**2. Results and Discussion**

### *2.1. Feedstock Cleanup with Continuous Liquid–Liquid Extraction*

In our prior work, a batch process was used to separate the target organics (carboxylic acids and alcohols) from the carbon treated stream [13]. The process worked well for high concentrations (~30 wt %) of carboxylic acids, however, we developed a continuous liquid–liquid extraction (LLE) (Figure 1) system to process the aqueous streams with lower concentrations of organics.

**Figure 1.** Continuous liquid–liquid extraction apparatus. Operation details can be found in Appendix A.

Details on the preparation of the PNNL-HTL feedstock were previously reported [13]. As received, the feedstocks had a different total organic concentration (based on LC composition), 26.2 wt % for KIT-SP, 14.6 wt % for USDA-FP, and 32.6 wt % for PNNL-HTL. For all feedstocks, acetic acid was the most abundant compound. The composition of the different feedstocks is summarized in Table 1. The full analysis can be found in the Supporting Information. A carbon treatment was performed first to remove color bodies and other compounds from the as received samples as it has been reported to cause deactivation during catalytic upgrading [13]. Following carbon treatment, the organic concentration decreased <10% due to adsorption onto the carbon surface, however, we also observed a loss of aqueous phase as it was retained in the porous structure of the carbon. The overall aqueous loss depended on the number of carbon treatments required for each sample. For example, the USDA-FP feedstock required two rounds of carbon treatment and lost 26.8% of the mass, while the KIT-SP feedstock required five rounds of carbon treatment and lost 69.6% of the mass (~13.6% during each carbon treatment). As previously shown, the carbon treatment removed all the color bodies from the sample and the feedstocks became water clear (see Supporting Figure S1) [13]. However, the carbon treatment increased the inorganic concentration of the feedstock, particularly for K, Mg, Na, and P from ≤100 ppm in all cases to as high as 1560 ppm K in USDA-FP and 4050 ppm K in KIT-SP.


**Table 1.** Summary of stream composition at the different stages of the clean-up process. PNNL-HTL is derived from corn stover, processed using hydrothermal

a S/C = molar ratio of steam-to-carbon. b BDL = below detection limit.

### *Catalysts* **2019**, *9*, 923

The carbon-treated aqueous phases were then placed inside a continuous LLE to remove and concentrate organic molecules from the aqueous phase. Addition of methyl tert-butyl ether (MTBE) to the carbon-treated aqueous samples created two phases, an aqueous phase (the ra ffinate) and an organic phase (the extract). Under batch LLE, this process was found to increase the total organic concentration from 34–80 wt %. The continuous LLE used here was found to generate extract (i.e., organic molecules extracted from aqueous phase) with the same concentration as the one obtained in the batch LLE. Further, both LLE systems generated the same amount of extract (e.g., ~0.60 gcarboxylic acids gaqueous phase fed<sup>−</sup>1). However, the removal of MTBE was less e ffective in the continuous LLE than in the rotary-vaporization used in the batch LLE. For example, the extract from the continuous LLE had about 10 times higher concentration of MTBE (20.9 and 35.2 wt % for USDA-FP and KIT-SP respectively) than the extract obtained in batch LLE (2.91 wt % for HTL). Therefore, an additional distillation step was necessary to remove the remaining MTBE from the extract generated in the continuous LLE to produce the final feedstock. The composition of the main constituents found in the streams at the di fferent steps of the clean-up process are summarized in Table 1. A full analysis of the stream can be found in the Supporting Information.

### *2.2. Catalytic Upgrading of Cleaned Up Aqueous Phase to Isobutene*

The final PNN-HTL and KIT-SP feedstocks were selected to explore the catalytic upgrading of carboxylic acids into olefins using the Zn1Zr2.5O catalyst. The product selectivity and catalyst stability were evaluated as a function of gas hour space velocity (GHSV). The final PNNL-HTL feedstock had a lower concentration of non-participating carbon compared to the final KIT-SP feedstock, 0.16 and 2.57 wt % respectively.

From these GHSV screening (Figure 2), the product distribution obtained with both feedstocks appeared similar to that previously observed with ethanol and propanol [22]. As depicted in Scheme 1 and in our previous work, [22] acetone and iso- and n-butene (i.e., iso-C4 = and n-C4 =) are direct products of acetic acid. However, methyl ethyl ketone (MEK) and methyl butane (C5 =) are produced from the combination of propanoic acid with acetic acid as depicted in Scheme 2. This mechanism also agrees with our recent study for conversion of MEK to olefins over ZnxZryOz catalysts [23]. As shown in Figure 2, the ketone and olefin selectivity are directly a ffected by the GHSV, emphasizing that the ketones are indeed reaction intermediates for the olefin production as depicted in Schemes 1 and 2. For example, at the highest GHSV (i.e., lowest contact time) there was already complete conversion of the carboxylic acids but we only observed ketone intermediates (e.g., acetone from acetic acid-acetic acid self-ketonization, MEK from acetic acid-propionic acid cross-ketonization, and 3-pentanone from propionic acid self-ketonization). However, the selectivity towards ketone formation decreased as the GHSV decreased (i.e., higher contact time). For example, at ~1200 h−<sup>1</sup> (i.e., the lowest SV studied) there was almost complete conversion of the ketone intermediates to their respective olefins, as illustrated in Schemes 1 and 2, with a combined ketone selectivity of 8.6 and 3.9% for the PNNL-HTL and KIT-SP feedstocks respectively. This suggested that Zn*x*Zr*y*O*z* is an e ffective catalyst for the direct production of olefins from carboxylic acids as well as alcohols.

**Figure 2.** Gas hour space velocity (GHSV) profile of (**A**) PNNL-HTL feedstock, S/C = 2.8, and (**B**) KIT-SP feedstock, S/C = 4.7. Temperature: 450 ◦C, catalyst loading 0.7 g, N2 50 vol%. Complete conversion of carboxylic acids was observed at all GHSV's studied. 3-pentanone was observed in trace (0.21% for PNNL-HTL and 0.10% for KIT-SP) amounts at ~5400 <sup>h</sup>−1, but was otherwise not observed.

**Scheme 2.** Reaction network of carboxylic acid to olefin ethanol to isobutene and major side products.

As shown in Figure 3, the overall product selectivity obtained with the three feedstocks favors olefin over ketone formation when operated at low GHSV and same reaction conditions, however, the individual olefin selectivity differs. For example, the PNNL-HTL and HIT-SP feedstocks were ~15% selective towards C5+ production, the USDA-FP sample was ≥5%. We hypothesize this difference in C5+ selectivity is due to the differences in acetic acid and propanoic acid concentration. For example, PNNL-HTL feedstocks had the highest overall concentration of carboxylic acids (33.9 wt %) and propanoic acid (4.79 wt %), while the USDA-FP feedstock had the lowest overall concentration of carboxylic concentration (10.9 wt %) and propanoic acid (1.68 wt %).

**Figure 3.** Major product selectivity of all feedstocks. Temperature: 450 ◦C, GHSV: 1200 <sup>h</sup>−1, catalyst loading 0.7 g, N2 50 vol%, S/C = 2.8, 11.2, 4.7 for PNNL-HTL, USDA-FP, and KIT-SP, respectively. The product selectivity was measured at 100% conversion of carboxylic acids.

Figure 4 depicts the stability of the Zn1Zr2.5O catalyst when tested with the PNNL-HTL and KIT-SP feedstocks containing different concentrations of non-participating carbon species, 0.16 and 2.57 wt %, respectively. The initial and final conversion and primary product selectivities are summarized in Table 2. The PNNL-HTL feedstock (with the lowest non-participating carbon concentrations) showed a stability profile like the one previously observed with model ethanol feedstocks [19,22]. While the higher GHSV did increase selectivity to the ketone products (~23.5 and ~10.0% for acetone and MEK respectively) overall selectivity was relatively stable for ~20 h time on stream (TOS), after which the ketone selectivity began increasing significantly while olefin selectivity decreased. This further supported our speculation that the ketones are the intermediate of the olefins. In contrast, the KIT-SP feedstock started with higher ketone selectivity, ~49% selectivity at 10 h TOS, and continued to increase for the duration of the test, ~65% selectivity by 22 h TOS. While the conversion was constant with the PNNL-HTL feedstock at 100% for the duration of the experiment, it decreased with the KIT-SP feedstock. As shown in Figure 4B, the conversion started to decrease at 26 h TOS and dropped to 82% by the end of the experiment. This difference in stability with the two feedstocks indicated that, while

the Zn*x*Zr*y*O*z* catalyst can accommodate a range of compounds, the catalyst will deactivate faster in the presence of more non-participating compounds in the feedstock [13].

**Figure 4.** High GHSV stability of (**A**) PNNL-HTL feedstock, S/C = 2.8, and (**B**) KIT-SP feedstock, S/C = 4.7. Temperature: 450 ◦C, GHSV: 5500 <sup>h</sup>−1, catalyst loading 0.7 g, N2 50 vol%.

**Table 2.** Comparison of initial and final activity and selectivity of PNNL-HTL and KIT-SP feedstocks shown in Figure 4.


The long-term stability of the Zn*x*Zr*y*O*z* catalyst (Figure 5) was tested with the USDA-FP feedstock as it had the lowest total carboxylic acid content and some non-participating carbon compounds, 10.9 and 1.55 wt %, respectively. We chose a low SV (1200 <sup>h</sup>−1) to maximize olefin production which is more representative of industrial targets. The Zn*x*Zr*y*O*z* catalyst was relatively stable for close to 100 h, and by 95 h TOS the isobutene selectivity was stable >50% (net olefins ~60%). The ketone selectivity, particularly acetone, slowly increased over this period from 3.4% at 47 h TOS to 8.9% at 95 h TOS. At 120 h TOS the Zn*x*Zr*y*O*z* catalyst showed catalytic deactivation as ketone breakthrough was observed, with ketone selectivity at 25.3% (23.9% and 1.37% for acetone and MEK respectively). Following in situ catalyst regeneration, the Zn*x*Zr*y*O*z* showed complete recovery of activity with isobutene selectivity increasing to 58.5% and acetone selectivity dropping back to 2.70%. The catalysts showed catalyst deactivation after another 50 h of reaction as acetones selectivity increased to 7.49%. A second in situ catalyst regeneration completely regenerated the Zn*x*Zr*y*O*z* activity.

**Figure 5.** Stability and regeneration of USDA-FP feedstock. Temperature: 450 ◦C, S/C = 11.2, GHSV: 1200 <sup>h</sup>−1, catalyst loading 0.7 g, N2 50 vol%. The catalyst regeneration was done by calcination in 5 vol% O2 in N2 at 500 ◦C for 4 h.

### *2.3. Catalytic Upgrading of Ethanol to Propene*

H2 is often used to improve catalyst stability, however the role of H2 on the olefin formation reactions has not been widely studied [1,12,22]. Whereas H2 could hydrogenate olefins into paraffins, it could also hydrogenate reaction intermediates and shift the product distribution towards different olefin products. As shown in Scheme 1, acetone can be hydrogenated to isopropanol which then can dehydrate to propene. To simplify this portion of our study, a 20 wt % ethanol in water feed was used rather than the more complex aqueous phase feedstocks discussed earlier. Propene production in the absence of H2 from the complex feedstocks (i.e., PNNL-HTL, USDA-FP, and KIT-SP) was negligible (<1% selectivity, typically ~0.2% selectivity).

As shown in Figure 6, the propene selectivity was enhanced by cofeeding H2. For example, when the N2 carrier gas was replaced by H2 in in 25 vol% increments, the propene (C3H6) selectivity increased roughly linearly with H2 composition from 5.40 to 18.6%, while the isobutene selectivity decreased from 43.1 to 32.6%. The change in product distribution is because the hydrogenation of acetone (and subsequent dehydration to produce propene) is enhanced in the presence of H2 while acetone self-condensation (to produce isobutene) is inhibited as depicted in Scheme 1 and described in our previous work [22]. Further, the CO2 selectivity also decreased as a result of inhibiting the isobutene formation as expected from the reaction network described in Scheme 1. The CH4 selectivity remained relatively constant at ~4%, indicating that changing the gas environment does not significantly impact decomposition or methanation reactions. We speculate that cofeeding H2 during the upgrading of the feedstocks explored in this work over the Zn*x*Zr*y*O*z* would cause a similar enhancement in propene production and shift the C5= production more towards C4= [23]. The mixed olefin product stream could be then oligomerized to higher value products as demonstrated by Saavedra Lopez et al. [14].

**Figure 6.** Effect of gas environment on product selectivity. Feedstock: 20 wt % ethanol in water, S/C = 5.1, Temperature: 450 ◦C, GHSV: 1200 <sup>h</sup>−1, catalyst loading 0.7 g, N2/H2 composition was constant at 50 vol%.
