Diversity in Atomic Structures of Zeolite-Templated Carbons and the Consequences for Macroscopic Properties.

Zeolite-templated carbons (ZTCs) are a family of ordered microporous carbons with extralarge surface areas and micropore volumes, which are synthesized by carbon deposition within the confined spaces of zeolite micropores. There has been great controversy regarding the atomic structures of ZTCs, which encompass two extremes: (1) three-dimensionally connected curved open-blade-type carbon moieties and (2) ideal tubular structures (commonly referred to as "Schwarzites"). In this study, through a combination of experimental analyses and theoretical calculations, we demonstrate that the atomic structure of ZTCs is difficult to define as a single entity, and it widely varies depending on their synthesis conditions. Carbon deposition using a large organic precursor and low-temperature framework densification generates ZTCs predominantly composed of open-blade-type moieties, characterized by low surface curvature and abundant H-terminated edge sites. Meanwhile, synthesis using a small precursor with high-temperature densification produces ZTCs with an increased portion of closed-strut carbon moieties (or closed-fullerene-like nodes), exhibiting large surface curvature and diminished edge sites. The variations in the atomic structure of ZTCs result in significant differences in their macroscopic properties, such as N2/CO2 adsorption, oxidative stability, work function, and electrocatalytic properties, despite the presence of comparable pore structures. Therefore, ZTCs demonstrate the potential to synthesize ordered nanoporous carbons with tunable physicochemical properties.

Design of Amine-Containing Nanoporous Materials for Postcombustion CO2 Capture from Engineering Perspectives.

ConspectusCarbon dioxide (CO2) capture and storage (CCS) is a means to enable the continued use of fossil fuels in the short term. In particular, postcombustion CO2 capture has attracted considerable attention because it can be retrofitted into existing power plants and industrial plants. Among various CO2 capture technologies, the absorption of CO2 using aqueous amines has been industrially employed for decades. However, such amine scrubbing technologies have inherent limitations of environmental and health concerns due to volatile amine loss, corrosion, and high energy demands for regeneration. To overcome these limitations, CO2 adsorption using solid adsorbents has emerged as a promising alternative due to its noncorrosiveness and low energy demand. Various amine-containing adsorbents have been synthesized and investigated for postcombustion CO2 capture. These materials are prepared by physically impregnating low-vapor-pressure amine polymers or by chemically grafting amines onto nanoporous materials. A wide variety of amine guests and nanoporous hosts (e.g., SiO2, Al2O3, zeolites, MOFs, and polymers) have been combined to develop advanced CO2 adsorbents.The design of CO2 adsorbents is a multifaceted puzzle that must ultimately consider integration with large-scale CO2 capture processes. Various engineering aspects need to be carefully considered. Unfortunately, a significant proportion of previous studies has primarily focused on the use of novel materials for improving the CO2 adsorption capacity. In this Account, we describe key challenges and solutions to develop energy-efficient and stable amine-containing adsorbents for postcombustion CO2 capture via temperature swing adsorption (TSA). We found that a high CO2 working capacity, often overemphasized in the literature, does not necessarily guarantee a low energy demand for CO2 capture. Suppressing coadsorption of H2O during the CO2 adsorption in humid flue gas is also a significant factor. Amine-containing adsorbents can be degraded through various pathways, including hydrothermal degradation of nanoporous hosts and chemical degradation of amine guests via urea formation and oxidation. To inhibit such degradation pathways, it is extremely important to properly design the nanoporous structures of the hosts and the molecular structures of the amine guests. By combining macroporous silica hosts, poly(ethylenimine) (PEI) functionalized with various alkyl epoxides, and phosphate-based oxidative stabilizers, we could synthesize adsorbents exhibiting low energy demands for CO2 capture and unprecedentedly high thermochemical stability under TSA conditions. The macroporous silica host synthesized by assembling fumed silica particles via spray-drying exhibited high hydrothermal stability and enabled uniform distribution of bulky amine polymers within its pores. The functionalization of PEI with alkyl epoxides converted its primary amines into hindered secondary amines, leading to a significant reduction in energy demand for TSA cycles and a remarkable improvement in long-term stabilities. The oxidative stability of amines could be drastically improved by adding phosphate metal-binding reagents, which can poison ppm-level metal impurities that catalyze amine oxidation. The present discussions will provide important insights into designing practical adsorbents for CO2 capture from engineering perspectives.

Hierarchical LTL Zeolite as an Efficient and Sustainable Solid Acid Catalyst for Replacing HCl in the Production of Polyurethane Intermediates.

In many industrially important reactions, caustic mineral acid catalysts have been successfully replaced with green solid acids such as zeolites. In this context, extensive efforts have been devoted to replacing HCl to produce methylenedianiline (MDA), a key intermediate in polyurethane production. Unfortunately, limited success has been achieved thus far due to low activity, selectivity towards the desired 4,4'-MDA, and rapid catalyst deactivation. Here we report that meso-/microporous hierarchical LTL zeolite exhibits unprecedentedly high activity, selectivity, and stability. The one-dimensional cage-like micropores of LTL promote the bimolecular reaction between two para-aminobenzylaniline intermediates to selectively produce 4,4'-MDA and inhibit the formation of undesired isomers and heavy oligomers. Meanwhile, the secondary mesopores alleviate mass transfer limitations, resulting in a 7.8-fold higher MDA formation rate compared to solely microporous LTL zeolite. Due to suppressed oligomer formation and fast mass transfer, the catalyst exhibits inappreciable deactivation in an industrially relevant continuous flow reactor.

Splitting of Hydrogen Atoms into Proton-Electron Pairs at BaO-Ru Interfaces for Promoting Ammonia Synthesis under Mild Conditions.

Ru catalysts promoted with alkali and alkaline earth have shown superior ammonia (NH3) synthesis activities under mild conditions. Although these promoters play a vital role in enhancing catalytic activity, their function has not been clearly understood. Here, we synthesize a series of Ba-Ru/MgO catalysts with an optimal Ru particle size (∼2.3 nm) and tailored BaO-Ru interfacial structures. We discover that the promoting effect is created through the separate storage of H+/e- pairs at the BaO-Ru interface. Chemisorbed H atoms on Ru dissociate into H+/e- pairs at the BaO-Ru interface, where strongly basic, nonreducible BaO selectively captures H+ while leaving e- on Ru. The resulting electron accumulation in Ru facilitates N2 activation via enhanced π-backdonation and inhibits hydrogen poisoning during NH3 synthesis. Consequently, the formation of intimate BaO-Ru interface without an excessive loss of accessible Ru sites enables the synthesis of highly active catalysts for NH3 synthesis.

First successful synthesis of an Al-rich mesoporous aluminosilicate for fast radioactive strontium capture.

Al-rich zeolites such as NaA (Si/Al = 1.00) have been widely applied to remove radioactive 90Sr2+ because of their high surface charge density enabling efficient ion-exchange of multivalent cations. However, due to the small micropore diameters of zeolites and large molecular size of strongly hydrated Sr2+, Sr2+-exchange with zeolites suffers from very slow kinetics. In principle, mesoporous aluminosilicates with low Si/Al ratios close to unity and tetrahedrally coordinated Al sites can exhibit both high capacity and fast kinetics in Sr2+-exchange. Nonetheless, the synthesis of such materials has not been realized yet. In this study, we demonstrate the first successful synthesis of an Al-rich mesoporous silicate (ARMS) using a cationic organosilane surfactant as an efficient mesoporogen. The material exhibited a wormhole-like mesoporous structure with a high surface area (851 m2 g-1) and pore volume (0.77 cm3 g-1), and an Al-rich framework (Si/Al = 1.08) with most Al sites tetrahedrally coordinated. Compared to commercially applied NaA, ARMS exhibited a dramatically improved Sr2+-exchange kinetics (>33-fold larger rate constant) in batch adsorption while showing similarly high Sr2+ capture capacity and selectivity. Due to the fast Sr2+-exchange kinetics, the material also exhibited 3.3-fold larger breakthrough volume than NaA in fixed-bed continuous adsorption.

Engineering nanoscale H supply chain to accelerate methanol synthesis on ZnZrOx.

Metal promotion is the most widely adopted strategy for enhancing the hydrogenation functionality of an oxide catalyst. Typically, metal nanoparticles or dopants are located directly on the catalyst surface to create interfacial synergy with active sites on the oxide, but the enhancement effect may be compromised by insufficient hydrogen delivery to these sites. Here, we introduce a strategy to promote a ZnZrOx methanol synthesis catalyst by incorporating hydrogen activation and delivery functions through optimized integration of ZnZrOx and Pd supported on carbon nanotube (Pd/CNT). The CNT in the Pd/CNT + ZnZrOx system delivers hydrogen activated on Pd to a broad area on the ZnZrOx surface, with an enhancement factor of 10 compared to the conventional Pd-promoted ZnZrOx catalyst, which only transfers hydrogen to Pd-adjacent sites. In CO2 hydrogenation to methanol, Pd/CNT + ZnZrOx exhibits drastically boosted activity-the highest among reported ZnZrOx-based catalysts-and excellent stability over 600 h on stream test, showing potential for practical implementation.

Bottom-up synthesis of two-dimensional carbon with vertically aligned ordered micropores for ultrafast nanofiltration.

Two-dimensional (2D) carbon materials perforated with uniform micropores are considered ideal building blocks to fabricate advanced membranes for molecular separation and energy storage devices with high rate capabilities. However, creating high-density uniform micropores in 2D carbon using conventional perforation methods remains a formidable challenge. Here, we report a zeolite-templated bottom-up synthesis of ordered microporous 2D carbon. Through rational analysis of 255 zeolite structures, we find that the IWV zeolite having large 2D microporous channels and aluminosilicate compositions can serve as an ideal template for carbon replication. The resulting carbon is made of an extremely thin polyaromatic backbone and contains well-defined vertically aligned micropores (0.69 nm in diameter). Its areal pore density (0.70 nm-2) is considerably greater than that of porous graphene (<0.05 nm-2) prepared using top-down perforation methods. The isoporous membrane fabricated by assembling the exfoliated 2D carbon nanosheets exhibits outstanding permeance and molecular sieving properties in organic solvent nanofiltration.

Sulfur-modified zeolite A as a low-cost strontium remover with improved selectivity for radioactive strontium.

Selective removal of radioactive strontium (90Sr) from the environment is important, and selective adsorption/ion exchange is appropriate for removal of trace amounts of 90Sr from large volumes of 90Sr-contaminated water. Although various inorganic ion-exchange materials, including zeolites, have been investigated intensively for removal of Sr2+ due to their excellent resistance to radiation and high ion-exchange capacity, their ion-exchange selectivity for Sr2+ is poor in the presence of competing ions such as Ca2+ and Mg2+. Here, sulfur-modified NaA zeolite (S-NaA) was prepared for low-cost, selective 90Sr removal because the elemental sulfur encapsulated in micropores provides additional Lewis acid-base interactions with Sr2+ during the Sr2+ ion-exchange. Our ion-exchange experiments revealed that S-NaA with 3 wt% sulfur (3 S-NaA) showed the highest Sr2+ selectivity among various S-NaAs containing up to 10 wt% sulfur because ion exchange involving bulky hydrated Sr2+ depends on the reduced micropore volume of S-NaA after sulfur loading. Most importantly, 3 S-NaA effectively and efficiently (>99.4%) removed 90Sr from groundwater containing 8.4 ppt 90Sr, demonstrating its excellent potential for practical application in the treatment of 90Sr-contaminated water.

Quantification of Active Site Density and Turnover Frequency: From Single-Atom Metal to Nanoparticle Electrocatalysts.

Single-atom catalysts (SACs) featuring atomically dispersed metal cations covalently embedded in a carbon matrix show significant potential to achieve high catalytic performance in various electrocatalytic reactions. Although considerable advances have been achieved in their syntheses and electrochemical applications, further development and fundamental understanding are limited by a lack of strategies that can allow the quantitative analyses of their intrinsic catalytic characteristics, that is, active site density (SD) and turnover frequency (TOF). Here we show an in situ SD quantification method using a cyanide anion as a probe molecule. The decrease in cyanide concentration triggered by irreversible adsorption on metal-based active sites of a model Fe-N-C catalyst is precisely measured by spectrophotometry, and it is correlated to the relative decrease in electrocatalytic activity in the model reaction of oxygen reduction reaction. The linear correlation verifies the surface-sensitive and metal-specific adsorption of cyanide on Fe-N x sites, based on which the values of SD and TOF can be determined. Notably, this analytical strategy shows versatile applicability to a series of transition/noble metal SACs and Pt nanoparticles in a broad pH range (1-13). The SD and TOF quantification can afford an improved understanding of the structure-activity relationship for a broad range of electrocatalysts, in particular, the SACs, for which no general electrochemical method to determine the intrinsic catalytic characteristics is available.

Controlled Synthesis of Metal-Organic Frameworks in Scalable Open-Porous Contactor for Maximizing Carbon Capture Efficiency.

Metal-organic frameworks (MOFs) are a class of microporous materials that have been highlighted with fast and selective sorption of gas molecules; however, they are at least partially unstable in the scale-up process. Here, we report a rational shaping of MOFs in a scalable architecture of fiber sorbent. The long-standing stability challenge of MOFs was resolved by using stable metal oxide precursors that are subject to controlled surface oxide dissolution-growth chemistry during the Mg-based MOF synthesis. Highly uniform MOF crystals are synthesized along with the open-porous fiber sorbents networks, showing unprecedented cyclic CO2 capacities in both flue gas and direct air capture (DAC) conditions. The same chemistry enables an in situ flow synthesis of Mg-MOF fiber sorbents, providing a scalable pathway for MOF synthesis in an inert condition with minimal handling steps. This modular approach can serve both as a reaction stage for enhanced MOF fiber sorbent synthesis and as a "process-ready" separation device.

The facet effect of ceria nanoparticles on platinum dispersion and catalytic activity of methanol partial oxidation.

The effect of platinum-supported nano-shaped ceria catalysts on methanol partial oxidation and methyl formate product selectivity has been investigated. A Pt-supported CeO2 nanocube catalyst had a higher turnover frequency than nanosphere catalysts; however, nanosphere catalysts showed higher selectivity towards methyl formate. The observed ceria shape effect in catalysis was associated with the shape-dependent Pt dispersion and its oxidation states. Furthermore, in situ studies revealed that the reduced platinum and mono-dentate methoxy group were responsible for the higher turnover frequency.

Tailoring a Dynamic Metal-Polymer Interaction to Improve Catalyst Selectivity and Longevity in Hydrogenation.

Controlling metal-support interactions is important for tuning the catalytic properties of supported metal catalysts. Here, premade Pd particles are supported on stable polymers containing different ligating functionalities to control the metal-polymer interactions and their catalytic properties in industrially relevant acetylene partial hydrogenation. The polymers containing strongly ligating groups (e.g., Ar-SH and Ar-S-Ar) can form a polymer overlayer on the Pd surface, which enables selective acetylene adsorption and partial hydrogenation to ethylene without deactivation. In contrast, polymers with weakly ligating groups (e.g., Ar-O-Ar) do not form an overlayer, resulting in non-selective hydrogenation and fast deactivation, similar to Pd catalysts on conventional inorganic supports. The results imply that tuning the metal-polymer interactions via rational polymer design can provide an efficient way of synthesizing selective and stable catalysts for hydrogenation.

Relationship between zeolite structure and capture capability for radioactive cesium and strontium.

Zeolites are widely used for capturing radioactive Cs+ and Sr2+, but the important structural factors determining their performance have not been clearly understood. To investigate the structure-property relationship, we prepared thirteen zeolites with various structures and Si/Al ratios. Ion-exchange experiments revealed that Cs+ exhibited an enhanced affinity to zeolites with high Si/Al ratios, which could be explained by the dielectric theory. Notably, zeolites containing 8-membered ring (8MR) showed extra-high Cs+ selectivity. Structural analysis using X-ray diffraction proved that Cs+ with an ionic diameter of 3.6 Å was selectively coordinated within 8MR having a cavity diameter of 3.6-4.1 Å. Such unique size-selective Cs+ coordination is analogous to ion complexation by macrocyclic organic ligands (e.g., crown ethers). Divalent Sr2+ showed decreasing affinity to zeolites as the Si/Al ratio increased, because of the increasing average Al-Al distance distribution. Sr2+ exchange exhibited an insignificant dependence on zeolite structures due to its strong hydration, which inhibited close interaction with zeolite frameworks. In terms of kinetics, Sr2+ exchange was significantly slower than Cs+ exchange because of the bulkiness of hydrated Sr2+ ions. Therefore, the micropore channels with large apertures (e.g., 12-membered ring) were beneficial for achieving fast ion-exchange kinetics, especially in the case of Sr2+.

Dynamic metal-polymer interaction for the design of chemoselective and long-lived hydrogenation catalysts.

Metal catalysts are generally supported on hard inorganic materials because of their high thermochemical stabilities. Here, we support Pd catalysts on a thermochemically stable but "soft" engineering plastic, polyphenylene sulfide (PPS), for acetylene partial hydrogenation. Near the glass transition temperature (~353 K), the mobile PPS chains cover the entire surface of Pd particles via strong metal-polymer interactions. The Pd-PPS interface enables H2 activation only in the presence of acetylene that has a strong binding affinity to Pd and thus can disturb the Pd-PPS interface. Once acetylene is hydrogenated to weakly binding ethylene, re-adsorption of PPS on the Pd surface repels ethylene before it is further hydrogenated to ethane. The Pd-PPS interaction enables selective partial hydrogenation of acetylene to ethylene even in an ethylene-rich stream and suppresses catalyst deactivation due to coke formation. The results manifest the unique possibility of harnessing dynamic metal-polymer interaction for designing chemoselective and long-lived catalysts.

Atomistic Insights into the Stability of Pt Single-Atom Electrocatalysts.

Single-atom catalysts (SACs) have quickly emerged as a new class of catalytic materials. When confronted with classical carbon-supported nanoparticulated catalysts (Pt/C), SACs are often claimed to have superior electrocatalytic properties, e.g., stability. In this study, we critically assess this statement by investigating S-doped carbon-supported Pt SACs as a representative example of noble-metal-based SACs. We use a set of complementary techniques, which includes online inductively coupled plasma mass spectrometry (online ICP-MS), identical location transmission electron microscopy (IL-TEM), and X-ray photoelectron spectroscopy (XPS). It is shown by online ICP-MS that the dissolution behavior of as-synthesized Pt SACs is significantly different from that of metallic Pt/C. Moreover, Pt SACs are, indeed, confirmed to be more stable toward Pt dissolution. When cycled to potentials of up to 1.5 VRHE, however, the dissolution profiles of Pt SACs and Pt/C become similar. IL-TEM and XPS show that this transition is due to morphological and chemical changes caused by cycling. The latter, in turn, is a consequence of the relatively poor stability of S ligands. As monitored by online ICP-MS and XPS, significant amounts of sulfur leave the catalyst during oxidation. Hence, in case catalysts with improved stability in the anodic potential region are desired, more robust supports and ligands must be developed.

SO2-Resistant Amine-Containing CO2 Adsorbent with a Surface Protection Layer.

Amine-containing solids are promising adsorbents for CO2 capture, but they suffer from irreversible poisoning by the highly acidic SO2 in flue gas. Here, we demonstrate a facile strategy to inhibit SO2 poisoning. We first prepared an amine-containing adsorbent by impregnating polyethyleneimine (PEI) into porous silica. The PEI located at the external surface of the adsorbent was selectively alkylated with epoxide so that amines were fully converted to tertiary amines. As opposed to that onto primary and secondary amines, SO2 adsorption onto tertiary amines is fully reversible. Therefore, during the flue gas adsorption, SO2 is reversibly captured by the tertiary-amine-rich layer and then desulfurized CO2 is adsorbed onto PEI beneath this layer. The resultant adsorbent showed insignificant loss of CO2 adsorption capacity (8.52%) even after 1000 CO2 adsorption-desorption cycles in the presence of 50 ppm SO2, whereas conventional PEI/silica showed severe capacity loss (65.1%) due to irreversible SO2 poisoning.

Carbon Monoxide as a Promoter of Atomically Dispersed Platinum Catalyst in Electrochemical Hydrogen Evolution Reaction.

Carbon monoxide is widely known to poison Pt during heterogeneous catalysis owing to its strong donor-acceptor binding ability. Herein, we report a counterintuitive phenomenon of this general paradigm when the size of Pt decreases to an atomic level, namely, the CO-promoting Pt electrocatalysis toward hydrogen evolution reactions (HER). Compared to pristine atomic Pt catalyst, reduction current on a CO-modified catalyst increases significantly. Operando mass spectroscopy and electrochemical analyses demonstrate that the increased current arises due to enhanced H2 evolution, not additional CO reduction. Through structural identification of catalytic sites and computational analysis, we conclude that CO-ligation on the atomic Pt facilitates Hads formation via water dissociation. This counterintuitive effect exemplifies the fully distinct characteristics of atomic Pt catalysts from those of bulk Pt, and offers new insights for tuning the activity of similar classes of catalysts.

Rational Design of the Polymeric Amines in Solid Adsorbents for Postcombustion Carbon Dioxide Capture.

Substantial efforts have been made to increase the CO2 working capacity of amine adsorbents for an efficient CO2 capture. However, the more important metric for assessing adsorbents is the regeneration heat required for capturing a fixed amount of CO2. In this work, we synthesized polyethyleneimine (PEI)/SiO2 adsorbents functionalized with various epoxides. This provided adsorbents with six different amine structures showing various CO2/H2O adsorption properties. Our studies revealed that the CO2 working capacity was not a decisive factor in determining the regeneration heat required for CO2 capture. This is because the benefit of large CO2 working capacity was canceled out by the difficulty of CO2 desorption. Instead, the suppression of H2O co-adsorption was critical for reducing the regeneration heat because substantial latent heat is required for H2O desorption. Consequently, the PEI/SiO2 functionalized with 1,2-epoxybutane required a much lower regeneration heat (2.66 GJ tCO2-1) than the conventional PEI/SiO2 (4.03 GJ tCO2-1) because of suppressed H2O co-adsorption as well as moderately high CO2 working capacity.

Oxidation-stable amine-containing adsorbents for carbon dioxide capture.

Amine-containing solids have been investigated as promising adsorbents for CO2 capture, but the low oxidative stability of amines has been the biggest hurdle for their practical applications. Here, we developed an extra-stable adsorbent by combining two strategies. First, poly(ethyleneimine) (PEI) was functionalized with 1,2-epoxybutane, which generates tethered 2-hydroxybutyl groups. Second, chelators were pre-supported onto a silica support to poison p.p.m.-level metal impurities (Fe and Cu) that catalyse amine oxidation. The combination of these strategies led to remarkable synergy, and the resultant adsorbent showed a minor loss of CO2 working capacity (8.5%) even after 30 days aging in O2-containing flue gas at 110 °C. This corresponds to a ~50 times slower deactivation rate than a conventional PEI/silica, which shows a complete loss of CO2 uptake capacity after the same treatment. The unprecedentedly high oxidative stability may represent an important breakthrough for the commercial implementation of these adsorbents.

Macroporous Silica with Thick Framework for Steam-Stable and High-Performance Poly(ethyleneimine)/Silica CO2 Adsorbent.

Poly(ethyleneimine) (PEI)/silica has been widely studied as a solid adsorbent for post-combustion CO2 capture. In this work, a highly macroporous silica (MacS), synthesized by secondary sintering of fumed silica, is compared with various mesoporous silicas with different pore structures as a support for PEI. The silicas with large pore diameter and volume enabled high CO2 adsorption kinetics and capacity, because pore occlusion by the supported PEI was minimized. The steam stability of the silica structures increased with the silica wall thickness owing to suppressed framework ripening. The silicas with low steam stability showed rapid leaching of PEI, which indicated that the PEI squeezed out of the collapsed silica pores leached more readily. Consequently, MacS that had an extra-large pore volume (1.80 cm3 g-1 ) and pore diameter (56.0 nm), and a thick wall (>10 nm), showed the most promising CO2 adsorption kinetics and capacity as well as steam stability.