Catalysis of Organic..

Liu, Cant and Smith 135Table 3. Apparent rate constants (k H2 ′) for diols based on H 2 productionreactantstructurek H2 ′, L g -1 h -1Cu catalyst Cu-Cr catalystethylene glycol HOCH 2 CH 2 OH 0.0065 0.0201,4-butanediol HOCH 2 CH 2 CH 2 CH 2 OH - 0.001diethanolamine HN(CH 2 CH 2 OH) 2 0.0050 0.011Figure 3 shows the NMR spectra of samples taken at several stages during thereaction of diethanolamine (DEA) over unpromoted copper. DEA exhibits triplets at~2.55 ppm due to the protons designated ‘a’ below and at ~3.5 ppm due to the ‘d’ones. The early stages of reaction are accompanied by the appearance of a singlet at3.02 ppm. This first grows in intensity and then declines as a second singlet appearsat 2.99 ppm and becomes dominant at high conversions. The 2.99 ppm line clearlyarises from the CH protons in the final product, iminodiacetic acid (IDA), denoted‘b’ below. The 3.02 ppm line can be attributed to the ‘c’ protons in the intermediate,2-hydroxyethylglycine (HEG). Its a′ and d′ protons are responsible for thesuperposition of additional lines on both of the DEA triplets as reaction proceeds.6.2h4.2h2.9h1.7h0h (DEA)3.6 3.5 3.4 3.3 3.2 3.1 3.0 2.9 2.8 2.7 2.6 2.5(ppm)Figure 3. NMR spectra for samples taken during the oxidative dehydrogenation ofdiethanolamine over unpromoted copper.