Dehydration of alcohols to olefins

Boron phosphate is used as an acid catalyst for dehydration of alcohols to olefins isomemization of olefins nitration of aromatic hydrocarbons polymerization of aldehydes and other synthetic reactions. It also is used as a flux in silica-based porcelain and ceramics special glasses and acid cleaners. 

Rate equations for the catalytic dehydration of alcohols to olefins... 

These represent hydrolysis of diethyl ether, synthesis of iV-mono-ethylaniline from alcohol and aniline, and dehydration of alcohols to olefins. Promoting the catalyst differently effects these reactions, in accord with the multiplet theory. Thus, the addition of the oxides of Fe, Ni, and Zn accelerates the first reaction and slows down the second and the third one 351). Incidentally, as has been found by the author and Sokolova 352), Nb20s and TaEOs are good catalysts in obtaining monoethylamine as well as in esterification of alcohols with acids, and they carry out the condensation of acetaldeyde into crotonaldehyde 353). 

Both unimolecular dehydration of alcohols to olefins and dehydrogenation to CO and aldehydes have been reported in the literature (Tolstopyatova et al., 1961 Tosun and Rase, 1972 Ganichenko, 1967 Minachev, 1973), but permanent dehydration activity was observed only at reaction temperatures higher than 300°C due to the contamination of the oxide surfaces with product water molecules. 

Aluminas are active and very selective catalysts for the dehydration of alcohols to olefins and to ethers (346,347). In particular, alumina-based catalysts are reportedly used in industrial processes for the production of DME from methanol, as shown in Equation (3.2). 

Dehydration of alcohols to olefins or ethers can be effected with most solid acid catalysts as well as with solid base catalysts. Solid acids are usuttlly more active than bases. Among acid catalysts, alumina is the most versatile. Metal phosphates, metal... 

Tertiary alcohols react either very slowly or do not react at all, and dehydration of alcohols to olefins takes place instead (20). 

For further dehydration, for example, of aldoximes and amides to nitriles, of alcohols to olefines, as well as the synthesis of heterocycles like oxiranes and aziridines see Section 18.5. 

An exhaustive review of dehydration reactions has been written recently by Winfield (3) and most of the relevant literature can be found there. The purpose of this chapter is to review some recent developments and to point out the resemblance of alumina-catalyzed dehydration of alcohols to solvolytic reactions. It will be demonstrated that by careful selection of model compounds, such as olefins and alcohols, it is possible to throw light on the catalytic action of alumina and to reveal the presence of active catalytic sites. 

Several insulator oxides, such as AI2O3 and Si02, can hydrate reversibly to give hydroxides of the type A1(0H)3, AIO(OH), or Si(OH)4, and, not surprisingly, can catalyze dehydration reactions such as the conversion of alcohols to olefins at elevated temperatures ... 

Consider, for example, the zeolite-catalyzed dehydration of alcohols to give olefins ... 

In general, dehydration means loss of water molecules from chemical substances, irrespective of their structure. Even if all cases where water is bonded in hydrate form are excluded, a number of reactions remain which also include formation of nitriles from amides, lactones from hydroxy acids etc. However, the present treatment will concentrate on the heterogeneous catalytic decomposition of alcohols in the vapour phase, which can be either olefin-forming or ether-forming reactions, and on the related dehydration of ethers to olefins. 

Hydration and Dehydration Reactions. Hydration and dehydration catalysts have a strong affinity for water. One such catalyst is AI2O3, which is used in the dehydration of alcohols to form olefins. In addition to aliunina, silica-alumina gels, clays, phosphoric acid, and phosphoric acid salts on inert carriers have also been used for hydration-dehydration reactions. An example of cm industrial catalytic hydration reaction is the synthesis of ethanol from ethylene ... 

MTO has also been claimed to be the first transition metal complex to catalyze the direct, solvent-independent formation of ethers from alcohols [30]. Aromatic alcohols give better yields than aliphatic ones and reactions between different alcohols have been used to prepare asymmetric ethers. Also catalyzed by 1 is the dehydration of alcohols to form olefins at room temperature. When primary or secondary amines, respectively, are used as the limiting reagents, direct amination of alcohols gives the expected secondary or tertiary amines in yields of ca. 95 %. Disproportionation of alcohols to carbonyl compounds and alkanes is also observed for aromatic alcohols in the presence of MTO as catalyst. 

Building on the proposed mechanism hy Hauffe, ° for metal oxide-catalysed dehydration of alcohols to form olefins, ethers and water which heavily relies on the assumption that the catalyst surface acts as semiconductor, Hasssan et al. purposively attempted to obtain a further insight into the mechanism of alcohol dehydration on pure cadmium oxide. On the basis of the experimental data involving kinetics of the dehydration reaction and the effect of pretreatment of the catalysts along with studies on lattice structure and specific surface areas, a mechanism for ethanol dehydration was put forward (Scheme 17.14). The proposed mechanism entirely depends... 

Side reactions are usually dehydration of alcohol to form olefins and ethers and sometimes polymerization. Dissolution of catalytically active sites on the surface (e.g., — SO3H group in the case of ion-exchange resin) and thermal stability, which shorten the life of catalysts, are other important considerations in the use of solid acids. 

In spite of the fact that alumina is an excellent and widely used catalyst for the dehydration of alcohols, there is no agreement in the literature with regard to the mechanism of this reaction or the nature of the olefinic products. For example, 1-alkenes have been obtained from primary alcohols such as 1-butanol (19-22), 1-pentanol (23), 1-hexanol (24-26), 1-heptano (27), and 1-octanol (25) but mixtures of olefins differing in the position of the double bond (13, 26, 28) or even in the carbon skeleton (29) have been reported from other primary alcohols. 

The dehydration of alcohols is mostly an acid-catalyzed reaction and much work has been done by Taft and co-workers to elucidate the mechanism (7 5-7 7). These investigators proved that the intermediate in the dehydration of tertiary alcohols or hydration of branched olefins in dilute acid solutions resembles the conjugate acid of the olefin and it is... 

The mechanism of dehydration of alcohols over acidic and non-acidic alumina is the same. In the presence of the acidic alumina, however, readsorption of the dehydrated product can occur, leading to either double bond migration or skeletal isomerization, depending on the strength of the acid sites, the structure of the olefins produced, and the experimental conditions. 

Silica-alumina mixtures are of great technological importance in the oil industry as catalysts for petroleum processing. The cracking activity is closely linked to surface acidity. Other typical reactions catalyzed by silica-alumina are the dehydration of alcohols and the polymerization of olefins. 

Occurs in nature in abundance the principal forms are bauxites and lat-erites. The mineral corundum is used to produce precious gems, such as ruhy and sapphire. Activated aluminas are used extensively as adsorbents because of their affinity for water and other polar molecules and as catalysts because of their large surface area and appropriate pore sturcture. As adsorbents, they are used for drying gases and liquids and in adsorption chromatography. Catalytic properties may be attributed to the presence of surface active sites (primarily OFT, 02, and AF+ ions). Such catalytic applications include sulfur recovery from H2S (Clauss catalysis) dehydration of alcohols, isomerization of olefins and as a catalyst support in petroleum refining. 

In contrast to olefin-forming dehydrations, the transformation of alcohols to ethers very probably includes surface alkoxides as intermediates. It is assumed that one molecule of the alcohol forms the alkoxide which is then attacked by the second alcohol molecule either from the gas phase or from a weakly adsorbed state. Again, cooperation of acidic and basic sites seems to be necessary [116,142,143]. The important step of ether forma-... 

A variety of organic reactions, including dehydration of alcohols, cleavage of ethers, many additions to olefins, a number of nucleophilic substitutions, and various rearrangements, are catalyzed by acids. Since the substrates in these... 

Dehydrations produce olehns from alcohols by the acid-catalyzed elimination of a water molecule from between two carbons. Acid-catalyzed dehydrations often give mixtures of products because the intermediate carbocation is prone to cationic rearrangements to more stable carbocations prior to formation of the olefin product. Moreover, even when the intermediate carbocation is not subject to skeletal rearrangement, as in file case of tertiary alcohols, mixtures of regioisomers are often produced during file loss of a proton from file carbocation. As a consequence, the acid-catalyzed dehydration of alcohols is generally not a viable synthetic method. 

Alcohols can undergo acid-catalyzed dehydration to give either the corresponding alkenes or the corresponding ethers. The product distribution of the dehydration of alcohols over Nafion-H catalyst shows temperature dependence187 (Table 5.40). Alcohols are thus efficiently dehydrated in the gas phase over Nafion-H under relatively mild conditions with no evidence for any side reactions such as dehydrogenation or decomposition. At higher temperature, olefin formation predominates. 

In aqueous acidic solutions of either Br nsted or Lewis acids, the dehydration of alcohols leads to the formation of Saytzeff olefins (48). Dehydration occurs most readily if the alcohol is tertiary. For example, the formation of 1,1-diphenylethylene from methyldiphenyl carbinol (Reaction XXX)... 

Two reactions for which specific poisoning experiments have contributed to the elucidation of the reaction mechanisms and permit evaluation of the possibilities and pitfalls of the technique are discussed as examples in this section. The first example is the dehydration of alcohols on alumina catalysts, and the second, the isomerization of olefins on the same type of catalyst. 

Olefins are a little more complicated to analyse than alcohols. They can be made by the dehydration of alcohols ... 

The name activated alumina is generally applied to an adsorbent alumina (usually an industrial product) prepared by the heat treatment of some form of hydrated alumina (i.e. a crystalline hydroxide, oxide-hydroxide or hydrous alumina gel). It has been known for many years that certain forms of activated alumina can be used as powerful desiccants or for the recovery of various vapours. It was apparent at an early stage that the adsorbent activity was dependent on the conditions of heat treatment. For example, in 1934 Bayley reported that the adsorption of H2S by a commercial sample of activated alumina was affected by prior heating of the adsorbent at different temperatures, the maximum uptake being obtained after heat treatment at SS0°C. During an investigation of the catalytic dehydration of alcohols, Alekseevskii (1930) found that a calcination temperature of c. 400°C was required to optimize the adsorption of the alcohol reactants, whereas calcination at 600°C was preferable for the adsorption of the olefine products. 

Where the rate is first-order in catalyst,an interpretation of the rate data is possible but always subject to uncertainties in our knowledge of the sorption isotherms of all species involved in the mechanism. Let us consider, for example, the decomposition of a single molecular species on a catalyst surface. An example might be the dehydration of alcohols on the surfaces of metal oxides such as alumina gel at 300 C to give an olefin plus water. This endothermic reaction, which is thermodynamically favored in the gas phase by an entropy increase of about 36 e.u., can be written as... 

Dit fereni observations led Ziesecke to the proposal that the homologation reaction proceeds via dehydration of the alcohols to olefins which are (hen carbonylatcd by a hydroformylalion-type process (56J. This theory is supported by the fact that r-butanol is homologated to give 3-methylbutanol and not neopemyl alcohol, as would be expected (cf. Equation (16))-... 

Reaction with alcohols. Primary alcohols are converted in high yield into alkyl chlorides by reaction with 1 at 80° (4-8 hours). Tertiary alcohols are dehydrated by 1 to olefins, again in high yield. Secondary alcohols on reaction with 1 give a mixture of alkyl chlorides and olefins. Displacement is favored in the case of less sterically hindered alcohols. The product of dehydration is usually the more thermodynamically stable isomer. 

The mechanism by which proton acids catalyze the dehydration of primary and secondary alcohols in water is not perfectly well understood (1). There is universal agreement that the dehydration of tertiary alcohols can be explained by an El mechanism (1,2) involving either a II complex ( ) or a symmetrically solvated carbonium ion (4) as the key reaction intermediate. Although an occasional text ( ) also describes the dehydration of primary alcohols by an El mechanism, authoritative reviews (1/4) conclude that a concerted E2 type mechanism is more probable. The dehydration behavior of secondary alcohols is presumed to be similar to primary alcohols (4). Discussions of the gas phase dehydration of alcohols by heterogeneous Lewis acid catalysts admit more possibilities. In their authoritative review Kut, et al. (1) consider E1-, E2-, and ElcB-like mechanisms, as well as the possible role of diethyl ether as a reaction intermediate, but they reach no conclusion concerning the relative importance of these mechanisms in the formation of olefins from alcohols. 

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