WO1999024383A1

WO1999024383A1 - A process for the catalytic oxidation of hydrocarbons

Info

Publication number: WO1999024383A1
Application number: PCT/DK1998/000488
Authority: WO
Inventors: Niels J. Bjerrum; Gang Xiao; Hans Aage Hjuler
Original assignee: Statoil Research Centre
Current assignee: Statoil Research Centre
Priority date: 1997-11-12
Filing date: 1998-11-11
Publication date: 1999-05-20
Anticipated expiration: 2000-05-12
Also published as: US6380444B1; EP1051379A1; DK128697A; AU1144599A

Abstract

A process for producing an alcohol from a gaseous hydrocarbon, e.g. a lower alkane such as methane, via oxidative reaction of the hydrocarbon in a concentrated sulfuric acid medium in the presence of a catalyst employs an added catalyst comprising a substance selected from iodine, iodine compounds, titanium, titanium compounds, chromium and chromium compounds.

Description

A PROCESS FOR THE CATALYTIC OXIDATION OF HYDROCARBONS

FIELD OF THE INVENTION

The present invention relates to the catalytic oxidation of hydrocarbons to give useful oxidation products thereof. In particular, an important aspect of the invention relates to a process for the production of alkanols, e.g. lower alkanols, such as C C₃ alkanols (e.g. methanol or ethanol), from the corresponding alkanes (i.e. methane and ethane, respectively, in the case of methanol and ethanol), wherein an alkanol is produced via the formation, and subsequent hydrolysis, of an intermediate (e.g. methyl bisulfate in the case of methane as starting hydrocarbon) formed by catalytic oxidation of the hydrocarbon in a liquid, essentially anhydrous sulfuric acid (H₂SO₄) medium containing a catalyst and preferably containing a significant proportion of dissolved sulfur trioxide (SO₃).

BACKGROUND OF THE INVENTION

Methane is a raw material of great synthetic importance and an abundant natural resource as the main constituent of natural gas. Nevertheless, it is primarily used only as fuel because two factors limit its use as a raw chemical. The first is that transporting methane gas or even liquefied natural gas is not economical. Therefore, it is highly desirable to transform methane into transportable raw materials or products. The second factor is that methane is a very stable molecule and its direct conversion to useful chemicals is very difficult. Today, over 90% of the produced methane is consumed as heating fuel. Because transportation of natural gas from remote sites is costly, it has often been suggested that natural gas, namely methane, should be converted to more easily transported liquid fuel.

As a raw material for chemical industries, the two leading uses of methane are in the production of methanol and ammonia. Methane must be first transformed into synthesis gas before usage in either ammonia or methanol synthesis. Clearly, this process makes conversion into synthesis gas the dominant process of methane upgrading. The term synthesis gas is generally used for a mixture of carbon monoxide and hydrogen, preferably at a 1 :2 or 1 :3 ratio. Today the dominant route to the production of synthesis gas is the methane steam reforming process. The reaction can be stoichiometrically expressed as

CH₄ + H₂O → CO + 3H₂

A considerable disadvantage of the steam reforming process is that it is an endothermic reaction. The endothermicity results from addition of steam in which a significant amount of energy is required to decompose water into its elements.

In addition to synthesis gas formation, several documents disclose a variety of methods for activating methane to produce other higher molecular weight materials. Mobil Oil Corporation is the assignee in several U.S. patents using sulfur or certain sulfur-containing compounds as the reactants in non-catalytic reactions with methane to produce methyl intermediates which can then be converted to higher molecular weight hydrocarbons.

In U.S. Patent No. 4,543,434, Chang teaches a process using the following steps:

CH₄ + 4S → CS₂ + 2H₂S

CS₂ + 3H₂ (Co or Ni) → CH₃SH + H₂S CH₃SH (HZSM-5) → [CH₂] + H₂S 4H₂S → 4H₂ + 4S

where "[CH₂]" is a hydrocarbon having at least two carbon atoms.

Another Mobil disclosure (U.S. Patent No. 4,864,073 to Han et al.) suggests a carbonyl sulfide-based process in which methane and carbonyl sulfide are contacted in the presence of ultraviolet light under conditions sufficient to produce CH₃SH. No other reaction initiators are said to be present. The reaction scheme is shown to be: CH₄ + COS → CH₃SH + CO

CH₃SH (HZSM-5) -> [CH₂] H₂S

(H₂S → S), a regeneration step

CO + S → COS

The selectivity of the first reaction is said to be high, i.e., around 81%, however, the conversion appears to be quite low.

A disclosure similar to that of Chang is found in Mobil's U.S. Patent No. 4,864,074 to Han et al. As in Chang, the methane is contacted with sulfur. The process conditions are changed, however, so that either CS₂ or CH₃SH is formed. These sulfur compounds may then be converted in the presence of the preferred HZSM-5 zeolite catalyst to produce hydrocarbons having two or more carbon atoms. Also, as was the case with "Chang", the step of contacting the methane to produce a methyl-sulfur compound is performed in the absence of a catalyst.

Other methods are known for producing substituted methanes which are suitable for further reaction to heavier hydrocarbons. A thermal methane chlorination process is shown in U.S. Patent No. 4,804,797 to Minet et al. A similar process is disclosed in U.S. Patent No. 3,979,470 to Fimhaber et al. (Although a preference for C₃ hydrocarbon feeds is expressed in the patent).

One method shown in U.S. Patent N.4,523,040 to Olah utilizes either a solid strongly acidic catalyst or a supported Group VIII metal (particularly platinum and palladium) in the gas phase halogenation of methane to produce methyl halides. The patent indicates that monohalides are produced with 85% to 99% selectivity. Olah suggests that subsequent or concurrent catalytic hydrolysis produces methyl alcohol and/or dimethyl ether. Production of methyl oxo-esters is not shown.

The reaction of methane with palladium (II) acetate in trifluoroacetic acid to effect the trifluoroacetoxolation of methane is shown in Sen et al., "Palladium (II) Mediated Oxidative Functionalization of Alkanes and Arenes", New Journal of Chemistry (1989), Vol. 13, No.10-11 , pp. 756-760. A yield of 60% based on palladium was reported when the reaction was practiced using methane as the reactant. Consequently, the reaction with methane utilized palladium as a reactant and not as a catalyst. The extent of methane conversion, selectivity, and reaction rates were not stated.

The Sen et al. article has been criticized in Vargaftik et al., "High Selective Partial Oxidation of Methane to Methyl Trifluoroacetate", Journal of the Chemical Society, Chemical Communications (1990), pp. 1049-1050, to the extent that the results were not found to be reproducible. Vargaftik et al. discloses the catalytic oxo- esterification of methane to methyl trifluoroacetate with cobalt in trifluoroacetic acid but shows that palladium is not even suitable for stoichiometric methane oxidation in the process. With Pd, less than 0.1% yield of methyl trifluoroacetate based on palladium (II) trifluoroacetate was obtained.

The Vargaftik et al. article discloses that although palladium is ineffective for the conversion of methane to methyl trifluoroacetate, Co(lll) can be used for this reaction. The Co(lll) is said to be catalytic in the presence of oxygen. The rate of the reaction was very low, 2.5x10^"11 mol/cc sec, (or four to five orders of magnitude away from typical commercial rates of about 10^~6 mol/cc.sec). Only four turnovers of the Co ion were disclosed. The extent of methane conversion was not stated. In addition to Co, other metals were suggested which were said to allow stoichiometric oxidation of methane to methyl trifluoroacetate in varying yields (based on amount of metal charged): Mn (30%), Cu(0.1%), and Pb(10%).

A later publication by Sen et al ("Homogeneous Palladium (II) Mediated Oxidation of Methane", Platinum Metals Review, (1991), Vol 35, No. 3, pp. 126-132) discloses a catalytic system using palladium as the catalyst, peroxytrifluoroacetic acid as the oxidant, and a mixture of trifluoroacetic acid and trifluoroacetic anhydride as the solvent. The reaction rate was low (4.2x10^"9 mol/cc.sec) and only 5.3 turnovers of Pd were observed. The extent of methane conversion and selectivity were not stated. There are several lesser routes to upgrade methane such as ammoxidation to HCN, chlorination to chloromethanes, carbon disulfide production and acetylene synthesis. Of great importance is the direct oxidation of methane to produce methanol, formaldehyde, or dimerized products, ethane and ethylene. None of these reactions, however, has yet found large-scale commercial application due to either limited catalytic activity, short catalytic life or low product selectivity. Despite the limited selectivities and yields, numerous studies have been reported on the direct partial or complete oxidation of methane.

A homogeneous system for the selective, catalytic oxidation of methane to methanol via methyl bisulfate was reported (R.A. Periana et al., International Patent Application WO 92/14738). This document describes a novel high-yield system for the catalytic conversion of methane to methanol. Homogeneous reaction takes place in concentrated sulfuric acid and is catalyzed by catalysts comprising metals such as Pd, Tl, Pt, and Au with Hg being most preferred. It is said that they have achieved a yield of 43% at a methane conversion of 50% and 85% selectivity to methyl bisulfate (R.A. Periana et al., Science, vol.259, 15 Jan. 1993). The activation of methane is proposed to occur through a net electrophilic displacement reaction with mercuric bisulfate to produce methyl mercuric bisulfate. This species then decomposes to the product and the reduced species, mercurous bisulfate, in the functionalization step. In the redoxidation step, the mercurous bisulfate is oxidized by sulfuric acid, regenerating mercuric bisulfate. The problems associated with this process are that most of the disclosed catalysts are expensive, and the most preferred catalyst, Hg(ll), is poisonous and environmentally damaging. Furthermore, the applied gas pressure necessary in order to achieve sufficient methanol yield is very high.

In the present invention we disclose a process which is related to that disclosed in WO 92/14738, but which employs other types of catalysts which are inexpensive, relatively non-poisonous and produces much higher yields of methanol from methane. In addition to this, to achieve the same methanol yield, the required methane gas pressure in the process according to the present invention is much lower than the pressures required in the process described by Periana et al. DESCRIPTION OF THE INVENTION

In the present invention, initial chemical reaction of a hydrocarbon - in general a hydrocarbon in the gas phase - takes place in a concentrated sulfuric acid (H₂SO₄) medium [preferably one containing dissolved sulfur trioxide (SO₃), in the present description denoted an H₂SO₄/ SO₃ medium] and is catalyzed by, in particular, substances containing iodine, titanium or chromium in various oxidation states; more generally, it would appear that species which have a standard reduction potential in the range of 0.5-1.4 volt versus the Standard Hydrogen Electrode are often well suited as catalysts in the context of the present invention.

In the case of alkanes, notably lower alkanes such as those already mentioned above, e.g. methane, an important major product of the reaction in sulfuric acid medium appears to be the corresponding alkyl bisulfate (alkyl hydrogen sulfate), e.g. methyl bisulfate in the case of methane, which may, if so desired, be isolated as an end product perse; such a process constitutes an aspect of the present invention.

Alternatively (and, particularly in the case of methane, importantly), the latter bisulfate product may subjected to a hydrolysis step (e.g. using water or another aqueous medium) to give the corresponding alkanol (alcohol), i.e. methanol in the case of methane.

The reactions of particular interest (illustrated here for methane) are shown below.

Step l : CH₄ + 2H₂SO₄ - CH₃OSO₃H + 2H₂O + SO₂ (1)

Step 2: CH₃OSO₃H + H₂O → CH₃OH + H₂SO₄ (2)

Overall: CH₄ + H₂SO₄ → CH₃OH + H₂O + SO₂ It is envisaged that in order to reduce consumption of the concentrated sulfuric acid medium in the process, a further step could be incorporated wherein SO₂ formed in the process is reoxidized [using, for example, an oxygen-containing gas (such as air or substantially pure oxygen)] to SO₃ which would then react with water present in the system to give sulfuric acid. Thus, a step such as the following is contemplated:

Step 3: SO₂ + ¹/₂O₂ + H₂O → H₂SO₄ (3)

With incorporation of such a step in a process of the invention, the overall reaction of particular interest could then be written as:

Overall: CH₄ + ¹/₂O₂ → CH₃OH (4)

The concentrated essentially 100% sulfuric acid used as the reaction medium in the initial stage of the process plays several roles. Firstly, it is a strong acid exerting a super-acidic function in the system which contributes to retention, absorption and/or binding of the hydrocarbon in the medium during the oxidative reaction in question. Secondly, sulfuric acid acts as an oxidant, being itself reduced to sulfur dioxide. Further, it functions as a reactant in its own right, combining e.g. with alkanes such as methane to initially form a sulfate ester species, and as a solvent for the hydrocarbon and the reaction products thereof.

Thus, as already indicated to some extent, an important aspect of the present invention relates to a process for producing an alcohol from a gaseous hydrocarbon - such as the production of an alkanol from a corresponding alkane, especially methanol from methane - via oxidative reaction of the hydrocarbon in a concentrated sulfuric acid medium in the presence of a catalyst, wherein the substance added as catalyst comprises a substance selected from the group consisting of iodine, iodine compounds, titanium, titanium compounds, chromium and chromium compounds. In preferred variants of the process, the latter reaction takes place in a pressurizable reaction vessel. Stirring or other effective mixing of the reaction medium during the reaction is generally to be preferred.

In order to form the desired alcohol (e.g. alkanol) from the product or products of the reaction of the hydrocarbon in the sulfuric acid medium, the product or products (e.g. an alkyl bisulfate and/or dialkyl sulfate) in question is/are suitably subjected to an hydrolysis step, e.g. using water or an aqueous medium.

The sulfuric acid medium employed in the process preferably contains dissolved SO₃ in an amount of from 0% by weight up to the solubility limit of SO₃ in H₂SO₄ under the particular reaction conditions (temperature, pressure, etc.) employed. The SO₃ content of the medium will, however, generally be in the range of 0-65% by weight (% w/w), and normally in the range of 30-65% w/w.

The reaction temperature in the sulfuric acid medium will normally preferably be in the range of 20-400°C, often around 200°C. With respect to pressure conditions, particularly when the gas-phase hydrocarbon in question is an lower alkane, e.g. methane, the reaction in the sulfuric acid medium will preferably take place in a pressurizable reaction vessel into which the hydrocarbon is introduced to an initial pressure in the range of 1-500 bar. It is, however, generally preferable to employ a initial pressure in the range of 20-80 bar, such as in the range of 40-70 bar.

With regard to catalytic substances of interest in relation to the present invention, the added catalyst employed is preferably one comprising:

iodine (elemental iodine) or a compound of iodine in any oxidation state (e.g. -1 or

+5); titanium (metal, e.g. in powder form) or a compound of titanium in any oxidation state (e.g. +3 or +4), such as titanium(IV) oxide (i.e. titanium dioxide); or chromium (metal, e.g. in powder form) or a compound of chromium in any oxidation state (e.g. +2, +3 or +6), such as a chromium (VI) compound in the form of, e.g., a chromate or dichromate (e.g. of an alkali metal such as sodium or potassium). Among catalytic substances of the latter types, particularly suitable substances are believed to include those having a standard reduction potential in the range of 0.5-1.4 volt versus the Standard Hydrogen Electrode (SHE).

The invention thus provides, inter alia, catalytic systems in a pressurized reactor vessel, comprising a catalyst such as a compound containing iodine, titanium or chromium, an active reaction medium comprising liquid H₂SO₄/SO₃, and a gaseous hydrocarbon as reactant. As already indicated, the resulting liquid containing oxo- esters (sulfate esters) may be hydrolyzed to produce alcohols or used to produce other compounds. The preferred parameters (temperature, pressure, SO₃ content, added catalysts etc.) for such catalytic systems are those already mentioned above in the context of processes of the invention.

EXAMPLES

The following examples are batch processes intended to illustrate parts of the overall inventive process, in particular the alkyl esterification reaction utilizing methane as the reactant. The remainder of the process steps are easily selectable from known processes. Continuous processes are also feasible.

A. A 200 ml pressure reactor was charged with 25 ml of fuming H₂SO (65% SO₃) and 0.00674 mol of KIO₃. The reactor was flushed with methane and heated to 200° C under 42 Bar of methane. The content of the reactor was stirred. After one hr the reactor was cooled to 25° C. A gas sample of the gas in the reactor was obtained for gas chromatographic analysis. An aliquot of the reaction mixture was first diluted with 19 volumes of water and the resulting solution heated in a sealed container for 4 hours. The resulting solution was quantitatively analyzed for the free CH₃OH by high pressure liquid chromatography (HPLC) using a Nucleosil C18 column and a refractive index detector. Based on the HPLC measurements, the yield of methanol was 36% (Table I, entry 4). Gas chromatographic analysis showed that fairly large amount of SO₂ and trace levels of CO₂ were produced. The amount of produced methanol is several orders of magnitude higher than the KIO₃.

B. To show the effect of KIO₃ concentration, the general procedure of Example A was repeated with 0.00169 mol and 0.01011 mol of KIO₃ for two hours under 42 Bar of methane. The results given in Table I (entries 1-3) show that 0.00169 mol of KIO₃ gives the highest yield of methanol.

C. To show the greater effectiveness of KIO₃ as compared to Hg(ll), the general procedure of Example A was repeated using 0.00674 mol of KIO₃ and 0.00674 mol of HgSO as the catalysts, respectively. The results shown in Table I show that both in the case of 25 ml acid (entries 4 and 5) and 50 ml acid (entries 6 and 7) KIO₃ gives better yields of methanol compared to HgSO .

D. As a "blank run", the procedure of Example A was repeated but without added catalyst. As may be seen from the results in Table I (entry 8) no methanol was produced in the reaction.

E. To show the effect of acid concentration, the general procedure of Example A was repeated with 100% H₂SO₄ and fuming H₂SO₄ (32.5% SO₃), 0.00674 mol KIO₃, and 42 Bar of methane for one hour. The results shown in Table I (entries 9 and 10) show that the yield of methanol was much higher in the case of higher acid concentration.

F. To show the effect of different acid amount, the general procedure of Example A was repeated with 25 ml, 50 ml and 75 ml of fuming H₂SO (65% SO₃) and KIO₃ as the catalyst. The best methanol yield was found with 50 ml acid and two hours of reaction time (84%), and with 75 ml acid and one hour of reaction time (80%) (entries 11 and 12). The same experiments were done using HgSO₄ as the catalyst. The yields of methanol were lower compared with KIO₃ (entries 7 and 5). G. To show the effect of temperature, the general procedure of Example A was repeated at 100° C, 120° C, 140° C, 160° C, 180° C, 190° C, 200° C, 210° C and 220° C with 0.00674 mol KIO₃ for one hour under 42 Bar of methane. The results shown in Table I (entries 13-20) show that increasing temperatures from 100° C - 210° C lead to increased yields of methanol. At 220° C, a loss in yield was observed.

H. To show the effect of reaction time, the general procedure of Example A was repeated with reaction time of 20 min., 40 min., 60 min., and 120 min. using KIO₃ as the catalyst. The results show that increasing reaction time lead to increased yields of methanol. However, 60 min. is perhaps the optimal duration considering the energy cost (entries 21 , 22, 4 and 1).

I. To show how the reaction proceeds with different forms of iodine, the general procedure A was repeated with Kl as the catalyst at 200 °C for 20 min. The result shows that I^"1 also works as a catalyst (entry 23).

J. To show that the reaction also proceeds with elementary l₂, the general procedure A was repeated with l₂ as the catalyst with 25 ml and 50 ml of fuming H₂SO (65% SO₃), respectively. The results show that l₂ works as a catalyst and gives methanol yields of 74% and 50%, respectively (entries 24 and 25).

K. To show that the reaction also proceeds with organic compounds containing iodine, the general procedure A was repeated with CH₃I as the catalyst with 25 ml and 50 ml of fuming H₂SO (65% SO₃), respectively. The results show that CH₃I works as a catalyst and gives methanol yields of 73% and 57%, respectively (entries 26 and 27).

L. To show that the reaction also proceeds with titanium compounds, the general procedure A was repeated with TiO₂ as the catalyst with 25 ml (1 hr) and 50 ml (2 hrs) of fuming H₂SO (65% SO3), respectively. The results show that TiO₂ (and Ti(IV) and Ti(lll) compounds since TiO₂ is dissolved in the strong acid) works as a catalyst and gives methanol yields of 71% and 49%, respectively (entries 28 and 29).

To show that the reaction also proceeds with compounds containing chromium, the general procedure A was repeated with K₂Cr₂O₇ as the catalyst with 50 ml of fuming H₂SO (65% SO₃) for two hours. The result shows that chromium in oxidation VI (and III since part of the chromium must be reduced) works as a catalyst and gives a methanol yield of 30% (entry 30).

Table I

5 Entry CataCatalyst Acid Temp Duration Yield lyst Amount (mol) CO (min.) (%)

1 KI0₃ 0.00674 25 ml H₂S0₄(65% S0₃) 200 120 37

10 2 KIO3 0.00169 25 ml H₂S0₄(65% S0₃) 200 120 41

3 KIO3 0.01011 25 ml H₂S0₄(65% S0₃) 200 120 35

4 KIO3 0.00674 25 ml H₂S0₄(65% S0₃) 200 60 36

5 HgS0₄ 0.00674 25 ml H₂S0₄(65% S0₃) 200 60 24

6 KIO3 0.00674 50 ml H₂S0₄(65% S0₃) 200 60 50

15 7 HgS0₄ 0.00674 50 ml H₂S0₄(65% SO3) 200 60 41

8 None - 25 ml H₂S0₄(65% SO3) 200 60 0

9 KIO3 0.00674 25 ml 100% H₂S0₄ 200 60 6

10 KIO3 0.00674 25 ml H₂S0₄(32.5% SO₃) 2 20000 60 13

11 KIO3 0.00674 50 ml H₂S0₄(65% SO3) 200 120 84

20 12 KIO3 0.01348 75 ml H₂S0₄(65% S0₃) 200 60 80

13 KIO3 0.00674 25 ml H₂S0₄(65% S0₃) 100 60 1

14 KIO3 0.00674 25 ml H₂S0 (65% S0₃) 120 60 9

15 KIO3 0.00674 25 ml H₂S0₄(65% SO3) 140 60 18

16 KIO3 0.00674 25 ml H₂S0₄(65% SO3) 160 60 20

25 17 KIO3 0.00674 25 ml H₂S0 (65% SO3) 180 60 27

18 KIO3 0.00674 25 ml H₂S0₄(65% S0₃) 190 60 30

19 KIO3 0.00674 25 ml H₂S0₄(65% SO3) 210 60 31

20 KIO3 0.00674 25 ml H₂S0₄(65% S0₃) 220 60 29

21 KIO3 0.00674 25 ml H₂S0₄(65% S0₃) 200 20 31

30 22 KIO3 0.00674 25 ml H₂S0 (65% SO3) 200 40 35

23 Kl 0.00674 25 ml H₂S0₄(65% SO₃) 200 20 26

24 I2 0.00674 25 ml H₂S0₄(65% S0₃) 200 60 74

25 I2 0.00674 50 ml H₂S0₄(65% S0₃) 200 60 50

26 CH3I 0.00674 25 ml H₂S0 (65% S0₃) 200 60 73

35 27 CH3I 0.00674 50 ml H₂S0₄(65% S0₃) 200 60 57

28 Ti0₂ 0.00674 25 ml H₂S0 (65% S0₃) 200 60 71

29 Ti0₂ 0.00674 50 ml H₂S0₄(65% S0₃) 200 120 49

30 K₂Cr₂0ι > 0.00674 50 ml H₂S0₄(65% S0₃) 200 120 30

40

Note: All experiments were carried out at a methane pressure of 42 bar. The above examples are merely illustrative and are not to be interpreted as limiting the scope of the disclosed invention in any manner.

Moreover, it should be clear that one of ordinary skill in this art could envisage equivalents to the processes described in the accompanying claims, and that such equivalents would be within the scope and spirit of the claimed invention.

Claims

1. A process for producing an alcohol from a gaseous hydrocarbon via oxidative reaction of said hydrocarbon in a concentrated sulfuric acid medium in the presence of a catalyst, wherein the substance added as catalyst comprises a substance selected from the group consisting of iodine, iodine compounds, titanium, titanium compounds, chromium and chromium compounds.

2. A process according to claim 1 , wherein said reaction takes place in a pressurizable reaction vessel.

3. A process according to claim 1 or 2, wherein said sulfuric acid medium is stirred during said reaction.

4. A process according to any one of the preceding claims, wherein the product or products of said reaction of said hydrocarbon is/are subjected to an hydrolysis step.

5. A process according to any one of the preceding claims, wherein said hydrocarbon is an alkane, and said alcohol is an alkanol.

6. A process according to claim 5, wherein said alkane is methane, and said alcohol is methanol.

7. A process according to any one of the preceding claims, wherein said sulfuric acid medium contains dissolved SO₃ in an amount of from 0% by weight up to the solubility limit of SO₃ in H₂SO₄ under the reaction conditions in question.

8. A process according to claim 7, wherein said sulfuric acid medium contains 0-65% w/w of dissolved SO₃.

9. A process according to any one of the preceding claims, wherein the reaction temperature in said sulfuric acid medium is in the range of 20-400°C, preferably around 200°C.

5 10. A process according to any one of the preceding claims, wherein said hydrocarbon is gaseous methane, and said reaction in said sulfuric acid medium takes place in a pressurizable reaction vessel under an initial gas pressure in the range of 1-500 bar, preferably in the range of 20-80 bar.

10 11. A process according to any one of the preceding claims, wherein said added catalyst comprises elemental iodine or a compound of iodine in any oxidation state, preferably elemental iodine.

12. A process according to any one of the preceding claims, wherein said added 15 catalyst comprises titanium or a compound of titanium in any oxidation state, preferably titanium dioxide.

13. A process according to any one of the preceding claims, wherein said catalyst comprises chromium or a compound of chromium in any oxidation state, preferably 0 a chromium (VI) compound, more preferably a chromate or dichromate of an alkali metal.

14. A process according to any one claims 11-13, wherein said iodine-containing, titanium-containing or chromium-containing substance has a standard reduction 5 potential in the range of 0.5-1.4 volt versus the Standard Hydrogen Electrode.