【作者】 王科；

【导师】 周小平；

【作者基本信息】 湖南大学 ， 化学工艺， 2009， 博士

【摘要】 作为一种清洁高效的新能源物质,氢能源引起了研究者越来越多的重视。近年来由于氢燃料电池的出现更是加剧了对氢能源的需求。目前氢气的制备主要是通过化石燃料的重整获得,但是该流程产生污染大气的COx。相比之下,天然气的直接分解制氢可以制备出高纯度的氢气,但是该流程存在催化剂的迅速失活问题。在本文中,我们设计出一种新型催化剂来解决催化剂的寿命问题。通过把特定的金属加热到熔融态,以此作为催化剂来分解碳氢化合物制取氢气。在经过独特设计的石英管反应器上,在700℃的条件下实现了23%甲烷转化率和100%的氢气选择性,而且催化剂的寿命大于67h,这在常规负载型催化剂上是很难实现的。我们还发现,进一步升高温度能够提高甲烷的转化率,但是催化剂的寿命会受到影响。增加催化剂用量在反应的前期能够提高转化率,但是催化剂量增加到一定程度后进一步提高催化剂用量没有明显的效果。在本课题的研究中,我们还对熔融态金属镁分解乙烷和丙烷制氢进行了研究,结果表明反应能够获得更高的烷烃转化率,但是产物中出现含碳的副产物,降低了反应的选择性。通过间接实验和原位红外光谱研究,发现反应机理为甲烷首先与镁生成碳化镁和氢气,然后碳化镁进一步分解生成金属镁和炭,完成整个反应循环。在反应过程中,还发现金属镁会出现自然蒸发并在反应器上端凝固的现象,这是导致催化剂失效的重要原因。在实际工作中,也可以利用这一性质把催化剂镁从产物炭粉中分离出来减少了催化剂的后处理程序,有利于降低能耗。在本研究工作中,还发现这种熔融态催化体系同样适合于大分子碳氢化合物的分解制氢。在聚乙烯塑料的降解实验中,获得可接近100%的氢回收率,在轮胎橡胶的降解过程中,氢的回收率为60%。这些研究工作为从废弃高分子材料中回收碳和氢提供了新的途径。实验中也考察了其它低熔点的金属如金属钠和金属锌的催化性能,结果发现在相同的条件下,金属钠对甲烷分解的活性低于金属镁；而金属锌活性太低,没有活化碳氢化合物的能力。本论文的另外一部分工作是合成一种重要的农药中间体3,3-二甲氧基丙酸甲酯。我们发现在Wacker催化剂PdCl2/CuCl2的作用下,丙烯酸能够被高效地氧化生成3,3-二甲氧基丙酸甲酯。在35℃和5个大气压下,反应6h(丙烯酸和甲醇的摩尔配比为1:4),丙烯酸的转化率为95.2%,主产物选择性为90.6%。在这个反应中,甲醇既是反应物又是溶剂。研究发现该反应的反应机理为：丙烯酸和甲醇首先进行酯化反应生成丙烯酸甲酯,然后丙烯酸甲酯在PdCl2和CuCl2催化剂作用下氧化生成醛类中间产物,接着醛类中间产物和甲醇发生醇醛缩合反应得到目标产物3,3-二甲氧基丙酸甲酯。由于后面氧化反应和醇醛缩合反应的不断进行,使前面的酯化反应平衡不断向生成丙烯酸甲酯的方向移动,进而导致整个反应能进行彻底。更多还原

【Abstract】 Currently, the hydrogen is mainly produced though the reformation of fossil fuels. However, great amount of COx is produced in these processes, which lead to serious environment pollution. Comparing with these processes, the direct decomposition of nature gas could produce hydrogen with high purity. However, the catalyst deactivation is still a problem.In the current investigation, we designed a type of new catalyst to resolve the catalyst deactivation problem. Certain metals were heated to a molten state to catalyze the decomposition of hydrocarbons to produce hydrogen and carbon. In a quartz reactor, a methane conversion of 23% with hydrogen selectivity of 100% was reached. A catalyst lifetime of more than 67 hours was achived, which is very difficult to obtain from regular solid state catalysts.It was found that higher temperature favors methane decomposition. However, the catalyst stability was not good and the lifetime of catalyst was shorted at relatively higher reaction temperature. The increasing of catalyst loading could enhance the decomposition of hydrocarbons, but when the amount of catalyst loading was higher than certain amount, there was no further increase in hydrocarbon conversion. At the same conditions, we investigated the decompositions of ethane and propane over molten magnesium. The results showed that even higher hydrocarbon conversions were achieved. However, carbon containing byproducts were formed, which led to the hydrogen selectivity decrease.Through the in-situ IR characterizations and catalytic reaction, it was thought that methane reacted with magnesium to form magnesium carbide intermediates and hydrogen, and then the magnesium carbide intermediates further decomposed to magnesium and carbon to complete the catalytic cycle. In the reaction, Magnesium was found evapourated from the bottom of the reactor and condensed in the up section of the reactor. The magnesium evapouration might be the reason of catalyst deactivation. However, the magnesium evapouration automatically separates catalyst from product, which makes the catalyst recovery easy.Besides the light alkanes, we found that the molten matel catalyst could also catalyze the decomposition of high molecular weight hydrocarbons, such as plastics and rubber. In an investigation of polyethene decomposition, almost 100% of hydrogen element recovery was obtained. In the decomposition of rubber,60% hydrogen recoverage was obtained. The current investigation offered altanative approach to recover hydrogen and carbon from waste hydrocarbon materials.We also examined the catalytic activities of other low melt point metals, such as sodium and zinc. However we found that sodium did not show obvious improvement comparing with magnesium at similar reaction conditions. Metal zinc was found not to be active for hydrocarbon decomposition reactions.As part of dissertation investigation, the pesticide intermediate methyl 3, 3-dimethoxypropionate was succeefually synthesized from the oxidation of acrylic acid in methanol. The reaction carried out efficiently over the Wacker catalyst PdCl2/CuCl2. Conducting reaction at 35℃and 5 atm pressure for 6 h with an acrylic acid to methanol mol ratio of 1:4,95.2% of acrylic acid conversion and 90.6% of 3, 3-dimethoxypropionate selectivity were obtained over PdCl2/CuCl2. In this process, it is proved that the methanol acted as both reactant and solvent. The investigation indicated that the possible reaction mechanism of the reaction could be that acrylic acid reacted with methanol to form methyl acrylate by self-catalyzed esterfication reaction, then the methyl acrylate was oxidized to aldehyde intermediate over PdCl2/CuCl2. The aldehyde intermediate reacts with methanol to form the target product. The oxidation and aldehyde-alcohol condensation reaction moves the self-catalyzed esterfication reaction equilibrium towards the formation reaction of methyl acrylate, which finally drives the reaction to the completion.更多还原