Weak Acid Centers Research Articles

Effect of Spectrum of Acidity of Zeolites of Various Structural Types on Their Catalytic Characteristics in the Synthesis of Ethyl tert -Butyl Ether

The catalytic properties of zeolites of various structural types in the liquid phase synthesis of ethyl tert-butyl ether from isobutylene and ethanol have been studied. The activity and selectivity of the catalysts depend on the concentration and strength of the acid centers. A possible mechanism for the synthesis of ethyl tert-butyl ether is proposed, suggesting that isobutylene and ethanol are activated on the weak and strong acid centers respectively.

Hydrodeoxygenation of Residual Fatty Acids Fraction over Ni-Mo / Al2O3 Catalyst

The hydrodeoxygenation of the residual fraction of fatty acids from sunflower oilwas performed on bimetallic Ni-Mo/-Al2O3. Experiments were carried out on a in a continuous system at temperatures below 340 �C and pressures less than 70 bar. The catalyst shows a total acidity of 1.192 meq / g, from which 0.714 meq / g represents the concentration of weak acid centers, 0.438 meq / g represents the concentration of medium strength acid centers, while the concentration of strong centers is very small, only 0.041 meq / g. The analysis of the reaction products mainly highlighted the presence of linear hydrocarbons.

Studies of the Surface of Fluorinated Carbon in the Aspect of its Catalytic Properties

Surface properties of fluorinated carbon were characterised from the point of view of its use as a material for catalytic purposes. Samples with different fluorine content (from 10 to 65 wt.%) were tested for their catalytic activity for a number of reactions of acid-base catalysis involving residual hydroxyl groups as active centres. It was found that they were capable of catalysing only such a reaction proceeding through the formation of carbenium ion for which the presence of very weak acid centres is sufficient. The generation of only weak acidity as a result of fluorination was unexpected, therefore calculations were performed using AM1 method to explain this fact. A simple molecule used as a model of fluorinated carbon surface enabled to predict acidity of residual OH groups which was in agreement with experimental findings.

Catalytic Properties of Fluorine Modified SmNa-Y Zeolites

It is known that mild fluorination of zeolites, including those modified with polyvalent cations, increases their activity for acid catalyzed reactions. In this study we report the reactivity of fluorine modified Sm(III) exchanged zeolite NaY. Mild fluorination of SmNaY reduces the activity of strong protonic acid sites and increases the contribution of weak aprotic acid centres. Results for cumene cracking and toluene disproportionation as well as for 1-hexene isomerization are reported.

Surface acidity relations of NiO-SiO 2

Solids composed of varying ratios of NiO to SiO 2 were prepared by mechanically mixing hydrated Ni(OH) 2 and silica hydrogel. The acid strengths and concentrations were estimated by the amine titration method on the products of thermal treatment at 500 °C, 600 °C and 800 °C. Most of the products showed an acid strength H o of +2.0 that was independent of the chemical nature or the chemical composition of the solids. However, most of the acid sites are very weak, with characteristic H o values of + 3.3 or less. At about 45 mol.% NiO the minimum acidity is obtained at 500 °C, but at 600 °C and above this solid is characterized by a maximum in the weak acidity and the presence of a small number of strong acid sites ( H o ⩽ + 2.0). A sample prepared by coprecipitating the two components did not differ much from the corresponding mechanically mixed solid, apart from the presence of an abundance of weak acid centres. Owing to the large separation of the moderate acid sites ( H o = + 2.0), which are calculated to have a distribution density of less than 1 site per 100 Å 2, it isdoubtful whether NiO-SiO 2 would be able to catalyse any reactions.

Acid-base and catalytic properties of iron-antimony oxide catalysts

Iron antimonate, whose strongest acidic sites (Fe3+) are blocked by weak acidic aprotic centers Sb3+, is an active and selective catalyst for the ammoxidation of propylene.

Synthesis of s-Butyltoluene by the Gas Phase Alkylation Reaction of Toluene with 1-Butene Using γ-Alumina-Boria Catalyst

For a selective synthesis of s-P-butyltoluene by an alkylation reaction of toluene with 1-betene, optimal reactioni conditions and suitable catalysts have been found by the authors.In this paper, the experimental results ofthe reaction in gas phase with the alumina boria catalysts using r-alumina as a base are reported. The conclusions may be summarized as follows:1) r-aluminareacted with boric gcid to form strongly acidic centers, as well as with tyand X-alumina, developing activity of toluene alkylation. The total acid amount of the catalyst was distributed uniformly over the acid centers with acid strength Hg ranging from +7.1 to -8.2. The catalysts containing O -vlO wt% boria had much more weak acid centers and Lewis acid ones than those containing more than 15 wt% boria. At the boria contents more than 15 wt% the greater parts of the acid centers were strong acids, considered as Bronsted type, and the acid amount showeda maximum at 15 wt 9Zer boria and this maximum value increased with inCreasing calcina temperature Qf the catalyst, 2) The activity was attributed to the acid centers at strength H, ;-3.0, being directly proportional to its acid amount, When the reaction proceeded with sIight decrease in activity as a result of co:keformation, as may be seen from the above fact and the result shown in 1), the catalyst containing about 15 wt%boria showed the maximum activity that became higher as the calcination temperature increased.3) At transient-state activity(about an hour after the start of run), the toluene conversions and s-butylto1uene(SBT)yields increased as the reaction temperature increased. While at stationary-state activity(five hours after the star of run)they decreased with increasing reac on temperature because of a cataryst decay. Alsg the isomer distribution of SBT at stationary-state activity was slightly dependent on reaction, condition, s, but independent of preparing conditions of the catalyst, and the SBT contained approxi:matly 20% of o-and m-isomer and 60% of p-one.

ChemInform Abstract: DEPOLYMERISATION OF PARALDEHYDE CATALYSED BY SOME SILICA AND ALUMINA BASED BINARY OXIDE CATALYSTS

Depolymerisation of paraldehyde in benzene catalysed by some silica and alumina based binary oxides has been studied. It is seen here that in all the cases, the acidity at pKa';; -3.0 best correlates with catalytic activity (first order rate constant K) and is cata­ lytically active for depolymerisation of paraldehyde. Mo03, W03 and VzOs supported on silica and alumina have acid sites at pKa';; -3.0, hence these are catalytically active. Silica and alumina based MgO, BeO and ThOz have very little acid strength at pKa';; -3.0 but have a number of weak acid centres at pKa';; +3.3, and these relatively weak acid sites have been found to be catalytically inactive. The observed rate constant values in case of silica supported catalysts are greater than those of the alumina supported catalysts. It is further observed that the rate of depolymerisation of paraldehyde is directly proportional to the acidity of the catalysts at pKa';; -3.0. The energies of activation have been found to be inversely proportional to the acidity of the catalysts.