CeO2 catalysts by a facile redox approach for high-efficiency hydrogenation of levulinic acid into gamma-valerolactone

Catalysis Communications 93 (2017) 10–14

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Self-assembled Pd/CeO2 catalysts by a facile redox approach for high-efficiency hydrogenation of levulinic acid into gamma-valerolactone Yong Zhang a,b, Chun Chen a,⁎, Wanbing Gong a,b, Jieyao Song a,b, Haimin Zhang a, Yunxia Zhang a, Guozhong Wang a, Huijun Zhao a,c,⁎ a Key Laboratory of Materials Physics, Centre for Environmental and Energy Nanomaterials, Anhui Key Laboratory of Nanomaterials and Nanotechnology, CAS Center for Excellence in Nanoscience, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, PR China b University of Science and Technology of China, Hefei, Anhui 230026, PR China c Centre for Clean Environment and Energy, Gold Coast Campus, Griffith University, Queensland 4222, Australia

a r t i c l e

i n f o

Article history: Received 27 October 2016 Received in revised form 28 December 2016 Accepted 10 January 2017 Available online xxxx Keywords: Pd/CeO2 catalysts One-pot redox method Hydrogenation Levulinic acid Gamma-valerolactone

a b s t r a c t A series of self-assembled Pd/CeO2 catalysts were synthesized by a facile one-pot redox method, demonstrating high catalytic performance in hydrogenation of levulinic acid. Under optimized conditions, almost 100% gammavalerolactone yield was achieved using Pd/CeO2 catalyst with only slight activity loss (ca. 4%) after five continuous runs. Combined with relevant characterization techniques, the superior catalytic hydrogenation performance could be attributed to Pd/CeO2 catalyst with noticeable advantages of high surface area beneficial to the exposure of Pd catalytic active sites and increase electron density of Pd favorable for the dissociation of H2 as well as the defects sites of CeO2 responsible for esterification of intermediate. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Currently, there is an urgent need to find sustainable resources to deal with the energy crisis of gradual depletion of traditional fossil resources and deterioration of environmental problems, leading to the prosperity of biomass utilization [1–4]. Biomass-derived levulinic acid (LA) can be used as low-cost and abundant precursor for synthesis of value-added chemicals, such as gamma-valerolactone (GVL). As an attractive intermediate of food and fuel additives, GVL is generally produced through LA hydrogenation using homogeneous or heterogeneous catalysts [5–7]. Among them, Ru-based catalysts have indicated excellent activity [5–10]. However, there are issues need to be noticed, including complicated synthesis procedure of catalyst, addition of exorbitant ligand, and relative poor catalytic recyclability [11,12]. Recent studies have shown that supported Pd-based catalysts exhibit an outstanding catalytic performance for LA hydrogenation to GVL [13]. Yan et al. developed recently a series of Pd-based catalysts including Pd/CNTs, Pd/SiO2 and Pd/MCM-41 [14–17], demonstrating a

⁎ Corresponding authors at: Centre for Clean Environment and Energy, Gold Coast Campus, Griffith University, Queensland 4222, Australia. E-mail addresses: [email protected] (C. Chen), h.zhao@griffith.edu.au (H. Zhao).

http://dx.doi.org/10.1016/j.catcom.2017.01.008 1566-7367/© 2017 Elsevier B.V. All rights reserved.

satisfactory catalytic performance of LA hydrogenation. However, the harsh reaction conditions including high pressure and temperature were necessary in their studies, possibly resulting in high energy consumption. In addition, a sophisticated calcination and/or reduction post-treatments were still needed to recover these used catalysts. Therefore, it is a huge challenge to obtain robust Pd-based catalysts with high activity, excellent recyclability by a facile method. Herein, we synthesized a series of self-assembled Pd/CeO2 catalysts via a facile one-pot redox method using Ce(III) and Pd(II) precursors. The main purpose of present work is to provide a flexible synthesis method for designing and developing a robust catalyst and reveal the intrinsic strong mutual effects of active metal and support for improving the catalytic performance and reusability in the hydrogenation of levulinic acid. Compared with the Pd-based catalysts reported in literatures, there are some merits of the Pd/CeO2 catalyst synthesized in this work as follows: (1) facile preparation method without high temperature pretreatments of calcination and reduction; (2) high catalytic activity and recyclability; (3) mild reaction conditions of LA hydrogenation. According to the relevant characterization results obtained by TEM, XRD, N2 adsorption-desorption and XPS measurements, the superior catalytic performance can be put down these factors: (I) high BET surface area favorable for exposure of Pd catalytic active sites; (II) increase of electron density of Pd surface beneficial to activation of H2 and LA; (III) defect sites of CeO2 favorable for esterification.

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2. Experimental section 2.1. Catalysts preparation A series of self-assembled Pd/CeO2 catalysts with different Pd loading of 0.58, 1.16, 2.91, 5.82, and 8.73 wt.% were synthesized via a facile one-pot redox method. Typically, 10 mL of 0.1 M Ce(NO3)3·6H2O aqueous solution was first added into flask at 348 K under N2 protection. After vigorous stirring for 15 min, 10 mL of 0.2 M NaOH was added into the above solution quickly, followed by addition of a certain amount of K2PdCl4 solution drop by drop. Stirring for another 108 min at 348 K, the slurry was washed, centrifuged and dried at 333 K for overnight in a vacuum oven. The Pd/CeO2 hybrid was formed as the follow+ PdCl2− ing formula: Ce3 + + 3OH− → Ce(OH)δx +, Ce(OH)δ+ x 4 → CeO2 + Pd + H2O. The as-obtained catalysts were denoted as Pd (χ)/CeO2, where χ was defined as Pd loading. For a meaningful comparison, we also prepared the conventional supported Pd/CeO2 catalyst (s-Pd/CeO2) by precipitation and chemical reduction method. The detail preparation procedure could be found in Supporting information. 2.2. Catalyst characterization TEM images and energy dispersive X-ray spectra (EDS) were conducted on a JEOL-2010 microscope operated at an accelerating voltage of 200 kV. The crystal phase structure of samples were analyzed by XRD equipped with Cu-Ka (λ = 0.154 nm) radiation operating at 40 kV and 40 mA for 2θ angles ranging from 10° to 75°. XPS measurement was performed on an ESCALAB 250 photoelectron spectrometer (Thermo-VG Scientific Co., Ltd.) with Al Kα X-ray radiation as the Xray source for excitation. The binding energy of C 1s peak (284.8 eV) was considered as a calibration. The Pd dispersion (DPd) and mean particle size (dPd) were determined by O2\\H2 pulse titration using an AutoChem 2090 apparatus equipped with a thermal conductivity detector (TCD) [18], provided by National Engineering Research Center of Chemical Fertilizer Catalyst of Fuzhou University. The textural properties including surface area and porosity of the as-prepared samples were probed by N2 adsorption-desorption measurements at 77 K using a Micrometrics tristar 3020M. Prior to measurements, the sample was degassed under vacuum at 523 K for 2 h. 2.3. Catalytic test All experiments were carried out in a 25 mL of batch reactor. Typically, LA (20 mg) was added to the Teflon-lined steel autoclave vessel with as-prepared catalysts (15 mg) and 5 mL of 2-proponal (2-PrOH) as reaction media. After purging with N2 for three times, hydrogen was pressurized to 4 bar at room temperature. Once the reactor was heated to the preset temperature, the reaction was executed for 90 min under continuous magnetic stirring. After reactions, the liquid products were identified by gas chromatography–mass spectrometry (GC–MS, Thermo Fisher Scientific-TXQ), and quantified by GC (Shimadzu, GC-2010 Plus) using n-octanol as an internal standard. The recycling test was executed to investigate the reusability of catalyst under a more harsh conditions of 363 K, 4 bar H2 for 10 min. Prior to each run, the self-assembled Pd/ CeO2 catalysts were used directly without any pre-reduction treatments. In comparison, the catalytic performance of 5 wt.% commercial Pd/C (purchased from Lanzhou Institute of Chemical Physics, China) and s-Pd/CeO2 was also surveyed.

Fig. 1. TEM image of as-obtained Pd(5.82)/CeO2 catalyst: (insert) EDX mapping analysis and HRTEM image.

facilitates the generation of nucleation center, further leading to insitu rapid formation of small-sized hybrids. The HRTEM image of Pd(5.82)/CeO2 shows that the lattice spacing of 0.31 nm and 0.27 nm is corresponding well with the(111) and (200) crystal planes of CeO2 NPs, respectively [19], and the lattice spacing of 0.225 nm may be ascribed to the (111) crystal plane of Pd NPs (inset in Fig. 1) [20]. The EDX mapping analysis is used to analyze the distribution of Pd and Ce elements in the hybrid (insets in Fig. 1). Apparently, the mapping signals of Ce, O and Pd elements are uniformly overlapped, suggesting Pd NPs embedded in CeO2 matrix with high dispesion. Such structure may be very beneficial for exposing the catalytic active sites of metallic Pd to enhance catalyst activity and improving the stability of catalyst. For comparison, the morphologies of self-assembled Pd/CeO2 with different Pd loadings were also investigated in this work and shown in Fig. S1. With increasing Pd loading amount from 0.58 wt.% to 8.73 wt.%, the morphologies of Pd/CeO2 hybrids gradually evolve from square-shape structure into sphere-shape structure, implying that Pd loading amount can effectively tune the morphology of hybrid which might be one of the factors to influence catalytic performance of LA hydrogenation. Fig. 2 shows the XRD diffraction patterns of as-prepared Pd/CeO2 catalysts with different Pd loading. As it is shown, all catalysts display main diffraction peaks at 28.5°, 47.3°, 56.3° which can be assigned to

3. Results and discussion 3.1. Structure and composition Fig. 1 shows the typical TEM images of self-assembled Pd(5.82)/ CeO2 hybrid with an average particle size of 8.0 nm. Under strong alkaline medium, the prompt redox reaction between Ce(III) and Pd(II)

Fig. 2. XRD diffraction patterns of self-assembled Pd/CeO2 catalysts: (a) Pd(0.58)/CeO2; (b) Pd(1.16)/CeO2; (c) Pd(2.91)/CeO2; (d) Pd(5.82)/CeO2; (e) Pd(8.73)/CeO2 (■: Cubic CeO2; ●: Cubic Pd).

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cubic CeO2 phase(JCPDS 43-1002). Furthermore, the samples of Pd(5.82)/CeO2 and Pd(8.73)/CeO2 present a new diffraction pattern at 2θ of ~ 40.3°, which can be indexed to the (111) crystal plane of cubic Pd phase (JCPDS 65-6174) [15]. Compared with cubic CeO2 phase, the intensity of cubic Pd phase is very weak, suggesting high dispersion of Pd NPs embedded in the matrix of CeO2. According to Scherrer equation, the crystal sizes of cubic CeO2 phase was calculated and listed in Table 1, essentially consistent with TEM characterization results. It is noteworthy that the crystallite size of CeO2 increases with Pd loading (≤ 1.16 wt.%), possibly attributing to the uncompleted redox reaction leading to the aggregation of ceria granule due to insufficient Pd. On the contrary, the trend is reversed when Pd loading is N 2.91 wt.%, suggesting that the prompt and absolute redox reaction occurs. It is beneficial to accelerating the nucleation rate and refraining the growth of CeO2 NPs. Compared with s-Pd/CeO2 catalyst, the commercial Pd/C sample displays obvious sharp peak of cubic Pd phase except for cubic CeO2, suggesting that larger crystal size of Pd NPs (Fig. S2). This is in line with the SEM characterization results (Fig. S3). O2\\H2 pulse titration is employed to determine the size and dispersion of Pd in Pd/CeO2 hybrids. As displaying in Table 1, the Pd/CeO2 hybrids with Pd loading of 2.91 wt.%, 5.82 wt.% and 8.73 wt.% show Pd dispersion of 21.3%, 28.6% and 29.8%, respectively. However, this values of Pd/CeO2 catalysts with low Pd loading, 0.58 wt.% and 1.16 wt.%, cannot be detected by this measurement. It may be attributed to the inefficient adsorption of H2 that gives very low dispersion. With the increasing of Pd loading, the abundant Pd active sites, which are derived from the complete redox reaction between Pd and Ce precursor, are responsible for H2 adsorption and dissociation. According to previous report [18], the mean particle size of Pd (dPd) is also calculated and the relevant results are listed in Table 1. Obviously, the dPd value reduces with increase of Pd loading. As illustration, the support CeO2 is formed by the mutual redox reaction of Pd and Ce precursor, and can encapsulate the Pd NPs into its matrix simultaneously, which can prevent migration and agglomeration of Pd NPs into large particles. Fig. 3 shows the N2 adsorption-desorption isotherms of Pd(0.58)/ CeO2 and Pd(5.82)/CeO2 catalysts. The samples present a typical IV adsorption isotherm and H1-type hysteresis loop, as classified by IUPAC [21]. Type H1 is often associated with narrow distribution of porous materials or the uniform micro-aggregate, confirming that the configuration of Pd/CeO2 is an aggregate of CeO2 NPS with Pd NPs. The BET surface area and pore volume of Pd(5.82)/CeO2 catalyst are calculated to be 102.83 m2/g and 0.27 cm3/g, respectively, which is larger than that of Pd(0.58)/CeO2 catalyst (Table S1). The higher surface area is favorable to dispersing of Pd NPs and exposure of catalytic active sites, thus improving the catalytic performance. To ascertain the surface chemical state of Pd(5.82)/CeO2catalyst, XPS analysis was carried out in this work, and the results were displayed in

Fig. 3. Adsorption and desorption isotherm curves of Pd(0.58)/CeO2 and Pd(5.82)/CeO2 catalysts: (insert) relevant distribution of pore size.

Fig. 4. In the Pd 3d XPS spectra, two obvious satellite peaks appeares at binding energies of 335.7 eV and 341.0 eV, which can be assigned to the 3d5/2 and 3d3/2 of Pd, respectively. Comparing with that of monometallic Pd on activated charcoal, the peak shifts to a lower value. It is inferred that partial electron donating from cubic CeO2 might be transfered to metallic Pd, leading to a higher shielding of the nuclear charge and a weaker binding effect of 3d electrons [22,23]. This mutual interaction of Pd-Ce is also verified from the Ce XPS spectra in Fig. S4, Fig. S5 and Table S2. The peaks assigned to Ce(IV) and Ce(III) are divided respectively [20–23]. According to the previous literature [24], The chemical state of the ceria nanoparticles in contact with metal could easily alternate between 3+ and 4+ oxidation. It can be inferred that the ratio of Ce3+ to Ce4+ is induced and regulated in the presence of Pd, i.e., the mutual interaction between Pd and ceria. Moreover, the ratio of Ce3 + is proportional to the amount of Pd, as listed in Table S2 in Supporting information. As we all known that the oxygen vacancies will be generated along with alternation of cerium valence state to maintain change balance [24,25]. It is accompanied on the basis of the equation as below: [2Ce4+, O2−] → [2Ce3+, Vo] + 1/2 O2,where the Vo represents oxygen vacancy [26]. The increase of metal electron density and oxygen defect sites is beneficial to improving LA hydrogenation activity [27]. This behavior is to be expected and will be further discussed by the results of

Table 1 Catalytic performance of various Pd-based catalysts for LA hydrogenation.

Entry

Catalystsa

DPd (%)d

dPd (nm)e

dCeO2 (nm)f

GVL yield (%)

TOF (h−1)g

1 2 3 4 5 6 7 8

Pd(0.58)/CeO2 Pd(1.16)/CeO2 Pd(2.91)/CeO2 Pd(5.82)/CeO2 Pd(8.73)/CeO2 Pd(5.82)/CeO2b s-Pd/CeO2c Pd/C

– – 21.3 28.6 29.8 – – –

– – 5.1 3.8 3.6 – – –

7.92 11.26 9.64 8.25 8.16 – – –

10.3 81.0 93.3 ≥99.9 ≥99.9 40.1 45.3 7.5

14.8 15.0 29.6 34.5 30.9 4.4 5.0 1.1

a b c d e f g

Prepared by a facile one-pot redox method. Determined by O2\ \H2 pulse titration measurement, DPd: the dispersion of Pd. Calculated by the equation: dPd (nm) = 1.08/D, d: mean particle size of Pd. Calculated from XRD. Prepared by conventional method (calcinated at 723 K and reduced at 673 K). TOF = molLA/(molPd·h) at conversion of 10– 20%. Prepared one-pot redox method and then reduced by H2 gas at 673 K.

Fig. 4. Pd 3d XPS spectra of Pd(5.82)/CeO2 catalyst.

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Fig. 5. Recycling catalytic performance of as-prepared Pd(5.82)/CeO2catalyst. Reaction conditions: LA (20 mg), catalyst (15 mg), 2-PrOH (5 mL) as a solvent, at 363 K, 4 bar H2 and 10 min.

catalytic performance at the following section. The other two peaks locate at binding energies of 338.2 eV and 343.2 eV, may be belonged to the oxidation state of Pd because of easy oxidation of surface Pd by oxygen in wet air during the characterization procedure [28].

Scheme 1. Catalytic mechanism of LA hydrogenation over self-assembled Pd/CeO2 catalysts.

3.2. Catalytic performance The catalytic activities of various Pd-based catalysts were evaluated and the results were shown in Table 1. For the self-assembled Pd/CeO2 catalysts, it is obviously found that the yield of GVL increases rapidly from 10.3% to 81.0% with increasing Pd loading from 0.58 to 1.16 wt.%. The poor catalytic performance of the catalyst with low Pd loading may be caused by the incomplete self-redox reaction, resulting in insufficient active sites and nanocrystal agglomeration (Fig. S1). Further increasing Pd loading from 1.16 wt.% to 5.82 wt.%, the yield of GVL is gradually heightened up to 100%. Moreover, the yield maintains almost constant when the Pd loading surpasses 5.82 wt.%. The superior catalytic performance can be put down to the strong synergistic effects including geometrical and electronic interaction between Pd and CeO2 NPs (Fig. 4), high surface area (Table S1 and Fig. 3), better dispersion and smaller particle size (Table 1 and Fig. 1) as well as modified surface electronic structure (Table S2, Figs. S4 and S5). Moreover, the Pd(5.82)/CeO2 −1 catalyst with the highest TOF value (34.5 molLA·mol−1 ) among all Pd h investigated Pd/CeO2 catalysts further confirms its outstanding LA hydrogenation activity. For comparison, both of s-Pd/CeO2 and commercial Pd/C catalysts exhibit poor activity, possibly owing to the larger particle sizes and weak interaction between Pd NPs and support. In addition, when the self-assembled Pd/CeO2 catalyst is further reduced by

H2 gas at 673 K, the activity of the further-treating catalyst deteriorates obviously to 40.1% yield of GVL (Table 1, entry 6), which is very close to the conventional prepared s-Pd/CeO2 catalyst (Table 1, entry 7). Besides the Pd loading amount, the activity of the self-assembled Pd/CeO2 catalyst is also influenced by the synthesis temperature (Table S3) and solvents (Table S4). As shown in Table S3, 348 K is the optimal synthesis temperature because the redox reaction of Pd and Ce precursor is sensitive to synthesis temperature. In Table S4, one can find that the catalyst shows an outstanding catalytic activity in water and isopropanol rather than in methanol, ethanol and nonpolar toluene. Interestingly, the selectivity toward target product of GVL can remain at ~ 100% in all the solvents. To further evaluate the recyclability of Pd(5.82)/CeO2 catalyst, the recycling measurements were carried out at low conversion level (around 30%). It is noteworthy that the GVL is still the sole product in each cycle, indicating high GVL selectivity over the Pd(5.82)/CeO2 catalyst. Further, the test results demonstrate that only slight loss of GVL yield (ca. 4%) has been detected after five continuous runs (Fig. 5). The crystal phase, dispersion and particle size of Pd(5.82)/CeO2 catalyst are almost unchanged after five continuous recycling (Fig. 6), demonstrating good applicable stability of this hybrid material.

Fig. 6. (left) TEM and (right) distribution of particle sizes for fresh and spent of Pd(5.82)/CeO2 catalyst (five recycling). Reaction condition: LA (20 mg), catalyst (15 mg), 2-PrOH (5 mL) as a solvent, at 363 K, 4 bar H2 and 90 min or 10 min.

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3.3. Plausible mechanism of LA hydrogenation There are multiple pathways for the transformation of LA into GVL (Fig. S6). Generally, LA can be hydrogenated to γ-hydroxyvaleric acid (γ-HVA) and subsequently undergoes ring-closing by intramolecular esterification or LA dehydration to generate α or β-angelica lactone (AL) followed by hydrogenation into GVL. GVL is probably generated through a γ-HVA mediated pathway under the condition of reaction temperature below 423 K [29,30]. Based on previous reports [15,30], one plausible explanation for this path has been put forward, as shown in Scheme 1. The substrate of LA and H2 molecule can be attracted and captured on the surface of electron-rich Pd simultaneously by hydrogen bonding between hydrogen with Pd and carbonyl group of LA [15,31]. The adsorbed H2 can be easily dissociated as active hydrogen on the surface of Pd and reacted with adsorbed LA to form an intermediate of γ-HVA. Meanwhile, the defect sites of CeO2 play an important role in subsequent intra-esterification of the intermediate into GVL. Therefore, the catalyst with sufficient defect sites displays higher LA hydrogenation activity. The effect of defect sites of CeO2 helping esterification has also reported in dehydration of diols over pure CeO2 and Ni/CeO2 catalysts [32,33]. 4. Conclusions In summary, we developed series of self-assembled Pd/CeO2 catalysts prepared by a facile one-pot redox method for evaluating the catalytic performance of liquid-phase LA hydrogenation into GVL. The experimental results demonstrate that Pd(5.82)/CeO2 exhibits superior LA hydrogenation performance and recycle stability under the mild conditions. The outstanding catalytic performance can be ascribed to high surface area beneficial to the exposure of Pd catalytic active sites and increase of electron density of Pd surface donating from CeO2 favorable for the dissociation of hydrogen molecule. In addition, the intra-esterification process of intermediate to target product can be promoted in the presence of defect sites of CeO2. Eventually, the probable pathway and mechanism of LA hydrogenation in our experiment are also illustrated briefly. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant No. 51372248, 51432009 and 51502297), the CAS/SAFEA International Partnership Program for Creative Research Teams of Chinese Academy of Sciences, China, the CAS Pioneer Hundred Talents Program, and the Users of Potential Program (2015HSC-UP006, Hefei Science Center, CAS). Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.catcom.2017.01.008. References [1] S. Xu, H. Sheng, T. Ye, D. Hu, S. Liao, Hydrophobic aluminosilicate zeolites as highly efficient catalysts for the dehydration of alcohols, Catal. Commun. 78 (2016) 75–79. [2] G. Mitran, O.D. Pavel, M. Florea, D.G. Mieritz, D.-K. Seo, Hydrogen production from glycerol steam reforming over molybdena–alumina catalysts, Catal. Commun. 77 (2016) 83–88. [3] C. Li, X. Zhao, A. Wang, G.W. Huber, T. Zhang, Catalytic transformation of lignin for the production of chemicals and fuels, Chem. Rev. 115 (2015) 11559–11624. [4] A. Corma, S. Iborra, A. Velty, Chemical routes for the transformation of biomass into chemicals, Chem. Rev. 107 (2007) 2411–2502. [5] J.M. Tukacs, M. Novák, G. Dibó, L.T. Mika, An improved catalytic system for the reduction of levulinic acid to γ-valerolactone, Catal. Sci. Technol. 4 (2014) 2908–2912.

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