Characterization of Chemisorbed Species and Active Adsorption Sites in Mg-Al Mixed Metal Oxides for High-Temperature CO2 Capture.

Mg-Al mixed metal oxides (MMOs), derived from the decomposition of layered double hydroxides (LDHs), have been purposed as adsorbents for CO2 capture of industrial plant emissions. To aid in the design and optimization of these materials for CO2 capture at 200 °C, we have used a combination of solid-state nuclear magnetic resonance (ssNMR) and density functional theory (DFT) to characterize the CO2 gas sorption products and determine the various sorption sites in Mg-Al MMOs. A comparison of the DFT cluster calculations with the observed 13C chemical shifts of the chemisorbed products indicates that mono- and bidentate carbonates are formed at the Mg-O sites with adjacent Al substitution of an Mg atom, while the bicarbonates are formed at Mg-OH sites without adjacent Al substitution. Quantitative 13C NMR shows an increase in the relative amount of strongly basic sites, where the monodentate carbonate product is formed, with increasing Al/Mg molar ratios in the MMOs. This detailed understanding of the various basic Mg-O sites presented in MMOs and the formation of the carbonate, bidentate carbonate, and bicarbonate chemisorbed species yields new insights into the mechanism of CO2 adsorption at 200 °C, which can further aid in the design and capture capacity optimization of the materials.

Acetate intercalated Mg-Al layered double hydroxides (LDHs) through modified amide hydrolysis: a new route to synthesize novel mixed metal oxides (MMOs) for CO2 capture.

Layered double hydroxide (LDH) based mixed metal oxides (MMOs) are promising high temperature CO2 capture sorbents. In order to improve their CO2 capture capacity, it is crucial to bring in changes to their physicochemical properties such as morphology, particle size, surface area and activity by tuning the synthesis method. Here we report a modified amide hydrolysis method to synthesize LDHs with a mixed morphology and better CO2 capture properties. Acetate intercalated Mg-Al LDHs with two different Mg/Al ratios (3 and 4) were synthesized by employing metal hydroxides as the starting precursors and acetamide as the hydrolysing agent. The resultant LDHs crystallized in a new morphology having a combination of both fibrous and sheet like crystallites. The MMOs derived from Mg-Al-acetate LDHs retained the mixed morphology observed in the precursor LDHs. The resultant MMOs showed almost a threefold increase in the BET surface area, 316 (Mg/Al = 3) and 341 (Mg/Al = 4) m2 g-1, compared to MMOs derived from anion exchanged Mg-Al-acetate LDH (118 m2 g-1). The MMOs synthesized by acetamide hydrolysis captured 1.2 mmol g-1 and 0. 87 mmol g-1 of CO2 at 200 and 300 °C (atmospheric pressure), respectively. The CO2 capture capacity realized was increased more than twofold compared to the CO2 capture capacity of MMOs derived from anion exchanged acetate LDH (0.57 mmol g-1) tested under similar conditions. The developed MMOs showed promising CO2 capture (1.0 mmol g-1) capacity at industrially relevant CO2 concentration (14%).

Advanced High-Temperature CO2 Sorbents with Improved Long-Term Cycling Stability.

Developing novel sorbents with maximum carbonation efficiency and good cycling stability for CO2 capture is a promising route to sequester anthropogenic CO2. In this work, we have employed a green synthesis method to synthesize CaO-based sorbents suitably stabilized by MgO and supported by in situ generated carbon under inert atmosphere. The varied amounts (10-30 wt %) of MgO were used to stabilize the CaO. The supported mixed metal oxide (MMO) sorbents were screened for high-temperature CO2 capture under CO2 rich (86% CO2) and lean (14% CO2) gas streams at 650 °C and atmospheric pressure. The MMO sorbents captured 53-63 wt % of CO2 per gram of sorbent under 86 and 14% CO2, accounting for about 98% carbonation efficiency, which outperforms the CO2 capture capacity of limestone derived CaO (L-CaO) sorbents (22.8 wt %). All of the synthetic MMO sorbents showed greater capture capacity and cyclic stability when compared to benchmark L-CaO. Because of the high carbonation efficiency and cycling stability of g-Ca0.69Mg0.3O sorbent, it was tested for 100 carbonation/regeneration cycles of 5 min each under CO2 lean conditions. The g-Ca0.69Mg0.3O sorbent showed exceptional CO2 capture capacity and cycling stability and retained about 65% of its initial capture capacity after 100 cycles.

The effect of the layer-interlayer chemistry of LDHs on developing high temperature carbon capture materials.

The layer-interlayer chemistry of layered double hydroxides (LDHs) offers a vast opportunity to develop mixed metal oxides (MMOs) as novel sorbents for high temperature CO2 capture. Here, we report the synthesis of adamantanecarboxylate intercalated Ca-Al LDH by employing a co-hydration method. Thermal decomposition under a controlled atmosphere converts the layered metal hydroxides to MMOs and an intercalated anion into a carbonaceous support. The higher thermal stability and polymeric nature of the intercalated adamantanecarboxylate ion act as a thermal/mechanical support for the layered MMOs. The resultant hybrid solid sorbent shows excellent high temperature CO2 capture and cycling performance under both CO2 rich and lean (industrial flue gas) conditions. The hybrid sample shows almost 100% carbonation of all the active phase present, leading to maximum atom efficiency. The hybrid sorbent also shows rapid kinetics for both the carbonation and regeneration steps. The employed synthetic strategy offers a new approach to develop improved novel sorbents based on LDHs for high temperature CO2 capture.

Reduced to Hierarchy: Carbon Filament-Supported Mixed Metal Oxide Nanoparticles.

We describe a one-pot synthesis method for carbon filament-supported mixed metal oxide nanoparticles. The thermal intracrystalline reaction of adamantanecarboxylate ions confined inside interlayer galleries of layered double hydroxide materials under a reducing atmosphere (H2) leads to carbon filaments forming in situ within the material. Raman spectroscopy and combined microscopy techniques show the formation of hybrid organic-inorganic carbon filaments with the mixed metal oxide particles interleaved.

Ultra-high aspect ratio hybrid materials: the role of organic guest and synthesis method.

Hybrid organic-inorganic layered double hydroxide materials have been prepared with an ultra-high aspect ratio via an environmentally friendly co-hydration approach where metal hydroxides and adamantane-carboxylic acid were used as the reagents. The method avoids use of either acidic or basic precipitation methods, or using a large excess of anion. The effect of organic anion, metal cation and synthesis method on developing ultra-high aspect ratio crystallites was studied.

Homogeneous precipitation by formamide hydrolysis: synthesis, reversible hydration, and aqueous exfoliation of the layered double hydroxide (LDH) of Ni and Al.

Homogenous precipitation by formamide hydrolysis results in the formation of a formate-intercalated layered double hydroxide (LDH) of Ni(II) and Al(III). The formate-LDH is sensitive to the atmospheric humidity and reversibly exchanges its intercalated water with atmospheric moisture. The hydration/dehydration cycle is complete within a narrow range of 0-30% relative humidity with significant hysteresis and involves a randomly interstratified intermediate phase. When immersed in water, the formate ion grows its hydration sphere (osmotic swelling), eventually leading to the exfoliation of the metal hydroxide layers into lamellar particles having in-plane dimensions of 100-200 nm and a thickness of 9-12 nm. These nanoplatelets restack to thicker tactoids again upon evaporation of the dispersion. The intercalated formate ion can be exchanged with nitrate ions in solution but not with iodide ions. These observations have implications for many applications of LDHs in the area of carbon dioxide sorption and catalysis.