Hydrothermal and conventional H₃PO₄ activation of two natural bio-fibers

Abstract

Two phosphoric acid activation procedures; Activation after Hydrothermal Impregnation (recently published) and Activation after Incipient Wetness Impregnation instead of conventional impregnation are analyzed in two natural bio-fiber precursors: banana pseudostem and coconut fiber matting. Both procedures are compared analyzing, in both precursors, the influence that variables such as H₃PO₄/precursor ratio, activation temperature and impregnation time have on the resulting activated carbons (ACs) properties. The work also pays special attention to the mesoporosity development and the application of these ACs to adsorb gasoline vapors.

Both H₃PO₄ activation procedures develop activated carbons having suitable activation yields and porosity developments, giving the Activation after Incipient Wetness Impregnation method better results than the Activation after Hydrothermal Impregnation. Both natural bio-fibers are good precursors, rendering the coconut fiber matting better results than the banana pseudostem. The variables studied affect the porosity development, being precursor and H₃PO₄/precursor ratio the variables that most affect. By a suitable selection of these variables, activated carbons having high adsorption capacities (BET above 2500 m² g⁻¹ and micropore volume above 1.00 cm³ g⁻¹) and well developed mesoporosity (reaching 1.41 cm³ g⁻¹), can be prepared. Most of the samples prepared perform very well for adsorbing gasoline vapors, showing a linear relationship with their resulting volumes.

Introduction

The world consumption of activated carbons (ACs) is steadily increasing and new applications are emerging, particularly those concerning environmental pollution remediation. There are multiple studies related to carbon activation [1], [2] and to activated carbon production [3]. To reduce production costs, there is a continuous research on classical precursors (e.g. wood [4], and coconut shell [5], as well as on other less used agricultural by-products and wastes (e.g. olive stones [6], palm seed [7], sugarcane bagasse [8], apple pulp [9] and others [10]).

Among other activating agents, phosphoric acid activation is widely used for production of activated carbons [11] because of its advantages in comparison with other activating agents (the activation with phosphoric acid is performed at lower temperatures [12], the yields obtained are high [13], this activating agent leads to the development of mesoporosity [14] and it is non-toxic [15]). Moreover, it has been used for carrying out carbon activation studies [16], [17], [18], [19], [20]. In the latter case, the variables of the conventional phosphoric acid activation method [17], [18], [19] or new activation method concepts [20] are analyzed. Thus, in a very recent publication, an interesting new preparation method has been reported [20]. In this study, the authors mixed the carbonaceous precursor and the phosphoric acid and submitted the mixture to a hydrothermal treatment in an autoclave [20]. In other words, this Activation after Hydrothermal Impregnation method is a reasonable extension of many reported hydrothermal carbonization treatments that demonstrated their usefulness to produce carbonaceous materials [21], [22], [23], [24].

Among the variables continuously investigated in the phosphoric acid activation process such as activation temperature, activating agent concentration and atmosphere [25], [26], [27], the carbonaceous precursor is one of the most important ones. It determines the structure and properties of the resulting ACs [28]. Additionally, for getting high activation yields, the precursor should have considerable cellulose and lignin contents [29]. Among potential precursors having suitable cellulose and lignin contents, there are abundant agricultural by-products and wastes that merit to be investigated because of their abundance. Two of these cheap natural bio-products have been selected for this study (coconut fiber matting and banana pseudostem) for analyzing their phosphoric acid activations.

In El Salvador the commercial production of coconuts (Cocos nucifera) is an important agricultural activity that causes different waste materials. One of them is the coconut fiber matting (CFM) which is constituted mainly of cellulose and lignin and accounts 30 wt.% of the waste materials. Currently, there are industrial research activities with these CFM waste materials in different areas. The fibers provide low heat conductivity, strength and stiffness for composite materials (building materials) [30], [31], high impact resistance to bacteria and water [32], [33], and can be used as additives to fertilizers (because of their ability to retain moisture) [33].

In Colombia the commercial production of banana (Musa acuminate) is an important agricultural activity that also causes different waste materials which are close to 2.4 Mt y⁻¹ [34]. Although a small fraction of these waste materials has traditionally been used for composting, they have also been investigated for the production of ethanol [34] and recently a preliminary study has been reported using raw banana tree pseudostem (BPS) for activated carbon preparation [35].

The aim of this paper is to compare two phosphoric acid activation methods in two bio-fiber carbonaceous precursors (coconut fiber matting and banana pseudostem), due the large production of these wastes. For such comparative purpose, the results of both activation methods have been obtained similarly. These two activation methods, thereafter termed Activation after Hydrothermal Impregnation (AHI) and Activation after Incipient Wetness Impregnation (AIWI) are, respectively, the above mentioned hydrothermal treatment and a slightly modified conventional activation (incipient wetness impregnation instead of conventional impregnation). The influence of H₃PO₄/precursor ratio, the activation temperature and the activation time on the porosity development has been analyzed with both activation methods, paying especial attention to the mesoporosity development for adsorbing gasoline vapors.