Structure-Directing Role of Molecules Containing Benzyl Rings in the Synthesis of a Large-Pore Aluminophosphate Molecular Sieve:  An Experimental and Computational Study

Abstract

We describe the synthesis of AlPO-5 and SAPO-5 materials (AFI topology) using five different tertiary amines or quaternary ammonium ions containing one or two benzyl rings as structure-directing agents (SDAs). All of the molecules successfully direct the crystallization of AlPO-5; however, only the most efficient templates are able to crystallize SAPO-5. The observed differences in template efficiency can be rationalized in terms of the interaction energy between these molecules and the AFI framework. In ranking the template molecules, we notice that a well-defined molecular shape enhances the templating ability, but molecules that are too rigid are not able to adapt to the AlPO framework, yielding an inferior templating ability. Results of atomic-level modeling show that templates with one benzyl ring self-assemble in the main AFI channel by forming dimers with the benzyl rings parallel to each other; templates with two benzyl rings assemble instead into longer chains in which the benzyl ring of one molecule faces the ring of the subsequent one. Both mono- and dibenzyl templates show a high space-filling ability in AFI. Kinetic and thermodynamic factors that might affect the structure-directing activity of the molecules are examined.

Insights into Structure Direction of Microporous Aluminophosphates: Competition between Organic Molecules and Water

“…The packing value of 1.33 is reached when rings are located parallel to each other to form dimers (see Figure 7, bottom), as we have already demonstrated. [21][22][23] This arrangement was studied for BP by loading four BP molecules (arranged as dimers, ss) in a 1 1 3 unit cell system, whereas for BPM, 24 BPM molecules were loaded in a 1 1 18 unit cell system (for details of these systems, see refs. [21,23]).…”

“…[30] This force field was originally developed for small organic molecules but has been developed for materials science applications, including the simulation of zeolite and related structures, to which it has been successfully applied recently. [21][22][23][31][32][33] The AFI framework atoms were kept fixed during all the calculations. Due to the very acidic pH of the synthesis gels (3-3.5), SDA molecules are expected to be protonated during the crystallisation of the AFI structure; therefore, protonated TEA, BP and BPM ammonium cations were studied.…”

Un-assemblable layered aluminophosphates from self-assembling structure-directing agents: Effect of fluorine

“…Despite the large number of SDAs of very different sizes and shapes used up to date, their choice as structure directing agents has almost invariably considered the features of single molecular units; supramolecular chemistry concepts have rarely been mentioned when studying structure direction of nanoporous frameworks by organic molecules. In this context, a new concept in structure direction of nanoporous materials was proposed by us [16][17][18][19][20][21] and by Corma et al [22], consisting in the use of supramolecular self-assembled molecules as SDAs. This can be achieved by using molecules containing aromatic rings that tend to self-assemble in water solution with their aromatic rings interacting through - type interactions; this synthetic strategy would lead to supramolecular large aggregates composed of several individual molecules that could potentially structure-direct the synthesis of very large-pore framework materials.…”

Mixed template effect adjusted by amine concentration in ionothermal synthesis of molecular sieves

“…To achieve these aims, researchers have focused much attention on guest species, such as designing the size and geometry of the template, 1 using mixed templates to interact with frameworks 2 or using supramolecular assembling of organic structure directing agents (SDA). [3][4][5] These methods have inspired us to develop a simpler route to control the synthesis of molecular sieves.…”