Halogen bonding has increasingly facilitated the assembly of diverse host-guest solids. Here, we show that a well-known class of organic salts, bis(trimethylammonium) alkane diiodides, can reversibly encapsulate α,ω-diiodoperfluoroalkanes (DIPFAs) through intermolecular interactions between the host9s I– anions and the guest9s terminal iodine substituents. The process is highly selective for the fluorocarbon that forms an I–···I(CF2)mI···I– superanion that is matched in length to the chosen dication. DIPFAs that are 2 to 12 carbons in length (common industrial intermediates) can thereby be isolated from mixtures by means of crystallization from solution upon addition of the dissolved size-matched ionic salt. The solid-state salts can also selectively capture the DIPFAs from the vapor phase, yielding the same product formed from solution despite a lack of porosity of the starting lattice structure. Heating liberates the DIPFAs and regenerates the original salt lattice, highlighting the practical potential for the system in separation applications.

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Nonporous Organic Solids Capable of Dynamically Resolving Mixtures of Diiodoperfluoroalkanes

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.

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

A series of diquaternary pyrazolium-derived organic templates (N,N′-dimethyl-N,N′-1,6-dihexylidenedipyrazolium, N,N′-diethyl-N,N′-1,6-dihexylidenedipyrazolium, N,N′-dipropyl-N,N′-1,6-dihexylidenedipyrazolium, denoted as 6C-DMP, 6C-DEP, 6C-DPP, respectively) with methyl, ethyl and propyl groups substituted on the N atom of pyrazole ring at both terminals have been used in the synthesis of high silica MTW and MFI zeolites. Through combining the characterization results, including XRD, NMR, elemental analysis, TG, XRF, FE-SEM, N2 sorption and FE-TEM, with molecular mechanics simulations to explore the location, orientation and the interaction energies of the three templates, we confirmed the state of templates in zeolite framework, carefully characterized their morphology/structure properties, and finally investigated their different spatial effects for the zeolite formation. The study found that 6C-DMP and 6C-DEP are able to produce MTW, while 6C-DPP is able to produce MFI. 6C-DMP, owing to a good match with the MTW framework and can be used to synthesize regular MTW zeolite with few defects. The MTW zeolite prepared by using 6C-DEP as a template presents more defects and irregular macromorphology due to a relatively poor match to the MTW framework. 6C-DPP can get MFI other than MTW due to a larger spatial hindrance, and it is located in the MFI framework with a special spatial orientation.

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Effect of pyrazolium-derived compounds as templates in zeolite synthesis

This chapter discusses the molecular modeling in Zeolite science. The methods of which there is no precise definition but in the present context, takes all methods that model systems at the molecular level, using an interatomic potential or forcefield, rather than an explicit quantum mechanical (QM) method. The distinction between molecular modeling and quantum chemistry, however, become blurred, especially as quantum chemical methods are commonly involved in developing interatomic potentials. The chapter emphasizes on modeling structure, sorption and diffusion, surface and defect properties, and problems related to synthesis, in particular nucleation and templating. Modeling of surfaces and defects offers particular challenges and rewards owing to the difficulty of experimental studies in these fields. Modeling has proved effective in template design and is beginning to throw light on the fundamental factors controlling synthesis. Illustrations of these and other applications have been given in this chapter, and the main methods employed in the field have been outlined.

Chapter 19 Molecular modelling in zeolite science

Molecular modelling in zeolite science

Computer simulation of the interactions of diquaternary cations with zeolites ZSM-50, NU-87, ZSM-23 and ZSM-12 is carried out to determine the templating ability of diquaternary cations in the formation of these zeolites. The calculated energies are a measure of “match” between the geometry of diquaternary cations and the topology of the zeolite framework. The systematic synthesis results of Mioni et al., especially the dependence of the products on the chain lengths of diquaternary cations are rationalized quantitatively. We infer that it is consistent with the templating theory of zeolite synthesis that ZSM-12 and ZSM-23 both can be formed by more than one type of diquaternary cations.

Lijun Wang

Papers

A Study on the Templating Ability of Diquaternary Cations for the Zeolite Synthesis in Terms of Energetics