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Sorption and diffusion in heteropolyoxometalates
Sorption and Diffusion in Heteropolyoxometalates 2. Aliphatic Saturated Hydrocarbons V. S. NAYAK AND J. B. MOFFAT 1 Department of Chemistry and Guelph-Waterloo Centre for Graduate Work in Chemistry, University of Waterloo, Waterloo, Ontario, Canada N2L 3GI Received January 7, 1987; accepted April 6, 1987 Sorption and diffusion properties of n-hexane, 3-methylpentane, cyclohexane, n-heptane, n-octane, and isooctane have been studied at 293, 308, and 323 K on three heteropoly acids, 12-tungstophosphoric acid, 12-tungstosilicic acid, and 12-molybdophosphoric acid, and their ammonium salts. The microporous salts show considerably larger sorption capacities than the nonporous acids. The sorption capacities of the microporous salts appear to be dependent on the pore size distributions and on the boiling points of the sorbates. The diffusivities of the aliphatic hydrocarbon are of the same order of magnitude for the heteropoly acids and the ammonium salt of 12-tungstophosphoric acid. In contrast, the ditfusivities are approximately 20 and 5 times higher for the ammonium salt of 12-tungstosilicic acid and 12-molybdophosphoric acid, respectively, than those for the acids. Heats of adsorption, in general, show no evidence of chemisorption effects. Little or no evidence was found for the penetration of aliphatic, saturated hydrocarbons into the bulk structure of the solid heteropoly acids. © 1988AcademicPros, Inc.
INTRODUCTION
Heteropolyoxometalates are ionic solids with large cagelike anions and cations ranging from protons to relatively large organic species. The solids of interest in the present work have anions customarily referred to as possessing Keggin structure. These anions have a central atom such as phosphorus surrounded by four oxygen atoms arranged tetrahedrally. The central tetrahedron is enveloped by 12 octahedra with a metal atom such as tungsten at their centers and oxygen atoms at their vertices, which are shared between each other and the central tetrahedron. Heteropolyoxometalates of such structure have been shown to be multifunctional in their catalytic properties, the functionality being primarily dependent on the nature of the peripheral metal atoms in the anion. Those containing tungsten are selective in the conversion To whom correspondence should be addressed.
of methanol and other alcohols to hydrocarbons while those with molybdenum have activity in oxidation reactions (1). In earlier work from this laboratory ammonium 12-tungstophosphate was shown to have an unexpected activity and selectivity in the conversion of methanol. In addition its surface area was found to be substantially higher (150 m2/g) than that measured for the parent solid acid (10 m2/g). However, in spite of the low surface areas of the heteropoly acids it has been shown with photoacoustic FTIR spectroscopy (PAS) that molecules such as ammonia and pyridine are capable of penetrating into the bulk structure of a low-area, nonporous solid acid such as H3PWl204o and interacting with the interior protons to form the ammonium and pyridinium salts (2, 3). It should be noted however that any water molecules present will restrict access of the sorbate molecules to the protons (4). Alcohols, such as methanol, have also been shown by PAS to interact stoichiometrically with surface and interior protons with the for-
475
JournalofColloidand InterfaceScience,Vol. 122,No. 2, April 1988
0021-9797/88 $3.00 Copyright© 1988by AcademicPress,Inc. All rightsof reproductionin any formreserved.
476
NAYAK AND MOFFAT
mation of protonated methanol initially but methylated anions and finally Cn hydrocarbons (n >/2) at higher temperatures (5, 6). Subsequent work on a series ofmonovalent salts of 12-tungstophosphoric (H3PWI2040), 12-tungstosilicic (H4SiW1204o), and 12-molybdophosphoric (H3PMolaO40) acids has shown that, with certain of the monovalent cations, for example, potassium, cesium, and ammonium, a microporous structure is produced (7-9). However, as shown from t plot analyses of nitrogen adsorption isotherms, the micropore radii extend over an approximately 5-A range. In view of the observations that certain gaseous species have the ability to penetrate into the bulk structure of solid heteropoly acids of Keggin structure and that various monovalent salts of these acids possess microporous structures apparently intrinsic to such elemental compositions, it appeared valuable to initiate studies of the sorption and diffusion of a series of molecules onto and into representative heteropoly acids and their microporous salts. Little or no information on this subject is currently available. The aforementioned three heteropoly acids and their ammonium salts have been chosen for this work. The first report was focused on aromatic hydrocarbons as sorbate species (10). The present study is concerned with aliphatic hydrocarbons. The present work employs a gravimetric method for the measurement of the rates of sorption and desorption as well as the sorption isotherms and sorption capacities.
nium carbonate solution, evaporating to dryness over a steam bath, and then heating in an air oven at 383 K for 3 h as described elsewhere (10). The sizes and shapes of the crystals of these heteropolyoxometalates were estimated from scanning electron microscopy (JEOL-JSM-840), while sorption equilibria and rates were measured by a gravimetric method using a Cahn electrobalance fitted with a high-vacuum system. Approximately 80-100 mg of the heteropolyoxometalates was used for the sorption equilibrium studies. For the measurement of sorption rates only approximately 15-20 mg of the solids was employed. This was spread in the form of single layers of crystals over a wide surface of the aluminum samples pan to minimize the external mass and heat transfer effects. The acids and salts were heated at 423 and 523 K, respectively, under vacuum for 2 h before measurements were initiated. The system could be evacuated to 10-4 Torr by the use of a Balzers mechanical pump and two mercury diffusion pumps connected in series. The pressure in the system was measured with a Texas Instruments fused quartz precision pressure gauge. With the sample tube of the balance immersed in a constant temperature water bath the solid sample could be maintained at the desired temperature +0. l K. The pressure in the system could be increased or decreased by opening to the storage bulb or to the vacuum bulb, respectively. Sorption equilibrium data were collected with both increasing and decreasing pressure.
EXPERIMENTAL
RESULTS
High-purity sorbents, n-hexane (Baker, analyzed), 3-methylpentane (Ventron), cyclohexane (Baker, analyzed), n-heptane (Fisher), n-octane (Fisher), and isooctane (Baker, analyzed) were employed without further purification. The solid acids HPW (BDH, Analar), HPMo (BDH, analar), and HSiW (Baker, analyzed) were recrystallized before use. The ammonium salts were prepared by treating aqueous solutions of the heteropoly acids with stoichiometric quantities of aqueous ammo-
In general sorption capacities of the heteropoly acids, HPW, HPMo, and HSiW, for the aliphatic saturated hydrocarbons are much lower than those of the microporous ammonium salts of these acids. The sorption capacities and rates as well as the shapes of the isotherms (of Type I) of the aliphatic saturated hydrocarbons, n-hexane, 3-methylpentane, cyclohexane, n-heptane, n-octane, and isooctane, appear to be dependent on the type of heteropoly compound. These results are sim-
Journal of Colloid and Interface Science, VoL 122, No, 2, April 1988
477
SORPTION AND DIFFUSION IN HETEROPOLYOXOMETALATES
ilar to those obtained earlier with the aromatic hydrocarbons (10). The adsorption of alkanes appears to be reversible at temperatures above 293 K. The sorption capacities of the heteropoly acids appear to be nearly independent of the size of the alkane molecule (Table I). However, although the differences are small, somewhat larger sorbate capacities are found for all molecules on HPMo as compared with HPW and HSiW. With all sorbates and all sorbents in the present work the isosteric heats of sorption are 9.7 + 0.6 kcal/mole. The sorption capacities of the ammonium salts of the heteropoly acids for the alkanes are 1 5-20 times higher than those of the heteropoly acids (Table I). The sorption capacities of a given ammonium salt for the various alkanes in general increase with the decreasing boiling point of the alkanes as would be expected where physical interaction forces are
involved. Where the boiling points are similar, as with n-heptane and isooctane, the higher sorption capacity is obtained with the sorbate molecule ofsmaUer diameter. Typical adsorption isotherms for the three ammonium salts are shown in Fig. 1. Of the three ammonium salts the highest sorption capacity for all alkanes is found with that of HSiW. Although the differences in sorption capacity between NHaPW and NH4PMo are small the latter generally shows somewhat larger capacities. The solution of the Fickian diffusion equation for a system of uniform spherical particles with a step change in surface concentration at time zero is given by (11)
-K21r2Dt nn~- - 1-~52 r ~ 2exp
r2
'
where n/n~ is the amount adsorbed at time t relative to that at equilibrium, D is the diffusivity, and r is the radius of the sorbent crystal.
TABLE I Sorption Capacities (mmole. g-l) of Heteropoly Compounds for Different Aliphatic Hydrocarbons Sorbent °
Temp. Sorbate
(K)
Pressure (mbar)
HPW
HSiW
HPMo
NHPW
NHSiW
NHPMo
n-Hexane
293 308 323
40
0.020 0.016 0.013
0.019 0.016 0.030
0.023 0.020 0.017
0.300 0.260 0.215
0.530 0.480 0.430
0.330 0.285 0.245
3-Methylpentane
293 308 323
40
0.021 0.016 0.011
0.020 0.016 0.013
0.024 0.020 0.016
0.333 0.290 0.250
0.510 0.475 0.433
0.323 0.260 0.215
Cyclohexane
293 308 323
40
0.026 0.019 0.014
0.027 0.022 0.017
0.031 0.026 0.020
0.320 0.275 0.235
0.523 0.490 0.458
0.325 0.280 0.240
n-Heptane
293 308 323
30
0.025 0.019 0.015
0.026 0.019 0.015
0.028 0.023 0.017
0.263 0.225 0.183
0.400 0.370 0.338
0.301 0.260 0.213
n-Octane
293 308 323
10
0.025 0.020 0.016
0.026 0.022 0.018
0.029 0.026 0.019
0.253 0.210 0.173
0.390 0.360 0.333
0.290 0.250 0.205
Isooctane
293 308 323
30
0.025 0.020 0.015
0.026 0.021 0.016
0.028 0.022 0.017
0.250 0.205 0.170
0.385 0.348 0.315
0.270 0.238 0.195
a Samples are designated as described in text. Journal of Colloid and Interface Science. Vol. 122, No. 2, April 1988
478
NAYAK
AND
MOFFAT
sorbate molecules into the solid spread in the form of a single layer of crystals. The relation
0,6
LN.s
S
d n/n ) lti~m0
2
d ~ - ~ L - 2 ]
0.4
¢o c 0.2
o.o
,'o
do
a'o
P(mbar)
FIG. 1. S o ~ t i o n i s o t h e r m f o r n - h e x a n e o n t h r e e hete r o p o l y a c i d s at 2 9 3 K .
Since n/noo was found to be linear in ~ up to a value of 0.5 for the former, the rate of sorption in this range may be taken as controlled only by intracrystalline diffusion of
where L is the characteristic length (V/a) with a the external surface area of the crystal and V the volume of the crystal, may then be employed to calculate the diffusivities. The values of L were calculated from the shapes and sizes of the crystals as obtained from scanning electron microscopy and are included in Table II. The particles were found to be approximately spherical and appropriate geometrical relations were employed. The rates of sorption data for the alkanes on HPW and NH4PW and of n-octane on the various heteropoly compounds are shown for illustrative purposes in Figs. 2 and 3, respectively. The diffusivities of the heteropoly acids are in general a factor of approximately 5 lower
T A B L E II Diffusivities ( c m 2. s -~) × l 0 n o f A l i p h a t i c S a t u r a t e d H y d r o c a r b o n s in D i f f e r e n t H e t e r o p o l y C o m p o u n d s a t Different T e m p e r a t u r e s Sorbent °
~n~c dmme~r (A)
Temp. (K)
HPW L = 0.083
HSiW L = 0.083
HPMo L = 0.083
NHPW L =0.117
NHSiW L = 0,583
n-Hexane
4.3
293
4.6
4.2
5.4
4.8
110
25
3-Methylpentane
5.5
293 308 323
4.3 ---
3.9 ---
5.0 ---
4.7 6.2 7.8
100 130 160
23 28 34
Cyclohexane
6.0
293 308 323
2.3 ---
2.1 ---
3.0 ---
2.6 3.4 4.3
47 62 78
10 13 16
n-Heptane
4.3
293 308 323
3.1 ---
3.0 ---
3.8 ---
4.1 5.6 7.1
78 99 120
18 23 29
n-Octane
4.3
293 308 323
2.3 ---
2.2 ---
3.1 ---
2.7 3.9 5.3
49 70 90
12 16 20
Isooctane
--
293 308 328
2.1 ---
1.9 ---
2.8 ---
2.2 3.0 4.0
45 59 78
8.8 11 15
~ffusing speci~
a S a m p l e s a r e d e s i g n a t e d as d e s c r i b e d in text. L is t h e c h a r a c t e r i s t i c l e n g t h (urn). Journal o f Colloid and Interface Science, Vol. 122, No. 2, April 1988
NHPMo
L = 0.208
479
SORPTION AND DIFFUSION IN HETEROPOLYOXOMETALATES
.
1.0 A
y
nln
HPW
/ 0.E
/
|
0.0 1.0
|
l
,
,
I
I
t
,
t
I
c
nln
0.5
¢
0.0
2
i
L
2
4
,
4
FtG. 2. Kinetics of sorption of n-hexane (e), n-heptane (A), n-octane (m), 3-methylpentane (O), cyclohexane (A), and isooctane ([:]) on HPW and NHPW at 293 K.
than those observed with NH4PMo and a factor of approximately 20 lower than those with NH4SiW. However, it is interesting to note
1,0,
nln 0.5
0.0
i
0
i
i
2
i
4
i
i
that the diffusivities with the ammonium salt of HPW are little different from those of the heteropoly acids. In contrast to the observations for sorption capacity the diffusivities now show a marked dependence on the kinetic diameters of the sorbate molecules. The lowest diffusivities are found for cyclohexane while the highest values are found with n-hexane. However, it is important to note that for a given heteropolyoxometalate the diffusivities are not monotonically dependent on the kinetic diameters of the sorbate molecules. Indeed, reasonable correlations may be obtained between the diffusivity values and the product of kinetic diameter and boiling of the sorbates (see Fig. 4).
6
DISCUSSION
q t{sec)
FIG. 3. Kinetics of sorption of n-octane on heterol~lyoxometalates at 293 K.
In this work the sorption capacity (mmole a particular temperature is defined as
g-t) at
Journal of Colloid and Interface Science, Vol.
122, N o . 2, A p r i l 1988
480
NAYAK AND MOFFAT
5
the m a x i m u m quantity of sorbate taken up at a pressure beyond which the sorption is essentially independent of the pressure of the sorbate. Since the relative pressures at which sorption capacities are reached are different for different sorbates, the sorption capacities are of necessity compared at different vapor pressures. The sorption capacities of the amm o n i u m salt of the heteropoly acids are in general higher by at least a factor of 10 than those observed with the parent acids for sorption of aliphatic hydrocarbons. In addition, with the solid acids the sorption capacities show relatively little variation for the different sorbates, and for a given sorbate, show only small differences between the sorbents (Fig. 5). There appears to be no systematic dependence of the m a x i m u m amounts sorbed on either sorbate molecular size or on sorbate normal boiling point. In contrast, with the a m m o n i u m salts, where the amounts taken up are m u c h larger, the sorption capacities depend markedly on
\\ o
\ \ \
4
o Alkanes • Aromatics
o
\
o \
\ \ \ \ \
'=o 3 o
\
o
\° \
=_o
\
x 2 =:3
\
\
\ \\ o\ \
.oo
\
\
z2oo
,8oo
26o0
3ooo
FIG. 4. Dependenceof diffusivity on the product of kinetic diameter and boiling point of sorbate for NHPW at
293 K. ((3)Alkanes,(e) aromatics(data from earlierresults (10)).
0.04 I~1 H 3 P W 1 2 0 4 0 [~
H4SiW12040
I~1
H3PMo12040
0.0,~
I=
=
E
E
"3 O 8
0.01
03
/ / / I / / / / /
o.oG
,'1 /'1
/
_
/
I
/
-
/1 /I
/ / /
/i
/'J
/ ]-
l
/
_
/ /
/
y
-
/
,'/
/ / / /
/1
/ /
/ I I / / / /
f/
/
/ / / /
7
0.02
/
-
i
t
/
n-Hex~e 3-Methyl- Cyclo- n-Hepiane n-Octane is(~Ociane Pentane Hexane 4.3
5.5
6.0
4.3
4.3
Kinetic Diameter, (7 (~)
FIG. 5. Sorption capacities of heteropoly acids for aliphatic saturated hydrocarbons at 293 K. Journal of Colloid and Interface Science, Vol. 122, No. 2, April 1988
SORPTION AND DIFFUSION IN HETEROPOLYOXOMETALATES
481
0.7
0.6
[~
(NH4)3 PW12040
[-1
(NH4)4 SIW12040
E]
(NH4)3 PMo12040
0.5
E ¢n >~ .',~
0.4
0.3
.-=1 .-4 "-I "-I
ca 0 0.2
V v
o)
0.1
0.0
--'1
"--f
"-'
-I
n-Hexane 3-Methyl- CycloPentane Hexane 4.3
5.5
6.0
v
V n-Heptane n-Octane iso-Octane 4.3
4.3
Kinet c Diameter, o- (~)
lOG. 6. Sorption capacitiesof ammonium salts of heteropolyacids for aliphatic saturated hydrocarbons at 293 K. the hydrocarbon being sorbed (Fig. 6). For a given salt, the capacities measured as the hydrocarbon is varied appear to be primarily dependent on the normal boiling point of the hydrocarbon. Marked differences are observed, however, for a given sorbate, between the sorption capacities for the three ammonium salts. With each of the aliphatic hydrocarbons the capacities for the ammonium salts of H P W and H P M o are approximately the same but that of (NH4)aSiW12040 is in each case substantially higher than that measured in the former cases. Although caution must be exercised in comparing the present results with those obtained from the analyses of nitrogen adsorption-desorption isotherms as reported earlier (7-9), it is nevertheless interesting to note that the micropore volumes calculated from the latter for NH4SiW12040 are similar to the sorption capacities found with the present sorbates, while with the remaining two sorbents, the hydrocarbon capacities are markedly lower than those found with nitrogen, the latter of
which are in turn higher than those for NH4SiW12040.It should be noted that the micropore distributions, as estimated from the nitrogen isotherms, for the a m m o n i u m salts show peaks centered about the 8- to 10-~,-radius range, although the a m m o n i u m salt of H P M o is binodal, with the broad second maximum at approximately 12-14 A. BET surface areas for the a m m o n i u m salts of HPW, HPMo, and HSiW were calculated as 128.2, 193.4, and 116.9 m E g-l, respectively (7-9). It is also interesting to compare the present results with those reported previously for the aromatic hydrocarbons as sorbates (l 0). With both the aromatic and the aliphatic hydrocarTABLE IIl Best-Fit n Values from Finite-LayerBET Equation Sorbent Sorbate
NHPW
NHSiW
NHPMo
n-Hexane Isooctane
2.6 2.3
1.7 1.3
2.4 2.0
Journal of Colloid and Interface Science. Vol. 122, No. 2, April 1988
482
NAYAK AND MOFFAT TABLEIV
Results Calculated from Isotherms for the Adsorption of Nitrogen at 78 K (9) Sorbent
Surfaceareas BET C Fmp n Micropore volume
NHPW
NI-I~W
NHPMo
128.2 760 10.3 2.0 50.0
116.9 1919 9.5 1.6 40.0
193.4 877 13.0 2.6 53.0
bons the sorption capacity of (NH4)4SiW12040 exceeded that of the remaining two solids. The values for the diffusivities, as calculated from the initial values of the rate of adsorption data, are of the same order of magnitude for all the heteropoly acids. Interestingly, however, those for the a m m o n i u m salt of H P W are indistinguishable from those for the solid acids while the diffusivity values for (NH4)4SiWI204o and (NH4)3PMol204o are higher by factors of approximately 20 and 5, respectively, than those of the acids. Although not exact, an approximately inverse relation is observed between the diffusivities and the product of boiling point and the size of the sorbate molecules with the a m m o n i u m salts of HSiW and HPMo. The diffusivities of the aliphatic hydrocarbons as measured in the present work on the heteropoly acids are nearly independent of the hydrocarbon examined. However, those of the aromatic hydrocarbons reported previously showed a dependence on the sorbate molecular size, from 5.8 to 7.5 A.
It is of interest to apply the n-layer BET equation to the isotherm data obtained in the present work. Preliminary calculations showed that relatively little error is introduced by employing c and Vm values from the infinite-layer BET equation in the finite-layer relation. Typical best-fit values for n calculated with these parameters are summarized in Table III. For comparison purposes data reported earlier with nitrogen as a sorbate are given in Table IV. The values of n calculated for hexane and isooctane are semiquantitatively consistent with the data obtained from nitrogen isotherms. As expected the number of layers calculated for isooctane is less than that for nhexane with each o f the sorbents and the values for NHSiW with either sorbate are smaller than those obtained with either N H P W or NHPMo, consistent with the smaller average micropore radius previously found from the nitrogen adsorption data (Table IV). In contrast, the n values for N H P M o are slightly smaller than those for NHPW, apparently inconsistent with the data in Table IV. However, with N H P M o the pore size distribution is binodal unlike that for N H P W or NHSiW. This may, at least in part, be responsible for the high value of r-rapThe kinetics of occlusion have recently been studied by Aharoni and Suzin (12) and an equation was proposed which related n~ and n~, the quantities sorbed at time r and at equilibrium, respectively, to the time z: nt_
n~
A
n~
)__~1 l n / t + t ~ ) bn~ ~lp '
TABLE V Values of Slopes of n/noo vs In r Curves of DifferentAliphatic Saturated Hydrocarbonson HeteropolyCompounds Sorbent Sorbate
HPW
HSiW
HPMo
NHPW
NHSiW
NHPMo
n-Hexane Cyclohexane Isooctane
0.21 0.28 0.29
0.20 0.29 0.26
0.19 0.24 0.23
0.30 0.30 0.33
0.34 0.31 0.34
0.24 0.28 0.31
Journal of Colloid and Interface Science, Vol. 122, No. 2, April 1988
SORPTION AND DIFFUSION IN HETEROPOLYOXOMETALATES where tr is taken as 0.333 and tp as 0 . 3 4 % with r = (1/~r2)(r2/D) and the values for r and D are those reported in the present work. r is a residence time related to the length o f the diffusion path and to the diffusion coefficient. Values o f the slopes obtained f r o m plots o f n t / n ~ vs In t fall in the range 0.24 ___0.05 and 0.29 + 0.05 for the solid acids and a m m o n i u m salts, respectively (Table V). A value o f 0.24 reflects the presence of a h o m o g e n e o u s pore system while a slope 0.24 is indicative o f a heterogeneous system o f pores. Although the data shown suggest the presence o f h o m o g e neous pore systems in b o t h the acids and the salts, the larger values for the latter are presumably a reflection o f the presence o f a micropore. ACKNOWLEDGMENT The financial support of the Natural Sciences and Engineering Research Council of Canada is gratefully acknowledged.
483
REFERENCES 1. Moffat, J. B., and Hayashi, H., "Catalytic Conversion of Synthesis Gas and Alcohols to Chemicals" (R. G. Herman, Ed.), p. 1395. Plenum, New York, 1984. 2. Highfield, J. G., and Moffat, J. B., J. Catal. 88, 177 (1984). 3. Highfield, J. G., and Moffat, J. B., J. Catal. 89, 185 (1984). 4. Moffat,J. B., "Catalysisby Acidsand Bases"(B. Imelik et al., Eds.), Vol. 157. Elsevier, Amsterdam, 1985. 5. Highfield, J. G., and Moffat, J. B., J. Catal. 95, 108 (1985). 6. Highfield, J. G., and Moffat, J. B., J. Catal. 98, 245 (1986). 7. McMonagle, J. B., and Moffat, J. B., J. Colloid Interface Sci. 101, 479 (1984). 8. Taylor, D. B., McMonagle, J. B., and Moffat, J. B., J. Colloid Interface ScL 108, 278 (1985). 9. Moffat, J. B., Polyhedron 5, 261 (1986). 10. Nayak, V. S., and Moffat, J. B., J. Colloid Interface Sci. 120, 301 (1987). 11. Crank, J., "The Mathematics of Diffusion," p. 91. Oxford Univ. Press, London, 1975. 12. Aharoni, C., and Suzin, Y., J. Chem. Soc. Faraday Trans. 1 78, 2313, 2321, 2329 (1982).
Journal of Colloid and Interface Science, Vol. 122, No. 2, April 1988
INTRODUCTION
Heteropolyoxometalates are ionic solids with large cagelike anions and cations ranging from protons to relatively large organic species. The solids of interest in the present work have anions customarily referred to as possessing Keggin structure. These anions have a central atom such as phosphorus surrounded by four oxygen atoms arranged tetrahedrally. The central tetrahedron is enveloped by 12 octahedra with a metal atom such as tungsten at their centers and oxygen atoms at their vertices, which are shared between each other and the central tetrahedron. Heteropolyoxometalates of such structure have been shown to be multifunctional in their catalytic properties, the functionality being primarily dependent on the nature of the peripheral metal atoms in the anion. Those containing tungsten are selective in the conversion To whom correspondence should be addressed.
of methanol and other alcohols to hydrocarbons while those with molybdenum have activity in oxidation reactions (1). In earlier work from this laboratory ammonium 12-tungstophosphate was shown to have an unexpected activity and selectivity in the conversion of methanol. In addition its surface area was found to be substantially higher (150 m2/g) than that measured for the parent solid acid (10 m2/g). However, in spite of the low surface areas of the heteropoly acids it has been shown with photoacoustic FTIR spectroscopy (PAS) that molecules such as ammonia and pyridine are capable of penetrating into the bulk structure of a low-area, nonporous solid acid such as H3PWl204o and interacting with the interior protons to form the ammonium and pyridinium salts (2, 3). It should be noted however that any water molecules present will restrict access of the sorbate molecules to the protons (4). Alcohols, such as methanol, have also been shown by PAS to interact stoichiometrically with surface and interior protons with the for-
475
JournalofColloidand InterfaceScience,Vol. 122,No. 2, April 1988
0021-9797/88 $3.00 Copyright© 1988by AcademicPress,Inc. All rightsof reproductionin any formreserved.
476
NAYAK AND MOFFAT
mation of protonated methanol initially but methylated anions and finally Cn hydrocarbons (n >/2) at higher temperatures (5, 6). Subsequent work on a series ofmonovalent salts of 12-tungstophosphoric (H3PWI2040), 12-tungstosilicic (H4SiW1204o), and 12-molybdophosphoric (H3PMolaO40) acids has shown that, with certain of the monovalent cations, for example, potassium, cesium, and ammonium, a microporous structure is produced (7-9). However, as shown from t plot analyses of nitrogen adsorption isotherms, the micropore radii extend over an approximately 5-A range. In view of the observations that certain gaseous species have the ability to penetrate into the bulk structure of solid heteropoly acids of Keggin structure and that various monovalent salts of these acids possess microporous structures apparently intrinsic to such elemental compositions, it appeared valuable to initiate studies of the sorption and diffusion of a series of molecules onto and into representative heteropoly acids and their microporous salts. Little or no information on this subject is currently available. The aforementioned three heteropoly acids and their ammonium salts have been chosen for this work. The first report was focused on aromatic hydrocarbons as sorbate species (10). The present study is concerned with aliphatic hydrocarbons. The present work employs a gravimetric method for the measurement of the rates of sorption and desorption as well as the sorption isotherms and sorption capacities.
nium carbonate solution, evaporating to dryness over a steam bath, and then heating in an air oven at 383 K for 3 h as described elsewhere (10). The sizes and shapes of the crystals of these heteropolyoxometalates were estimated from scanning electron microscopy (JEOL-JSM-840), while sorption equilibria and rates were measured by a gravimetric method using a Cahn electrobalance fitted with a high-vacuum system. Approximately 80-100 mg of the heteropolyoxometalates was used for the sorption equilibrium studies. For the measurement of sorption rates only approximately 15-20 mg of the solids was employed. This was spread in the form of single layers of crystals over a wide surface of the aluminum samples pan to minimize the external mass and heat transfer effects. The acids and salts were heated at 423 and 523 K, respectively, under vacuum for 2 h before measurements were initiated. The system could be evacuated to 10-4 Torr by the use of a Balzers mechanical pump and two mercury diffusion pumps connected in series. The pressure in the system was measured with a Texas Instruments fused quartz precision pressure gauge. With the sample tube of the balance immersed in a constant temperature water bath the solid sample could be maintained at the desired temperature +0. l K. The pressure in the system could be increased or decreased by opening to the storage bulb or to the vacuum bulb, respectively. Sorption equilibrium data were collected with both increasing and decreasing pressure.
EXPERIMENTAL
RESULTS
High-purity sorbents, n-hexane (Baker, analyzed), 3-methylpentane (Ventron), cyclohexane (Baker, analyzed), n-heptane (Fisher), n-octane (Fisher), and isooctane (Baker, analyzed) were employed without further purification. The solid acids HPW (BDH, Analar), HPMo (BDH, analar), and HSiW (Baker, analyzed) were recrystallized before use. The ammonium salts were prepared by treating aqueous solutions of the heteropoly acids with stoichiometric quantities of aqueous ammo-
In general sorption capacities of the heteropoly acids, HPW, HPMo, and HSiW, for the aliphatic saturated hydrocarbons are much lower than those of the microporous ammonium salts of these acids. The sorption capacities and rates as well as the shapes of the isotherms (of Type I) of the aliphatic saturated hydrocarbons, n-hexane, 3-methylpentane, cyclohexane, n-heptane, n-octane, and isooctane, appear to be dependent on the type of heteropoly compound. These results are sim-
Journal of Colloid and Interface Science, VoL 122, No, 2, April 1988
477
SORPTION AND DIFFUSION IN HETEROPOLYOXOMETALATES
ilar to those obtained earlier with the aromatic hydrocarbons (10). The adsorption of alkanes appears to be reversible at temperatures above 293 K. The sorption capacities of the heteropoly acids appear to be nearly independent of the size of the alkane molecule (Table I). However, although the differences are small, somewhat larger sorbate capacities are found for all molecules on HPMo as compared with HPW and HSiW. With all sorbates and all sorbents in the present work the isosteric heats of sorption are 9.7 + 0.6 kcal/mole. The sorption capacities of the ammonium salts of the heteropoly acids for the alkanes are 1 5-20 times higher than those of the heteropoly acids (Table I). The sorption capacities of a given ammonium salt for the various alkanes in general increase with the decreasing boiling point of the alkanes as would be expected where physical interaction forces are
involved. Where the boiling points are similar, as with n-heptane and isooctane, the higher sorption capacity is obtained with the sorbate molecule ofsmaUer diameter. Typical adsorption isotherms for the three ammonium salts are shown in Fig. 1. Of the three ammonium salts the highest sorption capacity for all alkanes is found with that of HSiW. Although the differences in sorption capacity between NHaPW and NH4PMo are small the latter generally shows somewhat larger capacities. The solution of the Fickian diffusion equation for a system of uniform spherical particles with a step change in surface concentration at time zero is given by (11)
-K21r2Dt nn~- - 1-~52 r ~ 2exp
r2
'
where n/n~ is the amount adsorbed at time t relative to that at equilibrium, D is the diffusivity, and r is the radius of the sorbent crystal.
TABLE I Sorption Capacities (mmole. g-l) of Heteropoly Compounds for Different Aliphatic Hydrocarbons Sorbent °
Temp. Sorbate
(K)
Pressure (mbar)
HPW
HSiW
HPMo
NHPW
NHSiW
NHPMo
n-Hexane
293 308 323
40
0.020 0.016 0.013
0.019 0.016 0.030
0.023 0.020 0.017
0.300 0.260 0.215
0.530 0.480 0.430
0.330 0.285 0.245
3-Methylpentane
293 308 323
40
0.021 0.016 0.011
0.020 0.016 0.013
0.024 0.020 0.016
0.333 0.290 0.250
0.510 0.475 0.433
0.323 0.260 0.215
Cyclohexane
293 308 323
40
0.026 0.019 0.014
0.027 0.022 0.017
0.031 0.026 0.020
0.320 0.275 0.235
0.523 0.490 0.458
0.325 0.280 0.240
n-Heptane
293 308 323
30
0.025 0.019 0.015
0.026 0.019 0.015
0.028 0.023 0.017
0.263 0.225 0.183
0.400 0.370 0.338
0.301 0.260 0.213
n-Octane
293 308 323
10
0.025 0.020 0.016
0.026 0.022 0.018
0.029 0.026 0.019
0.253 0.210 0.173
0.390 0.360 0.333
0.290 0.250 0.205
Isooctane
293 308 323
30
0.025 0.020 0.015
0.026 0.021 0.016
0.028 0.022 0.017
0.250 0.205 0.170
0.385 0.348 0.315
0.270 0.238 0.195
a Samples are designated as described in text. Journal of Colloid and Interface Science. Vol. 122, No. 2, April 1988
478
NAYAK
AND
MOFFAT
sorbate molecules into the solid spread in the form of a single layer of crystals. The relation
0,6
LN.s
S
d n/n ) lti~m0
2
d ~ - ~ L - 2 ]
0.4
¢o c 0.2
o.o
,'o
do
a'o
P(mbar)
FIG. 1. S o ~ t i o n i s o t h e r m f o r n - h e x a n e o n t h r e e hete r o p o l y a c i d s at 2 9 3 K .
Since n/noo was found to be linear in ~ up to a value of 0.5 for the former, the rate of sorption in this range may be taken as controlled only by intracrystalline diffusion of
where L is the characteristic length (V/a) with a the external surface area of the crystal and V the volume of the crystal, may then be employed to calculate the diffusivities. The values of L were calculated from the shapes and sizes of the crystals as obtained from scanning electron microscopy and are included in Table II. The particles were found to be approximately spherical and appropriate geometrical relations were employed. The rates of sorption data for the alkanes on HPW and NH4PW and of n-octane on the various heteropoly compounds are shown for illustrative purposes in Figs. 2 and 3, respectively. The diffusivities of the heteropoly acids are in general a factor of approximately 5 lower
T A B L E II Diffusivities ( c m 2. s -~) × l 0 n o f A l i p h a t i c S a t u r a t e d H y d r o c a r b o n s in D i f f e r e n t H e t e r o p o l y C o m p o u n d s a t Different T e m p e r a t u r e s Sorbent °
~n~c dmme~r (A)
Temp. (K)
HPW L = 0.083
HSiW L = 0.083
HPMo L = 0.083
NHPW L =0.117
NHSiW L = 0,583
n-Hexane
4.3
293
4.6
4.2
5.4
4.8
110
25
3-Methylpentane
5.5
293 308 323
4.3 ---
3.9 ---
5.0 ---
4.7 6.2 7.8
100 130 160
23 28 34
Cyclohexane
6.0
293 308 323
2.3 ---
2.1 ---
3.0 ---
2.6 3.4 4.3
47 62 78
10 13 16
n-Heptane
4.3
293 308 323
3.1 ---
3.0 ---
3.8 ---
4.1 5.6 7.1
78 99 120
18 23 29
n-Octane
4.3
293 308 323
2.3 ---
2.2 ---
3.1 ---
2.7 3.9 5.3
49 70 90
12 16 20
Isooctane
--
293 308 328
2.1 ---
1.9 ---
2.8 ---
2.2 3.0 4.0
45 59 78
8.8 11 15
~ffusing speci~
a S a m p l e s a r e d e s i g n a t e d as d e s c r i b e d in text. L is t h e c h a r a c t e r i s t i c l e n g t h (urn). Journal o f Colloid and Interface Science, Vol. 122, No. 2, April 1988
NHPMo
L = 0.208
479
SORPTION AND DIFFUSION IN HETEROPOLYOXOMETALATES
.
1.0 A
y
nln
HPW
/ 0.E
/
|
0.0 1.0
|
l
,
,
I
I
t
,
t
I
c
nln
0.5
¢
0.0
2
i
L
2
4
,
4
FtG. 2. Kinetics of sorption of n-hexane (e), n-heptane (A), n-octane (m), 3-methylpentane (O), cyclohexane (A), and isooctane ([:]) on HPW and NHPW at 293 K.
than those observed with NH4PMo and a factor of approximately 20 lower than those with NH4SiW. However, it is interesting to note
1,0,
nln 0.5
0.0
i
0
i
i
2
i
4
i
i
that the diffusivities with the ammonium salt of HPW are little different from those of the heteropoly acids. In contrast to the observations for sorption capacity the diffusivities now show a marked dependence on the kinetic diameters of the sorbate molecules. The lowest diffusivities are found for cyclohexane while the highest values are found with n-hexane. However, it is important to note that for a given heteropolyoxometalate the diffusivities are not monotonically dependent on the kinetic diameters of the sorbate molecules. Indeed, reasonable correlations may be obtained between the diffusivity values and the product of kinetic diameter and boiling of the sorbates (see Fig. 4).
6
DISCUSSION
q t{sec)
FIG. 3. Kinetics of sorption of n-octane on heterol~lyoxometalates at 293 K.
In this work the sorption capacity (mmole a particular temperature is defined as
g-t) at
Journal of Colloid and Interface Science, Vol.
122, N o . 2, A p r i l 1988
480
NAYAK AND MOFFAT
5
the m a x i m u m quantity of sorbate taken up at a pressure beyond which the sorption is essentially independent of the pressure of the sorbate. Since the relative pressures at which sorption capacities are reached are different for different sorbates, the sorption capacities are of necessity compared at different vapor pressures. The sorption capacities of the amm o n i u m salt of the heteropoly acids are in general higher by at least a factor of 10 than those observed with the parent acids for sorption of aliphatic hydrocarbons. In addition, with the solid acids the sorption capacities show relatively little variation for the different sorbates, and for a given sorbate, show only small differences between the sorbents (Fig. 5). There appears to be no systematic dependence of the m a x i m u m amounts sorbed on either sorbate molecular size or on sorbate normal boiling point. In contrast, with the a m m o n i u m salts, where the amounts taken up are m u c h larger, the sorption capacities depend markedly on
\\ o
\ \ \
4
o Alkanes • Aromatics
o
\
o \
\ \ \ \ \
'=o 3 o
\
o
\° \
=_o
\
x 2 =:3
\
\
\ \\ o\ \
.oo
\
\
z2oo
,8oo
26o0
3ooo
FIG. 4. Dependenceof diffusivity on the product of kinetic diameter and boiling point of sorbate for NHPW at
293 K. ((3)Alkanes,(e) aromatics(data from earlierresults (10)).
0.04 I~1 H 3 P W 1 2 0 4 0 [~
H4SiW12040
I~1
H3PMo12040
0.0,~
I=
=
E
E
"3 O 8
0.01
03
/ / / I / / / / /
o.oG
,'1 /'1
/
_
/
I
/
-
/1 /I
/ / /
/i
/'J
/ ]-
l
/
_
/ /
/
y
-
/
,'/
/ / / /
/1
/ /
/ I I / / / /
f/
/
/ / / /
7
0.02
/
-
i
t
/
n-Hex~e 3-Methyl- Cyclo- n-Hepiane n-Octane is(~Ociane Pentane Hexane 4.3
5.5
6.0
4.3
4.3
Kinetic Diameter, (7 (~)
FIG. 5. Sorption capacities of heteropoly acids for aliphatic saturated hydrocarbons at 293 K. Journal of Colloid and Interface Science, Vol. 122, No. 2, April 1988
SORPTION AND DIFFUSION IN HETEROPOLYOXOMETALATES
481
0.7
0.6
[~
(NH4)3 PW12040
[-1
(NH4)4 SIW12040
E]
(NH4)3 PMo12040
0.5
E ¢n >~ .',~
0.4
0.3
.-=1 .-4 "-I "-I
ca 0 0.2
V v
o)
0.1
0.0
--'1
"--f
"-'
-I
n-Hexane 3-Methyl- CycloPentane Hexane 4.3
5.5
6.0
v
V n-Heptane n-Octane iso-Octane 4.3
4.3
Kinet c Diameter, o- (~)
lOG. 6. Sorption capacitiesof ammonium salts of heteropolyacids for aliphatic saturated hydrocarbons at 293 K. the hydrocarbon being sorbed (Fig. 6). For a given salt, the capacities measured as the hydrocarbon is varied appear to be primarily dependent on the normal boiling point of the hydrocarbon. Marked differences are observed, however, for a given sorbate, between the sorption capacities for the three ammonium salts. With each of the aliphatic hydrocarbons the capacities for the ammonium salts of H P W and H P M o are approximately the same but that of (NH4)aSiW12040 is in each case substantially higher than that measured in the former cases. Although caution must be exercised in comparing the present results with those obtained from the analyses of nitrogen adsorption-desorption isotherms as reported earlier (7-9), it is nevertheless interesting to note that the micropore volumes calculated from the latter for NH4SiW12040 are similar to the sorption capacities found with the present sorbates, while with the remaining two sorbents, the hydrocarbon capacities are markedly lower than those found with nitrogen, the latter of
which are in turn higher than those for NH4SiW12040.It should be noted that the micropore distributions, as estimated from the nitrogen isotherms, for the a m m o n i u m salts show peaks centered about the 8- to 10-~,-radius range, although the a m m o n i u m salt of H P M o is binodal, with the broad second maximum at approximately 12-14 A. BET surface areas for the a m m o n i u m salts of HPW, HPMo, and HSiW were calculated as 128.2, 193.4, and 116.9 m E g-l, respectively (7-9). It is also interesting to compare the present results with those reported previously for the aromatic hydrocarbons as sorbates (l 0). With both the aromatic and the aliphatic hydrocarTABLE IIl Best-Fit n Values from Finite-LayerBET Equation Sorbent Sorbate
NHPW
NHSiW
NHPMo
n-Hexane Isooctane
2.6 2.3
1.7 1.3
2.4 2.0
Journal of Colloid and Interface Science. Vol. 122, No. 2, April 1988
482
NAYAK AND MOFFAT TABLEIV
Results Calculated from Isotherms for the Adsorption of Nitrogen at 78 K (9) Sorbent
Surfaceareas BET C Fmp n Micropore volume
NHPW
NI-I~W
NHPMo
128.2 760 10.3 2.0 50.0
116.9 1919 9.5 1.6 40.0
193.4 877 13.0 2.6 53.0
bons the sorption capacity of (NH4)4SiW12040 exceeded that of the remaining two solids. The values for the diffusivities, as calculated from the initial values of the rate of adsorption data, are of the same order of magnitude for all the heteropoly acids. Interestingly, however, those for the a m m o n i u m salt of H P W are indistinguishable from those for the solid acids while the diffusivity values for (NH4)4SiWI204o and (NH4)3PMol204o are higher by factors of approximately 20 and 5, respectively, than those of the acids. Although not exact, an approximately inverse relation is observed between the diffusivities and the product of boiling point and the size of the sorbate molecules with the a m m o n i u m salts of HSiW and HPMo. The diffusivities of the aliphatic hydrocarbons as measured in the present work on the heteropoly acids are nearly independent of the hydrocarbon examined. However, those of the aromatic hydrocarbons reported previously showed a dependence on the sorbate molecular size, from 5.8 to 7.5 A.
It is of interest to apply the n-layer BET equation to the isotherm data obtained in the present work. Preliminary calculations showed that relatively little error is introduced by employing c and Vm values from the infinite-layer BET equation in the finite-layer relation. Typical best-fit values for n calculated with these parameters are summarized in Table III. For comparison purposes data reported earlier with nitrogen as a sorbate are given in Table IV. The values of n calculated for hexane and isooctane are semiquantitatively consistent with the data obtained from nitrogen isotherms. As expected the number of layers calculated for isooctane is less than that for nhexane with each o f the sorbents and the values for NHSiW with either sorbate are smaller than those obtained with either N H P W or NHPMo, consistent with the smaller average micropore radius previously found from the nitrogen adsorption data (Table IV). In contrast, the n values for N H P M o are slightly smaller than those for NHPW, apparently inconsistent with the data in Table IV. However, with N H P M o the pore size distribution is binodal unlike that for N H P W or NHSiW. This may, at least in part, be responsible for the high value of r-rapThe kinetics of occlusion have recently been studied by Aharoni and Suzin (12) and an equation was proposed which related n~ and n~, the quantities sorbed at time r and at equilibrium, respectively, to the time z: nt_
n~
A
n~
)__~1 l n / t + t ~ ) bn~ ~lp '
TABLE V Values of Slopes of n/noo vs In r Curves of DifferentAliphatic Saturated Hydrocarbonson HeteropolyCompounds Sorbent Sorbate
HPW
HSiW
HPMo
NHPW
NHSiW
NHPMo
n-Hexane Cyclohexane Isooctane
0.21 0.28 0.29
0.20 0.29 0.26
0.19 0.24 0.23
0.30 0.30 0.33
0.34 0.31 0.34
0.24 0.28 0.31
Journal of Colloid and Interface Science, Vol. 122, No. 2, April 1988
SORPTION AND DIFFUSION IN HETEROPOLYOXOMETALATES where tr is taken as 0.333 and tp as 0 . 3 4 % with r = (1/~r2)(r2/D) and the values for r and D are those reported in the present work. r is a residence time related to the length o f the diffusion path and to the diffusion coefficient. Values o f the slopes obtained f r o m plots o f n t / n ~ vs In t fall in the range 0.24 ___0.05 and 0.29 + 0.05 for the solid acids and a m m o n i u m salts, respectively (Table V). A value o f 0.24 reflects the presence of a h o m o g e n e o u s pore system while a slope 0.24 is indicative o f a heterogeneous system o f pores. Although the data shown suggest the presence o f h o m o g e neous pore systems in b o t h the acids and the salts, the larger values for the latter are presumably a reflection o f the presence o f a micropore. ACKNOWLEDGMENT The financial support of the Natural Sciences and Engineering Research Council of Canada is gratefully acknowledged.
483
REFERENCES 1. Moffat, J. B., and Hayashi, H., "Catalytic Conversion of Synthesis Gas and Alcohols to Chemicals" (R. G. Herman, Ed.), p. 1395. Plenum, New York, 1984. 2. Highfield, J. G., and Moffat, J. B., J. Catal. 88, 177 (1984). 3. Highfield, J. G., and Moffat, J. B., J. Catal. 89, 185 (1984). 4. Moffat,J. B., "Catalysisby Acidsand Bases"(B. Imelik et al., Eds.), Vol. 157. Elsevier, Amsterdam, 1985. 5. Highfield, J. G., and Moffat, J. B., J. Catal. 95, 108 (1985). 6. Highfield, J. G., and Moffat, J. B., J. Catal. 98, 245 (1986). 7. McMonagle, J. B., and Moffat, J. B., J. Colloid Interface Sci. 101, 479 (1984). 8. Taylor, D. B., McMonagle, J. B., and Moffat, J. B., J. Colloid Interface ScL 108, 278 (1985). 9. Moffat, J. B., Polyhedron 5, 261 (1986). 10. Nayak, V. S., and Moffat, J. B., J. Colloid Interface Sci. 120, 301 (1987). 11. Crank, J., "The Mathematics of Diffusion," p. 91. Oxford Univ. Press, London, 1975. 12. Aharoni, C., and Suzin, Y., J. Chem. Soc. Faraday Trans. 1 78, 2313, 2321, 2329 (1982).
Journal of Colloid and Interface Science, Vol. 122, No. 2, April 1988