Preparation of selective carbonaceous adsorbents by ammoxidation of lignite: influence of the demineralisation by acids

Full text of publication follows: The aim of this work is to prepare low-cost carbonaceous adsorbents with the propensity to adsorb large quantity of pollutants like Volatile Organic Compounds (VOC's). Moreover, they are wished to be selective towards pollutants, acidic in our case. For this purpose, it was necessary to modify the starting material from a chemical point of view in order to obtain interesting adsorbents taking into account economical and scientific criteria. Indeed, our precursors, which are made of lignite, cheap and functionalized, have a rate of mineral matter of 6,15% however all the catalytic effects related to ashes are not well known and can disturb the process of preparation of such selective activated carbons and compromise its reproducibility. Several methods coexist to remove minerals from coal: To compare this study with another works, a standardised method, the ISO 602 norm [1], was chosen (reaction with HCl 5 N and HF 5 N under heating). Another protocol was tested [2] which comprises stronger acids concentration and three steps: HCl (5 N), HF (22 N) and again HCl (12 N). After these manipulations, the surface chemistry becomes largely acidic so, in order to create some basic nitrogenated functions on the surface, the samples undergo a heating treatment with a flow of ammonia and air. In this work, the influence of the demineralisation and the ammoxidation or both of them on the surface properties is followed throughout the process. Thus, the surface properties of different samples of the same starting lignite are studied before, after demineralisation and ammoxidation, and alter carbonization/activation. These investigations are performed using different analytical tools: The Boehm's titration method [3], elemental analysis, infra-red and X-ray spectroscopies, Inverse Gas Chromatography coupled with L.S.E.R. (Linear Solvation Energy Relationship) modelling. All these results are compared and discussed concluding on the acidic treatment on the selective performances of the final carbonaceous adsorbent. Initially it appeared both demineralisation treatments have both caused a fall of the near total of the minerals present in the lignite [4], inducing the same chemical modifications on surface [5]. The ammoxidation, as for it, involves on the raw and demineralized lignites a decrease of the concentration of oxygenated functions, in particular lactones supposed to react with ammonia to form amides. However the results obtained concerning the carboxylic acids and the phenolic groups are opposed for the raw and demineralized lignite: Indeed, some analyses according to Boehm show that there remains alter ammoxidation, in the raw ammoxidated lignite, more carboxylic acids than in the demineralized ammoxidated lignite but less phenols, perhaps this phenomena implies the catalytic activity of minerals. The characterization by model LSER (inverse gas chromatography) and the XPS study in progress will give further information. [1] International standard ISO 602: Charbon -Determination du taux de matieres minerales. (1983-02-01); [2] G. Skodras et al. Fuel Process Technol 2002;77-78:151-8; [3] HP. Boehm. Carbon 1994;32:759-69; [4] J. Starck et al. Fuel 2004;83:845-50; [5] Starck, J., Burg, P., Cagniant, D., Magri, P., Gruber, R, Tascon, JMD., Martinez-Alonzo, A. Jumelage materiaux carbones et catalytiques pour l'environnement, Zakopane (Pologne), 23-29 septembre 2004. ISBN 83 916351-6-3, pp 7-14. (authors)