Methane aromatisation based upon elementary steps: Kinetic and catalyst descriptors

Microporous and Mesoporous Materials 164 (2012) 302–312

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Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso

Methane aromatisation based upon elementary steps: Kinetic and catalyst descriptors Kae S. Wong a, Joris W. Thybaut a,⇑, Elisabeth Tangstad b, Michael W. Stöcker b, Guy B. Marin a a b

Laboratory for Chemical Technology, Ghent University, Krijgslaan 281-S5, B-9000 Ghent, Belgium SINTEF Materials and Chemistry, Process Chemistry Department, N-0314 Oslo, Norway

a r t i c l e

i n f o

Article history: Available online 14 July 2012 Keywords: Microkinetic modelling Methane aromatisation MCM-22 ZSM-5 Molybdenum

a b s t r a c t An elementary steps-based kinetic model was developed for the ‘steady-state’ reaction kinetics of methane aromatisation over HMCM-22 and HZSM-5 supported Mo. The model adequately describes the experimentally observed trends over a wide range of operating conditions, i.e., temperatures from 873 to 973 K, space times between 32 and 161 kgcat s mol1 and methane inlet molar fractions from 0.2 to 0.98 at atmospheric pressure. A benzene selectivity of 80 mol% can be achieved at 6% of methane conversion over Mo/HMCM-22 at 973 K. The reaction proceeds via similar pathways over Mo/HMCM-22 and Mo/HZSM-5: methane is first dimerised into ethene on Mo sites followed by ethene oligomerisation and cyclisation into benzene on the acid sites. Confinement effects induced by the larger pore mouth and near circular channel structure of ZSM-5 enhance the Mo dispersion, leading to a higher methane dimerisation rate coefficient over Mo/HZSM-5. Similar protonation enthalpies on MCM-22 and ZSM-5, i.e., -56 kJ mol1 for hexene, indicate that both zeolites have acid sites with a comparable strength. Nevertheless, confinement effects result in a more pronounced reactant stabilisation by chemisorption on Mo/HZSM-5 compared to Mo/HMCM-22. Ó 2012 Elsevier Inc. All rights reserved.

1. Introduction In the last decades, advanced techniques are being developed to exploit alternative energy sources and, hence, to gradually replace the rapidly exhausting resources of crude oil [1]. Natural gas seems to be one of the most promising alternatives to replace oil and to bridge the gap between a fossil fuel- and renewable-based technology [2]. The conversion of methane, which is the main constituent of natural gas, into added-value products, has been and still is one of the most interesting and challenging topics in the field of natural gas exploitation [3]. New breakthroughs in C1 chemistry and related technology will expand the utilisation of methane both as raw material for liquid fuels and as alternative feedstock for the petrochemical industry [1]. The catalytic conversion of methane may proceed via direct or indirect routes. The latter involve the production of synthesis gas followed by Fischer–Tropsch Synthesis (FTS) or methanol production, where the CO/H2 mixture is further converted into the desired products [4,5]. The direct conversion of methane into chemicals can proceed via oxidative and non-oxidative routes [6]. The non-oxidative aromatisation of methane into aromatics was first documented in 1993 [7], following the intensive study of oxidative

⇑ Corresponding author. E-mail address: [email protected] (J.W. Thybaut). 1387-1811/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.micromeso.2012.07.002

coupling of methane (OCM) into more valuable hydrocarbons such as ethene since 1982 [8]. The stable and symmetrical methane molecule, containing four C–H bonds with a bond energy of 435 kJ mol1, is difficult to activate without entirely decomposing the molecule [9]. The transformation of methane to aromatics in the absence of oxygen is thermodynamically more favourable than its conversion to alkenes at oxidative conditions, i.e., an in principle high selectivity towards higher hydrocarbon products can be obtained. The main aromatic products that can be obtained, i.e., benzene, toluene and xylenes, are important intermediates in the chemical industry and can be easily separated from the methane feed. Additionally, valuable H2 is formed as a by-product at typical aromatisation conditions, i.e., 973 K and 101,325 Pa [1,9,10]. In spite of research on catalyst composition, synthesis method and reaction conditions, the maximum reported methane conversion at typical methane aromatisation conditions remains low: 10–15%. The equilibrium conversion for the transformation of pure methane to benzene, based on Eq. (1), only amounts to 5% at 873 K, 11% at 973 K and 16% at 1023 K.

6CH4  C6 H6 þ 9H2

ð1Þ

Typical catalysts used, such as Mo/HZSM-5 and Mo/HMCM-22, exhibit low stability and severely deactivate due to coke deposition [3,4,6–8,11–13]. Over the years, much of the research related to non-oxidative methane dehydrogenation and aromatisation focused on

K.S. Wong et al. / Microporous and Mesoporous Materials 164 (2012) 302–312

303

Nomenclature Roman symbols – transition state A pre-exponential factor CN carbon number Ea activation energy (J mol1) F molar flow rate (mol s1) k kinetic coefficient of an elementary step (Pa1 s1 or s1) K equilibrium coefficient (Pa1 or Pa) m, n reaction orders P partial pressure (Pa) R net production rate (mol kgcat1 s1) r reaction rate (mol kgcat1 s1) S selectivity (mol%) SS sum of squares ((mol s1)2) SSQ objective function T absolute temperature (K) w response weight W catalyst mass (kg) X conversion (%) y molar fraction Y molar yield (mol%)

optimisation of catalytic materials [11]. The location and oxidation state of the metal species as well as the pore structure and acidity of the microporous materials are important properties that determine the catalytic behaviour. It is generally accepted that bifunctional Mo-based zeolite catalysts, comprising a transition metal and an acidic function, effectively catalyse methane aromatisation in the temperature range of 973–1073 K [9,14–16]. Zeolites are commonly used as supports for metal and metal oxide catalysts. Next to improving the metal dispersion, the well defined channel dimensions and structure of zeolites lead to shape selectivity which may play a crucial role in the formation of the desired products [17,18]. The activation of the C–H bonds in methane and the formation of the initial C–C bonds occur on the Mo species, followed by oligomerisation of C2 species and cyclisation of heavier intermediates to aromatics over the Brønsted acid sites of the zeolite. Benzene is not the final product as the formation of naphthalene was frequently reported as well as the production of toluene and xylene isomers [13–15,19–21]. Wang et al. [7] and Zhang et al. [22] showed that the Brønsted acid sites of ZSM-5 are crucial for methane aromatisation since Mo/NaZSM-5, Mo/Al2O3 and unsupported Mo2C catalysts show low conversions (0.2%) with high ethane selectivity (80%). Zhang et al. [18] established that H-type silica-alumina zeolites, such as ZSM-5, ZSM-8, ZSM-11 and b zeolite, with a two-dimensional channel structure and a pore diameter approximating the kinetic diameter of benzene, are promising supports. The HZSM-5 supported Mo catalyst exhibiting 60% benzene selectivity at 11% of methane conversion was shown to be the most promising catalyst among a series of Mo supported catalysts investigated by Xu and Lin [16]. The shape selectivity of the 10 member-ring channels of HZSM-5 promotes the selective formation of benzene despite the thermodynamically favourable formation of heavy aromatics and coke [23]. Next to Mo/HZSM-5, HMCM-22 supported Mo catalysts are most widely used. Similar catalyst activity and selectivity were observed for HZSM-5 and HMCM-22 supported Mo catalysts. However, Mo/HMCM-22 showed higher benzene selectivity, i.e., 80% benzene selectivity at 10% of methane conversion, and stron-

Constants h Planck constant (6.626  1034 m2 kg s1) Boltzmann constant (1.381  1023 m2 kg s2 K1) kB R universal gas constant (8.314 J mol1 K1) Subscripts ave average alk alkylation calc calculated exp experiment for forward phys physisorption chem chemisorption HT hydride transfer i.s. internal standard olig oligomerisation pdeh protolytic dehydrogenation ply protolysis prot protonation REG regression rev reverse RES residual resp response tab tabulated

ger tolerance towards coke at comparable reaction conditions [15]. These differences were attributed to the different channel structures [9,17,24–26]. The performance of Mo/HZSM-5 depends essentially on the Brønsted rather than the Lewis acidity [23]. Liu et al. [27,28] proposed that a small number of Brønsted acid sites per unit cell, was sufficient to accomplish the aromatisation reaction. On the other hand, a higher number of Brønsted acid sites could favour the formation of carbonaceous deposits leading to catalyst deactivation [21,29]. In this work, a kinetic study of oxygen-free methane aromatisation over HMCM-22 supported Mo catalysts has been carried out aiming at a better understanding of the methane aromatisation reaction mechanism and at identifying opportunities for improvement. Kinetic data obtained in a fixed-bed reactor under ‘steadystate’ conditions are used to construct a kinetic model based on elementary steps. This model relies on kinetic and catalyst descriptors. The effect of a different support topology on the latter is investigated by the assessment of a literature reported methane dehydroaromatisation data set on ZSM-5. 2. Procedures A methane aromatisation kinetic data set in which the operating conditions were systematically varied has been acquired on Mo/HMCM-22. The experimental details are outlined below. The kinetic study of methane aromatisation over Mo/HZSM-5 was carried out using literature reported kinetic data [30]. Yao et al. [31] reported 21 experiments on methane aromatisation over 3 wt.%Mo/HZSM-5. 2.1. Materials A 5.3 wt.%Mo/HMCM-22 catalyst with Si/Al ratio of 15.5 was prepared by incipient wetness. The solid samples were pressed, crushed and sieved to obtain catalyst granules in the size range 210–297 lm for subsequent use in the aromatisation reaction. Prior to feed gas exposure, the catalyst was dried at 373 K and

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heated gradually in air to 773 K. Finally, the temperature was elevated further to 973 K in argon and maintained at this temperature for 4 h. 2.2. Kinetic measurements and data analysis Kinetic measurements were carried out in a fixed-bed reactor at atmospheric pressure. The experimental setup consists of a 10 mm i.d. tubular quartz reactor with a thermocouple well inside the catalyst bed for controlling the reaction temperature. The reactor loaded with 0.5 g of catalyst was connected to the feed section and analytical equipment. The feed section consists of several feed lines, where a premixed gas of methane with 2% nitrogen is fed through the main line. The total pressure is monitored at the entrance of the reactor and is never found to deviate significantly from atmospheric pressure. At a space time of 54 kgcat s mol1, kinetic measurements were carried out at five different temperatures, i.e., 873, 898, 923, 948 and 973 K. At the highest temperature, i.e., 973 K, the space time has been varied from 32, over 40, 54, and 81 to 161 kgcat s mol1. Methane inlet partial pressures amounting to 20, 40, 60 and 98 kPa have been applied and were balanced with nitrogen to achieve a total pressure of 0.1 MPa, at 973 K and a space time amounting to 54 kgcat s mol1. The gaseous components were analysed on-line by a gas chromatograph (Agilent technologies 7890 A). The operating conditions and catalyst properties of Mo/ HMCM-22 and Mo/HZSM-5 [31] are listed in Table 1. Molar outlet flow rates are calculated based on an internal standard added to the feed, i.e., N2 and Ar for the reaction performed on Mo/HMCM-22 and Mo/HZSM-5, respectively:

F out ¼

F in yi:s:;in yi:s:;out

ð2Þ

The conversion of methane is calculated as follows:

dF j ¼ Rj dW

ð6Þ

with initial conditions:

F j ð0Þ ¼ 0; when W ¼ 0 The net production rate of gas phase component j is calculated using Eq. (7).

Rj ¼

X n ki P observable hm surfacespecies

The pseudo-steady state approximation is applied to all surface species, i.e., their net production rate as calculated from Eq. (7) is set equal to zero, resulting in a set of non-linear algebraic equations. The subroutine DASPK [33] is used to solve the corresponding set of differential algebraic equations composed by Eqs. (6) and (7). The estimation of kinetic parameters is performed by minimising the objective function SSQ(k), which is the residual sum of squares of the component molar flow rates, with respect to the model parameter vector k [34].

SSQðkÞ ¼ ð3Þ

The molar yield of a hydrocarbon product i, based on the molar feed of methane is calculated using Eq. (4).

CN Fi 100 F CH4 ;in

ð4Þ

with CN the carbon number. The selectivity to a hydrocarbon product i is calculated by

Y i ð%Þ CN Fi Si ð%Þ ¼ 100 100 ¼ X CH4 ð%Þ F CH4 ;in  F CH4 ;out

ð5Þ

2.3. Model regression A pseudo-homogeneous one-dimensional model was implemented to describe the kinetic data. The flow was assumed to be perfectly mixed in radial direction and the ratio of internal reactor Table 1 Comparison of experimental operating conditions and catalyst properties for Mo/ HMCM-22 and Mo/HZSM-5 [31]. Mo/HMCM-22

Mo/HZMS-5 [31]

Pressure (Pa) Temperature (K) Space time (kgcat s mol1) Reactor i.d. (mm) Methane inlet partial pressure (kPa) Catalyst mass (g)

101325 873–973 32–161 10 20–98 0.5

101325 913–973 38–115 12 90.5 0.4

Catalysts Mesh size (lm) Molybdenum loadings (wt.%) Si/Al

210–297 5.3 15.5

125–177 3 40

ð7Þ

i

nresp nexp X X k wj ðF p;j  F^p;j Þ2 !minimum j¼1

F CH4 ;in  F CH4 ;out X CH4 ð%Þ ¼ 100 F CH4 ;in

Y i ð%Þ ¼

diameter to the pellet diameter was 10. A plug flow behaviour is further confirmed by obtaining a Bodenstein number of 60. Experimentally measured molar outlet flow rates of the observed products, such as methane, ethane, ethene, benzene, toluene, naphthalene and hydrogen, were used as responses in the regression. At the examined conditions, the Weisz modulus was always around 2  102, showing that the obtained kinetic data are not being diffusion limited and, hence, can be considered as intrinsic [32]. The molar flow rate of a reference product component j is calculated using the following continuity equation.

ð8Þ

p¼1

where Fp,j is the j-th experimental response in the p-th experiment and Fp;j is the j-th response value calculated for the p-th experiment and wj are the weights for each response. The latter are calculated from

Pnexp 1 i¼1 F j;i wj ¼ P 1 nresp Pnexp j¼1 i¼1 F j;i

ð9Þ

The initial minimisation of the objective function in the model regression was carried out using the Rosenbrock method [35]. Then, the obtained result was optimised by applying the ODRPACK package [33], in which the Levenberg–Marquardt algorithm was implemented [36]. The global significance of the regression is assessed by applying the F test, in which the regression sum of squares, SSREG, and the residual sum of squares, SSRES, are divided by their respective degrees of freedom.

F calc ¼

SSREG p SSRES nexp nresp p

Pnresp Pnexp ^ 2 ½ j¼1 wj p¼1 ðF p;j Þ =p ¼ Pnresp Pnexp ½ j¼1 wj p¼1 ðF p;j  F^p;j Þ2 =ðnexp nresp  pÞ

ð10Þ

with p the number of adjustable parameters. The calculated F value is compared to the tabulated F value (Ftab) for a probability level of 1  a. When Fcalc > Ftab (p, nexpnresp  p, 1  a), the regression is considered as globally significant. The significance of each of the estimated model parameters is assessed by the t test as shown in Eq. (11).

tcalc ¼

j jbi  b i ^ rðbi Þ

ð11Þ

 , which is Each parameter is tested against a reference value, b i typically taken as zero. Hence, tcalc is the ratio between the esti^ ðbi Þ. The resulting mated value of bi and its standard deviation r

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a

^ ðbi Þ 6 bi bi  t tab ðnexp nresp  p; 1  Þr 2

10

5

0 8000

10000

12000

14000

16000

14000

16000

TOS (s)

(b) 100 80

ð13Þ

3. Experimental results

60

40

20

0 8000

10000

12000

TOS (s) Fig. 1. (a) Methane conversion and (b) benzene selectivity over Mo/HMCM-22 as a function of time on stream. (j) T = 948 K, W/FCH4,in = 54 kgcat s mol1, P CH4 = 98 kPa; (d) T = 973 K, W/FCH4,in = 40 kgcat s mol1, P CH4 = 98 kPa; (N) T = 973 K, W/ FCH4,in = 54 kgcat s mol1, P CH4 = 20 kPa.

15

20

Equilibrium methane conversion

15 10

10

YieldC6H6 (%)

In this work, a microkinetic model was developed to determine kinetic and catalyst descriptors for methane aromatization. As a result, ‘steady-state’ methane conversion into products such as benzene, toluene and naphthalene is focused on while dynamic catalyst behaviour will be addressed in future work. Sobalik et al. [38] divide the reaction into 3 stages. Stage 1 corresponds to the development of the active catalyst form; stage 2 to the optimal catalyst performance and stage 3 to catalyst deactivation. During the period of steady catalyst performance, carbon balances ranging from 80% to 95% were observed. Deviations from the carbon balance, which can be attributed to the formation of active molybdenum carbide and/or coke, have no apparent effect on activity and selectivity. For the purpose of model validation a ‘steady-state’ kinetic regime, where no significant changes in methane conversion and product concentrations with time on stream occur, has been identified, see Fig. 1. Four data points were taken between 2.5 and 4 h on stream and within the experimental error, both the methane conversion and the benzene selectivity over Mo/HMCM-22 remain unchanged. Because the behaviour represented in Fig. 1 is representative for what has been observed over the entire range of operating conditions, the data acquired between these times on stream in all experiments are considered as being in the steady state. Figs. 2–4 show the effects of the operating conditions on the methane conversion by and the benzene yield over Mo/HMCM-22 and Mo/ HZSM-5. Benzene yields rather than selectivities are shown in Figs. 2– 4 as the model developed here is a steady state kinetic model, i.e., coke is not accounted for in the model, as explained above. Typical space time curves as the one represented in Fig. 2 were observed. For Mo/ HMCM-22, the methane conversion increases almost linearly from 4% to 7.6%, which corresponds with a space time increase from 32 to 82 kgcat s mol1. At a space time of 161 kgcat s mol1, 8.6% of methane was converted, which is approximately 4% lower than the equilibrium methane conversion. The benzene yield steadily increases up to a space time of 82 kgcat s mol1. In this regime, the reactions leading to benzene formation are dominating. At higher space times, no further improvement of the benzene yield was obtained over Mo/ HMCM-22 regardless the increasing methane conversion. A constant benzene yield of 5.4 mol% was obtained between space times of 82 and 161 kgcat s mol1, in which methane conversion rises 1.5%. On

SelC6H6 (%)

   Ea 1 1  kT ¼ kave exp R T T ave

15

ð12Þ

It must be noted that there is a one-to-one relationship between the statistical significance of an adjustable parameter according to its calculated t value and its corresponding confidence interval not including the value of zero. A pre-exponential factor and an activation energy are used to calculate the rate coefficient. The reparameterised Arrhenius equation was used to avoid correlation between the preexponential factors and the activation energies and, hence, obtain better estimates for the parameters as documented by Froment and Bischoff [37]:

20

XCH4 (%)

a

^ ðbi Þ 6 bi þ ttab ðnexp nresp  p; 1  Þr 2

(a)

XCH4 (%)

value of tcalc is compared with a tabulated value obtained for nexpnresp  p degrees of freedom and a probability level of 1  a. When tcalc > ttab (nexpnresp  p, 1  a/2), the parameter is considered to be significant. The confidence interval of the estimated parameters bi delimits the range in which the optimal parameter value bi is located within a selected probability level of 1  a, as shown in Eq. (12). The probability is chosen as 95% for all statistical tests, i.e., a = 0.05.

5 5

0

0 50

100

150 -1

W/FCH4,in (kgcat s mol ) Fig. 2. Methane conversion and benzene yield as a function of space time for methane aromatisation over Mo/HMCM-22 and Mo/HZSM-5 [31] at 973 K, P CH4 = 98 kPa for Mo/HMCM-22, P CH4 = 90.5 kPa for Mo/HZSM-5. Methane conversion on (j) Mo/HMCM-22 and (h) Mo/HZSM-5; benzene yield on (d) Mo/HMCM22 and (s) Mo/HZSM-5. Symbols represent the experimental data. The bold line represents the equilibrium methane conversion based on Eq. (1). The solid and dashed lines represent the methane conversion and benzene yield calculated with the parameter values reported in Table 2 and 3 for Mo/HMCM-22 and Mo/HZSM-5.

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K.S. Wong et al. / Microporous and Mesoporous Materials 164 (2012) 302–312 25

20

20

15 10 10

YieldC6H6 (%)

XCH4 (%)

15

5 5

0 850

900

0 1000

950

Temperature (K) Fig. 3. Methane conversion and benzene yield as a function of temperature for methane aromatisation over Mo/HMCM-22 and Mo/HZSM-5 [31], W/ FCH4,in = 54 kgcat s mol1, P CH4 = 98 kPa for Mo/HMCM-22, P CH4 = 90.5 kPa for Mo/ HZSM-5. Methane conversion on (j) Mo/HMCM-22 and (h) Mo/HZSM-5; benzene yield on (d) Mo/HMCM-22 and (s) Mo/HZSM-5. Symbols represent the experimental data. The bold line represents the equilibrium methane conversion based on Eq. (1). The solid and dashed lines represent the results calculated with the parameter values reported in Tables 2 and 3 for Mo/HMCM-22 and Mo/HZSM-5.

50

15

40

20 5

YieldC6H6 (%)

XCH4 (%)

10 30

10

0

0 25

50

75

100

Methane inlet partial pressure (kPa) Fig. 4. Methane conversion and benzene yield as a function of methane inlet partial pressure for methane aromatisation over Mo/HMCM-22, at 973 K, W/ FCH4,in = 54 kgcat s mol1. (j) Methane conversion on Mo/HMCM-22; (d) benzene yield on Mo/HMCM-22. Symbols represent the experimental data. The bold line represents the equilibrium methane conversion based on Eq. (1). The solid lines were results calculated with the parameter values reported in Tables 2 and 3 for Mo/HMCM-22.

the other hand, heavier aromatics such as toluene and naphthalene were observed. This indicates that benzene could be further consumed by consecutive reactions, forming heavy aromatics at high space times. A better catalyst performance was achieved over Mo/ HZSM-5, see Fig. 2. At a space time of 40 kgcat s mol1, similar amounts of methane were converted over Mo/HMCM-22 and Mo/ HZSM-5, however, a slightly higher benzene yield was achieved over Mo/HZSM-5, i.e., 4.2 mol%. For a space time of 115 kgcat s mol1, 5.4 mol% of benzene yield was obtained over both Mo/HMCM-22 and Mo/HZSM-5, at methane conversions of 8.1% and 7.3%, respectively. Fig. 3 shows that both methane conversion and benzene yield increase with increasing temperature. The conversion of methane is nearly half of the equilibrium conversion, in the temperature range of 873–973 K at a space time of 54 kgcat s mol1. At such conditions, the reaction is kinetically limited and further increase in

space time improves the methane conversion. Similar catalyst performances between Mo/HMCM-22 and Mo/HZSM-5 were observed in the investigated temperature range. At 873 K, the methane conversion over Mo/MCM-22 is 2.2% and the corresponding benzene yield equals 1.4 mol%. The methane conversion slightly increases with the temperature. For example, at 923 and 973 K the methane conversion amounts to 5% and 6% with corresponding benzene yields of 2.7 and 5 mol%, respectively. The minor difference in methane conversion can be attributed to the different methane inlet partial pressure over Mo/HMCM-22 and Mo/HZSM-5, i.e., 98 and 90.5 kPa, respectively. Higher methane conversions are observed at lower methane inlet partial pressures, see Fig. 4. For a methane inlet partial pressure of 20 kPa, a benzene yield of 9.5 mol% was achieved at 12% of methane conversion. A further increase in methane inlet partial pressure to 40 kPa has led to a decrease in methane conversion to 7.8%. Nearly 5 mol% of benzene yield was obtained at 6% of methane conversion at a methane inlet partial pressure of 98 kPa. The experimental methane conversion is nearly half of the equilibrium methane conversion at any methane feed composition. 4. Reaction network development The detailed bifunctional mechanism of methane aromatisation over Mo-based supported zeolite catalysts is still under debate. In particular, the mechanism of methane activation on MoCx species has been discussed extensively in the literature [16,22,23,25,39]. Some authors suggested that ethene or acetylene is the primary intermediate, which will be directly oligomerised into benzene. Other works proposed that methane is initially coupled into ethane, which needs to be dehydrogenated into ethene prior to oligomerisation into benzene. The feature common to these alternative views is that an unsaturated C2 species plays a key role in benzene formation from methane aromatisation over Mo-based supported zeolite catalysts. In this work, this species will be represented as ethene. Moreover, several rate-determining steps have been proposed. Liu et al. [27] proposed that methane dehydrogenation and dimerisation on MoCx species are rate determining since the ethene or ethane aromatisation proceeds easily in the temperature range of 573 and 873 K over HZSM-5 or transition metal modified HZSM5. On the other hand, Iglesia et al. [40] monitored the catalyst initiated reaction networks for methane pyrolysis and concluded that the net forward rate of methane conversion reaction and the product distribution in the chain transfer regime is controlled by the rate of ethene transformation to products. Finite reaction rates for both methane dimerisation and C2 oligomerisation will be considered, cfr. infra. 4.1. Methane dimerisation This work will focus on the acid catalysed mechanism which will be explored in terms of elementary steps to investigate the effect of different acid supports on methane aromatisation. Hence, a more global type of step is used for the metal catalysed mechanism to reduce the number of parameters to be determined from regression. Eq. (14) describes formally the dimerisation of methane into ethene and hydrogen over metal sites.

2CH4  C2 H4 þ 2H2

ð14Þ

The corresponding reaction rate is calculated according to:

r1 ¼ k1 P2CH4 ð1  c1 Þ

ð15Þ

in which c1, the approach to equilibrium for this reaction, can be obtained from:

K.S. Wong et al. / Microporous and Mesoporous Materials 164 (2012) 302–312

c1 ¼

PC2 H4 P2H2 1 K1 P2CH

ð16Þ

4

c1 approaches 0 for an irreversible reaction and 1 at thermodynamic equilibrium. Iglesia et al. [40] showed that this simple, but mechanistically reasonable, rate expression adequately describes methane dimerisation reaction pathways on Mo-based supported zeolite catalyst. 4.2. Ethene to aromatics in elementary steps Based on the information available in the literature and the experimental data analysis discussed in Section 3, a reaction mechanism for methane aromatisation over Mo-based supported zeolite catalyst is proposed, see Fig. 5. After ethene has been formed on the Mo sites via methane dimerisation, see also Section 4.1, it subsequently migrates to the acid sites where protonation occurs as a result of the alkene double bond interaction with the Brønsted acid proton. During protonation, the proton moves from the Brønsted acid site to one of the carbon atoms of the alkene double bond. The p electrons of the double bond neutralize the positive charge of the proton at the carbon atom which is receiving this proton from the Brønsted acid site, hence, inducing a positive charge, c.q., carbenium ion, at the other carbon atom of the double bond. In the model protonation is accounted for via a chemisorption step that includes the protonation as described above as well as the physical adsorption of the protonated hydrocarbon within the zeolite framework because of typical Van der Waals interactions [41]. The latter are typically referred to as physisorption [42]. A similar methodology was used in the kinetic modelling of alkane hydroconversion [43] and propane aromatisation [44]. The chemisorption of alkenes and aromatics, as described above, corresponds to steps 2, 5, 12, 18 and 23 in Fig. 5. The chemisorbed light alkenes undergo acid-catalysed chain growth within the zeolite channels by oligomerisation. During oligomerisation, an alkyl carbenium ion reacts with a gas phase alkene molecule forming larger alkyl carbenium ions. These large alkyl carbenium ions may recrack into smaller alkenes, i.e., steps 3 and 4 in Fig. 5. Light alkenes oligomerisation is expected to proceed easily at methane aromatisation operating conditions, which are more severe than typical oligomerisation conditions [45]. The carbenium ions involved in hydride transfer from alkenes to carbenium ions. The hydride transfer reaction mechanism starts with an attack of the alkene on the carbenium ion. This results in the formation of an activated complex which will decompose into an alkane and an unsaturated carbenium ion [46–49]. In the present model, hydride transfer is mainly responsible for ethane and

307

hexenyl carbenium ion formation (step 6). The latter undergoes cyclisation into a cyclohexyl carbenium ion. It is assumed that the olefinic carbenium ion, C6H11+, instantaneously cyclises into a cyclic carbenium ion. Hence, no finite rate for step 7 in Fig. 5, is considered in the model. The cycloalkyl carbenium ions undergo a series of desorption (steps 8 and 10) and protolysis (steps 9 and 11) to remove hydrogen and form adsorbed cyclic unsaturated species. The latter can subsequently desorb to finally produce the experimentally observed aromatic products. Protolysis, rather than hydride transfer, is expected to play a significant role in these hydrogen removal steps as the reaction is carried out at high operating temperature (973 K) over catalyst of high acidity (Al/ Si = 15.5) [49]. Since no gas phase intermediates involved in this step were observed, such as cyclohexene and cyclohexadiene, it is assumed that these species are converted instantaneously after their formation [44]. Hence, a reaction family called protolytic dehydrogenation is assigned to describe this sequence of rapid one-step carbenium ion desorption step followed by protolysis. In the developed model, the aromatic products are restricted to benzene, toluene and naphthalene as observed experimentally. Methyl ions, formed from methane protolysis (step 21), can attack a free or weakly adsorbed benzene forming the adsorbed toluene species (step 22). The protolysis of butene forms C4H7+ species, which alkylate benzene into an alkyl cyclodienes carbenium ion (step 13). Through a similar series of cyclisation, protolytic dehydrogenation and desorption reactions adsorbed bicylic aromatic species are formed. The model developed here acknowledges that the alkylation of benzene with C4H7+ species can be more facile than that of CH3+ species and, hence, an individual activation energy is assigned for each the two aromatic alkylation steps. The resulting elementary step-based mechanism involves the reaction families as summarized in Table 2: chemisorption, desorption, oligomerisation, b-scission, protolysis, hydride transfer, protolytic dehydrogenation and hydrogenation, alkylation and dealkylation reactions. Cyclization is assumed to be instantaneous and, hence, is not considered separately. 4.3. Pre-exponential factors To each reaction family discussed in Section 4.2 corresponds a pre-exponential factor and activation energy. The pre-exponential factors can be calculated a priori, using transition state theory, to reduce the number of adjustable parameters. This methodology has been widely applied in various heterogeneously catalysed reactions including catalytic and steam cracking [50–52] and methane oxidative coupling [53,54]. Pre-exponential factor values are assessed a priori rather than the activation energy values, given the higher sensitivity of the model simulations to the latter

Fig. 5. Proposed reaction mechanism for methane aromatisation over Mo/HMCM-22.

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Table 2 Reaction families considered in the acid catalysed reaction network for methane aromatisation over Mo-based supported zeolites, see also Fig. 5. Reaction families

Pre-exponential factors (Pa1 s1 or s1) Afor

Arev

Chemisorption/desorption

106

1016

Oligomerisation/b-scission

106

1016

Protolysis

106

106

6

10

106

1016

106

Hydride transfer

Protolytic dehydrogenation/hydrogenation

  hCþ r prot ¼ kchem P C6 hHþ 1  K chem P6C h þ 6 H   hCþ r olig ¼ kolig P C4 hCþ 1  K olig PC6 h þ 2 4 C 2  hCH þ PH r ply ¼ kply P CH4 hHþ 1  K ply P3CH h2 þ 4 H   hC þ PC r HT ¼ kHT P C2 hCþ 1  K HT2PC h6 þ 6 2 C 6   PH2 hAþ r pdeh ¼ kpdeh hCþ 1  K pdeh h 6þ 6

106

1016

Alkylation/dealkylation

⁄

Rate equation

r alk

C

6

 hAþ ¼ kalk P A6 hCHþ 1  K alk PA 7 h 3

6

 CHþ 3

Cyclisation is assumed to be potentially instantaneous and is not considered separately.

compared to the former, see also the supporting information. According to transition state theory the pre-exponential factor of an elementary reaction can be expressed in terms of the molecular partition functions of each reactant (Qi) and the activated complex (Q–):

A¼

kB T Q –  n h Y Qi

ð17Þ

i¼1

kB and h are the Boltzmann and Planck constant, respectively, and the partition functions consist of contributions from translational, rotational and vibrational degrees of freedom. By applying reasonable assumptions on variations in the mobility and chemical bonding of various species in the occurring elementary steps, adequate pre-exponential factor values for each reaction family can be obtained as listed in Table 2. These values are in agreement with those reported by Dumesic et al. [55]. For example, a pre-exponential factor of 106 molecules site1 Pa1 s1 is obtained for the chemisorption by assuming a mobile transition state. Using a typical value for the site density [55], 1019 sites m2, and an average surface area for the Mo-based supported zeolite catalyst of 210 m2 gcat1 [17,27,56], the pre-exponential factor for chemisorption becomes 3.5  106 mol kgcat1 Pa1 s1. For unimolecular reactions, such as desorption, more rotational and translation freedom in the transition state is assumed, leading to pre-exponential values of 1016 molecules site1 s1. By using the same catalyst properties, a pre-exponential factor of 3.5  1016 mol kgcat1 s1 is obtained for desorption. 5. Regression results and discussion 5.1. Kinetic and catalyst descriptors on Mo/HMCM-22 Data at all five temperatures and space times were regressed simultaneously to estimate 17 model parameters. These parameters include two activation energies for the chemisorption of ethene and aromatics and three activation energies for the desorption of adsorbed aromatics, ethyl and hexyl carbenium ions. The model developed here acknowledges that the carbon number effect on the chemisorption energy is more significant than the effect of aromaticity. Hence, the activation energy of hexene chemisorption is set equal to the activation energy of benzene chemisorption and the number of adjustable parameters is reduced by one correspondingly. One activation energy is assigned to each of the following reaction families: oligomerisation, b-scission, protolytic hydrogenation and dehydrogenation, alkylation and dealkylation of toluene and naphthalene. The forward and reverse reactions of hydride transfer and protolysis are described by four activation energies.

The model is found to describe the methane aromatisation kinetics over a wide range of reaction conditions and to adequately capture the major observed trends as shown in Figs. 2, 3 and 6A–L. To ensure the model quality, experimental data obtained for different methane inlet partial pressures were predicted using the established kinetic model, see Figs. 4 and 6M–R, which resulted in a good agreement. The estimated activation energies, along with their corresponding 95% individual confidence intervals, are listed in Table 3, while the rate coefficient for methane dimerisation into ethene and hydrogen, i.e., k1 in Eq. (15), amounted to 4.0  104 s1 at 973 K. Table 3 shows that the activation energies are statistically significant at the 95% probability level. The rather narrow confidence intervals are indicative of strong gradients in the objective function around the optimum parameter values. The F value for the global significance of the regression exceeds the tabulated value by 2 orders of magnitude, i.e., 550, which is a typical range for models exhibiting the agreement with the experimental data as observed in this work. Correlation coefficients between the individual parameter estimates were all lower than 0.5, suggesting no correlation between the estimated parameters. All these analyses indicate a satisfactorily statistical significance of regression justifying a further assessment of the physical significance of the model parameter values. The estimated activation energy for ethene chemisorption is 103.9 kJ mol1 while a lower activation energy is obtained for chemisorption of hexene and aromatics, i.e., 80.4 kJ mol1. Both values lie within the reported range, 80–125 kJ mol1 [57–59]. This difference in chemisorption activation energy indicates that long chain alkenes are more reactive and can be chemisorbed easier on acid sites forming alkyl carbenium ions. A high activation energy is required for the desorption of an ethyl carbenium ion, i.e., 190.8 kJ mol1. The obtained value is in agreement with ab initio calculated values for alkyl carbenium ion desorption from an active Bronsted site in a zeolite framework, 171–213 kJ mol1 [59]. Desorption of hexyl carbenium ions and adsorbed aromatics have activation energies amounting to 199.6 and 127.9 kJ mol1, respectively. The lower desorption activation energy for adsorbed aromatics can be related to the high stability of the product. Adsorbed aromatics, such as protonated benzene, desorb preferably into thermodynamically stable aromatics [60]. The chemisorption enthalpy of hexene is 119.2 kJ mol1, which is more negative than the one obtained for ethene chemisorption, i.e., 86.9 kJ mol1. Its higher carbon number results in more pronounced physical van der Waals interactions of the hexyl ion with the catalyst surface. Moreover, the hexyl ion potentially has more carbon atoms in b position with respect to the charged carbon atom, which also leads to a more pronounced stabilization.

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900 950 Temperature (K)

1000

900 950 Temperature (K)

1000

E 0.2

-2

6 4 2 0 850

900 950 Temperature (K)

1000

0.1 0.0 850

900 950 Temperature (K)

1000

Ethane 10 (μmol/s)

0.6

0

15

50 100 150 -1 W/FCH4,in (kgcat s mol )

200

J 10

0.0

0

0.6

50 100 150 -1 W/FCH4,in (kgcat s mol )

200

K

0.4

-2

-2

0

0.3

5 0

0

50 100 150 -1 W/FCH4,in (kgcat s mol )

200

0.2 0.0

0

50 100 150 -1 W/FCH4,in (kgcat s mol )

200

N

2 25 50 75 Methane inlet partial pressure (kPa)

100

10

P

8

4 2 0

0.0

0

25 50 75 Methane inlet partial pressure (kPa)

100

0.3

Q 0.2

-2

6

Toluene 10 (μmol/s)

0

0.2

-2

4

0

25 50 75 Methane inlet partial pressure (kPa)

100

900 950 Temperature (K)

1000

0.3

F 0.2 0.1 0.0 850

900 950 Temperature (K)

1000

I 0.6 0.3 0.0

0

0.6

50 100 150 -1 W/FCH4,in (kgcat s mol )

200

L

0.4 0.2 0.0

0

50 100 150 -1 W/FCH4,in (kgcat s mol )

200

0.6

O 0.4

-2

0.4

-2

6

Ethane 10 (μmol/s)

Ethene 10 (μmol/s)

0.6

M

8

0

Naphthalene 10 (μmol/s)

5

0.0 850

-2

-2

10

0.2

0.9

H

0.1 0.0

Naphthalene 10 (μmol/s)

Ethene 10 (μmol/s)

0.9

G

C 0.4

-2

0.0 850

-2

D

8

Toluene 10 (μmol/s)

Methane (μmol/s)

0.2

0.3

10

-2

Benzene 10 (μmol/s)

Methane (μmol/s)

Ethane 10 (μmol/s)

2

10

-2

0.4

Naphthalene 10 (μmol/s)

4

15

Benzene 10 (μmol/s)

B

-2

Ethene 10 (μmol/s)

6

0 850

0.6

0.6

A

8

Toluene 10 (μmol/s)

-2

Benzene 10 (μmol/s)

Methane (μmol/s)

10

0

25 50 75 Methane inlet partial pressure (kPa)

100

0.2 0.0

0

25 50 75 Methane inlet partial pressure (kPa)

100

0.3

R 0.2 0.1 0.0

0

25 50 75 Methane inlet partial pressure (kPa)

100

Fig. 6. Product molar flow rates as a function of temperature, space time and methane inlet partial pressure for methane aromatisation over Mo/HMCM-22. Symbols represent the experimental data. The solid lines represent the product molar flow rates calculated with the parameter values reported in Tables 2 and 3 for Mo/HMCM-22.

The low activation energy of oligomerisation, 10.6 kJ mol1, shows that oligomerisation reactions readily take place once small alkenes are protonated on the Bronsted acid sites. The b-scission requires an activation energy of 81.1 kJ mol1, which is slightly lower than the values reported in literature, typically ranging between 115 and 219 kJ mol1 [61–64]. This indicates that, although no pronounced correlation between the parameters was evident from the binary correlation coefficients, i.e., a maximum binary correlation coefficient of 0.5 was obtained between the activation energy of hydride transfer and protolytic hydrogenation, cfr. supra, the energy difference between forward and reverse reaction

steps may be more significant than the exact value. The estimated activation energy for protolysis was 118.3 kJ mol1. This is within the range of reported value for protolysis on acid catalysts, 115– 134 kJ mol1 [62,65]. The high activation energy of protolysis can be attributed to the breaking of C–H bond in methane and forming the unstable methyl ion. The activation energy of benzene alkylation leading to toluene formation was 95.1 kJ mol1, which is 10 kJ mol1 higher than benzene alkylation leading to naphthalene formation. The dealkylation of toluene possesses high activation energy, 190.1 kJ mol1, suggesting that the dealkylation of toluene forming benzene and methyl ion is unfavourable.

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Table 3 Estimated activation energies with 95% confidence interval for methane aromatisation over Mo/HMCM-22 and Mo/HZSM-5. Reaction family

Mo/HMCM-22 Ea,for (kJ mol

Chemisorption/desorption (C2) Chemisorption/desorption (C6) Chemisorption/desorption (A6) Oligomerisation/b-scission Hydride transfer Protolytic dehydrogenation/hydrogenation Protolysis Alkylation/dealkylation (toluene) Alkylation/dealkylation (naphthalene)

1

)

103.9 ± 0.9 80.4 ± 0.5 80.4 ± 0.5 10.6 ± 0.1 43.1 ± 0.4 132.3 ± 1.2 118.3 ± 0.7 95.1 ± 0.7 85.3 ± 0.6

5.2. Assessment of zeolite framework effects The kinetic model for methane aromatisation over Mo/HZSM-5 was validated using literature reported data [31]. Three products, i.e., methane, hydrogen and benzene, were reported at seven temperatures and three space times. No toluene and naphthalene yields were included in the results. Due to the limited number of reported responses, the network considered here is restricted to benzene formation, resulting in 11 adjustable model parameters. In analogy to the kinetic model for Mo/HMCM-22, these parameters include two activation energies for the chemisorption of ethene and benzene, three activation energies for the desorption of adsorbed benzene, ethyl and hexyl carbenium ions, two activation energies for the hydride transfer reactions and four activation energies for oligomerisation, b-scission, protolytic dehydrogenation and hydrogenation. The six parameters excluded from the kinetic model of Mo/HZSM5 are the activation energies for the forward and reverse methane protolysis and benzene alkylation into toluene and naphthalene. Table 3 shows that the model parameters possess sufficiently low standard errors to ensure narrow confidence intervals. The activation energies of C2 desorption and b-scission posses highest binary correlation coefficient, i.e., 0.65, which indicates that the estimated parameters are not correlated. The parameters are globally significant (F test value = 367) and all t values of individual parameters are 2 orders of magnitude higher than the tabulated t value. A rate coefficient of 3.0  102 s1 was obtained for methane dimerisation into ethene and hydrogen over Mo/HZSM-5 at 973 K. This rate coefficient is comparable to the one reported by Iglesia et al. [66], i.e., 4.0  102 s1, for methane dimerisation over Mo/HZSM-5 at 950 K and a methane partial pressure of 0.5 bar. Fig. 7 shows that the developed model describes adequately the experimentally observed results. The main discrepancy between the simulated and experimental data is an underestimation of the hydrogen flow rate, which can, however, be attributed to the absence of heavier aromatics and/or carbon deposits in the available data set. The model, which assumes hydrogen is produced only from methane aromatisation into benzene, does not account for hydrogen production from consecutive and/or side reactions. Hence, the predicted hydrogen flow rate is lower than the experimentally obtained value. A ZSM-5 zeolite has a channel structure consisting of a 10-ring straight channel of 0.53  0.56 nm diameter, as well as a 10-ring sinusoidal channel of 0.51  5.5 nm diameter. MCM-22 consists of an interconnected building unit forming two independent pore systems: two-dimensional, sinusoidal 10-ring interlayer channels of 0.40  0.59 nm and 12-ring interlayer supercages of 0.71  0.18 nm with 0.40  0.59 nm entrance aperture [15], see Fig. 8. Xu et al. [17,56] show that the surface area as well as micropore volume of ZSM-5 are modified to a greater extent, compared to MCM-22, by the incorporation of molybdenum. The micropore volume of MCM22 was reduced only from 0.16 to 0.14 cm3 g1 by incorporating 6 wt.% of molybdenum, while impregnating the same amount of

Mo/HZSM-5 1

Ea,rev (kJ mol

)

190.8 ± 1.0 199.6 ± 0.8 127.9 ± 1.0 81.1 ± 0.4 50.4 ± 0.3 100.8 ± 0.5 20.6 ± 0.1 190.1 ± 0.8 116.8 ± 0.8

Ea,for (kJ mol1)

Ea,rev (kJ mol1)

88.2 ± 4.9 59.3 ± 4.5 59.3 ± 4.5 40.4 ± 0.5 33.1 ± 0.6 122.3 ± 4.5 – – –

216.2 ± 5.7 227.0 ± 6.5 135.7 ± 5.1 109.4 ± 3.8 40.1 ± 0.8 89.4 ± 4.1 – – –

molybdenum to ZSM-5 leads to a change in micropore volume from 0.11 to 0.03 cm3 g1. This suggests that the larger pore mouth of ZSM-5 allows the incorporation of molybdenum in the channels, leading to a better Mo dispersion. Compared to the molybdenum clusters formed without restriction on the external surface of the zeolite, the molybdenum formed in the channels is expected to exhibit a small particle size due to channel dimension limitations, hence, resulting in a higher Mo dispersion. The higher rate coefficient of methane dimerisation over Mo/HZSM-5 indeed reflects this enhanced Mo dispersion on the ZSM-5 zeolite, overcompensating the lower Mo content. The ethene chemisorption enthalpy on Mo/HZSM-5 was 128 kJ mol1, which is in close agreement with the DFT calculation of ethene chemisorption on HZMS-5, i.e., 135 kJ mol1 by Nguyen et al. [67]. The hexene chemisorption enthalpy on Mo/ HZSM-5 amounting to 167.7 kJ mol1 is comparable to the DFT calculated hexene chemisorption on HZSM-5, 170 kJ mol1 reported by the same authors [67]. The activation energy of ethene chemisorption on Mo/HZSM-5 was 15.7 kJ mol1 lower than on Mo/HMCM-22, see Table 3. The activation energy of ethyl carbenium ion desorption on Mo/HZSM-5 was 25.4 kJ mol1 higher than on Mo/HMCM-22. The low activation energy for chemisorption on Mo/HZSM-5 suggests that the chemisorption of a small alkene is more favourable on Mo/HZSM-5. The activation barrier for hexene chemisorption on Mo/HZSM-5 amounts to 59.3 kJ mol1, which is 21.1 kJ mol1 lower than hexene chemisorption on Mo/HMCM-22. The enthalpies corresponding to the different interactions between the gas-phase hydrocarbon and the zeolite are illustrated in Fig. 9. The reaction enthalpy of alkene protonation, DHprot, can be obtained by the enthalpy difference between the chemisorbed r-complex and physisorbed p-complex [41]:

DHprot ¼ DHchem  DHphys

ð18Þ

The physisorption enthalpy of hexene on ZSM-5 can be calculated by applying Eq. (19), leading to a reaction enthalpy of 60 kJ mol1 for hexene protonation on ZSM-5.

DHphys ¼ 9:5CN  46:2  5

ð19Þ

Using the physisorption enthalpy of hexane on MCM-22 reported by Denayer et al. [68], i.e., 63 kJ mol1, a protonation enthalpy of 56 kJ mol1 was obtained for hexene on MCM-22. The similar protonation enthalpy for hexene on ZSM-5 and MCM-22 indicates that the acid strength of the sites on both catalysts considered in this work is comparable. Observed differences between the two zeolites are, hence, mainly related to topological effects rather than to differences in acidity. The reaction enthalpies of oligomerisation over Mo/HMCM-22 and Mo/HZMS-5 value also comparable, i.e., 70.5 and 69 kJ mol1 respectively. The reaction enthalpy of hydride transfer over Mo/ HMCM-22 was 7.3 kJ mol1, which was very close to the reaction enthalpy of hydride transfer over Mo/HZSM-5, 7.0 kJ mol1.

311

5 0 900

925 950 975 Temperature (K)

1000

D 10

-2

Methane (μmol/s)

15

5 0

0

50

100 -1 W/FCH4, in (kgcat s mol )

150

B

4 2 0 900

925 950 975 Temperature (K)

1000

10

E

8 6 4 2 0

C 100

-2

6

150

50 0 900

925 950 975 Temperature (K)

1000

150

F 100

-2

-2

10

8

Hydrogen 10 (μmol/s)

Benzene 10 (μmol/s)

A

Benzene 10 (μmol/s)

Methane (μmol/s)

15

Hydrogen 10 (μmol/s)

K.S. Wong et al. / Microporous and Mesoporous Materials 164 (2012) 302–312

0

50

100

150 -1

W/FCH4, in (kgcat s mol )

50 0

0

50 100 -1 W/FCH4, in (kgcat s mol )

150

Fig. 7. Product molar flow rates as a function of temperature and space time for methane aromatisation over Mo/HZSM-5. Symbols represent the experimental data. The solid lines represent the product molar flow rates calculated with the parameter values reported in Tables 2 and 3 for Mo/HZSM-5.

ZeOH + alkene(g) ΔHphys ΔHchem

ZeOH + alkene(phys) ΔHprot

ZeO- + carbenium ion Fig. 9. Enthalpies corresponding to the physisorption, chemisorption and protonation between the gas phase hydrocarbon and the zeolite.

considered as catalyst descriptors. On the other hand, the reaction enthalpies of protonation, oligomerisation, hydride transfer and protolytic dehydrogenation are the kinetics descriptors, i.e., are independent of the catalyst. 6. Conclusions

Fig. 8. The framework structure of (a) ZSM-5 and (b) MCM-22. Zeolite channels are computed using the software, CAVER [69], shown in blue.

Similar reaction enthalpies were observed for protolytic dehydrogenation over Mo/HMCM-22 and Mo/HZMS-5, i.e., around 32 kJ mol1. Hence, the physisorption enthalpies, which describe the physisorption of gas phase molecules on catalyst acidic sites, can be

An elementary steps-based kinetic model has been constructed for methane aromatisation over Mo-based supported zeolite catalysts. Methane aromatisation over Mo/HMCM-22 and Mo/HZSM-5 proceeds via a similar reaction mechanism: methane is first dimerised into ethene on Mo sites followed by ethene oligomerisation into benzene on acid sites. The acid catalysed elementary steps are grouped into: chemisorption, desorption, oligomerisation, b-scission, hydride transfer, protolytic dehydrogenation and hydrogenation, protolysis, alkylation and dealkylation of toluene and naphthalene. Narrow confidence intervals are obtained for all estimated parameters and the activation energies for chemisorption, desorption, b-scission and protolysis agree with literature values. A better Mo dispersion within the HZSM-5 crystal, thanks to near circular channel structure and larger pore mouth size of ZSM-5, results in a methane dimerisation rate coefficient on Mo/HZSM-5 which is 2 orders of magnitude higher than on Mo/MCM-22. The acid strength of the sites on both catalyst considered in this work is comparable as a similar hexene protonation enthalpy was obtained on ZSM-5 and MCM-22. The main differences with respect to the acid catalysed elementary steps is situated in the physical interaction of the hydrocarbons with the zeolite walls. Also similar reaction enthalpies were obtained for oligomerisation, b-scission, hydride transfer, protolysis, protolytic dehydrogenation and hydrogenation. Hence, the observed differences between the two zeolites are mainly related to topological effects rather than differences in acidity.

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