Sorption Kinetic Investigation of NaCaA-type Zeolite Ageing

J. Rouqucrol, F. Rodriguez-Reinoso, K.S.W. Sing and K.K. Unger (US.) Characterization of Porous Solids [N Studies in Surface Sciencc and Cahlysis, Vol. 87 0 1994 Elscvicr Scicncc B.V. All rights rcscrvcd.

55 1

Sorption Kinetic Investigation of NaCaA-ty pe Zeolite Ageing M. Biilow a and P. Struve a The BOC Group

Technical Center, 100 Mountain Ave., Murray Hill, N.J. 07974, U.S.A.

Center of Heterogeneous Catalysis, Rudower Chaussee, D 12489 Berlin, Germany Abstract The sorption kinetic method enables one to study phenomenologically the process of zeolite ageing from the point of view of rate-limiting processes. This is exemplified for gaseous phase sorption uptake of the probe molecule n-decane by NaCaA-type molecular sieves The main feature of ageing which becomes visible is the generation of surface barriers exhibiting a complex nature with synergistic features: (1) Structural surface barriers due to thermal/hydrothermal damaging and, thus, amorphization of zeolite crystals accompanied by a retardation of molecular uptake and reduction of sorption capacity prior to complete breakdown of the zeolite structure; ( 2 ) Presorption and accumulation of organic trace compounds - out of the feed stream - in the solid-fluid interface. These compounds may be sensitive to catalytic transformations and promote, therefore, the generation of immobile and, finally, coke-type species; (3) Chemical reaction of feed components in the interface region of zeolite crystals and then in their intracrystalline void volume.

To differentiate between various influences on the sorption uptake behaviour of n-decane, its effective transport constants in a temperature range from x 520 to = 670 K on various NaCaAtype zeolites (SAMS) of different origin - synthesized under large- and mini-scale conditions were obtained. To relate these data to the intracrystalline diffisivity, the size of zeolite crystals was varied. Sorption rate behaviour of n-decane with respect to the nature of different trace compounds presorbed and to the concentration of sorbing species are compared. The information allows one to propose a tentative model for zeolite ageing under industrial conditions. 1. INTRODUCTION

One of the important challenges of the application of sorbents is how to delay their ageing. Recognition of chemical and physico-chemical mechanisms of sorbent ageing is a way toward higher sorbent efficiency. Much effort is being dedicated to improve the stability and to maintain the starting activity of such solids in large-scale processes as long as possible. In general, such investigation asks for the complex use of both instrumental analysis and sorption methods.

552 NaCaA-type zeolites have been used for many years as sorbents for stereo-selective separation of n-paraffins from their mixtures with iso-paraffins, e . g by means of the PAREX Process for n-/iso- parafin separation [ 11. In this high-temperature process, the zeolite maybe exposed to extreme thermal conditions and, simultaneously, to the influence of both water vapour and traces of poisoning by-products of different chemical nature. Although the feed product to be separated is being prepurified before contacting the zeolite. traces may be accumulated on or in the sorbent particles where they may undergo chemical transformation. To contribute to understanding sorbent ageing which takes place in sorption plants, the following phenomena were investigated: (1 ) Concentration and temperature dependences of effective diffusivities for n-decane representing n-parafins with chain length Clo .., in various SAMS samples comniercially available or synthesized on the lab scale - both hydrothermally treated and untreated; (2) Influence of presorption of trace compounds, such as pyridine, a-methylnaphthalene, tetralene, decalene, mixture of primary n-alkylamines (n-Clo . .. n-C 18, mainly n-C 12). n-heptyland n-decylmercaptane, i.e. characteristic impurities of various industrial feed streams, on the rate of sorption uptake of n-decane to follow; (3) Catalytic reactivity of SAMS-binder composites (containing two- and three-layered clay mineral structures) in connection with presorption of poisoning species and modification of zeolite interface region during exposure to extreme thermalhydrothermal conditions and their role in the process of SAMS ageing. For a consecutive approach to the problem, the region of intracrystalline diffusion will be identified, the extend to which thermalhydrothermal treatment generate surface barriers will be shown and the enhancement of such effects by presorption of by-products will be considered.

2. EXPERIMENTAL AND DATA EVALIJATION

Sorption kinetics and equilibrium measurements were performed for n-decane and - in a few experiments - for n-tetradecane on SAMS crystal monolayers (10 .. 20 mg of particles spread over an area of x 15 cm2) by the piezometric system with a response time in the region of up to 0.05 s in a differential concentration mode [2]. Since the response time of the Baratron capacitance pressure sensor was 2 25 ms, pressure I’S time curves expressing sorption rate behaviour could be measured unambiguously with time constants in the range of seconds. Data evaluation was based on the statistical moment model for diffusion-controlled uptake for linear sorption isotherm with negligible sorption heat effects [3]. A valve-effect correction included into that model [4]was utilized. The linearity of the sorption isotherm during uptake could be guaranteed by measuring kinetics over small pressure steps (0.1 ... 1 Pa). Isothermicity could be proven to exist by realizing that, in the case of uptake by untreated crystals, ( i e . in the fastest sorption uptake where, if at all, non-isothermicity is most likely to occur), the diffisivities determined agreed satisfactorily with data obtained previously for larger SAMS crystals, where heat effects were definitely excluded [5,6]. Favourable external thermal conditions [7,8] were also maintained during experiments. Sorption uptake experiments in the presence of trace compounds (cf Table 1) were carried out after presorption of the latter ones to an exactly measurable extent ensuring complete coverage of the external surface area of SAMS crystals by at least one molecular layer. The influence of trace compounds which are either able to enter the micropores or excluded due to

553 steric constraints, on kinetic behaviour of n-decane/SAMS systems was investigated. Since, in general, presorption represents a phenomenon of multi-component sorption, strict data evaluation should consider both sorption isotherms and mobilities for the mixture case [ 9 ] . However, for experimental reasons, the total pressure recorded was hlly ascribed to n-decane. Conclusions on the mobility of that specie were drawn presuming both constant and known values of equilibrium pressure and sorbed amount of trace compounds during corresponding uptake runs. Calculating diffusivities, as well as analyzing these data by means of the Darken equation, which is somewhat arbitrary, therefore, serve only as a comparative estimation of various influences on sorption properties rather than as a quantitative characterization. 3. RESULTS AND DISCUSSION Table 1 includes lists of SAMS samples and of trace compounds considered as well as the ranges of both experimental parameters maintained and diffusivity data calculated. Sorption equilibria data for both ndecane and n-tetradecane on lab-synthesized S A M S samples cx ma+ vs Ca2+ 93%, H,O capacity 0 32 g/g zeolite) and a'(Na+ 17s Ca2+ 95%, HzO capacity 0 32 g/g zeolite), respectively, are given as both isothenns in Figure 1a and 1 b and isosters in Figure 2 (for n-Clo) The isosteric sorption heats of nClo and "-CIA amount to 110 120 kJ/mol and 130 140 kJ/mol, respectiveley Figure 1a. Somtion isotherms of n-decane on NaCaA (a) The sorption uptake behaviour of n-Cro and "-CIA for samples a and a' is characterized by the data shown in Figures 3 ... 5. These data 10.0 reflect intracrystalline diffusion of those systems, cf [5,6].The 80. 0 concentration dependences can 623 K c 6.0 . well be described by a modified version of the Eyring theory [ l O , l l ] . However, the temperature dependence of the Do values for both sorbates v . 1 (for n-Clo, cf Figure 4 and 1.0 ID 3.0 40 5.0 6.0 7.0 8.0 9.0 1QO

: >

-

P 10L/Torr

Figure Ib. Sorption isotherms of n-tetradecane on NaCaA (a')

554

f t

I

0 P

h 15 re 2P

1 0 ' ~IT-' Figure 2. Sorption isosters of n-decane on NaCaA (a).

L

.

.

o

-

.

.

z

.

o

n / md/g

.

4

-

0

.

J

b

~

Figure 3. Concentration dependence of di&sivity Do for n-decane on NaCaA (a) (Do: values D treated by the Darken eq.).

lo

c

0

A E

\

I3

10

A

A

-12 ,

*

0.02

Figure 4. Arrhenius plot of intracrystalline diffusivity Do for n-decane on NaCaA (a).

A 0

I

-131

10

A

0

,

ON

.

.

,

0,s ODB

0.l

,

j

.o.a aIf+

n/ mmol/g

Figure 5. Concentration dependence of diffusivity Dofor n-tetradecane on NaCaA (a').

555

Figure 3 in [ 12]), shows a feature which indicates a transition from intracrystalline diffusion to surface barrier limited transport 1131 at T > 600 K (cf[5]). For the intracrystalline region, the energies of activation, E,, amount to approximately 115 kJ/mol and 160 kJ/mol, respectively. Compared to these diffusion data, the uptake of n-ClO by industrially prepared crystals, cf samples p and 6, is at least one order of magnitude slower, though there is only small deviation in sorption equilibrium characteristics. For these samples, the difference may be attributed to both surface barriers and retardation of transport rate by heterogeneities within the intracrystalline bulk of the latter zeolites as shown for other n-parafid5AMS systems, such as n-hexane and n-decane on NaMgA-type zeolite, by quantitative analysis of primary uptake rate data [ 131 utilizing a model for complex rate mechanisms [ 14,151. To model, phenomenologically, the influence of both thermal and hydrothermal treatments on the uptake behaviour under industrial conditions, large-scale prepared zeolites were treated as indicated in Table 1. The thermaVhydrotherma1 stabilities as deduced from uptake features correspond with well-known properties of A-type zeolite, cf [ 16,171. The presence of water in the actual process is crucial for a decrease of both uptake rate and sorption capacity. As shown elsewhere by S A X S [18], HREM [19], X ray photoelectron spectroscopy [20] and by traditional chemical investigations [211, zeolite lattice distortions and phase transitions (including loss of cristdlinity) occur. These processes proceed simultaneously with a retardation of uptake rate (cf Table 1, e.g. samples y and y'), with a decrease of both sorption capacity and sorption heat (decrease from = 1 10 ... 120 kJ/mol to 2 90 ... 30 kJ/mol). The latter peculiarities will be in detail discussed ebewhere [22]. NMR tracer desorption studies [23] gave additional clear evidence that this can be assigned to formation of surface barriers. Such state of zeolite crystals should be considered as transition toward the complete collapse of the microporous system. At the end, the uptake rate strongly increases and approaches the rate in macropores, however, the capacity becomes negligible. This sequence of events can be derived from Debye-Scherrer difiactograms as shown for different duration of hydrothermal treatment of sample p and, in a more quantitative manner, from Guinier X ray difiaction patterns [21]. These patterns also allow one to identifjl new intermediate crystalline phases, e.g CaA12Si208 or analogous ones if other bivalent cations were present in the original samples [2 1,241. Table 1 Diffusivity ranges, D,,l ... DOC( i e . between lowest and highest concentration), for systems ndecand5AMS differently pretreated and with trace compounds presorbed (in brackets of column 3: presorbed amounts; sample origin: a and a'... synthesized by the authors, p and y ... CK Bitterfeld, Germany, 6 and E ... Laporte, England, q and ( ... UOP, U.S.A.) Sorption system Sorbatehorbent n-Cln/NaCaA (a) 2R x 3 2 p m Ca2+ 4 93 %

Temperature

T/K

Concentration Diffusivity n(oo)h/mmol/g Dn.p..Dn.h/1012Cm2/S

n(m)l

473 523 573 623 646 673

...

0.427 ... 0.841 0.300 ... 0.657 0.186 ... 0.451 0.156 ... 0.365 0.049 _ . .0.314 0.029 ... 0.230

4.0 200

... 8000 ... 3000 ... 800 ._.1400

I

556

Sorption system Sorbatehorbent n-Ct,,/NaCaA (a') .. 2R = 12 pm Ca2+ z >90 % n-Cltt/NaCaA(y) 2R = 2 p m Ca2+--61 YO n-C,o/NaCaA (y') y treated: 873 K, 24 hours,

100 kPa H20. shallow bed n-Cln/NaCaA (y")

treated as y' except time: 80 h n-Ct..dNaCaA (y"') y treated: 873 K, 24 h, no

extra H20, shallow bed n-Ctn/NaCaA (6)

2K =2.2pm n-Cln/NaCaA (E)

pellets were ground sieved (= 2 pm) after utilization in PAREX plant Schwedt n-C1@aCaA (E') treated: 650K, 7 h, 80 kPa H20 in H2 stream, deep bed n-CldNaCaA (<) pellets ground and sieved (= 2 pm) after utilization in PAREX plant Schwedt n-Cln/NaCaA (E") after utilization in PAREX plant Schwedt (regime A2D) n-C,,,/NaCaA (E"') .. after utilization in PAREX E

Temperature

Concentration

523 573 623

0.036 ... 0.213 0.069 ... 0.278 0.035 .,. 0.105

3.0 . _ . 1.0 2.0 ... 2.0 30.0 ... 20.0

523 573 623

0.026 _ . _0.290 0.019 .._0.251 0,008 ... 0.106

1.9 10.9 _ _ _ 16.3 _ _ _ 20.1 30.9 ... 17.5

523 573 623

0.024 ... 0.306 0.027 .., 0.235 0.012 ... 0.108

0.04 .., 0.12 0.33 ... 0.31 1.21 ... 13.2

523 573 623

0.0001... 0.001 out of sensitivity region 0.033 ... 0.367 0.024 ... 0.266 0.01 1 ... 0.1 15

2.4 . _ . 0.54 out of sensitivity region 15 ... 3.0 18 _ . _ 7.0 72 ... 40

573 623

0.017 ... 0.299 0.006 _..0.144

523

0.018 ... 0.372 0.015 ... 0.236 0.009 ... 0.103

4.2 ... 1.6 7.4 ... 5.5 29 ... 13

523 573 623

0.021 ._.0.351 0.016 ... 0.21 1 0.008 ... 0.090

6.0 _ _ . 1.8 12 ... 5.2 36 ... 9.5

523 573 623

0.014 ... 0.316 0.010 _..0.256 0.008 ... 0.104

22 16 22

573 623

0.011 ... 0.134 0.007 ... 0.066

0.44 ... 0.36 1.3 . _ . 0.95

573 623

0.014 ,., 0.156 0.005 ._.0.059

0.59 ... 0.98 ...

573 > 573

DifTusivity

90

360

... 18 __.110

... ,.. .,

0.85 3.0

6.0

0.36 0.6

557

ISorption system

ITemperature

Ioriginal spheres were ground I Iand sieved (= 2 itm)

n-C1,,/NaCaA (a)+ a-methylnaphthaline

(brackets: presorbed amounts) n-Clo/NaCaA (a) + n-heptylmercaptane n-ClflaCaA (a) + n-decylmercaptane n-Cll,/NaCaA (a)+ pyridine n-Cln/NaCaA (a)+ decalene n-Cln/NaCaA (a)+ tctriiene n-Cll)/NaCaA(p) + n-alkyl-(Cl,) C18)-amines mixture n-Cln/NaCaA (p') + n-alkyl-(ClO- ClR)-arnines

-

treated: 873 K, 24 hours, 100 kPa HzO, shallow bed n-Cl@aCaA (a) + n-alkyl-(Cll, - C1R)-amines n-Clo/NaCaA (6) + n-alkyl-(Clo - Cls)-amines n-Cl,,/NaCaA (q)+ n-alkyl-(Cln - C+rrnines

623

I

Concentration

1

Diffusivity

I

0.004 ... 0.099

I

79

.,.

14

523 573 623

0.051 ... 0.445 (0.009) 0.037 ... 0.281 (0.031) 0.022 ... 0.144 (0.029)

414 54 312

... 19

523 573 623 523 573 623 523 573 623 523 573

0.067 ... 0.433 0.029 .._0.317 0.014 ... 0.179 0.057 . _ .0.316 0.044 _ . _0.268 0.014 ... 0. I18 0.093 ... 0.231 0.029 ... 0.112 0.008 ... 0.046 0.047 ... 0.409 0 . 0 2 8 .. 0.317

(0.003) (0.007) (0.005) (0.053) (0.029) (0.029) (0.068) (0.068) (0.056) (0.014) (0.007)

32 32 37 4.8 2.0 16 62 51 140 281 399

... 41 ... 108 ... 581 ... 17 ... 39 ,.. 48 .,. 63 ... 211 .,. 51 ... 10 ,. 18

523 573 623 523 573 623 573 623

10.059 ... 0.398 0.045 ... 0.275 0.028 ... 0.168 0.035 ... 0.284 0.019 . _ .0.209 0.014 ,.. 0.078 0.084 ._.0.214 0.023 ... 0.083

(0.021) 1 (0.056) (0.024) (0.025) (0.019) (0.017) (0.022) (0.014)

523 573 623 573 623 523 573 623

0.036 .._0.281 0.028 .._0.207 0.014 ... 0.076 0.029 ... 0.237 0.011 . . _0.102 0.027 ... 0.368 0.024 ... 0.214 0.013 ... 0.109

(0.049) (0.072) (0.041) (0.029) (0.020) (0.019) (0.017) (0.017)

... 52

... 80

12 ... 79 ... 78 ... 0.74 ... 1.0 ... 5.4 ... 0.65 . _ . 1.5 .,.

8.6 10 25 0.65 0.89 2.0 0.18 0.60

0.78 . . _ 1.2 1.04 ... 1.8 5.1 ... 3.9 5.7 ... 5.1 7.3 ... 15 2.0 ... 1.2 3.2 ... 1.8 4.0 ... 8.0

558

Diffusivity Temperature Concentration T/K n(oo)l n(w)h/mmol/g D,,Ap.D,,.t,/1012cm2/s

Sorption system Sorbate/sorbent n-ClO/NaCaA (E") + n-alkyl-(Cl(t Cl&amines after utilization in PAREX plant Schwedt (regime A2D) n-Cln/NaCaA (E"') + n-alkyyl-(Ct" - Cl+amines

-

after utilization in PAREX plant Schwedt (regime 2AD) n-C1@aCaA ( ~ 4 ' )+ n-alkyl-(Cln - Cl&amines after 10 months utilization in PAREX plant Sysran (Russia) n-Cl(I/NaCaA(q1)+ n-alkyl-(Clo C,R)-amines

-

after 36 months utilization in PAREX plant Kirishi (Russia) n-Cln/NaCaA (6')+ n-alkyl-(Clo Cl&amines

-

...

I

I

0.017... 0.125 (0.013) 0.008 ... 0.053 (0.012)

0.46 ... 0.37 0.66 ... 1.4

573 623

0.006_..0.038 (0.014)

0.28 ... 0.80

573 623

0.019... 0.110 (0.015) 0.006. _ .0.036 (0.014)

0.58 ... 0.54 0.82 ... 2.6

573 623

0.024... 0.161 (0.015) 0.013 ... 0.069 (0.015)

0.83 ... 1.1 1.3 ... 2.7

573 623

0.014... 0.201 (0.012) 0.008... 0.077 (0.011 )

6.7 ... 2.4

573 623

original spheres were ground and sieved (i.2 pm) The molecular uptake of n-decane by 5 A M S zeolite after presorption of trace compounds on the differently pretreated samples shows the following main features: (1) The presence of any trace compound in the system retards the uptake rate, cf Figure 6. (2) The sorption capacity of n-Clo during subsequent uptake steps decreases differently depending on whether or not the presorbed trace compound is able to enter the micropores. (3) The decrease of the uptake rate (blockage of transport paths) depends on the chemical nature of the presorbing species and, thus, on both the strength of their interaction with and their localization in the zeolitic micropores. (4)The rate retarding effect decreases with increasing concentration of n-C1, in the system, cf Figure 7. This is, probably, due to molecular replacement, although "pure" single gas sorption conditions for n-Ci0 could never be achieved in this investigation. (5) For chemically stable presorbing species, the most strong retardation effect can be attributed to pyridine. The presorptive uptake of this trace by 5 A M S has been proven to be an activated process during first steps. This result gives evidence of strong heterogeneity of the interfaces of crystals considered. ( 6 ) The mixture of n-alkyl-(Clo - Clg)-amines revealed the strongest influence on n-Clo uptake rate, altogether, qf Figure 8.This result is connected with decomposition of amines in contact with the 5 A M S . The effect is enhanced in presence with non-zeolitic binding material especially at temperature 2 600 K.

559

T IK

I 623

A A A

573

523

nP'

rnmol/g

A A

0,005 0.007 0,003

623K

A & A

0

c solid symbols : n-alkylamincs presorbcd

n/ mrnoI/g

Figure 6. Comparision of intercrystalline dffisivity for n-decane on NaCaA (a') (empty symbols) with apparent diffisivitiesfor this system after presorption of small amounts npr of n-heptylmercaptene (solid symbols).

lo"

16'

d

n/md/9

Figure 7. Comparision of apparent diflisivity data for n-decane on NaCaA (6) (empty symbols) with apparent diffisivities for this system after presorption of alkylamines (solid symbols).

(7) The amines do not only enter the micropores of zeolite crystals, but their sorption is also remarkably enhanced in the outermost regions of the latter, especially for SAMS-p samples. (8) In the w e of industrial SAMS samples for which thermal pretreatment at w 870 K leads to significant effects on n-Clo sorption capacity and kinetics only at comparativelylow temperature (i. 570 K), the presorption of amines retards that process by almost one order of magnitude over the full temperature range considered. If the sieve has already been demaged hydrothermally, subsequent treatment by amines has an extremely strong additional retardation effect reaching about two orders of magnitude in terms of rate constants, cf Figure 9. (9) The comparison of the behaviour of both crystals and ground pellets of samples p and 6 in the presence of amines suggests the important role of the crystal stability which has to be attributed to the chemical nature and behaviour of the binder. Best behaviour could be reached with binder transformed into zeolitic material during pelletization process. (10) Small-sized crystalline SAMS samples of different origin show quite a similar uptake rate behaviour with respect to any type of treatment, and also without special pretreatment. Although their sorption equilibrium properties remain still comparable after pelletization, uptake properties 'nay differ remarkably if binding materials are present. Thus, sorption kinetics allow one to evaluate the quality of binders, of the binding technology and they are also prospective for ageing.

With respect to SAMS deactivation in PAREX plants, the sorption kinetic measurements reveal the nature of the pellet binder (together with the process of manufacturing as well as primary activation of pellets) and the presence of both water and alkylamines - probably of any structure under process conditions to be the most crucial factors. Sorption experiments for primary n-alkylamine mixtures on SAh4S crystals, pellets, samples with binder material admixed (after pellet grinding) and on binding materials alone - performed in the

560

temperature region 500 ._.675 K at pressures 2 0 1 Pa - gave evidence of the decomposition of nalkylamines. This phenomenon is most significant at temperature 2 600 K. The presence of hydrogen, traces of ammonia and olefins could be shown in the gaseous phase (increase of pressure) together with formation of oligomeric coke precursors on the solid. Acid centres in the clay binders of several S A M S samples used lead to synergistic effects of attacking the aluminosilicate lattice, decomposition of amines and generation of coke-precursors accompanied by hydrothermal destruction of zeolite. In this case, accelerated pore blocking, which starts at the outer crystal shell, leads to a deactivation of the sorbent which could, in general terms, be imagined to proceed analogously to the model proposed in [25] on the basis of self-diffusion data for molecular sieve coking. If the binding material is inert or has been transformed into a zeolitic phase, deactivation may take place as a mere and stochastic process over the whole pellet phase

A

all sphcror: 573 K ati +riangke:613 K 1 8

n/ mmot/g

Figure 8. Comparision of apparent diffusivities n-decane on NaCaA-type crystals of samples (0) (with crosses) and (6) (without crosses): empty symbols: n-C,,, ; solid symbols: sorption of n-C,,, after presorption of alkylamines.

,

, 0 - 1 3 L , 10-1 ._

n/

,

, ,,,,,I

,

, I

10’’ ._

rnrnoi/g

Figure 9. Comparision of apparent diffusivities of n-C,, on NaCaA (p) at 623 K: A,A ... standard activation (s.a.): 673 K, Torr, 12 h V,V ... 4 h shallow bed, 873 K, then s.a.; 0,. ... 4 h deep bed, 873 K, then s.a O,+ _ . _ 4h shallow bed, 873 K, then 8h 24Torr H,O at 463 K, then s.a. 0,. ... pretreatment as for the preceding sample after 4 h deep bed; empty symbols: pure n-C,, ; solid symbols: n-C,, after presorption of n-alkylamines.

4. CONCLUSIONS As exemplefied for sorption kinetics of the probe molecule n-decane on various NaCaA-type molecular sieves, sorption uptake measurements enable one to study phenomenologically the

56 1

process of zeolite ageing from the point of view of rate-limiting processes. The methodology of such investigation is the consideration of properties of differently pretreated zeolite crystals and pellets. Attention has to be paid to the chemical nature of pellet constituents and of by-products in the product stream. For the PAREX process of n-/iso-para€h separation by NaCaA zeolites, the influence of water and alkylamines on the sorbent stability was found to be most crucial. Pieces of evidence were collected suggesting that the ageing mechanism proceeds viu generation and strengthening of surface barriers with a complex nature and deactivation of the crystals fiom their interfaces toward inner cores. REFERENCES I. 2. 3. 4

5 6. 7. 8. 9.

10. 1I.

12. 13. 14. 1s. 16.

17. 18. 19. 20. 21. 22. 23.

24. 25.

W. Schirmer, K. Fiedler, A. Stach and M. Suckow, Stud. Surf. Sci. Catal., 46 (1989) 439. M. Bulow, P. Struve and W. Mietk, Z. physik. Chern., Leipzig, 267 (1986) 613. M. Bulow, W. Mietk, P. Struve and P. Lorenz, J. Chem. SOC.Faraday Trans. I, 79 (1983) 2457. P. Struve, M. Kocirik, M. Bulow, A. Zikanova and A.G.Bezus, Z. physik. Chem., Leipzig, 264 (1983) 49. M. Bulow, P. Struve, C. Redszus and W. Schirmer, in L.V.C. Rees (ed.): Proc. 5th Intern. Conf Zeolites, Naples, 1980, Heyden, London, 1980, p. 580. M. Bulow, P. Struve and L.V.C. Rees, Zeolites, 5 ( 1 985) 1 13. M. Bulow, P. Struve and W. Mietk, J. Chem. SOC.Faraday Trans. I, 79 (1984) 813. M. Bulow, J. Caro, B. RZihl-Kuhn and R. Zibrowius, Stud. Surf. Sci. Catal., 46 (1989) 505. M. Bulow, Proc. SEIKEN Symposium (Japan), 9 (I 99 1 ) 207. K.J. Laidler, Chemical Kinetics, McGraw-Hill, New York, 1965. J. Kiirger and D.M. Ruthven, Diffusion in Zeolites and Other Microporous Solids, J. Wiley, New York, 1992. M. Kocirik, A. Zikanova, P. Struve and M. Bulow, Stud. Surt: Sci. Catal., 49 (1989) 925. M. Bulow, P. Struve, G. Finger, C. Redszus, K. Ehrhardt, W. Schirmer and J. Karger, J. Chem. SOC.Faraday Trans. I, 76 (1980) 597. R.M. Marutovsky and M. Bulow, Colloid J. (Russ.), 46 (l984), 43. M. Kocirik, P. Struve, K. Fiedler and M. Bulow, J. Chem. SOC.Faraday Trans. I, 84 (1988) 3001. W. Lutz, B. Fahlke, R. Seidel, M. Bulow and W. Wieker, Chem. Techn., Leipzig, 40 ( I 988) 121. M. Suckow, W. Lutz, J. Kornatowski, M. Ronvadowski and M. Wark, Gas Sep. Purif, 6 (1992) 101. M. Bulow, P. Struve and S. Pikus, Zeolites, 2 (1982) 267. J. Karger ,M. Bulow, G.R. Millward and J.M. Thomas, Zeolites. 6 (1986) 146. J. Kiirger ,H. Heifer, R. Seidel, B. Staudte and T. Gross, Zeolites, 7 (1987) 282. H. Fichtner-Schmittler, W. Lutz, S. Amin, A. Dyer and M. Wark, Zeolites, 12 ( I 992) 750. M. Bulow and P. Struve, Langmuir, to be submitted. J. Karger , H. Pfeifer, F. Stallmach, M. Bulow, P. Struve, R. Entner, H. Spindler and R.Seidel, AIChE J., 36 (1990) 1500. H. Fichtner-Schmittler, M. Bulow and W. Lutz, unpublished data. M. Bulow, J. Caro, J. Volter and J. Ktirger , Stud. Surf. Sci. Catal., 34 (1 987) 343.