Microporous Carbon Adsorbents Prepared by Activating Reagent-Free Pyrolysis for Upgrading Low-Quality Natural Gas

Separation of methane and nitrogen is an essential matter of concern in upgrading low-quality natural gases such as biogas, coalbed methane, shale gas, and so forth. However, similar weak polarities and the close kinetic diameter of methane and nitrogen make the separation of these two species very challenging. Considering that micropores play an extremely important role in adsorption selectivity, in this work, carbon adsorbents with homogeneous pore widths were controllably prepared from poly(vinylidene chloride) resins by activating reagent-free pyrolysis. Compared to traditional activated carbon prepared by pyrolysis with the presence of chemical reagents such as potassium hydroxide and zinc chloride, the obtained microporous carbon adsorbents are free of mesopores and macropores and demonstrate efficient separation of CH4 and N-2. These materials were comprehensively characterized by PXRD, SEM, TEM, TGA, XPS, Raman spectra, contact angle measurement, and water vapor adsorption. The synthesized carbon adsorbents have an ultrahigh specific surface area (>1100 m(2)) and a large micropore volume (>0.37 cm(3)/g). Adsorption performances of these materials were demonstrated by static adsorption tests and breakthrough experiments. The methane uptake up to 1.57 mmol/g at ambient conditions outperforms most top-performing adsorbents reported to date. Moreover, the optimal microporous carbon, denoted C-PVDC-700, exhibits the highest LAST selectivity (5.5-14.7 at 298 K) of methane over nitrogen. The facile preparation strategy, ultrahigh adsorption capacities, record-high selectivity, and robust stability guarantee that this material is promising for upgrading low-quality natural gas.