowed the chemical shift tensor to be precisely measured and related to the unit cell (or molecular) reference frame of cinnamic acid. The line widths of the (13)C resonances observed at different goniometer rotations were utilized to estimate the orientational dispersion of the cinnamic acid species, which ultimately provides a measure of disorder in the single crystal. The photoreacted sample, a solid solution of cinnamic and truxillic acids, maintained its single-crystal nature, even up to 44% conversion to truxillic acid, keeping its P2(1)/n symmetry. Upon photoirradiation, however, a slight loss of order was observed in the cinnamic acid species as evidenced by an increase in the (13)C NMR line widths, demonstrating that NMR can be used to monitor subtle orientational imperfections in single crystal to single crystal photoreactions.

Silica-Supported Sterically Hindered Amines for CO2 Capture

Most studies exploring the capture of CO2 on solid-supported amines have focused on unhindered amines or alkylimine polymers. It has been observed in extensive solution studies that another class of amines, namely sterically hindered amines, can exhibit enhanced CO2 capacity when compared to their unhindered counterparts. In contrast to solution studies, there has been limited research conducted on sterically hindered amines on solid supports. In this work, one hindered primary amine and two hindered secondary amines are grafted onto mesoporous silica at similar amine coverages, and their adsorption performances are investigated through fixed bed breakthrough experiments and thermogravimetric analysis. Furthermore, chemisorbed CO2 species formed on the sorbents under dry and humid conditions are elucidated using in situ Fourier-transform infrared spectroscopy. Ammonium bicarbonate formation and enhancement of CO2 adsorption capacity is observed for all supported hindered amines under humid conditions. Our experiments in this study also suggest that chemisorbed CO2 species formed on supported hindered amines are weakly bound, which may lead to reduced energy costs associated with regeneration if such materials were deployed in a practical separation process. However, overall CO2 uptake capacities of the solid supported hindered amines are modest compared to their solution counterparts. The oxidative and thermal stabilities of the supported hindered amine sorbents are also assessed to give insight into their operational lifetimes.