Molybdate catalysis

On treatment with molybdic acid, in addition to the primary C-2 epimeric products, the C-4, C-5 threo aldoses also similarly provide the complementary C-3 epimers. However, a proportion of these C-2, C-3 diastereoisomers, especially idoses and to some extent guloses and taloses, are transformed into more stable 2-ketoses, namely to sorboses and tagatoses in the case of hexoses [9] and their 6-deoxy derivatives [6] or to g/uco-glyco-2-uloses and mawwo-glyco-2-uloses in the case of higher aldoses [6,13]. The formation of these 2-ketoses from their parent aldoses is not associated with the molybdate catalysis and is obviously ascribed to the general acidity [41] of the reaction medium. 

The mechanism and rate of hydrogen peroxide decomposition depend on many factors, including temperature, pH, presence or absence of a catalyst (7—10), such as metal ions, oxides, and hydroxides etc. Some common metal ions that actively support homogeneous catalysis of the decomposition include ferrous, ferric, cuprous, cupric, chromate, dichromate, molybdate, tungstate, and vanadate. For combinations, such as iron and... 

Oxidation Catalysis. The multiple oxidation states available in molybdenum oxide species make these exceUent catalysts in oxidation reactions. The oxidation of methanol (qv) to formaldehyde (qv) is generally carried out commercially on mixed ferric molybdate—molybdenum trioxide catalysts. The oxidation of propylene (qv) to acrolein (77) and the ammoxidation of propylene to acrylonitrile (qv) (78) are each carried out over bismuth—molybdenum oxide catalyst systems. The latter (Sohio) process produces in excess of 3.6 x 10 t/yr of acrylonitrile, which finds use in the production of fibers (qv), elastomers (qv), and water-soluble polymers. 

In the case of selective oxidation catalysis, the use of spectroscopy has provided critical Information about surface and solid state mechanisms. As Is well known( ), some of the most effective catalysts for selective oxidation of olefins are those based on bismuth molybdates. The Industrial significance of these catalysts stems from their unique ability to oxidize propylene and ammonia to acrylonitrile at high selectivity. Several key features of the surface mechanism of this catalytic process have recently been descrlbed(3-A). However, an understanding of the solid state transformations which occur on the catalyst surface or within the catalyst bulk under reaction conditions can only be deduced Indirectly by traditional probe molecule approaches. Direct Insights Into catalyst dynamics require the use of techniques which can probe the solid directly, preferably under reaction conditions. We have, therefore, examined several catalytlcally Important surface and solid state processes of bismuth molybdate based catalysts using multiple spectroscopic techniques Including Raman and Infrared spectroscopies, x-ray and neutron diffraction, and photoelectron spectroscopy. 

Shropshire JA. 1965. The catalysis of the electrochemical oxidation of formaldehyde and methanol by molybdates. J Electrochem Soc 112 465-469. 

Furthermore, the same sol-gel matrices have been used in a system where acid and base catalysis occur in the same pot without quenching either catalyst [29]. In this case, the acids were either entrapped Nafion (perfluorinated resin sulfonic super acid, a3) or entrapped molybdic acid (M03-Si02, a2), while the bases were two ORMOSILs (organically modified silica sol-gel materials), one with H2N (CH2)2NH(CH2)3 groups (bi) and the other guanidine base residues (b2) (Scheme 5.12). 

Acrolein Production. Adams et al. [/. Catalysis, 3,379 (1964)] studied the catalytic oxidation of propylene on bismuth molybdate catalyst to form acrolein. With a feed of propylene and oxygen and reaction at 460°C, the following three reactions occur. 

The analysis by X-ray diffraction after catalysis showed only the presence of the a or p phase of the mixed iron and cobalt molybdates depending upon heat treatment, 380 or 430 C respectively. No phase suspected to be present in the conditions of the catalysis reaction have been detected. This was confirmed by IR spectroscopy and EPR which did not detected any new ferric species (9). 

XPS has been used to characterize the three mixtures containing respectively 7,25, and 50 weight % of Bi2Mo30i2 (Table II samples J,K and L). These samples have been characterized before and after catalytic reaction (table III). Bi, Mo, Fe, Co and O have been analyzed. The Mo/0 ratio remains equal to 0.25 for all the samples, before and after catalysis which confirms that no new phase was formed since the molybdates suspected to have formed, have a much lower Mo/0 ratio (0.17 for Bi2Mo06 and Bi3FeMo20i2). Concerning the Bi/(Fe+Co) ratio, it can first be observed that before catalysis this ratio was always lower than that calculated from chemical analysis. This can be explained by the difference between the particles size of the bismuth molybdate and the iron and cobalt molybdates which is in a ratio of more than 30 as calculated from differences in surface area values, 0.3 and 9 to 22 m. g Secondly the Bi/(Fe+Co) ratio increased systematically after catalysis which could be explained by the decrease in size of the bismuth molybdate particles or by the covering of the iron and cobalt molybdate particles by the bismuth molybdate or by both effects. 

The same synergy effect between bismuth molybdates and mixed iron and cobalt molybdates on the mechanical mixture of both particles was reported by Millet et al. (98). However, it was also found that the surface of mixed iron and cobalt molybdate particle was changed during catalysis and a thin layer of bismuth molybdate was formed on the surface of mixed iron and cobalt molybdates after the reaction. It is doubtful that pure mechanical mixture shows the synergy effect for propylene oxidation, and it seems likely that propylene was mainly oxidized on the thin layer of bismuth molybdates formed on the mixed iron and cobalt molybdate in the experiment reported by Millet et al. (98). 

At this stage, it is still difficult to determine whether the conclusion is appropriate for the fundamental part of the multicomponent bismuth molybdate catalyst. Unfortunately, we have no available information on the number of active reaction sites on the catalyst system. In the heterogeneous catalysis, apparent activation energy does not necessarily correspond to the real energy barrier of the elementary slow step of the reaction. Multicomponent bismuth molybdate catalyst has been established industrially, whereas only parts of the fundamental structure and working mechanism have been elucidated. In addition, important roles of alkali metals and other additives such as lanthanides remain unknown. Apparently, further investigations should be done to clarify the complete working mechanism of the multicomponent bismuth molybdate catalyst. 

The oxidation of propene is at present the most extensively studied gas phase heterogeneous oxidation process. The selective production of acrolein over cuprous oxide has been known for a very long time. However, the discovery of bismuth molybdates as highly active and selective catalysts for the oxidation to acrolein, and particularly the ammoxidation to acrylonitrile, has opened a new era in oxidation catalysis. 

Villa et al. [340] have shown that the bismuth tungstates are comparable with bismuth molybdates with respect to dehydrogenation catalysis, although activities and selectivities are somewhat lower. Although the phase structures are different, interesting catalysts are formed in a similar composition range Bi/W = 2/3 to 2/1. (Note that, in case of propene (amm)oxidation, tungstates are definitely inferior to molybdates.)... 

A redox mechanism for oxidation catalysis was proposed by Mars and van Krevelen (34) for the oxidation of aromatics over V205. This mechanism introduced the concept that lattice oxygen of a reducible metal oxide could serve as a useful oxidizing agent for hydrocarbons. Moreover, it formed the basis for the early work at SOHIO which led to the development of the bismuth molybdate catalyst. Since that time there have been many reports which support the redox concept. 

The combination of these experimental findings indicate that active bismuth molybdate catalysts undergo phase transformations when exposed to reducing conditions similar to the conditions of catalysis. The phase transformations are highly dependent upon both temperature and the severity of the reducing atmosphere. However, the occurrence of solid state reactions in the catalysts suggests that the bulk structure of the catalysts plays an important role in catalytic reaction. 

Manassen, J., Khalif, Sh. Organic Polymers Correlation between their Structure and Catalytic Activity in Heterogeneous Systems. IV. Oxydative Dehydrogenation. A Comparison between the Catalytic Activity of an Organic Polymer and that of some Molybdate Catalysts. J. Catalysis 13, 290 (1969). 

Over the past 30 years, our laboratory has examined the synthesis of nanostructured materials for advanced catalysts. The first process for the synthesis of fine grains of cobalt molybdates for hydrodesulfurization catalysis... 

Firsova et al. (122) reported that the room temperature Mossbauer spectrum of supported tin molybdate, which had been aged in vacuo at 723 K, showed the presence of tetravalent tin. Only after exposure to oxygen at 473 K did the sample act as an adsorbent for propylene. It then gave a Mossbauer spectrum that showed the reduction of the tetravalent tin to the divalent state. Reduction without exposure to oxygen was achieved at 673 K but supported tin in the absence of molybdenum was not reduced. The results were interpreted in terms of the proposals (123) for the synergistic oxidation-reduction during catalysis. 

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