Ethane Aromatization over Zn-HZSM-5: Early Stage Acidity

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Kinetics, Catalysis, and Reaction Engineering

Ethane Aromatization over Zn-HZSM-5: Early Stage Acidity/ Performance Relationships and Deactivation Kinetics Tingyu Liang, siavash fadaeerayeni, Junjun Shan, Tao Li, Hui Wang, Jihong Cheng, Hossein Toghiani, and Yizhi Xiang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b03012 • Publication Date (Web): 02 Sep 2019 Downloaded from pubs.acs.org on September 2, 2019

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Industrial & Engineering Chemistry Research

Ethane Aromatization over Zn-HZSM-5: Early Stage

Acidity/Performance

Relationships

and

Deactivation Kinetics Tingyu Liang,1 Siavash Fadaeerayeni,1 Junjun Shan,2 Tao Li,3,4 Hui Wang,2 Jihong Cheng,2 Hossein Toghiani,1 and Yizhi Xiang1* 1

Dave C. Swalm School of Chemical Engineering, Mississippi State University, Starkville, MS

39762, USA 2

NICE America Research, Inc., Mountain View, CA 94043, USA.

3

Department of Chemistry and Biochemistry, Northern Illinois University, DeKalb, IL 60115,

USA.

4

X-ray Science Division, Argonne National Laboratory, Lemont, IL 60439, USA.

*

Corresponding authors. Tel.: +1 662-325-0037; Fax: +1 662-325-2482. E-mail address:

[email protected] (Y. Xiang)

1 ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT: Catalytic conversion of ethane to aromatics (BTX) over metal/HZSM-5 catalysts involve significant catalyst deactivation due to coking. Consequently, true acidity/performance relationships escaped if the intrinsic catalyst acidity was correlated to the “steady-state” performance. Here the effect of acidity on the early-stage performance and time-dependent deactivation kinetics has been investigated. The early-stage ethane conversion and BTX selectivity both increased with decreasing Si/Al2 ratio. Specifically, the space-time yields of BTX increase linearly with increasing Brønsted acidity, indicating Brønsted acid as the main active sites for BTX. Further evidence can be found from the transient experiment (C2H6/Ar ↔ C2H6/NH3) and C2H4-TPSR. A promotion effect of the Zn (II) sites (mainly responsible for ethane dehydrogenation) on BTX formation was also observed. With time-on-stream, the catalytic performance attenuated due to coking, which can be modeled as “r(t) = r0/(1 + ktα)” kinetically and the parameters (for aromatics) k decrease and α increase with decreasing acidity. KEYWORDS:

Ethane

aromatization,

Zn-H-ZSM-5,

Early-stage,

relationships, Deactivation kinetics


Acidity/performance

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1. INTRODUCTION

According to the US Energy Information Administration, the production of natural gas in the United States will continue to grow during the coming decades. As cheaper and abundant natural gas compounds continuously flooding the market, the feedstock for chemicals and energy production is shifting from oil to gas.1 Upgrading of the natural gas compounds into the valueadded olefins and aromatics, therefore, has become of great industrial importance. Although natural gas contains a significantly larger fraction of methane (about 80-90%) than the other light alkanes (known as natural gas liquids (NGLs)), the latter has so far had more influence on the chemical industry. For example, the largest component of the NGLs, ethane, can be easily converted into ethylene via steam cracking, which, however, is a highly endothermic reaction and requires a high reaction temperature (up to 900°C) and a short contact time (milliseconds) to maintain high ethylene selectivity. Therefore, a catalytically driven aromatization process, which produces liquid transportable BTX (benzene, toluene, and xylene), appears to have significant benefits for the chemical industry due to the reduced energy use, capital costs, and carbon emissions.

Early research about the formation of aromatic BTX from light (C3-C5) alkanes was pioneered by Csicsery who performed the reaction over a Pt/Al2O3 catalyst and referred the



process as “dehydrocyclodimerization”.2-6 However, the selectivities to aromatics were low and the products spectra were extremely complex due to the lack of shape selectivity for the Al2O3. Later, the MFI type ZSM-5 (the 10-member ring pore channels of the ZSM-5 are very close to the size of a benzene molecule) based catalysts were found to be ideal for the production of BTX from various feedstocks, such as light paraffins,7-10 olefins,11 and alcohols.12, 13 With respect to the ethane aromatization, the HZSM-5 modified with Pt,14-17 Zn,18-22 Ga,23-27 Re,28,29 and Mo30-32 catalysts have been found to be active. Nevertheless, the yield of aromatics and stability of the catalyst require further improvements. The catalyst deactivation due to coking is inevitable in such high-temperature non-oxidative process, which actually has been a common industrial obstacle with zeolites catalyzed hydrocarbon processing (hence, call for the development of deactivation kinetic models for the purpose of process optimization). Among various catalysts investigated in literature, Pt and Zn have shown more efficiency. While the Pt-based catalyst has the advantage of high activity (for dehydrogenation), it shows higher selectivity towards methane (formed through hydrogenolysis or cracking) than the Zn-based catalyst. Here, we are focusing on the Zn modified HZSM-5 for the ethane aromatization and will demonstrate specifically the effect of Brønsted acidity on this catalytic system.

The acidity (especially the Brønsted acidity) of the catalyst is definitely essential for the catalytic aromatizations. In the ZSM-5 zeolite, the incorporation of aluminum into the SiO4 4 ACS Paragon Plus Environment

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tetrahedron causes a negative lattice charge, which is balanced with a proton (H+) or a metal cation, where the former is a Brønsted acid and the latter is a Lewis acid. Therefore, the acidity of the catalyst can be tuned through varying the Si/Al2 ratios of the ZSM-5. Different from the Pt modified HZSM-5 catalyst on which the initial ethane dehydrogenation supposedly to be driven by the Pt (0) nanoparticles,16,

17

the exchanged Zn (II) metal cations (or to less extent ZnO

nanoclusters) seems to be responsible for the initial ethane dehydrogenation on the Zn/HZSM-5 catalyst. To this issue, Mehdad and Lobo21 did a systematic study on the effect of Lewis and Brønsted acid sites density on the aromatization of ethane and ethylene. They suggested that Zn in the Lewis acid site position is active for aromatization, but ZnO clusters can only catalyze dehydrogenation. The authors also suggested that the balance between the number of Zn as Lewis acid and the remaining Brønsted acid sites to be important for ethane aromatization: the formation of ethylene is favored when Zn/BAS < 0.8.21 It is most likely that ethane aromatization is directed by dual-functional mechanism: exchanged metal cation (single Lewis acid site) or metal nanocluster is responsible for the initial ethane dehydrogenation, whereas the synergistic effect of Brønsted acid site and Lewis acid pairs (Brønsted/Lewis acid synergy) catalyzed the oligomerization and cyclization. Quite similar to ethane aromatization, Zn sites were also found to be important in the propane aromatization over the Zn/HZSM-5. Based on the isotopic analysis according to Biscardi and Iglesia,33 they suggested that the Zn sites facilitated the 5 ACS Paragon Plus Environment


desorption of hydrogen after the initial C-H bond activation on the Brønsted acid sites. Although the effect of Lewis and Brønsted acidity on the aromatization of light alkanes has been extensively studied, here we would like to provide new insight into this issue based on the earlystage acidity/performance relationships. We suggest that the true relationships between the (Lewis and Brønsted) acidy and the catalytic performance might be concealed due to inevitable fast coking on the acid sites. In the present paper, we are focusing on the effect of acidity (tuned by varying the Si/Al2 ratios from 30 to 80) on the early-stage (before significant coking) catalytic performance, as well as the time-dependent long-term deactivation kinetics during ethane aromatization on the zinc exchanged HZSM-5 catalysts. The physicochemical properties of the catalysts were extensively characterized by N2 physisorption, scanning transmission electron microscopy (STEM-EDS), and in-situ X-ray absorption near edge structure (XANES). Specifically, the total acid density and Brønsted acidity of the catalysts were quantified by the ammonia temperature-programmed desorption (NH3-TPD) and n-propylamine temperature-programmed decomposition (C3H7NH2TPDec). Then, a more detailed relationship between the Brønsted acidity and the early-stage catalytic performances was discussed. On the basis of the transient response of various products after fast partial pressure change from C2H6/Ar (50%/50%) to C2H6/NH3 (50%/50%) and vice versa, the effect of acidity on the mechanistic reaction steps involved in ethane aromatization 6 ACS Paragon Plus Environment

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was also discussed in detail. The effect of Zn (II) Lewis acid site on the ethane aromatization was demonstrated through comparing the ethylene-temperature programmed surface reaction (C2H4TPSR) profiles between the Zn-HZSM-5 and HZSM-5. Additionally, the deactivation (due to coking) kinetic model was developed and employed to understand the time-dependent process for products attenuation. 2. EXPERIMENTAL SECTION

Catalyst Preparation. The NH4-ZSM-5 zeolites with different Si/Al2 ratios (30, 50, and 80) were purchased from VWR International. The constraint index of the zeolites are between 1.31.7 according to the n-hexane and 3-methylpentane cracking, which indicating the acid sitting for the three zeolites is similar and mainly located at the channel intersections. All of the rest chemicals were purchased from Sigma-Aldrich. All of the chemicals were used as obtained. Znmodified HZSM-5 (Si/Al2 = 30, 50, and 80) catalysts were prepared through aqueous-phase ionexchange. Typically, 2.5 g of NH4-ZSM-5 zeolite was ion-exchanged with 0.05M zinc nitrate aqueous solution at 80°C for 7 h and repeated for three times. The obtained slurry was centrifuged and washed with water three times. The obtained sample was finally dried at 120oC overnight and calcined in air at 550oC for 6 h.



Catalyst Characterization. NH3-TPD and C3H7NH2-TPDec experiment was performed in a quartz reactor with a volume of 2 ml (ID, Φ = ½ inches). Prior to NH3 and n-propylamine preadsorption, The sample (0.1 g for NH3-TPD and 0.06 g for C3H7NH2-TPDec) was first activated in 10% H2/Ar at 600oC, then decreased to 120oC for NH3 or n-propylamine adsorption (under flowing of pure NH3 for 30 min or through pulsing n-propylamine injection until saturation). For NH3-TPD, the influent gas was then switched from NH3 to Ar (20 ml/min) at the same temperature and kept for 2 h to remove physically adsorbed NH3. Finally, the temperature of the sample was increased from 120 to 620oC at a ramp of 10oC/min under flowing of Ar at 20 ml/min. Desorption of NH3 was measured by an online mass spectrometer (Agilent 5973). For C3H7NH2-TPDec, the temperature of the pre-adsorbed sample increased to 200oC and kept for 2 h under flowing of Ar at 20 ml/min to remove physically adsorbed C3H7NH2. Finally, the temperature of the sample was increased from 200 to 550oC at a ramp of 10oC/min under the same flowing of Ar. Desorption of NH3 (m/z=17), C3H6 (m/z=41), benzene (m/z=78), toluene (m/z=92), and xylene (m/z=106) were measured by an online Agilent 5973mass spectrometer (EI source, Mass Range 10-800 amu) equipped with MS Sensor 2.0 software (Multi-Sensor Process Analysis Data System for Real-time Gas Analyzers, Diablo Analytical, Inc.).

The BET specific surface of the catalyst was measured by Micromeritics 3Flex High Resolution, High-throughput Surface Characterization Analyzer. The sample was degassed at 8 ACS Paragon Plus Environment

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350oC for 3 h prior to N2 physisorption at −196oC. Transmission electron microscopy (TEM) images and STEM-EDX chemical mapping of the sample were obtained using a JEOL 2100TEM (accelerating voltage 200kV) equipped with Gatan camera. The chemical state of zinc was characterized by in-situ X-ray absorption near edge structure (XANES) experiments conducted at the Advanced Photon Source (APS) beamline 12-ID using a Vortex detector; the sample was set at a 90 degree angle to the incoming beam and detector. Powder samples were pressed to disk and mounted in the Linkam stage equipped with a gas supply. Spectra were recorded for Zn K edge at room temperature under Ar flow, at 500oC under 10% H2/Ar, and at 500oC under 10% C2H4/Ar for 30 min, respectively. The obtained raw data were analyzed by Athena; the absorption signals were then normalized. Catalytic Performance Evaluation. The catalytic performance of zinc exchanged H-ZSM-5 catalysts for ethane aromatization was performed in a home-built flow reactor system equipped with an online Agilent 5973 mass spectrometer. Before catalytic tests, 0.1 g of the catalysts were pretreated in 10% H2 (20 ml/min) at 600oC (ramp 10 oC/min) for 30 min and then switched to pure Ar (20 ml/min) for 10 min at the same temperature so as to remove the residue H2 adsorbed on the surface of the catalyst. Prior to each experiment, bypass feed spectra were recorded for mass spectrometer calibration and used as a reference for activity calculation. Finally, the



reaction was initiated by abrupt switching of the influent gas from Ar to C2H6 (20 ml/min) at 550oC under atmospheric pressure at a gas hourly space velocity (GHSV) of 8000 h-1. All the connection gas lines between the reactor and mass spectrometry were heated to 150oC to avoid the condensation of liquid products (the boiling points of benzene, toluene, and xylene are 80.1, 110.6, and 138.4oC, respectively). A more detailed method for products quantification with the mass spectrometer has been shown in our previous paper.22 The conversion of ethane was calculated based on:

x=

Fethane,

in

― Fethane,

Fethane,

out

× 100%

in

The carbon-based selectivity to each product was calculated based on:

Si =

The amount of ethane converted into product i × 100% Total amount of ethane reacted Fi, out × 𝑛 = × 100% (Fethane, in ― Fethane, out) × 2

where the n represents the carbon number in the molecular of the product i.

The space-time yield of different products was calculated based

𝑆𝑇𝑌𝑖 =

𝐹𝑖 𝑚𝐶𝑎𝑡

where mCat is the mass of catalyst used for the reaction.

TOF =

𝑆𝑇𝑌 𝐶Brønsted acid 10

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where CBrønsted acid is the concentration of Brønsted acid in the catalyst (mol/gCat).

The reliability of the calibration and calculation was indicated by the small deviation of the total carbon balance (ΔC=

|

∑𝐶𝑖𝑛 ― ∑𝐶𝑜𝑢𝑡 ∑𝐶𝑖𝑛

| ≤ 5%).

Transient Response Experiment. The transient response experiment was performed on the same home-built flow reactor system. 0.4 g of the zinc exchanged HZSM-5 (Si/Al2 = 50) catalyst was activated according to the same procedure mentioned for catalytic performance evaluation. Transient response of various products from steady-state ethane aromatization to ethane “aromatization” with NH3 co-feed and vice versa at 550oC and GHSV 2000 h-1 were evaluated. Experimentally, the partial pressure of the gas flow was abruptly switched from C2H6/Ar (50%/50%) to C2H6/NH3 (50%/50%) and vice versa at a constant total flow rate 40 ml/min. Replacing Ar with NH3 in the gas flow will poison the acid sites of the catalyst through strong chemisorption of NH3, which will deactivate the catalyst on one hand, and will promote the desorption of the chemisorbed reaction intermediates on the other hand. Switching back to C2H6/Ar (50%/50%) provides information about the time-dependent catalytically active acid sites recovery through NH3 desorption. It might be expected that the weak acid site can be immediately recovered after removing NH3 from the reactor flux, but the strong acid site



(Brønsted acid) takes a longer time to recover due to the strong chemisorption of NH3 on the strong acid sites of the catalyst. Temperature programmed surface reaction. C2H4-TPSR experiment was also performed on the same home-built flow reactor system. 0.1 g of the HZSM-5 and Zn-HZSM-5 (Si/Al2 = 30) catalyst was activated according to the same procedure mentioned for catalytic performance evaluation. Then the temperature of the reactor was cooled to 50oC before introduce 10% C2H4/Ar to the reactor at 30 ml/min. Finally, the temperature of the sample was ramped at 10oC/min to 630oC. The obtained results were evaluated according to the same method mentioned for catalytic performance evaluation. 3. RESULTS AND DISCUSSION

Catalysts characterization The physicochemical properties of the zinc exchanged HZSM-5 catalysts with different Si/Al2 ratios were characterized by N2 physisorption, STEM-EDX, in-situ XANES, NH3-TPD, and C3H7NH2-TPDec. As shown in Table 1, the BET specific surface areas of the catalysts are all around 320 m2/g, which is about 100 m2/g lower than the host HZSM-5. STEM-EDX elements mappings of the zinc exchanged HZSM-5 (Si/Al2 = 30, 50, and 80) catalysts are shown in Figure 1. A comparison of the Zn Kα and Al Kα mappings demonstrated the homogeneous


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distribution of the zinc specious on the zeolite. Aggregated ZnO nanoparticles were not observed from the TEM, which suggested that the exchanged zinc (II) cations (i.e., Zn (II) bonded to the zeolite framework aluminum (AlF) to be responsible during ethane aromatization. Quantitative analysis of the EDX spectrum suggested that the zinc loadings are around 1 wt% for samples with Si/Al2 ratios of 50 and 80 and 1.3 wt% for the sample with Si/Al2 ratios of 30. In other words, the Zn/Al atomic ratio increase from 0.17 to 0.24 to 0.34 with an increase of Si/Al2 ratios from 30 to 50 to 80. More detailed chemical properties of the zinc species can be found from further in-situ XANES characterization (see Figure 2) as well as the EXAFS results of the same catalyst reported in the literature. According to Biscardi et al.,34 the main peak in EXAFS for the exchanged Zn/HZSM-5 catalyst is at 1.6 Å, indicating the presence of Zn-O nearest neighbors. The peak at 3.0 Å, which corresponding to Zn-Zn next nearest neighbors in ZnO was not observed, hence, further indicating that in our catalyst (prepared through ion-exchanged as the literature) the exchanged zinc (II) isolated cations dominates the ZnO crystallites. We suggested that different exchanged Zn (II) sites, such as “…Al-O-Zn(OH), …Al-O-Zn-O-Zn-O-Al…” and “…Al-O-Zn-O-Al…” might be formed.34 The effect of Si/Al2 ratio doesn’t seem to affect the homogeneity of Zn distribution according to the STEM-EDX, however, it is anticipated to affect the formation of different Zn (II) Lewis acid sites since the ratio of Si/Al2 could affect the concentration of paired Al sites. 13 ACS Paragon Plus Environment


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Table 1. Physicochemical properties of HZSM-5 and zinc exchanged HZSM-5 catalysts. Zn loading Catalysts

Si/Al2

Total acid

Brønsted acid

BET surface

(μmolNH3/g)

(μmol/g)

area (m2/g)

Zn/Al (wt%)

Zn-ZSM-5

30

1.3

0.17

890

279

313

HZSM-5

30

/

/

1225

531

400

Zn-ZSM-5

50

0.9

0.24

550

246

311

HZSM-5

50

/

/

735

387

425

Zn-ZSM-5

80

1.0

0.34

515

187

320

HZSM-5

80

/

/

606

224

425

Figure 1. STEM-EDX chemical mapping of Al and Zn Kα for zinc exchanged HZSM-5 with different Si/Al2 ratio. Results of the XANES measurements were performed at either room temperature or 500oC during reduction or reaction. The normalized spectra (Zn K edge) of Zn-ZSM-5, the spectra of


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fresh, reduced, and after reaction samples were obtained under Ar flow at room temperature, 10% H2/Ar flow at 500oC for 30 min, and 10% ethylene/Ar flow (ethylene is an important intermediate during ethane aromatization) at 500oC for 30 min, respectively. From a comparison with Zn foil spectrum, the adsorption edge of Zn-ZSM-5 shifted to higher energy, which implies the presence of positively charged zinc species.35 Since the 3d-subshell in the Zn (II) system is completely filled, it is not surprising that the pre-edge spectra originated from 1s→3d transition is absent in the K-edge spectra. Upon high-temperature treatment in 10% H2 at 500oC, the adsorption edge slightly shifted to lower energy, and remain largely unchanged after reaction (ethylene aromatization) for 30 min at the same temperature (Figure 2 (A)). The edge shift denotes the formation of partially reduced zinc species or the decreased oxidation number of absorbing atoms, the electronegativity of ligands and number of ligands.36, 37 Additionally, upon high-temperature treatment, the main peak splits into two peaks35 and the white line decrease (edge widths decrease), which indicated the transition of Zn species from octahedral to tetrahedral coordination due to dehydration (see Figure S1 for in-situ XANES spectra at different temperatures under 10% H2/Ar).36 In Figure 2 (B)-(D), the in-situ XANES spectra for the ZnZSM-5 catalysts with different Si/Al2 ratios exhibit only slight decrease of the white line with increasing Si/Al2 ratios. After high-temperature reduction and reaction, the adsorption edge doesn’t change with respect to the Si/Al2 ratios for both fresh catalysts and the catalysts, which 15 ACS Paragon Plus Environment


indicate that chemical state of zinc species on the HZSM-5 catalysts remains the same in spite of the changes of the Si/Al2 ratios. 1.5

Normalized absorption(a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Si/Al2=30

(A)

(B)

Fresh

1.0

0.0 1.5

Zn foil Si/Al2 = 30

Zn foil Fresh Reduced After reaction

0.5

(C)

Reduced

Si/Al2 = 50 Si/Al2 = 80

(D)

After reaction

1.0 0.5 0.0 9630

Zn foil Si/Al2=30

Zn foil Si/Al2=30

Si/Al2=50

Si/Al2=50

Si/Al2=80

9660

9690

9720

Si/Al2=80

9630

9660

9690

9720

Energy (eV)

Energy (eV)

Figure 2. Normalized in-situ XANES spectra (Zn K edge) of Zn exchanged HZSM-5 (with different Si/Al2 ratios) catalysts. (A) Comparison of fresh, reduced and after reaction samples of Si/Al2 = 30 with Zn foil; (B)-(D) Comparison of samples with different Si/Al2 ratio for the fresh (B), reduced (C), and after reaction (D), which were obtained under Ar flow at room temperature, 10% H2/Ar flow at 500oC, and 10% C2H6/Ar flow at 500oC for 30 min, respectively. The acidity of the HZSM-5 and Zn-ZSM-5 (with different Si/Al2 ratios) catalysts were characterized by both NH3-TPD and n-propylamine TPDec (see Figure S2 for the detailed NH3 desorption and n-propylamine decomposition profiles for Zn-ZSM-5). Generally, the host HZSM-5 shows higher numbers of total acid sites and Brønsted acid sites, indicating the 16 ACS Paragon Plus Environment

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exchange of zinc (II) cations. From the NH3-TPD profiles, two distinct NH3 desorption peaks are identified at 278 and 467oC, respectively, where the former represents the weak acid sites while the latter was responsible for strong acid sites. As shown in Table 1, with increasing Si/Al2 ratios, the total acid density decrease. The total amounts of NH3 desorbed from the catalyst during the course of TPD was quantified to be 890, 550, and 515 μmol/gcat for Si/Al2 = 30, 50, and 80, respectively. The Brønsted acid density of the catalysts was determined by quantifying the NH3 desorbed from n-propylamine decomposition. With this method, propylamine is protonated with the Brønsted acid site to form “C3H7NH3+----ZSM-5−”, which will decompose to NH3 and propylene via a reaction similar to the Hofman-elimination in a well-defined temperature range.38-41 Though quantification of olefins were typically employed to count the Brønsted acid sites, we suggested that the quantification based on NH3 rather than the olefins, because olefins (propylene) produced from the alky-amine decomposition could inevitably be converted to secondary products, such as higher olefins, aromatics, and even cokes over the zeolites (see Figure S2), making the quantification process complex. Based on the formation of NH3 during C3H7NH2-TPDec, the Brønsted acid density of the catalysts was quantified to be 279, 246, and 187 μmol/gcat for the catalysts with Si/Al2 = 30, 50, and 80, respectively. The temperature for the formation of propylene is between 400-450oC, which is the same as in the literature.41 However, the temperature for NH3 formation was shifted to the low temperature, which suggested that the 17 ACS Paragon Plus Environment


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real decomposition temperature is ⁓350oC over the zinc exchanged HZSM-5 catalyst, lower than the pure zeolites. Si/Al2=30

30

Si/Al2=50

Ethane conv. (%)

20

0 15

Ethylene sel. (%)

5

30

0 Butylene sel. (%)

Benzene sel. (%)

30

6

15

3 0 Toulene sel. (%)

30 15 0

Propylene sel. (%)

10

60

9

Methane sel. (%)

30 10

10 0 90

Si/Al2=80

40

0

40

80

120

160

0 4 3 2 1 0

t (min)

Xylene sel. (%)

0

40

80

120

160

t (min)

Figure 3. Time-resolved catalytic activity (conversion) and selectivity of ethane aromatization over Zn modified HZSM-5 catalysts. Reaction was performed over 0.1 g of catalyst (GHSV = 8000 h-1) at 550oC under atmospheric pressure. Effect of Si/Al2 ratio (acidity) The effect of Si/Al2 ratio on the catalytic performance with respect to ethane conversion and products selectivity in ethane aromatization over zinc exchanged HZSM-5 catalysts were shown in Figure 3 (see also Figure S3 for the early-stage (0-5 min) catalytic behavior). Generally, all of the Zn exchanged HZSM-5 catalysts are active in ethane aromatization, the early-stage (initial 218 ACS Paragon Plus Environment

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5 min) ethane conversion is between 10-20% and the selectivities to benzene and toluene are both around 20-30%. The selectivity to methane is below 10% and the selectivities to xylene, propylene, and butylene are all below 5%. The sum selectivity to aromatics and olefins is more than 90%. More detailed activity and products selectivity are depending highly upon the Si/Al2 ratios (acidity). With increasing Si/Al2 ratios from 30 to 80, the initial ethane conversion is decreased from 20% to 13%; accordingly, the selectivities to benzene, toluene, xylene, and methane are also clearly decreased (ethylene selectivity was increased). However, the selectivities to propylene and butylene remain the same for all of the investigated catalysts (also do not change with time-on-stream), which suggested that the formation of propylene and butylene might be limited by the thermodynamics. Since the increase of Si/Al2 ratios corresponds to a decrease of the acidity (see Table 1), it is not surprising that the selectivity to aromatics decreases (and ethylene selectivity increases) with increasing Si/Al2 ratios because the conversion of ethylene to aromatics requires the synergistic effect of Brønsted acid sites and Lewis acid sites. In order to demonstrate the relationships between the acidity and catalytic performance, the quantitative information of the acid density of the zinc exchanged HZSM-5 (with different Si/Al2 ratios) were then correlated with the early-stage (first few seconds) space-time yields (STYs) of the products. As a result, the effect of coking induced catalysts deactivation can be eliminated, 19 ACS Paragon Plus Environment


therefore, the results demonstrated the true relationships of acidity/performance. As shown in Figure 4, linear relationships between the Brønsted acid density and the STYs for benzene, toluene, and xylene were observed. The early-stage turnover frequency (TOF) for the formation of BTX calculated on each Brønsted acid site are 0.26 min-1 for benzene, 0.25 min-1 for toluene, and 0.026 min-1 for xylene, which are largely independent of the total acidity (Si/Al2 ratio) and the Lewis acidity (Zn/Al ratio). It is also important to mention that no clear connection between the early-stage STYs of BTX and the density of the total acid sites and Lewis acid sites were observed. While these results clearly suggested the importance of the Brønsted acidy on the formation of BTX during ethane aromatization, we can’t exclude the synergistic effect between the Brønsted and Lewis acid on the aromatization under current conditions. 80 70

70

Benzene

50

50 7.5

Xylene

Methane

300

6.0

150

4.5 500

Toluene

60

60

STY (mol/gcat/min)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0 32

Ethylene

400

28

300

24 180 200 220 240 260 280

Propylene

180 200 220 240 260 280

Bronsted acid density (mol/gcat)


Page 20 of 38

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Figure 4. Relationships between the early-stage catalytic performance (maximum space-time yield during the first few seconds) and the Brønsted acid density. Error bars were obtained from the deviation of the mass spectrometer signal within 5 s. The relationships between the Brønsted acid density and the early-stage STYs of methane, ethylene, and propylene are also shown in Figure 4. The STY of methane increased from 78 to 298 μmol/gcat/min and the STY of ethylene decreased from 466 to 322 μmol/gcat/min with increasing Brønsted acid density from 187 to 279 μmol/gcat. However, the STY of propylene (27 μmol/gcat/min) remains largely independent of the Brønsted acid density. The nonlinear increase of the methane STY with increasing Brønsted acid density suggested that different types of active acid sites and mechanisms might be involved for the methane formation. Our recent transient kinetic study suggested that methane is originated from ethylene rather than ethane through either hydrogenolysis (C2H4 + 2H2 → 2CH4) or cracking (C2H4 → C + CH4).22 Since ethylene and higher olefins (produced from the ethylene oligomerization) are important direct precursors for aromatics,22 the decreased ethylene STY (or unchanged propylene STY) with increasing Brønsted acidity could be rationalized by the enhanced consumption of ethylene (or propylene) for the formation of aromatics and methane. Both ethylene and propylene can be considered as intermediate products for reactions in series, where for propylene the rates of formation and consumption both positively related to the Brønsted acidity. Indeed, in a case of 21 ACS Paragon Plus Environment


all the carbons in the other products were balanced to ethylene, an increase of the STY of “total ethylene” with increasing Brønsted acidity will be observed, which, however, doesn’t really indicate that the formation of ethylene from ethane dehydrogenation directly involves the Brønsted acid sites (the catalytically active site involved for ethylene formation will be discussed later). The equilibrium conversion of ethane dehydrogenation at 550oC is only 8.8%, the formation of BTX and methane (from ethylene) will certainly increase the STY of “total ethylene” by breaking the thermodynamic equilibrium limit. In order to further discuss the effect of acid function on the ethane aromatization over the zinc exchanged HZSM-5 catalyst, a transient experiment using ammonia co-feed has been performed over the catalyst with Si/Al2 = 50. The transient responses of light hydrocarbons and BTX after fast partial pressure change from C2H6/Ar (50%/50%) to C2H6/NH3 (50%/50%) and vice versa are shown in Figure 5. After switching from C2H6/Ar to C2H6/NH3 (see Figure 5 (A)), the signal of Ar attenuates immediately (indicate the reactor system response), but NH3 takes more than 50 s to appear in the gas phase, which suggested that all of the NH3 was chemisorbed (“poison” the acid sites of the catalyst) during the first 50 s. It takes about 90 s for the dynamic NH3 adsorption/desorption to reach equilibrium. Although the temperatures for reaction (550oC) is higher than for NH3-desorption (278 and 467oC according to the NH3-TPD), the strong chemisorption of NH3 on both acid sites can easily compete the chemisorption of other species. 22 ACS Paragon Plus Environment

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Page 23 of 38

Significant transient responses of methane, ethylene, and BTX are observed during 10-100 s. Both methane and BTX first reach a climax, then follow by decay at apparent different time constants. After switching, the formation of initial peaks originated from the desorption of the surface chemisorbed intermediates, which was attributed to their failed competition with the strong chemisorption of NH3 on the catalyst. The acid sites (both Brønsted and Lewis acid) of the catalyst were selectively “poisoned” due to the NH3 chemisorption, which resulted in the decay of the products. All of the products were then completely absent from the spectra after 120 s.

NH3

2.0

C2H4 Benzene Toluene Xylene

1.5

1.0

0.5

0.0

Ar

Normolized outlet flow (a.u.)

CH4

(A)

Normolized outlet flow (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(B)

1.0

Ar

0.8 NH3

0.6

C2H4 C3H6

0.4

C4H8 Benzene Toluene Xylene

0.2 0.0

0

20

40

60

80

100

120

0

Time (s)

5

10

15

20

25

30

Time (min)

Figure 5. Transient response of various products (light hydrocarbons and BTX) during ammonia co-feed experiment over the Zn-ZSM-5 (Si/Al2 = 50) catalyst. The time axis is set to zero at the point which the reactor flux is switched from C2H6/Ar to C2H6/NH3 (A) and C2H6/NH3 back to



C2H6/Ar (B). The signal of Ar (green) in both experiments represents the characteristics of the reactor response. Remove NH3 from the reactor, namely, switching from C2H6/NH3 (50%/50%) back to C2H6/Ar (50%/50%), shows how the active sites (acid sites) recover. As shown in Figure 5 (B), the first effluent product is ethylene, which appears in the gas phase within a minute after switching and requires 7 min to be completely recovered. The immediate recovery of the active site for ethylene formation suggested that the weak acid sites (related to Lewis acid) are involved in ethane dehydrogenation. Then ethylene, propylene and butylene appear simultaneously in the gas after about 3 min and completely recover after about 25 min. Therefore, we suggest both weak (related to Lewis acid) and strong (related to Brønsted acid) acid sites are involved in the formation of higher olefins from the oligomerization of ethylene. They might function either independently or synergistically. Finally, the BTX appear in the gas phase, simultaneously, after more than 5 min and continues to recover almost linearly with time-on-stream, which once again suggested that the active sites for converting olefins to BTX are highly related to the Brønsted acids. However, the effect of Lewis acid sites on the BTX formation can not be completely ruled out, as will be discussed later, the Zn (II) Lewis acid sites show promotion effect. It must be noted that the transient period (up to 30 min) is significantly longer than our previous transient


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kinetic study where the reactor flux was switched from Ar to ethane.22 Therefore, the kinetic effect on the delayed formation of various products could be neglected. While it is clear from the above studies that the formation of BTX relies highly on the Brønsted acid sites, we are still skeptical about the effect of Zn (II) Lewis acid sites and ZnO. As already mentioned, different exchanged Zn (II) sites, such as “…Al-O-Zn(OH), …Al-O-Zn-OZn-O-Al…” and “…Al-O-Zn-O-Al…” could be formed,34 and their amounts varied with changing Si/Al2 ratios. However, the quantitative information on the concentration of different Zn species is not available in the present study. Although different Lewis acid sites should have different dehydrogenation/aromatization activities, an overarching effect of the Zn species can be discussed by comparing the C2H4-TPSR profiles over the Zn-HZSM-5 and HZSM-5 catalysts. As shown in Figure 6, both catalysts show the formation of methane, ethane, propylene, butylene, benzene, toluene, and xylene during C2H4-TPSR and the rate of formation of these products dependent highly upon the Zn promotion. The formation of propylene and butylene occurs at temperatures as low as 200oC over both catalysts, and at the temperature between 200-300oC, the rates of propylene and butylene formation are similar, which indicating that the Zn has a minor effect on the ethylene oligomerization at low temperature. When the temperature ramped up to more than 300oC, aromatics and other products are observed. The rate for the BTX formation on the Zn-HZSM-5 is significantly higher than that on the HZSM-5, indicating the presence of Zn 25 ACS Paragon Plus Environment


species promoted the aromatization reaction. We also noticed that the formation of ethane is almost absent over the HZSM-5 even at temperatures above 600oC, but over the Zn-HZSM-5, ethane became the main products when the temperatures are above 500oC. This result is consistent with our initial assumption that Zn species are mainly responsible for the ethane dehydrogenation (Absence of Zn species (HZSM-5) only show negligible activity in ethaneTPSR (results not shown here)). Consequently, we suggest in our Zn exchanged HZSM-5 catalysts with different Si/Al2 ratio, the formation of BTX mainly rely on the strong Brønsted acid sites (BTX absents when the Brønsted acid sites are covered by the NH3, see Figure 5B), nonetheless, the weak Lewis acid Zn (II) sites (with different coordination structure) show a promotion effect.

1.5

1.0

2.0

(A) Zn-HZSM-5

Methane Ethane Propylene Butylene Benzene Toluene Xylene

Rate (mol/gcat/s)

2.0

Rate (mol/gcat/s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 38

0.5

0.0

200

300

400

500

1.5

1.0

(B) HZSM-5

0.5

0.0

600

Methane Ethane Propylene Butylene Benzene Toluene Xylene

200

o

300

400

500 o

Temperature ( C)

Temperature ( C)


600

Page 27 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6. Temperature-programmed surface reaction of ethylene on (A) zinc exchanged HZSM5 catalyst (0.1 g) and (B) pure HZSM-5 catalyst. The temperature of the sample was ramped from 50 to 630oC (10oC/min) under 20 ml/min of 10% C2H4/Ar. Time-dependent catalyst deactivation and its kinetics Catalyst deactivation from coke deposition is a common industrial obstacle during hydrocarbon processing.42 Minimizing coke formation on the catalyst surface during ethane aromatization is extremely challenging due to both ethane aromatization and coke formation being thermodynamically favored at high temperature, low pressure and in the absence of H2 in the feed. The catalytic performance as a function of time-on-stream (TOS) was also shown in Figure 3. The results exhibit that both ethane conversion and aromatics (BTX) selectivity attenuate with TOS and the behavior of such decay dependent highly upon the Si/Al2 ratios. Deactivation of the catalyst was mainly originated from coking, and as mentioned in the introduction, is a common problem in light alkanes aromatization. In order to identify the effect of Si/Al2 ratios on the catalyst deactivation, the activity coefficient “a” (defined as the ratio of the real-time reaction rate to the initial reaction rate on the fresh catalyst (t = 0)) is plotted against TOS. As shown in Figure 7, the overall activity coefficient for the products changes with Si/Al2 ratios in sequence of Si/Al2 50 > Si/Al2 80 > Si/Al2 30. However, a different behavior was found for ethane (conversion), which Si/Al2 30 first shows the same “a” value as Si/Al2 80 then it 27 ACS Paragon Plus Environment


approaches to similar values as Si/Al2 50 with TOS. More generally, the “a” values decrease significantly with TOS for methane, ethane and BTX, respectively. The activity coefficient for ethylene remains largely unchanged within 2 hours, which suggested that the active sites (mainly the Lewis acid sites according to the former discussion) for ethane dehydrogenation remain intact in spite of the coking. As discussed in the Introduction, the ethane dehydrogenation and ethylene cyclization are most probably catalyzed by the different active sites, i.e., the exchanged metal cation (single Lewis acid site) catalyzes the initial ethane dehydrogenation, whereas the synergistic effect of Brønsted acid site and Lewis acid is responsible for the oligomerization and cyclization. Therefore, the relative stable “a” value for ethylene versus the rapid decreases of the “a” value of aromatic products suggested different coking rate on these two different acid sites. We anticipate that the rate of coking on the Brønsted acid of the HZSM-5 to be significantly higher than that on the exchanged Zn(II) Lewis acid sites. The activity coefficients “a” for propylene and butylene are not presented, but it might be expected that the decay behaviors for propylene and butylene could be similar to that of overall ethane conversion since the selectivities to propylene and butylene are not changed with TOS. In addition, the change of “a” values for benzene, toluene, xylene and methane shows almost identical tendency in Figure 7, which suggests that the same active sites, i.e., a synergy between exchanged zinc cations (Lewis acid) and the Brønsted acid, are involved for the formation of these products. 28 ACS Paragon Plus Environment

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Page 29 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Si/Al2 = 30

Si/Al2 = 50

Si/Al2 = 80

0.9 0.6 0.3

Methane Ethane

0.0 Benzene

0.9 0.6 0.3

Ethylene

0.0 Xylene

Toluene

0.9 0.6 0.3 0.0

0

40

80

120

0

t (min)

40

80

120

t (min)

Figure 7. Effect of Si/Al2 ratios on the catalyst decay period. The activity coefficient “a” is plotted against time, where “a” is defined as the ratio of the real-time reaction rate to the initial reaction rate on the fresh catalyst (t = 0). The value of “a” during the first two minutes was omitted since the reaction is still under the induction period (see Figure S3). The time-dependent attenuation of activity coefficient “a” promoted us to think about the deactivation kinetics due to the coking. According to Voorhies,43 the amount of coke deposition on the surface of the catalyst can be expressed as a function of time: “CC = Atn”, where CC is the concentration of carbon on the surface of the catalyst (g/m2). The activity coefficient then can be expressed as a function of CC (a = 1/(1+k0CCβ)), which can be converted into the following empirical deactivation kinetics model:



𝑎=

𝑟(𝑡) 1 = 𝑟(0) 1 + 𝑘𝑡𝛼

The kinetics parameters k and α were calculated from the change of a-value as a function of time for different catalysts and products. The deactivation kinetics model was first converted into a linear form: ln[(1-a)/a] = lnk + αln(t) (y = b + ax). The values of “(1-a)/a” for different products were plotted against time for different catalysts (see Figure 8 row 6). The “(1-a)/a” values increase with time-on-stream, which suggests a decrease of the activity coefficient. More detailed values of “(1-a)/a” dependent highly upon the catalysts and types of products (see also Figure 7).


Page 30 of 38

Propylene ln [(1-a)/a]

4 3 2 1 0

Si/Al2 = 30

0   

1

2

0

Benzene ln [(1-a)/a]

Toluene ln [(1-a)/a]

-2 lnk = -0.46 -3 4 5 1 0

2

3

2 1 0 -1    lnk = -3.67 -2 -3 4 5 1

-2

  

1 1

2

lnk = -2.4 4 5

3

-1 -2

  

-4 01

2

3

-1

0

1

2

3

1 0 -1 -2

  

-2

lnk = -1.78 4 5

-3

0

1

2

3

-1

2

3

  

-2

lnk = -2.14 4 5

-3

ln (t)

0

  

1

2

3

lnk = -2.18 4 5

1 0

2

3

-4 1

2

3

4

  

-2

lnk = -4.6 4 5

-4

=

C3

0

t (min)

120

160

0

40

80

3

lnk = -3.66 4 5

  

2

3

lnk = -3.87 4 5

ln (t)   

1

2

3

lnk = -4.1 4 5

ln (t)

1

80

lnk = -4.47 4 5

3

2

Benzene

2 40

lnk = -3.49 4 5

  

0

2

0

2

ln (t) =

C2

6

3

  

lnk = -4.4 -4 4 5 11 0 -1    -2 lnk = -3.47 -3 1 4 5 1 0 -1    -2 lnk = -3.8 -3 4 5 21

ln (t)

ln (t) C1

0

2

-2

-2

-2

  

0

-2

1 2

Xylene ln [(1-a)/a]

3

Si/Al2 = 80

Si/Al2 = 50

1 -1

-4

(1-a)/a

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Methane ln [(1-a)/a]

Page 31 of 38

120

160

t (min)

Toluene

4 3 2 1 0

0

40

Xylene

80

120

160

t (min)

Figure 8. Deactivation kinetics of ethane aromatization calculated based on a typical coking induced deactivation kinetic model “a = 1/(1+ktα)”. In order to obtain kinetic parameters of k and α, the model was transformed into a linear form: ln[(1-a)/a] = lnk + αlnt. According to the linear relation between ln[(1-a)/a] and ln(t) (see Figure 8 rows 1-5), the slop (α) and intercept (lnk) values for various products on different catalysts are inserted into the Figures, and the obtained final values for k and α are shown in Table 2. It is clearly seen that the α values increase, while the k values decrease with increasing Si/Al2 ratios from 30 to 80, for all



Page 32 of 38

of the products (methane, propylene, benzene, toluene, and xylene). We also noticed that the α and k values for BTX change according to the following orders: αBenzene < αToluene < αXylene and kBenzene > kToluene > kXylene, which are independent of the catalysts. A decrease of α value suggests that coke deposition is less sensitive to the TOS, which means the rate for coking decrease with TOS (a self-poison process). On the other hand, the kinetic parameter k can be understood as the initial “rate” of coke deposition, which is closely related to the acidity of the catalyst. Table 2. Deactivation kinetic parameters k and α for methane, propylene, benzene, toluene, and xylene on zinc exchanged HZSM-5 catalysts with different Si/Al2 ratios. The results are obtained based on Fig. 7.

Si/Al2

Methane

Propylene

Benzene

Toluene

Xylene

α

k

α

k

α

k

α

k

α

k

30

0.73

0.630

0.49

0.091

0.60

0.170

0.67

0.120

0.74

0.110

50

0.83

0.025

0.68

0.012

0.64

0.031

0.72

0.022

0.92

0.010

80

0.94

0.030

0.90

0.011

0.86

0.026

0.92

0.021

1.04

0.016

4. CONCLUSIONS To summarize, we demonstrated the effect of acidity on the early-stage catalytic performance and time-dependent deactivation kinetics during ethane aromatization over the zinc exchanged HZSM-5 catalysts. The acidity of the catalysts was tuned through varying the Si/Al2 ratios of the


Page 33 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ZSM-5 zeolite. Both the total acid density (from NH3-TPD) and the Brønsted acidity (from C3H7NH2-TPDec) decrease with increasing Si/Al2 ratios from 30 to 50 to 80. The initial catalytic performance (STYs) of the catalyst in ethane aromatization is highly dependent on the acidity. The STYs of benzene, toluene, and xylene increase linearly with increasing Brønsted acidity, which suggested that the formation of BTX related highly to the Brønsted acid sites. The TOF for the formation of BTX (calculated on each Brønsted acid site) is 0.26 min-1 for benzene, 0.25 min-1 for toluene, and 0.026 min-1 for xylene (independent of the Si/Al2 and Zn/Al ratios). The STY of byproduct methane increases nonlinearly with increasing Brønsted acidity, which suggested that the formation of methane might involve different mechanistic steps and active sites. However, the STY of ethylene, the intermediate for the formation of higher olefins and aromatics in ethane aromatization, decreases with increasing Brønsted acidity, which suggested that the initial ethane dehydrogenation is directed by the Lewis acid sites (exchanged Zn(II) cations). We conclude that the weak Lewis acid sites are responsible for ethane dehydrogenation, but the formation of BTX (from mixed olefins) dependent highly upon the strong Brønsted acid sites with the promotion of Lewis acid sites, and both types of acid sites might be involved in the ethylene oligomerization. Such a conclusion was further supported by the C2H4-TPSR (over both Zn-HZSM-5 and HZSM-5) and the transient response of various products from steady-state ethane aromatization to ethane “aromatization” with NH3 co-feed and vice versa. The catalyst 33 ACS Paragon Plus Environment


was deactivated with prolonged time-on-stream, both ethane conversion and BTX selectivity decrease due to coking. After modeling the kinetics of deactivation as “r(t) = r0/(1+ktα)”, the kinetic parameters (for aromatics) k decreases and α increases with decreasing acidity. Based on the obtained kinetic model and the parameters, the recalculated products yield corresponds well to the experimental data (see Figure S4), which could optimize the process based on the initial catalytic performance. ASSOCIATED CONTENT Supporting Information Available: In-situ XANES spectra of Zn-HZSM-5 at different temperature under 10% H2/Ar (Fig. S1); NH3-TPD and n-propylamine-TPDec profiles (Fig. S2); Time-resolved early-stage catalytic performance (Fig. S3); A comparison between the recalculated benzene yield (from the deactivation kinetics) and the experimental data (Fig. S4). Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS Use of the Advanced Photon Source, Office of Science user facilities, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. We also gratefully acknowledge the TEM work (supported by the National Science Foundation (MRI-1126743)) at Institute for Imaging and Analytical Technologies (I2AT) at Mississippi State University.


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Ethane Aromatization over Zn-HZSM-5: Early Stage Acidity

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