Gas-Phase Hydrodeoxygenation of Benzaldehyde, Benzyl Alcohol, Phenyl Acetate, and Anisole over Precious Metal Catalysts

Abstract

The aim of this work is to study the gas-phase hydrodeoxygenation (HDO) of bio-oil-related platform molecules over precious metal catalysts (Pt and Pd on alumina) at moderate temperature (325 °C) and low pressure (0.5 MPa). Bio-oils consist of a complex mixture of compounds, many of which are aromatic oxygenates, and have been modeled here by four model compounds with different oxygenated functional groups: benzaldehyde (aldehyde), benzyl alcohol (alcohol), phenyl acetate (ester), and anisole (ether). First, the stability and activity of the catalysts in the HDO of the different compounds have been studied in an isothermal fixed-bed reactor. The performance of the Pt catalyst is better than that of the corresponding Pd catalyst (highest activity and selectivity). The compound more refractory to HDO over Pt was benzyl alcohol. Both catalysts suffer from strong deactivation at the beginning of the tests, mainly by the formation of carbonaceous deposits. The deactivation is particularly strong for the HDO of benzyl alcohol over the Pd catalyst. Second, a kinetic study has been carried out varying the space time (0−1 (kg cat s)/mol). Kinetic models, based on reaction networks derived from the product distributions, have been proposed, and the kinetic constant fit to the experimental data by least-squares regression.

Hydrodeoxygenation of Lignin-Derived Phenols: From Fundamental Studies towards Industrial Applications

“…In the work of Prasomsri et al [36] the corresponding reactivity was the following: diphenyl > anisole > guaiacol > phenol > o-cresol [36]. Thus, dehydroxylation of the phenyl ring is difficult [37,39,48,61]. Reactivity of anisole higher than guaiacol reported in [39] similarly to the work of Prasomsri et al [36], can be explained by a higher bond dissociation energy required for the aromatic hydroxyl compared to methoxyl (Table 3).…”

“…Promising results have been obtained in HDO of phenyl acetate over Pt/Al 2 O 3 at 325 • C under 5 bar hydrogen. After 25 h time-on-stream a stable period of 40 h with constant activity followed [61]. Phenol formation was rapid from phenyl acetate, whereas the consecutive benzene formation was rather slow.…”

“…Transanthole contains both propyl and methoxy substituents and it is easy to deoxygenate (Table 1, entry 18). Phenylacetate formed in bio-oil via a reaction between acetic acid and phenol underwent HDO at 325 °C under 5 bar hydrogen over Pt/Al2O3 catalyst [61]. At elevated temperature due to thermodynamic limitations the main product was phenol ( Figure 4, Table 1, entry 19) [61].…”

“…It was also stated in [37] that ReOx/CNF is a promising catalyst favoring C-O bond cleavage and forming phenol at 300 • C under 50 bar hydrogen (Figures 5 and 6) without breaking C-C bond being thus hydrogen efficient and carbon atom economical catalyst. Demethoxylation was faster compared to transalkylation in continuous HDO of a phenylacetate and anisole mixture at 325 • C under 5 bar hydrogen over Pt/C catalyst [61] and thus it was proposed that anisole is converted more preferentially to phenol than to m-cresol.…”

“…Catalyst stability, activity as a function of time-onstream [31,33,36,39,42,43,47,48,58,59,61], reuse [30,54,[56][57][58]63], regeneration [34,36], origin of the catalyst deactivation [30], chemical composition of coke [31], thermogravimetrical analysis of the spent catalysts [62], modelling of deactivation [33] are also very important from the industrial point of view. Since the real bio-oils contain in addition to lignin derived compounds also water [59], acetic acid or furfural [56] as well several catalyst poisons, such as chlorine, potassium and sulphur containing compounds [47], it is very important not only to investigate HDO of model compounds but also the real feed [30] using different approaches, such as a single component feed, co-feed studies, real bio-oil [30], simulated bio-oil [40], adsorption studies [48,79] as well as continuous operation [33,36,39,42,43,48].…”