Origin of carbon in aromatic and olefin products derived from HZSM-5 catalyzed co-pyrolysis of cellulose and plastics via isotopic labeling

Authors: Dorado, Christina; Mullen, Charles; Boateng, Akwasi

Submitted to: Applied Catalysis B: Environmental
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/1/2014
Publication Date: 7/23/2014
Citation: Dorado, C., Mullen, C.A., Boateng, A.A. 2014. Origin of carbon in aromatic and olefin products derived from HZSM-5 catalyzed co-pyrolysis of cellulose and plastics via isotopic labeling. Applied Catalysis B: Environmental. 162:338-345.

Interpretive Summary: The properties of bio-oil produced from the fast pyrolysis of biomass are not compatible with current petroleum based fuels and efforts to improve these properties can be accomplished by incorporation of a catalyst in the fast pyrolysis process. Unfortunately, catalysts lose their activity overtime in the presence of biomass as it is converted to bio-oil due to the accumulation of carbon and oxygen based molecules found in the biomass. Sources of hydrogen rich carbon, such as plastics, have been utilized to reduce the deactivation of the catalyst and produce bio-oils with a higher yield of desirable compounds. Understanding how this hydrogen rich carbon source comes together with biomass to produce these desirable compounds is limited. In an effort to reveal the mechanisms involved in the production of pyrolysis oil from the catalytic fast pyrolysis of mixtures of biomass and plastic, we used cellulose labeled with a carbon isotope, 13C, for the biomass portion of the blend. This makes the carbon that comes from cellulose distinct from carbon that comes from plastic when using mass spectroscopy. This data was analyzed and allowed us to determine how much of the fuel product came from cellulose and how much came from plastic. These distributions were then used to draw conclusions about the mechanisms involved in the product formation. The results of this work will be useful to those interested in converting biomass to fuels using thermochemical processes.

Technical Abstract: Catalytic pyrolysis over HZSM-5 is an effective method for the conversion of biomass to aromatic hydrocarbons, albeit with low yield and short catalyst lifetimes. Addition of co-reactants rich in carbon and hydrogen can enhance yield and possibly increase catalyst lifetimes by reducing coke formation. Particularly, the catalytic co-pyrolysis of plastic and biomass has been shown to enhance conversion to aromatic hydrocarbons, and also offers a method for productive disposal of waste agricultural plastics. In an effort to determine the origin of the carbon (plastic or biomass) in the products from this catalytic co-pyrolysis, mixtures of uniformly labeled 13C cellulose and non-labeled plastic including polyethylene terephthalate, polypropylene, high density polyethylene, low density polyethylene and polystyrene were subjected to catalytic fast pyrolysis (CFP) at 650 oC in the presence of HZSM5. A micro pyrolyzer coupled with GC/MS (py-GC/MS) advised product distributions and mass spectral data was used to determine the distribution of biogenic carbon and plastic derived carbon in the products. The results demonstrate that aromatic hydrocarbon products formed from the CFP of mixtures of cellulose and plastic are composed mostly of molecules containing carbon of mixed origin. Data on the distribution of 13Cx12Cy from the products followed in this study show that polyolefin mixtures with cellulose favor the formation of alkyl benzenes that incorporate carbon from both sources. Utilization of aromatic polymers (polystyrene or polyethylene terephthalate) is more selective for formation of naphthalenes with carbon derived from both products. The distribution of various13Cx12Cy products is used to suggest active mechanisms that result in the formation of the observed products.