Abstract
This study provides new insights into sustainable fuel production by upgrading bio-derived oxygenates by catalytic hydrodeoxygenation (HDO). HDO of ethylene glycol (EG), cyclohexanol (Cyc), acetic acid (AcOH), and phenol (Phe) was investigated using a Ni-MoS2/MgAl2O4 catalyst. In addition, HDO of a mixture of Phe/EG and Cyc/EG was studied as a first step towards the complex mixture in biomass pyrolysis vapor and bio-oil. Activity tests were performed in a fixed bed reactor at 380–450 °C, 27 bar H2, 550 vol ppm H2S, and up to 220 h on stream. Acetic acid plugged the reactor inlet by carbon deposition within 2 h on stream, underlining the challenges of upgrading highly reactive oxygenates. For ethylene glycol and cyclohexanol, steady state conversion was obtained in the temperature range of 380–415 °C. The HDO macro-kinetics were assessed in terms of consecutive dehydration and hydrogenation reactions. The results indicate that HDO of ethylene glycol and cyclohexanol involve different active sites. There was no significant influence from phenol or cyclohexanol on the rate of ethylene glycol HDO. However, a pronounced inhibiting effect from ethylene glycol on the HDO of cyclohexanol was observed. Catalyst deactivation by carbon deposition could be mitigated by oxidation and re-sulfidation. The results presented here demonstrate the need to address differences in oxygenate reactivity when upgrading vapors or oils derived from pyrolysis of biomass.
Highlights
Fast pyrolysis is a well-known method for converting solid, lignocellulosic biomass, such as wood and straw, into bio-oil, a potential liquid hydrocarbon fuel [1]
As the conversion of cyclohexanol in the presence of ethylene glycol was performed at a four times higher residence time compared to the experiment with pure cyclohexanol, these results show that ethylene glycol inhibited the conversion of cyclohexanol significantly
The results demonstrate a pronounced different reactivity of the compounds in HDO, ranging from the highly reactive acetic acid, which rapidly coked up the reactor inlet, to phenol, which only underwent limited alkyl substitution reactions with low yields
Summary
Fast pyrolysis is a well-known method for converting solid, lignocellulosic biomass, such as wood and straw, into bio-oil, a potential liquid hydrocarbon fuel [1]. Different catalytic processes can be coupled with fast pyrolysis in order to improve the heating value and the stability of bio-oil. Catalysts 2019, 9, 521; doi:10.3390/catal9060521 www.mdpi.com/journal/catalysts (HDO) [5,6] This downstream hydroprocessing can be performed either on pyrolysis oil vapors or on the condensed bio-oil. Several HDO studies on various model compounds, in particular phenolic compounds, have shown that a broad range of catalysts can be used for bio-oil HDO. An overview of these studies can be found in recent review articles [5,6,7]. The reactivity, and the instability, of these functionalities has been mapped out in various studies [5,11,12], indicating that the most reactive bio-oil constituents originate from the cellulosic part of biomass
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