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Platinum Supported on WO-doped Aluminosilicate: A Highly Efficient Catalyst for Selective Hydrogenolysis of Glycerol to 1,3-Propanediol Shanghua Feng, Binbin Zhao, Lei Liu, and Jinxiang Dong Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02951 • Publication Date (Web): 11 Sep 2017 Downloaded from http://pubs.acs.org on September 12, 2017

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Industrial & Engineering Chemistry Research

Platinum Supported on WO3-doped Aluminosilicate: A Highly Efficient Catalyst for Selective Hydrogenolysis of Glycerol to 1,3-Propanediol Shanghua Fenga,b, Binbin Zhaoa, Lei Liua,*, Jinxiang Donga a

College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan

030024, Shanxi, China. b

School of Chemistry and Chemical Engineering, Taishan University, Tainan 271021, Shandong

China. ABSTRACT: Platinum-loaded WO3-Al2O3-SiO2 catalysts, denoted as Pt/WAlSi, in glycerol hydrogenolysis to 1,3-propanediol(1,3-PDO) with a batch reactor were investigated, wherein the WO3-doped silica–alumina material with a homogeneous dispersion was prepared by sol-gel method and Pt was loaded onto it by incipient wetness impregnation. The catalysts were characterized by powder XRD, N2 adsorption-desorption, TEM, CO chemisorption, NH3-TPD, Py-IR, and XPS. The WO3/Al2O3 ratio and Pt loading amount are shown to be of importance in determining the activity and directing the selectivity to 1,3-PDO. It was found that 2% Pt/WAlSi with the ratio of WO3 to Al2O3 at 2 has a suitable Brønsted acidity and contribute to a high selectivity to 1,3-PDO close to 56% at 48% glycerol conversion, and the space time yield for 1,3-PDO in an optimized reaction condition could reach 18.34 g/(gPt·h), much higher than the best result reported in literature. A plausible mechanism of glycerol hydrogenolysis was proposed from the reaction results. Keywords: glycerol, hydrogenolysis, 1,3-propanediol, sol-gel

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1. INTRODUCTION Glycerol as a by-product is abundantly obtained in the production of biodiesel, the search for synthesis routes aiming the transformation of glycerol into high value products is of importance to deal with the surplus of glycerol. The hydrogenolysis of the C-O bond is one of the most attractive methods for glycerol conversion to value-added chemicals. Among the glycerol hydrogenolysis products, 1,3-propanediol(1,3-PDO), as a monomer of polypropylene terephthalate (PTT), is now most valuable. Nevertheless, the production of 1,3-PDO by glycerol hydrogenolysis is not a straightforward task, such difficulty originates from that the second hydroxyl group in glycerol is more difficult to access than the primary one1. In recent years, the supported Pt and Cu catalysts had been employed for glycerol hydrogenolysis to 1,3-PDO, and some progress has been achieved (Table S1). Among these studies, the reaction process for the production of 1,3-PDO from glycerol generally involves the dehydration with Brønsted acid or Lewis acid sites and the hydrogenation with precious metal2-5. Therefore, the bi-functional metal/acid catalysts with oxophilic metal are the major categories in this reaction, mainly including the Pt/WOx-promoters6-20, Pt/HPA4,

21-25

, ReOx added

Rh(Ir)/SiO226-29 and some other catalyst systems30-33. It has been found that the W-modified Pt catalyst are very effective in improving the selectivity of 1,3-PDO from glycerol, because tungsten is considered as a strong site anchoring the primary hydroxyl group20, and it also can be involved in the redox reaction with H2 to produce Brønsted acid. Zhu et al.16 reported the glycerol hydrogenolysis over Pt/WOx-Al2O3 catalyst with a 1,3PDO yield of 42.4%, attributed to the high concentration of Brønsted acid sites arising from the reaction between WOx and the spillover H16. S. García-Fernández14,

20

also revealed that

tungstate species were responsible to produce Brønsted acidity in Pt/WOx/Al2O3 catalyst and the


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tungsten oxide provided the sites anchoring the primary hydroxy group in glycerol, and the 1,3PDO yield was 38.5% after only 4 h reaction time. Regarding the WOx-containing solid acid, the acid site nature and strength (Brønsted and / or Lewis) are closely related to the structure of the WOx polydomain, the dispersion degree and reducibility of WOx species, as well as the WOx cluster size34 , which are often affected by the preparation procedure and preparation approach35. To date, the Pt-loaded catalyst containing WOx for glycerol hydrogenolysis are generally prepared by the impregnation method, and it is frequent to observe WO3 aggregates at the outer surface of the support particles what has a significant effect on the activity or selectivity. It is well known that the amorphous aluminosilicate with Brønsted acid sites are often used as solid acid catalysts, herein, we report the synthesis of WO3 doped silica–alumina material with a homogeneous dispersion of all components of the mixed oxide, which was used as support to load platinum for glycerol hydrogenolysis to 1,3-PDO. Details of the catalysts were characterized by N2-adsorption, powder XRD, TEM, Py-IR, NH3-TPD, and XPS technologies to reveal the relationship between the catalytic performance and the property of catalysts. The reaction conditions were also discussed thoroughly for the optimal reactions to improve the selectivity of the 1,3-PDO. 2. EXPERIMENTAL SECTION 2.1 Catalysts Preparation The WO3-Al2O3-SiO2 support were prepared from ammonium hydroxide (NH4OH, Aladdin, 25.00 % ammonia), aluminum nitrate (Al(NO3)3·9H2O, Aladin,99.99%), ammonium metatungstate (AMT) ((NH4)6H2W12O40·xH2O, Sigma-Aldrich, >99.00%) water solution and colloidal silica (6.05 mol/L SiO2, Qingdao Ocean Chemical CO., Ltd, China). A typical procedure (the weight ratio of WO3 to Al2O3 is 2:1) was as following. 10mL of an NH4OH solution was added dropwise (≈0.5 mL/min) to colloidal silica (5.51 mL,


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diluted with deionized water to 100 mL) containing Al(NO3)3·9H2O (7.35 g) up to pH 10.00 monitored continuously by a pH meter under vigorous stirring. The AMT (3.13 g) solution was added to the above hydrogel when it was still in solution. The resulted hydrogel was aged for 24 h at room temperature. Then the as-prepared hydrogel was filtrated and washed with deionized water until the filtrate was neutral，then dried at 110 ℃ overnight and calcined at 600 ℃ in air for 3 h. The as-prepared WO3-Al2O3-SiO2 supports of which the SiO2 was fixed at 40 wt% with a varied weight ratio of WO3 to Al2O3, were impregnated with an aqueous solution containing the desired amount of chloroplatinic acid hexahydrate (H2PtCl6·6H2O, Aladdin, ≥37.5% Pt). The samples were dried overnight at 110 ℃ in an oven after impregnation and then reduced at 350 ℃ for 3 h in flowing 10% H2/N2(V/V), and passivated with 1% O2/N2(V/V) for 4 h at room temperature. The Pt loading varied from 0.5 to 4.0 wt%. The as-prepared catalysts were denoted as xPt/yWAlSi, where x and y referred to Pt content (%) and the weight ratio of WO3 to Al2O3, respectively. 2.2 Catalyst Characterization N2 adsorption–desorption experiments were carried out at 77K (Micromeritics ASAP 2020 instrument) to determine the Brunauer–Emmett–Teller (BET) surface area and micropore volume. Powder X-ray diffraction (XRD) patterns were collected using a Rigaku Miniflex II X-ray diffractometer with Cu Kα radiation(λ=1.5418 Å) in the 2θ range of 10–80°. CO chemisorption was carried out in Auto Chem. II 2920 Mircromeritics with a pure He and pure CO gas. TEM images were collected with a Hitachi H-600 microscope with an accelerating voltage of 120 kV. FT-IR spectra were conducted in transmission mode in a Shimadzu Affinity-1 with a KBr wafers. X-ray photoelectron spectroscopy (XPS) was tested on


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a VG MiltiLab 2000 spectrometer with Al Kα radiation and a multichannel detector. NH3-TPD was carried out in an Auto Chem 2910 Micromeritics using a 5%H2/95%Ar and 5% NH3/95% He gas mixtures respectively. Py-IR were obtained using a Bruker Equinox 55 spectrometer equipped with a DTGS detector. 2.3 Catalytic Tests The catalytic tests for glycerol hydrogenolysis were carried out in a 100 mL Teflon-lined stainless-steel autoclave equipped with a thermo couple and a magnetic stir bar inside. In a typical run, 500 mg of catalyst and 25 mL of glycerol aqueous solution (3 wt%) were put into the reactor, then the reactor was purged with H2 three times and charged to 6MPa, quickly heated to 160 ℃ for 12 h under stiring of 600 rpm. After the reaction, the reactor was cooled down to room temperature quickly with an ice bath. The liquid phase was centrifuged to remove solid catalyst and analyzed by an HPLC(Shimadzu, LC-20AD) equipped with a RID detector. The column used was a 4.6×250 mm C18 column with the filler size of 5 μm (Inertsil ODS-3, Catalog No.5020-01732). The column was run using a mobile phase of Milli-Q water at a flow rate of 0.5mL/min at 35oC. The amounts of consumed glycerol and produced products were quantified with an external calibration method. The conversion of glycerol and the selectivity of the liquid product were calculated by the following equations. Conversion (%)=(moles of glycerol converted)×100%/(initial moles of glycerol) Selectivity (%)=(carbon moles of one specific product)×100%/(all the carbon moles in glycerol converted) 3. RESULTS AND DISCUSSION 3.1 Catalyst Characterization 3.1.1 Physicochemical Properties The textual properties of xPt/2WAlSi catalysts with different Pt loading were listed in Table 1. The WO3-SiO2-Al2O3 support prepared by sol-gel


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method exhibited an excellent texture, with a large specific surface area (310 m2 g−1) and large pore volume (0.32 cm3 g−1). After loading Pt, it is observed that the BET surface area of support gradually decreased with increasing Pt content from 0.5 to 4wt%. This could be possibly attributed to the formation of large platinum evidenced from their powder XRD patterns (Figure 1) which blocks or fills the the pore channel of the support, which is the case for the Pt supported on SBA-15 or H-mordenite zeolite15,33. The dispersion and particle size of Pt determined by CO chemisorption were also summarized in Table1. It was observed that Pt particle size increased and the Pt dispersion decreased with the improvement of Pt loading, respectively. The Pt dispersion is closely related to the Pt particle size on the support13, which was also consistent with the acidity of catalysts obtained by Py-IR and NH3-TPD (Table 3 and 4). Table 1 N2-physisorption and CO-chemisorption results of the impregnated catalysts BET Surface area

Pore size

Pore volume

Dispersion

Particle size

(m2/g)

(nm)

(cm3/g)

(%)

(nm)

2WAlSi

310.24

7.37

0.32

/

/

0.5Pt/2WAlSi

298.67

6.93

0.32

44.3

3.1

1Pt/2WAlSi

237.75

5.59

0.31

44.1

3.4

2Pt/2WAlSi

179.13

5.35

0.28

44.1

3.4

3Pt/2WAlSi

178.12

2.76

0.31

42.1

3.4

4Pt/2WAlSi

130.19

2.23

0.24

39.6

3.6

Catalyst

Figure 1 showed the XRD patterns of catalysts mentioned in Table 1. There were no any peaks ascribed to Al2O3 or WO3 for xPt/2WAlSi, which suggested that the 2WAlSi support was amorphous or the Al2O3 or WOx were dispersed well on SiO2.


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For xPt/2WAlSi, there was only a broadening peak ascribed to the amorphous support at about 22° which represented a good dispersion of Pt on these support except for the 4Pt/2WAlSi, which clearly displayed the characteristic peak of metallic Pt(diffraction line at 39.8 o, 46.2 o and 67.5 o, JCPDS 04-080222, 36). It might be the synergism of WO3 and Al2O3 for dispersing Pt well.

Figure 1 XRD patterns of xPt/2WAlSi catalysts (x=0.5-4)

Figure 2 shows the TEM images of Pt/2WAlSi catalysts with different Pt loading. The Pt nanoclusters were dispersed well and the particle sizes was less than those calculated from CO chemisorption. This discrepancy may be attributed to that some Pt nanoparticles were encapsulated or covered by the metal oxides derived from the support, leading to the reduction of the CO adsorption amount. Consequently, the particle sizes of Pt determined by CO chemisorption are larger than those observed on TEM, and this case is commonly observed in previous studies on Pt-loaded catalysts14 .


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Figure 2 TEM image of the xPt/2WAlSi catalyst showing the homogeneous support and high-dispersion of Pt nanocluster. (a) 0.5%Pt, (b) 1%Pt, (c) 2%Pt, (d) 3%Pt, (e) 4%Pt

3.1.2 FT-IR analysis Figure 3 shows the FT-IR spectra of 2Pt/yWAlSi with various WO3 /Al2O3 ratio in support. The peak at 1120 cm-1 corresponded to Si-O-Si vibration in the SiO2 network37, 38. The peak around 800 cm-1 was attributed to the W-O stretching vibrations in tungsten oxide and the peak intensity gradually increase with the enhancement of WO3 content in the support, where the oxygen-tungsten network presented some distortion. These similar vibration bands has also been observed in the Keggin structures after their partial reaction with SiO239, 40. This evidence leads us to believe that the W-O-Si linkages extensively exist in WO3Al2O3-SiO2 support.


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Figure 3 FT-IR spectra of 2Pt/yWAlSi with various WO3 /Al2O3 ratio, (1) 5:1, (2) 2:1, (3) 1:1 (4) 1:2, (5) 1:5

3.1.3 XPS analysis XPS technology was used to investigate the surface chemical states of Pt and W and surface composition of the catalysts after reduction, and all the data were calibrated so that the C1 line was at 284.6 eV. Because the binding energies(BE) of Pt 4f overlapped with W 5s (69-77eV) and Al 2p (71-76 eV)41, it is difficult to decouple the spectrum of Pt element from the W and Al. But the BE of Pt 4d (290.4-350.4 eV) clearly showed that there was an obvious increase in the peak intensity with the Pt loading increments (Figure S1.), indicating that the intensified interaction between the Pt nanoclusters and the support. The Si 2p BE of 2Pt/2WAlSi (102.3 eV, 103.1 eV and 104.0 eV) (Figure 4) reveals the different coordination environment for Si, indicating that the Si atom was bonded with W or Al by bridging oxygens40,

42-45

. The W-O-Si formation could also be confirmed by the FT-IR

vibration modes as shown in Figure 3.


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Figure 4 XPS spectra of Si 2p for 2Pt/2WAlSi

Figure 5 W 4f XPS spectra for the typcial 2Pt/2WAlSi catalyst

Figure 5 shows the XPS profile of W 4f core level region for the typical 2Pt/2WAlSi, and the others with different Pt conten have a similar XPS profile to 2Pt/2WAlSi and were given in supporting information (Figure S2). The BE data for xPt/2WAlSi with various Pt content were summarized in Table 2. We assume that these bands (38.1, 37.3, 36.0 and 35.2 (±0.1) eV) are ascribed to different oxidation states and coordination environment of Wn+ in WO3-Al2O3-SiO2 support, which are due to the modification of Auger parameter of Si and W40, 46, 47. Comparing


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the pure Si or W oxides, the same oxidation state of the mixed oxide network got an increase for the BE about 1eV because of the different polarizabilities in Wn+ and Si. The slightly different BE from the fitted values were related to Pt content for the tungsten oxide mixture. These values were different obviously from the pure ones because of the step reduction of WO3, the Pt-WOx interaction and the modification of the Auger parameter40, 46-49. It also suggested that there was a complex mixture of tungsten oxide presented on the catalyst surface, and the percentage of W5+ was listed in Table 2.

Table 2 The binding energies and the percentage of W5+in W5+ and W6+ of xPt/2WAlSi W6+ (eV)

W5+(eV)

W5+/(W5++W6+)

Catalyst 4f5/2

4f7/2

4f5/2

4f7/2

(%)

0.5Pt/WAlSi

38.14

35.99

37.21

35.09

18.62

1Pt/WAlSi

38.05

35.96

37.21

35.09

22.27

2Pt/WAlSi

38.02

35.92

37.21

35.06

30.05

3Pt/WAlSi

38.25

36.13

37.43

35.37

20.70

4Pt/WAlSi

38.16

36.08

37.31

35.28

18.57

3.1.4 Py-IR and NH3-TPD The acid site nature and strength (Brønsted and / or Lewis) of the catalysts is related to the selectivity in glycerol hydrogenolysis, which was generally characterized by the Py-IR and NH3-TPD methods. Figure 6 shows the Py-IR profiles of the catalysts with different ratio of WO3 to Al2O3. The Brønsted and Lewis acid sites were centered at 1540–1548 cm−1 and 1445–1460 cm−1 separately, and the amounts of the B acids and L acids sites listed in Table 3 were calculated by the integration of the adsorption bands approximately 1515-1565 cm-1 and 1435-1470 cm-1 respectively50.


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Figure 6 Py-IR spectra for 2Pt/yWAlSi catalysts with different W/Al ratio: (1) y=5:1, (2) y=2:1, (3) y=1:1, (4) y=1:2 and (5) y=1:5.

As shown in Table 3, the ratio of WO3 to Al2O3 in WAlSi support has significant effects on the Brønsted and Lewis acid sites amounts, leading to the difference in B/(B+L). As the ratio of the WO3 to Al2O3 decreased from 5:1 to 1:5, the B acid sites and the B/(B+L) gave a volcano like tendency, and the maximum for the B acid sites appeared at the ratio of 2:1, at which the catalytic activity and 1,3-PDO selectivity could reach the maximum (Figure 8). This result shows that the Brønsted acid sites were responsible for the dehydration of the second hydroxyl. For Ptloaded catalysts containing WO3, the H2 was split into H atoms on Pt surface and the H atoms spillovered onto WO3 surface. The Brønsted acid sites were usually ascribed to the reduction of WO3 by the spillovered H51, 52, but the reduced WOx had different structures with different WOx surface density including monolayer, polytungstate, and polytungstate/crystalline WO353. The polytungstate domains grew when the WOx surface density increased and the WOx species became easier to reduce but harder to access for the reactant. The Brønsted acid sites formed on catalysts with a slight polytungstate domain reduction and an electron delocalization which resulted in H+δ(Brønsted acid site)49, 53. For the polytungstate-like WOx clusters domain with intermediate surface density, it could reach the balance between reducibility and accessibility, at


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which the glycerol conversion and the 1,3-PDO selectivity got a maximum when the ratio of WO3 to Al2O3 was 2:1. The results are in agreement with the XPS, which had shown that the W species are mixtures of WOx49, 53. Table 3 Acidities of different catalysts measured from Py-IRa B acidity

L acidity

Total

acidity

(μmol/g)

(μmmol/g)

(μmmol/g)

2Pt/5WAlSi

50.8

120.7

171.5

29.6

2Pt/2WAlSi

68.2

148.5

216.7

31.5

2Pt/1WAlSi

59.3

114.2

173.5

34.2

2Pt/0.5WAlSi

37.4

87.4

124.8

30.0

2Pt/0.2WAlSi

10.1

29.7

39.8

25.4

Catalyst

aThe

B/(B+L) (%)

amount of acid sites were calculated from the quantity of the desorpted pyridine from Py-IR.

Figure 7 shows the NH3-TPD results for the catalysts with different Pt loading amount. The TPD peaks represented the acidity strength and the amounts of the acid sites could be calculated from the NH3 desorbed. According to the desorption temperature, the acidity strength is classified into three types, weak (lower than 300 °C), medium (300-500 °C) and strong (>500 °C)54. The Brønsted acid sites were responsible for the dehydration of the second hydroxyl, and the acid sites were most abundant in 2Pt/2WAlSi, as listed in Table 4. These profiles in Figure 7 showed that the xPt/2WAlSi catalysts had only medium and strong acid sites. These results also showed that the Pt can improve the acidity by a spillover of H2 to produce proton with reducing H atom5.


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Figure 7 NH3-TPD profiles for xPt/2WAlSi catalysts with different Pt loading: (1) x=0.5 (2) x=1 (3) x=2 (4) x=3 (5) x=4 Table 4 Acidities of different catalysts measured from NH3-TPDa Desorption

Desorption

Total

Acidity Catalyst

a

temperature

Acidity temperature

acidity

℃

mmol/g

℃

mmol/g

mmol/g

0.5Pt/2WAlSi

316

1.46

518

0.24

1.70

1Pt/2WAlSi

346

1.64

690

0.23

1.87

2Pt/2WAlSi

348

0.68

690

1.02

1.70

3Pt/2WAlSi

330

0.66

610

1.01

1.67

4Pt/2WAlSi

310

0.81

600

0.13

0.94

The amount of acid sites were calculated from the quantity of the desorpted NH 3 from NH3-TPD.

3.2 Hydrogenolysis Reaction 3.2.1 Effect of the ratio of WO3 to Al2O3 Regarding the glycerol hydrogenolysis, the WO3 species in the catalysts seem to be crucial to achieve a high 1,3-PDO selectivity15,16, herein, the effect of WO3 content in the support on the catalytic performance was investigated. As shown in Figure 8, the glycerol conversion and 1,3-PDO selectivity over the WO3-free catalyst are very low, however, after WO3 was introduced into the support, the catalytic activity and the selectivity for 1,3-PDO were significantly improved. The glycerol conversion and 1,3-PDO


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selectivity have a similar tendency in the range of WO3/Al2O3 used for this study, and both of them reach the maximum with the ratio of WO3 to Al2O3 at 2:1. When the WO3/Al2O3 ratio varied from 5:1 to 1:5, the WOx surface density decreased, and there was a volcano like curve between the ratio of WO3 to Al2O3 and the conversion. Barton et al49, 53 had proved that the acid density had the maximum when the WOx had the intermediate content, which can be seen from Table 3 and 4. The WOx species was a mixture of tungsten oxide with different valance and crystalline styles, including octahedra (monoclinic) and tetrahedral(tetragonal)53. On the other hand, when the ratio of WO3 to Al2O3 was 2:1, the surface density and the species of WOx were coordinated well to provide sufficient acidity in obtaining high activity and 1,3-PDO selectivity. The change of the ratio for WO3 to Al2O3 resulted in an unsuitable surface density of WOx, which gave a negative influence on the catalytic performance of the catalyst49, 53.

Figure 8 The reaction results for glycerol hydrogenolysis over 2%Pt/WAlSi catalysts with different WO3/Al2O3 ratio. Reaction conditions: 25g of 3wt% glycerol aqueous solution, 0.5g of catalyst, 6MPa H2 pressure, stirring rate of 600 rpm, reaction temperature of 160 oC, and reaction time of 12 h.

3.2.2 Effect of Pt Loading As shown in Figure 9, the glycerol conversion rose up from 13.5% to 47.68% with Pt content increasing from 0.5 to 2 wt %, and the selectivity of 1,3-PDO


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had a highest value of 55.92% when the Pt loading was 2 wt %. In addition, the space time yield of 1,3-PDO with 2Pt/2WAlSi catalyst reached 18.34 g/(gPt ﹒h) when dealting with the 40% glycerol aqueous solution, much higher than the best result reported in a batch reactor in literature (Table S1). The high activity and selectivity with 2Pt/2WAlSi catalyst could be ascribed to the high dispersion of Pt and suitable acidity. The CO chemisorption results revealed that the 2Pt/2WAlSi catalyst gave a high dispersion (Table 1), which was in agreement with the TEM (Figure 2). Both of the glycerol conversion and the 1,3-PDO selectivity declined with the increase of Pt loading amount (3Pt/2WAlSi and 4Pt/2WAlSi), that is related to the decrease of the acidity, dispersion, and B/(B+L). Therefore, the 2Pt/2WAlSi catalyst was chosen for the succeeding reaction.

Figure 9 Effect of Pt loading amount on glycerol hydrogenolysis. Reaction conditions: 25g of 3wt% glycerol aqueous solution, 0.5g of catalys t(W:Al=2:1), 6MPa H2 pressure, stirring rate of 600 rpm, reaction temperature of 160 oC, and reaction time of 12 h.

3.2.3 Effect of Pressure Figure 10 shows the influence of the H2 pressure on the glycerol hydrogenolysis. Glycerol conversion was improved remarkably when the H2 pressure increased from 2.0 to 6.0 MPa, meanwhile, the selectivity of 1,3-PDO increased from 44.2% to 55.9%. Further improving the H2 pressure to 7.0 MPa, the conversion rate maintains the comparable


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level at 6.0 MPa, however, the 1,3-PDO selectivity decrease abruptly to 22%. For 1,2-PDO, its selectivity decreased gradually with the H2 pressure increase except at the middle pressure range(4.0-5.0 MPa). The 1-PO selectivity also increased from 3.0 MPa to 6.0 MPa, attributing to the further hydrogenolysis of 1,2-PDO and 1,3-PDO to 1-PO. Based on these reaction results, we can conclude that the high H2 pressure is beneficial to produce 1,3-PDO in the hydrogenolysis of glycerol, which is also observed on the previous study21, 22.

Figure 10 Effect of pressure on glycerol hydrogenolysis. Reaction conditions: 25 g of 3 wt.% glycerol aqueous solution, 0.5g of 2Pt/2WAlSi catalyst, stirring rate of 600 rpm, reaction temperature of 160 oC, and reaction time of 12 h.

3.2.4 Effect of Reaction Time As shown in Figure 11, glycerol conversion gradually increased with reaction time and reached 47.7% with a 1,3-PDO selectivity of 55.92% after 12 h. When the reaction time was prolonged to 18 h, a higher glycerol conversion of 55.3% was achieved but the selectivity declined. As the reaction time was raised up to 20 h, the conversion also dropped a little, so the optimal reaction time was 12 h. It was obvious that longer contact time allows for more glycerol to react with the catalyst active sites by diffusing, and glycerol conversion was increased23, 33.


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Figure 11 Effect of reaction time on glycerol hydrogenolysis. Reaction conditions: 25 g of 3 wt.% glycerol solution, 0.5g of 2Pt/2WAlSi catalyst, 6MPa H2 pressure, stirring rate of 600 rpm, and reaction temperature of 160 o

C.

3.2.5 Effect of Glycerol Content Water is one of the products of glycerol hydrogenolysis, so it is unavoidable for this reaction. It is also important from a techno-economical point to carry out the reaction with an appropriate water content because the dilute glycerol solution results in an increase in both capital and operation costs. On the other hand, water can also decrease the viscosity and increase the mass transfer performance. Figure 12 shows the catalyst conversion efficiency for glycerol and the space time yield of 1,3-PDO with different water content. As the glycerol content increased up to 40% (water content 60%), the TOF of the glycerol and the space time yield of 1,3-PDO was increased. The maximum for 1,3-PDO was 18.34 g/(gPt·h) at 40% glycerol content, and the TOF of the glycerol reached 101.73 g/(gPt·h) at the glycerol content of 90%. As the glycerol content increased, the more glycerol molecules could reach the active sites of the catalyst by diffusing, the higher glycerol conversion and 1,3-PDO selectivity could be reached. On the contrary, the viscosity of the system was also increased with an increase of glycerol content, which slowed down the mass transfer rate of 1,3-PDO from catalyst surface to


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the bulk55,

56

and facilitated the further hydrogenolysis of 1,3-PDO to 1-PO, therefore, the

selectivity of 1,3-PDO was lowered.

Figure 12 TOF of glycerol and the space time yield of 1,3-PDO with different glycerol content. Reaction conditions: 25 g of glycerol solution, 0.5g of 2Pt/2WAlSi catalyst, 6MPa H2 pressure, stirring rate of 600 rpm, reaction temperature of 160 oC, and reaction time of 12 h.

3.3 The initial selectivity and the effect of SiO2 To achieve a high selectivity of 1,3-PDO in glycerol hydrogenolysis, the further reaction of 1,3-PDO should be inhibited. In fact, the 1,3PDO could be further catalyzed into 1-PO57 because of the  f Gm,1PO   f Gm,1,3PDO 58, so we could evaluate the effectiveness of the catalyst to second hydroxyl by the selectivity ratio of 1,3PDO to 1-PO and the initial selectivity calculated from the selectivity sum of 1,3-PDO and 1-PO. The initial selectivity was a reflection of activity for the catalyst to the second hydroxyl, and the ratio of 1,3-PDO to 1-PO reflected the possibility aimed at the second hydroxyl belong to glycerol or 1,2-PDO. The two values for the catalyst systems in catalyzing glycerol to 1,3-PDO were shown in Table S1, including the results of this work. It can be seen that most of the systems displayed an initial selectivity larger than 50%, so the catalysts were more powerful for the second hydroxyl of glycerol than the primary one. Another value listed in Table S1 was the space time yield of 1,3-PDO, which exhibited the effectiveness of the catalyst used in this hydrogenolysis process.


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The hydrogenolysis of the second OH group was crucial for glycerol hydrogenolysis to 1,3PDO. The activity of catalyst to the second hydroxyl could be fairly compared by the initial selectivity and the ratio of 1,3-PDO to 1-PO. In Table S1, the SiO2 doped systems had a better performance than the other systems comparing with the catalysts without SiO2. Concluded from the above, SiO2 was helpful in improving the selectivity of 1,3-PDO in glycerol hydrogenolysis. One of the reason was the strong affinity for glycerol to silica mentioned in Section 3.2.159, 60. Another reason could be drawn from the binding energy of Si 2p, Al 2p, and O 1s in these catalysts. These binding energies were listed in Table S2. All the level range of Si 2p, Al 2p, and O 1s from the start BE to the end BE broadened, and the peak BE shifted to a higher value. For the Al 2p, the peak BE shifted from 74.44 to 75.15 eV with the Pt loading increasements. Concerning the level range broadening and the BE shifted to a higher value, there was a more and more powerful metal-support interaction as the Pt content increasing33. As was for Si 2p and O 1s, the same conclusion could be derivated from the BE shift, that was the formation of the metal-oxygen bond. Considering the W4f in Section 3.1.3, it could be figured out that the doped SiO2 promoted the interaction of Pt and the other elements in these catalysts and improved the reducibility of the WO3 to increase Brønsted acidity, which was important for dehydration of the second hydroxyl in glycerol. 3.4 The Hydrogenolysis Mechanism The glycerol hydrogenolysis experienced two steps via acid-catalyzed dehydration and hydrogenation on metal sites4, 31, 61. The nature and the strength of the acid sites were essential for dehydration to form carbenium intermediate, and it was proved that Brønsted acid sites were responsible for the formation of 1,3-PDO, while Lewis acid sites played a key role for 1,2-PDO formation16.


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Although it was more favorable to eliminate the second hydroxyl to form a stable secondary carbenium, the steric hindrance restricted the proton to access the second hydroxyl, and the intermediate, 3-hydroxypropionaldehyde (3-HPA), was less stable than the acetol formed from eliminating a primary hydroxyl5, 62, 63, so it was important to restrict the primary hydroxyl and to expose the second one in glycerol hydrogenolysis reaction. Kaneda10, 11 et al proposed a metal alkoxide process, wherein the primary alcohol was restricted in an aluminum alkoxide form, and the proton diffused to the second hydroxyl to proceed the dehydrate reaction. But the alkoxide is sensitive to water and easy to hydrolyze, and it was difficult to be generated under the reaction conditions64.

Scheme 1. Reaction mechanism of glycerol hydrogenolysis to 1,3-PDO over Pt/WAlSi catalyst

Based on the above experimental results and discussion, a plausible mechanism for glycerol hydrogenolysis to 1,3-PDO on Pt/WAlSi catalyst was proposed, as illustrated in Scheme 1. There are five steps involved in the reaction process. as follows. (1) The adsorbed H2 molecules on Pt


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nanoparticles were splited into H atoms, which could spillover onto the surface of WAlSi support and form a proton by donating an electron. Meanwhile, the glycerol was adsorbed onto the surface of support by a hydrogen bond between the primary alcohol of glycerol and the hydroxyl located on the SiO2 and Al2O3 surface, as a result, the second hydroxyl was exposed. (2) The protons derived from spillover and reduction move to the second hydroxyl for protonation and dehydration to form a second carbenium, and then hydrogen transfer reaction proceeds on the catalyst. (3) The 3-hydroxypropenol was generated following the hydrogen transfer reaction by step 2. (4) 3-HPA was formed via a tautomerization of 3-hydroxypropenol. (5) Hydrogenation of 3-HPA was quickly finished to produce 1,3-PDO on the active Pt sites, wherein the fast hydrogenation is necessary to prevent the sequence dehydration of 3-HPA to produce acrolein, because the dehydration of 3-HPA to acrolein was thermodynamically favorable63, 65. The formed 1,3-PDO was desorbed from the sruface of catalyst, and then the new reaction proceeds from the step 1 on the catalyst. In addition, the stability of the catalyst was investigated by three successive runs (Figure S3) under the identical condition. It was observed that the conversion of glycerol decreased negligibly from 47.7% in the first run to 44.5% in the third run and the product distribution had no obvious changes in the three successive runs. Based on this results, the slight drops of conversion ratio could be attributed to the loss of catalyst during the recovery of catalyst from the last run. Therefore, the excellent catalytic performance of the Pt/WAlSi catalyst shows promising potential for practical applications. 4. CONCLUSION The glycerol hydrogenolysis was studied over Pt-loaded WO3-Al2O3-SiO2 support prepared by sol-gel method, which exhibited a high glycerol activity and 1,3-PDO selectivity. The high TOF


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of 101.73 g/(gPt﹒h) for glycerol and the high 1,3-PDO space time yield of 18.34 g/(gPt﹒h) were achieved over the 2% Pt/WAlSi catalyst. A plausible mechanism of the Pt/WAlSi catalysts in the selective production of 1,3-PDO from the glycerol was proposed. The hydrogen bonds play a key role in bridging the primary hydroxyl of glycerol with the surface OH groups derived from Si-OH or Al-OH in the support. The Brønsted acid sites are responsible for dehydration and the loading platinums as the active centre for the hydrogenation of the intermediate to 1,3-PDO. ■ ASSOCIATED CONTENT Supporting Information. Table showing the reported reaction results for glycerol hydrogenolysis and figures related to the XPS specra of catalysts, and the stability of catalyst in successive runs. ■ Author Information Corresponding Author * Tel : +86-351-6010550, Fax: +86-351-6010550, Email: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work was financially supported by the National Natural Science Foundation (21276174

and

21322608),

Shanxi

Province

Science

Foundation

for

Youths

(2014021005), Program for the Innovative Talents of Higher Learning Institutions of Shanxi, and A Foundation for the Author of National Excellent Doctoral Dissertation of PR China (No.201350).


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The platinum-loaded WO3-Al2O3-SiO2 catalysts with a homogeneous dispersion show a high selectivity and space time yield for 1,3-Propanediol.


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A Highly Efficient Catalyst for Selective ... - ACS Publications

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