Aminosilanes Grafted to Basic Alumina as CO₂ Adsorbents—Role of Grafting Conditions on CO₂ Adsorption Properties

Abstract

Solid oxide-supported amine sorbents for CO2 capture are amongst the most rapidly developing classes of sorbent materials for CO2 capture. Herein, basic γ supports are used as hosts for amine sites through the grafting of 3-aminopropyltrimethoxysilane to the alumina surface under a variety of conditions, yielding the expected surface-grafted alkylamine groups, as demonstrated by FTIR spectroscopy and (29)Si and (13)C cross-polarization magic-angle spinning (CPMAS NMR) spectroscopy. Grafting amine sites on the surface in the presence of water leads to a high density of amine sites on the surface whereas simultaneously creating a unique type of aluminum species on the surface, as demonstrated by both 1D and 2D (27)Al MAS NMR spectroscopy. The thus prepared sorbents result in higher CO2 adsorption capacities and amine efficiencies compared to sorbents prepared in the absence of water or similar amine loading sorbents prepared using silica supports. In situ FTIR spectra of the sorbents exposed to CO2 at various pressures show no distinct difference in the nature of the adsorbed CO2 species on alumina- versus silica-supported amines, whereas water adsorption isotherms show that the improved performance of the amine-grafted alumina support is not a consequence of retained water on the more hydrophobic aminoalumina materials. The findings demonstrate that amine-grafted, basic alumina materials can be tuned to be more efficient than the corresponding silica-supported materials at comparable amine loadings, further demonstrating that the properties of amine sites can be tuned by controlling or adjusting the support surface properties.

Role of Alumina Basicity in CO2 Uptake in 3‐Aminopropylsilyl‐Grafted Alumina Adsorbents

“…This indicates large mesopores are present in these materials (Figure 2a). [15] We further note that the values for the amine density differed betweent he two samples, with the D-Al 2 O 3 series showingg enerally lower values. As imilar trend was observed for the D-Al 2 O 3 series (Figure 2b), again suggesting the APS is being deposited inside the mesopores of these materials instead of purely on the exterior surface of the supports.…”

“…[15,17,52] The pattern was less crystalline than is typicallyo bserved for commercial g-alumina, which may be owed to the templated mesoporosity in the sample. The pattern for the ordered alumina sample demonstrated that it had crystallinity and was in good agreement with patternsi nt he literature, exclusively showing peaks associated with the cubic Fd3m space group of g-alumina.…”

“…For the O-Al 2 O 3 systemt he hysteresis between the adsorption and desorption steps occurs betweent he 0.85 and 0.95 relative pressure range. [15] The differences between O-Al 2 O 3 and D-Al 2 O 3 are likely ar esult of the larger pores in the O-Al 2 O 3 system faclitating more polymerization of the organosilane species. Grafting APS onto the www.chemsuschem.org 2017 Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim materials resulted in lower N 2 uptakes.T he hysteresis between the adsorption and desorption steps occurs at al ower relative pressure as the APS quantity is increased, shifting from 0.85-0.95 to 0.80-0.90, suggesting ad ecrease in the pore diameter, consistentw ith propylc hains extending into the pore.…”

“…[1,2] Presently,a queous amine solutions are used to captureC O 2 from large-scale gas streamsi n natural gas processing and they are the benchmark technology for CO 2 removal from exhaust flue-gas streams( typically 5-15 %C O 2 ), traditionally with monoethanolamine (MEA). [15][16][17] These systemsh ave shown good efficacy and resilience in conditions simulating both post-combustion and direct-air-capture (DAC) conditions. [3] Ar ange of solid sorbents for use in adsorptiveC O 2 separations currently exists, and one of the most common sorbent types utilizes amines introduced into porouss upports including silica, [4][5][6][7][8][9][10] carbon or relatedm ain group materials, [11,12] metal-organic frameworks, [13,14] and alumina materials.…”

“…[3] The volatile and corrosiven ature of such chemicals could be minimized through the use of as olid sorbents and novel adsorption processest oa ccomplish such separations. [15] The types of supported amine sorbents have been groupedi nto four classes (FigureS1i nt he Supporting Information). [15][16][17] These systemsh ave shown good efficacy and resilience in conditions simulating both post-combustion and direct-air-capture (DAC) conditions.…”