Number Of Acid Centers Research Articles

Jonska izmjena natrijuma sa hlorovodoničnom kiselinom na zeolitu ZSM-5

ZSM-5 zeolites are highly silicate materials that have significant application in catalytic processes in petrochemistry, especially due to their high selectivity. Most reactions in the petrochemical industry are acid-catalyzed. The acidic properties of zeolite depend on the number of acid centers, i.e. the presence of hydrogen ions, and therefore, in this paper the possibility of reducing the sodium content in the pores of high silicate zeolite ZSM-5 with the modulus (SiO2 / Al2O3 = 1000) will be investigated, by applying ion exchange with hydrochloric acid. Chemical analysis of samples before and after ion exchange, and application of instrumental methods of X-Ray diffraction, FT-IR spectroscopy, and SEM analysis monitored the influence of the quantity of hydrogen ions on the chemical composition and the structure of ZSM-5 zeolite at different acid concentrations and at different exchange times. It has been shown that the application of ion exchange with hydrochloric acid can reduce the sodium content in zeolite. Even with the application of 5% HCl for 6 hours, the content of sodium in the zeolite is reduced by over 98%. A similar effect is achieved by applying more concentrated hydrochloric acid solutions for a shorter ion exchange time. By prolonging the ion exchange time, there are no significant changes in terms of the final ion exchange. On the other hand, the application of HCl solutions of higher concentrations leads to a slight decrease in the aluminum content in the zeolite, which may partially affect the structural stability of the zeolite. The results obtained by FT-IR and SEM analysis and X-Ray diffraction confirm the possibility of ion exchange with hydrochloric acid, without significant changes in the crystal structure of the zeolite.

Kaolins of high iron-content as photocatalysts: Challenges of acidic surface modifications and mechanistic insights

Surface modifications of natural kaolins (Felsőpetény, Hungary) with iron-content up to 9% (m/m) were carried out by varying acid concentration (5, 8 and 11 M HCl) and treatment time (1, 3 and 6 h) in order to evaluate the influence of treatment conditions on photochemical activity, amount of low coordinate Al defect sites, porosity, and acid/base character of the surface. Nitrogen adsorption and temperature-programmed desorption measurements showed that the acid treatment slightly reduced the pore volume and the surface area, while the average pore diameter and the number of acidic centers were increased. Solid-state 27Al NMR spectroscopy demonstrated the presence of mechanistically relevant Al defect sites. The distribution, concentration, and particle size of the iron oxide co-minerals were found to be influential to photochemical activity. Greater than 90% photochemical degradation efficiency of oxalic acid can be achieved by acid-treated samples upon 3 h exposure to 8 M HCl solution with good reproducibility.

Sulfated Halloysite Nanoscrolls as Superacid Catalysts for Oligomerization of Hexene-1

Possibility of creating superacid catalysts on the basis of sulfated hydrosilicate nanoscrolls with halloysite structure and catalytic activity of the resulting materials in the model reaction of hexene-1 oligomerization were studied. The sulfation was performed with sulfuric acid solutions at concentrations of 0.25–1 M. The number of acid centers on the surface of the scrolls decreases with increase of the scid concentration, most probably due to the selective dissolution of the aluminum oxide sheet. In this case, the composition of the reaction products also changes, with the content of hexene-1 isomers increasing as compared with the oligomers. It was possible to obtain, at the lower boundary of the concentration range, an increased content of heavy fractions in the oligomerization products.

Effect of MgO Addition on the Properties of Ni/Al2O3 and Its Catalytic Activity in Hydrogenation of N-(2`,3`-dimethoxy benzyl)-3,4-dioxy-methylene-phenylethylamine

the nickel based alumina-supported catalysts modified or promoted by magnesia were prepared by wet impregnation and successfully used for hydrogenation of N-(2`,3`-dimethoxy benzyl)-3,4-dioxy-methylene-phenylethylamine, the 20Ni-6MgO/74Al2O3 samples exhibits the highest BET surface area, the largest pore volume, and the largest pore diameter in all of the samples such as Ni/Al2O3 and 20Ni-xMgO(80-x)Al2O3 excepted the highest BET surface area of Ni/Al2O3. the average pore diameter of the 20Ni-6MgO/74Al2O3 samples were two times as large as Ni/Al2O3, it was indicated that the function of expanding role or the mesoporosity was increased by addition of MgO, and MgO might be regarded as pore-enlarge agent for the bare Al2O3 support and benefit for the transport of large molecules reactants and products The weak formation of MgO-Al2O3 and MgO-NiO solid solution as a result of competing interaction of MgO with Al2O3 support and NiO precursors restrained the strong interaction of NiO species with Al2O3 support, which favored the dispersion of active Ni centers and improved the reducible degree of NiO species on the surface of the catalysts. The improvement of basicity or the decrease in the number of acid centers in the catalysts avoid the secondary reactions, and subsequently resulted in high catalytic activity. The utilization of meso-porous 20Ni-6MgO/74Al2O3 for catalytic hydrogenation of N-(2`,3`-dimethoxy benzyl)-3,4-dioxy-methylene-phenylethylamine(Shiff’s base) with the highest selectivity of 99.70% and yields of 94.36% implied that the instead of Raney Ni was feasible.

Adsorption properties of palladium particles supported on aluminum oxides with varied acidity in hydrogenation of butadiene-1,3

Influence exerted by acid-base modifiers of aluminum hydroxide on the texture and acid characteristics of the aluminum oxide support was studied. The effect of the modified aluminum oxides on the charge state of the active component of aluminum-palladium catalysts was determined. It was found that catalysts based on a support with high concentration of strong acid centers are characterized by low conversion of butadiene-1,3 due to the firm chemisorption of the diene on electron-deficient palladium particles. Their formation is due to the strong metal-support interaction. By contrast, catalysts synthesized with supports having a small number of acid centers provide a high conversion of butadiene-1,3.

Effect of coke formation on the transformations of butylenes on a high-silica zeolite catalyst

The transformations of butylenes on a high-silica zeolite catalyst (HHSZC) at 423–773 K were studied. Liquid reaction products formed above 443 K. The liquid phase of the reaction products mainly consisted of aliphatic hydrocarbons at 448–523 K and aromatic hydrocarbons at 623–723 K. The catalyst activity in the formation of liquid products did not show itself until 4–6% consolidation products (CPs) accumulated on its surface. DTA and gravimetric studies showed that CPs were nonuniform in composition. Some of the CPs were removed by decomposition under nitrogen, while others by oxidation with oxygen. It was inferred from ammonia adsorption data that the accumulation of CPs on the surface of HHSZC led to a decrease in the number of acid centers.

The physicochemical properties of micro/mesoporous materials prepared by the recrystallization of zeolite BEA

Micro-mesoporous materials with various ratios between micro- and mesopores were prepared by the recrystallization of zeolite BEA in an alkaline medium in the presence of cetyltrimethylammonium bromide. The materials were characterized by X-ray diffraction, low-temperature nitrogen adsorption, and IR spectroscopy. Recrystallization under mild conditions did not cause substantial changes in the number of acid centers but increased the accessibility of acid centers to large-sized molecules because of the creation of mesopores. An increase in the degree of recrystallization caused first partial and then complete zeolite transformation into MSM-41 mesoporous aluminosilicate, which was accompanied by a decrease in the number of acid centers. The IR spectra were used to determine the diffusion coefficients of cumene in the initial and recrystallized samples. Recrystallization increased the diffusion coefficient by 3–4 times.

Characterization of the reactivity of a silica derived from acid activation of sepiolite with silane by 29Si and 13C solid-state NMR

The mechanism of the reaction between a silica sample coming from acid treatment of sepiolite (denominated Silsep) and an organosilane, namely bis(triethoxysilylpropyl)tetrasulfane (TESPT), has been evaluated by solid state NMR spectroscopy, being compared with the silanization reaction of a commercial silica. The effect of the silane concentration and temperature on the course of the reaction was considered. Experimental results indicate that the silanization reaction is more effective in the case of Silsep, favoring both the reaction of silane molecules with the filler surface and the reaction between neighboring silane molecules. This different behavior is attributed to structural factors, moisture, and number of acid centers on silica surface. Environmental scanning electron microscopy (ESEM) was used to deposit micrometric water drops on the surface of these samples and to evaluate the proportion and distribution of the organophylization process.

Surface acidity diagnosis and catalytic activity of AlSBA materials obtained by direct synthesis

The objective of this work is to give a characterization of aluminosilicate AlSBA materials in particular with respect to acidity and the nature of acid sites. The number of acidic centers and the strength of these sites are determined by ammonia adsorption and temperature-programmed desorption (TPD). The Brönsted acidity of AlSBA is also assessed by measuring the relative intensity of the laser-induced fluorescence (LIF) quinolinium ion band peaking at 390 nm when quinoline is adsorbed on the surface of the material. When Lewis acid sites are present, a complex formed between these sites and quinoline is observed. The AlSBA samples showed a high and durable activity in the cumene cracking.

Thermogravimetric Analysis for Acidity Determination in Pillared Clays

Thermogravimetric analysis of pyridine adsorption was applied to study the acidity at different temperatures of clays pillared with Al pillars and mixed Al-Ga pillars, in relation to the starting montmorillonite. These results were compared with those obtained by means of a pulse-chromatographic technique. The pillaring process produces a large number of acid centers in the samples. Al-Ga-PILC has a higher acidity than Al-PILC.

Acid activation of a Spanish sepiolite: II. Consideration of kinetics and physico-chemical modifications generated

AbstractSepiolite from Vallecas (Spain) was treated with HCl solutions (1.25; 2.5; 5.0; 10.0 and 20.0 wt%) at 25°C for 2, 6, 24 and 48 h. The solutions obtained after washing the acid treated samples were analysed and a kinetic model was applied to the cation dissolution data. The development of porosity and number of acid centres was studied. The experimental conditions which yield products with the highest surface activity were established. The size of the silica fibres, obtained after the treatment, decreases when the intensity of the treatments is increased. These characteristics are important for the adsorptive properties and the catalytic potential of a solid.

Activity and surface characteristics of several alkali-doped iron catalysts for nttrile hydrogenation

Abstract A non-doped and several alkali (Na, K, Cs)-doped iron catalysts were prepared by reduction of the impregnated iron oxide. The surface characterization of the catalysts was accomplished by use of BET, XRD, XPS, and surface acid determination. The surface areas of the catalysts decrease with the increase of the alkali atomic volume (BET). Alkali migration towards the surface is always found (XPS). The surface acid determination shows an increase in the number of acid centres per m2 of catalyst with the atomic volume of the alkali promoter. The curves of base adsorption show the presence of acid sites of different strengths in all the reduced catalysts. The characterization results are correlated with those of catalytic activity for hydrogenation of adiponitrile at 200–250 °C. The results indicate correlations between conversion and the type and concentration of surface acid sites.

Modification of the surface acidity of γ-alumina

A non-doped and several alkali (Na, K, Cs)-doped iron catalysts were prepared by reduction of the impregnated iron oxide. The surface characterization of the catalysts was accomplished by use of BET, XRD, XPS, and surface acid determination. The surface areas of the catalysts decrease with the increase of the alkali atomic volume (BET). Alkali migration towards the surface is always found (XPS). The surface acid determination shows an increase in the number of acid centres per m2 of catalyst with the atomic volume of the alkali promoter. The curves of base adsorption show the presence of acid sites of different strengths in all the reduced catalysts. The characterization results are correlated with those of catalytic activity for hydrogenation of adiponitrile at 200–250 °C. The results indicate correlations between conversion and the type and concentration of surface acid sites.

Properties of modified aluminas

Surface acidity and basicity, and catalytic activity and selectivity in 1-butanol dehydration at 573 K were measured with pure alumina and with aluminas modified by the addition of 0.6–4.8 wt % of selected ions. The number of acidic centres on the catalysts depended on the introduced ion in the order SO 2− 4 &> F − &> PO 3− 4 &> Cl − &> Zn 2+ &> none &> Mg 2+ &> Na +. The activity of the catalysts, the rate and selectivity of primary olefin formation and the extent of 1-butene isomerization correlated with the total acidity of the catalysts. The other reactions of the network were less sensitive or insensitive to catalyst acidity. The primary olefinic product of 1-butanol dehydration was 1-butene on an catalysts and, in the products of its isomerization, the thermodynamically less stable cis-2-butene always prevailed. It seems that only the activity and relative reaction rates, but not the mechanism of the reactions, are affected by the presence of the above ions in alumina.

Catalytic isomerization of olefins on BeO 3. Adsorption of pyridine on the catalyst

The number of acidic centers on BeO increases with increasing calcination temperature, i.e. with increasing dehydroxylation. Upon calcination above 973 K, the strongest acidic centers disappear. The majority of acidic centers seems to be of the Lewis type.

Heat of adsorption of pyridine on synthetic faujasites

1. At 30° pyridine is adsorbed with close heat values on all of the studied zeolites of type X and Y. Neither replacing the Na+ ions by Ca2+ nor decationization affects the magnitude of the heat values. 2. The method of poisoning zeolite catalysts with pyridine cannot be used to determine the number of acid centers in view of the absence of selective adsorption on these centers.

Chemisorption of ammonia on aluminosilica gels

1. The chemisorption of ammonia on aluminosilicate catalysts was studied to determine the number of active centers participating in the catalytic process. The amount of chemisorbed ammonia decreases with increasing temperature according to an empirical equation. 2. On samples poisoned with sodium, ammonia is chemisorbed at low temperatures not only on the acid centers, but also on the cationic centers. To determine the acid centers only high-temperature chemisorption should be used. 3. The method of determining acid centers according to the chemisorption of ammonia is free of subjective errors, which are unavoidable in the determination of the transition of the nonionized state of the indicator to the ionized state. Another advantage of the method is the fact that the determination is conducted under conditions closer to those under which the catalyst exists during the chemical reaction (high temperatures, gas phase). 4. The use of the chemisorption of ammonia to determine the number of centers leads to the same results as other methods, namely, to a linear dependence of the catalytic activity on the number of acid centers and to constancy of the activities of the individual centers, regardless of the method of their preparation and the total surface of the sample.

Investigation of the relationship between the acid nature of zeolites and their catalytic activity in reactions of dehydration of pentanol-1 and isomerization of pentene-1

1. In. the reactions of dehydration of pentanol-1 and isomerization of pentene-1, the catalytic activity of zeolites with divalent cations (zinc and magnesium) is higher than that with univalent cations (sodium). Zinc forms of zeolite are more active than magnesium forms. 2. A correlation is observed between the catalytic activity of zeolite in the dehydration of pentanol-1 and the degree of cationic exchange of sodium for zinc or magnesium. 3. The catalytic activity of zeolite in the reaction of displacement of the double bond of pentene-1 is a linear function of the degree of cationic exchange. 4. When the degree of cationic exchange is increased, the number of acid centers of the zeolites increases proportionately. The distribution of the centers according to their maximum acid strength varies little in this case. 5. There is a direct correlation between the activity of zeolites in the reactions of dehydration of alcohol and isomerization ofα-olefin and their acidity, determined by the method of indicator titration. 6. The acid nature of the activity of zeolites is also confirmed by the deactivating action of pyridine.