Robert K. Grasselli

Fundamental Principles of Selective Heterogeneous Oxidation Catalysis

Seven principles (“seven pillars”) underpinning selective heterogeneous oxidation catalysis, comprising lattice oxygen, metal-oxygen bond strength, host structure, redox, multifunctionality of active sites, site isolation, and phase cooperation are described. Their intended utilization towards a deeper understanding of metal oxide catalyst behavior and application towards catalyst design are indicated.

Multifunctionality of active centers in (amm)oxidation catalysts: from Bi-Mo-Ox to Mo-V-Nb-(Te, Sb)-Ox

Catalytic centers in selective (allylic) oxidation and ammoxidation catalysts are multimetallic and multifunctional. In the historically important bismuth molybdates, used for propylene (amm)oxidation, they are composed of (Bi3+)(Mo6+)2 complexes in which the Bi3+ site is associated with the α-H abstraction and the (Mo6+)2 site with the propylene chemisorption and O or NH insertion. An updated reaction mechanism is presented. In the Mo–V–Nb–Te–Ox systems, three crystalline phases (orthorhombic Mo7.5V1.5NbTeO29, pseudohexagonal Mo6Te2VO20, and monoclinic TeMo5O16) were identified, with the orthorhombic phase being the most important one for propane (amm)oxidation. Its active centers contain all necessary key catalytic elements (2V5+/Mo6+, 1V4+/Mo5+, 2Mo6+/Mo5+, 2Te4+) for this reaction wherein a V5+ surface site (V5+ = O ↔ 4+V•–O•) is associated with paraffin activation, a Te4+ site with α-H abstraction once the olefin has formed, and a (Mo6+)2 site with the NH insertion. Four Nb5+ centers, each surrounded by five molybdenum octahedra, stabilize and structurally isolate the catalytically active centers from each other (site isolation), thereby leading to high selectivity of the desired acrylonitrile product. A detailed reaction mechanism of propane ammoxidation to acrylonitrile is proposed. Combinatorial methodology identified the nominal composition Mo0.6V0.187Te0.14Nb0.085Ox for maximum acrylonitrile yield from propane, 61.8% (86% conversion, 72% selectivity at 420 °C). We propose that this system, composed of 60% Mo7.5V1.5NbTeO29, 40% Mo6Te2VO20, and trace TeMo5O16, functions with a combination of compositional pinning of the optimum orthorhombic Mo7.5V1.5±xNb1±yTe1±zO29±δ phase and symbiotic mop-up of olefin intermediates through phase cooperation. Under mild reaction conditions, a single optimum orthorhombic composition might suffice as the catalyst; under demanding conditions this symbiosis is additionally required. Improvements in catalyst performance could be attained by further optimization of the elemental distributions at the active catalytic center of Mo7.5V1.5NbTeO29, by promoter/modifier substitutions, and incorporation of compatible cocatalytic phases (preferably epitaxially matched). High-throughput methods will greatly accelerate the rational catalyst design processes.

Structural aspects of the M1 and M2 phases in MoVNbTeO propane ammoxidation catalysts

Abstract The structures of M1 and M2 in MoVNbTeO propane ammoxidation catalysts have been solved using a combination of TEM, neutron powder diffraction, and synchrotron X-ray powder diffraction. The unit cell of M1 is Pba2 (No. 32) with a = 21.134(2) Å, b = 26.658(2) Å, c = 4.0146(3) Å and Z = 4. The formula unit is Mo7.8V1.2NbTe0.937O28.9. The unit cell of M2 is Pmm2 (No. 25) with a = 12.6294(6) Å, b = 7.29156(30) Å, c = 4.02010(7) Å and Z = 4. The formula unit is Mo4.31V1.36Te1.81Nb0.33O19.81. Tellurium sites in hexagonal channels of both phases are displaced toward vanadium-occupied framework sites, whereas Te in the heptagonal channel of M1 is near the channel center. The chemical topology resulting from oxidation states and Madelung site potentials presents active moieties for the ammoxidation of propane in M1 and propene in M2. EPR confirmed the presence of V4+ and possibly Mo5+ in M1 and V4+ in M2.

Structural Characterization of the Orthorhombic Phase M1 in MoVNbTeO Propane Ammoxidation Catalyst

The structure of the orthorhombic phase in the MoVNbTeO propane ammoxidation catalyst system has been characterized and refined using a combination of TEM, synchrotron X-ray powder diffraction (S-XPD), and neutron powder diffraction (NPD). This phase, designated as M1 by Ushikubo et al. [1], crystallizes in the orthorhombic space group Pba2 (No. 32) with a = 21.134(2) Å, b = 26.658(2) Å, and c = 4.0146(3) Å. The formula unit is Mo7.5V1.5NbTeO29. Bond valence sum calculations indicate the presence of d1 metal sites neighbored by d0 metal sites. The d1 sites are occupied by a distribution of Mo5+ and V4+, whereas the d0 sites are occupied by a distribution of Mo6+ and V5+. Out-of-center distortions in d0 octahedra are consistent with the second-order Jahn–Teller effect and lattice effects. We argue that the V5+–O–V4+/Mo5+ moieties adjacent to Te4+ and Mo6+ sites in the [001] terminal plane provide a spatially isolated active site at which the selective ammoxidation of propane occurs.

Genesis of site isolation and phase cooperation in selective oxidation catalysis

Two of the important principles governing selective oxidation catalysis are site isolation and phase cooperation. They are discussed here in light of their inception, development, historic perspective and current acceptance in the field of selective oxidation and ammoxidation catalysis.

Catalytic dehydrogenation (DH) of light paraffins combined with selective hydrogen combustion (SHC): I. DH → SHC → DH catalysts in series (co-fed process mode)

Abstract Sb2O4, In2O3, WO3 and Bi2O3, supported on 50% SiO2, were found to be highly selective hydrogen combustion (SHC) catalysts. Their respective selectivities are 99.8, 99.7, 98.5 and 98.1% at 500°C, atm pressure, WHSV 2xa0h−1 and C°3/C3=/H2/O2xa0=xa080/20/20/10. Their activities vary greatly, reflected by the first-order hydrogen combustion constants (kH2, s−1) which are: In2O3 1.57, Bi2O3 0.53, WO3 0.36, Sb2O4 0.22. Among the SiO2, Al2O3, TiO2 and ZrO2 supports tested, ZrO2 was found to be the best overall carrier for the highly active In2O3. When a 0.7xa0wt% Pt-Sn-ZSM-5 dehydrogenation (DH) catalyst and a 10xa0wt% In2O3/ZrO2 SHC catalyst are used in a sequential microreactor DHxa0→xa0SHCxa0→xa0DH co-fed process mode, higher than equilibrium yields of light olefins are obtained from the corresponding paraffins. At 550°C, atmospheric pressure and WHSV of 2xa0h−1 propylene yields of 29.7% at 97% selectivity (0.3 air/propane in SHC), and 33% at 89% selectivity (0.6 air/propane in SHC) are realized, compared to the equilibrium yield of 25% at 99% selectivity when only the DH catalyst is used. The yield improvements over equilibrium dehydrogenation are 19 and 32%, respectively. Under the same operating conditions but 0.2 air/isobutane ratio in the SHC stage, the isobutylene yield from isobutane is 47.5% at 99+% selectivity as compared to 40% and 99+% selectivity when only the DH catalyst is used; i.e. an 18% yield improvement over equilibrium. The above metal oxide SHC catalyst systems might find application to improve conventional DH processes such as Oleflex and Catofin.

A Study of the Functionalities of the Phases in Mo–V–Nb–Te Oxides for Propane Ammoxidation

Catalysts belonging to the Mo–V–Nb–Te–O system have been prepared with both a slurry method and hydrothermal synthesis and were tested for propane and propylene ammoxidation to acrylonitrile. All samples were characterized with BET, XRD, ICP and XPS. The catalysts were found to consist of three phases, to which activity and selectivity correlations were made. The results indicate that both an orthorhombic phase and a hexagonal phase are needed to have an active and selective catalyst. The orthorhombic phase is the most active for propane conversion although less selective than the hexagonal phase for the conversion of formed propylene to acrylonitrile.

Catalytic dehydrogenation (DH) of light paraffins combined with selective hydrogen combustion (SHC): II. DH+SHC catalysts physically mixed (redox process mode)

Abstract When a physical mixture composed of a 0.7xa0wt.% Pt–Sn-ZSM-5 dehydrogenation (DH) catalyst and a 42xa0wt.% Bi 2 O 3 /SiO 2 selective hydrogen combustion (SHC) catalyst is subjected to a DHxa0+xa0SHC redox process operation, it gives substantially higher than equilibrium propylene yields from propane. At 540°C, atmospheric pressure and WHSV of 2xa0h −1 , initial microreactor propylene yields from neat propane are 48.2% at about 90% selectivity, compared to equilibrium olefin yields of 20.0% at about 95% selectivity when only the DH catalyst is used. The overall propylene yield improvement is 140%. In this novel redox process arrangement no gaseous oxygen is cofed to the physical mixture; the lattice oxygen of the SHC catalyst is the sole oxidizing agent to selectively combust the hydrogen produced in situ by the dehydrogenation of the propane, catalyzed by the commingled DH catalyst. Periodic regeneration with air is necessary to replenish the lattice oxygen of the SHC catalyst depleted in the redox reaction. Although still significantly (i.e., 65%) above equlibrium, the high initial DHxa0+xa0SHC propylene yields are not sustainable and decline from 48.2% (first cycle) to 33.0% (tenth cycle); i.e., 2.1%/cycle; they are ascribed to the loss of Bi 2 O 3 dispersion on the SiO 2 support caused by the deep redox cycling. Shortening the redox cycle or increasing the SHC/DH catalyst ratio lessen the decline. However, the identification of a more redox stable catalyst is imperative to make the process practical. Some suggestions are provided. It is surmised that the DHxa0+xa0SHC redox process approach will ultimately (i.e., once a sufficiently redox stable SHC catalyst is identified) be preferred over the DH-xa0>xa0SHC-xa0>xa0DH cofed approach to improve conventional dehydrogenation processes such as the Oleflex and Catofin processes.

Promoted NiO Catalysts for the Oxidative Dehydrogenation of Ethane

Metal oxide promoted NiO catalysts with a Ni/(Mexa0+xa0Ni) atomic ratio 0.92xa0have been investigated for the oxidative dehydrogenation of ethane. These materials have been characterized by several techniques (N2-adsorption, X-ray diffraction, X-ray photoelectron spectroscopy and Fourier transformed infrared spectroscopy of adsorbed CO and ethylene). The nature of surface sites is strongly influenced by the valence and the acid/base characteristics of the metal oxide promoters, which have a great impact on the selectivity to ethylene. Accordingly, a clear correlation between selectivity to ethylene and the valence of the promoter has been observed in the present work. Additionally, the acidity of the catalyst also enhances the selectivity to ethylene.

How the Yield of Maleic Anhydride in n -Butane Oxidation, Using VPO Catalysts, was Improved Over the Years

Key patents for the oxidation of n-butane to maleic anhydride (MA) using V/P mixed metal oxides (VPO) have been analyzed to evidence the important parameters needed to optimize the catalytic behaviour. The important aspects for the optimization of the catalyst performance are the preparation of the precursor (VO)HPO4·0.5H20, its activation to form the active catalyst VO2P2O7 and a small amount of VOPO4, the methodology of the addition of promoters, and the shaping of the catalyst. Even small, sustainable improvements in the MA yield (>1xa0%) achieved by improved, modified or optimized catalysts are significant on an industrial scale with the world production of MA being a respectable 2.7 Million tons/year. Although substantial improvements in MA yield have been achieved industrially over the past 40xa0years by improving the VPO catalyst composition and by optimizing process operations; the MA process remains, as practiced today, one of the least efficient industrial selective oxidation processes. Therefore, a huge incentive exists in the field to improve the MA catalyst and process, which led us to search for clues towards this end by analyzing the pertinent patent literature, as reported here.