Recent Advances in Heterogeneous Catalytic Hydrogenation of CO2 to Methane

With the accelerating industrialization, urbanization process, and continuously upgrading of consumption structures, the CO2 from combustion of coal, oil, natural gas, and other hydrocarbon fuels is unbelievably increased over the past decade. As an important carbon resource, CO2 gained more and more attention because of its converting properties to lower hydrocarbon, such as methane, methanol, and formic acid. Among them, CO2 methanation is considered to be an extremely efficient method due to its high CO2 conversion and CH4 selectivity. However, the CO2 methanation process requires high reaction temperatures (300–400°C), which limits the theoretical yield of methane. Thus, it is desirable to find a new strategy for the efficient conversion of CO2 to methane at relatively low reaction temperature, and the key issue is using the catalysts in the process. The advances in the noble metal catalysts, Ni-based catalysts, and Co-based catalysts, for catalytic hydrogenation CO2 to methane are reviewed in this paper, and the effects of the supports and the addition of second metal on CO2 methanation as well as the reaction mechanisms are focused

Characterization of Modified Mechanically Activated Cassava Starch Magnetic Porous Microspheres and Its Adsorption for Cd(II) Ions

The magnetic polymer microsphere is a promising adsorbent due to its high adsorption efficiency and good regeneration ability from wastewater. Cassava starch magnetic porous microspheres (AAM-MSMPMs) were synthesized by graft copolymerization in inverse emulsion. Mechanically activated cassava starch (MS) was used to graft skeletons, vinyl monomers [acrylic acid (AA) and acrylamide (AM)] as copolymerized unsaturated monomers, methyl methacrylate (MMA) as the dispersing agent, and polyethylene glycol/methanol (PEG2000/MeOH) as the porogen. It was found that the AAM-MSMPM adsorbent is superparamagnetic, the saturation magnetization is 14.9 emu·g–1, and it can be rapidly and directionally separated from Cd(II) ions in aqueous solution. The FTIR indicated that the carboxyl and hydroxyl groups were grafted into MS. The AAM-MSMPM had good speroidization and a uniform size. After the porogen was added, the particle size of the AAM-MSMPM decreased from 19.00 to 7.00 nm, and the specific surface area increased from 7.00 to 35.00 m2·g–1. The pore volume increased from 0.03 to 0.13 cm3·g–1. The AAM-MSMPM exhibited a large specific surface area and provided more adsorption active sites for Cd(II) ions. The maximum adsorption capacity of the AAM-MSMPM for Cd(II) ions was 210.68 mg·g–1, i.e., 81.02% higher than that without porogen. Additionally, the Cd(II) ion adsorption process on the AAM-MSMPM can be described by Langmuir isothermal and pseudo-second-order kinetic models. A chemical reaction dominated the Cd(II) ion adsorption process on the AAM-MSMPM, and chemisorption was the rate-controlling step during the Cd(II) ion adsorption process. The AAM-MSMPM still had excellent stability after five consecutive reuses

Zr-Modified ZnO for the Selective Oxidation of Cinnamaldehyde to Benzaldehyde

ZnO and Zr-modified ZnO were prepared using a precipitation method and used for the selective oxidation of cinnamaldehyde to benzaldehyde in the present study. The results showed that physicochemical properties of ZnO were significantly affected by the calcination temperature, and calcination of ZnO at 400 °C demonstrated the optimum catalytic activity for the selective oxidation of cinnamaldehyde to benzaldehyde. With 0.01 g ZnO calcined at 400 °C for 2 h as a catalyst, 8.0 g ethanol and 2.0 g cinnamaldehyde reacted at an oxygen pressure of 1.0 MPa and 70 °C for 60 min, resulting in benzaldehyde selectivity of 69.2% and cinnamaldehyde conversion of 16.1%. Zr was the optimal modifier for ZnO: when Zr-modified ZnO was used as the catalyst, benzaldehyde selectivity reached 86.2%, and cinnamaldehyde conversion was 17.6%. The X-ray diffractometer and N2 adsorption–desorption characterization indicated that doping with Zr could reduce the crystallite size of ZnO (101) and increase the specific surface area of the catalyst, which provided more active sites for the reaction. X-ray photoelectron spectrometer results showed that Zr-doping could exchange the electrons with ZnO and reduce the electron density in the outer layer of Zn, which would further affect benzaldehyde selectivity. The results of CO2 temperature-programmed desorption showed that Zr-modification enhanced the alkalinity of the catalyst surface, which caused the Zr–ZnO catalyst to exhibit higher catalytic activity

Enhanced Water Absorbency and Water Retention Rate for Superabsorbent Polymer via Porous Calcium Carbonate Crosslinking

To improve the water absorbency and water-retention rate of superabsorbent materials, a porous calcium carbonate composite superabsorbent polymer (PCC/PAA) was prepared by copolymerization of acrylic acid and porous calcium carbonate prepared from ground calcium carbonate. The results showed that the binding energies of C–O and C=O in the O 1s profile of PCC/PAA had 0.2 eV and 0.1–0.7 eV redshifts, respectively, and the bonding of –COO− groups on the surface of the porous calcium carbonate led to an increase in the binding energy of O 1s. Furthermore, the porous calcium carbonate chelates with the –COO− group in acrylic acid through the surface Ca2+ site to form multidirectional crosslinking points, which would increase the flexibility of the crosslinking network and promote the formation of pores inside the PCC/PAA to improve the water storage space. The water absorbency of PCC/PAA with 2 wt% porous calcium carbonate in deionized water and 0.9 wt% NaCl water solution increased from 540 g/g and 60 g/g to 935 g/g and 80 g/g, respectively. In addition, since the chemical crosslinker N,N′-methylene bisacrylamide is used in the polymerization process of PCC/PAA, N,N′-methylene bisacrylamide and porous calcium carbonate enhance the stability of the PCC/PAA crosslinking network by double-crosslinking with a polyacrylic acid chain, resulting in the crosslinking network of PCC/PAA not being destroyed after water absorption saturation. Therefore, PCC/PAA with 2 wt% porous calcium carbonate improved the water-retention rate by 244% after 5 h at 60 °C, and the compressive strength was approximately five-times that of the superabsorbent without porous calcium carbonate

Mn Modified Ni/Bentonite for CO₂ Methanation

To enhance the low-temperature catalytic activity and stability of Ni/bentonite catalyst, Ni-Mn/bentonite catalyst was prepared by introducing Mn into Ni/bentonite catalyst and was used for CO2 methanation. The results indicated that the addition of Mn enhanced the interaction between the NiO and the bentonite carrier, increased the dispersion of the active component Ni and decreased the grain size of the active component Ni, increased the specific surface area and pore volume of the Ni/bentonite catalyst, and decreased the average pore size, which suppressed the aggregation of Ni particles grown during the CO2 methanation process. At the same time, the Mn addition increased the amount of oxygen vacancies on the Ni/bentonite catalyst surface, which promoted the activation of CO2 in the methanation reaction, increasing the low-temperature activity and stability of the Ni/bentonite catalyst. Under the reaction condition of atmospheric pressure, 270 °C, V(H2):V(CO2) = 4, and feed gas space velocity of 3600 mL·gcat−1·h−1, the CO2 conversion on the Ni-Mn/bentonite catalyst with 2wt% Mn was 85.2%, and the selectivity of CH4 was 99.8%. On the other hand, when Mn was not added, the CO2 conversion reached 84.7% and the reaction temperature only raised to 300 °C. During a 150-h stability test, the CO2 conversion of Ni-2wt%Mn/bentonite catalyst decreased by 2.2%, while the CO2 conversion of the Ni/bentonite catalyst decreased by 6.4%