Formation of active phases in MoVTeNb oxide catalysts for ammoxidation of propane

Abstract

The method of drying, heat evaporation or spray drying, of the aqueous suspension of starting chemicals has a pronounced effect on the phase composition of the final MoVTeNb catalyst, which ultimately influences the catalytic properties in propane ammoxidation reaction. The sample synthesized by spray drying is active and selective; it contains two main crystalline phases, orthorhombic M1 and hexagonal M2. The activity of the sample prepared by heat evaporation is low. This sample does not contain the active M1 phase and consists of hexagonal M2, TeMo₅O₁₆, and Mo_5−x(V/Nb)_xO₁₄ phases. The different mechanisms of phase composition formation in the samples synthesized by heat evaporation or spray drying arise from the different chemical nature of corresponding solid precursors.

Introduction

Selective heterogeneous catalytic oxidation of alkanes is a promising method for the production of value-added products from inexpensive and abundant raw materials. During the last decade, the MoVTeNb oxide catalysts developed by Mitsubishi Chemical Corporation have received a lot of attention [1], [2], [3]. These catalysts have shown high efficiency in selective oxidation and ammoxidation of propane. Their unique catalytic properties are attributed to the presence of orthorhombic M1 phase, which is responsible for activation of propane [4], [5], [6], [7]. Formation of the active phases responsible for selective catalytic transformations of propane is determined by the synthesis conditions and post-synthesis thermal treatment of the material.

High sensitivity of M1 phase formation to some parameters of synthesis such as slurry pH, presence of oxoacids in the starting solution, method of slurry drying, redox conditions during heat treatment [8], [9], [10], [11], [12] points to the importance of the nature of precursors in the catalyst preparation process.

The pH of the slurry strongly influences both the crystallinity and the nature of the precursors. Oliver et al. [9] have demonstrated that (NH₄)₆TeMo₆O₂₄·nH₂O, an Anderson-type heteropolyanion, was formed in samples prepared at pH 3.0–4.5. At pH < 3 the heteropolyanion partially decomposed with the formation of mostly amorphous materials. At a very low pH 1–1.5 polyoxovanadates were also present in the precursors.

Beato et al. [12] studied early stages of the catalyst formation by in situ Raman spectroscopy. The interaction between molybdenum, vanadium, and tellurium resulted in favored formation of an Anderson-type heteropolyanion [TeM₆O₂₄]ⁿ⁻ (M = Mo, V; n ≥ 6) and protonated decavanadate species [H_xV₁₀O₂₈]^(6−x)−. Raman analysis showed that the nature of the Anderson-type tellurate was preserved after addition of the aqueous niobium oxalate solution and the subsequent spray drying of the material. According to the authors [12], niobium affected the crystallinity of the polyoxometallates and acted as a link connecting [TeM₆O₂₄]ⁿ⁻ (n ≥ 6) units even in the solution. The conditions of the drying process were critical as well [8].

In this work we have studied the influence of the method of drying, heat evaporation or spray drying, of aqueous suspension of starting materials on the chemical and phase composition of the solid precursors and of final MoV_0.3Te_0.23Nb_0.12O_x catalyst calcined for 2 h in He flow at 600 °C.