Ciencia

Liquid-Phase Oxidation of p-Xylene using N-Hydroxyimides
H. Falcona, J.M. Campos-Martina, S.M. Al-Zahranib, J.L.G. Fierroa*
a

Sustainable Energy and Chemistry Group. Instituto de Catálisis y Petroleoquímica, CSIC,

c/Marie Curie, 2 Cantoblanco, 28049 Madrid, Spain, http://www.icp.csic.es/eqs/; bChemical Engineering Department, College of Engineering, King Saud University, Riyadh, Kingdomof Saudi Arabia * Corresponding author: jlgfierro@icp.csic.es, FAX: +34 915 854 760

ABSTRACT In this communication, we describe p-xylene oxidation with molecular oxygen at 373 K and atmospheric pressure using N-hydroximide catalysts. p-Xylene conversion was rather high over the first 2 h of reaction and complete by the end of the experiment. The product distribution curves versus reaction timeare typical of consecutive reactions. The main intermediate product is p-toluic acid. Peak concentration and the shape of its concentration profile depend heavily on the N-hydroxyimide catalyst used. This effect seems to be related to the NO–H bond dissociation energy (BDE) of the N-hydroxyimides employed.

KEYWORKS p-Xylene; Terephthalic acid; Oxidation; Homogeneous catalysis; OrganocatalysisINTRODUCTION Autoxidation reactions are becoming increasingly important in industrial oxidation processes for several reasons: they involve the most abundant and cheapest oxidizing reagent (molecular oxygen), and they usually require only mild temperatures and pressures. The use of molecular oxygen as an oxidant prevents the generation of pollutants, as occurs with stoichiometric oxidants.Molecular oxygen is a very convenient and potent oxidant, but its direct use is restricted by the spin conservation due to the triplet ground structure. Hydrocarbon autoxidations therefore occur via a free radical chain mechanism that hinders the control of selectivity [1]. 1

Terephthalic acid (TPA) is the main raw material for polyester synthesis. It is produced by the liquid-phase catalyticoxidation of p-xylene (PX). Several processes have been developed for the oxidation of PX in liquid phase, but about 70 % of the terephthalate feedstock used worldwide is produced with a catalyst system discovered by Scientific Design (usually called the Amoco process), and almost 100 % of new plants use this process [2]. Typically, PX is oxidized using acetic acid as solvent, one or two multivalentmetal salts (mainly Co or Mn), and bromine as source of free radicals. The reaction temperature ranges from 160 to 200 °C, air is used as the oxygen source and the pressure is 1-2 MPa [2]. This reaction involves several intermediates: p-tolualdehyde (TALD), p-toluic acid (p-TOA), and 4-carboxybenzaldehyde (4CBA), as shown in Figure 1. N-Hydroxyphthalimide (NHPI) promotes the oxidation of a variety ofhydrocarbons with dioxygen in the presence or absence of transition metal salts [1, 3-5]. In this oxidation, the phthalimide N-oxyl radical (PINO) generated in situ from the NHPI and dioxygen abstracts the hydrogen atom from the hydrocarbon to form an alkyl radical, which is readily trapped by dioxygen, affording oxygenated-compounds. NHPI is a cheap, nontoxic catalyst that is easily prepared bythe reaction of phthalic anhydride and hydroxylamine. It acts as a precursor of the phthalimido-N-oxyl (PINO) radical, which is the effective abstracting species in all of the free radical processes mediated by this N-hydroxy derivative. NHPI is thought to catalyze oxidation through the initial generation of the PINO radical by abstracting the hydrogen in NHPI. The PINO radical then abstracts theH-atom from the hydrocarbon to form an alkyl radical, which is readily trapped by dioxygen and then forms oxygenated compounds [6, 7]. For NHPI to act as an efficient oxidation catalyst, it requires an O-H bond of similar strength so that the abstraction of H from the hydrocarbons is either exothermic or close to thermoneutral. However, it is also important for the O-H bond in NHPI to be weak...