Deactivation and thermal regeneration of zeolite HZSM-5 for methanol conversion at low temperature (260–290°C)

Microporous and Mesoporous Materials 29 (1999) 205–218

Deactivation and thermal regeneration of zeolite HZSM-5 formethanol conversion at low temperature (260–290°C)

Hans Schulz *, Ming WeiEngler-Bunte Institute, University of Karlsruhe, Kaiserstraße 12, D-76128 Karlsruhe, Germany

Received 18 June 1998; received in revised form 16 September 1998; accepted 17 September 1998

Abstract

Methanol conversion on zeolite HZSM-5 has been performed at relatively low temperature (260–290°C ) and thechanges of conversion and selectivity measured as a function of duration of the experiment. The catalysts used wereregenerated by thermal and oxidative treatment and the products of the regeneration analysed in dependence of timeand temperature, respectively. The yield of organic substance retained by the catalyst – the yield of ‘‘retardate’’ – wasalso determined as a function of time. Four episodes with different kinetic regimes were discriminated: preinitiation,initiation, acceleration and retardation. It is concluded that the retardate here plays a dominant role in controllingreaction rate and selectivity. With increasing temperature of reaction as well as of regeneration, the regime ofreanimation is established, where the formation and accumulation of deactivating compounds is forbidden by spatialconstraints in combination with fast reversible conversions between bigger and smaller molecules. The reactions ofretardate formation, transformation and decomposition are deduced from the time-resolved selectivity data. © 1999Elsevier Science B.V. All rights reserved.

Keywords: Methanol conversion; Retardate-forming reactions; Selectivity–time resolution; Thermal regeneration;Transient episodes; Zeolite HZSM-5

1. Introduction formation during methanol conversion on zeoliteHZSM-5, and also used for elucidating the reac-tions of retardate decomposition and thereby itsIn this work, time-resolved selectivity is used aschemical nature through a temperature-pro-a key for understanding the reactions of retardate1grammed thermal treatment. The nature of theretardate and the reactions of retardate trans-

* Corresponding author formation are influenced by spatial contraints1In this paper we use the term ‘‘retardate’’ for the organic mate-within the zeolite pore system. Results concerningrial that has been retained on the catalyst. In the literature, thethe mechanism of methanol conversion on zeoliteterms ‘‘coke’’, ‘‘green coke’’ or ‘‘white coke’’ are commonly

used irrespective of whether the retained material is coke-like HZSM-5 have been published extensively (e.g.,or not. So we think use of the term ‘‘coke’’ to be frequently Chang et al. [1–3] and Guisnet et al. [4,5]). Inmisleading and propose the term ‘‘retardate’’, which includes our work we have developed methods specificallyno material relation and only means that it is organic matter for the differential (instantaneous) determinationretained on the catalyst, irrespective of its molecular weight,

of the product composition, including the retar-composition or structure, and the mechanism of its bonding todate. The reactions of retardate formation arethe catalyst, whether by physical or chemical adsorption or by

inclusion in cavities. specified through the coupled formation of volatile

1387-1811/99/$ – see front matter © 1999 Elsevier Science B.V. All rights reserved.PII: S1387-1811 ( 98 ) 00332-1

206 H. Schulz, M. Wei / Microporous and Mesoporous Materials 29 (1999) 205–218

Their domain of validity assumed is often toobroad or even not specified. In our approach wesearch for the basic reactions that are the elementsof the kinetic schemes. They are not so many. Thecatalyst is an acid and the intermediates are carbe-nium ions (except initial oxonium ions). However,through combinations of reactions and reactants,the multiplicity of product composition is large(e.g., more than 100 peaks in a chromatogram).This means a wealth of information about ‘‘whatis happening on the catalyst surface’’. So, we groupthe product composition for fractions that corre-spond to distinct reaction steps and combine themto the kinetic regime.

Fig. 1. Conversion of methanol plus dimethylether and yields 2. Experimentalof volatile hydrocarbons and retardate as a function of time.Catalyst HZSM-5, reaction temperature 270°C (dimethylether

The sample of zeolite HZSM-5 used in this workis regarded as an educt together with methanol ).was prepared by the group of Professor C.T.O’Connor at the University of Cape Town. Itscompounds [6–10]. From these investigations itSi/Al atomic ratio was 100, a value considerablyturned out that the organic matter retained in thehigher than that of a catalyst used in an earlierpores plays a dominant role. Clearly, there is notstudy [8]. The zeolite crystals of about 2 mm diame-only one reaction mechanism of methanol conver-ter were coated on fused silica particles (averagesion but several kinetic regimes which change withdiameter ca. 0.25 mm) as an adhering layer oftime on stream and also with temperature. Thusabout 10 mm thickness. The mass ratio of zeoliteearlier mechanistic pictures appear too simple.to silica particles was 1:10. The catalyst was placedin a fused silica tube, which was mounted into asteel reactor for applying elevated reaction pres-sures. Thus no catalyst binder was used, a homo-geneous flow of the gas phase through the catalystbed established, any noticeable deviation fromisothermicity avoided, and any intraparticle masstransfer effects – except those in the zeolite crystalsthemselves – avoided. The conditions of methanolconversion were 5 bar ( pMeOH=2.5 bar, pArgon=2.5 bar), WHSV=1 h−1 (0.5 g of zeolite, 0.5 g/hbeing the methanol educt stream). A referencestream of 0.5 vol% of neopentane in nitrogen wasadded to the product stream for the direct determi-nation of yields and conversion from the chroma-tograms. The yield of retardate at a distinct timewas calculated as the difference between conversionand yields of volatile products. At the end of anexperiment, the amount of retardate accumulatedFig. 2. Conversion of methanol plus dimethylether on zeolitewas measured by temperature-programmed ther-HZSM-5 as a function of time at four different reaction

temperatures. mal and oxidative treatment of the catalyst.

207H. Schulz, M. Wei / Microporous and Mesoporous Materials 29 (1999) 205–218

I PreinitiationIncubationHII Initiation

III Acceleration

IV Deactivation(Retardation)

At the beginning of the experiment no noticeableconversion of methanol to hydrocarbons occurs(Episode I, preinitiation). Then small concen-trations of hydrocarbons in unusual compositionappear in the reactor effluent (Episode II, initia-tion). At this state the yield of retardate (YRet) is

Fig. 3. Selectivity (top) and yield (bottom) of retardate as afunction of time during methanol conversion on zeoliteHZSM-5.

Samples from the hot gaseous product stream weretaken in small glass ampoules (sampling durationca. 0.1 s [7]) and analysed later by temperature-programmed capillary gas chromatography (tem-perature range of gas chromatograph: −80 to260°C). This ampoule sampling was also used fordetermining the composition of the exit streamfrom temperature-programmed regeneration of thecatalyst. A detailed description of the reactor isgiven elsewhere [11].

3. Results and discussion

3.1. Time-dependent kinetic regimes of methanolconversion

Fig. 4. Fraction of methane among the volatile hydrocarbonsWhen passing the methanol vapor at 270°C overas a function of normalized time (top) and yield of methane asthe HZSM-5 crystallites, the conversion and selec-a function of normalized time at four different reaction temper-

tivity as a function of time change. Four episodes atures during methanol conversion on zeolite HZSM-5 (timewith different kinetic regimes are discriminated normalization is related to the duration of the experiment until

the maximum of conversion is obtained).(Fig. 1):


higher than that of the volatile products (YVol). probable species from which ‘‘first carbon–carbonbond formation’’ originates [12].Clearly, this retardate plays the role of a reactive

intermediate. In Episode III – the accelerationregime – the conversion increases from ~15 to 3.3. Episode II, initiation~97% with increasing retardate accumulationon/in the catalyst. The final Episode IV of deactiva- The initial hydrocarbon formation in Episode

II is linked to an unusual selectivity. The earlytion is marked by a gradual decline of conversion,as to be expected from extensive pore filling with hydrocarbon product is mainly the retardate (more

than 50% selectivity, Fig. 3) and among the volatileimmobile organic matter (see below, Tables 1 and2). hydrocarbons methane is a major constituent

(Fig. 4).These episodes of methanol conversion arestrongly temperature-controlled (Fig. 2). At low This means that the hydrocarbons formed early

(as ethene and propene via alkyloxonium iontemperature (260°C) the durations of the episodesare extended, the maximum conversion is lowered rearrangement [12]) preferentially do not leave the

zeolite crystals, but are converted further intoand attained later. At higher temperature(≥290°C) the episode of initiation is shortened larger and more strongly adsorbed molecules. The

reaction rate increases in this episode and it isand even no longer noticeable. Then completeconversion is observed for a long time. Catalyst concluded that the methanol reacts fast with the

retardate molecules. Here, alkylation of olefinsdeactivation is slower at higher recation temper-ature than at lower reaction temperature. with methanol to give larger (branched) olefin

molecules is established (e.g. see Eq. (1)).The reaction can be termed ‘‘polymethylation’’.3.2. Episode I, preinitiation

As fast double bond shift will be possible, amulticompound mixture of olefins is to be expectedIt is well documented in the literature [2,12]

that the formation of dimethylether from methanol as well. The size of the olefin molecules will belimited by the available space. According to carbe-is much faster than that of hydrocarbons. The

equilibrium between methanol, dimethylether and nium ion cracking rules [13,14], hydrocarbonsC6 and particularly C5 and smaller ones are ratherwater is commonly assumed to have been estab-

lished via oxonium ion intermediates. In our stable. However, olefins C7, C8 and C9 will easilysplit (see Eq. (2)).experiments in Episode I, the reversible conversion

of methanol to dimethylether and water was This reaction sequence produces more and morereactive olefins, and the methanol conversion rateobtained only, with no formation of hydrocarbons.

Dimethyl- and trimethyloxonium ions are the most increases progressively.

(1)

(2)

(3)


A particular problem is the understanding of support this reaction sequence it is mentioned thatthe 1,2,3,5-tetramethylbenzene product has beenthe high initial methane selectivity. It can be

anticipated that under the conditions applied ( low found as a main compound among the aromaticsrecovered from a ZSM-5 catalyst after use for low-temperature <300°C, acid catalyst, sufficient par-

tial pressure of methanol and availability of temperature methanol conversion. The zeolite wasdissolved for this purpose in hydrofluoric acid [16].reactive hydrogen as hydrogen bonded on carbon

atoms in the b-position to unsaturated carbon–car-bon bonds) the methane is produced from metha- 3.4. The regime of acceleration, Episode IIInol in an ionic reaction. The positively chargedmethyl of the methanol reacts with a b-hydride of After some retardate is collected in the pores,

the selectivity changes again and the reactionthe olefin and the negatively charged OH with theproton from the allyl cation to form H2O (see becomes increasingly faster. Fig. 5 shows the com-

position of the volatile products grouped as par-Eq. (3)).So this reaction produces a diene from an olefin affins, olefins, aromatics and methane as a function

of time. (The time axis has been normalized to theand in a next step a triene from a diene, makingthe retardate more deficient in hydrogen and more time of maximum conversion.)

It can be noticed that, in this regime of accelera-stable against cracking. It has been reported in theliterature [15] that the deactivated HZSM-5 cata- tion in the range of normalized time on stream of

about tnorm=0.5 to 1, the methane selectivity islyst, after methanol conversion at low temperature,exhibited ultraviolet (UV ) bands of highly unsatu- low. Here the paraffins C2+ (mainly propane and

i-butane) are the hydrogen-rich coproducts to therated hydrocarbons. Correspondingly we havenoticed that the ZSM-5 catalyst after use at low hydrogen-poor products – volatile aromatics and

retardate. This means that, in the case of availabil-temperature attains a yellow color.In consequence, the high initial methane selectivity of small olefins C3 and C4, the reactions of

hydrogen transfer to these olefins are favoured inity is associated to the formation of the unsaturatedretardate: the hydrogen-rich methane is coupled comparison to the hydrogen transfer to the metha-

nol (to form methane and water from the metha-to the hydrogen-poor retardate. A typical unsatu-rated compound could be the decatriene isomer nol ). In general, it is concluded that at this kinetic

regime the dehydrogenation to produce volatilewith a structure highly suited for cyclization. Theobtained cyclopentadiene derivative would react aromatics together with aromatic retardate from

olefins proceeds on similar routes via carbocationfurther to a tetramethylbenzene (Scheme 1). To

Scheme 1.


olefin secondary reactions are increasinglyhindered.

3.6. Higher alcohols as reaction products

The small yields of ethanol, methylethyletherand diethylether have been determined and arepresented as a function of time for the reactiontemperatures 270 and 280°C, respectively, in Fig. 6.

The yields attain values below 1% C. They aregenerally higher in the regime of deactivation thanin the regime of acceleration. This may beexplained as having a higher chance of survivalwhen the catalyst is less active and secondaryreactions are less probable.

3.7. Thermal regeneration

Catalysts that have been deactivated by cokingare commonly regenerated by burning off the coke.A typical example is the continuous catalyst regen-eration in the FCC (fluid catalytic cracking) pro-

Fig. 5. Composition of the volatile hydrocarbon productsgrouped as paraffins, olefins, aromatics and methane, as a func-tion of the normalized reaction time for three different reactiontemperatures. Methanol conversion on zeolite HZSM-5.

intermediates with i-butane and propane as thecoupled products.

3.5. Regime of deactivation, Episode IV

Later in this paper it is shown how the zeolitepores are increasingly filled up during this episodeIV, and it is concluded that this accumulation ofbulky and poorly mobile compounds creates masstransfer hindrance and, consequently, the decrease Fig. 6. Yields of ethanol, methylethylether and diethlyether asof methanol conversion rate. The volatile products a function of the normalized reaction time for methanol conver-

sion on zeolite HZSM-5 at 270, 280 and 290°C, respectively.are now becoming more olefinic with time. The


cess. Thermal treatment of the used catalyst would one has to assume a mixture of aliphatic, naph-thenic and aromatic compounds for the retardateresult in making the organic matter on the catalyst

more and more carbonaceous by splitting off small composition. The H/C ratio of 1.64 indicates theformula C11H18. As an aliphatic compound thishydrogen-rich compounds such as ethene, methane

or even H2. In contrast to this, it was observed corresponds to an undecatriene, e.g.,that the ZSM-5 catalyst, which had been deacti-vated during methanol conversion at low temper-ature (270°C), was regenerated completely by athermal treatment in a flow of inert gas (argon ornitrogen, see Table 1). It is concluded that in this As a naphthenic compound it would correspondcase the thermal treatment of the used catalyst to a C6-alkylated cyclopentadiene as:does not involve ‘‘coking’’ of the retardate butsolely its ‘‘complete’’ conversion to volatile com-pounds. This also implies that the retardate on thecatalyst is not a coke-like material.

It is to be seen in Table 1 that with increasingor a C5 alkylated cyclohexadiene with the highreaction temperature the amount of retardate onpropensity for dehydrogenation to an aromaticthe catalyst decreases. At the higher reaction tem-compound, such as the 1-ethyl-2,3,5-tri-perature of 290°C, a small fraction of the retardatemethylbenzene with the molar H/C rato of 1.54:of ~7–8% C is not thermally removable. This

behaviour of the catalyst, to be decreasingly sensi-tive to deactivation at increasing temperature, hasbeen termed ‘‘reanimation’’ [11]. It is explainedby a mechanism assuming that at elevated temper-ature the retardate in the pores of the zeoliteHZSM-5 is converted to smaller and slimmer This 1-ethyl-2,3,5-trimethylbenzene has been

observed as the most common constituent amongmolecules which leave the pore system, whereasthe formation of larger molecules in the pores is the aromatics recovered from HZSM-5 catalyst

which had been used for methanol conversion atspatially constrained.The H/C molar ratios of retardates were deter- 270°C via dissolving the zeolite in hydrofluoric

acid [8]. All these molecules would stick withinmined as 1.68, 1.65 and 1.64 at the three reactiontemperatures, respectively (see Table 1). Of course, the ZSM-5 pore system.

Table 1Thermal and oxidative regeneration of a HZSM-5 catalyst after use for methanol conversion at 270°C, 280°C and 290°C, respectively(reaction time, texp=180 min). The specific pore volume, VPore, of the zeolite was calculated from the framework density [17]. Thevolume of the retardate, VRet, was calculated assuming a density of the retardatea of 0.7 g ml−1TReaction (°C) 270 280 290Temperature at maximum rate of volatile product formation, Tmax,regn (°C) 310 323 340Total amount of retardate (g carbon/g catalyst) 0.053 0.051 0.040

By thermal treatment 0.053 0.047 0.037By following oxidative treatment <0.0001 0.004 0.003

VRet/VPore 0.75 0.72 0.58Retardate composition, calculated from decomposition products (%C)

Aromatic carbon 39 43 44Aliphatic carbon 61 57 56

H/C (mol/mol ) 1.68 1.65 1.64

aThe value of retardate density was estimated as the density of a typical retardate component (1-isopropyl-2,3,4-trimethylbenzene)at 270°C.


The thermal treatment decomposes and pounds during thermal treatment of the used cata-lysts (Fig. 7) show maxima at 310, 325 and 340°C,rearranges the retardate molecules in such a way

that they are converted into sufficiently small/slim respectively. With increasing temperature of themethanol conversion (270, 280 and 290°C) themolecules which can move within the pore system

and leave it (on the other hand, no substantially retardate is becoming more stable and less earlierconverted. It is consistent with the H/C ratio inhigher-molecular-weight compounds and particu-

larly coke-like material can be formed due to Table 1 which indicates that the retardate is ofmore aromatic nature at the higher reactionspatial constraints). This thermal removal of the

organic matter from the pores of the zeolite temperature.The composition of the volatile productsHZSM-5 is complete at ~475°C (see Fig. 7). Of

course, the organic substance on the outer surface grouped as paraffins, olefins and aromatics fromthe temperature-programmed treatment of theof the crystallites will be removed not all. A

consequence of this behaviour is that, during meth- used catalysts as a function of temperature isshown in Fig. 8. Fig. 9 shows the distribution ofanol conversion on the zeolite HZSM-5 at much

higher temperature (375–475°C), the catalyst the aromatics and Fig. 10 the distribution of thealiphatics.shows a very long life span: in the zeolite pore

structure now the regime of reanimation is estab- After the maximum of formation of volatileproducts, these consist almost exclusively of aro-lished. The formation of polynuclear aromatic

compounds is not possible due to spatial con- matics and olefins. At high temperature the mainproducts are xylenes and ethene. This reflects astraints. Slow catalyst deactivation proceeds

through coking on the external surface of the dealkylation reaction of the retardate, proceedingon the acid sites, such ascatalyst crystallites. The question remains as to

what extent the binuclear aromatics, which areformed in trace amounts, could be accumulated inthe pores and then how far their formation isreversible, so that they would be included into theregime of reanimation.

The formation rate curves of the volatile com-The ethyl substituent could already exist in the

original retardate [8] or be formed via isomeriza-tion, e.g.,

At the temperature of the maximum rate of volatileproduct formation from the retardate, dimethyl-and trimethylbenzenes and propene are the maincompounds. This indicates depropylation as:

Fig. 7. Formation rate of volatile hydrocarbons as a functionof time/temperature during temperature-programmed thermal

1-Isopropyl-2,4-dimethylbenzene was observed intreatment of HZSM-5 catalyst after use for methanol conver-sion at 270, 280 and 290°C, respectively. an earlier investigation, where the catalyst was


Fig. 8. Composition of volatile hydrocarbons as a function ofFig. 9. Composition of the aromatic fraction of the volatile

time/temperature during temperature-programmed thermalhydrocarbons as a function of time/temperature during temper-

treatment of HZSM-5 catalyst after use for methanol conver-ature-programmed thermal treatment of HZSM-5 catalyst after

sion at 270, 280 and 290°C, respectively.use for methanol conversion at 270, 280 and 290°C, respectively.

dissolved after use and the entrapped aromaticretardate recovered and analysed, as one of themain retardate components at low reaction temper-ature (270°C) [10]. Its formation can be explainedby alkylation of the benzene ring with propene, Deisopropylation is a much easier reaction thanthe most common olefin in the reaction product de-ethylation.of methanol conversion on the zeolite HZSM-5. In the lower temperature range, before the tem-In addition, the C3 substituent could be formed perature of maximum volatile product formation,

there is a relatively large portion of paraffinsby isomerization via:


Scheme 2.

among the products consisting mainly of i-butane during methanol conversion, because the com-pounds collected undergo further reactions andand propane. This implies an ongoing hydrogen

transfer coupled with cyclization and aromatiza- the retardate compounds formed later are ofdifferent composition. This arises because, in thetion, for making the aliphatic fraction of the

product more unsaturated (Scheme 2). partially retardate-filled pores, the spatial con-straints will be more severe than at the beginningWith regard to the composition of the retardate

and its reactions, these thermal treatment experi- of the experiment.As can be seen in Table 2, the amount of retar-ments show the following. The retardate mainly

date on the catalyst (reaction temperature=270°C)consists of alkylated monoring aromatics and theirincreases with time for the first 180 min; however,precursors, the maximum size being controlled bythen no further increase happens until 840 minthe pore dimensions of the ZSM-5 zeolite. Withhave elapsed. The composition changes in this lateincreasing temperature the retardate componentsperiod. It is remarkable that the H/C molar ratioundergo reactions on the acid sites of the catalyst.of the retardate increases with reaction time, itsOnly reactions leading to smaller or slimmer com-nature becoming less aromatic. Correspondingly,pounds are successful, and the products diffusethe products from the thermal catalayst regenera-out from the zeolite crystals. Reactions towardstion are less aromatic with increasing time oflarge molecules are forbidden because of lackingmethanol conversion. The percentage of aromaticspace. As the active sites are of an acidic nature,carbon decreases from 45% at 70 min to 39% atthe reactions proceed via carbenium ion intermedi-180 min on stream, the explanation being the laterates and the prevailing kinetic regime in the high-alkylation of the retardate molecules by reactiontemperature range ≤375°C is that of ‘‘reanima-with methanol (or with propene, the most reactivetion’’ as controlled by shape selectivity.and most common olefin).

The formation rate of volatile products during3.8. Influence of reaction time on the catalysttemperature-programmed thermal treatment of theregeneration behaviourcatalyst samples (Fig. 11) for different times on

It may be anticipated that the chemical nature stream shows the peak of the formation rate tosharpen with increasing time on stream and toof the retardate changes with time on stream

Table 2Thermal and oxidative regeneration of a HZSM-5 catalyst after use for methanol conversion at 270°C for different reaction timeson stream. The specific pore volume, VPore, of the zeolite was calculated from its framework density [17]. The volume of the retardate,VRet, was calculated assuming a density of the retardatea of 0.7 g ml−1. By oxidation after thermal treatment, no carbon was detectedon the catalyst

Reaction time, texp (min) 70 90 180 840Total amount of retardate by regeneration (g/g) 0.019 0.032 0.053 0.051VRet/VPore 0.27 0.46 0.75 0.72Retardate composition, calculated from decomposition products (%C)

Aromatic carbon 45 41 39 38Aliphatic carbon 55 59 61 62

H/C (mol/mol ) 1.48 1.64 1.68 1.66

aThe value of retardate density was estimated as the density of a typical retardate component (1-isopropyl-2,3,4-trimethylbenzene)at 270°C.


Fig. 11. Formation rate of volatile hydrocarbons as a functionof time/temperature during temperature-programmed thermaltreatment of HZSM-5 catalyst used for methanol conversion at270°C for different times on stream.

particular, the composition of the aliphatic fractionchanged from 180 min to 840 min, even if theamount of retardate did not increase. The aliphaticcompounds from the thermal treatment are smallmolecules (mainly C2–C5) and, as such, not constit-uents of the retardate, but are formed by crackingreactions of larger molecules. Even if the amountof retardate and its molar H/C ratio have notchanged during this period, compositional changeshave taken place. In particular, a sharp peak ofC3 (propene) formation is seen at 300°C, thetemperature where the volatile product formationrate accelerates to approach its maximum value.

As observed earlier [8,9], the ‘‘older’’ retardatecontains much isopropyldimethylbenzene and theFig. 10. Composition of the aliphatic fraction of the volatileincreased propane formation should be due to ahydrocarbons as a function of time/temperature during temper-

ature-programmed thermal treatment of HZSM-5 catalyst after depropylation reaction. The changes of retardateuse for methanol conversion at 270, 280 and 290°C, respectively. composition with time then should be the result

of the ageing reactions: alkylation of the aromaticshift to higher temperature. Obviously, with time ring with propene, transformation (isomerization)on stream, the retardate is becoming more uniform of unsaturated five-membered ring compoundsand less reactive. It is suggested that in this period into six-membered ring compounds (aromatics)some of the unsaturated aliphatic retardate reacts and cyclization of unsaturated aliphaticto form aromatic compounds. compounds.

The composition of the volatile compounds fromthermal treatment of HZSM-5 samples that hadbeen used for different times on stream at 270°C 4. Conclusionsreaction temperature, is characterized in Figs. 12 –14. It is seen that the shapes of the curves change Transient kinetic regimes are informative about

the dynamic features of catalytic processes andwith the time the catalysts had been on stream. In


Fig. 12. Composition of the volatile hydrocarbons as a function of time/temperature during temperature-programmed thermal treat-ment of HZSM-5 catalyst used for methanol conversion at 270°C for different times on stream.

Fig. 13. Composition of the fraction of aromatics of the volatile hydrocarbons as a function of time/temperature during temperature-programmed thermal treatment of HZSM-5 catalyst after use for methanol conversion at 270°C for different times on stream.


Fig. 14. Composition of the fraction of aliphatics among the volatile hydrocarbons as a function of time/temperature during temper-ature-programmed thermal treatment of HZSM-5 catalyst used for methanol conversion at 270°C for different times on stream.

can contribute to their understanding. The particu- sufficiently bulky for being no longer mobile inthe pore system. A main hydrogen-rich volatilelar spatial constraints on the reactions in the pores

of the zeolite HZSM-5 are extremely useful in product hydrocarbon is methane in this kineticregime. It is concluded that the fast increase ofcommercial processes. Methanol conversion is a

very elucidative reaction owing to its versatility in methanol consumption during this episode is dueto the much higher probability for the methanolleading to a complex product composition with a

high content of information. The multiplicity of to react with unsaturated hydrocarbons than withanother methanol molecule to form further hydro-product composition is then converted into kinetic

data about basic reactions as part of the reaction carbon products.The reaction rate then increases about exponen-mechanisms, and then into kinetic regimes.

Thus it is shown through this investigation how, tially (regime/episode of acceleration) as theamount of the reactive retardate in the poreswith increasing time at low reaction temperature

(260–280°C, initially even with the most active increases.A maximum of reaction rate is obtained andfresh catalyst), no noticeable formation of hydro-

carbons occurs (regime of preinitiation). Then first soon followed by a decrease of the rate of methanolconsumption (episode/regime of retardation). Thisvolatile hydrocarbons are observed (regime of initi-

ation). From their composition and the carbon is explained by the increasingly inhibited masstransfer due to increasing pore filling with retar-mass balance it follows that most of the product

is retained on/in the catalyst as an unsaturated date. Estimated values of the degree of pore fillingof the deactivated catalyst samples (from regenera-product (retardate). The ZSM-5 pore size con-

straints limit the size of these molecules to carbon tion measurements) were of the order of 75%.From the detailed data of the product composition,numbers of about only 10–12. However, they are


individual steps of reaction and their (with time- Referenceschanging) importance in the kinetic schemes of

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[11] H. Schulz, S.-W. Zhao, H. Kusterer, Stud. Surf. Sci. Catal.Thus, only decomposition reactions of the retar-60 (1991) 281.date are successful, leading to smaller molecules

[12] J.H.C. van Hooff, J.P. van den Berg, J.P. Wolthuizen, A.which diffuse out the pore system. This behaviour Volmer, in: D. Olson, A. Bisio (Eds.), Proc. 6th Int. Zeoliteis made use of in the high-temperature application Conf., Butterworths, Guildford, 1984.

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[15] H.G. Karge, H. Darmstadt, A. Gutsze, H.-M. Vieth, G.interplay of acid-acitvated reactions and spatialBuntkowsky, Stud. Surf. Sci. Catal. 84 (1994) 1465.constraints establishing distinct kinetic episodes

[16 ] S. Zhao, Dissertation, University of Karlsruhe, 1991.and thus kinetic regimes within the zeolite pore [17] W.M. Meier and D.H. Olson, Atlas of Zeolite Structure

Types, Butterworth-Heinemann, Oxford, 1992.system.

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Deactivation and thermal regeneration of zeolite HZSM-5 for methanol conversion at low temperature (260–290°C)