The Effect of Catalytic Structure Modification on Hydrogenolysis of

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The Effect of Catalytic Structure-modification on Hydrogenolysis of Glycerol into 1,3-propanediol over Platinum Nanoparticles and Ordered Mesoporous Alumina Assembled Catalysts Minyan Gu, Zheng Shen, Long Yang, Boyu Peng, Wenjie Dong, Wei Zhang, and Yalei Zhang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02899 • Publication Date (Web): 31 Oct 2017 Downloaded from http://pubs.acs.org on November 4, 2017

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Industrial & Engineering Chemistry Research

The Effect of Catalytic Structure-modification on Hydrogenolysis of Glycerol into 1,3-propanediol over

Platinum

Nanoparticles

and

Ordered

Mesoporous Alumina Assembled Catalysts Minyan Gu,† Zheng Shen,*,† Long Yang,† Boyu Peng,† Wenjie Dong,† Wei Zhang,† and Yalei Zhang*,† †

State Key Laboratory of Pollution Control and Resources Reuse, Key Laboratory of Yangtze River Water Environment of MOE, Tongji University, Shanghai 200092, China.

* Corresponding [email protected].

Author:

[email protected];

ABSTRACT: To increase the Brønsted acid sites and the dispersion of Pt, and decrease

the

loss

of

Pt

during

reuse,

a

structurally

modified

PtNPs-HSiW/mAl2O3 catalyst was synthesized by assembling platinum nano-particles

(PtNPs)

adsorption-desorption, significantly

into XRD

different

ordered and

mesoporous

TEM, structure

alumina.

PtNPs-HSiW/mAl2O3 from

By

N2

exhibited

impregnated

Pt-HSiW/γ-Al2O3—short-range ordered mesopores, large surface area and special structure where PtNPs were assembled in the mesopores of alumina. Further pyridine-IR, CO adsorption and ICP showed that with the structural modification, the Brønsted acidity increased from 12.0 to 30.3 µmol/g, the Pt dispersion increased from 15.0 to 35.4% and the loss of Pt decreased from 4.54 to 0.59 wt% during reuse. Finally, PtNPs-HSiW/mAl2O3 exhibited a 13.8% higher 1,3-PDO selectivity and more stable yields over reuse than Pt-HSiW/γ-Al2O3. It provided a reference that structural modification influences

1

ACS Paragon Plus Environment


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1,3-PDO production by altering Brønsted acidity, Pt dispersion and loss of Pt during reuse.

1. INTRODUCTION 1,3-propanediol (1,3-PDO) is not only an industrially important chemical terminal product but also a platform molecule in the synthesis of a wide range of chemicals, such as the staple polytrimethylene-terephthalate (PTT).1,2 Currently, the industrial production of 1,3-PDO typically involves three approaches—acrolein hydratation3, ethylene oxide formylation4, and biological fermentation5,6. Compared to the former two approaches, the renewable biomass (e.g., starch and glycerol), rather than fossil products, is usually used as the fermentation substrate in the biological fermentation. However, at present, the biological fermentation typically needs a long downstream process such as a complex 1,3-PDO recovery process from the fermentation broth.

7

As a substitute, the 1,3-PDO production process will perform both

cleaner and faster by utilizing glycerol as the reactant in the catalytic hydrogenolysis

1,8

. A large number of studies have made a contribution to the

selective conversion of glycerol into 1,3-PDO, instead of 1,2-PDO6,8-15. For hydrogenolysis of glycerol into 1,3-PDO, acid sites and hydrogenation sites of the catalyst are the key points to overcoming the obstacles brought by the low removal probability and strong steric hindrance of the middle hydroxyl. What’s more, these sites ensure the selective dehydration (glycerol to 3-hydroxypropanal on acid sites) and hydrogenation (3-hydroxypropanal to 1,3-PDO on hydrogenation sites) could occur simultaneously. On one hand, the acid sties play an important role in dehydration step because it decides the conversion of glycerol and the distribution of intermediate products (glycerol to 3-hydroxypropanal). It has been reported that Brønsted acid tends to remove the secondary hydroxyl group of glycerol to generate 1,3-PDO, while Lewis acid favors the formation of 1,2-PDO13. A 2


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series of Brønsted acidic source were studied, such as WOx8,9, H2SO4 and H2WO41. Hence, silicotungstic acid (HSiW), a typical Brønsted acid, was loaded onto catalysts to enhance the Brønsted acidity16-18, and the loading amount of HSiW (wt%) was regarded as the primary factor influencing the catalytic Brønsted acidity. Besides optimizing the loading amount of W species13, other ways should be explored to further enhance the catalytic Brønsted acidity, such as catalytic structure modification. On the other hand, in existing hydrogenating metals19-22, platinum (Pt) is well known for its high hydrogenation activity in glycerol hydrogenolysis. On the hydrogenation sites of Pt particles, H2 homolytically dissociates to two hydrogen atoms, and subsequently hydrogenate the dehydrated intermediate formed during hydrogenolysis to target products (3-hydroxypropanal to 1,3-PDO)8. Based on its essential role, the dispersion of Pt 19and the loss of Pt during reuse, as representative parameters, strongly influence the activity and stability of catalyst for 1,3-PDO yield. Various methods have been taken to promote the dispersion of Pt19,23, but it is tough to realize the uniform dispersion because the complex surface chemistry and mesoporous structure of preformed support sometimes limit the diffusion of Pt during the preparation processes (such as ion exchange and incipient wetness impregnation)24. As a substitute, if Pt particles are put into the solution of the support-precursor, rather than onto the preformed support, Pt might be more uniformly mixed with support-precursor and more firmly assembled with support via the AcHE process (a sol–gel solution composed of acetic acid, hydrogen chloride, and ethanol) 25. Herein, in order to promote the production of 1,3-PDO, modification of catalytic

structure

from

Pt-HSiW/γ-Al2O3

(traditional

impregnation)

to

PtNPs-HSiW/mAl2O3 (AcHE process) was tried to improve three aspects of the catalyst simultaneously: (1) the Brønsted acidity ; (2) the dispersion of Pt; (3) the loss of Pt during reuse. The two catalysts were characterized 3



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systematically to verify the role of structure modification, and the possible relationships between the structure and the above three aspects of catalyst were explained. We also investigated the catalytic performances of Pt-HSiW/γ-Al2O3 and PtNPs-HSiW/mAl2O3 in a tank reactor, and the latter showed higher catalytic activity and stability in hydrogenolysis of glycerol into 1,3-PDO. Hence, we hope to provide a reference that modification of catalytic structure is an effective way to promote catalytic activity and stability for hydrogenolysis of glycerol into 1,3-PDO.

2. EXPERIMENTAL SECTION 2.1. Catalyst preparation PtNPs/mAl2O3 preparation. There are two steps in the preparation of PtNPs/mAl2O3, including synthesis of the platinum nanoparticles and assembling of the PtNPs and mAl2O3. Platinum nanoparticles were synthesized via the ethylene glycol reduction process26,27. Typically, the Pt particles were synthesized by adding a total of 6.0 mL of 0.375 M PVP-ethylene glycol solution (Mw=55 000, Sigma-Aldrich) and 3.0 mL of H2PtCl66H2O-ethylene glycol solution (6.25×10-2 M, PVP/Pt salt 12:1, Aladdin) in refluxing ethylene glycol every 30 s over 16 min. The mixture of solutions was refluxed for an additional 5 min at 200°C. The particles were precipitated by adding a triple volume of acetone, and re-dispersed in 10.0 mL of PVP-ethanol solution (4×10-4 M) after centrifugation (8500 r, 15 min). PtNPs/mAl2O3 was synthesized via AcHE process25, as shown in Figure 1. Typically, 1.26 g of triblock copolymer Synperonic® F 108 (0.1 mmol, Mw≈12600, Sigma-Aldrich), 2.0 mL of hydrochloric acid (24 mmol), 2.4 mL of acetic acid (40 mmol), 2.46 g of aluminum tri-sec-butoxide (10 mmol, C12H27AlO3, 97%, Sigma-Aldrich), and 6.8 mL of PtNPs-ethanol solution (0.065 mmol) were dissolved in 30 mL of ethanol, and the mixture was stirred for 15 min by super sonication to form a transparent solution which was transferred 4


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into a glass Petri dish. After drying at 40°C for 3 h, the mixture was then aged at 65°C for 24 h. Finally, PtNPs/mAl2O3 was obtained by calcination (400°C-700°C, 6 h, ramp rate 2 °C/min, in air) to remove the template. Pt/γ-Al2O3 preparation. Pt/γ-Al2O3 was prepared by loading platinum on to preformed γ-Al2O3 via the incipient wetness impregnation method. The saturated water absorptivity of γ-Al2O3 (Aladdin) was tested before impregnation. Typically, 0.108 g H2PtCl66H2O (0.21 mmol) was dissolved in 1.2 mL of water, and then the solution was added dropwise to 1 g γ-Al2O3. The mixture was aged for 12 h after stirring evenly, and calcined at 450°C in air for 3 h.

PtNPs-HSiW/mAl2O3

and

Pt-HSiW/γ-Al2O3

preparation.

PtNPs-HSiW/mAl2O3 and Pt-HSiW/γ-Al2O3 were synthesized via the incipient wetness impregnation described above, where 0.186 g of silicotungstic acid (0.06 mmol, HSiW, SCRC) was added in for every 1 g of PtNPs/mAl2O3 or Pt/γ-Al2O3, and calcined at 350°Cin air for 4 h.

Figure 1. Synthetic process of PtNPs-HSiW/mAl2O3 and the assembly mechanism between PVP capped PtNPs and F108. 2.2. Catalyst characterization N2 adsorption–desorption isotherms were recorded on a Micromeritics 5



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ASAP 2020 adsorption analyzer. Each sample was purged in a vacuum at 300°C for 3 h before measurement. The Brunauer-Emmett-Teller (BET) method was used to calculate the specific surface areas of catalysts28, and the pore volume was calculated by the Barrett-Joyner-Halenda (BJH) model29. Physicochemical parameters of the catalysts were defined as follows: the mesoporous surface area (Smeso) was calculated by the BJH method from the desorption branches; the total pore volume (Vtotal) was measured at P/P0=0.995; the pore diameter (Dpore) was calculated by the BJH method from the desorption branches.30 Powder X-ray diffraction (XRD) patterns were collected on an X-ray diffractometer (Bruker D8 Advance) with Cu Kα radiation, operated at 40 kV 40 mA and recorded in 2θ range (10°to 90°). Transmission electron microscopy (TEM) images and EDS patterns were recorded on a JEOLJEM-1230 instrument operated at 80 kV. The inner morphology of the catalysts was observed by TEM of the catalyst slices after freezing sections. Slicing-TEM: the catalyst was embedded by spurr epoxy and after aggregation at 70°C overnight, the catalyst was sliced by the microtome (LEICA EM UC7, slice thickness 100 nm) and then one slice of catalyst will be tested by TEM. A Fourier transform infrared spectrophotometer was used to detect the functional groups located on the catalysts. The acidic properties of the catalysts were tested by adsorption of pyridine by Fourier transform infrared spectroscopy (Py-IR). The Py-IR spectra were recorded on a Perkin Elmer Frontier FT-IR in the 1400-1700 cm-1 range with a spectral resolution of 2 cm-1. A 10 mg of sample was pressed into a self-supported wafer with a diameter of 13 mm. The wafer was set in a quartz IR cell that was sealed with CaF2 windows and connected to a vacuum system. The samples were dried at 400°C for 2 h under vacuum. After cooling, pyridine vapor was admitted into the cell and adsorption lasted for 0.5 h. Subsequently, desorption steps at 150°C (1 h), 250°C (1 h), 350°C (1 h), 400°C (1 h) were performed. The acidity 6


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of Brønsted and Lewis acid sites was calculated by using the adsorption coefficient values reported in the previous paper31. The hydrogenation reactivity of catalysts and the dispersion of Pt was tested by CO pulse chemisorption. Sample (0.05 g) was firstly pretreated in hydrogen (50 cm3/min) at 150°C for 1 h and purged with helium (50 cm3/min) for 1 h at the same temperature. And then, the catalyst was cooled to room temperature and CO pulses were injected from a calibrated online sampling valve. CO adsorption was complete after three successive peaks showed the same peak areas. 23 The contents of Pt in the catalysts and the reaction solutions were determined by inductively coupled plasma optical emission spectroscopy (ICP-OES, Perkin Elmer Optima 2100 DV). Prior to the measurements, samples were digested in an acidic mixture (HCl-HNO3) at 150°C for 12 h. 2.3. Catalytic reaction Heterogeneous catalytic reaction. Heterogeneous catalytic reactions were performed in a 100 mL stainless steel reactor with an inserted Teflon vessel and magnetic stirring (400 r/min), as shown in Figure S1. Firstly, catalysts

were

reduced.

Typically,

150

mg

PtNPs-HSiW/mAl2O3

or

Pt-HSiW/γ-Al2O3 catalysts, 15 mL of water and a magnetic spinner were placed into the reactor. After sealing the reactor, the air was purged thrice by 4 MPa hydrogen. The reactor was inflated to 4 Mpa by hydrogen and then heated to 200°C. After catalysts were reduced for 2 h, the reactor was cooled down and the H2 was removed. Secondly, glycerol (0.1 M) was added to the reactor. After the reactor was sealed and purged thrice by hydrogen at 4 MPa, it was injected with 4 MPa hydrogen and then heated to 200°C. After an appropriate reaction time, the reactor was cooled down and the liquid products were analyzed by GC (Agilent 7820A, J&W125-7332, 30m×530µm×1µm) with a FID detector. The used catalysts were separated from the mixture by centrifugation, washed 5 times with water, and finally dried at 180°C for 3 h. The conversion, 7



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yield, and selectivity were calculated as follows: molarity of glycerol before − molarity of glycerol after × 100 molarity of glycerol before molarity of one product after × number of carbon atoms in this product Yield % = molarity of glycerol before × 3 Conversion % =

× 100 Selectivity % =

yield of one product × 100 conversion of glycerol

In catalytic reuse, the loss of Pt (Ln/wt%) and the cumulative loss of Pt (Cn/wt%) of the catalysts were calculated as follows (Ln stands for the loss of Pt after the nth cycle of reuse, and Cn stands for the cumulative loss of Pt when the catalyst has been used n cycles): L# = The loading amount of Pt on catalyst &after the n'( cycle of reuse) − The loading amount of Pt on catalyst before the n'( cycle of reuse C# = C#*+ + L#*+ ; n ≥ 2; C+ = 0

Semi-heterogeneous catalytic reaction. Semi-heterogeneous catalytic reactions were performed in the same reactor as the heterogeneous reaction. Typically, 15 mL of glycerol solution (0.1 M), 150 mg of PtNPs/mAl2O3 or Pt/γ-Al2O3 catalysts, and 22.5 mg of silicotungstic acid (HSiW) were added to the reactor. Subsequent operations were the same as those in the semi-heterogeneous reaction.

8


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3. RESULTS AND DISCUSSION 3.1. Morphology and crystalline structure N2 adsorption–desorption, XRD, and TEM were used to investigate the structure differences between the structurally modified mAl2O3-based catalysts (PtNPs/mAl2O3 and PtNPs-HSiW/mAl2O3) and traditional γ-Al2O3-based catalysts (Pt/γ-Al2O3 and Pt-HSiW/γ-Al2O3). Mesoporous properties. Table 1 lists the physicochemical properties of a series of mAl2O3-based and γ-Al2O3-based catalysts. The SBET as well as the Vtotal of mAl2O3 (Entry1) synthesized by the AcHE process (without PtNPs) were higher than those of γ-Al2O3 (Entry7), indicating that the AcHE process might be beneficial for the formation of mesopores. The Dpore of mAl2O3 was relatively larger (7.8 nm) compared with that of γ-Al2O3 (3.9 nm) and the reported alumina materials (6.7 nm

32

, 1.9 nm

33

, and 4.0-6.0 nm

34

). This

phenomenon can be explained by two reasons: on the one hand, the template F108 has a large molecular size in the template family, due to which the template-transferred pores are correspondingly large in size. On the other hand, the mesopores made via the AcHE process might be larger than from the other process in general because of the special performance of the colloid. As displayed in Table 1 (Entries 2 to 5), Figure S2 and Figure S3, the template

removal

temperature

have

a

significant

impact

on

the

physicochemical properties of PtNPs/mAl2O3. As shown in Figure S2, catalysts calcined at different temperatures all show type IV isotherms with hysteresis loops, which indicates the existence of mesopores.35 Table 1 (Entry 2-5) shows that the SBET values of the catalysts calcined at 400°C, 500°C, 600°C, and 700°C were 363, 213, 281, and 207 m2/g, respectively, and the Vtotal values were 0.73, 0.54, 0.49, and 0.37 cm3/g, respectively. The decreasing tendency of SBET and Vtotal can be attributed to pore collapse caused by increasing temperature. Figure S3 exhibits the relationship of the template removal 9



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temperature and the yield of 1,3-PDO in the semi-heterogeneous reaction, indicating that 400°C was the optimal template removal temperature in the experimental group. Thus, the catalysts below were only prepared at the template removal temperature of 400°C. The physicochemical properties of PtNPs-HSiW/mAl2O3 (Table 1, Entry 6) and Pt-HSiW/γ-Al2O3 (Table 1, Entry 9) were also very different. The SBET as well

as

Vtotal

of

PtNPs-HSiW/mAl2O3

were

higher

than

those

of

Pt-HSiW/γ-Al2O3. This result was mainly caused by two reasons: the excellent structure of mAl2O3 support, as discussed above; and the fewer number of calcinations. Calcination was performed twice in the process for γ-Al2O3 to Pt-HSiW/Al2O3, and in contrast, only once in the process of mAl2O3 to PtNPs-HSiW/mAl2O3 because the synthesis of alumina and the assembling of PtNPs and mAl2O3 were simultaneous.

10


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Table 1. Physicochemical properties of mAl2O3- and γ-Al2O3-based catalysts. Template Surface

Total Pore

Pore

Area

Volume

Diameter

(m2g-1)

(cm3g-1)

(nm)

369

0.74

7.8

Removal Entry

Catalyst

Temperature (°C)

1

mAl2O3

2

400

363

0.73

8.0

3

500

213

0.54

10.1

4

600

281

0.49

7.0

5

700

207

0.37

7.2

PtNPs/mAl2O3

6

PtNPs-HSiW/mAl2O3

311

0.65

8.0

7

γ-Al2O3

271

0.45

3.9

8

Pt/γ-Al2O3

243

0.43

3.5

9

Pt-HSiW/γ-Al2O3

202

0.34

3.9

Crystalline structure. To investigate the difference in crystalline structure of mAl2O3- and γ-Al2O3-based catalysts, XRD was employed and the patterns are displayed in Figure 2. As shown in Figure 2(a), the mAl2O3 has no large-angle diffraction peaks (10°≤2θ≤90°). The result indicates that mAl2O3 was amorphous, which agrees with the results of FT-IR (Figure S4). There were also no peaks corresponding to tungsten species in the XRD patterns, suggesting that the tungsten species primarily existed as polytungstate with 15 wt% and calcination at 350°C.36 In this amorphous mAl2O3 and HSiW, Pt diffraction peaks were clearly observed and fully matched the standard card (2 θ=39.76°, 46.24°, 67.45°, 81.29°, 85.71°). Additionally, as seen in Figure S5, there was no significant difference between the reduced mAl2O3-based 11



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catalysts and the unreduced mAl2O3-based catalysts. The result may be attributed to the first step of the synthesis (AcHE), where Pt (IV) was reduced to Pt (0) before addition into the synthetic system. As shown in Figure 2(b), the crystal form of Al2O3 matched γ-Al2O3 fully, quite different form mAl2O3-based catalysts, and Pt was hardly observed because the diffraction peaks of Pt0 was partially covered by γ-Al2O3.

Figure 2. The XRD patterns of (a) mAl2O3- and (b) γ-Al2O3-based catalysts.

Catalyst morphology and assembly mode of platinum/alumina. The TEM images of mAl2O3- and γ-Al2O3-based catalysts are displayed in Figure 3. As shown in Figure 3(a), mAl2O3 synthesized by the AcHE process (without 12


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PtNPs) had short-range ordered mesopores ( 7.953.6 *3.4 nm in diameter and

127569 *78 nm in length). The diameter of mesopores (~7.9 nm) in mAl2O3 agrees with the N2 adsorption-desorption behavior described above (Table 1) and the length of the mesopores (~127 nm) in mAl2O3 can be explained by the ‘rod-like micelle assembly’ theory

37

. Studies have shown that spherical micelles are

formed at the first critical micellar concentration (CMC), and the structural transition from spherical to rod-like micelles occurs at the second CMC.38,39 In the synthetic system of mAl2O3, with the evaporation of the solvent, the F108 concentration changed from higher than the first CMC to the second CMC, indicating that the micelles structure changed from spherical to rod-like. With the assembly of rod-like micelles and sequential template removal, tubular mesopores were generated. However, limited by the short length of the rod-like micelles (generally less than 200 nm), mesopores finally exhibited a short range (~127 nm). As shown in Figure 3(b), PVP-capped PtNPs had the size of 7.15:.; *:.: nm, which matched the size of mesopores (~7.9nm) and ensured the firm assembly of PtNPs and alumina. In order to ensure whether the PtNPs were assembled inside mAl2O3, the PtNPs/mAl2O3 catalyst was sliced into pieces, and one of these slices is displayed in Figure 3(c). Using the slicing technique, PtNPs could be observed clearly inside mAl2O3, and not on the surface. The result fulfils the purpose of using the special synthetic process, and indicates that AcHE is an effective way to assemble metal particles into the support. The possible mechanism of the structure that PtNPs assembled into the mesopores of alumina is displayed in Figure 1 (bottom). Firstly, PVP adheres to the nanoparticles through a charge-transfer interaction between the pyrrolidone rings and surface Pt atoms.40 Because of the hydrophobic association effect, the hydrophobic part of PVP (methylene chain) and the hydrophobic part of F108 (ethylene oxide unit) are associated. After the hydrophilic part of F108 (propyleneoxide unit) adheres to the hydroxyl group of 13



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alumina, PtNPs are finally assembled into the mesopores of alumina. Thus, PtNPs could be dispersed in the mesopores of alumina after template removal.41 As shown in Figure 3(d), no HSiW particles were observed, and EDS verified the existence of W species. The TEM images of γ-Al2O3 and Pt-HSiW/γ-Al2O3 are displayed in Figure 3(e) and (f). In contrast to mAl2O3 based catalysts, γ-Al2O3 had disordered mesopores indicating that differences in synthetic methods had a significant impact on the morphology and structure of the catalysts, as seen from a comparison of (a) and (e). In brief, a structurally modified PtNPs-HSiW/mAl2O3 had been synthesized and characterized,

which

had

the

significantly

different

structure

from

Pt-HSiW/γ-Al2O3.

14


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Figure 3. TEM images of (a) mAl2O3; (b) PtNPs; (c) PtNPs/mAl2O3 and the mesoporous size; (d) PtNPs-HSiW/mAl2O3 and EDS; (e) γ-Al2O3; (f) Pt-HSiW/γ-Al2O3.

15



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3.2. Catalytic acidity and Brønsted acid sites In order to investigate the type and quantity of acid sites in the catalysts, the mAl2O3- and γ-Al2O3-based catalysts were characterized by Py-IR. According the results of pre-experiment (Table S1), the optimal HSiW loading of mAl2O3and γ-Al2O3-based catalysts were both 15 wt% (theoretical content). As shown in Figure 4, PtNPs/mAl2O3 and Pt/γ-Al2O3, as well as γ-Al2O3 and mAl2O3, had abundant Lewis acid sites but no Brønsted acid sites. However, after HSiW loading, PtNPs-HSiW/mAl2O3 and Pt-HSiW/γ-Al2O3 both showed Brønsted acid sites, reasoning from that the HSiW loaded on catalysts could express Brønsted acidity. As figure 4 (a) showed, the pure HSiW possessed Brønsted acid sites which could further verify that Brønsted acidity in these catalysts was provided by HSiW.

16


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Figure 4. The Py-IR profile of (a) mAl2O3-based catalysts and HSiW; (b) γ-Al2O3-based catalysts.

The details of acid information are listed in Table 2. The Lewis as well as Brønsted acidity of all catalysts was gradually weakened with the increase of desorption temperature. Due to the abundance of Lewis acid sites, these were still present in all four types of catalysts when the desorption temperature was 450°C. In contrast, the Brønsted acid sites in these catalysts disappeared at a high desorption temperature because the Brønsted acid sites provided by HSiW in these catalysts were not strong enough. With the modification of catalytic structure from Pt-HSiW/γ-Al2O3 to PtNPs-HSiW/mAl2O3, the Brønsted acid sites increased from 12.0 to 30.3 µmol/g (150°C), indicating that structure modification is an effective way to enhance the Brønsted acidity of the catalyst. Moreover, the relative increment ratio of the quantity of Brønsted acid sties in this paper was 152.5% larger than 33.3% in a previous study by changing the loading amount of WOx from 5 to 25 wt%13. The big increase might attribute to the fact that PtNPs-HSiW/mAl2O3 synthesized by the AcHE process had a specific structure--a high surface area (311 m2/g, Table 1), order mesopores, and large mesoporous size (~7.8 nm, Figure 3). The high surface area of PtNPs-HSiW/mAl2O3 might help HSiW have a wide distribution and avoid the overlap of Brønsted acid sites. The ordered and large-sized mesopores in the catalyst were beneficial for mass transfer in the HSiW loading process. In previous studies, the type and quantity of acid sites have been proven to play a critical role in determining the product distribution13. In brief, the increase of Brønsted acid sites by catalytic structure modification was realized, but the role of Brønsted acid sites and the expression of HSiW in 1,3-PDO yield needs further investigation (see below).

17



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Page 18 of 33

Table 2. Acidity type and quantity in mAl2O3- and γ-Al2O3-based catalysts.

a

Tem.

b

L/B

150

250

L

350

400

B

L

B

L

B

L

B

PtNPs/mAl2O3

91.8

0

38.5

0

17.1

0

11.8

0

PtNPs-HSiW/mAl2O3

44.7

30.3

28.4

12.1

19.6

0

10.9

0

Pt/γ-Al2O3

84.3

0

39.4

0

22.3

0

21.9

0

Pt-HSiW/γ-Al2O3

50.8

12.0

30.5

0

14.9

0

14.5

0

HSiW

2.4

144.5

2.1

137.4

1.9

123.9

1.8

85.8

a

Desorption temperature (°C)

b

The amount of Lewis or Brønsted acid sites per unit mass of the catalysts

(µmol/g)

3.3. The dispersion of Pt and hydrogenation sites To

investigate

the

promotion

of

Pt

dispersion

that

caused

by

structure-modification, the number of catalytically active sites and dispersion of Pt for hydrogenation were determined by CO chemisorption.23,42 The CO adsorption capacity and Pt dispersion of mAl2O3 and γ-Al2O3-based catalysts are displayed in Table 3. The CO adsorption capacity of PtNPs/mAl2O3, PtNPs-HSiW/mAl2O3, Pt/γ-Al2O3, and Pt-HSiW/γ-Al2O3 were 24.2, 22.7, 10.0, and 9.6 µmol/g, respectively. The higher CO adsorption capacity of mAl2O3-based catalysts could be deemed to the evidence of that they had more active Pt sites (active hydrogenation sites) than γ-Al2O3-based catalysts, because the CO adsorption capacity was decided by the quantity of C-Pt bonds, which depended on the active Pt sites. The Pt dispersion of PtNPs/mAl2O3, PtNPs-HSiW/mAl2O3, Pt/γ-Al2O3, and Pt-HSiW/γ-Al2O3 were 18


Page 19 of 33

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37.6%, 35.4%, 15.6%, and 15.0%, respectively. The mAl2O3-based two catalysts synthesized by the AcHE process showed higher Pt dispersion than the γ-Al2O3-based ones, indicating that different synthetic methods could influence the active Pt sites by increasing the Pt dispersion. The higher Pt dispersion of mAl2O3-based catalysts attributed to that they were synthesized by the special synthetic process, i.e. PtNPs were uniformly mixed with precursor before the formation of alumina. In this way, PtNPs were dispersed completely in the liquid, and then dispersed well in the support after condensation and calcination. In addition, the Pt dispersion of mAl2O3- and γ-Al2O3-based catalysts both decrease slightly with the load of HSiW, which could be explained by the aggregation of Pt particles and active site coverage from mesoporous collapse during the impregnation and calcination processes. It is commonly accepted that higher Pt dispersion improves the activity of catalysts 23 and increase of the Pt dispersion is an effective way to promote the 1,3-PDO production. Shanhui Zhu et al.19 increased the Pt dispersion (rose from 23% to 41%) by changing the loading amount of SiO2. In this paper, without introducing other catalytic component in, the Pt dispersion was successfully increased by modification of catalytic structure (rose from 15.0% to 35.4%). The increment by this way was also more significantly than that by support-pretreatment way via H2O2 (rose from 29.1% to 45.3%)23. As expected, PtNPs-HSiW/mAl2O3 showed higher catalytic activity in the hydrogenolysis of glycerol into 1,3-PDO according to the following catalytic reaction tests (Table 4). It could be speculated that a higher Pt dispersion might be beneficial for 1) H2 homolysis into two hydrogen atoms; 2) uniform distribution of Pt and W species, thus promoting the electronic donor-acceptor interaction between Pt0 and W +x species and consequently, promoting catalytic activity.

19



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Page 20 of 33

Table 3. The dispersion of Pt measured by CO adsorption-desorption Pt dispersion CO adsorption Catalysts

Pt area

Dispersion

(m2/g)

(%)

24.2

103.6

37.6

22.7

97.2

35.4

Pt/γ-Al2O3

10.0

42.8

15.6

Pt-HSiW/γ-Al2O3

9.6

41.2

15.0

(µmol/g)

PtNPs /mAl2O3 PtNPs-HSiW/mAl2O 3

3.4. Catalytic performance of catalysts Catalytic activity. To investigate the catalytic activity, PtNPs/mAl2O3, PtNPs-HSiW/mAl2O3, Pt/γ-Al2O3, and Pt-HSiW/γ-Al2O3 were used for the reaction of glycerol hydrogenolysis into 1,3-PDO (Table 4). The results for mAl2O3-based catalysts in semi-heterogeneous and heterogeneous catalytic reactions are listed in Entries 1 to 3. As shown in Entry 1, PtNPs/Al2O3 showed 0% 1,3-PDO selectivity and 69.48% 1,2-PDO selectivity. This result suggests that PtNPs/mAl2O3 had no catalytic activity for glycerol hydrogenolysis into 1,3-PDO, but had preferable catalytic activity for the conversion of glycerol into 1,2-PDO. To promote the catalytic activity for 1,3-PDO production, HSiW was added as a co-catalyst in the reacting solution in the semi-heterogeneous catalytic system (Entry 2), and was loaded onto PtNPs/mAl2O3 in the heterogeneous catalytic system (Entry 3), respectively. The result showed that more 1,3-PDO was generated in the presence of HSiW, in both PtNPs/mAl2O3 + HSiW and PtNPs-HSiW/mAl2O3 catalytic systems. This suggests that HSiW played an important role in 1,3-PDO generation, which agreed with previous studies

12,13,43

. The role of HSiW may be connected with the catalytic acidity 20


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(Table 2), in that PtNPs/mAl2O3 and Pt/γ-Al2O3 had no Brønsted acidity, and correspondingly, had no activity in 1,3-PDO production. After HSiW was loaded, the catalysts showed catalytic activity and also showed Brønsted acidity. Therefore, increasing the quantity of Brønsted acid sites is an effective way to promote the 1,3-PDO yield. Moreover, the conversion of glycerol over HSiW-modified catalysts (Entry 2, 3) was higher than that over PtNPs/mAl2O3 (Entry 1). The result suggests that HSiW had an important effect, not only on the selectivity but also on glycerol conversion. The higher conversions of PtNPs/mAl2O3 + HSiW and PtNPs-HSiW/mAl2O3 were attributed to the larger quantities of total acidic sites as compared to PtNPs/mAl2O3 (Table 2). As shown in a previous study 44, both types of acidic sites played an important role in the conversion of glycerol, especially in glycerol dehydration. Additionally, the 1,3-PDO yield over PtNPs-HSiW/mAl2O3 (Entry 3) was significantly higher than that over PtNPs/mAl2O3+HSiW (Entry 2). This suggests that the bimetallic heterogeneous catalyst has its advantages in glycerol hydrogenolysis into 1,3-PDO, and this may be because the electronic donor-acceptor interaction between the Pt0 and W +x species promotes the hydrogenation of the intermediate to 1,3-PDO.13,45 The results for γ-Al2O3-based catalysts in semi-heterogeneous and heterogeneous catalytic reactions are listed in Entries 4 to 6, and the HSiW promotion effect and bimetallic effect are similar to those of mAl2O3-based catalysts. Finally, large differences were also observed between the performance of PtNPs-HSiW/mAl2O3 and Pt-HSiW/γ-Al2O3 catalysts. The selectivity to 1,3-PDO over PtNPs-HSiW/mAl2O3 and Pt-HSiW/γ-Al2O3 were 33.3% and 19.5%, respectively, and the corresponding the yield of 1,3-PDO were 20.1% and

12.1%,

respectively.

As

expected,

the

structurally

modified

PtNPs-HSiW/mAl2O3 which had more Brønsted acid sites and higher Pt dispersion showed higher catalytic activity than Pt-HSiW/γ-Al2O3, suggesting that the catalytic structure influenced the 1,3-PDO yield by altering the catalytic acid and Pt sites. According to the existing reaction pathway (Figure S6

17,46

)

21



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Page 22 of 33

and the data of this paper, acidity (Brønsted acidity) play a role in dehydration step and Pt particles work in hydrogenation step, indicating that increase of Brønsted acid sites and Pt dispersion could work together to improve 1,3-PDO yield. Thus, these results indicate that modification of the catalytic structure is a feasible way to promote catalytic activity for 1,3-PDO yield.

Table 4. Catalytic performance for hydrogenolysis of glycerol over mAl2O3and γ-Al2O3-based catalysts. Selectivity (%)a Conv. Entry

Catalysts

1,3-

1,2-P

(%)

1-PO

2-PO

Othersb

PDO

DO

35.5

0

69.5

9.2

5.0

16.3

61.9

25.2

14.7

36.2

8.0

15.9

1

PtNPs/mAl2O3 c

2

PtNPs/mAl2O3+HSiW

3

PtNPs-HSiW/mAl2O3 c

60.5

33.3

7.4

36.5

4.2

18.6

4

Pt/γ-Al2O3 c

31.8

0

68.8

13.4

6.1

11.7

5

Pt /γ-Al2O3+HSiW

58.9

17.8

9.8

53.9

11.6

7.0

6

Pt-HSiW/γ-Al2O3 c

62.0

19.5

9.1

51.2

9.2

11.0

a

d

d

1,3-PDO=1,3-propanediol;

1,2-PDO=1,2-propanediol;

1-PO=1-propanol;

2-PO=2-propanol. b Others: acetol, acetone, ethylene glycol, ethanol, methanol, methane, and so on. c Reaction conditions: 473K, 150 mg catalysts, 15 ml 0.1 M glycerol, 15 h, 4 MPa H2 pressure. d Reaction conditions: 473K, 150 mg catalysts, 22.5 mg of silicotungstic acid, 15 ml 0.1 M glycerol, 15 h, 4 MPa H2 pressure.

To further investigate the reaction over PtNPs-HSiW/Al2O3 and determine the optimal reaction time, the products obtained at different reaction times are listed in Figure 5. With increasing reaction time, glycerol conversion increased and the selectivity of 1,3-PDO decreased. The decrease of selectivity to 22


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1,3-PDO might be caused by continuous catalytic dehydrogenation to PrOH and other products. The result was confirmed by the increasing tendency of PrOH in Figure 5 and Table S2. As showed in Table S2, different reactants (glycerol, 1,3-PDO, and 1,2-PDO) were used in the reaction, and the results show that the 1,3-PDO was mainly converted into 1-PO (97.8%). By balancing the increasing tendency of glycerol conversion and the decreasing tendency of 1,3-PDO selectivity, the maximum yield of 1,3-PDO was 22.61% at 25 h. Generally, due to the side reaction (1,3-PDO into PrOH) and metal leaching problem caused by prolonged contact between the reaction system and the catalyst, the selectivity of 1,3-PDO in the batch reactors (14%47, 28%43, 29%48,49, 36.3%8) exhibited an extremely low level than that in the fixed-bed reactors (48.1%12, 52%19, 66.1%13). Here, in the batch reactor, we mainly focused on modifying the catalytic structure so as to improve 1,3-PDO production. The results showed that the yield of 1,3-PDO was promoted from 12.1% to 22.6%, which was higher than most of the existed results in batch reactors (7.1%50, 21.7%8, 13.7%43) and some results in fixed-bed reactors (11.6%12) catalyzed by Pt-W.

23



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Figure

5.

The

time

course

of

the

glycerol

hydrogenolysis

Page 24 of 33

over

PtNPs-HSiW/mAl2O3. Reaction condition: catalyst (150 mg), glycerol (0.1 M, 15 ml) and H2 (initial 4 MPa) at 473K.

Catalytic stability during reuse. To test the catalytic stability of mAl2O3and γ-Al2O3-based catalysts, PtNPs-HSiW/mAl2O3 and Pt-HSiW/γ-Al2O3 were reused five cycles, and the results are shown in Figure 6. Although the yield of 1,3-PDO over PtNPs-HSiW/mAl2O3 catalysts decreased from 22.6% to 18.0% with an increasing number of reuse cycles, it was still higher than that of Pt-HSiW/γ-Al2O3 (from 13.3% to 3.3%). The decrease in catalytic activity during reuse could be due to a combination of the deposition of carbon species in the mesopores and mesoporous collapse, together with the loss of metal species.

51

Among above possible reasons, the loss of metal species was

confirmed in Figure S7. Although Pt and W should be tested individually, considering the experimental condition, we eliminated the influence of W by using semi-homogeneous reaction system (HSiW was added to the reaction liquid instead of being loaded on catalysts) to test Pt individually. The changes of Pt content in PtNPs/mAl2O3 and Pt/γ-Al2O3 during reuse are listed in Table 24


Page 25 of 33

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S3. The Pt element in every reuse system composes of the Pt element in the liquid products and the Pt element in the solid catalysts, and the former shows the lost part. As shown in Table S3, the quantity of Pt loss in PtNPs/mAl2O3 in all reuse cycles were significantly lower than those of Pt/γ-Al2O3, which well explained the more stable activity of PtNPs/mAl2O3. As mentioned above, PtNPs were assembled into the channels of alumina by the charge-transfer interaction between PtNPs-PVP, hydrophobic association between PVP-F108, and hydrogen bonding between F108-Al2O3. This type of assembly mode between PtNPs and mAl2O3 might be beneficial for firm contact with the metal and support, thus reducing the loss of Pt during catalytic reuse.

Figure 6. The yield of 1,3-PDO during reuse over PtNPs-HSiW/mAl2O3 and Pt-HSiW/γ-Al2O3. Reaction condition: catalyst (150 mg), glycerol (0.1 M, 15 ml) and H2 (initial 4 MPa) at 473K for 25 h

25



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Page 26 of 33

To make the impact of Pt loss on 1,3-PDO yield definitely, the concept of “cumulative loss”, showing the accumulated effect of Pt loss, is introduced in this paper. The relationships between the cumulative loss of Pt and the yield of 1,3-PDO over PtNPs/mAl2O3 and Pt/γ-Al2O3 are displayed in Figure S8. A negative linear correlation was observed between the cumulative loss of Pt and the yield of 1,3-PDO. This result shows that the yield of 1,3-PDO decreased with increasing of the cumulative loss of Pt. This indicates that controlling the Pt cumulative loss is an effective way to control the catalytic reuse stability. Hence, we successfully modified the catalytic structure to decrease the Pt loss during catalytic reuse and consequently, promoted catalytic stability in hydrogenolysis of glycerol into 1,3-PDO.

4. CONCLUSIONS A structurally modified PtNPs-HSiW/mAl2O3 exhibited a significantly different structure from Pt-HSiW/γ-Al2O3, which consequently led to a higher Brønsted acidity (30.3 vs 12.0 µmol/g, 150°C), a better dispersion of platinum particles (35.4% vs 15.0%) and less loss of platinum during reuse (0.59 wt% vs 4.54 wt%, five cycles of reuse). The results of N2 adsorption-desorption and XRD characterization suggested that PtNPs-HSiW/mAl2O3 exhibited a larger surface area and pore diameter than Pt-HSiW/γ-Al2O3 and the amorphous form of support and HSiW. N2 adsorption-desorption and TEM images showed that PtNPs-HSiW/mAl2O3 possessed ordered mesopores and the PtNPs were assembled into alumina. The possible mechanism of the assembly mode between PtNPs and mesopores was that PtNPs were linked to alumina successively by the charge-transfer interaction between PtNPs and PVP, the hydrophobic association between PVP and F108, and the hydrogen bond between F108 and Al2O3. In catalytic reaction tests, the PtNPs-HSiW/mAl2O3 catalyst afforded a higher yield of 1,3-PDO (20.14% vs 12.09%) and exhibited more stable yields over five cycles of reuse (stayed above 18.0% vs 3.3%). 26


Page 27 of 33

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The results suggested that structural modification influenced 1,3-PDO production by altering the Brønsted acidity of catalyst, the dispersion of Pt and the loss of Pt during reuse, which ultimately affected the catalytic acid and hydrogenation sites.

AUTHOR INFORMATION

Corresponding Authors * Fax/Tel: +86 021 65985811 * E-mail: [email protected]; [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS

The authors acknowledge sponsorship of the National Natural Science Foundation of China (NO. 21376180 and 21676205), the National Science Fund for Distinguished Young Scholars (NO. 51625804) and the Fundamental Research Funds for the Central Universities (NO. 2870219028)

SUPPORTING INFORMATION

Diagram of stainless steel reactor; N2 adsorption-desorption isotherms of PtNPs/mAl2O3 calcined at different temperature; The impact of template removal temperature on Vtotal×SBET, Smeso/SBET and 1,3-PDO yield; FT-IR spectra of mAl2O3 based catalysts; XRD patterns of catalysts before and after reduction; Reaction pathway in acidic condition; The changes of content of Pt and W element in PtNPs-HSiW/mAl2O3 and Pt-HSiW/γ-Al2O3 during reuse; The impact of the cumulative loss of Pt on the yield of 1,3-PDO over 27



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Page 28 of 33

PtNPs/mAl2O3 and Pt/γ-Al2O3; The effect of HSiW loading on 1,3-PDO production based on mAl2O3 and γ-Al2O3-based catalysts; Results of different reactants over PtNPs-HSiW/mAl2O3; The change of Platinum content during five cycles of reuse

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The Effect of Catalytic Structure Modification on Hydrogenolysis of

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