Hydroisomerization of n-hexane over gallium-promoted sulfated zirconia

Gallium-promoted sulfated zirconia (GSZ) catalysts were prepared by impregnation of zirconium hydroxide with aqueous Ga₂(SO₄)₂ followed by calcination. Isomerization of n-hexane was studied over GSZ at 150 °C, 2.0 MP, WHSV 2 and H₂/hexane (molar) ratio of 5. In comparison to sulfated zirconia (SZ), the conversion of n-hexane over Gallium-promoted sulfated zirconia (GSZ) was greatly improved and it remained stable at 85%. In particular, almost all the products were isomers of hexane and the selectivity of 2,2-DMB reached 20%. The results of characterization indicated that the addition of gallium onto SZ catalyst showed little difference in acid strength between SZ and GSZ catalysts while the redox properties of the SZ catalyst changed with addition of gallium. The transformation of SZ crystalline from metastable tetragonal phase, the more active phase, to monoclinic phase was retarded with the addition of gallium. Furthermore, the simultaneous promotion of Pt and Ga brings the production distribution very close to the equilibrium one.     

In situ 13C MAS NMR Study on the Mechanism of Butane Isomerization Over Catalysts with Different Acid Strength

Reaction mechanism of skeletal isomerization of n-butane over sulfated zirconia (SZ), Cs_2.5H_0.5PW¹²O₄₀ (Cs_2.5) and H-form mordenite (H-MOR) catalysts was studied using ₁₃C MAS NMR with ₁₃C-labeled n-butane. The isomerization of n-butane over SZ type catalysts proceeds predominantly via a monomolecular mechanism below 333 K and gradually changes to a bimolecular alkylation-β-scission mechanism as the reaction temperature is increased to 423 K. Iron promoter in SZ catalyst facilitates the bimolecular process. The n-butane isomerization over Cs_2.5 also proceeds mainly via a monomolecular mechanism below 373 K. The bimolecular mechanism becomes significant as the reaction temperature is increased to 423 K. On both SZ and Cs_2.5 catalysts hydrogen inhibits the isomerization reaction, in particular the bimolecular process. In contrast, the n-butane isomerization over H-MOR with relatively moderate acid strength proceeds mainly via a bimolecular mechanism at 473 K. The kinetics of n-butane isomerization on SZ below 333 K and Cs_2.5 below 373 K are well represented by the Langmuir–Hinshelwood equation for a reversible first order surface reaction, further supporting that a monomolecular mechanism proceeds primarily on SZ and Cs_2.5 catalysts at early reaction stage. All results suggest that the stronger the acidity of the catalyst the lower the reaction temperature of n-butane isomerization and the more contribution of the monomolecular mechanism. The overall mechanism of 1−₁₃C-n-butane reaction on SZ, Cs_2.5 and H-MOR catalysts including ₁₃C scrambling and butane isomerization is proposed.     

Liquid-phase adsorption and isomerization of C<Subscript>5</Subscript>-C<Subscript>8</Subscript><Emphasis Type="Italic">n</Emphasis>-paraffins

The liquid-phase adsorption of n-pentane, n-hexane, n-heptane, and n-octane from natural gasoline on zeolite CaA and their catalytic isomerization has been investigated experimentally and theoretically with the aim of increasing the octane number of a low-octane gasoline. An integrated process flowsheet combining processes in an adsorber and in an isomerization reactor has been developed. The basic results are as follows: the ultimate activity of CaA with respect to n-pentane, n-hexane, n-heptane, and n-octane in the case of their simultaneous adsorption at 25.0°C is 4.2, 4.7, 5.1, and 6.3 kg/100 kg, respectively. Kinetic and outlet adsorption data are also presented. The maximum yield of C₅, C₆, C₇, and C₈ iso-paraffins is 62.0, 70.0, 66.0, and 47.0%, respectively. A mathematical model of the processes has been developed, and their parameters have been calculated. Calculated and experimental data are in satisfactory agreement.     

n-Hexane Isomerization over NiO/SO2− 4/ZrO2 Catalysts. Effect of Nickel Loading

The effect of nickel addition to the sulfated ZrO₂ catalysts for n-hexane isomerization was investigated. A series of catalysts with different nickel concentration (from 0 to 9.6 wt%) were synthesized by supporting nickel on sulfated zirconia by incipient wetness method. After a calcination procedure the catalysts were characterized by X-ray diffraction, nitrogen physisorption (BET method) and its acidic properties were determined by NH₃ adsorption. (TPD) and FT-IR of pyridine adsorption. The n-hexane isomerization reaction was carried out in a fixed bed microreactor at 673 K under 345 kPa of total pressure. Results showed that nickel content between 1.5 and 4.5 wt% improved the catalytic activity and favored the formation 2,3-dimethylbutane, which is a highly desired product. X-ray diffraction results showed no evidence of any NiO phase at NiO concentration below 9.6 wt% suggesting a NiO phase highly dispersed on the surface of ZrO₂–SO^2? ₄ with crystallite sizes lesser than 3nm. Catalytic activity and 2,3-dimethylbutane selectivity seems to be correlated with the NiO well dispersed phase rather than with their acidic properties.     

Activation and Deactivation of Pt Containing Sulfated Zirconia and Sulfated Zirconia Studied byin SituIR Spectroscopy

In situinfrared spectroscopy (IR), thermogravimetric measurements (TG) and temperature-programmed evolution of gases (TPE) have been used to study activation and regeneration of sulfated zirconia with and without Pt (SZ, Pt/SZ). Isomerization and cracking ofn-heptane was used as test reaction. The first activation of Pt/SZ is independent of the atmosphere, air or He and N₂, in each case the catalyst was active. But it was necessary to regenerate the catalyst in air to obtain the initial activity and selectivity. When regeneration was done in helium or nitrogen the catalyst lost its acid properties and only hydrogenolysis ofn-heptane to propane andn-butane was observed. This can be explained by a loss of a part of sulfates between 400 and 500 °C measured by TG and TPE. In the IR spectrum the intensity of the SO band at 1400 cm^?1decreased and shifted to lower wavenumbers whereas the band of terminal Zr-OH groups at 3740 cm^?1, which was absent in the first spectrum after activation increased. The activation conditions of SZ are independent of the atmosphere, no loss of sulfate groups was observed. The reason for deactivation of SZ is coke formation during alkane conversion.     

Liquid–liquid equilibrium for the systems hydrocarbon–thiophene or pyridine–1-hexyl-3,5-dimethylpyridinium bis(trifluoromethylsulfonyl)imide

Liquid–liquid equilibrium for eight ternary systems involving one hydrocarbon (n-hexane, n-heptane, i-octane or toluene), thiophene or pyridine and an ionic liquid (1-hexyl-3,5-dimethylpyridinium bis(trifluoromethylsulfonyl)imide) was experimentally determined at atmospheric pressure and 25°C. Equilibrium data are presented with binodal curves as well as with tie lines. The suitability of ionic liquid (IL) for extractive desulfurization and denitrification was evaluated in terms of solute distribution ratio and selectivity. Extraction experiments with three-component and seven-component (n-hexane, n-heptane, i-octane, toluene, thiophene, pyridine and IL) systems have been performed. The equilibrium data in three-component systems were well described with Non-Random Two-Liquid (NRTL) and Universal Quasi-Chemical (UNIQUAC) models.     

Influence of hydrogen on n-butane isomerization over sulfated zirconia catalysts

This work investigates the influence of hydrogen on the catalytic activity of Fe- and Mn-promoted sulfated zirconia catalysts. It was found that the effect of hydrogen on the activity of Fe-promoted SZ in n-butane isomerization significantly depended on the Fe content of the catalyst. It was also discovered that the negative effect of hydrogen is more significant at lower temperatures. The reason for the decreased activity in hydrogen is thought to be due to the interaction of hydrogen with reaction intermediates.     

Well-Dispersed Gallium-Promoted Sulfated Zirconia on Mesoporous MCM-41 Silica

Gallium-promoted sulfated zirconia (SZ) was confined inside pure-silica MCM-41 (abbreviated as SZGa/MCM-41), where the latter served as a host material. It was prepared by direct dispersion of metal sulfate in the as-synthesized MCM-41 materials, followed by thermal decomposition. The SZGa/MCM-41 catalysts were characterized by XRD, N₂ adsorption, HRTEM, DRIFT, NH₃-TPD, and TPR. The experimental results showed that the ordered porous host structure was still maintained in the catalyst. SZ was in meta-stable tetragonal phase and highly dispersed on the interior surface of MCM-41 even at a high loading of 50 wt%. Additionally, a small fraction of SZ nanoparticles on the external surface of MCM-41 was obtained. The catalytic activity of SZGa/MCM-41 was examined in n-butane isomerization. In comparison to SZ/MCM-41 without promoter, the catalytic activities of the Ga-promoted catalysts were greatly improved. The reason proposed for the higher activity of the Ga-promoted catalysts was that Ga enhances the oxidizing ability of the catalysts.     

Exploiting Adsorption/Diffusion Synergy in MFI-Catalyzed Hexane Isomerization Reactor

The isomerization of n-hexane to produce mono- and di-branched isomers is an important process for octane enhancement of gasoline. In a fixed bed reactor, packed with MFI catalyst, there is synergy between mixture adsorption equilibrium and intra-crystalline diffusion. This synergy is best exploited by operating the reactor in a transient manner, whereby the desired products consisting of the branched isomers can be recovered from the exiting gas mixture during the early stages of transience. This strategy essentially results in a combination of reaction and separation in a single fixed bed device.     

Synthesis and characterization of zirconia-based catalyst for the isomerization of n-hexane

Sulfated zirconia is a very strong solid acid catalyst which can be utilized for various reactions. The present study focuses on synthesis of zirconia-based catalyst with high acidity and high surface area, particularly for isomerization reaction. Sulfated zirconia has been obtained by sulfation of zirconia prepared by hydrothermal route. The catalyst was developed by impregnating tungstophosphoric acid on sulfated zirconia by wet incipient method. The catalyst was characterized through Brunauer–Emmett–Teller (BET) surface area, temperature-programmed desorption of ammonia, temperature program reduction of hydrogen, Fourier transmission infrared spectroscopy, and thermogravimetric analysis. The results revealed that the catalyst is crystalline in nature with surface area 190–225?m² g^?1 and acidity 0.135–0.558?mmol?g^?1. Twenty-five percent conversion was obtained (as confirmed by gas chromatography) at 225°C using n-hexane as model hydrocarbon in fixed-bed microreactor.     

Zeolite beta catalysts for n-C7 hydroisomerization

Zeolite β with Si/2Al ratios of 60, 100, and 200 were synthesized using tetraethlammonium hydroxide (TEAOH) as the structure-directing agent (SDA) in the absence of alkali metal cations. Pt, Pd and Pt-Pd catalysts supported on the zeolite β samples were studied in n-heptane (n-C7) hydroisomerization. The Pt/β catalysts showed a higher catalytic activity than the Pd/β catalysts. For the Pt/β with a Si/2Al ratio of 100, its n-C7 conversion and selectivity of C7 isomers were observed to be 87.06% and 75.48% respectively at 250^∘C. The activity of n-C7 conversion was stable for at least 82 h. However, the selectivity of C7 isomers was gradually decreased with the reaction time. Experimental data also showed that the addition of Pd to catalyst Pt/β enhanced the n-C7 conversion, but lowered the selectivity of C7 isomers. Pd catalyst was also observed to minimize the formation of aromatics in comparison with Pt catalyst.     

The role of shape selectivity and intrinsic selectivity of acidic sites of the catalysts in the skeletal isomerization of n-butenes

The effect upon kinetic behaviour (activity and selectivity) of the physical properties (pore volume distribution) and acidic characteristics (nature of acidic sites and acidic strength distribution) of acidic catalysts in the skeletal isomerization of n-butenes has been studied. The catalysts studied are the following: γ-alumina, alumina modified by introduction of F, Cl and Br, silica–alumina, HZSM-5 zeolite-based catalyst and SAPO-34-based catalyst. It has been proven that the selectivity of acidic catalysts towards the skeletal isomerization of n-butenes is the result of a compromise between pore shape selectivity and intrinsic selectivity of the acidic sites of different strength, this latter property being more significant. Strong acidic sites are needed for the skeletal isomerization, but sites which are too strong produce important amounts of byproducts. A chlorinated alumina is a highly active and selective catalyst for skeletal isomerization. © 1998 SCI.     

The Effect of the Addition of Re and Ge on the Properties of Pt/Al2O3

The effect of Re and Ge addition to Pt/Al₂O₃ was studied. Mono-, bi- and a trimetallic catalysts were prepared and characterized by TPR, XPS, TPO and by the n-pentane, cyclohexane (CH) and n-octane reactions. It was found that the trimetallic was the most active and stable catalyst and showed selectivities to aromatics and isomers very similar to the bimetallic germanium-based catalyst.     

Enthalpies of mixing and heat capacities of mixtures containing alcohols and <Emphasis Type="Italic">n</Emphasis>-alkanes with corn oil at 298.15 K

Enthalpies of mixing for mixtures containing alcohols (methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, and 2-butanol) plus corn oil or n-alkanes (n-hexane, n-heptane, and n-octane) plus corn oil were measured at 298.15 K and atmospheric conditions. The enthalpy was measured in the range of concentrations in which alcohols were miscible with the vegetable oil. Results were correlated by the Redlich-Kister equation. The mixing of alcohols with corn oil was strongly endothermic, whereas values obtained for the binaries containing n-alkanes were slightly endothermic. Thus, heat capacities were calculated for all the systems studied.     

Flash points of mixtures of flammable and non-flammable liquids

A general theory is developed which enables closed-cup flash-points of mixtures of flammable and non-flammable liquids to be predicted from a knowledge of certain properties of a flammability diagram. The theory illustrates, quantitatively, the effect of both the flame inhibiting properties of the vapour of the non-flammable component and the relative volatility of that component. Experimental results for some mixtures of carbon tetrachloride and dichlorodifluoromethane with n-hexane, n-heptane and n-octane are correlated by the theory. The theory also enables a flammability diagram to be partially constructed from standard flashpoint measurements. Such measurements for the n -hexane/carbon tetrachloride system are in good agreement with a published flammability diagram.     

Olefin as an Intermediate in n-Butane Isomerization on Sulfated Zirconia. An In Situ 13C MAS NMR Study of n-Octene-1 Conversion

By using in situ ¹³C MAS NMR and ex situ GC-MS, the analysis of hydrocarbon products formed from n-octene-1 adsorbed on sulfated zirconia catalyst (SZ) has been performed. It is shown that a mixture of alkanes and stable alkyl substituted cyclopentenyl cations (CPC) is formed as the basic reaction products. Formation of both alkanes and CPC from n-octene-1, a precursor of C₈ ⁺ cation, the key intermediate in n-butane isomerization via a bimolecular pathway, implies that formation of the isomerized alkane occurs by a complex process of conjunct polymerization, rather than isomerization itself. CPC deposited on the SZ surface can be in charge of the catalyst deactivation.     

Isomerization of n-Heptane over Ni-WO x /SiO2-Al2O3 Catalysts. Effect of Operating Conditions, and Nickel and Tungsten Loading

Isomerization of n-heptane over Ni-WO _x /Al₂O₃-SiO₂ catalysts was carried out in a continuous flow fixed-bed reactor under atmospheric pressure. The first part of this study deals with the preparation of two series of catalysts (A and B) by the sol-gel method, while the second part deals with the isomerization of n-heptane. The principal objective of this study was to choose the catalyst giving the best isomer yield (di- and tri-branched ones) with optimum reaction conditions (reaction temperature, reduction temperature and time on stream). From the results obtained, the optimum nickel content was found to be 15 wt% and it seems that the incorporation of tungsten (B series) leads to a significant enhancement in the activity of the prepared catalysts. After running on stream for 100 min, the catalyst with 15% nickel and 10% tungsten (B4 catalyst) gives the best results (29% conversion and 70% selectivity) at 250 °C.     

n-Hexane isomerization over copper oxide-promoted sulfated zirconia supported on mesoporous silica

Copper oxide-promoted sulfated zirconia (CuSZ) was supported on MCM-41 by the direct impregnation method. n-Hexane isomerization was investigated over the CuSZ/MCM-41 catalysts. 2-MP, 3-MP and 2,3-DMB are the major isomerization products, besides a small amount of 2,2-DMB. The product distribution is comparable to that reported for Pt based catalysts. The optimal CuO loading in these catalysts calcined at 700 °C is around 3.2 wt% and only leads to the formation of catalytic active metastable tetragonal ZrO₂. The improved performance of CuO-promoted SZ/MCM-41 is a trade-off between the sulfur amount and the content of tetragonal ZrO₂ phase.     

Neopentane cracking catalyzed by iron- and manganese-promoted sulfated zirconia

Cracking of neopentane was catalyzed by a sulfated oxide of zirconium promoted with iron and manganese. Reaction at 300–450°C, atmospheric pressure, and neopentane partial pressures of 0.00025–0.005 bar gave methane as the principal product, along with C₂ and C₃ hydrocarbons, butenes, and coke. The order of reaction in neopentane was determined to be 1, consistent with a monomolecular reaction mechanism and with the formation of methane andt-butyl cations; the latter was presumably converted into several products, including only little isobutylene. At 450°C and a neopentane partial pressure of 0.005 bar, the rate of cracking at 5 min onstream was 5×10^–8 mol/(g of catalyst s). Under the same conditions, the rates observed for unpromoted sulfated zirconia and USY zeolite were 3×10^–8 and 6×10^–9 mol/ (g of catalyst s), respectively. The observation that the promoted sulfated zirconia is not much more active than the other catalysts is contrasted to published results showing that the former catalyst is more than two orders of magnitude more active than the others forn-butane isomerization at temperatures <100°C. The results raise a question about whether the superacidity attributed to sulfated zirconia as a low-temperature butane isomerization catalyst pertains at the high temperatures of cracking.     

Characterization and selective poisoning of acid sites on sulfated zirconia

Microcalorimetric measurements and infrared spectroscopy of ammonia adsorption were used to characterize the acidic properties of sulfated zirconia catalysts. Reaction kinetic measurements forn-butane isomerization were conducted over catalysts that were selectively poisoned with controlled amounts of ammonia. Initial heats of ammonia adsorption on the strong acid sites of sulfated zirconia were 150–165 kJ/mol, and these sites contain Brønsted acid and possibly Lewis acid centers. Sulfated zirconia samples that show high activity for the isomerization ofn-butane possess Bransted acid sites of intermediate strength, with differential heats of ammonia adsorption between 125 and 140 kJ/mol. The results of selective poisoning of sulfated zirconia with ammonia confirm that Bransted acid sites of intermediate strength are active forn-butane isomerization at 423 K while not discounting a possible role of the stronger acid sites.