Insight into the Formation of Nanostructured MFI Sheets and MEL Needles Driven by Molecular Recognition

Abstract

Mesoporous and nanostructured zeolite-based catalysts experience prolonged lifetimes due to increased mass transfer and reduced micropore obstruction by coke formation as compared to their bulky microporous counterparts. Diquaternary ammonium structure-directing agents (SDAs) can be used to synthesize hierarchical MFI sheet-like and MEL needle-like zeolites. An explanation of the underlying molecular-level details of the synthesis of these nanostructured zeolites is presented on the basis of non-covalent interactions between the template and zeolite surfaces as well as silicate oligomers studied by means of classical molecular dynamics. Use was made of Si11 and Si33 silicate oligomers that contain structural features of the framework to be formed as originally proposed by the Leuven group. Molecular recognition is driven by a combination of strong electrostatic and weaker dispersion interactions. An analysis of the early stage of zeolite formation is necessary, as the template adsorption energies in the fully formed zeolite crystals cannot explain the preferential growth of the MFI sheets or MEL needles. Specifically, it is found that the differences in dispersion interactions between the SDA alkyl chains and the silicate oligomers are decisive in the formation of particular zeolite structures.

Environment, Stability and Acidity of External Surface Sites of Silicalite-1 and ZSM-5 Micro and Nano Slabs, Sheets, and Crystals

“…2 of the (010) orientation appears on both sides, in full agreement with the bulk cleavage approach (Figure 2b) and with previous structural considerations. 95 With regard to the (101) orientation, construction with the Si 33 precursors leads to a surface that is very similar (but not strictly identical) to cleavage no. 9, obtained using the bulk cleavage approach.…”

Hierarchically‐Ordered Zeolites: A Critical Assessment

“…The very early ones are FAU [ 143 ] and MFI. [ 141,145 ] Recently, this route has been applied also to the synthesis of aluminophosphate‐, [ 146 ] BEA‐, [ 147 ] MEL‐, [ 148 ] and MWW‐type [ 149 ] materials.…”

Toward the Atomic Scale Simulation of Intricate Acidic Aluminosilicate Catalysts

“…This has been undertaken by static and AIMD approaches, 20,[295][296][297] but due to the computational cost of this method, the very first steps of the growth could be described, up to the formation of a 4MR, 296 even if larger models have recently been built to understand the interaction between the structure-directing agent and possible Si 33 precursors in solution. 298 A scale change is clearly required to simulate the growth of particles of realistic sizes. Kinetic Monte Carlo may be an option to propagate the ab initio data obtained for small entities to bigger assemblies.…”

“…The most relevant simulation would reproduce the synthesis procedure of zeolites, starting from a model of the precursors solution, in the presence of the template. This has been undertaken by static and AIMD approaches, ,− but because of the computational cost of this method, the very first steps of the growth could be described, up to the formation of a 4MR, even if larger models have recently been built to understand the interaction between the structure-directing agent and possible Si 33 precursors in solution . A scale change is clearly required to simulate the growth of particles of realistic sizes.…”