Low-temperature selective ethanol upgrading to primary or secondary alcohols by homogeneous catalysis

doi:10.21203/rs.3.rs-1972018/v1

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Low-temperature selective ethanol upgrading to primary or secondary alcohols by homogeneous catalysis

https://doi.org/10.21203/rs.3.rs-1972018/v1

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Ethanol is one of the most promising renewable resources for the production of key industrial commodities. Herein, we present the first direct and selective conversion of ethanol to either primary or secondary alcohols, or to hydrocarbons, using ruthenium PNP pincer complexes [(RPNP)RuHXCO] (R= iPr, Ph, Cy, tBu; X = Cl, H-BH3) as catalysts. Using phenyl substituted phosphines leads to the selective produc-tion of secondary alcohols. Hence, employing [(PhPNP)RuH(Cl)CO] (Ru-1) as a catalyst in ethanol, containing 20 mol% of NaOEt, at 115 ⁰C leads to 89% selective production of secondary alcohols over pri-mary alcohols. A yield of 12% of 2-butanol, and in total 22% of secondary alcohols, was achieved. In addition, minor amounts of 2 butenes/butane (≤5%) were observed in the gas phase. On the contrary, when using bulky phosphine substituents, such as t-butyl, the selectivity completely shifts toward primary alcohols. Thus, using [(tBuPNP)RuH(Cl)CO] (Ru 5) leads to >99% selectivity of 1 butanol (13% yield) over secondary alcohols at 115 ⁰C. In fact, the catalytic system is highly competitive for producing 1-butanol with 22% yield obtained at 130 ⁰C, a temperature significantly lower than previously re-ported systems. Our methodology unveils the potential for using bulk bio-alcohols to selectively produce primary or secondary alcohols and hydrocarbons under mild conditions.

Current research within sustainable chemistry aims for solutions that provide renewable carbon sources as well as clean energy alternatives¹. The substitution of conventional fossil fuels is necessary to reduce greenhouse gas emissions in the transportation sector². Hence, alternative and benign fuels derived from biomass conversion represent promising sustainable options³.

Despite the promising applications of ethanol as a biofuel additive⁴, there are several drawbacks, such as low energy density (70% of that of gasoline)⁵, high water solubility⁶, and corrosion effects on the engine⁷. Recently, some alternative fuels from renewable resources have been investigated, such as 1-butanol and higher primary alcohols⁸. These biofuels arise not only as more favourable alternatives than ethanol but also as less susceptible to phase separation being more hydrophobic⁹.

Previous reports of ethanol upgrading (Supplementary Fig. 1) are based on the Guerbet reaction¹⁰ for carbon-chain growth, with selective production of 1-butanol and higher primary alcohols (Fig. 1, upper pathway). Wass¹¹ reported the use of 0.1 mol% of [RuCl₂(η⁶-p-cymene)]₂and INDOLPhos¹² ligand for the upgrade of ethanol to 1‑butanol in 28% yield (93% selectivity) and TON =314 at 150 ⁰C in the presence of NaOEt (5 mol%). The same author used Ru-MACHO (Ru-1) as the precatalyst for 1-butanol production, leading to only 13% conversion and merely 2% yield of the primary alcohol. In another attempt, Milstein¹³ reported a highly efficient conversion of ethanol (73%) with [RuHCl(Acr-ⁱ^PrPNP)(CO)] (200 ppm) and NaOEt (20 mol%) at 150 ⁰C for 40 h (Supplementary Fig. 1). The desired product 1-butanol was formed in 36% yield. To the best of our knowledge, there are no reports on ethanol upgrading to 2-butanol nor hydrocarbons under mild catalytic conditions (Fig. 1, lower pathway).

The industrial production of 2‑butanol exceeds 800,000 tons per year¹⁴, and is manufactured from glucose by fermentation^15,16. 2-butanol is widely used as a pharmaceutical standard¹⁷, solvent¹⁸, and fuel additive¹⁹. Among these applications, it is a crucial intermediate in the chemical production of 1-butene, 2‑butene²⁰, butyl acetate^21,22, sec-butyl acetate²¹, and methyl ethyl ketone (MEK)²³. Moreover, it has a large market potential as a biofuel and other synthetic applications. Nevertheless, investigations for catalytically upgrading ethanol report the production of 1-butanol with 2-butanol as byproduct²⁴.

The direct production of alkenes/alkanes from ethanol represents a potentially more economic route to jet- and diesel-range hydrocarbon fuels relative to the state-of-the-art technology^25,26, rendering the selective production of 2-butenes from renewable feedstock a critical challenge²⁷. Moreover, these hydrocarbons are also essential feedstock for the production of high-value-added products, such as rubbers, polymers, and synthetic oils²⁸. Indeed, recently the production of butenes from bioethanol has attracted more attention in sustainability research fields, with yields exceeding 60% at reaction temperatures between 300-400 ⁰C^29,25. As a significant component of liquefied petroleum gases (LP gases), butane is industrially derived from natural gas and crude oil³⁰. It may alternatively be obtained from ethanol using heterogeneous catalysis at reaction temperatures between 200-300 ⁰C, typically resulting in low yields of linear butane (<5%) and higher yields of the branched isomers (up to 85%)^31,32. However, the use of homogeneous catalytic systems and mild conditions for all the transformations of ethanol to 2-butanol, 2-butenes, or butane remains undisclosed.

Herein, we demonstrate the selective catalyzed transformation of ethanol to 2‑butanol with the pincer complexes Ru-MACHO (Ru-1) and Ru-MACHO-BH (Ru-2)³³, as well as the formation of low amounts 2-butenes/butane under mild conditions (Fig. 1, lower pathway). Simply shifting to the analogous complex Ru-5, with the P-substituents changing to t-butyls, leads to selective production of 1‑butanol (Fig. 1, upper pathway).

We commenced our studies by studying the ruthenium PNP pincer complexes Ru-1 to Ru-5 shown in Table 1 for the catalytic upgrading of ethanol. These complexes are robust catalysts for a wide range of sustainable chemical transformations under mild reaction conditions^33–38. However, in support of the findings made by Wass¹¹, Liauw recently suggested that consumption of ethoxide additive, via formation of NaOAc by ethyl acetate saponification, is a major reason for low yields of ethanol upgrading to 1-butanol catalyzed by Ru-2³⁹. Indeed, NaOEt is often used for Guerbet reactions promoting the aldol condensation of acetaldehyde and selective formation of 1-butanol and higher primary alcohols^10,11,13. Thus, we set to study the outcome in the solid-, liquid-, and gas phase of the upgrading reaction of ethanol containing 20 mol% NaOEt using Ru-1 to Ru-5 as catalysts.

Corroborating the findings by Wass¹¹, carrying out the upgrading reaction with 1000 ppm of Ru-1 for 96 h led to merely 3% of 1‑butanol. In fact, 12% of unprecedented 2-butanol and 22% of total secondary alcohols (C3-C7) were formed instead (Table 1, Entry 1). This corresponds to a unique 89% selectivity towards secondary alcohols over primary ones. Furthermore, in the gas phase, a mixture of 2-butenes and butane was observed. We speculate that 2-butanol dehydrates to the 2-butenes under these reaction conditions^40,20. A similar outcome is observed with 830 ppm and 250 ppm of Ru-1 (Entries 2 and 3, respectively). Interestingly, increasing the reaction temperature to 130 °C led to lower production of the longer-chain alcohols (Entry 4).

Likewise, consistent with Liauw’s results³⁹, with Ru-2 we found poor coupling of ethanol to 1-butanol. On the contrary, 830 ppm of Ru-2 catalyzed 2‑butanol production with a yield of 8% and an 86% selectivity towards secondary alcohols (Entry 5). Lowering the catalyst loading to 420 ppm still allowed for producing 7% and 10% of 2-butanol after 48 and 96 h, respectively (Entries 6 and 7). Further lowering the catalyst loading to 250 ppm even provided 12% of 2-butanol (TON = 480) and 18% of total secondary alcohols (TON of 720) after 96 h (Entry 8). Longer-chain alcohol production decreased at higher temperature (Entry 9). As expected, the ethanol conversion and 2-butanol formation decreased when using only 83 ppm Ru-2 (24% conversion and 5% yield, Entry 10). Extending the reaction time to 168 h gave 32% conversion, with 8% 1-butanol yield and with the observation of hexane albeit not quantified (Supplementary Table 17). Further increasing the temperature to 160⁰C did not improve the butane yield (5%) (Supplementary Table 3, Entry 13).

Using Ru-3 (250 ppm) for 96 h resulted in a low ethanol conversion (27%) along with the unselective production of 1‑butanol (5% yield) and 5% yield of combined secondary alcohols (Entry 11). The complex Ru‑4 seemingly also showed a lack of selectivity affording 5% of 1-butanol and 6% of combined secondary alcohols after 96 h (Entry 12). Interestingly, the complex Ru-5 showed >99% selectivity towards primary alcohols, generating 1‑butanol in 13% yield while no 2-butanol and higher secondary alcohols were detected after 96 h under optimized conditions (Entry 13). Besides the main primary alcohol, 2-ethyl-1-butanol (<1%), and 1‑hexanol (1%) were also observed (Supplementary Table 5, Entry 3). In fact, when heating to 130 °C, 1-butanol production increased to 18% after 24 h and to 22% after 96 h (Entries 14 and 15).

The time-dependent conversion of ethanol with 250 ppm of Ru-2 and the production of 2-butanol, combined with secondary alcohols is depicted in Fig. 2. The plot displays a steady increase in the formation of secondary alcohols, with 2-butanol as the major product in solution, until it reaches a plateau after approximately 72 h. Likewise, ethanol is steadily converted, and its conversion is at all times higher than the amount of formed liquid-phase products and continues its conversion after 72 h. Apart from the main secondary alcohols produced, small amounts of EtOAc, diethoxymethane, ketones, C5-C7 branched alcohols, and C9+ secondary alcohols and even some aromatics were produced. In the gas phase, a trace of methane and/or ethane were also observed, depending on the reaction conditions (Supplementary section 10). We speculate that formaldehyde might arise from a retro-aldol reaction of 4-hydroxy-2-butanone, which simultaneously leads to acetone (Fig. 3). Further hydrogenation of acetone leads to the observed 2‑propanol. The formation of diethoxymethane might be promoted by an acetalization reaction of the formaldehyde with ethanol. Moreover, these observations corroborate the formation of carbon compounds in other phases. We therefore also analyzed the solid phase and the gas phase.

Often, NaOAc is observed as a side product in traditional ethanol upgrading⁴¹. The Cannizzaro or Tishchenko mechanisms¹¹ or the dehydrogenative pathway⁴² could be responsible for its formation. Indeed, we also detect NaOAc. It precipitates under our reaction conditions, and after 96 h, NaOAc was observed with a yield of 11% when using Ru-2 as a catalyst (Table 2, Entry 3). It is worth mentioning that the formation of NaOAc starts in the early stage of the reaction (Supplementary Table 5, Entry 4), indicating that water formation, likely from the Aldol condensation, also occurs rapidly. Moreover, the results in Table 2 suggest a less straightforward relationship between catalyst structure and NaOAc selectivity than what was observed for alcohol production.

In all the reactions, a drastic pressure increase was observed to 10 bar in the first 30 minutes, which reached a maximum pressure of approximately 30 bar after 24 h (Supplementary Fig. 12). In all the reactions, a significant amount of H₂ was observed corresponding to around 65 – 95% of the total gas composition. Often, the H₂ yield roughly equals double that of NaOAc (Table 2), suggesting that the formation of NaOAc is the main source of H₂ production. Finally, small amounts of organic products (≤5%) were also detected. Whereas they are mixtures of 2-butenes and butane when the reactions are conducted at 115 °C, only butane is observed at 130 °C, both when using the 2-butanol selective Ru-1 or the 1-butanol selective Ru-5 (Entries 2 and 7).

We then turned our attention to the reaction mechanism. As Fig. 3 shows, the selectivity between 1° and 2° alcohols changes drastically depending on the choice of catalyst. For instance, the catalysts Ru-1 and Ru-2 containing less bulky phenyl substituted phosphines afford mainly secondary alcohols, whereas Ru‑3 and Ru-4 containing semi-bulky i-propyl (-iPr) and cyclohexyl (-Cy) P-substituents, respectively, provide practically no selectivity. Finally, Ru-5 containing the bulky t‑butyl (-tBu) P-substituents gives almost exclusively primary alcohols.

As in the typical Guerbet-type ethanol upgrading, our system also relies on generating acetaldol by the Aldol reaction of two acetaldehyde molecules, initially formed by ethanol dehydrogenation (Fig. 4). Typically, acetaldol then proceeds to dehydrate to crotonaldehyde and water followed by hydrogenation to 1-butanol (grey-colored route). However, here we observe a competing reaction that likely involves a dehydrogenation/hydrogenation process to the novel key intermediate 4-hydroxy-2-butanone. This intermediate then undergoes dehydration to yield methyl vinyl ketone (MVK) and water, and hydrogenation of MVK yields 2-butanol. We suggest that the given reaction conditions are, to some extent, capable of inducing dehydration of 2-butanol to 2-butenes, which are then finally hydrogenated to butane. The water is likely responsible for the observation of NaOAc, which also provides the necessary excess of H₂ for hydrogenating the 2-butenes to butane.

To further validate our hypothesis of 4-hydroxy-2-butanone comprising a novel key intermediate towards secondary alcohols and hydrocarbons, some qualitative test reactions were performed with this compound as substrate and Ru-2 as the catalyst at 115 ^oC in the presence of 22 bar of H₂. Using 2.6 equivalents of 4-hydroxy‑2‑butanone to H₂ led to the formation of MVK and a minor amount of 2‑butanol along with almost complete consumption of H₂ (Supplementary Table 18, Entry 1). There was a large amount of 4-hydroxy-2-butanone remaining, and 2-butanone and acetone were also observed. As a note, several different long-carbon products were observed as well. On the contrary, when 4‑hydroxy‑2‑butanone and H₂ are present in lower excess (1.3 equivalent), 2-butanol and 1,3-butanediol were produced as two main products, and trace amounts of butenes/butane were found in the gas phase (Supplementary Table 18, Entry 2). Moreover, there was still H₂ left at the end of the reaction (7 bar pressure). Finally, the addition of 20 mol% NaOAc was also tested while the two substrates were kept in 1.3 equivalent of 4-hydroxy-2-butanone (Supplementary Table 18, Entry 3). Acetone, 2-butanone, and 2-butanol were produced and almost no H₂ pressure was left while some 4‑hydroxy-2-butanone remained. These results corroborate the hypothesis that 4-hydroxy‑2‑butanone is the key intermediate towards 2-butanol or butenes/butane.

In conclusion, we present a previously undisclosed homogeneous catalytic ethanol upgrading pathway to secondary alcohols and hydrocarbons. Using a range of ruthenium pincer catalysts with varying ancillary ligand bulkiness, significant differences in the selectivity of the upgrading were achieved, with the less bulky catalyst favouring secondary alcohols and hydrocarbons, and the bulkier catalyst selectively steering the production towards primary alcohols. Hence, we disclose fundamentally new insights into the carbon chain growth of ethanol upgrading, providing a novel pathway leading to highly valuable secondary alcohols and hydrocarbons. We demonstrate that complexes with phenyl substituted phosphines lead to produce secondary alcohols, low catalyst loading of [(^PhPNP)RuH(HBH₃)CO] (250 ppm) with NaOEt (20 mol%) at 115 ⁰C produced up to 12% of 2-butanol (TON of 480), 4% of 3-hexanol (TON of 160) and a combined 18% of all secondary alcohols (TON of 720). Employing 1000 ppm [(^PhPNP)RuH(Cl)CO] under the same conditions got in total 22% of secondary alcohols. Complex [(^t^BuPNP)RuH(Cl)CO] primarily follows the traditional Guerbet reaction and produces 22% of 1-butanol already at 115 ⁰C. This work represents the first example of a homogeneous catalytic system to produce 2-butanol as well as higher secondary alcohols and hydrocarbons from ethanol. Finally, a few control experiments support the suggestion of a new mechanism for the selective production of these novel products from ethanol upgrading.

General procedure for a catalytic reaction. A 22.8 mL stainless steel container of a high-pressure reactor (reactor A) or a 16.0 mL Alloy 600 container of a high-pressure reactor (reactor B) provided with a Teflon cup and stirrer was loaded with the catalyst (8.3-1000 ppm) and base (10-25 mol%) inside of a glovebox. The sealed container was removed from the glovebox. Then, degassed EtOH (2.5, 3.5, or 5.0 mL) was added with a precise syringe. The reactor was quickly purged three times with N₂ before carrying out the experiments at the desired temperature (85-130 ^oC) for 1-168 h at a stirring rate of 600 rpm.

After the reaction time, the reactor was cooled down to room temperature, using an ice bath. The gas was slowly released into a gas sampling bag to analyze the gas phase. The remnant reaction mixture was neutralized with NH₄Cl (equimolar with the added base, 458 mg-1145 mg). Tridecane (100 µL) was added as the internal standard for the quantification of 2-butanol, 1-butanol, 2-propanol, diethoxymethane, ethyl acetate, 2-pentanol, 3-hexanol, and 4-heptanol. Decane (20 µL) was used as an internal standard for the quantification of ethanol and dimethyl sulfoxide (50 µL) was also added as an internal standard to quantify the amount of sodium acetate. Finally, the resulting solution was diluted with dichloromethane (10-15 mL) and analyzed by NMR, GC-FID, and GC-MS. The gas phase was analyzed by GC-MS, GC-TCD, and Micro-GC. The remnant solid product was analyzed by ¹H and ¹³C NMR using D₂O as solvent.

A control experiment with EtOH and NaOtBu without a catalyst was carried out at 105 ^oC for 1 h (Supplementary Table 2, Entry 1), and no pressure was detected after this time. The reaction of EtOH with NaOtBu produces NaOEt and tBuOH. An additional benchmark reaction of EtOH in presence of Ru-1 and absence of base was also carried out under similar reaction conditions (Supplementary Table 2, Entry 2). A low pressure (1-2 bar) was observed in the reactor manometer. The control experiment with Ru-1 (0.05 mol%) and NaOtBu (10 mol%) showed 2-butanol as the main product (Supplementary Table 2, Entry 3). If we only use Ru-2 without base, the reaction liquid product was EtOAc (Supplementary Table 2, Entry 4).

Acknowledgments

The authors thank VILLUM FONDEN (19049), Novo Nordisk Foundation (NNF20OC0064560), and COWI Foundation (A149.10) for generous research funding. ZN thanks China Scholarship Council (201906230349) for generous scholarship funding. LSM thanks Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – Brazil (CAPES) (88887.364521/2019-00) for generous scholarship funding.

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