Impact of ZSM-5 Deactivation on Bio-Oil Quality during Upgrading of Straw Derived Pyrolysis Vapors

In this work, we provide detailed information on the change in product distribution and bio-oil quality during extended feeding of biomass derived fast pyrolysis vapors over ZSM-5. The effect of catalyst deactivation by coking on the resulting oil product characteristics was clarified in order to determine when the vapor upgrading should be stopped and the regeneration initiated. Obtaining a stable catalyticfast pyrolysis (CFP) oil while maintaining good energy recovery is important within the context of potential coprocessing of these oils with petroleum feedstocks via fluid catalytic cracking (FCC) or hydrotreatmentof the whole CFP oil. Wheat straw derived fast pyrolysis vapors were upgraded in an ex-situ fixed bed reactor containing a steamed ZSM-5 catalyst at 500 °C. Oils were collected bothfor runs starting the upgrading over a fresh (or regenerated) catalyst and for runs which were continued over an increasingly coked zeolite. The oils were characterized for water content, elemental analysis, total acid number (TAN), chemical composition by gas chromatography mass spectrometry with flame ionization detection (GC-MS/FID), size exclusion chromatography (SEC), evaporation characteristics by thermogravimetric analysis (TGA), ¹H nuclear magnetic resonance (NMR), ¹³C NMR, and two-dimensional heteronuclear single-quantum correlation (2D HSQC) NMR. With increasing biomass-to-catalyst mass ratio (B:C), the yield of deoxygenated hydrocarbons decreased, accompanied by a breakthrough of primary pyrolysis vapors leading to an increasing organic liquid yield. The oxygen content of the condensed, phase separated oil fraction increased and the molar O/C ratio of 0.05 and TAN of6 mg KOH/g for oil collected during B:C = 0–1.1 increased to O/C = 0.18 and TAN = 14 mg KOH/g for oil collected during B:C = 3.6–6.2. Oil produced at 90% reduced catalyst amount and B:C = 0–6.5 and 0–12.9 increased the carbon recovery into the oil product to 23% and 27%, respectively but led to an increase in O/C ratio from 0.18 to 0.22, thus approaching the noncatalytic reference case (SiCbed at 500 °C) of O/C = 0.24. Clear differences in the evaporation behavior of the collected oils were observed, with a shift to morevolatile fractions and less charring for products obtained at low B:C ratio. Characterization of the upgraded oils with ¹³C NMR and ¹H NMR indicated a clear enhancement of thearomatics content and a reduction of sugar and aldehyde compounds.The concentration of carbon within carbonyl, carbohydrates, and methoxy/hydroxylgroups was effectively reduced for oils obtained at low B:C ratios. Catalyst characterization was performed with X-ray fluorescence (XRF), ammonia temperature-programmed desorption (NH₃-TPD), N₂ and Ar-physisorption, transmission electron microscopy (TEM),and X-ray diffraction (XRD). After steaming and four repeated upgrading/regenerationcycles corresponding to an accumulated B:C ratio of 40, the zeolite’s concentration of strong acid sites measured by NH₃-TPD(T_des > 275 °C) reduced from 0.43mmol/g for the calcined version to 0.07 mmol/g and the Brunauer–Emmett–Teller (BET) surface area decreased from 468 to 385 m²/g. Thehot gas filter upstream of the zeolite bed was found effective in preventing accumulation of potassium on the catalyst.