Stability of functionalized activated carbon in hot liquid water
Introduction
Activated carbons are comparatively inexpensive materials with tunable textural properties, and they can be prepared in a variety of forms including pellets, powders, extrudates, and cloths. The carbon structure is composed primarily of disordered graphene sheets oriented parallel to the particle surface [1], [2]. Edge sites of these sheets are typically saturated with inert CH moieties, but they can also bear various oxygen containing functional groups as depicted in Fig. 1. Carboxylic acid, lactone, and phenol groups give rise to the pronounced acidity of the material. These groups possess different acidic strengths, with carboxylic acid groups being the most acidic and phenolic groups the least acidic [2].
Activated carbons can also contain basic functional groups. It has been suggested that pyrone-like surface sites (Fig. 1b) account for the basicity of activated carbons. In addition, activated carbon may contain oxygen-free carbon sites located at π-electron rich regions of the basal planes of activated carbons, which can absorb protons from solution and, thus, impact the basicity of the carbon [3], [4].
The surface chemical properties of carbons such as hydrophobicity [5], [6] and acidity [7], [8] can be modified by treating the materials with different chemical agents. The modification methods include the reactions of activated carbons with nitric [5], [6], [8], sulfuric or phosphoric acid [5], [9], [10], hydrogen peroxide [8], [11], sodium hydroxide [5], ammonia [12], and hot air [11], [53]. A number of studies devoted to the application of such modified materials in the areas of gas separation [5], [13], water purification [14], [15], heterogeneous catalysis [10], [16], [17], [18], and electrochemistry [19], [20] were reported. Catalytic materials based on activated carbons are particularly attractive for applications in the field of biomass conversion, such as the conversion of cellulose, due to their structural integrity in aqueous environments with acidic or basic pH, among other properties.
Cellulose is one of the major constituents of lignocellulosic biomass. It is composed of glucose linked by β-1,4-glycosidic bonds. Hydrolysis of the cellulose is an essential step in its valorization, but the robustness of the cellulose structure is a challenge for the transformation [21], [22], [23], [24], [25]. Enzymatic hydrolysis of cellulose selectively produces glucose, but suffers from slow reaction rates [26]. Alternatively, more active mineral acids (e.g. sulfuric acid) have been used as catalysts [27], [28]. Solid acid catalysts are more attractive for this process due to the improved recyclability and easier separation from the reaction mixture, which provides improved control of the contact time. Recently, acid-modified activated carbon catalysts were proposed to be effective catalysts in cellulose transformation [29], [30], [31], [32], [33]. In addition to the adjustable acid-base properties and high surface area of some of these materials, the low price of carbon materials contributes to their attractiveness compared to other solid acids applied for biomass conversion in water, such as zeolites, metal oxides, cation-exchange resins, supported metal catalysts and heteropoly-acids [34], [35], [36], [37], [38], [39]. Activated carbon catalysts can be also utilized for wet peroxide oxidation [17], or as solid supports for metal catalysts for various reactions including aqueous-phase biomass transformations [40], [41], [42], [43].
Many biomass-derived compounds have considerable solubility in water. Therefore, water is an effective and also “green” medium for biomass conversion. However, it has to be taken into account that the aqueous-phase environment poses new challenges for process development, such as the stability of solid catalysts [9], [44], [45], [46], [47], [48], [49]. Onda et al. investigated the hydrolysis of ball-milled cellulose over an acid-modified activated carbon catalyst and reported leaching of 33% of the sulfonic surface groups under hydrothermal conditions [29]. However, data on the stability of surface groups at different temperatures and times on stream remains limited. High activity and stability of sulfonated carbon in cellobiose hydrolysis and oleic acid esterification was reported by Nakajima et al. [32], but it is unclear whether leaching of some of the functional groups occurred, and the reaction was catalyzed by the remaining surface groups.
Surface functional groups have also significant influence on the performance of carbon as a catalyst support. It is known that carbon surface groups act as binding sites for highly dispersed supported metal particles, while the removal of surface groups results in sintering of the supported metal particles [50]. Ketchie et al. utilized in-situ X-ray absorption spectroscopy to show that platinum particles on activated carbon sinter upon exposure to typical aqueous-phase biomass conversion conditions, and attributed this observation to the removal of oxygen-containing functionalities [44].
Instability of carbon-based catalysts in aqueous-phase reactions has also been related to their structural changes. For example, during lignin gasification in supercritical water at 400 °C, the catalytic activity of Ru/C decreased in sequential reaction runs due to a significant decrease of the surface area [51]. In the wet oxidation of phenol, an activated carbon catalyst was found to experience a significant reduction of its micropore volume upon repeated exposure to hydrothermal reaction conditions (160 °C, 16 bar) [52].
The stability of functional groups and structural integrity of carbons are essential for their catalytic activity. There have been a plethora of publications reporting on the effectiveness of acid-modification techniques on changing the surface chemistry of activated carbons [5], [6], [8], [10], [11], [12], [17], [53], [54], but the stability of these imparted functionalities under biomass conversion conditions was not comprehensively investigated. However, understanding their stability is imperative for understanding the performance of these catalysts and for developing appropriate preparation techniques.
This work provides a thorough investigation of the stability of modified activated carbon catalysts under hydrothermal conditions that are representative of cellulose hydrolysis and wet oxidation reactions reported elsewhere [52], [55]. Activated carbons are modified by using nitric and sulfuric acid, hydrogen peroxide and by calcination (at 300 and 400 °C) in order to incorporate surface functionalities. The hydrothermal stability of carbon surface functional groups at different temperatures and times of exposure is investigated in detail.
Section snippets
Materials
Activated carbon was purchased from Sigma Aldrich (C2889) with particle sizes of 8–20 mesh. Nitric acid (70%, 438073), hydrochloric acid (37%, 258148), and sulfuric acid (95–98%, 258105), sodium hydroxide (>97%, 221465), sodium carbonate (>99.9999%, 71347), cellobiose (>98%, C7252) and sodium bicarbonate (99.7–100.3%, S6014) were purchased from Sigma Aldrich. Hydrogen peroxide (35%, BDH8814-3) was purchased from BDH Chemicals.
Acidification procedures
Activated carbon was modified using liquid or gas phase methods.
Structural changes of activated carbons
The Langmuir surface areas of activated carbon catalysts were in the range of 900–1100 m2/g (Table 1). The surface area of the activated carbon increased slightly after modification with acids (AC-H2SO4, AC-HNO3) and calcination in air at 400 °C (AC-Air-400). Calcination in air at 400 °C resulted in the greatest surface area increase (16%) compared to the parent carbon material. The hydrothermal treatment of all modified materials resulted in an additional increase of the surface area by 1–6%.
The
Conclusions
The concentration of acid sites in activated carbon can be increased by treatments with sulfuric acid, nitric acid, and hydrogen peroxide. The sulfuric acid and nitric acid modified carbons are the most acidic catalysts. Sulfuric acid treated carbon had strong acid sites in the form of sulfuric and carboxylic acid groups, whereas the strong acid sites in the other carbon samples are carboxylic acids. Treatment of carbon by air at 400 °C results in the formation of base sites, which could be
Acknowledgements
Financial support from the Brook Byers Institute for Sustainable Systems is grateful acknowledged. The authors would like to thank the Institute of Paper Science and Technology for the use of their facilities. The authors would like to acknowledge John R. Copeland, John C. Crittenden for fruitful discussions and Benjamin Sauk, Luis Rendon, and Daniel Krötschel for experimental support.
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