Transient Kinetic Study of Ethane and Ethylene Aromatization over

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Kinetics, Catalysis, and Reaction Engineering

Transient Kinetic Study of Ethane and Ethylene Aromatization over Zinc Exchanged HZSM-5 Catalyst Tingyu Liang, Hossein Toghiani, and Yizhi Xiang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b03735 • Publication Date (Web): 23 Oct 2018 Downloaded from http://pubs.acs.org on October 24, 2018

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Industrial & Engineering Chemistry Research

Transient Kinetic Study of Ethane and Ethylene Aromatization over Zinc Exchanged HZSM-5 Catalyst Tingyu Liang, Hossein Toghiani and Yizhi Xiang* Dave C. Swalm School of Chemical Engineering, Mississippi State University, 39762 MS, U.S. *

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

Abstract The conversion of ethane and ethylene to aromatics over a zinc exchanged HZSM-5 catalyst has been studied by chemical transient kinetic analysis in a flow reactor under atmospheric pressure. Time-dependent early-stage/build-up catalytic behavior and the kinetic parameters of products decay during the back-transient have been discussed extensively at different reaction temperatures. For both ethane and ethylene aromatization, the delay times (relevant to the initial appearance of the reactant in the gas phase) of the formation of aromatics are 10-15 s during the build-up, which decreases with increasing temperature. On the basis of quantitative back-transient analysis, the rate constants for the formation of benzene, toluene, and/or xylene are ranged from 5-30×10-3 s-1 dependent upon the reaction temperatures. According to the Arrhenius equation, the activation energies for the formation of benzene and toluene during ethane aromatization are 57 and 73 kJ/mol, respectively; which is relatively higher than the activation energies from ethylene aromatization.


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1. Introduction The production of Natural Gas Liquids (NGLs), a mixture of C2-C5 light alkanes, has increased dramatically and surpassed its consumption due to the current shale gas revolution. According to the Energy Information Administration (EIA), in 2017 the U.S. produced 1.35 billion barrels of NGLs, where ethane comprised about 40% of the NGLs barrels.1 As a result, the price of ethane and its direct downstream product ethylene (in

current industry, ethane is consumed almost exclusively for ethylene production via high temperature endothermic steam cracking) has dropped significantly during the past years, which thus has urged the transformation of ethane and ethylene into the more valuable chemical feedstocks. Aromatic compounds, such as benzene, toluene, and xylene (BTX) are important petrochemical feedstocks (the benzene market alone was expected to surpass 46 million metric tons by 2020), which are mainly derived from the catalytic reforming and steam cracking of naphtha in the current petroleum refinery industry.2 The development of a catalytic process for the conversion of natural gas products (ethane and ethylene) to BTX could be lucrative in the near future. Due to the special shape selectivity of the MFI topology, ZSM-5 zeolite has been found to be an ideal catalyst for the production of BTX from various feedstocks, such as light alkanes,3−8 alkenes,9−12 and alcohols.13,14 Research on light alkanes aromatization started with the pioneering work of Csicsery (from Chevron) who demonstrated the “dehydrocyclodimerization” of C3-C5 alkanes to aromatics with Pt/Al2O3as the catalyst in 1970 when the ZSM-5 zeolite was not yet developed.9−12However, the total yield of aromatics on the Pt/Al2O3 catalysts was low. Until 1990s, the development of metal functionalized HZSM-5 catalysts made various aromatization processes, such as M2 forming, Cyclar process, Aroforming, and Z-forming possible.5 However, a similar catalytic process for ethane conversion remained ambiguous due to slow progress in the development of an active, selective, and stable catalyst. The first patent on ethane aromatization was filed by Mobil Oil Corporation

employed

Cu,

Zn,

and

Pt-functionalized

HZSM-5

as

the

catalysts.13About 19% aromatics yield with ethane conversion of 31% were obtained 3 ACS Paragon Plus Environment


over a bimetallic copper/zinc-modified HZSM-5 catalyst (0.25%Cu-1%Zn/HZSM-5). After that, Ga/HZSM-5 catalyst was also patented by the Mobil company;14 and bimetallic Ga-Rh, Ga-Re, and Ga-Pt functionalized HZSM-5 catalysts were later patented by The Standard Oil Company.15 More recently, bimetallic Pt-Sn, Pt-Ge, Pt-Ga, and Pt-Fe functionalized HZSM-5 catalysts were patented by the Shell Oil Company.16−18 Besides the aforementioned patents, various journal papers also demonstrated the successful conversion of ethane to aromatics using Pt, Zn, Ga, Re, and Mo functionalized zeolites catalysts,19−39 among which the Pt and Zn functionalized HZSM-5 catalysts were more efficient: While the Pt/HZSM-5 catalyst has the advantage of higher activity in ethane aromatization, the Zn/HZSM-5 catalyst exhibits higher aromatics selectivity (at the expense of CH4 formation) than the Pt based catalysts.13 It has been widely accepted that ethane aromatization is directed by dual-functional Mechanism: exchanged metal cation (single Lewis acid site) is responsible for the initial ethane activation (dehydrogenation), whereas the synthetic effect of Brønsted acid site and Lewis acid pairs (Brønsted/Lewis acid synergy) catalyzed the oligomerization and cyclization. The catalytic performance of zinc modified HZSM-5 catalyst in ethane and ethylene aromatization has been demonstrated in numerous literature,21,22, 24, 28, 32, 35, 38−43 and the effect of Zn (Lewis acid) has been found to be critical especially in ethane aromatization since the Brønsted acid sites of the zeolite alone cannot effectively activate the C1-C3 light alkanes. According to Pierella et al.,32 the ethane conversion and aromatics selectivity were increased with increasing Zn loading (Zn/(Zn+H)), but ethylene selectivity remained almost unchanged. As aromatization is catalyzed by the synergistic effects of Brønsted acid site and Lewis acid, and the Zn2+ was presented mainly in form of Lewis acid pairs at high Zn loading,44 it is reasonable that aromatics selectivity increased with high Zn loading. The unchanged ethylene selectivity may suggest that the active site (single Lewis site) for ethane dehydrogenation was less affected by the Zn loading. However, among literature, a detailed kinetic study of ethane aromatization has been less concerned. To 4 ACS Paragon Plus Environment

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the best of our knowledge, the only kinetic description of ethane aromatization was conducted on a gallium-doped HZSM-5 catalyst,30 but the steady-state kinetic models were based on assumptions, which requires further validation through other experimental methods. Relaxation-type transient kinetic analysis experiment, such as the Steady State Isotopic Transient Kinetic Analysis (SSITKA)45−48 and Chemical Transient Kinetic (CTK),49−51 is one of the most efficient techniques for the study of intrinsic reaction kinetics on the catalytic active sites, such as the abundancy of catalyst-bound reaction intermediates at a molecular level and their reactivity (presented by the rate constant “k”). Although transient kinetic analysis has been employed extensively in Fischer-Tropsch synthesis,45−51 such studies have merely been examined in other reactions, especially in ethane aromatization. The present paper aims at demonstrating the early-stage/build up (a transient from clean catalyst surface to dynamic catalytic reaction) catalytic behavior and kinetic parameters of the reactivity of the surface intermediates (obtained from quantitative back-transient analysis: a transition from steady-state catalytic reaction back to clean catalyst surface) in ethane and ethylene aromatization on a zinc exchanged HZSM-5 catalyst. Specifically, on the basis of the build-up experiment, the initial products distribution and the delay times (relevant to the initial appearance of the reactant in the gas phase) related to the appearance of various products were discussed. The rate constants (k) and activation energies (Ea) for the formation of benzene, toluene and xylene from their corresponding surface intermediates were calculated based on the quantitative back-transient decay analysis.

2. Experimental 2.1 Catalyst Preparation The NH4-ZSM-5 zeolite (specific area 400 m2/g) with a SiO2/Al2O3 ratio of 30/1 was purchased from VWR International. All of the rest chemicals were purchased from Sigma-Aldrich. All of the chemicals were used as obtained. Zn-modified HZSM-5 5 ACS Paragon Plus Environment


catalysts were prepared through ion-exchange. 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.

2.2 Catalyst characterization Ammonia temperature-programmed desorption (NH3-TPD) experiment was performed in a quartz reactor with a volume of 2 ml (ID, Φ = ½ inches). 100 mg of sample was first activated in 10% H2/Ar at 600oC, then decreased to 120oC for NH3 adsorption for 30 min. The influent gas was then switched from NH3 to Ar (30 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 600oC at a ramp of 10oC/min under flow of Ar at 30 ml/min. Desorption of NH3 was measured by an online mass spectrometer (Agilent 5973). 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. State of zinc was characterized by in-situ X-ray absorption near edge structure (XANES) experiments carried out at the Advanced Photon Source (APS) beamline 12-ID using a Vortex detector with the sample at a 45 degree angle to the incoming beam and detector. Powder samples were pressed to disk and mounted in the Linkam stage equipped with 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 using Athena, and the normalized absorption signals were presented in the present paper.

2.3 Catalytic Performance Evaluation Chemical transient kinetic experiments over zinc exchanged HZSM-5 catalysts for ethane and ethylene aromatization were performed in a home build gas supply unit 6 ACS Paragon Plus Environment

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equipped with an online Agilent 5973 mass spectrometer. Before transient kinetic experiment, the catalyst (0.1 g) was pretreated in 10% H2 (20 ml/min) at 600oC (ramp 10oC/min) for 30 min, 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. The temperature of the sample was then adjusted to targeted temperatures for transient kinetic experiment. Prior each experiment, by-pass 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 10% C2H6/Ar (or 10% C2H4/Ar for ethylene aromatization). The initial stage (until steady-state) of the reaction process refers to build-up or early-stage transient. Switching back from steady-state (10% C2H6/Ar or 10% C2H4/Ar) to Ar refer to back-transient, which shown the decay performance of both reactants and products. The gas hourly space velocity (GHSV) was ~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.

2.4 Catalytic Results Quantification In order to make quantitative analysis, various mass to charge ratio, such as m/z=2, m/z=15, m/z=16, m/z=18, m/z=26, m/z=27, m/z=28, m/z=29, m/z=30, m/z=32, m/z=40, m/z=41, m/z=56, m/z=78, m/z=91 and m/z=106, were monitored simultaneously. The intensity of the m/z signal (Im/z) then converted to the intensity of each molecule (Imolecule) based on the quantitative calibration. Apparently, IH2=I2, ICH4=I16, IH2O=I18, IAr=I40, IO2=I32, Ibenzene=I78, Itoluene=I92, and Ixylene=I106. The quantification of IC2H6, IC2H4, IC3H6, and IC4H8were somewhat different because they have significantly overlapped m/z signal. Therefore, IC4H8=I56 (the second most intense signal), IC2H6=I30 (the second most intense signal), IC3H6=I41-2.5×IC4H8 (most intense signal

of

C3H6

minus

contribution

of

C4H8

to

signal

41),

and

IC2H4=I27-0.291×IC2H6-0.75×IC4H8-2.5×IC3H6. The numbers in these equations were obtained based on the calibration by flowing the pure gas separately into the mass spectrometer, the ratio between the intensity of each fragment to the most intense signal 7 ACS Paragon Plus Environment


was calculated. The obtained intensity of each molecule was then converted to partial pressure, which finally calculated to each molecules’ mole flow rate (mol/s) based on the ideal gas equation (P v = F RT), where Pi is the partial pressure of selected molecules (Pa), vtot is the total volumetric flow rate (ml/s), Fi is the mole flow rate (mol/s), R is the ideal gas constant (J/mol/K), and T is the temperature (K). The conversion of ethane was calculated based on: x=

F , − F , × 100% F ,

The carbon-based selectivity of each product was calculated based on: S =

The amount of ethane converted into product i × 100% Total amount of ethane reacted F, × ' = × 100% (F , − F , ) × 2

Where the n represents the carbon number in the molecular of the product i. The results show in Table 1 was the normalized selectivity based on the above calculation. The reliability of the calibration and calculation was indicated by the small deviation ∑ -./ 0∑ -123

of the total carbon balance (∆C=+

∑ -./

+ ≤ 7%).

2.5 Back-transient kinetic evaluation The back-transient experiment (a transition from steady-state catalytic reactions to clean catalyst surface), namely to remove the reactant (ethane or ethylene) abruptly from the system, provides the kinetics information of the final products formation from their corresponding catalytic surface intermediates. Assume a first order kinetic for ethane and ethylene aromatization (see Scheme 1), the formation rate of benzene can be expressed as rB = k3CA·S, where k3 is the formation rate of benzene from the catalytic surface intermediate A and CA·S is the concentration of catalyst chemisorbed A. Note that the formation of BTX from ethane or ethylene is considered as thermodynamically irreversible due to the large equilibrium constant (see Figure S3). The general mole balance of species A can be written as: 8 ACS Paragon Plus Environment

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678∙: = 6?@;A>' =B;C >D E − @>'F?G' =B;C >D E 6;

While at steady-state, there is no net change of the CA·S, namely the rate of production of A equals to the rate of consumption of A. During back-transient experiment, the rate of production of A can be neglected since the reactants are absent from the system. 678∙: = −@>'F?G' =B;C >D E = −HI 78∙: 6;

which can be integrated into: CA·S = CA·S,0exp (-k3t), where time 0 is indicated by the disappearance of ethane and ethylene in the gas phase. Since the consumption rate of A equals the formation rate of benzene, t =J = HI 78∙: = HI 78∙:,K exp (−HI t) = =JK exp (−HI t) = =JK exp L− N M

τ is the time constant of decay in the production of benzene during back-transient. The outlet mole flow rate of BTX then can be model as ; OP = OP,K exp (− ) M

According to which, typical outlet flow profiles during back-transient ethane aromatization are shown in Figure S4. A linear relation will be obtained if we plot ln(Fi) as a function of time, and the slop is –k or –1/τ, which stands for the formation of products i from their corresponding catalyst-bound intermediates. Based on Arrhenius equation (lnk = lnA −

ST V

U W

), lnk is then plotted as a function of

1/T. The activation energy Ea could then be determined from the slope value (-Ea/R).

3. Results and discussion 3.1 Physicochemical properties of Zn exchanged HZSM-5 catalyst The physicochemical properties of the zinc exchanged HZSM-5 catalyst employed in the present kinetic study has been extensively characterized by TEM, STEM-EDX, NH3-TPD, and in-situ XANES. The HRTEM image (Figure 1-a) shows the lattices of ZSM-5 without any aggregation of zinc species. STEM-EDX elements mappings of 9 ACS Paragon Plus Environment


the selected region shown in Figure 1-c demonstrated the homogeneous distribution of zinc on the zeolite. Quantitative analysis of the EDX spectrum suggested that the weight percent of Si, Al, Zn, and O are 52.2, 3.1, 1.3, and 43.4 wt%, respectively. Ammonia-TPD profile of the zinc exchanged HZSM-5 catalyst is shown in Figure 1-d. Two distinct NH3 desorption peaks are identified at 278 and 467oC, respectively. The total amounts of NH3 desorbed from the catalyst during the course of TPD was

40

d)

278oC

600 500

30

400

467oC

20

300 10 200

893 µmol/gcat

0

100 0

15

30

45

60

Normalized absorption(a.u.)

NH3 desorption (µmol/gcat/min)

quantified to be 893 µmol/gcat.

Temperature (oC)

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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e) 1.5

1.0

0.5

Zn foil Zn-ZSM-5 (fresh) Zn-ZSM-5 (reduced) Zn-ZSM-5 (after reaction)

0.0 9630

9660

Time (min)

9690

9720

9750

Energy (eV)

Figure 1.Chemical properties of the zinc exchanged HZSM-5 catalysts employed in the present study. a) and b) TEM images of fresh Zn-ZSM-5; c) STEM-EDX elements mapping of the area shown in panel b; d) NH3-TPD profile of Zn-ZSM-5; and e) normalized in-situ XANES spectra (Zn K edge) of Zn-ZSM-5, the spectra of fresh, reduced and after reaction samples were obtained under flowing of Ar at room temperature, flowing of 10% H2/Ar at 500oC, and flowing of 10% ethylene/Ar at 500oC for 30 min, respectively.


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Results of in-situ XANES measurements at either room temperature or 500oC are presented in Figure 1-e. The normalized in-situ XANES spectra (Zn K edge) of Zn-ZSM-5, the spectra of fresh, reduced and after reaction samples were obtained under flowing of Ar at room temperature, flowing of 10% H2/Ar at 500oC, and flowing of 10% ethylene/Ar at 500oC for 30 min, respectively. From a comparison with Zn foil spectrum, the adsorption edge of Zn-ZSM-5 shifted to a higher energy, which implies the presence of positively charged zinc species.52 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. The edge is shifted towards lower energies at higher temperature due to the formation of partially reduced zinc species or the decreased oxidation number of absorbing atoms, electronegativity of ligands and number of ligands.53

3.2 Steady-state catalytic performance Table 1 shows the steady state catalytic performance of ethane and ethylene aromatization over zinc exchanged HZSM-5 catalyst at various reaction temperatures. The catalytic performance in terms of the ethane (or ethylene) conversion and selectivities to methane, ethylene (or ethane), propylene, benzene, toluene, and xylene were calculated from the average data obtained during the initial 5-15 min time-on-stream (see Figures S1-S2 for the original time dependent catalytic performance). The formation of other light hydrocarbons and heavier aromatics are not discussed in the present paper since their selectivities are negligible under current reaction conditions. For ethane aromatization at reaction temperatures between 525 to 600oC, the conversion and total BTX selectivity increase significantly with increasing temperature. A trade-off between the selectivities to BTX and ethylene is clearly identified. At 600oC, 48% of BTX selectivity was obtained at an ethane conversion of 41.9%. The selectivity to methane is only 7.8% and the overall selectivity of BTX and 11 ACS Paragon Plus Environment


ethylene is 90%. It is noteworthy that the selectivity to benzene increases more significantly with increasing temperature than the selectivity to toluene, while xylene selectivity (1%) remain unchanged in spite of the increasing temperatures. For ethylene aromatization, lower reaction temperatures between 425 to 500oC were employed due to the high reactivity of ethylene aromatization. The selectivities to BTX, methane and ethane increase (the selectivity of propylene decreases) with increasing temperature. Similarly to ethane aromatization, benzene selectivity increases significantly with increasing temperature, toluene selectivity remain almost unchanged at the temperatures between 450-500oC, while xylene selectivity decreases slightly with increasing temperature from 450 to 500oC (xylene selectivity remains the same at 425 and 450oC). In terms of the ethylene conversion, the increase of reaction temperature from 425 to 500oC doesn’t monotonically increase the conversion. Furthermore, over the investigated temperature range, the conversion of ethylene doesn’t change significantly with the change of temperature, which is not surprising if the reaction approaches the thermodynamic equilibrium. However, more complicated reasons were involved in the present case and can be demonstrated by the ethylene TPSR profiles shown in Figure S3: the conversion of ethylene run into two plateaus at the temperatures between 350 to 450oC and 550 to 600oC, respectively. The first plateau, which is closely related to the non-monotonically change of ethylene conversion with increasing temperature from 425 to 475oC (Table 1), is observed due to the presence of multiple reactions in the system. The first type of reaction, namely ethylene oligomerization, is thermodynamically inhibited at the temperatures between 300 to 500oC. As Figure S3 exhibits, the equilibrium constants for the formation of C3H6 and C4H8 from ethylene decrease with increasing temperature. The Log (K) is below zero for C3H6 at temperatures above 300oC and close to zero for C4H8 at the temperature between 450 to 500oC. Therefore, the oligomerization of ethylene is thermodynamically limited at high temperatures above 450oC. On the other hand, the second type of reaction, namely aromatization for the formation of benzene and toluene from ethylene, is kinetically inhibited at low 12 ACS Paragon Plus Environment

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reaction temperatures. As a result, the conversion of ethylene reach a minimum at the temperature around 450oC during the TPSR profiles.

Table 1. Steady-state catalytic performance of ethane and ethylene aromatization over Zn exchanged HZSM-5 catalyst. Selectivity (%)

T (oC) Feed Conv. (%) ∆C

CH4 C2H4 C3H6 C6H6 C7H8 C8H10 BTX 525

C 2 H6

11.7

0.1

3.7

66.8

3.8

14.4

10.6

0.7

25.7

550

C 2 H6

20.8

0.3

5.2

52.0

3.2

22.1

16.4

1.0

39.6

575

C 2 H6

30.9

3.4

7.7

42.9

2.5

27.9

17.9

1.1

46.9

600

C 2 H6

41.9

6.5

7.8

42.1

2.1

30.0

17.0

0.9

48.0

CH4 C2H6 C3H6 C6H6 C7H8 C8H10 BTX 425

C 2 H4

58.8

3.9

2.1

4.2

36.5

6.8

38.3

12.1

57.2

450

C 2 H4

55.7

6.1

2.4

3.7

28.0

10.9

42.8

12.2

65.9

475

C 2 H4

61.6

3.1

2.9

12.3

14.1

15.9

44.8

9.8

70.6

500

C 2 H4

63.1

5.7

3.2

13.3

11.7

21.0

43.3

7.5

71.9

3.3 Build-up analysis: early-stage catalytic behavior The build-up stage of the transient kinetic experiment is defined as the initial run-in of the reaction and led to the construction of the catalytically active surface or a transient from clean catalyst surface to steady-state catalytic reactions.51 On the basis of the products distribution analysis during the build-up, time dependent dynamic changes of activity and products selectivity can be obtained for the early-stage of the catalytic reactions. As shown in Figure 2, the products profiles are clearly delayed relevant to the appearance of ethane in the gas phase. The first product appears in the gas phase is ethylene

for

all

the

investigated

temperatures,

which

suggests

that

the

dehydrogenation of ethane is the first step involved in the whole reaction process. After ethylene, the formation of methane is immediately followed, which typically reaches a maximum at around 25-30 s then decreasing slightly over the whole investigated time period (see also Figure S1). The simultaneous formation of methane and ethylene indicates that methane is originated from ethylene rather than ethane 13 ACS Paragon Plus Environment


through either hydrogenolysis (C2H4 + 2H2 → 2CH4) or cracking (C2H4 → C + CH4). The formation of propylene, benzene, and toluene are further delayed by a time of 10 s,10s, and 20 s, respectively. The occurrence of delay could be originated from the restructuring of the catalytic active sites or dynamic accumulation of the different types of surface adsorbed intermediates. We anticipate that the later could be the main reason for the delay observed in the present study. Therefore, it is not surprising that the delay times for propylene and BTX decrease with increasing temperature, as at higher temperature, the reaction rate for the formation of olefins from ethane is high, which resulted in a rapid building of the adsorbed intermediates. However, a quantitative relationship between the reaction activity and the delay time remains difficult. It is also noteworthy that the formation of other light hydrocarbons, xylene, and heavier aromatics are not discussed due to their low intensity. o

Outlet flow (molecules/s)

15

o

525 C

550 C

6.0x10

=

16

C2

1.2x10

15

C2

C2

4.0x10

C1

=

C1 15

8.0x10

C2 B

B

15

2.0x10

=

C3

15

4.0x10

=

0.0

0

10

20

30

40

16

4x10

o

16

T

C3

T 0.0


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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575 C

2.0x10

C2

0

10

20

C1

=

30

40

o

600 C C1

=

16

3x10

C2

16

1.5x10

C2

16

C2

16

2x10

B

1.0x10

B

16

15

5.0x10

1x10

T C3

0.0 0

10

20

30

=

T =

C3

0 0

40

Time (s)

10

20

30

40

Time (s)

Figure 2. Early-stage catalytic behavior of ethane aromatization. Outlet flows (molecules/s) of the methane (C1), ethane (C2), ethylene (C2=), propylene (C3=), benzene (B) and Toluene (T) during the (early-stage) transient experiment at different temperatures over 0.1 g zinc exchanged HZSM-5 catalyst under atmospheric pressure. At t = 0, the reactor influent gas was switched abruptly from He to 10% C2H6/Ar.


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The early-stage catalytic behavior of ethylene aromatization reveals a different delay behavior from the ethane aromatization. As shown in Figure 3, there is no clear delay on the formation of methane, ethane, and propylene (relevant to the appearance of ethylene), which suggest that these products can be readily produced from ethylene over the zinc exchanged HZSM-5 catalyst. Both methane and ethane reach a maximum at 20 s then decreasing slightly to steady-state after 40 s. It may indicate the presence of two different types of catalytically active surface for the formation of methane and ethane from ethylene. With respect to the formation of BTX, clear delay times are observed at low temperatures (around 10 s at 425 and 450oC). Similarly in ethane aromatization, such delay times are clearly decreased at high temperature. Additionally, the formation of benzene seems to reach a maximum at 30-40 s, the formation of toluene and xylene increase monotonically during the build-up process. With the increase of temperature, the formation of propylene is largely suppressed, whereas the formation of ethane from hydrogen transfer is significantly enhanced. o

o


425 C

450 C

17

17

2.0x10

C2

2.0x10

=

=

C2

=

C3

17

17

1.5x10

1.5x10

C2

17

1.0x10

T

=

C3

T

17

1.0x10

C1

16

5.0x10

C2 C1

16

5.0x10

B

B X

0.0

0

20

40

X

0.0

60

80

0

o

20

40

60

80

o

475 C


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


500 C

17

17

2.0x10

C2

2.0x10

=

C2

17

1.5x10

17

1.0x10

T

5.0x10

=

C3

B

0.0 20

40

60

5.0x10

C3

T

C1

16

80

=

B

0.0

X 0

C2

17

17

C1

=

1.5x10

1.0x10

16

C2

X 0

Time (s)

20

40

60

80

Time (s)

Figure 3. Early-stage catalytic behavior of ethylene aromatization. Outlet flows (molecules/s) of the methane (C1), ethane (C2), ethylene (C2=), propylene (C3=), 15 ACS Paragon Plus Environment


butylene (C4=), benzene (B), Toluene (T) and Xylene (X) during the (early-stage) transient experiment at different temperatures over 0.1 g Zn-ZSM-5 catalyst under atmospheric pressure. At t = 0, the reactor influent gas was switched abruptly from He to 10% C2H4/Ar.

3.4 Kinetic parameters from back-transient analysis The back-transient kinetic experimental results for both ethane and ethylene aromatization at different temperatures are shown in Figure 4, and the rate constants for the formation of BTX from back-transient experiment are shown in Table 2. A linear relation between the natural logarithm of molecular flow rate of BTX and time (t) is observed for all the measured temperatures in both ethane and ethylene aromatization, which suggest that the kinetic model for back-transient experiment is first order, and indicate that the supply suspend of reactant (ethane or ethylene) does not affect the rate constants. The rate constants (k) for the formation of BTX (originate from their corresponding catalyst-bound surface intermediates) increase with increasing temperature (Table 2). During ethane aromatization, kbenzene value is slightly smaller than ktoluene at the same temperature, but for ethylene aromatization, kbenzene was found to be larger than ktoluene. Additionally, the kxylene for ethane aromatization is inaccessible due to the low selectivity to xylene, and the kxylene for ethylene aromatization was found to range from 5.8-10.7 10-3 s-1. It must be noted that a relative short TOS (time-on-stream) (20 min) was employed before back-transient experiment in order to minimize the effect of coke deposition on the kinetic parameters. Total coke deposition during 20 min TOS was quantified to be 7 mgC/gcat (see Figure S5). We also compared the effect of time-on-stream on the kinetic rate constants k for benzene and toluene formation during ethane aromatization at 600oC (see Figure S6). The rate constants k after 2 min TOS are almost the same as that after 20 min TOS. It is also noteworthy that the rate constants k obtained from the back-transient kinetics are the overall values from different surface intermediates. As mentioned in Scheme 1, the intermediates for the formation of BTX can be varied 16 ACS Paragon Plus Environment

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Page 17 of 25

when different olefins (such as ethylene and propylene) are involved in the reaction. The surface concentration of the different intermediates dependent on the partial pressure of olefins.

33 32 31 35 -0.01

-0.014

-0.018

34

-0.027

ln (FToluene)

ln (FBenzene)

-0.019

34

ln (FXylene)

33 32 31 30

0

100 200

0

80

160

0

80

160 0

80

36 35 34 33 38 37 36 35 36 35 34 33

160

o

o

475 C

450 C

425 C

600 C

-0.014

o

o

o

575 C

-0.0124

-0.0086

35

o

550 C

ln (FBenzene)

o

o

525 C

36

ln (FToluene)

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


500 C

-0.0056

-0.0079

-0.011

-0.0134

-0.0056

-0.008

-0.0096

-0.012

-0.0058

-0.0069

-0.008

-0.0107

0 100 200

0 100 200

0

80 160

0

80

160

Time (s)

Time (s)

Figure 4. Outlet molecular flow of BTX as a function of time during back-transient experiment. (left) ethane aromatization at different temperatures, (right) ethylene aromatization at different temperatures. According to first order kinetic ln(Fi) = ln(Fi0) – kt, a linear relation between ln(Fi) and t can be obtained, and the inset numbers represent the slop of the linear equation.

The Arrhenius plots of the rate constant as a function of 1/T are shown in Figure 5. The activation energies for the formation of benzene and toluene from ethane aromatization are 57 and 73 kJ/mol, respectively. For ethylene aromatization, the activation energies for the formation of benzene, toluene, and xylene are 53, 44, and 35 kJ/mol, respectively. The activation energies for the BTX formation are lower than the apparent activation energy obtained from steady-state kinetics analysis in literature.30 As aforementioned, the rate constants obtained from the back-transient kinetic analysis represent the reactivity of the formation of BTX from their corresponding catalyst-bound intermediates. Therefore, the activation energies obtained in the present paper represent the final dehydrogenation/desorption of the chemisorbed BTX precursors (see Scheme 1). During ethane and ethylene aromatization, the activation energies for the formation of benzene and toluene from ethylene aromatization are lower than that from ethane aromatization, indicating 17 ACS Paragon Plus Environment


different types of chemisorbed intermediates are responsible for the formation of aromatics. Since the rate constants obtained from the back-transient kinetic analysis are the overall values from different surface intermediates, the activation energies obtained during ethane aromatization largely represent the reaction for ethylene derived intermediate to BTX because the partial pressure of other olefins during ethane aromatization is relatively low (see Table 1). On the other hand, during ethylene aromatization, a large amount of ethylene was converted into propylene (see Table 1 and Figure S3). As a result, we obtain the mean activation energy for the conversion of ethylene and propylene intermediates to BTX. Therefore, the activation energies for benzene and toluene during ethane aromatization is higher than that during ethylene aromatization.

Table 2. Rate constant k for the formation of BTX from the back-transient experiment of ethane and ethylene aromatization. o

T ( C)

Rate constant k (10-3 s-1) Feed C6H6

C7H8

C8H10

525

C2H6

8.6

10.0

/

550

C2H6

12.4

14.0

/

575

C2H6

14.0

18.0

/

600

C2H6

19.0

27.0

/

425

C2H4

5.6

5.6

5.8

450

C2H4

7.9

8.0

6.9

475

C2H4

11.0

9.6

8.0

500

C2H4

13.4

12

10.7


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Page 19 of 25

Benzene Toluene

A

-4.2

Benzene Toluene Xylene

B

-3.6 -4.5

-4.4

−5346.5 −6403.8

-4.0

ln (k)

−8808.1

ln (k)

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


-4.8 −4266.1

−6825.4

-5.1 -4.8 0.00115

0.00120

0.00125

1/T

Figure 5. Arrhenius plots (lnk = lnA −

0.00132

0.00138

0.00144

1/T

ST V

U W

) of the formation of BTX from ethane

(A) and ethylene (B) aromatization.

Scheme 1. Main reaction pathways in ethane and ethylene aromatization. Intermediates (A), (B), and (C) for the formation of BTX can be varied based on the combination of different olefins, and their surface concentration dependent on the partial pressure of olefins.

4. Conclusion Chemical transient kinetic (CTK) analysis has been performed for ethane and ethylene aromatization over a zinc exchanged HZSM-5 catalyst. Such model catalyst has been extensively characterized by means of TEM, STEM-EDS, NH3-TPD, and 19 ACS Paragon Plus Environment


in-situ XANES, which suggested a stable and homogeneous distribution of zinc species in the zeolite. Time dependent dynamic catalytic behaviors during the three critical stages in the CTK experiment, namely, early-stage/build-up (transient from inert to reactant), steady state, and back-transient (from reactant to inert), have been extensively discussed. During the build-up stage, the delay times (relevant to the initial appearance of the reactants in the gas phase) of the formation of aromatics are 10-15 s for both ethane and ethylene aromatization. After comparison between the early-stage catalytic behavior of the ethane and ethylene aromatization, it might be concluded that ethylene was the first produced from ethane through dehydrogenation, which followed by hydrogenolysis (forming CH4), oligomerization (forming C2+ olefins) and cyclization (see Scheme 1 for the proposed reaction pathways). The surface chemisorbed intermediates for the formation of BTX are composed of olefins and their formation might be delayed due to the limited partial pressure of the desired olefins. Therefore, a high initial dehydrogenation activity of the catalyst is very crucial for ethane aromatization. Steady-state catalytic results suggest that higher reaction temperature favors the formation of aromatics (at a low temperature, ethylene oligomerization is prevailing). According to the back-transient kinetic analysis, the formation of BTX from their corresponding surface chemisorbed intermediates following the first order kinetics. The rate constants for the formation of benzene, toluene or xylene during both ethane and ethylene aromatization at different temperatures have been calculated, on the basis of which the activation energies were calculated to be 57 and 73 kJ/mol, respectively for benzene and toluene during ethane aromatization; and 53, 44, and 35 kJ/mol, respectively for benzene, toluene and xylene during ethylene aromatization. We conclude that different surface chemisorbed intermediates, which derived from different olefins are involved in the formation of BTX. Therefore, the BTX products distribution might be tuned by altering the partial pressure of olefins through co-feeding of olefins or other natural gas liquids components such as propane.


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Supporting Information Outlet molecular flow of reactant and various products during ethane aromatization; Outlet molecular flow of reactant and various products during ethylene aromatization; Temperature-programmed surface reaction of ethylene on zinc exchanged HZSM-5 catalyst; Typical outlet flow profiles of ethane and different products during back-transient experiment; TPO of Zn exchanged HZSM-5 catalyst after ethane aromatization at 600oC for 20 min; Outlet molecular flow of benzene and toluene as a function of time during back-transient experiment at 600oC. This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgment 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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Transient Kinetic Study of Ethane and Ethylene Aromatization over

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