Mordenite, Aluminum-Deficient Mordenite, and Faujasite Catalysts in

Mordenite, Aluminum-Deficient Mordenite, and Faujasite Catalysts in the Cracking of Cumene Paul

E.

Eberly, Jr., and Charles N. Kimberlin, Jr.

Esso Research Laboratories, Humble Oil & Refining Co., Baton Rouge, La.

70821

The adsorption and catalytic properties of H-mordenite with a conventional SiO*/A1203 ratio of 12 were compared to those of a highly aluminum-deficient mordenite with

a ratio of 64. Cumene adsorption studies at 200" F show that H-M(64) has a considerably larger adsorption capacity and a greatly decreased resistance to adsorptive diffusion. Cumene cracking was studied at 400", 450", 500", and 550°F for extended periods of time to determine deactivation rates. The activity ( A ) of both catalysts decreases with time according to the relationship A

= afn, where n i s approximately equal to -0.5 at temperatures of 450" and above. The aluminum-deficient mordenite, H-M(64),

i s considerably more active than H-M(12), which i s attributed to its lower diffusion resistance. The apparent activation energy of 1 1 kcal per mole i s the same for both catalysts and i s lower than that observed on other catalyst systems indicating diffusional limitations. Steaming of catalysts at 1400" F greatly reduces activity. Comparable results with hydrogen-faujasite indicate unusually high deactivation at 400" F but relatively low deactivation at 525" F.

M o r d e n i t e - t y p e zeolites have become of increasing interest as catalysts and adsorbents in the petroleum industry (Burbidge et al., 1970). The structure of the sodium form of natural mordenite has been described by Meier (1961). Basically, the pore structure consists of parallel adsorption tubes having an approximately elliptical opening with major and minor diameters of 6.95 and 5.81 A, respectively. There are smaller side pockets perpendicular to the main tubes which can adsorb only molecules smaller than n-butane (Barrer and Peterson, 1964). These side channels have restrictions which essentially prohibit motion of molecules from one main tube to the other. Because of the two-dimensional nature of its pore structure, mordenite is susceptible to loss of much of its adsorptive capacity by the presence of small amounts of impurities (Beecher et al., 1968). By varying synthesis conditions, it is possible to prepare mordenites which have different adsorptive properties due to some, as yet undefined. change in structure. Mordenites have been classified as "large-port'' or "small-port," depending on whether or not they adsorb large molecules such as benzene and cyclohexane (Sand, 1968). Further varieties of mordenite can be produced by removing aluminum from the structure by strong acid treatment with HC1 (Dubinin et al , 1968). The present investigation compares the cracking and sorption properties of H-mordenite with a conventional S i 0 2 / A l r 0 ratio ~ to those of a highly aluminum-deficient mordenite. Cumene cracking was selected as the test reaction to elucidate the differences in catalytic properties of the mordenites. For comparison, some results on H-faujasite are also presented.

The catalytic properties of mordenite have been reviewed by Burbidge et al., (1970). In general, mordenites have high activity initially but activity maintenance is poor. In regard to catalytic cracking, results with various reactants have been reported: n-butane and n-pentane (Benesi, 1967), n-hexane (Weller and Bauer, 1969), n-heptane, 2,4-dimethylpentane, and methylcyclohexane (Adams et al., 1965), cumene (Keough, 1963; Topchieva et al., 1968), n-decane (Keough, 1963), and East Texas light gas oil (Adams et al., 1965). Hydrocracking on Pd-H-mordenites has also been reported using n-decane and Decalin feeds (Beecher et al., 1968). The latter present data on both conventional and aluminum-deficient mordenites. The adsorptive and diffusion properties of mordenite can vary widely, depending on whether the mordenite is "large-port" or "small-port,'' and on the Si02/A1201ratio. I n earlier papers, the adsorption of C,-C, paraffins (Satterfield and Frabetti, 19671, benzene, n-hexane, and cyclohexane (Eberly, 1963), and n-octane, toluene, and Decalin (Beecher et al., 1968) was reported. Barrer and Peterson (1964) studied the adsorption of inert and other light gases. Diffusion properties were measured by a gas chromatographic technique using Ar, Kr, and SFs (Eberly, 1969). Katzer (1969) observed no counterdiffusion in the liquid phase for benzene and cumene in the pores of mordenite. Experimental

Materials. The chemical compositions of the zeolites used in the study are given in Table I. H-M(12) was Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, No. 3, 1970

335

Table 1. Composition of Mordenite and Faujasite Catalysts Catalyst

H-M(l2)

H-Y(4 7)

H-M(64) Composition, Wi. %

NarO

0.1 7 12.5 87.4 11.9

A1203

SiO? Si02/Al?Oqratio

0.0 2.6 97.4 63.6

1.29 26.3 72.4 4.67

Table II. X-Ray Diffraction Patterns of H-M( 12) and H-M( 64)

(CUKa radiation) H-M(12)

28

37 36.5 35.7 35.1 34.7 34.1 33.1 31.0 30.5 28.8 28.4 27.6 26.3 25.7 24.5 23.7 23.2 22.3 21.4 21.0 19.6 18.3 17.6 17.3 15.3 14.7 13.8 13.5 9.8 8.7 6.5

H-M(64) 1/10

x 100 3 4 8 4 2 1 3 18 4 4

4 44 33 80 2 9 16 61 3 2 38 1 4 a

16 8 20 50 100 30 61

29

l / l , > x 100

37.1 36.8 36.9 35.4 34.8 34.3 33.2 31.1 30.7 29.0 28.5 27.8 26.5 25.9 24.7 23.8 23.4 22.5 21.5 21.2 19.8 18.4 17.7 17.5 15.4 14.8 14.0 13 6 9.9 8.7 6.6

2 4 7 3 2 1 2 16 4 3 4 36 27 66 1 10 15 41 2 1 35 1 3 4 18 8 15 63 100 24 38

occur are shifted by several tenths of a degree to higher values. This indicates a slight decrease in the unit cell dimensions. The relative intensities of the lines ( I lox 100). however. are essentially the same for the two materials. To obtain a measure of the relative crystallinity. the sum total of all the diffraction lines of H - M ( l 2 ) calcined at 1000°F was arbitrarily set equal to 100. Data on the other materials are compared t o this value and are listed in Table TI1 along with other physical and chemical properties. H-M(64) is seen to have 9 0 5 of the crystallinity of H - M ( l 2 ) . These general observations from the x-ray diffraction patterns are similar to those reported by Dubinin et al , 1968. When standard N, adsorption techniques are used, H-M(64) is seen to have a higher Langmuir surface area than H - M ( l 2 ) . Its pore volume is nearly 1.8 times as great. Samples were also steamed at 1400°F for 16 hours. The results show significant losses in crystallinity and surface area and no change in the pore volume. The last column in Table 111 lists comparable data on H-Y(4.5), which has nearly twice the surface area of the mordenites. The cumene was obtained from Matheson. Coleman and Bell and was passed through a column of SiOl gel to remove any peroxides which might be present and which are known to affect catalytic activity (Prater and Lago. 1956). Adsorption Measurements. A Cahn microbalance was used to measure the adsorption of cumene. About 20 mg of catalyst were placed on one arm of tk balance. This was evacuated and heated to 800°F to remove adsorbed impurities. The sample was then cooled to the desired temperature (200'. 400". or 550°F) and small volumes of cumene were pipetted into the system through a Hg-covered glass frit. Weight changes were continuously recorded, Pressures were read with a Model 145, precision pressure gage (Texas Instruments). ('umene Cracking Studies. The catalysts were compressed into pills. which were then cracked and a 28-35mesh fraction was isolated. A 0.5-cc portion (-400 mg) was calcined in air at 1000°F. The catalyst was then transferred under nitrogen to a nominal l-inch stainless 4.0

3.5

prepared from the sodium form of mordenite by exchange with ",No? and subsequent calcination in air a t 1000' F. Theoretically, the ratio of SiOl / A1201in conventional mordenite has been frequently reported as 10 (Burbidge et al., 1970; Meier. 1961). Actually. several samples of nonacid-extracted mordenite have been found t o have higher values in the range of 10.5 to 15 (Adams et a1 , 1965: Eberly, 1963; Weller and Bauer, 1969). To prepare the aluminum-deficient mordenite H-M (64). the hydrogen form was extracted for several hours with 5lv HC1. The material was then washed. dried, and calcined a t 1000"F. The N a 2 0 content was negligible. Hydrogen-faujasite [H-Y(4.7)] was prepared from the Na form by several exchanges with ",NO,. followed by drying and calcining a t 1000°F. The soda content was considerably higher than that of the mordenites. The removal of 8 0 5 of the aluminum from H-mordenite by acid extraction results in surprisingly little change in the x-ray diffraction pattern (Table 11). In the aluminumdeficient material, the angles at which the diffraction lines 336 Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, No. 3, 1970

3.0 2.5 Y

c? E

2

2.0

Y

5

1.5 1.0

0.5

Table Ill. Physical and Chemical Properties Catalyst Calcination conditions Relative x-ray crystallinity, 5 Surface area, m g Pore volume cc g Adsorption capacity fur cumene at LOOd F and 2 mm. wt. r c Activity ( Aj for cuniene cracking a t 500" F

H-R.1i12 i 16 hrs at 1000 100

F

H- M ( 1 2 1 16 hrs a t 1400- F (1 a t m H>Oj

H-Mi64) 16 hrs at 1000° F

64

90

16 hrs a t 1000° F

71

540 0 27 4.0

456 0 29 0.40

602 0 49 6.46

1.20

0.12

2.27

steel, 20-gage tubular reactor, which was placed in a constant temperature fluidized sand bath. After equilibration with helium carrier gas, the cumene was introduced to the reactor by permitting the carrier gas to bubble through a cumene saturator maintained at 65°F. This resulted in a feed rate of about 0.33 w . / h r . / w . Samples of the product gases were collected with a syringe and analyzed immediately by GC techniques, approximately 10 minutes being required for analysis. Conversion was measured by the mole per cent of benzene in the aromatic fraction. Six-hour runs were made to determine catalyst deactivation rates.

H-Y(4.7),

H-M (64) 16 hrs a t 1400' F (1 atm H1O)

952 0.54 20.1

533 0 48 30

2.91

0.035

DESORPTION

40SORPTION

4

,-

3

2 1

Results

Cumene Adsorption Studies. H-M(l2). Adsorption equilibria on hydrogen-mordenite with a conventional silica-alumina ratio of 12 are not rapidly attained. Results are shown in Figure 1 for the adsorption of cumene. At 200°F. the weight increase is due entirely to cumene adsorption. since no chemical reaction occurs a t these conditions. The numbers on the figure refer to the pressures (millimeters) imposed on the sample a t various times. The results a t 200°F show an initial rapid adsorption of a major portion of the cumene, followed hy a slow adsorption process which prevents ready attainment of equilibrium. This effect had been observed with other hydrocarbons (Eherly. 1963). The desorption curves show that the material is very difficult t o remove. At 400' and 550°F. cracking of cumene is known to occur and the weight increase can be due to polymer formation or coke deposition as well as adsorption of both cumene and benzene. The slow adsorption process occurs at a faster rate a t 550" F than at 300' F, probably because of coke deposition which deactivates the catalyst. Even a t these elevated temperatures, the adsorbed material is difficult to remove. H-M(64). Aluminum-deficient mordenite obtained by acid extraction has totally different adsorptive properties. as shown by the results in Figure 2 . At 200°F. where no reaction occurs. the adsorption capacity is significantly higher. Equilibrium is rapidly attained and the adsorbed easily removed. Past results on Decalin adsorplustrated this effect (Reecher et a / . , 1968). The slow adsorption process is absent a t 200" F but noticeable a t 400' and 550"F, where cumene conversion occurs. Analogous to H - M ( l 2 ) . the rate of the slow adsorption is faster at the higher temperature.

20

0

43

60

80

100

0

20

40

60

80

100

T I M , MIN.

Figure 2. Adsorption of cumene on H-M(64)

0 0 A

Numbers are pressure of cumene vapor in mm. Adsorption experiments at 200°,400", and 550" F, respectively. Desorption data obtained by exposing solid to vacuum

Figure 3. Adsorption of cumene at 200'F on H-mordenite steamed at 1400°F 0 Numbers are pressure of cumene vapor in mm.

0

A

Data on H-M(64) and H-M( 12), respectively Desorption data obtained by exposing solid to vacuum

Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, No. 3, 1970

337

i

peratures on H-M(12). Conversion is measured by the production of benzene rather than propylene. Rates computed from propylene production were shown to be in error due to side reactions (Pansing and Malloy, 1965). In these 6-hour periods, it is not possible to obtain a steady-state conversion a t practical levels because of catalyst deactivation. The activity ( A ) of the catalyst was defined as

i

where W is the weight of the catalyst and C is the mole per cent of benzene in the aromatic fraction. The activity was found to obey the equation

DESORPTION

ADSORPTION

1

4

N

Li

1.5 1.0

I

0

20

1

I

I

I

I

I

I

40

60

SO

0

20

40

60

TIME,

I 80

I

100

A = at"

MIN

Figure 4. Adsorption of cumene at 200" F on H-M(64) catalyst discharged from 500" F cracking run Numbers are pressure of cumene vapor in mm. Desorption data obtained by exposing solid to vacuum 100

1

0

40

80

120

160 TIME,

Figure 5 . Cumene cracking on

200

240

280

320

360

MIN.

H-M(12) at 0.33 w./hr/w.

0 400°F

500°F

0 450°F

A 550" F

Steamed Mordenite. Steaming of mordenite catalysts causes losses in crystallinity and adsorptive capacity. H-M(12) after 1400°F steaming has only a tenth of its original capacity (Figure 3 and Table 111). The cumene cannot be readily desorbed. H-M(64) is more stable to steam, at least with respect to adsorption capacity. Only half the capacity is lost and most of the adsorbed material is readily removable. USED H-M(64). A previous investigation had shown that mordenite is susceptible to clogging of its pore structure by small amounts of irreversibly adsorbed material (Beecher et ul., 1968). T o ascertain to what extent this occurred in cumene cracking, a sample of H-M(64) was discharged for adsorption studies after a cracking experiment at 500' F. Reversibly adsorbed material was removed by evacuation at 500°F. Adsorption data a t 200" F are shown in Figure 4. The carbon deposit resulting from cracking caused a loss of over half the material's original adsorptive capacity. This effect would probably be accentuated with H-M(12). Cumene Cracking Studies. H-M(12). Figure 5 gives the results for the cracking of cumene at four different tem338 Ind. Eng. Chern. Prod.

Res. Develop., Vol. 9, No.

3, 1970

(2)

This relationship has been reported for amorphous silicaalumina catalysts by Voorhies (1945) and Eberly et al. (1966). The results from Figure 5 were expressed in this manner and are plotted in Figure 6. Values for the constants in Equation 2 are given in Table IV. The activity a t the three higher temperatures declines approximately with the square root of time. At 400"F, the activity decline is less and the exponent is only -0.3. This reflects the results obtained in the adsorption studies in the microbalance, where the slow weight increase due to carbon deposits deactivating the catalyst was less at 400" than at 550°F. In agreement with previous studies (Adams et al., 1965), this activity decline is greater than that observed on amorphous silica-alumina. H-M(64). Cracking runs were made at the same conditions on aluminum-deficient mordenite (Figure 7). The over-all activity of this catalyst is considerably higher than that of H-M(12). This can be attributed in part to a lowering of diffusion resistance, as illustrated by the adsorption-desorption experiments discussed above. The activity decline follows the same relationship as that observed with H-M(12). The values of n are essentially the same (Table IV). STEAMEDMORDENITES. Severe steaming of mordenite catalysts at 1400" F results in substantial loss of catalytic activity. As seen in Table 111, with H-M(12), the loss in activity is directly proportional to loss in adsorption

4

2

1

0.6

d. 4 0.2

0.1

1 10

20

40

60

100

200

400

TIME, MIN.

Figure 6. Cumene cracking on H-M(12) at 0.33 w./hr./w.

:t

Table IV. Cumene Cracking Results at 0.33 W./Hr./W. Catalyst

Calcination Temp, ' F

Cracking Temp., F

a,

G-I

n

H-M(I2)

1000

400 450 500 Zi0

1.3 8.4 14.3 31.3

-0.30 -0.55 -0.54 -0.59

14-M( 1% )

1400 (H,O)

500

1.1

-0.48

tI-hI(64 )

1000

400 450 500 560

j.0 8.5 23.8 43.7

-0.28 -0.44 -0.61 -0.50

H-R/I (64 )

1400 (H,O)

500

0.2

-0.34

H-Y (4.7)

1000

400 526

30.1 6.9

-0.73 -0.09

0 0 2

-13

l

0.6

--

0.4L 7

10

20

40

60

100

200

400

TIME, MIN.

Figure 7. Cumene cracking on H-M(64) at 0.33 w./hr./w.

capacity. However, with steamed H-M(64), the low activity appears to be caused by a loss of active sites, since this material still has a reasonable adsorption capacity for cumene. COMPARISON WITH H-Y(4.7). Runs were also made on hydrogen-faujasite [H-Y(4.7)],which is a key component in many of the newer, commercial cracking catalysts (Figure 8 ) . At 400"F, the activity undergoes a severe decline wit.h time on stream. H-Y (4.7) has a considerably higher adsorption capacity (Table 111) and a t this low temperature, we suspect that the propylene formed is being rapidly adsorbed and covering the active sites. A previous report demonstrated that propylene on HY undergoes polymerization, dehydrogenation, and cyclization to form aromatic ring structures a t 400°F (Eberly, 1967). This reaction is believed to be primarily responsible for the rapid loss in activity. Apparently, this does not occur to the same extent on mordenite catalysts. At 525" F, the activity is high and deactivation is much less severe. as evidenced by the low value of n in Table I V . Similar effects have been reported for a 10x zeolite (Topchieva et nl., 1963). Discussion

Acid treatment of inordenite t o produce an aluminunideficient sample results in marked changes of adsorption and catalytic properties. The fact that most of the aluminum can be removed with only minor changes in the X-ray diffraction pattern is somewhat surprising. This suggests that perhaps the lattice has reformed, so that the original alumina tetrahedra are not replaced by silica tetrahedra. This process would give a crystal having the same diffraction pattern, with the peaks shifted slightly to larger angles, corresponding to a lower unit cell dimension (the Si-0 bond is shorter than the A1-0 bond). This removal o f aluminum from the lattice eliminates the need for cations or protons for charge neutralization and would therefore be expected to lower acidity. At the same time, however, acid leaching removes extraneous materials from the morderiite channels, resulting in larger adsorptive capacity and a greatly decreased resistance to adsorptive diffusion. This decreased resistance is also aided by lowering the acidity of the surface. The activity decline of mordeiiite catalysts at 450" to 500" follows the relationship, A = at", where n is equal to approximately -0.5. At 400" F, the rate of deactivation is less, with TI. = -0.3. This may be due to some alkylation of the cumene to form diisopropylbenzene which occurs

a t this temperature. This would tend to depress deposition of carbonaceous deposits and thereby result in a less rapid loss of activity. Deactivation of mordenite is greater than that observed on silica-alumina. I n a two-dimensional pore structure such as exists in mordenite, carbon deposits can have a more pronounced effect on activity. The higher activity of H-M(64) can be largely attributed to a lowering in diffusion resistance. At any given time in the cracking cycle, the activity follows an Arrheniustype relationship with temperature and the apparent activation energy amounts to 11 kcal per mole for both H-M(12) and H-M(64). This indicates the reaction is strongly diffusion-limited, since values for other more open catalysts are considerably higher. Pansing and Malloy (1965) obtained 20.6 kcal per mole for cumene cracking on silica-alumina. On various faujasite catalysts, values of 28.2 (Richardson, 1967), 27 to 34 (Topchieva et al., 1968) and 20 (Eberly and Kimberlin, 1970; Topchieva et nl., 1969) have been reported. Diffusion resistance in H-M(64) is considerably less than that in H-M(12), as seen by the adsorption measurements, but this does not increase the apparent activation energy above that for H-M (12). Katzer (1969) observed that counterdiffusion of benzene and cumene did not occur on H-mordenite with conventional SiO2lAlLO3ratio. This is in line with our results on the difficulty of desorbing material from H-M(12) and suggests a low effectiveness factor for this

0.6 0.4

1 1 h

i i

8 10

20

I

1

40

60

100

200

T I M E , MIN

Figure 8. Cumene cracking on H-Y(4.7) at 0.33 w./hr./w. Ind. Eng. Chem Prod. Res. Develop., Vol. 9, No. 3, 1970

339

catalyst. I t would be interesting to determine whether counterdiffusion occurs in H-M(64). Steaming strongly deactivates mordenite catalysts, more than can be accounted for by loss in crystal structure. Perhaps the steaming has resulted in lattice dislocations which diminish adsorption capacity. The activity of H-M(64) is unexpectedly low. The acid sites responsible for activity have apparently been destroyed, since considerable adsorption capacity remains. Acknowledgment

The authors express sincere appreciation t o Robert Lucas for his excellent experimental assistance. Literature Cited

Adams, C. E., Kimberlin, C. N., Jr., Shoemaker, D. P., Proceedings of Third International Congress on Catalysis, 1964, p. 1310, Wiley, New York, 1965. Barrer, R. M., Peterson, D. L., Proc. Roy. Soc. (A) 280, 466 (1964). Beecher, R., Voorhies, A., Jr., Eberly, P. E., Jr., IND. ENG.CHEM.PROD. RES. DEVELOP.7, 203 (1968). Benesi, H. A., J . Catal. 8, 368 (1967). Burbidge, B. W., Keen, I. M., Eyles, M. K., International Conference on Molecular Sieve Zeolites, Worcester, Mass., 1970. Dubinin, M. M., Federova, G. M., Plavnik, G. M., Piguzova, L. I., Prokofeva, E . N., Izu. Akad. Nauk SSSR, Ser. Khim 11, 2429 (1968). Eberly, P. E., Jr., Ind. Eng. Chem. Fundam. 8, 25 (1969). Eberly, P. E., Jr., J . Phys. Chem. 67, 2404 (1963). Eberly, P. E., Jr., J . Ph,ys. Chem. 71, 1717 (1967).

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Eberly, P. E., Jr., Kimberlin, C. N., Jr., International Conference on Molecular Sieve Zeolites, Worcester, Mass., 1970. Eberly, P. E., Kimberlin, C. N., Jr., Miller, W. H., Drushel, H. V., Ind. Eng. Chem. Process Des. Develop. 5,193 (1966). Katzer, J. R., thesis digest, Massachusetts Institute of Technology, September 1969. Keough, A. H., Division of Petroleum Chemistry, 144th ACS Meeting, Preprint 8, 65 (1963). Meier, W. M., 2. Krist. 115, 439 (1961). Pansing, W. F., Malloy, J. B., Ind. Eng Chem. Process Des. Develop. 4, 181 (1965). Prater, C. D., Lago, R . M., Aduan. Catal. 8, 293 (1956). Richardson, J. T., J . Catal. 9, 182 (1967). Sand, L. B., Conference on Molecular Sieves, Society of Chemical Industry, p. 71, 1968. Satterfield, C. N., Frabetti, A. J., A.1.Ch.E. J . 13, 731 (1967). Topchieva, K. V., Romanovskii, B. V., Piguzova, L. I., Thoang, H., Bizreh, Y. M., Fourth International Congress on Catalysis, Moscow, Preprint 57, 1968. Topchieva, K. V., Romanovskii, B. V., Tkhoang, K. S., Dokl. Akad. Nauk SSSR 149, 644 (1963). Topchieva, K. V., Rosolovskaya, E . N., Zhavoronkov, M. N., Razanova, 0. N., Parmenova, A. S., Dokl. Ahad. Nauk SSSR 185, 132 (1969). Voorhies, A,, Ind. Eng. Chem. 37, 318 (1945). Weller, S. W., Bauer, J. M., “Studies of the Catalytic and Chemical Properties of Acid-Extracted Mordenite,” Preprint, 62nd Annual Meeting A.I.Ch.E., Washington, D. C., 1969. RECEIVED for review April 23, 1970 ACCEPTED July 2, 1970