Gas-phase hydrodeoxygenation of guaiacol over Fe/SiO₂ catalyst

Abstract

Lignin could be an important green source for aromatic hydrocarbon production (benzene, toluene and xylenes, BTX). Catalytic hydrodeoxygenation (HDO) of guaiacol was studied over Fe/SiO₂ as a model reaction of lignin pyrolysis vapours hydrotreatment. The catalytic conditions were chosen to match with the temperature of never-condensed lignin pyrolysis vapours. The catalyst was characterised by XRD, Mössbauer spectroscopy, N₂ sorption and temperature programmed oxidation. A comparison is made with a commercial cobalt-based catalyst. Cobalt-based catalyst shows a too high production of methane. Fe/SiO₂ exhibits a good selectivity for BT production. It does not catalyse the aromatic ring hydrogenation. Temperature (623–723 K) and space time (0.1–1.5 g_cat h/g_GUA) influence the aromatic carbon–oxygen bond hydrogenolysis reaction whereas H₂ partial pressure (0.2–0.9 bar) has a minor influence. 38% of BT yield was achieved under the best investigated conditions. Reaction mechanisms for guaiacol conversion over Fe/SiO₂ are discussed.

Introduction

Lignin is a natural polymer of methoxylated phenylpropane units and it is the second most important compound in lignocellulosic biomass (about 25 wt.%) [1]. It is industrially available at low prices from cellulose pulping processes (fuel value of about $0.04 per pound on a dry basis depending on the technical–economical scenario) [2]. Lignin is today mainly converted to heat and/or power by combustion. Lignin valorisation into aromatic hydrocarbons (benzene, toluene and xylene, BTX) represents a key route for lignocellulosic biorefinery (see Fig. 1) [1], [2], [3], [4].

BTX are widely used as fuel additives and solvents and are starting blocks for several molecules [1], [2], [5]. Conventional technologies for BTX production are: naphtha reforming (C₆–C₁₂; over Pt/Al₂O₃), C₂–C₆ paraffins conversion on Gallium promoted ZSM-5, coal pyrolysis and methanol-to-gasoline Mobil process on ZSM-5.

Catalytic pyrolysis was proposed as a single-step lignin conversion process [6], [7], [8], [9], [10], [11]. The catalyst is mixed with lignin before pyrolysis. Different lignins were mixed and pyrolysed with NH₃-treated H-ZMS-5 or CoO-MoO₃/Al₂O₃ in a pyroprobe [8]. With the zeolite catalyst lignin is first cracked and deoxygenated to C₂–C₆ olefins which then are converted into BTX. With CoO-MoO₃/Al₂O₃ catalyst, lignin is thought to depolymerise to phenols, and then phenols are deoxygenated on CoO-MoO₃ sites to aromatic hydrocarbons [8].

One of the main drawbacks of the direct catalytic pyrolysis is the difficulty to recover and regenerate the catalyst (mixed with the char) [12]. Moreover, catalytic pyrolysis involves many physical–chemical phenomena that are difficult to be monitored in the same reactor.

Lignin can also be valorised in two-step processes consisting in a depolymerisation of lignin and a treatment of depolymerised lignin (lignin bio-oil). Several techniques were proposed for lignin depolymerisation: pyrolysis, liquefaction (with a huge variety of solvents and catalysts), oxidation, electrochemical treatments, etc. [1]. Pyrolysis and liquid phase hydrogenation were the most studied [1]. Liquid phase hydrogenation consists on the high pressure (50–100 bar, 523–723 K) treatment of lignin in a solvent like an aliphatic alcohol, water [13], [14] or phenol [15], [16] under H₂ and/or a hydrogen donor (like tetralin). The main disadvantages of liquid treatments are: the consumption of solvent, the cost of high pressure equipment and the difficulties to separate catalyst, char, products, solvent and non-reacted lignin from the reaction media. Lignin pyrolysis produces permanent gases (H₂, CO, CO₂, CH₄), a fluffy char and a complex mixture of condensable compounds rich in OMACs (oxygenated monoaromatic compounds) [12], [17], [18], [19]. Faix et al. [20] studied the products of the non-catalytic pyrolysis of lignin at 600 °C in a fixed bed reactor. 15–30 wt.% of OMACs was obtained using different lignins.

The catalytic conversion of lignin pyrolysis oils (or model compounds like guaiacol) was extensively investigated in the liquid phase under H₂ high pressure [12], [13], [21], [22], [23]; and in the gas phase with H₂ [24], [25], [26], [27], [28], [29], [30] or without H₂ [31].

OMACs are generated in the gas phase during lignin pyrolysis in a reactor set at a temperature of about 773 K [17], [18], [19], [32]. Once condensed, the re-evaporation of bio-oils is difficult since bio-oils are very reactive and cause fouling of pipes. In addition, working on the liquid phase requires high pressures (that means costly equipment) and the condensation step after pyrolysis is detrimental for the heat recovery of the entire process [17]. For these reasons, the vapour phase hydrotreatment of OMACs before condensation seems a better route.

This paper is a contribution on the hydrodeoxygenation (HDO) of OMACs, using guaiacol as a model compound. Guaiacol is a minor but significant component in a very complex mixture in lignin bio-oils [32]. It has been chosen as a model compound because its elemental composition is close to the overall elemental composition of lignin bio-oils (see H/O/C ratios in Fig. 2) and its hydroxyl and methoxyl functions on the aromatic ring are significant and key functions for aromatics species in lignin bio-oil.

The hydrogenation should be soft in order to favour BTX production at the expense of the ring hydrogenation (Fig. 2). The catalyst should act selectively towards the hydrogenolysis of the oxygen–aromatic bonds of hydroxyl or methoxyl functions, i.e. it should be able to activate molecular hydrogen and the C(aromatic)O bond. Moreover, it should be resistant to lignin pyrolysis products and have the desired aspects of any catalyst: low price, low deactivation, easy regeneration and environmentally friendly disposal.

Phenol could also be an interesting product, but allowing some oxygen atoms linked to aromatic ring could produce an endless list of isomers difficult to be separated from the different compounds in bio-oils. Consequently, in our opinion, a complete hydrodeoxygenation to BTX seems a better route instead of a partial HDO to phenols.

Whilst undertaking our work, few papers appeared on the subject using similar reaction conditions but different catalysts (Ni₂P/SiO₂, Fe₂P/SiO₂, MoP/SiO₂, Co₂P/SiO₂ and WP/SiO₂ [24]; CoMoS over alumina, zirconia or titania [25], [26]; Pt-Sn/CNF [29]). Aims and practical results of these papers are discussed in this manuscript. From the academic point of view, they indicated that the catalysts activity and selectivity strongly depend on the nature of the active phase and that of the support. These factors particularly influence the two main competitive reactions, namely the HDO and the aromatic ring hydrogenation.

Transition (Fe, Co, Ni) and precious metals (Pt, Pd, Ru, Rh, Ir, etc.) are known to catalyse hydrogenation [34], but unfortunately most of them also catalyse benzene hydrogenation in the temperature range 473–673 K [35]. Emmett and Skau [34] detected no activity for benzene hydrogenation at 673 K working with Fe. Lately, Yoon and Vannice [35] measured a relatively low activity of Fe for benzene hydrogenation compared to other transition or precious metals (Ni, Co). Fe is a trade-off between activity and selectivity. This is the reason why we chose iron as the active phase in the guaiacol HDO, expecting minimum aromatic ring hydrogenation.

Metal–support interactions and support acidity play a crucial role in complex chemistry of transition metal supported catalysts. It was studied for hydrogenation of aromatics hydrocarbons [36], [37], [38], [39], [40] as well as for aromatic alcohols [25], [26], [41], [42], [43], [44], [45]. Benzene is hydrogenated with higher rates on Ni catalysts when silica was used as a support instead of alumina [45]. For HDO reaction on supported Mo catalysts, alumina gave higher activity but rapid deactivation due to coke deposit [41], [42], [45]. In contrast, the use of the less acidic active carbon led to lower activity but a better selectivity in HDO products [41], [43], [44], [45]. We concluded that silica would be a good iron carrier due to its low acidity.

Cobalt was chosen for comparison. Cobalt is an inexpensive active phase compared to precious metals. It is a component of typical CoMoS/Al₂O₃ hydrotreating catalyst and it is active for HDO [46]. Our group performed a screening of catalyst at 623 K [47]. It was shown that cobalt is able to catalyse the production of BT from guaiacol.

For those reasons, our work focuses on the gas phase HDO of guaiacol to BTX as a model compound of lignin pyrolysis vapours using Fe/SiO₂ and Co/Kieselguhr catalysts, at the temperature of interest (623–723 K) for coupling the HDO catalytic reactor to the pyrolysis reactor. The catalysts were characterised by BET, XRD, Mössbauer spectroscopy and temperature programmed oxidation (TPO).