Toward Selective Dehydrogenation of Glycerol to Lactic Acid over

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Towards Selective Dehydrogenation of Glycerol to Lactic Acid over Bimetallic Pt-Co/CeOx Catalysts Guangyu Zhang, Xin Jin, Yanan Guan, Bin Yin, Xiaobo Chen, Yibin Liu, Xiang Feng, Honghong Shan, and Chaohe Yang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b01918 • Publication Date (Web): 15 Jul 2019 Downloaded from pubs.acs.org on July 22, 2019

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Industrial & Engineering Chemistry Research

Towards Selective Dehydrogenation of Glycerol to Lactic Acid over Bimetallic Pt-Co/CeOx Catalysts

Guangyu Zhang, Xin Jin*, Yanan Guan, Bin Yin, Xiaobo Chen, Yibin Liu, Xiang Feng, Honghong Shan, Chaohe Yang* State Key Laboratory of Heavy Oil Processing, College of Chemical Engineering, China University of Petroleum, No. 66 Changjiang West Road, Qingdao, Shandong Province 266580, China. Corresponding Authors: [email protected], [email protected]

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract: Selective synthesis of lactic acid from bio-derived substrates still remains a grand challenge. We reported a series of unique bimetallic Pt-Co/CeOx catalysts, prepared by fractional precipitation method. Strong electronic coupling for Pt-Co is found to enhance activity towards dehydrogenation (C-H bond cleavage) of glycerol, as evidenced from XPS and TEM studies of the catalysts. Detectable electro affinity of PtCo trends to gain electron from CeOx support, thus some side reactions (such as C-C and C-O bond cleavage) are largely restrained. Such hybridized structures lead to remarkable activity (TOF: 1533.9 h-1) with good selectivity towards lactic acid (87.7%) and total organic acids (> 95%) at 200 oC. In addition, the effect of reaction temperature and substrate concentration on initial reaction rate reveals that dehydrogenation of glycerol possibly follows dual-site mechanism, where both glycerol and base need to be activated on bimetallic Pt-Co surface. Furthermore, the plausible reaction pathway is detailed discussion. Keywords: Lactic acid; Glycerol; Dehydrogenation; Bimetallic catalyst; electronic coupling; dual-site mechanism

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1. Introduction The catalytic conversion of renewable biomass feedstocks for the sustainable production of fuels and chemicals has gained increasing attention in the past decade. Glycerol, a by-product from biodiesel production, can be potentially converted to a variety of value-added chemicals for downstream applications.1 In the past decade, extensive theoretical and experimental studies have confirmed that, several industrially important intermediates can be derived from glycerol, including lactic acid (LA), glyceric acid (GLYA), glycolic acid (GA), 1,2-propanediol (1,2-PDO), 1,3-propanediol (1,3-PDO) and ethylene glycol (EG). They are essential building blocks for synthesis of polyester, antifreeze, resin and other biodegradable materials.2-5 There is no doubt that glycerol-to-chemical route will play a critical role in future biorefinery.6, 7 Among various bio-derived chemicals, LA is very unique intermediate, which has been traditionally used in food, medicine and plastic industry because of its excellent biocompatibility and degradability.8-10 Up to date, fermentation of carbohydrates is still the major conversion route for LA production.11-13 However, several bottlenecking issues such as long fermentation time, low concentration of substrates, high cost and complicated separation and purification processed limit the sustainable development of fermentation technologies.14 It demands more cost-effective technologies for sustainable production of LA with less energy and capital intensity.

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Figure 1 Comparison of various LA production processes

There are three routes for chemical conversion of glycerol to LA (see Figure 1). Hydrothermal conversion ((a) in Figure 1)15-17 and selective oxidation ((b) in Figure 1)18-20 of glycerol to LA has been demonstrated to be feasible. However, harsh reaction condition (high reaction temperature and highly concentration alkali) for hydrothermal conversion method and still highly alkali concentration even at a lower reaction temperature for selective oxidation method hinder further industrial application.20-22 Dehydrogenation of glycerol provides an alternative route for sustainable production of LA, which display several unique features thus receiving extensive interests from both academia and industry in recent years.23-26 For example, dehydrogenation of glycerol leads to formation of H2 as a valuable co-product, rather than forming H2O in the presence of O2, thus showing good atom efficiency. However, most existing studies are primarily focused on monometallic noble catalysts in literature. Auneau and coworkers27 studied Ir/C for glycerol dehydrogenation to LA, with the selectivity of 77% towards LA at a glycerol conversion of 90% in an alkaline condition at 180 oC. Urbano

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and coworkers28 investigated monometallic Pt, Rh, Pd and Au supported on ZnO for glycerol conversion in an alkaline condition at 200 oC under a 2 MPa H2 pressure. Rh/ZnO catalyst showed highest yield of 68% to LA at 100% conversion. Penil and coworkers29 studied the properties of Pt on several supports (ZrO2, TiO2 and C) for glycerol conversion to LA under inert atmosphere. Pt/ZrO2 catalyst exhibited remarkable selectivity of 84% at 94% conversion at 180 oC under He atmosphere after 24 h. Shen and coworkers30 synthesized hydroxyapatite-supported Pd catalysts for glycerol conversion to LA, which showed excellent activity (TOF=1274 h-1) with a high selectivity (95%) towards LA at 99% conversion, but, at a high temperature (230 oC). Shimizu coworkers23 reported that Pt/C catalyst showed an excellent activity (TOF: 684 h-1) in dehydrogenation of glycerol to LA at 160 oC. These results suggest that Pt-based catalysts can work well for dehydrogenation of glycerol to LA than other candidates, however, the activity and selectivity need to further improve for reducing noble metal content. To reduce the cost of catalyst, some attempts on non-noble metal catalytic materials, such as Cu NPs (S: 65.3%, X: 60.2%),31 CuO/ZrO2 (S: 94.6%, X: 100%),32 CuO/CeO2 (S: 74%, X: 87%),33 CuO/Al2O3 (S: 78.6%, X: 97.8%),34 Cu-Zn-Al (S: 96%, X: 99%),35 Ni0.3/graphite (S: 92.2%, X: 97.6%)5 and Co3O4/CeO2 (S: 79.8%, X: 85.7%),9 were developed for dehydrogenation of glycerol. However, these non-nobel metal based materials in general show poor intrinsic activity even with good yield at elevated reaction time and temperature. Our previous works have reported that bimetallic CuPd/rGO catalyst show enhanced activity (114.2 h-1) compared with monometallic Cu/rGO (33.0 h-1) for dehydrogenation of glycerol to LA.36 Shen et al. reported that bimetallic CuAu show an improvement activity (9.4 h-1) compared with monometallic Cu catalyst (5.8 h-1) at 200 oC.10 Furthermore, Heeres and co-workers also reported that 5



bimetallic Au-Pt supported on nanocrystalline CeO2 show excellent activity (1170 h-1) on conversion of glycerol to LA at oxidation atmosphere.22 These results indicated that bimetallic catalyst is a feasible route for improving activity of glycerol conversion, however, the poor activity is still a major issue for conversion of glycerol to LA. Rational design of novel cost effective and high efficiency catalysts with both noble and non-noble metal species could be beneficial for further implementation of glycerol conversion to LA technology. Combining the high activity of Pt metal and low cost as well as good selectivity towards dehydrogenation over non-noble metals, we propose bimetallic Pt-Co catalysts supported on CeOx to enhancing activity of selective dehydrogenation of glycerol to LA. Unique electronic coupling effect in Pt-Co composite and lattice distortion of CeOx induced by Co incorporation were detailed studied. The influence of experimental parameters, such as temperature and substrate concentration, on catalyst activity and product distribution were studied over selected bimetallic Pt-Co/CeOx catalyst. Based on experimental studies, dual-site mechanism and a comprehensive reaction pathway for dehydrogenation of glycerol were proposed and discussed in details. 2. Methods 2.1. Chemicals and materials. Chemicals including glycerol (>99.5%), 1,2-PDO (>99.5%), EG (>99.8%), LA (>85%), glyceraldehyde (95%), GLYA (>95%), GA (98%), methanol (>99.8%), ethanol (>99.5%), Na2CO3, NaOH, KOH, Ba(OH)2, Ca(OH)2, BaCl2, CaO, and metal precursors including chloroplatinic acid hexahydrate (H2PtCl6∙6H2O), cerium nitrate hexahydrate (Ce(NO3)3∙6H2O), cobalt nitrate hexahydrate (Co(NO3)2∙6H2O), were purchased from Sigma-Aldrich and Sinopharm Chemical Reagent Co., I. td.

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2.2. Preparation of the Pt-Co/CeOx catalysts. Pt-Co/CeOx catalysts were prepared by fractional precipitation (F.P.) method. In a typical procedure (Figure S1), solution A (6.98 mmol of Ce(NO3)3∙6H2O and different amounts of Co(NO3)2∙6H2O dissolved in 50 mL deionized (DI) water) and solution B (NaOH: 0.8 mol/L, Na2CO3: 0.25 mol/L) were added to solution C (100 mL DI water) dropwise simultaneously, when fleshcolored slurry was formed. The pH value of the solution C was kept at about 10.2 throughout the preparation process. After stirred for 12 h at room temperature, solution D (10 mL aqueous solution containing H2PtCl6·6H2O (0.06 mmol)) and solution B were simultaneously added drop by drop to solution C. The mixture solution was further stirred another 30 min, then reduced with 100 mL of NaBH4 solution (0.013 mol/L, solution E). After stirred 10 h, the slurry was filtered and the solids were washed 4-5 times with large amounts of DI water. Solid sample was dried overnight at 70 oC. Asprepared catalysts with Pt/Co molar ratio being 1/z (z: 0.5, 0.75, 1, 1.5 and 2) were denoted as Pt1-Coz/CeOx. The Pt1-Co1/CeOx catalysts calcined at 400 oC (Figure S2) under different atmosphere (N2, H2, air) and ramping rate (1 oC/min or 5 oC/min). For example, Pt1-Co1/CeOx-N1 represents a sample calcined under N2 environment with a ramping rate of 1 oC/min. (N2, H2, air denoted as N, H and A, respectively). Actual metal content for Pt and Co in Pt1-Co1/CeOx catalyst is approximately 0.7 wt% and 0.2 wt%, respectively (ICP results), respectively. Monometallic Pt/CeOx and Co/CeOx catalysts were also prepared with similar procedure for comparing with bimetallic Pt-Co/CeOx. 2.3. Catalysts characterization. Thrtmogravimetric analysis (TGA) analysis was carried out on a PerkinElmer TGA Pyris 1 instrument from 50 oC to 700 oC with a ramping rate of 5 oC/min in a flow of

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N2/O2 = 8:1. N2 adsorption-desorption isotherms were obtained at -196 oC Micromeritics ASAP 2020 instrument (Quadrasorb SI Automate Surface Area & Pore Size Analyzer). The samples were outgassed at 300 oC and 1 mTorr for 4 h prior to the measurements. BET surface areas were evaluated from the adsorption isotherm within the pressure range 0.05 < P/P0 < 0.3. Pore size distribution was obtained from the adsorption isotherm using the Barrett-Joyner-Halenda (BJH) method. X-ray diffraction (XRD) was performed on an X’pert PRO MPD diffractometer instrument using Cu-Kα radiation with a scanning angle (2θ) of 10o-80o, operated at 40 KV and 40 mA. A proper amount of power sample was pretreated by an agate mortar to get a fine powder. Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) analysis was used to determine Pt and Co loadings of the catalysts by a VARIAN 720ES ICP-OES spectrometer (America Varian technologies). Transmission electron microscopy (TEM) analysis was done using a JEOL JSM-2100F microscope. Solid catalysts were suspended in ethanol and treated with an ultrasonic bath. The samples were placed onto a Cu mesh grid and dried in air for several minutes prior to being examined. X-ray photoelectron spectroscopy (XPS) was performed on a Thermo ESCALAB 250Xi spectrometer equipped with an Al anode (Al Kα = 1846.6 eV), operated at 15 kV and 10.8 mA. The background pressure in the analysis chamber was lower than 1 × 10-5 Pa. the survey and spectra were acquired at a pass-energy of 20 eV. Energy calibration was carried out using the C 1s peak of adventitious C at 284.6 eV. 2.4. Activity tests Evaluation of catalyst performances was carried out similarly as previously described.3 Conversion of glycerol and product distribution were obtained in slurry stainless-steel autoclaves equipped with Teflon-liners. In a typical procedure, known amounts of 8


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glycerol and alkalis (e.g., NaOH) were dissolved in 15 mL aqueous solution. This solution and predetermined amounts of catalyst were charged to the reactor and purged three times with N2 at 1 MPa. After a fixed reaction time at desired temperature and 1 MPa N2 pressure, the reactor contents were cooled to ambient temperature. Both gas and liquid phase products were collected and analyzed in a GC (SCION 456-GC) and Shimadzu HPLC LC-20AT system equipped with Phenomenex chromatographic column (Rezex ROA-Organic Acid H+ (8%), 300 × 7.8 mm) and refractive index (RID10A) detectors, for quantitative analysis of gaseous and liquid phase composition. A typical liquid chromatograph from dehydrogenation of glycerol was shown in Figure S3. Conversion (X) of glycerol, selectivity (S) of target product, carbon balance (C%) and turnover frequency (TOF based on Pt atom) were calculated based on carbon number according to the following equations:

X (%) =

S (%) =

C (%) =

𝑓𝑖𝑛𝑎𝑙 𝑁𝑖𝑛𝑖𝑡𝑖𝑎𝑙 𝑚𝑜𝑙𝑒𝑠. 𝑔𝑙𝑦𝑐𝑒𝑟𝑜𝑙 ― 𝑁𝑚𝑜𝑙𝑒𝑠. 𝑔𝑙𝑦𝑐𝑒𝑟𝑜𝑙

𝑁𝑖𝑛𝑖𝑡𝑖𝑎𝑙 𝑚𝑜𝑙𝑒𝑠. 𝑔𝑙𝑦𝑐𝑒𝑟𝑜𝑙

× 100

𝑚 ∗ 𝑁𝑓𝑖𝑛𝑎𝑙 𝑚𝑜𝑙𝑒𝑠. 𝑝𝑟𝑜𝑑𝑢𝑐𝑡 𝑓𝑖𝑛𝑎𝑙 3 ∗ (𝑁𝑖𝑛𝑖𝑡𝑖𝑎𝑙 𝑚𝑜𝑙𝑒𝑠. 𝑔𝑙𝑦𝑐𝑒𝑟𝑜𝑙 ― 𝑁𝑚𝑜𝑙𝑒𝑠. 𝑔𝑙𝑦𝑐𝑒𝑟𝑜𝑙)

𝑚 ∗ 𝑁𝑓𝑖𝑛𝑎𝑙 𝑚𝑜𝑙𝑒𝑠. 𝑝𝑟𝑜𝑑𝑢𝑐𝑡𝑠 𝑓𝑖𝑛𝑎𝑙 3 ∗ (𝑁𝑖𝑛𝑖𝑡𝑖𝑎𝑙 𝑚𝑜𝑙𝑒𝑠. 𝑔𝑙𝑦𝑐𝑒𝑟𝑜𝑙 ― 𝑁𝑚𝑜𝑙𝑒𝑠. 𝑔𝑙𝑦𝑐𝑒𝑟𝑜𝑙)

TOF =

𝑓𝑖𝑛𝑎𝑙 𝑁𝑖𝑛𝑖𝑡𝑖𝑎𝑙 𝑚𝑜𝑙𝑒𝑠. 𝑔𝑙𝑦𝑐𝑒𝑟𝑜𝑙 ― 𝑁𝑚𝑜𝑙𝑒𝑠. 𝑔𝑙𝑦𝑐𝑒𝑟𝑜𝑙

𝑁𝑚𝑜𝑙𝑒𝑠 𝑃𝑡 ∗ 𝑡𝑖𝑚𝑒

× 100

× 100

× 100

The m refers to the number of C atom in the target products; TOF was defined as the moles of glycerol consumed normalized with respect to Pt atom content (mole) in the catalysts and batch time (h-1), while the TOF of Co/CeOx

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normalized with respect to Co atom content based on experimental data at conversion < 20%. Turnover frequency (TOF) was defined as the moles of glycerol consumed normalized with respect to Pt atom content (mol) in the catalyst and batch time (h-1), based on experimental data at conversion < 20%, while the TOF of Co/CeOx normalized with respect to Co atom content. The formation rate of a certain product (such as LA, 1,2PDO and EG) was defined as the moles of target product formed per metal content per reaction time (h-1). 3. Results and discussion 3.1. Synergistic bimetallic Pt-Co/CeOx catalysts

Figure 2 Benchmark data on mono Pt and bimetallic Pt-Co catalysts. (a) activity and production distribution on Pt/CeOx-N1, Co/CeOx-N1, Pt/CeOx-N1 + Co/CeOx-N1 admixture (including 0.072 g Pt/CeOx and 0.072 g Co/CeOx) and bimetallic Pt-Co/CeOx-N1 catalysts (Glycerol: 0.3 mol/L H2O solution, 15 mL, 0.072 g catalyst, OH-/glycerol (mol/mol): 1.0, PN2: 1 MPa, T: 200 oC,

t: 4 h); (b) XPS spectra of Pt/CeOx-N1 and Pt-Co/CeOx-N1 catalysts; (c) TEM image of Pt-

Co/CeOx-N1 catalyst.

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We compared catalytic performances of monometallic Pt and Co catalysts with bimetallic Pt-Co to understand possible bimetallic interaction in glycerol conversion to LA. It is observed in Figure 2(a) that bimetallic Pt-Co/CeOx-N1 (Pt/Co: 1/1) catalyst shows significantly enhanced activity of 1533.9 h-1 with improved LA selectivity of 87.7%, while monometallic Pt/CeOx-N1 catalyst exhibits an activity of 780.0 h-1 with LA selectivity being 81.2% and Co/CeOx-N1 catalyst shows poor activity (12.7 h-1) and low LA selectivity of 40.3% at 200 oC. The performance of Pt/CeOx-N1 and Co/CeOxN1 admixture (including equivalent contents of Pt and Co atom for bimetallic PtCo/CeOx-N1 catalyst) was also tested under identical condition. It is observed that such admixture shows negligible enhancement in TOF (772.5 h-1) compared with monometallic Pt/CeOx-N1 catalyst. XPS spectra (Figure 2(b)) indicates that metalmetal interaction induces electronic coupling behaviors, leading to slightly higher binding energy of Pt species, while HR-TEM characterization (Figure 2(c)) of the bimetallic catalyst suggest there exist lattice distortion of CeOx structure with no detectable Pt and Co particles. The benchmark data demonstrates that bimetallic PtCo/CeOx-N1 catalyst exhibits both enhanced catalytic activity and LA selectivity with unique structural properties. We also compared the benchmark data with previously published results (see Table 1), including monometallic and bimetallic noble Pt, Ru, Ir, Rh and Pd catalysts, as well as non-noble based Cu, Ni and Co catalysts. It is clear that bimetallic Pt-Co/CeOx-N1 catalyst displays leading performances in both activity and LA selectivity, thus chosen for further detailed studies in this paper. Table 1 Comparison of catalyst performances for glycerol conversion to LA

Catalyst

Reaction conditions

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X (%)

S (%)

Ref.


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(TOF(h1))

Pt-Co/CeOx

Glycerol: 0.3 mol/L H2O solution, 15 mL, 0.072 g catalyst (0.7 wt% for Pt, 0.2 wt% for Co), OH-/glycerol

85(1534)

88

This work

95

93

23

100

80

29

99

74

37

~100

95

38

92(720)

48

39

100(1080)

37

40

99

74

37

99(1274)

95

30

~100

68

28

35(33)

93

41

99(9)

94

10

87

74

33

(mol/mol): 1.0, PN2: 1 MPa, T: 200 oC, t: 4 h

Pt/C

Pt/C

Glycerol: 6.84 mmol H2O solution, 0.0021 mmol metal, OH-/glycerol (mol/mol): 1.1, PN2: 0.1 MPa, T: 160 oC, t: 18 h Glycerol: 5 wt% H2O solution, 100 mL, substrate/Pt = 2000, OH-/glycerol (mol/mol): 1.0, PHe: 3 MPa, T: 180 oC, t: 24 h

Pt/C

Glycerol: 0.5 mol/L, 100 mL, 0.2 g for Pd/C (10 wt%) or 0.4 g for Pt/C (5 wt%), OH/glycerol (mol/mol): 1.1, T: 230 oC, t: 4 h

Pt/C

Glycerol: 17.046 mmol, 0.05 g catalyst (2.7 wt%), OH-/glycerol (mol/mol): 1.2, PC2H4: 60 bar,T: 140 oC, t: 3 h

Pt/C PtRu/C

Pt/C

Glycerol: 1 wt% H2O solution, 150 mL, substrate/surface metal = 700, OH-/glycerol (mol/mol): 0.8, PH2: 4 MPa, T:200 oC, t: 1.5 h Glycerol: 0.5 mol/L, 100 mL, 0.2 g for Pd/C (10 wt%) or 0.4 g for Pt/C (5 wt%), OH/glycerol (mol/mol): 1.1, T: 230 oC, t: 4 h

Pd/HAP

Glycerol: 2 mol/L H2O solution, 100 mL, glycerol/total Pd intake: 1926, OH-/glycerol (mol/mol): 1.1, T: 230

Rh/ZnO

oC,

t: 1.5 h

Glycerol: 1 wt% H2O solution, 100 mL, 0.03 g catalyst (2.37%), OH-/glycerol (mol/mol): 0.8, PH2: 20 bar, T: 180 oC, t:.12 h

Cu/MgO

Glycerol: 1 mol/L H2O solution, 100 mL, 0.46 g catalyst (16 wt%), OH-/glycerol (mol/mol): 1.1, PN2: 0.1 MPa, T: 200 oC, t: 2 h

CuAu

Glycerol: 2 mol/L, 100 mL, 0.736 g catalyst (26.1 wt%), OH-/glycerol (mol/mol): 1.1, PN2: 0.1 MPa, T: 200 oC t: 2 h

CuO/CeO2

Glycerol: 5 wt% H2O solution, 100 mL, 0.6 g catalyst (26.1 wt%), OH-/glycerol

12


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(mol/mol): 1.1, PN2: 2 MPa, T: 220 oC, t: 8 h Cu/SiO2

75

82

CuO/Al2O3

Glycerol: 1.1 mol/L H2O solution, 30 mL, Cu: 3.5 mmol, OH-/glycerol

98

79

Cu2O

(mol/mol): 1.1, PN2: 1.4 MPa, T: 240 oC, t: 6 h

94

78

CuO/ZrO2

Glycerol: 1.4 mol/L H2O solution, 10 mL, 0.2 g catalysts (10 wt%), OH-/glycerol

18(8)

77

32

99

96

35

97(34)

92

5

86

80

9

34

(mol/mol): 1, PN2: 1.4 MPa, T: 160 oC, t: 6 h Cu-Zn-Al

Glycerol: 44 wt% H2O solution, 40 g, 1.2 g catalyst (50.7 wt%), OH-/glycerol (mol/mol): 1.5, T: 175

Ni0.3/graphite

oC,

t: 4 h

Glycerol: 1 mol/L H2O solution, 100 mL, 0.552 g catalyst (17.3 wt%), OH-/glycerol (mol/mol): 1.1, PN2: 0.1 MPa, T: 230 oC, t: 2 h

Co3O4/CeO2I

Glycerol: 4.7 wt% H2O solution, 120 mL, 0.6 g catalyst (21.8 wt% for DP, 18 wt% for I), OH-/glycerol (mol/mol): 1.1, PN2: 6 MPa, T: 250 oC, t: 8 h

Motivated by preliminary results, we also studied the influence of Pt/Co atomic ratio on catalyst activity and product distribution (see Table 2). With the introduction of Co species to Pt catalysts, it is clear that Pt1-Co0.75/CeOx-N1 catalyst exhibits relatively higher TOF value (1642.4 h-1) at 200 oC. Meanwhile, selectivity towards LA can be improved, while other side reactions such as the formation of 1,2-PDO and EG were restrained under reaction conditions. It is interesting to find that GLYA and GA, which are often obtained from glycerol oxidation, also formed (S: ~ 10%) in the presence of bimetallic Pt-Co catalysts, while it is minor over monometallic Pt catalyst. Table 2 Effect of Pt/Co molar ratio on catalytic activity and production distribution

S (%)

Pt/Co

X (%)

TOF (h-1)

1-0

66.3

780.0

81.2

2.6

5.2

0

2.0

91.0

1/0.5

75.1

716.2

81.4

3.0

3.6

4.2

6.0

98.2

LA

1,2-PDO EG GLYA GA

13


C balance (%)


Page 14 of 33

1/0.75

82.1

1642.4

82.7

3.9

3.5

3.0

4.2

97.3

1/1

85.3

1533.9

87.7

0

2.3

5.6

3.0

98.6

70.6 1/1.5 718.5 87.9 0 0 5.3 5.1 98.3 Glycerol: 0.3 mol/L H2O solution, 15 mL, 0.072 g catalyst, OH /glycerol (mol/mol): 1.0, PN2: 1 MPa, T: 200 oC, conversion: ~20%.

3.2. Effect of calcination atmosphere on catalyst performances To further inspect how Pt-Co and metal-support interaction affect catalyst activity and product distribution, the bimetallic Pt-Co/CeOx catalyst was calcined under different atmosphere (including H2, N2 and Air). Figures 3(a), 3(b) and 3(c) presents the XPS spectra for Pt 4f core-level region of Pt1-Co1/CeOx catalyst treated at H2, air and N2, respectively. Details of peak area analysis and summary of Pt0, Pt2+ and Pt4+ (Pt0%, Pt2+% and Pt4+%) in each sample was calculated to quantify concentration of various Pt species on catalyst surface (Tables S1-S3). We found that the most intense doublet of Pt1-Co1/CeOx-H1 catalyst with binding energy (BE) of 71.1 eV (Pt 4f7/2) and 74 eV (Pt 4f5/2) were attributed to Pt0 species, while the strong doublet of Pt1-Co1/CeOx-A1 and Pt1-Co1/CeOx-N1 catalysts are Pt2+ species as in either PtO or Pt(OH)2 form. The results reveal that Pt species mainly exist in the form of Pt0 (56.5%) in Pt1-Co1/CeOx-H1 catalyst, which is much higher compared with Pt1-Co1/CeOx-N1 (19.8%) and Pt1Co1/CeOx-A1 (15.1%).

14


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Figure 3 XPS analysis of Pt 4f and schematic description of electron coupling for the Pt1Co1/CeOx catalysts calcined at H2, Air and N2 atmosphere and effect of calcined atmosphere on catalyst activity and product distribution (Glycerol: 0.3 mol/L H2O solution, 15 mL, 0.072 g catalyst, OH-/glycerol (mol/mol): 1.0, PN2: 1 MPa, T: 200 oC, t: 4 h).

Metal-metal interaction. Furthermore, XPS spectra reveals that BE of Pt0 4f5/2 of Pt1Co1/CeOx-H1 has slightly shift towards lower value of 74.0 eV. The negative shift of BE suggests that reduced Pt0 species have much higher electron density on outer shell due to electron transfer from reduced Co species (Figure 3(g)).18 Conversely, BE of Pt0

15



Page 16 of 33

4f7/2 and Pt0 4f5/2 for Pt1-Co1/CeOx-A1 sample is slightly higher than bulk Pt foil (71.1 eV) and (74.5 eV),42 which may be caused by decreased electron density of d orbitals in Pt due to the electron transfer from Pt to Co2+ species (Figure 3h).18,

43

Similar

behavior is also observed for Pt1-Co1/CeOx-N1 catalyst. Metal-support interaction. The possible metal-support interaction also contributes to enhanced catalytic activity for dehydrogenation.43-45 We studied XPS spectra of Ce 3d core-level region of Pt1-Co1/CeOx-H1, Pt1-Co1/CeOx-A1 and Pt1-Co1/CeOx-N1 (Figure 3(d), 3(e) and 3(f)), respectively. The Ce 3d region was divided into 10 components and further categorized into two groups (2Ce3+ 3d5/2: X1 and X2, 2Ce3+ 3d3/2: Y1 and Y2, 3Ce4+

3d5/2:

X3, X4 and X5, 3Ce4+

3d3/2:

Y3, Y4 and Y5) referring to methodologies

reported before.43 It is calculated (Table S4-S6) that the concentrations of Ce3+ for three bimetallic samples are 49.6% (Pt1-Co1/CeOx-H1), 48.3% (Pt1-Co1/CeOx-N1) and 47.2% (Pt1-Co1/CeOx-A1) respectively, suggesting insignificant difference of Ce3+/Ce4+ ratio among the three catalyst samples. But we did observe BE shift for Ce 3d electrons for the three samples. As seen from Figures 3(d), 3(e) and 3(f), we find that BE of Ce 3d for Pt1-Co1/CeOx-H1 and Pt1-Co1/CeOx-A1 samples are slightly lower than Pt1Co1/CeOx-N1. The red shift of BE ascribes to the increased electron density of surface Ce species possibly induced by electron transfer from metal to support during calcination. Such electron transfer is consistent with the concentration changes of different Pt valence. Because the large fraction of Ce3+ species in CeOx framework distorts local structures of -O-Ce-O- and induces red shift of BE on catalyst surface.46 We further compared the performances of bimetallic Pt1-Co1/CeOx-H1, Pt1-Co1/CeOxA1 and Pt1-Co1/CeOx-N1 catalysts for glycerol conversion at 200 oC. It is found in Figure 3i that, the overall activity of calcined Pt1-Co1/CeOx catalysts (TOF: 1126.3-

16


Page 17 of 33

1831.0 h-1) is superior to as-prepared Pt1-Co1/CeOx catalyst (TOF: 717.9 h-1). Specifically, Pt1-Co1/CeOx-H1 catalyst shows highest activity (1831.0 h-1) at 200 oC, approximately 2.6-fold higher than unroasted Pt1-Co1/CeOx catalyst. As already mentioned in XPS analysis, Pt species in Pt1-Co1/CeOx-H1 catalyst show relatively higher electron density due to electron transfer from Co0 to Pt0 and strong Pt-Ce interaction. Such structure is favorable for enhanced dehydrogenation activity for glycerol conversion. Because electron rich Pt0 center tend to attack terminal -CH2-OH site thus C-H bond cleavage occurs. Although Ce3+/Ce4+ ratio does not change under different gas atmosphere, distorted CeOx lattice tends to release more electrons thus CO and C-C cleavage reactions are reduced to large extent as these two reactions need electron deficient sites on catalyst surface. Therefore, it is clear that Pt-Co and metalCeOx interaction are keys for enhanced catalytic performances of bimetallic catalysts for glycerol conversion to LA. 100 Others

80

GA

X and S (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60

GLYA EG

40

1,2-PDO LA

20 0

Fresh

Recycle-1

Recycle-2

Recycle-3

Figure 4 Recycle of Pt1-Co1/CeOx catalyst for aqueous-phase glycerol conversion to LA (Glycerol: 0.3 mol/L H2O solution, 15 mL, 0.072 g catalyst, OH-/glycerol (mol/mol): 1.0, PN2: 1 MPa, T: 200 oC, t: 4 h).

Pt1-Co1/CeOx catalyst was selected to test the reusability of bimetallic Pt-Co catalyst for glycerol conversion, at 200 oC and 4 h. As shown in Figure 4, recycle studies of

17



Page 18 of 33

bimetallic Pt1-Co1/CeOx catalyst show slightly decrease in conversion and selectivity for LA. Furthermore, elemental analysis for Pt species via ICP-AES confirms that there are negligible Pt leaching during our experimental studies. This results indicate that the proposed bimetallic Pt1-Co1/CeOx catalyst is stable under our reaction condition. 3.3. Other structural features Surface and structural characterization of mono Pt and bimetallic Pt-Co catalysts were carried out using BET, XRD, TEM to understand surface physical and chemical properties of solid catalyst samples, which will be discussed with catalytic activity and selectivity in later sections. N2 adsorption/desorption results of mono Pt, Co and bimetallic Pt-Co catalysts are presented in Table 3. In general, surface areas of CeOx supported Pt and Pt-Co catalysts are in the range of 108-140 m2/g. It is found that Pt/CeOx sample displays relatively lower surface area (108 m2/g) with slightly larger pore size (4.8 nm) compared with other catalysts. Bimetallic Pt1-Co1/CeOx catalyst shows relatively higher surface area (140.5 m2/g) than others. In general, surface areas of Pt1-Co1/CeOx catalysts calcined at different atmosphere are in the range of 137-145 m2/g. It is also found that Pt1Co1/CeOx-H1 sample displays relatively higher surface area (144.8 m2/g) with slightly larger pore size (5.35 nm) compared with other catalysts. Table 3 Textural property of different Pt-based catalysts Sample Pt/CeOx-N1

SBET (m2/g)

Pore volume (cm3/g)

Pore size (nm)

108.8

0.13

4.79

-N1a

125.0

0.15

4.68

-N1b

140.5

0.16

4.42

Pt1-Co1.5/CeOx

-N1c

116.8

0.14

4.77

Pt1-Co1/CeOx-N5b

138.0

0.13

3.78

Pt1-Co1/CeOx-H1b

144.8

0.19

5.35

Pt1-Co0.5/CeOx Pt1-Co1/CeOx

18


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Pt1-Co1/CeOx-A1b 137.1 0.14 4.11 o o N1: rampting rate 1 C/min in N2 atmosphere; N5: rampting rate 5 C/min in N2 atmosphere; H1: rampting rate 1 oC/min in H2 atmosphere; A1: rampting rate 1 oC/min in Air atmosphere. Actual Pt content: 0.7 wt%; Co content: 0.1 wt% (a), 0.2 wt% (b), 0.3 wt% (c).

The wide-angle XRD patterns of mono Pt and bimetallic Pt-Co/CeOx catalysts calcined at different atmosphereare shown in Figure 5(a). The diffraction peaks observed in the XRD patterns were attributed to the Ce (111), (200), (220), (311), (222) planes of the CeOx support, indicating that the CeOx support are of only the pure face-centered cubic phase, a fluorite-type structure with the space group Fm-3m (JCPDS 01-075-0076).33 And the XRD pattern of catalysts indicated their nanocrystalline nature. The TEM images in Figure 5(b), 5(c) and 5(d) show that the Pt-Co/CeOx catalysts are polyhedron nanoparticles with mainly Ce (111) and (200) surface facets. The average particle sizes are 3.55 ± 0.08 nm (Pt1-Co1/CeOx-N1), 3.56 ± 0.03 nm (Pt1-Co1/CeOx-A1) and 3.60 ± 0.09 nm (Pt1-Co1/CeOx-H1), respectively. The HR-TEM images confirm the existence of Ce (111) and (200) lattice fringes with the interplanar spacing of ~ 0.31 nm and ~ 0.26 nm, respectively. The Pt and Co species cannot be observed in TEM images and XRD patterns, which might be due to high dispersion and low content of Pt (0.7 wt%) and Co (0.2 wt%) species, suggesting good metal dispersion on CeOx support. To further confirm the good dispersion of Pt and Co species on CeOx support, additional element mapping has also been provided and shown in Figure 5(e). Pt and Co elements are mapped for selected region and presented in blue and yellow colors, respectively. It is clearly that both Pt and Co elements are are highly dispersed in catalyst samples.

19



Figure 5 XRD patterns of (a) mono Pt/CeOx, Co/CeOx and bimetallic Pt-Co/CeOx catalysts calcined at different atmosphere; TEM images of (b) Pt1-Co1/CeOx-N1, (c) Pt1-Co1/CeOx-A1, (d) Pt1-Co1/CeOx-H1,; (e) element mapping of Pt1-Co1/CeOx-N1 sample (white bar indicate 20 nm, yellow bar indicate 5 nm).

3.4. Temporal reaction profiles on Pt1-Co1/CeOx catalyst 20


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We further investigated how product distribution alters along with reaction time. As shown in Figure 6(a), the typical temporal product distribution profiles confirm that LA is the major product, and its concentration first rapidly increased and then gradual reached 0.22 mol/L after 4 h. The concentrations of glycols, such as 1,2-PDO and EG, as well as GLYA and GA only display a marginal increase even after 4 h. It is important to mention that dehydrogenation of glycerol over bimetallic Pt-Co catalysts displays good atom efficiency towards valuable liquid products, as remarkably high selectivity towards liquid phase products (96-98%) was achieved in our study.

Figure 6 Catalytic conversion of glycerol catalyzed by the bimetallic Pt1-Co1/CeOx catalysts. (a) Concentration-time profiles, (b) Reaction rate and products formation rate-glycerol concentration profiles, (c) Reaction rate and products formation rate-NaOH concentration profiles, (reaction conditions: T: 200 oC; t: 4 h for (b), 2 h for (c); glycerol: 0.3 mol/L × 15 mL for (a) and (c); NaOH/glycerol molar ratio: 1 for (a) and (b)) and (d) The dehydrogenation

reaction of glycerol to form LA; (e) Derivation of surface reactions in Model iii; (f) Rate equation and kinetic parameters.

21



The promotional effect of alkali species in dehydrogenation of glycerol in the presence of metal catalysts is still debatable.5,

30, 41

However, kinetic behaviors of glycerol

conversion on Pt-based catalysts are still yet to be fully understood. We conducted experimental studies on the influence of glycerol and NaOH concentration on initial reaction rates, with the aim to reveal possible mechanism of glycerol dehydrogenation on bimetallic Pt-Co catalysts. The influence of glycerol and NaOH concentration on initial reaction rate was first studied and relevant results are shown in Figures 6(b) and 6(c), respectively. It is observed in Figure 6(b) that, by varying glycerol concentration from 0 to 1.0 mol/L, initial reaction rate is enhanced from 0.045 to 0.08 mol/L/h. Interestingly, as glycerol concentration further increases from 1.0 to 2.0 mol/L, we found that initial reaction rate seems to be independent of glycerol concentration. A similar trend was also observed for the formation rate of LA, but its formation rate displays a slight decrease when glycerol concentration is higher than 1.0 mol/L. The formation rate of 1,2-PDO displays an almost linear dependence on glycerol concentration. As a result, LA selectivity exhibit an observable decrease from 83% to 66%, while 1,2-PDO selectivity is found to increase from 4% to approximately 11% with increasing glycerol concentration. The effect of NaOH concentration on reaction rate of glycerol and formation rate of LA was studied for a range of 0-0.4 mol/L at 160 oC. The experiment data (Figure 6(c)) reveal that the initial of glycerol, LA and other acids (including GA and GLYA) increases significantly as NaOH concentration increases from 0 to 0.3 mol/L. As concentration of NaOH further increases to 0.4 mol/L, there exists an obvious decrease in conversion rate of glycerol. Similar trends were also found for formation rate of LA, GLYA and GA. These results suggest that higher NaOH concentration in reaction medium might block surface sites for glycerol conversion. Taking into account the fact 22


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that high glycerol concentration inhibited LA formation (Figure 6(b)), it is clear that glycerol activation requires two adjacent sites (dual-site mechanism), where NaOH is adsorbed on catalysts surface to facilitate C-H cleavage of glycerol for dehydrogenation reactions. This result suggests that dehydrogenation is a metal catalyzed C-H cleavage reaction promoted by OH- species, which agrees well with literature results.24, 30, 41 Detailed kinetic modeling for dehydrogenation of glycerol to form LA were conducted to gain more insight into possible reaction mechanism. Two types of models were proposed (Table S7 and Scheme 1):47 (a) “dual-similar-site” mechanism models (Model i and ii: OH- and glycerol adsorbed on two adjacent similar sites); (b) “two-different-site” models (Model iii and iv: glycerol is adsorbed on l while OHon ll sites). The dehydrogenation reaction of glycerol to form LA shown in Figure 6(d) was assumed as basis for deriving the four proposed rate models. The detailed surface reactions are presented in Scheme 1: C-H and O-H bond of adsorbed glycerol were activated on Pt surface, promoted by adsorbed OH-. Once glyceraldehyde (GLA, a dehydrogenation product (-2H) from glycerol) was formed on catalytic site, it dehydrates and undergoes Cannizzara rearrange instantaneously to form lactate. The rate constants were calculated based on data shown in Table S8, the results of which are listed in Table S9. It is found that both Models i and iii show better fits, compared with Models ii and iv. Based on the reaction profiles shown in Figures 6(b) and 6(c), Model iii best represents the overall data (see Figure 6(e) and 6(f)).

23



Page 24 of 33

Scheme 1 Proposed reaction mechanism on Pt-Co/CeOx catalyst surface Influence of reaction temperature on product distribution was further investigated to establish plausible reaction network (Table 4). It is found that glycerol conversion increased significantly with increased reaction temperature from 130 oC to 200 oC (Entries #1, #3, #5), where LA selectivity also show slight enhancement (72.5%, 80% and 79.2%, respectively). Increasing reaction temperature, also leads to slightly higher 1,2-PDO and EG selectivity, with decreasing selectivity towards GLYA and GA (Entries #1, #2, #4). Table 4 Aqueous-phase glycerol conversion at different temperature S (%) Entry # T

(oC)

t (h) X (%) LA

1,2-PDO EG GLYA GA

LA yield (%)

1

130

4

4.6

72.5

1.7

2.2

13.8

2.7

3.3

2

160

4

53.2

80.0

2.6

2.3

10.4

2.8

42.6

3

160

8

68.2

75.9

9.3

2.8

6.9

2.9

51.8

4

200

1

64.9

81.4

6.5

3.7

3.7

2.7

52.8

5

200

4

86.5

79.2

8.7

3.7

3.5

3.4

68.5

PN2: 1 MPa, Glycerol: 0.3 mol/L × 15 mL, Solvent: H2O, NaOH/glycerol molar ratio: 1, on Pt1Co1/CeOx-N5 catalyst: 0.072g. 24


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3.5. Glycerol conversion in the presence of various alkali promoters Since the introduction of NaOH significantly facilitates dehydrogenation of glycerol to LA, we were motivated to conduct further studies to understand the promotional effect of different alkaline species during glycerol conversion. The performances of Pt1Co1/CeOx-N5 catalyst promoted by NaOH, KOH, Ba(OH)2 and Ca(OH)2 are compared in Table 5. Specifically, NaOH (Entry 1) shows the most significant promoting effect (TOF: 792.9 h-1) with good selectivity toward LA (78.1%) at 160 oC, in comparison with other alkalis. Compared with Na+, K+ cation has slightly larger radius but higher electronegativity. Larger ion radius can increase steric hindrance for dehydrogenation step. Furthermore, higher electronegativity will promote electron transfer from Pt species to K+, which will reduce the charge density of Pt species48. Thus the promotional effect by KOH (Entry 2, 413.7 h-1) is slightly lower than that of NaOH. The promotional effect of Ca(OH)2 (Entry 3) and Ba(OH)2 (Entry 4) is also insignificant compared with NaOH, which is possibly due to poor solubility of the alkalis. To further verify the possible steric effect, a control experiment with both NaOH and BaCl2 addition (Entry 5) was conducted. The result show that TOF values over Ba2+ (TOF: 427.3 h-1) is lower than Entry 1 (TOF: 792.9 h-1) but still higher than Ba(OH)2 (TOF: 271.6 h-1). Furthermore, the solid base of CaO was selected to facilitate glycerol conversion reaction. Compared with liquid bases, a lower TOF (42.8 h-1) is obtained, which could be ascribed to poor CaO solubility, although LA selectivity is much higher than literature results.3 Table 5 Glycerol conversion to LA with various bases

Entry #

alkalia

TOF (h-1)

S (%) LA

Glycolsb

25


other acidsc

liquid


a

Page 26 of 33

1

NaOH

792.9

78.1

0.7

18.3

97.1

2

KOH

413.7

74.4

6.6

13.6

94.6

3

Ba(OH)2

271.6

71.0

0

19.8

91.4

4

Ca(OH)2

283.7

82.1

0

9.6

91.7

5

NaOH+BaCl2

427.3

78.5

1.1

15.3

94.9

6 CaO 42.8 88.5 0 7.9 96.5 T: 160 oC, PN2: 1 MPa, Glycerol: 0.3 mol/L, Solvent: H2O, OH-1/glycerol molar ratio: 1, Pt1-

Co1/CeOx-N5 catalyst: 0.072g. b glycols: 1,2-PDO and EG. c other acids: GLYA and GA.

3.6. Possible reaction network for glycerol conversion The experimental results have implied unique performances of bimetallic Pt-Co catalysts for dehydrogenation of glycerol to LA. Taking into account the fact that GLYA and glycols are formed as important co-product, it is important to understand the possible reaction network of glycerol conversion under our reaction condition. We have conducted control experiments to confirm the critical role of NaOH in dehydrogenation of glycerol over bimetallic Pt-Co catalysts, as negligible conversion was observed when no NaOH is added (Figure 6(c)). In addition, hydrothermal induced dehydrogenation cannot occur without Pt-Co catalysts at 200 oC (Table S10). LA selectivity is found to be very high (~80%) with conversion of 86.5% in the presence of Pt-Co catalysts and NaOH promoter. Interestingly, the overall selectivity towards 1,2-PDO and EG (generated via in-situ hydrogenolysis) is very low (~ 9%) in most of our experimental results. However, previous reports show monometallic noble catalysts such as Pt, Rh and Ir often show relatively better selectivity (~ 20%) towards glycols. This difference suggests that the introduction of Co species restrain catalytic activity towards C-O cleavage thus formation of glycols was low. Based on the observations presented above, the following major three pathways will be critically discussed (Scheme 2):

26


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(a) Route 1 presents the base-catalyzed dehydration of glyceraldehyde to form pyruvaldehyde, which can be converted to LA in the presence of base. Alternatively, pyruvaldehyde can also be hydrogenated to 1,2-PDO in the presence of H2 formed insitu. This part is well agreed with various previous works.49-51 (b) Route 2 shows the base-catalyzed retro-aldolization route. The C-C bond cleavage of glyceraldehyde to form intermediate formaldehyde and glycol aldehyde, which could be further hydrogenated to form EG and methanol over metal catalyst.49 Furthermore, some GLYA and GA were also detected in liquid product. (c) Route 3 illustrates the possible pathway for the formation of GLYA. It is interesting to observe significant formation of GLYA and GA even without O2 in the system. The hydration of glyceraldehyde formed a gem-diol, which is subsequently converted to GLYA through dehydrogenation.22, 29, 32 GLYA could be further converted, via C-C bond splitting, to GA and formaldehyde, the latter of which can be converted methane or CO2.29, 52 The methane was detected with small quantities in our all gaseous products. CO2 was also detected in very small amount, as CO2 formed during glycerol conversion would be absorbed to form carbonate in alkaline medium.

Scheme 2 Possible reaction pathways of glycerol conversion on Pt1-Co1/CeOx-N5 catalyst 27



4. Conclusion In summary, the monometallic Pt or Co and bimetallic Pt-Co with different Pt/Co ratios supported on CeOx catalysts were synthesized by a simple fractional precipitation method and tested in the dehydrogenation of glycerol. The results reveal that bimetallic Pt1-Co1/CeOx catalysts display remarkable activity (TOF: 1533.9 h-1) and leading selectivity (87.7%) for glycerol conversion to LA at 200 oC. The Pt1-Co1/CeOx-H1 catalyst shows highest activity (1831.0 h-1) at 200 oC, induced by electron transfer from Co0 to Pt0 and strong Pt-Ce interaction. The influence of glycerol and NaOH concentration on initial reaction rates further suggests that, dehydrogenation of glycerol possibly follows a dual-site mechanism, where both glycerol and NaOH need to be activated on bimetallic Pt-Co surface. Based on experimental studies and surface characterization data, a comprehensive reaction network was finally proposed and discussed with regard to the formation of lactic acid, glycols, glyceric acid and glycolic acid in the aqueous medium. The experimental results presented in this work will provide useful information for rational design of active and selective dehydrogenation catalysts for various other energy and environmental applications. Supporting Information: Experimental details of the preparation and characterization of bimetallic Pt-Co catalysts, additional results (scheme of catalysts synthesis, TGA and DSC curves of Pt1Co1/CeOx catalyst, typical liquid chromatograph, XPS analysis, kinetic model and control experiments) Acknowledgements The authors want to acknowledge financial supports National Natural Science

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Foundation of China (21706290), Natural Science Foundation of Shandong Province (ZR2017MB004), Innovative Research Funding from Qingdao, Shandong Province (17-1-1-80-jch), “Fundamental Research Funds for the Central Universities” (17CX02017A) and New Faculty Start-Up Funding from China University of Petroleum (YJ201601059). References (1). Quispe, C. A. G.; Coronado, C. J. R.; Carvalho Jr, J. A., Glycerol: Production, consumption, prices,

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85x37mm (300 x 300 DPI)


Toward Selective Dehydrogenation of Glycerol to Lactic Acid over

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