Catalytic transformation of the marine polysaccharide ulvan into rare sugars, tartaric and succinic acids
Graphical abstract
Introduction
The species Ulva rigida is among the most common and widespread green algae. This alga is characterized by fast growth rates and low lignin content. However, except its use as food in Japan and other countries or in poultry nutrition in order to improve the egg yolk and meat coloration in Europe, U. rigida does not have at the moment other commercially important nutritional applications and, therefore, could be considered as a non-edible feedstock [1]. It is noteworthy that U. rigida is currently largely ignored as a feedstock in biorefineries. This fact is especially remarkable considering the high content of complex polysaccharides in U. rigida, including the water-soluble anionic sulfated polysaccharide ulvan, the insoluble cellulose, and, in less proportion, the alkali-soluble linear xyloglucan and glucuronan [2]. Ulvan is mainly composed of sulfated rhamnose, xylose, glucuronic and iduronic acids, but the exact composition can depend on factors such as the harvesting period, environmental and growth conditions and post-collection treatment [2,3]. The chemical structure of ulvan is composed of two different disaccharide units denoted as type A3s glucuronorhamnose and type B3s iduronorhamnose, arranged in regular sequences within the heteropolymer chain [2] (Fig. 1).
The interest in ulvan also stems from its rather abundant proportion of rare sugars that can be further valorized by their transformation in various chemicals. These rare sugars and derivatives display notable biological and physicochemical properties useful for a number of different applications [4,5]. The rare sugar rhamnose, for instance, is used in the synthesis of meat-like flavour [6] and for the treatment of gastric ulcer [2]. Ulvan can also be considered as a potential source of iduronic acid, another rare sugar found in mammalian glycosaminoglycans [7]. To date, iduronic acid is synthesized in several steps [8,9], but its isolation from ulvan could be a competitive alternative.
Due to the similarity in the bonds connecting the monosaccharides, hydrolysis of ulvan mimics that of hemicellulose. However, although the catalytic hemicellulose hydrolysis has been extensively studied [10], analogous hydrolysis of ulvan has received comparatively much less interest. Due to its abundance and chemical composition, ulvan can be a convenient source of rare sugars, such as rhamnose, as well as other common sugars, like xylose, mannose, arabinose, or galactose. In spite of this, studies in the literature dealing with the hydrolysis of ulvan are scarce. In a recent report [4], it has been shown that Amberlyst-70 can produce rhamnose from ulvan in approximately 40 % yield at 130 °C. This yield corresponds to the recovery of almost all of the sulfated rhamnose units present in ulvan.
The heterogeneous catalytic hydrolysis of biopolymers requires solid acids with several characteristics, including stability, water tolerance and high selectivity to guarantee the cost-effectiveness of the process [11]. In this context, Hilpmann et al. [12] demonstrated that at 90 °C the catalytic hydrolysis of glucuronoxylan to xylose takes place with higher yield in the presence of Smopex 101 (an ion exchange sulphonic fiber) as solid catalyst than using homogeneous mineral acids (0.01 M [H+]). However, deactivation caused by the leaching of sulfonic groups greatly affected the performance of the solid catalyst. Leaching of sulfonic acid groups grafted on organic or inorganic polymers has also been observed during the hydrolysis of polysaccharides from terrestrial plants [13]. This undesired leaching could be minimized by replacing sulfonate by triflate groups. Indeed, various triflate-based catalysts have already been reported [14,15], including also a triflate-modified graphene [16], but they have not been tested as catalysts for polysaccharide hydrolysis. Besides strong Lewis acidity, triflates possess high water tolerance [17]. The anchoring of triflate groups on the support has been achieved using different procedures, such as embedding of molecular triflates [18,19], chemical neutralization of OH groups [20,21] or simple impregnation of trifluoromethanesulfonic acid (ie, triflic acid) [[22], [23], [24]].
Based on this state of the art, the present study reports two different processes. On one hand, self-catalyzed hydrolysis of ulvan under hydrothermal conditions at 130 °C that occurs due to the abundant presence of sulfate groups in this polysaccharide is described. The results of autocatalyzed hydrolysis serves, on the other hand, as blank control to compare the activity of triflate anchored on reduced graphene oxide (GO@SO3CF3) as catalyst to promote hydrolysis and transformation of ulvan. Formation of significant amounts of possible platform chemicals in biomass transformations, namely tartaric and succinic acids, when ulvan is treated with GO@SO3CF3 either in the absence and presence of oxygen is herein reported. The synthesis of platform molecules from renewable marine raw materials represents an attractive alternative to the use of polysaccharides from terrestrial plants.
Section snippets
Experimental
All the chemicals and reagents were of analytical purity grade, purchased from Sigma-Aldrich and used without any further purification. Algal specimens of U. rigida were collected in Nafplion bay, Greece, at a depth of 0.5–1 m in July 2015. The specimens were cleaned from epiphytes, rinsed with seawater and fresh water, and dried in continuous air flow. The dried alga was then cut in 1–5 mm pieces with a mill and stored until used at room temperature in paper bags in a dry, dark place. A
Catalyst preparation and characterization
The preparation and characterization of the GO@SO3CF3 catalyst has been reported in detail in our previous work [16]. DRIFT spectra (Fig. 2S) and XRD pattern (Fig. 3S) performed in the present work confirm a random packing of the graphene sheets, on which the triflate groups are covalently bonded. The presence in the GO@SO3CF3 DRIFT spectrum of the CF3 vibration band centered at 1234 cm−1 and observation of a significant decrease in the intensity of the IR bands attributed to −OH and CO groups
Conclusions
The present data demonstrate the use of the marine polysaccharide ulvan as feedstock for the production of biomass-derived platform compounds, complementing those obtained from cellulose and hemicellulose of terrestrial origin, with the additional advantage of the low lignin content. Importantly, while ulvan valorization using metal catalysts in catalytic wet oxidation has proven to be very difficult due to the presence of the sulfate groups that produce a fast and notable deactivation of the
Funding
Financial support by the Spanish Ministry of Economy and Competitiveness (Severo Ochoa SEV2016-0683, RTI2018-890237-CO2-R1) and Generalitat Valenciana (Prometeo 2017−083) is gratefully acknowledged. Vasile I. Parvulescu kindly acknowledges UEFISCDI for financial support (project PN-III-P4-ID-PCE-2016-0146, Nr. 121/2017).
CRediT authorship contribution statement
Iunia Podolean: Investigation, Methodology, Writing - original draft. Simona M. Coman: Conceptualization, Data curation, Writing - original draft, Writing - review & editing. Cristina Bucur: Investigation, Resources. Cristian Teodorescu: Methodology, Validation. Stefanos Kikionis: Investigation, Methodology, Resources. Efstathia Ioannou: Methodology, Validation. Vassilios Roussis: Supervision, Validation, Writing - review & editing. Ana Primo: Methodology, Resources. Hermenegildo Garcia:
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
Authors are grateful to Dr. Bogdan Cojocaru for the infrared spectroscopy and NH3-TPD measurements. The authors would like to acknowledge networking support by the COST Action CM1407 “Challenging organic syntheses inspired by nature - from natural products chemistry to drug discovery”, while part of this work was supported by a STSM Grant from COST Action CM1407 to Stefanos Kikionis.
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2022, Algal ResearchCitation Excerpt :Till date, the degradation methods of ulvan mainly include chemical hydrolysis, physical degradation, and enzymatic degradation. It is generally believed that the reaction conditions of chemical hydrolysis and physical degradation are harsh, which will destroy the structure of degradation products in varying degrees, thus reducing their biological activity [23,24]. On the contrary, enzymatic degradation of ulvan has the advantages of mild reaction conditions and high substrate specificity, which is the best way to prepare Ulva oligosaccharide in the future [25].
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2022, Journal of Drug Delivery Science and TechnologyCitation Excerpt :The resulting precipitate was filtered through a cotton cloth, washed exhaustively with ethanol and sonicated for 1 h in an ultrasonic bath, filtered under vacuum, and finally freeze-dried overnight to afford ulvan (ULV) as an off-white powder, which was milled prior to use. Characterization of ulvan (MW distribution centered at approx. 1000 kDa; 48.3% sulfate, 39.8% carbohydrates; among carbohydrates, rhamnose and uronic acids represented 26.2% and 17.6%, respectively) was performed as previously described [56]. The FTIR spectrum exhibited characteristic absorption bands at 3352, 1607 and 1031 cm−1 attributed to the stretching vibrations of –OH, –CO carboxylic groups and C–O–C ether glycosidic linkage, respectively.
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2021, Algal ResearchCitation Excerpt :The degraded ulvans, also called Ulva oligosaccharides, has a variety of biological activities, such as: anti-viral [10], anti-inflammation [11], anti-coagulation, anti-tumor [12,13] and anti-oxidant [14]. In addition, the degraded ulvan can be used for fermentation and can provide rare sugar (such as rhamnose and iduronic acid) [15,16]. As a result, the preparation of Ulva oligosaccharides has received more and more attention.