Synthesis of Tetrahydropyran from Tetrahydrofurfuryl Alcohol over Cu – Zno / Al 2 O 3 under a Gaseous-Phase Condition

Tetrahydropyran (THP) represents an O-containing hetero-cyclic compound that can be used as a promising solvent or monomer for polymer synthesis. In this work, Cu–ZnO/Al2O3 catalysts have been prepared by a facile precipitation–extrusion method and used for the synthesis of THP through gaseous-phase hydrogenolysis of tetrahydrofurfuryl alcohol (THFA). The effect of the molar ratio of Cu/Zn/Al, reaction temperature, and hydrogen pressure was investigated. An 89.4% selectivity of THP was achieved at 270 ◦C and 1.0 MPa H2. Meanwhile, the optimum molar ratio of Cu/Zn/Al was determined to be 4:1:10. The Cu–ZnO/Al2O3 catalyst exhibited high catalytic activity and stability for 205 h on-stream. A possible reaction mechanism involving several consecutive reactions was proposed: THFA was firstly rearranged to 2-hydroxytetrahydropyran (2-HTHP), followed by the dehydration of 2-HTHP to 3,4-2H-dihydropyran (DHP) over acid sites; finally, the DHP was hydrogenated to THP. The synergy of acid sites and metal sites of Cu–ZnO/Al2O3 played an important role during the production of THP.

Recently, the catalytic conversion of furanics has been extensively studied over noble metal catalysts.However, due to the high cost and depleting resources of noble metals, it is of great importance to explore catalytic systems over non-noble metals.Among non-noble metals, copper-based catalysts were widely used in hydrogenation and hydrogenolysis reactions due to their good catalytic performance and low cost.Müller et al. [34] found that dimethyl succinate could be completely converted to THF over CuO-ZnO/Al 2 O 3 at 220/Allely MPa.Guo et al. [35] also reported that a selectivity of THF as high as 94% could be obtained over Cu-B/Al 2 O 3 during the hydrogenolysis of dimethylsuccinate.The selective conversion of furanic-derived compounds over Cu catalysts is particularly encouraging and needs further investigation.Soghrati et al. [36] reported that a 70% yield of THP was achieved via hydrogenolysis of THFA over Ni/HZSM-5 catalyst, encouraging us to investigate the probability of producing THP from THFA over Cu catalysts.
In this paper, we have prepared a bi-functional Cu-ZnO/Al 2 O 3 catalyst by a facile precipitation extrusion method.The physic-chemical properties were characterized by XRD, BET, TEM, NH 3 -TPD, and H 2 -TPR analysis.The role of Cu metal sites during the direct conversion of THFA to THP over Cu-ZnO/Al 2 O 3 catalysts was studied.The effect of temperature, pressure, and catalysts on THFA to THP along with side reactions was investigated.The current work was beneficial to develop the downstreams of TFHA over non-noble catalysts.

BET Characterization
The porosity and pore-size distribution of the calcined Cu-ZnO/Al 2 O 3 extrudate catalysts were determined by N 2 adsorption-desorption.The γ-Al 2 O 3 (boehmite particles behaving as a binder and supplying acid sites) exhibited a 12 nm mean diameter (Figure S1).Accordingly, the volume-size distribution of Cu-Zn particles was characterized as a bimodal distribution with a mean diameter of 2.7 nm.The other peak resulting from 8.2 nm particles was possibly derived from symbiotic fragments.Therefore, the extrudate's pore size distribution might be related with the solid composition, because the respective catalysts particle sizes differed significantly; therefore, the effect of the amount of γ-Al 2 O 3 on the main peak of the size distribution was not negligible.Lamelleted mesoporous materials exhibited a typical type-III shaped isotherm, obtained by nitrogen adsorption-desorption measurements (Figure S2).The surface areas (S BET ) of the calcined catalysts increased monotonically from 196.4 to 261.7 m 2 •g −1 with the increase in the content of γ-Al 2 O 3 , though they were generally lower than that of γ-Al 2 O 3 (A 0-0-01 ) (Table 1).This is consistent with the study by Kraushaar-Czarnetzki et al. [34].

XRD Characterization
The XRD patterns of calcined catalysts are shown in Figure 1.Diffuse diffraction peaks at 2θ = 35.5 • , 38.9 • , and 64.2 • (JCPDS 05-0661) were attributed to crystalline CuO, and the XRD patterns were associated with the copper crystalline size and the copper species [37].The peak intensities of both CuO and ZnO were weakened with the increase in aluminum content, possibly due to the increased dispersion of Cu and Zn species after the addition of γ-Al 2 O 3 (Figure 2).The copper crystallite size of all catalysts were determined to be 34.4 nm, 26.6 nm, 25.8 nm, 25.0 nm, 24.3 nm, and 22.3 nm by the Scherrer equation, based on the full width at maximum Cu diffraction of 35.5 • .The peak of γ-Al 2 O 3 was not observed due to the high dispersion of γ-Al 2 O 3 species.

XRD Characterization
The XRD patterns of calcined catalysts are shown in Figure 1.Diffuse diffraction peaks at 2θ = 35.5°,38.9°, and 64.2° (JCPDS 05-0661) were attributed to crystalline CuO, and the XRD patterns were associated with the copper crystalline size and the copper species [37].The peak intensities of both CuO and ZnO were weakened with the increase in aluminum content, possibly due to the increased dispersion of Cu and Zn species after the addition of γ-Al2O3 (Figure 2).The copper crystallite size of all catalysts were determined to be 34.4 nm, 26.6 nm, 25.8 nm, 25.0 nm, 24.3 nm, and 22.3 nm by the Scherrer equation, based on the full width at maximum Cu diffraction of 35.5°.The peak of γ-Al2O3 was not observed due to the high dispersion of γ-Al2O3 species.

NH3-TPD Characterization
The acidity of samples was determined by NH3-TPD.The acid distribution was obtained by Gaussian fitting of NH3-TPD curves (Table 2 and Figure S3).Three peaks centered at 104 °C, 225 °C, and 335 °C were fitted for the NH3-TPD curve, attributing to the NH3 desorption from the weak, medium, and strong acid sites, respectively [38].The total acid sites of γ-Al2O3 (A0-0-01) (7.2 μmol•g −1 ) was more than three times that of Cu-ZnO.Moreover, a ratio of strong acid sites of A0-0-01, larger than that of Cu-ZnO, was also obtained.When γ-Al2O3 was mixed with Cu-ZnO, the total acid sites increased along with the formation of more medium acid sites, while the ratio of strong acid sites kept relatively constant.The generation of medium acid sites may be due to the interaction between Cu-ZnO and γ-Al2O3 during the preparation process.The amount of strong acid sites increased with the increase in γ-Al2O3, in good agreement with previous reports [39].

NH 3 -TPD Characterization
The acidity of samples was determined by NH 3 -TPD.The acid distribution was obtained by Gaussian fitting of NH 3 -TPD curves (Table 2 and Figure S3).Three peaks centered at 104 • C, 225 • C, and 335 • C were fitted for the NH 3 -TPD curve, attributing to the NH 3 desorption from the weak, medium, and strong acid sites, respectively [38].The total acid sites of γ-Al 2 O 3 (A0-0-01) (7.2 µmol•g −1 ) was more than three times that of Cu-ZnO.Moreover, a ratio of strong acid sites of A0-0-01, larger than that of Cu-ZnO, was also obtained.When γ-Al 2 O 3 was mixed with Cu-ZnO, the total acid sites increased along with the formation of more medium acid sites, while the ratio of strong acid sites kept relatively constant.The generation of medium acid sites may be due to the interaction between Cu-ZnO and γ-Al 2 O 3 during the preparation process.The amount of strong acid sites increased with the increase in γ-Al 2 O 3 , in good agreement with previous reports [39].Figure 2 shows the H 2 -TPR profile of the calcined catalysts, and an asymmetrical peak can be observed.Generally, the reduction temperature of copper species changes with particle size, chemical environment, and the metal support interaction of Cu oxide species [40,41].The intensity of reduction peak was reduced with the increased addition of γ-Al 2 O 3 due to the decrease in Cu content.However, the peak area of A 4-1-10 is higher than that of A 4-1-07 , although the Cu content in A 4-1-10 is lower, possibly due to the hydrogen spillover.The main reduction peak shifted from 286 • C for A 4-1-03 to 268 • C for A 4-1-07 , indicating an increase in copper oxide dispersion due to the interaction between Cu-ZnO and γ-Al 2 O 3 .However, a further increase in the content of γ-Al 2 O 3 resulted in a reduction peak at 285 • C for A 4-1-10 .Evidently, the change in the H 2 -TPR peak position indicated the strength of interaction (Cu-ZnO and γ-Al 2 O 3 ), suggesting that γ-Al 2 O 3 had a great impact on the copper oxide reduction.Moreover, when γ-Al 2 O 3 was added to different catalysts, the reduction process of catalysts became difficult and slower.This resulted from smaller copper oxide particles or the better copper oxide dispersion on the γ-Al 2 O 3 surface.

H2-TPR Characterization
Figure 2 shows the H2-TPR profile of the calcined catalysts, and an asymmetrical peak can be observed.Generally, the reduction temperature of copper species changes with particle size, chemical environment, and the metal support interaction of Cu oxide species [40,41].The intensity of reduction peak was reduced with the increased addition of γ-Al2O3 due to the decrease in Cu content.However, the peak area of A4-1-10 is higher than that of A4-1-07, although the Cu content in A4-1-10 is lower, possibly due to the hydrogen spillover.The main reduction peak shifted from 286 °C for A4-1-03 to 268 °C for A4-1-07, indicating an increase in copper oxide dispersion due to the interaction between Cu-ZnO and γ-Al2O3.However, a further increase in the content of γ-Al2O3 resulted in a reduction peak at 285 °C for A4-1-10.Evidently, the change in the H2-TPR peak position indicated the strength of interaction (Cu-ZnO and γ-Al2O3), suggesting that γ-Al2O3 had a great impact on the copper oxide reduction.Moreover, when γ-Al2O3 was added to different catalysts, the reduction process of catalysts became difficult and slower.This resulted from smaller copper oxide particles or the better copper oxide dispersion on the γ-Al2O3 surface.

HRTEM Characterization
To confirm the dispersion and morphology of catalysts particles, HRTEM images of reduced samples (A4-1-00, A4-1-10) were obtained (Figure 3).The dispersion of copper zinc nanoparticles was lower than that of the copper zinc aluminum catalyst.In addition, the Cu particles with sizes <30 nm were observed in the image of the copper zinc aluminum catalyst.Therefore, XRD and HRTEM results are in good agreement with each other.

HRTEM Characterization
To confirm the dispersion and morphology of catalysts particles, HRTEM images of reduced samples (A 4-1-00 , A 4-1-10 ) were obtained (Figure 3).The dispersion of copper zinc nanoparticles was lower than that of the copper zinc aluminum catalyst.In addition, the Cu particles with sizes <30 nm were observed in the image of the copper zinc aluminum catalyst.Therefore, XRD and HRTEM results are in good agreement with each other.

Effect of Cu/Zn/Al Ratio
The catalytic performances of the bi-functional Cu-ZnO/Al2O3 catalysts are summarized in Table 3.During the catalytic conversion of THFA, reactions such as rearrangement and hydrogenation could occur, leading to the formation of DHP and THP, respectively.However, DVL and 1-pentanol represent the main by-products during the conversion of THFA, as is shown in Table 3, indicating the existence of strong acid sites.Both A4-1-00 and A0-0-01 showed a low conversion of THFA.The main two products over A4-1-00 were 3,4-2H-dihydropyran (DHP) (Sel.34.5%) and THP (Sel.31.3%) at 255 °C and 0.6 MPa H2, suggesting that Cu-ZnO could provide active sites for the rearrangement of THFA and further hydrogenation.In contrast, the main product was DHP over A0-0-01 with 74.4% selectivity, indicating that the γ-Al2O3 could only provide acid sites for THFA rearrangement.For the Cu-ZnO/Al2O3 catalysts, a significant increase in THFA conversion and THP selectivity was observed compared with Cu-ZnO.Furthermore, the conversion of THFA was increased from 11.5 to 78.6%, while the selectivity of THP was improved from 31.3 to 77.8%, when the γ-Al2O3 content increased from A4-1-00 to However, a further increase in alumina resulted in a decreased THFA conversion (57.4%) and THP selectivity (63.6%).NH3-TPD results suggested that γ-Al2O3 provided more middle and strong acid sites than Cu-ZnO.These acid sites promoted the rearrangement of THFA to DHP, and DHP can be further converted to THP at hydrogenation sites.However, δ-valerolactone (DVL) was observed as the main

Effect of Cu/Zn/Al Ratio
The catalytic performances of the bi-functional Cu-ZnO/Al 2 O 3 catalysts are summarized in Table 3.During the catalytic conversion of THFA, reactions such as rearrangement and hydrogenation could occur, leading to the formation of DHP and THP, respectively.However, DVL and 1-pentanol represent the main by-products during the conversion of THFA, as is shown in Table 3, indicating the existence of strong acid sites.Both A 4-1-00 and A 0-0-01 showed a low conversion of THFA.The main two products over A 4-1-00 were 3,4-2H-dihydropyran (DHP) (Sel.34.5%) and THP (Sel.31.3%) at 255 • C and 0.6 MPa H 2 , suggesting that Cu-ZnO could provide active sites for the rearrangement of THFA and further hydrogenation.In contrast, the main product was DHP over A 0-0-01 with 74.4% selectivity, indicating that the γ-Al 2 O 3 could only provide acid sites for THFA rearrangement.For the Cu-ZnO/Al 2 O 3 catalysts, a significant increase in THFA conversion and THP selectivity was observed compared with Cu-ZnO.Furthermore, the conversion of THFA was increased from 11.5 to 78.6%, while the selectivity of THP was improved from 31.3 to 77.8%, when the γ-Al 2 O 3 content increased from A 4-1-00 to A 4-1-10 .However, a further increase in alumina resulted in a decreased THFA conversion (57.4%) and THP selectivity (63.6%).NH 3 -TPD results suggested that γ-Al 2 O 3 provided more middle and strong acid sites than Cu-ZnO.These acid sites promoted the rearrangement of THFA to DHP, and DHP can be further converted to THP at hydrogenation sites.However, δ-valerolactone (DVL) was observed as the main byproduct over both γ-Al 2 O 3 and Cu-ZnO/Al 2 O 3 catalysts, while Cu-ZnO with a low content of strong acid sites showed a low selectivity of DVL.Therefore, the formation of DVL may be due to the strong acid sites.The selectivity of THP changed with the Al 2 O 3 content in Cu-ZnO/Al 2 O 3 .Thus, the synergy between γ-Al 2 O 3 (acid sites) and Cu-ZnO (metal hydrogenation sites) is the key for a high yield of THP.
In order to investigate the effects of γ-Al 2 O 3 and Cu-ZnO, we compared the catalytic performance of Cu-ZnO/Al 2 O 3 prepared via physical mixing and extrusion.As shown in Table 4, the THP selectivity over Cu-ZnO/Al 2 O 3 prepared via extrusion was lower than that over Cu-ZnO/Al 2 O 3 prepared via physical mixing at 260 • C, 0.6 MPa.However, the THP selectivity over Cu-ZnO/Al 2 O 3 prepared via extrusion was obviously higher than that over Cu-ZnO/Al 2 O 3 prepared via physical mixing at 280 • C, 0.6 MPa.Since the interaction between Cu-ZnO γ-Al 2 O 3 after extrusion is stronger than that after physical mixing when the temperature was higher, a strong and close interaction between hydrogenation sites (Cu-ZnO) and acid sites (γ-Al 2 O 3 ) favored the generation of THP from THFA.Therefore, the active sites of hydrogenation and hydrogenolysis reactions were provided by metallic copper, ZnO was used as the adsorbent for THFA reaction, and the acid sites were provided by γ-Al 2 O 3 .

Effect of Reaction Temperature and H 2 Pressure
The catalytic performance of the A 4-1-10 catalyst for THFA conversion at different reaction temperatures (0.6 MPa H 2 ) is shown in Table 5.At a low temperature (230 • C), the selectivity for THP was 55.2%, and the main by-product was DVL.When the reaction temperature increased from 230 to 270 • C, the selectivity of THP increased rapidly from 55.2 to 91.4%.However, the selectivity of THP decreased to 85.9% after further increasing the temperature to 290 • C, due to the intensive formation of 1-pentanol.Generally, 1-pentanol was generated from excessive hydrogenolysis of THFA via the acid-catalyzed ring-opening of THP coupled with metal-catalyzed hydrogenation [42].Table 6 shows the effect of hydrogen pressure on the activity and selectivity in THFA hydrogenolysis over the A 4-1-10 catalyst.At low pressure (<0.4 MPa), the main by-product is DVL.When the pressure increased from 0.2 to 0.8 MPa, THFA conversion increased, while the selectivity of THP increased from 45.5 to 90.5%.On the contrary, the DVL selectivity decreased from 17.7 to 2.6%.The changes in selectivity for THP and DVL indicated that formations of these compounds were competitive reactions.The high pressure of hydrogen promoted the generation of THP by accelerating the hydrogenation of DHP; in the meantime, the dehydrogenation of 2-HTHP was suppressed.As shown in Figure 4, the Cu-ZnO/Al 2 O 3 catalyst exhibited activity and stability during 205 h on-stream at 270 • C and 1.0 MPa H 2 .The THFA conversion and THP selectivity were 99.7% and 89.4%, respectively, after 205 h.Moreover, polymeric materials were not observed in the catalyst bed or colder parts of the unit after the stability test, suggesting that the deposit of polyesters was suppressed over the Cu-ZnO/Al 2 O 3 catalyst.were competitive reactions.The high pressure of hydrogen promoted the generation of THP by accelerating the hydrogenation of DHP; in the meantime, the dehydrogenation of 2-HTHP was suppressed.As shown in Figure 4, the Cu-ZnO/Al2O3 catalyst exhibited high activity and stability during 205 h on-stream at 270 °C and 1.0 MPa H2.The THFA conversion and THP selectivity were 99.7% and 89.4%, respectively, after 205 h.Moreover, polymeric materials were not observed in the catalyst bed or colder parts of the unit after the stability test, suggesting that the deposit of polyesters was suppressed over the Cu-ZnO/Al2O3 catalyst.

Reaction Paths
Scheme 1 summarizes the proposed reaction paths during the conversion of THFA to THP over Cu-ZnO/Al2O3, which was in good agreement with prior reports.Sato and coworkers [28] reported that the THFA could be rearranged into 2-hydroxytetrahydropyran (2-HTHP), which was then dehydrated to DHP over acid sites.In the current reaction, DHP was further converted to THP via a C=C bond hydrogenation.The effect of γ-Al2O3 suggested that a close interaction of metal sites and medium acid sites promoted the conversion of THFA to THP.Major by-products, such as DVL and 1-pentanol, were formed through the dehydrogenation of 2-HTHP and over-hydrogenation of THP, respectively.1-Pentanol was generated at high temperature, and DVL was generated at low temperature and pressure.Therefore, a proper control on both temperature and pressure is needed for the suppressing of byproducts.

Reaction Paths
Scheme 1 summarizes the proposed reaction paths during the conversion of THFA to THP over Cu-ZnO/Al 2 O 3 , which was in good agreement with prior reports.Sato and coworkers [28] reported that the THFA could be rearranged into 2-hydroxytetrahydropyran (2-HTHP), which was then dehydrated to DHP over acid sites.In the current reaction, DHP was further converted to THP via a C=C bond hydrogenation.The effect of γ-Al 2 O 3 suggested that a close interaction of metal sites and medium acid sites promoted the conversion of THFA to THP.Major by-products, such as DVL and 1-pentanol, were formed through the dehydrogenation of 2-HTHP and over-hydrogenation of THP, respectively.1-Pentanol was generated at high temperature, and DVL was generated at low temperature and pressure.Therefore, a proper control on both temperature and pressure is needed for the suppressing of byproducts.Scheme 1. Reaction pathway in the hydrogenolysis/hydrogenation of tetrahydrofurfuryl alcohol over the Cu-ZnO/Al2O3 catalyst in the gas phase.

Catalyst Preparation
The Cu-ZnO/Al2O3 catalysts were prepared by means of the precipitation-extrusion method [34].γ-Al2O3 produced from boehmite (>400 mesh and 26% m/m water, Shandong Aluminum Corp., Zibo, China), Cu(NO3)2•3H2O (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China), and Zn(NO3)2•6H2O (purchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) were dissolved in water with a final total metal concentration of 1.4 M. Aqueous Na2CO3 (0.3 M) was used as a precipitating agent.During the synthesis procedure, the Na2CO3 solution was added dropwise to a mixed solution of Cu 2+ and Zn 2+ under vigorous stirring at 70 °C, and the pH was adjusted to 7.0.The resulting precipitate was then aged for 3 h at 80 °C and cooled down to room temperature.The copper/zinc carbonate precursor was obtained after filtration, washing with water for several times, and drying at 120 °C for 4 h.
Before extrusion, the boehmite was mixed with the previously prepared dried copper/zinc carbonate precursor in an Rheo-kneader at room temperature and the rotation speed was kept at 100 r•min-1.The Cu/Zn/Al molar ratio (for instance, 0:0:10, 4:1:00, 4:1:03, 4:1:05, 4:1:07, 4:1:10, and 4:1:15) was modulated by changing the proportion of metal carbonates and boehmite.Herein, the Cu-ZnO/Al2O3 catalyst with a molar ratio of a:b:c is abbreviated as Aabc.For example, the catalyst name of "A4-1-10" means that the molar ratio of CuO/ZnO/Al2O3 was 4:1:10.The paste was then submitted to a piston extruder, and cylindrical green strips with diameters of 3 mm were formed.The green strips were dried at 120 °C for 4 h before calcination at 450 °C for 5 h with a heating rate of 5 °C•min −1 .

Catalytic Reaction
The catalytic performance of the catalysts was investigated in a fixed-bed reactor (i.d.20 mm, length 300 mm).The catalysts were used in cylinder state with a particle size of 3 mm in diameter.Before reaction, the calcined catalysts were reduced at 275 °C in a 20 vol % H2/N2 atmosphere at a flow rate of 100 mL•min −1 for 3 h.After that, pure THFA with hydrogen was pumped into the reactor at a rate of 0.12 mL•h −1 .Afterwards, the reactor was pressurized to 0.2~2.0MPa with H2.The reaction temperature was in the range of 230-300 °C.The liquid products were analyzed by a gas chromatography (Agilent 6890N GC, Santa Clara, CA, USA) equipped with a flame ionization detector (FID) and an AB-FFAP capillary column (30 m × 0.32 mm × 0.25 μm).The THFA conversion and product selectivity were calculated based on the following equations:

Catalyst Preparation
The Cu-ZnO/Al 2 O 3 catalysts were prepared by means of the precipitation-extrusion method [34].γ-Al 2 O 3 produced from boehmite (>400 mesh and 26% m/m water, Shandong Aluminum Corp., Zibo, China), Cu(NO 3 ) 2 •3H 2 O (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China), and Zn(NO 3 ) 2 •6H 2 O (purchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) were dissolved in water with a final total metal concentration of 1.4 M. Aqueous Na 2 CO 3 (0.3 M) was used as a precipitating agent.During the synthesis procedure, the Na 2 CO 3 solution was added dropwise to a mixed solution of Cu 2+ and Zn 2+ under vigorous stirring at 70 • C, and the pH was adjusted to 7.0.The resulting precipitate was then aged for 3 h at 80 • C and cooled down to room temperature.The copper/zinc carbonate precursor was obtained after filtration, washing with water for several times, and drying at 120 • C for 4 h.
Before extrusion, the boehmite was mixed with the previously prepared dried copper/zinc carbonate precursor in an Rheo-kneader at room temperature and the rotation speed was kept at 100 r•min −1 .The Cu/Zn/Al molar ratio (for instance, 0:0:10, 4:1:00, 4:1:03, 4:1:05, 4:1:07, 4:1:10, and 4:1:15) was modulated by changing the proportion of metal carbonates and boehmite.Herein, the Cu-ZnO/Al 2 O 3 catalyst with a molar ratio of a:b:c is abbreviated as Aabc.For example, the catalyst name of "A 4-1-10 " means that the molar ratio of CuO/ZnO/Al 2 O 3 was 4:1:10.The paste was then submitted to a piston extruder, and cylindrical green strips with diameters of 3 mm were formed.The green strips were dried at 120 • C for 4 h before calcination at 450 • C for 5 h with a heating rate of 5 • C•min −1 .

Catalytic Reaction
The catalytic performance of the catalysts was investigated in a fixed-bed reactor (i.d.20 mm, length 300 mm).The catalysts were used in cylinder state with a particle size of 3 mm in diameter.Before reaction, the calcined catalysts were reduced at 275 • C in a 20 vol % H 2 /N 2 atmosphere at a flow rate of 100 mL•min −1 for 3 h.After that, pure THFA with hydrogen was pumped into the reactor at a rate of 0.12 mL•h −1 .Afterwards, the reactor was pressurized to 0.2~2.0MPa with H 2 .The reaction temperature was in the range of 230-300 • C. The liquid products were analyzed by a gas chromatography (Agilent 6890N GC, Santa Clara, CA, USA) equipped with a flame ionization detector (FID) and an AB-FFAP capillary column (30 m × 0.32 mm × 0.25 µm).The THFA conversion and product selectivity were calculated based on the following equations: conversion (%) = (moles of THFA charged-moles of THFA left)/moles of THFA charged × 100% selectivity (%) = moles of a product generated/(moles of THFA charged-moles of THFA left) × 100%.

Characterization of Catalyst
XRD data was recorded on an XRD-7000 (Kyoto, Japan) using Cu Kα radiation (λ = 0.154 nm) produced by an X-ray source and operated at 40 kV and 30 mA.A scanning angle (2θ) ranged from 10 to 80 • .
The BET surface areas of the catalysts were determined through N 2 adsorption-desorption at −190 • C using a Quantachrome SI Instrument (Boynton Beach, FL, USA).Prior to measurements, all catalysts were degassed under vacuum at 100 • C for 10 min and at 300 • C for 3 h.Then, the specific surface area and BJH pore size distribution were calculated on the basis of the desorption branch of the isotherms.
The reducibility of the calcined catalysts was determined by H 2 -TPR, which was conducted on an Auto Chem.II2920 (Mircromeritics, Atlanta, FA, USA).Typically, 100 mg catalysts were outgassed in N 2 at 300 • C for 1 h to remove impurities and then cooled down to room temperature.A mixture of 5 vol % H 2 and 95 vol % He was passed through the catalyst bed (20 mL•min −1 ), while the temperature was increased from 40 to 800 • C at 15 • C•min −1 .
The NH 3 -TPD analysis was also conducted on an Auto Chem.II 2920 (Mircromeritics, Atlanta, FA, USA).Approximately 100 mg catalysts were added to a quartz tube, pre-treated with He gas at 300 • C for 1 h, and then cooled down to room temperature.The catalysts were saturated with pure NH 3 for 1 h.The samples were then cleaned with He gas at 50 • C to remove the physically adsorbed NH 3 .During the measurement, the temperature was raised to 800 • C at 10 • C•min −1 with the flow of He (50 mL•min −1 ) to desorb NH 3 .TEM were investigated using a JEM-2100F electron microscope (Akishima, Tokyo, Japan) at 200 kV.

Conclusions
The highly selective vapor-phase hydrogenolysis of THFA to THP was realized over a Cu-ZnO/Al 2 O 3 catalyst prepared via precipitation-extrusion.The yield of THP is closely related to the ratio of Cu/Zn/Al, reaction temperature, and H 2 pressure.At 270 • C and 1.0 MPa H 2 , the THP selectivity from THFA was up to 89.4% over the optimum Cu-ZnO/Al 2 O 3 catalyst (the molar ratio of Cu/Zn/Al being 4:1:10).During the conversion of THFA to THP, it was found that three consecutive reactions were involved: (1) the rearrangement of THFA into 2-HTHP, (2) the dehydration of 2-HTHP to DHP, and (3) the hydrogenation of DHP to THP.The synergy of metal sites and medium acid sites is the key for a high catalytic activity for the production of THP from THFA.

Figure 1 .
Figure 1.XRD patterns of calcined catalysts samples with different amount of γ-Al2O3.

Figure 1 .
Figure 1.XRD patterns of calcined catalysts samples with different amount of γ-Al 2 O 3 .

Figure 3 .
Figure 3. TEM image of the catalyst after reduction by H2.

Figure 3 .
Figure 3. TEM image of the catalyst after reduction by H 2 .

Scheme 1 .
Scheme 1. Reaction pathway in the hydrogenolysis/hydrogenation of tetrahydrofurfuryl alcohol over the Cu-ZnO/Al 2 O 3 catalyst in the gas phase.

Table 1 .
The physicochemical properties of the catalysts. 2

Table 2 .
Acidity distribution and chemical composition of the catalysts.

Table 2 .
Acidity distribution and chemical composition of the catalysts.

Table 4 .
Conversion of THFA over mechanical mixing and extrusion methods.

Table 5 .
Effect of reaction temperature on the conversion of THFA over A 4-1-10 .

Table 6 .
Effect of pressure on the conversion of THFA over A 4-1-10 .

Table 6 .
Effect of pressure on the conversion of THFA over A4-1-10.