Hierarchical porous nanosilica derived from coal gasification fly ash with excellent CO2 adsorption performance

https://doi.org/10.1016/j.cej.2022.140622Get rights and content

Highlights

  • Hierarchical porous nanosilica was prepared from CGFA via an efficient method.

  • Intergranular pores larger than 10 nm formed without additional pore swelling agent.

  • The formation mechanism for the hierarchical structure was revealed.

  • The CO2 adsorption capacity of HPNS was much higher than mSiO2.

  • The enhanced CO2 adsorption capacity of the HPNS was investigated and revealed.

Abstract

The preparation of mesoporous silica from solid waste is an effective method to realize high-value utilization of solid waste and reduce the synthesis cost of mesoporous silica. However, the silica derived from solid waste often suffers from small pore size and weak adsorption capacity. Besides, the traditional method of extracting silicon from solid waste requires high-temperature melting in the presence of a large amount of alkali, resulting in equipment corrosion. In this work, hierarchical porous nanosilica was prepared via microwave-assisted alkaline extraction followed by hydrothermal synthesis, using coal gasification fly ash (CGFA) as silicon source. The microwave assisted alkali-dissolution process is fast in silicon extraction and induces less impurity metal ions. Besides, the Fixed Carbon in CGFA was degraded into C20H24O3, C5H10O and C16H35NO3. These compounds acted as swelling agent, inducing intergranular pores with sizes greater than 10 nm in the nanosilicas. The isosteric heat of adsorption of the hierarchical porous nanosilica was −30.06 kJ/mol, indicating that CO2 was adsorbed by the hierarchical porous nanosilica via both physisorption and chemisorption. Chemisorption was induced by the surface silanol and its density increased with the intergranular pores. Because the intergranular pores and surface silanol group provided more adsorption sites, the CO2 adsorption capacity of the hierarchical porous nanosilica was significantly improved, which reached 1.63 mmol/g at 30 ℃ and 1 bar.

Introduction

Coal gasification fly ash (CGFA) is one of the solid wastes generated by coal gasification, which is recognized as one of the leading clean coal technologies. With the development of this technology, CGFA keeps increasing in recent years. In China, more than one million tons of CGFA is discharged annually according to He’s report [1]. At present, much of this solid waste is disposed in ash pond or landfill [2], not only occupying a great deal of land, but also posing potential environmental problems. A small amount of CGFA is used for the admixture of concrete or as a bulk building material [3], but these products have no market in remote areas, which are just the sites of coal chemical plants. CGFA mainly consists of silica and alumina, and the former accounts for about half of the total weight, making it a potential cost-effective source of silica.

Owing to the regular mesoporous structure, large specific surface area, adjustable pore size, high concentration of surface silicon hydroxyl used for amine immobilization by covalent or hydrogen bonds and excellent chemical stability, mesoporous silica has gained considerable attention in CO2 capture [4], [5]. Mesoporous silica was generally synthesized using silicon alkoxide or silica aerosol as silica sources, but these materials are expensive. Searching for low-cost silica sources to synthesize ordered mesoporous silica has become a hot research topic. Recent studies have reported the synthesis of mesoporous silica using industrial wastes as alternative silica sources, such as coal fly ash [6], bottom ash [7], iron tailings [8] and coal gasification slag [9], [10]. These beneficial attempts all successfully synthesized mesoporous silica. However, the synthesized mesoporous silica, as well as conventional mesoporous MCM-41 silica exhibits a typical pore size within the ranges of 3 to 4 nm, and 0.7–1.0 cm3/g, respectively [11]. The small pores lead to their low adsorption capacity towards CO2. Increasing the chain length of surfactants, using copolymers as templates or expanders such as alkanes, amines and trimethylbenzene, and postsynthesis hydrothermal treatment proved to be effective pore expansion methods. But addition of copolymers or expanders adds to the preparation costs, and postsynthesis hydrothermal treatment will undoubtedly makes the process more complicated. Besides, traditional silicon extraction methods involve acid leaching process or alkali-fusion process. The former is not environmentally friendly due to the high consumption of acid, and the latter suffers from high energy consumption and equipment corrosion.

Herein, hierarchical porous nanosilica with a specific surface area of 1117 m2/g was successfully synthesized from coal gasification fly ash via an environmentally friendly and efficient synthesis strategy. The morphologies and structures of the prepared nanosilica were systematically characterized. Secondly, the formation mechanism of the hierarchical structure was analyzed. Finally, the CO2 adsorption properties of the prepared nanosilica were studied and the mechanism for the enhanced adsorption capacity was revealed.

Section snippets

Materials

The coal gasification fly ash was provided by Sinopec Anqing Petrochemical Company. The samples were first dried at 105 ℃ for 2 h to remove water, then were ground and sieved to obtain particles with a size less than 178 μm (CGFA). The NaOH (Beijing Chemical Factory), 36 % acetic acid solution (Sinopharm Chemical Reagent Co., ltd) and hexadecyl trimethyl ammonium bromide (CTAB, Sinopharm Chemical Reagent Co., ltd) used in this investigation were all analytical grade. They were used without

Synthesis of hierarchical porous nanosilica and regulation of the pore structure

Hierarchical porous nanosilica was prepared from CGFA via a two-step method, microwave-assisted alkali-dissolution process to extract Si, followed by hydrothermal process to synthesize nanosilica. CGFA mainly consisted of ash and Fixed Carbon, accounting for 92.64 % and 4.99 %, respectively (Table 1), and the others were moisture and volatile matter. As can be seen from the ash composition, the inorganic compounds of CGFA were mainly SiO2 and Al2O3, the sum of which was about 70 %. The wide

Conclusions

Hierarchical porous nanosilica has been successfully synthesized from coal gasification solid waste via a two-step method, namely microwave assisted alkali-dissolution process, followed by hydrothermal synthesis. The microwave assisted alkali-dissolution process is fast in extracting silicon, induces less impurity metal ions, and brings about the degradation of Fixed Carbon in CGFA. C20H24O3, C16H35NO3 and C5H10O are the intermediates produced in the degradation process. They act as pore

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was financially supported by China Petrochemical Corporation Scientific Research Projects (417002-4, 418020-5).

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