HPZSM-5 catalyst

Journal of Catalysis 382 (2020) 1–12

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Journal of Catalysis journal homepage: www.elsevier.com/locate/jcat

Efficient liquid-phase hydrogenation of cinnamaldehyde to cinnamyl alcohol with a robust PtFe/HPZSM-5 catalyst Guimei Wang, Huiyue Xin, Qixiang Wang, Peng Wu, Xiaohong Li ⇑ Shanghai Key Laboratory of Green Chemistry and Chemical Processes, School of Chemistry and Molecular Engineering, East China Normal University, 3663 North Zhongshan Rd., Shanghai 200062, China

a r t i c l e

i n f o

Article history: Received 24 October 2019 Accepted 3 December 2019

Keywords: Hierarchical porous ZSM-5 Pt-Fe catalyst Liquid-phase Selective hydrogenation Cinnamaldehyde

a b s t r a c t HPZSM-5 with hierarchical porous structure obtained via partial desilication of commercial ZSM-5 was proved to be a suitable support for Pt nanoparticles towards the liquid-phase selective hydrogenation of cinnamaldehyde (CAL) to yield cinnamyl alcohol (COL). Compared with Pt/ZSM-5, Pt/HPZSM-5 showed improved catalytic performance although the improvement seems to be very limited. After doping Fe to Pt/HPZSM-5 with a Fe/Pt molar ratio of 0.25, the PtFe/HPZSM-5 catalyst gave much further enhanced CAL conversion. Under the optimized conditions, a 97.9% CAL conversion with 87.6% selectivity to COL was achieved with the PtFe/HPZSM-5 catalyst. The initial activity (in terms of TOF value, defined as molecules of CAL converted per surface atom of Pt per second) reached about 3.41 s
1 with the PtFe/HPZSM-5 catalyst, one of the highest values up to now to our best knowledge. Moreover, the PtFe/HPZSM-5 catalyst can be recycled for at least 9 times without obvious loss in activity or selectivity to COL. The effect of partial desilication and Fe doping on the catalytic performance of PtFe/HPZSM-5 was discussed based on the characterization results using a series of techniques. Ó 2019 Elsevier Inc. All rights reserved.

1. Introduction Chemoselective hydrogenation of a,b-unsaturated aldehydes to corresponding a,b-unsaturated alcohols has attracted intensive attention, due to its scientific and economic concerns [1–3]. Cinnamaldehyde (CAL), as a typical a,b-unsaturated aldehyde, has been usually adopted as a model reacatant for the selective hydrogenation and intensively studied during the last decades. However, due to multifunctional groups, the selective hydrogenation of CAL can lead to different products thus the selectivity cannot be easily controlled (Scheme 1) [4,5]. Moreover, owing to the conjugation effect of C@O and C@C bonds, the selective hydrogenation of C@O bonds to yield cinnamyl alcohol (COL), the a,b-unsaturated alcohol, is not thermodynamically favored [6,7]. Therefore, the selective hydrogenation of CAL to COL is still a challenging issue [8,9]. In order to realize the selective hydrogenation of CAL to yield COL, heterogeneous catalysts are highly desirable due to their compliance with the principles of green chemistry. It has been reported in the literature that Pt is the most promising catalyst for the selective hydrogenation of CAL to COL [10,11]. However, the catalytic

⇑ Corresponding author. E-mail address: [email protected] (X. Li). https://doi.org/10.1016/j.jcat.2019.12.004 0021-9517/Ó 2019 Elsevier Inc. All rights reserved.

properties of Pt alone are not perfect yet. Introduction of the second metal such as Fe has been proved to be effective to promote the activity and selectivity to COL due to that the surface electronic properties of Pt species can be regulated by Fe doping [12–17]. Alternatively, a special support can also adjust the electronic properties of Pt species via their interaction. For instance, because of the strong metal-support interaction, electrons transferred from CeO2-ZrO2 composite support to Pt0 species, leading to formation of Pt0 atoms with high electron density, so that the higher selectivity toward COL was achieved [18]. Another example is that a Pt/ mesoTiO2-SiO2-M catalyst afforded an excellent selectivity toward COL at nearly complete conversion of CAL. The explanation is that electrons can transfer from the reducible support to the metal, resulting in enrichment of electrons on the Pt surfaces. As a result, the carbonyl group of CAL can be adsorbed and activated preferentially [19]. ZSM-5 zeolites with MFI topology are widely used in petrochemical, fine chemical and other industries for their unique properties and advantages, including a wide range of Si/Al ratio, good hydrothermal stability, good shape selectivity, three-dimensional channels, large specific surface area and large pore volume [20– 25]. However, due to the microporous nature of ZSM-5, the application of ZSM-5 in some reactions involving the relatively larger molecules is very restricted [26]. Therefore, the hierarchical porous ZSM-5 (HPZSM-5) is required for this purpose. According to the lit-

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G. Wang et al. / Journal of Catalysis 382 (2020) 1–12

O

+H2 +H2

hydrocinnamaldehyde, HCAL

O

OH

+H2 +H2

hydrocinnamyl alcohol, HCOL

cinnamaldehyde, CAL

OH

cinnamyl alcohol, COL Scheme 1. The liquid-phase selective hydrogenation of cinnamaldehyde (CAL).

erature, partial desilication of commercial ZSM-5 in a basic solution is a simple but effective method to obtain HPZSM-5 [26,27]. Of particular note is that HPZSM-5 still maintains the original framework of ZSM-5, but a hierarchical porous structure including mesoporous and microporous channels are formed so that the mass transportation would become easier for some reactions involving larger molecules. Meanwhile, the acidity of HPZSM-5 is probably modified after partial desilication [26–28]. In addition, the specific confinement effect of mesoporous materials on nanoparticles was also reported in the literature [29]. Therefore, HPZSM-5 with hierarchical porous structure and modified acidity is anticipated to behave very well after loading Pt nanoparticles for the selective hydrogenation of CAL to yield COL [30]. Herein, a Pt/HPZSM-5 catalyst was prepared via an impregnation method for the selective hydrogenation of CAL to obtain COL. The results showed that the catalytic performance of Pt/ HPZSM-5 was obviously improved compared with the Pt/ZSM-5 catalyst. In order to further improve the catalytic ability, the second metal like Fe was doped to Pt/HPZSM-5. Correspondingly, the catalytic performance, particularly for the catalyst activity with the PtFe/HPZSM-5 catalyst, was greatly enhanced for the selective hydrogenation of CAL. 2. Experimental 2.1. Chemicals Chloroplatinic acid hexahydrate (H2PtCl66H2O) (Pt  37%) and other chemicals were of analytical grade and used as received. ZSM-5 (Si/Al2 = 168) was purchased from Shanghai Fuxu Molecular Sieve Co., Ltd. CAL was purchased from Alfa Aesar and used as received. Other reagents including ferric trichloride, sodium hydroxide and isopropanol were purchased from Sinopharm Chemical Reagent Co., Ltd. and used as received. 2.2. Catalyst preparation HPZSM-5 was obtained by partial desilication of ZSM-5 with an aqueous solution of sodium hydroxide [30,31]. Briefly, 1.5 g of ZSM-5 (the Si/Al2 ratio of 168) was added to 30 mL of an aqueous solution containing 0.25 g of sodium hydroxide and stirred at 75 °C for 5 h, then filtered and washed with plenty of deionized water, followed by drying at 80 °C overnight. According to ICP-AES analysis result, the resultant HPZSM-5 has a Si/Al2 ratio of 132 and the residue Na content is 0.46 wt%. 5 wt% Pt/HPZSM-5 catalyst was prepared by a traditional impregnation method. Typically, a certain amount of HPZSM-5 was impregnated with an ethanolic solution containing chloropla-

tinic acid and stirred at room temperature for 4–6 h till it became sticky. Afterwards, the catalyst precursors were dried at 80 °C overnight and followed by calcination in static air at 400 °C for 4 h. Finally, the catalyst precursor was reduced in flowing hydrogen at 400 °C for 2 h. For comparison, a 5 wt% Pt/ZSM-5 was also prepared using the similar method. The Fe doped PtFe/HPZSM-5 catalyst containing 5 wt% Pt with a Fe/Pt molar ratio of 0.25 was also prepared via the similar method using an ethanolic solution containing chloroplatinic acid and ferric trichloride as precursors.

2.3. Catalyst characterization The powder X-ray diffraction (XRD) patterns of samples were collected on a Bruker D8 ADVANCE instrument using Cu Ka radiation. The N2 sorption of samples were measured at
196 °C on a Quantachrome Autosorb-3B system after the samples were evacuated at 300 °C for 4 h. The Brunauer-Emmett-Teller (BET) specific surface area was calculated using the adsorption data in the relative pressure range from 0.01 to 0.10. The pore size distribution curves were calculated from analysis of the adsorption branch of the isotherm using the Barrett-Joyner-Halenda (BJH) algorithm. The scanning electron microscopy (SEM) was measured using a Hitachi S4800 electron microscope with an accelerating voltage of 20 kV. The transmission electron microscopy (TEM) images were taken on an FEI Tecnai G2-TF30 microscope at an accelerating voltage of 300 kV. The Si, Al and residual Na content, the amount of Pt atoms leached into the filtrate during the recycling was detected by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) using a Thermo Elemental IRIS Intrepid II XSP. The Xray photoelectron spectra (XPS) of the samples were acquired with a Thermo Fisher Scientific ESCALAB 250Xi spectrometer with Al Ka radiation (1486.6 eV) as the incident beam. The samples were pretreated in situ in flowing hydrogen at 400 °C for 2 h in a reactor attachment of the XPS spectrometer. The binding energy (BE) was calibrated using C-C binding energy at 284.6 eV in order to compare the BEs with the data from the literature. The spectra shown in the figures have been corrected by Shirley background subtraction. Spectral fitting and peak integration was performed using XPSPEAK software. Temperature-programmed reduction with hydrogen (H2-TPR) was conducted to investigate the interaction between Pt and support or Fe species with a Micromeritics AutoChem II Chemisorption Analyzer. Prior to H2-TPR experiments, each sample (about 100 mg) was outgassed under flowing He (99.999%, 30 mLmin
1) at 150 °C for 30 min and then cooled to ambient temperature. The H2-TPR profiles were obtained with 10% H2-Ar (30 mLmin
1) from 50 °C to 800 °C with a ramping rate of 10 °Cmin
1. The rate of H2 consumption was monitored by a gas chromatograph (GC) with a

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G. Wang et al. / Journal of Catalysis 382 (2020) 1–12

The selective hydrogenation of CAL was carried out in a 100 mL autoclave. For a standard reaction, 25 mg of Pt catalyst was placed in a specially designed quartz tube and pretreated in flowing hydrogen (99.999% purity, 25 mLmin
1) at 400 °C for 2 h. Then, the catalyst was transferred to the autoclave without exposure to air. Subsequently, 11.25 mmol of CAL and 10 mL of mixed solvent containing isopropanol and water with a volume ratio of 9/1 was added to the reactor and then purged with 2 MPa of H2 for at least 3 times. The reaction began at 90 °C and stirred at 1000 rpm. The reaction was stopped after an appropriate time, and then the products were analyzed using a GC (GC-2014, Shimadzu Co.) equipped with a flame ionization detector (FID) and capillary column (DMWAX, 30 m  0.25 mm  0.25 lm). The response factor of each component was calculated using the standard sample and used to calculate the conversion and selectivity. It should be pointed out that the absence of internal and external mass transfer limitation in the reaction was verified by WeiszPrater criterion and Mears criterion, respectively. The absence of heat transfer limitation in the reaction was verified by a Mears criterion [32]. At the maximum reaction rate, the CWP = 1.09  10
5 < 1 and the CM = 0.0857 < 0.15, which assured the absence of mass transfer limitation. Mears criterion for external (interphase) 0 Þq RE A b < 0:15, demonstrating no heat heat transfer gave
DHrhð
r 2 t T b Rg transfer limitations (see the Supporting Information for the details). For the recycling reactions, the catalyst was recovered by centrifugation, washed with isopropanol for several times to remove

3. Results and discussion 3.1. General characterization of the Pt-based catalysts ZSM-5, HPZSM-5 and the related Pt catalysts were firstly characterized by XRD. As shown in Fig. 1a, all the samples showed the diffraction pattern at 2 theta of 23.1°, 23.9° and 24.4°, which are characteristics of the typical MFI structure [33]. Furthermore, the intensity of the peaks did not change significantly for HPZSM-5 and related Pt catalysts, indicating that the alkali treatment did not damage the crystallinity of ZSM-5 seriously. In addition, the peaks at 2 theta of 39.8°, 46.4° and 67.6° assigned to the Pt (1 1 1), Pt(2 0 0) and Pt(2 2 0) facets were also observed for all the Pt catalysts [12]. Obviously, the Pt diffractions became wider and weaker after doping Fe, demonstrating that doping Fe is helpful for the dispersion of Pt nanoparticles. However, no typical XRD pattern assigned to Pt-Fe alloy can be observed, indicating that PtFe alloy might not form or exists as amorphous phase. To understand the porous nature including the specific surface area and the pore volume of HPZSM-5, related materials were characterized using nitrogen sorption (Fig. 2a). ZSM-5 and Pt/ZSM-5

MFI

a Pt (200) Pt (111)

Intensity (a.u.)

2.4. Catalytic test

the residual substrate and product after each run and then submitted to the next run with the fresh solvent and reactant.

Pt (220) PtFe/HPZSM-5

Pt/HPZSM-5 Pt/ZSM-5 HPZSM-5 ZSM-5

20

30

40

50

60

70

80

2 Theta (degree)

b Pt (111) Pt (200)

Pt (220)

Intensity (a.u.)

thermal conductivity detector (TCD). The acidic property of the samples were characterized by NH3-TPD (temperatureprogrammed desorption) measurements with a Micromeritics AutoChem II Chemisorption Analyzer as well. Before each measurement, the catalyst samples were pretreated at 400 °C under a He flow (99.999%, 50 mLmin
1) for 1 h and then exposed to a 10% NH3-He flow (50 mLmin
1) at 80 °C for 0.5 h, and then heated linearly from 80 °C to 600 °C at a ramping a rate of 10 °Cmin
1 in a He flow (99.999%, 50 mLmin
1). The desorbed NH3 was detected on-line using a TCD, and the TCD signals were calibrated by a given volume of NH3. The acidic property of the samples were characterized by FT-IR spectroscopy using pyridine as probe molecules. The IR spectra were collected using a Nicolet iS50 FT-IR Spectrometer in absorbance mode with a spectral resolution of 2 cm
1. The sample was pressed into a self-supported wafer without using any binder, which was set in a quartz cell sealed with CaF2 windows and connected to a vacuum system. After evacuation at 450 °C for 2 h, pyridine adsorption was carried out by exposing the pretreated wafer to a pyridine vapor at 25 °C for 0.5 h. After that, the physicallyadsorbed pyridine was evacuated with a flash desorption at a ramp rate of 50 °C min
1 and the spectra were collected once the temperature reached 100 °C and 200 °C, respectively. In addition, the electronic property of the related Pt catalysts was also characterized using FT-IR spectroscopy with CO as probe molecules using a Nicolet iS50 FT-IR Spectrometer in absorbance mode. Before measurement, the sample was pressed into a self-supported wafer without using any binder and put into the IR cell. Prior to CO adsorption, the samples were in situ pre-treated under H2 atmosphere at 400 °C for 2 h and then cooled down to 35 °C for CO adsorption. The IR spectra of the chemisorbed CO was recorded after physically adsorbed CO was evacuated. All of the FT-IR spectra were collected using 32 scans at a resolution of 4 cm
1.

O

500 C O

400 C

O

300 C

O

200 C 20

30

40

50

60

70

80

2 Theta (degree) Fig. 1. XRD patterns of (a) ZSM-5, HPZSM-5 and related Pt-based catalysts; (b) XRD patterns of the PtFe/HPZSM-5 catalyst after calcined at different temperatures.

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G. Wang et al. / Journal of Catalysis 382 (2020) 1–12

ZSM-5 HPZSM-5 Pt/ZSM-5 Pt/HPZSM-5 PtFe/HPZSM-5

Adsorbed Volume (cm /g, STP)

420

3

360

a

300 240 180 120

0.0

0.2

0.4

0.6

0.8

1.0

P/P0 0.10

b ZSM-5 HPZSM-5 Pt/ZSM-5 Pt/HPZSM-5 PtFe0.25/HPZSM-5

0.06

3

DV (d)(cm /g)

0.08

0.04

0.02

0.00

-0.02 3

4

5

6

7

remained upon desilication of ZSM-5. Compared with ZSM-5, HPZSM-5 had an increased mesoporous volume of 0.30 cm3g
1 from 0.14 cm3g
1. Meanwhile, the total specific surface area after desilication did not change considerably. Correspondingly, ZSM-5 and HPZSM-5 had a specific surface area of 361 m2g
1 and 373 m2g
1, respectively. The morphology of the samples were observed by SEM. As can be seen from Fig. 3, the particle size of HPZSM-5 is uniform. The partial desilication treatment or the subsequent impregnation with Pt precursor as well as the high-temperature calcination did not destroy the morphology of ZSM-5. Nevertheless, the particles are intact and have smooth outer surface for ZSM-5; while for HPZSM-5 related samples, the external surface becomes somewhat rough and there are defects on some particles, which may be caused by alkali corrosion. In addition, there are many bright dots on the surface of HPZSM-5 after Pt nanoparticles were loaded, which were confirmed to be Pt nanoparticles by SEM-EDS. The Pt particle size and dispersion was also characterized using TEM. Fig. 4 shows the TEM images and the Pt particle size distributions of the Pt catalysts. The Pt nanoparticles on conventional ZSM5 were not uniform and there were some relatively larger aggregations. As a result, the Pt/ZSM-5 catalyst showed a broad Pt particle size distribution ranging from 2.0 to 9.2 nm with an average Pt particle size of about 5.3 nm (Fig. 4a). After partial desilication of ZSM5, the Pt/HPZSM-5 catalyst had more uniform dispersion of Pt nanoparticles with an average particle size of about 3.2 nm. In addition, some bright and white holes appeared on Pt/HPZSM-5, which might be caused by alkali desilication (Fig. 4b). After Fe was doped, the average Pt particle size was further decreased to approximately 3.0 nm, and the Pt particles dispersion was further increased. The high-resolution TEM (HRTEM) image of the PtFe/ HPZSM-5 catalyst in the inset of Fig. 4c also displays the Pt (1 1 1) facets with a crystal plane spacing of 0.226 nm, in agreement with the XRD results.

Pore Diameter (nm) Fig. 2. (a) N2 adsorption-desorption isotherms and (b) BJH pore size distributions of ZSM-5, Pt/ZSM-5, HPZSM-5, Pt/HPZSM-5 and PtFe/HPZSM-5. The isotherms of Pt/ ZSM-5, HPZSM-5, Pt/HPZSM-5 and PtFe/HPZSM-5 were vertically translated by 30, 60, 90 and 120, respectively.

showed type I isotherms, conforming the microporous nature of ZSM-5. After partial desilication, Pt/HPZSM-5 and PtFe/HPZSM-5 displayed type IV isotherms with H1 hysteresis loops, demonstrating the presence of mesoporous structure. This indicates that after partial desilication of ZSM-5 with NaOH solution, the mesoporous and microporous hierarchical structure was formed for HPZSM-5. After loaded with Pt and Fe, no significant change was observed in the N2 sorption isotherms. The pore size distribution determined using BJH method was centered at nearly 4 nm for both HPZSM-5 and its supported Pt samples (Fig. 2b). For clarity, the physicochemical properties of the samples are listed in Table 1. As expected, both the mesoporous volume and the total pore volume were enhanced remarkably; while the microporous volume

3.2. Catalytic performance of the Pt-based catalysts With these Pt-based catalysts at hand, we are earger to investigate their catalytic performance for the selective hydrogenation of CAL to yield COL. Firstly, the monometallic Pt catalysts supported on different materials were investigated. Table 2 lists the detailed results obtained with different Pt catalysts. The Pt/ZSM-5 catalyst afforded a 27.4% conversion of CAL and 73.3% selectivity toward COL (entry 1). In contrast, the Pt/HPZSM-5 catalyst was more efficient. Both the CAL conversion and the COL selectivity were increased. As a result, 35.6% CAL conversion and 88.9% COL selectivity were achieved with the Pt/HPZSM-5 catalyst (entry 2). The improvement can be attributed to the hierarchical porous structure and the smaller Pt particle size. Additionally, the changed acidity of HPZSM-5 after partial desilication would adjust the surface geometry of Pt nanoparticles, which will be discussed thereafter and the adjusted surface geometry of Pt nanoparticles might influence the adsorption and activation of CAL, so that the selectivity would be affected as a result. In control experiments,

Table 1 Physico-chemical properties of relevant samples.

a b

Sample

Vmicro (cm3g
1)

Vmeso (cm3g
1)

SBET (m2g
1)

Pt sizea (nm)

Pt disp.

ZSM-5 HPZSM-5 Pt/ZSM-5 Pt/HPZSM-5 PtFe/HPZSM-5

0.14 0.13 0.15 0.14 0.14

0.14 0.30 0.12 0.27 0.33

361 373 367 355 396

– – 5.3 3.2 3.0

– – 21.3 35.3 37.7

Average Pt particle size (nm) was measured from the TEM investigation. Pt dispersion was calculated from the formula: D = 1.13/d (d is the mean diameter of Pt particle in nm, which was measured from the TEM investigation).

b

(%)

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G. Wang et al. / Journal of Catalysis 382 (2020) 1–12

a

ZSM-5

b

c

d

Fig. 3. SEM images of (a) HPZSM-5, (b) Pt/ZSM-5, (c) Pt/HPZSM-5 and (d) PtFe/HPZSM-5. The inset of Fig. 3a is the SEM image of prinstine ZSM-5.

Pt/ZSM-5

Distribution (%)

a

>8

2-3

3-4 4-5 5-6 Particle size (nm)

6-7

Distribution (%)

Pt/HPZSM-5

b

7-8

<2

2-3

PtFe/HPZSM-5

<2

2-3

3-4 4-5 5-6 Particle size (nm)

6-7

7-8

6-7

7-8

Distribution (%)

PtFe/HPZSM-5-used

d

Distribution (%)

c

3-4 4-5 5-6 Particle size (nm)

<2

2-3

3-4 4-5 5-6 Particle size (nm)

6-7

7-8

Fig. 4. TEM images and particle size distribution of (a) Pt/ZSM-5, (b) Pt/HPZSM-5, (c) PtFe/HPZSM-5 and (d) used PtFe/HPZSM-5.

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G. Wang et al. / Journal of Catalysis 382 (2020) 1–12

Table 2 The catalytic results for the liquid-phase selective hydrogenation of CAL over the various Pt catalysts. Entry

Catalyst

Conv. (%)

1 2 3 4 5 6

Pt/ZSM-5 Pt/HPZSM-5 PtFe/HPZSM-5 Fe/HPZSM-5 Pt/Al2O3 Pt/SBA-15

27.4 35.6 97.9 0.0 22.5 10.4

Selectivity (%) COL

HCAL

HCOL

73.3 88.9 87.6 – 77.7 85.9

23.1 9.8 6.8 – 1.3 0.7

3.6 1.3 5.6 – 21.0 13.4

Reaction conditions: 25 mg of the Pt-based catalyst, 11.25 mmol CAL, PH2 = 2 MPa, 9 mL isopropanol + 1 mL H2O, 90 °C, 1000 rpm, 1 h.

100

Conv. & Sel. (%)

80

60

CAL Conv. COL Sel. HCAL Sel. HCOL Sel.

40

20

0 200

250

300

350

400

450

500

o

Calcination temperature ( C) Fig. 5. The catalytic results for the selective hydrogenation of CAL over the PtFe/ HPZSM-5 catalyst after calcined at different temperatures before reduction. Reaction conditions: 25 mg of the PtFe/HPZSM-5 catalyst, 11.25 mmol CAL, PH2 = 2MPa, 9 mL isopropanol + 1 mL H2O, 90 °C, 1000 rpm, 1 h.

it almost kept the same level despite of the different calcination temperatures for the catalyst precursor. Therefore, the calcination temperature was set as 400 °C in the following studies. Furthermore, the hydrogen pressure effect on the selective hydrogenation of CAL over the PtFe/HPZSM-5 catalyst was also studied (Fig. 6). With the hydrogen pressure rising from 0.1 to 1.0 MPa, the CAL conversion distinctly increased from 23.6% to

120

10 HCOL Sel.

HCAL Sel.

COL Sel.

8 90

-1

-1

MSR (molCAL gPt h )

pure silica (ordered mesoporous SBA-15) and pure alumina (cAl2O3) were also adopted to support Pt nanoparticles and then submitted to the target reaction. The Pt/Al2O3 catalyst only gave 22.5% CAL conversion with 77.7% COL selectivity (entry 5); while the Pt/ SBA-15 catalyst showed much lower CAL conversion of 10.4% accompanied with 85.9% COL selectivity (entry 6). Compared with those obtained over the Pt catalysts supported on pure silica or pure alumina, the results with the Pt/HPZSM-5 catalyst are much higher. The possible reason would be the special acidity of HPZSM-5, which is rather different from that in SBA-15 or in Al2O3 and will be characterized and discussed in the following studies. Although the catalytic performance of Pt/HPZSM-5 was obviously improved when compared with that of Pt/ZSM-5, the activity of the Pt/HPZSM-5 catalyst still has big room to enhance. Inspired by our previous studies that doping Fe to the Pt-based catalysts with an optimal Fe/Pt molar ratio would greatly improve the catalytic performance for the selective hydrogenation of CAL to yield COL, we prepared the PtFe/HPZSM-5 catalyst by doping Fe to the Pt/HPZSM-5 catalyst with a Fe/Pt molar ratio of 0.25. As anticipated, the activity of the PtFe/HPZSM-5 catalyst was remarkably enhanced for the selective hydrogenation of CAL, whereas the improvement in the COL selectivity was very limited under the same conditions. Correspondingly, a 97.9% conversion of CAL was achieved in accompany with about 88% selectivity toward COL (entry 3). To discriminate the catalysis nature, Fe alone supported on HPZSM-5, Fe/HPZSM-5 catalyst, was also prepared using the similar method as that for Pt/HPZSM-5 and investigated in the selective hydrogenation of CAL. As a result, Fe/HPZSM-5 did not work at all as no any product was detected after 1 h (entry 4). Therefore, we can conclude that Fe is not a co-catalyst but a promoter for the selective hydrogenation of CAL. To explore the catalytic performance of the PtFe/HPZSM-5 catalyst to the greatest extent, some reaction parameters were optimized subsequently. As well known, the interaction between Pt and Fe speices was affected strongly by the calcination temperature for the catalyst precursor before reduction. Therefore, the PtFe/HPZSM-5 catalyst precursors were calcined at different temperatures before reduction in flowing hydrogen at 400 °C for 2 h and then investigated in the target reaction. As displayed in Fig. 5, as the calcination temperature for the PtFe/HPZSM-5 catalyst precursors raised from 200 to 400 °C, the CAL conversion increased distinctly from 46.6% to 97.9%. However, when the calcination temperature further increased to 500 °C, the CAL conversion decreased dramatically to 29.1% instead. This may be attributed to the varied Pt particle size caused by different calcination temperatures, which can be confirmed by the XRD patterns (Fig. 1b). With increasing the calcination temperature, the diffraction intensity of Pt(1 1 1) facets became stronger and stronger, indicating that the Pt particles aggregated remarkably with the increasing calcination temperature. In addition, this also reflects that the support HPZSM5 does not have a significant confinement effect on Pt nanoparticles. The Pt nanoparticles are mainly located outside the mesopores or at the pore mouth. With regards of the COL selectivity,

6 60 4 30 2

0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

0 3.5

PH2 (MPa) Fig. 6. The catalytic results for the liquid-phase selective hydrogenation of CAL over the PtFe/HPZSM-5 catalyst under different hydrogen pressures. Reaction conditions: 25 mg of the PtFe/HPZSM-5 catalyst, 11.25 mmol CAL, 9 mL isopropanol + 1 mL H2O, 90 °C, 1000 rpm, 1 h.

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G. Wang et al. / Journal of Catalysis 382 (2020) 1–12

94.1%. Further increasing the hydrogen pressure to 3.0 MPa, the CAL conversion hardly increased anymore. In order to understand the hydrogen effect directly, the mass specific rate (MSR, defined as converted CAL moles per gram of Pt per hour) versus hydrogen pressure plot was given in Fig. 6. The MSR increased promptly with hydrogen pressure when it was below 1.0 MPa. When hydrogen pressure was higher than 1.0 MPa, the influence of hydrogen pressure on MSR was tiny. This indicates that when hydrogen pressure was high enough, the MSR was independent of hydrogen pressure. As for the COL selectivity, it also increased with the hydrogen pressure from 0.1 to 0.5 MPa and almost kept the same level at the hydrogen pressure in the range of 0.5 to 2.0 MPa. When the hydrogen pressure further increased to 3.0 MPa, the COL selectivity slightly decreased while the HCAL selectivity was slightly increased. This suggests that too low hydrogen pressure cannot provide enough hydrogen for CAL hydrogenation. Relatively higher hydrogen pressure is beneficial to increase hydrogen concentration in the solvent [18]. However, too much high hydrogen pressure gave rise to the selective hydrogenation of CAL to yield HCAL. Therefore, 2.0 MPa of hydrogen pressure was adopted as a standard parameter in the following studies. 3.3. The catalytic performance of the PtFe/HPZSM-5 catalyst After the reaction parameters were optimized, the kinetic behaviors of the PtFe/HPZSM-5 catalyst for the selective hydro-

genation of CAL were investigated at different reaction temperatures. As revealed in Fig. 7a, the CAL conversion on the PtFe/ HPZSM-5 catalyst increased steadily with reaction time at 70 °C, so did that performed at 90 °C. Moreover, the CAL conversion obtained at 90 °C was higher than that obtained at 70 °C. Based on the CAL conversions within 5 min, the turnover frequency, TOF (defined as molecules of CAL converted per surface atom of Pt per second) was calculated. Accordingly, TOF values of 1.95 s
1 and 3.41 s
1 was reached with the PtFe/HPZSM-5 catalyst at 70 °C and 90 °C, respectively. Regarding of the COL selectivity, it was hardly affected by the reaction temperature. Above 80% selectivity toward COL was afforded by the PtFe/HPZSM-5 catalyst despite of at 70 °C or 90 °C. With the reaction going on, the selectivity toward HCAL decreased a little as a result of the selectivity toward the total hydrogenation product HCOL increased at relatively higher CAL conversions. We deduce that a large proportion of HCOL is derived from the further hydrogenation of HCAL because the COL selectivity almost kept unchanged even at higher CAL conversions. In addition, ln(C0/Ct) was plotted as a first-order function of reaction time with respect to CAL for the selective hydrogenation of CAL with the PtFe/HPZSM-5 catalyst, as shown in Fig. 7b. k90 and k70 are 3.3  10
2 min
1 and 2.3  10
2 min
1, respectively and the ratio of k90 and k70 is 1.4. This result suggested that the temperature effect indeed existed in the selective hydrogenation of CAL with the PtFe/HPZSM-5 catalyst. With regards of the reaction order with respect of CAL, both first-order and near

100

CAL Conv. & COL Sel. (%)

a 80

60 o

70 C o 70 C o 70 C o 70 C

40

20

o

90 C CAL Conv. o 90 C COL Sel. o 90 C HCAL Sel. o 70 C HCOL Sel.

0 0

10

20

30

40

50

60

Reaction time (min) 100

CAL Conv. & Sel. (%)

Product distribution (%)

d

c

70 60 50 40 30 20

HCAL COL HCOL

10

80 Pt/ZSM-5 Pt/HPZSM-5

60

CAL Conv. COL Sel. HCAL Sel. HCOL Sel.

40

20

0

0 0

5

10

15

20

Reaction time (min)

25

30

0

1

2

3

4

5

Reaction time (h)

Fig. 7. (a) Kinetic profiles of the selective hydrogenation of CAL with the PtFe/HPZSM-5 catalyst at different reaction temperatures. Reaction conditions: 25 mg of the PtFe/ HPZSM-5 catalyst, 11.25 mmol CAL, PH2 = 2 MPa, 9 mL isopropanol + 1 mL H2O, 1000 rpm, 1 h, 90 °C or 70 °C. (b) The rate constants (k) and 95%-confidence intervals for the selective hydrogenation of CAL over the PtFe/HPZSM-5 catalyst at different temperatures. C0 represents the initial molar concentration of CAL while Ct represents the CAL concentration varied with reaction time. (c) Kinetic investigation of the hydrogenation of the mixture containing the equal moles of HCAL and COL on the PtFe/HPZSM-5 catalyst. The reaction conditions are identical to (a) except that 11.25 mmol HCAL and 11.25 mmol COL were submitted. (d) Kinetic profiles of the selective hydrogenation of CAL with Pt/ZSM-5 and Pt/HPZSM-5, respectively. The reaction conditions are identical to Table 2 except for reaction time.

8

G. Wang et al. / Journal of Catalysis 382 (2020) 1–12

zero-order were reported in the literature, which might be related to the adsorption strength of CAL on the catalyst surface. Fast chemisorption would result in zero-order reaction [34,35]; while slow chemisorption leads to the first-order reactioin [16,17,36]. In this work, the first-order reaction with resepct of CAL was observed, suggesting that adsorption of CAL was a slow step. Moreover, in order to compare the independent hydrogenation rate of C@C and C@O bond, the kinetic behavior was further investigated with the PtFe/HPZSM-5 catalyst for the hydrogenation of a mixture containing an equal molar ratio of HCAL (only containing C@O double bond) and COL (only containing C@C double bond). As clearly shown in Fig. 7c, the total hydrogenation product HCOL, which was derived from both the hydrogenation of HCAL and COL, increased almost linearly with reaction time. However, when comparing the hydrogenation rate of HCAL and COL alone, the hydrogenation of HCAL is much faster than that of COL. This demonstrates that the hydrogenation rate of C@O double bond is much faster than that of C@C double bond with the PtFe/HPZSM5 catalyst. It also implies that the C@O double bond was preferentially adsorbed and activated on the PtFe/HPZSM-5 catalyst so that higher selectivity toward COL can be furnished with the PtFe/ HPZSM-5 catalyst for the selective hydrogenation of CAL, which might be influenced by the support acidity and the electronic properties of the Pt species. Moreover, in order to compare the Pt catalysts supported on different materials more fairly, the kinetic behaviors of the selective hydrogenation of CAL over Pt/ZSM-5 and Pt/HPZSM-5 were also conducted, respectively. As displayed in Fig. 7d, although the reaction went smoothly over the Pt/HPZSM-5 catalyst, the reaction rate was rather low. The complete conversion of CAL needed nearly 5 h, which is much longer than that over the PtFe/HPZSM-5 catalyst, where the reaction almost finished within 1 h. As for the Pt/ZSM5 catalyst, the reaction nearly stopped after about 2 h. This is very similar to our previous observation [30], which might be caused by pore block and thus the mass transportation became quite difficult. Nevertheless, the COL selectivity was kept almost at the same level with reaction going on despite of the different Pt catalysts. Furthermore, the recycability of the PtFe/HPZSM-5 catalyst was also investigated. As clearly shown in Fig. 8, the PtFe/HPZSM-5 catalyst can be recycled for at least nine times without obvious loss in MSR or in COL selectivity. To check the catalyst stability, the filtrate was detected using ICP-AES. As a result, the leached Pt amount in the filtrate is below the detection limit of ICP-AES. Moreover, the used PtFe/HPZSM-5 catalyst was also characterized by TEM. As

already displayed in Fig. 4d, the Pt nanoparticles for the used PtFe/HPZSM-5 catalyst did not aggregate obviously and the average Pt particle size was also comparable with the fresh one. This indicates that the PtFe/HPZSM-5 catalyst was stable enough during the recycling processes, which might be caused by the strong interaction between Pt nanoparticles and HPZSM-5 support. In order to understand the PtFe/HPZSM-5 catalyst deeply, the relevant comparison with the results in the literature was made. As clearly displayed in Table 3, only 100 h
1 TOF was achieved with the PtFe NWs catalyst although the selectivity toward COL was about 95.5%, mainly due to lower temperature and lower hydrogen pressure [37]. The Pt3Fe/CNT, MIL-101@Pt@FeP-CMPsponge and Pt/ FeFe-LDH catalysts furnished medium TOF values in the range of 1000–1600 h
1 with over 90% selectivity toward COL [16,38,39]. In our previous studies, the PtFe0.25/15AS catalyst using Al2O3@SBA-15 composites as support gave a TOF of 1.54 s
1 (5544 h
1) with the selectivity toward COL of 76.9% [12]; while the PtFe0.25/15TS catalyst using TiO2@SBA-15 composites as support afforded a TOF of 3.06 s
1 (11016 h
1), accompanied with a 86.4% selectivity toward COL under the similar conditions [13]. Very recently, the Pt catalyst supported on YCo0.3Fe0.7O3 perovskite composites showed a extremely high TOF of 15163 h
1 with about 95% COL selectivity [17]. By comparison, the PtFe/HPZSM-5 catalyst in this study gave a TOF value of 3.41 s
1 (12276 h
1) under the similar reaction conditions, which is slightly lower than that of the Pt/YCo0.3Fe0.7O3 catalyst, but much higher than those on Pt3Fe/CNT, MIL-101@Pt@FeP-CMPsponge and Pt/FeFe-LDH. With regards to the COL selectivity with the PtFe/HPZSM-5 catalyst, it is also a little lower than those obtained with other PtFe bimetallic catalysts. The possible reason might be the difference in support acidity for different materials. As already proved in the above studies, Fe is not a co-catalyst in PtFe bimetallic catalsyts, but a promoter. In another word, Fe promoter affects the catalytic performance via the interaction between Pt nanoparticles and Fe promoter, where electron transfer would occur either from Fe speices to Pt atoms or from Pt atoms to Fe species. However, support materials with acidic property also influence the electron properties of Pt nanoparticles through their interaction. Therefore, it is a combinational effect of acidic support and Fe promoter on the corresponding catalytic performance of the PtFe bimetallic catalysts supported on an acidic support material. We deduce that for the Pt catalysts supported on materials with somewhat acidity such as 15AS, 15TS and HPZSM-5, the interaction of Pt-support is opposite to that of Pt-Fe. As a result, the selectivity toward COL was not improved after Fe was doped to Pt/HPZSM-5. 3.4. Understanding the PtFe/HPZSM-5 catalyst and general discussion

HCOL Sel.

HCAL Sel.

120

COL Sel.

100

8

-1

-1

MSR (molCAL gPt h )

10

60 4

Sel. (%)

80 6

40 2

20

0

0 0

1

2

3

4

5

6

7

8

9

Number of run Fig. 8. Reusability of the PtFe/HPZSM-5 catalyst for the selective hydrogenation of CAL. Reaction conditions: 25 mg of the PtFe/HPZSM-5 catalyst, 11.25 mmol of CAL, PH2 = 2 MPa , 9 mL isopropanol + 1 mL H2O, 90 °C, 1000 rpm, 1 h.

When recalling the catalytic performance of the Pt-related catalysts for the selective hydrogenation of CAL in this work, the Pt/ HPZSM-5 catalyst was more active and selective than that supported on pristine ZSM-5. Moreover, after Fe was doped to the Pt/HPZSM-5 catalyst, the catalytic performance of the PtFe/ HPZSM-5 catalyst was greatly improved. In order to deeply understand the influence of partial desilication of ZSM-5 and Fe doping on the catalytic performance of the PtFe/HPZSM-5 catalyst, the relevant catalysts were further characterized using a series of techniques. The NH3-TPD profiles were measured to characterize the acidic property of the relevant materials (Fig. 9A). For ZSM-5 and Pt/ZSM5, there are two NH3 desorption peaks at about 160 °C and 380 °C and the total acid amount was 0.189 and 0.225 mmolg
1, respectively. As for HPZSM-5 related samples, the NH3-desorption peak at around 380 °C disappeared, but a new peak appeared at 273 °C, demonstrating that the acidity on HPZSM-5 was weaker than that on ZSM-5. Nonetheless, the total acid amount of HPZSM-5, Pt/

9

G. Wang et al. / Journal of Catalysis 382 (2020) 1–12 Table 3 Comparison of the PtFe bimetallic catalysts for the selective hydrogenation of CAL to yield COL. Catalyst

T (°C)

PH2 (MPa)

Time (h)

Conv. (%)

COL Sel. (%)

TOF (h
1)

Ref.

PtFe0.25/15AS PtFe0.25/15TS PtFe NWs Pt3Fe/CNT MIL-101@Pt@FeP-CMPsponge Pt/FeFe-LDH Pt/YCo0.3Fe0.7O3 PtFe/HPZSM-5

90 90 70 60 RT 110 90 90

2.0 2.0 0.1 2.0 3.0 1.0 2.0 2.0

1 0.5 2.5 0.5 0.25 2 0.5 1

77.4 68.0 95.7 62.1 97.6 90.0 98.9 97.9

76.9 86.4 95.5 97.2 97.3 92.0 94.9 87.6

5544 11,016 100 1200 1516 1026 15,163 12,276

12 13 37 16 38 39 17 This study

Fig. 9. (A) NH3-TPD curves of (a) ZSM-5; (b) Pt/ZSM-5; (c) HPZSM-5; (d) Pt/HPZSM-5 and (e) PtFe/HPZSM-5. (B) FT-IR spectra of pyridine-adsorbed on ZSM-5 and HPZSM-5 after evacuated at 100 °C and 200 °C, respectively.

HPZSM-5 and PtFe/HPZSM-5 was 0.422, 0.332 and 0.379 mmolg
1, respectively. This suggests that after alkali partial desilication, the total acidity of HPZSM-5 slightly increased, although the stronger acidity decreased instead. In order to further discriminate the Lewis acidity or Brönsted acidity of ZSM-5 before and after alkali partial desilication, the related samples were characterized by FT-IR spectroscopy using pyridine as probe molecules. Fig. 9B displays the FT-IR spectra of pyridine adsorbed on ZSM-5 and HPZSM-5 after evacuation at 100 °C and at 200 °C, respectively. As well known, the IR band at 1545 (1637) cm
1 and 1455 cm
1 can be ascribed to pyridine adsorbed on the Brönsted acid sites (BAS) and the Lewis acid sites (LAS), respectively [40–42]; while the one at 1490 cm
1 was the combinational contribution of pyridine adsorbed on BAS and LAS. As for the IR bands at 1446 cm
1 and 1598 cm
1, they can be attributed to adsorbed pyridine via the hydrogen bond [43]. For pyridine adsorbed on ZSM-5 after evacuation at 100 °C, the IR bands at 1446, 1490, 1545, 1598 and 1637 cm
1 can be clearly detected. In addition, a shoulder band at 1455 cm
1 can also be observed. After partial desilication of ZSM-5 with sodium hydroxide, the FT-IR spectra of pyridine adsorbed on HPZSM-5 still exhibit the BAS and LAS. Besides, a new IR band at 1444 cm
1 was detected. When the evacuation temperature increased to 200 °C, the IR band at 1446 cm
1 for pyridine adsorbed on ZSM-5 disappeared, which is the typical feature of pyridine adsorbed via the hydrogen bond; while the one at 1444 cm
1 for pyridine adsorbed on HPZSM-5 can be still detected. Thus, these two IR bands are quite different although the position difference is so tiny. Correpondingly, the IR band at 1444 cm
1 can be assigned to pyridine adsorbed on Na+ ions [44], demonstrating that Na+ ions on HPZSM-5 surface were not completely removed after partial desilication with sodium hydroxide. In fact, the ICP-AES results also confirm that 0.46 wt% Na was retained for HPZSM-5.

To make clear the Na+ residue on the catalytic performance of the resultant Pt catalyst, we also investigated the selective hydrogenation of CAL using Pt/ZSM-5 as catalyst after adding the equal weight amount of NaOH. As listed in Table S1, with addition of trace amount of NaOH, the catalytic performance was indeed enhanced and both CAL conversion and COL selectivity were slightly increased when compared with those obtained over the Pt/ZSM-5 catalyst. However, when compared with those obtained with the Pt/HPZSM-5 catalyst, the improvement either in CAL conversion or COL selectivity was very limited. In addition, we also removed the residual Na after alkaline desilication of ZSM-5 with an aqueous solution of ammonia chloride according to the literature. As a result, removal of Na+ residue led to decreased COL selectivity. This also implies that the residual Na+ on the HPZSM-5 surface can indeed be helpful for adsorption and activation of CAL. Nevertheless, the adjusted acidity after partial alkaline desilication of HPZSM-5 also played a vital role in determining the interaction with Pt nanoparticles. To get some insights about the interaction of Pt with support and Fe promoter, the reduction behaviors of the as-calcined Pt catalyst precursors were investigated using H2-TPR technique (Fig. 10A). For as-calcined Pt/ZSM-5, three reduction peaks can be distinguished, the distinct one at 181 °C and two broad ones at 370 °C and 475 °C, respectively. The hydrogen uptake at about 181 °C can be ascribed to the reduction of PtOx species with lower dispersion while the one at 370 °C can be assigned to the reduction of PtOx species having interaction with the LAS of the support, while the one at 475 °C can be attributed to the reduction of PtOx species with interaction with BAS of the support. After alkali desilication, three reduction peaks for as-calcined Pt/HPZSM-5 shifted to higher temperatures. The hydrogen uptake at 370–400 °C increases significantly, prossibly because of the higher dispersion of Pt nanoparticles on HPZSM-5 when compared with that on

10

G. Wang et al. / Journal of Catalysis 382 (2020) 1–12

399

B

487

2062

385

276

Pt/HPZSM-5 370

PtFe/HPZSM-5

2058

471

257

0.1

2089

PtFe/HPZSM-5

Absorbance

Hydrogen uptake (a.u.)

A

474

Pt/ZSM-5

2087

Pt/HPZSM-5 2092

181

2043

100

200

300

400

500

600

700

800

2200

2100

o

Pt/ZSM-5

2000

1900

1800

-1

Temperature ( C)

Wavenumber (cm )

Fig. 10. (A) H2-TPR curves of Pt/ZSM-5, Pt/HPZSM-5 and PtFe/HPZSM-5. (B) FT-IR spectra of CO adsorbed on Pt/ZSM-5, Pt/HPZSM-5 and PtFe/HPZSM-5.

ZSM-5. After Fe was doped to Pt/HPZSM-5, the reduction peaks were further shifted to higher temperatures, indicating that there is a strong interaction between Pt and Fe species as well as the support. Furthermore, the surface geometry of the different Pt catalysts were also characterized by FT-IR spectroscopy using CO as probe molecules in transmittance mode after in situ pretreated in hydrogen flow at 400 °C for 2 h. From Fig. 10B, we can clearly see that the bridged CO adsorption band at 1900–1800 cm
1 for three Pt catalysts is extremely weak and the linear CO adsorption is dominant. For CO linearly adsorbed on Pt/ZSM-5, there is a predominant IR band at around 2090 cm
1 with a shoulder at around 2040 cm
1. After partial desilication, the CO linearly adsorbed on the Pt/ HPZSM-5 catalyst showed two separate IR bands at 2087 and 2058 cm
1. With respect to CO linearly adsorbed on the PtFe/ HPZSM-5 catalyst, two linear CO bands shifted to 2089 and 2062 cm
1. According to the literature [45], the IR bands at around 2090 and 2060 cm
1 can be ascribed to CO adsorbed on Pt(1 1 1) and Pt(1 1 0) facets, respectively. As for the IR band at 2040 cm
1, it was attributed to CO adsorbed on low coordinated Pt sites at step-edges, corners, and defects. Furthermore, it was reported that the hydrogenation activity and selectivity toward COL are enhanced in the order Pt(1 1 1) facets > Pt(1 0 0) facets > rounded (stepped) surface [46]. As mentioned above, the IR band at around 2060 cm
1 appeared for CO linearly adsorbed on Pt/HPZSM-5 or PtFe/HPZSM-5. This suggests that the surface geometry of Pt species was remarkably changed after partial desilication of ZSM-5. For Pt/ZSM-5, Pt(1 1 1) facets are dominant with a few low coordinated Pt sites at step-edges, corners, and defects; while after partial desilication, both Pt(1 0 0) and Pt(1 1 1) facets are predominant. Doping Fe to Pt/HPZSM-5, the distribution of Pt (1 1 1) and Pt(1 0 0) facets almost unchanged. Moreover, to further understand the surface chemical states of Pt species, some related catalysts were also characterized using XPS after in situ pretreated in a hydrogen flow at 400 °C for 2 h. Fig. 11a shows the fitted Pt4f and overlapped Al2p XPS spectra. For Pt/ZSM-5, the spectrum can be deconvoluted to Pt0 (Pt4f 7/2 at 71.1 eV) and Pt2+ (Pt4f 7/2 at 72.8 eV) with very weak Al2p due to low content of Al in the sample [47]. After ZSM-5 was partially desilicated, there are also two Pt species on the surface of the Pt/ HPZSM-5 catalyst. As for the PtFe/HPZSM-5 catalyst, the BEs for two Pt speices shifted to lower values, confirming that there is electron donation from Fe species to Pt atoms. In addition, we also calculated the relative amount of the surface Pt species. As also displayed in Fig. 11a, the relative content of Pt0 species and Pt2+ species in Pt/ZSM-5 catalyst was 74.3% and 25.7%, respectively. The

relative content of Pt0 species in Pt/HPZSM-5 catalyst increased slightly to 78.4%. For PtFe0.25/HPZSM-5 catalyst, the relative content of metallic Pt0 is 82.2%, which is higher by 7.9% than that of Pt/ZSM-5 catalyst. This indicates that there is strong interaction between Pt and Fe speices and electron transfer from Fe species to Pt sepcies occurred [19,48]. Due to overlap of Al2p with Pt4f [12], the core level of Pt4d was also measured to exclude the interference of Al2p. As displayed in Fig. 11b, the Pt4d XPS spectra also showed the similar results as Pt4f spectra. Despite of low signal to noise of Fe2p XPS spectrum mainly due to low content of Fe, the Fe2p XPS spectrum was deconvoluted to Fe3+ and its shakeup satellites (Fig. 11c). This demonstrates that Fe3+ species are dominant on the catalyst surface. As already discussed above, the surface acidity of support would affect the surface geometry of Pt nanoparticles. Based on the FT-IR spectra of CO adsorbed on different Pt catalysts (Fig. 10B), Pt(1 1 1) facets are dominant for Pt/ZSM-5; while for Pt/HPZSM-5, both Pt (1 1 1) and Pt(1 0 0) facets are predominant on the surface. As well known, chemisorption of CAL will be influenced by different Pt facets. Although Pt(1 1 1) facets are more selective than other Pt facets, much more Pt(1 1 1) and Pt(1 0 0) facets were exposed for Pt/HPZSM-5 catalyst, so that the selectivity was slightly improved with the Pt catalyst supported on ZSM-5 after partial desilication. After Fe was doped to Pt/HPZSM-5, the acidic property was hardly changed and the surface geometry of Pt nanoparticles is hardly changed, so that the selectivity was also comparable. The H2-TPR results show that the interaction of Pt species with HPZSM-5 became stronger. After Fe was doped, the strong interaction of Pt and Fe resulted in much higher reduction temperature of as-calcined PtFe/HPZSM-5. Additionally, XPS spectra can further confirm that electron transfer from Fe species to Pt atoms indeed took place, so that the electron rich Pt atoms were detected on the catalyst surface. The higher electron density of Pt atoms on the catalyst surface are helpful of the adsorption of C@O bond in CAL via a parallel mode because the high electron density of Pt atoms can repulse the adsorption of C@C bond. 4. Conclusions In summary, through simple desilication of conventional ZSM-5 in NaOH aqueous solution, HPZSM-5 with enlarged pore size and pore volume was obtained. HPZSM-5 was proved to be a better support for Pt or Pt-Fe in the selective hydrogenation of CAL to yield COL. A robust PtFe/HPZSM-5 catalyst with 5 wt% Pt and a Fe/Pt molar ratio of 0.25 was prepared using an impregnation method. Under the optimized reaction conditions, the PtFe/

G. Wang et al. / Journal of Catalysis 382 (2020) 1–12

a

71.1 70.8

Al 2p 74.6

72.6

0

Pt :82.2% 2+ Pt :17.8%

72.2

PtFe/HPZSM-5

Intensity (a.u.)

Pt4f+Al2p

0

Pt :78.4% 2+ Pt :21.6%

Pt/HPZSM-5

0

Pt :74.3% 2+ Pt :25.7%

Pt/ZSM-5

11

transfer from Fe speices to Pt atoms. Therefore, most Pt atoms are in an electron-rich state and beneficial to hydrogen activation. Furthermore, the surface geometry of Pt nanoparticles is also altered after partial desilication of ZSM-5 and as a result, Pt (1 0 0) and Pt(1 1 1) facets are dominant so that the catalyst activity and selectivity toward COL was remarkably improved by comparison with Pt/ZSM-5. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements

80

78

76

74

72

70

68

Binding Energy (eV)

This work was supported by the National Natural Science Foundation of China (Grant No. 21872052 and 21872054).

Pt4d

b

Appendix A. Supplementary material

314.2 315.6

Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcat.2019.12.004.

Intensity (a.u.)

PtFe/HPZSM-5

References Pt/HPZSM-5

Pt/ZSM-5

340

330

320

310

Binding Energy (eV) 3+

Fe

c

Fe2p

Intensity (a.u.)

satellite

740

730

720

710

700

Binding Energy (eV) Fig. 11. (a) Pt4f + Al2P and (b) Pt4d XPS spectra of Pt/ZSM-5, Pt/HPZSM-5 and PtFe/ HPZSM-5 catalysts, and (c) Fe2p XPS spectrum of PtFe/HPZSM-5 catalyst.

HPZSM-5 catalyst showed a COL selectivity of 87.6% at 97.9% CAL conversion, furnishing a TOF of 3.41 s
1. The PtFe/HPZSM-5 catalyst also showed excellent stability and can be recycled for at least nine times without obvious loss in activity or selectivity toward COL. The interaction of Pt-support became stronger after partial desilication when compared with conventional ZSM-5. Moreover, doping of Fe greatly enhanced the catalyst activity via electron

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