Olga B. Lapina

Progress on the mechanistic understanding of SO2 oxidation catalysts

Abstract For almost a century vanadium oxide based catalysts have been the dominant materials in industrial processes for sulfuric acid production. A vast body of information leading to fundamental knowledge on the catalytic process was obtained by Academician [G.K. Boreskov, Catalysis in Sulphuric Acid Production, Goskhimizdat (in Russian), Moscow, 1954, p. 348]. In recent years these catalysts have also been used to clean flue gases and other SO − 2 containing industrial off-gases. In spite of the importance and long utilization of these industrial processes, the catalytic active species and the reaction mechanism have been virtually unknown until recent years. It is now recognized that the working catalyst is well described by the molten salt/gas system M 2 S 2 O 7 –MHSO 4 –V 2 O 5 /SO 2 –O 2 –SO 3 –H 2 O–CO 2 –N 2 (M=Na, K, Cs) at 400–600°C and that vanadium complexes play a key role in the catalytic reaction mechanism. A multiinstrumental investigation that combine the efforts of four groups from four different countries has been carried out on the model system as well as on working industrial catalysts. Detailed information has been obtained on the complex and on the redox chemistry of vanadium. Based on this, a deeper understanding of the reaction mechanism has been achieved.

Effect of Potassium Doping on the Structural and Catalytic Properties of V/Ti Oxide in Selective Toluene Oxidation

Small addition of potassium to V/Ti-oxide catalyst (K:VD0.19), consisting of 3.7 monolayer VOx , increased activity and selectivity in partial oxidation of toluene. In order to elucidate the nature of vanadia species formed on the surface of V/Ti-oxide upon potassium doping, the catalysts were studied by transient kinetics method. The transient product responses during toluene oxidation by the oxygen present in the catalyst were compared for K-doped and non-doped samples. The formation of CO2 decreased and formation of benzaldehyde increased with addition of potassium. This suggests a lower surface concentration of electrophilic oxygen (O ,O 2 ), which is usually responsible for the deep oxidation, and a higher concentration of nucleophilic oxygen (O 2 ), responsible for the partial oxidation. The catalysts were characterised by means of HRTEM, FT-Raman spectroscopy and 51 V NMR. Potassium addition introduces a disorder in the crystalline structure of bulk V 2O5 particles resulting in better spreading of V 2O5 over TiO2 surface. The interaction of V2O5 with TiO2 was facilitated upon K-doping, leading to the increased formation of monomeric vanadia species, which are the active sites in toluene partial oxidation to benzaldehyde.

Characterisation of strongly bonded V(V) species in VOx/TiO2 catalyst by static and MAS solid-state 51V NMR spectroscopy

Abstract 51 V static and MAS NMR spectroscopy of central transition is shown to be an effective method for the characterisation of strongly bonded V(V) species in VO x /TiO 2 catalysts. Simultaneous determination of quadrupole and CSA tensors parameters (i.e., C Q , η Q , δ σ , η σ ) from both static and MAS NMR spectra of central transition permits the extraction of all NMR parameters with a reasonable accuracy. For the first time, a large 51 V quadrupolar constant (14–16 MHz) has been obtained for strongly bonded V(V) species in a supported vanadia catalyst, whereas CSA tensor principal components were found to be very close to those for bulk vanadium (V) oxide.

Mechanism of sulphur dioxide oxidation over supported vanadium catalysts

The mechanism of SO2 oxidation to SO3 over industrial vanadium catalysts has been elucidated on a molecular level using 51V, 17O and 23Na n.m.r., i.r. and relaxation kinetic methods. The following reaction scheme has been shown to describe quantitatively the whole set of experimental data: [graphic omitted] According to this scheme three types of binuclear vanadium(v) complexes are involved in the catalytic cycle: the oxocomplex, sulphite complex and peroxocomplex. The rate constants for all the steps of the catalytic cycle have been determined. The data obtained provide guidelines for further improvement of vanadium catalysts as applied to particular conditions of their operation.

Solid-state NMR for characterization of vanadium-containing systems

Abstract This overview paper includes both published and original data of the current state of the field of 51 V NMR in solid-state chemistry. Advantages and shortcomings of different NMR techniques in their applications to vanadium are discussed on the examples of their application to various vanadia based systems (including individual highly crystalline compounds, solid solutions, glasses, catalysts). New correlations between local structure of vanadium atoms and NMR parameters allowing to discriminate at least seven different types of vanadium sites (tetrahedral sites of Q0, Q1 and Q2 types; trigonal pyramids of 3=1 and 3=2 (V2O5 like) types; tetragonal pyramids of 4=1, 4=2 types) are proposed. It is demonstrated that competent combination of different NMR approaches permits now not only to describe different vanadium sites in highly crystalline and amorphous materials, but also to insight into the structural aspects of disorder in crystallinity as well as to reveal the behavior of different functional groups at elevated temperatures. The influence of low valence vanadium atoms on 51 V NMR spectra is also discussed.

51V and 31P NMR studies of VOx/TiO2 catalysts modified by phosphorous

Phosphorous-doped VOx/TiO2 catalysts prepared by the spray-drying method and treated under catalytic reaction, as well as individual phases (αI-, αII-, β-) of VOPO4 were studied using modern high-resolution solid-state NMR techniques, including fast magic angle spinning (MAS), combined with the analysis of rotational satellites intensities by the satellite transition spectroscopy (SATRAS) method; 2D triple-quantum, quintuple-quantum MAS NMR, and spin mapping echo experiments. The simultaneous determination of chemical shielding anisotropy and quadrupole tensor parameters, as well as their distributions, permits to draw a conclusion on the local environment of vanadium sites in the catalysts. The formation of a triple V–P–Ti compound in phosphorous-doped VOx/TiO2 catalysts has been revealed. Only one type of slightly distorted tetrahedral vanadium atoms bound via oxygen to phosphorous was found in this compound. The very large distribution of the quadropole constant points to the irregular structure of this compound.

Glass fiber materials as a new generation of structured catalysts

Abstract Molecular structure of Zr-silicate glass fiber materials was studied to evaluate their potentiality in catalysis. Basing on NMR and IRS data the framework structure where Zr(IV) cations serve as a connectors linked with a few SiO4 tetrahedra was proposed. The effective ways of transition ions (Pt, Pd, Co) incorporation into the glassmatrix and their stabilization in highly dispersed state (clusters) were found. The obtained glass fiber based catalysts showed high activity and selectivity in oxidation of hydrocarbons and selective hydrogenation of acetylene-ethylene feedstock. The example of successful design of structured bed and commercialization of VOC removal process is presented.

Catalytically active complexes and influence of SiO2 on the catalytic properties of the active component of vanadium catalysts for SO2 oxidation

Abstract The complexes present in the active form of vanadium catalysts for SO 2 oxidation have been studied with use of 51 V, 17 O, and 23 Na NMR at room temperature and at 500 °C in reaction mixture media. In the V 2 O 5 K 2 S 2 O 7 melt the coordination of vanadium(V) with 2 or 3 pyrosulfate anions takes place. At vanadium concentrations above 1.5 mol/liter associated complexes are formed. These species are most active in the oxidation of SO 2 . This conclusion follows from measurements of the catalytic activity of V 2 O 5 K 2 S 2 O 7 supported on Pyrex tubes. 51 V NMR shows that the interaction of vanadium with SO 2 results in the formation of tetracoordinated surface complexes. These tetracoordinated vanadium complexes which are Catalytically inactive form active species upon interaction with monomeric vanadium-pyrosulfate complexes in the melt.

Multinuclear NMR study of silica fiberglass modified with zirconia.

Silica fiberglass textiles are emerging as uniquely suited supports in catalysis, which offer unprecedented flexibility in designing advanced catalytic systems for chemical and auto industries. During manufacturing fiberglass materials are often modified with additives of various nature to improve glass properties. Glass network formers, such as zirconia and alumina, are known to provide the glass fibers with higher strength and to slow down undesirable devitrification processes. In this work multinuclear (1)H, (23)Na, (29)Si, and (91)Zr NMR spectroscopy was used to characterize the effect of zirconia on the molecular-level fiberglass structure. (29)Si NMR results help in understanding why zirconia-modified fiberglass is more stable towards devitrification comparing with pure silica glass. Internal void spaces formed in zirconia-silica glass fibers after acidic leaching correlate with sodium and water distributions in the starting bulk glass as probed by (23)Na and (1)H NMR. These voids spaces are important for stabilization of catalytically active species in the supported catalysts. Potentials of high-field (91)Zr NMR spectroscopy to study zirconia-containing glasses and similarly disordered systems are illustrated.

High-temperature multinuclear magnetic resonance studies of vanadia catalysts for SO2 oxidation

Abstract Multinuclear 23 Na, 39 K, 133 Cs, 17 O, 51 V magnetic resonance studies of the M 2 S 2 O 7 -V 2 O 5 (M=Na, K, Cs) systems in the temperature range 20–650°C have been performed for vanadium oxide mole fractions, X(V 2 O 5 ), in the range 0–0.5. At ambient temperature the melt-quenched glassy samples exhibit a three-dimensional network of vanadium oxosulfate complexes. Octahedral coordination of vanadium atoms is found in the glassy samples at all compositions studied, in accordance with 51 V NMR spectra. Alkali cations are distributed randomly within an anion network. At high vanadium concentration the structure of vanadium sites in the glasses is very similar to that found in Cs 4 (VO) 2 O(SO 4 ) 4 , whereas for small vanadium contents the vanadium sites are separated by additional sulfate ligands. Heating to the glass-transition temperature, T g , and above, leads to jumps of the alkali cations between different sites. The mobility of pyrosulfate groups is accompanied by dissociation to SO 4 2− and SO 3 . At the elevated temperature the mobility of SO 3 molecules is sufficient to participate in chemical exchange with the sulfate groups of the network. Addition/splitting mechanism involving SO 3 has been proposed to be responsible for random fluctuations of the 51 V nuclear quadrupole tensor at given vanadium network sites with characteristic correlation time τ c . For 10 −8 τ c −6 s the 51 V NMR line became unobservable. For Cs-containing samples the increase of the temperature is accompanied by fast crystallization. In this case a cooperative motion of the anion network, caused by bond breaking and bond formation, dominates at temperatures around T g . The NMR spectra of alkali metals were found to be very characteristic for the structure of the network formed in melts between V 2 O 5 and M 2 S 2 O 7 . 17 O, 23 Na, 39 K, 133 Cs spectra recorded at 500°C point to the formation of different species and rapid exchange between them. A change of the local vanadium environment in melts takes place at X(V 2 O 5 )∼0.1 and 0.3 most probably due to the formation of dimeric and polymeric V(V) complexes, possibly (VO) 2 O(SO 4 ) 4 4− and (VO 2 SO 4 ) n n − . Correlation time of 51 V quadrupole tensor fluctuations for samples with X(V 2 O 5 )∼0.1–0.5 is higher than 10 −8 s, which makes 51 V NMR spectra unobservable in the region 400–500°C, whereas for more dilute samples, τ c is determined mainly by the size of the vanadium-sulfate species making 51 V spectra of these samples observable. The dependence of 51 V chemical shift on the vanadium concentration indicates a change of coordination number in the system M 2 S 2 O 7 -V 2 O 5 from tetrahedral in pure V 2 O 5 to octahedral in dilute samples. The structure of supported catalysts is very similar to the structure of bulk melts (M 2 S 2 O 7 -V 2 O 5 ), the main difference revealed is lower mobility of all structural units (such as metal cations, SO 4 2− and SO 3 ) for the supported melts.