Other Solid-acid Catalysts

2 Other Solid-acid Catalysts - NiS04/y-Al203 catalyst was used in oligomerization of C2-C4 alkenes. It was presumed that two different characteristic active sites in the NiS04/y-Al203 catalyst accounted for the olefinic reactions. 

The coordination sites contributed to the dimerization of ethylene, while oligomerization of propene proceeded both via coordination and acidic sites. However, but-l-ene only oligomerized via acidic sites. 

1997 Zr02-supported WOj using impregnation and co-precipitation methods 

Molecular sieve ion-exchanged with ammonium chloride, ammonium nitrate or ammonium hydroxide 

180-250 C, and addition of small amount of water to increase the activity 

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Zeolite catalysts play a vital role in modern industrial catalysis. The varied acidity and microporosity properties of this class of inorganic oxides allow them to be applied to a wide variety of commercially important industrial processes. The acid sites of zeolites and other acidic molecular sieves are easier to manipulate than those of other solid acid catalysts by controlling material properties, such as the framework Si/Al ratio or level of cation exchange. The uniform pore size of the crystalline framework provides a consistent environment that improves the selectivity of the acid-catalyzed transformations that form C-C bonds. The zeoHte structure can also inhibit the formation of heavy coke molecules (such as medium-pore MFl in the Cyclar process or MTG process) or the desorption of undesired large by-products (such as small-pore SAPO-34 in MTO). While faujasite, morden-ite, beta and MFl remain the most widely used zeolite structures for industrial applications, the past decade has seen new structures, such as SAPO-34 and MWW, provide improved performance in specific applications. It is clear that the continued search for more active, selective and stable catalysts for industrially important chemical reactions will include the synthesis and application of new zeolite materials. 

Due to the high conversion obtained with SAC 40 and especially with SAC 80, only a small amount of catalyst contamination was observed and the catalyst could be reused several times or regenerated by a washing procedure with acetone and diluted nitric acid solutions. Compared with the conversion of resorcinol using lower amounts of other solid acid catalysts such as Amberlyst 15 (74%), H-BEA (36%), H-US-Y (10%) and H-ZSM-5 (3%), the SAC 40 and SAC 80 Nafion/silica nanocomposites are the favoured catalysts. 

There are many other examples of useful applications of zeolite and mesoporous materials, as well as other solid acid catalysts, for developing sustainable chemical processes. Several have been reviewed by Corma [233] and other authors [234—236]. 

Traffic fuels account for about one-third of oil use and thus development of improved and more sustainable traffic fuels is a priority worldwide. We will not discuss this problem here, but instead take only two examples to evidence that not only zeolite or mesoporous materials are used as solid acid catalysts but that other solid acid catalysts (ion-exchange resins in these specific cases) are also vhdely applied [218]. 

By integrating optimized acid sites with superior mass transport characteristics and a pore architecture that reduces pore-mouth plugging, a catalyst with enhanced performance can be created. Figure 5 demonstrates that both the catalyst selectivity and lifetime are significantly improved. As shown in figure 6, which compares the performance of Exelus solid-acid catalyst with other commercially available systems, the new catalyst system is easily able to achieve a step-change in performance over other solid-acid catalysts. 

Figure 6. Relative performance of an engineered catalyst to other solid acid catalysts.

A remarkable synthesis of oxalate salts using a supercritical mixture of CO2 and CO under very drastic conditions (400 bar, 380°C) in the presence of solid 082(003) was reported [71]. Friedel-Crafts-type alkylations on zeolites [72] or other solid acid catalysts [73] have been studied using SCCO2 as the medium. Performing the reaction in the supercritical state was found to be superior to either liquid or gas phase processes. Using scCOj lead to enhanced catalyst life-... 

Using the catalyst 1, the alkylation reaction (3) which is a probe reaction for the synthesis of fragrances proceeded with better yields compared to various other solid acid catalysts (see figure 6). The drying method of the catalyst and the amount of residual water is critical for the reaction rate. 

Nafion-silica composite has proved to be a promising catalyst in acid catalysis. Different studies with this catalyst revealed very high activities per weight of catalyst compared with the pure Nafion and with other solid-acid catalysts, e. g. zeo-... 

For continuous processing it is necessary that the catalyst does not deactivate and that it is possible to regenerate the catalyst for reuse. If not, catalyst disposal will lead to waste production and other solid-acid catalysts, e. g. zeolites, might be more attractive. This aspect must still be investigated in detail, as must the reproducibility of the production of the composites. Finally, it should be mentioned that the selectivity of the reactions can be affected by reducing the acidity of the acid sites, because of an interaction between the silica matrix and the polymer backbone. 

Other Solid-acid Catalysts - Y-AI2O3 supported with different amounts of sulfate ion, and other carriers such as iron oxide, zinc oxide, silica, and silica-alumina impregnated with sulfate ion were used in the oligomerization reaction of a C4 olefin-containing material.Conversion of butene passed through a maximum at 80% when the catalyst sulfate ion content was around 20% by weight. Butene conversion was low for the different supports except for silica-alumina-supported sulfate ion, for which both the conversion and yield of trimer and tetramer were improved. 

Application of montmorillonite clays for the synthesis of coumarins has been systematically investigated. It has been shown that phenols with electron-withdrawing groups such as NO2 and CHO at the para-position do not lead to the expected Pechmann-cyclized product. This method offers easy separation of products, consistent yields and reusability of the clay catalyst (Tong-Shuang et al. 1998). Other solid acid catalysts employed are presented in Table 10.2. 

In an attempt to combine acidity, stability, and easy catalyst regeneration, other solid acid catalysts, among which zeolites have been of particular interest, have been looked to. The following part of this article reviews some of the alkylation works carried out using zeolites as well as other solids with stronger acidities such as sulfated transition metal oxides and heteropolyacids (HPAs) and related compounds. 

MTBE is commercially produced by the reaction of isobutylene with methanol in the presence of an acidic ion-exchange resin as catalyst, usually in the liquid phase and at temperatures below 100°C. A typical catalyst is sulfonated styrene/divinylbenzene resin catalyst. Other solid acid catalysts such as bentonites are also effective and other novel catalysts have recently been discovered. Isobutylene is obtained from field butane by initial isomerization of n-butane to isobutane, followed by dehydrogenation to isobutylene. Commercial preparations of MTBE are 95.03 to 98.93% pure. Impurities are methanol (<0.43%), t-butyl alcohol (<0.80%), and diisobutylene (<0.25%). 

The liquid phase rearrangement of 5 to 6 was performed for the first time on arenesulfonic acid-functionalized SBA-15 (SBA-Ar-SOsH) mesoporous silica. The catal)hic activity was compared with the activities of other solid acid catalysts such as propylenesulfonic acid-functionalized SBA-15, H-ZSM-5, H-mordemite, Al-MCM-41 and Al-SBA-15. The results revealed that SBA-Ar-SOsH has higher catalytic activity and lactam selectivity. 

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