**1. Introduction**

Biomass has received much attention as a renewable source for both fuels and chemicals. While there are a wide range of approaches for biomass conversions, a commonality is high cost of processing leading to a high minimum fuel selling price [1–4]. Direct liquefaction of biomass is an appealing approach as much of the hydrocarbon structure can be preserved [5–7]. Processes with direct liquefaction include hydrothermal liquefaction (HTL) and di fferent permutations of pyrolysis that includes fast pyrolysis (FP), catalytic fast pyrolysis (CFP), and intermediate screw pyrolysis (SP) [8–11]. Regardless of approach, aqueous and organic phases can be segregated by gravity separation. While the organic phase has a lower oxygen content, and consists of molecules typically more suited for fuel, the aqueous phase typically comprises a mixture of light oxygenates that are di fficult to separate [1,12]. Thus, the aqueous phase is usually considered a waste stream. Several recent works have reported how the aqueous phase can be converted to di fferent co-products, in order to improve the overall carbon efficiency of the liquefaction process [4,12,13]. For example, we recently reported the co-production of either H2 or propene, from the aqueous phase, and the resulting process economics for producing fuel from the organic phase [13]. From this work, it was found that the largest impact on minimum fuel selling price is increasing the utilization of carbon from the biomass, including as the sale of co-products [13].

With one pathway to co-products utilizing the aqueous phase from biomass liquefaction already demonstrated, the next step is to expand the platform of potential product compounds to make the biorefinery more versatile. Olefins are a promising group of co-products to produce with isobutene being particularly valuable as it is easily converted to fuel additives, solvents, and butyl rubber products [14–18]. Recently, Zn*x*Zr*y*O*z* catalysts have been identified as having a unique combination of surface acidic and basic sites, generating a cascade reaction network (Scheme 1) [19–22]. This cascade network allows for the direct conversion of ethanol to isobutene in a single reactor bed [15,22]. Ethanol first undergoes ethanol dehydrogenation and ketonization reactions thus producing acetone. ZnO addition o ffers the necessary basic sites while also suppressing most of the strong acid sites responsible for undesirable ethanol dehydration. Acetone then undergoes aldol condensation and C–C cleavage over acid sites, while the formation of acetone decomposition products (CH4 and CO2) is largely suppressed [22]. Recently, a complete loop starting from syngas and ending with isobutene oligomerization to jet fuel was also demonstrated [15]. In addition, it has been demonstrated that the Zn*x*Zr*y*O*z* catalyst is able to convert larger alcohols, and also carboxylic acids and ketones, into mixed olefin streams [22–24]. As previously demonstrated, the reaction mechanism involves the conversion of alcohols into carboxylic acids before undergoing subsequent reaction steps [22]. This makes the Zn*x*Zr*y*O*z* catalyst promising for the direct conversion of carboxylic acids present in the aqueous phase from biomass liquefaction to olefin products.

**Scheme 1.** Reaction network of ethanol to isobutene and major side products adapted from Smith et al. [22].

In this work we examine the feasibility of directly using biomass liquefaction-derived aqueous phases for the direct production of olefins. Three di fferent aqueous phases are studied, derived from hydrothermal liquefaction (supplied by PNNL, PNNL-HTL), a modular fast pyrolysis (supplied by USDA-ARS, USDA-FP), and a screw pyrolysis (supplied by KIT, KIT-SP). The Zn*x*Zr*y*O*z* catalyst (specific composition Zn1Zr2.5O) was used for the direct production of C4+ olefins. Finally, the e ffect of a gas environment was also investigated to study the effects on product selectivity with a model feedstock of ethanol.
