Ind. Eng. Chem. Res. 1994, 33,600-606
600
Selective Aromatization of C3- and C4-Paraffins over Modified Encilite Catalysts. 3. Coking Mechanism and Deactivation Kinetics of n-Butane Aromatization Apurba K.Jana and Musti S . Rao' Department of Chemical Engineering, Indian Institute of Technology, Kanpur 208 016, U.P., India
The deactivation of Zn-encilite catalyst during the aromatization of n-butane to benzene, toluene, and xylenes (BTX) has been investigated as a function of time on stream. The coke responsible for the deactivation was extracted and analyzed by 'H NMR, HPLC, and MS. The mode of coking was established on the basis of the carbonaceous compounds responsible for deactivation. These compounds were found to be retained at the channel intersections due to trapping. A small amount of deactivation was observed due t o pore-mouth poisoning after 10 h reaction time. From the product inhibition experiments a coking mechanism was proposed. The coke precursor was found to be formed from the intermediate butene rather than from the product BTX. In order words the deactivation occurred parallel with the main reaction. A kinetic model has been presented using the Langmuir-Hinshelwood approach, The kinetic parameters in the rate equations and the activation energies have been determined. Introduction
The selective transformation of lower alkanes and alkenes into more useful compounds like benzene, toluene, and xylenes (BTX) is of considerable importance since this expands the sources of aromatic hydrocarbons. In the first part of this work (Jana and Rao, 1993a), a comparison of the activities of propane and n-butane over Zn-encilite catalyst for the aromatization reaction, the effects of the operating variables on various responses, viz., the conversion of n-butane, selectivity, and yield of BTX, and some insight into the mechanism of the reaction were studied. In the kinetic study carried out in the second part of our work (Jana and Rao, 1993b),it was proven that the aromatization of n-butane was significant in comparison to the deactivation of the catalyst. It was also found that the reaction followed Langmuir-Hinshelwood type kinetics. The problem inherent in this aromatization process is that the rate of the main reaction decreases with time on stream due to the formation of carbonaceous compounds in the cavities, at channel intersections or on the outer surface (Delmon and Grange, 1980). The deactivation of the catalyst results in the loss of catalytic activity which is highly common for bifunctional acid zeolites in several industrial processes (Barbier, 1986; Best and Wojciechowski, 1973;Lin et al., 1983). Several studies have been made of the kinetics of the deactivation process (Bamwenda et al., 1991;Nam and Kittrell, 1984;Noharaet al., 1992;Olazar et al., 1989; Wojciechowski and Corma, 1986). Corella and Asua (1981,1982)generalizedtheoretically the kinetics of deactivation by coking, relating the activity directly to the deactivation reaction, and obtained a deactivation equation of the form -Pg
= (-rB)oa = f @ i , T ) U
(1)
-= $(pi,T)3ad
- da
dt where +(pi,T) is the deactivation function and d = ( m + h - l ) / m , where m and h are the number of active sites involved in the rate-controlling step of the main reaction and the deactivation reaction, respectively. This method
* To whom correspondence should be addressed.
has been successfully applied to various studies of catalyst deactivation by coke formation in the dehydration of isoamyl alcohol over silica-alumina catalyst (Corella and Asua, 1981),in the furfural decarboxylation on Pd-Al203 (Srivastava and Guha, 1985), and in the methylation of toluene over H-ZSM-5 and hydrogen mordenite catalyst (Mantha et al., 1991). The above approach is based on the mechanism of formation of coke precursor. Since various deactivation mechanisms can lead to the same deactivation kinetic equation, kinetic analysis alone is not adequate to find the true deactivation mechanism. I t is necessary to do a chemical analysis of the deposited coke on the catalyst which gives insight into the coke precursor and also the coking mechanism. In the past a few researchers attempted to identify the nature of coke (Langner, 1981; Magnoux and Guisnet, 1989). Although Fouch et al. (1990) established the coking mechanism for the hydrogenation of benzene over Pt-USHY and PtH-mordenite catalysts, investigations on deactivation kinetics coupled with the coking mechanism are rather scarce in the literature. It, therefore, seemed necessary to study the deactivation kinetics based on the reaction mechanism of formation of the coke precursor. The present investigation was carried out with the following objectives: (i) to isolate the components in the coke responsible for the deactivation of the catalysts, (ii) to propose a deactivation mechanism by coking, (iii) t o develop a kinetic model on the lines proposed by Corella and Asua (19821, and (iv) to estimate the deactivation parameters with statistical data interpretation. Experimental Section Catalysts. The reaction has been studied over Znexchanged encilite catalyst with a surface area of 419 m2 g'. The details of the catalyst preparation and the characterization techniques are given in part 1of this series. Apparatus and Procedure. The deactivation experiments were carried out in a fiied-bed tubular reactor under differential conditions. Details of the experimental setup and catalyst pretreatment procedure were reported in part 1of this series. The operating conditions were as follows: catalyst particle size, 0.1-0.24 mm; weight of the catalyst, 0.06-0.13 g; and n-butane concentration in the feed, 40-
0 1994 American Chemical Society 0888-5885194/2633-Q6QQ$Q4.5Q/Q
Ind. Eng. Chem. Res., Vol. 33, No. 3, 1994 601 60% (rest was nitrogen as diluent). The experiments were conducted at atmospheric pressure at four different temperature levels of 480, 500, 520, and 540 OC, respectively. The products were analyzed by using two gas chromatographs, viz., a NUCON gas chromatograph (fitted with a thermal conductivity detector for the analysis of aliphatic hydrocarbons) and a Hewlett-Packard 5890Agas chromatograph (fitted with a flame ionization detector for the analysis of aromatic hydrocarbons). The details of the analytical procedure were also reported in part 1of the series. The reaction products detected in reasonable quantities were BTX and very small quantities of methane, propane, and butene. For the identification of the nature of coke, experiments were conducted under the following experimental conditions: reaction temperature, 520 "C; n-butane concentration in the feed, 60% (rest was nitrogen as diluent); weight of the catalyst, 0.8 g. The coke analyses were performed separately using deactivated catalyst after 5 and 10 h reaction time on stream. We have applied the same definition of coke and its method of extraction from the deactivated catalyst as described by Guisnet and Magnoux (1989). The sample was dissolved in a minimum quantity of 40 % hydrofluoric acid solution in order to liberate the coke trapped inside the catalyst pores. The resulting mixture with insoluble coke was diluted with distilled water and then filtered. The filtrate was then leached with methylene chloride, and the methylene chloride layer was separated from the aqueous layer by using a separating funnel so that the dissolved soluble coke (if any) in the hydrofluoric acid solution comes out in the methylene chloride phase. The separated methylene chloride was used as a solvent to extract the soluble portion of the filtered coke. The extraction was done using the Soxhlet apparatus for a duration of 48 h. The pale yellow extract was then analyzed using the following techniques. A lH NMR (400 MHz; tetramethylsilane as standard) spectrum of the extract in CDC13 solvent was taken on a BRUKER WM 400 spectrometer. Low-energy (10 eV) mass spectrometry (MS) (JEOL D300 mass spectrometer) was carried out in order to avoid fragmentation of the samples. High-performance liquid chromatography (HPLC) (PYE UNICAM PU 4003) analysis was performed with a UV detector (PYE UNICAM PU 4025) in a 4.6-mm X 25-cm column of ZORBAX ODS which separates the components of the extract according to their polarity, and methanol was used as a mobile phase. The flow rate of the mobile phase (85% methanol and 15% double distilled water) was 2.4 mL/ min at 261-bar pressure. The different components were identified by calibration with the reference substances. The H/C ratio of the insoluble coke in methylene chloride was measured using the C, H elemental analyzer (CARLOERBA 1108). Transmission electron microscopic (TEM) (JEOL 2000 FX 11) characterization of the deactivated zeolite was carried out in order to study pore blocking with reaction time. Powder samples were finely ground in a mortar and pestle in order to break the soft agglomerates. A small portion of the sample was ultrasonicated in acetone, and a drop of the dispersed suspension was poured on a Formvar-coated Cu grid of 400mesh size. Acetone quickly evaporates, and the dried dispersed powder was ready for TEM observation. Results and Discussion Coke Composition. To get information about the chemical nature of coke and also the coke precursor with
Table 1. Analysis by 'H NMR of the Extract and H/C Ratio of the Insoluble Coke Formed with Time on Stream (Reaction Temperature = 620 OC; P,,.bu&pN, = 1.5; W/F = 0.3 g/(mol/h)
coke (wt % ) solubilization yield (wt %) lH NMR ( % ) Ar CH3 Ar CH2 Ar CH H/Cratio of insoluble coke
reaction time, h 5 10 1.9 3.2 100 82 53.6 8.9 37.4
41.5 3.6 54.9 0.38
time on stream, the deactivated catalyst samples after 5 and 10 h reaction time were removed from the reactor and the soluble portion of the coke dissolved in methylene chloride was analyzed. lH NMR spectra of the extract were taken in order to get information regarding the nature of the proton present in the soluble coke. lH NMR spectroscopic results (Table 1)show that the coke formed at 5 h time on stream was completely aromatic. The low solubility of the coke produced at 10 h time on stream indicates that the residue consists of macromolecular, insoluble components, whereas at 5 h time on stream the residue consists of soluble material of low molecular weight. The tendency of dealkylation of methyl group with reaction time was also observed from the 'H NMR spectra of the extracts as seen in Table 1. The aromaticity (aromatic protons from the 'H NMR spectra) also increases with reaction time probably due to the dealkylation and subsequent tendency to form oligoaromatics by cycloadditions. The low H/C ratio of insoluble coke may be due to the presence of high boiling aromatics or perhaps due to the graphite-like coke growth at the macropores of the catalyst. The differences between the coke samples at different reaction times are even more evident by their highperformance liquid chromatograms. Major components were identified by comparison with reference compounds. The existence of these components were further confirmed by the low-energy mass spectral analysis in order to get the molecular ion peaks only. The identified components along with their boiling points and molecular sizes are shown in Table 2 at 5 and 10h reaction times, respectively. The boiling points and molecular sizes are obtained from the literature (Magnoux and Guisnet, 1989). Mode of Deactivation. The coke components found at 5 h coking time comprise volatile components whose boiling points were much lower than the reaction temperature. Because of the high volatility, they would diffuse in the gas phase provided they were not blocked inside the pore structure of the catalyst. The analyzed components of coke were not observed in the product stream. Furthermore, all the components have a larger kinetic diameter than the pore dimension but comparable to the size of the channel intersections of the Zn-encilite catalysts. With 10 h time on stream a major part of the soluble coke was composed of components having two to four condensed aromatic rings. A significant amount of coke was insoluble with poor H/C ratio suggesting that perhaps graphite-like coke grows at the pore mouths by the reaction of coke precursor, which is formed within the pore and migrates to the exterior surface (Langner, 1981). Figure 1 shows the transmission electron micrographs of the catalyst at different values of time on stream. The white patches in Figure l a indicate that the particles are porous. The electron beam passes through the pores and does not expose the photographic film. The pores are being blocked by
602 Ind. Eng. Chem. Res., Vol. 33, No. 3, 1994
Table 2. Molecular Size and Formulas of the Main Comwnents of the Coke Soluble in Methylene Chloride reaction time, h
structure
formula
bp, 'C
Ite.
size, A
5
180
1.8
5
265
6x1
5
318
6 X 9.5
10
245
6x1
10
340
6 X 9.5
10
340
6.5 X 8.5
10
400
8.5 x 8.5
0-17p
H
w
carbonaceous particles as a function of reaction time. Figure l b and Figure IC,taken at the same magnification, demonstrate the change of catalyst morphology as a function of reaction time on stream. Figure l b shows the micrograph of a single particle after 5 h reaction time on stream. The pores were partially blocked. The white patches show the open pores whereas the blocked regions of the particle turned blackish. Figure ICis the micrograph of the catalyst after 10 h reaction time on stream. Here almost all the pores are blocked as indicated by the completely dark micrograph. In fact, with the increase of reaction time on stream the pores are gradually filled up and the particles appear to be totally dark. These results suggest that the deactivation is definitely due to the blockage of the channel intersections of the catalyst by the coke molecules. Mode of Coke Formation. The analysis of coke composition and the mode of deactivation reveals that thecoke formationcould be theresult ofreactionsbetween butene and BTX molecules. To investigate the participation of the BTX molecules in the coking reaction, BTX molecules were individually injected in varying amounts along with the feed stream. The observed rates for differentialoperation were computed from the conversion as follows:
-rob = FAxl W (3) where Ax and (W/F) represent conversion and contact time, respectively. The effect of product inhibition on rate is shown in Figure 2. There was no appreciable product inhibition on the rate of reaction. These results suggest the possibility of intermediate butene to be responsible for the formation of coke precursor. The components which were detected in low-energy mass spectroscopyand confirmed by HPLC are mesitylene, 1,4-
(C)
0045 p H Fignre 1. Transmission electron micrograph of (a) frenh catalyst at 12 500 magnification, (b) deactivated catalyst after 5 h of mking time at 50 OOO magnification. and (c) deactivated catalyst aft81 10 h of coking time at 50 OOO magnification (reprodud at 80% of the original).
dimethylnaphthalene, and 1-methylfluorene at 5 h and 1-methylnaphthalene, 1-methylanthracene,phenanthrene, and pyrene a t 10 h reaction time on stream. Figure 3 shows the probable reaction mechanism on the basis of the above detected compounds for the formation of coke in the catalyst. The butene formed from the dehydro-
Ind. Eng. Chem. Res., Vol. 33, No. 3, 1994 603 r
0
01
A
5
A 0
E 25.6-
enough, the demethylation from the intermediates occurs by a free-radical mechanism forming 1-methylanthracene. Kinetics of Deactivation. The results of product inhibition experiments show that the addition of BTX individually in the n-butane feed stream has no appreciable effect on the rate of reaction. These results indicate that the deactivation reaction is of parallel type. In simple form the reaction can be represented as
Benzene p-Xylene rn-Xylene o -Xylene Toluene
N
0 X C
.-
$1
f
n-butane n-butane W/FBo=0 . 4 6 9 Cntlmollh Temperature i 5 2 0 O C Conc. of n - B u t a n e s 60%
5
10
-
BTX
coke1
(4)
In fact, n-butane does not form coke directly. The intermediate butene and cycloalkane are the responsible species for the coke formation. The deactivation phenomenon can be represented by
25.2
2 5.0 0
-
15
20
Vol. of BTX Injected, pL
Figure 2. Effect of product inhibition on rate of main reaction.
genation of n-butane is highly reactive on the Lewis acid sites and forms carbocation. The cation formed reacts with another molecule of butene producing an intermediate, which in turn reacts with a molecule of butene via hydrogen transfer followed by internal cyclization and subsequent hydrogen transfer by alkyl radical, giving 1,4dimethylnaphthalene (Figure 3a). The hydrogen transfer from the cyclic compounds takes place probably by CH3CH2-CH2+, which may be formed due to cracking on the acid sites (as propane was observed in the product stream), or CH~-CH~-CHZ-CH~+ radical. The probable mechanism for the formation of 1-methylanthracene is shown in Figure 3b. 1,4-Dimethylnaphthalene reacts with primary butane carbocation forming an intermediate which, on internal cyclization and hydrogen transfer, gives l-methylanthracene. Because the reaction temperature is high
-rb = (-'&a (5) where activity a = (rate of reaction at any time t)/(rate of reaction at time t = 0). Equation 2 has been used to describe the deactivation by coke. It has already been established that the number of active sites involved in the controlling step of the main reaction is 2 Le., m = 2. The possible isothermal rate equations for the deactivation reaction (eq 4) were derived on the basis of singlesite and dual-site mechanisms by the method described by Corella and Asua (1982) and are given in Table 3. All these derivations are based on the Langmuir-Hinshelwood approach, assuming a coke precursor forming step (step 2 in Table 3) as the rate-governing step and considering butene to be adsorbed on the Zn2+site in a different way in the deactivation step than in the main reaction, Le., similar to the d-2 mechanism as proposed by Corella and Asua (1982). In order to determine the order of deactivation d , we proceed assuming a different number of active sites
I
Figure 3. Mechanism of formation of coked material: (a) formation of 1,4-dimethylnaphthalene from butene and (b) formation of 1-methylanthracene from 1,4-dimethylnaphthalene.
604 Ind. Eng. Chem. Res., Vol. 33, No. 3, 1994 Table 3. Isothermal Deactivation Rate Equations Assuming Coke Precursor Formation Is Rate-Controlling* Step1: O + T + O * T step 2: nO(g) + hO*T P 1 ~ h -+
Step 3: ( P 1 ~ h+ ) ( P 2 ~ h ) (P,T,)--Coke model
m
h
n
DEM-1
2
1
0
DEM-2
2
1
DEM-3
2
2
DEM-4
2
2
rate equations
1
K20 = K4*/K4.
E= 2’5
0.46 g cat I moll h
Temperature = 5 2 O O C
Conc. of n-Butane = 60%
P
Time, t ( h )
Figure 4. Test for finding out the deactivation order d.
involved in deactivation reaction (Le., h) and obtain the corresponding value of d. In order to determine the value of d, different relations of a vs t obtained from eq 2 were tested for linearity. With the experimental data it is found that, for d = 1.5,a linear plot was obtained, which is shown in Figure 4. The models DEM-1 and DEM-2 (in Table 3) were rejected because it was found in the previous section that two active sites were involved in the deactivation reaction Le., h = 2. In order to evaluate the parameters of the deactivation rate equations given in Table 3, the value of K16 (Kls = K I K ~ K ~ / K for~ the ) main reaction (part 2 of this series) has been used. K4*/K4 was considered as parameter K2o. The Davidon-Fletcher-Powell (DFP) method of unconstrained minimization technique (Fox, 1971;Edgar and Himmelblau, 1988;Arora, 1989)has been employed for the estimation of the parameters. Timeon-stream data were taken under the differential mode of operation of the reactor. The reaction rate was calculated at different operation times using eq 3. Then the activity at any time t was determined employing eq 5. The deactivation function J/@i,T) was evaluated from the slope of ( u - O . ~- 1)vs t by using the linear regression method. The responses of the experiments are given in Table 4. The values of the concentrations used in the parameter
estimation of the models were the average values of the inlet and outlet concentrations of each component. The parameter values of the models DEM-3 and DEM-4 are shown in Table 5. The model DEM-4 was rejected because of irregularities and some negative values of the parameters as well as poor residual sum of square (RSS) errors. The model which described the experimental results most satisfactorily is model DEM-3, which is given by
This model was based on the assumption that two adjacent adsorbed molecules are involved in the formation of the coke precursor P172, which ultimately leads to the formation of coke through several equilibrium steps. The formation of coke precursor is the rate-determining step. The butene was adsorbed at the active sites in the main reaction in a different way than that in the deactivation reaction. The total mechanism can be represented by
(7)
The same phenomenon has been reported by Jodra et al. (1976) for the dehydrogenation of benzyl alcohol over a Cu-Cr2Odasbestos catalyst, where benzyl alcohol was adsorbed in a different way in the main reaction as compared to the deactivation reaction. The confidence limits of the parameter values of eq 6 are given in Table 6.
The dependency of deactivation constant with temperature can be given by the equations
k, = 5.41 X
lo8 exp(-18000/T)
K,, = 0.0127 exp(3944/7‘)
(8)
(9)
The activation energy for deactivation was calculated as 149.6 kJ/mol, whereas the same for the main reaction was found to be 121 kJ/mol. The higher value of activation
Ind. Eng. Chem. Res., Vol. 33, No. 3, 1994 606 Table 4. Deactivation Function Values
inlet concn of n-butane x 108, mol/L
outlet concn of n-butane x 103, mol/L
17.857 20.089 22.321 24.554 26.785
17.000 18.943 20.780 22.565 23.999
17.857 20.089 22.321 24.554 26.785
16.909 18.823 20.692 22.368 23.866
17.857 20.089 22.321 24.554 26.785
16.803 18.683 20.557 22.147 23.725
17.857 20.089 22.321 24.554 26.785
16.660 18.522 20.379 22.074 23.463
av partial press. of n-butane x IO2, atm
av partial press. of BTX x 103, atm Temperature = 480 "C 42.589 5.146 47.691 6.875 52.663 9.140 57.570 11.803 62.049 16.337 Temperature = 500 "C 42.479 6.013 47.543 8.041 52.554 10.238 57.330 13.731 61.886 18.142 Temperature = 520 "C 42.348 6.620 47.372 8.836 52.390 10.957 57.060 14.952 61.624 19.438 Temperature = 540 O C 42.174 7.518 47.175 9.846 52.171 12.202 56.970 15.409 61.394 20.638
Table 5. Values of Parameters and RSS for Models DEM-3 and DEM-4 temp, "C model DEM-3 model DEM-4 480 kd = 0.0225 k d = -1.2477 K4* 0.2113 Km = 2.3203 K4 = -0.1361 RSS = 1.2952 X lO-9 RSS = 1.3579 X le7 500 kd 0.0417 kd = 0.0005 Kzo K4* = 0.4682 ~.= 2.0910 K i = 0.0123 RSS = 2.2474 X 10-9 RSS = 3.4123 X 10-7 520 kd = 0.0730 kd = 7.1109 Km = 1.9131 K4* = 2.0006 K4 = -1.0678 RSS = 1.0016 X RSS = 8.6686 X 540 kd = 0.1356 kd = -1.4782 K,* 1.8915 K20 = 1.6247 K4 = -0.9043 RSS = 1.0465 X 10-8 RSS = 1.5878 X 10-6 Table 6. Kinetic Constants for the Deactivation Reaction temp, "C
480 500 520 540
h-' 0.0225 f 0.0044 0.0417 0.0058 0.0730 i 0.0037 0.1356 0.0134 kd,
*
Kzo 2.3203 f 0.6691 2.0917 f 0.3903 1.9131 f 0.1218 1.6247 f 0.1775
energy for the deactivation reaction indicates that the deactivation reaction is more sensitive to temperature.
Conclusion The present study on the coke formation in the reaction of n-butane to BTX over Zn-encilite catalyst at different coking times revealed that the deactivation occurs due to formation of polyaromatics in the channel intersections. At higher values of time on stream the deactivation also occurs due to pore-mouth blocking by the macromolecular, graphite-like species. There is no site poisoning by coke but blockage of the access to the sites of the pores in which there are no coke molecules. It was established that butene was responsible for the formation of coke precursor and it was adsorbed on the active sites in a different way in the deactivation reaction as compared to the main reaction.
init rate of n-butane depletion X lo2, mol/(gh)
WtT) X
10'
9.791 10.100 10.378 10.598 10.714
0.9623 0.9705 0.9802 0.9882 0.9945
14.842 15.110 15.909 16.254 16.456
1.6098 1.6305 1.6508 1.6654 1.6794
22.237 23.438 24.335 24.937 25.463
2.5132 2.5604 2.6035 2.6323 2.6555
32.603 34.473 35.926 37.200 38.197
3.8820 3.9739 4.0586 4.1256 4.1747
The rate equation and mechanism of deactivation have been determined. A comparison of activation energy revealed that the deactivation reaction had a higher activation energy in comparison to that of the main reaction.
Nomenclature a = catalyst activity A = benzene, toluene, and xylenes (BTX) B = n-butane d = deactivation order F = molal flow rate of n-butane, mol/h h = number of active sites involved in the controlling step of deactivation reaction k d = rate constant of the deactivation reaction, h-l K1 = n-butane adsorption equilibrium constant, atm-1 Kz = thermodynamic equilibrium constant for n-butane dehydrogenation K3 = butene adsorption equilibrium constant on acid sites of Zn-encilite catalyst, atm-1 K4 = butene adsorption equilibrium constant on 7 sites for main reaction, atm-l K4* = butene adsorption equilibrium constant on 7 sites for deactivation reaction, atm-1 Ks = BTX adsorption equilibrium constant, atm-1 K16 = ratio of adsorption and desorption rate constants, atm-1 KZO= ratio of adsorption equilibrium constants of coke precursor formation (i.e., deactivation) and main reactions rn = number of active sites involved in the main reaction n = number of moles of butene in the gas phase which are involved in the coke precursor formation 0 = butene PIQ, P17h = coke precursor P 3 7 h = different forms of coke in the coking PZQ,P ~ QP27h, , sequence PA = partial pressure of BTX, atm PB = partial pressure of n-butane, atm pi = partial pressure of ith component, atm -rg = rate of reaction of n-butane, mol/(gh) (-rg)o = rate of reaction of n-butane at zero time -robs = observed rate, mol/(gh) t = time, h T = temperature, K
606 Ind. Eng. Chem. Res., Vol. 33, No. 3, 1994
W = weight of t h e catalyst, g Ax = conversion Greek Letters 7 = Zn2+ site on Zn-encilite catalyst + ( p i , T ) = deactivation functions as in
eq 2
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Jana, A. K.; Rao, M. S. Selective Aromatization Of C3- and C4-Paraffins over Modified Encilite Catalysta. 1. Qualitative Study. Ind. Eng. Chem. Res. 1993a, 32, 1046-1052. Jana, A. K.; Rao, M. S. Selective Aromatization of C3- and C4-Paraffins over Modified Encilite Catalysts. 2. The Kinetics of n-Butane Aromatization. Ind. Eng. Chem. Res. 199313, 32, 2495-2499. Jodra, L. G.; Romero, A.; Corella, J. Kinetics of the Catalytic Dehydrogenation of Benzyl Alcohol. An. Quim. 1976,72,833-40. Langner, B. E. Coke Formation and Deactivation of the Catalyst in the reaction of Propylene on Calcined NaNH4-Y. Ind. Eng. Chem. Process Des. Dev. 1981,20, 326-331. Lin, C.; Park, S. W.; Hatcher, J., Jr. Zeolite Catalyst Deactivation by Coking. Ind. Eng. Chem. Process Des. Dev. 1983,22,609-614. Magnoux, P.; Guisnet, M. Coking, Aging and Regeneration of Zeolites: X-Nature of Coke Formed on H-Erionite During n-Heptane Cracking, Mode of Deactivation. Zeolites 1989,9,329335. Mantha, R.; Bhatia, S.; Rao, M. S. Kinetics of Deactivation of Methylation of Toluene Over H-ZSM-5 and Hydrogen Mordenite Catalysts. Ind. Eng. Chem. Res. 1991,30, 281-286. Nam, I.; Kittrell, J. R. Use of Catalyst Coke Content in Deactivation Modelling. Ind. Eng. Chem. Process Des. Deu. 1984,23,237-242. Nohara, D.; Sakai, T. Kinetic Study of Model Reactions in the Gas Phase at the Early Stage of Coke Formation. Ind. Eng. Chem. Res. 1992, 31, 14-19. Olazar, M.; Aguayo, A. T.; Arandes, J. M.; Bilbao, J. Polymerization of Gaseous Benzyl Alcohol. 3. Deactivation Mechanism of a SiOd A1203 Catalyst. Ind. Eng. Chem. Res. 1989,28, 1752-1756. Srivastava, R. D.; Guha, A. K. Kinetics and Mechanism of Deactivation of Pd-Al203 Catalyst in the Gaseous Phase Decarboxylation of Furfural. J. Catal. 1985,91, 254-262. Wojciechowski, B. W.; Corma, A. Catalytic Cracking: Catalysts, Chemistry and Kinetics; Marcel Dekker: New York, 1986.
Received for review March 5, 1993 Revised manuscript received October 12, 1993 Accepted November 7, 1993'
* Abstract published in Advance A C S Abstracts, February 1, 1994.
