Generalized Mechanochemical Synthesis of Biomass‐Derived Sustainable Carbons for High Performance CO₂ Storage

Abstract

absorbents; [ 2 ] the better effi ciency of solid state absorbents is related to the ease (with respect to energy penalty) of their regeneration via energy effi cient pressure or temperature swing changes. Porous solid state materials, namely, zeolites, [ 3 ] metal organic frameworks (MOFs), [ 4 ] covalent organic frameworks (COFs), [ 4 ] and carbons [ 5,6 ] are currently the most studied CO 2 absorbers for postcombustion capture and storage. Of the three classes of porous materials, carbons are attractive due to their abundance, low cost, and rich availability in activated [5][6][7] or templated [ 8,9 ] form. Activated carbons are particularly attractive as they may be prepared from biomass via valorisation processes that convert low value waste into valuable carbonaceous materials. [ 5f ] Recent studies have shown that KOH activated carbons, including biomass-derived examples, can achieve some of the best CO 2 uptake under conditions (i.e., 0.1-1 bar and 25-50 °C) relevant to industrial postcombustion capture. [ 2,[5][6][7] It is now well established that pore size is the main factor that determines CO 2 uptake of activated carbons under postcombustion capture conditions. [5][6][7]9 ] The critical importance of pore size arises from the fact that the CO 2 -carbon interaction emanates from short-range attractive forces which are maximized when CO 2 adsorption takes place in very narrow pores where overlapping potential fi elds from neighboring walls exert a positive infl uence. For slit-shaped pores, such as those present in activated carbons, enhancement of the adsorption potential may be maximized for micropore widths that are two times the diameter of the CO 2 molecule, [ 10 ] which translates to an optimal pore size of 6-7 Å. Such pores can be obtained in lowly or mildly activated carbons prepared at KOH/carbon ratio of 2 and between 600 and 700 °C. [ 2,[5][6][7]9,11 ] Indeed, to date, such activated carbons are among materials that show the highest CO 2 uptake at 25 °C and low pressure (up to 1 bar). [ 2,[5][6][7]9 ] Conversely, though, such lowly activated carbons tend to have low surface area, which limits the maximum amount of CO 2 that they can store. For this reason, there appears to be a limit on the amount of CO 2 that can be stored by biomass-derived or other activated carbons at 25 °C of ≈1.5 mmol g −1 at 0.15 bar, and 4.5 mmol g −1 at 1 bar. [ 2,[5][6][7]9,12 ] Increase in surface area can be achieved at higher activation levels (higher KOH amounts Novel mechanochemical activation generates biomass-derived carbons with unprecedented CO 2 storage capacity due to higher porosity than analogous conventionally activated carbons but similar pore size. The mechanochemical activation, or so-called compactivation, process involves compression, at 740 MPa, of mixtures of activating agent (KOH) and biomass hydrochar into pellets/disks prior to thermal activation. Despite the increase in surface area and pore volume of between 25% and 75% compared to conventionally activated carbons, virtually all o...

High yield and high packing density porous carbon for unprecedented CO2 capture from the first attempt at activation of air-carbonized biomass

“…The CNL1-2T carbons, therefore, show a very high gravimetric CO2 uptake that is one of the highest ever reported for porous carbons. 14,29,56,[72][73][74][83][84][85][86][87][88] In all cases, compactivated carbons have higher CO2 uptake than their activated analogues, 56,73 with sample CNL1-2700P…”

“…This is surprising because most previous studies have noted a widening of the adsorption knee at higher activation temperature. 29,56,57,[61][62][63][64][65][66][67][68][69][70][71][72][73][74] The level of porosity generated, as judged by the amount of nitrogen adsorbed, increases modestly for samples prepared at higher activation temperature despite the non-changing nature and shape of the isotherms. The isotherms of the compactivated carbons are similar to those of analogous conventionally activated carbons except that they exhibit higher amounts of nitrogen sorption.…”

“…The size of pores (or pore size distribution) is identical for conventionally activated and compactivated carbons ( Figure 3B and Table 1), which confirms that the higher porosity of the compactivated carbons is not accompanied by pore enlargement. 56,73 The nitrogen sorption isotherms obtained for activated CNL1-4T carbons prepared at KOH/CNL1 carbon ratio of 4 are shown in Figure 4A. Despite the increase in amount of KOH used, the resulting CNL1-4T samples exhibit isotherms that are still remarkable typical of microporous materials.…”

“…The packing density of the activated CNL1 carbons is comparable to that of lowly activated carbon monoliths or activated carbons. 73,91 The volumetric uptake of the activated CNL1 carbons is given in Table 2. At low pressure of 0.15 bar, which mimics the conditions for post combustion CO2 capture from flue gas streams, the CNL1-2T carbons have volumetric CO2 uptake of between 44 and 63 g l -1 (i.e., 1 -1.43 mmol cm -3 ), while for CNL1-4T carbons it varies from 24 to 40 g l -1 (0.55 to 0.9 mmol cm -3 ).…”

Polyhydroxyalkanoates: opening doors for a sustainable future

“…Lignin is an amorphous macromolecule that is composed of a phenylpropane repeat unit and possesses aliphatic and aromatic hydroxyl groups, as well as carboxylic acid groups. 17 These active functional groups and their amorphous nature render lignin a suitable candidate for blending with PHA. The amorphous nature of lignin can reduce the formation of large spherulites crystals and secondary nucleation, which has a significant impact on PHB brittleness.…”