I n d . E n g . C h e m . Res. 1989, 28, 387-393
387
Mechanism and Active Sites for Methanol Oxidation to Methyl Formate over Coprecipitated Vanadium-Titanium Oxide Catalysts Ahmed S. Elmi, Enrico Tronconi, Cinzia Cristiani, Juan P. Gomez Martin, and Pi0 Forzatti* Dipartimento di Chimica Zndustriale ed Zngegneria Chimica "G. Natta" del Politecnico, Piazza Leonard0 da Vinci 32,Z-20133 Milano, Italy
Guido Busca Zstituto di Chimica, Facoltci di Zngegneria dell'Uniuersit6, Fiera del Mare, Padiglione D, 1-16129 Genoua, Italy
The mechanism of methanol oxidation to methyl formate over coprecipitated vanadium-titanium oxide catalysts previously proposed is confirmed and further clarified, and the nature of active sites in this reaction is assessed by using surface sensitive techniques in the catalyst characterization, by performing a series of flow reactor experiments where intermediates and reaction products are added to the reactor feed, and by comparing the adsorption of methanol and formaldehyde on titanium and vanadium-titanium oxides. The reaction mechanism is proved to consist of successive oxidation steps, and the active sites for each step are identified. An alternative route to methyl formate by a Cannizzaro-type disproportionation reaction of dioxymethylene species is found to be of minor relevance. The capability of coprecipitated vanadium-titanium oxide to catalyze the formation of methyl formate is correlated to the existence of dioxymethylene species with an intermediate stability as compared to pure titania and vanadia. At present, methyl formate is produced on a large scale as an intermediate in the synthesis of formic acid and formamides. However, this ester is a versatile and attractive intermediate for a number of chemicals, as detailed by Roeper (1984), and may have more extensive chemical utilization in the future. Methyl formate is currently manufactured by carbonylation of methanol in the liquid phase in the presence of basic catalysts, typically sodium methoxide, at low temperatures and under moderate-to-high CO pressures (BASF, 1925; Leonard Process Co., 1979). Alternative routes to methyl formate have recently been proposed, involving either the gas-phase dehydrogenation of methanol over Cu-based catalysts (Tonner et al., 1984) or the gas-phase oxidation of methanol over different metal oxide catalysts (Ai, 1982; van Hengstum et al., 1984; Forzatti et al., 1987; Louis et al., 1988). In previous papers we have studied the oxidation of methanol to methyl formate, 2CH30H + O2 HCOOCH, + 2H20 (1) over vanadium-titanium oxide catalysts, prepared either by coprecipitation or by impregnation (Forzatti et al., 1987; Tronconi et al., 1987; Busca et al., 198713). Productivities to methyl formate of about 200 g/(L h) with methyl formate concentrations higher than 470, HCOOCH3/CH20 weight ratios of about 20, and global selectivities well in excess of 90% have been obtained using a coprecipitated vanadium-titanium oxide catalyst with a V/Ti atomic ratio of 0.0375. These results in perspective make the industrial implementation of an oxidative route for the synthesis of methyl formate interesting. A detailed process variable study performed on the above catalytic system and a combined FT-IR study of the interaction of methanol and its oxidation products with the catalyst surface led us to formulate a tentative reaction mechanism. The proposed mechanism involves the following steps: (i) condensation of methanol with surface VOH groups; (ii) oxidation of methoxy groups leading to coordinated formaldehyde; (iii) formation of dioxymethylene species through interaction
-
* Author to whom correspondence should be addressed. 0888-5885/89/2628-0387$01.50/0
of adsorbed formaldehyde with nucleophilic sites of the catalyst surface; (iv) reaction of adsorbed dioxymethylene species with gaseous methanol to give dimethylformal (DMFL); (v) successive transformation of dioxymethylene groups to formate ions either by oxidation or by a Cannizzaro-type disproportionation reaction, which also results in the production of methoxy species; (vi) reaction of formate ions either with methanol to give methyl formate or with water to produce formic acid; (vii) decomposition of formate species to give carbon monoxide. A similar mechanism of methanol oxidation has recently been proposed by Feil et at. (1987) for a V206/Ti02monolayer catalyst on the basis of an IR investigation. Still, a few mechanistic aspects remain not completely clear and need to be confirmed and clarified further. First of all, the relevance of the surface reactions detected in the FT-IR study with respect to the mechanism operating under flow reactor conditions is always questionable, although in the present case this has been argued on the basis of the observation that the various reactions were observed in the same temperature range by IR spectroscopy and during flow reactor measurements. Also, the relative importance of the oxidation route with respect to the disproportionation route in the formation of methyl formate from formaldehyde, as well as the mechanism governing the origin of carbon dioxide, was not fully specified. Finally, the role of the catalyst constituents in the different reaction steps has not been completely elucidated. In this work our understanding of the reaction path has been improved by performing a series of flow reactor measurements where the reaction products and the reaction intermediates were employed as reagents. The experiments were performed over both vanadium-titanium oxide and Ti02 catalysts in order to elucidate the role of the catalyst constituents in the different reaction steps. Additional information on the nature of the active sites were obtained by comparing the FT-IR spectra of adsorbed methanol and adsorbed formaldehyde recorded on pure titania and on vanadia-titania surfaces and by using surface-sensitive techniques in the catalyst characterization. 0 1989 American Chemical Society
388 Ind. Eng. Chem. Res., Vol. 28, No. 4, 1989
t
Table I. Results of Chemical Analysis for the Vanadium-Titanium Oxide Catalyst 70V total at the surface interacting expect. detect. %V5+ %V4+ %V5+ %V4+ 2.3 2.32 1.32 0.07 0.94
Experimental Section The Ti02 and the vanadium-titanium oxide samples (V/Ti atomic ratio (a.r.) = 0.0375) employed in the present study are the same as those already used by Tronconi et al. (1987). Ti02was prepared by hydrolysis of TiC1, (pH 8) at room temperature, followed by drying and calcination at 600 "C. The vanadium-titanium oxide catalyst was prepared by coprecipitation from VOCl, and T i c 4at room temperature, dried a t x 100 OC, and then calcined to 600 "C. The two samples were previously characterized by XRD, UV-visible diffuse reflectance, ESR and IR spectroscopies, adsorption of probe molecules, and surface area measurements (Tronconi et al., 1987). In the present study the samples were further characterized by chemical analysis and laser Raman spectroscopy. Chemical analysis for surface and interacting V was performed according to the procedure described by Busca et al. (1986). The overall V content has been determined by titration with KMn04. Raman spectra were recorded with a Dilor multichannel spectrometer (Omars-891,equipped with an array of 512 diodes and with a microscope (Olympus BH-2), using an Ar+ Spectra Physics laser with the excitation line at 514.6 nm. The apparatus and the procedures used for the flow reactor experiments have already been described (Tronconi et al., 1987); however, the feed section was enhanced by using a liquid metering micropump (Gilson Model 302). For all of the flow reactor runs, the catalyst load was 2 g, the feed flow rate was 48 mL/min, and the axial temperature profiles, measured by a thermocouple sliding in a capillary tube immersed in the catalyst bed, were found to be essentially flat. Experiments with the following feed compositions were performed: (1)HCOOH + HzO in He or O2 + N,; (2) HCOOCH, + HzO and HCOOCH, + HzO + CH30H + HCOOH in He; (3) formalin (CH20 + HzO + CH,OH as a stabilizing agent or intentionally added to the mixture) in He or Nz + 0,; (4) CH20 (obtained by thermal decomposition of paraformaldehyde) in He; ( 5 ) DMFL + H 2 0 in He; (6) CH,OH in air. Concentrations of the reagents are given in the text. IR spectra were recorded by using a Nicolet MX1 Fourier transform spectrometer (Busca et al., 1986). Results Catalyst Characterization. Previous characterization work (Tronconi et al., 1987) has shown that the Ti02 sample employed in the present investigation is made of an anatase phase only. The BET surface area was 41 m2/g. As for the vanadium-titanium oxide catalyst (Tronconi et al., 1987; Busca et al., 1987a),the XRD patterns exhibited only reflections corresponding to the anatase phase. UVvisible, ESR, and IR techniques provided evidence for the existence of a solid-state solution characterized by the incorporation of V4+ in the bulk as the paramagnetic species. The surface characterization by adsorption of probe molecules demonstrated the presence of both V and Ti centers on the catalyst surface (Tronconi et al., 1987). The measured surface area of the vanadium-titanium oxide catalyst was 37 m2/g. The results of the chemical analysis of the vanadiumtitanium oxide sample (Table I) indicate that a considerable quantity of V interacts with the support in the form
1
I n 0
,
I 700
9% l . . C
YAVF MM IF I
n\
Figure 1. Raman spectrum of the vanadium-titanium oxide catalyst.
0 1 4000
. 3800
. - * - / *
3200
2000
2400
ZOO0
1600
i200
W A V E N U M B E R S (cm-')
Figure 2. FT-IR spectra of the vanadium-titanium oxide catalyst: activated in vacuo at 400 O C (solid line) and after adsorption of water (dotted line).
of V4+,but a significant amount of V is also present at the surface as V5+. Further information on the surface characteristics of the vanadium-titanium oxide catalyst are provided by the laser Raman microscopy study, which is reported in detail elsewhere (Cristiani et al., 1989). The laser Raman microscopy spectrum (Figure l) shows a sharp band at 1030 cm-', assigned to the V=O stretching of the monooxovanadyl species with a coordinative unsaturation. In the presence of water, the coordination sphere of V becomes saturated, and the band shifts to 995 cm-l. Indeed, this has been observed on a reference vanadium-titanium oxide monolayer catalyst exposed to laboratory atmosphere upon lowering the laser power (Cristiani et al., 1989). A similar laser beam power effect has been reported by Payen et al. (1986) on molybdena-alumina and by Le Coustumer et al. (1988) on vanadia-alumina and was interpreted as due to hydration of the molybdenyl and vanadyl centers, respectively. The shift of the Raman band from 1030 cm-l to lower frequencies was not observed on the coprecipitated vanadium-titanium oxide sample upon decreasing the laser power. As a matter of fact, this sample absorbs strongly a t the wavelength of the laser excitation line, which eventually may cause the dehydration of the surface vanadyl species already a t low laser power because of the stronger temperature rise. However, the shift of the V=O stretching mode is argued from FT-IR data taken in the overtone region, where a corresponding shift of the 2 v V = O band was observed from 2050 cm-' to near 1980 cm-' upon exposure to water vapor (Figure 2). Flow Reactor Experiments. Decomposition of HCOOH. The results of HCOOH decomposition in O2 + N2on vanadium-titanium oxide and Ti02catalysts are presented in Figures 3 and 4, respectively. Total con-
Ind. Eng. Chem. Res., Vol. 28, No. 4, 1989 389 @ C O N V E R S I O N OF C H 2 0 O S E L E C T I V I T Y T O HCOOCH3 V S E L E C T I V I T Y TO ~ C H ~ O H V S E L E C T I V I T Y TO C 0 2 T O co ASELECTIVITY
140
1eo
150 T
a 3
*
170
3c" CH30H
("C)
Figure 3. Decomposition of HCOOH over the vanadium-titanium oxide catalyst. Feed compositions: 1.7% HCOOH, 5% H20,8% O2 + He. .CONVERSION
O F HCOOH
140
150
V S E L E C T I V I T Y TO C 0 2
170
T
--
180
190
--
of HCOOH was observed under inert atmosphere (He). The data indicate that over both catalysts HCOOH decomposes according to the reactions, HCOOH C02 + H2 (2)
CONVERSION
c
-+
E
HCOOH
>
4
CO + HzO
H Y
V
w
rl Y Ln
50
(3)
and that reactions 2 and 3 dominate at low and high temperatures, respectively. Hydrolysis of HCOOCH3. This reaction was investigated by feeding to the reactor, loaded with Ti02, HCOOCH3 H20 in He and HCOOCH3 H20 + CH30H HCOOH in He. In the former case, formation of hydrolysis products was observed to take place already in the evaporator (kept at about 160 "C) according to the reaction HCOOCH3 + H2O * CH30H + HCOOH (4)
> I c
K
200
("C)
Figure 5. Disproportionationof CH20 over the vanadium-titanium oxide catalyst. Feed compositions: 1.6% CH20,0.5% CH,OH, 6% H 2 0 + He.
A S E L E C T I V I T Y TO CO
-
lclo
-
+
-
z
0
Lc K W
> 0 V
01
150
160
170
-
T ('C)
Figure 4. Decomposition of HCOOH over the Ti02catalyst. Feed compositions: 1.07% HCOOH, 8.9% H20, 8% O2 + He.
version of HCOOH was always measured; at low temperature the major product is C02 (H2was also detected but it could not be analyzed quantitatively), while at high temperature CO is more favored. A similar decomposition
+
+
In the latter case, condensation of methanol and formic acid to methyl formate was detected and compositions approaching chemical equilibrium for reaction 4 were measured between 160 and 180 "C. In this temperature range, the equilibrium constant of reaction 4 is estimated to vary from 1.7 X to 1.9 X (Reid et al., 1987). Decomposition of formic acid to CO and COz was also monitored. It appears, therefore, that the hydrolysis of methyl formate is a facile and reversible reaction that can proceed also in the absence of heterogeneous catalysts. Disproportionation of CHzO and Oxidation of CH20. In Figures 5 and 6 we compare the results obtained over the vanadium-titanium oxide catalyst with formalin as a reagent in the absence and in the presence of oxygen, respectively.
390 Ind. Eng. Chem. Res., Vol. 28, No. 4, 1989 @ C O N V E R S I O N OF C H 2 0
@ C O N V E R S I O N OF C H 3 0 H A C O N V E R S I O N O F HCHO 0 S E L E CT T V I T Y T 0 H C00C H
O S E L E C T I V I T YT O H C O O C H ~ V S E L E C T I V I T Y TO J CH30H D S E L E C T I V I T Y T O CO:, A S E L E C T I V I T Y TO CO
A S E L E C T I V I T Y TO CO Y 7 S E I . E C T I V I T Y T O CO;,
1 CONVERSION
i 40
-
160
I50 T
CO
b 170
( O f ' )
Figure 6. Oxidation of CHzO over the vanadium-titanium oxide catalyst. Feed compositions: 1%CHzO,0.3% CH,OH, 7.4% HzO, 8% O2 + He.
In the absence of oxygen, the conversion of CHzO increases slightly with temperature, but it is never complete; HCOOCH3 is the only reaction product at low temperature, while larger quantities of CH,OH, CO, and COz are produced at higher temperatures. CH30H produced in the course of the reaction is referred to as ACH30H (CH30H in the outlet - CH30H in the feed). The observed changes in selectivity with increasing temperature can be explained by invoking the hydrolysis of HCOOCH3(reaction 4) and the decomposition of HCOOH (reactions 2 and 3). Noticeably, a similar catalytic behavior was observed for pure titania, as documented in Figure 7. Besides, comparable conversion levels and similar selectivities were measured when using dry formaldehyde + He as reagents. This eventually rules out the possibility that water vapor inhibits the disproportionation of CH20 over vanadiumtitanium oxide. In the presence of oxygen, complete conversion of CHzO is always measured, whereas the conversion of methanol increases with temperature and approaches 100% only at about 170 "C. Complete conversions were measured also for higher concentrations of CHzO,up to 1.5-1.7%. The following products are observed: HCOOCH3,CO, and COP Selectivities to COz and to methyl formate decrease with temperature; on the other hand, the selectivity to CO increases markedly with temperature and becomes very high above 160 "C. Again, the observed changes in selectivity with increasing temperature are due to the hydrolysis of HCOOCH, accompanied by the decomposition of HCOOH. The selectivity to methyl formate can be enhanced by increasing the concentration of CH,OH in the feed according to the reverse of reaction 4, as demonstrated
0
' 140
150
T
160 ("C)
170
Figure 7. Disproportionationof CH20over the TiOz catalyst. Feed compositions: 1.6% CH20, 0.5% CH,OH, 6% HzO + He.
in Figure 8. The lack of detected formic acid during these experiments is due to thermodynamic reasons (unfavorable equilibrium of reaction 4). The above data clearly indicate that the oxidation of CHzO, CH20 + YzO2 HCOOH (5) is faster than the disproportionation of CH20, 2CHz0 HCOOCH, (6) over the vanadium-titanium oxide catalyst, and that the latter reaction is catalyzed by pure titania as effectively as by vanadia-titania. Also, the data confirm that the oxidation of CHzO is faster than the oxidation of CH30H over the vanadium-titanium oxide sample. Hydrolysis of CH,(OCH3),. The hydrolysis of DMFL over the vanadium-titanium oxide catalyst was tested by employing feeds consisting of DMFL + HzO + He with different DMFL/HzO ratios. As shown in Figure 9, DMFL is hydrolyzed almost quantitatively if an excess of water vapor is present. When the DMFL/HzO ratio in the feed increases, the conversion of DMFL drops, in line with the stoichiometry of the reaction CHZ(OCH3)z + H2O CH2O + 2CHBOH (7) In addition to CH30H and CHzO,HCOOCH,, CO, and COz are detected among the reaction products. These species are believed to originate from the disproportionation of formaldehyde and the hydrolysis of methyl formate followed by HCOOH decomposition (reactions 6 + 4, and 2 + 3). Oxidation of Methanol. As already discussed by Tronconi et al. (1987),the conversion of methanol is much higher on vanadium-titanium oxide catalyst than on pure titania, so that activity in methanol oxidation appears to
-
+
+
Ind. Eng. Chem. Res., Vol. 28, No. 4, 1989 391 A C O N V F R S I O N OF C H z O .CONVERSION OF CH3OH 0 S E L E C T I V I T Y TO H C O O C H 3 V S E L E C T I V I T Y TO C O 7 O S E L E C T I V I T Y T O CO
.CONVERSION OF A S E L E C T I V I T Y TO O X E L E C T I V I T TYO 0:;ELECTIVITY TO A S E L E C T I V I T Y TO O S E L E C T I V I T Y TO
100
CH7(OCH9)2 CH30ti HCOOCH~
cop CO HCtiO
a
-
-
100
CONVERSION OF C H 3 0 t l
t
a
' CONVERSION
0
0.04
0.1
0.2
C H J O H / H20
Figure 8. Effect of CH30H/H20 feed ratio on the oxidation of CHzOover the V-Ti catalyst at T = 143 O C . Feed compositions: 1% CH20, 7.4% H20, 8% O2 + CH30H + He.
be primarily related to the presence of V at the catalyst surface. Adsorption of CHsOH and CH20. In Figure 10 the FT-IR spectra of methanol adsorbed on pure titania and on the vanadia-titania surfaces evacuated at increasing temperatures are reported. As discussed in detail elsewhere (Busca et al., 1985, 1987b; Rossi and Busca, 1985; h i s et al., 1987),upon evacuation at room temperature, molecularly adsorbed methanol (bands marked with stars) and methoxy groups (bands marked with triangles) are observed. At higher temperatures, bands due to formate groups (marked with open circles) become manifest. The FT-IR spectra recorded on pure titania and on vanadia-titania show the following: (i) Greater amounts of undissociated methanol are present on the TiOz surface; for both catalytic systems these species desorb or are destroyed faster than methoxy groups. (ii) The intensity ratio CH stretchings (2939, 2830 cm-')/CH bendings (= 1450 cm-l) of methoxy groups is much higher for Ti02than for vanadium-titanium oxide. This indicates that the electronic structure of these species on the two surfaces is rather different. Accordingly, it has been assumed that methoxy groups are bonded to V on the vanadia-titania surface and to Ti on the titania surface, which eventually results in a much more covalent bond in the former case. (iii) The oxidation of methoxy groups produces formate ions on both catalysts. This reaction is much faster on vanadium-titanium oxide than on Ti02. Indeed, at 180 OC the oxidation of methoxy species has not yet started on titania (Figure 10, b), whereas the intensity of the bands of methoxy groups are markedly lowered and those of formate ions are already strong on vanadia-titania (Figure
0.06
5 CH,lOCH,l7
10
/ H20
Figure 9. Effects of CH2(OCH3)2/H20 feed ratio on the hydrolysis of CH2(OCH3)2over the V-Ti catalyst at T = 170 OC under inert atmosphere (He). Different feed compositions were used to obtain complete miscibility of DMFL in H20.
10, e). On pure vanadia, methoxy groups are oxidized also at very low temperatures, producing mainly formaldehyde, while formates are not detected (Busca, 1988). (iv) Water is formed (band near 1610 cm-l, H 2 0 scissoring) upon methanol adsorption on both systems (Figure 10, a and c). These data indicate that in the case of the vanadium-titanium oxide catalyst methoxy groups are formed by methanol condensation with VOH groups and are involved in the oxidation of methanol. In Figure 11 the spectra obtained after adsorption of formaldehyde on the two surfaces are compared. In the case of Ti02, dioxymethylene species (bands marked with full circles) are very strongly bonded and stable at room temperature. They evolve, transforming into formate and methoxy groups at about 100 OC. In the case of vanadium-titanium oxide, on the other hand, dioxymethylene species are less stable, and either desorb as CH20or transform into formate and methoxy groups already at room temperature (Figure 11, b and c). On pure vanadia, dioxymethylene species are very unstable and desorb quickly at 0 OC (Busca, 1988). It appears therefore that vanadium-titanium oxide provides sites with peculiar characteristics for the adsorption of formaldehyde in the form of dioxymethylene species: on such sites these species exhibit an intermediate stability as compared to pure titania and pure vanadia. Finally, formate species are much less stable over vanadia-titania than over titania. In fact, the intensity of the characteristic bands of formate ions decreases strongly
392 Ind. Eng. Chem. Res., Vol. 28, No. 4, 1989 I
I
Figure 12. Mechanism of methanol oxidation over vanadium-titanium oxide. V
V
3200
3000
moo
im
moo
WAVENUMBERS
(CII-'
1400
)
Figure 10. FT-IR spectra of species arising from methanol adsorption on the pure Ti02 sample (a,b) and on the V-Ti sample (c-e): (a, c) evacuation at room temperature; (d) evacuation at 110 O C ; (b, e) evacuation at 180 O C . (*) Adsorbed undissociated CH,OH; (v)methoxy groups; (0) formate species
1
, '
I
-__k1 > 1600 1400 I200 YAVrNIIMRtH
1000
1rm-l
Figure 11. FT-IR spectra of species arising from the adsorption of formaldehyde on the pure TiOz sample (a, evacuation at room temperature) and on the V-Ti sample (b, adsorption at room temperature; c, evacuation at room temperature). ).( Dioxymethylene species.
during evacuation at about 220 "C in the former case, as opposite to 300 " C in the latter one. Discussion The results of catalyst characterization, flow reactor experiments, and CH30H and CHzO adsorption confirm and clarify further the mechanism of methanol oxidation over coprecipitated vanadium-titanium oxide already proposed (Tronconi et al., 1987; Busca et al., 1987b) as depicted in Figure 12. The flow reactor data clearly indicate that the active species for the oxidation of methanol to formaldehyde (step 2) are associated with the vanadium oxide surface species. The IR investigation provides evidence that methoxy groups are involved in the oxidation of methanol and are formed at V sites upon methanol adsorption. FT-IR data
suggest that, on dry surfaces, these groups are produced by condensation of methanol with surface VOH groups (step l),water being formed as a byproduct. In reaction conditions, an alternative mechanism may involve the displacement of dissociatively adsorbed water by dissociative adsorption of methanol. Both mechanisms are consistent with the observed inhibiting effect of water on the overall methanol conversion (Tronconi et al., 1987). They also agree with the picture provided by laser Raman and infrared spectroscopies,which indicate that vanadium monooxo species with a coordinative unsaturation are present at the surface of the dry sample and that they can easily be hydrated. Accordingly, methoxy groups are formed at V sites where also a V=O entity is present. Previous studies have clarified that hydrogen abstraction from methoxy groups is rate deteremining over molybdate-methanol oxidation catalysts (Machiels et al., 1985). This is likely to apply also to vanadium-titanium oxide, considering that the oxidation of formaldehyde is faster than the oxidation of methanol, as shown by flow reactor data. If so, the V=O group could favor the hydrogen abstraction from the coordinated methoxy species along the lines originally proposed by Trifir6 and Pasquon (1968). The IR study showed that the oxidation of surface methoxy groups produces formate ions. However, upon adsorption of CH20 on the vanadium-titanium oxide catalyst, dioxymethylene species are observed as a rather stable intermediate, in line with similar results obtained on other oxides (Busca et al., 1987~).The above species are believed to be formed on the vanadium-titanium oxide catalyst by interaction of formaldehyde with a surface cation-anion couple and are easily oxidized to formate ions at temperatures lower than or similar to those where surface methoxy groups undergo oxidation. This indicates that dioxymethylene are intermediate species in the oxidation of formaldehyde to formate ions, and it is further consistent with the hypothesis that step 2 is rate determining, the subsequent steps 4 and 6 being faster. Previous catalytic data obtained on catalysts with different V loadings and by varying CH30H/02feed ratio, temperature, and contact time suggested that DMFL should be regarded as an intermediate product and that it can be formed by condensation of methanol and formaldehyde a t the catalyst surface (Tronconi et al., 1987). The present data confirm that the hydrolysis reaction of DMFL (step 5 reverse) occurs as well. As for the route by which methyl formate is produced from formaldehyde, IR spectroscopy (Busca et al., 1987b) provides evidence for a Cannizzaro-typedisproportionation of dioxymethylene, leading to formate and methoxy groups (step 11). However, the results of the oxidation of formaldehyde in the presence and absence of oxygen show that the oxidation route (step 6) is definitely faster under
Ind. Eng. Chem. Res., Vol. 28, No. 4, 1989 393 typical reaction conditions. It is worth noticing that methyl formate can be produced by contacting methanol with surface formate ions at room temperature (step 7; Busca et al. (1987b)) and that high selectivities to methyl formate were measured when the experimental conditions, namely CH30H/02feed ratio, reaction temperature, and contact time, were properly chosen so as to minimize parallel or consecutive reactions, leading to HCOOH and carbon oxides (Tronconi et al., 1987). This has been taken as an indication that the esterification of formate groups is faster than their decomposition provided that methanol is available in the reaction mixture. The present catalytic data further indicate that reverse step 7 can occur under reaction conditions. Finally, the flow reactor data of HCOOH decomposition prove that CO and COPoriginate through steps 9 and 10 upon dissociative adsorption of HCOOH both at V and Ti sites. This eventually indicates that the reaction of surface formate with water to give formic acid (step 8) is reversible. Other authors suggested that methyl formate is produced either by the dimerization of formaldehyde over various Mo- or W-based oxides (Ai, 1982) or by the heterogeneous analogue of the Tischenko reaction via a surface hemiacetal intermediate over Mo03/SiOz (Louis et al., 1988). However, the relative importance of the oxidation route with respect to the disproportionation route in the formation of methyl formate from formaldehydewas not addressed by Ai. Louis et al. reported that the conversion of formaldehydeis higher in the presence of oxygen than under inert atmosphere over molybdena-silica, which agrees with our results over vanadium-titanium oxide. As for the data supporting the mechanism via an hemiacetal intermediate, they were obtained on pure silica and in the presence of oxygen. Accordingly, the conclusions of Louis et al. (1988) might not apply to different catalytic systems and to different experimental conditions. The capability of vanadium-titanium oxide to catalyze the formation of methyl formate via oxidation of formaldehyde to formate ions can be correlated with the presence of dioxymethylene species, exhibiting an intermediate stability as compared to pure titania and vanadia. Along these lines the much greater lability of these species on V P 0 5(Busca, 1988) is possibly responsible for the high selectivity to formaldehyde, whereas their stability on Ti02, which is greater than or similar to that of surface formates, may account for the formation of CO and COPvia decomposition of formate ions. Besides, the high surface area of the vanadium-titanium oxide sample is expected to favor the consecutive oxidation of formaldehydeto formate ions as compared to pure vanadia. Acknowledgment
This work was supported by Minister0 Pubblica Istruzione (Roma). J.P.G.M. gratefully acknowledges a grant from Junta Andalucia (Spain). Registry No. HC02H, 64-18-6; TiOz, 13463-67-7; CH30H, 67-56-1; CHz(OCH3)2, 109-87-5; HCOQCH,, 107-31-3; HCHO, 50-00-0; vanadium oxide, 11099-11-9;titanium oxide, 51745-87-0.
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Received f o r review June 13, 1988 Accepted November 21, 1988
