The dehydrogenation of methanol to methyl formate

Chemical Engineering and Processing 44 (2005) 393–402

The dehydrogenation of methanol to methyl formate Part I: Kinetic studies using copper-based catalysts X. Huang, N.W. Cant, M.S. Wainwright∗ , L. Ma School of Chemical Engineering and Industrial Chemistry, The University of New South Wales (UNSW), Sydney 2052, Australia Received 5 September 2003; accepted 26 May 2004 Available online 20 August 2004

Abstract Kinetics of the dehydrogenation of methanol to methyl formate (MF) have been determined for a commercial copper-chromite catalyst and for a skeletal copper catalyst that has undergone deactivation to a steady-state activity. The activation energy over the copper-chromite catalyst was found to be around 78 kJ/mol, considerably lower than the approximately 120 kJ/mol observed for the skeletal copper catalyst. The reaction order with respect to methanol was found to be approximately 0.5 for both catalysts whilst hydrogen and methyl formate both inhibited the reaction significantly. This inhibition is consistent with a Langmuir–Hinshelwood model in which methanol is adsorbed dissociatively and the rate-determining step is the loss of hydrogen from the resultant methoxy species. The model parameter values imply that a significant fraction of the copper sites on the skeletal catalyst are occupied by formaldehyde while the coverage is low on copper-chromite. These differences can be used to explain the presence of deactivation and the considerably higher activation energy for dehydrogenation observed for the skeletal copper catalyst. © 2004 Elsevier B.V. All rights reserved. Keywords: Kinetics; Methanol dehydrogenation; Methyl formate; Skeletal copper catalyst; Copper-chromite catalyst

1. Introduction Dehydrogenation of methanol to methyl formate (MF): 2CH3 OH → CH3 OCHO + 2H2

(1)

is important as a first step in the synthesis of acetic acid and formamide [1]. Copper-based catalysts appear to be uniquely effective. Extensive research has been conducted to improve catalyst activity and selectivity [2–4]. Amongst copper-based catalysts, pure skeletal copper catalysts exhibit very high initial activity for the reaction [6]. However, rapid deactivation is observed with two-thirds of the initial activity lost due to fouling attributed to polymerization of formaldehyde on the surface of the copper [7]. Recent studies have shown that the addition of Cr2 O3 to the skeletal ∗

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copper system improves the structure of skeletal copper and thereby promotes the stability of the catalysts for dehydrogenation of methanol [8]. The improvement of performance was attributed to the presence of Cr2 O3 on the surface of copper, which minimised polymerization of adsorbed species on active copper sites [8]. However, detailed support for this interpretation was lacking. This study addresses the kinetics of the gas phase dehydrogenation of methanol to methyl formate over a significantly deactivated (stable) skeletal copper catalyst and a commercial copper-chromite catalyst, as a way to elucidate the role of chromia in improving the stability of copper catalysts.

2. Experimental The gas phase dehydrogenation reaction (1) was conducted in a conventional stainless steel micro-reactor system operated at atmospheric pressure. Methanol was in-

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troduced to the reactor by passing helium through saturators (inert material) connected in series. Methyl formate was introduced when required in the same way via separate saturators. The gas flow were controlled by mass flow controllers (MFCs). The products were analyzed using two on-line gas chromatographs (GCs) equipped with thermal conductivity detectors and a Porapak Q column in GC-1 (for analysis of methanol and methyl formate) and a CTR-1 column in GC-2 (for CO, CO2 and H2 ). An ice-bath between GC-1 and GC-2 was used to condense liquid products from the gas stream to improve the performance of the CTR-1 column. Prior to the kinetic measurements a blank test was performed with ␣-Al2 O3 (the diluent used in the normal runs) over the temperature range used for the catalytic measurements. There was no detectable reaction due to the reactor walls. In each catalytic experiment, approximately 0.03 g of the skeletal copper, prepared as described in [5], or 0.06 g of Harshaw-type 0203 copper-chromite catalyst particles (211–325 ␮m) were diluted with six times the volume of ␣Al2 O3 of the same particle size in order to ensure bed isothermicity and charged to the constant temperature zone of the stainless steel reactor. The ratio of bed length to diameter was greater than 7. Prior to use, the catalysts were slowly reduced in stages in hydrogen starting from room temperature with a hold for 2 h at 403 K and a final period of 4 h at 513 K. In order to efficiently measure the kinetics of methanol dehydrogenation over skeletal copper, it was necessary to stabilize the catalyst. Tonner et al. [6] showed that skeletal copper was highly active for the reaction but rapidly deactivated due to the fouling. In this study, we have allowed the copper surface to be fouled by the polymers until a stable steady state operating condition was obtained. The pretreatment conditions used to stabilize the skeletal copper catalyst were: reaction temperature: 468 K, time on stream: 2 h, methanol vapor flow rate: 10 cm3 /min. After the process of quick deactivation, the activity was significantly lower than that obtained on a fresh catalyst but the stable skeletal copper catalyst enabled kinetic measurements to be made with minimal further change in activity. Kinetic measurements were made under steady-state conditions with a total pressure of around 115 kPa and conversions below 15%. The total flow rate through the reactor was maintained constant using He as diluent gas and the flow rates were high enough to avoid mass transfer limitations.

The range of conditions explored were: 448–468 K, GHSV 150,000–200,000 h−1 , mol% methanol from 2 to 10%, added methyl formate from 0 to 5% and added hydrogen from 0 to 10%. Rates were calculated using the differential reactor approximation: −r = Fin

X W

(2a)

where −r is the reaction rate of methanol dehydrogenation, mol/h g-cat; Fin flow rate of methanol entering the reactor, mol/h; X the fractional conversion of methanol and W the mass of catalyst, g. In experiments without added products, or with hydrogen alone added, the conversion was calculated from the composition of the outlet stream in mol% (Y), i.e. X=

2YCH3 OCHO + YCO2 YCH3 OH + 2YCH3 OCHO + YCO2

(2b)

With methyl formate added, the rate was calculated directly from the difference between methanol entering and leaving the reactor, Fin − Fout , with the outlet molar flow obtained from the corresponding values for YCH3 OH with adjustment for small differences in volumetric flow rate.

3. Results 3.1. Thermodynamic analysis Under the low conversion conditions used here, the selectivity of the conversion of methanol to methyl formate and hydrogen was always >90% with CO2 the only significant by-product. The latter is probably formed by the methanol steam reforming reaction as a result of the trace amounts of water in the feed. The main reaction (1) is equilibrium limited. Table 1 summarises the equilibrium conversions, Xe , for the extremes of the conditions used here. The equilibrium constant was calculated using standard methods for ideal gases. The value for ∆Hfo (CH3 OCHO) was taken from the measurements of Hall and Baldt [9], and that for methanol from Chao et al. [10] and for hydrogen from the tabulations of Barin [11].

Table 1 Equilibrium conversions calculated for methanol dehydrogenation T (K) 448 448 448 468 468 468 a b

Ka 0.015 0.015 0.015 0.027 0.027 0.027

Initial partial pressure (kPa) CH3 OH CH3 OCHO

Conversions (%) Xe Xmax b

Xe /Xmax

H2

10.1 10.1 10.1 10.1 10.1 10.1

0.0 0.0 11.0 0.0 0.0 11.0

45.0 37.8 14.3 50.9 39.9 20.3

0.08 0.02 0.01 0.22 0.03 0.14

0.0 2.2 0.0 0.0 4.2 0.0

The equilibrium constants are for reaction (1) as written (i.e. with 2 mol of CH3 OH). Maximum conversions obtained when using the copper-chromite catalyst.

3.8 0.8 0.1 11.2 1.2 2.8

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395

Fig. 1. log–log plots of the rates of methanol consumption vs. methanol partial pressure for skeletal copper (a) and copper-chromite (b) catalysts.

Fig. 2. Arrhenius plots of the rate constants for methanol dehydrogenation using power law equation (Eq. (3b)) over skeletal copper (
) and copper-chromite (䊉).

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Table 2 Power law kinetic parameters for methanol dehydrogenation Catalyst Skeletal copper

Copper-chromite

T (K)

k × 100 (mol/h g-cat kPa−␣ )

453 458 463 468

1.27 1.96 2.71 3.45

448 458 468

1.13 1.81 2.76

The equilibrium conversion was then obtained by solution of the cubic expression for the equilibrium constant. The values for Xmax are the maximum conversions reached in the present work for the conditions shown. As may be seen, Xe is in the range 45–51% when no products are added compared with a maximum experimental conversion of 11.2% at

A (mol/h g-cat kPa−␣ )

Ea (kJ/mol)

α

4.2 × 1011

117 ± 10

0.50 ± 0.02

1.3 × 107

78 ± 1.5

0.46 ± 0.01

468 K. Thus, the conversion reached did not exceed 22% of the equilibrium value, with most values much less, and thus the differential reactor approximation used to obtain rates is justified. With products in the feed equilibrium conversions fell substantially (to 14% when 11 kPa of H2 were added at 448 K). However, as described later, the rate declined more steeply than this so the reaction was operated further from

Fig. 3. Plots of calculated rate using the power law model (Eq. (3)) vs. observed rate for methanol dehydrogenation over skeletal copper (a) and copper-chromite (b) catalysts.

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397

Table 3 Summary of Langmuir–Hinshelwood models in terms of reaction steps Model

CH3 OH adsorption

Rate-determining step

Route to methyl formate

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Molecular Molecular Molecular Molecular Molecular Molecular Molecular Molecular Dissociative Dissociative Molecular Molecular Molecular Molecular Dissociative Dissociative

CH3 OH* + * → CH3 O* + H* CH3 OH + * → CH3 OH* CH3 OCHO* → CH3 OCHO + * H* + H* → H2 + 2* CH3 OH* + * → CH3 O* + H* CH3 OH* + * → CH3 O* + H* 2CH2 O* → CH3 OCHO* + * CH3 O* + * → CH2 O* + H* CH3 O* + * → CH2 O* + H* CH3 O* + * → CH2 O* + H* CH3 O* + * → CH2 O* + H* CH3 O* + * → CH2 O* + H* CH3 O* + * → CH2 O* + H* CH3 O* + * → CH2 O* + H* CH3 O* + * → CH2 O* + H* CH3 O* + * → CH2 O* + H*

2CH2 O* → CH3 OCHO* + * 2CH2 O* → CH3 OCHO* + * 2CH2 O* → CH3 OCHO* + * 2CH2 O* → CH3 OCHO* + * CH2 O + CH2 O* → CH3 OCHO* CH3 O* + CHO* → CH3 OCHO* + * 2CH2 O* → CH3 OCHO* + * CH3 O* + CHO* → CH3 OCHO* + * 2CH2 O* → CH3 OCHO* + * CH2 O + CH2 O* → CH3 OCHO* CH2 O + CH2 O* → CH3 OCHO* CH2 O* + CH3 OH* → CH3 OCHO + 2H* 2CH2 O* → CH3 OCHO* + * CH3 OCH2 O* + * → CH3 OCHO* + H* CH3 OCH2 O* + * → CH3 OCHO* + H* CH3 OCH2 O* + CH2 O* → CH3 OCHO* + CH3 O*

* Indicates bound to surface.

Fig. 4. Effects of methyl formate partial pressure on the rate of dehydrogenation of methanol (ca. 8.0 kPa) over skeletal copper (a) and copper-chromite (b) catalysts.

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Fig. 5. Effects of hydrogen partial pressure on the rate of dehydrogenation of methanol (ca. 10 kPa) over skeletal copper (a) and copper-chromite (b) catalysts.

equilibrium when products were added than when they were not.

3.2. Kinetic studies An empirical power law expression (Eq. (3)) was used to assess kinetic parameters. α −r = kPCH 3 OH

(3a)

α −r = Ae−Ea /RT PCH 3 OH

(3b)

where k is the apparent rate constant in the power law model, mol/h g-cat kPa−α ; A the pre-exponential factor, mol/h g-cat kPa−α ; Ea the activation energy, kJ/mol; PCH3 OH the partial pressure of methanol, kPa; and α the reaction order of methanol. The influence of methanol over both catalysts was explored by changing the partial pressure. The plots used to determine reaction order with respect to methanol are shown in Fig. 1. The slopes are all close to 0.5. Similar behavior has been reported by Ai [12], who varied the initial concentration of methanol from 1.6 to 16.5 vol.%, while fixing the other reaction conditions.

Table 4 Langmuir–Hinshelwood model kinetic parameters for methanol dehydrogenation over the skeletal copper catalyst T (K)

k (mol/h g-cat)

K1 × 102 (kPa−1 )

K3

K4 (kPa)

K5 (kPa)

R2

Ea (kJ/mol)

458 463 468

0.52 0.78 1.06

9.8 0.9 0.2

3.66 0.09 0.47

0.53 12.53 8.03

0.50 9.41 113.7

0.97 0.95 0.96

127 ± 9.4

R2 is the regression coefficient for the fit to the linearised rate expression.

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Table 5 Formaldehyde coverage calculated using kinetic model parameters for the skeletal copper catalyst T (K)

Feed methanol partial pressure (kPa)

Feed methyl formate partial pressure (kPa)

Feed hydrogen partial pressure (kPa)

Coverage by formaldehyde (%)

463 463 463 468 468 468

9.30 9.30 8.37 9.30 9.30 8.37

0.00 0.00 4.00 0.00 0.00 4.00

0.00 5.00 0.00 0.00 5.00 0.00

16.0 7.91 39.0 16.1 9.91 34.5

Arrhenius plots for the reaction rate constants are shown in Fig. 2. The slopes, and hence the activation energies are quite different for the two catalysts. The values for the constants in the power law Eqs. (3a) and (3b) are listed in Table 2. A likely reason for the difference in activation energy is discussed later. Fig. 3(a–b) is a plot of the rates calculated using the power law equation versus the observed rates. They show that the power law model fits the experimental data very well for both catalysts. 3.3. Effect of products The effect of the principal products of the dehydrogenation of methanol on the reaction rate was also investigated. As noted previously, the presence of additional methyl formate or hydrogen reduces the equilibrium conversion but the observed effect on the rate was greater still since, as shown in Table 1, the maximum conversions obtained experimentally were further from equilibrium when either product was included in the feed. The effect of added methyl formate on rate is illustrated in Fig. 4. With both catalysts, the apparent order is −0.5 within the limits of the accuracy of the measurements. The negative order implies that methyl formate is adsorbed more strongly on copper than is methanol. This agrees with the conclusions of previous studies of the dehydrogenation of higher alcohols to aldehydes or ketones over copper where the adsorption coefficients for the carbonyl compounds are several times those for the alcohols [13]. The retarding effect of hydrogen on the rate of methanol dehydrogenation is shown in Fig. 5. The inhibition is somewhat greater than for methyl formate with apparent kinetic orders of −0.8 for skeletal copper and −0.7 for copperchromite. Such inhibition has been previously noted for a Cu/ZnO catalyst although the authors did not rule out a thermodynamic explanation [14].

3.4. Mechanistic considerations The inhibition data in Figs. 4 and 5 were tested jointly with those for the dependence on methanol in Fig. 1 for conformity to 16 Langmuir–Hinshelwood-type models listed in Table 3. They differed according to the mode of methanol adsorption (molecular or dissociative), the rate-determining step (conversion of methanol to a methoxy species, dehydrogenation of methoxy to formaldehyde, methyl formate formation or desorption) and the chemistry of methyl formate formation (dimerisation of adsorbed CH2 O, CH3 O + CHO or CH3 OH + CH2 O). Initial testing was carried out with linearised versions of each rate expression using the Excel function Linest. Most models could be excluded immediately because they gave one or more negative adsorption coefficients, sometimes in combination with a negative rate constant, both of which are physically unreasonable. In the case of the skeletal catalyst, the only model giving realistic solutions was as follows: K1

CH3 OH(g) + 2∗ ←→ CH3 O∗ + H∗

(4a)

k

CH3 O∗ +∗ −→ CH2 O∗ + H∗

(4b)

K3

2CH2 O∗ ←→ CH3 OCHO∗ +∗

(4c)

K4

CH3 OCHO∗ ←→ CH3 OCHO(g)+∗

(4d)

K5

H∗ + H∗ ←→ H2 (g) + 2∗

(4e)

This scheme assumes that methanol is adsorbed dissociatively with the subsequent conversion of the resultant methoxy species to adsorbed formaldehyde as the ratedetermining step. The rate expression is: −1/2

1/2

−r =

kK1 K5 PCH3 OH PH2 −1/2

1/2

(1 + K1 K5 PCH3 OH PH2 −1/2

+K3

−1/2

K4

+ K4−1 PCH3 OCHO −1/2

1/2

PCH3 OCHO + K5

(5)

−1/2 2 )

PH2

Table 6 Langmuir–Hinshelwood model kinetic parameters for methanol dehydrogenation over the commercial copper-chromite catalyst T (K)

k (mol/h g-cat)

K1 × 102 (kPa−1 )

K4 (kPa)

448 458 468

0.28 0.43 0.78

0.03 0.48 0.22

1.55 0.60 1.25

R2 is the regression coefficient for the fit to the linearised rate expression.

K5 (kPa) 643 30 104

R2

Ea (kJ/mol)

0.99 0.99 0.99

89 ± 9.5

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Fig. 6. Plots of calculated rate using Langmuir–Hinshelwood model (Eq. (5)) vs. observed rate for methanol dehydrogenation over skeletal copper (a) and copper-chromite (b) catalysts.

Table 4 shows the calculated values for the five parameters in the scheme with the fit between predicted and observed rates given in Fig. 6(a). The average deviation overall is ca. 14%. This is rather good bearing in mind that the concentrations of methyl formate and hydrogen vary by almost two orders of magnitude between the lowest produced by reaction and the maximum included in the feed. A slightly better fit (average deviation 12%) could be obtained by a further iterative non-linear refinement using the Excel tool Solver. However, a statistics package operated in conjunction with Excel [15] did indicate that some of the parameters had quite large uncertainties. Thus, the values in Table 4 should be regarded as indicative rather than accurate Langmuir–Hinshelwood coefficients. The parameter values in Table 4 were used to calculate the coverage of formaldehyde on the surface. Table 5 shows that at 463 and 468 K, the coverage of formaldehyde is more than 15%, indicating that the severe deactivation observed for skeletal copper is due to

fouling of the surface by polymerized formaldehyde. Furthermore, the presence of methyl formate in the feed significantly increases the coverage by formaldehyde. On the other hand, the presence of hydrogen in the feed halves the coverage by formaldehyde. The results indicate that the deactivation of the skeletal copper catalyst appears to be associated with surface coverage by formaldehyde and/or polymers derived from it. These results are consistent with the experimental findings of Evans, who showed that the deactivation of skeletal copper catalyst could be reversed by passing hydrogen instead of methyl formate over the deactivated catalyst during a study of methyl formate hydrogenolysis [16]. By contrast, the data for the copper-chromite catalyst gave physically reasonable parameters according to the above rate expression, Eq. (5), only if the second last term in the denominator, which arises algebraically from coverage of the surface by formaldehyde, was very small (i.e. K3 was very large). The improvement to the stability for methanol dehydrogenation

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over the copper-chromite catalyst, which was studied recently [8], can be explained by the presence of chromium oxide restricting formation of a polymer that can bind to the surface of active copper. Making the assumption that this term (CH2 O) could be neglected gave the parameters shown in Table 6 and the good fit between calculated and observed rates shown in Fig. 6(b). The data for the copper-chromite catalyst could be equally well fitted to a second model in which methanol initially adsorbed as molecules with the conversion to a methoxy species being rate determining and with the additional assumption that coverage of sites by both methoxy and formaldehyde species was very low. This model could be excluded on the basis that it was not compatible with the existence of a large deuterium kinetic isotope effect which shows that C H bond breaking is rate-determining over this catalyst [17]. It also required high hydrogen coverage to duplicate the observed inhibition. That is not reasonable given the weakness of hydrogen adsorption. It may be noted here that in the scheme described by Eqs. 4(a) and 4(e), inhibition by hydrogen arises primarily from its mass action effect on step (4a) which reduces the concentration of the surface methoxy species. From Fig. 6(a–b), it can be seen that good correlations were obtained for both catalysts. Tables 4 and 6 showed good fit to the Arrhenius plots of rate constants using the Langmuir–Hinshelwood model Eq. (5) and the activation energies of the reaction were close to the results which were found using the power law equation for skeletal copper and copper-chromite catalysts respectively.

4. Discussion The reaction scheme above is consistent with other knowledge. Methanol is readily adsorbed as methoxy groups on copper surfaces [18,19] and they have also been observed by infrared spectroscopy on supported copper catalysts [20–22]. Reversal of the adsorption facilitates exchange between D2 and the OH group of CH3 OH and this occurs at a rate that is much faster than that of methanol dehydrogenation over both the catalysts studied here. By comparison, deuterium exchange into the methyl group is considerably slower than that of dehydrogenation and the existence of a large deuterium kinetic isotope effect when the CH3 group in normal methanol is substituted by CD3 indicates that breaking of a C H bond is rate determining during dehydrogenation [17]. The formation of methyl formate by the formaldehyde dimerisation (the Tischenko reaction, 4c) is also consistent with suggestions from isotope studies at low pressure [23] and with catalysts similar to those used here [17]. In the latter case, the interpretation was based on detailed arguments concerning the deuterium distribution in the methyl formate formed by reaction of CH3 OH/CD3 OD mixtures and may not be totally definitive. Other authors have favoured a hemi-acetal route involving the combination of adsorbed methoxy and formaldehyde followed by reacting with adsorbed formalde-

401

hyde to form methyl formate [24]: CH3 O∗ + CH2 O∗ → CH3 OCH2 O∗ +∗

(6a)

CH3 OCH2 O∗ + HCHO∗ → CH3 OCHO∗ + CH3 O∗

(6b)

Substitution of this route in place of step (4c) in the earlier scheme led to a rate expression that did not fit the present data for either catalyst with all parameters positive. The most interesting aspect of the modeling is the inference of significant amounts of formaldehyde on the surface of the skeletal catalyst but with negligible amounts for copperchromite. The former is consistent with the observation of Tonner et al. [6] that trace amounts of gaseous formaldehyde are seen as a product with a skeletal catalyst when it is substantially deactivated. The presence of formaldehyde can provide an explanation for the existence of deactivation with the skeletal catalyst and none for the chromite one. If coverage by formaldehyde is higher on the extended copper planes on the skeletal catalyst then the probability of polymerisation to foulant will be enhanced and deactivation will be faster. Infrared studies using Cu/SiO2 are consistent with the conversion of formaldehyde to polyoxymethylene and this is directly associated with deactivation [25]. On the other hand, if chromia were to scavenge gaseous formaldehyde, then the probability and extent of fouling and deactivation would be reduced and the catalyst should be more stable as observed. The higher activation energy observed with the skeletal catalyst can be rationalised in a similar way. If formation of foulant is to some extent a reversible polymerization–depolymerisation process, then increasing temperature could lead to less coverage of the surface by foulant and an increased number of active sites. In this case, one would expect the apparent activation energy to be higher for the skeletal catalyst, due to an increase in the number of active sites with temperature, than for the chromite system where this effect is significantly reduced or absent.

5. Conclusions Kinetic studies have shown that the dehydrogenation of methanol over copper-based catalysts is near half order with respect to the partial pressure of methanol. The activation energy is ca. 120 kJ/mol for skeletal copper and ca. 78 kJ/mol for a commercial copper-chromite catalyst. Methyl formate and hydrogen, the reaction products, significantly inhibit dehydrogenation. The kinetic data can be fitted to a Langmuir–Hinshelwood model in which the rate-determining step is the dissociation of a C H bond in a methoxy species formed by dissociative adsorption of methanol with methyl formate then formed by a Tischenkotype reaction between two formaldehyde molecules. The implied coverage by formaldehyde is significant for the skeletal copper but negligible with copper-chromite catalyst. The difference can explain the initial deactivation seen with the skeletal catalyst and can also account for the higher

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