Production and Characterization of Carbon Molecular Sieves from Bituminous Lafia-Obi Nasarawa Coal by Pore Size Modification with Spent Engine Oil

RESEARCH ARTICLE E. Bala, O.R. Momoh, B.O. Aderemi and B.J. El-Yakubu, S. Afr. J. Chem., 2019, 72, 16–20, <http://journals.sabinet.co.za/content/journal/chem/>. 16 Production and Characterization of Carbon Molecular Sieves from Bituminous Lafia-Obi Nasarawa Coal by Pore Size Modification with Spent Engine Oil E. Bala* § , O.R. Momoh, B.O. Aderemi and B.J. El-Yakubu Department of Chemical Engineering, Ahmadu Bello University Zaria, Nigeria. Received 13 February, revised 16 October 2018, accepted 29 November 2018. ABSTRACT In this work, a carbon molecular sieve (CMS) was produced from a bituminous Lafia-Obi Nasarawa coal. An initial activated carbon (AC) sample was prepared via chemical activation from the coal, from which the CMS samples were then produced through heat treatment processes and carbon deposition from spent engine oil. Spent engine oil was pyrolyzed in order to deposit carbon at the pore mouth of coal-based AC to yield CMS. The effect of reaction temperature and holding time variation on the surface area, micropore pore volume and pore size of CMS was studied. Reaction temperature was varied at 400, 550 and 700 °C while holding time was varied at 30 and 60 min. The Brunauer–Emmett–Teller (BET) method was used to calculate the surface areas, while the Dubinin–Radushkevich (DR) and Horváth-Kawazoe (HK) methods were used to determine the micropore volumes and pore sizes of the AC and CMS, respectively. The CMS sample with the largest BET surface area (5.824 m2 g–1), DR micropore volume (0.172 cm3 g–1) and HK pore size (6.317 Å) were obtained at 700 °C reaction temperature and 60 min holding time. In addition to this, a molecular sieving ability test to separate benzene from a mixture of benzene and o-xylene in solution was carried out on the AC and CMS, with the CMS having a selective benzene percentage uptake of 61.36 %. KEYWORDS Activated carbon (AC), carbon molecular sieves (CMS), carbon deposition, reaction temperature, holding time. 1. Introduction The separation of molecules from mixtures into their various components constitutes a significant cost in industry from an energy and environmental perspective.1 Some very common and important molecular separations in the chemical industry today involve the separation of air, petrochemicals and alcohols by processes requiring large amounts of energy with its attendant environmental pollution implications. Owing to this, there is an increasing interest in the use of adsorbents operating at moderate process conditions to achieve these molecular separations with reduced energy consumption, and minimal negative impact on the environment. CMS are a special type of AC able to discriminate molecules on the basis of their size and shape.2 They are microporous materials, capable of carrying out molecular separations based on the differences in the rates of adsorption of the adsorbate molecules.3 The choice of the raw material constitutes the first important step for CMS production. Generally, certain bituminous coal qualities and coked coconut shells have proven to be suitable raw materials for industrial production.4 CMS can be synthesized from various carbon-containing precursors such as lignocellulosic materials, coals, carbon fibres and pitch by different methods like: (i) pyrolysis (ii) controlled gasification of chars to increase the pore size, (iii) thermal treatment of carbon precursor to modify the pore size, (iv) chemical vapour deposition (CVD) of carbon in the mouth of the pores and, (v) modification of coals by mixing with tars and resins and subsequent carbonization.5 Another approach for narrowing the pore size is by the pyrolysis of an organic precursor previously impregnated in the AC. AlcañizMonge et al.4 analyzed this approach and prepared CMS from the co-carbonization of a bituminous coal impregnated with a * To whom correspondence should be addressed. E-mail: slurry of coal tar pitch. Bituminous coal, due to its microstructure and surface composition, has been proposed as a precursor for CMS by many researchers.4,6–7 To optimize the preparation process, the modification of the porosity (pore blocking) of the starting AC, the molecular sieving properties of the CMS obtained by this method are analyzed. A report by the World Energy Council 8 puts Nigeria’s total proven coal reserves at 344 million tons, most of which fall within the rank of bituminous or sub-bituminous. Of the total Nigerian coal deposits, the Lafia-Obi basin has an estimated 21 million tons of bituminous coal reserves.9 This makes it an attractive economically viable precursor for the production of CMS. This study investigates the effect of reaction temperature and holding time variation on the surface parameters of CMS made from bituminous Lafia-Obi Nasarawa coal using spent engine oil as a carbon deposition agent. 2. Experimental The schematic diagram of the experimental setup is shown in Fig. 1. The main component of the system is a stainless steel reactor (23.5 cm internal diameter). The reactor is designed for a batch operation and it is heated within an enclosing electronic tube furnace. Prior to the carbonization process, a proximate and elemental analysis was carried out on the coal raw material to determine its ranking and the results obtained are presented in Tables 1 and 2. All samples were characterized in terms of surface area, micropore volume and pore size by N2 adsorption at 77 K in a Quantachrome Autosorb Automated Gas Sorption System. The sample was placed inside a tube and a glass bulb was inserted inside the tube. The bulb was inserted by slanting the sample tube almost to a horizontal position. Before an experiment began, the ISSN 0379-4350 Online / ©2019 South African Chemical Institute / http://saci.co.za/journal DOI: https://doi.org/10.17159/0379-4350/2019/v72a3 RESEARCH ARTICLE E. Bala, O.R. Momoh, B.O. Aderemi and B.J. El-Yakubu, S. Afr. J. Chem., 2019, 72, 16–20, <http://journals.sabinet.co.za/content/journal/chem/>. 17 Figure 1 Experimental set-up for the production of the AC and CMS samples. adsorbents were degassed (10–4 mm Hg) at 393 K. The surface area of the samples was measured based on the BET method. The DR method was applied to calculate the micropore volume, and the pore size was investigated based on the HK method. 2.1. Preparation of Activated Carbon (AC) A size reduction of the raw coal material was carried out to a 2 mm aperture. A 100 g coal sample was then weighed and mixed with 400 g of phosphoric acid (H3PO4 85 % v/v), continuously stirred on a constant temperature magnetic stirrer at 80 °C for a period of 3 h to ensure maximum contact and mixing between the acid and sample. The resulting slurry was then placed in an oven to dry at 105 °C for a period of 24 h.10 50 g of the phosphoric acid impregnated coal was packed into a tubular stainless steel vessel (ID 24 mm, L 30 cm) and placed in an electronic furnace. The furnace was heated under nitrogen atmosphere at a flow (...truncated)