The hydrogenation of bio-derived levulinic acid (LA) and its esters ^{[1, 2, 3, 4]} to γ-valerolactone (GVL) is a key reaction in sustainable biomass conversion because GVL can be widely used for fuel additive, food ingredient and as an intermediate for fine chemical production ^{[5, 6, 7, 8]}. Both homogeneous and heterogeneous catalytic systems based on precious metal (Rh, Pd, Ir, Au, Ru) catalysts have been developed for efficient LA hydrogenation. Wright et al. ^[8] highlighted in their review that GVL production relied strongly on the use of noble metals, which could be an issue for the scale-up of the process due to their cost and the uncertain future availability. Although a few base metal heterogeneous catalysts have been successfully developed for this process ^{[9, 10, 11]}, achieving a comparable activity and durability comparable with those of a low loading precious metal catalyst still remains a challenge due to their irreversible metal leaching and sintering, especially under liquid hydrogenation conditions ^[12].

We recently reported an ordered mesoporous carbon supported Ru-Ni bimetallic nanoparticle catalyst for the efficient conversion of LA into GVL ^[13]. We described here that a supported Co catalyst can be a suitable alternative for this interesting transformation. Co is widespread in the Earth, and Co-based catalysts have found wide application in the well-known Fisher-Tropsch synthesis ^{[14, 15]} and other hydrogenation reactions ^{[16, 17]}. Hydrotalcite-like compounds can be suitable precursors for the preparation of uniformly dispersed metal catalysts due to their tunable composition and confinement effect endowed by the interlayer galleries ^{[18, 19, 20, 21, 22]}. In the present work, we utilized an easily fabricated Co/Al-hydrotalcite (HT), a hydrated Co-Al hydroxide with a lamellar structure, as the single precursor to prepare an Al₂O₃ supported Co particle catalyst (Co/Al₂O₃), which possessed a relatively high surface area and strong interaction between the Co and Al species. The catalyst had a unique core-shell structure with amorphous alumina dominating the shell. These outstanding characteristics gave this Co/Al₂O₃ catalyst a comparable catalytic efficiency and recyclability with those of the representative noble metal catalyst Ru/C (LA conversion 80%; GVL selectivity 90%) in the conversion of LA to GVL ^[6].

In the present work, we used M/Al-HT (M = Co, Fe, Cu, Ni) as the precursor to prepare Al₂O₃ supported transition metal particles. The 4Co/Al-HT (n_Co/n_Al = 4) catalyst was prepared by a constant pH co-precipitation by following a reported procedure with a modification ^{[23, 24]}, which included the dropwise addition of an aqueous solution (100 mL) of Co (II) and Al (III) nitrates (total concentration of cations was 0.5 mol/L with a Co/Al ratio of 4 into 200 mL solution of Na₂CO₃ ([Al³⁺] = [CO₃^2-]). The suspension pH was kept at 10.0 by the addition of NaOH aqueous solution (1.0 mol/L). The slurry was vigorously stirred at 80 ℃ during the solution addition. After the solution of metal cations was completely added, the suspension was stirred at 80 ℃ for 1 h, then followed by the step of precipitate aging at the same temperature for 18 h without stirring. The prepared hydrotalcite sample (denoted as 4Co/Al-HT) was reduced under H₂ (50 mL/min) at 700 ℃ for 2.5 h, affording the Co particle supported catalyst denoted as 4Co/Al₂O₃. The other four catalysts (5Co/Al₂O₃, 3Co/Al₂O₃, 2Co/Al₂O₃, 1Co/Al₂O₃) were prepared following the same procedure but with the molar ratios of Co/Al = 5, 3, 2 and 1. The Fe/Al₂O₃, Cu/Al₂O₃ and Ni/Al₂O₃ catalysts were synthesized following a similar procedure but using Fe (III), Cu (II) or Ni (II) nitrate as the precursor. The Co₃O₄ supported on the γ-Al₂O₃ precursor (Co/Al = 4) prepared by incipient wetness impregnation was denoted as Co₃O₄/γ-Al₂O₃, and the reduced catalyst 4Co/γ-Al₂O₃ was prepared with the above reduction procedure. The 4Co/Al-HT hydrotalcite was first calcinated at 600 ℃ in air, then reduced under H₂ at 700 ℃ for 2.5 h. The sample was denoted as 4Co/Al₂O₃-CR.

Table 1 Catalytic performance of the base metal catalysts for the conversion of LA. Entry Catalyst Conversion (%) Selectivity (%) 1 2Fe/Al2O3 0.5 >99 2 2Cu/Al2O3 2 >99 3 2Ni/Al2O3 39 >99 4 2Co/Al2O3 43 >99 5 1Co/Al2O3 46 >99 6 3Co/Al2O3 43 >99 7 4Co/Al2O3 100 >99 8 5Co/Al2O3 93 >99 9 4Co/γ-Al2O3 29 >99 10 4Co/Al2O3-CR 69 >99 11 γ-Al2O3 — — 12 Blank — — 13 a 4Co/Al2O3 6 >99 14 b Ru/C 100 >99 Reaction conditions: LA 0.02 mol, catalyst Co/LA = 1.5% (molar ratio), 1,4-dioxane 30 mL, agitation rate 1000 r/min, H2 5 MPa, 180 ℃, 3 h. a Water used as solvent. b Ru/LA = 1.5% (molar ratio).

Table 1 Catalytic performance of the base metal catalysts for the conversion of LA.

The conversion of levulinic acid to GVL was carried out in a 100 mL stainless steel autoclave equipped with a mechanical stirrer (Dalian Sanlin Instrument Co., China). A typical procedure was the following: LA 2.32 g (0.02 mol), catalyst Co/LA = 1.5% (molar ratio), solvent 30 mL, H₂ 5 MPa, 180 ℃, stirring rate 1000 r/min, 3 h. The liquid product was analyzed by a gas chromatograph (Agilent GC-7890A) equipped with a capillary column AT-SE-54 (60 m × 0.32 mm × 0.1 μm) and FID detector using diethylene glycol dimethyl ether as an internal standard. The qualitative identification of the products was achieved by GC-MS (Agilent 5975C/7890A).

The results of the hydrogenation of LA to GVL over the HT-derived Co, Fe, Ni, Cu catalysts are summarized in Table 1. The 2Fe/Al₂O₃ and 2Cu/Al₂O₃ catalysts were almost inactive for LA hydrogenation. 2Ni/Al₂O₃ showed moderate hydrogenation activity with 39% LA conversion. 2Co/Al₂O₃ gave higher conversion (43%) of LA with >99% selectivity for GVL under the same reaction conditions, with no other hydrogenation product being detected in the product. Then we tested a series of HT-derived Co catalysts in which the Co/Al ratio was varied in the range of 1-5 (entries 5-8). The catalytic performance of the HT-derived Co catalyst was strongly dependent on the Co/Al ratio. The 4Co/Al₂O₃ catalyst exhibited by far the highest catalytic activity (100% conversion) with a low catalyst loading (1.5% molar ratio of LA) and short time (3 h). Also, the selectivity to GVL remained >99%. Notably, the reference catalyst, Co supported on γ-Al₂O₃ prepared by impregnation with a Co/Al ratio of 4 only gave 29% of LA conversion (entry 9), suggesting that the catalyst preparation protocol greatly affected the catalytic performance. Furthermore, 4Co/Al₂O₃-CR showed a lower LA conversion (69%) than 4Co/Al₂O₃ in the hydrogenation reaction (entry 10), so we prepared the catalyst sample by directly reducing the hydrotalcite without the calcination step. The possible reason for this phenomenon is discussed below together with Fig. 7. GVL was not formed when pure γ-Al₂O₃ or no Co-catalyst was used in the reaction (entries 11 and 12), which confirmed that the presence of dispersed Co particles was essential for the high activity in the hydrogenation of LA. In particular, the conversion of LA dropped to 6% when water was used as the solvent (entry 13). The commercial Ru/C catalyst used in the hydrogenation of LA achieved excellent LA conversion and GVL selectivity (entry 14). By comparison, 4Co/Al₂O₃ showed comparable catalytic performance with Ru/C in the liquid hydrogenative transformation to GVL.

Fig. 1. Procedure for catalyst separation. (a) Mixture of catalyst and reaction solution; (b) magnetic catalyst was separated from the mixture by a magnet; (c) catalyst was suspended in water when the magnet was removed.

Fig. 2. Reusability of 4Co/Al2O3 at 50% LA conversion. Reaction conditions: LA 0.06 mol, Co/LA = 0.4% (molar ratio), 1,4-dioxane 30 mL, agitation rate 1000 r/min, H2 5 MPa, 180 ℃, 3 h.

One of the major issues for supported metal catalysts in the hydrogenation conversion of carboxylic acids in the liquid phase is their deactivation due to metal leaching and/or sintering, which are serious for the non-precious metal catalysts. Since carbonyl groups adsorb on metal nanoparticles, aggregation or even the formation of soluble metal-carbonyl complexes can occur especially when the interaction between the metal and support is not strong enough. To examine the stability of the optimal 4Co/Al₂O₃ catalyst for GVL production, the reusability experiment of 4Co/Al₂O₃ was carried out. To recycle the magnetic catalyst, a simple separation method was designed shown in Fig. 1, where a glass tube equipped with a permanent magnet was used. The spent 4Co/Al₂O₃ was easily separated from the reaction mixture by the magnet. Since the surface of a Co nanoparticle was easily oxidized to cobalt oxide in air during the catalyst separation, before each run the separated catalyst was washed with distilled water and then reduced under H₂ flow (50 mL/min) at 700 ℃ for 1 h. After this treatment, the activity of the recovered catalyst was almost the same as in the first run (Fig. 2). The concentration of leached Co ion in the filtrate was measured as 8 ppm by ICP-AES, indicating that the Co/Al₂O₃ catalyst was stable enough in the liquid phase reaction and did not suffered severe deactivation under the relatively harsh conditions.

Table 2 Physicochemical properties of the Co/Al2O3 catalysts. Sample ABET (m2/g) Bulk Co/Al a (molar ratio) Surface Co/Al b (molar ratio) Total acid sites c (mmol/g) Co loading d (%) Co particle size e (nm) 1Co/Al2O3 120.4 1.72 0.27 0.031 53.6 25.1 2Co/Al2O3 86.7 2.07 0.31 0.020 69.8 25.4 3Co/Al2O3 88.7 3.06 0.33 0.023 77.6 25.1 4Co/Al2O3 74.9 4.04 0.72 0.021 82.4 25.5 5Co/Al2O3 89.2 4.60 0.35 0.027 85.3 27.3 a Calculated by ICP-AES. b Determined from XPS. c Determined from NH3-TPD. d Calculated by ICP-AES. e Calculated from TEM image.

Table 2 Physicochemical properties of the Co/Al2O3 catalysts.

The HT-derived Co catalysts were characterized by N₂ adsorption isotherm, inductively coupled plasma atomic emission spectroscopy (ICP-AES), X-ray photoelectron spectroscopy (XPS), NH₃ temperature programmed desorption (NH₃-TPD), powder X-ray diffraction (XRD), H₂ temperature-programmed reduction (H₂-TPR), and transmission electron microscopy (TEM) to elucidate the origin of their high LA hydrogenation activity and stability. Typical data of the BET surface area, Co/Al ratio, acid amount, Co loading, and mean particle size are summarized in Table 2.

It can be seen that there was no apparent relationship between the BET surface area or total acid sites and the catalytic performance of the Co/Al₂O₃ samples, indicating the activity of the HT-derived samples was less associated with the external texture and acidity. However, associating the bulk and surface Co/Al ratio in Table 2 showed a good correlation between the Co content and the catalytic performance of the Co/Al₂O₃ catalysts. The surface Co/Al ratio of each sample was less than the corresponding bulk value, suggesting the catalyst surface was enriched in Al. The fact that the specific composition of the 4Co/Al₂O₃ sample maximized the exposure of active Co species (highest surface Co/Al ratio) was crucial for achieving a high efficiency in the LA hydrogenation.

As for the five Co/Al-HT precursors, the XRD patterns showed the characteristic peaks of the hydrotalcite structure (Fig. 3) ^{[25, 26]}. All samples have sharp diffraction peaks at 11.7°, 23.5°, 34.8°, 39.4°, 47.2°, 60.2°, 61.6°, which were indexed to the (003), (006), (009), (015), (018), (110), and (113) lattice planes of hydrotalcite, respectively. For the samples with the higher Co/Al molar ratio (5,4,3, and 2), only the hydrotalcite crystal phase was observed, suggesting the high purity of the hydrotalcite. However, there were both the hydrotalcite phase and Bayerite phase in the sample with the Co/Al molar ratio of 1, due to that the excess Al ions precipitated as an additional Bayerite phase ^{[27, 28]}.

Fig. 3. XRD patterns of Co/Al-HT precursors. (1) 5Co/Al-HT; (2) 4Co/Al-HT; (3) 3Co/Al-HT; (4) 2Co/Al-HT; (5) 1Co/Al-HT.

Fig. 4. XRD patterns of samples reduced with H2. (1) 5Co/Al2O3; (2) 4Co/Al2O3; (3) 3Co/Al2O3; (4) 2Co/Al2O3; (5) 1Co/Al2O3; (6) spent 4Co/Al2O3.

Fig. 5. TEM images of 1Co/Al2O3-HT (a), 2Co/Al2O3-HT (b), 3Co/Al2O3-HT (c), 5Co/Al2O3-HT (d), 4Co/γ-Al2O3 (e), used 4Co/Al2O3-HT (f).

After the reduction at 700 ℃, the characteristic diffraction peaks of the hydrotalcite structure disappeared. Meanwhile, the major diffraction peaks with 2θ values of 44.3°, 51.6° and 76.1° (JCPDS 01-1255) ascribed to metal Co appeared (Fig. 4), indicating that the Co ions were reduced to metallic Co. During the catalyst screening, we found 4Co/Al₂O₃ was the most efficient catalyst in the hydrogenation reaction of LA, so we looked for the explanation for its superior performance. When comparing the XRD patterns from these five Co/Al₂O₃ catalysts, obvious and wide diffraction peaks ascribed to the Co(100) and Co(101) planes were clearly observed in 4Co/Al₂O₃. These results indicated that the Co(100) and Co(101) planes were highly active in the hydrogenation reaction, and the Co/Al₂O₃ catalyst exposed more of these two planes when the molar ratio of Co/Al = 4. We found that the used 4Co/Al₂O₃ catalyst also maintained the Co and CoAl₂O₄ phases, which further confirmed the catalyst was stable under the reaction conditions.

The micro-structure and particle size distribution of the fresh and spent Co-based catalysts were characterized by TEM. As displayed in Fig. 5, the HT-derived Co catalysts showed the well dispersed nature of the Co nanoparticles. All of the fresh catalysts had particle sizes in the range of 25-30 nm (Table 2), indicating that the particle size of Co was not affected by its amount. However, the average Co particle size of 4Co/γ-Al₂O₃ was larger than 200 nm (Fig. 5(5)). In view of the similar Co content in 4Co/γ-Al₂O₃ and 4Co/Al₂O₃, it was deduced that the difference in Co particle size was caused by the different preparation procedures. The remarkable difference in the microstructure between the 4Co/Al₂O₃ and 4Co/γ-Al₂O₃ catalysts manifested that the hydrotalcite was a good precursor for preparing well dispersed Co with a high loading. The TEM image of the used 4Co/Al₂O₃ catalyst is shown in Fig. 5(6), and the mean particle size was calculated to be 25.6 nm, demonstrating no significant aggregation of Co particles occurred under the reaction conditions.

The structure of the most efficient 4Co/Al₂O₃ catalyst was studied in detail to investigate its activity and stability (Fig. 6). It was clear that the Co particles have a core-shell structure (Figs. 6(a) and (b)). In the high resolution TEM (HRTEM) image, a dark core was surrounded by a light shell. The fast Fourier transform (FFT) patterns of the core are displayed in Fig. 6(d). It is common that the surface of the reduced Co nanoparticle is quickly oxidized to Co₃O₄ in air at room temperature. As for 4Co/Al₂O₃, the surface composition also changed during the TEM sample preparation, and the corresponding FFT patterns are in good accordance with the Co₃O₄ (400), (222), and (331) planes (JCPDS 43-1003). In addition, no FFT dot was observed in the shell area, indicating that the shell was dominated by amorphous Al₂O₃. The composition of the Co/Al₂O₃ core-shell structure was further supported by that no impurity was observed in the energy dispersive X-ray (EDX) analysis (Fig. 6(f)). The above observations were consistent with the reported core-shell structure of Co₄N-Al₂O₃-HT, which also could be prepared with a high metal loading ^[29]. The surface Co/Al ratio of each sample (Table 2) was less than the corresponding bulk value, suggesting the catalyst surface was enriched in Al. Therefore, it was deduced that the particle core was composed of metal Co species while the shell was made up of amorphous Al₂O₃. This special core-shell structure prevented the sintering and leaching of the Co particles and resulted in the small particle sizes. In addition, compared with the fresh 4Co/Al₂O₃, the XRD pattern (Fig. 4(f)) and TEM image (Fig. 5(f)) of the used catalyst did not show any obvious change. Therefore, both the core-shell structure and strong interaction of Co and Al species were responsible for the stability of the catalyst.

Fig. 6. TEM images of 4Co/Al2O3. (a), (b) low magnification image, (c) HRTEM image, (d) FFT pattern of the red framed area in (c), (e) FFT pattern of the blue framed area in (c), (f) EDX image.

Fig. 7. TEM images of 4Co/Al2O3 (a) and 4Co/Al2O3-CR (b).

As discussed above with Table 1, entries 7 and 10, 4Co/Al₂O₃-CR showed lower activity than 4Co/Al₂O₃ in the hydrogenation of LA. TEM was used to compare their microstructure. The Co particle of 4Co/Al₂O₃ was embedded in the Al shell and the average particle size was 25 nm (Fig. 7(a)). However, 4Co/Al₂O₃-CR showed no core-shell structure, and the Co particle size was 54 nm (Fig. 7(b)). So we propose that the high activity for the reduced 4Co/Al₂O₃ catalyst sample can be ascribed to its unique core-shell structure and small particle size.

To gain a deeper insight into the interaction between the supported phase and the carrier, H₂-TPR analysis was performed to observe the oxide reduction process. Figure 8 shows the H₂-TPR profiles of the Co₃O₄/γ-Al₂O₃ and Co/Al-HT catalyst precursors with different Co/Al ratios. Two sharp peaks (T_max = 220 and 311 ℃) were observed with Co₃O₄/γ-Al₂O₃. Since Al₂O₃ cannot be reduced in this temperature range, the reduction peaks were due to the reduction of Co species. Co₃O₄/γ-Al₂O₃ was reduced in two steps at 220 and 311 ℃ yielding CoO and Co⁰, respectively ^{[30, 31]}. However, all the HT-derived samples showed one or two broad peaks from 500 to 700 ℃. The T_max of the Co/Al-HT samples were located at 551, 605, 620, 631 and 647 ℃ for 5CoAl-HT, 4CoAl-HT, 3CoAl-HT, 2CoAl-HT, 1CoAl-HT, respectively. The shift to higher temperature revealed that the increase in Al amount in the samples hampered the reduction of Co ions ^[25]. In other words, there was a strong interaction between the Co ions and Al ions, which would prevent the sintering and leaching of Co nanoparticles.

The TPR results also reflected the strong interaction between Co and Al species in that the reduction temperature of 4Co/γ-Al₂O₃ was below 350 ℃, while those of the HT-derived Co catalysts were higher than 550 ℃. From these observations, during the thermal treatment of the precursor under H₂ flow, the reduction of Co and the formation of the Co/Al core-shell structure simultaneously occurred. Consequently, the embedded Co nanoparticle is more difficult to reduce, so a broad TPR peak appeared at higher temperatures.

Fig. 8. H2-TPR profiles of the Co/Al-HT precursors. (1) 5Co/Al-HT; (2) 4Co/Al-HT; (3) 3Co/Al-HT; (4) 2Co/Al-HT; (5) 1Co/Al-HT; (6) Co3O4/γ-Al2O3.

In summary, we developed an inexpensive and magnetic recyclable Co-based catalyst for the hydrogenation of LA to GVL. A unique core-shell Co/Al₂O₃ catalyst derived from hydrotalcite with a high Co loading was fabricated. The core-shell structure and strong interaction between Co and Al made the Co-based catalyst stable under the reaction conditions. These results also shed light on the rational design of efficient non-precious metal alternatives as heterogeneous catalysts.