Metal dispersion and distribution in Pd-based PTA catalysts
Introduction
Polyethylene terephthalate (PET) is a polymer commonly made into fibers, resins, films, etc, and it is one of the highest tonnage and fastest growing plastic materials. Actually PET is mainly manufactured by the purified terephthalic acid (PTA) whose process has been jointly developed by Scientific Design and Amoco [1] and it is based on the liquid-phase oxidation of p-xylene using as a homogeneous catalyst Co(OAc)2 with a promoter, HBr, and a cocatalyst Mn(OAc)2.
The obtained crude terephthalic acid (CTA) contains 4- carboxy-benzaldehyde (4-CBA) as the main impurity (2000–6000 ppm) and several coloured polyaromatic products, and must be purified. In the Amoco process and the like (ICI; Inca-Dow, Mitsui) CTA is dissolved in water at 270–280 °C and its impurities are hydrogenated on a granular Pd(0.5%)/C catalyst in a trickle bed reactor.
The PTA catalyst is very likely the fastest growing and one of the most important catalysts in petrochemistry. However, very few papers have been published on this catalytic system [2], [3], [4], [5], mainly dealing with catalyst deactivation, while some information are available in the patent literature [1]. Palladium is the metal commonly used, even if it has recently been reported that bimetallic Pd–Ru catalysts increase the sintering resistance of metal particles with respect the monometallic samples [6], [7]. For the industrial 0.5 wt%Pd/C catalyst the importance of achieving the most suitable metal dispersion and distribution has been previously reported [5]: the dispersion has to be as high as possible and the metal distribution in the active carbon granules (2–4 mm size) has to be optimized to a suitable egg-shell distribution that makes the active phase more accessible to the reactants, but not easily removable by abrasion. Both parameters can be tailored, but not independently, through suitable manufacturing procedures. Moreover in the industrial PTA plants different batches of the same catalyst sometimes give very different performances [8] and this is due to the several manufacturing variables, thus giving problems of reproducibility.
To collect knowledge useful for the PTA catalyst improvement, the influence of the pH of the impregnating H2PdCl4 solution has been studied in this work.
Section snippets
Methods
Pd(0.7 wt%)/C catalysts were prepared by wet impregnation of a commercial granular active carbon with H2PdCl4 aqueous solutions having different pH values (pH in the range 0.15–2.8). Reduction was performed in the slurry phase by addition of sodium formate. After filtration and washing free from ionic chlorine (AgNO3 test), samples were dried at 110 °C for 15 h.
BET surface area was obtained from N2 adsorption at −196 °C (Micromeritics ASAP 2000 Analyser). Samples were pre-treated at 350 °C for 15 h
Support characterization
The active carbon used in this work has been characterized by N2 physisorption analyses and pH measurements. As the metal dispersion is a key factor in the synthesis of an active PTA catalysts, a high surface area of the support is desirable. In fact, as a general rule, when the surface area of the carrier is high, small metal nanoparticles are usually obtained [11]. The N2 physisorption isotherm for the granular active carbon used as support is a type I isotherm, which is typical of
TPR analysis
After Pd deposition and drying, a TPR analysis was performed in order to investigate the metal oxidation state in relation with the pH of the impregnating solution. TPR set up requires that the reducing mixture flows through the sample to be analyzed prior to start the temperature ramp. This procedure can cause PdO reduction, since PdO can be reduced already at −15 °C and leads to the formation of Pd β-hydride if Pd metal particles are big enough [11], [12], [13]. Pd β-hydride decomposes at
Metal distribution
The PTA catalysts that we have studied are supported over irregular carbon granules of size in the 2–4 mm range. Through a simple procedure [10], it is possible to calculate the Pd penetration depth into the support granules, that show a typical external metal distribution [5]. This so-called “egg-shell type” makes the active phase more accessible to the reactants, therefore improving activity and selectivity and minimizing the palladium content.
In Fig. 2 the relationship between Pd penetration
Metal dispersion
Metal dispersion, namely the ratio between surface and total metal atoms, is a critical factor for several catalytic reactions and generally it has to be as high as possible. However deposition of noble metals with high dispersion on active carbon is not a trivial task, due to the tendency of metal towards agglomeration.
Results of pulse flow CO chemisorption measurements are reported in Table 1. In accordance with a previous investigation [9], calculations for determining the average Pd
Kinetic data
4-CBA hydrogenation has been performed in conditions strictly similar to the industrial ones. In Table 1 and Fig. 3 it is evident that the highest activity is reached with samples impregnated at pH 1.5–2.0, while the two samples prepared at the lowest impregnation pH (0.15 and 1.0) present a very low activity. The sample prepared at pH 2.8 behaves in the middle way.
The previously reported [5] linear correlation between catalytic activity and Pd surface area has been confirmed (Fig. 4). Moreover
Conclusions
In conclusion, among the different variables influencing the performance of the Pd/C PTA catalyst, the pH of the impregnating H2PdCl4 solution has been found to strongly influence both Pd surface area and catalytic activity. On the contrary the pH has a relatively small influence on the Pd distribution in the carbon granules.
A linear relationship between catalytic activity for 4-CBA hydrogenation and Pd surface area has been confirmed. The highest activity and Pd surface area are reached with
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