Bošáček, V.

Isakov, Â. I.

Isakova, T. A.

Jelínková, J.

Minahev, Khabib Minachevič

English

Bošáček V.

Jelínková J.

Isakov Â. I.

Isakova T. A.

Minahev Khabib Minachevič

University of Szeged

CATALYTIC PROPERTIES OF STABILIZED ZEOLITES Y IN THE REACTIONS OF DIMETHYL ETHER AND OF THE TOLUENE DISPROPORTIONATION V. BOSACEKa, J. JELfNKOV£a, YA.I. ISAKOVb, T.A. ISAKOVAb, K.M. MINA-CHEVb The J. Heyrovsk^ Institute of Physical Chemistry and Electrochemistry, 121 38 Prague 2, Czechoslovakia (a) The Zelinsky Institute of Organic Chemistry, Academy of Sciences, Moscow, USSR (b) ABSTRACT The catalytic properties of zeolites Y, stabilized under self-steaming conditions, were studied in the reactions of the dimethyl ether decomposition -to hydrocarbons and of the toluene disproportiona-2+ tion. It has been found that Cu cations introduced into the lattice of a stabilized, partially decationized zeolite enhance the zeolite 2+ activity in both the reactions. The favourable effect of the Cu ca-tions is mainly due to their easy reducibility leading to additional acidity. The Bronsted acidity form substantially increases the cata-lyst activity, while the Lewis form decreases the activity. INTRODUCTION Stabilized forms of decationized zeolites of the Y type exhibit some properties that differ from those of nonstabilized decationized zeolites /1-5/. The main advantages of stabilized samples involve the material stability in the processes of activation and regeneration at high temperatures. An interesting, property, which is important for catalysis, is the localization of exchangeable cations in the large cavities in the crystal structure of these zeolites /4,5/. The possibilities in the use of these zeolites as catalysts in various processes have only rarely been described. In the present paper, the catalytic conversion of dimethyl ether and the catalytic disproportionation of toluene were studied on H- and Cu-forms of stabilized zeolites Y. This study was stimula-ted by the fact that Cun+ cations exert positive effects on the acti-vity of zeolitic catalysts in many reactions involving carbonium ions /6,7/. 309 EXPERIMENTAL A series of decationized zeolites NH^Na-Y with varying degree of the Na+ exchange for NH* (15-80%) was studiëd. The materials were subjected to a stabilizing process in the form of powders, in a rota-ry quartz furnace. The ammonium zeolite form was decomposed in a clo-sed system with a water vapour pressure of ca. 1.8 kPa (13 torr) , by increasing the temperature up to 570°C at a rate of 4 degrees per minute. The H,Na-Y zeolite thus obtained and stabilized under hydro-2+ thermal conditions was further converted into the Cu form in a CuSO^ solution. The samples were washed, dried and their crystal-Unity was checked by argon adsorption, IR spectra and x-ray diffrac-tion. The state of the Cu cations in the zeolite was followed using the IR spectra of adsorbed CO and by photoelectron spectroscopy. The IR spectra were measured on a Nicolet MX-1 interferometer, using 2 sample pellets 10-15 mg/cm thick, in the transparent arrangement. The main characteristics of the samples are given in Table 1. Tab. 1: Characteristics of zeolites Y Zeolite Chemical ccmpsition of original zeolite mole Sorption capacity ,Ar itmol/g IP Symbol Na20 nh4 2o cuo ai2o3 Si02 Unstabi-lized in vac. 350 ° Stabilized 1 NaY 0,94 - 1 5,4 10,4 -2 NH4Y-15 0,78 0,14 1 5,9 10,6 9,9 3 NH4Y-37 0,61 0,35 1 5,8 11,1 10,1 4 'NH4Y-53 0,42 0,55 1 4,6 11,6 9,5 5 NH4Y-62 0,31 0,61 1 4,3 11,3 10,7 6 NH.Y-77 1/ 7 Cu,HY-St 0,18 0,19 0,75 _2/ 0,11 1 4,3 4,5 11,1 10,5 9,1 8 CUjHY-St1/ 0,1« _3/ 0,17 1 4,6 - 9,1 1/ .Sample prepared from the stabilized sample No 5 2/ 0,35 H20 structural OH groups 3/ 0,49 H20 dtto The catalytic tests were carried out in flow-through apparatus at atmospheric pressure. Dimethyl ether was injected in a mixture with nitrogen. The products of its conversion were analyzed chromato-graphically on a column packed with Carbopack-C (Supelco) with 0.02% 310 picric acid /8/. Chromosorb P with 10% polydiethyleneglycol succinate 78°, helium was used in the analysis of the products of toluene disproportionation. The composition of the isomers of aromatic hydro-carbons was determined on a chromatographic column packed with Benton 245 with a paraffin oil /9/. The catalysts were activated for 8 hours in a stream of dry ni-trogen at 350°C prior to the reaction of dimethyl ether (dme). Before the disproportionation reaction of toluene, the samples were activa-ted for 5 hours at 500 °C in the air stream. The toluene dispropor-tionation was studied without the use of a carrier gas. RESULTS AND DISCUSSION 1. Conversion of Dimethyl Ether into Hydrocarbons The dimethyl ether conversion was studied at 300°C and WHSV = 1.2 h-"*" on a number of NH^,Na-Y zeolites with various NH^ con-tents, in nonstabilized form (the ammonium form was decomposed at 350°C in a stream of dry nitrogen) and in stabilized form (decomposi-tion under hydrothermal conditions, 570°c).The DME conversion' yielded ca. 45-55% of the C^ - C^ aliphatic hydrocarbons; the main reaction products included ethylene and propylene, and saturated hydrocarbons, isobutane, isopentane and n-butane. The time course of the reaction and the composition of the products were analogous to those corre-sponding to the methanol decomposition on HY zeolites /10/. The cata-lyst activity is strongly affected by the presence of Bronsted acidic sites. These sites are more acidic in nonstabilized H,Na-Y zeolites than in stabilized zeolites /2,3,11/. In agreement with these facts, a lower DME conversion was found on stabilized zeolites see Fig. 1 . For example, a H,Na-Y St catalyst with a 15% decationization is inac-tive at 300°C, whereas its nonstabilized form with the same degree of decationization exhibits a 50% conversion of DME into hydrocarbons. 2+ The positive effect of Cu cations introduced by ion exchange into the stabilized form, H,Na-Y St, with a 70% decationization, is ma:.i-fisted by a substantial increase in the catalyst activity: the conver-sion reaches 90% and the selectivity remains unchanged /12/. It should be pointed out that H,Na-Y St zeolites lose their acti-vity much faster than nonstabilized zeolites or stabilized zeolites containing copper cations, apparently because of rapid coking. As all stabilized zeolites contain extra-lattice, nontetrahedrally bonded aluminium that was released from the lattice T-positions during the stabilizing process /13,14/, it can be assumed that this form of alu-minium is responsible for the intense formation of polyaromatics and Fig. 1: Dependence of the conversion K of DME 30 min on the decatio-nization degree at 300°C O . . . nonstabilized Y zeolites activated at 350°C nonstabilized Y zeolites dehvdroxylated at 500°C • ... stabilized Y •... Cu, H-Y St,(sample 7) coke and that it hastens the loss af activity of these catalysts /15/. The presence of these A1 species is probably not the only cause of zeolite deactivation, but it is known that these forms of aluminium, which have the cl^racter of a "Lewis acid, strongly catalyze the oligo-marization of ethylene, propylene and other unsaturated compounds /2, 3/. It cannost be excluded that the favourable effect of copper cati-ons is related not only to the formation of additional Bronsted acidi-but also to the displacement of %H> 100 2+ + ty after the reduction of Cu to Cu , a certain small fraction of readily exchangeable cationic forms of aluminium Al^+, AlO+ that also exhibit the highest Lewis acidity /16/. These readily exchangeable Al species may affect the reaction in the large cavities of a faujasite, while a substantial part of nontetra-hedrally bonded aluminium remains in the lattice apparently in the so-dalite units or in the form of associates eith a low Lewis electron-acceptor strength /16.17/. 2. Toluene disproportionation A decationized, stabilized H,Na-Y St zeolite and a Cu,H-Y St zeolite prepared from the former were studied in this reaction at various temperatures see Table 2 and with various methods of activa-tion Table 3. A certain period, during which the activity increased, was observed with both the catalysts. The time after which a maximum toluene conversion and the maximum degree of disproportionation were attained the pertinent values are given in Tables 2 and 3 depended on the zeolite composition and the reaction temperature. This time was usually 2 to 4 hours and decreased at higher reaction temperatu-res. This phenomenon is apparently connected with the kinetic proper-ties of the process, adsorption-desorption factors and with the effect of the reaction mixture on the formation of catalytically active sites. Toluene demethylation also occured to a very small extent under the reaction conditions and was observable at higher reaction tempe-ratures. Tab. 2: Results of toluene disproportionation on stabilized Y zeolites LHSV =1,2 h"1 Conver- Dispro-Q sion of portio-T, C toluene nation % degree % Products w.% fron the injected toluene Composition of the CqH. fraction % Composition of ethyl-toluenes % Benzene xylenes ethyl̂  toluenes trime-thylben- o-xy- m-xy- p-xy- ethyl- o-zene lene lene lene benzene m-Zeolite H,Na-Y 62 St 350 1,15 0,5 0,28 0,17 0,02 0,05 15 34 40 11 - - -400 7,1 5,85 2,6 3,06 0,17 0,07 16 48 35 1 11 62 27 450 24,4 20,6 10,4 10,6 0,22 0,74 21 51 26,8 1,2 14 59 27 Zeolite Cu,H-Y 62 St (sample 8) 350 15,0 13,2 5,76 6,81 0,16 0,44 19,0 52 29 traces 24 41 35 400 25,0 21,0 . 11,8 10,6 0,15 0,96 20 52,5 27 , 0,5 15 60 25 450 20,0 12,4 9,4 6,5 0,1 0,4 19 52 29 - 5 55 40 Tab. 3: Influence et the treatment on the catalytic properties of Cu,H-Y St /8/ zeolite in the reaction of toluene disproportionation Temperature of reaction t = 350°C LHSV s 1,2 h_1 Product in w. % from the Composition et the Composition of injected toluene C3H10 fraction % ethyltoluenes Sanp- Conditions of Toluene Dispro- — le the sample oonver- portio- ben- xyle- ethyl- trime- o-xy- m-xy- p-xy- ethyl- o- m- p-No. treatment sion nation zene nes tolue- thyl- lene lene lene benze-% degree nes benze- ne % nes 1 Air, 500°C, 5h 15,0 13,2 6,73 6,81 0,16 0,44 19 52 29 traces 24 41 35 2 Air, 500°C, 5h and CO, 400°C 5h 11,1 10,1 5,01 5,35 0,12 0,22 17 49 34 - 15 50 35 3 Treatment of sample No 2 after experiment Air, 500°C, 5h 10,8 10,2 4,9 5,45 0,14 0,18 17 52 31 - 13 55 35 4 Air, 500°C, 5h and H^ 400°C, 5h 25,0 23,5 10,0 10,9 0,83 1,18 18 50,6 27 4,4 14 58 28 5 Treatment of the sample No 4 Air, 500°C, 5h 16,6 14,2 7,0 7,25 0,2 0,5 18 53 28,5 0,5 20 56 24 The fact that ethyltoluenes were formed with both catalysts in addition to sylenes, trimethylbenzenes and benzene is worth attention. This indicates the formation of carbene-type intermediates in the sys-tem, whose interaction with the CH3 groups from xylenes produces ethyltoluenes. The possibility of the formation of ethylene by recom-bination of carbenes followed by alkylation of toluene cannot be ex-cluded, although it seems that this recombination is less probable than the direct interaction /18/. These considerations are in agree-ment with the fact that the CL - C4 hydrocarbons were found dissolved in the liquid reaction product. After treatment of the Cu,H-Y St zeolite with carbon monoxide 400°C, 5 h the zeolite activity decreased by 25%, but the original value was not attained on reoxidation. On the other hand, the catalyst reduction by hydrogen 400°C, 5 h increased the conversion by 70%. The conversion attained the original value on reoxidation. It seems that copper cations have a direct effect on the formation of the zeo-lite active sites in this reaction. As can be seen from the above results, the Cu,H-Y St zeolite is more active in the two reactions than the corresponding decationized stabilized zeolite. The increase in the activity is apparently caused by the formation of additional protonic acidity due to the reduction 2+ + of Cu cations to Cu by hydrogen that is often present in trace amounts in the reaction products. In this way OH groups are formed in the zeolite structure, whose protonic acidity is higher than that of the OH groups in the stabilized H,Na-Y St zeolite from which the Cu,H-Y St zeolite was prepared. The relatively small amount of protons formed in the Cu,N-Y St zeolite can considerably increase the catalyst activity, if the steep dependence of the (cH3)20 conversion on the degree of decationization Fig. 1. is taken into consideration. The infrared spectra of a reduced zeolite given in Fig. 2 confirm certain changes in the spectrum of OH groups after treatment with hy-drogen at 400°C. It can be seen that a higher reduction temperature leads to partial dehydfOXylation of the sample, which is apparently also the reason of a mild decrease in the toluene conversion at a tem-perature of 450°C see Table 2, Cu,H-Y St zeolite . It can also be seen from the spectra of adsorbed carbon monoxide given in Fig. 3 that an intense band appears at 2140 cm-1 after the reduction with hydrogen; the band is characteristic of the Cu+... CO complex. On the other harid, a zeolite treated with oxygen exhibits an intense band at 2160 cm-1. Easy reducibility self-reducibility of copper ions is well known and has also been confirmed in our zeolites by photoelectronic spectra. 315 Fig. 2. Infrared spectra of Cu,H-YSt sample 8 in OH stretching region: 1.. Activated in vacuo 350 , 8h 2.. Reduced with H2, 3 50° 8h, vac.lh 3.. Reduced with H2, 600° 8h, vac. 350°, lh 2200 2000 2200 2000 v cm1 V tin1 Fig. 3. Infrared spectra of adsorbed carbon monoxide on Cu,H-YSt (sample 8). A. Sample treated with oxygen, pQ = 2 Torr, 45C°C, 8h in vacuo at 350° lh - adsorption of CO, p C Q = 1 Torr. B. Sample treated with hvdrogen at 450°C, 4h, p„ =250 Torr Hi CO adsorption: 1.. p = 0,1 Torr, 2.. p = t Torr -2 u -4 CO desorption: 3.. p = 4.10 ' Torr, 4.. p = 10 Torr. 316 2+ + It should be pointed out that Cu was reduced to Cu very readily in the stabilized forms; at higher temperatures e.g. during the activa-tion in vacuo at 350°C or at higher temperatures the presence of Cu2+ could not be detected using shake-up satellite lines /20/. During the reduction with hydrogen, only the Cu 2—Cu + reduction was observed, but not the reduction to metallic copper. This reduction process was monitored by photoelectron spectroscopy and by the IR spectra of adsor-bed carbon monoxide at 25 °C; absorption bands at 2140, 2160 and, at a CO pressure above 25 Torr (3 kPa), also a band at 2180 cm 1 were only found. These bands correspond to valence vibrations of CO in the co-ordination sphere of copper cations. No absorption band characteristic of CO adsorbed on copper metal Cu° /21/ or of Cu carbonyls /22/ and located at lower wavenumbers was found with the studied zeolites. The positive effect of Cun+ cations need not be caused only by the formation of additional protonic acidity during the reduction, but may also be connected with coordination unsaturation of these cations. These coordination-unsaturated sites can ensure, together with the existence of protons in the lattice, the optimal configuration of the active sites on which active complexes can be formed that take part in the alkylation of the aromatic ring, isomerization and disproportio-nation /23/. REFERENCES 1. Scherzer J.; Catalytic Materials - Relationship Between Structure and Reactivity ACS 248, 157 /1984/ 2. TvaruSkovd Z., Patzelovd V., Bosdfiek V., React. Kinet. Catal. Lett. 6, /4/, 433 /1977/ 3. 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Catalytic properties of stabilized zeolites Y in the reactions of dimethyl ether and of the toluene disproportinaion

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Catalytic properties of stabilized zeolites Y in the reactions of dimethyl ether and of the toluene disproportinaion

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