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Ind. Eng. Chem. Res. 1997, 36, 3116-3125
Microkinetics Modeling of the Hydroisomerization of n-Hexane Annemieke van de Runstraat,* Joop van Grondelle, and Rutger A. van Santen Laboratory of Inorganic Chemistry and Catalysis, Department of Chemical Engineering, Eindhoven University of Technology, P.O. Box 513, 5625 NM Eindhoven, The Netherlands
The hydroisomerization of n-hexane over platinum-loaded mordenite and ZSM-5 was simulated using a microkinetics approach. Alkoxy species were assumed to be the reactive intermediates. Carbenium ions were considered to be part of the transition state. This leads to true activation energies of isomerization of approximately 130 kJ/mol for both zeolites. The model explicitly takes into account the variation in micropore filling. This enables modeling of the orders of the reaction over a wide pressure and adsorption enthalpy regime. The resulting model describes experimental measurements very well. It is shown by both methods that ZSM-5 is more active per acid site than Mordenite due to its higher adsorption enthalpy for n-hexane. The effect is small because of the low hexane order. The adsorption entropy plays a decisive role in determining the overall activity. Introduction We will report the simulation of the kinetics of the hydroisomerization of n-hexane over Pt/H-mordenite and Pt/HZSM-5 catalysts. The simulated results will be compared to the results of our own experiments as well as data from the literature. We will focus on the question of whether intrinsic properties of Brønsted acid sites can be unraveled from physical adsorption energy effects. Therefore, we propose a modeling scheme that explicitly separates physical adsorption from site reactivity. The two different zeolites were chosen because of their 10 kJ/mol difference in adsorption enthalpy of n-hexane. We performed kinetic experiments to measure apparent activation energies and reaction orders (Van de Runstraat, 1997; Van de Runstraat et al., 1997). A model is developed that enables us to analyze the differences in the orders of a reaction as a function of the degree of micropore filling and adsorption enthalpy. The predicted micropore filling degrees can be compared with experimental values from the in situ positron emission profiling (PEP) technique, as reported elsewhere (Van de Runstraat et al., 1996). A combination of theoretical and experimental data is used to design a reaction energy scheme for the acid-catalyzed hexene isomerization part of the reaction. Important is our conclusion that elementary reaction steps can be considered independent of the zeolite. Hence, quantum chemical cluster calculations can be used to compute these parameters. No such calculation is yet available, and therefore, we had to use a combination of experimental and theoretical information to deduce the activation energy and preexponential factor of this reaction in our work. The different approaches toward kinetic modeling used previously can be largely divided into two groups: firstly, true simulation procedures with parameters fitted to experiments to model complex reactions over a wide range of conditions (Baltanas et al., 1989; Froment, 1975; Svoboda et al., 1995); and secondly, those that use relatively simple kinetic expressions to deduce critical reaction steps and describe apparent kinetics (Guisnet et al., 1991; Spivey and Bryant, 1982). Since we are interested in molecular mechanistic aspects, in this paper, we will report simulation results in the spirit of the former approach but with a model * E-mail:
[email protected]. S0888-5885(96)00661-6 CCC: $14.00
containing explicitly all elementary steps and without making an assumption which is the rate-determining step. This approach is similar to the microkinetics method (Dumesic et al., 1993) and is based on a reaction model containing elementary reaction steps. These steps are deduced from knowledge or postulates of the reaction intermediates, for instance, obtained from spectroscopy or theory. They therefore describe the processes involved on a molecular scale. In heterogeneous catalysis, these steps involve adsorption and desorption and reactions in the adsorbed state. The reaction rate of each elementary reaction step is then described by a preexponential factor and an activation energy. In this way, a complete set of parameters is built describing the reaction. These are then fed into conventional catalytic engineering equations. We will use a model to describe the plug-flow reactor (PFR). A set of differential equations is generated that must be solved to obtain outlet concentrations as a function of inlet concentrations and reaction conditions. In this way, the overall kinetics can be calculated, whose validity can be checked experimentally. In the final stage, the model can be adjusted to the experiments to obtain final values for the activation energies and preexponential factors. Constraints have to be used, and they are used to simplify the procedure. For example, the difference between the activation energies of the forward and reverse reactions must be equal to the reaction enthalpy. From the simulation, information is gained about which steps are reversible, at equilibrium, kinetically significant, and the rate-determining step. Moreover, it generates information about surface coverages of reactants, intermediates, and products. In the case of a plug-flow reactor, intermediate gas-phase and surface concentrations are also obtained. Thus, information as a function of length along the reactor bed is gained. Details of the method followed to estimate the parameters will be presented in the next section. A comparison between the experimental and simulated results as well as an analysis of the consequences of our model will be made in the Results and Discussion section. Method Building the Model. The mechanism was constructed from elementary reaction steps, based on the © 1997 American Chemical Society
Ind. Eng. Chem. Res., Vol. 36, No. 8, 1997 3117
Figure 2. Transfer scheme.
Figure 1. Mechanism of hydroisomerization used in the model.
Weisz mechanism (Weisz, 1962). Adsorption/desorption steps and molecular-transfer steps between micropore sites and acid and metal sites were added. Micropore sites are those siliceous sites in a zeolite on which physisorption of hydrocarbons takes place. The molecular-transfer steps were written as a reaction. For example:
n-C6(ads) + empty platinum site f n-C6(Pt) + empty micropore site The resulting kinetic scheme consisted of the following groups of steps: (1) adsorption/desorption on the micropore site; (2) transfer to and from micropore sites to platinum sites; (3) dehydrogenation/hydrogenation on platinum; (4) transfer to and from micropore sites to acid sites; (5) protonation/deprotonation on Brønsted acid sites; (6) isomerization on Brønsted acid sites; (7) adsorption/ desorption of hydrogen onto platinum sites. A full reaction scheme is given in Figure 1. The three columns represent the three types of sites. Between these sites/columns, transfer takes place. Notice that all transport must proceed via a micropore site; there is no direct transfer between platinum and acid sites. As we will see later, this aspect of the model turns out to be critical. Figure 2 illustrates the essential features of the model in a schematic way. A total of 23 gas-phase species was used: all hexane and hexene isomers possible as well as hydrogen. All the corresponding adsorption intermediates on the platinum, acid, and micropore sites were also taken into account. We assumed that olefin molecules were adsorbed on acid sites as alkoxy species (Senchenya et al., 1985; Kazansky, 1994). Step 5 in the mechanism above must therefore be read as alkoxy formation/dissociation. Carbenium ions can be considered to be transition states
relative to these stable intermediates. This means that classical carbenium ion chemistry can still be used, provided that the activation into this species is taken into account. As already mentioned, the rates of elementary reaction steps, both forward and reverse, were calculated from fundamental data such as preexponential factors and activation energies. The preexponential factors and activation energies were either taken from the literature, estimated from theory, or deduced from experimental characterization data. An elaborate discussion of the these parameters will be given in the next paragraphs. Usually, the activation energies are described first, followed by estimation of the preexponential factors. Thermodynamic energy diagrams were used to ensure that each reaction cycle was closed; that is, conservation of energy was maintained. Adsorption and Desorption: Hydrocarbons. The activation energy of adsorption from the gas phase onto a micropore site in the zeolite was estimated to be 10 kJ/mol, taking into account the reaction temperature of approximately 240 °C and the pore rim surface diffusion barrier as described by Derouane et al. (1988). In the case of somewhat more difficult adsorption (dimethylbutanes on ZSM-5), the activation energy was estimated to be 15 kJ/mol. The adsorption enthalpies, the difference between the activation energies of adsorption and desorption, of the paraffins on the two zeolites were either measured (Van Well et al., unpublished results) or averaged from different literature sources and references cited therein (Derouane et al., 1988; Derouane, 1987; Blum et al., 1993; Stach et al., 1986). This resulted for monobranched isomers in slightly higher adsorption enthalpies than n-hexane in mordenite and slightly lower adsorption enthalpies in ZSM-5. The adsorption enthalpy of 1-hexene on the siliceous part of the zeolite (micropore site) was assumed to be equal to that of n-hexane (Stach et al., 1986). This assumption agrees with the data of Lechert and Schweitzer (1983) for butane/butene. Adsorption enthalpies of the other olefins were extrapolated from an article of Harlfinger et al. (1983), also on butane/butene. The assumption was made that 3-hexene behaves like 2-hexene, and 2-hexene in turn was comparable to 2-butene. It was assumed that the preexponential factor of adsorption for each gas-phase molecule was the same for all zeolites. This seems reasonable since the transition-state partition functions will not change much from zeolite to zeolite. Since most of the relevant adsorption experiments in the literature were performed on MFI zeolites, these were used to calculate this preexponential factor of adsorption. The values for the preexponential
3118 Ind. Eng. Chem. Res., Vol. 36, No. 8, 1997
factor of desorption of a hydrocarbon from a ZSM-5 catalyst were estimated from an article of Zhdanov et al. (1988). The values were chosen to be close to the lower limit of the range given (i.e., 1015 s-1 for paraffins, and 1014 s-1 for olefins due to the more restrained transition state). The value for the preexponential factors of adsorption of a molecule was then calculated from the equilibrium constant for adsorption, the corresponding desorption preexponent, and the activation energies for adsorption and desorption:
(-ERT ) ) A -E A exp( ) RT
Aads exp Kads ) Ades
act,ads
ads
act,des
(
)
-∆Hads exp RT des
(1)
The equilibrium constant for adsorption was calculated from equilibrium adsorption data (Prinz and Riekert, 1986; Eder, 1996). Due to the commensurate freezing of n-hexane in MFI-type zeolites (Smit and Maesen, 1995), high-temperature data were considered to be the most reliable source for calculating the preexponential factors of adsorption. Since the difference between the activation energies of adsorption and desorption is equal to the enthalpy of adsorption, the preexponential factor of desorption was changed due to a change in entropy of adsorption. This is an example of the compensation or isokinetic effect which is a well-known effect in organic chemistry (Leffler and Grunwald, 1963). Adsorption and Desorption: Hydrogen. The values of the data needed for the elementary steps of dissociative adsorption and associative desorption of hydrogen on platinum were taken from an article by Norton et al. (1982). They found the adsorption to be slightly activated and the corresponding sticking probability to be approximately 0.2 (Ertl, 1980). The dissociative adsorption enthalpy of hydrogen on platinum sites, resulting in strongly adsorbed hydrogen atoms, was deduced from data given by Dumesic et al. (1993) and Campbell et al. (1991). This enthalpy has a value of 125 kJ/mol. Transfer Reactions. The rate of molecular transfer of a species from a micropore site to an acid or metal site was calculated from the rate of intracrystalline diffusion for an almost empty zeolite. The actual transfer rate was suppressed when either of the types of sites involved became filled with sorbates: vacant sites were required for a reactant to (re)adsorb. The activation energy of transfer of a molecule from an micropore site to a platinum or an acid site was put equal to the activation energy of diffusion. The activation energies for transfer in ZSM-5 were deduced from different sources and literature cited therein (Post, 1991; Heink et al., 1992; Choudhary and Akolar, 1989). When an activation energy could not be found, this value was estimated on the basis of transferring species (Donnet et al., 1992) and zeolite dimensions and structure (Meier et al., 1996). The change in enthalpy from a paraffin adsorbed on an micropore to an acid site was calculated from the difference in the adsorption enthalpy on all-silica and acidic zeolites. In order to calculate the change in enthalpy between a hydrocarbon adsorbed on an micropore and a platinum site, the relative adsorption coefficients of olefins and paraffins on platinum were calculated. This was done applying an equation given in a paper by Cerveny and Ruzicka (1981), using values
Figure 3. Energy diagram (kJ/mol) of dehydrogenation to form cis-2-hexene.
for Es and σ* as given by Taft (1956).The parameter Es accounts for the steric effects of a substituent on a molecule, relative to a methyl group. The parameter σ* accounts for the polarity of a substituent. As starting points, the adsorption enthalpy of 1-hexene on platinum was calculated from the equilibrium constant for adsorption as given by Brehm et al. (1993).The adsorption enthalpy of n-hexane was taken from the article of Campbell et al. (1991). The adsorption enthalpies for the other molecules could be calculated from these two data by drawing parallels with 1-hexene and butenes (Maurice and Minot, 1989). Thus, we obtained all activation energies necessary for the transfer steps. The preexponential factors of transfer were then calculated by “fitting” on diffusion data in zeolites. These data were obtained from the same sources as used for the estimation of activation energy of diffusion. The activation energies and the surface coverages were then used to calculate a “rate” of diffusion. In the case of olefin transfer to and from an acid site, the situation was different since the molecule was then chemisorbed into an alkoxy species. These parameters will be dealt with in another subsection. Hydrogenation and Dehydrogenation. The hydrogenation and dehydrogenation steps could be described with a two-step reaction: the hydrogen atoms are added or abstracted one by one (Horiuti and Polanyi, 1934). To simplify the simulation, a one-step mechanism was assumed:
Pt + PtC6H14 f Pt2C6H12 + H2 A paper by Campbell et al. (1991) was used as a basis to calculate the different preexponential factors and activation energies for dehydrogenation. From this paper, we obtained the activation energies of steps 1 and 2 in Figure 3 for cis-2-hexene. An estimation of the activation energy for the corresponding backward step 1 was taken from an article by Brehm et al. (1993). The reaction energies involved in the complete hydrogenation/dehydrogenation cycle were then made consistent using the dehydrogenation data as well as the adsorption data (see paragraph about transfer reactions) and thermodynamics (API 44 Tables, 1968). The result is shown in Figure 3 for cis-2-hexene. The activation energies for the dehydrogenation to form cis-2-hexene are shown schematically in Figure 4. All other hydrogenation/dehydrogenation activation energies could then be calculated from the ones we had obtained for the cycle involving cis-2-hexene using relative reaction rates. The relative dehydrogenation rates of the different paraffins to the corresponding olefins were taken from two papers by Cerveny (Cerveny and Ruzicka, 1969; Cerveny et al., 1977). Since the activation energies were given for the platinum reaction
Ind. Eng. Chem. Res., Vol. 36, No. 8, 1997 3119 Table 1. Data of Alkoxy Species Formation and Decomposition To Form Adsorbed Hexene
Figure 4. Activation energies of surface dehydrogenation to form cis-2-hexene.
Figure 5. Energy diagram (kJ/mol) of hexene adsorption to a secondary alkoxy species.
Figure 6. Energy diagram (kJ/mol) of hexene adsorption to a tertiary alkoxy species.
in the liquid phase, the adsorption enthalpies of the alkanes were used to correct these data toward surfacereaction activation energies. In order to obtain the correct surface species equilibrium, it was taken into account that olefins are adsorbed on two platinum sites, as opposed to one in the case of paraffins. The reaction rates given were corrected in a similar way using the adsorption equilibria. The preexponential factors of hydrogenation could then be calculated from the equilibrium dehydrogenation constants (as deduced from thermodynamics) and from the rate constants. In this calculation, it was then assumed that the adsorbed paraffin is in “gas-phase equilibrium” with its adsorbed olefins. The probability that a particular compound is formed was also taken into account. The preexponential factor for dehydrogenation of n-hexane to form 1-hexene was therefore twice as large as that for the formation of 3-hexene. Alkoxy Formation and Dissociation. It is well accepted that primary carbenium ions in the gas and liquid phases are far less stable than secondary or tertiary ions. However, according to Kazansky (1994), carbenium ions do not exist as stable intermediates in zeolites; they are adsorbed on acid sites as an alkoxy species. Therefore, primary species may exist in the adsorbed state. Since the isomerization takes place in the “dissociated” state, in a carbenium ion-like transition state, only secondary and tertiary carbenium ions were accounted for in the simulation model (Kazansky, 1994; Zicovich-Wilson et al., 1994; Van Santen and Kramer, 1995). In Figure 5 and Figure 6, energy schemes for the formation of a secondary and a tertiary C6 alkoxy species are given. These data were estimated from quantum chemical calculations by Kazansky et al. (1996), by subtracting a value of approximately 30 kJ/ mol from their calculated values to account for the
type of species
Aform, mol/(m2 s)
Eact,form, kJ/mol
Adec, mol/(m2 s)
Eact,dec, kJ/mol
secondary tertiary
2.8 × 105 2.8 × 105
50 37
2.1 × 1011 9.7 × 1011
130 120
systematic overestimation of energies by these calculations. The values used were the same for each zeolite. The adsorption enthalpy (-∆Hads), defined by physical adsorption, did vary as a function of pore structure and, hence, zeolite. This seemed a reasonable assumption because the contribution of the physical adsorption to the interaction of the hydrocarbons will not change by alkoxy formation. An important feature to notice is that the activation energies to form tertiary alkoxy species are lower than for a secondary species. This is contrary to Baltanas et al. (1989), who applied lower activation energies for secondary species. These authors used, however, the concept that carbenium ions are the reactive intermediates. The preexponential factors of alkoxy formation and dissociation were calculated from Dumesic et al. (1993) . Their data on the protonation starting from the gas phase were recalculated by taking into account the number of acid sites, Kazansky’s activation energies, and the adsorption equilibrium constants for the alkenes on the zeolite. The results are listed in Table 1. The activation energies given apply to conversion of an C6 olefin on a micropore site into an alkoxy species. Isomerization. The isomerization itself was assumed to take place according to the protonated cyclopropane (PCP) mechanism (Chevalier et al., 1977; Weitkamp, 1981; Sie, 1993). Nonbranching isomerization was assumed to take place via alkyl shifts. Only conversions between secondary and tertiary carbenium ions in the transition state were taken into account. The data collected by Brouwer (1980) on superacids were used as input but adapted to the special case of the zeolite. Transition-state theory could be used to derive an equation for the rate constant of isomerization (Dumesic et al., 1993). Activation of the alkoxy species into a carbenium ion-like species involves σ-bond lengthening between the carbon atom of the molecule and the oxygen atom of the oxygen atom of the zeolite. The actual isomerization takes place in these carbenium ionlike transition states. This gives the following:
kiso ) (ABrAlength) exp(-[Eact,Br + Eact,length]/RT) (2) The activation energies and preexponential factors of isomerization in the superacid liquid phase are denoted here as “Brouwer” (Eact,Br and ABr). The former is of the order of 30 kJ/mol. The activation energy of alkoxy dissociation is the upper limit for the activation energy of bond lengthening (130 kJ/mol). From this, we calculated an activation energy of isomerization of 160170 kJ/mol. Previously we estimated from experiments and elementary kinetic expressions, of the form of eq 3, a value of approximately 130 kJ/mol (Van de Runstraat et al., 1996, 1997). We therefore concluded that full alkoxy dissociation is not required for isomerization and subtracted 20 kJ/mol from the appropriate activation energy. The resulting activation energies are listed in Table 2. Note that these values are much higher than those used by other authors (Baltanas et al., 1989; Svoboda et al., 1995; Dumesic et al., 1993). This difference is due to the explicit use of alkoxy-type intermediates as initial reactants for isomerization.
3120 Ind. Eng. Chem. Res., Vol. 36, No. 8, 1997 Table 2. Activation Energies of Isomerization branching “Brouwer”, lengthening, sum, [y/n] kJ/mol kJ/mol kJ/mol
type of isomerization secondary to secondary secondary to tertiary tertiary to secondary tertiary to tertiary secondary to tertiary secondary to secondary tertiary to secondary
y y y y n n n
22.0 30.4 76.5 71.0 2.0 8.4 56.5
110.0 110.0 100.0 100.0 110.0 110.0 100.0
132.0 140.4 176.5 171.0 112.0 118.4 156.5
An estimation of ABrAlength was derived from an article by Guisnet et al. (1991). Assuming that isomerization is the rate determining step, the rate of branching must be approximately 1 × 10-3 kmol/(m3 s). The final preexponential factor of isomerization was obtained from “tuning or fitting” it in factors of 10. A plausible conversion for a 2.0 wt % mordenite catalyst at standard conditions (240 °C, H2/n-C6 ) 28, WHSV 5.94, atmospheric pressure) was used as the criterion. This is the only parameter that has been adjusted to fit our experiments. The same value was used for all other simulations. The resulting energy diagram of hexene isomerization over ZSM-5 is given in Figure 7. A summary of the complete parameter sets used in the model for mordenite (MOR) and ZSM-5 (MFI) is given in Tables 3-5. The micropore adsorption parameters for the olefins were taken to be similar to the corresponding paraffins. Using the Model To Calculate Gas-Phase Concentrations and Surface Coverages. The model consists of a system of differential equations describing the change in concentration of each species as a function of time. These equations have to be solved simultaneously. At steady-state conditions, this system reduces to a set of 85 algebraic equations. There are only eight critical parameters: the preexponential factors and
Figure 7. Energy (kJ/mol) scheme of hexene isomerization.
activation energies of adsorption, hydrogenation, alkoxy formation, and isomerization since the forward and reverse of these steps are constricted by thermodynamics. The applied simulation routine uses a zero finder to calculate relative surface coverages from gas-phase concentrations and rate constants. Gas-phase concentrations along the length of the plug-flow reactor are calculated applying equations for a cascade of CSTRs (continuously stirred tank reactors). Input data for these equations are gas-phase concentrations and surface coverages at z, leading to concentrations at a point somewhat further in the reactor (z + ∆z). Since external diffusion is not taken into account in the model, intrinsic kinetics determine the reaction. Other input parameters are reaction conditions, such as temperature, molar flow, composition of the feed, and bed dimensions, concentrations of sites, and other catalyst characteristics (see Table 6). Although surface coverages are used instead of concentrations, the site balance can still be accounted for. We concluded from the factor of 3 difference in liquid nitrogen (BET) and hexane pore volume that in the case of the mordenite, only one-third of the internal surface of the zeolite was available for reaction (Van de Runstraat, 1997). This
Table 3. Reaction Parameters Used in the Simulations preexponential factor, m3/(m2 s)
activation energy, kJ/mol
reaction
forward
reverse
forward
reverse
adsorption n-C6, MOR adsorption n-C6, MFI adsorption MP’s, MOR adsorption MP’s, MFI adsorption DMB’s, MOR adsorption DMB’s, MFI
14.6 14.6 5.25 × 10-4 5.25 × 10-4 3.25 × 10-3 3.25 × 10-4
1.5 × 108 1.0 × 109 1.0 × 109 1.0 × 109 1.0 × 109 1.0 × 109
10 10 10 10 10 15
81.9 92 84 86 59/63 59/63
preexponential factor, mol/(m2 s)
activation energy, kJ/mol
reaction
forward
reverse
forward
reverse
alkoxy formation (s) alkoxy formation (t) branching isomerization (s;s) branching isomerization (s;t) branching isomerization (t;t) nonbranching isomerization (s;s) nonbranching isomerization (s;t) dissociative adsorption H2 on Pt
1.5 × 108 3.4 × 108 4.1 × 103 4.1 × 103 4.1 × 103 4.1 × 103 4.1 × 103 1.0 × 10-3
2.1 × 1011 9.7 × 1011 4.1 × 103 4.1 × 103 4.1 × 103 4.1 × 103 4.1 × 103 1.7 × 109
50 37 132.0 140.4 171.0 118.4 112.0 2.4
130 120 132.0 176.5 171.0 118.4 156.5 125
Table 4. Transfer Parameters Used in the Simulations preexponential factor, mol/(m2 s) transfer step H+
or Pt n-C6 from Zeo to n)C6 from Zeo to Pt i)C6 from Zeo to Pt 2-MP from Zeo to H+ or Pt 3-MP from Zeo to H+ or Pt DMB)’s from Zeo to Pt, MOR DMB)’s from Zeo to Pt, MFI DMB’s from Zeo to H+ or Pt, MOR DMB’s from Zeo to H+ or Pt, MFI
activation energy, kJ/mol
forward
reverse
forward
reverse
2.9 × 1.2 × 1012 5.7 × 1015 1.4 × 1011 1.4 × 1011 5.5 × 1017 5.5 × 1016 1.2 × 1014 1.2 × 1012
2.9 × 1.2 × 1012 5.7 × 1015 1.4 × 1011 1.4 × 1011 5.5 × 1017 5.5 × 1016 1.2 × 1014 1.2 × 1012
23.74 23.74 40 37 35 60 60 66 66
25 25 40 37 35 60 60 66 66
108
108
Ind. Eng. Chem. Res., Vol. 36, No. 8, 1997 3121 Table 5. Hydrogenation/Dehydrogenation Parameters Used in the Simulations dehydrogenation reaction alkane n-C6
2-MP
3-MP
22-DMB 23-DMB
preexponential factor
activation energy, kJ/mol
alkene
forward, mol/(m2 s)
reverse, m3/(m2 s)
forward
reverse
1-hexene cis-2-hexene trans-2-hexene cis-3-hexene trans-3-hexene 2-methyl-1-pentene 2-methyl-2-pentene 4-methyl-cis-2-pentene 4-methyl-trans-2-pentene 4-methyl-1-pentene 3-methyl-1-pentene 3-methyl-cis-2-pentene 3-methyl-trans-2-pentene 2-ethyl-1-butene 3,3-dimethyl-1-butene 2,3-dimethyl-1-butene 2,3-dimethyl-2-butene
3.7 × 105 1.2 × 108 1.2 × 108 5.32 × 107 5.32 × 107 2.5 × 105 3.7 × 107 6.91 × 107 6.91 × 107 1.18 × 105 3.2 × 105 7.0 × 107 7.0 × 107 1.24 × 105 1.73 × 105 5.5 × 105 2.7 × 107
2.7 × 109 9.69 × 107 1.63 × 108 6.5 × 109 5.1 × 109 3.06 × 108 1.87 × 109 5.95 × 109 6.72 × 109 1.98 × 109 5.31 × 109 3.77 × 109 2.75 × 109 1.99 × 108 4.68 × 109 1.7 × 109 7.44 × 109
40.35 55.23 55.28 55.22 55.29 40.34 55.27 55.24 55.22 40.36 40.36 55.28 55.25 40.34 40.29 55.29 55.22
36.50 36 39.4 41.3 44.3 30.20 44.50 42.3 45.2 38.4 26.8 45.43 45.4 30.30 40.60 56.50 57.2
Table 6. Characteristics of the Zeolites
density, kg/m3 specific surf area, m2/kg porosity, m3/m3 concn of Pt, mol/m2 no. of atoms in a Pt cluster concn of active acid sites, mol/m2 concn of micropore sites, mol/m2
2.0 wt % Pt/HMOR
0.5 wt % Pt/MFI
1368 1.95 × 105 0.26 1.75 × 10-7 5 1.88 × 10-6 2.79 × 10-6
1368 6.55 × 105 0.26 3.91 × 10-8 5 4.38 × 10-7 2.11 × 10-6
conclusion was supported by the work of Stockenhuber et al. (1995). The gas-phase concentrations of n-hexane along the reactor bed are used to calculate the conversions. TOFs (turnover frequencies) are also calculated and are defined as the number of moles of n-hexane converted per acid site per hour. In the case of mordenite, all sites are taken into account for calculating the TOF, as was done for the experimental data. Selectivities for the different products are calculated from their respective gas-phase concentrations. Nitrogen is used as the diluting agent when necessary. Comparison with Experiments. The standard conditions in the simulations were the same as we had used in our experiments (Van de Runstraat, 1997; Van de Runstraat et al., 1997). The simulation data can therefore be directly compared to the experimental data. This means a temperature of 513 K, a hydrogen partial pressure of 966 mbar, and a n-hexane partial pressure of 33.8 mbar (H2/n-C6 ) 28). The catalyst loading was 200 mg, while the total molar flow (including hydrogen, excluding nitrogen) was 1.12 × 10-4 mol/s. The apparent activation energy was simulated and measured between 493 and 533 K. Two extra points at 553 and 573 K were also calculated to investigate the linearity of the Arrhenius activation energy. The order of reaction in hydrogen at a given temperature was calculated by simulating the reaction at different hydrogen pressures. The applied partial pressure range was 677-966 mbar at a fixed n-hexane partial pressure of 33.8 mbar. The order of reaction in n-hexane at a given temperature was calculated by simulating the reaction in a n-hexane partial pressure range of 19.2-42.3 mbar at a fixed hydrogen partial pressure of 846 mbar. Simulations at high pressure were compared to the literature data reported by Guisnet et al. (1991). Standard conditions were 30 bar of total pressure, H2/n-C6 molar ratio of 9, and a WHSV of 3 h-1. Platinum
loadings of 0.5 wt % were used. The activation energies were calculated in the same temperature range as was used at atmospheric pressure. The applied partial pressure ranges to simulate the order of reaction in hydrogen at a total pressure of 33 bar were 10-30 bar of hydrogen at a fixed n-hexane partial pressure of 3 bar. The order of reaction in n-hexane was calculated by simulating the reaction in a n-hexane partial pressure range of 1.3-6 bar at a fixed hydrogen partial pressure of 27 bar. Results and Discussion Atmospheric Pressure. At low conversions (< 20%), a linear dependence of the simulated conversions on the axial position (z) in the reactor was observed. Above these conversions (for example, in the case of the reaction on ZSM-5 at 260 °C), nonlinear behavior is observed. A decrease in surface coverages and activity (TOF) as a function of position z is observed. In the case of very high activities (leading to reactor outlet conversions above 20%), they decrease linearly with z. In Table 7, the simulated activities of mordenite and ZSM-5 are given at three temperatures. The experimental values are given in parentheses. The surface coverages of n-alkoxy species and n-hexane on a micropore site are also included in the table. The simulated apparent activation energies are 111 and 114 kJ/ mol for ZSM-5 and mordenite, respectively. These are approximately 10% higher than the experimental values. The simulated activities per acid site (TOF) are close to one-half of the values obtained experimentally. It was already mentioned that the preexponential factor of the isomerization steps was adjusted to obtain a reasonable conversion at standard conditions. The values for these preexponential factors were underestimated by a factor of 2. The TOF of the ZSM-5 catalyst was experimentally and theoretically a factor of 5 larger than that of the mordenite catalyst. This stems mainly from the correction for the one-third usage of the mordenite acid sites. The noncorrected difference is a factor of 2. The coverage of acid sites with reactive intermediates is hardly different. The lower apparent activation energy on the ZSM-5 catalyst is responsible for the small difference in reaction rate. In Figure 8, this is schematically shown for an almost empty zeolite. The higher the adsorption enthalpy, the deeper the well in which the reactant molecule falls. When the true activation energy from the adsorbed state is the same
3122 Ind. Eng. Chem. Res., Vol. 36, No. 8, 1997 Table 7. Simulated Surface Coverages at Standard Conditions 2.0 wt % Pt/H-mordenite
0.5 wt % Pt/HZSM-5
T, K
θn-hexane
θn-alkoxy
TOF, h-1
θn-hexane
θn-alkoxy
TOF, h-1
493 513 533
0.77 0.62 0.44
0.98 0.91 0.73
1.41 (3.5) 4.54 (9.7) 11.32 (24.9)
0.86 0.72 0.53
0.98 0.90 0.72
7.45 (20.3) 23.83 (54.0) 57.13 (113.6)
Table 8. MFI Data as a Function of Adsorption Enthalpy at 240 °C at 240 °C
-∆Hads, kJ/mol
θn-hexane
68 78 82 88 110
0.088 0.50 0.72 0.91 0.97
θn-alkoxy
Eact,app, kJ/mol
TOF, h-1
TOFnorm, h-1
0.36 0.78 0.90 0.98 0.98
60 106 111 127 129
9.9 20.7 23.6 25.7 25.8
27.5 26.5 26.2 26.2 26.3
activation energy was again a constant (approximately 130 kJ/mol). One concludes that not only the adsorption enthalpy but also the adsorption entropy (preexponential factor) is very important in determining surface coverages and activities. Since a compensation effect exists between adsorption enthalpy and entropy of real zeolites (Eder, 1996), these parameters can never be decoupled experimentally to check the conclusion from the simulations. Orders of Reaction. In order to understand the simulated and measured orders of reaction, two extreme situations can be distinguished. (1) At low pressures and high temperatures, when the micropore coverage is low, the gas phase and micropore are at equilibrium. The rate expression for isomerization then becomes
KadsKdehydrKprot R ) kisoθn-alkoxy ) kiso Figure 8. Scheme of apparent and true activation energy (θ ≈ 0). A, reactant; B, product. Table 9. Orders of Reaction at Atmospheric Pressure mordenite sims expt
ZSM-5
n-C6 order
H2 order
n-C6 order
H2 order
0.19 0.13
-0.12 ∼0
0.21 0.51
-0.13 -0.25
for both zeolites, the apparent activation energy will be lower. Generally, when the rate-determining step of a reaction is a surface reaction, the relation between the activation energies is given by (Van Santen and Niemantsverdriet, 1995)
Eact,app ) Eact,true + n∆Hads
(3)
Since the values of the reaction orders are low (see Table 9), the actual difference in apparent activation energy is only 3 kJ/mol between the MFI and MOR catalysts. However, this can account for the factor of 2 difference in activity per acid site. Influence of Adsorption Enthalpy and Entropy Individually. When normalized TOFs are calculated (the activity per acid site covered by a n-alkoxy species) from model simulations using different adsorption enthalpies but the same zeolite, we found them to be independent of the adsorption enthalpy (Table 8). Therefore, a relation between apparent TOF and nalkoxy coverage exists. When in these simulations the acid sites are fully covered with n-alkoxy species (at an adsorption enthalpy of 85 kJ/mol using MFI data), the apparent TOF and the apparent activation energy reach a plateau. The normalized activation energy or true activation energy of isomerization, calculated from the Arrhenius plot using normalized TOFs, is 132 kJ/mol. We found that a decrease in adsorption entropy leads to lower coverages. In the case of n-alkoxy coverage, this leads to an increase in the normalized TOF and a decrease in apparent activation energy. The normalized
( ) ( ) pn-C6 pH2
1 + KadsKdehydrKprot
pn-C6
(4)
p H2
The absolute values of the orders in hexane and hydrogen are equal and between 0 and 1. (2) At high pressures and low temperatures, especially for long-chain molecules, the micropores may be completely filled. In this case, the other extreme situation is obtained. Now an absence of equilibrium between gas-phase and micropore sorbates composition may arise:
θn-hexane ≈ R ) kisoθn-alkoxy ) kisoKprotKdehydr θH2 (Kadspn-C6)0
kisoKprotKdehydr
pH2
(5)
This leads to zero order in the hydrocarbon and an order of -1 in hydrogen. These values are indeed reported at high pressures and longer alkanes (Froment, 1987). At lower pressures, a positive order in n-hexane and larger than -1 order in hydrogen is found (Guisnet et al., 1991). Table 9 summarizes the simulated and measured orders of reaction at atmospheric pressure. They are found to depend strongly on the reaction conditions. Extreme situation 1 (eq 4) is found in the experiments and is well reproduced by our model. A discussion to explain this feature is given in the subparagraph about the orders of reaction simulated at high pressure. High Pressure. Activity and Activation Energy. In Table 10, the results of the high-pressure simulations at three temperatures are given. A lower TOF compared to atmospheric pressure was observed for both zeolites (their relative values are given in parentheses). Our simulations for mordenite showed that the higher activity at atmospheric pressure was not due to a difference in platinum loading since the activity of a 0.5 wt % loaded sample was also higher than at elevated
Ind. Eng. Chem. Res., Vol. 36, No. 8, 1997 3123 Table 10. Simulated Activities and Surface Coverages at 30 bar 0.5 wt % Pt/H-mordenite T, K θn-hexane θn-alkoxy 493 513 533
0.997 0.992 0.985
0.78 0.67 0.56
0.5 wt % Pt/HZSM-5
TOF, h-1
TOF, h-1
θn-hexane θn-alkoxy
1.12 (83%) 3.29 (80%) 8.16 (83%)
0.998 0.995 0.990
0.78 0.67 0.56
5.93 (80%) 17.56 (74%) 42.59 (75%)
Table 11. Simulated Normalized TOFs (h-1) as a Function of Pressure and Temperature mordenite
ZSM-5
T, K
1 bar
30 bar
1 bar
30 bar
493 513 533 553 573 Eact,norm
1.4 5.0 15.4 44.4 117.7 129.2
1.4 5.0 15.0 40.1 78.4 119.4
7.6 26.2 79.9
7.6 26.0 76.4
128.0
126.4
Table 12. Simulated Orders of Reaction at 30 bar of Hydrogen, 3 bar of n-Hexane, and 240 °C order in n-hexane order in hydrogen n-alkoxy coverage
mordenite
ZSM-5
0.06 -0.63 0.69
0.05 -0.59 0.69
pressure. Competitive adsorption between n-hexane and n-hexene for the acid sites causes a lowered coverage of the reactive intermediates, the n-alkoxy species, at elevated pressure. This leads to the lower apparent activity per acid site. Guisnet et al. (1991) found a factor of 5 decrease in activity when using a mordenite catalyst with a Si/Al ratio of 5. This large decrease can be attributed to preferential adsorption of hexane onto the strongest acid sites, thus poisoning the most active sites first. Froment (1987) also found that the activity of a Pt/USY catalyst in isomerization of n-decane was lower at higher pressures. Because of the lower reaction orders, the simulated activation energies at high pressure are slightly lower than at atmospheric pressure. A value of 108.8 kJ/mol is calculated for mordenite; ZSM-5 simulations give 107.4 kJ/mol. Note that ZSM-5 is still the more active catalyst. When normalized TOFs were calculated, the same values are obtained at low and high pressure for ZSM-5 (Table 11). On mordenite, the low- and highpressure normalized TOFs start to deviate in the hightemperature range. This leads to a lower normalized activation energy which is due to a decreased occupancy of the micropore sites in mordenite at high temperature. Orders of Reaction. Table 12 contains the simulated high-pressure orders of reaction as well as the n-alkoxy coverage at standard conditions. A large change in orders as a function of partial pressure was observed. The order in hexane decreases to zero at increasing n-hexane pressure. The order in hydrogen became increasingly negative at increasing hydrogen pressure. These effects may also be seen as an extrapolation of the difference between low and elevated pressure orders of reaction. The values simulated resemble the “classically” found orders 0 in hydrocarbon and -1 in hydrogen (extreme 2, eq 5). This can be explained by the fact that communication between gasphase and active sites occurs via the micropore sites (Figure 2). The higher this occupancy (controlled by the free energy of adsorption), the higher the suppression of equilibration between gas-phase and micropore processes.
Guisnet et al. (1991) measured under elevated pressure on mordenite (Si/Al ) 8) an order of 0.7 in n-hexane and of -0.8 in hydrogen. These values were, however, obtained at 573 K. At this temperature, the micropore coverage decreases, and hence, an approximate equilibrium of gas-phase and micropore olefin and paraffin becomes possible (extreme 1, eq 4). The order in hexane will increase and become equal to the absolute value of the order in hydrogen. Conclusions Simulations using activation energies of approximately 130 kJ/mol for the elementary rate constant of isomerization with respect to alkoxy species reproduce experimental data quite well. This is consistent with the picture that in zeolites the reaction involves alkoxy species as ground-state intermediates and carbenium ion-like transition states. Also the experimentally found difference between the absolute values of the orders of reaction in hydrogen and hexane are reproduced. This is due to explicit consideration of transfer between micropore and acid and metal sites. Moreover, the model predicts the “classical” orders of 0 in the hydrocarbon and -1 in hydrogen at elevated pressure when micropore coverage of a zeolite has become nearly 1. Even at temperatures as high as 513 K, the zeolite micropores remain almost completely filled. Under elevated pressure, competitive adsorption of n-hexane and n-hexenes on the acid sites results in lower alkoxy coverages and lower turnover frequencies. The enhanced activity per acid site of ZSM-5 relative to mordenite is due to the higher adsorption enthalpy for n-hexane in ZSM-5, resulting in a lower activation energy. This effect is not very pronounced since the order of reaction in hexane is low. Both adsorption enthalpy and entropy play a decisive role in determining the catalyst activity. The differences in free energy of adsorption of different zeolites appear to have the largest effect on the relative orders of the reaction in hexane and hydrogen. Acknowledgment The work described in this paper was funded by the Dutch Organization for Scientific Research (NWO) through its Foundation for Chemistry (SON). Nomenclature 2-MP ) 2-methylpentane 22-DMB ) 2,2-dimethylbutane 23-DMB ) 2,3-dimethylbutane 3-MP ) 3-methylpentane Aads ) preexponential factor of adsorption ABr ) preexponential factor of monomolecular isomerization Adec ) preexponential factor of alkoxy decomposition Ades ) preexponential factor of desorption Aform ) preexponential factor of alkoxy formation Alength ) preexponential factor of C-O bond lengthening (ads) ) adsorbed on micropore site CA ) concentration of component A CSTR ) continuously stirred tank reactor DMB’s ) dimethylbutanes DMB)’s ) dimethylbutenes Eact,ads ) activation energy of adsorption Eact,app ) apparent activation energy Eact,Br ) activation energy of monomolecular isomerization Eact,dec ) activation energy of alkoxy decomposition Eact,des ) activation energy of desorption Eact,form ) activation energy of alkoxy formation Eact,length ) activation energy of C-O bond lengthening
3124 Ind. Eng. Chem. Res., Vol. 36, No. 8, 1997 Eact,norm ) activation energy calculated using normalized TOFs Eact,true ) true activation energy relative to the adsorbed state (g) ) gas phase H+ ) acid site H2 ) hydrogen ∆Hads ) adsorption enthalpy i)C6 ) iso-hexenes k1 ) rate constant of forward reaction 1 k-1 ) rate constant of reverse reaction 1 kiso ) rate constant of isomerization Kads ) equilibrium constant of adsorption Kdehydr ) equilibrium constant of dehydrogenation Kprot ) equilibrium constant of protonation or alkoxy formation MFI ) ZSM-5 MOR ) mordenite MP’s ) methylpentanes n ) order of reaction in the reactant (n-hexane) n-C6 ) n-hexane n)C6 ) n-hexenes p ) partial pressure PFR ) plug-flow reactor Pt ) platinum of platinum site R ) rate of reaction s ) secondary t ) tertiary t ) time TOF ) turnover frequency WHSV ) weight hourly space velocity Zeo ) micropore site Greek Symbols θA ) fractional surface coverage of component A θn-hexane ) fractional coverage of micropore sites by nhexane θn-alkoxy ) fractional n-alkoxy coverage of acid sites
Literature Cited API 44 Tables. American Petroleum Institute Research Project; Thermodynamic Research Center, Texas A&M University: 1968; Vols. V and VI. Baltanas, M. A.; Van Raemdonck, K. K.; Froment, G. F.; Mohedas, S. R. Fundamental Kinetic Modelling of Hydro-isomerization and Hydrocracking on Noble-metal-loaded Faujasites. Ind. Eng. Chem. Res. 1989, 28, 899. Blum, Z.; Hyde, S. T.; Ninham, B. W. Adsorption in Zeolites, Selfenergy and Gaussian Curvature. J. Phys. Chem. 1993, 97, 661. Brehm, A.; Busch, V.; Wilken, J. Kinetics of Hydrogenation of 1-Olefins Having Intermediate Chain Lengths. Chem.-Ing.-Tech. 1993, 65 (5), 573. Brouwer, D. M. Reactions of Alkylcarbenium Ions in Relation to Isomerization and Cracking of Hydrocarbons. In Chemistry and Chemical Engineering of Catalytic Processes; Prins, R., Schuit, G. C. A., Eds.; Sijthoff & Noordhoff: Amsterdam, 1980; p 137. Campbell, C. T.; Sun, Y.-K.; Weinberg, W. H. Trends in Preexponential Factors and Activation Energies in Dehydrogenation and Dissociation of Adsorbed Species. Chem. Phys. Lett. 1991, 179 (1/2), 53. Cerveny, L.; Ruzicka, V. Investigation of the Hydrogenation in the Liquid Phase VI: Influence of the Reactivity of the Olefin in the Catalytic Hydrogenation on Platinum Catalysts. Collect. Czech. Chem Commun. 1969, 34, 1570. Cerveny, L.; Ruzicka, V. Solvent and Structure Effects in Hydrogenation. In Advances in Catalysis; Eley, D. D., Pines, H., Weisz, P. B., Eds.; Academic: New York, 1981; Vol. 30, p 335. Cerveny, L; Nevrkla, V.; Ruzicka, V. Hydrogenation of Olefinic Substrates in the Absence of a Solvent. Collect. Czech. Chem. Commun. 1977, 42, 2890. Chevalier, F.; Guinet, M.; Maurel, R. Tracer Studies of the Isomerization of of Paraffins on Bifunctional Catalysts. In Proceedings of the 6th International Congress of Catalysis; Bond, G. C., Wells, P. B., Tomkins, F. C., Eds.; The Chemical Society: London, 1977; p 478.
Choudhary, V. R.; Akolar, D. B. Shuttlecock-shuttlebox Model for Shape Selectivity of Medium-pore Zeolites in Sorption and Diffusion. J. Catal. 1989, 116, 130. Derouane, E. G. The Energetics of Sorption by Molecular Sieves: Surface Curvature Effects. Chem. Phys. Lett. 1987, 142 (3/4), 200. Derouane, E. G.; Andre, J.-M.; Lucas, A. A. Surface Curvature Effects in Physisorption and Catalysis by Microporous Solids and Molecular Sieves. J. Catal. 1988, 110, 58. Donnet, J.-B.; Qin, R.-Y.; Wang, M.-J. J. A New Approach for Estimating the Molecular Areas of Linear Hydrocarbons and their Derivatives. Collet. Interf. Sci. 1992, 153 (2), 572. Dumesic, J. A.; Rudd, D. F.; Aparicio, L. M.; Rekoske, J. E.; Trevin˜o, A. A. The Microkinetics of Heterogeneous Catalysis; ACS Professional Reference Book; American Chemical Society: Washington, DC, 1993. Eder, F. A. Thermodynamics and Siting of Alkane Sorption in Molecular Sieves. Thesis, Twente University, 1996. Ertl, E. Surface Science and Catalysis on Metals. In Chemistry and Chemical Engineering of Catalytic Processes; Prins, R., Schuit, G. C. A., Eds.; Sijthoff and Noordhoff: Alphen aan den Rijn, 1980; p 278. Froment, G. F. Model Discrimination and Parameter Estimation in Heterogeneous Catalysis. AIChE J. 1975, 21 (6), 1041. Froment, G. F. Kinetics of the Hydroisomerization and Hydrocracking of Paraffins on a Platinum Containing Bifunctional Y-zeolite. Catal. Today 1987, 1, 455 Guisnet, M.; Fouche, V.; Belloum, M.; Bournonville, J. P.; Travers, C. Isomerization of n-Hexane on Platinum Dealuminated Mordenite Catalysts, II Kinetic Study. Appl. Catal. 1991, 71, 295. Harlfinger, R.; Hoppach, D.; Quachik, U.; Quitsch, K. Adsorption of C4 Hydrocarbons on X-zeolites Containing Li+, Na+, K+, Rb+ and Cs+ Cations. Zeolites 1983, 3, 123. Heink, W.; Ka¨rger, J.; Pfeifer, H.; Datema, K. P.; Nowak, A. K. High-temperature Pulsed Field Gradient Nuclear Magnetic Resonance Self-diffusion Measurements of n-alkanes in MFItype Zeolites. J. Chem. Soc., Faraday Trans. 1992, 88 (23), 3505. Horiuti, I.; Polanyi, M. Exchange Reactions of Hydrogen on Metallic Catalysts. Trans. Faraday Soc. 1934, 30, 1164. Kazansky, V. B. The Mechanism of Proton Transfer and the Nature of Protonated Intermediates in Heterogeneous Acid Catalysis. In Acidity and Basicity of Solids, Theory, Assessment and Utility; Fraissard, J., Petrakis, L., Eds.; Kluwer Acadamic: Dordrecht, 1994; p 335. Kazansky, V. B.; Frash, M. V.; Van Santen, R. A. Quantum Chemical Study of the Isobutane Cracking on Zeolites. Appl. Catal. 1996, 146, 225. Lechert, H.; Schweitzer, W. Gas Chromatographic Sorption Studies of Hydrocarbons in Pentasils with Different Si/Al Ratios. In Proceedings of the International Conference on Zeolites, Reno; Olson, D., Bisio, A., Eds.; Guildford-Butterworth: New York, 1983; p 210. Leffler, J. E.; Grunwald, E. Rates and Equilibria of Organic Reactions; Dover Books on Physics and Chemistry; Dover: New York, 1963. Maurice, V.; Minot, C. A Theoretical Investigation of the Adsorption Modes of Ethene, 1,3 Butadiene, 1-Butene and cis- and trans-2-Butenes on the Unreconstructed (110) Surface of Platinum. Langmuir 1989, 5, 734. Meier, W. M.; Olson, D. H.; Baerlocher, Ch. Atlas of Zeolite Structure Types, 4th revised ed.; Elsevier: London, 1996. Norton, P. R.; Davies, J. A.; Jackman, T. E. Absolute and Isosteric Heat of Adsorption of Deuterium on Pt(111) by Nuclear Micro Analysis. Surf. Sci. 1982, 121, 103. Post, M. F. M. Diffusion in Zeolite Molecular Sieves. In Stud. Surf. Sci. Catal.; Van Bekkum, H., Flanigen, E. M., Jansen, J. C., Eds.; Elsevier: Amsterdam, 1991; Vol. 58, p 391. Prinz, D.; Riekert, L. Observation of Rates of Sorption and Diffusion in Zeolite Crystals at Constant Temperature and Pressure. Ber. Bunsenges. Phys. Chem. 1986, 90, 413. Senchenya, I. N.; Kazansky, V. B. Quantum Chemical Studies of Ethylene Interaction with Zeolite OH-Groups. Catal. Lett. 1991, 8, 317. Senchenya, I. N.; Chuvilkin, N. D.; Kazansky, V. B. Comparative Quantum-chemical Analysis of the Electronic Structure of the Ethyl Fragment in Ethylsulfuric Acid and in an Ethoxy Structure on the Surface of High-silicon Zeolite. Kinet. Catal. 1985, 26 (5), 926.
Ind. Eng. Chem. Res., Vol. 36, No. 8, 1997 3125 Sie, T. Evidence for the Protonated Cyclopropane Mechanism from Hydrocracking/ Hydroisomerization Experiments. Ind. Eng. Chem. Res. 1993, 32, 403. Smit, B.; Maesen, T. L. M. Commensurate ‘Freezing’ of Alkanes in the Channels of a Zeolite. Nature 1995, 374, 42. Spivey, J. J.; Bryant, P. A. Hydro-isomerization of n-C5 and n-C6 Mixtures on Zeolite Catalysts. Ind. Eng. Chem. Process. Des. Dev. 1982, 21, 750. Stach, H.; Lohse, U.; Thamm, H.; Schirmer, W. Adsorption Equilibria of Hydrocarbons on Highly Dealuminated Zeolites. Zeolites 1986, 6, 74. Stockenhuber, M.; Eder, F.; Lercher, J. A. Calorimetric and IR Spectroscopic Study of the Sorption of Light Alkanes on the Hydrogen Forms of ZSM-5 and Mordenite. In Zeolites: A Refined Tool for Designing Catalytic Sites; Bonneviot, L., Kaliaguine, S., Eds.; Elsevier Science BV: Amsterdam, 1995; p 495 Svoboda, G. D.; Vynckier, E.; Debrabandere, B.; Froment, G. F. Single-Event Rate Parameters for Paraffin Hydrocracking on a Pt/US-Y Zeolite. Ind. Eng. Chem. Res. 1995, 34, 3793. Taft, R. W. Separation of Polar, Steric and Resonance Effects in Reactivity. In Steric Effects in Organic Chemistry; Newman, M. S., Ed.; John Wiley and Sons: Chichester, 1956; p 556. Van de Runstraat, A. Adsorption Effects in Acid Catalysis by Zeolites. Thesis, Eindhoven University of Technology, 1997. Van de Runstraat, A.; Stobbelaar, P. J.; Van Grondelle, J.; Anderson, B. G.; Van IJzendoorn, L. J.; Van Santen, R. A. Influence of Zeolite Pore Structure on Catalytic Reactivity. In Stud. Surf. Sci. Catal. 105: Progress in Zeolite and Microporous Materials; Chon, H., Ihm, S.-K., Uh, Y. S., Eds.; Elsevier Science: Amsterdam, 1996; p 1253. Van de Runstraat, A.; Van Grondelle, J.; Van Santen, R. A. Hydroisomerization of n-hexane over platinum loaded high-silica zeolites. J. Catal. 1997, submitted.
Van Santen, R. A.; Kramer, G. J. Reactivity Theory of Zeolitic Brønsted Acidic Sites. Chem. Rev. 1995, 95 (3), 637-660. Van Santen, R. A.; Niemantsverdriet, J. W. Chemical Kinetics and Catalysis; Plenum: New York, 1995. Van Well, W. J. M.; Wolthuizen, J. P.; Van Hooff, J. H. C.; Van Santen, R. A. Unpublished results. Weisz, P. B. Polyfunctional Heterogeneous Catalysis. In Advances in Catalysis and Related Subjects; Eley, D. D., Selwood, P. W., Weisz, P. B., Eds.; Academic: New York, 1962; p 137. Weitkamp, J. New Evidence for a Protonated Cyclopropane Mechanism in Catalytic Isomerization of n-Alkanes. In Stud. Surf. Sci. Catal. 7: New Horizons in Catalysis; Seiyama, T., Tanabe, K., Eds.; Elsevier Science BV: Amsterdam, 1981; p 1404. Zhdanov, V. P.; Pavlı´cek, J.; Knor, Z. Preexponential Factors for Elementary Surface Processes. Catal. Rev.-Sci. Eng. 1988, 30 (4), 501 Zicovich-Wilson, C. M.; Corma, A.; Viruela, P. Electronic Confinement of Molecules in Microscopic Pores. A New Concept which Contributes to Explain the Catalytic Activity of Zeolites. J. Phys. Chem. 1994, 98, 10863.
Received for review October 17, 1996 Revised manuscript received February 10, 1997 Accepted March 4, 1997X IE960661Y
X Abstract published in Advance ACS Abstracts, June 15, 1997.
