The Nature of Surface CrOx Sites on SiO2 in Different Environments

Anisha Chakrabarti 0 Israel E. Wachs 0 UV-Vis Polymerization 0 0 A. Chakrabarti I. E. Wachs (&) Operando Molecular Spectroscopy and Catalysis Laboratory, Department of Chemical Engineering, Lehigh University , Bethlehem, PA 18015, USA This perspective article critically reviews the catalysis literature on the nature of the surface CrOx sites present on SiO2 in different environments. The recent application of in situ spectroscopic techniques that directly monitor the surface chromia sites on silica in different environments has significantly improved our fundamental understanding of supported CrOx/SiO2 catalysts. 1 Introduction In the early 1950s, J. P. Hogan and R. L. Banks of Phillips Petroleum Company made the discovery that ethylene can be converted to polyethylene by a chromium oxidesilica alumina catalyst [1] that was subsequently commercialized. The Phillips type catalyst system, CrOx supported on an amorphous support, such as silica, is one of three types of catalysts currently used for olefin polymerization. The other two types are ZieglerNatta and single-site homogeneous catalysts or supported homogeneous catalysts, both of which require an activator. The appeal of the Phillips catalyst lies in its many advantages: (i) yielding over 50 different types of polyethylene, (ii) functioning without activators, and (iii) operating at low temperatures and pressures [13]. The original catalyst system has since been fine-tuned and ethylene polymerization by silica supported CrOx catalysts is now responsible for *4050 % of all high-density polyethylene (HDPE) produced [3]. In spite of the extensive research studies that have been performed about the supported CrOx/SiO2 catalyst system over the past six decades, the same fundamental structural and mechanistic questions are still being debated [14]. For example, the initial molecular structure of the oxidized surface Cr?6Ox site has been proposed to be present as isolated surface dioxo CrO4, isolated surface mono-oxo CrO5, and dimeric surface Cr2O7, while the chromia oxidation state during ethylene polymerization has been proposed to be reduced Cr?2 and Cr?3. This perspective focuses on the nature of the surface chromia sites on silica in the different environments [oxidizing (hydrated and dehydrated) and reducing (CO, H2 and C2H4)] to stress what is currently known and what more needs to be done to fully understand the nature of the surface CrOx sites present for the silica-supported chromium oxide catalysts. 2 Advantages and Disadvantages of Characterization Techniques for Supported CrOx/SiO2 Catalysts Many characterization techniques have been used to investigate supported CrOx/SiO2 catalysts, but each method has its advantages and disadvantages. The typical techniques have been ultravioletvisible diffuse reflectance spectroscopy (UVVis DRS), electron paramagnetic resonance (EPR) spectroscopy, infrared (IR) spectroscopy, Raman spectroscopy, X-ray absorption spectroscopy (XAS), and temperature programmed surface reaction (TPSR) spectroscopy. UVVis spectroscopy probes electronic transitions, allowing for determination of the coordination and oxidation state of charge transfer (CT) Cr?6 bands and dd Cr?5,?3 transition bands. The broad UVVis bands, however, make it difficult to distinguish between multiple species and the dd transitions are weak and may overlap nearby CT or dd bands. The UVVis edge energy, Eg, provides direct information about the extent of oligomerization of the surface CrO?6 sites (monomer, dimer, etc.). EPR spectroscopy can detect paramagnetic (containing unpaired electrons) Cr?5 and Cr?3 sites and provide their coordination information. Although Cr?2 sites are also paramagnetic, its EPR signal is too weak to be detected with conventional EPR spectrometers. IR spectroscopy is a very powerful technique that provides molecular vibrational information and is more sensitive to asymmetric vibrations due to its selection rules. IR has been valuable in determining the anchoring sites of the surface CrOx species since the SiOH vibrations are rather intense. The strong IR absorption by the SiO2 support, however, only allows the monitoring of CrO vibrations in the *850970 cm-1 window. IR spectroscopy also provides vibrational information about surface intermediates and reaction products formed on the catalyst surface. Raman spectroscopy also probes molecular vibrations and is complementary to IR but is more sensitive to symmetric vibrations. The weak Raman bands from the SiO2 support allows monitoring vibrations of surface CrOx sites from 0 to 4000 cm-1 for the supported CrOx/SiO2 catalyst system. XAS includes X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). XANES allows determination of oxidation and coordination states by observing the pre-edge energy and selection rules. The position of the pre-edge energy reflects the oxidation state. For CrO4 coordinated sites, there is a strong pre-edge, but the pre-edge is forbidden for CrO6 coordinated sites with inverse symmetry due to the XANES selection rules. EXAFS allows determination of CrO bond distances. The shortcoming of XAS is that it averages over multiple sites if a single site is not present, which complicates analysis since the concentration of sites may not be known. Furthermore, the XAS signal reflects the dominant species if two sites are present with one having a high concentration and the other being a minority species (e.g., CrOx and Cr2O3 or Cr?6 and Cr?3). Thus, it is critical to make sure that Cr2O3 nanoparticles are not present since they will complicate analysis of the surface CrOx species. The presence of multiple Cr oxidation states also complicates the analysis. Temperature Programmed Surface Reaction (TPSR) spectroscopy chemically probes the reactivity of the supported chromia phase on silica. During TPSR, the catalyst temperature is ramped in the presence of a chemical reactant and the gaseous reaction products are continuously monitored with an online mass spectrometer. TPSR has the capability to discriminate between multiple surface CrOx sites on silica, quantify their relative concentrations and determine their reaction kinetics. No single characterization technique is sufficient to fully characterize the supported CrOx sites on SiO2 given the limitation of each characterization method. Applications of multiple characterization techniques, however, can tease out the desired fundamental information. Consequently, as will be seen below, the most informative studies employ multiple characterization techniques to yield a comprehensive model of the supported CrOx/SiO2 catalyst system. 3 Hydrated Supported CrOx/SiO2 in Initial Oxidized Catalyst under Ambient Conditions The supported CrOx on silica catalyst system is prepared by the incipient-wetness impregnation method employing a chromia precursor that is soluble in the solvent being employed. The impregnated catalyst is initiall (...truncated)