Porous inorganic–organic hybrid polymers derived from cyclic siloxane building blocks: Effects of substituting groups on mesoporous structures

https://doi.org/10.1016/j.micromeso.2018.11.016Get rights and content

Highlights

  • Hybrid polymers with siloxane frameworks having both micro- and mesopores are synthesized by Friedel-Crafts alkylation.

  • The structures of mesopores are found to depend solely on the organic substituting groups of the siloxane building blocks.

  • The Si–phenyl bonds are selectively cleaved during polymerization, leading to the formation of different pore structures.

  • The polymers from the methyl and phenyl-substituted siloxanes show higher surface areas and much larger pore volumes.

Abstract

Porous inorganic–organic hybrid polymers (PHPs) with cyclic siloxane frameworks are synthesized via Friedel–Crafts alkylation between siloxane molecules and formaldehyde dimethyl acetal. The synthesized polymers with high specific surface areas (up to 1310 m2 g−1) show a combination of type I(b) and type IV(a) isotherms, as determined by argon adsorption–desorption measurements, suggesting that they contain both micropores and mesopores. Interestingly, two different types of hysteresis loops are observed, depending solely on the organic substituting groups of the siloxane building blocks. PHPs synthesized from cyclic siloxane building blocks substituted with one methyl group and one phenyl group per one silicon atom show type H3 hysteresis loop; while those synthesized from cyclic siloxane building blocks substituted with two phenyl groups show type H2(a) hysteresis loop. This indicates that the resulting PHPs possess different porous networks. 29Si MAS NMR spectra indicate that the Si–phenyl covalent bonds are selectively cleaved during polymerization; while the Si–methyl bonds are not, leading to the formation of different pore structures.

Introduction

Porous organic polymers have attracted considerable attention recently partly due to their low-density characteristic, diverse functionalities, and ease of synthesis, providing novel platforms for several applications such as gas storage and catalysis [1]. In general, they can be synthesized via standard synthetic techniques in organic chemistry. As a special case, porous polymers can be formed as crystalline materials, known as covalent organic frameworks (COFs), under reversible synthesis conditions [[2], [3], [4]]. Since crystallinity is not always necessary for applications, several amorphous porous polymers have been reported, including polymers of intrinsic microporosity (PIMs) [5,6], conjugated microporous polymers (CMPs) [7], and hypercrosslinked polymers (HCPs) [[8], [9], [10]].

To form new carbon–carbon bonds during polymerization of porous polymers, metal-catalyzed reactions such as Sonogashira–Hagihara coupling with Pd catalyst, Suzuki–Miyaura coupling with Pd catalyst, and Yamamoto coupling with Ni catalyst have often been employed. However, these reactions typically require expensive metal catalysts and/or organic ligands. As an alternative, Friedel–Crafts alkylation, which is catalyzed by much cheaper Lewis acid such as aluminum chloride and iron (III) chloride, has been used to synthesize microporous organic polymers [11]. A simple and one-pot method, named knitting, based on Friedel–Crafts alkylation of rigid aromatic building blocks using an external crosslinker has been shown to be applicable for synthesis of microporous organic polymers with versatile functional groups [12]. The aromatic building blocks range from simple compounds such as benzene and biphenyl, through substituted compounds such as binaphthol, to heterocyclic compounds such as thiophene and pyrrole [[12], [13], [14]].

To diversify the functionality of organic frameworks, inorganic moieties have been introduced to the frameworks. Polyhedral oligomeric silsesquioxane (POSS) is among representative and ideal inorganic building blocks used for construction of porous materials owing to its rigidity, high symmetry, highly connectable structure, and ease of functionalization [[15], [16], [17], [18], [19], [20], [21]]. Interestingly, it was reported that, during Friedel–Crafts alkylation, polymerization and destruction of functionalized POSS units occurred simultaneously, yielding porous inorganic–organic hybrid polymers with ultrahigh surface area [22]. Moreover, two porous polymers derived from the same POSS unit were synthesized via Friedel–Crafts alkylation with external linkers and homopolymerization by Scholl coupling, respectively. In the case of Friedel–Crafts alkylation, the siloxane framework collapsed partially, and the resulting polymer had a higher specific surface area than that of the polymer synthesized by Scholl coupling [23]. These findings suggest that the partially collapsed POSS units may lead to porous networks with higher surface areas, offering the idea of using cyclic siloxane molecules as the building blocks for construction of highly porous polymers [24].

Gas adsorption–desorption measurements have been used widely for characterization of porous materials [25]. The shape of sorption isotherms is directly correlated to the size and shape of pores. Although most of porous polymers showed type I isotherms, indicative of micropores, some of them possessed mesopores and thus showed type IV isotherms, especially the polymers synthesized from large building blocks (monomers) [[26], [27], [28]]. Alternatively, mesopores can be introduced to the polymer networks by simultaneous assembly and polymerization in the presence of block copolymers [29].

In contrast to POSS that has only one organic functional group at each silicon atom, cyclic siloxane can be substituted with two functional groups. Therefore, the degree of crosslinking can be tuned by the type and number of the substituting groups. In the present study, cyclic siloxanes are employed as the building blocks for construction of porous inorganic–organic hybrid polymers (PHPs) by Friedel–Crafts alkylation using formaldehyde dimethyl acetal as external linkers (see Fig. 1; POSS is also used for comparison), yielding the polymers containing both micropores and mesopores. Effects of the shape of siloxane units, namely, three-membered or four-membered rings, and the types of substituting groups at each silicon (i.e., methylphenyl or diphenyl) on the porous characteristics of the obtained polymers are also reported. In particular, we found that the structures of the mesopores are strongly depended on the types of substituting groups.

Section snippets

Chemicals

All reagents were used as received. 2,4,6-trimethyl-2,4,6-triphenylcyclotrisiloxane (a mixture of cis and trans isomers), 2,4,6,8-tetramethyl-2,4,6,8-tetraphenylcyclotetrasiloxane, hexaphenylcyclotrisiloxane, octaphenylsilsesquioxane, formaldehyde dimethyl acetal (FDA; absolute, over molecular sieve), and 1,2-dichloroethane (DCE; anhydrous) were purchased from Sigma–Aldrich Co. LLC. Octaphenylcyclotetrasiloxane was purchased from Tokyo Chemical Industry Co., Ltd.. Iron(III) chloride (FeCl3;

Results and discussion

Fig. 2 show FT-IR spectra of the obtained PHPs. All polymers exhibited a Csingle bondH stretching band at 2800–3000 cm−1 [30], suggesting that the siloxane building blocks are crosslinked with the methylene (single bondCH2single bond) bridges during the Friedel–Crafts reaction. A strong band observed near 1100 cm−1 was attributed to asymmetric Sisingle bondOsingle bondSi stretching [31], indicating that the original siloxane frameworks are retained after polymerization. PHP-1 and PHP-2 showed sharp bands at 1272 and 782 cm−1, which are attributed

Conclusions

Porous inorganic–organic hybrid polymers containing cyclic siloxane frameworks were synthesized by Friedel–Crafts alkylation using formaldehyde dimethyl acetal as external crosslinkers. The resulting hybrid polymers contained both micropores and mesopores with high specific surface areas. It was found that the shape of the hysteresis loops in the sorption isotherms, directly reflecting the structures of mesopores, depended only on the organic substituting groups at silicon atoms. The siloxane

Acknowledgement

Part of this work was conducted at the Center for Nano Lithography & Analysis at The University of Tokyo, which is supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT).

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    Present Address: Research and Services Division of Materials Data and Integrated System (MaDIS), National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan.

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