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Ind. Eng. Chem. Res. 1998, 37, 4560-4569
Catalysts for the Isomerization of C7 Paraffins Jerry F. Kriz,* Tim D. Pope, Maria Stanciulescu, and Jacques Monnier CANMET Energy Technology Centre, Natural Resources Canada, 1 Haanel Drive, Nepean, Ontario K1A 1M1, Canada
Catalysts having similar chemical compositions were tested for the isomerization of C7 paraffins, mainly branching of n-heptane. The principal objective was to maximize isomerization at minimal hydrocracking levels. Catalysts were prepared using commercially available zeolites with some modifications. Metals (mainly platinum) were incorporated in the conventional way (dispersed metal) but also as organometallic complexes. Initial activity tests used a stirred batch autoclave while time on stream tests used an automated continuous flow tubular microreactor. Products were analyzed by gas chromatography. Model compounds included other C7 hydrocarbons such as 2,4-dimethylpentane, methylcyclohexane, and toluene. A prototype catalyst provided 66% n-heptane isomerization with only 4% cracking per pass at high space velocities (LHSV ) 5). Its selectivity was similar to that of a highly active commercial alumina-based catalyst while showing an improved tolerance for aromatic hydrocarbons and a longer catalyst life. Simple kinetic models were examined and used to simulate the sequence of events on the catalyst surface. Introduction
Experimental and Analytical Procedures
In the petroleum industry, C5 and C6 straight-chain paraffins are typically used to obtain their branched isomers in catalytic isomerization units. The branched isomers are much more desirable in gasolines (higher octane number) than their straight-chain counterparts. Thus, catalytic isomerization provides a quality gasoline blending stock. Paraffins larger than C6 are present in streams used for catalytic reforming. Although reformates are highoctane products, they contain over 60% aromatics. In light of concerns about the availability of “clean fuels” and the toxicity of aromatic components such as benzene, reformulated gasolines will likely contain less reformate. The challenge faced by refiners would then be to produce a high octane gasoline containing less reformate. One potential solution is to isomerize the straightchain paraffins separately without forming aromatics. Further, the possible introduction of feedstocks from alternative sources might lead to an excessive proportion of straight-chain paraffins larger than C6 which could be preferably upgraded by isomerization. For these applications, we studied catalyst formulations based on X and Y zeolites of various pore size and Si/Al ratio, having Pt or other metals loaded (about 1%) by impregnation, by exchange, and as organometallic complexes. Such formulations were then screened using two different reactor systems and compared with an alumina-based commercial sample. One aspect of isomerization this study addresses is the extent of cracking. When cuts heavier than naphtha such as gas oils are hydrocracked, the isomerization increases the product octane rating and is thus beneficial. On the other hand, hydrocracking a light naphtha cut would decrease the gasoline yield and consume more hydrogen, neither of which would add to this concept of isomerization.
A high-pressure continuous-flow hydroprocessing system (model 8800 Micropilot Plant manufactured by AE/ CDS Autoclave Inc.), fully automated, was capable of nonstop operation for extended periods. A 10-mL (30.5 cm × 0.635 cm) tubular reactor placed in an enclosure with heating blocks was operated in a fixed-bed catalyst arrangement and an upflow configuration. A matrix of conditions was applied for each catalyst load. Each set of conditions was normally maintained for at least 24 h to ensure steady-state operation for the set, but 12-h intervals were sufficient for high space velocities. The “base” set was repeated to check for catalyst stability. A 300-mL magna-drive stirred autoclave, designed for up to 2000 psig and 300 °C, had a convenient sampling arrangement and good temperature control which allowed reproducible measurements of initial catalyst performance. Typically 100 mL of liquid hydrocarbons and 10 g of catalyst were charged and then pressurized with hydrogen to 500 psig (3.5 MPa). The temperature was raised at a fixed rate to a predetermined level. Six liquid samples were taken at regular intervals during a 2-h test. The final mixture (gas and liquid) was analyzed after cooldown. Two different gas chromatography systems were used to gain system-independent feedstock-product data. A Hewlett-Packard model 5890 provided a complete analysis of mixtures of hydrocarbons smaller than C6 in the product gas. A Perkin-Elmer Autosystem GC with Turbochrom Workstation determined all liquid product hydrocarbons, larger than C4. The work consisted of catalyst preparation, batch, and continuous-flow reactor catalyst tests using n-heptane and mixtures. The microreactor (continuous-flow) tests used hydrogen pressures within 200-1000 psig, liquid feed space velocities of 1-10 h-1, and hydrogen/liquid feed ratios of 500-5000 scf/bbl. Depending on the type of catalyst tested, the temperature was raised (adjusted) to a level at which only a small extent of cracking occurred. This was done to maximize isomerization at the threshold of cracking and to prevent overcracking
10.1021/ie9803366 CCC: $15.00 Published 1998 by the American Chemical Society Published on Web 11/10/1998
Ind. Eng. Chem. Res., Vol. 37, No. 12, 1998 4561 Table 1. Catalyst Identification support
precursor
nominal loading (wt %)
P1Z1 P1Z1* P2Z1 P3Z1 P4Z1 P5Z1 P/Z1
CBV-500 CBV-500 CBV-500 CBV-500 CBV-500 CBV-500 CBV-500
H2PtCl6 H2PtCl6 H2PtCl6 H2PtCl6 H2PtCl6 (NH4)2PtCl4 Pt(AcAc)2
1.0 Pt 1.0 Pt 0.5 Pt 0.2 Pt 1.5 Pt 0.5 Pt 1.0 Pt
P1Z2 P1Z3 P5Z3 P1Z4 P/Z5 PA PAZ6
NZ1
CBV-500
Ni(NO3)2
1.0 Ni
P1Z6
CZ1
CBV-500
Co(AcAc)2
1.7 Co
P1NZ6
FCZ1
CBV-500 CBV-500
1.0 Fe 1.7 Co 1.0 Ni 0.5 Pt
P2NZ6
P2NZ1 a, b, and ca
Fe(CO)5 Co2(CO)8 Ni(NO3)2 H2PtCl6
catalyst
catalyst
P1LZ6
support
precursor
nominal loading (wt %)
CBV-712 SK-500 SK-500 CBV-760 UX-400 γ-alumina LZY-84 γ-alumina LZY-84 bent. clay LZY-84 bent. clay LZY-84 bent. clay LZY-84 bent. clay
H2PtCl6 H2PtCl6 (NH4)2PtCl4 H2PtCl6 Pt(AcAc)2 (NH3)4PtCl2 (NH3)4PtCl2
1.0 Pt 1.0 Pt 1.0 Pt 1.0 Pt 1.0 Pt 0.5 Pt 0.5 Pt
(NH3)4PtCl2
1.0 Pt
Ni(NO3)2 (NH3)4PtCl2 Ni(NO3)2 (NH3)4PtCl2 La(NO3)3 (NH3)4PtCl2
3.0 Ni 1.0 Pt 3.0 Ni 0.5 Pt 1.0 La 1.0 Pt
a P2NZ1a: Ni and Pt loaded simultaneously. P2NZ1b: Ni loaded first, then catalyst dried, and then Pt loaded. P2NZ1c: Ni loaded first, then catalyst dried and calcined, and then Pt loaded.
which would be economically prohibitive. Thus, temperatures can range from 100 to 300 °C but are typically within 30 °C intervals for a particular catalyst. The autoclave tests were suitable for catalyst prescreening since autoclave conversions relate to microreactor conversions and more information is obtained per test. Each test was run at a predetermined (final) temperature and initial conditions. Liquid samples taken at regular intervals indicated the extent of reaction with time. Eventually, an optimum temperature was sought at which the extent of cracking had a certain (small but well-identifiable) value such as 2.5-5%. Catalyst performance was then compared (rated) at this temperature. Catalyst Selection and Preparation Bifunctional catalysts presently used for processes where isomerization plays an important role (such as C5-C6 isomerization, naphtha reforming, hydroisomerization of gas oils) contain platinum as the component which provides the most effective balance of metal/acid catalytic sites. Through its ability to hydrogenate, platinum is primarily deemed to protect the acid sites of the supporting material whose properties are crucial. In addition to the support’s surface acidity needed for chemisorption and skeletal rearrangement, there are a host of other functionalities such as high surface areas on which platinum is dispersed and a pore structure affecting access to reaction sites. Pore structures can be amorphous, crystalline (zeolitic), or a combination. Components providing extra acidity are often added. As mentioned, an important aspect of the present application is to limit the extent of cracking. The desirable catalytic selectivity would be such that in an optimal temperature range maximum isomerization would occur at the threshold of cracking. This feature is demonstrated in the bar charts shown later. Formulations with ingredients affecting the acid strength and concentration of sites could prove beneficial. Since metal complexes have been used1 to control metal distribution and surface morphologies, performance improvements were sought by incorporating organometallic structures into zeolites having different pore size distributions. Samples of catalysts prepared using conventional procedures and one commercial catalyst (platinum on acidified alumina C5-C6 isomerization
catalyst further referred to as PAX) were tested for comparison. Materials. Platinum(II) 2,4-pentanedionate [or Pt acetylacetonate, Pt(AcAc)2, a neutral compound with four Pt-O bonds in one plane, 49.6 wt % Pt] and ammonium tetrachloroplatinum(II) [(NH4)2PtCl4, 52.3 wt % Pt] were purchased from Colonial Metals Inc. Chloroplatinic acid [H2PtCl6, 4.76 wt % Pt in 10 wt % aqueous solution and solid PtCl4‚2HCl‚6H2O, 37.5 wt % Pt], nickel nitrate [Ni(NO3)2‚6H2O], and HPLC-grade methanol were purchased from Fisher Scientific. Tetraammineplatinum(II) chloride [(NH3)4PtCl2‚xH2O], cobalt(II) 2,4-pentanedionate [Co acetylacetonate, Co(AcAc)2], and iron pentacarbonyl [Fe(CO)5] were obtained from Aldrich. Cobalt carbonyl [Co2(CO)8] was obtained from Fluka Chimie. Lanthanum nitrate [La(NO3)3‚xH2O] and γ-alumina [13-2500, 225 m2/g] were obtained from Strem Chemicals Inc. Zeolites CBV-500-X-16 (SiO2/Al2O3 molar ratio 5.4), CBV-712-X-16 (SiO2/Al2O3 molar ratio 12), and CBV760-X-16 (SiO2/Al2O3 molar ratio 60) were obtained from PQ Corp. Zeolites SK-500 (rare-earth-doped, SiO2/Al2O3 molar ratio 4.9), UX-400 (SiO2/Al2O3 molar ratio 3.4), and LZY-84 (SiO2/Al2O3 molar ratio 5.8) were obtained from UOP. Bentonite clay (USP lab grade) was obtained from Anachemia. Preparation Procedures and Catalyst Identification. The catalysts were prepared by the introduction of metallic components to the zeolite supports in extrudate form. The zeolite supports, metallic precursors, and nominal metal loading are presented for each catalyst in Table 1. The majority of the catalysts were prepared by ion exchange/impregnation with a very dilute solution of the precursor. Loading involved repeated refluxing steps. Catalyst P1Z1* differs from P1Z1 in that ion exchange was not followed by impregnation. The catalysts with acetylacetonate precursors were impregnated from a methanol solution, and those with carbonyl precursors, from a pentane solution. The calcining step is expected to generate oxides of Pt(II) and Pt(IV), which are then reduced to Pt(0) although isolated Pt atoms have been found to be reoxidized by zeolitic protons.2 The majority of the catalysts were calcined in air for 12 h at 550 °C and reduced in flowing hydrogen for 4 h at 450 °C before testing. The catalysts based on Z6 were calcined and
4562 Ind. Eng. Chem. Res., Vol. 37, No. 12, 1998
Figure 1. n-Heptane isomerization, P1Z1 catalyst, and autoclave test at 250 °C, with profiles for straight-chain, total branched, monobranched, and dibranched isomers and cracked products
Figure 3. Effect of metals N (nickel), F (iron), C (cobalt), and P (platinum) on catalyst performance.
Figure 2. Isomerization on selected catalysts, autoclave, and temperatures indicated.
Figure 4. Effect of various supports, Z (zeolite) and A (alumina), on Pt-containing catalysts.
reduced in air and flowing hydrogen for 12 and 4 h, respectively, both at 350 °C.
can perform similarly if tested at optimal temperatures although testing within the same temperature range may not reveal their potential. Figure 3 shows the influence of different metals. The same Y-zeolite support, loading by exchange, calcining, and reduction conditions as well as operating temperatures were used. While iron and cobalt do not appear to add to the support’s functionality, nickel increased its overall activity but not selectivity, whereas platinum substantially increased its activity and selectivity. When bimetallic combinations of platinum with nickel and zinc were used (zinc not shown), these performed similarly to platinum alone provided that the Pt loading (same support) was 1 wt %. For 0.5 wt % Pt loading, nickel (more than zinc) again enhanced the overall activity rather than selectivity (if calculated at the same conversion level) as shown in Figure 3 for results at 250 and 255 °C. Figure 4 shows the effect of different supports used in similar procedures to prepare platinum-containing catalysts. The alumina support stands out as being ineffective compared with zeolites, although a very high activity can be achieved by incorporating strongly acidic components as in the case of the commercial catalyst PAX. Mixed supports did not improve the functionality
Autoclave Results More than 70 samples of different catalyst formulations or modifications were tested in the autoclave for n-heptane isomerization. Some were tested at several temperatures and others only at one. A whole spectrum of selectivities was observed, ranging from negligible to extensive isomerization/cracking ratios. Isobutane and propane were always the predominant (hydro)cracked products. Figure 1 shows data generated during a test of a prototype catalyst. It would not serve practical purposes to enumerate (graphically or otherwise) all results especially those identifying poor selectivities for some of the catalysts tested. However, general trends as well as significant effects of some modifications will be noted. Figure 2 gives observed selectivities for nine different catalysts each tested at suitable temperatures. The temperature range differs for each catalyst to arrive at comparable cracking. When the extent of cracking is small, all nine catalysts provide good isomer yields. This example was chosen to show that (different) catalysts
Ind. Eng. Chem. Res., Vol. 37, No. 12, 1998 4563 Table 2. Equilibrium Distribution of Heptane Isomers at Various Temperatures by SimSci’s PRO/II and by Experiment isomer
100 °C
250 °C
250 °Ca
300 °C
n-heptane (n-C7) 2-methylhexane (2MHX) 3-methylhexane (3MHX) 3-ethylpentane (3EPN) 2,2-dimethylpentane (22DMP) 2,3-dimethylpentane (23DMP) 2,4-dimethylpentane (24DMP) 3,3-dimethylpentane (33DMP) 2,2,3-trimethylbutane (223TMB)
7.2 22.6 15.3 1.6 22.1 9.3 9.6 7.3 5.0
12.3 25.1 22.2 2.5 11.4 10.9 7.2 5.6 2.8
14.4 23.9 25.0 1.9 9.3 9.5 7.1 6.5 2.5
14.8 25.0 23.8 2.8 9.0 11.0 6.1 5.0 2.2
a
By experiment.
Figure 5. Role of the Pt origin: P1 and P2 from Pt(4+), P5 from Pt(2+), and P/ from the Pt complex.
Figure 7. n-Heptane isomerization, P1Z1 catalyst, and autoclave equilibrium test, with profiles for the formation of individual monobranched isomers.
Figure 6. Effect of the presence of toluene and methylcyclohexane (MCH).
of the zeolite component. The incorporation of mesoporous structures (not shown) retrograded performance severely. Even the various zeolites showed a significantly different influence on the catalyst performance. Geometric (crystalline) and chemical (e.g., Si/Al ratio) structures were both varied, and both appeared to have an effect. Varying the Si/Al ratio (P1Z1/P1Z4) or having the presence of other atoms affecting acidity (PAX), was particularly consequential. Depending on the support used, the procedures for metal loading and pretreatment may influence the catalyst performance. The influence of the original compound was notable for loading platinum on zeolites as shown in Figure 5. The compounds used to load Pt were different salts and complexes as described earlier. Chloroplatinic acid containing Pt4+ is shown to provide good results. Several catalysts tested for n-heptane isomerization alone were also tested for blends with other C7 hydrocarbons. Our (model) blends had either methylcyclohexane (MCH) or toluene since the candidate hydrocarbon stream might contain a small amount of naphthenes and aromatics. The content of ringed hydrocarbons did not appear to vary under the conditions used so that their ring-opening reaction was slow; however, toluene
readily converted to MCH by hydrogenation. The impeding (inhibiting) effect of these on the n-heptane isomerization is shown in Figure 6. This effect depended on the catalyst type. The zeolite catalysts responded to the presence of methylcyclohexane in a way similar to that of the commercial PAX. However, their tolerance to toluene was much better. The continuous-flow tests provide more evidence for this behavior. Equilibrium Tests Equilibrium concentrations of all C7 saturated isomers were first computed using PRO/II with PROVISION software by Simulation Sciences Inc. Table 2 shows concentrations so identified for 100, 250, and 300 °C. Thus, at 250 °C a distribution of 50% monobranched, 35% dibranched, 12% normal, and 3% tribranched can be reached. The multibranched isomers are favored at lower temperatures, so that the equilibrium distribution at 100 °C would be 48% dibranched, 40% monobranched, 7% normal, and 5% tribranched. Attempts in reaching the equilibrium concentrations experimentally can be constrained kinetically, i.e., the reaction temperature is too low so that it would take a very long time, or the catalyst selectivity is such that the (hydro)cracking reaction would interfere. Neither of these constraints could be avoided in a practical way. The equilibrium concentrations were approached from two ends, one being n-C7 and the other 24DMP, as shown in Figures 7-10. The figures identify concentrations of individual isomers relative to each other. These autoclave reactions proceeded for up to 10 h in separate
4564 Ind. Eng. Chem. Res., Vol. 37, No. 12, 1998
Figure 8. 2,4-Dimethylpentane on P1Z1 catalyst and autoclave equilibrium test, with profiles for the formation of n-heptane and individual monobranched isomers.
mentally obtained values (using the two different isomers) was closer, within 5% of the individual values. The time-dependent isomer distribution patterns were more difficult to reproduce than the end points but were reproduced with some scatter in separate experiments for both n-C7 and 24DMP (starting isomers). Figures 7-10 indicate that the formation of 2MHX and 3MHX proceeds identically, irrespective of the original reactant. Both (monobranched) isomers are formed somewhat faster when starting with n-C7 than when starting with 24DMP. The formation of 23DMP dominates in the early stages of 24DMP reaction when 23DMP rapidly exceeds its equilibrium concentration. However, the formation of the rest of the dibranched isomers is as gradual as that when starting with n-C7. The patterns for the formation of 23DMP and 22DMP starting with n-C7 seem to cross in the later stage of the process, with 23DMP being more abundant initially. The extent of (hydro)cracking is not shown. It was difficult to measure reliably during these experiments. The extent of cracking did not exceed 12% when starting with n-C7 and 3% when starting with 24DMP. The lesser extent of 24DMP cracking was reproducible and the onset of this may have correlated with the appearance of monobranched isomers or the appearance of n-C7. Isobutane-propane represented more than 75% of cracked products found in each case, with the isobutane/n-butane ratio being more than 20. There was no evidence of methane or ethane formed but some evidence of C5, C6, and C7+ hydrocarbons formed. Microreactor Results
Figure 9. n-Heptane isomerization, P1Z1 catalyst, and autoclave equilibrium test, with profiles for the formation of individual multibranched isomers.
Figure 10. 2,4-Dimethylpentane on P1Z1 catalyst and autoclave equilibrium test, with profiles for the formation of individual multibranched isomers.
experiments at 250 °C, which produced similar mixtures of heptane isomers at the end. The (end) averages are indicated in Table 2 for comparison with the computed values. The agreement is within about 15% of the individual values. The agreement between the experi-
Microreactor tests were used to evaluate the functionality of candidate catalysts under realistic processing conditions. About 6 mL of catalyst in ground form (20-50 U.S. mesh size) was held inside the tubular reactor by quartz wool and chips. The platinum loading on catalysts chosen for these tests varied between 0.2 and 1.5 wt %. Operating conditions were pressures of 200-1000 psig (1.5-7.0 MPa), liquid space velocities of 1-10 h-1, and hydrogen flow of 500-5000 scf/bbl (80800 L/L at STP). On the basis of autoclave results, the temperature was chosen such that the set of operating conditions would not cause excessive cracking. However, occasional excessive cracking did indicate that the chosen temperature was too high. Each set of conditions was run for about 24 h while samples for analysis were collected in 4-h intervals. Steady state was typically noted after feeding about four reactor volumes of liquid. Catalyst stability (or deactivation) was checked by repeating conditions already tested. Activity points are shown in Figure 11 for catalyst P1Z1 with 1% Pt, in Figure 12 for the commercial PAX, and in Figure 13 for P3Z1 with 0.2% Pt. P1Z1 was run for more than 1000 h using various mixtures, but only one set of conditions is shown in Figure 11 to avoid confusion. It is seen that the initial activity level was maintained throughout so that deactivation was not significant. The other catalysts (P2Z1, P3Z1, and P4Z1) differed in Pt loading (0.5, 0.2 and 1.5 wt % Pt, respectively), and each batch was tested using about eight different sets of conditions as explained earlier. Selected results are shown in Figure 14. Excessive cracking where the temperature was set too high is shown in some cases. Excellent selectivity and reasonable conversions were obtained for the catalysts containing 1 and 1.5 wt % Pt. Lower Pt loading, 0.5 and
Ind. Eng. Chem. Res., Vol. 37, No. 12, 1998 4565
Figure 11. P1Z1 catalyst in continuous-flow tests feeding n-C7 or a mixture of n-C7 with MCH and operating conditions 250 °C, 500 psig, LHSV ) 1, and 5000 scf/bbl.
Figure 13. P3Z1 catalyst in continuous-flow tests feeding n-C7 or a mixture of n-C7 with toluene and operating conditions 240 °C, 500 psig, LHSV ) 1, and 5000 scf/bbl.
Figure 12. PAX catalyst in continuous-flow tests feeding n-C7 or a mixture of n-C7 with toluene and operating conditions 500 psig, LHSV ) 1, 5000 scf/bbl.
0.2 wt %, provided progressively lower conversions for which good selectivity could be maintained. Processing parameters important for optimizing and efficiency are liquid space velocity (high) and gas rate (low). The results indicate that space velocity 5 h-1 and 1000 scf/ bbl hydrogen are (comparatively) very effective in combination with 500 psig system pressure and 260 °C. It was previously noted (autoclave tests) that an important phenomenon in the process of forming branched isomers is the interference caused by the presence of other hydrocarbons. The evidence generated using the microreactor is broadly consistent with the autoclave results in terms of this interference. The activity patterns for the commercial PAX (Figure 12) indicate slow but significant deterioration with time on stream that may have been caused by traces of impurities (such as moisture) to which this catalyst tends to be sensitive. As the experiment progressed, the temperature was decreased from 105 to 100 °C, which somewhat lowered but stabilized the activity level for the next 20-h operation. At that point feed containing a mixture of 10% toluene and 90% n-heptane was introduced in place of pure n-heptane. This change caused a sharp decrease in the extent of isomerization
Figure 14. Continuous-flow tests of Pt/zeolite catalysts and various Pt loading. y axis numbers from left to right: pressure (100’s psig), LHSV, gas rate (1000’s scf/bbl), temperature (°C).
that did not improve much by increasing the temperature back to 105 °C. It should be noted that further temperature increases would cause faster deactivation
4566 Ind. Eng. Chem. Res., Vol. 37, No. 12, 1998
and more extensive cracking. When pure n-heptane was finally reintroduced, some of the previous activity was regained although lowered by further deactivation. Undoubtedly, the toluene presence strongly inhibited isomerization of n-heptane on this catalyst. Discussion 1. Contribution to the Reaction Mechanism and Kinetics. Several reaction parameters measured in the present study can be used for modeling the n-heptane reaction. These are the overall conversion, the yield of branched isomers, the yield of cracked products, and the ratio of multi-/monobranched isomers obtained. As mentioned, a technically important catalytic function is the selectivity to isomerization vs cracking. In an oversimplified approach these two reactions can proceed either simultaneously or consecutively. A (triangular) model incorporating both follows: Figure 15. Catalyst performance as simulated by the triangular model.
i–C7
ki
kic
n–C7 kc
cracked
It is evident that such a model will not be applicable when approaching isomer equilibrium concentrations since only irreversible steps are considered. However, this model can provide direction in the development of a more realistic model since the cracking reactions will be for all practical purposes irreversible. The kinetic expressions can be obtained by an “analytical” solution assuming the usual circumstances prevailing in a batch autoclave or plug-flow regimes. Analogous approaches have been used in this journal3 and elsewhere.4
xT ) xI + xC ) 1 - exp[-(ki + kc)t] xI ) xi )
ki [exp(-kict) - exp(-(ki + kc)t)] ki + kc - kic
ki x ki + kc T
also
for each rate constant,
xc )
kc x ki + kc T
(1) (2)
and xic ) xi - xI
k ) A exp(-E/RT)
(3)
The measurable conversions for isomerization, xI, and cracking, xC, are different from those associated with the individual rate constants, xi, xc, and xic. The Arrhenius parameters in (3) will depend on the chosen catalyst and can be estimated experimentally. Approximate or relative values are sufficient for a phenomenological treatment. Errors will develop because of the model limitations and the simultaneous occurrence of isomerization and cracking during measurements. Rigorous determinations would demand excessive experimental efforts and work within low conversions. Figure 4 includes data indicating how temperature affects conversions while other conditions are kept constant. For catalyst PAZ6 an Arrhenius plot with three-temperature points (averaged over multiple time equivalents as obtained during the autoclave tests) indicated ki activation energy of 135 kJ/mol. This is consistent with values obtained by Remy et al.,5 who used a more rigorous approach and catalysts very similar to some in this study but different conditions
(atmospheric pressures, low conversions at 185 °C, continuous flow). Their values for activation energies varied between 125 and 140 kJ/mol. For illustration, zeolite-based catalysts are assigned activation energies of about 125 kJ/mol for ki and associated preexponential factors (h-1) of about 12 in their order of magnitude. The cracking rate constants are assigned 170 kJ/mol (values higher than those for isomerization would be expected), and their preexponential factors, orders of magnitude of about 15. Using these values and expressions (1) and (2), conversion patterns can be simulated for a “desirable” or an “undesirable” performance. Such patterns are shown in Figure 15, where all Arrhenius parameters are identical except for the preexponential factor for ki. The difference in preexponential factors for ki (desirableundesirable case) was (arbitrarily) chosen to be 1 order of magnitude. The agreement with experiment is only in principle, indicating how the yield of isomers can be affected when overall conversions are increased either by the time of reaction (or space velocity) or by the reaction temperature. A good catalyst will isomerize selectively until high conversions are reached, whereas a poor catalyst will crack even at low conversions. In the exemplified case the desirable performance was associated with consecutive-like cracking and the undesirable performance with simultaneous-like cracking. This, however, cannot be generalized since the catalyst performance as depicted by the triangular model will also depend on the magnitude of the rate constant kic. Among the possible catalytic properties that could cause simultaneous cracking would be the rate of desorption of the (various) adsorbed species as indicated by the approach to the example shown in Figure 15. As mentioned previously, the triangular model is oversimplified mainly because it ignores the process reversibility. A more realistic model, which was used in this study to accommodate product yields as well as isomer distributions, is presented in Figure 16. Individual rate constants are expressed as relative probabilities of reaction to fit the experimental results. The aim was to simulate the reaction progress in time at optimum conditions for the chosen catalyst. The reaction progress was simulated by the Monte Carlo method, which in this case used the initial reactant molecule/ catalyst site ratio of 1000 and averaged the outcome of each turnover over 1000 sites.
Ind. Eng. Chem. Res., Vol. 37, No. 12, 1998 4567
Figure 16. Reaction model for the simulation of isomerization and cracking of heptanes over P1Z1 catalyst in the autoclave at 250 °C.
Figure 17. Simulation of experimental data (1000 molecules of n-heptane/site initially).
The molecule-site ratio is an approximation based on the initial number of heptane molecules per number of platinum atoms in the autoclave. As the number of available catalytic sites could be much smaller than the number of platinum atoms in place, this ratio could actually be much higher. However, it is assumed that the ratio will not influence the relative product distribution (statistically sampling a large number of sites). Furthermore, since the cracking reaction is irreversible, only relative isomer equilibrium concentrations can be reached. A calculation using the model data in Figure 16 would confirm equilibrium levels commensurate to those in Table 2 at 250 °C if cracking were excluded (system and hydrogen partial pressures have no effect). Both model (Figure 16) and experimental (Figure 1) data are plotted in Figure 17. Although the model can provide a reasonable fit for the experimental data, its application is not meant to prove mechanistic details. Instead, catalyst functions can be examined by altering the probability magnitudes in Figure 16 so that both experimental patterns and equilibrium are accommodated. It is evident that the experimentally obtained patterns cannot be satisfied using only one type of adsorbed species, and thus two (*a1* and *a2*) are used in the model. Only consecutive steps can account for the initial yield of isomers. When starting with n-heptane, the monobranched isomers initially exceed dibranched isomers by a large margin. Similarly, starting with 24DMP, the monobranched isomers initially exceed n-heptane by a large margin. Thus, at least two different adsorbed intermediates are indicated, with a limited direct transition between them. The reaction is indicated to proceed mainly via the formation of isomers in consecutive steps.
The catalyst performance can be strongly affected by rates of adsorption and desorption. It is obvious that a sufficient ability to adsorb the reactants will be a prerequisite for any catalyst to function. The skeletal rearrangement step is considered to be rate-determining.6 However, in the optimum temperature range the skeletal rearrangement will be activated, and the rate of desorption of the rearranged species could play a role. The adsorbed intermediate, while at the surface, prevents further turnovers from taking place. The reactivity of 24DMP was found to be greater than that of n-heptane considering the formation of the other dibranched isomers, whereas it was smaller considering the formation of monobranched isomers and cracked products. Although hydrogenolysis of linear paraffins is known to be faster than that of multibranched paraffins, the products obtained (e.g., isobutane) were not typical for hydrogenolysis. However, cracking is expected to be faster at the tertiary carbon. The difference in turnover times may be caused by both skeletal rearrangement and desorption rates. The good reactivity of 24DMP suggests that diffusion and possible steric effects were unimportant. Relatively slow desorption could also lead to more extensive cracking (lower selectivity). As isobutanepropane are the predominant products of cracking n-heptane, the first skeletal rearrangement has probably taken place before cracking occurred (true for both good and poor catalysts). The direct change from *a1* to *a2* would probably involve difficult shifts which may occur more easily via consecutive adsorption. Thus, slow desorption of adsorbed species could enhance the undesirable simultaneous cracking. Accelerating desorption (surface “cleaning”) through hydrogen transfer is probably the primary role of platinum. However, platinum can also accelerate the skeletal rearrangement associated with initial adsorption. Although this functionality may distinguish platinum from other metals, palladium has been found by Blomsma et al.6 to promote its performance. A more rapid skeletal rearrangement could enhance direct changes from *a1* to *a2*, etc., producing more multibranched isomers initially. The synergy of a variety of effects of both the support and metals probably determines the catalyst performance. The mechanism of skeletal rearrangement on the catalyst surface leading to (acid-catalyzed) isomerization and cracking has been the subject of numerous studies, building on the now classical carbenium ion mechanism.7,8 A more recent reformulation by Sie9 incorporated an intermediate cyclopropyl species previously considered in dealing with skeletal isomerization of paraffins.10,11 Sie’s theory claims a (nonclassical) carbonium ion of a cyclopropenyl ring structure (a protonated cyclopropane intermediate) to be a predecessor common to both cracking and isomerization type reactions. In effect this intermediate replaces the β-scission postulate of the classical mechanism for cracking. The theory is shown to provide an improved account for experimental results related to cracking,12 hydrocracking, and hydroisomerization.13 In the present study, the cyclopropenyl ring formation can be considered to occur in *a1* as well as in *a2*. Arguably, the direct reversible transition from *a1* to *a2* or any other secondary surface rearrangement could involve a different intermediate, so the lack or absence of the direct transition would lend the theory some support. Furthermore, since the β-scission pos-
4568 Ind. Eng. Chem. Res., Vol. 37, No. 12, 1998
tulate predicts 24DMP cracking more than n-C7, the observed opposite is also in the theory’s favor. However, there are problems in accounting for the equal distribution of the monobranched 2MHX and 3MHX and for the rapid formation of 23DMP starting from 24DMP. The primary carbons are considered less likely to participate in the cyclopropenyl ring formation than nonprimary carbons, while a side chain to the cyclopropenyl ring with at least three atoms is required before cleavage can occur. Only n-C7 adsorption can provide such a species without methyl shifts. These considerations are consistent with our results since the extent of cracking was more pronounced when starting with n-C7 than when starting with 24DMP and in the latter case the onset of cracking may have correlated with the appearance of n-C7. On the other hand, using the same considerations regarding the appearance of monobranched isomers, 3MHX ought to be favored over 2MHX when starting from n-C7, which evidently does not occur. The conversion of 24DMP to monobranched isomers requires the participation of primary carbons (in the absence of methyl shifts), in which case 2MHX ought to be favored over 3MHX. This did not occur either. Among the dibranched isomers, the formation of 23DMP was favored starting from either end. Some of this could be explained by dibranched isomers reacting in consecutive steps where the formation of 23DMP was initially favored. However, the rapid initial formation of 23DMP starting from 24DMP (in addition to the other observations above) suggests a fast methyl shift, not predicted by the cyclopropenyl ring mechanism. The extent of skeletal rearrangement per turnover can be influenced by the ease of this rearrangement relative to cracking. At the threshold of cracking, none of the studied catalysts seemed to provide extensive rearrangement per turnover or cracked products other than isobutane and propane. This indicates that the intermediate species formed upon n-C7 adsorption were relatively stable, easier to desorb or crack than further rearrange. Again, it would appear that the relative rate of desorption could account for the difference in selectivity of various catalysts used under optimum conditions. 2. Interference of Naphthenes and Aromatics. Negative influence has been observed to be caused by cyclic hydrocarbons over Pt/mordenite catalysts14,15 and has been reported for a xylene co-feed over Y zeolite.16 In this study the ringed hydrocarbons inhibited very strongly the catalyst PAX. This was noted using both autoclave (Figure 6) and continuous-flow tests (Figure 12). The ringed hydrocarbons condense at the catalyst surface more easily than paraffins. They adsorb more strongly at the acidic sites. The inhibition by ringed hydrocarbons while using the prototype zeolite catalysts was less pronounced. It can be assessed using the patterns in Figures 11 and 13. The presence of methylcyclohexane (Figure 11) decreased the extent of nheptane isomerization only marginally, and the presence of toluene (Figure 13) caused an even smaller inhibition. This finding is commensurate with the autoclave results (Figure 6), both providing evidence that the zeolite isomerization catalysts have a much improved tolerance to the presence of aromatic hydrocarbons. It should be noted that under the conditions used for isomerization aromatic hydrocarbons (alkylbenzenes) hydrogenate to form respective cycloparaffins (naphthenes). The conversion of alkylbenzenes is more rapid
than isomerization of paraffins or naphthenes. This conversion consumes hydrogen and reduces the gasoline octane rating associated with the difference between octane numbers for alkylbenzenes and naphthenes. The naphthene rings can be rearranged or opened during the process, but the ring-opening reactions proceed relatively slowly. The hydrogenation of toluene proceeded to completion rapidly whereas ring opening of methylcyclohexane was marginal compared with nheptane isomerization. Conclusion New processing and fundamental findings are revealed. The application of platinum/zeolite catalysts to alkane isomerization in unconventional hydrocarbon blends is technically feasible. With n-heptane, 66% isomerization with only 4% cracking per pass was achievable at LHSV ) 5. Our catalyst formulations showed selectivity similar to that of a commercial nonzeolite catalyst but much improved tolerance for aromatic and naphthenic hydrocarbons and a longer catalyst life. The results indicate that the inhibition by ring-containing hydrocarbons can be controlled. The content of aromatic hydrocarbons in such blends would have an impact on both economics and environmental goals, since aromatics would be hydrogenated in the process. The catalyst support properties strongly influenced the overall performance, with specific effects tied to acid site strength and concentration. The issue of selectivity is discussed in some detail, applying simple reaction models for illustration. The extent of isomerization at the threshold of cracking (achieved at optimal conditions) is the most important feature of the catalyst performance. Rates of both skeletal rearrangement and adsorption-desorption steps may play a role. Acknowledgment The authors acknowledge the contribution of Professor R. Le Van Mao of Concordia University who provided a number of catalysts used in this study. Notation A ) preexponential factor, h-1 E ) activation energy, kJ mol-1 kc, ki, kic ) first-order rate constants in the triangular model, h-1 R ) gas constant, 8.314 J mol-1 K-1 t ) reaction time, h T ) temperature, K xc, xi, xic ) fractional conversions associated with the individual steps (triangular model) xC, xI, xT ) overall conversions for cracking, isomerization, and total, respectively
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Received for review June 2, 1998 Revised manuscript received September 18, 1998 Accepted October 8, 1998 IE9803366
