Kinetic regimes of zeolite deactivation and reanimation

ELSEVIER Applied Catalysis A: General I32 ( 1995) 2940

Kinetic regimes of zeolite deactivation and reanimation

Hans Schulz *, Kerstin Lau, Michael Claeys EtlRler-Bunte-Institut, Universiy of Karlsruhe, Kuiser.straPe 12, 76128 Karlsruhe, Germany

Received 28 December 1994; revised 18 May 1995; accepted 18 May 1995

Abstract

Hydrocarbon formation from methanol on zeolite H-ZSM-5 has been investigated as a function of time at different temperatures. Thus results of time resolved yield of deposit on the catalyst as well as of yield and detailed selectivity of volatile products have been obtained. The experimental data show the eminent role of amount and composition of the compounds encaged in the zeolite pores. Kinetic regimes of preinitiation, incubation, acceleration and retardation are discriminated and characterized. Temperature-programmed release of the organic material, encaged in the zeolite, leads to the characterization of the regime of zeolite reanimation. Deactivation through coking at elevated temperature is also explained briefly.

Keywords: Kinetics; Zeolite; Deactivation; Reanimation

1. Introduction

Selectivity of hydrocarbons conversion - which may also be formed in-situ from methanol - is specifically controlled by geometric, respectively spacious demands of the zeolite. These constraints are attributed to limiting channel diameters and cage dimensions. However, the earlier models appear now too simple. The zeolite ‘pore architecture’ regarding as well the interlinkage of the channels and cages has also to be taken into account. A serious limitation of most investigations is that only the stationary state of conversion has been studied and a further limitation concerns the unspecified kind and degree of pore filling as depending on temperature and time and its controlling involvement in the kinetic schemes.

In our preceding work [ l] we have developed methods for measuring the time dependence of conversion and selectivity (sampling duration ca. 0.01 s; possible

* Corresponding author.

0926-860X/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDIO926.860X(95)00128-X

30 H. Schul; et 01. /Applied Catalwis A: Grrteral132 (1995) 29%40

sampling sequence every second) and as well a method for determining the yield of compounds, entrapped in the zeolite, on a time resolved basis. These encaged compounds have been found to participate decisively in the conversions. This is shown below on the basis of experimental results. The kinetic regimes of incubation, acceleration, retardation, reanimation and coking have been observed/discriminated during methanol conversion on the zeolite H-ZSM-5. In particular the regime of reanimation will be regarded as related to encaged compounds and as well to the carbocationic elemental reactions on the acidic sites.

2. Experimental

The catalyst bed consisted of fused silica spheres, coated with a thin layer ( N 10 pm) of the H-ZSM-5 (0.5 g zeolite, Si/Al-ratio = 15, average crystal diameter N 2 pm) [ 21. The H-ZSM-5 /silica mass ratio was 0.1. The catalyst bed was held in a fused silica tube which was placed in a steel reactor, allowing for elevated reaction pressure. Methanol conversion was performed at 5 bar (inlet partial pressures of methanol and nitrogen 2.5 bar each) at WHSV 1 h ~ ‘. A well measured reference gas stream of 0.5 vol.-% of neopentane in nitrogen was added to the product stream for direct determination of hydrocarbon yields from the chromatograms. Amounts of retained compounds on the catalyst were calculated from methanol conversion and yields of volatile product. Instantaneous product sampling was performed with the ampoule technique [ 1,2]. Organic compounds were determined via GC/FID.

Thermal catalyst reanimation was investigated at a heating rate of 2”Umin applying an argon flow of 80 ml/min (25°C 1 bar) and adding the neopentane reference stream to the exit gas. Ampoule samples were taken for determination of yield and composition of the hydrocarbons released from the catalyst. After this thermal treating, temperature-programmed oxidation ( 100 ml/min of air, 2”C/ min) was performed. In the effluent gas the concentrations of 02, CO, CO2 and H,O were continuously measured and recorded.

3. Results and discussion

3.1. Time resolved examination of methanol conversion on zeolite H-ZSM-.5

Yields of volatile compounds and deposit Results of experiments performed at 250,270 and 290°C are reported. Ca. 250°C

is the minimum temperature to react methanol to hydrocarbons on zeolite catalysts. Fig. 1 shows the yield of volatile hydrocarbons and also the yield of hydrocarbons, which are retained by the catalyst as a function of time.

Generally it is observed that the fraction of retained hydrocarbons is amazingly high, amounting to about 40% of all hydrocarbons. At higher temperature (350-

H. Schulz et al. /Applied Catalysis A: General 132 (1995) 2940 31

” -

50 100 150 200 250 300 350 50 100 150 200 250 300 350 Duration of experiment, min Duration of experiment, min

Fig. 1. Yield of volatile hydrocarbons (left) and of hydrocarbon deposit (right) as a function of time at the reaction temperatures 250, 270 and 290°C. Methanol conversion on zeolite H-ZSM-5.

400°C) the selectivity of organic deposit on the catalyst becomes very low (about 0.2%) [ 31. Obviously, these retained hydrocarbons play a dominant role in the mechanism of methanol conversion. When removing the fused silica reactor tube from the reactor, after catalyst deactivation, the colour of the catalyst had changed from white to yellow as indicative for the unsaturated nature of the retained organic material. Consistently IR-spectroscopy investigations of ZSM-5 samples deactivated at low temperature by Lange et al. showed adsorption bands, typical for unsaturated carbon/carbon bonds [4].

The yield curves in Fig. 1 can be divided into 3 sections: ( 1) an initial time of preinitiation/incubation, (2) a period of strong acceleration of reaction and (3) a period of fast retardation/deactivation.

Selectivity changes during the periods of incubation, acceleration and retardation With the very active catalyst at the beginning of an experiment (particularly at

250°C) no hydrocarbons could be detected in the reactor effluent, which then consists only of dimethyl ether, water and unconverted methanol. This regime of preinitiation is characterized by the kinetic Scheme 1 with only two kinds of revers- ible reactions ( 1) chemisorption/desorption of CH,OH, Hz0 and ( CH3)*0 on Briinsted sites and (2) exchange of CHC and H+ between chemisorbed oxonium ions. An indication of chemisorbed trimethyloxonium species to occur on zeolite H-ZSM-5 exposed to methanol at relatively low temperature has obtained by 13C MAS NMR investigations [ 51.

Hz0 CH ,OH CH,-0 -CH,

-ti+ II

-H’ t I -H’

I 1

H H H CHa ‘+

+ CHOOH I + CHBOH I + CHOOH

-Hz0 +

,O, -H&J + +

0, H’p H - CH, 0, H _ CHa’- -WJ /O,

CHa _ CH3 CH3

Oxonium Methyl-oxonium Dimethyloxonium Trimethyloxonium (Hydronium)

Scheme 1. Reaction scheme for the kinetic regime of preinitiation. Methanol conversion on zeolite H-ZSM-5 at low temperature ( - 250°C).

32

$ 0 0.5 1 1.5 2 2.5

_ ’ Olefins I

Y

h lT,ction=27O”C

0 0 0.5 1 1.5 2 :

Normalized reaction time, tN 5

Fig. 2. Selectivity (S’ ) for methane, paraffins C2+, alkenes and aromatics as a function of normalized duration of experiment (q.,) for the reaction temperatures 250°C (top), 270°C (middle) and 290°C (bottom) Methanol conversion on zeolite H-ZSM-5. The time axis was normalized for easier comparison of the experiments: t, = f3<, / t,,, COn,, , where t, = normalized reaction time, t,,, = actual reaction time, and t,,,,, il,nV = time at maximum conversion.

Selectivity of formation of volatile hydrocarbons (S*, fraction of volatile HC- product compounds, respectively compound groups on carbon basis) is shown in Fig. 2 and Fig. 3. In the regime of first hydrocarbon formation (incubation) the volatile hydrocarbons contain almost no aromatics. The main compounds are methane, propene, propane, ethene and some isobutane. In particular the pronounced initial methane formation is of high theoretical interest. The high hydrogen content of the volatile product corresponds to a low hydrogen content of the earliest formed hydrocarbons which are retained by the catalyst.

The proposed principle kinetic scheme of the regime of initiation/incubation is presented as Scheme 2.

First C/C-bonds in methanol conversion on acidic catalysts are preferentially assumed in the literature to be formed by trimethyl oxonium ion rearrangement [ 61

H. Schulz et al. /Applied Catalysis A: General 132 (1995) 2940 33

Conv.,X,&WC Conv.mWBU-2

12 I phnax270~c COnV.,&LPC : 290°C i

270°C;

sl;.o: j 250°C

0 :,: 50 100 150 200 250 300 350 50 100 150 200 250 300 350

Duration of experiment, min Duration of experiment, min

Fig. 3. Selectivities (S,’ ) as a function of time at the reaction temperatures 250,270 and 290°C for methane (A), ethylene (B), isobutane (C) and the sum of aromatics ( D). Methanol conversion on zeolite H-ZSM-5.

(reaction ( 1) in Scheme 2). Ethylene is alkylated easily by methanol to yield propene (reaction (2) in Scheme 2). Because in addition to ethylene also propene is always initially observed it may be admitted, that not all &-species which are formed from methanol are released as ethylene, but part of them react directly (in the chemisorbed state) with a C, (CHC from methanol) to yield a C3 which is desorbed from the catalyst as propene. Alkene oligomerization again is a fast reaction under these conditions (reaction (3) in Scheme 2).

Now methane formation (from a positively charged methyl group of a methylated oxonium ion) as a coproduct of formation of highly unsaturated retained compounds is proposed as reaction of hydrogen (hydride) abstraction of reactive hydrogen in P-position to an olefinic double bond to produce a comparatively very stable allylic carbenium ion, which in turn yields a 1,3 dialkene via proton elimi- nation (reaction (4) in Scheme 2).

The initial formation of bulky unsaturated hydrocarbons, which are retained in the H-ZSM-5 pore system tremendously accelerates the rate of methanol conversion (Fig. 1). The composition of the volatile hydrocarbon product in this regime of acceleration exhibits the features of carbenium ion reactions. Major constituents are propane, propene, i-butane, butenes and i-pentane (catalytic cracking associated with skeleton rearrangement, hydride transfer to small (C?, C,) carbenium ions in addition to alkene and aromatics alkylation with methanol and alkene oligomerization) . The increasing propane/propene molar ratio (Fig. 4) indicates the increasing hydrogen transfer capacity in the system and corresponds to the unsaturated

34 H. Schulz et al. /Applied Catalysis A: General 132 (1995) 2940

(1) First C/C-bonds:

C”3 H H

I I

CH3- 0+-CH3+-’ CH3- O+-CzHg - CH.- 0+-H

_ CH2= CH2

(2) Olefin methylation:

+ CH3 OH C”2= CH2 F

- Hz0 CH3- CH = CH2

(3) Oligomerization

CH,- C” = CH, + CH,-CH = CH, _ CH,- CH - CH= CH-CH,

I C”3

(and isomers)

(4) Ionic dehydrogenation associated with methane formation: (formation of unsaturated deposit)

cH3 ‘L-Cl+

+/ 3 CH3 (+)

CH,- CH - CH= CH-CH, - CH,- C - CH=CH-CH, +CH, I -WJ*O ,

CH, CH,

L -H’

CH,= C - CH=CH-CH, I

CH3 L_. deposit Scheme 2. Principle reactions in the regime initiation/incubation.

nature of the hydrocarbons retained in the catalyst pore system. Selectivity of volatile aromatic compounds remains low (Sa*romatics less than ca. 5 C-%). The increasing fast filling of the zeolite pore system prevents further increase of reaction rate which then attains a maximum value - however no stationary state - and now drastically declines due to rapidly increasing mass transfer limitation (kinetic scheme of retardation).

290°C

i0 100 150 200 250 300 35 Duration of experiment, min

Fig. 4. Molar ratio propane/propene as a function of time for the reaction temperatures 250, 270 and 290°C. Methanol conversion on zeolite H-ZSM-5.

H. Schulz et al. /Applied Catalysis A: General 132 (1995) 2940

0

‘r Time, min

50 100 150

T reaction a? . 270°C ::

0 290°C

0:

P

cl .

-... 0 N..... J_. 100 200 300 400

Temperature, “C

Time, min (max. rEdease rate, 50 100 150

20°..~~,~,~~ w!...‘.... 2

I ::

% j; gi5- TfWti0"

::

;5: - ii

Temperature, “C

35

IO

IO

Fig. 5. Temperature-programmed reanimation of zeolite H-ZSM-5 after deactivation during methanol conversion at 270 and 290°C. Left: Release rate of hydrocarbons as a function of temperature. Right: Molar propaneipropene ratio of released hydrocarbons as a function of temperature.

In the kinetic regime of retardation selectivity of paraffins declines and that of alkenes increases strongly (Fig. 2, Fig. 3 and Fig. 4). Selectivity of aromatics increases gradually to 8-10%. As compared with the regime of acceleration, obviously the hydride transfer between hydrocarbon species is being inhibited. This reaction is assumed to be spaciously demanding. Hydride abstraction in combina- tion with methane formation is being favoured again (see Fig. 3).

3.2. Catalyst reanimation

After deactivation through methanol conversion, the catalysts were subjected to a temperature-programmed thermal treatment at a well measured argon flow and samples of the effluent gas were taken for GC-analysis. Fig. 5 left shows the rate of hydrocarbons release from the zeolite pore system as a function of temperature and Fig. 5 right the respective molar propane/propene ratio.

The composition of the released hydrocarbons is shown in detail in Fig. 6, Fig. 7 and Fig. 8. Hydrocarbon release begins at a temperature of 275°C. This temperature

Time min

250 300 350 4c Temperature, “C

Time, min i0 50

1 o. (max. release rate) 1001 - j ‘A

,

IO 200 250 300 350 1 Temperature, “C

Fig. 6. Group composition (paraffins, alkenes, aromatics) of volatile released hydrocarbons as a function of temperature. Temperature-programmed reanimation of zeolite H-ZSM-5 after deactivation during methanol conversion. Left: Methanol conversion at 270°C. Right: Methanol conversion at 290°C.

Temperature, “C 350 1

Temperature, “C

36 H. Schulz er al. /Applied Catalysis A: Grnernl 132 (1995) 2940

Time, min

$ log 100

(max. reIeasB raq 0 250 , !

,oo (max. release rate) , I 150 _ _

,? 200 250 300- 350 - 400

Time, min (max. release rate)

lop0 . -1 I ! 100 150

Xylene -v

; Benzene w&&c,-

200 250 300 350 ”

Temperature, “C ”

Temperature, “C Fig. 7. Composition of the fraction of aliphatic hydrocarbons (top) and of the fraction of aromatic hydrocarbons (bottom) as a function of temperature (time). Thermal temperature-programmed reanimation of zeolite H-ZSM- 5 after deactivation at 270°C (left) and 290°C (right) during methanol conversion.

is much higher than that to be expected for removal of adsorbed compounds and indicates that this release of the compounds encaged in the zeolite pores implies chemical reactions. The rate of release increases drastically with temperature and attains its maximum value at about 3 15°C. At about 400°C the formation of volatile hydrocarbons is terminated. After this temperature treatment a temperature-programmed oxidation with air was performed for coke determination. The effluent gas was monitored very sensitively by IR-technique for CO and CO?, however, no remaining coke was detected. This result proves the complete thermal removal of all hydrocarbon material from the H-ZSM-5 zeolite pore system. Thus the catalyst has been reactivated (reanimated) without any coking just via thermal treatment. This behaviour is unique for the pore system of the zeolite H-ZSM-5, where the widest ‘cavities’ are the channel crossings with diameters of about 0.7 nm which is much too small for hosting any coke like material. Then the reanimation reactions are of fundamental interest and will be evaluated from the selectivity data given in the figures.

The composition of the released hydrocarbons proves their origin from carbenium ion reactions: no methane is observed (even at low detection level) and C3 and C4 compounds are dominating substances in the fraction of aliphatic products (Fig. 7).

T readion . 270°C 0 290°C

H. Schul; et al. /Applied Catalysis A: General 132 (1995) 2940

max. release rate)

Temperature, “C Temperature, “C

. : Treadion ‘\

‘. j . 270°C ‘. : 0 290°C

1 ‘p ; t . 270°C

250

Tempezure, or?

,I”““““‘:“““’ 200 250

Temoe%re, $.f Fig. 8. Selectivity of H-ZSM-5 reanimation. Dependence on temperature. Top left: para-isomer among ethyltoluenes; top right: mass ratio durene/pseudocumene; bottom left: mass ratio ethyltoluenes/pseudocumene; bottom right: mass ratio xylenesitoluene.

The molar propane/propene ratio (Fig. 6 right) varies in a wide range and exhibits a sharp maximum. At the beginning of the conversion at low temperature the total of encaged organic material is quite rich in hydrogen and the Cz carbenium ions which are formed in cracking reactions have increasingly more access to reactive hydrogen which is removable as hydride. However, the situation changes soon (before attaining the maximum release rate) and the molar propane/propene ratio declines due to available hydrogen scarcity in the system.

The initial product is preferentially aliphatic (ca. 80 C-%) and mainly paraffinic. This indicates the conversion to start with catalytic cracking of saturated C/C- bonds of sufficiently big molecules C,, of mainly aliphatic nature. At the same time, part of the unsaturated hydrocarbons in the zeolite pores will undergo ring closure, ring widening and dehydrogenation via ionic hydrogen transfer. Xylenes and pseudocumene are the dominating primarily observed aromatic compounds. A reasonable exemplifying and simplifying pathway of their formation would be the following.

This mechanism takes into account the features of product composition as well the peculiarities of carbenium ion reactions and primarily observed cyclization products

Table I Composition (C-R ) of the fraction of aromatic compounds which has been recovered from the zeohte pore system

after use for methanol converGon at 270°C

I compound

Duration of experiment, min I

22 1.9 2.7 4.6 10.4 0.8 21.6 4.9 36.1 2.3 15.8

36 6.1 1.9 0.7 16.4 3.4 9.4 13.7 26.4 9.3 12.2

60 I 5.1

Dominating structures are

1.4 1.3 3.0 10.0 13.3 7.9 27.5 23.4 6.7

in accordance with the literature. However, it cannot be discussed here further. Towards the end of reanimation the hydrocarbon product turns to be dominantly

aromatic. Interconversion reactions of aromatic compounds do now play a major role and only the small aromatic molecules will easily (fast) diffuse out of the pore system. Fig. 8 shows the amount of the para-isomer among the product fraction of all 3 ethyltoluenes. Its fraction increases with reanimation temperature continuously and strongly. The corresponding decline of the durene-to-pseudocumene-ratio is attributed to durene demethylation via trans alkylation. In a preceding investigation [3] we have analyzed the composition of the aromatic compounds entrapped in zeolite H-ZSM-5 after conversion of methanol at 270°C and different durations of experiments via dissolution of the zeolite in hydrofluoric acid and extraction with CH,C12 (Table 1).

Several of them are too big to escape from the pore system. Bulky 2-ring aromatic compounds have as well been recovered from the deactivated H-ZSM-5 zeolite by Guisnet and Magnoux [ 71.

In the view of our findings the kinetic regime of zeolite H-ZSM-5 reanimation is pictured as shown in Scheme 3.

With increasing reaction temperature the aromatic compounds undergo reversi- ble interconversions and from these equilibria (respectively steady state composition) the smaller and tinier compounds move out of the pores. Thus at a sufficiently high reaction temperature (e.g. 400°C) the pore system remains always partially filled and does not deactivate due to accumulation of bulky aromatics. This mechanism explains as well the results of NMR investigations with used ZSM-5 samples [ 81. The material frequently addressed as ‘internal coke’ has now to be regarded as frozen stationary state composition of the interconverting substances with a preference for compounds being to big for easy diffusion from the channel crossings through the channels.

Slow deactivation of zeolite H-ZSM-5 in high temperature methanol conversion ( > 350°C) via coking has been investigated in our earlier studies [ 21. The deac-

H. Schulz et al. /Applied Catnlysis A: General 132 (1995) 2940 39

Scheme 3. Zeolite H-ZSM-5 reanimation.

tivated catalyst is actually black from its external coverage with coke and the formation of this external coke which finally closes all pore entrances was attributed to the direct reaction of methanol with coke seeds on the outer surface of the zeolite crystals.

Coke seed + 2CH,OH -+ C - C + CH, + 2H,O coke ~ \

The co-produced methane has been proposed for tracing this high temperature coking reaction and for its discrimination from coking via carbenium ion mechanisms on acidic sites with particularly propane, i-butane and i-pentane among the volatile compounds. Among the earlier work on high temperature ( > 375°C) coking of zeolite H-ZSM-5 in methanol conversion (which is not the objective of this work) particularly by the investigations of Bibby et al. should be regarded [ 9,101. The review of Chang on ‘Hydrocarbons from methanol’ is being cited as picturing the chemistry of the MTG-process [ 111.

4. Concluding remarks

Time resolved determination of product composition, including the formation of organic compounds encaged in the zeolite, yields detailed information about tran- sient reaction mechanisms (as incubation, acceleration, retardation) and particularly the operative spacious constraints of the zeolite pore architecture and the strong involvement of the organic ‘deposit’ retained in the zeolite pores in the course of reaction. All these processes/reactions/kinetic schemes are not only time- but also temperature-dependent and individual kinetic regimes prevail at different

levels of temperature. This situation requires a much more specific treatment of the subject than presently common in literature.

In particular ‘coking’ of zeolites is a matter of high complexity which needs much specification (e.g. in our understanding coke is a black carbonaceous solid

material and organic compounds, as at low temperature formed unsaturated aliphatic substances or at high temperature formed bulky mono- or bi-cyclic aromatic compounds which are entrapped in the zeolite and cannot grow further due to spacious limitations, should never be addressed as coke because the term coke is material related [ 121 and should not be used for any kind of undesirable/deacti- vating organic compound/matter which is retained by the catalyst). Correspond- ingly this investigation shows, that the initially at low temperature in the zeolite encaged organic material acts highly activating and is an intrinsic constituent of the reaction mechanism (until it starts to deactivate via mass transfer limitations). On the other hand the compounds found in the used H-ZSM-5 zeolite pore-system, which are formed during high temperature conversion, are not at all coke but quenched reaction intermediates which relate to the dynamic state of conversion, where they participate in the kinetic schemes and their dynamic concentrations are controlled by the reaction conditions, particularly the reaction temperature, and their diffusivities, respectively spacious constraints.

Another concluding remark concerns the view that a high complexity of the reaction product offers a multiplicity of information about the ongoing reactions and from these the spacious constraints, as ruled by the zeolite pore architecture, may be evaluated. The reactions of formation of the entrapped compounds/matter may be deduced from the selectivity/composition of the volatile coproducts.

References

[ I] H. Schulz and S. Nehren, Erdnl Kohle Erdgaa Petrochem., 39 ( 1986) 93. [ 2 1 H. Schulz, Zhao Siwei and H. Kusterer, Stud. Surf. Sci. Catal., Vol. 60, Elsevier, Amsterdam. 199 I, p. 781. [3] H. Schulz, D. Barth and Zhao Siwei, Stud. Surf. Sci. Catal., Vol. 68. Elsevier. Amsterdam. 1991, p. 783. [4] J.-P. Lange, A. Gutsze and H.G. Karge. J. Catal.. 114 (1988) 136. 151 H.G. Karge, H. Darmstadt.A. Gutsze. H.-M. Vieth.G. Buntkowky, Stud. Surf. Sci. Catal., Vol. 848. Elaevier.

Amsterdam, 1994, p. 1465. [6] J.P. van den Berg. J.P. Wolthuizen. J.H.C. van Hoff, in L.V.C. Rees (Editor). Proc. 5th Int. Conf. Zeolitca,

Heyden, London. 1980, p. 649. [7] M. Guisnet, P. Magnoux, Appl. Catal., 54 ( 1989) I. 181 J. Kirger, H. Pfeifer, J. Caro, M. Biilow H. Schlodder. R. Mostowicz and J. Volter, Appl. Catal.. 29 ( 1987)

21. [9] D.M. Bibby. N.B. Milestone, J.E. Patterson, L.P. Aldridge. J. Catal., 97 ( 1986) 493.

[IO] D.M. Bibby,C.G. Pope. J. Catal., 116 (1989) 407. [II] C.D. Chang, Catal. Rev. Sci. Eng., 25( 1) ( 1983) p. I. [ 121 Riimpps Chemie-Lexikon, 8. Aufl.. Bd. 3, 2652, Franckhsche Vrrlagshandlung, Stuttgart, 1983.

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Kinetic regimes of zeolite deactivation and reanimation