400
Ind. Eng. Chem. Res. 1989, 28, 400-406
Satterfield, C. N.; Ozel, F. Some Characteristics of Two-Phase Flow in Monolithic Catalyst Structures. Znd. Eng. Chem. Fundam. 1977, 16, 61-67. Shiraishi, F.; Kawakami, K.; Kato, K.; Kusunoki, K. Hydrolysis of Soluble Starch by Glucoamylase Immobilized on Ceramic Monolith. Kagaku Kogaku Ronbunshu 1983, 9, 316-323. Shiraishi, F.; Kawakami, K.; Kusunoki, K. Saccharification of Starch in an Immobilized Glucoamylase Monolithic Reactor. Kagaku Kogaku Ronbunshu 1986,12, 492-495. Shiraishi, F.; Kawakami, K.; Kojima, T.; Yuasa, A.; Kusunoki, K. Maltose Production from Soluble Starch by @Amylase and Debranching Enzyme Immobilized on Ceramic Monolith. Kagaku Kogaku Ronbunshu 1988, 14, 288-294. Shiraishi, F.; Kawakami, K.; Kono, S.; Tamura, A.; Tsuruta, S.; Kusunoki, K. Continuous Production of Free Gluconic Acid by
Gluconobacter Suboxydans Immobilized on Ceramic Honeycomb Monolith. Biotechnol. Bioeng. 1989, in press. Tsukamoto, T.; Morita, S.; Okada, J. Oxidation of Glucose on Immobilized Glucose Oxidase in a Trickle-Bed Reactor: Effect of Liquid-Solid Contacting Efficiency on the Global Rate of Reaction. Chem. Pharm. Bull. 1982, 30, 1539-1549. Weibel, M. K.; Bright, H. J. The Glucose Oxidase MechanismInterpretation of the pH Dependence. J. Biol. Chem. 1971,246, 2734-2744. Yamane, T. On Approximate Expressions of Effectiveness Factors for Immobilized Biocatalysts. J . Ferment. Technol. 1981, 59, 375-381. Received for review June 9, 1988 Accepted December 5, 1988
Synthesis of Phthalic and Maleic Anhydrides from n -Pentane. 1. Kinetic Analysis of the Reaction Network Gabriele Centi, Jose Lopez Nieto,+Davide Pinelli, and Ferruccio Trifirb" Department of Industrial Chemistry and Materials, University of Bologna, V.le Risorgimento 4, 40136 Bologna, Italy
The kinetics of n-pentane oxidation over a vanadyl pyrophosphate catalyst is described by a Langmuir-Hinshelwood mechanism; four parallel reactions leading to maleic anhydride, phthalic anhydride, CO, and CO,; and two consecutive reactions for the formation of carbon oxides from the two anhydrides. The rate-determining step is a surface reaction between one adsorbed n-pentane molecule and one oxygen molecules, indicating that the reaction leading to C-C bond formation in phthalic anhydride synthesis occurs after the rate-determining step. Phthalic anhydride selectivity is higher for the lower reaction temperatures and n-pentane or oxygen concentrations. The relevance of the kinetic information with regard to the analysis of the mechanism for the formation of the C8 anhydride from the C5alkane is also discussed. The C5 cut is a relatively low-cost hydrocarbon stream in the oil-refining industry, and the straight-chain fraction in particular is not specifically utilized in the petrochemical industry (Weissermel and Arpe, 1978). Therefore, there is moderately large interest in the development of new routes to upgrade the value of these hydrocarbons, especially n-pentane. This alkane is one of the principal components of the C5 cut together with cyclopentadiene and isoprene, but its functionalization is much more difficult. Heterogeneous catalytic oxidation processes are a powerful method for the functionalization of raw hydrocarbons (Chinchen et al., 1987; Hucknall, 1974), which have been successfully applied in recent years also to the conversion of light alkanes. The large-scale application of the process of n-butane selective oxidation to maleic anhydride is a typical example. The active phase for this reaction is vanadyl pyrophosphate (Centi et al., 1988; Hodnett, 1985; Busca et al., 1986). Recently we have shown (Centi et al., 1987; Centi and Trifirb, 1987) that, by the use of vanadyl pyrophosphate as the catalyst, n-pentane can be selectively transformed to phthalic and maleic anhydrides. The global selectivity is comparable to that of maleic anhydride in the oxidation of n-butane. Using different catalysts (supported Mo/V/P mixed oxides), Honicke et al. (1987a,b) also have found the formation of maleic and phthalic anhydrides from C.5 hydrocarbons, particularly from cyclopentene. The synthesis of the C8 anhydride from the C5hydrocarbon
* To whom correspondence should be addressed. On leave from the Instituto d e Catalisis y Petroleoquimica, Serrano 119, 28006 Madrid, Spain.
0888-5885/89/2628-0400$01.50/0
is a new type of oxidation reaction involving the formation of multiple C-C bonds and aromatization, an unusual effect in the presence of gaseous oxygen and an oxidation catalyst (Chinchen et al., 1987; Hucknall, 1974). Previous investigations on the oxidation of C5 hydrocarbons have been focused on the conversion of branched- or straightchain pentenes on V205 (Butt and Fish, 1966a,b; Butt et al., 1966),of l,&pentadiene on CuO and cobalt molybdate catalysts (Mattson and Sasser, 1984), of C5 olefins on V,05-Mo03 catalysts (Seiyama et al., 1977) or on other mixed oxides of transition metals (Hucknall, 1974). In all cases, the formation of reaction products with a number of carbon atoms higher than the starting hydrocarbon was never observed. It is thus interesting to study the kinetic behavior of n-pentane oxidation on vanadyl pyrophosphate in order to learn more about the dynamic aspects of this new type of heterogeneous synthesis by selective oxidation. In addition to this specific aspect, a more general interest lies in the study of the kinetic and mechanistic characteristics of alkane functionalization, in order to understand the key factors responsible for the ability of the catalysts to activate and selectively convert paraffin feedstocks. In recent years, more attention has been focused on these problems, both for the purpose of upgrading basic knowledge regarding surface reaction mechanisms and for the more practical aspect of reducing costs in the substitution of olefins or aromatics with paraffinic feedstocks. This paper presents a systematic study of the kinetics aspect of the reactions of formation of phthalic and maleic anhydrides from n-pentane using an highly active/selective vanadyl pyrophosphate catalyst for n-butane oxidation to 0 1989 American Chemical Society
Ind. Eng. Chem. Res., Vol. 28, No. 4, 1989 401 maleic anhydride. The aim is to contribute to a better understanding of reaction mechanisms and to single out the experimental conditions in which the formation of the single anhydrides is maximized.
Experimental Section (VO)zP20,Preparation. Vanadium pentoxide (50 g from Merck) was stirred into 0.6 L of a 1:2 mixture of technical grade benzyl and isobutyl alcohols. Orthophosphoric acid (in such an amount as to give a final atomic P:V ratio of 1.1)dissolved in 0.1 L of isobutyl alcohol was then added to form a slurry which gradually darkened upon heating for several hours to the reflux temperature (approximately 378 K). After the mixture was cooled with continued stirring, the light-green product was isolated by filtration, washed, and dried overnight at 333 K and then at 393 K for 24 h. Both X-ray diffraction patterns and infrared examination indicated the solid to be [VOHPO4lZ-H20 (Busca et al., 1986; Johnson et al., 1984; Torardi and Calabrese, 1984). The vanadyl hydrogen phosphate hemihydrate was then activated at 673 K in a mixture of 1.5% butanelair until steady-state conversion of butane to maleic anhydride was well established. The reactor was then cooled and the solid recovered and shown to be pure (V0)2Pz07(Busca et al., 1986; Bordes et al., 1984) by chemical, infrared, and X-ray diffraction analyses. Further details on the characterization of the catalyst and its redox properties as well as catalytic behavior in n-butane oxidation have been previously reported (Busca et al., 1986; Cavani et al., 1985a,b; Centi et al., 1984,1985). The surface area of the catalyst after the activation procedure was 17.8 m2/g. Apparatus. The experimental investigations were conducted by using a tubular fixed bed integral down-flow reactor (Centi et al., 1985a,b; Cavalli et al., 1987). nPentane was added to the preheated (473 K) gaseous feed composed of calibrated amounts of helium and oxygen by means of a high-precision infusion pump. The organic feed was vaporized in a chromatographic-like injector. These vapors were carried by the incoming gas mixture, and the mixed feed was led to the reactor. The reactor consists of a 80-cm-long (0.49-cm-i.d.) stainless steel tube. This tube is encased in a cylindrical copper block, thus providing rather uniform heating of the reactor. Five electrical resistances with separate heating controls maintain an isothermal axial temperature profile. The reactor is built in such a way as to allow rapid exchange of the reaction heat. The axial temperature profile is measured by a thermocouple which slides inside a capillary tube immersed in the catalyst bed. The catalyst is placed on a stainless steel porous plate between two layers of glass wool, and the gas flow fed from the top. The catalyst charges used were in the 0.8-5-g range, with total flow at STP conditions usually higher than 60-80 X loT3L min-' in order to avoid interphase diffusional limitations. The exit stream led into a furnace heated at 503 K, where it could be sampled and sent directly into the oven of a gas chromatograph (flame ionization detector; carrier gas, nitrogen) for the analysis of the organic products. The column was Porapak QS (1.5 m long). The oven temperature was programmed from 333 to 493 K at a rate of 30 K min-' after the initial 5 min in isothermal conditions. L. Additional tests with The sample size was 1.4 X a 5-m-long 23% SP-1700 on Chromosorb P column in isothermal conditions (343 K) were performed for the detailed analysis of hydrocarbons. The gas outlet after the furnace was led to a cooled condenser, where the organic products were trapped; alternatively they could be bubbled
n-pentane.%
' 0
/
Figure 1. Experimental design for the analysis of the rates of formation of products from n-pentane.
into a solvent. Tests using a gas chromatograph-mass quadrupole Hewlett-Packard 5995 A system were performed on the condensed products dissolved in anhydrous ethanol in order to confirm the identification of the reaction products. A secondary line was led from the first cooled condenser to carry the noncondensable gases (02, N P ,CO, and COz) to a second gas chromatograph (thermal conducibility detector; carrier gas, helium). A Carbosieve S-II,lO0-2OO-mesh column was used. The oven temperature was programmed from room temperature to 498 K at a rate of 30 K min-'. Both gas charomatographs are interfaced with data analysis system and process computer, which elaborate experimentalresults verifying the material (C and 0) balances in each test. Experimental Setup. By assessing the influence of mass and heat transport with the appropriate criteria (Carberry, 1976,1987;Turner, 1984; Madon and Boudart, 1982; Mears, 1971), it was ensured that no hidden factors due to transport limitations were influencing the kinetic parameters under the present experimental conditions. In addition, experimental verifications of the absence of interphase diffusion phenomena by varying the volume of the catalyst and the gas flow rate were done. The role of interphase diffusion phenomena was also experimentally verified to be neglectable in the range of the parameters used in this study. The absence of homogeneous combustion reactions of the reactants at the reaction temperature was verified in the empty reactor. Repeated tests were carried out to confiim that both the activity and the selectivity of the catalyst were not altered during the kinetic experiments. Each new catalyst already activated in a flow of n-butanelair was conditioned in situ with the reactant mixture (n-pentane/oxygen/nitrogen) at 633 K for 4 h, and then the reaction temperature decreased to the selected value. The catalytic behavior was then analyzed up to the steady-state conditions. In selected experimental points, independent tests were repeated in order to estimate the internal error variance. The actual experiments for the determination of the kinetic parameters were performed according to a modified design, where the temperature and oxygen and n-pentane concentrations were chosen as the independent variables. The strategy for the experimentation was to investigate a representative experimental grid and to explore the dependence of the rates on the single variables in order to study better the form of the reaction model (Figure 1). The experimental grids intersect in the region considered most interesting (2.5% n-pentane, 20% oxygen).
402 Ind. Eng. Chem. Res., Vol. 28, No. 4, 1989 r
I
I
,
I
I
I
1
I
temperature, K
Figure 2. Conversion of n-pentane ( 0 )and yields of maleic anhydride (O),phthalic anhydride (A),and COX (0)as a function of reaction temperature. Experimental conditions: 2.5% n-pentane; 20% 0,; W/F= 780 g.h/mol of Cg. Symbols: experimental values. Solid lines: calculated values.
The parameters were obtained in a step strategy. The starting values for estimation of the parameters were assessed from plots of the experimental values in linear form according to some suitable transformations. The data utilized in this step were obtained in differential conditions (conversion lower than approximately 8-1070) in order to directly evaluate the rate of formation of all products. The optimization of the parameters was achieved with a robust search procedure utilizing an efficient program for multiresponse nonlinear regression analysis (Donati and Buzzi Ferraris, 1970, 1974; Villa et al., 1985). The parameter estimates were obtained by minimizing the sum of the squares of weighted residuals over all responses,
where r is the number of responses; m the number of experiments, yi, the experimental rates of maleic anhydride, phthalic anhydride, CO, and C02formation; and Yi, the model responses. The weight (wij) in eq 1was set equal to the inverse of the experimental variance determined in repeated runs. The parameters evaluated by this procedure were utilized as starting values for further optimization using all data obtained both in differential and integral conditions. In this case, the experimental dependent variables were the measured outlet mole fractions of the reaction product observed ( y i j ) , while the model responses were the yields of the same species calculated from integration of the respective rate expressions. An isothermal monodimensional pseudohomogeneous plugflow reactor model was utilized for the numerical integration coupled with atomic balances for C, H, and 0 based on the following reaction stoichiometries: C5H12 5.002 C4H2O3+ coz + 5Hz0 (2)
-+
CSH12
+ 4.2502
1/2C8H403+ COO+ 5H20
(3)
5CO + 6H20
(4)
5CO2 + 6Hz0
(5)
C5H12 -I- 5.502 C5H12 + 8.002
-+
--+
A final optimization and statistical evaluation was then followed with a suitable reparametrization (in particular, Arrhenius dependence on the reaction temperature) in order to improve the conditioning of the objective function (Himmelblau, 1968, 1970; Froment and Hosten, 1986).
Figure 3. Conversion of n-pentane (e)and yields of maleic anhyphthalic anhydride (A),and COX(0) as a function of W/F. dride (O), Experimental conditions: 2.5% n-pentane; 20% 02; temperature, 563 K. Symbols: experimental values. Solid lines: calculated valTotal yield of carbon oxides (inues. ( - - -) Residual oxygen. cluding COXfrom reactions of formation of maleic and phthalic anhydrides). (-e-)
The yields were calculated according to the following equations:
YMA= (outlet moles of MA)/(inlet moles of PE)
(6)
YPa = (2 x outlet moles of PA)/(inlet moles of PE) (7)
Yco, = [(outlet moles of CO + C02)/ 5 - (outlet moles of carbon oxides from reactions 2 and 3) J / [inlet moles of PE] (8) where PE, MA, PA, and COXare n-pentane, maleic anhydride, phthalic anhydride, and carbon oxides, respectively. The conversion was calculated by dividing the converted moles of n-pentane by the initial moles of hydrocarbon, and selectivities were calculated by dividing the yields calculated according to eq 6-8 by the conversion.
Results Reaction Network. Just as for n-butane oxidation on vanadium-phosphorus oxides (Centi et al., 1985; Schneider et al., 1987), in n-pentane oxidation, in addition to carbon oxides only the formation of anhydrides was found. There was no evidence for the formation of intermediate products such as olefins. However, contrary to what happens with n-butane, two anhydrides were synthesized, one with a lower carbon atom number than that of the starting C5 alkane (C4H203,maleic anhydride) and one with a higher carbon atom number (C8H403,phthalic anhydride). The analysis of the effect of the space velocity over the whole range of reaction temperatures investigated (Figures 3-5) shows that the two anhydrides form in parallel reactions. Tests where phthalic anhydride was fed directly into the reactor in a flow of air indicated that the consecutive oxidation of the phthalic anhydride on the vanadyl pyrophosphate leads to the formation of carbon oxides, with minor amounts of benzoic acid. At the higher reaction temperatures, the rate of consecutive oxidation to carbon oxides of the phthalic anhydride is higher than that of maleic anhydride. This explains the apparent effect of formation of maleic anhydride from phthalic anhydride as shown in Figure 2. On the basis of these results, general reaction network in Scheme I may be assumed for the kinetic investigation. The rate of oxidaton to CO to C02 was not taken into consideration.
Ind. Eng. Chem. Res., Vol. 28, No. 4,1989 403
'.
Lo
e
I
'
OO-
400
I
1 800
3K
'0
1200
n-pentane %
W/ F , g.h/molC5
Figure 4. Conversion of n-pentane ( 0 )and yields of maleic anhydride (O), phthalic anhydride (A),and COX(0) as a function of W/F. Experimental conditions: 2.5% n-pentane; 20% 0,; temperature, 598 K. Symbols: experimental values. Solid lines: calculated values. (- - -) Residual oxygen. (-.-) Total yield of carbon oxides (including COXfrom reactions of formation of maleic and phthalic anhydrides).
Figure 6. Rates of formation of maleic anhydride (0) and phthalic anhydride ( 0 )as a function of n-pentane concentration (a, left) and of 0,concentration (b, right) a t different reaction temperatures. (a) O,,20% molar; (b) n-pentane, 2.5% molar. Calculated values: (-) maleic anhydride; (- - -) phthalic anhydride.
"t
K
O,,%
n.pentane. % I
I
I
800
400
'0
1200
W/F, g.h/molC5
Figure 5. Conversion of n-pentane ( 0 )and yields of maleic anhyas a function of W/F. dride (O),phthalic anhydide (A),and COX(0) Experimental conditions: 2.5% n-pentane; 20% 0,; temperature 623 K. Symbols: experimental values. Solid lines: calculated values. (- -) Residual oxygen. Total yield of carbon oxides (including COXfrom reactions of formation of maleic and phthalic anhydrides).
-
(-e-)
Scheme I. Kinetic Reaction Network in n -Pentane Oxidation over Vanadyl Pyrophosphate Catalyst maleic anhydride
-
I
-.
3'
CO
n-C5H12
\
I
CO
/
phthalic anhydride
Kinetic Modeling. The dependence of the rates of formation of single products from the oxygen and n-pentane inlet concentrations was studied a t high flow rates. In these conditions, maintaining the conversion of both reagents under approximately 8-10% , the initial concentration of the reactants along the catalytic bed may be assumed constant (Differential reactor). Moreover, due to the short contact time and low concentrations of maleic and phthalic anhydrides, their reactions of consecutive oxidation (steps 5 and 6 of Scheme I) were neglected. Reported in Figures 6 and 7 are the experimental values of the rates of maleic and phthalic anhydrides and of
Figure 7. Rates of formation of CO (w) and C 0 2 (0) as a function of n-pentane concentration (a, left) and of 0,concentration (b, right) at different reaction temperatures. (a) O,,20%; (b) n-pentane, 2.5%. Calculated values: (-) CO,; (- - -) CO.
carbon oxides formation as a function of the n-pentane and oxygen concentrations at different reaction temperatures. The rates of CO and C 0 2 formation are those experimentally determined and thus include the carbon oxides formed according to eq 2 and 3. The rates of maleic and phthalic anhydrides formation have a different dependence on the reagent concentrations and the reaction temperature. In particular, in the range investigated, the rate of maleic anhydride formation had a linear dependence on both the n-pentane and oxygen concentrations. The rate of phthalic anhydride formation was more strongly affected by the saturation of the active sites. Reaction Model. According to the Langmuir-Hinshelwood treatment (Carberry, 1976), based on the hypothesis that (i) the rate-determining step is the surface reaction between adsorbed paraffin and oxygen and (ii) different active sites for the adsorption of n-pentane and oxygen are involved, the following rate equation can be derived: kcF0
= (1
+ KJ')(1 + K,O)
(9)
where k , is the global kinetic constant, K p and KO the equilibrium constants for n-pentane and oxygen adsorption, and P and 0 the concentrations of n-pentane and oxygen, respectively.
404
Ind. Eng. Chem. Res., Vol. 28, No. 4, 1989
Table I. Indexes for Model Responses for the Rate of Phthalic Anhydride Formation residual sum mean error, correlation model ea of sauares 70 index 9 8.1175 X lo4 8.55 0.9979 10 1.4969 X loe8 15.35 0.9962
In the hypothesis that the rate-determining step is the surface reaction between two adsorbed hydrocarbon molecules with adsorbed oxygen, the resulting rate equation is k p 2O r = (10) (1 + K$)2(1 K,O)
+
The assumption of different active sites for the adsorption of n-pentane and oxygen is based on the different chemical and electronic features of the two reactants that would cause a different type of interaction with the surface. The involvement of both reagents in the rate-determining step is self-evident from the data of Figures 6 and 7. Due to the low reactivity of the alkanes, possible consecutive rate-determining steps were not taken into consideration. For the synthesis of maleic anhydride and carbon oxides, the kinetic parameters associated with rate eq 9 were evaluated. For the synthesis of phthalic anhydride, due to the higher carbon atom number of the final product as compared with that of the starting alkane, the kinetic parameters associated with both rate eq 9 and 10 were determined by regression analysis. The discrimination between the two rival models (eq 9 and 10) was made based on the residual sum of the squares, the F-test, and the analysis of the residuals (Donati and Buzzi Ferraris, 1970, 1974; Froment, 1975; Froment and Bischoff, 1976; Himmelblau, 1968, 1970). The residual sum of squares, the mean percent error on the data in differential conditions (Figure 6), and the correlation index of the two rival
models are compared in Table I. Rate 9 is more adequate for the description of the formation of phthalic anhydride from n-pentane. For the rates of decomposition of maleic and phthalic anhydrides (steps 5 and 6 of Scheme I), the following simplified rate equations were utilized r5 = k,MAO (11) r6 = k,PAO (12) where MA and PA are the concentrations of maleic and phthalic anhydrides, respectively. Parameter Estimation. A step strategy as described in the Experimental Section was used to estimate the kinetic parameters. A total number of 39 experimental points in differential conditions (direct evaluation of reaction rates) and 34 experimental points in integral conditions (n-pentane conversion higher than 10%) were utilized for the multiresponse nonlinear regression analysis. The integration of the reaction rates was made on the basis of an isothermal monodimensional pseudohomogeneous plug-flow reactor model coupled with atomic balances for C, H, and 0. An Arrhenius dependence of the kinetic parameters on the reacton temperature was reparametrized in order to reduce parameter correlation, according to the model reported in Table I1 (Himmelblau, 1970). In the regression analysis, physical constraints on the rate coefficients and adsorption equilibrium constants (Froment and Hosten, 1986) were assumed. The parameter estimates for steps 1-6 of Scheme I together with their 95% individual confidence limits are reported in Table 11. The goodness of fit is shown in Figure 2-6 from the comparison of the experimental values (symbols) and calculated values (solid lines). The proper trend of the values of the constants with temperature confirms the validity of the kinetic expression selected. Moreover, good reliability of the adsorption constants was
Table 11. Parameter Estimates and 95% Individual Confidence Limits for Model Equation 9 (Rate of Formation of Maleic Anhydride, Phthalic Anhydride, CO, and COz) and Model Equations 11 and 12 (Rates of Maleic Anhydride and Phthalic Anhydride Decomposition, Respectively)" individual confidence intervals parameter step optimal estimate lower limit upper limit 1 k,: A*, mol/(p.h) 5.26037 X 10' 5.06731 X 10' 5.46342 X 10' 1.72370 X lo4 E/R,K' 1.67682 X lo4 1.76593 X lo4 Kp: A*, mol/L 2.18132 X 10' 1.96057 X 10' 2.40032 X lo2 -8.07310 X lo" -9.07739 X lo3 -7.04620 X lo3 EIR, K K,: A*, mol/L 3.448 51 3.222 63 3.676 82 -9.22445 X lo3 -1.04430 X lo4 -7.90259 X l o 3 EIR, K 2 k,: A*, L2/(g.h.mol) 1.26023 X lo2 1.25968 X 10' 1.33017 X lo2 7.83338 X lo3 1.04378 X lo3 8.62533 X lo3 EIR, K K,: A*, mol/L 9.15852 X lo2 8.43500 X lo2 9.88113 X lo2 EIR, K -3.45425 x 103 -3.97273 x 103 -2.92920 X 10' K,: A*, moljL 1.10292 X 10' 1.04392 X 10' 1.16226 X 10' -1.13983 X lo' -1.23341 X lo' -1.04659 X lo4 EIR, K A*, Lz/(g-h.mol) 1.94978 X 10' 1.88817 X IO2 2.01178 X 10' 3 k,: 1.42245 X lo4 1.38575 X lo4 1.45901 X IO4 EIR, K K,: A*, mol/L 1.40794 X lo3 1.36232 X lo3 1.45482 X lo3 4.46322 X lo2 1.90490 X 10' 7.28442 X lo2 EIR, K A*, mol/L K,: 1.26343 x 10' 1.31776 X 10' 1.20923 X 10' -8.28659 X lo3 -7.65184 X lo3 -8.92797 X lo3 EIR, K 4 k,: A*, L2/(gh.mol) 2.64695 X lo2 2.62683 X 10' 2.66760 X lo2 1.59764 X lo4 1.61889 X 10' 1.57671 X lo' EIR, K Kp: A * , mol/L 6.54264 X lo2 6.41964 X lo2 6.66760 X lo2 -2.71459 X lo3 -3.07889 X IO3 -2.35599 X IO3 EIR, K K,: A*, mol/L 1.15481 0.97281 1.33692 -7.81834 X lo3 -1.03257 X lo4 -5.30709 X IO3 EIR,K 5 k,: A*, L2/(gh-mol) 3.54493 x 102 2.71648 X lo2 4.43683 X 10' 7.21307 X lo3 9.48302 X IO3 4.82122 X lo3 EIR, K 6 k,: A*, L2/(g-h.mol) 3.752 96 4.47465 3.047 78 1.921 17 x 104 257494 X I O 4 1.23666 X lo4 EIR, K "Arrhenius dependence of the kinetic parameters was reparametrized according to k , = A,* exp(-E,/RT*), where A,* = .4: exp(--E,/R(603)) and 1 / T * = 1 / T - 1,'603, 603 K being the mean reaction temperature in the range investigated.
Ind. Eng. Chem. Res., Vol. 28, No. 4, 1989 405
- 563 K
Figure 8. Initial selectivities (at 1% of conversion) to maleic anhydride and phthalic anhydride as a function of feed composition for two reaction temperatures. Calculated values from kinetic data.
observed. The curve fit taken together with the analysis of residuals and the statistical indexes indicates that the kinetic model provides a good representation of the data over the region investigated.
Discussion Reaction Mechanism. The kinetic analysis provides some interesting information about this new type of mechanism for the oxidation of n-pentane involving the formation of C-C bonds together with 0 insertion. The effect of the space velocity indicates that the two reactions for the synthesis of maleic anhydride and phthalic anhydride are parallel reactions. This is further supported by the analysis of the reaction rates which clearly indicate distinct behaviors with respect to n-pentane and oxygen concentrations and to reaction temperature. The higher value of the adsorption constants (K, and KO)for the case of phthalic anhydride synthesis as compared with that of maleic anhydride suggests a very limited number of active sites for the former. Thus, two different types of specific sites of selective oxidation are present on the vanadyl pyrophosphate, one for the synthesis of maleic anhydride and one for the synthesis of phthalic anhydride. A further suggestion on the mechanism of phthalic anhydride formation derives from the analysis of reaction models 9 and 10. Even with all the limitations present in a mechanistic extrapolation from the kinetic models, the good fitting of the experimental data suggests that the stage of formation of C-C bonds occurs after the rate-limiting step, probably on already transformed and oxidized intermediates. In a previous paper (Busca and Centi, 1989) we have suggested, on the basis of FT-IR evidence, that a surface template reaction between adsorbed pentadiene and oxocyclo-3en-2-one is a key step in the formation of phthalic anhydride. The present kinetic results are in agreement with this hypothesis. A more detailed study of the mechanism of phthalic anhydride synthesis is in progress. Analysis of the Kinetic Results. The kinetic analysis of n-pentane oxidation may be used to predict in what conditions the reaction must be carried out to increase the selectivity to phthalic anhydride or to maleic anhydride. Phthalic anhydride selectivity is higher at the lower reaction temperatures and n-pentane and oxygen concentrations. In the present study, we have utilized a vanadyl pyrophosphate catalyst active at temperatures around 600 K. These reaction temperatures are about 100 K lower than that utilized by Volta et al. (1988) in a comparative study of linear and branched alkanes on vanadium-
phosphorus oxides. In the case of n-pentane, these authors did not find the formation of phthalic anhydride, only that of maleic anhydride. The significantly higher value of the activation energy for the maleic anhydride synthesis as compared with that for phthalic anhydride may explain the observation, in addition to possible differences in the nature of the catalysts. A further indication from the kinetic analysis is that phthalic anhydride has a higher rate of consecutive oxidation than maleic anhydride. The selectivity is thus maximized at low conversion and relatively short contact times. Figure 8 summarizes the effect of the feed composition on the selectivity to phthalic and maleic anhydrides at different reaction tempeatures, evidencing in what experimental conditions the selectivity to phthalic or maleic anhydide is maximized.
Conclusions The principal results of the kinetic study of n-pentane selective oxidation to phthalic and maleic anhydrides can be summarized as follows. Kinetic experiments were performed in a tubular flow reactor operating both in differential and integral conditions. The reaction network may be modeled by using a Langmuir-Hinshelwood approach, four parallel formation reactions (phthalic anhydride, maleic anhydride, CO, and COJ, and two reactions of consecutive decomposition to carbon oxides of the two anhydrides. The rate-determining step is the surface reaction between absorbed n-pentane and oxygen. In a three-step procedure, the correspondingkinetic parameter estimates were evaluated. Model discrimination criteria for the rate of phthalic anhydride formation indicated a better fit for a rate-determining step involving the surface reaction between one adsorbed n-pentane molecule and adsorbed oxygen rather than a surface reaction between two adsorbed n-pentane molecules and adsorbed oxygen. This indicates that the reaction leading to C-C bond formation occurs after the rate-determining step. Acknowledgment Financial support from the Minister0 Pubblica Istruzione is gratefully acknowledged. The authors thank Prof. G. Buzzi Ferraris for providing his nonlinear regression program. Registry No. Phthalic anhydride, 85-44-9; maleic anhydride, 108-31-6; pentane, 109-66-0; vanadyl pyrophosphate, 58834-75-6.
Literature Cited Bordes, E.; Courtine, J. C.; Johnson, J. W. On the Topotactic Dehydration of Vanadyl Hydrogen Phosphate Hydrate (VOHP040.5H20) into Vanadyl Pyrophosphate. J. Solid State Chem. 1984, 55, 270-279. Busca, G.; Centi, G . Surface Dynamics of Adsorbed Species on Heterogeneous Oxidation Catalysts: Evidence from the Oxidation of C4 and C5 Alkanes on Vanadyl Pyrophosphate. J.Am. Chem. SOC. 1989, in press. Busca, G.; Cavani, F.; Centi, G.; Trifirb, F. Nature and Mechanism of Formation of Vanadyl Pyrophosphate; Active Phase in n-Butane Selective Oxidation. J. Catal. 1986, 99, 400-414. Butt, N. S.; Fish, A. Vanadium Pentoxide-Catalyzed Oxidation. I. Branched-chain Pentenes. J. Catal. 1966a, 5 , 205-217. Butt, N. S.; Fish, A. Vanadium Pentoxide-Catalyzed Oxidation. 11. Straight-chain Pentenes. J. Catal. 1966b, 5 , 494-507. Butt, N. S.; Fish, A.; Saleeb, F. Z. Vanadium Pentoxide-Catalyzed Oxidation. 111. Role of the Catalyst. J. Catal. 1966,5,508-516. Carberry, J. J. Chemical and Catalytic Reaction Engineering; McGraw-Hill: New York, 1976. Carberry, J. J. Physico-Chemical Aspects of Mass and Heat Transfer in Heterogeneous Catalysis, In Catalysis-Science and Technology; Anderson, J. R., Boudart, M., Eds.; Springer-Verlag: Berlin, 1987; Vol. 8, pp 131-172.
406
Ind. E n g . Chem. Res. 1989,28, 406-413
Cavalli, P.; Cavani, F.; Manenti, I.; Trifirb, F.; El-Sawi, M. Kinetic and Mechanistic Analysis of Toluene Ammoxidation to Benzonitrile on Vanadium-Titanium Oxides. Ind. Eng. Chem. Res. 1987,26, 804-810. Cavani, F.; Centi, G.; Manenti, I.; Trifirb, F. Catalytic Conversion of C4 Hydrocarbons of Vanadium-Phosphorus Oxides: Factors Influencing the Selectivity of 1-Butene Oxidation. Ind. Eng. Chem. Prod. Res. Deu. 1985a, 24, 221-226. Cavani, F.; Centi, G.; Trifirb, F. Structure Sensitivity of the Catalytic Oxidation of n-Butane to Maleic Anhydride. J. Chem. SOC., Chem. Commun. 1985b, 492-494. Centi, G.; Trifirb, F. Functionalization of Paraffinic Hydrocarbons by Heterogeneous Vapor-Phase Oxidation. 111. Conversion of the C,-C7 Alkane Series. Catal. Today 1988, 3, 151-162. Centi, G.; Burattini, M.; Trifirb, F. Oxi-Condensation of n-Pentane to Phthalic Anhydride. Appl. Catal. 1987, 32, 353-356. Centi, G.; Fornasari, G.; Trifirb, F. On the Mechanism of n-Butane Oxidation to Maleic Anhydride: Oxidation in Oxygen-Stoichiometry-Controlled Conditions. J. Catal. 1984, 89, 44-51. Centi, G.; Fornasari, G.; Trifirb, F. n-Butane Oxidation to Maleic Anhydride on Vanadium-Phosphorous Oxides: Kinetic Analysis with a Tubular Flow Stacked-Pellet Reactor. Ind. Eng. Chem. Prod. Res. Deu. 1985, 24, 32-37. Centi, G.; Trifirb, F.; Ebner, J. R.; Franchetti, V. M. Mechanistic Aspects of Maleic Anhydride Synthesis from C4 Hydrocarbons over Phosphorus Vanadium Oxide. Chem. Reu. 1988,88, 55-80. Chinchen, G.; Davies, P.; Sampson, R. J. The Historical Development of Catalytic Oxidation Processes. In Catalysis-Science and Technology; Anderson, J. R., Boudart, M., Eds.; Springer-Verlag: Berlin, 1987; Vol. 8, pp 1-68. Donati, G.; Buzzi-Ferraris, G . Parameter Estimation for a HougenWatson Kinetic Model. Zng. Chim. It. 1970, 6, 139-149. Donati, G.; Buzzi Ferraris, G . Powerful Method for Hougen-Watson Model Parameter. Estimation with Integral Conversion Data. Chem. Eng. Sci. 1974,29, 1504-1509. Froment, G. F. Model Discrimination and Parameter Estimation in Heterogeneous Catalysis. AZChE J.,1975,21, 1041-1045. Froment, G . F.; Bishoff, K. B. Chemical Reactor Analysis and Design; Wiley: New York, 1976. Froment, G. F.; Hosten, L. H. Catalytic Kinetics: Modelling. In Catalysis-Science and Technology; Anderson, J. R., Boudart, M., Eds.; Springer-verlag: Berlin, 1986 Vol. 2, Chapter 3, pp 97-170. Himmelblau, D. M. Process Analysis and Simulation: Deterministic Systems; Wiley: New York, 1968. Himmelblau, D. M. Process Analysis and Statistical Methods; Wiley: New York, 1970. Hodnett, B. K. Vanadium-Phosphorus Oxide Catalysts for Selective Oxidation of C4 Hydrocarbons to Maleic Anhydride. Catal. Reus-Sci.Eng. 1985, 27, 373-424.
Honicke, D.; Griesbaum, K.; Yang, Y. Heterogen Katalysierte Selektivoxidation von n-Pentenen und n-Pentan. Chem.-Ing. Technol. 1987a, 59, 222-223. Honicke, D.; Griesbaum, K.; Newrzella, A. Mixed Oxide Mo/V/Pcatalysts for the Partial Oxidation of Cyclopentene. In Abstracts of Papers, 194th National Meeting of the American Chemical Society, Div. of Colloid and Surface Chem., New Orleans, 198713. Hucknall, D. J. Selective Oxidation of Hydrocarbons; Academic Press: London, 1974. Johnson, J. W.; Johnston, D. C.; Jacobson, A. J.; Brody, J. F. Preparation and Characterization of VOHP04*0.5H20and Its Topotactic Transformation to (VO)2P20,. J. Am. Chem. SOC.1984,106, 8123-8128. Madon, R. J.; Boudart, M. Experimental Criterion for the Absence of Artifacts in the Measurement of Rates of Heterogeneous Catalytic Reactions. Ind. Eng. Chem. Fundam. 1982,21, 438-447. Mattson, G.; Sasser, D. The Vapour-Phase Oxidation of 1,3-Pentadiene. Oxid. Commun. 1984, 7, 333-345. Mears, D. E. Tests for Transport Limitations in Experimental Catalytic Reactors. Ind. Eng. Chem. Prod. Des. Deu. 1971, IO. 541-547. Schneider, P.; Emig, G.; Hofmann, H. Kinetic Investigation and Reactor Simulation for the Catalytic Gas-Phase Oxidation of nButane to Maleic Anhydride. Ind. Eng. Chem. Res. 1987. 26, 2236-2400. Seiyama, T.; Nita, K.; Maehara, T.; Yamazoe, N.; Takita, Y. Oxyhydrative Scission of Olefins. I. Oxidation of Lower Olefins. J . Catal. 1977, 49, 164-173. Torardi, C. C.; Calabrese, J. C. Ambient and Low-Temperature Crystal Structure of Vanadyl Hydrogen Phosphate (VO),H,P,09. Inorg. Chem. 1984,23, 1308-1310. Turner, J. C. R. An Introduction to the Theory of Catalytic Reactors. In Catalysis-Science and Technology; Anderson, J. R., Boudart, M., Eds.; Springer-Verlag: Berlin, 1984; Vol. 1, Chapter 2, pp 43-96. Villa, P. L.; Forzatti, P.; Buzzi-Ferraris, G.; Garone, G.; Pasquon, I. Synthesis of Alcohols from Carbon Oxides and Hydrogen. 1. Kinetics of the Low-Pressure Methanol Synthesis. Ind. Eng. Chem. Prod. Res. Deu., 1985, 24, 12-19. Volta, J. C.; Aguero, A.; Sneeden, R. P. A. Selective-Oxidation on Vanadyl Pyrophosphate Catalysts: oxidation of Linear and Branched Alkanes. In Heter. Catalysis and Fine Chemicals; Guisnet, M., Barrault, J., Bouchoule, C., Duprez, D., Montassier, C., Perot, G., Eds.; Elsevier Pub.: Amsterdam, 1988; pp 353-360. Weissermel, K.; Arpe, H.-J. Industrial Organic Chemistry; Verlag Chemie: Weinheim-New York, 1978. Receiued for reuiew J u n e 3, 1988 Accepted November 28, 1988
Effect of Particle Size on the Activity of a Fused Iron Fischer-Tropsch Cata1yst William H. Zimmerman, Joseph A. Rossin, and Dragomir B. Bukur* Kinetics, Catalysis, a n d Reaction Engineering Laboratory, Department of Chemical Engineering, Texas A&M University, College Station, Texas 77843
The effect of particle size on the activity of a fused iron catalyst used for the Fischer-Tropsch synthesis has been studied. The significant resistance in our tests was found to be intraparticle mass transfer, which was caused by the low diffusivity of reactant in wax-filled catalyst pores. Particles of 30/60 and 60/ 100 mesh show strong mass-transport limitations with lower than expected activities and activation energies, while catalyst activity approaches its intrinsic value for 170/230-mesh particles. Catalyst effectiveness factors were calculated assuming first-order reaction kinetics and single reaction stoichiometry and were compared to those obtained by experiment. The calculated and experimental results were in good agreement, although the calculated values were consistently higher than the experimental values. The overprediction may be caused by basing the Thiele modulus on the diffusivity of H2 or by neglecting rate inhibition by water. Fixed bed reactors have often been employed in kinetics studies of the Fischer-Tropsch synthesis (FTS). Stirred
* Author t o whom correspondence should be addressed. 0888-5885/89/2628-0406$01.50/0
tank slurry reactors are better suited for detailed kinetic investigations since they are able to ensure uniform concentrations and temperature, but fixed beds are convenient for use in preliminary catalyst studies, as thev are inex1989 American Chemical Society
