Before jumping to the discussion on parameters effect, it’s better to take a look on catalyst characterizations. The first characterization is N2 adsorption-desorption analyzer, which could give information regarding characteristics of porous material covered surface area, and pore size. From figure below, combination of no sharp peak only broad obtuse distribution and hysteresis loop, indicate carbon matrix has different pore sizes (micro and mesopore).
Figure 11 N2 adsorption-desorption isotherm of Rh/C
Figure 12 shows the XPS spectra of Rh/C, which contains information of valence state of rhodium metal inside the Rh/C. After fitting process, information shows that not all rhodium metal inside the carbon matrix are in zero valence. Different valence could effect the performance of Rh/C used as catalyst.
0 100 200 300 400
0 0.2 0.4 0.6 0.8 1
Volume Adsorbed @ STP (cm3 g-1 )
Relative Pressure (P/Po)
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Figure 12 XPS spectra of Rh/C
Figure 13 shows TEM image, indicating black spot represented by rhodium metal on the carbon support. From the image, there is a concentrated black spot indicated the rhodium metal not distributed quite well.
Figure 13 TEM image of Rh/C
100 200 300
319.70 318.20 316.70 315.20 313.70 312.20 310.70 309.20 307.70 306.20 304.70 303.20 301.70 300.20
Count / s
Binding Energy (eV)
Original data Rh(0)
Rh(I) Rh(II)
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Figure 14 EDS elemental mapping
Figure 14 shows the qualitative analysis of rhodium metal. Actually, there are several techiques that can be used to determine the amount of metal in solid material, but for one and other reason, author couldn’t use any of these techniques.
Techique Advantages Disadvantages Current situation
ICP-OES/MS
AAS Fast, accurate, well documented
26 XRF
Relatively simple sample preparation, few
interferences
Very slow XRF standard very expensive UV-Vis Simple to use Poor in selectivity
and sensitivity
For organometallic only
Table 2 Different analytical techniques to quantify metal content
Hydrogenation of furfuryl alcohol to THFA isn’t new reaction. So far, there is no report on this particular reaction using NaBH4 as hydrogen-generator. Author designed the system not only in order to avoid high pressure (open system), and perform at 30°C, but also the reaction will be occurred in liquid-phase reaction.
Figure 15 Screening metal for FFA hydrogenation to THFA (NaBH4 method) Reaction condition : 30°C, 1 bar, 2 h, 0.005 g catalyst (5 wt%), 5 mL of DMA, 10.4 µL
FFA (0.12145 mmol), 8 µL H2SO4(conc), 0.5 mL of 1 wt% NaBH4(aq), dropwise rate = 0.25 mL/h
Commercial 5wt% rhodium metal embedded on carbon was selected to this particular reaction because there was a report by Antoine Gaset’s group in 1989 using same catalyst to perform FFA hydrogenation to THFA. This group achieved 53%
conversion and 30% yield of THFA, made 57% of selectivity in 26 minutes. Also author
0
Conversion of FFA Yield of THFA
63%
20% 20%
17%
60%
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did perform screening several catalyst started Ru/AC, Pd/AC, and In/AC before decided to use the commercial Rh/C, but none of these could produce THFA as author wanted.
Since the % yield isn’t equal to % conversion, there would be by-products other than THFA. Author tried to confirm these by-product using their standards in GC, and methyl tetrahydro furan (MTHF), 1-pentanol, 1,5-pentanediol were not in these by-products category.
Figure 16 The effect of FFA/Rh molar ratio on THFA production (NaBH4 method) Reaction condition : 30°C, 1 bar, 2 h, 0.005 g catalyst (5 wt%), 5 mL of DMA, 10.4 µL
8 µL H2SO4(conc), 0.5 mL of 1 wt% NaBH4(aq), dropwise rate = 0.25 mL/h
Rhodium/carbon was selected for further optimization. Figure 16 shows the increment of yield but then drop start from 50 molar ratio. 10 molar ratio shows almost all 42% of FFA converted to THFA. This particular condition reaction shows high selectivity. Highest yield of THFA was achieved at 20 molar ratio, which indicated amount of FFA quite low in system so the Rh/C catalyst quite helpful converting FFA to THFA, but trade-off occurred between yield and selectivity. 50 molar ratio shows
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62% conversion, but low as 21% yield of THFA. As the molar ratio increases, and the amount of catalyst’s active sites are fixed, make the catalyst itself get saturated quickly.
Turnover frequency (TOF) represents catalytic ability of catalyst, which means the turnover per unit time in definition. The best yield was achieved at 20 molar ratio, 0.022 mmol of THFA was produced using 0.002429 mmol of rhodium, resulted 0.00126 s-1. Nevertheless, since no molar ratio could produce 100% conversion, so author decided to select 50 molar ratio for further optimization.
The amount of NaBH4 addition was studied using 50 molar ratio. On procedure section, the preparation of aqueous solution pH 14 is to inhibit the production rate of H2
gas. The meaning of ―value wt%‖ on the figure doesn’t show real amount of NaBH4 in the aqueous, because the nature of self-hydrolysis. Since environment inside the vial was acid, the moment NaBH4(aq) made contact to system, OH- would react to excess H+ made the production rate of H2 increased, and the hydrogenation could be occurred.
Figure 17 shows the inclining trend of conversion and yield. The increase amount of NaBH4, means production of H2 also increases, make more FFA could react with H2. The best yield was achieved at 5 wt% NaBH4. As the amount of NaBH4 increases to 8 wt% NaBH4, yield of THFA drops.
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Figure 17 The effect of different NaBH4 amount on THFA production Reaction condition : 30°C, 1 bar, 2 h, 0.005 g catalyst (5 wt%), 10.4 µL FFA (0.12145 mmol),5 mL of DMA, 8 µL H2SO4(conc), dropwise rate = 0.25 mL/h
The amount of H2SO4 addition was also studied, showed on figure 19. Inside the vial, there were FFA, catalyst, DMA (solvent), and H2SO4. In such very acid environment, FFA get protonated. One report was published in 2011 by Peter C. Stair’s group related to acid-catalyzed furfuryl alcohol polymerization.[32] They calculated free energy of every possible protonated state of FFA. Based on the thermodynamics of protonation, all sites were found to be thermodynamically unfavorable at room temperature, with positions 2, 3, and 5 being significantly endothermic (52.3 – 128.9 kJ mol-1). Protonation at positions 1 and 6 are more probable than other positions as per assessment of their thermodynamic feasibilities, at which the change in free energy of protonation is 1.3 and 19.2 kJ mol-1, respectively.[32] In author’s system, there was a possibility that protonated states in all position 1, 2, 3, 4, 5, and 6 occurred, because the 100% conversion and unfavourable 100% yield of THFA.
0
Conversion of FFA Yield of THFA
33%
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Figure 18 Six protonated FFA molecular structures[32]
Figure 19 The effect of different H2SO4 amount on THFA production (NaBH4 method).
Reaction condition : 30°C, 1 bar, 2 h, 0.005 g catalyst (5 wt%), 10.4 µL FFA Conversion of FFA Yield of THFA
100% 100% 100% 100%
35%
79%
51% 56%
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The yield of THFA over time was also studied. Figure 20 shows there is no inclining of THFA yield after 2 hours. At 1 hour reaction, it produced 28% yield of THFA, but the showed 100% conversion. There is possibility that FFA still in the intermediate state to become THFA before 2 hours reaction, and later after 2 hours reaction, yield of THFA increases dramatically 80%. As the reaction time increases, the yield of THFA is almost constant around 80%, indicating no more intermediate state that would be transformed into THFA.
Figure 20 The yield of THFA over time (NaBH4 method)
Reaction condition : 30°C, 1 bar, 0.005 g catalyst (5 ww%), 10.4 µL FFA (0.12145 mmol), 8 µL H2SO4(conc), 0.5 mL of 5 wv% NaBH4(aq),dropwise rate = 0.25 mL/h
After all, catalyst Rh/C couldn’t achieve high yield (> 90%) after optimization, so next experiment was to test the commercial catalyst Rh/C using H2 gas at open system.
The optimization results for each method would be compared to each other. Figure 21 shows increasing yield of THFA in the ambient and in short period of reaction time Conversion of FFA Yield of THFA
100% 100% 100% 100%
28%
79% 74%
83%
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environment not really quite harsh compare to NaBH4 method. This indicates good performance for this hydrogenation using H2 gas. At 100 molar ratio, the turnover frequency is 0.01504 s-1 which is better than previous method for initial step. 100 molar ratio was selected for further optimization because of selective consideration.
Figure 21 The effect of Rh/FFA molar ratio on THFA production (H2 gas method) Reaction condition : 30°C, 1 h, 0.005 g catalyst (5 wt%), 5 mL of DMA,
H2 gas at 1 bar, gas flow rate = 100 cc/min
The yield of THFA over different temperature was studied. Figure 22 shows there is incremental of yield at 50°C, and drops as the temperature increases. Not only it shows that commercial catalyst Rh/C is well performed under ambient temperature, but also shows high selectivity toward THFA at 50°C. The selectivity decreased as temperature increased. Author suspects that the reason why using NaBH4 could give lower selectivity, maybe because since the NaBH4 could perform self-hydrolysis (production of H2 gas increases), and the reaction quite is exothermic (-210 kJ mol-1), make the temperature increased suddenly inside the vials.
0
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Figure 22 The effect of temperature on THFA production (H2 gas method) Reaction condition : 1 h, 0.005 g catalyst (5 wt%), 20.8 µL of FFA (0.2429 mmol)
5 mL of DMA, H2 gas at 1 bar, gas flow rate = 100 cc/min
Figure 23 The yield of THFA over time (H2 gas method)
Reaction condition :50°C, 0.005 g catalyst (5 wt%), 20.8 µL of FFA (0.2429 mmol), 5 mL of DMA, H2 gas at 1 bar, gas flow rate = 100 cc/min Conversion of FFA Yield of THFA
0
34
Figure 23 shows the increasing yield of THFA as reaction time increases. The high selectivity keep the pace with the conversion. The 100% conversion couldn’t be achieved whatever reaction time progressed.The maximum of FFA conversion is ~94%
followed by ~93% yield, make this result the highest selectivity for author could achieve so far.
Figure 24 Cycled test for catalyst (H2 gas method)
Reaction condition :1h, 50°C, 0.005 g catalyst (5 wt%), 20.8 µL of FFA (0.2429 mmol), 5 mL of DMA, H2 gas at 1 bar, gas flow rate = 100 cc/min
Since the best achievement was gotten from using H2 gas, so author decided to perform the recyclability test using this method. Commercial catalyst Rh/C was tested several runs to check its reusability. First, the quantity of catalyst and FFA was scaled up to 10 times, and performed the reaction. After the reaction finished, solution was centrifuged to separate the catalyst and the solution. Catalyst was washed using water for 6 times to remove the substituents (reactant, product, by-product). Catalyst was dried in the oven at 100°C overnight. As amount as 0.005 g of catalyst was used with equal 100 molar ratio for reaction. The results was analyzed using GC. This result was
0
Conversion of FFA Yield of THFA
62%
35
counted for run 2. The remaining catalyst was exposed to FFA with equal ratio, and run for 1 hour. This steps was done till 4 times. Figure 20 shows that yield of THFA gradually drops in the consecutive runs. There is possibility that there are still a lot of constituents attached to the Rh/C surface. It made this commercial Rh/C bad on this recyclability test.
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