Selective catalytic hydrogenation of 2-butyne-1,4-diol to cis-2-butene-1,4-diol in mass transfer efficient slurry reactors

The hydrogenation of 2-butyne-1,4-diol over a commercial 5% Pd/charcoal and a 5% Pd/TiO2 catalyst prepared in-house was investigated and the selectivity towards cis-2-butene-1,4-diol under various reaction conditions is reported. The experiments were carried out in a glass stirred tank reactor (STR) and a cocurrent downflow contactor reactor (CDCR). It was found that besides cis-2-butene-1,4-diol and 1,4-butanediol some side products such as cis-and trans-crotyl alcohol, n-butanol, and n-butyraldehyde were also observed. A reaction scheme is proposed. In the STR, the effects of catalyst loading, temperature, base, poisons and metal additives on the selectivity were studied. The results indicate that triethyl phosphite and lead acetate led to inhibition of further hydrogenation. The use of lead poisoned Pd/charcoal catalyst gave partial hydrogenation with selectivity towards cis-2-butene-1,4-diol of 97.4-98.8%. The use of Pd/TiO2 catalyst exhibited higher selectivity, more than 99%. In the reaction with Pd/charcoal, CDCR has shown its potential giving increased selectivity (up to 100%) to cis-2-butene-1,4-diol at the end of first hydrogenation step, even with no addition of poison and with much higher reaction rates.