This study focused on CO₂-triggered phase separation in concentrated dispersions of mono-, di-, and triaminated silica for CO₂ capture, based on the hypothesis of reduced regeneration energy coupled with the formation of a CO₂-rich, water-lean sediment. The sedimentation of the CO₂-rich and CO₂-lean dispersions was studied using time-resolved optical transmittance measurements. The CO₂ capacity, reaction rate, diffusivity, and solubility of the dispersions were also studied. The involved CO₂-amine chemistry was studied by using infrared (IR) and cross-polarization solid-state ¹³C and ¹⁵N NMR spectroscopy, and the fluid behavior of the dispersions was studied by rheology. The aminated silica was of the SBA-15 type and characterized by N₂ adsorption and desorption experiments, thermogravimetric analysis, powder X-ray diffraction, scanning and transmission electron microscopy, and crosspolarization solid-state ²⁹Si NMR spectroscopy. The derived energy balances for a simplified process indicated that the regeneration of the CO₂-rich sediments results in an energy demand similar to that of 30 wt % ethanolamine (MEA) in water. The bounds of the energy balances were found to be limited by the somewhat low CO₂ capacities of the dispersions, which underscores the need for increasing the amino group density of the dispersions in future efforts. The observed changes in transmittance between the CO₂-lean and CO₂-loaded dispersions showed that sedimentation occurred within the first 10 min for the CO₂-loaded dispersions, while the CO₂-lean dispersions exhibited no change in transmittance after 60 min. The analysis of the pressure decay curves of the partial CO₂ pressure showed that the absorption rates of the dispersions were smaller than those of monoethanolamine (MEA) in water but were similar to the absorption rates of 2-amino-2-methyl-1-propanol (AMP) in water. The IR spectroscopic analysis was consistent with the formation of ammonium carbamates at a low CO₂ loading and the subsequent formation of HCO₃^– at a higher loading. The flow curves displayed rich and complex fluid behavior, which was strongly affected by the capture of CO₂ by the dispersions. Phenomena such as shear thinning, jamming, and thixotropy were observed.

Efficient and durable biocatalysts are important for sustainable CO₂ capture technologies, but enzyme stability often limits their use under harsh process conditions. Here, we evaluate carbonic anhydrases (CAs) adsorbed onto aminated mesoporous SBA-15 as biocatalysts for CO₂ capture under the hypothesis of adsorption-induced thermal stabilization. Carbonic anhydrase from the thermophilic bacterium Persephonella marina (pmCA) and commercial bovine erythrocyte carbonic anhydrase (bCA) were used. Enzyme adsorption isotherms for pmCA and bCA onto the aminated SBA-15 were established, along with desorption tests. Adsorbed and free pmCA and bCA were incubated at 40–90 °C for 14 d. The structural integrity and possibility of amine leaching of the incubated (90°, 14 d) aminated SBA-15 were analyzed by X-ray diffraction (XRD) and NMR spectroscopy. The reaction product speciation in CO₂-loaded catalyzed and uncatalyzed dispersions was monitored using infrared (IR) spectroscopy. The maximum enzyme adsorption capacities were established to be 1.4 ± 0.2 g pmCA·g-aminated SBA-15^–1 and 2.1 ± 0.5 g bCA·g-aminated SBA-15^–1, with no detectable desorption. Adsorbed pmCA and bCA maintained high activity for 14 d at 40–65 °C and for 4 d at 90 °C, whereas free enzymes lost activity within 4 d at all temperatures. The XRD patterns of the heat-treated (90 °C, 14 d) aminated SBA-15 indicated a full collapse of the mesostructure. IR spectroscopy confirmed enhanced HCO₃^– formation in the presence of immobilized CA. Overall, enzyme adsorption onto the aminated SBA-15 significantly improved the thermal stability and activity of pmCA and bCA compared to the free enzymes, demonstrating the potential of adsorbed CAs for biocatalysis.

Carbonic anhydrases (CA-s) are an attractive type of catalyst for CO₂ capture system, offering a high hydration and dehydration activity for CO₂. However, reduced enzyme stability limits the industrial application of CA-s. Bovine erythrocyte CA (bCA) is one of the most active variants of CA because of its high enzyme activity and commercial scale production, but the lifetime at elevated temperature is limited because of its mesophilic nature. This led us to explore the hypothesis that adsorption-induced thermal stabilization would enhance the properties of bCA. A thermally stable zeolitic imidazole framework (ZIF) with large pores ZIF-90 was chosen, and bCA was encapsulated withing the ZIF-90 crystals (bCA@ZIF-90) via a water-based synthesis. The as-synthesized bCA@ZIF-90 was characterized by X-ray diffraction (XRD), N₂ adsorption, scanning electron microscopy (SEM), and infrared (IR) and Raman spectroscopy and the data was compared with that of regular ZIF-90. The catalytic hydration and dehydration activity of CO₂ by bCA@ZIF-90 was studied using a stopped-flow technique using UV-vis absorption spectrometry and colorimetric responses of pH-sensitive dyes. The concentration of bCA@ZIF-90 was intentionally kept low to reduce interference with the UV-vis absorption of the dyes. This resulted in an effective bCA concentration of 65 ± 3 nM  (1.95 ± 0.09 μg bCA/mL), up to three orders of magnitude lower than that reported in previous literature. The thermal stability of bCA@ZIF-90 was evaluated by incubation at pH 9.4 and 7.1 at 60°C for three weeks. ZIF-90 itself was incubated at pH 9.4 and 7.1 at 60°C for two weeks, and boiled at pH 7.1 for 24 h. The enzyme-containing bCA@ZIF-90 exhibited a well resolved diffractogram consistent with as-synthesized and simulated ZIF-90 structure, indicating that crystallinity was preserved during introduction of bCA. BET analysis of the N₂ adsorption of the regular ZIF-90 sample revealed a surface area of 890 m²/g, and the N₂ adsorption/desorption traces exhibited a hysteresis loop in the region of p/p⁰ = 0.3–0.4, which we ascribed to interparticle mesopores with a Barrett-Joyner-Halenda (BJH) analysis of the desorption branch. We hypothesize that the bCA in bCA@ZIF-90 resides in these crystal defects. IR spectral analyzes revealed bands consistent with amide I and amide II modes of the CA polypeptide backbone in bCA@ZIF-90. Analyzes of the reaction kinetics showed that bCA@ZIF-90 catalyzed the hydration of CO₂ at pH 9.4 and 7.1 at temperatures 30–60°C, and that hydration kinetics was enhanced by a factor 1.2 ± 0.1 (95% CI [1.17, 1.23]) at pH 9.4 and 2.1 ± 0.6 (95% CI [1.9, 2.3]) at pH 7.1 when using bCA@ZIF-90 compared to the uncatalyzed reaction. As the effective enzyme concentration was kept very low, only moderate rate enhancements were observed. Dehydration by bCA@ZIF-90 exhibited a more complicated behavior, where catalytic effects were observed at temperatures T ≥ 40°C.  In relation to thermal stability, enzyme activity of bCA@ZIF-90 was maintained after incubation for 3 weeks at 60°C. However, the XRD patterns of incubated samples of ZIF-90 revealed that it had transformed into another crystalline compound with a large unit cell when incubated at 60°C. The XRD pattern of ZIF-90 boiled at 100°C pH 7.1 for 24 h was consistent with a full recrystallization into a compound with likely a smaller unit cell. The phase-transformation of ZIF-90 also appeared to be pH-dependent, and it is concluded that bCA is stabilized by the adsorption and that more stable porous materials than ZIF-90 could be explored.