Supported Bimetallic Catalysts for the Solvent-Free Hydrogenation of Levulinic Acid to γ -Valerolactone: E ﬀ ect of Metal Combination (Ni-Cu, Ni-Co, Cu-Co)

: γ -valerolactone (GVL) is an important value-added chemical with potential applications as a fuel additive, a precursor for valuable chemicals, and polymer synthesis. Herein, different monometallic and bimetallic catalysts supported on γ -Al 2 O 3 nanoﬁbers (Ni, Cu, Co, Ni-Cu, Ni-Co, Cu-Co) were prepared by the incipient wetness impregnation method and employed in the solvent-free hydrogenation of levulinic acid (LA) to GVL. The inﬂuence of metal loading, metal combination, and ratio on the activity and selectivity of the catalysts was investigated. XRD, SEM-EDS, TEM, H 2 -TPR, XPS, NH 3 -TPD, and N 2 adsorption were used to examine the structure and properties of the catalysts. In this study, GVL synthesis involves the single-step dehydration of LA to an intermediate, followed by hydrogenation of the intermediate to GVL. Ni-based catalysts were found to be highly active for the reaction. [2:1] Ni-Cu / Al 2 O 3 catalyst showed 100.0% conversion of LA with > 99.0% selectivity to GVL, whereas [2:1] Ni-Co / Al 2 O 3 yielded 100.0% conversion of LA with 83.0% selectivity to GVL. Moreover, reaction parameters such as temperature, H 2 pressure, time, and catalyst loading were optimized to obtain the maximum GVL yield. The solvent-free hydrogenation process described in this study propels the future industrial production of GVL from LA. hydrogenation of LA to GVL. The γ -Al 2 O 3 nanoﬁber support was found to be important as the reaction between metal precursor and OH group on the surface of the γ -Al 2 O 3 nanoﬁber results in a strong metal–support interaction, providing additional Lewis acid sites, as conﬁrmed by the NH 3 -TPD. Of the examined metals, Ni was the most active metal for LA conversion, but its selectivity to GVL was found to be deﬁcient. Co has lower conversion with the lowest selectivity to GVL, whereas Cu has the lowest conversion with the highest selectivity to GVL. These results suggest that the monometallic catalysts need improvement to achieve both high conversion and selectivity. The bimetallic catalysts showed improved activity and selectivity. The high activity of the bimetallic catalysts can be attributed to the cooperative e ﬀ ect between M 1 -M 2 species yielding both metal and acid sites responsible for the conversion of LA to AL and the conversion of AL to GVL, respectively. It was shown that a suitable Ni / Cu, Ni / Co, and Co / Cu ratio was valuable to improve the conversion of LA and the selectivity to GVL in the solvent-free hydrogenation by enhancing the balance between metal and acid sites on the surface of the bimetallic catalysts. In all metal combinations, a 2:1 M 1 / M 2 ratio was found to be ideal. Values of > 99.0%, ~83.0%, and ~65.0% GVL yield were obtained over [2:1] Ni-Cu / Al 2 O 3 , [2:1] Ni-Co / Al 2 O 3 , and [2:1] Co-Cu / Al 2 O 3 , respectively. [2:1] Ni-Co / Al 2 O 3 , and [2:1] Co-Cu / Al 2 O 3 showed promising selectivities to 2-MTHF and 1,4-PDO, correspondingly. These ﬁndings provide a new perspective in the solvent-free hydrogenation of LA, with new possibilities for applications in the solvent-free production of 1,4-PDO and 2-MTHF.


Introduction
Due to the concomitant increase of the global consumption of fossil fuels and the rapid depletion of its reserves, research attention has shifted to the investigation and development of sustainable alternatives for fossil fuels as our primary energy source [1]. Lignocellulosic biomass is considered to be one of the most promising replacements of fossil fuels since it is a highly renewable, naturally abundant, carbon-neutral energy source [2]. Levulinic acid (LA), a five-carbon molecule that can be produced from both the C5 and C6 sugars of the lignocellulose, was recognized by the US Department of Energy as one of the top 10 biomass-derived compounds that can potentially replace fossil fuels [3]. A broad spectrum of value-added chemicals can be derived from LA, including γ-Valerolactone (GVL) [4]. GVL can be synthesized from the hydrogenation of LA, and it offers a tremendous number of applications as a food ingredient, green solvent, fuel additive, and a platform chemical for the production of consumer goods [5,6]. Considering the accessibility of LA as a feedstock and the potential industrial applications of GVL, the hydrogenation of LA to GVL was found to be a promising pathway in biomass conversion reactions. As represented by Scheme 1, GVL can be synthesized from two reaction In this work, we prepared γ-Al2O3 fiber-supported bimetallic (Ni-Cu, Ni-Co, Cu-Co) catalysts by incipient wetness impregnation for the transformation of LA to GVL. It was demonstrated that the alumina support provided additional Lewis acid sites by the strong metal-support interaction resulting from the reaction between metal precursor and OH group on the surface of the nanofiber. Reactions were conducted over monometallic catalysts and bimetallic catalysts with [1:1], [2:1], and [1:2] Ni/Cu, Ni/Co, and Co/Cu ratios. The conversion of LA and the selectivity to GVL in the solventfree hydrogenation was enhanced by adjusting the metal and acid sites on the surface of the bimetallic catalysts. [2:1] Ni-Cu/Al2O3, [2:1] Ni-Co/Al2O3, and [2:1] Co-Cu/Al2O3 yielded >99.0%, ~83.0%, and Scheme 1. Reaction pathway for the conversion of LA to GVL. In this work, we prepared γ-Al 2 O 3 fiber-supported bimetallic (Ni-Cu, Ni-Co, Cu-Co) catalysts by incipient wetness impregnation for the transformation of LA to GVL. It was demonstrated that the alumina support provided additional Lewis acid sites by the strong metal-support interaction resulting Catalysts 2020, 10, 1354 3 of 20 from the reaction between metal precursor and OH group on the surface of the nanofiber. Reactions were conducted over monometallic catalysts and bimetallic catalysts with [1:1], [2:1], and [1:2] Ni/Cu, Ni/Co, and Co/Cu ratios. The conversion of LA and the selectivity to GVL in the solvent-free hydrogenation was enhanced by adjusting the metal and acid sites on the surface of the bimetallic catalysts. [2:1] Ni-Cu/Al 2 O 3 , [2:1] Ni-Co/Al 2 O 3 , and [2:1] Co-Cu/Al 2 O 3 yielded >99.0%,~83.0%, and~65.0% GVL, respectively. The influence of reaction parameters, such as H 2 pressure, temperature, and time was also studied. Several characterization techniques, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), ammonia temperature-programmed desorption (NH 3 -TPD), and hydrogen temperature-programmed reduction (H 2 -TPR), were employed to assess the relationship between the catalyst's chemical and physical properties and its activity.

Catalyst Characterization
N 2 adsorption-desorption isotherms and pore size distributions performed to ascertain the structure of the catalysts are presented in Figure 1. All the isotherms obtained for the catalysts display a type IV isotherm with H3 hysteresis loops. The specific surface area of catalysts was calculated by the Brunauer-Emmett-Teller (BET) method based on the adsorption data in the relative pressure range of 0.05-0.25. Pore volume was evaluated by the adsorption quantities at a relative pressure of 0.99. Mesopore size distribution was calculated from the desorption branch by the Barrett-Joyner-Halenda (BJH) method. These isotherms, together with the pore size analysis of all catalysts, suggest that they are mesoporous in nature. The higher surface area of the γ-Al 2 O 3 nanofiber support (259.0 m 2 /g) can be attributed to the arbitrary arrangement of the nanofibers, resulting in a well-established porous structure. After the impregnation of active metals, the surface area decreased due to pore blockage, indicating that the metals were effectively deposited. The significant loss of surface area for the monometallic Ni catalyst compared to all the synthesized catalysts, coupled with the >70.0% pore volume loss, may suggest that Ni deposited within the pores of the γ-Al 2 O 3 nanofiber. Monometallic Cu and Co showed a 46.0% and 52.0% pore volume loss with increased pore diameter, which may indicate the Cu deposit on the surface of the γ-Al 2 O 3 nanofiber. A similar phenomenon was observed in previous studies [14]. For Ni-based catalysts, the surface area and pore volume improved with the addition of both Cu and Co, which can be a result of improved metal dispersion. The textural properties of the monometallic catalysts and catalysts with [2:1] M1/M2 are summarized in Table 1. The textural properties of catalysts with other ratios are reviewed in Table  S2. The experimental active metal contents (Ni, Co, Cu) in both monometallic and bimetallic catalysts were determined by ICP-OES, and the results are summarized in Table 1. The crystallite sizes of the catalysts were calculated using the Scherrer equation based on the peak with the highest intensity. The total metal content and the metal ratio of the catalysts were found to be comparable with the  The textural properties of the monometallic catalysts and catalysts with [2:1] M 1 /M 2 are summarized in Table 1. The textural properties of catalysts with other ratios are reviewed in Table S2. The experimental active metal contents (Ni, Co, Cu) in both monometallic and bimetallic catalysts were determined by ICP-OES, and the results are summarized in Table 1. The crystallite sizes of the catalysts were calculated using the Scherrer equation based on the peak with the highest intensity. The total metal content and the metal ratio of the catalysts were found to be comparable with the given metal contents of catalysts. The XRD diagrams of the reduced catalysts are presented in Figure 2 and Figure S1. The characteristic peaks of the nickel phase were at 2θ = 44.6, 51.7 and 76.3 for Ni/Al 2 O 3 , whereas a copper phase at 2θ = 43.5, 50.5 and 74.5 and a cobalt phase at 2θ = 44.3, 51.4, and 74.3 were identified for Cu/Al 2 O 3 and Co/Al 2 O 3 , respectively [17]. The Co/Al 2 O 3 catalyst showed a smaller diffraction peak due to the superior dispersion of the Co species on the γ-Al 2 O 3 nanofibers. The sharper diffraction peaks observed for Ni diffraction peaks in the monometallic Ni compared to the monometallic Cu and monometallic Co catalyst indicate that Ni particles were significantly larger because of metal aggregation. The existence of the Ni-Cu, Ni-Co, and Co-Cu mixed-species was also confirmed for the reduced bimetallic catalysts. Although the XRD patterns of the bimetallic catalysts appear similar, a closer look at the diffraction peaks of the catalysts shows a shift for the peak linked to metallic Ni to lower angle values for Ni-Co-based (2θ = 44.3) and Ni-Cu-based (2θ = 44.2) catalysts. This slight peak shift indicated the Co and Cu integration into the Ni crystal structure. In the case of Co-Cu/Al 2 O 3 , the characteristic peak of metallic Co has shifted to a lower angle, indicating the formation of a Co-Cu alloy. The intensity of nickel peaks decreased for Ni-Cu/Al 2 O 3 and Ni-Co/Al 2 O 3 catalysts with different metal ratios in the order [2:1] > [1:1] > [1 :2], as can be seen in Figure S1, which can be attributed to the improved metal dispersion when compared to the monometallic Ni/Al 2 O 3 catalyst. Thus, further characterization was performed for the monometallic catalysts and the bimetallic catalysts with a [2:1] metal ratio. Similar results can be seen from the works of Daniel et al. and Mahdi et al. [18,19].
alloy. The intensity of nickel peaks decreased for Ni-Cu/Al2O3 and Ni-Co/Al2O3 catalysts with different metal ratios in the order [2:1] > [1:1] > [1 :2], as can be seen in Figure S1, which can be attributed to the improved metal dispersion when compared to the monometallic Ni/Al2O3 catalyst. Thus, further characterization was performed for the monometallic catalysts and the bimetallic catalysts with a [2:1] metal ratio. Similar results can be seen from the works of Daniel et al. and Mahdi et al. [18,19]. The reducibility of the selected catalysts, together with the active metal−support interaction, was assessed by the H2-TPR analysis, as shown in Figure 3. Ni/Al2O3, Cu/Al2O3, and Co/Al2O3 exhibited reduction peaks at low-and high-temperature ranges. For monometallic Ni catalyst, the two peaks are at 180 °C and 520 °C. For the monometallic Cu catalyst, the peaks are at 200 °C and 520 °C. For the monometallic Co catalyst, the peaks are at 220 °C and 520 °C. The low-temperature peaks are allocated to the reduction of metal oxide with no interaction with γ-Al2O3 nanofibers, whereas the high-temperature peaks are for the reduction of spinel NiAl2O4, CuAl2O4, and CoAl2O4. The significant high-temperature peak of the monometallic Ni catalyst may suggest NiO species were embedded further inside the γ-Al2O3 nanofibers, supporting the observation from the N2 adsorptiondesorption data. [2:1] Ni-Cu/Al2O3 and [2:1] Ni-Co/Al2O3 catalysts show two peaks positioned below 300 °C and 550 °C with a shoulder peak at 600 °C. In [2:1] Ni-Cu/Al2O3, the peak at 200-300 °C relates to the reduction of support-free NiO. The peak at 500 °C can be allocated to the reduction of crystal NiO species, whereas the high-temperature shoulder peak at a range of 550-600 °C relates to the reduction of NiAl2O4 from the robust metal−support interaction. The peak around 250 °C for [2:1] Ni-Co/Al2O3 describes the reduction of NiO with no interaction with support. The peak above 500 °C can be attributed to the reduction of CoO species and tetrahedral Ni 2+ located on γ-Al2O3 with a metal−support interaction [18,20]. In the TPR profile of [2:1] Ni-Cu/Al2O3 and [2:1] Co-Cu/Al2O3 catalysts, the low-temperature reduction peak at around 150 °C is correlated with the reduction of CuO species. The presence of Cu enhances the reducibility of the NiO and CoO by behaving as an activation site for hydrogen molecules, which, in turn, enhances the reduction of NiO and CoO [17,21,22]. The decreased intensity of the shoulder peak for bimetallic catalysts at high temperature indicates better metal dispersion. The H2-TPR results show that the intensity of the high-temperature peak associated with the spinel MAl2O4 decreased from [2:1] Ni-Cu/Al2O3 to [2:1] Ni-Co/Al2O3, followed by [2:1] Co-Cu/Al2O3, confirming that the bimetallic catalysts compared to the monometallic catalysts retained more reduced metallic sites similar to the previously reported studies [14,23]. The reducibility of the selected catalysts, together with the active metal−support interaction, was assessed by the H 2 -TPR analysis, as shown in Figure 3. Ni/Al 2 O 3 , Cu/Al 2 O 3 , and Co/Al 2 O 3 exhibited reduction peaks at low-and high-temperature ranges. For monometallic Ni catalyst, the two peaks are at 180 • C and 520 • C. For the monometallic Cu catalyst, the peaks are at 200 • C and 520 • C. For the monometallic Co catalyst, the peaks are at 220 • C and 520 • C. The low-temperature peaks are allocated to the reduction of metal oxide with no interaction with γ-Al 2 O 3 nanofibers, whereas the high-temperature peaks are for the reduction of spinel NiAl 2 O 4 , CuAl 2 O 4 , and CoAl 2 O 4 . The significant high-temperature peak of the monometallic Ni catalyst may suggest NiO species were embedded further inside the γ-Al 2 O 3 nanofibers, supporting the observation from the N 2 adsorption-desorption data. [2:1] Ni-Cu/Al 2 O 3 and [2:1] Ni-Co/Al 2 O 3 catalysts show two peaks positioned below 300 • C and 550 • C with a shoulder peak at 600 • C. In [2:1] Ni-Cu/Al 2 O 3 , the peak at 200-300 • C relates to the reduction of support-free NiO. The peak at 500 • C can be allocated to the reduction of crystal NiO species, whereas the high-temperature shoulder peak at a range of 550-600 • C relates to the reduction of NiAl 2 O 4 from the robust metal−support interaction. The peak around 250 • C for [2:1] Ni-Co/Al 2 O 3 describes the reduction of NiO with no interaction with support. The peak above 500 • C can be attributed to the reduction of CoO species and tetrahedral Ni 2+ located on γ-Al 2 O 3 with a metal−support interaction [18,20]. In the TPR profile of [2:1] Ni-Cu/Al 2 O 3 and [2:1] Co-Cu/Al 2 O 3 catalysts, the low-temperature reduction peak at around 150 • C is correlated with the reduction of CuO species. The presence of Cu enhances the reducibility of the NiO and CoO by behaving as an activation site for hydrogen molecules, which, in turn, enhances the reduction of NiO and CoO [17,21,22]. The decreased intensity of the shoulder peak for bimetallic catalysts at high temperature indicates better metal dispersion. The H 2 -TPR results show that the intensity of the high-temperature peak associated with the spinel MAl 2 O 4 decreased from [2:1] Ni-Cu/Al 2 O 3 to [2:1] Ni-Co/Al 2 O 3 , followed by [2:1] Co-Cu/Al 2 O 3 , confirming that the bimetallic catalysts compared to the monometallic catalysts retained more reduced metallic sites similar to the previously reported studies [14,23].  The SEM images of the selected catalysts are shown in Figure 4. The SEM images exhibited similar morphologies with the bare γ-Al2O3 nanofibers. When compared with the monometallic Ni/Al2O3 catalyst, the [2:1] bimetallic catalysts showed smaller particle sizes, revealing that the consistency of metal particle distribution was improved with the addition of Cu and Co for the Nibased catalysts. The incorporation of Cu and Co could inhibit agglomeration, boosting the dispersion of the nickel species on the γ-Al2O3 nanofibers. The elemental mapping of the catalyst showed welldispersed metal particles over the Al2O3 support for all the catalysts except Ni/Al2O3, in good agreement with the XRD results. The mapping in the case of Ni/Al2O3 shows a more pronounced Ni presence on the surface of the γ-Al2O3 nanofibers, suggesting agglomeration of the Ni particles. The metal wt.% of catalysts obtained from the EDX analysis was analogous with the values acquired from the ICP-OES and the theoretical value.
The TEM images in Figure 5 show dark particles, which can be attributed to the successful impregnation of metals over the support. All the bimetallic catalysts showed excellent interaction between metals and support with good dispersion. It is possible to note that the active metal is unequally dispersed on the surface of the Ni/Al2O3 catalyst, showing a formation of aggregates with altered morphology. With the addition of a second metal (Cu and Co) to the monometallic Ni catalyst, the dispersion of the active metal species was enhanced. Particularly, for Ni-Co/Al2O3 catalysts, a uniform distribution of smaller metallic particles can be seen when compared to Ni-Cu/Al2O3. This result is in line with the XRD data. A more prominent particle distribution can be seen for the Co-Cu/Al2O3 catalyst, indicating that Ni-based catalysts are more prone to the formation of aggregates. A closer look is shown in Figure S2. The SEM images of the selected catalysts are shown in Figure 4. The SEM images exhibited similar morphologies with the bare γ-Al 2 O 3 nanofibers. When compared with the monometallic Ni/Al 2 O 3 catalyst, the [2:1] bimetallic catalysts showed smaller particle sizes, revealing that the consistency of metal particle distribution was improved with the addition of Cu and Co for the Ni-based catalysts. The incorporation of Cu and Co could inhibit agglomeration, boosting the dispersion of the nickel species on the γ-Al 2 O 3 nanofibers. The elemental mapping of the catalyst showed well-dispersed metal particles over the Al 2 O 3 support for all the catalysts except Ni/Al 2 O 3 , in good agreement with the XRD results. The mapping in the case of Ni/Al 2 O 3 shows a more pronounced Ni presence on the surface of the γ-Al 2 O 3 nanofibers, suggesting agglomeration of the Ni particles. The metal wt.% of catalysts obtained from the EDX analysis was analogous with the values acquired from the ICP-OES and the theoretical value.
The TEM images in Figure 5 show dark particles, which can be attributed to the successful impregnation of metals over the support. All the bimetallic catalysts showed excellent interaction between metals and support with good dispersion. It is possible to note that the active metal is unequally dispersed on the surface of the Ni/Al 2 O 3 catalyst, showing a formation of aggregates with altered morphology. With the addition of a second metal (Cu and Co) to the monometallic Ni catalyst, the dispersion of the active metal species was enhanced. Particularly, for Ni-Co/Al 2 O 3 catalysts, a uniform distribution of smaller metallic particles can be seen when compared to Ni-Cu/Al 2 O 3 . This result is in line with the XRD data. A more prominent particle distribution can be seen for the Co-Cu/Al 2 O 3 catalyst, indicating that Ni-based catalysts are more prone to the formation of aggregates. A closer look is shown in Figure S2.  XPS analysis was conducted for the selected catalysts after reduction to investigate the effect of the addition of a second metal on the surface properties of the monometallic catalysts. The XPS results of the Ni 2p, Cu 2p, and Co 2p spectra are presented in Figure 6. Based on the deconvolution of Ni 2p spectra, the binding energies at 852.5 eV in the Ni 2p3/2 spectra of the reduced Ni/Al 2 O 3 catalyst is assigned to the metallic Ni. The XPS spectra of Ni/Al 2 O 3 also presented two peaks in the Ni 2p3/2 and Ni 2p1/2 spectra at about 855.4 eV and 873.8 eV with corresponding satellite peaks at 862.7 eV and 880 eV, respectively, which are assigned the Ni 2+ species [23]. Ni 0 binding energies in the Ni 2p3/2 spectra in the bimetallic catalysts Ni-Cu/Al 2 O 3 and Ni-Co/Al 2 O 3 are 853.3 eV and 853.7 eV, respectively. XPS analysis was conducted for the selected catalysts after reduction to investigate the effect of the addition of a second metal on the surface properties of the monometallic catalysts. The XPS results of the Ni 2p, Cu 2p, and Co 2p spectra are presented in Figure 6. Based on the deconvolution of Ni 2p spectra, the binding energies at 852.5 eV in the Ni 2p3/2 spectra of the reduced Ni/Al2O3 catalyst is assigned to the metallic Ni. The XPS spectra of Ni/Al2O3 also presented two peaks in the Ni 2p3/2 and Ni 2p1/2 spectra at about 855.4 eV and 873.8 eV with corresponding satellite peaks at 862.7 eV and 880 eV, respectively, which are assigned the Ni 2+ species [23]. Ni 0 binding energies in the Ni 2p3/2 spectra in the bimetallic catalysts Ni-Cu/Al2O3 and Ni-Co/Al2O3 are 853.3 eV and 853.7 eV, respectively.
(a) (b) (c)  XPS analysis was conducted for the selected catalysts after reduction to investigate the effect of the addition of a second metal on the surface properties of the monometallic catalysts. The XPS results of the Ni 2p, Cu 2p, and Co 2p spectra are presented in Figure 6. Based on the deconvolution of Ni 2p spectra, the binding energies at 852.5 eV in the Ni 2p3/2 spectra of the reduced Ni/Al2O3 catalyst is assigned to the metallic Ni. The XPS spectra of Ni/Al2O3 also presented two peaks in the Ni 2p3/2 and Ni 2p1/2 spectra at about 855.4 eV and 873.8 eV with corresponding satellite peaks at 862.7 eV and 880 eV, respectively, which are assigned the Ni 2+ species [23]. Ni 0 binding energies in the Ni 2p3/2 spectra in the bimetallic catalysts Ni-Cu/Al2O3 and Ni-Co/Al2O3 are 853.3 eV and 853.7 eV, respectively. The peaks attributed to the Ni 2+ Ni 2p3/2 and Ni 2p1/2 spectra were also present for the bimetallic catalysts at 856.3 eV and 856.7 eV with satellite peaks for Ni-Cu/Al 2 O 3 and Ni-Co/Al 2 O 3 , respectively. It is possible to note that all the peaks in the bimetallic catalysts shifted to higher binding energies in comparison with Ni/Al 2 O 3 [22]. The results suggest that the addition of second metals improved the reducibility of nickel species and the interaction between nickel particles and γ-Al 2 O 3 nanofiber support, which was also demonstrated in the H 2 -TPR. The existence of Cu 2+ species in the Cu/Al 2 O 3 catalysts was confirmed by the Cu 2p spectra. The Cu 2p spectra for Cu 2p3/2 and Cu 2p1/2 gave two main peaks at about 932.6 eV and 953.6 eV with lower satellite peaks at around 943.4 eV and 963.4 eV. The binding energies for the Ni-Cu/Al 2 O 3 and Co-Cu/Al 2 O 3 catalysts shifted positively, indicating that there are strong electronic interactions between Ni-Cu and Co-Cu atoms in Ni-Cu/A l2 O 3 and Co-Cu/Al 2 O 3 , respectively. From the XPS spectra of Co 2p, for reduced Co/Al 2 O 3 , Ni-Co/Al 2 O 3 , and Co-Cu/Al 2 O 3 , the binding energies at 781.7 eV and 797.6 eV with corresponding satellite peaks in the Co 2p3/2 and Co 2p1/2 spectra are attributed to the Co 2+ and Co 3+ species. The weak peak at 777.2 eV in the Co 2p3/2 spectra of Co/Al 2 O 3 is attributed to the metallic Co shifted to 878.1 eV with increased intensity for Ni-Co/Al 2 O 3 and Co-Cu/Al 2 O 3 , indicating that metal species are rich in bimetallic catalysts, as has been presented in previous studies of bimetallic catalysts [17,22,24]. NH 3 -TPD was performed to ascertain the acidity of the three selected bimetallic catalysts, and the result is presented in Figure 7. The existence of weak and moderate to strong acidic sites was confirmed for all the catalysts by the presence of the three peaks between the temperatures 100 and 350 • C, 350 and 550 • C, and 550 and 900 • C. The intensity of the peaks originating from the Ni-Co/Al 2 O 3 catalysts was higher compared to Ni-Cu/Al 2 O 3 , indicating that Ni-Co/Al 2 O 3 retains the highest density of acidic sites among all the catalysts. This might be because of its higher surface area developed from the enhanced metal dispersion, which is advantageous to the accessibility of the acidic sites. Even though the surface area of Co-Cu/Al 2 O 3 is higher than the other bimetallic catalysts, it exhibits the lowest acidity, especially at a higher temperature. This shows that the presence of Ni in Ni-Co/Al 2 O 3 and Ni-Cu/Al 2 O 3 , in addition to the surface area, may significantly contribute to the densities of acidic sites by providing a more prominent metal−support interface. The work reported by Mengran et al. also demonstrated the same conclusions [14,17].
The peaks attributed to the Ni 2+ Ni 2p3/2 and Ni 2p1/2 spectra were also present for the bimetallic catalysts at 856.3 eV and 856.7 eV with satellite peaks for Ni-Cu/Al2O3 and Ni-Co/Al2O3, respectively. It is possible to note that all the peaks in the bimetallic catalysts shifted to higher binding energies in comparison with Ni/Al2O3 [22]. The results suggest that the addition of second metals improved the reducibility of nickel species and the interaction between nickel particles and γ-Al2O3 nanofiber support, which was also demonstrated in the H2-TPR. The existence of Cu 2+ species in the Cu/Al2O3 catalysts was confirmed by the Cu 2p spectra. The Cu 2p spectra for Cu 2p3/2 and Cu 2p1/2 gave two main peaks at about 932.6 eV and 953.6 eV with lower satellite peaks at around 943.4 eV and 963.4 eV. The binding energies for the Ni-Cu/Al2O3 and Co-Cu/Al2O3 catalysts shifted positively, indicating that there are strong electronic interactions between Ni-Cu and Co-Cu atoms in Ni-Cu/Al2O3 and Co-Cu/Al2O3, respectively. From the XPS spectra of Co 2p, for reduced Co/Al2O3, Ni-Co/Al2O3, and Co-Cu/Al2O3, the binding energies at 781.7 eV and 797.6 eV with corresponding satellite peaks in the Co 2p3/2 and Co 2p1/2 spectra are attributed to the Co 2+ and Co 3+ species. The weak peak at 777.2 eV in the Co 2p3/2 spectra of Co/Al2O3 is attributed to the metallic Co shifted to 878.1 eV with increased intensity for Ni-Co/Al2O3 and Co-Cu/Al2O3, indicating that metal species are rich in bimetallic catalysts, as has been presented in previous studies of bimetallic catalysts [17,22,24].
NH3-TPD was performed to ascertain the acidity of the three selected bimetallic catalysts, and the result is presented in Figure 7. The existence of weak and moderate to strong acidic sites was confirmed for all the catalysts by the presence of the three peaks between the temperatures 100 and 350 °C, 350 and 550 °C, and 550 and 900°C. The intensity of the peaks originating from the Ni-Co/Al2O3 catalysts was higher compared to Ni-Cu/Al2O3, indicating that Ni-Co/Al2O3 retains the highest density of acidic sites among all the catalysts. This might be because of its higher surface area developed from the enhanced metal dispersion, which is advantageous to the accessibility of the acidic sites. Even though the surface area of Co-Cu/Al2O3 is higher than the other bimetallic catalysts, it exhibits the lowest acidity, especially at a higher temperature. This shows that the presence of Ni in Ni-Co/Al2O3 and Ni-Cu/Al2O3, in addition to the surface area, may significantly contribute to the densities of acidic sites by providing a more prominent metal−support interface. The work reported by Mengran et al. also demonstrated the same conclusions [14,17].

Catalytic Activity
To determine the appropriate metal loading for the hydrogenation reaction, [2:1] Ni-Cu/Al 2 O 3 catalysts with a 10-50 wt.% total metal content were synthesized. The results summarized in Table  S3 show 35 wt.% gives the maximum GVL yield; thus, 35 wt.% was fixed for all the catalysts in this study. The catalytic activity of several monometallic and bimetallic catalysts with varying metal ratios (M1/M2) were successfully synthesized and applied in the solvent-free hydrogenation of LA to GVL. The hydrogenation activity of the catalysts was studied in terms of metal combinations at different ratios. The selected catalysts were further tuned by investigating the activities at different reaction parameters.

Effect of Metal Ratio and Combination
LA can be converted to GVL via two different pathways, as shown in Scheme 1. The pathway can be via hydrogenation of the carbonyl group in LA, resulting in the formation of the intermediate 4-4-HPA. Consequent dehydration and esterification result in cyclization, generating GVL or dehydration of LA to AL followed by AL hydrogenation to GVL. In all pathways, the hydrogenation step takes place on the metal catalyst, whereas dehydration and cyclization occur on the acid sites. The 4-HPA pathway of GVL has been shown to kinetically take over at low temperatures [5]. However, the hydrogenation activity of transition metals is low compared to noble metals; therefore, higher temperatures are needed to start the hydrogenation of LA. At high temperatures (~220 • C), LA could be easily dehydrated to AL, suggesting that the AL pathway is responsible for GVL production in this work [25][26][27].
The Ni/Al 2 O 3 catalyst showed a 100.0% conversion of LA with 75.0% selectivity to GVL. The final reaction mixture showed 25.0% selectivity to AL. The result presented in Figure 8 suggests that Ni/Al 2 O 3 favors AL, which can be explained by the characteristics of the Ni/Al 2 O 3 catalyst observed in the XRD, TPR, and XPS data. γ-Al 2 O 3 has Lewis acid sites with different acid strengths. The reaction between the Ni precursor and OH group on the surface of the γ-Al 2 O 3 nanofiber results in a strong metal-support interaction, leading to the formation of more NiAl 2 O 4 with medium to strong Lewis acid sites, decreasing the metallic Ni sites. Because AL is produced by the acid-catalyzed dehydration of LA, and since Ni/Al 2 O 3 has more acid sites, LA is completely converted to AL. In the conversion of AL to GVL, active metal sites proficient at dissociating the supplied hydrogen and hydrogenating the C=C of AL to obtain GVL are crucial. However, the Ni/Al 2 O 3 catalyst does not have enough metal sites to fully convert AL to GVL, resulting only in 75.0% selectivity to GVL.
To improve product selectivity, the second metals (Co or Cu) were introduced into the Ni-based catalysts since bimetallic catalysts have a synergistic effect that enhances both metal and acid sites.  The findings signify the importance of the bimetallic Ni-Cu-based catalysts in improving GVL selectivity. The balance between the metal and acid sites suitable for GVL production can be achieved through the Ni/Cu ratio, demonstrating the role of interaction between Ni-Cu species and the support in the catalyst system [17,28].
A 71.0% conversion of LA with 76.0% and 23.0% selectivities to GVL and 2-MTHF, respectively, was achieved over Co/Al 2 O 3 . No significant AL yield was obtained, indicating that all the AL formed was completely converted into GVL. The significant amount of 2-MTHF in the reaction mixture indicates a higher hydrogenation activity of Co towards 2-MTHF. In the case of Ni/Al 2 O 3 , a~24.0% AL selectivity was obtained. Upon the addition of the equal weight of Cu and Co, the AL selectivity was reduced to 15.0% and 8.0%, respectively. The enhanced selectivities to GVL and 2-MTHF after Cu and Co addition demonstrate the respective selectivities of the secondary metal species. The improved acidity noticed in the NH 3 -TPD and the presence of both Ni and Co metal sites observed from the XPS data allow Ni-Co/Al 2 O 3 to exhibit higher activity, increasing the conversion of the more stable GVL molecule to 2-MTHF. 2-MTHF is the result of intramolecular 1,4-PDO dehydration, which resulted from the hydrogenation of GVL. For the reaction over [1:2] Ni-Co/Al 2 O 3 , a substantial decrease in the LA conversion was detected, confirming once more that Ni is responsible for the conversion of LA. The hydrogenation activity of the [2:1] Ni-Co/Al 2 O 3 catalysts was sufficient to facilitate the GVL ring-opening to 1,4-PDO. This was then followed by 1,4-PDO dehydration to 2-MTHF, which was possible due to the heightened acidic function of the [2:1] Ni-Co/Al 2 O 3 catalyst [25]. A small amount of 2-butanol and 2-pentanol were also detected in the final reaction mixture. These findings emphasize that water, generated in situ from the hydrogenation of LA, may hinder the formation of 2-MTHF. GVL was hydrogenated to 1,4-PDO over the [2:1] Ni-Co/Al 2 O 3 catalyst, whereas the presence of water slightly affected the dehydration of 1,4-PDO to 2-MTHF. Despite the formation of 2-butanol and 2-pentanol, the [2:1] Ni-Co/Al 2 O 3 catalyst shows promising results for the one-pot solvent-free production of 2-MTHF from LA [29].
The reaction result from the monometallic Cu catalyst showed >99.0% selectivity to GVL with 64.0% LA conversion, supporting the conclusion made from the bimetallic Ni-Cu catalyst regarding the potential of Cu in enhancing GVL selectivity. As can be seen in the deconvoluted XPS and NH 3 -TPD profiles, Co-Cu/Al 2 O 3 catalysts possess a higher number of metal sites and fewer acid sites. The presence of unreacted LA in the final reaction mixture indicates that the first step of the reaction (acid-catalyzed dehydration of LA to AL) was slow. This result confirms the presence of Ni is critical for the conversion of LA, whereas the secondary metals are responsible for the product selectivity. No residual AL was present in the reaction mixture, implying that the hydrogenation of AL to GVL was a fast reaction due to the plentiful metal sites. On all the reactions over Co-Cu-based catalysts, there is a formation of 1,4-PDO from the hydrogenation of GVL on the metal sites, whereas only a trace amount of 2-MTHF was obtained. This phenomenon again signifies that Co-Cu-based catalysts have enough metal sites left to hydrogenate GVL to 1,4-PDO due to the higher number of metal sites, whereas not enough acid sites were left to convert 1,4-PDO to 2-MTHF. The monometallic Cu/Al 2 O 3 showed a lower conversion of LA with high selectivity to GVL. Upon the addition of Co, the conversion of LA slightly increased, whereas there was a significant decrease in the selectivity to GVL due to the formation of 1,4-PDO. The change in the selectivity towards the hydrogenation products suggests that the addition of Co increased the number of active metal sites on the catalyst surface, facilitating the hydrogenation of GVL to 1,4-PDO. The absence of 1,4-PDO to 2-MTHF suggested that there are no active acid sites left for the dehydration reaction [30]. Thus, Co-Cu-based catalysts are not well suited for the hydrogenation of LA to GVL production but showed the potential to produce 1,4-PDO.

Effect of Reaction Temperature
Evidently, in the hydrogenation over [2:1] Ni-Cu/Al 2 O 3 , the LA conversion increased substantially with the elevated reaction temperature up to 220 • C, whereas the GVL selectivity remained >99.0%, as shown in Figure 9. Above 220 • C, a decrease in GVL selectivity is observed, with an increased selectivity to AL and an exceedingly small increase in the 1,4-PDO and 2-MTHF selectivity, confirming that GVL can be hydrogenated and dehydrogenated over [2:1] Ni-Cu/Al 2 O 3 . not well suited for the hydrogenation of LA to GVL production but showed the potential to produce 1,4-PDO.

Effect of Reaction Temperature
Evidently, in the hydrogenation over [2:1] Ni-Cu/Al2O3, the LA conversion increased substantially with the elevated reaction temperature up to 220 °C, whereas the GVL selectivity remained >99.0%, as shown in Figure 9. Above 220 °C, a decrease in GVL selectivity is observed, with an increased selectivity to AL and an exceedingly small increase in the 1,4-PDO and 2-MTHF selectivity, confirming that GVL can be hydrogenated and dehydrogenated over [2:1] Ni-Cu/Al2O3. The conversion of LA to 2-MTHF is more difficult owing to the chemical stability of GVL. Thus, it requires a high reaction temperature and pressure with active metal and acid sites on a catalyst. The increased selectivity towards AL compared to 1,4-PDO suggests that the catalyst [2:1] Ni-Cu/Al2O3 does not have enough active sites to catalyze the further conversion of GVL, making it ideal for the production of GVL [31]. The dehydrogenation of GVL back to AL is an unwanted reaction in the production of GVL and for the catalyst stability considering that AL is a typical coke precursor. A reaction temperature of 220 °C is the most appropriate for the hydrogenation of LA to GVL [17]. Up to 90.0%, GVL yield was obtained over [2:1] Ni-Co/Al2O3 at a lower temperature of 200 °C, indicating that the Ni-Co maintains catalyst improved metal and acid sites. Unlike the [2:1] Ni-Cu/Al2O3 catalyst, [2:1] Ni-Co/Al2O3 shows a significant increase in the selectivity to 2-MTHF with the increase in temperature again confirming the characterization results. Therefore, catalysts with The conversion of LA to 2-MTHF is more difficult owing to the chemical stability of GVL. Thus, it requires a high reaction temperature and pressure with active metal and acid sites on a catalyst. The increased selectivity towards AL compared to 1,4-PDO suggests that the catalyst [2:1] Ni-Cu/Al 2 O 3 does not have enough active sites to catalyze the further conversion of GVL, making it ideal for the production of GVL [31]. The dehydrogenation of GVL back to AL is an unwanted reaction in the production of GVL and for the catalyst stability considering that AL is a typical coke precursor. A reaction temperature of 220 • C is the most appropriate for the hydrogenation of LA to GVL [17]. Up to 90.0%, GVL yield was obtained over [2:1] Ni-Co/Al 2 O 3 at a lower temperature of 200 • C, indicating that the Ni-Co maintains catalyst improved metal and acid sites. Unlike the [2:1] Ni-Cu/Al 2 O 3 catalyst, [2:1] Ni-Co/Al 2 O 3 shows a significant increase in the selectivity to 2-MTHF with the increase in temperature again confirming the characterization results. Therefore, catalysts with balanced metal sites and acid sites are required to achieve the maximum LA conversion, with high selectivity to the desired product.

Effect of Reaction Pressure
The impact of hydrogen pressure on the catalytic activity of [2:1] Ni-Cu/Al 2 O 3 and [2:1] Ni-Co/Al 2 O 3 was assessed, and the results are depicted in Figure 10. The H 2 pressure showed a strong effect on the conversion of LA when the H 2 pressure increased from 0 to 30 bar for [2:1] Ni-Cu/Al 2 O 3 and 0 to 20 bar for [2:1] Ni-Co/Al 2 O 3 . Increasing the pressure above the optimum demonstrated no change in the conversion of LA, whereas the effect was prominent for the selectivity of GVL. At 30 bar, 100.0% conversion of LA and >99.0% selectivity to GVL was obtained over [2:1] Ni-Cu/Al 2 O 3 . Increasing the pressure to 40 bar resulted in a 10.0% decrease in GVL selectivity. The findings from the reaction over [2:1] Ni-Co/Al 2 O 3 imply that at lower pressure, the solubility of H 2 is low, and increasing the pressure above 20 bar leads to enhanced conversion of LA. Further increasing the pressure provides a suitable reaction condition to disrupt the stable C=O group in GVL, leading to further ring-opening to 1,4-PDO and the subsequent cyclization to 2-MTHF. Here, again, [2:1] Ni-Cu/Al 2 O 3 -based catalysts were the most selective for the LA conversion to GVL.
Increasing the pressure to 40 bar resulted in a 10.0% decrease in GVL selectivity. The findings from the reaction over [2:1] Ni-Co/Al2O3 imply that at lower pressure, the solubility of H2 is low, and increasing the pressure above 20 bar leads to enhanced conversion of LA. Further increasing the pressure provides a suitable reaction condition to disrupt the stable C=O group in GVL, leading to further ring-opening to 1,4-PDO and the subsequent cyclization to 2-MTHF. Here, again, [2:1] Ni-Cu/Al2O3-based catalysts were the most selective for the LA conversion to GVL.

Effect of Reaction Time
The effect of reaction time on the conversion of LA and selectivity to GVL was analyzed for the reactions conducted over [2:1] Ni-Cu/Al2O3 and [2:1] Ni-Co/Al2O3 to confirm the suggested reaction pathway with intermediates produced at initial reaction time and progressively, as specified in Figure 11. Over [2:1] Ni-Cu/Al2O3, the conversion of LA increased rapidly, from 56.0% (2 h) to 100% (6 h). The conversion of LA remained constant with the increase in the reaction time to 10 h while the selectivity to GVL decreased. AL concentrations became significant at reaction times greater than 6 h after total LA conversion was attained. Thus, the identified AL is most likely produced from the synthesized GVL or by AL desorption from the [2:1] Ni-Cu/Al2O3 surface. Given the results for both temperature and pressure studies, the initial option is deemed to be the reason. Only trace amounts of 2-MTHF were detected in the reaction mixture, showing that [2:1] Ni-Cu/Al2O3 system is insufficient for its production. Over [2:1] Ni-Co/Al2O3, the conversion of LA increased rapidly from 65.2% (2 h) to 100% (6 h). Moreover, the selectivity to 2-MTHF increased at the expense of the selectivity to GVL with increased reaction time. A trace amount of 2-butanol and 2-pentanol was

Effect of Reaction Time
The effect of reaction time on the conversion of LA and selectivity to GVL was analyzed for the reactions conducted over [2:1] Ni-Cu/Al 2 O 3 and [2:1] Ni-Co/Al 2 O 3 to confirm the suggested reaction pathway with intermediates produced at initial reaction time and progressively, as specified in Figure 11. Over [2:1] Ni-Cu/Al 2 O 3 , the conversion of LA increased rapidly, from 56.0% (2 h) to 100% (6 h). The conversion of LA remained constant with the increase in the reaction time to 10 h while the selectivity to GVL decreased. AL concentrations became significant at reaction times greater than 6 h after total LA conversion was attained. Thus, the identified AL is most likely produced from the synthesized GVL or by AL desorption from the [2:1] Ni-Cu/Al 2 O 3 surface. Given the results for both temperature and pressure studies, the initial option is deemed to be the reason. Only trace amounts of 2-MTHF were detected in the reaction mixture, showing that [2:1] Ni-Cu/Al 2 O 3 system is insufficient for its production. Over [2:1] Ni-Co/Al 2 O 3 , the conversion of LA increased rapidly from 65.2% (2 h) to 100% (6 h). Moreover, the selectivity to 2-MTHF increased at the expense of the selectivity to GVL with increased reaction time. A trace amount of 2-butanol and 2-pentanol was detected at about 8 h and above. This can be attributed to the effect of water released with the synthesized GVL affecting the acid sites on the [2:1] Ni-Co/Al2O3 surface required for the cyclization of 1,4-PDO. The effective formation of 2-MTHF despite the low pressure indicates that [2:1] Ni-Co/Al2O3 is a favorable catalyst for the one-pot solvent-free production of 2-MTHF from LA.

Effect of Catalyst Loading
The conversion of LA increased with the available active site up to 1.0 g [2:1] Ni-Cu/Al 2 O 3 , whereas increasing the loading beyond 1.0 g exhibited no change in LA conversion ( Figure 12). A total of 0.8 g catalyst loading was found to be important for the selective production of GVL over [2:1] Ni-Co/Al 2 O 3 insight of its high activity. Further increasing the catalyst loading gave~15.7% 2-MTHF. With >99.0% GVL yield, [2:1] Ni-Cu/Al 2 O 3 is once again proving to be the best catalyst for GVL production.

Effect of Catalyst Loading
The conversion of LA increased with the available active site up to 1.0 g [2:1] Ni-Cu/Al2O3, whereas increasing the loading beyond 1.0 g exhibited no change in LA conversion ( Figure 12). A total of 0.8 g catalyst loading was found to be important for the selective production of GVL over [2:1] Ni-Co/Al2O3 insight of its high activity. Further increasing the catalyst loading gave ~15.7% 2-MTHF. With >99.0% GVL yield, [2:1] Ni-Cu/Al2O3 is once again proving to be the best catalyst for GVL production.

Effect of Catalyst Loading
The conversion of LA increased with the available active site up to 1.0 g [2:1] Ni-Cu/Al2O3, whereas increasing the loading beyond 1.0 g exhibited no change in LA conversion ( Figure 12). A total of 0.8 g catalyst loading was found to be important for the selective production of GVL over [2:1] Ni-Co/Al2O3 insight of its high activity. Further increasing the catalyst loading gave ~15.7% 2-MTHF. With >99.0% GVL yield, [2:1] Ni-Cu/Al2O3 is once again proving to be the best catalyst for GVL production.

Metal Leaching
Metal leaching was examined by measuring Ni, Cu, and Co concentrations in the reaction mixture after the reaction was completed and the result is summarized in Table S4. In general, no significant metals were found in the reaction mixture, implying negligible leaching. Ni was the least leached metal. The % Co leached was low compared to % Cu leached [32]. The strong interaction between Co and Al species was shown to stabilize the Co particles against leaching and sintering [33]. Results suggest that low metal leaching can be achieved if the LA conversion is 100% and no AL

Metal Leaching
Metal leaching was examined by measuring Ni, Cu, and Co concentrations in the reaction mixture after the reaction was completed and the result is summarized in Table S4. In general, no significant metals were found in the reaction mixture, implying negligible leaching. Ni was the least leached metal. The % Co leached was low compared to % Cu leached [32]. The strong interaction between Co and Al species was shown to stabilize the Co particles against leaching and sintering [33]. Results suggest that low metal leaching can be achieved if the LA conversion is 100% and no AL is present in the reaction mixture. The recyclability of [2:1] Ni-Cu/Al 2 O 3 was assessed through three consecutive cycles, indicating the high stability of the catalyst ( Figure S3).

Comparison with Previous Literature
A comparison of the catalysts in this study and other previously reported catalytic systems for the solvent-free hydrogenation of LA to GVL is presented in Table 2. Most of the existing literature reported the high activity of Ru-based catalysts for carbonyl hydrogenation. Ru-based homogeneous catalysts also exhibited excellent catalytic performance for this reaction. However, these types of catalysts suffer from serious drawbacks concerning their homogeneity, high cost, and limited reserves. To solve these problems, transition-metal-based catalysts were developed. Nonetheless, their disadvantages involved metal leaching and catalyst recyclability, hence hindering their industrial application for GVL production.

Catalyst Synthesis
Preparation of γ-Al 2 O 3 Nanofibers γ-Al 2 O 3 nanofibers were synthesized according to a formerly reported procedure [38,39] with some modifications. In a typical synthesis of γ-Al 2 O 3 nanofibers 50 g aluminum, isopropoxide was dissolved in 300 mL isopropyl alcohol at 80 • C for 2 h. A 40 mL volume of 0.28 M acetic acid was added dropwise, and the solution refluxed for 4 h under stirring. The following solution was then moved to a Teflon-lined stainless-steel reactor and heated in an oven at 120 • C for 24 h. The solid obtained after filtration was washed multiple times with deionized water and ethanol and dried at 110 • C overnight. Subsequent calcination at 600 • C for 5 h in air with a ramping rate of 2 • C/min resulted in γ-Al 2 O 3 nanofibers.

Catalyst Preparation
The catalysts were synthesized by the incipient impregnation method. In a typical preparation of 35 wt.% Ni-Cu/Al 2 O 3 catalyst, an aqueous solution containing the calculated amounts of the required metal precursors (Ni (NO 3 ) 2 ·6H 2 O and Cu (NO 3 ) 3H 2 O) with the desired ratio of metals was firstly impregnated on the as-synthesized γ-Al 2 O 3 fibers. This mixture was vigorously stirred for 12 h and subsequently dried at 110 • C overnight. The resulting powder was calcined in static air at 500 • C for 4 h. For the H 2 reduction, the catalyst was subjected to a 10% H 2 /Ar gas flow at 500 • C for 2 h at a ramping rate of 10 • C/min in a tube furnace. The Ni-Co and Cu-Co-based catalysts were also prepared with this procedure but with the use of their corresponding metal nitrates.

Catalyst Characterization
To evaluate the textural properties (surface area, pore volume, and pore size) of the catalysts, the nitrogen physical adsorption-desorption method was performed using the BELSORP-miniII instrument (Bel Japan Inc., Osaka, Japan). Before the analysis, the catalysts were degassed in a vacuum at 110 • C for 4 h. To study the crystallinity, X-ray diffraction (XRD) measurements, in the 2θ range of 20 • to 80 • , were performed using a Rigaku D-Max2500-PC diffractometer (Rigaku International Corporation, Tokyo, Japan). To investigate the morphology, transmission electron microscopy (TEM) was conducted in a JEOL JEM-3011HR microscope (JEOL GmbH, Freising, Germany). The samples were prepared by sonicating a small amount of the catalysts in ethanol and depositing it onto a JEM-3011HR microscope. To examine the elemental composition, X-ray photoelectron spectroscopy (XPS) was performed using XPS Thermo, Esc lab 250 xi (Thermo Fisher Scientific, Waltham, MA, USA). To investigate the reducibility and acidity of the catalysts, hydrogen temperature-programmed reduction (H 2 -TPR) and ammonia temperature-programmed desorption (NH 3 -TPD) were performed, respectively. For the pretreatment of the catalysts for both H 2 -TPR and NH 3 -TPD, the samples were subjected to 50 mL/min helium flow at 100 • C for 2 h. The H 2 -TPR profiles of the calcined samples were recorded in the range of 100-800 • C at a heating rate of 10 • C/min under a 10%H 2 /Ar gas flow. The consumption of hydrogen was monitored using a thermal conductivity detector (TCD). For NH 3 -TPD, the reduced samples were subjected to a 50 mL/min flow of 5% NH 3 /He to adsorb the ammonia. For the desorption of the ammonia, the furnace temperature was increased from 100 to 800 • C at a heating rate of 5 • C/min under a helium flow of 30 mL/min. The elemental contents of the catalysts and metal leaching were determined by inductively coupled plasma optical emission spectroscopy (ICP-OES) iCAP 7400DUO (Thermo Fisher Scientific, Waltham, MA, USA). The catalysts (20 mg) were digested in aquaria (HNO 3 : HCl; 1:3) for 2 h and diluted to 25 mL by distilled water.

Hydrogenation of Levulinic Acid
The solvent-free hydrogenation of LA to GVL was conducted in a 100 mL stainless steel (SUS 316 L) high-pressure batch reactor. In a representative reaction run, 20 mL of LA and 1 g of catalyst were loaded into the reactor. The reactor was then sealed and initially purged three times with argon gas to eliminate traces of O 2 and air before pressurizing it with H 2 gas to the desired reaction pressure. After reaching the desired pressure, the reactor was heated up to the set temperature. The start of the reaction was the time when the target temperature was attained. After the reaction, the reactor was cooled to room temperature, and the catalyst was separated from the reaction mixture through filtration.

Product Qualification and Quantification
For the quantification of the products, the purified product was analyzed in Waters High-Performance Liquid Chromatography (Milford, MA, USA) equipped with an Agilent Hi-Plex C-6 column (7.7 mm × 300 mm × 8 µm) and a refractive index detector (Santa Clara, CA, USA). A 5 µm H 2 SO 4 solution was used as the mobile phase. The conversion of LA and selectivity towards GVL were calculated using the following equations:

Conclusions
Monometallic Ni, Cu, Co, and bimetallic Ni-Cu, Ni-Co, and Co-Cu catalysts supported on γ-Al 2 O 3 nanofibers were synthesized by the incipient impregnation method and applied for the solvent-free hydrogenation of LA to GVL. The γ-Al 2 O 3 nanofiber support was found to be important as the reaction between metal precursor and OH group on the surface of the γ-Al 2 O 3 nanofiber results in a strong metal-support interaction, providing additional Lewis acid sites, as confirmed by the NH 3 -TPD. Of the examined metals, Ni was the most active metal for LA conversion, but its selectivity to GVL was found to be deficient. Co has lower conversion with the lowest selectivity to GVL, whereas Cu has the lowest conversion with the highest selectivity to GVL. These results suggest that the monometallic catalysts need improvement to achieve both high conversion and selectivity. The bimetallic catalysts showed improved activity and selectivity. The high activity of the bimetallic catalysts can be attributed to the cooperative effect between M 1 -M 2 species yielding both metal and acid sites responsible for the conversion of LA to AL and the conversion of AL to GVL, respectively. It was shown that a suitable Ni/Cu, Ni/Co, and Co/Cu ratio was valuable to improve the conversion of LA and the selectivity to GVL in the solvent-free hydrogenation by enhancing the balance between metal and acid sites on the surface of the bimetallic catalysts. In all metal combinations, a 2:1 M 1 /M 2 ratio was found to be ideal. Values of >99.0%,~83.0%, and~65.0% GVL yield were obtained over  Figure S1: The XRD diffraction diagrams of the bimetallic catalysts at a different metal ratio, Figure S2: TEM images of selected catalysts at 500 nm and 50 nm, Figure S3: The recyclability test over [2:1] Ni-Cu/Al2O3. Reaction conditions: 20 mL LA, 1g catalyst, 220 ºC, 30 bar H2 pressure and 6 h reaction time. Table S1: List of the synthesized catalysts, Table S2: The textural properties of the bimetallic catalysts at a different metal ratio, and Table S3: Metal leaching analysis