Effect of Oxide Supports on the Activity of Pd Based Catalysts for Furfural Hydrogenation

We investigated the effect of oxide supports on the hydrogenation of furfural over Pd catalysts on various supports (Al2O3, SiO2, TiO2, CeO2, and ZrO2). Pd catalysts (5 wt%) prepared by chemical reduction on various supports. The dispersion and uniformity of Pd were affected by the properties of the support and by the nucleation and growth of Pd. The conversion of furfural was enhanced by greater Pd dispersion. The selectivity for cyclopentanone and tetrahydrofurfuryl alcohol was affected by physicochemical properties of Pd catalyst and reaction parameters. High Pd dispersion and high acidity of the catalyst led to greater C=C hydrogenation, thereby, generating more tetrahydrofurfuryl alcohol. The Pd/TiO2 catalyst showed the highest cyclopentanone yield than other catalysts. The Pd/TiO2 catalyst exhibited the >99% furfural conversion, 55.6% cyclopentanone selectivity, and 55.5% cyclopentanone yield under the optimal conditions; 20 bar of H2, at 170 °C for 4 h with 0.1 g of catalyst.


Introduction
Lignocellulosic biomass is considered a potential source of renewable energy for mitigating the energy crisis created by the depletion of crude oil. In recent decades, furfural formed from biomass-derived resources has been used as a raw material to produce highly valuable materials, such as tetrahydrofurfuryl alcohol (THFAL), furfuryl alcohol (FAL), and cyclopentanone (CPO) [1][2][3][4].
CPO is regarded as an important fine chemical intermediate material for the synthesis of medicine, perfume, and rubber chemicals. CPO is produced by the rearrangement of FAL. Many researchers have investigated the hydrogenation of furfural to CPO, which is generally performed over Co/ZrO 2 -La 2 O 3 , CuZnAl, Cu/CNT, Pd/C, and Ru/C catalysts; the reaction pathways are shown in Figure 1 [5][6][7][8].
Attempts have been made to use non-noble-metal-supported catalysts for furfural hydrogenation. However, non-noble metals carry disadvantages, such as leaching and deactivation, and noble metal catalysts remain as mainstay catalysts. Certain factors affect the activity and selectivity of a catalyst, such as the metal dispersion, acid/base properties, surface chemical state, and metal-support interaction. These factors are mainly influenced by the support properties, which affect the catalyst activity and selectivity.  for selective hydrogenation of furfural to THFAL [9]. The Pd/TiO 2 catalyst provided the highest yield of THFAL. Li et al. reported that hydroxyapatite-bound Pd (HAP) catalysts exhibited 100% furfural conversion and 100% THFAL yield [10]. The physicochemical properties of support affected to metal dispersion and electron density which improved the catalytic activity. Bhogeswararao et al. investigated Pd and Pt supported catalyst for selective hydrogenation of furfural to FAL or THFAL [11]. The products selectivity changed with hydrogenolysis and ring opening depending on the support acidity.
Only a few studies, however, have reported the effects of oxide supports on the catalytic performance of Pd catalysts for furfural hydrogenation. Most of the previous researches, furfural hydrogenation was performed on a relatively small scale. Herein, we prepare Pd supported on different oxide supports (Al 2 O 3 , SiO 2 , TiO 2 , CeO 2 , and ZrO 2 ) by a chemical reduction method for the catalytic hydrogenation of furfural to produce CPO. We performed relatively large scale of furfural hydrogenation. The results confirm that the Pd dispersion affects the conversion of furfural. Furthermore, the physicochemical properties of the Pd catalysts affect the selectivity of the resulting product distribution. Table 1 summarizes the physical properties of the supports and Pd catalysts. The Al 2 O 3 and SiO 2 supports showed relatively high specific surface areas of 135 and 170 m 2 /g and pore volumes of 0.73 and 0.33 cm 3 /g, respectively. The specific surface areas changed after deposition of Pd onto the supports. Especially, the specific surface areas of the Pd/Al 2 O 3 and Pd/CeO 2 catalysts increased from 135 to 150 m 2 /g and 69 to 73 m 2 /g, respectively. In contrast, that of the Pd/SiO 2 and Pd/TiO 2 catalyst decreased from 179 to 161 m 2 /g, and 51 to 43 m 2 /g. However, the Pd/ZrO 2 catalysts did not change significantly. These results suggested that the Pd particle size affected the specific surface area. Smaller Pd particles were well-dispersed on the catalysts supports, thereby, increasing the specific surface area. A decreased specific surface area was caused by large Pd particles blocking the support pores. After the Pd loading, the Pd/SiO 2 and Pd/TiO 2 catalysts showed relatively large pore volume and pore diameter than other catalysts. During the preparation of catalysts, precipitating agent of NaOH may be fills small pore of support. We suggested that support pores filled with NaOH was collapsed into small pore structure, resulting in the produce of large pores.  Figure 2 shows the nitrogen adsorption-desorption isotherms and pore size distributions of the supports and Pd catalysts. The Al 2 O 3 , SiO 2 , TiO 2 , and ZrO 2 supports showed type IV isotherms with H1 hysteresis loops, which indicate mesoporous materials and cylindrical pores. The CeO 2 support showed a type IV isotherm with an H3 hysteresis loop, which represents a mesoporous materials and slit-shaped pores [12]. All the supported Pd catalyst nitrogen adsorption-desorption isotherms exhibited results similar to the catalyst supports. The Pd/Al 2 O 3 and Pd/SiO 2 catalysts had relatively larger pore volumes than the other catalysts.

Characterization of Supports and Pd Catalysts
The XRD patterns of the prepared supports and Pd catalysts are shown in Figure 3. As shown in Figure 3a, TiO 2 exhibited a mix of anatase (JCPDS 71-1166) and rutile phases (JCPDS 73-1763). The Al 2 O 3 , SiO 2 , CeO 2 , and ZrO 2 supports exhibited γ-Al 2 O 3 (JCPDS 29-0063), amorphous silica, cubic fluorite CeO 2 (JCPDS 34-0394), and monoclinic ZrO 2 (JCPDS 37-1484) phases, respectively. As shown in Figure 3b, all the prepared Pd catalysts showed characteristic Pd peaks at 2θ = 40.2 • , 46.6 • , and 67.6 • , which correspond to the (111), (200), and (220) planes of the face-centered cubic crystal structure, respectively. The main Pd peak of the Pd/Al 2 O 3 catalyst overlapped with the Al 2 O 3 phase at 2θ = 40.2 • . The Pd/SiO 2 catalyst exhibited the largest Pd peaks, whereas the Pd/CeO 2 catalyst exhibited the smallest. The intensities of the Pd peaks differed depending on the support, which is related to two causes: Crystalline sizes of Pd and Pd nanoparticles or Pd ions incorporated into the crystal lattices of the supports. Small Pd peak intensities in Pd catalysts indicated that small Pd particles were homogeneously dispersed on the support.
supports and Pd catalysts. The Al2O3, SiO2, TiO2, and ZrO2 supports showed type IV isotherms with H1 hysteresis loops, which indicate mesoporous materials and cylindrical pores. The CeO2 support showed a type IV isotherm with an H3 hysteresis loop, which represents a mesoporous materials and slit-shaped pores [12]. All the supported Pd catalyst nitrogen adsorption-desorption isotherms exhibited results similar to the catalyst supports. The Pd/Al2O3 and Pd/SiO2 catalysts had relatively larger pore volumes than the other catalysts.  The XRD patterns of the prepared supports and Pd catalysts are shown in Figure 3. As shown in Figure 3a, TiO2 exhibited a mix of anatase (JCPDS 71-1166) and rutile phases (JCPDS 73-1763). The Al2O3, SiO2, CeO2, and ZrO2 supports exhibited γ-Al2O3 (JCPDS 29-0063), amorphous silica, cubic fluorite CeO2 (JCPDS 34-0394), and monoclinic ZrO2 (JCPDS 37-1484) phases, respectively. As shown in Figure 3b, all the prepared Pd catalysts showed characteristic Pd peaks at 2θ = 40.2°, 46.6°, and 67.6°, which correspond to the (111), (200), and (220) planes of the face-centered cubic crystal structure, respectively. The main Pd peak of the Pd/Al2O3 catalyst overlapped with the Al2O3 phase at 2θ = 40.2°. The Pd/SiO2 catalyst exhibited the largest Pd peaks, whereas the Pd/CeO2 catalyst exhibited the smallest. The intensities of the Pd peaks differed depending on the support, which is related to two causes: Crystalline sizes of Pd and Pd nanoparticles or Pd ions incorporated into the crystal lattices of the supports. Small Pd peak intensities in Pd catalysts indicated that small Pd   To verify the reduction properties of the Pd catalysts, H2-TPR analysis was performed, and the results are shown in Figure 4. All the prepared Pd catalysts showed negative peaks in the range of 60 to 90 °C, which is related to formation of PdHx before the analysis began [13][14][15]. Each Pd catalyst showed a different intensity of this negative PdHx peak in the 60 to 90 °C range, where the intensity was the smallest for Pd/CeO2 and the largest for Pd/SiO2. The results indicated that larger Pd particles lead to a greater formation of PdHx. The Pd/CeO2 and Pd/SiO2 samples exhibited positive peak regions at approximately 300 and 700 °C, and 750 °C, respectively. These positive peaks indicated that bulk PdOx species were reduced to Pd 0 or hydrogen spillover [15,16]. To verify the reduction properties of the Pd catalysts, H 2 -TPR analysis was performed, and the results are shown in Figure 4. All the prepared Pd catalysts showed negative peaks in the range of 60 to 90 • C, which is related to formation of PdH x before the analysis began [13][14][15]. Each Pd catalyst showed a different intensity of this negative PdH x peak in the 60 to 90 • C range, where the intensity was the smallest for Pd/CeO 2 and the largest for Pd/SiO 2 . The results indicated that larger Pd particles lead to a greater formation of PdH x . The Pd/CeO 2 and Pd/SiO 2 samples exhibited positive peak regions at approximately 300 and 700 • C, and 750 • C, respectively. These positive peaks indicated that bulk PdO x species were reduced to Pd 0 or hydrogen spillover [15,16].  The acid properties of the Pd catalysts were analyzed by NH3-TPD, as shown in Table 1, Figure  5. The Pd catalysts showed two desorption peak around 100-250 °C and 250-600 °C, corresponding to weakly acid sites and medium and strong acid sites, respectively. The order of decreasing total acidity of Pd catalyst was Pd/Al2O3 > Pd/CeO2 > Pd/ZrO2 > Pd/SiO2 > Pd/TiO2. The Pd/Al2O3 catalyst showed the higher acidity than other catalysts.  The acid properties of the Pd catalysts were analyzed by NH 3 -TPD, as shown in Table 1, Figure 5. The Pd catalysts showed two desorption peak around 100-250 • C and 250-600 • C, corresponding to weakly acid sites and medium and strong acid sites, respectively. The order of decreasing total acidity of Pd catalyst was Pd/Al 2 O 3 > Pd/CeO 2 > Pd/ZrO 2 > Pd/SiO 2 > Pd/TiO 2 . The Pd/Al 2 O 3 catalyst showed the higher acidity than other catalysts.  The acid properties of the Pd catalysts were analyzed by NH3-TPD, as shown in Table 1, Figure  5. The Pd catalysts showed two desorption peak around 100-250 °C and 250-600 °C, corresponding to weakly acid sites and medium and strong acid sites, respectively. The order of decreasing total acidity of Pd catalyst was Pd/Al2O3 > Pd/CeO2 > Pd/ZrO2 > Pd/SiO2 > Pd/TiO2. The Pd/Al2O3 catalyst showed the higher acidity than other catalysts.   The XPS (X-ray photoelectron spectroscopy) spectra of the Pd catalysts in the Pd 3d region are shown in Figure 6. All the Pd catalysts showed both metallic Pd and Pd 2+ speices. The Pd 3d 5/2 binding energy at 335.2 eV and 336.0 eV, which is corresponding to metallic Pd and Pd 2+ [17,18]. In the case of Pd/CeO 2 catalysts, the Pd 3d 5/2 peak positions observed positive shifts in binding energy, which is could be ascribed to the strong metal-support interaction. In the case of Pd/TiO 2 and Pd/ZrO 2 catalysts, the Pd 3d 5/2 peak positions observed negative shifts in binding energy, which is could be ascribed to high ratio of metallic Pd [19]. The surface atomic ratio of Pd catalysts are summerized in Table 2. The atomic ratio of Pd/support decreased as following orders; Pd/CeO 2 > Pd/Al 2 O 3 > Pd/TiO 2 > Pd/ZrO 2 > Pd/SiO 2 . The surface atomic ratio of Pd on Pd/CeO 2 catalyst is at least 2 times higher than Pd/Al 2 O 3 , suggesting that the Pd nanoparticles are well dispersed on CeO 2 surface.
Catalysts 2020, 10, x FOR PEER REVIEW 7 of 19 The XPS (X-ray photoelectron spectroscopy) spectra of the Pd catalysts in the Pd 3d region are shown in Figure 6. All the Pd catalysts showed both metallic Pd and Pd 2+ speices. The Pd 3d5/2 binding energy at 335.2 eV and 336.0 eV, which is corresponding to metallic Pd and Pd 2+ [17,18]. In the case of Pd/CeO2 catalysts, the Pd 3d5/2 peak positions observed positive shifts in binding energy, which is could be ascribed to the strong metal-support interaction. In the case of Pd/TiO2 and Pd/ZrO2 catalysts, the Pd 3d5/2 peak positions observed negative shifts in binding energy, which is could be ascribed to high ratio of metallic Pd [19]. The surface atomic ratio of Pd catalysts are summerized in Table 2. The atomic ratio of Pd/support decreased as following orders; Pd/CeO2 > Pd/Al2O3 > Pd/TiO2 > Pd/ZrO2 > Pd/SiO2. The surface atomic ratio of Pd on Pd/CeO2 catalyst is at least 2 times higher than Pd/Al2O3, suggesting that the Pd nanoparticles are well dispersed on CeO2 surface.    Figure 7 shows the FE-SEM (Field mission scanning Electron Microscope) images and EDX (energy dispersive X-Ray spectroscopy) spectra of the Pd catalysts; the EDX spectra confirmed their Pd distributions. It appeared that the distribution of Pd particles differed depending on the support. The Pd/Al 2 O 3 , Pd/TiO 2 , and Pd/CeO 2 catalysts showed uniform distributions, whereas the Pd/SiO 2 and Pd/ZrO 2 catalysts showed non-uniform distributions. Figure 8 shows the FE-TEM images and particle size distributions of the Pd catalysts. The Pd/CeO 2 and Pd/Al 2 O 3 catalysts exhibited well-dispersed Pd on the support and narrower particle size distributions than the other catalysts. The Pd/SiO 2 and Pd/ZrO 2 catalysts exhibited poorly dispersed Pd on the support and aggregation of Pd particles. Average particle sizes were estimated over 200 spherical Pd particles. The order of increasing Pd particle size was Pd/CeO 2 (2.7 nm) < Pd/Al 2 O 3 (2.9 nm) < Pd/TiO 2 (4.3 nm) < Pd/ZrO 2 (5.2 nm) < Pd/SiO 2 (9.2 nm).    To estimate the catalyst Pd dispersions, CO chemisorption analysis was performed, and the results are summarized in Table 2. The Pd/CeO 2 catalyst had the highest Pd dispersion of 32%, whereas the Pd/SiO 2 catalyst had the lowest Pd dispersion of 5%. We also estimated Pd dispersion based on the FE-TEM images, which indicated much higher Pd dispersion values than the CO chemisorption analysis. These results suggested that some Pd particles were covered by the support. However, both CO chemisorption and FE-TEM analyses yielded the same order of Pd dispersion: Pd/CeO 2 > Pd/Al 2 O 3 > Pd/TiO 2 > Pd/ZrO 2 > Pd/SiO 2 . These results are in agreement with other analyses. We considered that Pd dispersion was affected by several factors, such as isoelectric point, nucleation and growth of Pd, ionic strength between Pd and support, etc. [20][21][22][23][24]. In particular, we focused on pH and support isoelectric point. Many researchers have reported that metal dispersion is affected by support properties and synthetic parameters such as pH values and solution temperature. Generally, Al 2 O 3 , CeO 2 , TiO 2 , and ZrO 2 show higher isoelectric points of 5 to 7 than that of SiO 2 (-2) [21,[25][26][27]. During Pd catalyst synthesis, the H 2 PdCl 4 precursor existed predominantly as PdCl 4 2− (at pH 2), and thus, the interactions between Pd ions and the SiO 2 support did not easily occur. The Pd dispersions differed because the nucleation and growth of Pd nanoparticles differed depending on the synthesis pH and support type. The Pd/CeO 2 catalyst had the highest dispersion and most uniform Pd distribution with a solution temperature of 0 • C and pH of 7.0.

Hydrogenation of Furfural
The effect of Pd dispersion and support properties on catalytic activity and selectivity of Pd catalysts were investigated with respect to the liquid-phase hydrogenation of furfural the results are summarized in Table 3. The reactions were performed 20 bar of H 2 at 150 • C for 1 h with 0.03 g of catalyst. The conversion of furfural decreased with decreasing Pd dispersion in the order: Pd/CeO 2 > Pd/Al 2 O 3 > Pd/TiO 2 > Pd/ZrO 2 > Pd/SiO 2 . FAL and CPO were the main products, generated via the hydrogenation of furfural or the rearrangement of furfural. The Pd/SiO 2 , Pd/TiO 2 , and Pd/ZrO 2 catalysts showed higher selectivity for CPO than the Pd/Al 2 O 3 and Pd/CeO 2 catalysts. The Pd/TiO 2 catalyst exhibited highest CPO yield as 12.7%. Note: a Reaction conditions: furfural (2.5 g), distilled water (100 mL), stirring rate (500 rpm). b Non-identified products.
Catalysts 2020, 10, 837 13 of 19 Figure 9 shows conversion and product distribution in furfural hydrogenation over Pd based catalysts under the following conditions: 20 bar of H 2 at 150 • C for 4 h with 0.1 g of catalyst. Under these conditions, CPO and THFAL were the main products, generated via the rearrangement of furfural or the hydrogenation of FAL. The conversion of furfural decreased with decreasing Pd dispersion in the order: Pd/CeO 2 > Pd/Al 2 O 3 > Pd/TiO 2 > Pd/ZrO 2 > Pd/SiO 2 . The Pd/SiO 2 , Pd/TiO 2 , and Pd/ZrO 2 catalysts showed greater selectivity for CPO than the Pd/Al 2 O 3 and Pd/CeO 2 catalysts. The Pd/Al 2 O 3 and Pd/CeO 2 catalysts exhibited greater selectivity for THFAL than the other catalysts. Furfural is converted to FAL, which can be further polymerized to oligomers by heat or strong acid sites [28,29]. Based on NH 3 -TPD analysis, the Pd/Al 2 O 3 catalyst was more acidic than the other catalysts. The acidity was comparable for the other catalysts. Therefore, the Pd/TiO 2 , Pd/SiO 2 , and Pd/ZrO 2 catalysts exhibited higher selectivity for CPO. However, the Pd/CeO 2 catalyst exhibit low selectivity for CPO and high selectivity for THFAL, despite the low acidity. THFAL can be produced by over-hydrogenation of FAL. The higher Pd dispersion in the Pd/Al 2 O 3 and Pd/CeO 2 catalysts led to high selectivity for THFAL. Non-identified materials may be produced by polymerization reaction [6,7]. The Pd/TiO 2 catalyst exhibited highest CPO yield as 45.2%. The Pd/TiO 2 catalyst was the most effective for furfural hydrogenation to CPO at 150 • C. Among the Pd-supported catalysts, the Pd/Al 2 O 3 and Pd/TiO 2 catalysts were selected for further evaluation of the effect of the reaction temperature and hydrogen pressure. summarized in Table 3. The reactions were performed 20 bar of H2 at 150 °C for 1 h with 0.03 g of catalyst. The conversion of furfural decreased with decreasing Pd dispersion in the order: Pd/CeO2 > Pd/Al2O3 > Pd/TiO2 > Pd/ZrO2 > Pd/SiO2. FAL and CPO were the main products, generated via the hydrogenation of furfural or the rearrangement of furfural. The Pd/SiO2, Pd/TiO2, and Pd/ZrO2 catalysts showed higher selectivity for CPO than the Pd/Al2O3 and Pd/CeO2 catalysts. The Pd/TiO2 catalyst exhibited highest CPO yield as 12.7%. Figure 9 shows conversion and product distribution in furfural hydrogenation over Pd based catalysts under the following conditions: 20 bar of H2 at 150 °C for 4 h with 0.1 g of catalyst. Under these conditions, CPO and THFAL were the main products, generated via the rearrangement of furfural or the hydrogenation of FAL. The conversion of furfural decreased with decreasing Pd dispersion in the order: Pd/CeO2 > Pd/Al2O3 > Pd/TiO2 > Pd/ZrO2 > Pd/SiO2. The Pd/SiO2, Pd/TiO2, and Pd/ZrO2 catalysts showed greater selectivity for CPO than the Pd/Al2O3 and Pd/CeO2 catalysts. The Pd/Al2O3 and Pd/CeO2 catalysts exhibited greater selectivity for THFAL than the other catalysts. Furfural is converted to FAL, which can be further polymerized to oligomers by heat or strong acid sites [28,29]. Based on NH3-TPD analysis, the Pd/Al2O3 catalyst was more acidic than the other catalysts. The acidity was comparable for the other catalysts. Therefore, the Pd/TiO2, Pd/SiO2, and Pd/ZrO2 catalysts exhibited higher selectivity for CPO. However, the Pd/CeO2 catalyst exhibit low selectivity for CPO and high selectivity for THFAL, despite the low acidity. THFAL can be produced by over-hydrogenation of FAL. The higher Pd dispersion in the Pd/Al2O3 and Pd/CeO2 catalysts led to high selectivity for THFAL. Non-identified materials may be produced by polymerization reaction [6,7]. The Pd/TiO2 catalyst exhibited highest CPO yield as 45.2%. The Pd/TiO2 catalyst was the most effective for furfural hydrogenation to CPO at 150 °C. Among the Pd-supported catalysts, the Pd/Al2O3 and Pd/TiO2 catalysts were selected for further evaluation of the effect of the reaction temperature and hydrogen pressure.  The effect of the reaction temperature (100-190 • C) on furfural hydrogenation over the Pd/Al 2 O 3 and Pd/TiO 2 catalysts was investigated under 20 bar of H 2 over the course of 4 h. FAL was generated with the use of the Pd/Al 2 O 3 and Pd/TiO 2 catalysts at 100 • C. However, the selectivity for THFAL was higher with the Pd/Al 2 O 3 catalyst having a high Pd dispersion, due to over-hydrogenation of FAL. The furfural conversion and CPO selectivity of the Pd/Al 2 O 3 and Pd/TiO 2 catalysts gradually increased as the reaction temperature was increased from 100 to 190 • C, whereas the selectivity for THFAL gradually decreased from 13.7% to 5% with the Pd/Al 2 O 3 catalyst. In contrast, the THFAL selectivity remained unchanged for the Pd/TiO 2 catalyst. Via hydrogenolysis over the metal sites, 2-MF was produced at a high temperature [30]. Therefore, an optimum reaction temperature of 170 • C was selected for further study.
The effect of the H 2 pressure (10-30 bar) on furfural hydrogenation over the Pd/Al 2 O 3 and Pd/TiO 2 catalysts at 170 • C for a reaction time of 4 h was investigated. When the H 2 pressure was increased from 10 to 20 bar, the furfural conversion, CPO selectivity and CPO yield increased with the Pd/Al 2 O 3 and Pd/TiO 2 catalysts. In the H 2 pressure range of 20-30 bar, the CPO selectivity increased slightly with the use of the Pd/Al 2 O 3 catalyst. However, the CPO selectivity decreased, whereas the THFAL selectivity increased, with the Pd/TiO 2 catalyst, due to differences in the adsorption strength. The Pd/Al 2 O 3 catalyst was more prone to rearrangement under high H 2 pressure than the Pd/TiO 2 catalyst. Hronec et al. reported that the selectivity for THFAL and CPO was affected by the hydrogen pressure and metal catalyst in the hydrogenation of FAL [1]. When Pd exposed to H 2 gas, α-PdH x (x < 0.03) is formed. Increasing the H 2 pressure, Pd absorbed a large amount of H 2 with formation of β-PdH x (x > 0.03) [31]. Increasing the H 2 pressure and metal concentration led to increased H 2 dissolution. The more H 2 dissolution in mixture can be more active and stabilized on the Pd catalyst surface, which is attributed to more formation of THFAL or CPO products.
The Pd/Al 2 O 3 and Pd/TiO 2 catalysts showed similar results in that the CPO selectivity decreased or changed slightly. Nandan et al. reported that the Lewis acid sites of PdO affect the rearrangement of FAL to CPO [32]. Based on the XPS results, the Pd 2+ /Pd ratio of the Pd/TiO 2 catalyst was lower than that of the Pd/Al 2 O 3 catalyst. We suggest that a larger amount of metallic Pd species on the catalyst surface leads to greater hydrogenation of the C=C bonds. Therefore, the Pd/TiO 2 catalyst showed higher selectivity for THFAL than the Pd/Al 2 O 3 catalyst under high H 2 pressure. The Pd/TiO 2 catalyst produced the highest furfural conversion and CPO selectivity at 170 • C under 20 bar of H 2 .
The effect of the reaction time (1-5 h) on furfural hydrogenation over the Pd/TiO 2 catalyst was investigated under 20 bar of H 2 at 170 • C, as shown in Figure 10. As the reaction time increased from 1 to 5 h, the furfural conversion increased from 85.7 to 100%. The CPO selectivity gradually increased from 40 to 55.6% when the reaction time was increased from 1 to 4 h. The CPO selectivity remained unchanged when the reaction was extended to 5 h. The selectivity for THFAL and 2-MF increased slightly with increasing reaction time. Under the optimal conditions of 20 bar of H 2 at 170 • C for 4 h, 100% furfural conversion, 55.9% CPO selectivity, and 56.6% CPO Yield were achieved with the Pd/TiO 2 catalyst.

Preparation of Pd Catalysts
Al2O3 (Aladin, γ phase), SiO2 (Degussa, Aerosil 200), TiO2 (Degussa, P25), CeO2 (Alfa-Aesar, nanopowder), and ZrO2 (Sigma-Aldrich, nanopowder) were used as catalyst supports. The supported Pd catalysts, containing 5 wt% Pd, were prepared by chemical reduction method. An H2PdCl4 precursor was prepared by dissolving PdCl2 (Sigma-Aldrich, 90%) in a 38% HCl solution. An aqueous solution containing 0.225 mmol of sodium tartrate dibasic dehydrate (Sigma-Aldrich, 99.0%) as a stabilizer was brought to 0 °C, and then the H2PdCl4 precursor was added to the solution. The support was added to the mixture and stirred with a high shear mixer. The Pd precursor solution containing the support was adjusted to a pH of 7.0 by adding 0.02 M NaOH (Sigma-Aldrich, 97.0%). The mixture was then stirred for 2 h at 0 °C. Reduction of the catalyst was carried out in the liquid phase using a 100 mL of 0.3 M sodium borohydride (Sigma-Aldrich, 99.0%) solution for 2 h at 0 °C. The suspension was then filtered and washed with distilled water. All catalysts were dried at 85 °C for 24 h.

Characterization of Supports and Pd Catalysts
The crystal structures of the supports and supported Pd catalysts were analyzed by x-ray diffraction (XRD) (D8 Focus, Bruker, Germany) at 50 kV and 50 mA using a Cu Kα radiation source (λ = 1.5406 Å). Specific surface areas and pore sizes/volumes were determined by BET and BJH analyses of N2 adsorption-desorption isotherms (ASAP 2020, Micromeritics, Norcross, GA, USA). The Pd contents were verified by inductively coupled plasma-optical emission spectroscopy (NexION 300D, PerkinElmer, Shelton, CT, USA) analysis. The reduction and acid properties of catalysts were tested using H2-temperature-programmed reduction (H2-TPR, AutoChem 2920, Micromeritics, Norcross, GA, USA) and NH3-temperature-programmed reduction (NH3-TPD, AutoChem 2920, Micromeritics, GA, USA) [33]. The H2-TPR measurements, 0.1 g of each catalyst was placed in a U-shaped quartz tube under He flow at 150 °C for 120 min, and then cooled to 50 °C.

Preparation of Pd Catalysts
Al 2 O 3 (Aladin, γ phase), SiO 2 (Degussa, Aerosil 200), TiO 2 (Degussa, P25), CeO 2 (Alfa-Aesar, nanopowder), and ZrO 2 (Sigma-Aldrich, nanopowder) were used as catalyst supports. The supported Pd catalysts, containing 5 wt% Pd, were prepared by chemical reduction method. An H 2 PdCl 4 precursor was prepared by dissolving PdCl 2 (Sigma-Aldrich, 90%) in a 38% HCl solution. An aqueous solution containing 0.225 mmol of sodium tartrate dibasic dehydrate (Sigma-Aldrich, 99.0%) as a stabilizer was brought to 0 • C, and then the H 2 PdCl 4 precursor was added to the solution. The support was added to the mixture and stirred with a high shear mixer. The Pd precursor solution containing the support was adjusted to a pH of 7.0 by adding 0.02 M NaOH (Sigma-Aldrich, 97.0%). The mixture was then stirred for 2 h at 0 • C. Reduction of the catalyst was carried out in the liquid phase using a 100 mL of 0.3 M sodium borohydride (Sigma-Aldrich, 99.0%) solution for 2 h at 0 • C. The suspension was then filtered and washed with distilled water. All catalysts were dried at 85 • C for 24 h.

Characterization of Supports and Pd Catalysts
The crystal structures of the supports and supported Pd catalysts were analyzed by x-ray diffraction (XRD) (D8 Focus, Bruker, Germany) at 50 kV and 50 mA using a Cu Kα radiation source (λ = 1.5406 Å). Specific surface areas and pore sizes/volumes were determined by BET and BJH analyses of N 2 adsorption-desorption isotherms (ASAP 2020, Micromeritics, Norcross, GA, USA). The Pd contents were verified by inductively coupled plasma-optical emission spectroscopy (NexION 300D, PerkinElmer, Shelton, CT, USA) analysis. The reduction and acid properties of catalysts were tested using H 2 -temperature-programmed reduction (H 2 -TPR, AutoChem 2920, Micromeritics, Norcross, GA, USA) and NH 3 -temperature-programmed reduction (NH 3 -TPD, AutoChem 2920, Micromeritics, GA, USA) [33]. The H 2 -TPR measurements, 0.1 g of each catalyst was placed in a U-shaped quartz tube under He flow at 150 • C for 120 min, and then cooled to 50 • C. The catalyst was heated to 800 • C at a heating rate of 10 • C/min under 5% H 2 /He flow. The NH 3 -TPD measurements, 0.1 g of each catalyst was placed in a U-shaped quartz tube under He flow at 150 • C for 120 min. Then, NH 3 (10% NH 3 in He) adsorption was allowed to proceed at 50 • C for 30 min. The catalyst was flushed under the He flow. Finally, the catalysts were heated to 500 • C at a heating rate of 10 • C/min under He flow. X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific, Waltham, MA, USA) measurements were performed using Al Kα radiation. Before the XPS measurement, all Pd catalysts were pre-reduced with hydrogen (4% H 2 in N 2 ) at 250 • C for 2 h.
The surface morphologies and Pd distributions of the Pd catalysts were analyzed by SEM (SU8020, Hitachi, Tokyo, Japan) with EDX at an accelerating voltage of 15 kV. Pd dispersions were measured by CO chemisorption (ASAP 2020C, Micromeritics, Norcross, GA, USA). Each catalyst (0.4 g) was reduced under H 2 flow at 250 • C for 2 h. The amount of adsorbed CO was evaluated by assuming a stoichiometry of one CO molecule to one surface Pd site [34,35]. The dispersion of Pd was calculated using Equations (1) and (2): V chem : CO chemisorption volume, M w : Pd atomic weight, SF: stoichiometry factor, and C/100: supported metal weight.
Particle sizes and Pd distributions were analyzed by FE-TEM (JEM-2100F, JEOL, Tokyo, Japan) at an accelerating voltage of 200 kV. Moreover, Pd dispersions were also evaluated with FE-TEM images using the following Equation (3) [36], where a m Å 2 : area occupied by a surface atom, v m (Å 3 ): volume occupied by an atom in the bulk metal, and d VA : the average particle size.

Hydrogenation of Furfural
Furfural hydrogenation was performed in a 250 mL stainless-steel autoclave using 2.5 g of furfural and 100 mL of distilled water as solvent. Prior to the start of the reaction, 0.1 g of each Pd catalyst was reduced with hydrogen (4% H 2 in N 2 ) at 250 • C for 2 h. The reaction mixture containing the reduced catalyst was rapidly transferred into the autoclave, and then the reactor was purged with nitrogen three times to remove air. The sealed reactor was stirred at 500 rpm. After the reaction temperature reached at desire temperature, hydrogen was pressured to 20 bar, and the reaction was carried out for 4 h. After completion of the reaction, the reactor was cooled down to room temperature. The products were centrifuged and added to 2-propanol (volume 1:10). The reaction products were analyzed by gas chromatography (GC, 7890A, Agilent, Santa Clara, CA, USA) using a flame ionization detector (FID) and a DB-Wax (30 m × 0.32 mm × 0.25 µm) [37]. The quantitative of reaction products was done by external standard method. The furfural (Sigma-Aldrich), FAL (Sigma-Aldrich), CPO (Sigma-Aldrich), THFAL (Sigma-Aldrich), and 2-MF (Sigma-Aldrich) standard were analytical grade and used without purification. The conversion and selectivity were calculated as follows: The conversion and product selectivity were calculated per the following equations:

Conclusions
Pd catalysts were prepared on various supports (Al 2 O 3 , SiO 2 , TiO 2 , CeO 2 , and ZrO 2 ) by a chemical reduction method. The dispersion and uniformity of Pd were affected by the properties of the support and by the nucleation and growth of Pd. The effects of the Pd dispersion and support properties on the catalytic activity and selectivity for furfural hydrogenation were investigated. The conversion of furfural was enhanced by greater Pd dispersion. The selectivity for CPO and THFAL was affected by the Pd dispersion, surface chemical state, acidity of the Pd catalyst, and reaction parameters (catalyst amount, temperature, H 2 pressure, and time). The Pd/TiO 2 catalyst showed the highest CPO yield than other catalysts. High Pd dispersion and high acidity of the catalyst led to greater C=C hydrogenation, thereby generating more THFAL. The Pd/TiO 2 catalyst exhibited the >99% furfural conversion, 55.6% CPO selectivity, and 55.5% CPO Yield under the optimal conditions; 20 bar of H 2 , at 170 • C for 4 h with 0.1 g of catalyst.