Topics in Catalysis Vol. 15, No. 2–4, 2001 139 Importance of site isolation in the oxidation of isobutyric acid to methacrylic acid on iron phosphate catalysts Jean-Marc M. Millet a and Jacques C. Védrine b,∗ a Institut de Recherches sur la Catalyse, CNRS, Université de Lyon 1, 2 avenue Albert Einstein, F-69626 Villeurbanne Cedex, France E-mail: millet@catalyse.univ-lyon1.fr b Leverhulme Centre for Innovative Catalysis, The University of Liverpool, Liverpool L69 7ZD, UK E-mail: vedrine@liv.ac.uk Isobutyric acid (IBA) oxidative dehydrogenation to methacrylic acid (MAA) reaction has been studied for a wide variety of iron (hydro- gen) phosphates catalysts and mechanical mixtures of several of these phosphates. It was observed that the reaction needs high amounts of water, typically 10–12 mol water per mol IBA, to get stable, active and selective catalysts and follows a Mars and van Krevelen mechanism, as schematised below: Fe 3+ 3 (PO 3 OH) 3 (PO 4 ) + (x/2)IBA → Fe 3+ 3−x Fe 2+ x (PO 3 OH) 3+x (PO 4 ) 1−x + (x/2)MAA Fe 3+ 3−x Fe 2+ x (PO 3 OH) 3+x (PO 4 ) 1−x + (x/4)O 2 → Fe 3+ 3 (PO 3 OH) 3 (PO 4 ) + (x/2)H 2 O The best catalysts were found to be composed of trimers of face sharing FeO 6 octahedra, isolated one from the others by cationic vacancies and bounded to the next row by PO 4 tetrahedra. Solids having dimers or long chain octahedra are also active but less selective. Theoretical calculations showed that this corresponds to an electron hopping from one Fe 2+ to the next Fe 3+ cation during the redox mechanism for a trimer, while no electron hopping occurs for dimers and whereas it occurs all over the Fe cations for infinite octahedra chains. The presence of such vacancies and, to a lesser extent, of excess phosphorus at the surface, as shown by XPS analysis, exemplifies the site isolation concept in partial oxidation reactions. At variance, the cooperation effect between several phases was not observed. KEY WORDS: oxidative dehydrogenation; methacrylic acid; site isolation concept; phase cooperation concept; iron phosphates; iron hydroxyphosphates 1. Introduction Methylmethacrylate (MMA) is an important intermedi- ate in several chemical processes with a world capacity of ca. 2.2 millions tons per year. It is synthesised all over the world using basically the same process known since 1934 as the acetone cyanhydrine process. The use of large quan- tities of sulfuric acid as well as the production of ammo- nium sulfate, as a by-product (2.5 kg per kg MMA) diffi- cult to recycle, has incited chemical industries to develop new processes, thought to be less detrimental to the environ- ment. One of them consists of the synthesis of isobutyric acid (IBA) by propene carbonylation followed by oxidative dehydrogenation (ODH) yielding methacrylic acid (MAA), which is then esterified by methanol to methylmethacrylate. Such an ODH reaction has been shown to be catalysed by iron phosphate based catalysts, as presented in details in a recent review by one of us [1] and by heteropolyoxometal- lates [2]. 2. Experimental 2.1. Catalyst preparation Two main procedures were used, namely the precipita- tion and the vivianite methods. For the first procedure sam- ∗ To whom correspondence should be addressed. ples were prepared by mixing ferric nitrate dissolved in wa- ter with an aqueous solution of phosphoric acid. The mix- ture was evaporated, dried and calcined between 400 and 500 ◦ C under an air stream. For some samples, Cs nitrate was also added to the solution. An industrial catalyst, pre- pared by the precipitation method following a patent pro- cedure [3], with tetraethylorthosilicate added (3 wt%) as a binder, was supplied by Orkem Co. Its chemical molar com- position was: Cs 2 O/Fe 2 O 3 /P 2 O 5 = 0.08/0.50/0.625. For the second method, vivianite, an hydrated ferrous iron phos- phate (Fe 3 (PO 4 ) 2 ·8H 2 O), was reacting with H 4 P 2 O 7 , in a P/Fe ratio equal to 4.5, in acetone under reflux for 15 h [4]. The solid was then filtered, washed with acetone and dried at 100 ◦ C. Several hydroxyphosphates were also prepared by hydrothermal synthesis, as described previously [5,6]. 2.2. Catalytic testing Oxidative dehydrogenation of isobutyric acid (IBA) was carried out in a flow differential microreactor at atmospheric pressure with on line GC analysis. Typical reaction condi- tions were O 2 /IBA/H 2 O/N 2 = 4.26/5.86/72.0/19.2 kPa with 30–50 mg catalyst. The role of water was studied by varying the water pressure from 0 to 72 kPa, keeping O 2 and IBA pressures constant. 1022-5528/01/0600-0139$19.50/0  2001 Plenum Publishing Corporation