Up to now, the focus of the solvent’s polarity has been on the overall LTMS reaction.
Even though the trends in activity as a result of the variation of solvent’s polarity has been consistent, the actual chemical effect is yet to be uncovered. The trend in activity with the different solvent was observed to be independent of the source and size of the Cu catalyst. The LTMS reaction involves two major steps, carbonylation to form MeF and hydrogenolysis of the MeF. Since traditionally, the Cu particles play a major role in the second step, it will be valuable to consider the effect of the solvent’s polarity on the MeF intermediate. MeF is known to undergo two major reactions in the presence of NaOCH3; (i) decarbonylation, which is a highly reversible reaction [39], illustrated in Equation (5.1), and (ii) nucleophilic substitution reaction [39, 151], illustrated in Equation (5.2).
Therefore, if the hydrogenolysis step is not fast enough, MeF can undergo multiple reactions in the presence of NaOCH3.
CH3OOCH ⇌ CO + CH3OH (5.1) CH3OOCH + NaOCH3 ⇌ CH3OCH3+ NaOOCH (5.2)
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To examine the role of the solvent’s polarity on possible side reactions, the MeF in the presence of NaOCH3 was heated to 100 oC for 1 h under 1 bar N2 gas in the different polar solvents. The pressure rise, equivalent CO released, amount of MeOH and MeF realised after cooling the mixture is shown in Table 5.2. In the GC gas phase analysis, MeOH, dimethyl ether (DME), MeF, N2 and CO were observed. However, it was difficult to distinguish between MeOH and dimethyl ether (DME) on the Porapak Q GC column, as their peaks overlapped. Hence the N2 and CO which was well separated on the Molecular Sieve GC column was quantified and reported in Table 5.2. The relative amount of CO with respect to N2 was reported as CO equivalent. The liquid portion showed that the amount of MeF drastically reduced from 33 mmol to below 4 mmol in all the solvent. Moreover, some traces of DME was observed in all the liquid analysis, however due to its high volatility, quantified amount will not be accurately represented.
In all, the pressure rise and CO equivalent released increased with decreasing solvent’s polarity as well as the amount of MeF and MeOH decreased with increasing solvent polarity.
Table 5.2: Solvent effect on MeF and NaOCH3 reaction, 20 ml solvent, MeF = 33 mmol. NaOCH3 = 18.75 mmol, MeOH = 49 mmol, in 20 ml solvent, N2 = 1 bar, CO equivalent=CO/(CO+N2) × pressure rise, CO and N2 was determined from gas analysis while the MeF was determined from liquid analysis.
Solvent Pressure rise
The total amount of products analysed in the GC showed that the total C counts were getting lower with increasing solvent polarity. This implied that not all the products were accounted for in the GC. Hence, the resulting mixture after the MeF reaction was further analysed using an FTIR instrument equipped with an ATR (attenuated total reflection)
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accessory cell to identify hidden molecules, which were IR active. Figure 5.7 shows the IR spectra of the resulting mixtures after the MeF and methoxide reaction in the various solvents. The grey lines represented the pure solvents’ spectra, while the black lines represent the resulting mixture spectra for the B-G spectra. For easy comparison, the spectra in A were pure MeOH, MeF and sodium formate (NaOOCH) spectra. In the spectra B-G, the bands 2830, 2700, 1650, 1570, 1360 and 770 cm-1 which is attributed to NaOOCH [152] were observed in all the mixtures. This indicated that NaOOCH was made in all the solvents.
Figure 5.7: ATR-IR spectra of solvent (B-G in grey), and reaction mixture (B-G, black with *). The spectra A is for MeOH, MeF and NaOOCH. The NaOOCH (in black) was adopted from NIST data base [152]
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The MeF reaction in the different polar solvents undertook two main reaction paths. All the products shown in Equations (5.1) and (5.2) were observed from the analysis of the resulting mixture in all the solvents. This indicated that the MeF was involved in the carbonylation reaction and the nucleophilic substitution reactions, resulting in the formation of CO + MeOH, and DME + NaOOCH respectively. However, only the CO + MeOH were quantified, while the DME + NaOOCH were qualitatively determined to be present. Assuming that the MeF undertook only the two reaction paths, then from the starting amount of MeF, the remaining C counts for a mass balance can be attributed to the nucleophilic reaction. In that case, the nucleophilic substitution reaction increased with solvent polarity.
The decarbonylation reaction was favoured in the less polar solvents. The relative higher amount of CO released, coupled with higher amount of MeOH in the less polar solvents suggested that carbonylation-decarbonylation which is a fast equilibrium step in the LTMS reaction exhibited preference for a less polar environment. This was not surprising as CO is relatively non-polar and will solubilize better in a less polar than more polar solvents [150]. Therefore, lowering solvent polarity increases the mass transfer of CO in the solvent, thereby enhancing MeOH production. The nucleophilic reaction on the other hand, was favoured in the more polar solvents. It is generally known that bimolecular reactions involving ionic intermediates are better stabilized in polar environment [153]. Hence, considering the fact that the formation of ionic salts was involved, it is not surprising that the nucleophilic pathway was favoured in the more polar solvents. However, this indicated that increasing solvent’s polarity will increasingly destabilize the MeF intermediate which can lead to side products outside the expected products in the LTMS reaction. Since the DME and the NaOOCH formed are not involved in the main LTMS steps, their formation could adversely reduce the overall MeOH production especially if the hydrogenolysis step is slower.
In the overall LTMS reaction studied for the different aprotic polar solvents in the previous Sections (5.1 & 5.3), the highest MeOH production was observed in diglyme with ɛ = 7.2. So far, while the decarbonylation-carbonylation step was favoured in less
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polar solvents, highly polar solvent reduces the MeF stability for nucleophilic reactions.
It therefore appears that the hydrogenolysis, decarbonylation and nucleophilic reactions have a comparable activation barrier, such that slight changes in polarity can favour one over the other reactions. As a result, irrespective of the solvent’s polarity, the amount of MeF in the LTMS remained relatively low. Hence, it appears that good compromise in the chemical environment is required to encourage both the carbonylation step and hydrogenolysis. Some level of polarity was required to achieve maximum conversion. This was because, since the hydrogenolysis is the rate determining step, any environment to destabilize the MeF will enhance the LTMS reaction. Nonetheless, this destabilization should be moderate otherwise it will lead to side reactions which will drastically reduce the MeOH production when the nucleophilic pathway is enhanced.
The formation of NaOOCH from the nucleophilic reaction poses an important catalyst deactivation pathway. So far, most of the deactivation in the LTMS reaction in relation to the methoxide catalyst has been attributed to the presence of H2O and CO2. However, it appears that irrespective of the solvents used, NaOOCH was formed. Even though, in the overall LTMS reaction, relatively low amount of this side reaction is expected to occur in a moderately polar solvent, the continuous formation can eventually consume all the starting methoxide present. Therefore aside Cu agglomeration as one path to catalyst deactivation in the LTMS reaction, NaOOCH formation is also another path to the methoxide deactivation, the importance of which increases with increasing solvent’s polarity.
5.5 Summary
The importance of solvent’s polarity in the LTMS reaction was studied using 8 different aprotic polar solvents. In the overall LTMS reaction, polar solvent with similar polarity as diglyme with ɛ= 7.2 produced maximum amount of MeOH production. This trend was independent of the Cu nanoparticle sizes. Furthermore, the differences in ether-solvent
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polarity and chain length did not have any major effect on the size of Cu nanoparticles formed in the once through system. Overall, the solvent polarity seemed to dictate the side reaction of the main intermediate in the LTMS reaction. MeF in the presence of NaOCH3 undergoes two main reactions (i) decarbonylation to form CO and MeOH and (ii) nucleophilic substitution reaction to form DME and NaOOCH. Decreasing solvent’s polarity increased the decarbonylation as non-polar CO solubilized better in less polar solvents, while nucleophilic substitution reaction was enhanced with increasing polarity since it involves polar ionic salt formation. The pathway of the nucleophilic substitution reaction presents a deactivation pathway for the methoxide catalyst in the LTMS reaction, the importance of which increased with increasing polarity of solvent. Overall, moderately polar solvents such as diglyme present a good compromise with carbonylation and a moderate polar environment for hydrogenolysis of MeF.
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6 Hydrogenolysis Reaction in the LTMS Reaction; a Synergistic Perspective
The LTMS reaction has been discussed so far based on a concurrent approach, where both carbonylation and hydrogenolysis occurred in one pot. However, previous reports have shown that, when the reactions are carried out in a step-wise approach, the rate of MeF hydrogenolysis is much slower compared to when both steps are combined [27, 74]. This implied that, the concurrent approach is not a simple summation of the main two reaction steps. Hence, in this section, our aim was to investigate the relationship between the Cu-based and the alkoxide catalysts involved in the LTMS, with particular focus on the hydrogenolysis step. Furthermore, we investigated to which extent the determined synergy can be applied to CO2 hydrogenation directly or indirectly via a carbonate intermediate.