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NMR Investigations of Self-Diffusion in Pentasils
NMR Investigations of Self-Diffusion
In
Pentasils
Karger, H. Pfeifer, D. Freude Sektion Physik der Karl-Marx-Universitat, Leipztg, German Democratic Republic ~. Caro, M. BO!ow, G. Ohlmann Zentralinstitut fOr physikalische Chemie der Akademie der Wissenschaften der DDR, Berlin, German Democratic Republic ~.
First results of nmr measurements on molecular selfdiffusion in zeolites of pentasil type (ZSM-5) are presented.With methane and propane as probe molecules, the influence of zeolite texture, of co-adsorbed molecules and of coke deposits on the transport properties of various ZSM-5 specimens are studied. Direct information on the strength and on the localization of transport obstacles may be deduced from the diffusion data.
INTRODUCTION Zeolites of pentasil type exhibit a number of remarkable properties /1, 2/ which are closely related to the diffusivity of the adsorbed molecules. So far, pentasil diffusion data obtained by con~entional techniques scatter over several orders of magnitude (cf., e.g. /2-4/). This situation is most likely to be referred to complications with the measuring procedure itself /5/ and to the difficulties on comparing different specimens including different sample pretreatments /6/. With the application of nmr spectroscopy to diffusion studies in zeolites, a direct observation of molecular migration within the adsorbate-adsorbent system has become possible /~, 8/. In this communication we present first results of nmr self-diffusion measurements performed on zeolites of pentasil type (ZSM-5). SINGLE COMPONENT ADSORPTION Fig. 1 shows experimental values for the self-diffusion coefficients of short-chain paraffins adsorbed in ZSM-5. As a consequence of the increasing mutual and steric hindrance, respectively, the absolute values are found to decrease with both increasing loading and chain length. In agreement with the order of increasing pore diameter, the diffusivities are smaller than in NaX, but larger than in NaCaA /9/. self-diffusion measurements performed on different ZSM-5 specimens (fig. 2) do not reveal a 633
634 (AD-8-4)
temperature I'e
°
z
,., "e
...
t
-so
.--
-100
10-' 4
10--
Z
J
- - - - . nu.. ter.F ...lIeu".
per II• • .• .
Fig. 1 Self-diffusion coefficient of methane (0). ethane (A) and propane (C) in ZSM-5 at 300 K in dependence on the sorbate concentration Fig. 3 Self-diffusion coefficient of water in ZSM-5 at 296 K and at a loading of about 35 ••• 40 mg/g in dependence on the.Si0 2/A120 (symbols as J.n fl:g. z) 3-ratio
Fig. 2 Temperature dependence of the self-diffusion coefficient of methane in ZSM-5 specimens of different origin /10/ an~ different Si02/A120~ratio (A ~ 10 ; ... 540; . . 216; ... 185; • 127; D 80; • 72 ; 0 50; A 42; 40)
<>
..
-t I
N
z
____ J~!j~!'~t:~
10"
o.. -0 A
II)
e
4
CI
fO-·
Y l!.
~
z 4 2
_
---
10'
102
SiOa
A'a O" -
103
.
ratIo
significant influence of the SiO /Al 0 3 ratio. This result is in agreement with the fact that sat~ratid hydrocarbons do not interact specifically with adsorption sites on zeolites /7. 10/. in contrast e.g. to wate~ where a decreasing mobility with decreasing SiO /Al 0 -ratio is observed. In complete agreement with this ten~enc~.3the self-diffusion coefficients of water in the hydrophilic zeolites NaX and NaA are distinctly smaller than in ZSM-5 (fig. 3/10/).
J. Karger et al.
635
INFLUENCE OF CO-ADSORBED MOLECULES Fig. 4 shows experimental values for the self-diffusion coefficient of methane in ZSM-5 under the influence of co-adsorbed benzene molecules. It can be seen, that the molecular mobility of methane is drastically reduced by the existence of the benzene molecules /11/. In view of the large difference in the mObilities of the two co-adsorbed molecular species, and the similarity in the cross-sections of the benzene molecules and of the intracrystalline channels, it should be possible to simulate the influence of the benzene molecules on the methane mobility by introducing rigid obstacles into the channel network, thereby assuming that the number of obstacles is equal to the number of benzene molecules. 4
Fig. 4 self-diffusion coefficient of methane in Z5M-5 at 296 K in dependence on the amount of co-adsorbed benzene
l
6 fO"'
..
~~A 6 6 6
•
t::,.
~
I
"l
E
A
2
6
<,
I:l
10·g
r
~
t::,.
A
6 6 A
I,
I
o
0,5 numberef co-lIlI..,.bed hn&ene molecu'" p~ ". u.c.
Fig. 5 shows theoretical results for the self-diffusion coefficient derived from a computer simulation of the random walk process in a two-dimensional channel network with obstacles distributed statistically over (i) the channel intersections and (ii) the channel segments. Comparing the experimental result with that of the computer simulation one has to conclude that the significant influence of the benzene molecules can only be explained by the assumption that (in the considered concentration range) the benzene molecules are preferably localized in the channel intersections. This conclusion is in agreement with nuclear magnetic relaxation studies and Raman scattering experiments /11/.
636 (AD-8-4)
~
'~
"
~a
~ ~
0.1
Q
1 0.01
o
o.s
- - . . . numbar of o'sloe/os
Ct!co' 0111"'011 ....zone u.c. mo/ecu/a/ por
'I.
Fig. 5 Theoretical results for the self-diffusion coefficient derived from a computer simulation of the random walk process in a two-dimensional channel network with obstacles statistically distributed (tJ) in the channel segments and in the channel intersections with transition probabilities of 0.004 (empty symbols) and 0.02 (full symbols) and comparison with experimental results for the self-diffuSion coefficient of methane in ZSM-5 under the influence of co-adsorbed benzene (~' cf. fig. 4)
(e»
INFORMATION ON STRUCTURAL PROPERTIES AND COKE DEPOSITIONS Combining the experimental data of nmr pulsed field gradient and of nmr tracer desorption measurements. one may derive quantitative results about the strength and the localization of transport resistances within zeolitic adsorbate-adsorbent systems. Comparing.' e.g •• methane and propane diffusivities in ZSM-5 mono- and poly,crystals /12/, for the latter specimen an enhancement by a factor of about 2 is observed. On has to conclude, therefore, that the boundaries between the individual crystallites within the polycrystalline compounds do not lead to a substantial reduction of the translational ~obility of the molecular compounds under study.
J. 'Karger et al.
637
As another example we have studied /13/ the influence of the coking conditions on the molecular transport properties of H 25M-5. In Fig. 6 values for the mean molecular life time 1intra in the interior of the zeolite crystallites as determined by nmr tracer desorption measurements, are compared with the limiting values 't'.an t ra, D··f f • which must be observed if there are no other l. transport resistance~ and which follow from the nmr pulsed field gradient measurements /8/. Applying n-hexane and mesitylene as starting chemicals for the coke production /14/, two distincly different dependences are observed: While with n-hexane ,; intra and 1r intra.Diff. coincide over a large range of coking times, with mesitylene only ~intra is found to decrease. while the intracrystalline mobility represented through ~i n t ra, D'ff remains unl. • affected. One has to conclude. therefore, that during mesitylene coking the carbonaceous compounds are exclusively deposited on the outer surface of the crystals, while for n-hexane coke is mainly deposited in the intracrystalline space, thus effecting a simultaneous retardation of intracrystalline self-diffusion and tracer desorption. This result is in agreement with the fact that only the n-hexane ~olecules may penetrate into the channel system of 25M-5. After sufficiently long coking times a divergency of 't'intra and ~intra,Diff. can be observed also for n-hexane, which proves that in a second stage even for this type of starting chemical, coke is predominantly deposited on the outer surface of the crystals. Fig. 6 Intracrystalline mean life times't'intra (0) and ~intr ,Diff. (~> for methane at 23 Be and at a sorbate concentration of 3 molecules per channel intersection in H25M-5 coked by n-hexane (full symbols) and mesitylene (open symbols), ~ respectively. in dependence on ...... the time on stream ll::
!
1.5
1.2 1.0
II)
0.8
'':;
'-
e
.'s
0.6
t
0.4
l->
0.3
L.....l._-L_.L.----l._-L_U---.J_ _--L-..J
o
2
4
6
8
- - . cOking time
16
638 (AD-8-4)
Fig. 7 Intracrystalline self-diffusion coefficients of methane in H ZSM-5 coked by n-hexane (" ) and by mesi tylene (Y ) under the influence of chemisorbed pyridine in dependence on the total amount of coke deposits and comparison with the diffusivity data for methane in the coked samples without chemisorbed pyridine (open symbols)
~
I N
Ul
E
t o
2 ----. mass
3
4
5
0/0 coke
Fig. 7 shows the results of diffusion studies /13/ on the combined effect of chemisorbed pyridine /15/ and coke deposits on the molecular transport properties of Z5M-5. The data for the uncoked material reflect the significant influence of the chemisorbed pyridine molecules (cf. /15/). In agreement with the preceding considerations, after mesitylene coking this influence remains unchanged. With n-hexane coking, however, the blocking effect of the chemisorbed pyridine is more and more reduced, corresponding to a decrease of its total amount. One has to conclude, therefore, that the coking process is initiated at these very positions, which act as sorption sites for the chemisorbed pyridine.
The authors thank W. Schirmer for his very helpful comments and discussions.
J. Karger et al.
639
REFERENCES 1. 2.
3.
4.
5.
6. 7. 8. 9.
10. 11.
12. 13. 14. 15.
Derouane. E.G •• Z. Gabelica. J. Catal. 65 (1980) 486 Doelle. H.-J •• J. Heering. L. Riekert. L7 Marosi. J. Catal. 71 (1981) 27 orson. D.H•• G.T. Kokotailo. S.L. Lawton. W.M. Meier. J. Phys. Chern. 85 (1981) 2238 Wu, P•• A. Debaoe. Y.H. Ma. Zeolites 3 (1983) 118 Bulow. M., Z. Chern. 25 (1985) 81 Ruthven. D.M •• A.M. Graham. A. Vavlitis. Proc. 5 th Internat. Conf. Zeolites. Naples 1980 Pfeifer. H•• Phys. Rep. 26 (1976) 293 Karger. J •• H. Pfeifer. W7 Heink. Proc. 6th Internat. Conf. Zeolites. Reno 1983. p. 184 Caro, J •• M. Bulo~. W. Schirmer. J. Karger. W. Heink. H. Pfeifer. S.P. Zdanov. J.C.S. Faraday I 81 (1985) 2541 Caro, J •• S. Hocevar. J. Karger. L. Rieker~ Zeolites. in press porste. C•• A. Germanus. J. Karger. H. Pfeifer. J. Caro, W. Pilz. A. Zikanov~. J.C.S. Faraday I. submitted Karger. J •• H. Pfeifer. J. Caro, M. Bulow. J.Richter-Mendau. B. Fahlke. L.V. Rees. Appl. Catal •• in press Karger. J •• H. Pfeifer. J. Caro, M. Bulow. H. Schlodder. R. Mostowicz. J. Volter. Appl. Catal •• submitted Gilson. J.P •• E.G. Derouane. J. Catal. 88 (1984) 538 Nayak. V.S •• L. Riekert. Proc. Int. symP7 Zeolite Catalysis. Siofok 1985. p. 157
In
Pentasils
Karger, H. Pfeifer, D. Freude Sektion Physik der Karl-Marx-Universitat, Leipztg, German Democratic Republic ~. Caro, M. BO!ow, G. Ohlmann Zentralinstitut fOr physikalische Chemie der Akademie der Wissenschaften der DDR, Berlin, German Democratic Republic ~.
First results of nmr measurements on molecular selfdiffusion in zeolites of pentasil type (ZSM-5) are presented.With methane and propane as probe molecules, the influence of zeolite texture, of co-adsorbed molecules and of coke deposits on the transport properties of various ZSM-5 specimens are studied. Direct information on the strength and on the localization of transport obstacles may be deduced from the diffusion data.
INTRODUCTION Zeolites of pentasil type exhibit a number of remarkable properties /1, 2/ which are closely related to the diffusivity of the adsorbed molecules. So far, pentasil diffusion data obtained by con~entional techniques scatter over several orders of magnitude (cf., e.g. /2-4/). This situation is most likely to be referred to complications with the measuring procedure itself /5/ and to the difficulties on comparing different specimens including different sample pretreatments /6/. With the application of nmr spectroscopy to diffusion studies in zeolites, a direct observation of molecular migration within the adsorbate-adsorbent system has become possible /~, 8/. In this communication we present first results of nmr self-diffusion measurements performed on zeolites of pentasil type (ZSM-5). SINGLE COMPONENT ADSORPTION Fig. 1 shows experimental values for the self-diffusion coefficients of short-chain paraffins adsorbed in ZSM-5. As a consequence of the increasing mutual and steric hindrance, respectively, the absolute values are found to decrease with both increasing loading and chain length. In agreement with the order of increasing pore diameter, the diffusivities are smaller than in NaX, but larger than in NaCaA /9/. self-diffusion measurements performed on different ZSM-5 specimens (fig. 2) do not reveal a 633
634 (AD-8-4)
temperature I'e
°
z
,., "e
...
t
-so
.--
-100
10-' 4
10--
Z
J
- - - - . nu.. ter.F ...lIeu".
per II• • .• .
Fig. 1 Self-diffusion coefficient of methane (0). ethane (A) and propane (C) in ZSM-5 at 300 K in dependence on the sorbate concentration Fig. 3 Self-diffusion coefficient of water in ZSM-5 at 296 K and at a loading of about 35 ••• 40 mg/g in dependence on the.Si0 2/A120 (symbols as J.n fl:g. z) 3-ratio
Fig. 2 Temperature dependence of the self-diffusion coefficient of methane in ZSM-5 specimens of different origin /10/ an~ different Si02/A120~ratio (A ~ 10 ; ... 540; . . 216; ... 185; • 127; D 80; • 72 ; 0 50; A 42; 40)
<>
..
-t I
N
z
____ J~!j~!'~t:~
10"
o.. -0 A
II)
e
4
CI
fO-·
Y l!.
~
z 4 2
_
---
10'
102
SiOa
A'a O" -
103
.
ratIo
significant influence of the SiO /Al 0 3 ratio. This result is in agreement with the fact that sat~ratid hydrocarbons do not interact specifically with adsorption sites on zeolites /7. 10/. in contrast e.g. to wate~ where a decreasing mobility with decreasing SiO /Al 0 -ratio is observed. In complete agreement with this ten~enc~.3the self-diffusion coefficients of water in the hydrophilic zeolites NaX and NaA are distinctly smaller than in ZSM-5 (fig. 3/10/).
J. Karger et al.
635
INFLUENCE OF CO-ADSORBED MOLECULES Fig. 4 shows experimental values for the self-diffusion coefficient of methane in ZSM-5 under the influence of co-adsorbed benzene molecules. It can be seen, that the molecular mobility of methane is drastically reduced by the existence of the benzene molecules /11/. In view of the large difference in the mObilities of the two co-adsorbed molecular species, and the similarity in the cross-sections of the benzene molecules and of the intracrystalline channels, it should be possible to simulate the influence of the benzene molecules on the methane mobility by introducing rigid obstacles into the channel network, thereby assuming that the number of obstacles is equal to the number of benzene molecules. 4
Fig. 4 self-diffusion coefficient of methane in Z5M-5 at 296 K in dependence on the amount of co-adsorbed benzene
l
6 fO"'
..
~~A 6 6 6
•
t::,.
~
I
"l
E
A
2
6
<,
I:l
10·g
r
~
t::,.
A
6 6 A
I,
I
o
0,5 numberef co-lIlI..,.bed hn&ene molecu'" p~ ". u.c.
Fig. 5 shows theoretical results for the self-diffusion coefficient derived from a computer simulation of the random walk process in a two-dimensional channel network with obstacles distributed statistically over (i) the channel intersections and (ii) the channel segments. Comparing the experimental result with that of the computer simulation one has to conclude that the significant influence of the benzene molecules can only be explained by the assumption that (in the considered concentration range) the benzene molecules are preferably localized in the channel intersections. This conclusion is in agreement with nuclear magnetic relaxation studies and Raman scattering experiments /11/.
636 (AD-8-4)
~
'~
"
~a
~ ~
0.1
Q
1 0.01
o
o.s
- - . . . numbar of o'sloe/os
Ct!co' 0111"'011 ....zone u.c. mo/ecu/a/ por
'I.
Fig. 5 Theoretical results for the self-diffusion coefficient derived from a computer simulation of the random walk process in a two-dimensional channel network with obstacles statistically distributed (tJ) in the channel segments and in the channel intersections with transition probabilities of 0.004 (empty symbols) and 0.02 (full symbols) and comparison with experimental results for the self-diffuSion coefficient of methane in ZSM-5 under the influence of co-adsorbed benzene (~' cf. fig. 4)
(e»
INFORMATION ON STRUCTURAL PROPERTIES AND COKE DEPOSITIONS Combining the experimental data of nmr pulsed field gradient and of nmr tracer desorption measurements. one may derive quantitative results about the strength and the localization of transport resistances within zeolitic adsorbate-adsorbent systems. Comparing.' e.g •• methane and propane diffusivities in ZSM-5 mono- and poly,crystals /12/, for the latter specimen an enhancement by a factor of about 2 is observed. On has to conclude, therefore, that the boundaries between the individual crystallites within the polycrystalline compounds do not lead to a substantial reduction of the translational ~obility of the molecular compounds under study.
J. 'Karger et al.
637
As another example we have studied /13/ the influence of the coking conditions on the molecular transport properties of H 25M-5. In Fig. 6 values for the mean molecular life time 1intra in the interior of the zeolite crystallites as determined by nmr tracer desorption measurements, are compared with the limiting values 't'.an t ra, D··f f • which must be observed if there are no other l. transport resistance~ and which follow from the nmr pulsed field gradient measurements /8/. Applying n-hexane and mesitylene as starting chemicals for the coke production /14/, two distincly different dependences are observed: While with n-hexane ,; intra and 1r intra.Diff. coincide over a large range of coking times, with mesitylene only ~intra is found to decrease. while the intracrystalline mobility represented through ~i n t ra, D'ff remains unl. • affected. One has to conclude. therefore, that during mesitylene coking the carbonaceous compounds are exclusively deposited on the outer surface of the crystals, while for n-hexane coke is mainly deposited in the intracrystalline space, thus effecting a simultaneous retardation of intracrystalline self-diffusion and tracer desorption. This result is in agreement with the fact that only the n-hexane ~olecules may penetrate into the channel system of 25M-5. After sufficiently long coking times a divergency of 't'intra and ~intra,Diff. can be observed also for n-hexane, which proves that in a second stage even for this type of starting chemical, coke is predominantly deposited on the outer surface of the crystals. Fig. 6 Intracrystalline mean life times't'intra (0) and ~intr ,Diff. (~> for methane at 23 Be and at a sorbate concentration of 3 molecules per channel intersection in H25M-5 coked by n-hexane (full symbols) and mesitylene (open symbols), ~ respectively. in dependence on ...... the time on stream ll::
!
1.5
1.2 1.0
II)
0.8
'':;
'-
e
.'s
0.6
t
0.4
l->
0.3
L.....l._-L_.L.----l._-L_U---.J_ _--L-..J
o
2
4
6
8
- - . cOking time
16
638 (AD-8-4)
Fig. 7 Intracrystalline self-diffusion coefficients of methane in H ZSM-5 coked by n-hexane (" ) and by mesi tylene (Y ) under the influence of chemisorbed pyridine in dependence on the total amount of coke deposits and comparison with the diffusivity data for methane in the coked samples without chemisorbed pyridine (open symbols)
~
I N
Ul
E
t o
2 ----. mass
3
4
5
0/0 coke
Fig. 7 shows the results of diffusion studies /13/ on the combined effect of chemisorbed pyridine /15/ and coke deposits on the molecular transport properties of Z5M-5. The data for the uncoked material reflect the significant influence of the chemisorbed pyridine molecules (cf. /15/). In agreement with the preceding considerations, after mesitylene coking this influence remains unchanged. With n-hexane coking, however, the blocking effect of the chemisorbed pyridine is more and more reduced, corresponding to a decrease of its total amount. One has to conclude, therefore, that the coking process is initiated at these very positions, which act as sorption sites for the chemisorbed pyridine.
The authors thank W. Schirmer for his very helpful comments and discussions.
J. Karger et al.
639
REFERENCES 1. 2.
3.
4.
5.
6. 7. 8. 9.
10. 11.
12. 13. 14. 15.
Derouane. E.G •• Z. Gabelica. J. Catal. 65 (1980) 486 Doelle. H.-J •• J. Heering. L. Riekert. L7 Marosi. J. Catal. 71 (1981) 27 orson. D.H•• G.T. Kokotailo. S.L. Lawton. W.M. Meier. J. Phys. Chern. 85 (1981) 2238 Wu, P•• A. Debaoe. Y.H. Ma. Zeolites 3 (1983) 118 Bulow. M., Z. Chern. 25 (1985) 81 Ruthven. D.M •• A.M. Graham. A. Vavlitis. Proc. 5 th Internat. Conf. Zeolites. Naples 1980 Pfeifer. H•• Phys. Rep. 26 (1976) 293 Karger. J •• H. Pfeifer. W7 Heink. Proc. 6th Internat. Conf. Zeolites. Reno 1983. p. 184 Caro, J •• M. Bulo~. W. Schirmer. J. Karger. W. Heink. H. Pfeifer. S.P. Zdanov. J.C.S. Faraday I 81 (1985) 2541 Caro, J •• S. Hocevar. J. Karger. L. Rieker~ Zeolites. in press porste. C•• A. Germanus. J. Karger. H. Pfeifer. J. Caro, W. Pilz. A. Zikanov~. J.C.S. Faraday I. submitted Karger. J •• H. Pfeifer. J. Caro, M. Bulow. J.Richter-Mendau. B. Fahlke. L.V. Rees. Appl. Catal •• in press Karger. J •• H. Pfeifer. J. Caro, M. Bulow. H. Schlodder. R. Mostowicz. J. Volter. Appl. Catal •• submitted Gilson. J.P •• E.G. Derouane. J. Catal. 88 (1984) 538 Nayak. V.S •• L. Riekert. Proc. Int. symP7 Zeolite Catalysis. Siofok 1985. p. 157