Structure-Oriented Design of Hyper-Crosslinked Porous Organic Nanomaterials

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1		Yang Xu	--	1581	2023-10-14 15:11:59	\|
2	Reference format revised.	Lindsay Dong	Meta information modification	1581	2023-10-15 13:39:09	\|

This entry is adapted from the peer-reviewed paper 10.3390/nano13182514

Hyper-crosslinked porous organic nanomaterials, especially the hyper-crosslinked polymers (HCPs), are a unique class of materials that combine the benefits of high surface area, porous structure, and good chemical and thermal stability all rolled into one. A wide range of synthetic methods offer an enormous variety of HCPs with different pore structures and morphologies, which has allowed HCPs to be developed for gas adsorption and separations, chemical adsorption and encapsulation, and heterogeneous catalysis.

hyper-crosslinked polymer porous organic materials hydrocarbons

1. Introduction

Porous organic nanomaterials, also known as hydrocarbons, are characterized by the presence of pores or voids. According to the IUPAC recommendations, pores in these materials, that have continuous pathways connecting them to the outer surfaces of the porous structure, are referred to as open pores ^[1]. In contrast, pores that are isolated or not connected to other pores are referred to as closed pores ^[2]. Porous organic nanomaterials have good stability, chemical resistance, and excellent hydrothermal resistance, which can be effectively applied in gas separation, catalysis, energy storage, and other important applications ^[2].

Currently, porous organic nanomaterials can be classified into covalent organic frameworks (COFs) ^[3]^[4]^[5], hyper-crosslinked polymers (HCPs) ^[6]^[7]^[8]^[9], conjugated microporous polymers (CMPs) ^[10]^[11], polymers of intrinsic microporosity (PIMs) ^[12]^[13], covalent triazine frameworks (CTFs) ^[14]^[15], and porous aromatic frameworks (PAFs) ^[16]^[17]^[18], depending on their composition and bonding nature. Among them, hyper-crosslinked polymers (HCPs) are a family of permanently microporous polymeric materials originally discovered by Davankov and which have gained increasing interest ^[19]. HCP is characterized by simple synthesis methods, low cost, a wide choice of monomers, high yields, and easy post-modification ^[20]^[21]. Due to their relatively stable porous structure and resistance to collapse, HCPs have been widely used in catalysis ^[22]^[23], adsorption ^[24], sensing ^[25]^[26], drug delivery ^[27]^[28], gas storage ^[29], and other fields ^[30]^[31]^[32].

2. Structure-Oriented Design of Hyper-Crosslinked Polymer Nanomaterials

Due to the monomeric nature of hyper-crosslinked polymers and the variety of methods used to synthesize them, an increasing number of studies focus on designing their pore structures and morphology to achieve better performance. Therefore, it is important to study and summarize the design rules in order to obtain the optimum structure more systematically and easily in subsequent studies.

2.1. Design Methods for Pore Size Modulation and Distribution

It is well known that pore sizes < 2 nm are known as micropores, while those between 2 nm and 50 nm are known as mesopores. The degree of crosslinking of a hyper-crosslinked polymer largely affects its pore size and volume. Pore size design can also generally be achieved by artificially adding pore-forming agents, templates, etc. The most common methods are the hard and soft template methods. Hard template methods such as, for example, Ag ^[33] or silica ^[34]^[35], can be followed by etching of the template with strong acids and other reagents to form a porous structure. The soft template method often adds surfactants as porogenic agents in the polymerization process, and the removal of porogenic agents is generally greener, they can be removed by solvent washing and other means ^[36]^[37]. Both of these methods form mostly macro/mesopores.

The hyper-crosslinking of suitable monomers can result in the formation of a microporous structure. When choosing monomers with a rigid structure, such as benzene rings, imidazole, and carbazole, which contain five-membered or six-membered rings, these rigid structures can form microporous structures through inter-crosslinking or crosslinking with crosslinking agents ^[38]^[39].

The method of removing functional groups from the polymer framework through etching and breaking of chemical bonds can adjust the size and number of micropores in the hyper-crosslinked polymer. In 2017, Li and colleagues reported a study on the preparation of novel porous materials using PMS as a starting material ^[40]. They crosslinked the benzene moieties on PMS through FDA to construct a HCP (highly crosslinked polymer) with moderate surface area. Then, they etched the Si-O bonds using hydrofluoric acid (HFA) to create additional pores within the HCP. Simultaneously, some uncrosslinked phenyl groups were removed, adjusting the conformation of the remaining polymer skeleton to generate micropores and mesopores. For samples with a feed ratio of 1.5, the number of micropores and mesopores in F-HCP-1.5 within the range of 1.0 to 9.3 nm was significantly higher than the number in HCP within the corresponding range. The extra micropores and mesopores were generated by removing PMS segments. Compared to the micropore and mesopore volumes of HCP, F-HCP exhibited an increase in micropore and mesopore volumes. Therefore, by adjusting the crosslinking and etching conditions, it is possible to easily modulate surface area, pore size distribution, and other properties.

Modifying functional groups can also achieve the purpose of altering the distribution of mesopores. In 2022, Tan et al. reported on the successful introduction of hydrazine (HZ) through post-modification into metalloporphyrin-based hyper-crosslinked polymers, resulting in the generation of abundant CO₂ chemical adsorption sites within the structure ^[41]. This enabled efficient adsorption (with a Qst value of up to 34.7 kJ/mol) and chemical transformation of carbon dioxide. HCP-TPP, HCP-TPP-SO₃H, and HCP-TPP-Co-SO₃H exhibited a microporous structure with primary pore sizes below 2 nm (peaks at 0.8 nm, 1.1 nm, and 1.5 nm), with some mesopores at 2.1 nm, indicating a hierarchical pore size distribution. After grafting with amino groups, the mesopores were blocked, and the main pore sizes of HCP-TPP-Co-HZ were concentrated around 0.5 nm and 1.3 nm. HZ-modified material (HCP-TPP-Co-HZ) exhibits high catalytic performance towards epoxy substrates of different molecular sizes, with excellent yields (reaching 98% for epichlorohydrin).

Mesopores are more easily formed by introducing a template or changing the degree of crosslinking between hyper-crosslinked polymers. In 2015, Wang et al. reported a study in which Fe₃O₄ superparticles were used as the core, and micro/mesoporous polyoctylene pimelate (POP) served as the shell ^[42]. They synthesized Fe₃O₄@PS microspheres with a core/shell structure and initiated solvation polymerization in the PS shell using a mixture of divinylbenzene (DVB) and vinyl chloride (VBC). The resulting poly(VBC-co-DVB) network mixed with PS in the shell, leading to significant phase separation.

In most cases, mesopores and micropores are simultaneously adjusted during the synthesis of hyper-crosslinked polymer nanomaterials. In 2018, Červený et al. reported the synthesis of PPC-type conjugated hyper-crosslinked poly(arylethynylene) networks through chain-growth copolymerization using 1,4-diethynylbenzene, 1,3,5-triethynylbenzene, and tetra(4-ethynylphenyl) methane. These PPC materials exhibit permanent microporous/mesoporous structures and a high specific surface area (S_BET) of up to 1000 m²/g. The PPC materials demonstrate activity in acid-catalyzed reactions such as aldehyde and ketone condensation and carboxylic acid esterification ^[43].

2.2. Design Strategies for Morphological Tailoring

Common catalysts with tailorized morphologies are generally yolk-shell, core-shell, honeycomb-like, and hollow tubular catalysts. By designing the morphologies of the HCPs, some special selective catalysis can be achieved by screening the reactants. The HCPs with hollow structure are more intriguing due to the presence of internal cavities. The internal cavities can serve as excellent enrichment sites, providing significant assistance in catalytic reactions.

In 2021, Tan et al. proposed a self-templating method for the preparation of monodisperse mesoporous hollow capsules ^[44]. By adding styrene and divinylbenzene (DVB) monomers at different stages of the polymerization reaction, dense core-shell spheres consisting of a pure polystyrene (PS) core and lightly crosslinked polystyrene-divinylbenzene (PS-DVB) shell were obtained in a one-pot synthesis. Control over particle size and hollow structure was easily achieved by varying the reaction conditions and the timing of DVB addition. These catalysts, immobilized with gold nanoparticles, exhibited excellent catalytic performance in the reduction of 4-nitrophenol model reaction. The exceptional catalytic performance is attributed to the catalyst’s high surface area and abundant hierarchical porosity.

Additionally, Tan’s team utilized divinylbenzene hyper-crosslinking to obtain microporous polymers and discussed the morphology, porosity, and applications of their derived hollow microporous carbon spheres (HCS) ^[6]. They controlled the various morphologies of core-shell microspheres by altering the content of divinylbenzene (DVB) and prepared HCS with eccentric core and central core configurations. It was demonstrated that the DVB content of the precursor hollow organic microporous carbon (HOMC) and the carbonization temperature played crucial roles in controlling the structural characteristics and maintaining the porous structure of HCS.

A honeycomb-like structure has also been found to be beneficial for mass transfer. In 2017, Huang’s team reported a strategy for the synthesis of honeycomb-like porous organic nanospheres using triphenylphosphine-guided hyper-crosslinking self-assembly. They further synthesized a catalyst encapsulating Pd nanoparticles through a simple impregnation-reduction method (Pd@HBP). The catalyst exhibited superior selective hydrogenation performance compared to similar homogeneous or heterogeneous counterparts, achieving a benzene amine separation yield of up to 99% in the hydrogenation of nitroaromatic compounds. They attributed this to the advantages of the three-dimensional (3D) honeycomb-like interconnected mesoporous structure, which allows accessible catalytic active sites to be efficiently exposed to the reactants, thereby promoting mass transfer more effectively. The size of the honeycomb-like mesopores could also be adjusted by varying the length of the degradable PLA domain ^[45].

Apart from the aforementioned structures, another interesting structure is hollow nanotubes. Huang and Rzayev firstly reported that the bottlebrush copolymers can act a as soft-template to form the cylindrical structures in solution ^[46]. By employing a crosslinking method, the original shape and size of the bottlebrush macromolecules can be preserved after core etching. In the approach, a hierarchical organic polymer tubular network can be directly obtained. Using this soft-template method, various functional groups such as amine, amino, sulfonic acid, porphyrin, and thiol can be incorporated or immobilized onto the tubular network with micropores, mesopores, and macropores, either in situ or post-synthesis ^[47]^[48]. They also synthesized a tubular nanomaterial with a hierarchical pore structure. By manipulating the distribution of micropores and mesopores within the structure, they confirmed the importance of the mesoporous structure for the highly efficient catalytic activity in the oxidation of benzyl alcohol. The absence of tubular mesopores might limit the mass transfer of reactants and products, as well as the accessibility of active sites ^[49].

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