Model for the Synthesis of Self-Assembling Template-Free Porous Organosilicas

High surface area solids are important materials in science and in many industrial applications but often are produced from expensive and inefficient combinations of materials and processes. New principles for the selection of molecular precursors that yield high surface area solids in simple and efficient sol–gel processes would be useful. Focusing on organosilicas, we show that an index based on rigidity theory can be used to quantify the relative strength of the gel and the level of condensation at which it is able to withstand the capillary stresses imposed by drying, thereby preventing loss of surface area. This index correctly orders precursors according to the surface area of the solid materials produced from them and provides, when correlated to a few data points, a predictive relationship between the index and the surface area. Precursor features leading to early formation of a highly connected rigid network include high ratios of nonhydrolyzing (e.g., methylene) to hydrolyzing (e.g., oxy) groups bridging silicate moieties, large SiOH/Si ratios in the hydrolyzed precursors, and low numbers of noncondensing terminal groups (e.g., methyl). These features explain the extremely high surface areas obtained from 1,1,3,3,5,5-hexaethoxy-1,3,5-trisilacyclohexane and high surface areas obtained by similar materials in aqueous, nontemplated syntheses, as shown in a related publication (DOI: 10.1021/acs.chemmater.7b04480)

EMM-23: A Stable High-Silica Multidimensional Zeolite with Extra-Large Trilobe-Shaped Channels

Stable, multidimensional, and extra-large pore zeolites are desirable by industry for catalysis and separation of bulky molecules. Here we report EMM-23, the first stable, three-dimensional extra-large pore aluminosilicate zeolite. The structure of EMM-23 was determined from submicron-sized crystals by combining electron crystallography, solid-state nuclear magnetic resonance (NMR), and powder X-ray diffraction. The framework contains highly unusual trilobe-shaped pores that are bound by 21–24 tetrahedral atoms. These extra-large pores are intersected perpendicularly by a two-dimensional 10-ring channel system. Unlike most ideal zeolite frameworks that have tetrahedral sites with four next-nearest tetrahedral neighbors (Q⁴ species), this unusual zeolite possesses a high density of Q² and Q³ silicon species. It is the first zeolite prepared directly with Q² species that are intrinsic to the framework. EMM-23 is stable after calcination at 540 °C. The formation of this highly interrupted structure is facilitated by the high density of extra framework positive charge introduced by the dicationic structure directing agent

High-Throughput Synthesis and Structure of Zeolite ZSM-43 with Two-Directional 8‑Ring Channels

The aluminosilicate zeolite ZSM-43 (where ZSM = Zeolite Socony Mobil) was first synthesized more than 3 decades ago, but its chemical structure remained unsolved because of its poor crystallinity and small crystal size. Here we present optimization of the ZSM-43 synthesis using a high-throughput approach and subsequent structure determination by the combination of electron crystallographic methods and powder X-ray diffraction. The synthesis required the use of a combination of both inorganic (Cs⁺ and K⁺) and organic (choline) structure-directing agents. High-throughput synthesis enabled a screening of the synthesis conditions, which made it possible to optimize the synthesis, despite its complexity, in order to obtain a material with significantly improved crystallinity. When both rotation electron diffraction and high-resolution transmission electron microscopy imaging techniques are applied, the structure of ZSM-43 could be determined. The structure of ZSM-43 is a new zeolite framework type and possesses a unique two-dimensional channel system limited by 8-ring channels. ZSM-43 is stable upon calcination, and sorption measurements show that the material is suitable for adsorption of carbon dioxide as well as methane