Synthesis of mesoporous carbons by a soft-templating routusing tannin as a carbon precursor

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Tannin extraction and economic aspects

Mimosa tannin is extracted from the barks of Acacia mearnsii trees, which are grown mainly in Brazil and Africa (South Africa and Tanzania). In this work, tannin was extracted in Tanzania and the extraction procedure is showed in Fig. 8. The process starts with the plantation of the Acacia trees that grow up first before being ready for cutting down [Molan et al. 2009]. After cutting down, barks are separated from wood and they are cut into smaller pieces for tanning extraction. In parallel, wood is used for pellets manufacturing and energy production. Tannin extraction/striping takes place in autoclaves in series, with hot water, at around 70°C, having a very small percentage of sodium bisulphite. The late additive reduces tannin autocondensation and improves tannin solubility [Sealy-Fisher and Pizzi 1992]. After striping, the solution contains 50% of extractives that are concentrated by spray-drying. The light-brown powder obtained generally contains 80 – 82% of polyphenolic flavonoid materials, 4 – 6% of water, 1% of amino and imino acids, the remainder being monomeric and oligomeric carbohydrates, in general broken pieces of hemicelluloses. The polyphenolic molecules have low molecular weight ranging from 500 to 3500 g mol-1 [Pizzi 1983]. The older is the bark, the darker is the colour of tannin. Its colour also changes because of the facility of hydroxyl groups present in tannin oxidize to form quinones [Feng et al. 2013].
Table 1 gathers some information on three different types of condensed tannins from Mimosa (Acacia mearnsii), Quebracho (Schinopsis balansaeand lorentzii) and Maritime pine (Pinus pinaster). Maritime pine tannin is more reactive than Quebracho and Mimosa tannins, these latter presenting similar reactivities. The condensed tannins are the most abundant and represent more than 90% of the world production [Stokke et al. 2014]. Other condensed tannins are those extracted from chestnut, tara or galls which represent together 27 300 tons/year*. The production of Mimosa tannin is of 60 000 tons/year and its price is remarkably low, estimated from 1500 to 2300 €/ton (1.5 to 2.3 €/kg). Maritime pine tannin from France has a much higher price, 29 000 €/ton, and it is not yet commercially available. Therefore, we used Mimosa tannin as materials precursor due to its availability in the market and low price.
Tannins Species of Reactivity Annual Production Price (€/ton) dominant flavonoid (tons/year) Mimosa Prorobinetinidine low 60 000* 1 500 à 2 300€/ton* Quebracho Profisetinidine low 60 000* 1 500€/ton* Maritime Pine Procyanidine high 300* 29 000€/ton** * Indicative figures of tannin’s market today provided by Silva Team (2014) ** Value obtained by DRT Company (2013).
Tannins were extracted and commercialised for being applied in leather tanning industry for long time. However, after the end of the Second World War, their commercialisation declined with the emergence of synthetic phenolic materials. Several scientific showed other possible uses of tannins due to the fact that these materials are quite reactive and have a great economical potential. The commercialisation of condensed tannin-formaldehyde adhesives started in 1970. These adhesives were successfully applied together with wood for producing particleboard, plywood, glulam (horizontal wood pieces glued together), among others to fabricate furniture and buildings [Pizzi 1983]. Apart from the classical application of tannin as leather conditioner [Sreeram and Ramasami 2003] other minor application coexist as: wine additives [Harbertson et al. 2012], dying agents [Sánchez-Martín et al. 2010], pollutant adsorbents [Chow 1972; Vazquez et al. 2007; Dalahmeh et al. 2012] or antioxidants in health supplements [Ku et al. 2007; Shahat and Marzouk 2013], among others. Indeed, tannin contains quite reactive polyphenols, which are useful to produce high added value materials as gels [Amaral-Labat et al. 2012a], foams [Szczurek et al. 2013] or carbons [Szczurek et al. 2011a; 2011b], etc. The latter materials can be then applied in separation/storage of gas, depollution or as electrodes for lithium batteries or supercapacitors.

Amorphous carbons production

The amorphous carbons produced in this work from tannin are hard carbons. There are two methods to produce them:
Carbonization or dry pyrolysis is the transformation of an organic material into a carbon matrix by heating in a flow of inert gas, i.e. nitrogen, in the absence of oxygen. Pyrolysis removes volatile species [Basu 2010]. If pyrolysis is carried out in an autoclave with a liquid at moderate temperature and pressure, is called as wet pyrolysis or hydrothermal carbonization (HTC) [Libra et al. 2011]. HTC will be explained in detail afterwards.
The precursor has an enormous influence on the resultant carbon yield. Materials containing large amounts of carbon or even aromatic structures will present a high carbon yield, calculated as the ratio of the final mass of the carbon residue and the organic material. While precursors containing a lower amount of carbon such as sugars, will present a lower carbon yield [Junpirom 2006; Marsh and Rodriguez-Reinoso 2006; Abdullah et al. 2011].
Pyrolysis conditions have an important effect not only on carbon yield but also on porosity. The way that gases evolve during pyrolysis controls the materials’ characteristics. Pyrolysis involves several reactions such as dehydrogenation, condensation, transfer and isomerisation. During these reactions, volatiles are released and new radicals are produced, which react with other molecules producing the rearrangement of stable molecules [Marsh and Rodriguez-Reinoso 2006; Abdullah et al. 2011]. Two competing phenomena could occur during the pyrolysis: i) a shrinkage of the materials and of its porosity may be observed by rearrangement of atoms in a more compact structure [Fischer et al. 1997]; ii) creation of porosity owing to the formation of volatiles species. Indeed, materials continue losing mass up to 1000°C and a concomitant creation of mesopores and micropores volume and an increase of the surface area (for example at 900°C) [Al-Muhtaseb and Ritter 2003; Matos et al. 2006; Job et al. 2008; Bruno et al. 2010]. Particles may also expand producing mainly macropores and therefore, the applications range for such materials would be reduced [Manocha 2003].
The heating rate is also a parameter that must be controlled to obtain high carbon yield [Saddawi et al. 2010]. At higher heating rate, the thermal degradation tend to be delayed, promoting gases released and reducing the carbon yield [Damartzis et al. 2011 and refs therein]. This effect has been seen in the pyrolysis of cherry stones [Gonzalez et al. 2003], rapeseed [Haykiri-Acma et al. 2006] or olive residue and sugar cane bagasse [Ounas et al. 2011]. The same trend has been seen for the pyrolysis of coal. Moreover, surface areas and total pore volume are decreased with the increase of heating rate such as 20-30 °C min-1 [Seo et al. 2011 and refs. therein]. Thus, the effect of heating rate must be taken into account and should remain slow enough to preserve high porosity.
Activation is a process that develops surface and porosity in carbon and it can be either physical or chemical. Physical activation consists in partial carbon gasification by reaction with steam, CO2, or air at temperatures up to 1000°C [Hernández-Montoya et al. 2012] and most reactive amorphous components are burned off. Chemical activation consists of heat treatment in the presence of alkali (NaOH, KOH), inorganic acids (H2SO4, H3PO4), or salts (ZnCl2) in an inert atmosphere at typical temperatures from 400 to 800°C. The advantage of the chemical activation is a more uniform pore structure of the resultant activated carbon. Chemical activation is also a one step process while physical activation requires a previous pyrolysis [Marsh and Rodriguez-Reinoso 2006].
Activated carbon (AC) is one type of amorphous or disordered carbon having been processed to make it extremely porous, and thus to have a very large surface area available for adsorption or chemical reactions. The term active or activated carbon refers to carbon materials manufactured by high temperature (500 to 1000°C) pyrolysis of various vegetable residues (i.e., wood chips, peat, nutshells, pits, etc) as well as pitch, coal or polymers, followed by activation to create desirable porous structure of the target materials. AC’s structure is formed by imperfect stacking of carbon layers, which are bonded together to build a three-dimensional structure, as shown in Fig. 12 [Oberlin 1984]. The basic structural unit of activated carbon may be approximated by that of graphite with, however, a higher interlayer spacing and the possibility of having some carbon layers domains rotated with respect to each other. Activated carbon may thus be considered as a carbon form allied to graphite, but highly disorganised due to impurities and to the method of preparation (activation process). ACs can be classified on the basis of their application (e.g. gas vs liquid phase), precursor (e.g. wood, coal, etc.), and final shapes (e.g. powder, grains, pellets, etc) [Marsh and Rodriguez-Reinoso 2006].

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Materials’ morphology and particle size

Fig. 31 gathers the data of specific surface areas, SBET, determined by Kr adsorption at – 196°C. The typical error for this kind of measurements is less than 5%. Two cases were considered: (a) different HTC temperatures at constant initial amount of tannin, and (b) different amounts of tannin at the same HTC temperature. For both cases, SBET of hydrothermal carbons were all below 10 m2 g-1, suggesting the absence of porosity, and exhibited a complex behaviour. At a given hydrothermal carbon yield, i.e., at a given reaction time, SBET decreased when the HTC temperature increased. Such a finding should be related to the growth of hydrothermal particles, leading to a lower surface area, and will be confirmed below by TEM observations in Fig. 35. For temperatures higher than 130°C, a shallow minimum of SBET was observed at values of carbon yield around 30 – 40%, see again Fig. 31(a). However, the main trend was an increase of SBET with the yield, and hence with the HTC time. Such findings may be interpreted as follows. When HTC goes on, carbon particles are continuously nucleated from the solution, hence the resultant, broad, distribution of sizes seen by TEM pictures in Figs. 31- 33. At the moment at which they appear, these particles are very small and hence present a rather high surface area. During HTC, an antagonistic effect also occurs, consisting in the growth and coalescence of hydrothermal carbon particles, and leading to a decrease of SBET. If the nucleation of carbon particles is faster than their growth and coalescence, then the surface area is expected to increase with time, on average, with a possible minimum when both effects compensate, as shown in Fig. 31. Finally, and according to Fig. 31(b), the behaviour remained qualitatively the same when the amount of tannin increased at the same HTC temperature, but SBET decreased. It can be indeed expected that a higher concentration of precursor leads to bigger carbon particles, thus exhibiting lower surface areas. All these assumptions agree well with TEM images (Figs. 31-33).

Proposed mechanism for particle formation

Some experiments carried out at 130°C (see Fig. 31(a)), showed unusual behaviour with a surface area that abruptly decreased from around 10 m2 g-1 to 0.1 m2 g-1 as the carbon yield exceeded 35%, i.e. for reaction times higher than 66 h. Fig. 36 shows TEM photos of such materials prepared at 130°C and for different HTC times: 48h (a and b) and 120h (c and d). Clear morphologic differences could be observed between those two samples: small particles (200-400 nm) highly agglomerated into micrometric particles for the short reaction time and a a glassy morphology composed of dense and homogenous distributed matter for long reaction time.

Table of contents :

Chapter 1: State of the art
1.1 Biomass as raw material for fuel and chemicals
1.2 Tannins
1.2.1 Tannins structure and reactivity
1.2.2 Tannin extraction and economic aspects
1.3 Carbon materials
1.3.1 Crystalline carbons
1.3.2 Amorphous carbon
1.3.2.1 Amorphous carbons production
1.3.2.2 Determination of textural properties
1.4 Hydrothermal carbonization (HTC)
1.4.1 Mechanism of hydrochars formation
1.4.2 Increase of HTC yield
1.4.3 Synthesis of N-doped carbon materials
1.4.4 Synthesis of N-doped carbon gels
1.4.5 Applications of HTC-derived carbon materials
1.5 Synthesis of ordered mesoporous carbons
Chapter 2: Mechanism and kinetics of hydrochar synthesis from Mimosa tannin
2.1 Hydrochar synthesis and yield determination
2.2 Chemical composition
2.3 Materials’ morphology and particle size
2.4 Effect of the reaction time
2.5 Effect of the tannin concentration
2.6 Effect of the temperature
2.7 Proposed mechanism for particle formation
2.8 Kinetics study
2.9 Conclusions
Chapter 3: Carbons produced from tannin by alteration of the HTC reactional medium: addition of H+, sucrose and Ag+
3.1 HTC synthesis
3.2 Effect of H+ addition
3.2.1 Textural and chemical properties of 100T materials at different pHs
3.3 Effect of sucrose addition
3.3.1 Textural and chemical properties of 50S50T materials at different pHs
3.3.2 Textural and chemical properties of 50S50T materials prepared at various temperatures and pHs
3.3.3 Textural and chemical properties of tannin/sucrose materials at different proportions and pHs
3.3.4 Van Krevelen diagram, TPD (Temperature Programmed Desorption) and XPS analysis
3.4 Effect of silver nitrate addition into HTC of tannin
3.5 Conclusions
Chapter 4: N-doped carbon materials and their use as electrodes for supercapacitors
4.1 Preparation of N-doped materials by HTC
4.2 Chemical structure of hydrothermally treated tannin
4.2.1 13C NMR studies
4.2.2 MALDI-ToF studies
4.2.3 Elemental analysis
4.2.4 XPS technique
4.3 Materials’ morphology and porous texture
4.3.1 Morphological characteristics: TEM and SEM photos
4.3.2 Textural properties
4.4 Comparison of NCM’s made from tannin and other precursors
4.5 Electrochemical performances of N-doped, tannin-based, hydrothermal carbons
4.5.1 Electrochemical experiments
4.5.2 Effect of oxygen and nitrogen-containing functional groups
4.5.3 Comparison with previous studies
4.6 Conclusions
Chapter 5: Synthesis of N-doped carbon gels and their use as electrodes for supercapacitors
5.1 Introduction to gel drying
5.1.1 Subcritical Drying or xerogels synthesis
5.1.2 Freeze-drying or Cryogels synthesis
5.1.3 Supercritical Drying or Aerogel synthesis
5.2 N-doped carbon gel synthesis by HTC
5.3 Morphology of carbon gels
5.4 Porous texture of organic and carbon gels
5.5 Elemental composition of organic and carbon gels
5.6 Electrochemical performances of carbon gels
5.7 Conclusions
Chapter 6: Synthesis of mesoporous carbons by a soft-templating routusing tannin as a carbon precursor
6.1 F-127 Pluronic® and OMC synthesis
6.2 Synthesis of OMC’s from tannin and F127
6.2.1 Effect of the pH
6.2.2 Effect of the carbonization temperature
6.3 Conclusions
Conclusions and Perspectives
Annexes
Annex 1: Techniques of characterisation
A1. Chemical characterisation
A1.1 Elemental Analysis (EA)
A1.2 Mass Spectrometry – MALDI ToF
A1.3 Nuclear Magnetic Resonance Spectroscopy (NMR) 13C
A1.4 Temperature Programmed Desorption (TPD)
A1.5 Thermogravimetric Analysis (TGA)
A1.6 X-ray photoelectron spectroscopy (XPS)
A2. Physical characterisation
A2.1 Adsorption – desorption of gases
A2.2 Scanning and Transmission Electron Microscopy (SEM and TEM)
A2.3 Small Angle X-rays Scattering (SAXS)
A2.4 Determination of the skeletal density and the particle size
A3. Electrochemical characterization
A3.1 Electrical Double-Layer Capacitors (EDLC)
Annex 2: Published materials
A2.1 List of Publications in scientific journals:
A2.2 Articles in Conference
A2.3 Oral Communications
A2.4 Communications by Poster
Bibliography

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