Ring Zeolites Research Articles

Origin of the Low Olefin Production over HZSM-22 and HZSM-23 Zeolites: External Acid Sites and Pore Mouth Catalysis

The methanol-to-olefin (MTO) reaction was studied over two one-dimensional 10-ring zeolites with similar pore structures: ZSM-22 (TON) and ZSM-23 (MTT). It was found that the initial low but detectable production of olefins over both zeolites was catalyzed by external and/or pore mouth acid sites through hydrocarbon pool mechanism. Evidence is listed as follows: The uncalcined HZSM-22 had similar MTO activity as the calcined catalyst. The HZSM-22 zeolite with smaller crystal size had higher MTO activity. Both zeolites treated by HNO3 to selectively leach the external acid sites showed significantly reduced initial MTO activity. Both zeolites coked through catalytic cracking of 1,3,5-triisopropylbenzene showed significantly lower initial MTO activity. This conclusion may also be suitable for other one-dimensional zeolites with pore size below 5.7 Å in the MTO reaction.

Effective Fabrication of Catalysts from Large-Pore, Multidimensional Zeolites Synthesized without Using Organic Structure-Directing Agents

An Al-rich zeolite beta with *BEA topology and a Si/Al ratio as low as 6–7 was synthesized without the use of an organic structure-directing agent (OSDA) and subsequently treated by steam followed by heating with nitric acid for the purposes of dealumination, so as to prepare a catalyst. The steaming process played an important role in stabilizing the *BEA framework, presumably by repairing site defects with migrating silicon species. Steaming at around 700 °C was observed to produce optimal stabilization of the zeolite and allowed subsequent acidic dealumination while maintaining an intact framework. A second demonstration of successful OSDA-free synthesis and effective catalyst fabrication through postsynthetic modification involved the fabrication of a 12–10–10-ring zeolite having an MSE-type framework. This represented the first successful synthesis of an Al-rich MSE-type zeolite (with a Si/Al ratio as low as 6–7) using seed crystals in the absence of any OSDA. The gel composition as well as the crystallization temperature and time were optimized for the purpose of this synthesis such that a pure MSE phase could be obtained in a relatively short crystallization period of only 45 h. Longer crystallization periods and inadequate aging times gave mordenite as an impurity and as a major phase, respectively. These results offer further support for the so-called “composite building unit” hypothesis. As with the zeolite beta, direct dealumination of the MSE-type zeolite by acid treatment resulted in the collapse of the framework, which was avoided by steaming at 700 °C. After stabilization by steaming, acidic dealumination without framework collapse became possible. The dealuminated versions of the Al-rich beta and MSE-type zeolites were shown to be effective catalysts for the hexane cracking reaction, affording propylene in high selectivity. The MSE-type zeolite exhibited a particularly high level of coking resistance in addition to a significant yield of propylene, indicating that zeolites synthesized without using an OSDA show promise for industrial applications as highly selective and long-lived catalysts.

Stability of the Reaction Intermediates of Ethylbenzene Disproportionation over Medium-Pore Zeolites with Different Framework Topologies: A Theoretical Investigation

The strain energies of the main reaction intermediates (i.e., monoethylated diphenylethane (mEDPE) and diethylated diphenylethane (dEDPE) derivatives), which can be formed during ethylbenzene (EB) disproportionation over six 10-ring zeolites with different framework topologies, as well as over the large-pore zeolite Y, were determined by the density functional theory calculations in order to more precisely investigate the effects of the pore structure of medium-pore zeolites on their formations. It was found that while the strain energies of mEDPE and dEDPE intermediates in zeolite Y, MCM-22 and TNU-9, were always lower than 19.6 kJ mol(-1), some of them were characterized by considerably higher energies (>32.8 kJ mol(-1)) when positioned in the intersection channels of ZSM-5 and ZSM-57. As expected, in addition, all the mEDPE and dEDPE derivatives embedded in TNU-10 and ZSM-22 with narrower 10-ring channels were strongly distorted, giving them much higher strain energies (>37.7 kJ mol(-1)), which were in excellent agreements with our recently reported experimental results (J. Phys. Chem. C 2010, 115, 16124). This led us to conclude that the size and shape of void spaces in the medium-pore zeolites play a crucial role in governing the type of mEDPE and dEDPE formations during the EB disproportionation. Our work also shows that the strain energies of various reaction intermediates confined within zeolites with different pore topologies could be regarded as a useful quantitative means in better understanding the shape-selective nature of this important class of microporous crystalline catalysts.

Selective synthesis of linear alkylbenzene by alkylation of benzene with 1-dodecene over desilicated zeolites

The alkylation of benzene with 1-dodecene to linear alkylbenzenes (LAB) was investigated over 12-ring zeolites MOR, BEA, and FAU with varying framework topologies and Si/Al ratios. The reaction was carried out under a high-pressure, 20bar, in a fixed-bed flow reactor at 140°C, using WHSV 4h−1, benzene/1-dodecene molar ratio of 6.0 and time-on-stream of 6.0h. In contrast to MOR and BEA zeolites, FAU exhibited the lowest selectivity (24%) to the desired 2-phenyl dodecane (2-LAB) due to its large cavities. The MOR and BEA with different Si/Al ratios were further desilicated using alkali-metal treatments (0.2M and 0.05M NaOH) to create hierarchical porous structure. The desilication of both zeolites improved the conversion of 1-dodecene and the selectivity to 2-LAB. The excellent stability resulting from desilication is attributed to a better diffusivity of the LAB isomers, shortening of real contact time, due to the enhanced mesporous structure in both zeolites and the higher Lewis acidity. The selectivity to 2-LAB increased to 70% over desilicated MOR (Si/Al ratio=20) compared with a selectivity of 35% over desilicated BEA (Si/Al ratio=24).

The impact of zeolite pore structure on the catalytic behavior of CuZnAl/zeolite hybrid catalysts for the direct DME synthesis

In this work, the influence of the pore structure of 10-ring zeolites used as the methanol dehydration function in CuZnAl(CZA)/zeolite hybrid catalysts was studied for the direct dimethyl ether (DME) synthesis. To this purpose, six different 10-ring H-zeolites (ZSM-5, FER, IM-5, TNU-9, MCM-22, ITQ-2) with alike bulk Si/Al ratios (in the 9–14 range) were employed. Additionally, the effect of crystallite size (for ZSM-5) and selective surface dealumination by treatment with oxalic acid (for MCM-22) was also investigated. While the initial activity of the zeolites for methanol dehydration was driven by the concentration of strong Brønsted acid sites, the extent of decay was dictated by the pore structure, which determined the amount and nature of the formed carbon species. When evaluated for direct DME synthesis under methanol synthesis-controlled conditions, all CZA/zeolite hybrid catalysts (prepared by grinding, CZA:zeolite mass ratio of 2:1) experienced a decline of CO conversion (and DME yield) with time-on-stream (TOS) due to a gradual loss of the methanol synthesis activity of the Cu-based component. Interestingly, the stability with TOS was the lowest for the hybrid catalysts comprising zeolites with large external surface areas (Sext) such as ITQ-2 and MCM-22. Moreover, for zeolites with similar Sext, the deactivation extent of the hybrid catalysts increased with the concentration of surface Al species (from XPS) in the zeolite. Thus, the delaminated ITQ-2 zeolite (Si/Alsurf=10.6, Sext=324m2/g) produced the less stable hybrid while that comprising zeolite TNU-9 (Si/Alsurf=17.9, Sext=12m2/g) displayed the highest stability during the syngas-to-DME experiments. These results suggest that the deterioration of the methanol synthesis activity of the CZA catalyst in the hybrid catalysts prepared by grinding is produced by detrimental interactions between zeolitic Al species and Cu sites at the surface-contact between zeolite and CZA particles.

Efficient and Accurate Methods for Characterizing Effects of Framework Flexibility on Molecular Diffusion in Zeolites: CH4 Diffusion in Eight Member Ring Zeolites

Molecular dynamics (MD) and transition state theory (TST) methods are becoming efficient tools for predicting diffusion of molecules in nanoporous materials. The accuracy of predictions, however, often depends upon a major assumption that the framework of the material is rigid. This saves a considerable amount of computational time and is often the only method applicable to materials for which accurate force fields to model framework flexibility are not available. In this study, we systematically characterize the effect of framework flexibility on diffusion in four model zeolites (LTA, CHA, ERI, and BIK) that exhibit different patterns of window flexibility. We show that for molecules with kinetic diameters comparable to (or larger than) the size of the window the rigid framework approximation can produce order(s) of magnitude difference in diffusivities as compared to the simulations performed with a fully flexible framework. We also show that simple recipes to include the effect of framework flexibility are not generally accurate. To account for framework flexibility effects efficiently and reliably, we introduce two new methods in which the flexible structure is approximated as a set of discrete rigid snapshots obtained from simulations of dynamics of an empty framework, using either classical or, in principle, ab initio methods. In the first method, we perform MD simulations of diffusion in a usual manner but replace the rigid structure with a new random snapshot at a certain characteristic frequency corresponding to the breathing motion of the window, while keeping positions of adsorbate molecules constant. In the second method, we directly compute cage to cage hopping rates in each rigid snapshot using TST and average over a distribution of snapshots. Excellent agreement is obtained between diffusivities predicted with these two new methods and direct MD simulations using fully flexible structures. Both methods are orders of magnitude more efficient than the simulations with the fully flexible structure. The new methods are broadly applicable for fast and accurate predictions of both infinite dilution and finite loading diffusivities of simple molecules in zeolites and other nanoporous materials, generally without the need for an accurate flexible force field.

Controlled and rapid growth of MTT zeolite crystals with low-aspect-ratio in a microwave reactor

The nucleation and growth of MTT zeolite, an important 10-membered ring (10 MR) zeolite, was controlled by a microwave-assisted hydrothermal synthesis. The effect of irradiation time (12,18, and 24h), seed-assisted synthesis with 1wt.% seeds for 6h and desilication under microwave irradiation on crystallinity and porosity were investigated. Under microwave-assisted synthesis, uniform crystal size, with lower crystal aspect-ratio and suppressed phase-impurity has been obtained. The tendency of column-like crystal to form bundle of columns was reduced significantly. Seeded growth further reduced synthesis time to 6h. Post-synthetic treatment was used to obtain larger pore volume of MTT zeolites. Some of the protonated MTT samples were tested in n-hexane cracking as model reaction. Characterization techniques such as XRD, FE-SEM, N2 adsorption–desorption, 27Al MAS NMR and NH3-TPD have been carried out to obtain insight in the physico-chemical properties of zeolite catalysts.

IZM-2: A promising new zeolite for the selective hydroisomerization of long-chain n-alkanes

IZM-2 (IFPEn Zeolitic Material number 2) is an aluminosilicate zeolite with unknown structure. The synthesis is performed using 1,6-bis(methylpiperidinium)hexyl dibromide template. According to SEM and TEM, IZM-2 particles consist of intergrowths of 20–60nm nanocrystals of various orientations. The nitrogen adsorption isotherm is characteristic of a nanosize microporous material. Characterization of the pore architecture using m-xylene isomerization and disproportionation and n-decane isomerization-hydrocracking test reaction suggests the presence of two types of pores of different diameter. The catalytic fingerprints of 10-membered ring pores are observed and concluded to be present next to wider pores, tentatively qualified as 12-membered ring pores. Pt/IZM-2 was evaluated in the hydroisomerization of n-hexadecane. Pt/IZM-2 is an excellent hydroisomerization catalyst especially suited for formation of multi-branched skeletal isomers. IZM-2 is a valuable addition to the family of 10-membered ring zeolites and may find application in hydroisomerization processes for selective hydroisomerization of long-chain n-alkanes.

The importance of channel intersections in the catalytic performance of high silica stilbite

Researchers at Chevron recently discovered a route for making high silica stilbite (STI), a zeolite with intersecting 10- and 8-ring channels. This zeolite, named SSZ-75, provides an interesting comparison to two other two-dimensional 10/8-ring zeolites, namely, ferrierite (FER) and ZSM-57 (MFS). The catalytic properties of these zeolites are characterized here using the Constraint Index test, adsorption measurements, and toluene disproportionation and its alkylation with isopropyl alcohol. Results are then compared to ZSM-5 (a well-characterized 3-dimensional 10-ring zeolite) and TON (a 1-dimensional 10-ring zeolite). While the micropore volumes for several of these five materials are similar, the Constraint Index, which considers the competitive cracking of n-hexane and 3-methylpentane, differs greatly. On the other hand, toluene conversions along with selectivity to p-xylene for MFI and STI are remarkably similar. This indicates that the reaction volume available at the intersections of the 10- and 8-ring channels in STI is comparable with that of the 10-ring channel intersections in MFI. However, in toluene alkylation with isopropyl alcohol, differences in intersection volume between MFI and STI are revealed. These results nicely demonstrate how the size of the pores and geometry of the channel intersections (rather than just the size of the individual pores) impact the catalytic properties of zeolites with multidimensional channel systems.

Shape Selectivity in the Conversion of Methanol to Hydrocarbons: The Catalytic Performance of One-Dimensional 10-Ring Zeolites: ZSM-22, ZSM-23, ZSM-48, and EU-1

The methanol-to-hydrocarbon (MTH) reaction, a process in which low-value carbon-rich feedstocks are converted to value-added petrochemical products, is studied over one-dimensional 10-ring zeolites: ZSM-22 (TON), ZSM-23 (MTT), ZSM-48 (*MRE), and EU-1 (EUO). The latter three are little studied as MTH catalysts and were expected to display interesting product-shape-selective properties. The influence of slight differences in channel systems of the materials (size and shape) on product distribution and stability is investigated under various reaction conditions. In addition, the influence of coke deposition on product selectivity is investigated. Temperatures between 350 and 500 °C and WHSV between 2 and 6 g g–1 h–1 are investigated using a fixed bed reactor. The products are analyzed using online GC, and hydrocarbons trapped in the channels of the material during the reaction were liberated using the standard HF dissolution procedure and analyzed using GC/MS. Despite the small differences in the channel sha...

Spectroscopic and catalytic characterization of extra large pore zeotype H-ITQ-33

The ITQ-33 structure comprises 18-ring channels, and this material constitutes one of very few zeolites/zeotypes with extra large pores. Such materials are of interest due to their ability to catalyze reactions involving bulky species and the prospect of novel shape selective properties. In this work, the recently discovered and little studied H-ITQ-33 catalyst has been synthesized and characterized. Emphasis has been placed on characterization of the fundamental properties of this material and its catalytic properties in the industrially relevant conversion of methanol to hydrocarbons. An unusually harsh calcination procedure (5 h at 650 °C) is required to fully remove the structure directing agent. Using FTIR, it is shown that the Brønsted acid sites become available once persistent NH 3 / NH 4 + formed during the decomposition is removed. It is demonstrated that H-ITQ-33 is an active catalyst in the conversion of methanol to hydrocarbons. The catalyst is fully regenerable during three deactivation/regeneration cycles. The presence of extra large 18-ring channels is not reflected in the observed product shape selectivity, which is similar to that commonly observed for 12-ring zeolites. This indicates that the formation of bicyclic species as undesired byproducts (coke) is a slow reaction step relative to the reaction leading to arenes. Remarkably, the H-ITQ-33 catalyst contains unusually little carbonaceous deposits (coke) when fully deactivated and the mode of catalyst deactivation has been studied in detail using in situ FTIR, UV–Vis, thermogravimetry and a catalyst dissolution/coke extraction protocol.

Post-synthesis modification of TUN zeolite: Textural, acidic and catalytic properties

Dealumination, desilication and silylation of TUN zeolite was investigated and related to the catalytic behavior in toluene disproportionation and its alkylation with isopropyl alcohol. Only negligible concentration of acid sites was identified on the “external” surface of parent TUN zeolite and this concentration increased with increasing desilication. Silylation resulted in channel narrowing and probably some plugging. As a result, significant increase in the para-selectivity in toluene disproportionation and its alkylation with isopropyl alcohol was achieved. No changes in toluene conversion were observed for all parent and modified samples in toluene alkylation confirming diffusion and transport of products as the rate determining step in 10-ring zeolites. Selectivity to cymenes and p-cymene decreased with dealumination as well as desilication. In contrast, silylation increased both cymene (80%) and p-cymene (90%) selectivity in toluene alkylation.

Conversion of methanol over 10-ring zeolites with differing volumes at channel intersections: comparison of TNU-9, IM-5, ZSM-11 and ZSM-5

Four 3D 10-ring zeolites, IM-5, TNU-9, ZSM-11 and ZSM-5, with Si/Al = 14-24 and crystal sizes below 2 microns, were tested as catalysts for the methanol to hydrocarbons reaction (MTH) at atmospheric pressure, 350 °C and WHSV = 9 h(-1). All catalysts gave initially full methanol conversion, and showed strikingly similar effluent product selectivities. However, their life-time duration differed significantly, and decreased in the order: ZSM-11 > ZSM-5 ≫ TNU-9 > IM-5. A main difference between the two groups of stability behaviour was the size of cavities formed by channel intersections; larger cavities in TNU-9 and IM-5 leading to polyaromatics formation and a more rapid deactivation compared to ZSM-5 and ZSM-11. Effluent yield-conversion plots suggested that polymethylated benzene intermediates were more important in IM-5 and TNU-9 than in ZSM-5 and ZSM-11, where alkene methylation and cracking reactions seemed to dominate product formation. However, this difference had only minor influence on effluent selectivity.

Synthesis of Open Zeolite Structures from Mixtures of Tetramethylammonium and Benzylmethylalkylammonium Cations: A Step Towards Driving Aluminium Location in the Framework

Three bulky organic cations, benzylmethylpyrrolidinium (BMP), (S,S)-2-hydroxymethyl-1-benzyl-1-methylpyrrolidinium (BML) and benzylmethylhexamethyleniminium (BMH), were used in combination with the small cation tetramethylammonium (TMA) in the synthesis of zeolite materials. Two of them are distinguished by the number of carbon atoms forming the cyclic amine, which varies from 4 in BMP to 6 in BMH. In addition, a –CH2–OH group has been introduced in the carbon atom belonging to the amine ring next to the nitrogen atom, to originate a chiral molecule, BML, the third SDA used in this work. It has been found that the combination of the small TMA with those three bulkier SDAs, promotes the crystallization of zeolite structures that contain cages and channels. BMP leads to the crystallization of the 10-ring zeolite ferrierite, where TMA is located inside the ferrierite cage. If the size of the N-cyclic amine moiety increases, the resulting cation cannot be accommodated in the relatively narrow 10-ring channels of ferrierite, and in these cases the zeolite MCM-22 crystallizes. This zeolite contains large cages whose cross section is delimited by a 12-ring, where both BMH and BML can be easily accommodated. In this system, cage-like, chlatrasil type structures often co-crystallize with the open structures, evidencing that an appropriate balance between TMA and the bulky SDA is required to achieve the crystallization of open-frameworks. When BMP is used, mixtures of structurally related ferrierite and cage-like RUB-10 are obtained if an excess of TMA is present in the synthesis gel, while MCM-22 crystallizes together with the structurally related cage-like MTN phase.

ChemInform Abstract: UTM-1: An Eight-Membered Ring Zeolite with the Basic Building Chains of the MFI Topology.

The structure of zeolite UTM-1 has been determined from synchrotron X-ray powder diffraction pattern by direct methods and Rietveld refinement. The framework of UTM-1 consists of pocket-like cages connected by sharing eight-membered rings forming one-dimensional channels. The structure can be completely described by the assembly of chains of 58 cages. Identical chains are found in MFI topology; the zeolites UTM-1 and ZSM-5 are distinguished from each another by the way the chains are assembled.

N-Hexaneactivation over zeolites: aromatic alkylation to 1-phenylhexane

Terminal 1-phenylhexane was observed during alkylation of benzene with n-hexane over 10-membered ring zeolites at moderate temperature, whereas it was not observed when 1-hexene was the reactant.

Acidity of MCM-58 and MCM-68 zeolites in comparison with some other 12-ring zeolites

Two novel zeolites MCM-58 and MCM-68 with 12-ring channels and containing the cages in the structure were investigated with respect to their acidic properties and compared with other two 12-ring zeolites, namely Beta and ZSM-12. The main objective was to characterize the accessibility and acid strength of Lewis and Brønsted sites using FTIR spectroscopy. Pyridine adsorption allowed for quantification of Brønsted and Lewis acid centres and pointed out that while in the structure of MCM-58 nearly all Al atoms form acidic Si OH Al bridges, only about half of Al is a source of the Brønsted acidity in MCM-68. The special attention was devoted to the origin and location of the hydrogen-bonded OH groups in the MCM-58 zeolite. The frequency lowering is caused by the across-the-ring interaction with the oxygen atom and not by the interaction with the nearby OH groups, as proved by high temperature activation and deuteration experiments. The location of the 3485 cm −1 OH groups in the five member ring was proposed. Carbon monoxide adsorption allowed proposing the location of the residual K + cations in the main channels of MCM-58 and in the 6 104 6 cages in MCM-68.

The role of the zeolite channel architecture and acidity on the activity and selectivity in aromatic transformations: The effect of zeolite cages in SSZ-35 zeolite

A series of zeolites differing in the channel architecture and acidity was investigated in toluene disproportionation, together with toluene and p-xylene alkylation with isopropyl alcohol. Zeolites with one- to three-dimensional 10-ring and 12-ring channels with and without cages, and those having 12–12–10 and 12–10–10-ring channel systems were studied. It was shown that general relationship of increasing zeolite activity with increasing pore diameter and pore connectivity is not valid as the size of some 12-ring channels (Beta, MCM-68) is comparable with 10-ring channels (ZSM-5, SSZ-35). In addition, the presence of cages in the structure of SSZ-35 and MCM-58 attributes the unusual catalytic behavior of these zeolites. SSZ-35 and MCM-58 zeolites behave in both toluene reactions such as three-dimensional large-pore zeolites. Subtle differences between zeolites of similar pore sizes and dimensionality can be usually explained based on the differences in the acidity of the individual zeolites. In p-xylene alkylation SSZ-35 exhibited high conversion with the highest selectivity to 1-isopropyl-2,5-dimethyl benzene and a low rate of deactivation. The presence of 18-ring cages in the channels of 10-ring zeolite SSZ-35 (STF) gives rise to an unusual catalytic behavior of this zeolite by comparison to other 10-ring zeolites. SSZ-35, possessing channels of 0.54 × 0.57 nm in diameter, exhibits catalytic activity in transformation of aromatic hydrocarbons that is similar to large-pore zeolites. 18-Ring cages enable the formation of relatively bulkier transition states while the diffusion of the product molecules out of the 10-ring channel system is not slowed down due to 10-ring windows. In addition, the channel system of SSZ-35 prevents the formation of coke precursors.

Synthesis, characterization of COK-5 with different Si/Al ratios and their catalytic properties for the tert-butylation of phenol

The synthesis, characterization and catalytic properties of COK-5 with different Si/Al ratios are described. Various key parameters (including the ratios of Si/Al and/or Na/Si, the type/concentration of alkali cations, and the structure of dialkylammonium) are investigated in detail for the successful synthesis of COK-5. Under our synthesis conditions, it is found that COK-5 zeolites could be synthesized only in a narrow range of Si/Al ratios and Na/Si ratios. The concentration of Na + in the initial gel should be carefully controlled for the successful synthesis of COK-5 with lower Si/Al ratios. Rising the crystallization temperature (<453 K) or adding seed (including heterogeneous seeds ZSM-57 and ZSM-22) can shorten the crystallization time of COK-5. N 2 adsorption–desorption results suggest that the COK-5 samples synthesized from gels with different Si/Al ratios show different texture properties. NH 3-TPD characterization demonstrates the presence of some relatively strong acid sites on the surface of H-form COK-5 zeolites. The resultant COK-5 zeolites with different Si/Al ratios are catalytic active in the liquid-phase tert-butylation of phenol. The acid sites located on the external surface and near the entrance of pore channels of COK-5 zeolites are the main active centers for the reaction. Compared with other 10-ring and/or 12-ring zeolites, different product selectivities are observed on COK-5. We speculate that the nature of less stronger acids and the unusual pore systems of COK-5 zeolites, especially with distorted 12-ring pore structures, should be highly correlated with their catalytic properties.

The Alkylation of Naphthalene over One-Dimensional Twelve-Membered Ring Zeolites. The Influence of Zeolite Structure and Alkylating Agent on the Selectivity for Dialkylnaphthalenes

The Alkylation of Naphthalene over One-Dimensional Twelve-Membered Ring Zeolites. The Influence of Zeolite Structure and Alkylating Agent on the Selectivity for Dialkylnaphthalenes