AN, Zengjian (457 Zhongshan Road, Dalian Institute of Chemical Physics, Dalian 3, 116023, CN)
PAN, Xiulian (Zhixin Yuan 58-2-1102, Wuyi Road Shahekou District,Liaoning, Dalian 1, 116021, CN)
LIU, Xiumei (Hanlinyuan 30-2-402, Zhongshan Road, Dalian 3, 116023, CN)
HAN, Xiuwen (Zhixin Yuan 54-1-301, Wuyi Road Shahekou District, Dalian 3, 116023, CN)
BP P.L.C. (1 St James's Square, London SW1Y 4PD, GB)
BAO, Xinhe (Xi'an Street 39, Dalian 1, 116011, CN)
AN, Zengjian (457 Zhongshan Road, Dalian Institute of Chemical Physics, Dalian 3, 116023, CN)
PAN, Xiulian (Zhixin Yuan 58-2-1102, Wuyi Road Shahekou District,Liaoning, Dalian 1, 116021, CN)
LIU, Xiumei (Hanlinyuan 30-2-402, Zhongshan Road, Dalian 3, 116023, CN)
HAN, Xiuwen (Zhixin Yuan 54-1-301, Wuyi Road Shahekou District, Dalian 3, 116023, CN)
| Claims
1. A catalyst for the oxidation of a hydrocarbon to an oxygenated hydrocarbon in the presence of oxygen, which catalyst comprises a redox active metal centre that can be present in an oxidised and in a reduced form, an acid, and an oxidant for oxidising the reduced form of the redox active metal centre, characterised in that the catalyst also comprises a source of nitrous oxide.
2. A catalyst as claimed in claim 1, in which the redox active metal centre is selected from Cu, Zn, Pd 5 Ag, In, Sn, Sb, Te, Pt, Au, Pb, Bi, Ga, Ge, As, Rh, Ir, Os and Ru.
3. A catalyst as claimed in claim 2, in which the redox active metal centre undergoes a two electron redox cycle when in use.
4. A catalyst as claimed in claim 2 or claim 3, in which the redox active metal centre is Ni, Rh, Pd or Pt.
5. A catalyst as claimed in any one of claims 1 to 4, in which the oxidant is selected from a second redox active metal centre, a peroxide, a peracid, or quinone or a derivative thereof.
6. A catalyst as claimed in claim 5, in which the oxidant is quinone or a derivative thereof.
7. A catalyst as claimed in any one of claims 1 to 6, in which the acid is selected from trifluoroacetic acid, oleum, sulphuric acid, methyl sulphonic acid, trifluoromethyl sulphonic acid or a heteropolyacid. 8. A catalyst as claimed in claim 6, in which the acid is trifluoroacetic acid.
9. A catalyst as claimed in any one of claims 1 to 8, in which the source of nitrous oxide is a nitrite salt.
10. A process for the oxidation of a hydrocarbon to an oxygenated hydrocarbon in the presence of oxygen, which process comprises contacting a hydrocarbon and oxygen with a catalyst according to any one of claims 1 to 9. |
OXIDATION CATALYST
This invention relates to the field of catalysis, more specifically to a catalyst for the catalytic and direct oxidation of methane to oxygenated hydrocarbons using oxygen as the oxidant.
Converting natural gas to oxygenated hydrocarbons is typically achieved industrially in two stages. First, the methane is converted to syngas (a mixture of carbon monoxide and hydrogen) by processes such as partial oxidation, steam reforming or autothermal reforming. The second stage is the conversion of the syngas into oxygenated hydrocarbons, for example the production of methanol using a Cu/ZnO/ Al 2 O 3 catalyst, or the production of ethanol and/or higher hydrocarbons using a rhodium catalyst.
In order to minimise the complexity of the process, the direct conversion of methane into oxygenated hydrocarbons using a single stage would be of a considerable advantage.
WO 92/14738 describes a process for reacting methane with a strong acid in the presence of a metallic catalyst and an oxidising agent. The product is the methyl salt or ester of the acid. The examples of WO 92/14738 include catalytic systems comprising palladium as the active metal, triflic acid or sulphuric acid as the acid, and oxygen as the oxidising agent.
Although oxygen is a desirable oxidant to use, due to its low cost and high abundance, the methane conversions achieved when it is used to be low. Other oxidants, such as SO 3 , persulphate or peracids, can improve conversions, but they are relatively expensive and constantly need to be replaced in order to maintain the catalytic reaction. WO 92/14738 describes how a mercury catalyst, in the presence of sulphuric acid, is able to oxidise methane more effectively than other metals, such as palladium, thallium, gold and platinum, in the presence of oxygen, and optionally hi the presence of SO 3 . However, as mercury is a highly toxic metal, there remains a need for a catalyst and process for the oxidation of a hydrocarbon with a high oxygenate yield, but which avoids the necessity for highly toxic components.
According to the present invention, there is provided a catalyst for the oxidation of a hydrocarbon to an oxygenated hydrocarbon in the presence of oxygen, which catalyst comprises a redox active metal centre that can be present in an oxidised and in a reduced
form, an acid, and an oxidant for oxidising the reduced form of the redox active metal centre, characterised in that the catalyst also comprises a source of nitrous oxide.
The catalyst of the present invention is capable of converting a hydrocarbon to an oxygenated hydrocarbon in the presence of oxygen. Nitrous oxide, generated in use by the source of nitrous oxide, provides superior catalytic activity and enhances yield of the oxygenated hydrocarbon. The catalyst may be a homogeneous catalyst, in which the components are mixed or dissolved in a liquid phase, for example being dissolved in a liquid acid. Alternatively, the catalyst may be heterogeneous, in which one or more of the components are in the solid phase, for example where the components are supported on a refractory metal oxide or a solid acid, such as an alumino silicate zeolite. Homogeneous catalysts are preferred, as they are typically more active than heterogeneous counterparts under milder conditions, and allow improved contact between the constituent components of the catalyst.
Sources of nitrous oxide (NO) include nitrous oxide itself, other oxides of nitrogen such as NO 2 , N 2 O 3 , N 2 O 4 and N 2 O 5 , salts comprising anionic oxides of nitrogen such as NO 2 " (nitrite), and salts comprising NO + (nitrosonium) cations. Suitable compounds comprising nitrite ions include alkali metal salts, alkaline earth metal salts and transition metal salts. In one embodiment, the cation of the nitrite salt is the redox active metal centre of the present composition. Suitable compounds comprising nitrosonium ions include tetrafluroborate (BF 4 " ) and perchlorate (ClO 4 " ) salts, and nitrosyl sulphuric acid. Conveniently, an alkali metal nitrite salt is the source of nitrous oxide, such as sodium or potassium nitrite, which can generate nitrous oxide in the presence of an acid. The source of nitrous oxide releases or produces nitrous oxide when the catalyst us in use. The nitrous oxide, when the catalyst is in use, is reversibly oxidised to NO 2 in the presence of oxygen, which in turn is able to regenerate oxidant that has been reduced in the reoxidation of reduced redox active metal centres. The use of a source of nitrous oxide in the catalyst of the present invention is advantageous, as the nitrous oxide/nitrogen dioxide cycle for regenerating the oxidant is stable under the acidic conditions prevalent when the catalyst is in use, unlike macrocyclic metal complexes such as iron-pthalocyanine or cobalt-porphyrin complexes.
The catalyst comprises a redox active metal centre which can exist in an oxidised and in a reduced form. In this context, the term "metal" includes those elements described as
metalloids, such as germanium, antimony, tellurium and the like. Most transition metals, lanthanides and actinides are capable of existing in more than one form, as are a number of main group metals. Examples of metals suitable for use as the redox active metal in the present invention include Cu, Zn, Pd, Ag, In, Sn, Sb, Te, Pt, Au, Pb, Bi, Ga, Ge, As, Rh, Ir, Os and Ru. Although metals such as Hg, Cd or Tl are also capable of being used in the present invention, they are preferably avoided due to their high toxicity. In a preferred embodiment, the redox active metal is selected from one or more of V, Fe, Co, Ni, Cu, Rh, Pd or Pt. One redox active metal centre or more than one redox active metal centre may be present in the catalyst. In embodiments having more than one redox active metal centre, one of the metal centres may act as the oxidant for the other. For example Cu can be used as an oxidant in a catalyst comprising both Pd and Cu, in which Cu(II) species can oxidise Pd(O) species to Pd(II), in the process being reduced to Cu(I).
The metal can be provided in any form such that, when in use, it is capable of cycling between two oxidation states. Thus, for example, the redox active metal can be introduced in the metallic (0 oxidation state) form, or as a compound or complex in which the metal is in a higher oxidation state. For example, the redox active metal centre can be added to the catalyst as a salt, such as a nitrate, sulphate, oxalate, halide, acetate. In one embodiment, the redox active metal centre can be coordinated to the anion and/or any other ligands, such as amines, phosphines, oximes, or macrocyclic ligands, such as crown ethers, porphyrins, salophens and the like. In another embodiment, the metal centre is added in the form of an oxide. In yet another embodiment the redox active metal centre is provided in a compound having more than one redox active metal centre, such as a heteropolyacid, for example in the form of molybdovanadophosphoric acid having general formula H 3+x PMo (12 . x) V x . where x is typically between 1 and 3. In this embodiment, the heteropolyacid can also function as the acid component of the catalyst.
When in use, the redox active metal centre is capable of being present in an oxidised form and a reduced form, such that the metal centre can cycle between two different oxidation states, for example Pd(O)ZPd(II), Pt(O)/Pt(II) and/or Pt(II)ZPt(IV), Rh(I)ZRh(III), Ni(O) and Ni(II) and Co(II)ZCo(III). In the oxidation of alkanes, such as methane oxidation, the redox active metal centre oxidises, or activates, the hydrocarbon by cleaving a carbon- hydrogen bond. This can be through a homolytic mechanism, via a free-radical pathway, or by a heterolytic mechanism. One-electron redox cycles tend to result in homolytic
cleavage of the C-H bond, which produces highly reactive free radicals which can attack or decompose one or more of the catalyst constituents. Therefore, two-electron redox cycles are preferred, which tend to promote heterolytic cleavage of C-H bonds. This prolongs the lifetime of the catalyst components, and improves selectivity to desired products. Preferred redox active metal centres with two- electron redox cycles are Ni 5 Rh, Pd or Pt.
The catalyst composition comprises an acid. The acid, which can act as a solvent for the other catalyst components in a homogeneous system, is able to form an ester with the oxidised hydrocarbon. In the case of methane oxidation, for example, the acid forms a methyl ester. Examples of acids suitable for use in the present invention are typically strong Brønsted acids, and include inorganic mineral acids, such as heteropolyacids (for example phosphotungstic acid, silicotungstic acid, phosphomolybdic acid, or silicomolybdic acid, or substituted analogues thereof such as molybdovanadophosphoric acid), sulphuric acid, oleum, methyl sulphonic acid, trfluoromethyl sulphonic acid, and organic acids such as trifluoroacetic acid. In use, during oxidation of the hydrocarbon, the redox active metal centre is reduced to a lower oxidation state. For catalysis to be maintained, the metal centre is reoxidised to a higher oxidation state by an oxidant. Although this can be achieved by oxygen alone in certain circumstances, it can be a very slow process. The presence of an oxidant in the catalyst enhances the rate of reoxidation of the metal centre. Examples of oxidants suitable for use in the present invention include a peroxide such as hydrogen peroxide, tert-butyl hydrogen peroxide or cumene hydroperoxide, a peracid such as peroxyacetic acid, quinone or derivatives thereof, or a second redox active metal. In one embodiment, the second redox active metal, when employed as an oxidant, is in the form of a porphyrin or salophen complex of, for example, Cu, Co or Fe. When the catalyst is in use, the source of nitrous oxide produces nitrous oxide.
Nitrous oxide is oxidised in the presence of oxygen to nitrogen dioxide. The nitrogen dioxide in turn can oxidise the reduced oxidant, and re-create the nitrous oxide.
An advantage of the present invention is that only catalytic amounts of the catalyst components are required, as opposed to stoichiometric amounts, and only oxygen and the hydrocarbon are consumed in the process.
In a particularly preferred embodiment, the oxidant is quinone or a derivative thereof. Quinone and its derivatives tend to be more resistant to deactivation compared to other
oxidants, such as transition metal macrocy ic complexes, when the catalyst is in use. Derivatives of quinone comprise the basic quinone unit (i.e. O=C 6 H 4 =O) with one or more of the carbon atoms having a functional group, such as an alkyl, aryl, halide, hydroxide, ester, ether and the like. When in use, the quinone or quinone derivative oxidises the reduced form of the redox active metal to form hydroquinone. This is achieved in the presence of acid, requiring two protons to balance the negative charges acquired on its reduction. When the hydroquinone is oxidised, the protons are re-released. Before use, the quinone or derivative thereof may be present in the catalyst in the oxidised or reduced form, i.e. as quinone or hydroquinone (or derivative thereof). The source of nitrous oxide is particularly beneficial when used in conjunction with quinone in the catalyst of the present invention. A high degree of reoxidation of the hydroquinone to quinone (or derivatives thereof) can be achieved, which in turn benefits the rate of catalysis and yield of oxygenated hydrocarbon when the catalyst is in use. Typically, the molar ratio of the redox active metal centre to the oxidant is in the range of from 1 : 100 to 100 : 1, preferably in the range of from 1 : 0.5 to 1 : 50. The molar ratio of redox active metal centre to the source of nitrous oxide is suitably in the range of from 1 : 100 to 100 : 1, preferably in the range of from 1 : 0.5 to 1 : 50.
The catalyst can be used in the oxidation of hydrocarbons to oxygenated hydrocarbons in the presence of oxygen. Oxygenated hydrocarbon products include alcohols, ethers, esters, carboxylic acids, epoxides, aldehydes and ketones. In one embodiment, the catalyst can be used to oxidise an alkane, for example a C 1 to C 4 alkane, to an alcohol. The catalyst shows surprisingly high activity towards the direct oxidation of methane to methanol. Temperatures typically used in methane oxidation reactions are in the range of from 50 to 25O 0 C, and pressures up to 100 barg, for example in the range of from 20 to 70 barg (2.1 to 7.1 MPa).
The invention will now be illustrated by the following non-limiting examples and by Figure 1, which shows a schematic overview of a methane oxidation mechanism using a catalyst in accordance with the present invention;
In Figure 1, a typical catalytic mechanism is illustrated for a homogeneously catalysed methane oxidation reaction in the presence of oxygen, in which the redox active metal centre is palladium, the acid is trifluoroacetic acid, the oxidant is quinone, and the source of nitrous oxide is a nitrite salt (in the form of sodium nitrite). In this embodiment,
the trifluoroacetic acid, in the presence of a Pd(II) redox active centre, reacts with methane to produce methyl trifluoroacetate and two protons, the palladium being reduced in the process to Pd(O). The Pd(O) is oxidised back to Pd(II) by quinone in the presence of the two protons to produce hydroquinone. In turn, the hydroquinone is reoxidised to quinone by the action of nitrogen dioxide, which in turn is reduced to nitrous oxide, releasing water. The nitrous oxide is oxidised to nitrogen dioxide by oxygen. Methanol is released from the methyl trifluoroacetate by hydrolysis with water (catalysed by acid). The net result of the process can be expressed by the formula:
CH 4 + Vi O 2 → CH 3 OH
Experiment 1
A 50 mL glass-lined autoclave was charged with a ptfe-coated magnetic stirrer, the desired quantities of palladium acetate and oxidant, and 3 mL trifluoroacetic acid. The autoclave was purged three times with methane at 30 atm, and then charged with 55atm methane. The autoclave was then heated in an oil bath held at 8O 0 C over a period of 10 hours under constant stirring, before being quenched in an ice bath and depressurising the autoclave.
The product identities were determined using GC-MS and NMR spectroscopy, and quantified by GC.
Experiment 2
A 5OmL glass-lined autoclave, equipped with a PTFE-coated magnetic stirrer bar, was charged with 3 mL trifluoroacetic acid, and the desired quantities of palladium acetate, para-quinone and sodium nitrite. The reactor was purged three times with methane at 30 atm. The autoclave was then charged with methane (54 atm partial pressure) and optionally oxygen (1 atm partial pressure), and then heated in an oil bath held at 8O 0 C with constant stirring. After 10 hours, the reaction was quenched by cooling in an ice bath and releasing the pressure. The product identities were determined using GC-MS and NMR spectroscopy, and quantified by GC, and the quantity of Pd(II) remaining in solution was determined by gravimetric analysis after precipitation.
Comparative Examples 1 to 7,
Conversions of methane to methyl trifluoroacetate in the presence of trifluoroacetic acid using a palladium catalyst were evaluated according to the procedure outlined in Experiment 1. These examples are not in accordance with the present invention as there was no source of nitrous oxide.
The results of methane oxidation experiments in the presence of different oxidants are shown in table 1. The results show the surprisingly superior yields of methyl trifluoroacetate achieved using para-quinone as the oxidant compared to other oxidants. Only stoichiometric conversions of methane were achievable, as no oxygen or other oxidant were provided to re-oxidise the quinone, and hence the palladium catalyst.
Comparative Examples 8 to 11
Conversions of methane to methyl trifluoroacetate using a palladium catalyst and a para-quinone oxidant were evaluated following the procedure of experiment 2. These examples are also not in accordance with the present invention as a source of nitrous oxide was neither present in the catalyst composition nor was added to the process. The results are shown in table 2.
Examples 12 to 16
These are examples in accordance with the present invention. Conversions of methane to methyl trifluoroacetate using a palladium catalyst and a para-quinone oxidant in the presence of sodium nitrite were evaluated using the procedure of experiment 2. Results are shown in table 2.
These examples demonstrate that the presence of a source of nitrous oxide in the catalyst composition, or added to the catalyst composition, can dramatically increase the concentration of the oxidised form of redox active metal centres, which can result in prolonged catalyst lifetime. The results also demonstrate that significantly improved yields of oxygenated hydrocarbon products are achievable using a combination of para-quinone as the oxidant and a source of nitrous oxide.
Table 1
Based on Pd(OAc) 2
Table 2
Percentage of palladium remaining in solution at the end of the reaction. 0 below detection