5. RESULTS AND DISCUSSION
5.1. HMF AS THE STARTING REACTANT
5.1.3. Effect of Reaction Time
5.1.2. Effect of Reaction Temperature
As shown in Figure 5.2, 353K proves to be the most suitable for the synthesis of DMF. Going against the goals of this research, temperature variables were still tested because yields would not exceed 15% at this point. Since most of the other works have conducted this reaction under higher temperatures, it was guessed that maybe temperature was the key to this reaction. After these tests, it was proven that temperature does effect the yield of this reaction, but the key to a massive increase of the yield for this reaction lies somewhere else.
5.1.3. Effect of Reaction Time
The results shown in Figure 5.3 suggest that a notable increase of DMF yield occurs at 3 hours. Figure 5.3 and 5.4 are conducted under different reaction containers.
Figure 5.4 shows higher yield due to the suspected higher pressure related with the smaller amount of reactor volume. Nonetheless, it can be concluded that even after 9 hours, the yield of DMF remains at about 10%, meaning 3 hours is the optimum reaction time. It is also favored to have a shorter reaction time.
52
Figure 5.3 DMF yield varying reaction time. 4.5 mL THF in 10 mL Vial, 0.05 g HMF, 0.06 g NaBH
4, 1 mL DI Water, 0.1 g Catalyst.
Figure 5.4 DMF yield varying reaction time. 4.5 mL THF in 7 mL Vial,
0.05 g HMF, 0.06 g NaBH
4, 1 mL DI Water, 0.1 g Catalyst.
53 5.1.4. Effect of NaOH addition
After trying out several obvious and easily tunable variables, the problem could be confirmed to be elsewhere, therefore the addition of NaOH was considered. Our source of hydrogen comes from NaBH4, and the rate of hydrogen production is determined by the acidity of solution it is dissolved in. Solutions of higher acidity will increase the rate of hydrogen production while lower acidity will decrease the production rate. The high hydrogen production rate could lead to unused hydrogen gas that would rise to the top of the reactor due to the low solubility of hydrogen gas in most solvents. When the hydrogenation rate is significantly lower than the hydrogen production rate, there would be less contact between hydrogen gas and the active site. Therefore it is thought that a slower hydrogen production rate will help achieve a steady state of hydrogen usage and production. It could be argued that increasing the amount of NaBH4 would lead to the same result, but it would be most efficient if the least NaBH4 could be used. It can be seen on Figure 5.5 that addition of NaOH does indeed slow down the production of hydrogen, which leads to lower yields under 3 hours of reaction. Temperature varying tests were also given to show that even under reaction at 353K, DMF yield could not exceed that of reactions without NaOH additions.
54
Figure 5.5 DMF yield with or without NaOH addition. 4.5 mL THF in 7 mL Vial, 0.05 g HMF, 0.06 g NaBH
4, 0.14 mL DI Water + NaOH, 0.1 g
Catalyst, 3 hr.
Figure 5.6 DMF yield with NaOH addition varying temperature. 4.5 mL
THF in 7 mL Vial, 0.05 g HMF, 0.06 g NaBH
4, 0.14 mL DI Water +
55
5.2. Comparison of Different Starting Reactants
With knowledge that supported palladium catalysts often show higher affinity for aromatic aldehyde substituents, it was then proposed that starting with a furan structure that contains only aldehyde groups would amount to a higher yield of DMF.
As the aldehyde substituents attach itself on the palladium surface, the hydrogenation of the aldehyde group to an alcohol group occurs, while the hydrogenolysis of the alcohol group instantly follows. This higher affinity between the palladium surface and the aldehyde group compared to the affinity between the palladium surface and an alcohol group could possibly amount to a higher yield of DMF. Two possible candidates were 5-Methylfurfural (MFAD) and 2,5-diformylfuran (DFF).
5-Methylfurfural has a similar structure to HMF, except that the alcohol group on HMF has been replaced with a methyl group. This means that MFAD needs to go through only two reaction steps as compared to three of HMF to be converted into DMF, which means that a higher yield should be anticipated as shown in Table 5.1.
One of the major benefits of using MFAD as the starting reactant lies in that it is significantly cheaper than HMF. At this point of research, these results were enough to
justify the use of MFAD as the starting reactant for this research. Using DMF as a starting reactant proves that there weren’t any further reactions that could occur in this
system. As shown in Table 5.1, BHMF having lower DMF yield compared to HMF
56
supports the idea that our catalyst shows higher affinity for aromatic aldehyde groups compared to aromatic alcohol groups.
Table 5.1 Yield of DMF starting from different reactants.
Reactant DMF Yield (%)
HMF
4.8
BHMF
3.7
MFAD
7.4
DMF
100
57
5.3. Palladium and Cobalt on Same or Different Supports
It was proposed in this thesis that the effect of Cobalt and Palladium supported on the same support would create a synergetic effect where the hydrogen produced on the cobalt would immediately be adsorbed by the palladium surface to conduct hydrogenation reactions, therefore tests on palladium and cobalt on different supports were conducted. It can be seen from Figure 5.7 that when Palladium and Cobalt are supported on the same supports, they do exhibit synergetic effects to give higher yield.
It could be because when Cobalt and Palladium are supported on different supports, the distance increases between these sites, so the hydrogen produced won’t be adsorbed by the Palladium surfaces, therefore it floats to the liquid surface before utilization.
58
Figure 5.7 DMF yield with Pd and Co location variation. 4.5 mL THF in 7 mL Vial, 0.05 g HMF, 0.06 g NaBH
4, 1 mL DI Water, 303K.
5.4. Reactions Starting with MFAD
As MFAD has been proven to be more efficient in terms yield and it is significantly cheaper than HMF (With MFAD being 3.96 US dollars per gram and HMF being 27.6 US dollars per gram on the Sigma Aldrich website as of June 11th, 2014), the following experiments have been conducted with MFAD as the starting reactant.
5.4.1. With or Without Acid Addition
As the research progressed, most of the variables have been tried. After a comparison with other works on DMF synthesis, it was found that most of the other
0
59
works have been based on either acidic or neutral environments.52 The system used in this research would become more basic as the reaction went on. As seen in Eqn. 1, sodium borohydride is converted into sodium metaborate after the release of hydrogen gas, and sodium metaborate is a basic compound. Therefore, externally added acid is needed to bring the reaction environment to neutral or acidic conditions. Sulfuric acid was chosen for this work because HCl could have leaching effects on our catalyst due to the tendency of Cobalt forming Cobalt chloride. It was mentioned in the experimental section that two different systems were explored in this experiment, because the timing for the addition of sodium borohydride could be crucial for determining the environment of the reaction. Therefore it can be seen in Figure 5.8 and 5.9 that the addition of acids together with sodium borohydride would negate the acid effect too soon which means the solution turns basic too soon. The slow addition of sodium borohydride results in the solution being acidic for a longer period of time, resulting in higher yields. It can be seen on Figure 5.10 that the addition of acid does in fact increase the yield significantly. Therefore it is important to find the optimum amount of acid needed. Figure 5.11 shows tests on the optimum amount of sulfuric acid needed for the highest yield.
60
Figure 5.8 Scheme for batch and semi-batch reactions determining the timing for the addition of sodium borohydride.
Figure 5.9 Comparison of Batch or Semi-Batch yields.
0 10 20 30 40 50 60 70
Batch Semi-Batch
Y iel d (% )
Batch or Semi-Batch Operations
61
Figure 5.10 DMF yield with or without H
2SO
4addition. 4.5 mL THF in 20 mL Vial, 39 μL MFAD, 0.06 g NaBH
4, 30 wt% NaOH solution in pump, 0.1 g Catalyst, 303K, 0.34 mL/hr for 3 hr, then 1 hr reaction time.
(8 μL H
2SO
4added if required.) 0
10 20 30 40 50 60 70
Without Acid With Acid
Y iel d (% )
Addition of Acid
62
Figure 5.11 DMF yield with addition of varying amounts of H
2SO
4. 4.5 mL THF in 10 mL Vial, 39 μL MFAD, 0.06 g NaBH
4, 30 wt% NaOH solution in pump, 0.1 g Catalyst, 303K, 0.34 mL/hr for 3 hr, then 1 hr
reaction time.
5.5. Different Starting Reactants under Optimum Conditions
After the optimum reaction conditions and the right system was found. Tests were done for HMF, DFF and MFAD to confirm once again that the Pd/CoNC catalyst shows higher affinity for aldehyde groups. As seen from the results in Figure 5.12, HMF and DFF shows similar yields, meaning that the yield of the four step reaction from DFF shows similar yields to the three step reaction from HMF.
However, using DFF as the starting reactant isn’t economic due to its high cost as of
0
63
now. All in all, using this system to compare with other group’s works, a DMF yield
of 54% could still be obtained under room temperature and atmospheric pressure, while using an aqueous source of hydrogen.
Figure 5.12 DMF yield starting with different reactants. 4.5 mL THF in 10 mL Vial, 0.06 g NaBH
4, 30 wt% NaOH solution in pump, 0.1 g Catalyst, 303K, 1 Atm, 0.34 mL/hr for 3 hr, then 1 hr reaction time.
(Equal mol of Reactant)
5.6. Effect of Atm or Non-Atm Pressure Tests
After the optimum condition was found, tests on atmospheric or non-atmospheric tests were conducted and it can be found that the atmospheric pressure tests result in
0
64
better selectivity overall, as shown in the GC-MS spectrums shown in Figure 5.13.
Figure 5.13 Atmospheric and Non-Atmospheric Pressure Tests.
5.7. Synergetic Effects of Cobalt and Palladium
The catalyst effect was tested by going through only using Palladium or Cobalt.
A comparison can be seen in Figure 5.14, where Palladium alone shows poor catalytic effects. It is proposed that it is because Palladium shows no catalytic effects for the production of sodium borohydride, or the competition between the usage of the active site for production of hydrogen or hydrogenation hinders the reaction.
65
Figure 5.14 Comparison of catalyst with Palladium or Cobalt only.
0 20 40 60 80 100
Pd/C Pd/CoNC
Y iel d (% )
Catalyst Synergy Effect
66
6. CONCLUSION
In this thesis, we aimed to produce DMF using a novel method never demonstrated before. Using a combination of Sodium Borohydride and our Pd/CoNC catalyst, we were able to provide hydrogen for hydrogenation in the solution under room temperature and atmospheric pressure. The catalyst Pd/CoNC shows bifunctional properties, with Cobalt used as a catalyst for hydrogen production from NaBH4 and palladium being the catalytic surface for the conversion of reactant to product.
It is also discussed in this thesis that our catalyst shows better affinity for aldehyde groups compared to alcohol groups, and MFAD is significantly cheaper than HMF, which makes it more suitable as a starting reactant when using our catalyst.
The highest yield achieved under room temperature and atmospheric pressure was 83.07% yield of DMF. When using the same starting reactant as other people have used, a yield of 54.1% could still be achieved. We propose that the high yield comes from the fact that hydrogen is produced near the active sites compared to traditional methods where hydrogen is externally added.
67
7. FUTURE PROSPECTS
As can be seen in the results and discussion section, most of the data were very rough and some of the results did not make sense. This was due to the fact that this work is still in the very early stage of discovery. Most of the experiments were done under the context of an urge to increase the yield.
Going through the whole research again, the catalyst amount could be optimized to have a higher efficiency, while neglecting tests on temperature. The perfect balance between reaction time and the amount of NaOH added could also be tested to obtain the best results. Other homogeneous acids could also be tested to see which of them could show the best effect. Even going further, a catalyst overhaul could be done, such as using other supports like mesoporous silica functionalized with acidic groups to obtain a heterogeneous acid, while depositing palladium and cobalt through methods such as impregnation. This would make even recycling of the catalyst possible, and prevent corrosion of reactor. Catalysts that have never been explored in this field such as ZIF could also be explored. A different hydrogen carrier could also be used, since there have been works using Ammonia Borane (AB) or Hydrazine Borane (HB) as hydrogen carriers and they also produce hydrogen gas rather than providing hydrogen through chemical means (Transfer hydrogenation).74,75 Lastly,
68
recycling tests and different metals could be tried to screen for the best candidate, such as replacing precious metals with abundant metals.76
69
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