Bismuth catalysis

BiCfr [34b] proved to be a better catalyst than scandium [76] or ytterbium triflates [77] for this rearrangement which is also catalyzed by Bronsted acids [78]. Other functionalities such as Boc, allyl, and propargyl are compatible with bismuth catalysis (Section 11.2.6). 

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. 

Abstract Several bismuth-catalyzed synthetic reactions, which proceed well in aqueous media, are discussed. Due to increasing demand of water as a solvent in organic synthesis, catalysts that can be used in aqueous media are becoming more and more important. Although bismuth Lewis acids are not very stable in water, it has been revealed that they can be stabilized by basic ligands. Chiral amine and related basic ligands combined with bismuth Lewis acids are particularly useful in asymmetric catalysis in aqueous media. On the other hand, bismuth hydroxide is stable and works as an efficient catalyst for carbon-carbon bond-forming reactions in water. 

Thus, it was found that selective propargylation or allenylation reactions of hydrazonoester with allenyl pinacol boronate proceeded smoothly in the presence of a catalytic amount of bismuth(III) or copper(II) hydroxide in aqueous media. The use of metal hydroxide as a catalyst in organic synthesis is rare, and it is noteworthy that efficient catalysis occurred in aqueous media. In addition, the allenyl adduct was produced with high selectivity in the presence of Bi(OH)3, whereas the propargyl adduct was obtained with high selectivity in the presence of Cu(OH)2 as a catalyst. 

We have previously reported that when the rearrangement of trans-stilbene oxide was carried out with CF3SO3H, the solution turned red and the product diphenylacetaldehyde was less pure than that obtained with bismuth triflate. This observation points to the role of bismuth(III) triflate as a Lewis acid in the rearrangement of epoxides and not to protic acid catalysis by triflic acid released by hydrolysis of bismuth triflate. 

Gaspard-Iloughmane H, Le Roux C (2008) Bismuth(III) Lewis acids. In Yamamoto H, Ishihara K (eds) Acid catalysis in modem organic synthesis. Wiley, New York, p 551... 

Keywords Bismuth(IH) salts Catalysis Green chemistry Lewis acidity Moisture tolerance Non-toxicity Organic transformations... 

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. 

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. 

After catalysis the two types of particles were always present. The smaller particles were exactly the same composition and no bismuth was detected. The larger particles had a size reduced by a factor 2 and were not any more composed exclusively of bismuth and molybdenum. The presence of cobalt and iron, in the same proportions... 

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.)... 

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