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Ind. Eng. Chem. Prod. Res. Dev., Vol. 17, No. 3, 1978
High Surface Area Molybdenum Catalysts: Preparation, Characterization, and Activity George A. Tsigdinos” and Wllbur W. Swanson climax Molybdenum company of Michigan, Research Laboratory, A Subsidiary of AMAX INC., Ann Arbor, Michigan 48105
Methods for preparing high surface area Mo/MoO,, Moo2, and Co-Mo and Ni-Mo bimetallic compositions are described. Methods for preparing metal molybdates, such as Fe,(MoO,),, CoMoO,, NiMoO,, and MnMoO, of moderately high surface area are also described. All of the materials prepared were characterized by chemical analysis, X-ray, and surface area. Passivation and regeneration procedures were also devised for the pyrophoric materials. The Mo/Mo02 composition was found to be active in the isomerization of n-hexane.
Introduction Over a period of years, molybdenum-containing materials have found uses as catalysts in several industrial applications. For example, iron molybdate is one of the standard catalysts for the oxidation of methanol to formaldehyde, cobalt and nickel molybdates are catalysts in the hydrotreating of petroleum, and bismuth molybdates reportedly are active in the ammoxidation of propylene to acrylonitrile. Although molybdenum trioxide and its reduction products have been extensively investigated as catalysts on carriers, relatively little work has appeared on the preparation and characterization of high surface area molybdenum trioxide and the lower oxides of molybdenum. It is known that established industrial processes (Climax Molybdenum Co.) produce molybdenum trioxide of low bulk density by carrier gas sublimation; however, the surface area of the sublimed material is in the vicinity of 8 to 10 m2/g. In an investigation of the sublimation process for molybdenum trioxide (Dietz and Klostermann, 19661, materials having a bulk density from 0.2 to 1.2 g/mL were produced, but the surface areas were not given. A technique for producing ultrafine powders by arc vaporization has been reported to yield platelike MooB particles having surface areas of 20 m2/g with equivalent sphere diameters of 700 A (Holmgren et al., 1964). More recently, molybdenum trioxide with surface areas up to 130 m2/g was produced from dispersions of condensed aerosols (Sutugin and Fuks, 1970). Finally, molybdenum trioxide of surface area 17 m2/g was obtained by the decomposition in air at 300 “C of anhydrous ammonium heptamolybdate (Fransen et al., 1976). Several methods for preparing metal oxides or compounds from highly dispersed mixtures of oxides that contain high surface areas have been described (Delmon et al., 1969; Courty et al., 1973). Such materials include refractory oxides like TiOz and Al2O3,spinels like MgA1204, or perovskite-type structures like LaCr03 They have been prepared by pyrolysis of precursors of the oxide(s) obtained from solutions containing all the required ions and an organic polyfunctional acid possessing at least one hydroxy and one carboxylic group such as citric, malic, tartaric, glycollic, or lactic acid (Delmon, 1968, 1969, 1970; Courty, 1973; Szabo and Paris, 1969; Paris and Paris, 1965). Of the metal molybdates, iron molybdate and the bismuth molybdates have been the most extensively investigated because of their use in selective hydrocarbon oxidation catalysis. Iron(II1) molybdate gels, stoichiometric or promoted by other metals, have been prepared by mixing Fe(N03)3.9H20 and (NH4)6M07024-4H20 (AHM) and 0019-7890/78/1217-0208$01.00/0
kneading at room temperature to give a very viscous solution that transforms to an elastic, transparent gel within 30 min. Gels prepared in this manner have surface areas of 0.5 to 50 m2/g. However, when these were used as catalysts in the oxidation of methanol to formaldehyde, the best selectivity was obtained with surface areas of 10 to 12 m2/g (Courty et al., 1970; Courty, 1971, 1973). As reported in the literature, methods for preparing high surface area molybdenum metal include primarily the reduction of molybdenum dioxide and molybdenum pentachloride by hydrogen. The reduction of molybdenum dioxide by hydrogen at 450 O C yielded molybdenum “metal” having a maximum surface area of 50 m2/g; however, the source of the starting material or the extent of its reduction was not given (Hillis et al., 1966). Nevertheless, the reduced materials prepared by this method have been found to have catalytic activity for the hydrogenation of absorbed nitrogen to ammonia (Hillis et al., 1966) and for the hydrogenolysis of ethane (Sinfelt and Yates, 1971). Molybdenum metal powder of 0.01 to 0.1 pm average particle diameter has been prepared via the reduction of MoC15 by hydrogen in an argon atmosphere at 800 “C. The pyrophoric metal contained 0.15% 0 and had a surface area of 11m2/g (Lamprey and Ripley, 1962). Aside from the work thus far cited, no other information on high surface area molybdenum-containing materials is available. Consequently, this study has centered on devising methods for preparing high surface area materials such as molybdenum oxides, metal molybdate gels, and mono- and bimetallic molybdenum-containing compositions. The procedures devised for preparing moderately high surface area metal molybdates depended on the use of hydrogen peroxide solutions of ammonium heptamolybdate, whereas the preparation of high surface area mono- and bimetallic molybdenum-containing compositions was based on procedures involving the use of molybdenum oxalate (Tsigdinos et al., 1975) or metal molybdate prepared from AHM/H202. These products were examined for surface area and sintering properties. Passivation and regeneration procedures were also devised for the pyrophoric materials. The activity of Mo/Mo02 toward the isomerization of n-hexane was also evaluated by others (Burch and Mitchell, 1976, 1977). Experimental Section Materials. All chemicals used were reagent grade unless otherwise specified. The aquaoxalatomolybdic(V1) acid (molybdenum oxalate), H2[Mo03C204.H20]-H~0, used was prepared as described in the literature (Killeffer and Linz, 1952). All air-sensitive materials were handled in a 0 1978 American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 17, No. 3, 1978 209 Table I. Analysis of Metal Molybdates ~found calcd metal molybdate %metal % MO %metal % MO
Table 111. Data on Supported Mo/MoO,
~~
19.0 25.3 28.5 25.6 41.7
Fe,(MoO,), MnMoO, CoMoO, NiMoO, Bi,(MoO,),
46.2 44.1 41.9 41.7 31.5
18.9 25.5 26.9 26.8 46.5
48.6 44.6 43.8 43.9 32.0
Table 11. Surface Areas of Metal Molybdates as a Function of Temperaturea metal molybdate Fe2(Mo04
MnMoO, CoMoO, NiMoO, Bi2(Mo04)3
13
surface area at indicated temp, m'k 450 C 550 a C 650 "C 8.1 20.7 24.4 (27.8)' 3.2
7.42 11.0 18.3 25.2d
3.Sb 3.0 11.3 19.7
Calcination at temperature was for 2 h unless otherRefers to green form of iron molybwise indicated. date. ' Impure phase (contains NiO). Calcined for 24 h to eliminate oxides.
drybox with an antechamber that could be evacuated and refilled with dry nitrogen and with a circulating 96% Nz-4% H2 atmosphere for removing traces of oxygen. A 25-W light bulb, with the shroud removed, served as an indicator for traces of oxygen and moisture. Preparation of Compounds. (a) Metal Molybdates. The cobalt, nickel, and manganese metal molybdates were prepared by the addition of powdered metal acetate to a highly concentrated (37 w t % Mo) ammonium heptamolybdate/ hydrogen peroxide solution. Iron nitrate was used for preparing ferric molybdate. In a typical example, CoMo04 was prepared by adding 87.5 mL of 30% HzOz t o 350 mL of water followed by the addition of 805 g of (NH4)6M07024-4H20 (AHM). When all the AHM was dissolved, 1136.1 g of C O ( C H , C O O ) ~ . ~ Hin ~ Opowdered , form, was added while the solution was stirred vigorously. After a 10-min period a gray unstirrable mass formed, which had the odor of acetic acid. The mass was heated in a vacuum oven at 120 "C for approximately 2 h, and the dried material which crumbles readily was then ground to a fine powder and calcined in air at 450 "C for 2 h. A 1-kg yield was obtained. The analyses of the metal molybdates prepared are given in Table I. The surface areas of the products as a function of temperature are given in Table 11. (b) Molybdenum Trioxide and Blue Oxides. For this preparation, 487 g of molybdenum oxalate (-30 mesh) was heated a t 300 "C for 1 h with vigorous stirring in a stirred-bed reactor having an air flow of 2 L/min. The heat-up time was 1.5 h. The yield of the slightly gray-blue powder obtained was 212 g. When the decomposition of molybdenum oxalate was carried out in nitrogen at 300 "C with vigrous stirring, bright blue solids were obtained. Decomposition under vacuum (300 "C, vigorous stirring) produced blue-black products. A sample of molybdenum oxalate was also decomposed under vacuum, with no stirring provided, to yield a blue powder. ( c ) Molybdenum Dioxide. All small-scale runs (employing 30 g or less of molybdenum oxalate charge) were carried out in a vertical Vycor tube, 3.8-cm i.d., provided with a coarse-porosity frit at the lower part so that the charge could be fluidized. A thermocouple imbedded in the center of the charge was used to record the internal
reduction at molybtotal sur500 'C, denum, carbon, face area, h wt% support wt% m'/g 260 48 9.49
