Catalyzing mesophase formation by transition metals
Introduction
In some petroleum refining processes aromatic rich streams are generated as by-product. clarified oil (CLO) and heavy extract (HE) are such by-products which are produced during fluidized catalytic cracking (FCC) process and Lube Base Oil refining process respectively. These aromatic streams have high C/H ratio and have potential for making industrial carbon materials like mesophase pitches and premium quality petroleum coke. It is well known that carbonaceous mesophase pitch has been extensively used as a precursor for making a variety of industrial as well as high performance carbonaceous materials such as premium quality needle coke, graphite electrodes [1], [2], [3], carbon fibres [4], C–C composites [5], fine-grained sintered carbons [6], [7], Li-ion battery anodes [8], mesocarbon microbeads (MCMB) [9], [10], carbon foam [11] and plasma-facing components for fusion devices [12].
Brooks and Taylor [10] first time observed formation of carbonaceous mesophase in pitch where liquid ‘isotropic phase’ and crystalline ‘mesophase’ remained in equilibrium. Since then, mesophase formation has seen many advances and is a subject of active research for carbon scientists and engineers because different feed stocks show different mesophase formation behavior. In literature, several studies have been reported in which mesophase pitches were prepared from different feed stocks (petroleum, coal tar, naphthalene and anthracene etc.) and under different experimental conditions such as thermal treatment temperature, heating rate, thermal soaking time and catalysts etc. [13].
Greinke and Singer [14] observed that during heat treatment of petroleum pitches chemical changes take place between isotropic and anisotropic phases. They also suggested that tiny mesophase spheres having round shape are formed due to polymerization of more reactive species present in the pitches. As thermal soaking time increases, the size of mesophase spheres also increases. Finally, the bigger mesophase spheres coalesce into bulk mesophase and finally converted into semicoke [10], [15].
Generally, mesophase formation in the feed stocks and isotropic pitches takes long thermal treatment time of several hours which makes the mesophase pitch process more energy – intensive. Therefore, several researchers studied the effect of various catalysts to enhance mesophase formation in pitches and reduce thermal treatment time. Braun et al. [16] studied the effect of iron benzoate and naphthoate catalysts on coal tar mesophase pitches. They observed that the nucleation and growth of mesophase spheres are strongly influenced by the catalyst. Further, mesophase spheres formed are almost of equal size having a reduced tendency to coalesce with each other. The mesophase content (MC) in the pitches was also high. Braunhauer et al. [17] reported the catalysis of mesogen formation by ferric chloride and ferrocene catalysts. They found that these catalysts are effective to promote mesophase formation in the pitches.
Song et al. [18] reported that size of mesophase spheres increases by the addition of ferrocene catalyst and some of the mesophase spheres are converted into coalesced mesophase which indicate that the formation and transformation behavior of mesophase is accelerated by the addition of ferrocene. Carreira et al. [19] reported a different finding that addition of boron compound into petroleum residue initially enhances development of mesophase in the solid but as the concentration of boron exceeds to a certain limit the size of anisotropic structures decreases. Obara et al. [20] observed that as the concentration of inert silica gel is increased, the size of the mesophase spheres decreases during carbonization of a petroleum pitch. In some studies, catalysts were found to be effective for enhancing mesophase formation but catalyst recovery from pitch was a problem. The presence of catalyst particles in pitch may pose problems of deteriorating the final carbon product quality.
In literature, several researcher taken naphthalene and anthracene as a starting material and used AlCl3 [21], [22], FeCl3 [23] and HF/BF3 [24], [25], [26] as catalysts for making mesophase pitches. Mochida et al. [21] used aluminium chloride (AlCl3) as a catalyst for making pitches which was very effective but main disadvantage of using AlCl3 is difficulty in recovering it from pitch. Moreover, it is not recyclable. Mochida et al. [24] overcome the catalyst recovery and recyclability problems by using another gas phase catalyst HF-BF3 for converting aromatic hydrocarbon feed stock into mesophase pitches. This strong Lewis acid catalyst (HF-BF3) was found very effective to get high yield of mesophase pitches (up to 90%) as well as capable of producing mesophase in pitch as high as 100%. The further advantage of using HF-BF3 catalyst is its easy recovery.
In literature, very few transition metals namely aluminium, iron have been used for catalyzing mesophase formation, therefore in the present study it was thought of using some unexplored transition metals such as cobalt and nickel for catalyzing mesophase formation in petroleum feed stocks. Further, in most of the prior research a very little or no work has been done on catalyst recovery aspect in spite of its importance in mesophase pitch application. In this work, we have also carried out work on recovery of catalyst (Co, Ni) used in preparation of pitches.
In the present work, we have prepared environmentally-benign mesophase pitches by thermal treatment of CLO and HE in presence of Co and Ni catalysts. Pitches prepared by providing different soaking time and in presence of different catalysts were examined to monitor the formation of poly-aromatics and increase of mesophase content. For detailed understanding of pitch composition mesophase pitches were characterized by various analytical techniques such as FT-IR, NMR, XRD, TG/DTG and scanning electron microscopy (SEM) combined with energy dispersive X-ray spectroscopy (EDX) and optical microscopic imaging etc.
Section snippets
Raw materials and catalysts
In this study, two petroleum feed stocks namely CLO and HE sourced from different petroleum refineries were used for preparing mesophase pitches. Physico-chemical properties of CLO and HE are given in Table 1. Transition metal salts (CoCl2·6H2O, NiCl2·6H2O) were used as catalysts. AR grade toluene and quinoline were used as solvents for determining insolubility in mesophase pitches.
Preparation of mesophase pitches
To prepare mesophase pitches 3 wt% percentage of transition metal salts (CoCl2·6H2O, NiCl2·6H2O) were added in each
Petroleum feed stocks analysis
Physico-chemical properties of both the petroleum feed stocks i.e., CLO and HE are given in Table 1. Comparison of properties of these two feed stocks showed that HE is more aromatic in nature than CLO which is evident from higher density, micro carbon residue (MCR) and bureau of mines correlation index (BMCI) values of HE. This fact is also confirmed by low pour point of HE (18 °C) which indeed indicates the presence of more aromatic hydrocarbons in HE. HE being more aromatic in nature than CLO
Conclusions
Various pitches having different mesophase content were prepared by thermal treatment of CLO and HE feed stocks in presence of catalysts Co and Ni. The Co and Ni catalysts promote mesophase formation in both the feed stocks as a result thermal treatment time to produce mesophase pitch get reduced. The size of mesophase spheres tended to be accelerated by the addition of Co and Ni catalysts due to faster ‘polymerization’ and ‘condensation’ reactions. The crystal assembly of mesophase pitches are
Acknowledgements
We kindly acknowledge the Director IIP for his kind permission to publish these results. The authors also thank to Mr. Sandeep Saran, Mr. Raghuvir Singh, Mr. Piyush Gupta, Mr. Shiva Kumar Konathala and Mr. Kamal Kumar for carrying out XRD & TGA, IR, NMR, SEM-EDS and Softening Point analysis respectively. Author SK thanks UGC for awarding fellowship.
References (33)
- et al.
The formation of mesophase microstructures during the pyrolysis of selected coker feed stocks
Carbon
(1974) - et al.
Formation scheme of needle coke from FCC-decant oil
Carbon
(1988) The chemistry of mesophase formation
(1986)- et al.
Carbon Fibres
(1998) - et al.
Chemistry of synthesis, structure, preparation and application of aromatic-derived mesophase pitch
Carbon
(2000) - et al.
Carbon disc of high density and strength prepared from synthetic pitch-derived mesocarbon microbeads
Carbon
(1999) Introduction to Carbon Technologies
Recent developments in lithium ion batteries
Mater. Sci. Eng.
(2001)- et al.
Characteristics of meso-carbon microbeads separated from pitch
Carbon
(1974) - et al.
The formation of some graphitizing carbons
High-thermal conductivity mesophase pitch-derived carbon foam
International SAMPE Symposium and Exhibition (Proceedings)
Preparation of mesophase pitch doped with TiO2 or TiC particles
J. Anal. Appl. Pyrolysis
Development of mesophase from a low-temperature coal tar pitch
Energy Fuel
Constitution of coexisting phases in mesophase pitch during heat treatment: mechanism of mesophase formation
Carbon
The formation of graphitizing carbons from the liquid phase
Carbon
Kinetics of mesophase formation in a stirred- tank reactor and properties of the products – VI. Catalysis by iron benzoate and naphthoate
Carbon
Cited by (29)
Hydrogenation of coal tar pitch for improved mesophase pitch molecular orientation and carbon fiber processing
2023, Journal of Analytical and Applied PyrolysisStudy of mesophase pitch based carbon fibers: Structural changes as a function of anisotropic content
2023, Journal of Analytical and Applied PyrolysisMesophase pitch production from fluorine-pretreated FCC decant oil
2022, FuelCitation Excerpt :In XPS data, one can see that there is no trace of remaining fluorine in the as-prepared pitches. This result indicates that fluorine pretreatment does not need any further removal of catalysts and purification of pitches, unlike other catalytic processes for mesophase production [16,44]. It seems that in case of using 20 vol% F2, which is sufficient condition for mesophase formation, there are no remaining fluorine in the resulting pitches.
Methods for modifying needle coke raw materials by introducing additives of various origin (review)
2022, FuelCitation Excerpt :In the case of organic substances, on the other hand, additives appear as differently structured radical donors or initiators, ensuring the mesophase development. A significant effect has been achieved when nickel oxide and cobalt oxide [97] where used as catalysts for the development of mesophase, which due to their properties are able to accelerate the polymerization and polycondensation reactions. As a result of these reactions, the resulting mesophase C/H ratio increases, which indirectly indicates the emergence of more condensed structures and a reduction in the proportion of long alkyl chains [50].
Accelerating the oxidative stabilization of pitch fibers and improving the physical performance of carbon fibers by modifying naphthalene-based mesophase pitch with C9 resin
2021, Journal of Analytical and Applied PyrolysisModified effect on properties of mesophase pitch prepared from various two-stage thermotreatments of FCC decant oil
2021, FuelCitation Excerpt :Hereafter, researchers [12–18] found that naphthenic structure and short alkyl group in molecules of feedstock not only suppress excessive reactions during thermal treatments, but also improve the fusibility of mesophase pitches by retaining part of these structural elements into the resulted mesogens. Hence, other modification methods such as co-carbonized process [12,13], premesophase process [14] and catalytic polymerized process [15–18], have been developed to introduce naphthenic structure and short-chain alkyl group to a raw material or mesophase pitch, achieving compromise between high mesophase content and low softening point for derived mesophase pitch. Besides, another factor, i.e., mesogenic molecule size and concentration, is worth to deserving our attention.