External Crystal Surface Research Articles

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.

Ruthenium-containing small-pore zeolites for shape-selective catalysis

Ruthenium-containing small-pore zeolites with the framework types LTA, KFI, MER and RHO were prepared by addition of ruthenium(III) chloride to the gel of the hydrothermal zeolite synthesis in the absence of organic templates. This synthesis strategy was described earlier for the large- and medium-pore zeolites Ru/Na-X (FAU) and Ru/Na-ZSM-5 (MFI), respectively. The majority of the ruthenium species in the modified small-pore zeolites has a diameter in the order of 1 nm or less and is located inside the pores of the frameworks. These species are available for shape-selective catalytic conversions as demonstrated by the competitive hydrogenation of 1-hexene and 2,4,4-trimethyl-1-pentene. This test reaction allows to differentiate between ruthenium particles within the zeolite pores and those on the external crystal surface. Consequently, LTA-type zeolites with intrazeolitic ruthenium species could be synthesized with ruthenium loadings of up to 4.0 wt.%. A further increase of the ruthenium content to 8.0 wt.% resulted in a significant fraction of the ruthenium (oxide) clusters on the external crystal surface. Ruthenium species on the outer crystal surface also exist after the hydrothermal zeolite synthesis. These can, however, be removed by ion exchange with an aqueous calcium nitrate solution.

Preparation of M 0 metal/alumina-pillared mica composites (M = Cu, Ni) by in situ reduction of interlayer M 2+ ions of alumina-pillared fluorine micas

Alumina-pillared fluorine micas form micropores in the interlayer region and exhibit cation exchangeability. Reduction of 3d transition metal cations (Cu 2+ and Ni 2+) in the interlayer regions of the cation exchanged alumina-pillared micas was attempted using ethylene glycol or diethylene glycol. The Cu 2+-exchanged pillared mica led to the formation of a pillared mica composite containing interlayer zero-valence Cu 0 metal clusters along with Cu 0 metal fine particles on external surfaces of pillared mica flakes when refluxed in either ethylene glycol or diethylene glycol. Ni 2+ cations in the interlayer region were also found to be reducible by refluxing in diethylene glycol. The zero-valence metal contained in the refluxed products occurs in three forms: M 0 clusters in the interlayer region, elongated fine particles sandwiched between silicate layers, and submicron spherical particles on external surfaces of mica crystals. The zero-valence metal/alumina-pillared mica composites thus obtained largely retained the micropore properties of the alumina-pillared micas.

Active phase of a nickel phosphide (Ni2P) catalyst supported on KUSY zeolite for the hydrodesulfurization of 4,6-DMDBT

Abstract Ni2P catalysts supported on potassium ion-exchanged ultrastable Y zeolites (KUSY) were prepared by temperature-programmed reduction (TPR), and the effect of Ni2P loading and initial Ni/P ratios on the hydroprocessing performance was studied. X-ray diffraction (XRD), and extended X-ray absorption fine structure (EXAFS) were used to obtain structural parameters. Transmission electron microscopy (TEM) analysis showed that the KUSY-supported Ni2P samples consisted of nanoparticles, which were likely situated in the mesoporous cavities or the external surfaces of the zeolite crystals. The catalytic activity was measured at 613 K and 3.1 MPa in a three-phase fixed bed reactor for hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) using a model liquid feed containing 500 ppm S as 4,6-dimethyldibenzothiophene (4,6-DMDBT), 500 ppm N as quinoline, and 3000–6000 ppm S as dimethyldisulfide (DMDS). Partial exchange with K enhanced the catalytic activity for the HDS of 4,6-DMDBT and resistance to N-compound inhibition. The Ni2P/KUSY had high activity with an HDS conversion of 99%, and an HDN conversion of 100%, which were much higher than those of a commercial Ni–Mo–S/Al2O3 catalyst with an HDS conversion of 80% and HDN conversion of 100%, based on equal sites (240 μmol) loaded in the reactor. The sites were counted by CO chemisorption for the phosphide and by low-temperature O2 chemisorption for the sulfide. Deficiency of P in the Ni2P resulted in deactivation, probably due to susceptibility to sulfidation. EXAFS analysis of the catalysts showed that the addition of extra P led to an increase in Ni–P coordination with lengthening of Ni–Ni bond distances, resulting in a high and stable catalytic activity.

Liquid chromatography method for quantification of surface connected mesoporosity in ultrastable Y zeolites

Liquid phase breakthrough experiments were performed in order to characterize the accessible mesoporosity of a dealuminated Y zeolite. The methodology presented herein shows that the use of a bulky molecular probe such as 1,3,5-triisopropylcyclohexane allows discrimination between the occluded mesoporosity accessible via the micropores only, and mesopores connected to the external surface of the zeolite crystals. The results, compared to nitrogen physisorption and mercury intrusion data, show that in commercial ultrastable Y CBV780 zeolite mesoporosity is mainly connected to the surface of the zeolite crystals.

The influence of the desorption barrier on the transport of molecules through the external surface of nanoporous crystals

Results of Molecular Dynamics simulations of tracer exchange of guest molecules between zeolite crystals and the surrounding gas phase are reported. The crystal surface is assumed to be free of structural defects that may additionally slow down molecular transport. The main focus of this study is on the influence of the potential energy difference between the intra- and intercrystalline space of the crystals on adsorption and desorption. Our results illustrate that such energy difference may be responsible for the appearance of flat ( i.e. adsorption- or desorption-limited) intracrystalline concentration profiles for sufficiently small zeolite nanocrystals present in novel hybrid materials.

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.

Layered Double Hydroxide Minerals as Possible Prebiotic Information Storage and Transfer Compounds

One of the fundamental difficulties when considering the origin of life on Earth is the identification of an emergent system that not only replicated, but also had the capacity to undergo discrete mutation in such a way that following generations might inherit and pass on the mutation. We speculate that the layered double hydroxide (LDH) minerals are plausible candidates for a proto-RNA molecule. We describe a hypothetical LDH-like system which, when intercalated with certain anions, forms crystals with a high degree of internal order giving rise to novel information storage structures in which replication fidelity is maintained, a concept we use to propose an explanation for interstratification in terephthalate LDHs. The external surfaces of these hypothetical crystals provide active sites whose structure and chemistry is dictated by the internal information content of the LDH. Depending on the LDH polytype, the opposing external surfaces of a crystal may give rise to reactive sites that are either complementary or mirror images of each other, and so may be chiral. We also examine similarities between these proposed "proto-RNA" structures and the DNA that encodes the hereditary information in life today, concluding with a hypothetical scenario wherein these proto-RNA molecules predated the putative RNA-world.

Structure and acidity of Mo/ZSM-5 synthesized by solid state reaction for methane dehydrogenation and aromatization

The Mo/ZSM-5 catalysts were synthesized by solid-state reaction with the mixture of MoO 3 and HZSM-5 under N 2 atmosphere at 500 °C, and characterized by XRD, MAS NMR, BET, NH 3-TPD, FT-IR and powder XRD structural analysis. The results showed that the Mo species were well dispersed on the internal and external surface of the zeolite ZSM-5, while the framework of ZSM-5 was retained. One part of Mo species inside the channels of ZSM-5 can be described as [Mo 5O 12] 6+ units in the Mo/ZSM-5 with 8 wt% MoO 3 loading. The [Mo 5O 12] 6+ units are situated at the cross-point of the zigzag channel and the straight channel. They interacted with the framework of ZSM-5, and decreased the Brönsted acid sites remarkably. There was about 0.47 [Mo 5O 12] 6+ unit in every cell of the ZSM-5 on an average. The other part of Mo species is distributed on the external surface of the ZSM-5 crystals, and can be described as crystallites of molybdenum oxides. The distribution of Mo species on HZSM-5 increased the Lewis acid sites in the catalysts. The [Mo 5O x C y ] n+ units inside the channels of ZSM-5 partially reduced by methane maintained the catalytic activity of the catalysts and were beneficial for the formation of benzene during methane dehydrogenation and aromatization at 700 °C.

A MAS NMR and DRIFT study of the Ga species in Ga/H-ZSM5 catalysts and their effect on propane ammoxidation

71Ga, 27Al, and 29Si MAS NMR and DRIFT spectroscopies were used to characterize the state of gallium in Ga/H-ZSM5 catalysts tested for their ability to catalyse the ammoxidation of propane. Ga species were observed in two different possible environments: octahedrally coordinated gallium in small Ga2O3 particles at the external surface of the zeolite crystals, octahedrally coordinated gallium in GaO(OH) or related species, and tetrahedrally coordinated gallium in cationic-exchange positions inside the zeolite. Redox (H2–O2) cycles promote the migration of gallium from the GaO(OH) or Ga2O3 species at the external surface of the zeolite crystals to cationic-exchange sites within the zeolite channels. The redox treatment definitely had a beneficial effect on its catalytic performance for the ammoxidation of propane, which is known to occur via a bifunctional mechanism. The main product was acetonitrile at high Ga and Al contents while acrylonitrile was also observed as a side product at high Ga and low Al contents.Key words: ZSM5, MFI, in situ studies, MAS NMR, DRIFT, propane activation, ammoxidation.

Combining macroscopic and microscopic diffusion studies in zeolites using NMR techniques

In this study the zero length column (ZLC) technique is extended to the case where the decay of the adsorbed phase concentration is observed directly by nuclear magnetic resonance (NMR). An adsorption-desorption apparatus compatible with a 400-MHz NMR spectrometer was developed. It operates with nitrogen or helium as the inert purge gas. The column of the adsorbent material is placed in the sensitive region of the superconducting magnet and the rf coil of the NMR spectrometer. The time scales of the adsorption and desorption processes depend on concentration, temperature and crystal shape and are found to be in the range of 1-10 min. From the desorption branch, the non-equilibrium ZLC-NMR measurements yield intracrystalline diffusion coefficients in the range of 10(-13) to 10(-11) m2/s for different alkanes in silicalite-1. These values are always found to be smaller than the values measured by pulsed field gradient NMR under equilibrium condition indicating that there must be additional transport resistance at the external surface of these silicalite-1 zeolite crystals.

Cesium salt of a heteropolyacid in nanotubular channels and on the external surface of SBA-15 crystals: preparation and performance as acidic catalysts

The Cs-salt of heteropolyacid with stoichiometry Cs 2.5H 0.5PW 12O 40 (CsHPW) was deposited selectively at the external surface of the SBA-15 silica microcrystals, inside its mesoporous channels and simultaneously at both location modes. The structure, texture and performance of these CsHPW/SBA-15 composites were compared with that of a reference bulk salt of the same composition. Location of CsHPW salt on the external surface of SBA-15 microcrystals leads to disintegration of its agglomerates increasing acidity of the catalytic phase. A novel preparation strategy consisting of grafting the basic Cs-species at the internal pores surface of SBA-15 stabilized the CsHPW phase inside the channels in form of 5–8 nm nanocrystals at 30–70 wt.% loadings. The catalytic tests demonstrated that insertion of the CsHPW catalytic phase inside the nanotubular channels of SBA-15 in combination with location of an additional amount of this phase at the external surface of SBA-15 microcrystals allows to increase the specific activity of this phase in MTBE synthesis, propionylation of anisole and alkylation of catechol with t-butanol by a factor of 1.5–3. This level of specific activity in combination with high total loading of catalytic phase >60 wt.% permit to get composite catalytic materials with catalytic activity higher by a factor of 1.2–1.5 with respect to the bulk CsHPW catalyst and stabilizing the catalytic phase against colloidization in polar media.

Influence of Defects on the External Crystal Surface on Molecular Uptake into MFI-Type Zeolites

The interference microscopy technique, which was recently introduced in our laboratory, is applied to study transient intracrystalline concentration profiles in ZSM-5 crystals during adsorption and desorption of isobutane. Two different zeolite samples were used, viz., samples of etched and of nonetched ZSM-5 crystals. Etching has been carried out to remove the outer layer of the crystal surface, which may contain large amounts of defects and impurities. Studying the transient concentration profiles in both samples provides unique information on the influence of surface defects on molecular uptake. It is shown that, depending on their type, the defects of the crystal surface can either increase or decrease the rate of adsorption/desorption. The former effect is associated with adsorption/desorption through cracks in the crystal surface. The latter has its origin in the blockage or structural changes of the external crystal surface, leading to the appearance of surface transport barriers. Owing to the ability of interference microscopy to gain direct insight into the influence of surface defects on molecular uptake, this technique gives more accurate information on the transport diffusivities in zeolite crystals than the classical uptake methods.

Exploratory study of mesopore templating with carbon during zeolite synthesis

Here we present an exploratory study on the generation of mesopores in zeolite crystals by templating with carbon during synthesis. With nitrogen physisorption, scanning electron microscopy and (three-dimensional) transmission electron microscopy we have studied the influence of the carbon source and zeolite synthesis conditions on the mesopores generated after the carbon has been burned off. In particular, silicalite-1 crystals have been synthesized in the presence of a carbon matrix. After burning off the carbon, zeolite samples with mesopore volumes between 0.05 and 0.71 ml/g were obtained. With dissolved SiO 2 at a crystallization temperature of 150 °C large crystals (15 μm length) were obtained, whereas the use of tetraethylorthosilicate at a crystallization temperature of 170 °C yielded crystals with a size between 0.5 and 6 μm. Low heating rates (0.5 °C/min) to a final crystallization temperature of 170 °C led to rather uniform small zeolite crystals (0.6–1 μm), whereas the crystals were less uniform in size when the autoclave was rapidly heated to the final temperature, i.e. also much larger crystals were found. Both carbon nanofibers and carbon black aggregates were capable to act as a template for cylindrical mesopores that start at the external surface of the zeolite crystals. However, the tortuosity of the mesopores templated by the carbon black aggregates was much higher than the cylindrical mesopores templated by the carbon nanofibers. With increasing zeolite crystal size the chance that carbon aggregates are completely surrounded by the zeolite increases, leading to inkbottle type mesopores. The results show for the first time carbon nanofiber skeins to be highly promising secondary templates to obtain cylindrical mesopores with a low tortuosity in zeolite crystals.

On the enhanced para-selectivity of HZSM-5 modified by antimony oxide

HZSM-5 was modified with antimony oxide by solid-state reaction at 773 K to enhance diffusional constraints. Crystalline phases of antimony oxide were not detected at a Sb 2O 3 loading up to 10.2 wt%. Antimony oxide was found to strongly interact with the hydroxyl groups of the zeolite, i.e., silanol and bridging hydroxyl groups, and with extra-framework aluminum oxide. Only a small amount of Sb 2O 3 penetrated the pores of the zeolite, while the main fraction of Sb 2O 3 was deposited on the external surface of the zeolite crystals. With increasing antimony oxide loading a pronounced pore narrowing of the zeolite was observed, which suppressed the further penetration of antimony oxide into the pores of the zeolite. The dispersion of antimony oxide on the surface of the zeolite led to complete removal of unselective Brønsted acid sites and to a narrowing/blocking of the pore mouth openings of the zeolite. Both effects induced enhanced para-selectivity for toluene disproportionation.

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.

Preparation and properties of lipase immobilized on MCM-36 support

Based on pure MCM-22 precursor, MCM-36 was produced by swelling and pillaring with SiO 2 pillars. These materials were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), thermogravimetric analysis/differential scanning calorimetry (TGA/DSC), MAS NMR, and N 2 adsorption isotherms. Compared with the MCM-22 sample, the resulting MCM-36 material contains a mesoporous region between the microporous layers, with a mesopore volume of 0.472 cm 3/g, and the surface area increased up to 671 m 2/g. Lipase from Candida antarctica B (CALB) was immobilized on both supports by physical adsorption at equilibrium. Twenty milligrams of CALB/g of MCM-22 was adsorbed on the external surface of crystals, while only 4 mg CALB/g support was adsorbed onto the mesoporous hybrid material MCM-36. The mechanism of adsorption was also discussed. The acylation of alcohols (1-butanol and 1-octanol) by vinyl esters (vinyl acetate and vinyl stearate) was used as a test reaction in order to evaluate the catalytic activity of MCM-36-immobilized lipase. The test reaction indicated MCM-36-immobilized enzyme as an active catalyst for the acylation and its activity was approximately two times higher than that of the free lipase (for the vinyl acetate/1-butanol system).

On the Enhanced Selectivity of HZSM-5 Modified by Chemical Liquid Deposition

The external surfaces and pore mouth regions of HZSM-5 samples with different crystal sizes were modified by chemical liquid deposition (CLD) with tetraethoxysilane (TEOS), which led to the passivation of unselective acid sites. The modification was found to be more effective for zeolite samples of larger crystal size. The diffusivity of o-xylene was substantially reduced after silylation, while the diffusivity of toluene hardly changed. Dealumination of the external surface of the zeolite crystals enhanced the silylation effects; this was related primarily to the removal of acid sites associated to extra-framework alumina. For the modified catalysts, a significant increase in the selectivity to p-xylene in the disproportionation of toluene was achieved.

A study of the localization and accessibility of Brønsted and Lewis acid sites of H-mordenite through the FT-IR spectroscopy of adsorbed branched nitriles

Adsorption of propionitrile (PrN), isobutironitrile (IBN) and pivalonitrile (PN) with H-MOR (Si/Al=10) has been investigated. The accessibility of IBN and, more, of PN are limited, unlike PrN. Terminal silanols ( 3745 cm −1 ), weakly acidic, are located, as Lewis sites, at the external crystal surface. Bridging hydroxy groups absorbing at 3588 cm −1 are inside the side pockets, those absorbing at 3605 cm −1 are in the main channels and those absorbing at 3609 cm −1 are at the intersection between side pockets and main channels. OH’s due to extra framework materials ( 3650 cm −1 ) are in the side pockets.

Dimethyl Branching of Long n-Alkanes in the Range from Decane to Tetracosane on Pt/H–ZSM-22 Bifunctional Catalyst

Single long n-alkanes in the range n-C10 to n-C24 were hydroisomerized at 233°C in a fixed-bed down-flow vapor-phase reactor loaded with Pt/H–ZSM-22 zeolite catalyst. The multibranched isomer fractions obtained by skeletal isomerization of the different n-alkanes were analyzed in detail with GC-MS. The large majority of multibranched isomers were dimethyl branched. In the preferred isomers, the tertiary carbon atoms carrying the methyl branchings were separated by 3 up to 14 carbon atom positions. Further peculiarities were the preferential formation of isomers with symmetric methyl positions on the chain. Among this family of “symmetric” isomers, the formation of 2,x−3-dimethyl–Cx−2 isomer was favored in conversions of n-Cx alkanes with x equal to or smaller than 17 and disfavored with longer molecules. In the family of 3,n-dimethyl–Cx−2 isomers obtained from the longest alkanes studied (n-C20, n-C22, n-C24), the preferred n positions were from 8 to 12 and from 14 to 18. In the 4,n-dimethyl–Cx−2 isomers, the preferred n positions were from 8 to 17. It was also found that the formation of a dimethyl-branched isomer occurs preferentially through methyl branching of a monomethyl-branched isomer having already the more centrally positioned methyl group rather than the one close to the extremity of the chain. These peculiar positional selectivities and reaction paths can be explained by key-lock catalysis in pairs of micropore openings on the external surface of the ZSM-22 zeolite crystals.