H2 CO2 Research Articles

Crystallite Phase Effect on the Redox Reactivity of Layered Double Hydroxide-Derived Iron-Based Oxygen Carriers for Chemical Looping CO2 Reduction

Layered double hydroxide (LDH) is an effective self-template to develop efficient iron-based oxygen carriers for chemical looping CO2 reduction. However, how the synthesis condition and metal ratio affect crystallite phase formation and redox reactivity is still unclear. In this work, a series of iron-based oxygen carriers are synthesized from thermal transformation of LDHs with varying metal ratios and calcination temperatures to reveal the crystallite phase formation mechanism and interpret the phase–activity relationship. Spinel MgAlδFe2−δO4 is formed below 800 °C, while MgFeδAl2−δO4 is transformed into MgAlδFe2−δO4 with increasing iron content at 800 °C. During H2–CO2 redox cycles, Fe3+/Fe2+ transition and iron segregation are observed for MgAlδFe2−δO4, but only the former occurs for MgFeδAl2−δO4. For a phase mixture, with a low MgAlδFe2−δO4 amount, Al migrates and substitutes Fe with the reincorporation of segregated iron to generate MgFeδAl2−δO4. MgAlδFe2−δO4 achieves higher oxygen storage capacity and weaker stability than MgFeδAl2−δO4 because the former favors the creation of more oxygen vacancies and induce a larger crystallite size. Due to the crystallite phase effect, Fe0.5Mg2Al0.5 exhibits highest redox rates (6 mol[O]·kgFe–1·min–1 and 10 mol[O]·kgFe–1·min–1 for the release and uptake of oxygen, respectively) and CO space-time yield (0.46 molCO·kgFe–1·s–1). These findings provide a new route to tailor efficient oxygen carriers for chemical looping applications.

Energy use of biogas through chemical looping technologies with low-cost oxygen carriers

Biogas is a renewable fuel generated in the anaerobic digestion of organic material. The expansion of biogas as a bioenergy source in the next decades leads to find new ways for its energy exploitation such as the Chemical Looping technologies. In this work, a low-cost Fe-based residue was used as oxygen carrier in two processes with biogas, namely Chemical Looping Combustion (CLC) and Chemical Looping Reforming (CLR). CLC process is focused on the energy production with CO2 capture. CLC experiments showed higher methane combustion efficiency (∼86%) than those referred in literature with other low-cost Fe-based materials. CLR process is dedicated to the production of syngas or hydrogen without CO2 emissions. Ni- or Cu-based oxygen carrier had been successfully used in order to catalyse the methane reforming. Here, the Fe-based material was also used for CLR. The biogas composition facilitated dry reforming of biogas to obtain syngas. In the dry-CLR of biogas, it was possible to reach about 55% methane conversion with a yield to syngas of 1.3 mol (CO + H2)/mol CH4. A suitable management of the effluent gas is proposed to maximize CH4 conversion and syngas yield, which includes CO2 separation, H2 purification and CH4 recirculation. Both results confirm the potential of the energy use of biogas under these technologies and the promising results with the new oxygen carrier developed.

Photosynthesis re-wired on the pico-second timescale.

Photosystems II and I (PSII, PSI) are the reaction centre-containing complexes driving the light reactions of photosynthesis; PSII performs light-driven water oxidation and PSI further photo-energizes harvested electrons. The impressive efficiencies of the photosystems have motivated extensive biological, artificial and biohybrid approaches to 're-wire' photosynthesis for higher biomass-conversion efficiencies and new reaction pathways, such as H2 evolution or CO2 fixation1,2. Previous approaches focused on charge extraction at terminal electron acceptors of the photosystems3. Electron extraction at earlier steps, perhaps immediately from photoexcited reaction centres, would enable greater thermodynamic gains; however, this was believed impossible with reaction centres buried at least 4 nm within the photosystems4,5. Here, we demonstrate, using in vivo ultrafast transient absorption (TA) spectroscopy, extraction of electrons directly from photoexcited PSI and PSII at early points (several picoseconds post-photo-excitation) with live cyanobacterial cells or isolated photosystems, and exogenous electron mediators such as 2,6-dichloro-1,4-benzoquinone (DCBQ) and methyl viologen. We postulate that these mediators oxidize peripheral chlorophyll pigments participating in highly delocalized charge-transfer states after initial photo-excitation. Our results challenge previous models that the photoexcited reaction centres are insulated within the photosystem protein scaffold, opening new avenues to study and re-wire photosynthesis for biotechnologies and semi-artificial photosynthesis.

Role of Hydride Formation in Electrocatalysis for Sustainable Chemical Transformations

The electrochemical potential of numerous reduction reactions corresponds to hydrogen pressures that thermodynamically favor the formation of many metal hydrides. Whether a catalyst remains metallic with hydrogen on the surface (Hads) or absorbs hydrogen into its lattice (Habs), forming a metal hydride, results from a balance between not only this thermodynamic driving force but also the kinetics of concurrent surface reactions and equilibrium surface coverage. Drawing parallels with thermal catalytic processes, we provide examples of how hydride formation impacts electrocatalysis in H2 evolution, organic and CO2 reduction, and N2 and NO3– reduction reactions. Hydride formation not only changes catalyst activity and selectivity but also can impact durability. We highlight techniques capable of identifying hydride formation under reaction conditions, imperative for an understanding of electrocatalyst kinetics. Hydrides offer many possibilities in electrocatalysis, including unique reaction mechanisms involving the catalyst lattice and innovative reactor architectures, beneficial for applications in chemical transformations and energy.

Effect of MgFe-LDH with Reduction Pretreatment on the Catalytic Performance in Syngas to Light Olefins

MgFe-layered double hydroxides (LDH) were widely used as catalysts for Fischer–Tropsch synthesis to produce light olefins, in which the state of Fe-species may affect the resulting catalytic active sites. Herein, the typical MgFe-LDH was hydrothermally synthesized and the obtained MgFe-LDH was pretreated with H2 at different temperatures to reveal the effects of the state of Fe-species on the catalytic performance in Fischer–Tropsch synthesis. MgFe-LDH materials were characterized by X-ray diffraction (XRD), N2 adsorption–desorption, scanning electron microscopy (SEM), transmission electron microscopy (TEM), H2 temperature-programmed reduction (H2-TPR), and X-ray photoelectron spectroscopy (XPS). It was found that a MgO-FeO solid solution would be formed with the increase of the reduction temperature, which made the electrons transfer from Mg atoms to Fe atoms and strengthened the adsorption of CO. The pre-reduced treatment toward Mg-Fe-LDH enabled the FeCx active sites to be easily formed in situ during the reaction process, leading to the high conversion of CO. CO2 temperature-programmed desorption (CO2-TPD) and H2 temperature-programmed desorption (H2-TPD) analysis confirmed that the surface basicity of the catalysts was increased and the hydrogenation capacity was weakened, the secondary hydrogenation of the olefins was inhibited, and therefore as were the enhancement of O/P in the product and the high selectivity of light olefins (42.7%).

Process Simulations of High-Purity and Renewable Clean H2 Production by Sorption Enhanced Steam Reforming of Biogas

Renewable clean H2 has a very promising potential for the decarbonization of energy systems. Sorption enhanced steam reforming (SESR) is a novel process that combines the steam reforming reaction and the simultaneous CO2 removal by a solid sorbent, such as CaO, which significantly enhances hydrogen generation, enabling high-purity H2 production. The CO2 sorption reaction (carbonation) is exothermic, but the sorbent regeneration by calcination is highly endothermic, which requires extra energy. Biogas is one of the available carbon-neutral renewable H2 production sources. It can be especially relevant for the energy integration of the SESR process since, due to the exothermic sorption reaction, the CO2 contained in the biogas provides extra heat to the system, which can help to balance the energy requirements of the process. This work studies different process configurations for the energy integration of the SESR process of biogas for high-purity renewable H2 production: (1) SESR with sorbent regeneration using a portion of the produced H2 (SESR+REG_H2), (2) SESR with sorbent regeneration using biogas (SESR+REG_BG), and (3) SESR with sorbent regeneration using biogas and adding a pressure swing adsorption (PSA) unit for hydrogen purification (SESR+REG_BG+PSA). When using biogas as fuel (Cases 2 and 3), these configurations were studied using air and oxy-fuel combustion atmospheres in the sorbent regeneration step, resulting in five case studies. A thermodynamic approach for process modeling can provide the optimal process operating conditions and configurations that maximize the energy efficiency of the process, which are the basis for subsequent optimization of the process at the practical level needed to scale up this technology. For this purpose, process simulations were performed using a steady-state plant model developed in Aspen Plus, incorporating a complex heat exchanger network (HEN) to optimize heat integration. A comprehensive parametric study assessed the effects of biogas composition, temperature, pressure, and steam to methane (S/CH4) ratio on the process performance represented by the selected key performance indicators, i.e., H2 purity, H2 yield, CH4 conversion, cold gas efficiency (CGE), net efficiency (NE), fuel consumption for the sorbent regeneration step, and CO2 capture efficiency. H2 with a purity of 98.5 vol % and a CGE of 75.7% with zero carbon emissions can be achieved. When adding a PSA unit, nearly 100% H2 purity and CO2 capture efficiency were achieved with a CGE of 77.3%. The use of oxy-fuel combustion during regeneration lowered the net efficiency of the process by 2.3% points (since it requires an air separation unit) but allowed the process to achieve negative carbon emissions.

Self-supported copper selenide nanosheets for electrochemical carbon dioxide conversion to syngas with a broad H2-to-CO ratio

Production of syngas from carbon dioxide (CO2) and water (H2O) is greatly attractive but very challenging for accomplishing tuneable/wide range of H2-to-CO ratios at current density. Herein, the anion-regulated self-supported Cu3Se2 with a hierarchical structure of nanosheet-assembled fibers, enabling multiple copper valence states and abundant in situ formed copper boundaries simultaneously regulating electrochemical CO2 reduction reaction (CO2RR) and H2 evolution reaction (HER). As a consequence, the delicate design of Cu3Se2 catalyst results in an outstanding electrocatalysis of syngas generation with a tuneable/wide range of H2-to-CO ratios (0.8 to 6.0), and high turnover frequency of 1303 h−1, which can achieve a high current density much higher than Cu foam and stable electrolysis with negligible attenuation of Faraday efficiency and current. The superior performance is attributed to the multiple copper valence states for activation of CO2, and the abundant boundaries for modifying the binding energy of intermediate for *COOH formation and *CO desorption and hydrogen adsorption for HER process. Therefore, the design of anion-regulated electrocatalysts with self-supported Cu3Se2 nanosheet-assembled fibers show great potential for the investigation of value-added chemicals/fuels from CO2RR.

Molecular design of two-dimensional graphdiyne membrane for selective transport of CO2 and H2 over CH4, N2, and CO

CO2 capture and H2 purification are the main challenges in syngas and flue gas processing. Two-dimensional graphdiyne (GDY) membrane, with intrinsic uniform pores, provides a promising candidate but lacks both a design strategy and an atomic understanding for these separations. In this study, for the first time, flexible GDYs are computationally designed with the engineering of various functional groups, namely GDY-H, GDY-F, GDY-OH and GDY-NH2, to gain molecular-level insights into CO2 capture and H2 purification from binary mixtures of CO2/CH4, CO2/CO, CO2/N2, H2/CH4, H2/CO and H2/N2. Gas separation performance is enhanced by the co-effect of size sieving and membrane-gas interactions. Both the hydroxylated GDY-OH and the aminated GDY-NH2 membranes exhibit unprecedented performance for both CO2 and H2 separation. Despite processing a small-sized aperture, the GDY-NH2 achieves the highest performance for CO2 separations with permeance above 8.9 × 104 GPU and selectivities over CH4, CO and N2 up to 13935, 12356 and 809, respectively, which far surpass the 2008 upper bound. Structural and energetic analyses show that the flexible GDY-NH2 tends to open its nanowindows and evoke concerted motions that enhance gas permeation, thereby promoting prompt diffusion. Additionally, the ultra-high H2 separation performance of the functional GDY-NH2 and GDY-OH is also several orders of magnitude higher than state-of-the-art membranes. This computational study reveals two dominant effects that govern the gas permeation process and suggests the great potential of hydroxylated and aminated GDYs for both CO2 capture and H2 purification.

Heteronuclear Dual Single-Atom Catalysts for Ambient Conversion of CO2 from Air to Formate

Single-atom catalysts (SACs) can achieve the maximum metal atom utilization, which exhibit great potential for the chemical transformation of CO2. However, an isolated single active site is less satisfactory for catalyzing multiple molecule reactions involving CO2 hydrogenation. In this work, we propose heteronuclear dual SACs composed of Pd and 3d transition metals supported by covalent triazine frameworks (CTFs). Among these catalysts, the optimized Pd1–Co1/CTF catalyst exhibits up to 84.6% conversion of the captured CO2 from air into formate at 30 °C and 1 bar. The in situ DRIFT characterization and density functional theory calculations reveal that CO2 in air is captured by the Et3N solution as bicarbonate, which is then hydrogenated into formate via the Pd1–Co1 heteronuclear dual single atom with an energy barrier as low as 17.2 kcal/mol. The heteronuclear Pd and Co metal atoms act as the active site for H2 activation and CO2 adsorption, respectively, thus exhibiting enhanced activity for formate synthesis with a synergistic effect. These findings present an insight into the synthesis and application of heteronuclear dual SACs and pave an avenue for conversion of CO2 in the air by heterogeneous catalysts.

Oxide based Heterostructured Photocatalysts for CO2 Reduction and Hydrogen Generation

AbstractGlobal warming and its cause and effects have necessitated the researchers to look beyond fossil and its derivatives. The scalability of CO2 reduction and H2 energy is one of the most enigmatic questions asked by the calamitous environmental changes happening across the world such as bushfires in Australia, Amazon and California or the melting of Arctic and Antarctic ice scalps. Thus, the research fraternity is keen to elevate the efficiency of CO2 reduction and H2 energy to the extent that the good chemical livestock produced by CO2 conversion and H2 fuel would become principal energy resources. In this quest, photocatalytic pathway envisions the environmentally benign protocol to bring about CO2 reduction and H2 production. Photo‐reduction of CO2 and H2 generation via photocatalytic water splitting by utilizing oxide based heterostructured photocatalysts are the promising approaches to carry out CO2 mitigation and H2 production efficiently ascribed to the advanced optoelectronic and structural superiority of oxides photocatalysts as discussed in this review.

Cryogenic Chemistry and Quantitative Non-Thermal Desorption from Pure Methanol Ices: High-Energy Electron versus X-Ray Induced Processes.

X-Ray irradiation of interstellar ice analogues has recently been proven to induce desorption of molecules, thus being a potential source for the still-unexplained presence of gaseous organics in the coldest regions of the interstellar medium, especially in protoplanetary disks. The proposed desorption mechanism involves the Auger decay of excited molecules following soft X-ray absorption, known as X-ray induced electron-stimulated desorption (XESD). Aiming to quantify electron induced desorption in XESD, we irradiated pure methanol (CH3 OH) ices at 23 K with 505 eV electrons, to simulate the Auger electrons originating from the O 1s core absorption. Desorption yields of neutral fragments and the effective methanol depletion cross-section were quantitatively determined by mass spectrometry. We derived desorption yields in molecules per incident electron for CO, CO2 , CH3 OH, CH4 /O, H2 O, H2 CO, C2 H6 and other less abundant but more complex organic products. We obtained desorption yields remarkably similar to XESD values.

Heterojunction nanoarchitectonics of WOx/Au-g-C3N4 with efficient photogenerated carrier separation and transfer toward improved NO and benzene conversion

Topics on effectively improving the photochemical CO2/benzene/NO oxidation conversion performances of g-C3N4 based materials via charge transfer and separation enhancement are still considered challenging, despite the growing popularity of applying these materials in a variety of energy conversion related applications. Based on the idea of nanoarchitectonics, a post-nanotechnology concept, WOx/Au-g-C3N4 heterostructures are synthesized using two-step thermal polymerization and solvothermal treatment methods in this paper. Small Au nanoparticles are incorporated in superior thin g-C3N4 via mechano-chemical pre-reaction and two-step thermal polymerization (treated at 500 and 700 °C). Enhanced photocurrent density is observed after incorporation of Au, which is also in good agreement with the photocatalytic activity (H2 generation and CO2 reduction) data. Layered WOx with abundant oxygen vacancies are further incorporated into Au-modified g-C3N4 nanosheets to form heterojunctions possessing excellent photocatalytic CO2 photo-reduction performances with CO and CH4 generation rate of 5.64 and 2.58 μmolg−1h−1, respectively, under full solar spectrum. The heterojunctions constructed via in-situ formation show direct Z-scheme charge transfer pathway with improved charge separation and transport efficiencies. These highly stable and recyclable hierarchical g-C3N4 hybrid nanostructures (WOx/Au-g-C3N4 heterojunctions) show outstanding conversion rate (88.1%) and selectivity (99.3%) for benzene to phenol conversion under full solar spectrum condition, as well as excellent NO removal rate (61%).

High H2 selective performance of Ni-Fe-Ca/H-Al catalysts for steam reforming of biomass and plastic

The development of a selective catalyst for the conversion of biomass and plastics into H2 by steam reforming can combat the energy crisis and global warming. In this work, support Ni-Fe-Ca/H-Al bifunctional catalysts were prepared by loading Ni and Fe into pretreatment CaO/Al2O3 (Ca/H-Al) carriers and showed high catalytic activity for the steam reforming of biomass and plastic. Moreover, the idea of bidirectional degradation was exploited to strengthen the pyrolysis of plastic with a high H/C and biomass with a high O/C. Interestingly, the products presented high H2 selective (1302.10 mL/g) and low CO2 yield (120.23 mL/g) in 7Ni-5Fe-Ca/H-Al(2:4) catalyst compared with current reports. Here, the abundant oxygen vacancies (Ov) in the H-Al carrier exhibited an electron-deficient nature, providing active sites for anchoring NiO. Meanwhile, NiO interacted with Ca2Fe2O5 to produce more defective Ov sites, which stabilized the NiO particles in the 7Ni-5Fe-Ca/H-Al (2:4) catalyst, and the interaction between the catalyst and the carrier was enhanced, leading to the reduction of weakly basic sites, this property promoted the strong adsorption of CO2 and H2O by the catalyst, contributing to the enhancement of efficient steam conversion and the promotion of conversion of by-products to H2. Notably, 7Ni-5Fe-Ca/H-Al(2:4) catalysts maintained structural integrity after regeneration and exhibited excellent regenerability in H2 selection and CO2 adsorption. The work provides a new idea for the study of efficient H2 production from steam reforming of biomass and plastics.

Modeling study on a two-stage hydrogen purification process of pressure swing adsorption and carbon monoxide selective methanation for proton exchange membrane fuel cells

A two-stage hydrogen purification process based on pressure swing adsorption (PSA) and CO selective methanation (CO-SMET) is proposed to meet the stringent requirements of H2-rich fuel for kW-scale skid-mounted or distributed proton exchange membrane fuel cell systems. The reforming gas is purified using dynamic adsorption model of PSA with activated carbon for initial purification and then kinetic model of CO-SMET with 50 wt% Ni/Al2O3 for CO deep removal. Sensitive analyses of the gas hourly space velocity, adsorption time and adsorption pressure etc. are studied. The results show that excellent H2 purity and CO concentration below 1000 ppm for the initial target using the three-bed and four-bed PSA system at shorter adsorption time and higher pressure, and then CO concentration below 10 ppm with H2 purity over 99.94% on CO-SMET. This work provides a small-scale and hydrogen-saving process for hydrogen purification can be achieved by the two-stage process.

Anodic Reaction in Syngas-Fueled Proton-Conducting Solid Oxide Fuel Cells

Anode-supported proton-conducting solid oxide fuel cells (PC-SOFCs) fabricated with two representative proton-conducting oxides, BaCe0.7Zr0.1Y0.1Yb0.1O3-δ (BCZYYb) and BaZr0.8Y0.2O3-δ (BZY), were compared to obtain the insight into the electrochemical performances when fueled with syngas at 700°C and the correlation between the anode thickness (0.4, 0.8, and 1.6 mm) and operational stability. We have demonstrated that, in stability tests, the BCZYYb cells exhibited significantly higher maximum power density (MPD) than the BZY cells when operating on H2, 1.22 and 0.48 W/cm2 for the BCZYYb and BZY cells, but the BCZYYb cells degraded more rapidly than the BZY cells when operating on syngas. In addition, decreasing the anode thickness significantly enhanced the stability of BCZYYb cells operating on syngas, a reduction of 81, 76, and 71% in MPD for 1.6, 0.8, and 0.4 mm anode after 30 min. The electrochemical impedance spectra and X-ray diffraction patterns indicated that the rapid degradation of cerate-based cells with syngas could be mainly attributed to the considerable increase in polarization resistance due to the phase decomposition of the electrolyte powder in the anode. Heterogeneous catalysis was performed to study the catalytic reaction of the H2–CO mixture over anode powders prepared with the two proton-conducting oxides (Ni-BCZYYb and Ni-BZY) in a fixed bed reactor, and the CO conversion and selectivity to CO2 and CH4 were determined. For all anode powders, continuous CO2 production was initially observed with CH4 formation, and no significant difference in catalytic activity trends was observed between both anode powders. An increased residence time substantially decreased the normalized CO2 yield, which was associated with the potential secondary reaction of CO2 with H2, CH4, or perovskite oxide. Based on the results of heterogeneous catalysis with both anode powders, the observed cell degradation on both cells during operation with syngas may be primarily attributed to carbon coking due to CO disproportionation; however, more than 4 times rapid degradation of the cerate-based cell when operating with syngas was clearly demonstrated to be attributed to the decomposition of the electrolyte powder in anode by the resulting CO2 from the catalytic reaction of the H2–CO mixture.

MOF-Derived Defective Co3O4 Nanosheets in Carbon Nitride Nanocomposites for CO2 Photoreduction and H2 Production

In photocatalysis, especially in CO2 reduction and H2 production, the development of multicomponent nanomaterials provides great opportunities to tune many critical parameters toward increased activity. This work reports the development of tunable organic/inorganic heterojunctions comprised of cobalt oxides (Co3O4) of varying morphology and modified carbon nitride (CN), targeting on optimizing their response under UV-visible irradiation. MOF structures were used as precursors for the synthesis of Co3O4. A facile solvothermal approach allowed the development of ultrathin two-dimensional (2D) Co3O4 nanosheets (Co3O4-NS). The optimized CN and Co3O4 structures were coupled forming heterojunctions, and the content of each part was optimized. Activity was significantly improved in the nanocomposites bearing Co3O4-NS compared with the corresponding bulk Co3O4/CN composites. Transient absorption spectroscopy revealed a 100-fold increase in charge carrier lifetime on Co3O4-NS sites in the composite compared with the bare Co3O4-NS. The improved photocatalytic activity in H2 production and CO2 reduction is linked with (a) the larger interface imposed from the matching 2D structure of Co3O4-NS and the planar surface of CN, (b) improvements in charge carrier lifetime, and (c) the enhanced CO2 adsorption. The study highlights the importance of MOF structures used as precursors in forming advanced materials and the stepwise functionalization of the individual parts in nanocomposites for the development of materials with superior activity.

De-Linker-Enabled Exceptional Volumetric Acetylene Storage Capacity and Benchmark C2H2/C2H4 and C2H2/CO2 Separations in Metal-Organic Frameworks.

An ideal adsorbent for separation requires optimizing both storage capacity and selectivity, but maximizing both or achieving a desired balance remain challenging. Herein, a de-linker strategy is proposed to address this issue for metal-organic frameworks (MOFs). Broadly speaking, the de-linker idea targets a class of materials that may be viewed as being intermediate between zeolites and MOFs. Its feasibility is shown here by a series of ultra-microporous MOFs (SNNU-98-M, M = Mn, Co, Ni, Zn). SNNU-98 exhibit high volumetric C2H2 uptake capacity under low and ambient pressures (175.3 cm3 cm-3 @ 0.1 bar, 222.9 cm3 cm-3 @ 1 bar, 298 K), as well as extraordinary selectivity (2405.7 for C2H2/C2H4, 22.7 for C2H2/CO2). Remarkably, SNNU-98-Mn can efficiently separate C2H2 from C2H2/CO2 and C2H2/C2H4 mixtures with a benchmark C2H2/C2H4 (1/99) breakthrough time of 2325 min g-1, and produce 99.9999% C2H4 with a productivity up to 64.6 mmol g-1, surpassing values of reported MOF adsorbents.

Computational Investigation of Dual Filler-Incorporated Polymer Membranes for Efficient CO2 and H2 Separation: MOF/COF/Polymer Mixed Matrix Membranes.

Mixed matrix membranes (MMMs) composed of two different fillers such as metal-organic frameworks (MOFs) and covalent-organic frameworks (COFs) embedded into polymers provide enhanced gas separation performance. Since it is not possible to experimentally consider all possible combinations of MOFs, COFs, and polymers, developing computational methods is urgent to identify the best performing MOF-COF pairs to be used as dual fillers in polymer membranes for target gas separations. With this motivation, we combined molecular simulations of gas adsorption and diffusion in MOFs and COFs with theoretical permeation models to calculate H2, N2, CH4, and CO2 permeabilities of almost a million types of MOF/COF/polymer MMMs. We focused on COF/polymer MMMs located below the upper bound due to their low gas selectivity for five industrially important gas separations, CO2/N2, CO2/CH4, H2/N2, H2/CH4, and H2/CO2. We further investigated whether these MMMs could exceed the upper bound when a second type of filler, a MOF, was introduced into the polymer. Many MOF/COF/polymer MMMs were found to exceed the upper bounds showing the promise of using two different fillers in polymers. Results showed that for polymers having a relatively high gas permeability (≥104 barrer) but low selectivity (≤2.5) such as PTMSP, addition of the MOF as the second filler can have a dramatic effect on the final gas permeability and selectivity of the MMM. Property-performance relations were analyzed to understand how the structural and chemical properties of the fillers affect the permeability of the resulting MMMs, and MOFs having Zn, Cu, and Cd metals were found to lead to the highest increase in gas permeability of MMMs. This work highlights the significant potential of using COF and MOF fillers in MMMs to achieve better gas separation performances than MMMs with one type of filler, especially for H2 purification and CO2 capture applications.

Engineered g-C3N5-Based Nanomaterials for Photocatalytic Energy Conversion and Environmental Remediation.

Photocatalysis plays a vital role in sustainable energy conversion and environmental remediation because of its economic, eco-friendly, and effective characteristics. Nitrogen-rich graphitic carbon nitride (g-C3N5) has received worldwide interest owing to its facile accessibility, metal-free nature, and appealing electronic band structure. This review summarizes the latest progress for g-C3N5-based photocatalysts in energy and environmental applications. It begins with the synthesis of pristine g-C3N5 materials with various topologies, followed by several engineering strategies for g-C3N5, such as elemental doping, defect engineering, and heterojunction creation. In addition, the applications in energy conversion (H2 evolution, CO2 reduction, and N2 fixation) and environmental remediation (NO purification and aqueous pollutant degradation) are discussed. Finally, a summary and some inspiring perspectives on the challenges and possibilities of g-C3N5-based materials are presented. It is believed that this review will promote the development of emerging g-C3N5-based photocatalysts for more efficiency in energy conversion and environmental remediation.

Economic and Environmental Performance of an Integrated CO2 Refinery.

The consequences of global warming call for a shift to circular manufacturing practices. In this context, carbon capture and utilization (CCU) has become a promising alternative toward a low-emitting chemical sector. This study addresses for the first time the design of an integrated CO2 refinery and compares it against the business-as-usual (BAU) counterpart. The refinery, which utilizes atmospheric CO2, comprises three synthesis steps and coproduces liquefied petroleum gas, olefins, aromatics, and methanol using technologies that were so far studied decoupled from each other, hence omitting their potential synergies. Our integrated assessment also considers two residual gas utilization (RGU) designs to enhance the refinery's efficiency. Our analysis shows that a centralized cluster with an Allam cycle for RGU can drastically reduce the global warming impact relative to the BAU (by ≈135%) while simultaneously improving impacts on human health, ecosystems, and resources, thereby avoiding burden-shifting toward human health previously observed in some CCU routes. These benefits emerge from (i) recycling CO2 from the cycle, amounting to 11.2% of the total feedstock, thus requiring less capture capacity, and (ii) reducing the electricity use while increasing heating as a trade-off. The performance of the integrated refinery depends on the national grid, while its high cost relative to the BAU is due to the use of expensive electrolytic H2 and atmospheric CO2 feedstock. Overall, our work highlights the importance of integrating CCU technologies within chemical clusters to improve their economic and environmental performance further.