N-butane Isomerization Activity Research Articles

Study of n-Butane Isomerization on Acid Catalysts Niobium and Lanthanum Promoted Tungstated Zirconia: n-Butane Isomerization Activity

Study of n-Butane Isomerization on Acid Catalysts Niobium and Lanthanum Promoted Tungstated Zirconia: n-Butane Isomerization Activity

Changes in Surface Acidity of Silica-Supported Dodecatungstosilicic Acid in Relation to the Loading Amount

Changes in surface acidity of H4SiW12O40/SiO2 in relation to the loading amount were investigated by using temperature-programmed desorption (TPD) of benzonitrile (BN), BN infrared (BN-IR) spectroscopy, and test reactions, including skeletal isomerization of n-butane and double-bond shift of 1-butene. From BN-TPD profiles and BN-IR spectra, H4SiW12O40/SiO2 had medium strength Brønsted (Mid-B site) and strong Brønsted acid sites (Strong-B site) on its outermost surface, whereas there was no evidence for Lewis acid sites regardless of the loading amount. The number of Strong-B sites correlated with the catalytic activity for the skeletal isomerization of n-butane and was a maximum when the loading amount was 50 wt %. On the other hand, the number of Mid-B sites was a maximum for 30 wt % H4SiW12O40/SiO2, which had the highest activity for the double-bond shift reaction of 1-butene. Mid-B sites of H4SiW12O40/SiO2 were present in the starting H4SiW12O40, whereas the Strong-B sites formed when H4SiW12O40 was supported on SiO2. There were no Strong-B sites on H4SiW12O40/SiO2 with low loadings, including 5 and 10 wt %. Thus, it was concluded that the direct interaction of H4SiW12O40 with the surface of SiO2 caused the Strong-B sites to form in the second-layer of H4SiW12O40 exposed on the surface.

Pd-promoted WOx/ZrO2 catalysts: Characterization and catalytic activity for n-butane isomerization

Abstract The need for environmental friendly catalysts for alkane isomerization has prompted research on the WO x /ZrO 2 system. Suitable promoters may improve its activity and stability. We investigated the effect of palladium on WO x /ZrO 2 redox properties and catalytic activity for n -butane isomerization. Pd favoured the reduction of W x O y clusters already at RT (redox cycles O 2 /H 2 ) and in reduced samples EPR identified W 5+ surface species. Increasing Pd dispersion increased the catalytic activity. Leached and pre-reduced catalysts resulted to have the highest activity, suggesting that the most active species are W x O y clusters anchored to zirconia surface and interacting with near Pd hydride. For n -butane isomerization Pd-promotion is as effective as Pt-promotion.

Enhanced catalytic activity for butane isomerization with alumina-promoted tungstated mesoporous zirconia

Mesoporous zirconia, synthesized from surfactant templating, was used to disperse alumina and tungstate as a catalyst (denoted as AW/m-ZrO2) for the isomerization of n-butane. At a relatively low temperature of calcination (700 °C), the catalyst (AW/m-700) performed exceptionally well, both in maximum rate and stability, at a temperature of 250 °C. This was lower than the temperature needed by other tungstated zirconia-based catalysts reported so far. This lower temperature of calcination leads to the presence of more Brönsted acid sites with higher acid strength. In comparison with the normal dense phase zirconia catalyst, AW/m-ZrO2 catalyst showed higher n-butane isomerization activity than the corresponding AW/d-ZrO2 catalyst, which has about the same surface area. The difference in catalytic activity was attributed to the higher degree of dispersion of tungstate on the surface of m-ZrO2.

Influence of calcination procedure on the catalytic property of sulfated zirconia

Calcination parameters, such as atmosphere, duration and catalyst bed depth have a marked influence on the catalytic and spectroscopic properties of sulfated zirconia. Sulfated zirconia calcined in nitrogen or synthetic airflow, in deep bed, exhibited comparable activity in n-butane isomerization at 373 K, which suggests that oxygen is not necessary for formation of active sites. Catalysts calcined in shallow bed are catalytically inactive. Thus, the bed depth is concluded to be crucial for the formation of active sites. The samples calcined in shallow bed possessed lower sulfate content and the S=O stretching vibration was located at lower frequency. Calcination in the presence of water vapor also led to lower catalytic activity, sulfate content, and BET area. Extended calcination reduced gradually the activity and the sulfate content, which underlines the labile property of the active sites. A new interpretation of the function of the calcination step is proposed and compared with models described in the literature.

Ga-promoted sulfated zirconia systems. II. Surface features and catalytic activity

Sulfated zirconia samples having a variable Ga 2O 3 content (in the 1–15% molar range) were synthesized. The promoting effect of gallium was studied in the catalytic isomerization of n-butane at 523 K, by feeding n-butane and H 2 (with a 1:4 ratio). Catalytic activity was found to be greatly dependent on gallium loading. Catalysts containing 3–5 mol% Ga 2O 3 doubled the activity of sulfated zirconia, whereas for a sample with 15 mol% Ga 2O 3 the catalytic activity was completely lost. Surface chemistry of these materials was studied by means of FTIR spectroscopy and adsorption microcalorimetry, using selected probe molecules (CO and 2,6-dimethylpyridine). IR spectroscopy showed that gallium-containing sulfated zirconia samples exhibit both Lewis and Brønsted acidity. Lewis acidity is attributed to coordinatively unsaturated Zr 4+ ions located in defective surface sites, whereas Brønsted acidity is associated to surface sulfate groups. Samples with a Ga 2O 3 content between 1 and 9 mol% show a combination of Lewis and Brønsted acidity that quantitatively decreases with increasing gallium oxide content. Samples having a Ga 2O 3 content equal to 9 mol% or greater show the following specific features: (i) these samples are much more difficult to dehydrate than those with smaller gallium content, and the hydroxy groups interact by hydrogen bonding, (ii) the sulfate groups progressively lose their covalent character, and (iii) both Lewis and Brønsted acidity of the samples decreases drastically. Couples of Lewis and Brønsted acid sites appear to be needed for catalytic activity in n-butane isomerization, and they present an optimum ratio when the catalyst is brought to a medium-high dehydration degree and when its Ga 2O 3 content is of about 3–5 mol%.

Solid Synthesis of CsxH3−xPW12O40 Salts and Their Catalytic Activity for the Isomerization of n-Butane

Abstract CsxH3−xPW12O40 salts prepared by a simple solid-synthesis method show good catalytic activity in n-butane isomerization as those analogues prepared by traditional liquid-preparation method.

Synthesis of highly active sulfated zirconia by sulfation with SO 3

Highly active sulfated zirconia (SZ) materials were prepared by sulfation of crystalline zirconia with gaseous SO 3. This direct method circumvents the use of liquid sulfation agents, as well as a final calcination step. The amount of sulfur species retained by SO 3-SZ samples was directly proportional to the fraction of monoclinic phase and the concentration of Brønsted acid sites. However, the catalytic activity in n-butane isomerization at 373 K was proportional to the concentration of labile chemisorbed SO 3, most likely in the form of pyrosulfate species showing a characteristic IR band at 1404 cm −1. Tetragonal zirconia favors the formation of these labile sulfur species, playing a key role in light alkane activation.

The effect of Al 2O 3-promotion of sulfated zirconia on n-butane isomerization: An isotopic transient kinetic analysis

This paper reports on an investigation using steady-state isotopic transient kinetic analysis (SSITKA) of Al 2O 3-promotion of sulfated zirconia (SZ) for n-butane isomerization. SZ promoted with Al 2O 3 showed an approximately 30% increase in n-butane isomerization activity at 250 °C. The average surface residence times of the reaction intermediates on unpromoted and Al 2O 3-promoted SZ measured by SSITKA suggest that the active sites on both catalysts were identical. There was, however, a significant increase in the concentration of surface intermediates upon Al 2O 3-promotion, leading to the increased activity. Al 2O 3-promotion also appeared to improve catalyst site stability resulting in a slower rate of deactivation.

Investigation of factors influencing catalytic activity for n-butane isomerization in the presence of hydrogen on Al-promoted sulfated zirconia

Two series of Al-promoted sulfated zirconia with 3 mol% Al 2O 3 content were prepared. In one case, zirconium–aluminum hydroxide has been aged at room temperature; in the other case, zirconium–aluminum hydroxide has been synthesized at 373 K under hydrothermal condition. For the two series of samples, the influence of the synthesis parameters such as ageing time, the amount of ammonia added, and zirconium concentration on catalytic activity for n-butane isomerization in the presence of hydrogen were investigated. Moreover, Al-promoted sulfated zirconia with a high surface area (174 m 2/g) can be obtained by properly adjusting synthesis parameters under hydrothermal condition at 373 K. The catalytic data showed that this material had similar catalytic activity to a conventional Al-promoted sulfated zirconia with a low surface area (98 m 2/g). The characterization of the samples revealed that the catalytic activity was directly correlated with sulfur content rather than surface area, sulfate density and the total number of Brønsted acid sites. The synthesis of the precursors of hydroxides before sulfation is a crucial step for catalytic activity of the samples in n-butane isomerization.

Iron and manganese promoted sulfated zirconia: acidic properties and n-butane isomerization activity

Sulfated zirconia catalysts promoted with Fe and Mn have been synthesized and the acidity characterized by IR spectroscopy of adsorbed pyridine. The catalytic isomerization of n-butane was investigated in a fixed bed reactor operated at low conversion at atmospheric pressure and 250 °C. The main effect of the promoters was to change the ratio Brönsted:Lewis acidity of the samples. The catalytic activity was found to be correlated with the number of strong Brønsted acid sites.

Improved catalytic activity and stability of mesostructured sulfated zirconia by Al promoter

Al-promoted mesostructured sulfated zirconia (Al-MSZ-5) has been successfully synthesized from the template of triblock polymer surfactant (P123) in aqueous solution. Mesostructured Al-MSZ-5 shows high thermal stability, and it can preserve its characteristic mesostructures until calcination at 650 °C. In contrast, the mesostructure of mesoporous sulfated zirconia (MSZ-5) will be destroyed during calcination at 600 °C. Catalytic data for n-butane isomerization show that the Al-MSZ-5 exhibits higher catalytic activity and better stability than both MSZ-5 and conventional Al-promoted sulfated zirconia (Al-SZ). The Al-MSZ-5 sample was characterized by XRD, bright-field and dark-field TEM images, N 2 adsorption isotherms, NMR, TG–DTA, IR, and numerous other techniques. The results suggest that Al-MSZ-5 has worm-like mesopores, and that the mesostructured walls contain tetragonal crystalline ZrO 2. Such unique structural features might be responsible for the observed large surface area, high catalytic activity for n-butane isomerization, and high thermal stability of mesostructured Al-MSZ-5.

Sulfated nanozirconia: an investigation on acid–base properties and n-butane isomerization activity

Hydrated zirconia was synthesized by an organo–inorganic route employing surfactant and was sulfated using aqueous ammonium persulfate, followed by drying at 110 °C. The sample thus obtained was calcined at 600 °C to obtain sulfated zirconia and was characterized by several physicochemical methods. Crystallite sizes of sulfated zirconia were calculated from X-ray line broadening using the Debye–Scherer equation and were found to be in the range of 25 nm. When pretreated in air, the catalyst was found to exhibit butane isomerization activity at a temperature as low as 35 °C under atmospheric pressure. It showed conversion as high as 37% at 100 °C under normal pressure when pretreated in air, whereas nitrogen-pretreated catalyst showed zero activity under similar conditions. NH 3 and CO 2 temperature-programmed desorption studies on air- and helium-pretreated samples indicated that the catalyst surface changes appreciably during air pretreatment. Results on butane isomerization in conjunction with TPD studies suggest that zirconium–oxy sites play an important role in butane activation during the reaction.

Preparation and Catalytic Evaluation of Macroporous Crystalline Sulfated Zirconium Dioxide Templated with Colloidal Crystals

Colloidal crystal templates were used to synthesize three-dimensionally ordered macroporous sulfated zirconia catalysts with pore diameters of ∼300 nm and less. Ordered arrays of uniformly sized poly(methyl methacrylate) latex spheres were infiltrated with clear precursor solutions containing varying SO4/Zr molar ratios. After solidification of the material in the void space between the spheres, the polymer templates were removed by calcination at various temperatures, producing crystalline sulfated zirconia replicas of the template arrays. The effects of changing sulfate content and calcination temperature on the physicochemical properties of the material (including shrinkage, grain size, surface area, and composition) were systematically studied. The presence of sulfate retarded the crystallization and crystal growth, which enabled greater control of macropore shrinkage and periodic order of the material. The combination of crystal growth inhibition and the inherent porosity of the PMMA latex are believed to be the major factors contributing to the observed BET surface areas of the materials, which were significantly larger than those of their nontemplated counterparts and passed through a maximum as a function of calcination temperature and initial SO4/Zr ratio. The maximum value of 123 m2/g was attained by a sample with SO4/Zr = 2, calcined at 650 °C, with a sulfate surface coverage of 3.1 nm-2. The n-butane isomerization activity of the material also passed through a maximum as a function of calcination temperature and initial SO4/Zr ratio, reaching its maximum value for a sample with SO4/Zr = 2, calcined at 600 °C.

Mesostructured sulfated zirconia with high catalytic activity in n-butane isomerization

A mesostructured sulfated zirconia with a large surface area (189 m2/g) has been successfully prepared using a triblock copolymer as a structure-directing agent. The resulting material was characterized by XRD, TEM, nitrogen adsorption, FTIR and TG/DTA, which suggested that the mesostructured sulfated zirconia was tetragonal crystalline. Catalytic testing showed that the mesostructured sulfated zirconia was much more active than conventional sulfated zirconia for n-butane isomerization.

Studies on Chlorided Pt/Al2O3 Catalysts: Preparation, Characterization and n-Butane Isomerization Activity

A series of chlorided Pt/Al2O3 (both γ and η) catalysts were prepared and characterized for various physicochemical properties. The chloride content of the catalysts was found to increase with chloride treatment time up to a certain level and then decrease owing to prolonged exposure at high temperature. The surface area and pore volume of the catalysts were decreased by chloride treatment. The activity of the prepared catalysts were tested in n-butane isomerization. The platinum content of the catalysts was found to have no effect on catalytic activity up to 0.2 wt% whereas the chloride content of the catalyst strongly influenced the activity and a >20-fold increase in activity was observed on chloriding Pt/Al2O3 catalysts. The catalyst activity was found to be directly related to its acidity.

Sulfated Zirconia Nanoparticles Synthesized in Reverse Microemulsions: Preparation and Catalytic Properties

Microemulsion-derived sulfated zirconia was prepared by addition of zirconium butylate to mixtures of aqueous H2SO4, nonionic surfactants, and heptane as the oil phase and subsequent drying and calcination at 873 K. The resulting catalysts have surface areas ranging from 40 up to 175 m2 g-1 depending on the microemulsion composition and show a high activity in n-butane isomerization (4.7 × 10-7 mol s-1 g-1, TOS = 60 min) comparable to that of other commercially available catalysts. The size of the inverse micelles in the microemulsion can be used to tailor the properties of the catalyst in a wide range. In situ X-ray diffraction studies demonstrate significant differences in the crystallization kinetics of materials derived from low and high water/surfactant ratios (Rw). Transmission electron microscope investigations show that a higher degree of crystallite agglomeration and sintering is responsible for lower surface areas of high Rw derived zirconias. According to thermogravimetric studies coupled with ...

Pt/SO 42−-ZrO 2 catalysts prepared from Pt organometallic compounds

New Pt/SO 4 2−-ZrO 2 catalysts were prepared using precalcined SO 4 2−-ZrO 2 and organometallic Pt precursors. The objective was to obtain a bifunctional catalyst with improved metal properties, which are mostly suppressed in Pt/SO 4 2−-ZrO 2 prepared in a standard fashion. The synthesis route used low temperatures to form the Pt particles in order to avoid sulfate decomposition and Pt poisoning. The use of precalcined zirconia decreased ionic diffusion, related to crystallization of the gel and encapsulation of Pt particles. As detected by TPR and cyclohexane dehydrogenation tests, after reducing at 270 °C, Pt on the new catalyst had metallic properties. Its dehydrogenation activity was higher than that of a standard Pt/SO 4 2−-ZrO 2 catalyst (prepared from H 2PtCl 6). After activating in air at 300 °C, its activity for n-butane isomerization (300 °C, 1 atm., H 2: n-C 4=6, WHSV=1 h −1) was low, very likely because of incomplete elimination of adsorbed water. After activation in air at 600 °C, the isomerization activity was almost similar to that of standard Pt/SO 4 2−-ZrO 2 but the dehydrogenation activity decreased to negligible values. The deleterious interaction between Pt and SO 4 2−-ZrO 2 that occurred during calcination was possibly related to oxidation of the metal. On the other side, high reduction temperatures did not enhance the metal activity beyond the level obtained after reducing at 270 °C. On the contrary, the higher the reduction temperature, the lower the activity for cyclohexane dehydrogenation that was obtained. This effect was seemingly related to poisoning by sulfur compounds. The new materials could be potentially useful for reacting systems needing a bifunctional catalyst and an acid function like SO 4 2−-ZrO 2 but their operation is limited if high temperatures in air or H 2 are necessary for activation.

Tetragonal structure, anionic vacancies and catalytic activity of SO 42−-ZrO 2 catalysts for n-butane isomerization

An assessment of the influence of the crystal structure, surface hydroxylation state and previous oxidation/reduction pretreatments on the activity of sulfate-zirconia catalysts for isomerization of n-butane was performed using crystalline and amorphous zirconia supports. Different sulfation methods were used for the preparation of bulk and supported SO 4 2−-ZrO 2 with monoclinic, tetragonal and tetragonal+monoclinic structures. Activity was important only for the samples that contained tetragonal crystals. The catalysts prepared from pure monoclinic zirconia showed negligible activity. SO 4 2−-ZrO 2 catalysts prepared by sulfation of crystalline zirconia displayed sites with lower acidity and cracking activity than those sulfated in the amorphous state. Prereduction of the zirconia samples with H 2 was found to greatly increase the catalytic activity, and a maximum rate was found at a reduction temperature of 550–600 °C, coinciding with a TPR peak supposedly associated with the removal of lattice oxygen and the creation of lattice defects. A weaker dependence of catalytic activity on the density or type of surface OH groups on zirconia (before sulfation) was found in this work. A model of active site generation was constructed in order to stress the dependence on the crystal structure and crystal defects. Current and previous results suggest that tetragonal structure in active SO 4 2−-ZrO 2 is a consequence of the stabilization of anionic vacancies in zirconia. Anionic vacancies are in turn supposed to be related to the catalytic activity for n-butane isomerization through the stabilization of electrons from ionized intermediates.

The Impact of Pt and H2 on n-Butane Isomerization over Sulfated Zirconia: Changes in Intermediates Coverage and Reactivity

Previous steady-state isotopic transient kinetic analysis (SSITKA) studies in our laboratory showed that the deactivation of an unpromoted sulfated zirconia (SZ) catalyst during n-butane isomerization at 150°C is due to a loss of active sites as well as a small decrease in the average site activity (Kim, S. Y., Goodwin, Jr., J. G., and Galloway, D., Catal. Today63, 21 (2000)). The mechanism by which SZ deactivates is linked to coke/oligomer formation and, possibly, surface sulfur reduction. The use of Pt promotion of SZ and H2 addition to the reaction mixture has been shown to greatly decrease deactivation during n-butane isomerization. In this study, the mechanisms by which H2 addition and Pt promotion affect n-butane isomerization over SZ were studied using SSITKA. H2 probably inhibits n-butane isomerization activity of the unpromoted SZ catalyst through inhibition of butene formation. Little effect on the average residence time for reaction was observed. An improvement in catalyst stability was linked to a reduction in coke/oligomer formation. In the absence of H2 in the feed stream, the presence of Pt did not improve the activity or the stability of the catalyst. An enhanced catalytic activity and stability, however, was evident when both H2 addition and Pt promotion were used, but only at the higher reaction temperature of 250°C. This enhanced catalytic activity at 250°C upon both Pt promotion and H2 addition was a result of an increase in the concentration of active surface intermediates leading to isobutane. This concentration did not change greatly during reaction, thus showing the improvement also in catalyst stability. Although the concentration of active intermediates increased significantly, the average reaction rate (inverse seconds) of an intermediate actually decreased, but not enough to prevent an increase in overall activity. Only 4% (SZ)–16% (PtSZ) of the intermediate to strong acid sites appeared to be involved in the reaction after 5 min time-on-stream, suggesting a reason why NH3 adsorption characterization results do not correlate well with catalyst activity for SZ-based catalysts. The implications of the obtained results on the roles of Pt and H2 on SZ in enhanced catalyst activity and decreased deactivation are discussed.