Catalyst hydrothermal deactivation and metal contamination during the in situ catalytic pyrolysis of biomass

Abstract

Lignocellulosic biomass contains small amounts of alkali and alkaline earth metals, which may volatilize during the in situ catalytic pyrolysis of biomass and deposit on the catalyst, affecting its properties. In addition, due to the presence of steam in the process and exposure of the catalyst to high temperatures, hydrothermal deactivation also plays a key role to the catalyst's life span. In this work we studied the effect of hydrothermal deactivation and deactivation by metal contamination of commercial ZSM-5 zeolite based catalyst formulations using two techniques. In the first technique, biomass metal nitrates were spray impregnated on the catalyst at different levels, followed by hydrothermal deactivation of the samples and characterization. In the second technique, hydrothermal deactivation and metal contamination were decoupled by using a hydrothermally stable ZSM-5 sample as a parent material for the preparation of catalyst samples that were exposed to different biomass amounts by carrying out biomass catalytic pyrolysis reaction–regeneration cycles in a bubbling fluidized bed reactor. During spray impregnation, the different metals accumulated on the catalyst at the same rate and increasing metal loading resulted in gradual loss of the surface area and pore volume of the catalyst, eventually leading to complete destruction of the zeolite after a certain threshold. During catalytic pyrolysis however, the biomass metals accumulated at different rates. Potassium accumulated very selectively on the catalyst, while sodium and calcium were less selective. Accumulation of magnesium and iron was not found to increase with increasing exposure to biomass. Cross section examination of the catalyst particles revealed that potassium was deposited evenly throughout the particles, while magnesium and calcium were only detected on the outer surface. Evaluation of the catalyst in pyrolysis tests showed that the presence of biomass metals on the catalyst altered the catalyst's functionality, likely by introduction of some basic sites, which led to the deterioration of its catalytic performance.