Development of mesoporous structure and high adsorption capacity of biomass-based activated carbon by phosphoric acid and zinc chloride activation

Abstract

This paper reports the preparation of activated carbon from two different types of agricultural biomass materials, sugar cane bagasse and sunflower seed hull, by phosphoric acid and zinc chloride activation. The experiments in this study vary the pre- and post-treatment procedures, the impregnation ratio of the activating agent, and the carbonization temperature. In recent years, the high surface area and high mesopore proportion of carbon have attracted a lot of attention for potential applications in the green resources such as hydrogen energy storage and carbon dioxide capture. However, the traditional methods for fabricating activated carbon produce a mainly microporous structure. The experimental results show that the activated carbon produced by base-leaching has a mostly mesoporous structure, which effectively enhances its adsorption capacity. The carbon materials obtained from zinc chloride activation of both sugar cane bagasse and sunflower seed hull have mesopore volumes as high as 1.07 and 0.95 cm³/g, and mesopore contents of 81.2 and 74.0%, respectively. The surface area and pore volume of carbon produced using zinc chloride activation were higher than that produced using phosphoric acid activation. The total activation process of bagasse and hull occurs in three reaction stages. This study also presents a corresponding pyrolysis mechanism that agrees well with the experimental results. The proposed method of preparing mesoporous activated carbon is not complicated, and is suitable to bulk production.

Introduction

Activated carbon is an extremely versatile material with a high surface area. Due to its excellent adsorption capability, it is widely used in industrial wastewater and gas treatment [1], [2]. The pore structure of activated carbon in conventional applications is mainly microporous. Recently, the potential use of activated carbon with a high proportion of mesoporous structures has attracted increasing attention because it possesses a high pore volume and a wide range of pore size. The mesoporous carbon materials are very suitable for use in large molecule adsorption, including battery capacitors, catalyst supports, biomedical engineering and adsorbents for bulky pollutants [3], [4], [5]. In the future, gasoline and diesel fuel for vehicles will probably be replaced for economy advantages. An important application of activated carbon is related to the gas storage for natural gas vehicles [6]. Moreover, researchers have paid significant attention to the use of carbon materials in hydrogen storage. Large pores are necessary for the fast mass transfer of adsorbate to the bulk of the carbon material [7]. Hence, the high surface area of activated carbon and its well-balanced micro/mesoporosity can provide an appropriate medium for energy storage. In particular, the CO₂ sorption of activated carbon plays an important role in reducing carbon emissions [8].

Sugar cane and sunflower are two of the most commonly cultivated plants in the world. Sugar cane bagasse is the waste material produced by sugar factories after sugar juice extraction. Sunflower seed hull is produced by oil extraction plants that manufacture bio-oil products. The production of sugar cane in Brazil, which is the largest global producer, totaled 425 million tons for the 2006/2007 season [9]. China and India are the two largest sugar cane producers in Asia. Together, these countries produced 437 million tons of sugar cane in 2008 [10]. On the other hand, global sunflower production reached 27 million tons in 2007 [11]. These two crops are abundantly available. Sugar cane and sunflower are also very good candidates for bio-ethanol and bio-diesel, which are used as alternative fuels [12], [13]. The production of sugar cane and sunflower can be anticipated to increase significantly in the future. The observations above show that sugar, oil, and ethanol manufacturing processes produce a large amount of sugar cane bagasse and sunflower seed hull wastes each year. These wastes are usually burnt in the open or left in the field, and only a small portion is used as paper pulp or fuel. This creates disposal and pollution problems. The activated carbon obtained from these two agricultural wastes can provide useful, value-added products in the sugar factories, oil refineries, and related industries. Activated carbon for commercial utilization can be obtained by thermal treatment of low-cost and low ash content materials. Sugar cane bagasse just conforms to the rule for it is a low ash content and availability. In contrast, sunflower seed hull contains a high proportion of ash content. This study compares the two carbon products obtained from these two biomass sources.

Mesoporous activated carbon can be synthesized by many methods. Traditional methods use carbon precursors such as lignocellulosic materials, coals, and phenolic resins in the presence of transition metals, followed by a combination of carbonization and physical activation with stream or carbon dioxide, to obtain mesoporous activated carbon powder [14], [15], [16]. Shen et al. [17] used commercially activated carbon produced by steam activation with cerium oxide as a catalyst to prepare mesoporous carbon, and found that the catalyst restricted the formation of mesoporous structures at temperatures of 800–870 °C. Besides, some recent studies have developed mesoporous carbon with pores that can be controlled using mesoporous ceramic templates, such as SBA or MCM series [18], [19]. Compared to the above methods, ZnCl₂ and H₃PO₄ are considered more effective and less expensive activating agents for producing mesoporous carbon [20], [21]. Hu and Srinivasan [22] produced mesoporous activated carbon using coconut shells and palm seeds as starting materials, with simultaneous activation by ZnCl₂ and CO₂. They found that at a low ZnCl₂ concentration, the main pore characteristic is microporous; whereas at a high ZnCl₂ concentration, the main pore characteristic is mesoporous. Kennedy et al. [23] studied the H₃PO₄ activation of rice husk through precarbonization and chemical activation. They found that pores created at a higher activation temperature were enlarged or widened, leading to the formation of mesoporosity. Generally, activated carbon has a mesopore volume and a mesopore surface area in the range of 0.1–0.5 cm³/g and 100–200 m²/g, respectively [22]. However, for advanced applications, carbon materials should possess not only a high surface area, but also a high mesopore content. In particular, treating sugar cane bagasse and sunflower seed hull with an adequate base-leaching procedure enlarges or widens the micropore structure of the resulting carbon during the activation process. This, in turn, increases the mesopore surface area and mesopore volume. After activation, the carbon precursor must be washed with water to remove the chemical activating agent. However, traditional water-washing cannot effectively remove all chemical residues, and may result in the reduction of pore volume. In this case, acid-washing procedure is an adequate alternative [24].

Previous studies have already reported the preparation of mesoporous activated carbon from lignocellulosic materials by physical or chemical activation. However, the mesopore surface area and mesopore volume of these materials are not high enough, and many of the proposed approaches are complicated or require expensive equipments. In addition, there is a lack of information in the literature about activated carbon prepared from sugar cane bagasse and sunflower seed hull using H₃PO₄ or ZnCl₂ in a base-leaching or acid-washing process. This lack in existing literature is a motivation for the present study. The primary objective of this study is to compare activated carbon prepared from sugar cane bagasse and sunflower seed hull through chemical activation to produce a highly mesoporous activated carbon. The study investigates the effects of base-leaching, acid-washing, the impregnation ratio of activating agent, the kind of activating agent, and the carbonization temperature on the development of pore characteristics of activated carbon. Furthermore, the reactants and products are examined by several forms of analysis, including SEM, XRD, FTIR, ICP-MS, EA, TGA and a N₂-adsorption analyzer. The method of activated carbon preparation proposed in this study is not complicated, and well suited to mass production. Simultaneously, it is an effective way to enhance the mesopore surface area and mesopore volume of activated carbon.