*2.1. Catalyst Characterisation*

Crystalline phases were characterised by powder X-ray diffraction (XRD). Figure S1 shows diffraction patterns for HZSM-5 as a function of Ga loadings to HZSM-5, and pure bulk Ga2O3. No reflections associated with gallium oxide phases were observed for any loadings, indicating either the presence of highly dispersed Ga2O3 nanoparticles throughout the HZSM-5 pore network, or the exchange of Ga3+ with Al3+ ions in the framework (or protons at the surface of the zeolite). It is well documented that protons play an important role in regulating the interaction of metal oxides with zeolite surfaces [49,50]. Fang and co-workers report that impregnation favours the formation of Ga2O3 and small amounts of GaO+ at surface of ZSM-5, thereby introducing weak Lewis acid sites [51]. In contrast ion-exchanged Ga/HZSM-5 prepared by refluxing the zeolite in aqueous Ga(NO3)3 at 70–100 ◦C [46,52] appears to favour framework dealumination through Ga ion-exchange. Although the latter syntheses resemble our impregnating conditions we cannot conclude whether Ga resides as surface GaO+ clusters or within the zeolite framework. The reference gallium oxide was phase-pure monoclinic (m-Ga2O3) with reflections at 2θ = 19.0◦, 30.4◦, 30.5◦, 31.8◦, 33.5◦, 35.2◦, 37.4◦, 38.5◦, 42.8◦, 45.9◦, 48.7◦ and 57.5◦ [53,54]. Crystallite sizes of the parent HZSM-5 (Table 1) were independent of Ga loading, and significantly smaller than the zeolite. Nitrogen porosimetry revealed type IV isotherms for xGa/HZSM-5 (Figure S2), with the observed mesoporosity attributed to interparticle voids [55]. Corresponding Brunauer-Emmett-Teller (BET) surface areas, total pore volumes and micropore volumes continuously decreased with increased Ga loading (Table 1), attributed to partial

a

pore blockage, possibly as a result of extra-framework gallium deposition within the micropores [56]. Bulk Ga2O3 exhibited a very low surface area <10 m2.g−1.


**Table 1.** Elemental analysis and physicochemical properties of catalysts.

ICP-OES, b total pore volume at P/Po = 0.98, c t-plot method, d XRD, e propylamine desorption, f XPS.

Elemental analysis revealed the surface Ga content was consistently lower than the bulk determined by XPS and ICP-AES analysis respectively (Table S1), consistent with the selective incorporation of gallium inside the HZSM-5 pore network. The formation of large Ga2O3 particles on the external surface of zeolite crystallites can be discounted due to the absence of associated XRD patterns. Note that the lower Ga surface versus bulk content for the m-Ga2O3 reference reflects oxygen termination of gallium surfaces [57]. O 1s XP spectra of HZSM-5 revealed a single broad peak with a 533 eV binding energy associated with Si–O–Si and Si–O–Al environments [58,59] (Figure 3a), which was una ffected by low levels of Ga doping, but shifted to lower binding energy for 10Ga/HZSM-5, approaching that of Ga2O3 at 530.7 eV [60,61]. A similar trend was observed for the Ga 2p3/2 XP spectra (Figure 3b), which exhibited a single broad peak at 1119.0 eV for low Ga loadings, whose binding energy decreased towards that of m-Ga2O3 at 1117.9 eV for 10Ga/HZSM-5 [62]. These data demonstrate that the local environment of gallium in HZSM-5 is chemically distinct from that in bulk Ga2O3, consistent with either highly dispersed Ga2O3 nanoparticles, or ion-exchange of Ga3+ into the zeolite framework [62,63]. Corresponding Al and Si 2p XP spectra of xGa/HZSM-5 (Figure S3a,b) each evidenced a single chemical environment with respective binding energies of approximately 75.1 eV and 103.8 eV, consistent with the literature for HZSM-5 [58,59]. Al and Si 2p peaks shifted to lower binding energy for 10Ga/HZSM-5 indicative of significant ion-exchange and concomitant formation of extra-framework alumina.

**Figure 3.** (**a**) O 1s and (**b**) Ga 2p XP spectra of xGa/HZSM-5 and Ga2O3.

The acid properties of xGa/HZSM-5 and m-Ga2O3 were first investigated by di ffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) following pyridine adsorption (Figure S4a). Strong bands at 1444 cm<sup>−</sup><sup>1</sup> and 1545 cm<sup>−</sup><sup>1</sup> were assigned to pyridine bound to Lewis and Brønsted acid sites respectively, the intense band at 1490 cm<sup>−</sup><sup>1</sup> to pyridine bound to both acid sites and the weak 1600 cm<sup>−</sup><sup>1</sup> band to pyridine bound to Lewis acid sites [56]. The relative Lewis/Brønsted acid character was quantified from the ratio of 1444 cm<sup>−</sup><sup>1</sup> and 1545 cm<sup>−</sup><sup>1</sup> band intensities (Figure S4b). The Lewis:Brønsted ratio exhibited a small increase for 10Ga/HZSM-5, in accordance with literature reports [56,64,65]. Corresponding DRIFTS for pyridine on m-Ga2O3 revealed two weak bands at 1452 cm<sup>−</sup><sup>1</sup> and 1614 cm<sup>−</sup><sup>1</sup> indicative of pure Lewis acid character as previously reported [66,67]. Acid strength was subsequently probed by temperature-programmed reaction spectroscopy (TPRS) of propylamine. Reactively-formed propene (arising from propylamine decomposition over acid sites) was evolved in two desorptions at ~480 ◦C and ~540–555 ◦C associated with strong and weak acid sites respectively (Figure 4); the former possibly arising from high-index facets or defects [68,69]. The desorption temperature of both peaks was independent of Ga loading, however the ratio of weak:strong acid sites decreased monotonically reaching ~0.83 for 10Ga/HZSM-5. The decreased acid strength was consistent with ion-exchange of less electronegative Ga3+ for Al3+ into the zeolite surface, which is expected to decrease hydroxyl polarisation and hence Brønsted acid strength [70]. Acid site loadings and weak:strong acid site ratio respectively decreased and increased with Ga loading (Table 1 and Figure S5), however the total acid site density was approximately constant at ~2.6 <sup>μ</sup>mol·m<sup>−</sup>2. The acid site density of m-Ga2O3 was significantly higher at 18.4 <sup>μ</sup>mol·m<sup>−</sup>2, with a weak:strong acid site ratio of 0.40 akin to 0.5Ga/HZSM-5, however the absolute Ga loading was far lower than any of the xGa/HZSM-5 materials.

**Figure 4.** Reactively-formed propene from propylamine temperature-programmed reaction spectroscopy (TPRS) over xGa/HZSM-5.

### *2.2. Catalytic Activity in Ketonisation*

Vapour phase acetic acid ketonisation was subsequently studied over xGa/HZSM-5 in a fixed-bed continuous flow reactor. Turnover frequencies (TOFs) were derived by normalising the steady state rate of acetic acid conversion to the acid site loadings from Table 1. At 350 ◦C, TOFs were almost independent of Ga loading, exhibiting only a small increase for 10Ga/HZSM-5. Increasing the reaction temperature to 400 ◦C increased TOFs for all catalysts as previously reported [19,71], with a monotonic rise with Ga loading now apparent (Figure 5). Catalytic reactivity mirrored the weak:strong acid site ratio for both reaction temperatures, indicating that ketonisation preferentially occurs over weak acid sites within xGa/HZSM-5. Limited deactivation (<15%) was observed for 5 h on-stream for all xGa/HZSM-5 catalysts (Figure S6), attributed to pore/site-blocking by coke or strongly bound

bidentate carboxylate species [72], or structural changes, whereas the Ga2O3 reference exhibited minimal deactivation. Powder XRD revealed negligible change zeolite structure following the reaction (Figure S7), however elemental analysis confirmed the presence of surface carbon post-reaction for all xGa/HZSM-5 catalysts (falling from 12 wt% for the parent HZSM-5 and xGa/HZSM-5 samples to only 1 wt% for Ga2O3, Table S2).

**Figure 5.** Turnover frequencies (TOFs) for acetic acid ketonisation over xGa/HZSM-5 and corresponding weak:strong acid site ratio. Reaction conditions: 200 mg catalyst, 0.2 mL·min−<sup>1</sup> acetic acid, 50 mL·min−<sup>1</sup> N2, 1 bar.

Acetone selectivity at iso-conversion increased with Ga loading at both 350 ◦C and 400 ◦C (Figure 6), concomitant with the rise in weak:strong acid site ratio and Lewis acidity [73]. Vervecken also reported an increase in acetone selectivity >350 ◦C for acetic acid ketonisation over HZSM-5(100) [42], attributed to a higher activation energy for ketonisation that competing aromatisation (which forms xylenols, phenolics and other aromatics). The maximum acetone selectivity for 10Ga/HZSM-5 was 30%; the principal by-products were CO2, xylenol, phenol and hydrocarbons [42]. The observation that weak Lewis acid sites and/or related acid-base pairs are the active species for vapour phase acetic acid ketonisation (Figure S8) is consistent with previous experimental [74–77] and computational studies [72]. As discussed in the Introduction, acidic protons in zeolites promote the formation of surface acyl species, which may couple with carboxylate species formed over weaker acid sites to yield an acid anhydride intermediate which in turn decomposes to liberate CO2 and acetone [25]. However, Chang et al report that ketonisation over HZSM-5 occurs via nucleophilic attack of an acylium ion by carboxylate species [48]; the acylium ion being formed by acid protonation and dehydration [48]. In the case of xGa/HZSM-5, Ga loadings >10 wt% may further increase acetone productivity (and to a lesser extent selectivity) at lower reaction temperature. Although all xGa/HZSM-5 catalysts were stable for 5 h on-stream at 400 ◦C, future extended ageing and recycling tests are required to optimise formulation and performance.

**Figure 6.** Correlation between acetone selectivity from acetic acid ketonisation at iso-conversion (23% and 29% at 350 ◦C and 400 ◦C, respectively) and weak:strong acid site ratio for xGa/HZSM-5. Reaction conditions: 200 mg catalyst, 0.2 mL·min−<sup>1</sup> acetic acid, 50 mL·min−<sup>1</sup> N2, 1 bar.
