Ind. Eng. Chem. Res. 1997, 36, 4207-4211
4207
Light Alkene Selectivity on Y Zeolite FCC Catalysts Marı´a C. Galiano and Ulises A. Sedran* Instituto de Investigaciones en Cata´ lisis y Petroquı´micasINCAPE (FIQ, UNL-CONICET), Santiago del Estero 2654, 3000 Santa Fe, Argentina
n-Hexadecane was used as a test reactant with a pulse technique over various types of commercial FCC catalysts, either equilibrated or steam-dealuminated, to study the changes in the selectivity to light C4-C6 alkenes over unit cell sizes ranging from 24.26 to 24.56 Å at 475-525 °C. The activity depended on the total catalyst acidity, while the unit cell size of the zeolite component was the factor controlling the relative alkene yields. Inside a given hydrocarbon group, alkenes decreased their relative yields when conversion increased, while for a given conversion, their consumption as a function of increasing unit cell sizes was dependent on the molecular size: the higher the molecular weight, the slower the depletion. Linear alkenes showed a steady rate of change along the whole range of unit cell sizes, while branched alkenes showed a much faster consumption when unit cell sizes were under about 24.31 Å. The yield of light alkenes was significantly augmented as a function of increasing temperature, linear alkenes being favored. The incidence of hydrogen-transfer reactions is discussed. Introduction The catalytic cracking of hydrocarbons (FCC) is one of the most important sources of transportation liquid fuels, mainly gasoline (Biswas and Maxwell, 1990). Recently, as a consequence of more restrictive legislation on gasoline composition, there has been growing concern about the aims of FCC units and product distributions obtained in gas oil cracking (Schipper et al., 1993). Among other issues, both a diminution of aromatic compounds and an increase in lower alkenes that are raw materials for downstream products are desirable in the FCC process. For a given feedstock, shifts in the product distributions can be obtained either through the catalyst used or the operational approach. Besides the fact that they always possess Y zeolite in its acidic form as the main component on a certain matrix, many catalyst options are available: conventional with rare earths (REY), ultrastable with (REUSY) or without (USY) rare earths, or various additives would produce different product slates. On the other hand, FCC units can be run under different modes, maximizing the middle distillates, gasoline, or alkene productions (King, 1993). Light alkenes are valuable products obtained from gas oil FCC since, from the gasoline standpoint, they add potential to FCC units because they can be used to produce ethers and alkylates. A continuously increasing demand on light branched alkenes to produce ethers, which are both octane promoters and oxygenate compounds used in gasoline blending, is being observed. While, particularly, isobutylene sources are foreseen as a bottleneck for the production of methyl tert-butyl ether (MTBE), etherification processes similar to the one leading to MTBE were developed to use C5 (leading to tert-amylmethyl ether, TAME) and even C6 branched alkenes (Pescarollo et al., 1993). Hydrogen-transfer reactions are very important in FCC chemistry because they stabilize products through the saturation of alkenes and the generation of aromatics from naphthenic compounds; however, the gasoline octane net balance accounted for by this overall conversion scheme is negative. Then, it is generally accepted that if alkene yields are to be maximized, it is necessary to depress hydrogen-transfer reactions. Indeed, octane catalysts that stabilize at lower zeolite unit cell sizes S0888-5885(96)00510-6 CCC: $14.00
were developed in order to decrease the relative significance of these reactions (Scherzer, 1989). Though there exists controversy about the mechanisms controlling these reactions (Sedran, 1994), they seem to depend on the acid site density through the need for closely located paired active sites. Indirect indexes have been used to study and evaluate hydrogen-transfer reactions by means of the conversion of test reactants (Corma et al., 1991; Suarez et al., 1990) which typically are light molecular weight alkene or naphthenic compounds. n-Hexadecane has been considered as a test reactant representative of, at least, long-chain alkanes present in standard FCC feedstocks. It is the objective of this paper to report the results obtained in the conversion of n-hexadecane over equilibrium and laboratory steam-dealuminated commercial FCC catalysts of various types in view of the changes in product distributions, focusing on C4-C6 light alkenes, and the incidence of hydrogen-transfer reactions. Experimental Section The catalysts used were two series (series I, with rare earths and series II, without rare earths) of laboratory steam-dealuminated samples derived from commercial ultrastable catalysts (REUSY and USY, respectively, Fabrica Carioca de Catalisadores, Rı´o de Janeiro, Brazil), two samples of equilibrium conventional REY catalysts obtained from running FCC units (E-CAT-a, Octydine 1160 BR; and E-CAT-b, PRE-50 AR, both from Engelhard Corp.), and one sample of a steam-dealuminated conventional REY catalyst (A-1, derived from fresh Octydine 1160 BR, Engelhard Corp., by steaming for 1 h). Dealuminated samples were treated under 100% steam in a fixed-fluid-bed reactor at 815 °C during different hourly time periods as indicated by the associated number in the sample’s designation (see Table 1). Catalyst-specific surface areas were determined from nitrogen adsorption isotherms and micropore volumes, and the zeolite contents were estimated by means of t plots and Johnson’s method (Johnson, 1978). The zeolite unit cell sizes (UCSs) were determined following the ASTM D-3942-85 method on a Shimadzu XD-1 X-ray diffractometer. The total catalyst acidity was measured by means of ammonia temperature-programmed desorption following a standard procedure © 1997 American Chemical Society
4208 Ind. Eng. Chem. Res., Vol. 36, No. 10, 1997 Table 1. Catalyst Propertiesa
sample
surf acidity, zeolite, area, micropore mequiv rare-earth wt % UCS, Å m2/g vol, mL/g of NH3/g oxides, wt %
E-CAT-a E-CAT-b A-1
9.1 11.5 12.7
24.31 24.29 24.41
175 147 230
0.030 0.033 0.050
0.065 0.069 0.097
1.30 1.42 1.30
I-0 I-0.5 I-1 I-2 I-4 I-8
27.1 22.2 18.9 17.9 16.5 17.0
24.56 24.34 24.31 24.28 24.28 24.28
Series I 279 0.092 194 0.077 172 0.073 152 0.063 154 0.044 150 0.050
0.643 0.094 0.092 0.054 0.045 0.045
1.60
II-0 II-0.5 II-1 II-2 II-4 II-8
25.6 22.5 22.3 22.1 20.3 17.6
24.55 24.32 24.29 24.27 24.26 24.23
Series II 225 0.082 181 0.076 182 0.074 178 0.071 167 0.063 165 0.052
0.518 0.080 0.069 0.047 0.041 0.034
0.00
Figure 1. Variation of zeolite unit cell size in catalysts from series I (b) and II (O) as a function of steaming time period. T ) 815 °C, 100% steam.
a The numbers in the samples denote the hourly steaming time periods.
that has been described elsewhere (de la Puente and Sedran, 1996). Two types of conversion experiments were performed in a fixed-bed reactor by using a pulse technique in which the reactant (n-hexadecane, Fluka, minimum 99 wt %) was injected in pulses of 3 µL into a dried nitrogen stream at 130 mL/min; after eluting the reactor, the pulse was sent directly to a gas chromatograph in order to analyze the reaction products. The catalyst samples were previously treated at 550 °C under flowing air during 20 min. The experiments were (a) activity runs conducted at 500 °C, using 100 mg of catalysts, and (b) selectivity studies performed at a fixed conversion, using selected samples from the series and additional catalysts, so as to cover a wide range of UCSs. According to the samples’ activities, catalyst weights were changed in the 25-400-mg range, with inert silica particles of the same size range always being added to a total amount of 460 mg. The selectivity test was also conducted at 475 and 525 °C on an equilibrium catalyst (sample E-CAT-a). The amount of coke was assessed from a weight difference method by burning off carbonaceous deposits after injecting a given number of pulses. Trial runs were performed to confirm that thermal cracking was negligible under these experimental conditions. Results and Discussion Catalyst Properties. The catalyst properties are summarized in Table 1. Zeolite UCSs of fresh, dealuminated, and equilibrium samples are in line with the expected values; if compared against those of fresh REY catalysts (about 24.71 Å), the UCSs of the base samples I-0 and II-0 (24.56 and 24.55 Å, respectively) indicate that they were previously treated, while those of the E-CATs are typical of equilibrated REY catalysts (Scherzer, 1989). The samples in the series that were subjected to steaming showed modifications in their properties that correspond to a fixed-temperature, varying-time treatment: surface area, micropore volume, zeolite content, UCS, and acidity all decrease as a function of treatment severity (duration), with steeper changes in the first steps (Wallestein and Alkemade, 1996). The most important difference between base catalysts I-0 and II-0 is their loading of rare earths: while catalyst I-0 shows an amount that can be considered
Figure 2. Conversion of n-hexadecane as a function of total catalyst acidity. T ) 500 °C. Symbols: (b) samples from series I; (O) samples from series II; (9) samples E-CAT-a and -b; (0) sample A-1.
typical for REUSY catalysts (Biswas and Maxwell, 1990), catalyst II-0 does not have any rare earths. The content of rare earths is reflected in the evolution of the UCSs as a function of treatment severity, since the samples from series I stabilize at 24.28 Å, while those in series II go to about 24.24 Å (refer to Figure 1), thus showing the stabilizing effect of rare earths on the Y zeolite component of FCC catalysts. As well, E-CAT samples show specific surface areas, pore volume, and rare-earth and zeolite contents that are characteristic of the REY catalysts under commercial operation. While the total catalyst acidity decreases strongly due to the loss of zeolite acid sites as a consequence of increasing dealumination treatment severity (refer to Table 1), the ammonia desorption profiles (not reported) do not present important qualitative variations, suggesting that the acid strength does not change considerably. Profiles are characterized by a single, wide desorption peak with the maximum located at about 320 °C. Both fresh base catalysts are very acidic as compared with the steamed samples, with sample I-0 showing a total acidity higher than that of sample II-0; this could be assigned to the differences in the matrices, since the zeolite contents and properties are similar, and/or to the rare-earth ions which are open to hydrolysis, thus being capable of adding acid sites (Scherzer, 1989). Conversion of n-Hexadecane. The activities of all the samples at 500 °C are shown in Figure 2 in terms of n-hexadecane conversion observed under the same conditions as a function of total catalyst acidity. It can
Ind. Eng. Chem. Res., Vol. 36, No. 10, 1997 4209 Table 2. Product Distribution (wt %) as a Function of Reaction Temperature at 70% Conversion (Catalyst E-CAT-a) temp, °C hydrocarbon groups
475
500
525
C1 + C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12 C13 C14 C15 i-C5)/T-C5a i-C5)/n-C5)b
0.31 10.22 22.37 17.08 10.67 3.59 2.66 0.43 1.07 1.03 0.24 0.22 0.00 0.00 33.48 2.02
0.43 10.44 20.06 16.90 10.99 3.47 2.24 0.55 0.74 0.42 0.15 0.00 0.00 0.00 33.74 1.75
0.50 11.53 22.72 17.97 11.17 2.58 1.85 0.55 0.77 0.23 0.17 0.00 0.00 0.00 35.34 1.70
a C branched alkenes/C total hydrocarbons. b C branched 5 5 5 alkenes/C5 linear alkenes.
be seen that for samples from series I and II except base catalystsswhich are far more acidic and convert the injected reactant pulse almost completelysthe activities decrease continuously as a function of increasing steaming treatment severity (acidities decrease), and they do not follow the stabilization trend observed in UCSs. Considering that UCS is a direct indication of Y zeolite framework aluminum atom concentration (and consequently of the associated acid sites) through very wellknown correlations (e.g., refer to Jorik (1993)), this phenomenon is not in line with the assumption that the zeolite is the only contributor to the activity of this type of catalyst; in fact, contributions from both extraframework aluminum species generated by steaming (Corma, 1989) and matrix acid sites are to be expected. Moreover, the observed linear relationship applies for ECATs and dealuminated A-1 samples as well. It can be mentioned that a dependency of the activity on total aluminum content had been observed by Cotterman et al. (1989) when cracking n-hexadecane on steam dealuminated pure Y zeolite. In the activity experiments, the amount of coke deposited on the catalysts was significant: typically about 0.2-0.3% (for two pulses), which implies that approximately 4-6% of the pulse mass remains on the catalyst surface. Coke formation followed a linear pattern with the amount of reactant injected, at least for a number of pulses between two and six: the more active the catalysts, the more coke they formed and with faster growth. As an example, with four pulses injected, sample II-4 (UCS 24.26 Å) formed 0.4% coke on the catalyst, sample I-1 (UCS 24.31 Å) formed 0.9%, and sample A-1 (UCS 24.41 Å) formed 1.0%. It is to be noted, however, that the conversion and product distributions kept essentially steady during the activity tests. The distribution of products as a function of carbon atoms per molecule observed at 500 °C (refer to Table 2) shows that they are mostly C5- hydrocarbons, typically about 65% of the total products, the highest contributions being those of C4 and C5 products (typically around 28% and 24%, respectively). No significant qualitative change was observed among the samples of different catalyst types. The cracking chemistry of n-hexadecane up to 40% conversion at 400 °C has been studied on Y zeolite (Abbott and Wojciechowski, 1988). We observed general agreements between our results and previously published works (Abbot and Wojciechowski, 1988; Cotterman et al., 1989), the only discrepancy
Figure 3. Relative alkene yields as a function of conversion in samples from series I: (a, top) C4 hydrocarbon group; (b, middle) C5 hydrocarbon group, and (c, bottom) C6 hydrocarbon group. Symbols: (b) sample I-0, (9) sample I-1, (2) sample I-4. T ) 500 °C.
being on the relative importance of the C3 compound fraction, which is higher in our case. Since light branched alkenes are potentially etherificable through well-established technology and propylene and butylene are alkylate feedstocks, then they are important products from FCC units and it is important to know about the factors controlling their production, either from the catalyst or operative standpoints. The analysis of the product slate in order to study changes in the relative yields of alkenes in the C4-C6 range shows that over all the samples tested, the relative amounts of alkenes in each of the groups in the range decrease continuously as conversion increases, thus indicating a higher reactivity than other hydrocarbon types with the same carbon number per molecule. This is shown in Figure 3 for samples from series I covering the whole range of UCSs tested, where it can be seen that for a given hydrocarbon group and catalyst, this
4210 Ind. Eng. Chem. Res., Vol. 36, No. 10, 1997
Figure 4. Relative alkene yields in the C4, C5, and C6 hydrocarbon groups observed at 70% conversion as a function of zeolite unit cell size. Symbols as in Figure 2.
Figure 5. Relative linear and branched C5 alkene yields observed at 70% conversion as a function of zeolite unit cell size. Symbols as in Figure 2.
pattern keeps steady in a wide conversion window, suggesting that the sets of the most important reactions (e.g., cracking, hydrogen transfer) hold their relative magnitude in most of the conversion range. Essentially the same behavior was observed on series II samples. Changes in the relative yields of C4-C6 alkenes as a function of zeolite UCS at the same conversion level (70%) are presented in Figure 4. It can be seen that all the alkene groups decrease significantly as the unit cell sizes increase. This fact can be ascribed to the higher incidence of hydrogen-transfer reactions, which is normally associated with the effect of higher acid site density (increase in the number of paired acid sites, or aluminum atoms with at least one next nearest neighbor, NNN (Pine et al., 1984). The overall dependency of the hydrogen-transfer reactions on paired acid sites occurrence has been shown on various reactants and FCC catalyst types (Cheng and Rajagopalan, 1989; Corma et al., 1991). Our results are clear in that there is a significant increase in the rate of alkene consumption at UCSs below about 24.31 Å, which is a singular point in this type of correlation. Certainly, theoretical distribution models for zeolite Y (e.g., Pine et al., 1984) predict that for values below this UCS, the density of paired active sites decreases more pronouncedly, and this has been associated with the sharper decrease in hydrogen-transfer rates that has been observed at UCS values close to 24.31 Å (Cheng and Rajagopalan, 1989; Jacquinot et al., 1990; de la Puente and Sedran, 1996). As shown by the slopes of the lines in the range of UCSs between 24.31 and 24.56 Å in Figure 4, the rate of consumption of alkene products as a function of increasing UCS depends on their molecular size: the larger the molecules, the less prone to hydrogen-transfer reactions they are. On cracking gas oils, a qualitatively similar observation was made by Bonetto et al. (1992) for hydrocarbons in the C5-C7 range and by Cheng et al. (1992) for C4 and C5 alkenes. This would confirm that the zeolitic character of commercial FCC catalysts, as expressed by the unit cell sizes of their zeolite component, is the controlling factor in hydrogen-transfer reactions (de la Puente and Sedran, 1996). Note that the results in Figure 4 belong to different FCC catalyst types (conventional REY, REUSY, and USY, also including actual equilibrium samples). It has to be mentioned that it was not possible to find an unambiguous correlation between the relative alkene yields and total catalyst acidity. While there is overall agreement about the higher consumption of alkenes as a function of increasing zeolite UCS, there are not ample details published
regarding the specific behaviors of branched and linear light alkenes. In effect, as shown in Figure 5 for the example of C5 compounds, it can be seen that considering the broad range of UCSs between 24.26 and 24.56 Å, branched alkenes control this depletion; while the relative concentration of linear alkenes increases steadily as long as the UCS decreases, branched molecules do increase at a significantly higher rate for UCSs lower than about 24.31 Å. Cheng et al. (1992) also had observed a more significant affectation of branched C4 and C5 alkenes as compared to linear ones when cracking gas oil over various catalysts with UCSs in the range of 24.26-24.39 Å, but contrary to our data, they observed that while branched compounds kept their rate of consumption, linear compounds flattened off at UCSs below 24.31 Å. The behavior of branched alkenes can be rationalized in terms of a mechanism postulated to explain the reactions of hydrogen transfer to alkenes (Sedran, 1994): a carbenium ion adsorbed on the surface (e.g., a protonated alkene molecule) might accept a hydride ion transferred from a donor molecule somewhat trapped in a nearby site and desorb saturated; this model has received theoretical support (Mota et al., 1996). Since the stability of the secondary carbenium ions is lower than that of the tertiary ones with the same carbon number per molecule, even in a stabilizing environment like the one a zeolite supercage may offer, then branched alkenes yielding tertiary ions are more propense to participate in hydrogen-transfer reactions. Additional experiments at 475 and 525 °C performed at the same conversion level of 70% on sample E-CAT-a showed that, as expected, light alkenes increase their relative yields as long as the temperature increases (refer to Table 2 and Figure 6). It is interesting to observe that the most important total hydrocarbon fractions (C3-C6) keep their yields approximately steady, thus confirming that the changes in product distribution, i.e., in the ratio between the saturated and insaturated compounds, are due to the relative decrease in hydrogen-transfer activity: cracking reactions have a higher energy of activation as compared to those of hydrogen-transfer and isomerization reactions (de la Puente and Sedran, 1996). This is certainly the basis for the operation of commercial units at higher temperatures when working in the “olefins mode” (King, 1993). As shown in Figure 6, the fact that relative alkene yields increase at higher temperatures in the order C4 . C5 > C6 could be considered to be another index of the higher relative cracking activity, yielding more light compounds.
Ind. Eng. Chem. Res., Vol. 36, No. 10, 1997 4211
Figure 6. Relative alkene yields in the C4, C5, and C6 hydrocarbon groups observed at 70% conversion as a function of reaction temperature. Catalyst E-CAT-a.
The distribution among alkenes for a given hydrocarbon group, as shown for C5 products in Table 2, indicates that although the relative yield of branched isomers increases at higher reaction temperatures, indeed linear ones expand their yields more significantly. This could be ascribed to the fact that isomerization reactions are unfavored at higher temperatures in relation to hydrogen-transfer ones, due to their lower energy of activation (de la Puente and Sedran, 1996). It can be assumed that in this reacting system, most of the “initial” carbenium ions formed after the first cracking steps are secondary, which would later isomerize to ternary ones; since this route would be hindered, linear alkenes would be preserved, their yields being higher in relative terms. Conclusions The conversion of n-hexadecane by means of a pulse technique is a useful tool to study variations in the product distribution, particularly in alkene yields, which are due to the changing properties of commercial FCC catalysts. While the activity is controlled by the total catalyst acidity, alkene selectivity at a given conversion is a direct function of the intensity of hydrogen-transfer reactions, in turn depending on the UCS and zeolitic component. For C4-C6 alkene groups, approximately 24.31 Å appears as a singular UCS below which the incidence of hydrogen-transfer reactions is more appreciable. In a given hydrocarbon group, branched alkenes are much more sensitive than linear isomers to hydrogen-transfer reactions. In terms of maximizing the alkene yields, high reaction temperatures and low UCS catalysts are necessary; however, higher temperatures favor linear alkenes selectively against branched ones, and in the case of lowering UCSs, a trade-off with activity must be considered. Acknowledgment This work was performed with the financial assistance of University of Litoral, Secretary of Science and Technology (Santa Fe, Argentina), Project 167. This project is within a Joint Study Project CENACA (National Catalysis Center)-JICA (Japan International Cooperation Agency) in the field of catalysis. The cooperation of Fa´brica Carioca de Catalisadores S. A. is gratefully acknowledged.
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Received for review August 15, 1996 Revised manuscript received November 18, 1996 Accepted July 12, 1997X IE960510V
Literature Cited Abbot, J.; Wojciechowski, B. W. The effect of temperature on the product distribution and kinetics of reactions of n-hexadecane on HY zeolites. J. Catal. 1988, 109, 274.
X Abstract published in Advance ACS Abstracts, September 1, 1997.
