Recent developments in computer hardware allow the extension of quantum chemical calculations to multiatom systems. Quantum chemical calculations can analyze the mechanism of catalysis by modelling the molecular orbitals on the substrate, the electron states of the cluster formed on the active site, and the interactions between the substrate and the active site of the catalyst. The hydrorefining reaction, which includes hydrodesulfurization (HDS) and hydrodenitrogenation (HDN), proceeds via three discrete steps: adsorption of the sulfur or nitrogen species and hydrogen atoms at the active site of the catalyst; cleavage of the C-S or C-N bond by the addition of hydrogen atoms to the substrate; and desorption of the products from the active site. However, the specific details of the steps are largely unknown.
In the present review, the mechanisms of the HDS of dibenzothiophene (DBT) and the HDN of acridine, a typical sulfur- and nitrogen-containing compound in middle distillate, were simulated based on molecular orbital calculations. Combination of the simulation and experimental data will allow more accurate evaluation of catalysis for hydrorefining.

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The stability of residual oil is considered to be one of the major factors in coke deposition onto the inner surfaces of furnace tubes in distillation units, such as the vacuum distillation unit (VDU). This preliminary study investigated the stability of a Middle East atmospheric residue oil and the coke deposition tendency on type 304 stainless steel surfaces and chromium molybdenum steel surfaces. The latter materials are widely used as furnace tube materials. Coke deposition rates were determined using a laboratory scale test apparatus at atmospheric pressure. The oil was fed in once-through mode into the inlet of the test section of the apparatus for 24h. During the feeding of the oil, the temperature of the metal surface of the test tube was controlled within a range from 500 to 600°C. The stability of the feed oil and effluent oil was evaluated by the P-value based on the Heithaus evaluation method, which indicates the tendency of asphaltene flocculation to form sludge. A decrease of P-value indicates decreased oil stability with the formation of sludge. Coke deposition experiments were carried out at higher temperatures than the conventional VDU operation to promote coke deposition. The results showed that the coke deposition rate was slightly lower for stainless steel than for chromium molybdenum steel. The coke deposition rates increased exponentially regardless of the materials. The P-values of the effluent oil gradually increased with the effluent temperature from 415 to 450°C. This indicates an increase of oil stability with heating temperature. The changes in P-values correspond to removal of unstable aggregates of asphaltenes from the oil. The unstable aggregates are thought to be one of the predominant precursors for coke deposition. The results suggest that the analysis of P-values of the feed and effluent oils together with coke deposition is useful to estimate the quantity of deposited coke on the furnace tubes before operation.

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Thermolytic behavior of asphaltene fractions derived from both vacuum residue of Arabian crude oil and Indonesian natural asphalt was investigated. At first, the asphaltenes were analyzed by ¹H- and ¹³C-NMR spectroscopy to obtain structural information. Structural differences between the two asphaltenes were found in the amount of naphthenic carbon and in the size of polycyclic aromatic units. The asphaltene obtained from Indonesian natural asphalt contains much larger amount of naphthenic carbon but smaller size of polycyclic aromatic units compared those from Arabian crude. Thermal treatments of the asphaltene were conducted in a glass tube at 420°C for 5min in the presence or in the absence of aromatic solvents. The molecular weight distribution of the samples and the products was evaluated by using a gel permeation chromatography. Among the solvents used, partially hydrogenated aromatic compounds such as 9, 10-dihydroanthracene (DHA) showed a slight effect on thermolysis of asphaltene, while polycyclic aromatic compounds such as anthracene (Ant) or acridine (Acr) were more effective. These results could be explained by referring to the radical acceptability of these aromatic solvents.

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Hydrogenation of CO was carried out over Pd catalyst supported on Al₂O₃, MgO, silica alumina, and silica gel supports. Addition of CeO₂ increased the activity of Pd/Al₂O₃ and the main product was dimethyl ether (DME). DME formation proceeded via dehydration of methanol on the acid sites of the supports. However, the yield of DME was much higher over Al₂O₃ than silica alumina. Therefore, Al₂O₃ has an inherently high activity for methanol production. Temperature programmed reduction experiments showed that CeO₂ on Al₂O₃ was reduced with H₂, whereas CeO₂ on MgO underwent little reduction as confirmed by a XAFS measurement. XAFS analysis indicated that Pd/CeO₂/MgO contained Pd⁰ plus Pd²⁺ owing to the presence of the reduction-resistant CeO₂ on MgO and the Pd on CeO₂/Al₂O₃ was in a more reduced state although Pd²⁺ still remained. The results of CO adsorption measurement and XAFS and TEM observation revealed that an addition of CeO₂ increased the dispersion of Pd metal particles on Al₂O₃. The intermediate strength interactions between Pd, CeO₂, and Al₂O₃ seems to account for the higher activity of the Pd/CeO₂/Al₂O₃ catalyst for O-containing compounds formation than Pd/CeO₂/MgO.

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Alumina catalysts containing Co were prepared under various conditions of Co loading, Co salt and calcination temperature to elucidate the factors affecting the activity for the selective reduction of NO with ethene in excess oxygen, and the acctive Co species. The optimum calcination temperature was 800°C regardless the type of Co salt and the loading of Co, at which formation of CoAl₂O₄ was maximum, formation of Co₃O₄ was minimum and the γ-type stmcture of alumina was retained. The catalytically active component is CoAl₂O₄, which acts as a cocatalyst with γ-alumina to complete the reduction of NO.

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The catalytic activities of pure CoAl₂O₄, Co₃O₄, physical mixture of CoAl₂O₄ and Al₂O₃, and Co₃O₄ free Co/Al₂O₃ for the reduction of NO with ethene in excess oxygen, the oxidation of NO, and the oxidation of ethene were investigated to elucidate the activities of CoAl₂O₄, Co₃O₄ and Al₂O₃. Pure CoAl₂O₄ was more active than Al₂O₃ for oxidation of NO, the initiation step of the selective reduction of NO over Co/Al₂O₃, but was less active than Co₃O₄ for oxidation of ethene. Well-mixed CoAl₂O₄-Al₂O₃ catalyst calcined at 800°C had higher activity for selective catalytic reduction of NO with ethene in excess oxygen than catalyst calcined at 500°C as well as roughly-mixed CoAl₂O₄-Al₂O₃ catalyst. The high activity of CoAl₂O₄-Al₂O₃ catalyst depends on the synergy of CoAl₂O₄ and Al₂O₃ particles at their interfaces. Selective removal of Co₃O₄ from cobalt-loaded alumina, Co₃O₄-free Co/Al₂O₃, enhanced the activity for selective catalytic reduction of NO with ethene in excess oxygen, indicating that Co₃O₄ inhibited the reaction, and that the residual CoAl₂O₄-like species was catalytically active. High activity of Co/Al₂O₃ calcined at 800°C for the selective reduction of NO with ethene in excess oxygen is due to the fomlation of CoAl₂O₄ on the Surface layer of γ-Al₂O₃.

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Improvement in efficiency of CO₂ exhaust reduction and in reduction of harmful exhaust gas from automotives is now in demanded. For further reduction of NO_x, fuel reformulation is necessary in addition to improving engine technology and improving after-treatment technology. Various additives are used in both gasoline and diesel fuel to meet environmental regulations and to improve performance. A change in the composition of the exhaust gases resulting from using fuel additives will change the impact of exhaust gases on atmospheric environment. In this study, a products model, which consists of the equilibrium calculation and the reaction calculation, was established to predict the change in the combustion products by addition of gasoline additives. As a result, the concentration of CO, CO₂, and NO were decreased by addition of oxygenates and increased by addition of a nitrous compound. However, because of the magnitude of the changes in the products due to the additives is small, effects of the oxygenates on the atmosphere would be also small since the products are removed with a three-way catalyst with an equivalence ratio of 1.0. The characteristics of the additives which affect the change in emissions were investigated by comparing the contribution of these factors to the emissions. It is shown that the C/H ratio and the enthalpy of formation of the additives affect the change in the emissions. As the range of enthalpy of formation of the actual additives is restricted and limited, especially among the compounds with similar compositions, the C/H ratio would be effective and influential.

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CO hydrogenation over RhVO₄ catalyst on SiO₂ support (RhVO₄/SiO₂) was investigated after H₂ reduction at 573K. RhVO₄ catalyst was formed on the SiO₂ support by calcination in air at 973K with an atomic ratio of V/Rh=1. RhVO₄ was decomposed to Rh metal particles and highly dispersed V oxide particles by H₂ reduction, and a strong metal-oxide (Rh-vanadium oxide) interaction (SMOI) was induced. The RhVO₄/SiO₂ catalyst after H₂ reduction showed higher activity and selectivity for C₂ oxygenates in CO hydrogenation, compared with unpromoted Rh/SiO₂ catalyst. The activity of the RhVO₄/SiO₂ catalyst decreased after intentional deactivation treatment (at 513K in a stream of the reaction gas for 10h; CO conversion of 100%). Temperature-programmed oxidation (TPO) of the deactivated RhVO₄/SiO₂ catalyst resulted in formation of CO₂. Therefore, the deactivation was probably due to carbon deposition. RhVO₄ on the SiO₂ support was regenerated by recalcination in O₂ at 873K, and the activity and selectivity for C₂ oxygenates were restored.

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The hydrodechlorination of chloromethanes over Pd/SiO₂ gave higher hydrocarbons selectively. The produced hydrocarbons followed well the Schulz-Flory distribution, indicating that the hydrocarbons were formed via polymerization of surface C₁ species. The probability of chain-growth was increased in the order of the reactivity of chloromethane, that is, CH₂Cl₂<CHCl₃<CCl₄.

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