Surface chemistry of phase-pure M1 MoVTeNb oxide during operation in selective oxidation of propane to acrylic acid

Abstract

The surface of a highly crystalline MoVTeNb oxide catalyst for selective oxidation of propane to acrylic acid composed of the M1 phase has been studied by infrared spectroscopy, microcalorimetry, and in-situ photoelectron spectroscopy. The acid-base properties of the catalyst have been probed by NH3 adsorption showing mainly Brønsted acidity that is weak with respect to concentration and strength of sites. Adsorption of propane on the activated catalyst reveals the presence of a high number of energetically homogeneous propane adsorption sites, which is evidenced by constant differential heat of propane adsorption qdiff,initial=57 kJ mol -1 until the monolayer coverage is reached that corresponds to a surface density of approximately 3 propane molecules per nm 2 at 313 K. The decrease of the heat to qdiff,initial=40 kJ mol -1 after catalysis implies that the surface is restructured under reaction conditions. The changes have been analyzed with high-pressure in-situ XPS while the catalyst was working applying reaction temperatures between 323 and 693 K, different feed compositions containing 0 mol.-% and 40 mol.-% steam and prolonged reaction times. The catalytic performance during the XPS experiments measured by mass spectrometry is in good agreement with studies in fixed bed reactors at atmospheric pressure demonstrating that the XPS results taken under operation show the relevant active surface state. The experiments confirm that the surface composition of the M1 phase differs significantly from the bulk implying that the catalytically active sites are no part of the M1 crystal structure and occur on all terminating planes. Acrylic acid formation correlates with surface depletion in Mo 6+ and enrichment in V 5+ sites. In presence of steam in the feed, the active ensemble for acrylic acid formation appear to consist of V 5+ oxospecies in close vicinity to Te 4+ sites with an element ratio Te/V=1.4. The active sites are formed under propane oxidation conditions and are embedded in a thin layer enriched in V, Te, and Nb on the surface of the structural stable self-supporting M1 phase.

Chemical, Structural, and Morphological Changes of a MoVTeNb Catalyst during Oxidative Dehydrogenation of Ethane

“…13,[23][24][25][26][27][28][29][30][31][32][33] In this sense, López-Nieto et al have reported an important tellurium loss during thermal treatment activation of catalyst, which is commonly performed at 600 °C under nitrogen. 34 Indeed, the Mo/Te ratio increases when increasing the niobium oxalate content, since, oxalate anions act as reducing agent, which enhances the reduction degree of tellurium cations, rendering it instable and easily removal from the crystalline structures.…”

Heterogeneous Catalysis on Metal Oxides

“…It was observed [26] that the surface composition of the M1 phase differs significantly from the bulk, implying that the catalytically active sites are not part of the M1 crystal structure and rather part of all terminating planes. It then appeared that the active sites are formed under propane oxidation conditions and are embedded in a thin layer enriched in V, Te, and Nb on the surface of the stable self-supporting M1 phase [23]. This example shows how complex a catalytic surface can be and how the surface structure and chemical composition may change under reaction (working) conditions.…”

“…In a study using microcalorimetry, Schlögl et al have shown [23] that, on an activated mixed oxide MoVTeNb-O catalyst, active for propene and propane selective oxidation and ammoxidation to acrylic acid and acrylonitrile, respectively, the adsorption energy of propene and propane are about constant vs. coverage (57 kJ·mol −1 ). However, after reaction the energy decreases and its distribution in strength becomes heterogeneous, as illustrated in Figure 4.…”

Characterization of MoVTeNbOx Catalysts during Oxidation Reactions Using In Situ/Operando Techniques: A Review

“…The mobility of Te in the presence of water vapor (steam) causes the enhanced selectivity to acrylic acid [17,75]. Adsorption of propane after catalysis shows a decrease of the heat of adsorption, which implies that the surface is restructured during the reaction [17]. This surface restructuring also indicates that active sites do not contain all the elements of the M1 phase and the surface composition of the M1 phase differs greatly from the bulk.…”