Enrichment of low grade CH4 from N2/CH4 mixtures using vacuum swing adsorption with activated carbon

Engineered Catalyst Based on MIL-68(Al) with High Stability for Hydrogenation of Carbon Dioxide and Carbon Monoxide at Low Temperature

Today, the importance of decreasing and converting COx gases from the atmosphere into value-added chemicals by catalytic hydrogenation reactions has become one crucial challenge. In the current work, to facilitate the hydrogenation of COx, several mesoporous alumina catalysts with high efficiency and stability were synthesized using the MIL-68(Al) platform, a nanoporous MOF with a high surface area as a precatalyst, encapsulating nickel or nickel-iron nanoparticles (NPs). After removing the organic linker of MIL-68(Al) by calcination in air, two types of catalysts, promoted and unpromoted, were obtained with various loads of nickel and iron. A set of analyses (PXRD, BET-N2, TEM, FE-SEM, ICP-OES, EDX-map, CO2-TPD, H2-TPR, and H2-TPD) were performed to evaluate the physicochemical properties of catalysts. Based on the analysis results, the promoted catalyst had smaller particles and pores due to the effective and uniform distribution of nickel NPs. Also, H2-TPR and CO2-TPD results in samples containing Fe promoter demonstrated the facilitation of the reduction process and the adsorption and activation of CO2, respectively. The results of CO2 methanation indicated an improved catalytic performance for promoted samples, especially at low temperatures (200-300 °C), compared to unpromoted catalysts. 5Fe·15Ni@Al2O3 MIL-68(Al) catalyst displayed the best performance compared to other catalysts, with a conversion of 92.4% and selectivity of 99.6% at 350 °C and GHSV = 2500 h-1. Moreover, the 5Fe·15Ni@Al2O3 MIL-68(Al) catalyst facilitated the CO2 methanation reaction by reducing the activation energy to 42.5 kJ mol-1 compared with other reported catalysts. Both types of catalysts performed 100% hydrogenation of CO to CH4 with full selectivity at 250 °C and exhibited high stability for at least 100 h at 300 °C. Notably, such high significant catalytic performance is only achieved by the usage of the "MOFs templating strategy" due to the high surface area for the effective distribution of NPs, the strong metal-support interaction, and the formation of nickel aluminate species, preventing the sintering of NPs.

Experimental Study on Flammability Limits Behavior of Methane, Ethane, and Propane with Dilution of Nitrogen

The dilution inerting process of multi-component flammable gaseous mixtures is an important emergency disposal technology that has been widely applied in the explosion-proof field of flammability gases (vapors). In this study, we examined the flammability limits (LFLs and UFLs) of mono and binary alkane mixtures of methane, ethane, and propane when nitrogen is used for dilution inerting. The HY12474B explosion limit test device was used to determine the flammability limits, and the obtained data were compared with the literature data and Chatelier's law. Additionally, the sensitivity coefficient of the chemical reaction chain for LFLs and UFLs of the binary alkane mixtures was analyzed. The minimum inerting concentration (MIC) of methane was found to be sequentially higher than that of ethane and propane when using nitrogen for dilution inerting, and the MIC of the binary alkane mixtures follows the rule of methane/ethane > methane/propane > ethane/propane. Chemical kinetics calculation revealed that the maximum positive sensitivity coefficient of methane/ethane, methane/propane, and ethane/propane are both R5 H + O2 ↔ O + OH, and the reaction with the maximum negative sensitivity coefficients are both R34 H + O2(+M) ↔ HO2(+M) and R43 CH3 + H(+M) ↔ CH4(+M), respectively. The limiting oxygen concentration (LOC) for both mono alkane and binary alkane mixtures ranged between 10 and 13%. The region of the triangular flammability diagram for methane and ethane was greater than the regions for methane/ethane and methane/propane. In contrast, propane had a smaller region compared to other mono alkane or binary alkane mixtures.

Screening of Alkali Metal-Exchanged Zeolites for Nitrogen/Methane Separation

Methane (CH4) is the primary component of natural gas and must be purified to a certain level before it can be used as pipeline gas or liquified natural gas (LNG). In particular, nitrogen (N2), a common contaminant in natural gas needs to be rejected to increase the heating value of the gas and meet the LNG product specifications. The development of energy-efficient N2 removal technologies is hampered by N2's inertness and its resemblance to CH4 in terms of kinetic size and polarizability. N2-selective materials are so rare. Here, for the first time, we screened 1425 alkali metal cation exchange zeolites to identify the candidates with the best potential for the separation of N2 from CH4. We discovered a few extraordinary zeolite frameworks capable of achieving equilibrium selectivity toward N2. Particularly, Li+-RRO-3 zeolite with a specific two-dimensional structure demonstrated a selective N2 adsorption capacity of 2.94 mmol/g at 283 K and 1 bar, outperforming the capacity of all known zeolites. Through an ab initio density functional theory study, we found that the five-membered ring of the RRO framework is the most stable cationic site for Li+, and this Li+ can interact with multiple N2 molecules but only one CH4, revealing the mechanism for the high capacity and selectivity of N2. This work suggests promising adsorbents to enable N2 rejection from CH4 in the gas industry without going for energy-intensive cryogenic distillations.

Upgrading Waste Activated Carbon by Equipping Micro-/Mesopore-Dominant Microstructures from the Perspective of Circular Economy

Equipping wastes with interesting properties in response to the circular economy could release environmental burdens by reducing resource exploitation and material manufacturing. In this study, we demonstrated that the waste regenerated activated carbon (RAC) could become micro-/mesopore-dominant through a simple surfactant/gel modification. This was achieved by associating carbon precursors, such as commercially available low-cost surfactants/methyl cellulose thickening reagents, with the pores of RAC. Following heat treatment, associated carbon precursors were carbonized, hence modifying the microstructure of RAC to be micro-/mesopore-dominant. The surfactant modification gave rise to a micropore-dominant RAC by increasing the micropore volume (PVmicro) together with significantly decreasing the mesopore volume (PVmeso) and macropore volume (PVmacro). In contrast, gel modification led to mesopore-rich RAC by blocking micropores with carbonized methyl cellulose and a surfactant matrix. Interestingly, both surfactant/gel modifications were insensitive to the properties of the surfactant applied, which provided a new alternative for waste/low-grade surfactant mixture disposal. Our results provide an important demonstration that waste could be effectively upgraded with a rational design by exhibiting new properties in response to the circular economy.

The adsorption mechanism of CF4 on the surface of activated carbon made from peat and modified by Cu

In order to find a way to deal with CF₄ with good removal effect and easy to promote. In this study, peat was used as raw material, and copper-loaded activated carbon (Cu/AC) was successfully prepared through nitric acid oxidation and copper chloride impregnation. Compared with commercial activated carbon and widely used metal organic frameworks (MOFs), it shows a fast adsorption rate and larger adsorption capacity for CF₄. The static experiment was used to study the influence of Cu/AC on the adsorption of CF₄ in the adsorbent dosage, reaction time, temperature, and initial concentration. SEM, FTIR, XPS, XRF, and BET were used to study the changes of physical and chemical properties before and after the adsorption. It was found that the oxygen-containing group was consumed during this process. Unsaturated sites on Cu can accelerate the adsorption of CF₄, and the adsorption process is reversible. For the first time, the kinetic model, adsorption isotherm, and thermodynamic model are used to analyze the adsorption mechanism of CF₄ on the Cu/AC surface from different angles. The results show that the adsorption of CF₄ on the Cu/AC surface is a process of exothermic entropy reduction. The static adsorption process conforms to the pseudo-first-order, the pseudo-second-order, and the Freundlish adsorption model. Through 5 adsorption and desorption processes, it is found that Cu/AC has excellent recycling and recyclability performance.