Elements Issue 1 - Chemistry

The Catalytic Conversion of Natural Gas

Julian Ross

Introduction

The economies of the developed countries of the world depend to a marked degree on the availability and cost of raw materials and energy. A substantial proportion of the products which we now take for granted (e.g. polymers, detergents, pharmaceuticals, synthetic fibres, etc.) depend on the supply of basic organic chemicals. These in turn are frequently derived from the same sources of carbon on which we rely for the majority of our energy: coal, oil and natural gas. Prior to the development of the petrochemicals industry in the second half of the twentieth century, most basic organic chemicals were derived from coal. However, with increasing supplies of crude oil from sources such as the TOP East and, later, the North Sea, these chemicals have been produced to a greater and greater extent from oil. Although the majority of oil-derived products are used for transportation and the production of energy, a substantial proportion is now used in the chemical and related industries, the most important intermediate product being ether produced by steam cracking of crude oil (Fig. 1, reaction I).

It has been recognised for many years that the reserves of oil are very limited and it can therefore be argued that the use of oil as a source of energy is an irresponsible squandering of a natural resource. This resource should be husbanded for use in the future in only those processes in which there was a chance of recycling the product, e.g. in the production of polymers or fibres, or in which the product could not be produced in an any other way, e.g. in speciality chemicals or pharmaceuticals. Such considerations have led to widespread research on possible alternative energy sources and on other subjects such as the development of various types of fuel cells, using conventional or unconventional fuels more efficiently, or coal gasification. (Fig. 1 reaction II) In parallel, interest has centred on developing other sources of hydrocarbon-based products, the most notable area being that of producing useful products from natural gas.

Uses of Natural Gas

Natural gas is composed largely of methane but it also contains varying quantities (depending on the source) of ethane, propane and other hydrocarbons as well as carbon dioxide, nitrogen and other inert gases. In some sources, particularly in Canada, it also contains substantial quantities of hydrogen sulphide. The world wide reserves of natural gas are quite substantial and it is estimated that they will survive well into the 21st Century. Natural gas is currently used almost exclusively as an energy source (Fig. 1, reaction III) and, as we have witnessed recently in the controversy over the planned closure of many British coal pits, it is in direct competition with coal (and oil) as a fuel for the production of electricity. Except in those cases when it has a high sulphur level, natural gas is a much cleaner fuel when burnt than either oil or coal, there being little or no emission of SO₂ and relatively low emission of NOx ( a mixture of the various oxides of nitrogen). The cost of producing electricity from it is claimed to be only 10% higher than that of producing it from coal.

The only important current use of natural gas other than in the production of energy (Fig. 1, reaction IV) is the production of "Synthesis gas" (CO + H2) by steam reforming:

This reaction, with the suitable addition of subsidiary reactions such as the water-gas shift reaction:

is used in the production of ammonia as an intermediate in the production of fertilisers (e.g. at I.F.I. in Cork) and of methanol (Fig. 1, reaction V) which is used as a precursor of formaldehyde (reaction Vl) and other important chemicals. Almost all the steps in these important industrial processes are carried out catalytically.

Developing New Catalysts

My own involvement in catalysis research began in 1970 at the time when natural gas had become available from the North Sea and there was an increased interest in these reactions. My first research student working on methane chemistry examined the relationship between the structure of various

catalysts and their behaviours for the steam reforming reaction; later students developed a new commercial steam reforming catalyst and another examined catalysts for use in molten carbonate fuel cells.

A break-through in the area of methane chemistry occurred in 1982 with the publication of a paper which demonstrated that it was possible to convert methane to C2 products (Fig. 1, reaction VII) by partial catalytic oxidation. The so-called "Oxidative Coupling" reactions can be represented by the following equations:

and

The initial work showed that the reaction was best carried out in a cyclic mode in which the catalyst was first oxidised and the oxidised material was then exposed to the methane, producing ethane and ethene. It was later shown that a co-feed mode could be used in which both methane and oxygen were fed simultaneously to the catalyst. In both modes of operation, the problem is that there is a simultaneous production of the oxides of carbon: e.g.

and

Such non-selective reactions not only decrease the efficiency of carbon usage but introduce a secondary problem: the separation of the "COx" from the relatively complicated reaction mixture already existing. My own involvement in the subject commenced in 1984 at the University of Twente in the Netherlands where a large research group working on the subject was gradually built up. This work is now continuing at the University of Limerick. The main aim of almost all the work of the group has been to achieve large enough yields of C₂ products to enable the reaction to be used commercially for the production of ethylene. It is currently felt by those working in the field that it is necessary to achieve a yield of C₂ products (conversion (in %) x selectivity (as % of total carbon converted) + 100) of at least 20%, preferably with a selectivity above 80%.

Work in Twente has concentrated on the development of new catalyst types and on an examination of reaction networks and mechanisms. Early work showed that catalyst stability was an important point (the reaction occurs at temperatures in the region of 800慢) and so much effort was directed towards the development of stable materials. Recent results have shown that catalysts containing the compound are not only active but very stable under reaction conditions. However, the yields obtained with these materials are not yet high enough for commercial application. Further work is therefore planned, with sponsorship of the European JOULE programme and with the collaboration with seven other European laboratories to develop such catalysts and the process to such an extent that it can become usable commercially. Another aim of the programme is to produce methanal directly from methane (Fig. 1, reaction VIII) and this possibility is being examine by Dr. Kieran Hodnett and his students at UL as part of the same programme.

Improving the Economics

The eventual commercial success of such a process will depend on the cost of the natural gas used in the process: this is illustrated in Figure 2 which shows schematically the cost of producing ethylene from natural gas as a function of the natural gas price, assuming yields of 20% (see above).

Figure 2 also shows a band representing the cost of production from crude oil. It is clear that only in remote areas such as Siberia and Alaska is the price of natural gas low enough for the process to be economical with presently achievable yields. The problem then being that the cost of transportation of the ethylene will be prohibitive unless a valuable product can be made from it in situ. A breakthrough for the process will thus depend either on great improvements of yield with existing price differentials or a substantial shift downwards of the price of natural gas relative to that of crude oil.

There has recently been a resurgence in interest in a number of other possible catalytic routes for the conversion of natural gas; these, which were also shown in Fig.1, include catalytic combustion, (reaction IX) which improves the efficiency of the combustion process and minimises NOx production and partial oxidation or CO₂ reforming (reaction X) to give synthesis gas according to the equations:

and

Both these reactions give more valuable synthesis gas mixtures than does the steam reforming reaction referred to above. The second reaction also provides a potential route for reduction of CO emissions to the atmosphere. Research on catalysts for each of these reactions is also being carried out in the Heterogeneous Catalysis Research Group at the University of Limerick

Concluding Remarks

The different methods of conversion of natural gas to useful intermediates discussed above offer potential routes to help conserve and optimise the use of natural resources both in Ireland and in other developed or developing countries. While the ultimate success of such developments will depend on factors outside the control of chemists and chemical engineers much will still depend on what occurs within the black box: the catalytic reactor. Research on catalysts for such reactions offers a challenge to academic scientists to apply scientific principles to the design of catalysts and the knowledge gained in these areas is equally likely to be applicable in all the other diverse areas in which catalysis is of importance. Current areas of research in the University of Limerick not discussed above include catalysis in environmental control and in the production of fine chemicals. The work is funded by the European Community (JOULE and BRITE/EURAM programmes) and also by EOLAS and private industry.