Molecular dynamics simulations of a coarse-grained model are used to study the formation mechanism of periodic mesoporous silica over a wide range of cationic surfactant concentrations. This follows up on an earlier study of systems with low surfactant concentrations. We started by studying the phase diagram of the surfactant–water system and found that our model shows good qualitative agreement with experiments with respect to the surfactant concentrations where various phases appear. We then considered the impact of silicate species upon the morphologies formed. We have found that even in concentrated surfactant systemsin the concentration range where pure surfactant solutions yield a liquid crystal phasethe liquid-crystal templating mechanism is not viable because the preformed liquid crystal collapses as silica monomers are added into the solution. Upon the addition of silica dimers, a new phase-separated hexagonal array is formed. The preformed liquid crystals were found to be unstable in the presence of monomeric silicates. In addition, the silica dimer is found to be essential for mesoscale ordering at both low and high surfactant concentrations. Our results support the view that a cooperative interaction of anionic silica oligomers and cationic surfactants determines the mesostructure formation in the M41S family of materials.

We have performed replica-exchange reaction ensemble Monte Carlo simulations to study the low-energy crystalline structures of a reactive model of silica. We have utilized a model of silica polymerization based on the reactive assembly of semiflexible tetrahedral units developed by us previously to reproduce silica bulk moduli as well as self-assembly of amorphous silica gels and nanoparticles. Our implementation of replica-exchange Monte Carlo involves simulating several system copies, each with its own value of the equilibrium constant controlling silica condensation/hydrolysis reactions, which are essential for building higher-order network structures and eventually crystals. These replica-exchange simulations were found to traverse energy landscapes from amorphous to crystalline phases, yielding the dense silica polymorphs α-cristobalite, β-cristobalite, and keatite, as well as the nanoporous silica materials SOD and EDI and nanoporous phosphates with DFT and ATT structures. Simulated crystal structures were confirmed by computing X-ray patterns for comparison with known XRD data. The behavior of this model opens the door to future simulation studies of the free energy barriers controlling these crystallization processes.

Although many porous materials, including metal-organic frameworks (MOFs), have been reported to selectively adsorb C 2 H 2 in C 2 H 2 /CO 2 separation processes, CO 2 -selective sorbents are much less common. Here, we report the remarkable performance of MFU-4 (Zn 5 Cl 4 (bbta) 3 , bbta = benzo-1,2,4,5-bistriazolate) toward inverse CO 2 / C 2 H 2 separation. The MOF facilitates kinetic separation of CO 2 from C 2 H 2 , enabling the generation of high purity C 2 H 2 (> 98 %) with good productivity in dynamic breakthrough experiments. Adsorption kinetics measurements and computational studies show C 2 H 2 is excluded from MFU-4 by narrow pore windows formed by ZnÀ Cl groups. Postsynthetic F À /Cl À ligand exchange was used to synthesize an analogue (MFU-4-F) with expanded pore apertures, resulting in equilibrium C 2 H 2 /CO 2 separation with reversed selectivity compared to MFU-4. MFU-4-F also exhibits a remarkably high C 2 H 2 adsorption capacity (6.7 mmol g À 1 ), allowing fuel grade C 2 H 2 (98 % purity) to be harvested from C 2 H 2 /CO 2 mixtures by room temperature desorption.

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