Background of the invention Hydrocarbonaceous feedstock are usually desulphurised in a hydrodesulphurisation process. In this process, the sulphur containing feedstock is contacted with a hydrodesulphurisation catalyst, typically a Co-Mo or Ni-Mo catalyst, in the presence of hydrogen, at elevated temperature and pressure. Hydrodesulphurised hydrocarbonaceous streams still contain sulphur compounds, in particular sulphur compounds that are difficult to remove such as heterocyclic sulphur compounds like thiophenes, benzothiophenes, substituted and condensed ring dibenzothiophenes.

It is known to use catalysts comprising nickel, zinc oxide and alumina for deep desulphurisation of hydrocarbon streams in the presence of hydrogen. These catalysts are able to remove"difficult"sulphur compounds and to achieve sulphur concentrations as low as 0. 1 ppm. In WO 01/15804, for example, a catalyst is disclosed having 5-25 wt% Ni, 30-70 wt% ZnO and the remainder alumina. The catalyst of WO 01/15804 has a double function: nickel catalyses the reaction of sulphur with hydrogen to form hydrogen sulphide and zinc oxide absorbs the hydrogen sulphide formed.

It is also known to desulphurise hydrocarbonaceous streams by oxidising the sulphur of the sulphur compounds to sulphur dioxide in a vapour phase process.

US 2,640, 010, for example, describes the oxidation of sulphur compounds like hydrogen sulphide, mercaptans and disulphides in petroleum hydrocarbons to sulphur dioxide, by passing the vapour of the hydrocarbon over an oxidation catalyst comprising cuprous sulphide.

US 2,361, 651 describes a process for sweetening and desulphurising sour hydrocarbon distillates by contacting vapours of said distillates in the presence of oxygen with a catalyst comprising copper oxide. The reaction involved in the process is an oxidation of mercaptans, sulphides and disulphides to sulphur dioxide by the action of the copper catalyst in the presence of oxygen.

A disadvantage of he processes disclosed in US 2,640, 010 and US 2,361, 651 is that"difficult"sulphur compounds like thiophenes are not removed.

Summary of the invention It has now been found that also the sulphur from "difficult"sulphur compounds like thiophenes in hydrocarbonaceous streams can be converted into sulphur dioxide by catalytic selective oxidation by using a catalyst comprising a Group VIII noble metal. The thus- formed sulphur dioxide can be removed by processes known in the art. Reference herein to selective oxidation of sulphur compounds is to the oxidation of sulphur compounds with no or minimal oxidation of the non-sulphur containing hydrocarbonaceous compounds.

An advantage of selective oxidation followed by removal of sulphur dioxide is that no hydrogen is needed for the desulphurisation. Another advantage of the vapour phase selective oxidation process is that the process can be performed at ambient pressure. Moreover, with the process according to the invention it is possible to achieve deep desulphurisation without using nickel- containing catalysts.

Accordingly, the present invention relates to a process for the catalytic selective oxidation of sulphur compounds in a hydrocarbonaceous feedstock to sulphur dioxide, wherein a gaseous feed mixture of the hydrocarbonaceous feedstock and a molecular-oxygen containing gas is contacted with a catalyst at a temperature of at most 500 °C, the catalyst comprising a group VIII noble metal on a catalyst carrier, wherein the oxygen-to-carbon ratio of the feed mixture is below 0. 15.

Detailed description of the invention Sulphur compounds that can be selectively oxidised by the process according to the invention are for example hydrogen sulphide, mercaptans, disulphides, or heterocyclic sulphur compounds such as thiophenes, benzothiophenes, or substituted and condensed ring dibenzothiophenes.

The hydrocarbonaceous feedstock is a hydrocarbonaceous feedstock that is gaseous under the conditions prevailing at the catalyst surface. Preferred feedstocks are feedstocks that are gaseous at standard <BR> <BR> temperature and pressure (STP; 0 °C, 1 atm. ) conditions such as methane, natural gas, LPG and other gaseous hydrocarbon streams. Further, feedstocks that are liquid under STP conditions but gaseous at the conditions prevailing at the catalyst surface such as naphtha, diesel or gasoline are suitable feedstocks.

The catalyst of the process according to the invention comprises as catalyst carrier an oxidising solid surface, typically in the form of solid particles.

Reference herein to an oxidising surface is to a surface that is able to activate molecular oxygen. Preferably, . the catalyst carrier comprises a refractory oxide.

Refractory oxides such as stabilised and partially stabilised zirconia, ceria, yttria, silica, alumina, titania and combinations thereof are particularly suitable. A catalyst carrier comprising stabilised or partially stabilised zirconia is most preferred.

Alternatively, the catalyst carrier may comprise a non-refractory oxide bulk material having an oxidising surface. Examples of such materials are a Fe, Cr and Al containing alloy (commercialised as FECRALLOY) with an alumina or zirconia surface layer (FECRALLOY is a trademark).

The catalyst comprises one or more catalytically active metals supported on the solid surface or carrier.

These catalytically active metals are Group VIII noble metals, more preferably platinum, rhodium, iridium or a combination of two or more thereof. Typically, the catalyst comprises the catalytically active metal (s) in a concentration in the range of from 0.02 to 10% by weight, based on the total weight of the catalyst, preferably in the range of from 0.1 to 5% by weight. The catalyst may further comprise a performance-enhancing inorganic metal cation selected from Al, Mg, Zr, Ti, La, Hf, Si, Ba, and Ce which is present in intimate association supported on or with the catalytically active metal, preferably a zirconium and/or cerium cation.

Catalysts comprising a noble metal on a carrier are also suitable for the catalytic partial oxidation of hydrocarbons, typical at temperatures above 700 °C. It has been found that at much lower temperatures, typically between 200 and 500 °C, the oxidation of sulphur compounds takes preferentially place as compared to the oxidation of hydrocarbons.

In order to prevent degradation of hydrocarbon compounds, the process temperature is maintained at at most 500 °C. Preferably, the process temperature is in the range of from 200 to 500 oC, more preferably of from 200 to 300 °C.

The pressure at which the feed mixture is contacted with the catalyst is preferably in the range of from 1 to 10 bar (absolute), more preferably of from 1 to 5 bar (absolute). Most preferably, the feed mixture is contacted with the catalyst at ambient pressure.

The molecular-oxygen containing gas may be oxygen, air or oxygen-enriched air. Preferably, air is used as molecular-oxygen containing gas.

It will be appreciated that the exact process conditions, such as the temperature at which the catalyst is maintained, pressure, gas or liquid velocity and the oxygen-to-carbon ratio in the feed mixture, will inter alia depend on the catalyst used, the required sulphur conversion and selectivity, and the boiling characteristics of the hydrocarbonaceous feedstock.

The oxygen-to-carbon ratio of the feed mixture is at most 0.15, preferably at most 0.10. Reference herein to the oxygen-to-carbon ratio is to the ratio of oxygen in the form of molecules (02) to carbon atoms present in the hydrocarbonaceous feedstock.

The process according to the invention is very suitable for deep desulphurisation of hydrocarbonaceous streams. It is particularly suitable for the removal of hydrogen sulphide from gaseous hydrocarbonaceous steams comprising up to 10% v/v hydrogen sulphide or the removal of"difficult"sulphur compounds from liquid hydrocarbonaceous steams comprising up to 1000 ppmw sulphur.

The sulphur dioxide formed may be removed by techniques known in the art. In liquid feedstocks, sulphur dioxide may for example be removed by distillation or stripping. Suitable techniques known in the art for the removal of sulphur dioxide from gaseous feedstocks are for example solvent extraction using an aqueous amine solution or an alkaline solution, absorption on copper, barium or cerium oxide, or reaction with lime to produce gypsum.

In order to remove larger amounts of hydrogen sulphide from gaseous hydrocarbonaceous feedstocks, the selective oxidation process according to the invention can suitably be applied in combination with a process for the conversion of H2S/SO2 mixtures into elemental sulphur according to the well-known Claus reaction: Part of the hydrogen sulphide, preferably about one third of the total volumetric amount of hydrogen sulphide, is then converted into sulphur dioxide by the catalytic selective oxidation process according to the invention.

The process according to the invention will be further illustrated by the following non-limiting examples.

EXAMPLE 1 Catalyst preparation Catalyst 1 Particles (1 mm average diameter) of zirconia partially stabilised with yttria (Y-PSZ) were coated with a zirconia paint (zirconium oxide partially-stabilised with 4 % wt CaO; type ZO ; ex. ZYP Coatings Inc. , Oak Ridge, USA) and provided with 0.9 wt% Rh, 0.9 wt% Ir, 0.6 wt% Zr, 1.9 wt% Ce by impregnating the painted particles with a solution containing rhodium tri chloride, iridium tetra chloride, zirconyl nitrate and cerium nitrate. The impregnated particles were dried at 140 °C during 2 hours and calcined at 700 °C during 2 hours.

Catalyst 2 (comparative) Particles (1 mm average diameter) of zirconia partially stabilised with yttria (Y-PSZ) were coated with a zirconia paint (see above under catalyst 1) and provided with 0.5 wt% Zr, 1.6 wt% Ce by impregnating the painted particles with a solution containing zirconyl nitrate and cerium nitrate. The impregnated particles were dried at 140 °C during 2 hours and calcined at 700 °C during 2 hours.

Catalyst 3 Particles (30-80 mesh) of Y-PSZ were coated with a zirconia paint (see above under catalyst 1) and provided with 1.6 wt% Rh, 1.0 wt% Zr, 1.6 wt% Ce by impregnating the painted particles with a solution containing rhodium tri chloride, zirconyl nitrate and cerium nitrate. The impregnated particles were dried at 140 °C during 2 hours and calcined at 700 °C during 2 hours.

Catalyst 4 Particles (30-80 mesh) of zirconia-toughened alumina partially stabilised with ceria (Ce-ZTA) were impregnated with a solution containing H2PtCl6 and zirconyl nitrate. The impregnated particles were dried at 140 °C during 2 hours and calcined at 700 °C during 2 hours. The resulting catalyst particles contained 5 wt% Pt and 7 wt% Zr.

Catalyst 5 Calcined (2 hours at 1000 °C) particles (30-80 mesh) of alumina stabilised with magnesium oxide were provided with 0.6 wt% Ir by impregnating the particles with a iridium tetra chloride containing solution. The impregnated particles were dried (2 hours at 120 °C) and calcined (2 hours at 700 °C).

Catalyst 6 Particles (1 mm average diameter) Y-PSZ were coated with a zirconia paint and provided with 0. 8 wt% Rh, 0.8 wt% Ir, 0.6 wt% Zr, 1.7 wt% Ce by impregnating the painted particles with a solution containing rhodium tri chloride, iridium tetra chloride, zirconyl nitrate and cerium nitrate. The impregnated particles were dried at 140 °C during 2 hours and calcined at 700 °C during 2 hours.

Catalytic selective oxidation Approximately 1 g of catalyst particles were loaded in a 6 mm inner diameter reactor tube. A gas mixture of air and H2S-containing methane was passed over the catalyst particles at elevated temperature and at ambient pressure.

In Table 1, the HzS concentration of the H2S- containing methane, the oxygen-to-carbon ratio of the air/methane mixture, the gas space velocity (N1 feed mixture per kg catalyst per hour), the temperature at which the catalyst is maintained, the H2S conversion and the selectivity are given for each catalyst. The selectivity is calculated as the quotient of the molar S02/CO2 ratio in the effluent and the S/C ratio in the feed.

It can be seen from the results in Table 1 that very high H2S conversions are obtained when the methane feed is oxidised over a catalyst comprising Pt, Rh and/or Ir (catalysts 1,3-6). When a catalyst without catalytically active metal is used (catalyst 2), the temperature has to be increased above 500 °C in order to achieve such a high conversion.

Table 1 Selective oxidation of H2S in methane: feed composition,<BR> process conditions and results. Catalyst No. 1 2 (comparison) 3 4 5 6 H2S (% v/v) 2. 8 2. 0 2. 5 2. 5 02 : C0. 060. 04'0. 050. 050. 050. 070. 050. 06 GSV (Nl/kg/h) 7, 000 7, 000 7, 000 7, 000 7, 000 7, 000 13, 000 7, 000 T (OC) 360 459 413 553 470 458 467 433 H2S conversion 99. 9 9. 7 73. 7 99. 9 97. 1 94. 7 95. 2 selectivity 222 373 5070 8400 53 57 57 57 EXAMPLE 2 0.95 g of particles of catalyst 1 were loaded in a 6 mm inner diameter reactor tube. A gas mixture of air and thiophene-containing methane was passed over the catalyst particles at elevated temperature and ambient pressure. Two different experiments with different feed composition and different process conditions were carried out.

In Table 2, the sulphur concentration of the methane, the oxygen-to-carbon ratio of the air/methane mixture, the gas space velocity (N1 feed mixture per kg catalyst per hour), the temperature at which the catalyst is maintained, the thiophene conversion and the selectivity are given for the two experiments. The selectivity is calculated as the quotient of the molar SO2/CO2 ratio in the effluent and the S/C ratio in the feed.

Table 2 Selective oxidation of thiophene in methane: feed composition, process conditions and results. experiment a b ppmw S 11, 400 300 02 : C 0. 05 0. 02 GSV (Nl/kg/h) 8, 000 7, 000 T (°C) 391 304 thiophenes conversion (%) 99.8 79. 9 selectivity'281816 EXAMPLE 3 0.94 g of particles of catalyst 1 were loaded in a 6 mm inner diameter reactor tube. A mixture of air and thiophene-containing naphtha was passed over the catalyst particles at a temperature of 320 °C and ambient pressure. The naphtha had a boiling range of 40-180 °C, a H/C ratio of 1.8, a density of 0.74 g/ml.

In Table 3, the sulphur concentration of the naphtha, the oxygen-to-carbon ratio of the air/naphtha mixture, the liquid space velocity (kg naphtha per kg catalyst per hour), the temperature at which the catalyst is maintained, the thiophene conversion and the selectivity are given. The selectivity is calculated as the quotient of the molar SO2/CO2 ratio in the effluent and the S/C ratio in the feed.

Table 3 Selective oxidation of thiophene in naphtha: feed composition, process conditions and results. ppmw S 590 02 : C 0. 003 LSV (kg/kg/h) 23.4 T (°C) 320 thiophenes conversion (%) 62.7 Selectivity 428 EXAMPLE 4 Catalyst preparation Catalyst 7 Particles (30-80 mesh average diameter) of zirconia partially stabilised with yttria (Y-PSZ) were coated with a zirconia paint (zirconium oxide partially-stabilised with 4 % wt CaO; type ZO ; ex. ZYP Coatings Inc. , Oak Ridge, USA) and provided with 2.26 wt% Ir, 0.98 wt% Zr, 1.56 wt% Ce by impregnating the painted particles with a solution containing iridium tetra chloride, zirconyl nitrate and cerium nitrate. The impregnated particles were dried at 140 °C during 2 hours and calcined at 700 °C during 2 hours.

Catalytic selective oxidation 2.04 g of particles of catalyst 7 were diluted with 2.13 g SiC (0.05 mm) to improve heat transfer and flow properties and loaded in a 15 mm inner diameter reactor tube. A gas mixture of air and thiophene-containing methane was passed over the catalyst particles at elevated temperature and ambient pressure. Three different experiments with the same feed composition and different temperatures were carried out. The sulfur content of the feed was 210 ppmv thiophene.

The 02/C ratio was 0.005 and the GHSV 2500 Nl/kg/hr.

In Table 4, the thiophene conversion and selectivity are given for the three experiments. The selectivity is calculated as the quotient of the molar SO2/CO2 ratio in the effluent and the S/C ratio in the feed.

Table 4 Selective oxidation of thiophene in methane experiment a b c temperature (°C) 220 250 300 thiophenes 99. 8 99. 8 99. 8 conversion (%) selectivity 2380 1590 676 EXAMPLE 5 2.03 g of particles of catalyst 7 were diluted with 2.2 g SiC (0.05 mm) to improve heat transfer and flow properties and loaded in a 15 mm inner diameter reactor tube. A gas mixture of air and LPG (5.9% v/v butane, balance propane) containing 50 ppmv each of H2S, COS, ethyl mercaptan, tetrahydrothiophene and diethyl disulfide was passed over the catalyst particles at elevated temperature and ambient pressure. The 02/C ratio was 0.002 and the GHSV 3600 N1/kg/hr.

In Table 5, the conversion of each of the sulphur species is given for a reactor temperature of 275 °C.

Table 5 Conversion of different sulphur species in LPG sulphur species conversion (%) H2S 99. 97 COS 99. 9 ethyl mercaptan 99. 97 tetrahydrothiophene 97 diethyl disulfide 99. 9 The selectivity (calculated as the quotient or the molar SO2/CO2 ratio in the effluent and the total S/C ratio in the feed) was 65.

EXAMPLE 6 Catalyst preparation Catalyst 8 Particles (20-30 mesh average diameter) of zirconia partially stabilised with yttria (Y-PSZ) were coated with a zirconia paint (zirconium oxide partially-stabilised with 4 % wt CaO ; type ZO ; ex. ZYP Coatings Inc. , Oak Ridge, USA) and provided with 0.81 wt% Rh, 0.78 wt% Ir, 0.98 wt% Zr, 1.57 wt% Ce by impregnating the painted particles with a solution containing rhodium tri chloride, iridium tetra chloride, zirconyl nitrate and cerium nitrate. The impregnated particles were dried at 140 °C during 2 hours and calcined at 700 °C during 2 hours.

Catalytic selective oxidation 1.93 g of particles of catalyst 8 were mixed with 1.81 gram SiC (0.05 mm) and loaded in a 15 mm inner diameter reactor tube. A mixture of air and a hydrocracked naphtha (ex Pernis refinery, initial boiling point 91 °C, final boiling point 195 °C, containing a total of 32 ppmw S, predominantly as (substituted) thiophenes, sulphides and disulfides) was passed over the catalyst particles at two different temperatures, 250 and 270 °C, an 02/C ratio of 0.003 and a liquid space velocity of 3.5 kg/kg/hr.

In Table 6, the sulphur conversion of the naphtha, and the selectivity are given. The conversion is based on the S analysis of the liquid product. The selectivity is calculated as the quotient of the molar SO2/CO2 ratio in the effluent and the S/C ratio in the feed.

Table 6 Selective oxidation of sulphur species in naphtha experiment a b temperature (°C) 250 270 S conversion (%) 88 92 selectivity 334 17