Alumina-embedded HZSM-5 with enhanced behavior for the catalytic cracking of biomass pyrolysis bio-oil: Insights into the role of mesoporous matrix in the deactivation by coke

Abstract

The behavior of HZSM-5 zeolite embedded in γ-Al2O3/α-Al2O3 matrix in the catalytic cracking of biomass pyrolysis bio-oil was studied with special focus on the role of the matrix in the deactivation by coke. The runs were conducted at 450 ºC, space-time of 0.35 gzeolite.h.gfeed−1 and 0.5 h, 1 h, and 2 h on stream. The analysis of reaction products was conducted by in-line GC and GC/MS, and deactivated catalyst samples were extensively characterized by N2 physisorption, NH3-TPD, TPD-TPO, TPD-GC/MS and LDI FT-ICR MS. The alumina-embedded catalyst (Cat−Z30) exhibited greater stability, keeping the bio-oil conversion above 88 % for 0.5 h on stream, in comparison to the bulk HZSM-5 that deactivated rapidly entailing a bio-oil conversion plunge (from 95 % to 40 %). Consequently, the yield of gasoline-range hydrocarbons (> 70 % MAH monoaromatics) was boosted from 18 % to 26 % for the same reaction time. The aqueous liquid byproduct concurrently produced has a suitable composition for H2 production by further steam reforming. The enhanced behavior was caused by the weak-acid mesoporous structure of the matrix playing a dual function: i) diffusive-shell (inwards) role by promoting fragmentation/deoxygenation of bulky molecules (e.g., saccharides and guaiacols) into reactive oxygenates, and deposition of thermally-induced coke away from the zeolite micropores; ii) spread-growth (outwards) role by enhancing the diffusion of hydrocarbons outwards from zeolite crystals, and by promoting the development of heavy polycyclic structures that delay the blockage of active sites in micropores. The results are useful to progress towards the integral valorization of raw bio-oil, the development of active and stable catalysts, and the proposal of regeneration strategies necessary for the scaling up.