Behavior of Molybdenum⁻Vanadium Mixed Oxides in Selective Oxidation and Disproportionation of Toluene.

Gheorghita Mitran,Mihaela Trandafir,Octavian Pavel,Mihaela Florea,Florentina Neaţu

doi:10.3390/ma12050748

Gheorghita Mitran, Mihaela Trandafir + Show 3 more

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https://doi.org/10.3390/ma12050748

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Abstract

This study deals with the behavior of molybdenum–vanadium (Mo/V) mixed oxides catalysts in both disproportionation and selective oxidation of toluene. Samples containing different Mo/V ratios were prepared by a modified method using tetradecyltrimethylammonium bromide and acetic acid. The catalysts were characterized using several techniques: nitrogen adsorption–desorption isotherms, X-Ray diffraction (XRD), ammonia temperature-programmed desorption (TPD-NH3), temperature-programmed reduction by hydrogen (H2-TPR), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, Fourier-transform infrared-spectroscopy (FTIR) and ultraviolet-visible spectroscopies (UV–VIS). The XRD results evidenced the presence of orthorhombic α-MoO3 and V2O5 phases, as well as monoclinic β-MoO3 and V2MoO8 phases, their abundance depending on the Mo to V ratio, while the TPD-NH3 emphasized that, the total amount of the acid sites diminished with the increase of the Mo loading. The TPR investigations indicated that the samples with higher Mo/V ratio possess a higher reducibility. The main findings of this study led to the conclusion that the presence of strong acid sites afforded a high conversion in toluene disproportionation (Mo/V = 1), while a higher reducibility is a prerequisite to accomplishing high conversion in toluene oxidation (Mo/V = 2). The catalyst with Mo/V = 1 acquires the best yield to xylenes from the toluene disproportionation reaction, while the catalyst with Mo/V = 0.33 presents the highest yield to benzaldehyde.

Highlights

Disproportionation of toluene is an important route to produce xylenes, as valuable intermediates for the production of more added value polymeric materials, such as polyesters or polyethylene terephthalates (PET) [1,2]
The X-ray diffraction (XRD) patterns of the mixed molybdena–vanadia catalysts, gathered in Figure 1, show specific diffraction lines of (i) orthorhombic molybdite phase of α-MoO3 (JCPDS card no. 76-1003), which consist of layered MoO6 octahedra, corner or edge-connected between them [29], (ii) monoclinic phase of β-MoO3 (JCPDS card no. 47-1081) containing corner of edged shared MoO6 octahedra arranged this time in a perovskite-like structure [30], and (iii) base-centered monoclinic V2 MoO8 phase (JCPDS card no. 20-1377), which was ascribed to a shear structure of octahedra integrated in a three-dimensional network of corner-linked octahedra [31]
It is obvious that the catalysts with low Mo/V ratios present higher amounts of Brønsted acid sites, while by increasing the Mo loading the amount of Lewis type sites became preponderant. Correlating these results found in NH3 -TPD measurements with the phases identified in XRD, we can assume that the addition of Mo, together with the formation of MoO3 phases, is strongly related to the presence of the strong acid sites, while the presence of V in higher amounts favors the formation of

Summary

Introduction

Disproportionation of toluene is an important route to produce xylenes, as valuable intermediates for the production of more added value polymeric materials, such as polyesters or polyethylene terephthalates (PET) [1,2]. This process has been intensively studied over the years using different zeolites as catalysts [3,4,5,6,7]. The addition of La2 O3 led to a decrease in the acidity of ZSM-5 [10] and of the activity, but the reaction took place with an enhancement in the p-xylene

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