Fullerenes, Nanotubes and Carbon Nanostructures

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To cite this Article: Mohammad Shakirullah, Imtiaz Ahmad, Mohammad Arsala. Khan, Mohammad Ishaq, Habib ur Rehman and Amjad Ali Shah , 'Study of.

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Fullerenes, Nanotubes and Carbon Nanostructures

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Study of Graphitic Crystallites in Some Carbonized Residues Prepared From Catalytic Cracking of Asphalt a

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Mohammad Shakirullah ; Imtiaz Ahmad ; Mohammad Arsala Khan ; a a a Mohammad Ishaq ; Habib ur Rehman ; Amjad Ali Shah a Department of Chemistry, University of Peshawar. N.W.F.P.. Pakistan

To cite this Article: Mohammad Shakirullah, Imtiaz Ahmad, Mohammad Arsala Khan, Mohammad Ishaq, Habib ur Rehman and Amjad Ali Shah , 'Study of Graphitic Crystallites in Some Carbonized Residues Prepared From Catalytic Cracking of Asphalt', Fullerenes, Nanotubes and Carbon Nanostructures, 14:4, 595 606 To link to this article: DOI: 10.1080/15363830600812134 URL: http://dx.doi.org/10.1080/15363830600812134

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Fullerenes, Nanotubes, and Carbon Nanostructures, 14: 595–606, 2006 Copyright # Taylor & Francis Group, LLC ISSN 1536-383X print/1536-4046 online DOI: 10.1080/15363830600812134

Study of Graphitic Crystallites in Some Carbonized Residues Prepared From Catalytic Cracking of Asphalt Mohammad Shakirullah, Imtiaz Ahmad, Mohammad Arsala Khan, Mohammad Ishaq, Habib ur Rehman, and Amjad Ali Shah Department of Chemistry, University of Peshawar, N.W.F.P., Pakistan

Abstract: This paper demonstrates the carbonization of asphalt collected from the Morgah Oil Refinery, Rawilpindi, Pakistan. The asphalt (80/90) was carbonized in a micro-autoclave under nitrogen environment at 3008C. To enhance cracking reactions of side chains, and condensation of polycyclic configurations, the sample was also loaded with catalysts such as Zeolite Socony Mobil No. 5 (ZSM-5), phosphotungstic acid, coal ash and Utmanzai clay (UTIMAC). Each carbonized residue was crushed with mortar and pestle and soxhlet extracted with n-pentane for removal of oil, dried and analyzed by X-ray diffractometery (XRD). The appearance of distinct bands correspond to crystallites in some samples particularly those loaded with UTIMAC has established graphitization. Keywords: Petroleum pitch, carbonization, graphitization, thermal, catalytic cracking

INTRODUCTION The black viscous residue rejected as asphalt during refining is enjoying popularity as a roofing and road-paving material and as a constituent of heatretardant paint. In recent years, asphalt has been considered to be a valuable commodity and is used as a feed stock for the production of some engineering materials that require superior properties such as high stiffness, high strength, high thermal conductivity and low density (1–5). High-strength materials Received 25 September 2005, Accepted 11 November 2005 Address correspondence to Mohammad Shakirullah, Department of Chemistry, University of Peshawar, N.W.F.P., 25120, Pakistan. E-mail: [email protected] 595

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prepared from petroleum asphalt has the potential for many applications (6, 7) and is therefore, in high demand. As reported elsewhere (8), the major components constituting asphalt are: asphaltenes and maltenes (resins and oils). These have been considered to be configurations comprised of saturated paraffins as both straight and branched chain hydrocarbons in addition to fused aromatic rings with aliphatic substituents. For conversion into a material of high strength, the asphaltenes play a vital role, whose carbon atoms can orient giving rise to graphite lattice configuration (9– 11). The various morphological changes occur during transformation to graphite has been underlined by many workers (12–14). Many analytical methods have been viewed for the analysis of carbonbased, high-strength materials (15 –18). XRD is the most widely used technique for the characterization of petroleum residues (19, 20). The present work is aimed at converting petroleum pitch into carbonized residue in the presence of nitrogen environment at 3008C. The effect of some catalysts on the condensation reactions is also discussed. EXPERIMENTAL Sample Collection The asphalt sample was collected from Attock Oil Refinery, Pakistan in a sealed container in order to avoid possible oxidation. The physicochemical characteristics of the sample used are provided in Table 1. HZSM-5 was purchased from the market. UTIMAC was obtained from a brick burning industry, Utmanzai, Charsadda. UTIMAC was characterized by EDX analysis. The EDX profile and the elemental composition are provided in Figure 1. Carbonization Procedure Asphalt was carbonized under nitrogen in a micro-autoclave. A 10 g portion of asphalt was placed in a Pyrex brand glass insert fitted tightly in the autoclave. The autoclave was bolted tightly and pressure-tested with nitrogen at room temperature after purging twice to remove air, then pressure-tested to 10 atm with N2. The furnace temperature was increased incrementally (heated at 58C min21) to the desired temperature in order to avoid shrinkage in the Table 1.

Properties of asphalt used

Penetration grade 80/90

Density (gm/cm3)

Carbon residue (%)

Ash (%)

Acid no. (mg KOH)

Pentane insolubles (%)

0.987

24.458

0.512

0.370

45

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Graphitic Crystallites in Some Carbonized Residues

Figure 1.

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EDX profile and elemental composition of UTIMAC.

product as a result of catastrophic collapse, held there for 1 hour, and cooled to room temperature overnight (18 hrs). The gaseous products were allowed to vent at room temperature. The residual asphalt, along with distillates, was collected and the cake (carbonized asphalt) was subsequently soxhletly extracted with n-pentane until clearance in the thimble compartment was achieved. The carbonized residue from the thimble was dried in a vacuum oven at 708C until constant weight, crushed using a pestle and mortar and used for XRD analysis. The yield was calculated as: Yield ð%Þ ¼ ½Weight of carbonized residue=Weight of the asphalt  100 High Temperature Carbonization The carbonized residues obtained from virgin asphalt and UTIMAC loaded asphalt were further carbonized at 10008C. XRD Analysis Powdered samples were analyzed on a JEOL X-Ray Diffractometer, JDX-3532, equipped with a divergence slit and scintillation counter with a preamplifier (photomultiplier tube). Each of the powder samples was placed in a silica sample holder, leveled and placed in the diffractometer. The conditions used were: tube voltage, 40 kV; count time, 0.3 seconds; tube current, 20 mA; step angle, 0.020 deg; measurement axis, 2u-u; target name, Cu; diverging slit, 1 degree; receiving slit, 0.2 mm; scattering slit, 1 degree. RESULTS AND DISCUSSION The removal of the substituents and the condensation of small aromatics into multi-ring systems helped in the transformation of asphalt into a material of high strength. This can be accomplished by severe heating of the feed material in an inert atmosphere. In order to convert the material under

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study, the carbonization was done in a micro-autoclave. To determine the effectiveness of carbonization and to evaluate its transformation to graphitic crystallites, the residues were analyzed by XRD. The XRD profile of the virgin asphalt sample is provided in Figure 2. Sharp and narrow bands corresponding to aliphatic and aromatic hydrocarbons can be seen in region 24 and 25 2u angle. This peak is originated due to the scattering of X-rays by the giant asphaltene molecules, which confirms the aromaticity. The material can be suitable for onward graphitization into useful products. The XRD profile after carbonizing these samples at 3008C for a time duration of 1 hour, features a band at 25.2 (Figure 3). This narrow distinct band corresponds to graphitized carbon (8). The presence of this band evidences the graphitic ordering. The broader the band, the poorer the crystallinity. The graphitization is a consequence of the thermal cracking of the aromatic lamellae adjoined by cleavable ethylene, propylene or butylenes linkages as well as aliphatic substituents. Thermal cracking of these linkages leads to the formation of free radicals. The resultant aromatic-free radicals react together to give aryl-aryl linkages, building up the polyaromatic molecules to form a carbonaceous mesophase. In order to view the effect of some foreign catalysts, the materials were loaded with HZSM-5, phosphotungstic acid, coal ash and UTIMAC. The charges were then carbonized. The XRD patterns are displayed in Figures 4– 9. The profiles feature a very intense band centered at 25 –26 2u angle particularly in case of UTIMAC. This reflects a larger degree of graphitization

Figure 2. XRD profile of asphalt (Virgin).

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Figure 3. XRD profile of asphalt carbonized at 3008C.

Figure 4.

XRD profile of asphalt loaded with HZSM-5 catalyst carbonized at 3008C.

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Figure 5. at 3008C.

Figure 6.

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XRD profile of asphalt loaded with phosphotungstic acid carbonized

XRD profile of asphalt loaded with coal ash and carbonized at 3008C.

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Figure 7. XRD profile of asphalt loaded with UTICAM (1%) and carbonized at 3008C.

Figure 8. at 3008C.

XRD profile of asphalt loaded with UTICAM (2.5%) and carbonized

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Figure 9. at 3008C.

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XRD profile of asphalt loaded with UTICAM (20%) and carbonized

and aromaticity due to the stimulation of aromatic ring condensation and favoring other reactions like hydrogenation, reforming, alkylation, fragmentation, dehydrogenation and dehydrocyclization leading to the formation of aromatic ring clusters. These reactions are helpful in turning the material under trial into graphitic crystallites. The mechanism in the case of catalysts is considered to be carbonium ion formation and hydride ion abstraction followed by dehydrogenative cyclization. This is due to the protonic sites that exist in these types of catalyst. The utilization of ZSM-type catalysts in aromatization of hydrocarbons is well established (21, 22). Phosphotungstic acid favors heterolytic dissociation leading to the formation of hydride ions (23). In this way, aromaticity is achieved and the material is graphitized. Coal ash, which is enjoying popularity as a cracking catalyst for carbonaceous material (24) was used next to view its effectiveness in graphitization of the sample under study. The existence of a pronounced reflection band in the XRD pattern (Figure 6) establishes its usefulness as a catalyst for aromatization/graphitization. Clays have been the focus of many workers with a view to use them as cracking catalysts (25 –28). The catalytic activity of clays is ascribed to acidic sites due to SiO2/Al2O3. To study the effect of the UTIMAC in more detail, the concentration of the clay was varied. The corresponding profiles are displayed in Figures 7 –9. It is evident from the patterns that by using 20% UTIMAC, the material attained an aromatic character and caused formation of crystallites as confirmed from the generation of some bands centered in the region 25– 26 ¼2u. The XRD profile displayed in

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Figure 10.

Figure 11.

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XRD profile of virgin UTIMAC.

XRD profile of virgin asphalt carbonized at 10008C.

Figure 9 was also compared with the XRD profile of virgin UTIMAC shown in Figure 10. In the case of Figure 9, shouldering in the region 25– 26 ¼2u is quite significant which shows that UTIMAC caused the formation of some crystallites. As it is well established that graphitization is favored at high temperatures, the temperature was increased to 10008C. The XRD profile of the virgin asphalt carbonized at 10008C is shown in Figure 11 and the XRD

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Figure 12. XRD profile of asphalt loaded with UTICAM (20%) and Carbonized at 10008C

profile of asphalt loaded with UTICAM (20%) and carbonized at 10008C is shown in Figure 12. The high temperature caused some changes in both the profiles and shoulders in the region 25– 26 ¼ 2u that were quite significant. This evidently causes more crystallite formation. Thus, if catalysts are employed, crystallites can be obtained even at the low temperature of 3008C, if the inert atmosphere is maintained; for effective and meaningful crystallites formation, however, temperatures must be high. CONCLUSIONS It is concluded from the study that all the samples used are graphitizable even at low temperatures. The presence of inert atmosphere is a must to avoid retrogressive reactions. The presence of all catalysts, particularly the UTIMAC, proved very effective in crystallites formation. The concentration of the catalyst was found very promising and high loadings caused graphitization in the case of UTIMAC. The increase in temperature was also effective in terms of the considerable crystallization. REFERENCES 1. Vix-Guterl, C., Saadallah, C., Vidal, L., Reda, M., Parmentier, J., and Patarin, J. (2003) Template synthesis of a new type of ordered carbon structure from pitch. Mater. Chem., 13 (10): 2535– 2539.

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