Comparison of Hydroisomerization and Hydrocracking Reactions of

The activity and reaction paths for the hydroisomerization and hydrocracking of n-C8 and various branched octane molecules over Pt/H−USY and Pt/H−...
6 downloads 0 Views 105KB Size
Download PDF
2950

Ind. Eng. Chem. Res. 2004, 43, 2950-2956

Comparison of Hydroisomerization and Hydrocracking Reactions of Normal and Branched Octanes over USY and ZSM-12 Catalysts Srikant Gopal, Wenmin Zhang, and Panagiotis G. Smirniotis* Department of Chemical and Materials Engineering, University of Cincinnati, Cincinnati, Ohio 45221-0012

The activity and reaction paths for the hydroisomerization and hydrocracking of n-C8 and various branched octane molecules over Pt/H-USY and Pt/H-ZSM-12 were studied. The relative sizes of the zeolite pore opening and the kinetic diameter of the molecule, as well as the degree of branching and position of the branches, all played a role in determining the rate of conversion of the molecule. Only n-C8 reacted at a faster rate over ZSM-12 than over USY; all the other isomers reacted faster over USY. The tribranched isomers 2,2,4-TMC5 and 2,3,4-TMC5 reacted several times faster than n-C8 over ZSM-12, and the formation of bulkier 2,2,3- and 2,3,3-TMC5 isomers was suppressed to a large extent, which indicated that the reaction was occurring inside the ZSM-12 pore. Even the distribution of the monobranched isomers was slightly affected by the shape selective effects in the ZSM-12 pore. Reactions that led to the removal of a branch from both monobranched and dibranched isomers were observed to a much larger extent over ZSM-12 than USY. These data, from catalytic reactions of molecules of various sizes, provide information about the true catalytic pore size of ZSM-12, which is greater than the crystallographic pore size. Over USY, 2,3,4-TMC5 reacted three times faster than 2,2,4-TMC5, even though it had to undergo an alkyl shift before the type A β-scission step. This indicates that the reaction of 2,2,4-TMC5 is significantly slowed due to diffusion even over USY. Introduction Hydroisomerization of n-paraffins to branched paraffins is of great interest since branched paraffins inherently possess high octane numbers and are ideal substitutes for aromatics and olefins in gasoline. Bifunctional zeolite catalysts have shown very high activity in hydroisomerization of n-paraffins.1,2 Pt/H-mordenite is used commercially for isomerization of C5 and C6 alkanes, but recently, some isomerization feeds also contain nonnegligible amounts of heptane, and the yield of branched paraffins becomes impractically low when heptane or higher molecular weight paraffins are involved.3 This is because high molecular weight multibranched paraffins are much more susceptible to cracking than low molecular weight ones.4 Cracking reactions can be suppressed by using one-dimensional zeolites with a smaller pore5 since shape selectivity prevents formation of multibranched isomers that are easily cracked. However, in these catalysts the product almost exclusively consists only of monobranched isomers. In our work with ZSM-12-based catalysts, we found that, in addition to possessing excellent time-stability characteristics,6 a significantly higher yield of monobranched as well as dibranched isomers can be obtained compared to other large pore zeolite catalysts.7 This behavior is clearly due to the somewhat smaller pore size of ZSM12 relative to the other large pore zeolites. In this work, to better understand the shape-selectivity characteristics of ZSM-12 in hydroisomerization reactions, we studied the hydroisomerization kinetics of various octane isomers over a ZSM-12 catalyst and compared it with a USY catalyst having a similar acid site density. Since the USY catalyst is considered to be an “open” * To whom correspondence should be addressed. Tel: +1513-556-1474. Fax: +1-513-556-3473. E-mail: panagiotis. [email protected].

structure and is not expected to be shape selective, the comparison would enable identification of the reactions that are suppressed inside the pores of ZSM-12 resulting in the higher isomer selectivity. The purpose of this work is also to investigate the effect of molecular sizes on the reaction rates and pathways over the catalysts for the hydroconversion of individual branched octanes. Experimental Section Catalyst Preparation and Characterization. The ZSM-12 (ZSM-12-31) used in this study was synthesized hydrothermally using tetraethylammonium hydroxide ((TEA)OH) as the template. The procedure used for the synthesis of ZSM-12 has been described in detail elsewhere.8 The synthesized zeolite was then calcined in air at 520 °C for 4 h to burn the occluded template. The USY sample (CBV-760) was obtained from Zeolyst International. The crystal sizes of the zeolites, from SEM observations, were found to be about 0.6 µm for USY and 0.7-0.8 µm for ZSM-12. Ammonium forms of the zeolites were obtained by cation exchange with 2 N NH4Cl solution at 90 °C for 4 h. Finally, the zeolites were converted to their protonated forms by calcination in air at 500 °C for 1 h. Platinum was loaded on the protonated form of the catalysts by wet impregnation using H2PtCl6 (Aldrich). A final Pt loading of 0.5 wt % was achieved. The impregnated catalysts were dried overnight in an oven at 120 °C. The catalysts were activated in situ before starting the reaction in flowing oxygen for 1 h at 450 °C, followed by purging with helium for 15 min, and reduction at 450 °C in hydrogen flow to disperse the metal. Crystallinity of the zeolites was estimated by X-ray diffraction on the basis of the heights of the main crystallographic peaks of each zeolite and comparison with a reference sample. Sorption capacities of n-C8 and 2,2,4-trimethylpentane on the zeolites were determined using a dynamic adsorp-

10.1021/ie030472x CCC: $27.50 © 2004 American Chemical Society Published on Web 12/24/2003

Ind. Eng. Chem. Res., Vol. 43, No. 12, 2004 2951 Table 1. Properties of Zeolite Samples Used in This Study

zeolite sample

source

bulk Si/Al ratio

USY-30 ZSM-12-31

Zeolyst International synthesized

30a 31

no. of acid sites,b mmol/g

XRD crystallinity, %

0.40 0.49

72 100

dynamic sorption capacity,c wt % n-C8 2,2,4-TMC5 6.0 (47) 5.2

5.3 (50) 1.6

Pt dispersion, % 95 79

a Nominal Si/Al ratio reported by the company. b The number of acid sites was determined by NH -STPD using the protonated form of 3 the zeolite, before platinum was loaded. c Equilibrium capacity from dynamic sorption experiments carried out at 100 °C. The numbers shown in parentheses for USY-30 are the percent sorption capacity with respect to the parent NaY sample.

tion method. The measurements were made using a Perkin-Elmer Pyris 1 TGA system; adsorption was carried out at 100 °C. Measurement of the zeolite surface acidity was carried out using the NH3-stepwise temperature-programmed desorption (NH3-STPD) technique.9 The platinum dispersion on the catalysts was determined by pulse hydrogen chemisorption using a Micromeritics Autochem 2910 automated catalyst characterization system.7 The physicochemical properties of the ZSM-12 and USY catalyst are given in Table 1. Catalytic Experiments. n-Octane, 2-methylheptane (2-MC7), 3-methylheptane (3-MC7), 4-methylheptane (4MC7), 2,2-dimethylhexane (2,2-DMC6), 2,4-dimethylhexane (2,4-DMC6), 2,2,4-trimethylpentane (2,2,4-TMC5), and 2,3,4-trimethylpentane (2,3,4-TMC5) (all from Aldrich) were used as the probe molecules. The impurities were mainly isomers; the exact amount of each impurity was determined and was accounted for when calculating the product selectivities. The experiments were carried in a flow reactor system incorporating a 1/4 in. o.d. stainless steel reactor, with the catalyst loaded on top of a glass wool plug. For the catalytic experiments, H2 was provided from the gas cylinder to the reactor while the pressure regulation was achieved with a backpressure regulator placed at the exit of the reactor tube. The feed was introduced into the reactor at a predetermined flow rate through a special septum injection port into the heated line using a liquid infusion pump (ColeParmer). For the n-C8 hydroconversion experiments, the reaction was carried out at a pressure of 100 psig (6.9 bar), WHSV of 7 h-1, and the H2/n-C8 (molar) ratio was 16; the conversion was varied by varying the reaction temperature. The kinetic experiments were carried out at a temperature of 220 °C, 100 psig pressure, and the H2/C8 (molar) ratio was 16; the conversion was kept below 10% by changing the WHSV, which was varied by changing both the catalyst amount and feed flow rates. The data were collected at 1 h on stream. Identification of the products was accomplished using a gas chromatograph (Hewlett-Packard, 5890 Series II) equipped with a mass spectrometer (Hewlett-Packard, 5972 Series II); separation was achieved using a capillary column (Supleco Petrocol DH50.2). The concentration of products was calculated by using a calibration factor for each product. From the data collected at less than 10% conversion, the rates were calculated from the rate expression for differential conditions Fx ) rW, where F is the flow rate of the reactant (mol/s), x the conversion, r the reaction rate (mol g-1 s-1), and W the weight of the catalyst (g). Results and Discussion Catalyst Performance in n-Octane Hydroisomerization. The Pt/H-ZSM-12-31 and the Pt/H-USY-30 catalysts were studied for n-octane hydroisomerization; the conversion was varied by changing the temperature but keeping the WHSV and H2/n-C8 ratio constant. For

Figure 1. Variation of the n-C8 conversion with temperature for the Pt/H-ZSM-12 and Pt/H-USY-30 catalysts: WHSV ) 7 h-1; H2/n-C8 (molar) ) 16; total pressure ) 100 psig; 0.5 wt % Pt loading.

Figure 2. Yield of monobranched (filled symbols) and dibranched (open symbols) C8 isomers as a function of n-C8 conversion over Pt/H-ZSM-12-31 (0) and Pt/ H-USY-30 (4): WHSV ) 7 h-1; H2/ n-C8 (molar) ) 16; total pressure ) 100 psig; 0.5 wt % Pt loading.

the sake of convenience, the catalysts will be referred to as ZSM-12 and USY in the remainder of the paper. The variation of the n-C8 conversion with temperature (Figure 1) over the two catalysts and the variation of the monobranched and dibranched i-C8 isomer yield with respect to the n-C8 conversion are compared in Figure 2. From Figure 1, the ZSM-12 catalyst is more active than the USY catalyst, requiring lower temperatures to achieve the same level of n-C8 conversion. The advantageous selectivity characteristics of the ZSM-12 catalyst are evident in Figure 2, the yield of both monobranched as well as dibranched C8 isomers being higher than that of the Y-zeolite catalyst. The interesting observation here is that even the dibranched isomers are produced in a slightly higher yield over the ZSM12 catalyst. It should be mentioned that the catalysts used in this study met the criteria for an “ideal bifunctional catalyst”. The absence of hydrogenolysis products and the high isomer yield indicated that the metal and acid functions were well balanced. In an ideal bifunctional catalyst, the metal phase hydrogenates and dehydrogenates rapidly, while the rate-determining

2952 Ind. Eng. Chem. Res., Vol. 43, No. 12, 2004 Table 2. Reaction Rates of C8 Isomers over Pt/H-USY-30 and Pt/H-ZSM-12-31a

n-C8 2-MC7 3-MC7 4-MC7 2,2-DMC6 2,4-DMC6 2,2,4-TMC5 2,3,4-DMC5

reacn rate × 107, mol g-1 s-1

rel rates (n-C8 over USY ) 1)

USY

ZSM-12

USY

ZSM-12

1.8 12.0 25.4 25.7 21.3 23.1 52.4 137.1

2.9 11.5 16.3 17.6 12.3 9.9 43.8 65.8

1.0 6.5 13.9 14.0 11.6 12.6 28.6 74.9

1.6 6.3 8.9 9.6 6.7 5.4 23.9 35.9

a Reaction conditions: temperature ) 220 °C; pressure ) 100 psig; H2/C8 (molar) ) 16; conversions < 10%.

event (rearrangement and breaking of C-C bonds) occurs on the acid sites.10 Effect of Probe Molecular Sizes on the Activity. As mentioned earlier, the objective of this study was to examine the effect of differences in zeolite pore size on the conversion of various octane isomers with different kinetic diameters. Table 2 lists the rates of hydroconversion of the various C8 isomers over the two catalysts. It can be seen that the order in which reactivity of the C8 isomers varies is approximately the same over both the catalysts; however, the reactivity of the isomers over the ZSM-12 catalyst is drastically affected by the degree of branching and the position of the methyl groups. It is interesting to note that the ZSM-12 catalyst is more active than USY only for the hydroconversion of n-C8 and the rate of conversion of 2-MC7 is similar over both the catalysts. All the other C8 isomers react faster over USY, suggesting that diffusion and steric hindrances significantly start affecting the reaction rate even for conversion of 3-MC7 over ZSM-12. The fastest reaction over both the catalysts is the conversion of 2,3,4-TMC5, which suggests that even in USY diffusion might be affecting the reaction of 2,2,4-TMC5, which should be the fastest. Although the reaction rates of all the C8 isomers except n-C8 are lower over ZSM-12 compared to the USY catalyst, they are still many times faster than the hydroconversion of n-C8 over ZSM-12, which suggests that the molecules are able to enter the zeolite and the reactions occur inside the zeolite pore. However, the data for sorption of 2,2,4-TMC5 on ZSM-12 (Table 1) does not conclusively establish that adsorption is intracrystalline, even though a significant amount is adsorbed. This means that, for 2,2,4-TMC5, the contribution of acid sites on the external surface may not be negligible. The hydroconversion of the individual isomers, the products obtained from their reactions, and the reaction pathways favored over the catalysts are discussed in detail in the following sections. Conversion of n-C8. Table 2 shows that the ZSM12 catalyst is more active than the USY catalyst for n-C8 conversion at the reaction conditions used in this study. Since the n-C8 molecule can easily enter the pores of both the zeolites, possible reasons for the higher activity of ZSM-12 are a greater number of acid sites in the ZSM-12-31 sample compared to the USY-30 sample (Table 1) and the high crystallinity of the ZSM-12 sample. The USY-30 sample is highly dealuminated, and the loss in crystallinity is evident from the sorption data, the adsorption capacity of the USY-30 sample, for both n-C8 and 2,2,4-TMC5 (Table 1), being only about 50% of that of the parent Na-Y sample (not shown). In addition, it has been shown that zeolites with smaller

Table 3. Product Selectivity (mol %) from the Hydroconversion of n-C8 and Mono- and Dibranched Octanes over Pt/H-USY-30a 2,22,4n-C8 2-MC7 3-MC7 4-MC7 DMC6 DMC6 conversion 3.1 (mol %) n-C8 2-MC7 30.4 3-MC7 48.6 4-MC7 18.3 3-EC6 2.8 2,2-DMC6 2,5-DMC6 2,4-DMC6 3,3-DMC6 2,3-DMC6 3,4-DMC6 3E,2M-C5 2,2,4-TMC5 cracking products i-C4/n-C4

7.1

81.9 8.1 3.4 0.8 0.9 2.1 0.1 2.4 0.3

7.4

3.8

6.3

6.8

35.0

5.4 68.2

0.2 0.8 0.1

1.5 2.3

42.5 18.9 0.2 0.5 0.5

26.0 0.3 1.5 14.4 5.1 66.1 7.4 2.9 0.2 1.3 4.5

8.8 31.5 4.8 30.7 14.4 3.5 2.5 11.5

a Reaction conditions: temperature ) 220 °C; pressure ) 100 psig; H2/C8 (molar) ) 16.

Figure 3. Reaction scheme showing the primary isomerization steps of various C8 isomers over the Pt/H-USY-30 catalyst. The numbers represent the reaction rate multiplied with the product selectivity. The value for the isomerization of n-C8 to 2-MC7 was arbitrarily set to unity, and the values for the other steps are given relative to this step.

pores generally have a higher adsorption enthalpy, which leads to a higher activity.11 It is likely that the adsorption enthalpy for n-C8 on ZSM-12 would be higher than on USY which could also contribute to a higher activity for ZSM-12. Table 3 and Figure 3 show the isomer selectivities and the primary isomerization reaction schemes respectively over the USY catalyst. The corresponding data for the ZSM-12 catalyst are shown in Table 4 and Figure 4. The three methylheptanes formed by type B isomerization from n-C8 are the primary products over both the catalysts, and some 3-ethylhexane is also observed. Details of the mechanisms involved in the isomerization and cracking of hydrocarbons over heterogeneous acid catalysts have been discussed by Martens and Jacobs.12 The protonated cyclopropane (PCP) mechanism for type B isomerization predicts a 2-MC7/3-MC7/4-MC7 ratio of 1/2/1 from noctane, since there are two routes for formation of 3-MC7. The product selectivities over USY (Table 3) show that the 3-MC7 amount is close to that predicted by the PCP mechanism but 2-MC7 is produced in higher amounts than 4-MC7. This is expected since 2-MC7 is thermodynamically favored over 4-MC7. Also, since 2-MC7 is the least bulkiest among the three monomethyl

Ind. Eng. Chem. Res., Vol. 43, No. 12, 2004 2953 Table 4. Product Selectivity (mol %) from the Hydroconversion of n-C8 and Mono- and Dibranched Octanes over Pt/H-ZSM-12-31a 2,22,4n-C8 2-MC7 3-MC7 4-MC7 DMC6 DMC6 conversion 6.3 (mol %) n-C8 2-MC7 37.4 3-MC7 45.3 4-MC7 14.3 3-EC6 3.0 2,2-DMC6 2,5-DMC6 2,4-DMC6 3,3-DMC6 2,3-DMC6 3,4-DMC6 3E,2M-C5 2,2,4-TMC5 cracking products i-C4/n-C4

11.7

4.8

2.6

2.0

0.9 49.3

1.4 24.6 61.7

72.2 12.4 4.1 0.5 2.8 3.9 1.8 0.3

33.1 13.1 0.3 0.7 2.0 0.5

8.7 0.7 0.8 2.0

3.7

2.9

3.3 2.4 0.5

7.3 5.2 0.9

7.5 22.1 1.6 52.7 5.0 1.2 1.4 3.3 2.2

8.1 35.9 1.8 25.9 8.9 1.3 4.5 7.0

a Reaction conditions: temperature ) 220 °C; pressure ) 100 psig; H2/C8 (molar) ) 16.

Figure 4. Reaction scheme showing the primary isomerization steps of various C8 isomers over the Pt/H-ZSM-12-31 catalyst. The numbers represent the reaction rate multiplied with the product selectivity relative to the isomerization of n-C8 to 2-MC7 over USY, which was arbitrarily set to unity. The numbers in parentheses indicate the values relative to the isomerization of n-C8 to 2-MC7 over ZSM-12.

isomers, its faster diffusion out of the zeolite pore would also result in a higher concentration of this isomer. Over ZSM-12, the 4-MC7 selectivity is somewhat lower than that observed over USY, indicating that the smaller pores of ZSM-12 are exerting some shape selective effects on even the monobranched isomers. In Figures 3 and 4, the net rate of formation of 2-MC7 from the conversion of n-C8 over USY (obtained by multiplying the selectivity to 2-MC7 with n-C8 conversion) has been arbitrarily set to unity and the values for the other primary reaction steps (obtained in a similar manner) are expressed relative to this rate. Comparison of the numbers in Figures 3 and 4 clearly show that transformation of n-C8 into 3-MC7 and 4-MC7 is slower over the ZSM-12 catalyst. Conversion of Monobranched Isomers. The conversion of methylheptanes yields the other two methylheptanes and 3-EC6. For all the three methylheptanes the selectivity to monobranched isomers was over 90% and no cracking products were observed over both the catalysts. Small amounts of dibranched isomers were the only other products observed. Type A isomerization

(methyl shift) is therefore the dominant reaction. Over both the catalysts, 3-MC7 and 4-MC7 react at similar rates while the reaction rate of 2-MC7 is lower. The higher reactivity of 3-MC7 and 4-MC7 is due to the existence of more reaction pathways for their isomerization. For 4-MC7, there are two type A isomerization paths leading to the formation of 3-MC7 and two type B isomerization paths leading to the formation of 3-EC6. Similarly for 3-MC7, there are two type A routes for forming 4-MC7 and 2-MC7 and a possible ethyl shift leading to 3-EC6. A methyl shift leading to 3-MC7 is the only possible type A isomerization step for 2-MC7, which is responsible for its lower reactivity. Over the USY catalyst, n-C8 is not observed in the product indicating that the backward debranching reaction via type B isomerization is less favored than the forward branching reaction to form other dibranched isomers. However, over the ZSM-12 catalyst, the conversion of all the three monobranched isomers yields small amounts of n-C8. This is clearly due to steric effects in the smaller pores of ZSM-12 which favors the formation of less bulky molecules. Other studies on conversion of branched alkanes over narrow pore systems have also reported higher amounts of n-C8 and n-C7 formation.5,13 The formation of 4-MC7 and 3-EC6 is also hindered over the ZSM-12 catalyst. The formation of 3-ethylhexane from the monobranched isomers is quite interesting, since it is obtained in fairly large amounts from the conversion of 3-MC7 and 4-MC7. Theoretically, 3-EC6 can be produced from all the three monomethyl isomers, from 4-MC7 via type B isomerization, from 3-MC7 via an ethyl shift as well as a type B path, and from 2-MC7 by a protonated cyclobutane (PCB) mechanism. In reality however, the amount of 3-EC6 produced from the conversion of 2-MC7 is quite small, thus ruling out the PCB mechanism. From Figure 3, it can be seen that the rate of formation of 3-EC6 from both 3-MC7 and 4-MC7 is substantially higher than those of the other dibranched isomers. This suggests that the particular type B isomerization step leading to the formation of 3-EC6 from 4-MC7 and 3-MC7 may be much faster than the other type B steps leading to the formation of other dibranched isomers. However, the more likely possibility is that the ethyl shift could be a primary reaction for formation of 3-EC6 from 3-MC7 in addition to the two PCP paths from 4-MC7 and one PCP route from 3-MC7. The reaction schemes shown in Figures 3 and 4 give the rate of formation of a certain isomer relative to the rate of formation of 2-MC7 from n-C8 over USY (arbitrarily set at unity). It should be noted that these are not the true rates. It can be seen from Figure 3 that, over the USY catalyst, the methyl shift in the monobranched isomers is about 10 times faster than the type B isomerization of n-C8 to form a monobranched isomer, which is in agreement with results reported in the literature. The conversion of 4-MC7 to 3-MC7 is the fastest isomerization step since two routes exist as mentioned earlier. If the number of paths is taken into account, the rates for a methyl shift to an adjacent carbon in the monobranched isomers is pretty similar. In Figure 3, the values for the conversion of 4-MC7 to 2-MC7 and vice-versa is also indicated (even though they are not primary steps) to obtain an idea about the combined rate of two methyl shifts. From these data it appears that the combined rate of two type A isomerization steps, to produce an isomer with the branch two

2954 Ind. Eng. Chem. Res., Vol. 43, No. 12, 2004

carbons away from the original position, is about the same or maybe slightly higher than one type B isomerization step. Over the ZSM-12 catalyst (Figure 4), the reaction rate of 4-MC7 as well as the selectivity to the 4-MC7 isomer from conversion of 2-MC7 and 3-MC7 is lower compared to the USY catalyst, providing further proof that the bulkier 4-MC7 isomer experiences some mild shape selective effects in the ZSM-12 pores. Conversion of Dibranched Isomers. The reaction of two dibranched isomers, 2,2-dimethylhexane and 2,4dimethylhexane, was examined at conversions less than 10% over both the catalysts. Both these dibranched isomers react at rates similar to that of the monobranched isomers over the USY catalyst, but these rates are halved over the ZSM-12 catalyst (Table 2). The reaction rates over USY are not surprising since over 90% of the products are isomers and the reaction is again dominated by the type A isomerization. Moreover, the number of possible routes available for the transformation of the isomer is greater. Over the ZSM-12 catalyst, the lower reaction rate indicates that diffusion is significantly hindering the reaction. It is interesting to note that 2,2-DMC6 reacts faster than 2,4-DMC6 over ZSM-12 even though 2,2-DMC6 should be the bulkier isomer since it has two methyl groups on the same carbon. The product distribution from the conversion of 2,2DMC6 and 2,4-DMC6 over USY (Table 3) shows that other dibranched isomers formed by type A isomerization constitute the majority of the products. The rates of the methyl shifts seem to be similar to that observed for the monobranched isomers (Figure 3). Comparison of the distribution obtained over ZSM-12 with USY shows some interesting trends. It is clear that the bulkier 3,3-, 3,4-, and 2,3-DMC6 isomers are not favored in ZSM-12, and due to this, the 2,3-DMC6 formed from 2,2-DMC6 is quickly transformed into 2,4-DMC6, resulting in significantly higher selectivities for the 2,4-DMC6 and 2,5-DMC6 isomers. A similar behavior is observed over ZSM-12 for the conversion of 2,4-DMC6 also, with the 2,5 isomer favored over the 2,3 isomer (Table 4 and Figure 4), while, over USY, 2,4-DMC6 is transformed into 2,5-DMC6 and 2,3-DMC6 at about the same rates (Table 3 and Figure 3). The formation of 3-ethyl,2methylpentane is also somewhat suppressed over ZSM12. The observation of shape selective effects hindering the formation of bulkier dibranched isomers over ZSM12 indicates that the reaction of 2,2-DMC6 and 2,4DMC6 does occur inside the zeolite pore. Interestingly, the formation of 2,2-DMC6 does not appear to be significantly affected over ZSM-12 since it is formed in amounts similar to that over USY from the conversion of 2,4-DMC6. A small amount of cracked products is obtained from the conversion of both the dibranched isomers, but tribranched isomers are hardly detected. Debranching to form the monobranched isomers also occurs to a small extent over USY, but the amounts of monobranched isomers obtained over ZSM-12 are much larger, again illustrating how the shape-selective character of the ZSM-12 pore drives the backward reaction favoring the less bulky molecules. Conversion of Tribranched Isomers. The conversion rates of 2,2,4-TMC5 (Table 2) and the product selectivities obtained (Table 5) over both the catalysts are quite similar. The only small, but noticeable, difference in the product distribution is the higher selectivity to 2,3,4-TMC5 and a lower selectivity to the

Table 5. Product Selectivities (mol %) from the Hydroconversion of Tribranched Octanes over Pt/ H-USY-30 and Pt/H-ZSM-12-31 Catalystsa USY

ZSM-12

2,2,4-TMC5 2,3,4-TMC5 2,2,4-TMC5 2,3,4-TMC5 conversion (%) isobutane butane 2,2-DMC6 2,5-DMC6 2,3-DMC6 3,4-DMC6 2,2,3-TMC5 2,3,3-TMC5 2,3,4-TMC5 2,2,4-TMC5

7.8 89.9 0.5 0.2 1.8 1.2 6.3

7.8 76.9 5.5 0.3 0.1

6.5 88.4 0.7 0.1 0.5 0.1

6.0 4.8

1.1 0.5 8.7

6.4

9.4 88.7 3.6 0.5 0.7 0.1 1.4 0.2 4.8

Reaction conditions: temperature ) 220 °C; pressure ) 100 psig; H2/C8 (molar) ) 16. a

bulkier 2,2,3- and 2,3,3-TMC5 over the ZSM-12 catalyst. Type A β-scission is the overwhelming reaction pathway over both catalysts, which results in the formation of isobutane molecules. This is expected, since it is wellknown that type A β-scission is many times faster than type A isomerization or other types of β-scission.10 As in the case of 2,2-DMC6, it is interesting to note that 2,2,4-TMC5 is also converted easily over ZSM-12, even though its critical molecular diameter of about 6.5-7.0 Å is larger than the pore size of ZSM-12. The amount of 2,2,4-TMC5 adsorbed on ZSM-12 in the dynamic sorption experiments (Table 1) suggests that it does enter the zeolite pore, but the data are not very conclusive. The suppression of the bulkier tribranched isomers over ZSM-12 provides an indication that the reaction of 2,2,4-TMC5 is taking place inside the pore. Wu et al.14 found evidence for intracrystalline adsorption of mesitylene (molecular dimensions of 3.7 × 7.8 × 8.5 Å) and suggested an effective pore size of 6.0 × 7.5 Å for ZSM-12. Even ZSM-5, which has smaller pore openings than ZSM-12, was found to be able to adsorb naphthalene molecules with a kinetic diameter of 7.4 Å into its pore system, due to the flexibility of the ring aperture.15 Other researchers16 have suggested a larger crystallographic pore size for ZSM-12 (5.6 × 7.7 Å) than what is commonly believed. This phenomenon of molecules larger than the crystallographic zeolite pore size being adsorbed and converted is well-known and has been attributed to the thermal vibrations of both the zeolite framework and hydrocarbon molecules.17 Although, there is evidence for the reaction of 2,2,4-TMC5 inside the ZSM-12 pore, the contribution of the acid sites on the external surface cannot be ruled out since this is a very fast reaction. The conversion of 2,3,4-TMC5 is the fastest reaction over both the catalysts, as mentioned earlier. 2,3,4TMC5 has the smallest kinetic diameter among the four trimethylpentanes, since the three methyl groups are located on different carbons. However, such a high rate of hydrocracking, yielding isobutane as the major product is surprising, since a methyl shift must occur before the type A β-scission as shown in Figure 5 (step 5.2). Propane and 2-methylbutane were not detected in the products, which indicates that the direct cracking of the 2,3,4-TMC5 carbenium ion (step 5.1) is not occurring. Therefore, all of the 2,3,4-TMC5 is undergoing a type A isomerization step before cracking. The fact that the reaction rate of 2,3,4-TMC5 is three times faster than that of 2,2,4-TMC5 over USY, despite undergoing a type A isomerization step before type A β-scission, clearly

Ind. Eng. Chem. Res., Vol. 43, No. 12, 2004 2955

Figure 5. Reaction routes for 2,3,4-TMC5.

Figure 6. Reaction scheme showing the main isomerization and cracking paths for tribranched C8 isomers over (a) Pt/H-USY-30 and (b) Pt/H-ZSM-12-31. Again, the values indicated are relative to the isomerization of n-C8 to 2-MC7.

illustrates the role of the diffusion rate of the reactant into the zeolite pore. This observation also suggests that even though 2,2,4-TMC5 should be easily accommodated inside the pores of Y-faujasite, its diffusion into the pore is significantly hindered. Butane can be produced from the conversion of 2,3,4-TMC5 according to step 5.3 in Figure 5. The observation of butane, but no propane or isopentane (via step 5.1), indicates that the type B1 β-scission is faster than the type B2 β-scission. The reaction rate of 2,3,4-TMC5 is only somewhat higher than that of 2,2,4-TMC5 over the ZSM-12 catalyst (Table 2). Comparison of the product distributions from the conversion of 2,3,4-TMC5 over USY and ZSM-12 (Table 5) shows some interesting observations. Over USY, all the other three trimethylpentanes are obtained in significant amounts, while, over the ZSM-12 catalyst, only the 2,2,4-TMC5 is obtained in similar amounts. It appears that, inside the ZSM-12 pores, the formation of only the bulkiest trimethylpentane molecules is hindered to a large extent. These details can be seen more clearly in Figure 6 where, again, the numbers for each step indicate the rate of reaction multiplied by the selectivity to the product. The selectivity for 2,2,3-TMC5 from conversion of 2,3,4-TMC5, even though it requires two methyl shifts, is higher than that of 2,3,3-TMC5 which can be formed from 2,3,4-TMC5 by a single methyl shift. Moreover, 2,2,3-TMC5 can be cracked more easily (via step 5.3 in Figure 5) than 2,3,3-TMC5 (step 5.4), since type B1 β-scission is much faster than type C β-scission.

Therefore, in both the catalysts, the relative sizes of the reactant molecule and the zeolite channel dimension have a big effect in determining the reaction rate of the octane isomer. In ZSM-12, shape selectivity clearly hinders the formation of the bulkier isomers. The number of alkyl groups and their location also affect the reaction rate, due to differences in the rates of the various isomerization and cracking steps. Even though the pore size of ZSM-12 is closer to that of the medium pore zeolites, its performance is different from the behavior observed over certain small and medium pore zeolites, where reaction rate is completely determined by the critical molecular diameters of the reactants.18,19 The data suggest that even a tribranched isomer like 2,2,4-TMC5 is able to enter and react inside the ZSM12 pore. The reaction rate of 2,2,4-TMC5 over ZSM-12 is much higher than that of 2-MC7 and 2,4-DMC6, whereas, over the medium-pore zeolites in studies mentioned above, the reaction rate of 2,2,4-TMC5 is lower than or comparable to these isomers. However, in the very fast reactions involving tribranched isomers, it is possible that part of the reaction is occurring on the external surface of ZSM-12. More work is necessary in the future to determine the contribution of the external surface. Conclusions The hydroconversions of n-C8, the three monobranched C8 isomers, and selected dibranched and tribranched isomers were studied over Pt-loaded acidic USY and ZSM-12 catalysts. ZSM-12 was more active than the USY catalyst for the conversion of n-C8, but all the other isomers reacted faster over USY. All the C8 isomers studied were able to enter and react inside the ZSM-12 pores, but the less bulky isomers were the more favored products due to shape selective effects. However, only the bulkiest tribranched isomers (2,2,3and 2,3,3-TMC5) are suppressed to a very large extent over ZSM-12. The backward debranching reaction to form n-C8 from the monobranched isomers, and monobranched isomers from the dibranched isomers, occurred to a much greater extent over ZSM-12 compared to the USY catalyst. Evidence for intracrystalline reaction of isomers larger than the ZSM-12 pore size confirmed that the “catalytic pore diameter” is larger than its crystallographic pore diameter. Finally, this study helps understand how mild suppression of the formation as well as the reaction of the various isomers ultimately

2956 Ind. Eng. Chem. Res., Vol. 43, No. 12, 2004

translates into a significantly higher yield of monobranched and dibranched isomers over the ZSM-12 catalyst from n-C8 hydroconversion. The three times higher rate of 2,3,4-TMC5 conversion than 2,2,4-TMC5 over USY indicates that diffusion of the bulkier molecules is significantly hindered even in Y zeolite. Acknowledgment This paper is dedicated to Professor George Gavalas of Caltech for his numerous and excellent contributions in chemical engineering over the period of his entire life. The authors wish to acknowledge the support of the National Science Foundation through an award (Grant CTS-9702081). Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this research through the Grant ACS-PRF 31606-G5. W.Z. thanks the University of Cincinnati for a University Distinguished Dissertation Fellowship Award in 1998-1999. Literature Cited (1) Giannetto, G. E.; Perot, G. R.; Guisnet, M. R. Hydroisomerization and Hydrocracking of Normal-Alkanes. 1. Ideal Hydroisomerization Pt/HY Catalysts. Ind. Eng. Chem. Prod. Res. Dev. 1986, 25, 481. (2) Martens, J. A.; Tielen, M.; Jacobs, P. A. Relation between paraffin isomerization capability and pore architecture of largepore bifunctional zeolites. In Zeolites as Catalysts, Sorbents and Detergent Builders; Karge, H. G., Weitkamp, J., Eds.; Elsevier: Amsterdam, 1989; Vol. 46, p 49. (3) Chica, A.; Corma, A. Hydroisomerization of Pentane, Hexane, and Heptane for Improving the Octane Number of Gasoline. J. Catal. 1999, 187, 167. (4) Martens, J. A.; Jacobs, P. A. Evidence for Branching of LongChain n-Alkanes via Protonated Cycloalkanes larger than Cyclopropane. J. Catal. 1990, 124, 357. (5) Nghiem, V. T.; Sapaly, G.; Meriaudeau, P.; Naccache, C. Monodimensional Tubular Medium Pore Molecular Sieves for Selective Hydroisomerisation of Long Chain Alkanes: n-octane Reaction on ZSM and SAPO type Catalysts. Top. Catal. 2001, 14, 131. (6) Zhang, W. M.; Smirniotis, P. G. Effect of Zeolite Structure and Acidity on the Product Selectivity and Reaction Mechanism for n-octane Hydroisomerization and Hydrocracking. J. Catal. 1999, 182, 400. (7) Gopal, S.; Smirniotis, P. G. Pt/H-ZSM-12 as a Catalyst for the Hydroisomerization of C5-C7 n-alkanes and Simultaneous Saturation of Benzene. Appl. Catal., A: 2003, 247, 113. (8) Gopal, S.; Yoo, K.; Smirniotis, P. G. Synthesis of Al-rich ZSM-12 using TEAOH as Template. Microporous Mesoporous Mater. 2001, 49, 149.

(9) Zhang, W. M.; Burckle, E. C.; Smirniotis, P. G. Characterization of the Acidity of Ultrastable Y, Mordenite, and ZSM-12 via NH3-Stepwise Temperature Programmed Desorption and Fourier Transform Infrared Spectroscopy. Microporous Mesoporous Mater. 1999, 33, 173. (10) Martens, J. A.; Tielen, M.; Jacobs, P. A. Attempts to Rationalize the Distribution of Hydrocracked Products. III. Mechanistic Aspects of Isomerization and Hydrocracking of Branched Alkanes on Ideal Bifunctional Large-Pore Zeolite Catalysts. Catal. Today 1987, 1, 435. (11) van deRunstraat, A.; Kamp, J. A.; Stobbelaar, P. J.; van Grondelle, J.; Krijnen, S.; van Santen, R. A. Kinetics of Hydroisomerization of n-Hexane over Platinum Containing Zeolites. J. Catal. 1997, 171, 77. (12) Martens, J. A.; Jacobs, P. A. Conceptual background for the conversion of hydrocarbons on heterogeneous acid catalysts. In Theoretical Aspects of Heterogeneous Catalysis; Moffat, J. B., Ed.; Van Nostrand Reinhold: New York, 1990; p 52. (13) Hochtl, M.; Jentys, A.; Vinek, H. Hydroisomerization of Heptane Isomers over Pd/SAPO Molecular Sieves: Influence of the Acid and Metal Site Concentration and the Transport Properties on the Activity and Selectivity. J. Catal. 2000, 190, 419. (14) Wu, E. L.; Landolt, G. R.; Chester, A. W. Hydrocarbon adsorption characterization of some high silica zeolites. In Studies in Surface Science and Catalysis 28; Murakami, Y., Iijima, A., Ward, J. W., Eds.; Elsevier: Amsterdam, 1986; p 547. (15) van Koningsveld, H.; Jansen, J. C. Single-Crystal Structure Analysis of Zeolite H-ZSM-5 loaded with Naphthalene. Microporous Mater. 1996, 6, 159. (16) Fyfe, C. A.; Gies, H.; Kokotailo, G. T.; Marler, B.; Cox, D. E. Crystal-Structure of Silica-ZSM-12 by the Combined Use of High-Resolution Solid-State MAS NMR Spectroscopy and Synchrotron X-ray Powder Diffraction. J. Phys. Chem. 1990, 94, 3718. (17) Bendoraitis, J. G.; Chester, A. W.; Dwyer, F. G.; Garwood, W. E. Pore size and shape effects in zeolite catalysis. In Studies in Surface Science and Catalysis 28; Murakami, Y., Iijima, A., Ward, J. W., Eds.; Elsevier: Amsterdam, 1986; p 669. (18) Choudhary, V. R.; Akolekar, D. B. Shuttlecock-Shuttlebox Model for Shape Selectivity of Medium-Pore Zeolites in Sorption and Diffusion. J. Catal. 1989, 117, 542. (19) Meriaudeau, P.; Tuan, V. A.; Nghiem, V. T.; Lai, S. Y.; Hung, L. N.; Naccache, C. SAPO-11, SAPO-31, and SAPO-41 Molecular Sieves: Synthesis, Characterization, and Catalytic Properties in n-octane Hydroisomerization. J. Catal. 1997, 169, 55.

Received for review June 2, 2003 Revised manuscript received October 6, 2003 Accepted October 7, 2003 IE030472X