Acidity, zeolite catalyst

A solution of trifluoroacetic acid in toluene was found to be advantageous for cydization of pyruvate hydrazoncs having nitro substituents[4]. p-Toluene-sulfonic acid or Amberlyst-15 in toluene has also been found to give excellent results in preparation of indole-2-carboxylale esters from pyruvate hydra-zoiies[5,6J. Acidic zeolite catalysts have been used with xylene as a solvent to convert phenylhydraziiies and ketones to indoles both in one-flask procedures and in a flow-through reactor[7]. 

Jacobs et al. employed an acidic zeolite catalyst for the racemization of sec-alcohols, which occurs through the formation of carbocations [44] (Figure 4.19). The KR is catalyzed by CALB in the presence of vinyl octanoate as acyl donor. DKR takes place successfully in a biphasic system (octane/H2O, 1 1) at 60 °C. 

The highly oxygenated bio oil can be de-oxygenated, and thereby upgraded, over acidic zeolite catalysts through the formation of mainly water at low temperatures and C02 and CO at higher temperatures [1-3], Successful catalytic pyrolysis of woody biomass over Beta zeolites has been performed in a fluidized bed reactor in [4]. A drawback in the use of pure zeolitic materials has been the mechanical strength of the pelletized zeolite particles in the fluidized bed. 

K. Smith, G. A. El-Hiti, and M. Al-Shamalia, Rearrangement of epoxides to carbonyl compounds in the presence of reusable acidic zeolite catalysts under mild conditions, Catal. Lett., 109 (2006) 77-82. 

The direct high-pressure animation of ethylene with ammonia to give ethylamine occurs in the presence of acidic zeolite catalysts such as H-elinoptilolite, H-erionite or H-offretite38. Primary amines R1NH2(R1 = Bu, cyclohexyl, PhCH2, Ph or Ar) have been monoalkylated by reaction with di-t-butyl dicarbonate, followed by successive treatment of... 

Because of the large demand for p-xylene, another method is now being used by Amoco to increase the percentage of the para isomer in mixed xylenes. They are heated at 300°C with an acidic zeolite catalyst, which equilibrates the three xylenes to an o,m,p ratio of 10 72 18%. The para isomer is separated by fractional crystallization, whereas the o,m mixture is reisomerized with the catalyst to produce more para product. Theoretically, all the xylenes could be transformed into the desired para isomer. The zeolite catalyst has the following structure. 

Selective hydroxylation of phenol with hydrogen peroxide was reported on acid zeolite catalysts [91-92]. Peroxonium ions, formed by H2O2 protonation, are the oxidizing species. When the reaction is carried out on a faujasite catalyst, a mixture of hydroxybenzenes and tars is obtained [91]. In the presence of H-ZSM-5 on the other hand, no tar formation was mentioned (which does not necessarily mean that it was absent) and p-selectivities close to 100% were reported for the hydroxylation [92]. These superior selectivities reflect the shape selective properties of ZSM type zeolites. 

By characterizing various zeolite catalysts under the same reaction conditions, the authors found weaker MAS NMR signals of alkoxy species for the less active zeolites HY and HZSM-5 than for the more active zeolite H-beta (250). This observation suggests that the alkoxy species observed under steady-state conditions act as reactive surface species in the MTBE synthesis from isobutylene and methanol on acidic zeolite catalysts. 

The main reactions of the MTG/MTO process can be summarized as follows the first is the dehydration of methanol to DME on acidic zeolite catalysts. The equilibrium mixture of methanol, DME, and water is then converted to light alkenes, which react further to form higher alkenes, n- and Ao-alkanes, aromatics, and naphthenes by hydrogen transfer, alkylation, polycondensation, isomerization, and other secondary reactions. 

In recent years alkylations have been accomplished with acidic zeolite catalysts, most nobably ZSM-5. A ZSM-5 ethylbenzene process was commercialized jointly by Mobil Co. and Badger America in 1976 (24). The vapor-phase reaction occurs at temperatures above 370°C over a fixed bed of catalyst at 1.4—2.8 MPa (200—400 psi) with high ethylene space velocities. A typical molar ethylene to benzene ratio is about 1—1.2. The conversion to ethylbenzene is quantitative. The principal advantages of zeolite-based routes are easy recovery of products, elimination of corrosive or environmentally unacceptable by-products, high product yields and selectivities, and high process heat recovery (25,26). 

Catalytic cracking Refining unit to produce distillates from crude oil using an acidic zeolite catalyst. 

Mitsubishi Rayon (formerly Nitto Chemical) has operated a process since 1984 in which the equilibrium of methylamine formation is shifted to make more dimethylamine by use of an acid zeolite catalyst with a particular pore structure. The product stream contains 7 mol% MMA, 86 mol% DMA and 7 mol% TMA, and its investment as well as its operating costs are lower than the conventional Leonard process that is used by most companies. Since DMA has the highest sales volume, the process might be appealing246. 

In order to get acidic zeolite catalysts, zeolites are ion exchanged with ammonium cations ... 

Heterogeneous catalysts are solid materials that sometimes consist of the bulk material itself, for example, acid zeolite catalysts [10] or fused catalysts [11], Or in other cases of an active component or components deposited, as a rule, on a highly developed area support, for example, silica, alumina, carbon or in some cases a zeolite. The function of the support is to enhance the catalyst properties, for example, the stability of the active component or components, or in some cases to be even included in the catalytic reaction, for example, by providing acidic sites in bifunctional zeolite catalysts [10],... 

The formation of coke on acid zeolite catalysts depends on i) the characteristics of the acid sites and of the pore structure of the zeolite and ii) the nature of the feed and the operating conditions (T,P). 

The production of LAB involves the liquid-phase alkylation of benzene with linear monoolefins or alkyl chlorides. Liquid HF is used as catalyst for linear monoolefins. And the A1C13 is used as the catalyst for alkyl chlorides. Nowadays, acidic zeolite catalyst is used for olefin alkylation which generates less waste and reduces manufacture cost. The alkylate is then sulfonated to produce linear alkylbenzene sulfonate for biodegradable detergents. The manufacture of detergents is described in detail in Chapter 27. 

The most important process involving a zeolite, Fluid Catalytic Cracking (FCC) uses a catalyst containing an acid FAU zeolite (Chapter 5). Other examples of processes using acid zeolite catalysts will be examined in this book, like Methanol to Olefins (Chapter 12), Acetylation (Chapter 14) etc. 

Acid zeolite catalysts offer a very good alternative for the clean synthesis of these sulfur containing substances. A suitable feed stock is the 4-isopropenyl-l-methyl-1-cyclohexene. In the presence of a commercial beta-zeolite (25) hydrogen sulfide is added to the autoclave at a reaction temperature of 50°C at a pressure of 17 bar. The conversion is 65.1% and the selectivity to 1-p-menthene-8-thiol is 43.9%. These are very promising results and they can be improved by using a commercial H-US-Y zeolite which rendered a conversion of 76.8% and a selectivity of 64.3% (62). 

A famous example, which illustrates the reaction mechanisms that happen with the acidic zeolite catalysts, is the chemisorption reaction of olefins (see Figure 4). 

Figure 5. The principle of the cluster approach method. The small cluster model (right) aims to model an acidic zeolite catalyst (left).

However, Michaud et al. and Landau et al reported that the use of an acid zeolite catalyst and classical HDS catalyst bifunctional catalyst was a very efficient way to desulfurize alkylated DBT. In this case, they observed that acid zeolite catalyst achieves isomerization of the more hindered sulfur atom alkylated DBT (viz. 46DMDBT) to compounds for which the sulfur atom is not anymore hindered by the methyl groups (viz. 37DMDBT). 

Modes of Coking and Deactivation of Acid Zeolite Catalysts... 

The deactivation of acid zeolite catalysts is mainly due to the coke deposit within... 

Despite the enormous commercial success and widespread use of acidic zeolite catalysts, important questions on the local structure of zeolitic materials remain unanswered. The local structure of aluminum in the zeolites has proven rather elusive to investigation. 

Bifunctional zeolite catalysts such as platinum loaded acid zeolite catalysts are applied in several petroleum refinery operations, designated as hydroconversion processes isomerisation of light n htha, iso-dewaxing and hydrocracking of heavy fractions [4]. Most experimental investigations in academic laboratories are typically performed with pure model components or simple mixtures thereof as feedstock, and using reaction conditions under which the hydrocarbon compounds are in the vapor phase. Industrial hydroconversion processes are mostly run under three phase, or even in some cases under liquid phase conditions and with feedstocks that are extremely complex mixtures of large numbers of different hydrocarbon compounds [4]. 

As emphasized earlier, the redistribution or transfer of hydrogen is one of the most dominant and recurrent reactions of olefins in the presence of acidic zeolite catalysts. The formation of hydrogen-rich paraffins and hydrogen-deficient aromatics is superimposed constantly on any reaction where a low molecular weight olefin—or olefin precursor — is either a reactant, a product, or an intermediate. Even more specifically, numerous examples of hydride-transfer processes during reactions over zeolite catalysts have been observed, and we will discuss some of these in detail. 

The next step in the determination of the mechanism of the catalytic action of the acid zeolite catalyst is elucidating the nature of the active... 

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