Deoxygenation of wheat straw fast pyrolysis vapors over Na-Al₂O₃ catalyst for production of bio-oil with low acidity

Catalytic upgrading of pyrolysis vapors from wheat straw over Na2CO3 impregnated γ-Al2O3 was studied as a promising route to biofuels. The Na species were homogenously distributed on the support and created a basicity of ~0.02 mmol CO2/g catalyst, at 80% lower catalyst acidity. Analytical pyrolysis using a micro-pyrolyzer showed that Na-Al2O3 particularly decreased the yield of acids via ketonization, which was confirmed by feeding carboxylic acid model compounds.

The presence of Na decreased the coke yield and catalyzed the coke combustion, decreasing the combustion temperature by ~100 °C. Subsequently, 100 g Na-Al2O3 catalyst was tested in an ablative bench scale fast pyrolysis unit where ~5 kg of wheat straw was pyrolyzed and the vapors passed the catalytic reactor during six reaction/regeneration cycles.

In agreement with the micro-pyrolyzer results, Na-Al2O3 was highly effective in reducing the acidity of the bio-oils. Total acid numbers (TAN) as low as ~1–4 mg KOH/g could be maintained up to high B:C ratios of ~13. For a given TAN, this allowed operating to higher B:C ratios and provided higher oil yields compared to using acidic catalysts such as γ-Al2O3 and HZSM-5 zeolite for vapor treatment.

At bio-oil energy recoveries of ~60–70% relative to raw bio-oil, the deoxygenation was comparable to the acidic catalysts. Operation to higher B:C ratios allowed increasing the energy recovery to ~85% relative to the non-treated bio-oil while still obtaining a good deoxygenation performance of ~60%.

Despite the hydrothermal conditions during reaction and oxidative regeneration, the activity of Na-Al2O3 was regained by coke combustion, the Na remained well dispersed on the support, and the catalyst maintained its capacity for CO2 adsorption at 500 °C after six reaction/regeneration cycles.