Enhanced production of γ-valerolactone from levulinic acid hydrogenation-cyclization over ZrxCe1-xO2 based Cu catalysts

international journal of hydrogen energy xxx (xxxx) xxx

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Enhanced production of g-valerolactone from levulinic acid hydrogenation-cyclization over ZrxCe1-xO2 based Cu catalysts Harisekhar Mitta a,**, Vijayanand Perupogu a, Rajender Boddula b,***, Srinivasa Rao Ginjupalli c, Inamuddin d,e,*, Abdullah M. Asiri d a

CSIR-Indian Institute of Chemical Technology, Department of Energy & Environmental Engineering Division, Hyderabad, 500007, Telangana, India b CAS Key Laboratory of Nanosystem and Hierarchical Fabrication, National Centre for Nanoscience and Technology, No. 11 Zhongguancun, BeiYiTiao, 100190, Beijing, PR China c Swarnandhra College of Engineering and Technology, Chemistry Division, Narsapur, 534280, India d Chemistry Department, Faculty of Science, King Abdulaziz University, Jeddah, Saudi Arabia e Advanced Functional Materials Laboratory, Department of Applied Chemistry, Faculty of Engineering and Technology, Aligarh Muslim University, Aligarh, India

highlights

graphical abstract


Monodisperse Cu2O nanocrystals prepared by thermal decomposition of copper acetate.
Zr to CeO2 had a significant influence surface area and enhanced acido-basic sites.
Loading of Cu in Zr0.8-Ce0.2 catalysts

by

colloidal-deposition

method.
Small

Cu particles have been

found to exhibit superior catalytic performance for GVL synthesis.
88.5% conversion of LA and 94.2% selectivity of GVL were obtained at 260  C for 12 h.

article info

abstract

Article history:

The production of high-value renewable fuel and industrial feedstocks from low-value

Received 14 May 2019

carbon sources is an ongoing scientific challenge. Herein we synthesized bifunctional

Received in revised form

ZrxCe1-xO2 with different zirconia/ceria (Zr:Ce) ratios ranging from 1  x  0, were

* Corresponding author. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (H. Mitta), [email protected] (R. Boddula), [email protected] ( Inamuddin). https://doi.org/10.1016/j.ijhydene.2019.11.149 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Mitta H et al., Enhanced production of g-valerolactone from levulinic acid hydrogenation-cyclization over ZrxCe1-xO2 based Cu catalysts, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.149

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international journal of hydrogen energy xxx (xxxx) xxx

29 September 2019

deposition with Cu content and characterized various spectroscopic techniques. Their

Accepted 13 November 2019

catalytic performance was investigated in the hydrogenation-cyclization of levulinic acid

Available online xxx

(LA) to g-valerolactone (GVL) in a fixed bed reactor operating at 0.5 MPa, here hydrogen source is formic acid that showed a tremendous interest in the chemical industry. From

Keywords:

catalysts characterizations demonstrated that achieve porous structure, enhanced support

Zirconia/ceria

surface area and increased surface acido-basic sites which should be influenced by the

Levulinic acid

proper zirconia content incorporated into cerium lattice. Interestingly the catalytic per-

g-valerolactone

formance of different copper loaded catalysts was examined and 3Cu/Zr0.8-Ce0.2 showed

Hydrogenation-cyclization

the highest LA conversion of 88.5% and maximum GVL selectivity to 94.2%. The catalyst

Acido-basic sites

stability, reusability and structure-activity relation were also described. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction Gamma-valerolactone (GVL), important basic synthetic blocks in the chemical industry, are widely used as a green solvent in fine chemicals synthesis, food additives, precursor of diesel and gasoline fuels [1]. GVL is an intermediate for the production of number of value-added chemicals such as a-methylene-g-valerolactone, ε-caprolactam, and adipic acid [2]. In this respect, catalytic hydrogenation-cyclization of levulinic acid (LA) to g-valerolactone (GVL) has recently attracted increasing attention owing to the promise for the future industrial applications [2] (see Scheme 1). Conventional, LA hydrogenation-cyclization has been studied through on transitions (non-noble) metals like Cu, Ni, and etc., using external hydrogen source [3e6]. Generally, external hydrogen source is not easy to handle while experiment. However, recently few groups have been progressed for producing GVL using an internal hydrogen source instead of the external in a homogenous medium [7e9]. Although, the homogenous catalyst systems apparently have some serious drawbacks in recovery, catalyst synthesis and recycling for applications. The literature information regarding the LA hydrogenation under formic acid used as hydrogen carrier under the heterogeneous is very limited [10,11]. The heterogeneous

LA hydrogenation to GVL is a desirable considering the characteristic of high efficiency. As per the several reports, specific number of heterogeneous catalysts were studied in this reaction. Most of the heterogeneous catalysts are on noble based catalyst like Ru [12], Pd [13] and Au [14]. However, disadvantage of noble based catalysts are extremely high precise, hard manufacture and abstraction to poison. At present, transitions metal-based catalysts were also found to be similar catalytic performance and also inexpensive compared to noble metals. In the current situation, as the demand of copper-based catalysts is predominantly more effective for the production of GVL. However, copper catalysts either in a bulk form or as supported materials have shown good activity. Lomate et al. [15] have reported in gas phase LA hydrogenation over 6 wt% CueSiO2 catalyst has shown LA conversion to 80% and GVL selectivity to 66% under atmospheric pressure using FA as a H2 source. Zhang et al. [16] demonstrated that direct the reaction pathway for hydrogenation of levulinic acid to g-valerolactone for Clean-Energy Vehicles over a magnetic CueNi catalyst. Mohan et al. [17] have investigated on alumina, magnesium oxide and hydrotalcite based Ni catalysts for vapor phase LA hydrogenation using FA as a H2 source. Upare et al. [18] also reported in the vapor phase LA hydrogenation using 20Nie60CueSiO2 catalysts with 99% of LA conversion and 96% GVL selectivity at

Scheme 1 e The schematic diagram to illustrate preparation of Cu nanocrystal supported on Zr0.8-Ce0.2 catalysts.

Please cite this article as: Mitta H et al., Enhanced production of g-valerolactone from levulinic acid hydrogenation-cyclization over ZrxCe1-xO2 based Cu catalysts, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.149

international journal of hydrogen energy xxx (xxxx) xxx

260  C under FA as hydrogen donor. Dhanalaxmi et al. [19] demonstrated on Nanohybrid PdeFe3O4/PPTPA-1 catalyzed for the LA hydrogenation to GVL using FA as H2 source. Moreover, FA can be used as an efficient and reversible storage for H2 even under ambient condition. Redondo et al. [20] have reported over FA decomposition clearly. Recently, a multifunctional catalyst has been a great interest in the synthesis of ZrxCe1-xO2(ZrxCe1-x) bifunctional catalyst. It provides high redox performance, thermally more stable, the maximum surface area and acido-basic sites of the catalysts. In case of Zr0.8-Ce0.2based catalysts, after introducing zirconia on ceria lattice, the resultant -Zr-O-Ce-O-Zrnetwork not only rendered adjustable porosity including larger surface area but also enhanced surface acidity. In view of their excellent properties of this material derived from Zr/Ce mixed oxides, this could be probably good alternatives for producing GVL. Recently several researchers reported on Zr based materials used for Meerwin-Ponndorf-Verley (MPV) reduction over LA hydrogenation-cyclization [21]. Li et al. [22] also demonstrated that applied Zr-graphitic carbon nitride complex as a catalyst for the MPV reactions. Yang et al. [23] also observed similar phenomena on TixZry sample (with different Ti/Zr molar ratios) for producing of GVL using 2propanol as hydrogen donor. Melero et al. [24] notifiable on ZreAl based Beta zeolite catalysts used for LA hydrogenation under 2-propanol. Jian et al. [25] demonstrated on AleZr based samples for ELA hydrogenation-cyclization to GVL. Although heterogeneous catalysts based on non-noble metals are preferred for the GVL production, the catalysts usually suffer from some obvious disadvantages such as low activity and selectivity. It is highly desirable to develop nonnoble metal catalysts for this reaction with high activity and selectivity. In this work, we prepared Cu/ZrxCe1-xO2 catalysts with various compositions. The catalytic performances of the catalysts for the LA hydrogenation-cyclization n into GVL using formic acid as an internal hydrogen source were investigated. It was shown that 3Cu/Zr0.8-Ce0.2 achieved outstanding catalytic performance in this reaction (GVL selectivity: 94.2%). The excellent catalytic performance is related to the high specific surface area, large pore volume and well-defined acido-basic properties of the ZrxCe1-xO2 supports, and active Cu sites also contributed to the enhanced catalytic properties. This implies, ZrxCe1-xO2 based copper catalyst will be performed better catalytic activity performance compared previously reported mixed metal oxides. In this perspective, no one has been reported yet on porous Zr0.8Ce0.2 based copper catalysts in hydrogenation-cyclization of LA in vapor phase using formic acid as an internal hydrogen source.

3

obtained from Merck chemicals (Hyderabad, India). Cerium (III) nitrate hexahydrate, zirconium (IV) nitrate hydrate, copper (II) nitrate trihydrate were purchased from Aldrich chemicals, India.

Catalyst synthesis The Zr1-xCexO2 supported Cu catalysts were prepared by a multi-step process. First Zr1-xCexO2 support synthesized by the hydrothermal process. Aqueous solutions including cerium (III) nitrate hexahydrate (99%, Aldrich) and zirconium (IV) nitrate hydrate (Aldrich) solution were dissolved into 50 cm3 distilled water and precipitated by the adding an aqueous ammonia at room temperature along with constant stirring to attain a pH to 9, subsequently the mixture was stirred for 5 h at same temperature. Next the slurry transferred into a 100 mL Teflon lined stainless-steel autoclave, heated to 100  C and kept it at same temperature for 24 h. Finally obtained precipitate was filtered by the hot distilled water, ethanol and next vacuum dried at 50  C for overnight followed by the calcined at 500  C for 5 h under static air condition. Various zirconia/ceria molar ratios (Zr1-xCexO2, x ¼ 0, 0.5, 0.8 and 1) supports were synthesized by a coprecipitation method but varying the contents of the starting materials.

Synthesis of Cu colloidal solution The copper colloids have been synthesized by thermal decomposition adapting a procedure described in the literature [26]. We used of Cu (I) acetate in instead of Cu (II) acetate as described in [26] because only Cu (I) acetate could be obtained as water-free salt. Copper (I) acetate (0.760 g) was dissolved in trioctylamine (14.8 mL) and oleic acid (1.7 mL) and then quickly heated to 180  C under nitrogen because the presence of oxygen or water is highly undesirable during the decomposition and nucleation stage. The color gradually changed from forest green to coffee brown before finally producing a gray colloid at this temperature. The solution was kept at 180  C for 1 h and then quickly heated to 260  C kept for 45 min. However, the gray colloid gradually changed to a deep burgundy solution. The Cu nanocrystals were precipitated by ethanol (300 mL) and separated through centrifugation in the range of 9000 rpm for 8 min. Then, the Cu nanoparticles were washed with 300 mL ethanol (2 times) and re-dispersed in the ethanol solution (5e10 mL). The colloid size was controlled by simply varying the experimental conditions and the exact size was determined by TEM analysis.

Synthesis of catalyst

Experimental section Materials and chemicals Copper acetate (I) (Cu (CH3COO), 99%, Aldrich), Tri-octylamine ((C8H14)3N, 98%, Aldrich) and Oleic acid (C18H34O2, Aldrich, 99%), Ethanol (C2H5OH, 99.87%, Analytical grade), Hexane (C6H6, 99%, HPLC grade) and cyclohexane (C6H12, 99%) and Nitrogen (N2, 99.99%). Levulinic acid (LA) and Formic acid were

CuO/Zr0$8Ce0$2O2 catalysts were prepared with various Cu loading (from 3 to 15 wt%) using colloidal solutions of different Cu NPs by a colloidal-deposition method, Thereafter, 1.8 g Zr0$8Ce0$2O2 was dispersed in 40e50 ml of ethanol and Cu NP colloidal solution corresponding with various weight percentage of Cu colloidal was added. The following suspension was stirred under reflux at 70e80  C (heating ramp rate, 2  C min1) for 4 h. The solid was subsequently collected by filtration, followed by drying at 50  C overnight under vacuum

Please cite this article as: Mitta H et al., Enhanced production of g-valerolactone from levulinic acid hydrogenation-cyclization over ZrxCe1-xO2 based Cu catalysts, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.149

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and calcination at 300  C (heating ramp rate, 2  C min1) in air for 5 h. This procedure results in Cu/Zr0$8Ce0$2O2 samples (see following TEM images). The samples were subsequently characterized. Furthermore, the samples were loaded in a reactor, reduced, and tested for their performance in GVL synthesis.

Catalytic reaction A traditional fixed-bed reactor run at 0.5 MPa pressure and made by the stainless-steel reactor which used for vapor phase hydrogenation-cyclization of LA, here formic acid (FA) as an internal hydrogen donor. For a typical run, 3Cu/Zr0.8Ce0.2 catalyst sample (500 mg, 40e60 mm particle size) diluted with an equal amount of glass bead was introduced into the middle of reactor. Prior the reaction, the sample must be reduced in situ at 220  C for 3 h under a 1%H2eAr flow (99.9%, 50 mL min1), heating ramp rate of 2  C min1. After the reduction, the reactor was cooled down to room temperature in 1% H2/Ar under the same flow. Then the vaporized feed was switched to the reaction mixture of LA and FA (molar ratio, 1:4) solution with a H2 flow rate of 20 ml min1and the pressure was raised to 0.5 MPa. Then the temperature was gradually raised to 265  C at a heating ramp rate of 2  C min1. The reaction was performed in the temperature range of 220e300  C. The obtained products collected in an ice-cold trap for each hour and analyzed through gas chromatography (Shimadzu, GC-2014) via flame ionization detector (FID) using a DB-wax column (30 m  0.32 mm width) at a ramping rate of 10  C min1 from 60 to 280  C. The confirmation to the liquid products was carried by using HP-5973 quadruple GC-MS equipped with HP-1MS capillary column (15 m  0.25 mm) with EI mode. The LA conversion Eq. (1) and desired product selectivity Eq. (2). LA Conversion ð%Þ¼

½moles of LA in  ½moles of LA  out X 100 ½moles of LA in (1)

GVL Selectivity ð%Þ¼

½moles of GVL out X 100 ½moles of LA in  ½moles of LAÞ out (2)

using N2O titration method Initially, the catalyst sample loaded in the sample holder and reduced at a 220  C about 3 h then dropped to 60  C. 5%N2O/He (flow amount 50 mL min1) was passed over the sample until saturation. Metallic Cu was oxidized to Cu2O through the decomposition of N2O. Definite number of Cu atoms over the surface measurement using a molar stoichiometry ratio i.e. N2O/Cu ¼ 0.5. The metal area, metal dispersion and Cu particle size measured from following Eqs. (3)e(5).
 Dispersion ð%Þ ¼ N2 O uptake mmole=gcat 
  100 total Cu metal mmole=gcat Size of particle ðnmÞ ¼ 6000

(3)


 Cu metal area m2 g M1  r (4)

Metal area ¼ metal cross  sectional area  No: of Cu atoms per area of surface

(5)

3

Whereas, r ¼ Cu density ¼ 8.94 g cm. Acido-basic properties of catalysts performed through the TPD of NH3/CO2 technique using a Micromeritics 2920 equipment. Prior to adsorption, the sample activated at 120  C for 12 h under a helium flow and then reduced at 220  C for 3 h under 1%H2eAr of a 50 mL min1. Subsequently, samples were saturated to NH3/CO2 gases at 60  C for 1 h. Prior to TPD analysis, physisorbed NH3/CO2 removed by stripping with He flow at 120  C about 2 h (flow rate 50 mL min1) to clean the catalyst surface. The NH3/CO2 analysis was run between 25 and 800  C (ramp scale, 10  C min1). To study property of being reducible of the catalyst’s executive from H2-temperature programmed reduction (H2-TPR) method on a Micromeritics 2920 instrument. Before to analysis, the sample activated under He flow to 50 mL min1 at 120  C about 2 h to move out moisture then drop to room temperature. TPR analysis was recorded from 50 to 650  C (ramp rate of 5  C min1) by using a 5% H2/Ar, amount of gas flow to 50 mL min1.

Results and discussion

Catalyst characterizations techniques

Characterization techniques

Powder diffraction (XRD) lines was performed on a Rigaku Xray diffractometer which provided by using the Ni-filtered Cu Ka radiation and the crystalline phases Identification confirm by JCPDS file. The exact Cu amount was determined from ICPAES on optima 7300 DV Instrument (Perkin Elmer, USA). Scanning electron microscopy provided energy dispersive spectrometer (SEM-EDS) images of the samples were obtained on Hitachi-S-520 unit. Morphology and average particle size of the samples was recorded on a JEOL 2010 operated 200 KV and the average Cu particle diameters were estimated on a nanoscale measurer 1.2. The samples specific surface area calculated from a Brunauer-Emmett-Teller (BET) using a N2 adsorption-desorption method which recorded at 77 K on ASAP 2020 instrument (Micromeritics, USA). The Cu metal surface, metal dispersion and average particle size measurement

The XRD results of pure Zr0.8-Ce0.2 and various Cu/Zr0.8-Ce0.2 samples were shown in Fig. 1. The crystallite size of CuO in each sample presented in Table 1. From XRD patterns of pure Zr0.8-Ce0.2, the diffraction peaks at 28.6 , 33.3 , 47.5 , 56.2 are assigned (111), (220), (200), and (311) index planes, respectively [27,28]. Which are the characteristic diffraction peaks to cubic fluorite structure of Zr0.8-Ce0.2 (PDF-ICDD No. 28e0271). However, no distinguishable XRD characteristic reflections of CeO2, ZrO2 detected in Zr0.8-Ce0.2, samples. Their results indicated that the formation of monophasic Zr0.8-Ce0.2 solid material with a cubic fluorite structure. In the case of the Zr0.8Ce0.2 supported Cu sample, no diffraction lines could be discover related to CuO, at lower Cu loading (3 wt% Cu). However, the absence of CuO diffraction lines inform the formation of extremely dispersed copper species. And more

Please cite this article as: Mitta H et al., Enhanced production of g-valerolactone from levulinic acid hydrogenation-cyclization over ZrxCe1-xO2 based Cu catalysts, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.149

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Fig. 1 e XRD patterns of pure Zr

0.8Ce0.2

and various Cu/Zr

0.8Ce0.2

based catalysts.

Table 1 e Physico-chemical properties of Zr0.8Ce0.2 support and Cu loaded/Zr0.8Ce0.2catalysts. Texture propertiesa,b Cu (wt.%) 0.0 3 7 10 15 a b c

N2O decompositionc

BET-surface area (m2g-1)a

Pore-diameter (nm)b

Pore volume (ccg-1)b

Metal dispersion (%)c

Metal area (m2g-1)c

ICP Cu (wt.%)

120 105 84 80 61

33 32.5 32.7 32.8 32.5

0.0873 0.0783 0.0769 0.0734 0.0583

0 24.8 17.1 11.3 10.1

0 160.1 110.3 73.1 65.3

0 2.7 6.5 9.3 14.6

BET surface area was calculated from nitrogen adsorption-desorption isotherm. Pore diameter and Pore volume were estimated by BJH method. Cu dispersion, metal area and particle size (nm) calculated by the N2O decomposition.

intense diffraction reflections corresponding to CuO, 2q values 35.3 , 38.6 and crystal lattice indices (002), (111) are visible while increasing copper loading (10 wt%) (JCPDS No. 800076). Therefore, these results suggested the maximum dispersed CuO formed at lower Cu loading [29,30]. Further, calculate the average CuO crystallite sizes through the Scherer equation whereas average CuO size range between 4.8 and 13.5 nm, results are demonstrated in Table 1 in detail. The scanning electron microscopy (SEM-EDS) analysis, morphology and the elemental composition of pure Zr0.8-Ce0.2 and various Zr0.8-Ce0.2 supported copper samples have shown a plate-type structure are shown Fig. 2. Actual compositions of various Cu loaded Zr0.8-Ce0.2 samples observed through ICPAES analysis which are adjacent to the theoretical values and results presented in Table 1. The transmission electron microscopy (TEM) images and particle distributions of various Cu loaded Zr0.8-Ce0.2 samples

are display in Fig. 3. 3Cu/Zr0.8-Ce0.2 consist of Cu particles are homogeneously spread throughout the Zr0.8-Ce0.2 surface where average Cu particle size found to be less than 5 nm. In addition, at higher Cu loadings i.e.  7 wt%, increased an average size of the Cu particle from 6.1 to 9.5 nm. It has to be noted that the average Cu particle size within the series of Cu/ Zr0.8-Ce0.2 samples increases with increasing surface Cu loading. These findings well correlated with results of XRD and N2O titration. N2O decomposition data of various copper loaded Zr0.8Ce0.2 samples presented in Table 1. In the principle, Cu dispersion, metal surface area and particle size can be estimated according to my previous reports [26,29]. From Table 1, 3Cu/Zr0.8-Ce0.2sample observed that maximum amount of active copper oxide phase dispersion (43%) and the highest metal surface area (386 m2/g Cu) being to a large extent covered by small Cu particles over Zr0.8-Ce0.2 surface

Please cite this article as: Mitta H et al., Enhanced production of g-valerolactone from levulinic acid hydrogenation-cyclization over ZrxCe1-xO2 based Cu catalysts, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.149

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Fig. 2 e SEM images of pure Zr

0.8Ce0.2

and various Cu loaded Zr

0.8Ce0.2

catalysts.

Please cite this article as: Mitta H et al., Enhanced production of g-valerolactone from levulinic acid hydrogenation-cyclization over ZrxCe1-xO2 based Cu catalysts, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.149

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Fig. 3 e TEM images and amount of Cu particles distribution of fresh various Cu loaded Zr

Fig. 4 e N2 adsorption-desorption isotherms and pore diameter for pure Zr loaded Zr0·8Ce0.2 catalysts.

0.8Ce0.2

0.8Ce0.2

catalysts.

support (insert figure) and various Cu

Please cite this article as: Mitta H et al., Enhanced production of g-valerolactone from levulinic acid hydrogenation-cyclization over ZrxCe1-xO2 based Cu catalysts, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.149

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respectively. Further, decreased from 12 to 8% at high Cu loadings (7 wt%) supposed to be mainly due to enhancement of the Cu particle size. The N2 isotherms curves and BJH distributions of pure Zr0.8-Ce0.2 and various Cu loaded Zr0.8-Ce0.2 samples displayed in Fig. 4(a) and average pore diameter, pore volume and specific surface areas presented in Table 1. As can be observed that all the samples found to be characteristic of the Type IV isotherms, a hysteresis loop a significantly nitrogen uptake between 0.35 and 0.80 of relative pressure, which is relatively typical a mesoporous material as evidence from IUPAC classification [25,27]. Notably, pure Zr0.8-Ce0.2 sample to show maximum specific surface area (120 m2 g1) with low porediameter 3.3 nm. After Cu introduced into Zr0.8-Ce0.2 support, the specific surface areas, pore diameter and pore volume significantly varied from 105 to 61 m2. g1, 0.0873e0.0583 cm3 g1 and 3.2e3.3 nm, respectively. This may due to active copper phase might be settle down on the pore channels which leads directly affected to textural properties of the samples. Fig. 5, an information of various Cu oxidation states and surface co-ordinations along with finding the d-d transitions and Oxygen-Cu ion charge transfer (CT) bands of various Cu loaded Zr0.8-Ce0.2 samples. Pure Zr0.8-Ce0.2 sample can be resolved two bands i.e. between 250 and 290 nm may be assigned to Ce3þ ) O2 and Ceþ4 ) O2 charge transfer transitions and around 320e350 nm is assigned to Zrþ4 ) O2 transition of the substituted fluorite structure, respectively [28]. When an enhancing copper loading on Zr0.8-Ce0.2 surface third adsorption band observed strongly, due to d-d transitions in Cuþ2 ions situated in the distorted octahedral

environment. Therefore, these results suggest that third band has appeared clearly at higher Cu loadings, as evidenced by the XRD analysis. The H2-TPR profiles of pure Zr0.8-Ce0.2 and various Cu loaded Zr0.8-Ce0.2 samples are illustrated in Fig. S1(supporting information). As it can be observed, all the samples hold the three distinct reduction peaks at ~116, 189 and ~218  C. At 3 wt % Cu/Zr0.8-Ce0.2exhibited two reduction peaks, one is lower temperature at 189  C (shoulder peak, denoted as a b) and the other is at higher temperature at 218  C (the main peak, denoted as a g), which were hold the reduction of highly dispersed surface CuO species neither in clusters or as an isolated (Cuþ2/Cuþ) [29,31]. While Cu content increases (i.e. above 7 wt %), resulting three resolved reduction peaks; extra peak (with a small peak, denoted as a a) at ~ 140  C was observed. However, during the reduction of ceria, coordinatively bind the unsaturated surface oxygen anions or diffusion of lattice oxygen to the surface can be removed easily even at low-temperature region, for bulk oxygen species were reduced at high temperatures, respectively [32]. Shen et al. [33] have reported similar phenomena on redox properties of ceria-zirconia nanorods. Therefore, these results are well correlate with XRD, UV-DRS analysis. The type of acidic strength of pure Zr0.8-Ce0.2 and various Cu loaded Zr0.8-Ce0.2 samples were studied by NH3-TPD and obtained NH3 profiles are shown in Fig. S2(a) (Supporting Information). The quantitative estimation of acid strength presented in Table S1(Supporting Information). From Fig. S2(a), the NH3-TPD profiles were distributed in two regions; i.e. at 138  C, ascribed the weak acidic strength and at 300  C, ascribed the moderate acidic strength, respectively. As it can

Fig. 5 e UV-DRS patterns of pure Zr0·8Ce0.2 and various Cu loaded Zr

0.8Ce0.2

catalysts.

Please cite this article as: Mitta H et al., Enhanced production of g-valerolactone from levulinic acid hydrogenation-cyclization over ZrxCe1-xO2 based Cu catalysts, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.149

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be observed, total surface acid strength (copper ions or OH) of the samples slightly increases from 0.152 to 0.187 mmol NH3 g1.cat, while increases the Cu loading from 0 to 3 wt% and then further higher Cu loadings decreases significantly from 0.158 to 0.117 mmol NH3 g1. In order to investigate the total basic strength of various Cu loaded Zr0.8-Ce0.2, as shown in Fig. S2(b). The CO2 uptake values presented in Table S1. From Fig. S2(b), all Zr0.8-Ce0.2 based copper samples hold the two main desorption band at 50e100  C, was assigned to physisorption and second desorption band at around 200e340  C was attributed to the pair). In addition, moderate chemical adsorption (i.e. - O2 2 third desorption band in the region between 400 and 600  C assigned to the strong chemical adsorption of CO2 [34,35]. As can be seen in Table S2, for pure Zr0.8-Ce0.2 exhibit less basic strength as compared to the various copper impregnated Zr0.8Ce0.2 samples. When Cu loadings enhance from 0 to 3 wt%, total basic strength raised from 0.82 to 0.92 mmol CO2 g1 cat. and then subsequently decreased further higher Cu loadings i.e. above 7 wt%, due to bulk CuO phase covered on surface of support. Overall, From CO2-TPD results, the 3 wt% Cu/Zr0.8Ce0.2 sample expose maximum amount of surface basic strength (0.92 mmolg1) with high Cu metal surface area. Overall, the results demonstrate that the Cu component introduced to material can improved the surface acido-basic strength of the catalyst.

Besides Angelica lactones (ALs) and valeric acid (VLs) increased slightly. These results indicated that the Cu addition may bring two advantages i.e. the new hydrogenation active site from well-dispersed Cu nanoparticles with smaller size (3.5 nm) along with exhibit appropriate acido-basic sites (Table S1) could be beneficial to enhance the activity. However, the further increase of Cu led to a lower GVL selectivity of 71.3% at LA conversion of 48.2% (Table 2). It is probably due to excessive addition of Cu that resulting in the coverage of larger metallic Cu species and thereby decreased the GVL selectivity. In addition, the well-dispersed active Cu particles and acido-basic sites were crucial to the high activity of the Cu-modified Zr0.8-Ce0.2 catalyst. Jian et al. [25] have also observed similar phenomena for catalytic transfer hydrogenation-cyclization of ethyl levulinate to GVL due to strong synergetic between acido-basic properties and metallic sites of catalyst by zirconia-based mixed oxides. Hu Li et al. [36]. have also observed same phenomena on acid-base bifunctionalized Fe-ZrOx nanocatalysts used for conversion of EL to GVL. Liu et al. [37] reported similar phenomena over Ru/C on hydrogenation reaction of LA. Some other researchers have also demonstrated same phenomena [38,39]. Therefore, these results demonstrated that the 3Cu/Zr0.8-Ce0.2 catalyst was the optimum catalyst for LA hydrogenation-cyclization to GVL, which showed 94.2% GVL selectivity in 30 h at 260  C using FA as a hydrogen source.

Catalyst screening

Reaction conditions optimization

The catalytic performance of the catalysts for the conversion of LA to GVL was carried out in using FA as the H-donor in 1,4 dioxane solvent. The better solubility of LA in FA solvent that is beneficial for the mass transfer during reaction. When Zr0.8Ce0.2 catalyst was performed to the LA hydrogenationcyclization reaction at 260  C under 0.5 MPa, the reaction gave 18% of LA conversion and no selectivity to GVL (Table 2). The introduction of 3 wt% Cu on Zr0.8-Ce0.2 gradually increased the LA conversion to 88.2% with an obvious increase of GVL selectivity by 94.2%. As the Cu weight percentage increased from 7 to 15 wt% in the Zr0.8-Ce0.2 catalysts, lower GVL selectivity was obtained, 73% and 48.2% for 7Cu/Zr0.8Ce0.2 and 15Cu/Zr0.8-Ce0.2 catalyst, respectively (Table 2).

The reaction temperature, WHSV and tuning Zr/Ce ratio have a large effect on the reaction rate. Thus, the catalytic LA hydrogenation-cyclization was investigated at different reaction temperature (ranging from 220  C, 240  C, 260  C, 280  C and 300  C, different WHSVLA (ranging from 0.1, 0.2, 0.5- and 1mL h1.) (Fig. 6 (b) and different zirconia/ceria volume ratio (ranging from ZrxCe1-xO2, X ¼ 0, 0.5, 0.8 and 1) (Fig. 6(c)).

Table 2 e Activity results f LA hydrogenation-cyclization using Zr0.8-Ce0.2 and various Cu loaded Zr0.8Ce0.2catalystsa. Copper loading (wt.%)

Conver. Of Levulinic acid Selectivity (%) (LA) (%) GVL AL Others e 88.5 73 65 48.2

Zr0.8-Ce0.2 3 7 10 15 1

e 94.2 84 75 71.3 1

e 3 10 14 16

e 3 1 1 e

H2 flow rate ¼ 30 mL min , WHSV ¼ 0.456 h , LA ¼ Levulinic acid (LA), g-Valerolactone (GVL), Valaeric acid (VA), Angelica lactone (AL) and others. a Reaction conditions: 0.5 g of catalyst, reaction temperature ¼ 260  C, pressure 0.5 MPa H2.

Effect of reaction temperature When the reaction carried out at a milder temperature (220 & 240  C) only  64% selectivity of GVL with a LA conversion of 58% was achieved (Fig. 6(a)). The LA conversion obviously increased to 88.2% when the reaction temperature was increased to 260  C. With the further increase of the reaction temperature, the LA conversion continuously increased until a maximum of 100% and decrease GVL selectivity to 78% was obtained at 300  C. However, the GVL product might undergo some side reactions and generate the other products such as valeric acid and angelica lactone. Therefore, the reaction temperature was kept at for 260  C the following reaction.

Effect of WHSVLA The LA hydrogenation-cyclization was also investigated to the effect of WHSVLA at 260  C under 0.5 MPa, as shown in Fig. 6(b). With the increase of LA feed flow rates from 0.1 to 1-mL h1, the LA conversion decreased from 95% to 38%, consequently GVL selectivity slightly increased from 89.6 to 94.2. It is may be due to the more amount of LA molecules have been passed

Please cite this article as: Mitta H et al., Enhanced production of g-valerolactone from levulinic acid hydrogenation-cyclization over ZrxCe1-xO2 based Cu catalysts, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.149

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Fig. 6 e a) Effect of Reaction Temperature b) Effect of LA conversion c) Time on stream (TOS) during on hydrogenationcyclization of LA activity and d) The effect of Cu loading over Turn over frequency (TOF) of 3Cu/Zr0·8Ce0.2 catalyst.

through the catalyst surface for adsorption. At 0.457 h1 WHSV, the GVL selectivity reached the maximum value.

Tuning Zr/Ce ratio Table S2 displays the activity results of various zirconia/ceria ratio of ZrxCe1-xO2 supported Cu catalysts carried out at 260  C under 0.5 MPa. Here the nominal wt.% of Cu loading is 3 wt% for all the samples Interestingly. 3Cu/Zr0.8Ce0.2 based copper catalyst exhibited the highest GVL selectivity (94.2%) and LA conversion (88.2%) among all samples. The performance of those samples decreased while decreasing ratio of Zr/Ce. When the Zr/Ce ratio around 4, the catalyst showed highest catalytic performance due to provides higher surface area (105 m2. g1) with low pore diameter (3.2 nm) and appropriate acido-basic sites (presented in Table S2) (supporting information). Therefore, the results of the 3Cu/Ze0.8Ce0.2 properties correlate well the catalytic activity towards GVL production.

Effect of time on stream and catalyst reproducibility studies Interestingly, most of the heterogeneous catalysts performed in vapor phase conditions which are endured with deactivation due to the deposition of carbonaceous species or larger particle size over catalyst surface. 3Cu/Zr0.8-Ce0.2 catalysts

were also suffered with deactivation but comparable with other catalysts studied at same this condition, due to these were less efficiently adsorbed carbonaceous species on surface of the catalyst. Therefore, the time on stream results was tested over 3Cu/Zr0.8-Ce0.2catalysts under optimized reaction conditions (at 260  C, 0.5 MPa). The reaction reached rapidly increased to 95% of GVL in 1 h and the LA conversion gradually increased to 88.2% as the reaction proceeded to 11 h. However, further prolonging the reaction time resulted in the slightly decrease of GVL selectivity. Then, this sample was successfully run 30 h (depicted in Fig. 6 (c)). To investigate further the stability of the 3Cu/Zr0.8-Ce0.2, which recovery and reuse test for the LA hydrogenationcyclization reaction was carried out under the optimized conditions (catalyst 0.5 g, 260  C, FA, 0.5 MPa, 30 h. After 30 h run, the catalyst was regenerated by the treating with air flow of 30 mL min1 at 300  C for 3 h to remove the weakly adsorbed carbon species. The results are presented in Table 3. As can be observed that though the catalyst activity showed a slow decrease, LA conversion slightly decreased from 88.2 to 71.2, with a maximum GVL yield of 82%. To figure out the structurally change of the catalyst during the hydrogenation-cyclization LA reaction, the spent catalyst 3Cu/Zr0.8-Ce0.2 catalyst after one cycle was collected

Please cite this article as: Mitta H et al., Enhanced production of g-valerolactone from levulinic acid hydrogenation-cyclization over ZrxCe1-xO2 based Cu catalysts, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.149

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Table 3 e Results of physico-chemical properties and activity of fresh and spent catalyst of 3Cu/Zr0.8-Ce0.2. Sample name 3 wt% Cu/ Zr0.8-Ce0.2 Used Spent a b

BET surface area (m2/g)

Total CO2 uptake (mmol/g)a

Total NH3uptake (mmol/g)b

Conver. Of LA (%)

Selectivity of GVL (%)

89 55

0.82 0.37

0.19 0.04

88.5 71.2

94.2 82

Measured from the temperature programmed desorption of NH3 analysis. Measured from the temperature programmed desorption of CO2 analysis.

Fig. 7 e Characterization of the fresh and spent 3Cu/Zr0·8Ce0.2 after 1 cycle: (a) XRD patterns, and (b) N2 adsorptiondesorption and (c&d) TEM images.

and characterized by XRD (Fig. 7(a)) N2 adsorptiondesorption (Fig. 7(b)), TEM (Fig. 7(c)) and CO2/NH3-TPD analysis (Table 3). TEM analysis of the spent 3Cu/Zr0.8-Ce0.2 catalyst showed that average Cu particle size slightly grew up from 4.9 to 6 nm due to the heat treatment prior to recycle experiment. It can be proved by XRD characterization, extra diffraction lines found at 2q ¼ 34.1 , 38.1 77.1 and 44.2 , 50.1 which corresponding to the Cu (þ1), Cu (0), as shown in Fig. 7(a). Another factor that due to the loss of activity of the catalyst could be the changes of acido-basic sites. CO2eNH3 TPD of fresh and spent 3Cu/Zr0.8-Ce0.2 catalyst, acido-basic sites slightly decreased from 0.19 to 0.12 mmol NH3 g1cat and from 0.82 to 0.57 mmol CO2 g1cat respectively. This may due to bulk copper particle covered on Zr0.8-Ce0.2 pores, thus acido-basic sites are inaccessible for the reactants (Fig. S1.). Therefore, as per the above measurements, 3Cu/Zr0.8-Ce0.2 catalyst could be an efficient

for catalytic hydrogenation-cyclization of LA into GVL. Moreover, thoroughly optimized the reaction conditions and preparation method of the catalyst. Fig. 6(d), shows the relation between the Cu loading vs turnover frequency (TOF) for the LA hydrogenationcyclization of 3Cu/Zr0.8-Ce0.2. Activity is expressed in terms of turnover frequency (sec1), whereas TOF is defined as the rate of LA molecules converted per unit time per exposed copper active site. In this study TOF values were calculated based on the Cu active sites per gram of the catalyst determined by using N2O decomposition (Table 1). As can be seen in the results, at 3Cu/Zr0.8-Ce0.2 catalyst exhibited the maximum TOF of 0.0304 sec1. As shown in Fig. 6(d), the TOF value was decreased (from 0.018 sec1 to 0.009 sec1) by the increasing copper loadings (7 wt%), which reveals the presence of an appropriate number of active copper sites on the support and further decreased at higher loaded catalysts.

Please cite this article as: Mitta H et al., Enhanced production of g-valerolactone from levulinic acid hydrogenation-cyclization over ZrxCe1-xO2 based Cu catalysts, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.149

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Proposed reaction pathway of LA hydrogenation-cyclization over Cu/Zr0.8-Ce0.2 catalyst

references

The structure-activity relationship indicated that the production of GVL over a bifunctional 3Cu/Zr0.8-Ce0.2 catalyst proceeds through hydrogenation-cyclization, as shown in Fig. 7. First LA hydrogenation takes place over active Cu sites to produce 4-hydroxy valeric acid which is an unstable intermediate and subsequently ring-closes through intramolecular condensation to produce GVL under acido-basic sites [4,40,41].

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Conclusions Present study described a novel approach to synthesis of 3e15 wt% copper particles deposited on bifunctional Zr0$8Ce0.2 catalyst, studied first time for a GVL production in LA hydrogenation-cyclization reaction, using formic acid as the in-situ hydrogen source and prepared catalysts were characterized through XRD, textural properties, SEMEDS, TEM, H2-TPR, UV-DRS, NH3 and CO2 TPD and N2O titration analyses. Enhanced the catalytic performance due to generate the acido-basic sites which were confirmed by the CO2eNH3 TPD analysis. Among the investigated samples, 3Cu/Zr0.8-Ce0.2 exhibited good performance with high LA conversion of 88% and maximum GVL selectivity of 94.2% at 260  C. The characterization and catalytic results revealed that highly dispersed active copper sites and acido-basic sites promoted hydrogenation through intramolecular condensation favoring the formation of GVL. The catalytic performance differentiated based on the optimizations and upon reaction conditions, then suitable reaction condition should be carried out the best activity results over the most active 3Cu/Zr0.8-Ce0.2 catalyst, at 260  C, 0.5 MPa H2, mL min1 H2 flow rate, WHSV of 0.457 h1. Moreover, the catalyst provided better stability and reused successfully in the consecutive cycle without a larger loss in the activity. Further studies should be focus on the nanomorphology of zirconia-based catalysts, zirconia coated copper particles to be tryout for LA hydrogenation-cyclization.

Acknowledgements This project was funded by the Deanship of Scientific Research (DSR), King Abdulaziz University, Jeddah, under grant No. (DF-650-130-1441). The authors, therefore, gratefully acknowledge DSR technical and financial support. H.M acknowledges the CSIR-Indian Institute of Chemical Technology technical supports.

Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.11.149.

Please cite this article as: Mitta H et al., Enhanced production of g-valerolactone from levulinic acid hydrogenation-cyclization over ZrxCe1-xO2 based Cu catalysts, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.149

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Please cite this article as: Mitta H et al., Enhanced production of g-valerolactone from levulinic acid hydrogenation-cyclization over ZrxCe1-xO2 based Cu catalysts, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.11.149