Stacey I. Zones

Zeolites and Catalysis: Synthesis, Reactions and Applications

Prof. AVELINO CORMA was born in Moncofar, Spain in 1951. He studied Chemistry at the Universidad de Valencia (1967-1973), and received his Ph.D. at the Universidad Complutense de Madrid in 1976. He was a Postdoctoral researcher in the Department of chemical engineering at the Queen?s University (Canada, 1977-79). Prof. Corma is the director of the Instituto de Tecnologia Quimica (UPV-CSIC) at the Universidad Politecnica de Valencia since 1990. His current research field is catalysis, and molecular sieves covering aspects of synthesis, characterization and reactivity in acid-base and redox catalysis. Avelino Corma is co-author of more than 700 articles and 100 patents on these subjects.

A combustion-free methodology for synthesizing zeolites and zeolite-like materials

Zeolites are mainly used for the adsorption and separation of ions and small molecules, and as heterogeneous catalysts. More recently, these materials are receiving attention in other applications, such as medical diagnosis and as components in electronic devices. Modern synthetic methodologies for preparing zeolites and zeolite-like materials typically involve the use of organic molecules that direct the assembly pathway and ultimately fill the pore space. Removal of these enclathrated species normally requires high temperature combustion that destroys this high cost component, and the associated energy release in combination with the formed water can be extremely detrimental to the inorganic structure. Here we report a synthetic methodology that avoids these difficulties by creating organic structure-directing agents (SDAs) that can be disassembled within the zeolite pore space to allow removal of their fragments for possible use again by reassembly. The methodology is shown for the synthesis of zeolite ZSM-5 using a SDA that contains a cyclic ketal group that is removed from the SDA while it is inside the zeolite without destruction of the inorganic framework. This approach should be applicable to the synthesis of a wide variety of inorganic and organometallic structures.

Zeolite SSZ-26

A crystalline zeolite SSZ-26 is prepared using a hexamethyl [4.3.3.0] propellane-8,11-diammonium cation as a template. Also disclosed is a process for converting hydrocarbons with crystalline zeolite SSZ-26.

SSZ-26 and SSZ-33: Two molecular sieves with intersecting 10- and 12-ring pores

The framework structures of two closely related molecular sieves, SSZ-26 and SSZ-33, are described. These materials possess a previously missing but desired structural feature in a group of industrially significant zeolites. They contain a three-dimensional pore system that provides access to the crystal interior through both 10- and 12-rings. This property is a consequence of the organic structure—directing agents used in the synthesis of these materials. These materials are examples of the purposeful design of a micropore architecture. Both SSZ-26 and SSZ-33 contain the 4=4–1 building unit that had been previously found only in natural zeolites.

Properties of organic cations that lead to the structure-direction of high-silica molecular sieves

Abstract A variety of new organic structure-directing agents (SDAs) are synthesized and used for the synthesis of high-silica molecular sieves. The hydrophobicity and rigidity of these new and other previously known SDAs are evaluated in terms of their phase transfer behavior from water to chloroform and the number of tertiary and quaternary connectivities, respectively. It is found that the phase transfer behavior of the organic cation SDAs is best measured when they are in their iodide form, in that the greatest ability for discrimination between differences in hydrophobicity is possible in this form of the salt. The phase transfer behavior of numerous SDAs shows the same trend as that of simple tetraalkylammonium iodides with respect to the correlation between C/N + values and percent transferred. The phase transfer results are correlated to the ability for structure-direction in molecular sieve synthesis. It is difficult to obtain a molecular sieve when using extremely hydrophobic SDAs. For hydrophobic, monocationic SDAs, introduction of a second charge into the molecule decreases the hydrophobicity and allows for structure-direction in molecular sieve synthesis. Thus, SDAs with intermediate hydrophobicity are found to be most useful for high-silica molecular sieve synthesis. In terms of SDA geometry, bulky, rigid molecules with limited conformational variability result in near unique formation of a great variety of new high-silica molecular sieves. The use of relatively flexible molecules with a minimum diameter of approximately 5 A gives more than one molecular sieve.

Ordered Silicon Vacancies in the Framework Structure of the Zeolite Catalyst SSZ-74

Physico-chemical characterization of the high-silica zeolite catalyst SSZ-74 (ref. 1) suggested that it, like the related materials TNU-9 (ref. 2) and IM-5 (ref. 3), has a multidimensional 10-ring channel system. Such pore systems are ideal for many petrochemical applications, and indeed SSZ-74 has been shown to be a good catalyst for a wide variety of reactions. The elucidation of its framework structure, however, proved to be difficult. Comparable problems were encountered with TNU-9 and IM-5, which were synthesized with related structure-directing agents. Their framework structures, which are the two most complex ones known, both have 24 Si atoms in the asymmetric unit, and were finally solved by combining high-resolution powder diffraction data with information derived from high-resolution electron microscopy images. Therefore, a similar approach, using the powder charge-flipping algorithm to combine the two types of data and molecular modelling to help to locate the structure-directing agent, was applied to SSZ-74. This procedure eventually revealed a most unusual 23-Si-atom framework structure (|(C(16)H(34)N(2))(4)&Si(92)(4)O(184)(OH)(8)]) with ordered Si vacancies.

Product selectivity in methanol to hydrocarbon conversion for isostructural compositions of AFI and CHA molecular sieves

Isostructural molecular sieves based upon AlPO4 and SiO2 chemistry were made for comparison of catalytic selectivity. The AFI and CHA structures were compared with B and Al substitution of SiO2 and Mg and Si substitution in the AlPO4 case. The conversion of methanol to hydrocarbons was studied. Materials were characterized for acidity by NH3 TPD and NH3 microcalorimetry. Methanol conversion was carried out with products analyzed by GC-MS and spent catalysts by 13C MAS NMR. Borosilicate sieves have acidity too low to carry out this catalytic transformation. Other substituting components were successful but product selectivity seemed to be governed by geometric features of the sieves, rather than by variable acidity due to different types of lattice substitution. Products from small pore molecular sieves SAPO-34 and SSZ-13 were largely olefinic and comprised of C5 and smaller. The large pore sieves, SAPO-5, MAPO-5, and SSZ-24, all produced aromatic-rich products. A considerable quantity of the recovered hydrocarbon was incorporated into penta- and hexamethylbenzene.

Synthesis and Catalytic Properties of Metal Clusters Encapsulated within Small-Pore (SOD, GIS, ANA) Zeolites

The synthesis protocols for encapsulation of metal clusters reported here expand the diversity in catalytic chemistries made possible by the ability of microporous solids to select reactants, transition states, and products on the basis of their molecular size. We report a synthesis strategy for the encapsulation of noble metals and their oxides within SOD (Sodalite, 0.28 nm × 0.28 nm), GIS (Gismondine, 0.45 nm × 0.31 nm), and ANA (Analcime, 0.42 nm × 0.16 nm) zeolites. Encapsulation was achieved via direct hydrothermal synthesis for SOD and GIS using metal precursors stabilized by ammonia or organic amine ligands, which prevent their decomposition or precipitation as colloidal hydroxides at the conditions of hydrothermal synthesis (<380 K) and favor interactions between metal precursors and incipient aluminosilicate nuclei during self-assembly of microporous frameworks. The synthesis of ANA requires higher crystallization temperatures (~415 K) and high pH (>12), thereby causing precipitation of even ligand-stabilized metal precursors as hydroxides. As a result, encapsulation was achieved by the recrystallization of metal clusters containing GIS into ANA, which retained these metal clusters within voids throughout the GIS-ANA transformation.

Diquaternary ammonium compounds in zeolite synthesis: cyclic and polycyclic N-heterocycles connected by methylene chains.

An additional dimension has been added to our long-standing studies in high silica zeolite synthesis via a guest/host synergism. We have created and studied the impact of making symmetric diquaternary ammonium compounds, by varying the chain length between nitrogen charge centers, and the heterocycle size and geometry containing the nitrogen. This allows the introduction of a second spatial parameter in the use of the charged organo-cation guest in the zeolite synthesis. The series of 15 diquaternary ammonium compounds (5 heterocycles synthesized onto chain lengths of C4-C6) were tested in a total of 135 zeolite syntheses reactions. Nine screening reactions were employed for each guest molecule, and the conditions built upon past successes in finding novel high silica zeolites via introduction of boron, aluminum, or germanium as substituting tetrahedral framework atoms for silicon. Eighteen different zeolite structures emerged from the studies. The use of specific chain lengths for derivatives of the pyrrolidine ring system produced novel zeolite materials SSZ-74 and 75.

m-Xylene reactions over zeolites with unidimensional pore systems

Reactions of m-xylene are performed over a series of unidimensional, high-silica zeolites. It is determined that zeolites with a unidimensional pore structure have a unique reaction selectivity when compared to multidimensional zeolites of the same pore size. Unlike multidimensional zeolites with 12 member ring (MR) pores that give a para/ortho (p/o) selectivity of >1, zeolites with unidimensional pores bounded by 14 MRs or large 12 MRs give a p/o <1 due to the influence of a bimolecular isomerization mechanism between a xylene and trimethylbenzene. The unidimensional, parallel pore zeolites (UTD-1, SSZ-31, CIT-5, SSZ-24, ZSM-12, ZSM-48) gave products with the lowest p/o ratio for the given pore size. Multidimensional zeolites and unidimensional zeolites with internal cavities larger than the pore openings all gave higher p/o selectivities. It is shown that at lower flow rates, where external diffusion becomes important, p/o selectivity can be lowered due to an increased amount of bimolecular isomerization occurring at or near the catalyst external surface. While the selectivity of the reactions of m-xylene give useful information for characterizing zeolite structures, it is imperative that all zeolites be compared at the same flow rate and that the flow rate be sufficiently high to negate significant external diffusion effects on reaction selectivity. Under the proper conditions, the reactions of m-xylene can give information enabling the characterization of medium, large and extra-large pore zeolites.