Larry W. Beck

Imaging the Assembly Process of the Organic-Mediated Synthesis of a Zeolite

The formation and consumption of precursors during zeolite crystallisation has been followed by small-angle X-ray scattering techniques. The nanometer-scale precursors are specific for the zeolite topology formed, and their aggregation is an essential step in the nucleation process shown here.

Influence of the hydrophobic character of structure-directing agents for the synthesis of pure-silica zeolites

Tetrapropylammonium, ethanoltripropylammonium and diethanoldipropylammonium cations were used as structure-directing agents for the synthesis of pure-silica, MFI-type zeolites. These molecules are employed to investigate the influence of the organic hydrophobicity/hydrophilicity on zeolite formation. Ethanoltripropylammonium cation can direct the formation of Si-ZSM-5 but at a much slower rate than tetrapropylammonium cation. Diethanoldipropylammonium alone is unable to influence the formation of Si-ZSM-5 but is able to do so with the addition of either seeds or a small amount of tetrapropylammonium. Thus diethanoldipropylammonium cannot elicit nucleation but can support crystal growth. Solid-state nuclear magnetic resonance results show that ethanolalkylammonium molecules are occluded intact within the zeolite pores; however, tetrapropylammonium (more hydrophobic) is preferentially occluded. The results show that control of the hydrophobicity of the organic structure-directing agent is necessary for the synthesis of pure-silica zeolites.

Alkylammonium polycations as structure-directing agents in MFI zeolite synthesis

Abstract Silicate and aluminosilicate MFI-type zeolites are synthesized with di- and triquaternary ammonium cations as structure-directing agents (SDA). Solid-state NMR spectra show that polycations are intact in the as-synthesized zeolites. Materials synthesized with polycationic molecules are compared to zeolites synthesized with tetrapropylammonium, the normal SDA for MFI-type zeolite. The zeolite nucleation rates and particle sizes decrease as the net charge of the organic cation increases, i.e. SDA n + where n =1, 2 or 3.

Synthesis of CIT-6, a zincosilicate with the BEA topology

A new, large pore zincosilicate denoted CIT-6 with the framework topology of zeolite beta has been synthesized from reaction mixtures containing Li+, Zn2+ and tetraethylammonium hydroxide (TEAOH) as a structure-directing agent (SDA). The effects of the concentrations of Li+, Zn2+ and TEAOH on the preparation of CIT-6 are investigated and it is shown that there are critical ranges in concentration for all three components for the crystallization of pure CIT-6. The incorporation of Zn2+ into the CIT-6 framework is confirmed by 29Si MAS-NMR and ion exchange results. CIT-6 is the first molecular sieve to contain framework zinc sites accessible to organic molecules.

Tailoring molecular sieve properties during SDA removal via solvent extraction

Abstract Pure-silica molecular sieves with the MFI topology and zincosilicates, aluminosilicates, borosilicates and pure silicates with the *BEA topology are synthesized and subjected to solvent extraction treatments in an effort to remove the organic structure-directing agents (SDAs) from the micropores. For both molecular sieve topologies, the amount of SDA that can be removed by extraction is found to be dependent on the size of the SDA and the strength of interaction of the SDA with the molecular sieve framework. Furthermore, the potential for extraction of SDAs from the micropores of the material is shown to correlate well with the temperature at which the SDA combusts in thermogravimetric analyses experiments. For materials with SDAs that are small relative to the size of the micropores, the fraction of SDA that can be removed is found to correlate well with the fraction of the SDA that decomposes below 400°C in the materials studied here. SDA that burns or decomposes at temperatures exceeding this value is strongly bound to the framework via ionic charge-balancing interactions. The ease of liberation of charge-balancing tetraethylammonium (TEA) cations from the various metallosilicates is shown to be Zn>B>Al, following the reverse trend of known Bronsted acidity of the various types of sites. It is shown that this tightly bound SDA is removed by extraction under conditions that simultaneously hydrolyze part of the framework. For example, TEA cations charge-balancing boron atoms in the silicate framework are removed with concomitant hydrolysis of the B–O–Si bonds, releasing the tightly bound TEA cation with subsequent desorption of the boron and TEA from the molecular sieve pores. A borosilicate with the *BEA topology synthesized with TEA fluoride as an SDA is shown to be a precursor to a variety of molecular sieves as was previously demonstrated for the zincosilicate with the *BEA topology, CIT-6.

Zeolite synthesis using 1,4-diazabicyclo[2,2,2]octane (DABCO) derivatives as structure-directing agents

Abstract A number of 1,4-diazabicyclo[2,2,2]octane (DABCO) derivatives are used as structure-directing agents in high-silica, zeolite syntheses in the presence and absence of Al 3+ , B 3+ and Zn 2+ . Several high-silica products including phases with unknown structure are synthesized. The results suggest that the product selectivity can be influenced by methyl-substitution of the DABCO moiety. However, when using DABCO-polymers, the number of methylene carbon atoms between the DABCO units rather than methyl-substitution of DABCO has the largest effect on the final products formed. 13 C-CP MAS NMR results show that the signal attributed to the carbon atoms in the DABCO unit in both a DABCO polymer and 1,4-diazoniabicyclo[2,2,2]octane,1,4-dimethyl are split into two signals when these organics are guests in zeolites. The data suggest that the ionic interactions between two cation sites of the DABCO unit and the zeolite framework are different.

Synthesis of 4,4'-trimethylenebis (1-benzyl-1-methylpiperidinium) diastereomers and their use as structure-directing agents in pure-silica molecular sieve syntheses

Abstract The diastereomers of 4,4′-trimethylenebis(1-methyl-1-benzylpiperidinium) are synthesized and used as organic agents in the synthesis of pure-silica molecular sieves in order to investigate further the nature of structure-direction. The diastereomer A that has two trans -1-benzyl- cis -1-methyl-4-(trimethylene)piperidinium moieties adopts a predominant conformation in solution phase at room temperature with N -benzyl groups in the equatorial positions. This molecule strongly directs the formation of the pure-silica analogue of zeolite beta. On the contrary, the diastereomer B that has two cis -1-benzyl- trans -1-methyl-4-(trimethylene)piperidinium moieties whose most stable conformer has N -benzyl groups in the axial positions appears not to direct the formation of materials with the BEA topology but rather the MTW topology. This is somewhat surprising since B is less cylindrical than A and appears more likely to form a multidimensional pore system. These results suggest that the true organic agent that structure-directs the BEA topology is the diastereomer that has N -benzyl(eq)- N -methyl(ax)-piperidinium moieties (diastereomer A). The fact that the diastereomer B is able to give the MTW topology with a rather long crystallization time suggests that cis -1-benzyl-4-(trimethylene)piperidinium manages to fit in the one-dimensional 12-membered ring pore possibly by changing conformation by piperidinium ring flipping.

Investigation of organosilanes as structure-directing agents in zeolite synthesis

Abstract MFI-type zeolites are synthesized with N -trimethoxysilylpropyl- N,N,N -trialkylammonium (TAASi) salts using the composition SiO 2 :0.1TAASiBr:0.25NaOH:40 H 2 O (molar ratios). Prehydrolysis of the organosilane precursor is necessary for zeolite formation. Quantitative NMR spectra show that the majority of the organsilicon carbon-silicon bonds are cleaved in the as-synthesized zeolite.