Active centres of decationated zeolites in oxidative transformation of methanol

Petrol. Chem. U.S.S.R. Vol. 25, No. 3, pp. 160-165, 1985 Printed in Poland

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ACTIVE CENTRES OF DECATIONATED ZEOLITES IN OXIDATIVE TRANSFORMATION OF METHANOL* G. P. TSINTSKALADZE,A. R. NEFEDOVA,Z. V. GRYAZNOVA,O. V. TSITSISHVILI and N. G. GIGOLASHVILI Moscow State University

(Received 23 April 1983) CATALYTIC properties of zeolites for oxidation with molecular oxygen are linked with the catalytic action of transition metal cations, or with acidic centres formed when these cations are introduced into, the zeolite lattice [1, 2]. Oxidative dehydrogenation of methanol on natural zeolites has been examined in recent years. The use of nickel, cobalt, copper and manganese forms of clinoptilolite in this process showed the important role of the form of the cation [3]. Changing the reaction product ratio, the temperature range of the reaction and activation energies of the process for various cationic forms enabled us to conclude that cations have a decisive role in the activity and selectivity of clinoptilolites in oxidative dehydrogenation of methanol. No ester-formation occurred on the cation forms of clinoptilolite, although the X type nickel and cobalt zeolites actively supported this process [4]. Special structural features of X type zeolite cannot be an explanation, since dimethyl ester is not formed on the copper form of this zeolite, but instead formaldehyde (yield 80.5 ~) with 90 ~ selectivity. The dependence of specific catalytic activity on the degree of exchange for a two-charge ion was shown to pass through a maximum. Study of methanol transformation on nickel, cobalt and L type copper zeolites and mordenite [5] showed that mordenites (except for copper) are more active than L type zeolites. The specific activity of the initial form of NaM exceeds by a factor of three, the activity of the initial form of KL, no dimethyl ether forming on KL. The introduction of cobalt ions increases the activity of both types of zeolites and the ability to form esters appaers in CoL. The formation of ester and formaldehyde on zeolites alone, takes place step-by-step, but in parallel on others, ester and formaldehyde yields showing an inverse dependence in this case, thus many parameters probably influence this process. However the form and amount of cation in the zeolite appears to have a decisive effect on the activity for oxidative dehydrogenation of methanol. The structure of cr3;stalline lattice and the modulus of zeolite are less important. Zeolites are known [6] to contain acid centres of varying strength, both aprotic (triplercoordinated aluminiura atoms, polycharged and single-charged cations) and protonic (hydroxyl groups combined with oxygen atoms of the lattice in different crystallographic positions - O1, Om and localized in the field ofpolycharged cations). * Ne,ftekhimiya25, No. 4, 530--535, 1985. 160

Active centres of decationated zeolites

161

To explain the role of protonic ceutres on oxidative transformation of methanol, the hydrogen forms of high-silica synthetic zeolites - L type erionite and natural zeolites.- erionite, chabazite, mordenite and clinoptilolite are examined here. The character of the adsorption centres of these zeolites, was explored using IR spectroscopy to examine the region for bond-stretching and deformation vibrations of H20 molecules. EXPERIMENTAL

Catalytic activity was examined under continuous conditions. The catalyst sample (0.2 g; grain size 0.25 to 0-5 mm) previously treated at 400°C in dry and purified air flow for 4 hr was placed between two layers of crushed quartz. Reagents were fed in the form of a steam-air mixture. Reaction products were sampled by a valve type metering device and analysed chromatographically. A column (2 m), containing porapak N, was used to analyse CH20, H20, (CI-I3)20, CH3OI.I and CO +CO2 and a column (3 m) containing SKT carbon, was used to analyse separately CO and CO2 contents. Analysis at 120°C, carrier-gas (helium) at 40 ml/min. IR spectra, from a UR-20 spectrophot0meter, sample vacuum treated (2 hr) at given temperature. The chemical compositions of initial and hydrogen forms of zeolites, which had been~ obtained from ammonium forms, are given below. Natural erionite (Enat). Initial 0"28 K20'0"33 Na20.0"46 CaO. 0.07 MGO.0-09 F%Oa "A1203-6.64 SiO2.1.9 H20. H-form 0.16 K20"0'16 Na20.0.17 CAO.13.05 F%O3"A1203"7"1 SiOz.4"3 H20. Chabazite (Ch). Initial 0.09 K20.0-43 Na20.0-42 CAO.0.13 MgO.0.1 Fe203" •A1203 •5.48 SiO2" 5.35 H20. H-form 0.08 K20.0-07 NazO .0.21 CaO .0"03 Fe203 "AlzOa .6.73 SiOz .4.26 H20. Clinoptilolite (Cl). initial 0.14 K20.0.42 NazO .0.55 CaO .0.29 MgO .0"08 FezO3•A1203.8.27 SiO2" 5.3 H20. H-form 0.25 K20.0.16 NaaO-0.45 CAO.0.33 MgO.0.01 F%O3-AI2Oa'10"49 SiOz'4-23 H20. Mordenite (M). Initial 0.13 K20'0"44 Na20"0.81 CAO.0.22 MgO.0.27 F%O3" AlzO3 • 11.9 SiO2" 4.89 HzO. H-form 0.24 Na20" 0.39 CaO. 0.18 MgO. 0.09 FezO 3"A1203 • 12.4 SiO2" 3.8 HzO. Synthetic erionite (E~). Initial 0.3 K20"0"56 Na20"A1203" 6.65 SIO2"5"4 I-I20. H-form 0.15 K20.0.12 Na20 "A1203.6.79 SiOz" 6"23 H20. L type zeolite (L). Initial 0.4 K20"0"l Na20 "A12Oa"5.5 SIO2"4-4 I-I20.] H-form 0.26 K20" 0.05 Na20 "Al~O3" 6.8 SiO2" 5.03 HzO. RESULTS

The IR spectra of zeolite catalysts provided some information concerning the nature of active centres involved in methanol conversion. As the vacuum treatment temperature was gradually increased, IR inspection

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of the ammonium forms of zeolites indicated the following mechanisms: at 20°-300°C bands are observed at t440 and 3230 cm- 1,. which correspond to ammonium ions, the intensity at these frequencies decreases as temperature increases. These bands practically disappear at 400°C, evidence apparently for the complete desorption of ammonia, the OH band group intensifies sharply at the same time, reaching maximum intensity at the temperature used to vacuum-treat each sample. In each case, (except for L type zeolite), IR bands characterizing an OH group formation show well

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FIG. 1. IR spectra of hydrogen forms of zeolites, vacuum-treated, 2 hr at 300 ° (a) and 400°C (b). i--Es; 2-Enat; 3 - C h ; 4--M; 5-C1; 6 - L . FIG. 2. Dependence on temperature of reaction product yield; hydrogen form of natural chabazite: 1-(CH3)zO, 2 - C H 2 0 ; 3 - C 0 ~ ; 4 - C 0 .

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defined maxima around 36!5 cm -1 and suggest indeticat proton acid centres in the high-silica zeotites examined (Fig. t). As the temperature is increased further dehydroxylation becomes pronounced and at 550°C the band examined is of very low intensity. Dehydroxylation of the hydrogen form of erionite begins somewhat sooner than it does for mordenite or clinoptilolite. A possible explanation is offered by a more open crystalline structure of erionite than for example, mordenite. wf.%

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A. somewhat different pattern is observed in IR spectra of the hydrogen form of L type zeolite. In this case vacuum treatment at 350°C results in the formation of a comparatively weak band at 3635 cm-1, due probably to the low ability of.potassium cations to exchange with other cations in this zeolite. Particular properties of synthetic and natural erionites may be of interest. Chemical analysis and IR sectroscopy confirm the loss of aluminium from the zeolite as transition to the H form is effected. As the aluminium content of aluminosilicates is reduced a shift towards low-frequency is experienced in the band of antisymmetric bond-stretching vibrations of Si-O-Si(A1) with a maximum in the range 10001100 cm- 1 [7]. During processing, when a chabazite zeolite reaches 600°C the maximum of this band in the spectrum is displaced, (compared with the spectrum of the original chabazite) from 1042 to 1061 cm-1. Similarly in the spectrum of C1 it is shifted from 1062 to 1069, in E - from 1065 to 1075 and in M - from 1069 to 1075 cm - I . Spectrometric data confirm the transition of aluminium atoms from their tetrahedron locations in the crystalline lattice of zeolite, to cationic positions. Transformation of alcohol in the presence of 02, on all the high-silica zeolites examined, takes place with the formation of dimethyl ether, formaldehyde, CO and CO2. The alcohol is dehydrated on all samples at temperatures lower than would be expected for oxidative dehydrogenation (Fig. 2). The yield curves for CH20 and (CHa)20 pass through a maximum as~ the temperature is increased.

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In relation to the temperature displacement range of these reactions, the samples examined may be placed in the following order: H - E s < H - En,t < H - C1 ~H - Enat> H - Ch~> H - CI> H - L > H - M .

The concentration of OH groups formed after heat treatment at 300°C calculated bearing in mind the "thickness" of samples, mg/cm 2 [8] decreases in practically the same order H-Es >H-En,t>H-

Ch > H - C I > H - M > H - L.

The catalytic activity for (CH3)20 formation, in the samples examined can thus be related to the relative concentration of OH groups characterizing their Br6nsted acidity. Some deviation may be due to structural features of the zeolites studied, since their catalytic activity may be determined by the presence and concentration of certain active centres and particularly by the availability of these centres. This may well explain precisely why the highest activity ~br (CH3)20 formation is to be found in the hydrogen form of erionite. Mordenite showed the lowest activity for this process, of all the samples examined. This behaviour may be explained by the partial blocking [9] of the main structural channels of mordenite, by cations localized at sites which prevent the diffusion of adsorbed molecules into large channels. The highest activity for formaldehyde formation was shown by the chabazite and L type zeolite samples, the C H 2 0 yield reaches 3 4 ~ with a ratio of 02 : : CHaOH = 3 : 1 with these zeolites. These samples are characterized by about the same ratio of SiO2/A1203 (minimum among the zeolites studied; i.e. maximum A1 content). The IR data also confirm maximum loss of aluminium during heat treatment of the samples and therefore, maximum aluminium cation formation is observed in chabazite. Zeolites are also observed to have a minimum number of OH groups, at the very temperatures where formaldehyde formation is maximized. Because of differences in structural types of zeolites studied, a clear dependence of the oxidative dehydrogenation Of methanol on the extent of dealumination could not be demonstrated. However, a clear correlation has been observed [10] between clinoptilolite activity in this process, and the extent of dealumination. A study of the effect of oxygen level in the reaction mixture, on the activity of samples examined, showed that a variation in 02 concentration changes the ratio of alcohol transformation and also displaces the temperature ranges of these reactions. Thus, on L type zeolite, chabazite and natural erionite, an increase of 02 content in tile mixture above a ratio of three, reduces formaldehyde yield. This may be due to blocked aluminium atoms resulting from the increased adsorption of oxy-

Active centres of decationated zeolites

165

gen. Increasing the contact time of methanol with the catalyst reduces the role of oxidative dehydrogenation and dehydration, whilst increasing the role of alcohol oxidation to CO and CO2. The catalytic activity of zeolites studied in methanol transformation, can be related by I R spectroscopy to certain catalytically active centres which control esterformation and the oxidative dehydrogenation of methanol. A correlation is observed between the relative number of struct'~wal OH-groups and dimethyl ether yield, thus Br6nsted acidic centres stimulate de1-~ydration of alcohol. During oxidative dehydrogenation of methanoi polychargcd cationa are of considerable significance. These cations may be formed in decationized zeolites during acid and heat treatment as skeletal aluminium is eliminated from the zeo!i:e lattice (dealumination) and during ion exchange with sodium cations. SUMMARY

1. Oxidation of methanol on hydrogen forms of synthetic zeolites, type L, and erionite and natural zeolites-erionite, chabazite, mordenite and clinoptilolite has been examined. Methanol is converted at various active centres to dimethyl ether, formaldehyde and carbonmonoxides. 2. Consideration of catalytic activity and I R spectra indicate that dehydration takes place mainly at Br6nsted acidic centres, whereas polycharged cations are mainly significant for oxidative dehydrogenation. REFERENCES

1. S. Z. ROGINSKII, O. V. AL'TSHULER, O. N. VINOGRADOVA, V. A. SELEZNEV and N. A. TSITOVSKAYA, Dokl. AN SSSR 196, 4, 872-875, 1971 2. V. F. ANUFRIYENKO, N. G. MAKSIMOV, V. G. SHINKARENKO, A. A. DAVYDOV, Yu. A. LOKHOV, N. N. BOBROV and K. G. IONE, In: Primeneniye tzeolitov v katalize (Use of Zeolites in Catalysis), pp. 133-154, Nauka, Novosibirsk, 1977 3. Z. V. GRYAZNOVA, G. V. TSITSISHVILI, Sh. I. SIDAMONIDZE, Z. M. KORIDZE and L. G. AKHALBEDASH¥ILI, In: Prirodnyye Tseolity (Natural Zeolites), pp. 203-208, Metsniyereba, Tbilisi, 1979 4. Z. V. GRYAZNOVA and L. G. ALKHALBEDASHVILI, Neftekhimiya 19, i, 101-106, 1979 5. L. G. AKHALBEDASHVILI, G. V. TSITSISHVILI, Z. V, GRYAZNOVA, Sh. I. SIDAMONIDZE and B. G. CHANKVETADZE, Soobshch. AN GSSR 94, 361-364, 1979 6. D. RABO, Khimiya tzeolitov i kataliz na tseolitakh (Zeolite Chemistry and Catalysis on Zeolites), 1, p. 489, Mir, Moscow, 1980 7. G. V. TSITSISHVILI, G. P. TSINKALADZE and M. K. CHARKVIANI, Dokl. AN SSSR 273, 6, 934-939, 1983 8. M. K. CHARKVIANI, G. V. TSITSISHVILI and G. P. TSINTSKALADZE, In: Adsorbenty, ikh polucheniye, svoistva i primeneniye, pp. 1t9-122, Nauka, Leningrad, 1978 9. G. V. TSITSISHVILI, Adsorbtsionnyye khromatograficheskiye i spektral'nyye svoistva vysokokremnistykh molekularnykh sit, p. 48, Metsniereba, Tbilisi, 1979 10. G. V. TSITSISHVILI, G. P. TSINTSKALADZE, M. K. CHARKVIANt, A. R. NEFEDOVA and Z. V. GRYAZNOVA, Dokl. na II Vses. konf. ,Issledovaniye ,i primeneniye tzeolitov

v narodnom khozaistve, Tbilisi, 1981