Enhanced dehydrogenative aromatization of propane by incorporating

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Kinetics, Catalysis, and Reaction Engineering

Enhanced dehydrogenative aromatization of propane by incorporating Fe and Pt into Zn/HZSM-5 catalyst wei zhou, Jiaxu Liu, Long Lin, Xiaotong Zhang, Ning He, Chunyan Liu, and Hongchen Guo Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b03865 • Publication Date (Web): 29 Oct 2018 Downloaded from http://pubs.acs.org on November 3, 2018

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Industrial & Engineering Chemistry Research

Enhanced dehydrogenative aromatization of propane by incorporating Fe and Pt into Zn/HZSM-5 catalyst Wei Zhou, Jiaxu Liu, Long Lin, Xiaotong Zhang, Ning He, Chunyan Liu, Hongchen Guo* State Key Laboratory of Fine Chemicals & School of Chemical Engineering, Dalian University of Technology, Dalian 116012, P. R. China

Corresponding Author E-mail: [email protected]

1

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Abstract In this study, an attempt was made to modify Zn/HZSM-5 aromatization catalyst by successively impregnating Fe and Pt. Multi-techniques including NH3 adsorption FT-IR, TEM, HAADF-STEM, XPS and operando dual beam FT-IR spectroscopy of propane aromatization were employed to investigate the interactions among Zn, Fe, Pt, and the synergetic catalysis among the Brønsted acid sites, Lewis acid sites and metallic sites of ZnFePt/HZSM-5

catalyst.

Results

show

that

an

optimized

trimetallic

Zn1.0Fe0.3Pt0.1/HZSM-5 catalyst has much lower dry gas production, and significantly enhanced propane transformation activity and aromatization selectivity compared with parent Zn/HZSM-5. The excellent performance of the trimetallic catalyst could be attributed to the high dispersion of metals, the formation of FePt bimetallic dehydrogenation active sites and the promotion effect of FePt sites for the recombinational desorption of H atoms for the [ZnOZn]2+ sites. Fixed-bed reaction indicates that the trimetallic Zn1.0Fe0.3Pt0.1/HZSM-5 catalyst also has excellent durability which is desirable for practical use. KEYWORDS: propane aromatization, Zn/HZSM-5, FePt bimetallic sites, synergetic dehydrogenation, dual beam FT-IR spectroscopy

2


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1.Introduction The catalytic conversions of short-chain alkanes to aromatics have of vital importance from both industrial and academic viewpoint due to the growing demand of BTX (benzene, toluene, xylene) 1-4. Recently, interest in studying these reactions has been renewed, due to new opportunities for short-chain alkane upgrading created by increased availability of shale gas 5, 6. Zn/HZSM-5 catalysts have been proven to be efficient in the activation and aromatization of short-chain alkanes, which has been applied to commercial processes, such as the Alpha process from Sanyo 7and the Topas process from Topsoe 8. It is well accepted that the incorporation of Zn into HZSM-5 could improve the catalytic performance of HZSM-5 in short-chain alkane aromatization even though the exact essence of a Zn species and zeolitic acid sites is still debatable

9-21.

aromatization over Zn/HZSM-5 follows bi-functional mechanism

Short-chain alkanes 22, 23.

The Zn related

Lewis acid sites are responsible for the dehydrogenation steps while the Brønsted acid sites (Si(OH)Al) are responsible for reactions such as oligomerization, cracking and cyclization. Numerous methods have been reported in the preparation of Zn/HZSM-5 catalysts. The isolated Zn2+ cations and [ZnOZn]2+ species inside the channels of zeolite

9

are widely

studied as the active sites of aromatization. According to literature reports and our previous 3



work 24-29, [ZnOZn]2+ is more active than other Zn species in the activation of C-H bonds of short-chain alkanes. However, the recombinative desorption of surface H atoms over [ZnOZn]2+ is rather difficult. The surface H atoms linger on the [ZnOZn]2+ sites would result in serious hydrogenolysis side-reactions, which eventually leads to the substantial formation of methane and ethane by-products. Thus, in order to accelerate the recombinative desorption of surface H atoms, the introduction of other metals with strong promotion effect of dehydrogenation is necessary. It is well known that supported Pt catalysts have been widely applied in the dehydrogenation reactions

30, 31.

Therefore, the combinative modification of Zn and Pt is

expected to be beneficial in accelerating the recombinative desorption of surface H atoms over [ZnOZn]2+. However, there is one thing worth mentioning. That is, Pt nanoparticles with poor dispersion could also catalyze the hydrogenolysis side reactions of short-chain alkanes 32. The addition of Sn and Re (more electropositivity than Pt) into Pt has proved to be an effective way to improve the catalytic performance of supported Pt catalysts by improving the dispersion of Pt 33. Most recently, single-atom catalysts have attracted lots of attention due to its superior performance

34, 35.

The isolated single-Pt atom catalyst

prepared by anchoring Pt ions onto iron oxide (Pt1/FeOx) exhibited unique and excellent performance in many reactions

34.

Iron oxide as support or additive can enhance the

dispersion of Pt particles via bimetallic particle formation, which has been described in the literature. This special interaction in-between Pt and iron oxide has been utilized in isobutane dehydrogenation reaction. According to Kobayashi et al. 36, the Pt/Fe2O3-Al2O3 4


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catalyst with approximately 7 wt% Fe2O3 in the support exhibited better performance in isobutane dehydrogenation than Pt/Al2O3. Zheng et al. 37 proposed that the bimetallic PtFe/KL catalyst prepared by Fe and Pt co-impregnation exhibited better performance than Pt/KL in n-hexane aromatization. They proposed that the addition of Fe2O3 increased the surface Pt atoms’ electron density through the formation of Pt-Fe bimetallic particle. In this paper, a tri-metallic Zn1.0Fe0.3Pt0.1/HZSM-5 catalyst was prepared by impregnating iron and platinum precursor compounds one by one onto Zn1.0/HZSM-5. Results showed that the tri-metallic catalyst possessed wonderful dehydrogenative aromatization performance. In order to gain insight into the catalysis of the trimetallic catalyst, multiple characterization methods including transmission electron microscopy (TEM), hydrogen temperature programmed reduction (H2-TPR), X-ray photoelectron spectroscopy (XPS) and dual beam Fourier transform infrared (FT-IR) spectroscopy were applied to catalysts and operando propane aromatization studies. The vital role of Fe in Pt dispersion on Zn/HZSM-5 zeolite was observed, and the synergy effect of FePt bimetallic particles with [ZnOZn]2+ lewis acid sites in the dehydrogenation aromatization of propane was concluded.

2. Experimental section 2.1. Catalyst preparation Nano-sized NH4-ZSM-5 zeolite (20-50 nm, Figure S1) with a Si/Al ratio of 15 was supplied by Dalian Ligong Qiwangda Chemical Technology (DQ-TECH). Hydrogen form zeolite was obtained via calcining the NH4-ZSM-5 zeolite in muffle at 540 °C for 6 h. 5



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Then, the HZSM-5 zeolite was leached with 0.6 M nitric acid (acid solution to zeolite ratio 30 mL g-1) at room temperature for 24 h. After leaching, the zeolitic powders were recovered by filtration and washed repeatedly until the pH value approached 7.0. Finally, the filter cake was dried at 110 °C for 24 h and then calcined at 540 °C for 3 h. 1.0 wt.% Zn modified HZSM-5 zeolite was prepared with ordinary incipient wet impregnation (IWI) method using Zn (NO3)2·6H2O solution. The IWI was carried out at 80 °C for 4 h, then the material was dried at 110 °C for 12 h and calcined at 540 °C for 6 h to obtain the Zn1.0/HZSM-5 parent catalyst. The trimetallic ZnFePt/HZSM-5 catalysts were prepared by successively loading Fe and Pt onto the Zn1.0/HZSM-5 parent catalyst with the same IWI method. The impregnation of Fe was carried out with an aqueous solution of Fe (NO3)3·9H2O in a water bath at 80 °C for 4 h. The impregnated sample was dried at 110 °C for 12 h and calcined at 540 °C for 6 h to obtain the Zn1.0Fe/HZSM-5 catalyst. In order to investigate the effect of Fe addition, different Zn1.0Fe/HZSM-5 catalysts with Fe loading ranged from 0.1 wt.% to 0.2 wt.%, 0.3 wt.%, 0.5 wt.% and 1.0 wt.% were fabricated. Then, 0.1 wt% Pt was deposited onto the Zn1.0Fex/HZSM-5 (x = 0.1 wt%, 0.2 wt %, 0.3 wt %, 0.5 wt % and 1.0 wt %) with H2PtCl6·6H2O (Sinopharm Chemical Reagent Co., Ltd) solution. The obtained materials were dried at 110 °C for 12 h and calcined at 540 °C for 6 h to obtain the Zn1.0FexPt0.1/HZSM-5 catalysts. For the purpose of comparison, both monometallic catalysts (Zn1.0/HZSM-5, Pt0.1/HZSM-5,

Fe0.3/HZSM-5)

and

bimetallic

catalysts

6


(Zn1.0Pt0.1/HZSM-5,

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Zn1.0Fe0.3/HZSM-5, Fe0.3Pt0.1/HZSM-5) were also prepared with the IWI method. 2.2. Characterization 2.2.1 Basic physicochemical properties Metal loading of all samples were determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES) which was performed on a Perkin-Elmer Optima 2000DV instrument. Nitrogen physisorption was conducted on a Micromeritics ASAP 2020 instrument at -196 °C to obtain textural information. Prior to the measurement, samples (380-830 μm sieve fraction) were degassed at 400 °C for 6 h. The surface area was calculated by the Brunauer-Emmett-Teller (BET) method using the adsorption branch in the p/po range from 0.10 to 0.15, the pore volumes were estimated at p/po of 0.99, while the micro- and mesoporosity was discriminated by the t-plot method. BET method applied to the N2 isotherm. The acidity characterization was made with NH3 adsorption FT-IR spectroscopy. The spectra was recorded with a Nicolet is 10 FT-IR spectrometer in the range from 4000 to 400 cm-1 with an optical resolution of 4 cm-1. To do this, the zeolitic samples were pressed into self-supporting thin wafers (approximately 15 mg) and decontaminated at 400 °C under vacuum (10-3 Pa) for 4 h in a quartz IR cell equipped with CaF2 windows. After the pretreatment, the cell was cooled down to room temperature for background spectrum measurement. The spectra of NH3 was obtained as follows: first, NH3 adsorption was carried out at 150 °C in continuous NH3 flow at the speed of 3 mL min-1 for 30 min; second, the evacuation treatment (10-3 pa) was conducted for 30 min at 150 °C; then, the cell was 7



cooled down to room temperature for sample spectrum measurement; finally, the spectra were obtained by subtracting the background spectrum from the measured sample spectra. The concentration of Brønsted and Lewis acid sites was calculated from the intensities of Brønsted acid (1450 cm-1 for NH4+ ions) and of Lewis acid (1630 cm-1 for NH3L adducts), by using the extinction coefficients of these bands. The integrated molar extinction coefficient (IMEC) of the band at 1450 cm-1 due to NH3 on a Brønsted acid site is 0.11 cm/μmol and that of the band at 1630 cm-1 due to NH3 on a Lewis acid site is 0.026 cm//μmol 38, 39. These values apply to Si/Al-based catalysts and a measurement temperature of 150 ℃. The concentration calculation equation of Brønsted and Lewis acid sites as followed: C (NH3 on Brønsted sites) = πR2·IA/(IMEC(Brønsted)·W); C (NH3 on Lewis sites) = πR2·IA/(IMEC(Lewis)·W); IA (B, L) = integrated absorbance of Bronsted or Lewis band (cm-1); R = radius of catalyst disk (cm); W = weight of disk (mg). 2.2.2 Metals dispersion and interactions TEM, high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) of the modified H-ZSM-5 zeolites along with EDS analysis were done on a FEI (Tecnai F30 G2, The Netherlands) microscopy. The H2-TPR measurements were carried out by Quantachrome ChemBET Pulsar TPR/TPD Automated Chemisorption Analyser. The calcined catalyst sample (0.2 g) was first purged with high purity He flow at 400 ℃ for 0.5 h, followed by cooling to room temperature. H2-TPR was performed by heating the samples from room temperature to 800 ℃ at a rate of 10 °C min-1 in a 5% H2/Ar mixture (30 mL min-1). XPS analysis was conducted with a VG ESCALAB MK2 8


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instrument using Al Kα radiation (1486.6 eV) to analysis chemical states of the sample. The voltage and power for the measurements were 12.5 kV and 250 W, respectively. The vacuum in the test chamber during spectrum collection was maintained at 2 × 10−10 mbar. The binding energies were calibrated for the surface charge by referencing to the C1S peak of the contaminant carbon at 284.6 eV. 2.2.3 Investigation of metal roles with pulse reaction and dual beam FT-IR spectroscopy The micro-reactor had an inner diameter of 6 mm. 0.15g catalyst was loaded each time (20-40 mesh particles). The micro-reactor was operated at following condition: the flow rate of N2 carrier gas was 20 mL s-1, the reaction temperature was in the range of 450600 °C, the input dosage of propane (99 wt% purity) was 0.1 mL each pulse. On-line GC (Techcomp GC-7900) was equipped with two detectors (FID and TCD) and two columns (a HP-alumina (PLOT) column (50 m × 0.32 mm) and a TDX-01 molecular sieve column (5 m × 0.3 mm)) were used to analyze the reaction products. Operando FT-IR spectroscopy was employed to obtain the information of surface species during the aromatization of propane over Zn, ZnPt and ZnFePt modified HZSM-5 catalysts. In this case, a self-developed dual beam FT-IR spectrometer, a dual beam IR-cell reactor and an on-line mass spectrometer (Omnistar, 1-200 amu, QMS 200) were used to construct the operando spectroscopy system. Catalyst samples were pressed into selfsupporting thin wafers (1 cm2) and placed in the sample beam of the dual beam IR cell, and the reference beam was left vacant. The experiment procedures and the method to do 9



spectrum subtraction were described elsewhere 47. In this study, samples were pretreated in the IR-cell reactor at 400 °C for 4 h under vacuum (10-3 Pa). A temperature- programmed propane aromatization was carried out in the temperature range of 50-400 °C under constant pressure of 0.1 MPa. Propane was fed continuously in a weight-hourly space velocity (WHSV) of 180 h-1 by using a dilute propane gas (1% propane in nitrogen). The spectra were recorded at a resolution of 4 cm-1 with 64 scans in the region of ṽ = 40001000 cm-1. 2.3. Fixed-bed catalytic tests A small fixed-bed reactor was used to evaluate the trimetallic catalyst’s industrialization prospect. The inner diameter of the fixed-bed reactor was 10 mm, 3 g catalyst (20-40 mesh particles) was loaded each run. The fixed-bed reaction was carried out in the range of 450600 °C, 0-0.7Mpa, and WHSV was from 0.13 to 1.10 h-1. The mass balance was obtained by weighing up the gross weight of feedstock, products and carbon deposits over catalyst. Liquid products were collected in a cooling trap and analyzed by using a Shimadzu GC14C gas chromatograph (OV-1 capillary column 50 m × 0.2 mm, FID detector), while gas products were analyzed by using an on-line GC (Techcomp GC-7890F, OV-1 capillary column 50 m × 0.2 mm, FID detector).

3. Results and discussion 3.1. Preparation and characterization of Zn1.0Fe0.3Pt0.1/HZSM-5 catalyst As it is mentioned in the experimental section, the trimetallic catalyst Zn1.0Fe0.3Pt0.1 /HZSM-5 was prepared by impregnating a nanometer HZSM-5 zeolite with the solutions 10


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of zinc nitrate, ferric nitrate and hydrogen hexachloroplatinate(IV) successively. Figure 1(a) and (a’) show that the metal species were highly dispersed on the Zn1.0Fe0.3Pt0.1/ HZSM-5 catalyst. The size of metallic particles almost completely fell in the narrow range of 1-3 nm. We have made a preliminary investiga-

Figure 1. TEM images of metals dispersion in trimetallic catalyst Zn1.0Fe0.3Pt0.1/HZSM-5 (a, a’) and the reference catalyst Zn1.0Pt0.1/HZSM-5 (b, b’) and Pt0.1/HZSM-5(c, c’)

tions on the effects of several impregnation factors on the dispersion of metals, such as metal loadings, loading sequence, co-loading and step by step loading. Results indicated that the dispersion of metals, especially that of noble metal Pt, was notably influenced by 11



multi-factors. However, compared with other factors, the introduction of ferric species onto Zn/HZSM-5 before impregnating hydrogen hexachloroplatinate (IV) seems to be crucial for obtaining highly dispersed and uniform metallic particles. This might easily understand if Figure 1 (a) and (a’) are compared with Figure 1 (b) and (b’) and Figure1 (c) and (c’). In Table 1, the textual data of the nanometer HZSM-5 zeolite and its modified counterparts obtained from nitrogen physisorption isotherms indicate that, reasonable decreases of the total specific surface area took place to the nanometer zeolite when small amount (no more than 1.0 wt.%) of metal species (zinc, ferric, and platinum) was loaded. However, it is interesting to note that the external surface area of the nanometer zeolite generally was reduced more pronouncedly than micropore surface area by metal loading. This might suggest that most of the metal species were loaded on the external surface of the nanometer zeolite. Table 1 also shows that the results of ammonia adsorption FT-IR spectroscopy characterization (Figure S2). There is one thing worth mentioning for these results. That is, the trimetallic catalyst Zn1.0Fe0.3Pt0.1/HZSM-5 and the reference catalysts all have BrØnsted acid sites (BAS) and Lewis acide sites (LAS). It is obvious that, only the dispersion of zinc species on zeolite generate Lewis acid sites. Both iron and platinum modification has little influence on the zeolitic acidity. Therefore, it is reasonable to believe that they merely have metallicity. Based on these results, we intend to think that the trimetallic catalyst Zn1.0Fe0.3Pt0.1 /HZSM-5 had three kinds of active sites on its surface. The metallic sites were produced by Fe and Pt species. According to the metal particle size indicated by Figure 1, they were expected to be find mainly on the external surface of the 12


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zeolite. The BrØnsted acid sites were inherently generated by framework Al. They should be mainly located in the micropores of the zeolite, since the loading of zinc ions on the external surface would consume most of the external BrØnsted acid sites. The Lewis acid sites were mainly produced by zinc ion species which should be largely located on the external surface of the zeolite. Figure S3 assures that Zn modification resulted in the formation of approximately 2-4 nm ZnO particles on the HZSM-5 zeolite. There is no doubt that such particles cannot enter the 10 member ring micropores of ZSM-5 zeolite. According to literatures27, 29, ZnO clusters have [ZnOZn]2+ type Lewis acid sites. Table 1. Textural and acidity data of trimetallic Zn1.0Fe0.3Pt0.1/HZSM-5 catalyst and the reference catalysts Brønsted

Lewis

Acid

Acid

(μmol g-1)

(μmol g-1)

0.12

30.26

0.21

0.22

0.12

29.42

0.43

0.30

0.20

0.10

23.36

5.43

69

0.30

0.20

0.10

23.01

5.19

316

67

0.28

0.20

0.08

20.55

5.21

380

315

65

0.27

0.20

0.07

18.54

5.22

Zn1.0Fe0.3Pt0.1/HZ

375

313

62

0.26

0.20

0.06

16.99

5.24

Zn1.0Fe0.5Pt0.1/HZ

357

310

57

0.25

0.20

0.05

12.37

5.31

Sample

BET

Smicro

Sexter

Vpores

Vmicro

(m2 g-1)

(m2 g-1)

(m2 g-1)

(cm3 g-1)

(cm3 g-1)

HZ

425

334

91

0.35

0.23

Pt0.1/HZ

415

330

85

0.34

Zn1.0/HZ

392

318

74

Zn1.0Pt0.1/HZ

385

316

Zn1.0Fe0.1Pt0.1/HZ

383

Zn1.0Fe0.2Pt0.1/HZ

Vmeso (cm3 g-1)

In order to investigate the possible interaction among Zn, Fe and Pt species, characterizations with HAADF-STEM/EDS, H2-TPR and XPS were conducted. In HAADF-STEM/EDS experiments, the EDS-mapping was not successful. This is because that the trimetallic catalyst sample had very low metal contents and it was unstable even if the electron beam adopted was pretty weak. As a result, the map scanning time was very limited and several attempts all failed in mapping the extremely low content of Fe and Pt 13



elements (0.1-0.3 wt.%). Therefore, we had to adopt EDS point scan. The EDS point scan was carried out with dozens of tiny metal particles (see Figure 2(a)), which showed the existence of Zn, Fe and FePt three kinds of metallic particles. Statistics indicated that the Zn, Fe and FePt particles had roughly uniform distributions. However, no more noble metal Pt was detected except FePt alloy particles. A representative EDS analysis on a FePt particle was shown in Figure 2 (a), (b) and (c). The scan analysis was done in two ways. One way was average scan which is similar to a large EDS point scan in a rectangular zone of 25 × 25 points (see zone (1) in Figure 2 (a)). The result would be a statistic 'averaged' value of 625 points (see Figure 2 (c)). Another way of point scan was just focused on the tiny metallic particle (see Figure 2 (a) point (2)). In this case, the result would mainly reflect the composition of the metallic particle itself (see Figure 2 (b)). Figure 2 (b) clearly shows that the targeted particle belonged to a Fe and Pt bimetallic alloy phase. Figure 2 (c) illustrates that there were ZnO nano-particles in the adjacent area of the FePt alloy particle.

Figure 2. HAADF-STEM/EDS analyses of Zn, Fe, Pt elements in trimetallic Zn1.0Fe0.3Pt0.1 /HZSM-5 catalyst: (a) HAADF-STEM image labeling a 25 × 25 points scan area (1) and a target particle (2); (b) EDS of area (1); (c) EDS of particle (2).

14


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The profiles of H2-TPR for the trimetallic catalyst Zn1.0Fe0.3Pt0.1/HZSM-5 and the reference catalysts were shown in Figure 3. It is easy to see that, there is no consumption of hydrogen for Zn/HZSM-5 at a broad temperature range from as low as 100 ºC to as high as 700 ºC, which implies that zinc species would exist in ZnO form during catalyst preparation and in the propane aromatization; the monometallic reference catalyst Pt0.1/HZSM-5 showed a weak and abroad peak at 240 ºC, which is usually ascribed to the oxychloroplatinum complex

40.

Another monometallic reference catalyst Fe0.3/HZSM-5

displayed a weak peak around 255 ºC and an intensive peak centered at 432 ºC, they are temporarily ascribed to the reduction of ferric oxide to ferroferric oxide and zero-valent iron, respectively. In the bimetallic catalyst Zn1.0Pt0.1/HZSM-5, a weak reduction peak of platinum species appears at such a high temperature of 558 ºC. This might imply the existence of strong interaction between zinc and platinum species. It is interesting to see that the bimetallic Fe0.3Pt0.1/HZSM-5 catalyst has only one obvious reduction peak at around 353 °C. This should be a clear indicator that both ferric and platinum species form a homogeneous phase. The new phase seems much easier to be reduced than the monometallic Fe0.3/HZSM-5 catalyst, due probably to the hydrogen spillover effect of noble metal Pt

41, 42.

In trimetallic ZnFePt/HZSM-5 catalysts (f, g, h), a relatively low-

temperature reduction peak at 198 °C (f), 216 °C (g) and 243 °C (h), and a relatively hightemperature reduction peak at 450 °C (f), 450 °C (g) and 477 °C (h) can be seen existence in all the catalysts with different iron content. Moreover, a middle-temperature reduction peak appears at around 349 °C when Fe loading increased to 0.30 wt.% and above. As far 15



as we know, there is no literature dealing with the H2-TPR of the same trimetallic catalyst system. So the attributions of these three peaks are mainly referred to the mono- and bimetallic catalysts of this study. Specifically, the low-temperature reduction peak and high-temperature reduction peak are temporarily attributed to the reduction of ferric oxide to ferroferric oxide and zero-valent iron, respectively. While the middle-temperature reduction peak at around 349 °C is temporarily ascribed to the reduction of ferric and platinum homogeneous phase to FePt alloy. The appearance of reduction peak that is related to FePt phase agrees well with the funding of EDS (Figure 2). In conclusion, the H2-TPR characterization confirms that there is strong interaction between Fe and Pt in the

Figure 3. H2-TPR profiles of trimetallic Zn1.0Fe0.3Pt0.1/HZSM-5 catalyst (g) and the reference catalyst Zn/HZSM-5(a); Pt/HZSM-5(b); Fe/HZSM-5(c); Zn1.0Pt0.1/HZSM-5(d); Fe0.3Pt0.1/ HZSM-5(e); Zn1.0Fe0.1Pt0.1/HZSM-5 (f), Zn1.0Fe0.3Pt0.1/HZSM-5(g), and Zn1.0Fe1.0Pt0.1/HZSM-5(h)

trimetallic catalyst. Besides, there should be mutual influence among Zn, Fe and Pt, because the support of Fe and Pt onto Zn/HZSM-5 has resulted in the appearances of a low-temperature reduction peak at 198-243 °C and a high-temperature reduction peak at 16


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450-477 °C, which belong to ferric phase alone. These reduction peaks are not present in the bimetallic catalyst Fe0.3Pt0.1/ HZSM-5(Figure 3 (e)). In addition, the H2-TPR characterization also clarifies that the metallic active sites of the trimetallic catalyst were actually provided by both Fe and FePt. Due mainly to the extremely low Pt loading (0.1 wt.%) and the interference of Al 2p from the zeolitic framework aluminum on Pt 4f

43-45,

no profiles related to Pt 4f

was

obtained by XPS characterization, although many efforts were made. But the XPS characterization did offer useful information related to Zn2p and Fe2p. Figure 4 shows that, the Zn 2p1/2 binding energy of trimetallic catalyst Zn1.0Fe0.3Pt0.1/HZSM-5 and Zn1.0Fe1.0Pt0.1/HZSM-5 is almost the same as that of monometallic catalyst Zn1.0/HZSM-5. The phenomenon indicates that the interaction ZnO species with Fe and FePt metal components was very weak. Figure 5 shows that, the binding energies of Fe 2p3/2 and Fe 2p1/2 in Fe0.3/HZSM-5 were 710.98 eV and 724.56 eV, respectively. These results indicates that the ferric species in the monometallic catalyst had +3 valence state, corresponding to Fe2O3 46. By contrast, these values shifted up to 711.12 eV and 724.92 eV in Zn1.0Fe0.3Pt0.1/HZSM-5, respectively; and further shifted up to 7711.32 eV and 725.08 eV in Zn1.0Fe1.0Pt0.1/HZSM-5, respectively. The chemical shift of the Fe 2p doublet in trimetallic catalysts moved to higher binding energy seems to suggest the occurrence of electron transfer from ferric species to Pt. Therefore, the XPS data agrees well with the abovementioned viewpoints which recognized the existence of FePt bimetallic particles and strong interaction between Fe and Pt in the trimetallic catalyst. 17



Page 18 of 36

Figure 4. XPS profiles of Zn1.0/HZSM-5(a), Zn1.0

Figure 5. XPS profiles of Fe0.3/HZSM-5(a),

Fe0.3Pt0.1/HZSM-5(b) and Zn1.0Fe1.0Pt0.1/HZSM

Zn1.0Fe0.3Pt0.1/HZSM-5(b)

-5(c) catalysts

HZSM-5(c) catalysts

and

Zn1.0Fe1.0Pt0.1/

3.2 Roles of metals demonstrated by pulse reaction and operando DB-FTIR spectroscopy Figure 6 and Table 2 show that, under the pulse reaction conditions, HZSM-5 zeolite is not an active and selective catalyst for propane aromatization although it has the largest number of BrØnsted acid sites (Table 1). Zinc modification dramatically enhances not only aromatics selectivity but also propane transformation activity. These results agrees well with common sense. The direct modification of HZSM-5 with Pt leads to a most active catalyst with enhanced aromatics selectivity. But the catalyst suffers from very high dry gas (C1 and C2 hydrocarbons) production especially under high reaction temperature. This is also understandable since the direct impregnation of HZSM-5 with H2PtCl6 resulted in the formation of very large Pt particles on surface of HZSM-5(Figure 1 (c)and (c’)). 18


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According to literatures32, larger Pt particles can catalyze hydrogenolysis which would break propane into methane and ethane. It is worth mentioning that the direct deposit of Pt with H2PtCl6 precursor onto zinc modified catalyst (Zn1.0/HZSM-5) cannot lead to a satisfying catalyst (see Zn1.0Pt0.1/HZSM-5). This is because that the introduction of Pt onto Zn1.0/HZSM-5 increases propane transformation activity but brings down aromatics selectivity. Again, the serious hydrogenolysis promoted by large Pt particle (Figure 1 (b) and (b’)) should be the main cause to be ascribed for the Zn1.0Pt0.1/HZSM-5 catalyst. Surprisingly, the deposit of small amount ferric species onto the Zn1.0/HZSM-5 before Pt modification makes the catalyst different. Ferric species itself has quite low propane transformation activity and aromatics selectivity, which can be easily seen from the performance of catalyst Fe0.3/HZSM-5. However, Zn1.0Fe0.3Pt0.1 /HZSM-5 catalyst shows not just the highest selectivity to aromatics and the lowest selectivity to dry gas by-product, but also very high propane transformation activity. These results clearly demonstrate a key role of Fe in the trimetallic catalyst. Table 2 also indicates that, zinc active sites have better dehydrogenation ability than the BrØnsted acid sites of HZSM-5 (see HZSM-5 and Zn1.0/HZSM-5). However, Pt

19



Page 20 of 36

Figure 6. Effect of reaction temperature on the performance of trimetallic Zn1.0Fe0.3Pt0.1/HZSM-5 (HZ is the abbreviated form of HZSM-5) catalyst and the reference catalysts evaluated by pulse microreactor (atmospheric pressure, flow rate of N2 carrier gas 20 mL s-1, propane pulse injection volume 0.10 mL.)

Table 2. Propane aromatization performance of Zn1.0Fe0.3Pt0.1/HZSM-5 catalyst and reference catalysts evaluated by pulse microreactor Product selectivity (wt%) Sample

X%-C3H8

H2

Dry Gas

Olefin

（CH4+C2H6）

(C2H4+C3H6+C4H8)

BTX

HZSM-5

9.82

1.05

31.25

58.28

6.14

Zn1.0/HZSM-5

34.62

3.48

8.63

43.70

43.85

Pt0.1/HZSM-5

69.47

4.37

30.32

29.21

35.75

Zn1.0Pt0.1/HZSM-5

58.10

4.30

26.99

30.37

38.64

Zn1.0Fe0.1Pt0.1/HZSM-5

54.82

4.05

10.43

43.63

41.89


56.42

4.11

2.68

37.88

55.32


60.24

4.42

4.87

39.81

48.90

Fe0.3/HZSM-5

22.02

1.46

17.11

50.78

30.65

Zn1.0Fe0.3/HZSM-5

43.58

1.81

14.44

44.07

39.01

Fe0.3Pt0.1/HZSM-5

49.54

5.37

7.55

58.24

28.31

20


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Reaction condition: T=550 ℃, atmospheric pressure, flow rate of N2 carrier gas 20 mL s-1, pulse injection volume 0.10 mL.

sites are much superior to zinc active sites with respect to propane dehydrogenation to H2 (see Zn1.0/HZSM-5 and Pt0.1/HZSM-5). Furthermore, the Pt-containing catalyst Pt0.1/HZSM-5, Zn1.0Pt0.1/HZSM-5 and Zn1.0Fe0.1~0.5Pt0.1/HZSM-5 all have similar hydrogen selectivity, regardless of Zn and Fe presence or not. This phenomenon implies that, the Pt sites, or to be more precisely, the FePt sites is the major dehydrogenation active sites in the trimetallic catalyst Zn1.0Fe0.3Pt0.1 /HZSM-5. Remember that the abovementioned HAADF-STEM/EDS characterization result assured the existence of FePt particles but excluded the presence of pure Pt particles in the trimetallic catalyst. It seems that Fe is a very important assistant ingredient in the trimatallic catalyst. The key role of Fe was played via promoting the dispersion of metals (especially noble metal Pt), forming FePt bimetallic particles and, most probably, donating electrons to Pt atoms. Operando DB-FTIR spectroscopy study was employed to know more about the roles of different metal components of the trimetallic catalyst in propane aromatization. To do this study, an operando spectroscopy system was built according to described method 47. Figure 7 shows that the surface dynamic processes of propane transformation over Zn1.0/HZSM5, Zn1.0Pt0.1/HZSM-5 and Zn1.0Fe0.3Pt0.1/HZSM-5 catalysts were completely recorded by the dual beam FT-IR spectrometer. The stripes caused by temperature-programmed reaction can be seen in all the three group of time-resolved spectrum. In addition, the difference of Zn1.0/HZSM-5 from Zn1.0Pt0.1/HZSM-5 and Zn1.0Fe0.3Pt0.1/HZSM-5 in C-H 21



stretching region (around 3000 cm-1) is obvious. These facts confirm that the time-resolved FT-IR spectra of the three catalysts belong to surface desorbed species. During the collection of surface information by the dual beam FT-IR spectrometer, the on-line Mass spectrometer was following the product changes at the outlet of the dual beam IR-cell reactor (Figure S5). Figure S5 confirms that the transformation of propane in the IR-cell reactor produced aromatics indeed. In other words, propane aromatization took placed during the operando FT-IR spectroscopy study. On account of high feed space velocity of propane (GHSV  8 h-1. Catalyst loading approximately 15 mg), considerable amount of propylene was also produced with aromatics. Results obtained by the online mass spectrometer indicate that the dehydrogenation aromatization performance of trimetallic catalyst Zn1.0Fe0.3Pt0.1/HZSM-5 is much superior to that of monometallic catalyst Zn1.0/HZSM-5. This conclusion, coincident with the results of pulse reactions, reflects that the IR-cell reactor was capable of distinguishing different aromatization catalysts. In general, both the IR spectra and Mass spectrometric profiles provided by the operando dual beam FT-IR spectrometer system are reliable.

22


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Figure 7. The dynamic surface processes of temperature-programmed propane aromatization over Zn1.0/HZSM-5 (a), Zn1.0Pt0.1/HZSM-5 (b) and Zn1.0Fe0.3Pt0.1/HZSM-5 (c) recorded by operando dual beam FT-IR spectrometer under real reaction conditions (50-400 °C, 0.1 MPa) in a flowing mixture of propane gas (1% propane in nitrogen, GHSV  8 h-1). The temperature program of 100 min operando FT-IR spectroscopy study was: 50 ºC (t = 0 min)150 ºC (10 min)150 ºC (20 min) 200 ºC (30 min) 200 ºC (40 min) 250 ºC (50 min) 250 ºC (60 min) 300 ºC (70 min)300 ºC (80 min) 350 ºC (85 min) 350 ºC (90 min) 400 ºC (95 min) 400 ºC (100 min).

23



Page 24 of 36

Figure 8. Selected surface operando FT-IR spectra of catalyst Zn1.0/HZSM-5(a), Zn1.0Pt0.1/HZSM 5(b), and Zn1.0Fe0.3Pt0.1/HZSM-5(c), see Figure 7 for more details.

Table 3. Assignments of the IR bands collected by the dual beam FT-IR spectrometer during an operando spectroscopy study of a temperature-programmed propane aromatization. Band (cm-1)

Assignment

Ref.

2983,2925,2887

Zinc methyl species

19, 20

2956,2875

Stretching vibration of CH3 group

18

2935,2850

Stretching vibration of CH2 group

18

2900

Stretching vibration of CH group

18

1585

Stretching vibration of a C=C bond linked to Zn

18, 20

1507

Stretching vibration band of benzene ring

18

1471,1395

Bending modes of CH2 group

20

1453,1375

Bending modes of CH3 group

20

The assignments of infrared bands are showed in Table 3. Figure 8 shows that both Zn1.0Pt0.1/HZSM -5 and Zn1.0Fe0.3Pt0.1/HZSM-5 have intensive and rich C-H stretching bands (2850-3000 cm-1). By contrast, there is almost no C-H stretching band in the spectra of Zn1.0/HZSM -5. These phenomena imply that there were abundant surface adsorbed species on catalyst Zn1.0Pt0.1/HZSM-5 and Zn1.0Fe0.3Pt0.1/HZSM-5, but there was little surface adsorbed species on catalyst Zn1.0/HZSM-5. These phenomena are also in line with the different propane transformation activity evaluated by the pulse reactions (Table 2). 24


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Additionally, by comparing Zn1.0Pt0.1/HZSM-5 and Zn1.0Fe0.3Pt0.1/HZSM-5 it is easy to find that the band of zinc methyl species (at 2983-2985 cm-1) of Zn1.0Fe0.3Pt0.1/HZSM-5 is much weaker than that of Zn1.0Pt0.1/HZSM-5. This phenomenon implies, on one hand, that there should be much more propane molecules being decomposed via hydrogenolysis on [ZnOZn]2+ sites of the Zn1.0Pt0.1/HZSM-5 catalyst owing to the difficulty of recombinative desorption of hydrogen atoms from Zn-O-Zn sites. On the other hand, the highly dispersed FePt bimetallic sites of the Zn1.0Fe0.3Pt0.1/HZSM-5 must have helped the [ZnOZn]2+ sites to break the strongly adsorbed hydrogen atoms away from them, consequently the hydrogolysis on [ZnOZn]2+ sites was largely avoided (thus less zinc methyl species were formed). Therefore, the band of zinc methyl species recorded by the dual beam FT-IR spectrometer is well matched with the dry gas selectivity obtained by the aforementioned pulse reaction. The effect of collaborative dehydrogenation of FePt sites with [ZnOZn]2+ sites in the trimetallic catalyst Zn1.0Fe0.3Pt0.1/HZSM-5 might be further seen in the bending vibration mode (1350-1700cm-1). Obviously, both Zn1.0Pt0.1/HZSM-5 catalyst and Zn1.0Fe0.3Pt0.1/ HZSM-5 catalyst have distinct adsorbed olefin band (1585 cm-1). However, only the trimetallic catalyst has 1507 cm-1 band which is related to benzene ring. This difference is also in line with the pulse reaction results (Table 2), and might account for the synergy between [ZnOZn]2+ Lewis acid sites and FePt metallic sites. The [ZnOZn]2+ Lewis acid sites are the major sites responsible for forming aromatics at last. However, in the bimetallic catalyst, due probably to the lingering of hydrogen atoms on them ([ZnOZn]2+ Lewis acid sites took part in the dissociative activation of propane, which 25



released propylene easily but had difficulty in hydrogen release), most of the active sites might be too congested to be used for aromatics formation. In the trimetallic catalyst, on other hand, probably because of the help of FePt, the [ZnOZn]2+ sites might be able to break the strongly adsorbed hydrogen atoms quickly and therefore most of the Lewis sites could engage in the final formation of aromatics. Based on the foregoing discussion, the synergy of [ZnOZn]2+ Lewis sites and FePt bimetallic sites in the initiative activation of propane is speculated (Schematic 1). According to this speculation, the aromatization mechanism over the trimetallic catalyst should still follow the well-accepted BrØnsted acid-Lewis acid synergetic mechanism if propylene is considered as true reactant.

Scheme 1. Speculated roles of FePt bimetallic active site in the metal- acid BrØnsted acid site (BAS) and Lewis acid site (LAS)) synergetic catalysis of propane aromatization over ZnFePt/ HZSM-5 catalyst

3.3 Excellent performance of Zn1.0Fe0.3Pt0.1/HZSM-5 catalyst shown by fixed-bed reactor 26


Page 26 of 36

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Since the trimetallic Zn1.0Fe0.3Pt0.1/HZSM-5 catalyst showed superior activity and selectivity in pulse reaction, we are eager to know if it has practical significance. Therefore, the trimetallic catalyst was subjected to a systematical study on a continuous flow small fixed-bed reactor. First, the effects of reaction conditions were investigated (Figure 9). Then, an as long as 900 hours durability examination was carried out (Figure 10). Results show that, under suitable conditions (550°C, 0.1Mpa, propane WHSV = 0.55 h-1), Zn1.0Fe0.3Pt0.1/HZSM-5 catalyst demonstrated approximately 50 wt.% propane conversion and 52-55 wt.% aromatics selectivity. Taking under consideration of the carrier gas influence in the pulse reaction, it is

Figure 9. Effect of reaction temperature (0.1Mpa, 0.55h-1), propane weight-hourly-space-velocity (550℃, 0.1Mpa) and reaction pressure (550℃, 0.55h-1) on the propane aromatization performance of trimetallic Zn1.0Fe0.3Pt0.1/HZSM-5 catalyst evaluated by small fixed-bed reactor

27



Figure 10. Durability of trimetallic Zn1.0Fe0.3Pt0.1/HZSM-5 catalyst in propane aromatization evaluated by fixed-bed reactor under condition of 550°C, 1 atm and propane WHSV = 0.55 h-1.

reasonable to think that both propane conversion and aromatics selectivity are identical with those obtained by pulse reaction. There is no doubt that trimetallic catalyst has very good activity and selectivity indeed. Perhaps the very impressive durability of the trimetallic catalyst is another attraction for practical use. The prominent anti-coking deactivation capability of the trimetallic catalyst allows propane aromatization to be implemented in commercial fixed-bed reactor, which is cheaper, more simple and reliable than moving-bed reactor used by previous Cyclar process48.

4. Conclusions In summary, a highly active and selective trimetallic catalyst Zn1.0Fe0.3Pt0.1/ HZSM-5 catalyst for propane dehydrogenation aromatization was fabricated by successively impregnating Fe and Pt onto a Zinc modified nanometer /HZSM-5 zeolite (Zn/HZSM-5). The trimetallic catalyst was found having not only BrØnsted acid sites and Lewis acid sites, but also metallic sites like ferric species alone and FePt bimetallic species. Both pulse 28


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reaction and operando FT-IR spectroscopic studies revealed that the trimetallic catalyst was superior to Zn/HZSM-5 and ZnPt/HZSM-5 catalysts with respect to promoting propane dehydrogenation aromatization and inhibiting propane hydrogenolysis to dry gas by-product. Results suggested that the FePt bimetallic sites had excellent dehydrogenation ability. This function should have be helpful for the propane dehydrogenation aromatization. In addition, the FePt bimetallic sites might also involve in promoting the recombinational desorption of surface hydrogen atoms from [ZnOZn]2+ sites. This function of the bimetallic sites could account for the remarkably reduced dry gas production over the trimetallic catalyst Zn1.0Fe0.3Pt0.1/ HZSM-5. Therefore, the propane dehydrogenation aromatization over the trimetallic catalyst was considered to be proceeded by the synergetic catalysis of Lewis acidic [ZnOZn]2+ sites, BrØnsted acidic (AlOHSi) sites and metallic FePt sites. AUTHOR INFORMATION Corresponding Author E-mail: [email protected] Notes Any additional relevant notes should be placed here. ACKNOWLEDGMENT

29



Page 30 of 36

This work is financially supported by the National Natural Science Foundation of China (NO. 21603023).

Supporting Information: SEM image of ZSM-5 with a Si/Al ratio of 15; NH3 adsorbed FT-IR spectra of Zn1.0Fe0.3Pt0.1/HZSM-5 catalyst and its references; Zn particles dispersion image and size distribution provided by TEM; Metal particles dispersion images; On-line

mass

spectra

of

propane

aromatization

on

Zn1.0/HZSM-5

and

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Enhanced dehydrogenative aromatization of propane by incorporating

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