Bio-sourced mesoporous carbon doped with heteroatoms (N,S) synthesised using one-step hydrothermal process for water remediation

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Highlights

  • Mesoporous carbon xerogel doped with N, S or dual doped have been fabricated.

  • Doping and mesoporosity are developed via one pot hydrothermal process.

  • The precursors are biobased chemicals containing the atomic building blocks.

  • The mesopore average diameter depends on the type of heteroatom.

  • Mesoporosity and doping enhanced the capture of bulky dye molecules in aqueous phase.

Abstract

Mesoporous carbon monoliths doped with nitrogen, sulphur or both have been prepared in one step hydrothermal carbonization from bio-based precursors. Glucose and pyrrole carboxaldehyde or glucose and thiophene carboxaldehyde aqueous solutions have been used as precursors for nitrogen and sulphur doped carbon materials, respectively. In the same hydrothermal process, a salt-templating approach was used to endow the material with mesoporosity. Additionally, the materials were also pyrolysed in N2 at 973 K. The average mesopore size of resulting xerogels is tuned by the used dopant. Both the doping and mesoporosity enhanced substantially the adsorption of large dye molecules. Accordingly, these metal-free, cost-effective and sustainable materials are excellent candidates for liquid phase environmental and energy applications where the dopant may play a role as catalytic active phase or electronic modulator.

Introduction

The rational design of high performance and cheap nanomaterials for multiple sustainable energy and environmental applications is extremely urgent but remains challenging. Carbons doped with heteroatoms (N, S) with tailored chemical composition are required for some applications in which the incorporation of heteroatoms showed some benefits such as metal-free catalysis or electrocatalysis. The doping element affects conductivity, active sites and wettability of the carbon material which is worth to explore. Top–down processes typically used for the synthesis of porous carbons (such as physical or chemical activation) does not allow a well-defined doping with heteroatoms. Hydrothermal carbonization (HTC) of carbohydrates is a bottom–up process that allows the preparation of carbon materials with tailored heteroatom content [1]. Nitrogen doped HTC [2] and Nitrogen and Sulphur co-doped HTC [3], [4] were prepared in one step using glucose plus a building block containing the heteroatom. These materials have been used in several applications such as electrocatalyst for oxygen reduction reaction [2], [3], [4] or for capacitive deionization [5]. Other important aspect is the development of porosity, especially mesoporosity, which is crucial for the adsorption or catalytic application in liquid phase involving large molecules (high weight compounds, dyes or biomolecules) [6], [7], [8]. Sometimes a sacrificial template such silica nanoparticles has been used to create mesoporosity, but this approach requires multiple steps and the use of hazardous reagents [9], [10], [11], [12], [13], [14], [15], [16]. It is beneficial if mesoporosity can be generated in one-pot during hydrothermal carbonization. The use of a special precursor (ovoalbumina) led to a mesoporous material [3] but the surface area is not very high (below 300 m2 g−1) even for the pyrolized material. Borax was used to prepare N-doped HTC with mesoporosity [4] and after pyrolysis at 1173 K the maximum surface area was 427 m2 g−1 formed by pores between 4 and 24 nm. A similar approach is the so-called “salt templating” method using ionic liquids or eutectic salt mixtures as porogen [17]. Using this approach, Fechler et al. prepared aerogel-like mesoporous bodies by using hypersaline conditions [18]. The basic concept behind is that the hypersaline conditions stabilize the surface of the as-formed primary small nanoparticles (<50 nm) to avoid Ostwald ripening or excessive particle growth. These primary particles at sufficiently high concentration then turn collectively unstable, undergoing spinodal phase separation and cross-linking toward the final porous carbon gels. The more salt is added, the smaller the primary particles are and hence the higher the surface area is. Thus, this method allows structural control by varying the salt concentration and salt type. Recycling of the reaction medium in all these cases is very simple: the salt is washed away with water, filtered, and can be reused after evaporation of the water. This procedure creates a mesoporous monolithic bodies in one single step. A variant of this approach is when ZnCl2 is not washed after hydrothermal carbonization and the material is pyrolised. During pyrolysis, the remaining ZnCl2 has a second role as an activating agent to develop further microporosity [19]. Following this later approach, other authors prepared mesoporous carbon materials introducing ZnCl2 during hydrothermal carbonization of coconut shell in a 2:1 weight ratio [20], [21]. In that case, the solid was not washed after hydrothermal synthesis but pyrolysed it at 1073 K.

As far as we know, salt templating approach has not been used to generate mesoporosity in heteroatom doped carbons. Herein, we explored how this approach allows tuning the dopant content and mesopore size in one pot. The prepared materials have showed enhanced performance in the adsorption of dyes even without the need of an activation step. It is foreseen that these materials will exhibit enhanced performance in liquid phase reactions of energy and environmental interest.

Section snippets

Experimental

For the preparations, anhydrous Glucose (panreac), Pyrrole-2-carboxaldehyde (Sigma–Aldrich), 2-Thiophenecarboxaldehyde (Sigma–Aldrich) were used.

For the synthesis of N-doped carbon materials, 3 g of anhydrous Glucose, 4.5 g of ZnCl2, 0.50 g (5.3 mmol) of Pyrrole-2-carboxaldehyde and 1.5 mL of H2O were thoroughly mixed. The mixture was transferred a glass vessel that was introduced in a Teflon-lined autoclave and kept at 463 K under autogenous pressure for 19 h.

For the synthesis of S-doped

Results and discussion

The hydrothermal carbonisation produced xerogel bodies which adopt the form of the mould in which they are synthetized (Fig. S1 supplementary material). The microscopy inspection of the materials (Fig. 1) revealed that the prepared materials have similar morphology, irrespective if they are undoped, N-doped or S-doped. They are formed by the aggregation of primary particles of size smaller than 20 nm (Fig. 2a,b) and the high magnification shows that they have an onion-like morphology. When the

Conclusions

Mesoporous carbon materials doped with nitrogen, sulphur or dual doped have been prepared via hydrothermal carbonisation of glucose and building blocks containing the heteroatom. In the same single step, mesoporosity has been developed in the materials by introducing ZnCl2 as easily removable template. The average mesopore size varies with the type of doping, providing materials with average mesopore sizes of 3.5 nm, 12 nm, 34 and 32 nm for un-doped, N-doped, S-doped, and dual N,S-doped

Acknowledgements

The financial support of European Commission (FREECATS project, FP7 Grant agreement nº 280658) from Spanish Ministry MINECO and the European Regional Development Fund (project ENE2013-48816-C5-5-R), and Regional Government of Aragon (DGA-ESF-T66 Grupo Consolidado) are gratefully acknowledged.

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