Bismuth Molybdate-Based Catalysts for Selective Oxidation of Hydrocarbons

Abstract

Bismuth molybdate materials (α-Bi 2 Mo 3 O 12 , β-Bi 2 Mo 2 O 9 , and γ-Bi 2 MoO 6 ) are well-known in the field of catalysis due to their excellent activity for one of the most important industrial processes: the oxidation/ammoxidation of lower olefins. These processes play an important role in society since the production of one quarter of the most important industrial organic chemicals and intermediates (such as acrolein, acrylic acid, propylene oxide, etc.) used in the manufacture of industrial products and consumer goods is based on these reactions. Although the materials were developed since the 1960s, the topic still attracts many attentions; new catalysts with different additive elements to enhance catalytic activity are still explored. Advanced researches on bismuth molybdate-based catalysts have been performed not only with the change in composition but also in the synthesis methods. This book chapter summarizes recent researches on the development of bismuth molybdate-based catalysts with new achievements in catalysis field.although there are still a lot of controversies. Among many other selective oxidation catalysts, bismuth molybdates are the most extensively studied and serve as the basis for today's many highly active and selective commercial catalyst systems [1]. There are many kinds of bismuth molybdates with different Bi/Mo atomic ratios, but those which have been quite thoroughly investigated and exhibit good catalytic properties are in the range of composition Bi/Mo = 2/1-2/3. Three phases of bismuth molybdates, α, β, and γ, exhibit good catalytic properties for oxidation of propylene. Their catalytic activities decrease in the following generally accepted order, Bi 2 Mo 2 O 9 (β) ≥ Bi 2 Mo 3 O 12 (α) > Bi 2 MoO 6 (γ), which is reported by many authors [1-5]. However, there are some authors who reported the opposite trend: γ ≥ β > α [6, 7], β > (α = γ) [8], α > β > γ [9].Catalysts based on bismuth molybdates can be divided into the following components: Primary bismuth molybdate system: These catalysts are mixed oxide constituted from Bi 2 O 3 and MoO 3 at specified ratios as mentioned above. Multicomponent bismuth molybdate system: These catalysts were developed by the modification of primary bismuth molybdates by replacing or adding other metal elements. The first replacement of half of the amount of the bismuth molybdate by iron in Bi 9 P 1 Mo 12 O 52 increased the catalytic activity for the ammoxidation of propylene noticeably. The Bi-Fe-Mo-O system consists of several different composite oxides including bismuth molybdate, iron molybdate, and Bi 3 FeMo 2 O 12 . Following this improvement, divalent transition metal cations such as Co 2+ and Ni 2+ were also found to enhance the catalytic activity and selectivity significantly [4].The catalyst Bi 1-x/3 ϕ x/3 V 1−x Mo x O 4 , which form a solid solution with composition limits of bismuth vanadate, x = 0, and the α phase of bismuth molybdate, x = 1, attracted many attentions [10, 11] since it is a good system to investigate the diffusion of l...