Characteristics of negative DC discharge in a wire–cylinder configuration under coal pyrolysis gas components at high temperatures

This work aims to provide a comprehensive understanding of negative DC discharge under coal pyrolysis gas components (CO₂, H₂, N₂, CH₄, CO) and air. The characteristics of negative DC discharge were studied in a wire–cylinder configuration at an ambient temperature range of 20–600 °C by analyzing V–I characteristics, discharge photographs, and gas composition. With increasing temperature, corona onset voltage, spark breakdown voltage and operational voltage range for corona discharge decrease, but discharge current and electron current ratio increase. Discharge current of CO₂ is higher than that of air due to the difference of electronegativity. During CO₂ discharge, with the increase of output voltage, three types of discharge are successively observed, namely corona, glow and arc. However, during H₂ discharge, only glow discharge is observed. Temperatures significantly affect the capability of CO to attach electrons. The discharge characteristic of CO is similar to the electronegative gas media at 20 °C and the non-electronegative gas media when the temperature exceeds 350 °C. Chemical reactions and carbon generation are observed during the CH₄ and CO discharge process. The product of carbon filaments under the CH₄ gas medium leads to discharge current volatility and short circuit. These results assist in understanding the property of ESP at high temperatures.

This work aims to provide a comprehensive understanding of negative DC discharge under coal pyrolysis gas components (CO₂, H₂, N₂, CH₄, CO) and air.     

One step synthesis of high-efficiency AgBr–Br–g-C3N4 composite catalysts for photocatalytic H2O2 production via two channel pathway

In this work, a two-component modified AgBr–Br–g-C₃N₄ composite catalyst with outstanding photocatalytic H₂O₂ production ability is synthesized. XRD, UV-Vis, N₂ adsorption, TEM, XPS, EPR and PL were used to characterize the obtained catalysts. The as-prepared AgBr–Br–g-C₃N₄ composite catalyst shows the highest H₂O₂ equilibrium concentration of 3.9 mmol L⁻¹, which is 7.8 and 19.5 times higher than that of GCN and AgBr. A “two channel pathway” is proposed for this reaction system which causes the remarkably promoted H₂O₂ production ability. In addition, compared with another two-component modified catalyst, Ag–AgBr–g-C₃N₄, AgBr–Br–g-C₃N₄ composite catalyst displays the higher photocatalytic H₂O₂ production ability and stability.

In this work, a two-component modified AgBr–Br–g-C₃N₄ composite catalyst with outstanding photocatalytic H₂O₂ production ability is synthesized.     

Unusual adsorption behaviours and responsive structural dynamics via selective gate effects of an hourglass porous metal–organic framework

An hourglass porous metal–organic framework, LIFM-12, constructed on a T-shaped flexible ligand with Cu²⁺ paddle-wheel clusters, shows temperature and gas adsorption responsive structural dynamics upon reversible molecular guest binding. Temperature-dependent single crystal and powder X-ray diffraction experiments show that the open gate status of the framework with adaptive behaviours facilitates kinetic diffusion of gas molecules resulting in the sequential filling of pores of different sizes, thus creating a breathing behaviour reminiscent of the observation of several steps in adsorption isotherms. In addition, adsorption studies revealed that LIFM-12 performs exceptional adsorption selectivity of 10–25 for CO₂versus light gases N₂, CH₄, and CO and up to 200 for C₃H₆versus CH₄.

A Cu²⁺ based metal–organic framework exhibits dynamic behaviour upon guest inclusion/release process, and performing stepwise sorption isotherms for various gas. We elucidate detailed mechanisms under its exceptional sorption behavior.     

Molecular simulation of CO2/CH4/H2O competitive adsorption and diffusion in brown coal

Carbon dioxide enhanced coalbed methane recovery (CO₂-ECBM) has been proposed as a promising technology for the natural gas recovery enhancement as well as mitigation of CO₂ emissions into the atmosphere. Adsorption and diffusion of CO₂/CH₄ mixture play key roles in predicting the performance of CO₂-ECBM project, i.e., the production of coalbed methane as well as the geological sequestration potential of carbon dioxide. In the present work, the mechanism of competitive adsorption and diffusion of CO₂/CH₄/H₂O mixture in brown coal were investigated by employing grand canonical Monte Carlo and molecular dynamics simulation. The effects of temperature and pressure on competitive adsorption and diffusion behaviours were explored. It is found that CO₂ has much stronger adsorption ability on brown coal than CH₄. The adsorption amounts of CO₂/CH₄ increase with pressure but have a decreasing trend with temperature. High adsorption selectivity of CO₂/CH₄ is observed with pressure lower than 0.1 MPa. In addition, the effects of moisture content in brown coal on the adsorption characteristics have been examined. Simulation results show that the adsorption capacities of CO₂/CH₄ are significantly suppressed in moist brown coal. The competitive adsorption of CO₂/CH₄/H₂O follows the trend of H₂O ≫ CO₂ > CH₄. Moreover, the results reveal that moisture content has great effects on the self-coefficients of CO₂/CH₄. Compared with dry coal, the self-diffusion coefficients of CO₂ and CH₄ reduce by 78.7% and 75.4% in brown coal with moisture content of 7.59 wt%, respectively. The microscopic insights provided in this study will be helpful to understand the competitive adsorption and diffusion mechanism of CO₂/CH₄/H₂O in brown coal and offer some fundamental data for CO₂-ECBM project.

Competitive adsorption and diffusion behaviours of CO₂/CH₄/H₂O in brown coal were explored by GCMC and MD simulations.     

Metal–organic framework-derived CeO2–ZnO catalysts for C3H6-SCR of NO: an in situ DRIFTS study

Metal–organic framework (MOF)-based derivatives have attracted an increasing interest in various research fields. Here, we synthesized CeO₂–ZnO catalysts through the complete thermal decomposition of the Ce/MOF-5 precursor. The catalysts were characterized using XRD, FTIR, TG-DSC, SEM and H₂-TPR. It is found that the as-prepared CeO₂–ZnO is favorable for strengthening the interaction between Ce⁴⁺ and Zn²⁺. A significant improvement in the catalytic performance for C₃H₆-SCR of NO was found over the Ce-doped catalysts with the highest N₂ yield of 69.1% achieved over 5% CeO₂–ZnO. In situ DRIFTS and NO-TPD experiments demonstrated the formation of monodentate nitrates, bidentate nitrates, chelating nitrite, nitro compounds, nitrosyl and C_xH_yO_z species (enolic species and acetate) on the surface, followed by the formation of hydrocarbonate or carbonate as intermediates to directly generate N₂, CO₂ and H₂O.

Metal–organic framework (MOF)-based derivatives have attracted an increasing interest in various research fields. Here, we synthesized CeO₂–ZnO catalysts through the complete thermal decomposition of the Ce/MOF-5 precursor.     

Effects of calcination temperatures on the structure–activity relationship of Ni–La/Al2O3 catalysts for syngas methanation

A series of Ni–La/Al₂O₃ catalysts for the syngas methanation reaction were prepared by a mechanochemical method and characterized by thermogravimetric analysis (TG-DTA), X-ray fluorescence (XRF), X-ray diffraction (XRD), N₂ adsorption–desorption, H₂ temperature-programmed reduction (H₂-TPR), and X-ray photoelectron spectroscopy (XPS). The calcination temperatures (350–700 °C) had significant impacts on the crystallite sizes and interactions between NiO and Al₂O₃. The catalyst calcined at 400 °C (cat-400) showed a 12.1% Ni dispersion degree and the maximum bound state of NiO (54%) through the Gaussian fitting of H₂-TPR. Cat-400 also achieved the highest CO conversion, CH₄ selectivity and yield. Cat-400 exhibited good stability and catalytic activity in a lifetime testing of 200 h. The deactivation of cat-400 was mainly caused by carbon deposition according to the data from XRD, TG-DTG and XPS.

Calcination temperature affects the existing types of NiO, and the influence of the three NiO types on the catalytic activity of samples is bound type ≫ free type > combined type.     

Mechanism of the evolution of pore structure during the preparation of activated carbon from Zhundong high-alkali coal based on gas–solid diffusion and activation reactions

Zhundong coal can significantly reduce the preparation temperature of activated carbon (AC) due to the high contents of alkali and alkaline earth metals (AAEMs) present in it. Moreover, because of its lower operating temperature and the presence of carbon matrix, Zhundong coal can effectively inhibit the release of AAEM during the preparation of AC. For these reasons, the preparation of AC from Zhundong coal is a promising approach for the clean utilization of Zhundong coal. Accordingly, this study was aimed to investigate optimum conditions for the preparation of AC from Zhundong coal. For this purpose, at first, Raman spectroscopy was used to determine the conditions for an optimal carbonization process using a coal sample; then, the evolution of the pore structure of AC under different conditions was examined by small-angle X-ray scattering (SAXS) and the N₂ adsorption analyser. Furthermore, environmental scanning electron microscopy (ESEM) was performed to analyze the surface morphology of AC. Finally, by dividing the activation process into gas–solid diffusion and activation reactions, a mechanism for the evolution of pore structure during the preparation of AC was proposed. The results showed that the char with an amorphous structure and less graphite-like carbon, which was obtained by heating Zhundong coal from room temperature to 600 °C at 5 °C min⁻¹ under the protection of N₂ and then maintaining it at this temperature for 60 min, is suitable for the subsequent activation process. At low temperatures, the diffusion of H₂O was dominant in the activation process, and the weak gas–solid reaction resulted in poor development of the pore structure; on the other hand, the CO₂ activation reaction mainly occurred on the surface of the char due to the poor diffusion of CO₂, and then, the produced pores could improve the diffusion of CO₂; this led to significant development of the pore structure. With an increase in temperature, the H₂O diffusion reaction was enhanced, and the pore structure of AC was completely developed; however, the diffusion of CO₂ reduced with an enhancement in the CO₂ activation reaction, leading to the consumption of carbon matrix by CO₂ gasification instead of pore formation by the CO₂ activation reaction. Therefore, proper utilization of the unique characteristics of H₂O and CO₂ during pore formation is important to control the activation process.

Decoupled the activation process into gas–solid diffusion and reaction, and revealed an evolution mechanism of pore structure during the preparation of activated carbon.     

Improved NO reduction by using metal–organic framework derived MnOx–ZnO

Derivatives based on metal frameworks (MOFs) are attracting more and more attention in various research fields. MOF-based derivatives x% MnO_x–ZnO are easily synthesized by the thermal decomposition of Mn/MOF-5 precursors. Multiple technological characterizations have been conducted to ascertain the strengthening interaction between Mn species (Mn²⁺ or Mn³⁺) and Zn²⁺ (e.g., XRD, FTIR, TG, XPS, SEM, H₂-TPR and Py-FTIR). The 5% MnO_x–ZnO exhibits the highest NO conversion of 75.5% under C₃H₆-SCR. In situ FTIR and NO-TPD analysis showed that monodentate nitrates, bidentate nitrates, bridged bidentate nitrates, nitrosyl groups and C_xH_yO_z species were formed on the surface, and further hydrocarbonates or carbonates were formed as intermediates, directly generating N₂, CO₂ and H₂O.

Derivatives based on metal frameworks (MOFs) are attracting more and more attention in various research fields.     

Scope of sulfur dioxide incorporation into alkyldiallylamine–maleic acid–SO2 tercyclopolymer

Alternate copolymerization of diallylamine derivatives [(CH₂CH CH₂)₂NR; R = Me, (CH₂)₃PO(OEt)₂, and CH₂PO(OEt)₂] (I)–maleic acid (MA) and (I·HCl)–SO₂ pairs have been carried out thermally using ammonium persulfate initiator as well as UV radiation at a λ of 365 nm. The reactivity ratios of ≈0 for the monomers in each pair I–MA and I·HCl–SO₂ ensured their alternation in each copolymer. However, numerous attempted terpolymerizations of I–MA–SO₂ failed to entice MA to participate to any meaningful extent. In contrast to reported literature, only 1–2 mol% of MA was incorporated into the polymer chain mainly consisting of poly(I-alt-SO₂). Quaternary diallyldialkylammonium chloride [(CH₂ CH–CH₂)₂N⁺R₂Cl⁻; R = Me, Et] (II) also, did not participate in II–MA–SO₂ terpolymerizations. Poly((I, R = Me)-alt-SO₂) III is a stimuli-responsive polyampholyte; its transformation under pH-induced changes to cationic, polyampholyte-anionic, and dianionic polyelectrolytes has been examined by viscosity measurements. The pK_a of two carboxylic acid groups and NH⁺ in III has been determined to be 2.62, 5.59, and 10.1. PA III, evaluated as a potential antiscalant in reverse osmosis plants, at the concentrations of 5 and 20 ppm, imparted ≈100% efficiency for CaSO₄ scale inhibition from its supersaturated solution for over 50 and 500 min, respectively, at 40 °C. The synthesis of PA III in excellent yields from cheap starting materials and its very impressive performance may grant PA III a prestigious place as an environment-friendly phosphate-free antiscalant.

Alternate copolymerization of diallylamine derivatives [(CH₂=CH–CH₂)₂NR; R = Me, (CH₂)_nPO(OEt)₂] (I)–maleic acid (MA) and (I·HCl)–SO₂ pairs have been carried out thermally using ammonium persulfate initiator as well as UV radiation at λ of 365 nm.     

Investigation of the reaction pathway for synthesizing methyl mercaptan (CH3SH) from H2S-containing syngas over K–Mo-type materials

The reaction pathway for synthesizing methyl mercaptan (CH₃SH) using H₂S-containing syngas (CO/H₂S/H₂) as the reactant gas over SBA-15 supported K–Mo-based catalysts prepared by different impregnation sequences was investigated. The issue of the route to produce CH₃SH from CO/H₂S/H₂ has been debated for a long time. In light of designed kinetic experiments together with thermodynamics analyses, the corresponding reaction pathways in synthesizing CH₃SH over K–Mo/SBA-15 were proposed. In the reaction system of CO/H₂S/H₂, COS was demonstrated to be generated firstly via the reaction between CO and H₂S, and then CH₃SH was formed via two reaction pathways, which were both the hydrogenation of COS and CS₂. The resulting CH₃SH was in a state of equilibrium of generation and decomposition. Decomposition of CH₃SH was found to occur via two reaction pathways; one was that CH₃SH first transformed into two intermediates, CH₃SCH₃ and CH₃SSCH₃, which were then further decomposed into CH₄ and H₂S; another was the direct decomposition of CH₃SH into C, H₂S and H₂. Moreover, the catalyst (K–Mo/SBA-15) prepared with co-impregnation exhibits higher catalytic activities than the catalysts (K/Mo/SBA-15 and Mo/K/SBA-15) prepared by the sequence of impregnation. Based on characterization of the oxidized, sulfided and spent catalysts via N₂ adsorption–desorption isotherms, XRD, Raman, XPS and TPR, it was found that two K-containing species, K₂Mo₂O₇ and K₂MoO₄, were oxide precursors, which were then converted into main K-containing MoS₂ species. The CO conversion was closely related to the amount of edge reactive sulfur species that formed the sulfur vacancies over MoS₂ phases.

The reaction pathway for synthesizing methyl mercaptan (CH₃SH) using H₂S-containing syngas (CO/H₂S/H₂) as the reactant gas over SBA-15 supported K–Mo-based catalysts.     

Synthesis and gas transport properties of polyamide membranes containing PDMS groups

A series of gas-separation polyamide-poly(dimethylsiloxane) (PA-PDMS) membranes containing PDMS groups were synthesized through the polycondensation reaction. The structural characteristics of polymers were evaluated by ¹H-NMR spectroscopy (NMR), Fourier-transform infrared spectroscopy (FTIR) and UV-vis absorption spectroscopy. The permeability and selectivity behavior was studied at different temperatures (25–55 °C) and pressures (1.0–3.0 atm), using various gases, such as H₂, O₂, CO₂, CH₄, and N₂. The effect of chemical structure, PDMS content, operating pressure and temperature on gas permeability was explored and discussed. Gas-permeation measurements showed that polyamides containing PDMS groups exhibited different separation performance. The PA-PDMS-20 membrane with 20 wt% PDMS exhibited the highest selectivity (CO₂/N₂ = 41.84 and O₂/N₂ = 7.01) at 35 °C and 3.0 atm while CO₂ and O₂ permeability was 29.29 barrer and 4.91 barrer, respectively.

PA-PDMS membranes were synthesized by polycondensation reaction and the gas permeability was found to increase with an increase of PPG content, with the gas permeability of PA-PDMS-20 membrane reaching 29.29 at 35 °C and 3.0 atm.     

Facile synthesis of aminated indole-based porous organic polymer for highly selective capture of CO2 by the coefficient effect of π–π-stacking and hydrogen bonding

A new aromatic aminated indole-based porous organic polymer, PIN-NH₂, has been successfully constructed, and it was demonstrated that the coefficient effect endows this porous material with outstanding CO₂ absorption capacity (27.7 wt%, 1.0 bar, 273 K) and high CO₂/N₂ (137 at 273 K and 1 bar) and CO₂/CH₄ (34 at 273 K and 1 bar) selectivity.

It was demonstrated that the coefficient effect endows POP PIN-NH₂ with outstanding CO₂ absorption capacity and high selectivity.

Today, one of the most serious environmental problems is climate change, such as global warming and sea-level rises, which are caused by increased concentrations of carbon dioxide (CO₂) in the atmosphere.^1–3 As we all know, CO₂ mainly arises from fossil-fuel combustion in power plants, and the flue gas is always mixed with other gases including nitrogen (N₂), methane (CH₄) and so on. Therefore, it is necessary to design materials for selectively separating and adsorbing CO₂ from these industrial and energy-related sources to improve the environmental problems.^4–6 Aqueous amine solutions are the most common adsorbents for CO₂ separation and capture,⁷ however, not only do these adsorbents degrade over time and are corrosive, toxic, and volatile, but also the regeneration process is highly energy demanding for these systems due to the chemical capture mechanism. As alternatives, porous organic polymers (POPs)^8–10 relying on physical adsorption have become the research focus due to their low density, large specific surface area, good thermal stability, and narrow pore size distribution, but the low uptake capacity, and especially, the poor selectivity are two urgent issues that need to be addressed that seriously restrict the commercialization of POP adsorbents.¹¹ Hence, in the past few years, many methods have been developed to improve the POP performance including increasing the surface area and adjusting the pore size.^12,13Recently, based on the rapid development of supramolecular interactions^14,15 and the unique advantage of POP materials, i.e., the structure designability, researchers found that introducing special active sites into the framework, such as heteroatoms and diverse organic groups, is a simple and effective way to ameliorate the adsorption performance by the formation of some special non-covalent interactions and various functional groups have been explored.^16–18 Recently, Chang et al.¹⁹ have designed and prepared an novel aerogel (PINAA) that contains both amide and indole groups and they demonstrated that the CO₂ can be rapidly adsorbed on the heteroaromatic ring of indole because of its relatively large binding area via strong π–π-stacking interactions, and then, the desorbed CO₂ molecule can be captured by an adjacent amide group because of “electrostatic in-plane” interaction. This synergistic effect of electrostatic in-plane and dispersive π–π-stacking interactions of amide and indole with CO₂ endows the resulting aerogel enhanced CO₂ adsorption capacity and CO₂/CH₄ and CO₂/N₂ selectivity. Inspired by this fascinating study, we hypothesized that when the indole group is aminated, the CO₂ can be rapidly adsorbed on the heteroaromatic ring of indole because of its relatively large binding area via strong π–π-stacking interaction (Fig. 1a), after that, the hydrogen bonding interactions between the O of the CO₂ and –NH of the aniline group would make the CO₂ to further form a stable conformation with the aminated indole system (Fig. 1b), as a result, the coefficient effect of π–π-stacking interactions and hydrogen bonding interactions would ensure the high CO₂ adsorption capacity and further enhanced CO₂/CH₄ and CO₂/N₂ selectivity.Open in a separate windowFig. 1Schematic representation showing the heteroaromatic ring of indole adsorbing CO₂via π–π-stacking interactions (a) and the CO₂ molecule is further stabled via the coefficient effect of π–π-stacking interactions and hydrogen bonding interactions (b). (c) Synthetic route of PIN-NH₂ aerogel.To verify our suppose, in this work, we tactfully designed and fabricated an aminated indole-based aerogel PIN-NH₂via Friedel–Crafts alkylation (Fig. 1c), and its CO₂ adsorption capacity and CO₂/CH₄ and CO₂/N₂ selectivity were immediately investigated. The successful preparation of PIN-NH₂ was confirmed by Fourier transform infrared spectroscopy (FT-IR) and ¹³C solid state cross-polarization magic-angle-spinning nuclear magnetic resonance (¹³C CP/MAS NMR) spectrometer, and the results are in good agreement with the proposed structures (Fig. S1 and S2, ESI^†). In the ¹³C CP/MAS NMR spectrum of PIN-NH₂, the peaks at 169–103 ppm are ascribed to the indole group carbons, and the signals located at 35–40 ppm are assigned to the methylene carbons (Fig. S1, ESI^†). For FT-IR spectrum (Fig. S2, ESI^†), the peak at 3438 cm⁻¹ is attributed to the stretching vibrations of N–H in amine unit and indole amine. The peaks at 2999 cm⁻¹ and 2927 cm⁻¹ are assigned to the stretching vibration of –CH₂– in the polymer network and the peaks at 1630 cm⁻¹ and 1480 cm⁻¹ are ascribed to the vibrations of the aromatic ring skeleton.The porosity of PIN-NH₂ was quantified by scanning electron microscopy (SEM), high-resolution transmission electron microscopy (TEM) and N₂ adsorption–desorption isotherms at 77 K. As shown in Fig. 2a, the SEM image displays that the PIN-NH₂ consists of aggregated particles with sub-micrometer sizes. And the microporous characteristic can be observed clearly from the TEM image as shown in Fig. 2b, the presence of porous structure provides the essential condition for CO₂ capture and separation. As shown in Fig. 2c, at a low pressure (0–0.1 bar), there is a rapid raise in the N₂ adsorption–desorption isotherm, indicating its microporous nature, and the increase in the N₂ sorption at a relatively high pressure (∼0.9 bar) shows the presence of meso- and macrostructures of the PIN-NH₂. The specific surface area calculated in the relative pressure (P/P₀) range from 0.01 to 0.1 shows that the Brunauer–Emmett–Teller (BET) specific surface area of PIN-NH₂ is up to 480 m² g⁻¹. Additionally, the pore-size distribution (PSD)²⁰ calculation result was shown in Fig. 2d and S3 ESI,^† which indicating the pore diameter is about 14 Å and further confirming the microporous feature of the PIN-NH₂. To gain further insight into the microstructural information, the powder wide-angle X-ray diffraction (PXRD) was further performed on the PIN-NH₂ polymer. As shown in Fig. S4, ESI,^† only a broad peak at 17.8° 2θ in the PXRD pattern is present, which clearly suggests that the polymer is mainly amorphous in nature. Additional, the thermogravimetric analysis (TGA) show that the microporous material is stable up to 370 °C indicating its potential in post combustion processes operated at high temperatures (Fig. S5, ESI^†).Open in a separate windowFig. 2The microstructures of PIN-NH₂ framework. (a) SEM, (b) TEM, (c) the nitrogen adsorption–desorption isotherms and (d) the pore size distribution of PIN-NH₂ framework.Owing to the artful structure design and particular preparation method, there is a reserved aniline group on the side of the indole group in the PIN-NH₂ network. It was expected that after the rapidly capture of the CO₂ molecule via the π–π-stacking interactions, the next aniline group would assist to further stabilize the CO₂ molecule via hydrogen bonding interactions, in other words, the coefficient effect of π–π-stacking interactions and hydrogen bonding interactions would make this porous organic polymer more efficiently attract CO₂ molecules, which inspires us to investigate the gas uptake capacity. Physisorption isotherms for CO₂ (at 273 K) measured with a pressure more than 1.0 bar indicated that the resulting PIN-NH₂ network exhibited a high carbon dioxide uptake of 27.7 wt% at 1.0 bar, as shown in Fig. 3a. Comparing with most of reported porous materials such as metal–organic frameworks,²¹ activated carbons,²² and microporous organic polymers,^23,24 the porous organic polymer PIN-NH₂ shows an enhanced CO₂ uptake (Table S1, ESI^†). The calculation of isosteric heat of adsorption of PIN-NH₂ shows that the heat of adsorption is 35.7 kJ mol⁻¹ (Fig. S6, ESI^†), which is higher than that of the reported azo-linked polymers (27.9–29.6 kJ mol⁻¹),²⁵ the acid-functionalized porous polymers (32.6 kJ mol⁻¹).^26,27 and the indole-based porous polymers.^28,29 The high value of the heat of adsorption indicated the strong physisorption effect owing to the coefficient effect of π–π-stacking interactions and hydrogen bonding interactions.Open in a separate windowFig. 3Gas adsorption isotherms of PIN-NH₂ for CO₂ at 273 K (a), adsorption and desorption isotherms of PIN-NH₂ for different gases at 273 K (b), a CO₂ molecule is adsorbed on the heteroaromatic ring of indole via π–π-stacking interaction (c) and the CO₂ molecule is further stabled via the coefficient effect of π–π-stacking and hydrogen bonding interactions (d), adsorption isotherms of PIN-NH₂ for different gases with 3% RH of water at 273 K (e), reversibility of the PIN-NH₂ polymer in CO₂ capture measured by TGA at 273 K (f).The application in CO₂ separation and adsorption field of the traditional POPs is limited in a great degree by the poor gas selectivity as the flue gas and natural gas are both mixed gas. Here, we believed that the CO₂ can be easily attracted by the heteroaromatic ring via the π–π-stacking interactions and then stabled with the assist of hydrogen bonding interactions, which leading an enhanced gas selectivity. Therefore, we urgently evaluated the selective gas uptake of the PIN-NH₂ network for small gases (CO₂/CH₄, CO₂/N₂). In the calculation, the ratio of CO₂/N₂ is 15/85 and the ratio of CO₂/CH₄ is 5/95, which is the typical composition of flue gas and natural gas, respectively, the test results were shown in Fig. 3b. It can be found there is a rapid increase for the CO₂ uptake while there is a negligible increase for the CH₄ and N₂ uptake with the increase of the pressure, which maybe due to the unique local dipole–π interactions between the porous organic framework PIN-NH₂ and CO₂ molecule. The test results shows that the CO₂ uptake of PIN-NH₂ is up to 5.92 mmol g⁻¹ at a pressure of 1.0 bar and a temperature of 273 K while the CH₄ and N₂ uptake of PIN-NH₂ is only 0.18 and 0.04 mmol g⁻¹, respectively. The estimated ideal CO₂/CH₄ and CO₂/N₂ adsorption selectivities are up to 34 and 137, respectively. Additionally, the selectivities of PIN-NH₂ toward CO₂ over CH₄ and N₂ at 291 and 303 K were also investigated, respectively, and the results indicated that the resulting polymer PIN-NH₂ still exhibited good selectivity at higher temperatures (Fig. S7 and S8, ESI^†).The high gas selectivities of this microporous framework may attribute to the strong affinity for CO₂ compared with N₂ and CH₄ arising from the coefficient effect of π–π-stacking interactions and hydrogen bonding interactions between the sorbent and CO₂ guest molecule, to further attest the above surmise, we used density functional theory (DFT)³⁰ at the M06-2X level with the aug-cc-pVDZ basis set to investigate the interaction of aminated indole system with CO₂ and the details of the calculation is shown in the ESI.^†Fig. 3c and d shows the snapshot for CO₂ capture by a model compound. The calculation result shows that owing to the electron-rich and large binding area, the CO₂ was very easily attracted by the indole plane at a distance of 3.706 Å, and the computational binding energy was 13.23 kJ mol⁻¹ (Fig. 3c). Soon, the balance structure was changed, the CO₂ molecular was moved towards the amino group till the distance between the amino group and CO₂ molecular was 3.037 Å, indicating a hydrogen bonding interaction was formed in this system. As a result, the distance between the indole plane and CO₂ molecular decreased to 3.257 Å from 3.706 Å, and the computational binding energy increased to 42.15 kJ mol⁻¹, which meaning a more steady system was formed (Fig. 3d). In the sense of computational chemistry, the expected strong coefficient effect of π–π-stacking interactions and hydrogen bonding interactions would favor the uptake of CO₂ of the PIN-NH₂ network. Additional, The DFT result also indicated that the interaction energy between CO₂ and the imine group of indole is relatively weak with a correlation distance at 4.041 Å.As we all know that the CO₂ adsorption property will be affected in a great degree for porous polymers in the presence of water.³¹ In real industrial applications, the flue gas from a power plant is a mixture of CO₂, water vapor, and others. As a result, it has very important practical significance to study the CO₂ capture performance under humid condition. Here, the CO₂ capture property of PIN-NH₂ was studied at a relative humidity of 3% RH, as shown in Fig. 3e, the CO₂ adsorption capacity of PIN-NH₂ decreased from 5.92 to 4.88 mmol g⁻¹ (1.0 bar, 273 K), however, the uptake of CH₄ and N₂ does not affected by the water. These results indicate that adsorption of water diminishes the CO₂ capture. Although the selectivity (CO₂/N₂ = 104, CO₂/CH₄ = 21) is decreased under humid condition, PIN-NH₂, to the best of our knowledge, still has very good CO₂ selectivity over other CO₂ capture materials in similar conditions.³² Moreover, the CO₂ adsorption process is fully reversible (Fig. 3f). Herein, the new aminated indole-based aromatic porous organic polymer PIN-NH₂ synthesized from easily available starting materials demonstrated not only remarkable CO₂ capture capacity, but also prominent CO₂/N₂ and CO₂/CH₄ selectivities. Further, the dynamic breakthrough separation experiments of gas mixture at 298 K using a fixed-bed column packed with PIN-NH₂ was carried out to evaluate the performances of PIN-NH₂ aerogel in an actual adsorption-based separation process. The details of the experiment process were described in ESI.^† As shown in Fig. S9 and S10, ESI,^† the CH₄ and N₂ penetrated through the bed firstly with a retention time for only 6.5 and 3.4 min, respectively, while PIN-NH₂ column can retain CO₂ until above 23 min, which means the high CO₂ adsorption capacity and selectivity of the PIN-NH₂ adsorbent in actual application.     

Hydrodesulfurization of dibenzothiophene using Pd-promoted Co–Mo/Al2O3 and Ni–Mo/Al2O3 catalysts coupled with ionic liquids at ambient operating conditions

Sulfur compounds in fuel oils are a major source of atmospheric pollution. This study is focused on the hydrodesulfurization (HDS) of dibenzothiophene (DBT) via the coupled application of 0.5 wt% Pd-loaded Co–Mo/Al₂O₃ and Ni–Mo/Al₂O₃ catalysts with ionic liquids (ILs) at ambient temperature (120 °C) and pressure (1 MPa H₂). The enhanced HDS activity of the solid catalysts coupled with [BMIM]BF₄, [(CH₃)₄N]Cl, [EMIM]AlCl₄, and [(n-C₈H₁₇)(C₄H₉)₃P]Br was credited to the synergism between hydrogenation by the former and extractive desulfurization and better H₂ transport by the latter, which was confirmed by DFT simulation. The Pd-loaded catalysts ranked highest by activity i.e. Pd–Ni–Mo/Al₂O₃ > Pd–Co–Mo/Al₂O₃ > Ni–Mo/Al₂O₃ > Co–Mo/Al₂O₃. With mild experimental conditions of 1 MPa H₂ pressure and 120 °C temperature and an oil : IL ratio of 10 : 3.3, DBT conversion was enhanced from 21% (by blank Ni–Mo/Al₂O₃) to 70% by Pd–Ni–Mo/Al₂O₃ coupled with [(n-C₈H₁₇)(C₄H₉)₃P]Br. The interaction of polarizable delocalized bonds (in DBT) and van der Waals forces influenced the higher solubility in ILs and hence led to higher DBT conversion. The IL was recycled four times with minimal loss of activity. Fresh and spent catalysts were characterized by FESEM, ICP-MS, EDX, XRD, XPS and BET surface area techniques. GC-MS analysis revealed biphenyl as the major HDS product. This study presents a considerable advance to the classical HDS processes in terms of mild operating conditions, cost-effectiveness, and simplified mechanization, and hence can be envisaged as an alternative approach for fuel oil processing.

Synergistic application of ionic liquids with Pd loaded Co–Mo@Al₂O₃ and Ni–Mo@Al₂O₃ catalysts for efficient hydrodesulfurization of dibenzothiophene at ambient conditions.     

Catalytic decomposition of N2O over Cu–Al–Ox mixed metal oxides

Cu–Al–O_x mixed metal oxides with intended molar ratios of Cu/Al = 85/15, 78/22, 75/25, 60/30, were prepared by thermal decomposition of precursors at 600 °C and tested for the decomposition of nitrous oxide (deN₂O). Techniques such as XRD, ICP-MS, N₂ physisorption, O₂-TPD, H₂-TPR, in situ FT-IR and XAFS were used to characterize the obtained materials. Physico-chemical characterization revealed the formation of mixed metal oxides characterized by different specific surface area and thus, different surface oxygen default sites. The O₂-TPD results gained for Cu–Al–O_x mixed metal oxides conform closely to the catalytic reaction data. In situ FT-IR studies allowed detecting the form of Cu⁺⋯N₂ complexes due to the adsorption of nitrogen, i.e. the product in the reaction between N₂O and copper lattice oxygen. On the other hand, mostly nitrate species and NO were detected but those species were attributed to the residue from catalyst synthesis.

Cu–Al–O_x mixed metal oxides with intended molar ratios of Cu/Al = 85/15, 78/22, 75/25, 60/30, were prepared by thermal decomposition of precursors at 600 °C and tested for the decomposition of nitrous oxide (deN₂O).     

Nanostructured poly(l-lactic acid)–poly(ethylene glycol)–poly(l-lactic acid) triblock copolymers and their CO2/O2 permselectivity

Biodegradable poly(l-lactic acid)–poly(ethylene glycol)–poly(l-lactic acid) (PLLA–PEG–PLLA) copolymers were synthesized by ring-opening polymerization of l-lactide using dihydroxy PEG as the initiator. The effects of different PEG segments in the copolymers on the mechanical and permeative properties were investigated. It was determined that certain additions of PEG result in composition-dependent microphase separation structures with both PLLA and PEG blocks in the amorphous state. Amorphous PEGs with high CO₂ affinity form gas passages that provide excellent CO₂/O₂ permselectivity in such a nanostructure morphology. The gas permeability and permselectivity depend on the molecular weight and content of the PEG and are influenced by the temperature. Copolymers that have a higher molecular weight and content of PEG present better CO₂ permeability at higher temperatures but provide better CO₂/O₂ permselectivity at lower temperatures. In addition, the hydrophilic PEG segments improve the water vapor permeability of PLLA. Such biodegradable copolymers have great potential for use as fresh product packaging.

Biodegradable PLLA copolymers, containing higher molecular weight and content of PEG present better CO₂ permeability and CO₂/O₂ permselectivity, have great potential for use as fresh product packaging.     

Effects of Sm on Fe–Mn catalysts for Fischer–Tropsch synthesis

Sm-promoted FeMn catalysts were prepared by the co-precipitation method and characterized by N₂ adsorption, XRD, CO-TPD, H₂-TPD, CO₂-TPD, H₂-TPR, XPS and MES. It was found that compared with the un-promoted catalyst, when Sm was added at a proper content, the catalyst showed a larger BET surface area and promoted the formation of iron particles with a smaller size. The presence of Sm could increase the surface charge density of iron, which enhanced the Fe–C bond and promoted the stability and amount of CO dissociated adsorption, as confirmed by XPS and CO-TPD. Furthermore, according to MES, Sm could promote the formation of Fe₅C₂, which was the active phase of FTS. In addition, Sm could also enhance the basicity of the catalysts and suppress the H₂ adsorption capacity, which inhibited the hydrogenation reaction and the conversion of olefins to paraffins, as verified by the results of CO₂-TPD and H₂-TPD. According to the FTS performance results, compared with the observations for the un-promoted catalysts, when the molar ratio of Sm to Fe was 1%, the CO conversion increased from 63.4% to 70.4%, the sum of light olefins in the product distribution increased from 26.6% to 32.6, and the ratio of olefins to paraffins increased to 4.18 from 4.09.

The effect of samarium on iron-based catalysts for Fischer–Tropsch synthesis was investigated.     

A H2O2/HBr system – several directions but one choice: oxidation–bromination of secondary alcohols into mono- or dibromo ketones

In this work we found that a H₂O₂–HBr(aq) system allows synthesis of α-monobromo ketones and α,α′-dibromo ketones from aliphatic and secondary benzylic alcohols with yields up to 91%. It is possible to selectively direct the process toward the formation of mono- or dibromo ketones by varying the amount of hydrogen peroxide and hydrobromic acid. The convenience of application, simple equipment, multifaceted reactivity, and compliance with green chemistry principles make the application of the H₂O₂–HBr(aq) system very attractive in laboratories and industry. The proposed oxidation–bromination process is selective in spite of known properties of ketones to be oxidized by the Baeyer–Villiger reaction or peroxidated with the formation of compounds with the O–O moiety in the presence of hydrogen peroxide and Bronsted acids.

Convenience of application, multifaceted reactivity, and compliance with green chemistry principles: H₂O₂–HBr(aq) system for preparation of bromo ketones with yields up to 91%.     

ZIF-95 as a filler for enhanced gas separation performance of polysulfone membrane

Metal–organic frameworks (MOFs) are found to be promising porous crystalline materials for application in gas separation. Considering that mixed matrix membranes usually increase the gas separation performance of a polymer by increasing selectivity, permeability, or both (i.e., perm-selectivity), the zeolitic imidazole framework-95 (ZIF-95) MOF was dispersed for the first time in polysulfone (PSF) polymer to form mixed matrix membranes (MMMs), namely, ZIF-95/PSF. The fabricated ZIF-95/PSF membranes were examined for the separation of various gases. The characterization of solvothermally synthesized ZIF-95 was carried out using different analyses such as powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), porosity measurements, etc. ZIF-95 was mixed with PSF at 8%, 16%, 24%, and 32% weight percent to form different loading MMMs. SEM analysis of membranes revealed good compatibility/adhesion between the MOF and polymer. The permeability of He, H₂, O₂, CO₂, N₂, and CH₄ were measured for the pure and composite membranes. The ideal selectivity of different gas pairs were calculated and compared with reported mixed matrix membranes. The maximum increases in permeabilities were observed in 32% loaded membrane; nevertheless, these performance/permeability increases were at the expense of a slight decrease of selectivity. In the optimally loaded membrane (i.e., 24 wt% loaded membrane), the permeability of H₂, O₂, and CO₂ increased by 80.2%, 78.0%, and 67.2%, respectively, as compared to the pure membrane. Moreover, the selectivity of H₂/CH₄, O₂/N₂, and H₂/CO₂ gas pairs also increased by 16%, 15%, and 8% in the 24% loaded membrane, respectively.

Metal–organic frameworks (MOFs) are found to be promising porous crystalline materials for application in gas separation.     

Effect of axial molecules and linker length on CO2 adsorption and selectivity of CAU-8: a combined DFT and GCMC simulation study

Density Functional Theory (DFT) and Grand Canonical Monte Carlo (GCMC) calculations are performed to study the structures and carbon dioxide (CO₂) adsorption properties of the newly designed metal–organic framework based on the CAU-8 (CAU stands for Christian-Albrechts Universität) prototype. In the new MOFs, the 4,4′-benzophenonedicarboxylic acid (H₂BPDC) linker of CAU-8 is substituted by 4,4′-oxalylbis(azanediyl)dibenzoic acid (H₂ODA) and 4,4′-teraphthaloylbis(azanediyl)dibenzoic acid (H₂TDA) containing amide groups (–CO–NH- motif). Furthermore, MgO₆ octahedral chains where dimethyl sulfoxide (DMSO) decorating the axial position bridged two Mg²⁺ ions are considered. The formation energies indicate that modified CAU-8 is thermodynamically stable. The reaction mechanisms between the metal clusters and the linkers to form the materials are also proposed. GCMC calculations show that CO₂ adsorptions and selectivities of Al-based MOFs are better than those of Mg-based MOFs, which is due to DMSO. Amide groups made CO₂ molecules more intensively distributed besides organic linkers. CO₂ uptakes and selectivities of MOFs containing H₂TDA linkers are better in comparison with those of MOFs containing H₂BPDC linkers or H₂ODA linkers.

Density Functional Theory (DFT) and Grand Canonical Monte Carlo (GCMC) calculations are performed to study the structures and CO₂ adsorption properties of the newly designed metal–organic framework based on the CAU-8 prototype.     

Constructing Rh–Rh3+ modified Ta2O5@TaON@Ta3N5 with special double n–n mutant heterojunctions for enhanced photocatalytic H2-evolution

A multiple core–shell heterostructure Rh–Rh³⁺ modified Ta₂O₅@TaON@Ta₃N₅ nanophotocatalyst was successfully constructed through nitriding Rh³⁺-doped Ta₂O₅ nanoparticles, which exhibited a much higher carrier separation efficiency about one order of magnitude higher than the Ta₂O₅@Ta₃N₅ precursor, and thus an excellent visible light photocatalytic H₂-evolution activity (83.64 μmol g⁻¹ h⁻¹), much superior to that of Rh anchored Ta₂O₅@TaON (39.41 μmol g⁻¹ h⁻¹), and improved stability due to the residual Rh–O/N in the Ta₃N₅ shell layer. Rh-modifying significantly extended light absorption to the overall visible region. Localized built-in electric fields with hierarchical potential gradients at the multiple interfaces including a Rh/Ta₃N₅ Schottky junction and double n–n Ta₃N₅/TaON/Ta₂O₅ mutant heterojunctions, drove charge carriers to directionally transfer from inside to outside, and efficiently separate. Enhanced photoactivity was ascribed to a synergetic effect of improved light absorption ability, increased carrier separation efficiency, and accelerated surface reaction. A promising strategy of developing excellent Ta₃N₅-based photocatalysts for solar energy conversion is provided by constructing double n–n mutant heterojunctions.

Localized built-in electric fields at multiple hierarchical interfaces facilitate the efficient separation and fast inside-out directional transfer of photogenerated carriers.