Real-time monitoring of the deactivation of HZSM-5 during upgrading of pine pyrolysis vapors†
The conversion of pine pyrolysis vapors over fixed beds of HZSM-5 catalyst was studied as a function of deactivation of the catalyst, presumably by coking. Small laboratory reactors were used in this study in which the products were identified using a molecular beam mass spectrometer (MBMS) and gas chromatography mass spectrometry (GCMS). In all of these experiments, real-time measurements of the products formed were conducted as the catalyst aged and deactivated during upgrading. The results from these experiments showed the following: (1) Fresh catalyst produces primarily aromatic hydrocarbons and olefins with no detectable oxygen-containing species. (2) After pyrolysis of roughly the same weight of biomass as weight of catalyst, oxygenated products begin to appear in the product stream. This suite of oxygen containing products appears different from the products formed when the catalyst is fresh and when the catalyst is completely deactivated. In particular, phenol and cresols are measured while upgrading pine, cellulose and lignin pyrolysis vapors, suggesting that these products are intermediates or side products formed during upgrading. (3) After the addition of more pyrolysis vapors, the product stream consists of primary vapors from pine pyrolysis. Catalyst samples collected at various points during deactivation were analyzed using a variety of tools. The results show that carbon build-up is correlated with catalyst deactivation, suggesting that deactivation is due to coking. Further, studies of nitrogen adsorption on the used catalyst suggest that coking initially occurs on the outside of the catalyst, leaving the micropores largely intact. From a practical point of view, it appears that based upon this study and others in the literature, the amount of oxygen in the upgraded products can be related to the level of deactivation of the HZSM-5 catalyst, which can be determined by how much pyrolysis vapor is run over the catalyst.