Acid Functional Mesoscopic Structures Catalyst Synthesis And Its Application In The Alkylation And Acylation Reactions

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.