Carbamate Formation in the System (2-Methylpiperidine + Carbon Dioxide) by Raman Spectroscopy and X-ray Diffraction

Abstract

In aqueous solution, 2-methylpiperine (2-MP) has been proposed as a phase-separating amine for carbon-capture applications, whose carbamate is considered to be unstable due to steric hindrance. This paper demonstrates, for the first time, that the carbamate can be synthesized as the salt of the 2-methylpiperidinium cation (2-MPH+) and the 2-methylpiperidine-N-carboxylate anion (2-MPCOO–) by adding carbon dioxide gas to anhydrous liquid 2-MP at CO2/amine ratios α > 0.32 to yield well-defined prismatic crystals. Raman spectra have been measured for anhydrous liquid 2-MP and aqueous solutions of 2-MP and 2-MPH+Cl– at 298 K. The spectra of anhydrous liquid 2-MP, containing dissolved CO2 at lower CO2/amine ratios,. clearly showed the presence of 2-MPH+ and several unidentified bands that were attributed to the carbamate, 2-MPCOO–. Quantum chemical calculations at the B3LYP/6-311++G­(d,p) level of theory yielded simulated Raman spectra consistent with the experimental spectra. The spectra show the methyl group of both liquid and aqueous 2-MP and that of aqueous 2-MPH+Cl– to be in the equatorial position. The crystal structure shows the same conformations in the solid state and confirms that Raman spectroscopy can be used to determine the conformation of 2-MP species in the liquid state and aqueous solutions.

Formation Constants and Conformational Analysis of Carbamates in Aqueous Solutions of 2-Methylpiperidine and CO2 from 283 to 313 K by NMR Spectroscopy

“…The conformational assignment from the 2D NMR results is in agreement with the conformations expected to occur based on minimizing steric hindrance with the methyl group and the DFT NMR calculations (Table ). It also corroborates the conformations previously reported by NMR, DFT molecular energy calculations, and Raman spectroscopy of the solutions as well as X-ray diffraction of the 2-MPH + /2-MPCOO – crystal structure …”

“…The predominant conformations of the neutral and ionized aqueous amine and carbamate species were determined by 2D NMR to have the methyl group in the equatorial position for 2-MP and 2-MPH + and in the axial position for 2-MPCOO – . These findings are consistent with the other studies in our laboratory that use Raman spectroscopy and DFT calculations to identify the structure–function relationships that might control the speciation and phase behavior of aqueous alkylpiperidines that are candidates for use as phase-separating carbon capture solvents …”

“…The CO 2 injection system consisted of a CO 2 gas cylinder and a Druck DPI 104 digital pressure indicator, connected in-line by a 3.175 mm o.d. stainless steel tubing and directly attached to a high-pressure NMR tube, with the tubing and fittings described elsewhere . The tubing from the gas cylinder to the NMR sample tube was purged at least three times before opening the valve on the NMR tube to inject CO 2 .…”

“…Recently, Robinson et al , observed the formation of the 2-MP carbamate in nonaqueous solvents by infrared and mass spectroscopic methods. A current work in our laboratory by mass spectroscopy has shown that carbamates can be formed in liquid 2-MP and has recorded solid 2-MPH + with 2-MPCOO – from which crystal structures could be obtained . Anhydrous conditions do not permit CO 2 to react with water to form carbonate and bicarbonate, which compete with the formation of carbamates.…”

Theoretical Assessment of Carbon Dioxide Reactivity in Methylpiperidines: A Conformational Investigation

“…Additionally, trans-ax 2-MP also shows preference for the carbamate pathway and this is in accordance with the previous experimental works based on IR and NMR data of the resulting carbamate. 27,49 Moreover, some of the other conformers of MPs such as the cisax 3-MP and trans-ax 4-MP also show low barrier heights for carbamate formation, which indicates that the axial methyl and equatorial COO − group could be an energetically viable and ideal orientation of the functional groups in the carbamate product of MPs. Due to notable conformational variations involved in the carbamate formation and its hydrolysis, the mechanistic parameters of each possible conformers of MPs have been explored.…”