Shape Selectivity Effects Research Articles

Zeolite HNU-87 : synthesis, characterization and catalytic properties in the shape-selective conversion of methylnaphthalenes

The synthesis of the high-silica zeolite NU-87 and its physicochemical characterization are reported. The catalytic properties of zeolite HNU-87 were investigated in the conversion of 1- and 2-methylnaphthalene. Both isomerization to the corresponding methylnaphthalene and transalkylation to naphthalene and dimethylnaphthalenes are observed. The influence of reaction temperature and the modified residence time on the product distributions is reported and discussed in terms of shape selectivity effects in the peculiar pore system of NU-87. For comparison, typical results obained with zeolite HMCM-22, which possesses a similar pore architecture, are also included. The results of methylnaphthalene conversion over HNU-87 are compared to those obtained in the alkylation of naphthalene with methanol on the same zeolite catalyst.

Controlling Acidity and Selectivity of HY-Type Zeolites by Silanation

Silanation, which is the chemisorption of silane, allowed modification of the acidity and the pore size of HY zeolites. Nitrous oxide was used to stabilize the remaining SiH bonds after silanation by converting them into SiOH groups. In the case of 3-methylpentane cracking, the silanated oxidized samples exhibited a higher activity and a reduced deactivation rate at longer times on stream. Silanation of the HY zeolite without subsequent oxidation allowed almost total suppression of deactivation, especially in the disproportionation of ethylbenzene but also less markedly in the cracking of 3-methylpentane. This suppression of deactivation required avoiding hydrolysis of the SiH group under reaction conditions, and therefore the reactants needed to be absolutely free of traces of water. The maximum deposition of silica that could be reached in one silanation/oxidation step was about 10 wt% at 573 K. Higher temperatures were not possible because of the uncontrolled formation of polysilanes. Simultaneous silanation and oxidation, which means that the oxidant was already present during silanation, allowed controlled deposition degrees higher than 10 wt%. The high-silanated HY samples were found to exhibit shape selectivity in ethylbenzene disproportionation which was not observed on the parent samples. It was shown that pore blocking is the main reason for the decreasing activity with increasing silica deposition in the case of the high-silanated samples. The observed shape selective effects were comparable to those of ZSM-5-type zeolites.

Shape selective methylation of biphenyl within zeolites: An example of transition state selectivity

Shape selective alkylation of binuclear aromatic compounds was studied using the methylation of biphenyl as a test reaction. The slurry phase reaction was carried out in a stainless steel autoclave using n-heptane as a solvent. By simultaneously varying the zeolite catalyst type and the alkylating agent shape selective effects to the least bulky para-isomers 4-methyl and 4,4′-dimethyl-isomers were observed. Alkylation using methanol turned out to be unselective, transalkylation of biphenyl using polymethylbenzenes and an ultrastable faujasite type zeolite produced mainly 4- and 4,4′-isomers. This effect was due to a supposed transition state selectivity. By reducing the amount of the acid sites on the outer as well as the inner surface of the zeolite catalyst through a treatment with phosphorous containing compounds and with water, undesired side reactions were suppressed and the yield of para-products was enhanced. Although the catalysts suffered deactivation during the reaction because of formation of carbonaceous residues, which mainly consisted of irreversibly adsorbed reaction partners, they could nearly be completely reactivated by burning these residues in a stream of synthetic air. To illustrate the potential industrial applicability of the reaction, the synthesis of the required alkylating agents polymethylbenzenes starting from xylenes and methanol was investigated and based on these results a possible 4,4′-dimethylbiphenyl production process was outlined.

Particular selectivity of m-xylene isomerization over MCM-41 mesoporous aluminosilicates

While over silica alumina and non-shape selective zeolite catalysts such as FAU, m-xylene isomerizes at similar rates into o-and p-xylenes as expected from the classical intramolecular mechanism, over MCM-41 mesoporous aluminosilicates o-xylene is preferably formed ( o/p≥2.5). A bimolecular pathway involving m-xylene disproportionation followed by transalkylation between trimethylbenzenes and m-xylene would be responsible for this preferential isomerization into o-xylene. In agreement with this proposal the addition of methylcyclohexane to the reactant causes an identical decrease in the rates of isomerization and disproportionation while over silica alumina the xylene isomerization which occurs through the classical intramolecular mechanism is not affected. The particular behaviour of MCM-41 aluminosilicates seems to be due to their unidirectional mesopore structure (shape selectivity effect) rather than their weak acidity.

Influence of Coke Deposition on Selectivity in Zeolite Catalysis

The selectivity of a complex reaction depends on a number of factors, such as the reaction mechanism, operating conditions, catalyst properties and catalyst deactivation. The present work discusses how the selectivity of a complex reaction depends on the formation of coke. For zeolite catalysts, changes in selectivity can be a result of intrinsic selectivity effects or shape selectivity effects. A method is suggested to analyze a complex reaction system with deactivation caused by coke formation, and different cases of selectivity change during deactivation of zeolites are discussed. Transition-state shape selective deactivation is proposed as a mechanism in addition to the deactivation mechanisms suggested by Guisnet and Magnoux (Guisnet, M.; Magnoux, P. Appl. Catal. 1989, 54, 1). By variation of the space velocity, the selectivities to the main products are measured as a function of conversion (optimum performance envelopes). Selectivities at given conversions can then be compared for results obtained with different contact time and with varying degree of catalyst deactivation (space velocity loops). Selective or nonselective deactivation is thereby distinguished. This type of selectivity plot is applied to two different types of reaction, i.e. ethene oligomerization over HZSM-5 and methanol conversion to light olefins (MTO) over SAPO-34. The selectivity of ethene oligomerization was affected only by the decrease in conversion due to coke formation; hence, this is an example of nonselective deactivation. Selective deactivation was found for methanol conversion over SAPO-34.

Thioether oxidation by hydrogen peroxide using titanium-containing zeolites as catalysts

Titanium substituted ZSM-5 (TS-1) and BEA (Ti-beta) are investigated as catalysts for the selective oxidation of thioethers with diluted H 2O 2, in a protic solvent, at 303 K. The performances of TS-1 and Ti-beta samples are analyzed in relation with structure of thioethers, nature of solvent and shape selectivity effect of catalysts. A mechanism for organic sulfides oxidation on Ti-containing zeolites is proposed.

Isomerization of dimethylnaphthalenes over zeolites

A dimethylnaphthalene (DMN) isomer mixture from a reforming unit was reacted at 350°C and atmospheric pressure over H-mordenites (Si/Al ranging from 5 to 100), a partially decationated NaY (Si/Al=2.4) and a slightly dealuminated HY (Si/Al=5), with the aim of increasing the 2,6- and 2,7-DMN content by isomerization. The best results were obtained on H-mordenite with Si/Al=10, where shape selective effects made possible to double the amount of the valuable isomers while limiting to a negligible extent the side reactions of disproportionation and dealkylation.

Methane activation without using oxidants over supported Mo catalysts

The catalytic properties of Mo/HSAPO-34, Mo/H-ZSM-5 and Mo/HY catalysts for methane conversion without using oxidants were compared. Mo/H-ZSM-5 catalyst has the best performance in terms of catalytic activity and stability. This catalyst is very selective for benzene production and the benzene yield is the highest of the three catalysts studied. The catalytic activity and benzene selectivity over the Mo/HSAPO-34 catalyst, however, decreased with increasing time on stream. As the catalyst deactivated, the selectivity to ethene increased. The Mo/HY catalyst is active for methane conversion but rapid deactivation of the catalyst resulted in negligible product yield. These results are discussed in terms of the shape-selectivity effect of the support taking into consideration the high reaction temperature. Preliminary studies on alkali metal doped Mo/HSAPO-34 catalysts showed no observed effect on catalyst deactivation.

Effect of pore size and aluminium content on the production of linear alkylbenzenes over HY, H-ZSM-5 and H-ZSM-12 zeolites: Alkylation of benzene with 1-dodecene

H-ZSM-5 (Si/Al = 15 and 30), H-ZSM-12 (Si/Al = 80) and HY zeolites (Si/Al ratios in the range 2.7-26.4) were evaluated in the benzene alkylation reaction with 1-dodecene for linear alkyl-benzene-LAB production. A series of dealuminated Y-zeolites was prepared under strictly controlled conditions by treatment with silicon tetrachloride. The materials thus obtained were washed and exchanged with ammonium chloride. The catalysts were characterized by X-ray diffraction, infrared spectroscopy of the framework vibrations, 29Si and 27Al-nuclear magnetic resonance. Temperature-programmed desorption of ammonia and elemental analysis. H-ZSM-5 and H-ZSM-12 catalysts showed very low activity. H-ZSM-12 zeolite exhibits both product and transition state type shapeselectivity, favouring the formation of the least bulky 2-phenyl isomer. With HY-zeolites, the alkylation activity increased linearly with the number of lattice Al per unit cell over a range of Si/Al ratios from 2.7 to 26.4, so a practically constant turnover frequency is obtained. The HY-zeolites also gave 97–98% selectivity towards LAB and produced branched dodecene, traces of branched alkylates and dialkylbenzenes as by-products. Neither dialkyltetralins nor dialkylindanes were found among the products. HY-zeolites did not exhibit shape-selectivity effects. The apparent activation energy was 15 kcal/mol. Small changes in the low Na content of the zeolites were responsible for considerable increases in activity. Dealuminated HY zeolites showed the highest selectivity towards 2-phenyldo-decane. In this case we suggest that the alkylation reaction competed effectively with the migration of the double bond across the chain, so that some of the initial alkylcarbenium ions formed alkylated benzene without undergoing isomerization. The isomerization of the phenylalkane was not observed after alkylation.

Shape selective methylamines synthesis: Reaction and diffusion in a CMs-SiO-AlO composite catalyst

Product shape selective effects, to date observed only with zeolites and a few other crystalline molecular sieves, should be generalizable and thus useful in the design of other novel catalyst materials. To be a viable approach in general for a given reaction, the only requirement for product shape selectivity is that the molecules involved be sufficiently different in size that they transport through micropore structures at different rates. Here, it is reported for the first time that non-crystalline composite catalysts, consisting of silica-alumina and amorphous carbogenic molecular sieves, are able to produce mono- and dimethylamines from methanol and ammonia with higher selectivities than the silica-alumina catalyst, currently used in the commercial practice. With the standard catalyst at ∼ 85% methanol conversion a ratio of (MMA + DMA)/TMA = 1 results, while the new catalyst, designated CMS-SiO 2 -Al 2 O 3 , at the same conditions and methanol conversion, produces ratios of at least (MMA + DMA)/TMA = 2.7. Although the rate of conversion per gram on the new catalyst is only one-third that of the standard catalyst, this is more than adequate activity for a commercial fixed-bed process. A model that accounts for reaction on silica-alumina and diffusion in the molecular sieve carbon is described. By adjusting the diffusivities of the three amines good agreement between prediction and experiment is obtained.

Shape selective methylamines synthesis: Reaction and diffusion in a CMs-SiO 2-Al 2O 3 composite catalyst

Product shape selective effects, to date observed only with zeolites and a few other crystalline molecular sieves, should be generalizable and thus useful in the design of other novel catalyst materials. To be a viable approach in general for a given reaction, the only requirement for product shape selectivity is that the molecules involved be sufficiently different in size that they transport through micropore structures at different rates. Here, it is reported for the first time that non-crystalline composite catalysts, consisting of silica-alumina and amorphous carbogenic molecular sieves, are able to produce mono- and dimethylamines from methanol and ammonia with higher selectivities than the silica-alumina catalyst, currently used in the commercial practice. With the standard catalyst at ∼ 85% methanol conversion a ratio of (MMA + DMA)/TMA = 1 results, while the new catalyst, designated CMS-SiO 2-Al 2O 3, at the same conditions and methanol conversion, produces ratios of at least (MMA + DMA)/TMA = 2.7. Although the rate of conversion per gram on the new catalyst is only one-third that of the standard catalyst, this is more than adequate activity for a commercial fixed-bed process. A model that accounts for reaction on silica-alumina and diffusion in the molecular sieve carbon is described. By adjusting the diffusivities of the three amines good agreement between prediction and experiment is obtained.

Epoxidation of Lower Olefins with Hydrogen Peroxide and Titanium Silicalite

The epoxidation of lower olefins, catalysed by titanium silicalite (TS-1) under mild conditions, is reported. The reaction may be performed at near room temperature, in dilute alcoholic or aqueous solutions of hydrogen peroxide. In methanol C4-C8 linear olefins, allyl chloride, and allyl alcohol show fast reaction rates and high selectivities (72-97% on H2O2). The solvolysis of the oxirane ring and the oxidation of the solvent are the main side reactions. Yields and kinetics are decreased by increasing the chain length or the cross-section of the olefin (n-Cn > n-Cn+1, 1-hexene ⪢ cyclohexene), by electron-withdrawing substituents (1-butene > allyl chloride > allyl alcohol), and by solvents in the order methanol > ethanol > t-butanol. The rate of reaction also depends on the position and steric configuration of the double bond and on the branching, as a result of inductive and shape selectivity effects: trans 2-butene < iso-butene < 1-butene < cis 2-butene, 2-methyl-1-butene < 1-pentene, and 3-methyl-1-butene < 2-methyl-1-butene < 2-methyl-2-butene. The epoxidation occurs with retention of configuration. Basic compounds at low concentration do not slow the kinetics, but do improve significantly the yields, up to 97% in the epoxidation of 1-butene. At higher concentrations, TS-1 activity is decreased and eventually inhibited by bases. No effect on kinetics is exerted by tetrapropylammonium hydroxide. Catalytic activity is improved by acids. A heterolytic peracid-like mechanism is envisaged in the oxygen-transfer step. A five-membered cyclic structure, formed by a titanium hydroperoxo moiety Ti-OOH, and a protic molecule ROH at Ti sites, is proposed as the active species.

Shape-Selectivity Effect of Highly Reduced Cobalt-Exchanged Zeolite in the Fischer-Tropsch Reaction

Abstract Cobalt-exchanged zeolites, when reduced by sodium injection followed by treatment in dihydrogen at 450 °C, showed an enhanced production of long chain and linear form of hydrocarbons with a bimodally distributed non Schultz-Flory distribution. Long chain linear hydrocarbons of C12–C20 were believed to be produced on cobalt clusters located inside zeolite cage.

Catalytic alkylation of phenol with methanol: factors influencing activities and selectivities: II. Effect of intracrystalline diffusion and shape selectivity on H-ZSM5 zeolite

Six samples of H-ZSM5 zeolites, differing in the size of crystallites and acidity, have been tested as catalysts in the alkylation of phenol with methanol. We observed that the size of crystallites strongly influences activity, even for very small crystallites (220–280Å), by the intervention of intracrystalline diffusion. Kinetic runs have been performed on the most active catalyst. A moderate shape selectivity has been observed only in the formation of p-methylanisole and xylenols. Catalyst deactivation occurs through two mechanisms which are independent of the contact time, the first evolving exponentially and the second linearly. The interpretation of this behaviour and of related consequences on the reaction is discussed in the paper together with the reaction path model, the mechanism and the kinetics.

Hydrocarbon synthesis from CO hydrogenation over Pd supported on SAPO molecular sieves

Palladium impregnated on silicoaluminophosphate molecular sieves (SAPO-5, SAPO-11, SAPO-34) and alumina has been studied during CO hydrogenation. Pd/SAPO-5, which exhibited the highest turnover frequencies, initially produced only methane and oxygenates (methanol and dimethyl ether), but catalyzed significant C 2 + hydrocarbon formation under the same conditions after temporary increase in gas contact time or exposure to propylene. While high yields of C 2-C 4 alkanes were observed over Pd/SAPO-34, reaction over Pd/SAPO-11 and Pd/Al 2O 3 yielded only oxygenates. Methane yields increased significantly with higher temperature, particularly over Pd/SAPO-5. Higher H 2 partial pressures increased oxygenate production and decreased C 2+ fractions. Many similarities between the results of this work and findings from studies of Pd/ZSM-5 indicate that hydrocarbon synthesis over Pd/SAPO catalysts occurs via a bifunctional pathway, involving methanol synthesis on Pd and autocatalytic conversion of oxygenates responsible for C 2+ hydrocarbons on the SAPO molecular sieves. Differences in the product distributions from Pd-impregnated molecular sieves arise mainly from the effect of acidity and shape selectivity in methanol conversion on the SAPO molecular sieves.

Chemioselectivity in cinnamaldehyde hydrogenation induced by shape selectivity effects in Pt-Y zeolite catalysts

Platinum and rhodium particles encapsulated in Y-type zeolite are much more selective for the hydrogenation of cinnamaldehyde to cinnamyl alcohol than active charcoal-supported platinum and rhodium catalysts. The good selectivity is interpreted in terms of molecular constraints in the zeolite micropores which favour an end-on adsorption of the molecule via the C = O group on the encapsulated metal.

Selective catalytic transformations of methanol to C2-C3 hydrocarbons over the new zeolite type LZ 132

The small pore zeolite HLZ 132 exhibits, in comparison with other zeolites, an increased selectivity for the transformation of methanol to ethylene in the reaction temperature range 350–500 °C: the weight ratio of C2H4 to C3H6 in the products ranges between 1 and 4 at WHSV=2 h−1. Besides the effect of the reactant shape selectivity this fact may be interpreted by the participation of the asymmetrical methoxy groups in the surface as well as by proton-donor centres of lower acidity which do not catalyze the oligomerization of ethylene but which do the more basic molecule of propylene, thereby generating polyene-type coke.

Shape selectivity of zeolite EU-1 in reactions of aromatic hydrocarbons

The shape-selective properties of H-EU-1 in the isomerization of m-xylene and the methylation and disproportionation of toluene were studied and compared with those of H-ZSM-48. The presence of an one-dimensional ten-ring channel system together with deep side pockets off the channels in EU-1 leads to peculiar shape selectivity effects. The concentration of the para isomer in the isomerization products decreases in the order ZSM-48>EU-1>ZSM-5. The presence of large side pockets, in which bimolecular reactions such as xylene disproportionation can occur, leads to the following selectivity for isomerization versus disproportionation: ZSM-48>ZSM-5>EU-1. It is concluded that apart from the shape and size of the channel system, the presence or absence of pore intersections and cavities can also significantly affect the shape selectivity of medium-pore zeolites in catalytic reactions. It is suggested that shape selectivity effects in the isomerization of m-xylene can be used as diagnostic tools to reveal not only the size of the pores but also the presence or absence of cavities and the dimensionality of the pores in zeolites of unknown structure.

Synthesis and shape-selective properties of ZSM-22

Zeolite ZSM-22 was synthesized in the presence of diaminohexane. From the characterization of the synthesized material, it could be deduced that the organic compound acts as a pore-filling agent during synthesis. The H-form of the zeolite contains surface hydroxyl groups, which are identical with those found in H-ZSM-5 and other high-silica zeolites. The intracrystalline voids of ZSM-22 were characterized with catalytic test reactions. The constraint index was determined and the n-decane test applied. ZSM-22 catalytically behaves as a ten-membered ring zeolite, with an effective pore size slightly smaller than that of ZSM-5. As a result of shape-selective effects, n-decane isomerization products are mainly branced at the ends of the hydrocarbon chain.

Structural order and orientation of organometallic pillars on synthetic fluorhectorite and montmorillonite surfaces

Pillard clays with controlled interpillar distances have been prepared by systematically varying the pillar and clay charges in metal-sarcophagine cage complex/clay systems. In some cases there is very high crystallinity - 14 orders of c-axis diffraction having been observed. Adsorption isotherms have been determined that show that two separate shape-selective effects occur in these materials: (a) molecular sieving due to the interpillar gaps; (b) enhanced adsorption of hydrogen due to the pillar shape allowing penetration under the pillar. These effects are discussed by using molecular modeling calculations to simulate the porosity and available adsorbing surface.