I n d . Eng. C h e m . Res. 1990, 29, 789-795 Probstein, R. F.; Hicks, R. E. Synthetic Fuels; McGraw-Hill: New York, 1982; pp 182-185. Ramachandran, p. A,; Smith, J. M. A Single Pore Model for GasSolid Noncatalytic Reactions. AIChE J . 1977, 23 (31, 353-361. Shadman, F.; Dombek, P. E. Enhancement of SO2 Sorption on Lime by Structure Modifiers. Can. J . Chem. Eng. 1988, 66, 930-935. Shearer, J. A.; Johnson, I.; Turner, C. B. Interaction of NaCl with Limestone During Calcination. Am. Ceram. Soc. Bull. 1980, 59 (51, 521-525.
789
Sherrington, P. J.; Oliver, R. Granulation; Heyden and Sons: London, 1981. Van Hecke, M. C.; Bartlett, R. W. Kinetics of Sulfation of Atlantic Ocean Manganese Nodules. Metall. Trans. 1973, 4 , 941-947.
Received for review March 8, 1989 Revised manuscript received August 10, 1989 Accepted January 16, 1990
Kinetics of Zeolite-Catalyzed Toluene Disproportionation Kerry M. Dooley,* Stephen D. Brignac, and Geoffrey L. Price Department of Chemical Engineering, Louisiana State University, Baton Rouge, Louisiana 70803
Y zeolites containing 0-6% Ni were prepared and tested as model disproportionation catalysts at 613-774 K and toluene partial pressures t o 0.43 bar with H2 as the carrier gas. At these conditions deactivation was slow, side reactions insignificant, and intraparticle concentration gradients negligible; similar results were obtained in some cases when disproportionation kinetics data from other studies were examined. All kinetics data shown t o be characteristic of the intrinsic disproportionation were fitted t o a set of 42 Langmuir-Hinshelwood rate models, which were derived assuming previously proposed alkyl-transfer and biphenylalkane mechanisms. Most of the data were adequately fit by a second-order rate expression with toluene inhibition, which is in accord with previous findings for non-zeolite solid acid catalysts. These results are in contrast t o previous work on Cu/AlF,-Y zeolites, which follow a first-order rate expression with toluene inhibition; possible reasons for t h e disagreement are discussed. Toluene can be selectively disproportionated to the xylene isomers in the vapor phase using a variety of acidic zeolites. Typical side reactions include xylene disproportionation, formation of higher aromatics, and hydrodealkylation to benzene aned methane in the presence of H2 (Matsumoto and Morita, 1967; Aneke et al., 1979a; Davidova et al., 1979; Olson and Haag, 1984; Chang et al., 1987). Highly shape-selective catalysts, producing mostly p-xylene, appear not to require excess H2 in the feed in order to maintain activity. Many such catalysts are ZSM-5 modified by the addition of compounds of magnesium, boron, and phosphorus (Kaeding and Young, 1977; Kaeding, 1978; Chen e t al., 1979; Kaeding et al., 1981; Meshram, 1987). However, Y and other large-pore zeolites that are loaded with transition metals, but are not shape-selective, retain activity for several turnovers in the presence of excess H, (Aneke et al., 1979a; Davidova et al., 1979; Wu and Leu, 1983). In these studies, at least some of the metal was apparently in the zero-valent state prior to the reactions. A natural question arising from this work is whether the reaction kinetics for the two stable classes of catalysts differ. This question is made complex because of the difficulty in assessing the contributions of mass transfer to the observed (global) rates or even in determining whether such contributions exist. In some instances, shape selectivity may arise from restrictions on the transition state rather than from diffusional limitations (Weisz, 1980; Haag et al., 1982; Olson and Haag, 1984). Apart from this question, the kinetics of toluene disproportionation are of interest because the reaction is a simple, convenient one with mild heat effects (a heat of reaction less than 1 kJ/mol), which is useful in probing for strong Bronsted acidity in acid catalysts (Benesi, 1967; Jacobs et al., 1974) and for shape selectivity of intermediate-pore zeolites, especially pentasils. In this study, both a pure Y zeolite and a Ni-containing Y zeolite were prepared and tested for toluene dispro0888-5885/90/ 2629-0%9$02.50/0
portionation activity at temperatures from 613 to 774 K and toluene partial pressures to 0.43 bar. The data for the pure Y zeolite can be compared to the kinetics data for other unmodified acidic zeolites (Yashima et al., 1970; Kaeding et al., 1981; Gnep and Guisnet, 1981; Olson and Haag, 1984; Nayak and Riekert, 1986; Schulz-Ekloff et al., 1987; Meshram, 1987; Beltrame et al., 19871, after suitable correction for the effects of heat and mass transfer on the global rates. Likewise, the data for the Ni-containing Y zeolites can be compared to previous studies using Cu/ A1F3-Y (Aneke et al., 1979b), Cu-Mordenites and Cu/ Pd-Mordenites (Wu and Leu, 1983), La-X (Matsumoto and Morita, 1967), and La- and Ti-Y (Chang et al., 1987). There also exists a substantial body of kinetics data for strongly acidic alumina-boria catalysts (Izumi and Shiba, 1964) that may be relevant to the kinetics of zeolite-catalyzed disproportionation.
Experimental Section Catalyst Preparation. Catalysts containing O % , 270, or 6% nickel by weight were prepared as follows. About 25 g of Linde LZ-Y-62 zeolite (ammonium form, Si02/ A120, molar ratio = 4.94) was dried under vacuum and then slurried in deionized water to which was added the required amount of Ni(N03)2.6H20(Aldrich, 99.999%). Ion exchange was allowed to take place at 40 "C for 12 h, after which the slurry was filtered and the cake dried overnight at 80 "C. The zeolites were pressed (2200 psi) into a hard cake, ground to 20-40 mesh and sieved to retain this fraction, and then calcined by slowly heating to 540 "C in dry air and holding for 2 h at the final temperature. The final activation took place in the reactor; the catalysts were dried by using helium at 300 "C and then reduced by H, at 300 "C for 12 h and 500 "C for 1 h. Spent catalysts were reactivated in a similar manner. Toluene Disproportionation Kinetics. The toluene disproportionation experiments were performed by using a differential reactor system, which is shown schematically C 1990 American Chemical Society
790 Ind. Eng. Chem. Res., Vol. 29, No. 5, 1990 5-
n 4 X v
z --I
I 3-
0 21
12.5
I
I
I
13.0
13.5
14.0
K/T x
Figure 1. Differential reactor system.
in Figure 1. All wetted parts are of 316 stainless steel. The reactor consists of 1.5 in. of 0.25-in.-0.d. tubing containing about 0.3 g of catalyst inserted between plugs of silanized glass wool and sintered metal frits. The carrier gas (usually H2) flow rate could be varied from 5 to 200 scc/min by using a Precision Model 914 mass flow controller, and the saturator and valve ovens were controlled by Omega Model 52 temperature controllers, accurate to within f 2 deg over the range used. The reactor oven was controlled by an Omega Model 53 temperature controller accurate to within fl deg over the range used. The reactor was interfaced to a Gow-Mac Model 550 gas chromatograph with a thermal conductivity detector. The reactor could be operated either continuously or in a pulsed mode. In either case, the toluene partial pressure was determined from the saturator temperature by using the Antoine equation (Reid e t al., 1977). When operated continuously, a steady stae was achieved in 10 min or less. The product stream was sampled online to the GC; the toluene and products were separated by using a 6-ft-long, 0.125-in.-0.d. SE-30/Bentone 34 column a t 80-150 "C (10 OC/min ramp). For pulsed operation, samples were trapped in the GC sample loop by freezing it by using liquid N,; the loop was subsequently reheated rapidly in flowing helium in order to inject an undispersed sample into the GC. Further details on the catalyst preparation and reactor operation are contained in a thesis (Brignac, 1984).
Result s For continuous reactor operation, the only products observed at steady state were benzene and the three xylene isomers. The catalysts deactivated, but slowly; a t typical conditions of 698 K, 3.04-3.08 atm total pressure, and toluene space velocities of (2.5-3.0) X s-l, there was less than 10% loss of initial activity in three complete sampling periods. When helium was substituted for H, as the carrier gas a t these same conditions, the rate of deactivation for the Ni-Y zeolites was more pronounced, but not for H-Y. Therefore, all data reported here were taken at short times on-stream using H2as the carrier gas. The declines in activity were reversible upon reactivation with H,. For differential reactor operation, observed rates were computed from the conversions as follows: robs
= Fox/
At 698 K, the temperature a t which most of the data were taken, the maximum fractional conversion of toluene was only 5.5%. At higher temperatures, conversions as high as 18.4% were measured, but in most experiments conversions were held to less than 10%. The higher tem-
I
io
4 14.5
I
1
15.0
15.5
Figure 2. Arrhenius plot for 2% Ni-Y catalyst. +
1.257
- i
/
1.0
:!
2
.75
0
6% Ni
E -
I
0.0
0.1
0.2
0.3
0.4
0.5
, bar
P T
Figure 3. Initial catalytic reaction rates for toluene disproportionation. The curves are the fits to the data given by eq 11 with the optimum regression parameters of Table V.
perature data were not used in determining rate expressions but only in the estimation of apparent activation energies. By varying the reactor temperature a minimum of 100 deg, these were estimated as 87 f 10,104 f 10, and 93 f 13 kJ/mol for the 0%, 2%, and 6% Ni catalysts, respectively. An example Arrhenius plot is shown as Figure 2. The rates a t 698 K as computed by eq 1,as a function of toluene partial pressure, are given in Figure 3. It should be noted that the backpressure regulator downstream of the reactor held the total pressure constant; therefore, for these experiments the H2 partial pressures did vary slightly, from 2.69 to 3.04 atm. A few tests for dehydrogenation activity were conducted by substituting methylcyclohexane for toluene and helium for H2 Reaction conditions and operating procedures were similar to those of the disproportionation experiments. The ratio of cracking to dehydrogenation rates decreased as the Ni content of the catalyst increased, but this ratio still exceeded 1.0 even a t temperatures as low as 623 K. No dehydrogenation reaction products were observed for the 0% Ni-Y catalyst, and for all catalysts this reaction decreased in importance for temperatures greater than 650 K.
Discussion Estimation of Temperature and Concentration Gradients. Equating the observed rates with the intrinsic rates of the reaction is possible only if significant intraparticle concentration and temperature gradients are absent. Because of the lack, in most cases, of detailed information on macropore size (in pressed pellets) and crystal size distributions, we excluded from further consideration all data, both our own and that from previous studies,
Ind. Eng. Chem. Res., Vol. 29, No. 5 , 1990 791 Table I. Sources of Crystal Size and Diffusivity Data kinetics studv zeolitek) this work Y, Ni-Y
crystal size Fu et al. (1986)
Aneke et al. (1976a,b) Chang et al. (1987) Matsumoto and Morita (1967)
Cu/AlF3/Y Ti-Y, La-Y La-X
Beltrame et al. (1987)
pentasils
Nayak and Riekert (1986) Olson and Haag (1984) Kaeding et al. (1981)
pentads ZSM-5, Mg-ZSM-5 ZSM-5, B-ZSM-5, Mg-ZSM-5
Schulz-Ekloff et al. (1987) Meshram (1987) Yashima et al. (1970)
pentasils pentasils Mordenite
Gnep and Guisnet (1981) Wu and Leu (1983)
Mordenite Cu-Mordenite
a a
Karger et al. (1978) Hyun and Danner (1985) a a a Chen et al. (1979) Olson and Haag (1984) Nayak and Riekert (1986) Beltrame et al. (1987) same as above same as above Satterfield et al. (1971) Hopper and Voorhies (1972) same as above a
diffusivity Karger et al. (1980) Ragaini et al. (1984) same as above same as above same as above Wu et al. (1983) Forni and Viscardi (1986) same as above same as aboveb same as aboveb
same as above same as above Satterfield et al. (1971)' same as above same as above
Experimental values from the paper. Modified the literature data assuming the time to adsorb o-xylene to 30% saturation (data from the paper) is inversely proportional to De. cFor the counterdiffusion of cumene and benzene.
unless conservative criteria on the estimated gradients were met. The maximum particle to bulk gas temperature gradient was computed by using the equation of Lee and Luss (1969):
T o- T b Tb
ag=
L2robsP
-
(4)
D,C
Rate data were excluded from further analysis if the maximum gradient was greater than 1% of the bulk temperature. For the isothermal data, the effectiveness factor, 7, was estimated by first calculating the observable (Weisz) modulus (Froment and Bischoff, 1979):
g(C) = r ( C ) p / k ,
(6)
The modulus was determined assuming zero-, first-, and second-order kinetics in toluene concentration, and for both intracrystalline and (where applicable) macropore transport through pressed pellets. For > 0.25, effectiveness factors deviate from 1.0, because the concentration gradients are significant. These data were also excluded from subsequent analysis. In the above calculations, the physical properties of the fluid mixture were determined from either experimental values or standard correlations (Reid et al., 1977). Typical values of A, and p for zeolites were estimated by using data from Satterfield (1980) and Meier and Olson (1978), respectively. The transport coefficients, k, and h were computed by utilizing correlations for uniform spgerical particles (Froment and Bischoff, 1979). Accurate diffusivities are crucial to the computation of both the maximum temperature gradient and @. For macroparticles, De was computed according to the parallel cross-linked pore model (Feng and Stewart, 1973), assum-
+
ing communicating, constant tortuosity pores. Mean multicomponent diffusivities were taken as averages of computed values a t the particle centerline and external surface. For intracrystalline diffusivities, literature values for either toluene or p-xylene (which are of similar critical diameter) were typically used with extrapolation according to an Arrhenius model. The sources for both De (intracrystalline) and L (crystal) data are given in Table I. The results of some temperature gradient and calculations for the zeolite-catalyzed toluene disproportionation appear in Table 11. These results are only a portion of those obtained a t different temperatures, concentrations, diffusivities, and average crystal sizes but are representative of "worst case" calculations. The experimental De values employed here are conservative, because they are in most cases based on uptake or chromatographic methods, which can be considered as the lower bounds of the true intracrystalline diffusivities (Karger and Ruthven, 1989). From Table 11, it is evident that our own and all previous data are roughly isothermal, with the largest calculated temperature gradients being less than 1 "C. Concentration gradients, as indicated by the @ values, are larger and show more variation. It was concluded that the data of Yashima et al. (19701, Olson and Haag (1984), and Schulz-Ekloff et al. (1987) were affected by rates of intraparticle mass transfer. This conclusion also holds for some of the data of Kaeding et al. (1981) and Nayak and Riekert (1986) and is a strong possibility for the data of Gnep and Guisnet (1981) and Meshram (1987). Rate Expressions for Toluene Disproportionation. Although in principle the effects of mass transfer in much of the above data could be accounted for and the intrinsic reaction rates computed, in practice this is quite difficult given the present understanding of diffusion in zeolites. In subsequent calculations, only those data that were shown to be free of the complicating effects of mass transfer (using Table 11) were employed. However, there are further problems associated with some of these remaining data. In the work of Matsumoto and Morita (1967; La-X catalyst), hydrodealkylation was a primary reaction, as evidenced by benzenejxylene ratios of 1.5-3.0. Because xylene disproportionation also took place, it is impossible to determine the toluene disproportionation rate from the conversion to either benzene or the xylenes. This problem was found for some other data as well.
792 Ind. Eng. Chem. Res., Vol. 29.No. ,5, 1990 Table 11. Results of Temperature Gradient and Weisz Modulus" Calculations kinetics study zeolites Tb,K __. this work T 748 Aneke et al. (1979a.b) ruj.4lF;1-1' ( 1 0 a t (7r CU, 18 wt R 723 AIF,i Chang et al. (1987) TiT 803 Matsumoto and Morita (1967) 1,a-X 823 pentasil (SiiAl = 12.2; 20 wt 70Si02) 5 7 :3 Beltranie et al, (1987) Nayak and Riekert (1986) pentasil (Si/A1 = 110) 723 pentasil (SiiAl = 22) 723 Olson and Haag (1984) ZSM-5 850 Mg-ZSM-5 ( 2 Mjt '5 MgO) 850 Kaeding et a1. 11981) ZSM-5 723
ZSM-5 B-ZSM-5 (10 wt 70B,03) Mg-ZSM-5 (18.2 w t 70MgO) pentasil (Si/AI = 40) pentasil (Si/Ai = 18) Mordenite hlordenite Cu-Mordenite 18.7 wt 70 Cui
Schulz-Ekloff et al. (1987) Meshram (1987) Yashima e t al. (1970) Gnep and Guisnet (1981) Wu and Leu (1983)
773 773 773 598 773 723 673 753
100(Tb- To),,, 0.059 15 0.0004 0.0003 0.0002 0.0014 0.0059 0.0024 0.0024 0.0002 0.0003 0.0001 0.00014 0.0001 0.0012 0.89 0.029 0.0011
0.066-0.19 0.14 x 10+-0.12 x 10-3 7.4 X 10-'-8.5 0.0065-0.0075 0.041-0.047 0.16-0.18 1.3-1.5 1.0-2.2 2.3-5.1 0.023-0.027 0.031-0.039 2.1-2.5 19-23 1.6-1.8 0.52-0.84 25-41 0.40-0.46 0.14-0.21
X
lo-*
The range of zero-. first-, and second-order moduli for intracrystalline and macroparticle transport
Table 111. Mechanisms of Toluene Disproportionation" 1. Alkyl Transfer (Pukanic and Massoth. 1973; Poutsma, 1976) T + S zy TS TS + T 2 B + XS
x + s xs *:
2. Biuhenylalkane (Poutsma, 1976; Aneke et ai., 1979a; Kaeding et ai., 1981)
T + H+S- . - T'S + H2 T+S- + rr :. D+SD+S- . : B + X+S-
" T = toluene, B = benzene, X = xylene. S = an unspecified surface site. D+ = biphenylalkane intermediate.
The remaining data were fit to the Langmuir-Hinshelwood rate expressions, which result from proposed mechanisms for acid catalysis of the disproportionation reaction. Reduced nickel crystallites can be ruled as primary sources of disproportionation activity, because of the quite low methylcyclohexane dehydrogenation activities observed in this work. For acid catalysis, both alkyl-transfer and biphenylalkane (or benzylic cation) pathways have been postulated, in both cases based primarily on kinetics evidence. The elementary steps are summarized in Table 111. In the alkyl-transfer mechanism, a nucleophilic aromatic species attacks a carbenium ion, transferring an alkyl group. In the biphenylalkane mechanisms, a 1,l-diarylmethyl carbenium ion is formed, and H2, toluene, or xylene reacts with carbenium ions to re-form initial B r ~ n s t e dacid sites. All three possible sources of hydride ions (H2,toluene, xylene) were considered in this work, in the relevant steps of a biphenylalkane mechanism. In both mechanisms the nature of toluene (T) adsorption is mostly unspecified; in the alkyl-transfer mechanism, TS could represent toluene adsorption on either Brmsted or Lewis sites, although previous results (Jacobs et al., 1974; Aneke et al., 1979a) suggest that the former are necessary in some capacity. All other proposed mechanisms for this reaction are essentially variants of these two, in most cases assuming more complete adsorption of toluene, benzene, or H,. For example, in one modification of the biphenylalkane mechanism (Gnep and Guisnet, 19811, it is assumed that two adsorbed benzyl cations react in the rate-determining step. Appropriately modifying mechanism 2 of Table I11 leads to the rate expression
Taking these 2 basic mechanisms and all proposed modifications as a basis, a set of 40 different rate expressions can be developed assuming that every elementary step could be rate-determining. To these 40,2 additional rate expressions were employed in order to be consistent with a set examined by Aneke et al. (197913). These two bear no relationship to any of the mechanisms in Table I11 but were useful in correlating the data of Aneke et al. and could be developed by assuming other types of toluene-H atom surface complexes. They are
Of these 42 rate expressions, 22 contained 3 adjustable parameters. These could be tested only when PHhad been varied extensively, i.e., only with the data of Aneke et al. (1979b). However, for the other intrinsic rate data, the H2 partial pressures were lower (Aneke e t al. used PH values as high as 10.7 bar), so H2 inhibition was less important; also, for much of these data, the ratio PTIPH was kept roughly constant, and therefore, a toluene inhibition term includes the effect of H2 inhibition as well. Therefore, all data other than that of Aneke et al. were fitted to two-parameter rate models only. In all cases, the reverse reaction was accounted for; the reverse rate expression was determined assuming the applicability of microscopic reversibility. The intrinsic rate data were fitted to the models by nonlinear regression, with the objective function
The unknown parameters were determined by using Box's complex method (Box, 1965). Our data a t 698 K were best f i t by a second-order expression: kPT2 r = (11) 1 + KTPT However, statistical analysis of F could not distinguish eq 11 from three other rate equations; these are kPT' r= (12) (1 + KTPT)'
Ind. Eng. Chem. Res., Vol. 29, NO. 5 , 1990 793
r=
~PT~/PH
(13)
1 + KTPT/PH
Table IV. Model Fits to Kinetics Data" model FIF,;.. model
FIF,;.
~PT~/PH"~ r=
(1
(14)
+ KTPT/PH)'"
Table IV lists several values of F/Fmin (698 K, 0% Ni-Y catalyst) for those models judged adequate. At these conditions, an F test excludes those models with F f Fmin > 3.2 a t the 95% confidence level; therefore, all except eqs 11-14 and one other model (which was excluded by using the results for the other two catalysts) are excluded on these F values alone. The fits of the best model (eq 11) to the data are seen as the curves of Figure 3; the parameter estimates and additional statistical data are in Table V. The models were discriminated on the basis of (i) the value of F (ii) the estimated 95% confidence limits on the parameters, as a percentage of the parameters themselves; (iii) the estimated k-KT correlation coefficient; and (iv) analysis of the residuals for randomness. The model chosen (eq 11) typically exceeded the others tested in all these criteria, especially in ii, iii, and iv. A similar result (eq 12) was obtained when the set of rate models were applied to the data of Beltrame et al. (19871, who worked with pentasil catalysts. The values of KT obtained for these data and for our own are such that there is less than 10% error in assuming simple second-order kinetics for PT5 0.1 bar. This result prompted an attempt to fit the limited intrinsic rate data of Nayak and Riekert (1986; pentasil catalysts) to power-law rate expressions; their data were taken at low PT. However, although these data are consistent with second-order kinetics, no firm conclusion can be drawn because second- and first-order models could not be distinguished by an F test at the 90% confidence level. This same procedure proved more successful when applied to the limited data of Wu and Leu (1983; 8.7 wt '70 Cu-Mordenite); a power n 1.5-1.7 fit these data well enough to be statistically distinguishable from n = 1.0 and n = 2.0. In contrast to these results, the data of Aneke et al. (1979b) are best fit by any of three rate expressions, all of which are first-order, with toluene and hydrogen inhibition. It is impossible to discriminate among these three expressions, which include eqs 8 and 9 along with
-
r=
kPT
1 + KHPH
+ KTPT
(15)
The estimated parameters for eq 15, along with some statistical results, are also given in Table V. Considering the regression results in their entirety, it is evident that the kinetics fall into two categories: (1)first order in toluene with toluene inhibition and H2 dissociation and inhibition, and (2) second order in toluene with toluene and possibly H2 inhibition. The data of Aneke et al. (1979b) are of type 1 and all other data [except for the data of Nayak and Riekert (19861, which were inconclusive] are apparently of type 2. The composite catalyst of type 1 differed from the other catalysts considered here in its high loading of Cu and the presence of the separate AlF3 phase; it was also strongly inhibited by H2. It is interesting to note that the extensive data of Izumi and Shiba (1964; 10% B203/A1,03catalyst) also followed type 2 kinetics. Fitting their data a t 753 K to the complete set of rate models gave eq 12 as the best fit; this model was statistically distinguishable from the best first-order model. Pukanic and Massoth (1973) could fit their data for mesitylene disproportionation (at 533-617 K), catalyzed by a Si02/A1203,
kPiifPx
1.10 1
1.00 18.6
+ KTPT2/Px ~PTPHIPx KXPH/PX
10.3 7.73 7.71 7.71
1.63 a?'his work, 698 K, 0% Ni-Y catalyst.
Table V. Regression Analysis Results catalyst" 106k, mol/(s.pbar) K,, bar-' 10.8 f 4.3 2.7 f 2.1 0% Ni-Y 6.2 f 2.0 2% Ni-Y 32 i 7 17 f 8 6% Ni-Y 93 f 28 107k, mol/ ( s . g temp,c K bar) KT, bar-' KH, bar-' 0.40 i 0.28 0.15 f 0.14 673 2.5 f 1.2 0.0 f 0.06 703 4.1 f 2.1 0.93 f 0.52 0.30 f 0.07 0.04 f 0.02 723 6.8 f 0.8 0.10 i 0.10 0.06 f 0.06 743 9.9 f 3.9
RMSDb 0.0327 0.0234 0.0173
RMSDb 0.0810 0.227 0.182 0.361
"This work, all data a t 698 K. bRoot-mean-squared deviation for the rate. 'Data of Aneke et al. (1979b), for 10 wt 70 Cu, 18 wt 90 A1F3-Y catalyst.
Table VI. Comparison of Reaction Rates" and Activation Energies rate, app act. mol/ ( g s ) energy, X lo6 kJ/mol kinetics study zeolite(s) this work Y 0.07 87 f 10 2% Ni-Y 0.12 104 f 10 6% Ni-Y 0.13 93 f 13 Aneke et al. CUIAlFq-Y 0.04 88 f 13 (1979b) Nayak and pentasil: Si/A1 = 36 0.01 53 f 5 Riekert (1986) Si/A1 = 110 4 X 56 f 7 Kaeding et al. ZSM-5 5 x 10-3 96 f 5 (1981) Wu and Leu Cu-Mordenite (8.7 wt 0.05 (1983) 5% CUI Izumi and Shiba 10% Al2O3/B2O3 2 x 10-4 122* (1964) I
"
Extrapolated or interpolated t o 698 K, atm. *From the paper.
PT
= 0.1 atm, PH= 0.9
to eq 12 as well. These results suggest that all purely acidic (no transition metal) catalysts follow a second-order rate expression for toluene disproportionation. If the differences between catalysts of types 1 and 2 are real, one might also expect to see differences in their overall activities and activation energies. These quantities are compared in Table VI, for this work and for all previous studies where the necessary data were available and observed rates free of mass-transfer complications. The activities were interpolated or extrapolated to a common basis of 698 K, PT = 0.1 bar, and PH = 0.9 bar. The interesting result of this exercise is the slightly low activity of the type 1 catalyst relative to the catalysts of this work. However, the apparent activation energies of the type 1 and type 2 catalysts are quite similar; the only zeolites showing unusual (low) activation energies are the pentasil
794 Ind. Eng. Chem. Res.. Vol. 29, No. 5 , 1990
catalysts of Nayak and Riekert (1986). Nevertheless, the distinct nature of type 2 kinetics is emphasized by the fact that an F test distinguishes between the best first-order model and eq 11 for all the catalysts of this type that we examined
Conclusions There is extensive, but not overwhelming, evidence that for toluene disproportionation most zeolite catalysts behave quite differently from the composite catalyst studied by Aneke et al. (1979a,b). These type 2 catalysts follow a second-order rate expression, which is consistent with the alkyl-transfer mechanism of Table I11 if it is assumed that the surface reaction is rate-determining and involves either one adsorbed toluene moiety (eq 11) or two of the same type (eqs 12 or 14). However, the rate expression is also consistent with certain biphenylalkane mechanisms (eq 13) if the second step in the example reaction sequence of Table I11 is rate-determining and the toluene molecule that 1s attacked by the benzylic cation is not adsorbed. Those biphenylalkane mechanisms involving two adsorbed toluene moieties (e.#., giving eq 7 ) are excluded on the basis of the regression results and observed effects of total pressure on the reaction, as taken from data of U'u and Leu (1983) and Beltrame et al. (1987). In each case, some of their data were taken at constant PT/PH: PT/PH = 2.3 at total pressures of 7.9-28.6 bar and PrlPH = 5 at 1.G2.0 bar, respectively. At constant PT/PH,eq 7 predicts constant toluene conversion at constant temperature and space velocity. However, Wu and Leu observed conversions of 0.114-0.463 (at 753 K) and Beltrame et al. observed 0.038 -0.064 (at 573 K) under these conditions. It was originally assumed that an alkyl-transfer mechanism would be characterized by an alkylbenzenium ion intermediate (TS in Table 111). The relevance of such an intermediate to transalkylation was disproved, at least for acid catalysis in solution, by the isotopic tracer work of Streitwieser and Reif (1960); they found that a hydride abstraction was the initial step of a transalkylation reaction and a biphenylalkane intermediate was formed. Amelse (1988) took this result to imply a biphenylalkane mechanism for zeolite-catalyzed transalkylation; he found that, for ethylbenzene disproportionation catalyzed by largepore zeolites (Y.mordenite), there was little scrambling of 14C from the d position. However, the observed lack of scrambling would also be true of an alkyl-transfer mechanism; therefore, no definite conclusion regarding alkyl transfer vs biphenvlalkane mechanisms for zeolites can be drawn. The kinetics of the type 1 catalyst are consistent with the biphenylalkane mechanism, with the decomposition of the biphenylalkane intermediate being the rate-determining step, if it is assumed that H, can adsorb; this modification does not constitute a radical departure from the basic mechanism, which already assumes that H2 can react with carbenium ions. Aneke et al. (1979a) postulated that the disproportionation sites of their Cu/A1F3-Y catalysts were relatively isolated, because NH, adsorption data indicated that only 11% of the total acid sites was active at typical reaction conditions. If so, the isolation may account both for the difference in rate-determining step and for the slightly lower activity of this catalyst (Table \'I). However, it must be noted that an F test cannot distinguish between eq 15 and the best secondorder model for the data of Aneke et al., a t temperatures of 673, 703, and 743 K and at the 90% confidence level. Although far more study would be needed to establish a complete rate expression for catalysts of both types and to correctly interpret these in terms of reaction mecha-
nisms, the present work in combination with careful study of past work establishes a rough kinetics framework for many zeolites that is in agreement with results on other acidic, but non-zeolite, catalysts.
Acknowledgment Stephen Brignac was the recipient of a fellowship provided by the Exxon Foundation. This work was supported in part by the National Science Foundation under Grant CBT-8504877.
Nomenclature = concentration, mol/cm3
C b. C s = bulk gas, external surface concentrations, mol/cm3 C, = equilibrium concentration, mol/cm3 De = effective diffusivity, cm2/s F = objective function, defined by eq 9 F,, = flow rate of limiting reactant, mol/s g = defined by eq 6 1H = enthalpy of reaction, J/mol h , = gas-phase heat-transfer coefficient, J / (cm2.s.K) k = reaction rate constant, mol/(g.bar") k , = gas-phase mass-transfer coefficient, cm/s KT, KH = toluene, hydrogen adsorption equilibrium constants 1, = catalyst diffusional half-width, cm ti = overall order of reaction PT,PH= toluene, hydrogen partial pressures, bar r = reaction rate, mol/(g.s) robs= observed or global rate of reaction, mol/(g-s) T o = temperature at centerline of catalyst, K Tb, T s = bulk gas, external surface temperatures, K W = catalyst weight, g X = fraction conversion
Greek SSmbols Jg= dimensionless adiabatic temperature rise, defined by eq
3
a, = Weisz (observable) modulus based on bulk concentration, defined by eq 4 = Weisz (observable) modulus, defined by eq 5 c = Thiele modulus, defined by eq 5 7 = catalyst effectiveness factor 1, = effective thermal conductivity, J/(cnmK) p = catalyst density, g/cm3
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Received for review February 21, 1989 Revised manuscript receiued October 13, 1989 Accepted J a n u a r y 30, 1990
Effect of Catalysts on the Kinetics of the Reduction of Barite by Carbont Satish B. Jagtap, Anita R. Pande, and Ashok N. Gokarn* Chemical Engineering Division, National Chemical Laboratory, Pune 411 008, India
Reduction of high-grade barite powder with particles of active charcoal has been studied both in t h e absence and presence of catalysts. Catalysts t h a t are known t o enhance Boudouard reaction bring about corresponding improvements in the reduction rate of barite also. T h e conversion-time data have been analyzed by using a modified volume reaction model, and the effect of catalysts on kinetic parameters has been elucidated. Reduction of barite by solid carbon is an important step for the recovery of barium chemicals from barite. In this so-called “black ash process” (McKetta, 19771, the reduction of barite is carried out in a rotary kiln or in a fluidized ‘ N C L Communication 4558.
OSSS-5885/90/2629-0795$02.50/0
bed at a high temperature of the order of 1100-1200 “C in the presence of reducing agents, mostly carbon. In the course of the reduction, the main reducing agent is carbon monoxide, which in turn is generated in situ by the reduction of carbon dioxide by carbon. The reactivity of coal is generally correlated by its reactivity with COz to generate G 1990 American Chemical Society
