Gregory J. Lewis

Experimental charge density matching approach to zeolite synthesis

Abstract An Experimental Charge Density Matching (ECDM) approach to zeolite synthesis is described. The method features the initial formation of a Charge Density Mismatch (CDM) aluminosilicate reaction mixture characterized by the mismatch between the charge density on the organoammonium structure directing agent (SDA) and the charge density on the potential aluminosilicate network that is expected to form. This amounts to creating an aluminosilicate reaction mixture using a large SDA and a low Si/A1 ratio, but also depends on hydroxide levels and crystallization temperature. The crystallization of a zeolite from such a reaction mixture may be difficult, or even impossible. Crystallization can be induced by the controlled addition of supplemental SDAs that have charge densities that are more suitably matched to that of the desired low ratio aluminosilicate network. Advantages of this approach are greater control over the crystallization process and reliable cooperation of multiple templates. The approach is demonstrated in the TEA-TMA template system, in which the new zeolites UZM-4, UZM-5, and UZM-9 are synthesized.

Formation Pathway for LTA Zeolite Crystals Synthesized via a Charge Density Mismatch Approach

A solid understanding of the molecular-level mechanisms responsible for zeolite crystallization remains one of the most challenging issues in modern zeolite science. Here we investigated the formation pathway for high-silica LTA zeolite crystals in the simultaneous presence of tetraethylammonium (TEA(+)), tetramethylammonium (TMA(+)), and Na(+) ions as structure-directing agents (SDAs) with the goal of better understanding the charge density mismatch synthesis approach, which was designed to foster cooperation between two or more different SDAs. Nucleation was found to begin with the formation of lta-cages rather than the notably smaller sod and d4r-cages, with concomitant incorporation of TMA(+) and Na(+) into a very small amount of the solid phase with a low Si/Al ratio (ca. 2.5). The overall characterization results of our work demonstrate that sod-cages are first built around the preorganized lta-cages and that d4r-cages are in turn constructed by the progressive addition of low-molecular-weight (alumino)silicate species, which promotes the formation and growth of embryonic LTA zeolite crystals. We also show that the crystal growth may take place by a similar process in which TEA(+) is also incorporated, forming a single LTA zeolite phase with a higher Si/Al ratio (ca. 3.3).

Combinatorial chemistry – The emperor's new clothes?

Considering the increasing pressures and constraints on research and development towards focussed innovation, the prospects of new fast and cost-effective approaches addressing the many challenges faced in more rapidly developing next generation products are expected to be very significant. The application of the new methodologies of combinatorial chemistry to materials discovery and development, though facing special challenges, has now been demonstrated in the hydrothermal synthesis of a series of aluminophosphate materials. Using this approach an extensive study of complex multi-component systems was easily achieved, demonstrating the highly reproducible experimental conditions that allows efficient screening of phase composition based on X-ray diffraction analysis.

Synthesis and characterization of the 12-ring zeolites UZM-4 (BPH) and UZM-22 (MEI) via the charge density mismatch approach in the Choline-Li2O-SrO-Al2O3-SiO2 system

Abstract The Charge Density Mismatch (CDM) approach to zeolite synthesis employs zeolitic reaction mixtures which cannot easily crystallize, which are then perturbed by the addition of small amounts of crystallization-inducing agents to achieve crystallization. This approach allows great control over crystallization and extends the utility of any organic template. In this work, choline hydroxide derived aluminosilicate solutions were treated with solutions containing Li + , Sr 2+ , or both, resulting in the isolation of useful forms of the structurally similar 12-ring zeolites UZM-4 (BPH) and UZM-22 (MEI). UZM-4 is isolated in the Li-Choline system at Si/Al = 2.66, the highest ratio achieved by direct synthesis. UZM-22 is the first example of an MEI aluminosilicate prepared with a simple organic template, making this an accessible material.

UZM-13, UZM-17, UZM-19 and UZM-25: synthesis and structure of new layered precursors and a zeolite discovered via combinatorial chemistry techniques

Abstract A combinatorial chemistry investigation screening the abilities of the diethyldimethylammonium (DEDMA), ethyltrimethylammonium (ETMA), and [Me 3 N(CH 2 ) 4 NMe 3 ] 2+ (diquat-4, DQ-4) cations to form zeolites over a range of conditions was carried out. Each of the templates formed a layered precursor; UZM-13, UZM-17, and UZM-19 forming in the DEDMA, ETMA, and DQ-4 systems, respectively. The layered materials are distinct from, but similar to MCM-47 by XRD. The structure of UZM-13 was solved from powder data and was confirmed to contain MCM-47-like aluminosilicate layers. Calcination of these layered materials led to condensation along the b-axis, forming similar 3-dimensional zeolitic species designated UZM-25. However, the UZM-25 derived from the UZM-13 material was superior in crystallinity and had the only XRD pattern that could be indexed. The structure of UZM-25 was determined to be of the CDO structure type, which contains 2-dimensional intersecting 8-ring pores, from both powder and single crystal data.

Synthesis and catalytic activity of UZM-12

Abstract UZM-12 is a microporous crystalline aluminosilicate with ERI topology synthesized via the Charge Density Mismatch approach. The Charge Density Mismatch approach features synthesis from clear aluminosilicate solutions with high organic template content that are subsequently treated with crystallization inducing solutions. The solution containing low amounts of alkali and a higher charge density organic “crystallization” template, yields UZM-12 with Si/Al > 5.5 and nano- to micron-sized crystallites of spherical, plate or rod morphology depending on crystallization template, K + /Al ratio and synthesis conditions. Crystallization templates include the diquat-6, diquat-4, and benzyltrimethylammonium cations. The thermal, steam stability, and acid properties of H-UZM-12 were determined using the combinatorial heptane microreactor.

A novel screening approach utilizing combinatorial methods in the ethyltrimethylammonium template system

Abstract A template screening approach which applies combinatorial methods is used to investigate the structure directing properties of the ethyltrimethylammonium (ETMA) template, both by itself and in combination with other structure directing agents (SDA), in the range Si/Al=2-48. A central theme of the approach is to decouple the elements of zeolite crystallization, namely the silica, alumina and hydroxide sources, from the sources of the additional SDAs that strongly influence crystallization pathways, to more easily elucidate the roles of each in synthesis. This is accomplished by using ETMA aluminosilicate solutions, ETMAOH as the sole hydroxide source and supplemental SDAs (alkali and alkaline earth cations) as salts. The resulting experiment is well integrated with combinatorial methods, yields the new zeolite species UZM-4, UZM-8, UZM-15, UZM-17, and demonstrates the broad potential of a single organic template to make both small and large pore materials.

Synthesis, characterization, and applications of the new zeolite UZM-5

Abstract A new zeolite, UZM-5, has been recently reported and assigned the IZA structure code UFI. Materials with different morphologies ranging from a plate-like structure to rosettes can be obtained by varying the synthesis conditions. Adsorption IR measurements McBain adsorption, and microreactor testing determined accessibility of external and internal acid sites consistent with the structure. Catalytic activity testing and multiple probe molecule adsorption studies indicate primarily external surface reactivity. Bronsted/Lewis acid site ratio and acid strength distribution can be varied through the control of morphology for optimized efficiency for targeted applications.

UZM-4: A stable si-rich form of the BPH framework type

Abstract The zeolite UZM-4 is an organic templated form of the large pore, 12-ring BPH framework type (Si/Al=1.5–2.5), that is more stable than the isostructural Linde Q (Si/Al=1.1). Powder X-ray diffraction and adsorption properties are consistent with this framework type. Occluded cations, both inorganic and organic, are readily ion-exchanged without calcination. Extensive modification attempts (ammonium fluorosilicate treatment, steaming, acidic NH 4 -exchange) have produced stable, active acid catalysts.

High Throughput Screening of Complex Hydrides for Hydrogen Storage

The discovery that dopants, such as Ti, cause NaAlH 4 to reversibly desorb H 2 at mild conditions has spurred a great deal of research into complex metal hydrides. However, no complex hydride meets the targets for automotive hydrogen storage. Our approach is to accelerate the rate of discovery of improved hydrides and dopants through the combination of Virtual High Throughput Screening (VHTS) and Combinatorial Synthesis and Screening (CSS). Our CSS methods will allow us to screen thousands of samples in a year. These samples will be prepared by ball milling mixtures of hydrides and dopants similar to the established method of preparing Ti doped NaAlH 4 . VHTS exploits a molecular mechanics method to screen a thousand phases in a month. The combination of combinatorial methods and VHTS will help us discover the most promising complex hydrides for hydrogen storage. We will show the results of our medium throughput CSS and VHTS as applied to the NaAlH 4 –LiAlH 4 – Mg(AlH 4 ) 2 mixed alanate compositions.