Kinetics of the oxidative dehydrogenation(ODH)of methanol to formaldehyde by supported vanadium-based nanocatalysts

摘要

The aim of the present contribution was to develop a detailed kinetic analysis of the oxidative dehydrogenation(ODH)reaction of methanol to formaldehyde on a nano-structured supported vanadium oxide catalyst,selected in a preliminary screening.The chosen vanadium catalyst,supported on TiO2/SiO2,has been prepared by grafting vanadyl alkoxide,dissolved in dioxane,and characterized by BET,XRD,Raman,XPS and SEM.An exhaustive set of experimental runs has been conducted in an isothermal packed bed tubular reactor by investigating several operative conditions,such as:temperature,contact time,methanol/oxygen feed molar ratio and water feed concentration.Depending on the operative conditions adopted,the main products observed were formaldehyde and dimethoxymethane while lower amounts of methyl formate and CO2 were also found.At low contact time,the main reaction product was dimethoxymethane which was then converted into formaldehyde through the reverse equilibrium reaction with water.As a confirmation of this observation,a peculiar behaviour was detected consisting in an increase of selectivity to formaldehyde by increasing methanol conversion.The obtained experimental data of methanol conversion and selectivity towards products were modelled by means of an integral reactor model and the related kinetic parameters were determined by non-linear regression analysis.The adopted reaction rate expressions were of the Mars van Krevelen-Langmuir Hinshelwood type and a good agreement was found between the model theoretical prediction and the experimental data.A reaction mechanism and a detailed reaction scheme(rake-type)were proposed for methanol ODH on a nano-structured catalyst that were able to interpret correctly the collected experimental observations.