Al Ratio and of the Zeolite Structure on the Performance

Ind. Eng. Chem. Res. 1996, 35, 3055-3066

3055

Effect of the Si/Al Ratio and of the Zeolite Structure on the Performance of Dealuminated Zeolites for the Reforming of Hydrocarbon Mixtures Panagiotis G. Smirniotis* and Wenmin Zhang Chemical Engineering Department, University of Cincinnati, Cincinnati, Ohio 45221-0171

Various 12-membered ring pore zeolites were employed for the reforming of “synthetic” hydrocarbon mixtures which simulate industrial naphthas. All the zeolites were dealuminated to various extents. It was found that, under the present conditions over the samples which are slightly dealuminated, bimolecular-condensation reactions followed by recracking are responsible for the relatively large selectivities of C4 paraffins. The monomolecular cracking (via pentacoordinated carbonium ions) of the latter hydrocarbons is responsible for the large generation of CH4 from the cracking of C4 paraffins. When the Si/Al ratio increases, the selectivity of methane passes through a steep minimum, while those of C3, C4, and C5 pass through a maximum. It was also found that the zeolite pore structure is a very important factor for the time on stream activity of zeolite-based catalysts. Zeolites with reduced aluminum content and pore structures, which do not favor the formation of coke precursors in their cavities, can lead to very promising catalysts for acid-catalyzed reactions. From our study a 12-membered ring pore zeolite, which demonstrates minimal coke deactivation, was identified. 1. Introduction Only recently zeolite-based catalysts have been employed in naphtha reforming reactions. Bernard (1980) reported originally that Pt/KL zeolite transforms nhexane to benzene very selectively at atmospheric pressure. Later, Hughes et al. (1986), Law et al. (1987), and Tamm et al. (1988) employed Pt on Ba/KL zeolite for the reforming of C6-C8 industrial paraffinic naphthas. The above catalyst, being monofunctional in nature, can transform the feedstock to the corresponding aromatics very selectively. Large pore zeolites, namely, ZSM-3, ZSM-20, Y, USY, and zeolite β, have been tested for the isomerization of long-chain nparaffins (Martens et al., 1989), and NaY and HNaY faujasite supported Pt or PtRe have also been employed for the reforming of n-heptane (Dossi et al., 1989). The performance of bifunctional Pt/Y faujasites with C6naphthenes and Pt/HZSM-5 with methylcyclopentane, methylcyclohexane, and ethylcyclohexane has been studied originally by Schulz et al. (1973) and Weitkamp et al. (1984). It was found that the zeolite pore structure in combination with their Brønsted acidity enhances bimolecular alkylation reactions between the naphthenes and light hydrocarbons, leading to the enlargement of the feed naphthenes. Chow et al. (1985) studied the ring enlargement and ring opening of methylcyclopentane. The reforming capabilities of protonated zeolite β loaded Pt were examined by Smirniotis and Ruckenstein (1993a) with n-hexane, methylcyclopentane, and cyclohexane under low severity conditions. A comparison with nonacidified Pt/γ-Al2O3 was carried out as well. It was found that β zeolite in comparison with alumina favors the skeletal isomerization of n-hexane to a great extent, the enlargement/isomerization of the C5-cyclic ring of methylcyclopentenes to a C6-cyclic ring, and bimolecular alkylation reactions between naphthenes or aromatics with light paraffins or olefins, respectively. * Author to whom correspondence should be addressed. Telephone: (1-513) 556-1474. Fax: (1-513) 556-3473. E-mail address: [email protected].

S0888-5885(95)00763-9 CCC: $12.00

This behavior is a combined result of its Brønsted acidity and its pore structure, which can accommodate molecules like those involved in reforming naphthenes and simultaneously increase the rate of effective bimolecular collisions, leading to the alkylations of naphthenes or aromatics. The catalyst functionality can be significantly altered by balancing its acidic function represented by the H/β zeolite with its metal function represented mainly by the Pt/Al2O3 (Smirniotis and Ruckenstein, 1993b, 1994a). As a result of this balancing, maximum and time-stable aromatization of alkylcyclopentanes (methylcyclopentane and ethylcyclopentane) was achieved over Pt/β zeolite with a SiO2/Al2O3 molar ratio of 130 and 0.5 wt % Pt (Smirniotis and Ruckenstein, 1994b,c). This indicates that the control of the Al sites/Pt sites ratio via a careful dealumination of β zeolite can suppress the rate of coke deactivation and hydrocracking. It seems that the moderate acidity of this sample provided the necessary isomerization capability while simultaneously keeping the rate of hydrocracking and coke formation at low levels. In the present study, we compare the performance of various large-pore Pt/zeolites for reforming type reactions with a paraffinic and a naphthenic hydrocarbon mixture. The effect of dealumination on the product selectivities and catalyst activity was investigated as well. 2. Experimental Section 2.1. Catalyst Preparation. In the present study, β zeolite with a Si/Al molar ratio of 14.5 was employed. It was synthesized hydrothermally from an aluminosilicate gel with a nominal composition of 30SiO2:Al2O3: 0.9Na2O:3TEAOH:250H2O (TEAOH stands for tetraethylammonium hydroxide). The reagents employed were colloidal silica (LUDOX HS-40, DuPont), sodium aluminate (Fisher Scientific), tetraethylammonium hydroxide (Aldrich 40 wt % solution in water), and distilled water. The final gel was prepared by vigorously mixing individual solutions in the appropriate proportions, it was then loaded in a Teflon-lined stainless-steel autoclave (Autoclave Engineers) and kept at 150 ( 1 °C for © 1996 American Chemical Society

3056 Ind. Eng. Chem. Res., Vol. 35, No. 9, 1996 Table 1. Si/Al (Framework and Extraframework Aluminum) Ratios of the Zeolites Employed in the Present Study As Determined by ICP Spectroscopy β Zeolite catalyst

acid concn (HCl), N

Si/Al (bulk)

crystallinity, %

B B25 B4 B5 B7

as synthesized 2.5 4 5 7

14.5 69.3 75.9 85.0 132.0

100 77 84 79 not determined

catalyst

acid concn (HCl), N

Si/Al (bulk)

crystallinity, %

ZEOL ZEOL1 ZEOL3 ZEOL5

as synthesized 1 3 5

34.8 40.4 53.9 68.6

100 ≈100 ≈100 ≈100

ZEOL

USYa catalyst

Si/Al (bulk)

Si/Al (framework)

crystallinity, %

USY USY1 (CBV-712) USY2 (CBV-760)

2.6 5.8 28.0

2.6 13.6 62.2

100 81 72

ZSM-5 catalyst

acid concn (HCl), N

Si/Al (bulk)

crystallinity, %

ZSM5 ZSM5-15 ZSM5-25 ZSM5-5

as synthesized 1.5 2.5 5

51.0 52.9 54.6 54.9

100 71 not determined 78

Mordenite catalyst

(NH4)2SiF6, g

Si/Al (bulk)

crystallinity, %

M M3

as synthesized 25.0

9.5 35.7

100 83

a All the USY catalysts were provided to us from PQ Corp. The dealumination was carried out by steam treatment.

10 days under autogeneous pressure. The second zeolite involved in this work is a large-pore (12-membered ring pore) high-silica zeolite. This zeolite was synthesized in our laboratory from the corresponding aluminosilicate gel by following the related patent. In the following text we will refer to this zeolite as ZEOL. The samples of USY zeolite were provided to us from PQ Corp. The Si/Al ratios of these catalysts are given in Table 1. Steaming was employed for the dealumination of these samples. Mordenite and L zeolite, which were used only for the time on stream experiments (samples M3 and L3), were kindly donated to us from UOP. The samples of the latter zeolites employed have Si/Al ratios of 35.7 and 4.0, respectively. For comparison purposes, ZSM-5 (10-membered ring-pore zeolite) was employed. The ZSM-5 sample was synthesized hydrothermally from an aluminosilicate gel (Chen et al., 1978) with a nominal composition of 30SiO2:Al2O3: 24Na2O:3TPA-Br:1200H2O (TPA-Br stands for tetrapropylammonium bromide). The reagents employed for the preparation of the gel were sodium silicate aqueous solution (Fisher Scientific, 40-42 °Be′), sodium aluminate (Fisher Scientific, 46.8 wt % Al2O3 and 28.4 wt % Na2O), TPA-Br (Aldrich), H2SO4, and NaCl. The crystallization took place for 25 days at 95 °C. The zeolites synthesized in our laboratory (β, ZEOL, and ZSM-5) were calcined in dry air at 520 ( 2 °C for 4 h in order to burn the occluded template, and they were then dealuminated to different extents. For the high-

silica zeolites, the dealumination was carried out with various concentrations of HCl (1.0-7.0 N). These zeolites can withstand dealumination/acid leaching with some strong acids without any significant loss of their crystallinity. The dealumination took place in a 1 L, three-necked flask under reflux at 90 ( 1 °C for 4 h (2 g of catalyst were loaded each time). The concentrations of the acid employed and the Si/Al ratios of the dealuminated zeolites are presented in Table 1. For the dealumination of mordenite (high aluminum content zeolite), the method of Breck et al. (1985) was employed. This method involves the reaction of the zeolite with aqueous solutions of (NH4)2SiF6. Various concentrations of (NH4)2SiF6 were employed in order to achieve different extents of aluminum extraction and silicon substitution. For each dealumination experiment, 2 g of the ammonium form of the zeolite, 7 g of ammonium acetate (Fisher), and 800 mL of distilled water were added in a three-necked flask. The ammonium form of mordenite was prepared with cation exchange of the alkali form of the zeolite (2 g) in a 1 L flask with 2.0 M NH4Cl at 85 °C for 4 h. The suspension was agitated vigorously and heated at 70 °C. At this point the solution of (NH4)2SiF6 was added dropwise. This solution was prepared by mixing 200 mL of distilled water with the necessary amount of (NH4)2SiF6 (Aldrich, 98%) in order to achieve a certain Si/Al ratio. At the end of the titration, the temperature of the solution was raised to 90 ( 1 °C. The dealumination took place for 24 h under vigorous agitation. At the end of this period, the content of the flask was filtered and washed thoroughly with distilled water. The protonated forms of all the zeolites after dealumination were prepared by cation exchange under reflux (about 1 g of dealuminated zeolite was loaded in a 1 L flask) with a 2.0 M NH4Cl solution at 90 °C for 4 h. The ammonium zeolites were calcinated in air at 450 ( 2 °C for 1 h. Wet impregnation with H2PtCl6 (Aldrich, 8 wt % sol.) was employed for loading the Pt on the catalysts; a final loading of 0.5 wt % was achieved. After the impregnation step, the catalysts were dried overnight at 120 °C. 2.2. X-ray Diffraction. For the identification of the synthesized zeolites as well as for the determination of the crystallinity of the dealuminated zeolites, X-ray analysis was employed. The X-ray patterns were collected with a Nicolet powder X-ray diffractometer equipped with a Cu KR source. MgO (peak at 2θ ) 43°) was added to each sample (20 wt %) in order to allow the quantitative comparison of the crystallinity between each nondealuminated zeolite and the samples of the same zeolite after various extents of dealumination. The estimation of the crystallinity was based on the areas under the main peaks of each zeolite. 2.3. Acidity Measurements. The total acidity (both Brønsted and Lewis) of the catalysts employed in the present study was measured with the butylamine titration method (Benesi, 1965). Details for the procedure and the available indicators for acidity investigations are presented elsewhere (Tanabe, 1970; Tanabe et al., 1989). However, the accuracy of this method is not very high due to inherent limitations. The protonated form of each zeolite was ground and dried at 135 °C. A few drops of the indicator (Neutral Red from Aldrich was employed for all the titrations (H0 ) +6.5)) were added to a suspension formed with a known amount of zeolite powder in benzene (nonpolar solvent). The suspension was agitated vigorously for 30 min at ambient temper-

Ind. Eng. Chem. Res., Vol. 35, No. 9, 1996 3057

ature. The titration was carried out with a 0.02 N solution of n-butylamine (Aldrich) in benzene. The end of the titration was reported when the color of the suspension containing the catalyst became yellow (base form of the indicator). Enough time was allowed for the detection of the end point. FT-IR studies and stepwise NH3-TPD experiments are being performed (Robb et al., 1996) in order to characterize qualitatively and quantitatively the types and concentrations of acid sites of each zeolite as a function of the extent of dealumination. 2.4. Catalytic Runs. The catalytic experiments were conducted in a differential plug flow high-pressure reactor. The catalysts were activated in situ by oxidation with high-purity oxygen (Wright Bros, 99.99%) for 1 h at 450 °C followed by the purging of the catalyst with ultra-high-purity He (Wright Bros, 99.998%) for 15 min. The reduction of the catalyst was carried out at 450 °C in high-purity H2 for 1 h. The purging of the catalyst with helium after the oxidation step was carried out in order to avoid the production of water due to the reaction of oxygen and hydrogen, something that could lead to the steam dealumination of the catalysts. The catalyst activation step was carried out at atmospheric pressure. The reaction temperature was 470 °C, and the total pressure was kept at 90 psig. High-pressure H2 was provided from the tank to the reactor, while the pressure regulation was achieved with a backpressure regulator placed at the end of the reactor tube. The reactor (a stainless-steel tube of 0.60 cm i.d.) was placed vertically inside a temperature-programmable furnace. The heating of the reactor was carried out at a rate of 15 °C/min. In all the experiments, 100 ( 2 mg of fresh catalyst was loaded on top of a glass wool plug located in the middle of the reactor. The feed mixtures were introduced into the reactor at a predetermined flow using a liquid infusion pump through a heated line. The identification of the reaction products was carried out with high-resolution gas chromatography. The reactor effluent stream was sent for analysis through a heated line (about 180 °C) to the gas chromatograph (Hewlett-Packard, 5890 Series II) equipped with a mass spectrometer (Hewlett-Packard, 5972 Series II). The GC/MS unit was attached to a PC unit for data acquisition and storage. The GC was equipped with a highperformance capillary column (HP-5, cross-linked 5% phenylmethylsiloxane, 0.32 mm i.d., 30 m length, and a film thickness of 0.25 µm), a molecular sieve (5A) column (10 ft length), and a fused-silica PoraPLOT capillary column (0.53 mm i.d., 28 m length, with a film thickness of 20 µm). 3. Results and Discussion The Si/Al ratios (bulk) of the dealuminated samples were measured with ICP spectroscopy after the powders were dissolved in a HNO3 solution (lithium metaborate was involved for the fusion) and are presented in Table 1. Only zeolite β and ZEOL were dealuminated effectively with HCl. This method was not very successful for ZSM-5, since the employment of even a 5 N HCl solution increased the Si/Al only slightly (compare ZSM-5 with ZSM5-5 in Table 1). Additional dealumination of USY2 was attempted with HCl solutions, but this method resulted in the destruction of the zeolite matrix even for the cases where very dilute HCl solutions were employed. The total acidity of the dealuminated samples was determined with the n-butylamine titration method (Table 2). As expected, the acidity of the dealuminated

Table 2. Acid Amount in mmol of n-Butylamine/g of Catalyst (First Column) and in Molecules of n-Butylamine/g of Catalyst (Second Column) as a Function of the Extent of Dealumination for the Various Catalysts (Neutral Red Was the Indicator Employed, H0 ) +6.5) acid amount (mmol of butylamine/ g of cat.)

catalyst

acid amount (molecules of butylamine/ g of cat.) × 10-20

B B25 B4 B5 B7

β Zeolite 2.67 1.73 1.57 1.16 1.07

16.1 10.4 9.5 6.9 6.4


ZEOL 2.87 2.26 1.55 0.77

17.3 13.6 9.3 4.6


USY 1.93 a 1.23

ZSM5 ZSM5-15 ZSM5-25 ZSM5-5

ZSM-5 2.07 1.77 1.53 1.09

11.6 7.4 12.4 10.6 9.2 6.5

a For the USY1 (CBV-712) sample no color change from the acid form of the indicator to its basic form was detected repeatedly.

samples decreases with increasing dealumination severity. 3.1. Catalytic Behavior. The catalytic performance of the zeolites employed with mixtures of hydrocarbons was investigated for reforming type reactions. Emphasis is given on the effect of the zeolite structure and the level of dealumination. Two hydrocarbon mixtures were employed (Table 3); the first one had a paraffinic character (60 mol % paraffins), while the second had a highly naphthenic character (70 mol % naphthenes). Representatives of paraffins, aromatics, and naphthenes are employed in various proportions. The experiments were performed at 470 °C under a total pressure of 90 psig. This pressure was selected because, in the earlier work of Smirniotis and Ruckenstein (1994b), it was shown that there is an optimum operating pressure in the range of 80-130 psig which favors the dehydroisomerization of alkylcyclopentanes. It is expected that lower operating pressures will result in faster deactivation of the catalyst due to coke formation, while higher pressures will favor to a large extent hydrocracking reactions. The conversion X and the selectivity Si of the product i are defined as: X)

[

1-

moles of unconverted reactant hydrocarbons moles of the reactant hydrocarbons fed into the reactor

]

× 100 (I)

Si )

of i species produced × 100 [moles moles of all the products ]

(II)

The above definition for the conversion is suggested by Hughes et al. (1986) when a mixture of hydrocarbons is employed as feeds. A maximum value for the conversion of 95% was observed for all our experiments because the benzene (5 mol % with both feed mixtures) did not react.

3058 Ind. Eng. Chem. Res., Vol. 35, No. 9, 1996 Table 3. Compositions of the Paraffinic and Naphthenic Mixtures Employed hydrocarbons

paraffinic mixture (mol %)

naphthenic mixture (mol %)

n-hexane n-octane benzene ethylbenzene methylcyclopentane methylcyclohexane propylcyclohexane

33 27 5 5 15 10 5

5 10 5 10 25 25 20

The purpose of this study is to identify the most promising catalysts (two different zeolites with the optimum Si/Al ratio) for future studies which will involve the evaluation of zeolite/alumina composites for industrial naphthas. From this comparison it is expected that zeolite catalysts with constant activity as a function of time on stream will be found. In addition, from the product distribution obtained, a deeper understanding about the various reactions occurring over the employed zeolites as a function of the Si/Al is gained. β-Zeolite. From the total product selectivities with the paraffinic mixture over zeolite β catalysts (Figure 1a), one can observe a minimum in the methane selectivity as a function of the Si/Al ratio. Moreover, the C3, C4, and C5 selectivities for the B4 and B5 catalysts pass through a maximum (Figure 1a,b). These trends are observed for the other zeolites, as will be shown later when the Si/Al ratio varies considerably with both mixtures at the high conversions employed in the present study. A possible explanation for this unexpected behavior is proposed. The paraffins from either the reacting feed mixture or the ones formed by opening of the cyclic ring of naphthenes can form a carbonium or carbenium ion over the Brønsted or Lewis sites, respectively. As demonstrated by Corma et al. (1985), a possible pathway for primary cracking can proceed via the formation of carbonium ions (pentacoordinated ions) over the Brønsted sites, followed by the protolytic cracking of the latter ions to form a gas-phase paraffin and a smaller carbenium ion (tricoordinated ion). The latter ion cracks further via β-scission to form an olefin and an even smaller carbenium ion. According to the second pathway, a carbenium ion can be formed from a paraffin via abstraction of a hydride ion by a Lewis site; this carbenium ion generates via β-scission a gas-phase olefin and a smaller carbenium ion. Carbenium ions can also be formed from the protonation of olefins over the Brønsted sites; olefins are generated by the fast dehydrogenation of the feed (both paraffins and naphthenes) over Pt sites. Reactions such as A-type rearrangements (via alkyl and hydride shifts), B-type rearrangements which take place via protonated cyclopropanes (Weitkamp, 1982; Gianneto et al., 1986), and β-scissions involving the above carbonium/carbenium intermediates can take place on the catalyst surface and are responsible for the cracking of the parent hydrocarbons. A typical example of β-scissions of a C5+ hydrocarbon is given as

CsCsC+sC- - - - -CsCsC f CsCsCdC + CsC+sC (1) Hence, the increased CH4 selectivity in the products at low Si/Al ratios cannot be a result of β-scissions of long

paraffins since CH4 is not favored by the above type of reaction (Abbot and Wojchiechowski, 1988). Evidently, some other type of reaction is responsible for this behavior. It was shown earlier (Lombardo and Hall, 1988; Corma et al., 1994; Yaluris et al., 1994) that isobutane cracking over the numerous Brønsted sites of zeolites can lead to methane (reaction 2.1). This

C H

H

C

C C

C + H+

C H

C

CH4 + C C

C

C H2 + C C

C

C

(2.1)

(2.2)

monomolecular cracking mechanism (Haag and Dessau, 1984; Krannila et al., 1992) proceeds via carbonium ions (pentacoordinated carbon atoms). According to this reaction, gas-phase isobutane is protonated on the Brønsted sites and results in the formation of a carbonium ion. The protolytic cracking of the latter ion results in CH4 and a C3 carbenium ion. The fact that for relatively low Si/Al ratios (B25 catalyst and B (the latter one is not shown in Figure 1)) the CH4 selectivity is higher than the C3, C4, and C5 selectivities indicates that the produced C3 carbenium ion (reaction 2.1) does not prefer to desorb from the Brønsted sites as propene (reaction 3); the latter molecule could be further hydro-

CsC+sC f H+ + CdCsC

(3)

genated to propane. If this was the case, the C3 selectivity would be significantly higher than that of CH4 since equimolar quantities of CH4 and propane would be generated for each i-butane molecule through reaction 2.1, and, in addition, propane would be generated by the direct center primary cracking of C6-C8 paraffins. Under the present operating conditions (elevated H2 pressure) reaction 2.2 is inhibited in comparison to reaction 2.1; thus, CH4 is the product of the monomolecular cracking of i-butane and n-butane. Additional CH4 can be generated not only from the cracking of i-butane but also from the monomolecular cracking of n-butane (Haag and Dessau, 1984) proceeding through a carbonium ion mechanism (Krannila et al., 1992). It seems that the C3 carbenium ion generated by reaction 2.1 over the Brønsted sites of the zeolites with relatively low Si/Al ratios participates under the present conditions preferably in bimolecular-condensation reactions (Smirniotis and Ruckenstein, 1994d) with C4+ olefins. The latter hydrocarbons are generated either from the dehydrogenation of paraffins on Pt or by β-scissions of bulkier carbenium ions (most likely). These reactions are favored by the relatively high density of aluminum sites, which implies that higher concentrations of the C3 carbenium ions (reaction 2.1) are closer to Pt sites (dehydrogenation sites which generate olefins) or other acid sites from which olefins desorb as products of β-scissions. The role of this “intimacy” effect is more pronounced for catalysts with a relatively low Si/Al ratio. In the work of Corma et al. (1990), it was shown that the ratio of cracking to hydrogen-transfer reactions increases very rapidly with the increase of the dealumination due to the inhibition of the latter type of reactions. A typical bimolecularcondensation reaction for the generation of [C7+] carbenium intermediates (formed from a C3 carbenium ion (reaction 2.1) and an olefin) followed by skeletal isomerizations and finally cracking which generates i-butane


Figure 1. Product selectivities with the paraffinic mixture over the Pt/β zeolite samples at 470 °C (WHSV ) 2.17 h-1, H2/oil ) 5.6, operating pressure ) 90 psig, 2 h on stream): (a) total hydrocarbons, (b) C4, C5, and C6 paraffins, (c) aromatics.

is presented in reaction 4. The produced i-butane can participate again in reaction 2.1,thus increasing the net production of CH4.

Table 4. Conversion (Estimated According to eq I) and Molar Ratios of the Branched to Normal C4 and C5 Hydrocarbons for Both Paraffinic and Naphthenic Feeds paraffinic mixture

C H C+C

C C H

C C H

C

C

C H

R

bimolecular condensation

H

hydride shift

H

C

C C (classical H carbenium ion)

C

C H C C

C

R

H C

H

C

C

C

C

catalyst

H

C

C

C

R

H methyl shift

C C C

C

C

C

hydride shifts

C R

C C C

C

C

C

C R

scission

C C C

+ C

C

C

R

C

hydride transfer from R′

C

Note: R stands for alkyl species R′ stands for gas-phase hydrocarbon

C + C C

C

C R′

C

R (4)

Evidence of the involvement of the generated i-butane from either reaction 4 or primary hydrocracking reactions in the monomolecular cracking reactions (reaction 2) is the molar ratios of i-C4/n-C4 presented in Table 4. These ratios are lower than unity (about 0.6) for the conditions employed in this study (conversion > 90%), indicating that the generated i-butane participates in other reactions. In the earlier studies of Gianetto et al. (1986) involving n-alkanes over bifunctional zeolites and of Lombardo et al. (1988) involving neopentane, it was found that the i-C4/n-C4 molar ratios in the initial product distribution were larger than unity.

naphthenic mixture

i-C5/ n-C5

X, %

i-C4/ n-C4

i-C5/ n-C5

B25 B4 B5 B7

94.8 94.2 93.9 94.9

Pt/β Zeolite 0.65 1.43 0.56 1.19 0.64 1.40 0.60 1.39

94.9 94.7 93.1 92.9

0.67 0.70 0.76 0.77

1.45 1.44 1.45 1.46


95.0 94.9 94.9 92.1

Pt/ZEOL 0.60 a 0.65 1.54 0.71 1.64 0.62 1.48

94.9 94.8 94.7 95.0

0.61 0.63 1.06 0.69

a 1.65 1.76 1.63


90.4 87.9 95.0

Pt/USY 0.68 1.29 0.86 1.83 0.77 1.44

72.8 89.9 93.8

0.54 0.99 0.91

0.91 1.60 1.48

ZSM5 ZSM5-15 ZSM5-5

95.0 95.0 95.0

Pt/ZSM-5 0.66 0.0b 0.54 1.38 0.65 0.0b

95.0 95.0 95.0

0.69 0.66 0.61

1.85 1.38 1.77

R

H

C

X, %

i-C4/ n-C4

a

No C5 paraffins are detected.

b

No i-C5 paraffins are detected.

It is well-known that, in general, the strength of the Brønsted sites increases with the extent of dealumination. Earlier IR studies of dealuminated β zeolite reveal that the number of Brønsted sites decreases with dealumination, but their strength increases (Maache et al., 1993). The same trend has been observed with other zeolites as well. For β zeolite, when the Si/Al ratio increases, the CH4 selectivity decreases while those of C3, C4, and C5 paraffins increase (Figure 1a,b). This behavior is a result of the decrease of the importance of reaction 2 due to the significant decrease of the number of acid sites with increasing the Si/Al ratio. Corma et al. (1994) observed during the cracking of i-butane that the rate of desorption of the C3 carbenium ion from the Brønsted sites leading to propene decreases

3060 Ind. Eng. Chem. Res., Vol. 35, No. 9, 1996

Figure 2. Product selectivities with the naphthenic mixture over the Pt/β zeolite samples at 470 °C (WHSV ) 2.32 h-1, H2/oil ) 5.4, operating pressure ) 90 psig, 2 h on stream): (a) total hydrocarbons, (b) C4, C5, and C6 paraffins, (c) aromatics.

when the reaction temperature decreases and when the strength of the Brønsted acid sites increases (case of high dealumination). It was suggested that the desorption of the C3 carbenium ion depends on its average lifetime, which is, of course, relatively low when the strength of the acid sites is low (case of low extents of dealumination). Lombardo and Hall (1988) observed with i-butane cracking that the increase of the lifetime of the carbenium ions associated with the increase of the strength of the acid sites results in the shifting of the product spectrum from methane, propene, and butenes to propane, n-butane, and pentanes. Under these conditions, the product distribution is mainly a result of secondary hydrocracking. A minimum of the CH4 and a maximum of the C3 and C4 paraffins occurs for the catalyst B5. For even higher extents of dealumination (catalyst B7 or more dealuminated zeolites) the CH4 selectivity increases again while those of C3, C4, and C5 paraffins drop. In order to justify the increase of the CH4 selectivity for the catalysts with the highest degree of dealumination, we propose that for this catalyst the effect of the hydrogenolysis of the reactants and of the primary and secondary products over the Pt sites becomes dominant. Under these conditions (low density of acid sites), the role of the acid sites on the product distribution decreases significantly. Similar trends for the methane and the C3, C4, and C5 hydrocarbons occur for the naphthenic feed as well. This is the case because the Si/Al ratio of all zeolites employed increases within a considerable range with the extent of dealumination (Table 1). No significant differences were observed for the C4 and C5 selectivities of the paraffinic and the naphthenic feed (Figure 2). It should be noted that the C4 and C5 (normal and iso-) paraffins (potential feeds for alkylation reactions) acquire a maximum for intermediary levels of dealumination. This information can be useful to refineries if

the target is the production of relatively large quantities of alkylates. In contrast to what is observed with β zeolite, as will be shown later for ZSM-5 zeolites which possess almost the same Si/Al ratios, the minimum of CH4 with respect to Si/Al cannot be detected. This is because the extents of the different reactions occurring simultaneously over our ZSM-5 catalysts do not vary significantly in this Si/Al range. Regarding the selectivities of the C6 paraffins, only 2- and 3-methylpentane were observed (Figure 1b). Paraffins with more than six carbon numbers were not detected as final products under these conditions. The selectivities of aromatics for both feeds (Figures 1c and 2c) remain at relatively low levels, indicating that the feed naphthenes are partially transformed into paraffins. Aromatics are generated over our catalysts via the direct dehydrogenation of naphthenes with C6-cyclic ring, dehydroisomerization of methylcyclopentane, disproportionations of large aromatics to lower molecular weight aromatics, and bimolecular alkylation reactions of light hydrocarbons with naphthenes or aromatics (Schulz et al., 1973; Smirniotis and Ruckenstein, 1993a). The effect of oligomerization of olefins for the generation of aromatics is minimal under the present conditions (increased H2 pressures). The fact that the benzene selectivity was the highest among all the aromatics and only traces of C9 aromatics were detected despite the fact that both feed mixtures had 5 and 20 mol % propylcyclohexane (C9 naphthene) indicates that the extent of disproportionation reactions of aromatics is relatively high. The aromatic selectivities with the naphthenic mixture are not as high as one would expect if one considers the composition of the feed mixtures (the naphthenic feed has 70% naphthenes). This indicates that a relatively high proportion of the naphthenic part of the feed under the present conditions is transformed to lighter hydrocarbons. ZEOL. The product distributions over ZEOL dealuminated catalysts are presented in Figures 3 and 4. As


Figure 3. Product Selectivities with the paraffinic mixture over the Pt/ZEOL samples at 470 °C (WHSV ) 2.17 h-1, H2/oil ) 5.6, operating pressure ) 90 psig, 2 h on stream): (a) total hydrocarbons, (b) C4, C5, and C6 paraffins, (c) aromatics.

Figure 4. Product selectivities with the naphthenic mixture over the Pt/ZEOL samples at 470 °C (WHSV ) 2.32 h-1, H2/oil ) 5.4, operating pressure ) 90 psig, 2 h on stream): (a) total hydrocarbons, (b) C4, C5, and C6 paraffins, (c) aromatics.

for the case of the dealuminated β zeolites, the CH4 selectivity acquires a minimum for some Si/Al ratio of about 54 with both feed mixtures (Figures 3a and 4a). For this catalyst, a maximum of the C3 selectivities of paraffins for the paraffinic mixture and of the C3 and C4 selectivities for the naphthenic mixture occurs. The C3 selectivity of paraffins passes through a maximum for the ZEOL3 catalyst. The Si/Al ratios of the dealuminated ZEOL where the maximum of the C3 selectivity occurs are shifted to lower values than those of zeolite β (Table 1). The proposed explanation for this type of

unexpected behavior was presented earlier for the case of β zeolite and can be used for ZEOL as well. A comparison of the C1-C6 paraffinic selectivities of ZEOL with β zeolites shows that the selectivities over the former zeolite acquire slightly higher values in average. With the ZEOL zeolites the spectrum of paraffins has been shifted to hydrocarbons with lower carbon numbers. One can observe that the C4 and C5 selectivities of ZEOL (Figures 3b and 4b) are lower than those of β zeolite probably because of the higher aluminum content of ZEOL. In addition, no C6 hydro-


Figure 5. Product selectivities with the paraffinic mixture over the Pt/USY samples at 470 °C (WHSV ) 2.17 h-1, H2/oil ) 5.6, operating pressure ) 90 psig, 2 h on stream): (a) total hydrocarbons, (b) C4, C5, and C6 paraffins, (c) aromatics.

carbons were observed over the ZEOL zeolite. However, the total acidities of ZEOL zeolites as determined with butylamine titrations (Table 2), which correspond to the total number of acid sites, are comparable with those of β zeolite. This is not as expected since the former zeolite has a lower Si/Al ratio. The isomerization capability of ZEOL seems to be similar to that of β zeolite since the i-C4/n-C4 and i-C5/n-C5 ratios for both zeolites are approximately the same (Table 4). The aromatic selectivities, as expected, are lower than the case of β zeolite for both the mixtures employed. Benzene acquires the highest selectivity for any Si/Al ratio. USY. A type of behavior similar to that of the other two zeolites described earlier is observed for USY. A very pronounced minimum of the CH4 selectivity and a maximum of the C3, C4, and C5 paraffinic selectivities is observed with respect to the Si/Al ratio (Figures 5a,b and 6a,b) for both mixtures. In contrast to the β zeolite and ZEOL, the reaction pathways which are responsible for this type of behavior occur for lower Si/Al (the Si/Al ratio of USY1 is 5.8, while those of the former zeolites are about 75 and 55, respectively). It seems that the occurrence of these reactions is not only a function of the Si/Al ratio of the zeolites but a function of the zeolite structure as well. The supercages of USY zeolite (12.2 Å) favor the accumulation of coke precursors, and therefore coke builds up to a much higher extent than the case of β zeolite and ZEOL which possess channel intersections with sizes of about 10 Å. The ratios of the iso-paraffins/normal-paraffins follow trends similar to those of β zeolite and ZEOL. In contrast to the ZEOL, the C4-C6 paraffinic selectivities are higher over USY probably due to the fact that the former zeolite favors the polystep cracking of the feed components to a higher extent. The aromatic selectivities over the USY catalysts are comparable with those of β zeolite and higher than those of ZEOL. ZSM-5. For comparison purposes, three ZSM-5 catalysts were evaluated with both feed mixtures. The Si/

Al ratio of all the ZSM-5 catalysts employed is about 50, and it does not increase significantly with the severity of dealumination. HCl leaching was employed for the dealumination of the synthesized zeolite; this method evidently is not very effective to dealuminate ZSM-5. From the product selectivities with both feeds, the CH4 selectivity does not pass through a minimum (Figures 7a and 8a), as observed with all the other zeolites. Moreover, the selectivity of propane remains almost unchanged as a function of the Si/Al ratio of the catalysts. This is as expected, since the extent of the various types of reactions occurring does not vary significantly in the given range of Si/Al ratios. The CH4 selectivity is surprisingly high and can be compared with that of propane for the paraffinic feed (Figure 7a). This is in contrast to what was observed with the other zeolites since for this range of Si/Al ratios one could expect that the CH4 would acquire its minimum value. This indicates that, under the present operating conditions and Si/Al ratios, for ZSM-5 reaction 2, which is responsible for the CH4 generation, is dominant. This observation is in agreement with the findings of Haag and Dessau (1984), who supported that over ZSM-5 the monomolecular cracking (through pentacoordinated ion) was favored while large-pore zeolites and amorphous SiO2/Al2O3 favored the cracking via carbenium ions. This was not the case for the naphthenic mixture (Figure 8a) since the CH4 selectivity was lower than that of propane probably because of the different nature of the latter feed mixture in comparison with the paraffinic one. The i-C4/n-C4 and i-C5/n-C5 ratios follow the same trends as the other zeolites. No C6 paraffins were detected, indicating that the acid sites of ZSM-5 are relatively stronger than those of β zeolite and USY; a similar behavior was observed for the ZEOL. A similar observation was reported by Abbot and Wojciechowski (1989), where the product distribution over ZSM-5 is shifted to products with lower carbon number in com-


Figure 6. Product selectivities with the naphthenic mixture over the Pt/USY samples at 470 °C (WHSV ) 2.32 h-1, H2/oil ) 5.4, operating pressure ) 90 psig, 2 h on stream): (a) total hydrocarbons, (b) C4, C5, and C6 paraffins, (c) aromatics.

Figure 7. Product selectivities with the paraffinic mixture over the Pt/ZSM-5 samples at 470 °C (WHSV ) 2.17 h-1, H2/oil ) 5.6, operating pressure ) 90 psig, 2 h on stream): (a) total hydrocarbons, (b) C4, C5, and C6 paraffins, (c) aromatics.

parison to Y faujasite. The aromatic selectivities are at very low levels especially with the paraffinic feed. 3.2. Time on Stream Behavior. The activity of all the large-pore zeolites employed in the present study with time on stream (TOS) was investigated as well. Only the naphthenic feed was employed since we preferred to evaluate our catalysts under the most favorable conditions for coke generation (its high naphthenic content favors coke formation to a higher extent than the paraffinic one). Dealuminated zeolites were also employed because it was found that the increase

of the Si/Al ratios can lead to time-stable activity of the catalyst as a function of time (Smirniotis and Ruckenstein, 1994b,c). It was shown that the activity of dealuminated β zeolite with SiO2/Al2O3 ) 130 and 0.5 wt % Pt loading remains unchanged after 3 days on stream. In addition, the selectivity of aromatics was increased after the initial 10 h on stream probably because of the selective poisoning of the strong Brønsted sites due to the deposition of small amounts of coke. A comparison of the activity of the zeolite β samples with TOS (Figure 9) shows that the nondealuminated


Figure 8. Product selectivities with the naphthenic mixture over the Pt/ZSM-5 samples at 470 °C (WHSV ) 2.32 h-1, H2/oil ) 5.4, operating pressure ) 90 psig, 2 h on stream): (a) total hydrocarbons, (b) C4, C5, and C6 paraffins, (c) aromatics.

Figure 9. Conversion versus time on stream for selected catalysts with the naphthenic mixture (operating pressure ) 90 psig, reaction temperature ) 470 °C, WHSV ) 2.32 h-1, H2/oil ) 5.4).

sample (B) loses its activity rapidly. The activity of the dealuminated β zeolite (B5) remains unchanged for about 1 day on stream, but it decreases slightly for longer periods of time. This is not as expected based on earlier experience (Smirniotis and Ruckenstein, 1994b,c) using dealuminated β zeolite with Si/Al > 50. It seems that the involvement of a mixture of different categories of hydrocarbons leads to significantly higher rates of generation of coke precursors which cover both the Pt and the acid sites and block the pore openings. In addition, the fact that a lower H2/oil molar ratio was employed in the present work in comparison with the earlier work (Smirniotis and Ruckenstein, 1994b), where the H2/oil molar ratio was 12, is another reason for the increased rates of deactivation due to coke. Higher H2 partial pressures can enhance the rates of hydrogenation of unsaturated olefinic coke precursors located on the active sites, which further desorb from the active sites and form gas-phase hydrocarbons. As expected,

the selectivities for light hydrocarbons decrease rapidly with time while those of aromatics increase. The activity of the USY2 catalyst (dealuminated USY with Si/Al ) 28) decreases with time continuously and reaches almost a plateau value after about 2 days on stream. The activity of USY2 decreases faster than that of B catalyst. It should be noted that the latter catalyst has a lower Si/Al ratio (Si/Al ) 14.5) than USY2 (Si/Al ) 28.0). Evidently, this is due to the different pore structures of the above zeolites and especially the dimensions of the channel intersections. The channels of Y faujasite are 7.4 Å and are comparable with those of β zeolite which are about 7.3 × 6.5 Å (straight channels) and 5.6 × 5.6 Å (tortuous channels) (Newsam et al., 1988). In contrast, the supercages of Y faujasite are about 12 Å, while the channel intersections of β zeolite are slightly less than 10 Å. This results in higher rates of accumulation of coke precursors in the supercages of the former zeolite than β zeolite. Hence, despite the fact that both are large-pore zeolites (12-membered ring pore), they exhibit much different behaviors in terms of coke generation. The activity of dealuminated mordenite (M3; Si/Al ) 35.7) decreases rapidly with time and reaches a plateau value which is lower than those of B and USY2 catalysts. Mordenite consists of combinations of 8-membered ring pores (2.6 × 5.7 Å) with 12-memberd ring pores (6.5 × 7.0 Å). It seems that its pore structure favors the generation of coke to a higher extent than β zeolite and USY. The dealuminated L zeolite (L3) possesses low initial activity which remains unchanged as the time on stream increases. The zeolite ZEOL demonstrates a superior behavior in comparison with the other zeolites. The nondealuminated ZEOL loses its activity slightly with time on stream in a period of 3 days. However, the activity of the dealuminated ZEOL (ZEOL3; Si/Al ) 53.9) under identical operating conditions with the other catalysts remains unchanged in a period of more than 250 h. This


unexpected observation was reproduced repeatedly. It is worth noting that the latter catalyst (large-pore zeolite) after the end of the run was slightly gray, in contrast to the other zeolite catalysts which were black due to excessive coke deposition. In addition, one should observe that the Si/Al ratio of ZEOL3 is lower than that of B5 (Si/Al ) 85), something that indicates that the exceptional behavior of the former zeolite is not due to the increased level of dealumination. Careful dealumination of zeolites pertaining to the decrease of acid sites (type of sites which are mainly responsible for coke formation) is not sufficient to ensure minimal coke formation in the zeolite pores. It is the combination of the Al content of the zeolite with its pore structure that can lead to zeolite-based catalysts with time-stable behavior for reforming or generally acid-catalyzed reactions. The activity of dealuminated ZSM-5 (medium-size pores) with a Si/Al ratio of 54.6 remains unchanged as a function of time on stream. 4. Conclusions In the present study it was found that the increase of the Si/Al ratio of large-pore zeolites can have a very important effect on the relative extent of the various secondary reactions occurring when mixtures of hydrocarbons were involved. A minimum of the CH4 selectivity which is associated with a maximum of the C3, C4, and C5 selectivities was observed for all the 12membered ring-pore zeolites employed for a certain range of Si/Al ratios. This behavior is a result of the variance of the extents of the following main reactions: (1) primary cracking of the components of the feed mixtures, (2) the monomolecular cracking (proceeding via carbonium ions) of C4 paraffins for the generation of CH4, (3) the bimolecular-condensation reactions followed by recracking which is responsible for the redistribution of the primary products of cracking, and (4) hydrogenolysis of the reactants and products on the Pt sites. The range of Si/Al ratios where the above behavior is observed varies with the zeolite structure. The employment of hydrocarbon mixtures as feeds results in relatively high coke generation. The decrease of the aluminum content of the zeolites is not sufficient to ensure low rates of coke deposition. It was found that the zeolite pore structure can play a very important role as well. It is the combination of the Si/Al and the zeolite pore opening which can lead to the time-stable activity under coke deactivation conditions. Dedication This paper is dedicated to Professor Eli Ruckenstein in recognition of his 70th birthday. I met him in the summer of 1990 when I started my Ph.D. research under his supervision at S.U.N.Y. at Buffalo. Dr. Ruckenstein’s dedication to science has been a standard to exemplify throughout my research work and my career. His valuable scientific insights, knowledge, and continued guidance as I pursued my doctorate were the most important ingredients which catalyzed my work. I enjoyed the close working relationship that culminated through endless hours that we spent together. He taught me through our research to be creative and that diligence and perseverance is what every scientist should possess. Through the years we formed a mutual bond of respect and admiration which I will carry with me

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Received for review December 18, 1995 Revised manuscript received April 18, 1996 Accepted April 19, 1996X IE9507639

X Abstract published in Advance ACS Abstracts, August 15, 1996.

Al Ratio and of the Zeolite Structure on the Performance

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