Lewis Acid-base Pairs Research Articles

Cycloaddition between propylene oxide and CO2 over metal oxide supported KI

A novel catalyst system composed of KI supported on metal oxides for cycloaddition of propylene oxide with CO2 was investigated. It was found that the activity of KI for cycloaddition was strongly enhanced by ZnO as both support and promoter, resulting in a high yield of propylene carbonate within a short reaction time. Based on the characterization of BET, XRD, TG, IR and XPS-AES, this enhancement could be attributed to both the interaction between KI and amphoteric ZnO, which gave rise to the concerted active Lewis acid–base pairs of Iδ-Mδ+ and Kδ+O-δ and the dispersion of KI on the surface of the supports. The catalytic mechanism was then proposed.

A computer modelling study on the interaction of -(CF 2CF 2O)- polyperfluorinated ethers with Lewis acid sites: perfluorodiethyl ether

Lewis acid catalysis significantly enhances the thermally induced degradation of polyperfluorinated ether lubricants. We have used CF 3CF 2OCF 2CF 3 and AlF 3 as models to investigate via ab initio theory, the Lewis acid-base interactions between polyperfluorinated ethers and Lewis acid sites. The results of these studies indicate that perfluorodiethyl ether, or PFDEE, may form a Lewis acid-base pair between the ether oxygen atom in PFDEE and the aluminum atom in AlF 3 provided that there is no steric hindrance limiting access to the ether oxygen atom. This may be achieved, for example, by PFDEE adopting a trans-gauche instead of a trans-trans conformation. In the trans-trans conformation, a terminal perfluoromethyl group sterically interferes with the accessibility of the ether oxygen atom to the AlF 3 surface. These results indicate that in lubricant molecules such as the ‘Z’ family of Fomblin fluids, composed of a copolymer of perfluoromethylene and perfluoroethylene oxide units, steric hindrance will limit AlF 3 access to the ether oxygen atom and would depend upon the ratio of -CF 2CF 2O and -CF 2O- structural units and their spatial distribution. Even when the -CF 2CF 2OCF 2CF 2- PFDEE backbone adopts a local trans-gauche conformation such as to spatially allow a Lewis acid-base interaction between the ether oxygen atom and aluminum, the interatomic distance is much greater, 2.5 Å compared to 1.8 Å for perfluoromethylene oxide. The binding energy is correspondingly smaller, −3 compared to −9 kcal/mol, respectively. Several degradation paths for the decomposition of PFDEE, originating within the -CF 2OCF 2- structural group, were computed. Thus, the decomposition of PFDEE to CF 3CF 3 and CF 3CFO via an initial cleavage of a C-O bond, and, alternatively, via a transition state, were investigated. In both cases, the activation energy to initiate the decomposition is rather high, near 80–100 kcal/mol. However, in the presence of an AlF 3 catalyst, the activation energy for the decomposition reaction via the transition structure is significantly reduced to 50 kcal/mol. Alternatively, a C-O bond scission leading to the formation of radical sites adjacent to -CF 2O- structural units may lead to mass loss via elimination of COF 2. Such a reaction proceeds with an activation energy of only 24 kcal/mol.

N-Butane isomerization on metal-promoted sulfated zirconia

The promoting effect of metal cations addition to SO 4 2− ZrO 2 in n-butane isomerization is studied. Several metal cations (Pt, W, Ni, Co, Fe, Cr) supported on SO 4 2− ZrO 2, or in an inert material mechanically mixed with SO 4 2− ZrO 2, produce an increase in the rate of n-C 4 isomerization. Cu has an inhibiting effect. The metals or their oxides provide the dehydrogenation capacity that can produce C 4 alkenes, which in turn can influence the mechanism of isomerization. These C 4 alkenes can be added to the pool generated by the strong Lewis acid-base pair of SO 4 2− ZrO 2 leading to an increase in the isomerization rate.

FT-IR Spectroscopic Studies of Methane Adsorption on Magnesium Oxide

adsorption of ch4 and coadsorption of ch4 and co on differently treated magnesium oxide have been studied by ft-ir spectroscopy at 173-273 k. five ir bands at 3008, 3000, 2900, 2890, and similar to 1306 cm(-1) were observed when well-outgassed mgo was exposed to ch4 at 173 k. the bands at 3008 and 3000 cm(-1) are assigned to a degenerate stretch vibration of ch4, and those at 2900 and 2890 cm(-1) originate from an infrared-forbidden mode at 2917 cm(-1) of free ch4. these bands became noticeably stronger with the outgassing temperature of mgd but could be sufficiently reduced as mgo was pretreated in air or under water vapor. adsorbed co on mgo gives two ir bands at 2161 and 2155 cm(-1) which exhibit a parallel variation with those bands of adsorbed ch4 with the different pretreatments of mgo. coadsorption of ch4 and co indicated that adsorbed co can deplete part of the adsorbed methane, particularly that with ir bands at 2900 and 2890 cm(-1). it is proposed that the adsorbed methane can be substituted by adsorbed co coordinated;l to a lewis acid-base pair site, mglc2+olc2-, and the methane unaffected by coadsorbed co interacts mainly with surface oxygen anions alone. the observed ir bands can be attributed to two types of adsorbed methane on mgo: one with ir bands at 3008 and 1305 cm(-1) due to weakly adsorbed methane on surface oxygen anions and another with bands at 3000, 2900, 2890, 1305 cm(-1) due to methane strongly interacting with the lewis acid-base pair sites of mgo. a possible mechanism of methane interaction with surface of mgo is discussed.

An NMR study of acid sites on chlorided alumina catalysts using trimethylphosphine as a probe

The MAS-NMR spectra of adsorbed trimethylphosphine (TMP) were used to determine the concentration of Bronsted and Lewis acid sites on pure and chlorinatedγ-Al2O3 samples. Chlorination with CHCl3,CCl4 or AlCl3 promoted the formation of Bronsted acid centers, which are characterized by the protonated adduct of TMP. This adduct has a31P chemical shift of ca. −3.8 ppm and a JP−H scalar coupling of 517 Hz. Additional resonances in the −44 to −54 ppm range are attributed to Lewis acid-base pairs. In some cases a partially resolved JP−Al coupling is observed, which confirms the assignment. Upon thermal treatment of a chlorinated sample at temperatures > 200°C, the concentration of Bronsted acid centers decreased; the concentration of one type of Lewis acid increased and another remained almost constant. In a parallel set of experiments the initial conversion ofn-hexane at 150°C and the yields of cracking and isomerization products were determined. Comparable functional relationships were observed between the loss of Bronsted acid sites and the decrease in yields of both cracking and isomerization products. These results suggest that Bransted acidity is responsible for the cracking and isomerization ofn-hexane over chlorided aluminas at 150°C.

A review of optical basicity and its applications to oxidic systems

A description is given of the theoretical background to the optical basicity concept, which is based on orbital expansion effects reflecting the nature of the chemical bonding between a Lewis acid-base pair. Optical basicity is particularly useful for oxidic network systems, such as silicate, phosphate, or borate in crystalline, vitreous, and liquid (molten) form. The experimental measurement of optical basicity, from the ultraviolet spectra of probe ions such as Tl + and Pb 2+, is described, and it is shown how optical basicity values can be assigned to individual oxides such as Na 2O, CaO, or SiO 2. These make it possible to calculate a “theoretical” optical basicity value, Λ th , from chemical constitution, thereby allowing correlations to be made with various properties of, for example, molten silicates as a function of composition. The correlations have important applications for redox behaviour, e.g., in glass making and in the performance of metallurgical slags, and should find application in geochemistry, for example, to Fe 2+ Fe 3+ and other redox ratios in magmas. Furthermore, in the chemical reactions between metal silicate, borates, etc., the Λ th values indicate how minimisation or elimination of optical basicity differences between the reactants is a major driving force.

Hydrogen isotopic exchange between D2 and H2O on alumina prepared by sputtering

Hydrogen isotopic exchange between D2 and H2O has been investigated on alumina prepared by sputtering at 803 K and at low pressures. The alumina is amorphous, consisting of aluminum with coordination numbers four, five, and six in the ratio 1:2:1 as determined by NMR. A kinetic model is proposed on the basis of the pressure dependence of the exchange rate and the heats of adsorption at adsorbed sites, evaluating by molecular orbital calculations. The adsorbed sites are assumed to be Lewis acid–base pairs composed of a coordinatively unsaturated aluminum and the adjacent unsaturated oxygen site. The site relating to the rate-determining step contains threefold aluminum, which is formed by unsaturation of fourfold aluminum at the surface. As a consequence, the exchange rate depends on the amount of fourfold aluminum when the alumina is heat treated and changes its phase.

Propene aromatization over alkali-exchanged ZSM-5 zeolites

The transformation of propene was carried out over alkali-exchanged ZSM-5 zeolites. Compared with H-ZSM-5, the alkali-exchanged ZSM-5 showed lower activity but higher selectivity to benzene among aromatics. Furthermore, they displayed activity for dehydrogenation of cyclohexane to form benzene. Infrared results using both pyridine and pyrrole as probe molecules revealed that these alkali-ZSM-5 samples do not contain Brønsted acid sites, however strong Lewis acid sites and also weak Lewis basic sites are present. The oligomerization of propene and the further dehydrocyclization are suggested to comprise the reaction route for benzene formation, the active sites for these reactions are Lewis acid sites, which should be regarded as acid-base pairs of alkali—cation and the neighbouring basic oxygen. A mechanism involving these Lewis acid—base pairs is proposed for the observed products of methylpentene.

Supported perovskites

The acid-base properties of perovskite-type LaMnO3 and LaFeO3 catalysts were adjustable by immobilizing them on silica. Unsupported LaMnO3 and LaFeO3 contained majorly basic sites on their surfaces. Dispersing LaMnO3 and LaFeO3 on silica suppressed their basicities and improved their acidities due to the increased concentrations of tetravalent B-site cations. Catalytic performances of ethanol conversion reflected the acid-base properties of bulk and silica-supported LaMnO3 and LaFeO3. Under differential analysis conditions, unsupported perovskites were active in base-catalyzed reactions such as reverse aldolization and Tishchenko reaction, while silica-supported perovskites were active in aldolization and dehydration. Furthermore, a higher aldolization activity of LaMnO3/SiO2 than that of LaFeO3/SiO2 was discovered. The different aldolization activities were attributed to excess mobile oxygen on the surface of LaMnO3/SiO2, forming aldolization-active Lewis acid (Mn4+)-base (nonstoichiometric oxygen) pairs. In contrast, the Lewis acid-base pair was absent in LaFeO3/SiO2, of which oxygen vacancies were enriched with. Oxygen nonstoichiometry was responsible for the varying aldolization activities of silica-supported LaMnO3 and LaFeO3 catalysts.

Formyl onium salts: synthesis and decomposition pathways

Isolable formyl-pyridinium and -imidazolium salts are generated by treating Lewis acid–base pairs of type Me3SiA–N-heterocycle(A = leaving group) with acetic formic anhydride; their thermal and base-catalysed decarbonylation is discussed in the light of theoretical data.

H-D exchange between benzene and the OH groups of alumina

H-D exchange of benzene with the OH groups of aluminas was investigated using FTIR spectroscopy. The reactions benzene- d 6-alumina and benzene-alumina- d are of first order with respect to OH and benzene concentrations ( E A = 112 kJ/mol). They mainly take place on centers—probably Lewis acid-base pairs—which are poisoned by CO 2 adsorbed as carbonate.

The effect of dehydroxylation of HNaY zeolites on the interaction with ethylene and propylene

The extent of dehydroxylation of HNaY zeolites of various degree of decationization was varied by the termal treatment in vacuo over the temperature range 620-890 K. The solid phase was investigated by means of infrared spectroscopy, the gaseous phase by means of mass spectrometry. Lewis acid-base pair centres were formed by the dehydroxylation and these initiated at 310 K the oligomerization of ethylene in zeolite cavities. The reaction occurred via the cationic mechanism. A certain ratio of Lewis centers to OH groups was necessary in order to achieve the maximum oligomerization rate on given type of zeolite. It was showed up in a shift of the maximum to higher activation temperatures with less decationized zeolites, more stable with respect to the dehydroxylation. The zeolite activity increased with increasing decationization. The same trend was found also for the oligomerization related to one Lewis center. The presence of Lewis centers affected the oligomerization rate of propylene, too, though Broensted centers played the dominant role in this case. After the thermal decomposition of the oligomers aliphatic and cyclic hydrocarbons, mostly saturated compounds, were found in the gaseous phase. The isotopic exchange of propylene-d6 at 570K occurred via a multiple mechanism, the value of its rate being of the same order of magnitude as with the hydroxylated forms. In view of the varying amount of slowly exchanging SiOH groups in various decationized zeolites, the dependence of the exchange rate of structural zeolites on the decationization could not be established.The isotopic exchange of ethylene-d4 occurred via a single mechanism with a slightly increased rate in comparison with the hydroxylated forms.