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Final Report Summary - BIOADSORB (Biomass-derived Microporous Carbon Adsorbents for CO2 Capture and Storage)

Final report for ‘BIOADSORB - Biomass derived microporous carbon adsorbents for CO2 capture and storage'

The project BIOADSORB started in September 2014. The main objective was to discover new carbon materials and investigate their fundamental insights in relation to capture and storage of CO2. BIOADSORB involved multiple work packages such as i) molecular design of materials for CO2 capture; ii) provide fundamental understandings and new insights at atomic resolution; iii) provide new materials produced using green and eco-friendly methods to obtain high surface area functionalized carbons; iv) studies that expose the limitations and the capacity of these materials for CO2 capture.

All of these work pacakges and associated tasks were achieved strategically based on laboratory experiments and in-silico experiments and executed in a timely manner.

On the experimental side, we have successfully discovered a new synthetic technique called ‘Salt templating with pore padding’ or STPP. STPP can produce high surface area carbons using environmentally friendly and cheap salts from waste biomass with the desired level (concentration of N) and type of N (graphitic N, pyridinic N or oxides of N) functionalities on the carbon surface. We tested the STPP carbons for the capture of CO2 and for the first time we exposed the need for mesoporosity in order to improve the CO2/N2 selectivity during the separation of these fluids from their bulk fluid mixtures.
The STPP carbons were characterized for their surface and pore properties using volumetric gas adsorption measurements, SEM, TEM, XRD, TEM, and RAMAN. Based on the measured adsorption excess, we showed at 273 K, the essentially repulsive N2 molecules are poorly confined in mesopores whereas the CO2 molecules tend to form either single to multiple layers of adsorbed phased, thereby ultimately showing remarkable CO2/N2 selectivity (> 40). These results are highly encouraging, as it will make an important research impact in the area of synthesis of carbon membranes for the separation of CO2/N2 from their gas mixtures or from flue gas. High selectivity in mesoporous N-doped carbons means less pressure drop during the operation of adsorption in a pressure swing mode and it will also allow the separation of CO2/N2 via simple permeation process; which mean a low operating cost. Another major finding is the higher CO2/N2 selectivity comes from a material that has a moderate surface area of 1000 m2/g. This obviously shows a higher selectivity material does not essentially need to possess a remarkable surface area closer to the ideal limit of graphene, rather selectivity is dictated by N functionality and the presence of mesopores. Apart from gas adsorption measurements, through a combination of techniques like TG-MS, RAMAN, XPS and volumetric gas adsorption experiments we showed how STPP technique could also help to hierarchically tailor the pore-size distribution of functionalized carbons and also to improve the surface area without the expense of microporosity. This is a much-desired design strategy in the area of material synthesis. The major findings can be found in one of our submitted manuscript ‘Salt templating with pore padding – a sustainable route for hierarchical tailoring of pore size distribution’ – the revised version is submitted to ChemSusChem and expected to be published in a special version celebrating the journal anniversary.

On the theoretical side we delivered a series of new findings and understandings that expose the role of N functionalities on the CO2 uptake, CO2/N2 selectivity, hydrophilicity, fluid confinement effects, overall binding energy of the carbon frameworks for CO2, the effect of pore structure and the need for a pore-size homogeneity to improve the CO2 uptake of carbon structures. Additionally, we also showed how carbons obtained in-silico can be used to expose the effect of N functionality in disordered pore structures pore structure can influence on the CO2 uptake and CO2/N2 selectivity. The key findings are (i) N doping practically does not show any significant improvement in the CO2 uptake, however it can massively increase the CO2/N2 selectivity due to electrostatic screening effects; (ii) N doping can ultimately make the carbon useless to host CO2 molecules during the adsorption of CO2 from a bulk fluid that contain water as impurity. Another major finding is the influence of pore-structure: an N doped carbon with disorder pore structure and a surface area of > 3000 m2/g exhibit CO2 capacity much lower than a N doped carbon with less surface area (2600 m2/g) but with a homogeneous pore structure (a structure analogue of ideal graphene with N functionalities). The results are already published in Journal of Physical Chemistry C (2015, 119 (39), pp 22310–22321; 2016, 120 (32), pp 18167–18179). These results are also presented as posters in Carbon 2015, Carbon 2016 and in the UKCCS meetings where a price for the best poster was obtained.

Another main objective of BIOADCAT is to study the limit of the pristine and N functionalized carbons on the CO2 uptake and also to bring out the capability of such structures for the kinetic separation of CO2/N2 mixtures; which is essential information for the process design of CCS system in industrial Pressure Swing Adsorption units. Additionally, as part of BIOADSORB, a new method called GeXM or gas expansion method that will rely on the concept of non-equilibrium molecular dynamics is developed (using open source code DL_POLY) and tested at the preliminary level to study the kinetic and transport properties of CO2 in porous media at the nanoscale. In GeXM, the compressed gas in the bulk solution will be artificially forced to expand and permeate via a porous material (pristine or functionalized carbon) towards an empty compartment, until an equilibrium is reached. This theoretical method exactly mimic the experimental setup of volumetric gas adsorption experiments. Through GeXM we showed several concepts involved in CO2 transport in pristine and functionalized carbons like, (i) how N doping can increase the transport properties of CO2 and in case if the bulk fluid contains water molecules, then the N doping especially that are located at the edges can practically condense the water molecule at specific locations thus blocking the pore volume or in some case might make a majority of the pore volume completely useless. GeXM method is tested with slit-shaped N doped carbon pores and currently it suffers from several limitations like its applicability only for fluids (CO2+water or CO2/N2+water) at higher pressures. Once this limitation is resolved (via introducing artificial force in the direction of the fluid flow), then this method will be documented in a reputed journal like J Phys Chem C or Langmuir. GeXM will receive a huge impact as it can also provide valuable information for researchers and industrialists who need the fundamental understanding of the gas separation or water purification (like desalination) using porous materials of any type.
Impact: The results of BIOADSORB will be a prime importance to both academics and industry. On the experimental side, our results exposed the truth that N doping is not a promising strategy to improve CO2 uptake, although it can increase the CO2/N2 selectivity. This is a very significant result, as it demands now the experimentalists to rethink before making any further investment on this topic. The STPP technique developed as part of BIOADSORB will receive huge attention from both industrialists and researchers as it provides a straightforward, one-step and simple route to obtain high surface area carbons with desired level and type of N functionalities, pore-size and pore volume at a much lower cost. The functionalized carbons obtained via this method are extremely light (voluminous) and thus offer the possibility to conform them to the desired shape or to the desired form like sheets (as in membranes). Such techniques are essential to obtain materials that can find applications not only in gas storage but also in other promising technologies like supercapacitors, fuel cell (for O2 reduction) and in water purification. Also, this technique is scalable and it can be done by simply varying the size of the pyrolysis chamber.
From the theoretical side, the GeXM simulation developed through BIOADSORB is a huge breakthrough as it allows now to predict the transport properties of gasses or gas mixtures in porous materials based on information obtained at the atomic level. Also, GeXM is built using the open source code DL_POLY that is available for free to academics (industries can use it after buying a license from Daresbury laboratories). Once our results are documented, it can be exploited to test the kinetic selectivity and understand the transport properties of both gasses and liquids in carbons or in the different class of porous materials like PIMs, MOFs, ZIFs. In addition GeXM method also allow solving the problem of estimating the actual adsorption in any porous material at least from the theoretical point of view. Estimation of the actual adsorption from experiments is almost an impossible task from experiments as it can only measure the adsorption excess. Thus any theory that will be developed out of this GeXM method will be useful to design or interpret experimentally obtained adsorption isotherms. We believe the adsorption equipment manufacturing industries will exploit this method in future to develop new adsorption kernels to better predict the pore size distribution of the porous materials.

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United Kingdom
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