ABSTRACT
Abstract The rates of oxidative dehydrogenation of a range of monoalcohols and diols, with and without amino groups, has been studied over skeletal copper catalysts in an autoclave that allows the reactants to be preheated to reaction temperature prior to mixing. Chromia, introduced by deposition during leaching, improves activity and stability. Some aspects of the reactivity order are consistent with the inductive effect of NH2 and OH groups in weakening CH bonds but steric effects on adsorption via the nitrogen atom appear necessary to explain the low rates observed for alkyl substitution at this position. NMR spectra of samples taken at different conversions show that the two OH groups in diethanolamine react in sequence forming 2-hydroxyethylglycine as an intermediate. Much of the progress of the reaction can be modeled assuming first order kinetics but the final stages proceed faster than predicted. Introduction The trail-blazing patent of Goto et al. (1) for the oxidative dehydrogenation of aminoalcohols to the corresponding aminocarboxylic acid salts over Raney® copper catalysts in strongly alkaline solutions was cast in terms of the general reaction R1R2NCH2CH2OH + OH-→ R1R2NCH2COO-+ 2 H2 [1] Published work since then has largely concentrated on two specific reactions, the conversion of ethanolamine to glycinate e.g. (2, 3) H2NCH2CH2OH + OH-→ H2NCH2COO-+ 2 H2 [2] and that of diethanolamine to iminodiacetate for use in the production of the herbicide, glyphosate, e.g. (4, 5) HN(CH2CH2OH)2 + 2 OH-→ HN(CH2COO-)2 + 4 H2 [3]
Patents describing improved catalysts for this class of reactions often claim suitability for a wide range of substrates (6-9). However there is no data comparing
rates of reaction for alcohols with different structure. In part this is because the emphasis is usually on yield but also because the reaction is normally carried out batchwise in stirred autoclaves under severe conditions (≥160°C, ≥9 bar, ≥6M NaOH). Kinetics are then difficult to measure since considerable reaction takes place during warm-up and the catalyst commonly deactivates during the course of reaction.
