煤焦表面官能团对甲烷裂解的影响

采用石英管固定床反应器,考察了1 123 K,CH_4/N_2=1∶4的状态下,煤焦表面官能团对甲烷裂解的影响。煤焦表面主要含有羟基、羰基和醚键等官能团,分别通过氢氧化钡、苯肼和碘化氢溶液对煤焦表面进行化学处理,除去煤焦表面相应的官能团,研究处理后煤焦的催化活性,得出煤焦中官能团对甲烷裂解的影响。氢氧化钡处理煤焦后,煤焦中部分羟基被氢氧化钡消耗,甲烷初始转化率和氢气初始收率分别为90.5%和65.2%,说明煤焦中的羟基不利于甲烷的裂解。通过苯肼处理煤焦中羰基并使之转化,甲烷初始转化率和氢气初始收率分别为55.4%和42.9%,说明煤焦中的羰基对甲烷裂解有利。碘化氢处理煤焦后,煤焦中的醚键转化成羟基,甲烷的转化率和氢气收率都明显下降,说明醚键的存在对甲烷裂解同样有利。随着反应时间的延长,甲烷的转化率和氢气的收率降低,催化剂逐渐失活。反应前后煤焦电镜扫描照片表明,甲烷裂解生成的积炭沉积在煤焦表面,堵塞煤焦的孔道,煤焦的比表面积减小,催化活性降低。

Role of coal surface functional groups in methane cracking over different chars

Methane cracking was studied over a set of Xiaolongtan lignite chars in a fixed bed reactor at 1123K and atmospheric pressure with a mixture of CH₄/N₂ (1:4). The chars were obtained by pyrolysis of raw Xiaolongtan coal in nitrogen at 1173K for 30min in a fixed bed reactor. The main functional groups in coal char were hydroxyl group, carbonyl group and ether bond and so on, which can be protected by dipping the char into solutions of barium hydroxide, phenyl hydrazine and hydrogen iodide, respectively. The catalytic activity of coal chars treated by barium hydroxide was lower than the raw chars, while that of the chars treated by phenyl hydrazine or hydrogen iodide were higher. Hydroxy in the coal char was expended by Ba(OH)₂. The initial methane conversion was achieved at about 90.5% for the Ba(OH)₂ -treated char with equivalent-volume impregnation. The corresponding hydrogen yield was at about 65.2%. The carbonyl group of the coal char was reduced by phenyl hydrazine, with the methane conversion and hydrogen yield about 55.4% and 42.9%, respectively. The methane conversion and hydrogen yield decreased, while the ether bond turned into hydroxy. It was speculated that hydroxyl group in coal char restrain the methane cracking, while carbonyl group and ether bond accelerate it. The methane conversion and hydrogen yield on the different coal chars decreased with increasing reaction duration. The char became deactivated at 123min following its exposure to methane. The main reason was that the carbon from methane cracking was deposited on the char, which is supported by scanning electron microscopy analysis.