褐煤热转化过程氢键分布特征及调控手段研究进展

Research progress on distribution characteristics and modulation methods of hydrogen bonds during thermal conversion of lignite

褐煤氧含量高且具备良好热反应性,是制备酚类、腐植酸等高附加值含氧化学品的重要原料。褐煤中的杂原子尤其是丰富的含氧官能团(酚羟基、羧基等)导致了大量氢键的存在,褐煤热转化过程(热解、干燥和直接液化等)中常见氢键包括煤中氢键和褐煤-溶剂间氢键等。煤中氢键和褐煤-溶剂间氢键在构型和强度上存在明显区别,目前可通过体积溶胀度、红外光谱等观测手段以及量子化学计算进行探究。煤中氢键和褐煤-溶剂间氢键广泛存在且明显作用于褐煤热解和直接液化等热转化过程。煤中氢键是维持褐煤大分子网络结构稳定的重要因素,能在热解过程中诱导酚羟基和羧基脱水,进而促进褐煤热转化过程中的低温交联反应,不利于焦油等轻质产物的生成。褐煤-溶剂间氢键是褐煤与溶剂相互作用的重要形式,其强度显著影响褐煤在热转化过程中的物理化学反应如萃取、溶胀以及脱氧等。充分认识褐煤相关氢键的存在形态和影响因素,并以此为基础进行氢键调控,对于褐煤清洁高效转化具有重要意义。现有的氢键调控主要目的在于破坏煤中氢键,从而在一定程度上抑制热转化过程中的交联反应。氢键调控方法主要包括低温预热以及使用吡啶、离子液体等强氢键受体进行溶剂预处理等。当前针对褐煤-溶剂间氢键的研究主要停留在定性层面,缺乏褐煤热转化过程中的原位观测以及定量分析。

With high content of oxygen and high thermal reactivity,lignite is an important feedstock for the preparation of high value-added chemicals,such as phenols and humic acids.The heteroatoms especially the abundant oxygen-containing functional groups(e.g.phenolic hydroxyl groups and carboxyl groups)result in the existence of a large number of hydrogen bonds in lignite.Hydrogen bonds are usually involved in thermal conversion(pyrolysis,drying and direct liquefaction)of lignite,mainly including hydrogen bonds in lignite and hydrogen bond between lignite and solvents.Those two kinds of hydrogen bonds apparently differ in geometry configuration and strength.To present,various hydrogen bonds can be figured out by experimental methods(such as volumetric swelling degree and infrared spectroscopy),as well as quantum chemistry calculations.Hydrogen bonds in coal and hydrogen bonds between lignite and solvent exist widely,and significantly affect thermal conversion of lignite,especially pyrolysis and direct coal liquefaction.Hydrogen bonds in lignite play important roles in the stabilization of macromolecular network of coal,which can promote dehydration of phenolic hydroxyl groups and carboxyl groups,as well as the low-temperature crosslinking reactions during thermal conversion and is not conducive to the formation of light products such as tar.The hydrogen bond between lignite and solvent is an important form of interaction between lignite and solvent.Its strength significantly affects the physical and chemical reactions of lignite in the process of thermal conversion,such as extraction,swelling and deoxidation.It is of great significance for the clean and efficient conversion of lignite to fully understand the existing forms and influencing factors of hydrogen bonds related to lignite,and to regulate the hydrogen bonds on this basis.To present,the main purpose of hydrogen bonds modulation is destroying hydrogen bonds which exist in lignite,so that the crosslinking reactions during thermal conversion can be suppressed to some degree.It can be realized by preheating treatment at relatively low temperature,as well as solvent pretreatment using stronger hydrogen bonds acceptors such as pyridine and ionic liquid.However,current study about hydrogen bonds formed by lignite and solvents are mainly qualitative researched,the in-situ observation and quantitative analysis of the lignite thermal conversion are lack.