C.N. Pillai

Catalytic dehydration of alcohols over alumina: Mechanism of ether formation

The dehydration of alcohols over alumina to form ethers and olefins was studied from a mechanistic point of view. The rates of both olefin formation and ether formation were independent of the partial pressure of the alcohol, beyond a certain partial pressure. This saturation partial pressure was higher for ether formation than for olefin formation. The conclusion was drawn that both reactions are surface reactions but requiring different types of active sites. Certain reagents, like phenol, when added to the alcohol in small quantities, caused a large increase in the rate of olefin formation and a large decrease in the rate of ether formation. Certain other reagents like pyridine caused decrease in the rates of both ether formation and olefin formation. An explanation is offered for the above observations assuming the presence of both acidic and basic sites on the surface of alumina.

Hydride transfer reactions. VIII: Reactions of benzyl alcohol over alumina: dehydration and disproportionation

The present study is a continuation of the study of the disproportionation of diphenylcarbinol over alumina ( 1). Landis and Heiba (2) had reported the disproportionation of benzyl alcohol over alumina to yield benzaldehyde, toluene, and water, in addition to the expected dehydration to the ether. In the present paper an attempt is made to differentiate the sites responsible for disproportionation and dehydration. Alumina catalysts were prepared by different methods of which three sources yielded catalysts with significantly different selectivities. These were the following: (1) Conventional catalyst, prepared by the calcination of precipitated aluminum hydroxide from either aluminum nitrate or aluminum isopropoxide at 600°C (3) (catalyst 1). (2) Catalyst prepared by the thermal decomposition of alumina isopropoxide (360°C) followed by calcination at 600°C (catalyst 2). (3) Catalyst prepared by the slow hydrolysis of aluminum isopropoxide by prolonged (24-48 hr) exposure to humid atmosphere (relative humidity 50-70%) followed by heating initially at 120°C for 6 hr followed by heating at 600°C for 6 hr (catalyst 3). All the catalysts had surface area of 200 -+ 20 m2/g. Further details of the preparation and characterization of catalysts 2 and 3 will be reported in another communication. Results of the reactions of benzyl alcohol over the three catalysts have been summarized in Table 1. It is clear that catalysts 2 and 3 are more active for alcohol conversion than catalyst 1. Catalyst 2 is selective for ether formation and catalyst 3 for disproportionation. However, it could be shown that the initially formed ether itself decomposed at increased contact times on all the catalysts. The results for catalyst 3 are represented in Fig. 1. It was further shown that dibenzyl ether itself underwent decomposition to benzaldehyde and toluene. Catalyst 3 was much more active for this reaction than catalyst 2 (Table 2). It could be seen from Table 1 that disproportionation increased and ether formation decreased with increase in reaction tempera-

Reactions of benzyl alcohol over oxide catalysts: A novel condensation to form anthracene

Abstract A novel heterogeneously catalyzed condensation of benzyl alcohol to form anthracene is reported. The reaction involves an orthoalkylation of a surface benzyl oxide species by the benzyl cation from a neighboring benzyl oxide to form o -hydroxymethyldiphenylmethane which undergoes cyclodehydration followed by aromatization to yield anthracene. Reactions have been carried out with alumina and modified alumina catalysts (fluorided and sodium ion-impregnated), HY-zeolite, phosphotungstic acid, and silicotungstic acid. Mechanistic features of the reaction are discussed. A simple and efficient synthesis of anthracene from benzyl alcohol over HY-zeolite is also described.

Reactions of phenols and alcohols over thoria: Mechanism of ether formation

Abstract The dehydration of phenols and alkylation of phenols by alcohols over thoria were studied at 400–500 °C and atmospheric pressure. Phenol and cresols, when dehydrated gave diaryl ethers as main products. With para -substituted phenols such as p -methoxy, p-t -butyl, p -chloro, and p -nitrophenol no ether formation was noticed. All the reactions were accompanied by considerable amount of coke formation. Alkylation of phenols by alcohols gave a mixture of O- and C-alkylated products under the same reaction conditions. O-alkylation and C-alkylation are parallel reactions. The mechanistic aspects of the reactions are discussed.

Reaction of carboxylic acids with carbonyl compounds over alumina

As part of a study to elucidate the mechanism of the ketonization of carboxylic acid, the reactions of carboxylic acids with carbonyl compounds over alumina at 350–400 °C were studied. Products arising from the transfer of alkyl group of the carboxylic acid to the carbonyl compounds as well as those arising from a Perkin-type condensation of carboxylic acid with carbonyl compound were detected. Mechanisms of these reactions are discussed.

Reaction of benzaldehyde with other carbonyl compounds over thoria and mixed oxides of thoria: Development of an effective catalyst and a process for the conversion of benzaldehyde to phenyl alkyl ketones

Aldol condensation of benzaldehyde with a variety of carbonyl compounds yielded phenyl alkyl ketones in addition to the expected condensation products over thorium oxide catalyst at 400 °C. The β-hydroxyketone formed underwent both dehydration to yield α,β-unsaturated carbonyl compound and dehydrogenation to yield 1,3-diketone. The 1,3-diketone cleaved into the phenyl alkyl ketone and an acid. Mixed oxide catalysts of thorium oxide with zinc oxide or magnesium oxide were tested for improving the yield of phenyl alkyl ketones. A mixture 80% magnesium oxide-20% thorium oxide was found to be an effective catalyst.

Reactions of acetophenone with alcohols over oxide catalysts: alkylation of ketones by alcohols

As part of a study of hydrogen transfer reactions over alumina, the reactions of acetophenone with methanol over η-alumina at 300–500 °C were studied. In addition to styrene arising from reductive dehydration, products arising from methylation (propiophenone, β-methylstyrene) were also observed. At higher conversion, secondary reactions like dealkylation of higher ketones and alkylation, dealkylation, and hydrogenation of olefins were also observed. Reactions have also been carried out with modified alumina (fluorided and Na+-doped) catalysts, HY-zeolite, phosphotungstic acid, and silicotungstic acid to study the effect of acidity on the selectivity for these reactions. Reactions of methanol with aralkyl ketones and with olefins were also studied. Acetophenone conversion was found to increase with increasing catalyst basicity while the selectivity for reductive dehydration decreased. Conclusions have also been drawn, on the basis of these studies, regarding the nature of the active catalytic sites responsible for the different reactions and mechanisms have been proposed for the surface reactions.

Hydrogen transfer reactions: IV. Substituent effect in the reduction of benzaldehydes by isopropyl alcohol catalyzed by alumina☆

A linear Hammett correlation is reported for the reduction of substituted benzaldehydes by isopropyl alcohol catalyzed by alumina containing 2.2% (by weight) sodium ions, at 300 °C in the vapor phase. The positive nature of the slope supports the hydrid transfer mechanism proposed for this reduction.

Studies on the isomerization of substituted allyl alcohols over Raney nickel

Abstract Substituted allyl alcohols, namely crotyl alcohol, 4-methylpent-3-en-2-ol, cinnamyl alcohol, 4-phenylbut-3-en-2-ol, 1,3-diphenylprop-2-en-1-ol, cyclopent-2-en-1-ol, and cyclohex-2-en-1-ol were found to isomerize to the corresponding carbonyl compounds in the presence of Raney nickel. The mechanism appears to involve the “half-hydrogenated state” followed by reversal as in the case of the well-known isomerization of olefins during hydrogenation. Tricyclo-[5.2.1.0 2,6 ]-dec-3-en-4-ols were also found to undergo isomerization to tricyclo-[5.2.1.0 2,6 ]-decan-3-one.

Reactions of phenols and alcohols over thoria

Abstract Alkylphenols when dehydrated over thoria at 500 °C gave diaryl ethers as main products. In addition to diaryl ethers, minor products were also formed, which were identified as xanthene and substituted xanthenes. Xanthenes were also formed in the reactions of phenol with alcohols over thoria. A mechanism is proposed for the formation of xanthenes.