Single-phase tetragonal zirconium oxides have been made by the incorporation of 5.4 mol-% of Y3+ or La3+ in ZrO2 to form solid solutions. The samples were prepared by controlled coprecipitation from aqueous solutions of the respective metal chlorides at room temperature and at a constant pH of 10, followed by calcination at 500°C (in the case of the Y3+ -doped sample) or 600°C (in the case of the La3+ -doped sample) to effectuate the crystallization into the tetragonal phase. The process of crystallization of the hydrous zirconia precursor was found to be retarded by the incorporation of Y3+ or La3+, the latter giving the greater effect. Upon crystallization, stabilized tetragonal samples were obtained with high specific surface areas (SBET ca. 88 m2 g−1 for both the samples) and well-developed mesoporous textures but without any microporosity. Both the Y3+ - and the La3+ -alloyed ZrO2 samples were found to fully retain the tetragonal phase upon calcination over the entire range of temperatures studied (up to 900°C). The thermal stability of the texture of zirconia was found to be considerably improved, in comparison with the undoped monoclinic material, by the stabilization of the crystal structure in the defect tetragonal form. In particular, incorporation of 5.4 mol-% of La3+ resulted in a support material which had a remarkable thermal stability. It is shown that the improvements in the thermal stability are derived from a strong inhibition of the processes of crystallite growth and the accompanying intercrystallite sintering and thus of the process of mass transport; the mass transport probably occurs by a mechanism of surface diffusion.
Both nominally pure and Li+-promoted MgO monoliths are catalysts for the oxidative dimerization of methane, with the MgO monoliths being the more active but less selective for the formation of ethane and ethylene. The high activity of the MgO is related, in part, to Ca2+ impurities that concentrate on the surface. The effectiveness of both types of monoliths, however, is not as great as the same materials in a conventional packed-bed configuration. Based on this observation and the low carbon monoxide concentrations it appears that homogeneous reactions in the channels of the monoliths do not contribute significantly to the conversion of methane.
27Al and 31P nuclear magnetic resonance studies have been carried out in order to investigate structural changes in connection with the thermally induced transformation of VPI-5 to AlPO4-8 as well as a structure indication of AlPO4-8 itself. Dramatic changes in the spectral pattern were observed during the transformation. Hydrated AlPO4-8 apparently consists of three different environments in the ratio of 1:2:6.
The suitability of various carbonaceous materials, modified by oxidation and/or defunctionalization in the vapor phase methanol carbonylation, was inquired. In addition to a comparison of activities and their dependence on process time, the catalysts were characterized by carbon monoxide and methyl iodide chemisorption as well as by temperature programmed reaction, XRD and XPS. While oxidized and subsequently defunctionalized carbon-supported catalysts achieved good performances, surface-oxidized carbons have been shown to be unsuitable carriers.
Temperature-programmed oxidation of nickel-molybdate, molybdate and nickel catalysts was carried out in thermogravimetric and fixed bed reactors. During the oxidation of sulfided Ni-Mo catalyst in air, the SO2 formation exhibited three maxima at about 433, 530 and 693 K, and about 11% of the total amount of sulfur remained after oxidation at 773 K. The SO2 formation for sulfided molybdenum and nickel catalysts indicated that the first and second maxima might be mainly from oxidation of nickel sulfide and MoS2-like sulfides, respectively. The promotion of nickel in molybdate catalysts resulted in an increase in the amount of residual sulfur, mainly as sulfate, after oxidation. The formation of nickel sulfate was inferred from thermodynamic data. XPS data revealed that the formation of the third SO2 maximum, accompanied by an exothermic reaction, might mainly result from oxidation of the unoxidized MoS2-like sulfides (Mo4+) and nickel sulfides remained in the bulk of the supported metals. The amount of residual sulfur could be reduced by decreasing the amount of oxygen in the oxidizing agent. A thermodynamic investigation supported this result.
The reaction of 2-propanol over various phosphorus-containing heteropolyacids was investigated as a function of pretreatment temperature and composition of the heteropolyacids. With catalysts pretreated at 100–300 °C, only propene was produced. However, after pretreatment at 450 °C, the selectivity changed in favor of acetone over some mixed molybdenum and tungsten heteropolyacids. From infrared (IR) and thermogravimetry-differential thermal analysis (TG-DTA) measurements, the improvement in acetone selectivity was ascribed to the destruction of Keggin units of the heteropolyacid. The maximum selectivity to acetone was observed for H3PMo12−xWxO40 whenx = 6.
Temperature-programmed sulfiding (TPS) of an oxidic Ni-Mo-P/Al2O3 catalyst and subsequent temperature-programmed oxidation (TPO) of the sulfided catalyst were performed. Treatments of the sulfided catalysts with ammonia-hydrogen, followed by TPO were also carried out, as well as reaction with n-butylamine-hydrogen at elevated pressure. The TPS results were in agreement with previous studies, showing three general stages of sulfiding: (1) initial exchange of catalyst surface oxygen for sulfur; (2) reduction of molybdenum accompanied by release of hydrogen sulfide; and (3) slow continued sulfiding of the residual oxygen. The TPO runs revealed the presence of hydrogen on the sulfided catalyst. Reaction of ammonia or butylamine with the sulfided catalysts provided evidence for strongly-adsorbed nitrogen species, probably as NH or NH2, on the catalyst.
The effect of adding fluoride at various stages of preparation of CoMo/γ-Al2O3 catalysts was investigated with X-ray photoelectron spectroscopy (XPS) and with a pulse reactor method for activity in hydrodesulphurization (HDS) of thiophene and deactivation by nickel and vanadium naphthenate (NiN and VN). Activity is lowered by increasing fluoride content, except for initial activity at a fluoride-to-aluminium (F/Al) ratio of 0.1. It may be correlated with a decreased surface area and dispersion of the molybdenum sulphide phase. Deactivation in HDS of thiophene strongly increases with the fluoride concentration, probably due to an increased acidity induced by the fluoride anions. XPS data suggest that fluoride is highly dispersed except at the highest concentration, i.e. F/Al=0.2. Fluorination of the catalysts has an inhibiting effect on deactivation by NiN and VN and the highest initial and final activity is observed at F/Al=0.1. XPS intensity data suggest that the dispersion of deposited metals decreases with increasing fluoride concentration and that the deposition occurs to a higher extent at the exterior of the particles than the interior under the conditions applied. Both sulphidic and oxidic forms are detected. The order of adding fluoride and the salts of cobalt and molybdenum results in minor differences in activity, except when metal naphthenates are deposited.
Conversion of primary, secondary and tertiary alkylamines were studied at 6732¯773 K over three conventional catalysts, SiO2-Al2O3 (SA), MgO, and ZrO2, which were a typical solid acid, a solid base and an acid-base bifunctional catalyst, respectively, and two new types of synthesized acid-base bifunctional catalysts, SiOx/MgO and hybrid SA/MgO, which consist of a silanization of MgO and a mechanical mixture of SA and MgO, respectively. The results indicate that acid and base bifunctional properties are necessary for an effective synthesis of nitriles from alkylamines, especially from tertiary and secondary alkylamines, in which the acidic functions mainly contribute to the dealkylation of the latter two amines to their corresponding primary ones while the basic site serves mainly the dehydrogenation of the resultant primary amine to nitrile. A detailed comparison of the behavior of SA/MgO, SiOx/MgO and ZrO2 suggests that the relative orientation of the acidic and basic sites greatly affects the product selectivity in the alkylamine conversion.
This work compares and contrasts wet versus dry co-impregnation of RhCl3 with hydrofluoric acid into the walls of honeycomb supports made from γ-alumina. The internal distributions of rhodium within the supports were measured by electron probe microanalysis (EPMA) and compared with respect to the transport mechanisms during wet and dry impregnation. The type of rhodium distribution produced was strongly effected by the type of impregnation (wet versus dry) and the impregnation time. With short impregnation times, the wet and dry co-impregnations produced subsurface bands (egg-whites) of rhodium. These subsurface bands were found to diffuse toward the support center with increasing impregnation times. Calculated characteristic times for diffusion during wet and dry impregnation are shown to be capable of accurately predicting the penetration depth of these bands during diffusion. Subsurface cores (egg-yolks) of rhodium were formed for both the wet and dry methods at long impregnation times (>5 min) by diffusion of the subsurface bands. At still longer impregnation times, these subsurface cores were washed out by back-diffusion and produced uniform distributions of rhodium. This back-diffusion process is modelled accurately as the diffusional decay of a square concentration profile.
Zinc-chromium oxide based catalysts have been tested for the synthesis of higher oxygenates from methanol and hydrogen at atmospheric pressure and in the temperature range 300–405dgC. It has been found that potassium-promoted zinc-chromium oxide, while decomposing a large part of methanol to carbon monoxide and hydrogen, also produces significant amounts of C2+ oxygenates. It is proposed that formation of higher oxygenates occurs by a slow C1→C2 step, which involves methanol-related C1 species, followed by rapid aldol-type condensation reactions and by fast hydrogenation to alcohols. Hydrogen is not necessary for the formation of higher oxygenates, but appears to prevent catalyst deactivation. Marked changes in the product selectivity have been observed when using unpromoted zinc-chromium oxide: higher oxygenates are no longer produced and the fraction of methanol not decomposed to carbon monoxide and hydrogen is converted to dimethyl ether and hydrocarbons. It is concluded that alkali addition plays a crucial role in the formation of C2+ oxygenates over zinc-chromium oxide catalysts.
In this paper we describe the comparative examination of samples of two catalysts, alumina impregnated with 15% sodium carbonate and a silicated alumina modified with polyvalent cations, having regard to their morphology, acid-base properties, isomerization tests, and a laboratory-scale modelling of the processing of an industrial C4 fraction. We conclude that both catalysts are suitable for application in the manufacture of 1-butene on an industrial scale.
The catalytic conversion of de-pitched tall oil to fuels and chemicals was studies over H-ZSM-5 in a fixed bed microreactor operated at atmospheric pressure and in a temperature range of 350–500°C. The effect of co-feeding steam on the product distribution was investigated also. In addition, experiments were carried out using a dual reactor system where two reactors, with different temperature levels, were used in series. The conversion of tall oil in the single reactor system was between 65–93 wt.-% and decreased by 10–15 wt.-% upon steam addition. The amount of organic distillate product was between 24 and 52 wt.-% of feed, which contained up to 65 wt.-% of aromatic hydrocarbons and 20 wt.-% of heavy fraction. The conversion to gaseous hydrocarbons increased from 9 wt.-% at 350°C to 40 wt.-% at 500°C. With the addition of steam, the coke formation decreased, especially at lower temperatures. When the dual reactor system was used, the concentration of heavy fraction in the organic distillate decreased to 6–8 wt.-%. Based on these results, a reaction scheme was postulated.
Styrene divinylbenzene copolymer (SDB) is an inert high surface area material suitable for supporting noble metal catalysts. Because SDB is hydrophobic, the resulting catalysts are free from capillary condensation and remain active in the presence of water. Although SDB supported catalysts are stable at moderate temperatures, it is desirable to enhance the thermal stability and hydrophobicity of SDB for reactions which require higher temperatures. The direct fluorination of SDB was studied by exposing it to fluorine gas. The results show that the degree of fluorination depends on reaction time, reaction temperature, and the concentration of fluorine in the gas stream. Fluorination improved the thermal stability and hydrophobicity, but the treatment had little effect on the surface area and pore structure of SDB. The extent of reaction was determined by comparing the intensity of the IR bands for C-H and C-F bonds. The optimum degree of fluorination occurred when the weight gain of the SDB sample equalled 100%. Studies on platinum catalysts supported on fluorinated and nonfluorinated SDB show that the fluorination results in similar dispersion and activity for the hydrogen-oxygen reaction at ambient conditions. Fluorinated SDB is a superior catalyst support for use under more severe reaction conditions due to the improvement in thermal stability and hydrophobicity.
The product compositions for the isopropylation of dibenzofuran over solid acid catalysts before and after reaching thermodynamic equilibrium (TE) were studied over H-silica-alumina and H-mordenite catalysts, and the shape selectivity of the latter was confirmed. For monoisopropylation over H-silica-alumina at 100–300°C for 2 h in an autoclave, the compositions of four isopropyldibenzofurans (MIPDBFs) before TE were in the following order: 2-MIPDBF=.4-MIPDBP > 62; 1-MIPDBF > 62; 3-MIPDBF. The compositions after TE were in the order: 4-MIPDBF > 62; 3-MIPDBF > 62; 2-MIPDBF > 62; 1-MIPDBF. Comparison between the two catalysts showed that the composition of 2-MIPDBF increased from 20–30% (over H-silica-alumina) to 48% (over H-mordenite). For diisopropylation under comparable conditions, the composition of 2,7-diisopropyldibenzofuran among its isomers also increased from 5–8 to 32%.
The dependence of aluminium hydroxide porous structure on the content of pseudoboehmite in precipitates was studied. It was found that more crystalline wet material had a higher total pore volume as it resisted to a greater extent the influence of mechanical forces during centrifugation. No decrease in the effect of the capillary forces acting in the drying stage with an increasing amount of pseudoboehmite was observed.
The cracking of C6-C8 alkenes has been studied on the ammonium salt of 12-tungstophosphoric acid ( NH4PW ). The products range from C3 hydrocarbons to aromatics, the latter appearing in amounts up to 30% of the products. The primary products from the alkenes provide strong evidence that condensation processes are strongly favoured on these catalysts. The cracking ability of the catalyst decreases rapidly with time on stream but attains a steady state after approximately three hours. The activity of the catalyst is shown to be dependent on the source of the ammonium ion employed in the preparation of the salt with the activity of NH4PW for hezene cracking decreasing in the order NH4Cl > NH4NO3 > (NH4)2CO3 > (NH4)2SO4. The cracking activity of NH4PW for the C6-C8 alkenes decreases in the order: octenes > heptenes > hexenes.
