**1. Introduction**

Hydrocarbons sourced from non-edible or waste lignocellulosic or algal biomass are an attractive source of sustainable liquid transportation fuels to mitigate current dependence on fossil fuels and associated anthropogenic climate change [1,2]. However, biomass-derived fuels are incompatible with existing distribution infrastructure and vehicle engines without (catalytic) upgrading to improve their physicochemical properties [2,3]. A range of thermochemical technologies exist for bio-oil production, including hydrothermal liquefaction [4,5] and pyrolysis [6,7], or gasification [8,9] and subsequent Fischer–Tropsch synthesis [10,11]. Pyrolysis routes have gained particular attention over the past 30 years, offering a high liquid (bio-oil) yield which can be used directly as a drop-in fuel, blended with conventional diesel, or as an efficient energy vector [12,13]. Pyrolysis bio-oils are mixtures of oxygenated compounds which typically comprise phenolics, furanics, carboxylic acids and other small oxygenates whose composition varies with biomass source and processing [6,14,15]. The high oxygen content of crude bio-oils results in a heating value half that of petroleum-derived fuels, while the presence of carboxylic acids renders the oils corrosive (pH 2–3) [15] and chemically unstable due to

presence of small reactive oxygenates (e.g., unsaturated aldehydes) which may undergo acid-catalysed polymerisation [2]. Crude bio-oils must therefore be upgraded to remove corrosive components and improve stability prior to subsequent hydrodeoxygenation to improve their calorific value.

A range of catalytic routes exist for upgrading pyrolysis bio-oils, including esterification [16], hydrodeoxygenation (HDO) [17], aldol condensation [18] and ketonisation [19]. Each route has advantages and disadvantages. For example, esterification operates at low temperature in the liquid phase but requires an external source of short-chain alcohols, and produces water by-product which must be separated [20]. HDO is e ffective for the production of cyclic and aliphatic alkanes as liquid fuels, but requires a renewable H2 input and precious metal catalysts that are stable in (often acidic) bio-oils and coke-resistant [21]. Aldol condensation stabilises bio-oils by converting some reactive oxygenates over solid base catalysts, but does not address the intrinsic acidity of bio-oils that can deactivate HDO catalysts [22]. Ketonisation a ffords an intermediate deoxygenation step that can be close-coupled to a pyrolysis reactor to upgrade vapours before then condense into a bio-oil, thereby improving bio-oil acidity, and achieving partial deoxygenation [19], although it is also accompanied by a small loss of carbon as CO2. Ketonisation [22–24], takes place through the condensation of two carboxylic (e.g., acetic) acid molecules to form a ketone (e.g., acetone), CO2 and H2O (Figure 1). An important advantage of ketonisation over esterification for acid neutralisation is that the former can be performed in the vapour phase without additional reactants, thus enabling close-coupling to a pyrolysis reactor to upgrade bio-oil vapours prior to their condensation as a bio-oil [24,25]. Ketonisation also facilitates bio-oil deoxygenation and concomitant hydrocarbon chain growth [26], and hence improves the calorific value of the resulting condensate (in addition to its pH and stability).

$$\text{a}^2 \stackrel{\text{Q}}{\underset{\text{O}\text{H}}{\underset{\text{Catalyst}}{\rightleftharpoons}}} \stackrel{\text{A}}{\underset{\text{Catalyst}}{\underset{\text{Catalyst}}{\rightleftharpoons}}} \stackrel{\text{Q}}{\underset{\text{Q}}{\text{ }}} \text{ + } \text{co}\_2 \text{ + } \text{H}\_2\text{C}$$

**Figure 1.** Acetic acid ketonisation.

Ketonisation is widely studied in organic synthesis [25,27], being catalysed by diverse heterogeneous catalysts including alkaline earth metal oxides such as BaO, MgO [28,29], transition metal oxides including MnO2 [24,30,31], TiO2 [32,33], Fe3O4 [22,24,34], CeO2 [35,36] and ZrO2 [37,38] and actinide oxides such as ThO2 [39]. The mechanism of ketonisation and corresponding sensitivity to catalyst properties remains the subject of ongoing debate [25,40]. Ketonisation over basic and reducible oxide catalysts proceeds via two distinct pathways depending on the lattice energy of the metal oxide, with lower energy lattice (stronger bases) forming stable carboxylates that thermally decompose at elevated temperature (>420 ◦C) [41] to yield ketones, whereas higher energy lattices favour a lower temperature surface catalysed route [25]. However, there are fewer reports of carboxylic acid ketonisation over zeolites, being limited to HZSM-5, HZSM-11, HZSM-34, HZSM-35, mordenite, erionite and zeolite Beta. Of these, HZSM-5(100) is the most favourable for acetic acid ketonisation to acetone [42], being very selective to xylenols and acetone at moderate reaction temperatures (320 ◦C) and forming acetone as the major product >350 ◦C [42]. Zeolite modification by transition metals and lanthanides such as Ce, Co, Ni and Ga increases aromatic product yields during catalytic pyrolysis [43–45], however to our knowledge Ga-promoted zeolites have never been investigated for carboxylic acid ketonisation. Gallium can be introduced into zeolites by incipient wetness impregnation and ion exchange [46], although the choice of preparation method had little impact on aromatic products from Ga/HZSM-5-catalysed fast pyrolysis [46].

HZSM-5 is an attractive catalyst for acetic acid ketonisation, owing to its corrosion resistance, high surface area, commercial availability and ability to stabilise undercoordinated cations and hence tune surface composition and resulting catalytic performance [25]. The presence of strongly acidic protons in zeolites reportedly promotes the formation of surface acyl species, rather than carboxylates, following acid adsorption. Subsequent coupling of a carboxylic acid and surface acyl yields an acid anhydride, which in turn dissociates to liberate CO2 and a ketone as illustrated in Figure 2 [25,47], although Chang et al propose that ketonisation proceeds by nucleophilic attack of an acylium ion by adsorbed carboxylate [48]. The latter is similar to a ketene intermediate pathway, in with the acylium ion is directly formed by acid protonation and water loss [48]. The relationship between Lewis/Brønsted acid character and ketonisation activity/selectivity over zeolites remains poorly established.


**Figure 2.** Proposed mechanism for acetic acid ketonisation to acetone over zeolites. Reproduced with permission from reference [47], Copyright © 2016, Elsevier.

Herein, we report the impact of Ga doping on the surface acidity of HZSM-5 and associated reactivity for the continuous vapour phase ketonisation of acetic acid to acetone. Activity and selectivity to acetone were proportional to Ga loading, reflecting the formation of weak Lewis acid sites and suppressed coking.
