Alumina ether formation

A recent report( ) on the use of iron carbonyl and potassium carbonate in a similar carboxyalkylation scheme to prepare methyl phenylacetate prompted us to examine the use of carbonate on alumina in a similar manner. It was suggested that if the amount of free base was less than the amount of iron carbonyl than ether formation would not occur being that iron carbonyl was a better electrophile than benzyl halide. Under our conditions, the metal carbonyl anion... 

The positive slope for ether formation found on alumina (series 47), which contrasts with the negative one for alkene formation, has been interpreted by Knozinger as evidence of different mechanisms for these two, often in parallel proceeding transformations of alcohols. It has been suggested that the first step of the dehydration to an ether is the formation of a surface alkoxide, which is then attacked by a weakly bonded alcohol molecule. 

The experiments with reversible poisoning of alumina by small amounts of bases like ammonia, pyridine or piperidine revealed [8,137,142,145, 146] relatively small decreases of dehydration activity, in contrast to isomerisation activity which was fully supressed. It was concluded that the dehydration requires only moderately strong acidic sites on which weak bases are not adsorbed, and that, therefore, Lewis-type sites do not play an important role with alumina. However, pyridine stops the dehydration of tert-butanol on silica—alumina [8]. Later, poisoning experiments with acetic acid [143] and tetracyanoethylene [8] have shown the importance of basic sites for ether formation, but, surprisingly, the formation of olefins was unaffected. 

This is again a direct analogy with ether formation from alcohols (see Sect. 2.2.4). The acidic sites might be the Al3+ ions because rehydration of the alumina surface does not enhance the rate, in contrast to deamination [149],... 

The carbon monoxide selectivity was well below 2% for all samples. As a by-product, substantial amounts of dimethyl ether were found for all samples the highest selectivity of 23% was detected over pure ceria. Only traces of another by-product, methyl formate, were measured. The dimethyl ether formation was attributed to separate dehydration of the methanol on the alumina surface. 

A solution of N-(4-pyridinyl)-lH-indol-l-amine (6 g) in 25 ml of dimethylformamide was slowly added to an ice-cooled suspension of NaH (1.3 g of 60% NaH dispersion in mineral oil was washed with hexanes, the liquid was decanted and the residual solid was dispersed in 5 ml of dimethylformamide). After anion formation, a solution of 1-bromopropane (4 g) in 5 ml of dimethylformamide was added. After one hour of stirring at ambient temperature, the reaction mixture was stirred with ice-water and extracted with dichloromethane. The organic extract was washed with water and saturated sodium chloride solution, was dried over anhydrous magnesium sulfate, filtered and concentrated to 8 g of oil. This oil was purified by HPLC (silica, ethyl acetate) and thereafter by column chromatography (alumina, ether) to give 6.4 g oil. This oil was converted to the maleate salt and recrystallized from methanol/ether to give 6.8 g of crystals, m.p. 115-116°C. 

It has been shown that only those alcohols that form detectable surface alco-holate species on alumina undergo bimolecular dehydration with ether and water as reaction products (340). Thus, ether formation is the dominant reaction direction of methanol and ethanol at low temperatures, and the tendency toward ether formation is reduced as the chain length increases and chain branching occurs (28, 340). The same trends are observed for the stability and surface concentrations of the surface alcoholate species (27, 28, 47, 340). Alcoholate formation is due to a dissociative chemisorption step of the alcohol that occurs on A1—O pair sites (47, 340, 354-358). One is, thus, led to the conclusion that incompletely coordinated Al3+ ions and O2- ions are both important sites in the ether formation from alcohols and that their participation should be detectable by specific poisoning. 

The temperature effect on the dehydration of alcohols in the presence of alumina as has been shown by the work of Sabatier and Mailhe,41 Brown and Reid,°°b and Pease and Yung08 was not checked by Adkins,° b who used what were presumably better conditions experimentally. The rate of dehydration increases in the order of butyl, propyl, isobutyl, ethyl, isopropyl, and secondary butyl alcohols. Although ethanol and ethyl ether give the same rate of dehydration, butyl alcohol gives a faster dehydration late than does butyl ether. Hence, the hypothesis advanced at one time that olefin formation from alcohols was through intermediate ether formation cannot hold. 

Characteristic examples of the dehydration of primary alcohols are collected in Table 1. Kaolinite containing alumina [67], aluminas, modified aluminas [68,69], silica-aluminas [70] and AIPO4 [26] have also been studied. Ether formation was found to be favored by a high concentration of sites of intermediate or weak acidity. 

A comparative study of the alumina-supported catalysts prepared from [H2FeOs3(CO)i3], [H20s3Rh(acac)(CO)io], and [Rli4(CO)i2] was performed and each catalyst was found to be active in the conversion of CO -I- H2. The major product observed in each experiment was methane and the hydrocarbon products were formed in approximately a Schulz-Flory-Anderson distribution. The heterogeneous [Os3Rh] catalyst was two orders of magnitude more active at 543 K than the [FeOs3] catalyst, but showed a lower selectivity for ether formation. ... 

The reversibility of ether formation has been demonstrated , but it is not known whether the ether is formed from the collision of gaseous and absorbed alcohol or from adsorbed alcohol molecules only. On alumina, ethyl alcohol... 

Men and coworkers investigated methanol steam reforming over Cu/Ce02/Al203 catalysts [12-14] in a 10-fold screening reactor developed by Kolb et al. [3]. At a reaction temperature of 250 °C and an S/C ratio of 0.9, the atomic ratio of copper to ceria was varied from 0 to 0.9, revealing the lowest conversion for pure ceria and a sharp maximum for a ratio of 0.1. The carbon monoxide selectivity was lower than 2% for all samples. As byproduct, substantial amounts of dimethyl ether were observed for all samples the highest selectivity of 23% was detected for pure ceria. The dimethyl ether formation was attributed to separate dehydration of methanol on the alumina surface. 

Dimethyl ether formation was also observed by Men et al. over copper/zinc oxide/ alumina catalysts at lower values of the weight hourly space velocity [ 163]. A low weight hourly space velocity of 10.9 L (h gcat) was required at a S/C ratio of 2 in order to gain full methanol conversion vtithout formation of any by-products such as dimethyl ether. Under these conditions, around 1.5 vol.% carbon monoxide was formed. 

The bimolecular ether formation from alcohols proceeds at much lower temperatures than the olefin formation.Over alumina, it occurs even at 400 — 410 The... 

The zinc oxide component of the catalyst serves to maintain the activity and surface area of the copper sites, and additionally helps to reduce light ends by-product formation. Selectivity is better than 99%, with typical impurities being ethers, esters, aldehydes, ketones, higher alcohols, and waxes. The alumina portion of the catalyst primarily serves as a support. 

Treatment of 2- p-hydroxyphenyl)ethyl bromide with basic alumina produces a white solid mp, 40-43°C IR, 1640cm Uy 282nm in H20,261 mm in ether NMR, two singlets of equal intensity at 1.69 and 6.44 ppm ftom TMS. Anah C, 79.97 H, 6.71. Suggest a reasonable structure for this product and a rationalization for its formation. 

The cobalt complex is usually formed in a hot acetate-acetic acid medium. After the formation of the cobalt colour, hydrochloric acid or nitric acid is added to decompose the complexes of most of the other heavy metals present. Iron, copper, cerium(IV), chromium(III and VI), nickel, vanadyl vanadium, and copper interfere when present in appreciable quantities. Excess of the reagent minimises the interference of iron(II) iron(III) can be removed by diethyl ether extraction from a hydrochloric acid solution. Most of the interferences can be eliminated by treatment with potassium bromate, followed by the addition of an alkali fluoride. Cobalt may also be isolated by dithizone extraction from a basic medium after copper has been removed (if necessary) from acidic solution. An alumina column may also be used to adsorb the cobalt nitroso-R-chelate anion in the presence of perchloric acid, the other elements are eluted with warm 1M nitric acid, and finally the cobalt complex with 1M sulphuric acid, and the absorbance measured at 500 nm. 

The reaction of alkyl sulfates with alkoxide ions is quite similar to 10-12 in mechanism and scope. Other inorganic esters can also be used. One of the most common usages of the reaction is the formation of methyl ethers of alcohols and phenols by treatment of alkoxides or aroxides with methyl sulfate. The alcohol or phenol can be methylated directly, by treatment with dimethyl sulfate and alumina in cyclohexane. Carboxylic esters sometimes give ethers when treated with alkoxides (Bal2 mechanism, p. 473) in a very similar process (see also 10-24). 

Ethers contain additives to stabilise them against peroxide formation. For instance, tetrahydrofuran is commonly stabilised by the addition of small amounts of hydroquinone. This absorbs uv radiation strongly and so interferes with uv absorbance detection. It can be removed by distilling the solvent from KOH pellets. If you use inhibitor-free tetrahydrofuran, it should be stored in a dark bottle and flushed with nitrogen after each use. Any peroxides that form should be periodically removed by adsorption onto alumina. 

Dasler, W. et al., Ind. Eng. Chem. (Anal. Ed.), 1946,18, 52 Like other monofunctional ethers but more so because of the four susceptible hydrogen atoms, dioxane exposed to air is susceptible to autoxidation with formation of peroxides which may be hazardous if distillation (causing concentration) is attempted. Because it is water-miscible, treatment by shaking with aqueous reducants (iron(II) sulfate, sodium sulfide, etc.) is impracticable. Peroxides may be removed, however, under anhydrous conditions by passing dioxane (or any other ether) down a column of activated alumina. The peroxides (and any water) are removed by adsorption onto the alumina, which must then be washed with methanol or water to remove them before the column material is discarded [1], The heat of decomposition of dioxane has been determined (130-200°C) as 0.165 kJ/g. 

Anionicallv Activated Alumina. At this time we had also developed an interest in anionically activated alumina. These basic reagents were active in promoting alkylation(42), condensation(43) and hydrolysis(44) reactions. Thus, we impregnated alumina with sodium hydroxide and used this combination both with and without a phase transfer catalyst (benzyltriethyl ammonium chloride). When BTEAC was added, the conversion to ether was decreased and the formation of ester was noted. In the absence of a phase transfer catalyst, the ether became a minor product and methyl phenylacetate became the major product with coproduction of phenylacetic acid. This ester does not result from esterification of the acid as simple stirring of phenylacetic acid with Na0H/A1203 in methanol does not produce methyl phenylacetate. 

The formation of byproduct methyl benzyl ether was the key reason for the low selectivity to ester in the absence of alumina. A more careful examination of the product distributions with time was made using the alkoxide, alkoxide on alumina and bicarbonate on alumina bases. The results from Table V indicate that the formation of ether was indeed the predominant pathway with alkoxide alone, while the presence of alumina retarded this conversion and promoted the carboxyalkylation pathway. The bicarbonate on alumina gave little ether product and excellent selectivity to the methyl phenylacetate. 

In discussing the mechanism, there has been a tendency to take as evidence the results obtained on alumina with a single reactant, mostly ethanol. Almost all of the deductions have hinged on the relationship between the formation of ether and of ethylene. Additionally, the various investigators failed to realize that the structure and the mode of preparation of the catalyst were important. 

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