AmericanChemicalSociety.com

The current project explores our recently proposed approach to the judicious selection of micelle expanders (swelling agents) in the surfactant-templated synthesis of ultra-large-pore ordered mesoporous materials (OMMs). The swelling agent candidates are selected on the basis of a hypothesis that substances that solubilize in micelles of a particular surfactant to a moderate extent promise to be micelle expanders leading to well-ordered OMMs with significantly enlarged mesopores, perhaps of size beyond that hitherto achieved. On the other hand, substances that solubilize too strongly are likely to afford non-uniform very-large-pore products (perhaps foams), while substances solubilizing to a small extent may lead to no appreciable pore size enlargement. The suggested way to identify swelling agents is to examine data on the extent of solubilization of organic compounds in surfactant solutions, with a particular attention to families of compounds for which the extent of solubilization varies in a wide range. If there are experimental data available on the micelle expander performance of one or more of the members of a particular swelling agent family in conjunction with a particular surfactant (or surfactant family), one can consider whether the experimentally observed swelling action was excessive or too weak. On this basis, one can attempt to identify a member of the swelling agent family that would solubilize in the considered surfactant in an optimal way. In particular, in the synthesis of ordered mesoporous silicas using Pluronic triblock copolymers (poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide), PEO-PPO-PEO), the use of EO₂₀PO₇₀EO₂₀ surfactant Pluronic P123 (with relatively large fraction of the hydrophobic block) in conjunction with 1,3,5-methylbenzene (TMB) was known to lead to large-pore high-pore-volume foams. Apparently, the swelling was excessive, and the use of a swelling agent that solubilizes to a smaller extent was predicted to be beneficial. Knowing that in case of alkyl-substituted benzenes, the extent of solubilization in Pluronics decreases as the number and/or size of alkyl substituants increases, 1,3,5-triisopropylbenzene (TIPB) was identified and it indeed afforded ultra-large-pore 2-D hexagonal SBA-15 silica with cylindrical mesopores (Chem. Mater. 2009, 21, 1144). On the other hand, it was noticed that EO₁₀₆PO₇₀EO₁₀₆ triblock copolymer Pluronic F127 with a low fraction of the hydrophobic block was swollen to a limited extent by TMB in the synthesis of LP-FDU-12 silica with face-centered cubic, fcc, structure (Fm3m symmetry) of spherical mesopores. Therefore, more strongly solubilized xylene and toluene were selected, indeed leading to significant unit-cell size and pore-diameter increases.

The work on the project was focused on: (i) the optimization of our syntheses of ultra-large-pore SBA-15 and FDU-12 that were elaborated earlier, (ii) the application of our micelle expander selection principles to a new class of materials, periodic mesoporous organosilicas (PMOs), and (iii) better understanding of the selection principles.

While our earlier work resulted in SBA-15 silicas with (100) interplanar spacing d₁₀₀ up to 26 nm, the degree of structural ordering was significantly reduced for d₁₀₀ above 24 nm. The optimization of the synthesis allowed us to significantly improve the ordering for SBA-15 with d₁₀₀ of 25-27 nm and also afforded less ordered SBA-15 with d₁₀₀ approaching 30 nm These materials have pore diameters of 25-30 nm. The way to dramatically reduce the synthesis time (to several hours) was also established for ultra-large-pore SBA-15 (L. Cao, Ph.D. Dissertation under PI’s guidance). Moreover, the synthesis of FDU-12 with spherical mesopores using xylenes as a micelle expander was optimized to achieve the unit-cell size up to ~55 nm and pore diameter up to ~35 nm. A comparable unit-cell size was achieved earlier only using custom-made surfactants.

Our earlier study identified TIPB and cyclohexane as swelling agents used in conjunction with EO₂₀PO₇₀EO₂₀ surfactant. This identification allowed us to develop a family of PMOs with 2-D hexagonal structures of unprecedented unit-cell size (d₁₀₀ from 14 to 26 nm) and methylene, ethylene, ethenylene, and phenylene bridging groups. Cyclohexane was identified as an extension of linear hydrocarbon series used by others in large-pore SBA-15 synthesis, as the solution data suggested that cyclohexane solubilizes in Pluronics to a larger extent than its linear counterparts. Notably, cyclohexane was found much more suitable for PMOs with aliphatic bridging groups than for pure silicas. While cyclohexane did not afford large-pore PMO with aromatic bridging group, TIPB was found highly suitable. The unit-cell size of PMOs varied with the initial synthesis temperature, allowing us to generate families of PMOs with a wide range of unprecedented unit-cell parameters and pore diameters. In addition, highly ordered ethylene-bridged organosilicas with Fm3m structure and unit-cell parameters of up to 43 nm as well as moderately or weakly ordered PMOs with unit-cell parameters up to ~50 nm were successfully obtained using EO₁₀₆PO₇₀EO₁₀₆ and xylene or toluene as swelling agents. Such large-unit-cell PMOs have not been reported before. For these cubic PMOs, the pore diameter was controllable by the adjustment of the inorganic salt concentration in the synthesis mixture.

Our research showed that our principle of the swelling agents selection is applicable to not only silicas, but also PMOs, confirming the promise of its applicability to different framework compositions. Even in cases where for a particular framework composition, the synthesis did not work well with our initial selection of a surfactant/swelling-agent pair, the replacement of a swelling agent with another one predicted to have similar properties was likely to afford high-quality large-pore materials.

Using large-pore silicas discussed above, we explored surface-initiated atom transfer radical polymerization and “click” chemistry attachment of preformed polymers as ways to introduce uniform thin layers of polymers on surfaces of high-surface-area porous supports.

The PRF award significantly facilitated PI’s advancement as an assistant professor and contributed to a favorable tenure decision. It provided funds to support two Ph.D. students, as well as two undergraduate students (one being a minority student). The funding also allowed Ph.D. students to participate in an ACS National Meeting. The two Ph.D. students defended their Ph.D. Dissertations in Summer 2010. The PRF award also provided much needed funding for chemicals and supplies.