Clay and ammonium catalyzed reactions of alkanols, alkanoic acids and esters under flash pyrolytic conditions

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Abstract

Pyrolysis of an alkanol, an alkanoic acid and a wax ester is shown to be affected by the presence of clay and/or ammonium. The alkanol dehydrates and is mainly converted into an 1-alkene, trans and cis 2-alkenes, and a variety of mid-chain alkenes due to double bond migration (‘scrambling’). The alkanoic acid is less affected than the alkanol, although a series of alkenes, alkanes and alkanones is produced in the presence of clay. Furthermore, a large portion is converted into the corresponding alkyl nitrile due to reaction with ammonium, which is promoted by clay. Pure clay does not cause bond breaking of the wax ester upon pyrolysis, but in the presence of ammonium ions the ester bond is cleaved to some extent yielding the alkanol and the alkanoic acid moieties. The products released (i.e. alkanol and alkanoic acid) undergo subsequent reactions. Comparison of pyrolysates of a mixture of grass material and clay with a soil sample taken from the grassland showed great resemblance in the aliphatic part consisting mainly of n-alkenes and n-alkanes, while a large difference with the pyrolysate of grass leaves (alkanols and alkanoic acids) alone was observed. This suggests that mineral particles, especially clay, present in soil samples, can generate compounds upon pyrolysis not directly derived from plant biopolymers.

Introduction

Analytical pyrolysis techniques are frequently used in the molecular study of a wide range of materials including plastics, bio- and geopolymers, coal and soil organic matter (SOM). Complex macromolecular mixtures can be rapidly analyzed due to the simple sample preparation. By contrast, interpretation of the obtained data is difficult and far from unequivocal [1]. Numerous sources of products can be distinguished in pyrolysates: evaporation (free, low molecular weight compounds), combustion (compounds derived from burning in the presence of oxygen (in the environment) after which the products are deposited and consequently present in a given soil sample) and pyrolysis (products split off from macromolecules in inert atmosphere) [1]. The latter can be further divided into thermal degradation, rearrangements, cyclizations and otherwise newly produced compounds. The presence of non-organic compounds, such as minerals, metals and salts influences pyrolysis patterns [2], [3], [4], [5], [6], [7], [8] and therefore complicates the interpretations even more.

The characterization of SOM is usually carried out on the (base and acid extractable) organic matter fractions of soil, such as humic acids and fulvic acids, to obtain samples with sufficient amounts of organic carbon and to prevent interaction and interference of mineral parts [9]. However, such extractions may not only alter to some extent SOM, i.e. hydrolysis [10], but also only the extractable part of SOM is analyzed in those cases. Consequently, the data and the resulting interpretation of the overall SOM composition will be biased even more. Thus, analysis of non-treated samples is prefered. Although in organic geochemical approaches sedimentary samples are usually extracted with an organic solvent to remove low molecular weight compounds prior to pyrolysis, such a pretreatment is rarely done for soil samples, and low molecular weight lipids are considered as part of SOM.

A pyrolysis study on unextracted samples derived from grassland and agricultural soils revealed that free n-alkanols did not appear as their expected evaporation products, as was observed after pyrolysis of the grass leaf material, but mainly as several n-alkenes [11]. Also a number of alkylnitriles were found with a similar distribution as the identified alkanoic acids. Previously, Evans et al. [12] showed that alkylnitriles were formed during pyrolysis of shale oils as a result of the reaction between carboxylic acids and ammonia liberated from minerals. Using tetramethylammonium hydroxide (TMAH), both the grass leaves and soil samples revealed a C26 methoxyalkane, the methylated analogue of the alkanol, and a series of methyl esters of n-alkanoic acids (C14–C32) but no alkenes or alkylnitriles were produced [11].

These observations suggest that inorganic soil components induced the dehydration of the alkanol to cause the generation of the alkenes. Some preliminary experiments, using a mixture of grass leaves and a soil sample, which originally contained very little SOM, suggested that pyrolysates of these samples were indeed strongly affected by the soil matrix. Extraction of soil samples by dichloromethane/methanol prior to pyrolysis decreased the signals corresponding to the alkenes, indicating that the alkanols were low-molecular weight and not covalently bound as part of a macromolecule.

A number of studies involving sedimentary OM have shown that clay minerals affect the pyrolysis patterns [13], [14], [15], [16], [17], [18]. By contrast, until now, only a few studies have been published dealing with the effects of inorganic particles on pyrolysis products of soil samples. The presence of elemental sulfur yielded a series of n-alkylthiophenes upon pyrolysis of humic acids, whereas these compounds were not detected in the absence of sulfur [19]. Pyrolysis of silicon-bounded n-octadecane and the sodium salt of an alkanoic acid produced both a series of n-alkenes and n-alkanes [6], [7]. Unsaturated alkanoic acids appear to cyclisize and to aromatize in the presence of sulfur or when present as sodium salts, giving alkylbenzenes and naphthalenes [20], [21]. To date, little attention has been given to the thermal decomposition of SOM in the (physical) presence of clay, silica and ammonium, which are all abundant in many (agricultural) soils. For soils, Schnitzer et al. [22] found that minerals affected the thermal evolution, and also slightly the distribution, of the various groups of pyrolysis components of physical mixtures of minerals and fulvic acid.

To determine to what extent common soil particles affect the formation and distribution of pyrolysis products, model compounds including n-alkanol, n-alkanoic acid and a wax ester were pyrolyzed in the presence and absence of clay minerals, silica (milled sea sand) and ammonium chloride. Because plant material is a major contributor to SOM, grass leaves were mixed with clay minerals and subsequently pyrolyzed to explore and interpret the results obtained with the model compounds. In addition, these results were compared with a pyrolysate of a soil sample beneath the grass vegetation with emphasis on alkanols and alkanoic acids.

Section snippets

Samples and sample preparation

Docosanol, octadecanoic acid, dodecyl hexadecanoate and ammonium chloride (NH4Cl) were obtained from Aldrich. Kaolinite and illite clay minerals were from the collection of the Laboratory of Soil Science and Geology, Wageningen University, and not treated before use. Sea sand (Merck) was ball-milled before use. The soil sample was collected from a permanent grassland (southwest Netherlands) and the grass leaves (predominantly rye grass, Lolium perenne) were removed from the top. Both were dried

Alkanol

To test which inorganic soil constituents may have caused the aforementioned effects experiments were performed using a n-alkanol as a model compound. First, pure docosanol (C22 alkanol) was pyrolyzed and yielded, as expected, only its evaporation product (Fig. 1a). Subsequently, pure alkanol was pyrolyzed after mixing with silica (milled sea sand), kaolinite and illite clay minerals, respectively. The mixture of alkanol and sea sand produced mainly the alkanol, and small amounts corresponding

Acknowledgements

The Laboratory of Soil Science and Geology, Wageningen University, is thanked for using their facilities. Dr G. Love is thanked for his constructive comments on an earlier version of the manuscript.

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