Chapter 6: Results and Discussion: Catalytic Testing
6.2. Water-impact Study
6.2.2. Effect of water on the hydrogenation of octanal using CuO/Cr 2 O 3
The conversion of octanal and the selectivity to octanol during the hydrogenation of octanal using fresh feed and water-spiked feed is shown in Figure 6.9. The conversion initially decreases from 89 % to 86 % and then increases to 99 % at 5 hours and remains steady at this value for 26 hours. The initial low conversion possibly suggests that the catalyst was not fully reduced prior to the start of the reaction. Hence, when the reaction was underway, some of the hydrogen present was initially used for further reduction of the CuO in the catalyst.
Water
Interaction via hydrogen bonding Surface hydroxyl on alumina
Upon introduction of the water-spiked feed, the conversion initially decreases to 94 % and thereafter reaches 99 % and remains at this value for the remaining period of the reaction. It is possible that the water oxidizes some of the Cu0 present and this causes the drop in conversion. However, as the reaction proceeds, the reaction conditions cause the reduction of these re-oxidized species and do not allow for any further changes to occur.
The selectivity to octanol reaches approximately 96.3 % during the first 31 hours of the reaction when using the fresh feed, whilst after introducing the water-spiked feed, only a minor increase in selectivity was observed (0.5 %).
Figure 6.9: Conversion of octanal and selectivity to octanol for the hydrogenation of octanal using fresh feed and water-spiked feed
A similar conversion (99 %) was obtained for this reaction when using CuO/Al2O3 as the catalyst (Section 6.2.1). From the TPR data obtained from the fresh catalyst characterization, it was determined that CuO/Al2O3 and CuO/Cr2O3 had similar degree of reducibility of 84 and 86 %, respectively. This accounts for the similar activity of these catalysts during the hydrogenation reaction. In addition, the BET surface area for CuO/Cr2O3 is low (25.3 m2 g-1) compared to the much higher surface area for CuO/Al2O3
(128.8 m2 g-1) indicating that the surface area of the catalyst does not influence their activity to a great extent under these operating conditions.
The reaction network in the hydrogenation of octanal when CuO/Cr2O3 is used as the catalyst is the same as that obtained when using CuO/Al2O3 as the catalyst. The by- products formed in this reaction are as listed in Figure 6.10. The selectivity to these by-
70 75 80 85 90 95 100
70 75 80 85 90 95 100
1 3 5 6 8 29 31 34 53 55 74 76 79 81 98 100 103 Selectivity to octanol/%
Conversion of octanal/%
Time-on-Stream/h Selectivity Conversion
Water-spiked feed Fresh feed
products formed during the reaction with the fresh feed and the water-spiked feed, is shown in Figure 6.10. The major by-product formed during the reaction with the fresh feed and the water-spiked feed is the C16 diol. The selectivity to the C16 diol when using the fresh feed is around 2.2 %, whilst a selectivity of 2 % is reached when the reaction is carried out with the water-spiked feed. Similarly, all other by-products showed a minor change in the selectivity after introducing the water-spiked feed into the system.
Figure 6.10: Selectivity to the various by-products formed during the hydrogenation of octanal using the fresh feed and the water-spiked feed
In Section 6.2.1, it was suggested that the presence of water in the feed stream can improve the selectivity to octanol by suppressing the formation of by-products (using CuO/Al2O3 as the catalyst). However, a key factor favoring this trend is believed to be the presence of surface hydroxyls on the catalyst support. Since a minor increase in the selectivity to octanol was obtained when using CuO/Cr2O3 as the catalyst, the effect of water on the hydrogenation of octanal is not as pronounced as when CuO/Al2O3 is used as the catalyst. It was expected for the trends observed with CuO/Al2O3 as the catalyst to be present for the reaction with CuO/Cr2O3 as the catalyst, since Armistead et al.6 state that all metal oxides contain varying amounts of surface hydroxyls. Since only a minor change in the selectivity to octanol and the selectivity to the C16 diol and the C24 acetal (by- products mainly formed due to the presence of the support) is obtained, it indicates that this catalyst contains a low concentration of surface hydroxyls. However, since the C16 diol and the C24 acetal form with selectivity values similar to those obtained when using
0 0.5 1 1.5 2
Alkanes Other
alcohols Ethers Octanoic acid Octyl
octanoate C16 diol C24 acetal Unknowns
Selectivity to other products/%
Other products Fresh feed Water-spiked feed
CuO/Al2O3 as the catalyst, it implies that there may be partial passivation of the acid and base sites responsible for their formation. It may also be that these acid and base sites (which usually exist due to surface hydroxyls) may not have functional groups that can interact with the water via hydrogen bonding and cause the by-product formation to decrease and thus favor the selectivity to octanol. Since the conversion of octanal and the selectivity to octanol essentially remain unchanged after the introduction of the water- spiked feed, it indicates that the presence of water in the reactant stream does not influence the hydrogenation of octanal for this system.