The Synthesis And Analysis Of Hemp Stem-Based Activated Carbons

Activated carbon is an effective adsorbent for several applications. One ofthe hot spot nowadays is to synthesize activated carbons with high specificsurface area and adjustable pore size, using cheap agricultural waste as thecarbonaceous materials and via simple preparing process. In this study, hempstem is used as raw material to synthesize various porous structural hempstem-based activated carbons, via chemical, physical and two stepchemical-physical activation processes. The optimized preparing parameterswere decided on the basis of specific surface area (S_BET) and yield of theresultant products. The skeletal microstructure, textile characteristics, surfacechemical properties and pyrolysis course were characterized by SEM, Nadsorption-desorption isotherm, XPS, FTIR and TG/DTA techniques. Besides,the influence of skeletal structure on the compressive strength, shrinkage ratio,electrical resistivity and textile of the carbonized products was discussed bycomparing hemp stem with bamboo culm.The results showed that the resultant hemp stem-based carbon had aporous skeleton, containing two types of pore channels with differentdiameters, all of which were end-to-end pore channels and were parallel toeach other. These pores were partitioned by the walls on which plenty of even smaller pores, i.e. micropores, existed. Compared with bamboo culm, hempstem didn't have the bamboo node-shaped structure, resulting in lowercompressive strength than bamboo and higher shrinkage ratio of hempstem-based charcoals than that of bamboo charcoals. Besides, the less lignincontent in hemp stem resulting in more sufficient improvement of porousstructure in hemp stem carbons activated by H₃PO₄．Both S and yield of hemp stem-based carbons by steam activation wererelatively low. The active reaction between steam and carbon became severely.S_BET of the steam activated carbon reached the maximum after activated at700℃, which was 830．59m²/g, however, the yield and microporosity were aslow as 3．21%å’Œ16%, respectively. H₃PO₄ and ZnCl₂ activation processes canobviously increased S_BET and yield. S_BET was 1388．4m²/g and 1691．5m²/g, andthe microporosity was 74% and 92%, for H₃PO₄ and ZnCl-2 activated carbons,respectively. Using H₃PO₄ as active agent, concentration and carbonizationtemperature had important influence on BET surface area, yield and theporous structure in the resultant activated carbons. By controlling theconcentration of H₃PO₄ at a lower level, hemp stem-based activated carbonswith more than 70% microporosity can be obtained, and the mesoporosityincreased as the temperature rises. Although through two stepchemical-physical continuous activation process can further increase S_BET ofthe resultant carbons,the yield of which was much lower. Micropores weredominance in the H₃PO-4-steam activated carbon, while ZnCl₂-steam activated carbon was dominated by mesopores. The adsorption property on methyleneblue and iodin of the resultant hemp stem-based activated carbons increasedwith the increasing of surface area.The impregnated H₃PO₄ and ZnCl₂ facilitate the decomposion of cellulose,hemicellulose and lignin which were almost decomposed at about 200℃,lowering the pyrolysis temperature. When using H₃PO₄ as active agent,aromatization started at 300℃and finished at 400℃. Phosphorus functionalgroups appeared above 400℃, resulting in acidity surface property of theH₃PO₄ activated hemp stem-based carbons.