Effects of C5-heterocyclic compounds on CO adsorption and crotonaldehyde hydrogenation over supported Cu and Co catalysts

Abstract

The modifying effects of pyrrole, dicyclopentadiene and furan on CO adsorption on a Cu/Al₂O₃ catalyst have been monitored by FTIR spectroscopy. Four bands at 2136 and 2120 cm⁻¹ due to CO on Cu⁺ sites and at 2089 and 2060 cm⁻¹ (Cu°) were weakened by coadsorption of furan but the sites were regenerated by furan desorption at 298 K. Cyclopentadiene completely poisoned Cu° sites although e-modified Cu⁺ sites remained, these effects not being reversed by evacuation at 473 K showing that chemisorption had occurred. Cu° sites were also heavily poisoned by the chemisorption of pyrrole. Removal of molecularly adsorbed pyrrole by evacuation revealed additional Cu⁺ sites, not present before treatment, arising through the dissociative adsorption of pyrrole giving adsorbed pyrrolate anions. The poisoning effects of the modifiers on crotonaldehyde hydrogenation over the same catalyst were in the order pyrrole>cyclopentadiene>furan. Furan and cyclopentadiene had little effect on catalyst selectivity, but pyrrole gave a small improvement in selectivity to crotyl alcohol. Cationic Co sites in Co/Al₂O₃ were largely poisoned by thiophene adsorption although some Co° sites were unpoisoned. Thiophene pretreatment also heavily poisoned crotonaldehyde hydrogenation, reducing the selectivity to butan-1-ol to zero but having little effect on the selectivity to crotyl alcohol. For Co/SiO₂ the activity was less than that for Co/Al₂O₃ , but the selectivity to crotyl alcohol was higher. However, thiophene treatment selectively poisoned CO hydrogenation and gave 100% selectivity to butanal. The rates of formation of butanal were the same for unmodified and thiophene-treated Co/SiO₂, although, in contrast, thiophene considerably decreased the rates of formation of butanal over Co/Al₂O₃. The relative proportions of Co°Co° and Co⁺Co° sites available for CC and CO hydrogenation have a significant influence on catalyst selectivities.