GB2252104A

GB2252104A - Hydrocarbon conversion

Info

Publication number: GB2252104A
Application number: GB9101770A
Authority: GB
Inventors: Robert William Judd; David Ian Bradshaw
Original assignee: British Gas PLC
Current assignee: British Gas PLC
Priority date: 1991-01-28
Filing date: 1991-01-28
Publication date: 1992-07-29
Also published as: GB9101770D0

Abstract

A method is described for converting a hydrocarbon into a higher hydrocarbon, for example, methane into ethane and ethene. A reducible metal oxide solid, such as potassium doped manganese dioxide, is contacted firstly with a reducing agent and a halogen-containing species under substantially oxidant-free conditions. The contacting with the halogen- containing species is discontinued and the solid is then contacted with a hydrocarbon and an oxidant to produce the higher hydrocarbon product(s).

Description

HYDROCARBON CONVERSION The present invention relates to the conversion of hydrocarbons into higher hydrocarbons, such as the conversion of methane into ethane and ethene.

Such reactions involving the oxidative coupling of methane are currently the subject of much attention both academically and industrially as it is envisaged that they may form a basis for another use of natural gas which is in plentiful supply and which contains methane.

Methane may be converted to products including ethane and ethene (C2 products) by contacting methane with a suitable reducible metal oxide based catalyst, for example some oxides of manganese, which can function as methane coupling catalysts. During contact with the methane the oxide is reduced and then no longer functions well as a methane coupling catalyst. In order to reactivate the catalyst to function properly again as a methane coupling catalyst the oxide has to be re-oxidised. It is however recognised that feeding the oxidant and reactant gases simultaneously (co-feeding) is a process that works equally satisfactorily. In such a case, the reduction and oxidation processes are superficial only and the contacting solid is acting as a conventional heterogeneous catalyst.

According to one aspect of the invention, a method for converting a hydrocarbon to a higher hydrocarbon product comprises contacting a solid which comprises at least one reducible oxide of at least one metal with a reducing agent and with a halogen-containing species under substantially oxidant-free conditions, discontinuing the contacting with the halogen-containing species, and then contacting the solid with the hydrocarbon and an oxidant to produce the higher hydrocarbon product.

The method may be carried out at a temperature of from 500 C to 1000 C, and preferably from 600 C to 85O0C, and may be carried out at a pressure of from 1 to 50 bar.

The hydrocarbon-preferably is methane and the at least one reducible oxide of at least one metal is preferably manganese dioxide (MnO2). The reducible oxide may be doped with or contain an alkali metal or a compound thereof such as lithium chloride, sodium chloride, potassium chloride or caesium chloride; or an alkaline earth metal or a compound thereof such as barium chloride or calcium chloride; or yttrium or yttrium chloride (YC13); or manganese or manganese chloride (MnC12); or a mixture of two or more of these elements or compounds.

In the case of potassium chloride, sodium chloride, lithium chloride or calcium chloride when used as dopant in manganese oxide, the content of the chloride compound may, for example, be present in an amount up to about 50% by weight.

The halogen-containing species may be chlorine, or an inorganic halide such as hydrogen chloride, or may be an organohalide for example an organochloride such as CH2C12,CHC13,CC14.

The reducing agent may be hydrogen, carbon monoxide or methanol, or the reducing agent may be a hydrocarbon and such 'reducing agent' hydrocarbon may be the same composition as the hydrocarbon to be converted, for example methane.

The hydrocarbon to be converted may be present with the halogen-containing species when the latter is contacting the solid.

The contacting of the solid with the halogen-containing species may be carried out in two or more separate or discrete stages.

Following the discontinuing of the contacting with the halogen-containing species and the reducing agent, the solid may be contacted with an oxidant-containing gas as the oxidant partially to re-oxidise the solid following at least partial reduction thereof as a result of the conversion of the hydrocarbon. When a hydrocarbon is being used as the reducing agent it is preferable to contact the solid with the oxidant-containing gas in the absence of any hydrocarbon to remove carbon deposits.

Preferably the duration of the contacting of the solid with the oxidant-containing gas is sufficient to allow substantial, or substantially complete, burning off of the residual carbon deposited in the vicinity of the solid as a result of the initial or pre-reduction in the presence of the halogen-containing species.

Where oxidant-containing gas is employed to re-oxidise the solid, fall in the production of oxide(s) of carbon or the appearance in the exit gases of oxidant may be detected and, in response to such detection, hydrocarbon to be converted may be added to the oxidant-containing gas to contact the solid. This hydrocarbon may be the same as, or different from, the initially used hydrocarbon. In the mixture of the hydrocarbon to be converted and the oxidant-containing gas, the ratio of hydrocarbon to oxidant-containing gas on a volume/volume basis is preferably from 1:1 to 100:1 and more preferably 5:1 to 25:1. From this point the gradual build up of higher hydrocarbons in the exit gases should be observed.

Preferably further said halogen-containing species is introduced into the hydrocarbon/oxidant-containing gas mixture to contact the solid. Moreover, when a predetermined proportionate rise in the amount of oxidant containing gas is detected in the presence of the higher hydrocarbon produced, the ratio of hydrocarbon to the oxidant-containing gas in the gas mixture may be increased for the purpose of maintaining product selectivity.

An illustrative procedure for carrying out a method of conversion according to the invention will now be described.

A catalyst composition was prepared for use in the method by dry mixing and grinding MnO2 with a halide, for example KC1, and then calcining the mixture at about 7500C for 8 to 10 hours. This method of catalyst composition preparation is only described by way of example and other methods are also suitable.

100mg of the calcined mixture (having a particle size of 150-250 pm) was introduced into a quartz reactor having a 4mm internal diameter to form a catalyst charge or bed contained between two 3mm plugs of quartz wool in the reactor. The reactor was operated at a pressure of approximately 1 bar.

The catalyst temperature was raised to 700 to 7900C (typically about 7500C) in a flow of non-oxidising gas (such as argon) or non-oxidising gas mixture including a reducing agent (such as argon and as the reducing agent methane). The flowrate of the non-oxidising gas or gas mixture was typically 10 to 30ml/min.

If not already present, methane was introduced with the non-oxidising gas into the reactor at a flowrate of typically 10 to 30ml/min.

A chloride-containing species, usually CHC13, was then injected directly over the catalyst, into the flowing gas stream, typically as a 5pl aliquot (analysis of the products formed at the exit of the reactor at this stage identified a pulse of product species including CO, CO2, C2H41 C2H6, 02, C12 and HC1 where NnO2/KC1, CHC13 and CH4/argon were employed in the method).

Further injections of the chloride-containing species each of an equivalent quantity as before were continued at intervals of 1 second to 45 mins (usually 5 mins to 15 mins) until (in the specific experiments carried out) at least three such injections were made.

After the first chloride-containing species addition, analysis showed that subsequent injections thereof resulted in a diminishing quantity of oxygen-containing products until eventually ethene was the major carbon containing species produced immediately upon injection.

Subsequently, the catalyst was re-oxidised in the same temperature range by the controlled addition of oxygen into the reactor, preferably with the discontinuation of the flow of methane into the reactor first to allow burn off of any residual carbon in the reactor. This required an oxygen flow rate of about 2 to 5 ml/min in an inert gas stream, for example argon and operated at substantially complete oxygen consumption. After a period of time when the quantity of carbon oxides produced began to fall, methane was re-introduced into the reactor such that the CH4/O2 ratio was adjusted to approximately 10:1. This resulted in substantially complete oxygen consumption and the conversion of methane to products with optimum steady state selectivity of C2 products > 90%, of which ethene was found to be as high as 80%.

It was convenient after the introduction of CH4 and adjustment of CH4/02 ratio, to inject a further pulse of chloride-containing species into the reactor to facilitate or allow rapid attainment of optimum selectivity as before.

During operation the oxygen content of the exit gas is monitored; the oxygen flow to the reactor is adjusted continuously to maintain the optimum selectivity whilst minimising oxygen in the product gas.

Under the conditions described the high C2 products selectivity mentioned above has been maintained for up to 24 hours. A specific example of a presently preferred catalyst and the conditions are shown in Table 1.

TABLE 1 Catalyst 100mg MnO2/25%KC1 Treatment Temperature 750 C CHCl3 treatment Flow Rates CHC13 3 x 5 1 aliquots CH4 10-3Oml/min Argon 10-30ml/min Methane oxidation Initial (CH4/02 ratio ~10) CH4 10-30ml/min 2 l-3ml/min Final (CH4/02 ratio 25) CH4 10-30ml/min 2 < lml/min Illustrative results of selectivities and yields obtained under steady state conditions (similar to those referred to earlier) for catalysts stabilised by the method described above are shown in Table 2.

TABLE 2 C-atom CATALYST TEMPERATURE C SELECTIVITY % C2 YIELD %** C2H4 C2H6 C2* MnO2 770 25 25 50 7 MhO2/25% KC1 770 71 19 90 10-20 Mn2/20% KC1 770 40 7 47 5 MnO2/20% KC1 650 59 12 71 8 MnO2/50% NaCl 770 40 3 43 6 Mn02/50% NaCl 670 48 5 53 5 * C2 = Aggregate of C2H4 and C2H6 No. of C-atoms converted to C2H6 + C2H4 ** C2 yield % - x 100 Total no. of C-atoms Applicants investigations have shown that the method specifically described above stabilises the manganese oxide based catalyst in a methane coupling mode for longer periods than was hitherto known by the Applicants for manganese oxide based catalysts and that this allows increased production from methane of C2 products, particularly ethene, at high selectivities and for longer durations. As can be seen from Table 2 good methane conversion results were obtained not only in the temperature range 750 to 7700C but also in the lower temperature range 650-6700C.

Claims

1 A method for converting a hydrocarbon to a higher hydrocarbon product comprising contacting a solid comprising at least one reducible oxide of at least one metal with a reducing agent and a halogen-containing species under substantially oxidant free conditions, discontinuing the contacting with the halogen-containing species and the reducing agent and then contacting the solid with the hydrocarbon and an oxidant to produce the higher hydrocarbon product.

2. A method as claimed in claim 1, in which the solid further comprises at least one alkali metal or compound thereof and/or at least one alkaline earth metal or compound thereof.

3 A method as claimed in claim 2, in which the solid further comprises potassium chloride and/or sodium chloride.

4. A method as claimed in claim 2, further comprising potassium chloride present in an amount of up to and including 50% by weight.

5. A method as claimed in any of the preceding claims, in which the reducing agent is a hydrocarbon.

6. A method as claimed in any of claims 1 to 4, in which the reducing agent is hydrogen, carbon monoxide or methanol.

7. A method as claimed in any of the preceding claims, in which the contacting of the solid with the halogencontaining species is performed in two or more discrete stages.

8. A method as claimed in any of the preceding claims, in which the oxidant comprises an oxidant-containing gas.

9. A method as claimed in claim 8, in which the contact of the solid with the hydrocarbon is discontinued for a period sufficient to allow substantial, or substantially complete, burning off by the oxidant-containing gas of residual carbon deposited in the vicinity of the solid.

10. A method as claimed in claim 8 or 9, further comprising detecting a fall in oxide(s) or carbon production or detecting the appearance in the exit gases of oxidant and then adding further hydrocarbon to be converted to the oxidant-containing gas to contact the solid.

11. A method as claimed in claim 10, in which in the mixture of the hydrocarbon and the oxidant-containing gas the ratio of hydrocarbon to oxidant-containing gas on a volume/volume basis is from 1:1 to 100:1.

12. A method as claimed in claim 10 or 11, in which further said halogen-containing species is introduced into the hydrocarbon/oxidant-containing gas mixture to contact the solid.

13. A method as claimed in claim 11 or 12, further comprising detecting a predetermined proportionate rise in the amount of oxidant obtained with the higher hydrocarbon product, and in response to such rise increasing the ratio of hydrocarbon to the oxidant-containing gas in the gas mixture.

14. A method as claimed in any of the preceding claims, in which the hydrocarbon to be converted is methane.

150 A method as claimed in any of the preceding claims, in which the halogen-containing species contacts the solid under said substantially oxidant-free conditions in the presence of an inert gas.

16. A method as claimed in any of the preceding claims, in which the halogen-containing species is an organohalide.

17. A method as claimed in claim 16, in which the organohalide is an organochloride.

18. A method as claimed in any of the preceding claims, in which the at least one reducible oxide of at least one metal is manganese dioxide (Mn02).

19. A method for converting a hydrocarbon to a higher hydrocarbon product according to claim 1 and substantially as hereinbefore described with reference to the illustrative examples.

20. A hydrocarbon product obtained by a method according to claim 1.