磷酸活化-热解炭化制备大麻秆活性炭的特性研究

CHARACTERISTIC STUDY OF HEMP-STALK ACTIVATED CARBON PREPARED BY PHOSPHORIC ACID ACTIVATION-PYROLYSIS CHARRING

采用磷酸活化法并辅以热解成炭法制备大麻秆活性炭,利用比表面积测试(BET)、扫描电子显微镜(SEM)等检测手段分析活性炭的性能。结果表明:磷酸能促进大麻秆中半纤维素、纤维素等物质的分解,并在磷酸介入下与纤维素反应形成C—O—P缔合结构,强化炭化过程的骨架强度。磷酸活化也可促进活性炭微孔的形成,从而进一步增大活性炭的比表面积。由实验可知,活性炭的比表面积最高可达到1465.58 m^(2)/g。活性炭的形成分为浸渍、干燥、预炭化、炭化4个阶段,期间大麻秆中不稳定的成分会发生分解,磷酸会与纤维素发生反应形成C—O—P缔合结构,作为碳骨架。若要用磷酸活化法成功制得大麻秆活性炭,炭化温度最低要达到350℃,且大麻秆活性炭的比表面积在650℃时达到峰值;350~650℃温度区间内,随着炭化温度的升高,不断有新的微孔生成,同时已有的微孔扩孔变为中孔,微孔孔容保持稳定,中孔孔容不断增加,平均孔径持续增大。若炭化温度继续升高,大麻秆活性炭的比表面积与孔容会减小。为了控制烧失率,炭化时间宜控制在约1.5 h。

Using the phosphoric-acid activation method,coupled with the pyrolysis char-forming method,hemp-stalk activated carbon was prepared,and the properties of the prepared activated carbon were characterized by the technical means of BET and SEM. The experimental results show that phosphoric acid is capable of promoting the decomposition of hemicellulose and cellulose in hemp stalk,while also reacting with cellulose to form C—O—P associated structure which leads to an increase of the framework strength in the carbonization process. Phosphoric acid activation can also promote the generation of more micropores in activated carbon and further improve the activated carbon specific surface area,which can reach the maximum value of 1465.58 m^(2)/g according to the experimental result. The four steps of impregnation,drying,pre-carbonization,and carbonization are needed to transform hemp stalk into activated carbon,in the process of which the decomposition of unstable hemp-stalk components and the generation of C—O—P associated structure for use as carbon framework will take place simultaneously. To guarantee the successful transformation of hemp stalk to activated carbon using the phosphoric acid activation method,carbonization temperature cannot be lower than 350 ℃. At 650 ℃,the specific surface area of hemp-stalk activated carbon will reach the peak. Within the temperature range between 350 ℃ and 650 ℃,new micropores will be constantly formed and expanded to mesopores as the temperature keeps rising,and the average pore diameter will increase continually while the pore volume of micropores remains stable and that of mesopores grows increasingly larger. If carbonization temperature continuously rises,the specific surface area and pore volume of hemp-stalk activated carbon will both decrease. Besides,the suggested carbonization time is 1.5 h in order to control the rate of loss on ignition.