0.5 wt.% Pd/C catalyst for purification of terephthalic acid: Irreversible deactivation in industrial plants

A systematic study on several spent catalysts, withdrawn from different terephthalic acid purification reactors loaded with the same fresh catalyst (0.5 wt.% Pd/C, type D3065, supplied by Chimet SpA), has been carried out. Spent catalysts characterized by different lifetimes, position in the catalytic bed, sintering degree, and types of contaminant (mainly S, Pb, and Mo) have been investigated by TEM coupled with EDS detection, XRPD, EXAFS spectroscopy, and CO chemisorption. The Pd sintering process involves all catalysts, irrespective of the nature and amount of contaminants that have no influence on sintering rate except for S poisoning. Pd sintering occurs following different steps, leading to the formation of larger crystals, aggregates, and agglomerates, the last being the primary cause of the loss of Pd surface area and, consequently, of the decrease in catalytic activity. Among the investigated contaminants, S and Pb are the worst, because they strongly interact with Pd, forming from surface adlayers (not detectable by XRPD, but visible by EDS mapping) up to bulk Pd4S or Pd3Pb alloys (easily detectable by XRPD). In both cases, the catalytic activity decreases. In contrast, Mo, Cr, Fe, Ti, and Al do not interact preferentially with Pd (no alloys have been detected, although the contaminants are present in relevant concentrations): They have been found to be spread on the whole catalyst surface. Accordingly, no direct effect of these contaminants on catalytic activity loss has been evidenced.