Hexane activation over vanadium modified zeolite ZSM-5

The influence on the form of ZSM-5, vanadium content and the elimination of the exterior surface, on the activity and selectivity of n-hexane oxidation was studied using a fixed bed reactor. Blank reactor studies (carborundum packed reactor) showed no conversion below 450 °C with the highest conversion (8 %) at 500 °C. The dominant products were found to be carbon oxides (Sel./% = 90) with minor selectivities to the hexene isomers (7 %) with the remainder being cracked products, THF and benzene. H-ZSM-5 with different SiO₂/Al₂O₃ ratios (100 and 320) and Na-ZSM-5 (SiO₂/Al₂O₃ ratio of 100) were tested under non-oxidative conditions. As the ratio of the SiO₂/Al₂O₃ increased the aluminium content decreases and so too does the cracking ability of the zeolite (i.e. yield of cracked products dropped from 36 to 8 %). However, the use of the Na- form of the ZSM-5 zeolite completely eliminated acid cracking and therefore this system was further investigated. Na-V-ZSM-5 (~1 % loading) was synthesized using the solid state ion-exchange method. Time on stream experiments (fresh batch of catalyst for each experiment with sampling at the same time for a period of 24 h) were conducted and temperature (350, 400 and 450 °C), contact time (0.5, 0.8, 1.1 and 1.5 s) and fuel/air ratios (0.7, 1.3 and 2) were varied. The optimum conditions (Conv./% = 39) for terminal functionalised products were found to be at 400 °C at a contact time of 1.1 s and a fuel air ratio of 1.3. With the lower fuel air ratio of 0.7 (oxygen rich conditions), hexanal formation was favoured.