2404
P. E. EBERLY, JR.
thermodynamic data are available for solid Ins, the results of the dissociation study cannot be compared to the results of the present investigation. If the slow step in the evaporation of indium sesquisulfide from a free surface is due entirely to an energy barrier, an a of 0.01 leads to an activation energy
Vol. 67
of 167 kcal./mole for the congruent sublimation a t 298’K. This value is about 20 kcal. more than the equilibrium heat of sublimation. Acknowledgments.-Dr. K. K. Kelley and Mr. K. C. Conway of the U. S. Bureau of Mines kindly provided the analysis of the solid indium sesquisulfide.
HYDROCARBOX ADSORPTION STUDIES AT LOW PRESSURES ON THE SODIUJl AND ACID FORMS OF SYNTHETIC MORDEKITE BY P. E. EBERLY, JR. Esso Research Laboratories, Humble Oil and Rejining Company, Baton Rouge Refinery, Baton Rouge, Louisiana Received May 98, 1963 The adsorption of benzene, cyclohexane, and n-hexane on both S a and H mordenite was investigated a t pressures of 0.010 to 6 mm. from 90 to 260’. Na mordenite has exceptionally high adsorptive capacity for the hydrocarbons at these conditions. The saturation value of adsorption expressed in liq. cc./g. is 0.0759 and is essentially independent of the nature of the hydrocarbon and temperature. This is of the same order of magnitude as the pore volume of 0.094 cc./g. determined from the crystallographic dimensions. The heats of adsorption of benzene, cyclohexane, and n-hexane on Na mordenite are nearly independent of coverage and amount to 25, 18, and 21 kcal./mole, respectively. The H form exhibits a slow irreversible adsorption process occurring simultaneously with a fast reversible adsorption. The reversible adsorption is considerably less than that observed on the Na form. Heats of the reversible adsorption of benzene, cyclohexane, and n-hexane are 12.5, 13.5, and 16.0 kcal./mole, respectively. There is evidence to suggest that n-hexane molecules have access to more of the intracrystalline surface than either benzene or cyclohexane. Entropy calculations indicate that the hydrocarbons have more degrees of freedom of motion on the H form than on the Na form.
I. Introduction I n recent years, several reports have appeared in the technical literature concerning the structure,l synthesis,2 and adsorptive properties of morde~iite.~-j This mineral is a crystalline sodium aluminosilicate having the general formula Ka8A18Si400s6~ 24H20. As reported by Meierl in his structure determination, mordenite is orthorhombic with unit cell dimensions of a = 18.13, b = 20.49, and c = 7.52 8. The main building blocks are four- and five-membered rings composed of Si04 and A104- tetrahedra. These rings are so arranged that the resulting crystal contains parallel adsorption tubes having an approximately elliptical opening with a major and minor diameter of 6.95 and 5.81 A., respectively. A schematic diagram of a cross section of the mordenite structure is shown in Fig. 1. The points of intersection represent the silicon and aluminum atoms and the lines represent atoms of oxygen. The indicated unit cell contains two of these elliptical tubes and extends a distance of 7.52 8. perpendicular to the plane of the page. The volume Aloilg the of these tubes per unit cell is Va = 480 walls of the tubes, other cavities occur a t periodic intervals. These have openings which can also be approximated by an ellipse having a major and minor axis of 4.72 and 3.87 A,,respectively. The volume of these cavities per unit cell is V b = 428 A.3. In spite of this fairly open structure, earlier reports on the adsorption properties of mordenite indicated that the material had an effective pore opening of only It adsorbed nitrogen and smaller moleabout 4 cules rapidly, whereas it took up methane and ethane slowly. This lack of capacity for larger molecules mas (1) W. hl. Meter, 2. Krzst , 116,439 (1961). (2) R. RI. Rarrer, J . Chem Soc., 2158 (1948). (3) R. M. Barrer, Brenns’of-C e m , 3.5, 325 (1954). (4) R.&I. Barrer and A . I3. Robins Trans Faiaday So&, 49, 920 (1958). ( 6 ) R M , Barrer and 4. B. Rohina itr.ld., 40, 807 (196a)n
attributed to stacking faults in the mordenite structure. 1 More recently, however, a synthetic form of mordenite has become commercially available which has considerable capacity for larger molecules such as n-heptane, cyclohexane, and benzene and, hence, is apparently more nearly free of stacking faults.6 This material is obtainable in both the Iia and so-called “hydrogenform” and has a surface area of 400-500 m.”g. as measured by the B.E.T. method with nitrogen adsorption. The H form is made by replacing the sodium with ammonium ions and subsequently calcining to liberate free ammonia or by direct treatment with dilute acid. The H mordenite has been found to be an active cracking catalyst.6 The present investigation deals with the study of benzene, cyclohexane, and n-hexane adsorption on both the Na and H forms of this new mordenite preparation in the low pressure region from 0.010 to 6 mm. As is characteristic of zeolites of this type, the adsorption isotherms are very steep and saturation is nearly complete a t low pressures. Use of an electromagnetic microbalance facilitated the determinatioii of these isotherms. 11. Experimental
’
Apparatus.-The Cahn electromagnetic microbalance was used for the adsorption measurements. The sample was placed in a small quartz bucket which was suspended from one end of the balance beam by a quartz fiber. Under the conditions of the experiment, the balance could detect a weight change of 0.001 mg. The entire balance and quartz suspension were enclosed in a glass system which could be evacuated to a pressure on the order of mm. The pumping system consisted of a three-stage oil diffusion pump backed up by a mechanical pump. The system was protected from oil vapo-s by a liquid nitrogen trap. The heating system consisted of an aluminum block heater which was placed around the glass tube containing the sample bucket. Two copper-constantan thermocouples were placed ( 6 ) A. H. Keough and L. R. Sand, J . A m . Chem. Soc., 83, 3.530 (1961). (7) Chem. Cng. S e i L a , 40, N o . 11, 52 (1862).
HYDROCARBON ADSORPTION ON SYXTHETIC NORDESITE
Nov., 1963
about 0.25 in. below the sample bucket. One couple was connected to the control system and the other to a millivolt recorder. Good temperature control was achieved by the combination of an on-off controller connected to a stepless, proportional controller. After closing off the adsorption system to the vacuum pumps, injections of liquid hydrocarbons were accomplished by inserting pipets of various sizes through a mercury layer to a glass frit. The volumes of these pipets were 0.0005, 0.001, 0.002, 0.003, 0.02, and 0.04 cc. Since the volume of the adsorption system was near 3 l., these pipets produced pressures in the desired range. Procedure.-Tor the adsorption measurements about 20 mg. of sample was placed an the quartz bucket and heated to 370". The sample was degassed at this temperature for 16 hr. The temperature was then lowered t o the desired value and the system closed off from the pumps. Successive injections of the hydrocarbon were made until the pressure reached 6-10 mm. During this process, the weight of the sample was continuously recorded. It normally took four to five hours for the measurement of one isotherm. The system was then opened to the vacuum pumps and the rate of desorption measured a t near vacuum conditions to determine the degree of reversibility of the adsorption. The sample was then heated again to 370" and degassed for 16 hr. in preparation for the next isotherm measurement. All determinations were made above 90". Below this temperature, the sample even under high vacuum conditions began to increase in weight a t the rate of about 1 pg./min. It was not determined whether this was merely an apparent weight change due to temperalure effects on the balance or a true weight change reflecting the adsorption of small amounts of mercury or water vapor. Above 90°, this effect did not occur. To correct the weight changes for thermomolecular flow and buoyancy effects, calibration runs were made with a 19.762-mg. sample of nonadsorptive quartz chips. The corrections for cyclohexane a t four different temperatures and various pressures are shown in Fig. 2. These were added to the weight measurements observed during the actual isotherm determination. I n most cases, they were negligible compared to the total weight change due to adsorption. Materials.-Specially purified samples of the Na and H forms of mordenite were obtained from the Sorton Co., Worcester, Mass. Chemicd analyses are shown in Table I. I n preparing the H form from the Na farm, nearly complete removal of the sodium was achieved. Benzene, cyclohexane, and n-hexane were obtained from the Phillips Petroleum Co., and had purities of 99.9, 99.8, and 99.9 mole %, respectively. Prior to use, they were filtered through columns of silica gel.
TABLE I COMPOSITION OF MORDENITE SAMPLES SiOz, wt. '% AlzOa, wt. % NazO, wt. % SiOz/Al~O3,molar ratio
NE form
H form
78.26 12.57 9.17 10.56
86.05 13 87 0.08 10.53
111. Results Adsorption Studies on Na Form of Mordenite. Benzene Isotherms.--Large amounts of benzene were adsorbed on Na mordenite at very low pressures and high temperatures. The rate of adsorption was rapid and equilibrium was established within a few minutes after injection. Figtire 3 shows a portion of three isotherms up to 1 mm. pressure obtained a t 223, 241, and 260'. The following La,ngmuir equation can be used to express the data. UXmp
1
or
+ up
(1)
= 20.49
Fig. l.-Schematic diagram of a cross section of mordenite. The points of intersection represent silicon or aluminum atoms and the lines represent oxygen atoms. The two volumes possible for adsorption are indicated by 'v, and Vb. I
0.03
A.
x=-
-b
2405
I
280%
(500'1.1
204%.
(40O.F. I
149.C.
0.02
I
93.C.
(300'F.
I
I
I
1
0 5
O B
0 7
0 8
8
(200'F.)
0.01
0
0 1
0 2
0 3
0 4
P m
Fig. 2.-Weight correction13 for cyclohexane. Data represent apparent weight losses observed with cyclohexane on a 19.762mg. sample of quartz chips.
where p is the pressure and z is the amount of material absorbed in mmole/g. The quantity 3, represents the amount of material necessary to cover the surface with a monomolecular layer. Thus, if p/x is plotted us. p , a straight line should result. The slope and intercept are equal to l/zm and l/ux,, respectively. Such Langmuir plots for these benzene isotherms are shown in Fig. 4 up to 6 mm. pressure. At all three temperatures, the data are best represented by two straight lines intersecting a t a pressure near 0.6 mm. Values of a and zmare listed in Table II for both pressure regions.
2406
0
0.1
0.3
0.2
0.4
0.5
0.0
0.7
0.8
0.9
1.0
1.1
p , mm.
Fig. 3.-Benzene
adsorption isothernis on S a mordenite.
6.0
P, mm.
Fig.4.-Langmuir
plots of benzene adsorption on ?I’a mordenite.
If A H , the heat of adsorption, is assumed to be independent of temperature, it can be evaluatcd for any given coverage, x,by the relatioilship -AH
RT2
(‘3)
These data are listed in Table 111. The heat of bcnzene adsorption is fairly independent of coverage varying only from 24.6 to 26.8 kcal./mole from 0.100 to 0.300 mmole/g., respectively. To calculate the entropy changcs accompanying the adsorption, use is made of cq. 4-6
2407
HYDROCARBON ADSORPTION ON SYNTHETIC MORDENITE
Nov., 1963
TABLE I1 LANGMUIR CONSTANTS FOR ADSORPTION O N SODIUX MORDENITE Adsorbate
Benzene Benzene Benzene
T,O C .
Press. range, mm.
a, mm.-1
Xm, mmole/g.
223 223
0-0.7 0.7-6
16.69 2.716
0.4280 .5938
.....
..
0,0750
0.91
24 1 24 1
04.9
0.9-6
7.344 1.967
.4126 ,5648
0.0757
260 260
0-0.9 0.9-6
3.877 1.273
.3738 .5456
....
..
0.0800
0.84 0.83
Liq. cc./g.a
.....
161
0-6
2.331
.5430
0.0725
Cyclohexane
183
0-6
0.8204
.5277
,0739
.81
Cyclohexane
203
0-6
0.3652
.5510
.0808
.84
n-Hexane
162 162
0-0.6 0.6-6
15.34 3.198
.3259 .4468
....
..
0.0765
0.68
182 182
0-0.6 0.6-6
5.958 2.309
.2945 .4011
....
..
0.0731
0.61
0-0.6 2.937 .2560 204 0.6-6 1.352 .3736 204 Calculated from the orthobaric densities. Having unit cell length of c = 7.52 A.
....
..
0.0754
0.57
n-Hexane
TABLE I11 THERMODYNAMIC DATAFOR BEXZEXEADSORPTION ON SODIUM MORDENITE -AaSO, #.go, SS, X, -AH, - AGO. cal./deg.- cal./deg.- oal./deg.mmole/ T, kcal./mo:le
cal./mole
mole
mole
mole
24.7
10,500 9,980 9,530
28.6 28.6 28.5
77.3 78.3 79.7
48.7 49.7 51.2
24.6
9,940 9,390 8,890
29.5 29.6 29.5
77.3 78.3 79.7
47.8 48.2 50.2
223 241 260
25.0
9,480 8,880 8,320
31.2 31.4 31.3
77.3 78.3 79.7
46.1 46.9 48.4
.250
[\ ; :! 260
25.6
9,000 8,370 7,720
33.4 33.5 33.6
77.3 78.3 79.7
43.9 44.8 46.2
.300
223 1241 260
26.8
8,490 7,810 7,070
36.8 36.9 37.0
77.3 78.3 79.7
40.5 41.3 42.7
g.
OC.
0,100
241 223
\ 260
\ 260
.200
0.86
Cyclohexane
n-Hexane
a
Molecules/ads. tubeb
[
AGO = AGO =
ASo
P -RTlnP AHo - TAX' AHo
= __
T
fR1n-
(4)
(3 P
P
(6)
where AGO, AH', and A S o are standard free energy, enthalpy, and entropy changes in the adsorption process. P and p are the equilibrium pressures of the adsorbed phase in its standard state and the state under study. The standard state pressure, P, is taken as one atmosphere. For isothermal conditions, neglecting gas imperfections, A H = AH'. Furthermore, the entropy change accompanying adsorption (ASo) can be expressed by AXo
=
Sgo -
S,
(7)
where figo is the molar entropy of the hydrocarbon vapor at the standard pressure P of one atmosphere is the partial molar entropy of the adsorbed and phase.
s,
Such thermodynamic data for benzene adsorption on Na modenite are given in Table 111. S, and consequently the mobility of the adsorbed phase tends to decrease with coverage and increase with temperature. The entropy of the adsorbed phase, S,, can be further analyzed in the manner originally proposed by Barrer, Bultitude, and Sutherland8 in their study of hydrocarbon adsorption on faujasite. I n this case, S, can be represented by
Ss=
6%
+
(8)
Sth
where 3, and &h denote the partial molar configurational and thermal entropies of the adsorbed phase, respectively. If adsorption is localized, as indicated by the near agreement with the Langmuir equation, S,h becomes independent of 0, the fraction of surface covered. For our purposes, 0 can be expressed as z/xm. The configurational entropy, &, is equal to
[-J
Sc'= R I n 1 - e
(9)
which a t 0 = 0.5, reduces to zero, and Sa becomes equal to St,,. For polyatomic hydrocarbon molecules, the thermal entropy of the adsorbed phase can be expressed as the sum of three terms Sth
=
32%
+ SI +
SR
(10)
where 3s"is the partial molar vibrational entropy of the adsorbed molecule as a whole; SI, the partial molar entropy associated with the internal vibrational and rotational degrees of freedom; and &, the partial molar rotational entropy of the adsorbed molecule as a whole. Following the procedure described by Barrer,* can be estimated by the equation
~ S=V3R[y(exp(y - 1>)-1 - In (1 - exp-y)] where y
=
(11)
hv/kT
(8) R. M. Barrer, F. W. Bultitude and J. W. Sutherland, Trans. Faraday Soc., 68, 1111 (1957).
P. E. EBERLY, JR.
2408
The quantity v is the vibrational frequency of the hydrocarbon molecule as a whole in its intracrystalline environment. This was estimated according to the procedure previously described.8 Knowing 3Sv,the SR can be evaluated for the adsorbed sum of 3, phase. This latter sum can be compared to a similar quantity . is obtained by subin the gas phase, XI, S R ~This tracting the standard translational entropy, d T g , from the standard entropy of the gaseous hydrocarbon.
+
Vol. 67
TABLE V THERMODYNAMIC DATA FOR CYCLOHEXA.NE ADSORPTION OK SODIUM MORDENITE X,
T,
-AUO, cal./ mole
18.0
7750 7150 6760
23.6 23.8 23.6
83.5 85.3 87.2
59.9 61.5 63.6
161 183 203
17.7
7300 6670 6260
23.9 24.2 24.0
83.5 85.3 87.2
59.6 61.1 63.2
,200
161 [\ 203 183
18.1
6930 6280 5870
25.7 25.9 25.7
83.5 85.3 87.2
57.8 59.4 61.6
,250
4 183
18.2
6600 5930 5512
26.7 26.9 26.6
83.b 85.3 87.2
56.8 58.4 60.6
17.4
6280 5580 5170
25.6 25.9 25.7
83.5 85.3 87.2
57.9 59.4 61.6
mmole/g.
OC.
( 161
+
log 1%' 3 8 ~ 6.864 ~
+ 11.439 log T - 2.314
(12) where M is the molecular weight of the hydrocarbon and T i s the absolute temperature. Summary calculations of the entropy of benzene in the adsorbed phase a t 0 = 0.5 are included in Table IV. As can be seen, there is a considerable decrease in the internal vibrational and rotational degrees of freedom of adsorbed benzene as compared to its vapor. This difference changes little over the temperature range studied. TABLE IV ENTROPIES FOR HYDROCARBON ADSORPTIONON SODIUMMORDENITE e = 0.500
&=
SIg
+
51
a8Tm
SRg,
SR,
cal./ deg.mole
oal./ deg.mole
Gal./ deg.mole
cal. deg.mole
Diff., Gal./ de&mole
11.1 11.2 11.2 11.3 11 4 11.6 11.2 11.3 11.4
41 5 41.7 41.9 41.1 41.3 41.5 41.1 41.1 41.3
35.8 36.6 37.8 42.4 44.0 45.7 66.9 69.4 71.3
29.4 30.2 31.4 45.5 47.0 49.1 66.4 68.2 70.3
-6.4 -6.4 -6.4 +3.1 +3.0 +3.4 -0.5 -1.2 -1.0
T,'C.
Benzene Benzene Benzene Cyclohexane Cyclohexane Cyclohexane %-Hexane %-Hexane +Hexane
223 241 260 161 183 203 162 182 204
40.5 41.4 42.7 56.8 58.4 60.6 77.5 79.4 81.7
161
[ 203
\ 161
