Direct Conversion of Sugars and Ethyl Levulinate into Î³-Valerolactone

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Direct conversion of sugars and ethyl levulinate into #-valerolactone with superparamagnetic acid-base bifunctional ZrFeOx nanocatalysts Hu Li, Zhen Fang, and Song Yang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.5b01480 • Publication Date (Web): 03 Dec 2015 Downloaded from http://pubs.acs.org on December 10, 2015

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ACS Sustainable Chemistry & Engineering

Direct conversion of sugars and ethyl levulinate into γ-valerolactone with superparamagnetic acid-base bifunctional ZrFeOx nanocatalysts Hu Lia,b, Zhen Fanga,*, Song Yangb

a

Chinese Academy of Sciences, Biomass Group, Key Laboratory of Tropical Plant Resources and

Sustainable Use, Xishuangbanna Tropical Botanical Garden, 88 Xuefulu, Kunming, Yunnan 650223, China

b

State-Local Joint Engineering Laboratory for Comprehensive Utilization of Biomass, Center for

R&D of Fine Chemicals, Guizhou University, Guiyang 550025, China

* Corresponding author: Prof. Zhen Fang, Tel.: +86 871 65137468; Fax: +86 871 65160916; E-mail address: [email protected]; URL: http://brg.groups.xtbg.ac.cn/

Revised for ACS Sustainable Chemistry & Engineering Dec. 2015

1

ACS Paragon Plus Environment


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Abstract

Acid-base bifunctional superparamagnetic FeZrOx nanoparticles were synthesized via a two-step process of solvothermal treatment and hydrolysis-condensation, and were further employed to catalyze the conversion of ethyl levulinate (EL) to γ-valerolactone (GVL) using ethanol as both H-donor and solvent. ZrFeO(1:3)-300 nanoparticles (12.7 nm) with Fe3O4 core covered by ZrO2 layer (0.65 nm thickness) having well- distributed acid-base sites (0.39 vs. 0.28 mmol/g), moderate surface area (181 m2/g), pore size (9.8 nm) and strong magnetism (35.4 Am2 kg−1) exhibited superior catalytic performance, giving a high GVL yield of 87.2% at 230 ºC in 3 h. The combination of the nanoparticles with solid acid HY2.6 promoted the direct transformation of sugars to produce GVL in moderate yield (around 45%). Moreover, the nanocatalyst was easily recovered by a magnet for six cycles with an average GVL yield of 83.9% from EL.

Keywords: Nanocatalyst; biomass; biofuels; heterogeneous catalysis; acid-base sites; magnetism

2


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Introduction Efficient production of fine chemicals and fuels from biomass is being considered as one of promising ways to alleviate the reliance of modern society and industry on fossil fuels 1. Among various biomass-derived molecules, γ-valerolactone (GVL), which is stable under neutral and air conditions as well as suitable for liquid fuel, perfume and food additives, has been identified as a versatile platform compound for the ambitious objective in recent years

2,3

. GVL can be used not

only as a precursor for the production of gasoline and diesel fuels (e.g., C8−C18 alkanes and valeric esters), but also as an intermediate to synthesize a variety of valuable chemicals such as methyl pentenoate and 1,4-pentanediol 1. Furthermore, GVL has been demonstrated to be a renewable and green solvent to improve biomass conversion by accelerating product initial formation rate, slowing down product degradation, and dissolving biomass and humins 4,5. All these properties make GVL be a good candidate for the production of biofuels and value-added chemicals. Attention has been placed on the synthesis of GVL from biomass derivatives.

The frequently used substrate for GVL production is levulinic acid (LA) that is able to be obtained from lignocellulosic biomass through multiple steps with 5-hydroxymethylfurfual (HMF) or furfuryl alcohol as the intermediate 6,7. A number of studies have been carried out for catalytic conversion of LA to GVL over noble metals (e.g., gold, platinum, palladium, iridium and ruthenium particles) by using hydrogen gas as hydrogen source

8-11

, and a very limited number of non-noble metal catalysts

are demonstrated to exhibit comparable activity under mild conditions

12-15

. Besides molecular

hydrogen, formic acid (FA) is also explored to be used as H-donor by catalytic decomposition of FA to provide H2 or by direct transfer hydrogenation over metal catalysts for the selective reduction and lactonization of LA to GVL

16

. In fact, both FA and LA can be formed from further hydration of

HMF in equimolar ratio, which renders this reaction process to be highly renewable and more attractive. Heterogeneous catalysts like ruthenium nanoparticles, silica immobilized Ru particles, Au/ZrO2 and Cu/ZrO2 were recently investigated to simplify catalyst recovery and produce high GVL yield (~90%) from LA 17-20. Nevertheless, only few non-noble metal catalysts are acid-resistant and can selectively decompose FA into H2 other than CO and H2O 8,20.

3



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Through Meerwein–Ponndorf–Verley (MPV) reduction, catalytic transfer hydrogenation (CTH) of LA and its esters to GVL by using inexpensive and abundant alcohols as H-donors in the absence of zero valence metal catalysts has attracted much attention; despite Raney Ni was found to be efficient for producing GVL from alkyl levulinates in 2-propanol

21,22

, the residual Al species (Lewis acid

sites) in Raney Ni were proposed to enhance GVL yield from levulinates with 4-hydroxypentanoates as the key intermediates

22

. A range of non-noble metal oxides with moderate basicity such as

ZrO(OH)2·xH2O 23, in situ formed ZrO(OH)2 24, ZrO2 25,26 and ZrO2/SBA-15 27, as well as modified zeolites bearing Lewis acidity (e.g., Ti-, Sn-, Zr- and Hf-Beta)

28-30

have been demonstrated to be

capable of catalyzing LA and its esters to produce GVL in moderate to high yields. Most recently, both acidic and basic sites are illustrated to be crucial to produce GVL from LA and its esters, in which basic sites (O2−) with the aid of Lewis acidic sites can synergistically activate the dissociation of the hydroxyl groups in isopropyl alcohol for MPV reaction to yield 4-hydroxypentanoate, and GVL is finally obtained through succeeding intramolecular esterification or transesterification (Scheme 1)

31,32

. It should be noted that the reduction potential of secondary alcohols such as

2-propanol and 2-butanol is lower than that of primary alcohols (e.g., ethanol and ethanol) 33, which determines their optimal reaction temperatures for the synthesis of GVL to be ~150 and ~240 ºC, respectively

24,34

. Among different types of alcohols, ethanol capable of being produced from

biomass is sustainable and shows great potential as hydrogen source 35. Importantly, solid catalysts separated and recovered from reaction mixtures can be facilitated by magnetic Ni or Fe particles and recycled with almost constant activity and stability

36,37

. Therefore, the development of acid-base

bifunctional magnetic catalysts active for CTH process with ethanol is highly in demand.

4


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Scheme 1. Catalytic pathways leading to GVL from LA or its esters via MPV reduction with acid and/or base catalysts

Instead of using LA or its esters as the starting material, the direct conversion of carbohydrates to GVL is more promising for GVL manufacture on the industrial scale, while the requirement of multiple sequential catalytic reduction and acid-mediated steps largely restricts the selectivity towards GVL 10. To improve product distribution, one-pot, two-step catalytic processes involving consecutive conversion of carbohydrates to LA or its esters and then GVL have been recently developed 24,38-41. Although one-step strategy is more economical, it is rarely efficient for direct conversion of sugars to GVL and the hydrogen source is exclusively FA or H2

42,43

. In contrast, the

use of secondary alcohol as H-donor is only shown to be active for GVL production from furfural other than carbohydrates, to the best of our knowledge

44

. The present study is to directly produce

GVL from sugars and EL in ethanol without using external hydrogen over as-prepared superparamagnetic acid-base bifunctional nanocatalysts (ZrFeOx) that can be easily separated for recycling.

Experimental Section Materials Chemical reagents Fe(NO3)3·6H2O (> 99.9%), ZrOCl2·8H2O (98.0%), urea (> 99.5%), NH4OH (AR, 25−28% in water) and isopropyl levulinate (98.0%) were purchased from Aladdin Industrial Inc. (Shanghai). Ethanol (> 99.5%), 2-propanol (> 99.5%), glucose (> 99.5%), fructose (> 99.0%), and sucrose (> 99.5%) were bought from Sigma-Aldrich (Shanghai). EL (99.0%) and GVL (98.0%) were from J&K Scientific Ltd. (Beijing). Zeolite HY (Si/Al = 2.6) was supplied by Zeolyst International (Conshohocken, PA). Other chemicals were of analytical grade and used as received, unless otherwise noted. Preparation of magnetic acid-base bifunctional nanocatalysts (ZrFeOx) Magnetic acid-base bifunctional nanocatalysts (ZrFeOx) were prepared by coating zirconia onto the surface of Fe3O4 nanopowders via a two-step process: (i) For synthesis of Fe3O4 nanoparticles, urea 5



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(4.9 g, 82 mmol) was slowly added to a solution of Fe(NO3)3·6H2O (5.0 g, 12 mmol) in 20 mL of ethanol, and stirred at room temperature for 2 h. The resulting precipitates were filtered and washed with ethanol (25 mL) for three times to give [Fe(CON2H4)6](NO3)3, which was subsequently transferred to a 25 mL autoclave with Teflon liner (YZPR-25, YanZheng Shanghai Experimental Instrument Co., Ltd.) with ~0.5 mL dead volume (pressure gauge was removed) for solvothermal treatment in ethanol (20 mL) at 220 ºC for 10 h to afford Fe3O4 particles by partial reduction of Fe3+ to Fe2+ (3Fe(NO3)3 + CON2H4 → Fe3O4 + CO2 + 4NO + 7NO2 + O2 + 2H2O)

45,46

. (ii) A

base-catalyzed hydrolysis-condensation approach was adopted to encapsulate Fe3O4 cores with zirconia. In a general synthetic procedure modified from a previous report

47

, ZrOCl2·8H2O and

Fe3O4 particles in a Zr/Fe molar ratio of 1/1, 1/3, or 1/9 were added into ethanol (50 mL) and vigorously stirred at room temperature to form a well-dispersed mixture. Then, NH4OH was injected using a syringe to adjust the solution’s pH value at 12, and the obtained reaction mixture was kept stirring for another 5 h. Upon completion, the encapsulated magnetic particles were isolated from the solution containing residue zirconia and basic ethanol by using a permanent magnet, and repeatedly washed with ethanol for 5−8 times until no chloride was detectable with AgNO3. The obtained slurry was dried overnight at 80 ºC in an oven (WFO-710, EYELA, Tokyo Rikakikai Co., Ltd.), then placed into a tubular furnace (SGL-1100, Shanghai Daheng Optics and Fine Mechanics Co., Ltd.) and heated to 110, 300 and 500 ºC at a heating rate of 2 ºC/min for 6 h calcination in air. Preparation of zirconia-HY2.6 Zirconia-HY2.6 hybrids [hereafter denoted as pZrY2.6, p = (ZrOCl2·8H2O/HY2.6 ×100%, wt%) = 15, 30, and 60 wt%] were prepared from HY2.6 and ZrOCl2·8H2O with different weight loadings by using deposition-precipitation method. In a general procedure, HY2.6 (1.0 g) was added into an aqueous solution containing a certain weight of ZrOCl2·8H2O (0.15−0.60 g, 25 mL) at a fixed pH of 9-10, which was controlled by NH4OH. After aged for 5 h under ambient conditions, the resulting slurry was filtered (0.45 µm pore size), washed with deionized-water to give a neutral filtrate, dried overnight at 110 ºC in the oven, and heated to 450 ºC at a heating rate of 2 ºC/min for 6 h calcination in air in the tubular furnace. All catalysts were ground and past through 200-mesh sieves before experiments. 6


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Catalyst characterization XRD (X-ray diffraction) patterns were recorded with D/max-TTR III X-ray powder diffractometer (Rigaku International Corp., Tokyo) using Cu Kα radiation source. XPS (X-ray photoelectron spectroscopy) measurements were fulfilled using a Physical Electronics Quantum 2000 Scanning ESCA Microprobe (Physical Electronics Inc., PHI, MN) equipped with a monochromatic AlKa anode. Magnified images of samples were obtained with TEM (transmission electron microscope; JEM-1200EX, JEOL, Tokyo), and the mean particle diameter and shell thickness of selected catalysts were estimated from TEM images by using a software Nano Measurer 1.2. The elemental compositions of catalysts after dissolved in water by concentrated acid were determined by ICP-OES (inductively coupled plasma-optical emission spectrometer) on an Optima 5300 DV instrument (PerkinElmer Inc., Waltham, MA). BET (Brunauer−Emmett−Teller) surface areas of the porous materials were determined from nitrogen physisorption measurements at liquid nitrogen temperature on a Micromeritics ASAP 2010 instrument (Tristar II 3020, Norcross, GA). The magnetic properties of powder catalysts were measured by VSM (vibrating sample magnetometer; HH-15, Nanda Instrument Plant, Nanjing) at room temperature. NH3 and CO2-TPD (temperature programmed desorption) were conducted on an automated Chemisorption analyzer (Quantachrome Instruments, Boynton Beach, FL) to assess the surface acidity and basicity of catalysts, respectively. Catalytic conversion of sugars and EL to GVL The production of GVL from different substrates was carried out in the 25 mL stainless steel autoclave. In a typical procedure, 0.2 g magnetic catalyst was added into a stock solution consisting of 0.65 g substrate and 15 mL ethanol (11. 8 g) with a constant substrate concentration of 5.5 wt% relative to ethanol. The autoclave was flushed with nitrogen 3 times to remove the air before the contained reaction mixture was heated to the desired temperature (150−230 ºC) in less than 50 min. The reactant mixture was magnetically stirred at 500 rpm for a specific reaction time (0.5−20 h), and the time zero was defined as the designated temperature reached. Reaction pressure in the autoclave measured by pressure gauge mainly caused by ethanol vapour at 150, 180, 210, 230 and 250 ºC was found to be 1.0, 2.1, 3.6, 5.2 and 7.1 MPa, respectively. After the reaction, the autoclave was quenched by cold water in a beaker. Liquid products and residue ethanol were directly decanted from 7



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the Teflon liner, where magnetic catalyst was attracted by a permanent magnet. The remaining catalyst in the liner was washed with ethanol for 3 times, dried at 80 ºC for 2 h (catalyst recovery rate of 92−96%) and directly used for next cycles. Analysis of products in ethanol phase Liquid products and major by-products were identified with GC-MS (Agilent 6890N GC/5973 MS, Santa Clara, CA). The concentrations of glucose, fructose, sucrose, ethyl fructoside/glucoside and furans (5-hydroxymethylfurfural and furfural) in ethanol phase (after diluted 10 times with deionized water) were determined by High Performance Liquid Chromatography (HPLC; LC-20A, Shimadzu, Kyoto) fitted with an Aminex HPX-87H column (Bio-Rad, Richmond, CA) and a refractive index (RI) detector as well as an ultraviolet (UV) detector at 280 nm. EL, GVL and other products (e.g., isopropyl levulinate and ethyl 4-ethoxypentanoate) were analyzed on GC (GC-2014, Shimadzu, Kyoto) with a Rtx-Wax capillary column (30 m × Ø0.25 mm × 0.25 µm) and a flame ionization detector. N2 was used as the carrier gas at a flow rate of 0.75 mL min−1, and a programmed temperature of 60 ºC (1 min) – 10 ºC min−1 – 230 ºC (5 min) was employed in the analysis. n-Butyl alcohol was added as an internal standard for quantitative analysis with relative response factors of 0.974 and 1.023 for EL and GVL, respectively. The conversion of EL and GVL yield were calculated on the basis of the standard curves (with R2 > 0.997) made from commercial samples with five different concentrations (e.g., 0.05, 0.1, 0.2, 0.3, and 0.6 mmol/mL). The yields of isopropyl levulinate, ethyl 4-ethoxypentanoate and GVL condensed by-products (GVLac) were obtained from GC by referring to the standard sample isopropyl levulinate, EL and GVL, respectively, which are also cross-checked by 1H NMR. EL or sugar conversion (X, mol%) and the yield of GVL, isopropyl levulinate, ethyl 4-ethoxypentanoate, GVLac, ethyl fructoside/glucoside, or furans (Y, mol%) were calculated as follows: X(%) = [1 − (mole of EL or sugar in products) / (mole of initial EL or sugar)] × 100% Y(%) = (mole of product) / (mole of initial EL or sugar monomers) × 100%

Results and Discussion Characterization of catalysts 8


(1) (2)

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XRD patterns of ZrO2 and ZrFeOx particles with different Zr/Fe molar ratios by calcination treatment at 300 ºC are shown in Figure 1a. The neat ZrO2 calcined at 300 ºC shows low crystallinity

48

with

only two wide peaks at 2θ of around 31° and 50°. For ZrFeOx or ZrFeO(m:n)-T (m:n is Zr/Fe molar ratio; T is calcination temperature) particles, a range of diffraction peaks at 30.3°, 35.7°, 43.4°, 53.9°, 57.4°, and 63.1° are assigned to the reflections of Fe3O4 crystalline structure from the (220), (311), (400), (422), (511), and (440) planes, respectively

45

. However, no distinguishable XRD

characteristic reflection of ZrO2 is detected in ZrFeOx smples, possibly due to ZrO2 powders primarily existent in non-crystalline phase with dimensions below the instrumental detection limit (~ 2 nm)

49

. The actual compositions of ZrFeOx particles determined by ICP-OES are consistent with

the controlled Zr/Fe molar ratios (Table S1), and the XPS spectrum of ZrFeO(1:3)-300 with photoelectron peaks located at 711 and 725 eV successively corresponding to the Fe2p3/2 (without satellite peak) and Fe2p1/2 (Figure 1b) indicates the formation of Fe3O4 other than γ-Fe2O3

45

.

Moreover, the primary metal signals detected by XPS belonged to Zr species despite the molar ratio of Fe is 3 times more than that of Zr in the case of ZrFeO(1:3)-300 sample, implying that the as-prepared ZrFeOx particles exhibit a Fe3O4 core–shell ZrO2 structure 50.

9



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Figure 1. (a) XRD patterns, (b) XPS spectrum, (c) BET analyses, and (d) VSM curves of ZrO2 and ZrFeOx catalysts with different Zr/Fe molar ratio.

The texture properties of ZrO2 and ZrFeOx particles were evaluated by N2 adsorption–desorption isotherms (Figure 1c), and related results including BET surface area and average pore size are shown in Table S1. For ZrO2, the hysteresis loop of isotherm located in the relative pressure ranging from 0.40 to 0.60 suggests the presence of both microporous and mesoporous structures. With the introduction of Fe3O4 cores in molar ratio increasing from 1 to 9, the relative pressure of the loop is gradually shifted to 0.6-0.95, which is in concert with the increased average pore size but decreased BET surface area (Table S1). In this respect, the porous ZrO2 shells are most likely to provide high surface areas by assembly or aggregation, while Fe3O4 cores encapsulated by ZrO2 appear to facilitate the formation of pores in relativly larger sizes through restraining the excessive accumulation of the particles. Saturation magnetization (Ms) of ZrFeOx samples was also investigated by VSM (Figure 1d). Surprisingly, all ZrFeOx particles exhibit apperent superparamagnetism, which is confirmed by the existence of zero magnetic coercive force and permanent magnetization

51

. The Ms for ZrFeO(1:1)-300, ZrFeO(1:3)-300, and ZrFeO(1:9)-300

samples is 22.3, 35.4, and 53.1 Am2 kg−1, respectively, which increases with the increase of Fe3O4 content. Meanwhile, the increase in calcination temperature from 110 to 500 ºC results in the slight decrease of Ms from 22.3 to 17.8 Am2 kg−1. In both situations, the magnetic samples in small particle size are probably prone to exhibit high Ms (Table S1)

52

. To elucidate its reliability, particle size

distributions of ZrFeOx samples were thus measured by TEM (Figure 2). Interestingly, all ZrFeOx 10


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samples are nano-sized particles, although they are overlapped each other in different degrees. The average particle diameters of ZrFeO(1:1)-300, ZrFeO(1:3)-300, and ZrFeO(1:9)-300 samples are found to be 17.9, 12.7 and 10.6 nm, respectively (Figure 2a-c). In addition, HRTEM (high resolution TEM) image of ZrFeO(1:3)-300 sample shows lattice plane spacings around 0.25 nm (Figure 2d), typical for cubic structures of Fe3O4 (311, hkl) 53. It also can be seen that the well-organized Fe3O4 nanoparticles encapsulated with ZrO2 in ~0.65 nm thickness have some open pores and defects, indicating that Fe3O4 cores with high surface area and small particle size are helpful to disperse ZrO2 despite zirconia particles are partially assembled in some places. Assuming that a ZrFeO(1:3)-300 nanoparticle is composed of Fe3O4 core covered by ZrO2 shell uniformly, Zr/Fe molar ratio of this core-shell particle is calculated as 1.14/3 by knowing its overall diameter (12.7 nm), thickness of ZrO2 layer (0.65 nm), and densities of Fe3O4 (5.0 g/cm3) and ZrO2 (5.89 g/cm3), which is very close to 1/3 in consideration of the porous structure of ZrO2 particles.

Figure 2. TEM images and particle size distributions of (a) ZrFeO(1:1)-300, (b) ZrFeO(1:3)-300, and (c) ZrFeO(1:9)-300, as well as (d) HRTEM image of ZrFeO(1:3)-300.

Furthermore, the acid strength and contents of ZrO2, Fe3O4 and ZrFeOx-300 nanoparticles were measured by NH3-TPD (Figure S1). All tested samples containing zirconium species possess a single 11



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NH3 desorption band at ~130 ºC, implying that the acidic centres of these samples are related to weak acidity due to Lewis acid sites located on the surface of ZrO2 54. Moreover, the opening pores of ZrO2 shells accessible to Fe3O4 cores (with low acid content of 0.12 mmol/g, but stronger acidity centered at ~275 ºC in NH3-TPD, Figure S1) by more NH3 may contribute to the slightly increased acid content of ZrFeOx-300 nanoparticles (Table S1). On the other hand, two CO2 desorption bands at ~175 and 300 ºC for ZrO2 and ZrFeOx-300 samples are observed in Figure S2, which are corresponding to the weak and moderate base strength, respectively. The gradually reduced base content with increasing the quantity of Fe3O4 cores that bear few base sites (0.002 mmol/g, Figure S2) should be ascribed to the decrease in surface area of the ZrFeOx-300 catalysts (Table S1).

Conversion of EL to GVL over different catalysts Initial experiments for EL-to-GVL conversion were carried out over different solid catalysts at 230 ºC for 0.5 h by using 2-propanol as H-donor. In Table 1, ZrO2 calcined at 110 ºC is very active for the production of GVL (81.9% yield) from EL (93.7% conversion), with a high GVL initial formation rate of 615.4 µmol g−1 min−1 (Entry 1). Nevertheless, only a moderate GVL selectivity of 87.4% along with almost no hydrogenated intermediate ethyl 4-hydroxypentanoate is observed (Figures S3 & S4), and isopropyl levulinate and ethyl 4-isopropoxypentanoate (~8% yield) are identified as domonant by-products (Figures S3 & S5) promoted by base and acid sites of ZrO2-110 catalyst through transesterification and etherification, respectively. When the calcination temperature for ZrO2 increases from 110 to 300 ºC, a higher GVL selectivity of 90.1% with a good initial formation rate of 597.4 µmol g−1 min−1 is obtained despite EL conversion and GVL yield are successively decreased to 88.2% and 79.5% (Table 1, Entry 2). Lower content of Lewis acid and base sites

26

, and the increased pore size from 2.2 to 3.5 nm appear to facilitate the production of

GVL with less by-products formation. Unfortunately, poor GVL yield of 33.4% is detected as calcination temperature for ZrO2 increases further to 500 ºC (Table 1, Entry 3), which may be attributed to the significant loss of BET surface area and hydroxyl groups of zirconia 28,55.

Table 1. Results of EL-to-GVL conversion with different zirconia-based catalysts a Entry

Catalystb

Solvent

EL Conv.

GVL Yield

GVL Selec.

Average Rate

12


Pore Size

BET Surf.

Ms (Am2

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(%)

(%)

(%)

(µmol g−1

(nm)d

area (m2 g−1)

kg−1)e

min−1)c 1

ZrO2-110

2-Propanol

93.7

81.9

87.4

615

2.2

286

--

2

ZrO2-300

2-Propanol

88.2

79.5

90.1

597

3.5

198

--

3

ZrO2-500

2-Propanol

41.8

33.4

79.9

251

7.4

67

--

4

Fe3O4

2-Propanol

9.7

1.5

15.5

11

5.3

112

62.6

5

ZrFeO(1:1)-110

2-Propanol

97.5

83.6

85.7

644

3.1

316

22.3

6

ZrFeO(1:1)-300

2-Propanol

94.2

86.7

92.0

652

6.9

195

20.3

7

ZrFeO(1:1)-500

2-Propanol

48.4

39.3

81.2

295

10.6

64

17.8

8

ZrFeO(1:1)-300

Ethanol

59.1

48.9

82.7

367

6.9

195

20.3

9

ZrFeO(1:3)-300

Ethanol

50.5

45.3

89.7

340

9.8

181

35.4

10

ZrFeO(1:9)-300

Ethanol

37.1

22.6

60.9

170

13.0

103

53.1

f

ZrFeO(1:3)-300

Ethanol

26.9

15.2

56.5

114

--

--

--

ZrFeO(1:3)-300

Ethanol

19.3

8.7

45.1

66

--

--

--

11

g

12

a

Reaction conditions: 0.65 g EL (5.5 wt%), 11.8 g solvent, 0.20 g catalyst, 230 ºC and 0.5 h.

b

100, 300 and 500 are catalyst calcination temperature (ºC); 1:1, 1:3 and 1:9 denote the molar ratio of Zr/Fe.

c

Average rate is defined as: GVL mole/(catalyst weight × time).

d

Average pore diameters were obtained from nitrogen isotherms by the BJH method.

e

Denoted as saturation magnetization.

f

ZrFeO(1:3)-300 was treated with 0.1 g tetraethoxysilane to cover Lewis acid sites 59.

g

ZrFeO(1:3)-300 was titrated with 0.1 g benzoic acid to poison Lewis base sites 60.

The introduction of Fe3O4 cores into ZrO2 improves physico-chemical properties of ZrO2, such as smaller particle diameters (< 20 nm), larger pore sizes (> 6.9 nm), almost constant Lewis acidity but decreased Lewis base contents, and strong paramagnetism for catalyst separation (Table S1). Although Fe3O4 with low acid and base contents (0.12 and 0.002 mmol/g, Figures S1 & S2) is almost inactive for GVL production (1.5% yield; Table 1, Entry 4), ZrFeOx nanoparticles show enhanced catalytic performance for the transformation of EL to GVL (39.3-86.7% yields; Table 1, Entries 5-7), which are even superior to corresponding single ZrO2 samples (Table 1, Entries 1-3). In particular, ZrFeO(1:1)-300 nanocatalyst exhibits the highest GVL yield (86.7%) and initial reaction rate (651.5 µmol g−1 min−1), possibly owing to its moderate pore size (6.9 nm), surface area (195 m2 g−1) and base content (0.32 mmol/g).

Ethanol is more promising as hydrogen source for EL-to-GVL conversion than 2-propanol, since it is renewable and avoidable to the possible side reaction of transesterification between alcohol and substrate. However, much lower catalytic activity is observed over the same catalyst ZrFeO(1:1)-300 13



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by using ethanol as both H-donor and solvent (48.9% vs. 86.7%; Table 1, Entry 8 vs. 6), owing to its higher reduction potential than 2-propanol. Notably, the selectivity of GVL is improved by increasing the amount of Fe3O4 (Table 1, Entry 9 vs. 8), while continuous increase of Fe species content leads to a sharp decrease in GVL yield (22.6%) and selectivity (60.9%) (Table 1, Entry 10). These results illustrate that appropriate acid-base contents, particle diameter, pore size, and surface area are crucial factors to enhance GVL production by facilitating the formation of ethoxides on zirconia surface

56

. Lewis sites have been demonstrated to promote ethanol deprotonation and

hydride transfer through generating surface ethoxides

57

. In this study, Lewis acid and base sites

seem to play a synergic role in MPV reduction like hydrotalcite 58, in combination with subsequent lactonization to boost the production of GVL from EL, which is verified by poisoning either Lewis acid or base sites of ZrFeO(1:3)-300 nanocatalyst (Table 1, Entries 11 & 12) with previously reported methods 59,60.

Effect of Zr/Fe ratio on EL-to-GVL conversion in ethanol Catalytic conversion of EL to GVL in ethanol over ZrFeOx nanoparticles with Zr/Fe ratios of 1:1, 1:3 and 1:9 was further carried out at 230 ºC for different reaction periods to study the distribution of products (Figure 3). EL conversions apparently increased with the extension of reaction time, and maximum GVL yields of 84.1% and 87.2% are obtained over ZrFeO(1:1)-300 and ZrFeO(1:3)-300, respectively. Ethyl 4-ethoxypentanoate (EEP; Figure S6) was observed as dominant byproduct (7-12% yield) in the early stage of reaction (e.g., 0.5-3 h), which proves that EL is mainly converted into GVL through ethyl 4-hydroxypentanoate although it is hardly detected by GC-MS. Nevertheless, further increasing reaction time to 5 h results in a slightly decreased or almost constant GVL yield through side reactions especially aldol condensation between GVL and acetaldehyde dehydrogenated from ethanol (~5% yield in total, denoted as GVLac; Figure S6). In contrast, ZrFeO(1:9)-300 with lower content of base sites (0.19 mmol/g) but higher content of acid sites (0.42 mmol/g) gives persistently increased GVL yields with the increase of reaction time from 0.5 to 5 h although EL conversions are relatively lower. It’s deduced that the presence of zirconia with more base sites [e.g., 0.32 and 0.28 mmol/g for ZrFeO(1:1)-300 and ZrFeO(1:3)-300, respectively] is most likely to 14


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facilitate MPV reduction but also contribute to the formation of by-products after a long reaction duration. On the other hand, a vast amount of acetal is formed from acetaldehyde and ethanol possibly assisted by Lewis acid sites

23

, which may restrain side reactions to largely decline GVL

yields. Overall, ZrFeO(1:3)-300 nanoparticles with well-assigned Lewis acid-base sites are more suitable for GVL production, and used for the following experiments.

Figure 3. Catalytic conversion of EL to GVL in ethanol over ZrFeOx nanoparticles with Zr/Fe ratios of 1:1 (a), 1:3 (b), and 1:9 (c) [0.65 g EL (5.5 wt%), 11.8 g ethanol (15 mL), 0.20 g catalyst and 230 ºC].

Effect of reaction temperature and time on EL-to-GVL conversion in ethanol By using ZrFeO(1:3)-300 as catalyst, the influence of reaction temperature with varying time on GVL production from EL is shown in Figure 4. It can be seen that raising temperature from 180 to

15



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230 ºC accelerates GVL yield and EL conversion rate. However, higher temperature of 250 ºC suppresses the formation of GVL despite EL conversion is superior to that at lower temperatures, and condensation products derived from GVL (i.e., GVLac) significantly increased from ~5% at 230 ºC to ~11% at 250 ºC in 5 h. For reactions at 180 and 210 ºC, much longer reaction time of 8 and 20 h is required for achieving comparable EL conversion, respectively. Unfortunately, lower GVL yields (82.7% & 81.5%) are obtained in both cases, showing that the extension of reaction time is beneficial to intermediates or by-products formation. Additional experiment was executed at an even lower temperature of 150 ºC for 20 h to examine the possible reaction pathway, and a high EL conversion of 97.6% is attained while GVL yield is quite low (69.3%). EEP is observed in large amount (~18% yield in total), but GVLac by-products (~2%) formed at 150 ºC are less than that at high temperatures. Therefore, the optimal temperature of 230 ºC and time of 3 h are helpful to produce GVL with less formation of intermediates and by-products.

16


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Figure 4. Catalytic conversion of EL to GVL in ethanol over ZrFeO(1:3)-300 nanoparticles by varying reaction time (0.5-5.0 h) and temperature: 250 ºC (a), 230 ºC (b), 200 ºC (c), and 170 ºC (d) [0.65 g EL (5.5 wt%), 11.8 g ethanol (15 mL) and 0.20 g ZrFeO(1:3)-300].

Effect of catalyst dosage on EL-to-GVL conversion in ethanol Catalyst dosage is one of important parameters to judge the activity of catalytic materials. In Figure 5, different weights (i.e., 0.05, 0.1, 0.2, and 0.4 g) of ZrFeO(1:3)-300 nanoparticles are employed for producing GVL from EL at 230 ºC. Both EL conversion and GVL yield increase with the increase of catalyst dosage, while an apparent decrease in GVL yield from 90.3 to 83.5% is observed with 0.4 g catalyst after 5 h, showing that the formation of GVLac by-products is mediated by excess base sites. Prolonging reaction time to 10 h seems to fulfill the catalytic process with improved EL conversions (99.8% and 97.3%) and GVL yields (85.3% and 79.5%) in the presence of lower ZrFeO(1:3)-300 loadings of 0.05 and 0.1 g, respectively; but the obtained results with respect to GVL yield still can not match with the system containing a high catalyst dosage of 0.2 g (87.2% yield) after a short reaction duration of 3 h. It is not difficult to conclude that GVL yield and EL conversion are highly sensitive to acid-base content, reaction time, temperature, and catalyst dosage.

17



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Figure 5. Catalytic conversion of EL to GVL in ethanol over ZrFeO(1:3)-300 nanoparticles with different catalyst dosages: 0.4 g (a), 0.2 g (b), 0.1 g (c), and 0.05 g (d) [0.65 g EL (5.5 wt%), 11.8 g ethanol (15 mL), and 230 ºC].

Catalyst recycle and magnetization Recycling study for EL-to-GVL conversion was carried out over magnetic ZrFeO(1:3)-300 nanoparticles at 230 ºC in 3 h with results provided in Figure 6. In a typical run, the magnetic catalyst attracted by a permanent magnet was separated from reaction mixture by decantation, washed with ethanol, dried, and used for the next cycle. The nanocatalyst exhibits almost constant activity in consecutive four cycles, with GVL yield slightly decreasing from 87.2% to 83.6%. Although the yield of GVL and EL conversion in the fifth run decrease to 78.9% and 85.4%, respectively, GVL selectivity is still high (92.4%). Interestingly, the catalytic activity of ZrFeO(1:3)-300 could be recovered by regeneration at 300 ºC for 6 h calcination in air, and a high GVL yield of 86.7% at EL conversion of 94.8 was achieved. To test the leachability of ZrFeO(1:3)-300, the nanocatalyst was separated from the mixture after 0.5 h and the residual solution further reacted for another 0.5 h. It was found that no significant difference in terms of EL conversion (50.5% vs. 50.9%) and GVL yield (45.3% vs. 46.2%) was detected for 0.5 and 1 h, respectively, indicating the heterogeneous nature of the catalyst. To examine the stability of ZrFeO(1:3)-300, XRD patterns and CO2-TPD profiles of fresh and reused (after four cycles) ZrFeO(1:3)-300 catalysts are provided in Figure S7. Slight change in structure and a little increase in base content are possibly resulted from adsorbed ethoxides or other organic species on the surface of

18


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the catalyst, which are in line with the results of recycling experiments as well as BET and elemental analyses (Table S2).

Figure 6. Recyclability of ZrFeO(1:3)-300 nanocatalyst for EL-to-GVL conversion in ethanol [0.65 g EL (5.5 wt%), 11.8 g ethanol (15 mL), 0.20 g ZrFeO(1:3)-300, 230 ºC and 3 h).

Direct conversion of sugars to GVL Direct conversion of sugars to GVL was tested in ethanol at 230 ºC for 3 or 6 h (Table 2). The addition of water did not apparently affect the reactivity of ZrFeO(1:3)-300 nanoparticles for EL-to-GVL conversion (82.9% vs. 87.2% GVL yield; Table 2, Entries 1 & 2). Only small amounts of GVL (3.5% and 6.8% yields) with 10.2% and 9.3% yields of EL were obtained from fructose (94.5% and 98.5% conversions) after 3 or 6 h, respectively (Table 2, Entries 3 & 4), possibly due to lack of strong acid sites for upstream synthesis of EL. HY2.6 zeolite capable of producing EL in a relatively high yield of 52.8% (Table 2, Entry 5), was thus introduced as a co-catalyst for fructose conversion by sequential or simultaneous addition with ZrFeO(1:3)-300 (Table 2, Entries 6 & 7). Gratifyingly, enhanced GVL yield of 44.7% or 36.5% was successively achieved, while the slightly decreased GVL yield by simultaneously adding two catalysts can be ascribed to the base sites in ZrFeO(1:3)-300 partially poisoned by [H+] derived from HY2.6. For the catalytic process conducted through sequential addition of HY2.6 and ZrFeO(1:3)-300, HY2.6 can be recovered by centrifugation after reaction in the first step and ZrFeO(1:3)-300 is able to be subsequently separated by a magnet for recycles. 19



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Page 20 of 26

Table 2. Direct conversion of sugars to GVL with zirconia-based catalysts a Entry

Catalyst

Time (h)

Substrate

Substrate

EL Yield

GVL Yield

Ether & Furans

Conv. (%)

(%)

(%)

Yield b (%)

Mass balance c (%)

1

ZrFeO(1:3)-300

3

EL

93.3

6.7

87.2

--

93.9

2d

ZrFeO(1:3)-300

3

EL

91.6

8.4

82.9

--

91.3

3

ZrFeO(1:3)-300

3

Fructose

94.5

10.2

3.5

38.9

58.1

4

ZrFeO(1:3)-300

6

Fructose

98.5

9.3

6.8

32.7

50.3

5

3

Fructose

98.9

52.8

--

18.3

72.2

3+3

Fructose

99.6

15.1

44.7

14.7

74.9

6

Fructose

100

18.5

36.5

10.7

65.7

8

HY2.6 HY2.6 + ZrFeO(1:3)-300 HY2.6 + ZrFeO(1:3)-300 15 wt% ZrY2.6

3

Fructose

98.7

38.2

8.9

13.5

61.9

9

15 wt% ZrY2.6

6

Fructose

100

29.3

16.5

9.8

55.6

10

30 wt% ZrY2.6

3

Fructose

96.7

35.4

10.1

15.4

64.2

6

e

7f

11

30 wt% ZrY2.6

6

Fructose

99.8

22.7

28.5

8.7

60.1

12

60 wt% ZrY2.6

3

Fructose

95.4

32.7

15.3

16.2

68.8

13

60 wt% ZrY2.6

6

Fructose

99.5

17.8

31.4

11.2

60.9

14

30 wt% ZrY2.6

6

Glucose

99.3

21.9

22.9

6.9

52.4

15

30 wt% ZrY2.6

6

Sucrose

98.8

23.2

26.7

6.4

57.5

a

Reaction conditions: 0.65 g substrate (5.5 wt%), 11.8 g ethanol, 0.20 g catalyst, and 230 ºC.

b

Ether is denoted as ethyl fructoside or glucoside, and furans as 5-hydroxymethylfurfural and furfural.

c

Mass balance is designated to the total yield of EL, GVL, ether, furans and residual substrate.

d

0.2 g water was added before reaction.

e

Sequential addition of HY2.6 (0.05 g; separated by centrifugation after 3 h) and ZrFeO(1:3)-300 (0.20 g; reacting

for another 3 h). f

Simultaneous addition of HY2.6 (0.05 g) and ZrFeO(1:3)-300 (0.20 g).

To further optimize the catalytic system, a series of bifunctional solid catalysts were prepared from HY2.6 and ZrOCl2·8H2O with different weight loadings by using deposition-precipitation method. As illustrated in Table 2 (Entries 8-13, 15-60 wt% refers to the weight percentages of ZrOCl2·8H2O to HY2.6), GVL yield increases with the increase of ZrO2 content, and a maximum total yield of EL plus GVL (51.2%) was achieved at 230 ºC for 6 h in the presence of 30 wt% ZrY2.6, while a much high Zr species content of 60 wt% was needed to attain a relatively higher yield of GVL (31.4%). The results indicate that the balance between Lewis acid-base and strong Brønsted acid is crucial to efficiently catalyze fructose to GVL. Glucose and sucrose were also used as substrates catalyzed by

20


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30 wt% ZrY2.6 for producing GVL and EL, and moderate yields of 44.8% and 49.9% in total could be obtained after 6 h at 230 ºC, respectively (Table 2, Entries 14 & 15). In all above experiments, more than 95% sugar conversions were observed with certain amounts of unidentified by-products and few insoluble humins formed.

Conclusions Acid-base bifunctionalized FeZrOx nanocatalysts with superparamagnetism were efficient for conversion of EL to GVL by using ethanol as both H-donor and solvent. Among these nanoparticles, ZrFeO(1:3)-300 with well-assigned distribution of acid-base sites, as well as moderate surface area and pore size exhibited superior activity for EL conversion, giving GVL in a high yield of 87.2% at 230 ºC in 3 h. Importantly, the nanocatalyst could be easily recovered from reaction mixtures by an external magnetic force and reused at least four times with almost constant activity, with slight decrease in GVL yield from 87.2% to 83.6%. Furthermore, the combination of ZrFeO(1:3)-300 with a solid acid HY2.6 could promote the direct conversion of sugar to GVL in a moderate yield of up to 44.7%.

Aknowledgements The authors wish to acknowledge the financial support from Chinese Academy of Sciences [CAS 135 program (XTBG-T02) and equipment R&D grant (No.YZ201260)], the Natural Science Foundation of China (No. 21576059), and the Yunnan Provincial Government (Baiming Haiwai Gaocengci Rencai Jihua).

Supporting Information The acid/base strength and contents of ZrO2, Fe3O4 and ZrFeOx-300 nanoparticles were measured by NH3/CO2-TPD (Figures S1 & S2). Physico-chemical properties of ZrO2 and ZrFeOx catalysts are supplied in Table S1. GC-MS is used to examine the tentative distribution of products, and the obtained GC and MS spectra are shown in Figures S3-S6. XRD patterns and CO2-TPD profiles of fresh and reused (after four cycles) ZrFeO(1:3)-300 catalysts are provided in Figure S7. BET and elemental analyses of fresh, reused and regenerated ZrFeO(1:3)-300 samples are illustrated in Table S2. This material is available free of charge via the Internet at http://pubs.acs.org

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Table of Contents:

Direct conversion of sugars and ethyl levulinate into γ-valerolactone with superparamagnetic acid-base bifunctional ZrFeOx nanocatalysts Hu Li, Zhen Fang*, Song Yang

ZrFeO(Zr:Fe = 1:3) nanoparticles (12.7 nm) with Fe3O4

core covered by ZrO2 layer (0.65 nm thickness) having

acid-base sites and strong magnetism exhibited superior catalytic performance in conversion of ethyl levulinate (EL) to γ-valerolactone (GVL, 87.2% yield). Moreover, the combination of the nanocatalyst with a solid acid HY2.6 could directly transform sugars to GVL (45% yield), and be easily recovered by a magnet for recycles.

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Direct Conversion of Sugars and Ethyl Levulinate into Î³-Valerolactone

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