Elsevier

Journal of Catalysis

Volume 285, Issue 1, January 2012, Pages 48-60
Journal of Catalysis

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

https://doi.org/10.1016/j.jcat.2011.09.012 Get rights and content

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 nm2 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 Mo6+ and enrichment in V5+ sites. In the presence of steam in the feed, the active ensemble for acrylic acid formation appears to consist of V5+ oxo-species in close vicinity to Te4+ sites in a Te/V ratio of 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.

Graphical abstract

The surface of a highly crystalline MoVTeNb oxide catalyst composed exclusively of the M1 phase has been studied before catalysis and during operation in oxidation of propane at 693 K. Increasing formation of acrylic acid in general goes along with surface depletion of Mo and accordingly the enrichment in Te, V, and Nb. A close correlation exists between the acrylic acid production and the increasing presence of surface V5+ species.

  1. Download : Download high-res image (70KB)
  2. Download : Download full-size image

Highlights

► The surface of MoVTeNb oxide is characterized by a high number of energetically equivalent sites for propane adsorption. ► Restructuring of the catalyst surface during propane oxidation has been monitored under working conditions. ► Acrylic acid formation correlates with surface depletion in Mo6+ and enrichment in V5+ sites. ► The active ensemble contains V5+ oxo-species in close vicinity to Te4+ sites.

Introduction

Alkanes are abundant in fossil resources, such as crude oil and natural gas, and accessible from coal or biomass via synthesis gas chemistry. Oxidative dehydrogenation and selective oxidation of alkanes represent prospective routes to the manufacture of chemical building blocks and intermediates, like olefins and oxygenates, by heterogeneous catalysis. Technical realization of alkane oxidation processes is generally limited due to insufficient productivity. Selectivity to the desired reaction product is the decisive factor in oxidation catalysis in view of an efficient usage of feedstock and energy. Sophisticated catalyst design is required to activate carbon–hydrogen bonds in the non-polar hydrocarbon molecule lowering the activation barrier such that adequate reaction temperatures for subsequent or concurrent oxygen insertion into generally more reactive unsaturated intermediates can be achieved while partial or total oxidation to carbon oxides is prevented. Vanadium in oxidation state 5+ has been suggested to be of essential relevance in alkane activation [1].

MoVTeNb oxides in the form of an orthorhombic phase, called M1 phase, are active and selective catalysts in ammoxidation of propane to acrylonitrile [2], oxidation of propane to acrylic acid [3], [4], and oxidative dehydrogenation of ethane to ethylene [5], [6]. MoVTeNb oxides have also been studied in oxidative dehydrogenation of propane [7], selective oxidation of butane to maleic anhydride [8], and alcohol oxidation [9]. The complex chemistry of the polycrystalline MoVTeNb oxide catalyst is reflected in its crystal structure [10], [11], [12]. The model of the M1 unit cell comprises 44 atoms. Metal–oxygen linkages in the polyhedral network give rise to a channel-like structure with 5-sided pillars filled with Nb, and 6-, and 7-sided channels partially occupied by tellurium oxide entities. Structural building blocks, like the pentagonal {(Mo)Mo5} unit, which consists of a central bipyramidal MO7 polyhedron sharing edges with five MO6 octahedra, are well known from supramolecular polyoxometalates representing a link between molecular species in solution and metal oxide networks [13]. Crystal growth of M1 occurs in c direction resulting in prismatic particle morphology with the {0 0 1} basal plane of the orthorhombic structure arranged perpendicular to the length axis of the cylindrical catalyst particles. Recent concepts directed toward the nature of the active sites on the surface of such catalysts were inspired by the structural specifics of the M1 phase assuming rigid links between MoO6 and VO6 octahedra as sites for propane activation [14]. The peculiar propane activation efficiency of the M1 phase has been attributed to its ability to host V5+ species, the presence of which has been ruled out for the less active M2 phase [14]. The efficiency of M1 in selective oxidation of propane to acrylic acid was originally attributed to the distortion of the octahedral units in the M1 crystal structure on its terminating basal plane arising in particular from the arrangement of heptagonal rings that causes strain [15]. The channels exhibit no detectable porosity due to occupation by tellurium. They open up at the basal plane and have been suggested to be of importance in the reduction and reoxidation of the surface during catalysis [16], [17]. Definite occupations of metal framework M1 positions have been proposed to result in the formation of site-isolated active ensembles on its terminating basal plane that are characterized by close proximity of the required catalytic functions, and, consequently, attributed to high selectivity [18], [19]. In this respect, vanadyl groups with vanadium at the crystallographic positions M3 and M7 have been considered to be particularly responsible for C–H activation of propane in the rate-determining abstraction of the first hydrogen atom.

Catalytic experiments trying to verify the assignment of the minority {0 0 1} faces as location of active sites gave conflicting results [20], [21], [22]. Approximately 80% of the catalyst surface area accounts for the lateral surface of the cylindrical M1 particles. Electron microscopy has shown that the latter is characterized by a stepped morphology presumably resulting in similar terminating metal-oxo arrangements like on the basal plane [23]. STEM of a single M1 crystal viewed in the projection along 〈0 0 1〉 revealed that the crystal periodicity is broken along the thinnest sections through the mesh of polyhedra. Consequently, the lateral surface of the cylindrical M1 particles is constituted of roughly half-pipes from the formerly closed channels and from wall fragments exposing the inner surface of the channels. This termination model provides a rational explanation for the enriched tellurium content generally observed when surface-sensitive methods are applied for analysis of M1 [4], [16], [24], [25], [26], [27], [28], [29]. The actual termination may differ depending on local cation site occupancy [30], synthesis method, pretreatment, and reaction conditions. In conclusion, a thorough characterisation of the M1 phase is required to obtain a detailed understanding of its functionality.

We reported on the catalytic properties of a number of phase-pure M1 catalysts with varying chemical composition in the M1 framework [29]. Vanadium and tellurium have been identified as key elements in selective oxidation of propane to acrylic acid, whereas surface enrichment of molybdenum is detrimental with respect to acrylic acid selectivity. These observations have prompted us to suggest a functional model in which the crystal structure of M1 is considered as a host that bears under operation an active thin layer comprising a thickness of about 1 nm that is connected by chemical bonds to the framework and that is composed of isolated VxOy moieties embedded in a matrix of TexOy surface species. Further elucidation of the actual electronic and molecular structure of the active ensembles on the surface of M1 requires in situ spectroscopic investigations of the catalyst under operation conditions. In the present work, a well-defined, phase-pure, and highly crystalline M1 catalyst was synthesized to study its surface properties before catalysis by adsorption of probe molecules using infrared spectroscopy and microcalorimetry. The response of the terminating layer of the M1 phase to the chemical potential of the gas phase during oxidation of propane to acrylic acid has been investigated applying in situ photoelectron spectroscopy.

Section snippets

Synthesis of M1 and reference compounds

Phase-pure M1 was synthesized using a precipitation–purification procedure [31]. First, 92.86 mmol of (NH4)6Mo7O24⋅4H2O was dissolved at 353 K in 1.5 L of MQ water, and then, 195.00 mmol of NH4VO3 was added as a solid to the solution and dissolved. Next, the Mo/V-containing solution was cooled to 313 K, and 149.50 mmol of Te(OH)6 was added in solid form and dissolved. In parallel, the Nb-containing solution was prepared by dissolution of 81.25 mmol of NH4[NbO(C2O4)2]⋅xH2O in 0.5 L of MQ water at 313 K.

General properties of M1 and reference compounds

Chemical, structural, and textural details of the M1 catalyst are summarized in Table 1. The phase purity was verified by XRD. The XRD patterns (not shown) are exclusively characterized by reflections that arise from the M1 phase. No peaks were detected related to the presence of any other crystalline phase, especially not the most likely M2 phase. The lattice parameters determined by Rietveld analysis are given in Table 1. Slight differences compared to lattice constants reported in the

Discussion

The oxidation of propane proceeds with high selectivity to the desired partial oxidation products propylene and acrylic acid over MoVTeNb oxide composed exclusively of the M1 phase. The well-defined and stable bulk structure of this phase provides an appropriate basis for studying surface dynamics necessarily associated with catalysis of redox reactions. Selective transformation of propane to acrylic acid involves the abstraction of four hydrogen atoms, the insertion of two oxygen atoms, and

Conclusions

Based on in situ investigations of an activated highly crystalline, phase-pure M1 MoVTeNb oxide catalyst before catalysis and under working conditions of propane oxidation to acrylic acid the following model of the active catalyst surface is proposed:

  • 1.

    The catalytic properties of the M1 phase of MoVTeNb oxide seem to be controlled by the redox chemistry of the solid, because the acidity, which mainly comprises Brønsted acid sites, is comparatively weak, but, probably contributes to the disruption

Acknowledgments

The authors thank G. Lorenz and D. Brennecke for their help with the N2 physisorption measurements, Dr. Wei Zhang for EDX, and Dr. G. Auffermann (MPI for Chemical Physics of Solids, Dresden, Germany) for chemical analysis. We thank Dr. Alexander Yu. Stakheev for stimulating discussions at the Second Geman-Russian Seminar on Catalysis, Kloster Seeon, March 2010. The HZB staff is acknowledged for their continual support of the high-pressure electron spectroscopy activities of the FHI at BESSY II.

References (63)

  • P. Botella et al.

    J. Catal.

    (2002)
  • W. Ueda et al.

    Catal. Today

    (2004)
  • P. Botella et al.

    J. Catal.

    (2004)
  • T. Katou et al.

    Catal. Today

    (2004)
  • B. Solsona et al.

    J. Catal.

    (2007)
  • F. Wang et al.

    Appl. Catal. A: Gen.

    (2008)
  • H. Murayama et al.

    Appl. Catal. A: Gen.

    (2007)
  • R.K. Grasselli et al.

    Catal. Today

    (2004)
  • K. Oshihara et al.

    Top. Catal.

    (2001)
  • J.M.M. Millet et al.

    Appl. Catal. A: Gen.

    (2002)
  • R.K. Grasselli

    Catal. Today

    (2005)
  • A. Celaya Sanfiz et al.

    J. Catal.

    (2008)
  • M. Baca et al.

    Appl. Catal. A: Gen.

    (2005)
  • P. Botella et al.

    Catal. Today

    (2005)
  • A. KnopGericke et al.

    Ray photoelectron spectroscopy for investigation of heterogeneous catalytic processes

  • M. Salmeron et al.

    Surf. Sci. Rep.

    (2008)
  • J.J. Yeh et al.

    At. Data Nucl. Data Tables

    (1985)
  • Y.V. Belokopytov et al.

    J. Catal.

    (1979)
  • Q. Sun et al.

    J. Mol. Catal. A: Chem.

    (2007)
  • P. Concepcion et al.

    Appl. Catal. A: Gen.

    (2004)
  • M. Baca et al.

    Catal. Commun.

    (2005)
  • N. Cardona-Martinez et al.

    Applications of adsorption microcalorimetry to the study of heterogeneous catalysis

  • A.L. McClellan et al.

    J. Colloid Interface Sci.

    (1967)
  • W. Ueda et al.

    Catal. Today

    (2005)
  • E. Balcells et al.

    Appl. Catal. A: Gen.

    (2004)
  • X. Rozanska et al.

    J. Phys. Chem. C

    (2007)
  • R.K. Grasselli et al.

    Top. Catal.

    (2003)
  • L. Yuan et al.

    Top. Catal.

    (2008)
  • M. Aouine et al.

    Chem. Commun.

    (2001)
  • P. DeSanto et al.

    Z. Kristallogr.

    (2004)
  • A. Müller et al.

    Chem. Commun.

    (1999)
  • Cited by (148)

    View all citing articles on Scopus
    1

    Present address: Department of Solar Energy Research, Helmholtz-Zentrum Berlin/BESSY II, Albert-Einstein-Str. 15, 12489 Berlin, Germany.

    2

    Present address: International Iberian Nanotechnology Laboratory, Avda Central 100, Edificio dos Congregados, 4710-229 Braga, Portugal.

    View full text