CO2 capture and MWCNTs synthesis using mesoporous silica and zeolite 13X collectively prepared from bottom ash

doi:10.1016/j.cattod.2011.09.032

Catalysis Today

Volume 190, Issue 1, 1 August 2012, Pages 15-22

https://doi.org/10.1016/j.cattod.2011.09.032 Get rights and content

Abstract

Ground power plant bottom ashes were alkali-fused at 600 °C for 1 h and silicate extraction was conducted in water at room temperature. The supernatant solution was then used to make mesoporous silicas after adding a triblock copolymer, Pluronic P123, as a template, whereas the solid precipitate was used to make zeolite 13X. This combined synthesis scheme enabled high levels of silicate recycling close to 100%. The mesoporous silica prepared using butanol as a swelling agent (MS-b) resulted in higher surface areas and higher pore volumes than those prepared using mesitylene (MS-m). MS-b, used as a support material for impregnating PEI (polyethyleneimine) for capturing CO₂, achieved excellent CO₂ capture capacity (218 mg/g-adsorbent with 70 wt% PEI) at 75 °C. Removal of a portion of silicate species by aqueous extraction rendered the composition of the remaining precipitate more favorable for 13X synthesis, and the lower Si/Al ratio in the prepared zeolite 13X led to increased carbon dioxide adsorption. Finally, high content of Fe impurity (ca. 12.6 wt%) remained in the porous structure of the zeolite 13X, enabling uniform growth of multi-wall carbon nanotubes (MWCNTs) by chemical vapor deposition in a quartz reactor using acetylene as a carbon source at 700 °C.

Graphical abstract

Highlights

► Mesoporous silica and zeolite 13X were collectively synthesized using bottom ash. ► The prepared samples had comparable textural properties to those produced using pure chemicals. ► Mesoporous silica was impregnated with PEI to make an effective CO₂ sorbent. ► The prepared zeolite 13X was applied to a CVD process as a catalyst to fabricate MWCNTs.

Introduction

Fly and bottom ashes are residual by-products of coal power plants in the production of electricity, and their disposal is causing serious environmental problems worldwide [1], [2]. Whilst fly ash is used in many parts of recycling systems and has a high recycling rate, approximately 79% of bottom ash goes to landfills and only 21% ends up in bulk fills or substitute aggregates in construction blocks or in asphalt [3]. Therefore, converting bottom ash into more valuable products with high recycling rates is desirable for global resource conservation and a clean environment. Researchers have thus attempted the conversion of coal fly ash [4], [5], [6] and bottom ash [7], [8] into either zeolites or mesoporous silica materials such as MCM-41, MCM-48, and SBA-15.

In this work, we propose a collective scheme to prepare mesoporous silica and zeolite 13X simultaneously using bottom ash as a raw material (Scheme 1); silicate extraction from the ash powder after alkali fusion was conducted, and the extracted supernatant solution was then used for the synthesis of mesoporous materials, whilst the precipitated solid part was used to produce zeolite 13X. By making these two different kinds of porous materials in this manner, it is expected that we can retain high recyclability of silicate species in bottom ashes. In addition, the Si/Al ratio of the precipitate would decrease after silicate extraction, which would lead to a precipitate composition that is more favorable for making zeolite 13X [9].

It would be highly desirable if an effective adsorbent could be prepared using power plant waste products and applied for carbon dioxide capture on the same site. The porous materials prepared in this work, despite a minor drop in physicochemical properties compared with materials prepared using pure chemicals, can be applied as an adsorbent for carbon dioxide; zeolite 13X often has been quoted as a standard material for post-combustion capturing of carbon dioxide [10]. High carbon dioxide capturing capacity of mesoporous silica materials after amine-functionalization has also been known [7]; amine species on the surface react with CO₂ to form a carbamate species during the chemisorption process (2R-NH₂ + CO₂ → RNH₃⁺ + R-NHCOO⁻), and immobilization of amine functional groups inside a mesoporous silica support in a highly dispersed state can produce an efficient adsorbent. In this work, we have attempted to improve the textural properties of mesoporous silicas by introducing a suitable pore-swelling agent, and impregnated polyethyleneimine (PEI) inside the pores in methanol to prepare a high performance hybrid material for carbon dioxide capture.

Synthesis of carbon nanotubes (CNTs) has been widely explored since their introduction by Iijima [11]. Among the diverse techniques available for the synthesis of CNTs [12], [13], chemical vapor deposition (CVD) via catalytic decomposition of hydrocarbons is currently employed for large-scale, high yield production of multi-walled carbon nanotubes (MWCNTs) [14]. Cracking of the carbon source at the surface of catalytic transition metal-impregnated particles leads to the growth of CNTs [15], [16]. Since the zeolite 13X synthesized in this work from power plant bottom ash also has a high content of Fe (12.6 wt%) in high dispersion, we envisaged that it can be used as a catalyst for making MWCNTs without further chemical treatment. In this work, we have selected acetylene as the hydrocarbon [17], [18] in a CVD unit, as shown in Scheme S1, to demonstrate the applicability of zeolite 13X as a catalyst for MWCNTs.

Section snippets

Alkali fusion and silicate extraction

Initially, a large granular mass of coal power plant bottom ash (Hadong Power Plant, Korea) was ground to obtain a fine powder using a commercial mixing-grinder, and the coarse bottom ash powder was sieved to particles of less than 100 μm size. Alkali fusion and silicate extraction were then conducted based on the procedure reported by Kumar et al. [19]. Bottom ash was mixed with NaOH powder at a 1:1.2 weight ratio. It was then fused at 600 °C for 1 h. The fused mixture was cooled to room

Bottom ash properties

Bottom ash is a coarse granular mixture having a particle size ranging from 0.07 to 4.75 mm. The chemical composition of bottom ash was analyzed by XRF, revealing that Si, Al, Na, K, Fe, and Ca were the major components. Its composition in oxide form is summarized in Table 1. Quartz mainly exists as a crystalline substance in coal bottom ash [22]. The supernatant solution extracted from the bottom ash was analyzed by atomic absorption spectroscopy, and Si, Al, Na, K, Fe, and Ca were found to be

Conclusions

Mesoporous silica and zeolite 13X were synthesized using power plant bottom ash powder as a raw material without adding an extra silica source, and possessed comparable textural properties to specimens prepared using pure chemicals. A novel strategy of making mesoporous silica using the extracted solution after alkali-fusion and preparing zeolite 13X using the precipitated solid phase was proposed. The prepared mesoporous silica was used as a support material for PEI to capture CO₂ with high

Acknowledgements

This work was supported by the Energy Resources Technology Development Program of the Ministry of Knowledge Economy and Inha University in Korea.

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CO2 capture and MWCNTs synthesis using mesoporous silica and zeolite 13X collectively prepared from bottom ash

Abstract

Graphical abstract

Highlights

Introduction

Section snippets

Alkali fusion and silicate extraction

Bottom ash properties

Conclusions

Acknowledgements

Miner. Eng.

Fuel

Fuel

Int. J. Greenhouse Gas Control.

Catal. Today

Chem. Phys. Lett.

Zeolites

Chem. Phys. Lett.

Fuel Process. Technol.

Microporous Mesoporous Mater.

Catal. Today

Powder Technol.

J. Organomet. Chem.

J. Chem. Technol. Biotechnol.

CO₂ capture and MWCNTs synthesis using mesoporous silica and zeolite 13X collectively prepared from bottom ash