Acid Properties Of Zeolites Research Articles

IR studies of the formation of ammonia dimers in zeolites TON

Ammonia may be a very convenient probe molecule to study the acid properties of zeolites, the pores of which are so narrow that they are inaccessible to pyridine. Ammonia molecules interact with acidic hydroxyls forming ammonium ions, however upon the neutralization of acidic hydroxyls next introduced ammonia molecules can react with ammonium ions (which also act as Brønsted acid sites), forming dimers NH 4 +·NH 3. If ammonia is adsorbed at low temperatures (room temperature or less), ammonia can react with ammonium ions even before the neutralization of all acidic hydroxyls. Based on the results of our IR studies of formation and decomposition of dimers in zeolite TON optimal experimental conditions of IR quantitative studies of Brønsted acid concentration with ammonia were performed. The concentration of Brønsted sites determined by this method in zeolites TON of various Si/Al was practically the same as the values calculated from chemical analysis.

Zeolites-catalyzed Alkylation of Isobutane with 2-butene: Influence of Acidic Properties

The influence of acidity properties of zeolites on the alkylation of isobutane with 2-butene was studied using ultrastable Y (USY) zeolites. Most zeolites possessed almost comparable hydrogen transfer activity except for USY-30, which has the lowest acid site density. On the other hand, the strength of acid sites increased with degrees of dealumination. During the alkylation, a maximum in catalytic activity for certain Si/Al ratios were observed. This behavior is believed to be a result of a balanced level of acidic properties between the concentration and strength of Bronsted acid sites. It can prohibit the formation of heavy carbonaceous materials that deactivate the zeolites.

Liquid-phase alkylation of phenol with t-butanol over various catalysts derived from MWW-type precursors

Three types of MWW-based catalysts (MCM-22, MCM-36 and ITQ-2) were studied in the liquid-phase alkylation of phenol by tert-butanol (TBA). Ammonia adsorption calorimetry was applied to investigate the acid properties of zeolites with different framework topology and crystallinity. It was established that acid properties of MCM-22 family depend mainly of the solid Al-content (the Si/Al ratio varied from 9.1 to 46.0). Delamination of MCM-22 precursor (MCM-22(P)) leading to ITQ-2 results in the decrease of the total acidity and the increase of the intermediate acid sites concentration, while the pillaring process yielding MCM-36 affected mainly the total concentration of acid sites, the acid strength distribution being similar to the corresponding MCM-22 zeolite. The catalytic activity and selectivity of zeolite-based catalysts are discussed. The most active catalyst for the studied reaction was MCM-22 B (Si/Al ~ 15). The results obtained with MCM-22 as catalyst revealed that, despite the expectations due to the presence of 10-MR pores in the structure of this zeolite, no enhanced selectivity to p- tert-butyl phenol ( p-TBP) is observed. This has been rationalized by the assumption that most of reactions occurring in this system take place on acid sites at or close to the external surface. This assumption is also supported by the similar catalytic selectivity of the MWW-derived structures (viz. MCM-36 and ITQ-2), which expose a higher concentration of external pockets. However, it is noticeable the high catalytic activity of MCM-22 zeolites for the liquid-phase alkylation of phenol by TBA that is comparable to the gas-phase activity of large pore zeolites like as HY or HM. To cite this article: E. Dumitriu et al., C. R. Chimie 8 (2005).

The Removal of Direct Orange 39 by Pulsed Corona Discharge From Model Wastewater

Untreated wastewater from the dye industry and dyehouses cannot be directly discharged into the environment due to the high content of organic matter and intensive colouration, even with low concentrations of dye. In this paper, the application of a high voltage pulsed electrical discharge in the aqueous phase has been assessed for the dye degradation. Experiments were conducted in a batch reactor using model wastewater of the commercial water-soluble monoazo dye C.I. Direct Orange 39 (DO39). The effects of zeolite and ferrous sulphate in combination with the corona discharge were examined. Experiments were conducted for a range of process parameters including pH, conductivity, type and amount of zeolite, and ferrous sulphate concentration. A mathematical model to describe the kinetics of DO39 degradation in the corona reactor was developed. Aqueous phase pulsed streamer corona discharge as a method for coloured wastewater treatment showed very high effectiveness in the case of iron salt addition (Fenton's reaction). Low pH enhanced dye removal by corona in the absence of zeolite, thus implying that the acid properties of zeolites are important in dye degradation. Ecological parameters such as COD, TC, IC, TOC and IC50 measured before and after corona treatment showed that the treated wastewater can be discharged into the environment or reused as process water.

Effects of channel structures and acid properties of large-pore zeolites in the liquid-phase tert-butylation of phenol

The liquid-phase alkylation of phenol with tert-butanol in the presence of the H + form of the zeolites with FAU, BEA, and MOR topologies was investigated. Over dried catalysts, tert-butyl phenyl ether is the main product of alkylation. The accumulation of water molecules on the catalyst surface suppresses this reaction. The irreversible C-alkylation of phenol becomes the main reaction only after an induction period. The catalytic activity and selectivity are controlled by the porous structure and acid properties of the zeolite. The three-dimensional interconnecting pore system of Beta and USY zeolites showed a higher catalytic activity than the monodimensional system of pores of the MOR zeolites. As concerns the catalytic selectivity, the USY zeolites were selective for the ortho-butylation, whereas BEA zeolites were the most efficient catalysts for the para isomer forming.

Effect of Treatment with NaAlO2 Solution on the Surface Acid Properties of Zeolite β

The influence of treatment with an NaAlO2 aqueous solution of NaAlO2 on the surface acid properties of zeolite β was studied as a method to increase the number of both Brønsted and Lewis acid sites. The Hβ samples, prepared by treating with an aqueous solution of NaAlO2, were characterized by XRD, NH3-TPD, pyridine-IR, 29Si MAS NMR, and 27Al MAS NMR. The surface properties of the Hβ zeolite changed after treatment with the aqueous NaAlO2 solution: the number of strong and weak acid sites, the total amount of acid, and the Brønsted and Lewis acid sites increased. The treatment of zeolite β with an aqueous solution of NaAlO2 may realuminate an Si(0Al)A site by the isomorphous substitution of Al(OH)4− anions.

Adsorption characteristics of HCA(Hydrocarbon Adsorber) catalysts for hydrocarbon and NOx, removals under cold-start engine conditions

To meet the SULEV (Super Ultra Low Emission Vehicle) regulations, we have focused on the utilization of HCA (Hydrocarbon Adsorber) in order to adsorb the excess hydrocarbons emitted during a period of engine cold-start. As main recipes of HCA materials, many types of zeolite as well as a combination of alumina and precious metals were used. Representative physico-chemical factors of zeolite such as acidic and hydrophobic properties have been characterized and tried to find the optimum recipe of HCA materials. Among the acid properties of zeolites, the Si/ Al ratio has been found to be the most important factor to achieve higher hydrocarbon adsorption capacity.

Effect of catalyst deactivation on the acid properties of zeolites used for isobutane/butene alkylation

Coke formation during the isobutane/butene alkylation reaction over zeolites decreases the acidity and acid strength of the catalysts. Microcalorimetric measurements of the differential heat of pyridine adsorption and FTIR spectroscopy of adsorbed pyridine were used to probe the changes in the acid properties caused by the deactivation processes. Specifically, fresh and deactivated commercial acid catalysts such as REY, USY, and Beta zeolite were studied. The adsorption microcalorimetry and FTIR spectroscopy results demonstrated that USY has the strongest acid sites (both Brønsted and Lewis) and the highest concentration of strong sites followed by REY and then by Beta zeolite. This order is the opposite of that observed for the alkylation catalytic performance of these zeolites. In particular, it seems that having a high concentration of strong Lewis sites promotes catalyst deactivation. The deposits formed during deactivation have a strong paraffinic character, but evidence of olefinic species is also observed. The degree of unsaturation of the surface species formed increases from Beta to USY zeolite, implying that the presence of a high concentration of strong Lewis-acid sites promotes the formation of unsaturated compounds. Brønsted sites with intermediate acid strength appear to be the appropriate sites for maintaining good alkylation catalytic performance. The best catalytic performance and the slowest deactivation were achieved with Beta zeolite, followed by REY and USY with low sodium content. Only butene isomerization was observed for USY with high sodium content. For the active catalysts, the alkylation global reaction route dominates initially, but the amount of alkylation products decreases as the catalyst starts deactivating when oligomerization predominates; and, finally, the catalyst loses most of its activity and isomerization is the only reaction observed. The product distribution obtained suggests that, instead of authentic alkylation, the initial prevalent mechanism is polymerization followed by β-scission. Two deactivation models are proposed to explain the deactivation. The direct obstruction of the alkylation active sites by irreversible adsorption of coke or coke precursors and the indirect obstruction of the active sites by pore blocking or pore filling.

Characterization of the acidic properties of zeolites by means of temperature-programmed desorption (TPD) of ammonia

Desorption energy distributions were calculated for temperature-programmed desorption (TPD) of ammonia from H zeolites of different type by means of regularization. This method does not require any limiting assumptions about the distribution function. It could be shown that the desorption energy distributions obtained are nearly independent of the experimental conditions and therefore they should represent a suitable measure for the distribution of the strength of acidic sites. The calculated desorption energy distributions for the ammonia desorption from the isolated bridging SiOHAl groups of H zeolites of different type significantly differ from each other in shape. The increase of the desorption energy of the main range of the distribution functions correlates well with the increase of the average acid strength of the SiOHAl groups with decreasing Al content of the zeolites.

The modelling of nucleophilic and electrophilic additions to organometallic complexes using molecular graphics techniques.

A new formalism has been developed in order to evaluate intermolecular interaction energies for inorganic and organometallic complexes in the framework of the extended Hückel method. In order to provide the shortest possible response time on an interactive computer graphics facility, this model should require the minimum amount of computer time, which explains why approximate procedures are used to evaluate electrostatic, charge transfer and exchange repulsion components. When applying this model to typical examples of electrophilic addition reactions to organometallic complexes, it is found that it is essential to take account of charge transfer interactions, the electrostatic component alone being not sufficient, even qualitatively, for a proper description of the reaction mechanism. The results, presented as color-coded dot molecular surfaces, show a very good agreement with experiment as to the site of attack, namely (i) on metal for the electrophilic attack on Fe(cp)2, Fe(CO)5 and X(cp)(CO)2, X = Co, Rh; (ii) on the cp ligand for the nucleophilic attack on Co(cp)2+ and Rh(cp)2+; (iii) on bz for the nucleophilic attack on Fe(cp)(bz)+. Finally, modellizations of the nucleophilic attack on a coordinated olefin and of the relation between structure and acidic properties of zeolites are presented and discussed.

Hydroxyl groups and acid sites in NaZSM-5 zeolites studied by i.r. spectroscopy

The decomposition of TPA (tetrapropylammonium ions) in NaZSM-5 zeolites, the localization of OH groups and acid properties of zeolites were studied by i.r. spectroscopy. NaZSM-5 zeolite in which TPA are not decomposed contain small amount of 3738 cm −1 OH groups. The decomposition of TPA results in the formation of considerable amounts of 3738 cm −1 OH groups situated inside zeolitic channels. These OH groups are not acidic at the reaction with pyridine. NaZSM-5 zeolites contain Brönsted and Lewis acid sites the amounts of which depend on the composition of the zeolite.

Hydrogenation of Carbon Monoxide on CO Reducing Catalyst and Solid Acid (III). The Effect of the Pore Size and Acid Properties of Zeolite on the Product Distribution

Abstract Synthesis gas reactions over hybrid catalysts composed of Pd/SiO2 and zeolites and methanol conversion on the same zeolites were studied. The selectivities to aromatic hydrocarbons were about 30% for the methanol conversions over ZSM-5, de-aluminated mordenite, and de-aluminated Y type zeolite (DAY), which possess strong acid sites. When ZSM-5 or mordenites were combined with Pd/SiO2 and subjected to synthesis gas conversion, the selectivities to aromatic hydrocarbons were between 10% and 30%. However, the synthesis gas reaction over the hybrid catalyst composed of Pd/SiO2 and DAY, gave few aromatic hydrocarbons, but aliphatic paraffins with 2 to 6 carbon atoms. Over large pore zeolites like DAY, the diffusion of olefins is so quick that most of the olefins come out of pores to be hydrogenated to paraffins on the Pd/SiO2 catalyst. In contrast, because of the slow diffusion of olefins in the medium size pores of ZSM-5 or mordenite, olefins stay in the pores for long enough to form aromatic hydrocarbons on the strong acid sites before being hydrogenated.