【作者】 任丽萍；

【导师】 范康年；

【作者基本信息】 复旦大学 ， 物理化学， 2004， 博士

【摘要】 甲醇直接脱氢制备无水甲醛的负载银催化剂及其反应机理的研究在无氧及各种催化剂条件下利用甲醇直接脱氢进行无水甲醛的生产是目前研究较多的生产方案,其中对催化剂的选择是该方法研究的关键。为了寻找甲醇直接脱氢的催化剂,各国学者对周期表中大量元素和化合物进行了研究,按催化剂性质不同可分为金属、氧化物、碳酸盐和沸石四大类,但是由于各种原因这些催化剂都没有实现工业化。另外,由于用于甲醇直接脱氢反应的催化剂种类繁多,使该反应的机理仍模糊不清,从而阻碍了人们对各种催化剂的进一步优化。银催化剂至今仍被广泛地应用于甲醇氧化脱氢制甲醛的反应过程中,所以人们自然会尝试将其应用于甲醇直接脱氢制无水甲醛的反应。本论文旨在开发几种甲醇直接脱氢制取无水甲醛的新型负载银催化剂,解决其它催化剂用于该反应时所产生的种种问题,为无水甲醛的工业化生产提供一定的参考。根据催化剂的表征结果发现,负载银催化剂中银元素的存在状态与载体组分中是否含铝密切相关,所以可将所研制的催化剂分为两类:非铝型催化剂和铝型负载银催化剂。另外,以结构最为简单的电解银作为探针,采用原位拉曼等手段,详细研究了电解银上氧物种的变化情况,探讨了甲醇直接脱氢反应的活性中心并试图揭示甲醇直接脱氢反应的机理,从而进一步完善甲醇氧化脱氢的反应机理,以期对工业生产提供理论指导。1、甲醇直接脱氢制无水甲醛的非铝型负载银催化剂研究采用溶胶凝胶法制备了一系列双组分载体的非铝型负载型银催化剂-Ag/SiO2-ZnO和Ag/SiO2-MgO,确定了最佳制备条件:水醇复合溶剂中的醇性溶剂选择为乙醇,水含量30%,于1000(C下用马福炉焙烧。Ag/SiO2-ZnO催化剂的最佳组成为Ag/Zn(摩尔比)=10:1;而对Ag/SiO2-MgO催化剂来说,银负载量为20%,硅镁比为10:1时活性最佳。Ag/SiO2-ZnO催化剂用于甲醇直接脱氢反应,在700℃反应时,甲醇转化率和甲醛得率分别为82.3%和73.2%,明显高于文献报道的结果;而对于Ag/SiO2-MgO催化剂,当反应温度达到650(C时,即可达到96%的甲醇转化率和75%的甲醛得率,此时达到最高活性。可以看出Ag/SiO2-MgO催化剂的反应温度稍低一些。通过一系列的常规表征方法,如:<WP=7>BET、XRD和SEM等,对催化剂的结构进行了表征,发现这些催化剂均为低比表面微孔结构,适合甲醇直接脱氢制甲醛的反应。另外,该类非铝型负载银催化剂中的银在反应前后均以金属银的形式存在于催化剂表面,不同的是由于Ag/SiO2-ZnO催化剂用于反应时温度过高,吸附氧物种消耗完以后表面孤立的银颗粒会发生烧结,而Ag/SiO2-MgO催化剂反应温度较低,反应后银颗粒只会稍微长大,不会烧结。银在催化剂表面的特殊存在状态决定了其具有类似于电解银的甲醇脱氢活性位――强相互作用的银氧物种,所以氧气气氛的高温焙烧是非铝型催化剂再生的有效途径,Ag/SiO2-ZnO催化剂由于失活后表面银颗粒的大面积烧结,致使再生后催化剂的活性只能达到新鲜催化剂的55%左右;而Ag/SiO2-MgO催化剂再生后的活性则可达到新鲜制备的催化剂的90%以上。所以,综合比较两种非铝型负载银催化剂,Ag/SiO2-MgO催化剂具有高于Ag/SiO2-ZnO催化剂的甲醇脱氢反应活性,而且反应温度较低,再生性能良好,是一个很有发展前景的甲醇脱氢制备无水甲醛的催化剂。2、甲醇直接脱氢制无水甲醛的铝型负载银催化剂研究Ag/SiO2-Al2O3是我们实验室开发的甲醇氧化脱氢反应的优良催化剂,它在甲醇直接脱氢反应中同样表现出了良好的催化性能,通过调变各制备参数,得到最佳性能的催化剂应符合硅铝(重量)比为9:1、银含量为20%,该催化剂用于甲醇直接脱氢反应,在温度为650℃时,甲醇转化率和甲醛得率分别达到95.0%和81.2%。将Ag/SiO2-Al2O3催化剂与上述两种非铝型负载银催化剂相互组合,我们得到两种性能优异的四组分催化剂Ag/SiO2-Al2O3-ZnO和Ag/SiO2-MgO-Al2O3。在含银20%的Ag/SiO2-Al2O3催化剂中添加15%的锌(相对于银),得到的Ag/SiO2-Al2O3-ZnO催化剂可使甲醛的选择性达到为87%,甲醇的转化率也可提高至99%。XRD和SEM结果表明:与Ag/SiO2-Al2O3催化剂相同,在Ag-SiO2-Al2O3-ZnO催化剂中,银仍以离子态存在,不同的是载体的骨架结构发生了明显的变化,可见载体结构对催化剂的活性影响很大。在制备Ag/SiO2-MgO-Al2O3的过程中,通过仔细调变载体各组份含量,我们得到镁含量为8.3%、铝的含量为16.3%的含银20%的Ag/SiO2-MgO-Al2O3催化剂活性最佳,其甲醇的转化率和甲醛选择性均可达到100%!NH3-TPD和CO2-TPD的表征结果证明,四组分的铝型催化剂具有明显弱于Ag/SiO2-Al2O3的酸性和远远小于相应的非铝型催化剂的碱性,这使得该类催化剂用于反应时,由酸碱中心引起的副产物DME和CO明显减少,从而使甲醛的选择性大幅提高,这和活性测试的结果是一致的。XRD、SEM、UV-Vis. DRS和TPR等结果表明:这类铝型负载银<WP=8>催化剂在反应后银元素的存在形态发生了根本的改变,银由反应前的离子态被还原为元素态,而且离子态银的含量与催化剂的活性密切相关,说明这种离子态的银可能是甲醇在该类催化剂上发生直接脱氢反应的活性中心。催化剂的超高热稳定性排除了银以简单的氧化物形式存在的可能,而氧气气氛的高温焙烧仍然是该类铝型负载银催化剂再?更多还原

【Abstract】 Studies on supported silver catalysts of methanol dehydrogenation to anhydrous formaldehyde and the reaction mechanismThe formation of anhydrous formaldehyde by methanol dehydrogenation under the no-oxygen & catalytic conditions is studying by many reasearchers presently and the selection of catalyst plays a key role in the research process. The reasearchers have studied many elements and compounds to look for the catalyst of methanol dehydrogenation, such as metal, oxide, carbonate and zeolite and so on. But none of the catalysts has been applied into industry for some reasons. In addition, the reaction mechanism of methanol dehydrogenation is not clear for the various catalysts, which hampering us to optimize the catalyst ulteriorly. Silver catalyst has been widely applied into the formaldehyde formation from oxidative dehydrogenation of methanol. So it may be naturally used into the formation of anhydrous formaldehyde from methanol dehydrogenation. Therefore, the main purpose of this dissertation is to develop some new kinds of supported-silver catalysts and resolve the problems by other catalysts and then realize the industrial production of anhydrous formaldehyde. All the catalysts can be divided into two types according to the characterization results, one of which is named as non-Al-type with no Al element exists in the catalyst and the other is Al-type with Al element being in the catalyst. In addition, in order to elucidate the molecular reaction mechanism of direct dehydrogenation of methanol, electrolytic silver catalyst has been used as a probe for its simplest structure. The translation process of adsorbed oxygen species has been studies by using in situ Raman method and the active center is found. Then the methanol direct dehydrogenation mechanism will be completed and the molecular reaction mechanism of the oxidative dehydrogenation of methanol to formaldehyde will also be integrated. These results will be helpful to the industrial process.1. Non-Al type catalyst used in the methanol dehydrogenation to formaldehydeA series of supported-silver catalysts---Ag/SiO2-ZnO and Ag/SiO2-MgO were prepared by sol-gel method and the optimal preparation conditions were confirmed as <WP=11>follows: mixed alcohol and water as the solvent, water content fixed at 30%, calcined at 1000(C with flow air. Ag/SiO2-ZnO catalyst with Ag/Zn(mol)=10:1 shows the highest activity. But for the Ag/SiO2-MgO catalyst,the best condition is Si/Mg=10:1 and the proper silver content was 20%(wt.). Methanol conversion and formaldehyde yield are 82.3%and 73.2%, respectively with Ag/SiO2-ZnO catalyst applied into the reaction of methanol direct dehydrogenation and reacted at 700℃, which obvious higher than the patent reported. As for the Ag/SiO2-MgO catalyst, the highest methanol conversion and formaldehyde yield are 96% and 75%, respectively with the reaction temperature as 650(C, which is lower than that of Ag/SiO2-ZnO catalyst. All the catalysts have been characterized by a series of general method, such as BET, XRD, SEM and so on. It is found that these catalysts all have low BET area, which very appropriate to be applied into the methanol dehydrogenation to anhydrous formaldehyde. Silver exists as Ag0 state either before or after reaction in all the non-Al type catalysts. The difference is that silver particles on the Ag/SiO2-ZnO catalyst will be sintered after reaction for the high reaction temperature. As for the Ag/SiO2-MgO catalyst, silver particles just become a little bigger than that before reaction but not sintering for the lower reaction temperature. The special state of silver indicates that the active center of the non-Al type catalyst is Ag-O species, which is similar to that of electrolytic silver catalyst. So an effective way to regenerate the inactive catalysts is to calcine it in oxygen atmosphere under high temperature. Ag/SiO2-ZnO catalyst gains only 55% activity of the fresh one after regeneration because of the sintering after reaction. But the Ag/SiO2-MgO catalyst can get above 90% activity of the fr更多还原