Dubru, M. L.

Jacobs, P. A.

Lerot, L.

Poncelet, G.

Somme, G.

Tielen, M.

Uytterhoeven, Jan-Baptist

English

Poncelet G.

Dubru M. L.

Somme G.

Lerot L.

Jacobs P. A.

Tielen M.

Uytterhoeven Jan-Baptist

University of Szeged

DECOMPOSITION OF ALCOHOLS OVER DIFFERENT FORMS OF GERMANIUM SUBSTITUTED FAUJASITE AND PHILLIPSITE By G. PONCELET, M. L. DUBRU and G. SOMME Groupe de Physico-Chimie Minérale et de Catalyse, Université Catholique de Louvain, Louvain-la-Neuve (Belgium) L. LEROT Solvay Research Center, Brussels (Belgium) P. A. JACOBS, M. TIELEN and J. B. UYTTERHOEVEN Centrum Oppervlaktescheikunde en Colloïdale Scheikunde, K. U. Leuven, Heverlee (Belgium) (Received 11th February, 1978) Several cation forms of germanium substituted X zeolite and phillipsite are tested for their dehydration/dehydrogenation properties of alcohols. Both types of sites are present and act inde-pendently, excluding E,cB type mechanism. No secondary reactions occur in striking contrast to their silicium homologues. Isotope effects measured on different samples indicate the mechanism is in between Ei and E2, depending on the zeolite and the cation. Upon ion exchange with polyvalent cations, the same behavior is observed as for other zeolites and other proton catalysed reactions. Dehydrogenation seems to occur at exposed surface germanium atoms. Very weak acid sites related to the presence of residual surface carbonate are shown to dehydrate only tertiary alcohols. Introduction The catalytic decomposition of alcohols is strongly dependent upon the alcohol structure but also on the nature of the catalyst and, therefore, may provide informa-tion on the general way a catalyst functions in elimination and even other reactions. Practically, the use of alcohols as source of fuels and basic chemicals nowaday's is being considered very seriously. The dehydration/dehydrogenation capability of silicic zeolites was investigated intensively and the knowledge was reviewed several times [1—3]. The dehydration reaction was found to be catalysed by very weak acid sites [1, 2, 4] and is therefore a measure of the overall Bronsted acidity, although for polyvalent ion exchanged zeolites the direct influence of cations cannot be excluded experimentally [3, 5]. The dehydrogenation reaction of alcohols was related to. the presence of cations, even at the impurity level [6]. It is the aim of the present work to test these ideas about reaction mechanism and active sites on the germanium homologues of faujasite and phillipsite zeolites. 275 / 2 7 4 G. PONCELET ET AL. Experimental The synthesis and physico-chemical characterization of germanium substituted faujasite and phillipsite were described in detail [7—10]. The samples are denoted as follows. The first capital indicates the type zeolite (X, Y or P, phillipsite). This is followed by the indication whether the zeolite is silicic (Si) or germanic (Ge). Then is given the parent cation (Na or K) followed by the exchanged cation and its degree of exchange in %. The parent zeolites have the following anhydrous unit cell com-position : XGeNa: Na96Al96Ge960384 PGeK: K16Al16Ge16Oe4 XSiNa: Na85Al85Si107O384 YSiNa: Na56Al56Si1360384 Other cations were exchanged into these zeolites, using diluted solutions and low solid to liquid ratios [4, 10]. The reactants were of analytical grade and deuterated alcohols were prepared according to a procedure found in literature [11]. Decomposition of alcohols was studied either in a conventional recirculation reactor or in a continuous flow reactor operating in the differential mode. Analysis of the products was done in situ using gas chromatography. In situ infrared spectra were recorded on a BECKMAN 1R—12 double beam grating spectrometer. Results Reaction selectivity for isopropanol decomposition Under the experimental conditions, the main reaction products from wopropanol were acetone and propylene. At the relatively high reaction temperatures (>200° C), only traces of di-i'jopropylether were found. They will be neglected further on. In Table I, it is shown that neither the reaction time in the recirculation reactor, the alcohol pressure, nor the reaction temperature have a pronounced influence on the reaction selectivity. This is confirmed in the flow reactor for a wide range of contact times (0.5—3.5 s). The data therefore indicate that the sites responsible for dehydra-tion and dehydrogenation act independently. Fig. .1 shows the evolution of the amount of secondary products formed at increasing degree of zsopropanol conversion in comparable conditions and for several catalysts. In the silicic faujasites (X and Y) they are of major importance, while on the germanic X and P zeolites, they are found in negligible amounts. This allows to state that in a wide range of reaction conditions, the germanium zeolites aré highly selective towards primary products. Dehydrogenation is favored on PGe zeolite while the XGe zeolite has more pronounced dehydration power (see Table I). Influence of the degree of cation exchange Fig. 2 shows the relative rates of wopropanol dehydration and dehydrogenation for XGe in which Ca2+ was exchanged for Na + , Ni2+ for Na + , and Ñi2+ for Ca2+ . In each case, there is a more than proportional increase of the dehydration activity DECOMPOSITION O F ALCOHOLS 2 7 5 Table I Influence of reactant pressure and reaction temperature on selectivity of isopropanol decomposition Catalyst Reaction p Selectivity11' in % for S<2> temp. °C k N m - 2 propylene acetone XGeNaNi (60) 250 3.733 x 76.5 23.5 2.3 280 3.733 76.9 23.1 2.5 300 3.733 77.0 23.0 1.8 300 1.333 78.9 21.1 1.2 XGeCaNi (60) 230 3.733 . 88.4 11.6 1.1 270 3.733 88.3 ' 11.7 0.5 300 3.733 89.3 10.7 1.2 300 1.333 91.3 8.7 2.2 350 3.733 85.1 14.9 2.4 PGeKNi (60) 230 3.733 4.9 95.1 0.8 300 1.333 11.4 88.6 1.1 300 3.600 10.6 89.4 1.3 (1> Calculated on the basis of primary products. <2> Standard deviation of selectivity during reaction. 50 3 T3 O 2.5 re TJ c o Ü l/> (CL^" p ® 1 / i \ 50 25 O \50 100 Isopropanol conversion / % Fig. 1. Amount of secondary products formed against the degree of isopropanol conversion over: a) PGeNaNi (46) at 300° C;- b) XGeCaNi (56) at 350° C; c) YSiNaCa (67) at 300° C; d) XSiNaCa (50) at 300° C at exchange levels exceeding 50%. A very similar relation has been observed for the silicic analogues X and Y, and also for many other reactions (for a review, see ref. [2]). In the case of nickel exchanged zeolites, the behavior during dehydrogenation is similar, indicating the participation of nickel. For the XGeNaCa catalysts, dehydro-genating sites with different nature.are present. 18* 2 7 6 G. PONCELET ET AL. Fig. 2. Influence of the degree of cation ex-change of XGeNa (A, B) or XGeCa (C) zeo-lites on the relative rate of propylene (P) and acetone (A) formation from isopropariol Fig. 3. Initial rate of alcohol dehydration (r) on Ca-exchanged XGeNa; a) (CH3)3COH at 200° C; b) (CH3)2CHOH at 300° C; c) CH3CH„OH at 300° C Influence of the nature of the alcohol The initial rate of alcohol dehydration over XGeNaCa catalysts is given in Fig. 3. The activity decreases in the following sequence: tert. butanols>wopropanol» methanol. This general order of reactivity parallels the generally observed order of stability of the corresponding carbonium ions [13]: tertiary »secondary »primary. Surprisingly, the parent sodium form of XGe is more active than the Ca exchan-ged forms in the case of dehydration of the tertiary alcohol. In the case of /sopropanol dehydration, the Na form is only slightly more active than the samples with low Ca content. This effect does no longer exist for the primary alcohol. This indicates that weak acid sites exist on XGeNa, which still catalyze the dehydration of tertiary and to a minor extent of secondary alcohols. Fig. 4 shows that these sites exist on the T i m e / n Fig. 4. Dehydration of tertiary-butanol on sodium X zeolites: XSiNa at 200° C (O) and 230° C ( • ) ; XGeNa at 200° C ( • ) , and 230° C ( • ) Table II Initial rate of isopropanol decomposition on Ge substituted faujasite and phillipsite at 300° C Catalyst •pXlO8 mol g ~ l s ~ l ' rAX10» PGeNa 3.4 4.8 XGeNa 3.7 5.6 PGeNi 2.5 23.7 PGeZn 2.2 196.0 DECOMPOSITION OF ALCOHOLS •8 2 7 7 germanium form, but not on the silicium homologue. Indeed, the dehydration acti-vity of (CH3)3COH is highest on the germanium form compared to the silicium X ..zeolite. The latter effect has to be related to the existence of residual carbonate species on the germanium sieve. Influence of nature of cation and of zeolite It was already pointed out that the parent germanium forms of phillipsite have stronger dehydrogenating properties than the faujasite. This is-confirmed in Table II. From this table it is further clear that upon exchange with Ni and Zn, the dehy-drogenating properties increase drastically. This is indicative of two. kinds of de-hydrogenating sites, one set being present on the parent zeolite, another appearing upon exchange with Ni2+ or Zn2+ . The dehydrogenation properties of the germanium zeolites could be poisoned gradually with carbon monoxide (Fig. 5). At the same time an IR band at 2190 c m - 1 with increasing intensity was observed. This band is due to chemisorbed CO. m Fig. 5. Carbon monoxide poisoning on rate of acetone formation (a) on XGeNa at 275° C and intensity chan-ges of the IR band (b) of adsorbed carbon monoxide at 200 °C Isotope effects In Table III are given the isotope effects in the dehydration of 2-butanol. A significant effect is observed in . the case of deuteration of the alcohol H atom over XSiNa, XGeCa and PGeNaCa. On the other zeolites this primary isotope effect is much less important, while a significant effect is observed when deuteration of the: H atoms on a carbon atom in /? position has occurred. 2 7 8 G. PONCELET ET AL. P . Table III Dehydration of 2-butanol over zeolite Zeolite Temperature °c <z"> ß<2) XSiNa (3) 200 2.9 1.10 XGeNa 275 1.8 1.90 XGeCa 250 2.4 1.25 PGeNa 300 1.4 2.10 PGeNaCa (60) 300 2.0 1.75 (1> Isotope effect for CH3—CH2—CHOD—CH3 . (2) Isotope effect for CD3—CD2—CHOH—CH3 . <3) From ref. [4]. Discussion Reaction mechanism Dehydration of alcohols may occur via a continuous sequence of possibilities, E1? E2 and EjcB eliminations being limiting cases [5]. In case of elimination near the EiCB side, reaction starts with rupture of the most acidic C—H bond and dehydrogena-tion is occurring as well as dehydration [3,5]. In the present case, there is strong evi-dence that distinct sites exist for dehydration and dehydrogenation (see Table 1). This means necessarily that the alcohol dehydration mechanism is between Ex and E2 on the germanium zeolites. For the silicic X and Y zeolites, it has been proved that the reaction is on the Ej side for most alcohols, methanol not taken into account [4]. When a primary isotope effect exists, the reaction has to shift to the Ex side. When the mechanism is concerted (E2 side) an isotope effect in H at should exist. In this respect, Taible III allows to conclude that: (i) upon germanium incorporation in the structure, the mechanism tends to shift more to the E2 side, this being more pronounced for the' germanium phillipsite; (ii) exchange with polyvalent cations (Ca2+) shifts the mechanism again to the Ei side, due to an increase of the electron pair acceptor properties of the structure [5], regardless whether the cations or protons act as active sites. The dehydrogenation over Y and XSi was found to occur over an acid-base pair of sites, the impurity iron species playing a dominant role: —C—C—O—H - —C—C = O + HOj + FeH H i I Oi~ Fex+ Oj represents lattice .oxygen. In the case of Ni2+ and Zn2+ exchanged samples, a similar mechanism may exist. The same mechanism can be invoked to explain the dehydrogenating activity of the parent unexchanged germanium sieves. The nature of the active site will be discussed later. DECOMPOSITION OF ALCOHOLS 2 7 9 Active sites for dehydration The activity sequence: tertiary > secondary;» primary alcohols indicates a proton catalyzed El type reaction, since it parallels the stability of the corresponding carbo-nium ions. The behaviour upon Ca2+ exchange (more than linear increase above 50% exchange) tends to favor this interpretation. Indeed, this is a common behavior observed for most zeolites and for most proton catalyzed reactions in zeolites. How-ever, since at this degree of exchange, Ca2+ ions become available in the supercage, it cannot be excluded that they act as active sites, since they have electron pair acceptor properties [3, 5]. At the moment, no direct experimental evidence is present, that could reject the generally accepted views in zeolite catalysis and favor the second hypothesis. The remarkable selectivities for primary reaction products observed on: the present zeolites, are also observed for olefin isomerization on XGeNaNH4 zeolites [13]. This suggests that in both cases protons may be the active sites. The parent XGeNa contains weak sites, not present on XSiNa and strong enough toi catalyze the dehydration of tertiary alcohols (Figs 3 and 4). This has to do with the presence of residual surface carbonate species in the germanic sieve [7, 10]. Upon addition of water under reaction conditions bidentate carbonate species (bands at 1580—1250 cm - 1) are replaced by monodentate species (1660—1375—1175 cm - 1) [14]. A similar observation was reported on MgO [15] and carbon dioxide was also found to enhance the dehydration properties of. silicic zeolites by similar surface reactions [2]. Active sites for dehydrogenation Besides the dehydrogenation sites on Ni and Zn exchanged germanium zeolites associated with a cation—lattice oxygen pair of sites, the parent germanium zeolites have dehydrogenating sites of distinct nature (Table II) which can be poisoned with CO (Fig. 5). This form of chemisorbed CO (2190 cm - 1) never observed on the silicium zeolites, is most probably bonded to germanium at oxygen deficient sites, and can be schematized as follows (I) I II \ | / C O o—c III c o o H H \ v X / Ge A1 Ge \ / \ Ge A1 Ge The reaction can occur over an acid-base pair of sites as illustrated in II. Acknowledgment P . A . JACOBS acknowledges a permanent research position as "Bevoegdverklaard Navorser" from N. F. W. O. (Belgium). 2 8 0 G. PONCELET ET AL.: DECOMPOSITION OF ALCOHOLS References [1] Rudham, R., A. Stockwell: Catalysis 1. The Chemical Society, London, 1976, p. 87. [2] Jacobs, P. A.: Carboniogenic Activity of Zeolites, Elsevier Scientific, Amsterdam, Oxford New York, 1977, p. 85. [3] Noller, H., W. Kladnig: Catalysis Reviews, Science and Engineering, 13, 149 (1976). [4] Jacobs, P. A., M. Tielen, J. B. Uytterhoeven: J. Catal. 50, 98 (1977). [5] Vi nek, N.. H. Noller, M. Ebel, K. Schwarz: J. C. S. Faraday I. 73, 734 (1977). [6] Jacobs, P. A.. J. B. Uytterhoeven: J. Catal. 50, 109 (1977). [7] Lerot.L., G. Poncelet, J. J. Fripiat: Mat. Res. Bull. 9, 979 (1974). [8] Poncelet, G.. M. L. Dubru, T. Lux: Mat. Res. Bull. 11, 813 (1976). [9] Poncelet. G.. M. Lauriers: Mat. Res. Bull. 10. 1205 (1975). [10] Lerot, L„ G. Poncelet, M. L. Dubru, J. J. Fripiat: J. Catal. 37, 396 (1975). [11] Knözinger, H., A. Scheg/iia: }. Catal. 17, 252 (1970). [12] Venuto, P. B.. P. S. Landis: Advan. Catalysis 18, 259 (1968). [13] Poncelet, G.. M. L. Dubru, P. A. Jacobs: Molecular Sieves II, J. Katzer, ed., ACS Symp. Ser. 40, 606 (1977). [14] Jacobs, P. A., G. Poncelet: to be published. [15] Evans, J. V.T. L. Whateley: Trans. Faraday Soc. 63, 2769 (1967). 0 

Decomposition of alcohols over different forms of germanium substituted faujasite and phillipsite

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Decomposition of alcohols over different forms of germanium substituted faujasite and phillipsite

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