Alkoxy-Functionalized Amines as Single-Component Water-Lean CO2 Absorbents with High Efficiency: The Benefit of Stabilized Carbamic Acid

Aqueous alkanolamine-based processes currently represent the most mature and widely employed CO2 capture technology. However, extensive energy input and severe equipment corrosion constitute their major and inherent drawbacks due to the involvement of vast amounts of water as the diluent. Water-lean absorbents are proposed to deliver potential benefits, such as higher capacity and enhanced energy efficiency, by abandoning the aqueous solvent or replacing it with organic counterparts. Great efforts have been devoted to the development of CO2 capture protocols under nonaqueous circumstance, but their industrial deployment is still challenged by the exponentially increasing viscosity during operation. In this work, a series of alkoxy-functionalized methylamines have been devised as single-component postcombustion CO2 absorbents under water-lean condition. These nonaqueous amines are capable of reversibly capturing CO2 with low viscosities (48–114 cP at 25 °C and 27–63 cP at 40 °C) at their maximal gravimetric capacities (15–21 wt % at 25 °C and 14–21 wt % at 40 °C). Comprehensive mechanistic studies by means of in situ Fourier transform infrared spectroscopy, density functional theory calculations, and control experiments revealed that the stabilization of sequestered CO2 via intramolecular hydrogen bonding between in situ formed carbamic acid and the flexible alkoxy side chain of the designed amines would play the key role in enhancing both the capacity and flowability. Meanwhile, thermal desorption of the captured CO2 could easily be carried out at a feasible temperature (75 °C) under ambient pressure, and the CO2-saturated absorbents have remained intact at 80 °C for 2 days within a closed system. Furthermore, these novel amines would exhibit considerable physisorption by operating at high-pressure conditions (20 and 30 bar), thanks to the inherent CO2-philicity of the alkoxy functionality. Hence, the integration of enhanced capacity, reduced operating viscosity, and mild regeneration makes such alkoxy-functionalized methylamine-type absorbent a compelling candidate for practical application.