Olga Ovsitser

Molybdenum oxide based partial oxidation catalyst: Part 3. Structural changes of a MoVW mixed oxide catalyst during activation and relation to catalytic performance in acrolein oxidation

The activation of a Mo9V3W1.2Ox catalyst was investigated in the partial oxidation of acrolein as function of reaction temperature and atmosphere. The activity and selectivity to acrylic acid considerably increased during activation in the acrolein oxidation reaction comparable to the recently reported activation after thermal pretreatment in inert gas. The activation during the catalytic acrolein oxidation, however, proceeds at about 200 K lower temperatures as compared to the inert gas pretreatment. The initial nanocrystalline catalyst structure changed during operation in the acrolein oxidation, as shown by X -ray diffraction (XRD), scanning electron microscopy (SEM), transition electron microscopy and energy dis- persive X-ray (EDX) analysis. A (MoVW)5O14-type mixed oxide was found to be the main crystalline phase in the active and selective catalysts. Hence, this (MoVW)5O14 phase crystallizes already during catalysis at the low acrolein oxidation reaction temperatures. The evolution of the catalytic performance is directly related to this low temperature formation of the (MoVW)5O14 phase. It is suggested that this (MoVW)5O14 phase has to be present in a detected, specific ordering state which is vital for optimum selective oxidation properties. In addition, other minority phases were identified by transmission electron microscopy (TEM) in the operating catalyst. The so-called bundle-type, and a new corona-type texture was detected, which show ordering in only one or two dimensions, respectively. These disordered structures are also relevant candidates for active catalyst phases as they are detected during the activation period of the MoVW mixed oxide.

The synthesis and structure of a single-phase, nanocrystalline MoVW mixed-oxide catalyst of the Mo5O14 type

The different preparation steps are characterized to the single phase, crystalline, ternary oxide (MoVW) 5O14, which is important for catalytic, mild selective oxidation reactions. For the synthesis of this oxide, solutions of ammonium heptamolybdate, ammonium metatungstate, and v anadyl oxalate were spray-dried followed by different thermal treatments. The structures of the materials formed at each preparation step, starting from the precursor to the final product, were studied using scanning and transmission electron microscopy, X -ray powder diffraction, thermal analysis, and Raman spectroscopy. Raman spectroscopy was also applied to shed some light into the aqueous chemistry of the mixed precursor solutions. Raman data indicates that a molecular structure is already formed in solution which seems to be closely related to that of the final crystalline Mo5O14-type oxide. X-ray diffraction revealed that the thermal treatment steps strongly affect the degree of crystallinity of the ternary Mo5O14 oxide. Transmission electron microscopy with energy dispersive microanalysis confirmed the presence of V and W in the molybdenum oxide particles and gave evidence for the (010) plane as the most developed face of the crystals of this phase. Details of the structural transformation of this system at the different preparation and calcination steps are discussed in relation to their performance in the selective partial oxid ation of acrolein to acrylic acid.

Catalytic Abatement of Nitrous Oxide Coupled with Selective Production of Hydrogen and Ethylene

In the past two decades, global warming caused by anthropogenic emissions of greenhouse gases has become an intensively discussed topic of public and scientific interest. The Kyoto protocol to the United Nations Framework Convention on Climate Change is a practical step to control the emissions of environmentally harmful gases. The protocol obliges participating countries to reduce emission of CO2 as well as non-CO2 greenhouse gases. One of the non-CO2 greenhouse gases is nitrous oxide (N2O), which has a 310 times greater potential than CO2 to warm up the atmosphere. Moreover, N2O contributes to the destruction of ozone in the stratosphere. One of the anthropogenic sources of N2O is the production of adipic and nitric acids, both of which are key components in the manufacture of a variety of commercial products. The estimated annual production (without abatement) of N2O in these processes [1,2] is approximately 1.3 Mton, which corresponds to about 20% of the overall anthropogenic N2O emissions. [3] There are several commercial N2O removal technologies [3–6] that are based on catalytic or thermal conversion of N2O to N2 and O2. When N2O is abated by selective catalytic reduction, the reducing agents, such as natural gas and/or ammonia, are converted into COx and N2, respectively. However, a process that combines N2O removal with the simultaneous production of important chemical products would be both more sustainable and economically attractive. In this regard Solutia Inc., in collaboration with the Boreskov Institute of Catalysis (BIC), developed a technology that employs pure N2O as an oxidant to produce phenol from benzene over Fe-MFI zeolites. Fe-MFI zeolites are also promising catalysts for the oxidative dehydrogenation of propane to propene using N2O as an oxidant. [9,10] These elegant approaches, however, suffer from fast catalyst deactivation and low selectivity when N2O is contaminated with O2 and NOx, which is the case with off-gases from the production of adipic and nitric acids. Unfortunately, the purification of off-gases results in a substantial cost increase of the above processes, which makes them uneconomical. Thus, better catalysts tolerant to O2 and NOx are key to allow for the development of green processes using waste N2O. Herein, we present a novel process and catalyst for N2O decomposition to N2 with simultaneous production of H2 and C2H4 from C2H6. Our concept is based on the use of the exothermic N2O decomposition for the thermal dehydrogenation of ethane. Calcium oxide doped with small amounts of sodium oxide (Na/CaO) was used as a catalyst. To determine whether the procedure for catalyst preparation is reproducible, two catalyst charges were prepared with sodium concentrations of 0.9 and 1.5 at.% (Na0.009CaO and Na0.015CaO). It was found that the catalysts are active for direct N2O decomposition. For example, a nearly complete (ca. 99%) N2O conversion was achieved when an N2O–Ne mixture (40 vol.% N2O in Ne) was fed over Na0.009CaO at 903 K with a contact time of 0.048 sgcatmL . As a result of the exothermicity of N2O decomposition, the catalyst temperature rose above 1100 K. The N2O conversion without catalyst did not exceed 5% at 903 K and no temperature increase was detected. However, the catalyst temperature increased above 1100 K when a mixture of N2O and C2H6 (N2O/C2H6/Ne= 40:40:20) was fed over Na/CaO at 903 K with a contact time of 0.048 sgcatmL . Moreover, N2O is almost completely (X(N2O)> 99%) converted into N2, while C2H6 is converted (X(C2H6)> 50%) into C2H4 and H2; CH4, COx, and H2O were also observed as reaction products. The conversion of N2O and C2H6 did not exceed 10 and 18%, respectively, when the same N2O–C2H6 mixture was fed to the reactor without catalyst (filled with 250–350-mm SiO2 particles) at 1023 K. Thus, Na0.009CaO and Na0.015CaO catalyze direct N2O decomposition and N2O abatement with C2H6. The catalytic performance of Na/CaO materials in N2O removal with simultaneous generation of C2H4 and H2 from C2H6 is shown in Table 1. The N2O conversion (X) to N2 was above 99%. No significant difference in the catalytic performance of Na0.009CaO and Na0.015CaO was observed, which indicates the good reproducibility of the method of catalyst preparation. Ethane is converted into ethylene with approximately 50% yield (Y) and 63% selectivity (S). Other C-

Molybdenum oxide based partial oxidation catalyst: 4. TEM identification of a new oxygen-reduced phase formed during acrolein partial oxidation under reducing conditions

A new Mo-based structure was detected by high resolution transmission electron microscopy (HRTEM) and electron diffraction techniques in a molybdenum based mixed oxide catalyst after operation in the partial oxidation of acrolein at temperatures between 533 and 573 K for 9 h. This new crystalline phase has an orthorhombic crystal system. The structure has presumably a space group of Pban, but Pmmn cannot be excluded. XRD analysis was further used to refine this new Mo2O5 structure. The lattice constants were estimated to be a=1217.5, b=374.7 and c=404.4 pm, respectively. The elemental composition of the crystal was determined by the EDAX system attached to the electron microscope to be about (Mo0.6V0.3W0.1)2O5-x. This new structure is discussed in relation to earlier reports on a comparable composition and with respect to its possible role for partial oxidation catalysis.