Thoria, dehydration

When esters are passed with ammonia over a contact catalyst such as alumina or thoria at 400—500°C, nitriles are obtained via dehydration of the intermediate amides ... 

An ethyl acetate yield of 24% is obtained using a copper oxide catalyst with 0.1—0.2% thoria at 350°C. Dehydration. Ethyl alcohol can be dehydrated to form ethylene or ethyl ether. 

The vapor-phase esterification of ethanol has also been studied extensively (363,364), but it is not used commercially. The reaction can be catalyzed by siUca gel (365,366), thoria on siUca or alumina (367), zirconium dioxide (368), and by xerogels and aerogels (369). Above 300°C the dehydration of ethanol becomes appreciable. Ethyl acetate can also be produced from acetaldehyde by the Tischenko reaction (370—372) using an aluminum alkoxide catalyst and, with some difficulty, by the boron trifluoride-catalyzed direct esterification of ethylene with organic acids (373). 

Cyclopentanecarboxaldehyde has been prepared by the procedure described above 2 3 by the reaction of aqueous nitric acid and mercuric nitrate with cyclohexene 6 by the action of magnesium bromide etherate 6 or thoria 7 on cyclohexene oxide by the dehydration of frarei-l, 2-cyclohexanediol over alumina mixed with glass helices 8 by the dehydration of divinyl glycol over alumina followed by reduction 9 by the reaction of cyclopentene with a solution of [HFe(CO)4] under a carbon monoxide atmosphere 10 and by the reaction of cyclopentadiene with dicobalt octacarbonyl under a hydrogen and carbon monoxide atmosphere.11... 

The steric effects may be more pronounced in heterogeneous catalysts than in homogeneous reactions in solution. The rigid, solid surface restricts the approach of the reactants to the active centers and interaction between the reactants. The steric requirements are quite stringent when a two-point adsorption is necessary and when, in consequence, the internal motion of the adsorbed molecules is limited. In this way, the stereoselectivity of some heterogeneous catalytic reactions, for example, the hydrogenation of alkenes on metals (5) or the dehydration of alcohols on alumina and thoria (9), have been explained. 

Metal molybdates421 and cobalt-thoria-kieselguhr422 also catalyze the formation of hydrocarbons. It is believed, however, that methanol is simply a source of synthesis gas via dissociation and the actual reaction leading to hydrocarbon formation is a Fischer-Tropsch reaction. Alumina is a selective dehydration catalyst, yielding dimethyl ether at 300-350°C, but small quantities of methane and C2 hydrocarbons423 424 are formed above 350°C. Heteropoly acids and salts exhibit high activity in the conversion of methanol and dimethyl ether.425-428 Acidity was found to determine activity,427 130 while hydrocarbon product distribution was affected by several experimental variables.428-432... 

Another method for the preparation of aromatic ethers which has received relatively little attention because of experimental difficulties and because it is not a general synthesis, was pioneered by Sabatier (68,69, 70, 71). This involved the vapor phase dehydration of phenols over alumina or preferably thoria at temperatures in the neighborhood of 400° C. 

Golden (28) unsuccessfully attempted to prepare polymers by elimination of acetyl chloride and acetic anhydride from p-acetoxy-chlorobenzene and diacetoxybenzene, respectively. He was also unable to dehydrate hydroquinone. In the course of his study on the dehydration of phenols to aromatic ethers over thoria, Briner (8) also attempted, unsuccessfully, to dehydrate the polyphenols pyrocatechol, resorcinol and hydroquinone. 

Sabatier and Maihle have stndied the catalytic action of various metallic oxides upon the vapours of certain organic compounds. They find that alcohols are oxidised to aldehydes by manganous oxide and that they are dehydrated by alumina, thoria or the blue oxide of tungsten, with the formation of olefines and ethers. Those changes are explained in the case of thoria by the following equations ... 

The evidence for the existence of the anhydrous salt is not conclusive. Some crystals were reported to have formed in the reaction between thoria and perchloric acid,122 but no analysis was carried out. The salt would certainly be very hygroscopic. Attempts to dehydrate the hydrate Th(C104)4.4 H20 resulted in the formation of the anhydrous oxysalt Th0(C104)2.123... 

The behaviour of the oxides of Group IV which contains thoria is quite complex. Titania can exist in two forms, anatase and rutile, the former usually giving selective dehydration with Saytzev alkene orientation from 2-alcohols. Hafnia also gives selective dehydration irrespective of any... 

Use of Model Alcohols in Mechanistic Studies. - Much use has been made of model alcohols of various types in order to elucidate the detailed mechanism of dehydration, and in so doing, most catalysts have been compared with either alumina or thoria representing respectively E1/E2 and ElcB mechanisms. 

Thomke has used model alcohols very effectively in his studies of dehydration mechanisms. For several oxides including thoria a number of deuteriated butanols were employed including d,l erthro(threo)-[3- Y i -butan-2-ol(a), [l,l,l,2,3,3- H6]butan-2-ol (b), and [2- Hi]butan-2-ol (c). 

In the case of thoria, 7-elimination was almost negligible, despite the latter s more basic nature with respect to 7-AI2O3. Again, geometric consideration led to the conclusion that dehydrations with thoria require the presence of 3-hydrogens and there is no suitable site-pairing (see Scheme 11 reproduced by permission from J. Catal, 1979, 59, 405) to allow 7-elimination with an alcohol such as neopentyl alcohol, which contains no 3-hydrogens and was unreactive on thoria. 

Vapor-phase dehydration of 2-alcohols with thoria as catalyst affords almost exclusively the 1-olefin ... 

Dehydration Alumina (see also Dihydropyrane, preparation). Boric acid. Boron triSuoride. N-Bromoacetamide-Pyridine-SOj. Dicyclohexylcarbodiimide. Diketene. Dimethylform-amide-Thionyl chloride. Dimethyl sulfoxide. Ethylene chlorophosphite. Florisil. Girard s reagent. Hydrobromic acid. Iodine. Mesyl chloride-Sulfur dioxide. Methyl chlorosulfite. Methylketene diethylacetal. Naphthalene-d-sulfonic acid. Oxalic acid. Phenyl isocyanate. Phosgene. Phosphorus pentoxide. Phosphoryl chloride. Phthalic anhydride. Potassium bisulfate. Pyridine. Thionyi chloride. Thoria. p-Toluenesulfonic acid. p-Toluenesulfonyl chloride. Triphenylphosphine dibromide. 

From this table it is evident that all monazite contains much more ceria than thoria and since the mantle is mainly thoria a very large part of the ceria is not needed for mantle manufacture. The residue which remains after removal of the thoria contains about 45 per cent Ce02,25 per cent La203, and 15 per cent didymia, the remainder being yttrium earths, samaria, etc. The residue represents 60-65 per cent of the original monazite. Since the total world s consumption of monazite has been estimated as being about 88,000 tons up to 1918, it is evident that the supply of cerium material has been very large. Some firms have stored enormous quantities of these rare earth salts, and others have thrown them away. The residues are transformed to the chlorides, which are carefully dehydrated to prevent the formation of basic salts. The purity of the chlorides is not important, but the phosphorus and sulfur content must be low, and iron and aluminium should not be present in more than small amounts. A mixture of the... 

Extraction. — From thorite and thorianite tjie extraction of thoria is easily accomplished. The minerals are easily dissolved in hydrochloric or sulfuric acid (nitric acid may be used for thorianite) and the solution evaporated to expel excess acid and dehydrate the silica. The residue is extracted with water, and the solution saturated with hydrogen sulfide to remove heavy metals. Separation from the rare earth metals may be accomplished by the carbonate, sulfate, or oxalate methods. 

Sabatier 42 attributes the dehydration of alcohols over the surfaces of the oxides of thoria, alumina, and tungsten to the formation on the surface of these oxides, with the elimination of water, of a thin layer of an ester comparable to ethyl hydrogen sulfate. This ester is decomposed by heat into an olefin and the regenerated oxide. Thus ... 

Here we describe the effect of chromia doping on three different catalysts viz. alumina, zirconia and thoria. The basis for choosing these catalysts and using chromia as a doping are two fold. First literature references clearly indicate that the first two materials when doped with chromia are active for the above mentioned reaction. Thoria whose thermal stability, high selectivity in dehydration reactions are well documented, was tried to compare the activities of acidic catalysts with that of an amphoteric one.(7 8)... 

A number of unexplained factors warrant mention. Orientation of elimination differs for secondary and tertiary structures. The peculiar predominance of cis- rather than /ra/ii-olefin may arise from the relative stabilities of the proton-olefin complexes. but a more certain conclusion would be possible if the stereochemistry of the dehydration in the acyclic series had been determined. Assumption of the anti stereospecificity known to be favoured by the cyclohexyl systems may be unsound especially in the light of the recent stereochemical findings in base-catalysed elimination reactions (Section 2..1.1(e)). The solution of the problem of the cis/trans ratios may lie in the duality of mechanism, namely the syn-clinallanti complexity. Certainly recent results on the dehydration of threo- and eo t/iro-2-methyl-4-deutero-3-pentanols on thoria show syn-clinal rather than anti stereospecificity as indicated by deuterium analysis of the cis- and /rn/iJ-4-methyl-2-pentenes, but in these cases the trans isomer was formed in a three-fold excess over the m-olefin . Of course, the dehydration reactions on the less acidic thoria may not be good models for alumina but a knowledge of stereochemistry in the acyclic series might prove an invaluable aid in the elucidation of the mechanism. There is obviously plenty of scope for future kinetic investigations which at the moment sadly lag behind preparative studies. 

Porous Oxides Silica, Activated Alumina, Titania, Thoria, Zirconia Porous inorganic oxides are made through a sol-gel process. The sol is converted into a hydrogel that is subjected to dehydration to form a porous xerogel. Special techniques have been developed to combine the sol-gel transition vith an aim to develop spherical shaped particles (Figure 3.2). 

The first C-unsusbstituted silole to be isolated, 1,1-dimethylsUole (4), was reported simultaneously by Dubac and coworkers , and Bums and Barton in 1981. This silole was prepared by dehydration of 1,1-dimethyl-l-sUacyclopentA-en-3-ol in the gas phase on alumina or thoria - (Scheme 2). In the presence of acidic reagents, the reaction results in C—Si rather than fi C—H bond cleavage, giving the dienic sUoxane (Me2SiC4H5)20. The Barton method consists of the flash vacuum p3Tolysis of 3-(benzoyloxy)-l,l-dimethyl-l-silacyclopent-4-ene at 540 °C (equation 2). 

Table 4.14 Thoria-catalyzed dehydration of secondary 2-alkanols...

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