Catalytic ethanol oxidation

The catalytic oxidation of ethanol was performed in a continuous-flow fixed-bed reactor. The catalytic products obtained were acetaldehyde, acetic acid and ethyl acetate and can be seen in

Scheme 5.1 in the introduction section of this chapter. The catalytic results showed inactivity of silica (KIT-6) as a pure support. The activities of the native mesoporous metal oxides were very low and increased in the following order, MnO2 < CeO2 < Co3O4 as summarized in Table

5.3. The mesoporous Co3O4 displayed higher activity than MnO2 and CeO2 because of its

relatively high surface area and lower reduction temperature. The difference between activities of MnO2 and CeO2 is small. The positive effect of low reduction temperature of MnO2 is

probably cancelled by the low surface area it possesses. While on the other hand, the positive effect that results from high surface area of CeO2 is cancelled by a high reduction temperature

Table 5.3: Ethanol oxidation using mesoporous metal oxides (MMO) as support for Pd28Au28 nanoalloys at 120 °C. Entry Metal oxide Metal(s) [Au] / % Temp / °C Pressure (bar) Conversion / % Selectivitya _/ % GHSV (mL·g-1·_hr-1₎ Ref 1 SiO2 - - 120 3 < 0.1 - - This wor k 2 SiO2 Pd28Au28 2.3 120 3 4.2 > 73 59676 3 Co3O4 - - 120 3 0.5 - 58375 4 Co3O4 Pd28Au28 2.3 120 3 1.6 > 81 59138 5 MnO2 - - 120 3 < 0.1 - 60148 6 MnO2 Pd28Au28 2 120 3 1.1 > 92 56132 7 CeO2 - - 120 3 < 0.1 - 59499 8 CeO2 Pd28Au28 2.2 120 3 1.5 > 92 55661 9 SiO2 Au 1 350 5 18 72 18000 [32] 10 SiO2 AuCu 3.6 200 - - 80 - [33] 11 CeO2 Au 1 350 5 34 84 18000 [32] 12 Co3O4 Au 11.4 140 - - 34 - [26]

a _{% selectivity towards acetaldehyde and the remaining percentage is for acetic acid and ethyl}

acetate.

A significant increase in ethanol percentage conversion was achieved upon immobilization of AuPd nanoalloy onto the MMOs. The AuPd on Co3O4 improved the activity by a factor of

three (see Entries 3 and 4) and the activity of MnO2 and CeO2 based catalysts was improved

by almost 20 (see Entries 5 – 8 in Table 5.3). The strongest improvement was observed for AuPd supported on silica, where the final conversion level was approximately 50 times higher compared to the native support (see Entries 1 and 2). However, this catalyst suffered significantly towards acetaldehyde selectivity at higher temperatures compared to AuPd/MMO catalysts. As a result, further studies were performed using MMO supported catalysts only. At 120 °C, formation of possible by-products acetic acid and ethyl acetate could not be observed for the native MMO materials due to low conversions. The AuPd/MMO catalyst showed high selectivity toward acetaldehyde at 120 °C, however, the selectivity decreased at higher temperatures as shown for Au28Pd28/Co3O4 in Figure 5.5.

Figure 5.5: Conversion (top) and selectivity (bottom) versus temperature for ethanol oxidation using Au28Pd28/Co3O4 catalysts. Reaction conditions: p = 3bar, flow rate = 0.2 L/min and 200

mg of catalyst.

From the temperature variations apparent activation energies, EA,app, have been calculated (see

Figure S5.5 in SI). For native Co3O4 and Au28Pd28/Co3O4 the values were almost identical with

74 kJ·mol-1 and 70 kJ·mol-1, respectively. In Table 5.4 the apparent activation energies are summarized. Mass transport limitations due to pore diffusion of ethanol and oxygen do not seem to limit the overall reaction rate. This is not surprising since powdered catalysts with < 90 µm particle size have been used.

Table 5.4: Apparent activation energies for native mesoporous metal oxides and Au28Pd28/MMO catalysts.

Entry Metal oxide EA,app / kJ·mol-1

1 Co3O4 74 2 MnO2 68 3 CeO2 74 4 Au28Pd28/Co3O4 70 5 Au28Pd28/MnO2 70 6 Au28Pd28/CeO2 58

Reaction condition: 3 bar, 0.2 L/min and 200 mg of the catalyst.

The apparent activation energies are almost the same in case of Co3O4 and MnO2 (see Entries

4 and 5, Table 5.4). Note that those two materials exhibited the lowest 1st reduction temperatures at 331 and 311 °C, respectively (see Table 5.2). In contrast, the CeO2 support and

the Au28Pd28 nanoalloy supported onto it required significantly higher reduction temperatures

around 520 and 510 °C. Therefore, the reducibility of Co3O4 and MnO2 is faster at lower

temperatures, especially in the regime between 120 and 160 °C. The low activity of cerium oxide is based on its lower reducibility as depicted by high reduction temperature and the few oxygen vacancies in the cerium oxide structure at the temperature at which the reaction was conducted. It has been reported that the highest concentration of lattice vacancies occur at higher temperatures for ceria and these oxygen vacancies enhance the reducibility of cerium oxide [34].

To study the contribution of the nanoalloy in more detail, MnO2 was used as support material.

Nanoalloys with different Au:Pd ratios were deposited onto the support, namely Au28Pd28 (1:1)

Au45Pd9 (5:1) and Au50Pd5 (10:1). The influence of the composition was studied at 1 bar in the

Table 5.5: Ethanol oxidation using different AuPd nanoalloys supported on MnO2. Catalyst Temp (°C) XEtOHa / % SAAb / % Au28Pd28/MnO2 120 1.1 96.2 140 5.7 86.7 160 7.8 81.9 Au45Pd9/MnO2 120 0.2 > 99.5 140 0.8 > 99.5 160 1.6 > 99.5 Au50Pd5/MnO2 120 0.1 > 99.5 140 0.3 > 99.5 160 0.8 > 99.5

Reaction conditions: p = 1 bar, volume flow = 0.2 L min-1, 200 mg of catalyst. a ethanol percentage conversion. b percentage selectivity towards acetaldehyde.

As expected, the activity increased with higher reaction temperature for all three catalysts and apparent activation energies between 70 kJ mol-1 (Au:Pd 1:1) and 87 kJ mol-1 (Au:Pd 10:1) were observed (see Figure S5.6 in SI). The activity of the different catalysts seems to be significantly influenced by the Au:Pd ratio and thus by the size of the Au surface-islands that form within the Pd matrix. We postulate a Langmuir-Hinshelwood mechanism where both the substrate and the oxidant adsorb on the surface of the catalyst prior to the reaction. At high Au:Pd ratios the size of these Au surface-islands is probably too large to effectively split the adsorbed oxygen molecule. This can be seen from the percentage conversions, where the catalyst with the highest amount of Au is the least active. This then leads to another assumption that the Pd surface effectively splits oxygen more than the Au surface. As a result, the larger the Au surface-islands the lower the conversion but exclusive formation of the acetaldehyde (AA) is observed. At Au:Pd ratios of 10:1 larger Au surface-islands can exist, the splitting of oxygen occurs slow but the desorption of acetaldehyde occurs fast. The fast desorption of acetaldehyde from the Au surface-islands ensure that there is minimal oxidation of acetaldehyde to acetic acid and other related products.