Introduction

Most of the petroleum hydrocarbon was generated and occurred in fine-grained sedimentary rocks (i.e., shales). Clay minerals are the predominant components in these rocks, especially for gas-bearing shales. These clay minerals include montmorillonite, illite, mixed-layer illite–smectite (I-Sm) minerals. Montmorillonite is a 2:1 expanding clay mineral that has two Si–O tetrahedral sheets sandwiching a central Al–O octahedral sheet. Considerable substitution occurred in the octahedral sheet (i.e., substitution of Mg2+ for Al3+) and tetrahedral sheets (substitution of Al3+ for Si4+). Due to isomorphous substitution in the octahedral sheet, some exchangeable hydrated ions may appear in the interlayer space of montmorillonite, i.e., Ca2+, Na+ [1]. Illite is a non-expanding 2:1 clay mineral, and the interlayer space of illite is mainly occupied by poorly hydrated K+. Mixed-layer I-Sm clay mineral is the intermediate composition between end-members of montmorillonite and illite. In different sedimentary environment, the assemblage pattern of these I-Sm clay minerals groups varies significantly [2].

I-Sm clay minerals had valuable geological participators in petroleum basin and played an important role in organic matters (OMs) accumulation [3] and petroleum hydrocarbon generation [4, 5], which thus have been focused by many researchers. For instance, I-Sm clay minerals were found to be closely related to the hydrocarbon accumulation in some sedimentary basins [6,7,8]. Especially, the occurrence of mixed-layer I-Sm minerals was related to petroleum accumulation [9]. In addition, there was a coincidence between the temperatures for the I-Sm minerals conversion from the random to ordered interstratified and those for the onset of oil generation peak in some sedimentary, showing that changes in the ordering of mixed-layer I-Sm minerals were particularly important in the exploration for hydrocarbons occurrence [10].

Considering this phenomenon, the thermal evolution or degradation of OMs with I-Sm clay minerals in experimental system has been studied during the last two decades. The experiments on heating petroleum source rocks or OMs (i.e., kerogen, fatty acids, and other organic compounds) are often used to simulate hydrocarbon generation processes and identify the organic–inorganic interactions within the formation of petroleum in nature [11, 12]. These studies indicated that I-Sm clay minerals have an influence on hydrocarbon generation by cracking of OM, controlling the pyrolysis maturity indicator, pyrolysis reaction (i.e., isomerization) gas-to-oil ratio, gas wetness, and fluid aromaticity pyrolysate [13,14,15,16,17,18]. The inherent solid acidity (the proton-donating capability or ability of being an electron pair acceptor) of clay minerals has been linked with the development of complex organic networks (i.e., kerogen) and cracking of kerogen or bitumen phase to produce hydrocarbons [19,20,21,22]. Thus, understanding the effect of I-Sm clay minerals on thermal evolution of OMs is essential for comprehending the processes and mechanisms of the evolution of OMs-involved clay minerals in the petroleum basins.

Previous work studied the effect of clay minerals on hydrocarbon generation or thermal degradation of OMs through various pyrolysis experiments in the open or closed laboratory heating system. In most pyrolysis experiments, the possibility that organics react with surface of minerals (including outer- and inner-surfaces) was not considered. In fact, OMs could be usually associated with clay minerals through several processes, such as adsorption, bounding, and intercalation [5, 8, 11, 17]. The multiple organic components (i.e., kerogen) were selected for these experiments, making it difficult to investigate the effect of surface properties (i.e., solid acidity) and microstructure of minerals on the hydrocarbon generation of pyrolyzed OM, and interface interaction of inorganic–organic became elusive. Even though some studies have considered the association between clay minerals and OMs and investigated the evolution of OM-clay associations with respect to thermal maturity in organic-rich source rocks [23]. Nevertheless, clay mineral assemblage (i.e., rocks) was commonly considered for these pyrolysis experiments. However, the clay mineral assemblage of natural samples possesses complex structure and mineral phase. Moreover, the coexistence of randomly and ordered interstratified mixed-layer I-Sm minerals made the analysis of the organic–inorganic interactions more difficult. The mixed-layer I-Sm mineral is a coherent stack of 2:1 layer having both illite and smectitic sheets [24]. As the mixed-layer I-Sm mineral became illitic, the interlayer arrangements gradually evolved from random to short-range ordered, and finally to long-range ordered [25]. According to ratios of smectite layer, the mixed-layer I-Sm minerals can be divided into two types: such as mixed-layer random I-Sm (R = 0; amounts of illite layer is less than 50%) and mixed-layer ordered I-Sm (amounts of illite layer is more than 50%). Hence, the microstructure is different from each other and differs from montmorillonite and illite. Therefore, it is necessary for us to use one kind of clay mineral alone to prepare the pyrolysis experiment.

In this work, we investigated the contribution of I-Sm clay minerals to thermal decomposition of OMs using an atmospheric pyrolysis experiment. We hypothesized that these clay minerals play an unaccounted role in gaseous hydrocarbon, where mixed-layer I-Sm mineral could enhance hydrocarbon formation. To test this hypothesis, we characterized the relationship between the microstructure and solid acidity of I-Sm clay minerals and the chemical and spectroscopic characteristics of thermal OMs. Thermogravimetry coupled with Fourier transform infrared spectroscopy (TG-FTIR) was applied for thermal degradation assessment of the OMs. It can detect changes in the mass loss of samples during heating and can simultaneously integrate evolved gas information (i.e., the gaseous products). Some analysis methods such as X-ray diffraction (XRD), FTIR, and solid acidity analysis were used to characterize the I-Sm clay minerals.

Materials and methods

Materials

Some aliphatic organic compounds were selected as the model OMs, including fatty acids, fatty amine, and alkane, for that this organic compound were regarded as some important precursors of hydrocarbons [26, 27]. Octadecanoic acid (OA, CH3(CH2)16COOH), octadecyl trimethyl ammonium bromide (OTAB, CH3(CH2)16CH2N(CH3)3Br), n-octadecane (ODE, CH3(CH2)16CH3) were chosen. These model organic material (with a purity of 95 mass%) were obtained from Aldrich, Milwaukee, WI, USA. These organic compounds were used based on the following considerations. First, they possess relatively simple structures, and thus it is easier to be analyzed than organic compounds with complex compositions and structures, such as kerogen. Second, this kind of OMs contains carboxyl groups, alkyl chains, and a carboxylate group attached to a long chain hydrocarbon, which are common in natural organics such as in low-rank coals [28,29,30]. Third, the decarboxylation and cracking of aliphatic OMs (i.e., fatty acid) are proposed to be closely related to petroleum generation [26, 31, 32]. Therefore, investigating its thermal evolution behavior is of great significance to the exploration of petroleum formation in clay-rich rocks.

Rectorite (Rec), the mixed-layer I-Sm minerals with 50% S%, was selected as the typical mixed-layer I-Sm minerals. It is regular interstratification of one layer of illite and one-layer smectite (…ISIS…) with the value of R = 1. In some shale or coal samples, Rec is one of the important mineralogical assemblages [33, 34]. Because this kind of mineral commonly occurred in clay-rich rocks and this clay mineral composition in sedimentary basins was found to be closely related to hydrocarbon accumulation. Particularly, it was found that the illitization of smectite corresponded with the evolution of OM that began to enter the threshold of oil generation [10]. Hower et al. (1976) [6] even showed the R1-ordered mixed-layer I-Sm minerals correlated with oil generation. The structural formula of Rec was (Na0.45K0.32Ca0.37Mg0.08) (Al3.78Fe0.11Ti0.10) [(Si6.22Al1.78) O20] (OH)4·nH2O [35]. The purity of Rec used in this study was very high, and less than 5% quartz occurred in Rec. Montmorillonite (Mnt) (with purity of > 95%) was collected from Inner Mongolia, China. Illite (Ilt) was sourced from Jilin Province, China. Based on the results of chemical analysis, the structural formula of Mnt was: Ca0.168Na0.025K0.013[Si3.984Al0.016][Al1.352Fe0.271Mg0.365Ti0.010]-O10(OH)2·nH2O. By purification and exchange of sodium, the product of Mnt was obtained. The structural formula of Ilt (with purity of > 95%) is as follows: K0.571Ca0.010Na0.028[(Al1.853Fe0.083Mg0.032Ti0.023P0.009)-(Si3.331Al0.667)O10] (OH)2·nH2O.

The OM-clay mixtures were prepared to make the mineral and OMs mix evenly. Clay mineral sample was mechanically mixed with organic compound at a ratio of 4:1 (mass ratio). The products were denoted as OM-clay, i.e., OA-Rec, OTAB-Rec, and ODE-Rec. For OTAB, an OM-clay composite was prepared according to previous studies through a cation exchange method [17]. The OM-clay composite was denoted OTAB-Reccom. The basal reflection obtained from XRD patterns and TOC content of each sample determined by elemental analysis are listed in Table 1. Compared with clay minerals alone, there was no obvious change in these d001 values in the (001) reflections of OM-clay mixture. These data indicated that the OM was not intercalated into the interlayer space of clay minerals for the OM-clay mixtures, and the organics only existed outside of clay minerals. However, for ODE-Rec and OTAB-Reccom, which showed d001 values of 2.54–3.13 nm, respectively. It implied that a small amount of ODE entered the interlayer space of Rec, and OTAB successfully intercalated into the interlayer space of Rec. The corresponding results are shown in Figure S1 and S2 in part I of Supplementary material (SM).

Table 1 The d001 values and the total organic content (TOC) of I-Sm clay minerals and OM-clay complexes
Full size table

Analysis methods

An elemental analyzer (Elementar Vario EL III Universal CHNOS) was used for detecting the TOC content of samples. X-ray diffraction (XRD) pattern was used to analysis the microstructure of clay minerals, OM-clay mixtures, and OM-clay composite. Powdered XRD analysis was performed with a Ni filter and Cu Kα radiation (40 kV and 40 mA) on a Bruker D8 Advance diffractometer. The patterns were collected from 1°2θ to 40°2θ at the scanning rate of 1°(2θ) min−1. FTIR spectra were used to investigate the composition and structural properties of clay minerals, as well as the bond of the elements. Sample in pressed KBr pellets were recorded on a Bruker Vertex-70 FTIR spectrometer. The spectra were collected at a resolution of 4 cm−1 with 64 scans for each measurement. The corresponding results are shown in Figure S3 in part II of SM.

Clay mineral influencing hydrocarbon generation is because the inherent surface solid acidity, which can catalyze reactions by energy transfer processes, redox reaction, and stabilization of intermediates [36]. Two types of surface solid acidity center distinguished in clay minerals, i.e., Brønsted acidity (B) and Lewis acidity (L). B and L acid sites were often differentiated by using diffuse reflectance Fourier transform infrared spectroscopy (DRIFT) with pyridine as a probe molecule [37]. The amount of total solid acid sites (QT) of clay minerals was determined by n-butylamine titration with Hammett indicators according to Liu et al. [37]. The amount of B acid sites (QB) of clay minerals was obtained from the product of the peak area ratio and the QT. The amount of Lewis acid (QL) of clay minerals was obtained by subtracting QB from QT. Detailed steps can be found in previous references [37]. The corresponding results are shown in Figure S4 in part III of SM.

Pyrolysis experiment

The pyrolysis experiments of samples were studied by using an in situ TG (NETZSCH STA 449C, Germany) coupled with a Bruker Vertex-70 FTIR. Approximately 10 mg of OM-clay complexes were heated from room temperature to 900 °C at a rate of 20 °C min-1 in an argon flow (99.99% purity, 40 cm3 min-1). Before testing, the instruments were temperature-corrected by the melting point of the standard metal. The spectrometer was connected to the TG analyzer through a polytetrafluoroethylene transfer line and a gas cell that were both heated to 200 °C. Three-dimensional (3D) spectrogram was obtained through real-time detection of the OMs pyrolysis process.

Results

The temperature at which maximum mass loss occurred was defined as the main thermal decomposition temperature (TM) in this study. One DTG peak was observed at 305 °C for pure OA (Fig. 1b), indicating that the thermal evolution of OA occurred in the range of 200–350 °C (Fig. 1a) and the TM of OA was 305 °C. The corresponding evolved gases at TM showed strong bands at 2934–2864 cm−1 (Fig. 2a), which were related to the antisymmetrical and symmetrical stretching of CH2 groups [38], respectively. These bands indicated that aliphatic hydrocarbon fragments were the major gaseous products at this stage. In addition, a band at 1108 cm−1 appeared (Fig. 2a), which was related to C–O–C stretching vibration [39]. This occurrence of this band indicated the presence of some small amounts of carboxylic acid. Carboxylic acid was also identified at 3573–1779 cm−1 [39, 40]. These bands suggested that the mass loss process of OA might be a degradation not a simple evaporation/desorption, even though some small mass loss of pure OA was possibly attributed to the sublimation of OA. Pure OA was decomposed via cleavage of C–C bonds, which led to the production of aliphatic hydrocarbons and carboxylic acid.

Fig. 1
figure 1

TG-DTG curves of OMs and OM-clay complexes. a TG curves of OA, OTAB, and ODE; b DTG curves of OA, OTAB, and ODE; c TG curves of OA-Rec, OA-Mnt, and OA-Ilt; d DTG curves of OA-Rec, OA-Mnt, and OA-Ilt; e TG curves of OA-Rec, OTAB-Rec, OTAB-Reccom, and ODE-Rec; f DTG curves of OA-Rec, OTAB-Rec, OTAB-Reccom, and ODE-Rec

Full size image
Fig. 2
figure 2

3D-FTIR spectra of OMs and FTIR spectra of the evolved gases from pyrolyzed OMs at temperature of the maximum evolution rate. a 3D-FTIR spectra of OA; b FTIR spectra of the evolved gases from pyrolyzed OA at 305 °C; c 3D-FTIR spectra of OTAB; d FTIR spectra of the evolved gases from pyrolyzed OTAB at 272 °C; e 3D-FTIR spectra of ODE; b FTIR spectra of the evolved gases from ODE at 164–247 °C

Full size image

For OA-Rec sample as shown in Fig. 1c, a DTG peak at 104 °C occurred below 200 °C owing to the release of the water from Rec (Figure S5 in part IV of SM). A sharp DTG peak was resolved at 276 °C in the DTG curve of OA-Rec (Fig. 1d). This result meant the TM in OA-Rec was lower than that of OA decreased by approximately 29 °C. This indicated that the thermal degradation of OA was strongly promoted by Rec. A similar phenomenon occurred in the OA–Mnt sample, the TM in OA-Mnt was 289 °C, which was 16 °C lower than the pure OA (Fig. 1d). Additionally, a smaller DTG peak was read at 435 °C, which may be due to some OM that was pyrolyzed spontaneously and was unaffected by clay minerals located in peripheral positions [41]. For the OA-Ilt sample, only one DTG peak occurred at 302 °C (Fig. 1d), indicating that Ilt had almost no effect on the TM of OA. The TM of OA in the OA-clay mixture was changed by the presence of clay minerals (Fig. 1c and d), and so was gaseous products such as CO2 (2323–2370 cm−1) and methyl gaseous hydrocarbons (Fig. 3). CO2 and methyl gaseous hydrocarbons were only occurred in OA-Rec and OA-Mnt (Fig. 3a and b). The results suggested that both C–C bond breakage and decarboxylation reactions were primary thermal reactions during the pyrolysis of OA in the presence of Rec and Mnt. The relative areas ratios of the FTIR bands at 2361 and ~ 1777 cm−1 at TM was 0.000 for OA, 0.294 for OA-Rec, and 0.802 for OA-Mnt. These relative areas ratio referred to the ratios of CO2 to carboxylic acid in their gaseous products, which implied that the decarboxylation of OA was significantly promoted by Mnt at TM.

Fig. 3
figure 3

FTIR spectra of the evolved gases from pyrolyzed OA-clay mixture samples at temperature of the maximum evolution rate. a FTIR spectra of the evolved gas from OA-Rec at 275 °C, 367 °C, and 399 °C; b FTIR spectra of the evolved gases from OA-Mnt at 289 °C, 435 °C, and 602 °C; c FTIR spectra of the evolved gases from OA-Ilt at 301–420 °C

Full size image

For OTAB, TM in OTAB alone occurred at 272 °C (Fig. 1b), while it in OTAB-Rec and OTAB-Reccom occurred at 241–428 °C, respectively (Fig. 1f). It indicated that the Rec had significant effect on the TM of OTAB. The small DTG peak of OTAB-Rec at 428 °C indicated that a small part of OTAB had intercalated into the interlayer space of Rec, which agreed with the XRD data (Figure S2). The DTG peaks at 269–342 °C in OTAB-Reccom implied that the pyrolysis of OTAB was delayed by the interlayer space of Rec. Aliphatic hydrocarbons were the mainly gaseous products that evolved from OTAB (Fig. 2b). The bands of antisymmetrical and symmetrical stretching of CH2 groups (2933–2863 cm−1), CH-stretching (2823 and 2775 cm−1), CH3-bending (1460 cm−1), and CH3-rocking (1289 cm−1) confirmed the generation of aliphatic hydrocarbons. However, for OTAB-Rec, a CO2 band (2361 cm−1) was occurred (Fig. 4a), which was due to the pyrolysis of residual OM in Rec. In addition to this, a band was observed at 1701 cm−1 (Fig. 4a, b), which was attributed to the C=C stretching vibration, indicating that there are alkenes in the products. However, the product of alkenes was almost only appeared in the case of OTAB-Reccom at higher temperature (342–428 °C) (Fig. 4b). These different products implied various reaction pathways existed during the pyrolysis of OTAB-Rec and OTAB-Reccom.

Fig. 4
figure 4

FTIR spectra of the evolved gases from pyrolyzed OM-Rec complexes at temperature of the maximum evolution rate. a FTIR spectra of the evolved gases from OTAB-Rec at 241–428 °C; b FTIR spectra of the evolved gases from OTAB-Reccom obtained at 269 °C, 342 °C, and 428 °C; c FTIR spectra of the evolved gases from ODE-Rec obtained at 233 °C

Full size image

For ODE, ODE decomposed completely 300 °C and one DTG peak observed at 247 °C (Fig. 1b). Aliphatic hydrocarbons were the main gaseous products, which showed characteristics bands at 2933, 2864, 1463 cm−1 (Fig. 2c). The antisymmetric CH3-stretching vibration (2967 cm−1) suggested the presence of methyl gaseous hydrocarbons. The TM of 233 °C were found in the case ODE-Rec (Fig. 1f), and the types of the main gaseous products were almost the same with that of the ODE alone (Fig. 4c). The relative areas of the bands at 2933 cm−1 at TM were evaluated to quantitatively compare the amounts of aliphatic hydrocarbons in the presence and absence of Rec. This relative area was 1.06/mg TOC for ODE and 2.54/mg TOC for ODE-Rec, which implied that Rec promoted the production of aliphatic hydrocarbons.

Discussion

Effect of clay mineralogy on the thermal evolution of OM-clay complexes

Previous studies demonstrated that hydrocarbon generation of OMs were largely depended on the clay mineralogy of I-Sm minerals [10, 17]. In this study, three types of I-Sm clay minerals including Rec, Mnt, and Ilt were used to evaluate the effect of clay mineralogy on the thermal evolution of OMs. The effect of Rec for pyrolyzed OA was greater than that of Mnt on that of TM. Ilt had no significant effect on the TM and gas products (Figs. 1d and 3c). The identified gaseous species (hydrocarbons and CO2) and temperature changes were analyzed (Fig. 5a and c). The result indicated that the hydrocarbons release peak temperature in OA-Mnt (440 °C) was higher than that of OA-Rec (294–376 °C), and the CO2 release peak temperature in the case of OA-Mnt (308 °C) was lower than in the case of OA-Rec (353 °C). It suggests the C–C cleavage reaction and decarboxylation of OA generally depended on the clay mineralogy. Abundant B acid sites in Rec are important factors influencing the temperature for generation of hydrocarbons [5, 11, 17]. As shown by the acidity determination, the QB in Rec (0.10 mmol/g) at room temperature was higher than that in Mnt (0.09 mmol/g). In particular, the QB in Rec was more than four times that of Mnt (Rec: 0.29 mmol/g; Mnt: 0.07 mmol/g) at the high temperature (350 °C) (Figure S4 in part III of SM).

Fig. 5
figure 5

FTIR absorbance vs. temperature curves of evolved hydrocarbons and CO2 from pyrolyzed OM-clay complexes. a the curves of hydrocarbons for OA-Rec, OA-Mnt, and OA-Ilt; b the curves of hydrocarbons for OA-Rec, OTAB-Rec, OTAB-Reccom, and ODE-Rec; c the curves of CO2 for OA-Rec and OA-Mnt

Full size image

Decarboxylation was reported to depend on L acid sites [22, 26], which mainly results from octahedral-coordinated Al3+ and/or Fe3+ ions exposed at the edges [42]. CO2 was produced during the pyrolysis of OA-Rec while not in OA alone, suggesting that decarboxylation was prevalent in pyrolyzed OA-Rec but not in pyrolyzed pure OA (Table 2). Nevertheless, the QL in Rec (0.51 mmol/g) was much higher than that in Mnt (0.07 mmol/g) (Figure S4). According to a previous study, the decarboxylation of stearic acid occurred more readily with the minerals when the more basic Mg and Li populate the octahedral sheets of the clays than by A1 and Fe ions [43]. In this study, Mnt contained some Mg–Al substitution structures compared with Rec according to FTIR analysis (Figure S3). Therefore, it can be inferred that the octahedral sheets of Mnt were more basic than that of Rec. Thus, an acid–base reaction on Mg-containing surfaces led to increased decarboxylation in the presence of Mnt. The above results seem to conclude that the degree of decarboxylation of fatty acid in OM-clay complexes still depended on the chemical composition of the octahedral sheets.

Table 2 The TM, the main pyrolysis products, and pyrolysis mechanism of OMs/OM-Rec complexes
Full size table

Interestingly, the presence of methyl gaseous hydrocarbons (a band at ~ 2966 cm−1) further manifested the effect of I-Sm clay mineralogy in the pyrolysis process of OA. There was a significant peak for methyl gaseous hydrocarbon in the case of OA-Rec, but it almost did not occur in OA and OA-Ilt (Fig. 3). It appeared that C–C bond rupture mainly occurred between alkyl C and carboxyl C for pyrolysis of OA alone or in OA-Ilt, which led to the formation of carboxylic acid and the generation of long-carbon hydrocarbons (Fig. 6a). In contrast, for pyrolysis of OA in OA-Mnt and OA-Rec, the C–C bond cleavage reaction was promoted through carbonium ion pathways by the catalysis of B acid sites, producing methyl gaseous hydrocarbons (such as CH4, C2H6, and C3H8). This result suggested that C–C bonds tend to break between alkyl C and alkyl C and away from carboxyl C during the pyrolysis of OA in OA-Rec (Fig. 6b). These results suggested that the effect of Rec and Mnt were beneficial for breaking the C–C bonds of fatty acids away from carboxyl C. This behavior was closely related to abundant B acid sites catalysis of I-Sm clay minerals.

Fig. 6
figure 6

Schematic representation of the possible processes of pyrolysis of OA and OA-clay mixture: a C–C bond cleavage reaction of pyrolyzed OA alone; b C–C bond cleavage reaction and decarboxylation of OA-clay mixture. b does not include a description of Ilt. The numerical symbols in the figure represent the locations of possible chemical bond breaks.

Full size image

Effect of OM characteristics on the thermal evolution of OM-clay complexes

The above data indicated that the thermal evolution of the OM-clay complexes was closely related to the types of OMs. Table 2 showed the TM, main pyrolysis products, and pyrolysis mechanism of OM/OM-Rec complexes. Three types of OMs (including OA, OTAB, and ODE) and one kind of clay mineral Rec were discussed. Rec had contrasting effect on the pyrolysis of these OMs. For all OMs located into the external surface of Rec, the TM of OMs was decreased by Rec. This result possibly depended on the catalytic activities of solid acid sites in clay minerals [44, 45]. B acid sites in Rec offer protons to trigger off the formation of carbocations [46, 47], which promoted the C–C bond breaking reaction, and it thus reduced the thermal temperature of the pyrolysis reaction [48]. The H3O+ ions electrostatically captured by the negatively charged basal surface were the possible B acid sites [49, 50], and also the silanol (Si–OH) and aluminol (Al–OH2OH) groups of clay minerals. Therefore, C–C cleavage by a carbocation mechanism was the pyrolysis reaction mechanism of OM-clay complexes, while C–C cleavage by a free radical mechanism was the pyrolysis reaction mechanism of OMs alone [5]. Probably for this reason, more hydrocarbons are present in the case of ODE-Rec. In addition to this, B acid sites could change the pyrolysis pathway of quaternary ammonium salt. For OTAB alone, the decomposition occurred following a SN2 nucleophilic substitution reaction. While for OTAB-Rec complexes, the pyrolysis of OTAB can be promoted by the B acid sites through a Hoffmann elimination [8, 17], which lead to some alkene production.

To further investigate the effect of OM characteristics on the pyrolysis of OM-clay complexes, the characteristic band of hydrocarbons (~ 2934 cm− 1) for OM-Rec complexes corresponded pyrolysis of carbon chain were tracked with respect to temperature (Fig. 5b). The temperatures at which the hydrocarbons released followed the order: ODE-Rec < OA-Rec < OTAB-Rec < OTAB-Reccom. The trend in the maximum hydrocarbon release temperature was ODE-Rec < OTAB-Rec < OA-Rec < OTAB-Reccom. Previous studies have found that the bond energy of C–C and C–H was higher than that of C–O and C=O [51, 52]. It thus followed that the pyrolysis of OMs in this work was generally affected by Rec, owing to the binding effects between functional groups of OMs and Rec. For OA, the –COOH group was able to interact with hydroxyl ion in Rec through ligand exchange, cation bridging, and water bridging to some extent [53]. In this case, the interaction between OA and Rec needed additional energy to overcome compared with ODE and Rec. While for OTAB, the positive charge groups ((CH3)3 N+) in OM was easily adhered to the negative charge surface site of Rec because of the electrostatic interaction [54], and it even intercalated into interlayer space of Rec through cation exchange [55]. It thus caused a higher TM and the difficulty of decomposition. It is because the electrostatic interaction and barriers effect of clay mineral sheet needed the additional energy to overcome [48]. For organic compound OTAB, however, the presence of a haloid anion (i.e., Br −) could decrease the thermal stability of quaternary alkyl ammonium-clay complexes [56, 57]. Therefore, compared with OA-Rec, the thermal evolution of OTAB-Rec was influenced, and the maximum hydrocarbon release temperature was lower than that of OA-Rec.

Conclusions

The results of our study showed that clay mineralogy had significant effects on the pyrolysis of OMs, which could lead to a decrease in the TM and a change in the thermal pathways of pyrolysis OM. I-Sm clay minerals exhibited different effects on the peak temperature of hydrocarbon and CO2 production. The thermal degradation of OA by effect of Rec was more pronounced than that by Mnt and Ilt. Rec-OA had lowest TM and Ilt-OA had a similar thermal behavior with that of neat OA. These results were owing to the solid acidity of these clay minerals. B acid sites could promote the C–C bond cleavage reaction. L acid sites were responsible for the decarboxylation of OMs. Clay minerals (Mnt and Rec) with B acidity may influence the locations of C–C bond breaks, which leading to C–C bonds tend to break away from carboxyl C during the pyrolysis of OA. In addition to the solid acidity, the functional groups of OMs affected the interaction between clay and OMs, which thus leading to a different thermal evolution behavior (i.e., gaseous products). The interlayer structures for I-Sm clay minerals appeared to act as a barrier effect on the pyrolysis of OMs with the positive charge groups (i.e., OTAB). The findings demonstrated that the thermal evolution of aliphatic OMs was affected by the types of I-Sm clay minerals and the chemical structure of organic compound. The combined influence of surface reactive sites (i.e., solid acidity) and microstructure controlled the ability of I-Sm clay minerals to thermal evolution of OM. Further experiments involving natural OMs (i.e., kerogen), other clay minerals (i.e., mixed-layer kaolinite-smectite (K-Sm) minerals), and water are required to unravel the role of OM-clay interactions in hydrocarbon generation in the future work.