Sodium alginate conversion into char via pyrolysis at the onset temperature
Graphical abstract
Introduction
In order to reduce the consumption of fossil fuels, the valorization of natural products as alternative feedstock for the production of useful materials (in accordance to the green chemistry principles) is becoming a very promising research field [1], [2], [3]. Interestingly, the exploitation of aquatic-derived substances as possible raw materials is one of the most appealing technological solutions, especially considering that the majority of our planet surface belongs to the hydrosphere [4].
Aquatic-derived substances comprise a large variety of biomasses from animal (residues from (shell)fish and other (in)vertebrates) and vegetable (aquatic plants and algae) origin [5]. In this context, alginates are a class of polysaccharides extractable from brown microalgae. These biopolymers are composed by two monomeric units, i.e., β-d-mannuronic acid (M) and ∝-l-guluronic acid (G), organized in either homo- or hetero-polymeric sequences [6]. This class of bio-based materials is widely used in biomedicine as hydrocolloids (in particular with calcium).
According to the literature, several studies are focused on the production of carbonaceous materials from alginates [7], [8], [9]. As suggested by the International Biochar Initiative (IBI), temperatures commonly selected for pyrolysis are higher than 700 °C [10], whereas milder conditions (below 550 °C) are usually preferred in hydrothermal conversions [11]. In our recent studies [12], [13], chitin (another aqueous-derived substance forming the crustaceans’ shells), municipal biowaste-derived humic-like substances and nanosponges from β -cyclodextrins were converted into biochar via pyrolysis carried out under mild conditions (in the range comprises between the onset temperature and 550 °C). The main advantage of performing controlled pyrolysis at low temperatures is twofold: from the chemical viewpoint, it is possible to maintain some residual functionalities exploitable for particular applications (e.g., adsorption of polar substrates) [14], while, from the energetic viewpoint, lower temperatures require lower consumption of energy.
In the present work, we investigated the effects induced by thermal treatment on sodium alginate (derived from brown algae) at the beginning of the degradation phenomenon (onset temperature), alone and in presence of zinc chloride (catalyst).
Section snippets
Reagents and chemicals
Alginic acid sodium salt from brown algae (Na alginate, CAS 9005-38-3, Sigma-Aldrich), either alone or in presence of the catalyst zinc chloride (ZnCl2, purity 99.999%, CAS 7646-85-7, Aldrich), was thermally treated under inert atmosphere. All chemicals were used without further purification.
Pyrolysis process
Pyrolysis of Na alginate (ca. 1 g) either alone or manually mixed in an agate mortar with ZnCl2 (ca. 1 g, ratio 1:1) was performed in a quartz tube reactor (LTF 12/38/500 Lenton) under inert atmosphere
Results and discussion
Fig. 1 reports the thermal degradation profiles of bare Na alginate (A0). As reported in the figure, the thermal degradation profile of Na alginate presents four weight losses: below 100 °C, it has been registered the loss of moisture from the biopolymer (ca. 15 wt%), whereas the main degradation phenomenon occurred at 211 °C (onset temperature) and it is a two-step process. The first one, over a temperature range of ca. 211–270 °C is due to the evolution of small molecules (H2O and CO2) with
Conclusions
In this study, it has been investigated the possibility of converting Na alginate (a natural biopolymer) into a char material by performing pyrolysis under very mild thermal condition (onset temperature). The degradation effects on Na alginate either alone or in presence of Zn-catalyst were evaluated, evidencing the possibility of modifying its chemical structure already at low temperatures, thus guaranteeing low energetic consumption.
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