节点文献
强酸功能化介观结构催化剂的合成及其在烷基化和酰基化反应中的应用
【作者】 顾怡；
【作者基本信息】 复旦大学 ， 物理化学， 2010， 硕士
【摘要】 异丁烷与C3-C5的烯烃在强酸催化下反应生成支链烷烃混合物,称为烷基化油(alkylate)。烷基化油具有高辛烷值(混合辛烷值为93),低Reid蒸汽压,而且不含芳香化物、烯烃和硫等物质,是一种理想的汽油组分。随着一些高辛烷值组分如MTBE (methyl-tertiary-butyl ether)和芳香化合物的禁用,汽油中烷基化油的比重将进一步提高。上世纪三十年代,UOP的Ipatieff小组发现异构烷烃与烯烃在AICl3/HCl和BF3/HF催化下发生烷基化反应,生成饱和支链烃。不久,UOP公司建立了生产装置,采用的催化剂是硫酸。1942年Phillips建立第一套以HF为催化剂的装置生产高辛烷值航空汽油。目前从全球的范围看,硫酸和氢氟酸两种均相催化路线拥有接近的产能。但两种液相催化剂都存在着很大的缺陷,HF是一种高毒性易挥发的液体,一旦泄漏,会在空气中形成稳定的气溶胶,可以随风在地面扩散数公里之远。硫酸催化路线的酸消耗量非常高,生产每吨烷基化油需消耗70-100kg硫酸,生产成本的1／3用于硫酸再生。液体酸催化剂的运输、储存、对设备的腐蚀及废酸的处理都对环境形成了很大的压力。1990年以来,异丁烷／烯烃烷基化面临着开发环境友好催化体系的巨大挑战。分子筛作为一种无毒、非腐蚀性的固体酸催化剂,得到了最广泛的关注,Mobil Oil的Garwood和Venuto及Sun Oil的Kirsch等在1960年代末就对稀土元素交换的八面沸石方面进行了研究。之后,又对其它沸石进行研究。总的来说,大孔沸石是有效的烷基化催化剂。其他的固体酸,如SO42-/MxOy(M=Zr,Ti,Fe)、杂多酸、各种磺酸树脂、负载型强酸催化剂等,也作为潜在的催化剂受到关注和研究。但至今为止,所有这些催化剂用于合成烷基化油时都伴随着快速失活,这从根本上影响了分子筛催化剂的工业应用。酰基化反应是另一个重要的酸催化反应,同样面临着开发高性能固体酸催化剂的巨大压力。本文从三个主要方面设计强酸功能化介观结构催化剂,包括强酸的引入、表面疏水性调变和多维孔道载体的选择。研究了这些因素对烷基化和酰基化反应的影响。Nafion全氟磺酸树脂是通过全氟磺酸醚单体和四氟乙烯的共聚反应制备而成的固体超强酸。它是一种多聚全氟磺酸,氟原子作为取代基有很强的电负性,可以增强磺酸的酸性,使得Nafion的酸性可与硫酸、全氟甲磺酸相媲美。Nafion树脂的强酸性以及化学稳定性使得它成为均相酸催化剂的理想替代品。但是Nafion单独使用也存在诸多局限性。因为它很容易团聚,使得酸性中心大大减少,此外它的比表面积很小仅为0.02 m2/g。为了提高Nafion树脂的催化活性,Harmer等人将Nafion负载于大表面积的氧化硅材料上(200 m2/g)。酰基化反应和烷基化反应证实这一方法十分有效。后来的研究者使用一步法将全氟甲基β亚砜嫁接到MCM-41上,这种材料的催化活性比起前者有了显著提高,而制备时采用的Nafion最高负载量仅为1.5wt%,就已显示出很好的催化活性。Fujiwara等人使用溶胶凝胶法合成Nafion-氧化硅复合物。相比较而言,负载法比溶胶凝胶法和表面嫁接法更为方便。Wang和Guin等的研究表明,负载法制备的催化剂表现出更好的烯烃醚化催化活性。本文的主要工作是使用负载法将Nafion负载于SBA-15和SBA-16载体上,制备强酸功能化催化剂应用于烷基化和酰基化反应。实验结果表明,随着Nafion载量由15wt%提高到30wt%,烷基化的初始转化率大幅上升,酰基化转化率也有明显提高,且酸性位越多,催化剂使用寿命越长。此外,本文还尝试使用全氟甲基β亚砜嫁接法制备新型强酸功能化介孔高分子,拟拓展强酸种类。催化剂的孔结构在酸催化反应中也起着重要角色。孔道结构直接影响分子的有效扩散。Corma等人指出三维的沸石孔道结构要优于一维的孔道。文献中尚未有关于烷基化活性与催化剂孔道结构的研究报道。本文采用研究较成熟的SBA-15作为酸催化剂载体。SBA-15可提供均一的孔径,大比表面和孔容,这些特质为Nafion在载体表面的分散提供便利。催化剂表征显示,Nafion载量达到30wt%时,也能在SBA-15表面很好的分散。另外,三维孔道结构的介孔材料,如SBA-16,FDU-14,MCF等也是潜在的烷基化催化剂载体。本文研究发现,在相同的Nafion载量下,三维SBA-16载体明显优越于一维SBA-15载体,无论是烷基化初始活性还是催化剂寿命都得到提高。之后,本文还对FDU-14,MCF等三维载体进行了初步研究,发掘出其中的潜在应用价值。催化剂表面的亲水/憎水平衡是一个影响反应物和产物在介孔内吸附、扩散的重要特性。而且,许多固体酸在使用中因强极性水分子存在而导致活性降低。因此,酸性位周围的憎水微环境将有利于降低水分子对活性的影响。可以预计调整催化剂的憎水性会大大改变催化表现。对于异丁烷/1-丁烯烷基化反应,催化剂失活的重要原因是烯烃的选择性吸附和孔阻塞。不同极性的墙壁会使材料有非常不同的吸附行为。墙壁中有机区域的疏水性使得反应条件下的烷烃浓度提高,从而有效抑制烯烃的聚合反应。而对于苯甲醚/苯甲酰氯酰基化反应,疏水表面可有效降低多聚芳烃副产物的生成。因此材料表面的化学特性是很重要的活性影响因素。周期性介孔有机硅(PMOs)是使用至少含有两个硅氧基为硅源作为前躯体合成的介孔框架结构与有机表面相结合的复合材料。Inagaki等证实苯桥双硅烷化试剂可在模板周围聚合得到一种在孔系统、孔壁都高度有序的晶体,苯桥是结构堆砌的基本单位。随后在苯桥硅中引入丙基巯基,经氧化处理获得磺酸功能化材料。该材料表现出很好的酯化反应活性。除了骨架疏水性的引入外,还可在材料的表面嫁接疏水性有机硅烷从而提高材料的憎水性能。Macquarrie等在全氟磺酸嫁接的介孔材料壁上引入丙基基团作为疏水基团,大大提高Friedel-Crafts酰基化反应的速率。鉴于烷基化和酰基化反应体系疏水性的要求,本文采用甲基硅烷化试剂(三甲基乙氧基硅烷)对氧化硅表面的硅羟基进行覆盖。该方法同时在SBA-15和SBA-16上得到实施。甲基修饰后的介孔表面能够提高异丁烷的吸附效率,增大表面的烷烃/烯烃比,反应初始转化率及寿命均有提高。且在较低烷烃／烯烃原料配比下,转化率仍较佳。在酰基化反应中,疏水催化剂表面可有效降低多聚芳烃副产物的生成。其次,本文首次将C-FDU-14系列材料用作催化剂载体。全碳介孔材料具有优良的表面疏水性,同时兼具三维介孔孔道的优点,将其负载或嫁接全氟磺酸活性基团后,是高效的烷基化催化剂。最后,文中还合成了PMO及有机-无机杂化PMO材料,并将其作为烷基化催化剂载体进行初步尝试。
【Abstract】 Alkylation of isobutane and butene in the presence of strong acids leads to the formation of mixtures of branched alkanes called alkylate. Alkylate has a high content of highly branched alkanes, such as trimethylpentanes, which gives it a high octane number (-93). Moreover, it has a low vapor pressure and a narrow distillation range and is free of olefins, aromatics, and sulfur. Therefore it is an excellent blending component for gasoline. With the increasing strictness of air regulations, other high octane number components such as methyl-tertiary-butyl ether (MTBE) and aromatics will be limited because of their potential hazards. Therefore, it is expected that the demand for alkylate will increase dramatically in the future.In the 1930s, Ipatieff’s group discovered that isoalkanes react with alkenes in the presence of strong acids, AICl3/HCl and BF3/HF to give saturated hydrocarbons. This process was first commercialized by UOP and sulfuric acid was used as catalyst at the early alkylation plants. In 1942, a plant based on HF as catalyst was constructed by Phillips as a result of the demand for high octane aviation fuel. Up to now, nearly equal amounts of alkylate are produced by HF and H2SO4 processes worldwide. Both processes suffer from substantial drawbacks. Sulfuric acid and HF pose significant corrosion hazards during transportation, storage and handling. The high volatility of HF makes it prone to forming mists which can drift downwind at ground level for several kilometers if released to the atmosphere. The spent acid which contains water and heavy hydrocarbons has to be regenerated in the H2SO4 process. The acid consumption can be as much as 70-100 kg of acid/ton of alkylate. About one-third of the operating cost of H2SO4 process can be attributed to acid consumption.Since 1990, there has been substantial pressure to develop a more environmentally friendly alkylation process. Zeolites, being noncorrosive, non-toxic and rather inexpensive materials, were the first solid acids tested as alternatives to sulfuric and hydrofluoric acid in isobutane/alkene alkylation. In the late 1960’s, two groups, Garwood and Venuto of Mobil Oil and Kirsch of Sun Oil did pioneering work on rare earth exchanged faujasitic zeolites. Later, other zeolites were also examined. In general, all large pore zeolites are active alkylation catalysts. Other materials studied are sulfated zirconia, Bronsted and Lewis acids promoted on various supports, heteropolyacids and organic resins, both supported and unsupported. However, the unacceptably rapid deactivation of the abovementioned ctalysts was and still is the obstacle to commercialization.On the other hand, acylation reaction of anisole and benzoyl chloride is another important acid catalyzed reaction, and the development of novel solid superacid catalysts is also demanded. The scope of this thesis is firstly to design and synthesis novel catalysts from three aspects, namely the incorporation of superacid active sites, the hydrophobicity modification of the silica surface and the introduction of multi-dimensional mesostructures, and secondly to use them in alkylation and acylation reactions.Nafion is a polymeric perfluoroalanesulfonic acid. The presence of electron-withdrawing fluorine atoms in the structure significantly increases the acid strength of the terminal sulfonic-acid groups, which becomes comparable to that of pure sulfuric acid and to that of trifluoromethanesulfonic acid. Nafion is an extremely acidic and stable acid as a substitute of homogeneous acids. However, the catalytic activity of pure Nafion was far from optimal due to its small surface area of ca.0.02 m2/g and the intention to aggregate during the reaction processes. To overcome this limitation, Harmer et al. synthesized Nafion-silica composites by entrapping Nafion particles in porous silica framework to increase the surface area. These nanocomposite materials have large surface areas (200 m2/g), and they were shown to be active for acylation and alkylation reactions. A further development was achieved by abcholoring perfluoromethylβ-sulfone with the free silanol groups of MCM-41. This activity increase is even more remarkable considering that the loading of sulfonic acids is only ca.1.5wt%. And Fujiwara et al. use sol-gel method to synthesize Nafion/silica composite materials for Friedel-Crafts reactions. In comparison, impregnation method is more facile than sol-gel and grafting ones. Wang and Guin reported that impregnation route is more beneficial than the sol-gel technique to make Nafion/silica catalyst for etherification of olefins. In this thesis, impregnation mothed was employed to synthesize Nafion/SBA-15 and Nafion/SBA-16 solid acids for alkylation reaction of isobutane/1-butene and acylation reaction of anisole/benzoyl chloride. The results showed that with the increasing of Nafion loading from 15wt% to 30wt%, the 1-butene conversion was promoted remarkably, and the same trend was shown in acylation reactions. Meanwile the higher amount of acid sites prolonged the lifetime. Besides, other attempts of grafting perfluoromethylβ-sulfone were also made to explore different acid types. The mesostructure also plays an important role in acid catalyzed reactions, influencing the efficiency of molecular diffusion directly. Corma et al. pointed out that three-dimensional pore structure was superior to one-dimensional counterparts, however, till now, few work on the relationship of the catalytic activity and pore sturture was reported in alkylation reaction of isobutane/1-butene. In this thesis, SBA-15 was firstly employed in two acid catalyzed reactions for its uniform pore size, large surface area and pore volume, all of which guaranteed an ideal support for Nafion impregnation. The Nafion resin was effectively dispersed over SBA-15 even at a higher loading of 30wt%. Other three-dimensional mesostructures, such as SBA-16, FDU-14 and MCF were also used as supports for alkylation. It was clearly demonstrated that SBA-16 was superior to SBA-15 not only in initial activity but in prolonged lifetime as well. At last, some primary work has been done on FDU-14 and MCF as alternative three-dimensional supports.The hydrophilic/hydrophobic character of the catalyst’s surface has important effects on the adsorption and diffusion of reactants and products within the mesopores. Moreover, most solid acids are poisoned by water, a highly polar molecule, in reactions which leads to deactivation. Thus, the hydrophobilization of the acid sites microenvironment is an important challenge to reduce poisoning by water molecules. It can be expected that the adjustment of the catalyst’s hydrophobicity will probably have strong implications in the catalytic performance. As regards isobutane/1-butene alkylation, an important reason of deactivation is the preferential adsorption and pore filling with the olefin. The different polarity of walls makes the materials have very different adsorption behaviours. The hydrophobicity of the organic moiety of its walls should allow a higher paraffin steady concentration in the reaction conditions, thus to lower the speed of oligomerization. And as for acylation reaction of anisole/benzoyl chloride, the hydrophobicity reduces the formation of polyacylated bulky byproducts. Therefore, the importance of control over surface chemistry is of great importance. Periodic mesoporous organosilicas (PMOs) are materials which utilise silane precursors containing at least two silane groups. Inagaki has demonstrated that phenylene bis-silanes can be condensed around a template to give materials which show a high degree of order not only of the pore system, but in the walls, where phenylene units stack up to give crystallinity. They incorporated mercaptopropylsilane into a phenylene, followed by oxidation, to get sulfonic acid-functionalised material. The material showed good activity for esterfication. Apart from pore wall modification, trimethylsilyl groups (or similar) are often grafted onto preformed catalysts to modify the hydrophilic/hydrophobic character of the catalyst’s surface, thus to adjust the surface adsorption properties. Incorporation of an organosilane in the synthesis of the material is a method we can apply. Macquarrie et al. introduced propyl groups to the wall of perfluorosulfonic acid grafted mesostructured material and enhanced the rate of a Friedel-Crafts acylation dramatically. Owing to the hydrophobic nature of alkylation and acylation reactions, ethoxytrimethylsilane was used to cap the surface-OHs on SBA-15 and SBA-16. The adsoption of isobutane was enhanced on the hydrophobic catalysts therefore increased the surface isobutane/butene ratio, thus increasing the initial activity and catalytic lifetime. The conversion rate was still significant even at a lower isobutane/butene ratio. Moreover, C-FDU-14 was firstly introduced as a support in alkylation. Mesoporous carbons combine the merits of hydrophobicity and three-dimensional pore structures. Their further treatment by Nafion impregnation or perfluorosulfone grafting is promising in alkylation reactions. Last, PMO and organic-inorganic hybrid PMO were tested as supports in this reaction for the first time.