SOME INVESTIGATIONS
ON THE CHEMICAL
NATURE OF KEROGEN
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SOME INVESTIGATIONS
ON THE CHEMICAL
NATURE OF KEROGEN
PROEFSCHRIFTter verkrijging van de graad van doctor in de technische wetenschappen
aan de Technische Hogeschool Delft, op gezag van de rector magnificus
ir. H.B. Boerema,
hoogleraar in de afdeling der elektrotechniek, voor een cominissie
aangewezen door het college van dekanen, te verdedigen op
woensdag 12 maart 1975 te 16.00 uur
door
Marcellinus Lambertus Jozef van den Berg
doctorandus in de geologie ^•?TT''^'i^>,,
/C^ 75 '''^^
geboren t e Apeldoorn
r^0 0 3 5 i l < i
D r u k k e r i j Princo B.V., Culemborg
1975
Prof. drs. P.A. Schenck
BIBLIOTHEEK TU Delft P 1866 7209
Organization for the Advancement of Pure Research (Z.W.O.) Drawings and Photos: P. Dullaart
I INTRODUCTION 1 General background 2 The r e c o n s t r u c t i o n p r i n c i p l e 3 The n e c e s s a r y r e q u i r e m e n t s 1 The k e r o g e n samples 2 The d e g r a d a t i o n procedure 4 Previous work on GRS kerogen 5 L i t e r a t u r e
I I INTRODUCTORY EXPERIMENTS 1 I n t r o d u c t i o n
2 Surface a r e a and p o r o s i t y 1 I n t r o d u c t i o n
2 Low temperature gas adsorption 1 Nitrogen
2 Carbon dioxide 3 Mercury porosimetry
4 Interpretation of the results 3 Solvent effects
1 Introduction
2 Solvent-ozone interactions 3 Solvent-substrate interactions 4 The reaction of ozone and Philblack 0
1 Introduction
2 Kinetic and other results
3 Mechanism of ozone attack and the chemical nature of the adsorption complex
1 A mechanism involving surface free radica 2 The Criegee mechanism
4 Interpretation of the results
5 The reaction of ozone and kerogen and lignite 1 Introduction
2 The kinetic results
3 Mechanisms of ozone attack 4 Interpretation of the results
1 Glass ware and reagents
2 Preparation of kerogen samples 3 Surface area determination 4 Ozone uptake measurement 7 Literature
III OZONIZATION OF GRS AITO MESSEL KEROGEN 1 Introduction
2 Green River shale kerogen 1 The oxidation fragments
1 The isolation and identification of ozonization products
2 Solvent effects
2 The structural elements in the kerogen 3 The contributing constituents
4 Some final considerations 3 Messel shale kerogen
1 The oxidation fragments
1 The isolation and identification of ozonization products
2 Solvent effects
2 The structural elements in the kerogen 3 The contributing constituents
4 Some comparisons between GRS and Messel kerogen 5 The experimental methods
1 Reaction conditions and working-up procedure 2 Analytical methods
3 Preparation of derivatives
4 Preparation of standard compounds 6 Literature
IV ORGANIC GEOCHEMICAL CLUES IN RELATION TO THE EXTENT OF ALTERATION OF THE ORGANIC HATTER
1 Introduction
2 Structure of the triterpenoid acids of Messel and GRS
3 Significance of the results 4 Experimental methods
5 Literature SUMMARY
INTRODUCTION
I.l General background
The deposition of plant and animal remains in marine and conti-nental environments and the alteration of this debris during geological time and increasing depth of burial by numerous reac-tions produced a wide variety of carbonaceous matter enclosed in sediments.
It has been calculated that 3.8 x 10-^^ tons of organic matter is embedded in sediments. About 25% of this occurs in organic-rich shales^ (organic matter content between 5 and 65%) . A very small part, about 6 x IQ-*-^ tons, occurs in a very concentrated form and is generally known as coal.'- Most of the organic matter occurs in a rather disseminated state and constitutes a complex macromolecular material that occurs as an amorphous and
uniden-tifiable mass between preponderant inorganic constituents. Various terms have been put forward to describe this organic matter like kerabitumen and kerogen. In recent organic
geochem-ical literature the term kerogen is usually applied to describe the insoluble part of the organic matter of sediments^ and this is also done in this thesis.
There are no reasons to suppose that the dispersed organic matter would have a totally different character from that occurring in a very concentrated form as in coal. As to source materials various types of coals can be distinguished but very roughly two main types can be considered: the humic and sapropelic type. The
former is the most important and is mainly composed of plant debris while the latter type is to a large extent composed of algal remains. Between these two extremes all other types are possible.
Coal is since a long time a subject of considerable research and a quite extensive knowledge has been built up through the years. However this knowledge is mainly confined to the humic type coals and is less well-developed for the sapropelic and other types.
Studies intended to elucidate the nature of kerogen have been hampered in the past due to its dispersed character, its amor-phous constitution and intrinsic complexity. The constitutional analysis of kerogen has been motivated by various reasons:
1) The kerogen embedded in the so-called oil shales represents an economic value. The shales can be considered as an energy resource either by the use as such or after retorting the shale by the production of oil and gas. Alternatively kerogen-rich rocks may be used for the production of interesting raw chemi-cals like ethylene."* It is clear that the possibilities for economic usage of the oil shales are directly dependent on the chemical constitution.
2) It is known that in nature petroleum is generated from the kerogen of the so-called source rocks. In a systematic study^ ^ on the constitution of kerogens isolated from argil-laceous shales of the Toarcian (Lower Jurassic) of the Paris basin it is found that the kerogen alters in chemical consti-tution with increasing depth of burial. This is demonstrated by an increase in carbon content, a decrease in oxygen and sulphur and a decrease in the intensities of aliphatic C-H bands in infra-red spectra, all with increasing depth. It is known that in a relatively small subsurface zone the generation of petro-leum hydrocarbons from the kerogen is rather strong. This has been demonstrated by various authors in studies on the hydro-carbon generation in several sedimentary basins. -^ ^
It is thus clear that knowledge of the nature of kerogen, its physical appearance, its chemical composition and constitution can help in evaluations of the petroleum potential of sedimen-tary basins.
3) Kerogens are also studied from a more fundamental point of view in order to elucidate the type of contributing materials and the type of chemical reactions that are active in the kerogen forming process. Few is known about these processes and in this light the present study was undertaken.
1.2 The reeonstruation principle
In organic geochemistry two general ways of approach of the central problem, which can be defined as the search to the fate of organic molecules in the geosphere, have been developed. In the first and oldest approach it is attempted to isolate and identify specific compounds from the sedimentary organic matter that are structurally related to common constituents of organ-isms. Reactions can then be proposed that make up for the
modifications noted between the two species. This approach can be called the reconstruction principle.
In the second approach simulation experiments are performed in which model compounds (often radio-labelled) are altered under quasi-geological conditions. By identification of the reaction products mechanisms can be proposed for the modifications noted. These mechanisms are then assumed also to occur in.the natural environment. This approach can be called the simulation prin-ciple .
The simulation principle has been succesfully applied in the past using various very simple model systems. It is however clear that its use in kerogen constitutional analysis is limited since it would be very hard to select a representative model system.
When the reconstruction principle is applied on kerogen it must be considered as being composed of two different stages.
In the first stage the nature of the kerogen structure has to be defined while in the second stage the original kerogen-forming constituents and their incorporation reactions must be proposed. By the use of statistical constitutional analysis, e.g. in a way as described by Van Krevelen^"* for coal, a structure model of the kerogen can be developed. The application of such a method will not provide detailed information as to the nature of indi-vidual structural elements but rather information with a more general character to describe mean structural units. Detailed information is however essential for the realisation of stage two of the above formulated reconstruction principle. We con-sequently have to define in a rather accurate way, and thus on a molecular level, the nature of structural elements in the kerogen. This however is only possible if some kind of degrada-tion method is applied.
In ordinairy organic constitutional analysis the structure of an organic compound is generally determined by a step-wise degradation of the substance to the point at which identifiable compounds can be obtained. These degradative fragments can be used to reconstruct the chemical structure of the original sub-stance which is then subsequently synthesized for confirmation. However as has been pointed out before for c o a l s , ^ confirma-tion of the reconstructed model is impossible since there would be few criteria for comparing the synthetic with the natural product. Forsman^ has stated that this also holds for kerogen. An obvious answer to the problem is to limit the reconstruction principle to only special parts of the kerogen structure. It
can be argued that such parts are not necessarily representative for the kerogen as a whole and this should be kept in mind when structure models are proposed. However by definition of the chemical structure of particular structural elements of the kerogen more light can be thrown on the processes that were involved in the formation and alteration of these elements.
I. 3 The necessary requirements
Two criteria should be met in order to use the reconstruction principle for our purpose succesfully.
1) The kerogen should contain sufficient chemical information. It should thus be relatively unaltered since such kerogens will mostly reflect the chemical structure of the original kerogen
forming material. Although not essential, an additi_onal prac-tical condition is the use of a kerogen-rich sample since then the kerogen is more readily accessible for investigation. 2) In order to isolate structurally significant degradation fragments it is of importance to use a sufficiently mild and selective degradation procedure.
1.3.1 The kerogen samples
On the basis of the above mentioned first criterium two oil shale samples were selected: the American Green River oil shale (GRS) and the German Messel oil shale. Both are of Eocene age and were deposited in a lacustrine environment. Their organic matter content is about 40% (GRS) and 50% (Messel) on a dry basis.
A lignite was also studied in the initial stage of the investi-gation in order to furnish experimental data on the behaviour of organic matter formed from an entirely different starting material.
1.3.2 The degradation procedure
Various possibilities exist for chemical degradation of the kerogen. The degradative methods usually applied for structure analysis of coals and kerogens are pyrolysis, hydrogenolysis, and oxidation. Functional group analysis, although not always having a degradative character, may also be included. In general it can be stated that pyrolysis and hydrogenolysis do not yield sufficient specific degradation fragments for an application of the reconstruction principle. Functional group analysis on the other hand is logically limited to very specific structural
elements and the results can be used for reconstruction purposes only in combination with one of the other mentioned techniques. Usually some type of oxidation procedure is chosen in coal and kerogen constitutional analysis. These include the use of nitric acid, potassium permanganate, oxygen, ozone, and chromic acid. The results obtained with these methods when applied on coal are reviewed and discussed in various textbooks ' while Forsman^ reviews the literature on kerogen oxidation up to
1962. Since then no new oxidation techniques were added but much advantage was made in the characterisation of single com-ponents in the complex mixtures resulting from oxidation of kerogen and coal.
As has been mentioned before it is important to use a mild and selective degradation technique for a succesful partial recon-struction of the kerogen. The use of ozone as an oxidative reagent in this investigation was to some extent inspired on the work of Bitz and Nagy.'^ ^'' From their studies it appeared that ozonization might be succesfully used in the kind of re-constructions pursued in this investigation. Since the mode of ozone attack on kerogen-like substrates is only poorly under-stood it was necessary to gain a better insight into the way of the action of ozone on the substrate. Such knowledge is required for a meaningful interpretation of the oxidation frag-ments in terms of the structural elefrag-ments of the kerogen from which they are derived. In a series of introductory experiments carried out under mild conditions the nature of the heterogene-ous ozone reaction is investigated. In this study the kinetic data on the ozone reaction as well as literature information on the way of ozone attack on simple substrates like synthetic polymers and carbon black helped considerably towards a better understanding of the type of reactions taking place at the kerogen surface. As a result it proved feasible to define specific structural elements of the kerogen.
I. 4 Previous work on GRS kerogen
Relatively much effort has been put in the elucidation of the chemical structure of GRS kerogen. At this moment it therefore provides one of the best studied kerogens. A detailed account of the results obtained by various techniques applied up to
1967 is given by Robinson. ^•^ In his summarizing section the kerogen is described as "a macromolecular material having predominantly a linearly condensed saturated cyclic structure with heteroatoms of oxygen, nitrogen and sulphur. Straight-chain and aromatic structures are a minor part of the kerogen."
The combination of oxidative degradation and sophisticated analytical techniques'^>^^ has provided more specific struc-tural information on GRS kerogen. These new methods have demon-strated that it is possible to isolate and identify single com-ponents from the very complex oxidation mixtures usually ob-tained. No partial reconstructions were endeavoured in these studies but rather structure models were proposed. In most degradation studies the yields of ultimately identified com-pounds are rather low and the question arises whether the identified fragments reflect the 'structure' of the kerogen as a whole. It has been recognized that although kerogen represents in its physical appearance an amorphous mass it does not repre-sent a homogeneous material. This can be shown by optical methods" while recently this was also suggested based on chemi-cal data.'"* Robinson'-*- has already mentioned before that GRS kerogen consists of two different types of organic material as seen under the microscope. In this light the proposition of even simplified structure models as suggested by Burlingame et a l . " and Djuricic et al.'^ seems of a rather limited value. The wealth of information available on GRS kerogen is advan-tageous for our purpose and this is an additional reason for selecting this particular kerogen in our investigation.
I.5 Literature
1 Degens, E.T., Geochemie der Sedimente, Enke Verlag Stutgart (1968) p. 162
2 Duncan, D.C. and V.E. Swanson, U.S. Geol. Survey Circ. 523 (1965)
3 Welte, D.H. in: Adv. Org. Geochem. '73, B. Tissot and F. Bienner, ed. p. 3(1974)
4 Cook, E.W., Fuel 53_, 146 (1974)
5 Vassojevic, N.B. et al., Z. angew. Geologie ]5_, 1 (1969) 6 Staplin, F.L., Bull. Canad. Petrol. Geol. \7_, 47 (1969) 7 Welte, D.H., J. Geochem. Expl. j_, 117 (1972)
8 Durand, B. et al., Rev. Inst, francais Petrole 27_, 865 (1972) 9 Espitalie, J. et al.. Rev. Inst, francais Petrole 28, 37
(1973)
10 Philipi, G.F., Geochim. Cosmochim. Acta 2_9, 1021 (1965) 11 Albrecht, P. and G. Ourisson, Geochim. Cosmochim. Acta 33,
138 (1969)
12 Durand, B. and Espitalie, J. in: Adv. Org, Geochem. '71 H.R, von Gaertner and H. Wehner, ed. p. 455 (1972) 13 Eglinton, G. in: Chemistry in Evolution and Systematics,
T. Swain, ed. p. 611 (1973)
15 van Krevelen, D.W. and J. Schuyer, Coal Science, Elsevier Amsterdam (1957)
16 Forsman, J.P. in: Organic Geochemistry, E. Breger, ed. p. 148 (1963)
17 Francis, W., Coal, Arnhold, London (1961)
18 Bitz, M.C. and B. Nagy, Proc. Nat. Acad. Sci. 5j6, 1383 (1966) 19 Bitz, M.C. and B. Nagy, Anal. Chem. 39.. '310 (1967)
20 Nagy, B. and L.A. Nagy, Nature 2_23_, 1226 (1969)
21 Robinson, W.E. in: Organic Geochemistry, G. Eglinton and M.T.J. Murphy, ed. p. 619 (1969)
22 Burlingame, A.L. et al, in: Adv. Org. Geochem. '68, P. P.A. Schenck and I. Havenaar, ed. p. 85 (1969)
23 Djuricic, M.V. et al., Geochim, Cosmochim, Acta, 35, 1201
(1971) ~ 24 Vitorovic, D., Djuricic, M.V, and B. Ilic in: Adv. Org.
INTRODUCTORY EXPERIMENTS
TT.1 Introduction
The kerogens are present in the form of particulate matter and consequently the reaction with ozone is complicated by factors arising from the heterogeneous character of the reaction. In this chapter especially the effect of solvent on the overall ozonization reaction is investigated since it was thought that this parameter could greatly influence the nature and the ex-tent of the reaction taking place at the surface of the sub-strate. A better insight into the underlying causes of such an influence was of interest because it was likely to yield infor-mation as to the nature of the substrate.
For this purpose a series of small-scale introductory experi-ments was designed. In these experiexperi-ments the overall course of ozonization was followed by continuous registration of the ozone uptake as a function of time in a continuous flow system. The ozone uptake can be considered as the ultimate result of all operating reaction parameters. The effect of n-hexane, methylenechloride and methanol was studied. The other para-meters were kept constant and were selected such as to increase
the specificity of the reaction. In view of the small-scale character of the introductory experiments an arbitrary low ozone concentration (approx. 200 ppm in air) was used. A low reaction temperature (-78°C) was chosen in order to suppress side effects and minimize possible chain degradation reactions resulting from the presence of oxygen in the system. Some other variables were quantitatively determined in order to evaluate their influence on the reaction behaviour.
In section II.2 the surface area available for reaction is determined using gas adsorption and mercury porosimetry. In section II.3 solvent effects that are related to ozone-solvent and substrate-ozone-solvent interaction are shortly discussed A straightforward interpretation of the kinetic results of the overall reaction of the kerogens and lignite with ozone proved
to be rather difficult and it seemed sensible to evaluate the results firstly for a well-defined model substance. A carbon black was chosen for this purpose and the interpretation of the ozonization of carbon black in various solvents is pre-sented in section II.4.
In the following section II.5 the considerable differences in reactivity found in the kerogen (or lignite)-ozone-solvent systems are interpretated.
II.2 Surface area and porosity
11.2,1 Introduction
Since the reaction between ozone and the kerogens is hetero-geneous it seemed relevant to gain a quantitative insight into the extent of the reactive surface area,
The most common method to determine surface areas is based on gas adsorption at low temperatures. Gas adsorption studies in general have provided much useful information as to the phys-ical structure of many porous materials including coals. In-formation on the physical structure of the kerogen samples can contribute to a better understanding of the overall con-stitution. It was therefore decided in the initial phase of this investigation that it might be rewarding to pay somewhat more attention to the interpretation of surface area and porosity determinations as would be strictly necessary. Nitrogen is the most common gas used in adsorption studies to determine surface areas and it was also used in this study, The use of carbon dioxide as an adsorbate for the determina-tion of surface areas of coals has since some time become favoured over the use of nitrogen. This follows from the fact that the areas calculated from nitrogen adsorption data meas-ured at 77 K are associated with the external particle area plus the area contained in pores with an entrance diameter greater than about 5 A. This is due to the occurrence of an activated diffusion effect which, at 77 K, effectively hinders the nitrogen molecule to enter pores smaller than about 5 A in diameter in a reasonable period of time. Coals are known to possess molecular sieve properties^ due to the presence of an extensive pore volume in ultramicropores (pore diameter 4-5 A ) . From the work of Lamond' it is known that carbondioxide is capable of penetrating type 4 A zeolites in the temperature range 195-293 K. On this basis carbon dioxide is increasingly used to determine surface areas and pore volumes of ultrami-croporous materials. It was argued that carbon dioxide adsorp-tion studies could also provide useful informaadsorp-tion on a possible
ultrafine structure present in the kerogens. This was the reason for a series of adsorption measurements in which carbon dioxide was used as an adsorbate.
A method to gain direct insight into the open porosity of the samples is based on high pressure mercury penetration. By this method no information is gained as to the presence of ultra-micropores. However it provides an independent method to deter-mine the surface area confined in pores with a diameter greater than (in our case) about 50 A.
The results obtained by low temperature gas adsorption and mercury porosimetry will be compared and discussed below.
11,2.2 Low temperature gas adsorption 11,2,2.1 Nitrogen
The most common method to determine surface areas is based on physical adsorption of nitrogen at 77 K. The adsorption iso-therm is measured and relevant data are processed using the BET equation:2
p J _ (^-')P/Po
V(Po-P) " V^c ' V^c
in which P/PQ is the relative pressure, p is the pressure during adsorption, p^ is the saturation pressure at the tem-perature of the experiment, V is the volume gas adsorbed at pressure p, Vjj, is the monolayer volume and c is a constant. The equation is usually applied in the pressure range p/po 0.05-0.35. By plotting P / P Q versus p/V(po-p), V^^^ and c can be determined from the slope and intercept of the linear function. From the monolayer volume thus obtained the surface area can be calculated using the standard cross sectional area of an adsorbed nitrogen molecule (16,2 A ' ) . The nitrogen adsorption isotherms measured for the kerogens, lignite and Philblack 0 are shown in figure 2.1. The monolayer volumes and the calculated surface areas are given in table 2.1. From the table it appears that the kerogens and the lignite only have a small surface area available for reaction in contrast with Philblack 0.
9 0 -• 70-
bO
3 0 -1U • V|vm (N2) Lignite _^ - ' 0 ' -0-,<f , " , 0 0 -0 ' ' o„---5>'°-°'°' / Messel , , 9 - ' , . ° - ' GRS / ,'0 ,0" ° ° , o ' - ' ! o ' ° ' 1.5 1.0 0.5-Messel P/Po 0.2 0.4 0.5 0.6 F i p . 2 . 1 . T h e adsorption isotherms o f n i t r o g e n at 7 7 K (solid l i n e s ) and carbon d i o x i d e s at 195 K (dotted l i n e s ) on G R S and M e s s e l kerogen, l i g n i t e and Philblack 0 ( P h ) . V o l u m e s adsorbed ( m l / g ) are divided by Vpj for n i t r o g e n ; f o r the values o f the l a t t e r see table 2 . 1 . T h e r e l a t i v e pressure (p/Pg) for carbon dioxide a d s o r p t i o n i s based on t h e extrapolated liquid v a p o u r p r e s s u r e .Table 2.1. gas adsorption N CO Hg porosimetry V S V S A m m Philblack 0 17.7 77 13.5 lh» 77 GRS 0.9 U - - 1* Messel 1.1 5 - - 1 Lignite 1.0 5 - - 3 Monolayer volumes (V in ml/g) and surface area (S in m /g) obtained by application of the BET equation on the nitrogen and carbon dioxide adsorption isotherms shown in fig. 2.1.
No calculations were made from the carbon dioxide isotherms of GRS, Messel and lignite.
The mercury porosimetry results are given as the surface areas (A in ttr/g,) calculated up to pressures corresponding with point A in fig.2.2, except for the lignite which was taken only to point B. K 2
0.205 nm was taken for the cross sectional area of carbon dioxide.
II.2.2.2 Carbon dioxide
In the experiments with the kerogens and the lignite it was found very difficult to determine the adsorption isotherms at
195 K with carbon dioxide because no definite equilibration times could be assigned to most adsorption points. Times up to 50 hours were sometimes needed for equilibration. Measure-ment of the desorption branch of the isotherm, to study any hysteresis effects, introduced even greater problems to the extent that these measurements were given up. In view of this it was considered senseless to calculate surface areas from the adsorption isotherms. The isotherms are shown in figure 2,1 and comparison with the nitrogen adsorption isotherms learns that substantial greater volumes of carbon dioxide are sorpted.
In the case of Philblack 0 the adsorption behaviour was normal and more or less the same as that found for nitrogen. The ad-sorption data could thus be used for determination of the sur-face area by application of the BET equation. The result is shown in table 2.! and is good agreement with that obtained from nitrogen adsorption.
II.2.3 Mercury porosimetry
A way to measure surface area and porosity directly is based on mercury porosimetry. In order to determine the pore size distribution down to pores of 25 X radius and the total sur-face area contained in these pores mercury porosimetry was applied up to pressures of 3000 atm.
The diameter d (A) of a pore filled with mercury is related to the absolute pressure p (atm.) exerted in the porosimeter by the Washburn equation:^
_ - 4 Y C O S 9
where y is the surface tension of mercury and 6 is the contact angle between mercury and the pore wall. A contact angle of
140° is used and for the surface tension of mercury 480 dyne/cm is taken.
The total surface area A (m^/g) of the samples can be calculated regardless of any specific pore geometry provided that no ink bottle pores are present according to the following equation derived by Rootare:^
V max
A =
—^
J PdV,
ym cos9 i.
The same values for the contact angle and surface tension are used as aoove and m is the sample weight in grams while V
is the volume of mercury penetrated. When this equation is used the upper integration limit (Pmax* ^max^ •'•^ normally taken at the beginning of the high pressure plateau as is indicated in figure 2,2 at A, This plateau illustrates that all pores are filled with mercury. By substitution of the pres-sure corresponding with point A in the Washburn equation men-tioned above the diameter of the smallest pores present in the examined pore size range is found. On this basis it could be established that there are no pores with a diameter smaller than about 500 X present in Messel kerogen and no pores smal-ler than 200 A in GRS kerogen. For the lignite it appears that already before the high pressure plateau at A is reached there exists a linear relationship between the volume of mercury penetrated and the exerted pressure as is indicated as the line BA in figure 2.2.
Such a linear relationship in about the same pressure range was also observed for coals of various ranks by other authors.^' It has been suggested that this represents the linear compres-sibility of these materials. In our case the comprescompres-sibility
1 8 -mer c ur y p e n e t r a t e d ( m l / g ) GRS p r e s s u r e ( a t m ) — I — 500 1000 1500 2000 2500 F i g . 2 . 2 . High p r e s s u r e p o r o s i m e t e r r e s u l t s f o r GRS k e r o g e n and l i g n i t e . P o i n t A d e n o t e s t h e b e g i n n i n g of t h e h i g h - p r e s s u r e p l a t e a u .
of the lignite determined from the slope of the line BA would then amount to 2.6 x 10 '•^ cm^/dyne which is in good agreement with other published values. > » ' It follows that in the lig-nite no pores are present that are filled at pressures greater than corresponding with point B. This means that no pores smaller than about 750 A are present.
The total surface areas calculated from the porosigrams by integration of the equation presented before up to pressures corresponding with point A (except for the lignite) are given also in table 2.1.
The interpretation of the mercury intrusion data for Philblack 0 which consists of essentially non-porous small diameter spheres must be in terms of the filling of inter-particle void which is dependent on the packing characteristics of the
spheres. The application of the Rootare equation on the porosi gram gives the surface area mentioned in table 2.1.
II.2.4 Interpretation of the results
The interpretation of the results of nitrogen adsorption and mercury porosimetry is rather straightforward. The results of both methods are essentially the same for the investigated
samples (except perhaps for Messel kerogen) and we can thus arrive at the following conclusion: The available surface area is confined to the external particle surface and to very large pores.
This information provides a sufficient evaluation of the avail-able surface area as a reaction parameter because fast gas-solid reactions are mainly confined to the outer surface and larger pores. However the results obtained with carbon dioxide as an adsorbate may indicate that this evaluation is of a too limited character as will be discussed below.
The interpretation of the carbon dioxide adsorption data pro-vides some difficulties. One can argue, as is sometimes done,
that the high sorption uptake of carbon dioxide as compared with nitrogen is due to the presence of an ultramicropore
(pores 4-5 A) system.^ ^^ The existence of such a pore system in high rank coals like anthracites is generally accepted because it is readily comprehensible from a crystallographic point of view and is more or less confirmed by X-ray diffrac-tion studies. Such an interpretadiffrac-tion of the sorpdiffrac-tion uptake of low rank coals is not readily understandable from an underlying structural point of view. Criticism has been directed towards carbon dioxide adsorption studies concerning mainly two points: 1) Chemical interaction with polar groups on the external surface. It has been noted that carbon dioxide adsorption can not be exclusively considered as a pure physical process in view of the fact that carbon dioxide interacts chemically with polar (oxygen containing) groups. > > This phenomenon leads to intensified adsorption of carbon dioxide. Quantitative in-sight of the effect is presented by a study using Graphon
surfaces'-'* that were deliberately covered with oxygen to varying extents. It is found that for this type of surface the effect increases with rising the pressure up to the adsorption of an extra carbon dioxide molecule for each 3-5 oxygen atoms on the surface. The results obtained with Philblack 0 that also con-tains oxygenated surface complexes indicate that the effect of chemical interaction on the total adsorption behaviour is fairly limited and does not influence the equilibration times. The nature of the polar groups on the surfaces of GRS and Messel kerogen and the lignite will be somewhat different from that of Philblack 0. Nevertheless the large differences in adsorption
behaviour and equilibration times seem to exclude that these are solely caused by chemical interaction with polar groups on the external surface.
2) Penetration of the surface. ^'•'-^ Unusual adsorption phenomena have been observed for polar sorbates as water and methanol on polymeric carbonaceous substances containing oxygen and
nitrogen.'''''-^ These materials include various natural products and a variety of synthetic poljmiers. It is suggested that the sorption of polar molecules is complicated by swelling and imbibition. For example the adsorption behaviour of methanol on coal very much resembles that of swelling gels.^» Although it thus is recognized that swelling can complicate the adsorp-tion behaviour most data are related to polar molecules capable of hydrogen bonding and no pertinent data could be found for
a non-polar molecule like carbon dioxide. However it seems acceptable to assume that also carbon dioxide can penetrate into the volume of suitable polymer-like substances by polar interaction during adsorption. Such behaviour can also account for the peculiar observation that for coals carbon dioxide has acces to a pore volume to which the smaller helium atom is restricted.^' The extent of the effect is rank dependent and increases with increasing oxygen content. This indicates that also in this case the sites causing the swelling are oxygen containing functions.
In view of the above it is assumed that in the case of the kerogens and the lignite carbon dioxide adsorption is accompa-nied by swelling of the adsorbent, rather than carbon dioxide penetration in an ultramicropore system.
II.2 Solvent effects
II.3.1 Introduction
Several ways in which a solvent can affect the course of ozo-nization are considered here. The solvents used in the experi-ments are: n-hexane, methylenechloride and methanol. The main
influences of solvents on the mode of ozone action are (1) solvent-ozone interactions, (2) solvent-substrate inter-actions and (3) the effect of solvent on the mechanisms involved in the reaction between ozone and the substrate. The latter point is related to mechanistic aspects of the reaction and will be considered in sections II.4.3 and II.5.3, The other mentioned effects will be considered in the following sections.
II.3.2 Solvent-ozone interactions
In order to gain quantitative insight into the extent of solvent-ozone interaction the solubility and the reactivity of solvent-ozone in the solvents used in the kinetic experiments is determined at -78 C under the dynamic flow-through conditions typical for all experiments. The results are presented in table 2.2.
Table 2.2. solvent n-hexane m e t h y l e n e c h l o r i d e m e t h a n o l m e t h y l e n e c h l o r i d e / m e t h a n o l 3/1 m e t h y l e n e c h l o r i d e / e t h a n o l 3/1 solubili ranol/lit 0.22 0.U2 0.23 0.3T 0.33 ty er r e a c t i v i t y m m o l / l i t e r / h o u r 0 11 0.12 0.52 0.I4I 1.10
Solubility and reactivity of ozone in various solvents under the dynamic flow through conditions typical for the experiments at -78 C.
The presented values are calculated from the ozone uptake curves as indicated in figure 2.8. From the table it appears that methylenechloride combines a good solubility and low reactivity
towards ozone which makes it a very suitable ozonization solvent, Other solvents or solvent combinations possess more unfavourable properties for ozonization. It is of interest to note that the tabulated values for the solvent reactivity appear to be related to the lowest C-H bond strength present in the molecules (see also sec. III.2.2).
II.3.3 Solvent-substrate interactions
In heterogeneous reactions interaction at the solid-liquid interface can have a profound influence on the efficiency of the overall reaction studied. This is especially true when it concerns effects that influence the surface area available for reaction. For porous rigid materials (inorganic solids) one may think of differences in wettability in going from one solvent to another. For non-porous non-rigid materials as various poly-mers and polymer-like materials as the kerogens and the lignite
one may think of differences in immersion swelling. In various solvent-polymer systems the volume may expand several hundred
per cent which could facilitate the diffusion of an attacking species when dissolved in the swelling medium resulting in an enhanced reactivity.
No quantitative data were gathered with regard to volume swell of the kerogens and the lignite and the literature does not provide any data on this point for the kerogens either. However on the basis of the carbon dioxide adsorption behaviour and the explanations given (sec. II.2.4) it seems safe to assume that suitable polar solvents can induce swelling of the kerogens and lignite. In the literature it is generally recognized that low rank coals and lignites swell appreciably when immersed in methanol and this should be kept in mind when interpreting the reaction behaviour of the lignite.
II. 4 The reaction of ozone and Philblack 0
II.4.1 Introduction
In the initial phase of the ozonization experiments Philblack 0 was used as a reference sample. It has a well-determined sur-face area, it does not swell appreciably upon immersion in liquids while the surface constitution of carbon blacks in general is reasonably well-known. »^ This provides a good basis for a first attempt to interprete the kinetic and other experimental results obtained for the Philblack O-ozone reac-tion.
Previous work on the reaction between ozone and carbon black or a related substance like adsorptive charcoal has provided information as to the kinetics of the reaction when conducted in the gas-solid system without solvents,'^^'^^»^^ in water^^'^^ and in organic solvents.^^ However all these reactions are carried out at relatively high temperatures (25°C or higher) and the kinetics are followed by measurement of weight increases or gas evolution. In a recent paper by Deitz and Bitner^'^ the ozone consumption was used as a reaction parameter in addition to measurement of gas evolution. Especially at these relatively high temperatures it seems essential to measure at least the ozone consumption as a reaction parameter since oxygen, which is always present in these types of experiments, may seriously interfere in the reaction especially above 100°C.
It can be expected that at low temperatures (-78°C) the kinetics of the reaction will be more related to actual ozone adsorption rather than to other effects that can be rate-determining at higher temperatures like thermal desintegration of the adsorp-tion complex, the presence of a diffusion regime and oxygen involvement.
II.4.2 Kinetic and other results
The kinetic plots of the raction between ozone and Philblack 0 in various solvents and in the absence of solvent at -78 C are shown in figure 2.3. mmol 03 g 1 5-10 05 3 6 9 ^^ours Fig. 2 . 3 . The cumulative ozone uptake of P h i l b l a c k 0 with
and without solvents at-78°C. ( ) methylenechloride (—.-^. ) methanol, ( ) n-hexane and ( ) g a s - s o l i d r e a c t i o n without s o l v e n t .
Two different kinds of curves can readily be distinguished one indicating continuous decay of ozone as in methanol and another one indicating a more limited reaction.
The ozone uptake behaviour in the gas-solid reaction and in the reactions when ozone is dissolved in methylenechloride and n-hexane is very much alike. In these reactions the ozone uptake approaches zero after a short period of time. Apparently no new reactive sites are available for reaction. The formed oxygenated surface complex seems an efficient barrier against further ozone attack.
Since the reaction has the character of a chemisorption reaction it was interesting to investigate the effect of the ozone con-centration. By increasing the ozone concentration in the gas stream in the gas-solid experiment (and thereby also changing the gas velocity) by more than a factor three, the ultimate
ozone uptake only showed a small increase (about 15%). This suggests that despite the low partial ozone pressures applied, chemisorption already approaches its maximal value.
Since it could be expected that part of the formed adsorption complexes would be of limited thermal stability it was decided to gain some quantitative information on this point. For this purpose the following experiments were conducted: The Philblack 0 surface was saturated with the adsorption complex at -78°C. Then the ozonized carbon black was brought to 0 C for 15 min. and was again cooled to -78 C. The sample was re-exposed to ozone and the ozone uptake was measured. The experiment was repeated using a new and fresh sample which was kept at 22°C for 15 min. In both experiments 0.07 mmol 03/g was re-adsorbed. This result shows that at least a part of the surface complexes have a fairly limited thermal stability.
In the next section we will treat the possible ways of ozone adsorption in order to get a better insight into the signifi-cance of the results presented in this section.
II.4.3 Mechanism of ozone attack and the chemical nature of the adsorption complex
It is often tacitly assumed that in the various heterogeneous reactions taking place at the carbon surface the common mecha-nisms known from homogeneous organic reactions are applicable and this concept is also assumed in this investigation. With this assumption we can extrapolate the mechanisms known
to occur in the reaction of ozone with polynuclear aromatics to the carbon black surface. These reactions, when conducted in solution, generally follow a course consistent with the ionic Criegee mechanism and they will be discussed in section II.4.3.2.
Another type of ozone attack on which no data are available for polycyclic aromatics but which may be of some importance at the carbon black surface involves surface free radicals. As yet this aspect of ozone attack has not gained much attention despite the fact that free radicals are known to exist on car-bon surfaces. The possibilities of a mechanism involving these surface free radicals are discussed below.
II.4.3.1 A mechanism involving surface free radicals
It is known that carbon surfaces like that of carbon black have an affinity to fix free radicals. This is explained by the fact that carbons contain surface free radicals that are stabilized
by the extensive delocalisation of the electrons in the graphite-like crystallites. The number of free radical acceptor sites on the surface has been measured by chemical methods while the total free radical content was measured by electron spin reso-nance spectroscopy (ESR). Since both methods gave comparable results it was once believed that all unpaired electrons were at the surface. However later this view appeared oversimplified. A detailed discussion on this point is given by Donnet.^
Anyhow it can be considered as quite certain that the carbon surface has an important free radical acceptor ability. It has been suggested that the free radical acceptor sites are mainly in the form of an aroxylic structure.^^'^^ Sites of this type have a certain affinity to react with molecular oxygen to form a new surface oxyde, possibly of a quinone-peroxide structure as suggested by Donnet.^^ It is found that below about - 40°C the reactivity of oxygen towards the carbon surface becomes insignificant. At this and lower temperatures ozone still may react with these free radical sites according to:
O3
R" —> RO" + O2
These newly formed surface radicals can isomerize by the forma-tion of a new aroxylic structure. In such a process the free spin concentration would remain essentially constant during ozonization. Contrary to previous results with oxidized carbons it was recently reported that upon ozone treatment of a carbon black in water the spin concentration remains constant and at the same level as before ozonization.^'* Another indication for a possible isomerization of RO' type surface radicals is that infra-red studies on carbon thin films exposed to ozone at 25°C show intensive formation of carbonyl functions.^^ The deviating kinetic behaviour of the reaction in methanol cannot easily be accounted for in a process involving surface free radicals. As yet no clear conclusions can be reached about the participation of surface free radicals in the ozone attack on the carbon surface and further research is needed to unravel this interesting problem.
II.4. 3.2. The Criegee mechanism
Some special parts of the carbon surface will have suitable unsaturated sites available for ozone and the reaction can proceed according to that of polyclic aromatic hydrocarbons. Such a type of ozone attack proceeds according to the mechnis-tic concepts as proposed by Criegee^^ which involves 1,3 bipolar
addition of ozone to a double bond. The initial transitory product is the molozonide (I) that rearranges to the zwitterion intermediate (II) and a carbonyl containing fragment. The sub-sequent reactions of the zwitterion very much depend on the polarity of the solvent, on whether or not the solvent is a reactive and participating one, on electronic effects of sub-stituents and on steric effects.
In inert solvents like n-hexane and methylenechloride the zwit-terion usually recombines to form the proper ozonide (1,2,4-trioxolane structure((III). At low temperatures ozonides are rather stable structures. For example the monoozonide of
I I I I I I
pyrene (IV) can be isolated and characterized^' and only slowly decomposes and/or rearranges at room temperature to produce a aldehydeacid (V) or isomeric a hydroxy-lactones.
IV V
In a participating solvent like methanol the reaction sequence is normally rather different because methanol reacts with the zwitterion (II) to form a methoxyhydroperoxyde (Via). A com-pound of structure (VI) only has the character of a transitory intermediate in the reactions of ozone with phenanthrene^°>^^ and naphtalene**"''*' in participating solvents like water and methanol. The compound usually isolated at room temperature is the cyclic peroxide (VII). At low temperatures however the
VI VII VIII a R = CH3 a R = CH3
b R = H b R = H
cyclisation reaction does not appear to occur. Under our exper-imental conditions the presence of the open chain compound (Via) is thus very probable. In the case water is used as a partici-pating medium the cyclic dihydroxyperoxide (Vllb) fairly easily decomposes at room temperature to the dialdehyde (VIII).^^ II.4.4 Interpretation of the results
From the above presented data it appears that when the reaction proceeds according to the Criegee mechanism the kinetics of ozonization in the various solvents can be understood. In the non-participating solvents like n-hexane and methylenechloride and at low temperatures parts of the surface are probably covered with a stable ozonide surface complex that forms a protective barrier against further ozone attack. As a result the ozone uptake will cease after a certain period of time.
When the reaction is conducted in a participating solvent like methanol at -78 C the aromatic rings are opened up by formation
of stable open chain peroxides and further oxidation of the sub-strate is facilitated. This results in the continuous consump-tion of ozone as shown in the kinetic plot.
In the reactions without solvents the kinetic behaviour is very much like that found for the inert solvents and possibly the Criegee mechanism is also applicable here. In view of this the
limited thermal stability of part of the surface complex as encountered in the re-adsorption experiments can easily be caused by decomposing ozonides.
Although ozone attack according to the Criegee mechanism can explain much of the kinetic behaviour, other types of ozone attack will play a role on sites where ozone addition according to the Criegee mechanism is impossible. This can include the formation of quinone functions by reaction with surface free radicals. The relative importance of these processes can not be evaluated.
It was mentioned before that despite the applied low partial ozone pressures the available reactive surface area was almost fully 'occupied'. This enables us to estimate a maximum area covered by ozone induced adsorption complex. This estimation is based on the assumption that all adsorbed ozone is in the form of ozonide structures. The ozonide structure is assigned a cross-sectional area of 25 A^. On this basis a maximum estimate of the covered area is obtained since some ozone will react to form quinone functions which have a much smaller cross-sectional area. It is found that in n-hexane and methylenechloride about 50% of the available area is covered. This partial coverage by ozone induced surface oxides can be explained by the following considerations:
1) Philblack 0 already contains oxygen complexes at the surface prior to ozone attack. On the basis of the oxygen content (see
table 2.4) and the assumption that per oxygen atom 8 A^ is covered it can be calculated that about 60% of the surface is already covered with oxygen complexes leaving limited space for ozone reaction.
2) Ozone attack by the formation of ozonides and to a lesser extent by the formation of quinones is restricted to specific sites on the carbon surface. For example the foirmation of ozonides is restricted to sites in between carbon atoms which can easily change their sp^ hybridization into sp^. This is only possible for special periferal sites.
It should be mentioned that the kinetics of the reaction of carbon black with ozone in methanol may show a peculiar temper-ature dependence. This follows from the observation that at low temperatures progressive oxidation appears to be facilitated by ring opening, while at intermediate temperatures (e.g. from -20 to +20 C) the oxidation rate may slow down because of the presence of cyclic peroxides like (VII). At higher temperatures these cyclic peroxides readily decompose which again accelerates oxidation. This behaviour can be relatively easy experimentally verified; however such experiments fall definitely outside the scope of the present study and have therefore not been carried out.
II. 5 The reaction of ozone and kerogen and lignite
II.5.1 Introduction
As was mentioned before the ozone uptake can be considered as the result of the combined effects of the most important reac-tion parameters.
In this section we will try to evaluate the most important single effects that contribute to the kinetic behaviour of the overall reactions performed in several solvents and at -78 C. For a good evaluation some basic information as to possible modes of ozone action on the substrate is required and this is considered in section II.5.3. In relation to this additional information is provided by the nature of the degradation prod-ucts but this is only treated in sections III.2 and 111,3 as far as GRS and Messel are concerned. For clarity some of the most important results from these sections will already be used here.
II.5.2 The kinetic results
The kinetic plots of the reaction between ozone and the kerogens and the lignite are shown in figure 2.4.
In the absence of solvent and when n-hexane is used the reaction behaviour is very much alike for all samples. The main differen-ces are the relative reactivities of ozone in either methanol or methylenechloride. nnmol03 9 15- 10-0
5-a
grs. ' " —
-mmol03 9 15- 1.0-05-b
/
messel f 1 t 1 1 1 1 1 1 1 mmol03 ~ 9 ~ lignite / / I I I I IFig. 2.1*. The cumulative ozone uptake of the kerogens and the lignite with and without solvents at -78°C. ( ) methylenechloride, ( ) methanol, (-n-hexane and ( ) gas-solid reaction without
solvent.
II.5.3 Mechanisms of ozone attack
Two modes of ozone action are already proposed in section II.4 for the reaction between ozone and Philblack 0. These mechanisms could be proposed on account of the fact that the surface con-stitution of carbon black is reasonably well-defined. In the case of the kerogens and the lignite the problem is somewhat different. It concerns natural complex materials on which no information on the detailed structure exists. However in broad terms such information is available at least for GRS (sec. 1.4) and lignite. These materials range from a predominant saturated aliphatic type material (GRS) to material having a more aromatic character (lignite). On this basis we may consider several ways of ozone attack.
1) Ozone attack involving surface free radicals can be neglected in this case since low rank kerogens as well as lignites show an insignificant paramagnetic activity .'*^ >'*^ >'*'*
2) Ozone attack according to the Criegee mechanism seems univer-sally applicable as long as suitable reactive sites are offered. Since double bonds are rather common structural elements it is possible that at least a part of the adsorbed ozone reacts ac-cording to the Criegee mechanism. This can especially be expec-ted for the lignite. We have treaexpec-ted some of the concepts of the Criegee mechanism in section II.4 and it will not be con-sidered further here.
3) A third possibility for ozone attack that has not been con-sidered in the previous sections is a type of ozone attack commonly encountered in the action on polymeric materials like rubber, polythylene and other organic materials. The presence of ozone in air, even in very small concentrations, markedly accelerates the ageing of such materials and leads to a rapid deterioration of the mechanical and electrical properties. The mechanisms involved are as yet not completely understood and may differ depending on the type of polymer.
The relatively simple case of polyethylene which is made up of a regular chain of CH2 groups has recently been studied by Kefeli and coworkers.'*^ In the investigation on the kinetics of the reaction of solid polyethylene and ozone in the temper-ature range 30-70°C and on the type of degradation products isolated, a mechanism is proposed in which ozone initiates the formation of peroxy radicals (ROO') as was substantiated by ESR, These radicals are considered as intermediates and they decompose by the formation of aldehydes, acids and carbonyl containing compounds. The formation of aldehydes and kerones
was also noticed by earlier workers on the ozonization of poly-thylene using infra-red spectroscopy."*^
The peroxy radicals were not thought to be derived from chain carrying reactions involving oxygen as
ROO' + R'H ^ ROOH + R " O2
R'• ^ R'OO'.
A similar mechanism in which the formation of peroxy and probably also phenoxy radicals is initiated has been proposed for the solid phase reaction of ozone and polystyrene.^''
Both in the case of polyethylene and polystyrene the formation of radicals mainly takes place at the surface."*® This is indi-cated from the fact that the concentration of free radicals is proportional to the surface area and independent of the weight or volume of the sample. It can be argued that for pro-gressive oxidation the decomposition of the surface complexes is essential and this then must be related to the decomposition of the peroxy radicals formed in the initial stage of the reac-tion. The formation of surface-oxide radical complexes as a result of ozone attack on simple synthetic polymers provides a further example as to what can happen as a result of ozone attack on the kerogens and the lignite.
II.5.4 Interpretation of the results
Since the ozone uptake behaviour of the reactions periormed without solvents and in n-hexane shows the same patterns we may
try to interprete this behaviour first. It must be taken into consideration that both the Criegee mechanism and a mechanism in which ozone initiates the formation of ROO" or RO' radicals are the types of ozone attack mostly anticipated. It can be expected that the initial intermediates of both types of reac-tion (ozonides and ROO' and RO' radicals) are of low reactivity and present rather stable species at -78°C. Such a stability of the adsorption complex will eventually lead to a decrease in the rate of ozone attack as also found for Philblack 0. The amount of ozone consumed up to the point on the curve beyond which there are only very small increases in the ozone
con-sumption (e.g. after about 3 hours in fig. 2.4 a/c) should then reasonably correspond with the total area available for reac-tion(table 2.1). When it is assumed that the main reaction is the formation of peroxy radicals and that the cross-sectional area of a peroxy complex is about 15 A^ then it can be calcu-lated that about 7 m^/g is covered, compared with a total available area of about 5 m^/g. This is a reasonable agreement
especially when it is considered that the excess area allows for instability of some primary reaction products, reaction of ozone with intermediates etc., diffusion of ozone into the kerogen etc. The reaction behaviour as well as the simple cal-culation above confirm that for GRS and lignite ozone attack is mainly confined to the external surface when the reaction is conducted without solvent and in n-hexane.
In the case of Messel kerogen (fig. 2.4 b) the reaction behaviour is somewhat different. After 3 hours still noticeable amounts of ozone are consumed and it seems that no effective barrier against ozone attack is built up during the time of the experi-ment. An acceptable explanation is that Messel kerogen contains extremely vulnerable sites towards ozone attack. In reacting with ozone these sites could facilitate a rapid penetration of ozone into the kerogen.
The large difference in ozone reactivity in methanol and methyl-enechloride as compared with n-hexane and as found for GRS and Messel kerogen (fig. 2.4 a/b) can be explained by the following
effects:
1) The primary reaction products at the external surface decom-pose more easily in a more polar solvent than in n-hexane. It may be noted as a trend that the reactivity of the investigated systems is greater in more polar solvents but it is not a general case since the lignite-methylenechloride-ozone system behaves differently.
2) Diffusion of dissolved ozone into the interior of the kerogen is facilitated as a result of solvent-substrate interaction
(sec. II.3.3).
A very special case of the first mentioned effect is that al-ready described for the Philblack O-methanol-ozone system (sec. II.4). For this type of surface degradation the presence of double bonds is required. Since methylenechloride is a non-participating solvent in these reactions such an effect is of limited importance in this medium. The absence of double bonds in GRS kerogen can be evaluated from the nature of the degrada-tion fragments (sec. 111.2,2) while in Messel kerogen such sites are present to a limited extent (sec. III.3.2). As a result the strong reactivity of both kerogens in methanol can not be ascribed to the presence of double bonds. When this type of ozone attack is relatively unimportant the decomposing sur-face complexes could be radical species. It is found (table 3,1 and 3.4) that for GRS and Messel kerogen in both methylene-chloride and methanol at -78°C rather stable surface complexes are formed that only decompose at an appreciable rate at higher temperatures. This seems to indicate that the effect of
decom-position of the primary reaction products on the external sur-face is of limited importance and of a value comparable with that assumed for n-hexane.
The second effect mentioned above should thus be taken into consideration. Since we do not posses any quantitative data on the extent of volume swell of GRS and Messel kerogen in methanol and methylenechloride the actual occurrence of the effect can as yet only be postulated. In view of the polymer-like charac-ter of GRS and Messel kerogen the effect would provide an acceptable explanation for the observed ozonization behaviour. It should be realised however that also in this case (minor) decomposition reactions will occur simultaneously.
The results for the lignite-ozone-methanol system very much resemble those found for Philblack 0. The reaction when con-ducted in the absence of solvent, in methylenechloride and n-hexane is confined to the external surface and a stable sur-face complex is formed that evidently is an effective barrier against further ozone attack. When methanol is used as an ozonization medium the reactivity increases dramatically. This can be explained by surface degradation due to participation of methanol in ozone attack on double bonds according to the Criegee mechanism as also encountered for Philblack 0. However lignites are known to swell appreciably when immersed in metha-nol and thus the possibility that the strong reactivity is partly caused by reaction of ozone with internal sites of the swollen lignite gel should be considered.
A possibility to gain some quantitative information on this point is provided by a comparison of the kinetic behaviour when binairy mixtures of methanol and ethanol in methylene-chloride are used. Since both are participating solvents in the reaction of ozone with double bonds the kinetic behaviour of both binairy mixtures should be comparable.
In figure 2.5 it is shown that in the case of Philblack 0 mix-tures of methanol and ethanol in methylenechloride show approx-imately the same behaviour while this is not the case for the lignite. Methanol is a much more efficient solvent for ozone attack in this case which suggests that methanol is much more effective in opening up the internal volume of the lignite as ethanol under the applied experimental conditions.
Binairy mixtures of methanol and ethanol in methylenechloride were used rather than the pure liquids since the differences
in viscosity of the latter are rather high at -78°C which would seriously influence the kinetic behaviour.
25% ethanol m CH2C12 25% methanol in CH2CI2
25%methanol i n C H 2 C l 2
25% ethanol in CH2CI2
CH2CL2
Fig. 2 . 5 . The cumulative ozone uptake of Philblack 0 and l i g n i t e in b i n a i r y mixtures of methanol and ethanol in methylenechloride a t -78°C.
II.8 Experimental methods
11.6.1 Glass ware and reagents
All reagents used were of analytical grade. Solvents used were from J.D. Baker Chemical Company and had the quality mark
'analysed reagent'. Water was distilled in an all glass appa-ratus. During the analytical procedure the samples never came into contact with materials that were not especially treated before usage to reduce the risk of organic contamination. In this way all glass ware was cleaned using chromic acid, Aluminium foil was used to prevent open pretreated glass ware
to become contaminated by laboratory dust prior to analytical usage.
11.6.2 Preparation of kerogen samples
Sample description
Philblack 0: A sample (about 500 g) of the carbon black Phil-black 0 (Phillips Petroleum Company) was obtained by kind
cooperation of Nefaboline N.V., Voorschoten, The Netherlands. Green River Shale: A sample (about 700 g) of the Eocene Green River oil shale (code KR52-351) was obtained through Dr. W.E. Robinson of the Laramie Energy Research Center, Laramie, Wyoming, USA. The sample, assaying 66 gallons of oil per ton of shale, was obtained from the Piceance Creek Basin from the Bureau of Mines demonstration mine near Rifle, Colorado from only a few hundred feet below the surface.
Messel Shale: Various samples of the Eocene Messel oil shale were used in this study.
The samples used in the ozonization experiments were kindly provided by the late Mr. J.A. Gransch of the Koninklijke Shell Exploration and Production Laboratory at Rijswijk, The Nether-lands. They were collected by P.A. Schenck in the oil shale quarry at Messel (near Darmstadt, German Federal Republic) in May 1967 and stored in special polyethylene sample bags until needed.
Other fresh samples of Messel oil shale were collected by J.W. de Leeuw and M.L.J, van den Berg in January 1974 at the Messel quarry.* These samples were used for a comparative study of the rock extract. For this purpose all samples were taken from a horizontal section of about 15 meters length in the same approximately 40 cm thick bed outcropping in the central north west area of the quarry. The sample used in this study
(chapter IV) was coded 3S2 and weighed about 1 kg.
Lignite: A sample (about 1 kg) of a Miocene lignite was kindly provided by the Herzogenrather Braunkohlen Werke A.G, at Herzogenrath, German Federal Republic through cooperation of the late Mr. J.A. Gransch of KSEPL Rijswijk.
Isolation of the kerogen
The procedural scheme for the isolation of kerogen from sedi-mentary rocks is shown in figure 2.6. It represents the basic scheme applied by most workers in the field. Although several other techniques have been developed in the past, e.g. sink float techniques and procedures based on differences in the wettability of the organic and inorganic part of the rock, these all result in low yields, while also there exist hazards of fractionating the kerogen. The acid digestion method used
In both cases the kind permission and cooperation of Ytong A.G., Messel is gratefully mentioned.
Fig. 2,6. Procedural scheme for the isolation of kerogen from sedimentary rocks.
in this study has the advantage that most mineral matter can be removed and that fractionation of the kerogen is not expected to occur.'*^ However by the action of strong mineral acids chem-ical alterations may be induced. The demineralisation procedure applied is mainly according to the method of Smith, " which was slightly modified in order to use a lower reaction temperature
(50°C).
The dried residue after demineralisation was repulverized in a mortar and subsequently extracted using benzene/methanol and methylenechloride/methanol mixtures and finally acetone until the residue was virtually free of extract. The combined extracts were evaporated to dryness and weighed. The residue after the last extraction was evaporated to dryness under reduced pres-sure and repulverized in a mortar. The powder was sieved yielding
two fractions. The fraction passing 6 ym was used for surface area measurement and the small scale ozonization experiments. The other fraction was repulverized and used for the larger scale ozonizations. The samples were stored in a vacuum desic-cator over phosphorouspentoxide until needed.
The yields of organic extract, the losses of inorganic constit-uents and the ultimate yields of kerogen concentrates for the analysed samples are listed in table 2.3.
Table 2.3. GRS Messel Lignite Benz/MeoH 3.0 1.3 3.6 HCl sol. % 12.6 n.a. U.5 HCl/HF % 38.5 US.9 n.a. sol. Residue % I4O.9 1*8.5 89.7
The yields of organic extract, losses of inorganic constituents ana the ultimate yields of kerogen concentrate (residue), (n.a. = not applied).
The elemental analysis of the extract free kerogen concentrates is shown in table 2.4. It is known that the residual inorganic matter after acid treatment can contain considerable amounts of unremoved pyrite.
The presence of pyrite in the GRS kerogen concentrate was con-firmed by X-ray diffraction. The residual inorganic matter of the other samples was not further analysed.
Table 2.1*. H N 0 CI F residue H/C
J
%_ %, % % %
Philblack GRS Messel Lignite 0 97.1* 61*.5 68.5 61.6 0.1* 8.5 7.9 1*.6 n.d. 2.3 2.1* 0.6 1.6 6.8 17.5 32.U n.d. 1.7 0.6 -n.d. 6.7 * 0.2 n.d. <! 12.8 1.3 <1 0.05 1.58 1.38 1.11Elemental analysis of the kerogens. n.d. = not determined. The determinations were performed at the Micro-analytical Department of the Institute of Organic Chemistry TNO, Utrecht, The Netherlands, under supervision of W.J. Buis.
11.6.3 Surface area determination
Gas adsorption and mercury penetration experiments were conducted at the Laboratory for Chemical Technology (T.H. Delft) under the supervision of Prof. dr. J.J.F. Scholten.
Gas adsorption
Prior to the adsorption measurements the samples were degassed overnight at 10 ^ mm Hg and at a low temperature (maximally 80°C) to prevent irreversible alterations to take place. Adsorp-tion measurements were performed in a convenAdsorp-tional constant volume apparatus. Nitrogen was used as the adsorbate at 77 K and carbondioxide at 195 K. The BET plots were drawn using the data of the adsorption branch of the isotherms.
Mercury porosimetry
Prior to mercury porosimetry the samples were treated as de-scribed above for the adsorption measurements. High pressure mercury penetration experiments were performed using a Micro-meritics Instrument Corp. model 905-1 porosimeter.
11.6.4 Ozone uptake measurement
Apparatus
A diagram of the continuous flow apparatus used for measurement of the ozone uptake of small samples is shown in figure 2.7. Along an ultra violet lamp (PCQ9G-1, Ultra Violet Products) a constant flow of air is passed at a rate of 2F. The ozonized air is supplied to a dual system. The actual measuring system in which the reaction vessel is placed and a reference system designed to provide a built-in check on the constancy of the ozone output. Both systems had an approximate flow F, the maximal relative difference being 10%. After the reactor the ozonized air stream is splitted to reduce the ozone concentra-tion to an acceptable level for the galvanic detecconcentra-tion cell. This reduction is realised by separating a small flow f via a capillary restriction from the main flow (rate F-f) which is passed through a system of manganese dioxide and activated charcoal filters in order to remove all ozone present. After
Mrs. L. de Wit and Mr. G. van Westen performed respectively the gas adsorption and mercury porosimetry measurements.
