Molybdena calcination

Figure 8.12 Raman spectra of alumina- and silica-supported molybdena catalysts after impregnation of the supports with solutions of ammonium heptamolybdate, (NH4)6Mo7024 4 H20 of different pH values, and after calcination in air at 775 K. See Table 8.3 for a list of characteristic Raman frequencies of molybdate species. The sharp peaks in the spectra of the calcined MoOySiOj catalyst are those of crystalline Mo03 (from Kim el at. [43J).

Fra. 29. Spectra of cobalt-molybdena-alumina catalyst and related compositions, C oMo04, and a coprecipitated cobalt alumina catalyst (all three samples were calcined in air) also spectrum of cobalt-alumina catalyst reduced in hydrogen. 

Initially tests were conducted in glass equipment at atmospheric pressure. It was discovered that a more durable catalyst could be made if the Group VI metal oxide were deposited on an alumina support. The best support found for this reaction was alumina, and the first commercial catalyst was made by impregnating a material very similar to activated alumina 1 with a molybdenum salt solution, followed by drying and calcining at a temperature above 1000° F. Interestingly enough, the supported chromia catalyst which showed a marked superiority over the supported molybdena catalyst at atmospheric... 

The earliest NMR studies of oxide surfaces (362-364) involved wide-line proton NMR of adsorbed organic species. For example, Petrakis and Kiviat (363), who studied the adsorption of pyridine and thiophene on molybdena-modified alumina, found that chemisorbed and physisorbed species can be readily distinguished. When physically adsorbed, both compounds exhibited liquid-like NMR behavior with high molecular mobility even at low temperatures. Chemisorbed pyridine was much more rigidly held with essentially only a rotation about the C2 molecular axis persisting to - 130°C. Pyridine was sorbed both physically and chemically, and pretreatment of the surface was not particularly significant in this respect. By contrast, thiophene was physisorbed only on surfaces previously reduced with hydrogen, and underwent a reaction on calcined but unreduced surfaces. 

Hydrogen (Takachiho Co., 99.999%) and He (Takachiho Co., 99.999%) were dried by passing them through a Deoxo unit (SUPELCO Co. Oxysorb) and a Linde 13X molecular sieve trap prior to use. NH3 (Takachiho Co., 99.999%) was used without further purification. The alumina-supported molybdena was prepared using a mixture of hexa-ammonium molybdate and y-alumina (Nikki Chemicals Co.) and calcined in air at 823 K for 3 h. 

The individual techniques used to characterize molybdena catalysts are now considered. Table II presents a listing of articles concerning the characterization of molybdena catalysts. Unless otherwise specified, we implicitly refer to Mo and/or Co supported on an activated alumina, commonly y-AlaOs. Most work has been done on the calcined (oxidized) state of the catalyst because of ease of sample handling. Reduced and sulfided catalysts are more difficult to work with since for meaningful results, exposure of these samples to air or moisture should be rigorously avoided. Therefore, sample transfer or special in situ treatment facilities must be provided. 

The most obvious choice to determine phases that may be present in the molybdena catalyst is XRD. Matching of diffraction lines obtained for the catalyst with those of pure bulk compounds gives unequivocal identification of phases present. This is one of the few techniques that yields positive results. The absence of matching diffraction lines, however, is not proof that the phase in question is not present in the catalyst. The XRD technique is limited to particle sizes of above approximately 40 A for oxides or sulfides, lower sized particles giving no discernible pattern over that of the broad alumina pattern. Thus, the presence of a highly dispersed phase, either as small crystallites or as a surface compound of several layers thickness will not be detected. Also, if the phase is highly disordered (amorphous), a sharp pattern will not be obtained, although some broad structure above that of the alumina may be detected. It is a moot point as to whether such a case is considered as a separate phase or a perturbation of the alumina structure. Ratnasamy et al. (11) have examined their CoMo/Al catalyst from the latter point of view, with particular emphasis on the effect of calcination temperature. 

The situation with respect to reduction of the CoMo/Al catalyst is more confusing. Various authors claim that the presence of cobalt at a low level accelerates (16), retards (27), or has no effect on (31) the reduction of the molybdena. Of course, at high Co loadings, more reduction is obtained than for the Mo/Al alone, due to reduction of the Co304 phase present, but it is difficult to assess whether the molybdena is itself affected by the cobalt reduction. It is well known that transition metals can catalyze reduction of oxides (32). It is probable that the different results obtained could be due in large part to differences in preparation or calcination temperature as pointed out earlier. 

Addition of Mo to A1203 has been reported to lower and shift the frequency of the Al-O-H band (57, 75). This has been interpreted as due to covering of the A1203 surface by a molybdena layer, or actual loss of OH by interaction during preparation or calcination of the catalyst. The latter view agrees with a suggested reaction put forth by Dufaux et al. (36),... 

Adsorption measurements on oxidized catalysts have been done at low temperatures to avoid reduction or reaction with the molybdena present. Adsorption of H2 appears to be negligible at room temperature (73). Dolli-more et al. (82) reported an adsorption ratio of 1.8H2S/Mo, indicative of a high Mo dispersion on their catalyst. In one study, thiophene was reported to be reversibly adsorbed (82) whereas another found appreciable irreversible adsorption (73). Chemisorption of 1-butene at 100° was found to be the same for a series of catalysts of varying Co levels with the same Mo content (61). One butene molecule was adsorbed per two Mo atoms. A calcined Mo catalyst gave 2H20/Mo (83) adsorbed at room temperature but this value may be incorrect as A1203 itself can adsorb considerable water. 

Figure 7. Raman spectrum of an 8 wt % molybdena on y-Al2Os (A) after impregnation at pH 6, (B) after drying at 393 K, (C) after calcination at 773 K. Redrawn from Ref. 10.

Indeed, Lund and Dumesic (5-8) reported that the water-gas shift activity of an iron oxide catalyst is reduced by several orders of magnitude when supported on silica. Strong interactions between molybdena and alumina have been documented for the calcined states of hydrotreating catalysts (e.g., 9-11). Also, interaction is manifested in many mixed oxides by enhanced acidity, compared to the acidities of the pure component oxides (12-14). 

Supported molybdena catalysts are prepared by impregnating alumina with ammonium molybdate, calcining in air at 500-600°C to form the oxide. 

The cogelled catalysts contained 0-33 % molybdena by weight and were prepared by mixing solutions of ammonium molybdate with acetic acid sols, drying, and calcining (I). For comparison catalysts containing 0-15% molybdena were prepared by impregnating dried and calcined sol with ammonium molybdate solutions. 

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