(80f) A First Principles Evaluation of the Role of Substituent Effects on the Interaction of Carbon Dioxide with Tertiary Amines

Introduction To effectively reduce carbon dioxide (CO2) emissions from post-combustion energy sources, there exists a need to develop an optimal sorbent for CO2 capture under flue gas conditions, and alternatively, a means of determining the optimal operating conditions for sorbents to be used under alternative environments. The use of solid amines, in contrast to the traditional liquid amines systems, allows us to analyze the system as a series of adsorption / desorption cycles which can be related to the thermodynamic and kinetic properties of the amine. For these gas phase interactions, the nitrogen acts as a basic site to interact with the acidic CO2, and substituent effects in its vicinity should directly influence both the basicity and nucleophilicity of the capture site. This influence ultimately results in changes to the driving forces involved in the interaction, meaning that each sorbent will require different operating conditions in order to maximize its capture efficiency. Here we investigate the role of substituent effects on the interaction of CO2 with the immobilized tertiary amidine N-Methyltetrahydropyrimidine (MTHP). Through modification of the functional group at the R1 position adjacent to the active nitrogen, we expect changes in the energies of desorption as well as the reaction rates. We use a combination of Density Functional Theory (DFT) calculations and experimental packed bed temperature programmed desorption (TPD) experiments in an attempt to develop a methodology capable of determining the adsorption energies of CO2 with various functionalities of MTHP and determining a range of optimal operating conditions for a particular sorbent. Materials and Methods Recent studies have shown that CO2 ? amidine preferentially create bicarbonate complexes over zwitterions complexes when in the presence of water [1]. We apply Density Functional Theory to finite basis set calculations to determine the structure of these complexes in addition to the adsorption energies. Structure optimization calculations were conducted using Dunning's correlation consistent basis sets. Also, the electronic energies obtained through our DFT calculations are adjusted for vibrational energetic and entropic terms derived from the partition function. Finally, a thermodynamic framework is then developed to relate our zero temperature calculations to those experimentally relevant. A plug flow reactor (PFR) was used to obtain packed bed adsorption / desorption experiments in which the exit gas could then be monitored by a mass spectrometer. Adsorption breakthroughs and desorption peak responses from a controlled temperature ramp were established from molar concentration measurements in the exit gas stream provided by the mass spectrometer. From these measurements, we establish both the capture efficiency (mol CO2/mol Amidine) and capture capacity (mol CO2/kg sorbent) in order to form some of the underlying thermodynamic properties of the sorbent for correlation with our DFT results. Results and Discussion Upon comparison of the energies formed in creation of both zwitterion and bicarbonate complexes, we confirm that the bicarbonate complex is more thermodynamically stable than the zwitterion complex. The range of substituent effects on MTHP includes charge transfer, hydrogen bonding, and steric hindrance. We find from our DFT calculations that while both steric hindrance and charge transfer play a role in the interaction, hydrogen bonding proved the strongest of the substituent effects. Additionally, the thermodynamic framework applied to our DFT calculations help to determine the driving forces required to surpass the desorption barrier. From these calculations we conclude that there exists a small range of adsorption energies that result in an optimal interaction for a given set of adsorption and desorption conditions. Conclusions This work provides insight into the role that substituent effects play on the gas phase interaction of CO2 and solid amines. An understanding of these effects, as well as the determination of adsorption energies from the combination of DFT and experimental analysis, provide a basis for determining the optimal conditions for a given sorbent. References 1. D. J Heldebrant et al., ?The Reaction of 1,8-Diazabicyclo[5.4.0]undec7ene (DBU) with Carbon Dioxide,? The Journal of Organic Chemistry 70, no. 13 (2005): 53355338. Figure 1. N-Methyltetrahydropyrimidine (R1=H), a tertiary amidine where the double bonded nitrogen atom could interact with CO2 in formation of (a) zwitterion complex or (b) bicarbonate complex