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Detailed Reaction Kinetics for the Dehydrogenation of Methylcyclohexane over Pt Catalyst Muhammad Usman,* David Cresswell, and Arthur Garforth School of Chemical Engineering and Analytical Science, The University of Manchester, Manchester M13 9PL, United Kingdom ABSTRACT: Detailed reaction kinetics of the dehydrogenation of methylcyclohexane were studied over an in-house-prepared 1.0 wt % Pt/γ-Al2O3 catalyst. Experiments were conducted in a fixed-bed reactor for a wide range of operating conditions including reactions without hydrogen in the feed. Kinetic model equations were developed, and the experimental data were analyzed according to the power-law, LangmuirHinshelwoodHougenWatson (LHHW), and HoriutiPolanyi kinetic mechanisms. The rate of loss of the first hydrogen molecule in the LHHW single-site surface reaction mechanism was found to be the ratecontrolling step. Experiments with 1-methylcyclohexene confirmed that the rate-controlling step does not lie after the loss of the first hydrogen molecule.
1. INTRODUCTION Developing a hydrogen economy is one possible approach for establishing a sustainable and clean energy supply. Howvere, the efficient and safe storage of hydrogen is probably the greatest hurdle in widespread commercialization of hydrogen as a fuel. The methylcyclohexanetoluenehydrogen (MTH) system is considered to offer a practical solution for the generation, storage and transportation, and utilization of hydrogen. Hydrogen is released in the forward reaction by the dehydrogenation of methylcyclohexane (MCH), whereas toluene is hydrogenated to MCH in the reverse reaction. The heart of the MTH system is the highly endothermic dehydrogenation reaction of MCH. Achieving the optimum rate of hydrogen release requires an efficient catalytic material with high activity, selectivity toward aromatics, and stability. Moreover, the design and scaleup of the dehydrogenation reactor require knowledge of the kinetics observed from experimental data for a wide range of operating conditions. Not only is the dehydrogenation of MCH an important reaction in hydrogen storage applications, but it is also considered an essential reforming reaction1 and a heat-sink reaction for aircraft cooling.2 Moreover, it can be considered as a model reaction in the refining of heavy crude oils of naphthenic nature. The kinetics of the dehydrogenation of MCH over supported Pt-containing catalysts has been studied by a number of researchers.118 Sinfelt et al.3 studied the dehydrogenation reaction over 0.3 wt % Pt/Al2O3 catalyst and suggested that the reaction follows a two-step nonequilibrium mechanism in which methylcyclohexane is adsorbed and converted to toluene on the surface in the first step and the toluene formed is desorbed from the surface in the following step. Desorption of toluene was reported to be the rate-controlling step, and toluene was observed to inhibit the reaction. Touzani et al.6 employed a PtSn/Al2O3 catalyst and assumed that the surface reaction was the rate-controlling step, but they reported no toluene inhibition. Pacheco and Petersen7 and Jothimurugesan et al.1 described the adsorption of MCH as the rate-controlling step and reported increased toluene inhibition with increasing temperature. This type of inhibition was also observed by Jossens and Petersen.5 For a Pt/Al2O3 catalyst, van Trimpont et al.8 observed the step in r 2011 American Chemical Society
which adsorbed methylcyclohexadiene (MCHde) is converted to adsorbed toluene to be the rate-controlling but found no toluene inhibition. However, for a PtRe/Al2O3 catalyst, the formation of adsorbed methylcyclohexene from adsorbed methylcyclohexane was described as the rate-determining step, and competitive adsorption was observed for toluene. Chai and Kawakami12 proposed the dehydrogenation of methylcyclohexene to methylcyclohexadiene to be the rate-controlling step. This review of the literature shows that there is significant disagreement in describing the kinetic mechanism of the dehydrogenation reaction of MCH. There is no consensus on the ratedetermining step or the inhibition caused by products. Moreover, no detailed kinetic investigation has been conducted over a wide range of operating conditions including experiments without H2 in the feed and under integral conditions. In previous studies in our research group,16,19,20 a number of commercial and in-housedeveloped catalysts were tested for their activity, selectivity, and stability. An in-house-produced 1.0 wt % Pt/γ-Al2O3 catalyst showed an activity and selectivity comparable to those of other catalysts, even without hydrogen in the feed, but exhibited exceptional stability.19 The present study was thus designed to conduct a detailed kinetic investigation of the dehydrogenation reaction of MCH for a wide range of operating conditions including experiments without hydrogen in the feed for the inhouse-developed (1.0 wt % Pt/γ-Al2O3) catalyst. Model rate expressions based on simple power-law kinetics, single-site and dual-site LangmuirHinshelwoodHougenWatson mechanisms, and competitive and noncompetitive HoriutiPolanyi mechanisms were developed and analyzed kinetically and statistically for the laboratory experimental data.
2. EXPERIMENTAL SECTION 2.1. Experimental Setup. The reactor was designed to operate as a fixed-bed reactor with reactants flowing from the Received: July 15, 2011 Accepted: November 16, 2011 Revised: November 4, 2011 Published: November 16, 2011 158
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Table 3. Mole Fractions in the Vapor at Conversion X
Table 1. Reactor Specifications for the Experimental Rig description
specification
body material
SS-316
reactor tube length (cm)
66
reactor bed length (cm)
55
tube outside diameter (cm)
1.27
tube inside diameter (cm)
1.02
tube wall thickness (cm)
0.1245
thermowell material
Al2O3
thermowell outside diameter (cm)
0.28800.3175 cm
Table 2. Properties of the Alumina Support property physical form extrudate size
extrudates /8 in. (3.175 mm)
1
208 m2/g
pore volume
0.58 cm3/g
median pore diameter
69 Å
representation
mole fraction
MCH
A
yA0 ð1 XÞ 1 þ 3yA0 X
toluene
B
yB0 þ yA0 X 1 þ 3yA0 X
H2
C
yC0 þ 3yA0 X 1 þ 3yA0 X
inert
I
yI0 1 þ 3yA0 X
but also as a heatervaporizersuperheater. The bottom zone was employed to reduce the empty volume of the reactor to support the catalyst bed and to retain any fine particles of coke. Experiments were performed under integral conditions for temperatures of 340380 °C, pressures of 1.0139 bar, H2/MCH ratios of 08.4, and N2/MCH ratios of 01.1. Methylcyclohexane of 99% purity (Sigma-Aldrich) was used to study the dehydrogenation reaction. Typically, on a single day, four experimental runs were conducted that constituted a series of experimental targets (SET). The sequence started with the middle flow rate of MCH (0.25 mL/min) and changed to the lowest flow rate (0.125 mL/min) after 90 min of operation. After an additional 90 min had passed, the conditions were changed to the highest flow rate (0.50 mL/min) of MCH. For each MCH flow rate, two experimental data points were recorded, after 60 and 90 min. At the end of 270 min, the flow rate of MCH was switched back to the middle value (0.25 mL/min). This was done to investigate the short-term deactivation that occurred during the total time of operation. The final run lasted only for 45 min, and a complete SET was ended after a total time of 315 min. At the completion of a SET, the temperature was raised to 450 °C, and the catalyst was reduced overnight under a hydrogen flow rate of 100 mL/min to reclaim the activity of the catalyst for the next SET. 2.4. Product Analyses. The products of the reaction were analyzed by gas chromatography (GC). The major products (containing toluene and methylcyclohexane) of the dehydrogenation were quantified on a Varian 3400 gas chromatograph containing a 100-m nonpolar capillary column (BP-5, 5% phenyl and 95% dimethylpolysiloxane) and equipped with a flame ionization detector. Samples with known compositions were prepared, and calibration curves were developed to check the response of the GC system. The same standards were rerun from time to time to check the variations in the response of the machine.
description
N2 BET surface
component
top to the bottom. It consisted of a stainless steel tube of 1.02-cm i.d. A concentric ceramic thermowell was installed in the reactor through which a K-type thermocouple was passed to measure the axial temperature distribution. Relevant information about the reactor assembly is listed in Table 1. The whole reactor assembly was placed within a three-zone tubular furnace (Carbolite) to maintain the required temperatures. The condensable organic products leaving the reactor were recovered in a surface condenser and analyzed. 2.2. Catalyst Preparation. γ-Al2O3 extrudates provided by Alfa-Aesar (Johnson-Matthey) were employed as a support material for the preparation of the catalyst. The properties of the support are listed in Table 2. To prepare the catalyst, 1.0 wt % Pt metal was loaded by the wet impregnation technique. Chloroplatinic acid (H2PtCl6 3 6H2O) from Sigma-Aldrich was used as the platinum precursor. The method of preparation of the catalyst was based on Alhumaidan et al.’s17 work, and details of the catalyst preparation are given elsewhere.21 The freshly prepared catalyst was calcined and then reduced. Both calcination and reduction were performed in situ. Calcination was performed under an air flow of 100 mL/min by raising the temperature of the catalyst bed at a rate of 3 °C/min to 500 °C, where it was maintained for 5 h. This was followed by reduction under a hydrogen flow of 100 mL/min at 450 °C for 16 h. Inductively coupled plasma emission spectrometry results for the catalyst prepared by Alhumaidan et al.17 using the same recipe provided Pt and Cl contents of 1.0 and 360 °C. Jothimurugesan et al.1 reported a value of 51.9 kJ/mol on a PtRe/alumina catalyst, and Pal et al.9 obtained an activation energy of 56.4 kJ/mol on the same catalyst as used by Jothimurugesan et al.1 For Pt over α- and γ-alumina catalysts, Tsakiris16 reported apparent activation energy values in the range of 44.160.0 kJ/mol, and for 0.3 wt % Pt/α-Al2O3 and 1.0 wt % Pt/γ-Al2O3, the activation energies were 45.7 and 49.8 kJ/mol, 165
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Figure 4. Scatter diagram for the preferred fitting model (M-1) based on single-site LHHW kinetics when loss of first H2 controls the rate.
kinetics, the denominator constants in LHHW-type kinetics usually do not equate to adsorption constants derived independently, meaning that the coefficients obtained here are not true adsorption coefficients but rather lumped empirical apparent coefficients. 3.4.4.4. Supporting Data for the Model Using Experiments with MCHe (Methylcyclohexene). In the kinetic schemes considered in this work, dehydrogenation was assumed to occur in a series of dehydrogenation steps, and MCHe and MCHde were considered as reaction intermediates in the overall dehydrogenation reaction of MCH to toluene. With this in mind, it was considered that dehydrogenation experiments starting with an intermediate as the feed would be valuable in contributing to a better understanding of the reaction mechanism. Thus, experiments were carried out with pure 1-methylcyclohexene (SigmaAldrich, 97 wt % purity) as the feed reactant rather than methylcyclohexane. Under virtually the same conditions as maintained for methylcyclohexane, experiments were carried out under hydrogen at 5 bar total pressure and reactor wall temperatures of 340 and 380 °C. Figure 5 shows the results and a comparison between the MCH and MCHe dehydrogenation reactions. As methylcyclohexene contains one unsaturated bond, it was expected that it would undergo both hydrogenation and dehydrogenation reactions. If the rate-determining step occurred after the loss of the first hydrogen molecule, that is, downstream of the formation of methylcyclohexene intermediate, then a toluene yield comparable to that obtained with MCH would be expected. However, as shown in Figure 5, under all of the conditions studied, the toluene yield was always higher when MCHe was the feed. This tends to support the hypothesis that either the rate-controlling step in the dehydrogenation of MCH occurred in the loss of the first hydrogen molecule or the adsorption of MCH. The same procedure was followed by Corma et al.4 to discriminate among the possible rate-controlling steps for a Pt/zeolite catalyst, and they reached the same conclusion. 3.5. HoriutiPolanyi Kinetics. The HoriutiPolanyi (HP) mechanism33 is an old but still notable mechanism for doublebond hydrogenation. In its present form, which is slightly different
Figure 5. Yields of toluene at 5 bar pressure, H2/HC ratio of 8:1, and reactor wall temperatures of (a) 340 and (b) 380 °C. HC stands for hydrocarbon (i.e., MCH or MCHe).
from the original, it can be written for a simple olefin such as ethylene as shown in Figure 6.34 The mechanism involves the dissociative adsorption of hydrogen on active sites and the nondissociative molecular adsorption of ethene. The adsorbed ethylene reacts with the adsorbed atomic hydrogen to form a half-hydrogenated ethylene species. Further atomic or halfhydrogenation leads to the formation of the saturated ethane. 3.5.1. Kinetic Schemes Based on HoriutiPolanyi Mechanism. Considering competitive and noncompetitive hydrogen adsorption, three separate mechanisms based on HP kinetics can be defined: a competitive HP mechanism, a noncompetitive HP mechanism, and a combined competitive and noncompetitive HP mechanism.35 In the competitive HP mechanism, both hydrogen and hydrocarbon compete for the same type of active sites,35 whereas in the noncompetitive HP mechanism, there are separate classes of sites for hydrogen and hydrocarbons. When hydrogen alone has access to both kinds of sites and not the hydrocarbon, the combination of competitive and noncompetitive mechanism occurs. van Trimpont et al.8 outlined the elementary steps for the dehydrogenation of MCH based on a competitive Horiuti Polanyi concept. The kinetic equations developed for the 166
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Figure 6. Present-day HoriutiPolanyi hydrogenation mechanism for a simple olefin.
Figure 8. Scheme IV, combined competitivenoncompetitive Horiuti Polanyi mechanism.
Table 10. Kinetic Equations Developed Based on the HP Mechanism model type
Figure 7. Scheme III, noncompetitive HoriutiPolanyi mechanism.
kinetic equation
HP-I (noncompetitive)
competitive HP mechanism are more or less the same as those obtained from scheme I (LHHW dual-site surface reaction). Similarly to van Trimpont et al.,8 Alhumaidan et al.17 outlined the elementary steps shown in scheme III (Figure 7) for the noncompetitive HoriutiPolanyi mechanism, different only in the hydrogen adsorptiondesorption step, which was taken to be elementary here as well. Kinetic scheme III starts with the usual molecular adsorption of MCH, which then undergoes dehydrogenation to produce an adsorbed half-hydrogenated MCHe intermediate and adsorbed atomic hydrogen. Two separate kinds of sites are involved for hydrocarbons and hydrogen adsorption: Site s is a hydrocarbon site, whereas site s* is a hydrogen site. Owing to the different categories of the two sites, the mechanism is non-Langmuirian in nature. The half-hydrogenated MCHe further dehydrogenates to produce a MCHe intermediate. Similar abstractions of hydrogen finally lead to adsorbed toluene. The adsorbed toluene is then desorbed from the hydrocarbon sites, and the adsorbed hydrogen is desorbed molecularly from the adjacent hydrogen sites in the final step. Scheme IV is shown in Figure 8 and is based on the combined competitivenoncompetitive HP mechanism. Unlike scheme III, scheme IV allows for hydrogen adsorption/desorption on both s and s* sites. 3.5.2. Development of Kinetic Expressions. Following mathematical steps similar to those performed for schemes I and II, two kinetic equations were developed based on the noncompetitive HP mechanism (scheme III) and the combined competitive noncompetitive HP mechanism (scheme IV). In the derivation of the model equations, the surface coverages of all intermediate species were assumed to be negligible, and loss of the first hydrogen atom was considered to be the rate-controlling step. The two model equations obtained are shown in Table 10. 3.5.3. Kinetic Treatment of the Rate Equations. Both the competitivenoncompetitive and noncompetitive HoriutiPolanyi models were fitted to the experimental data. The regression of both the competitivenoncompetitive and noncompetitive
HP-II (combined competitivenoncompetitive)
kKA pA ð rÞ ¼
pB pC 3 1 KpA
!
ð1 þ KA pA þ KB pB Þð1 þ
qffiffiffiffiffiffiffiffiffiffiffiffi KCr pC Þ
! pB pC 3 pA K qffiffiffiffiffiffiffiffiffiffiffiffiffi ð rÞ ¼ pffiffiffiffiffiffiffiffiffiffiffi ð1 þ KA pA þ KB pB þ KC pC Þð1 þ KCr pC Þ kKA pA 1
HP models resulted in extremely large values of adsorption coefficients for both MCH and toluene with similar SSE and F values. This suggests the need to incorporate an assumption of high surface coverage (HSC) into the model equations. The original equations were then modified by the introduction of a new constant: the ratio between the two adsorption equilibrium constants KB and KA. The modified equations resulted in one fewer parameter and led to correspondingly improved F values, as well as more realistic values of the kinetic coefficients ! pB pC 3 k pA K pffiffiffiffiffiffiffiffiffiffiffi ð13Þ ð rÞ ¼ 0 ðpA þ KB pB Þð1 þ KC pC Þ because KA p A þ K B p B . 1
ð14Þ
and 0
KB ¼
KB KA
ð15Þ
In the modified form shown in eqs 1315, the noncompetitive HP model resulted in a higher F value and was preferred to the combined competitivenoncompetitive HP model. For the noncompetitive HP model, the model F value increased from 620.2 to 778.7 when the HSC inequality (eq 14) was applied. An Arrhenius temperature dependency term was introduced into 167
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Table 11. Results of Regression for the Model Based on the Noncompetitive HP Mechanism when Loss of the First Hydrogen Atom Controls the Rate Kinetic Model
!
pB pC 3 K qffiffiffiffiffiffiffiffiffiffiffi ð1 kd td Þ ð rÞ ¼ 0 ðpA þ KB pB Þð1 þ KC pC Þ k pA
KA pA þ KB pB . 1 0
KB ¼
KB KA
KC ¼ KCr p2 Tr k ¼ kr exp B 1 T B ¼
E RTr Kinetic Parameters with Parameter Statistics 95% confidence intervals t values
parameter
values
units
lower
upper
kr 105
4.63 ( 0.57
8.11
3.51
5.75
B
9.92 ( 0.91
mol 3 (g of catalyst)1 3 s1
10.9
8.13
11.7
(E)
(50.9)
kJ/mol
K0B
1.02 ( 0.17
5.96
0.68
1.35
K/Cr kd
0.09 ( 0.01 1.27 ( 0.19
bar3 day1
9.07 6.57
0.07 0.89
0.11 1.66
N
m
210
5
Overall Statistics Adj(R2) 0.950
SSE
F
0.569
1008.2
virtually same value of SSE. Moreover, the 95% confidence limits for the heat of adsorption term included 0. In the next step, the previous temperature dependence term was dropped, and instead a nonlinear total pressure dependence was introduced as shown in eq 16. c p KC ¼ KCr
ð16Þ
A nonlinear pressure dependency was attempted, and KC was taken as a function of pressure raised to the power c. The new model was better fitted, as the SSE value decreased from 0.726 to 0.569 and the F value increased a small amount from 778.7 to 802.6, with the exponent c approximately 2.0. In the subsequent analysis, the value of the pressure exponent c was fixed as 2.0. This was done to increase the overall effect of the hydrogen partial pressure in the denominator. The equation then resembled the forms of the best kinetic equations based on the LHHW mechanism. The new model produced the highest F value of 1008.2, and all of the parameters were well-defined. No 95% confidence limits included 0, and all of the t values were well above 3.0. The results of the best kinetic treatment are reported in Table 11, and a scatter diagram to provide a comparison between the observed and model conversion values is provided in Figure 9.
Figure 9. Scatter diagram for the model based on noncompetitive HP kinetics when loss of first H atom controls the rate.
4. CONCLUSIONS A detailed kinetic investigation of the dehydrogenation of methylcyclohexane was carried out using the power-law model,
K/Cr which led to the addition of a new parameter. The model F value decreased from 778.7 to near the original value (i.e., 621.6) with 168
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p = pressure, Pa pA = partial pressure of methylcyclohexane, Pa pB = partial pressure of toluene, Pa pC = partial pressure of hydrogen, Pa (r) = rate of the dehydrogenation reaction, mol 3 kg1 3 s1 (r0 ) = rate of the dehydrogenation reaction involving deactivation kinetics, mol 3 kg1 3 s1 R = universal gas constant, J 3 mol1 3 K1 s = active or empty site s* = hydrogen-only active or empty site td = online reaction deactivation time, s T = temperature, K Tr = reference temperature, K Tw = reactor wall temperature, K Tz = temperature at any position in the axial direction, K W = weight of catalyst, kg X = conversion (fractional conversion) of MCH Xmod = model or calculated conversion of MCH Xobs = observed or measured conversion of MCH yA0 = initial mole fraction of MCH in the vapor phase yB0 = initial mole fraction of toluene in the vapor phase yC0 = initial mole fraction of hydrogen in the vapor phase yI0 = initial mole fraction of inert in the vapor phase YTol = yield of toluene, mol/100 mol of hydrocarbon (MCH or MCHe) fed
LHHW kinetics, and the HoriutiPolanyi kinetic mechanism. Poor fitting results were obtained with the power-law model. When subjected to LHHW single-site and dual-site mechanisms, the experimental data were best fitted by single-site LHHW kinetics considering loss of the first hydrogen molecule as the ratecontrolling step and surface coverage by the methylcyclohexadiene intermediate as negligible. The noncompetitive HP mechanism also provided a good fit to the overall data but with an empirical insertion of the total pressure dependence of the hydrogen equilibrium constant. Containing two fewer parameters, the noncompetitive HP model led to a somewhat higher value of SSE and was less efficient in fitting the overall data.
’ AUTHOR INFORMATION Corresponding Author
*Address: Institute of Chemical Engineering and Technology, University of the Punjab, Lahore (54590), Pakistan. Tel.: 0092429230462, Ext 114. Fax: 0092429231159. E-mail:
[email protected].
’ ACKNOWLEDGMENT M.R.U. acknowledges the Higher Education Commission of Pakistan for funding this project. ’ NOMENCLATURE B = dimensionless activation energy B0 = dimensionless heat of adsorption for lumped equilibrium constant defined in Table 9 BCr = dimensionless heat of adsorption for KCr defined at Tr c = exponent in eq 16 e = equilibrium approach factor E = activation energy, J/mol FA0 = initial molar flow rate of MCH, mol/s FHC0 = initial molar flow rate of hydrocarbon (MCH or MCHe), mol/s Δh0 = lumped heat of adsorption defined in Table 9 k = rate constant for the MCH dehydrogenation reaction kd = apparent short-term deactivation constant, s1 kr = rate constant at the reference temperature k0 = frequency factor in the Arrhenius equation K = equilibrium constant of the MCH dehydrogenation reaction, Pa3 KA = adsorption equilibrium constant for methylcyclohexane, Pa1 KB = adsorption equilibrium constant for toluene, Pa1 KC = adsorption equilibrium constant of hydrogen, Pa1 KCr = adsorption equilibrium constant of hydrogen at Tr, Pa1 Ki = surface equilibrium constant of the ith step in a reaction sequence, Pa1 / KC = adsorption equilibrium constant of hydrogen adsorbed on site s*, Pa1 K/Cr = constant in eq 16, Pa3 K0r = lumped adsorption equilibrium constant defined at Tr K0 = lumped equilibrium constant K0B = ratio of KB to KA defined in eq 15 K00 = lumped equilibrium constant K000 = lumped equilibrium constant m = number of parameters n = order of the reaction N = number of data points
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’ NOTE ADDED AFTER ASAP PUBLICATION After this paper was published online December 2, 2011, a correction was made to the year in reference 20. The revised version was published December 7, 2011.
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