Porous carbons from sustainable sources and mild activation for targeted high-performance CO2 capture and storage

Carbon activation typically involves corrosive activating reagents such as KOH and NaOH. In this report, a less corrosive activating agent, i.e., potassium oxalate (PO), was explored for the preparation of activated carbons from biomass (hydrochar from sawdust, SD), polypyrrole (Ppy), or pre-mixed (SD + PPy) precursors. The resulting activated carbons have surface area of up to 2740 m2 g−1 and pore volume up to 1.7 cm3 g−1, and depending on activation conditions and/or type of precursor, are either highly microporous (up to 91% surface area from micropores) or highly mesoporous (up to 96% of surface area from mesopores). Unlike hydroxide activation, varying the PO/precursor ratio, within the range of 2–6, does not have a significant effect on porosity; carbons activated at PO/precursor ratio of 2, 4, 5 or 6 have comparable surface area. In contrast, activation temperature plays a critical role in determining the textural properties at any PO/precursor ratio. In this regard, a low PO/precursor ratio of 2 can achieve the full range of porosity, which not only offers a more eco-friendly and sustainable activation process but also allows easier control of the porosity by simple choice of activation temperature or type of precursor. The porosity of the activated carbons may be tailored towards 6 to 8 Å pore channels, which are excellent for CO2 storage at low pressure (i.e., post-combustion CO2 capture) where at 25 °C, the carbons capture up to 1.4 and 4.4 mmol g−1 of CO2 at 0.15 bar and 1 bar, respectively. The porosity can also be tailored towards mesoporosity, which is suited for moderate to high pressure (pre-combustion) CO2 storage, which reaches 20.3 mmol g−1 at 20 bar and 30.1 mmol g−1 at 40 bar at 25 °C. For high surface area samples, the porosity is favourable for pre-combustion CO2 capture, via pressure swing adsorption processes at an adsorption pressure of up to 40 bar and desorption at 1 bar, from gas mixtures (e.g., 60 : 40 H2/CO2) where the carbons achieve excellent working capacity

Testing the limits of SMILES-based de novo molecular generation with curriculum and deep reinforcement learning

Deep reinforcement learning methods have been shown to be potentially powerful tools for de novo design. Recurrent-neural-network-based techniques are the most widely used methods in this space. In this work we examine the behaviour of recurrent-neural-network-based methods when there are few (or no) examples of molecules with the desired properties in the training data. We find that targeted molecular generation is usually possible, but the diversity of generated molecules is often reduced and it is not possible to control the composition of generated molecular sets. To help overcome these issues, we propose a new curriculum-learning-inspired recurrent iterative optimization procedure that enables the optimization of generated molecules for seen and unseen molecular profiles, and allows the user to control whether a molecular profile is explored or exploited. Using our method, we generate specific and diverse sets of molecules with up to 18 times more scaffolds than standard methods for the same sample size; however, our results also point to substantial limitations of one-dimensional molecular representations, as used in this space. We find that the success or failure of a given molecular optimization problem depends on the choice of simplified molecular-input line-entry system (SMILES)

Optimization of the pore structure of biomass-based carbons in relation to their use for CO2 capture at low and high pressure regimes

A versatile chemical activation approach for the fabrication of sustainable porous carbons with a pore network tunable from micro- to hierarchical micro-/mesoporous is hereby presented. It is based on the use of a less corrosive and less toxic chemical, i.e. potassium oxalate, than the widely used KOH. The fabrication procedure is exemplified for glucose as precursor, although it can be extended to other biomass derivatives (saccharides) with similar results. When potassium oxalate alone is used as activating agent, highly microporous carbons are obtained (SBET ~ 1300 - 1700 m2 g-1). When a melaminemediated activation process is used, hierarchical micro-/mesoporous carbons with surface areas as large as 3500 m2 g-1 are obtained. The microporous carbons are excellent adsorbents for CO2 capture at low pressure and room temperature, being able to adsorb 4.2 - 4.5 mmol CO2 g-1 at 1 bar and 1.1 - 1.4 mmol CO2 g-1 at 0.15 bar. On the other hand, the micro-/mesoporous carbons provide record-high room temperature CO2 uptakes at 30 bar of 32 - 33 mmol g-1 CO2 and 44 - 49 mmol g-1 CO2 at 50 bar. The findings demonstrate the key relevance of pore size in CO2 capture, with narrow micropores having the leading role at pressures < 1 bar and supermicropores/small mesopores at high pressures. In this regard, the fabrication strategy presented here allows fine-tuning of the pore network to maximize both the overall CO2 uptake and the working capacity at any target pressure

Experimental Demonstration of Dynamic Temperature-Dependent Behavior of UiO-66 Metal–Organic Framework: Compaction of Hydroxylated and Dehydroxylated Forms of UiO-66 for High-Pressure Hydrogen Storage

High-pressure (700 MPa or ∼100 000 psi) compaction of dehydroxylated and hydroxylated UiO-66 for H2 storage applications is reported. The dehydroxylation reaction was found to occur between 150 and 300 °C. The H2 uptake capacity of powdered hydroxylated UiO-66 reaches 4.6 wt % at 77 K and 100 bar, which is 21% higher than that of dehydroxylated UiO-66 (3.8 wt %). On compaction, the H2 uptake capacity of dehydroxylated UiO-66 pellets reduces by 66% from 3.8 to 1.3 wt %, while for hydroxylated UiO-66 the pellets show only a 9% reduction in capacity from 4.6 to 4.2 wt %. This implies that the H2 uptake capacity of compacted hydroxylated UiO-66 is at least three times higher than that of dehydroxylated UiO-66, and therefore, hydroxylated UiO-66 is more promising for hydrogen storage applications. The H2 uptake capacity is closely related to compaction-induced changes in the porosity of UiO-66. The effect of compaction is greatest in partially dehydroxylated UiO-66 samples that are thermally treated at 200 and 290 °C. These compacted samples exhibit XRD patterns indicative of an amorphous material, low porosity (surface area reduces from between 700 and 1300 m2/g to ca. 200 m2/g and pore volume from between 0.4 and 0.6 cm3/g to 0.1 and 0.15 cm3/g), and very low hydrogen uptake (0.7–0.9 wt % at 77 K and 100 bar). The observed activation-temperature-induced dynamic behavior of UiO-66 is unusual for metal–organic frameworks (MOFs) and has previously only been reported in computational studies. After compaction at 700 MPa, the structural properties and H2 uptake of hydroxylated UiO-66 remain relatively unchanged but are extremely compromised upon compaction of dehydroxylated UiO-66. Therefore, UiO-66 responds in a dynamic manner to changes in activation temperature within the range in which it has hitherto been considered stable

Co-pelletization of a zirconium-based metal-organic framework (UiO-66) with polymer nanofibers for improved useable capacity in hydrogen storage

We report on a concept of co-pelletization using mechanically robust hydroxylated UiO-66 to develop a metal-organic framework (MOF) monolith that contains 5 wt% electrospun polymer nanofibers, and consists of an architecture with alternating layers of MOF and nanofiber mats. The polymers of choice were the microporous Polymer of Intrinsic Microporosity (PIM-1) and non-porous polyacrylonitrile (PAN). Co-pelletized UiO-66/PIM-1 and UiO-66/PAN monoliths retain no less than 85% of the porosity obtained in pristine powder and pelletized UiO-66. The composition of the pore size distribution in co-pelletized UiO-66/PIM-1 and UiO-66/PAN monoliths is significantly different to that of pristine UiO-66 forms, with pristine UiO-66 forms showing 90% of the pore apertures in the micropore region and both UiO-66/nanofiber monoliths showing a composite micro-mesoporous pore size distribution. The co-pelletized UiO-66/nanofiber monoliths obtained improved useable H2 capacities in comparison to pristine UiO-66 forms, under isothermal pressure swing conditions. The UiO-66/PIM-1 monolith constitutes the highest gravimetric (and volumetric) useable capacities at 2.3 wt% (32 g L−1) in comparison to 1.8 wt% (12 g L−1) and 1.9 wt% (29 g L−1) obtainable in pristine UiO-66 powder and UiO-66 pellet, respectively

Pore Characteristics for Efficient CO2 Storage in Hydrated Carbons

Development of new approaches for carbon dioxide (CO2) capture is important in both scientific and technological aspects. One of the emerging methods in CO2 capture research is based on the use of gas-hydrate crystallization in confined porous media. Pore dimensions and surface functionality of the pores play important roles in the efficiency of CO2 capture. In this report, we summarize work on several porous carbons (PCs) that differ in pore dimensions that range from supermicropores to mesopores, as well as surfaces ranging from hydrophilic to hydrophobic. Water was imbibed into the PCs, and the CO2 uptake performance, in dry and hydrated forms, was determined at pressures of up to 54 bar to reveal the influence of pore characteristics on the efficiency of CO2 capture and storage. The final hydrated carbon materials had H2O-to-carbon weight ratios of 1.5:1. Upon CO2 capture, the H2O/CO2 molar ratio was found to be as low as 1.8, which indicates a far greater CO2 capture capacity in hydrated PCs than ordinarily seen in CO2-hydrate formations, wherein the H2O/CO2 ratio is 5.72. Our mechanistic proposal for attainment of such a low H2O/CO2 ratio within the PCs is based on the finding that most of the CO2 is captured in gaseous form within micropores of diameter less than 2 nm, wherein it is blocked by external CO2-hydrate formations generated in the larger mesopores. Therefore, to have efficient high-pressure CO2 capture by this mechanism, it is necessary to have PCs with a wide pore size distribution consisting of both micropores and mesopores. Furthermore, we found that hydrated microporous or supermicroporous PCs do not show any hysteretic CO2 uptake behavior, which indicates that CO2 hydrates cannot be formed within micropores of diameter 1–2 nm. Alternatively, mesoporous and macroporous carbons can accommodate higher yields of CO2 hydrates, which potentially limits the CO2 uptake capacity in those larger pores to a H2O/CO2 ratio of 5.72. We found that high nitrogen content prevents the formation of CO2 hydrates presumably due to their destabilization and associated increase in system entropy via stronger noncovalent interactions between the nitrogen functional groups and H2O or CO2