MONTMORILLONITE CATALYSTS FOR ETHYLENE HYDRATION

Transcription

1 Clay Minerals (1983) 18, MONTMORILLONITE CATALYSTS FOR ETHYLENE HYDRATION M. P. ATKINS, D. J. H. SMITH AND D. J. WESTLAKE BP Research Centre, Chertsey Road, Sunbury-on-Thames, Middlesex TW16 7LN (Received 17 June 1983) A B S TRA C T: Ion-exchanged montmorillonites behave as solid acid catalysts and are effective and selective catalysts for the hydration of ethylene. The reaction proceeds predominantly in the interlamellar environment of the clay catalyst. Ethylene conversions were found to be markedly dependent on the choice of exchangeable cation, with A1 being the best of those examined. The detailed structure of the smectite clay mineral group is still the subject of debate but it is generally believed that the framework structure of Wyoming bentonite is predominantly that suggested by Hofmann, Endel & Wilm (1933). There is, however, evidence that there is also a contribution from an alternative structure proposed by Edelmann & Favejee (1940). Both structures (Fig. 1) utilize the basic 'sandwich' consisting of an octahedrally-coordinated cation-hydroxyl sheet between two tetrahedrally-coordinated cationoxygen sheets. However, the Edelman & Favejee structure has the silica tetrahedra (a) (b) O hydroxyl o aluminium 9 silicon 0 oxygen ~) hydroxyl o aluminium 9 silicon FIG. 1. Proposed structures for montmorillonite. (a) After Hofmann el al. (1933); (b) after Edelmann & Favejee 1983 The Mineralogical Society

2 424 M.P. Atkins et al. aluminosilicate layer H hydrated cation \ 0 jh H/" 0 M+ \ HI~ H/0 H H free water H \0 fh water interactions with silicate surface H \ jh 0 aluminosilicate layer FIG. 2. Water environments in the interlamellar region of montmorillonite. inverted relative to the Hofmann structure and proposes free Si-OH groups projecting into the interlamellar environment. Montmorillonite catalysts posses both Lewis and Br6nsted acid sites and these functions are responsible for the diverse catalytic behaviour of these minerals in reactions such as ether formation from alcohols and alkenes, and ester formation from a carboxylic acid and alkene (Adams et el., 1983; Bylina et el., 1980, Ballantine et el., 1981 ; Thomas 1982). The montmorillonite layers carry an overall negative charge arising from isomorphous substitution of Mg and Fe for A1 in the octahedral sheet and, to a lesser extent, substitution of Si in the tetrahedral sheet by A1. The negative charges on the layers are balanced by hydrated cations in the interlamellar space (usually Na + or Ca 2+ in the natural form) which can be readily exchanged. The cation exchange capacity of montmorillonites is typically meq/100 g clay. By suitable choice of cation in the interlamellar space, the acidity and polarizability of water in the interlamellar environment can be varied according to the charge density of the cation. The water in the interlamellar environment is present in three distinct phases: (i) free water able to diffuse readily through the clay structure; (ii) bound water present as a hydration sphere around the cations in the interlamellar space; (iii) water hydrogen-bonded to the silicate surface. These are illustrated in Fig. 2. The Br6nsted acidity of the cation-exchanged clays can be accounted for by dissociation of coordinated water in the interlamellar space according to the general equilibrium: M(H20)x n+ ~ [M(OH)(H20)x_]] (n-l)+ + H + Lewis acid sites on the interlamellar or external surfaces may arise by dehydroxylation of the framework hydroxyl groups: OH t I boat I + I o i-o

3 Ethylene hydration in clays 425 and it is likely, therefore, that the number of Lewis sites available increases with increasing reaction temperature and decreases with increasing water content in the environment. Recent solid-state 27A1 nuclear magnetic resonance spectroscopic studies in our laboratories have demonstrated the presence of 5-10% tetrahedral A1 in the structure of Wyoming bentonite. The reaction described in this paper is the continuous hydration of ethylene to ethanol using a number of ion-exchanged montmorillonite catalysts. The generally accepted mechanisms for the hydration of ethylene in terms of Br6nsted and Lewis acid sites are shown in the following equations: H+ + " CH2-CH 3 " CH2--CH 3 + H + (1) CH2=CH 2 CH2--CH 3 9 H H H H CH2=CH 2. ' CH2--CH 2 ~ -LA CH2-CH 2. C H2--CH 3 (2) +I~A %. L-A [O~..H~ ~- " OH A H ont OH Two important parameters which govern the use of clay catalysts in organic reactions are (i) catalyst activity and (ii) selectivity to the desired product, and these were investigated. Catalyst preparation EXPERIMENTAL Ion-exchanged montmorillonites were prepared by routine procedures. H-bentonite was made by ion-exchanging Wyoming bentonite with cold 5 M sulphuric acid, washing to remove all extraneous ions and drying at 80~ in air. AI-, Fe(III)- and Cr(III)-bentonites were made in a similar manner by exchange with 0.5 M solutions of aluminium sulphate, iron(iii) nitrate and chromium (III) nitrate, respectively. Apparatus The reactions were carried out in a conventional medium-pressure, continuous flow unit using a fixed-bed reactor; a simplified flow diagram for this is shown in Fig. 3. Ethylene and water were supplied to the reactor and the products collected in a catch pot for subsequent analysis. Method A residence time of 30 min was used throughout the study and the experimental runs were generally carried out for 24 h. Temperatures were kept in the range ~ and pressure was maintained at 50 bar by a slow bleed of ethylene. Reaction products were analysed by gas chromatography and catalysts were examined by XRD and Raman spectroscopy.

4 426 M. P. A tkins et al. pump ; I ~ water reservoir I,r / ~ inert packing,i~~ catalyst bed / / ethylene cylinder furnace [~ condenser ~~gas meter to vent I product pot liquid producls FIG. 3. Apparatus for the continuous flow production of ethanol from ethylene and water. RESULTS AND DISCUSSION Several ion-exchanged montmorillonites (H +, A13+, Fe 3+, Cr 3+) were tested for ethylene hydration under steady-state conditions where the rate of reaction is proportional to the concentration of ethanol in the product stream. Ethanol was produced with a selectivity of 95% (based on water) with diethyl ether as the major by-product. Al-montmorillonite gave the highest conversions of ethylene and water to ethanol (Fig. 4). A 5% conversion (based on water) per pass was obtained with this catalyst. Most current commercial processes for ethylene hydration operate with low conversions of ethylene per pass and this is subsequently recycled for further reaction. The order of reactivity of the various ion exchanged montmorillonites was found to be: A13+ ~ Fe 3+ > Cr 3+ > H + > calcined clay. A clay catalyst whose structure had been collapsed by calcination was shown to give very poor conversions of ethylene to ethanol, suggesting that the reaction is, at least in part, an interlamellar process and access to the clay interior is a prerequisite for efficient catalysis. The unexpectedly low activity of the H-montmorillonite was explained by XRD analysis of the spent catalyst (Table 1). This revealed that the catalyst had collapsed under the reaction conditions, since the basal d001 spacing had dropped from 15.2 to 9.4/~: it was thus behaving in a manner identical to the deliberately calcined catalyst.

5 Ethylene hydration in clays Ala o ~ 2.0 Fe~+ Cr ~+ I-0 ~ catalyst FIG. 4. Activity of various ion-exchanged montmorillonites for ethanol production. A few experiments were attempted in the liquid phase at lower temperatures but the clay granules were unstable in contact with liquid water and washed out of the catalyst bed. The choice of cation in the interlamellar space, apart from influencing the polarizability of the interlamellar environment, is also of paramount importance in binding the alcohol to the cation. Two types of interaction between ion-exchanged montmorillonites and alcohols influence the process of adsorption of the alcohol: 1. The interaction of the empty electronic orbitals of the cation with the lone pair of electrons of the alcohol molecule. 2. The strength of the hydrogen bonding between the alcohol molecules. A recent paper (Annabi-Bergaya et al., 1981) indicates a substantial difference in the number of alcohol molecules coordinated to various cations in smectite layers, and the idealized packing of methanol around Li, Na, Ba and Ca ions is shown in Fig. 5. The favoured conformation adopted by methanol itself is a zig-zag pattern with hydrogen bonding between adjacent alcohol molecules holding the structure together. With Li (atomic radius 0.7 A) as the interlamellar cation the zig-zag conformation of methanol is TABLE 1. Basal spacings (d000 of clay catalysts. Major d0o 1 Catalyst Treatment basal spacings Comments Al-bentonite Freshly prepared catalyst dried 11.4 A Mainly monolayer of water in the at 80~ C interlamellar region of clay Spent catalyst as retrieved from 15-1 A Mainly bilayer of water present in ethanol run (temperature, 340~ clay H-bentonite Freshly prepared sample dried 15.2 A Mainly bilayer of water present at 80~ Spent catalystas retrieved from 9.4 A Totally collapsed clay catalyst ethanol run (temperature, 350~

6 428 M. P. Atkins et al. (a) Li + (b) Na + cation (~ methanol (c) Ba 2+ (d) Ca 2+ FIG. 5. Idealized bonding of methanol around various cations in smectites. unperturbed. However, as the size of the exchangeable cation increases (e.g. Na, atomic radius 0.97 ) the favoured hydrogen bonding arrangement in the zig-zag conformation is disturbed, leading to randomly distributed chains of alcohols surrounding the interlamellar cations. Similar effects are likely to be found with ethanol and various interlamellar cations. These may have an effect on the overall rate of the hydration reaction because, depending on the strength of the alcohol--cation dipole attraction, the reactants could be hindered in their approach to the interlamellar catalytic sites. Finally, small amounts of coke were observed on the catalyst surface after most runs were completed. A recent review (Trim, 1983) describes some of the possible mechanisms

7 Ethylene hydration in clays 429 of coke formation with alkanes and alkenes over supported nickel on alumina cracking catalysts and gas-phase coke formation is postulated via free radical reactions. In the case of acidic oxides such as clays, coke formation is believed to originate from polymerization reactions involving carbonium ions produced from olefins: RCH=CH 2 + HA ~ RCHCH A RCH = CH2 tsrx-hch2ch2r / coke CH3 where R =H, Me, Et. CONCLUSIONS 1. Ethylene hydration is largely an interlamellar reaction and access to the interlamellar space is a prerequisite for efficient catalysis. Calcined catalysts show little activity arising from the acidic surface sites on the catalyst. 2. The activity of the clay catalysts is strongly dependent on the choice of exchangeable cation in the interlamellar space; A1, the most active one examined, produced ethanol with a selectivity of >95% and a yield of 5% (based on water) per pass. ACKNOWLEDGMENTS The authors wish to thank the British Petroleum Company plc for permission to publish this paper. REFERENCES ADAMS J.M., CLEMENT D.E. & GRAHAM S.H. (1983) Synthesis of methyl-t-butyl ether from methane and isobutene using a clay catalyst. Clays Clay Miner. 30, ANNABI-BERGAYA F., CRUZ M.I., GATINEAU L. & FRIP1AT J.J. (1981) Adsorption of alcohols by smectites: IV. Models. Clay Miner. 16, BALLANTINE J.A., DAVIES M.E., PURNELL J.H., RAYANAKORN M., THOMAS J.M. & WILLIAMS K.J. (1981) Chemical conversions using sheet silicates: Facile ester synthesis by direct addition of acids and alkenes. J. C. S. Chem. Comm BYLINA A., ADAMS J.M., GRAHAM S.H. & THOMAS J.M. (1980) Chemical conversions using sheet silicates: Simple method for producing methyl-t-butyl ether. J. C. S. Chem. Comm EDELMAN C.H. t~ FAVEJEE J.C.L. (1940) On the crystal structure of montmorillonite and halloysite. Z. Krist. Al02, HOFMANN U., ENDELL K. & WILM D. (1933) Kristalstruktur und Quellung yon Montmorillonit. Z. Krist. 86, THOMAS J.M. (1982) Sheet silicate intercalates: New agents for unusual chemical conversions. Pp in: Intercalation Chemistry (M. S. Whittingham & A. J. Jacobson, editors). Academic Press, New York. TRIM D.L. (1983) Catalyst design for reduced coking (Review). App. Catalysis 5,