Drewinski M.

Dzwigaj S.

Haber J.

Ritter G.

English

University of Szeged

STUDIES ON THE INITIAL STAGES OF TRANSFORMATION OF C 1»C 4 ALCOHOLS ON ZSM-5 ZEOLITES M. DEREWINSKI®, S. DZWIGAJ3, 3. HABER3 , G. RITTERb Institute of Catalysis and Surface chemistry.Polish Academy of Scien-ces .Krakow,Poland (a) Institut of Physical Chemistry, Technical University, Vienna, Austria ( b ) ABSTRACT Catalytic activity of ZSM-5 zeolite poisoned by adsorption of pyridine was studied in order to elucidate the role of C 2 and C^ olefins in the transformation of methanol. Consecutive steps in the sequence : alcohol-olefins-oligomers-cyclopolyenes-arom8tics/paraffins are catalysed by acid centers of increasing strength, but in case of methanol the initiation of the chain by the formation of the first C-C bond from C^ species requires the presence of strongest acid centers. Ethylene is the first product in Cj conversion and its appearance creates a new facile route consisting in methylation and bypassing the formation of C-C bond. Propylene and higher olefins are methylated more readily than ethylene and are responsible for the increasing rate of methanol consumption, but a parallel route to develop the reaction network is operating through oligomerization of propylene. INTRODUCTION It is well known that high silica type ZSM-5 zeolites show high catalytic activity in the conversion of alcohols and olefins [ 1 - 4 ] . Reactions of different alcohols in the presence of hydrogen form of ZSM zeolites result in the formation of the whole spectrum of hydro-carbon products containing olefins,cycloolefins, paraffins and aromatics. It is the characteristic feature of these catalysts that the composition of the products obtained is practically independent of the type of alcohols used what hints to the common reaction path of the transformations of all alcohols. The main difficulty in the identification of the elementary steps and in the construction of the reaction network is due to the very high rate of these transformation in the presence of the very active hydrogen form of the zeolite. It seems thus of interest to reduce the activity of this catalyst by poisoning the active centers of acid character with appropriate bases adsorbed before the catalytic tests in order to study the mechanism of the raactions of C^-C^ alcohols and to determine the role of C 2 and C2 olefins in the transformation of alcohols, in particular of methanol, into higher hydrocarbons. EXPERIMENTAL Zeolite z-79 of the ZSM-5 type was synthesized according to the patent description [ 5 ] and its structure was identified as that of ZSM-5 by X-ray analysis. The hydrogen form was obtained by three fold ion exchange in 0.1 M NH^Cl solution at 363 K and subsequent heating in the stream of nitrogen at 823 K for 5 hr. Preparations with reduced acidity were obtained by adsorption of pyridine, which was introduced into the carrier gas passing through the reactor with catalyst bed heated to the reaction temperature of 673 K until pyridine appeared in the gas at the outlet. Catalytic activity in the reaction of methanol ( MeOH ), ethanol ( EtOH ), isopropanol (iso-PrOH) and butanol ( n-BuOH ) was determined in the pulse microreactor 8 mm in diameter, filled with 0.28 g samples of the catalyst and kept at reaction temperature of 673 K. The flow rate of nitrogen as the carrier gas was 30 ml/min at the atmospheric pressure. Products were analysed using the on-line gas chromatograph with flame ionization detector and a 4 m column filled with Chromosorb w ( 60/80 mesh ) and silicon oil DC 550 with 5 % addition of stearic acid as the active phase. Reagents were of analytical grade and were used without further purification. Pyridine (Fluka) used for poisoning of the cetalyst was of spectroscopic grade. In the Temperature Programmed Desorption ( t p d ) experiments the sample of 100 mg of the zeolite was first activated at 823 K for 3 hr at the pressure of 5.10~3 Torr (6.63*10"1-N/m2). After cooling to room temperature 15 yl of alcohol was introduced end its adsorption carried out for 15 min. The sample was then outgassed for 1 hr and tpd experiment was carried out in the temperature renge 303-923 K at the heating rate of 10 K/min. Desorbing products were analysed with a mass-spectrometer. RESULTS AND DISCUSSION Our earlier studies showed [ 6 ] that adsorption of pyridine at CH30H/ZSM-5-Pyridine o methanol Fig. 1. Product distribution as function of number of pulses of MeOH introduced on ZSM-5 type zeolite poisoned by pyridine. the surface of ZSM-5 zeolite results in drastic changes of the product distribution of the reaction of alcohols. Fig. 1 shows the changes of the composition of products obtained when pulses of MeOH were consecutively introduced on the initialy poisoned zeolite. In the first pulses dimethylether (DME)is observed as the only product. Only after 10 pulses of MeOH traces of higher hydrocarbons appear. As the first products small amounts of C 2 and C 3 olefins are observed after 8 pulses, when more important amounts of propylene are formed immedietely the whole spectrum of both paraffins and aromatics appears in the products. This is accompanied by the rapid decrease of the amount of unreacted MeOH and DME. When reaction of EtOH was carried out in identical conditions, 100 % conversion to ethylene was observed already in first pulses, ethylene remaining as the only product through the whole experiment. Zeolite, which after partial removal of pyridine by consecutive pulses of the alcohol due to competitive adsorption of the reaction products and eluating effect of the carrier gas at high temperature developed the activity in the transformation of MeOH hot only to DME but also hydrocarbons, remained inactive for the transformation of ethylene. Apparently it still leeks acid centers strong enough to convert ethylene into higher hydrocarbons. Their first trace amounts appeared only in the conditions in which methanol was already transformed into higher hydrocarbons to a considerable degree. After 20 pulses the conversion of ethylene was only 5 % the main product(2 %) being propylene. Fig. 2 illustrates the results obtained when pulses of iso-PrOH were injected into the reactor with poisoned zeolite. At variance with the beheviour of C^ and C2 alcohols already after introduction of the first pulses of iso-PrOH not only propylene as the direct product of the dehydrstion of this alcohol on weak acid centres, but also the whole spectrum of hydrocarbons including paraffins and aromatics was observed, the conversion of iso-PrOH being 100 %. At the begining mainly propylene,products of its oligomerization and of the oligomer cracking are formed as Indicated by high content of C^-Cg olefins. It may be mentioned that studies of the adsorption of propylene on HY zeolite [ 7 ] revealed the oligomerization of 4 molecules of propylene, and measurements of the adsorption of butene also on HY zeolite [ 8 ] indicated the formation of the oligomer from 3 molecules of butene. Further removal of adsorbed pyridine by pulses of iso-PrOH results in gradual increase of the amount of products of cyclization Fig. 2. product distribution as function of number of pulses of iso-PrOH introduced on ZSM-5 type zeolite poisoned by pyridine. and hydride transfer i.e. paraffins and aromatics. It is noteworthy that in the esse of iso-PrOH all these products appear in conditions in which in the case of MeOH and EtOH only products of their dehydra-tion to DME and ethylene respectively are observed, but no compounds which require the formation of C-C bonds between Cj or C 2 species. Results obtained with n-BuOH were similar to those with iso-PrOH. Comparison of the behaviour of MeOH and iso-PrOH clearly indicates that formation of the first C-C bond from C^ species, necessary for further transformations and development of the whole reaction network, requires the presence of acid centers of the catalyst stronger than those, which are involved in the development of this network from propylene. It also shows that transformations of higher alcohols such as iso-PrOH or n-BuOH consist of a series of consecutive steps, catalysed by acid centers of increasing strength: very weak a o a weuk acid i centers centers alcohol o olefins — — o l i g o m e r s strong acid / \ strong acid st iung centers / \ centers acid y centers r ^ aromatics cyclopolyenes paraffins It can be namely concluded from the results presented in Fig. 2 that oligomerization and cracking, which are the first step in the series, require the presence of weaker acid centers and are therefore regenerated in the first instance on pyridine desorption. Further removal of pyridine incovers stronger acid centers of the catalyst responsible for cyclization and hydrogen transfer. The condition necessary to start the reaction chain in the case of methanol is the removal of pyridine from strongest acid centers required for the formation of the first C-C bond. Appearance of even small amount of such centers makes possible the formation of first portion of ethylene. Once they have been formed they undergo methyla-tion by MeOH, which as shown before [6] proceeds with the participa-tion of much weaker acid centers, from which pyridine has already been desorbed earlier. The absence of the products of dimerization or trimerization of ethylene at the early stages with the simultaneous presence of propylene indicates that no oligomerization of C 2H 4 takes 539-place, but only its methylation with MeOH. It may be concluded, that once small amounts of ethylene have been formed in the reaction of MeOH, the initial step consisting in the formation of C-C bond from C^ species becomes bypassed by the much easier step of the methylation of the resulting carbon chain. It is obvious that also other products of the reaction sequence, higher olefins, cycloolefins, aromatics may undergo methylation by MeOH. Thus, in the case of methanol the reaction network may be represented by a following scheme: very weak acid MeOH very strong acid centers DME tsmsiL^ ethylene weak acid centers j weak acid centers methylation aromatics strong acid centers methylation cyclopolyenes propylene strong acid centers weak acid centers V oligomers paraffins . strong aad cenfers In the absence of MeOH, ethylene remains unreacted and therefore is the only product of the reaction of EtOH on zeolite catalysts which have not available very strong acid centers. The low reactivity of ethyl alcohol and ethylene was also postulated by other euthors[3 ,4,9] We have noticed that besides the intramolecular dehydration of EtOH also its intermolecular dehydration to diethylether (DEE) is possible in the first stages of the reaction. This is illustrated by Fig. 3, which shows the results of an TPD experiment after adsorption of EtOH on hydrogen form of the ZSM-5 zeolite, which has been previously activated at 823 K in vacuum of (6.65.10"1 N/m2 ) 5.10"^ torr. The values in brackets by the TPD curves of different m/e fragments indicate the sensitivity range used by registration. The curve m/e=45 corresponding to the fragment CH-jCHeOH with maximum at 395 K represents the desorption of EtOH as confirmed by the appearance of a similar pe8k on curve m/e=3l (ch2=0H), m/e=29 ( HC=0 ) and m/e=46 (CH3CH2OH). The curve m/e=28 shows two maxima. The first, at 395 K is apparently related to the fragmentation of 539-Temperature , K of EtOH on may be assigned presence of olefin e.g. the desorption of ethylene at this temperature. The curve illustrating the formation of a fragment with m/e»59 registered at the sensitivity 21 times high-er than that of ethylene indicates the presence of diethylether (m/e=59 corresponds to the fragment CH2=0-CH2CH3). The characteristic fragment m/e=3l, which usualy serves for identification of DEE is in our experiment screened by fragments originating from EtOH. Results of TPD experiment clearly indicate that on the inter-action of EtOH with ZSM-5 zeolite not only its intramolecular but also intermolecular dehydration with the formation of DEE takes place. The studies of catalytic properties of the series of boron and aluminium phosphate of different composition [ 10 ] have shown that in the composition range, in which BrSnsted acidity is present, only dehydration of EtOH to ethylene takes place, whereas in the composi-tion range, in which pairs of Lewis acid-base centers are generated, ethanol is dehydrated mainly to DEE. It is possible that preactivation of the zeolite sample before the TPD run in comparitively severe conditions resulted in generation of some Lewis acidity. In normal conditions this reaction path is marginal in comparison with intra-Fig. 3. TPD spectrum obtained after adsorption ZSM-5 zeolite. desorbed alcohol, whereas the second maximum at 509 K to the desorption of ethylene (CH2=CH2 f ragment). The other peak characteristic for the mass spectrum of c~ + + + e/m=27 { c h = c h 2 )m/e=25 ( c h e c), m/e=l4 ( c h 2 ), confirms 541-CH30H/ZSM-5— NH3 molecular dehydration. In order to confirm the role of ethylene as the intermediate ivhich enables the bypassing of the first most difficult step in the transformation of MeOH-the formation of the first C-C bond,a series of expe-riments was carried out in which the state of the reaction was registered on the zeolite poisoned with ammonia (Fig. 4). The first product appearing besides the DME at the begining of the reaction is ethylene. The presence of ethylene does not yet lead to the appearance of higher hydrocarbons. The next rapidly growing product is propylene. As already mentioned the absence of C 4 and Cg hydrocarbons as products of ethylene oligomerization and appearance of propylene indicates that it is methylation of the first portion of ethylene which is responsible for initiation of the reaction sequence. The appearance of propylene results in rapid acceleration of the reaction and increase of the conversion of methanol which is consumed mainly through the methylation of the growing hydrocarbon chains. Simultaneously, as may be concluded from the results obtained with iso-PrOH, propylene alone reacts rapidly, developing through oligomerization and cracking the whole reaction network. This contributes to further consumption of MeOH and results in its total conversion. On studing the influence of various alcohols Fig. 4. Results of gas-chromatorgaphic analysis after five consecutive pulses of MeOH introduced on 2SM-5 type zeolite poisoned with ammonia. 542-and olefins added in small amounts to MeOH on its conversion at the surface of the zeolite catalyst poisoned by pyridine [ 11 ] it was found that indeed propylene much easier undergoes methylation than ethylene and therefore accelerates the transformation of MeOH to a much greater extent. In later stages of the reaction ethylene is formed not only by direct formation of C-C bond from c^ species.which is a difficult step and requires the presence od strong acid centers, but also as the result of cracking of larger olefins. Temperature , K Fig. 5. TPD spectrum obtained after adsorption of MeOH on ZSM-5 type zeolite. Additional information on the role of lower olefins in the transformation of MeOH was obtained from TPD experiments. Fig. 5 illustrates the TPD curves registered after adsorption of methanol on active HZSM-5. The curve m/eo29 illustrates the desorption of the unreacted MeOH, curves m/e=18 and m/e=17 - desorp-tion of water. Maximum at 481 K is related to the dehydration of methanol. The curve m/e«45 represents the formation of DME (maximum desorption temperature 523 K ] and its disappearance on further heating. The magnified part of the temperature range 643 K - 823 K (shown in the insert) illustrates the desorption of ethylene m/eo27 (CH2»CH), m/e-26 ( CH5CH) and propylene m/eo4l (CH3-CH=CH) It may be seen that methylation of ethylene is not difficult. On the other hand the very rapid disappearance of propylene at higher ter.peratures with simultaneous accumulation of ethylene shows that propylene reacts further more rapidly than ethylene, through methylstion and oligoraerization. Apparently, the competition from higher olefins and aromatics for rcethylation results in the accumulation of less reactive ethylene. It is noteworthy that some amount of methane is also formed (curve representing the fragment with rn/e=16). REFERENCES 1. Chang, C.D., Silvestri, A.3. . 3. Catal. , 47, 249 (1977). 2. Derouane, E.G., Nagy, 3.B. , Dejaifve, P., van Hooff, 3.H.C. , Spekman, B.P. , Vedrine, 3.C. , Naccache, C. , 3. Catal., j53, 40 (1978). 3. Anderson, 3.R. , Foger, K., Mole, T., Rajadhyaksha, R.A., Sanders 3.V. , 3. Catal., 58, 114 (l979). 4. Dejaifve, P., Vedrine, 3.C. , Bolis, V., Derouane, E.G., 0. Catal. 63, 331 (1980 ). 5. Argauer, R.3. , Landolt, G.R., US Patent 3 702 886 (1972 ) Mobil Oil Corp. 6. Derewinski, M., Dzwigaj, S., Haber, 3., Zeolites, 4, 214 (1984). 7. Haber, 3. , Komorek, 3. , Romotowski, T., prepared for Inter. Symp, on Zeolite Catalysis, Siofok, May 1985. 8. Datka, 3., 3. Chem. Soc. Faraday I, 76, 2437 (1980). 9. Ahn, B.3., Armando, 3., Perst, G., Guisnet, M., C.R. Acad. Sci. Paris 288c, 245 (1979). 10. Haber, 3., Szybalska, U., Disc. Faraday Soc., 72, 263 (l98l). 11. Dzwigaj, S., Haber, 3., Derewinski, M. , in press. 544-

Studies on the initial stages of transformation of C1-C4 alcohols on ZSM-5 zeolites

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Studies on the initial stages of transformation of C1-C4 alcohols on ZSM-5 zeolites

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