Role of surface oxygen-containing functional groups in liquid-phase adsorption of nitrogen compounds on carbon-basedadsorbents

The carbon-based adsorbents are promising for adsorptive denitrogenation (ADN) of liquid hydrocarbon streams. The objective of the present study is to develop a fundamental understanding of the role of surface oxygen-containing functional groups on carbon-based adsorbents in adsorption of nitrogen compounds that are known to be present in liquid fuels. The adsorption properties of four representaive activated carbons were evaluated in a batch adsorption system for removing quinoline and indole respectively from decane. The adsorption was found to obey the Langmuir adsorption isotherm. The adsorption isotherms were obtained and the adsorption parameters (the maximum capacity and adsorption constant) were estimated. The surface chemical properties of the adsorbents were characterized by temperature-programmed desorption (TPD) technique with a mass spectrometer to identify and quantify the type and concentration of the oxygen-containing functional groups on the basis of the CO_2- and CO-evolution profiles. It was found that both the type and the concentration of surface oxygen-containing functional groups play an important role in determining the ADN performance. Higher concentration of the oxygen-containing functional groups on the adsorbents resulted in higher adsorption capacity for the nitrogen compounds. A fundamental insight was gained on the contributions of different oxygen functional groups by analyzing the changes in the monolayer maximum adsorption capacity qm and the adsorption constant K for nitrogen compounds on different carbon adsorbents. The acidic functional groups, such as carboxyl and carboxylic anhydride groups, appear to contribute more for adsorption of quinoline, whereas the basic oxygen-containing groups such as carbonyl and quinone groups may have more contribution to adsorption of indole