Regimes of methanol conversion on zeolites

Studies in Surface Science and Catalysis, volume 154 E. van Steen, L.H. Callanan and M. Claeys (Editors) 9 2004 Elsevier B.V. All fights reserved.

2133

R E G I M E S OF M E T H A N O L C O N V E R S I O N ON Z E O L I T E S Schulz, H. and Wei, M. University of Karlsruhe, Engler-Bunte Institute, Kaiserstrasse 12, 76128 Karlsruhe, Germany.

ABSTRACT The temporal changes of selectivity and rate of methanol conversion to hydrocarbons on the zeolites HZSM5, HY and H-Beta have been studied. With the zeolite HZSM5 at low temperature (270~ kinetic regimes of initiation, acceleration and retardation were observed. During initiation, the zeolite pores collect first reactive hydrocarbons. Dehydrogenation of this retainate performs with methanol as the reactant, methane being the hydrogen-rich co-product. During acceleration, the rate of methanol conversion increases drastically, because of reactive retainate accumulation in the pores, and during retardation conversion decreases, as caused by ample pore filling with molecules which are too big to diffuse sufficiently fast to allow their escape from the pore system. With increasing temperature, up to 400~ reanimation of the HZSM5 is noticed. The at low temperature deactivating retainate undergoes cracking reactions at higher temperature. Stable bigger species, as multi-ring aromatics - and the more coke - are not formed, because of spatial constraints. At high temperature (475~ the HZSM5 deactivates slowly. Coke is formed on the crystallite surfaces by methylation and dehydrogenation of "coke seeds". Spatial constraints in the HZSM5 pores allow no big molecules to form, and larger molecules of the reaction mixture do not accumulate, as they are in "pseudo equilibrium" with smaller ones. With the zeolite HY at 475~ coke selectivity is high. The HY-super-cages can host multi-ring aromatics. Coke methylation and coke dehydrogenation are finally the only methanol reactions. With the zeolite H-Beta, the behavior is intermediate when comparing with the zeolites HZSM5 and HY. The composition of the volatile aromatics is unique, as even hexamethyl-benzene is obtained.220 Keywords: Methanol to hydrocarbon conversion, zeolites INTRODUCTION Changes of rate and selectivity of catalytic reactions can be used for exploring the steady state rules. The changes may be provided by sudden alteration of pressure or feed composition. Such changes occur when starting experiments. The steady state surface of coverage with reactants, intermediates, products and further species that might promote or suppress distinct reactions is established asymtotically. Today, restructuring of catalyst surfaces under reaction conditions is also seen as a phenomenon to cause temporal changes. In case of conversions with zeolite catalysts, temporal effects may be caused by spatial constraints, applying to distinct reactions or to diffusion of distinct compounds (reactants, intermediates and products). In this paper, temporal changes during conversion of methanol to hydrocarbons on three zeolites are reported. The conversion of methanol to hydrocarbons is highly informative about ruling catalytic principles, because of the complex composition of the product, which reflects the catalytic events. However, research tools must be available, to determine the product composition in detail at high temporal resolution, including the retained m a t t e r - respectively the retained compounds - in the zeolite pores. EXPERIMENTAL The zeolites were applied as a thin layer of crystallites, adhering on the surface of fused silica particles (dp - 0.3 mm) packed in a fused silica tube, which was mounted in an outer steel reactor [1,2,3]. The mass ratio of zeolite to silica particles was 1:10. No binder was used and an isothermal catalyst bed with good flow characteristics was obtained. The conditions of methanol conversion were 5 bar (PMeOH= 2.5 bar, Pargon = 2.5 bar), WHSV = 1 h -1 (0.5 g zeolite in the reactor, 0,5 g/h methanol flow). The reaction temperatures are presented together with the results. Temporal resolution of conversion and selectivity was achieved by applying the ampoule sampling technique [4] and feeding a reference flow of 0.5 vol-% neopentane in

2134 nitrogen to the product stream. Thermal and oxidative catalyst regeneration was performed in the reactor [3 ]. The ampoule samples were analyzed by gas chromatography, temperature programmed f r o m - 80 to 262~ The Si/A1 molar ratios of the zeolites HZSM5, HY and H-Beta were 100, 5.1 and 35 respectively. R E S U L T S AND D I S C U S S I O N Initiation When passing methanol vapor through a catalyst bed of zeolite HZSM5 at 270~ "no" formation of hydrocarbons is noticed in the beginning (Figure 1 left). The fresh catalyst is not active until about 30 minutes on stream (texp = 30 min). This is the episode la ofpre-initiation. The problem is" What stimulates activity? The question can be answered from the changes of product composition and the changes of differential mass balances. The zeolite collects (reactive) hydrocarbon species. Hydrocarbons are noticed in the product flow after about 20 minutes. During the next 20 minutes most hydrocarbons are retained in the zeolite pores (YRet is much higher than Yvol; see Figure 1 left). This hydrocarbon retainate reacts with methanol (Episode Ib, regime of initiation). The composition of the volatile products during this episode (texp, ,o~m= 0,3- 0,4 in Figure 1 right) is dominated by methane. The retainate will be correspondingly deficient in hydrogen. Inspecting the catalyst shows its color to have become intensively yellow. This indicates highly unsaturated hydrocarbons [5], respectively their carbenium ions, as obtained by addition of a proton from the catalyst.

EPISODES E; P ! S 0

C, E S

.im" i ' ! ~ ! '~' ~,L~,~_._----:;

d _J iLU ,,.,.,.

~ ~

Volali|e$

Re~ain ate

(I

0 o

2c~)

TIME,, texp,

rain

0

1

lIME,

2

texp, nowm

Figure 1. Left: Yields of retainate and volatile hydrocarbons as a function of time (texp), Right: Group composition of the volatile products (S'group) as a function of normalized time (texp. . . . ). texp. . . . = texp/texp,fin(tex0;~niS the time of methanol-breakthrough at the catalyst bed end), Zeolite HZSM5, 270 o

In conclusion, the important reactions in the regime of initiation are: 9 Formation of C2-species (ethene) via oxonium ion rearrangement as proposed by van Hoof [6], 9 Reaction of C2- and higher hydrocarbons with methanol for increase of their size, 9 Dehydrogenation of retainate by reaction with methanol, methane being the hydrogen-rich co-product (Scheme 1). Episode I ends, as noticed by declining methane selectivity, the yield of volatile products (yvol in Figure 1 left) now surpassing the yield of retainate (Yret). Acceleration In Episode II (texp ca. 40 to 76 minutes in Figure 1 left, respectively texp,nonnca. 0.60 to 1.0 in Figure 1 right) the conversion of methanol - as equal to the yield of volatile plus retained hydrocarbons (yvol, YRet) increases drastically to almost 100 % . There are more and more reactive hydrocarbons present in the pores, and the rate of methanol consumption increases as for their alkylation.

2135

+CH3OH

-: CH~. :, H20

C~

.~,:H20

Scheme 1. Example of proposed retainate dehydrogenation with methanol. Methane is the hydrogen-rich co-product. E P:l $ o o E

, i,,! 0

E;Pi ~ ODE ,,,

L '

s

6

Ta~:~ 27o'0-6i

....R. *

0

< ~;,~-

S

~J

i..... ':~

~ .<" . . . . . . xyle.es ..... Tfi-MB /'

1

T~ilM E, ~ p , . ~

!

2

L

E o

Q

T I M E, ~ x p . ~

Figure 2. Composition of the fractions of volatile aromatics (left) and aliphatics (right) as a function of normalized reaction time. Methanol conversion on zeolite HZSM5 at 270~ The average size of the volatile aliphatic products (Figure 2 right) increases, but remains of limited carbon number. This reflects their stability against cracking, because reactivity for cationic cracking is low up to C6, but increases drastically with carbon number from C6 to C7 to C8, [7], as explained by the stability of the participating carbenium ions. In this episode, the selectivity of volatile aromatic compounds increases from zero to the final value of ca. 20 %. In conclusion, the episode of acceleration is characterized by: 1. Increasing methanol conversion, due to increasing amounts of reactive olefins and aromatics in the pores, 2. Increasing concentrations of aromatics in the product stream, together with co-produced paraffins, as resulting from secondary reactions of olefins by cyclization and hydrogen transfer [8].

Retardation Acceleration is followed by retardation, (Figures 1,2). Now the pore filling with bulky retainate becomes so high to generally reduce diffusion rates. As seen in Figure 2, the selectivity of the bigger volatile aliphatic molecules (C6, C7, C8) declines, whereas that of the smaller ones (C3, C4) increases. The composition of the aromatics retained in the ZSMS-zeolite pores during the early episodes at low temperature, determined by gas chromatography of the extract after dissolving the zeolite in hydrofluoric acid [2], is shown in Table 1. The dominating compounds are 1-ethyl-,2,3,5-trimethyl-benzene (No. 8 in Table 1 and Scheme 2) and 1-isopropyl-,2,4-dimethyl-benzene (No. 9 in Table 1 and Scheme 2). As proposed in Scheme 2, they are formed in reactions of alkylation with methanol, ethene or propene. This composition of aromatics can be regarded as depending on the following parameters (1) The structure of the primary cyclization product, (2) Relative diffusion rates of compounds to leave the pores, (3) Spatial demands of individual reactions, respectively their transition states and (4) Degree and kind of pore filling with further compounds, as depending on time (texp) and temperature.

2136 Table 1. Composition of the aromatic compounds recovered from the pores of zeolite HZSM5 after methanol conversion at 270~ for different reaction times [2]. Corn ~osition in C-%

&

R :|~,l:n

(1)

(2)

(3)

(4)

(5)

(6)

(7)

(8)

(9)

(1o)

22

2

3

4

10

1

21

5

36

2

16

36

6

2

1

16

3

10

14

27

9

12

60

5

1

1

3

10

13

8

28

24

7

The following rules are concluded to apply: The only small fraction of naphthalene derivates among the aromatics is explained by their big size, merely fitting into the channel crossings of the zeolite HZSM5. Its relative amount decreases with increasing total amount of retainate in the p o r e s - e.g. from 16 to 7 C%. The aromatic molecules retained in the pores are very preferentially alkylated benzenes. It would be a misunderstanding (sometimes found in literature [9]) to term these retainate molecules "coke" (which is a solid, carbon-rich, amorphous phase to which no distinct molecular weight applies).

~et~.,--7- (~.

CH3OH

CH30H !

(e i 8

"Ct12=C1t2 +CH2=CH-CH3

t91

tAI

Scheme 2. Alkylation reactions with methanol and small olefins to form ZSM5 retainate molecules, Reanimation Amazingly, with increasing reaction temperature, with the zeolite HZSM5, the catalyst lifetime does not decrease, but increases drastically, from e.g. 0.1 to 100 hours (Figure 3). Simultaneously, the retainate selectivity (as equal to the retainate yield at 100% methanol conversion) decreases from e.g. 20 C% at 270~ to less than 1 C% at about 400~ reaction temperature [2]. This is a unique behavior of the zeolite HZSM5, based on its particular pore architecture. The explanation is that the at low temperature deactivating retainate, undergoes cracking reactions at higher temperature and the formed smaller molecules now diffuse out from the pore system. Such cracking reactions are normal for acidic catalysts, and are accompanied by the formation larger aromatic molecules and amorphous phases of lower H/C-ratio. But the narrow ZSM5 pore architecture allows no formation of multi-ring aromatic compounds and even the more, no formation of a coke-alike material. This view is supported by the catalyst regeneration results (see Table 2). The zeolite HZSM5 was used for methanol conversion at 270, 280 and 290~ each for 180 minutes and for the durations (texp) 70, 90, 180 and 840 minutes at 270~ Afterwards the catalyst was treated thermally and consecutively in an oxidizing manner. The yields and compositions of desorbed products were determined in detail. It is seen in Table 2 that the catalyst used at 270~ reached a retainate loading of 0.053 g per gram of catalyst at texp - 180 min. The loading increased not further, up to 840 minutes. This retainate was completely removed by thermal treatment, as the following oxidative treatment produced no CO2. The conclusion is that the thermal treatment of the used catalyst proceeded without coke formation from the retainate. The TPD spectra in Figure 4 left exhibit their maxima at 310-340~ the desorption being complete at ca. 400~ From these results we understand the regime of reanimation in the following way: At ca.

2137

400~ pseudo-equilibriums of cracking-, alkylation-, isomerization- and trans-alkylation are established. The smaller molecules diffuse out from the pores of the catalyst particles. In other words, reanimation means, the catalyst re-attains its activity with increasing temperature [ 10].

E P iS: 0 D E

..,

',

IV

~ : .

20

Y~et~ C ::%

~

"

~

,

S

V

.............................]...... '

. . . . .

~

i ~,e: i i m e

10 i ~0 ~

104

....': i! 200

30~

'~ 4~

!!

1~ =

:5~::

R E:ACT:iON T E M P E R AT U R ~; T+~ ~n::...,, '~

Figure 3. Yield of retainate and catalyst lifetime as a function of reaction temperature during methanol conversion on the zeolite HZSM5. It is seen in Table 2 that the degree of pore filling with retainate (Vret/Wpore) decreases with reaction temperature from 75 to 58% (for Trctn = 270 to 290 ~ and increases with time from 27 to 72% (for texp = 70 to 840 minutes). The aromaticity of the retainate increases with reaction temperature (Carom/Caliph = 0.64 to 0.79 for Trctn = 270 to 290 ~ and decreases with reaction time from 0.82 to 0.61 for texp = 70 to 840 min, as explained by continuing alkylation (methylation) with methanol. The H/C-ratio of the retainate (at 270~ is low in the beginning, as reflecting the initial formation of unsaturated hydrocarbons (H/C = 1.48 at texp = 70 min) and then increases to the common value of ca. 1.65. Table 2. Thermal regeneration (reanimation) and oxidative treatment of zeolite HZSM5 after use for methanol conversion at different durations and temperatures. Te,em..+ ~'C

270

290

~3" ()

.27.~}

%rpD, m,ax~':'C 310 3~1 (3;| O) mR.e.L~me~..,~g ii0s 0:~037 O~(!:190:0321).(}53 0=05i mRe::~:~x:~l.....Ng i{ ,o :0.0)3 : = : VRa]V:m.~e,: m.~[ml { {/i:,75 [).58 0~27 ()A6 0/75 0.72 El.e~n.t~ m~tai.n.alecpmg~aiti~:m, r D=omT.{~ :ms.::ult~; C=~'*m/C*~,'a~ph / 0..64 0:79 t 0=:82 0.70 0 . ~ 0o6;I rexP =Du,:mt~onof experin:~nt (rain), Tn,+~+=~I:emp, c~f~ c t i o n ('~C}, V Ret=Volume ~f retaina~e ~ r ~ a m of catalyst (~tg)~ ~sumi~g a ~m:ina~e denshy of O,:7giml (i~mpy/4:f~hyl=~.frz~u~e ~u:27/(FC)+ Vp~;~e=Pom volume o:f I g zeolite (,mUg)=,eal=cula~ from |he value o~' f r ~ e ~ k de:~si~y+ T!r},o:m~..~ =Peak.+m.axi~m mm~:au~re .during TPD of used ,e.amlysts. The reanimation reactions have been characterized further by the composition of the products from thermal regeneration (Figures 4,5) and taking also in account the results in Table 1 about the recovered aromatics from used catalysts. At the high temperature of regeneration (ca. 400 ~ the last (most difficultly formed) products are xylenes and trimethyl-benzenes as aromatics (Figure 5 left) together with ethene and propene as aliphatics (Figure 5 right), indicating the dealkylation reactions shown in Scheme 3.

2138 At low temperature, in the beginning of catalyst regeneration, the aliphatic fraction of the desorption products contains much saturated compounds, in relation to catalytic cracking of larger paraffin- and olefin molecules, in combination with hydrogen transfer in the formation of aromatics. The regime of reanimation is fully established at ca. 400~ (see Figures 3 and 4).

a~

Scheme 3. ZSM5-Reanimation by dealkylation reactions. ~,,

!00

+"+a,la, a~+ r~+

,/T.o,.

- aTa+c mm~ics

50 &

do 6

0 200

3~0

500

400

efin:s _ . ~ a r a f ~ n s ..... 0 200

:

~

3(}0

i

,

,

400

D e s o r p t i o n T e m p e r a t u r e , "C Figure 4. Thermal regeneration (reanimation) of the HZSM5 catalyst after use for methanol conversion at low temperature (270,280, 290~ Left 9Hydrocarbon release rate as a function of desorption temperature Right: Group selectivity (aromatics, o|efins, paraffins) of the TPD-products as a function of desorption temperature i" r

T R c , . = 270"C

+6 50

..

T R,e,,',, " 270;C

6 Xyfer~e .....

~~/Tetra.-MB

lie84 <

C

~~

................~ ' + : + '

0 200

300

400

Desorption

.'.z.+~.

~....+

<

500

~].,:;)

temperature,

300

~

500

"C

Figure 5. Composition of the fractions of aromatic (left) and aliphatic (right) hydrocarbons during thermal treatment of the HZSM5 catalyst, used for methanol conversion at 270 ~

Regime of deactivation by coking At high reaction temperature (Trct, - 400~ the HZSM5 catalyst deactivates faster when increasing the reaction temperature. Simultaneously, the retainate selectivity increases (Figure 3). When inspecting the catalyst bed visually after about half of its useful lifetime, one sees a grey - not so broad - reaction zone in about half of the catalyst bed length. Behind this zone, the catalyst is dark-black, being covered with coke and before the front, the particles show a light blue color, indicating external sites coking. Selectivity of methanol conversion at high temperature (475~ has comparatively been investigated with the zeolites HZSM5, HY and H-Beta. Some principal results are reported in Table 3. As for results with the zeolites ZSM58, Erionite, Mordenite, EU1, MCM41 and MCM 22 reference [3] is being cited.

2139

Table 3. Results of methanol conversion on the Zeolites HZSM5, HY and HBeta at 475~ H-ZSM5 5.1 x5.4 (10-ring) 5.4 x 5.6 (10-ring) 9.5 100 0.106

HY 7.4 (12-ring) 13.4 5.1 0.340

H-Beta 6.7 x 7.6 (12-ring) 5.5 x 5.5 (12-ring) 9.2 35 0.217

Life time, min 3) (at 50% MeOH-convers. ) Ultimate MeOH-convers 4) g(MeOH/g(zeolite) . ..

7910

177

133

123

1.9 ,,,

1.8

Coke load, g(C)/g(zeolite )

0.25

0.26

0.13

Rel.coke vol. ~) mlCcoke)/gCzeol.) Coke vol./Pore vol., ml/ml . . . .

0.23 2.1

Channels, A Cage/Crossing, A Si/A1-Atomic ratio Pore volume, ml/g 2)

,

0.24 0.70

.

.

.

.

.

,

0.12 0.55

.

Note: 1) Reaction conditions: 475~ PMeOH=2.5 bar, WHSV- lh -~, fixed bed, 1g of zeolite 2) Calcualted from framework density. 3) Life time defined as time when conversion has decreased to 50%. 4) Total of methanol converted at 50% breakthrough time. Coke density assumed as 1.1 g/ml. The HZSM5-zeolite deactivates very slowly, in about 8000 minutes, as compared with 177 minutes for HY, and 133 minutes for H-Beta. 123g of methanol were converted to hydrocarbons before deactivation, as compared with only 1.9, respectively 1.8g. During its long life span, the HZSM5 accumulates not more coke than the zeolite HY in its short life (0.25 against 0.26 g of coke per gram of zeolite). The estimated coke volume is much higher than can be hosted in the HZSM5 pore volume (by a factor of 2.2). Clearly, the coke on HZSM5 (at least very preferentially) is located externally on the catalyst particles. With the zeolites HY and H-Beta the coke volume is smaller than the pore volume (0.70 and 0.55 of the pore volume) and the coke can be placed inside the pores.

....~

.....

j~.+~.

9

::

++

+

0

~.~.....

~++~.+++e++ ,::

;Oi::+fin~

.......~ ~

~ + T . : ...............................

o o

+:+

1

No'rrna~ized

ca.ta+y:st

li,~e t i m e

Figure 6. Group selectivity of methanol to hydrocarbon conversion on the zeolites HZSM5, HY, and HBeta at high temperature (475~ as a function of normalized time. Ber~ene

H :Z ~ M

j•

H- 8 o ~,

......

0 E o

o

o

g,5

I

Norma.;tized

catalyst

life

time

Figure 7. Composition of the fraction of aromatic compounds during methanol conversion at 475~

on the 3 zeolites.

2140

9

tOO

.........~ H:Y ,,,( ....

~ ~. , ... . . . ~. : , + ~ ~

0

~.. ................... +~:!:.

.

~.~

[:

~S"

E o

,:;

!ii ~'

~.~ . . . . . . : ~

~ :,~ ~ . ~ ~ "

E1

,:: ' ~ i ~ t ~ .

o

o +5

Normalized

c a t a l y s t life t i m e

Figure 8. Composition of the fraction of paraffins during methanol conversion at 475~ on the 3 zeolites. From the selectivity of hydrocarbon formation, the spatial constraints within the zeolite pores, as applying to distinct reactions or compounds diffusion, might be perceived. The regimes of coking with the three catalysts are best discriminated when beginning with the results obtained with the wide pore Y-zeolite (Figures 6 and 8, middle). Initially (texp.norm< 0.10), the product consists preferentially of coke and propane. Clearly, the primarily formed products react further to coke retained on the catalyst and propane as volatile co-product, this obtained by hydrogen transfer to propene. .::::.. : . . . . . . . . . . . . . .

=========================== ........

.:.,,

: .........

:.::.:: ........................

:..~:~o~::::::9

F ........................................ .......HZ5 . M 5..........

c5

50

c ~

o

~

.

autenesI

"%,o-~

E o ~

9

++0

0

0,5

I

0

O+

0,5 ~" P e n t - - 1

N o r m a l i z e d c a t a l y s t life time Figure 9. Composition of the fraction of olefins during methanol conversion at 475 ~ on the 3 zeolites. With the zeolite HZSM5 at short time on stream (texp. norm--< 0.1, Figures 6 and 8, left), mono-ring aromatics are obtained instead of coke, together with propane as the dominating hydrogen-rich co-product, the propane formation being similar as in case of the HY-zeolite. Convincingly, we see the spatial constraints in the pores of the HZSM5 to act against the formation of multi-ring aromatics and any higher coke-alike material.

Dohydrogenalion by reaellon

with rneihanol"

i l

"'~

+ CH:~OH

+c~

-.___) Methy~alion with met~a~O!:

....

:Reat:Nngemeni and e:ye~!:zailou t,eac:~ioes

Scheme 4. Coke methylation and dehydrogenation with methanol.

+HzO

2141 With ongoing accumulation of coke, selectivity changes. With the zeolite HY at break-through time of methanol (texp,norm= 1) the (almost) only products are coke and methane. It can be assumed that the active sites of the HY are now covered with non-volatile aromatics, respectively coke, and the methanol reacts with the coke for its growth by methylation and dehydrogenation (Scheme 4). It is stated that CH4-selectivity is indicative for the growth of retainate at the active sites.

Scheme 5. Consecutive methylation of benzene to obtain hexamethylbenzene on zeolite Hbeta. With the zeolite HZSM5, the changes of product composition with time are different. Methane selectivity remains very low, even at methanol break-through (texp, norm = 1.0), as consistent with the only very low selectivity of coke (less than 1 C%), which could be dehydrogenated. As known from measuring the methanol concentration at the reactor exit, a narrow deactivation zone moves slowly through the catalyst bed. This proves a short remaining active catalyst bed-length at methanol break-through (texp, norm = 1.0). Accordingly, only a low degree of secondary conversion of the hydrocarbon products is noticed, and primarily formed olefins are now the main products (Figure 6,1eft). With HZSM5, very initially (texp, norm --< 0,03), an episode of fast selectivity changes is noticed. The external sites on the crystallites, all along the catalyst bed, are expected to deactivate by coking with olefins. The secondary hydrocarbon reactions are extensive. The fraction of aromatics (Figure 7, left) contains much benzene (ca. 30 C%) and toluene (ca. 40 C%). This is explained by secondary isomerization and dealkylation of alkylbenzenes (see above the reactions of thermal HZSM5 regeneration). Deactivation of the zeolite Beta can be understood from the behavior of the extremes, the zeolites HY and HZSM5. Both, volatile aromatics and retainate are formed Figure 6, right), indicating less spatial constraints as compared with zeolite HZSM5. Temporal selectivity trends of methane, paraffins C2+ and olefins are intermediate. The composition of volatile aromatics (Figure 7, right) exhibits a unique time dependence: With increasing time (texp, norm), concentration maxima of toluene, xylenes, trimethyl-benzenes, tetramethyl-benzenes, pentamethyl-benzene and hexamethyl-benzene are observed. The channels of zeolite Beta are sufficiently wide to allow the diffusion even ofhexamethylbenzene. It appears that with the partially deactivated catalyst and a broad reaction zone (at about texp,norm= 1.0) the further methylation of mono-ring aromatics with methanol prevails to even obtain hexamethyl-benzene. In its fresh state (texp,norm < 0,2) the opposite is true for the catalyst zeolite Beta: Benzene and toluene here are the main compounds as explained by dealkylation after isomerization (see above the thermal regeneration reactions). CONCLUSIONS The findings of our investigations can be generalized. Adding the proton from the catalyst to the oxygen of the methanol, yields the methyl-oxonium ion: CH3OH + H + ---> [CH3OH2] + . The carbon/oxygen bond will be polarized to make the methyl-group positively charged. So it can react like a methyl cation "CH3 +''. It appears from the selectivity data that this is the basic intermediate of methanol-to-hydrocarbon-conversion on acidic catalysts. The principal alternative reactions of the "methyl-cation" then are: 9 Reaction with methanol to form ethanol, respectively ethene. This reaction is comparatively difficult (slow), however important to provide first reactive hydrocarbon species in the episode of initiation at low temperature. 9 Reactions with unsaturated hydrocarbons (olefins, aromatics, and retainate) for growth (increase of their molecular weight) e.g.: R-CH=CH2 + CH3 + --->R-CH+-CHz-CH3 --> R-CH=CH-CH3 + H +. This is the main methanol consumption reaction. In terms of stoichiometry, the alkylation by methanol is the addition of a CH2- group. Accordingly, the carbene species has also been imagined as intermediate [11,12]. 9 Reaction of the methyl cation to dehydrogenate olefins, 6-ring naphthenes or retainate with methane being the co-product, e.g.: R-CH2-CH=CH-CH3 + CH3 + --~ R-CH+-CH=CH-CH3 + CH4 --~ R-CH=CH-CH=CH2 + H +

2142 This reaction is important in the episode of initiation, to build up the unsaturated (yellow) retainate at low temperature and it is dominant with the already much with coke loaded zeolite HY to make the coke more deficient in hydrogen and thus more susceptible for further methylation. In addition to their reactions with methanol, the hydrocarbons themselves undergo various (in principle known) reactions on the acidic catalyst, as isomerization (double bond shift, skeleton rearrangement of olefins, alkylaromatics and paraffins) disproportionation (hydride transfer, methyl transfer) cracking (of particularly aliphatics C7 and higher, dealkylation of aromatics) and cyclization (to form aromatics and build up the aromatic retainate). All the reactions on/in acidic zeolites might be controlled by spatial constraints, imposed by the pore architecture. With the zeolite HZSM5, these constraints rule selectivity very favorably if the reaction temperature is sufficiently high (>_ 350 ~ and the reanimation regime is actively working. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

Schulz, H., Zhao, S., Kusterer, H. Stud.Surf. Sci. Catal., 60 (1991) 281. Zhao, S. Dissertation, University Karlsruhe, 1991. Wei, M. Dissertation, University Karlsruhe, 1998. Schulz, H., BOhringer, W., Kohl, C.P., Rahman, N.M.,WiI1, A. DGMK-Forschungsbericht 320, DGMK, Hamburg, 1984. Germain, J. E. "Catalytic Conversion of Hydrocarbons", Academic Press, London, New York (1969). van Hoof, J.H.C., van den Berg, J.P., Wolthuizen, J.P., Volmer, A., in D. Olsen, A. Bisio (Eds.), Proc. 6th Int. Zeolite Conf., Butterworths, Guildford, 1981. Schulz, H., Weitkamp J., IEC, Prod. Res. Dev. 11 (1972) 46. Schulz, H., Wei, M., Microporous a. Mesoporous Matter. 29 (1999) 205. Guisnet, M., Magnoux, P., Appl. Catal. 54 (1989) 1. Schulz, H., Lau, K., Claeys, M., Appl. Catal. A: General 132 (1995) 29. Chang, C.D., Silvestri, A.J., J.Catal. 47 (1977) 249. Chang, C.D., Catal. Rev. Sci. Eng. 25 (1983) 1.