0.5 wt.% Pd/C catalyst for purification of terephthalic acid: Irreversible deactivation in industrial plants R. Pellegrini a,⇑ , G. Agostini b , E. Groppo b , A. Piovano b , G. Leofanti a,c , C. Lamberti b a Chimet SpA—Catalyst Division, Via di Pescaiola 74, I-52041 Viciomaggio (Arezzo), Italy b Department of Inorganic, Physical and Materials Chemistry, INSTM Reference Center and NIS Centre of Excellence, Università di Torino, Via P. Giuria 7, I-10125 Torino, Italy c Consultant, Via Firenze 43, 20010 Canegrate (Milano), Italy article info Article history: Received 18 October 2010 Revised 10 March 2011 Accepted 17 March 2011 Available online 29 April 2011 Keywords: Supported Pd catalyst Terephthalic acid Catalyst deactivation Sintering Poisoning Pd 4 S Pd 3 Pb XRPD TEM EDS abstract A systematic study on several spent catalysts, withdrawn from different terephthalic acid purification reactors loaded with the same fresh catalyst (0.5 wt.% Pd/C, type D3065, supplied by Chimet SpA), has been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, sinter- ing 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 Pd 4 S or Pd 3 Pb 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. Ó 2011 Elsevier Inc. All rights reserved. 1. Introduction Terephthalic acid (TA) is an important intermediate used for the manufacture of polyethylene terephthalate (PET), which is mainly applied in the production of fibers and bottles [1]. In the Amoco MC process, TA production starts from the homogeneous phase oxida- tion of p-xylene in acetic acid solvent, using Co, Mn, and Br as con- stituents of the catalytic system [2]: CH 3 CH 3 O 2 COOH COOH Co - Mn - Br (TA) The crude terephthalic acid (CTA) so obtained contains around 3000 ppm of 4-carboxybenzaldehyde (4-CBA) as well as colored polyaromatic compounds (yellow compounds) [2,3]. Being a monofunctional compound, the 4-CBA lowers the polymerization rate and the average molecular weight of the polymer; on the other hand, colored impurities confer a polymer color that is undesired for fiber manufacture [4,5]. Therefore, CTA is purified by hydroge- nation in water at 270–290 °C over a granular 0.5 wt.% Pd/C cata- lyst. In this way, the 4-CBA is converted into p-toluic acid that is more soluble than 4-CBA and thus remains in the mother liquid after crystallization; at the same time, the yellow compounds are hydrogenated to colorless compounds [4]: CHO COOH CH 3 COOH H 2 0.5% Pd/C (4-CBA) PTA is a fast-growing material: Worldwide production went from 24 million tons in 2000 [6] to about 42 million tons in 2006 [7]. The worldwide consumption of granular 0.5 wt.% Pd/C used for the final purification step well exceeds the 1000 tons per year. This catalyst is composed of nanodispersed palladium particles 0021-9517/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.jcat.2011.03.012 ⇑ Corresponding author. Fax: +39 0575 441424. E-mail address: riccardo.pellegrini@chimet.com (R. Pellegrini). Journal of Catalysis 280 (2011) 150–160 Contents lists available at ScienceDirect Journal of Catalysis journal homepage: www.elsevier.com/locate/jcat