Hydroisomerization of Cyclohexane and n-Pentane over Series of Mordenite Catalysts of Varying Silica-Alumina Ratio Jack R. Hopper' and Alexis Voorhies, Jr. Chemical Engineering Department, Louisiana State University, Baton Rouge, L A 70803
Four palladium-hydrogen mordenite catalysts of varying silica-alumina mole ratio were evaluated in the hydroisomerization reactions of cyclohexane and n-pentane. A kinetic model was developed for the cyclohexane reaction over these catalysts. One batch of sodium mordenite was used as the source from which each of the catalysts was prepared. The standard or parent mordenite preparation was caustic-leached to remove silica, producing a catalyst with a lower silica-alumina ratio than the standard mordenite, and two other catalysts with higher silica-alumina ratio than the standard mordenite were made b y acid-extracting palladium. alumina from the standard mordenite preparation. Each of the catalysts contains about '/2 wt Isomerization rates were temperature sensitive for all of the catalysts, and energies of activation were presented for both the cyclohexane and n-pentane reactions. The effect of the partial pressure of the hydrocarbons and hydrogen was represented b y a dual-site adsorption model. Isomerization activity was observed to initially increase with an increase in silica-alumina ratio and then decrease with additional increase in the ratio. It i s felt that the reduction in surface area and active acid sites a t the higher alumina extraction i s primarily responsible for the loss in activity a t the higher silica-alumina ratio. A general relationship for both reactions i s not apparent.
70
M o l e c u l a r sieve zeolites have in the past decade been demonstrated to be outstanding catalysts for isomerization reactions (Voorhies and Bryant, 1968; Beecher and Voorhies, 1969; Rabo et al., 1961). Nordenites particularly are active isomerizat'ion catalysts (Voorhies and Bryant, 1968; Beecher and Voorhies, 1969; Benesi, 1965). Thus, these catalytic qualities have prompted a number of kinetic st'udies by use of zeolites for isomerization react'ions. Voorhies and Bryant' (1968) have reported on the isomerization of n-pentane, and Beecher and Voorhies (1969) on the isomerizat'ion of hexanes. Modifications of the si02/&03 ratio in t'he mordenite are also particularly interesting because of the changes in catalytic activity which have result,ed. Beecher et al. (1968) have investigated activity changes for decalin and decane hydrocracking fora change in Si02jh1203 from 12 to 66. The crackingactivity x a s greater a t the higher ratio. Weller and Brauer (1969), however, observed that. n-hexane cracking activity went through a maximum as the si02/&03 ratio was increased over a much narrower range. Eberly and Kimberlin (1970) reported t h a t cumene cracking act,ivity increased with Sio2/Lk1203ratio. However, Kranich et al. (1970) and Bierenbaum et al. (1971) observed from differential conversions a loss of cracking act'ivity for cumene at, high Sio~!~k1?03rat8ios. Eberly e t al. (1971) reported on the effect' of si02!~&03 ratio on acidic properties of mordenite and isomerization activity for n-pentane; Kranich et, al. (1970) investigated activity changes with Si02 :Xls03 ratios for butene isomerization; and Piguzova et al. (1969) on the change in isomerization activity of n-pentane and o-xylene with Sio,!.kl203 ratio. Present address, Chemical Engineering Department, Lamar University, Beaumont, TS 77710. To whom correspondence should be addressed. 294
Ind. Eng. Chem. Prod. Res. Develop., Vel. 1 1 , No. 3, 1972
Isomerization activity decreased inversely with Si02/&03 rat~iofor n-pentane and o-xylene, but a n optimum value was observed with butene. I n addition to the int,erest in these catalysts, the isomerization reaction itself is also useful to investigate because it is an integral part of many of the desirable conversions in petroleum processing. I n this study t'he isomerization of anaphthene, cyclohexane, and n-pent'ane has been investigated. The kinetic model for the iiaphthene reaction has been developed for a series of modified mordenites. The modification of the SiOzi *k1203rat.io has been made by either extract'ing silica with caustic or extract,ing alumina with mineral acid. The physical propert'ies of 111203, SiOe, Ya, Pd, and surface area have been determined t o characterize the catalysts. Experimental
Kinetic System. T h e system consisted of a fixed-bed reactor operated under isothermal and plug-flow conditions. The diameter of t h e catalyst bed was 0.62 in. and normally contained a volume of 15 cc of catalyst. Catalyst particles were contained in the downflow react,or between tlTo micrometallic porous frits. Isothermal operations were achieved with temperature control of a fluidized sand bath, and reactor bed temperature was measured with a l/'lG-in. diameter ironconstantan met,allic-sheathed therniocouple protruding into the catalyst bed. Feed mixtures of hydrogen and hydrocarbon were preheated to the reactor temperature before ent,ering the bed. Phillips pure grade liquid hydrocarbons were metered with a Ruska pump, and dry electrolytic hydrogen was fed t'hrough an orifice flow meter. Products were removed through a Grove back-pressure regulator. Product gas vias measured with a \yet
Table 1. Schematic Diagram of Preparation Steps of Various Mordenite Catalysts
Aqueous solutions of sodium aluminate, sodium silicate, and sodium hydroxide Crystallized
J.
Sodium mordenite Severely HCl extracted
.1
Palladium e x hanged
Ammonium exchanged
Moderately HC1 extracted
.1
.1
Palladium exchanged
Palladium exchanged
Caustic leached
.1
Ammonium exchanged
J.
Palladium exchanged Oven dried
J. .1 J. J.
Pd-H-mordenite (sioL/-k1203= 52 111)
J. .1
Pilled
JCrushed
4
and sized Calcined
Pd-H-mordenit e (sio2/211203= 25 5/1)
test meter, and any lilquid product was weighted after being separated. Analyses were made with a 10-ft gas chromatograph column of 10% silicon rubber, SE-30, on 90% chromosorb. Catalyst Preparatio'n. T h e standard or parent niordeiiite and three modifications were selected for this investigation. Two of the modified mordenites were made with a hiyher silica-alumina ratio t,hiin the parent' mordenite, and one with a lower ratio. A schematic of the preparation steps is shown in Table I. X single batch of Norton Co. sodium mordenite was used at, the Esso Reseitrch Laboratories, Humble Oil and Refining Co., Baton Roug:e, LA, to prepare these four catalyst,s. Sodium mordenite is prepared from a highly reactive, aluminosilicate "gel." Typical gels are prepared from solutions of sodium aluminate, ::odium silicate, and sodium hydroxide. The sodium mordenite results when the gels are crystallized a t temperatures of 25-1OO0C, producing a final product of finely divided white powder of small crystals. Sodium mordenite is usually received from t'he S o r t o n Co. in the form of 1-5 p crystallites. The standard mordenite was convert'ed to the hydrogen form by a n ion-exchange reaction between the sodium ions and ammonium ions as ammonium nitrate or chloride. The palladium metal component, is incorporated into the ammonium mordenite from solutions of t.he amine complex: [Pd(KH3)4]C1?.T h e catalyst' is then oven dried, arid t h e crystallites are pilled, crushed, and screened to the desired size for use. The catalyst is activated with heat before being placed in the react'or by heating to 350°F. This t'emperature is maintained for about. 16 hr. The temperature is increased to 1000°F a t a rate of 10O"Fihr and is maint,ained for 2 hr. The temperature is then reduced, sample removed, transferred to glass vials with covers, and stored in a desiccator.
.1 .1 .1 .1
Pd-H-mordenite (siO2/.1l2o3 = 10 8/1)
.1 J. J. .1
Pd-H-mordenite ( S O j -1I2O3= 9 0/1)
Mordenite of higher silica-alumina ratios are prepared by extraction of the alumina with mineral acids such as HCl. The severity of the extraction is controlled by the t'emperature, pH of t'he solution, and the time of exposure. The design values of t,he SiO,/Al,O, ratio of 25/1 and 60;'l were select,ed on the basis of previous unreported experience wit.11 extracted catalysts prepared from this standard mordenite. The level of t'he alumina removal which destroys t'he structure of the catalyst cannot be predicted. This level has been observed t,o depend on t,he parent mordenite. Good cat,alytic act,ivit#yhas been observed with catalysts with Si02!-11203 ratio values as high as 100/1 by t'his laboratory, and values of 600!1 have recently been reported by Kranich et al. (1970). It is not necessary to give the extracted catalysts an ammonium exchange as the sodium is removed in the ext,raction. Palladium metal was incorporated, and the catalysts were completed as described above. Xordenite can also be modified by leaching silica from t.he structure wit,li alkali-metal hydroxide such as sodium hydroxide (Young, 1967, 1968). X S O 2 ,h 1 2 0 3 ratio of 911 was selected, based on this reported lit,erature. Conversion of the silica-deficient mordenite t,o the hydrogen form is achieved by replacement of the sodium ions with ammonium ions, palladium metal is added, and t,he cat'alyst,is finished as previously described. -knalytical measurements of silica (Siol), alumina (&03), sodium ( S a ) , palladium (Pd), and surface area viere determined on each catalyst a t the Esso Research Laboratories, Baton Rouge, Lh. These values are shown in Table 11. The silica-alumina mole ratio ranged from 9.011 for the silicondeficient sample to 52.1 for the most severely extracted alumina-deficient, sample. The vt % sodium ranged from 0.03 for the most severely acid-extracted catalyst to 0.10 for the Ind. Eng. Chem. Prod. Res. Develop., Vol. 1 1 , No. 3, 1972
295
Table II. Properties of Catalysts Catalyst description
Silica deficient
Si02, tvt % 83 2 A1203, wt % 15 8 SiOz/A1203, mol 9 0 ratio Na, wt % 0 10 Pd, wt % 0 65 Langmuir surface 451 area, m2/g
Standard
Alumina deficient
Alumina deficient
83 9 13 3 10 8
94 1 6 3 25 5
94.6 3.1 52.1
0 05 0 55 532
0 09 0 57 554
0.03 0.61 249
-0.5
-1,o
.
-1.5
.
r
3
-2.0
'
'
I
0.41
0.95
I
1
Lo5
1.10
I
l,a,
\ I 15
lOa/Twp, 1/"R
Figure 2. Summary of Arrhenius plots for cyclohexane
The rate of reaction, expressed as g-mol of M C P produced per hour (N,) per gram of catalyst ( W J ,is represented by: rate
dN, d Wc
= -=
k(C,
- C,/K)
with C, and C, the concent,ration of C H and MCP, and K the thermodynamic equilibrium constant. Integration of this equation results in an expression in terms of the hydrogen-free mole fraction of RlCP (Ym),t,he corresponding equilibrium value ( Y E ); pG, the molar densit.y of reactor gases; M, the molecular weight of M C P ; p , the part.ia1 pressure of C H and M C P ; P , the total pressure; and W , the weight of feed per hour per weight of catalyst: Figure 1. Reversible first-order reaction
caustic-leached catalyst. Prior experience indicated this range was not significant a t this sodium level. W t yo palladium ranged from 0.55 to 0.65. The Langmuir surface area ranged from a low of 249 m2/g for the most severely acid-extracted catalyst to 554 m2jg for the 2611 silica-alumina mole ratio sample. Hydroisomerization of Cyclohexane
Kinetic Results. Initial variable studies had shown t h a t gas-to-particle mass transfer was an insignificant resistance to the overall rate. Intraparticle diffusion was also shown not to be rate limiting for the 10.8/1 silica-alumina Pd-H-mordenite. It was assumed this would be the case for the other catalysts as the pores nould a t least be as large as those of the parent mordenite. The importance of micropore diffusion was not determined, and these kinetic rates include any effect of the diffusional rates in micropores. -% first-order kinetic model, with k and k' the forward and reverse reaction rate constants, respectively, was based on the following reaction: I
cyclohexane (CH) 296
methylcyclopentane (JICP)
Ind. Eng. Chem. Prod. Res. Develop., Vol. 1 1, No. 3, 1972
IC
=
-PWYEln(l
- Y,/YE)pcMp
(2)
This equation can be used to determine values for k . This model was evaluated for each of the four catalysts. These evaluations are illustrated in Figure 1. These data show that each cat,alyst can be satisfactorily represented by this initial model. Each cat,alyst, evaluat.ion was carried out a t separate conditions, depending on t8hecatalyst activity, and these conditions are designated on the figure. The effect of temperature on the cyclohexane isomerization rate constant was evaluated wit'h Equation 2. These data are shown in Figure 2 in Arrhenius-type plots. The temperature was varied to obtain a wide range of conversion, and all other conditions were held constant. The temperature range selected depended on the catalyst activity. Each set of dat'a was linearly regressed t o obtain the activat,ion energy. The activation energy values and the 95% confidence limits are shown in Table 111. The lowest value is 23.9 kcal/mol for the 5211 Si02/-%1203catalyst, and the highest value is 35.5 kcal/mol for the 10/1 Si02/X1202catalyst. Literature values of 30 kcal/mol (Levitskii and Gonikberg, 1962) and 35.4 kcal/mol (Maslyanskii, 1963) have been reported for TVSS between 608-716'F and for XoS2between 698-806'F1 respectively. Effect of Pressure, T h e effect of pressure can often be correlated by a Langmuir-Hinshelwood type adsorption model. The overall rate constant for the first-order reaction is related t o the adsorpt'ion-model constants by
Table 111. Activation Energy for Cyclohexane H yd roiso mer izat ion C a ta lyrt SiO*/A1203 ratio
9/ 1 10 8 1 25 5,1 52 111
Activation energy f 2 u, kcal/mol
Temp range, O F
18
460-540 402-502 427-520 546-655
33 35 29 23
18 1 52 =k 2 42 i 2 87 =k 2
40 08 87
Table IV. Effect of Total Pressure for Cyclohexane Hydroisomerization on 9/1 SiO2/Al203Mordenite
Temp, “F Total press, psia HI, cyclohexane, mol ratio k , cc g-see
390
0 0770
482 465 9 8 0 0618
565
0 0400
Table VI. Adsorption-Model Constants for Cyclohexane Hydroisomerization Catalyst sio2/d1203 9 1 10 811 25 5 1 52 111 Temp, O F 482 460 475 651 k0 39 04 12 92 201 2 47 37 KO 0 0944 0 0400 0 6255 0 0179 K H 0 00503 0 00203 0 0107 Table VII. Activation Energy for n-Pentane Hydroisomerization Catalyst Si02/AI*O$ ratio
Total press., psia
HS partial press., psia Hydrocarbon partial press., psia k , cc g-sec
485 417 68 0 0330
665 620 45 0.0379
Temp range, O F
69 + 2 01 =t2 41 =k 3 84 + 1
520- 6 10 491-581 500- 580 572- 669
35 38 49 36
9/ 1 10 8/1 25 5 , l 52 1 / 1
Table V. Effect of Partial Pressure on 9/1 SiO2/AI2O3 Nlordenite, 482°F
Activation energy f 2 u, kcal/mol
-0.5
71 11 00 95
,
465 422 43 0 0618 -1,o
where ko is the rate (Ionstant for the adsorption model, and p A k A , p R k R , and p H K H are the partial pressure-adsorption constant products for CH, XCP, and hydrogen, respectively. Surface reactions for a reversible unimolecular reaction have generally been correlated with a single-site or a dual-site surface reaction mechanism hich corresponds to a value of 1 and 2 for the exponent “n” in Equation 3. .kccording to the theory, a single-site mechanism describes a reaction involving only a single active site, whereas a dual-site mechanism describes a reaction of a n adsorbed molecule only if it is adjacent to a n occupied active site. A reasonable assumption that the adsorption constants for the hydrocarbons are essentially equal has been made, and both are designated by K O .The expressions in Equation 3 can then be rearranged to: Single site:
Dual site:
Experiments were made a t various partial pressures of ~ holding the total pressure cycloheyaiie and hydrogen ‘ i hile constant Experimeii cs were also made a t constant partial pressure of hydrocarbon or hydrogen. The contact time TT as held coiistant in this latter case by holding the ratio of hydrocarbon partial pressure to feed-rate coiistant. Examples of the data for the 9 1 Si02 .\1203 niordenite a t 482°F are shonn in Tables IV and T.’. The rate decreased with a n increase in either hydrocarbon or hydrogen partial pressure. Similar results for the 10.8 1 Si02 /X120g mordeiiite have previously been piesented (1-oorhies and Hopper, 19T1). hnalysis of the effect of total pressure and hydrocarbon partial pressure for the 25 511 and 52 1 1 SiOi/X1203 mordenites alqo s h o ~similar results,
4
3
-1.5
-2.0
0,so
0.95
1,c
1.05
1633/im~, Y R
Figure 3. Summary of Arrhenius plots for n-pentane
but the rate for the 52.1/1 Si02/Ak1+& mordenites n a s independent of the hydrogen partial pressure. .Inalysis for these latter t n o catalysts is less complete than with the 9’1 and 10.8/1 si02/&03 mordenites. The adsorption-model constants for the single-site and dualsite models were determined by use of the variable pressure data nit11 Equation 4.The dual-site model has been accepted as the more correct model in each case, because ph~sically realistic values (nonnegative) for the constants ere obtained. Values for the constants are shown in Table VI. Hydroisomerization of n-Pentane
Effect of Temperature on Overall Rate for Pentane Hydroisomerization. T h e effect of temperature on the overall isomerization rate of n-pentane was evaluated by use of a previouslj developed expression for this reaction (T’oorhies and Biyant, 1968). The resultq are illustrated i i i Xirheiiiustype plots in Fiqure 3 for each catalyzt Each bet of data n a s linearly regresqed, and the activation energy from each of these curves is shon 11 in Table VI1 Ind. Eng. Chem. Prod. Res. Develop., Vol. 1 1 I No. 3, 1972
297
Acknowledgment
Special appreciation is expressed to the Esso Research and Engineering Co. and to the research staff a t the Esso Research Laboratories in Baton Rouge, LA, for the sponsorship and assistance in conducting this research in the Petroleum Processing Laboratory of the Chemical Engineering Depart ment a t Louisiana State University, Baton Rouge, LA.
1,o
E>
E $
0.8
Y
E d 9
Nomenclature
0,6
CH C,
C,
0.4 ’
k
Om4
t
0
PENWNE
JY
t
Figure 4. Catalytic activity as function of alumina content
reaction = first-order rate constant for cyclohexane adsorption model k , = first-order rate constant for overall pentane reaction K = thermodynamic equilibrium constant KA = adsorption constant for reactant K R = adsorption constant for product K H = adsorption constant for hydrogen K O = adsorption constant for hydrocarbons, KO = KA =
ko
.I! The activation energies of the 9/1, 10.811, and 52.111 SiOn/ A1203mordenites are not significantly different, but the value for the 25.5/1 SiOJAi1203 mordenites is significantly different a t the 95% confidence level. Literature values have been reported which cover a range greater than reported here (Carr, 1959; Hutchings, 1962). Effect of SiO?/A1203 Ratio for Hydroisomerization of Cyclohexane and n-Pentane
The variation of mordenite activity with percentage alumina is shown in Figure 4. Data for both cyclohexane and n-pentane are shonm. The activity is shown as relative activity of each catalyst. Relative activity is the ratio of the overall rate constant for each catalyst to the overall rate constant of the most active catalyst; values for each catalyst were compared a t the same temperature. The rate constants were determined a t 492’ and 550°F for cyclohexane and npentane, respectively. The activity of the catalyst increased as the Si02/.%O~ ratio was initially increased (.%03% decreased) for both reactions. However, the activity passed through a maximum as the ratio was further increased. A maximum in activity has been observed by Kranich et al. (19iO) for the isomerization of 1-butene over modified mordenites and also by Keller and Brauer (1969) from n-hexane hydrocracking. The reduction in acid sites r i t h the reduction in alumina content for the acidextracted catalysts is a plausible explanation for the decrease in activity of the acid-extracted catalysts. In fact, a correlation between catalyst acidity and q e n t a n e isomerization activity has been reported by Eberly et al. (1971). The reason for the decrease in activity for the causticleached catalyst is not apparent. The evistence of other unknown parameters no doubt must eventually be uncovered to completely explain these activity changes. A similar result (Le., maximum activity) is obtained when the adsorptionmodel rate constant k~ is used t o establish the relative activity. Xote that the activities shown are dependent on the temperature selected. This dependence could be eliminated by using the logarithm of the frequency factor, in the Xrrhenius rclationship, in place of the rate constant. However, the results of such a correlation add no i i e insight. ~ 298
Ind Eng. Chern. Prod Res. Develop, Vo‘ 1 1 No 3, 1972
= cyclohexane = cyclohexane concentration = methylcyclopentane concentration = first-order rate constant for overall cyclohexane
K R
= molecular weight of cyclohexane M C P = methylcyclopentane 5, = molar flow rate of methylcyclopentane n = exponent in adsorption model P = total pressure p , P A , p , , p,, PO = partial pressure of CH and LICP, reactant, product, hydrogen, and hydrocarbon T V = cyclohexane feed rate per unit weight of catalyst TT, = weight of catalyst in reactor Y E = hydrogen-free equilibrium mole fraction of methylcyclopentane Y , = hydrogen-free mole fraction of methylcyclopentane pc = molar density of reactor gases a t reactor conditions literature Cited
Beecher, R. G., Voorhies, Jr., A., Ind. Eng. Chem. Prod. Res. Develop., 8, 366 (1969). Beecher, R. G., Yoorhies, Jr., A., Eberly, Jr., P. E., ibid., 1, 203 11968) ,-” --
Benesi, H. A. (to Shell Oil Co.), TJ.S. Patent 3,190,939 (June 22, 1963) . Bierenbaum, H. S.,Chiramongkol, S., Weiss, A. €I., J . Catal., 23, 61 (1971). Carr, X. L., dmer. Chem. Soc., Diu. Petrol. Chem., Prepr., 4 (2), 4-29 (1059) _ - -,\ -
Ed&, Jr., P. E., Kimberlin, Jr., C. N., Ind. Eng. Chem. Prod. Res. Develop., 3 , 335 (1970). Eberlv. Jr.. P. E.. Kimberlin. Jr.. C. N.. Voorhies. Jr.. A , . J . Catal., 22 ( 3 ) , 419 (1971). Kranizh, W.L., Ma, Y. I%., Sand, L. B., Weiss, A. H., Zweibel, I., International Conference on Molecular Sieve Zeolites,” Preprints, 802, Worcester, JIA, September 8-1 1, 1970. Hutchings, J. P., PhD dissertation, University of Wisconqin, IIadison, KI, 1962. Levitskii, I. I., Gonikberg, 31. G., Chem. Abstr., 5 6 , 1097e (19621. Naslvanskii. G. 5’.. J . Gen. Cheni.. 13. ,54011963). Piguiova, L’. J., Piokof’eval E. h., Dubinin, Ai. X., Bursian, N. R., Shavandin, 5’. A., Kznct. Catal. CSSR, 10, 252 (1969). Rabo, J. -4., Pickert, P. E., Mays, R. L., Ind. Eng. Chem., 53, 733 ilQf3l)
Voorhies, Jr., A., Bryant, P. A., AIChE J., 14, 852 (1968); Voorhies, Jr., A., Hopper J. R., Advan. Chem. Ser. 102, Molecular Sieve Zeolites-11,” R. F. Gould, Ed., American Chemical Society, Washington, DC, KO. 72, p 410, 1971. Weller, S. W.,Brauer, J. >I., “Studies of the Catalytic and Chemical Properties of Acid-Extracted Nordenite,” Preprint, 62nd Annual Meeting AIChE, Washington, DC, 1969. Young, D. .4. (to Union Oil Co. of California), U.S. Patent 3,326,707 (June 20 1967). . Young, 11. A. (to Gnion oil ~ o of. California), LT .e ~ Patent 3,374,182 (11arch 19, 1968). RECEIVED for review November 8, 1971 ACCEPTED June 9, 1972 Presented at the Symposium on Recent Advances in Reaction Kinetics and Catalysis 11, 64th Annual Meeting, AIChE, San Francisco, CA, Sovember 28 to December 2, 1971.
