Olefin impurity effect on n-pentane bimolecular isomerization over WOx/ZrO2

Depending on the catalyst used, the reaction conditions and the molecules converted, alkane isomerization proceeds via a monomolecular or a bimolecular mechanism. n-Pentane isomerizes via both mechanisms which makes it an excellent probe molecule. In this study, n-pentane isomerization was investigated using WO_x/ZrO₂. Propylene and 1-pentene were co-fed separately to study the effect on n-pentane isomerization and it was identified that only 1-pentene increases the isomerization selectivity to isopentane. Our results support the model of a bimolecular mechanism that proceeds via a C₁₀ intermediate on Zr-WO_x Brønsted sites.     

1-Pentene Isomerization over Zeolites Studied by in situ IR Spectroscopy

The skeletal isomerization of 1-pentene at 1.2 bar total pressure and 30 mbar 1-pentene partial pressure was investigated by in situ IR measurements over FER and BEA zeolites. If the reaction was carried out isothermally a small loss of Brønsted acid sites due to coke formation, a constant conversion and a high selectivity to isopentenes was found over FER. A loss of acid sites and a decrease in conversion was found over BEA, but the selectivity to isomerization increased remarkably. The pore size of FER favored the monomolecular reaction of 1-pentene and suppressed bimolecular reactions. In the bigger pores of BEA initially dimerization–cracking and hydrogen transfer reactions leading to a coke layer took place then the skeletal isomerization of 1-pentene prevailed.     

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.     

The isomerization and metathesis of n-butenes: I. Unreduced molybdena-alumina catalysts

A significant change in the catalytic characteristics of an unreduced molybdena-alumina catalyst occurred at room temperature with increasing duration of contact with the reactant cis-2-butene. Butene isomerization over the fresh catalyst was effected by an acidic mechanism with the concomitant intermolecular transfer of H and D. The chief diagnostic characteristics for this mechanism were maintained throughout pulse experiments, but changed to those characteristic of isomerization by metathesis after soaking in butene for 15 min or when a static recirculation reactor was used, i.e., with increasing contact time, the acidic character of the catalyst decreased and the activity for metathesis increased. Thus, in the later stages of the reaction, equimolar mixtures of cis-2-butene d₀ and d₈ isomerized to produce cis- and trans-C₄H₄D₄, C₄H₈, and C₄D₈ in ratios of 2:1:1 without other hydrogen scrambling.     

Kinetic modelling of the liquid-phase dimerization of isoamylenes on Amberlyst 35

Selectivity, conversion, yield and kinetics of the liquid-phase dimerization of 2-methyl-1-butene and 2-methyl-2-butene mixture have been studied in a batch stirred tank reactor in the temperature range 60–100 °C catalysed by the acid resin Amberlyst 35 and using ethanol as a selectivity enhancer. Selectivity to dimers showed a maximum at R_IA/EtOH = 20. Obtained diisoamylenes consisted mainly of 3,4,4,5-tetramethyl-2-hexene, 2,3,4,4-tetramethyl-1-hexene and 3,4,5,5-tetramethyl-2-hexene. LHHW type kinetic model was derived for the dimerization reaction as a whole. The kinetic model assumes that three active sites take part in the rate-limiting step of dimerization; the apparent activation energy for the dimerization reaction being 65 kJ mol⁻¹. A pseudohomogeneous kinetic model was used to describe the dimerization reaction network. The apparent activation energy for each dimerization reaction was found to be in the range 61–81 kJ mol⁻¹.     

Selectivity for dimers in pentene oligomerization over acid zeolites

The reactions of 1-pentene over acid zeolites were investigated in the liquid phase at 473 K. The primary reactions were isomerization, dimerization, and subsequent cracking of dimers. Zeolites consisting of only 10-membered (MFI) or 12-membered rings (FAU, BEA) behaved similarly, with dimerization and subsequent cracking products observed. Zeolites possessing 8-membered rings (MOR, FER) showed very different selectivity from each other and from other zeolites. MOR showed almost complete conversion of C₁₀ olefins, such that hexene and butene from cracking were the dominant products. FER showed high activity and selectivity for dimerization, with very small amounts of cracking products observed.     

Catalytic cracking and skeletal isomerization of n-hexenes on HY zeolite

The catalytic cracking and skeletal isomerization of n-hexenes on 80/100 mesh HY zeolite has been studied in the temperature range 350–405°C, and compared with results previously obtained on ZSM-5 zeolites. Species with less than three carbon atoms were not observed as primary cracking products, with traces of ethylene formed only as a secondary product. Although propylene and propane may be formed partially by monomolecular cracking of n-hexenes, the dominant cracking process is bimolecular. Dimerization, followed by disproportionation gives stable C₃, C₄ and C₅ species, in addition to C₉, C₈ and C₇ fragments. The probability of these larger fragments undergoing further cracking before desorption increases with temperature, but is significantly less than found on ZSM-5 zeolites. The main products of skeletal isomerization were monomethylpentenes, with those isomers which can originate from tertiary carbonium ions dominant. Dimethylbutenes were formed mainly as secondary products. The rate of the dimerization-cracking process in which two n-hexene species participate is ? six times slower than the reaction involving a monomethylpentene species and a linear hexene. The increase in the rate of the latter process with respect to the former is reduced on ZSM-5, and this can be attributed to the narrower pore size within this zeolite. In contrast to ZSM-5, there is significant formation of alicyclics, paraffins and aromatic species. The residual coke was found to be considerably poorer in hydrogen content than comparable material formed on ZSM-5.     

Polymerizations of α-olefins and styrene with MgCl2-supported titanium catalyst system: MgCl2/TiCl4/PhCO2Et with AlEt3PhCO2Et

Summary Kinetic analysis was performed in a short time polymerizations of 1-butene,4-methyl-1-pentene and styrene by using a catalyst system composed of MgCl₂/TiCl₄/PhCO₂Et with AlEt₃/PhCO₂Et which is known as a highly active and highly stereospecific catalyst system in olefin polymerization. The concentration of the active centers, [C ^*], the propagation rate constant, k _p, and the chain transfer rate, r _tr, were determined for each monomer. It was found that the values of [C ^*] were almost same for every monomer, but the values of k _p changes widely in the following order: propylene>1-butene>4-methyl-1-pentene>styrene.     

The catalytic characteristic and synthesis technique for 4-methyl-1-pentene over a potassium-supported superbase catalyst

The dimerization of propene to 4-methyl-1-pentene (4MP1) was studied in a fixed-bed reactor at 150?°C temperature, 8?MPa pressure, and 1?h^?1 liquid hourly space velocity (LHSV) in the presence of solid superbase with 20?wt% one-pass conversion to hexene and 88% selectivity toward 4MP1. Based on the reaction results, propene competes with 4MP1 for adsorption on the catalyst surface. A higher reaction pressure or a lower liquid rate of 4MP1, controlled by a suitable propene LHSV or reaction temperature, was beneficial to the adsorption of propene, which reduced the isomerization of 4MP1. Under optimized conditions, propene dimerization was stable for 200?h, and isomerization was limited to less than 5%, yielding products with about 19% one-pass conversion to hexene by weight and 87% selectivity toward 4MP1.     

Comparative catalytic studies on the conversion of 1-butene and n-butane to isobutene over MCM-22 and ITQ-2 zeolites

The catalytic properties of H-MCM-22 and Pt-MCM-22 for the skeletal isomerization of 1-butene and the dehydroisomerization of n-butane were compared with those obtained from their delaminated derivatives, H-ITQ-2 and Pt-ITQ-2. The overall results of our study strongly suggests that the desired 1-butene and n-butane conversions to isobutene in steady state occur mainly near the pore mouth inlets of the sinusoidal 10-ring channels retained in both of MCM-22 and ITQ-2 and inside this intralayer void space, respectively, which may be attributed to the unique geometric constraints imposed by the two-dimensionality of the pore system with a suitable degree of ellipticity of 10-ring channels. Whereas coke formation on the sinusoidal channels during the 1-butene skeletal isomerization can occur in such a way as to make most of the intrachannel Brønsted acid sites less available for nonselective dimerization-cracking reactions, it appears that the presence of H₂ in the n-butane dehydroisomerization stream would prevent extensive coke buildup, thereby allowing pore diffusion of reactants and products even in steady state.     

Catalytic activities and acid strengths of Nio-Zro2 catalysts modified with acids

A series of NiO-ZrO₂ catalysts were prepared by coprecipitation from the mixed aqueous solution of nickel chloride-zirconium oxychloride, and were modified with acids, H₂SO₄, H₃PO₄ and H₃BO₃. It was found that the NiO-ZrO₂ catalyst containing 25 mole percent of nickel oxide and evacuated at 400‡C for 1.5 hr showed maximum catalylic activities for 1-butene isomerization and ethylene dimerizalion. The catalytic activities for both isonierization and dimerizalion were correlated with the acid strengths of the catalysts. The isonierization activity of 1-butene ran parallel with the dimerization activity of ethylene, although the isomerization look place on relatively weak acid sites as compared with the dimerization.     

The skeletal isomerization of C4-C7 1-olefins over ferrierite and ZSM-5 zeolite catalysts

The skeletal isomerization of C₄-C₇ 1-olefins was studied on ferrierite (FER) and ZSM-5 (MFI) zeolites to elucidate the effect of the molecular distribution in zeolite pores on the selectivity foriso-olefin formation. Regardless of the difference in molecular length of 1-olefins, the FER zeolite showed high selectivity foriso-olefins, while the selectivity became slightly low at the skeletal isomerization of long olefin molecules. The drastic decrease in the selectivity of MFI zeolites by increasing the conversion is concurrently observed in the skeletal isomerization of C₄-C₇ 1-olefins. The high selectivity of FER zeolites is explained by their sparse distributions of olefin molecules in pores, which induces a high preference for monomolecular skeletal isomerization.     

Isomerization in olefin formation in radiation degradation of poly(olefin sulphone)s

Summary Gamma irradiation of poly(3-methyl-1-butene sulphone) in the solid state at 20°C produced a mixture of isomers,viz. 3-methyl-1-butene (61%), 2-methyl-1-butene (2%) and 2-methyl-2-butene (37%). Similarly, poly(1-butene sulphone) yielded 1-butene (65%) and 2-butene (35%). A hydride shift reaction in a polymer cation produced by main-chain scission is proposed to account for the isomerization.     

Influence of the fluorine loading level on the skeletal isomerization of 1-butene over fluorine-modified alumina

γ-alumina catalysts modified with different weight loadings of fluorine have been used for skeletal isomerization of 1-butene in order to investigate the effects of the fluorine loading level on the conversion of 1-butene and the selectivity to isobutene formation. Increasing the actual loading of fluorine up to 0.012 wt% led to an increase in conversion of 1-butene over fluorine-modified γ-alumina catalysts, while the high selectivity to isobutene remains almost unchanged. On the other hand, a clear trend of increasing 1-butene conversion with a decreasing selectivity to isobutene is observed for the γ-alumina catalysts with higher loadings of fluorine. An analysis of the results from the thermal analysis, NH₃ temperature-programmed desorption, infrared and the 1-butene sorption measurments clearly indicates that the number of strong acid sites in the modified γ-alumina catalysts is greatly enhanced at fluorine loadings higher than 0.012 wt%, leading to the acceleration of 1-butene oligomerization followed by cracking to light hydrocarbons. Therefore, the 1-butene isomerization selectivity from fluorine-modified γ-alumina catalysts can be understood in terms of a competition between the monomolecular and bimolecular reaction pathways, which highly depend on the concentration of strong acid sites. This revised version was published online in July 2006 with corrections to the Cover Date.     

Direct evidence of intramolecular rearrangement in skeletal isomerization of n-butane over bifunctional catalysts

Mechanisms of skeletal isomerization of n-butane over bifunctional catalysts, Pt–Cs_2.5H_0.5PW₁₂O₄₀ and Pt-sulfated ZrO₂, as well as the corresponding solid acids were studied using 1,4-¹³C₂-n-butane. The isotopic distributions of the reactant and product were analyzed with field ionization mass spectrometry, by which the parent peak patterns were obtained. It was found that 1,4-¹³C₂-n-butane was selectively isomerized to ¹³C₂-isobutane over these catalysts in the presence of H₂ at 423–523 K, while the corresponding solid acids gave isobutane with binomial distributions of ¹³C. These results clearly demonstrate that the skeletal isomerization of n-butane proceeded mainly via a monomolecular path with intramolecular rearrangement on both the bifunctional catalysts, while it occurred through a bimolecular path with intermolecular rearrangement on the solid acids. This difference in reaction mechanism is reflected on that in the selectivity to isobutane.     

Equilibrium of the simultaneous etherification of isobutene and isoamylenes with ethanol in liquid-phase

The simultaneous etherification of isobutene and isoamylenes with ethanol has been studied using macroreticular acid ion-exchange resins as catalyst. Most of the experiments were carried out over Amberlyst-35. In addition, Amberlyst-15 and Purolite CT-275 were also tested. Chemical equilibrium of four chemical reactions was studied: ethyl tert-butyl ether formation, tert-amyl ethyl ether formation from isoamylenes (2-methyl-1-butene and 2-methyl-2-butene) and isomerization reaction between both isoamylenes. Equilibrium data were obtained in a batchwise stirred tank reactor operated at 2.0 MPa and within the temperature range from 323 to 353 K. Experimental molar standard enthalpy and entropy changes of reaction were determined for each reaction. From these data, the molar enthalpy change of formation of ethyl tert-butyl ether and tert-amyl ethyl ether were estimated. Besides, the chemical equilibrium between both diisobutene dimers, 2,4,4-trimethyl-1-pentene and 2,4,4-trimethyl-2-pentene, was evaluated. A good agreement between thermodynamic results for the simultaneous etherification carried out in this work and those obtained for the isolated ethyl tert-butyl ether and tert-amyl ethyl ether systems was obtained.     

Synthesis of tertiary amyl methyl ether (TAME): Equilibrium of the multiple reactions

The octane enhancer tertiary amyl methyl ether (TAME) is produced by liquid phase synthesis from methanol and a mixture of isoamylenes, namely 2-methyl-1-butene and 2-methyl-2-butene, using a sulfonic acid ion exchange resin as catalyst. Three reactions take place simultaneously in TAME synthesis: etherification of the two methylbutenes and their isomerisation. In order to study the equilibrium of the multiple reactions in TAME synthesis, the thermodynamic properties of the compounds in the liquid phase and equilibrium constants were calculated using a modified UNIFAC method to describe the nonideality of the system. Four parameters influencing the equilibrium conversion were derived and discussed in detail. Supplemental experiments were performed at three temperatures in the range from 303 to 343 K and at different initial molar ratios of educts. Equilibrium conversions of methanol were determined from these experiments and compared with calculated values. At 298 K the predicted activity based equilibrium constant was 22.9 for TAME synthesis from 2-methyl-1-butene and 1.6 for TAME synthesis from 2-methyl-2-butene; for isomerisation of 2-methyl-1-butene to 2-methyl-2-butene a value of 14.3 was obtained.     

Synthesis and catalytic properties of substituted AlPO4-31 molecular sieves

Molecular sieve AlPO₄-31 and substituted analogues (MnAPO-31,SAPO-31, MnAPSO-31) have been tested for their catalytic properties in the isomerization of 1-butene over 6 h time on stream at 743 K. The conversion of 1-butene proceeds selectively by either double bond or skeletal isomerization. MnAPSO-31 with a molar MnO/P₂O₅ ratio adjusted to 0.01 yields the highest percentage of isobutene whereas the parent AlPO₄-31 leads almost completely to a double bond shift with only minor skeletal isomerization. The results are related to the acidity characteristics that were determined by ammonia thermodesorption. Deactivation is accompanied by a loss of selectivity for the skeletal isomerization.     

Skeletal isomerization of 1-butene over mesoporous materials

On the skeletal isomerization of 1-butene, mesoporous materials with mesopores too large to expect any shape selectivity have been used in order to investigate the effects of the concentration of acid sites on the conversion of 1-butene and the selectivity for isobutene. The concentrations of acid sites can be varied through the control of the Si/Al ratio. The conversion of 1-butene increases with increasing the aluminium content of mesoporous materials, while the selectivity for isobutene decreases. The results of ammonia TPD, IR measurement of 1-butene adsorption, and TG analysis of used catalysts indicate that distant location of activated 1-butene molecules induces the monomolecular reaction over the mesoporous materials with low aluminium content, resulting in high selectivity for skeletal isomerization. On the mesoporous material with high aluminium content, however, the high concentration of activated 1-butene molecules accelerates the multimolecular oligomerization and, thus, reduces the selectivity. This revised version was published online in July 2006 with corrections to the Cover Date.     

Etherification of C5- and C8-Alkenes with C1- to C4-Alcohols

Etherification of two alkenes, 2-methyl-1-butene and 2,4,4-trimethyl-1-pentene, was studied with seven different C₁- to C₄-alcohols. Although etherification was of primary interest, the isomerisation of the alkenes was the main reaction to occur. For the primary alcohols the etherification and isomerisation rates correlated well with the properties of the alcohols. Both rates increased with decreasing polarity and with increasing carbon number, acidity and Mulliken charge of the oxygen atom of the alcohol. It is difficult to distinguish the effect of each property separately, and probably the differences in the reactivities are not due to any one property alone but rather the synergy of the properties affects the reactivities. The secondary alcohols behaved in a different way than the primary ones: the etherification was almost negligible. The effect of alcohol on the isomerisation of alkenes was notable even though alcohol does not directly react in the reaction, which was concluded to be due to the stronger adsorption of the more polar alcohols which hinders the reactions of other components.