Mesoporous organic-inorganic hybrid materials built using polyhedral oligomeric silsesquioxane blocks.

It has been well-recognized that many natural organisms exhibit elegant hierarchical porous architectures, which endow them with very unique properties. Inspired by nature, substances that are “made porous” have been one of the most intensively investigated topics involved in the approaches to construct hierarchically dimensioned materials. Vast potential applications can be envisaged for such well-organized porous materials in the fields of catalysis, adsorption, photonics, electronics, and so on. Among the various approaches, the stepwise assembly of predefined nanoscale building blocks is an intriguing strategy for building hierarchical porous materials that can be finely designed and synthesized by tuning the primary building blocks with specific functionality and structure. Polyhedral oligomeric silsesquioxanes (POSS) are ideal building blocks for constructing organic–inorganic hybrid materials. Built through stable siloxane bonds and surrounded by organic peripheries, POSS compounds embody both organic–inorganic characteristics and cagelike structures in one small nanoentity. The hybrid materials made from POSS are usually prepared by blending or covalently bonding POSS to a polymer matrix, which usually results in phaseseparated composites. Early attempts to build homophase materials from sole POSS units mainly used the X-ray radiation induced polymerization, coupling of different POSS cages by hydrosilylation, or by using R2SiCl2 and other organic molecules as cross-linkers. However, it is highly anticipated but still a challenge to use POSS as the sole building blocks for constructing hierarchical organic–inorganic hybrid materials with well-defined porous architectures and specific functions. Herein, we present a new strategy for synthesizing organic–inorganic hybrid materials with hierarchical structure by using predefined POSS units as the only building blocks. We weaved the POSS units into an infinite mesoporous structure by a block-copolymer-assisted coassembly method. A POSS precursor, OVPOSS-SILY (compound 2, Scheme 1), bearing hydrolyzable peripheries, was synthesized by Pt-catalyzed hydrosilylation of an octavinyl-substituted POSS octamer (compound 1, OctavinylPOSS, denoted hereafter as OVPOSS, Scheme 1) with triethoxysilane (HSi(OEt)3). [9] A substoichiometric amount of triethoxysilane relative to the vinyl groups was employed (6:1 HSi(OEt)3/1), with the intention of retaining the vinyl groups in OVPOSSSILY so that they could be further modified. NMR and MALDI-TOF mass spectra show that compound 2 appears as a mixture of POSS with different degrees of vinyl substitutions. The average number of the remaining vinyl groups per cage is estimated to be 2.3 (Figure S1–S3 in the Supporting Information). The hybrid material was synthesized by coassembly of OVPOSS-SILY with P123 and subsequent condensation of OVPOSS-SILY around the P123 micelles (Scheme 1). Extraction of the occluded P123 is expected to generate mesopores penetrating the polymerized POSS framework. The resulting hybrid material is denoted as MesoPOSS. Figures 1a and 1b display the nitrogen-sorption isotherms and the corresponding pore-size-distribution curve of MesoPOSS, respectively. A type IV isotherm with a steep hysteresis loop at relative pressure P/P0 of 0.5–0.6 can be observed, indicating that this sample has mesoporous structure with uniform pore size. The Barrett–Joyner–Halenda (BJH) poresize-distribution curve further confirms the uniform mesopore size centered at 4.40 nm. The sample exhibits a BET surface area as high as 960 mg 1 with a total pore volume of 0.91 cmg . The microporous volume (micropore size: ca. 0.51 nm) is estimated to account for about 35% of the total pore volume from micropore analysis based on the Horvath–Kawazoe (HK) method. However, it is difficult to distinguish the micropores between the densely cross-linked POSS units from the ones that originate from the polyethylene oxide chains of the template. Thus, the porosity from [*] L. Zhang, Prof. Dr. Q. Yang, Prof. Dr. C. Li State Key Laboratory of Catalysis Dalian Institute of Chemical Physics Chinese Academy of Sciences 457 Zhongshan Road, Dalian, 116023 (P.R. China) Fax: (+86)411-84694447 E-mail: [email protected] [email protected] Homepage: http://www.hmm.dicp.ac.cn http://www.canli.dicp.ac.cn