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Adsorption behaviors of sugars and sulfuric acid on activated porous carbon

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Abstract

To investigate the suitability of activated porous carbon as the adsorbent for separation of sugars and sulfuric acid, the relevant adsorption equilibrium data were obtained by using a staircase frontal analysis method in the range of sulfuric acid concentration (0–13.61 g/L), glucose concentration (0–5.86 g/L), and xylose concentration (0–34.96 g/L). It was found from the resultant adsorption data that sugars had considerably higher adsorption affinities to activated porous carbon than sulfuric acid, which indicates that activated porous carbon has the potential to be utilized as an economical adsorbent for separation of sugars and sulfuric acid. It was also found that the resultant adsorption data could be well predicted by the Langmuir isotherm model.

Introduction

One of the noteworthy developments in the field of renewable chemical industries was to utilize biomass such as corn stover for producing bio-ethanol and lactic acid, which have the potentials to be exploited as an alternative source of energy and a starting material of the biodegradable plastic polylactide respectively [1], [2], [3], [4], [5], [6], [7]. An important intermediate step in such production of biomass-derived chemicals is to hydrolyze the primary polysaccharide components of biomass into sugars (glucose and xylose) by using sulfuric acid as hydrolysis catalyst [1], [5], [6], [7], [8], [9]. To ensure high fermentability of the hydrolyzed sugars into the aforementioned products (bio-ethanol and lactic acid), it is essential to separate sulfuric acid from sugars before fermentation.

To address the necessity for such separation, several ion-exclusion resins from major resin-production companies have been utilized as an effective adsorbent [1], [5], [6], [7], [8], [9]. In addition, poly-4-vinyl pyridine (PVP) resin from Reilly Industries Inc. was also found to be useful for such a separation task [6], [7]. All of these polymeric adsorbents are known to have an advantage in the aspect of regeneration and re-use [8]. However, all of them are relatively on the expensive side, which are approximately in the range of $118–$2260 per kilogram on the basis of the retail price in Korea. This has sometimes caused the necessity for exploring another adsorbent that can be much more advantageous in terms of price competitiveness while maintaining its suitability for the aforementioned separation task. In regard to this issue, it is worth paying attention to a recent technical report [10], which mentioned the feasibility of using activated porous carbon as the adsorbent for separation of sulfuric acid and sugars.

It is well known that activated porous carbon possesses highly developed internal surface area and porous structure, and hence a large capacity for adsorbing chemicals from gases and liquids [11], [12]. Furthermore, its retail price in Korea is approximately in the range of $8–$9 per kilogram, which is much cheaper than other polymeric adsorbents ($118–$2260 per kilogram) [10], [11], [12]. Due to such a well-established structure and such a low cost, activated porous carbon has attracted a lot of attention as a versatile and economical adsorbent in many industries.

Therefore, it is quite worth considering the development of an adsorption separation process based on activated porous carbon that is aimed at separating sulfuric acid and sugars. One of the essential prerequisites for such a process development is to investigate the adsorption behaviors of sulfuric acid and sugars on activated porous carbon and then to obtain the relevant adsorption equilibrium data. However, such work has not been performed so far in the literature.

The goal of this study is to accomplish the aforementioned prerequisites that can serve as essential information in the stage of designing the adsorption process based on activated porous carbon for separation of sulfuric acid and sugars. For this purpose, the single-component adsorption equilibria of sulfuric acid and sugars on activated porous carbon will be measured on the basis of a staircase frontal analysis (SFA) method [13], [14], [15], which has been recognized to be highly accurate in acquiring adsorption equilibrium data. For such work, a model solution based on reagent-grade sulfuric acid and sugars will be used instead of a real hydrolyzate, and a maximum liquid-phase concentration for each compound will be set to be the same as in the actual corn-stover hydrolyzate, which was reported to contain 13.61 g/L of sulfuric acid, 5.86 g/L of glucose, and 34.96 g/L of xylose [6]. Although other concentration ranges of sulfuric acid and sugars in biomass hydrolyzate can be found in several previous publications [16], [17], only the concentration range in the reference [6] will be taken into account in this work. In addition, other byproducts from the acid hydrolysis of corn-stover hydrolyzate, which include degradation byproducts (furfural and hydroxymethylfurfural) and several other sugar components (cellobiose, mannose, galactose, and arabinose) [6], will not be covered in this study because of the following two reasons: (1) such by-products were contained in the hydrolyzate with small amounts and (2) the separation between sulfuric acid and sugars has been a main subject in most of the related literate publications [5], [6], [7], [8], [9].

The relevant adsorption experiments for implementation of the aforementioned SFA method will be carried out at the temperature of 65 °C, which is the same as in the previously reported adsorption processes based on other adsorbents (ion-exclusion and PVP resins) for separation of sulfuric acid and sugars [1], [5], [6], [7]. Furthermore, we will suggest an appropriate adsorption model for predicting the measured equilibrium data over the investigated range of liquid-phase concentration, which is set to cover the feed concentration of the actual biomass hydrolyzate [6]. Finally, the relevant parameters of the suggested adsorption model will be determined.

Section snippets

Materials

Glucose and xylose were purchased from Sigma-Aldrich Co. (St. Louis, MO), and 1 N sulfuric acid aqueous solution was supplied from Yakuri Pure Chemicals Ltd. (Osaka, Japan). Distilled deionized water (DDW) was obtained from a Milli-Q system by Millipore (Bedford, MA) and used as an eluent in the SFA experiments performed. Activated porous carbon (CPG® LF 12 × 40) was purchased from Calgon Carbon (Pittsburgh, PA), and sieved into particles with diameter from 425 to 710 μm before its use as an

Preliminary testing of activated porous carbon as the adsorbent for separation of sugars and sulfuric acid

Before the investigation of adsorption behaviors of sugars and sulfuric acid, it was a matter of primary concern to clarify whether the porous material of our interest (i.e., activated carbon) had the potential to function as the adsorbent for removal of sulfuric acid from sugars. For this task, a preliminary pulse test was carried out, where the solution containing each component was injected through the column packed with the activated porous carbon while maintaining the injection volume at

Conclusions

The single-component adsorption equilibria of sugars and sulfuric acid on activated porous carbon were measured at the temperature of 65 °C by using a staircase frontal analysis (SFA) method. It was found that there was a marked difference between the equilibrium solid-phase concentrations of sugars and sulfuric acid i.e. between the adsorption affinities of sugars and sulfuric acid for activated porous carbon. This indicated that activated porous carbon could be sufficiently qualified as the

Acknowledgments

This research was supported by Basic Science Research Program through the National Research Foundationof Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (grant number NRF-2015R1A2A2A01003455).

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