Embedded Mo/Mn Atomic Regulation for Durable Acidity-Reinforced HZSM-5 Catalyst toward Energy-Efficient Amine Regeneration.

Metal-molecular sieve composites with high acidity are promising solid acid catalysts (SACs) for accelerating sluggish CO2 desorption processes and reducing the energy consumption of CO2 chemisorption systems. However, the production of such SACs through conventional approaches such as loading or ion-exchange methods often leads to uncontrolled and unstable metal distribution on the catalysts, which limits their pore structure regulation and catalytic performance. In this study, we demonstrated a feasible strategy for improving the durability, surface chemical activity, and pore structure of metal-doped HZSM-5 through bimetallic Mo/Mn modification. This strategy involves the immobilization of Mo-O-Mn species confined in an MFI structure by regulating MoO42- anions and Mn2+ cations. The embedded Mn/Mo species of low valence can strongly induce electron transfer and increase the density of compensatory H+ on the MoMn@H catalyst, thereby reducing the CO2 desorption temperature by 8.27 °C and energy consumption by 37% in comparison to a blank. The durability enhancement and activity regulation method used in this study is expected to advance the rational synthesis of metal-molecular sieve composites for energy-efficient CO2 capture using amine regeneration technology.

Use of La-Co@NPC for Sulfite Oxidation and Arsenic Detoxification Removal for High-Quality Sulfur Resources Recovery in Desulfurization Process.

Ammonia desulfurization is a typical resource-recovery-type wet desulfurization process that is widely used in coal-fired industrial boilers. However, the sulfur recovery is limited by the low oxidation rate of byproduct (ammonium sulfite), leading to secondary SO2 pollution due to its easy decomposability. In addition, the high toxic arsenic trace substances coexisting in desulfurization liquids also reduce the quality of the final sulfate product, facing with high environmental toxicity. In this study, nitrogen-doped porous carbon coembedded with lanthanum and cobalt (La-Co@NPC) was fabricated with heterologous catalytic active sites (Co0) and adsorption sites (LaOCl) to achieve sulfite oxidation and the efficient removal of high toxic trace arsenic for the recovery of high-value ammonium sulfate from the desulfurization liquid. The La-Co@NPC/S(IV) catalytic system can generate numerous strongly oxidizing free radicals (·SO5- and ·O2-) for the sulfite oxidation on the Co0 site, as well as oxidative detoxification of As(III) into As(V). Subsequently, arsenic can be removed through chemical adsorption on LaOCl adsorption sites. By using the dual-functional La-Co@NPC at a concentration of 0.25 g/L, the rate of ammonium sulfite oxidation reached 0.107 mmol/L·s-1, the arsenic (1 mg/L) removal efficiency reached 92%, and the maximum adsorption capacity of As reached up to 123 mg/g. This study can give certain guiding significance to the functional material design and the coordinated control of multiple coal-fired pollutants in desulfurization for high-value recovery of sulfur resources.

In Situ Reconstruction of Active Catalysis Sites Triggered by Chromium Immobilization for Sulfite Oxidation.

Hexavalent chromium (Cr(VI)) is a highly toxic substance in wastewater, triggering grievous detriment to aquatic life and human health. Magnesium sulfite is spawned along with the desulfurization process in coal-fired power plants, which is usually disposed of as solid waste. Here, a "waste control by waste" method was proposed upon the redox of Cr(VI)-sulfite, in which highly toxic Cr(VI) is detoxicated and sequent enriched on a novel biochar-induced cobalt-based silica composite (BISC) due to the forced electron transfer from chromium to surface hydroxyl. The immobilized Cr on BISC gave rise to the reconstruction of catalytic active sites "Cr-O-Co", which further enhance its performance in sulfite oxidation by elevating O2 adsorption. As a result, the sulfite oxidation rate increased by 10 times compared with the non-catalysis benchmark together with the maximum chromium adsorption capacity being 120.3 mg/g. Therefore, this study provides a promising strategy to simultaneously control highly toxic Cr(VI) and sulfite, achieving high-grade sulfur resource recovery for wet magnesia desulfurization.

MOF-Derived Robust and Synergetic Acid Sites Inducing C-N Bond Disruption for Energy-Efficient CO2 Desorption.

Amine-based scrubbing technique is recognized as a promising method of capturing CO2 to alleviate climate change. However, the less stability and poor acidity of solid acid catalysts (SACs) limit their potential to further improve amine regeneration activity and reduce the energy penalty. To address these challenges, here, we introduce two-dimensional (2D) cobalt-nitrogen-doped carbon nanoflakes (Co-N-C NSs) driven by a layered metal-organic framework that work as SACs. The designed 2D Co-N-C SACs can exhibit promising stability, superhydrophilic surface, and acidity. Such 2D structure also contains well-confined Co-N4 Lewis acid sites and -OH Brønsted acid sites to have a synergetic effect on C-N bond disruption and significantly increase CO2 desorption rate by 281% and reduce the reaction temperatures to 88 °C, minimizing water evaporation by 20.3% and subsequent regeneration energy penalty by 71.7% compared to the noncatalysis.

Zero-valent bimetallic catalyst/absorbent for simultaneous facilitation of MgSO3 oxidation and arsenic uptake.

Cobalt (Co)-based catalysts can efficiently reduce the heat waste from sulfate concentration by enhancing sulfite oxidation during wet flue gas desulfurization system. However, arsenic (As) can poison such catalysts and migrate into the sulfate by-products, resulting in severe secondary pollution. In this study, a zero-valent Co/iron (Fe)-based nanoparticle (NZV-Co2Fe1) was fabricated and applied as a bifunctional catalyst/adsorbent. The catalytic stability of the Co-based catalyst was enhanced by the introduction of Fe because the poisonous effect of As was substantially suppressed because of the high adsorption capacity of Fe for As. Compared with the noncatalytic benchmark, the presence of 0.5 g/L NZV-Co2Fe1 can increase the rate of MgSO3 oxidation by approximately 12-fold even at a high concentration of As (2.5 mg/L). The Langmuir model was fitted to the As adsorption isotherms, indicating that As uptake is a single-layer adsorption process. The pseudo-second-order kinetic model indicated that As was removed through chemisorption. The oxidation pathway of As(III) involves reactive radicals (mainly OH, SO4- and SO5-) and ligand-to-metal charge transfer between SO32- and Co2+. The availability of MgSO3 improved the removal efficiency at high concentrations of As(III) (1 mg/L). These results indicate that using NZV-Co2Fe1 as a catalyst to purify the by-products of flue gas desulfurization can effectively prevent secondary pollution.

Construction of Confined Bifunctional 2D Material for Efficient Sulfur Resource Recovery and Hg2+ Adsorption in Desulfurization.

Substantial energy penalty of valuable sulfate recovery restricts the efficiency of wet desulfurization and increases the risk of Hg0 reemission. Although the enhanced sulfite oxidation rate with cobalt-based materials can increase the energy efficiency, inactivation and poisoning of catalyst due to the competition of reactant must be addressed. Here we obtained a superwetting two-dimensional cobalt-nitrogen-doped carbon (2D Co-N-C) nanosheet featuring confined catalysis/adsorption sites for the energy-efficient sulfite oxidation and Hg2+ adsorption. The designed structure exhibits enhanced surface polarity, availability and short reactant diffusion path, thus enabling the significant catalytic TOF value of 0.085 s-1 and simultaneous mercury removal ability of 143.26 mg·g-1. The catalyst nanosheets present regenerating stabilities to improve cost-efficiency. By deployment of the Co-N-C catalysts, a marked reduction of heat penalty up to 69% can be achieved, which makes this catalytic pathway for sulfur resource recovery economically feasible in real industry scenario.

TiO2 Coating Strategy for Robust Catalysis of the Metal-Organic Framework toward Energy-Efficient CO2 Capture.

High energy duty restricts the application of amine-based absorption in CO2 capture and limits the achievement of carbon neutrality. Although regenerating the amine solvent with solid acid catalysts can increase energy efficiency, inactivation of the catalyst must be addressed. Here, we report a robust metal-organic framework (MOF)-derived hybrid solid acid catalyst (SO42-/ZIF-67-C@TiO2) with improved acidity for promoting amine regeneration. The TiO2 coating effectively prevented the active components stripping from the surface of the catalyst, thus prolonging its lifespan. The well-protected Co-Nx sites and protonated groups introduced onto the TiO2 surface increased the amount and rate of CO2 desorption by more than 64.5 and 153%, respectively. Consequently, the energy consumption decreased by approximately 36%. The catalyzed N-C bond rupture and proton transfer mechanisms are proposed. This work provides an effective protection strategy for robust acid catalysts, thus advancing the CO2 capture with less energy duty.

Selenium uptake and simultaneous catalysis of sulfite oxidation in ammonia-based desulfurization.

Accelerating the (NH4)2SO3 oxidation gives rise to the reclaiming of byproduct, while there are secondary environmental risks from reduction of the coexisted selenium species by sulfite. In this study, a bi-functional Co-SBA-15-SH, were synthesized through Co impregnation and sulfhydryl (-SH) decoration, which can simultaneously uptake Se and accelerate sulfite oxidation efficiently. Meanwhile, the adsorption kinetics and migration mechanism of Se species were revealed through characterization and density functional calculations, with maximum adsorption capacity of 223 mg/g. The inhibition of Se0 re-emission and poisonous effect of Se on sulfite oxidation was also investigated. Using the findings of this study, the ammonia desulfurization can be improved by enabling purification of the byproduct and lowering the toxicity of effluent by removing toxic pollutants.

Short-range ordered Co(OH)2/TiO2 for boosting sulfite oxidation: Performance and mechanism.

Catalysis of magnesium sulfite (MgSO3) oxidation is crucial for the flue gas desulfurization by magnesium oxide. Here, a short-range ordered Co(OH)2/TiO2 hybrid with a direct Z-scheme band structure, was fabricated for the reaction of MgSO3 oxidation. The abundant defects of Co(OH)2/TiO2 induced the generation of more surface adsorbed oxygen species, and therefore greatly facilitated the formation of the key radical of SO5- to enhance MgSO3 oxidation. The oxidation rate of MgSO3 was enhanced 6.1-fold higher than the uncatalyzed benchmark with this neat Co(OH)2/TiO2 catalyst. Moreover, the oxidation rate of MgSO3 could be further improved 11.3-fold by the assistance of light irradiation, which broadened the pathways for MgSO3 oxidation by inspiring more active radicals (O2-, OH, SO4-, SO5-) and photogenerated holes to participate into MgSO3 oxidation. The photogenerated holes were proved to play key role to accelerate reaction by directly oxidized MgSO3 or inducing active radicles, and the highly efficient utilization of photogenerated holes was guaranteed by the Z-scheme charge transfer in the Co(OH)2/TiO2 interface. The results provided by this study could serve as an environmentally friendly approach to improve the overall performance of wet desulfurization.

Cobalt-based metal-organic frameworks promoting magnesium sulfite oxidation with ultrahigh catalytic activity and stability.

In wet magnesia desulfurization, the ultrahigh stable catalysis of sulfite oxidation is a crucial step in byproduct reclaimation. In this study, a robust and efficient catalyst, Co-MOF-74(4), was synthesized through a facile solvothermal method with an optimal Co/ligand ratio of 4:1. The oxidation rate of magnesium sulfite catalyzed by Co-MOF-74(4) was >2.6 times higher than that of previously reported cobalt-based heterogeneous catalysts. Crucially, almost no attenuation of the catalytic activity was observed even after three reuse cycles. The properties of Co-MOF-74(4) before and after the reaction were characterized. Density functional theory calculations demonstrate that the unsaturated cobalt site on the open framework of Co-MOF-74(4) provides greater opportunity for active Co to be attacked by sulfite ions, resulting in the ultra-high catalytic activity of Co-MOF-74(4). In contrast to the conventional impregnated catalysts, the robust combination of active cobalt with ligands prohibits its stripping from the surface of Co-MOF-74(4) particles. The bond length, angle and lattice parameters have only slight changes after sulfite adsorption, which supports the stability of the catalyst in the reaction process.

Suppressing Ammonia Re-Emission with the Aid of the Co3O4-NPs@KIT-6 Catalyst in Ammonia-Based Desulfurization.

The re-emission of NH3 and SO2 caused by the decomposition of (NH4)2SO3 is a crucial concern in ammonia-based desulfurization. In this study, a novel Co3O4-NPs@KIT-6 catalyst with a three-dimensional two-helix structure is proposed for converting (NH4)2SO3 into (NH4)2SO4. The oxidation rate of (NH4)2SO3 with the catalyst was 7.5 times that without any catalyst, and this improvement was attributed to appropriately dispersed Co3O4 nanoparticles with a size of 4-10 nm that interacted with the KIT-6 support. Therefore, the number of active sites with substitution and hole defects was substantially increased, which is advantageous for high catalytic activities. Consequently, the amount of NH3 and SO2 re-emission during (NH4)2SO3 oxidation was reduced by 43.9%, which considerably reduced potential environmental risks. The results of this study serve to advance ammonia desulfurization by improving the desulfurization efficiency, downsizing the oxidation tank, and generating considerable profit from efficient reclaiming of (NH4)2SO4 as a fertilizer.

Co-site substitution by Mn supported on biomass-derived active carbon for enhancing magnesia desulfurization.

Oxidation of magnesium sulfite (MgSO3) is a crucial step for reclaiming the product in wet magnesia desulfurization processes. Here, for enhancing this reaction, a bimetallic catalyst was developed by loading CoOx and MnOx species on a biomass-derived active carbon (AC) support to minimize the costs and potential environmental risks during catalyst application. The substitution effect of Mn to Co sites was investigated, and a comparison of the catalyst with plain cobalt suggested that the ratio of Co/Mn must be greater than 3. A series of catalyst characterizations was performed to reveal the synergistic effect of Co and Mn in the bimetallic catalyst. The introduction of Mn species not only improved the dispersion of CoOx-MnOx mixed oxide but also generated abundant Co3+ species and surface-adsorbed oxygen, both of which acted as the main active sites for sulfite oxidation. Notably, in the bimetallic catalyst, the presence of Mn4+ species assisted regeneration of Co2+ to Co3+ species, further accelerating sulfite oxidation. Besides, the partial substitution of Co sites by Mn also suppressed the losing of Co species during reaction, favoring to decrease the environmental risk, as well as to save the cost of catalyst.

Uniform dispersion of cobalt nanoparticles over nonporous TiO2 with low activation energy for magnesium sulfate recovery in a novel magnesia-based desulfurization process.

The forced oxidation of magnesium sulfite (MgSO3) aims to not only reclaim the by-product in the magnesia desulfurization, but also lower the risk of secondary pollution. The non-porous titanium dioxide nanoparticle was used as a support to prepare the cobalt catalyst (Co-TiO2) in order to expedite the oxidation rate. This fabricated Co-TiO2 was characterized by inductively coupled plasma optical emission spectroscopy (ICP-OES), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), and energy dispersive spectroscopy (EDS) to figure out its catalytic mechanism. The results revealed that the cobalt nanoparticles were uniformly dispersed on the surface of the TiO2 in forms of Co3O4 and Co2O3. The kinetics of the MgSO3 oxidation catalyzed by the prepared Co-TiO2 was investigated in a bubbling tank reactor, indicating that the oxidation rate was dependent on the catalyst concentration, oxygen partial pressure, pH value, and the reaction temperature. Compared with the reported porous catalyst (Co-CNTs), the activation energy with the Co-TiO2 (17.29kJmol-1) decreased by 50.9%, resulting in a good catalytic performance in sulfite oxidation. The findings will help advance the industrial application of the novel magnesia desulfurization process.

Inhibiting Mercury Re-emission and Enhancing Magnesia Recovery by Cobalt-Loaded Carbon Nanotubes in a Novel Magnesia Desulfurization Process.

Mercury re-emission, because of the reduction of Hg2+ to form Hg0 by sulfite, has become a great concern in the desulfurization process. Lowering the concentrations of Hg2+ and sulfite in the desulfurization slurry can retard the Hg0 formation and, thus, mitigate mercury re-emission. To that end, cobalt-based carbon nanotubes (Co-CNTs) were developed for the simultaneous Hg2+ removal and sulfite oxidation in this work. Furthermore, the thermodynamics and kinetics of the Hg2+ adsorption and effect of Hg2+ adsorption on catalytic activity of Co-CNTs were investigated. Experimental results revealed that the Co-CNTs not only accelerated sulfite oxidation to enable the recovery of desulfurization by-products but also acted as an effective adsorbent of Hg2+ removal. The Hg2+ adsorption rate mainly depended on the structure of the adsorption material regardless of the cobalt loading and morphological distribution. The catalytic activity of the Co-CNTs for sulfite oxidation was not significantly affected due to the Hg2+ adsorption. Additionally, the isothermal adsorption behavior was well-fitted to the Langmuir model with an adsorption capacity of 166.7 mg/g. The mercury mass balance analysis revealed that the Hg0 re-emission was decreased by 156% by adding 2.0 g/L of Co-CNTs. These results can be used as a reference for the simultaneous removal of multiple pollutants in the wet-desulfurization process.