Partial Oxidation of Methanol on a Sn−Mo−O Catalyst. A Kinetic Study

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Ind. Eng. Chem. Res. 2000, 39, 2902-2909

Partial Oxidation of Methanol on a Sn-Mo-O Catalyst. A Kinetic Study Daniel E. Ardissone, Norma G. Valente, Luis E. Cadu ´ s, and Luis A. Arru ´ a* INTEQUI, Instituto de Investigaciones en Tecnologı´a Quı´mica (UNSL-CONICET), Chacabuco y Pedernera, 5700 San Luis, Argentina

A Sn-Mo-O catalyst was prepared, characterized, and tested in the partial oxidation of methanol under different experimental conditions. A kinetic model which assumes that the reaction involves different active sites such as terminal and bridged oxygen vacancy sites, terminal oxygen (ModO), and dual dioxo sites (two adjacent surface dioxo units) has been selected. The experimental observations and the predictions of the model show a rather good agreement. Introduction The partial oxidation of methanol (MeOH) is an important chemical process used in formaldehyde (FA) production. The catalysts most commonly used in the ODH of MeOH are mixtures of Fe2(MoO4)3 and MoO3 usually modified by different additives.1,2 MeOH is also selectively oxidized to formaldehyde in the presence of a large excess of air on MoO3- and V2O5-based catalysts. In studying these catalysts, Ai3 has observed a clear correlation between catalytic activity for FA production and the number of acidic sites on the catalytic surface. However, when studying Sn-Mo-O catalysts obtained by coprecipitation,4-6 he has observed that the ODH of MeOH also produces formic acid, methyl formate (MF), CO2, and water (W). This activity is attributed to acidic and basic sites. Valente et al.7 have reported that by employing mechanical mixtures of SnO2 and MoO3 the reacting mixture (MeOH + O2) plays an important role in the activation process of the catalysts. The mechanism of MeOH ODH would indicate that the transference of protons and electrons between alcohol and the catalyst is necessary and in order for this to happen the catalyst must have not only the suitable redox properties but also acid-base ones.8-10 There exist various kinetic studies of MeOH oxidation on different catalysts.11-18 The main product mentioned in the literature is FA because of its industrial interest. Nevertheless, important quantities of dimethyl ether (DME), dimethoxymethane (DMM), and MF have been reported. The apparent kinetic orders found in O2 and MeOH have been positive in most cases. Their values depended on the kind of catalyst and on the experimental conditions in which the kinetic study was performed (conversion, temperature, and pressure range).13-16 However, Pernicone et al.1,12 have reported that by using pure MoO3 and MoO3-Fe2(MoO4)3 catalysts the reaction rate of MeOH oxidation does not depend on the partial pressures of the reactants, except for the lowest ones. The apparent activation energies reported are about 20 kcal/mol.1,13,15,17 The rate-determining step (RDS) in FA formation determined by isotopic kinetic measurement is the breaking of the C-H bond from an adsorbed methoxy group to produce an adsorbed FA form.18 Several authors have investigated the effect of W as an inhibitor of MeOH oxidation. In an early work, Jiru * Telephone/Fax: 54-2652-426711.E-mail: [email protected].

et al.11 reported that FA acts as an inhibitor while W has no effect. Holstein and Machiels19 studied the kinetics of this reaction over an iron molybdenum oxide catalyst in a continuous-flow reactor with external recycling at temperatures of 473-573 K. They described the kinetics by power law rate expressions. They found that the inhibition by W occurs through kinetics coupling, whereby W vapor chemisorbs dissociatively to form hydroxyl groups, which contribute to the reduction of the steady-state concentration of methoxy groups on the catalyst surface by reacting with them to reform MeOH. Cheng20 has performed experiments of competitive adsorption of MeOH, FA, and W over molybdenum oxide. This study reveals that adsorbed FA is readily displaced by W. This suggests that W and FA adsorb on the same site in MoO3. W can also be displaced by MeOH. Further experiments showed that W and MeOH can displace each other. Therefore, in addition to W, MeOH could also suppress the adsorption of FA. Thus, Cheng20 concluded that the high selectivity to FA in the partial oxidation reaction of MeOH over MoO3 (and presumably molybdates in general) can be attributed to W, a reaction product, and MeOH, which both suppress the further oxidation of FA, probably by blocking FA adsorption sites. Pernicone et al.1 also concluded that FA and especially W act as inhibitors in the oxidation of MeOH. According to Tatiboue¨t,17 the MeOH reactions can follow two parallel pathways: (i) oxidation reaction in which O2 is required (molecular or provided by the catalyst) and (ii) dehydration reaction which does not need O2. It is difficult to determine if the MeOH oxidation to give FA, MF, and DME is carried out through consecutive and/or parallel surface reactions, with or without adsorption of intermediate species, because there are no precise data about the mechanism followed. However, from a kinetic point of view the general reaction network can be considered as a series of consecutive surface reactions produced from the intermediaries adsorbed.18 In the present work a Sn-Mo-O catalyst is prepared and characterized by means of several techniques, and its behavior in the partial oxidation of MeOH is studied under different experimental conditions. After several kinetic models developed on the basis of previous findings reported in the literature were tested, a combined model is proposed. This model is supported on our experimental observations, thermochemical results from

10.1021/ie990427i CCC: $19.00 © 2000 American Chemical Society Published on Web 07/13/2000

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ab initio quantum chemical calculations,21 and the mechanism suggested by Chung et al.22 which involves different oxygen vacancies as active sites. Experimental Section Catalysts Preparation. The reagents used were SnCl2‚2H2O (May and Baker AR grade with As < 0.0001%) and (NH4)6Mo7O24‚4H2O (Merck AR grade with Fe < 0.0005%, Pb < 0.001%, and Cu < 0.001%). The catalysts were prepared by precipitating tin hydroxide in excess of urea in aqueous ammonia solution at pH 8.5. After being filtered and washed, it was mixed with a saturated solution of (NH4)6Mo7O24‚4H2O in an adequate amount to obtain a molar ratio Sn/Mo of 7/3. The solvent was evaporated by using a rotary evaporator at 323 K and at reduced pressure. Then, the solid was dried at 373 K overnight and calcined in air at 823 K for 8 h. Catalyst Characterization. The specific surface area (SBET) of the catalyst was determined from nitrogen adsorption isotherms at 77 K by the Brunauer-Emmett-Teller (BET) method. A Micromeritics Accusorb 2100E was used. The X-ray diffraction (XRD) patterns were obtained with a Rigaku diffractometer operated at 35 kV and 30 mA by using Cu KR radiation (λ ) 0.514 18 nm) with a Ni filter. Temperature-programmed reduction (TPR) experiments were performed in a conventional TPR unit equipped with a thermal conductivity detector. Samples of about 80 mg were first oxidized in a 30 mL/min flow of 20% O2 in He at 877 K for 30 min and then cooled to room temperature. After that, helium was admitted at room temperature to remove the O2. The samples were subsequently contacted with a 30 sccm. Flow was of 5 vol % H2 in N2, and samples were heated, at a rate of 10 K/min, to a final temperature of 823 K, while hydrogen consumption was monitored after W removal by mean a cold trap. The amount of hydrogen consumed was evaluated by integration of the TPR peak areas up to 100 min of previous calibration. Electronic paramagnetic resonance (EPR) spectra were taken at room temperature in a Bruker spectrometer operated at X-band frequencies. A Klystron frequency of 9.7 GHz and a magnetic field modulation of 100 Hz were used. The isopropyl alcohol (IPA) decomposition was used for determining the acid-base properties of the catalyst. The reaction was performed between 353 and 373 K in a fixed-bed reactor with continuous flow under atmospheric pressure. The feed consisted of 4.5% IPA in He at a gas flow rate of 40 sccm. The conversion of IPA was MeOH/N2 = H2/N2. The IPA decomposition results showed that pure tin oxide did not produce reaction of IPA under the employed conditions. Pure molybdenum oxide produces a large amount of propene but not acetone. The SnMo-O catalyst produces both propene and acetone, with the latter in a lower proportion. Results of catalytic tests corresponding to the 54 experimental levels are given in Figures 3-5. The catalytic activity has been expressed as MeOH conversion (X ) mol of converted MeOH/mol of fed MeOH) and yields of FA, MF, DME, and CO2 (Yi ) mol of ith product produced/mol of fed MeOH). Then, the respective selectivities are defined as SFA ) YFA/X, SMF ) 2YMF/X, SDME ) 2YDME/X, and SCO2 ) YCO2/X. Between the operative conditions studied, the influence of neither external nor internal diffusion was observed. No homogeneous reaction was detectable at the conditions used. The catalytic values reported were obtained after running the reaction for at least 2 h, when the steady state was essentially reached. During experiments, the carbon balance was always better than 100 ( 6%. The largest deviations between conversion and the addition of product yields were observed in experiments whose conversions were lower than 10%. Although changes in the catalytic activity were not detected with time on stream, experiments were performed in order to investigate possible coke formation on the catalyst surface. The CO2 evolution was examined by interrupting the steady-state reaction by diverting the N2-O2 flow from the MeOH saturators. In all tests, the CO2 concentration fell abruptly to unmeasurable amounts when MeOH flow was stopped. The axial temperature difference through the catalytic bed was less than 2.5 K even at the more severe reaction conditions. The values of MeOH conversion (empty symbols in Figure 3) show a moderate sensitivity to changes in the

Figure 3. Experimental (symbols) and calculated (lines) conversions of MeOH (empty symbols) and yields of DME (filled symbols) as a function of W/F at different reaction temperatures: (0,9-) 463 K; (O,b- - -) 448 K; (4,2‚‚‚) 433 K.

Figure 4. Experimental (symbols) and calculated (lines) yields of FA as a function of W/F at different reaction temperatures: (9-) 463 K; (b- - -) 448 K; (2‚‚‚) 433 K.

molar fraction of O2. These changes in MeOH conversion can be accounted mainly by variations in FA yields (symbols in Figure 4), considering that MF yields (filled symbols in Figure 5) and DME yields (filled symbols in Figure 3) are almost not sensitive to changes in the molar fraction of O2. The CO2 yields (YCO2) were very low for all of the experiments performed. CO2 was detected mainly at the highest reaction temperatures and W/F ratios. Further analysis of experimental results from Figures 4 and 5 shows that YFA/YMF ratios (which are propor-

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Figure 5. Experimental (symbols) and calculated (lines) yields of CO2 (empty symbols) and yields of MF (filled symbols) as a function of W/F at different reaction temperatures: ( 0,9-) 463 K; (O,b- - -) 448 K; (4,2‚‚‚) 433 K.

tional to SFA/SMF ratios) have a general trend to decrease when the W/F ratio rises. In the complementary experiments performed by feeding Me without O2, the only reaction product observed was a small amount of FA during a short time. No DME formation (MeOH dehydration) was observed. The appearance of MF, FA, DME, CO2, and W as products was observed when O2 was added to the feed. When FA (without MeOH-O2) was fed, a very low conversion (lower than 10%) to MF, CO, CO2, and W was observed. When FA-O2 (without MeOH) was fed, a conversion of near 40% was measured. The detected products were CO2, formic acid, and W. No MF production was observed, even at different W/F ratios. On the contrary, when a mixture of FA-MeOH-O2 was fed, large amounts of MF and W were detected together with DME in a lower proportion. When FA-MeOH (without O2) was fed, an initial conversion of MeOH on the order of 20% was measured. The detected products were MF and W. DME and CO2 were not observed. The conversion decreased clearly with time. Discussion Even after the reaction the XRD spectra only revealed the crystalline phases SnO2 (casiterite) and MoO3 (orthorhombic). The crystalline phase corresponding to MoO2 was not observed. The used catalyst showed a sharper signal of Mo5+ by EPR compared with the fresh one, which indicates that under the reaction conditions some reordering of the Mo5+ arrangement is produced, at least during the first minutes. The EPR results obtained with pretreated catalysts showed that the reacting mixture which includes O2 produces a higher concentration of Mo5+ than the treatment with reducing agents such as MeOH or hydrogen. It cannot be disregarded that molybdenum is being reduced to Mo4+ (MoO2) when reducing agents are used.

Decomposition of IPA is regarded as a test in determining the presence of acid and/or redox centers on a given surface.24 The IPA decomposition proceeds by two parallel routes: dehydration to propene on acidic (rather weak) sites and dehydrogenation to acetone on redox (basic) sites. This reaction cannot distinguish between the Bro¨nsted and Lewis sites. Its advantage lies in the fact that it can be applied to low-specific surface area catalysts for which spectroscopic techniques cannot provide reliable data. From results of the IPA decomposition, it can be concluded that at least two types of active sites are present on the surface of the Sn-Mo-O catalyst: isolated acidic sites (responsible for propene formation) and redox or basic sites (responsible for acetone formation). An analysis of catalytic experimental data shows a general trend to decrease the YFA/YMF ratio as W/F increases. This could mean that MF is formed from FA. When the complementary experiment was performed by feeding FA plus O2, MF was not produced. The principal products were CO2 and W. However, when MeOH was added to the feed (MeOH, FA, O2), a large amount of MF was formed, with DME, CO2, and W as secondary products. Taking into account these results, it can be concluded that, from MeOH plus O2, FA is primarily formed. Then, it is necessary to have FA plus MeOH to produce MF. When MeOH was fed alone (without O2) a small amount of FA for a short time was only observed as a product. After that, no reaction was detected. This result means that, in the absence of O2, the catalyst is partially reduced (as evidenced by the formation of FA) and it loses acidity which is necessary to convert MeOH into DME. At all of the experimental conditions studied, the catalyst showed the production of several products (mainly FA and MF). Chung et al.22 have pointed out that more FA is produced with a less reduced catalyst (high O2/MeOH ratio) while the formation of high-order products increased with a more reduced catalyst (low O2/MeOH ratio). The latter could be our case because the O2/MeOH ratio used ranged from 0.38 to 1.07. Allison and Goddard21 have used thermochemical results from ab initio quantum chemical calculations to examine the reaction mechanism for the MeOH oxidation to FA catalyzed by MoO3. They found that surface dioxo sites are critical to activating the MeOH. However, they also concluded that an important catalytic site involves two adjacent surface dioxo units called the dual dioxo site >Mo(dO)2(Od)2Mo< (referred to as DDOS), with each dioxo site extracting one H in a sequence of steps. During the MeOH reaction to produce FA, the dual dioxo site is partially reduced to >Mo(dO)O(Od)Mo< (referred to as DDOSred). Then the site is reoxidized by O2 from the gas phase or from the lattice. The required dual dioxo site exists on the (010) surface of MoO3 but does not exist on the other low-index surfaces. The mechanism proposed by Allison and Goddard21 is in agreement with the experimental results obtained by Tatiboue¨t17 when studying the MeOH oxidation as a probe reaction on single crystals of MoO3. Niwa et al.25 have found that the rate of MeOH oxidation on MoO3 catalysts supported on SnO2 is proportional to the square of the number of molybdenum atoms and thus a second-order reaction with respect to molybdenum atoms. It is thereby suggested that two molybdenum atoms play the role of the active site simultaneously. Activity for MeOH oxidation may be generated when two adjacent molybdenum atoms are loaded on the

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surface of tin oxide. This assessment is also in agreement with the requirement of the dual site proposed by Allison and Goddard.21 Chung et al.22 studied the partial oxidation of MeOH over molybdenum oxide at low temperature, where highorder products (DME, MF, and DMM) were formed in small amounts. Correlation of the activity data with data on the surface states makes it possible to suggest a reaction mechanism involving MeOH adsorption on terminal oxygen vacancy sites -O-Mo(0)-O- (referred to as Vt) and bridged oxygen vacancy sites -Mo-0-Mo(referred to as Vb), interacting with adjacent oxo sites gMo(dO) (referred to as OS). FA could be mainly produced from methoxy intermediates chemisorbed on terminal oxygen vacancy -O-Mo(~-CH3)-O- (referred to as H3COVt, where ~ is O) while methoxy intermediates chemisorbed on bridged oxygen vacancy -Mo~(CH3)-Mo- (referred to as H3COVb) could be responsible for the production of high-order products, especially DME. They also found that the bond strength of chemisorbed methoxy is greatly affected by the electronic state of the oxygen vacancy. Reduced oxygen vacancy sites weaken the C-H bond but strengthen the C-O and chemisorption bond in the methoxy, resulting in the formation of more hydrogen-abstracted products. The decrease in the formation of high-order products at high reaction temperature could be caused by the decrease in the concentration of bridged oxygen vacancy sites by the formation of shear planes in the surface layer. Thus, the Vt involved in FA production was characterized as a terminal oxygen vacancy on Mo5+, owing to the moderate bond strength in the VO and CH, and the previous study (Chung and Bennett26) shows that the most abundant species at reaction temperatures above 423 K is methoxy chemisorbed on this kind of vacancy. Taking into account the conclusions achieved from our experiments but also the different mechanisms proposed in the literature (i.e., Allison and Goddard,21 Chung et al.,22 Holstein and Machiels,19 and Machiels et al.27), a combined model was proposed. The kinetic model assumes the following: (a) MeOH chemisorbs dissociatively on the previously described DDOS, Vt, and Vb sites, leading to the formation of different methoxy intermediates and hydroxyl groups. (b) FA is produced in two ways. In both cases the RDS is the breaking of a C-H bond in the methyl group of the adsorbed methoxy. (c) FA can chemisorb dissociatively on two oxo sites (OS). (d) The other RDSs were the surface reactions of MF, DME, and CO2 formation, respectively. (e) Steps other than RDSs were assumed to be equilibrated. (f) DME formation requires OS and bridged oxygen vacancy sites (Vb) existing on Mo6+. Both sites are present in the oxidized catalyst. (g) The reoxidation of sites is by gas-phase O2 or by the diffusion of lattice O2. The following pathways were proposed: KMeOH

1. H3COH + DDOS 798 H3CO-DDOS-H k2

2. H3CO-DDOS-H 98 H2CO-DOS-2H KFA

3. H2CO-DDOS-2H 798 H2CO + DDOS-2H

KH

2O

4. DDOS-2H 798 H2CO + DDOSred K*MeOH

5. H3COH + Vt + OS 798 H3COVt + H-OS K** MeOH

6. H3COH + Vb + OS 798 H3COVb + H-OS k7

7. H3COVt + OS 98 H2CO + H-OS + Vt * KFA

8. H2CO + 2OS 798 HOC-OS + H-OS k9

9. H3COVt + HOC-OS 98 HCOOCH3 + Vt + OS k10

10. H3COVt + H3COVb 98 H3COCH3 + Vt + OVb k11

11. HOC-OS + OS 98 CO2 + Vt + H-OS * KH

2O

12. 2H-OS 798 H2O + Vt + OS KO

2

13. DDOSred + 1/2O2 798 DDOS * KO

2

14. Vb + 1/2O2 798 O-Vb ** KO

2

15. Vt + O-Vb 798 OS + Vb The rate equations are

r2 ) r7 )

k2KMeOHpMeOH DEN1

/ k7K/MeOHpMeOH(KO p 1/2)1.5 2 O2

xK

2 / H2OpH2ODEN2

/ k9K/FAK/MeOHKO p p pO21/2 2 FA MeOH

r9 )

/ KH p DEN22 2O H 2O

r10 )

/ 2 / 1/2 k10K// MeOHKMeOHKO2pMeOH pO2 / KH p DEN2DEN3 2O H2O

r11 )

/ p 1/2)1.5 k11K*FApFA(KO 2 O2

xK

/ 2 H2OpH2ODEN2

with

DEN1 ) 1 +

(

)

1 [1 + KH2OpH2O(1 + KO2pO21/2

/ DEN2 ) 1 + KO p 1/2 + 2 O2

(x

1

)

/ KH p 2O H 2O

DEN3 ) 1 +

KFApFA)] + KMeOHpMeOH

xK

/ 1/2 O2pO2

[

xK

/ H 2O p H 2O

+

/ ( K/MeOHpMeOH + K/FApFAKO p 1/2) 2 O2

// KO p 1/2 2 O2

+

K// MeOHpMeOH

x

/ p 1/2 KO 2 O2

/ KH p 2O H2O

]

Ind. Eng. Chem. Res., Vol. 39, No. 8, 2000 2907 Table 1. Kinetic Parameters parameter cat-1 s-1)

k2 (gmol g k7 (gmol g cat-1 s-1) k9 (gmol g cat-1 s-1) k10 (gmol g cat-1 s-1) k11 (gmol g cat-1 s-1)

frequency factor

Ea (cal gmol-1)

3.8498 × 1.1197 × 109 3.4481 × 1010 1.2765 × 109 3.5592 × 109

23 542 29 802 19 816 18 736 21 571

1010

parameter

preexponential factor

KMeOH (atm-1) KH2O (atm-1) KFA (atm-1) KO2 (atm-1/2) * KO (atm-1/2) 2 * KH (atm-1) 2O K*MeOH (atm-1) K*FA (atm-1) -1 K** MeOO (atm ) ** -1/2 KO2 (atm )

0.990 337 1.677 01 × 10-3 0.097 814 0.182 634 0.028 627 4.139 42 × 10-3 1.248 96 × 10-4 2.859 87 × 10-4 1.861 60 × 10-3 0.031 881

-∆Ha° (cal

gmol-1)

5938 4408 8412 6431 4348 4480 7560 7469 4655 6059

The kinetic analysis of data was carried out by using the integral method. Parameter estimates were obtained by a nonlinear least-squares fit, using a combination of both the direct search method (OPTNOV28) and the Marquardt29 method. The continuity equations were numerically integrated by means of a fourth-order Runge-Kutta method. Parameter estimation based on the data at all temperatures was carried out. Thus, frequency factors, activation energies, preexponential factors, and heats of adsorption (30 parameters) were estimated in one step. The best set of parameter values with their confidence limits are given in Tables 1 and 2. As can be seen, the signs of the obtained activation energies (Ea) have good

physicochemical significance. Their magnitudes are in general agreement with the values reported in the literature for this reaction. The equilibrium constants K which appear in the rate expressions can be written as

K ) e∆S°a/Re-∆H°a/RT where ∆Ha° is the standard enthalpy of adsorption and ∆Sa° is the standard entropy of adsorption. Because adsorption is exothermic, the adsorption enthalpy has to satisfy the inequality

-∆H°a > 0 and the adsorption entropy has to satisfy

∆S°a < 0 then, the preexponential factor (A) of the equilibrium constants Kj must be

0