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Formation of ethene and propene from methanol on zeolite ZSM-5
Applied Catalysis, 37 (1988) 139-154 Elsevier Science Publishers B.V., Amsterdam -
139 Printed in The Netherlands
Formation of Ethene and Propene from Methanol on Zeolite ZSM-5 I. Investigation of Rate and Selectivity in a Batch Reactor D. PRINZ and L. RIEKERT* Znstitut fiir Chemische Verfahrenstechnik der Universittit Karlsruhe, D-7500 Karlsruhe (F.R.G.) (Received 8 May 1987, accepted 7 October 1987)
ABSTRACT The catalytic conversion of methanol to hydrocarbons on zeolite ZSM-5 was investigated in the range 550 to 625 K in a well mixed batch reactor system, which permits observation of product composition as a function of conversion accurately for isothermal reaction conditions. Interconversion of methanol and dimethyl ether rapidly attains equilibrium. Conversion to hydrocarbons is preceeded by an induction period. This induction period and the selectivity for formation of ethene and propene increase with the Si: Al ratio in the zeolite and decrease with crystal size. Ethene and propene appear simultaneously as gaseous products if water vapour is present.
INTRODUCTION
The catalytic conversion of methanol to hydrocarbons in the C.&I!,, range by zeolites of the pentasil-structure was announced by the Mobil Research and Development Corp. in 1976 [ l] and has since been investigated in many respects. The abundant literature has been reviewed in detail by Chang [ 21; acid catalysis by pentasil-zeolites in general has been treated in perspective by Haag [ 31. Light olefins appear to be key intermediates in the methanol to gasoline process and they are potentially valuable products in their own right. The present communication is concerned with the kinetics of methanol conversion to hydrocarbons and with the selectivity of this reaction for the formation of ethene and propene. The experimental investigation was carried out in a well mixed batch reactor, wherein the mass of zeolite catalyst is m, and the mass of starting material A initially (at t = 0) is ( mA) o. The entire system is kept at a temperature well above the condensation point of reactants or any product, and the temperature
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of the catalyst is controlled within + 1 K, so that reaction takes place under strictly isothermal conditions. At different times t, samples of the reacting gas are withdrawn and analyzed by gas chromatography (GC) , so that the composition of the gas in contact with the catalyst can be established as a function of normalized time t = t m,/ ( mA) o. It follows from elementary chemical reactor theory that this normalized time r is equivalent to the modified residence time m,/ ( tiA) o in an ideal plug flow reactor. The same compositions of the reacting gas (reaction path) will be observed in both systems as a function of conversion of reactants, normalized time z or modified residence time, respectively, provided that feed composition, pressure and catalyst temperature are identical and that the properties of the catalyst are constant in time. Observation of the reaction path in a well-mixed batch reactor offers several advantages compared to experiments with an integral fixed bed reactor: catalyst temperature can be controlled precisely and thus isothermal operation secured; the reaction path for a wide range of conversion can be observed in one continuous experiment and it is possible to obtain a “slow motion picture” easily by using a smaller amount of catalyst under otherwise identical conditions. It must be noted, however, that in the batch reactor a fresh or regenerated catalyst initially interacts with the reactants, whereas with an integral fixed-bed reactor observations are, in general, made after a nearly steady state of the system has been reached. The properties of the catalyst may then be different from those of a fresh catalyst as charged to the reactor. EXPERIMENTAL
Zeolites
All zeolites used in this study were hydrogen forms belonging to the pentasil family, with the ZSM-5 structure predominating. Crystallinity was 98-100% as shown by X-ray diffraction (XRD ) . Templates used in the synthesis, composition and crystal sizes of the materials used are shown in Table 1. The designations given in the first column are used in this article; references to the literature, where the method of preparation and XRD data for the same preparations have already been communicated, are listed in the last column. Sample d was supplied by Mobil in the H-form. Zeolites a-c where obtained in the H-form through calcination (at 650°C in air) of crystals, synthesized by W. Holderich (BASF) from an alkali-free gel. Zeolites e, f and g were ionexchanged six times with 1 M ammonium nitrate solution at 8O”C, then calcined at 500°C in air for 6 h. Residual sodium content was less than 0.05 wt%. The zeolites were pressed into pellets without any binder, the pellets were crushed and a particle-size fraction of 0.5-l mm was obtained by sieving.
141 TABLE t Zeoiites Designation
: : ; g
--
Template used in synthesis
SiO,: Al&
Crystal size in pm
Ref.
NH,(CH,),NH, NH,(CH,),NH, NH,(CH,),NH, TPAX TPABr TPABr TPABr
34 48 45 100 72 220 660
%2 0.5 2X2x6 0.5iO.l 6.Oil 7.0+1 8.011
4 5
6 I
Catalytic reaction The batch reactor system is shown schematically in Fig. 1. A gas volume of 7 1 (0.007 m3) is circulated in a loop through a magnetically driven turbine of 0.21 m diameter at a rate of 1.8 1 s-l. This loop and the turbine (the mixer) are made of stainless steel and are heated to 433 K to prevent condensation of reactants or products. Total pressure in the system was always near atmospheric, with nitrogen as the diluent of reactants and products. A gas stream of
Ni
Nz , MEOH
. AIR I
1
in
OJt
t REACTOR1wP
MIX!W
TURBINE
Fig. 1. Well-mixed batch reactor system, schematic.
142
200 ml s-l flows from the mixer through the quartz reactor, containing the catalyst heated to temperature T, and back to the mixer. This flux is driven by a membrane pump which is also heated to 423 K, as are the connecting lines. The reactor loop and the mixer can be separated, so that both parts can be flushed, filled or evacuated independently. Gas samples can be withdrawn from the mixer through a heated gas sampling valve and are analyzed chromatograpically (two l/8” columns, 4 m Chromsorb 104 and 3 m Porapak N, 80-120’ C, flame ionization detector). A detailed description of the apparatus has been given elsewhere [ 71. The condition of perfect “mixedness” of the gas phase was considered to be sufficiently approximated if relative differences in mole fractions &/xi were kept below 3%. For a flow-rate of 200 ml s-l through the reactor loop this requirement will be fulfilled for any species whose observed rate of change of concentration in the system is characterized by a relaxation time of 20 min or more. The time constant of mixing in the large volume of the turbine mixer was less than 10 s. Before each experiment the zeolite catalyst was calcined in air at 850 K for 30 min, then brought to reaction temperature T under nitrogen in the reactor. All results were thus obtained for zeolite catalysts which are free from sorbed hydrocarbons or carbonaceous deposits at the beginning of an experiment. The initial composition of the reacting gas phase was prepared in the 7-l volume of the mixer and checked by GC while the reactor loop was separated from the mixer. At time t=O the reactor loop was then connected to the mixer. Two time scales are used in the following. Clock time t, counted from the beginning of an experiment, and normalized time r r=t(m,,,+lm;“g
(rnDM-&)()
(1)
where ( mM) o = mass of methanol ( M) charge at t= 0; ( mDME) o = mass of dimethyl ether ( DME ) charge at t= 0; m, = mass of zeolite catalyst. The factor 1.39 converts the mass of DME to the equivalent mass of methanol; the normalized time r corresponds to the mass-specific modified space time in an ideal plug flow reactor with methanol as feed. Gas-phase composition is described by carbon fractions yi Yi =
amount of carbon in compound i total amount of carbon in the system
(2)
The sum of carbon fractions of oxygenates, methanol (M) and dimethyl ether ( DME ) , is designated yA YA=YM+YDME
Conversion X means conversion of oxygenates to hydrocarbons
(3)
143
915
OJ
0.2 W
PL
05
O$ -
07 T/h
+
T/h
Fig. 2. Composition in batch reactor as a function of time t (upper scale) and reduced time z (lower scale) for two catalyst temperatures in terms of carbon fractions y,. ( PCHBOH) o= 95mbar; catalyst, 100 mg zeolite c. Left hand ordinate: ( 0) methanol; (0 ) dimethyl ether. Right hand ordinate: ( l ) ethene; (+ ) propene; (A ) propane.
x=1-y,
(4)
The integral selectivity, Si, of the conversion of oxygenates to hydrocarbons with respect to hydrocarbon i
will be a function of conversion X, if the product hydrocarbons undergo secondary reactions [ 201. The differential or point selectivity dSi ds, = 1
_dy”=s_+xds, dyz+ L
CI‘X
(6)
can be obtained from the integral selectivity and its variation with conversion X. The sum of the selectivities for the light olefins ethene and propene is designated S= in the following: S= = SC2H4+SCRH6 The relative amount of ethene in the light olefins (ethene +propene) as EO (ethene in olefins) in the figures EO=
is shown
YC2H4 YCzH4
+YChH6
Fig. 2 shows the change of composition of the gas phase in contact with 0.1
144
g of zeolite c as a function of clock time t and of reduced time r, for catalyst temperatures of 275’ C and 305 “C, respectively, as an example of raw results. Conversion X as function of time, as well as the reaction path (product distribution as function of conversion), were obtained from these figures, which are reproduced extensively elsewhere [ 71. RESULTS
Interconversion of methanol and dimethyl ether The relaxation time for establishment 2CH,OH=
of the equilibrium
(CHs)zO+HzO
(7)
in the system was in most experiments less than 10 min. The kinetics of the interconversion of methanol and dimethyl ether is thus influenced by the rate of gas circulation and it was therefore not investigated. Fig. 3A shows the reaction path in the quasi-ternary system methanol-dimethyl ether -hydrocarbons, starting methanol. Fig. 3B shows reaction paths starting from dimethyl ether with and without addition of water vapour. The equilibrium 7 is established for methanol as starting material when the conversion X to hydrocarbons is still below 5%; afterwards the ratio of methanol to dimethyl ether follows the equilibrium line as conversion X= y~cH2) increases. ycn30u remains nearly constant for X< 0.6 because additional water vapour from the dehydration of oxygenates to hydrocarbons shifts equilibrium 7 to the left. Equilibrium lines in Fig. 3 were calculated with an equilibrium constant K= 10.2 at T= 578 K
----r-X
Fig.‘3. Reaction paths in the ternary system CH,OH- (CH,) 20- (CH,); (CH,) designating the sum of hydrocarbons. (A) T=578 K, zeolite a; @M)O = 95 mbar. (B) T=578 K, zeolite a; (Pn&O=46mbar;o= (PH~O)~/(PDME ) O. ( 0 ) Observed compositions; (0 ,o ) compositions calculated under the assumption that equilibrium 7 is established at 578 K.
145
100
50
0
1
, 0.5
lb1 j X
1
150
t/min
200
O.11 , 0.5
1
(C)
X
1
Fig. 4. Influence of catalyst temperature. (a) Conversion Xversus t and 7. (b ) Integral cumulative se1ectivit.y of formation S_ of ethene +propene as a function of conversion X. (c) Distribution of light olefins (EO : ethene in ethene + propene) as a function of conversion; (0 ) T= 548 K; (+ ) T=578K; (0) T=623 K.
for reaction 7, which was obtained from available thermodynamic data [ 8,9]. The calculated ratio of y bME: yM at equilibrium should be reliable within - 2 % . The equilibrium line is also approached but not passed over if the reaction starts from dimethyl ether; y(cH3J20 then always exceeds the value which has to be expected if dimethyl ether is in equilibrium with methanol and water. Comparison of the reaction paths in Fig. 3A and B seems to indicate that the dehydration reaction leading to hydrocarbons preferentially removes methanol from the gas phase. Conversion of methanol and dimethyl ether to hydrocarbons A mixture of methanol and dimethyl ether which is in or near equilibrium according to eqn. 7 has to be considered as the reactant generating hydrocarbons. Conversion and selectivities as defined in eqns. 4-6 are therefore based on the sum of oxygenates (methanol plus dimethyl ether) converted to hydro-
146
0
X
Fig. 5. Influence of
X
water vapour added to methanol as starting material. T= 578 K; zeolite c. (pM),,=%i mbar, (PH20~O0, (pMvl)O=95mbar, (~~~0)0=228mbar.
carbons. Consideration pene as products.
+,
of selectivities shall be restricted to ethene and pro-
Influence of temperature and reactant composition Fig. 4 shows conversion X as a function of clock time (upper abscissa) and normalized time (lower abscissa) for three catalyst temperatures on zeolite c, starting from methanol. Conversion versus time is S-shaped with an initial induction period, which decreases with increasing temperature. Product distribution is not very strongly affected by catalyst temperature within the interval investigated here, as can be seen from Fig. 4b and c. The induction period is increased and the rate of conversion observed thereafter is decreased by addition of water vapour to the gas phase, as shown in Fig. 5. Addition of water vapour increases the selectivity for the formation of light olefins at low conversion and it also leads to an increase in the ratio ethene/propene in the products. When dimethyl ether is used as reactant then the partial pressure of water in the system will be as low as possible [ PHzO= ( PDME)OX], vanishing at X= 0. As shown in Fig. 6 no induction period was observed with dimethyl ether as reactant. Selectivity for (ethene + propene) is about the same as for methanol as reactant; however, only propene and no
147
X
0
0.4
0.2
01
1
T/h
0.6
0.6 0.5-
Q ,~+-o+--ol-cm-~~_+ + +
EO "0, 0.2 (b)
0'
0.5
X
1
Fig. 6. Methanol versus dimethyl ether as starting material. T = 578 K, zeolite a. + , mhar; 0, (pDM~),,=47mbar.
Fig.
7. Product
+,
(PM)"=40
(PM) o= 95
distribution for two initial pressures of methanol as reactant. T=578 K, zeolite C. mhar; 0, (pILIjO=285mbar.
ethene is produced from dimethyl ether as reactant at very low conversion, when pHzO is zero. Selectivity of formation of light olefins and olefin distribution for two initial pressures of methanol is shown in Fig. 7. Increasing (pM) o leads to an increase of pH20 in the system due to the equilibrium 7 and also to an increase in the length of the induction period. No induction period preceding the onset of conversion to hydrocarbons was observed if a small amount of propene was initially added to the reacting methanol (Fig. 8).
148
Fig. 8. Effect of addition of propene on conversion of methanol versus time. T= 578 K, zeolite.a, (pMj0=95 mbar. C+) no C,H, added; (0) (pC:IHsio=0.93 mbar.
0.6 EO
Fig. 9. Effect of Si: AI ratio in the zeolite at T= 578 K, (pM),=95 mbar. ( + ) Zeolite e (Si: Al=36); (0) zeolite f (Si:Al=llO); (0) zeolite g (Si:A1=330).
Influence of zeolite composition (Si:Al ratio) The effect of zeolite composition on the rate of conversion and product distribution was studied on zeolites e-g, which were prepared with TPABr as a
149
100
200
Vmin
Fig. 10. Effect of crystal size for zeolites with Si:A1=22f Zeolite c (2 X 2 x 6 ,um) ; (0 ) zeolite b (diameter 0.5 pm).
template.
1. T=578
K, (pM)0=95 mbar. (+)
They exhibit the same crystal morphology, are roughly of the same crystal size and differ only in the Si: Al ratio. Fig. 9a shows conversion of methanol versus normalized time at 578 K, Fig. 9b and c selectivity, yield and CZ/C3distribution for ethene and propene, The aluminium content affects the activity in two ways: it influences the induction period and (for Si:Al< 100) the rate of reaction thereafter. Increasing Si: Al from 110 to 330 causes a substantial increase in the induction period, which appears to be roughly proportional to the Si: Al ratio; otherwise the curves X vs. z remain essentially parallel if Si : Al is increased from 110 to 330. Selectivity and yield of formation of ethene and propene as well as the ratio of propene: ethene increase at low conversion (Xc 0.6) with decreasing aluminium content of the zeolite. A maximum of the yield of light olefins at high conversion appears to occur at intermediate Si : Al ratios around 100.
150
Influence of crystal size The effect of crystal size was studied on zeolites b and c which had been synthesized with the same template (1,3 diaminopropane, PDA) and have nearly the same composition. They differ substantially with respect to crystal size. Sorptive properties and micrographs of these crystals have already been communicated together with observations on their activity in the cracking of hexane [ 51. Fig. 10 shows conversion versus time for these zeolites under otherwise identical conditions. The induction period is substantially longer for the smaller crystals; the rate of conversion at X= 0.5 (where it reaches a maximum in both cases) for the larger crystals is 1.9 times the value for the smaller crystals. At low conversion (Xc 0.7) selectivity for light olefins is higher for the smaller crystals, where also relatively more propene is generated than on the larger crystals. The same observations were also made with two samples synthesized with tetrapropylammonium (TPA) as template, having roughly the same composition but different crystal size. DISCUSSION
The Thiele number characterizing interaction of intercrystalline mass transfer in the zeolite pellets and reaction of oxygenates to hydrocarbons was smaller than 0.8 throughout. The results concerning the conversion of oxygenates to hydrocarbons therefore are not influenced by external mass transfer and reflect the kinetics of intracrystalline processes. It is not possible, however, to infer any firm conclusion concerning intracrystalline concentration gradient from the results, since neither the nature of the molecular species prevailing in the crystals nor their coefficients of diffusion under reaction conditions are known. Only the composition of the gas phase in contact with the zeolite catalyst and its change with time, which means the fluxes of different compounds entering or leaving the crystals, has been observed. Therefore we do not intend to enter what has been called by Chang [ 21 the “mechanistic sweepstakes”. The results presented here concern reaction on zeolite catalysts which contain initially no sorbed hydrocarbons or other carbonaceous material and are thus in a virgin state. Two conclusions can be drawn from these observations which may be of general interest. (i) The onset of conversion of oxygenates to hydrocarbons is always preceded by an induction period, which varies in length with temperature, p&o, Si : Al ratio in the zeolite and crystal size. This phenomenon indicates that the hydrogen-form of ZSM-5 in itself is not sufficient as a catalyst for the conversion of methanol to hydrocarbons. It is catalytically active only after some entity, which is an intermediate in the sequence of reactions leading to gaseous hydrocarbons, has been generated in the zeolite. Several authors [ lo-121 have concluded that C-C bonds are formed by homologation of C3+ -olefins or ole-
151
finic residues in the zeolite with methanol. Van den Berg et al. [ 131 observed that C,-C, olefins oligomerize in H-ZSM 5 at temperatures between 300 and 600 K and that at temperatures above 500 K a dynamic situation prevails, where different olefins coexist. The same authors have also shown that the presence of water has an inhibiting effect on the oligomerization of light olefins in ZSM-5 [ 141. The induction period observed here in a closed system and its dependence on temperature,zeolite composition and &no can thus be understood to result from the formation of a system of reactive hydrocarbons in the zeolite, which are necessary as intermediates for the catalytic reaction. (ii) Crystal size does have a pronounced influence on the rate of conversion of methanol-dimethyl ether to hydrocarbons and on product distribution. For larger crystals the catalytic activity is higher and the induction period is shorter than for smaller ones. This phenomenon can be understood if it is assumed that the concentration and/or effectiveness of the oligomer which sustains the catalytic conversion of oxygenates decreases towards the surface of the zeolite crystals, where it continuously depolymerizes into smaller molecules which evaporate into the gas phase. The catalytic activity in the crystal would thus increase towards the centre and the mass specific catalytic activity would then be higher for larger crystals. The S-shaped curves of conversion X vs. r observed on zeolites with different ratios Si : Al > 100 under otherwise identical conditions (Fig. 9) appear to be parallel but displaced with respect to each other along the r-axis. In the interval 0.1
(8) An initial increase of dX/dt with X (“autocatalysis”) has also been found in an integral reactor by Chen and Reagan [ 151. Together these observations indicate that the rate of formation and the properties of the dynamic system which serves as intermediate in the conversion of oxygenates to hydrocarbons depend on zeolite and gas-phase composition, crystal size and time. It will therefore be difficult if not impossible to describe the kinetics in the conventional way by a system of rate equations with constant coefficients. A compilation of rates of conversion of oxygenates, observed on different zeolites at X=0.5, are given in Table 2 in the form of K=--
1 -dnA 1 m, dt CA
(9)
where nA=non$on+2n(cn_&o; cA=nA/V= (cA),)‘O.5 (gas-phase concentration of oxygenates) ; 172,= mass of zeolite. It is obvious from the data in Table 2 that crystal size as well as Si : Al ratio has an influence on the observed activity, besides temperature. It appears remarkable, however, that ZSM-5 zeolites from different sources and prepared
152 TABLE 2 Rates of conversion of oxygenates, characterized by first-order coefficients as defined by eqn. 9 Si:Al
Zeolite
Crystal size in pm
22
C
6x2x2
b d z
22 50 110 36
0.5 0.5 621 721
g a
330 17
8Ifil 1
PO
T
K
mbar
K
cm3 gg’ s-’
95 95 95 40 95 95 95
548 578 623 578 578 578 578 578 578 578 578
5.7 33 140 35 20 17 27 9.0 7.8 16 16
CH,OH CH,OH CH,OH CH,OH CH,OH CH,OH CH,OH CH,OH 95 CH,OH 95 CH,OH 47 (‘X,),0
by different methods agree rather well with respect to catalytic activity in the conversion of methanol to hydrocarbons. This observation is not a general experience in heterogeneous catalysis and one can speculate that here it is due to the self-organization of the catalytic system, which has to be considered as a dissipative structure as understood by Nicolis and Prigogine [ 161. The selectivity of formation of the light olefins ethene and propene is generally assumed to depend on the relative degrees of advancement of formation of light olefins from oxygenates methanol and dimethyl ether and their consecutive conversion to paraffins and aromatics [ 21. The results obtained here indicate that not only reactions 1 and 2 but also reaction 3 in the triangular scheme CZHA .G% 1
2
/ [2
CHxOH
+
\
(CH3)>0] 3
-
other hydrocarbons
occurs in the zeolite crystals as seen from the gas phase, the selectivity SCc2=+tc3=)at low conversion X being smaller than unity in most cases. Ethene and propene from the gas phase are converted through reaction 2 to other hydrocarbons and therefore reaction 3 must also be expected to occur in the scheme describing the gas-phase kinetics. Ethene and propene generated in the crystal will be subject to some degree to further conversion before they reach the gas phase, the net result being that reaction 3 will be observed in the material balance based on gas phase composition. This effect will be the more pronounced the longer the diffusion path in the crystal, which explains the lower selectivity for ethene and propene observed with large crystals, compared to smaller ones of the same composition (Fig. 10). The rate of reaction 2 and
153
A CH30H+(CH,),0
x
,.'
c othe: hydrocarbons
Fig. 11. Triangular representation of composition in carbon fractions nol+dimethyl ether; (B) C,H, +C,
(y) for (A) methaT=578 K, (phi +2
therefore also that of reaction 3 appear to be relatively more sensitive to an increase in acid-site concentration (lower Si : Al ratio) than reaction 1, as has already been observed by Chang et al. [ 171. Selectivity for light olefins is therefore enhanced by a high Si: Al-ratio, as has been shown previously by Wu and Kaeding [ 181, and also by small crystallite size. On all catalysts the ethene : propene ratio in the products increases with conversion, propene being much more reactive in reaction 2 than ethene, as has been shown previously [ 10, 13, 191. Only when pure dimethyl ether was used as the starting material was the ratio of ethene: propene zero in the products at the beginning of conversion. When methanol was present as reactant, ethene and propene were always found simultaneously in the gaseous products, even at very low conversion. All compositions observed with different zeolite samples as a function of conversion at T= 578 K and (pM + 2 PDME)O= 95 mbar without addition of water fall into a restricted region of composition space, which is indicated in Fig. 11 by the hatched area. In this diagram, lines of constant integral selectivity SCn=+C:l=are straight lines from the lower left corner of the triangular simplex. At full conversion (X= 1) yield and selectivity of ethene + propene will not exceed 30%. On the other hand selectivities for ethene+propene between 50 and 80% appear to be possible at incomplete conversion (Xc 0.7) with a suitable zeolite as catalyst. A process aiming at the production of ethene and propene from methanol should therefore operate at intermediate degrees of conversion in a fixed bed reactor and with recycling of unconverted reactants.
154
A pilot plant study of such a process is described in the following communication. The data presented here can only indicate in an approximate way suitable catalysts and operating conditions for a continuous fixed bed operation. The observation of induction periods shows clearly that here the catalytic properties of the zeolite depend on time. Therefore the normalized time ‘s will not be equivalent to modified space time in a fixed bed operating at steady state. ACKNOWLEDGEMENTS
We thank Ms. A. Weisz for skillful, reliable help in tedious experimentation and Dr. W. Holderich and Dr. V.S. Nayak for providing zeolite samples. This work was supported by the Bundesminister fur Forschung und Technologie. We also thank the Fonds der Chemischen Industrie and the Max-BuchnerForschungsstiftung for financial assistance.
REFERENCES 1
2 3
9 10 11 12 13 14
15 16 17 18 19 20
S.C. Meisel, J.P. McCullough, C.H. Lechtaler and P.B. Weisz, Chemtech, 6 (1976) 86. C.D. Chang, Catal. Rev., 25 (1983) 1. W.O. Haag, in D. Olson and A. Bisio (Editors), Proc. of the 6th Internat. Zeolite Conference, Butterworths, London, 1984, pp. 466-478. L. Marosi, J. Stabenow and M. Schwarzmann, German Pat. DOS 30 06 471 (1981). W. Hiilderich and L. Riekert, Chem. Ing. Tech., 58 (1986) 412. V.S. Nayak and L. Riekert, Appl. Catal., 23 (1986) 403. D. Prinz, Dissertation, Karlsruhe, 1986. Landolt-Biirnstein, Zahlenwerte und Funktionen, 6. Auflage, II. Band, 4. Teil, Springer, Berlin, 1961. R.C. Reid, J.M. Prausnitz and T.K. Sherwood, The Properties of Gases and Liquids, McGraw-Hill, New York, 3rd ed., 1977. J.R. Anderson, K. Foger, T. Mole, R.A. Rajadhyaksha and J.V. Sanders, J. Catal., 58 (1979) 114. R.M. Dessau and R.B. Lapierre, J. Catal., 78 (1982) 136. T. Behrsing, T. Mole, P. Smart and R.J. Western, J. Catal., 102 (1986) 151. J.P. van den Berg, J.P. Wolthuizen and J.H.C. van Hooff, J. Catal., 80 (1983) 139. J.P. Wolthuizen, J.P.van den Berg and J.H.C. van Hooff, in B. Imelik, C. Naccache, Y. Ben Taerit, J.C. Vedrine, G. Coudurier and H. Praliand (Editors) Catalysis by Zeolites, Elsevier, Amsterdam, 1980, pp. 85-92. N.Y. Chenand W.J. Reagan, J. Catal., 59 (1979) 123. G. Nicolis and I. Prigogine, Self-Organization in Nonequilibrium Systems, Wiley-Interscience, New York, 1977. C.D. Chang, C.T.-W. Chu and R.F. Socha, J. Catal., 86 (1984) 289. M.M. Wu and W.W. Kaeding, J. Catal., 88 (1984) 478. H.-J. Doelle, J. Heering, L. Riekert and L. Marosi, J. Catal., 71 (1981) 27. L. Riekert, Appl. Catal., 15 (1985) 89.
139 Printed in The Netherlands
Formation of Ethene and Propene from Methanol on Zeolite ZSM-5 I. Investigation of Rate and Selectivity in a Batch Reactor D. PRINZ and L. RIEKERT* Znstitut fiir Chemische Verfahrenstechnik der Universittit Karlsruhe, D-7500 Karlsruhe (F.R.G.) (Received 8 May 1987, accepted 7 October 1987)
ABSTRACT The catalytic conversion of methanol to hydrocarbons on zeolite ZSM-5 was investigated in the range 550 to 625 K in a well mixed batch reactor system, which permits observation of product composition as a function of conversion accurately for isothermal reaction conditions. Interconversion of methanol and dimethyl ether rapidly attains equilibrium. Conversion to hydrocarbons is preceeded by an induction period. This induction period and the selectivity for formation of ethene and propene increase with the Si: Al ratio in the zeolite and decrease with crystal size. Ethene and propene appear simultaneously as gaseous products if water vapour is present.
INTRODUCTION
The catalytic conversion of methanol to hydrocarbons in the C.&I!,, range by zeolites of the pentasil-structure was announced by the Mobil Research and Development Corp. in 1976 [ l] and has since been investigated in many respects. The abundant literature has been reviewed in detail by Chang [ 21; acid catalysis by pentasil-zeolites in general has been treated in perspective by Haag [ 31. Light olefins appear to be key intermediates in the methanol to gasoline process and they are potentially valuable products in their own right. The present communication is concerned with the kinetics of methanol conversion to hydrocarbons and with the selectivity of this reaction for the formation of ethene and propene. The experimental investigation was carried out in a well mixed batch reactor, wherein the mass of zeolite catalyst is m, and the mass of starting material A initially (at t = 0) is ( mA) o. The entire system is kept at a temperature well above the condensation point of reactants or any product, and the temperature
0166-9834/88/$03.50
0 1988 Elsevier Science Publishers B.V.
140
of the catalyst is controlled within + 1 K, so that reaction takes place under strictly isothermal conditions. At different times t, samples of the reacting gas are withdrawn and analyzed by gas chromatography (GC) , so that the composition of the gas in contact with the catalyst can be established as a function of normalized time t = t m,/ ( mA) o. It follows from elementary chemical reactor theory that this normalized time r is equivalent to the modified residence time m,/ ( tiA) o in an ideal plug flow reactor. The same compositions of the reacting gas (reaction path) will be observed in both systems as a function of conversion of reactants, normalized time z or modified residence time, respectively, provided that feed composition, pressure and catalyst temperature are identical and that the properties of the catalyst are constant in time. Observation of the reaction path in a well-mixed batch reactor offers several advantages compared to experiments with an integral fixed bed reactor: catalyst temperature can be controlled precisely and thus isothermal operation secured; the reaction path for a wide range of conversion can be observed in one continuous experiment and it is possible to obtain a “slow motion picture” easily by using a smaller amount of catalyst under otherwise identical conditions. It must be noted, however, that in the batch reactor a fresh or regenerated catalyst initially interacts with the reactants, whereas with an integral fixed-bed reactor observations are, in general, made after a nearly steady state of the system has been reached. The properties of the catalyst may then be different from those of a fresh catalyst as charged to the reactor. EXPERIMENTAL
Zeolites
All zeolites used in this study were hydrogen forms belonging to the pentasil family, with the ZSM-5 structure predominating. Crystallinity was 98-100% as shown by X-ray diffraction (XRD ) . Templates used in the synthesis, composition and crystal sizes of the materials used are shown in Table 1. The designations given in the first column are used in this article; references to the literature, where the method of preparation and XRD data for the same preparations have already been communicated, are listed in the last column. Sample d was supplied by Mobil in the H-form. Zeolites a-c where obtained in the H-form through calcination (at 650°C in air) of crystals, synthesized by W. Holderich (BASF) from an alkali-free gel. Zeolites e, f and g were ionexchanged six times with 1 M ammonium nitrate solution at 8O”C, then calcined at 500°C in air for 6 h. Residual sodium content was less than 0.05 wt%. The zeolites were pressed into pellets without any binder, the pellets were crushed and a particle-size fraction of 0.5-l mm was obtained by sieving.
141 TABLE t Zeoiites Designation
: : ; g
--
Template used in synthesis
SiO,: Al&
Crystal size in pm
Ref.
NH,(CH,),NH, NH,(CH,),NH, NH,(CH,),NH, TPAX TPABr TPABr TPABr
34 48 45 100 72 220 660
%2 0.5 2X2x6 0.5iO.l 6.Oil 7.0+1 8.011
4 5
6 I
Catalytic reaction The batch reactor system is shown schematically in Fig. 1. A gas volume of 7 1 (0.007 m3) is circulated in a loop through a magnetically driven turbine of 0.21 m diameter at a rate of 1.8 1 s-l. This loop and the turbine (the mixer) are made of stainless steel and are heated to 433 K to prevent condensation of reactants or products. Total pressure in the system was always near atmospheric, with nitrogen as the diluent of reactants and products. A gas stream of
Ni
Nz , MEOH
. AIR I
1
in
OJt
t REACTOR1wP
MIX!W
TURBINE
Fig. 1. Well-mixed batch reactor system, schematic.
142
200 ml s-l flows from the mixer through the quartz reactor, containing the catalyst heated to temperature T, and back to the mixer. This flux is driven by a membrane pump which is also heated to 423 K, as are the connecting lines. The reactor loop and the mixer can be separated, so that both parts can be flushed, filled or evacuated independently. Gas samples can be withdrawn from the mixer through a heated gas sampling valve and are analyzed chromatograpically (two l/8” columns, 4 m Chromsorb 104 and 3 m Porapak N, 80-120’ C, flame ionization detector). A detailed description of the apparatus has been given elsewhere [ 71. The condition of perfect “mixedness” of the gas phase was considered to be sufficiently approximated if relative differences in mole fractions &/xi were kept below 3%. For a flow-rate of 200 ml s-l through the reactor loop this requirement will be fulfilled for any species whose observed rate of change of concentration in the system is characterized by a relaxation time of 20 min or more. The time constant of mixing in the large volume of the turbine mixer was less than 10 s. Before each experiment the zeolite catalyst was calcined in air at 850 K for 30 min, then brought to reaction temperature T under nitrogen in the reactor. All results were thus obtained for zeolite catalysts which are free from sorbed hydrocarbons or carbonaceous deposits at the beginning of an experiment. The initial composition of the reacting gas phase was prepared in the 7-l volume of the mixer and checked by GC while the reactor loop was separated from the mixer. At time t=O the reactor loop was then connected to the mixer. Two time scales are used in the following. Clock time t, counted from the beginning of an experiment, and normalized time r r=t(m,,,+lm;“g
(rnDM-&)()
(1)
where ( mM) o = mass of methanol ( M) charge at t= 0; ( mDME) o = mass of dimethyl ether ( DME ) charge at t= 0; m, = mass of zeolite catalyst. The factor 1.39 converts the mass of DME to the equivalent mass of methanol; the normalized time r corresponds to the mass-specific modified space time in an ideal plug flow reactor with methanol as feed. Gas-phase composition is described by carbon fractions yi Yi =
amount of carbon in compound i total amount of carbon in the system
(2)
The sum of carbon fractions of oxygenates, methanol (M) and dimethyl ether ( DME ) , is designated yA YA=YM+YDME
Conversion X means conversion of oxygenates to hydrocarbons
(3)
143
915
OJ
0.2 W
PL
05
O$ -
07 T/h
+
T/h
Fig. 2. Composition in batch reactor as a function of time t (upper scale) and reduced time z (lower scale) for two catalyst temperatures in terms of carbon fractions y,. ( PCHBOH) o= 95mbar; catalyst, 100 mg zeolite c. Left hand ordinate: ( 0) methanol; (0 ) dimethyl ether. Right hand ordinate: ( l ) ethene; (+ ) propene; (A ) propane.
x=1-y,
(4)
The integral selectivity, Si, of the conversion of oxygenates to hydrocarbons with respect to hydrocarbon i
will be a function of conversion X, if the product hydrocarbons undergo secondary reactions [ 201. The differential or point selectivity dSi ds, = 1
_dy”=s_+xds, dyz+ L
CI‘X
(6)
can be obtained from the integral selectivity and its variation with conversion X. The sum of the selectivities for the light olefins ethene and propene is designated S= in the following: S= = SC2H4+SCRH6 The relative amount of ethene in the light olefins (ethene +propene) as EO (ethene in olefins) in the figures EO=
is shown
YC2H4 YCzH4
+YChH6
Fig. 2 shows the change of composition of the gas phase in contact with 0.1
144
g of zeolite c as a function of clock time t and of reduced time r, for catalyst temperatures of 275’ C and 305 “C, respectively, as an example of raw results. Conversion X as function of time, as well as the reaction path (product distribution as function of conversion), were obtained from these figures, which are reproduced extensively elsewhere [ 71. RESULTS
Interconversion of methanol and dimethyl ether The relaxation time for establishment 2CH,OH=
of the equilibrium
(CHs)zO+HzO
(7)
in the system was in most experiments less than 10 min. The kinetics of the interconversion of methanol and dimethyl ether is thus influenced by the rate of gas circulation and it was therefore not investigated. Fig. 3A shows the reaction path in the quasi-ternary system methanol-dimethyl ether -hydrocarbons, starting methanol. Fig. 3B shows reaction paths starting from dimethyl ether with and without addition of water vapour. The equilibrium 7 is established for methanol as starting material when the conversion X to hydrocarbons is still below 5%; afterwards the ratio of methanol to dimethyl ether follows the equilibrium line as conversion X= y~cH2) increases. ycn30u remains nearly constant for X< 0.6 because additional water vapour from the dehydration of oxygenates to hydrocarbons shifts equilibrium 7 to the left. Equilibrium lines in Fig. 3 were calculated with an equilibrium constant K= 10.2 at T= 578 K
----r-X
Fig.‘3. Reaction paths in the ternary system CH,OH- (CH,) 20- (CH,); (CH,) designating the sum of hydrocarbons. (A) T=578 K, zeolite a; @M)O = 95 mbar. (B) T=578 K, zeolite a; (Pn&O=46mbar;o= (PH~O)~/(PDME ) O. ( 0 ) Observed compositions; (0 ,o ) compositions calculated under the assumption that equilibrium 7 is established at 578 K.
145
100
50
0
1
, 0.5
lb1 j X
1
150
t/min
200
O.11 , 0.5
1
(C)
X
1
Fig. 4. Influence of catalyst temperature. (a) Conversion Xversus t and 7. (b ) Integral cumulative se1ectivit.y of formation S_ of ethene +propene as a function of conversion X. (c) Distribution of light olefins (EO : ethene in ethene + propene) as a function of conversion; (0 ) T= 548 K; (+ ) T=578K; (0) T=623 K.
for reaction 7, which was obtained from available thermodynamic data [ 8,9]. The calculated ratio of y bME: yM at equilibrium should be reliable within - 2 % . The equilibrium line is also approached but not passed over if the reaction starts from dimethyl ether; y(cH3J20 then always exceeds the value which has to be expected if dimethyl ether is in equilibrium with methanol and water. Comparison of the reaction paths in Fig. 3A and B seems to indicate that the dehydration reaction leading to hydrocarbons preferentially removes methanol from the gas phase. Conversion of methanol and dimethyl ether to hydrocarbons A mixture of methanol and dimethyl ether which is in or near equilibrium according to eqn. 7 has to be considered as the reactant generating hydrocarbons. Conversion and selectivities as defined in eqns. 4-6 are therefore based on the sum of oxygenates (methanol plus dimethyl ether) converted to hydro-
146
0
X
Fig. 5. Influence of
X
water vapour added to methanol as starting material. T= 578 K; zeolite c. (pM),,=%i mbar, (PH20~O0, (pMvl)O=95mbar, (~~~0)0=228mbar.
carbons. Consideration pene as products.
+,
of selectivities shall be restricted to ethene and pro-
Influence of temperature and reactant composition Fig. 4 shows conversion X as a function of clock time (upper abscissa) and normalized time (lower abscissa) for three catalyst temperatures on zeolite c, starting from methanol. Conversion versus time is S-shaped with an initial induction period, which decreases with increasing temperature. Product distribution is not very strongly affected by catalyst temperature within the interval investigated here, as can be seen from Fig. 4b and c. The induction period is increased and the rate of conversion observed thereafter is decreased by addition of water vapour to the gas phase, as shown in Fig. 5. Addition of water vapour increases the selectivity for the formation of light olefins at low conversion and it also leads to an increase in the ratio ethene/propene in the products. When dimethyl ether is used as reactant then the partial pressure of water in the system will be as low as possible [ PHzO= ( PDME)OX], vanishing at X= 0. As shown in Fig. 6 no induction period was observed with dimethyl ether as reactant. Selectivity for (ethene + propene) is about the same as for methanol as reactant; however, only propene and no
147
X
0
0.4
0.2
01
1
T/h
0.6
0.6 0.5-
Q ,~+-o+--ol-cm-~~_+ + +
EO "0, 0.2 (b)
0'
0.5
X
1
Fig. 6. Methanol versus dimethyl ether as starting material. T = 578 K, zeolite a. + , mhar; 0, (pDM~),,=47mbar.
Fig.
7. Product
+,
(PM)"=40
(PM) o= 95
distribution for two initial pressures of methanol as reactant. T=578 K, zeolite C. mhar; 0, (pILIjO=285mbar.
ethene is produced from dimethyl ether as reactant at very low conversion, when pHzO is zero. Selectivity of formation of light olefins and olefin distribution for two initial pressures of methanol is shown in Fig. 7. Increasing (pM) o leads to an increase of pH20 in the system due to the equilibrium 7 and also to an increase in the length of the induction period. No induction period preceding the onset of conversion to hydrocarbons was observed if a small amount of propene was initially added to the reacting methanol (Fig. 8).
148
Fig. 8. Effect of addition of propene on conversion of methanol versus time. T= 578 K, zeolite.a, (pMj0=95 mbar. C+) no C,H, added; (0) (pC:IHsio=0.93 mbar.
0.6 EO
Fig. 9. Effect of Si: AI ratio in the zeolite at T= 578 K, (pM),=95 mbar. ( + ) Zeolite e (Si: Al=36); (0) zeolite f (Si:Al=llO); (0) zeolite g (Si:A1=330).
Influence of zeolite composition (Si:Al ratio) The effect of zeolite composition on the rate of conversion and product distribution was studied on zeolites e-g, which were prepared with TPABr as a
149
100
200
Vmin
Fig. 10. Effect of crystal size for zeolites with Si:A1=22f Zeolite c (2 X 2 x 6 ,um) ; (0 ) zeolite b (diameter 0.5 pm).
template.
1. T=578
K, (pM)0=95 mbar. (+)
They exhibit the same crystal morphology, are roughly of the same crystal size and differ only in the Si: Al ratio. Fig. 9a shows conversion of methanol versus normalized time at 578 K, Fig. 9b and c selectivity, yield and CZ/C3distribution for ethene and propene, The aluminium content affects the activity in two ways: it influences the induction period and (for Si:Al< 100) the rate of reaction thereafter. Increasing Si: Al from 110 to 330 causes a substantial increase in the induction period, which appears to be roughly proportional to the Si: Al ratio; otherwise the curves X vs. z remain essentially parallel if Si : Al is increased from 110 to 330. Selectivity and yield of formation of ethene and propene as well as the ratio of propene: ethene increase at low conversion (Xc 0.6) with decreasing aluminium content of the zeolite. A maximum of the yield of light olefins at high conversion appears to occur at intermediate Si : Al ratios around 100.
150
Influence of crystal size The effect of crystal size was studied on zeolites b and c which had been synthesized with the same template (1,3 diaminopropane, PDA) and have nearly the same composition. They differ substantially with respect to crystal size. Sorptive properties and micrographs of these crystals have already been communicated together with observations on their activity in the cracking of hexane [ 51. Fig. 10 shows conversion versus time for these zeolites under otherwise identical conditions. The induction period is substantially longer for the smaller crystals; the rate of conversion at X= 0.5 (where it reaches a maximum in both cases) for the larger crystals is 1.9 times the value for the smaller crystals. At low conversion (Xc 0.7) selectivity for light olefins is higher for the smaller crystals, where also relatively more propene is generated than on the larger crystals. The same observations were also made with two samples synthesized with tetrapropylammonium (TPA) as template, having roughly the same composition but different crystal size. DISCUSSION
The Thiele number characterizing interaction of intercrystalline mass transfer in the zeolite pellets and reaction of oxygenates to hydrocarbons was smaller than 0.8 throughout. The results concerning the conversion of oxygenates to hydrocarbons therefore are not influenced by external mass transfer and reflect the kinetics of intracrystalline processes. It is not possible, however, to infer any firm conclusion concerning intracrystalline concentration gradient from the results, since neither the nature of the molecular species prevailing in the crystals nor their coefficients of diffusion under reaction conditions are known. Only the composition of the gas phase in contact with the zeolite catalyst and its change with time, which means the fluxes of different compounds entering or leaving the crystals, has been observed. Therefore we do not intend to enter what has been called by Chang [ 21 the “mechanistic sweepstakes”. The results presented here concern reaction on zeolite catalysts which contain initially no sorbed hydrocarbons or other carbonaceous material and are thus in a virgin state. Two conclusions can be drawn from these observations which may be of general interest. (i) The onset of conversion of oxygenates to hydrocarbons is always preceded by an induction period, which varies in length with temperature, p&o, Si : Al ratio in the zeolite and crystal size. This phenomenon indicates that the hydrogen-form of ZSM-5 in itself is not sufficient as a catalyst for the conversion of methanol to hydrocarbons. It is catalytically active only after some entity, which is an intermediate in the sequence of reactions leading to gaseous hydrocarbons, has been generated in the zeolite. Several authors [ lo-121 have concluded that C-C bonds are formed by homologation of C3+ -olefins or ole-
151
finic residues in the zeolite with methanol. Van den Berg et al. [ 131 observed that C,-C, olefins oligomerize in H-ZSM 5 at temperatures between 300 and 600 K and that at temperatures above 500 K a dynamic situation prevails, where different olefins coexist. The same authors have also shown that the presence of water has an inhibiting effect on the oligomerization of light olefins in ZSM-5 [ 141. The induction period observed here in a closed system and its dependence on temperature,zeolite composition and &no can thus be understood to result from the formation of a system of reactive hydrocarbons in the zeolite, which are necessary as intermediates for the catalytic reaction. (ii) Crystal size does have a pronounced influence on the rate of conversion of methanol-dimethyl ether to hydrocarbons and on product distribution. For larger crystals the catalytic activity is higher and the induction period is shorter than for smaller ones. This phenomenon can be understood if it is assumed that the concentration and/or effectiveness of the oligomer which sustains the catalytic conversion of oxygenates decreases towards the surface of the zeolite crystals, where it continuously depolymerizes into smaller molecules which evaporate into the gas phase. The catalytic activity in the crystal would thus increase towards the centre and the mass specific catalytic activity would then be higher for larger crystals. The S-shaped curves of conversion X vs. r observed on zeolites with different ratios Si : Al > 100 under otherwise identical conditions (Fig. 9) appear to be parallel but displaced with respect to each other along the r-axis. In the interval 0.1
(8) An initial increase of dX/dt with X (“autocatalysis”) has also been found in an integral reactor by Chen and Reagan [ 151. Together these observations indicate that the rate of formation and the properties of the dynamic system which serves as intermediate in the conversion of oxygenates to hydrocarbons depend on zeolite and gas-phase composition, crystal size and time. It will therefore be difficult if not impossible to describe the kinetics in the conventional way by a system of rate equations with constant coefficients. A compilation of rates of conversion of oxygenates, observed on different zeolites at X=0.5, are given in Table 2 in the form of K=--
1 -dnA 1 m, dt CA
(9)
where nA=non$on+2n(cn_&o; cA=nA/V= (cA),)‘O.5 (gas-phase concentration of oxygenates) ; 172,= mass of zeolite. It is obvious from the data in Table 2 that crystal size as well as Si : Al ratio has an influence on the observed activity, besides temperature. It appears remarkable, however, that ZSM-5 zeolites from different sources and prepared
152 TABLE 2 Rates of conversion of oxygenates, characterized by first-order coefficients as defined by eqn. 9 Si:Al
Zeolite
Crystal size in pm
22
C
6x2x2
b d z
22 50 110 36
0.5 0.5 621 721
g a
330 17
8Ifil 1
PO
T
K
mbar
K
cm3 gg’ s-’
95 95 95 40 95 95 95
548 578 623 578 578 578 578 578 578 578 578
5.7 33 140 35 20 17 27 9.0 7.8 16 16
CH,OH CH,OH CH,OH CH,OH CH,OH CH,OH CH,OH CH,OH 95 CH,OH 95 CH,OH 47 (‘X,),0
by different methods agree rather well with respect to catalytic activity in the conversion of methanol to hydrocarbons. This observation is not a general experience in heterogeneous catalysis and one can speculate that here it is due to the self-organization of the catalytic system, which has to be considered as a dissipative structure as understood by Nicolis and Prigogine [ 161. The selectivity of formation of the light olefins ethene and propene is generally assumed to depend on the relative degrees of advancement of formation of light olefins from oxygenates methanol and dimethyl ether and their consecutive conversion to paraffins and aromatics [ 21. The results obtained here indicate that not only reactions 1 and 2 but also reaction 3 in the triangular scheme CZHA .G% 1
2
/ [2
CHxOH
+
\
(CH3)>0] 3
-
other hydrocarbons
occurs in the zeolite crystals as seen from the gas phase, the selectivity SCc2=+tc3=)at low conversion X being smaller than unity in most cases. Ethene and propene from the gas phase are converted through reaction 2 to other hydrocarbons and therefore reaction 3 must also be expected to occur in the scheme describing the gas-phase kinetics. Ethene and propene generated in the crystal will be subject to some degree to further conversion before they reach the gas phase, the net result being that reaction 3 will be observed in the material balance based on gas phase composition. This effect will be the more pronounced the longer the diffusion path in the crystal, which explains the lower selectivity for ethene and propene observed with large crystals, compared to smaller ones of the same composition (Fig. 10). The rate of reaction 2 and
153
A CH30H+(CH,),0
x
,.'
c othe: hydrocarbons
Fig. 11. Triangular representation of composition in carbon fractions nol+dimethyl ether; (B) C,H, +C,
(y) for (A) methaT=578 K, (phi +2
therefore also that of reaction 3 appear to be relatively more sensitive to an increase in acid-site concentration (lower Si : Al ratio) than reaction 1, as has already been observed by Chang et al. [ 171. Selectivity for light olefins is therefore enhanced by a high Si: Al-ratio, as has been shown previously by Wu and Kaeding [ 181, and also by small crystallite size. On all catalysts the ethene : propene ratio in the products increases with conversion, propene being much more reactive in reaction 2 than ethene, as has been shown previously [ 10, 13, 191. Only when pure dimethyl ether was used as the starting material was the ratio of ethene: propene zero in the products at the beginning of conversion. When methanol was present as reactant, ethene and propene were always found simultaneously in the gaseous products, even at very low conversion. All compositions observed with different zeolite samples as a function of conversion at T= 578 K and (pM + 2 PDME)O= 95 mbar without addition of water fall into a restricted region of composition space, which is indicated in Fig. 11 by the hatched area. In this diagram, lines of constant integral selectivity SCn=+C:l=are straight lines from the lower left corner of the triangular simplex. At full conversion (X= 1) yield and selectivity of ethene + propene will not exceed 30%. On the other hand selectivities for ethene+propene between 50 and 80% appear to be possible at incomplete conversion (Xc 0.7) with a suitable zeolite as catalyst. A process aiming at the production of ethene and propene from methanol should therefore operate at intermediate degrees of conversion in a fixed bed reactor and with recycling of unconverted reactants.
154
A pilot plant study of such a process is described in the following communication. The data presented here can only indicate in an approximate way suitable catalysts and operating conditions for a continuous fixed bed operation. The observation of induction periods shows clearly that here the catalytic properties of the zeolite depend on time. Therefore the normalized time ‘s will not be equivalent to modified space time in a fixed bed operating at steady state. ACKNOWLEDGEMENTS
We thank Ms. A. Weisz for skillful, reliable help in tedious experimentation and Dr. W. Holderich and Dr. V.S. Nayak for providing zeolite samples. This work was supported by the Bundesminister fur Forschung und Technologie. We also thank the Fonds der Chemischen Industrie and the Max-BuchnerForschungsstiftung for financial assistance.
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