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. 2020 Nov 10;5(46):30257–30266. doi: 10.1021/acsomega.0c04736

Total Hydrogenation of Furfural under Mild Conditions over a Durable Ni/TiO2–SiO2 Catalyst with Amorphous TiO2 Species

Jiamin Xu 1, Qianqian Cui 1, Teng Xue 1, Yejun Guan 1,*, Peng Wu 1
PMCID: PMC7689957  PMID: 33251460

Abstract

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One-step total hydrogenation of furfural (FAL) toward tetrahydrofurfuryl alcohol in continuous flow using cheap transition metals still remains a great challenge. We herein reported the total hydrogenation of FAL over Ni (∼5 nm) nanoparticles loaded on TiO2–SiO2 composites with long-term stability. The TiO2–SiO2 composites comprise amorphous TiOx which was grafted on the silica aerogel by acetyl acetone-aided controlled hydrolysis of tetrabutyl titanate. The catalysts were characterized by several techniques including Brunauer–Emmett–Teller, X-ray diffraction, transmission electron microscopy, H2-temperature-programmed reduction, and H2-temperature-programmed desorption. The hydrogenation performances were systematically explored in terms of TiO2 content, Ni loading, liquid hour space velocity, and so forth. Ni nanoparticles in contact with amorphous TiOx showed strengthened interaction with the C=O bond of FAL as well as enhanced hydrogen dissociation and desorption ability, hence benefiting the overall hydrogenation process.

Introduction

Upgrading biomass feedstock to widely used fuels and chemicals has attracted increasing attention due to the fast depletion of fossil fuels and large CO2 emission caused by the usage of fossil carbon.1 Furfural (FAL) is considered to be one of the “Top-10” platform molecules in the biorefinery approach, which is produced from xylose via acid-catalyzed hydrolysis.2−5 FAL can be converted to biofuels and chemical intermediates such as furfuryl alcohol (FA), 2-methylfuran, tetrahydrofurfural alcohol (THFA), cyclopentanone, and so forth.6−12 The production of THFA via total hydrogenation of C=C and C=O groups in FAL has attracted much attention because THFA serves as a green solvent and biofuel due to its low toxicity and biodegradable nature. Moreover, THFA has been also regarded as a raw material for the production of other higher-valuable chemicals such as diols, dihydropyran, pyridine, and so forth.13−15

THFA is commercially produced by a stepwise hydrogenation of FAL via the combination of Cu–Cr and Ni catalysts.16,17 This route meets with two obvious drawbacks such as high-energy consumption and the usage of toxic Cu–Cr catalyst. Therefore, it is desirable to develop catalysts for direct hydrogenation of FAL to THFA in one-step. To this end, noble metals such as Pd, Pt, Ir, and Ru or their mixtures deposited on specific supports have been investigated.18−23 Compared with expensive noble metals, Ni as a transition metal is more promising due to its large abundance and low cost and thus have attracted wide attention for hydrogenation of unsaturated functional groups including C=O and C=C bonds in FAL.24−29 Great efforts have been devoted to elucidate the structure–activity relationship between supported Ni catalysts and their hydrogenation performance. Ni nanoparticles less than 4 nm gave higher turnover frequency values than other supported Ni catalysts.24 Introducing basic metals such as alkaline earth metals (Ca, Sr, Mg, and Ba) into Ni/Al2O3 catalysts is also beneficial to the full hydrogenation of FAL.29 Wei and co-workers30 found that high exposure of the Ni(111) plane in Ni/MMO-CO3 promotes activated adsorption of both the furan ring and C=O group and favors the production of a fully hydrogenated product. However, Ni catalysts always underwent deactivation with time on stream (TOS) either in the gas phase or in the liquid phase due to the coke formation by oligomerization and deoxygenation, regardless of the support categories (SiO2, TiO2, and CeO2).31 The authors proposed that tuning the selectivity of the nickel by expense of some of its high hydrogenation activity may be helpful for developing stable Ni catalysts.31 We noticed that one of the commonly used supports to tune the electronic property of Ni is TiO2, which plays a dual role by activating the C=O bond as well as stabilizing the metal nanoparticles via so-called strong-metal–support interaction (SMSI).32,33 Stimulated by these studies, we herein reported the long period continuous hydrogenation of FAL over a TiO2–SiO2 composite-supported Ni catalyst, which showed high stability and selectivity toward THFA within 120 h studied. The nanocrystalline TiO2 was loaded on SiO2 by controlled hydrolysis of tetrabutyl titanate (TBT) with the aid of acetyl acetone (acac),34,35 while Ni was supported in the presence of ethylene glycol (EG).36 By combining several characterization techniques, we speculate that the highly dispersed TiO2 species play a determining role in enhancing the carbonyl activation of FAL and significantly retard the coking process.

Results and Discussion

Characterization of TiO2–SiO2 Supports and Supported Ni Catalysts

The textural properties including specific surface area (SSA) and total pore volume (Vtotal) of TiO2–SiO2 materials are shown in Table 1. The SSA and pore volume of pristine fumed SiO2 are 230 m2/g and 1.007 cm3/g, respectively. The grafting of TiO2 results in a slight decrease of SSA ranging from 230 to 214 m2/g. The pore volume of TiO2–SiO2 materials also decreases from about 1–0.8 cm3/g with the increase in TiO2 loading. Figure 1 shows the X-ray diffraction (XRD) patterns of TiO2–SiO2 supports prepared via hydrolysis of TBT in the presence of acac. Amorphous TiO2 species were obtained when TiO2 loading was up to 10 wt % regardless of the amount of acac used. When TiO2 loading was up to 20 wt %, a weak peak at 25.2° assigned to the (101) plane of anatase phase was observed for samples with Ti/acac = 5.37

Table 1. Textural Properties of SiO2 and TiO2–SiO2 Materials.

support Ti/acaca SSAb (m2/g) Vtotal (cm3/g)
SiO2   230 1.007
5TiO2–SiO2(5) 5 211 0.908
10TiO2–SiO2(25) 25 214 0.873
10TiO2–SiO2(5) 5 177 0.796
10TiO2–SiO2(2) 2 184 0.858
20TiO2–SiO2(5) 5 202 0.816
10TiO2–SiO2   199 0.843
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a

The molar ratio of TBT and acac.

b

Calculated using BET method.

Figure 1.

Figure 1

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XRD patterns of TiO2–SO2 materials with various TiO2 loading and Ti/acac ratio. (a) 5TiO2–SiO2(5), (b) 10TiO2–SiO2(5), (c) 20TiO2–SiO2(5), (d) 10TiO2–SiO2(25), (e) 10TiO2–SiO2(2), and (f) 10TiO2–SiO2.

We then investigated the reduction behavior of the obtained NiO/yTiO2–SiO2(x) catalysts by H2-temperature-programmed reduction (TPR) (Figure 2). For NiO supported on fumed SiO2, three reduction peaks centered at 323, 354, and 443 °C were observed. Previous study has shown that the low reduction peaks (323 and 354 °C) are attributed to NiO crystallites weakly interacting with the SiO2 surface, while a higher reduction peak at 443 °C is assigned to the complete reduction of NiO species strongly interacting with SiO2.38 The interaction strength of NiO and SiO2 could be affected by the location of NiO in pores with various diameters or the formation of nickel silicates. As for 20NiO/TiO2, the reduction of NiO also gives a strong peak at 356 °C along with a small shoulder at 443 °C. When NiO was supported on TiO2–SiO2 mixtures, the reduction intensity of NiO at lower temperatures decreases significantly and a very broad peak in the range of 400–650 °C was observed. The latter reduction peak with a broad feature clearly points to the stronger interaction between NiO and TiO2–SiO2 surface, which was not observed on NiO supported on TiO2 or SiO2. We have calculated the H2/Ni ratio by calibrating the H2 assumption at a reduction temperature up to 650 °C, and the results are shown in Table 2. It can be seen that the H2/Ni ratio for NiO/SiO2 is close to 1.0, while those catalysts containing TiO2 give a H2/Ni ratio higher than 1.0. This result suggests that the reduction of NiO is almost accomplished at 650 °C and partial reduction of TiO2 may occur under the reduction conditions. We thus chose 650 °C as the reduction temperature for the preparation of all catalysts.

Figure 2.

Figure 2

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H2-TPR profiles of 20NiO/TiO2, 20NiO/SiO2, 20NiO/10TiO2–SiO2, 20NiO/10TiO2–SiO2(25), 20NiO/10TiO2–SiO2(5), 20NiO/10TiO2–SiO2(2), and 30NiO/10TiO2–SiO2(5) catalysts.

Table 2. Textural Properties and Hydrogen Consumption of Supported Ni Catalysts.

catalyst Nia (wt %) SSAb (m2/g) Vtotal (cm3/g) H2 (mmol/g)c H2/Ni ratio
20Ni/SiO2 19.2 158 0.63 1.8 1.0
20Ni/TiO2 21.3 8.4 0.085 1.9 1.1
20Ni/5TiO2–SiO2(5) 20.2 178 0.61 1.8 1.1
20Ni/10TiO2–SiO2(25) 19.1 144 0.55 1.9 1.1
20Ni/10TiO2–SiO2(5) 20.6 140 0.48 2.2 1.3
20Ni/10TiO2–SiO2(2) 19.3 159 0.53 1.9 1.1
20Ni/20TiO2–SiO2(5) 20.6 163 0.56 2.2 1.3
20Ni/10TiO2–SiO2 20.6 141 0.53 2.1 1.2
10Ni/10TiO2–SiO2(5) 10.6 163 1.0 1.0 1.2
30Ni/10TiO2–SiO2(5) 31.5 157 0.55 2.9 1.1
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a

Determined by ICP-AES.

b

Calculated by liquid nitrogen adsorption.

c

Calculated by H2-TPR.

The XRD patterns and N2 adsorption–desorption curves of supported Ni catalysts are displayed in Figure 3. The textural properties of reduced Ni catalysts are listed in Table 2. The SSA of 20Ni/SiO2 is 158 m2/g. The decreased surface area can be explained by the pore-blockage effect due to the incorporation of Ni nanoparticles. As a result, the total pore volume also decreased. This phenomenon is also observed for Ni/TiO2–SiO2 catalysts. The diffraction peaks at 44.5, 51.7, and 76.4° are assigned to the metallic Ni nanoparticles. The Ni particle size was estimated to be about 5 nm. The small particle size confirms previous finding that EG is a promising protecting reagent to control the particle size in catalyst synthesis by strengthening the interaction between the nickel cations and supports.39 To provide the particle size distribution in more detail, we have performed transmission electron microscopy (TEM) analysis on several Ni catalysts, and the results are shown in Figure 4. The average Ni particle sizes on 20Ni/SiO2, 20Ni/10TiO2–SiO2, 20Ni/10TiO2–SiO2(5), and 30Ni/10TiO2–SiO2(5) are 6.5, 5.3, 5.0 and 5.3 nm, respectively. One can see that the average Ni particle size on four catalysts does not differ much from each other. Moreover, the average particle size retained to be 5.3 nm even after increasing the Ni loading to 30 wt % (Figure 4d) in the presence of TiOx clusters.

Figure 3.

Figure 3

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XRD patterns (left) and N2 adsorption–desorption isotherms (right) at 77 K of reduced catalyst: (a) 20Ni/SiO2, (b) 20Ni/10TiO2–SiO2, (c) 20Ni/10TiO2–SiO2(25), (d) 20Ni/10TiO2–SiO2(5), and (e) 20Ni/10TiO2–SiO2(2).

Figure 4.

Figure 4

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TEM images and particle size distribution of typical Ni catalysts: (a) 20Ni/SiO2, (b) 20Ni/10TiO2–SiO2, (c) 20Ni/10TiO2–SiO2(5), and (d) 30Ni/10TiO2–SiO2(5).

Catalytic Performance of Total Hydrogenation of FAL over Ni Catalysts

Supported Ni nanoparticles are readily oxidized by air-exposure during sample transfer, thus a reduction pretreatment is always required for the evaluation of its catalytic hydrogenation performance. In this study, the pre-reduction temperature was chosen to be 250 °C according to the TPR tests of several Ni samples, as shown in Figure S1. As a starting test, we have performed several screening tests by varying the reaction temperature under 1 MPa (not shown) over Ni/SiO2 catalyst. The main findings are that reaction temperature lower than 90 °C led to very low reactivity because of strong adsorption of FAL, while reaction temperature higher than 100 °C also results in fast deactivation and low carbon balance in liquid phase due to both C–C cracking and coking. Therefore, in this study, the reaction conditions were chosen to be 90 °C and 1 MPa. Under this reaction condition, Ni/SiO2 catalyst gave 94% FAL conversion in the first hour and then dropped to 30% with TOS for 8 h with a flow rate of 0.2 mL/min (Figure S2A). The selectivity toward THFA also decreased from 90 to 28%. This deactivation is consistent with the result observed in gas-phase hydrogenation of FAL over Ni/SiO2 catalysts.24,31 The coke deposition on some highly active Ni species is proposed to be responsible for the deactivation.31Figure 5 shows the catalytic performances of 20Ni/10TiO2–SiO2 catalysts with different Ti/acac ratios in terms of FAL conversion and THFA selectivity. The incorporation of TiOx significantly improves the stability of Ni catalysts toward total hydrogenation. The highest activity was observed for Ni supported on TiO2–SiO2(5) with a Ti/acac ratio of 5. The initial FAL conversion and THFA selectivity were 90 and 95%, respectively. Moreover, the FAL conversion did not change within 8 h, while THFA selectivity decreased slightly from 95 to 70%. Compared with 20Ni/10TiO2–SiO2(5) catalyst, other catalysts underwent faster deactivation. The optimum activity of 20Ni/10TiO2–SiO2(5) indicates that the Ti/acac ratio is a key parameter in enhancing the catalytic performance. It should be noted that Ni/TiO2 gave much lower FAL conversion as well as lower selectivity toward THFA (Figure S2B) than 20Ni/10TiO2–SiO2(5). The conversion of FAL dropped from 33 to 10% with a TOS of 8 h, while the selectivity toward THFA dropped from 31 to 2%, with FA being the predominant product. We then investigated the effect of the TiO2 content on the catalytic performance of hydrogenation while keeping the ratio of Ti/acac unchanged. As shown in Figure S3, the yield of THFA follows the trend of 20Ni/10TiO2–SiO2(5) > 20Ni/5TiO2–SiO2(5) > 20Ni/20TiO2–SiO2(5). This result suggests that the optimized TiO2 loading was about 10 wt %, further emphasizing the importance of titania loading in promoting the hydrogenation efficiency of Ni nanoparticles on a SiO2 support. We next investigated the effect of Ni loading (10, 20 and 30 wt %) on the catalytic activity of Ni/10TiO2–SiO2(5) catalysts (Figure 6). Apparently, catalysts with higher Ni loading showed greater FAL conversion. Moreover, the stability and selectivity toward total hydrogenation were also significantly improved for catalysts with 30 wt % Ni loading. 30Ni/10TiO2–SiO2(5) gave complete FAL conversion and THFA selectivity higher than 95% within 8 h. In our previous study,20 we have reported the catalytic performance of bilayer noble metal-based catalysts in a fixed-bed reactor comprising Pd/Al2O3 (1 g, top) and Ru/ZrO2 (0.5 g, bottom) at 50 °C and 1 MPa. When the liquid flow rate was about 0.03 mL/min, total conversion of FAL and 96% selectivity of THFA were achieved within 8 h, while further increase of liquid flow resulted in a significant decrease of THFA selectivity. These results suggest that Ni as a cheaper metal showed comparable activity with noble metals for total hydrogenation of FAL.

Figure 5.

Figure 5

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Total hydrogenation of FAL over 20Ni/10TiO2–SiO2 catalysts with various Ti/acac ratios. Reaction conditions: 1.0 g catalyst, 0.2 mL/min of 1 vol % FAL aqueous, 1 MPa H2, 90 °C.

Figure 6.

Figure 6

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Total hydrogenation of FAL over Ni/10TiO2–SiO2(5) catalysts with various Ni loadings. Reaction conditions: 1.0 g catalyst, 0.2 mL/min of 1 vol % FAL aqueous, 1 MPa H2, 90 °C.

Effect of Hydrogen Pressure on the Activity of 30Ni/10TiO2–SiO2(5) and Its Stability

Given the good performance of 30Ni/TiO2–SiO2(5) catalyst, we then tested the effect of H2 pressure on the hydrogenation activity. To avoid full conversion of FAL, higher liquid hour space velocity (LHSV, 0.3 g catalyst and 12 mL/min liquid flow) was employed. The H2 pressure investigated ranged from 0.1 to 4 MPa with a H2 flow of 60 mL/min. Figure 7 shows the FAL conversion and THFA selectivity at TOS of 2 h under different H2 pressure. We can clearly see that the FAL conversion increased from 31.1 to 75.1% when the H2 pressure increased from 0.1 to 1 MPa. Meanwhile, the selectivity toward THFA increased remarkably from 20.4 to 62.8%. Further increase of H2 pressure could improve the FAL conversion while did not change the product distribution significantly. These results suggest that activation of H2 plays a key role in FAL hydrogenation. We surmise that the hydrogen adsorption and dissociation is likely competing with the adsorption of FA formed and/or the coke deposited, which results in fast deactivation of supported Ni catalysts with TOS. We then chose 1 MPa to further determine the stability of 30Ni/10TiO2–SiO2(5) at unsaturated FAL conversion. Figure 8 shows the catalytic performance of 0.3 g 30Ni/10TiO2–SiO2(5) with a liquid flow rate of 0.2 mL/min. The FAL conversion underwent a slight decrease in the first 3 h and then increased in the next 5 h. In contrast, the THFA selectivity gradually decreased from 68.7 to 41.7% after reaction for 8 h. A similar trend was noticed when 1 g catalyst and 0.4 mL/min FAL solution were employed under otherwise identical conditions (Figure S4). We also tested the stability of 30Ni/10TiO2–SiO2(5) for FAL total hydrogenation at a flow rate of 0.2 mL/min by using 1 g catalyst for longer TOS. The result (Figure 9) shows that the FAL conversion always attained 99% while the THFA selectivity decreased very slowly from 98 to 83% after reaction for 60 h. The catalyst was then treated in flowing H2 at 300 °C for 5 h, and the activity was tested again. One can see that its hydrogenation activity was partially recovered. After running for another 60 h, the FAL conversion and the THFA selectivity dropped to 91 and 69%, respectively. To clarify the deactivation reasons, we have analyzed the spent catalyst after 120 h reaction with XRD, TEM, and thermogravimetric analysis (TGA) techniques. XRD patterns (Figure S5) of the fresh and spent catalysts clearly showed the aggregation of Ni particles after long-term reaction. The average size increased from 6 to 20 nm estimated by Scherrer equation. From the TEM image (Figure S6A) of the spent catalyst, one can also clearly see that the average particle size of Ni changed from 5.3 to 9.2 nm with some large particles even in 12 nm. It should be also noted that the morphology of the SiO2 support also changed significantly after the long-term run. The morphology change might be explained by the slow dissolution of SiO2 species in flowing hot water.40 This assumption has been confirmed by the Si inductively coupled plasma (ICP) analysis of the effluent, in which 65 ppm of Si was detected. In line to this finding, the Ni loading increased to 42% according to the ICP analysis for the spent catalyst because of the loss of the SiO2 support. It should be noted that no leaching of TiO2 was found according to the ICP analysis. The dissolution of SiO2 in water may also contribute to the aggregation of Ni species. Another adverse effect of SiO2 dissolution might be related to the coking effect since the Si–OH groups may contribute to the polymerization of FAL or FA under reaction conditions. Indeed, the TG analysis (Figure S6B) on the spent catalyst showed a remarkable weight loss (3.5 wt %) at a temperature from 200 to 500 °C, which was much less than that in an early report.31

Figure 7.

Figure 7

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Effect of hydrogen pressure (1–4 MPa) on the catalytic performance of 30Ni/TiO2–SiO2(5) catalyst for total hydrogenation of FAL at TOS of 2 h. Reaction conditions: 0.3 g catalyst, 0.2 mL/min of 1 vol % FAL aqueous solution, and 60 mL/min H2, 90 °C.

Figure 8.

Figure 8

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Catalytic performance of 30Ni/10TiO2–SiO2(5) for total hydrogenation of FAL with TOS. Reaction conditions: 0.3 g catalyst, 0.2 mL/min of 1 vol % FAL aqueous solution, 90 °C, and 1 MPa H2, 60 mL/min H2.

Figure 9.

Figure 9

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Life-time study of 30Ni/10TiO2–SiO2(5) catalyst for the total hydrogenation of FAL in water. Reaction conditions: 1.0 g catalyst, 0.2 mL/min 1 vol % FAL aqueous, 1 MPa H2, 90 °C.

Role of TiO2–SiO2 Composite in Enhancing the Stability of Ni Catalysts

TiO2–SiO2 composite is a well-known catalyst that has found wide applications in the epoxidation of alkenes.41,42 Several studies have shown that isolated TiO4 species prepared by controlled hydrolysis of TBT in the presence of acac demonstrated good oxidation activity. A suitable Ti/acac ratio ensures an optimum condensation rate of Ti–OH and Si–OH, thus allowing for the formation of isolated TiOx while preventing the self-condensation of Ti–OH species. This assumption has been confirmed by UV–vis spectra, as shown in Figure S7. The adsorption edge of supported TiOx down-shifted to a wavelength of about 320 nm compared with that of the bulk anatase TiO2 (400 nm), pointing to the amorphous nature of TiO2 species. These TiO2 species may provide an O-vacancy site on which the adsorption of C=O bond takes place.43 The promotion role of TiOx in hydrogenation of unsaturated carbonyl compounds has also been recognized by the so-called SMSI effect.44 The hydrogenation performance of Ni catalysts is directly related to the ability of H2 activation over the Ni surface.45−48 We have followed the desorption behavior of surface adsorbed hydrogen on 20Ni/SiO2, 20Ni/10TiO2–SiO2, 20Ni/10TiO2–SiO2(5), and 30Ni/10TiO2–SiO2(5) below 400 °C via H2-temperature-programmed desorption (TPD), and the results are shown in Figure 10. The H2 desorption over all Ni catalysts with Ni loading of 20 wt % gave a broad peak centered at 111 °C, while the amount of hydrogen desorbed varied to each other depending on the supports used. It should be noted that a bare SiO2 support did not give any hydrogen desorption at a temperature below 400 °C. We have calibrated the amount of H2 desorbed below 300 °C over all Ni catalysts and calculated the H2/Ni molar ratio. For Ni supported on SiO2, the H2/Ni is 0.024. Much higher H2/Ni ratio was noticed for Ni/TiO2–SiO2 catalysts though with a similar Ni particle size distribution. This phenomenon points to the promotion effect of TiOx clusters on Ni nanoparticles toward H2 adsorption, which can be explained by the strong interaction between Ni and TiO2 via the creation of a Ni–Ov–Ti interface site.49 The H2 desorption temperature shifted to 159 °C for 30Ni/10TiO2–SiO2(5), which implies that a stronger Ni–H bond formed for higher Ni loading catalyst. As expected, the stronger Ni–H interaction may weaken the adsorption strength of organic compounds on the Ni surface and thus render the possible coke deposition.

Figure 10.

Figure 10

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H2-TPD on SiO2, 20Ni/TiO2, 20Ni/SiO2, 20Ni/10TiO2–SiO2, 20Ni/10TiO2–SiO2(5), and 30Ni/10TiO2–SiO2(5) catalysts.

To better understand the role of surface active sites in FAL hydrogenation, we also carried out the in situ Fourier transform infrared (FT-IR) study of FAL adsorption on 20Ni/SiO2, 20Ni/10TiO2–SiO2, and 20Ni/10TiO2–SiO2(5), and the results are shown in Figure 11. The C=O stretching vibration of FAL adsorbed on 20Ni/SiO2 and 20Ni/10TiO2–SiO2 catalysts was observed at 1657 cm–1. The C=O stretching vibration mode of gas-phase FAL is centered at 1720 cm–1. The downshift of C=O stretching IR for surface adsorbed C=O has been noticed previously on Cu/SiO2 catalyst, indicating that the C=O bond is weakened via the formation of η1(O) species.50 Even lower wavenumber of 1650 cm–1 was observed for 20Ni/10TiO2–SiO2(5) catalyst, meaning that Ni coordinated to highly dispersed TiOx showed stronger interaction with the C=O bond. The enhanced interaction between the carbonyl group and reduced TiOx has also been demonstrated by FT-IR spectroscopy for crotonaldehyde, wherein a downshift of C=O IR band from about 1693–1660 cm–1 was noticed.51 Another feature worthy of noting is that a shoulder peak centered at 1636 cm–1 was observed for FAL adsorbed 20Ni/10TiO2–SiO2(5) catalyst. This band can be assigned to the C=C vibration of the furan ring of adsorbed FAL, indicating a coordination of the furan ring with the Ni metal. This interaction may favor the hydrogenation of C=C bond in the furan ring. Taking into account the FTIR results, the Ni–Ov–Ti species showed strong coordination with both C=C and C=O bonds in FAL, thus leading to the highest selectivity in total hydrogenation.

Figure 11.

Figure 11

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FTIR spectra after FAL adsorption and following degassing at 120 °C over (a) 20Ni/SiO2, (b) 20Ni/10TiO2–SiO2, and (c) 20Ni/10TiO2–SiO2(5).

Conclusions

In summary, a stable catalyst based on Ni, a transition metal, has been prepared for total hydrogenation of FAL in continuous flow. Ni/SiO2 catalyst with Ni nanoparticles with an average size of 5 nm in 20 wt % showed high activity and selectivity toward THFA in the beginning of the reaction but deactivates quickly with TOS. By loading highly dispersed TiOx species on the SiO2 surface, the stability of Ni nanoparticles toward THFA production was significantly enhanced in continuous flow. The presence of highly dispersed TiOx grafted on the silica aerogel by controlled hydrolysis of TBT plays an important role in catalysis, which can be explained in several aspects: promoting H2 activation and strengthening the interaction of unsaturated functional groups in FAL with Ni metals. The (30 wt %)Ni/(10 wt %)TiO2–SiO2(5) catalyst showed >90% FAL conversion and 70% THFA selectivity after reaction for 120 h on stream at 90 °C and 1 MPa H2 with LHSV of 12 mL·h–1·gcat.–1. The loss of catalytic activity is likely due to the low stability of the silica matrix in hot aqueous solution, which results in the reconstruction of the catalyst surface and aggregation of Ni nanoparticles.

Experimental Section

Preparation of TiO2–SiO2

TiO2–SiO2 composites were prepared by controlled hydrolysis of TBT by using acac as the ligand.34,35 In a typical synthesis, 1 g of fumed silica was dispersed in 60 mL of isopropanol and stirred for 1 h. A certain amount of TBT/acac solution with a different Ti/acac molar ratio (2, 5, and 25) was then added into the isopropanol. The resultant mixture was stirred for about 6 h allowing for the complete hydrolysis of TBT. Then, the mixture was filtered, washed with deionized water, dried at 60 °C for 12 h, and calcined at 500 °C for 3 h with a ramp rate of 3 °C/min in air. The sample was denoted as yTiO2–SiO2(x), where x and y stand for the Ti/acac ratio and titania loading in wt %, respectively. For comparison, a TiO2–SiO2 material with TiO2 loading of 10 wt % in the absence of acac was also prepared by the direct hydrolysis of TBT and denoted as 10TiO2–SiO2. Commercial TiO2 in anatase phase (Aldrich) was also chosen as a catalyst support.

Preparation of Ni Catalysts

Supported Ni catalysts were prepared according to previous report.36 In a typical synthesis, Ni(NO3)2·6H2O was incipiently impregnated on the support in the presence of desired amount of EG. The mixture was dried and subsequently calcined at 500 °C in air for 2 h with a heating rate of 2 °C/min. After calcination, the catalysts were reduced at 650 °C in hydrogen for 2 h with a heating rate of 5 °C/min. The Ni catalyst was denoted as zNi/yTiO2–SiO2(x), where z denotes the loading of Ni in wt %.

Characterization

Powder XRD patterns were collected on a Rigaku Ultima IV X-ray diffractometer using Cu Kα radiation (λ = 1.5405 Å) operated at 35 kV and 25 mA. TEM images were taken on a FEI Tecnai G2 F30 microscope operated at 300 kV. The ultraviolet–visible diffuse reflectance spectra (UV–vis) were obtained using a Shimadzu-UV2700 spectrophotometer in the range of 190–800 nm. Nitrogen adsorption–desorption isotherms were obtained on a BELSORP-Max equipment at −196 °C. Prior to the measurement, the samples were first degassed at 150 °C under vacuum for 6 h. SSAs were calculated by the Brunauer–Emmett–Teller (BET) method using five relative pressure points in the interval of 0.05–0.30. The pore size distribution was obtained by applying the BJH model to the adsorption branch of the isotherm. The Ni loading was determined by a Thermo Elemental IRIS Intrepid II XSP ICP emission spectrometer (AES). Prior to analysis, a certain amount of catalyst was dissolved in 10 mL of aqua regia by heating in an autoclave at 180 °C for 6 h.

H2-TPR experiments were performed on a Micromeritics AutoChem 2920 TPR/TPD analyzer equipped with a thermal conductivity detector. About 10 mg of sample was placed in a quartz tubular reactor. The reactor was heated to 150 °C with a ramp rate of 10 °C/min and held at this temperature for 30 min in N2. After cooling to room temperature, a gaseous mixture of 5% H2 in Ar was fed at a flow rate of 30 mL/min, and the sample temperature was increased to 800 °C with a ramp rate of 10 °C/min. H2-TPD was performed to determine the hydrogen adsorption and desorption behavior on different nickel catalysts. In each experiment, the sample (200 mg) was reduced by 5% H2/Ar mixture at 300 °C for 1 h and cooled to room temperature. After that, He was passed through the sample for about 20 min in order to sweep the physical adsorbed hydrogen. The sample was then heated to 500 °C at a ramp rate of 10 °C/min in He with a flow rate of 50 mL/min.

The carbonaceous deposition on the spent catalyst after the lifetime test was studied by thermogravimetric (TG) analysis using a NET2SCH STA449F3 TGA analyzer with a ramp rate of 10 °C/min from 25 to 800 °C in a N2 flow. To exclude the effect of Ni oxidation on the weight change of the spent catalyst, N2 was employed in TG analysis instead of air. FT-IR spectroscopy of FAL adsorption on supported Ni catalysts was performed on a Nicolet iS50 spectrometer equipped with a vacuum cell. Catalysts were pressed into self-supporting wafers (about 10 mg in diameter of 11 mm) and reduced in flowing hydrogen at 450 °C. After reduction, the samples were cooled to room temperature and evacuated to 1 × 10–3 Pa allowing for the introduction of FAL vapors at room temperature. A spectroscopy of the reduced sample after evacuation was recorded as the background for subtraction. After FAL adsorption, the IR cell was evacuated at 120 °C for 5 min to remove the physisorbed FAL before the spectra were recorded.

Catalytic Activity Measurements

The catalytic hydrogenation was conducted in a high pressure fixed-bed reactor with a H2 pressure of 1 MPa at 90 °C, which allows a good carbon balance. Higher reaction temperature would lead to a fast coke deposit as discussed in early literature.31 Before each reaction, the catalysts were pretreated in flowing H2 at 300 °C for 1 h and then cooled to 90 °C. When the reactor temperature was stable, the aqueous solution of FAL (1 vol %) was introduced into the reactor with a Series-II pump at a flow rate of 6–12 mL/h. The hydrogen flow was kept at 60 mL/min. The liquid phase product was collected in the gas–liquid separator at room temperature. No significant formation of volatile species occurred under the selected reaction conditions. The hydrogenation products were periodically withdrawn from the separator and analyzed by gas chromatography (GC, Tianmei) equipped with a flame ionization detector and a capillary column of DB-FFAP (30 m length and 0.25 mm internal diameter). Only two products, namely, FA and tetrahydrofurfuryl alcohol (THFA), were detected under the reaction conditions investigated. The FAL conversion and product selectivity were defined as follows by using the internal standard method, and the GC relative response factors of FAL, FA, and THFA were established over a known concentration of mixture.

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Acknowledgments

This work was financially supported by the China Ministry of Science and Technology under contract 2016YFA0202804, National Natural Science Foundation of China (21773067, 21533002, and 21573073) and the Open Research fund of Shanghai Key Laboratory of Green Chemistry and Chemical Processes.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.0c04736.

  • H2-TPR profiles of reduced Ni catalysts after air exposure at 30 °C, total hydrogenation of FAL over 20Ni/SiO2 and 20Ni/TiO2(anatase), effect of TiO2 loading on hydrogenation activity of 20Ni/TiO2–SiO2(5), catalytic performance of 30Ni/10TiO2–SiO2(5), characterization of spent 30Ni/10TiO2–SiO2(5), and UV–vis spectra of TiO2 containing supports (PDF)

Author Contributions

† J.X.and Q.C. contributed equally.

The authors declare no competing financial interest.

Supplementary Material

ao0c04736_si_001.pdf (335.3KB, pdf)

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