Supercritical Water Assisted Zeolite Catalyzed Upgrading of Hydrocarbons

Azadeh Zaker, Worcester Polytechnic Institute


Previous studies have successfully used near and supercritical water (SCW) for cracking and desulfurization of heavy crude oil and bio-oil, suppressing coke formation as a low-value by-product. Some of these studies benefited from using zeolite catalysts to increase the activity and selectivity toward targeted products; however, in depth studies are required to identify the role of water on zeolite catalysis under supercritical condition. Using three common zeolites, ZSM-5, HY, and β for supercritical water cracking of dodecane at 400°C, 24±2 MPa (in a 100 ml batch reactor), we showed that ZSM-5 is the only catalyst that partially retains its crystalline structure and activity under hydrothermal conditions. Further characterization of the ZSM-5 (used under 50/50 wt% SCW/dodecane feed) revealed 95% decrease in Brønsted acid site (BAS) density and 80% decrease in microporous area after 2 h reaction time. However, compared to the runs where SCW was absent, the apparent dodecane cracking rate constant in SCW decreased only by a factor of 2.6. Examining catalytic activity of ZSM-5 degradation products and re-using ZSM-5 showed that the unexpected activity cannot be ascribed to ZSM-5 degradation products. Using a group-type model, we showed that SCW accelerated gas and suppressed coke formations. Additionally a coke gasification pathway was suggested to account for formation of CO and CO2 in the presence of SCW. Additional experiments with two different ZSM-5 particle sizes suggested that dodecane cracking reaction is diffusion-limited in the absence of SCW and reaction-limited in its presence. Zero length chromatography of calcined and hydrothermally treated ZSM-5 showed 10 times greater apparent diffusivity for un-treated catalyst. This, according to Weisz-Prater analysis, suggested a 250 times greater dodecane surface concentration in the absence of SCW. We successfully optimized the water content of feed (5-15 wt%) to decrease the destructive effects of SCW on the structure, increase the selectivity toward BTEX products and eliminate coke formation.