Energy Procedia 00 (2008) 000–000
3URFHGLD
www.elsevier.com/locate/XXX
GHGT-9
Templated polymeric materials as adsorbents for the post- combustion capture of CO
2C. Pevida
a, C.E. Snape
band T.C. Drage
b*
aInstituto Nacional del Carbon, CSIC, Apartado 73, 33080 Oviedo, Spain.
bDepartment of Chemical and Environmental Engineering, University of Nottingham, NG7 2RD, UK.
Elsevier use only: Received date here; revised date here; accepted date here
Abstract
Amine solvents have long been used by industry as absorbents for acid gas (CO2, H2S) removal and are the current technology of choice for post-combustion carbon capture from fossil fuel power plants. However, these technologies are energy intensive and have a number of short comings. In an effort to reduce the capital cost and energy penalty of carbon capture alternative technologies are being explored, of which solid sorbents have shown good potential. Adsorbents containing basic nitrogen functional groups to increase adsorbent / adsorbate interaction have been demonstrated to be effective for post-combustion capture. These materials are usually synthesised by the impregnation of basic amine polymers or bonding of amine groups to the surface of inorganic porous substrates. In this work the development of a range of novel high nitrogen content activated carbon adsorbents will be described. The aim of this research is to introduce basic nitrogen directly into the matrix of activated carbon to yield high capacity adsorbents with high thermal stability in terms of volatile and thermal loss of nitrogen.
Novel nitrogen enriched activated carbons have been synthesised by a templating or nanocasting technique by which a removable inorganic template is used to generate porous polymers with high surface area. A range of adsorbents have been synthesised by templating of melamine-formaldehyde resin with silica followed by activation over a range of temperatures from 400 – 700 °C.
The resultant adsorbents have been characterised in terms of their textural properties, elemental composition and surface chemistry, with materials containing up to 42 wt.% nitrogen and 880 m2 g-1 surface area generated. Adsorption capacities up to 2.25 mmol g-1 of CO2 at 25 ºC were measured using thermogravimetric analysis and will be discussed in terms of the textural and surface chemical properties of the carbons as determined by X-ray Photoelectron Spectroscopy (XPS). Both texture and surface chemistry influence the CO2 capture performance of the adsorbents. The activation temperature used during the synthesis step controls the nitrogen functional groups present, as determined by XPS, with the loss of triazine nitrogen with increasing activation temperature proposed to account for the decreased CO2 affinity. Finally the stability and regeneration of the carbons over numerous thermal swing adsorption cycles will be described.
© 2008 Elsevier Ltd. All rights reserved
Keywords: Activated Carbon; Carbon Capture;Adsorption.
* Corresponding author. Tel.: +44 115 951 4099; fax: +44 115 951 4115.
E-mail address: [email protected].
2009 Elsevier Ltd. All rights reserved.c
Energy Procedia 1 (2009) 869–874
www.elsevier.com/locate/procedia
doi:10.1016/j.egypro.2009.01.115
1. Introduction
Increasing awareness of the influence of greenhouse gases on global climate change has led to recent efforts to develop strategies for the reduction of carbon dioxide (CO2) emissions. In 2000, the burning of coal generated 37.8% of all CO2 arising from fossil fuels [1] and as a result the strategy that is receiving the most attention involves the capture of CO2 from large point sources (such as fossil fuel-fired power plants). The greenhouse gas can then be stored underground or in the ocean over the long term. To achieve this economically CO2 must be in a relatively pure high pressure form, requiring the capture and compression of the CO2 emitted by the power plant.
The CO2 capture step is projected to account for the majority (ca. 75%) of the expense for the carbon capture and subsequent storage process. Aqueous solutions of amines, for example monoethanolamine, have long been used by industry as absorbents for acid gas (CO2, H2S) removal, and in fact provide a large percentage of the natural gas sweetening operations [2]. However, regarding their application to flue gases, these technologies need significant modification and this ultimately leads to high capital and running costs [3]. Therefore, the development of alternative low cost technologies is crucial in the long term to provide a more cost and environmentally effective route CO2 capture and storage on a global scale.
Adsorption is considered to be one of the more promising technologies for the efficient capture of CO2 from flue gases. Activated carbons are well known as adsorbents of gases and vapours [4]. Some of the most successful adsorbents for CO2 have been developed via the alteration of the surface chemistry of porous substrates by impregnation with amine polymers, for example polyethylenimine [5,6]. Another method is the modification of the surface chemistry of the carbon matrix by the incorporation of heteroatoms such as nitrogen to enhance the specific adsorbate-adsorbent interaction. Nitrogen incorporation has previously been demonstrated to enhance the adsorptive properties of activated carbons for hydrogen sulphide, SOx, NOx and acetaldehyde [7] and recently CO2
[8,9]. However, the complexity of the carbon structure prevents the conventional activation techniques from generating carbon materials with a strictly controlled pore structure. Activation also involves heating of the precursor material to high temperatures (~900 °C), destroying original functionality, with the most potent functional groups for the adsorption of CO2, such as amine groups, lost or converted to pyridinic groups [8,9].
Nanocasting or templating is a route by which high surface area adsorbents can be generated without the requirement for thermal treatment or activation. In nanocasting an inorganic diluent such as a silica is used to shape the in-growing polymer. On dissolution of the inorganic template the polymer remains as a mirror image, inheriting the porosity of the template. This paper describes recent studies [10] using this novel templating or nanocasting technique for the generation of porous nitrogen containing polymers without high temperature activation for CO2
capture.
2. Experimental
All reagents and solvents were supplied be Sigma-Aldrich, UK, were reagent grade and used without further purification. Porous melamine-formaldehyde (MF) resins were synthesized in the presence of 7 nm and 14 nm fumed silica as a templating / pore creating agent. Full description of the synthesis techniques has been previously reported (ref). Characterisation of the adsorbents has also previously been reported [10]. Briefly, the organic carbon, hydrogen and nitrogen content of the MF derived active carbon adsorbents were determined using a Thermo 1112 Series Flash EA, textural characterisation was carried out using a ASAP 2010 with N2 adsorption isotherms measured at -196 °C, surface chemistry was determined using XPS analysis.
Assessment of the CO2 adsorption and desorption potential was determined using a TA Q500, thermogravimetric analyser (TGA). Temperature-programmed analysis was used to evaluate the influence of temperature upon the CO2
adsorption capacity of the adsorbents. Temperature resolved CO2 adsorption capacity was determined by holding the adsorbent at 25 °C for 2 hours, and the weight increase measured used to derive the CO2 adsorption capacity of the materials at room temperature. The temperature was then increased gradually at a rate of 0.25 °C min-1, the slow heating rate allowing for equilibrium adsorption capacity to be attained, up to 100 °C [10].
3. Results and discussion
3.1. Adsorbent properties
The nanocasting technique has been applied successfully to generate a range of CO2 adsorbents with no, or mild thermal treatment. All of the MF derived carbons have high nitrogen content (Table 1), greatest at 500 °C activation temperature (30 mol.%), but remaining high after 700 °C activation (20 mol.%). Surface area and pore volume increase concomitantly with carbonization temperature up to a maximum of 876 m2g-1 when carbonised at 700 °C (Table 1). This has demonstrated the techniques ability to generate high surface area materials from MF resin, which has previously been demonstrated to be difficult using chemical activation techniques [9].
Elemental Analysis (daf) N2 adsorption at -196 ºC
Samples N
mol .%
C
mol .%
H
mol .%
O a
mol .%
N/
C
SBET
(m2/ g)
CBE T
Vp
(cm3 /g)
Vmeso
(cm3 /g)
MFA-500 25.6 48.3 23.5 2.6 0.5 283 165 0.79 0.60 MFB-500 27.2 45.1 23.6 4.0 0.6 336 200 0.70 0.45 MFC-500 29.3 45.0 22.0 3.7 0.7 302 178 0.71 0.5 MFD-500 29.6 42.5 23.7 4.1 0.7 284 193 0.58 0.38
MFA-600 23.3 49.3 20.2 7.2 0.5 520 252 1.06 0.63 MFB-600 24.4 47.4 20.3 7.9 0.5 490 261 0.91 0.49 MFC-600 26.0 46.4 19.4 8.3 0.6 488 260 0.99 0.59 MFD-600 26.4 46.2 19.4 8.1 0.6 476 260 0.72 0.32
MFA-700 19.0 57.1 17.2 6.7 0.3 829 252 1.66 0.93 MFB-700 20.5 54.9 16.3 8.2 0.4 791 228 1.30 0.60 MFC-700 21.5 57.3 15.2 6.0 0.4 876 195 1.64 0.92 MFD-700 21.4 53.9 16.3 8.4 0.4 818 207 1.05 0.41 Table 1 Chemical characteristics and textural parameters determined for the carbon materials studied [10].
3.2. CO2 adsorption performance
Temperature resolved adsorption capacities were determined by TGA analysis. Figure 2 presents the adsorption capacities of two MF resin adsorbents. Adsorption capacity is highest in both cases at 25 °C and decreases gradually with increasing temperature. The capacity and decrease in adsorption of CO2 at elevated temperature is superior to the performance of a standard commercial activated carbon. The decrease in capacity with temperature differs from the performance of polyethylenimine based adsorbents that remain constant up to approximately 90 °C [11]. The difference chemistry of the nitrogen of the MF resin and PEI based adsorbents is proposed to account for this difference in performance.
Figure 1. a.) CO2 adsorption capacity of templated melamine-formaldehyde resin adsorbents from 25 - 100 C, heated at 0.25 °C min-1, b.) Temperature programmed desorption profiles, heating at 5 °C min-1 [10].
The samples with the largest narrow micropore volumes (MF-700) show the greatest CO2 uptakes per mmol of N while the samples with the smallest micropore volumes (MF-500) present the lowest values. MF-500 carbons capacities are strongly influenced by the chemistry present in these samples, for MF-600 samples both chemistry
a.)
b.)
and narrow microporosity play a significant role in CO2 capture, improving the capacity of the sorbents to capture CO2 and MF-700 carbons capacities, controlled by the adsorption on the narrow micropores, are the least influenced by the chemistry of the adsorbents, most probably because the nitrogen functionalities in these samples present less affinity to reaction with CO2. Therefore, the amount of CO2 adsorbed on the prepared MF sorbents may not depend on the amount of nitrogen in the sorbent, nitrogen contents above 20 mol%, but on the effectiveness of the nitrogen functionalities towards CO2 adsorption. The difference in the affinity of the surface of the adsorbents to CO2 can be further illustrated by the TPD profiles of the carbons generated at different carbonisation temperatures (Fig. 1b). The temperature at which CO2 is desorbed from the carbons occurs at lower temperature for the adsorbents carbonised at higher temperatures, which as previously discussed are proposed to have a reduced chemical affinity to CO2. This is most apparent when comparing the TPD curves for the carbons synthesised between 500 and 600 °C to those prepared at 700 °C. It is proposed that this difference in surface affinity of CO2 is as a result of nitrogen functional groups present on the surface of the adsorbents.
Analysis of the adsorbents by XPS [10] was performed to determine surface chemistry at different activation temperatures [10]. A principal component of melamine–formaldehyde resin is the triazine resin structures of melamine. Triazine rings have been demonstrated to be stable under pyrolytic conditions up to 700 °C with a series of condensation reactions postulated [12,13]. Therefore, it is likely that such structures would be present in the adsorbents. The decomposition of triazine rings has previously been reported during the pyrolysis of melamine, being lost or converted to more thermostable forms of nitrogen [13], whilst pyridinic nitrogen has been reported to be stable up to 1000 °C [14]. Examination of the composition of surface nitrogen reveals a change in nitrogen composition from carbons containing triazine ring nitrogen to more stable pyridinic and tertiary nitrogen with increasing carbonisation temperature. These functionalities present less affinity for CO2. Both chemistry and texture play a significant role in CO2 capture.
4. Conclusions
A range of MF highly porous adsorbents for CO2 capture have been generated by templated synthesis of MF resin, using silica as template material, and carbonisation at different temperatures. This technique has been demonstrated to be effective at generating porous polymeric materials. The prepared adsorbents presented CO2
capture capacities up to 2.25 mmol g-1 at 25 °C, out performing many commercial activated carbons. Both texture and surface chemistry influence the CO2 capture performance of the prepared adsorbents: If an adequate and well developed porosity is joined to a favourable chemistry, the CO2 adsorption capacity is considerably enhanced.
Research is underway to explore different polymer classes to optimise adsorption capacities
Acknowledgements
TD would like to thank the EPSRC for funding this research (EPSRC Advanced Research Fellowship EP/C543203/1). CP acknowledges a postdoctoral fellowship from the Plan I+D+I – Gobierno del Principado de Asturias.
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