H-ZSM-5

Applied Catalysis A: General 266 (2004) 55–66

CH4 –C2 H6–CO2 conversion to aromatics over Mo/SiO2 /H-ZSM-5 Michael C.J. Bradford a,∗ , Mure Te a , Mahesh Konduru b , Digna X. Fuentes a a

b

CeraMem Corporation, 12 Clematis Avenue, Waltham, MA 02453, USA Institut de Recherches sur la Catalyse-CNRS, 02 Avenue Albert Einstein, 69626 Villeurbanne Cedex, France Received in revised form 23 January 2004; accepted 28 January 2004 Available online 20 March 2004

Abstract The influence of carbon dioxide and ethane on the conversion of methane to aromatics over Mo/SiO2 /H-ZSM-5 was investigated at 973 K and 1.2 bar. The immediate effects of carbon dioxide introduction (CO2 /CH4 = 0.04) include a slight increase in the rates of methane conversion and hydrogen production, but a slight decrease in the rates of C2 + production. In addition, carbon dioxide introduction improved catalyst stability for methane conversion and C2 + production. Conversely, methane conversion was completely suppressed in the presence of carbon dioxide and ethane (CO2 /C2 H6 /CH4 = 0.04/0.11/1). Characterization of as-prepared and spent catalyst samples via X-ray diffraction, 27 Al MAS NMR, nitrogen adsorption, and temperature-programmed oxidation (TPO) reveals that the accumulation of coke decreases the accessible micropore surface area and volume, distorts the zeolite lattice, and ultimately, deactivates the catalyst. While carbon dioxide addition inhibits the rate and consequences of coke accumulation, ethane addition accelerates the rate and consequences of coke accumulation. © 2004 Elsevier B.V. All rights reserved. Keywords: Methane; Ethane; Carbon dioxide; Aromatization

1. Introduction The efficacy of transition metals (such as molybdenum, tungsten, and rhenium) loaded onto zeolite supports (such as H-ZSM-5, H-ZSM-11, and H-MCM-22) for the conversion of methane to aromatics has been thoroughly examined and demonstrated [1–17], as summarized in two recent reviews [1,2]. Silanation of the zeolite prior to modification with molybdenum has been shown to increase selectivity to benzene and to suppress the accumulation of coke [13]. Carbon dioxide addition, in dilute concentrations, has been reported to increase selectivity to benzene, as well as increase catalyst stability [14,15]. Ethane addition has been reported to promote methane conversion and the formation of aromatic products [16]. A commercially viable process for the conversion of natural gas to aromatics will likely necessitate the separation and recycle of non-aromatic C2 + products (such as ethane), such that a regenerable catalyst is required that exhibits moderate activity, selectivity and stability for methane conversion in the presence of C2 +. Therefore, the investigation reported herein focused on the influence of car∗ Corresponding author. Tel.: +1-781-899-4495; fax: +1-781-899-6478. E-mail address: [email protected] (M.C.J. Bradford).

0926-860X/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2004.01.026

bon dioxide and ethane on the conversion of methane to aromatics over Mo/SiO2 /H-ZSM-5.

2. Experimental 2.1. Material synthesis H-ZSM-5 was prepared from NH4 + -ZSM-5 (CBV5524G, Zeolyst International, Si/Al = 27.8 ± 3.3) via calcination at 773 K for 8 h in flowing oxygen (O2 /Ar = 1/9; GHSV = 600 cm3 /h·gcat ). Silanated H-ZSM-5, hereafter referred to as SiO2 / H-ZSM-5, was prepared according to the procedure reported by Ding et al. [13]. Ten grams of H-ZSM-5 were added to a well-mixed solution containing 1.46 g of 3-amino propyl triethoxy silane (Gelest Inc.) in 100 ml of ethanol (Aldrich, 99.5%) and mixed thoroughly. The resultant slurry was subsequently stirred at 333–373 K on a hot plate until all of the solvent had evaporated, after which the dry material was calcined in flowing oxygen (O2 /Ar = 1/9; GHSV = 600 cm3 /h·gcat ) for 16 h at 773 K, cooled to room temperature, and stored in a dessicator. Mo/SiO2 /H-ZSM-5 (Mo/Al = 0.43) was synthesized by the solid-state mixing of molybdenum(VI) oxide and

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M.C.J. Bradford et al. / Applied Catalysis A: General 266 (2004) 55–66

Table 1 Bulk particle size distribution of the as-prepared Mo/SiO2 /H-ZSM-5 catalyst Particle size range (␮m)

Fraction (wt.%)

d > 250 250 ≥ d > 125 d ≤ 125

14 76 10

SiO2 /H-ZSM-5 followed by calcination, according to a procedure adapted from that reported by Borry et al. [17]. That is, molybdenum(VI) oxide (Aldrich, >99.5%, ACS Reagent) and SiO2 /H-ZSM-5 were physically mixed with an alumina mortar and pestle for 15–30 min, loaded into a quartz tube, and exposed to flowing oxygen (O2 /Ar = 1/9; GHSV = 600 cm3 /h·gcat ) at room temperature. Then, the catalyst temperature was ramped from 298 to 623 K at 3 K/min, held at 623 K for 24 h, ramped from 623 to 773 K at 10 K/min, held at 773 K for 24 h, ramped from 773 to 973 K at 10 K/min, held at 973 K for 2 h, soaked from 973 to 383 K at 5 K/min, and cooled to room temperature. The calcined material was then removed from the quartz tube, ground and sieved (Table 1), and stored in a dessicator. 2.2. Material characterization As-prepared and spent material samples were characterized using several techniques. • Direct current plasma emission spectroscopy (DCPES) was performed with a Beckman Spectrospan VI Emission Spectrometer (Luvak Co. Inc.) to measure the molybdenum and aluminum contents of as-prepared, calcined materials; • 27 Al magic angle spinning (MAS)–nuclear magnetic resonance (NMR) was performed with a 360 MHz Tecmag spectrometer using a sample pretreatment with ammonium sulfate at a relative humidity of 81% (Spectral Data Services) to determine the relative amount and coordination of aluminum species in as-prepared and spent materials. All spectra were recorded at a spectrometer frequency of 94.7 MHz using a magic angle spinning rate of 10 kHz and a 1.1 ms pulse; • X-ray diffraction (XRD) was performed with a Rigaku Geigerflex diffractometer using filtered Cu K␣ radiation (Materials Characterization Laboratory at the Pennsylvania State University) to probe bulk material structure; • Nitrogen adsorption isotherms for as-prepared and spent materials were obtained from 0.08 ≤ P/P0 ≤ 0.30 with a Nova 3200 (Quantachrome Corporation) and analyzed using the t-plot method in order to estimate sample micropore volume and micropore surface area [18]; • Combustion-infrared (IR) analysis of as-prepared and spent samples was performed with a LECO Combus-

tion Infrared Analyzer (Luvak Co. Inc.) to quantify total carbon content; and • Temperature-programmed oxidation (TPO) of as-prepared and spent samples was performed ex-situ in a tubular quartz reactor with flowing 9.7% O2 in He (3680 cm3 /h) from 293 to 973 K at 4 K/min, using an on-line gas chromatograph (SRI) equipped with a Carbosphere 80/100 column and a thermal conductivity detector for oxygen and carbon dioxide analysis. 2.3. Experimental apparatus for methane conversion High purity methane (99.998%), nitrogen (99.999%), ethane (99.96%, balance ethylene), and carbon dioxide (99.99%) were used as-received without further purification and introduced independently using mass flow controllers (Porter) to a quartz reactor. The reactor was placed within a vertical split-tube furnace (Lindberg) equipped with a thermocouple for external reactor temperature control and another thermocouple inserted into the catalyst bed to monitor catalyst temperature. Pressure was monitored through the use of transducers (DJ Instruments) located upstream and downstream of the reactor. The temperature of the reactor effluent line was typically maintained at 473–503 K. The effluent from the reactor was fed to the sampling valve of an on-line gas chromatograph (SRI model 8610C), in which samples were subjected to simultaneous analysis with flame ionization (FID), thermal conductivity (TCD), and helium ionization (HID) detectors. Hydrocarbons were separated using a 30 m MXT-1 capillary column and analyzed via FID; hydrogen, nitrogen, methane, carbon monoxide and carbon dioxide were separated with a HayeSep D column and analyzed via TCD and HID. The absolute rates of methane and carbon dioxide conversion were determined with the TCD via reference to nitrogen, and the rates of hydrocarbon formation (and ethane conversion) were quantified with the FID via reference to methane. Measured TCD and HID response values were used with FID relative sensitivity values available in the literature [19] during data reduction. 2.4. Experimental procedures 2.4.1. CH4 conversion In a typical experiment, ca. 1.5 g of catalyst was packed into the reactor between plugs of quartz wool, and thereafter purged for 15 min at 293 K in flowing nitrogen. The catalyst was then exposed to flowing nitrogen from 298 to 973 K at 10 K/min, followed by a switch to CH4 /N2 (CH4 /N2 = 1/1; GHSV = 1500 cm3 /h·gcat ) at 973 K and 1.2 bar. 2.4.2. CH4 –CO2 conversion Each experiment was initiated using the procedure described for “CH4 conversion.” However, after 5 min time-on-stream, carbon dioxide was introduced (CH4 /N2 / CO2 = 25/25/1; GHSV = 1530 cm3 /h·gcat ).

M.C.J. Bradford et al. / Applied Catalysis A: General 266 (2004) 55–66

2.4.3. CH4 –CO2 –C2 H6 conversion Each experiment was initiated using the procedure described for “CH4 –CO2 conversion.” However, after an additional 5 min time-on-stream, ethane was introduced (CH4 /N2 /C2 H6 /CO2 = 25/25/2.8/1; GHSV = 1614 cm3 /h·gcat ). 2.5. Thermodynamic equilibrium calculations All equilibrium calculations were performed using HSC Chemistry v.4.1 (Outokumpu Research).

3. Results 3.1. Characterization of as-prepared materials As-prepared materials were characterized via DCPES (Table 2), combustion-IR/TPO (Table 3), nitrogen adsorption (Table 4), 27 Al MAS NMR (Fig. 1A), and XRD (Fig. 2A). The atomic Mo/Al and C/Al ratios of the as-prepared Mo/SiO2 /H-ZSM-5 catalyst are 0.43 (Table 2) and 0.10 (Table 3), respectively. Application of the t-plot method to the nitrogen adsorption data (as recommended Table 2 Material composition as determined via direct current plasma emission spectroscopy (DCPES) Material

H-ZSM-5 SiO2 /H-ZSM-5 Mo/SiO2 /H-ZSM-5

Composition (wt.%) Mo

Al

(Mo/Al)atomic

<0.003 <0.003 2.32

1.79 1.63 1.53

<0.0005 <0.0005 0.43

Table 3 Carbon contents of as-prepared materials determined via combustioninfrared (C-IR) and temperature-programmed oxidation (TPO) Material

H-ZSM-5 SiO2 /H-ZSM-5 MoO3 /SiO2 /H-ZSM-5

Carbon content TPO (wt.%)

C-IR (wt.%)

(C/Mo)atomic

0.000 0.000 0.000

0.047 0.038 0.028

– – 0.10

a

a Calculated from carbon content via C-IR and Mo content via DCPES (Table 2).

57

by Derouane [18]) revealed that the micropore volume and surface area of H-ZSM-5 decreased after modification by silica and molybdenum oxide (Table 4). 27 Al MAS NMR spectra of the H-ZSM-5 and SiO2 /H-ZSM-5 samples revealed only tetrahedral, framework aluminum species (+54.8 ppm); however, peaks in the NMR spectrum of the Mo/SiO2 /H-ZSM-5 catalyst at −0.1 ppm and +13.6 ppm revealed the presence of octahedral, extraframework aluminum species, and a hydrated aluminum molybdate phase [20], respectively. The XRD spectrum of the Mo/SiO2 /H-ZSM-5 catalyst is not significantly different from those of either H-ZSM-5 or SiO2 /H-ZSM-5 (Fig. 2A). Presumably, the octahedral aluminum and aluminum molybdate phases are either highly dispersed throughout the zeolite matrix, and/or at a concentration limit below the lower detection limit of the diffractometer employed. 3.2. Performance of Mo/SiO2 /H-ZSM-5 for CH4 –CO2 –C2 H6 conversion to aromatics Reactant conversions and product formation rates are compared as a function of time-on-stream and reactor inlet composition in Figs. 3–7 and Table 5. Inspection of these data reveals several general trends. Specifically, carbon dioxide addition: • Increases and stabilizes methane conversion (Fig. 3); • Stabilizes ethane formation (Fig. 5) at the expense of ethylene formation (Fig. 6); • Decreases propane formation (Fig. 7); • Initially decreases but stabilizes benzene (Fig. 8) and toluene formation (Table 5); • Increases and stabilizes hydrogen production (Table 5); and • Results in continuous carbon monoxide production (Table 5). The presence of ethane in the CH4 –CO2 feed stream. • Suppresses methane conversion to the extent that methane formation (indicated by negative conversion) is observed (Fig. 3); and • Increases the formation rates of all C2 + hydrocarbons (Figs. 6–8) and hydrogen. However, ethane conversion decreases continuously with time-on-stream (Fig. 4).

Table 4 Micropore surface areas and volumes of as-prepared and spent catalysts determined via application of the t-plot method to N2 adsorption isothermsa Material

Condition

Micropore surface area (m2 /g)

H-ZSM-5 SiO2 /H-ZSM-5 Mo/SiO2 /H-ZSM-5 Mo/SiO2 /H-ZSM-5 Mo/SiO2 /H-ZSM-5 Mo/SiO2 /H-ZSM-5

As-prepared As-prepared As-prepared After CH4 conversion After CH4 –CO2 conversion After CH4 –CO2 –C2 H6 conversion

305.1 295.2 250.0 206.2 222.7 105.1

a

± ± ± ± ± ±

0.8 2.2 0.3 7.9 5.5 6.8

Micropore volume (cm3 /g) 0.1273 0.1222 0.1039 0.0859 0.0937 0.0427

± ± ± ± ± ±

0.0002 0.0010 0.0000 0.0030 0.0018 0.0028

All experimental data are based on two independent measurements and are reported as an average ± non-biased standard deviation (±1σ n−1 ).

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M.C.J. Bradford et al. / Applied Catalysis A: General 266 (2004) 55–66

Fig. 1. 27 Al MAS NMR spectra of (A) as-prepared H-ZSM-5, SiO2 /H-ZSM-5 and Mo/SiO2 /H-ZSM-5; and (B) Mo/SiO2 /H-ZSM-5 after exposure to CH4 , CH4 /CO2 , or CH4 /CO2 /C2 H6 for ∼24 h at 973 K.

3

3

(A)

(B)

H-ZSM-5

CH4/CO2/C2H6

2 Normalized Intensity

Normalized Intensity

2

SiO2/H-ZSM-5

1

CH4/CO2

1

Mo/SiO2/H-ZSM-5 CH4

0

0 20

22

24

o

2θ

26

28

30

20

22

24

o

26

28

30

2θ

Fig. 2. Normalized XRD spectra of (A) as-prepared H-ZSM-5, SiO2 /H-ZSM-5 and MoO3 /SiO2 /H-ZSM-5; and (B) Mo/SiO2 /H-ZSM-5 after exposure to CH4 , CH4 /CO2 , or CH4 /CO2 /C2 H6 for ∼24 h at 973 K.

Inlet flow rate (␮mol C/s·gcat )

Conversion (%)

Consumption rate (␮mol C/s·gcat )

Formation rate of carbon-containing molecules (␮mol C/s·gcat )

Formation rate of H2

CH4

CO2

C2 H6

CH4

CO2

C2 H6

CH4

CO2

C2 H6

CO

C 2 H4

C2 H6

C3 H8

C6 H 6

C7 H8

(␮mol/s·gcat )

9.31 9.28 9.30

0.00 0.37 0.37

0.00 0.00 2.08

8.0 ± 0.8 9.1 ± 0.8 −1.9 ± 0.5

– 100 100

– – 93 ± 3

0.74 ± 0.08 0.84 ± 0.08 −0.17 ± 0.04

– 0.37 0.37

– – 1.93

0.01 0.58a 0.62a

0.054 0.044 0.130

0.015 0.015 –

0.001 0.001 0.007

0.55 ± 0.07 0.44 ± 0.03 1.20 ± 0.07

0.051 0.034 0.166

1.2 ± 0.1 1.4 ± 0.1 1.8 ± 0.1

Reaction conditions: CH4 GHSV = 750 ± 1 cm3 /h·gcat , CH4 /N2 ratio = 1/1. a Note than a system oxygen balance based entirely on the rate of CO conversion and the exclusive formation of CO as the only oxygenate product results in a small discrepancy which is due to 2 the formation of other oxygen-containing products (like water), and/or to experimental error.

M.C.J. Bradford et al. / Applied Catalysis A: General 266 (2004) 55–66

Table 5 Performance of Mo/SiO2 /H-ZSM-5 for CH4 , CH4 –CO2 , and CH4 –CO2 –C2 H6 conversion to aromatics during the initial 1–4 h on-stream at 973 K and 1.2 bar

59

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M.C.J. Bradford et al. / Applied Catalysis A: General 266 (2004) 55–66

16% 14%

CH4 Conversion

12% 10% 8% 6% 4% 2% 0% -2% -4% 0

5

10

15

20

25

Time-on-Stream (h) Fig. 3. CH4 conversion over Mo/SiO2 /H-ZSM-5 at 973 K and 1.2 bar during catalyst exposure to (䊏, 䊐) CH4 /N2 , (, 䉱) CH4 /CO2 /N2 , or (䊊, 䊉) CH4 /C2 H6 /CO2 /N2 . Negative CH4 conversion indicates CH4 formation. Reaction conditions are provided in Table 5. Curves are only intended as visual guides.

conversion to carbon monoxide increases, and the extent of methane conversion to higher hydrocarbons decreases; and • Selectively suppresses naphthalene (C10 H8 ) formation, thereby shifting the equilibrium composition to favor benzene and ethylene (Fig. 10).

3.3. Thermodynamic equilibrium calculations The influence of carbon dioxide on the conversion of methane to hydrogen, acetylene, ethylene, ethane, benzene, naphthalene, carbon monoxide and water at thermodynamic equilibrium was calculated at conditions comparable to those used experimentally (973 K, 1.2 bar, CH4 /(N2 + CO2 ) = 1/1). The calculations indicate that carbon dioxide addition:

The influence of ethane on the conversion of methane at thermodynamic equilibrium was also calculated at conditions comparable to those used experimentally (973 K, 1.2 bar, CH4 /CO2 /(N2 +C2 H6 ) = 16/1/20.8). Calculations indicate that ethane addition:

• Does not significantly influence the total methane conversion for CO2 /CH4 < 0.10 (Fig. 9); however, as carbon dioxide concentration increases, the extent of methane

• increases the apparent C2 + hydrocarbon yield (Fig. 11);

C2H6 Conversion

100%

80%

60%

40% 0

5

10

15

20

25

Time-on-Stream (h) Fig. 4. C2 H6 conversion over Mo/SiO2 /H-ZSM-5 at 973 K and 1.2 bar during catalyst exposure to (䊊, 䊉) CH4 /C2 H6 /CO2 /N2 . Reaction conditions and results are provided in Table 5. The curve is only intended as a visual guide.

M.C.J. Bradford et al. / Applied Catalysis A: General 266 (2004) 55–66

61

C2H6 Formation ( mol C/s gcat)

0.025

0.020

0.015

0.010

0.005

0.000 0

5

10

15

20

25

Time-on-Stream (h)

Fig. 5. C2 H6 formation over Mo/SiO2 /H-ZSM-5 at 973 K and 1.2 bar during catalyst exposure to (䊏, 䊐) CH4 /N2 , and (, 䉱) CH4 /CO2 /N2 . Reaction conditions are provided in Table 5. Curves are only intended as visual guides. 0.40

C2H4 Formation ( mol C/s gcat)

0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00 0

5

10

15

20

25

Time-on-Stream (h)

Fig. 6. C2 H4 formation over Mo/SiO2 /H-ZSM-5 at 973 K and 1.2 bar during catalyst exposure to (䊏, 䊐) CH4 /N2 , (, 䉱) CH4 /CO2 /N2 , or (䊊, 䊉) CH4 /C2 H6 /CO2 /N2 . Reaction conditions are provided in Table 5. Curves are only intended as visual guides.

C3H8 Formation ( mol C/s gcat)

0.03

0.02

0.01

0.00 0

5

10

15

20

25

Time-on-Stream (h) Fig. 7. C3 H8 formation over Mo/SiO2 /H-ZSM-5 at 973 K and 1.2 bar during catalyst exposure to (䊏, 䊐) CH4 /N2 , (, 䉱) CH4 /CO2 /N2 , or (䊊, 䊉) CH4 /C2 H6 /CO2 /N2 . Reaction conditions are provided in Table 5. Curves are only intended as visual guides.

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M.C.J. Bradford et al. / Applied Catalysis A: General 266 (2004) 55–66

C6H6 Formation ( mol C/s gcat)

1.40 1.20 1.00 0.80 0.60 0.40 0.20 0.00 0

5

10

15

20

25

Time-on-Stream (h)

Fig. 8. C6 H6 formation over Mo/SiO2 /H-ZSM-5 at 973 K and 1.2 bar during catalyst exposure to (䊏, 䊐) CH4 /N2 , (, 䉱) CH4 /CO2 /N2 , or (䊉) CH4 /C2 H6 /CO2 /N2 . Reaction conditions are provided in Table 5. Curves are only intended as visual guides.

Fig. 9. The influence of CO2 on CH4 conversion to hydrocarbons (C2 H2 , C2 H4 , C6 H6 and C10 H8 ) and CO at thermodynamic equilibrium. Initial conditions: T = 973 K; P = 1.2 bar; CH4 /(CO2 + N2 ) = 1/1. 80 70

C6H6

C10H8

Selectivity (%)

60 50 40 30 20 C2H 4

10 0 0.00

0.05

0.10

0.15

CO2 / CH4

Fig. 10. The influence of CO2 on hydrocarbon selectivity (C2 H4 , C6 H6 and C10 H8 ) at thermodynamic equilibrium on an atomic carbon (rather than molecular) basis (selectivity to C2 H2 is <0.1%). Initial conditions: T = 973 K; P = 1.2 bar; CH4 /(CO2 + N2 ) = 1/1.

M.C.J. Bradford et al. / Applied Catalysis A: General 266 (2004) 55–66

63

16

Apparent C2+ Yield (%)

15 14 13 12 11 10 9 8 0.00

0.05

0.10

0.15

C2H6 / CH4 Fig. 11. The influence of C2 H6 on apparent hydrocarbon yield (moles of C converted to C2 H2 , C2 H4 , C6 H6 and C10 H8 , normalized to the moles of C at the initial condition) at thermodynamic equilibrium. Initial conditions: T = 973 K; P = 1.2 bar; CH4 /CO2 /(C2 H6 + N2 ) = 16/1/20.8.

• inhibits methane conversion, resulting in negative methane conversion (i.e., net methane formation) for C2 H6 /CH4 > 0.117 (Fig. 12); and • decreases selectivity to benzene, while increasing selectivity to naphthalene (Fig. 13). Qualitatively, these results are consistent with experimental observations (Section 3.2). 3.4. Characterization of spent catalysts Spent catalyst samples were characterized via TPO, nitrogen adsorption, 27 Al MAS NMR, and XRD. TPO spectra of the catalyst after exposure to methane at 973 K reveal

gradual increases in the amount of coke deposited and the temperature required for coke removal with increasing time-on-stream (Fig. 14), in agreement with results reported by Ding et al. [21]. Additional TPO spectra obtained after catalyst exposure to either CH4 , CH4 –CO2 , or CH4 –CO2 –C2 H6 for 24 h at 973 K reveal that while the introduction of carbon dioxide into the reactor reduces the rate and amount of coke accumulation on the catalyst, the rate and amount of coke accumulation doubled when ethane was present in the reactor feed (Fig. 15). A comparison of catalyst carbon content (Fig. 15) with catalyst micropore volume after reaction (Table 4) demonstrates that the loss in catalyst pore volume during reaction is associated with the accumulation of coke within the catalyst pores. That is,

20

CH4 Conversion (%)

15

10 C2H6 / CH4 = 0.117 5

0

-5 0.00

0.05

0.10

0.15

C2H6 / CH4 Fig. 12. The influence of C2 H6 on total CH4 conversion to hydrocarbons (C2 H2 , C2 H4 , C6 H6 and C10 H8 ) and CO at thermodynamic equilibrium. Initial conditions: T = 973 K; P = 1.2 bar; CH4 /CO2 /(C2 H6 + N2 ) = 16/1/20.8. Ethane conversion at equilibrium exceeds 98% for C2 H6 /N2 > 0.024.

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M.C.J. Bradford et al. / Applied Catalysis A: General 266 (2004) 55–66

70 C10H8 60

Selectivity (%)

50 C6H6

40 30 20 10

C2H4

0 0.00

0.05

0.10

0.15

C2H6 / CH4 Fig. 13. The influence of C2 H6 on hydrocarbon selectivity (C2 H4 , C6 H6 and C10 H8 ) at thermodynamic equilibrium on an atomic carbon basis (selectivity to C2 H2 is <0.1%). Initial conditions: T = 973 K; P = 1.2 bar; CH4 /CO2 /(C2 H6 + N2 ) = 16/1/20.8.

the catalyst C/Mo ratios after exposure to CH4 –CO2 (22.7), CH4 (29.0) and CH4 –CO2 –C2 H6 (46.3) correlate with the observed reductions in micropore volume (0.0102, 0.0180, and 0.0612 cm3 /g, respectively).

A reduction in accessible pore volume due to coke accumulation is also associated in part with catalyst deactivation. That is, while the observed decrease in micropore volume for Mo/SiO2 /H-ZSM-5 after exposure to CH4 –CO2 –C2 H6 for 24 h from 0.104 to 0.043 cm3 /g (Table 4) correlates well

6 C/Mo 56.9

25

37 h

29 h

38.6

20

16 h

15.1

4

3 12.5

4h 2

8.3

2h 1

Rate of CO2 Formation (µmol/s gcat)

Normalized Rate of CO2 Formation

5

C/Mo 15

CH4/CO2/C2H6

46.3 +/- 0.7

CH4/CO2

22.7 +/- 0.7

CH4

29.0 +/- 3.0

10

5 4.0

1h 0 0

200

400

600

800

o

Temperature ( C) Fig. 14. TPO spectra of Mo/SiO2 /H-ZSM-5 as a function of exposure time to CH4 /N2 at 973 K and 1.2 bar. Coke accumulation for each sample, expressed as the molar ratio of carbon to molybdenum (C/Mo), was determined by normalizing sample carbon content (determined ex situ via combustion-IR) to catalyst molybdenum content (see Table 2). TPO conditions: β = 4 ◦ C/min; 9.74% O2 in He; GHSV = 61,600–98,200 cm3 /h·gcat . The curves are intended only as visual guides.

0 0

200

400

600

800

o

Temperature ( C) Fig. 15. TPO spectra of Mo/SiO2 /H-ZSM-5 after catalyst exposure to CH4 /N2 , CH4 /CO2 /N2 , or CH4 /C2 H6 /CO2 /N2 for 24 h at 973 K and 1.2 bar. TPO conditions: β = 4 ◦ C/min; 9.74% O2 in He; GHSV = 50,000–71,000 cm3 /h·gcat . The curves are intended only as visual guides.

M.C.J. Bradford et al. / Applied Catalysis A: General 266 (2004) 55–66

with the observed decrease in ethane conversion from 99 to 53% (Fig. 4), the relative decrease in methane conversion during catalyst exposure to CH4 for 24 h from 8.9 to 3.4% (Fig. 3) cannot be accounted for solely by the observed decrease in accessible pore volume from 0.104 to 0.086 cm3 /g (Table 4). 27 Al MAS NMR spectra of spent catalyst samples, relative to the spectrum of the as-prepared Mo/SiO2 /H-ZSM-5 catalyst (Fig. 1), exhibit lower signal-to-noise ratios and broader peaks for tetrahedral framework aluminum species (near +50 to +51 ppm), and lack the sharp peaks at −0.1 ppm and +13.6 ppm indicative of octahedral, extraframework aluminum species, and a hydrated aluminum molybdate phase. Therefore, these 27 Al MAS NMR spectra reveal that coke accumulation blocks access to sites and causes a distortion of the zeolite lattice. In addition, XRD spectra of spent catalyst samples after exposure to either CH4 or CH4 –CO2 –C2 H6 reveal a significant decrease in intensity in the range of 23.2–23.4◦ 2θ (associated with the 5 0 1 planes [22]), indicating that reaction and coke accumulation result in a distortion of the zeolite structure.

4. Discussion Several aspects of the reaction mechanism for methane conversion to aromatics over Mo/H-ZSM-5 have been investigated and elucidated [1–17], and the results reported herein are consistent with most of these findings. An initial induction period is observed prior to the formation of aromatic products during which methane reduces MoOy moieties to generate molybdenum carbide (␤-Mo2 C or ␣-MoC1−x ) and/or molybdenum oxycarbide (MoOx Cy ) species [4], e.g., via: 2CH4 + 2MoO3 → Mo2 C + CO2 + 4H2 O

(1)

The reduced molybdenum surface then catalyzes the reforming of methane to yield a transient production of synthesis gas: CH4 + H2 O
CO + 3H2

(2)

CH4 + CO2
2CO + 2H2

(3)

Once the concentrations of CO2 and H2 O decrease below a threshold (that is dependent on the reaction temperature and contact time [14]), a distribution of CHx fragments then accumulates on the molybdenum carbide (or oxycarbide) surface: CH4 + (8 − 2x)∗
CHx− ∗ (4−x) + (4 − x)H∗

(4)

where (*) is a molybdenum atom on the catalyst surface and (∗ #) is the number of surface atoms (#) in the active site ensemble (see [23]). As the surface coverage of CHx (most likely CH [24]) species increases, the coupling of CHx occurs: 2CHx− ∗(4−x)
C2 H2x + (8 − 2x)∗

(5)

65

C2 H2x species (such as acetylene [24]) presumably then oligomerize on the molybdenum carbide (or oxycarbide) surface—such as is observed on Ni(1 1 1) [25]—as well as migrate to Brönsted acid sites, where oligomerization can also occur, yielding primarily benzene (C6 H6 ), naphthalene (C10 H8 ), and coke (Cx Hy ): 3C2 H2 → C6 H6

(6)

5C2 H2 → C10 H8 + H2
 x x−y C 2 H 2 → Cx H y + H2 2 2

(7) (8)

The formation of ethylene and ethane can also occur via acetylene and ethylene hydrogenation, respectively, on the molybdenum carbide (or oxycarbide) surface: C2 H2 + H2
C2 H4

(9)

C2 H4 + H2
C2 H6

(10)

The introduction of ethane into the feed to the reactor during CH4 –CO2 conversion results in notable increases in the production of ethylene (Fig. 6), benzene (Fig. 8) and coke (Fig. 15), as well as a cessation of methane conversion and a net production of methane (Fig. 3). Therefore, the elementary steps that comprise reactions (4), (5), (9) and (10) are likely reversible. In contrast to the observation in this study that methane conversion is suppressed in the presence of a relatively high concentration of ethane (in agreement with expectations derived from thermodynamic equilibrium calculations), other researchers have reported that ethane addition promotes methane conversion to aromatic products [16,26]. For example, the observed enhancement of methane conversion to aromatics at ≤873 K over H-GaAlMFI zeolite in the presence of higher alkanes has been reported to proceed via a combination of alkane dehydrogenation to the corresponding alkene, carbonium ion formation from the alkene via hydrogen transfer from the zeolite, and methane activation by the carbonium ion [27]. Nevertheless, a possible explanation that reconciles this apparent discrepancy (regarding the influence of ethane on methane conversion) is presently unavailable. The accumulation of coke species at the pore mouths and/or within the pores of the catalyst (Fig. 14)—through direct methane decomposition (Eq. (4)) and/or C2 H2x oligomerization (Eq. (8))—decreases the accessible micropore surface area and volume (Table 4), distorts the zeolite lattice (Figs. 1 and 2), and ultimately, deactivates the catalyst (Figs. 3, 4 and 8). However, the introduction of carbon dioxide into the feed (CO2 /CH4 = 0.04) can inhibit the extent of coke formation and accumulation (Table 4 and Fig. 15), resulting in an increase and stabilization of methane conversion (Fig. 3), and an initial decrease but stabilization of the rate of benzene formation (Fig. 8). Similar results have been reported by Liu et al. [14], Shu et al. [15], Tan et al. [28], and Osawa et al. [29].

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The subtle influence of carbon dioxide on the rate of methane conversion is explicable (at least qualitatively) if Eq. (4) is the rate determining step for methane conversion in both the presence and absence of CO2 (and H2 O), such that: 4−x rCH4 = k4 PCH4 θη − k−4 θCHx θH

deactivates the catalyst. While carbon dioxide addition inhibits the rate and consequences of coke accumulation, ethane addition accelerates the rate and consequences of coke accumulation.

(11)

where k4 and k−4 are the forward and reverse reaction rate constants for Eq. (4); PCH4 is the partial pressure of methane; ␪i is the surface coverage of species i; and η = (8 − 2x)∗ (the active site ensemble for methane dissociation). This rate expression indicates that the observed rates of methane conversion (Table 5) are not intrinsic, forward rates but apparent, net rates. Therefore, conditions which decrease the surface coverages of hydrogen atoms and CHx moieties—such as a reduction in conversion by increasing the space velocity (decreasing the contact time), or the consumption of surface hydrogen and/or CHx species via reaction with carbon dioxide (Eq. (12)) or an intermediate derived from carbon dioxide (Eq. (13)), respectively—can increase the apparent, observed rate of methane conversion. CO2 ∗ + H∗
CO∗ + OH∗

(12)

CH–∗3 + OH∗
CO∗ + 2H∗ +∗

(13)

The influence of carbon dioxide on the rates of aromatic and coke formation may also be a consequence of the same mechanism. That is, surface oxygen and/or hydroxyl species generated from the dissociation and/or reaction of carbon dioxide might react with surface CHx moieties and simply reduce the surface coverage of said species, thereby inhibiting CHx –coupling (reaction (5)) and C2 H2x oligomerization (reactions (6)–(8)). Regardless, the formulation of definitive conclusions beyond the hypothesis presented herein about the roles of carbon dioxide and ethane on the mechanism of methane conversion over Mo/SiO2 /H-ZSM-5 requires additional experimental evidence.

5. Summary The influence of carbon dioxide and ethane on the conversion of methane to aromatics over Mo/SiOx /H-ZSM-5 was investigated at 973 K and 1.2 bar. The immediate effects of carbon dioxide introduction (CO2 /CH4 = 0.04) include a slight increase in the rates of methane conversion and hydrogen production, but a slight decrease in the rates of C2 + production. In addition, carbon dioxide introduction improved catalyst stability for methane conversion and C2 + production. Conversely, methane conversion was completely suppressed in the presence of carbon dioxide and ethane (CO2 /C2 H6 /CH4 = 0.04/0.11/1). Characterization of as-prepared and spent catalyst samples via X-ray diffraction, 27 Al MAS NMR, nitrogen adsorption, and temperature-programmed oxidation reveals that the accumulation of coke decreases the accessible micropore surface area and volume, distorts the zeolite lattice, and ultimately,

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