External Surface Of Zeolite Crystals Research Articles

Influence of Localization of Acid Sites on Deactivation of Zeolite MFI in Oligomerization Process of Light Alkenes

A series of samples of zeolite MFI with the same morphology, but different concentrations of Bronsted acid sites, have been obtained by means of ion exchange. Methods of temperature-programmed desorption of ammonia (TPD NH3) and IR spectroscopy of absorbed 2,6-di-tert-butylpyridine have demonstrated that the samples differ in the ratio of acid sites in the bulk of zeolite and on the external surface. Stability of the samples against deactivation has been determined in the oligomerization reaction of butylenes using the accelerated deactivation test under severe conditions. It has been shown that the main contribution to deactivation is made by acid sites located on the external surface of zeolite crystals.

Humidity-tolerant H2O2 recombination platinum catalyst for mitigating hydrogen using silicalite-1 as support

Abstract Passive autocatalytic recombiner (PAR) applied in the nuclear plant containments is to use catalytic H2 O2 recombination to lower the explosion risk associated with hydrogen release. Deactivation by water vapor must be taken into account and overcome for the recombination catalyst. For supported metal catalyst, the nature of support plays an important role in such water-poisoning effect. In this work, the pure silica MFI zeolite, silicalite-1, was used as the support for Pt to enhance the humidity tolerance (Pt/sil). Steaming of silicalite-1 is an effective way to further enhance its hydrophobicity (sil-S) due to the healing of silanol defects. A serial of supports with different water adsorption ability (Al2O3 > SiO2 > SiO2-500 » sil > sil-S) were compared and their hydrophobicity is correlated with their stability in H2 O2 recombination. Over hydrophilic Pt/Al2O3 and Pt/SiO2, the activity declines continuously especially under wet conditions. Hydrophobic Pt/sil and Pt/sil-S are both stable in wet conditions. Because Pt nanoparticles are located on the external surface of zeolite crystals and silanol defects are mainly in the inner part of zeolite crystals, Pt/sil and Pt/sil-S exhibit minor difference in resistance to water vapor in spite of their different hydrophobicity.

Core–Shell Zeolite Y@γ‐Al2O3 Nanorod Composites: Optimized Fluid Catalytic Cracking Catalyst Assembly for Processing Heavy Oil

AbstractCompositions, structures, and distributions of the active phases in fluid catalytic cracking (FCC) catalysts have an essentially important impact on their porosities, acidities, and thus cracking capabilities for processing heavy oil fractions. Here, we report a facile approach for fabrication of core–shell zeolite Y@γ‐Al2O3 nanorod composites by controlling the attachment of boehmite nanorods on the external surface of zeolite crystals, and thus regulating the dispersion and combination of the active alumina matrix on zeolite components in the FCC catalyst. The synthesized discrete boehmite nanorods possess a large quantity of positive charges and can directly electrostatically interact with the negative surface of zeolite crystals, leading to the formation of core–shell zeolite Y@γ‐Al2O3 nanorod composites (nY@Al2O3). The optimized 4Y@Al2O3 composite (Y/γ‐Al2O3 weight ratio of 4) with zeolite crystals fully embedded by alumina nanorods shows enhanced textural properties, acidity accessibilities, and improved cracking ability of 1,3,5‐triisopropyl benzene. Taking it as the main active component, the produced FCC catalyst shows better contaminant resistant ability and especially higher gasoline yield in the cracking of heavy oil, which could potentially bring practical economic profits for the oil‐refining industry.

Oxidation of furfural in aqueous H2O2 catalysed by titanium silicalite: Deactivation processes and role of extraframework Ti oxides

Titanium silicalites (TS-1) with different Ti/Si atomic ratio (0.01–0.08) have been prepared, characterised by different techniques (X-Ray Diffraction, Transmission Electron Microscopy and Raman, Diffuse Reflection Infrared Fourier Transform and X-Ray Photoelectron spectroscopies), and their catalytic properties were investigated for the liquid phase oxidation of furfural with aqueous H2O2. For samples prepared with an initial Ti/Si>0.01, extraframework Ti oxide is present and become more important as Ti loading increases. Two types of extraframework Ti have been identified: (i) anatase Ti oxide located at the external surface of zeolite crystals and (ii) Ti oxide nanodomains occluded within the channels and cavities of the zeolite. The latter can block the access of reactants to the active framework Ti(IV) sites for samples prepared with an initial Ti/Si>0.04 and their removal results in an improvement in the selective oxidation of furfural to valuable products. Fouling by deposition of heavy by-products and Ti leaching were identified as two main causes of catalyst deactivation. The former can be reverted by removal of the deposits through calcination under air at 773K. Leaching affects both the extraframework Ti and the active framework Ti species. During the first runs of reutilization, the Ti leaching is severe, affecting mainly to extraframework Ti oxide. Initially this leaching may be positive because its removal leads to the elimination of species blocking the access of reactants to the active Ti species; but in the longer term, the continuous leaching of active framework Ti irreversibly deactivates the catalyst.

A ZSM-5 zeolite-based catalyst for oligomerization of the butane–butylene fraction

The effect of localization of acid sites and the nature of the modifier metal on the activity, selectivity, and operation stability of a ZSM-5 zeolite-based catalyst in the oligomerization of the butane–butylene fraction (BBF) has been examined. It has been shown that the selective poisoning of acid sites on the external surface of zeolite crystals reduces coking and increases the yield of the desired gasoline fraction. Introduction of a promoter metal insignificantly affects the catalytic properties of the zeolite. Among Zn, Ga, and La, gallium appears to be the best modifier, which provides an increase in the yield of the desired gasoline fraction by 0.9%. As a result of the study, a BBF oligomerization catalyst has been developed that ensures a threefold increase in the catalyst on-stream time and a 7% increase in the yield of the gasoline fraction as compared with its commercial counterpart.

The assessment of pore connectivity in hierarchical zeolites using positron annihilation lifetime spectroscopy: instrumental and morphological aspects

Recent studies demonstrated the power of positron annihilation lifetime spectroscopy (PALS) to characterise the connectivity and corresponding effectiveness of hierarchical pore networks in zeolites. This was based on the fractional escape of ortho-positronium (Ps), formed within the micropore framework, to vacuum. To further develop this technique, here we assess the impact of the positron implantation energy and of the zeolite crystal size and the particle morphology. Conventional measurements using fast positrons and beam measurements applying moderated positrons both readily distinguish purely microporous ZSM-5 zeolites comprised of single crystals or crystal aggregates. Unlike beam measurements, however, conventional measurements fail to discriminate model hierarchical zeolites with open or constricted mesopore architectures. Several steps are taken to rationalise these observations. The dominant contribution of Ps diffusion to the PALS response is confirmed by capping the external surface of the zeolite crystals with tetraethylorthosilicate, which greatly enhances the sensitivity to the micropore network. A one-dimensional model is constructed to predict the out-diffusion of Ps from a zeolite crystal, which is validated experimentally by comparing coffin-shaped single crystals of varying size. Calculation of the trends expected on the application of fast or moderated positrons indicates that the distinctions in the initial distribution of Ps at the crystal level cannot explain the limited sensitivity of the former to the mesopore architecture. Instead, we propose that the greater penetration of fast positrons within the sample increases the probability of Ps re-entry from intercrystalline voids into mesopores connected with the external surface of zeolite crystals, thereby reducing their fractional escape.

Hydroisomerization of Biomass Derived n-Hexadecane Towards Diesel Pool: Effect of Selective Removal External Surface Sites from Pt/ZSM-22

Abstract Influence of dealumination of zeolite ZSM-22 (Si/Al ratio of 45) by treating it with oxalic acid on its catalytic performance in n-hexadecane hydroisomerization reaction was studied. This reaction is an attempt in the direction of green and sustainable source of diesel via improving the cold-flow properties of deoxygenated vegetable oils. Pt (0.5 wt%) on ZSM-22 treated with 1 M oxalic acid afforded highest yields of the mono-branched paraffins. This improved is attributed to selective removal of active sites on external surface of zeolite crystals (responsible for undesired cracking reactions) using the bulkier dealuminating agent, oxalic acid. Thus, pore-mouth key-lock mechanism was brought to play the role to cause high selectivity to mono-branched isomers. Preferential external site deactivation was inferred from mesitylene cracking results. Effects of operating parameters such as temperature, and space velocity on product distribution also were studied. Also, kinetics of the reactions involved too has been in brief reported.

The role of acid sites induced by defects in the etherification of HMF on Silicalite-1 catalysts

Defects in Silicalite-1 (structure type MFI) generate acid sites in the form of (i) hydroxyl nests present inside the channels or (ii) Brønsted–Lewis acid pairs, associated with terrace step kink sites present on the external surface of zeolite crystals. The amount of the sites has been changed by controlling the thermal treatment and pH value during preparation, or by post-synthesis treatments (ionic-exchange, silylation). These sites have been quantified by NMR, XRD and FT-IR pyridine adsorption/desorption at room temperature and 150°C, and correlated with the catalytic etherification of 5-hydroxymethyl-2-furfural (HMF) with ethanol, a relevant reaction to produce diesel additives. The results show that the surface acidity of Silicalite-1 materials presents an heterogeneous distribution of Lewis and Brønsted acid sites with different degrees of strength. The amount and distribution of such active centers determine the catalytic performance in HMF etherification. It is also shown that adsorption of water, produced in situ by ethanol condensation, has a promoter effect at lower concentration and high temperature, due to the dissociation of S2R siloxanes bonds, and formation of Lewis and Brønsted acid centers with medium-high strength, responsible for the activation pathways of HMF conversion to 5-(ethoxymethyl)furfan-2-carbaldehyde (EMF) and ethyl-4-oxopentanoate (EOP), respectively.

Selective photocatalytic degradation of aquatic pollutants by titania encapsulated into FAU-type zeolites

The selective photocatalytic degradation of charged pollutants in water was achieved on titania encapsulated into FAU-type zeolites. The electrostatic attraction of cationic substrates and repulsion of anionic substrates by the negatively charged zeolite framework facilitated the selective photocatalytic degradation of charged substrates. The hybrid zeolite–titania photocatalysts were prepared through the ion-exchange method. The titania clusters were mainly well distributed within the cavities of FAU-type zeolites whereas no TiO 2 nanoparticles aggregates were observed on the external surface of zeolite crystals. The hybrid zeolite–titania photocatalysts were characterized by diffuse reflectance UV–visible spectroscopy, transmission electron microscopy, energy-dispersive X-ray analysis and X-ray photoelectron spectroscopy. The selective degradation of charged pollutants was investigated by employing three pairs of oppositely charged substrates. The comparison between the cationic and anionic substrates clearly showed that the degradation rates for the cationic substrates on the hybrid photocatalysts are markedly higher than those for the anionic substrates. Among the cationic substrates, the smaller cations such as tetramethylammoniums were preferentially degraded. This enabled the selective removal of cationic substrates among the mixture. Such a selective photocatalytic degradation of water pollutants may provide a useful strategy for the development of economical photocatalytic process by targeting only the most recalcitrant pollutant.

Aromatic Transformations Over Mesoporous ZSM-5: Advantages and Disadvantages

Catalytic behavior of mesoporous ZSM-5 was investigated in toluene disproportionation, toluene alkylation with isopropyl alcohol, and p-xylene alkylation with isopropyl alcohol to understand the effect of the presence of mesopores. Three ZSM-5 zeolites (conventional one and two mesoporous differing in the mesopore volume) having similar Si/Al ratio were synthesized and characterized as for their acidity (internal and external) as well as their micropore/mesopore volume. No substantial differences among three samples were observed as for the type and concentration of Bronsted and Lewis acid sites as well as their location in zeolite channels or on external surface of zeolite crystals. Conversions of toluene and p-xylene increased with increasing volume of mesopores in ZSM-5 zeolite while the selectivity to individual products depended on the type of reaction. In general, selectivity to sum of xylenes in toluene disproportionation, sum of isopropyltoluenes in toluene alkylation and to 1-isopropyl-2,5-dimethylbenzene in p-xylene alkylation increased due to a shorter contact time molecules spent in mesoporous ZSM-5 catalysts. In contrast, para-selectivity decreased as diffusion pathways were shorten due to the presence of mesopores.

Sonochemical synthesis of silver nanoparticles in Y-zeolite substrate

Silver ions can be reduced by 24 kHz ultrasonic waves in ion-exchanged Ag+–Y zeolite. In this research, silver ions were introduced into the nano-porous (1.2 nm) zeolite lattice by ion-exchange route. After the reduction process, silver nanoparticles were placed in the cavities, with a size of about 1 nm and also on the external surfaces of the zeolite, with sizes about less than 10 nm. Fast and simple lab-scale reduction of silver ions in the zeolite is important for researchers who work on catalytic properties of metallic silver–zeolite. Several reduction methods have been reported but reduction by ultrasonic waves is a new, simple, and size-controllable method with a high practical value which does not need any complicated facilities. In a sonochemical process, a huge density of energy is provided by the collapse of bubbles which formed by ultrasonic waves. The released energy causes the formation of reducing radicals that consequently reduce the silver ions. It is concluded that the higher silver content may result in the formation of larger silver crystals on the external surface of zeolite crystals. Also, the addition of 1-propanol and 2-propanol to the aqueous reaction medium does not cause better reduction. In addition, increasing the irradiation time and ultrasonic power does not affect the silver crystal growth significantly but the extent of silver ion reduction increases when the power of ultrasonic waves increases. All samples were irradiated under the same ultrasonic conditions. The samples were analyzed by XRD, EDS, SEM, and TEM.

Quantification of the mass-transfer coefficient of the external surface of zeolite crystals by molecular dynamics simulations and analytical treatment

Analytical treatment of the problem of molecular transport through the external surface of zeolite crystals was applied to develop an expression for the mass-transfer coefficient of the external crystal surface ( α) under the conditions of sorption equilibrium. The underlying model used in the derivation of this expression takes into account the possibility of strong adsorption of sorbate molecules on the external crystal surface. The new expression allows estimating the value of α if the following quantities are known: (i) the sorbate concentrations on both sides of the external crystal surface and, (ii) the probability for a sorbate molecule located on this surface to enter the zeolite lattice. The validity of the expression for α was confirmed by molecular dynamics simulations of the tracer exchange of methane molecules between the membrane of a silicious A-type zeolite and the surrounding gas phase.

Influence of zeolite crystals external surface modification on toluene para- disproportionation selectivity

A series of catalysts with the deactivated external surface on the basis of HZSM-5 have been prepared by extracting aluminium atoms from zeolite crystals or by covering this zeolite surface by the products of tetraethoxysilane thermal destruction. The external surface deactivation has been found to be not the main factor of increasing the para-selectivity in the reaction of toluene to benzene and p-xylene disproportionation: decreasing the diffusion of o- and p-xylene from the catalyst pores on the external surface of zeolite crystals seems to be as the determinant.

Coke Alternate Movement in Faujasite Based Catalysts Deactivated from Butene Alkylation

Background: Examples of alternate dynamics phenomenon observed in zeolites were listed. A special attention was paid to coke alternate dynamics phenomena in deactivated HZSM5 zeolite catalysts and shortcomings of its interpretation. Present investigation deals with a coke movement of different vectors observed in deactivated catalysts based on X faujasite. Methods: Two catalysts on the base of X zeolite by means of isomorphic partial substitution of aluminium atoms for silicon and ion exchange were synthesized. Catalysts were deactivated in isobutane with butenes alkylation in flow-circulating system and then were divided into three (catalyst 1) and two (catalyst 2) portions, which were aged for different time. Coke on samples obtained was subjected to investigation by the method of discrete-successive coke microoxidation. Results: Dynamics of coke microoxidation in the samples of catalysts was obtained. Differences in such dynamics for variously aged samples were treated as indicating the alternate movement of coke in zeolite structure. Conclusion: The interpretation of the nature of observed coke alternate movement was proposed. It bases on being of coke on the external surface of zeolite crystals by turn as continuous layer, which fully envelop the crystals, and as separate clusters. These states differ in chemical potential. Such difference is considered as the driving force of coke transformation from one form to another and thus a coke alternate movement in catalysts deactivated. Keywords: Chemical potential, Clusters, Coke, Coke alternate dynamics, Disperse particles, Zeolite catalysts deactivated.

Propane as alkylating agent for benzene alkylation on bimetal Ga and Pt modified H-ZSM-5 catalysts: FTIR study of effect of pre-treatment conditions and the benzene adsorption

FTIR spectroscopy was used to study the influence of pre-treatment on OH groups of H-ZSM-5, Ga-ZSM-5, Pt-ZSM-5 and GaPt-ZSM-5 catalysts and the confinement of benzene molecules in these catalysts pre-treated under different conditions. Types of the hydroxyl groups in GaPt-ZSM-5 and Ga-ZSM-5 are very sensitive to the pre-treatment of the samples. Besides the IR bands at 3745, 3665 and 3612 cm−1 assigned to terminal silanol Si–OH, OH group attached to extra-framework aluminum and bridged Si–(OH)–Al groups, respectively, gallium loaded samples show additional IR band at 3699 cm−1 which has been attributed to OH groups attached to Ga, such as Ga2(OH)xO3−x. Dose-dependent measurements have shown that upon benzene loading of 1.7 molecules/u.c. (m/u.c.) for metal modified H-ZSM-5, the reagent molecules are located in the channel intersection. Above 1.7 m/u.c., part of benzene molecules enter the straight or sinusoidal channels and form pseudo liquid phase. The formation of pseudo liquid phase begins at low benzene loading for H-ZSM-5 zeolite. With increasing concentration of benzene molecules in zeolite channels, each molecule undergoes perturbation from the surrounding molecules and framework, which leads to a modification in electronic state and in the dipole moment of different vibrations. Abnormal adsorption isotherms for pure zeolite and gallium loaded samples have been correlated with this modification. This effect is not observed for Pt loaded samples, probably due to possible adsorption of benzene molecules on platinum particles dispersed on the external surface of zeolite crystals. The present paper provides important information on the effect of pre-treatment on the state of active species and the confinement and interaction of benzene molecules in a series of modified H-ZSM-5 catalysts.

ChemInform Abstract: Selective Catalytic Reactions at the External Surface of Zeolite Crystals

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Selective Catalytic Reactions at the External Surface of Zeolite Crystals

The arrangement of the pore openings serves as a template for the positions of the branchings in the isomerization of n‐alkanes—this is the explanation proposed by Martens et al. (Angew. Chem. Int. Ed. Engl. 1995, 34, 2528) to rationalize their results in the isomerization of n‐heptadecane on the zeolite ZSM‐22. This reaction would thereby be the first example of key–lock catalysis on the external surface of a zeolite.