Effect of pore structure on the adsorption capacities to different sizes of adsorbates by ferrocene-based conjugated microporous polymers

Highlights

• Two conjugated microporous polymers based on carbazole and ferrocene units were synthesized successfully.

• In comparison with PDCPF, PDCF has similar pore size distribution in the region below 3 nm and extra pores in the region between 10 and 100 nm.

• The extra mesopores in PDCF exhibit better adsorption capacities to adsorbates with larger sizes.

• Dye molecules adsorbed in the micropores are harder to desorption than that in the mesopores.

Abstract

Conjugated microporous polymers (CMPs) are potential materials for gas uptake, pollutant adsorption and photocatalysis. However, their relatively wide pore size distribution (PSD) makes it hard to establish the relationship between adsorption capacities and pore structure. Here, we synthesized two CMPs with similar chemical structure based on carbazole and ferrocene units. Poly[1,1′-di(9-carbazolyl)ferrocene] (PDCF) showed hierarchical pore structure with pore sizes distributed in three regions: 0.7–0.85 nm, 1–3 nm and 10–100 nm, while the pore sizes of poly{1,1′-di[4-(9-carbazolyl))phenyl]ferrocene} (PDCPF) were mostly less than 3 nm. The results of gas uptake and dye adsorption capacities show that the extra mesopores of PDCF exhibit better adsorption capacities to adsorbates with larger sizes. Besides, dye molecules adsorbed in the micropores are harder to desorption than that in the mesopores. The intermolecular forces between porous polymers and adsorbates were used to explain these results successfully. Our findings indicate that designing the right pore sizes for porous polymers when adsorbing gases or dyes with a specific molecules size is very important in the future.

Introduction

Porous organic polymers (POPs) have potential applications in gas uptake and separation [[1], [2], [3]], pollutant adsorption [4,5], heterogeneous catalysis [6], etc [7]. During the past few decades, a series of POPs, such as conjugated microporous polymers (CMPs) [8], polymers of intrinsic microporosity (PIMs) [9], hyper-cross-linked polymers (HCPs) [10] and covalent organic frameworks (COFs) [11], have been intensively studied. Among them, CMPs are three-dimensional amorphous materials that combine extended π-conjugation with a permanently microporous skeleton [8]. The conjugated structure offers CMPs stable pore structure and special photoelectric performance.

Carbazole is a commonly used building unit for linear conjugated polymers [12] and CMPs [13]. Han et al. reported the formation of microporous polycarbazole via oxidative coupling polymerization in 2012. The Brunauer–Emmett–Teller (BET) specific surface area of microporous polycarbazole was higher than 2000 m²/g [14]. After that, a series of carbazole-based POPs have been successfully synthesized, and they exhibited good gas uptake and dye removal capacities [[15], [16], [17], [18]].

Ferrocene has a sandwich structure consisting of two cyclopentadienyl rings bound on opposite sides of a central metal atom [19], which makes it easy to form a three-dimensional structure when the hydrogen atoms on the cyclopentadienyl rings are replaced. Therefore, ferrocene is a promising building unit for POPs. Several groups have used ferrocene to build POPs, and the products showed good gas uptake and pollutant adsorption capacities [[20], [21], [22], [23], [24], [25]]. The cyclopentadienyl ring in ferrocene is a conjugated structure, and thus ferrocene could form CMPs when connected by other conjugated units. For example, Yu et al. chose ferrocene and s-triazine as the building units, and the resulting CMP showed good gas uptake capabilities for H₂, CH₄ and CO₂ [26]. Yu et al. also found that CMP based on ferrocene had a high affinity towards I₂ [27]. Besides, like many CMPs without ferrocene, CMPs based on ferrocene also had good photocatalysis performance, as shown by Wang et al. [28].

In addition to chemical structure, the pore structure also has a big influence on the adsorption capacity of POPs [2]. Microporous materials (with pore sizes smaller than 2 nm) are generally regarded as promising gas adsorbing materials because their pore sizes are on the order of the molecular size of small gases and their adsorption capacities are high due to their large surface areas [2,29]. For porous carbons, Gogotsi's group showed that smaller pores increase both the heat of adsorption and total volume of adsorbed H₂ [30]. Gogotsi's group also found that CO₂ sorption is limited by pores smaller than a certain diameter, which decreased from 0.8 nm to 0.5 nm when the pressure decreased from 1 bar to 0.1 bar [31]. Teng et al. studied phenol, iodine and tannic acid adsorption capacities of carbons with different mesopore volumes, and found that the adsorption capacity is an increasing function of the mesopore volume [32]. Thus, understanding the relationship between the adsorption capacity and pore structure is very important. However, most POPs (except COFs) are amorphous structures without exact chemical structure, which makes it hard to study the relationship between the adsorption capacity and pore structure accurately. The synthesized POPs generally have a wide pore size distribution (PSD) in the range of micropore and mesopore [23,33,34].

Herein, two CMPs based on carbazole and ferrocene units were synthesized. In comparison with poly{1,1′-di[4-(9-carbazolyl))phenyl]ferrocene} (PDCPF), poly[1,1′-di(9-carbazolyl)ferrocene] (PDCF) has a similar PSD in the micropore region and a much wider PSD in the mesopore and macropore regions. The results of gas uptake and dye adsorption show that the extra mesopores of PDCF are beneficial to the adsorption of adsorbates with larger sizes. Moreover, dye molecules adsorbed in the micropores are harder to desorption than that in the mesopores.