Chemical fixation of carbon dioxide (CO2) may be very important in the future as a solution for the problem of increased atmospheric CO2 levels. Recent developments for chemical fixation of CO2 to cyclic carbonate, dimethyl carbonate (DMC), cyclic urea and cyclic urethane are reviewed. Synthesis of cyclic carbonate via CO2 addition to epoxide has been already applied on the industrial scale, but catalyst development continues. Direct oxidative carboxylation of olefin is preferable for the synthesis of cyclic carbonate, but requires the development of catalysts for the epoxidation step in the presence of CO2. Direct synthesis of DMC is not in practical use at present because of the reaction equilibrium and chemical inertness of CO2. The preferred alternative is transesterification of ethylene carbonate and methanol for converting CO2 to DMC indirectly. Moreover, combination of this reaction with CO2 addition to epoxide or reaction of ethylene glycol and urea to synthesize DMC is promising. Application of these ideas depends on the development and optimization of catalysts and reaction conditions. Synthesis of cyclic urea and urethane without catalyst should use either CO2 or urea depending on the structures of diamine and amino alcohol.
Catalytic properties of Ni- and Co-based intermetallic compound (IMC) were assessed for the CO2 reforming of methane. Single-phase IMCs were prepared by arc-melting mixtures of stoichiometric amounts of nickel or cobalt and a second element. IMCs were crushed into particles and used as a catalyst for the CO2 reforming of methane at 1073 K. Ni- and Co-IMCs containing Ta, Hf and Sc showed higher initial activity than Ni and Co powders. Stability of these IMCs was examined for the reaction at 923 K because lower temperatures favor coke formation. Among Ni-IMCs, Ni7Hf2, NiHf and NiHf2 showed high stability for CO2 conversion. However, temperature programmed oxidation revealed that a relatively large amount of coke was formed on these catalysts. Filamentous carbon was observed by TEM on NiHf. High activity for the hydrogenation of ethene suggested that the surface of Ni-Hf IMCs is separated into Ni particles and Hf species. Among Co-IMCs, Co2Sc and CoHf2 showed stable CO2 conversion at 923 K. Low ethene hydrogenation activity suggested that the surface of these Co-IMCs consists of each IMC phase. CoHf2 formed less coke than Co2Sc. Only a slight decrease in CO2 conversion was observed on CoHf2 after 100 h of the reforming reaction at 923 K. CoHf2 is a good catalyst for the CO2 reforming of methane with high stability for reforming activity and low activity for coke formation.
129Xe NMR (nuclear magnetic resonance), a useful analytical tool for the investigation of zeolite pores, was evaluated as a novel technique for the analysis of active sites on Mo/Al2O3 hydrodesulfurization catalyst. 129Xe NMR spectroscopy of Mo/Al2O3 catalyst detected a single peak attributed to xenon migrating in a few micropores on the surface of Al2O3. When a chemical shift δ of the peak was plotted against the amount of adsorbed xenon N in the NMR measurement, a nonlinear variation of δ appeared for sulfided Mo/Al2O3 catalyst. This result indicates that xenon strongly interacts electronically with molybdenum species on the surface. In addition, the term δ0 was calculated which mainly depends on collisions between xenon and the catalyst surface from the fitting of the plot to a theoretical equation. As a result, δ0 became larger with increased the molybdenum content. This result shows that migration of xenon was inhibited by molybdenum species on the surface. Increase in the sulfurization temperature also caused δ0 to increase and almost corresponded to the sulfurization degree of molybdenum measured by XPS (X-ray photoelectron spectroscopy). This indicates that δ0 is sensitive to formation of MoS2 crystallites on the surface. 129Xe NMR can be a powerful tool for analysis of the formation of MoS2 crystallites on Mo/Al2O3 catalyst.
Ni/SiO2 catalysts were prepared by fume pyrolysis of Ni-silica sols prepared under different conditions, and the porosities and Ni dispersions were evaluated by comparison with catalysts prepared by vacuum drying. Fume pyrolysis of Ni-doped silica polymers with Ni-O-Si bonds created by the sol-gel process resulted in the formation of microporous and highly dispersed catalysts. Similar features were also obtained from a mixed solution of pure silica polymers and Ni ions without Ni-O-Si bonds. However, the pore structure was independent of the sol viscosity in the former, whereas an increase in the viscosity led to an increase in the pore volume in the latter. The catalysts obtained by vacuum drying of the same sols had mesopores and the viscosity effect on the porosity was different. In addition, the Ni particles were larger. The differences in the pore structure were probably attributable to whether or not the precursory particles in the solutions grew during the drying processes.
Conversion of methanol and/or dimethyl ether into LPG, previously proposed as a potential route for fuel synthesis from natural gas, was evaluated over H-ZSM-5 and H-FeAlMFI-silicate catalysts to optimize LPG fractional selectivity at high conversion, to simulate C2 recycling by co-feed conversion of ethene with oxygenates, and to assess deactivation and regeneration. Selectivity for the desired product depended not only on catalyst performance but also on reaction conditions. The effect of feedstock partial pressure on product distribution could be explained by the concentrations of the active olefin intermediates. Ethene was selectively converted into the LPG fraction by co-feed reaction with oxygenates over H-ZSM-5 catalyst. The large scale process yield of LPG could be increased by recycling C2 components according to the available recycling ratio. However, H-FeAlMFI-silicate catalyst was effective for one-pass conversion but not for recycled ethene conversion. The two catalysts deactivated during one-pass conversion could be regenerated well by a moderate carbon combustion process. Good reproducibility of fractional selectivity was achieved by the regenerated catalyst. H-FeAlMFI-silicate catalyst suffered relatively little deactivation compared with H-ZSM-5 catalyst, and showed high resistance to deactivation after ageing. The improvement in stability of H-FeAlMFI-silicate catalyst may originate from the elimination of some strong acid sites via the ageing process. H-FeAlMFI-silicate catalyst was superior to H-ZSM-5 catalyst except for recycled ethene conversion.
HY-Al2O3-supported Co-Mo catalysts were prepared with citric acid and phosphorus for the hydrodesulfurization of diesel fractions. The HDS activity was measured with straight-run light gas oil feedstocks under actual hydrotreating conditions. CoMoP/HY-Al2O3 catalyst had three times higher HDS activity compared with the conventional CoMoP/Al2O3 catalyst. The long-term stability test of ultra-deep HDS of the developed catalyst was carried out in a pilot plant under conditions maintaining 10 ppm sulfur content in the product. This catalyst enables deep HDS of diesel fuel and shows no significant deactivation. Commercial operation with the developed catalyst has successfully demonstrated high performance. The developed catalyst has super high activity, which enables <10 ppm sulfur content in products in a commercial hydrotreater designed to produce 500 ppm sulfur diesel fuels. The reaction kinetics of the HDS over the developed catalyst in the diesel hydrotreater fit 1.2-order kinetics. The apparent activation energy calculated from the Arrhenius plots is 100 kJ/mol.
Mo K-edge EXAFS, TEM and FT-IR of adsorbed NO were performed to further investigate the nature of the active sites on the CoMo/HY-Al2O3 catalyst containing citric acid and phosphorus, which had significantly higher HDS activity compared to the conventional CoMoP/Al2O3 catalyst. The results showed that the new catalyst has multiple layers of MoS2 slabs and the edges of MoS2 are mainly occupied by Co-Mo-S phases. XPS and FT-IR were used to investigate the sulfiding behavior of Co and Mo in the formation process of the active sites during sulfidation. The results showed that addition of citric acid to the impregnation solution postponed the sulfidation of Co at low temperatures, thereby increasing formation of the Co-Mo-S phase.