Kohichi Segawa

Hydrodesulfurization of dibenzothiophene derivatives over TiO2Al2O3 supported sulfided molybdenum catalyst

Abstract Composite TiO2Al2O3 supports with different loadings of TiO2 have been prepared by chemical vapor deposition (CVD), using TiCl4 as the precursor. The supports have been characterized by N2 adsorption measurements, X-ray photoelectron spectroscopy (XPS), X-ray fluorescence analysis (XRF) and IR spectroscopy. The specific surface area of each TiO2Al2O3 support is comparable to that of γ-Al2O3. High dispersion of TiO2 on the Al2O3 surface has been obtained, and no cluster formation has been detected. Mo TiO 2  Al 2 O 3 and Ni Mo TiO 2  Al 2 O 3 catalysts have been synthesized by impregnation and co-impregnation method. We studied the conversion of Mo and Ni from the oxidic to the sulfidic state by XPS. Furthermore, the catalytic behavior of Mo and NiMo supported on Al2O3, TiO2 and TiO2-Al2O3 has been investigated for the deep hydrodesulfurization (HDS) of dibenzothiophene (DBT) and methyl substituted DBT derivatives. The conversion over the TiO2Al2O3 supported catalysts, in particular for the HDS of 4-methyl-DBT (4-MDBT) and 4,6-dimethyl-DBT (4,6-DMDBT), is much higher than the conversion obtained over Al2O3 supported materials. In particular with regard to the NiMo catalysts, the ratio of the corresponding cyclohexylbenzene (CHB)/biphenyl (BP) derivatives is increased over the composite support, indicating that the prehydrogenation of an aromatic ring is important for the HDS of DBT derivatives over TiO2Al2O3 supported catalysts.

Catalysis and surface chemistry: III. The adsorption of pyridine on molybdena-alumina catalysts

Abstract Only Lewis-bound pyridine (LPy) was detected in infrared spectra from pyridine which had been adsorbed on Ketjen Alumina (CK-300) and then evacuated to successively higher temperatures. Exposure to H 2 O lowered the intensity of the LPy bands. On the oxidized forms of molybdena-, chromia-, rhenia-, and tungsta-alumina catalysts (made with CK-300) both LPy and pyridinium ion (BPy) were observed. The amount of BPy present was increased by added-back H 2 O; lowering the pretreatment temperature had the same effect. On reduction, the ability of the catalyst to generate BPy was eliminated although a sufficient extent of reduction was difficult to achieve with the tungsta-alumina catalyst. Added-back H 2 O did not produce BPy on reduced catalysts. Coverage with LPy and BPy and heats of adsorption were estimated in separate experiments. These findings appear to be in conflict with our earlier assumption that the facile isomerization of cyclopropane over reduced molybdena-alumina is effected by acid catalysis.

Site selective chemisorptions on molybdena-alumina catalysts

Abstract NO was found to adsorb selectively on the molybdena portion of reduced molybdena-alumina catalysts and CO 2 on the uncovered alumina portion of the surface. These two chemisorptions had little effect on each other. The chemisorption of NO was correlatable with coordinative unsaturation developed during reduction (by the removal of oxygen as H 2 O or CO 2 ); NO molecules were chemisorbed as pairs on sites which also dissociatively adsorbed H 2 and catalyzed hydrogenation. The chemisorption amounted to NO/□ = 0.43 or to less than 1 NO pair for every 10 Mo (depending upon the extent of reduction). Alumina hydroxyl groups were regenerated as the catalyst was reduced and the CO 2 chemisorption increased concomitantly. At 500 °C these quantities reached maximum values of about 50% recovery of the hydroxyls lost from the alumina when the catalyst was prepared and to a CO 2 chemisorption of about 80 μmol/g. The intensities of the CO 2 bands of the ir spectra were lower for reduced molybdena-alumina catalysts than for the parent alumina, but were otherwise identical. These bands remained unchanged when NO was added to the system and the NO bands remained unchanged when the previous spectrum for the CO 2 was subtracted. Within the experimental error the reverse experiment, where CO 2 was added to a catalyst on which NO had been previously chemisorbed, gave the same results. Interestingly, a bicarbonate band about 3610 cm −1 appeared on CO 2 adsorption, while the highest frequency band from the alumina OH (at 3792 cm −1 ) was strongly diminished. An attempt was made to use these tools to determine the parts of the surface on which pyridine adsorption occurs. When CO 2 was adsorbed first, it was eluted by pyridine and Lewis bound pyridine (LPy) was formed on the alumina surface. When NO was chemisorbed first its concentration was not greatly affected by the chemisorption of pyridine, but the two NO bands shifted to lower frequency by about 50 cm −1 and their relative intensity was altered. When pyridine was adsorbed first, the NO bands still formed but with lower intensity; the same shift to lower frequency was observed.

Infrared and Mössbauer spectroscopic studies of the interaction of nitric oxide with FeY zeolite

Abstract The time evolution of infrared spectra obtained from NO adsorbed on reduced FeY zeolite at room temperature is reported. Bands near 1845 and 1870 cm− formed quickly and remained invariant while bands at 1917 and 1815 cm− grew slowly over periods of hours. The intensity ratio of the latter two bands remained constant during growth and decay, and they were therefore attributed to a dinitrosyl species formed within the supercages. On evacuation at room temperature, the dinitrosyl species was converted to a mononitrosyl species with an absorption band at 1767 cm−. All bands disappeared on evacuation above 520 K. The dinitrosyl bands also disappeared on heating in NO but the absorption in the region 1855 to 1870 cm− did not, showing that the oxidized form of this material chemisorbed NO, presumably as a mononitrosyl species. The ir bands characteristic of NO adsorbed on reduced FeY were also eliminated by exposure of the sample to O2 at room temperature. In both of these cases, however, the extent of NO adsorption on oxidized FeY zeolite was significantly smaller than that on reduced FeY. Gravimetric studies showed that the total adsorption of NO grew slowly over a period of 20 h at room temperature to a value more than twice its initial value. This corresponded to the formation of increasing amounts of the dinitrosyl species. The data suggested that new sites were being formed in the supercages concomitant with the slow diffusion of iron to an accessible location. Mossbauer spectroscopy was used to probe those sites in the zeolite structure responsible for the different nitrosyl complexes observed in the infrared spectra. Reduced FeY zeolite showed two Fe2+ doublets due to iron cations of different coordination. Upon exposure to NO at room temperature, those Fe2+ cations of low coordination which were accessible to gases (at Sites II and II′) formed nitrosyl complexes. The infrared bands at 1845 and 1870 cm−1 are assigned to these complexes. Ferrous cations of high coordination (presumably at Site I) also formed nitrosyl complexes by migrating to sites of high accessibility. The dinitrosyl bands are assigned to these complexes. The mononitrosyl species at 1767 cm−1, which was formed from the dinitrosyl species upon evacuation, gave rise to a Mossbauer spectrum characteristic of highly coordinated iron. It is suggested that this low-frequency mononitrosyl species is due to NO adsorbed on iron cations at Site III′ in the supercages.

Physicochemical properties of MoO3TiO2 prepared by an equilibrium adsorption method

The adsorption phenomena of molybdena species onto titania surfaces and the surface properties of the catalysts have been studied by using an equilibrium adsorption method. 95Mo NMR and UV spectroscopic studies show that the aqueous molybdena species vary as a function of the pH of the impregnating solution. For acidic pH values, polymeric species, Mo7O246 ions, are present, while in the basic solutions it is the monomeric MoO42− ions that are present. The adsorbed amounts of molybdate anion are strongly dependent on the pH of the impregnating solution and increase as an inverse function of the pH. XRD, Raman, and XPS data of the calcined samples show that monolayer coverage of molybdenum oxide is established at pH 3.98 (6.6 wt%). The Raman studies reveal that the molybdenum oxide monolayer is composed of distorted octahedra. At more acidic pH regions, pH < 3.98, crystalline MoO3 is formed above monolayer coverage. The results of catalytic oxidation of methanol show that the catalysts up to monolayer coverage of surface molybdate species possess higher turnover numbers than the catalysts possessing more than monolayer coverage (presence of crystalline MoO3. The primary methanol oxidation product is dimethoxymethane at low conversions; methyl formate is next in abundance. The selectivity for dimethyl ether, which occurred as a side reaction on the acidic sites of catalysts, increases as the Mo loading increases.

Hydrodesulfurization of dibenzothiophenes over molybdenum catalyst supported on TiO2–Al2O3

Abstract Composite types of TiO 2 –Al 2 O 3 supports, which are γ-aluminas coated by titania, have been prepared by chemical vapor deposition (CVD), using TiCl 4 as a precursor. Then supported molybdenum catalysts have been prepared by an impregnation method. As supports, we employed γ-alumina, anatase types of titania, and composite types of TiO 2 –Al 2 O 3 with different loadings of TiO 2 . We studied the conversion of Mo from oxidic to sulfidic state through sulfurization by X-ray photoelectron spectroscopy (XPS). The obtained spectra unambiguously revealed the higher reducibility from oxidic to sulfidic molybdenum species on the TiO 2 and TiO 2 –Al 2 O 3 supports compared to that on the Al 2 O 3 support. Higher TiO 2 loadings of the TiO 2 –Al 2 O 3 composite support led to higher reducibility for molybdenum species. Furthermore, the catalytic behavior of supported molybdenum catalysts has been investigated for hydrodesulfurization (HDS) of dibenzothiophene (DBT) and methyl-substituted DBT derivatives. The conversion over the TiO 2 –Al 2 O 3 supported Mo catalysts, in particular for the 4,6-dimethyl-DBT, is much higher than that obtained over Al 2 O 3 supported Mo catalyst. The ratio of the corresponding cyclohexylbenzene (CHB)/biphenyl (BP) derivatives is increased over the Mo/TiO 2 –Al 2 O 3 . This indicates that the prehydrogenation of an aromatic ring plays an important role in the HDS of DBT derivatives over TiO 2 –Al 2 O 3 supported catalysts.

Selective catalytic reduction of nitrous oxide over Fe-MFI in the presence of propene as reductant

Abstract The direct decomposition and the selective reduction of nitrous oxide (N 2 O) over Fe-MFI have been investigated in the presence of water and oxygen, using propene (C 3 H 6 ) as reductant. Comparative studies have been performed over H-MFI, Cu-MFI and Na-MFI zeolite catalyst. Although Fe-MFI showed the highest catalytic activity among all materials under investigation in the presence of C 3 H 6 , no significant deactivation in the presence of water vapor and oxygen has been detected. Furthermore, in the case of Fe-MFI, the oxidation of C 3 H 6 is directly correlated to the conversion of N 2 O, indicating the selective catalytic reduction being the preferred reaction. The conversion of N 2 O over Fe-MFI as well as the number of hydroxyl groups detected by FTIR spectroscopy increases with increasing Fe loading, and even Fe-MFI samples with rather low exchange levels exhibit remarkably high N 2 O conversions. FTIR experiments applying C 3 H 6 and pyridine as probe molecules have been carried out in order to elucidate the adsorption behavior, as well as the nature of the active sites and the acidity of the different ion-exchange zeolite catalysts. The studies indicate the surface properties of the Fe-MFI catalysts, with regard to acidity and adsorption behavior, as being strongly depending on the ion-exchange level.

Characterization of crystalline zirconium phosphates and their isomerization activities

The catalytic activities for ring-opening isomerization of cyclopropane and isomerization of butenes have been examined on crystalline zirconium phosphates. e-Zr(HPO4)2 catalyst was highly crystallized during the dehydration of zirconium phosphate gel with concentrated phosphoric acid solution under reduced pressure. This catalyst, which was evacuated at higher temperatures (above 770 K), exhibited higher catalytic activities (based on unit surface area) than α-Zr(HPO4)2 · H2O catalyst or other conventional solid acid catalysts. The coisomerization of d0− and d8−-1-butene suggests that isomerization would proceed on protonic acid sites even after heat treatment at 1100 K. After evacuation at 773 K, most of phosphate groups were removed, with consequent loss of water, due to the condensation of phosphate groups between each zirconium atom layer. However, a trace amount of residual phosphate groups still remained on the surface. After heat treatment at higher temperatures, the stretching and bending vibration of POP appeared in infrared studies; their intensities increased with increasing temperatures of evacuation. Even though the protonic concentrations decreased, the reaction rates for isomerization were drastically enhanced, because of the presence of POP bonds which could withdraw the electrons from the residual phosphate groups on the surface. Thus some enhancement of acid strength of protons of phosphate groups may occur.

Development of new catalysts for deep hydrodesulfurization of gas oil

Abstract TiO2–Al2O3 composite supports have been prepared by chemical vapor deposition (CVD) over γ-Al2O3 substrate, using TiCl4 as the precursor. High dispersion of TiO2 overlayer on the surface of Al2O3 has been obtained, and no cluster formation has been detected. The catalytic behavior of Mo supported on Al2O3, TiO2 and TiO2–Al2O3 composite has been investigated for the hydrodesulfurization (HDS) of dibenzothiophene (DBT) and methyl-substituted DBT derivatives. The conversion over the Mo catalysts supported on TiO2–Al2O3 composite, in particular for the HDS of 4,6-dimethyldibenzothiophene (4,6-DMDBT) is much higher than that of conversion obtained over Mo catalyst supported on Al2O3. The ratio of the corresponding cyclohexylbenzenes/biphenyls is increased over Mo catalyst supported on TiO2–Al2O3 composite support. This means that the reaction rate of prehydrogenation of an aromatic ring rather than the rate of hydrogenolysis of C–S bond cleavage is accelerated for the HDS of DBT derivatives. The Mo/TiO2–Al2O3 catalyst leads to higher catalytic performance for deep HDS of gas oil.

Highly selective methylamine synthesis over modified mordenite catalysts

Methylamine syntheses from methanol and ammonia on various zeolite catalysts have been studied over a temperature range of 573–673 K at atmospheric pressure. The mordenite catalysts were treated with silicon tetrachloride in sodium form and then ion-exchanged to the acid form; such catalysts have been shown to suppress trimethylamine production almost completely and to generate enhanced yields of dimethylamine and/or monomethylamine. The formation of TMA has been extremely retarded because the pore openings of the catalyst are smaller than TMA molecules. The sorption data of methylamines and IR spectroscopy studies suggest that the more bulky base molecules, such as trimethylamine or pyridine, cannot penetrate into mordenite channels after treatment with silicon tetrachloride. The 29Si- and 27Al-MASNMR studies showed that silicon tetrachloride treatment caused less dealumination of zeolite framework for sodium mordenite than for H mordenite.