Single‐Crystalline‐Phase Mo3VOx: An Efficient Catalyst for the Partial Oxidation of Acrolein to Acrylic Acid

From a chemical point of view, acrylic acid (AA) is the simplest unsaturated monocarboxylic acid; it shows excellent propensity to undergo polymerization, and it is mainly used in the manufacture of superabsorbing materials, disposable diapers, and low-phosphorous detergents. An increase in the rate of consumption of AA is the most pronounced of all unsaturated organic acids in recent years. There are several pathways to produce AA, but the most common one involves the two-step oxidation of propylene as a raw material : acrolein (ACR) is obtained as the first oxidation product, and it undergoes further oxidation to AA. 2] Recently, with the fast development in the utilization of biomass resources, a new, raw, bio-based ACR material emerged that comes from the catalytic dehydration or oxidative dehydration of glycerol. Glycerol is a significant byproduct of the biodiesel process, and this may supply a complementary or substitutive route for the production of AA in the future. Much attention should be paid to the common second step of both production routes, that is, the partial oxidation of ACR to AA. Though a high-enough conversion can be obtained for this reaction at high reaction temperatures ( 300 8C), it is remains a challenge to obtain satisfactory conversion under mild conditions and to understand the relationship between the properties of the catalyst and its performance. Owing to the commercialization of the production of AA over the past 50 years, a great number of catalysts have been developed, most of which are multicomponent metal oxides. Among those complex metal oxides, MoV-based oxides have attracted much attention for their high efficiency and for their use in industry. During the long period of optimization, a large number of metal promoters (Me) such as W, Cu, Mn, Fe, Sb, Cr, and Sr were introduced to modify the catalytic performance of these complex oxides. Though these investigations facilitated an increase in the efficiency of these catalysts, they provided little in terms of the understanding of the underlying mechanisms as a result of their complexity. However, for these MoVMe complex oxides, it seems as though there are limited and often contradictory data concerning the active phase for the partial oxidation of ACR to AA. A mixed oxide with the composition of Mo3VO11 was considered to be the active and selective phase by Andrushkevich, whereas Mestl et al. suggested (MoVW)5O14 with a tetragonal structure as the active and selective phase. It was also illustrated that the catalytic efficiency was proportional to the fraction of the catalyst that remained uncrystallized. Again, it was concluded by Vogel et al. that only X-ray amorphous Mo/V mixed oxides contained selective oxidation centers in contrast to the crystalline samples. Recently, they developed their opinion and stated that the prepared mixed oxides were not necessarily the catalytically active species and that the catalyst was formed under the reaction conditions. Consequently, the situation still seems complicated owing to the diversity of components and structures of the different catalysts. One of the possible ways to elucidate this problem is to make a catalyst that is much simpler in terms of both the component and the structure. Recently, four different pure-crystalline-phase Mo3VOx materials, including trigonal, orthorhombic, tetragonal, and amorphous systems (denoted as Tri-Mo3VOx, Orth-Mo3VOx, TetraMo3VOx, and Amor-Mo3VOx, respectively) without containing a third metal species were successfully synthesized in our group. 9] Owing to the simplicity of the components and the uniformity of the structures of these materials, study of the structure–activity relationship between the properties of the catalysts and their performance was opportune. In the present work, these four different pure-crystalline-phase Mo3VOx species were separately used as catalysts in the partial oxidation of ACR to AA. Some understanding of the active phase and the factors affecting the catalytic activity was obtained. These four catalytic materials all present a long rod-shaped morphology with an average diameter of approximately 0.2– 0.3 mm (Figure S1, Supporting Information). XRD (Figure 1) was utilized to determine the crystalline structure, and the corresponding structures of these four materials are shown in Scheme 1. It can be observed in Figure 1 that one strong diffraction peak (22.28) and one weak diffraction peak (45.38)