Relation between the catalytic and acidic properties of zeolite catalysts

Influence of the strength and concentration of non protic centers on the catalytic activity of metal-zeolite catalysts in isomerization of n-butane has been studied. A relationship between the specific activity and the center strength is suggested.

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Raman spectroscopy and dioxygen adsorption on Cs-loaded zeolite catalysts for butene isomerization

Cesium-exchanged zeolite X was impregnated with cesium acetate (Cs(Ac)/CsX) or cesium carbonate (Cs(2)CO(3)/CsX) and subsequently calcined to yield a basic catalyst. The Raman spectra of calcined Cs(Ac)/CsX and Cs(2)CO(3)/CsX exhibited a new peak at 1036 cm(-1) associated with the occluded species. No evidence for cesium peroxide or superoxide was observed. The occluded cesium species in both samples is proposed to be an oxycarbonate, which is a metastable intermediate between cesium carbonate and cesium oxide. The isomerization of 1-butene to cis- and trans-2-butene was catalyzed by cesium-loaded zeolite X. Although CO(2) readily poisoned the active base sites for catalysis, pretreatment of a basic zeolite with O(2) at 373 K did not. Co-feeding O(2) with 1-butene at 373 K, however, completely deactivated the base sites. Analysis of the reactor effluent at 473 K and the IR spectrum of the catalyst indicated the formation of carbon dioxide, which irreversibly adsorbed on the basic sites of the catalyst. Deactivation of basic catalysts by O(2) is proposed to occur through a low temperature oxidation of 1-butene to carbon dioxide, which strongly adsorbs on the active sites.     

The dimerization of isoprene on complex nickel catalysts

Summary Eight-membered dimers, 1,5(1,6)-dimethylcyclooctadiene-l,5, were obtained as the main products by dimerizing isoprene on a complex nickel catalysts.     

2-phosphanylphenolate nickel catalysts for the polymerization of ethylene

The previously unknown methallylnickel 2-diorganophosphanylphenolates (R=Ph, cHex) were synthesized and found to catalyze the polymerization of ethylene. To explore the potential for ligand-tuning, a variety of P-alkyl- and P-phenyl-2-phosphanylphenols was synthesized and allowed to react with [Ni(cod)(2)] (cod=1,5-cyclooctadiene) or with NiBr(2).DME and NaH. The complexes formed in situ with [Ni(cod)(2)] are generally active as ethylene polymerization catalysts with all the ligands tested, whereas the latter systems are inactive when 2-dialkylphosphanylphenols are applied. M(w) values, ranging from about 1000 to about 100000 g mol(-1), increase for various R(2)P groups in the order R=Ph相似文献   

Double-bond isomerization and hydrogenation in polyethylene with soluble nickel catalysts

Polyethylene exhibits an increase in melt viscosity (melt index drop-off) when subjected to extreme thermal treatment. Although stabilizers and antioxidants do not prevent this change it has been possible to stabilize high-density polyethylene by isomerization or hydrogenation of the polymer unsaturation. A soluble nickel octoate–triethylauminum complex was an active catalyst system for these reactions. A triethyl-aluminum/nickel ratio of three appeared optimum for maximum activity. Of particular interest regarding this catalyst system was the fact that the terminal vinyl unsaturation of the polymer had to be isomerized to some internal position before hydrogenation commenced. However, when this catalyst system was impregnated on a predried silica or when butyllithium was substituted for triethylaluminum, both isomerization and hydrogenation occurred simultaneously. The addition of a light olefin such as ethylene to the reaction mixture slowed the overall reaction until most of the ethylene had been hydrogenated. At this point the isomerization–hydrogenation reaction path previously mentioned commenced.     

Solid aluminiumalkyl-based isomerization catalysts. Surface acidity and catalytic activity

Isomerization catalysts with high activity and selectivity were prepared by chemical anchoring of aluminiumethyldichloride on the surface of inorganic oxides. Surface acidity was studied by calorimetric and IR spectroscopy methods, and was found to correspond well with catalytic activity for m-xylene isomerization.

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A comparison between ethylene dimerization and Diels-Alder reaction

The reaction pathways for the two reactions 2C₂H₄C₄H₈ and C₂H₄+C₄H₆ C₆H₁₀ were investigated. The transition state geometries and activation energies were determined with the SINDO method and compared. Both reactions were found concerted with different significance of diradical character at the transition state. Whereas the ethylene dimerization showed that a diradical will be encountered along the reaction pathway, we did not find one in the Diels-Alder reaction.     

Synthesis of benzoxazolylpyridine nickel complexes and their efficient dimerization of ethylene to alpha-butene

A series of new nickel complexes bearing benzoxazolylpyridines was synthesized and characterized by FT-IR spectroscopic and elemental analysis. The molecular structures of two representative complexes were determined by single-crystal X-ray diffraction. The complex [NiCl2[2-(2-benzoxazolyl)-6-methylpyridine]] (5) is a centrosymmetric dinuclear compound with two penta-coordinated Ni(II) centers, whereas the complex [NiCl2[2-(5-methyl-2-benzoxazolyl)-6-methylpyridine}] (6) is mononuclear exhibiting a distorted octahedral-coordination geometry around the nickel atom. Upon activation with diethylaluminium chloride (Et2AlCl), all the complexes exhibited moderate to good catalytic activity for ethylene oligomerization (27-415 g mmol(-1)(Ni) h(-1) bar(-1)) with high selectivity for ethylene dimerization to form alpha-butene. The observed variance in the catalytic activities of the complexes is attributed to the different ligand environments and effects of reaction parameters.     

Tuning of nickel 2-phosphinophenolates - catalysts for oligomerization and polymerization of ethylene

This short personal account summarizes recent work concerning the syntheses and structural aspects of neutral and cationic (organo)nickel and (organo)palladium 2-phosphinophenol(ate) complexes and catalysts for the oligo- or polymerization of ethylene.     

Catalytic and acidic properties of different metal-zeolite catalysts

Catalytic and acidic properties of bifunctional catalysts containing faujasite, superhigh-silica zeolite and mordenite have been studied in isomerization of C₈-alkylaromatic hydrocarbons.

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Cooperative effect through different bridges in nickel catalysts for polymerization of ethylene

A series of mononuclear (M₁ and M₂) and dinuclear (C₁–C₆) Ni α‐diimine catalysts activated by modified methylaluminoxane were used in polymerization of ethylene. Catalyst C₂ bearing the optimum bulkiness showed the highest activity (1.6 × 10⁶ g PE (mol Ni)^?1 h^?1) and the lowest short‐chain branching (32.5/1000 C) in comparison to the dinuclear and mononuclear analogues. Although the mononuclear catalysts M₁ and M₂ polymerized ethylene to a branched amorphous polymer, the dinuclear catalysts led to different branched semicrystalline polyethylenes. Homogeneity and heterogeneity in the microstructure of the polyethylene samples was observed. Different trends for each catalyst were assigned to syn and anti stereoisomers. In addition, thermal behavior of the samples in the successive self‐nucleation and annealing technique exhibited different orders and intensities from methylene sequences and lamellae thickness in respect of each stereoisomer behavior. Higher selectivity of hexyl branches obtained by catalyst C₂ showed a cooperative effect between the centers. The results also revealed that for catalysts C₅ and C₆, selectivity of methyl branches led to very high endotherms and crystalline sequences with melting temperatures higher than that of 100% crystalline polyethylene indicating ethylene/propylene copolymer analogues. For catalysts C₃ and C₄, more vinyl end groups were a result of the long distance between the Ni centers. Kinetic profiles of polymerization along with a computational study of the precatalysts and catalysts demonstrated that there is a direct relation between rate constant, energy interval of catalyst and precatalyst, and interaction energy of Et···methyl cationic active center (Et···MCC or π–Comp.). Based on this, narrow energy interval (activation energy) of precatalyst and catalyst leads to fast and higher activation rate (catalyst M₂), and strong interaction of ethylene and catalyst leads to high monomer uptake and productivity (catalyst C₂). Moreover, theoretical parameters including electron affinity, Mulliken charge on Ni, chemical potential and hardness, and global electrophilicity showed optimum values for C₂.     

Copolymerization of 2‐butene and ethylene with catalysts based on titanium and zirconium complexes

The polymerization of 2‐butene and its copolymerization with ethylene have been investigated using four kinds of dichlorobis(β‐diketonato)titanium complexes, [ArN(CH₂)₃NAr]TiCl₂ (Ar = 2,6‐ⁱPr₂C₆H₃) and typical metallocene catalysts. The obtained copolymers display lower melting points than those produced of homopolyethylene under the same polymerization conditions. ¹³C NMR analysis indicates that 9.3 mol‐% of 2‐butene units were incorporated into the polymer chains with Ti(BFA)₂Cl₂‐MAO as the catalyst system. With the trans‐2‐butene a higher copolymerization rate was observed than with cis‐2‐butene. A highly regioselective catalyst system for propene polymerization, [ArN(CH₂)₃NAr]TiCl₂ complex using a mixture of triisobutylaluminium and Ph₃CB(C₆F₅)₄ as cocatalyst, was found to copolymerize a mixture of 1‐butene and trans‐2‐butene with ethylene up to 3.1 mol‐%. Monomer isomerization‐polymerization proceeds with typical metallocene catalysts to produce copolymers consisting of ethylene and 1‐butene.     

Metal complexes in the catalytic reactions of olefins. 1. Comparison of the catalytic properties of triphenylphosphine nickel complexes both individually and heterogenized on Al2O3 in the dimerization of ethylene

Conclusions By studying the liquid-phase dimerization of ethylene in the presence of Cat-Et₃Al₂-Cl₃ catalytic systems based on a nickel complex heterogenized on Al₂O₃ and a number of model nickel complexes, a similar activity and selectivity of the process has been established (Cat=NiPPh₃(CO)₂L, where L=PPh₃, CO, Al₂O₃; Ni(PPh₃)₂(CO)₂; Ni(PPh₃)₂(₂-C₂H₄); Ni[P(C₆H₁₁)₃]₂(H)Cl and Ni(PPh₃)(Et)Cl).The results of the investigation agree with the hypothesis that mono- and diolefinic nickel complexes are formed as the active intermediates in the reaction.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 11, pp. 2466–2469, November, 1988.     

The acidic properties of stoichiometric and nonstoichiometric B-P-O catalysts

A spectrophotometric method has been employed to measure the surface acidities of stoichiometric and nonstoichiometric B-P-O catalysts. Three phosphorus/boron ratios, 0.7, 1.0, 1.3, and three pretreatment temperatures, 200, 375, and 600°C, were employed. The data obtained appear to be consistent with earlier measurements of surface acidity by the Hammett indicator method and by infrared spectroscopy and with the results of studies of probe reactions.     

The effect of alkyl substituents in nickelocenes on the catalytic dimerization of ethylene

(C₅H₄ i-Pr)₂Ni exhibits the highest catalytic activity in the dimerization of ethylene among the nickelocenes, (C₅H₄R)₂Ni (R = H, Et,n-Pr,i-Pr, ori-Bu) and their analogs (C₅H₄R)Ni(C₃H₅) (R = H,i-Pr). The higher activity is accompanied by lower selectivity with respect to 1-butene and with higher yields of 1-hexene. It is suggested that the introduction of an alkyl substituent in the cyclopentadienyl ring of nickelocene favors the generation of hydride sites involving the nickel atom. These sites participate in the process of ethylene dimerization.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 970–972, May, 1993.