Porous Carbon Materials from Sustainable Precursors

Robin J White

• Publisher: Royal Society of Chemistry
• Year: 2015
• Print ISBN:
  9781849738323
• Edition: 1
• eBook ISBN:
  9781782624431
• Topic: Biological Sciences
• Subtopic: Environmental Science
• Index: Biological Sciences

Porous carbon materials are at the heart of many applications, including renewable energy storage and generation, due to their superior physical properties and availability. The environmentally-friendly production of these materials is crucial for a sustainable future.
This book focuses on the transformation of sustainable precursors into functional, porous carbonaceous materials via the two most significant approaches: Starbon® and Hydrothermal Carbonisation. Covering cutting-edge research and emerging areas, chapters cover applications of porous carbon materials in catalysis and separation science as well as in energy science. Moreover, the challenges of characterization of these materials and their commercialization are explained by worldwide experts.
The content will be accessible and valuable to post-graduate students and senior researchers alike and it will serve as a significant reference for academics and industrialists working in the areas of materials science, catalysis and separation science.

PART 1
STARBONS^®

CHAPTER 2

From Polysaccharides to Starbons^®

VITALIY L. BUDARIN^a, PETER S. SHUTTLEWORTH^b, ROBIN J. WHITE^*c AND JAMES H. CLARK^a

^aGreen Chemistry Centre of Excellence, University of York, Department of Chemistry, Heslington, York, YO10 5DD, UK; ^bDepartamento de Física de Polímeros, Elastómeros y Aplicaciones Energéticas, Instituto de Ciencia y Tecnología de Polímeros, CSIC, c/ Juan de la Cierva, 3, 28006, Madrid, Spain; ^cUniversität Freiburg, FMF - Freiburger Materialforschungszentrum, Stefan-Meier-Straße 21, 79104 Freiburg im Breisgau and Institut für Anorganische und Analytische Chemie, Albertstrasse 21 79104 Freiburg, Germany
*E-mail: [email protected]

Abstract

Many commercially employed carbon materials are typically hydrophobic, chemically inert and microporous. Therefore, with an eye to the future, there is a need to develop new, carbon-based porous materials, the properties of which can be easily tuned to address the catalytic and separation challenges of future energy and chemical provision schemes (e.g. the Methanol Economy or Biorefinery schemes). In this regard, the synthesis of such materials must be conducted in as sustainable a manner as possible, ideally providing a flexible platform upon which to tailor properties such as functionality, porosity at different length scales (e.g. micro-, meso-, and macroporosity), hydrophilic character and macrophology (e.g. monoliths, particulates, etc.) amongst others. This chapter therefore aims to introduce one top-down synthetic approach to this end, the Starbon® materials concept. An accompanying material development history will be provided followed by a review of the variety of interesting functionally rich, highly mesoporous, high surface area (e.g. > 0.5 cm³ g^–1; > 200 m² g^–1) carbonaceous materials that are accessible via the development of porous polysaccharide-derived materials and their subsequent carbonaceous derivatives. The chapter intends to provide the reader with an overview of the exciting opportunities that are open to the carbon materials chemist based on the discussed synthetic approach.

2.1 Introduction

Taking into account their natural abundance and general low cost, coupled with a well-known capability to form thermoreversible “expanded” aqueous gels, polysaccharides can be considered by the materials chemist as excellent precursors for the preparation of functional materials. Preservation of the expanded gel phase can be achieved by careful drying to produce porous polymer-based cryo-, xero- and aerogels, is one approach to the opening up of the often compact polysaccharide state to generate high surface area, high volume, typically hydroxyl rich functional porous phases.¹ Furthermore, such “gels” can be viewed as excellent precursors for the preparation of porous carbon-based materials and it is in this context that this chapter is discussed.

The successful elaboration of “porous polysaccharide-to-carbon” synthetic schemes provides the opportunity to the green materials chemist to valorise typically low value and often waste biomass for the preparation of new porous, higher-value media. As will be briefly discussed, whilst extremely interesting materials in their own right, “soft” porous polysaccharides gels often suffer from poor mechanical/chemical resistance and in turn applications can be rather limited although many of these structures are extremely attractive as metal (nanoparticle) catalyst support media, although a number of recent reports elude to the synthesis of cellulose and chitin-based aerogels with exceptional mechanical properties.^2–7 The theme of this chapter is the transformation of such porous polysaccharide-based gels to produce more stable porous carbonaceous forms to circumvent problems associated with, e.g., chemical resistance/thermal stability. This opens new synthetic pathways to the synthesis of a variety of nanostructured sustainable carbon-based materials, the properties of which can in principle be directly applied to a specific application. The highly functional and often, “noncondensed” chemistry of the porous polysaccharide precursors (e.g. amylose, starch, alginic acid, chitosan, etc.) also enables the development of a carbon materials platform to synthesise highly functional more condensed structures, the physiochemical, bulk and surface properties can be tuned and directed. This renders materials with properties that essentially fill a “materials” void between conventional activated carbons and porous inorganic materials. As will be shown, this can be achieved using relatively simple, controllable synthesis parameters.

The conversion of photosynthetic products – polysaccharide biomass – to more thermochemically condensed, carbonised forms also potentially contributes to environmental benefits, as this process may represent a form of carbon sequestration particularly if the polysaccharide is derived from fast-growing plants. The utilisation of such saccharide-based products of photosynthesis, for the production of new functional, nanoporous materials (e.g. cryo- and aerogels), is receiving increasing amounts of interest both academically and commercially due to the range of economic/process/chemistry advantages offered by such synthetic approaches (e.g. in biomaterials/medicine). From a sustainability standpoint, the synthesis of such sustainable materials can, if conducted correctly, potentially represent a holistic approach to the production of novel, inexpensive and highly applicable “soft” polymeric and carbonaceous materials. This chapter will introduce the reader to the Starbon^® concept, its history and the variety of interesting carbonaceous materials that are accessible via the development of porous polysaccharide-derived materials (PPDMs). The chapter intends to provide the reader with an overview of the area and highlight the exciting opportunities open to the materials chemist based on the discussed synthetic approaches.

2.2 Porous Polysaccharide-Derived Materials

Nature provides a wide range of biosynthetic sugar-based polymers – the polysaccharides (Figure 2.1). These renewable resources are readily available, inexpensive and functionally rich (e.g. –OH, –C(O)OH, –NH₂).⁸ They are the products of natural processes (e.g. photosynthesis) and perform a wide range of biological functions, including as membrane and cell-wall components, storage of photonic energy and as sequestering agents for water, nutrients and metals in the cell environment.^8–12

Figure 2.1 The chemical structures of some common polysaccharides; (A) Chitin (deacetylation leads to Chitosan); (B) hemicellulose; (C) cellulose; and (D) a basic α-polyglucopyranose structure forming the basis for the branched amylopectin and the linear amylose.

From a materials point of view, polysaccharides are known to self-associate or order into particular structures, physical forms or shapes in nature (e.g. the starch granule, plant cell structures, etc.).^8,13 They are also known, perhaps more significantly in the context of this chapter, to form aqueous “expanded” gels, which if desired can be dried to yield a porous solid.² This “expanded” phase provides the opportunity to the materials chemist to access a range of novel porous materials including, cryo-, xero-, and aerogels. In their native form, polysaccharides have a low surface area and little developed porosity. The “expansion” of these compact (often semicrystalline) polymeric structures is therefore vital for the development of porous materials (e.g. sustainable porous carbons) that are relevant in applications where mass transport/diffusion (e.g. chromatography) and surface interactions (e.g. liquid-phase catalysis) are critical to function. In this respect, the early work of Glenn et al. and Te Wierik et al. in the 1990s, based on starches demonstrated the preparation of xerogels (S_BET < 145 m² g⁻¹), prepared via a sol-gel-like process involving the thermal gelation and recrystallisation (often to referred to as “retrogradation”) of starch, followed by the careful replacement of pore-entrapped H₂O for a lower surface tension solvent (e.g. CH₃CH₂OH) and eventually air (e.g. via supercritical extraction).^14–16

This work was revisited in the 2000s at the Green Chemistry Centre of Excellence, University of York, where Clark and coworkers demonstrated the potential of corn starch (∼73% amylopectin) in the production of low-density, high surface area starch xerogels (S_BET ∼ 120 m² g⁻¹). The resulting porous starches were employed in stationary media in normal phase chromatography separations,¹⁷ and in the preparation of solid acid catalysts (e.g. starch-SO₃H).^18,19 This work was extended to the microwave-assisted preparation of high surface area (S_BET > 180 m² g⁻¹), highly mesoporous starch-derived materials (V_meso > 0.6 cm³ g⁻¹; > 95% mesoporosity).²⁰ This research demonstrated that the key to the formation of...

Table of contents

• Cover
• Title
• Copyright
• Contents
• Introduction
• Part 1: Starbons®
• Part 2: Hydrothermal Carbonisation (HTC)
• Part 3: Characterisation of Porous Carbonaceous Solids
• Part 4: Commercialisation
• Subject Index