Origin of the structure-directing effect resulting in identical topological open-framework materials

Abstract

In the synthesis of zeolites and related crystalline materials with open-frameworks, a single structure is obtained in the presence of many different templates, known as the “one-structure/multiple-templates” phenomenon. However, the reasons behind this phenomenon have yet to be elucidated. By analyzing the possible starting point of crystallization in several “one-structure/multiple-templates” systems and applying the molecular dynamics simulation to such systems, we found that the template-framework binding free energy level or charge transfer (exchange) degree was the key to the structure-directing effect of a template. This discovery explains why the structure-directing effect of a template can be affected by many variables, such as the nature of the source materials, molar composition of the initial reaction mixture (recipe), mineralizers, type of solvent, and heating temperature. In the synthesis of zeolites and related crystalline materials with open-frameworks, the template or organic additive played a topological structure-directing role instead of a structure-directing role.

A Multinuclear NMR Study of Six Forms of AlPO-34: Structure and Motional Broadening

“…Relating our results to those of Xin et al, who examined AlPO-34 as a model for understanding the origin of the structure-directing effect, 35 we have shown how three further SDAs, not considered in their work, are effective for forming the same framework.…”

“…[29][30][31]33,34 However, although AlPO-34(pip) has been prepared several times before, 32,44,45 its structure was only reported very recently, during the preparation of this manuscript. 35 At the time of our work the structure was unknown, and so high-resolution powder synchrotron X-ray diffraction measurements were undertaken in order to determine a crystal structure for this form of as-made AlPO-34. Selected crystallographic parameters for AlPO-34(pip) are summarized in Table 1 The presence of H2O in two of the hydrothermally-prepared samples is perhaps not surprising, although the presence of water was not determined previously by diffraction in either AlPO-34(mor) 33 or AlPO-34(cyclam), 34 which are also prepared in aqueous media.…”

“…Xin et al 35 concluded that the SDA-framework binding free energy is the key origin of the structure directing effect. Our work also reveals that water may be included in the porous structure, along with the SDA, depending on synthesis route, and that the amount of water may vary with time once the sample has been isolated and exposed to the atmosphere.…”

“…33 It is, therefore, possible that the mor exhibits greater conformational or orientational flexibility than indicated in the published structures, both of which contain a single cation conformation. 33,35 Another possibility, as will be discussed in more detail below, is that water is also included in the structure depending on either the synthesis route or the conditions under which the sample is studied. The 1 H NMR spectrum of AlPO-34(cyclam) is of very low resolution, even at a MAS frequency of 55 kHz.…”

“…During the course of our work and the preparation of this article, Xin et al have reported as a model system to examine the origin of the structure directing effect in the formation of microporous materials. 35 They studied five forms of the material, with three SDAs the same as we have studied here (morpholine, piperidine and pyridine) and two other examples (diethylamine and propylamine) and used single-crystal X-ray crystallography and molecular dynamics simulations to understand SDA-framework interactions. Our work, using a combination of different experimental and theoretical approaches, is highly complementary to the work of Xin et al, since we consider local structural disorder, and we additionally study three further SDAs not considered in their work (dimethylimizadole, diethylimizadole and cyclam).…”