The influence of water vapour on the photocatalytic oxidation of cyclohexane in an internally illuminated monolith reactor

Abstract

This paper discusses effects of humidity on photocatalytic cyclohexane oxidation performed in an internally illuminated monolith reactor equipped with an immobilised layer of approximately 3 µm titania (Hombikat uv100). Using dry nitrogen containing 10-20% of oxygen, cyclohexanone is produced with high selectivity (> 90%) over cyclohexanol. The photocatalytic monolith deactivates within 80 minutes of operation. Regeneration of activity of such deactivated monolith is possible by air treatment at 450°C.When the applied nitrogen/oxygen gas is humidified, stable ketone production rates are obtained around 5 · 10 −6 mol h −1 at an optimised relative humidity of 65 %. Ketone over alcohol selectivity is lower in humidified conditions, the ratio of the cyclohexanol/cyclohexanone production rates increases from 0.4 to 1.0 as a function of increasing humidity from 30% to 90%.Rapid transients in water vapour content lead to relatively slow changes in concentration of in situ produced cyclohexanone and cyclohexanol. The observation of these changes are used to explain the effect of humidity on reactor performance.

Recent Progress in Semiconductor Photocatalysis for Organic Fine Chemical Synthesis

“…In studies conducted at dry nitrogen containing 10-20% of oxygen, cyclohexanone is produced with high selectivity (>90%). In humidified nitrogen/oxygen gas, the cyclohexanol selectivity was increased [23]. This suggests the participation of surface hydroxyl group in the reaction mechanism.…”

Towards Scaling-up Photocatalytic Process for Multiphase Environmental Applications: A Review

“…More conventional geometries adopting honeycomb monoliths have been already commercialized for the photocatalytic purification of air (Figure 7a,b). Dijk et al [120] have designed an internally illuminated honeycomb-based photoreactor for air treatment (Figure 7d) with high surface-to-volume ratio for air purification. This reactor is made for multiphase use [110], wherein the photocatalyst is coated via sol-gel in the inner walls of monolith channels.…”

“…(c) TiO2-coated transparent monolith for air purification, reproduced with permission from reference [124]. (d) Photographs of monolith reactor, equipped with light emitting fibers inserted into the monoliths, for multiphase photocatalytic treatment, adapted with permission from [120]. (e-f) Fluidized bed photoreactor with external (e) and internal (f) irradiation, reproduced with permission from reference [119].…”

“…The deactivation of photocatalyst systems is a serious issue that should be solved for a continuous processing [34]. It takes place during the photocatalytic reaction due to irreversible adsorption of recalcitrant by-products or due site blocking by carbonaceous residues or dust particles on the surface [120]. In gas phase photocatalysis, deactivation is a more pressing concern than in liquid phase, as there is no water solvent to help to remove products and intermediates from the surface.…”

“…Deactivation is most severe in the presence of aromatic compounds [135,136], however it has been reported for a wide range of species [137]. For instance, van Dijk et al reported photocatalyst deactivation within 80 min of photocatalytic operation for the oxidation of cyclohexane [120]. Despite the importance of this topic for the commercial application of photocatalytic technologies, comparatively few studies have investigated the mechanisms of deactivation, regeneration and the photocatalyst lifetime.…”