Review: the development and applications of thermally assisted hydrolysis and methylation reactions

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Abstract

This review outlines the developments of the thermally assisted hydrolysis and methylation (THM) technique in analytical pyrolysis over the past 12 years. The principal reagent used to effect the thermochemolysis reaction is tetramethylammonium hydroxide (TMAH) but the use of other quaternary ammonium hydroxides is discussed. The mechanisms of the reaction and the effect of experimental variables are also incorporated. The wide variety of applications reviewed include synthetic and natural resins, lipids, waxes, wood products, polysaccharides, proteins, soil and humic substances, sediments, kerogen and dyes. On-line and off-line THM reactions are involved.

Introduction

Analytical pyrolysis is a well-established method for the chemical structure characterisation of intractable materials [1], [2]. The perceived shortcomings of the technique have been largely overcome now [3] and the method has many useful applications in areas such as forensic science [4], [5], [6], natural organic polymers [7], soil chemistry [8] and petroleum geochemistry [9].

In the last decade or so, modifications to the pyrolysis process, which involve high-temperature chemical reactions other than conventional thermolysis of macromolecules, have been reported. These processes have the potential to provide additional chemical structure information not readily obtainable by conventional pyrolysis gas chromatography (Py-GC) and pyrolysis mass spectrometry (Py-MS) techniques. The purpose of this paper is to review the developments of these modifications which involve the high-temperature chemical reaction of macromolecules in order to determine chemical structure information and, in particular, those reactions involving tetramethylammonium hydroxide (TMAH) and related components.

In earlier work, other modifications to the identification of materials by pyrolysis techniques have employed catalysis. Schomberg et al. [10] developed the technique of on-line hydrogenation in reactors that are coupled to GC capillary columns providing a method that can yield additional information about the composition of unknown mixtures. The structural elucidation of olefinic and aromatic hydrocarbons and compounds containing polar functional groups may be carried out. The GC profiles of the mixtures, with and without this hydrogenation step, are then compared. Hydrogenation of unsaturated pyrolysis products of polyethylene and polypropylene has been used to determine the degree of branching in the polymer [11]. The flash pyrolysis products, entrained in hydrogen carrier gas, are passed over a palladium catalyst. More recently, sequence distributions in polyacetals were studied by employing the pyrolysis of the finely divided polymer in the presence of a cobalt catalyst [12]. The ethylene oxide content and distributions, in relation to the basic polyoxymethylene structure, were evaluated on the basis of the cyclic ethers. These processes necessarily involved the thermolysis of the polymer with the production of smaller molecular units that were then changed by a catalytic reaction.

In many thermolytic processes, recombination of the pyrolysis products also occurs resulting in the production of new, and perhaps unexpected, products. Theoretically it may be possible to carry out a pyrolysis reaction with an additive compound which would combine with the pyrolysis products of the analyte polymer to form products which could lead to the determination of the chemical composition of the polymer. Till now there appears to have been no such useful reactions reported in the scientific literature.

Alternatively, chemical degradation reactions can occur when compounds and macromolecules are flash heated with a reactant. A well-known example of this process is the ‘pyrolytic methylation’ reaction reviewed by Kossa et al. [13] where a compound which contains acidic functional groups is mixed with a tetraalkylammonium hydroxide (TAAH) and injected into the heated injected port of a GC. The products are the respective alkyl derivatives of the compounds with the acidic functional groups. The reaction is particularly suitable for the analysis of fatty acids in triglycerides which are determined as the methyl esters when TMAH is employed [14]. The optimal experimental conditions for the process have been studied [15].

The use of an external heating source to execute these chemical reactions with polymers was developed by employing the reaction of a TMAH solution with a number of synthetic polymers [16]. The reaction involved flash heating of an intimate mixture of the finely divided polymer with a 25% w/w aqueous TMAH solution in a Curie-point pyrolyser using a 770°C pyrolysis wire to hold the mixture. Any type of pyrolyser unit may be employed. The 770°C pyrolysis temperature was chosen so that conventional pyrolysis products, where appropriate, could be observed.

The reaction was termed simultaneous pyrolysis methylation (SPM) suggesting that the process was a gas-phase recombination reaction in which the pyrolysis products were converted to methyl derivatives. The experimental results reported later in this paper contradict this hypothesis, although this mechanism could apply in some cases. The mechanism appears to involve a high-temperature hydrolysis process, with hydrolytic scission occurring at ester and ether bonds by the reaction of the strongly basic TAAH reagent. TAA salts are formed rapidly and these salts undergo pyrolysis to the respective alkyl derivatives. In the light of this proposed mechanism, and to avoid confusion generated by the term SPM, the reaction was renamed thermally assisted hydrolysis and methylation (THM), where TMAH was employed as the derivatisation reagent [17], [18]. The following scheme outlines the THM mechanism, where AB represents the hydrolysable analyte molecule:

HydrolysisOH+AB→A+BOH

Formation of TAA saltsA+R4N+OH→R4N+A+OHBOH+R4N+OH→R4N+OB+H2O

Thermal dissociation to alkyl derivativesR4N+A→AR+R3NR4N+OB→BOR+R3NIn the case of a typical polyester, polyethylene terephthalate (PET), the mechanism of hydrolysis and alkylation with a TAAH is as follows:

While methyl esters and ethers are useful derivatives, the higher alkyl homologues of TAAHs produce compounds which assist in the identification of the components of polymers, for example, in compounds having sites of pre-existing methylation.

In many cases, pyrolysis processes and derivatisation mechanisms occur concurrently. The products from the THM reaction of some phenolic polymers, for example, cannot be explained solely on the basis of hydrolysis and alkylation reactions. Homolytic scission of carbon–carbon bonds must occur to give the observed products.

The choice of solvent for TMAH also could have an influence on the mechanism [19]. The mechanism would be expected to be a transesterification process, for example, where methanolic TMAH is employed for the reaction.

The use of methanol as a solvent for TMAH and the influence of alkali salts as contaminants were discussed by Ishida et al. [19]. The contribution of the solvent was evaluated by using a deuterated methanol solution. Methyl derivatives are formed not only through hydrolysis by TMAH but also through methanolysis to some extent. They also claim that the presence of alkali salts in the TMAH hinders the quantitative conversion of aromatic polyesters into the methyl derivatives.

Since the original work was published other terms have been employed to describe the reaction. Thermochemolysis, on-line methylation, reactive pyrolysis and pyrolysis (TMAH)-GC–MS are such terms, but, in the opinion of the author, are not as mechanistically descriptive as THM. Care should be taken to avoid the term ‘pyrolytic methylation’ as this is confused easily with the injector port reaction and also could introduce confusion about the mechanism (e.g. free radical process). Thermally assisted hydrolysis and derivatisation techniques, in contrast to the method termed ‘pyrolytic methylation’ reviewed by Kossa et al. [13], rely on the use of an external pyrolyser to provide the flash heating capability [16]. The analyte, which must be intimately mixed with the TAAH derivatising reagent, is rapidly heated to a temperature which provides sufficient thermal energy for the reaction to proceed. Temperatures exceeding 300–400°C are not absolutely necessary to achieve a successful reaction; however, where pyrolysis processes occur concurrently, a higher temperature is necessary to achieve homolytic scission of carbon–carbon bonds.

This reaction proceeds successfully in pyrolysers with pyrolysis zones set at ambient temperature or slightly above. However, at higher pyrolysis zone temperatures, e.g. above 100°C, the solvent in the solvent-borne TMAH can evaporate leading to separation of the TMAH from the analyte. On flash heating, the reagent and the analyte pyrolyse independently resulting in a failure of the hydrolysis and alkylation reaction. Prior reaction of the TAAH reagent with the analyte before placing in the pyrolysis zone would be necessary to avoid this problem. Venema and Boom-van Geest [20] discussed the benefit of examining model compounds when carrying out THM experiments with particular polymers.

Section snippets

Applications

The THM reaction, linked to GC, GC–MS and MS, has been applied to the chemical characterisation of a number of synthetic and natural materials including synthetic resins, lipids, waxes, natural resins, wood products, soil, sediments, proteins and carbohydrates. The reaction can be used for the chemical characterisation of those materials that undergo some chemical degradation by base-hydrolysis cleavage mechanisms.

Conclusions

In conclusion, it is clear that there is a broad spectrum of applications for THM. It will be interesting to see the development of other thermally assisted on-line chemical reactions. TMSH, TMAAc and BSTFA reagents provide specific advantages when compared to TMAH where particular applications are required.

Acknowledgements

This paper is published with the approval of the Director of the Chemistry Centre (WA), Department of Minerals and Energy, Perth, Western Australia.

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