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The interplay of catalytic and gas-phase stages at oxidative conversion of methane: A review

Dedicated to the 70th Anniversary of Prof. G.B. Shul’pin.
https://doi.org/10.1016/j.molcata.2016.08.008Get rights and content

Highlights

  • Temperature determines contributions of catalytic and gas-phase stages.

  • Reactor surface is important “catalytic” factor in gas-phase oxidation.

  • C2 hydrocarbon yield at catalytic OCM is limited by gas-phase process.

  • Why catalysts are unimportant in chain-branched gas-phase DMTM.

  • Catalytic control of fast gas-phase processes is possible.

Abstract

The effective functionalization of Csingle bondH bond in methane, the main hydrocarbon component in Earth's crust and the most real source of energy for mankind in the nearest observable future, is undoubtedly can be regarded nowadays as one of the most important scientific and technological tasks. However, the usual practice of considering catalytic and gas-phase processes as independent technological branches seriously restricts the possibilities for a deeper understanding and technological optimization of real processes. The development of more effective technologies to convert methane and other gas-phase hydrocarbons into more valuable and demanded chemicals needs a complex approach based on the combined heterogeneous–homogeneous chemistry of these processes. A number of examples are used to illustrate the interconnection between heterogeneous catalytic and gas-phase processes in oxidative functionalization of methane. A number of potential possibilities for the optimization of the real heterogeneous–homogeneous chemistry of these processes are discussed.

Introduction

The effective and inexpensive conversion of methane, the principal component of natural gas, into more valuable and demanded chemicals is a long-sought goal not only for relatively young gas chemistry, but also for organic chemistry as a whole. Search for appropriate solutions of this task continues for more than a century. Nevertheless, the global significance of its successful solution has become particularly evident only recently, after it had been established that the overwhelming majority of hydrocarbons in the Earth's crust, that is global future energy and petrochemical resource of mankind, is presented by different kinds of gas-phase hydrocarbons [1].

The permanent decrease of available liquid hydrocarbons resources, which are undoubtedly the most attractive and convenient stuff for use and processing, makes the transition of the world economy to gas-phase stuff, namely methane, inevitable. Today, at the beginning of the new century it is evident that the 21st century will be the century of natural gas, and as a consequence, the century of gas chemistry. A significant decrease in the gas price relative to that of oil makes natural gas very attractive not only as a fuel, but also as a petrochemical raw material. That is why a very high activity in developing new gas chemical enterprises is observed, particularly in the USA and Middle East, two regions with huge resources of inexpensive shale or natural gas.

Many new gas chemical plants were announced to use as a raw stock ethane and heavier hydrocarbons extracted from natural gas. However, the production of olefins by dehydration of ethane and its heavier homologues, which constitute only a minor part of natural gas, is an established industrial-scale petrochemical process. A more ambitious goal for gas chemistry is to build a methane-based analog of petrochemistry.

Note, however, that methane-based gas chemistry defers fundamentally from traditional “destructive” petrochemistry by its “constructive” trend, with the aim to produce from CH4, the most simple and stable hydrocarbon molecule, a variety of more complex and less stable products that are now produced by petrochemistry. This task is difficult to realize because methane is a low-reactive hydrocarbon, with a thermodynamic stability exceeding not only that of all other hydrocarbons, but also that of the majority of target chemicals to produce from it.

Traditionally, the overwhelming part of industrial methane processing is based on high-temperature heterogeneous catalysis [2], [3]. Despite of intense attempts to develop appropriate low-temperature homogeneous catalytic systems and numerous excellent researches in this field describing new trends in oxidative functionalization of Csingle bondH bonds [4], [5], [6], [7], [8], [9], [10], [11], [12], at present they can hardly be considered as a real base for industrial applications and show “concepts rather than recipes” [10]. The possibility of using for this purpose homogeneous gas-phase processes of direct conversion of methane into methanol or ethylene, although intensively investigated [2], [3], [13], [14], [15], [16], [17], [18], has only limited scope of practical applications. The same applies to the halogenation of methane. That is why the analysis of the most promising ways for the industrial-scale functionalization of the Csingle bondH bond in methane, as well the mechanisms of these processes and possibilities of their optimization, remain one of the most important practical tasks.

In relation with this task, it is necessary to highlight one very important peculiarity of the high-temperature heterogeneous catalytic processing of methane: in most cases, it involves hetero-phase processes that take place in conditions of an intense competition between catalytic and parallel gas-phase reactions. This situation was clearly elucidated by investigations of the oxidative coupling of methane (OCM), for which the heterogeneous generation of methyl radicals and their subsequent homogeneous gas-phase reactions constitute a combined indivisible mechanism [19].

Unfortunately, until recently researchers working in catalysis and gas-phase chemistry had applied very different methodologies to solving their problems. Therefore, it is desirable to bridge this gap. The practical realization and optimization of catalytic processes of methane conversion and an active control of it require detailed information on the interaction between catalytic and gas-phase stages of these complex radical processes. Only an optimal coupling of catalytic and gas-phase chemistry can provide the best use of the advantages of both: a combination of a high rate and high selectivity of catalytic activation with a high productivity and technological simplicity of homogeneous gas-phase processes.

The aim of this review is to attract the attention to very important and interesting problem of the interplay between the heterogeneous and homogeneous stages of the catalytic activation of methane and demonstrate the most interesting examples and emerging possibilities. It was done mainly by the examples of the well known processes of oxidative Direct conversion of Methane To Methanol (DMTM) and Oxidative Coupling of Methane (OCM) to ethane and ethylene.

Section snippets

On the interaction between the catalytic and gas-phase stages of methane oxidation

The oxidative activation of methane is one of the most evident and energy effective methods of its activation, which involves only a small external energy input. Outside the production of halogenated derivatives of methane, the practical application of which are now restricted for the well-known reasons, the oxidative activation of methane can be considered as the main method for its activation with subsequent conversion into various chemicals. Firstly, the oxidative conversion makes it

Gas-phase nature of the limit of the yield of C2 hydrocarbons for the at catalytic oxidative coupling of methane

One of the most striking examples of an inseparable relation between heterogeneous and homogeneous processes at oxidative conversion of methane is the oxidative coupling of methane (OCM), which was discovered at the beginning of the 1980s [34], [35]. Evidently, OCM was the first heterogeneous–homogeneous process the nature of which has been studied in detail. In this example, many substantial features of this class of processes were rationalized.

The direct observations of free radicals and

Why catalysts do not play any role in chain-branched gas-phase DMTM process?

Those who work in the field catalysis generally believe that catalysts can improve practically any process, only if appropriate catalyst and the relevant reaction conditions are selected. DMTM process is a graphic example disapproving this opinion. It shows that, under conditions where the yield of target products is not determined by the thermodynamic of the process but rather by its kinetics, the situation can be very different. Dozens of studies had been performed in attempts to improve the

The reactor surface as an important “catalytic” factor in gas-phase processes

Nevertheless, the low probability of improving DMTM by means of heterogeneous catalyst does not mean the absence of the influence of heterogeneous factors on its behavior. The indivisible homogeneous-heterogeneous nature of complex high temperature processes of methane conversion needs not only the accounting of homogeneous processes at catalytic methane conversion, but also no less thorough accounting of the processes occurring on the reactor surface during homogeneous conversion.

The effect of

The role of the surface in the final distribution of products

The nature of the reactor surface affects not only the kinetics of the formation of the products during the reaction, but also their subsequent transformations, especially in experiments with a long residence time of the reactants in the reactor. Note, however, that, even at residence times not exceeding a few seconds, the selectivity and yield of methanol and, especially, formaldehyde decrease with increasing residence time after oxygen conversion completion. However, the available data on the

A strong effect of temperature on the relative contributions of the catalytic and gas-phase processes

A key factor that determines the relative effects of the catalytic and gas-phase processes on the partial oxidation of hydrocarbons is, undoubtedly, temperature. It can be clearly demonstrated by the example of the interplay between heterogeneous and homogeneous processes during the oxidative pyrolysis of ethane. Although the oxidation of ethane somewhat differs from that of methane there are many common features between these processes.

It is well known that, in the case of the gas-phase

Short-time catalysis as an essentially multi-phase process

An interesting example of possible coupling of catalytic and gas-phase processes in a combined technology is provided by the rapidly developing branch of hydrocarbon oxidation known as short-time catalysis. In this case, the gaseous reactants contact with a very active catalyst for a very short time, typically several milliseconds. After that the reaction proceeds further in a gas phase. One of pioneers of such approach L. Schmidt has shown the advantages of hydrocarbons oxidation on monolith

Is the catalytic control of fast gas-phase processes possible?

In general, the possibilities of high-temperature catalysis in the oxidative conversion of alkanes are not limited to a trivial increase in the rate of generation of active radicals. Such oxidative processes have fundamentally nonlinear character due to numerous cross-reactions involving stable and unstable intermediates in the gas phase and on the surface. Therefore, there always exists a possibility to direct, by relatively weak impact, their evolution to different pathways, leading to

Conclusions

The main point of this review is to draw attention to the fact that the conventional practice of separating and independently investigating catalytic and gas-phase processes of conversion of methane and other light hydrocarbons is not especially productive. Much wider possibilities for deeper understanding and for developing more effective methods for their conversion into valuable chemicals are offered by integrated studies of such processes within the framework of combined gas-phase–catalytic

Acknowledgment

This work was supported by Ministry of Education and Science of Russian Federation under Agreement No. 14.607.21.0131 (Unique. identifier RFMEF160715X0131).

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