Production of aromatic hydrocarbons via catalytic pyrolysis of biomass over fe-modified HZSM-5 zeolites

Authors: Mullen, Charles; Boateng, Akwasi

Submitted to: ACS Sustainable Chemistry & Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/21/2015
Publication Date: 5/22/2015
Publication URL: https://handle.nal.usda.gov/10113/61043
Citation: Mullen, C.A., Boateng, A.A. 2015. Production of aromatic hydrocarbons via catalytic pyrolysis of biomass over fe-modified HZSM-5 zeolites. ACS Sustainable Chemistry & Engineering. 3:1623-1631.

Interpretive Summary: The largest source of renewable feedstock to produce green fuels and chemical products is biomass, including woody materials, herbaceous grasses (e.g. switchgrass) and crop residues (e.g. corn stover, straws). However, in order to use these materials as a feedstock for these products the biomass must first be converted into a more dense material to lower transportation costs. One promising process is to liquefy biomass via fast pyrolysis which produces a product called pyrolysis oil (bio-oil). Pyrolysis oil can be further refined to “green” gasoline and diesel fuels or chemicals that are indistinguishable from those produced from petroleum. However, bio-oil is not a stable liquid, it becomes highly viscous upon storage or heating due to a high concentration of oxygen containing components, complicating its refining. To reduce these components often a catalyst is added to the process and the process is then called catalytic fast pyrolysis (CFP). The most effective class of catalysts found to date for CFP are materials called HZSM-5 zeolites whose acidity and microscopic shape make them ideal for producing non-oxygenated compounds called aromatic hydrocarbons. These compounds can be used as components of gasoline or as chemical feedstocks. In an attempt to improve these catalysts for CFP we added iron to them in small amounts to give them a second chemically active ingredient to aid in the conversion to aromatic hydrocarbons. We found that small amounts of iron can increase the yield of aromatic hydrocarbons from CFP of cellulose (the major component of biomass) and other similar materials. We also found that the selectivity to particular aromatic hydrocarbons such as benzene and naphthalenes increased when iron was added to the catalysts. This information will be of value to those considering using various biomass sources as a feedstock for a pyrolysis based biorefinery producing fuels and chemicals.

Technical Abstract: Iron modified HZSM-5 catalysts were prepared by partial ion exchange of NH4ZSM-5 with Fe (II) at three different loadings (1.4, 2.8 and 4.2 wt%), and their effectiveness for producing aromatic hydrocarbons from cellulose, cellobiose, lignin and switchgrass by catalytic pyrolysis were screened using a microscale pyrolysis reactor coupled with gas chromatography-mass spectrometry (py-GC/MS). Two different catalyst to biomass ratios of 10/1 and 5/1 (w/w) were studied to determine the varying effects at full and partial conversion of the primary oxygenated pyrolysis vapors. Among the four catalysts screened (including the parent HZSM-5), the one loaded with iron at 1.4 wt% Fe [Fe-HZSM-5 (1.4)] produced the largest increase in production of aromatic hydrocarbons from cellulose, cellobiose and lignin. From cellulose, a carbon yield of selected aromatics (benzene, toluene, o,p-xylenes, ethylbenzene, 1,2,4-trimethylbenzene, naphthalene and 2-methylnapthalene) of ~18 % was achieved with Fe-HZSM-5 (1.4), and for cellobiose the carbon yield of selected aromatics using Fe-HZSM-5 (1.4) was 25%. For switchgrass, Fe-HZSM-5 (1.4) catalyst produced a similar carbon yield of aromatics as the standard HZSM-5 (~17%) but higher loadings of Fe decreased the yield. However, for all of the starting materials studied, the chemical selectivity of the aromatic products changed with addition of Fe to the catalyst. Benzene and naphthalenes were favored for the iron containing catalysts compared with the standard HZSM-5, while the selectivities for p-xylene, ethylbenzene and trimethylbenzene were decreased with the addition of iron.