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One-Pot Template-Free Synthesis of Cu-MOR Zeolite towards Efficient Catalyst Support for Aerobic Oxidation of 5-Hydroxymethylfurfural under Ambient Pressure Wei Zhang, Jingyan Xie, Wei Hou, Yangqing Liu, Yu Zhou, and Jun Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b07675 • Publication Date (Web): 13 Aug 2016 Downloaded from http://pubs.acs.org on August 18, 2016

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ACS Applied Materials & Interfaces

One-Pot Template-Free Synthesis of Cu-MOR Zeolite towards Efficient Catalyst Support for Aerobic Oxidation of 5-Hydroxymethylfurfural under Ambient Pressure Wei Zhang, Jingyan Xie, Wei Hou, Yangqing Liu, Yu Zhou∗ and Jun Wang∗

State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemical Engineering, Nanjing Tech University (former Nanjing University of Technology), Nanjing, 210009, P. R. China *Corresponding author, Tel: +86-25-83172264, Fax: +86-25-83172261 E-mail: [email protected] (Y. Zhou), [email protected] (J. Wang)

KEYWORDS: copper-containing zeolite, heterogeneous catalysis, 5-hydroxymethylfurfural, hydrothermal synthesis, biomass conversion. ABSTRACT: Supported catalysts are widely studied and exploring new promising supports is significant to access more applications. In this work, novel copper (Cu) containing MOR type zeolites Cu-MOR were synthesized in a one-pot template-free route and served as efficient supports for vanadium oxide. In the heterogeneous oxidation of 5-hydroxymethylfurfural (HMF) to 2, 5-diformylfuran (DFF) with molecular oxygen (O2) under ambient pressure, the obtained catalyst demonstrated high yield (91.5%) and good reusability. Even under the ambient air pressure, it gave the DFF yield of 72.1%. Structure-activity relationship analysis indicated that the strong interaction between the framework Cu species and the guest V sites accounted for the remarkable performance. This work reveals that the Cu-MOR zeolite uniquely acts as the robust support towards well-performed non-noble metal heterogeneous catalyst for biomass conversion.

1

ACS Paragon Plus Environment


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1. INTRODUCTION Supported heterogeneous catalysts are widely applied both in industry production and academic study.1-3 Porous materials with large surface area are the commonly adopted supports because they endow high dispersion of the guest and provide fast mass transfer.4-10 As a result, many efforts have been devoted to develop stable and efficient porous supports for fabricating the versatile target catalysts.11-14 In this context, zeolites with large-scale synthesis and extensive applications in industry have already attracted long-term and growing attentions due to their high thermal/hydrothermal stability, low cost, tunable porosity and acid/basic/redox properties.15,16,17 Doping transition metals into zeolites is an efficient strategy to adjust their physicochemical properties, which have been used as heterogeneous catalysts for many reactions.18,19 Nonetheless, rare study is related to the application of heteroatomic zeolites as catalyst supports, despite that heteroatom doping provides the nearly isolated metal ions in the framework. Moreover, those isolated metal ions in the framework of zeolites may greatly alter their surface state, affording the required host-guest interaction in preparing supported catalysts.20,21 Herein, a new kind of Cu-containing MOR zeolite is synthesized and used as the catalyst support to load vanadium oxide and the obtained supported catalyst act as an efficient heterogeneous catalyst for biomass conversion. Zeolites with Cu species are highly promising heterogeneous catalysts for a variety of chemical reactions, such as NOx reduction,22 selective oxidation of methane to methanol23 and aerobic oxidation in liquid phase.24 Normally, Cu species are introduced into zeolites by post-preparation methods like ion-exchange and impregnation.25,26 Only several reports adopted the direct synthesis to prepare Cu-containing zeolites, such as CuSSZ-1327,28 and Cu-X, Cu-Y, Cu-ZSM-5.29 Mordenite (MOR) zeolites with parallel 12-membered ring (0.65×0.70 nm) and Cmcm space group are 2


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industrially applied in alkylation, reforming, hydroisomerization, dewaxing, cracking and so on. Great efforts have been made on the synthesis and modification of MOR zeolite, regrettably, no Cu-containing MOR zeolite is directly synthesized up to now. Development of biomass-derived chemicals interests many researchers, among which 5-hydroxymethylfurfural (HMF) is considered as an important bridge molecule between biomass and specialty chemicals.30-34 Selective oxidation of HMF to 2, 5-diformylfuran (DFF) has received particular attentions, because DFF is a valuable intermediate for manufacturing pharmaceuticals, fungicides, heterocyclic ligands and furan-based biopolymers.35-37 Heterogeneous catalysts are preferred due to their facile separation and recycling performance. Hydrotalcite-supported ruthenium catalyst (Ru/HT) converted HMF to DFF with O2 flow (20 mL min-1) with the yield of 92%.5 γ-Fe2O3@HAP-Ru was used in the oxidation of HMF with O2 flow (20 mL min-1) in N,N-dimethylformamide (DMF), achieving the DFF yield of 89.1%.35 As non-noble metal catalysts, vanadium based catalysts were developed for the oxidation of HMF to DFF with O2;6,37,38 nevertheless, V2O5 supported on zeolite like H-Beta was only active at a high pressure of O2 with the disadvantage of rapid deactivation,6 which hindered the application of zeolites in this important biomass conversion reaction. In this work, novel Cu-containing MOR zeolites (Cu-MOR) are directly synthesized in a one-pot route without using any organic template. The preparation process involves the unusual co-hydrolysis/condensation of silica precursor (tetraethoxysilane, TEOS) and Cu salt (copper (II) acetate monohydrate, Cu(CH3COO)2·H2O) under mild acid condition, which is a key step towards in situ formation of framework Cu species in the followed base-conditioned hydrothermal zeolite crystallization. The obtained Cu-MOR zeolites can act as an efficient support to load vanadium oxide, 3



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wherein the framework Cu species provide strong interaction towards vanadium species. Owing to such host-guest interaction, the obtained vanadium oxide supported on Cu-MOR exhibits high yield and well reusability in the selective oxidation of HMF to DFF under ambient O2 pressure (balloon). Systematic control experiments, including Cu-ion exchanged and CuO impregnated MOR zeolites, indicate that the counterparts lacking such strong V-Cu interaction show much lower activities. Further catalytic assessment reveals that the catalyst is also active even under ambient air pressure. Possible lattice oxygen vacancy-mediated mechanistic pathway is proposed based on reaction and characterization results.

Scheme 1. Schematic diagram for the preparation of V2O5@Cu-MOR samples.

4


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2. EXPERIMENTAL SECTION

2.1. Preparation of samples 2.1.1. Synthesis of Cu-containing MOR zeolites All chemicals were of analytical grade and employed as received. The syntheses of copper (Cu) containing mordenite (Cu-MOR) zeolites were carried out in an acidic hydrolysis route in the absent of any organic templates. In a typical procedure, 6.34 g tetraethoxysilane (TEOS, Sinopharm) was mixed with 30 g deionized water. Under vigorous stirring, the concentrated hydrochloric acid was slowly dropped to reach pH=1.0, followed by the addition of pre-calculated amount of copper (II) acetate monohydrate (Cu(CH3COO)2·H2O, Aladdin). The resulted mixture was stirred at 20 °C for 20 h to obtain a complete co-hydrolysis/condensation of TEOS with the copper salt. Next, 1.34 g aluminium sulfate octadecahydrate (Al2(SO4)3‧18H2O, Xilong) was introduced. After the mixture became clear, aqueous NaOH solution (12.5 M) was slowly drop-wise added to induce the gelation at pH=5.0-6.0 and the mixture was stirred for 0.5 h. Finally, the pH value of the mixture was regulated to 12.0 by the addition of NaOH solution (12.5 M). The gel molar composition was xCuO: 1SiO2: 1/15Al2O3: 60H2O (x=0.003, 0.008, 0.015). After aging at room temperature for 20 h, the slurry was transferred to a Teflon-lined stainless steel autoclave and left to statically crystallize at 180 °C. Solid was separated by centrifugation, washed with water and air-dried. The obtained as-synthesized products were designated as Cu-MOR(n), in which n was the index of the Si/Cu molar ratio in the gel (n=1/x). The Cu-free sample obtained with the similar procedure was labelled as MOR. For comparison, the conventional “basic co-hydrolysis route” was utilized to prepare a counterpart of Cu-MOR(125). 6.34 g TEOS and 0.0484 g cupric acetate were co-hydrolysed in basic

5



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media (using an aqueous solution of NaOH instead of concentrated HCl), with the other conditions similar to Cu-MOR(125). The obtained sample was named as Cu-MOR-basic. Cu2+-ion-exchanged MOR (Cu(Ex)-MOR) and CuO-impregnated MOR (Cu(Im)-MOR) were synthesized, and the details of the preparation processes are available in Supplementary Information.

2.1.2. Synthesis of V2O5@Cu-MOR zeolites The vanadium oxide supported Cu-MOR zeolites, shorten as V2O5@Cu-MOR(n), were obtained by incipient wetness impregnation. In a typical preparation, ammonium metavanadate (NH4VO3, Zhejiang Yuda) (0.1298 g) was dissolved in water (1 mL) and then mixed with Cu-MOR (1 g), the mixture was evaporated at 80 °C to remove water. After that, the solid was dried overnight in an oven at 100 °C. Finally, the sample was calcined at 550 °C for 5 h in air, giving the sample 10V2O5@Cu-MOR(n) with the V2O5 loading amount of 10 wt%. Two

other

Cu

and

V

containing

MOR

zeolites,

0.8%Cu-10%V2O5@MOR

and

0.8%Cu@10%V2O5@MOR were prepared. The former was prepared by simultaneously impregnating CuO (loading amount 1.0 wt% based on Cu) and V2O5 (loading amount 10 wt% based on V2O5) on MOR zeolite. The latter was achieved by impregnating CuO (loading amount 1.0 wt% based on Cu) on 10V2O5@MOR sample.

2.2. Characterizations X-ray diffraction (XRD) patterns were collected on a SmartLab diffractmeter (Rigaku Corporation) equipped with a rotating anode Cu source (9 kW) at 45 kV and 100 mA, from 5 to 50° with a scan rate of 0.2° s-1. The morphologies of the samples were tested with a field emission 6


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scanning electron microscope (SEM) instrument of Hitachi S-4800. Energy dispersive X-ray spectrometry (EDS) analysis and elemental mapping were obtained on this instrument with an acceleration voltage of 20 kV. Morphology and microstructure of zeolites were determined by transmission electron microscopy (TEM) using a JEM-2100 F. The metal contents were detected by X-ray Fluorescence (XRF, Rigaku ZSX Primus II) with Rh end-window tube and “SQX” calculation software. The nitrogen sorption isotherms were measured by using a BELSORP-MAX analyzer with the samples degassed at 573 K for 3 h prior to the measurements. The pore-size distribution curves were achieved by Horvath-Kawazoe (HK) methods. FT-IR spectra were recorded on an Agilent Cary 660 instrument FT-IR instrument (KBr disks) in the 4000-400 cm-1 region. Solid UV-vis spectra were measured with the Shimadzu UV 2600 spectrometer and BaSO4 was used as an internal standard. Raman Spectra were obtained using a Horiba HR 800 spectrometer. As a source of excitation, the 514 nm line of a Spectra Physics 2018 Argon/Krypton Ion Laser system was focused through an Olympus BX41 microscope equipped with a 50 magnification objective. Electron spin resonance (EPR) spectra were recorded on a Bruker EMX-10/12 spectrometer at the X-band at ambient temperature. X-ray photoelectron spectra (XPS) were conducted on a PHI 5000 Versa Probe X-ray photoelectron spectrometer equipped with Al Kα radiation (1486.6 eV). The

29

Si and

27

Al MAS

NMR spectra were recorded on a Bruker Avance III spectrometer. Temperature-programmed reduction (TPR) was carried out using the glass flow system. TPR runs were performed in flowing 10% H2/Ar (30 cm3 min-1), ramping the temperature at 10 K min−1 and using a Gow-Mac thermal conductivity detector (TCD).

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2.3. Catalytic activity tests Catalytic

performances

were

assessed

in

the

selective

aerobic

oxidation

of

5-hydroxymethylfurfural (HMF) into 2, 5-diformylfuran (DFF) using O2 or air (balloon) as oxidant. In a typical run, HMF (0.4 mmol, 0.05 g) and catalyst 10V2O5@Cu-MOR(125) (0.1 g) was added into dimethyl sulfoxide (DMSO, 5 mL) with magnetic stirring. Then the system was vacuumized and equipped with an oxygen balloon. The reaction was carried out at 120 °C for 7 h with vigorous stirring. After the reaction, ±2-methyl-1-butanol (0.05 g) was added into as an internal standard. The mixture was analyzed by gas chromatography (Agilent GC 7890B) equipped with a hydrogen flame ionization detector and a capillary column (HP-5, 30 m × 0.25 mm × 0.25 µm). After reaction, the catalyst was separated by filtration and thoroughly washed with ethanol. Then, the catalyst was dried at 80 °C for 12 h in an oven. At last, the catalyst was calcined at 550 °C for 5 h and then reused for the next run. Both direct recovered and regenerated catalysts were assessed in a five run recycling tests. Regenerated catalyst was obtained by reloading small amount (0.7 wt%) V2O5 on the recovered catalyst through the wet-impregnation method.

3. RESULTS AND DISCUSSION

3.1. Synthesis and textural property of Cu-MOR and V2O5@Cu-MOR Following the procedure in Scheme 1, Cu-containing MOR zeolites Cu-MOR(n) are synthesized in template-free route by varying the molar ratio of Si to Cu (n) in the initial gel. Various synthesis factors are investigated including the silica sources, SiO2/Al2O3, H2O/SiO2, crystallization time, crystallization temperatures and crystallization pH values (Figure S1-S6). The XRD patterns in 8


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A

B

CuO

k

Intensity (a.u.)

e

Intensity (a.u.)

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


d

c

b (202) (310) (111) (200) (020) (110)

(130)

10

(330)

i h g

a

(350) (150) (002) (511) (241)

j

(402)

(680) (732) (004)

(352)

f

20

30

40

10

50

20

30

40

50

2 Theta (degree)

2 Theta (degree)

Figure 1. XRD patterns of (A) Cu-MOR(n) and (B) V2O5@Cu-MOR(n): (a) MOR, (b) Cu-MOR(333), (c) Cu-MOR(125), (d) Cu-MOR(67), (e) Cu-MOR-basic, (f) 10V2O5@MOR, (g) 10V2O5@Cu-MOR(333),

(h)

10V2O5@Cu-MOR(125),

(i)

10V2O5@Cu-MOR(67),

(j)

5V2O5@Cu-MOR(125) and (k) 15V2O5@Cu-MOR(125).

Figure 1A indicate that all Cu-MOR(n) samples (n=333, 125 and 67) exhibit typical peaks associated with MOR zeolite, which is identified by using the joint committee on powder diffraction standards (JCPDS) file (43-0171 for mordenite zeolite). Notably, CuO (2 theta at 35.6 and 38.8°) and Cu2O (2 theta at 36.44°)26 phases cannot be detected in these samples, suggesting high dispersion of Cu species. Further increasing the Cu content in the synthesis mixture will hinder the formation of the zeolite structure. For instance, in the case of n=50, the obtained product presents low crystallinity and

quartz

impurity

(Figure

S7).

The

sample

Cu-MOR-basic

synthesized

from

co-hydrolysis/condensation of silica and copper precursors under the conventional base condition 9



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Table 1. Textural properties and catalytic performances of various catalysts in aerobic oxidation of HMF to DFF.a

Entry

Catalyst

SiO2/Al2O3[b] Cu[b] V2O5[b] SBET VP CHMF[c] YDFF[d] TON[e] (mol/mol) (%) (wt%) (wt%) (m2g-1) (cm3g-1) (%)

1f

V2O5

-g

-

100

3.9

0.04

98.0

67.1

2.5

2

10V2O5@MOR

14.64

-

9.95

271

0.16

>99.0

63.4

2.4

3

5V2O5@CuMOR(125)

14.12

1.01

5.12

298

0.17

>99.9

73.1

5.3

4

10V2O5@Cu-MOR(125)

14.41

0.98

9.98

282

0.16

>99.9

91.5

3.3

5h

10V2O5@Cu-MOR(125)

14.41

0.98

9.98

282

0.16

>99.9

72.1

2.5

i

10V2O5@Cu-MOR(125)

14.41

0.98

9.98

282

0.16

>99.9

18.3

0.7

7

10V2O5@Cu-MOR(333)

14.50

0.25

9.96

270

0.16

>99.9

80.8

2.9

8

10V2O5@Cu-MOR(67)

14.52

1.38

9.93

246

0.18

>99.9

63.8

2.3

9

15V2O5@Cu-MOR(125)

14.61

0.95

14.45

240

0.14

>99.9

81.8

3.0

10

10V2O5@Cu(Ex)-MOR

14.48

0.94

9.95

225

0.14

>99.9

61.2

2.2

11

10V2O5@Cu(Im)-MOR

14.92

1.07

9.42

237

0.14

>99.9

79.3

2.9

12

0.8%Cu-10V2O5@MOR

14.78

1.09

9.16

285

0.16

>99.9

78.1

2.8

13

0.8%Cu@10V2O5@MOR 13.46

1.05

9.68

116

0.08

>99.9

66.1

2.4

6

[a] Reaction conditions: 0.05 g HMF, 0.1 g catalyst, 5 mL DMSO, 120 °C, 7 h, O2 balloon. [b] Cu and V2O5 content Measured by XRF. [c] Conversion of HMF. [d] Yield of DFF; 2, 5-furandicarboxylic (FDCA) is identified as the byproduct detected by 6540 UHD Accurate-MassQ-TOFLC/MS. [e] Turnover number (TON): mole of DFF produced over per molar V in the catalyst. [f] Catalyst amount: 0.01 g V2O5. [g] Not applicable. [h] Reaction conditions: 0.05 g HMF, 0.15 g catalyst, 8 mL DMSO, 125 °C, 6 h, air balloon. [i] Reaction conditions: 0.05 g HMF, 0.1 g catalyst, 5 mL DMSO, 120 °C, 7 h, N2 balloon.

leads to the detection of CuO phase (Figure 1), suggesting that the initial co-hydrolysis/condensation step plays a crucial role for synthesis of Cu-MOR zeolites, similar to previously reported other heteroatomic zeolites.39-41 Table S1 lists the unit cell parameters of these Cu-MOR(n) series that are calculated from the XRD patterns. The values become larger after incorporation of Cu ions, reaching the maximum at n=125. The incorporation of bulky metal cations with ionic radius larger than Si4+

10


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and Al3+ will enlarge the unit cell parameter.39 The ionic radius of Cu2+ (0.073 nm) is larger than those of Si4+ (0.04 nm) and Al3+ (0.053 nm), thus the increase of unit cell parameters may reveal the incorporation of Cu2+ cations into the framework sites of MOR structure. On the contrary, Cu(Ex)-MOR does not lead to such significant increase in unit cell parameters compared to Cu-MOR(125), suggesting that only partial amounts of copper may locate at framework sites.

(a)

(f)

(k)

(b)

(c)

(d)

(e)

(g)

(h)

(i)

(j)

(m)

(l)

1 nm

Figure 2. SEM images of (a, b) Cu-MOR(125) with EDS elemental mapping images of (c) Si, (d) Al and (e) Cu element. SEM images of (f) 10V2O5@Cu-MOR(125) with EDS elemental mapping images of (g) Si, (h) Al, (i) Cu and (j) V element. TEM images of (k, l) Cu-MOR(125) and (m) 10V2O5@Cu-MOR(125). 11



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Furthermore, no increment of the unit cell volume is observed for the impregnated sample Cu(Im)-MOR, indicating that the Cu species disperses outside of zeolite skeleton in this case. Figure 1B displays the XRD patterns of the vanadia supported samples V2O5@Cu-MOR(n). All of them show well-preserved crystal structure of MOR zeolite. No significant diffraction peaks derived from other phases such as Cu5V2O10 crystallites (2 theta at 29.8 and 43.0°) is observed, reflecting the high dispersion of vanadium (V) species.42 The chemical compositions of Cu-MOR and V2O5@Cu-MOR samples are measured by XRF (Table 1), confirming the existence of Cu and V species. The Cu content increases with the initial Cu ions added in the synthesis mixture, while the SiO2/Al2O3 ratios are all around 14.5, close to the value in the gel (15). The morphologies of these samples are collected by SEM images (Figure S8). All of them own almost identical morphology with nearly circular pie-shaped particles sized of 40-50 µm, similar to the neat MOR. Magnified SEM images illustrate that the particles are composed of small primary rods. Similar morphology is observed on the V2O5@Cu-MOR samples. The elemental mappings of Cu-MOR(125) and 10V2O5@Cu-MOR(125) confirm the relatively uniform dispersion of Cu and V species (Figure 2a-j). TEM images for typical samples Cu-MOR(125) and 10V2O5@Cu-MOR(125) exhibit the well zeolitic crystalline structure (Figure 2k-m). No V2O5 bulk crystal is observed on 10V2O5@Cu-MOR(125), confirming the high dispersion of V species on MOR zeolite (Figure 2m).

29

Si MAS NMR spectrum for the typical sample Cu-MOR(125) (Figure

3A) presents two strong signals at -105 and -112 ppm, respectively assigned to Si(1Al) and Si(0Al). The peak at -99 ppm is due to the Si-OH.43 27Al MAS NMR spectrum (Figure 3B) shows only one signal at 57 ppm corresponding to the tetrahedral coordinated aluminum species, while no signal is observed at 0 ppm for the extra-framework octahedral coordinated aluminum. 12


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A

B

-105

57

Intensity (a.u.)

Intensity (a.u.)

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


-112

-99

120

-80 -100 -120 -140 -160

80

40

0

-40

Chemical Shift (ppm)

Chemical Shift (ppm)

Figure 3. (A) 29Si and (B) 27Al MAS NMR spectra of Cu-MOR(125).

Quantitative porosity is analyzed by N2 sorption experiment (Figure 4 and S9) with selected samples measured at high accuracy mode (relative pressure p/p0 of 10-7 Pa) to insight into their pore size distribution in the micropore range (Figure 4). N2 sorption isotherms of Cu-MOR(n) samples exhibit mixed type of I and IV with a sharp uptake at low relative pressures and a slowly increasing one at higher relative pressures (Figure 4A and S9), indicative of typical micro-/mesoporous materials possessing mainly zeolitic micro-channels and small amount mesopores derived from the aggregations of primary particles. Similar isotherms are observed on the V2O5@Cu-MOR samples. As showed in Table 1, all the samples show the large surface area and pore volume, suggesting the existence of abundant open pores. Compared to each Cu-MOR(n), the corresponding V2O5@Cu-MOR(n) samples demonstrate decreased surface area and pore volume due to the occupation of the host V species in pores. Further, compared to Cu(Ex)-MOR and 13


Volume adsorption (cm3 g-1)

A d

300

c

200

b a

100

0 0.0

0.2

0.4

0.6

0.8

1.0

Page 14 of 40

B

250

h

200

g f

150

e

100 50 0 0.0

Relative pressure (p/p0)

0.2

0.4

0.6

0.8

1.0

Relative pressure (p/p0)

C

D d

h

c

g

dVp/ dW

dVp/ dW

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

Volume adsorption (cm3 g-1)


b

f

e a

0.5

1.0

1.5

2.0

2.5

3.0

0.5

1.0

W (nm)

1.5

2.0

2.5

3.0

W (nm)

Figure 4. (A, B) Nitrogen sorption isotherms and (C, D) pore size distribution curves of Cu-MOR(n) and V2O5@Cu-MOR(n): (a) MOR, (b), Cu-MOR(125), (c) Cu(Ex)-MOR, (d) Cu(Im)-MOR, (e) 10V2O5@MOR,

(f)

10V2O5@Cu-MOR(125),

(g)

10V2O5@Cu(Ex)-MOR

and

(h)

10V2O5@Cu(Im)-MOR. The isotherms are collected at high accuracy mode (relative pressure p/p0 of 10-7 Pa) and the pore size distributions are calculated by HK method.

14


Page 15 of 40

A

B

250

300

250

300

f

680

300

Absorbance (a.u.)

Absorbance (a.u.)

260

750 e

300

250

750 d

250 300

c

480 260 260 260 260

600-800

l 300

k

300

480

j

300

i 300

h

270 320

400

b a

300 400 500 600 700 800

g V2O5

300 400 500 600 700 800

Wavelength (nm)

Wavelength (nm) D

C 972

Transmission (a.u.)

f

Transmission (a.u.)

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


536

e 960

d c

960

b

960

a

960 800 720 580 560 1225

l 990

k j

963

i 990

h

963

g

963 990

450

1050 1000

1400 1200 1000 800

600

-1

400

1400 1200 1000 800

600

-1

400

Wavenumber (cm )

Wavenumber (cm )

Figure 5. (A, B) UV-Vis and (C, D) IR spectra of Cu-MOR(n) and V2O5@Cu-MOR(n): (a) MOR, (b) Cu-MOR(333), (c) Cu-MOR(125), (d) Cu-MOR(67), (e) Cu(Ex)-MOR, (f) Cu(Im)-MOR, (g) 10V2O5@MOR,

(h)

10V2O5@Cu-MOR(333),

(i)

10V2O5@Cu-MOR(125),

(j)

10V2O5@Cu-MOR(67), (k) 5V2O5@Cu-MOR(125) and (l) 15V2O5@Cu-MOR(125).

Cu(Im)-MOR samples, Cu-MOR(125) sample almost remains the similar pore size dispersion as its 15



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

parent MOR (Figure 4B), suggesting that the in situ synthesized Cu-MOR zeolite benefits the preservation of open zeolitic channels due to the incorporation of Cu species into the framework. The obtained 10V2O5@MOR(125) sample also exhibits narrow pore size dispersion (Figure 4D), reflecting the high dispersion of V species that may come from their strong interaction with the support.

3.2. Surface state of Cu and V species Figure 5A shows the UV-vis spectra of Cu-MOR(n). The strong band at ~250 nm is assigned to the charge transition from oxygen to isolated Cu ion (O2−→Cu2+) (LMCT, ligand to metal charge transfer), implying the existence of mononuclear Cu2+ ions interacting with the lattice oxygen. Such signal is absent in MOR and depicts weak intensity over Cu(Ex)-MOR and Cu(Im)-MOR, because they have none or rare isolated Cu species. No adsorption between 300-400 nm is observed on Cu-MOR(333), excluding the formation of oligomeric species [Cu2+-O2--Cu2+]. Tail peak above 300 nm emerges in Cu-MOR(125), suggesting the emergence of slight [Cu2+-O2--Cu2+] species. Such band becomes visible in Cu-MOR(67), Cu(Ex)-MOR and Cu(Im)-MOR, suggesting the presence of [Cu2+-O2--Cu2+] species. Additional broad band around 750 nm for Cu-MOR(67) and Cu(Ex)-MOR can be assigned to the typical d-d transition of Cu2+ with distorted octahedral or square pyramidal coordination.44 The wide absorption centered at 680 nm is attributed to the bulky CuO aggregation in Cu(Im)-MOR,45 but lack in Cu-MOR. Figure 5B depicts the UV-vis spectra of V2O5@Cu-MOR samples. V2O5 gives a broad band ranging from 220 to 550 nm for the V5+ species in pentahedral or distorted octahedral coordination. 10V2O5@MOR presents two major adsorption bands centered at 270 and 320 nm, assignable to LMCT for the isolated tetrahedral VO4 and oligomeric tetrahedrally 16


Page 16 of 40

Page 17 of 40

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


coordinated V species, respectively.46 The broad tail observed above 400 nm reflects the occurrence of distorted octahedral V species.47 Therefore, 10V2O5@MOR contains V species in forms of monomers, chains, and microcrystallites. Significant variation of adsorption peaks happens by loading V species onto Cu-MOR. 10V2O5@Cu-MOR(333) and 5/10V2O5@Cu-MOR(125) exhibit the strong peak at 260-270 nm accompanying with a weak shoulder above 300 nm, indicating that these V species are mainly in isolated and oligomeric tetrahedral V5+ state but rarely in the square pyramidal or distorted octahedral coordination.48 An apparent peak around 480 nm emerges on 15V2O5@Cu-MOR(125) and 10V2O5@Cu-MOR(67), revealing the existence of polymerized octahedral V species. The above results reflect a strong interaction between the framework Cu and supported V species. On the contrary, Cu2+ ion-exchanged and impregnated CuO lack such interaction. Large amount of polymerized octahedral V species exist on 10V2O5@Cu(Ex)-MOR and they become dominating on 10V2O5@Cu(Im)-MOR, as demonstrated by the strong adsorption around 480 nm. Notably, 15V2O5@MOR(125) presents a band around 600-800 nm attributable to the V3+/V4+ d-d transition. The band for Cu species becomes murky due to the signal overlapping with V species, as well as possible state change of Cu species by the strong host-guest interaction. 0.8%Cu@10%V2O5@MOR and 0.8%Cu-10%V2O5@MOR obtained from impregnation method (Figure S10) show strong adsorption around 480 nm for the large amount of polymerized octahedral V species, further proving the uniqueness of framework Cu ions in Cu-MOR zeolites that allow high and even isolated dispersion of V species through intimate interaction. IR spectra of Cu-MOR (Figure 5C) exhibit typical vibrations of MOR framework: external and internal stretching at 1225 and 1050 cm-1 (asymmetric) and 800 and 720 cm-1 (symmetric), double ring at 580 and 560 cm−1, and T-O bending at 450 cm-1. The band at ~1000 cm-1 for the framework 17



stretching vibrations of MOR (Si-O-Si, or Si-O-Al) shifts to 960 cm-1 for Cu-MOR. Such shift can be assigned to the formation of Si-O-Cu bond, further revealing the existence of framework Cu species.49 No such shift occurs for Cu(Ex)-MOR, indicating that the dissociative Cu species slightly affect the framework vibration. The band shifting to 970~980 cm-1 for Cu(Im)-MOR is attributed to the stretching vibrations of surface Si-O-Cu.50 The impregnated CuO is reflected by the small hump peak appearing at 536 cm-1.49 As shown in Figure 5d, the V-loaded samples all keep the typical vibrations

for

MOR

structure.

The

~990

cm-1

band

for

15V2O5@Cu-MOR(125),

10V2O5@Cu-MOR(67), 10V2O5@MOR and other control samples (Figure S11) suggests the existence of polyvanadate V-O-V species.51 10V2O5@Cu-MOR(333) and 5-10V2O5@Cu-MOR(125)

9E6

A

g = 2.08

× 400 for e

8E7

B

2500

6E6 d

g = 2.12

5E6 c

g = 2.12

3000

3500

4E6 g = 2.12

3E6 b 2E6

g = 2.12

1E6 a 0

k 5E7

3000

3500

4000

i h g

g =1.96 g =1.96 g =1.96

f 1E7 V O 2 5

CuO

g=1.96

j

4E7

2E7

g=1.96

g =1.96

6E7

3E7

g=2.12

and j (dark-blue)

7E7

4000

Intensity (a.u.)

7E6

e

g=2.12

×4 for i (green)

8E6

Intensity (a.u.)

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

Page 18 of 40

0 2000

4000

6000

Magnetic Field (G)

2000

4000

6000

Magnetic Field (G)

Figure 6. EPR spectra of (A) Cu-MOR(n) and (B) V2O5@Cu-MOR(n): (a) Cu-MOR(333), (b) Cu-MOR(125), (c) Cu-MOR(67), (d) Cu(Ex)-MOR, (e) Cu(Im)-MOR, (f) 10V2O5@MOR, (g) 10V2O5@Cu-MOR(333),

(h)

10V2O5@Cu-MOR(125),

(i)

5V2O5@Cu-MOR(125) and (k) 15V2O5@Cu-MOR(125). 18


10V2O5@Cu-MOR(67),

(j)

Page 19 of 40

give the 960~970 cm-1 band attributable to V-O-Cu or V-O-Si.52 The comparison further indicates that rare polyvanadate V-O-V species exist in these samples due to the strong host-guest interaction. EPR spectra allows the detection of paramagnetic Cu2+ with a d9 electronic configuration. Only isolated Cu2+ can be observed, while Cu+, [Cu2+-O2--Cu2+] and CuO species are silent.53 As shown in Figure 6A, Cu-MOR(n) series present strong isotropic signal without resolvable Cu2+ splitting, characteristic of isolated Cu2+ species.54 The strongest intensity for Cu-MOR(125) suggests its largest amount of such isolated Cu species, while the very weak signal on Cu(Ex)-MOR and Cu(Im)-MOR reveals slight isolated Cu species and large amount of [Cu2+-O2--Cu2+] or CuO species. EPR is also suitable to characterize the structure of surface V4+ species. Both Cu2+ and V4+ species have EPR signals, but give different g values of 2.12 and 1.96. The EPR signals of V4+ and Cu2+ species are centered at about 3500 and 3100 Gs, thus can be used to differentiate Cu2+ and V4+

B

A

915

312 395 441 468 312 400 441 470 312 395 441 468 312

200

h 915

d

824

824

c

824

b

Intensity (a.u.)

297 395 347 441468

Intensity (a.u.)

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


a

824

850

g 930

f 942 995 917

e 400

600

800 1000 1200 1400 -1

Raman Shift (cm )

200

400

600

800 1000 1200 1400 -1

Raman shift (cm )

Figure 7. Raman spectra of (A) Cu-MOR(n) and (B) V2O5@Cu-MOR(n): (a) MOR, (b) Cu-MOR(125),

(c)

Cu(Ex)-MOR,

(d)

Cu(Im)-MOR,

(e)

10V2O5@MOR,

10V2O5@Cu-MOR(125), (g) 10V2O5@Cu(Ex)-MOR and (h) 10V2O5@Cu(Im)-MOR. 19


(f)


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

species.55 In the EPR spectra of the V-loaded samples (Figure 6B), the signal refers to the V4+ species with higher intensity corresponding to higher V4+ content. V2O5 shows a weak signal due to the small amount of V4+ impurity. The strong signal for 10V2O5@MOR reflects the existence of large amount V4+ species. Interestingly, weak signals indicating slight V4+ species also occur for 10V2O5@Cu-MOR(333) and 5/10V2O5@Cu-MOR(125) together with disappearance of the initial signal of the isolated Cu species. The reason can be assigned to the strong interaction between Cu and V species. 10V2O5@Cu-MOR(67) shows a stronger signal of V4+ species, since it has less isolated Cu ions but more [Cu2+-O2--Cu2+] species than its above three counterparts. For similar reason, large amount of V4+ species exist in 10V2O5@Cu(Ex)-MOR, 10V2O5@Cu(Im)-MOR, 0.8%Cu@10%V2O5@MOR and 0.8%Cu-10%V2O5@MOR (Figure S12). 15V2O5@Cu-MOR(125) contains enhanced amount of V4+ species due to the high V loading that will weaken the interaction between out-layer V species and surface Cu species. EPR results again evidence that the framework isolated Cu species can provide strong interaction to V species. Raman spectra (Figure 7) exhibit the typical bands of MOR framework at 312 cm-1 (bending mode of 6-membered rings), 395 cm-1 (bending mode of 5-membered rings), 441 and 468 cm-1 (bending mode of 4-membered rings), and 824 cm-1 (symmetric stretching motions of T-O bonds).56,57 For Cu(Im)-MOR, two additional signals are observed at 297 and 347 cm-1 for the vibrations of the lattice oxygen atoms in CuO structure.23 However, they are absent for Cu-MOR(125), suggesting that their Cu species may be in the framework or homogeneously dispersed in zeolite lattice. Besides, Cu-MOR(125) demonstrate broader and weaker peaks at 400 and 470 cm-1 than MOR, Cu(Im)-MOR and Cu(Ex)-MOR. This phenomenon arises from the distortion of the harmonic vibration of the MOR framework due to the incorporation of Cu ions into 20


Page 20 of 40

Page 21 of 40

the framework.58 For 10V2O5@MOR, the small band at 995 cm-1 corresponds to trace quantities of crystalline V2O5 due to its strong Raman cross-section.59 The band at 930 cm-1 for 10V2O5@Cu-MOR(125) is assigned to the V=O symmetric stretching of the highly dispersed oligomerized

vanadium

oxides

in

extra-framework.60

For

10V2O5@Cu(Ex)-MOR

and

10V2O5@Cu(Im)-MOR, the band at 915 cm-1 presents the V-O-V polymeric (metavanadate-like) species.61 The signal at 850 cm-1 for 10V2O5@Cu(Ex)-MOR is assigned to V-O-V clusters.62 The absence of the terminal V=O stretching bands for V-loaded samples ranging 990-1050 cm-1 is due to the hydration under ambient conditions.63 Temperature programmed reduction of hydrogen (H2-TPR) is performed on the selected samples (Figure 8). MOR is silent, and Cu(Ex)-MOR presents two peaks at about 189 and 548 °C attributable to the two-step reduction process of the isolated Cu2+ species (from Cu2+ to Cu+ ions and then to

B

A

637

284

TCD (UV)

d

c

189 548 552

593

488

h

TCD (UV)

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


528

415

g

248

343

621 670

561 629

f

523

563

e

601

662 718

654

689

b a

V2O5

100 200 300 400 500 600 700 800 o

100 200 300 400 500 600 700 800 o

Temperature ( C)

Temperature ( C)

Figure 8. H2-TPR curves of (A) Cu-MOR(n) and (B) V2O5@Cu-MOR(n): (a) MOR, (b) Cu-MOR(125),

(c)

Cu(Ex)-MOR,

(d)

Cu(Im)-MOR,

(e)

10V2O5@MOR,

10V2O5@Cu-MOR(125), (g) 10V2O5@Cu(Ex)-MOR and (h) 10V2O5@Cu(Im)-MOR. 21


(f)


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

Cu0).64 Cu(Im)-MOR shows only one peak at 284 °C corresponding to the reduction of CuO or [Cu2+-O2--Cu2+] to Cu0.65 By contrast, Cu-MOR(125) shows the sole high-temperature peak at 551 °C, because the framework Cu species are more difficult to be reduced than the ion-exchanged and impregnated ones. Figure 8B shows the H2-TPR curves of V-containing samples. V2O5 gives two peaks at high temperatures of 654 and 689 °C. 10V2O5@MOR presents four peaks, in which the one at 560 °C is related to the reduction of tetrahedral coordinated V species, while the three at 601, 662 and 718 °C come from the reduction of polymeric V5+ species.66 10V2O5@Cu-MOR(125) demonstrates three reduction peaks at 523, 561 and 629 °C, demonstrating the existence of isolated and oligomeric tetrahedral V5+ species. The lower temperatures than those over 10V2O5@MOR and the disappearance of the signal derived from Cu species suggest the strong interaction of support to V species, which changes the surface V state and will ultimately improve the mobility of oxygen species. Six reduction peaks can be observed on 10V2O5@Cu(Ex)-MOR. The peak at 248 and 343 °C is assigned to the direct reduction of octahedral Cu(II) and/or polynuclear Cu2+ into Cu0, and the reduction of aggregates CuO to Cu0. The emergence of these peaks reflects a weak interaction of the ion-exchanged Cu species to the supported V species. 10V2O5@Cu(Im)-MOR gives a strong peak at 637 °C as it mainly contains highly polymeric V species.67 These comparisons further confirm the special host-guest interaction between the supported V species and the framework Cu species in 10V2O5@Cu-MOR(125). The XPS spectrum of Cu-MOR(125) reveals two strong signals at 933 (Cu 2p3/2) and 953 eV (Cu 2p1/2) (Figure 9A). The former is attributable to the well dispersed pseudo-tetrahedral Cu(II) species while the latter comes from the slight octahedral Cu(II) species. Such phenomenon is in accordance

22


Page 22 of 40

Page 23 of 40

962.2

935

954.5

g

V 2p 517.2 516.1

f

943

963.6

935.2 955.8

b

C

Cu 2p

933.2

944.3 933

Intensity (a.u.)

c

B

Cu 2p

e

Intensity (a.u.)

A Intensity (a.u.)

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


933.6

f

517.3

e

517.2

516.1

516.4 515.4

517.2

933

a

953

960

950

d

935

940

d

930

Binding energy (eV)

960

950

940

525 522 519 516 513

930

Binding energy (eV)

Binding energy (eV)

Figure 9. Cu 2p XPS spectra of (A) Cu-MOR(n) and (B) 10V2O5@Cu-MOR(n) and V 2p XPS spectra of (C) 10V2O5@Cu-MOR(n): (a) Cu-MOR(125), (b) Cu(Ex)-MOR, (c) Cu(Im)-MOR, (d) 10V2O5@Cu-MOR(125),

(e)

10V2O5@Cu(Ex)-MOR,

(f)

10V2O5@Cu(Im)-MOR

and

(g)

10V2O5@MOR.

with UV and EPR results. The absence of satellite peaks (943-944 and 962-963.6 eV) also implies the highly dispersion of Cu species. By contrast, Cu(Ex)-MOR exhibits the strong peak at 935 eV and weak one at 933 eV, suggesting the existence of mainly octahedral Cu species and small amount tetrahedral ones. Cu(Im)-MOR shows a small peak at 933.2 eV attributable to the presence of CuO.68,69 The Cu 2p signal for the V-loaded samples disappears except for 10V2O5@Cu(Ex)-MOR (Figure 9B), due to the coverage of V species on the surface. The observation of Cu 2p signal in 10V2O5@Cu(Ex)-MOR reveals the exposure of Cu species on the surface, as demonstrated by H2-TPR curve. The V 2p3/2 peaks centered at 517.0-517.3, 516-516.5 and 515.9 eV (Figure 9C) are characteristics of V5+, V4+ and V3+ species.70 10V2O5@Cu-MOR(125) presents the sole one at about 23



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

517.2 eV, suggesting that it mainly contains V5+ species; by contrast, all the other samples possess certain amount V4+ species. Especially, V3+ is observed in 10V2O5@Cu(Ex)-MOR. The above results additionally confirm the existence of framework Cu species in Cu-MOR that enable the supported V species mainly in its highest valence state (V).

3.3. Catalytic performance in HMF conversion to DFF The catalytic performances of V2O5@Cu-MOR(n) and various controls are assessed in the aerobic oxidation of HMF to DFF under ambient pressure (Table 1 and S2). CuO is almost inert with DFF yield of 8.3% (Table S2, entry 1). Cu-MOR(n) samples give DFF yields of 13.5, 33.6 and 24.5% for

n=333, 125 and 67, respectively, superior to those over Cu(Ex)-MOR (2.4%) and Cu(Im)-MOR (7.5%) (Table S2, entry 2-6). V2O5 presents the DFF yield of 67.1% (Table 1, entry 1), higher than those over Cu-containing samples, suggesting that V species are more active for this reaction. No promotion of activity is observed on 10V2O5@MOR (DFF yield of 63.4%, Table 1, entry 2). In clear contrast, target samples of V2O5@Cu-MOR exhibit much improved activity (Table 1, entry 3-9). 10V2O5@Cu-MOR(125) offers a high DFF yield of 91.5% with the TON of 3.3 (performances under different conditions are seen in Figure S15-S18). As has been revealed by systematic characterizations, the higher content of isolated Cu species causes more highly dispersed tetrahedral V5+ species, which makes it understandable that 10V2O5@Cu-MOR(125) presents the highest activity among V2O5@Cu-MOR series. It is the first zeolite catalyzed conversion of HMF to DFF with O2 under ambient pressure, and the DFF yield of 91.5% even exceeds the one over previous zeolitic catalysts under high pressure.6 Also, the yield over 10V2O5@Cu-MOR(125) is higher than or at least comparable to previous heterogeneous catalysts under ambient pressure or O2 flow (Table 24


Page 24 of 40

Page 25 of 40

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


S2). Furthermore, 10V2O5@Cu-MOR(125) is still able to give well catalytic performance even at ambient air pressure, achieving 72.1 % yield of DFF (Table 1, entry 5; Figure S19-22). All the results suggest that the in situ synthesized Cu-MOR zeolite for supporting V2O5 benefit much enhanced activity, which comes from the framework Cu species that strongly interact with the V species and ultimately alter their surface state. In order to further understand above promotion effect, two other post-prepared Cu-containing MOR samples, Cu(Ex)-MOR and Cu(Im)-MOR, are used as catalyst supports. The obtained catalysts exhibit lower yields (Table 1, entry 10 and 11) than 10V2O5@Cu-MOR(125), despite that they have similar Cu and V contents. This phenomenon indicates that the framework Cu species in Cu-MOR(125) greatly promote the activity of loaded V species, while rare or weak improvement is observed on ion-exchanged and impregnated counterparts. Simultaneously impregnating Cu and V species on MOR gives the sample 0.8%Cu-10%V2O5@MOR with similar activity to 10V2O5@Cu(Im)-MOR (entry 12). In the case of impregnation in the order of V2O5 and CuO, the obtained sample 0.8%Cu@10%V2O5@MOR shows inferior yield (entry 13), which is similar to 10V2O5@MOR. These two MOR-supported Cu-V bimetallic samples also contain certain amount V4+ species due to the lack of framework Cu species (Figure S10-12). Such comparison further proves the special role of framework Cu species, whose interaction to the V species is an interface effect rather than mixture of Cu and V oxides. Combining the above catalytic results and spectra analyses, one can conclude that it is the MOR-framework Cu2+-related tetrahedral V5+ species (rather than square pyramidal or distorted octahedral coordination V5+ species) that bring about the high activity.

25



DFF Yield (%)

1

2

3

1 5

10.

Catalytic

2

3

reusability

of

(A)

2

4

75.4 25.8

31.5

40.0

3

90.2

96.7

99.9

99.0

1 5

52.9

100 80 60 40 20 0

63.4

91.0 99.9

91.2 99.9

4

Catalytic cycle

Catalytic cycle

Figure

C 91.3 99.9

99.9 80.4

82.2

4

HMF Conversion (%)

91.5 99.9 91.0 99.9

100 80 60 40 20 0

99.9

B 100 80 60 40 20 0

87.6 99.9

A 91.5 99.9 89.7 99.9

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

Page 26 of 40

5

Catalytic cycle

10V2O5@Cu-MOR(125),

(B)

V-reloaded

10V2O5@Cu-MOR(125) and (C) 10V2O5@MOR in aerobic oxidation of HMF to DFF. Reaction conditions: 0.05 g HMF, 0.1 g catalyst, 5 mL DMSO, 120 °C, 7 h, O2 balloon.

After reaction, the catalyst can be facilely separated by filtration. Reusability assessment of 10V2O5@Cu-MOR(125) is performed in a five-run recycling test and compared with 10V2O5@MOR (Figure 10). If the recovered catalyst without reloaded vanadium is directly reused in the next run (Figure 10A), it still keeps about 90% of initial activity in the 5th run, suggesting that it can be reused for several times with only slight deactivation due to the well preservation of its original structure (Figure S23-S26). The slow decrease in activity can be assigned to the slight leaching of the V species (∼0.7 wt% V in each run), which is detected by XRF for reused catalysts. Very stable activity is obtained by reloading the small amount V species (0.7 wt%) for each run. By contrast, 10V2O5@MOR displays a sharp deactivation: about 60% activity losses in 5th run. Such comparison suggests that the framework Cu species not only promote the activity of supported V species but also enhance the stability due to the strong host-guest interaction. 26


Page 27 of 40

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


Scheme 2. Possible reaction mechanism for aerobic oxidation of HMF to DFF over 10V2O5@Cu-MOR(125) catalyst.

Based on above results, V2O5@Cu-MOR-catalyzed HMF oxidation to DFF is proposed to follow the Mars-van Krevelen mechanism by referencing the previous related studies.71-73 As shown in Scheme 2, HMF is adsorbed on the catalyst surface and oxidized to generate DFF mainly by lattice oxygen (V5+-O2-). The formed V4+-□ (□ denotes the lattice oxygen vacancy) transition active site is further re-oxidized with O2 to regenerate V5+-O2- species. The mechanism is reflected by the fact that certain amount DFF (18.3%, Table 1, entry 6) can be obtained over 10V2O5@Cu-MOR(125) even in N2 atmosphere, suggesting that the lattice oxygen (V5+-O2-) species start the reaction. However, without O2 supplement, formed lattice oxygen vacancies cannot be regenerated into V5+-O2-, thus the reaction is stopped when the lattice oxygen is used up. In order to prove the formation of lattice 27



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oxygen vacancies due to the involvement of lattice oxygen atoms in the HMF oxidation, we conduct the comparison of the anaerobic and aerobic experiments on 10V2O5@Cu-MOR(125) under N2 and O2 atmosphere (balloon), respectively.73 As shown in Figure S27, the initial reaction rates follow pseudo first order kinetics at first 2 min under both O2 and N2 atmosphere, exhibiting more or less the same initial reaction rates (10.86 vs. 9.97 (mmol HMF)/(mmol V) h-1). This phenomenon confirms the explicit activity of the lattice oxygen atoms for catalyzing this reaction through Mars-van Krevelen mechanism. Figure S28 compares the O1s XPS spectra of fresh and reused catalyst after reaction under N2 atmosphere. There exists three types of surface oxygen species of lattice oxygen atoms (OI) (529.3-529.7 eV), oxygen vacancies or surface adsorbed O (OH) groups (OII) (530.8-531.1 eV) and adsorbed water (OIII) (532.6-533.6 eV), respectively.74,75 Decreased surface lattice oxygen atoms accompanied with increased oxygen vacancies are observed on the reused sample after reaction under N2 atmosphere. This result further demonstrates the consumption of lattice oxygen atoms and the formation of lattice oxygen vacancies. Owing to the strong interaction to the framework Cu species, the surface V species of V2O5@Cu-MOR are mainly in the tetrahedral V5+ state, thus benefit to improve the mobility and reducibility of the lattice O species, which ultimately enhances the activity in the reaction. Cu-MOR(125) demonstrates high amount of isolated Cu species that can provide strong interaction to the V species, as a result, 10V2O5@Cu-MOR(125) exhibits high activity. Further, owing to such host-guest interaction, not only activity but stability of the surface V species is improved, promoting the reusability of the heterogeneous catalyst.

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4. CONCLUSIONS Novel Cu-containing MOR (Cu-MOR) zeolites are synthesized from a one-pot template-free strategy and served as highly efficient catalyst supports. The key step towards Cu-MOR is the acidic co-hydrolysis of TEOS with copper (II) acetate monohydrate, followed by basic aging and hydrothermal crystallization at 180 °C for 9 d. The obtained V2O5-loaded Cu-MOR zeolites exhibit high DFF yield (91.5%) and good reusability for the selective aerobic oxidation of HMF to DFF with atmospheric O2 (1 bar) at 120 °C for 7 h in DMSO. The remarkable performance comes from the strong interaction of framework Cu species with V guest that promotes both the activity and stability of the supported V active sites. This work suggests a new alternative to fabricate efficient catalyst supports from heteroatom zeolites.

ASSOCIATED CONTENT

Supporting Information. Electronic Supplementary Information (ESI) is associated with this article, including detailed experimental procedures for control samples and additional characterization and catalytic data.

AUTHOR INFORMATION Corresponding Authors [email protected] (Y. Zhou), [email protected] (J. Wang) Tel: +86-25-83172264, Fax: +86-25-83172261. ACKNOWLEDGMENT This work is supported by the National Natural Science Foundation of China (Nos. 21136005, 29



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21303084 and 21476109), Jiangsu Provincial Science Foundation for Youths (No.BK20130921), Specialized Research Fund for the Doctoral Program of Higher Education (No.20133221120002).

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