A Partner of Concorde CA and of Idecat NoE, 6FP of the EU) b Lonza SpA, via E. Fermi 51, 24020 Scanzorosciate (BG), ItalyThis review describes recent findings in the oxidation of n-butane to maleic anhydride. The process is commercial since the 80's, but yet the yield is far from being optimised. Therefore, it represents an emblematic example of how several scientific disciplines, from solid-state science to reactor technology, can contribute to the improvement of the process performance.KEY WORDS: Vanadyl pyrophosphate; n-butane; selective oxidation; maleic anhydride 1. The oxidation of n-butane catalysed by VPO: How to improve process performance?The activation of light alkanes by high-temperature contact with a redox catalyst represents one way to transform these hydrocarbons to valuable chemicals (a few reviews and books dealing with this topic, published after 1998, are refs [1-18]). The most successful example is the oxidation of n-butane to maleic anhydride (MA), catalysed by a V/P/O-based catalyst (VPO), which starting from the 80's has been replacing in part the commercial synthesis from benzene. This reaction, and the unique chemical-physical properties of the vanadyl pyrophosphate (VO) 2 P 2 O 7 (VPP), the active and selective phase for this reaction, are amongst the most studied topics in catalysis during the latest decades, initially with the aim of understanding which catalyst peculiarities make this complex transformation possible, and later on with the aim of improving the process performance.MA finds its major use in the production of unsaturated polyesters and of butanediol. The yearly world consumption exceeds 1.3 · 10 6 metric tons [19]. Approximately 70% is produced by n-butane oxidation, the remaining still being obtained from benzene. There are several reactor technologies available, including fixed-bed (Scientific Design, Huntsman, BASF, Pantochim), fluidised-bed (Lonza, BP, Mitsubishi), and transported-bed (DuPont). Best process performances reported in patent literature range from 53 to 65% molar yield to MA [20][21][22][23][24][25], with a conversion of the hydrocarbon not higher than 85-86%. An excellent result of more than 70% yield is reported [26], which however refers to a recycle process. In lab reactors best yields reported are lower than 50%.The best performance for a fixed-bed reactor does not exceed 65% per-pass yield, while that in a fluidised-bed is typically somewhat lower; in fact, back-mixing phenomena are responsible for the consecutive combustion of MA. Moreover, with fluidised-bed operation n-butane-richer conditions can be used (up to 5 molar % in feed); under the latter conditions, a worsening of selectivity is expected, which however is compensated by a considerable improvement in MA productivity. In the fluidised-bed process developed by Lonza and Lummus (shown in figure 1), the loss of selectivity is also compensated by the higher amount of high-pressure steam produced, due to the more efficient removal of the reaction heat and to the more CO x produced.The maximu...
