Screening of transition metal doped ZSM-5 zeolite for catalytic cracking of biomass pyrolysis oil

CRESPO, I.; CHIOSSO, M.E.; MERLO, A.; GAYUBO, A. G.; VALLE, B.

BUENOS AIRES

Congreso; 11th World Congress of Chemical Engineering (2023); 2023

ASOCIACION ARGENTINA DE INGENIEROS QUIMICOS

The development of new processes for obtaining hydrocarbons from renewable resources must consider the integral use of biomass as a competitive replacement for petroleum-derived products. The biomass pyrolysis oil (bio-oil) is a complex mixture of oxygenated compounds that offers a wide range of alternatives, among which the catalytic cracking (CC) is an attractive process in terms of operation requirements since it is conducted at atmospheric pressure without H2 supply. Besides, it is a versatile process can be targeted towards production of fuels and/or chemicals (C2-C4 olefins and monoaromatics) depending on the catalyst and operating parameters. Zeolites have revealed as effective catalysts in the conversion of biomass pyrolysis vapors/bio-oil into hydrocarbons [1]. The zeolite acidity (usually modified by incorporating metal cations) is a key factor that determines the products selectivity. The upgrading of bio-oil follows complex metal- and acid-catalyzed reaction pathways (hydrogenation, dehydration, isomerization, cracking, etc.) which are strongly affected by the selection of the metal and zeolite functionalities [2].In this work, we evaluate the activity and product selectivity of ZSM-5 zeolite doped with transition metals (Cr, Fe, Zn) in the catalytic cracking of raw bio-oil. Unlike most of the studies conducted with bio-oil model compounds, this work deals with real bio-oil feed, which represents a contribution towards the industrial implementation of the CC process.The raw bio-oil (stabilized with 20 wt% methanol) was fed into the continuous two-step catalytic cracking (TS-CC) equipment operating at atmospheric pressure. The volatile oxygenates exiting the first thermal unit were converted in-line in the fluidized bed reactor (at 450 ºC), where the catalyst is located (space-time 0.7 gcatalysth/gfeed).Three catalysts (named Cr-Z30, Fe-Z30 and Zn-Z30) were prepared by loading the bulk ZSM-5 zeolite (SiO2/Al2O3 =30) with 2 wt% of each metal by incipient wetness impregnation with nitrate solutions as metal precursors. Each metal-doped zeolite was embedded in -Al2O3 mesoporous matrix, then sieved (150-300 m) and calcined (575°C). Experiments with in-situ reduced catalysts (10 ºC/min up to 450 ºC in H2/N2 flow) were also conducted in order to analyze the effect of metal state and gain knowledge on the active metal species for CC of bio-oil. The conversion of oxygenates, and the yield and selectivity of hydrocarbon lumps (C2-C4 and C5+) were quantified from the on-line GC analyses of the products stream (Agilent 490 micro-GC) and GC/MS analyses (Shimadzu GC/MS QP2010) of the liquid collected.The results reveal poor activity of Zn-Z30, with notably lower feed conversion (56 %) compared with bulk zeolite (96%), whereas the conversion achieved with Cr-Z30 and Fe-Z30 is just slightly lower ( 89 %). Metal doping causes changes in product distribution so that selectivity to C2-C4 olefins is increased (by 18 % with Cr and 14 % with Fe). Besides, both yield and selectivity to monoaromatics (BTXE + alkylbenzenes) are increased (by 11 %) with Fe-Z30 catalyst. The reduced catalysts promote the dehydrogenation and decarbonylation of oxygenates, at the expense of dehydration and hydrocarbon formation reactions.