Moderate pressure boosts the aromatization activity of HZSM-5 zeolite during catalytic pyrolysis of lignocellulosic materials

Lignocellulose pyrolysis has been deeply investigated in the past years as a potential route for the production of advanced biofuels and biochemicals from residues and wastes. However, the effect of pressure has rarely been addressed in catalytic pyrolysis processes. Accordingly, the current work reports a comprehensive study on the pressure influence during the catalytic pyrolysis of different lignocellulosic materials (oak wood, cellulose, hemicellulose, and lignin) using nanocrystalline HZSM-5 zeolite as a catalyst and a downflow fixed-bed reactor. In thermal pyrolysis tests, pressure increase affects negatively the bio-oil* (organic components in the liquid products) since it reduces its yield in favor of the production of char and gases. However, in the case of the catalytic pyrolysis tests, working under moderate pressure (10 bar) has very positive effects on bio-oil upgrading, minimizing its oxygen concentration (< 10 wt.%) and sharply increasing the carbon yield of aromatic hydrocarbons, while the undesirable coke deposited on the catalyst surface can be even reduced. This finding denotes the occurrence of a remarkable synergistic effect between the pyrolysis pressure and the catalyst loading. These outstanding results are attributed to a more efficient activity of the zeolite under mild pressurization since both oxygenated species and light olefins are more strongly adsorbed over the catalyst surface, facilitating their subsequent transformation into aromatic hydrocarbons. The sharp enhancement of the deoxygenation and aromatization activity does not involve higher coke deposition over the zeolite surface, thus attenuating one of the major limitations of the lignocellulose catalytic pyrolysis process.