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Oxyesterification of Methanol To Methylformate Over V-Ti Oxide Catalysts
G. Centi and F. Trifiro’ (Editors),New Developments in Selective Oxidation 0 1990 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands
OXYESTERIFICATION OF KETHANOL TO ME-RMATR
305
OVER V-Ti OXIDE
CATALYSTS
A.S. EL MI^, G. BUSCA~,c. CRISTIANI~,P. FORZATTI~ and E. TRONCONIl
Dipartimento di Chimica Industriale e Ingegneria Chimica del Politecnico, P.zza L. Da Vinci 32, 20133 Milano (Italy) Dipartimento di Chimica, Facolth di Ingegneria dell ’ Universith, 16129 Genova (Italy) SUMMARY Based on previous work and on new data for various V-Ti oxide systems, generalized results are presented concerning their physico-chemical characterization, their catalytic behavior in the oxyesterification of methanol to methyl formate, and the related reaction mechanism. The feasibility of industrial process configurations for the the production of methyl formate, possibly combined with formaldehyde, are discussed. INTRODUCTION Methyl formate is regarded as a convenient intermediate in the synthesis of several chemicals. The current technology for its production involves carbonylation of methanol in the liquid phase in the presence of basic catalysts, typically sodium methylate, at low temperatures and under moderate CO pressures (ref.11, CH30H + CO --> HCOOCH3 A route using the gas-phase dehydrogenation of methanol over Cubased catalysts has been recently proposed (ref.21, 2 CH30H --> HCOOCH3 + 2 H2 for which attractive yields in methylformate have been claimed. An alternative oxidative route, based on the reaction 2 CH30H + 02 --> HCOOCH3 + 2 H20 was studied by Ai (ref.3) over various Mo- and W- based catalysts. We have reported that this reaction occurs also over V-Ti oxide catalysts prepared either by coprecipitation (refs.4-5) or by impregnation (ref.6) techniques. Encouraging preliminary results concerning global selectivities and yields of methyl formate (ref.4) prompted us to perform a complete process variable study (ref.5), and to address the mechanistic features by an FT-IR study
306
on the interaction of methanol and its oxidation products with the V-Ti02 surface (ref.6). The characterization of the catalyst samples was also fully investigated (refs.4-9). Recently, our understanding of the reaction mechanism has been completed and refined by the results of a series of flow reactor experiments where reaction products and intermediates were used as reagents, which have confirmed the oxidative nature of the reaction step leading to methyl formate ("oxyesterification") as compared to the disproportionation mechanism previously suggested (ref.7). Based on our previous work as well as on new data for various coprecipitated V-Ti systems, in this paper we present generalized results concerning their physico-chemical characteristics, their catalytic behavior and the related mechanistic features. The effects of the catalyst preparation parameters (V/Ti a.r. and calcination temperature) and of the operating conditions is discussed in connection with the selection and the feasibility of alternative process configurations (production of methyl formate only versus coproduction of formaldehyde and methyl formatel. EXPERIMENTAL
V-Ti oxide samples with V/Ti atomic ratios (a.r.1 0 - 0.5 were prepared by coprecipitation from VOCl3 and Tic14 at r. t., followed by drying and calcination. Different samples were obtained varying the calcination temperatures between 500 and 700 "C. The procedures and the equipment used in catalyst characterization and in flow reactor experiments have been described elsewhere (refs. 4-9). RESULTS AND DISCUSSION Catalyst Characterization Coprecipitated V-Ti oxide catalysts have been characterized with respect to the influence of both calcination temperature and V/Ti a.r. Samples with low V/Ti a.r. and low activation temperature are constituted by the anatase phase only. XRD, W-visible diffuse reflectance, ESR, FT-IR and chemical analysis provide evidence for the presence of a solid state solution characterized by the incorporation of V4+ in the bulk (ref.8). For the samples with V/Ti a.r.=0.0375, on increasing the calcination temperature the rutile phase becomes predominant (Tc= 6 5 0 "C), and a sudden drop in surface area is observed. On the other hand, on increasing the
307
V/Ti a.r. at a calcination temperature of 600 "C, V2O5 appears in the samples with VITi a.r.2 0.0625, again causing a drop in surface area. Appearance of a rutile phase of Ti02 is detected in the sample with V/Ti a.r.= 0.5. For a calcination temperature of 700 O C , the rutile phase is first detected at V/Ti a.r. 20.0125, while V2O5 is observed at higher V loadings. Table 1 presents the specific surface area of the coprecipitated V-Ti02 catalysts and the detected phases other than the anatase phase as functions of the sample calcination temperature and of the V/Ti a.r.. TABLE 1 Effects of calcination temperature and V/Ti a.r. on the surface areas (m2/g) and on the phase composition of V-Ti oxide samples. V = V2O5 detected. R = rutile phase detected. CALCINATION TEMPERATURE ("C) V/Ti (a.r.) 500" 0 0.01 0.0125 0.025 0.0375 0.05 0.0625 0.125 0.25 0.50
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70 82 83 78
-
80 30
550"
-
600" (ref.5)
--
53
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59 54 64 62
-
-
48 44
37 28 5 4 6 7
625"
39 34 v v V R+V
--
-
650"
675"
700"
(ref.4) 20 16 9 R 9 R 6 R
-
1 R
-
3 R+V
-
The boundary between the samples constituted by the anatase phase only, and those where also the rutile phase and/or V2O5 are detected is seen to correspond to a dramatic reduction of the surface area. The results of elemental chemical analysis further indicate that V interacts with the support in the form of V4+ and that it is also present at the surface as V5' (ref.7). The characterization by adsorption of probe molecules and a combined FT-IR and Laser Raman microscopy study demonstrate that both V and Ti centers, and specifically mono-oxo vanadyl centers with a coordinative unsaturation, are present at the catalyst surface (refs. 5,9).
308
Catalytic behavior of V-Ti02 samples: effects of V/Ti a.r. and of calcination temperature During the flow reactor experiments, the observed reaction products included HCHO, methyl formate, water, dimethylformal (DMFL), CO, C02 and formic acid (refs. 4 - 5 ) . Varying the V content of the catalysts was found to affect significantly both the conversion of methanol and the distribution of products. For the samples calcined at 600 OC, the overall conversion is seen to go through a maximum (V/Ti a.r.= 0.0625-0.1251, which can be attributed to the contrasting effects resulting from increasing the V loading: while this enhances the oxidizing capacity of the
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V l T i atomic ratio
Fig.1 - Effects of the calcination temperature Tc and of the V/Ti a.r. on the HCOOCH3/HCHO molar ratio in the oxidation of methanol over V/Ti oxide catalysts. catalyst, it also reduces its surface area (see Table 1). For the same calcination temperature, Fig. 1 shows that a maximum is evident also in the HCOOCH3/HCHO molar ratio. As discussed in a later section, this ratio is of specific interest for the implementation of an industrial process for the production of methyl formate: depending on its value, different process designs have to be considered. As compared to the optimal V content in Fig. 1, the low selectivities to HCOOCH3 corresponding to low and high V contents appear to be associated with poorly active systems due to deficiency of oxidizing capacity, and to deficiency of surface area and excess of V, respectively. This interpretation is consistent with a reaction mechanism where formation of HCOOCH3 occurs consecutively to the formation of HCHO, requiring V-related
309
catalytic centers and adequate surface areas. In this work the influence of changes in the catalyst calcination temperature has also been studied, as shown in Fig. 1. Higher calcination temperatures correspond to lower surface areas for the same V/Ti a.r., as indicated in Table 1, and also to greater amounts of V at the surface: accordingly, the HCOOCH3/HCHO ratio is seen only to decrease in the case of the catalysts calcined at 700 OC, where the optimal HCOOCHJ/HCHO ratio is shifted to lower V loadings; on the other hand, for the catalysts calcined at 550 OC, associated with higher surface areas and lower V contents at the surface, only the rising branch of the curve is apparent, the maximum being shifted to greater V/Ti a.r.. Catalytic behavior of V-Ti02 - samples: effects of the process variables The effects of the main process variables, including methanol and water feed concentrations, space velocity, temperature and pressure have been investigated over various catalysts. The 02 feed molar concentration was fixed at = 10% in all runs in order to remain below the flammability limits of methanol/oxygen mixtures.
30
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Fig.2 - Effect of methanol feed content on % HCOOCH3 and on the HCOOCH3/HCHO molar ratio. Catalyst: sample with V/Ti a.r.= 0.0625 calcined at 55OOC. Reaction conditions: T= 165 OC, 10% 0 2 feed, F/Wc= 10 cc/g min.
310
For a catalyst with V/Ti a.r. = 0.0625, Tc= 550 'C, Fig. 2 illustrates the effects of the methanol feed level on the concentration of methylformate in the products and on the HCOOCH3/HCHO molar ratio. Distinct optimal values of the methanol feed content exist for the output concentration of methylformate and for the HCOOCH3/HCHO ratio. Selectivities to valuable products (HCOOCH3+ HCHO+ DMFL) in excess of 90% were achieved with methanol concentrations greater than 15%. An excess of methanol enhanced DMFL with respect to HCHO, and almost suppressed the formation of CO and CO2. The effect of reaction temperature at two space velocities on the HCOOCH3/HCHO is presented in Fig. 3 for a catalyst with V/Ti a.r.= 0.0375 calcined at 600 OC. Both high temperatures and long contact times are seen to favor methyl formate with respect to formaldehyde, in line with the consecutive nature of the reaction scheme. At temperatures higher than 180 OC, however, a sudden drop in the selectivity to methyl formate has been observed for prolonged contact times. The addition of H20 to the feed was found to depress the overall conversion of methanol, and also reduced the ratio HCOOCH3/HCHO.
Fig.3
-
T ('C
1
Effect of reaction temperature and of contact time on the HCOOCH3/HCHO molar ratio. Catalyst: sample with V/Ti a.r.= 0.0375 calcined at 600 OC. Reaction conditions: 10% CH30H, 10% 0 2 feed; F/Wc = 12 cc/g min (curve A ) and 2 4 cc/g min (curve BI.
311
Similar effects of the process variables had observed with other V-Ti catalysts (refs. 4,5), to be representative of the general catalytic systems. They are interpreted in the following light of our findings on the reaction mechanism.
been previously so that they seem
behavior of such section in the
Mechanism of the oxidation of methanol over V/TI oxide catalysts The mechanistic features of the oxidative route to methyl formate over V-Ti oxide catalysts have been studied by FT-IR techniques, investigating the interaction of methanol and its oxidation products with the catalyst surface (ref.61, and by running a series of flow reactor experiments where intermediates and reaction products were used as reactants (ref.7). The results are supportive of the reaction scheme presented in Fig. 4 , consisting essentially of successive oxidation steps. Each of these steps has received experimental validation by FT-IR and/or specifically designed flow reactor runs. Thus, in the case of the route leading from formaldehyde to methyl formate, IR spectroscopy has provided evidence for a Cannizzaro-type disproportionation of dioxymethylene (step 111, and the occurrence of this reaction has been confirmed by flow reactor experiments with a HCHO + He feed, where HCOOCH3 was produced. However, the results of similar experiments with a HCHO + 02 + He feed show that the oxidation route (step 6) is considerably faster under typical, oxidizing reaction conditions.
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Fig.4 Reaction mechanism for the oxidation of methanol over V/Ti oxide catalysts.
312
Flow reactor runs with HCOOH in the feed have proved that CO and C02 originate through decomposition of formate groups. The esterification of formate groups to methyl formate appears however to be faster than their decomposition, provided that methanol is available in the reaction mixture. All of the effects of the operating variables can be interpreted on the basis of the scheme in Fig.4. Thus, step 1 is consistent with the observed inhibiting effect of water on the conversion of methanol. The effect of the CH30H feed concentration can be rationalized by observing that, for CH30H < lo%, the excess of oxygen favors the oxidation steps, leading preferentially to the terminal products HCOOCH3, CO and HCOOH. On increasing the methanol feed content, the steps involving gaseous methanol are beneficially affected, resulting first in a decreased selectivity to CO and HCOOH, corresponding to an increased selectivity to HCOOCH3, and finally in enhanced selectivities to HCHO and particularly to DMFL. The data in Fig. 3 are explained considering that an increase in temperature and contact time results in enhanced methanol conversions, and reduces the concentration of gaseous methanol. Accordingly, first the selectivity to HCOOCH3 grows at the expense of HCHO + DMFL, then the selectivity to CO is favored at the expense of methyl formate. Catalytic tests for the oxidation of methanol over pure Ti02 (ref. 5) have confirmed the fundamental role of Vanadium in the oxidative steps of the mechanism (steps 2 and 6 in Fig. 4). Process considerations The general results, of the flow reactor experiments indicate that the ratio HCOOCH3/HCHO in the products can be adjusted within a wide range of values by appropriate choices of both the catalyst preparation parameters and of the reaction conditions, depending on the desired features of the reaction product. One possible goal is to design a process aimed at the production of methyl formate only. Fig. 1, 2 and 3 illustrate a few examples where the production of methyl formate can be optimized by a suitable selection of either V/Ti a.r. and calcination temperature, or of the methanol feed concentration or of the reaction temperature. Along these lines we have achieved values of HCOOCH3/HCHO as high as 20, corresponding to weight ratios =40/1, with productivities to HCOOCH3 exceeding 200 g/Lh.
313
Alternatively, the V-TiOz catalysts appear suitable also for the industrial coproduction of HCOOCH3 and HCHO by the mild gas-phase oxidation of methanol. In this case, the ratio HCOOCH3/HCHO is expected to have a strong impact on the design of the separation section of such a process, for which a tentative schematic diagram is given in Fig.5.
ASES
ri C H30H
'"
YHC HO
Fig.5 - Tentative process scheme for the coproduction of methyl formate and formaldehyde by mild gas-phase oxidation of methanol. Units 1 and 2 are devoted to the separation of HCHO, which is dissolved in water, and to the concentration of the resulting aqueous solution. The remaining separation units effect removal from the gaseous stream of the inert gases (unit 3 1 , which may be in part recycled to dilute the oxygen in the air feed stream, of methyl formate (unit 4 ) , and eventually of unreacted methanol (unit 5 ) , which is recirculated to the synthesis reactor. In this scheme, the trickiest section is that effecting separation of formaldehyde, its target being the production of a commercial aqueous solution of HCHO. If concentration of the solution is required, a lower bound exists on the acceptable content of formaldehyde in the reaction products. This implies that it may be desirable to design operation of the reactor without necessarily maximizing the HCOOCHJ/HCHO ratio. Preliminary calculations of the separation section were performed assuming a reactor outlet stream containing 10.5% H20
314
and 1.5% HCHO. Results demonstrate that concentrations of HCHO of 20% w/w and more in the final solution are feasible by autothermal operation of units 1 and 2 under slight pressure. Polymerization of HCHO can be prevented by allowing a small concentration of residual methanol in the final solution. The final choice of the reactor working conditions, however, is controlled by a balance between the increase in revenues expected from maximization of the HCOOCH3 production and the increased costs resulting from concentration of more dilute aqueous solutions of HCHO, for which a detailed economic analysis is required. ACKNOWLEDGEMENTS This work was supported by Minister0 Pubblica Istruzione (Roma). REFERENCES 1 The Leonard Process Co. - Kemira OY, Formic Acid, Hyd. Process., November (1983). 2 S.P. Tonner, D.L. Trimm, M.S. Wainwright and N.W. Cant, Dehydrogenation of Methanol to Methyl Formate over Copper Catalysts, I&EC Prod.Res.Dev., 23 (1984) 384. 3 M. Ai, The Production of Methyl Formate by the Vapor Phase Oxidation of Methanol, J. Cat., 77 (1982) 279. 4 P. Forzatti, E. Tronconi, G. Busca and P. Tittarelli, Oxidation of Methanol to Methyl Formate over V-Ti Oxide Catalysts, Cat. Today, 1 (1987) 209. 5 E. Tronconi, A.S. Elmi, N. Ferlazzo, P. Forzatti, G. Busca and P. Tittarelli, Methyl Formate from Methanol Oxidation over Coprecipitated V-Ti-0 Catalysts, I&EC Res., 26 (1987) 1269. 6 G. Busca, A.S. Elmi and P. Forzatti, Mechanism of Selective Methanol Oxidation over Vanadium Oxide - Titanium Oxide Catalysts: A FT-IR and Flow reactor Study, J. Phys. Chem., 91 (1987) 5263. 7 A.S. Elmi, E. Tronconi, C. Cristiani, J.P. Gomez Martin, P. Forzatti and G. Busca, Mechanism and Active Sites for Methanol Oxidation to Methyl Formate over Coprecipitated VanadiumTitanium Oxide Catalysts, I&EC Res., 28 (1989) 387. 8 G. Busca, P. Tittarelli, E. Tronconi and P. Forzatti, Evidence for the Formation of an Anatase-Type V-Ti Oxide Solid State Solution, J. Solid State Chem., 67 (1987) 91. 9 C. Cristiani, P. Forzatti and G. Busca, On the Surface Structure of Vanadia-Titania Catalysts: Combined Laser-Raman and FT-IR Investigation, J. Cat., 116 (1989) 586.
315
V. CORTES CORBERAN (Instituto de Catalisis y Petroleoquimica, CSIC, Madrid, Spain): Concerning the composition of the different V-Ti-0 catalysts, have the authors experimental evidence of the surface composition ? And, if s o , does the bulk composition represent the actual surface composition of samples having different V/Ti atomic ratios ? G. BUSCA (University of Genova, Italy): A qualitative analysis of the catalyst surface composition has been performed for one of the most active catalysts (V/Ti a.r. 0.0375, calcined at 6 0 0 OC ( 1 ) ) using FT-IR spectroscopy. It has been shown that vanadyl species (resaonsible for a well evident surface-sensitive IR band the V=O stretching) and of at 2050 cm , first overtone coordinatively unsaturated Ti ions (responsiblf for the formation of carbonyl species absorbing at 2195 cm when the catalyst is put into contact with CO gas) are both present on the surface. By measuring the intensities of these bands and by compearing them with those are measured on pure Ti0 and on VTi02 catalysts prepared by impregnation with measure2 amounts of vanadium compounds, a quantitative evaluation can be obtained ( 2 ) . In this case, a surface enrichment of vanadium seems evident.
af
1. E. Tronconi, A.S. Elmi, N. Ferlazzo, P. Forzatti, G. Busca and P. Tittarelli, Ind. Eng. C h e m . Res., 2 6 (1987) 1269. 2. G. Ramis, G. Busca and V. Lorenzelli, 2. Folge, 153 (1987) 189.
Phys. C h e m . ,
Neue
OXYESTERIFICATION OF KETHANOL TO ME-RMATR
305
OVER V-Ti OXIDE
CATALYSTS
A.S. EL MI^, G. BUSCA~,c. CRISTIANI~,P. FORZATTI~ and E. TRONCONIl
Dipartimento di Chimica Industriale e Ingegneria Chimica del Politecnico, P.zza L. Da Vinci 32, 20133 Milano (Italy) Dipartimento di Chimica, Facolth di Ingegneria dell ’ Universith, 16129 Genova (Italy) SUMMARY Based on previous work and on new data for various V-Ti oxide systems, generalized results are presented concerning their physico-chemical characterization, their catalytic behavior in the oxyesterification of methanol to methyl formate, and the related reaction mechanism. The feasibility of industrial process configurations for the the production of methyl formate, possibly combined with formaldehyde, are discussed. INTRODUCTION Methyl formate is regarded as a convenient intermediate in the synthesis of several chemicals. The current technology for its production involves carbonylation of methanol in the liquid phase in the presence of basic catalysts, typically sodium methylate, at low temperatures and under moderate CO pressures (ref.11, CH30H + CO --> HCOOCH3 A route using the gas-phase dehydrogenation of methanol over Cubased catalysts has been recently proposed (ref.21, 2 CH30H --> HCOOCH3 + 2 H2 for which attractive yields in methylformate have been claimed. An alternative oxidative route, based on the reaction 2 CH30H + 02 --> HCOOCH3 + 2 H20 was studied by Ai (ref.3) over various Mo- and W- based catalysts. We have reported that this reaction occurs also over V-Ti oxide catalysts prepared either by coprecipitation (refs.4-5) or by impregnation (ref.6) techniques. Encouraging preliminary results concerning global selectivities and yields of methyl formate (ref.4) prompted us to perform a complete process variable study (ref.5), and to address the mechanistic features by an FT-IR study
306
on the interaction of methanol and its oxidation products with the V-Ti02 surface (ref.6). The characterization of the catalyst samples was also fully investigated (refs.4-9). Recently, our understanding of the reaction mechanism has been completed and refined by the results of a series of flow reactor experiments where reaction products and intermediates were used as reagents, which have confirmed the oxidative nature of the reaction step leading to methyl formate ("oxyesterification") as compared to the disproportionation mechanism previously suggested (ref.7). Based on our previous work as well as on new data for various coprecipitated V-Ti systems, in this paper we present generalized results concerning their physico-chemical characteristics, their catalytic behavior and the related mechanistic features. The effects of the catalyst preparation parameters (V/Ti a.r. and calcination temperature) and of the operating conditions is discussed in connection with the selection and the feasibility of alternative process configurations (production of methyl formate only versus coproduction of formaldehyde and methyl formatel. EXPERIMENTAL
V-Ti oxide samples with V/Ti atomic ratios (a.r.1 0 - 0.5 were prepared by coprecipitation from VOCl3 and Tic14 at r. t., followed by drying and calcination. Different samples were obtained varying the calcination temperatures between 500 and 700 "C. The procedures and the equipment used in catalyst characterization and in flow reactor experiments have been described elsewhere (refs. 4-9). RESULTS AND DISCUSSION Catalyst Characterization Coprecipitated V-Ti oxide catalysts have been characterized with respect to the influence of both calcination temperature and V/Ti a.r. Samples with low V/Ti a.r. and low activation temperature are constituted by the anatase phase only. XRD, W-visible diffuse reflectance, ESR, FT-IR and chemical analysis provide evidence for the presence of a solid state solution characterized by the incorporation of V4+ in the bulk (ref.8). For the samples with V/Ti a.r.=0.0375, on increasing the calcination temperature the rutile phase becomes predominant (Tc= 6 5 0 "C), and a sudden drop in surface area is observed. On the other hand, on increasing the
307
V/Ti a.r. at a calcination temperature of 600 "C, V2O5 appears in the samples with VITi a.r.2 0.0625, again causing a drop in surface area. Appearance of a rutile phase of Ti02 is detected in the sample with V/Ti a.r.= 0.5. For a calcination temperature of 700 O C , the rutile phase is first detected at V/Ti a.r. 20.0125, while V2O5 is observed at higher V loadings. Table 1 presents the specific surface area of the coprecipitated V-Ti02 catalysts and the detected phases other than the anatase phase as functions of the sample calcination temperature and of the V/Ti a.r.. TABLE 1 Effects of calcination temperature and V/Ti a.r. on the surface areas (m2/g) and on the phase composition of V-Ti oxide samples. V = V2O5 detected. R = rutile phase detected. CALCINATION TEMPERATURE ("C) V/Ti (a.r.) 500" 0 0.01 0.0125 0.025 0.0375 0.05 0.0625 0.125 0.25 0.50
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70 82 83 78
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80 30
550"
-
600" (ref.5)
--
53
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59 54 64 62
-
-
48 44
37 28 5 4 6 7
625"
39 34 v v V R+V
--
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650"
675"
700"
(ref.4) 20 16 9 R 9 R 6 R
-
1 R
-
3 R+V
-
The boundary between the samples constituted by the anatase phase only, and those where also the rutile phase and/or V2O5 are detected is seen to correspond to a dramatic reduction of the surface area. The results of elemental chemical analysis further indicate that V interacts with the support in the form of V4+ and that it is also present at the surface as V5' (ref.7). The characterization by adsorption of probe molecules and a combined FT-IR and Laser Raman microscopy study demonstrate that both V and Ti centers, and specifically mono-oxo vanadyl centers with a coordinative unsaturation, are present at the catalyst surface (refs. 5,9).
308
Catalytic behavior of V-Ti02 samples: effects of V/Ti a.r. and of calcination temperature During the flow reactor experiments, the observed reaction products included HCHO, methyl formate, water, dimethylformal (DMFL), CO, C02 and formic acid (refs. 4 - 5 ) . Varying the V content of the catalysts was found to affect significantly both the conversion of methanol and the distribution of products. For the samples calcined at 600 OC, the overall conversion is seen to go through a maximum (V/Ti a.r.= 0.0625-0.1251, which can be attributed to the contrasting effects resulting from increasing the V loading: while this enhances the oxidizing capacity of the
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V l T i atomic ratio
Fig.1 - Effects of the calcination temperature Tc and of the V/Ti a.r. on the HCOOCH3/HCHO molar ratio in the oxidation of methanol over V/Ti oxide catalysts. catalyst, it also reduces its surface area (see Table 1). For the same calcination temperature, Fig. 1 shows that a maximum is evident also in the HCOOCH3/HCHO molar ratio. As discussed in a later section, this ratio is of specific interest for the implementation of an industrial process for the production of methyl formate: depending on its value, different process designs have to be considered. As compared to the optimal V content in Fig. 1, the low selectivities to HCOOCH3 corresponding to low and high V contents appear to be associated with poorly active systems due to deficiency of oxidizing capacity, and to deficiency of surface area and excess of V, respectively. This interpretation is consistent with a reaction mechanism where formation of HCOOCH3 occurs consecutively to the formation of HCHO, requiring V-related
309
catalytic centers and adequate surface areas. In this work the influence of changes in the catalyst calcination temperature has also been studied, as shown in Fig. 1. Higher calcination temperatures correspond to lower surface areas for the same V/Ti a.r., as indicated in Table 1, and also to greater amounts of V at the surface: accordingly, the HCOOCH3/HCHO ratio is seen only to decrease in the case of the catalysts calcined at 700 OC, where the optimal HCOOCHJ/HCHO ratio is shifted to lower V loadings; on the other hand, for the catalysts calcined at 550 OC, associated with higher surface areas and lower V contents at the surface, only the rising branch of the curve is apparent, the maximum being shifted to greater V/Ti a.r.. Catalytic behavior of V-Ti02 - samples: effects of the process variables The effects of the main process variables, including methanol and water feed concentrations, space velocity, temperature and pressure have been investigated over various catalysts. The 02 feed molar concentration was fixed at = 10% in all runs in order to remain below the flammability limits of methanol/oxygen mixtures.
30
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Fig.2 - Effect of methanol feed content on % HCOOCH3 and on the HCOOCH3/HCHO molar ratio. Catalyst: sample with V/Ti a.r.= 0.0625 calcined at 55OOC. Reaction conditions: T= 165 OC, 10% 0 2 feed, F/Wc= 10 cc/g min.
310
For a catalyst with V/Ti a.r. = 0.0625, Tc= 550 'C, Fig. 2 illustrates the effects of the methanol feed level on the concentration of methylformate in the products and on the HCOOCH3/HCHO molar ratio. Distinct optimal values of the methanol feed content exist for the output concentration of methylformate and for the HCOOCH3/HCHO ratio. Selectivities to valuable products (HCOOCH3+ HCHO+ DMFL) in excess of 90% were achieved with methanol concentrations greater than 15%. An excess of methanol enhanced DMFL with respect to HCHO, and almost suppressed the formation of CO and CO2. The effect of reaction temperature at two space velocities on the HCOOCH3/HCHO is presented in Fig. 3 for a catalyst with V/Ti a.r.= 0.0375 calcined at 600 OC. Both high temperatures and long contact times are seen to favor methyl formate with respect to formaldehyde, in line with the consecutive nature of the reaction scheme. At temperatures higher than 180 OC, however, a sudden drop in the selectivity to methyl formate has been observed for prolonged contact times. The addition of H20 to the feed was found to depress the overall conversion of methanol, and also reduced the ratio HCOOCH3/HCHO.
Fig.3
-
T ('C
1
Effect of reaction temperature and of contact time on the HCOOCH3/HCHO molar ratio. Catalyst: sample with V/Ti a.r.= 0.0375 calcined at 600 OC. Reaction conditions: 10% CH30H, 10% 0 2 feed; F/Wc = 12 cc/g min (curve A ) and 2 4 cc/g min (curve BI.
311
Similar effects of the process variables had observed with other V-Ti catalysts (refs. 4,5), to be representative of the general catalytic systems. They are interpreted in the following light of our findings on the reaction mechanism.
been previously so that they seem
behavior of such section in the
Mechanism of the oxidation of methanol over V/TI oxide catalysts The mechanistic features of the oxidative route to methyl formate over V-Ti oxide catalysts have been studied by FT-IR techniques, investigating the interaction of methanol and its oxidation products with the catalyst surface (ref.61, and by running a series of flow reactor experiments where intermediates and reaction products were used as reactants (ref.7). The results are supportive of the reaction scheme presented in Fig. 4 , consisting essentially of successive oxidation steps. Each of these steps has received experimental validation by FT-IR and/or specifically designed flow reactor runs. Thus, in the case of the route leading from formaldehyde to methyl formate, IR spectroscopy has provided evidence for a Cannizzaro-type disproportionation of dioxymethylene (step 111, and the occurrence of this reaction has been confirmed by flow reactor experiments with a HCHO + He feed, where HCOOCH3 was produced. However, the results of similar experiments with a HCHO + 02 + He feed show that the oxidation route (step 6) is considerably faster under typical, oxidizing reaction conditions.
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Fig.4 Reaction mechanism for the oxidation of methanol over V/Ti oxide catalysts.
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Flow reactor runs with HCOOH in the feed have proved that CO and C02 originate through decomposition of formate groups. The esterification of formate groups to methyl formate appears however to be faster than their decomposition, provided that methanol is available in the reaction mixture. All of the effects of the operating variables can be interpreted on the basis of the scheme in Fig.4. Thus, step 1 is consistent with the observed inhibiting effect of water on the conversion of methanol. The effect of the CH30H feed concentration can be rationalized by observing that, for CH30H < lo%, the excess of oxygen favors the oxidation steps, leading preferentially to the terminal products HCOOCH3, CO and HCOOH. On increasing the methanol feed content, the steps involving gaseous methanol are beneficially affected, resulting first in a decreased selectivity to CO and HCOOH, corresponding to an increased selectivity to HCOOCH3, and finally in enhanced selectivities to HCHO and particularly to DMFL. The data in Fig. 3 are explained considering that an increase in temperature and contact time results in enhanced methanol conversions, and reduces the concentration of gaseous methanol. Accordingly, first the selectivity to HCOOCH3 grows at the expense of HCHO + DMFL, then the selectivity to CO is favored at the expense of methyl formate. Catalytic tests for the oxidation of methanol over pure Ti02 (ref. 5) have confirmed the fundamental role of Vanadium in the oxidative steps of the mechanism (steps 2 and 6 in Fig. 4). Process considerations The general results, of the flow reactor experiments indicate that the ratio HCOOCH3/HCHO in the products can be adjusted within a wide range of values by appropriate choices of both the catalyst preparation parameters and of the reaction conditions, depending on the desired features of the reaction product. One possible goal is to design a process aimed at the production of methyl formate only. Fig. 1, 2 and 3 illustrate a few examples where the production of methyl formate can be optimized by a suitable selection of either V/Ti a.r. and calcination temperature, or of the methanol feed concentration or of the reaction temperature. Along these lines we have achieved values of HCOOCH3/HCHO as high as 20, corresponding to weight ratios =40/1, with productivities to HCOOCH3 exceeding 200 g/Lh.
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Alternatively, the V-TiOz catalysts appear suitable also for the industrial coproduction of HCOOCH3 and HCHO by the mild gas-phase oxidation of methanol. In this case, the ratio HCOOCH3/HCHO is expected to have a strong impact on the design of the separation section of such a process, for which a tentative schematic diagram is given in Fig.5.
ASES
ri C H30H
'"
YHC HO
Fig.5 - Tentative process scheme for the coproduction of methyl formate and formaldehyde by mild gas-phase oxidation of methanol. Units 1 and 2 are devoted to the separation of HCHO, which is dissolved in water, and to the concentration of the resulting aqueous solution. The remaining separation units effect removal from the gaseous stream of the inert gases (unit 3 1 , which may be in part recycled to dilute the oxygen in the air feed stream, of methyl formate (unit 4 ) , and eventually of unreacted methanol (unit 5 ) , which is recirculated to the synthesis reactor. In this scheme, the trickiest section is that effecting separation of formaldehyde, its target being the production of a commercial aqueous solution of HCHO. If concentration of the solution is required, a lower bound exists on the acceptable content of formaldehyde in the reaction products. This implies that it may be desirable to design operation of the reactor without necessarily maximizing the HCOOCHJ/HCHO ratio. Preliminary calculations of the separation section were performed assuming a reactor outlet stream containing 10.5% H20
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and 1.5% HCHO. Results demonstrate that concentrations of HCHO of 20% w/w and more in the final solution are feasible by autothermal operation of units 1 and 2 under slight pressure. Polymerization of HCHO can be prevented by allowing a small concentration of residual methanol in the final solution. The final choice of the reactor working conditions, however, is controlled by a balance between the increase in revenues expected from maximization of the HCOOCH3 production and the increased costs resulting from concentration of more dilute aqueous solutions of HCHO, for which a detailed economic analysis is required. ACKNOWLEDGEMENTS This work was supported by Minister0 Pubblica Istruzione (Roma). REFERENCES 1 The Leonard Process Co. - Kemira OY, Formic Acid, Hyd. Process., November (1983). 2 S.P. Tonner, D.L. Trimm, M.S. Wainwright and N.W. Cant, Dehydrogenation of Methanol to Methyl Formate over Copper Catalysts, I&EC Prod.Res.Dev., 23 (1984) 384. 3 M. Ai, The Production of Methyl Formate by the Vapor Phase Oxidation of Methanol, J. Cat., 77 (1982) 279. 4 P. Forzatti, E. Tronconi, G. Busca and P. Tittarelli, Oxidation of Methanol to Methyl Formate over V-Ti Oxide Catalysts, Cat. Today, 1 (1987) 209. 5 E. Tronconi, A.S. Elmi, N. Ferlazzo, P. Forzatti, G. Busca and P. Tittarelli, Methyl Formate from Methanol Oxidation over Coprecipitated V-Ti-0 Catalysts, I&EC Res., 26 (1987) 1269. 6 G. Busca, A.S. Elmi and P. Forzatti, Mechanism of Selective Methanol Oxidation over Vanadium Oxide - Titanium Oxide Catalysts: A FT-IR and Flow reactor Study, J. Phys. Chem., 91 (1987) 5263. 7 A.S. Elmi, E. Tronconi, C. Cristiani, J.P. Gomez Martin, P. Forzatti and G. Busca, Mechanism and Active Sites for Methanol Oxidation to Methyl Formate over Coprecipitated VanadiumTitanium Oxide Catalysts, I&EC Res., 28 (1989) 387. 8 G. Busca, P. Tittarelli, E. Tronconi and P. Forzatti, Evidence for the Formation of an Anatase-Type V-Ti Oxide Solid State Solution, J. Solid State Chem., 67 (1987) 91. 9 C. Cristiani, P. Forzatti and G. Busca, On the Surface Structure of Vanadia-Titania Catalysts: Combined Laser-Raman and FT-IR Investigation, J. Cat., 116 (1989) 586.
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V. CORTES CORBERAN (Instituto de Catalisis y Petroleoquimica, CSIC, Madrid, Spain): Concerning the composition of the different V-Ti-0 catalysts, have the authors experimental evidence of the surface composition ? And, if s o , does the bulk composition represent the actual surface composition of samples having different V/Ti atomic ratios ? G. BUSCA (University of Genova, Italy): A qualitative analysis of the catalyst surface composition has been performed for one of the most active catalysts (V/Ti a.r. 0.0375, calcined at 6 0 0 OC ( 1 ) ) using FT-IR spectroscopy. It has been shown that vanadyl species (resaonsible for a well evident surface-sensitive IR band the V=O stretching) and of at 2050 cm , first overtone coordinatively unsaturated Ti ions (responsiblf for the formation of carbonyl species absorbing at 2195 cm when the catalyst is put into contact with CO gas) are both present on the surface. By measuring the intensities of these bands and by compearing them with those are measured on pure Ti0 and on VTi02 catalysts prepared by impregnation with measure2 amounts of vanadium compounds, a quantitative evaluation can be obtained ( 2 ) . In this case, a surface enrichment of vanadium seems evident.
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1. E. Tronconi, A.S. Elmi, N. Ferlazzo, P. Forzatti, G. Busca and P. Tittarelli, Ind. Eng. C h e m . Res., 2 6 (1987) 1269. 2. G. Ramis, G. Busca and V. Lorenzelli, 2. Folge, 153 (1987) 189.
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