Molecular Conformations of Protonated Dipropylamine in AlPO4-11, AlPO4-31, SAPO-34, and AlPO4-41 Molecular Sieves

In Situ Multinuclear Magic-Angle Spinning NMR: Monitoring Crystallization of Molecular Sieve AlPO4-11 in Real Time

Molecular sieves are crystalline three-dimensional frameworks with well-defined channels and cavities. They have been widely used in industry for many applications such as gas separation/purification, ion exchange, and catalysis. Obviously, understanding the formation mechanisms is fundamentally important. High-resolution solid-state NMR spectroscopy is a powerful method for the study of molecular sieves. However, due to technical challenges, the vast majority of the high-resolution solid-state NMR studies on molecular sieve crystallization are ex situ. In the present work, using a new commercially available NMR rotor that can withhold high pressure and high temperature, we examined the formation of molecular sieve AlPO₄-11 under dry gel conversion conditions by in situ multinuclear (¹H, ²⁷Al, ³¹P, and ¹³C) magic-angle spinning (MAS) solid-state NMR. In situ high-resolution NMR spectra obtained as a function of heating time provide much insights underlying the crystallization mechanism of AlPO₄-11. Specifically, in situ ²⁷Al and ³¹P MAS NMR along with ¹H → ³¹P cross-polarization (CP) MAS NMR were used to monitor the evolution of the local environments of framework Al and P, in situ ¹H → ¹³C CP MAS NMR to follow the behavior of the organic structure directing agent, and in situ ¹H MAS NMR to unveil the effect of water content on crystallization kinetics. The in situ MAS NMR results lead to a better understanding of the formation of AlPO₄-11.

Conformation of intrazeolitic choline ions and the framework topology of zeolite hosts

A close relationship between the conformation of choline and the structure type of zeolites crystallized using this organic species was found.

The host–guest interactions in as-made zeolites Y, UZM-4, UZM-22, offretite, ferrierite, phillipsite, EU-12 and levyne, all of which were synthesized using choline as an organic structure-directing agent, have been investigated by a combination of different experimental techniques, including Raman, ¹H-¹³C CP MAS NMR and variable-temperature IR spectroscopies, together with theoretical calculations. The conformation of this asymmetric quaternary ammonium cation was shown to differ significantly according to the pore topology of the zeolite host and the intrazeolitic location of the organic guest molecule. Theoretical calculations using the pure-silica zeolite model reveal that among its three representative conformers (i.e., gauche, trans and trans′ forms), the conformer, which was experimentally found to dominantly or exclusively exist in zeolite structures studied, always has a lower interaction energy with the surrounding zeolite framework. Our work provides the first example in which the conformation of organic structure-directing agents plays an important kinetic role in governing the phase selectivity during zeolite nucleation.

Study on the synthesis of MFI and FER in the presence of n-butylamine and the property of n-butylamine in a confined region of zeolites

N-Butylamine restricted in MFI and FER was oxidized by hydrogen peroxide.

Influence of template content on selective synthesis of SAPO-18, SAPO-18/34 intergrowth and SAPO-34 molecular sieves used for methanol-to-olefins process

The synthesized products were switched from SAPO-5/18, to SAPO-18, SAPO-18/34, and finally to SAPO-34 with the increasing of TEAOH amount.

SAPO-34 templated by dipropylamine and diisopropylamine: synthesis and catalytic performance in the methanol to olefin (MTO) reaction

This paper describes a detailed investigation on SAPO-34 synthesis templated by isomeric amines and the catalytic performances in the MTO reaction.

High-yield nanosized (Si)AlPO-41 using ethanol polarity equalization and co-templating synthesis approach

Control of the crystallite dimensions of the microporous aluminophosphate AlPO-41 (AFO-type framework structure), and the Si-containing analogue SAPO-41, was attained down to the nanometer scale under stable hydrothermal conditions. The combined application of a tetraalkylammonium co-template (tetrapentylammonium hydroxide) along with an amine structure directing agent (n-dipropylamine) stabilized through the use of ethanol in the initial suspension enables a crystallization medium, which remains homogeneous throughout the entire synthesis. As a direct consequence of the optimized homogeneity of the suspension, the AFO-type microporous nanocrystals (AlPO-41 and SAPO-41) with a size in the range of 30-500 nm with yields surpassing 50% are obtained. The feasibility to obtain nanosized AlPO-41 and SAPO-41 crystals using ethanol as a polarity equalizing agent, resulting in a scalable hydrothermal synthesis from non-colloidal starting mixtures without the use of other assisting methods, is presented.