A novel approach to understanding and modelling performance evolution of catalysts during their initial operation under reaction conditions – Case study of vanadium phosphorus oxides for n-butane selective oxidation

In heterogeneous catalysis, catalyst precursor activation and subsequent initial operation (‘conditioning’) under reaction conditions are often highly dynamic steps which are important in delivering effective catalyst performance over time scales of years. In many cases however, the phenomena occurring in both steps are often poorly understood. A novel approach to assess the conditioning step, comprising advanced catalyst testing methods in micro-reactors, catalyst characterisation and detailed kinetic and activity modelling techniques is demonstrated for a model system: the selective oxidation of n-butane to maleic anhydride (MA) using a pre-activated vanadium phosphorus oxide (VPO) catalyst. A transient kinetic model for the conditioning step is presented which describes the decline of population of active sites for three reaction pathways on the catalyst surface over time. Two reaction pathways, n-butane --> MA and n-butane --> COx decay similarly with time whilst a third, MA --> COx declines very sharply. The approach used provides important mechanistic information on catalyst reaction kinetics whilst also providing understanding the impact of reaction conditions on the catalyst during conditioning.

Performance evolution of an integral operation VPO catalyst bed during initial operation under reaction conditions was assessed experimentally using a novel combination of parallel difference testing and kinetic modelling. A sector profile with time on stream was realised, providing an extensive evolution history. Subsequent kinetic and activity modelling of these data allowed the active site evolution of different reaction pathways during initial operation to be understood (see below), enhancing fundamental mechanistic understanding of the system.Figure optionsDownload high-quality image (39 K)Download as PowerPoint slideHighlights
► Catalysts often show transient reaction performance during early stages of life.
► Vanadium phosphorus oxides for n-butane selective oxidation used as a case study.
► Transient, integral operation data used to understand fundamental reaction mechanisms.
► Intrinsic selectivity of a VPO catalyst evolves during initial reactor operation.
► Kinetic and activity modelling suggests two distinct active sites on the catalyst.