Sucesión secundaria y dinámica de claros

Experiments were carried out using different stirrer speeds (Figure 3.13) and catalyst mass (Figure 3.14) to assess the impact of gas-liquid and liquid-solid mass transport limitations respectively. It is seen that for stirrer speed values of 800 rpm or greater, rate of reaction with respect to PBN conversion is roughly constant. This led to a choice of 1400 rpm for the bulk of the experimental programme (see section 3.2.1). A reasonably linear trend was seen between initial PBN conversion rate and catalyst mass suggesting that liquid-solid mass transport effects are negligible within the reactor setup. A similar, linear observation is made if different starting PBN concentrations are used. This further verifies the absence of liquid- solid mass transport limitations and consolidates the kinetic findings in Figure 3.4, which demonstrated that product desorption effects have an impact on reaction performance.

Figure 3.13: Effect of stirrer speed on initial reaction rate of PBN (100 mg catalyst, 5 bar H2 pressure, 343 K, 0.26 mol L-1 initial PBN concentration)

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Figure 3.14: Effect of catalyst mass on initial reaction rate of PBN (1400 rpm stirrer speed, 5 bar H2 pressure, 343 K, 0.26 mol L-1 initial PBN concentration)

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Chapter 4:

A novel approach to understanding and modelling the

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.8

8

Material presented in this chapter has been published: Wilkinson S.K., Simmons M.J.H., Stitt E.H., Baucherel X., Watson M.J., Journal of Catalysis, (2013), 299, 249-260

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