10-MR Channels Research Articles

Unique trans-Selectivity of Ti-MWW in Epoxidation of cis/trans-Alkenes with Hydrogen Peroxide

The catalytic properties of Ti-MWW in the epoxidation of linear cis/trans-alkenes with hydrogen peroxide has been studied extensively, and a mechanistic investigation into its unique trans-selectivity has been carried out by comparing with TS-1 and Ti-Beta. Ti-MWW exhibits a singularity never observed on conventional titanosilicates, that it selectively epoxidizes the trans-isomer to give a selectivity of ca. 80% for corresponding trans-epoxide from an alkene mixture with a cis/trans ratio of 50:50. The trans-selective nature of Ti-MWW is affected neither by the reaction conditions nor by the kinds of alkenes. Ti-MWW retains the stereochemistry for both the substrates and the epoxides during the epoxidation reactions. Liquid-phase adsorption and the epoxidation reaction with bulky organic oxidant and position-selective poisoning reagents have revealed that the unique trans-selectivity of Ti-MWW originates mainly from its sinusoidal 10-MR channels.

The adsorption of a series of aromatics in ITQ-1: grand canonical Monte Carlo simulations

Grand canonical Monte Carlo (GCMC) simulations employing a rigid framework are performed to explore the adsorption behaviors of some aromatics in the purely siliceous MCM-22 zeolite, ITQ-1. Benzene, toluene, m-xylene and o-xylene are separately simulated at 315 K, investigating a series of pressure from 0.1 to 14 kPa. The potential adsorbed sites of the studied molecules are determined from the mass clouds. The comparison of the different mass clouds indicates that as the kinetic diameters change, the distributions of the adsorbate molecules present regular alterations. The predicted adsorption isotherms of two kinds of adsorbates, including toluene and m-xylene are in good agreement with the experimental results. Furthermore, the activation energies of xylenes migrating through the 10-MR windows interconnecting the 12-MR supercages are determined. The energy profiles distinctively suggest that near the 10-MR windows, the potential barrier of o-xylene is significantly larger than that of m-xylene, which will block the diffusion of o-xylene into the interior of the zeolite. The theoretical calculations and experimental results show that at relatively low pressure and temperature, benzene, toluene and m-xylene can migrate through the 10-MR windows and reach the adsorption equilibria easily, while o-xylene can not overcome the potential barriers along the 12-MR supercages to enter the interior of the zeolite. These results can be well understood by the existence of micropores or cavities of two different sizes: (1) the narrow 10-MR channels, which only benzene and toluene (or smaller molecules) can enter; (2) the wider 12-MR supercages, in which benzene as well as toluene and m-xylene can penetrate; (3) the much lower uptake of o-xylene or bigger molecules can be ascribed to the difficulty of the entrances of micropores and cavities.

The localization and adsorption of benzene and propylene in ITQ-1 zeolite: grand canonical Monte Carlo simulations

Grand canonical Monte Carlo (GCMC) simulations have been performed to investigate the localization and adsorption behavior of benzene and propylene, which are involved in the cumene synthesis process, in purely siliceous MWW zeolite (ITQ-1). From the mass clouds of GCMC simulations, it can be seen that the benzene and propylene molecules show different localization and adsorption behavior in the zeolite cavities. In the 10-MR channels, both benzene and propylene show high localization. In the 12-MR supercages, the propylene molecules cannot only almost fill all the possible positions in one supercage, but also can be steadily located in the short 10-MR conducts interconnecting the 12-MR supercages, where the benzene molecules are adsorbed close to three adsorption sites. By analyzing the location of benzene and propylene in ITQ-1, it can be deduced that the alkylation of benzene and propylene will happen mainly in 12-MR supercages. Moreover, a series of simulations have been performed to predict the adsorption isotherms of benzene and propylene at 315 K and 0–1.4 kPa. The results for benzene generally are in agreement with the trend from experiments on a series of aromatic compounds. The results reveal that at low pressures, the loading of propylene is lower than that of benzene, which seems to be caused by the relatively unfavorable potential interactions between propylene/zeolite and propylene/propylene.

Preparation, characterisation and catalytic activity of ITQ-2, a delaminated zeolite

ITQ-2 is the first member of the class of delaminated zeolite precursors. Here we present data to corroborate its proposed structure. Sorption measurements show that ITQ-2 no longer contains large 12-membered-ring (12MR) cavities typical for the non-delaminated MWW-type zeolite, and only retained the 10-membered-ring (10MR) system inside the layer. Adsorption and high resolution transmission electron microscopy (HR-TEM) experiments confirm that delamination allows the ITQ-2 layer to retain most of its structural integrity. The catalytic properties of ITQ-2 are in agreement with those expected from an aluminosilicate that combines a large, catalytically active external surface with the 10MR channel system.

RUB-23: a new microporous lithosilicate containing spiro-5 building units

A new microporous lithosilicate, Cs 10(Li,H) 14[Li 8Si 40O 96]·12H 2O, was synthesised under hydrothermal conditions. The crystal structure has been solved and refined from single-crystal data in the space group P2 1 to R=14%. The three-dimensional (3D) silicate framework contains lithium as cations on T sites in the centre of spiro-5 units that are charge-balanced by non-framework caesium, lithium and hydrogen cations. The hydrate water in the pores can be removed, and the structure is stable up to ∼300°C. The experimental density of the as-synthesised crystal is 2.62 g cm −3; the framework density is calculated to be 17.7 T/1000 Å. The channel system is one-dimensional for 10-membered ring (10-MR) channels which are connected to 8-MRs running parallel to the 10-MRs. The connection is established by 8-MR pore openings. In principle, this allows the 3D diffusion of (at least) small molecules. Chemical analysis, 29Si, 7Li and 133Cs solid-state nuclear magnetic resonance (NMR) spectroscopy, thermal analysis, infrared (IR) spectroscopy and X-ray structure analysis are all consistent and agree with crystal–chemical reasoning based on the flexibility of [TO 4] units in silicates which are required to build 3-MRs as observed in spiro-5 units.

Crystallization of MAZ-type zeolites using tetramethylammonium, sodium and n-hexane derivatives as structure- and composition-directing agents

An attempt was made to synthesize MAZ-type zeolites with enhanced Si/Al ratio by adding organic molecules to conventional aluminosilicate hydrogels for MAZ zeolites containing sodium and tetramethylammonium cations. Molecular models of candidate templates were built and optimized inside the 12-membered ring (12-MR) pore of the MAZ structure. The hexamethonium cation, 1,6-diaminohexane and 1,6-hexanediol show the most favorable calculated Lennard-Jones potential. The crystallization in presence of these organic molecules led to the formation of MAZ-type zeolite. The samples were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive analysis of X-rays (EDAX), chemical analysis, nitrogen adsorption, thermogravimetric analysis/differential thermal analysis (TGA/DTA), 29Si and 27Al magic-angle spinning (MAS) nuclear magnetic resonance spectroscopy (NMR) and 13C cross-polarization (CP)/MAS NMR. The hexamethonium cation and 1,6-diaminohexane are incorporated into the MAZ crystals, whereas 1,6-hexanediol is not. Incorporation of 1,6-diaminohexane caused a higher incorporation of silicon in the framework, especially in the T1 sites delineating the 12-MR channels. The upper limit of silicon incorporation was investigated by systematically reducing the aluminum content of the gel. The highest Si/Al ratio achieved by this approach was 5.3. A molecular crystallization mechanism for mazzite is proposed, explaining this upper limit as well as the crystal morphology changes depending on the aluminum content of the synthesis gel.

Influence of the Nature of the Exchanged Ion on n-Hexane Aromatization Activity of Pt–M–ETS-10: Ab Initio Calculations on the Location of Pt

The activity of Pt–M–ETS-10 samples exchanged with different alkali metals (M=Li, Na, K, Rb, or Cs) in the aromatization of n-hexane depends on the nature of the exchanged metal ion (M). A distinct relationship between the intermediate electronegativity (Sint) of the different metal-exchanged ETS-10 samples and benzene yield is reported, suggesting the activation of Pt by the basicity of the exchanged metal. Results of ab initio Hartree–Fock calculations on cluster models representing Pt–M–ETS-10 corresponding to the presence of Pt nearer to [TiO6]2− and [SiO4] groups reveal that the presence of Pt nearer to [TiO6]2− is energetically more favored and results in a gain of electron charge by Pt. The studies also suggest an increasing amount of electron transfer from the model clusters of ETS-10 to the Pt with increase in the electropositive character of the exchanged metal. It appears therefore, that small Pt clusters residing nearer to [TiO6]2− accessible from the 12-MR channels are the likely active sites in n-hexane aromatization over Pt–M–ETS-10.

Reactions of m-xylene on zeolites with intersecting medium and large pores : Part 2: aluminum population in structures with CON topology

A series of modified SSZ-33 molecular sieves (CON topology) are used as catalysts for the reactions of m-xylene. Borosilicate SSZ-33 is transformed into the aluminosilicate form by a low-temperature hydrothermal treatment. Aluminosilicate SSZ-33 samples with different aluminum populations behave similarly in the reactions of m-xylene, suggesting that the intracrystalline pore space acts as an ensemble of cages connected by 10- and 12-member rings (MR) rather than intersecting 10-MR and 12-MR channels. Additionally, at low m-xylene flow rates, SSZ-33 samples with high Si/Al ratios show an increased amount of transalkylation and the para/ ortho ( p/ o) and isomerization/disproportionation (i/d) selectivities are lower. Reaction over external-surface deactivated SSZ-33 at low flow rates results in unaltered p/ o ratios. In contrast, reaction over SSZ-33 which contains only aluminum at or near the surface has a greatly decreased p/ o ratio. The loss in p/ o and i/d selectivity at low flow rates can be attributed to the occurrence of ortho-selective bimolecular isomerization at or near the catalyst external surface.

Selective formation of p-xylene with disproportionation of toluene over MCM-22 catalysts

Disproportionation of toluene has been studied over dealuminated MCM-22 zeolites. The dealumination of MCM-22 was performed by acid reflux and the combination of calcination or steaming with the subsequent acid reflux. These treatments effectively eliminated Al atoms from the framework site but left most of them in the crystallites as non-acidic extra-framework species. The activity of MCM-22 for the toluene disproportionation measured at relatively low temperatures (473–573 K) was comparable to that of mordenite and much higher than that of ZSM-5. Toluene disproportionation was found to occur mainly within the supercages of MCM-22 but not in the 10-membered ring (10-MR) channels of MCM-22 at these temperatures. The fraction of p -xylene in xylene isomers ( para -selectivity) formed on MCM-22 was higher than its equilibrium value, and increased further by the dealumination. p -Xylene formed within the supercages as a primary product would be isomerized to o - and m -xylene by the acid sites not only on the external surface but also within the 10-MR channels. The improvement in para -selectivity by the dealumination resulted from the suppression of isomerization activity probably because the dealumination treatments eliminated framework Al atoms predominantly on the external surface and inside the 10-MR channels compared with those inside the supercages.

Sorption, diffusion and catalytic properties of zeolites containing 10- and 12-member ring pores in the same structure

The structure, adsorption, diffusion and catalytic properties of zeolites containing 10- and 12-member ring pores in the same structure, i.e. SSZ-33, SSZ-26, CIT-1, MCM-22, NU-86, NU-87, are presented. The adsorption and diffusion properties are well explained on the basis of the pore topologies. In those structures in which the 10- and 12-MR channels cross, the catalytic behaviour is strongly influenced by the void space left in the crossing points. This is not the case in the structures in which the pores are not connected. In this case, the catalytic performance is the average of the primary processes occurring in each one of the channels.

Synthesis and Structure of As-Prepared ITQ-4, A Large Pore Pure Silica Zeolite: The Role and Location of Fluoride Anions and Organic Cations

Pure silica ITQ-4, a zeolitic material with large one-dimensional 12MR channels and an unexpectedly large microporous void volume, can be synthesized in the presence of fluoride ions and N-benzylquinuclidinium cations within a wide range of synthesis pH. Substitution of the tertiary carbon of the quinuclidine moiety by a N atom still renders the organic cation as a suitable structure-directing agent for this material. The role of F- anions and the influence of the pH of the synthesis on both the phase selectivity of the crystallization and the presence of connectivity defects in the final material are discussed. In the presence of F-, defect formation is observed to be pH-dependent, and there appears to be a threshold pH for the generation of a constant defect concentration. A plausible explanation for this observation based upon the ionization state of the condensing species is also presented. The structure of as-made ITQ-4 has been obtained and refined from high-resolution powder XRD data, and the organ...

Effect of metal/acid balance in Pt-loaded large pore zeolites on the hydroisomerization ofn-Hexane andn-heptane

Large-pore zeolites H-Beta, H-Mordenite and H-Omega were loaded with platinum and applied for the hydroisomerization of n-hexane andn-heptane. The catalytic activity of Pt-loaded zeolite showed good correlation with the effective acidity probed by pyridine rather than with the total acidity probed by ammonia when equal amounts of metal sites are present. The selectivity to multibranched isomers over three different catalysts was found dependent upon the conversion ofn-paraffin. At low conversion, the selectivity to multibranched isomers was higher over Pt-load-ed zeolites with high acid strength. At high conversion level, however, the selectivity was mainly governed by the metal/acid balance. On H-Mordenite and H-Omega, large amount of strong acid sites are located in the small pore or cage. In Pt/H-MOR, the metal/acid balance was poor because there was detrimental loss of metal sites caused by isolation of Pt in the side pockets of 8-MR and/or pore blockage of the linear 12-MR channel. Even though the acidity of H-Beta was very low compared to H-MOR and H-Omega, Pt/H-Beta yielded the best performance for the hydroisomerization ofn-paraffin because of better metal/acid balance in Pt/H-Beta.

Cracking Behavior of Zeolites with Connected 12- and 10-Member Ring Channels: The Influence of Pore Structure on Product Distribution

n-Heptane has been cracked on a CIT-1 zeolite which has connected 12- and 10-member ring (MR) channels, and its behavior was compared with that of MCM-22 with nonconnected 12- and 10-MR channels, and SSZ-24 and BETA with unidirectional and tridirectional 12-MR channels, respectively. The kinetic rate constant is highest for CIT-1, and the decay constant is lowest. From the selectivity point of view, its behavior can be better represented by a system with large cavities (the intersections between the 12- and 10-MR) connected by windows. This gives a behavior typical of that of large pore zeolites. CIT-1 produces a remarkably high selectivity toi-C4, and specially to isobutane. This zeolite shows promise as an additive in FCC operations.

Molecular shape selectivity of EUO zeolites

The micropores of the EUO zeolite consist of 10-membered ring (10-MR) channels with deep side-pockets circumscribed by 12-MRs. In decane isomerization and hydrocracking EUO zeolites crystallized in the presence of hexamethonium (HM) and dibenzyldimethylammonium (DBDMA) exhibit different molecular shape selective properties. The catalytic sites of EUO specimens crystallized with DBDMA are located in more spacious cavities than those of EUO crystals synthesized with HM. This disparity in molecular shape selectivity is explained by a different location of the framework aluminium atoms in the two materials, viz. by the absence of aluminium in the 10-MR channels in EUO-DBDMA. Such aluminium siting is in agreement with the location of the organic template cations in the pores of the as-synthesized zeolites suggested in literature and revealed by a molecular graphics modeling.

Skeletal Isomerization of 1-Butene on 10-Member Ring Zeolite Catalysts

Effects of zeolite pore structure on skeletal isomerization of 1-butenes are investigated. Three 10-member ring zeolites, ferrierite, ZSM-23, and ZSM-5, are studied and compared. Results show that ferrierite, with intersecting 10-MR and 8-MR channels, has better isomerization activity than ZSM-23, with monodirectional 10-MR channels, ZSM-5 with two intersecting 10-MR channels is not a selective isomerization catalyst.

Investigation of the process of poisoning of nickel catalysts by thiophene

The effect of time-on-stream TOS (from 1 min to 21.5 h) on the rate and selectivity of n-butene transformation at 350 °C was determined over a series of HFER zeolites with different Si/Al ratios (6, 9.5, 10, 19 and 32). The porosity and acidity of the fresh and aged samples were characterized by nitrogen adsorption and by NH3 adsorption at 50 °C followed by FTIR, respectively. The main differences between the samples were the following: (i) increase with Si/Al in the activity of the protonic sites (turnover frequency) of fresh samples and in their selectivity for isobutene production, (ii) decrease in the rate of isobutene production with increasing TOS for high Si/Al ferrierites as could be expected from coke formation and complex change for low Si/Al ferrierites with in particular an unexpected increase. Furthermore with all the samples, the turnover frequency for isobutene production increases with TOS whereas the one for propene + pentenes decreases or remains constant. All these observations can be explained by considering the zeolite pore system as a series of non-interconnected nanoreactors (the 10-MR channels) in which the molecules diffuse without any possibility of desorption before their exit while undergoing successive bimolecular reactions. In those with few protonic sites, n-butene isomerizes selectively through an autocatalytic process. In those with many sites a thermodynamic equilibrium mixture of propene, butenes and pentenes is formed.