Allen W. Burton

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.

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.

Structure-directing roles and interactions of fluoride and organocations with siliceous zeolite frameworks.

Interactions of fluoride anions and organocations with crystalline silicate frameworks are shown to depend subtly on the architectures of the organic species, which significantly influence the crystalline structures that result. One- and two-dimensional (2D) (1)H, (19)F, and (29)Si nuclear magnetic resonance (NMR) spectroscopy measurements establish distinct intermolecular interactions among F(-) anions, imidazolium structure-directing agents (SDA(+)), and crystalline silicate frameworks for as-synthesized siliceous zeolites ITW and MTT. Different types and positions of hydrophobic alkyl ligands on the imidazolium SDA(+) species under otherwise identical zeolite synthesis compositions and conditions lead to significantly different interactions between the F(-) and SDA(+) ions and the respective silicate frameworks. For as-synthesized zeolite ITW, F(-) anions are established to reside in the double-four-ring (D4R) cages and interact strongly and selectively with D4R silicate framework sites, as manifested by their strong (19)F-(29)Si dipolar couplings. By comparison, for as-synthesized zeolite MTT, F(-) anions reside within the 10-ring channels and interact relatively weakly with the silicate framework as ion pairs with the SDA(+) ions. Such differences manifest the importance of interactions between the imidazolium and F(-) ions, which account for their structure-directing influences on the topologies of the resulting silicate frameworks. Furthermore, 2D (29)Si{(29)Si} double-quantum NMR measurements establish (29)Si-O-(29)Si site connectivities within the as-synthesized zeolites ITW and MTT that, in conjunction with synchrotron X-ray diffraction analyses, establish insights on complicated order and disorder within their framework structures.

Chapter 5 - Organic Molecules in Zeolite Synthesis: Their Preparation and Structure-Directing Effects

This article reviews historical trends in the use of organic structure-directing agents (SDA) for the syntheses of high-silica zeolites. Methods for the preparation of amines and quaternary ammonium molecules are illustrated with examples of known SDA molecules. Recent developments in the synthesis of zeolites are described: synthesis in concentrated fluoride media, synthesis of germanosilicate zeolites, synthesis in ionothermal media, synthesis using the concept of charge density mismatch, and synthesis with “recyclable” SDA molecules. Examples are also provided that illustrate how molecular modeling is used to understand the structure-directing effects of SDA molecules.

Diquaternary structure-directing agents built upon charged imidazolium ring centers and their use in synthesis of one-dimensional pore zeolites

A series of diquaternary organo-cations, structure-directing agents (SDA), were prepared from imidazole rings connected by methylene chains of various lengths. These were tested in zeolite syntheses to determine if four different one-dimensional pore system zeolites (MTT, TON, MTW and ZSM-48) could accommodate the changes in the positioning of the imidazolium ring that bears the charged nitrogen. We were able to demonstrate that there is flexibility in the methylene chain length for all of the zeolites except MTT, which apparently has more rigid demands concerning the fit of the SDA. While subsequent calculations showed which chain lengths should provide better fits of the SDA within the MTT framework, the syntheses with these SDA still did not yield MTT phases. This raised some issues concerning the dominance of kinetic factors over our thermodynamic considerations in zeolite synthesis.

EMM-23 : A Stable High-Silica Multidimensional Zeolite with Extra-Large Trilobe-Shaped Channels.

Stable, multidimensional, and extra-large pore zeolites are desirable by industry for catalysis and separation of bulky molecules. Here we report EMM-23, the first stable, three-dimensional extra-large pore aluminosilicate zeolite. The structure of EMM-23 was determined from submicron-sized crystals by combining electron crystallography, solid-state nuclear magnetic resonance (NMR), and powder X-ray diffraction. The framework contains highly unusual trilobe-shaped pores that are bound by 21-24 tetrahedral atoms. These extra-large pores are intersected perpendicularly by a two-dimensional 10-ring channel system. Unlike most ideal zeolite frameworks that have tetrahedral sites with four next-nearest tetrahedral neighbors (Q(4) species), this unusual zeolite possesses a high density of Q(2) and Q(3) silicon species. It is the first zeolite prepared directly with Q(2) species that are intrinsic to the framework. EMM-23 is stable after calcination at 540 °C. The formation of this highly interrupted structure is facilitated by the high density of extra framework positive charge introduced by the dicationic structure directing agent.

Strategies in developing routes to commercialization of novel high silica zeolites

In this paper we will contrast the exciting initial work of finding new molecular sieves and the follow-up items needed to address moving forward with application development of a material. Principally, to arrive in the marketplace, the economics must be attractive for introducing new technology. A key hurdle in the production of high silica zeolites, metallophosphates, metalloorganic frameworks, and many novel silicogermantes, is the management of the costs concerning the guest organo-cations and amines used as structure directing agents (SDAs). In this discussion we will describe several novel routes attempting to create a positive impact on this issue.

Control of zeolite framework flexibility and pore topology for separation of ethane and ethylene

Purifying ethylene with flexible zeolites Ethylene is a key feedstock for many chemicals and polymers, but its production requires cryogenic separation from ethane, an energy-consuming step. In theory, pure silica zeolites are well suited to separate olefins from paraffins. Bereciartua et al. synthesized a pure silica zeolite with very small pores, which, if static, would not adsorb either of these hydrocarbons. However, molecular dynamics suggested that the pores should be flexible. Indeed, in competitive adsorption experiments, the zeolite preferentially adsorbed ethylene from a mixed stream of ethylene and ethane. Science, this issue p. 1068 A pure silica zeolite has small, flexible pores that preferentially adsorb ethylene over ethane. The discovery of new materials for separating ethylene from ethane by adsorption, instead of using cryogenic distillation, is a key milestone for molecular separations because of the multiple and widely extended uses of these molecules in industry. This technique has the potential to provide tremendous energy savings when compared with the currently used cryogenic distillation process for ethylene produced through steam cracking. Here we describe the synthesis and structural determination of a flexible pure silica zeolite (ITQ-55). This material can kinetically separate ethylene from ethane with an unprecedented selectivity of ~100, owing to its distinctive pore topology with large heart-shaped cages and framework flexibility. Control of such properties extends the boundaries for applicability of zeolites to challenging separations.

The role of barium cations in the synthesis of low-silica LTL zeolites

Abstract The structure and extraframework atoms of the dehydrated zeolites (Ba,K)-GL (Ba 10 K 16 Si 36 Al 36 O 144 ) and (Ba,Li)-GL (Ba 18 Si 36 Al 36 O 144 ) are characterized by powder neutron diffraction. The frameworks have the LTL topology and are refined in the hexagonal space group P 6/m c c (no. 192). Barium cations fully occupy the centers of the cancrinite cages (site B) and the positions between adjacent cancrinite cages (site C). In (Ba,K)-GL, potassium cations fully occupy sites above the centers of the eight-rings (site D), which line the pores of zeolite (Ba,K)-GL, whereas in (Ba,Li)-GL, barium cations occupy two-thirds of the site D positions. The syntheses and structural refinements of LTL zeolites show that barium and potassium play analogous structure-directing roles in LTL zeolites with different Si/Al ratios. The nearly identical sizes of barium and potassium allow these cations to have optimal coordination with framework oxygens in sites B, C, and D. Barium (a divalent cation) is used in the synthesis of low-silica LTL zeolites, and its role is equivalent to potassium (a monovalent cation) in the synthesis of higher-silica (Si/Al=3) LTL zeolites where potassium also occupies sites B, C, and D. In the absence of a sufficient amount of barium in the synthesis gel, other phases are formed immediately after all the barium cations have been consumed by the zeolite GL crystals.

A comparative study of three closely related unsolved zeolite structures

Abstract In this report we discuss the synthesis and physicochemical characterizations of three zeolites with unsolved crystal structures: SSZ-57, SSZ-74, and IM-5. The diffraction data of SSZ-57 are similar to, but distinct from, the data for ZSM-5, ZSM-11, or ZSM-5/11 intergrowths. The powder diffraction data of SSZ-74 can be indexed in a unit cell with dimensions similar to those found in ZSM-5 and ZSM-11. The organic structure directing agents (SDA) used to prepare SSZ-74 and IM-5 are similar to SDA molecules that often yield multidimensional 10-ring zeolites. The micropore volumes, adsorption uptake rates of 2,2-dimethylbutane, and the constraint index tests of these three unknown materials are also consistent with those expected for multidimensional 10-ring zeolites.