The fossil oil depletion and environmental issues have stimulated the exploration of renewable fuels [1, 2]. Ethanol is the most promising biofuel to partially replace the conventional gasoline thanks to the well-established sugar fermentation processes and the developed catalytic methods [3-6]. Its production reached ca. 30 billion gallons in 2017 with an annual increase of 4%–6% in the past five years [7]. Nevertheless, ethanol has a relatively low energy density compared to gasoline (19.6 MJ/L vs 32 MJ/L), and the mixed ratio of ethanol in gasoline is confined to < 15 v%. Additionally, ethanol readily absorbs water under most conditions, not only causing the separation and storage trouble in the existing fuel infrastructures, but also inducing corrosion problems to the current engines of vehicles [8]. In contrast, butanol has a higher energy density close to gasoline (29.2 MJ/L), and is immiscible with water and noncorrosive to the engines. These improved features endow it a promising candidate to be used as an advanced fuel additive [9, 10]. In spite of the progress in ABE (acetone-butanol-ethanol) fermentation, the synthesis of butanol from renewable feedstocks remains a challenge due to the self-inhibition of butanol (1%–3%) [11]. Considering the great amount of ethanol production and the excellent performance of butanol, it is highly attractive to upgrade ethanol into butanol [12-15].

The catalytic conversion of ethanol to butanol is a multiple step reaction, known as the Guerbet reaction [16, 17]. In general, ethanol is first dehydrogenated to acetaldehyde, and then to acetaldol via the aldol condensation. After the dehydration and hydrogenation reactions, butanol is formed eventually. A variety of catalysts have been developed for this reaction [18]. Homogeneous catalysts of Ru, Ir, and Mn complexes were used to convert ethanol into butanol in the presence of different bases and ligands [19-23]. The Ru or Ir complexes participate in the dehydrogenation and hydrogenation reactions, and the base or some non-noble hydroxide complexes catalyze the aldol condensation of acetaldehyde to C₄ products. By precisely controlling the micro reaction environment, the butanol selectivity reached > 99% with ca. 30% ethanol conversion at 423 K in the presence of different additives [21]. On the other hand, recyclable heterogeneous catalysts of hydroxyapatite [24, 25], mixed oxides [26, 27]and metal based catalysts [28-32] have been widely exploited for ethanol conversion. However, it is still a challenge to balance the ethanol conversion and butanol selectivity for the heterogeneous catalysts. In most cases, the selectivity to butanol is lower than 60% at > 20% ethanol conversions. The reason lies in the uncontrollable distribution and strength of acid and base sites in different reaction conditions, which cause serious side reactions to produce methane, ethylene, ethyl acetate, butyraldehyde and > C₁₀ carbon alcohols. Recently, binary catalysts were developed to combine the advantages of homogeneous and heterogeneous catalysts. For instance, Zhang et al. [33] employed commercial cobalt powders and NaHCO₃ for converting ethanol to butanol at 473 K in aqueous solution. The selectivity to butanol reached 69%, but unfortunately the ethanol conversion was less than 10% with a very low reaction efficiency (3–20 d reaction). Therefore, it is highly desirable to explore more efficient and reusable catalytic systems to reach the high ethanol conversion and butanol selectivity. Herein, a novel binary catalytic system of FeNiO_x and LiOH was first developed for the conversion of ethanol to butanol. The butanol selectivity reached > 70% at 28% ethanol conversion under the optimal reaction conditions of 493 K for 24 h (butanol productivity: 0.33 g h^–1 g_cat^–1). The FeNiO_x played the key role in hydrogen transfer reaction, while the LiOH catalyzed the aldol condensation reaction and also enhanced ethanol conversion due to the synergistic effect with FeNiO_x. The FeNiO_x catalyst was rather stable, and able to be used for seven times without deactivation.

Dehydrogenation is the first and foremost reaction step for the one-pot conversion of ethanol to butanol. To activate the ethanol, a variety of metal catalysts were screened in the presence of NaHCO₃. As shown in Table 1, entries 1–3, the selectivity to butanol reached ca. 53.0% at 4.4%–4.6% ethanol conversions over the NiO_x and CuO_x catalysts (the XRD patterns are shown in Fig. S1). Large amounts of gaseous products (methane, carbon monoxide, and carbon dioxide; ca. 10.3% carbon yield) and ethyl acetate were observed over the NiO_x and CuO_x catalysts, respectively. This should be attributed to the random cleavage of C–C bonds in ethanol and excessive dehydrogenation reaction of acetaldehyde [30, 34]. In contrast, over the FeNiO_x catalyst, the butanol selectivity reached 68.3% with more than 80% overall high carbon alcohols at the same ethanol conversion. The main by-product is ethyl acetate (10.5%) with decreased gaseous products, which endows it as the promising catalyst for ethanol conversion. From the results of XRD and XRF (Fig. S1 and Table S1), the FeNiO_x catalyst is composed of FeNi oxides (possible phases are NiFe₂O₄, Fe_1.85Ni_1.25O₄, Fe₃O₄, and a small amount of metallic Fe, Ni, or FeNi alloy) with the Fe/Ni ratio of 4:1. Different supported metal catalysts were also used for this reaction. As shown in entries 4–7, the butanol selectivities were less than 43% at ca. 3% ethanol conversions over the Ru/C, Pd/C, Rh/TiO₂, and Ni/CeO₂ catalysts, which also gave a low carbon balance in the reaction due to by-products. NiIr/MC (meso porous carbon) catalyst was tried in this reaction (entry 8) due to its high activity in the hydrogenation reaction as we previously reported [35]. Although the catalyst demonstrated high selectivity to butanol (64.7%), the low ethanol conversion and the scarcity of noble metal stimulated us to employ cheap and readily available FeNiO_x catalysts for ethanol upgrading.

Liquid or solid base/salt catalysts are widely used for the aldol condensation reaction, and the base types, strength or strength distributions greatly influence the final product selectivity [17, 36, 37]. Therefore, besides the benchmark salt of NaHCO₃, different salts or bases with same weight ratio were tested for ethanol conversion in the presence of FeNiO_x catalysts under mild reaction conditions. As shown in Fig. 1, the butanol selectivities were less than 60% over the alkali metal carbonates, following an order of Na > Cs > Li > K. Differently, the main product shifts to ethyl acetate with lower ethanol conversion when using the alkaline earth metal carbonates like CaCO₃ and BaCO₃ in the reaction. The selectivity to butanol over CaCO₃ is much lower than that of metal oxide such as CaO, indicating that suitable base sites are needed to balance the ethanol dehydrogenation and aldol condensation reactions. Following the strategy of homogeneous catalysts, C₂H₅ONa was tested for ethanol conversion in combination with FeNiO_x. It gave 54.9% butanol selectivity at 9.4% ethanol conversion, being more efficient than the above mentioned salts and bases. Afterwards, hydroxide bases were tested in ethanol conversion. Over Mg(OH)₂ and NaOH, low ethanol conversion (< 3%) with ca. 30% selectivity to butanol were obtained. Interestingly, LiOH significantly improved the ethanol conversion to 11.2% with 56.1% selectivity to butanol, even much higher than that over the C₂H₅ONa or NaOH at the same mole of LiOH. In accordance with previous results about the high activity of Li based catalysts in condensation reactions [38-40], LiOH herein provided the base sites to promote both aldol condensation and ethanol dehydrogenation reactions, and afforded the elevated butanol yield. Therefore, the binary catalyst of FeNiO_x and LiOH was identified as the optimal candidate for the further study in the following sections.

To balance the rates of dehydrogenation and aldol condensation reactions, the amounts of metal and base catalysts were optimized. As shown in Fig. 2, the amount of LiOH greatly affected the ethanol conversion and butanol selectivity. As the amount of LiOH increased from 0.05 g to 0.2 g, the ethanol conversion gradually increased, and maximized to ca. 28%. Then, the ethanol conversion remained constant in the range of 0.2–0.6 g, and decreased to 23.2% with 0.8 g LiOH. The butanol selectivity first increased and then levelled off at around 71% with LiOH amount increase. In contrast, the yield of ethyl acetate gradually decreased as increasing the LiOH amount. The base sites are crucial to the aldol condensation reaction. At low base loadings, the ethanol was preferentially transformed to ethyl acetate due to the further conversion of acetaldehyde over the FeNiO_x catalyst. The presence of abundant base sites promoted the condensation reaction, and shifted the reaction equilibrium to butanol with elevated ethanol conversion. However, the excessive base sites changed the interaction between ethanol and FeNiO_x, and inhibited the ethanol activation. It is worth noting that this reaction has a wide catalyst operation window. At the FeNiO_x and LiOH weight ratio of 0.5–1.5, the butanol selectivity reached 71% with > 90% high carbon alcohols at 28% ethanol conversion. Additionally, the ethanol conversion, butanol selectivity, and high carbon alcohols selectivity are comparable to the homogeneous catalysts results (in autoclave), and far higher than the results of most heterogeneous catalysts [22].

The effect of reaction temperature (473–523 K) and reaction time (4–50 h) on ethanol conversion and high carbon alcohols selectivity were investigated. As shown in Fig. 3(A), the ethanol conversion was 13.2% with 77.9% selectivity to butanol at 473 K over the optimized FeNiO_x (0.3 g) and LiOH (0.3 g) catalysts. The ethanol conversion was linearly increased to 43.5% as increasing the reaction temperatures to 523 K. The butanol selectivity levelled off 75% at temperatures between 473 and 503 K, and then decreased dramatically to 44.2% and 28.9% at 513 and 523 K, respectively. Large amounts of gaseous products were detected due to the decomposition of products or intermediates at higher temperatures. Fig. 3(B) depicts the effect of reaction time on the butanol yield. Prolonging the reaction time improved the butanol yield to ca. 19.0% after the 24 h reaction (butanol productivity reached 0.33 g h^–1 g_cat^–1), and then remained constant in the time range of 24–50 h. The ethanol conversion kept increasing in the whole time range, but the butanol selectivity was as high as ca. 72% in 24 h, and then gradually decreased to 40.2% in the 50 h reaction. The butanol product is metastable, and able to be decomposed to methane and carbon oxides. It also serves as the reaction feedstock for the further aldol condensation to produce higher carbon alcohols (> C₁₀). To get a high butanol yield, the reaction temperature and reaction time were optimized to 493 K and 24 h, respectively.

The stability of the FeNiO_x catalyst was evaluated. As shown in Fig. 4, the initial butanol selectivity was 75% at 29.1% ethanol conversion. After four cycles running, the butanol selectivity still reached 72% at 25% ethanol conversion. The ethanol conversion was slightly decreased with the run number increase. Then, the FeNiO_x was reduced by NaBH₄ after the fourth run and then used for further cycle experiments. After the reduction, the butanol selectivity reached 71% and the ethanol conversion kept at 24%–25% in the following four cycles. The slightly decrease in ethanol conversion should be attributed to the surface oxidation and physical loss of fine FeNiO_x particles during the recycle procedures. The XRD patterns and surface area of the FeNiO_x catalyst before and after reaction are shown in Fig. S1 and Fig. S2, which demonstrated the same phases and structures before and after reaction. The SEM and TEM images of FeNiO_x before and after reaction were shown in Fig. S3. The FeNiO_x catalysts had identical layer structures with thickness of 30–60 nm before and after reaction. These results indicate that the FeNiO_x is rather stable under reaction conditions. Additionally, the FeNiO_x is conveniently to be recovered from the reaction solution by the magnetic separation method, as shown in Fig. S4. For LiOH, it was recovered after the removal of alcohols by the distillation method. Based on the XRD patterns of LiOH before and after reaction in Fig. S5 and the reaction results in Fig. S6, one can find that the LiOH is also very stable, which can be directly reused in this reaction for three times. During the reaction, less active salt of Li₂CO₃ formed, which decreased the ethanol conversion to some extent. However, the LiOH can be regenerated through the calcination process, indicating the recyclable of LiOH in this reaction.

To identify and quantify the iron phases in FeNiO_x, ⁵⁷Fe Mössbauer spectrum was collected after reaction with ethanol protection. As shown in Fig. 5, the spectrum was fitted by three sextets, corresponding to the main phase of Fe₃O₄ (octa, blue line, 60%), Fe₃O₄ (tetra, red line, 28.1%) and Fe⁰ (green line, 11.9%). The corresponding spectral parameters were given in Table S2. The isomer shift (IS) of Fe₃O₄ (octa) was 0.63 mm/s, much higher than the pure Fe₃O₄ (octa, 0.56 mm/s) in the literature [41-43], indicating the presence of Ni²⁺ in the Fe₃O₄ phase. Additionally, about 11.9% of metallic Fe was detected, which was nearly invisible in the XRD due to its high dispersion and the formation of FeNi alloy [44, 45]. It is generally accepted that the hydrogen transfer is crucial to the ethanol to butanol process [46]. The presence of metallic Fe, Fe²⁺, and Fe³⁺ provides the feasibility of multiple sites for ethanol dehydrogenation, hydrogen storage, and hydrogenation. This conjecture is confirmed by the XPS results in Fig. S7. Different Fe species including Fe₂O₃, Fe₃O₄, and Fe were detected (Fig. S7, A) along with NiFe₂O₄ and Ni₂O₃ species (Fig. S7, B) over the FeNiO_x catalyst. From the results of H₂-TPR and TPD experiments (Fig. S8), although negligible metallic Ni and Fe was observed by XRD and XPS, the great hydrogen consumption at 420–571 K in Fig. S8(A) indicating the presence of metallic Ni and Fe under reaction conditions (493 K in ethanol, vs 420–571 K for reduction in H₂-TPR). Additionally, strong hydrogen adsorption was found over the FeNiO_x catalyst during the wide desorption window (500 to 750 K) in Fig. S8(B), indicating the strong interaction of hydrogen and the FeNiO_x catalyst for the hydrogen transfer reaction.

To disclose the reaction mechanism of the binary catalytic system in ethanol conversion, some conditional experiments were conducted. As shown in Fig. 6, dehydrogenation products of acetaldehyde and ethyl acetate were preferably produced at 2% ethanol conversion over the FeNiO_x catalyst. In contrast, butanol and hexanol were the main products over the LiOH catalyst even though the ethanol conversion was < 1%. Interestingly, both the ethanol conversion and butanol selectivity increased sharply over the binary catalyst of FeNiO_x and LiOH. The FeNiO_x had the mixed phases of FeNi alloy and FeNi oxides, which promoted the ethanol dehydrogenation, and intermediates hydrogenation. LiOH enhanced the aldol condensation reaction due to its suitable base sites. Moreover, it also changed the reaction equilibrium of ethanol dehydrogenation, and correspondingly enhanced the ethanol conversion. Synergistic effect of FeNiO_x and LiOH was present to afford the high ethanol conversion and butanol selectivity.

According to the catalyst characterizations and conditional experiments, the reaction pathway was summarized in Scheme 1. The metallic Fe, Ni, and FeNi oxides formed the hydrogen storage sites, which dehydrogenated the ethanol to acetaldehyde and transferred the hydrogen to butanol via the intermediate of butanal. Meanwhile, the LiOH promoted the ethanol conversion by changing the reaction equilibrium of ethanol conversion. With the synergistic effect of FeNiO_x and LiOH, the ethanol was converted to butanol with > 70% selectivity at ca. 28% ethanol conversion.

In conclusion, ethanol was converted to butanol over the binary catalytic system of FeNiO_x and LiOH. At 503 for 24 h reaction, the butanol selectivity reached 71% at 28% ethanol conversion with the butanol yield of ca. 19.0%. The FeNiO_x was efficiently transferred the hydrogen from ethanol to butanol, and LiOH catalyzed the aldol condensation reactions and promoted the ethanol conversion. The heterogeneous FeNiO_x catalyst was conveniently recycled by the magnetic separation method and cycled for seven times without deactivation. The LiOH was also recyclable, and could be regenerated by the calcination process. These results make the binary catalytic system of FeNiO_x and LiOH very attractive for practical applications.