Applied Catalysis A: General 228 (2002) 95–101
Mechanism of selective oxidation of methanol over stannic oxide-molybdenum oxide catalyst V. Lochaˇr∗ , J. Machek, J. Tichý Department of Physical Chemistry, University of Pardubice, nam. Cs. Legii 565, 532 10 Pardubice, Czech Republic Received 13 June 2001; received in revised form 7 November 2001; accepted 7 November 2001
Abstract The mechanism of the selective oxidation of methanol on Sn-Mo oxide catalyst with molar ratio Mo/(Mo + Sn) = 0.3 is investigated. The interaction of methanol and its oxidation products (i.e. formaldehyde and methyl formate) is studied by FT-IR spectroscopy. The data are interpreted on the basis of a reaction mechanism which contains 10 elementary steps. The behaviors of the surface species (methoxy, formate and dioxymethylene groups) are compared with those monitored on other systems and the catalyst requirements for the title reaction are discussed. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Stannic; Molybdenum; Catalysts; Oxidation; Methanol; FT-IR spectroscopy
1. Introduction The oxidation of methanol can lead to various products depending on the catalyst used, the reaction temperature, conversion and partial pressure of reactants. The main products given in literature include formaldehyde, methyl formate, dimethyl ether, dimethylal, CO, CO2 and water. Rarely observed is the formation of formic acid [1]. Judging from literature references, the greatest attention was given to two catalytic systems in studies of oxidative esterification of methanol to methyl formate, namely V2 O5 -TiO2 and SnO2 -MoO3 . Owing to the papers by Forzatti et al. the properties of the oxide system V2 O5 -MoO3 are well known now. The authors carried out a detailed investigation of the catalyst characteristics inclusive of evaluation of the effect of reaction conditions. On the basis of ∗ Corresponding author. Tel.: +42-406-037041; fax: +42-406-037068. E-mail address:
[email protected] (V. Lochaˇr).
the studies of surface complexes by means of FT-IR spectroscopy they also suggested the reaction mechanism [2]. The production of methyl formate by the vaporphase oxidation of methanol over the SnO2 -MoO3 catalyst is also dealt with in a paper by Ai [3]. Tests of various binary oxide catalysts indicated that the best results for both activity and selectivity were obtained with the molar ratio of Mo/(Mo + Sn) = 0.3. It was proposed that methyl formate is formed via formaldehyde as follows: 2CH3 OH → 2CH2 O → HCOOCH3 , and the first reaction is rate limiting. Carbon oxides and formic acid were found as the side products. In connection with this reaction, also the surface complexes formed by interaction of methanol, formaldehyde and methyl formate with the surface of the oxide system Mo/(Mo + Sn) = 0.3 were studied with the help of FT-IR spectroscopy. The aim was again a contribution to better understanding of the mechanism of these two reactions, i.e. oxidation of methanol and dimerization of formaldehyde over the above-mentioned catalyst, and comparison of the
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results with those obtained by Busca et al. [4], and Machiels and Sleight [5].
2. Experimental 2.1. Catalyst: preparation and characteristics The catalyst with the molar ratio of Mo/(Mo+Sn) = 0.3 was prepared in the following way: 216 g SnCl2 ·2H2 O p.a. was dissolved in water. The solution was adjusted at pH 8 by an addition of 26% aqueous ammonia. The precipitated hydroxide was collected by filtration and mixed with an aqueous solution of 106 g (NH4 )6 Mo7 O24 ·4H2 O p.a. The mixture formed was evaporated on a water bath at 80 ◦ C to give a paste, which was then dried at 135 ◦ C 10 h and annealed at 300 ◦ C 5 h and at 500 ◦ C 8 h. The content of Mo and Sn in the catalyst was estimated by means of X-ray fluorescence analysis using an XRF spectrometer Spectroscan. The specific surface of the catalyst (53 m2 g−1 ) was determined from the adsorption of nitrogen using a Micrometrics Rapid Surface Area Analyser 2200 A. The X-ray structural analysis was performed with a Krystaloflex apparatus, and the IR spectra of crystals were measured with an FT-IR apparatus Nicolet Protege 460 using the KBr disc technique. In agreement with literature [6] the solid solution of molybdenum in stannic oxide was proved. 2.2. Apparatus The IR spectra of both the surface complexes and the gas phase were measured in the interval of 400–4000 cm−1 using the FT-IR spectrometer Nicolet Protege 460 equipped with a MCT/B detector cooled with liquid nitrogen. The spectra were recorded and treated first with the Omnic program supplied by Nicolet Inst. Corp. at a resolution power of 2 cm−1 with the number of readings equal to 256. The transparent self-supporting plates of catalyst with the density of 10–25 mg cm−2 were placed in the evacuated apparatus with a cell equipped with KBr windows. The experimental measurements were carried out with methanol for spectroscopy, methyl formate p.a., and formaldehyde obtained by sublimation of paraformaldehyde p.a. at 100 ◦ C. Before the measurement proper, the catalyst plate was oxidized in air at
400 ◦ C for 3 h. At the beginning of evacuation, the air was pumped out of the cell by means of a rotary air pump whereupon the pressure was further decreased by means of charcoal cooled with liquid nitrogen. The adsorption of individual components proceeded at room temperature and the sorbate pressure of 1.5 kPa. For the desorption, temperatures in the interval of 100–200 ◦ C were chosen. The spectrum of pure catalyst sample (as background) was subtracted from each IR spectrum of the adsorbed substance and the result was standardized with respect to uniform plate density equal to 15 mg cm−2 . The spectra were analyzed with the help of the Origin.4.1 program into individual bands with Gaussian profiles.
3. Results 3.1. Adsorption of methanol The experimentally obtained IR spectra of adsorbed methanol presented in Fig. 1 can be divided into two groups. In the wave number region above 2500 cm−1 there are several absorption bands at 2961, 2940, 2889, and 2829 cm−1 . In accordance with literature [4,7,8], these absorption bands correspond to the bond vibrations of ν as (CH3 ), ν s (CH3 ), ν(C–H), and the Fermi resonance 2δ s (CH3 ) in the methoxy complex (I) formed. Moreover, the IR spectrum contains a broad absorption band of the valence vibration ν(OH) lying in the wave number region about 3400 cm−1 . Another absorption band at 2837 cm−1 belongs to ν s (CH3 ) in the non-dissociated methanol. The second wave number region of IR spectrum below 1800 cm−1 contains several low-intensity absorption bands at 1452 and 1433 cm−1 belonging to the deformation vibrations δ as (CH3 ), and δ s (CH3 ) of the methoxy complex formed. Beside that it contains a distinct absorption band at 1616 cm−1 that can be assigned to the deformation vibrations δ(H2 O) and an absorption band at 1370 cm−1 characteristic of δ(CH) of non-dissociated methanol. At 1053 cm−1 there is a distinct absorption band of valence vibration ν(CO). Heating at 100–130 ◦ C for a period of 10 min with concomitant evacuation of the gas phase was accompanied by a considerable decrease in intensity of the absorption bands due to the methoxy complex, and at
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Fig. 1. IR spectra of methanol adsorbed at room temperature (RT) on catalyst with the molar ratio of Mo/(Mo + Sn) = 0.3 (a) and evacuated at RT (b), 373 K (c), 403 K (d), 433 K (e) and 473 K (f).
the same time there appeared new absorption bands in the spectrum at the wave numbers of 1543, 1378, and 1357 cm−1 , which are characteristic of the group vibrations ν as (COO− ), δ(CH2 ), and ν s (COO− ) of formate complex (II) [4]. After increasing the desorption temperature to 160 ◦ C, these distinct absorption bands attain their maximum intensity. The intensity of the absorption bands due to the methoxy complex strikingly decreased and at 200 ◦ C these absorption bands practically disappear from the spectrum. At the last mentioned temperature, the absorption bands characterizing the formate complex decrease, too. In the wave number region above 2500 cm−1 it is possible to observe a decrease of all the absorption bands in the temperature interval from 100 to 200 ◦ C. 3.2. Adsorption of formaldehyde The experimentally obtained IR spectra of adsorbed formaldehyde are presented in Fig. 2. As the absorption bands in the wave number region above 2000 cm−1 were not readable and badly resolved, only the part involving absorption bands below 1800 cm−1 was used for the following analysis. This region contains several distinct absorption bands at 1730, 1640, 1543, 1378, and 1357 cm−1 . Furthermore, there are here a number of low-intensity absorption
bands at 1482, 1468, 1433, 1405, 1306, 1272, and 1230 cm−1 . The absorption bands at 1730, 1640, 1468, and 1272 cm−1 can (according to [7,9]) be assigned to the vibrations ν(C=O), δ(CH2 ), and r(CH2 ) of molecularly adsorbed formaldehyde. The formate complex formed as early as at room temperature can be identified by the absorption bands at 1453, 1378, and 1357 cm−1 . The absorption bands with the wave numbers 1482, 1405, and 1306 cm−1 confirm the existence of the groups with the vibrations δ(CH2 ), ω(CH2 ), τ (CH2 ) in dioxymethylene complex (III) [4]. The presence of methoxy complex is characterized by the absorption bands at 1453 and 1433 cm−1 . The spectra of gas phase after adsorption of formaldehyde at room temperature show, before evacuation, the absorption bands at 1754, 1207, 1178, and 941 cm−1 due to gaseous methyl formate [10]. After heating at 100–130 ◦ C with simultaneous evacuation of gas phase, the spectra did not show absorption bands corresponding to adsorbed formaldehyde and dioxymethylene complex. The decomposition of this complex is connected with an increase in intensity of the absorption bands at 1453 and 1433 cm−1 ascribed to the methoxy complex. At the same time, an intensity decrease can be observed in the absorption bands at 1543, 1378, and 1357 cm−1 belonging to the formate complex. The spectrum also
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Fig. 2. IR spectra of formaldehyde adsorbed at room temperature on catalyst with the molar ratio of Mo/(Mo + Sn) = 0.3 (a) and evacuated at RT (b), 373 K (c), 403 K (d), 433 K (e) and 473 K (f).
exhibits a new absorption band at 1315 cm−1 corresponding to the vibration δ(COH) and confirming the existence of formic acid at the catalyst surface [9]. 3.3. Adsorption of methyl formate The IR spectra (Fig. 3) of adsorbed methyl formate show (in the wave number region below 1800 cm−1 ,
which is the most valuable for unambiguous interpretation) several distinct absorption bands at 1705, 1641, 1629, 1540, 1305, and 1164 cm−1 . Moreover, there is a non-negligible number of low-intensity absorption bands there, namely at 1450, 1433, 1377, 1355, 1324, 1272, and 1176 cm−1 . In accordance with [3], the absorption bands at 1705, 1641, 1176, and 1164 cm−1 correspond to the vibrations ν(C=O), ν(C–O), r(CH3 )
Fig. 3. IR spectra of methyl formate adsorbed at RT on catalyst with the molar ratio of Mo/(Mo + Sn) = 0.3 (a) and evacuated at RT (b), 373 K (c), 403 K (d) and 433 K (e).
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in molecularly adsorbed methyl formate. The formate complex, which appears in the spectrum of adsorption as early as at room temperature, is identified with the help of absorption bands at 1540, 1377, and 1355 cm−1 . The absorption bands at 1450 and 1433 cm−1 confirm the presence of methoxy complex. After heating at 100, 130, and 160 ◦ C with concomitant evacuation, the spectral absorption bands belonging to molecularly adsorbed methyl formate and methoxy complex decreased. The drop in these absorption bands is connected with an intensity increase in absorption bands of formate complex.
4. Discussion The observed absorption bands (Fig. 1) in the region of 3400 and 2837 cm−1 in the spectrum obtained at room temperature indicate that the first step of the whole catalytic cycle is adsorption of methanol. The subsequent detection of an absorption band at 1616 cm−1 due to water and the simultaneous appearance of absorption bands at 1450 and 1435 cm−1 characterizing the methoxy complex support the opinion that the second elementary step of the reaction is condensation of adsorbed methanol with the surface OH group, whose existence at the catalyst surface is supported by the vibration ν(OH) lying in the region about 3400 cm−1 . In accordance with the published data [7], the methoxy complex can realistically be ascribed a monodentate structure with a covalent-type bond to molybdenum. The formation of water as the oxidation reaction side product is a result of interaction of adsorbed methanol with the Mo–OH group. This finding simultaneously explains the negative effect of water vapor in the gaseous reaction mixture upon the conversion degree of methanol over the Sn-Mo-Ox catalyst, which was observed already in the cited work [3]. Increase of desorption temperature above 100 ◦ C results in decreasing intensity of the absorption bands ascribed to the methoxy complex, and at the same time there appear spectral absorption bands at 1540, 1375, and 1355 cm−1 due to bond vibrations in formate complex, as well as an absorption band at 1272 cm−1 characterizing the adsorbed formaldehyde. The increased temperature necessary for splitting off of hydrogen atom from methoxy group indicates that the process is energetically demanding. An
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unambiguous analogy has been observed with the mechanism of oxidation of methanol over an iron-molybdenum oxide catalyst presented by Machiels and Sleight [5]. The reduction–oxidation mechanism of oxidation of methanol over the said catalyst was experimentally proved for the first time ◦ in a paper by J´ıru et al. [11]. In accordance with the results of both cited papers it can realistically be presumed that the oxidation of methoxy complex is most probably the rate-limiting step of the whole catalytic process, and the splitting of C–H bond is assisted by a lattice oxygen of the catalyst with concomitant enrichment of the catalyst with two electrons. The oxidation products, formaldehyde and water, are desorbed into gas phase. The desorption of formaldehyde has a competing reaction in surface dimerization of formaldehyde. The last elementary step consists in re-oxidation of the catalyst by molecular oxygen from gas phase. The mechanism of dimerization of formaldehyde can be suggested from the spectra of adsorption of formaldehyde and methyl formate. The absorption bands (Fig. 2) at 1482, 1405 and 1306 cm−1 obtained at room temperature indicate that the formation of dioxymethylene group from adsorbed formaldehyde is a rapid process. The simultaneous occurrence of surface formate and methoxy complex supports the hypothesis that the subsequent elementary step of the reaction sequence is disproportionation of the labile dioxymethylene complex at the catalyst surface. The detection of methyl formate with the help of absorption bands at 1705, 1641, 1176, and 1164 cm−1 supports the idea that the next step consists in esterification of methoxy group by the formate complex. The occurrence of methyl formate in gas phase indicates that the last step of the whole catalytic cycle is desorption of the produced ester from the catalyst surface. The study of adsorption of methyl formate (Fig. 3) by means if IR spectra showed absorption bands due to methoxy complex, formate complex, and formaldehyde. This fact confirms a fast course of all the equilibrium elementary steps of dimerization of formaldehyde, and at the same time it supports the opinion that the rate-limiting step of both subsequent reactions is the oxidation of methoxy complex. The formate complex is the most stable out of all the surface complexes observed. Its existence at the catalyst surface was also proved at 200 ◦ C, i.e. above the reaction temperature. It can be deduced that this
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complex is also a transition complex of formation of carbon oxides and formic acid. The proportion of these two side products of oxoesterification probably depends on the presence of H2 O or OH surface groups and the presence of oxygen in gas phase.
5. Conclusions On the basis of experimental results of this paper and with the help of data known from literature [4,5,12] it is possible to suggest the following reaction scheme (s: surface, g: gas): 1. Adsorption of methanol (CH3 OH)g (CH3 OH)s 2. Condensation (CH3 OH)s + (OH)s (CH3 O)s + (H2 O) 3. Oxidation—redox mechanism − (CH3 OH)s + O2− s (CH2 O)s + (OH)s + 2e
4. Desorption of formaldehyde (CH2 O)s (CH2 O)g
10. Desorption of methyl formate (HCOOCH3 )s (HCOOCH3 )g The suggested reaction scheme presumes the participation of OH groups and two types of catalyst oxygen in the reaction. One of the oxygen species has nucleophilic nature and enters the product of redox reaction, i.e. water. The other one takes part in the formation of transition complexes of formaldehyde dimerization. Formaldehyde dimerization (Tischenko reaction) was proved by independent catalytic test (integral flow-through reactor with fixed bed, temperature range 100–160 ◦ C, reaction mixture: helium and formaldehyde). The selectivity to methyl formate was higher then 90%. According to studies carried by Ai [3] the direct reaction, i.e. oxidation of methanol, needs the presence of acidic centers whereas the other reaction, i.e. dimerization of formaldehyde, requires the presence of basic centers. The maximum content of both types of centers at the catalyst with molar ratio of Mo/(Mo + Sn) = 0.3 was also confirmed from a study of adsorption of pyridine and acetic acid carried out by Švachula et al. [13]. As confirmed by Machiels and Sleight [5], and Davydov [6], the first acidic center is connected with molybdenum in analogy with the paper by Busca et al. [4], the other (basic) center arising from SnO2 phase or Sn/Mo interphase.
5. Desorption of water (H2 O)s (H2 O)g + vacancy 6. Re-oxidation of catalyst −
2e + vacancy +
1 2 (O2 )g
→
O2− s
7. Formation of dioxymethylene complex − 2(CH2 O)g + O2− s (H2 COO)s + 4e
Acknowledgements Financial support for this work has been provided by The Ministry of Education, Youth and Sports of the Czech Republic under research project no. CZ310008/2010/3340. References
8. Disproporcionation 2(H2 COO)s + 4e− (CH3 O)s + (HCOO− )s + e− + O2− s 9. Esterification (CH3 O)s + (HCOO− )s + e− (HCOOCH3 )s + O2− s
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