Reactions of primary and secondary alcohols over palladium-Cab-O-Sil catalysts in the presence of hydrogen: Variables affecting ether formation

Abstract

Palladium oxide on Cab-O-Sil (10% Pd) reduced at 300 °C was found to be an effective catalyst for the conversion of primary alkanols to the corresponding ethers. The reaction was carried out in a flow type microreactor attached to a gas chromatograph through which a molar ratio of 3.1 to 1.0 of hydrogen and alcohol were passed. The selectivity toward ether formation at 160 °C was of the order of about 93% in the case of 1-propanol and 1-butanol. The yield of dineopentyl ether under similar conditions amounted to 73%. Secondary alkanols produced both ethers and alkanes, the relative amounts depending on the temperature to a very pronounced degree. Thus the amount of diisopropyl ether obtained from 2-propanol dropped from 52 mol% to 3.5 when the temperature was raised from 160 to 190 °C. This behavior could be correlated with the influence of Pd-catalyst on diisopropyl ether and di- n-butyl ether at 190 °C. While the former decomposes entirely, the latter remains virtually unaffected. The substitution of hydrogen by helium reduces the total conversion to about 20%. This occurs at the expense of ether formation. On the other hand, a drop in the activity of the Pd-catalysts results if the catalyst is left to cool in a current of hydrogen. This effect is reversed on heating. It thus seems that it is connected with adsorption of hydrogen on the active sites, which is in accord with the particular chemistry of Pd and hydrogen. Injection of air increases the selectivity toward ether, e.g., to 89% in diisopropyl ether formation at 160 °C. This effect is explained by the formation of new basic sites. It is reversed by subsequent treatment in a current of hydrogen.