Patent Description:
H<NUM>S is a common component in off gases from a high number of industries including refineries and viscose fiber production. H<NUM>S may commonly be present together with CS<NUM>, COS and CO especially in waste gases from viscose production. A common method for abatement of sulfides has been the elimination by thermal incineration and by catalytic oxidation involving the following chemical reactions.

<NUM>)     H<NUM>S + <NUM> O<NUM> -> SO<NUM> + H<NUM>O.

<NUM>)     CS<NUM> + <NUM> O<NUM> -> COS + SO<NUM>.

<NUM>)     COS + <NUM> O<NUM> -> SO<NUM> + CO.

The thermal incineration is costly, as it requires the addition of a support fuel and incineration at temperatures above <NUM>, whereas catalytic oxidation according to the prior art has been in the presence of a material comprising a noble metal typically with the oxidation of H<NUM>S taking place at a temperature above <NUM>, and the oxidation of CO in the presence of sulfur requiring even higher temperatures up to <NUM>. Catalytic oxidation may thus also require support firing.

Vanadium based catalysts are well known from e. g selective reduction of NOx (<CIT>) or oxidation of SO<NUM> to SO<NUM> (<CIT>). A vanadium based catalyst was also used in a process for the reaction H<NUM>S+CO = COS+H<NUM>; i.e. production of H<NUM> from H<NUM>S by reduction of CO to COS without changing the oxidation state of sulfur, at <NUM>-<NUM>. According to <CIT> catalytic activity in the oxidation of <NUM> ppm H<NUM>S, <NUM> ppm COS and <NUM> ppm CS<NUM> was observed over a catalyst consisting of copper, vanadium, silica and titania at an elevated temperature of <NUM>. In other experiments, H<NUM>S oxidation on catalysts comprising copper, long term tests have shown sulfatization of copper to copper sulfate which has reduced stability. In addition <CIT> showed that the combined presence of copper and vanadium in this material decreased the conversion over a material with copper alone.

<CIT>, <CIT> and <CIT> disclose oxidation of H<NUM>S at temperatures above <NUM> and <CIT> and <CIT> disclose other catalytic reactions of sulfur compounds.

However, according to the present disclosure a method for the oxidation of hydrogen sulfide over a a catalyst stable against sulfate formation at a temperature as low as <NUM> is provided together with a further variant of said method additionally active in the oxidation of carbon monoxide and carbonyls at temperatures down to <NUM> with the effect of reducing the amount of support fuel required for catalytic oxidation.

The catalytically active materials are based on vanadium and tungsten on a porous support; and in the variant active in carbon monoxide oxidation additionally comprises a noble metal. The material may beneficially be provided on a monolithic substrate with a porous washcoat.

The term catalytically active material shall herein be understood a material having the ability to reduce the activation energy of a reaction, compared to the gas phase reaction. Catalytically active material shall not be construed as having a specific physical structure, but rather than that understood as having a capability in a chemical reaction.

A catalytically active material typically consists of an active constituent, which is providing the chemical interaction with the reactants, and a porous support which has the primary function of distributing the active constituent over a high area and typically in many individual clusters. In addition a structural support may also be present with the main function of providing a defined structure with physical stability to the catalytically active material. Furthermore, additional constituents such as stabilizers reducing the sintering or similar deactivation of crystals structures and/or particles of active constituents and further active constituents may be present in the catalytically active material.

A monolithic catalytically active material or a catalyst monolith is a specific physical configuration of a catalyst, in which a structural substrate (with little or no contact with the reacting gas) is covered by a porous support, on which the active material is deposited.

Where reference is made to an unpromoted catalytically active material this shall be understood as a material not comprising promoters of CO oxidation, i.e. palladium or platinum.

Where concentrations are stated in % or vol% this shall be understood as volumetric % (i.e. molar percentages for gases).

Where concentrations are stated in ppm this shall be understood as volumetric parts per million (i.e. molar ppm for gases).

For a structural catalyst the void vol% corresponds to the relative volume accessible by air.

Where concentrations are stated in wt% this shall be understood as weight/weight %.

In a broad form the present invention relates to a method for oxidation of a gas comprising one or more species comprising sulfur, such as H<NUM>S, and/or S<NUM> vapor, to SO<NUM> said method comprising the step of contacting the gas and an oxidant being O<NUM> in at least the stoichiometric amount for oxidation of sulfur containing compounds to SO<NUM> with a catalytically active material consisting of1 wt% to <NUM> wt% V<NUM>O<NUM>, and from <NUM> wt% to <NUM> wt% WO<NUM>, optionally with the presence of other elements in a concentration below <NUM> wt%, and one or more supports taken from the group consisting of Al<NUM>O<NUM>, SiO<NUM>, SiC, and TiO<NUM>, at a temperature between <NUM> and <NUM>, with the associated benefit of such a temperature being highly energy effective, and the benefit of said elements having a low tendency to form sulfates under the conditions, with the related benefit of an increased stability of the catalytically active material. The other elements present may be catalytically active noble metals or impurities in the listed materials.

In a further embodiment, said catalytically active material comprises from <NUM> wt% or <NUM> wt% to 4wt%, 5wt% or <NUM> wt%V<NUM>O<NUM>, with the associated benefit of balancing low cost at low concentrations and high activity at low temperatures at higher concentrations.

In a further embodiment the catalytically active material or the porous support comprises TiO<NUM> preferably in the form anatase with the associated benefit of TiO<NUM> and especially anatase being highly porous and thus active as catalyst supports.

In a further embodiment the porous support comprises SiO<NUM> preferably being in the form of diatomaceous earth or a highly porous artificial silica with the associated benefit of SiO<NUM> and especially diatomaceous earth and highly porous artificial silica being highly porous, and thus active as catalyst supports.

In a further embodiment the catalytically active material further comprisesfrom 3wt% to <NUM> wt% or <NUM> wt% WO<NUM>, with the associated benefit of stabilizing the active crystal structure such as the vanadium/anatase structure thus giving a longer life time of the catalytically active material.

In a further embodiment the catalytically active material is in the form of a monolithic catalyst, preferably comprising a structural substrate made from metal, high silicon glass fibres, glass paper, cordierite and silicon carbide and a catalytic layer with the associated benefit of providing a stable and well defined physical shape.

In a further embodiment the monolithic catalyst has a void of from <NUM> vol% or <NUM> vol% to <NUM> vol% or <NUM> vol%, with the associated benefit of a good balance between the amount of catalytic material and an open monolith with low pressure drop.

In a further embodiment the catalytic layer of said monolithic catalyst has a thickness of <NUM>-<NUM> with the associated benefit of providing a catalytically active material with high pore volume.

In a further embodiment the catalyst further comprises from <NUM> wt%, <NUM>. 02wt% or <NUM>. 05wt% to 1wt% of a noble metal, preferably Pd or Pt, with the associated benefit of noble metals, and especially Pd and Pt being catalytically active in the oxidation of CO and COS.

A further aspect of the present disclosure relates to a method for the oxidation of a gas comprising one or more species comprising sulfur in an oxidation state below +<NUM>, such as H<NUM>S, CS<NUM>, COS and/or S<NUM> vapor to SO<NUM> said method comprising the step of contacting the gas and an oxidant with such a catalyst at a temperature between <NUM> and <NUM>, with the associated benefit of such a method requiring a lower temperature of the feed gas, compared to similar processes with traditional catalysts.

As the oxidant is O<NUM> and said oxidant is present in at least the stoichiometric amount for oxidation of sulfur containing compounds to SO<NUM>, with the associated benefit of providing substantially all sulfur in an oxidated form convertible to e.g. sulfuric acid by well-known processes.

Vanadium is known as a material catalytically active in oxidation, but it has not been applied at high concentration for the oxidation of sulfur compounds to sulfur dioxide. In the present context the concentration of vanadium oxide in the catalytic material is above <NUM> wt % or 2wt%.

The high activity of catalytically active vanadium requires that vanadium is well distributed on a porous support. This support may according to the present disclosure preferably comprise titania (TiO<NUM>) possibly in combination with silica (SiO<NUM>), and preferably in highly porous forms. For titania, anatase is the preferred form, and silica is preferably provided as diatomaceous earth or artificial high porosity silica.

The structure of vanadium and titania may be stabilized by an appropriate stabilizer. A preferred stabilizer is tungstenate WO<NUM>, but other materials are also known to the skilled person.

Without being bound by theory, specifically vanadium provides high catalytic activity and is stabilized and/or promoted by combination with the presence of tungsten, cerium and molybdenum, which share the benefit of a low tendency to form sulfates from SO<NUM> produced oxidation of sulfur, contrary to e.g. Cu or Mn (see e.g. <NPL>)). The absence of sulfates results in an increased physical stability of the catalytically active material.

To enable the oxidation of carbonyls and carbon monoxide at low temperatures noble metal is required. Preferred noble metals are palladium and platinum. These materials are only required to be present in concentrations between <NUM> wt% and <NUM> wt%.

To ensure a low pressure drop the catalytically active material may be provided on a monolithic support. As it is known to the person skilled in the art, many variants of monolithic support exist. These are predominantly chosen as inert materials according to their mechanical and production characteristics. Some examples are metal grids, high silicon glass fibres, glass paper, cordierite and silicon carbide.

High silicon content glass may contain <NUM>-<NUM>% by weight SiO<NUM>, <NUM>-<NUM>% by weight Al<NUM>O<NUM> and some Na<NUM>O, these fibres have a density of <NUM>-<NUM>/l with a fibre diameter is <NUM>-<NUM>. An example is the commercially available SILEX staple fiber.

E-glass (electrical grade glass fibre) may contain <NUM>-<NUM>% by weight SiO<NUM>, <NUM>-<NUM>% by weight Al<NUM>O<NUM>, <NUM>-<NUM>% by weight B<NUM>O<NUM>, <NUM>-<NUM> % by weight TiO<NUM>, <NUM>-<NUM>% by weight MgO, <NUM>-<NUM>% by weight CaO, <NUM>-<NUM>% by weight K<NUM>O/Na<NUM>O and <NUM>-<NUM>% by weight Fe<NUM>O<NUM>.

The catalytic material may be applied on a monolithic substrate, which may have the form of plane or corrugated plates. The monolithic substrate comprises a structural substrate such as sheets of E-glass fibres, sheets of a glass with high silicon content, cordierite or silicon carbide and a porous layer comprising TiO<NUM> and/or SiO<NUM>. The porous layer may be applied by dipping the structural substrate in an appropriate slurry. Solvents for said slurries may be water or a mixture of organic solvents (alcohols, alifatic or aromatic solvents) depending on the binder solubility. Binders may be soluble in water (e.g. PVA, PEG) or organic solvents (e.g. PVP, resins) and serve as rheology modifiers as well as binders after evaporation of solvents. Alternatively low viscosity slurries may be shaped to catalyst supports by dip-coating of a pre-shaped structure, i.e. a pre-wash-coated structure. Concentrated slurries or pastes may be shaped to catalyst support by extrusion or calendering into metal wire mesh or glass fibre paper.

<NUM> anatase TiO<NUM> powder was suspended in <NUM> of a solution of tetra-isopropyl-titanate in butanol containing <NUM> % by weight of Ti and <NUM> % by weight of water. This slurry was mixed thoroughly in a laboratory dissolver in order to secure intimate mixture of the constituents and to break down any agglomerate to be smaller than <NUM> mesh. An Erichsen Grindometer was used to control this. Glass fibre mats having a thickness of approximately <NUM> were cut to dimensions of approximately <NUM> by <NUM>. These mats were dipped into the above mentioned slurry to obtain a fully wetted fibre mat. After drying, the material was calcined at <NUM> for <NUM> hours.

After calcination, the catalyst support material was impregnated with solutions made from NH<NUM>VO<NUM> and (NH<NUM>)<NUM>H<NUM>W<NUM>O<NUM> and treated at <NUM> in air to give a final catalyst containing <NUM> wt% V<NUM>O<NUM> and <NUM> wt% WO<NUM>, having a void of approximately <NUM>%.

The catalyst produced according to Example <NUM> was further impregnated with palladium by suspension in a solution of palladium tetra-ammine bicarbonate in nitric acid. The resulting Pd concentration was approximately <NUM> wt%.

The unpromoted catalyst prepared according to Example <NUM> was tested for the oxidation of hydrogen sulfide, H<NUM>S.

A stream of <NUM> ppm H<NUM>S, and <NUM>% O<NUM> was directed to contact the unpromoted catalyst at temperatures from <NUM> to <NUM> in an oven, with a NHSV of <NUM> Nm3/h/m<NUM>. To simplify the evaluation, the relative amount of H<NUM>S found as SO<NUM> in the product gas is tabulated.

Table <NUM> shows the results of these examples, according to which the ignition temperature for oxidation of H<NUM>S to SO<NUM> was found to be around <NUM>.

The Pd promoted catalyst produced according to Example <NUM>, was tested for the oxidation of carbon disulfide, CS<NUM>, which produces CO as an intermediate product.

A stream of <NUM> ppm CS<NUM>, <NUM> ppm SO<NUM>, <NUM>% H<NUM>O and <NUM>% O<NUM> was directed to contact the Pd-promoted catalyst at temperatures from <NUM> to <NUM> in an oven with a NHSV of <NUM> Nm3/h/m<NUM>.

Under similar conditions the unpromoted catalyst according to Experiment <NUM> showed a good sulfur oxidation activity, but insufficient carbon monoxide and carbonyl oxidation activity below <NUM>.

In Table <NUM> the experimental results are shown. At <NUM> full oxidation of CS<NUM> to CO<NUM> and SO<NUM> occurs. At lower temperatures some indication of deactivation with respect to CO oxidation was observed but this deactivation was reversible.

Claim 1:
A method for the oxidation of a gas comprising one or more species comprising H<NUM>S and/or S<NUM> vapor to SO<NUM> said method comprising the step of contacting the gas and an oxidant being O<NUM> in at least the stoichiometric amount for oxidation of sulfur containing compounds to SO<NUM> with a catalytically active material consisting of from <NUM> wt% to <NUM> wt% V<NUM>O<NUM>, and from <NUM> wt% to <NUM> wt% WO<NUM>, optionally with the presence of other elements in a concentration below <NUM> wt%, and one or more supports taken from the group consisting of Al<NUM>O<NUM>, SiO<NUM>, SiC, and TiO<NUM>, at a temperature between <NUM> and <NUM>.