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.
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.
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.
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.
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.
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.
Improvement in efficiency of CO2 exhaust reduction and in reduction of harmful exhaust gas from automotives is now in demanded. For further reduction of NOx, 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.
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.
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.