Synthesis and Testing of Catalysts for the Production of Maleic

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Ind. Eng. Chem. Res. 1996, 35, 663-671

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Synthesis and Testing of Catalysts for the Production of Maleic Anhydride from a Fermentation Feedstock Sanjay K. Yedur,† Joel Dulebohn,‡ Todd Werpy,§,| and Kris A. Berglund*,†,§,⊥ Department of Chemical Engineering, Michigan State University, East Lansing, Michigan 48824, Grand River Technologies, 3900 Collins Roads, Lansing, Michigan 48909, and Michigan Biotechnology Institute, 3900 Collins Roads, Lansing, Michigan 48909

It is necessary to develop alternate pathways for the production of chemicals that are traditionally produced from fossil fuels to reduce our dependency on nonrenewable energy sources. In this paper, an alternate technology is presented for producing maleic anhydride from a fermentation feedstock. The process involves the catalytic oxydehydrogenation of fermentation-derived succinic anhydride to produce maleic anhydride. Various catalysts have been synthesized and tested for the oxydehydrogenation reaction. Iron phosphate based catalysts are found to be the best on the basis of high conversions and selectivities obtained. The effects of temperature, oxygen concentration, contact time, and the total time on stream on the performance of the catalyst are investigated, and an optimum set of conditions for the operation of the bench-scale reactor is presented. The bulk and surface compositions, the surface areas, and the bulk crystallographic structure of the catalysts are also reported. 1. Introduction With the ever-increasing cost of fossil fuels, especially that of petroleum, there is a need to develop newer and more efficient technologies that are better able to use alternate feedstocks. Most of the chemical technologies available today are based on nonrenewable energy and feedstock sources like fossil fuels. As the consumption of various chemicals increases, a strain is placed on the fossil fuel sources due to increased production. Furthermore, it is well-known that the worldwide stock of fossil fuels has a finite lifetime; therefore, it is imperative to reduce our dependence on nonrenewable feedstocks. One option is to replace conventional nonrenewable feedstocks with renewable ones. The production of maleic anhydride from a completely domestic, renewable, fermentation-derived source affords one such opportunity. Maleic anhydride is an unsaturated dibasic acid anhydride used extensively in the chemical industry. The anhydride and the corresponding acid are industrially important raw materials in the manufacture of alkyl and polyester resins, surface coatings, lubricant additives, plasticizers, copolymers, and agricultural chemicals. The primary end use of maleic anhydride is as unsaturated polyester resins. Most existing maleic anhydride production technologies are based on nonrenewable feedstocks like benzene or butane. The need to shift to renewable feedstocks, coupled with the expansion in the market, suggests the opportunity for a fermentation-derived feedstock to compete with the existing technologies. This paper describes the development of a catalytic process for the production of maleic anhydride from a fermentationderived feedstock: succinic acid. Several patents have been issued that describe different processes for the fermentation production of * Author to whom all correspondence should be addressed. † Michigan State University. ‡ Grand River Technologies. § Michigan Biotechnology Institute. | Current address: Battelle, P.O. Box 999 (K2-40), Richland, WA 99352. ⊥ Current address: Departments of Chemical Engineering and Chemistry, Michigan State University, East Lansing, MI 48824. E-mail: [email protected].

0888-5885/96/2635-0663$12.00/0

Figure 1. Idealized conversion of succinic acid to maleic anhydride.

succinic acid (Datta, 1992; Berglund et al., 1991; Datta et al., 1992; Glassner and Datta, 1992). The feedstock for this fermentation may be derived from corn or other agricultural materials that represent an abundant, renewable, low-cost feedstock. The conversion to succinic acid utilizes an anaerobic fermentation process with an approximate product yield of 90% based on carbohydrate feedstock. The separation and purification processes involved are also of relatively low cost, making it a viable technology. The overall reaction from succinic acid to maleic anhydride proposed in this work is depicted in Figure 1. It shows the simple dehydration of succinic acid to succinic anhydride, which is believed to be an intermediate, followed by the dehydrogenation of succinic anhydride to maleic anhydride. This second reaction occurs only in the presence of a catalyst, and it is the aim of this work to select a catalyst suited for this reaction. A vapor-phase reaction is feasible for the proposed conversion if the melting and boiling points of each of the compounds involved are sufficiently high. The melting point of succinic acid is 185-187 °C, and the boiling point is 235 °C; the corresponding values for succinic anhydride are 119.6 and 261 °C, respectively. Maleic anhydride melts at 52.8 °C and boils at 202 °C (Robinson and Mount, 1983; Winstrom, 1978). These numbers indicate that a vapor-phase reaction would suit the proposed reaction scheme. In order to draw an analogy from available literature, a reaction scheme reasonably similar to the one of interest was sought. The conversion of isobutyric acid to methacrylic acid via oxydehydrogenation, depicted in Figure 2, was chosen due to several reasons. The similar dehydrogenation of a carbon-carbon bond adjacent to a carboxylic group and sufficient success in the development of appropriate catalysts are some of the attractive features of this reaction scheme. Further© 1996 American Chemical Society

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Figure 2. Catalytic dehydrogenation of isobutyric acid to methacrylic acid.

more, the temperature range involved is moderate, vapor-phase reaction is possible, catalysts used (usually iron phosphate or molybdenum oxide based) are relatively inexpensive, and the conversions and selectivities achieved are reasonably good. Oxidative dehydrogenation is chosen over conventional dehydrogenation since a study of the thermodynamics reveals a much more favorable pathway for the reaction in the presence of oxygen than without oxygen. In the dehydrogenation of butane, for example, the heat of formation and the free energy change drop to negative values in the presence of oxygen, whereas in the absence of oxygen, they are positive. Also, the free energy change tends to be negative in the presence of oxygen and positive in its absence, which is unfavorable. Theoretically, in some cases, the presence of oxygen also greatly increases the equilibrium constant, which is a positive attribute. In the dehydrogenation of butene, for example, the equilibrium constant increases from an order of 10-4 to 1013 going from the absence of oxygen to conducting the reaction in the presence of oxygen. An idealized view of the proposed mechanism of oxidative dehydrogenation is shown in Figure 3. The example chosen is that of the oxidative dehydrogenation of butane to butene over an iron-based catalyst. The first step shows the adsorption of the reactant molecule onto an active site on the catalyst. The next two steps show the release of the hydrogens and their subsequent reaction with surface oxygen resulting in dehydrogenation. The last two steps show the removal of water and the regeneration of the oxygen sites on the catalyst surface. As is evident, the process goes through a number of steps, indicating numerous avenues for possible optimization of the process.

Figure 3. Idealized mechanism of oxidative dehydrogenation.

Obviously, the most important step in the analogous reaction scheme being studied is the catalytic oxidative dehydrogenation of isobutyric acid. Numerous patents have been issued for different catalysts developed for this reaction. One catalyst developed for the oxydehydrogenation was a calcined residue of the mixed phosphates of iron and lead (Watkins, 1974), while another was composed of a calcined residue of a mixture of bismuth oxynitrate, iron phosphate, and lead phosphate (Watkins, 1975). A catalyst containing alkali metal, chromium, iron, lead, phosphorus, and oxygen was patented for catalyzing the oxidative dehydrogenation of isobutyric acid (Statz and Doty, 1980). Conversions up to 60% and selectivities up to 85% were achieved. A group working at Ashland Oil, Inc., developed catalysts composed of calcined phosphates of iron containing silver, tellurium, niobium, cobalt, or lanthanum as a modifier (Daniel and Brusky, 1981a,b; Daniel, 1982). These resulted in conversions of up to 87% and selectivities to methacrylic acid of about 70%. A catalyst composed of the calcined oxides of iron and at least two members selected from a group consisting of antimony, niobium, tantalum, and tungsten was also patented (Ruszala and Weeks, 1983). A host of supports was tried that included alumina, pumice, silicon carbide, zirconia, titania, silica, alumina-silica, etc. Ruszala also patented a catalyst composed of the calcined oxides of uranium and tungsten (Ruszala, 1983). Both of these resulted in relatively low conversions and selectivities of less than 50%. A catalyst, with good selectivity and conversion of about 90% and 70%, respectively, composed of the calcined phosphates of chromium, molybdenum, and tungsten was patented as was a catalyst composed of the calcined phosphomolybdate of potassium and cerium (Daniel, 1983a,b). Supports tried included silica, alumina, quartz, titanium dioxide, carbon, silicon carbide, etc. This approach gave lesser conversions and selectivities. Another catalyst tried was composed of the calcined phosphates of iron, silver,

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and chromium and a support material consisting of any one of silica, alumina, quartz, titanium dioxide, carbon, and silicon carbide (Daniel, 1983c). Calcined phosphates of iron and cerium were also experimented with some success (Daniel, 1983d). High conversions of up to 96% were obtained with these. A catalyst composed of the calcined mixtures of salts of titanium and iron was developed in which various supports were tried including colloidal silica or any other form of silica, alumina, pumice, zirconia, quartz, carbon, silicon carbide, etc. (Ruszala, 1984). The conversions and selectivities were lower than 50%, though. A catalyst composed of iron, phosphorus, silicon, vanadium, and molybdenum for the conversion of isobutyric acid to methacrylic acid was developed which had as support one of silica, alumina, quartz, titanium dioxide, carbon, or silicon carbide (Daniel, 1984). This resulted in a conversion of about 81% and a selectivity toward methacrylic acid of about 74%. The reaction has also been conducted over a series of ion-exchange-modified 12-heteropolyoxometalate catalysts (McCarvey and Moffat, 1991). Based on these numerous patents available in the literature for the oxidative dehydrogenation of isobutyric acid, two general categories of catalysts were selected to test their effectiveness for the proposed conversion of succinic acid to maleic anhydride. One class of catalysts prepared was iron phosphate based, whereas the other was molybdenum oxide based. 2. Experimental Methods 2.1. Preparation of the Catalysts. Two general categories of catalysts were prepared. The first were catalysts that had an iron phosphate base, whereas the second had a molybdenum oxide base. In the former category, four different types of catalysts were prepared, each having a different composition: the first was the base iron phosphate catalyst, the second had a support (Ludox HS 40%) added to the base, the third had a promoter (lanthanum or cerium) added to the base, and the fourth had both the support and the promoter added to the base. Three different types of molybdenum oxide based catalysts were also prepared. A wide-mouthed flask was used as the reaction vessel with a refluxing column and a condenser attached to it. The reaction mixture consisting of the raw materials was constantly stirred and the vapor refluxed using cooling water in the reflux column. Refluxing was continued for about 24 h, after which the reaction was considered to be complete. The excess water in the system was distilled off. The resulting catalyst (formed as a thick paste) obtained was dried in an oven overnight at about 110 °C. The catalyst was removed from the reaction vessel and stored for calcination in a high-temperature oven. Calcination was carried out in air at 450 °C for 6 h and further calcined in a mixture of nitrogen and oxygen at 500 °C for 2 h. A detailed preparation scheme for each of the nine catalysts prepared and tested in this work is given below. All chemicals used in this project were bought and used as obtained from Aldrich Chemical Co., except for the silica support Ludox HS 40% (DuPont Chemicals), iron nitrate nonahydrate (Columbus Chemical Industries, Inc.), and heavy water (Cambridge Isotope Laboratories). 1. Base Iron Phosphate. The base iron phosphate catalyst was prepared by dissolving 48.5 g of Fe(NO3)3‚9H2O (iron nitrate nonahydrate) and 15 mL of H3PO4 (85%) in 120 mL of water. The solution was

refluxed with constant stirring and heating; after 2 h of refluxing, a cream-colored slurry was formed. Refluxing was carried on for about 20 h, after which the slurry was distilled to form a thick paste. The catalyst paste obtained was dried in an oven at 110 °C for about 24 h. The catalyst was calcined at 450 °C for 6 h in a forced-air oven. A second calcination was performed in a quartz reactor tube at 500 °C for 2 h with an oxygen flow rate of 10 mL/min and a nitrogen flow rate of 10 mL/min. 2. Base Iron Phosphate with Silica Support. The base iron phosphate catalyst with a silica support was prepared exactly as in preparation 1, but with the addition of 10 mL of Ludox HS 40% to the aqueous solution. The catalyst was calcined in a similar fashion. 3. Cerium-Promoted Iron Phosphate. The base iron phosphate catalyst with a promoter (5% cerium) was prepared exactly as in preparation 1, but with the addition of 2.5 g of Ce(NO3)3‚6H2O (cerium nitrate hexahydrate) to the aqueous solution. The catalyst was calcined in a similar fashion. 4. Lanthanum-Promoted Iron Phosphate. The base iron phosphate catalyst with a lanthanum promoter was prepared exactly as in preparation 1, but with the addition of 9.9 g of lanthanum pentaoxide to the aqueous solution. The catalyst was calcined in a similar fashion. 5. Cerium-Promoted Iron Phosphate Catalyst Supported on Silica. The base iron phosphate catalyst with a promoter (5% cerium) and a silica support was prepared exactly as in preparation 1, but with the addition of 2.5 g of Ce(NO3)3‚6H2O and 10 mL of Ludox HS 40% to the aqueous solution. The catalyst was calcined in a similar fashion. 6. Lanthanum-Promoted Iron Phosphate Catalyst Supported on Silica. The base iron phosphate catalyst with a lanthanum promoter and a silica support was prepared exactly as in preparation 1, but with the addition of 2.5 g of lanthanum pentaoxide and 10 mL of Ludox HS 40% to the aqueous solution. The catalyst was calcined in a similar fashion. 7. Molybdenum Oxide Based Catalyst I. The catalyst was prepared by adding 0.71 mL of H3PO4 (85%), 1.17 g of NH4VO3, 19.5 g of (NH4)2Mo2O7, and 0.48 g of Cu(NO3)‚2.5H2O to 300 mL of water. A fine suspension was formed. The water was evaporated off to form a thick paste. The paste was dried in air in an oven for 12 h at 155 °C. The catalyst was calcined at 400 °C in a forced-air oven for 6 h. A second calcination was performed in a quartz tube at 400 °C for 2 h with an oxygen flow rate of 10 mL/min and a nitrogen flow rate of 10 mL/min. The resulting catalyst was a complex mixture of molybdenum, phosphorus, vanadium, copper, and oxygen. 8. Molybdenum Oxide Based Catalyst II. The second molybdenum oxide based catalyst was prepared by adding 1.6 mL of H3PO4 (85%), 30 g of MoO3, 2.1 g of V2O5, and 10.7 g of WO3 to 200 mL of distilled water. The suspension formed was refluxed for 72 h. It was filtered after being cooled to room temperature. The filtrate was evaporated to dryness at 120 °C and dried in an oven at 115 °C for 16 h. The catalyst was calcined at 400 °C in a forced-air oven for 6 h. A second calcination was performed in a quartz tube at 400 °C for 2 h with an oxygen flow rate of 10 mL/min and a nitrogen flow rate of 10 mL/min. The resulting catalyst was a complex mixture of molybdenum, phosphorus, vanadium, tungsten, oxygen, and hydrogen.

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Table 1. Commercial Catalysts Tested catalyst

constituents; form

United Catalyst G-84C United Catalyst G-65 United Catalyst G-13 United Catalyst G-22 United Catalyst G-41

iron, potassium, chrome; 1/8 in. extrusions nickel on refractory; 1/8 in. extrusions copper chromite; 3/16 × 3/16 in. tablets copper chromite; 3/16 × 3/16 in. tablets chromium on alumina; 1/8 in. extrusions

9. Molybdenum Oxide Based Catalyst III. The third molybdenum oxide based catalyst was prepared by adding 27 g of (NH4)6Mo7O24‚4H2O, 8.1 g of (NH4)2Ce(NO3)6, 1.73 g of KOH, 3.2 mL of H3PO4 (85%), and 60 mL of concentrated HCl to 200 mL of distilled water. The resulting slurry was heated at 125 °C to a thick paste. The paste was dried in an oven at 115 °C for 18 h. The catalyst was calcined at 400 °C in a forced-air oven for 6 h. A second calcination was performed in a quartz tube at 400 °C for 2 h with an oxygen flow rate of 10 mL/min and a nitrogen flow rate of 10 mL/min. The resulting catalyst was composed of molybdenum, cerium, potassium, phosphorus, and oxygen. Commercial Catalysts. Commercially available standard dehydrogenation catalysts were also procured from United Catalysts Inc. Table 1 lists these catalysts along with their constituents and form. Exact compositions are proprietary information. 2.2. Testing of the Catalysts. The reactor used for testing the effectiveness of the catalysts was a 1-m quartz tube with a glass frit attached at the center that served to act as a support for the catalyst. The reactor tube was enclosed within a three-zone furnace (Series 3210, manufactured by Applied Test Systems, Inc.). The temperature of each zone of the furnace was independently controlled using Digi-Sense temperature controllers (Model 2186-10, marketed by Cole-Parmer Instrument Co.). A syringe infusion pump with adjustable flow rates (Model 11, manufactured by Harvard Apparatus) at the top of the reactor served to inject the succinic acid feed solution into the reactor. Glass wool was used to prevent any contamination on the frit. Glass beads were loosely packed on the glass wool up to the top of the reactor to provide a uniform flow of the feed onto the catalyst. Nitrogen was used as an inert carrier gas through the reactor. A provision was made to introduce oxygen into the reactor along with the nitrogen. The flow of the two gases was regulated by Brooks Series 5850E mass flow controllers. The product gases from the reactor were condensed into a collection flask placed in an ice bath. The reaction products deposited on the sides of the collection vessel were dissolved in water and taken for analysis. Carbon dioxide production was monitored by routing the effluent gases to a flask containing a buffer solution of pH 5 into which an Orion (Model 95-02) CO2 electrode was immersed. A schematic of the whole apparatus is shown in Figure 4. 2.3. Product Analysis. Product analysis was done on a VXNMR 300 machine located at the Max T. Rogers facility at Michigan State University. 1H nuclear magnetic resonance was the technique of choice because of the ease in identifying products and determining their composition. This technique permits the identification of all compounds regardless of their functionality. However, due to the nature of the experiment here, maleic anhydride could not be detected because of the large amounts of water in the system, which caused the hydration of the anhydride to the acid form. So, the final detectable product was maleic acid. This was not considered a problem, since the maleic acid can easily

Figure 4. Reactor setup: (1) nitrogen gas, (2) oxygen gas, (3) syringe feed pump, (4) reactor tube, (5) catalyst, (6) furnace, (7) collection vessel, (8) ice bath, (9) temperature controllers.

Figure 5. NMR spectrum showing 5% succinic acid and 5% maleic acid standards.

Figure 6. Typical NMR spectrum of the reaction products of reaction of succinic acid to maleic acid over a supported iron phosphate catalyst.

be converted back to the anhydride simply by heating and dehydrating the acid. As NMR required a presence of a deuterated solvent to lock on, the succinic acid was dissolved in heavy water, D2O, instead of regular water. Typical NMR spectra are shown in Figures 5 and 6. Figure 5 shows succinic and maleic acid standards at a concentration of 5% in D2O, while Figure 6 shows the reaction products from a typical run over an iron phosphate catalyst. D2O is highly hygroscopic, absorbing atmospheric water to form HDO. This shows up in the NMR spectra as the large solvent peak. Decoupling techniques were used to reduce the size of the large solvent peaks expected in all analyses. As is evident

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from comparing the two spectra, the formation of maleic acid is confirmed. 1H NMR, as the name suggests, is a technique to measure the number of protons present in the sample. As maleic acid has two protons less than succinic acid, the peak of maleic acid is half that of succinic acid at the same concentrations. Therefore, to determine the relative amounts of the two, the area of the maleic acid peak is multiplied by 2 to equate it with the succinic acid peak. In Figure 6, the numbers alongside the peaks indicate the integrated area as obtained by the analysis. Since initially there was no maleic acid in the system, all maleic acid present is as a result of conversion of succinic acid. As an example, the conversion to maleic acid is given as

% conversion ) (0.19 × 2)/{(0.19 × 2) + 1.09} × 100 ) 25.8% The absence of any other detectable peak in the spectrum also indicates the high selectivity of the reaction. The only other reaction product possible that is not detectable by 1H NMR is carbon dioxide which could be produced if the succinic acid was being directly combusted to CO2 in the presence of oxygen. This possibility was monitored by the CO2 electrode. The presence of CO2 is indicated by a large change in the voltage reading by the electrode. Experiments were conducted with the commercially available catalysts and those prepared in the laboratory to determine their effectiveness in the proposed conversion. Thereafter, the effects of various parameters on the conversion of succinic acid to maleic anhydride were studied. These parameters included temperature, contact time, oxygen concentration, and water concentration. Also, the effect of time on stream on catalyst performance was evaluated. Temperature dependence was studied by changing the temperature of the middle zone of the furnace that enclosed the catalyst. Contact time was varied by changing the height of the bed of catalyst, keeping the flow rates constant. A range of oxygen concentrations was achieved by controlling oxygen flow rate through the reactor by means of a mass flow controller. Long time runs were conducted to study the possible deactivation of the catalysts with time. In order to keep all the reactions on an equal footing, a constant-weight hour space velocity, defined as the mass of feed per mass of catalyst per hour, was maintained throughout in all the runs. 2.4. Characterization of the Catalysts. Surface area of the catalysts was measured by nitrogen adsorption experiments done on a Micrometrics PulseChemisorb 2700 instrument. The well-known Brunauer, Emmett, and Teller (BET) analysis was used to determine the actual surface area. Multipoint analysis was used to ensure better accuracy. The X-ray diffraction experiments were conducted on a Rigaku X-ray diffractometer having a rotating anode. Cu KR X-rays generated by the machine were used for the analysis. Powder X-ray diffraction analysis was used to determine the crystallinity of the catalyst. Once that was established, an attempt to determine the exact crystalline phase of the material was made by comparing the peaks obtained to the standard spectra available. The role of calcining conditions on the formation of additional phases was of special interest, and the diffraction spectra of catalysts calcined at different tempera-

tures were compared to determine the effect of calcining temperature on the catalysts. Elemental analysis of the catalyst was done to determine the bulk composition. However, due to the inherent nature of the assays, it was not possible to determine the concentration of oxygen in the presence of metals. Bulk composition analyses of all catalysts were obtained from Galbraith Laboratories Inc. Detailed descriptions of the techniques used are given elsewhere (Yedur, 1992). The surface compositions of the catalysts were determined by using X-ray photoelectron spectroscopy (XPS). XPS data were obtained on a Perkin-Elmer Surface Science instrument equipped with a Model 10-360 precision energy analyzer and an omnifocus small-spot lens. X-rays were generated by an Al (1486.6 eV) anode operated at 15 kV and 20 mA. XPS binding energies were referenced to the C 1s line (284.6 eV) and were measured with a precision of (0.2 eV or better. The samples were outgassed at 400 °C for 24 h to remove any surface moisture accumulated during storage. 3. Results and Discussion The experiments reported here were conducted to ascertain the effectiveness of the catalysts synthesized in the laboratory to convert succinic acid into maleic anhydride. As such, the parameters studied were chosen more to study general trends than to simulate exact conditions in an industrial-scale reactor. Further optimization work is required for each of the parameters. 3.1. Testing of Commercial Catalysts. Five commercially available catalysts were tested at two different temperatures to ascertain their suitability to the reaction being studied. The temperatures chosen were 375 and 475 °C. Each run was conducted under the following conditions: (a) contact time ) 4 s; (b) weight hourly space velocity (WHSV) ) 0.8-0.9 g of reactant/g of catalyst/h; (c) feed concentration ) 40 g/L; (d) oxygen to succinic acid mole ratio ) 10:1; and (e) time on stream ) 15 min. These conditions were maintained throughout in all experiments, unless otherwise noted. Although the results indicated conversions of over 90% for all five commercial catalysts, as evidenced by the relative size of the peaks on the NMR spectra, the selectivity to maleic acid was less than 1% in each case. The commercial catalysts thus showed little promise as a catalyst for the conversion of succinic acid to maleic anhydride. The poor selectivity was confirmed by the CO2 electrode measurements that showed an extremely high percentage of CO2 formation during the reaction. It was thus evident that most of the reactants were being directly combusted to CO2 over the catalyst with no other byproducts such as ethylene. The extremely small intensity of the peaks on the NMR spectrum made it impossible to measure the conversion with any greater accuracy than that reported. The presence of a large amount of CO2 indicates an acid-catalyzed reaction. This can be explained on the basis of the metal oxides present in these catalysts. Metal oxides are active in the dissociation of steam at high temperatures. This leads to the dissociation of water and the subsequent formation of acid and base sites on the catalyst surface which significantly increases decarboxylation. Thus, it is unlikely that any acidic or basic catalyst will be suitable for the oxidative dehydrogenation reaction. 3.2. Testing of Synthetic Molybdenum Oxide Based Catalysts. The three molybdenum oxide based

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catalysts synthesized in the laboratory were analyzed under the conditions described above except that the catalysts were evaluated at a single temperature of 475 °C, and the total time on stream was 30 min. All the molybdenum oxide based catalysts yielded conversions of less than 10%; however, the selectivity toward maleic anhydride was always greater than 95%. This is confirmed by the NMR spectra obtained and by the CO2 sensor that did not indicate the production of any measurable amount of carbon dioxide. The lack of CO2 formation indicates that the postulated mechanism is operating and that, in this case, decarboxylation is not the predominant pathway. Unfortunately, the molybdenum oxide based catalysts possess an inherent low thermal stability that reduces their potential for commercial application to operating temperatures of less than about 400 °C. An irreversible phase change occurs at this temperature that makes it impossible to use these catalysts at higher temperatures, which is necessary to obtain higher conversions. Based on these observations, it was decided to discontinue further testing of molybdenum oxide based catalysts and to concentrate on those which would be better suited for commercial application. 3.3. Testing of Synthetic Iron Phosphate Based Catalysts. Preliminary studies were done to test the ability of the iron phosphate based catalysts prepared in the laboratory to convert succinic acid to maleic anhydride. These tests revealed an improved conversion using catalysts promoted by cerium compared to those promoted by lanthanum under identical conditions. This observation resulted in limiting further testing to only those catalysts promoted by cerium. 3.3.1. Effect of Temperature on Conversion. All the catalysts were tested for the conversion obtained and the selectivity toward maleic acid. Three different temperatures of 375, 425, and 475 °C were tested. These temperatures were the broad ranges described in patent literature for the conversion of isobutyric acid to methacrylic acid (Daniel, 1981b; Ruszala, 1983; Daniel, 1983a). Other reaction conditions were the same as mentioned before for the testing of the molybdenum oxide based catalysts. Figure 7 shows the results obtained for these catalysts. The results show a very promising trend for the iron phosphate based catalysts. From the results obtained, it is evident that temperature is a very important variable in the performance of the catalysts. For all the catalysts, at 375 °C, the conversions obtained were between 16% and 33%, whereas, at 475 °C, the conversions obtained were between 49% and 59%. This establishes that a higher temperature is more favorable for the reaction within the chosen range. Also, the selectivities obtained were all greater than 95%. Almost no production of carbon dioxide or any other byproducts was observed. Less than 5% of succinic acid feed was converted to acrylic acid and CO2. This high selectivity obtained with the iron phosphate based catalysts is more than that achieved for the analogous reaction of isobutyric to methacrylic acid reported in patent literature. These results conclusively prove that succinic acid can be converted to maleic anhydride using the iron phosphate based catalysts. The high selectivity obtained is believed to be a consequence of the inherent stability of the five-membered ring of succinic anhydride. The absence of any free acid sites offers little opportunity for decarboxylation and subsequent production of carbon dioxide to occur. This is in contrast to the commercial

Figure 7. Effect of temperature on the conversion of succinic acid to maleic acid using iron phosphate catalysts under the following conditions: (a) contact time ) 4 s, (b) WHSV ) 0.8-0.9 g of feed/g of catalyst/h, (c) feed concentration ) 40 g/L, (d) oxygen to feed mole ratio ) 10:1, (e) time on stream ) 30 min.

catalysts, wherein even though the succinic acid was converted to the more stable succinic anhydride, the presence of acidic sites on the catalyst surface itself caused rapid decarboxylation of the reactants, making them unsuitable for this application. Based on these results, it was decided to conduct further studies on these four catalysts only at 475 °C in order to optimize other conditions for the best operation of the reactor. 3.3.2. Effect of Contact Time on Conversion. The effect of contact time on the conversion of succinic acid to maleic acid over two iron phosphate based catalysts was studied in order to establish the optimum contact time to be used. Conditions of the experiments were the same as before except that the oxygen to succinic acid feed ratio was 25:1 and that the total time on stream was 15 min. The results of the experiments are shown in Figure 8. For both the pure base catalyst and the base catalyst with support and promoter, the optimum contact time was determined to be 4 s. All the iron phosphate catalysts tested showed approximately 15% less conversion at a contact time of 2 s as compared to a contact time of 4 s. A contact time of 6 s shows virtually no increase in the conversions obtained. It is to be noted that, under all conditions, the selectivities obtained for the iron phosphate based catalysts were higher than 95%. Since no further increase in conversion was achieved beyond a contact time of 4 s (as is evident from Figure 8), it appears that equilibrium is achieved in about 4 s or less. Based on this, it was decided to conduct all future experiments at a contact time of 4 s. 3.3.3. Effect of Oxygen Concentration on Conversion. The amount and purity of oxygen required for the oxidative dehydrogenation reaction to take place is an important parameter since the cost of oxygen will be a major factor in the economics of the process. Experiments were carried out with two catalysts by increasing the relative concentrations of oxygen to the

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Figure 8. Effect of contact time on the conversion of succinic acid to maleic acid using iron phosphate catalysts under the following conditions: (a) temperature ) 475 °C, (b) WHSV ) 0.8-0.9 g of feed/g of catalyst/h, (c) feed concentration ) 40 g/L, (d) oxygen to feed mole ratio ) 25:1, (e) time on stream ) 15 min.

Figure 10. Effect of time on stream on the conversion of succinic acid to maleic acid using iron phosphate catalysts under the following conditions: (a) contact time ) 4 s, (b) WHSV ) 0.8-0.9 g of feed/g of catalyst/h, (c) temperature ) 475 °C, (d) feed concentration ) 40 g/L, (e) oxygen to feed mole ratio ) 25:1. Table 2. Surface Areas of Iron Phosphate Based Catalysts

Figure 9. Effect of oxygen concentration on the conversion of succinic acid to maleic acid using iron phosphate catalysts under the following conditions: (a) contact time ) 4 s, (b) WHSV ) 0.80.9 g of feed/g of catalyst/h, (c) feed concentration ) 40 g/L, (d) time on stream ) 15 min.

feed, keeping all other reaction conditions the same as before and with a feed concentration of 40 g/L. Effects were measured at two different temperatures of 375 and 475 °C. The results of the experiments are shown in Figure 9. In all experiments, the selectivities obtained were over 95%. The results indicate that, when the mole ratio of oxygen to succinic acid was greater than 10:1, there was no substantial increase in the conversions obtained. If the proposed mechanism for oxidative dehydrogenation was working, the total oxygen requirement for conversion of succinic acid to maleic anhydride would be stoichiometric with respect to succinic acid. In all the experiments conducted, the oxygen concentra-

catalyst

surface area, m2/g

base iron phosphate base iron phosphate + Ludox HS 40% base iron phosphate + Ce promoter base iron phosphate + Ce + Ludox HS 40%

2.66 2.73 3.20 6.38

tion used was always above at least 10 times the minimum required; so the result obtained, i.e., no change in conversion with an increase in oxygen concentration, is only to be expected and further substantiates the proposed mechanism. This low oxygen requirement strongly suggests that pure oxygen is not required for large-scale production and that air should be an adequate source of oxygen, thereby reducing the operating cost substantially. 3.3.4. Effect of Time on Stream on Conversion. Experiments were conducted on two catalysts to test their performance after different time intervals. This information is of interest because it would give an indication of the catalysts’ active life. The effect of time on stream on catalyst performance is shown in Figure 10. Selectivities achieved were greater than 95%. From the results, it is seen that the base iron phosphate catalyst shows a slight decrease in conversion with time, whereas the catalyst promoted with cerium and supported with silica shows a slight increase in conversion after 60 min. However, there is no indication of any significant change in the catalyst performance with time. Although the runs need to be conducted for a longer time, indications are that catalyst poisoning and regeneration are not significant problems for these catalysts. 3.4. Results of Characterization Experiments. 3.4.1. Surface Area Measurements. As discussed previously, the linearized BET relation was used to calculate the surface areas of the catalysts. The slope and the intercept of the linear plot together give the surface area. Table 2 shows the calculated surface area for each of the four catalysts.

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Table 3. Bulk Compositions of the Iron Phosphate Based Catalysts catalyst

iron (%)

phosphorus (%)

nitrogen (%)

base iron phosphate base iron phosphate + Ludox HS 40% base iron phosphate + Ce promoter base iron phosphate + Ce + Ludox HS 40%

26.82 21.19 29.13 22.23

25.47 23.93 21.85 21.04