**4. Conclusions**

A family of Ga-doped HZSM-5 materials were synthesis by wet impregnation as solid acid catalysts for the vapour phase ketonisation of acetic acid, a potential route to upgrading pyrolysis bio-oil vapours. XRD indicates that Ga is either incorporated into the parent zeolite framework or highly dispersed across the zeolite surface as GaO+ clusters for loadings spanning 0.3–10 wt% Ga doping has little impact on the zeolite textural properties but increased the Lewis acid character concomitant with a decrease in acid strength relative to HZSM-5. Turnover frequencies for acetic acid ketonisation, and acetone selectivity at iso-conversion, were both proportional to the weak:strong acid site ratio, evidencing that ketonisation over xGa/HZSM-5 preferentially occurs over weak (Lewis) acid sites. The most active catalyst was 10 wt% Ga/HZSM-5, which was stable for 5 h on-stream despite significant carbon laydown, with an acetone selectivity of 30%. Acetic acid ketonisation is an attractive route to upgrading biomass pyrolysis vapours through close-coupling with Ga/HZSM-5 catalysts derived from earth abundant elements.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2073-4344/9/10/841/s1, Figure S1: XRD patterns of xGa/HZSM-5 and bulk Ga2O3,; Figure S2: N2 adsorption-desorption isotherms of xGa/HZSM-5 and Ga2O3; Figure S3: (a) Al 2p and (b) Si 2p XP spectra of xGa/HZSM-5; Figure S4: (a) DRIFT spectra of pyridine-saturated xGa/HZSM-5 and Ga2O3 and (b) corresponding Lewis:Brønsted acid site ratio (1444 cm<sup>−</sup>1: 1545 cm<sup>−</sup><sup>1</sup> bands) for xGa/HZSM-5; Figure S5: Density of strong and weak acid sites for xGa/HZSM-5 from propylamine TPRS; Figure S6: Acetic acid conversion over xGa/HZSM-5, and Ga2O3 vs time on stream. Reaction conditions: 200 mg catalyst, at 400 ◦C, 0.2 mL·min−<sup>1</sup> acetic acid, 50 mL·min−<sup>1</sup> N2, 1 bar; Figure S7: XRD patterns of (a) fresh and (b) post-reaction xGa/HZSM-5 and Ga2O3; Figure S8: Correlation between acetone selectivity from acetic acid ketonisation at iso-conversion (23% and 29% at 350 ◦C and 400 ◦C, respectively) and acid strength from propylamine temperature-programmed reaction spectroscopy (higher temperatures indicate weaker acidity) over xGa/HZSM-5, and Ga2O3; Table S1: Surface and bulk composition of xGa/HZSM-5 and Ga2O3; Table S2: Carbon content of used xGa/HZSM-5 and Ga2O3 after 5 h reaction.

**Author Contributions:** Conceptualisation, A.H., A.F.L. and K.W.; Formal analysis, H.J., A.O., A.F.L. and K.W.; Investigation, H.J.; Methodology, H.J.; Supervision, M.O., A.H., A.F.L., K.W.; Writing-original draft, H.J., A.O. and M.O.; Writing-review & editing, A.F.L. and K.W.

**Funding:** We thank the EPSRC (EP/K036548/2, EP/K014676/1, EP/N009924/1) for financial support. K.W. thanks the Royal Society for the award of an Industry Fellowship.

**Conflicts of Interest:** The authors declare no conflict of interest.
