N-butane Isomerization Reaction Research Articles

Role of Pt in Pt/SO42−-ZrO2 catalyzed n-butane isomerization reaction: Hydrogen spillover rather than bifunctional “metal-acid” catalysis

The understanding of reaction mechanism is the key to develop the highly efficient catalyst. The introduction of Pt on SO42−/ZrO2 surface greatly improves its catalytic stability in n-butane hydroisomerization, but the function of Pt is still in controversy. The introduction of Pt on sulfated zirconia surface only promotes the hydrogenation of coke precursor and slows catalyst deactivation during n-butane hydroisomerization, but has no effect on the initial conversion rate of n-butane and selectivity to isobutane. H2-TPR and H2-TPD investigations reveal that Pt functions via hydrogen spillover mechanism, rather than classical bifunctional “metal-acid” catalysis mechanism. Our findings provide a new perspective for the rational design and synthesis of “metal-acid” bifunctional catalysts for alkane isomerization.

Investigation of Operating Conditions and Catalytic Performances of Metal-Bearing (Pt, Pd, and CE) Phosphotungstic Acid and Silicotungstic Acid Supported on Mesoporous Kit-6 in N-Butane Isomerization Reaction

Investigation of Operating Conditions and Catalytic Performances of Metal-Bearing (Pt, Pd, and CE) Phosphotungstic Acid and Silicotungstic Acid Supported on Mesoporous Kit-6 in N-Butane Isomerization Reaction

Nature of active sites and deactivation mechanism for n-butane isomerization over alumina-promoted sulfated zirconia

The deactivation of n-butane isomerization on alumina-promoted sulfated zirconia (SZA) was studied at 473K under atmospheric pressure. Under N2, the regeneration temperature influences the isomerization activity greatly. The change of the sulfate species during the deactivation process was determined quantitatively by a temperature-programmed evolution mass spectrometer. Two active sites, reducible sites and acid sites, exist on the catalyst surface and show a synergistic effect on the n-butane isomerization reaction. The reducible sulfate species play an important role in butene formation via oxidative dehydrogenation of n-butane, while the acid sites are responsible for the isomerization steps. The reduction of sulfate species and the accumulation of sulfite species on the surface, which cause the decrease of butene concentration in the catalyst bed, are the main reasons for catalyst deactivation. Based on these findings, a new reaction model and two possible ways to improve the catalytic stability of SZA catalyst are proposed.

An aluminum promoted cesium salt of 12-tungstophosphoric acid: a catalyst for butane isomerization

Aluminum doped cesium salts of tungstophosphoric acid, AlxCs2.5–3xH0.5PW12O40 (x = 0.05, 0.1 and 0.15), were prepared via a solid state synthesis method and their catalytic activity in n-butane isomerization reaction was tested using the 13C MAS NMR technique and 1-13C-n-butane as the reactant. All AlxCs2.5–3xH0.5PW12O40 catalysts show higher catalytic activity than Cs2.5H0.5PW12O40. Among them, Al0.1Cs2.2H0.5PW12O40 exhibits the best performance. The MAS NMR technique was further employed to characterize the acidity of the catalysts using trimethylphosphine and 2-13C-acetone as the basic probe molecules. Al0.1Cs2.2H0.5PW12O40 possesses higher Bronsted acid strength than Cs2.5H0.5PW12O40, which parallels the activity of the catalysts. For a catalyst used in n-butane isomerization reaction, to enhance its Bronsted acid strength rather than to increase its acid density is more important for improving the activity of the catalyst.

Surface characterization and catalytic activity of sulfated-hafnia promoted zirconia catalysts for n-butane isomerization

Sulfated-hafnia promoted zirconia containing various compositions of hafnia (1–10wt.%) were prepared by precipitation method in an attempt to ultimately carry n-butane isomerization reaction. The catalyst samples were calcined under dry air flow at 600°C. The structural and crystal changes were monitored by FTIR and XRD whereas the textural changes were estimated by low temperature N2 adsorption. FTIR spectroscopy has been used to characterize the hydroxyl groups and to determine the concentration of Brönsted and Lewis acid sites from pyridine adsorption. The catalytic activity, stability and selectivity of the catalyst samples were tested for n-butane isomerization at 250°C. The results reveal that the existence of a small content of hafnia increases the surface sulfate density, stabilizes the tetragonal phase of zirconia, increases the amount and strength of Brönsted acid sites and enhances the initial catalytic activity of the samples. Increasing hafnia content to 5 and 10wt.% is accompanied with the formation of hafnia monoclinic phase which causes a drastic decrease in the amount of Brönsted acid sites and the initial activity of the catalyst. The isomerization of n-butane occurred through bimolecular pathway with the formation of iso-butane, propane and pentanes as primary products. In spite of the high activity of the samples, the catalyst deactivates rapidly during the first hour of the reaction due to coke formation, retreating the catalyst at 450°C in the presence of dry air before the catalytic reaction lead to regeneration of the initial catalytic activity of the catalyst which implies that the coke formation is the main source for catalyst deactivation.

Effect of Pt Incorporation into Ni/ZrO2–SO4=/Al2O3 Catalyst for n-Butane Isomerization

Three Ni/ZrO2–SO 4 = /Al2O3 catalysts with different concentrations of platinum (0.2, 0.3 and 0.4 wt%) were prepare and tested for n-butane isomerization reaction at 338 K, in absence and in presence of hydrogen. The results shown that, at low temperature, platinum contributes to the olefin or butyl ion formation and the reaction follows a bimolecular pathway. However, when the reaction occurs in the presence of hydrogen, the formation of butyl ions is inhibited. The main feature of platinum addition is the stabilization of the catalytic activity, which is indicated by the slow deactivation constants compared to that of the unpromoted catalyst.

Platinum- and iron-doubly promoted tungstated zirconia catalyst for n-butane isomerization reaction

The catalysis of n-butane isomerization over iron-promoted tungstated zirconia (F 1.2WZ) and platinum–iron-promoted tungstated zirconia (P/F 1.2WZ) catalysts was studied and correlated to the catalyst characterization results. In the singly promoted F 1.2WZ catalyst, we suggest that the iron species promotes the n-butane isomerization reaction through a redox effect. The formation of W–O–Fe linkages optimized the surface reduction of WO x , thereby facilitating the formation of Brønsted acid sites necessary for the generation of carbenium ion. In addition, Fe 3+ itself can also act as a redox site. In the case of the doubly promoted P/F 1.2WZ catalyst, its catalytic activity is much higher than the sum of the singly promoted P/WZ and F 1.2WZ catalysts. The exceptionally high activity of P/F 1.2WZ catalyst in n-butane isomerization reaction is explained by a cooperative effect. The diffusion of activated n-butane species stabilized by iron promoter to the Brønsted acid sites created by platinum promoter will be improved as both promoters are in close proximity to each other. The interaction between iron promoter and n-butane or its activated species is shown by the influence of iron on the propane selectivity of the reaction. In addition, the close proximity of platinum and iron promoters in P/F 1.2WZ catalyst is shown by TPR. The location and nature of iron promoter on WZ were also characterized. Results revealed that the iron promoter is located on the surface of F 1.2WZ catalyst. XANES, EXAFS, and EPR studies suggest that the iron promoter exists as highly dispersed Fe 3+ species either bound to WO x surface or located at the surface vacant sites of zirconia. In addition, EPR study indicates the presence of fine α-Fe 2O 3 clusters on the surface of catalyst; the TPR peak at 421 °C is probably due to Fe–O–Fe of this species.

Aluminum-promoted tungstated zirconia catalyst in n-butane isomerization reaction

The promotion effect of Al on tungstated zirconia (WZ) catalyst was studied in a butane isomerization reaction and compared with Fe-promoted catalyst (FWZ). XRD and EXAFS studies revealed an interaction between Al and WO x , whereas XANES analysis indicates that the octahedral symmetry around W sites remained unchanged. Two Al 3+ sites are present on co-precipitated Al-promoted WZ (AWZ C) catalyst; one is formed by the substitution of Al species in the WO x structure and the other is the terminal Al species anchoring on the WO x surface. Only the terminal Al 3+ sites are evident in impregnated AWZ catalyst. Our NH 3-TPD and DRIFT results suggest that these Al 3+ sites are Lewis acid in nature with at least moderate acid strength. They are stronger than those Brönsted and Lewis acid sites originally present in WO x . Exposure of the catalyst to atmospheric water results in the poisoning of these Al 3+ sites by chemisorbed water, forming octahedral species. In an n-butane isomerization reaction, Al addition onto WZ increases both the activity and the stability of the catalysts. The catalytic activity of fresh catalysts decreases in the order FWZ I>AWZ C⩾WZ C, whereas regenerated catalysts follow the order AWZ C⩾FWZ I>WZ C. The improved activity of AWZ C over WZ C catalyst is related to the increased acidity created by the Al promoter. Most importantly, a great enhancement in catalytic activity was observed for regenerated catalyst relative to the fresh catalyst. The regenerated AWZ C catalyst is as active as the FWZ I catalyst.

Isomerization of n-butane by gallium-promoted sulfated zirconia supported on MCM-41

The n-butane isomerization reaction on sulfated zirconia (SZ) supported on MCM-41 mesoporous molecular sieve (SZ/MCM-41) was studied at various reaction temperatures in the presence of hydrogen. The catalytic activity was significantly improved with the addition of an appropriate amount of gallium as a promoter. The best conversion was achieved when the catalyst was promoted by 1.7 wt% gallium and calcined at 680 °C. Although the deactivation of these catalysts in the initial stage was observed the deactivated Ga-promoted SZ/MCM-41 catalysts (SZG/MCM-41) could be completely regenerated in air at 450 °C. The Pt-impregnated SZG/MCM-41 catalyst exhibited higher steady conversion compared with the Pt-free form. These halogen-free catalysts have advantages over other strong acid catalysts, in overcoming corrosion and environmental problems.

Butane isomerization over persulfated zirconia

A study on n-butane isomerization reaction over persulfated zirconia was performed under atmospheric pressure varying pretreatment gas, butane diluents, concentration of butane, contact time and reaction temperatures. It was found that pretreatment of the catalyst in various atmospheres such as nitrogen, hydrogen and air altered the properties of the catalyst as well as its performance towards n-butane isomerization. Air, as a pretreatment gas enhanced the initial activity of the catalyst; whereas, hydrogen had a negative effect. Among the pretreatment gases used, air and hydrogen were the most and the least deactivating carrier gases, respectively. Increase in conversion with increasing butane concentration, the reaction product distribution, and the induction phenomenon together strongly supported the existence of a surface stabilized C 8 + species and a bimolecular mechanism of n-butane isomerization reaction.

Manganese, iron and sulfur K edge XAFS of promoted sulfated zirconia catalysts

Promoted sulfated zirconia samples were prepared by the incipient wetness technique to produce isomerization catalysts which were active for the conversion of n-butane to isobutane at 338K (up to 10% conversion of 1% n-butane, 1atm., 0.25 h(-1) WHSV). The local structure of Fe and Mn in promoted sulfated zirconia was investigated using fluorescence yield XAS. Spectra were taken of calcined samples, activated samples, and samples after reaction with n-butane (maximum activity and deactivated). Factor analysis reveals that the Mn K edge XANES can be described by a linear combination of the spectra of two separate components, and that the ratio of these components changes with activation of the catalyst, and during use in the n-butane isomerization reaction. The change in ratio of the Mn species during activation and reaction results in a reduction of the average Mn valence from 2.4 to 2.2. The Fe K edge XANES was not similarly affected by activation and reaction with n-butane.

A new catalyst for n-butane isomerization: persulfate-modified Al 2O 3–ZrO 2

The n-butane isomerization reaction on Al-promoted zirconia catalysts modified by persulfate was studied both at low temperature in the absence of hydrogen and at high temperature in the presence of hydrogen. The addition of Al shows a significant enhancement in the activity and stability of the samples for n-butane isomerization at 250°C. After running on stream for 200 h, the n-butane conversion on the catalyst containing 3 mol% Al 2O 3 remains steady at 72% of its equilibrium conversion and no observable trend of further deactivation has been observed. The different behavior of the with or without Al-promoted catalysts at low temperature and high temperature is probably associated with a change of reaction mechanism from bimolecular to monomolecular. Microcalorimetric measurements of ammonia adsorption on catalysts reveal that the remarkable activity and stability of the Al-promoted catalysts are caused by an appropriate distribution of acid site strengths and an enhancement in the number of acid sites with intermediate acid strengths effective for the n-butane isomerization reaction.

TGA/FT-IR studies of the deactivation of sulfated zirconia catalysts

The deactivation of sulfated zirconia catalysts in the isomerization of n-butane has been studied using a combined TGA/FT-IR technique. Following deactivation at 200°C in the n-butane isomerization reaction, two SO 2 desorption maxima centered at 600°C and at 850°C were observed under a flow of N 2. Coincidental with the low temperature desorption maximum of SO 2 centered at 600°C, a CO 2 desorption maximum was also observed. Under an oxygen flow, the coke could be selectively burned off without removing the low temperature sulfur, which results in the complete restoration of the catalytic activity. In a N 2 flow, both CO 2 and SO 2 (low temperature) were removed. The loss of the low temperature sulfur results in a catalyst which is inactive in the isomerization of n-butane. A change in the crystal phase was not observed by XRD as result of the loss of the sulfur evolved at 600°C.

Characterization of palladium supported on sulfated zirconia catalysts by DRIFTS, XAS and n-butane isomerization reaction in the presence of hydrogen

Palladium supported on sulfated zirconia (PdSZ) has been characterized by the n-butane isomerization reaction in the presence of hydrogen, X-ray absorption spectroscopy (XAS) and diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) of adsorbed carbon monoxide. Catalyst calcination at 873 K followed by hydrogen reduction at 513 K results in the formation of 30–40 Å Pd metal clusters, but the surface can only weakly adsorb CO, though stronger than Pd-free, sulfated zirconia catalysts. In the presence of hydrogen, PdSZ has a lower n-butane isomerization activity than SZ, and the Pd function cannot stabilize the reaction at low H 2/n-butane ratios.

Acidic Properties of Sulfated Zirconia: An Infrared Spectroscopic Study

Sulfated zirconia with S content of 2 wt.% equivalent to complete coverage of its surface was studied by infrared spectroscopy. At least four sulfated species were identified and exhibited an important and reversible sensitivity to water as schematized below: (SO 3) ads + H 2O ⇆ (H 2SO 4) ads (H 2SO 4) ads + 2(H 2O) ads ⇆ (H 3O +) ads + (HSO − 4 ads + (H 2O) ads ⇆2(H 3O +) ads + (SO 2− 4) ads. These equilibria were demonstrated to exist by the study of adsorption of incremental amounts of water. D 2O and (H 2 18O isotopically enriched water molecules were used to assist interpretation of IR spectra. To characterize acidity features, the probe molecules butane, CO, and H 2O (as weak bases) or pyridine (as a strong base) were adsorbed. Two Lewis acid sites (L 1 and L 2) were observed and one Brønsted site (B) related to the zirconia support (L 1) and the sulfated species (L 2, B). They were evidenced by pyridine adsorption which was shown to partly displace adsorbed sulfate species. With the help of previous theoretical calculations using an ab initio method and representing the zirconia surface by a mononuclear zirconium complex, it is emphasized that the sulfated zirconia can be visualized as a H 2SO 4 compound grafted onto the surface of zirconia in a way which makes it very sensitive to water but in a reversible way. Its acidity is similar to that of sulfuric acid but it is not really superacidic. Comparison with other oxides leads us to suggest that the cationic charge borne by the metallic cation is of prime importance for the acidity strength. The role of water on the acidic and catalytic properties for n-butane isomerization reaction is emphasized.

Influence of ZrO2 Crystalline Structure and Sulfate Ion Concentration on the Catalytic Activity of So2−4- ZrO2

The catalytic behavior of SO2−4ZrO2 in the n-butane isomerization reaction was studied. Both ZrO2 crystalline structure and SO2−4 concentration were analyzed. ZrO2 samples were obtained from ZrCl4 in different reaction media, thus allowing us to obtain tetragonal and monoclinic phases separately. The SO2−4 concentration over ZrO2 was the same by sulfating amorphous or crystalline materials. The sulfate addition over amorphous Zr(OH)4 generated strong acid sites, which are active in n-butane isomerization. The catalytic activity showed a maximum at calcination temperatures between 873 and 893 K, where the Lewis acidity is higher. The tetragonal phase is present under these conditions, as shown by XRD patterns. The same results were obtained with samples synthesized from another precursor. Coke deposition or water presence produced catalyst deactivation.

Influence of hydrogen chloride addition on the catalytic isomerization activity of chlorinated alumina and chlorinated platinum–alumina solids. Superacid behaviour

Alumina and alumina-supported platinum have been submitted to chlorination by carbon tetrachloride at 573 K. The addition of HCl during the n-butane isomerization reaction at 573 K strongly increases the catalytic activity when HCl is added to the reaction mixture during the early stage of the reaction. Preadsorption of hydrogen chloride at 573 K leads to the conversion of the strongest Lewis acid sites into superacid ones able to perform the isomerization of n-butane at temperatures as low as 373 K. In such conditions the reaction rate is not dependent on the presence of platinum: a superacid behaviour is proposed. At 573 K, the role of platinum in the isomerization reaction would be to produce important amounts of HCl by dechlorination of the support and to contribute to the formation of superacid centres. In the absence of platinum, the formation of HCl would be limited leading to a less active catalyst.

N-Butane isomerization on solid superacids

n-Butane isomerization reaction was studied on H-mordenite and ZrO 2 treated with sulphate ion. ZrO 2/SO = 4 was calcined at different temperatures ranging 773–933 K; when calcining at 893 K, an optimum in catalytic activity was found. Both catalysts isomerize n-butane in different ways. On H-mordenite, disproportionation and cracking reactions occur simultaneously with isomerization. On the other hand, ZrO 2/SO = 4 is a very selective catalyst for iso-butane formation, being the other reactions less favored. Differences in acid strength distribution can be mentioned to explain different catalyst behaviours. Catalysts deactivate with time on stream due to coke formation; H-mordenite activity diminishes continuously, and ZrO 2/SO = 4 reaches a steady state.

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n-Butane isomerization reaction was studied on H-mordenite and ZrO2 treated with sulphate ion. ZrO2/SO=4 was calcined at different temperatures ranging 773–933 K; when calcining at 893 K, an optimum in catalytic activity was found. Both catalysts isomerize n-butane in different ways. On H-mordenite, disproportionation and cracking reactions occur simultaneously with isomerization. On the other hand, ZrO2/SO=4 is a very selective catalyst for iso-butane formation, being the other reactions less favored. Differences in acid strength distribution can be mentioned to explain different catalyst behaviours.Catalysts deactivate with time on stream due to coke formation; H-mordenite activity diminishes continuously, and ZrO2/SO=4 reaches a steady state.