Production of low-cost adsorbents with tunable surface chemistry by conjunction of hydrothermal carbonization and activation processes S. Román a,⇑ , J.M. Valente Nabais b , B. Ledesma c , J.F. González a , C. Laginhas b , M.M. Titirici d a Departamento de Física Aplicada y, Universidad de Extremadura, Avda. Elvas s/n, CP: 06006, Spain b Departamento de Química e Centro de Quimica de Évora, Universidade de Évora, Rua Romao Ramalho, 7000 Évora, Portugal c Departamento de Ingeniería mecánica, Energética y de los Materiales, Universidad de Extremadura, Avda. Elvas s/n, CP: 06006, Spain d Colloid Chemistry Department, Max Planck Institute of Colloids and Interfaces, 14424 Potsdam, Germany article info Article history: Received 13 June 2012 Received in revised form 3 August 2012 Accepted 7 August 2012 Available online 16 August 2012 Keywords: Hydrothermal carbonization Biomass Adsorption Surface chemistry abstract We report a new methodology to produce activated carbons from biomass-derived hydrothermal carbons using air and carbon dioxide. The activation step is crucial to develop porosity in the hydrothermal car- bons. Additionally different surface functionalities are also introduced on the surface of the final materi- als. Our method based on initial hydrothermal carbonization of lignocellulosic biomass (walnut shell, sunflower stem and olive stone) represents a more energy-efficient tactic as compared with the tradi- tional pyrolysis. The final yield is higher and the initial hydrothermal treatment allows a better control over the resulting porosity. The produced activated carbons show a higher porosity development when activated with carbon dioxide. The activation with air produced carbon materials with acidic surface chemistry. Ó 2012 Elsevier Inc. All rights reserved. 1. Introduction The worldwide market of activated carbons (ACs) is increasing as recent reports announce total world demand for ACs to rise by nearly 10% per year, reaching an estimated quantity near 1.36Mt in 2015 [1], which is the driving force for the research on new pre- cursors and manufacturing processes to produce ACs. Traditionally, ACs are produced by chemical or physical activa- tion processes frequently using biomass materials as precursors [2]. By means of physical activation, a material is first pyrolyzed (usually at temperatures above 500 °C) and then activated by ther- mal treatment with an activating agent (air, carbon dioxide and water steam) at high temperature (700–900 °C). In the case of chemical activation, the precursor is treated with chemicals and then pyrolyzed. The possibility of improving the energy efficiency of the process would be very interesting to decrease the production costs. Recently, the use of hydrothermal carbonization processes (HTC) to produce carbon nanomaterials was reported [3,4]. This process consists in the hydrothermal treatment of a carbohy- drate-rich precursor under soft temperature conditions (in the range 150–250 °C) under self-generated pressures and in some cases under additional pressurised conditions. The HTC process is very attractive due to its simplicity, low-cost and energy and CO 2 efficiency; it can also be classified as ‘‘green’’ since it does not in- volve organic solvents, catalysts or surfactants [5]. During HTC, the carbohydrate components from biomass are broken up and dis- solved in the water, following a complex cascade of aldol-reac- tions, cycloadditions and condensations, a carbon rich solid product is obtained. The remaining liquid phase contains some unreacted sugars and/or oligomers that can be used for a variety of practical purposes such as for example the production of bio- plastics and biofuels. This process is energetically favourable when compared to tra- ditional pyrolysis, because it uses milder conditions, does not need previous drying of the precursors and is exothermic; in fact the heat released by the hydrocarbonization reactions contributes to 1/3 of the energy needed to complete the process. Previous studies [6] have reported that chars produced from HTC of biomass, called hydrochars or hydrothermal carbons, have good self-binding properties, which is very interesting for their subsequent pelletization. Moreover, HTC processes avoid gas emis- sions and the formation of tars, which are unavoidable during pyrolysis. Finally, economic studies have shown the advantages of HTC in comparison with other carbonization treatments due, but not limited, to the needless of using gases and the high effi- ciency of the process since most of the carbon stays bounded to the carbonaceous material [7]. Recently, some studies dealing with the preparation of hydroch- ars from biomass resources have highlighted that these materials show an incipient porosity and special morphologies such as 1387-1811/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.micromeso.2012.08.006 ⇑ Corresponding author. Address: Applied Physics Department, University of Extremadura, Avda. Elvas s/n, 06071 Badajoz, Spain. Tel.: +34 924289600; fax: +34 924289601. E-mail address: sroman@unex.es (S. Román). Microporous and Mesoporous Materials 165 (2013) 127–133 Contents lists available at SciVerse ScienceDirect Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso