Porous Carbon with Ultrahigh Surface Area for CO 2 Capture at Elevated Pressure and Developing Methods for Measuring CO 2 Adsorption Dynamics

Natural gas is the cleanest fossil fuel source. However, natural gas wells typically contain considerable amounts of CO2, with on-site CO2 capture necessary. Solid sorbents are advantageous over traditional amine scrubbing due to their relatively low regeneration energies and non-corrosive nature. However, it remains a challenge to improve the sorbent’s CO2 capacity at elevated pressures relevant to natural gas purification. Here, we report the synthesis of porous carbons derived from a 3D hierarchical nanostructured polymer hydrogel, with simple and effective tunability over the pore size distribution. The optimized surface area reached 4196 m g, which is among the highest of carbon-based materials, with abundant microand narrow mesopores (2.03 cm g with d < 4 nm). This carbon exhibits a record-high CO2 capacity among reported carbons at elevated pressure (i.e., 28.3 mmol g total adsorption at 25 °C and 30 bar). This carbon also showed good CO2/CH4 selectivity and excellent cyclability. Molecular simulations suggested increased CO2 density in microand narrow mesopores at high pressures. This is consistent with the observation that these pores are mainly responsible for the material’s high-pressure CO2 capacity. This work provides insights into material design and further development for CO2 capture from natural gas. In addition, we discuss a quantitative thermodynamic analysis for a series of aminefunctionalized SBA-15 materials using simultaneous thermal analysis (STA), a technique that combines differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) in one simultaneous measurement. Values of ΔH° ads avg determined experimentally by STA are coupled to independent equilibrium adsorption isotherm measurements to provide ΔG° ads avg and ΔS° ads avg values using the well-known Langmuir adsorption model. Introduction Global annual energy-related CO2 emission reached a record high of 31.2 gigatonnes (Gt) in 2012[1], and is expected to rise continuously given the growing energy demands and the remaining fossil fuel-dependent energy infrastructure. The mitigation of CO2 emission has been recognized as a crucial necessity, as CO2 is a major contributing greenhouse gas that gives rise to global warming and associated consequences, including sea level rise, significant variation in weather patterns and threats to human health and wildlife habitats[2]. Natural gas has been viewed as the cleanest fossil fuel, as its combustion leads to the lowest carbon dioxide (CO2) emission per unit of energy generated, as well as negligible emissions of sulfur oxides (SOx), nitrogen oxides (NOx) and mercury compounds[3]. In particular, for the equivalent energy, combusting natural gas produces ~30% and ~45% lower CO2 emissions than combusting petroleum and coal, respectively[3]. However, natural gas wells contain considerable amounts of CO2 (i.e., typically 10-20 mol% and as high as 70 mol% in some locations)[4]. which require on-site CO2 capture. Amine scrubbing has been used previously for CO2 removal from natural gas, but its drawbacks are prominent, some of which include the corrosive nature of the amine solution, significant energy penalty associated with material regeneration, and difficulty in the implementation of off-shore capture units[5, 6]. Background High-Pressure CO2 Capture The development of novel materials and technologies for CO2 capture has attracted tremendous interest from the scientific community, while solid sorbent technologies have shown great promise[7, 8]. Among the broader class of solid sorbents, porous carbons are of particular interest due to their exceptional chemical and physical stability, high surface area and flexibility in terms of tunable pore structure and surface functionality[9-11]. However, it is still challenging to improve the CO2 capacity at high pressures relevant to natural gas purification. An interesting strategy of heteroatom-induced CO2 polymerization has been explored[12]; however, it has shown relatively low CO2 capacity at elevated pressures compared to some commercial activated carbons[13]. Another attractive approach is to optimize the pore properties including surface area, pore volume and pore size[14]; however, the synthesis of these high-surface area carbons involves the use of sacrificial templates of metal-organic frameworks (MOFs), which adds complexity and increases material cost. In our recent work, we reported a low-cost and template-free synthesis of a highly porous 3D carbon with superior performance for energy storage applications[9]. Herein, we extend the synthetic approach with efficient tunability on the surface area and pore size distribution. Our strategy is based upon the thermal annealing of a 3D hierarchical nanostructured polymer hydrogel (polyaniline, or PAni) without any sacrificial templates, followed by chemical activation. The surface area and pore size distribution can be tuned simply by varying the carbonization temperature. The final porous carbon (denoted as SU-AC) materials have ultrahigh surface areas up to 4196 m g and total pore volume as large as 2.26 cm g. More importantly, the SU-AC materials have abundant microand narrow mesopores (d < 4 nm) up to 2.03 cm g (~90% of the total pore volume), which are beneficial for CO2 adsorption at elevated pressures. The maximum CO2 capacity of the SU-AC materials (28.3 mmol g total adsorption at 25 °C and 30 bar) outperforms the highest reported value for porous carbon materials at identical conditions[14]. Furthermore, the hierarchical porous structures and highly interconnected pore networks of the SU-AC materials facilitate efficient gas diffusion. The SU-AC materials also show excellent CO2/CH4 selectivity, easy CO2 regenerability and multicycle stability. In addition, the precursors used for synthesis are commercially available and have relatively low cost. All these desired properties render the SU-AC materials promising candidates for high-pressure CO2 capture from natural gas. We also performed molecular simulation investigations on the CO2 capacity relationships, which provides insight into material design and further development for CO2 capture from natural gas. A diverse range of solid CO2 sorbents have been studied in the literature, with emphasis placed typically on maximizing the equilibrium CO2 capacities of materials. The fundamental thermochemistry of the adsorption/desorption reactions under conditions relevant to flue gas capture and, specifically, how the structure and surface distribution of immobilized sorbent molecules affect the free-energy of both adsorption and desorption reactions, is not well understood. The most common approach to assessing the strength of CO2-sorbent interactions reported in the literature is to calculate isosteric heats of adsorption (ΔH°ads) from equilibrium adsorption isotherm data measured at multiple temperatures using empirical isotherm models[15-18]. Reports of directly measured calorimetric heats of adsorption are far less common[19]. Results High-Pressure CO2 Capture The separation of CO2 from natural gas typically occurs at elevated CO2 partial pressures up to 30 bar[12]. It has been reported that the high-pressure CO2 capacity largely correlates to the pore volumes of microand narrow mesopores[14, 20]. As was shown in the previous sections, the SU-AC materials possess large pore volumes in the pore size regime below 4 nm, which indicates their potential for high-pressure CO2 capture applications. Figure 1a shows the CO2 total adsorption isotherms of the SU-AC materials at 25 °C and 0 – 50 bar. SU-AC-400 exhibits the highest total CO2 capacity, i.e., 28.3 mmol g (124.5 wt%) at 30 bar and 35.0 mmol g (154.2 wt%) at 50 bar. Due to its ultrahigh surface area and large pore volume, the CO2 adsorption has not reached saturation even at 50 bar. The CO2 capacity decreases as the carbonization temperature increases from 400 to 800 °C, i.e., from SU-AC-400 to SU-AC-800. This trend is in good agreement with that of surface area. For all samples tested, no hysteresis was observed between the adsorption and desorption data points, which suggests the reversible nature of CO2 adsorption even up to 50 bar. To evaluate the recyclability of the sorbent, CO2 adsorption was repeated for multiple cycles on SU-AC-400. The adsorption-desorption cycles were conducted as a pressure swing adsorption (PSA) process, with 30 bar and 1 bar as the adsorption and desorption pressure, respectively. No heat was applied to the system during the desorption process. The CO2 capacity over 10 cycles is plotted in Figure 2. No reduction in the CO2 capacity was observed, which indicates the stability and regenerability of the sorbent material. Figure 1b shows the comparison of the CO2 adsorption isotherm (25 °C) of our SUAC-400 sample with the best-performing literature carbon materials with surface areas larger than 2000 m g. It can be seen that the total CO2 uptake of SU-AC-400 at 30 bar (28.3 mmol g) is superior to those of the previously reported carbon materials[12-14, 20-22]. Table 1 lists the CO2 and CH4 adsorption uptakes and CO2/CH4 selectivities for the SU-AC samples. Table 5 summarizes the textual properties and high-pressure CO2 capacities of SU-AC-400 in comparison with previously reported carbons. The promising CO2 capacity along with high CO2/CH4 selectivity of the SU-AC materials makes them excellent candidates for high-pressure CO2 capture from natural gas. In addition, the potential applicability of the SU-AC materials in precombustion CO2 capture was also evaluated using the high-pressure CO2 uptake at 50 °C. As shown in Figure 1c, the SUAC-400 sample still exhibits a high total CO2 capacity of 25.3 mmol g-1 (111.3 wt%), which far exceeds that of the best-performing literature carbon (20.4 mmol g-1) under identical conditions[14]. The adsorption-desorption of CO2 is also fully reversible at 50 °C in a PSA process. The exceptional high-pressure CO2 u