Deoxygenation of Wheat Straw Fast Pyrolysis Vapors using HZSM-5, Al₂O₃, HZSM-5/Al₂O₃ Extrudates, and Desilicated HZSM-5/Al₂O₃ Extrudates

Abstract

19 HZSM-5 extrudates, its two constituents (HZSM-5 zeolite and alumina binder), and SiC for 20 reference were tested after steam treatment for the upgrading of wheat straw fast pyrolysis (FP) 21 vapors from an ablative bench scale system. In addition, mesoporosity was added to the HZSM-5 crystals of the zeolite/Al 2 O 3 extrudates by desilication, which decreased the microporous volume 23 and led to enhanced weak acidity and less strong acidity compared to the parent extrudates. For 24 increasing biomass-to-catalyst ratios (w/w, B:C), oils were collected and analyzed for elemental 25 composition, total acid number (TAN), moisture, molecular weight, evaporation characteristics, 26 and chemical composition by gas chromatography mass spectrometry with flame ionization 27 detection (GC-MS/FID), 1 H nuclear magnetic resonance (NMR), 13 C NMR, and two-dimensional 28 heteronuclear single-quantum correlation (2D HSQC) NMR. Compared to Al 2 O 3 , catalysts 29 containing HZSM-5 promoted aromatization and limited the coke formation due to its shape 30 selective micropores. Nevertheless, Al 2 O 3 was effective in deoxygenation. At B:C ~7, 23 wt-% 31 carbon/25 % energy recovery in the oil fraction was obtained while reducing the oxygen content 32 by 45 % relative to a thermal reference oil fraction obtained over a SiC bed. As such, Al 2 O 3 offers 33 certain benefits compared to HZSM-5 based catalysts due to its lower cost and better hydrothermal 34 stability with respect to acidity. At a catalyst temperature of 500 °C, the introduction of mesopores 35 to HZSM-5 extrudates led to higher energy recovery as oil compared to the parent HZSM-5 36 extrudates. At B:C = 6.3, 23 wt-% carbon/26% energy recovery in the oil phase was achieved 37 while removing 45% of the oxygen functionalities relative to the thermal reference bio-oil.38 Compared to deep deoxygenation for direct hydrocarbon production, mild deoxygenation 39 improved the energy recoveries of the oil fractions and appears viable for pretreating pyrolysis 40 vapors before co-processing bio-oils with fossil oil in refineries.

Counteracting Rapid Catalyst Deactivation by Concomitant Temperature Increase during Catalytic Upgrading of Biomass Pyrolysis Vapors Using Solid Acid Catalysts

“…The performance of a steam-treated HZSM-5/γ-Al 2 O 3 extrudate as a catalyst for the deoxygenation of wheat straw fast pyrolysis vapors has been reported previously [28,39]. For the present work, HZSM-5/γ-Al 2 O 3 extrudates were modified with 0.5 wt% phosphorus in order to improve the hydrothermal stability of the HZSM-5 component, as reported in several studies [40][41][42][43][44][45][46].…”

“…The physicochemical properties of γ-Al 2 O 3 and HZSM-5/γ-Al 2 O 3 extrudates were detailed in previous work [28]. Table 1 provides an overview of the textural properties and the acidity of the different catalysts (steamed) that were tested in the present work.…”

“…The slightly higher mesoporous volume compared to the total pore volume (directly determined from adsorption data) for Al 2 O 3 is attributed to the uncertainties of the BJH model calculations. The parent HZSM-5/γ-Al 2 O 3 contained 0.12 cm 3 /g micropores [28]. Although a slight narrowing of the micropore width was observed from the high-resolution Ar physisorption data ( Figure S1a), the microporous volume remained similar after the addition of phosphorus and the steamed P/HZSM-5/γ-Al 2 O 3 , and HZSM-5/γ-Al 2 O 3 had similar acidity (see Table 1 and Figure S2).…”

“…The coke combusted under an oxidizing atmosphere in the temperature range 350-650 °C, as shown by the differential thermogravimetric (DTG) curves in Figure 7. Coke on γ-Al2O3 combusted more readily (main weight loss around 475 °C), while the coke on P/HZSM-5/γ-Al2O3 that combusted at higher temperatures, is attributed to coke in the zeolite component [28,57]. Table 2 provides an overview of the carbon recovery of different product groups.…”

“…In the present work, the focus lies on upgrading biomass-derived fast pyrolysis vapors outside the pyrolysis reactor in a close-coupled catalyst reactor prior to vapor condensation (often termed ex situ catalytic fast pyrolysis). This process configuration can prevent the poisoning of catalytic active sites due to ash species [27][28][29] and allows for the independent temperature control of the pyrolysis and catalytic reactor. Using HZSM-5 as a catalyst, the selectivity of oxygen-free hydrocarbons (HCs) is highest in the initial upgrading phase over a fresh catalyst (at high rates of coke and light gas formation from cracking reactions and thus low organic liquid yield), and then gradually deteriorates due to the incomplete conversion of oxygenates.…”