(86i) Materials for Direct Air Capture of CO2 Via Particle Molecular Layer Deposition

As the adverse impacts of anthropogenic climate change continue to become more prevalent and intense, adaptation and mitigation technologies are becoming vital. Among the mitigation technologies, direct air capture (DAC) of CO₂ addresses emissions from non-stationary sources and could aid in addressing the existing atmospheric concentration. Further implementation of DAC technology in conjunction with an array of other fields that use CO₂ such as solar thermal chemical processing or reverse water gas shift reactions could produce liquid fuels and other desirable products. However, DAC faces complications due to the high processing volume of air and the low concentration of CO₂. Improvements in the CO₂ sorbents are needed to reduce cost and politically motivate DAC implementation. Traditional CO₂ capture technology relied on liquid amines which require substantial energy, and therefore substantial cost, to regenerate. To lower sorbent regeneration cost, solid adsorbent materials have been a focus of research for the past few decades. In particular, supported amines are attractive for DAC as they can operate under ambient conditions, regenerate at relatively low temperatures, and are tolerant of humidity. The current techniques of functionalization through liquid methods face limitations; grafting tends to have lower adsorption capacities while impregnation faces poor regeneration stability (Unveren 2017). We aim to introduce a novel technique to generate supported amine adsorbents with high adsorption capacity and stable regeneration.

Here, we employ particle molecular layer deposition (MLD) as a vapor-phase functionalization method. MLD is closely related to atomic layer deposition (ALD), involving a series of stepwise reactions between gaseous reactants on a solid surface to create thin films (Weimer 2019). We use MLD to deposit a covalently bonded aminopropylsiloxane network on particle supports. Two MLD chemistries have been employed: (3-aminopropyl)triethoxysilane (mono-amine) and N1-(3-trimethoxysilylpropyl)diethylene triamine (tri-amine). Sorbent materials were generated on varying support materials with varying amine composition based on a design of experiments. For comparison, materials were also generated through traditional liquid phase grafting. Characterization of the sorbents included BET surface area analysis, LECO elemental analysis, CO­₂ chemisorption, and thermogravimetric analysis (TGA). The adsorption capacity was shown to increase as the number of MLD cycles increased for both chemistries. To understand the efficacy of the MLD functionalization method, we took the adsorption capacity on the basis of surface area. For example, the 25 cycle tri-amine on fumed silica MLD sample achieved 0.005 mmol/m² which is competitive with other surface functionalization methods and well performing adsorbents in literature. Ambient adsorption and low regeneration temperatures, below 100°C, were demonstrated by the mono-amine and the tri-amine materials. These materials appeared robust over cycling conditions between 25°C and 80°C and appeared stable for adsorption/desorption cycling temperatures below the deposition temperature of 150°C. This work demonstrates a new functionalization method and capture material to the field of solid CO₂ capture research that could aid in widespread CO₂ DAC implementation.

Ünveren, E. E.; Monkul, B. Ö.; Sarıoğlan, Ş.; Karademir, N.; Alper, E. Solid amine sorbents for CO 2 capture by chemical adsorption: A review. Petroleum 2017, 3 (1), 37

Weimer, A.W. Particle atomic layer deposition. J. Nanopart. Res. 2019, 21:9