Separation of propylene from propane and nitrogen by Ag(I)-doped nanoporous carbons obtained from hydrothermally treated lignin

Abstract

Separation of propylene (C3H6) from propane (C3H8) is highly important to produce polymer (polypropylene)-grade C3H6. The separation of C3H6 from nitrogen (N2) is also industrially important to recover unreacted propylene from the polypropylene plant. Compared to the cryogenic separation that is usually employed to separate C3H6, an adsorption process is sustainable and inexpensive provided a suitable adsorbent is developed. Lignin is an inexpensive, sustainable, and natural precursor of porous carbons. In this work, we have hydrothermally treated lignin followed by its carbonization, activation and Ag(I) doping to synthesize the Ag(I)-functionalized microporous carbons. These carbons were characterized with pore textural properties, scanning electron microscopic imaging (SEM), EDX-mapping, and X-ray photoelectron spectroscopy (XPS). The Ag(I)-doped nanoporous carbons possessed the BET surface areas of 440–1146 m2/g and Ag(I) contents 1.5–5.9 at.%. The adsorption isotherms at 298 K and pressure upto 760 Torr suggested that all the adsorbents were selective to C3H6 compared to C3H8 and N2. Ideally adsorbed solution theory (IAST) was employed to calculate the selectivity of binary mixtures of 50/50 C3H6/C3H8 and 30/70 C3H6/N2 mixtures at 298 K. The selectivity of C3H6/C3H8 lies within 3–7 whereas the selectivity of C3H6/N2 lies within 259–2600 that is higher than that of a large number of adsorbents reported in the literature. It is observed that the carbon with a lower Ag(I) content is more selective to C3H6 that may be attributed to the competition between Ag(I) and oxygen functionalities. The fixed-bed dynamic breakthrough simulations suggested that C3H6 can be separated from N2 by all the carbons whereas C3H6 can be separated from C3H8 by the carbon with a lower Ag(I) content. The overall results suggested these carbons can potentially be used for C3H6 separations.