Preparation of Highly Active Cu/SiO 2 Catalysts for Furfural to 2-Methylfuran by Ammonia Evaporation Method

: Biomass plays an important role in the green manufacture of high value-added chemicals. Among them, the conversion of furfural (FFA) into 2-methylfuran (2-MF), catalyzed by a copper-chromium catalyst, is important in its industrial application. However, the use of chromium is limited due to its toxicity and pollution of the environment. In this paper, a Cu/SiO 2 catalyst, prepared by the ammonia evaporation method, shows a better catalytic performance compared with that prepared by the co-precipitation method for the vapor-phase hydrodeoxygenation of FFA. The selectivity of 2-MF is higher than 80% with almost a complete conversion of FFA. Combined with the characterizations, the superiority of the ammonia evaporation method is attributed to the reduction of highly dispersed copper species and the increased Cu + /(Cu + + Cu 0 ) ratio due to the formation of a large content of copper phyllosilicate during the preparation. Moreover, Cu + sites can act as a weak acid site, which improve the surface acidity of the catalyst and facilitate the formation of 2-MF. This new catalytic system provides a feasible and promising strategy for the industrial preparation of 2-MF from FFA, and effectively utilizes biomass resources to promote the development of biomass industry.


Introduction
Furfural (FFA), an important bio-based platform molecule, has been industrially produced. The catalytic conversion of FFA into high value-added products, such as furfuryl alcohol, 2-methylfuran (2-MF), 2-methyltetrahydrofuran, 3-acetylpropanol and cyclopentanone [1][2][3][4][5], is an important method of biomass utilization. Additionally, 2-MF is an important organic raw material, which can be used to synthesize a series of chemical products such as pesticides and antimalarial drugs. At the same time, 2-MF can also act as a substitute or additive agent of fossil fuels [6,7].
The earliest production of 2-MF was through the oxidation of 1,3-pentadiene from fossils, which is undesirable from a green chemistry perspective [8]. Therefore, the production of green chemicals from biomass conversion has received great attention, which not only limits the mitigation of fossil resources, but effectively utilizes the abundant biomass resources. 2-MF could be obtained from FFA by vapor-phase and liquid-phase hydrogenation [9]. For the liquid-phase method, Pd-, Pt-, Cu-, and/or Ni-based catalysts are commonly used [10][11][12], which have been limited in their industrial applications due to a low yield of 2-MF and difficult separation [12]. Therefore, vapor-phase hydrogenation has received great attention.
It is reported that Cu has a good hydrogenation ability of C-O bonds, while it does not favor the hydrogenolysis of C-C bonds [13,14], thus showing good catalytic activity in the hydrodeoxygenation of FFA. Copper-chromium catalysts are commonly used in the hydrogenation of aldehydes, ketones or esters [15][16][17][18][19]. Adkins et al. [20] found that the earliest copper-chromium catalyst, Cu 2 Cr 2 O 5 , could be utilized in catalytic hydrogenation. However, industrial manufacture is nowadays limited due to the use of Cr, which is toxic and pollutive to the environment. In recent years, researchers have paid more attention to developing new green copper-based catalysts to replace Cu/Cr catalysts. Yang et al. [21] reported the yield of 2-MF as 94.5% when using a Cu/ZnO catalyst in FFA conversion. They concluded that the higher catalytic performance was benefited from the high dispersion of copper, surface acidity, and the synergistic effect between Cu 0 and ZnO. Huang et al. [22] reported a combined yield of 83% from FFA to 2-MF and 2-MTHF using a bimetallic Cu-Pd catalyst with 2-propanol as the hydrogen donor. They concluded that the yield of both products can be improved by adjusting the ratio of the two metals of the bimetallic catalyst, and the synergistic effect of Cu-Pd suppressed side reactions. Moreover, the in situ reduction of metal particles also helped to maintain reactivity. Lobo et al. [23] found that the Fe-containing Cu/SiO 2 catalysts had a higher activity from FFA to 2-MF, and the selectivity of 2-MF was 85%. They found that the addition of Fe increased the dispersion of Cu. Moreover, the partial reduction of Fe species enhanced the rapid hydrogenation of furfuryl alcohol to 2-MF. Park S et al. [24] reported the enhanced hydrodeoxygenation of furfural to 2-MF by a mesoporous Cu/Al 2 O 3 catalyst, and the yield of 2-MF was 74%. The special pore structure, improved Cu accessibility, Cu 0 /Cu + synergy and metal-support interaction enhanced activity and stability. Li et al. [25]. prepared Cu 0 /Cu 2 O·SiO 2 catalysts and found that a hydrogen-reduced copper phyllosilicate sample comprising Cu 0 /Cu 2 O·SiO 2 sites with interfaces demonstrated high activities. In conclusion, the preparation method of the Cu/SiO 2 catalyst has greatly affect the electronic structure of Cu and catalytic performance. Therefore, the acid sites, Cu 0 and Cu + , in the catalysts are crucial for FFA to 2-MF.
In this work, we synthesize two catalysts by the ammonia evaporation method and the co-precipitation method, which are denoted as Cu/SiO 2 (AE) and Cu/SiO 2 (CP), respectively. The Cu/SiO 2 (AE) catalyst shows a better catalytic performance for the vapor-phase hydrodeoxygenation of FFA to 2-MF versus the Cu/SiO 2 (CP) catalyst. The high activity of the Cu/SiO 2 (AE) catalyst is realized due to the high dispersion of Cu nanoparticles. In addition, copper phyllosilicate is formed during the preparation process using the ammonia evaporation method, which increases the ratio of Cu + /(Cu + + Cu 0 ) and strengthens the synergistic effect between Cu 0 and Cu + . Moreover, Cu + can enhance oxophilicity and act as a weak acid site to improve the selectivity of 2-MF [24,26]. In conclusion, the Cu/SiO 2 (AE) catalyst provides a new route for the vapor-phase hydrogenation of FFA to 2-MF and a feasible strategy for the industrialized green synthesis of 2-MF.

Characterization of Catalysts
The ICP-AES results show that 20 wt% Cu/SiO 2 catalysts are all close to the theoretical values, and the Cu/SiO 2 (AE) catalyst has a larger specific surface area versus the Cu/SiO 2 (CP) catalyst (Table 1). TEM images of Cu/SiO 2 catalysts are shown in Figure 1. For the calcined Cu/SiO 2 (AE) catalyst (Figure 1a), the typical filandrous morphology of copper phyllosilicate is observed [14,27], while no similar structure is observed for Cu/SiO 2 (CP) catalyst. The Cu/SiO 2 (AE) catalyst after reduction (Figure 1b,c) shows that the filandrous morphology of copper phyllosilicate is eliminated substantially [27] and the copper species are highly dispersed. The average diameter of the Cu/SiO 2 (AE) catalyst is about 3.2 nm. However, the Cu/SiO 2 (CP) catalyst (Figure 1d) shows the aggregation of Cu particles and lower dispersion. The average diameter of Cu/SiO 2 (CP) catalyst is about 5.4 nm, which indicates that the formation of copper phyllosilicate may be beneficial for the dispersion of Cu nanoparticles.   FT-IR is further used to determine the existence of copper phyllosilicate. The FT-IR spectra of calcined Cu/SiO2 catalysts are shown in Figure 2. The absorption bands at 1110, 800 and 470 cm −1 correspond to various vibration modes of the Si-O-Si bonds of amorphous silica [28]. The support of the catalyst prepared by the co-precipitation method and ammonia evaporation method exists in amorphous form. The band at 1640 cm −1 corresponds to the vibrational mode of the -OH of adsorbed water [29,30]. The existence of absorption bands at 1040 cm −1 and 670 cm −1 indicates the existence of the copper phyllosilicate [31]. The absorption band at 1040 cm −1 is ascribed to the vibration mode of Si-O bonds, and the absorption band at 670 cm −1 results from the vibration mode of -OH bond. The relative amount of copper phyllosilicate of the δOH band at 670 cm −1 and the νSi-O band of amorphous silica at 800 cm −1 (I670/I800) [32] are shown in Table 1, which indicates the ratio of copper phyllosilicate. Clearly, the Cu/SiO2 (AE) catalyst shows more copper phyl- FT-IR is further used to determine the existence of copper phyllosilicate. The FT-IR spectra of calcined Cu/SiO 2 catalysts are shown in Figure 2. The absorption bands at 1110, 800 and 470 cm −1 correspond to various vibration modes of the Si-O-Si bonds of amorphous silica [28]. The support of the catalyst prepared by the co-precipitation method and ammonia evaporation method exists in amorphous form. The band at 1640 cm −1 corresponds to the vibrational mode of the -OH of adsorbed water [29,30]. The existence of absorption bands at 1040 cm −1 and 670 cm −1 indicates the existence of the copper phyllosilicate [31]. The absorption band at 1040 cm −1 is ascribed to the vibration mode of Si-O bonds, and the absorption band at 670 cm −1 results from the vibration mode of -OH bond. The relative amount of copper phyllosilicate of the δ OH band at 670 cm −1 and the ν Si-O band of amorphous silica at 800 cm −1 (I 670 /I 800 ) [32] are shown in Table 1, which indicates the ratio of copper phyllosilicate. Clearly, the Cu/SiO 2 (AE) catalyst shows more copper phyllosilicate. The formation of copper phyllosilicate indicates the metalsupport interaction between copper and the support, which is corroborated by the report that copper can insert into the framework of the silica support and connect to it through chemical bonds [14]. The existence of this structure is related to the formation of a catalyst with a high dispersion, which is consistent with the results of TEM ( Figure 1).
Catalysts 2022, 12, x FOR PEER REVIEW 4 of 12 [14]. The existence of this structure is related to the formation of a catalyst with a high dispersion, which is consistent with the results of TEM ( Figure 1).
The XRD patterns of calcined Cu/SiO2 catalysts are shown in Figure 3a. The two Cu/SiO2 catalysts have similar diffraction patterns to pure SiO2. For the Cu/SiO2 (CP) catalyst, clear CuO diffraction peaks at 35.3° and 38.7° (JCPDS 05-0061) [14,33] are found, which indicate larger CuO nanoparticles. However, no clear CuO diffraction peaks are found for Cu/SiO2 (AE) catalysts, indicating that no aggregated CuO particles exist. Additionally, the peaks for Cu/SiO2 (AE) catalysts at 31.0°, 57.0° and 64.1° are assigned to the copper phyllosilicate [14], indicating that copper phyllosilicate is formed after calcination, which is consistent with the results of FT-IR.  XRD patterns of the reduced Cu/SiO2 catalysts are shown in Figure 3b. The peak at 38.7° for Cu/SiO2 (CP) catalysts shows the existence of CuO after reduction, indicating that the catalyst prepared by the co-precipitation method is not fully reduced. Cu/SiO2 (AE) catalyst has no CuO peak at 38.7°, indicating that CuO is reduced to Cu 0 and Cu + . The disappearance of the peak may be caused by the high catalyst surface area and high metal dispersion. The peak at 43.4° ascribed to Cu is observed for the Cu/SiO2 (CP) catalyst, indicating larger Cu nanoparticles [34,35]. The weaker peak at 43.4° for Cu/SiO2 (AE) catalyst may be related to the high dispersion and small size of Cu nanoparticles. The two catalysts show the diffraction peaks at 36.5° and 61.8°, which are attributed to Cu2O, indicating that Cu + is present in both catalysts.  [14,33] are found, which indicate larger CuO nanoparticles. However, no clear CuO diffraction peaks are found for Cu/SiO 2 (AE) catalysts, indicating that no aggregated CuO particles exist. Additionally, the peaks for Cu/SiO 2 (AE) catalysts at 31.0 • , 57.0 • and 64.1 • are assigned to the copper phyllosilicate [14], indicating that copper phyllosilicate is formed after calcination, which is consistent with the results of FT-IR.
Catalysts 2022, 12, x FOR PEER REVIEW 4 of 12 [14]. The existence of this structure is related to the formation of a catalyst with a high dispersion, which is consistent with the results of TEM ( Figure 1).
The XRD patterns of calcined Cu/SiO2 catalysts are shown in Figure 3a. The two Cu/SiO2 catalysts have similar diffraction patterns to pure SiO2. For the Cu/SiO2 (CP) catalyst, clear CuO diffraction peaks at 35.3° and 38.7° (JCPDS 05-0061) [14,33] are found, which indicate larger CuO nanoparticles. However, no clear CuO diffraction peaks are found for Cu/SiO2 (AE) catalysts, indicating that no aggregated CuO particles exist. Additionally, the peaks for Cu/SiO2 (AE) catalysts at 31.0°, 57.0° and 64.1° are assigned to the copper phyllosilicate [14], indicating that copper phyllosilicate is formed after calcination, which is consistent with the results of FT-IR. XRD patterns of the reduced Cu/SiO2 catalysts are shown in Figure 3b. The peak at 38.7° for Cu/SiO2 (CP) catalysts shows the existence of CuO after reduction, indicating that the catalyst prepared by the co-precipitation method is not fully reduced. Cu/SiO2 (AE) catalyst has no CuO peak at 38.7°, indicating that CuO is reduced to Cu 0 and Cu + . The disappearance of the peak may be caused by the high catalyst surface area and high metal dispersion. The peak at 43.4° ascribed to Cu is observed for the Cu/SiO2 (CP) catalyst, indicating larger Cu nanoparticles [34,35]. The weaker peak at 43.4° for Cu/SiO2 (AE) catalyst may be related to the high dispersion and small size of Cu nanoparticles. The two catalysts show the diffraction peaks at 36.5° and 61.8°, which are attributed to Cu2O, indicating that Cu + is present in both catalysts. XRD patterns of the reduced Cu/SiO 2 catalysts are shown in Figure 3b. The peak at 38.7 • for Cu/SiO 2 (CP) catalysts shows the existence of CuO after reduction, indicating that the catalyst prepared by the co-precipitation method is not fully reduced. Cu/SiO 2 (AE) catalyst has no CuO peak at 38.7 • , indicating that CuO is reduced to Cu 0 and Cu + . The disappearance of the peak may be caused by the high catalyst surface area and high metal dispersion. The peak at 43.4 • ascribed to Cu is observed for the Cu/SiO 2 (CP) catalyst, indicating larger Cu nanoparticles [34,35]. The weaker peak at 43.4 • for Cu/SiO 2 (AE) catalyst may be related to the high dispersion and small size of Cu nanoparticles. The two catalysts show the diffraction peaks at 36.5 • and 61.8 • , which are attributed to Cu 2 O, indicating that Cu + is present in both catalysts. The H 2 -TPR profiles of Cu/SiO 2 catalysts are shown in Figure 4. All samples are calcined in flowing air at 450 • C for 4 h to ensure oxidized Cu species. H 2 -TPR shows that the reduction temperature for CuO over the Cu/SiO 2 (CP) catalyst is at 300-420 • C higher than the reduction temperature for the reported Cu-based catalyst [14,16]. This may be due to the reduction of bulk CuO [27]. The Cu/SiO 2 (AE) catalyst shows a reduction peak at 180-230 • C, which is related to the reduction of the high dispersion and small size of CuO according to the literature [14,27,36]. The XRD results of the Cu/SiO 2 (CP) catalyst show that CuO is present after reduction. However, no CuO exists for the Cu/SiO 2 (AE) catalyst after the same reduction process, which is consistent with the H 2 -TPR results. TEM results show that the particle sizes of the Cu/SiO 2 (AE) catalyst and Cu/SiO 2 (CP) catalyst are 3.2 nm and 5.4 nm, respectively. The easy reduction of Cu/SiO 2 (AE) may be due to its small particle size and uniform dispersion. The H2-TPR profiles of Cu/SiO2 catalysts are shown in Figure 4. All samples are calcined in flowing air at 450 °C for 4 h to ensure oxidized Cu species. H2-TPR shows that the reduction temperature for CuO over the Cu/SiO2 (CP) catalyst is at 300-420 °C higher than the reduction temperature for the reported Cu-based catalyst [14,16]. This may be due to the reduction of bulk CuO [27]. The Cu/SiO2 (AE) catalyst shows a reduction peak at 180-230 °C, which is related to the reduction of the high dispersion and small size of CuO according to the literature [14,27,36]. The XRD results of the Cu/SiO2 (CP) catalyst show that CuO is present after reduction. However, no CuO exists for the Cu/SiO2 (AE) catalyst after the same reduction process, which is consistent with the H2-TPR results. TEM results show that the particle sizes of the Cu/SiO2 (AE) catalyst and Cu/SiO2 (CP) catalyst are 3.2 nm and 5.4 nm, respectively. The easy reduction of Cu/SiO2 (AE) may be due to its small particle size and uniform dispersion. XPS spectra of the Cu/SiO2 catalysts are shown in Figure 5a. No peak at 940-945 eV is found for the Cu/SiO2 (AE) catalyst, indicating that Cu 2+ is totally reduced to Cu + or Cu 0 [14,16,29], and the two peaks with binding energies at ~932 eV and 952 eV are assigned to the 2P3/2 and 2P1/2 of Cu + or Cu 0 [29,37]. The Cu/SiO2 (CP) catalyst shows a peak at 940-945 eV related to Cu 2+ , indicating the incomplete reduction of CuO. This is consistent with the results of XRD and H2-TPR. The peak with binding energy at ~935 eV may be related to the 2P3/2 of Cu 2+ [22,27]. The peak with binding energy at 952 eV is assigned to the 2P1/2 of Cu 0 [22]. To further analyze the content of Cu + and Cu 0 , the Cu LMM auger spectra is investigated (Figure 5b). For the Cu/SiO2 (CP) catalyst, the peak between 910-925 eV is divided into two peaks at 916 eV and 919 eV, which are ascribed to Cu + and Cu 0 species [26,38]. Accordingly, the Cu/SiO2 (AE) catalyst shows two peaks for Cu + and Cu 0 species at 915 eV and 920 eV, respectively [38]. For the Cu/SiO2 (AE) catalyst, the Cu + /(Cu 0 + Cu + ) intensity ratio, obtained by fitting the Cu LMM peaks, is 32%, higher than that of Cu/SiO2 (CP) catalyst (22%) ( Table 1) [14]. XPS spectra of the Cu/SiO 2 catalysts are shown in Figure 5a. No peak at 940-945 eV is found for the Cu/SiO 2 (AE) catalyst, indicating that Cu 2+ is totally reduced to Cu + or Cu 0 [14,16,29], and the two peaks with binding energies at~932 eV and 952 eV are assigned to the 2P 3/2 and 2P 1/2 of Cu + or Cu 0 [29,37]. The Cu/SiO 2 (CP) catalyst shows a peak at 940-945 eV related to Cu 2+ , indicating the incomplete reduction of CuO. This is consistent with the results of XRD and H 2 -TPR. The peak with binding energy at~935 eV may be related to the 2P 3/2 of Cu 2+ [22,27]. The peak with binding energy at 952 eV is assigned to the 2P 1/2 of Cu 0 [22]. To further analyze the content of Cu + and Cu 0 , the Cu LMM auger spectra is investigated (Figure 5b). For the Cu/SiO 2 (CP) catalyst, the peak between 910-925 eV is divided into two peaks at 916 eV and 919 eV, which are ascribed to Cu + and Cu 0 species [26,38]. Accordingly, the Cu/SiO 2 (AE) catalyst shows two peaks for Cu + and Cu 0 species at 915 eV and 920 eV, respectively [38]. For the Cu/SiO 2 (AE) catalyst, the Cu + /(Cu 0 + Cu + ) intensity ratio, obtained by fitting the Cu LMM peaks, is 32%, higher than that of Cu/SiO 2 (CP) catalyst (22%) ( Table 1) [14].
The NH 3 -TPD profiles of Cu/SiO 2 catalysts are shown in Figure 6. For the Cu/SiO 2 (CP) catalyst, the peaks at 90-240 • C and 330-530 • C are attributed to the desorption of NH 3 at weak and strong acid sites. The acid amounts of weak acid and strong acid are 0.03 mmol·g −1 and 0.12 mmol·g −1 , respectively. Additionally, the NH 3 desorption peaks at 90-230 • C and 235-370 • C are attributed to the desorption of NH 3 at weak and moderate acid sites for the Cu/SiO 2 (AE) catalyst [26], and the acid amounts of the weak and moderate sites are 0.08 mmol·g −1 and 0.06 mmol·g −1 . The report shows that pure CuO and SiO 2 have almost no acidity, and the acid sites of Cu/SiO 2 may be related to the low state copper species, such as Cu + [26,39]. Cu/SiO 2 (AE) catalyst shows more weak acid sites versus the Cu/SiO 2 (CP) catalyst, suggesting that the increased acid sites for the Cu/SiO 2 (AE) catalyst may originate from the higher content of Cu + confirmed by XPS results. The NH3-TPD profiles of Cu/SiO2 catalysts are shown in Figure 6. For the Cu/SiO2 (CP) catalyst, the peaks at 90-240 °C and 330-530 °C are attributed to the desorption of NH3 at weak and strong acid sites. The acid amounts of weak acid and strong acid are 0.03 mmol•g −1 and 0.12 mmol•g −1 , respectively. Additionally, the NH3 desorption peaks at 90-230 °C and 235-370 °C are attributed to the desorption of NH3 at weak and moderate acid sites for the Cu/SiO2 (AE) catalyst [26], and the acid amounts of the weak and moderate sites are 0.08 mmol•g −1 and 0.06 mmol•g −1 . The report shows that pure CuO and SiO2 have almost no acidity, and the acid sites of Cu/SiO2 may be related to the low state copper species, such as Cu + [26,39]. Cu/SiO2 (AE) catalyst shows more weak acid sites versus the Cu/SiO2 (CP) catalyst, suggesting that the increased acid sites for the Cu/SiO2 (AE) catalyst may originate from the higher content of Cu + confirmed by XPS results.

Catalytic Activity and Stability
The catalytic performance of Cu/SiO2 catalysts for the vapor-phase hydrogenation of FFA to 2-MF is further studied (Figure 7). The conversion of FFA into 2-MF is achieved through two-step reaction as follows. Firstly, FFA is hydrogenated into furfuryl alcohol (FA), then FA is further converted into 2-MF by the cleavage of C-O bonds through hydrogenolysis (Scheme 1) [25,40]. In this work, the metal loading of Cu is chosen based on the previous work, and two catalysts prepared by different methods are compared [24,26]. Interestingly, the selectivity of 2-MF, up to 86% (Figure 7a), is obtained for the Cu/SiO2 (AE) catalyst, a good yield obtained so far in the gas hydrogenation of FFA into 2-MF [24,41]. However, only 80% FA and 5% tetrahydrofurfuryl alcohol (THFA) are produced  The NH3-TPD profiles of Cu/SiO2 catalysts are shown in Figure 6. For the Cu/SiO2 (CP) catalyst, the peaks at 90-240 °C and 330-530 °C are attributed to the desorption of NH3 at weak and strong acid sites. The acid amounts of weak acid and strong acid are 0.03 mmol•g −1 and 0.12 mmol•g −1 , respectively. Additionally, the NH3 desorption peaks at 90-230 °C and 235-370 °C are attributed to the desorption of NH3 at weak and moderate acid sites for the Cu/SiO2 (AE) catalyst [26], and the acid amounts of the weak and moderate sites are 0.08 mmol•g −1 and 0.06 mmol•g −1 . The report shows that pure CuO and SiO2 have almost no acidity, and the acid sites of Cu/SiO2 may be related to the low state copper species, such as Cu + [26,39]. Cu/SiO2 (AE) catalyst shows more weak acid sites versus the Cu/SiO2 (CP) catalyst, suggesting that the increased acid sites for the Cu/SiO2 (AE) catalyst may originate from the higher content of Cu + confirmed by XPS results.

Catalytic Activity and Stability
The catalytic performance of Cu/SiO2 catalysts for the vapor-phase hydrogenation of FFA to 2-MF is further studied (Figure 7). The conversion of FFA into 2-MF is achieved through two-step reaction as follows. Firstly, FFA is hydrogenated into furfuryl alcohol (FA), then FA is further converted into 2-MF by the cleavage of C-O bonds through hydrogenolysis (Scheme 1) [25,40]. In this work, the metal loading of Cu is chosen based on the previous work, and two catalysts prepared by different methods are compared [24,26]. Interestingly, the selectivity of 2-MF, up to 86% (Figure 7a), is obtained for the Cu/SiO2 (AE) catalyst, a good yield obtained so far in the gas hydrogenation of FFA into 2-MF [24,41]. However, only 80% FA and 5% tetrahydrofurfuryl alcohol (THFA) are produced

Catalytic Activity and Stability
The catalytic performance of Cu/SiO 2 catalysts for the vapor-phase hydrogenation of FFA to 2-MF is further studied (Figure 7). The conversion of FFA into 2-MF is achieved through two-step reaction as follows. Firstly, FFA is hydrogenated into furfuryl alcohol (FA), then FA is further converted into 2-MF by the cleavage of C-O bonds through hydrogenolysis (Scheme 1) [25,40]. In this work, the metal loading of Cu is chosen based on the previous work, and two catalysts prepared by different methods are compared [24,26]. Interestingly, the selectivity of 2-MF, up to 86% (Figure 7a), is obtained for the Cu/SiO 2 (AE) catalyst, a good yield obtained so far in the gas hydrogenation of FFA into 2-MF [24,41]. However, only 80% FA and 5% tetrahydrofurfuryl alcohol (THFA) are produced for the Cu/SiO 2 (CP) catalyst. The conversions of FFA over the two catalysts are both close to 100%, indicating that the hydrogenation of FFA is easy and would not be the rate-limiting step. The reaction rates of the Cu/SiO 2 (AE) catalyst and Cu/SiO 2 (CP) catalyst are 10.4 mmol·h −1 and 9.8 mmol·h −1 , respectively. The conversion rate of FFA for the Cu/SiO 2 (AE) catalyst is a little higher than that for the Cu/SiO 2 (CP) catalyst, which may benefit from the high dispersion of Cu nanoparticles and the higher reduction degree of CuO. It is interesting that the further conversion of FA is totally different. FA is further hydrogenated into THFA for the Cu/SiO 2 (CP) catalyst by the further hydrogenation of the C=C bond. No FA or THFA is found for the Cu/SiO 2 (AE) catalyst, suggesting that C-O bond hydrogenolysis is preferred rather than the hydrogenation of C=C bond. Park S et al. [24] ascribed the high selectivity and stability of 2-MF over the Cu/Al 2 O 3 catalyst to the smaller size of active copper, the acidity, and the synergy between Cu 0 and Cu + . They concluded that Cu 0 and Cu + were beneficial for activating molecular H 2 on the surface of metallic copper, and Cu + can enhance oxophilicity. Moreover, Li et al. [26] proposed that the high selectivity of the Cu/SiO 2 catalyst to 2-MF was due to the synergistic effect of metal and weak acid sites. They pointed out that weak acid sites could promote the hydrogenolysis of C-O bonds [42]. Combined with the FT-IR and XPS results, the Cu/SiO 2 (AE) catalyst improved the ratio of Cu + /(Cu + + Cu 0 ) due to the formation of copper phyllosilicate during its preparation. The NH 3 -TPD results also confirmed that the Cu/SiO 2 (AE) catalyst shows more weak acid sites, which may be favorable for the formation of 2-MF [26]. Additionally, it is also interesting to find that the carbon balance for Cu/SiO 2 (AE) is satisfying, while that for Cu/SiO 2 (CP) is lower than 90%, which may be caused by the polymerization of FFA or FA over the strong acid sites.
is interesting that the further conversion of FA is totally different. FA is further hydrogenated into THFA for the Cu/SiO2 (CP) catalyst by the further hydrogenation of the C=C bond. No FA or THFA is found for the Cu/SiO2 (AE) catalyst, suggesting that C-O bond hydrogenolysis is preferred rather than the hydrogenation of C=C bond. Park S et al. [24] ascribed the high selectivity and stability of 2-MF over the Cu/Al2O3 catalyst to the smaller size of active copper, the acidity, and the synergy between Cu 0 and Cu + . They concluded that Cu 0 and Cu + were beneficial for activating molecular H2 on the surface of metallic copper, and Cu + can enhance oxophilicity. Moreover, Li et al. [26] proposed that the high selectivity of the Cu/SiO2 catalyst to 2-MF was due to the synergistic effect of metal and weak acid sites. They pointed out that weak acid sites could promote the hydrogenolysis of C-O bonds [42]. Combined with the FT-IR and XPS results, the Cu/SiO2 (AE) catalyst improved the ratio of Cu + /(Cu + + Cu 0 ) due to the formation of copper phyllosilicate during its preparation. The NH3-TPD results also confirmed that the Cu/SiO2 (AE) catalyst shows more weak acid sites, which may be favorable for the formation of 2-MF [26]. Additionally, it is also interesting to find that the carbon balance for Cu/SiO2 (AE) is satisfying, while that for Cu/SiO2 (CP) is lower than 90%, which may be caused by the polymerization of FFA or FA over the strong acid sites.  from the high dispersion of Cu nanoparticles and the higher reduction degree of CuO. It is interesting that the further conversion of FA is totally different. FA is further hydrogenated into THFA for the Cu/SiO2 (CP) catalyst by the further hydrogenation of the C=C bond. No FA or THFA is found for the Cu/SiO2 (AE) catalyst, suggesting that C-O bond hydrogenolysis is preferred rather than the hydrogenation of C=C bond. Park S et al. [24] ascribed the high selectivity and stability of 2-MF over the Cu/Al2O3 catalyst to the smaller size of active copper, the acidity, and the synergy between Cu 0 and Cu + . They concluded that Cu 0 and Cu + were beneficial for activating molecular H2 on the surface of metallic copper, and Cu + can enhance oxophilicity. Moreover, Li et al. [26] proposed that the high selectivity of the Cu/SiO2 catalyst to 2-MF was due to the synergistic effect of metal and weak acid sites. They pointed out that weak acid sites could promote the hydrogenolysis of C-O bonds [42]. Combined with the FT-IR and XPS results, the Cu/SiO2 (AE) catalyst improved the ratio of Cu + /(Cu + + Cu 0 ) due to the formation of copper phyllosilicate during its preparation. The NH3-TPD results also confirmed that the Cu/SiO2 (AE) catalyst shows more weak acid sites, which may be favorable for the formation of 2-MF [26]. Additionally, it is also interesting to find that the carbon balance for Cu/SiO2 (AE) is satisfying, while that for Cu/SiO2 (CP) is lower than 90%, which may be caused by the polymerization of FFA or FA over the strong acid sites.

Catalyst Preparation
For the preparation of Cu/SiO 2 catalyst by co-precipitation (Figure 8), 5 g Cu(CH 3 COO) 2 ·H 2 O and 20 mL H 2 O were mixed and put under ultrasonic for 30 min. Additionally, 16 g Na 2 SiO 3 ·9H 2 O was added into 40 mL H 2 O to form a solution and kept ultrasonic for 30 min. Then, 4 mL of ammonia water was added to the Cu(CH 3 COO) 2 ·H 2 O solution to form a dark blue clear copper ammonia solution. Next the copper ammonia solution was poured into Na 2 SiO 3 ·9H 2 O solution to form a mixed solution with PH = 12.8. Then, 8 g CH 3 COOH was added dropwise to the mixed solution, and white mist continued to emerge during the process. After stirring for 2 h, the solution was transferred to the hydrothermal reactor and kept at 100 • C for 12 h. Then, the solid was filtered, washed, and dried in a vacuum drying oven at 80 • C for 5 h. The sample was ground and calcined in flowing air at 450 • C for 4 h to obtain a light green powder, which was denoted as Cu/SiO 2 (CP) with the theoretical loading of Cu at 20 wt%. ditionally, 16 g Na2SiO3·9H2O was added into 40 mL H2O to form a solution and kept ultrasonic for 30 min. Then, 4 mL of ammonia water was added to the Cu(CH3COO)2·H 2O solution to form a dark blue clear copper ammonia solution. Next the copper ammonia solution was poured into Na2SiO3·9H2O solution to form a mixed solution with PH = 12.8. Then, 8 g CH3COOH was added dropwise to the mixed solution, and white mist continued to emerge during the process. After stirring for 2 h, the solution was transferred to the hydrothermal reactor and kept at 100 °C for 12 h. Then, the solid was filtered, washed, and dried in a vacuum drying oven at 80 °C for 5 h. The sample was ground and calcined in flowing air at 450 °C for 4 h to obtain a light green powder, which was denoted as Cu/SiO2 (CP) with the theoretical loading of Cu at 20 wt%. For the preparation of Cu/SiO2 catalyst by ammonia evaporation method (Figure 9), 5 g Cu(CH3COO)2·H 2O, 2 g of urea and 200 mL H2O were dissolved and put in a 25 °C water bath. To the mixed solution, 10 g of silica sol (40%) was added and constantly stirred. Then, 10 mL of ammonia water was used to obtain pH = 12. After stirring at 25 °C for 1 h, the temperature was raised to 90 °C and stirred for 1 h. The ammonia was steamed openly to PH = 8, and the solution was a light blue turbid liquid. The solution was transferred to the hydrothermal reactor and kept at 100 °C for 12 h. The sample was filtered, washed, and dried. The sample was ground and calcined in flowing air at 450 °C for 4 h to obtain a light blue powder, which was denoted as Cu/SiO2 (AE), with the same metal loading as Cu/SiO2 (CP).

Characterization
The X-ray diffraction (XRD) patterns of samples were analyzed on the D8 X-ray diffractometer of Bruker, Germany, with a Cu Kα radiation, λ = 1.541 nm Å . N2 adsorption and desorption were performed on the physical adsorption analyzer Micromeritics ASAP 2420 at a temperature of 77 K. The content of copper in the sample was determined by For the preparation of Cu/SiO 2 catalyst by ammonia evaporation method (Figure 9), 5 g Cu(CH 3 COO) 2 ·H 2 O, 2 g of urea and 200 mL H 2 O were dissolved and put in a 25 • C water bath. To the mixed solution, 10 g of silica sol (40%) was added and constantly stirred. Then, 10 mL of ammonia water was used to obtain pH = 12. After stirring at 25 • C for 1 h, the temperature was raised to 90 • C and stirred for 1 h. The ammonia was steamed openly to PH = 8, and the solution was a light blue turbid liquid. The solution was transferred to the hydrothermal reactor and kept at 100 • C for 12 h. The sample was filtered, washed, and dried. The sample was ground and calcined in flowing air at 450 • C for 4 h to obtain a light blue powder, which was denoted as Cu/SiO 2 (AE), with the same metal loading as Cu/SiO 2 (CP). ditionally, 16 g Na2SiO3·9H2O was added into 40 mL H2O to form a solution and kept ultrasonic for 30 min. Then, 4 mL of ammonia water was added to the Cu(CH3COO)2·H2O solution to form a dark blue clear copper ammonia solution. Next the copper ammonia solution was poured into Na2SiO3·9H2O solution to form a mixed solution with PH = 12.8. Then, 8 g CH3COOH was added dropwise to the mixed solution, and white mist continued to emerge during the process. After stirring for 2 h, the solution was transferred to the hydrothermal reactor and kept at 100 °C for 12 h. Then, the solid was filtered, washed, and dried in a vacuum drying oven at 80 °C for 5 h. The sample was ground and calcined in flowing air at 450 °C for 4 h to obtain a light green powder, which was denoted as Cu/SiO2 (CP) with the theoretical loading of Cu at 20 wt%. For the preparation of Cu/SiO2 catalyst by ammonia evaporation method (Figure 9), 5 g Cu(CH3COO)2·H2O, 2 g of urea and 200 mL H2O were dissolved and put in a 25 °C water bath. To the mixed solution, 10 g of silica sol (40%) was added and constantly stirred. Then, 10 mL of ammonia water was used to obtain pH = 12. After stirring at 25 °C for 1 h, the temperature was raised to 90 °C and stirred for 1 h. The ammonia was steamed openly to PH = 8, and the solution was a light blue turbid liquid. The solution was transferred to the hydrothermal reactor and kept at 100 °C for 12 h. The sample was filtered, washed, and dried. The sample was ground and calcined in flowing air at 450 °C for 4 h to obtain a light blue powder, which was denoted as Cu/SiO2 (AE), with the same metal loading as Cu/SiO2 (CP).

Characterization
The X-ray diffraction (XRD) patterns of samples were analyzed on the D8 X-ray diffractometer of Bruker, Germany, with a Cu Kα radiation, λ = 1.541 nm Å. N2 adsorption and desorption were performed on the physical adsorption analyzer Micromeritics ASAP 2420 at a temperature of 77 K. The content of copper in the sample was determined by

Characterization
The X-ray diffraction (XRD) patterns of samples were analyzed on the D8 X-ray diffractometer of Bruker, Germany, with a Cu Kα radiation, λ = 1.541 nm Å. N 2 adsorption and desorption were performed on the physical adsorption analyzer Micromeritics ASAP 2420 at a temperature of 77 K. The content of copper in the sample was determined by inductively coupled plasma atomic emission spectrometry (ICP-AES). X-ray photoelectron spectroscopy (XPS) and X-ray Auger electron spectroscopy (XAES) of the catalysts were performed on a PHI Quantera SXM spectrometer with Al Kα = 1486.6 eV as the excitation source, and the binding energy was corrected by setting that of C 1s at 284.8 eV. Transmission electron microscopy (TEM) images were obtained using a FEI Tecnai G2 F20 electron microscope operated at 200 kV. Fourier transform infrared (FT-IR) spectroscopy was performed on a Thermo Fisher Nicolet 380. The spectral resolution was 2 cm −1 and 32 scans were recorded for the spectrum. The experiments of temperature-programmed reduction by hydrogen (H 2 -TPR) were performed on a Quantachrome Autosorb-IQ gas adsorption analyzer. In the experiment, the sample was calcined in air at 450 • C for 4 h and cooled to room temperature. Then, the TCD signal was collected when the temperature increased to 600 • C at 10 • C min −1 under 10 vol% H 2 /Ar. The temperature-programmed desorption of ammonia (NH 3 -TPD) was performed on a Quantachrome Autosorb-IQ gas adsorption analyzer. The sample was heated from 50 • C to 400 • C at 50 • C·min −1 in He for 30 min and cooled to 100 • C, and 10 vol% NH 3 /He was introduced to achieve saturated adsorption. Then, after purging with He for 1 h to remove NH 3 , which is physically adsorbed, the sample was heated from 100 • C to 700 • C at 10 • C·min −1 .

Catalytic Performance Evaluation
The activity test was conducted on a continuous flow unit equipped with a stainlesssteel fixed-bed tubular reactor (Figure 10), where 1 g Cu/SiO 2 catalyst was pressed into tablets and sieved to 20-40 mesh, then loaded into a stainless-steel reactor with an inner diameter of 8 mm. The position of the catalyst bed was fixed with a thin layer of quartz wool to ensure that the catalyst bed was located in the constant temperature section of the stainless-steel reactor. Feed H 2 and the stainless-steel reactor temperature were set to 200 • C, and the vaporizer temperature was set to 260 • C. Furfural was pumped into the vaporizer through a syringe pump for vaporization, and then hydrogen flowed through the vaporizer and was mixed with furfural in the stainless-steel reactor. Among them, the hydrogen space velocity is 400 h −1 , and the molar ratio of H 2 and furfural is 10:1. The product is analyzed online by gas chromatography (Aglient GC-7820A) and equipped with a FID detector and HP-INNOWAX column (30 m × 0.32 mm, 0.5 µm).

Conclusions
In this work, Cu/SiO2 catalysts were prepared for vapor-phase hydrodeoxygenation of FFA to 2-MF by the co-precipitation method (CP) and modified ammonia evaporation method (AE). The Cu/SiO2 (AE) catalyst shows a superior catalytic performance with a high selectivity of 2-MF versus Cu/SiO2 (CP) catalyst; 2-MF selectivity still maintains above 80% within 25 h. The Cu/SiO2 (AE) catalyst improves the selectivity of 2-MF mainly through the following aspects: (1) The Cu/SiO2 (AE) catalyst has a high dispersion and small size of copper nanoparticles, due to the formation of the copper phyllosilicate structure. The formation of the copper phyllosilicate structure improves the ratio of Cu + /(Cu + + Cu 0 ) and enhances the synergy between Cu 0 and Cu + . (2) Cu + sites enhance oxophilicity and act as a weak acid site, which improve the surface acidity of the catalyst, favoring the formation of 2-MF. This work puts forward an efficient strategy and provides a feasible route for the industrial production of 2-MF, which is of great significance in biomass conversion.

Conclusions
In this work, Cu/SiO 2 catalysts were prepared for vapor-phase hydrodeoxygenation of FFA to 2-MF by the co-precipitation method (CP) and modified ammonia evaporation method (AE). The Cu/SiO 2 (AE) catalyst shows a superior catalytic performance with a high selectivity of 2-MF versus Cu/SiO 2 (CP) catalyst; 2-MF selectivity still maintains above 80% within 25 h. The Cu/SiO 2 (AE) catalyst improves the selectivity of 2-MF mainly through the following aspects: (1) The Cu/SiO 2 (AE) catalyst has a high dispersion and small size of copper nanoparticles, due to the formation of the copper phyllosilicate structure. The formation of the copper phyllosilicate structure improves the ratio of Cu + /(Cu + + Cu 0 ) and enhances the synergy between Cu 0 and Cu + . (2) Cu + sites enhance oxophilicity and act as a weak acid site, which improve the surface acidity of the catalyst, favoring the formation of 2-MF. This work puts forward an efficient strategy and provides a feasible route for the industrial production of 2-MF, which is of great significance in biomass conversion.