Selective Conversion of Renewable Furfural with Ethanol to Produce

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Selective conversion of renewable furfural with ethanol to produce furan-2-acrolein mediated by Pt@MOF-5 Liangmin Ning, Shengyun Liao, Hongge Cui, Linhao Yu, and Xinli Tong ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b01929 • Publication Date (Web): 07 Nov 2017 Downloaded from http://pubs.acs.org on November 7, 2017

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ACS Sustainable Chemistry & Engineering

Selective conversion of renewable furfural with ethanol to produce furan-2-acrolein mediated by Pt@MOF-5 Liangmin Ning, Shengyun Liao,* Hongge Cui, LinhaoYu and Xinli Tong* Tianjin Key Laboratory of Organic Solar Cells and Photochemical Conversion, School of Chemistry and Chemical Engineering, Tianjin University of Technology, No 391, Binshuixi Road, Tianjin 300384, People’s Republic of China. E-mail address: [email protected]. (X. T.). [email protected]. (S. L.).

KEYWORDS: Oxidative condensation, Platinum nanoparticles, Furfural, Ethanol, Biomass Transformation

ABSTRACT: The selective conversion of furfural (FUR) with ethanol to produce furan-2acrolein has been successfully performed in the presence of O2, in which metal organic framework (MOFs) supported platinum nanoparticles (Pt@MOF-5, Pt@UIO-66 and Pt@UIO66-NH2) were used as the catalysts. Under the optimal conditions, 84.1% conversion of FUR and 90.1% selectivity of furan-2-acrolein was obtained with Pt@MOF-5 as the catalyst. Moreover, the catalysts were characterized by XRD, H2-TPR, HRTEM, HRSEM and XPS techniques, synergetic effect of well-dispersed platinum nanoparticles in the MOF-5 channel. In addition, the

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results of recycling experiment exhibited that there was no significant loss after the catalyst being reused for 5 times. INTRODUCTION As an important biomass platform chemical, furfural can be converted into many kinds of fine chemicals via the catalytic transformation processes such as selective oxidation, oxidative esterification, partial hydrogenation, aldol condensation and oxidative condensation.1-6 In particular, the condensation procedures are of great important techniques for catalytic upgrading the small furanic compounds to a desirable component gasoline and stabilizing furanics. The oxidative condensation of FUR with alcohol in the presence of O2 is a promising green approach, which can impel two carbon molecules together to produce longer hydrocarbon chains using as low volatile liquid transport fuels. Our group has explored the oxidative condensation of FUR with n-propanol in the presence of molecular oxygen and found that the precious metal catalysts displayed the good catalytic performance in oxidative condensation process.7-9 However, the catalytic systems such as Au/Fe3O4 、Pt/FH etc. were always restricted to the conventional catalyst carriers, the size of the particles and the non-uniform distribution of the metal particles. Generally speaking, it would make sense if we could find a new supporting system that could control the size of the metal particles. The metal organic frameworks (MOFs) have shown the tremendous advantages in confining metal-nanoparticles (NPs) and preventing particles from aggregating due to their structure and properties. 10-14 When the metal or metal oxide particles are confined in the cavities or the porous channel of MOFs, the as-prepared catalytic materials have uniform particle dispersion with controllable size distribution.15-17 The previous studies exhibited that the MOFs supported metal catalysts are more efficient on the oxidation of CO, hydrogenation of aromatic compound and C-


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C coupling reactions than the traditional metallic catalysts.18-23 Very recently, Zhao et al reported that Pd@IRMOF-3 catalysts could efficiently promote the cascade reaction involving Knoevenagel condensation and subsequent hydrogenation processes.24 As yet, there are limited reports on an application of MOFs as catalysts for biomass valorization, which mainly focus on acid-functionalized MOFs as solid acid catalysts for dehydration and hydrolysis of biomass. 25-28 Chen et al reported the catalytic transformation of cellobiose and cellulose into sorbitol was achieved with bifunctional acid-metal catalyst [Ru-PTA/MIL-100(Cr)].29 In addition, Yuan et al reported that selective hydrogenation of furfural to furfuryl alcohol under mild conditions was evaluated over Ru nanoparticles supported on a series of zirconium based metal organic frameworks.30 There is thereby an urgent need but it is still a significant challenge to synthesize new NPs@MOFs catalysts and broadening its application in catalytic field. Some recent studies demonstrated types of metal nanoparticles, nature of MOFs and modes of loading are the key factors for determining the catalytic performance of NPs@MOFs catalysts.31-35 As far as MOFs are successfully employed in catalytic applications, they must be stable and resistant to degradation under the established conditions, which can include changes in chemical and thermal environments. MOF-5, UIO-66 and UIO-66-NH2 serve as prototypes for an extensive family of metal-organic frameworks due to the high stability under a range of conditions. Their high BET surface, well-distributed pores and the abilities for reversible adsorption and desorption make them acquire catalytic activity in a variety of ways.36,37 Therefore, as shown in Figure 1, we designed platinum nanoparticles in the pores of MOFs (Pt@MOF-5, Pt@UIO-66 and Pt@UIO-66-NH2). The rational considerations for design of Pt@MOFs catalyst are as follows: (i) MOF-5, UIO-66 and UIO-66-NH2 can keep the integrity of the framework in the established reaction medium. At the same time, the substrate and product


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molecules may diffuse smoothly in and out. (ii) The traditional supported Pt nanoparticles show some activities in the oxidation transformation of biomass. (iii) The precursor of Pt particles can be penetrated into the pores of MOF-5, UIO-66 and UIO-66-NH2 well. (iv) The suitable pore sizes and 3D framework nets can control the sizes of the nanoparticles and prevent the metallic nanoparticles from aggregating. Within these discussions, all of them are consistent with the observation from characterization as followings. Very interesting, with Pt@MOF-5 catalyst, the maximum conversion (up to 84.1%) of FUR and selectivity of furan-2-acrolein (1) (about 90.1%) was achieved at 150 oC for 4 h with oxygen (O2) as the oxidant and Pt@MOF-5 as catalyst (see Scheme 1). Particularly, the selectivity of 1 is higher than the previous Au/Al2O3 and Pt/FH catalytic system. However, the lower conversion of FUR and poor selectivity for 1 in the presence of UIO-66 or UIO-66-NH2 catalytic system affirmed the nature of MOFs could not be ignored. In addition, the synergistic effect among Pt nanoparticles, MOFs and additives should be responsible for the good performance of Pt@MOF5 catalyst. EXPERIMENTAL SECTION Chemicals. All solvents including N,N-dimethyl formamide (DMF), dichloro methane (CH2Cl2), ethanol with AR purity (analytical reagent grade) and ZrCl4, terephthalic acid (H2BDC), K2CO3, Zn (NO3)2·6H2O, Li2CO3, Na2CO3, NaOH, Cs2CO3, KHCO3, KOH, Ca(OH)2, n-butylamine, K3PO4, EtONa, FUR and H2PtCl6 were purchased from Sigma-Aldrich. Oxygen supplied with a high-pressure cylinder was used through a reducing valve without further treatment. Preparation of MOF-5. MOF-5 was prepared by using the reported method according to the literature.38 Zn (NO3)2·6H2O (0.0608 g, 0.2 mmol), H2BDC (0.0255 g, 0.15 mmol) and distilled DMF (16 mL) were placed in a 20 mL a Teflon-lined lid and stirred vigorously for


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clarifying the reaction mixture. Then the reaction mixture was heated to 100 oC and kept for 5 h. After cooling to room temperature in a nitrogen-filled glove bag, cube-shaped crystals (~2 mm) were washed six times with 15 mL of anhydrous DMF and 15 mL of anhydrous CH2Cl2, respectively. The samples were dried in a vacuum drying oven at 373 K for 12 h. This procedure produced a pure material MOF-5. The yield of as-obtained MOF-5 was 80 % based on the Zn (NO3)2·6H2O. UIO-66 and UIO-66-NH2 was synthesized according to the method of literature, respectively.39,40 Preparation of Pt@MOF-5 catalyst. Pt@MOF-5 was prepared by an incipient-wetness impregnation method combined with a H2 reduction procedure.21 Under N2 atmosphere, 100 mg of activated MOF-5 was immersed in 1 mL solution of H2PtCl6 (containing ca. 1.0 wt.% of Pt). After being sonicated for 5 min at room temperature, the mixture was placed in a vacuum oven and dried at 60 oC for 3 h. Finally, the dried precursor catalyst was treated in a fixed-bed stainless steel reactor with an inner diameter of 6 mm under H2 with a total flow rate of 50 ml min-1 and maintained at 220 oC for 2 h to obtain 1.0 wt.% Pt@MOF-5 catalyst. The characterization of catalysts. The XRD patterns were recorded on Rigaku D/Max 2400 diffractometer using Cu/Kα radiation. FT-IR spectra were recorded using Bruker EQUINOX55 infrared spectrometer. The platinum contents of the samples were determined quantitatively by Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES: Varian 700-ES). High resolution scanning electron microscopy (HRSEM) was performed in a JSM 6490LV JEOL microscope at 25 kV. High resolution transmission electron microscopy (HRTEM) was performed in a Phillips CM200 at 200 kV. X-ray photoelectron spectroscopy (XPS) was carried out in a 5700 model Physical Electronics apparatus. The BET surface areas and pore


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volumes were measured by N2 adsorption-desorption at 77 K on a Micromeritics ASAP 2020 instrument at -196 °C. Hydrogen Temperature programmed reduction (H2-TPR) analysis were carried out using a Micromeritics ChemiSorb 2720 Pulse Chemisorption System, 50 mg of each catalyst was pre-treated in 5 % H2/N2 (100 mL/min) at 70 oC for 1 h, and then TPR was performed over the sample with the temperature increasing from 70 oC to 700 oC at a speed of 10 °C /min. Ammonia temperature programmed desorption (NH3-TPD) experiments were carried out using a Micromeritics 2920 Autochem II Chemisorption Analyzer. The samples were first pretreated at 200 °C for 1 h under Ar at a flow rate of 30 mL min−1 and cooled to room temperature, then adsorption proceeded under NH3 at a flow rate of 30 mL min−1. During the TPD experiments, the temperature was set to 200 °C by means of a temperature ramp of 10 °C min−1 using helium as the carrier gas flowing at 60 mL min−1. The effluent gas was dried by powder KOH and the concentrations of ammonia were recorded by using a thermal conductivity detector. General procedure for catalytic transformation of furfural (FUR). Typically, the selective oxidation of FUR was carried out in a 120 mL stainless steel autoclave with a polytetrafluoroethylene liner. FUR (1 mmol, 0.1 g), Pt@MOF-5 (0.025 g), K2CO3 (0.025 g) and alcohol (15 mL) were added into the reactor. After being charged with 0.3 MPa O2, the reaction mixture was stirred and heated to 140 oC for 4 h. Then, the reactor was cooled down to room temperature and the products were diluted with alcohol. The feed and reaction products have been analyzed offline by means of comprehensive gas chromatography (GC×GC) using an Agilent 7890A apparatus provided with a flame ionization detector (FID), and coupled in line with mass spectrometry (Agilent 5975C inert XL MSD).


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RESULTS AND DISCUSSION The Characterization of platinum supported catalysts. Several MOFs materials including MOF-5, UIO-66 and UIO-66-NH2 were chosen as the host matrix for encapsulating Ptnanoparticles. The impregnation process combined with gas phase reduction was used to fabricate the Pt@MOFs catalyst. A simple schematic diagram is presented in Figure 1, where Pt nanoparticles have been obtained at 220 ºC after being treated under H2 atmosphere. Particularly, the loading amount of platinum on MOF-5 is changed from 0.5 wt.% to 3.0 wt.%. Figure 2 shows the XRD patterns of MOF, Pt@MOF-5 and used Pt@MOF catalysts. As a result, the XRD patterns of as-obtained MOF-5 material overlaps well with the simulated one from the crystallographic result (see Figure 2a and 2b). After Pt nanoparticles being introduced into MOF materials, the framework of MOF-5 keeps almost well-defined (Figure 2c-2f). However, along with the increase of Pt loading from 0.5 wt.% to 3.0 wt.%, the relatively diffraction intensity of peak at (311) crystal plane is gradually elevated, which indicates the occurence of numerous platinum can affect the diffraction of the specific crystal face. As it can be seen from the results of Figure 2g, the whole structure of used Pt@MOF-5 catalyst keeps consistent with that of fresh one. Therefore, it is concluded that the catalyst is stable during the reaction. On the other hand, the evidence from FT-IR spectra of MOF-5, fresh Pt@MOF-5 and used Pt@MOF-5 also verifies the integrity framework of MOF-5 could be kept under the established preparation and catalytic reaction conditions (see Figure S1 in the Supporting Information). In addition, the XRD patterns of Pt@UIO-66 and Pt@UIO-66-NH2 catalysts are provided in supporting information (Figure S2). According to the characterization results, the framework structure of these MOFs materials kept excellent after Pt nanoparticles being encapsulated into their frameworks.


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Figure 1. Schematic chart for the fabrication of Pt@MOF-5 catalysts through the impregnation process combined with gas phase reduction.

Figure 2. XRD patterns of synthesized materials: (a) simulated pattern of MOF-5; (b) as-synthesized MOF-5; (c) 0.5 wt.% Pt@MOF-5; (d) 1 wt.% Pt@MOF-5; (e) 2 wt.% Pt@MOF-5; (f) 3 wt.% Pt@MOF-5; (g) 1 wt.% Pt@MOF-5 used.

As shown in Figure 3, the H2-TPR experiments have been performed on the as-synthesized MOFs and the corresponding Pt@MOFs catalyst precursors (H2PtCl6@MOFs). The profiles of MOF-5, UIO-66 and UIO-66-NH2 show only one peak in the high temperature range of 350-


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600oC, which confirms the collapse of framework under those conditions. In other words, these MOFs must be employed at below 350 ºC for utilization their framework effects. Moreover, the new H2 consumption peaks can be observed in the low temperature range of 100-260 oC for the H2PtCl6/MOFs materials, which may be ascribed to the reduction peaks of H2PtCl6. The aforementioned results help us confirm 220 oC is the optimal reduction temperature for preparation Pt@MOF-5 catalysts.

Figure 3. H2-TPR result of the as-synthesized MOFs (a) and H2PtCl6@MOFs (b) materials

Figure 4 displays typical high resolution transmission electron microscopy (HRTEM) images of fresh and used Pt@MOF-5 catalysts, respectively. Clearly, a high dispersion of Pt NPs with the particle sizes ranging from 1-3 nm was observed (Figures 4a and 4b). Meanwhile, as shown in HRSEM images, there are no Pt nanoparticles agglomeration found on the surface of MOF-5 materials (see Figure 4c and 4d). These show that the porous tunnels of MOF-5 may


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provide the suitable environment for the growth of Pt nanoparticles and restrict them in tunnel with the small particle sizes distribution. Also, the rigid frameworks can prevent Pt nanoparticles from sliding and agglomerating. Compared to our precious metal supporting system, the size of Pt particles is smaller and the distribution of the Pt particles is more uniform.9, 10 In addition, as shown in the HRTEM images of Pt@UIO-66 and UIO-66-NH2 (see Figure S3 in supporting information, the size of Pt nanoparticle is ca. 2-6 nm in the Pt@UIO-66-NH2 materials. Moreover, in Pt@UIO-66 material, the agglomeration is very serious. Thus, the nature of MOFs as the supports plays an important role on the size and distribution of Pt nanoparticles, which lead to the different catalytic performance.

Figure 4. HRSEM and HRTEM images of Pt@MOF-5 catalyst: (a) HRTEM of fresh Pt@MOF5; (b) HRTEM of used Pt@MOF-5; (c) HRSEM of fresh Pt@MOF-5; (d) HRSEM of used Pt@MOF-5.


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The surface chemical states of fresh Pt@MOF-5 and used Pt@MOF-5 have been analyzed by X-ray photoelectron spectroscopy (XPS) measurements. The high-resolution XPS spectrum of the Pt4f could be deconvolved into double peaks at binding energies about at 71.8 and 75.2 eV (see Figure 5(a) and (b)), corresponding to Pt4f7/2 and Pt4f5/2 of metallic Pt species, respectively.41 So it demonstrated Pt(IV) in H2PtCl6@MOF-5 precursor catalysts has been successfully reduced Pt(0) after being treated with H2 at 220 oC for 2 h.

Figure 5. XPS high resolution Pt spectra of 1.0 wt.% Pt@MOF-5: (a) fresh Pt@MOF-5 catalyst; (b) used Pt @MOF-5 catalyst. In addition, the chemical composition of the as-synthesized bulk 1.0 wt% Pt@MOF-5 was investigated by ICP-OES. It was found that the content of main active element (Pt) in the fresh and used Pt@MOF-5 catalysts are 0.935 wt.% and 0.803 wt.%, respectively, which is very closed to the theoretical value. The Brunauer-Emmett-Teller (BET) area of MOF-5 and Pt@MOF-5 was carried by N2 adsorption/desorption measurements (see Figure S4 in the Supporting Information). Compared with bare MOF-5, the specific surface area of Pt@MOF-5 decreases greatly (see Table S1 in Supporting Information), which can prove that Pt nanoparticles were successfully introduced into the MOF-5 support.


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Catalytic oxidative condensation of furfural (FUR) with ethanol. The catalytic activities of different Pt@MOFs were tested on the selective oxidation of FUR with molecular oxygen as oxidant in ethanol (see Scheme 1). The oxidation equation of FUR was given as Scheme 1, where the generated products include furan-2-acrolein (1), 2-(2, 2-diethoxythyl) furan (2), 2furoic acid and ethyl 2-furoate. Based on the GC and GC-MS spectra, the main product is compound 1 and the by-products contain ethyl 2-furoate and compound 2 when Pt@MOFs was employed as catalysts (see Figure S5, S6, S7, S8, S9 and S10 in Supporting Information). Pt@MOF-5

O

O

O

O+

O

OEt

O

O 2, EtOH

+

OEt 1

O

O

+

OH

O OEt

2

Scheme 1. The oxidative condensaton of FUR with ethanol in the presence of molecular oxygen. Table 1. The results for the oxidative condensation of FUR with different catalysts a

Entry

a

Catalytic System

Conversion (%)

selectivity (%) b

b

1

2

Others c

1

None (baseline)

8.8

0

>99

0

2

Pt@MOF-5+K2CO3

78.4

88.5

0

11.5

3

Pt@UiO-66+K2CO3

20.2

>99

0

0

4

Pt@UiO-66-NH2+K2CO3

51.3

74.8

0

25.2

5

Pt@MOF-5

36.9

0

>99

0

6

MOF-5

18.0

0

>99

0

7

K2CO3

29.7

62.7

0

37.3

8

MOF-5+K2CO3

22.3

>99

0

0

Reaction conditions: 0.1 g FUR, 0.025 g Pt@MOF-5 (1.0 wt.% Pt loading) catalyst, 0.025 g additive, 15 mL of ethanol, 0.3

MPa of O2, 4 h, 140 oC. b

The results are obtained by GC analysis with the internal standard technique.

c

Other product generally refers to ethyl-2-furoate as by-product.


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As shown in Table 1, 78.5% conversion of FUR and 88.5% selectivity of 2 were obtained when Pt@MOF-5 and K2CO3 were used as the catalyst. In the case of Pt@UiO-66, a 20.2% conversion of FUR and more than 99% selectivity of 1 was acquired. When Pt@UiO-66-NH2 and K2CO3 were employed, the conversion and selectivity was 51.3% and 74.8%, respectively. These results indicate that the combination of Pt@MOF-5 and K2CO3 is excellent for the transformation of FUR to 1 in the presence of molecular oxygen. Obviously, the nature and the internal micro-circumstances of MOFs are very crucial. As comparison, only 8.8% conversion of FUR and over 99% selectivity of 2 was obtained in absence of any catalyst. Moreover, when a Pt@MOF-5 or MOF-5 in the absence of additives was used as catalyst, the conversion of FUR is respectively 36.9% and 18.0% and the main product is compound 2 formed through the acetalization. For the catalysis of K2CO3 or MOF-5 + K2CO3 system, the main product is compound 1, while only 29.7% or 22.3% conversion was attained, respectively. In order to test the basic and acidic properties of these three MOFs, NH3-TPD technique was employed and the results were shown as Figures S11 and S12 in Supporting Information. It was found that the amount of acidic sites of Pt@MOF-5 is greater than that of MOF-5, which shows that numerous weak Lewis acidic sites of Pt@MOF-5 are originated from the interaction of Pt with MOF-5 support. Moreover, from Figure S12, it can be seen that the amount of acidic sites of catalysts was listed in sequence of Pt@UIO-66 < Pt@UIO-66-NH299

0

2

Na2CO3

24.6

>99

0

0

3

K2CO3

78.4

88.5

0

0

4

Cs2CO3

36.6

76.5

23.5

0

5

KHCO3

23.8

42.4

0

57.6

6

KOH

84.8

55

0

45

7

NaOH

84.3

64.4

0

35.6

8

Ca(OH)2

13

66.7

33.3

0

9

n-butylamine

75.2

39.5

0

60.5 d

10

K3PO4

31

>99

0

0

11

EtONa

57.9

80

0

20

Reaction conditions: 0.1 g FUR, 0.025 g Pt@MOF-5 (1.0 wt.% Pt loading) catalyst, 0.025 g additive, 15 mL of ethanol, 0.3

MPa of O2, 4 h, 140 oC. b


c


d

It is the selectivity of N-(furan-2-ylmethylene)butan-1-amine as the main product.

In view of aforementioned promotion effect from the additive, the Pt@MOF-5 with different additives including alkali carbonate, alkali bicarbonate, metal hydroxide, organic base such as butylamine and EtONa, alkali metal phosphate were investigated in the oxidation condensation of FUR and the results were summarized in Table 2. It was observed that


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Pt@MOF-5+K2CO3 and Pt@MOF-5+EtONa displayed the high catalytic efficiency in the oxidation of FUR. For Pt@MOF-5+K2CO3 and Pt@MOF-5+EtONa, the conversion of FUR was 78.4% and 57.9%, the selectivity of desired product (1) reached 88.5% and 80%, respectively (see Entries 3 and 11 in Table 2). The other alkali carbonate, metal bicarbonate and K3PO4 didn’t show any promotion effect (see Entries 1, 2, 4, 5 and 10 in Table 2). Pt@MOF-5 with some strong basic metal hydroxide could improve the transformation of FUR, but the less selectivity of desired products was unsatisfactory (see Entries 6-7 in Table 2). The effects of reaction temperature, time and Pt loading. In order to improve the conversion of FUR and selectivity of the desired product, the catalytic reaction conditions was further optimized and the results were shown in Figure 6 and Figure 7. With increasing the reaction temperature to 150 oC, 84.1% conversion of FUR and 90.1% selectivity of 1 was acquired. Then further increasing the reaction temperature, the conversion and the selectivity of 1 decreased. At the same time, prolonging the reaction time led to a higher conversion of FUR with a lower selectivity of 1. Furthermore, effect of Pt loading was shown in Table 3. It was found that Pt@MOF-5 with 1.0 wt.% loading was proved to be an excellent catayst for oxidation condensation of FUR with ethanol (see Table 3).


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Figure 6. Trend of conversion and selectivity as a function of reaction temperature over 1.0 wt.% Pt@MOF-5 (reaction conditions: 0.1 g FUR, 0.025 g Pt@MOF-5 as catalyst, 0.025 g K2CO3 as additive, 15 mL ethanol, 0.3 MPa O2, 4 h)

Figure 7. Trend of conversion and selectivity as a function of reaction time over 1.0 wt.% Pt@MOF-5 (reaction conditions: 0.1 g FUR, 0.025 g Pt@MOF-5 as catalyst, 0.025 g K2CO3 as additive, 15 mL ethanol, 0.3 MPa O2, 150 oC)


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Entry

a

Table 3. Effect of Pt loading on conversion of FUR and selectivity of 1. a Selectivity (%) b b Pt loading (%) Conversion (%) 1 2

Othersc

1

0.5

44.8

>99

0

0

2

1

84.1

90.1

0

9.9

3

2

79.9

85.1

0

14.9

4

3

70.4

89

0

11

Reaction conditions: 0.1 g FUR, 0.025 g Pt@MOF-5 (1.0 wt.% Pt loading) catalyst, 0.025 g K2CO3, 15 mL of ethanol, 0.3 MPa

of O2, 4 h, 150 ◦C. b


c


The recycling of catalyst. The recycling experiment of of 1.0 wt.% Pt@MOF-5 was further performed in the reaction of FUR with ethanol and the corresponding results are shown in Figure 8. After being recycled for 5 times, the selectivity of furan-2-acrolein kept almost unchanged and the conversion of FUR exhibited a slight decrease, which demonstrated that Pt@MOF-5 catalyst has excellent recyclability under the suitable catalytic reaction conditions. Moreover, the used Pt@MOF-5 catalyst was also characterized by XPS, HRSEM, HRTEM ans FT-IR techniques, and the obtained results further confirmed the Pt@MOF-5 catalyst was stable in the oxidative condensation reaction.


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Figure 8. Recyclability of 1.0 wt% Pt@MOF-5 in the catalytic oxidation condensation of FUR (reaction conditions: 0.1 g FUR, 0.025 g Pt@MOF-5 as catalyst, 0.025 g K2CO3 as additive, 15 mL ethanol, 0.3 MPa O2, 150 oC for 4 h) Possible mechanism for oxidative condensation of frufural (FUR) with ethanol. Based on the results of control experiments and catalytic principle of reaction, a possible reaction mechanism for the oxidative condensation of FUR with ethanol is provided in Scheme 2. First, a small amount of ethanol is oxidized by O2 to produce a small amount of molecular acetaldehyde; in the following, the rapid condensation process happens between FUR and very little in situ generated acetaldehyde in the solution (see route a in Scheme 2). In addition, another catalytic route should be mentioned in this research. Firstly, the hydrogen transfer process occurs between FUR and ethanol, in which a certain amount of acetaldehyde and furfuryl alcohol can be generated; in the following, the condensation is performed between the left FUR and the generated acetaldehyde; meanwhile, the furfuryl alcohol can be oxidized to FUR by oxygen (see route b in Scheme 2). Correspondingly, in this reaction, the Pt@MOF-5 catalyst should be responsible for the selective oxidation of ethanol or furfuryl alcohol to the corresponding


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aldehyde. The role of K2CO3 is promoting the next condensation reaction to produce compound 1. In order to verify the catalytic reaction mechanism, FTIR, GC and GC-MS were used to monitor the generation of acetaldehyde, respectively, and the results are respectively presented in Figures S13, S14 and S15 of Supporting Information. Actually, a small amount of acetaldehyde has been detected successfully.

Scheme 2. The possible reaction mechanism for the oxidative condensation of FUR with ethanol. CONCLUSION In summary, Pt nanoparticles have been confined in the pores of MOF-5 using impregnation process combined with gas phase reduction, and tested as an efficient heterogeneous catalyst for selective oxidation of biomass-derived FUR to furan-2-acrolein in ethanol. The size of the highly dispersed Pt particles confined in Pt@MOF-5 was about 1-3 nm, which was confirmed by HRTEM and HRSEM. 1.0 wt%@MOF-5 catalyst showed the best catalytic performance in oxidation of FUR, in which 84.1% conversion of FUR and 90.1% selectivity of 1 was obtained at 150 oC for 4 h. More importantly, Pt@MOF-5 catalysts were very stable under the reaction condition used for oxidation condensation of FUR. The catalyst could be reused for 5 times without any loss in activity and selectivity. Under established reduction and catalytic reaction


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conditions, the crystalline structure of MOF-5 and the morphology of Pt nanoparticles remained intact by measuring the XRD patterns and HRTEM images of used catalysts. The research thus highlights new perspectives on application of MOFs materials in the field of biomass transformation. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: xxxxxxxxx. Supplementary data include the FTIR spectra, XRD patterns, TEM images and BET measurements of catalysts, and catalytic results of the as-synthesized catalysts, NH3-TPD profiles of the catalysts, the GC spectra and GC-MS spectra of products. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Tel./Fax: (+86)-22-6021-4259 (X. T.). *E-mail: [email protected]. Tel./Fax: (+86)-22-6021-4259 (S. L.). Notes The authors declare no competing ﬁnancial interest. ACKNOWLEDGMENT We are grateful for the ﬁnancial support from National Natural Science Foundation of China (No.21601135), Tianjin Program of Application Foundation and Advanced Technology (No.


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17JCYBJC20200) and the Key Program of National Natural Science Foundation of China (No.21336008).

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For Table of Contents Use Only

The efficient oxidative condensation process of renewable furfural with ethanol has been developed in the presence of molecular oxygen.


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Selective Conversion of Renewable Furfural with Ethanol to Produce

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