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.     

Alkane isomerization over sulfated zirconia and other solid acids

Some solid acids, including sulfated zirconia and certain industrial isomerization catalysts, catalyze two types of n-butane isomerizations, avoiding primary carbenium ions or carbonium ions: (1) an internal rearrangement of the C atoms in n-butane and (2) skeletal isomerization of n-butane to iso-butane. No superacid sites are required for these reactions. The skeletal isomerization is an intermolecular reaction, involving a C₈ intermediate. Easily accessible Brønsted acid sites and small amounts of olefin are crucial. Spectroscopic examination of the acid sites on sulfated zirconia shows that they are not stronger than the acid sites in zeolites such as HY. The butane isomerization rate is suppressed by CO, even when no CO is adsorbed on Lewis sites; formation of oxocarbenium ions is likely. The decisive role of Brønsted acid sites is demonstrated by results on deuterated catalysts.     

Dehydrocyclization of 1-hexene to benzene on Cu3Pt(111)

The disproportionation ofn-butane (and of isobutane) was catalyzed by sulfated zirconium oxide containing 1.5 wt% Fe, 0.5 wt% Mn, and 4.0 wt% sulfate at 2.0 atm and temperatures in the range of 30–60C. The reaction accompanies isomerization, which under some conditions is as much as one or two orders of magnitude faster than disproportionation. The conversion to each of the products increased with time on stream in a flow reactor, and then declined. The time on stream for maximum conversion was the same for each product. The results suggest that the disproportionation and isomerization reactions proceed through a common C₈ intermediate. Rates of the disproportionation reaction were determined at the time on stream corresponding to the maximum conversion at each temperature; for example, the rate of conversion ofn-butane into isopentane at 60C with ann-butane partial pressure of 0.58 atm was about 1×10^–7 mol/(g of catalyst s).     

n-Butane conversion on sulfated zirconia: in situ 13C MAS NMR monitoring of the kinetics of the 13C-label scrambling and isomerization

The kinetics of the conversion of ¹³C-labeled n-butane adsorbed on sulfated zirconia (SZ) were monitored by in situ ¹³C MAS NMR spectroscopy. Rate constants of n- to isobutane isomerization and of the ¹³C-isotope scrambling from the primary to the secondary carbon atoms in n-butane were determined. The monomolecular scrambling of the ¹³C-label in adsorbed n-butane has an activation energy of 17 ± 3 kcal mol^–1 and occurs faster than the bimolecular process of n-butane isomerization which has an activation energy of 15.1 ± 0.2 kcal mol^–1. The transfer of the selective ¹³C-label from the primary to the secondary carbon atom in the adsorbed n-butane seems to consist of two reaction steps: (i) a hydride abstraction by SZ leading to the formation of sec-butyl cations and (ii) a label scrambling in the sec-butyl cations. This two-step process with the formation of sec-butyl cations as intermediate increases the apparent activation energy for the ¹³C-label scrambling, which is almost twice as large compared with the activation energy for carbon scrambling of sec-butyl cations in a superacidic solution.     

New catalyst of SO 4 2− /Al2O3–ZrO2 for n-butane isomerization

The catalytic behavior of Al-promoted sulfated zirconia for n-butane isomerization at low temperature in the absence of H₂ and at high temperature in the presence of H₂ was studied. The addition of Al enhances the activity and stability of the catalysts for reaction at 250°C and in the presence of H₂ significantly. After on stream for 120 h, the n-butane conversion of the catalyst containing 3 mol% Al₂O₃ keeps steadily at 88% of its equilibrium conversion and no observable trend of further deactivation has been observed. The difference in behavior of the promoted and unpromoted catalysts at low and high temperature is associated with a change of reaction mechanism from bimolecular to monomolecular. Experimental evidence is presented to show that the promoting effect of Al is different from that of the transition metals. Microcalorimetric measurements of NH₃ adsorption on catalysts reveal that the remarkable activity and stability of the Al-promoted catalysts are caused by an enhancement in the number of acid sites effective for the isomerization reaction. This revised version was published online in June 2006 with corrections to the Cover Date.     

Solid superacid catalysis: kinetics of butane isomerization catalyzed by a sulfated oxide containing iron,manganese, and zirconium

Kinetics of the isomerization ofn-butane and of isobutane catalyzed by sulfated zirconium oxide containing 1.5 wt% Fe, 0.5 wt% Mn, and 4.0 wt% sulfate at 60°C are well represented by a Langmuir-Hinshelwood equation accounting for the reaction equilibrium and for adsorption of both butanes. The adsorption equilibrium constants estimated from the kinetics data are nearly the same for the two butanes. The form of the rate equation and the observation that disproportionation accompanies isomerization suggest that the reaction proceeds via a C_g intermediate.     

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.     

ZrO2 promoted with sulfate,iron and manganese: a solid superacid catalyst capable of low temperaturen-butane isomerization

A sulfated oxide of zirconium, iron and manganese is prepared and shown to isomerizen-butane to isobutane at 35°C with rates approximately 2–3 orders of magnitude greater than sulfated zirconia as claimed by workers at Sun Refining and Marketing Company. Temperature programmed desorption of benzene is used to investigate the acidity of this remarkable catalyst. Adsorbed benzene is oxidized to CO₂ by the triply promoted oxide catalyst; CO₂, SO₂ and O₂ are found to desorb at 525, 575 and 560°C, respectively. Sulfated zirconia does not adsorb benzene in a similar manner. The results from the temperature programmed desorption of benzene cannot be correlated with then-butane isomerization activity.On leave from The University of Tokyo.     

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.     

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.     

Studies on SO 4 2− promoted mixed oxide superacids

Sulfated binary and trinary oxide solid superacids were prepared and characterized. The incorporation of Cr, Fe, Mn and V into sulfated zirconia increases its superacidity and catalytic activity forn-butane isomerization at 35°C significantly. The catalytic activity of sulfated oxides of Cr-Zr, Fe-Cr-Zr and Fe-V-Zr is 2–3 times greater than that of the well-known sulfated Fe-Mn-Zr oxide. A negative effect was observed in the cases of sulfated Mn-Zr, Sn-Zr, W-Zr and Mo-Zr oxides. The origin of the enhancement in superacidity and activity is discussed in the light of experimental results obtained by XRD, IR and chemical analysis.     

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.     

A comparison of cesium-containing heteropolyacid and sulfated zirconia catalysts for isomerization of light alkanes

The heteropolyacid H₃PW₁₂O₄₀ and its cesium salts Cs_xH_3-x PW₁₂O₄₀ (x = 1, 2, 2.5, 3) were synthesized, characterized and tested as catalysts for hydrocarbon reactions. All samples were characterized by a variety of techniques including elemental analysis, X-ray diffraction, dinitrogen adsorption, thermal gravimetric analysis and ammonia sorption. Results from these methods confirmed that pure cesium salts were prepared without significant contamination by amorphous oxide phases. Incorporation of cesium into the heteropolyacid decreased the acidic protons available for catalysis, increased the specific surface area, and increased the thermal stability. The heteropolyacids were tested as catalysts for butane skeletal isomerization, pentane skeletal isomerization and 1-butene double bond isomerization. For comparison, the activity of sulfated zirconia, a well-studied strong acid catalyst, was also evaluated for the three probe reactions. On a per gram basis, the Cs₂HPW₁₂O₄₀ sample was the most active heteropolyacid, presumably due to its high surface area. This sample was more active than sulfated zirconia for pentane skeletal isomerization and 1-butene double bond isomerization. However, sulfated zirconia was more effective for butane skeletal isomerization. Since the pentane and 1-butene reactions were monomolecular in nature, whereas butane isomerization was bimolecular, restrictions inside the micropores of the heteropolyacid may inhibit the formation of long chain intermediates. Interestingly, trace butenes were required to initiate butane isomerization reactions on sulfated zirconia, whereas heteropolyacids catalyzed the reaction in the absence of butenes. This revised version was published online in June 2006 with corrections to the Cover Date.     

Catalytic Behavior of Alumina-Promoted Sulfated Zirconia Supported on Mesoporous Silica in Butane Isomerization

Alumina-promoted sulfated zirconia was supported on mesoporous molecular sieves of pure-silica MCM-41 and SBA-15. The catalysts were prepared by direct impregnation of metal sulfate onto the as-synthesized MCM-41 and SBA-15 materials, followed by solid state dispersion and thermal decomposition. Measurements of XRD and nitrogen adsorption isotherms showed that the structures of resultant materials retain well-ordered pores, even with ZrO₂ loading as high as 50 wt%. The characterization results indicated that most of the promoted sulfated zirconia were well dispersed on the internal surface of the ordered mesopores. The catalytic behavior of the alumina-promoted sulfated zirconia supported on mesoporous silica was studied in n-butane isomerization. The supports of mesoporous structures led to high dispersion of sulfated zirconia in the meta-stable tetragonal phase, which was the catalytic active phase. The high performance of alumina-promoted catalysts was ascribed to the sulfur retention by alumina.     

n-Butane Isomerization Catalyzed by Sulfated Zirconia Nanocrystals Supported on Silica or γ-Alumina

Supported sulfated zirconia catalysts with zirconia contents of 10, 20 and 50 wt% were prepared by impregnation of SiO₂ and γ-Al₂O₃ supports with H₂SO₄/ Zr(SO₄)₂ solutions followed by calcination at 923 K. The catalysts were characterized by X-ray diffraction, extended X-ray absorption fine structure measurements, thermal analysis, UV–vis spectroscopy, and electron microscopy. Tetragonal zirconia was detected in all silica-supported samples but only in the 50 wt% zirconia-containing alumina-supported sample, indicating high dispersion of zirconia on alumina. Alumina-supported samples retained additional sulfate, at least in part as Al₂(SO₄)₃. All samples were active in n-butane isomerization (1 kPa n-butane, 378 K). There was no relation between the presence of tetragonal zirconia in these samples and the catalytic performance.     

Effect of preparative conditions of Fe and Mn sulfated zirconia catalysts on their activities for η‐butane isomerization

This work investigates the effect of preparative conditions of Fe‐ and Mn‐promoted sulfated zirconia catalysts on their activities for low‐temperature η‐butane isomerization. It was found that the active species on a promoted catalyst can be successfully regenerated in an oxidative treatment at 450°C after the catalyst was deactivated either during the reaction or under a high‐temperature treatment in helium. The loading sequence of Fe and Mn does not significantly affect the catalyst activity. Both Fe and Mn can individually promote the activity of sulfated zirconia catalysts. However, the promoting effect of Fe is much stronger than that of Mn; the catalyst containing only Fe is significantly more active than that containing both Fe and Mn and does not deactivate any more rapidly. The optimum Fe content was found to be 4 wt%. This revised version was published online in July 2006 with corrections to the Cover Date.     

Isomerization of n-Butane over Ultrastable H-Y Zeolites with Different Si/Al Atomic Ratio

Ultrastable H-Y zeolites with different Si/Al atomic ratios (3n-butane isomerization. The initial activity of these catalysts is lower than that measured on tungsta supported on zirconia catalysts (WO _x /ZrO₂) and acidic mordenite catalysts; however, the Brønsted acid sites of the ultrastable H-Y zeolites are stable and selective towards isobutane. No deactivation of the catalysts was observed after 5 h of time on stream. In contrast, WO _x /ZrO₂ and acidic mordenite catalysts under the same experimental conditions are largely deactivated in less than 1 h of time on stream. The stability of the ultrastable H-Y zeolite in comparison to H-mordenite catalysts may be due to the three-dimensional structure of H-Y made of large supercages interconnected by apertures of 12 oxygen atoms. This structure may favour the diffusion of reactant and product decreasing the residence time and the ensuing degradation to coke. Acidic molecular sieves with monodimensional structure may favour the formation of the precursors of the coke responsible of the catalyst deactivation.     

Sulfated Zirconia with Ordered Mesopores as an Active Catalyst for n-Butane Isomerization

Zirconia/surfactant composites were hydrothermally synthesized in aqueous sulfuric acid at 373 K using Zr(O-nPr)₄ as oxide precursor and hexadecyl-trimethyl-ammonium bromide as template. Mesostructural features similar to those of MCM-41 were detected by X-ray diffractometry, with d=4.6 nm. A sample obtained from a starting mixture with Zr:S:CTAB = 2:2:1 was stable enough for removal of occluded organics. After calcination at 813 K, the d-value was 3.6 nm, the surface area 200 m²/g, and the mean pore diameter estimated by the BJH method 2.2 nm. Extended X-ray absorption fine structure analysis suggests Zr to be in a short-range structure (<4 Å) similar to that of Zr in monoclinic ZrO₂. Scanning electron microscopy including energy dispersive X-ray analysis showed 1-5 m sulfur-containing ZrO₂ spheres. The material catalyzes the isomerization of n-butane to i-butane at 378 K with a steady activity in the order of magnitude of commercial sulfated ZrO₂.     

Isomerization ofn-butane on sulfated zirconia: Evidence for the dominant role of Lewis acidity on the catalytic activity

The catalytic activity of a ZrO₂/SO₄ catalyst in the isomerization ofn-butane at 423 K is reversibly suppressed by addition of CO. IR analysis of the adsorption of CO indicates that the only -coordination of CO onto coordinatively unsaturated surface Zr⁴⁺ cations occurs in the 300–473 K interval.     

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.