Template-free synthesis of hierarchical mesoporous carbon: based on functional coal tar pitches with carboxylatiaon and diketone structures

An effective and simple template-free method on the preparation of hierarchical porous carbons (HPCs) was successfully developed by using low-cost coal tar pitch (CP) as starting material.

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Template-free synthesis of hierarchical mesoporous carbon: based on functional coal tar pitches with carboxylatiaon and diketone structures

An effective and simple template-free method on the preparation of hierarchical porous carbons (HPCs) was successfully developed by using low-cost coal tar pitch (CP) as starting material.

research, mesoporous carbon

Template-free synthesis of hierarchical mesoporous carbon: based on functional coal tar pitches with carboxylation and diketone structures

Haiyang Wang,a,b Hongzhe Zhu,b Debang Qi,a Shoukai Wang,b Kaihua Shen,*a

a The State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116023, China

b Sinosteel Anshan research institute of themo-energy company limited, Anshan 114044, China

* Corresponding author. No.2 lingong road high-tech zone, Dalian city, Liaoning province, Dalian 116024, China. Email: [email protected]

ABSTRACT

An effective and simple template-free method on the preparation of hierarchical porous carbons (HPCs) was successfully developed by using low-cost coal tar pitch (CP) as starting material. By applying the diketone (-COCO-) radical, which provides a carbonyl methylene group for the polycyclic aromatic hydrocarbons by condensation, and generates a good micro-mesoporous network with high specific surface area. Through an easy controllable Friedel-Crafts reaction and oxidation, carboxyl-functionalized CP was obtained as the stratified porous CP precursor and high-temperature decarboxylation (>230 °C) facilitated the generation of well-developed micro-mesoporous carbon. Total pore volume of porous carbon of 1277.3 m2 g-1 and 1.45 cm3 g-1 and a maximum BET surface area respectively indicated that a rapid electrolyte ion exchange and charge transport ability would be better for the electrochemical capacitor.

Keywords: Coal tar pitches Hierarchical porous carbons Carboxylation Electrochemical properties

1. Introduction

Porous carbon materials are the core of many technological applications, such as electrochemical energy storage and gas separation, purification. Among them, the controlled porous carbons is the most commonly used electrode material for commercial available super capacitors due to its large surface area, adjustable pore size and acceptable cost [1,2]. In addition to its high conductivity and large accessible surface area, the ideal carbon material should also exhibit a hierarchical porous texture which combined mesopores, macropores and micropores [3,4]. It has been suggested that macropores would serve as ion-buffering reservoirs inside carbon materials, while mesopores would provide channels for fast ion transport; furthermore micropores offer locations for charge accommodation [5,6]. For these reasons, a sustainable method for the preparation of highly porous carbons has drawn significant research interest, and it remains a challenge to be overcome [7,8]. Recently, several approaches have been proposed to synthesize hierarchical porous carbons, such as the activation process, the template method, polymer blend carbonization and organic aerogel carbonization [9,10]. However, these studies did not provide a clear picture of the overall scope of the process of the activation mechanism that responsible for the generation of porosity.

Template strategies have also been applied to prepare porous carbons using either a hard or a soft template [11,12]. Various inorganic materials such as silica nanoparticles, mesoporous silica materials, zeolites, anodic alumina membranes and metal-organic frameworks have been successfully used as hard templates. However, many limitations such as the sacrificial use and difficulties of synthesis of various templates additions, or the removal process that would usually increase processing complexity and the costs [13,14]. A feasible synthesis method for preparing porous carbon materials should be effectively regulated with a controlled pore network and offer convenient application.

In the coal industry, coal tar pitch (CP) is a by-product of the coking process and is often used as carbonaceous precursors for carbon materials due to its rich sources, high carbon output and good cost-efficiency. Since it contains a large number of aromatic heterocyclic compounds, the pitch would produce carbons with different characteristic properties and structures. The Friedel-Crafts reaction would be the main strategy to produce highly porous hyper-cross-linked materials [15,16]. CP is composed of hundreds of polyaromatic molecules with different molecular sizes and its chemical structure varies in a wide range. When CP is charred for the synthesis of porous carbon materials, it is difficult to propose a clear point on each single component and its fine structure, and an ordering process would be necessary for the carbonation of CP.

In this paper, an effective template-free method has been successfully developed to prepare hierarchical porous carbons (HPCs) in which low-cost CP is used as a carbonaceous precursor. As shown in Scheme 1, a controllable Friedel-Crafts reaction of CP via introducing diketone. A diketone (-COCO-) bridge between polycyclic aromatic hydrocarbons would increase the crossing density, and would confer a high level of controllability during the charring process. This study investigated the C-C cleavage of diketones for its carbonyl methylene condensation, and also the subsequent carboxyl-functionalized tar pitch with decarboxylation during the elastic-plasticide formation phase.

2. Experimental

2.1Materials

The Anshan Iron and Steel Group Co. Ltd produced the raw CP (Anshan, China). A refined coal tar pitch (RF-CP) with a softening point of 33°C was obtained by a mixed solvent-extraction method and its compositions. The main properties of the RFCP and DKCP are shown in Table 1. Anhydrous aluminum chloride (AlCl3) was from Tianjin Guangfu Fine Chemical Reagent Co. Ltd. (Tianjin, China). Oxalyl chloride (OC), hydrochloric acid (HCl), hydrogen peroxide(H2O2) and dichloroethane (DCE) were from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). And all chemical reagents were of analytical grade.

Table 1 Compositons of refined coal tar pitch and raw coal tar pitch

Name α fractions(%) β fractions(%) γ fractions(%) Softening point(℃) QI(%)

Raw CP 2.05 6.10 91.85 46 2.07

RF-CP 0.08 5.95 93.97 33 0.08

2.2. Synthesis of the DK-CP and carboxylic

To synthesize carbonyl-functional CP, a refined coal tar pitch (RF-CP) was obtained with a softening point of 33 °C. As an external cross-linker, Oxalyl chloride (OC) was used. As shown in Scheme 1, a 50 g RF-CP was dissolved in 500 ml dichloroethane and then a 50 g AlCl3 solution with oxalyl chloride was dropwise added under argon atmosphere. The resulting mixture was stirred for 6 h at 40 °C and the reaction was terminated by adding ethanol solution. After filtering and washing with ethanol solution and hydrochloric acid, the product was subjected to heat drying. To study the effects of polymerization, different combinations of coal tar pitch (DK-CP) with OC referred to as DK-CP-1,-2,-3 respectively, in 0.3, 0.6, and 0.9 weight ratios, and the obtained diketone-functionalized DK-CP-3 were decomposed to carboxylic CP (C-CP) with H2O2 at 40 °C. Under N2 atmosphere, the dark brown carbonized samples were heated in a tube furnace. Carbonation was finished within 2 h with a heating rate of 2 °C min-1, up to 800 °C.

Scheme 1. Structural variation of RF-CP reacting with OC during the carbonization process

By the Friedel-Crafts reaction, the functional CP (DK-CP-1, 2 and 3) with diketone (-COCO-) exhibited a variety of crosslinking bridges bevtween the polycyclic aromatic hydrocarbons and the higher intensity of the C=O stretching peak (1760 cm−1 and 1735 cm−1) as confirmed via FTIR spectra. As illustrated (Scheme 1), this showed that the carboxylic tar pitch could be prepared with diketone functionalized pitch after oxidation and fine porous carbons would then be obtained after the charring process. In this experiment, oxidation ended in solution and a pair of polycarboxylic acid (or anhydride) was formed. Dense sintering of the tar pitch and its functional derivatives were then conducted to detect different porous structure formations. Peak changes of C=O stretching were found around 1740 cm−1 (-COOH), which formed the best evidence for tracing the presence of diketone and its cleave. The cleavage mechanism of diketone might be explained by the key carbonyl methylene condensation during the charring process. The IR spectra showed that the diketone group of functional tar pitch was a constant process of change and the subsequent SEM test demonstrated its strong influence upon pore formation in charring. For the case, other precursor starting with carboxylic tar pitch but appeared differently for CP with a diketone group. The decarboxylation of carboxylic derivatives was conducted at 200 to 300 °C and its mechanism had been suggested before [17,18]. In fact, the carbonation of pitch and its derivatives could be accessible to elastic-plasticide formation phase above 200 °C and the decarboxylation of carboxyl polycyclic aromatic rings were favorable for the generation of hierarchical mesoporous carbons.

With the aim of simulating the decarboxylation of carboxyl tar pitch, stearic acid as a model compound was heated in the atmosphere of nitrogen and the thermal gravimetric process was carried out to 600 °C. Thermal gravity-differential analysis and NMR spectrum were used for better understanding the influence on the pore generation of carboxyl tar pitch (Shown in Fig. 1). Other studies have been reported that the high-temperature pyrolysis would take place the decarboxylation of carboxyl group. Thermal analysis of stearic acid showed that it began to decompose at 220 °C. The structure of stearic acid and its decomposed product above 200 °C were characterized by 1H-NMR and found its decarboxylation. It would readily see that heat decarboxylation occurred first and the carbonazition reaction would then proceed by raising the pyrolysis temperature.

Fig. 1. (a) 1H-NMR spectra of stearic acid and its decomposer above 200 °C; (b) Thermo gravimetric an

Template-free synthesis of hierarchical mesoporous carbon: based on functional coal tar pitches with carboxylatiaon and diketone structures
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Tags Research, Mesoporous carbon
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Published 01/02/2021, 13:34:47

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