Ammonia-activated mesoporous carbon membranes for gas separations

Porous carbon membranes, which generally show improved chemical and thermal stability compared to polymer membranes, have been used in gas separations for many years. In this work, we show that the post-synthesis ammonia treatment of porous carbon at elevated temperature can improve the permeance and selectivity of these membranes for the separation of carbon dioxide and hydrocarbons from permanent gases. Hierarchically structured porous carbon membranes were exposed to ammonia gas at temperatures ranging from 850°C to 950°C for up to 10min and the N₂, CO₂, and C₃H₆ permeances were measured for these different membranes. Higher treatment temperatures and longer exposure times resulted in higher gas permeance values. In addition, CO₂/N₂ and C₃H₆/N₂ selectivities increased by a factor of 2 as the treatment temperature and time increased up to a temperature and time of 900°C, 10min. Higher temperatures showed increased permeance but decreased selectivity indicating excess pore activation. Nitrogen adsorption measurements show that the ammonia treatment increased the porosity of the membrane while elemental analysis revealed the presence of nitrogen-containing surface functionalities in the treated carbon membranes. Thus, ammonia treatment at high temperature provides a controlled method to introduce both added microporosity and surface functionality to enhance gas separations performance of porous carbon membranes.