Reaction Mechanisms and Production of Hydrogen and Acetic Acid from Aqueous Ethanol Using a Rn-Sn/TiO 2 Catalyst in a Continuous Flow Reactor

: The catalytic reforming of bioethanol can produce green hydrogen (H 2 ) and acetic acid (AcOH). In the present study, the conversion of aqueous ethanol (EtOH) over 4 wt%Ru-4 wt%Sn/TiO 2 in a flow reactor was investigated at different temperatures at 0.1 MPa or at various pressures at 260 ◦ C. The ethanol conversion was rather slow in liquid water, while the reactivity increased significantly when water was evaporated. Under gas-phase conditions at 0.1 MPa, the conversion rate increased with increasing reaction temperature, but the AcOH yield and H 2 purity decreased due to by-production of CH 4 , CO, and CO 2 . The CH 4 and CO generated by fragmentation of acetaldehyde (AA), an intermediate, were suppressed by increasing reaction pressure, although the formation of CH 4 and CO 2 generated from AcOH was pressure independent. Thus, the highest-pressure conditions in steam at a given reaction temperature are preferred for the production of pure H 2 . The initial step, EtOH → AA, was the rate-determining reaction, and the model experiments using AA as a substrate showed that the Cannizzaro reaction of two AA molecules to form EtOH and AcOH occurred preferentially. This oxidation system was confirmed to be effective at EtOH concentrations of up to 500 g/L in water.


Introduction
The efficient use of biomass as an alternative to fossil resources is attracting attention as a means of mitigating climate change caused by increasing emissions of greenhouse gases.Specifically, bioethanol produced by the fermentation of sugars derived from biomass can be used as a substitute for gasoline.However, the demand for bioethanol as a fuel is expected to decline as electric and fuel cell vehicles become more widespread in the future.Therefore, it is important to develop alternative utilization technologies for bioethanol.
Because bioethanol produced by fermentation contains a large amount of water, it would be preferable to develop techniques that allow the utilization of aqueous ethanol (EtOH) solutions without removing this water.The reforming of EtOH with water is a potential method of converting aqueous EtOH to hydrogen (H 2 ) and acetic acid (AcOH) via the reaction shown in Equation (1).
Hydrogen is a clean energy carrier, producing only water as a by-product during combustion, but the extent to which this fuel is carbon neutral depends on the manner in which it is produced.Carbon dioxide is emitted in the process of producing H 2 from fossil resources, whereas green H 2 production without CO 2 emissions would be highly desirable.The reforming of bioethanol to generate H 2 is therefore a promising means of reducing CO 2 emissions and the H 2 produced in this manner also has numerous applications such as a clean fuel and in the synthesis of various chemicals.AcOH is an important industrial chemical feedstock with a global demand of 16.1 million tons in 2022 [1] and can be used as a raw material for the generation of vinyl acetate, cellulose acetate, and other compounds, the majority of which are currently produced from petroleum-based AcOH.
Various organometallic complexes [2,3] and metal catalysts [4][5][6][7][8] have been investigated for the reforming of EtOH with water.Iridium complexes with bipyridonate ligands bearing N,N-dimethylamino substituents have been reported to convert aqueous EtOH to H 2 and AcOH with high yields and good selectivity [3].In this catalytic system, the oxidation is able to proceed under mildly basic conditions, providing a 99% sodium acetate yield (based on the amount of EtOH used) and a 95% theoretical H 2 yield under reflux conditions.Even so, the required reaction time is very long (over 18 h) and the resulting sodium acetate must be protonated to generate pure AcOH.In addition, the spent catalyst must be dried under reduced pressure and then extracted with dichloromethane in preparation for reuse.
Due to their great advantages in industrial applications, many attempts have been made to use supported metal catalysts [4][5][6][7][8].Cu/ZnO-ZrO 2 -Al 2 O 3 has been found to catalyze the conversion of aqueous EtOH to AcOH and H 2 with a maximum AcOH yield of 73.3% [4].However, the selectivity of this process for AcOH decreases with increases in the EtOH concentration as by-products such as ethyl acetate, acetaldehyde (AA), methyl ethyl ketone, and butanol are produced.This previous work also did not report the purity of the H 2 obtained.CuCr can provide almost complete EtOH conversion but gives a reported AcOH yield of only 48.6% based on the formation of a high proportion of AA (approximately 50%) as an oxidation intermediate [5].The temperature range used for these metal catalysts is also generally high (250-300 • C for Cu/ZnO-ZrO 2 -Al 2 O 3 and 350 • C for CuCr).
Supported precious metal catalysts have been used to lower the reaction temperature.Nozawa et al. [6] used Ru/TiO 2 for aqueous-phase reforming of EtOH at 200 • C, but the main products were CH 4 and CO 2 rather than AcOH, resulting in the lower H 2 purity.Such high selectivity of gaseous products in aqueous-phase reforming of EtOH has also been reported for carbon black-and carbon nanotube-supported Pd and PdZn catalysts at 250 • C [7].The addition of rhenium to the TiO 2 supported Ru, Rh, Ir, and Pt catalysts was reported to improve the selectivity of AcOH and H 2 , but the conversion of EtOH was only less than 37% even after 10 h at 200 • C in a batch reactor [8].
For all these reasons, the catalytic reforming of aqueous EtOH to H 2 and AcOH is not yet sufficiently mature to allow for practical applications as an approach to green H 2 production.On this basis, the present study assessed the performance of Ru-Sn/TiO 2 as an efficient and selective catalyst for the production of H 2 and AcOH in a flow reactor and discusses the reaction mechanisms involved in this conversion.In previous studies by the authors on the hydrogenation of AcOH to EtOH while investigating bioethanol production via AcOH fermentation, Ru-Sn/TiO 2 was developed as an effective catalyst capable of functioning in aqueous media [9,10].The formation of the gaseous products was effectively suppressed by the addition of Sn to Ru/TiO 2 [9], and the hydrogenolysis efficiency of AcOH to EtOH was significantly improved by using a continuous flow reactor [10].The 4 wt%Ru-4 wt%Sn/TiO 2 (based on the weight of TiO 2 ) was selected in accordance with the previous investigation to optimize the Ru and Sn contents and to minimize the production of gaseous by-products from the hydrogenolysis of aqueous AcOH [9].

Effect of Temperature at 0.1 MPa
The oxidation of a 10 g/L aqueous EtOH solution over 4 wt%Ru-4 wt%Sn/TiO 2 was performed over the temperature range of 190-260 • C at atmospheric pressure (0.1 MPa) in a flow-type reactor.The yields of liquid-phase products, of gaseous products other than H 2, and of H 2 , together with the H 2 purity, are summarized in Figure 1A-C, respectively.Under all conditions, water vapor was present in the gas phase.The data show that two moles of H 2 were obtained from each mole of EtOH (as expected according to Equation (1)) and so the H 2 yields are reported herein relative to the expected theoretical yields.That is, a theoretical yield of 100% H 2 would be equivalent to a yield of 200 mol% based on the initial amount of EtOH.Yields of other products are given as mol% based on the actual amount of EtOH used in the trial.
The oxidation of a 10 g/L aqueous EtOH solution over 4 wt%Ru-4 wt%Sn/TiO2 w performed over the temperature range of 190-260 °C at atmospheric pressure (0.1 MP in a flow-type reactor.The yields of liquid-phase products, of gaseous products other th H2, and of H2, together with the H2 purity, are summarized in Figure 1A-C, respective Under all conditions, water vapor was present in the gas phase.The data show that tw moles of H2 were obtained from each mole of EtOH (as expected according to Equati (1)) and so the H2 yields are reported herein relative to the expected theoretical yield That is, a theoretical yield of 100% H2 would be equivalent to a yield of 200 mol% bas on the initial amount of EtOH.Yields of other products are given as mol% based on t actual amount of EtOH used in the trial.The effects of reaction temperature on the conversion of aqueous EtOH (10 g/L) to AcOH and H 2 , based on the outputs of (A) liquid-phase products, (B) gaseous products other than H 2 and (C) H 2 .The graph in (C) also plots H 2 purity in the gaseous products as a function of temperature.Data were acquired with a reactor column having a 3.9 mm diameter and 100 mm length with a pump flow rate of 0.3 mL/min at a pressure of 0.1 MPa.
The EtOH conversion efficiency increased from 22.4% to 92.8% as the reaction temperature was increased from 190 to 260 • C. The yields of H 2 and AcOH also increased, from 23.2% to 99.4% (on a theoretical basis) and from 16.0 mol% to 71.2 mol% (on an EtOH basis), respectively.The yield of AA, the intermediate in EtOH oxidation, remained low (from 4.6-8.7 mol% based on EtOH) regardless of the reaction temperature, indicating that the second step in this process (AA → AcOH) occurred more efficiently in the sequential oxidation scheme (EtOH → AA → AcOH).
The yields of all gaseous products, including H 2 , increased with increases in the reaction temperature.However, the CH 4 , CO, and CO 2 formation rates all increased exponentially, whereas that of H 2 exhibited a linear increase (Figure 1B,C).Consequently, the purity of H 2 decreased from 97.9 mol% at 190 • C to 84.1 mol% at 260 • C at 0.1 MPa.The total combined yield of CH 4 , CO, and CO 2 reached 37.4 mol% (EtOH basis) at 260 • C, while the total combined yield of the liquid products (EtOH, AcOH and AA, Figure 1A) decreased from 98.1 mol% at 190 • C to 83.5 mol% at 260 • C.Although the EtOH conversion was low at the lower temperatures, the selectivities for H 2 and AcOH were relatively high, giving almost pure products.At higher temperatures, the conversion of EtOH was increased but the selectivity decreased as a result of the increased fragmentation of AA and AcOH.

Effect of Pressure at 260 • C
Additional reactions of 10 g/L aqueous EtOH were performed over the pressure range of 0.1 to 6.0 MPa at a fixed temperature of 260 • C (Figure 2).The aqueous phase transitioned from steam to liquid water upon increasing the reaction pressure to the saturated vapor pressure of water, which was estimated to be 4.7 MPa at 260 • C using the Soave-Redlich-Kwong model with the ProII 2022 process simulator.The residence time as shown in this figure was also increased up to 3.1 s at 4.0 MPa by compressing the steam, based on increasing the reaction pressure.The residence time was significantly longer in the liquid aqueous phase (93 s at 6.0 MPa).The amount of unreacted EtOH increased with increases in the reaction pressure, and this trend became more pronounced at 6.0 MPa, at which the water vapor was converted to liquid water even though the residence time increased with increasing pressure.Thus, the oxidation of EtOH was more efficient in the gas phase and a lower pressure improved the conversion of EtOH in the steam.The H 2 yields showed a similar trend to the EtOH conversion throughout the pressure range used in this study.
In contrast, the AcOH yield was almost constant at approximately 70 mol% over the pressure range of 0.1-4.0MPa based on gas-phase reactions, while the H 2 purity gradually increased from 84.1 mol% at 0.1 MPa to 93.2 mol% at 4.0 MPa.The latter effect is ascribed to decreases in the yields of CO and CH 4 derived from the fragmentation of AA (Equation ( 3)).The yield of CO became almost zero at pressures of 3 to 4 MPa, indicating that the fragmentation of AA was completely suppressed under such conditions.Conversely, the CO 2 yield was constant over the pressure range of 0.1-4.0MPa, whereas the CH 4 yield plateaued, suggesting that the fragmentation of AcOH (Equation ( 2)) also occurred but the progress of the reaction was independent of pressure.Therefore, a reaction pressure slightly lower than the saturated vapor pressure of water at a given reaction temperature was evidently optimal with regard to obtaining high-purity H 2 .

Figure 2.
The effects of reaction pressure on the conversion of aqueous EtOH (10 g/L) to AcOH an H2, based on the outputs of (A) liquid-phase products, (B) gaseous products other than H2 and ( H2.The graph in (C) also plots H2 purity in the gaseous products as a function of pressure.Da were acquired with a column having a 3.9 mm diameter and 100 mm length with a pump flow ra of 0.3 mL/min at a temperature of 260 °C.
In contrast, the AcOH yield was almost constant at approximately 70 mol% over t pressure range of 0.1-4.0MPa based on gas-phase reactions, while the H2 purity gradual increased from 84.1 mol% at 0.1 MPa to 93.2 mol% at 4.0 MPa.The latter effect is ascribe to decreases in the yields of CO and CH4 derived from the fragmentation of AA (Equatio (3)).The yield of CO became almost zero at pressures of 3 to 4 MPa, indicating that th fragmentation of AA was completely suppressed under such conditions.Conversely, t The effects of reaction pressure on the conversion of aqueous EtOH (10 g/L) to AcOH and H 2 , based on the outputs of (A) liquid-phase products, (B) gaseous products other than H 2 and (C) H 2 .The graph in (C) also plots H 2 purity in the gaseous products as a function of pressure.Data were acquired with a column having a 3.9 mm diameter and 100 mm length with a pump flow rate of 0.3 mL/min at a temperature of 260 • C.
The yield of AA was consistently low, as can be seen from Figure 1, but tended to decrease with increases in reaction pressure.From this outcome, it appears that the conversion of AA to AcOH was more efficient at higher pressures.The opposite trend was observed for the fragmentation of AA to CH 4 and CO.The effects of reaction pressure on the various steps involved in the conversion of EtOH in the vapor phase at 260 • C are summarized in Table 1.

Cannizzaro Reaction
To better understand the reactivity of the AA intermediate, a 10 g/L aqueous solution of AA was reacted over 4 wt%Ru-4 wt%Sn/TiO 2 at pressures between 0.1 and 4.0 MPa at 260 • C. The results are shown in Figure 3.At all pressures, the AA was almost completely reacted (with recoveries of 0.0-1.0%)and AcOH and H 2 were formed.The H 2 yields were high (82.2-93.2% on a theoretical basis), indicating that the conversion of AA to AcOH and H 2 was highly selective.Note that, in these trials, a 100% theoretical yield of H 2 was equivalent to 100 mol% based on the amount of AA used.Unexpectedly, EtOH was found to have been produced from the AA, although the yields were less than 10.2 mol%.Evidently, the Cannizzaro reaction occurred.In this reaction, two molecules of a non-enolizable aldehyde give a primary alcohol and a carboxylic acid (Equation ( 4)) [11,12].
The EtOH produced by the Cannizzaro reaction in these trials was subsequently converted to AA (Figure 4).
In addition to the Cannizzaro reaction, the direct oxidation of AA (Equation ( 5)) was also expected to take place during the conversion of EtOH to AcOH (Figure 4).To evaluate this phenomenon, the amount of EtOH produced by the Cannizzaro reaction, EtOH (Cannizzaro), was estimated based on Equation (6), assuming that the EtOH obtained from the original AA (Figure 3) had a reactivity similar to that of EtOH during the trials used to generate the data in Figure 2.
In this equation, Recovery rate (EtOH ex) is the percentage of the original EtOH recovered following the EtOH oxidation reaction used to generate the data in Figure 2. EtOH (O) is the amount of EtOH observed in the experiment of AA as a substrate (Figure 3).The estimated EtOH (Cannizzaro) yield values are plotted in Figure 5.Because the theoretical EtOH (Cannizzaro) yield had a maximum of 50 mol% based on the original amount of AA, the approximately 50 mol% values estimated for the trials at 0.1, 0.5, and 1.0 MPa indicate that the conversion of AA occurred primarily via the Cannizzaro reaction rather than the direct oxidation pathway.In contrast, the estimated EtOH (Cannizzaro) yields were lower (30-40 mol%) in the pressure range of 2.0-4.0MPa, demonstrating that the direct oxidation of AA became competitive with the Cannizzaro reaction at these pressures, although the Cannizzaro reaction was still preferred even under these conditions.

Figure 3.
The effects of reaction pressure on the conversion of aqueous AA (10 g/L) to AcOH and based on the outputs of (A) liquid-phase products, (B) gaseous products other than H2 and (C The graph in (C) also plots H2 purity in the gaseous products as a function of pressure.Data w acquired using a reactor column with dimensions of 3.9 mm diameter and 100 mm length w pump flow rate of 0.3 mL/min at 260 °C.The graph in (C) also plots H 2 purity in the gaseous products as a function of pressure.Data were acquired using a reactor column with dimensions of 3.9 mm diameter and 100 mm length with a pump flow rate of 0.3 mL/min at 260 • C.

Figure 3.
The effects of reaction pressure on the conversion of aqueous AA (10 g/L) to AcOH and H2, based on the outputs of (A) liquid-phase products, (B) gaseous products other than H2 and (C) H2.
The graph in (C) also plots H2 purity in the gaseous products as a function of pressure.Data were acquired using a reactor column with dimensions of 3.9 mm diameter and 100 mm length with a pump flow rate of 0.   The composition of the gaseous by-product mixture provided insights into the fragmentation reactions of AA and AcOH.Similar to the trials using EtOH (as shown in Figure 2), the yields of CO and CH4 resulting from the fragmentation of AA decreased with increases in reaction pressure (Figure 3B).Interestingly, the yields of CO and CH4 obtained from the AA experiments were approximately half those obtained from trials using EtOH solutions (CO: 14.5 → 6.0 mol%, CH4: 18.7 → 9.1 mol%).This change can be explained by considering that the fragmentation reaction of AA occurred only during the conversion of EtOH to AA on the surface of the 4 wt%Ru-4 wt%Sn/TiO2, as shown in Figure 6.In the trials using the AA solutions, the amount of EtOH involved in the conversion process was approximately half the amount of AA because the Cannizzaro reaction converted AA according to the relationship 2AA → EtOH + AcOH.It is not presently clear why the fragmentation of AA did not occur in conjunction with the Cannizzaro reaction catalyzed by 4 wt%Ru-4 wt%Sn /TiO2.The composition of the gaseous by-product mixture provided insights into the fragmentation reactions of AA and AcOH.Similar to the trials using EtOH (as shown in Figure 2), the yields of CO and CH 4 resulting from the fragmentation of AA decreased with increases in reaction pressure (Figure 3B).Interestingly, the yields of CO and CH 4 obtained from the AA experiments were approximately half those obtained from trials using EtOH solutions (CO: 14.5 → 6.0 mol%, CH 4 : 18.7 → 9.1 mol%).This change can be explained by considering that the fragmentation reaction of AA occurred only during the conversion of EtOH to AA on the surface of the 4 wt%Ru-4 wt%Sn/TiO 2 , as shown in Figure 6.In the trials using the AA solutions, the amount of EtOH involved in the conversion process was approximately half the amount of AA because the Cannizzaro reaction converted AA according to the relationship 2AA → EtOH + AcOH.It is not presently clear why the fragmentation of AA did not occur in conjunction with the Cannizzaro reaction catalyzed by 4 wt%Ru-4 wt%Sn /TiO 2 .
In contrast, the CO 2 yields obtained from the fragmentation of AcOH to give CO 2 and CH 4 were approximately 5 mol% when using either AA or EtOH.This outcome suggests that the yields of these compounds were dependent on the amount of AcOH that was produced.The yields of CO 2 and CH 4 during the AA trials (Figure 3B) tended to increase along with pressure on going from 2 to 4 MPa, showing that the fragmentation of AA was more efficient at higher pressures.However, this effect was not as evident during the experiments using the EtOH solution (Figure 2B).
of EtOH to AA on the surface of the 4 wt%Ru-4 wt%Sn/TiO2, as shown in Figure 6.In the trials using the AA solutions, the amount of EtOH involved in the conversion process was approximately half the amount of AA because the Cannizzaro reaction converted AA according to the relationship 2AA → EtOH + AcOH.It is not presently clear why the fragmentation of AA did not occur in conjunction with the Cannizzaro reaction catalyzed by 4 wt%Ru-4 wt%Sn /TiO2.Because the Cannizzaro reaction did not produce H 2 and the contribution of the direct oxidation of AA to AcOH was minimal, the majority of the H 2 would have been generated during the oxidation of EtOH to AA on the catalyst (Figure 6).

Oxidation of High-Concentration Aqueous EtOH Solutions
The oxidation of more highly concentrated aqueous EtOH solutions (100 and 500 g/L) was also attempted at 260 • C and 4 MPa or 320 • C and 10 Mpa, with the results presented in Table 2.The latter conditions were the most extreme that could be obtained with the flow-type reactor system used in this study.The conversions obtained using the 100 g/L EtOH were 26.7% and 62.8% at 260 • C (4 Mpa) and 320 • C (10 Mpa), respectively.The corresponding yields of AcOH/H 2 were 21.4 mol%/30.1% and 50.7 mol%/72.3%,along with H 2 purities of 98.2% and 86.1%, respectively.Although the reactivity was not as high (with an EtOH conversion of just 13.1%) in trials with the 500 g/L EtOH at 320 • C and 10 MPa, the reacted EtOH was selectively converted to both AcOH and H 2 (10.4 mol% AcOH with a H 2 purity of 90.9%).These results suggest that this catalytic system could represent a viable approach to producing green H 2 and AcOH, although further development is needed to produce a larger-scale system capable of processing highly concentrated aqueous EtOH.
The 4 wt%Ru-4 wt%Sn/TiO 2 catalyst was prepared by a sol-gel precipitation technique [10].In this process, RuCl 3 (0.82 g, corresponding to 4 wt% Ru based on 10 g TiO 2 ) and SnCl 2 •2H 2 O (0.76 g, corresponding to 4 wt% Sn based on 10 g TiO 2 ) were added to hot water (100 mL, 60 • C).Following this, a mixture of 2-propanol (20 mL) and titanium isopropoxide (37.2 mL) was added dropwise with stirring.After waiting 30 min for TiO 2 to precipitate, an aqueous NaOH solution (100 mL, having a concentration sufficient to neutralize the metal chlorides) was added and the mixture was stirred for 30 min.The mixture was subsequently allowed to stand for 12 h and the resulting precipitate was filtrated and then washed five times with water, oven-dried in air at 105 • C for 12 h, calcined at 450 • C for 1 h, and then reduced under a H 2 flow (100 mL/min) at 400 • C for 5 h.Following this, the material was ground to a powder having an average particle size in the range of approximately 50-70 µm and then packed into a stainless-steel column (internal diameter, 3.9 mm; length, 100 mm).
According to a previous study [9], the crystal structure of the TiO 2 is mostly anatase type, and the BET surface area and crystallite diameter of TiO 2 of the 4 wt%Ru-4 wt%Sn/ TiO 2 catalyst were reported to be 84.5 m 2 /g and 7.7 nm, respectively.Regarding the stability of the catalyst, no difference in catalytic activity was observed even after the continuous use in the temperature range of 140-320 • C for 60 h in the hydrogenolysis of aqueous AcOH [10].

Catalytic Conversion and Product Analysis
A flow-type reactor (H-cube Pro™, ThalesNano Inc., Budapest, Hungary) equipped with an electric furnace (Phoenix, ThalesNano Inc.), as shown in Figure 7, was used for these trials.In each experiment, an aqueous solution of EtOH, AcOH, or AA (10 g/L in each case) was fed into the catalyst column at a flow rate of 0.3 mL/min at a temperature in the range of 190-260 • C and a pressure in the range of 0.1-6.0MPa.The resulting mixture was fractionated into aqueous and gaseous phases.
Catalysts 2024, 14, x FOR PEER REVIEW 10 of precipitate, an aqueous NaOH solution (100 mL, having a concentration sufficient to ne tralize the metal chlorides) was added and the mixture was stirred for 30 min.The mixtu was subsequently allowed to stand for 12 h and the resulting precipitate was filtrated a then washed five times with water, oven-dried in air at 105 °C for 12 h, calcined at 450 for 1 h, and then reduced under a H2 flow (100 mL/min) at 400 °C for 5 h.Following th the material was ground to a powder having an average particle size in the range of a proximately 50-70 µm and then packed into a stainless-steel column (internal diamet 3.9 mm; length, 100 mm).
According to a previous study [9], the crystal structure of the TiO2 is mostly anata type, and the BET surface area and crystallite diameter of TiO2 of the 4 wt%Ru wt%Sn/TiO2 catalyst were reported to be 84.5 m 2 /g and 7.7 nm, respectively.Regardi the stability of the catalyst, no difference in catalytic activity was observed even after t continuous use in the temperature range of 140-320 °C for 60 h in the hydrogenolysis aqueous AcOH [10].

Catalytic Conversion and Product Analysis
A flow-type reactor (H-cube Pro™, ThalesNano Inc., Budapest, Hungary) equipp with an electric furnace (Phoenix, ThalesNano Inc.), as shown in Figure 7, was used f these trials.In each experiment, an aqueous solution of EtOH, AcOH, or AA (10 g/L each case) was fed into the catalyst column at a flow rate of 0.3 mL/min at a temperatu in the range of 190-260 °C and a pressure in the range of 0.1-6.0MPa.The resulting m ture was fractionated into aqueous and gaseous phases.The aqueous phase was analyzed by high-performance liquid chromatography (L 20 system, Shimadzu Corp., Kyoto, Japan) using an Aminex HPX-87H column (300 mm 7.8 mm, Bio-Rad Laboratories, Inc., Hercules, CA, USA) with 5 mM sulfuric acid in wa as the mobile phase at a flow rate of 0.6 mL/min and a column temperature of 60 °C, e ploying a refractive index detector (RID-20A, Shimadzu Corp.).The total yield of gaseo The aqueous phase was analyzed by high-performance liquid chromatography (LC-20 system, Shimadzu Corp., Kyoto, Japan) using an Aminex HPX-87H column (300 mm × 7.8 mm, Bio-Rad Laboratories, Inc., Hercules, CA, USA) with 5 mM sulfuric acid in water as the mobile phase at a flow rate of 0.6 mL/min and a column temperature of 60 • C, employing a refractive index detector (RID-20A, Shimadzu Corp.).The total yield of gaseous products was quantified by the water displacement method using a graduated cylinder.The composition of these products was determined by micro gas chromatography (CP-4900, Varian Medical Systems, Palo Alto, CA, USA).These analyses used a 10 m MS5A column with argon as the carrier gas and a column temperature of 100 • C and inlet pressure of 170 kPa along with a thermal conductivity detector or a 10 m PoraPLOT Q column with helium as the carrier gas and a column temperature of 80 • C and inlet pressure of 190 kPa with a thermal conductivity detector.

Conclusions
The oxidation of aqueous EtOH (10 g/L) over 4 wt%Ru-4 wt%Sn/TiO 2 was investigated at various temperatures at a pressure of 0.1 MPa and at various pressures at a temperature of 260 • C. The following conclusions were obtained.

1.
The conversion of aqueous EtOH to AcOH and H 2 over 4 wt%Ru-4 wt%Sn/TiO 2 was more efficient in the gas phase than in the liquid aqueous phase.

2.
The selectivities for AcOH and H 2 were generally high.

3.
Methane, CO, and CO 2 were produced as gaseous by-products via the fragmentation of AA to give CO and CH 4 and of AcOH to give CO 2 and CH 4 , thus reducing the H 2 purity.4.
At a fixed reaction temperature of 260 • C in the gas phase, the EtOH conversion decreased with increasing pressure, whereas the H 2 purity increased because of the less-efficient reaction of AA to generate CO and CH 4 at the higher pressure.

5.
The fragmentation of AcOH to CO 2 and CH 4 was independent of the reaction pressure.6.
The first reaction, EtOH → AA, was the rate-determining step in this process and AA was converted to AcOH and EtOH by the Cannizzaro reaction rather than undergoing direct oxidation to AcOH. 7.
The fragmentation of AA occurred during the conversion of EtOH on the surface of the 4 wt%Ru-4 wt%Sn/TiO 2 catalyst prior to the removal from the surface.8.
Highly concentrated aqueous EtOH solutions (100 and 500 g/L) could also be employed with this process to selectively produce H 2 and AcOH.9.
The reaction pressure should be slightly less than the saturation vapor pressure of water to selectively obtain AcOH and H 2 .10.The use of the 4 wt%Ru-4 wt%Sn/TiO 2 catalyst in a flow reactor was found to be an effective method for the aqueous-phase reforming of EtOH to AcOH and H 2 with high purity.This provides insight into the industrial production of green hydrogen and AcOH.

Figure 1 .
Figure 1.The effects of reaction temperature on the conversion of aqueous EtOH (10 g/L) to AcO and H2, based on the outputs of (A) liquid-phase products, (B) gaseous products other than H2 a (C) H2.The graph in (C) also plots H2 purity in the gaseous products as a function of temperatu

Figure 2 .
Figure 2.The effects of reaction pressure on the conversion of aqueous EtOH (10 g/L) to AcOH and H 2 , based on the outputs of (A) liquid-phase products, (B) gaseous products other than H 2 and (C) H 2 .The graph in (C) also plots H 2 purity in the gaseous products as a function of pressure.Data were acquired with a column having a 3.9 mm diameter and 100 mm length with a pump flow rate of 0.3 mL/min at a temperature of 260 • C.

Figure 3 .
Figure 3.The effects of reaction pressure on the conversion of aqueous AA (10 g/L) to AcOH and H 2 , based on the outputs of (A) liquid-phase products, (B) gaseous products other than H 2 and (C) H 2 .The graph in (C) also plots H 2 purity in the gaseous products as a function of pressure.Data were acquired using a reactor column with dimensions of 3.9 mm diameter and 100 mm length with a pump flow rate of 0.3 mL/min at 260 • C.

Figure 4 .
Figure 4.A diagram showing the Cannizzaro reaction and direct oxidation of AA, which represent competing pathways during the oxidation of EtOH.

Figure 5 .
Figure 5.The effects of reaction pressure on the yield of EtOH produced by the Cannizzaro reaction during the reaction of AA.EtOH (Cannizzaro) refers to EtOH produced from AA by the Cannizzaro reaction.Data were acquired using a reaction column having dimensions of 3.9 mm diameter and 100 mm length with a pump flow rate of 0.3 mL/min at a temperature of 260 °C.

Figure 5 .
Figure 5.The effects of reaction pressure on the yield of EtOH produced by the Cannizzaro reaction during the reaction of AA.EtOH (Cannizzaro) refers to EtOH produced from AA by the Cannizzaro reaction.Data were acquired using a reaction column having dimensions of 3.9 mm diameter and 100 mm length with a pump flow rate of 0.3 mL/min at a temperature of 260 • C.

Figure 6 .
Figure 6.A proposed mechanism for the conversion of aqueous EtOH to AcOH and H2 with the formation of gaseous by-products over a 4 wt%Ru-4 wt%Sn/TiO2 catalyst.

Figure 6 .
Figure 6.A proposed mechanism for the conversion of aqueous EtOH to AcOH and H 2 with the formation of gaseous by-products over a 4 wt%Ru-4 wt%Sn/TiO 2 catalyst.

Figure 7 .
Figure 7.The flow-type reactor used for catalytic conversion trials.

Figure 7 .
Figure 7.The flow-type reactor used for catalytic conversion trials.

Table 1 .
Summary of the effects of reaction pressure on the steps involved in the conversion of aqueous EtOH in steam over 4 wt%Ru-4 wt%Sn /TiO 2 at 260 • C.

Table 2 .
Conversions of high-concentration aqueous EtOH solutions (10, 100 or 500 g/L) to AcOH and H 2 using a column with dimensions of 3.9 mm diameter and 100 mm length with a pump flow rate of 0.3 mL/min at various temperatures and pressures.The yields of AcOH, AA, CH 4 , CO and CO 2 are on an EtOH basis, and the yields of H 2 are on a theoretical basis. *