Performance-screening of metal-impregnated industrial HZSM-5/γ-Al₂O₃ extrudates for deoxygenation and hydrodeoxygenation of fast pyrolysis vapors

Screening of catalysts for deoxygenation and atmospheric hydrodeoxygenation of fast pyrolysis vapors from biomass (wheat straw) was performed using a micro-scale setup. Steam-treated HZSM-5/Al2O3 extrudates were impregnated with metals by incipient wetness impregnation. 18 different metal modifiers (Pt, Pd, Ru, B, Li, Nb, Ga, Ca, Mg, Ce, Zn, Fe, Ni, Cu, Mo, Co, La, Zr) were tested, of which 14 were tested under inert atmosphere and 10 were tested in H2 containing atmosphere.

The catalysts were evaluated based on the yield of individual product groups (incl. gas and coke), the oxygen content of the vapors, and the yield of vapor compounds containing, zero, one, and two-or-more oxygen atoms. The deactivation was monitored during 16 consecutive vapor pulses and cumulative product properties at B:C ∼4 were compared.

In He atmosphere, a slight improvement of the deactivation behavior and the resulting cumulative vapor properties was observed for impregnation with Mg, B, Fe and Zr, but generally, the benefits were moderate or absent. Ga, Co, Ni, Fe, Cu, Mo, Nb, Pt, Ru, and Pd-modified HZSM-5/Al2O3 were tested in H2-containing atmosphere.

A clear enhancement in catalytic activity resulted for Co, Fe, and Pd, achieving 21–26 % lower oxygen content compared to the unmodified HZSM-5/Al2O3. Even higher deoxygenation activity resulted for Ni, Mo, and Pt impregnated HZSM-5/Al2O3, achieving 35 %, 40 %, and 46 % lower oxygen content compared to HZSM-5/Al2O3.

The results are attributed to a more favorable reaction chemistry such as hydrodeoxygenation and hydrogenation of coke precursors. Coke yields were clearly decreased compared to the parent HZSM-5/Al2O3 for the majority of the metal modifiers under H2 atmosphere, while under He atmosphere coke yields tended to be higher.

Based on the high yields of gasoline-range hydrocarbons and considering catalyst costs, Mo-promoted HZSM-5/Al2O3 appears to be a promising catalyst that should be tested in hydrogen-containing atmosphere at larger scale allowing for further assessment of the liquid yield and properties.