A study of molybdena catalysts in ethanol oxidation. Part 1. Unsupported and silica‐supported MoO₃

Abstract

Background: Acetaldehyde is a main organic intermediate for manifold chemical products. In the near future, its production using renewable raw materials is rapidly becoming highly desirable. In this paper, investigations on pure and silica-supported molybdenum oxide (MoO 3 ) as catalysts for the ethanol oxidative dehydrogenation process to acetaldehyde are reported.Results: Acicular pure ⊍-MoO 3 crystals and silica-gel supported MoO 3 (1, 5 and 12% wt MoO3 /wt support ) were prepared by the thermal decomposition method and by incipient wetness impregnation, respectively. Catalysts were studied and extensively characterized assessing structural, morphological and chemical properties. The samples were tested in ethanol oxidative dehydrogenation through Temperature Programmed Surface Reaction (TPSR) and steady-state measurements. MoO 3 /SiO 2 samples were constituted by MoO 3 particles weakly interacting with the support, but also by some molybdate species entering the silica framework and significantly modifying the silica morphology. High catalyst acidity limits oxydehydrogenation yield, catalyzing the competitive dehydration reaction to ethylene. Thus, the highest obtained acetaldehyde yield was ≈60%. Molybdenum loss by MoO 3 volatilization was found on MoO 3 /SiO 2 . Conclusion:The produced and characterized catalysts are active, and allow quite a high yield to acetaldehyde. Slight deactivation was observed and also investigated.

A study of molybdena catalysts in ethanol oxidation. Part 2. Alumina‐supported and silica‐doped alumina‐supported MoO3

“…The alumina-supported molybdenum oxide species, identified as monomeric and polymeric molybdate species, clearly gave rise to higher catalytic activity than both pure MoO 3 and MoO 3 /SiO 2 ; where a large part of molybdenum oxide species is in the form of 'isolated' MoO 3 particles almost not interacting with the silica support, together with molybdate species interacting with the support. 23 As discussed previously, both the alumina based supports and the molybdenum oxide species act as acid catalysts, 47 catalyzing the parallel dehydration of ethanol and possibly contributing to carbon deposition, as detected for Mo12S5 after the full steady-state run. Thus, the enhanced performances of MoO 3 /Al 2 O 3 based systems are due to the stronger redox activity of alumina-dispersed molybdate species with respect to MoO 3 particles that allows conversion to acetaldehyde at low temperatures, where dehydration is still very slow.…”

“…45 Pure silica has a very low dehydration activity, according to its poor acidity. 23,46 The addition of molybdena to the supports (Fig. 8) introduces relevant oxidative dehydrogenation activity in all cases, and this clearly increases with molybdenum content.…”

“…On the other hand, by looking at BS Electron micrograph, it is possible to observe that, in all cases, Mo is homogeneously distributed over the whole support without segregation of Mo-rich species; this agrees with the above spectroscopic results and contrasts with what was observed for silica-supported molybdena. 23 Catalytic activity in ethanol conversion in the presence of oxygen: TPSR studies In Fig. 7 (supports) and 8 (Mo based catalysts), the results of Temperature Programmed Surface Reaction experiments realized on the catalytic materials under study are reported.…”

“…This loss is, in any case, much lower than that observed on pure silica supported catalysts. 23 FE-SEM images of all the catalysts tested in steady state conditions have also been recorded. Taking advantage of Back Scattered Electrons, it is possible to observe that Mo is still homogeneously dispersed over Mo12SØ, Mo12S1, and Mo12S5.…”

“…However, acetaldehyde yield is limited by the excessive acidity of the same MoO 3 species, producing ethylene as a main byproduct. 23 For these reasons, MoO 3 /γ-Al 2 O 3 catalysts were prepared in order to shed light on the effect of the addition of small amounts of silica in catalysts formulation; this goal can be achieved through a deep catalyst characterisation coupled with catalytic tests that allow the exploration of eventual deactivation phenomena in oxidative dehydrogenation conditions by reducing active phase volatilization.…”