Showing papers in "Studies in Surface Science and Catalysis in 1989"
TL;DR: In this article, the surface characterization of silicalite and titanium-silicalite is made by combined use of IR and UV-Vis reflectance spectroscopy, and the nature of the IR modes and of the electronic transitions associated with framework titanium is discussed in detail.
Abstract: Surface characterization of silicalite and titanium-silicalite is made by combined use of IR and UV-Vis reflectance spectroscopy. The nature of the IR modes and of the electronic transitions associated with framework titanium is discussed in detail. The perturbation caused by the adsorption of small polar molecules (H 2 O, NH 3 , CH 3 OH) on both the IR and UV-Vis spectra is discussed in terms of the formation of six-coordinated complexes by ligand insertion in the Ti(IV) coordination sphere. All the Ti(IV) is accessible to the interaction with the molecules.
392 citations
TL;DR: In this article, the structure of a TMA-Co-Mn-Ge-S phase was shown to have a tetrahedral framework with adamantine (Me4S6) secondary building units linked together by MeS3SH tetralayer units, and the substantial void fraction of the framework was filled with tetramethylammonium template cations.
Abstract: Crystalline microporous metal sulfides have been discovered. These novel materials are based on germanium(IV) and tin(IV) sulfide frameworks, and are synthesized hydrothermally in the presence of alkylammonium templating agents. Multinary metal thiogermanate frameworks have been synthesized which contain one or more of the following incorporated framework elements: Mn, Fe, Co, Ni, Cu, Zn, Cd, and Ga. X-ray powder diffraction patterns of these materials indicate that their structures are completely novel, spanning a family of at least twelve unique framework structure types which possess no analogues in microporous oxide chemistry. A single crystal x-ray study of a TMA-Co-Mn-Ge-S phase reveals a tetrahedral framework with adamantine (Me4S6) secondary building units linked together by MeS3SH tetrahedral units. The substantial void fraction of the framework, 0.58, is filled with tetramethylammonium template cations, 0.4 TMA per T-atom. Details of synthesis, compositions, and structures of this extensive new family of microporous sulfide materials are reported.
103 citations
TL;DR: The Co edge atoms in promoted Co-Mo (Ni-Mo) catalysts are present in two types of sites having square and tetragonal pyramidal coordination, respectively as mentioned in this paper.
Abstract: The nature of the active phases and sites, the reaction mechanisms, and the role of additives and promoters are discussed in the light of new spectroscopic and kinetic results. The active phases in promoted Co-Mo (Ni-Mo) catalysts are the socalled Co-Mo-S (Ni-Mo-S) structures. A description of these structures is given. It is proposed that the Co edge atoms in Co-Mo-S are present in two types of sites having square and tetragonal pyramidal coordination, respectively. These represent the free and occupied sites which interconvert during a catalytic cycle. The chemical and catalytic properties of the sites associated with the active phases are influenced by many parameters and changes in these may, for example, give rise to either Type I or Type II Co-Mo-S structures with different catalytic properties. Also the presence of an additive like P influences the properties of the surface sites as revealed by infrared and Mossbauer spectroscopies and kinetic studies. Simultaneous hydrodesulfurization (HDS), hydrodenitrogenation (HDN), and hydrogenation (HYD) studies have also yielded new insight. The results indicate that under typical reaction conditions the active sites are predominantly covered by atomic and not molecular species. Furthermore, in spite of the structural complexities it is found that for a given catalyst, it may often be sufficient to consider the HDS, HYD, and HDN reactions occurring on the same sites. This is in contrast to the conclusions from earlier studies. Infrared spectroscopy has also provided the first direct evidence for the existence of Bronsted acid sites in sulfided hydrotreating catalysts but the role of these sites in HDS, HDN, and HYD appears to be small. Based on the description of the active sites and their genesis, catalyst design criteria are discussed.
100 citations
TL;DR: In this article, a vapor phase Beckmann rearrangement of cyclohexanone oxime to τ-caprolactam over a high silicious ZSM-5 (Si/Al ≧ 1, 000) was studied in detail in relation to the hydrothermal synthesis of ZSM and to its surface silanol groups.
Abstract: A vapor phase Beckmann rearrangement of cyclohexanone oxime to τ-caprolactam over a high silicious ZSM-5 (Si/Al ≧ 1, 000) was studied in detail in relation to the hydrothermal synthesis of ZSM-5 and to its surface silanol groups. Two important factors were found to enhance the lactam selectivity; external surface area (ESA) and modification of the surface silanols with trimethylchlorosilane (TMCS). Thus the excellent results (oxime conversion=100%, lactam selectivity=95%) were obtained over the high silicious (Si/Al≧1,000) and high ESA (≧10m 2 /g) ZSM-5 which was further modified with TMCS (ref.1).
80 citations
TL;DR: In this paper, the authors examined whether these concepts are valid when other than hydrocarbon substrates are converted on the mentioned shapes elective zeolites and related materials, and concluded that they are suitable for data rationalization and property prediction not only on acid catalysts but on metal, bifunctional and even basic catalysts o f zeolitic origin as well.
Abstract: Publisher Summary With the advent of acid 10-membered ring (10-MR) zeolites as heterogeneous catalystsin hydrocarbon transformations, several new concepts on selectivity such as transition state and product shape s electivity, to name t h e most important ones, were developed. In the meantime it was established that these concepts were suitable for data rationalization and property prediction not only on acid catalysts but on metal, bifunctional and even basic catalysts o f zeolitic origin as well. The aim o f this chapter is to examine whether these concepts are valid when other than hydrocarbon substrates are converted on the mentioned shapes elective zeolites and related materials.
75 citations
TL;DR: In this paper, the role of cationic species for the formation of aromatic hydrocarbons was investigated in the reactions of ethane, propane, pentane, hexane, and hexane over H-ZSM-5, Ga−ZSM−5, and Zn-MSM-5 zeolite.
Abstract: The reactions of ethane, propane, pentane, and hexane over H–ZSM-5, Ga–ZSM–5, and Zn-ZSM-5 zeolite were carried out to find the role of cationic species for the formation of aromatic hydrocarbons. In the ethane conversion, zinc cations act as dehydrogenation center for ethane. For other hydrocarbons, the cracking of the starting hydrocarbons on acid centers is the first step of the conversion, and metal cations act as catalyst for the dehydrogenation of the olefinic intermediates. The high activity of the zinc or gallium cations for the olefin dehydrogenation is demonstrated in the conversion of lower olefins and methanol.
61 citations
TL;DR: In this article, the results of EXAFS studies suggested the possibility of the existence of gold atoms coordinated with iron atoms, and the chemical shifts in XPS showed that the ultrafine gold particles were electron deficient.
Abstract: Highly dispersed gold catalysts have been prepared by calcining the coprecipitates obtained from an aqueous solution of HAuCl4 and the nitrate of Fe, Co, Ni, or Be. They are active for the oxidation of CO even at such a low temperature as–70°C and become more active in the presence of moisture. The gold particles are hemispherical in shape and are nearly homodispersed with a mean diameter smaller than 10nm. Most of them are strongly attached at their flat planes to the metal oxide support exhibiting a specific crystal orientation. The chemical shifts in XPS showed that the ultrafine gold particles were electron deficient. The results of EXAFS studies suggested the possibility of the existence of gold atoms coordinated with iron atoms.
56 citations
TL;DR: Isomorphic substitution of Al-P pairs for Si occurs in siliceous SAPO-37 crystals as discussed by the authors, which are composed of aluminosilicate and silicoaluminophosphate domains.
Abstract: Isomorphic substitution of Al-P pairs for Si occurs in siliceous SAPO-37 crystals. 29Si MAS NMR shows that such crystals are composed of aluminosilicate and silicoaluminophosphate domains. The aluminosilicate parts have unusual Si, Al ordering. Si-enriched SAPO-37 has enhanced catalytic activity.
54 citations
TL;DR: In this article, the use of IR, Mossbauer, ESR, UV-vis and DTA techniques allowed to show that majority of iron ions are in tetrahedral environments, very probably at lattice position, even after calcination at 500°C.
Abstract: Isomorphous substitution of iron for silicon in ZSM-5 zeolite has been studied for samples prepared either in basic medium or in neutral fluoride medium. The use of IR, Mossbauer, ESR, UV-vis and DTA techniques allowed to show that majority of iron ions are in tetrahedral environments, very probably at lattice position (95%) even after calcination at 500°C. The Si : Fe ratio varied from 31 to 87. Three major environments of iron were identified. The first one corresponded to the formation of Bronsted sites associated to iron as charge balance cation and was identified by IR of pyridinium ion, ESR (g eff π 4.28) and Mossbauer spectrum (∂dL = 0.34 ± 0.04 mm.s −1 , QS = 0.7 = 0.4 mm.s −1 at room temperature). The other two were more difficult to assign. One was very sensitive to hydration-rehydration process, not sensitive to reduction by H 2 even at 400°C, not exchangeable by NH 4 cation even several times at 80°C and yielded Lewis type IR band upon pyridine adsorption. Their Mossbauer parameters at room temperature were δ = 0.18 ± 0.02 mm.s −1 and QS = O and in ESR g eff = 2.002 and g eff π2→4 depending upon hydration extent. We tentatively assign the latter one as lattice iron exhibiting free bonding rather than iron in exchangeable site as it is usually proposed. All samples exhibit catalytic activity for methanol conversion at 350°C and for toluene methylation reaction at 400°C. Large crystals (π 150 × 60 × 40 μm) resulted in low toluene adsorption rate and therefore in enhanced direct methanol conversion and lower toluene methylation while p-xylene selectivity was strongly enhanced and life time on stream was decreased with respect to small crystallites. This was assigned to crystallite size effect rather than due to the presence of iron, while catalytic activity was attributed to the moderately strong acidity induced by iron substitution for Si in ZSM-5 framework.
53 citations
TL;DR: The first serious advance in commercial catalytic cracking was achieved by the Houdry group in the late thirties as discussed by the authors, who made the synthesis of faujasite zeolites, and found that they had a very high cracking activity.
Abstract: Publisher Summary The first serious advance in commercial catalytic cracking was achieved by the Houdry group in the late thirties. In the early sixties Union Carbide made the synthesis of faujasite zeolites, and found that they had a very high cracking activity. When they were later introduced by Mobil into commercial operation, a dramatic change in FCC was produced. Modern cracking catalysts are made of some 20% zeolite dispersed in a matrix, which can be more or less active depending on formulation. In these catalysts most of the activity and selectivity comes from the zeolite, and therefore much attention is given to the study and optimization of this component. In order to find cut the key parameters in a zeolite which determine its behavior in a cracking process, one should see the conditions that the catalyst will encounter when in use.
52 citations
TL;DR: In this article, the preparation of cyclic and heteroaromatic compounds as well as reactions (e.g., alkylation, isomerization and rearrangement) of these compounds in the presence of zeolite catalysts are described.
Abstract: Nitrogen-containing intermediates and fine chemicals play an important role in the chemical industry, e. g. in the synthesis of active substances and the production of dyestuffs as well as solvents. Thus, attempts are continually being made to synthesize new nitrogen-containing compounds and to devise improved routes to known products. In this connection zeolites with their numerous adjustable catalytic properties can be an useful instrument. This article describes in particular the preparation of cyclic and heteroaromatic compounds as well as reactions (e. g. alkylation, isomerization and rearrangement) of these compounds in the presence of zeolite catalysts.
TL;DR: A review of the developments in zeolite catalysis during the last decade is presented in this paper, and examples illustrate the advantages of using zeolitic catalysts in the selective synthesis of organic compounds.
Abstract: Heterogeneous catalysis is gaining in importance in the synthesis of chemical intermediates and fine chemicals. In the shift away from homogeneous catalysis, zeolites play a major role. The experience gained thereby with zeolitic catalysts in the petrochemical industry has, in recent years, been increasingly applied to the selective synthesis of organic compounds. The developments in zeolite catalysis during the last decade are reviewed, and examples presented which illustrate the advantages of zeolitic catalysts.
TL;DR: In this article, the synthetic procedures used to crystallize aluminophosphate and element substitutions of VPI-5 are described, consistent with the fact that organic species do not act as templates or spacefillers and that aqueous phase ion transport is most likely not involved.
Abstract: The synthetic procedures used to crystallize aluminophosphate and element substituted aluminophosphate VPI-5 are described. The crystallization process for aluminophosphate VPI-5 is consistent with the fact that organic species do not act as templates or space-fillers and that aqueous phase ion transport is most likely not involved. Cobalt substitution into aluminophosphate VPI-5 is similar to the substitution observed for cobalt in CoAPO-5. Mono and bi-liquid phase reaction mixtures are capable of crystallizing silicon containing VPI-5. Silicon appears to substitute for both phosphorus and aluminophosphate pairs.
TL;DR: In this article, O'Reilly et al. used magic-angle-spinning (MAS) NMR to measure highly resolved solid-state spectra of zeolites and obtained quantitative information about Bronsted acidity, structural defects and also about dealumination and realumination.
Abstract: Publisher Summary An early attempt to obtain information about OH groups on surfaces from 1 H NMR line shape and chemical shift was made 30 years ago by O'Reilly et al. . But it was only with the advent of superconducting magnets and the development of magic-angle-spinning (MAS) that it became possible to measure highly resolved solid-state spectra of zeolites. 1 H MAS NMR yields quantitative information about Bronsted acidity, structural defects and also about dealumination and realumination of zeolites, since in the H-form of zeolites the number of framework aluminium atoms is equal to the number of acidic OH groups. This chapter highlights NMR methods, acidity of zeolites, studies on sites with enhanced catalytic activity, OH groups in alkaline earth exchanged zeolites, OH groups in framework defects, and dealumination and realumination of zeolites.
TL;DR: In this paper, long-chain n-paraffins in the range from nonane to heptadecane were converted over bifunctional zeolite catalysts in the presence of hydrogen.
Abstract: In this study long-chain n-paraffins in the range from nonane to heptadecane were converted over bifunctional zeolite Y, USY, ZSM-3, ZSM-20 and BETA catalysts in the presence of hydrogen. n-Paraffins can be selectively converted into branched isomers, provided the hydrogenation-dehydrogenation function is in balance with the acid function of the catalyst. The equilibration of the two catalytic functions depends on the catalyst as well as on the carbon number of the feed. For each zeolite there exists an optimum loading with noble metal which depends on the chain length of the n-paraffin feed. If the geometry and dimensions of the zeolite pores are such that hydrocracking is suppressed by molecular shape-selectivity, the yield of isomerisation is improved substantially. The presence of mesopores in zeolite Y crystals is reflected in the isomerisation yield.
TL;DR: In this paper, the diffusion of benzene and several linear paraffins in large crystals of silicalite using the zero length column (ZLC) method has been studied.
Abstract: The zero length column (ZLC) method has been applied to study diffusion of benzene and several linear paraffins in large crystals of silicalite. The ZLC diffusivity data are broadly consistent with recently reported membrane, piezometric and frequency response values but the values (for propane) are much smaller than the NMR self diffusivities. The trends of diffusivity with carbon number for 5A, 13X and silicalite are qualitatively similar, with the diffusivities for silicalite lying between the corresponding values for 5A and 13X. For carbon numbers greater than 5, activation energies for silicalite are essentially the same as for NaX.
TL;DR: The infrared wavenumber of linear CO adsorbed on Pt-faujasites decreases in the order PtHY > PtHNaY > PtNaY> PtNaX as the zeolite acidity decreases and the basicity increases as discussed by the authors.
Abstract: The infrared wavenumber of linear CO adsorbed on Pt-faujasites decreases in the order PtHY > PtHNaY > PtNaY > PtNaX as the zeolite acidity decreases and the basicity increases. New bands at 1960-2000 cm −1 assigned to linear CO are formed which also parallel the zeolite basicity. The results suggest a strong Pt/zeolite interaction involving the negatively charged framework oxygen.
TL;DR: In this paper, the means of regenerating or disposing of spent catalysts are reviewed, with selective removal of Ni, V and Fe being achieved by leaching with different reagents.
Abstract: Hydrotreating heavy oils produces catalysts that are contaminated with coke and with nickel, vanadium and iron. Regeneration may be possible but sooner or later irreversible deactivation occurs. Means of regenerating or disposing of spent catalysts are reviewed. Regeneration may or may not involve decoking, with selective removal of Ni, V and Fe being achieved by leaching with different reagents. Leaching of all metals from the spent catalyst may be achieved if disposal is required and the economic justification exists. The solid wastes must be encapsulated or stabilized before final disposal in order to meet environmental standards.
TL;DR: In this paper, the formation of coke in methanol and ethylene reaction over two well-characterized industrial H-ZSM-5-based catalysts was studied mainly by UV-visible spectroscopy.
Abstract: Coke formation upon methanol and ethylene reaction over two well-characterized industrial H-ZSM-5-based catalysts was studied mainly by UV-visible spectroscopy. The investigations were complemented by TGA/GC determination of the H/C ratio of the carbonaceous deposits as well as by in-situ IR and conversion measurements. Various stages of coke formation as a function of reaction temperature, catalyst acidity and morphology were distinguished and correlated to catalytic performance.
TL;DR: In this article, the same authors showed that in the as-synthesized blue materials Co 2+ is in the lattice in a tetrahedral environment of oxygen atoms with a ligand field strength of 3796 cm −1.
Abstract: CoAPO-5 was synthesized hydrothermally in the presence of the templates triethylamine and N,N-diethylethanolamine. On the basis of the TO 2 formula the cobalt content was varied between 0.0004 and 0.04. Diffuse reflectance spectroscopy showed that in the as-synthesized blue materials Co 2+ is in the lattice in a tetrahedral environment of oxygen atoms with a ligand field strength of 3796 cm −1 . Upon calcination in O 2+ the tetrahedral environment is distorted, part of the structural Co 3+ is oxidized to Co 3+ and the samples turn green. This structural Co 3+ is reduced with H 2 at 773°K to the blue CoAPO-5 with Co 2+ in a distorted tetrahedral environment.
TL;DR: The medium pore aluminophosphate based molecular sieves with the -11, -31 and -41 crystal structures are active and selective catalysts for 1-hexene isomerization, hexane dehydrocyclization and C8 aromatic reactions as mentioned in this paper.
Abstract: Medium pore aluminophosphate based molecular sieves with the -11, -31 and -41 crystal structures are active and selective catalysts for 1-hexene isomerization, hexane dehydrocyclization and C8 aromatic reactions. With olefin feeds, they promote isomerization with little loss to competing hydride transfer and cracking reactions. With C8 aromatics, they effectively catalyze xylene isomerization and ethylbenzene disproportionation at very low xylene loss. As acid components in bifunctional catalysts, they are selective for paraffin and cycloparaffin isomerization with low cracking activity. In these reactions the medium pore aluminophosphate based sieves are generally less active but significantly more selective than the medium pore zeolites. Similarity with medium pore zeolites is displayed by an outstanding resistance to coke induced deactivation and by a variety of shape selective actions in catalysis. The excellent selectivities observed with medium pore aluminophosphate based sieves is attributed to a unique combination of mild acidity and shape selectivity. Selectivity is also enhanced by the presence of transition metal framework constituents such as cobalt and manganese which may exert a chemical influence on reaction intermediates.
TL;DR: In the presence of gaseous O2 or of oxygen sources (N2O), pure and isomorphic-substituted zeolites show an enhancement of the rate of propane transformation due to the formation of specific centers able to selectively convert propane to propylene as discussed by the authors.
Abstract: In the presence of gaseous O2 or of oxygen sources (N2O) pure and isomorphic-substituted zeolites show an enhancement of the rate of propane transformation due to the formation of specific centers able to selectively convert propane to propylene. Experiments with HZSM5 zeolites with different Si/Al ratios, doped with K or with different mean diameters of the crystals indicate that two types of centers in the zeolite can activate oxygen: Broensted sites and structural defects. Strong Broensted sites are also responsible for unselective oxidation to COx and cracking activity. The control of surface acidity by isomorphic substitution of Al and/or the insertion of transition metals such as V with one-electron redox properties allows control of secondary reactions and/or an increase in the rate of activation, with highly selective synthesis of propylene from propane.
TL;DR: In this paper, the amount of different silicon and aluminium species, formed during steaming, could be determined from deconvoluted MAS NMR spectra, and the chemical nature of which was discussed.
Abstract: The hydrothermal dealumination of zeolite Y was followed by 29Si and 27Al MAS NMR. From deconvoluted MAS NMR spectra the amount of the different silicon and aluminium species, formed during steaming, could be determined. Next to well documented 27Al signals at 60 ppm of tetrahedral framework Al and at 0 ppm of octahedral Al, lines at 50 ppm, 30 ppm and in the region of 4–0 ppm are observed, the chemical nature of which is discussed. Zeolite Y with framework Si/Al ratio from 3.5 to 17 contains non-negligible amounts of extra-framework silicon and aluminium species, which complicate the determination of the framework Si/Al and non-framework:framework Al ratios. A considerable amount of extra-framework aluminium is missing in the 27Al MAS spectra. This ‘NMR-invisible’ aluminium, assigned to oligomeric alumina species, can be made ‘NMR-visible’ by complexation with acetylacetone under specific conditions.
TL;DR: In this paper, it was shown that propane aromatization of physical mixtures of HZSM5 (25 mg) and Ga2O3 (5 to 200 mg) are much greater than the sum of the activities of pure components.
Abstract: The activities for propane aromatization of physical mixtures of HZSM5 (25 mg) and Ga2O3 (5 to 200 mg) are much greater than the sum of the activities of the pure components. This synergic effect is characteristic of a reaction in which catalytic sites of the two components (bifunctional catalysis) participate. Experiments with model reactants propane, propene, 1-hexene, 1-heptene, methylcyclohexene and methylcyclohexane confirm that gallium oxide catalyzes the dehydrogenation of alkanes into olefins and of naphthenes into aromatics but is not active for oligomerization and cyclization reactions. The aromatization of propane on the mixture occurs in the following way: on gallium oxide propane is dehydrogenated into propene; propene diffuses from gallium oxide to HZSM5, on which it undergoes oligomerization and then cyclization; naphthenic compounds diffuse from HZSM5 to gallium oxide where they are dehydrogenated into C6-C8 aromatics.
TL;DR: In this article, the authors measured the stretching frequency and molar heat of adsorption on Y-zeolite and found that the stretching frequencies were higher than those on ionic surfaces, probably because of the low coordination assumed by the cations in the supercage.
Abstract: Heats of adsorption and IR spectra of CO on Na-Y, 72% Ca-Y and 15% Zn-Y zeolites have been measured at room temperature. The data fit a correlation between stretching frequency and molar heat of adsorption already found for non-d cations in ionic systems. For the same cation, both stretching frequency and molar heat of adsorption measured on Y-zeolite are higher than the corresponding ones on ionic surfaces, probably because of the low coordination assumed by the cations in the Y-zeolite supercage.
TL;DR: In this article, the synthesis of microporous beryllophosphates, zeolite-type materials containing extra-framework cations and water, is described, and properties of these materials, such as thermal stability and ion exchange capacity are also discussed.
Abstract: The synthesis of a range of microporous beryllophosphates, zeolite-type materials containing extra-framework cations and water, is described. The frameworks are made up of strictly alternating beryllium and phosphorus and the resulting negative charge is balanced by alkali metal cations. The presence of the cation in the synthesis mixture leads to a clear structure-directing effect; each alkali metal cation promotes the formation of a different structure type. The properties of these materials, such as thermal stability and ion-exchange capacity are also discussed.
TL;DR: In this article, high Ga-contained pentasil type Ga-silicates having Si/Ga atomic ratios 20, 15, and 10 were synthesized by adopting the rapid crystallization method.
Abstract: High Ga-contained pentasil type Ga-silicates having Si/Ga atomic ratios 20, 15, and 10 were synthesized by adopting the rapid crystallization method. The conversion rate of ethane was lower than that of propane, however since the selectivity to aromatics increased very much on the Pt/H-Ga-silicate, which involved Ga as Si/Ga ratio 15, the space-time yield of aromatics from ethane was rather higher than that from propane. Addition of O2 or H2 in the feed gas for ethane conversion retarded the reaction, indicating that the balance of dehydrogenation and hydrogenation activities is necessary to advance the aromatization. Although the conversion was very low, methane to aromatic conversion was observed on this catalyst.
TL;DR: In this article, the alkylation of anisole and phenol with methanol was measured in a microreactor at 473 K and at different contact times, using as catalysts pentasil and H-USY zeolites.
Abstract: The alkylation of anisole and phenol with methanol was measured in a microreactor at 473 K and at different contact times, using as catalysts pentasil and H-USY zeolites. Activity and selectivity results showed that monomolecular reactions are favoured on pentasil and bimolecular reactions on large-pore zeolites. The high selectivity of o-cresol observed on the ten-membered-ring compared to H-USY zeolites is explained by the intramolecular rearrangement of anisole to o-cresol.
TL;DR: In this article, a template-free synthesis of ZSM-5 was achieved in both static and stirred autoclaves employing sodiumwaterglass and pyrogenic silicas as silica sources.
Abstract: The fast template-free synthesis of ZSM-5 was accomplished in both static and stirred autoclaves employing sodiumwaterglass.silica hydrogel and pyrogenic silicas as silica sources. Highly pure crystalline ZSM-5 was obtained in the sodiumwaterglass reaction system in the stirred autoclaves without seeds within 3 hours. Addition of 2% (w/w) of seeds reduced the reaction time to 2 hours. The catalytic activity and shape selectivity of the H-ZSM-5, tested by the disproportionation of ethylbenzene, was found to be largely controlled by the synthesis parameters and by the posttreatments. The homogeneous aluminum distribution across the crystals achieved by this synthetis route resulted in a greater shape selectivity compared to those with an uneven aluminum distribution being prepared by adding templates. Various kinds of posttreatments, e.g. “alkaline calcination”, led to an additional enhancement of the shape selectivity yielding up to 99% of p-isomer in the ethylbenzene disproportionation reaction.