Chapter 6: Results and Discussion: Catalytic Testing
6.2. Water-impact Study
6.2.3. Effect of water on the hydrogenation of octanal using CuO/SiO 2
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.
Figure 6.12 shows the selectivity to the various by-products formed during the reaction with fresh feed only. The reaction network in the hydrogenation of octanal using CuO/SiO2 is similar to that obtained with CuO/Al2O3 and the by-products formed are as listed in Figure 6.12. The C16 diol is initially the major by-product formed, however, as the reaction progresses the selectivity to dioctyl ether (represented as ethers), C24 acetal and heptane (represented as alkanes) is seen to increase, whilst the selectivity to the C16 diol decreases. The selectivity to the C16 diol reaches approximately 2 % after 8 hours on stream, after which time it steadily decreases to 0 % by the end of the reaction. The selectivity to the C24 acetal is less than 0.1 % at the start of the reaction and it progressively increases to 7.5 % after 55 hours on stream. The selectivities to heptane and dioctyl ether show an overall increase of around 0.2 % by the end of the reaction. The increase in the selectivity to dioctyl ether may due to the increase in the hydrocracking of the C24 acetal, as more of this compound is formed during the course of reaction. It is also possible that the aldol condensation of octanol is favored, thus increasing the selectivity to dioctyl ether. The increase in the selectivity to the C24 acetal and heptane indicates that the conversion of octanal to octanol becomes less preferred as the reaction proceeds. This possibly implies that the required adsorption of octanal on the catalyst to allow for the hydrogenation to octanol becomes less favored during the course of the reaction.
Figure 6.12: Selectivity to the various by-products formed during the hydrogenation of octanal using fresh feed
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1 3 5 7 8 26 28 29 31 50 51 55
Selectivity/%
Time-on-Stream/h
Alkanes Other alcohols Ethers Octyl octanoate
C16 diol C24 acetal Unknowns
The trends observed with the conversion of octanal and the selectivity to octanol and the by-products (as discussed above) may be explained based on the surface charge of silica.
Due to the nature of silica and its isoelectric point of 2 (the pH at which the surface of the metal oxide is not charged)14, the adsorption of cations primarily occurs.14 Since octanal has a positive dipole on the carbonyl carbon and a negative dipole on the carbonyl oxygen, the adsorption via the delta positive end will be more likely. The required orientation of octanal on the catalyst surface to promote the conversion of octanal to octanol is via the positive and negative ends. Hence, since the adsorption of cations is preferred, it is most likely that the probability of the required orientation of octanal to form octanol decreases as the reaction progresses. Furthermore, as the reaction proceeds, the surface of the silica becomes polarized and charged. As a result, the support no longer enriches the active metal with electrons and the backbonding (discussed in Section 6.1.1) is no longer favored. Since the adsorption of cations is preferred and the catalyst is comprised of acid sites (determined using NH3-TPD, Section 5.7), the formation of the C24 acetal is promoted since the protonation of octanal yields a cation (Scheme 6.3). The decrease in the selectivity to the C16 diol supports the notion that cation adsorption is preferred since an octanal nucleophile must interact with the octanal electrophile on the catalyst so that aldol condensation of octanal can occur (Scheme 6.2).
Due to the nature of the adsorption of octanal and the dipolar nature of octanol, desorption of octanol occurs much slower in comparison to the reactions using CuO/Al2O3
and CuO/Cr2O3. This allows for longer residence times and therefore the formation of the heavy by-products increases with time-on-stream. Since alumina and chromia have isoelectric points greater than 7 and can adsorb cations and anions14, the effect of surface charge is not observed and catalyst deactivation does not occur.
Since steady state could not be reached and catalyst deactivation set in, the hydrogenation of octanal was carried out using water-spiked feed over fresh CuO/SiO2. Figure 6.13 shows the conversion of octanal and the selectivity to octanol obtained during the reaction. The conversion is initially at 84 % and gradually decreases to 73 % after 55 hours on stream. This indicates that the presence of water in the feed does not prevent the catalyst deactivation but slows down the process. The selectivity to octanol reaches approximately 98 % and remains steady at this value for the duration of the reaction.
Figure 6.13: Conversion of octanal and the selectivity to octanol during the hydrogenation of octanal using water-spiked feed
The selectivity to the various by-products formed during the hydrogenation of octanal using fresh feed and water-spiked feed is shown in Figure 6.14. In the presence of the water-spiked feed, the C16 diol is the major by-product formed with a selectivity of around 1.3 %. The selectivity to all other by-products, in the presence of water, is less than 0.3 %. These selectivity values are similar to those obtained with CuO/Al2O3 and CuO/Cr2O3 using fresh feed only. This indicates that in the presence of water the catalyst is able to maintain a selectivity to octanol that is similar to that obtained with the more stable and active catalysts. In comparing these selectivity values with those obtained from the reaction with the fresh feed only (Figure 6.14), it is clearly seen that in the presence of water, the selectivity to all by-products, except the C16 diol, is noticeably reduced. These selectivity trends demonstrate the beneficial effect of water in the reactant stream.
The improved selectivity to octanol obtained in the presence of water can be explained using the argument presented in Section 6.2.1. As with Al2O3, silica contains surface hydroxyls6, 8, 11, 15 (which allow for by-product formation) and the presence of water will passivate or neutralize these hydroxyls. Hence, the selectivity to octanol is improved by preventing by-product formation. Furthermore, the interaction of water with the surface hydroxyl would prevent any changes in the surface charge. This will allow for the adsorption of octanal to occur so that the conversion to octanol is promoted. Since a high
70 75 80 85 90 95 100
70 75 80 85 90
1 3 4 6 8 25 27 29 31 50 51 55 Selectivity to octanol/%
Conversion of octanal/%
Time-on-Stream/h
Selectivity Conversion
selectivity to octanol is maintained throughout the reaction, it indicates that the surface hydroxyls on silica are most likely responsible for the influence of surface charge and the trends observed during the reaction with the fresh feed. However, since there is still a decline in the conversion of octanal, it indicates that there is a catalyst deactivation mechanism that plays a significant role in the loss of catalytic activity. The decrease in the BET surface area and other changes in the used catalyst indicate that mechanical failure is most likely contributing to the loss of activity over time. The characterization of the used CuO/SiO2 catalyst is discussed in greater detail in Section 6.3.
Figure 6.14: Selectivity to the various by-products formed during the hydrogenation of octanal using fresh feed and water-spiked feed