Transport, Zwitterions, and the Role of Water for CO2 Adsorption in Mesoporous Silica-Supported Amine Sorbents

The uptake of CO2 in highly loaded, silica-supported, polyethylenimine (PEI)-impregnated sorbents was investigated in a reaction-diffusion model of the CO2 adsorption process. The model successfully replicated the pseudoequilibrium behavior experimentally observed in thermogravimetry (TGA) experiments. A parametric study and sensitivity analysis of the model revealed that the stability and mobility of diffusive intermediatesassumed in the model to be zwitterionseffectively control the observable capacity of the sorbent. A subsequent quantum chemical study called into question the stability of zwitterions in PEI but suggested that physically bonded moieties involving water, amines, and CO2 may be better candidates for diffusive intermediates. The implications are a strong dependence of the observable CO2 capacity of the sorbent on the presence of water in the gas stream, which was found to be consistent with TGA results. ■ INTRODUCTION Given the enormous role of fossil fuels in global power generation and the slow pace of the scale-up of renewable resources, carbon capture and storage (CCS) is increasingly recognized as a key strategy in the fight against global climate change, but CCS faces its own challenges. One of the principal challenges is that the current state-of-the-art postcombustion separation technologyaqueous amine scrubbingreduces the thermal efficiency of the generation process to which it is attached by approximately 30%, with a corresponding increase in the cost of electricity in the neighborhood of $0.07/kWh. New separation technologiessuch as solid sorbents or ionic liquidscould decrease these numbers, but they remain unproven at industrial scales. The Department of Energy’s Carbon Capture Simulation Initiative (CCSI) is a project aimed at reducing the typical 20−30 year timeline for development and deployment of new technologies in the power generation sector through the use of science-based simulation. Solid sorbents have been the technology of primary focus in the first two years of the project; this article contains results on the physicochemical modeling and theoretical study of a particular class of sorbentsmesoporous silica-supported amine (SSA) sorbentsobtained during the course of the project’s first two years. A review of the development history for SSA sorbents can be found in a broad review of carbon capture technology written by Choi et al. Generally amine groups are loaded onto a mesoporous silica substrate with either ordered porosity (such as SBA-15) or disordered porosity (silica xerogel). The amines may be covalently tethered to the substrate through the reaction of an aminosilane with silanol groups on the silica surface or through the direct impregnation of amines into the silica support. It has long been clear that transport of CO2 through the amine bulk is the key limiting factor for the reaction rate (and thereby the working capacity for uptake of CO2) for SSA sorbents with a relatively high amine loading. Evidence comes in the form of the frequently reported maximum in capacity as a function of temperature, as well as the observed dependence of capacity on the internal surface area of the sorbent. However, previously reported kinetic models for SSA sorbents have utilized semiempirical kinetic approaches which are not suitable for describing these effects. The uptake of CO2 in aqueous alkanolamines (which is the most intensively studied amine−CO2 interaction) has most often been described as the formation of alkylammonium carbamates through a zwitterion mechanism. The zwitterion mechanism does not enjoy universal support, however, and an alternative termolecular mechanism commonly represented as a reaction between gaseous CO2 and an amine that is already in a hydrogen-bonded relationship with a free basehas been proposed. The distinction is a subtle one. The zwitterion mechanism is written + ⇌ + − R NH CO (g) R NH CO 2 2 2 2 (1) + ⇌ + + − − + R NH R NH CO R NCO R NH 2 2 2 2 2 2 2 (2) whereas the termolecular mechanism reads Received: July 31, 2013 Revised: November 25, 2013 Published: November 26, 2013 Article