Langmuir 1991, 7, 2213-2218
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Preparative Effects on the Surface Area and Pore Structure of Microporous Heteropoly Oxometalates D.Lapham and J. B. Moffat' Department of Chemistry and Guelph- Waterloo Centre for Graduate Work in Chemistry, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1 Received February 8, 1991. In Final Form: May 1, 1991 The ammonium, potassium, and cesium salts of the heteropol oxometalates of Keggin structure, in particular 12-molybdophosphoricand 12-tungstophosphoric aci s have previously been shown to have high surface areas and microporous structures. In the present work the effect of the source of the cation, the stoichiometryof the preparation, the nature of the cation, the peripheral metal atom of the anion, and the thermal stability of a number of heteropoly oxometalates of Keggin structure has been evaluated through measurements of adsorption-desorption isotherms of nitrogen and comparisonsof the calculated surface areas and pore structures. The source of the cation has relatively little effect, except where the sulfate is the anion while a deficit of ammonium carbonate employed in the preparation produces a substantial change. The surface areas and contribution of the micropores generally decrease markedly with increase in temperature but the extent of such decreases depends significantly on the nature of the cation.
B
Introduction Heterogeneous catalysts containing pore networks and particularly microporous structures offer considerable advantages in comparison with those where pores are absent.' Porosity almost inevitably increases the surface available for reactant molecules and in some cases may act as collectors for unwanted materials in the feed streams.2 The presence of micropores may introduce both geometric and kinetic advantages as has been well documented with various zeolites.3 Recent work in this laboratory has shown that heteropoly oxometalates may be prepared with intrinsic microporous structures.4-12 In these solids, network structures in the sense of those found in zeolites are absent. Instead, the anion is a discrete unit, and those considered here have the familiar Keggin structure with, in the present case, a central PO4unit surrounded by 12 octahedra with oxygen atoms at their vertices and a peripheral metal atom (here either W or Mo) at their centers. Two types of bridging oxygen atoms are present, those between two peripheral metal atoms and those joining the latter with the central atom. Twelve terminal oxygen atoms are also present. With lbtungstophosphoric acid (H3PW12040, HPW) the (1) Spencer, M. S.In Selected Developments in Catalysis; Jennings,
J. R., Ed.;Blackwell: Oxford, U.K., 1985; p 64. (2) Wei. J. In Catalyst Design; L. L., Ed.;Wiley-Interscience: - Heaediea, New York, 1987; p 245. (3) Derouane, E. G. In Guidelines for Mastering the Properties of
Molecular Sciences; Barthomeuf, D., Derouane, E. G., Hblderich, W., Plenum: New York, 1990; p 225. (4) Moffat, J. B.; McGarvey, G. B.; McMonagle,J. B.; Nayak, V.; Nishi, H. In Guidelines for Mastering the Properties of Molecular Sciences; Bncthomenf, D., Derouane, E. G., Holderich, W., Eds.; NATO ASI Ser. B VOl. 22. (5) Moffat, J. B. J. Mol. Catal. 1989,52, 169. (6) McMonagle,J. B.;Nayak, V.; Taylor, D.; Moffat, J. B. Proceedings, 9th International Congress on Catalysis; Phillips, M. J., Tervan, M., Ede.; Chemical Institute of Canada; Ottawa, ON, Canada, 1988. (7) McGarvey, G. B.; Moffat, J. B. J. ColloidInterface Sci. 1988,125, 51. (8) Moffat, J. B.; McMonagle,J. B.;Taylor,D. Solid State Ionics 1988, 26,101. (9) Moffat, J. B. Studies in Surface Science and Catalysis; Ward, J., Ed.;Elsevier: Amsterdam, 1988; Vol. 38. (10) Moffat, J. B. Polyhedron 1986,5, 261. (11) Taylor, D. B.; McMonagle,J. B.; Moffat, J. B. J. ColloidZnterface Sci. 1985, 108, 278. (12) McMonagle, J. B.; Moffat, J. B. J. Colloid Interface Sci. 1984, 101, 479.
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secondary or crystallographic structure is cubic with each proton surrounded by four water molecules, which are in turn hydrogen bonded to the terminal oxygen atoms of the anions. While neither HPW nor the corresponding molybdenum compound has a porous structure (surface areas are of the order of 5 m2/g), certain of the monovalent salts of these have been shown to have high surface areas and microporous structure~.~-'~ Both the catalytic properties and the porous structures are evidently dependent upon the type of peripheral metal atom, the nature of the cation, and to the lesser extent the anionic central a t ~ m . The ~*~ peripheral metal atom has a significant effect on the catalytic properties of the heteropoly oxometalates with Keggin structure. Heteropoly oxometalates containing tungsten are most effective in catalyzing processes requiring the presence of acidic sites while those containing molybdenum are active in oxidation processes.13 The influence of the cation is evident from a comparison of the properties of HPW and ita ammonium salt. While HPW has a low surface area and no pore structure, ita ammonium salt is found to have a relatively high surface area and a microporous structure.4*6J2 The ammonium salt shows a high catalytic activity and selectivity, relative to the parent acid, HPW, for the production of hydrocarbons in the methanol conversion process.l4JS A combination of physisorption and X-ray diffraction data has provided information on the source of the differences between HPW and NHQW. In the nonporous heteropoly oxometalates, such as HPW and HPMo, interstitial voids are separated from one another by terminal oxygen atoms. The voids are aligned with each other parallel and normal to the XRD [110] plane of the crystal. For monovalent salts possessing a microporous structure, the secondary structure is altered by the translation and rotation of the anions such that the voids form a continuum of channels running both parallel and normal to the XRD [ 1101plane, thusproducing a porous network and increasingthe surface area. Correlations of the micropore volumes and the XRD [110] intensity with cation size have demonstrated that cation size has an influence on the secondary structure (13) Moffat, J. B. Chem. Eng. Commun. 1989,83, 9. (14) Hayashi, H.; Moffat, J. B. J. Catal. 1985,83, 192. (15) Hayashi, H.; Moffat, J. B. J. Catal. 1982, 77, 473.
0 1991 American Chemical Society
Lapham and Moffat
2274 Langmuir, Vol. 7, No. 10,1991 and related porous structure of heteropoly ox om eta late^.^^^ It should also be noted that there is strong evidence that the salts of heteropoly oxometalates are not stoichiometricand that the presence of residual protons in these solids is important to their catalytic properties and porous structure.''3J7 There are a number of factors associated with the preparation and properties of a number of microporous heteropoly oxometalates. The factors include the following: the peripheral metal atom, the stoichiometry of the preparation, the source of the cation, the nature of the cation, and the thermal stability of the prepared solids. The thermal stability of the pore structures was investigated with the ammonium salts of HPW and HPMo prepared from stoichiometric quantities of ammonium carbonate and the appropriate parent acid. The influence of the stoichiometry of the preparation was evaluated by varying the relative quantities of the preparative reagents for these solids. The effect of the source of the cation employed in the preparation was examined with the stoichiometrically prepared NHQW. In addition, the thermal stabilities of KPW and CsPW prepared stoichiometrically were investigated toevaluate the influence of the cation on this property.
Experimental Section HPW and HPMo (BDH) were recrystallized from aqueous solution by a variation on the etherate method.18 The acids were dissolved in a minimum of deionized water and were extracted with excess diethyl ether. The etherate layer was drawn off and heated over a water bath to near dryness. The resulting solids were dried overnight at 110 "C. The cation sources (NH&COs, NHICl, NHdNO,, (NH&SOr, KzCOs, and CszCOs were reagent grade purity and were used without further purification. With the exception of (NH&COs, these salts were dried overnight under vacuum at 110 "C. In the studyof the effects of cation source,the four ammonium sources were used. The carbonate sourceswere used for all other preparations. In the study of the effect of stoichiometry, 0.85, 1.00, and 1.15 equiv of ammonium carbonate were employed. The heteropolyoxometalatesaltswere prepared fromsolutions of the cation sourceof concentrationsof approximately0.13 mol/L in deionized water. The solutions of the cation sourcewere added dropwise for 1h at ambient temperatures to a stirred solution of the heteropoly acid dissolved in a minimum of deionizedwater. The suspensions were stirred for an additional 1.5 h at ambient temperaturesafter the desired amount of cation source had been added. During the preparative process the pH remained below 2.5 except for the cesium salts, for which the pH did not exceed 4.2. All salts were heated over a water bath until near-dryness followed by drying in air at 110 "C. Pretreatment of the solidsconsisted of evacuation (
