Stability of Supported Amine Adsorbents to SO₂ and NO_<i>x</i> in Postcombustion CO₂ Capture. 2. Multicomponent Adsorption

Packed bed CO₂ adsorption breakthrough experiments using both amine-impregnated and amine-grafted silica adsorbent materials in the presence of SO₂, NO and NO₂ impurities are reported. The effects of temperature, feed concentration and adsorbent amine loading on the dynamic adsorption capacity of the adsorbents are evaluated by performing dual component SO₂/CO₂, NO/CO₂ and NO₂/CO₂ coadsorption experiments as well as three component SO₂/NO/CO₂ adsorption experiments. Although SO₂ is found to significantly influence the dynamic CO₂ capacity of aminosilica adsorbents, the obtained results confirm the long-term stability of the adsorbents during SO₂/CO₂ coadsorption runs when the bed is not allowed to fully saturate with SO₂. On the other hand, little competitive effect of NO on CO₂ adsorption is observed in any case. This is due to the decreased affinity of amine-based adsorbents toward NO as opposed to SO₂. The more reactive nitrogen oxide, NO₂, is shown to have a minimal impact on CO₂ adsorption when it is present at low levels in the simulated flue gas. Among the adsorbents investigated, the results demonstrate that secondary amine containing adsorbents are more stable to SO_<i>x</i> and NO_<i>x</i> impurities in CO₂ capture processes than those that contain primary amine groups

Amine-Functionalized Porous Silicas as Adsorbents for Aldehyde Abatement

A series of aminopropyl-functionalized silicas containing of primary, secondary, or tertiary amines is fabricated via silane-grafting on mesoporous SBA-15 silica and the utility of each material in the adsorption of volatile aldehydes from air is systematically assessed. A particular emphasis is placed on low-molecular-weight aldehydes such as formaldehyde and acetaldehyde, which are highly problematic volatile organic compound (VOC) pollutants. The adsorption tests demonstrate that the aminosilica materials with primary amines most effectively adsorbed formaldehyde with an adsorption capacity of 1.4 mmol_HCHO g^–1, whereas the aminosilica containing secondary amines showed lower adsorption capacity (0.80 mmol_HCHO g^–1) and the aminosilica containing tertiary amines adsorbed a negligible amount of formaldehyde. The primary amine containing silica also successfully abated higher aldehyde VOC pollutants, including acetaldehyde, hexanal, and benzaldehyde, by effectively adsorbing them. The adsorption mechanism is investigated by ¹³C CP MAS solid-state NMR and FT-Raman spectroscopy, and it is demonstrated that the aldehydes are chemically attached to the surface of aminosilica in the form of imines and hemiaminals. The high aldehyde adsorption capacities of the primary aminosilicas in this study demonstrate the utility of amine-functionalized silica materials for reduction of gaseous aldehydes

Insights into Azetidine Polymerization for the Preparation of Poly(propylenimine)-Based CO₂ Adsorbents

The cationic ring-opening polymerization of azetidine to form branched poly­(propylenimine) (PPI) is investigated for the purpose of evaluating the utility of PPI/silica composite adsorbents for CO₂ capture. The polymerization kinetics and primary:secondary:tertiary amine distribution are monitored with ¹H NMR during reaction with varied synthesis conditions (i.e., reaction time 20–150 h), temperature (343–353 K), and monomer to acid initiator (here, HClO₄) ratio. It is found that primary amines are converted to tertiary amines with increased polymerization time, while the addition of monomer over the first 6 h of polymerization increases the primary amine content. This suggests a mechanism where the monomer is rapidly consumed, leaving dimers or small oligomers that still contain rings as key reaction centers. The synthesized polymer is neutralized with either NH₄OH or a basic resin and impregnated into mesoporous silica (SBA-15). The CO₂ capture properties of these composite adsorbents are investigated, giving information about the effectiveness of the acid neutralization processes

Steam Induced Structural Changes of a Poly(ethylenimine) Impregnated γ‑Alumina Sorbent for CO₂ Extraction from Ambient Air

Poly­(ethylenimine) (PEI) impregnated mesoporous γ-alumina sorbents are utilized for CO₂ adsorption from dry and humid simulated ambient air, and the sorbents are regenerated under an environment of flowing steam for times ranging from 5 min to 24 h of continuous exposure. The sorbents are compared on the basis of equilibrium CO₂ capacities from simulated air at 400 ppm of CO₂, 50% relative humidity, and 30 °C as well as their physiochemical characterization by means of X-ray diffraction (XRD), ²⁷Al NMR spectroscopy, IR spectroscopy, Raman spectroscopy, N₂ physisorption, and elemental analysis. The sorbents retain better than 90% of the initial equilibrium capacity of ∼1.7 mmol/g at steam exposure times up to 12 h; however, PEI leaching reduced the capacity of the sorbent to 0.66 mmol/g after 24 h of continuous treatment. It is demonstrated that the γ-alumina support partially hydrates to form a boehmite crystal phase at steam times of 90 min and longer but that this phase transition occurs predominately between 90 min and 12 h of steam treatment, slowing at longer times of 12 and 24 h of treatment. Evidence is presented to suggest that the presence of boehmite on the sorbent surface does not significantly alter the amine efficiency of impregnated PEI. The collected results suggest that γ-alumina/PEI composite materials are promising sorbents for CO₂ capture from ambient air with regeneration in flowing steam

Recyclable Silica-Supported Iridium Bipyridine Catalyst for Aromatic C–H Borylation

A mesoporous silica (SBA-15)-supported bipyridine iridium complex is prepared by grafting of bipyridine onto the silica support, followed by complexation of an iridium­(I) precursor in the presence of HBpin and cyclooctene. Structural analyses by X-ray powder diffraction, nitrogen physisorption, FT-IR, and solid-state NMR spectroscopy demonstrate that the 3-dimensional, hexagonal pore structure of SBA-15 is maintained after the immobilization. In particular, as a heterogeneous catalyst, this silica-supported iridium complex shows moderate to good catalytic activity in the aromatic C–H borylation of a variety of substrates. More importantly, the heterogeneous catalyst is recovered easily and reused repeatedly by simple washing without chemical treatment and exhibits good recycling performance with a modest decrease in the catalytic rate, showing good potential for increasing the overall turnover number of this synthetically useful catalyst

Hybrid Polymer/UiO-66(Zr) and Polymer/NaY Fiber Sorbents for Mercaptan Removal from Natural Gas

Zeolite NaY and metal organic frameworks MIL-53­(Al) and UiO-66­(Zr) are spun with cellulose acetate (CA) polymer to create hybrid porous composite fibers for the selective adsorption of sulfur odorant compounds from pipeline natural gas. Odorant removal is desirable to limit corrosion associated with sulfur oxide production, thereby increasing lifetime in gas turbines used for electricity generation. In line with these goals, the performance of the hybrid fibers is evaluated on the basis of sulfur sorption capacity and selectivity, as well as fiber stability and regenerability, compared to their polymer-free sorbent counterparts. The capacities of the powder sorbents are also measured using various desorption temperatures to evaluate the potential for lower temperature, energy, and cost-efficient system operation. Both NaY/CA and UiO-66­(Zr)/CA hybrid fibers are prepared with high sorbent loadings, and both have high capacities and selectivities for <i>t</i>-butyl mercaptan (TBM) odorant sorption from a model natural gas (NG), while being stable to multiple regeneration cycles. The different advantages and disadvantages of both types of fibers relative are discussed, with both offering the potential advantages of low pressure drop, rapid heat and mass transfer, and low energy requirements over traditional sulfur removal technologies such as hydrodesulfurization (HDS) or adsorption in a pellet packed beds

Cooperative Catalysis with Acid–Base Bifunctional Mesoporous Silica: Impact of Grafting and Co-condensation Synthesis Methods on Material Structure and Catalytic Properties

The structural and cooperative catalytic characteristics of acid and base co-functionalized mesoporous silica synthesized through grafting and co-condensation methods are investigated. It is shown that incorporation of the mutually reactive amine and carboxylic acid functional groups is aided by a protecting group in the grafting method. Using a thermally cleavable protecting group on the carboxylic acid organosilane, the differential effect of silanol removal and acid group functionalization on catalytic activity is studied. For samples prepared here by both the co-condensation and grafting procedures, the removal of silanols and the introduction of the carboxylic acid has a negative impact on activity of the catalyst in aldol condensations under the conditions used here. These results demonstrate that a weaker Brønsted acid silanol is more effective in cooperatively catalyzing the aldol condensation in combination with an amine base than the stronger carboxylic acid for all the materials prepared in this study

Linking CO₂ Sorption Performance to Polymer Morphology in Aminopolymer/Silica Composites through Neutron Scattering

Composites of poly­(ethylenimine) (PEI) and mesoporous silica are effective, reversible adsorbents for CO₂, both from flue gas and in direct air-capture applications. The morphology of the PEI within the silica can strongly impact the overall carbon capture efficiency and rate of saturation. Here, we directly probe the spatial distribution of the supported polymer through small-angle neutron scattering (SANS). Combined with textural characterization from physisorption analysis, the data indicate that PEI first forms a thin conformal coating on the pore walls, but all additional polymer aggregates into plug(s) that grow along the pore axis. This model is consistent with observed trends in amine-efficiency (CO₂/N binding ratio) and pore size distributions, and points to a trade-off between achieving high chemical accessibility of the amine binding sites, which are inaccessible when they strongly interact with the silica, and high accessibility for mass transport, which can be hampered by diffusion through PEI plugs. We illustrate this design principle by demonstrating higher CO₂ capacity and uptake rate for PEI supported in a hydrophobically modified silica, which exhibits repulsive interactions with the PEI, freeing up binding sites

Adsorption Microcalorimetry of CO₂ in Confined Aminopolymers

Aminopolymers confined within mesoporous supports have shown promise as materials for direct capture of CO₂ from ambient air. In spite of this, relatively little is known about the energetics of CO₂ binding in these materials, and the limited calorimetric studies published to date have focused on materials made using molecular aminosilanes rather than amine polymers. In this work, poly­(ethylenimine) (PEI) is impregnated within mesoporous SBA-15, and the heats of CO₂ adsorption at 30 °C are investigated using a Tian-Calvet calorimeter with emphasis on the role of PEI loading and CO₂ pressure in the compositional region relevant to direct capture of CO₂ from ambient air. In parallel, CO₂ uptakes of these materials are measured using multiple complementary approaches, including both volumetric and gravimetric methods, and distinct changes in uptake as a function of CO₂ pressure and amine loading are observed. The CO₂ sorption behavior is directly linked to textural data describing the porosity and PEI distribution in the materials

Mesoporous Alumina-Supported Amines as Potential Steam-Stable Adsorbents for Capturing CO₂ from Simulated Flue Gas and Ambient Air

Carbon management by a means of CO₂ capture from large stationary sources such as coal-fired power plants or from ambient air is a significant global issue. In the context of steam-stripping as a regeneration process for solid CO₂ adsorbents, new adsorbent materials robust enough for direct contact with low temperature steam are needed. Here, mesoporous γ-alumina-supported poly(ethyleneimine) composite materials are prepared and evaluated as effective CO₂ adsorbents, using dilute CO₂ streams such as simulated flue gas (10% CO₂) and ultradilute streams such as simulated ambient air (400 ppm CO₂). In comparison to the silica-supported amine adsorbents typically utilized for CO₂ capture applications, the alumina-supported amine adsorbents give better performance in terms of both capture capacity and amine efficiency, in particular, at ambient air conditions. In addition, the alumina-supported amines are stable over short multicycle temperature swing tests and, more importantly, appear to be more robust than the silica-based counterparts upon direct contact with steam. Thus, the resulting alumina-supported amines are suggested to be promising new materials for CO₂ capture processes equipped with steam-stripping regeneration, especially from ultradilute gas streams