摘要
煤直接液化残渣(简写为CLR)是一种高碳、高灰、高硫的混合物,约占煤直接液化工艺使用原煤的20～30wt.%,有效利用CLR对提高煤直接液化工艺的经济性、实现煤炭清洁高效利用具有重要意义。本文以CLR为原料,采用KOH活化法制备了多种炭材料,并研究了其作为催化甲烷裂解制氢催化剂时的催化性能以及作为超级电容器电极材料时的电化学性能。
本文系统研究了KOH活化法制备CLR基炭材料的工艺条件(包括KOH/CLR质量比、KOH与CLR混合时采用的溶剂种类、碳化温度程序、碳化后的洗涤方式、CLR组成等)对所制备炭材料的孔结构、催化甲烷裂解性能及其作为超级电容器电极时的电化学性能等方面的影响,提出并实现了以CLR内在矿物质及其与KOH反应生成的无机盐作为炭材料成孔过程中的模板,直接制备出了孔径集中分布在3-5nm的CLR基介孔炭。
通过索氏抽提和脱灰的方式将CLR分为重油(8.0wt%)、沥青烯(29.2wt.%)、前沥青烯(12.1W.%)、复杂碳基质(29.0wt.%)和矿物质(21.7wt.%)五部分,研究了各部分对炭材料孔结构和性能的影响。结果显示：在KOH活化过程中,CLR中的有机质作为碳源时趋向于形成微孔炭,而在CLR内在矿物质的模板作用下,可直接制得介孔炭(介孔孔隙率高达92%)。这种介孔炭材料具有比微孔炭材料更高且更稳定的催化甲烷裂解的活性。经过KOH活化、碳化及碳化后的洗涤等步骤,可将CLR基炭材料中的硫和铁等杂质含量降低到0.2%以下。相对于商业活性炭、炭黑BP2000以及文献报道的同类炭材料,CLR基介孔炭在催化甲烷裂解反应中表现出更高的催化活性和稳定性。
CLR基炭材料的孔结构特征与其催化性能之间的关系表明：较大的比表面积和孔容促使炭材料具有较高且较稳定的催化甲烷裂解的活性。“催化-再生”性能测试实验显示脉冲式再生方式比连续式的效果更好,但是再生过程中产生的CO或C02会使催化甲烷裂解制氢反应丧失了相对于甲烷水蒸汽重整反应的优势。
采用KOH活化与外加添加剂相结合的方法,可实现CLR基炭材料孔结构的调控,制备出多级孔道炭材料。研究了不同硅源材料(Si02、正硅酸乙酯、Na2SiO3和SBA-15)、金属氧化物(A1203和MgO)和有机物(蔗糖、尿素和CTAB)三类添加剂对炭材料的孔结构、催化活性和电化学性能的影响。结果发现：不同种类的添加剂会导致炭材料成孔机理的不同。以硅源材料或A1203作为添加剂时,它们通过与KOH反应生成无机盐,进而充当炭材料成孔过程中的模板；MgO通过自身占位的方式,可直接充当炭材料成孔过程中的模板；而有机添加剂通过在碳化过程中释放气体来影响炭材料的孔结构。通过优化添加剂的用量,可显著降低炭材料电极的等效电阻,进而将其电容值提高30%以上。当以MgO为添加剂时,所制备的炭材料电极性能优异；其电容值在5mV/s的扫描速率下高达186F/g,在10A/g的电流密度下为137F/g,在200mV/s扫描速率下经6000次的充放电循环后仍然保持在118F/g左右。以A1203为添加剂所制备的CLR基炭材料催化剂,通过甲烷裂解的方式首次实现了同时制备出纤维炭和氢气的优异效果；且生成的纤维炭也具有一定的催化活性。
采用KOH活化结合外加Fe(NO3)3或Ni(NO3)2两种添加剂的方式,利用炭的高温还原性,可直接制备出掺杂Fe或Ni单质的炭材料,省略了传统制备工艺所必需的氢气还原步骤,简化了制备过程。当以掺杂Ni的炭材料作为催化甲烷裂解的催化剂时,甲烷转化率随着反应时间逐渐增高。这主要归因于生成的积碳中分散着粒径较小的Ni活性成分；该积碳可再次用作催化剂使用,且有利于提高Ni活性成分的利用率。
选用具有不同灰含量的4种煤和2种油页岩为碳源,探讨了将制备与应用CLR基炭材料的方法拓展至煤或油页岩基炭材料领域的可行性,结果显示：在KOH活化过程中,引入添加剂可显著改进低灰煤基炭材料的孔结构和电化学性能；但却会破坏高灰煤基炭材料的孔结构,进而抑制其电化学性能。油页岩灰分含量较高,使得炭材料的收率较低；但可直接利用它的内在矿物质及其与KOH反应生成的无机盐为模板,制备出介孔炭。
Direct coal liquefaction residue (CLR) is rich in carbon, ash and sulfur contents, sharing about20-30wt.%of the raw coal used in the direct coal liquefaction process. It is necessary to effectively utilize CLR for the benefits of the process and developing clean and efficient coal technologies. In this study, various CLR based carbons were prepared by KOH activation and used as catalysts for hydrogen production by catalytic methane decomposition (CMD) and as supercapacitor electrodes.
Effects of carbon preparation conditions including the weight ratio of KOH/CLR, solvents for mixing KOH and CLR, carbonization procedure and solvents for washing the carbonized samples and CLR compositions, were investigated on the resultant carbon pore structure, catalytic activity and electrochemical performance. The results show that CLR based mesoporous carbons, with the pore sizes centred at3-5nm, can be directly prepared by KOH activation by using the mineral matters contained in the CLR and their salts formed with KOH as the templates.
The CLR were separated into five fractions, including oil, asphaltene, preasphaltene, complex carbon matrix and mineral matter, by Soxhlet extraction and demineralization. They share8.0,29.2,12.1,29.0and21.7wt.%of the CLR. As for their effects on the resultant carbons, the organic matter alone is conducive to developing a microporous structure, while the mineral matter plays a positive role on the KOH activation process of the CLR and can serve as the template for mesoporous structure (the mesoporosity up to92%). The resultant mesoporous carbon shows higher and more stable activity for CMD than the microporous. The contents of the impurities(including sulfur and Fe) in the resultant carbon can be down to less than0.2%after KOH activation, carbonization and washing after carbonization. And the CLR-based mesoporous carbon has higher and more stable activity for CMD than commercial coal-based activated carbon, carbon black BP2000and the similar carbons reported in literatures.
The relationship between carbon catalytic activities and the textural properties indicates that larger surface area and higher pore volume correspond to higher and more stable catalytic activity. With respect to the decomposition-regeneration cycles, pulsed regeneration is better than the continuous. However, CO and/or CO2will release during the regeneration process, resulting in that the CMD reaction will wipe out its advantage compared with the traditional steam methane reform process.
The porous structures of CLR-based carbons can be adjusted and hierarchical porous carbons can be obtained by KOH activation with addition of some additives. It was investigated the effects of three kinds of additives, including different silica materials (SiO2, TEOS, Na2SiO3and SBA-15), metal oxides(AI2O3and MgO) and organic materials (sugar, urea and CTAB), on the resultant carbon pore structure, catalytic performance and electrode capacitance. The results show that different additives correspond to different mechanisms for the carbon porous structures. Some nanoparticles, formed by the reaction of the silica materials (or AI2O3) and KOH, can serve as space fillers of nanopores in the carbonized carbon. So can MgO particles themselves. The gases, produced by the decomposition of the organic additives, can develop and/or widen some pores. Equivatent resistances of the carbon electrodes can be reduced by the optimum dosage of the additive, with the capacitance increased by more than30%. When MgO was used as the additive, the resultant carbon electrode shows excellent capacitive performance, with the capacitance up to186F/g at the scan rate of5mV/s or137F/g at the current density of10A/g and good cycle stability (keeping the capacitance of about118F/g) after6000cycles at200mV/s. When Al2O3was used as the additive, the resultant carbon can serve as the carbon catalyst of CMD for simultaneous production of hydrogen and fibrous carbons, which was reported for the first time. And the produced fibrous carbons show good catalytic activity for CMD.
Fe (or Ni)doped carbons can be directly prepared from CLR by KOH activation with addition of Fe(NO3)3(or Ni(NO3)2). The preparation method can utilize the carbon reducibility at high temperature, eliminating hydrogen reduction process required by the traditional synthesis process. When Ni doped carbon was used as the catalyst for CMD, methane conversion increased with the reation time. It was mainly attributed to that the carbon deposits produced by CMD were mixed with small particles of active Ni. So they can serve as the catalysts for CMD, increasing the active Ni utilization.
4coals and2oil shales, as carbon precursors with different ash contents, were used to prepare carbons by the methods mentioned for CLR-based carbons. The results show that the method combined with KOH activation and the additive can improve the porous structure and electrochemical performance of the carbons from coal with low ash content, but do harm to those from coal with high ash content. Due to the high ash content, carbons from oil shales had a low yield. But the mineral matters contained in oil shales and their salt nanoparticles formed by the reaction with KOH can serve as the template for mesoporous carbons.
引文
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