Heterogeneous catalytic oxidation of isobutylene in vapour-phase Ph.D. Thesis

Abstract

Kinetics of the catalytic vapour-phase oxidation of isobutylene to methacrolein, an important raw material for polymer industries, was investigated in a fixed-bed isothermal flow reactor. Out of a number of catalysts prepared initially, bismuth molybdate and ferric molybdate catalysts were selected for kinetic studies of this vapour-phase oxidation reaction. Bismuth molybdate and ferric molybdate catalysts were found to have good activity and selectivity towards methacrolein formation in the temperature range (425°–500°C) studied. The effect of various operating variables, namely, (i) time factor, (ii) mole ratio of oxygen to isobutylene, (iii) temperature, (iv) partial pressure of isobutylene, (v) partial pressure of oxygen, on conversion and reaction rate of isobutylene, was studied. Under the reaction conditions studied, the oxidation products were mainly methacrolein, carbon monoxide, carbon dioxide and water. It was established from the analysis of experimental data that these products were formed by a parallel reaction scheme. Kinetic data were collected in the region of experimentally determined conditions where resistances due to physical transport phenomena (internal and external diffusion) do not influence the rate data. Absence of significant mass and heat gradients in the reactor under the experimental conditions was also confirmed by theoretical calculations. To elucidate the mechanism and kinetics of this reaction, twelve different models based on redox mechanisms, Rideal mechanisms and Langmuir–Hinshelwood mechanisms and some empirical equations were employed. Amongst these models, the two-stage redox model - that is, the substance to be oxidized reduces the catalyst, which in turn is oxidized by oxygen from the feed - with the order of reaction with respect to isobutylene as 1.0 and that with respect to oxygen as 0.5 was found to represent the rate data well. Model discrimination studies were made using both classical criteria and non-intrinsic parameter method. The rate constants of the selected model were estimated both by linear and nonlinear least squares regression techniques and were correlated through Arrhenius equation whose parameters were also evaluated by linear and nonlinear least squares regression techniques. From the values of rate constants, it was found that the re-oxidation of the reduced catalyst was the rate-controlling step.