Light olefins from biomass-derived butyric acid by tandem deoxygenation reactions

Abstract

The feasibility of using a series of Zn-Zr mixed oxides for the gas-phase production of light olefins from butyric acid, a platform molecule derived from biomass processing, was investigated. These materials with Zn/Zr molar ratios of 0.11–0.66 were prepared by incipient wetness impregnation of Zn on Zr(OH)4 and characterized by BET, XRD and CO2 and NH3 TPD. At full butyric acid conversion conditions, a range of C2-C7 olefins was obtained. The molecular weight of the olefin pool as well as the total olefin yield depend on the catalyst composition and experimental conditions. The catalyst acid-base properties can be tuned for optimal olefin yield by generating a high number of Zn-O-Zr species and oxygen vacancies resulting from incorporation of Zn2+ to the ZrO2 lattice. The best olefin yield coincides with the catalyst showing the highest acid site number and moderate basic properties. Also, the reaction temperature and reactor contact time were varied to improve the olefin yield. A total olefin yield of 60.7% was obtained with a catalyst having 11.1 wt% Zn (Zn/Zr=0.25) at 723 K and 835 h g cat./mol. Ethylene (52%) and isobutene (33%) were the main components of the olefin fraction, the other 15% being pentene, isohexene and heptene. The olefin yield can be further improved to 65.8% by increasing the contact time to 1500 h g cat./mol. Butyric acid transformation proceeds with the intermediate formation of different ketones (heptanone, pentanone and acetone), each of which gives rise to a particular olefin. Isobutene forms through tandem butyric acid ketonization/C-C bond rupture by McLafferty rearrangement/aldol condensation/breaking of the aldol adduct reactions. Ethylene is generated during the McLafferty step. The pathways toward other olefins may involve deoxygenation steps such as reduction of unsaturated bonds by in-situ generated hydrogen species, followed by dehydration. Acetic and propionic acids, formed in minor amounts, participate in further tandem sequences. No catalyst deactivation was observed during standard and extended catalytic tests because water co-feeding and hydrogen generation prevent heavy product formation and active site loss.