Chemically feasible hypothetical crystalline networks

Abstract

Our systematic enumeration of 4-connected crystalline networks (that is, networks in which each atom is connected to exactly four neighbours) used recent advances in tiling theory to evolve over 900 topologies. The results are relevant to the structures of zeolites and other silicates, aluminophosphates (AlPOs), oxides, nitrides, chalcogenides, halides, carbon networks, and even to polyhedral bubbles in foams. Given their importance as molecular sieves, ion exchangers, catalysts and catalyst supports, we have applied the results to microporous aluminosilicates and aluminophosphates (zeolites). Zeolite chemistry has to date produced 152 distinct types of structure. However, it was always clear that although many further structures can be synthesised, only a fraction of the mathematically generated networks would be chemically feasible (many are 'strained' frameworks requiring unrealistic bond lengths and bond angles), and that an effective 'filtering' process is needed to identify the most plausible frameworks. Here, we describe the use of computational chemistry methods to calculate optimized structural parameters, framework energies relative to alpha-quartz, volumes accessible to sorption, and X-ray diffraction patterns for systematically enumerated hypothetical 4-connected crystalline frameworks. Structures were treated as silica polymorphs with the empirical formula SiO(2), and their energies were minimized.

Design of a “green” one-step catalytic production of ε-caprolactam (precursor of nylon-6)

“…We could have, in practice, invoked variants of the popular practical protocols embracing fast, high-yielding syntheses (27) and high-speed testing (28) of catalytic performance recently adopted by others. However, the number of options open to such a combinatorial approach is forbiddingly large: several dozen candidate AlPO structures and nearly 100 siliceous zeolites (10) would have been open to us. Moreover, the need to work with only scrupulously phase-pure solid catalysts with the considerable care and time needed to secure such purity rendered such an approach impracticable.…”

“…In each case the ion at the T site is connected to its neighboring T site by bridging oxygens. The most recent theoretical and computational analyses (10), employing advances in tiling theory (11), reveal that, in principle, 887 distinct kinds of open structures, based on 3D networks of TO 4 tetrahedra, exist (with unit cell volumes less than Ϸ30,000 Å 3 ). However, very many of these are not chemically plausible because of restrictions imposed by bonding theory.…”

Ultralow-Density Nanocage-Based Metal-Oxide Polymorphs

“…Considering typical excess energies with respect to the ambient ground state of synthesized amorphous oxide phases (e.g., amorphous ZrO 2 : 0:35 0:04 eV=ZrO 2 [29]) together with typical energetic criteria for the feasibility of hypothetical nanoporous silicate frameworks (<0:31 eV=SiO 2 [30]), the energetic expense of forming these new low-density nanoporous oxides does not seem prohibitive. The (nano)-technological value of nanoporous oxides is undoubted with the widespread use of SiO 2 -based zeolites.…”

Apparent Scarcity of Low-Density Polymorphs of Inorganic Solids

“…The rich polymorphism of silica, especially in its low-density forms, allows for fine-tuning of applications (e.g., gas separation membranes) by choosing the best suited polymorph. In addition, 100 000þ hypothetical silica polymorphs have been predicted theoretically as fourfold-connected networks (4CNs) [3][4][5][6][7], a large fraction of which, after accurate theoretical evaluation as silica materials, were found to have comparable energetics to experimentally prepared polymorphs [8,9]. Similarly rich polymorphism has only been further observed for a select group of other inorganic solids, for example, aluminophosphates (AlPO 4 ).…”