Efficient and selective catalytic hydrogenation of furanic aldehydes using well defined Ru and Ir pincer complexes

Rosa Padilla, Sakhitha Koranchalil and Martin Nielsen*
Department of Chemistry, Technical University of Denmark. Kemitorvet 207, DK-2800 Kgs. Lyngby, Denmark. E-mail: marnie@kemi.dtu.dk

Received 5th May 2020 , Accepted 18th June 2020

First published on 20th June 2020


We report the homogeneous catalytic hydrogenation of biomass derived furanic aldehydes to furfuryl alcohols using low loadings of PNP metal complexes under mild conditions. Our strategy represents an efficient and selective approach to the direct hydrogenation of furan derivatives to promising platform chemicals.


Developing efficient processes for the valorizations of biomass-derived substrates is imperative for a future sustainable production of chemicals and fuels.1 As such, particularly the last decade has witnessed the developments of a plethora of effective and selective biomass transformations using homogeneous organometallic catalysis under mild conditions.2

One of the more recent additions to the list of substrates includes furanic aldehydes, mainly represented by hydroxymethyl furfural (HMF),3 which is derived from celullose.1c,3b,4 However, the inherent difficulty of handling HMF induces considerable challenges for its selective synthetic modifications.5 Thus, to access more suitable liquid biofuels, further chemical transformations of HMF are required. The majority of these synthetic modifications focus on transforming the furan ring itself.3 Selective reduction of the aldehyde functionality to products such as 2,5-bis(hydroxymethyl)furan (DHMF) has been more scarcely reported. This product type is a highly important starting molecule for various polymerization or etherification processes.3b,4,6 The selective conversion of HMF to DHMF has been mainly achieved by various hydrogenation methodologies, such as electrocatalytic hydrogenation,7 transfer hydrogenation,8 biocatalysis,9 and heterogeneous catalysis.10

The gradual progress of selective homogeneous organometallic catalytic systems for HMF hydrogenation to DHMF is pioneered by Elsevier11a Mazzoni,11b Beller,11c and Hashmi.3i Mazzoni used 0.1 mol% of the dimeric Shvo's catalyst to reach a practically quantitative NMR yield of DHMF after 2 hours under 10 bar H2 at 90 °C in a 29[thin space (1/6-em)]:[thin space (1/6-em)]1 mixture of toluene/H2O. Beller used pure toluene and 1 mol% of an iPrPNP-Mn complex to afford 64% of isolated DHMF after 24 hours of reaction time under 30 bar H2 at 100 °C (see ESI).

Hence, the challenge remains to produce the desired product highly selectively under mild and sustainable conditions. This drawback is likely due to the labile nature of HMF, which significantly affects its potential in a bio-based industry.

Toward this end, the fructose derived 5-methyl furfural (MF) has been proposed as an alternative substrate for biofuels development due to its high stability, excellent synthetic utility and reduced oxygen content.12,13 MF is industrially produced from biomass13a as an important intermediate for the production of pharmaceuticals,14 food flavoring component15 and agricultural chemicals.16 Furthermore, 5-methyl furfuryl alcohol (MFA) is also interesting as an industrially important component and bio-diesel precursor.4b,c,j Moreover, to the best of our knowledge a homogeneous catalytic MF hydrogenation to MFA remains elusive in the literature.

Likewise, only recent reports have emerged with furfural (FAL) as substrate.17 FAL, derived from hemicellulose,4b,h is a key platform compound which can be widely converted to a variety of chemicals and biofuels.18 However, selective hydrogenation of FAL to furfuryl alcohol (FA) is challenging due to undesired side reactions.19 FA is the most significant derivative of FAL with high demand in the manufacture of foundry resins and feedstock for the production of levulinic acid.20

Transition metal pincer complexes are known for their robustness and efficacy in catalyzing both dehydrogenation as well as hydrogenation reactions.21 In this regard, several concrete studies on mechanistic investigations for the hydrogenation of carbonyl functionalities are known. In particular, the outer-sphere stepwise mechanism of cooperating pincer ligands describes the catalyzed hydrogenation of aldehydes.21f Hence, we were prompted to study this type of complexes for the transformation of biomass-derived furanic aldehydes.

Herein, we show the effective and selective conversion of all three furanic aldehydes to their corresponding alcohols under mild conditions using low catalyst loadings (Scheme 1).


image file: d0gc01543a-s1.tif
Scheme 1 This work: selective catalytic hydrogenation of furanic aldehydes to their corresponding alcohols.

Our initial work concentrated on testing the conversion of HMF to DHMF using the PNP complexes Ru-MACHO (Ru-1),22 its iPrPNP congener (Ru-2), and the Abdur-Rashid iPrPNP-Ir(H)2Cl complex (Ir-1)23 (ESI, Table S1). Thus, with 0.1 mol% of Ru-1 or Ir-1 and 5 mol% of base under 10 bar of H2 in EtOH, the conversion towards DHMF was highly selective, affording 76% and 93% conversion after 1.5 h at 25 °C, respectively. Interestingly, Ru-2 led to a significant increase in conversion, with 0.05 mol% affording >95% after 15 minutes and 2 mol% of NaOEt under 10 bar H2 at 25 °C. Control experiments without any base additive led to no conversion, suggesting that the presence of a strong base seems to be necessary for the reaction to occur, which is in line with the typically necessary activation of the chlorido PNP complexes. Interestingly, the reaction rate seems to also be affected by the loading of the base. Thus, when lowering the NaOEt loading from 2.0% to 0.5% in the presence of 0.05 mol% Ru-2, the initial reaction rates dropped significantly. Nevertheless, both reactions reach full conversion after 20 min and 60 min, respectively (ESI, Fig. S2).

The effect of concentration of HMF in EtOH was investigated with 0.05 mol% of complex Ru-2 by using 0.79 mmol of HMF and 10 bar H2 at 25 °C in EtOH volumes ranging from 0.25–5.00 mL. The reaction afforded full conversion within 10 min in the solvent range 0.50–5.00 mL, but in 0.25 mL a minor drop to 91% conversion was observed (ESI, Table S2), showing that a highly concentrated solution is slightly detrimental for catalytic activity. Moreover, the reaction is at all concentrations entirely selective (>99%), towards DHMF according to 1H- and 13C-NMR analysis as well as the absence of any humins by simple visual inspection.

Increasing the hydrogen pressure to 30 bar reduced the reaction time to 1 min before reaching >95% conversion of HMF, which corresponds to a turnover frequency (TOF) of >1900 min−1 (Table 1, entry 1). To the best of our knowledge, this system constitutes the first example of homogeneous catalytic HMF hydrogenation to DHMF at room temperature. In addition, the catalytic rate is more than a 200 fold improvement to the previous state-of-the-art.11b

Table 1 Hydrogenation of HMF to DHMF using Ru-2[thin space (1/6-em)]a

image file: d0gc01543a-u1.tif

Entry Ru-2, mol% EtOH/H2O ratio Time, min Conv.,b % TON TOF, min−1
a Standard reaction conditions: 0.79 mmol HMF, Ru-2, 2 mol% base, 30 bar H2, 25 °C.b Determined by 1H-NMR. Selectivity ≥99%.c Base is NaOEt: 2.0 M/EtOH.d Base is LiOH.e 4.36 mmol HMF.
1c 0.05 EtOH 1 >95 >1900 >1900
2c,e 0.01 EtOH 120 ≥99 10[thin space (1/6-em)]000 83
3d 0.05 H2O 120 ≥99 2000 17
4d 0.05 95[thin space (1/6-em)]:[thin space (1/6-em)]5 15 >95 >1900 >127
5d 0.05 80[thin space (1/6-em)]:[thin space (1/6-em)]20 15 >95 >1900 >127


We then scaled up to 1 g of HMF using 0.01 mol% (100 ppm) of Ru-2 at 25 °C and 30 bar H2 (entry 2). After 120 min, we isolated a quantitative yield of DHMF after a simple filtration through a silica gel. Further decreasing the catalyst loading to 50 ppm caused a sharp drop in conversion. Thus, 32% conversion was achieved after 6 h, and practically no further conversion was observed after 24 h, suggesting catalyst inhibition or even degradation.

Next, we tested the tolerance of the catalytic protocol by performing the reaction in H2O in the presence of various additives. A number of common bases were evaluated, and LiOH was found to be optimal (ESI, Tables S6 and S7). Thus, employing 2 mol% LiOH, 0.05 mol% of Ru-2, and 30 bar of H2 afforded full conversion after 2 hours (entry 3).

We also carried out the HMF hydrogenation in varying ratios of EtOH/H2O mixtures. Thus, >95% conversion was achieved after 15 min in both 95[thin space (1/6-em)]:[thin space (1/6-em)]5 and 80[thin space (1/6-em)]:[thin space (1/6-em)]20 EtOH/H2O ratios using 0.05 mol% Ru-2 under 30 bar H2 at 25 °C (Table 1, entries 4 and 5), suggesting the feasibility of using bioethanol as solvent.

Finally, we attempted to reuse Ru-2 for the hydrogenation of HMF through consecutive addition using 0.79 mmol of HMF per loading and an initial 0.05 mol% of catalyst (30 bar H2, 25 °C, 2 h per run, ESI, Fig. S26). The experiment shows a detrimental effect in the conversion after the third run, where the overall catalyst loading is 0.0125 mol%. As such, we observed 75% overall conversion in the last run, and we were unable to carry out the additions to the point where the overall catalyst loading goes below our best results with batch reactions.

To shed light on the fate and stability of the catalyst during the consecutive additions, we carried out some crude NMR studies for the characterization of the resting species. The catalytic hydrogenation of HMF in EtOH with 1 mol% of Ru-2 at 25 °C and 30 bar of H2 was monitored by 1H-NMR (ESI, Fig. S40). Based on the hydride region, we suggest the expected presence of an alkoxide complex, Ru-OR, overlapping with remnant Ru-2 at −16.5 to −16.7 ppm. These Ru-OR species might correspond to coordinated DHMF. Interestingly, Ru–OR is still found after carrying out the first consecutive addition of HMF under similar reaction conditions, suggesting to some extent the stability of the catalyst.

As much as the result points to the feasibility for conducting consecutive addition reactions, we speculate whether a behavior similar to what was suggested by Mazzoni for their hydrogenation of HMF[thin space (1/6-em)]11b is occurring in our system as well, i.e. that the presence of two hydroxyl units in DHMF is particularly responsible for the catalyst inactivation.

We then explored the catalytic activity for the transformation of MF. Interestingly, Ir-1 is more active than Ru–2 for hydrogenating MF in EtOH as well as in EtOH/H2O mixtures (ESI, Tables S9 and S10). A slight increase in reaction temperature was found necessary to reach effective catalytic turnover rates. In fact, under identical reaction conditions (0.1 mol% catalyst, 30 bar H2, 60 °C, 2 mol% NaOEt, EtOH as solvent, 10 min reaction time), both Ru–1 and Ru-2 facilitates <10% conversion whereas Ir-1 leads to ≥99% conversion (TOF = 100 min−1). Moreover, further lowering the Ir-1 loading to 0.05 mol% requires 150 min until full conversion is observed (Scheme 2, upper reaction). In 95[thin space (1/6-em)]:[thin space (1/6-em)]5 and 80[thin space (1/6-em)]:[thin space (1/6-em)]20 EtOH/H2O mixtures, excellent conversion rates were obtained as well.


image file: d0gc01543a-s2.tif
Scheme 2 Hydrogenation of MF to MFA.

At this stage, we performed a benchmark reaction employing MF under neat conditions (Scheme 2, lower reaction). Surprisingly, Ru-1 showed superior catalytic activity over Ir-1, whereas Ru-2 merely reached 21% conversion (ESI, Table S11). From these observations, we speculate whether the diminished activity and low conversion is a result of catalyst deactivation or a detrimental change in solubility of Ru-2 in the neat conditions. Thus, employing 0.005 mol% of Ru-1 or Ir-1 led to high conversions (≥95% and 91%, respectively) with TONs of 19[thin space (1/6-em)]000 and 18[thin space (1/6-em)]200 after 5 h. Decreasing the catalyst loading to 0.0005 mol% gratifyingly led to 17% conversion after 5 h when using Ru-1, corresponding to a TON of 34[thin space (1/6-em)]000 and TOF of 113 min−1 (ESI, Table S12). On the contrary, Ir-1 exhibited a somewhat inferior TOF of 40 min−1. Extending the reaction time to 48 h resulted in 74% conversion in the Ru-1 system, corresponding to a TON of 148[thin space (1/6-em)]000 and an overall TOF of 51 min−1. Under identical conditions, Ir-1 provided 56% conversion. Scaling up the reaction to 7.9 mmol of MF with 0.01% Ir-1 under 30 bar and 120 °C for 2 h allowed to isolate the product MFA in 97% yield.

Finally, we turned our attention to hydrogenating FAL to FA. In the literature, impressive results have been achieved by several research groups (see ESI).17 For example, Kirchner, Hoffmann, and Bica demonstrated that the 2,6-diaminopyridine based PNP complexes of the base metals Fe17c–f and Mn17h are highly competent catalysts for FA production, with catalyst loadings as low as 0.005 mol% still affording quantitative NMR yields under relatively mild conditions (EtOH as solvent, 1.0 mol% DBU additive, 30 bar H2, 40 °C, 16 h, TOF = 21 min−1).17c

Interestingly, whereas Ir-1 was superior for hydrogenating MF to MFA when a solvent is present, Ru-2 is again the most competent catalyst for the transformation of FAL to FA. Thus, full conversion is achieved after 30 min with 0.05–0.1 mol% Ru–2 in solvent mixtures ranging from 100[thin space (1/6-em)]:[thin space (1/6-em)]0 to 80[thin space (1/6-em)]:[thin space (1/6-em)]20 of EtOH/H2O under 30 bar H2 at 25 °C (Table 2, entries 1–3). These results corresponds to TONs ranging from 1000–2000 and TOFs ranging from 33–67 min−1. Next, the isolation of the product was carried out under similar reaction conditions using 7.90 mmol of FAL and 0.1 mol% Ru-2 in EtOH. Then, the reaction mixture was filtered over silica gel affording 84% yield of FA.

Table 2 Hydrogenation of FAL to FA using Ru-2[thin space (1/6-em)]a

image file: d0gc01543a-u2.tif

Entry Ru-2, mol% EtOH/H2O ratio Conversionb, % TON TOF, min−1
a Standard reaction conditions: 0.90 mmol FAL, Ru-2, 2 mol% base (NaOEt: 2.0 M/EtOH), 30 bar H2, at 25 °C, 30 min.b Determined by 1H-NMR. Selectivity ≥99%.c Formation of an insoluble dark solid in the reaction (humins) observed by visual inspection.
1 0.05 EtOH >99 2000 67
2 0.1 95[thin space (1/6-em)]:[thin space (1/6-em)]5 ≥99 1000 33
3 0.05 80[thin space (1/6-em)]:[thin space (1/6-em)]20 ≥99 2000 67
4 0.1c H2O ≥99 1000 100
5 0.05c H2O 93 1860 186


On the other hand, when the catalyst Ru-1 (0.1 mol%) was evaluated in the presence of EtOH (30 bar H2 at 25 °C), the reaction lead to low conversion (24%, ESI, Table S13).

Finally, the reaction in water afforded full conversion in 10 min (Table 2, entry 4) albeit along with a clearly observable formation of an insoluble brown solid (humins).

Furthermore, we carried out a consecutive addition experiment under standard reaction conditions (25 bar H2, 25 °C, 10 min) using 0.90 mmol of FAL per loading and an initial 0.1 mol% of Ru-2 in water. The conversion dropped from ≥99% to 56% already after the second addition. This observation suggests the inhibition of Ru-2 due to the presence of humins (ESI, Fig. S26). In fact, humins formation are frequently observed from FAL in aqueous conditions.24

Moreover, comparing with the mentioned literature precedence, our method allows to combine the use of relatively low catalyst loading with effective catalytic conversion rates of FAL to FA while still employing mild conditions and green solvents.

Further insight into the formation of DHMF was obtained from deuterium-labeling experiments using the catalyst system Ru-2 in presence of 30 bar of D2 (Scheme 3).22b,25 In EtOH, practically exclusively d1 labeled product, DHMF-d1, was formed. When changing the solvent to H2O, the D-incorporation is diminished to approximately 80%, the remainder being simply DHMF. The observation might be explained by the fact that the reaction is significantly faster in EtOH than in H2O. Thus, for the reaction in EtOH, we suggest that when the active catalyst is loaded with deuterium, it is delivered to HMF before any scrambling with the protic proton on the EtOH alcohol unit occurs. This scenario is corroborated by previous results we have obtained for the hydrogenation of ethyl levulinate.22b In this case, 24 hours under 30 bar D2 at 60 °C led to a ∼2[thin space (1/6-em)]:[thin space (1/6-em)]1 mixture of labeled/unlabeled products.


image file: d0gc01543a-s3.tif
Scheme 3 Deuterium-labeling experiments.

When conducted in H2O, both the higher acidity of the solvent compared to EtOH as well as the different catalytic rate might contribute to the lower degree of deuterium labeling. Previous work by Dumeignil and Gauvin strongly suggest that for the same catalyst family, temperatures significantly higher than 25 °C are needed to facilitate hydride/deuteride exchange.25b However, those studies were conducted with ∼10 equivalents D2O in toluene, and not in an all-aqueous solvent, which might explain the somewhat diverging observations. Finally, a Cannizaro type reaction could explain the presence of non–labeled DHMF. However, no conversion was observed in the absence of the catalyst, strongly suggesting that this option can be ruled out.

Conclusions

In conclusion, we demonstrate the highly effective and selective hydrogenation of the furanic aldehydes HMF, MF, and FAL under mild reaction conditions toward the corresponding alcohols catalyzed by PNP-Ru and PNP-Ir complexes. Moreover, our method allows to achieve a TOF >1900 min−1 or a TON = 10[thin space (1/6-em)]000, as well as isolating a quantitative yield of DHMF and MFA. Unfortunately, the yield is somewhat diminished for FA due to humins formation. Furthermore, we show for the first time the homogeneously organometallic catalyzed hydrogenation of neat MF to MFA with a TOF = 100 min−1 or a TON = 148[thin space (1/6-em)]000. In addition, our method allows for converting FAL to FA under mild conditions using low catalyst loading with a TOF = 67 min−1. Importantly, we demonstrate the feasibility of employing “green” solvents or even neat conditions. Finally, we shed light on the involvement of the solvent in the hydrogenation process via deuterium-labeling experiments.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We gratefully thank Kasper Enemark-Rasmussen and Associate Professor Charlotte Held Gotfredsen for valuable help in performing 2H-NMR experiments.

Notes and references

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/d0gc01543a

This journal is © The Royal Society of Chemistry 2020