Chemistry of the co-pyrolysis of an aromatic petroleum residue with a pyridine–borane complex

Carbon 41 (2003) 549–561

Chemistry of the co-pyrolysis of an aromatic petroleum residue with a pyridine–borane complex a b ´ ´ P. Carreira a , M. Martınez-Escandell , J.M. Jimenez-Mateos , a, * ´ F. Rodrıguez-Reinoso a

´ ´ , Facultad de Ciencias, Universidad de Alicante, Apartado 99, E-3080 Alicante, Spain Departamento de Quımica Inorganica b ´ de Tecnologıa ´ , REPSOL YPF. Ctra N-V, Km 18. E-28930, Mostoles ´ , Madrid, Spain Direccion Received 10 August 2002; accepted 12 October 2002

Abstract Synthesis of boron-doped mesophase by controlled co-pyrolysis of an aromatic petroleum residue and a pyridine–borane complex, PB, as a boron source, was carried out. Pyrolysis was performed at temperatures ranging from 400 to 440 8C, under 1 MPa nitrogen atmosphere. Soaking time was varied between 1.5 and 6 h, and the boron concentration in the pyrolysis mixtures ranged from 0 to 1 wt.%. The effect of the presence of boron upon solid yield, insoluble content, mesophase development and microstructure was studied. PB reacts with molecules of the petroleum residue and creates B–C bonds, B substituting C in the polyaromatic molecules, with sp 2 hybridisation, and probably also creating B crosslinking between polyaromatic units. The enhanced reactivity caused by B addition is reflected in an increment of viscosity; as a result, the development of mesophase is strongly increased, but the size of the mesophase structures is decreased to a mosaic texture; B also causes an increase of the interlayer spacing and a noticeable enhancement of insoluble material.  2002 Elsevier Science Ltd. All rights reserved. Keywords: A. Doped carbons; Mesophase; Petroleum pitch; B. Pyrolysis; C. Optical microscopy

1. Introduction Mesophase is the liquid crystal system that appears during heat treatment of graphitizable carbons. It is mainly constituted by planar aromatic molecules and can be considered a preform of the graphite structure. Mesophasebased materials can be used for the production of a great number of high performance carbon materials, such as carbon fibres [1], C / C composites [2], fine-grained sintered carbons [3,4], Li-ion battery anodes [5], etc. Addition of heteroatoms in these carbon materials can improve some properties; specifically, Si and B have been reported to increase oxidation resistance [6–8]; B promotes graphitisation [9], whereas Si hinders graphitic order [10]; B improves flexural strength and strain to failure [11] and Si increases strength and wear resistance of carbon materials [12]. On the other hand, the inclusion of boron into a *Corresponding author. Tel.: 134-96-590-3544; fax: 134-96590-3454. ´ E-mail address: [email protected] (F. Rodrıguez-Reinoso).

graphitic material increases the electrochemical properties (capacity and efficiency) when it is used as Li-ion battery anode [13,14]; likewise, the Si-doped carbons have been also studied in this application, but the effect of silicon on the electrochemical behaviour of the material is not clear [15–17]. Some common methods to obtain heteroatom doped carbonaceous materials are: (i) chemical vapour deposition [18,19]; (ii) high temperature heat treatment of mixtures of solid carbon and a precursor of the heteroatom (elemental state, oxide or carbide forms [20–23], organoboranes [24], acids [25]); (iii) infiltration of the solid carbonaceous material with molten species of the heteroatom [17,26]; (iv) pyrolysis of organic compounds or materials (precursors) containing foreign atoms (N, O, Si, etc.) [27]; and (v) chemical treatment of the carbon [27]. During recent years, some groups have reported the synthesis of doped carbons via co-pyrolysis of mixtures of pitch and organic soluble heteroatom precursors [28–30]. This is an advantageous method because it can be performed in a conventional pyrolysis reactor and the re-

0008-6223 / 02 / $ – see front matter  2002 Elsevier Science Ltd. All rights reserved. PII: S0008-6223( 02 )00365-2

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sulting solids have an homogeneous distribution of the heteroatom, which is intimately bonded to the carbon matrix. In a previous paper [31], we described the synthesis of a silicon doped material via co-pyrolysis of an aromatic petroleum residue and triphenylsilane as a silicon source. The present study employs a pyridine–borane complex as a boron source to obtain boron doped materials through a similar procedure. Unlike silicon, boron can substitute carbon atoms in the planar molecules of mesophase because it can form three planar bonds through sp 2 hybrid orbitals [32]. In fact, preceding studies on the pyrolysis of the pyridine–borane complex concluded that boron [28,33] and nitrogen [33] remain substituted in the graphitic structures of the resulting solid. In this work, the chemical behaviour of the boron compound in the polyaromatic system is studied, as well as its effects upon the stacking of macromolecules during mesophase formation and molecular growth.

2. Experimental

2.1. Pyrolysis An aromatic petroleum residue, R1, previously described [31] was mixed with the pyridine–borane complex (PB) diluted in pyridine (BH 3 concentration |8 M), to give mixtures with boron contents of 0, 0.4 and 1 wt.% B. Table 1 shows some physical properties of PB. Pyrolysis was carried out in a laboratory-sized pilot plant as previously described [34], with 350 g of the mixture, at temperatures of 400, 420 and 440 8C, soak time from 1.5 to 6 h and a pressure of 1.0 MPa (supplied by nitrogen). Release of pressure from 1.0 to 0.1 MPa occurred only at the end of a pyrolysis experiment, at reaction temperature, causing a second distillation.

2.2. Solid yield Solid yield is calculated on the basis of the mixture, that is the mass of the residue R1 (m R1 ) and the PB mass (m PB ). The applied formula is: ms Ys (%) 5 ]]] ? 100 m R1 1 m PB

Table 1 Physical properties of PB complex MP (8C)

B.P. (8C)

Molecular weight (u)

11

Decomposes

93

where Ys is the solid yield, and m s is the mass of the solid remaining in the reactor.

2.3. Analysis of liquids 2.3.1. 1 H-NMR A spectrometer Brucker, AC250 series, 300 MFIz was used. Spectra were obtained from solutions of 0.4 g of liquid distillates in 1 g of DCCl 3 . TMS was used as the internal standard. 2.3.2. ICP-ES ( inductively coupled plasma emission spectrophotometer) The amount of B in the liquid distillates was obtained according to the ASTM D 5185 method; samples were previously dissolved in kerosene. 2.4. Solid analysis 2.4.1. Boron content The percentage of B was determined by ashing 2-g samples at 1200 8C for 3 h, in a dry air atmosphere. A residue of B 2 O 3 remains after complete burning the carbon. Results are expressed as wt.% of B referred to the original carbon. Additionally, boron yield, YB , is calculated according to %B ? m s YB (%) 5 ]] ? 100 m B,O where %B is the percentage of B in the pyrolysis solid, m s is the mass of the pyrolysis solid remaining in the reactor, and m B,O is the initial amount of B added to the feedstock.

2.4.2. Nitrogen content The amount of nitrogen in the pyrolysis solids was determined by elemental analysis using a Carlo Erba, CHNS-O EA1108 analyser with phenantrene as standard. 2.4.3. Optical microscopy Samples were mounted in a resin block and optically polished surfaces were examined by reflected polarised light. Plastic solids from the milder pyrolysis conditions could not be observed by optical microscopy. The percentage content of anisotropic phases (mesophase spheres and domains) was measured by analysing 25 fields of the solids, a total of 2500 points being counted. 2.4.4. Solvent sequential extraction The pyrolysis solids were separated in fractions of different solubility. Assuming that solubility is a function of molecular size, the sequential extraction can be used as a measure of the progress of the chemical transformations leading to molecular growth. Three solvents were used: heptane (H), toluene (T) and N-methyl-2-pyrrolidinone (NMP), and each solid was separated in three soluble

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fractions (HS, TS, NMPS) and one insoluble fraction (NMPI). The experimental procedure is as previously described [31]. The boron content of the soluble and insoluble fractions was determined following the same method used for the pyrolysis solids.

2.4.5. X-ray photoelectron spectroscopy ( XPS) The analyses were carried out with a V.G. Scientific Microtech Multilab using 1253.6 eV MgKa radiation. The binding energy scale was calibrated by assigning the C(1 s) peak at 284.6 eV. Experimental peaks, recorded with a constant pass energy of 50 eV were decomposed into theoretical bands (70% Gaussian and 30% Lorentzian). 2.4.6. X-ray diffraction XRD patterns of the powders (particle size below 63 mm) were obtained in a Seifert FSO 2002 Debye Flex system, using the Bragg–Brentano geometry and the CuKa radiation operating at 35 mA and 42 kV. The coherent height of the stack, LC , was calculated from the broadening of the Gaussian profiles of the (002) peak, using the Scherrer equation with a shape factor of 0.94 [35].

3. Results and discussion

3.1. Solid yield Fig. 1 shows the solid yield versus soak time for several B concentrations and pyrolysis temperatures. Solid yield is not affected much by soak time, thus indicating that the production of low-molecular-weight cracking products is scarce as this petroleum residue is mainly aromatic. The decrease of solid yield with increasing temperature mainly takes place in the heating-up stage, indicating that differences originating with increasing temperature are due to distillation of light material. When analysing the influence of the heteroatom it was observed that the yield is similar in the pyrolysis of the undoped residue and mixtures with 1 wt.% B, but the pyrolysis of mixtures with 0.4 wt.% B gives a slightly lower solid yield, more noticeable at the higher pyrolysis temperature. This result seems to indicate that PB addition induces two opposite effects on the evolution of the solid yield: (i) an increase in the amount of material distilled during heating up (by partial distillation of PB or pyridine), thus decreasing the solid yield; and (ii) an enhancement of the molecular growth which leads to a decrease in the amount of material distilled during the final high temperature depressurisation, and to an increase in the solid yield. Therefore, according to this hypothesis, the first of these effects would predominate at 0.4 wt.% B, but the distillation losses would be compensated by an enhanced molecular growth when using 1 wt.% B. In order to corroborate this hypothesis, pyrolysis experi-

Fig. 1. Solid yield against soak time, for different B concentrations in the mixture and temperatures of: (a) 400 8C; (b) 420 8C; (c) 440 8C.

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ments increasing the concentration of boron were tried, but when the concentration of this element is .1 wt.%, using the current experimental design, the resulting material was very pyroforic, decomposing and burning just when it was exposed to air. The liquid fraction was characterised in order to determine the nature of the distillates and the contribution of PB. The results are described in the next section.

3.2. Analysis of the liquid fraction Fig. 2 shows the 1 H-NMR spectra of the liquids obtained at 400 8C, 3 h and different B concentrations, as well as the spectrum of the pure PB complex. The PB spectrum presents four bands between 2 and 3.5 ppm, corresponding to the hydrogen of the borane (BH 3 ), the chemical shift being independent of the pyridine–BH 3 interaction. However, the hydrogen atoms of pyridine provide different signals depending whether the molecule of pyridine is or not complexed to BH 3 . The relative areas of the peaks due to the free pyridine and those due to the complex in the spectrum of BP (Fig. 2a) indicate that most of the pyridine of BP is complexed to BH 3 . The spectra of the liquid distillates from the feedstock doped with PB (Fig. 2b and c) show an increment of the relative intensity of the peak at 7.1 ppm with increasing B concentration, thus indicating a contribution of free pyridine to the distillates. On the other hand, there is a characteristic peak at 8.6 ppm, the relative intensity of which increases as the initial B concentration is increased. This signal seems to be related in some way with the presence of pyridine, and it could correspond to either complexed pyridine or free pyridine and related organic compounds. To solve this dilemma, the B content in the liquid fraction has been quantified by ICP-ES. Table 2 shows the B percentage determined by ICP in the distillates (B ICP ) and the B percentage expected if the band observed at 8.6 ppm (Fig. 2b and c) was entirely originated by BP (B calc ). As can be observed, almost no B is incorporated into the liquid distillates, this indicating that the band observed at 8.6 ppm cannot be assigned to complexed pyridine, but to any related pyridinic organic compound. Consequently, it can be assumed that the complex PB is dissociated during pyrolysis. To determine the amount of pyridine in distillates, the area under the peak at 8.6 ppm has been integrated, assuming that this band is entirely caused by free pyridine. The calculated percentage (PYr max ) is compared in Table 3 with the theoretical maximum percentage of pyridine in the liquid fraction if all the pyridine in the feedstock is distilled (Pyr max ). The theoretical amount of pyridine distilled is also calculated (PYr HNMR ?100 / Pyr max ), and indicated in Table 3. Just a small fraction (12–22%) of the pyridine from the mixture is incorporated into the distillates.

Fig. 2. 1 H-NMR spectra of (a) PB complex; and liquids distilled in pyrolysis (400 8C, 3 h) of mixtures with (b) 1 wt.% B; (c) 0.4 wt.% B and (d) 0% B. Table 2 B concentration in distillates from pyrolysis at 400 8C, 3 h and different amounts of B in the mixture B mixture (%)

B ICP (%)

B calc (%)

0.4 1.0

0.02 0.05

0.24 0.33

P. Carreira et al. / Carbon 41 (2003) 549–561 Table 3 Pyridine percentage in distillates obtained at 400 8C, 3 h and different amounts of B in the mixture B mixture (%) 0.4 1

Pyr in distillates (%) Pyr H-RMN

Pyr max

1.6 2.3

7.4 18.8

T (8C)

t (h)

B (%) 0.4 % B

1% B

0.4% B

1% B

22 12

400

1.5 3 6

0.31 0.42 0.36

1.34 1.35 1.32

37 50 41

64 61 60

420

1.5 3 6

0.48 0.60 0.65

1.58 1.45 1.52

51 65 70

69 64 64

400

1.5 3 6

0.67 0.75 0.66

1.75 1.74 1.43

61 68 66

76 76 62

3.3.1. Nitrogen content Quantification of nitrogen in the solid fraction was carried out to determine if the pyrolysis solid shows an increase of nitrogen content due to reaction of pyridine within pyrolysis. Table 4 shows the nitrogen content obtained by elemental analysis of some pyrolysis solids, obtained under different conditions. When PB is added to the feedstock in mixtures with 1 wt.% B, the pyrolysis solids contain around 2 wt.% N, for all pyrolysis conditions studied, whereas nondoped solids have a maximum nitrogen content of around 0.2 wt.%. Some nitrogen may also be lost as volatile species. Thus, it can be concluded that an important percentage of pyridine, or at least the N from this molecule, is retained in the solid after pyrolysis. This result is opposed to that of a previous study carried out with coal-tar pitch [28], in which the authors found that distillation of pyridine was extensive. 3.3.2. Boron content Table 5 shows the percentage of B in the solids, for different pyrolysis conditions. B concentration is almost unchanged with soak time, but slightly increases with temperature. Table 5 also includes the B yield; the fact that B yield does not decrease with temperature indicates that the complex PB is not distilled during heating to the reaction temperature, but mainly during depressurisation. The boron complex decomposes and participates in the pyrolysis reactions, leading to boron-containing molecules of a large molecular weight, thus preventing them from Table 4 Nitrogen content (%) of pyrolysis solids T (8C)

t (h)

B mixture (%)

N (%)

400

1.5

1

1.97

420

1.5

0 1 0 1

0.22 2.00 0.16 2.08

0 1

0.18 1.91

440

1.5

Table 5 B concentration and yield, for different soak time and temperature, and initial amounts of B in the mixture

Pyr H-RMN ?100 / Pyr max (%)

3.3. Analysis of the solid fraction

6

553

B yield (%)

being distilled. This becomes more evident as soaking time increases. B yield is in all cases less than 100%; as almost no B is accumulated in the liquid fraction (Table 2), the B that escapes from the solid fraction should be lost in the gases, maybe as B 2 H 6 or other volatile boranes.

3.3.3. Optical microscopy Fig. 3 shows the amount of anisotropic structures (mesophase) determined by optical microscopy against soak time, for different temperatures and initial amounts of B added (as PB) to the residue. Nondoped solids from pyrolysis at 400 and 420 8C, could not be polished due to their plasticity, and only the value for the solid at 6 h and 420 8C could be measured. Soaking time and temperature favour the formation of mesophase, as expected. Addition of B strongly enhances development of mesophase in the solids, to a larger extent as its concentration is increased. However, the size of the anisotropic structures decreases as the B concentrations increases (Fig. 4). The spheres do no grow, because when they get in contact they have difficulties in coalescing and remain as a fine mosaic. This indicates a high viscosity in the B-doped systems, both in the anisotropic and isotropic phases. The increase in viscosity can be explained if it is assumed that B increases reactivity and enhances molecular growth. In this way, molecules rapidly reach a molecular weight high enough to stack by Van der Waals forces and they form mesophase. Because of the high rate of the process, the stacking does not occur orderly and the macromolecules are not totally planar; as a consequence, coalescence and incorporation of new molecules is more difficult than in a nondoped system. For a given amount of B, the increase in the temperature leads to a larger amount of anisotropic structures (mesophase), and to a slightly larger size of the structures, which maybe due to: (i) increase of nucleation, as the B reactivity is increased; and (ii) limited growth of the existing spheres

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by incorporation of molecules from the isotropic phase. Although not shown in Fig. 4, increasing soak time has a similar effect. Optical microscope resolution does not detect structures with a size smaller than 1 mm, so it cannot detect molecular transformations implying structures under this size. Consequently, additional techniques are employed, such as insolubles development and infrared spectroscopy, as described below.

3.3.4. Sequential solvent extraction Fig. 5 shows the development of the soluble (HS, TS, NMPS) and insoluble (NMPI) fractions with carbonisation soaking time, for different temperatures and initial amounts of B of 0 and 1 wt.% B. In all cases, it is observed that increasing soak time enhances molecular growth and the lighter fractions (HS, TS, NMPS) evolve to the heavier one (NMPI). Increasing temperature has a similar effect, and strongly favours the conversion to NMPI. Addition of B also noticeably enhances the insolubility, and even at short times (1.5 h) the solid from the feedstock with 1 wt.%B has developed a great amount of NMPS molecules, whereas the non doped solid is mainly constituted by the lighter HS molecules. The influence of B as promoter of the formation of insoluble material could be due to: (i) acceleration of the molecular growth; and (ii) development of structures more insoluble than those in the nondoped solids. Probably, both factors should be considered. As mentioned above, B enhances molecular growth and molecules can suffer a premature stacking by Van der Waals forces; as a consequence, light molecules and molecules containing aliphatic chains, can be trapped in the mesophase structure, and they can be beyond the reach of the solvent, thus contributing to the insoluble fraction. On the other hand, it is interesting to learn about the evolution of B during the molecular growth processes. Fig. 6 shows the B yield for each fraction (percentage of B in each fraction with respect to the amount of B in the pyrolysis solid) against soak time for the three temperatures studied. In spite of the fact that the HS and TS fraction of the solid obtained at 400 8C–1.5 h–1% PB constitutes more than 40% of the solid (Fig. 5b), there is almost no B in these fractions. Even under these mild conditions, B is mainly in the NMPS fraction and it progressively incorporates into the NMPI fraction as the temperature and time are increased. At 440 8C and 6 h, the whole of the B in the solid is in the NMPI fraction. Under these conditions, the amount of mesophase is close to 100%, meaning that B containing molecules must be forming the mesophase structure.

Fig. 3. Amount of mesophase determined by optical microscopy versus soak time, for different B concentrations in the mixture and temperature of: (a) 400 8C; (b) 420 8C; and (c) 440 8C.

3.3.5. XPS XPS analysis was carried out to determine the nature of the interaction between B in the PB complex and molecules of the residue. Fig. 7 shows B(1s), C(1s), and N(1s) spectra of a boron-doped solid. The binding energies in the

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Fig. 4. Optical micrographs of the pyrolysis solids obtained at 6 h soak time and different temperatures, with initial amounts of B of 0, 0.4 and 1.0 wt.%B.

B(1s) spectrum (Fig. 7a), around 189.5 and 191.2 eV, are characteristic of the B–C bonding in a partially oxidised trigonal geometry, BC 2 O and BCO 2 , respectively [18,19]. These B–C bonds were not present in the PB complex, but they were formed during pyrolysis. Boron has a strong affinity for oxygen, and B–O bonds are probably formed during pyrolysis, with residual oxygen from the petroleum residue, or at room temperature when the solid is in contact with the air. The C(1s) spectrum (Fig. 7b) shows a main peak at around 284 eV, that is usually assigned to C in graphitic structures [18] but in this case it may correspond to C in

polyaromatic molecules constituting mesophase, and a slow intensity peak at a higher binding energy of 286 eV, due to C in higher oxidation state. Deconvolution of the N(1s) spectrum (Fig. 7c) leads to a band centred at 399.6 eV, which can be assigned to ‘pyridinic’ nitrogen (N atoms contributing to the p-band with one electron in extended polyaromatic molecules) [36]. Nitrogen in this solid comes from pyridine in PB complex, as the petroleum residue used has a very low content of this heteroatom (Table 4). The binding energy of the second deconvolved peak is 402 eV; this high binding energy has been traditionally assigned to quater-

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Fig. 5. Evolution of HS, TS, NMPS and NMPI with soak time, for different pyrolysis temperatures and amount of B in the mixture of 0 and 1 wt.%B.

nary nitrogen (R 4 N 1 ; BE¯401.5 eV [36]) or oxidised nitrogen (–NO; BE 403.3 eV [36]), but it has been experimentally [36] and theoretically [37] demonstrated that the most correct assignation of this high BE is to highly coordinated N atoms substituting inner C atoms within the ‘graphene layers’, contributing with two electrons to the p-band (‘graphitic’ nitrogen). In consequence, it seems that the N furnished by the pyridine–PB complex to the carbonised material is incorporating to the mesophase structure, either on the edges of the lamellae (BE 399.6 eV) or just within them (BE 402 eV).

3.3.6. XRD The effect of B on the molecular structure was studied by XRD. Fig. 8 shows the interlayer spacing, d 002 , against soak time for different temperatures and B concentration.

Data corresponding to nondoped solids at 400 8C are not included because they are too plastic to prepare them properly for XRD analysis. As the amount of B is increased, d 002 is also increased, specially with 1 wt.%B. The parameter d 002 decreases with temperature and time, the tendency being less noticeable as the B concentration is increased. As has been deduced before, the presence of B inhibits stacking of planar molecules because of the strong induced reactivity, and this is reflected in an increase of the d 002 . In our point of view, the reaction of boron containing molecules accelerates the formation of polyaromatic molecules of a critical size to piled up (mesogen) and form mesophase. Dealkylation reaction of these molecules does not seem to be accelerated, as denoted by the amount of gases and liquids evolved, and seems to proceed at a

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Fig. 6. Evolution of B yield in the soluble and insoluble fractions of pyrolysis solids with 1 wt.% B in the mixture and temperatures of (a) 400 8C and (b) 440 8C.

similar rate of the undoped system. The existence of a larger amount of aliphatic chains bonded to the polyaromatic molecules constituting mesophase would lead to larger values of d 002 . Fig. 9 shows the coherent height of the stack, Lc , versus soak time, for different B concentration and pyrolysis temperatures. Data at 400 8C are not included because there was a large error when determining the peak width and Lc . Under mild pyrolysis conditions, the height of the stack increases with boron content, being higher for semicokes obtained from pyrolysis mixtures with 0.4 wt.% B than with 1 wt.% B, probably due to the small d 002 of the former. The size of the stack increases with temperature and soak time, but at 440 8C the growth of Lc is hindered by the presence of boron. Consequently, in spite of Lc being higher for the doped solids at short soak time (1.5 h), the enhancement of Lc with is faster for the nondoped solids, slower for the solids obtained from

Fig. 7. XPS spectra of: (a) B(1s); (b) C (1s) and (c) N (1s) of the pyrolysis solids obtained 440 8C, 6 h, and 1 wt.% B in the mixture.

mixtures with 0.4 wt.% B, and nonexistent for the solids from mixtures with 1 wt.% B. The XRD results show that, initially, the presence of B enhances the stacking of a greater number of lamellae. But

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Fig. 9. Evolution of Lc with soak time, for the pyrolysis solids obtained at (a) 420 8C and (b) 440 8C, and different B concentration in the mixture.

the extent of the stacking is lower as B concentration increases (d 002 increases with B concentration), this fact limiting the Lc growth, which decreases with increasing boron content in the feedstock. When the pyrolysis temperature is increased, reactivity is also increased and nondoped systems increase their Lc , but the disorder in stacking in the B-doped system inhibits the Lc growth. This apparent tendency of Lc can be also interpreted as a consequence of the catalytic effect induced by boron, i.e. the presence of boron is making ‘detectable’ by XRD (although with low crystallinity) part of the material that in the case of the nondoped sample would be considered ‘amorphous’ (nondetectable by XRD).

3.4. Proposed reaction scheme

Fig. 8. Evolution of d 002 with soak time, for the pyrolysis solids obtained at: (a) 400 8C; (b) 420 8C; (c) 440 8C, and different B concentration in the mixture.

The elemental analysis and XPS of the semicokes indicate the incorporation of N-containing molecules to mesophase. Previous studies of the synthesis of polyaromatic mesophase by co-pyrolysis of a coal-tar and PB,

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360–420 8C and 1 MPa, reported extensive distillation of pyridine, as a result of the decomposition of PB at low temperature, and borane reacting with aromatics mainly via hydroboronation [28]. The results obtained in this work seem to indicate that decomposition of PB is not so extensive and distillation of pyridine is low. Amino– borane complexes, like PB, can act as catalysts in radical reactions [38]. The hydrogen atom in the borane is rapidly abstracted by a radical initiator, thus forming an amine– boryl radical, which subsequently captures H from organic molecules and increases concentration of organic radicals. Based on the above information a simple free radical scheme for the reaction of PB with residue molecules is proposed (Fig. 10) One of the major reactions in pyrolysis of aromatic hydrocarbons (like petroleum residue R1) is the thermal C–H, C–C bond cleavage to form reactive free radicals [39], as presented in Fig. 10, stage (1), X being a H or C

559

atom. Abstraction of H from PB complex by a radical is a fast step (2), and the resulting pyridine–boryl radical can easily remove a H from aromatic molecules (3), thus increasing radical concentration. The higher the amount of PB, the more radicals will be formed and the system reactivity will increase. Pyridine–boryl radicals can react with aromatic radicals and form new B–C bonds (4). Subsequent H abstraction from borane group leads to boryl radicals than can further react with aromatic radicals (5). Consequently, high-molecular-weight B-containing species are formed, R a and R b , being typical molecules of the residue. Due to their fast formation, the resultant molecules still contain a large amount of aliphatic side chains, and additionally, their planarity is restricted by the presence of boron. This molecules have difficulties to pile up, thus restricting the formation of large mesophase units, as denoted from the large d 002 and small Lc values. These polyaromatic molecules can further react to give more planar molecules. They can evolve through molecular rearrangement reactions [(6), (7) and (8)], to aromatic structures where B and N atoms are in trigonal geometry (like C). In order to simplify the scheme, simple aromatic molecules, R a 5 –C 10 H 7 and R b 5 –C 8 H 7 , have been selected [Fig. 10 (5)], but petroleum residue constituents are usually larger, less aromatic, containing a larger amount of aliphatic side chains, which restricts the planarity of the molecules. In addition to these scheme of reaction, the borane formed by decomposition of the PB complex can also react with aromatic molecules through hydroboration mechanism [28], and the effect of BH 3 from the PB complex as a Lewis acid has to be considered [28,40]. Probably, chemical behaviour of PB complex is a combination of all these reactions, and as a result it enhances the formation of macromolecules, more or less aromatic and more or less planar, where B is both substituting in planar polyaromatics and also creating boron linkage between layers.

4. Conclusions

Fig. 10. Proposed scheme for the reaction between PB complex and molecules of petroleum residue R1, based on a radical mechanism.

Addition of a B precursor such as PB complex in the pyrolysis system of an aromatic petroleum residue increases the molecular growth, through radical reactions, hydroboration and acidic catalysis. Resulting macromolecules are not completely aromatic and they include aliphatic radicals and B crosslinkings; the hasty stacking of these molecules is disordered, and some aliphatic fragments and low-molecular-weight species can be trapped in mesophase structure. As a consequence: (i) viscosity is increased and growth and coalescence processes are disrupted, thus leading to a mosaic texture; (ii) insoluble development is enhanced; (iii) microstructure is less ordered, with a high interlayer spacing and a restricted stacking of layers. B in this solids is located in the more

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insoluble molecules, substituted in planar macromolecules and probably also forming three-dimensional crosslinking.

Acknowledgements This work was supported by the BRITE-EURAM Programme (Project No. BRPRCT97-04829). PC acknowledges a grant from Conselleria de Cultura, Educacio´ y ` Ciencia de la Generalitat Valenciana. Also we acknowl´ edge Repsol Petroleo for supplying the petroleum residue.

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