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Preparation of lignosulfonate-based carbon foams by pyrolysis and its use in the microencapsulation of a Phase Change Material Herve Deleuze, Martin A Palazzolo, Marie-Anne Dourges, Anthony Magueresse, Patrick Glouannec, and Laurent Maheo ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03900 • Publication Date (Web): 21 Dec 2017 Downloaded from http://pubs.acs.org on December 26, 2017

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ACS Sustainable Chemistry & Engineering

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Preparation of lignosulfonate-based carbon foams by pyrolysis and its use in the

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microencapsulation of a Phase Change Material

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Martín A. Palazzolo1,2, Marie-Anne Dourges1, Anthony Magueresse3, Patrick Glouannec3,

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Laurent Maheo3, Hervé Deleuze1,*

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1

Cours de la Libération, F-33405 Talence, France.

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2

Instituto de Investigaciones en Tecnología Química (INTEQUI), Universidad Nacional de San Luis, CONICET, 1455 Almirante Brown, D5700HGC, San Luis, Argentina.

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University of Bordeaux, Institut des Sciences Moléculaires (ISM), UMR-CNRS 5255, 351

3

Université de Bretagne Sud, Institut de Recherche Dupuy de Lôme (IRDL), FRE-CNRS 3744, rue de Saint-Maudé - BP 92116, F-56321 Lorient, France.

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*

Corresponding author. E-mail: [email protected]

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Abstract

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Currently, further research on the valorization of lignin is needed to shift biorefineries from the

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conceptual basis to the profitable practice. Providing global warming is a major concern as well,

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the use of lignin as the solely precursor to elaborate materials with Thermal Energy Storage

20

(TES) applications is especially welcomed in the search for new sustainable solutions. To this

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end, the preparation of on-demand macroporous carbon foams from calcium lignosulfonate

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(CaLS) by pyrolysis is described herein, and their capability to microencapsulate phase change

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materials (PCMs) dedicated to the passive refrigeration of buildings by TES is further assessed

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as a proof of concept.

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The as-produced CaLS-based foams were found to be efficient containers for this purpose,

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displaying any appreciable leakage of PCMs. Furthermore, the thermal properties of the final

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materials were satisfactory as well, showing that the support does not affect the PCM

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performance negatively. Considering the process to produce such materials is not only

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straightforward but also relies on an inexpensive, widely-available carbon precursor, it is

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expected that it serves as a starting point for pilot studies in TES projects.

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Keywords

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Carbon foam; Macroporous material; Lignosulfonate; Pyrolysis; Phase Change Materials; Butyl

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stearate; Thermal Energy Storage

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Introduction

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In recent years, the need to push further the biorefinery concept for the sake of a biobased

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economy has fostered the development of processes leading to the complete harnessing of

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lignocellulosic biomass. Currently, special focus is put on the valorization of the lignin

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fraction1,2, which is still a major challenge. By taking paper mills as model example, massive

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lignin-containing wastes are produced from the pulp bleaching step. These are usually burnt to

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produce energy while recycling the pulping chemicals, but these demands are already met with a

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minor part of the total residue3. Although a variety of low-value products and some specialties

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are elaborated from lignins4-6, it is believed that only 2% of the total amount of lignin available

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worldwide reaches commercial end-use markets3. In particular, lignosulfonates –actually a

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generic name given to technical grade lignins from sulfite pulping- are complex feedstocks since

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they not only contain counterions and sugars to some extent, but also they exhibit a broad

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polydispersity (Đ = 4-9, Mw = 1-140k) and have 3-8% sulfur content as well7, which hampers

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their use in high-value applications through chemistry8.

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The pyrolysis of lignin is considered as a procedure displaying the potential to lead to profitable

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biorefineries since all the generated fractions can be transformed into fuels, chemicals and

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energy9. By definition, the pyrolysis is a procedure by which a carbon precursor is decomposed

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at a temperature above its softening point –usually 200-700ºC for a range of feedstocks, but

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around 170ºC for lignins10- in absence of oxygen to yield liquid, solid and gas fractions8,11.

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When particular attention is given to the thermally stable solid fraction, and provided the

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obtained structure is a porous monolith, the name foam is often preferred over the term char. The

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mechanism through which foams are formed by pyrolyzing different carbon precursors has been

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described early. Originally, coal was used as the starting material to produce carbon foams12-14.

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Therefore, it has also traditionally served as the model to rationalize the pyrolysis process and to

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extend it to other feedstocks like mesophase pitch, birch bark, etc.15-19. It is understood that the

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pressurization of the vessel plays a major role when attempting to control pyrolysis.


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Furthermore, temperature, heating rate and time may impact on it as well. Nevertheless, these

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parameters must be modulated according to the carbon feedstock.

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Given the complex nature of lignosulfonates, these are not used as the sole carbon precursor but

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often blended with different synthetic polymers –and/or mixed with additives- upon pyrolysis to

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yield porous carbons meeting a variety of applications. Therefore, the literature on the pyrolysis

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of as-produced, raw lignosulfonates is currently scarce. The first preparation of glassy carbon by

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lignosulfonate pyrolysis was addressed by Törmälä and Romppanen in 1981, but their apparent

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porosities were not competitive20. Three decades later, Chen et al. addressed the production of

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activated carbon with capacitive applications from lignosulfonate21. A report describing the

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thermal preparation of non-activated carbon foams under N2 pressure from sodium

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lignosulfonate is available from the patent literature, but data on the procedure and on the

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features of the obtained foams are not fully disclosed therein22. In a preliminary work, our group

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addressed the production and the characterization of carbon foams by means of pyrolyzing

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different lignosulfonates as carbon precursors23.

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The use of non-renewable sources to produce energy and chemicals led not only to availability

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issues but foremost to sustainability concerns. To face global warming, alternative solutions for

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the thermal energy storage (TES) are needed. To this end, the phase change materials (PCMs)

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represent a promising technology24. Nowadays, their prototype applications in the passive

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refrigeration of buildings –a field known as free cooling- are attractive enough so as to dedicate

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continuous efforts on this matter25,26. It is well documented that PCMs exhibit their best

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performances when microencapsulated. However, the containers available currently do not often

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meet key chemical, physical or kinetic specifications beyond the thermal ones, like for instance,

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resistance to corrosion and ignition, phase segregation, renewability and cost30. Therefore, it is of

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great interest to develop new suitable, green containers for PCMs. Considering their chemical

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nature and vast industrial availability, lignosulfonates might serve as a starting material meeting

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these criteria.

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In this work, we aimed to produce lignosulfonate-based foams on demand by pyrolysis and to

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further test them in a TES-related application such as the microencapsulation of PCMs. Calcium

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lignosulfonate (CaLS) was selected as carbon precursor to study the influence of different

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variables independently, and subsequently to set the limits where the features of the carbon

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foams are tailorable. Finally, proof-of-concept experiments were run to test the use of the 3 ACS Paragon Plus Environment


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lignosulfonate-based foams as supporting matrices for the microencapsulation of PCMs in the

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attempt of challenging the preparation new materials dedicated to free cooling. A sustainable

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strategy is described herein, comprising the novel combination of an inexpensive renewable

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carbon precursor and ecofriendly PCMs as feedstock through biorefinery-compatible techniques

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like pyrolysis and vacuum microencapsulation.

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Materials and methods

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Chemicals

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The calcium lignosulfonate (CaLS) feedstock was purchased from Wintersun Chemical (Ontario,

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CA, USA). Batches of it were oven-dried at 105°C, applying periodic reductions of 200 mbar of

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the atmosphere until constant weight. These were further kept permanently in the oven and

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samples were removed just before its use. Butyl stearate (> 97%) and polyethylene glycol 600

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were acquired from TCI Europe (Zwijndretch, Belgium), n-hexadecane (99%) from Acros

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Organics (Geel, Belgium), dimethyl sebacate (97%) and 1-dodecanol (98%) from Alfa Aesar

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(Karlsruhe, Germany), Rubitherm RT 24 from Rubitherm (Berlin, Germany), n-octadecane

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(99%) and decanoic acid (98%) from Sigma Aldrich (Lyon, France). All were employed as

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received.

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Material Characterizations

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The thermal decomposition of the carbon precursor was studied by thermogravimetric analysis-

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mass spectrometry (TGA-MS) on a STA 409 device (Netzsch, Selb, Germany) coupled to a

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Thermostar quadrupolar mass spectrometer (Balzers Instruments, Balzers, Liechtenstein). With

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this purpose, a 10-mg sample in average was degraded at 5ºC.min-1 in the range of 30-1000ºC

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under a 60 mL.min-1 N2 flow. The m/z intensities of selected ions (2.00, H2; 15.97, CH4; 17.97,

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H2O; 44.06, CO2; 47.06, CH3S+; 64.06, SO2) and the thermogravimetric parameters were

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registered every 15 sec, and then normalized to 1 mg sample size.

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Fourier Transformed Infrared (FTIR) spectra were obtained by the KBr transmission technique

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using a Thermo Nicolet Magna-IR 750 spectrometer (Waltham, MA, USA). A total of 20 scans

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per spectrum were recorded in the mid-infrared region at 4 cm-1 resolution.


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The foam morphology was analyzed by Scanning Electronic Microscopy (SEM) using a TM-

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1000 microscope (Hitachi, Tokyo, Japan) by mounting a piece of foam of approximately 36 mm2

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and 2 mm in height on a carbon tab to ensure good conductivity. Scans were done at 25X, 30X,

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50X, 100X, 200X, 300X, 500X and 1000X. Two-dimensional (2D) circular cross sections cell

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diameter (d2D) was estimated from SEM micrographs after image processing with ImageJ

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freeware (NIH, USA). Experimental data were obtained by manual measurements of diameters

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from a population of at least 100 cells. Several methods have been devised to find a simple factor

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to convert the mean size of such 2D size distribution to the actual 3D mean size of the spheres

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without a consensus. In this work, the average cell size (Cd) was estimated by applying the eq 1:

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d3D/d2D = 4/π ≈ 1.27

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where d3D is the mean diameter of set of spheres and d2D are its the 2D intercepts irrespective of

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the particular distribution of the 3D sizes. The detailed method is reported elsewhere27,28.

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The experimental open porosity of each sample and average window size (Wd) were calculated

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from Mercury Intrusion Porosimetry (MIP) analysis using an Autopore IV 9500 porosimeter

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(Micromeritics, Norcross, GA, USA), with the following parameters: contact angle = 130°, Hg

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surface tension = 485 mN.m−1, maximum intrusion pressure = 124 MPa.

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The specific surface area was determined by nitrogen sorption measurements with a

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Micromeritics ASAP 2010 analyzer (Micromeritics, Norcross, GA, USA). The specific (BET)

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surface areas were calculated using the Brunauer, Emmett and Teller method.

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Thermal analyses were conducted using a µDSC3 EVO instrument (SETARAM, Caluire,

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France). Samples of about 60 mg were encapsulated in a Hastelloy C cell. Modulated DSC

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measurements were carried out in the temperature range from 15 to 35°C. Each test was done

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under N2 atmosphere at a heating and cooling rate of 0.2 K.min−1.

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Uniaxial compressive loadings have been performed using a universal testing machine Inspekt

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50 (Hegewald & Peschke, Nosse, Germany). A load cell of 5 kN has been used to get force and

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to calculate nominal stress. Nominal strain was calculated thanks to the stroke, considering the

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rigidity negligible compared to the one of the specimen. A digital camera was positioned in front

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of the specimen to follow its deformation at a frequency of one frame per second. The specimen

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was cylindrically-shaped with a diameter of 21 mm and a height of 7 mm.

(1),

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Carbon foam preparation

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The standard condition for pyrolysis was defined on the basis of our initial work23. Briefly, 2 g of

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CaLS was loaded in a conventional glass test tube (1.40 cm internal diameter), and then placed

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inside a 250-mL stainless steel autoclave (Prolabo, Paris, France). Air was removed by vacuum,

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and then N2 was added to a final pressure of 30 bar. Afterwards, the autoclave was placed in a

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heating vessel at room temperature which was immediately set to rise up to 350°C at 2.8°C.min-1

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with the aid of a Pyrolabo-600 heater (Prolabo, Paris, France). When reached, a residence time of

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3 h was allowed at constant temperature. Then, the heater was unplugged and the autoclave was

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left to cool down for 18 h. The pressure was released and reached the normal value within 5 sec

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approximately. The difference in weight between the initial and the final stage was registered so

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as to calculate the mass loss. After, the foam was removed from the tube by turning it upside

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down and finally oven-dried at 105°C for 18 h. This condition was modified when assaying the

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effect of the different selected pyrolysis variables. Three independent experiments were run in

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every case, therefore three carbon foams were produced under a given condition and further

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characterized.

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PCM microencapsulation procedure

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In a typical run, a 400-mg CaLS-based foam was placed inside a 500-mL Schlenk tube and then

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20 mL of PCM were loaded. A 5-g weight was placed on top of the foam piece to ensure

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complete immersion. Afterwards, the system was placed under vacuum (about 1 mm Hg)

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safeguarded with a trap. The encapsulation proceeded for 8 h at a thermostated temperature of

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26°C. Finally, the Schlenk tube content was filtered to remove the excess of liquid and the foam

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containing the PCM was allowed to drip for 18 h at 18-24°C. By comparing the final and the

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initial weight of the foam, the encapsulation extent was calculated and assessed by weight

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fraction. Immediately, the foam-PCM material was incubated at 50°C for 24 h so as to address

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leakage. In the end, the encapsulation extent was eventually corrected regarding this matter. The

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day-night cycles experiment was performed by simply incubating an encapsulated PCM material

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at 50°C during daylight (9 h) and then at 15°C during the night (15 h) for 10 days while

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assessing the loss of the PCM by the difference in weight in between each cycle.

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Results and Discussion

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Lignosulfonate pyrolysis

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Impact of the water content on CaLS pyrolysis

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Regarding the hygroscopic character of lignosulfonates, it was necessary to study the effect the

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moisture content in the carbon precursor might have on its foaming. To this end, two CaLS

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batches, one of them taken from the as-received stock and the other from an oven- and vacuum-

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dried preparation, were pyrolysed under identical conditions ‒hereafter defined as the standard

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ones-. As observed by SEM, the starting material was prevented from foaming properly when

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containing 15.4±0.3% of water adsorbed from air during shelf storage as determined by mass

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loss on drying. It should be noted that the given amount might account for low molecular weight

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volatiles as well.

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To further understand the thermal behavior of the feedstock in a wet and dry basis, the above-

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mentioned samples were studied by TGA (Figures 1 and 2).

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From the TGA curve shown in Figure 1A, it is observed that the wet sample exhibited a typical

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onset temperature (To) value of 45°C. The temperature at which a 50% of decomposition was

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achieved (T50%) is 402°C. Finally, the endset temperature (Te) was registered at 1000°C and

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showed the complete degradation of the sample. On the other hand, the DTG plot in Figure 1A

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displays six peak temperatures (Tp) within the entire range, clearly showing that the

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decomposition of the wet CaLS comprises multiple stages. To gain knowledge on the nature of

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these events, MS was coupled to identify key gaseous products31-32 as they evolved (Figure 1B).

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Two main decomposition steps could be identified. Regarding the first one, a significant peak is

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observed at Tp1 = 112°C, indicating the removal of the absorbed water. Then, a broader, more

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prominent peak at Tp2 = 265°C is denoting the loss of SO2, and a smaller proportion of CH3S+ as

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well, both proceeding fast. The T50% value points the limit between the stages. The second one is

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characterized by more losses in quantity albeit with minor intensities. At this point, it is

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interesting to highlight that the T50% value matches with the highest intensity found for the CO2

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emanations, therefore showing that the largest decomposition of C-rich portions –i.e. the alkyl

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side-chains and the methoxy groups- occurs at this temperature29. Five Tp define the second

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stage, the first two belonging to relative small SO2 further emanations, Tp3 = 455°C and Tp4 = 7 ACS Paragon Plus Environment


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617°C. As follows, a little peak is observed for a short increase in the CO2 loss at Tp5 = 747ºC.

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The last two peaks corresponds to the third main event of SO2 elimination, which occurred at Tp6

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= 852°C and Tp7 = 970°C. Finally, the Te met with a drop in the CO2 emanations as expected.

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Likewise, it is worthy to mention that both CH4 and H2O were the most quantitative losses, with

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almost constant rates throughout the thermal process. The same observations were made on H2,

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albeit with one order of magnitude less.

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When comparing the thermal degradation results of the dry CaLS with the ones obtained for the

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as-received, wet sample, some interesting observations rise. Figure 2a shows the TGA curve

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corresponding to the dry sample. At first sight, it stands out that the carbonous feedstock

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decomposes smoothly in a dry basis. There is initial evidence of the absence of absorbed water

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as the To value, 70ºC, is much higher than the one exhibited by the wet sample. Noteworthy, the

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T50% is dramatically shifted up to 558ºC, being the absence of the plasticizing effect of water

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revealed by this fact30. Thus, it can be assumed that the hydrogen bonding between the lignin

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chains strengthen without absorbed water. Therefore, considerably more energy was demanded

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to ease the restriction molecular mobility of the polymer. In addition, it is believed that the

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thermal degradation of the dry sample proceeded at a lower rate in comparison with the wet one

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also owing to this fact. As the mass loss did not reach a plateau before 1000ºC, the Te could not

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be determined. From the DTG plot, the absence of absorbed water is confirmed as no peak is

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detected in the typically agreed range for its elimination, 25-150ºC29. Furthermore, the main

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quick, large loss is the one observed at Tp1 = 280°C –a very close value to the one registered for

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the wet sample-, whereas at Tp2 = 573°C a second but significantly minor degradation event

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occurs. Interestingly, the course of the selected evolved gases detected by MS (Figure 2B)

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shows a very similar profile as observed for the wet CaLS sample (Figure 1B), both

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quantitatively and qualitatively. From Figure 2B, it can be appreciated that the most intense SO2

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elimination is addressed by Tp1. Then, shortly after Tp2 the second SO2 emission occurs as well

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as an increase of the CO2 loss, which remains high until the end of the process. On the other

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hand, almost no variation is registered for the CH3S+ losses neither for the H2 ones. In addition,

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the most quantitative, regular loss accounted for CH4. Finally, it is observed that H2O is

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eliminated at a very small rate, and it reaches a plateau around 700°C. It is believed that, given

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the dry-proved nature of the sample, water molecules might arise not only from the combination


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of different hydroxylated compounds while being degraded, but also from the so-called non-

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freezing water fraction31.

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From the thermal degradation experiments, it was confirmed that the absorbed water does not

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substantially modify the nature neither the course of the evolved products from the feedstock.

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While considering that the pyrolysis procedures comprise the thermal decomposition of the

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carbon precursor in an inert atmosphere as well, albeit at high pressure, the former conclusions

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could be extrapolated so as to analyze the impact of the absorbed water content on the final

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macrostructure of the foams. Essentially, it is believed that the loss of the absorbed water exerts

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a masking effect on the bubbling generated by the thermal elimination of gaseous products from

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the carbon precursor, which results in a vague, gross porosity development. Nevertheless, not to

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be ignored is the large shift in the T50%, due to it is of paramount importance to control the

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carbon yield of the foam. Therefore, to further assess the influence of the temperature and other

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crucial variables on the pyrolysis procedure, a dry, ground-powdered CaLS batch was employed

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as carbon precursor.

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Influence of pyrolysis individual variables on the foam features

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To study whether the CaLS-based foams could be tailored in a range of practical operative

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conditions, pyrolysis was performed by modifying different variables independently, namely N2

263

pressure, heating rate, peak temperature as well as the residence time at that temperature. A set

264

of magnitudes was taken as the standard condition on the basis of our preliminary work23 to

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prepare the first foam –thereafter, experiment 1-. The features of the foams produced under each

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new experimental condition were compared to the ones of the standard (Table 1). The

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morphologies of the foams were analyzed by SEM. For a matter of simplicity, Figure 3 shows

268

only images at different augmentations corresponding to the foam obtained in the experiment 2

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(Table 1, entry 2) . Sphere-like cells and circular windows can be easily observed from it, which

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is in agreement with the mechanism proposed for their formation. Although not quantified, a

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high cell density can be regarded, as well as a rather high number of interconnections (windows)

272

between them. Noteworthy, no cracks are detected. All others samples were analyzed by the

273

same way (images not shown).



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By modifying the heating rate (Table 1, entries 1-3), neither the precursor mass loss nor the

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foam size were significantly affected. Indeed, the mass loss was in agreement with the value

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typically found for the slow pyrolysis of different lignins, that is 60%8. A small variation was

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observed for the open porosities, which ranged around 73%. On the other hand, the heating rate

278

did smoothly alter Cd and Wd, denoting a Gaussian-like profile. However, the opposite fashion

279

was seen for the open porosities. These results therefore evidenced that the evolution of the

280

decomposition gases is tightly controlled by the heating rate and thus suggest that producing

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foams on demand by modifying just this variable might be struggling.

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As expected from the mechanism through which the foams rise from the carbon precursor, the

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impact of varying the loaded N2 pressure on the foam characteristics was found to be

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significantly high (Table 1, entries 1 and 4-6). First, the foam relative size falls exponentially in

285

the assayed range (15-90 bars). Then, up to three different Cd can be obtained modifying the N2

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load. Indeed, these can be reduced to the half by pyrolysis with six times the lowest N2 pressure

287

(Table 1, entries 4 and 6), whereas a plateau is observed when doing in-between. Thus, the

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minimum Cd was 70±3 µm (Table 1, entry 6). Moreover, Wd can be reduced in the same way,

289

but it follows an exponential decay, exhibiting a minimum value of 17±1 µm (Table 1, entry 6).

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Remarkably, the open porosity was also modified under an exponential fashion, with average

291

values in the range of 59-86%. From the mechanism17,18, it can be proposed that the coalescence

292

between the gas bubbles released from the precursor may have been enhanced by larger shear

293

forces –aside from that originated by the surface tension on the precursor matrix- given by the

294

higher N2 internal pressures, hence smaller foams exhibiting smaller pores as well were obtained.

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This shows that the thermal decomposition of the precursor can be effectively controlled so as to

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yield, a carbon foam exhibiting a single kind of open macroporosity by simply pyrolyzing at low

297

N2 pressure. However, at this point it is worthy to mention that both extremes yielded foams with

298

unsatisfactory balances of porosity and hardness. Naturally, no changes on the mass loss during

299

the process were observed when the N2 pressure varied.

300

Surprisingly, by varying the pyrolysis temperature, only the mass loss was affected (Table 1,

301

entries 1 and 7-9). In particular, it was determined that as the temperature increased, the CaLS

302

degraded the least in a linear fashion. This trend could be partially explained from the results

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obtained from TGA, which showed the fastest mass loss occurs around Tp1 = 280°C. Then,

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regarding these pyrolysis processes were conducted under 30 bar N2 atmospheres, the actual Tp1 10 ACS Paragon Plus Environment

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value, defined as Tp1*, might have been shifted. In any case, from the pyrolysis results, it was

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inferred that Tp1* may be closer to 300°C rather than to 500°C. As a consequence, it can be

307

reasoned that the longest the residence time is around 300ºC, the higher the mass loss will be. On

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the other hand, an open porosity of 74% was observed, with Cd of 87 µm and Wd of 20 µm, in

309

average values.

310

By holding the pyrolysis maximum temperature for different times, the features of the foams

311

were found to be slightly affected (Table 1, entries 1, 10 and 11), following the same patterns

312

as described for the variation in the heating rate. In particular, the smallest Cd and Wd as well as

313

open porosities were obtained both above and below the magnitudes defined as standard for each

314

parameter (Table 1, entries 10 and 11). Although there is virtually no need to maintain the

315

maximum temperature once reached to achieve good features, from a practical perspective it

316

might be easier to deal with this parameter, rather than with the heating rate.

317

Overall, it was found that among all the assayed variables, the N2 pressure was the only one

318

offering to modulate the foam features with the highest impact and under easy-to-control

319

operative conditions. Nevertheless, as these variables are affected one by each other, an

320

experimental design should be applied in order to maximize the response of the pyrolysis

321

variables. In this way, we selected the simplex optimization method –proved to be effective in

322

material chemistry, among other disciplines- due to its mathematical simplicity and the short

323

time needed to achieve the optimal conditions by means of it, given that it is a self-directing

324

procedure32. However, all the attempts were unfruitful, thus suggesting that every selection of

325

our initial experiments was already enclosing the optimal values (data not shown). Therefore, we

326

arrived to the conclusion that, without considering the variables time and cost, the best pyrolysis

327

results were obtained by pyrolyzing CaLS under 30 bar N2 while heating up to 350ºC at a

328

heating rate of 1.8ºC.min-1 and keeping this temperature constant for 3 h, as described in Table

329

1, entry 2.

330

Finally, it is worthy to mention that all the foams prepared in this study exhibited very low

331

specific (BET) surface area ( 70 µm). This means that the foam walls do not exhibit any porosity and that all the

333

pore volume of the material comes from the cavities reported in Table 1.

334



335

Page 12 of 31

FTIR analysis of CaLS and the foam obtained by its pyrolysis under standard conditions

336 337

Evidences of thermal degradations were collected by FTIR (Figure 4). By comparing the plots

338

from the CaLS dry batch and the standard foam, the disappearance of the typical band at 1510

339

cm-1 corresponding to the aromatic skeletal vibration confirmed the occurrence of thermal

340

degradation33. Then, the broad signal at 1600 cm-1 including an aromatic skeletal vibration band

341

superimposed by the C═O stretching mode, denotes the limited extent of the precursor

342

decomposition at 350°C. However, from the significant reduction of the band centered at 1212

343

cm-1, it possible to verify that C‒C, C‒O and C═O bonds were thermally cleaved as well.

344

Furthermore, the removal of the sulfonate groups could be accounted from the differences in the

345

signals at 1050 cm-1 and 660 cm-1. Finally, from the signal at 1390 cm-1, we verified the methyl

346

groups were not affected since these are only removed under superior temperatures according to

347

literature data34.

348 349

Thermal performance of carbon foam microencapsulated PCM

350 351

First, from a TES application perspective, we aim to select PCMs covering the melting point

352

range dedicated to free cooling. Since organic PCMs are often preferred over the inorganics to

353

this end25, we thus focused on the former. It became interesting to assess a set of compounds

354

with (i) high specific heat and different (ii) densities, (iii) viscosities and (iv) chemical natures as

355

well. Nevertheless, to offer a real thermal solution, the PCMs should be environmentally friendly

356

and commercially available at a low cost35. Overall, eight different PCMs were chosen (Table

357

2).

358

To analyze the capability of the foams to act as containers for PCMs, and to evaluate the

359

behavior of different PCMs in the minimum number of experiments, we selected n-hexadecane

360

as reference because it is has a low, sharp melting point and it is therefore well characterized

361

throughout the literature36. To determine what were the features the foams should gather to be

362

effectively capable of encapsulate n-hexadecane, we first took three foams exhibiting from the

363

smallest to the broadest Cd (entries 1, 2 and 4, Table 2). In the three cases the

364

microencapsulation was successful, achieving above 60% by mass fraction, but no significant

365

effect was exerted by the Cd. With this consideration, and by comparing two foams with almost 12 ACS Paragon Plus Environment

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366

the same Wd but different open porosities, it came evident that the latter was ruling the

367

microencapsulation (entries 1 and 2, Table 2). Afterwards, we increased the temperature of the

368

encapsulation procedure by a 50% approximately, but no significant change was observed

369

(entries 2 and 3, Table 2). Intrigued by the effect both density and viscosity might have on the

370

PCM encapsulation by the CaLS-based foams, we selected different compounds so as to assay a

371

broad range for both (entries 5-11, Table 2). Once again, the encapsulation rate was observed to

372

be only controlled by the porosity. It is also worthy to note that PCMs of different chemical

373

nature were employed throughout, but this had no impact on their microencapsulation. On

374

average, it was observed that by using a foam having 70% open porosity, a material with 65%

375

PCM by mass fraction can be prepared. This shows that the foams’ macropores are occupied to a

376

very high extent. Furthermore, the PMC leakage from the matrix was inferior to 1% in every

377

case. Overall, the as-produced CaLS-based foams were probed to meet these key

378

physicochemical requirements and thus considered suitable containers for organic PCMs.

379

Due to butyl stearate meets key features so as to be applied in industrial fashion –it can be easily

380

produced out of butanol and stearic acid, both of them renewable, and it is commercially

381

available at a low cost- we selected it to address the thermal performance of the final material.

382

These studies were performed using foams obtained according to the conditions of experiment 2

383

(Table 1, entry 2). It was found that, under the conditions used, the CaLS-based foam (porosity

384

76%) can encapsulate 61% by weight fraction of butyl stearate. Furthermore, the PMC leakage

385

from the matrix stored at 50°C for 24h was always inferior to 1%.

386

Pure carbon matrix, encapsulated butyl stearate and pure butyl stearate were analyzed by DSC

387

(Figure 5). The endothermic peak (melting, lower) and exothermic peak (freezing, upper) were

388

clearly observed for the PCMs in both cases.

389

The DSC curve of the pure butyl stearate shows a fusion temperature of 26.9°C (Figure 5, lower

390

blue curve) and a solidification temperature of 25.8°C (Figure 5, upper blue curve). These

391

values are in agreement with the data reported in the literature37. The plot corresponding to the

392

pure carbon matrix (Figure 5, yellow curve) clearly shows that the matrix has no effect on the

393

phase change of the PCM. The latent heat of fusion (∆Hm) of the pure butyl stearate calculated

394

from the DSC curve (∆Hm = 122 J.g-1) is in agreement with data reported on the literature (∆Hm =

395

124 J.g-1)38. Considering the encapsulated butyl stearate, the estimated fusion and solidification

396

temperature from DSC (Figure 5, lower and upper red curves, respectively) are 28.1°C and 13 ACS Paragon Plus Environment


Page 14 of 31

397

24.6°C respectively. The larger gap on the supercooling of the encapsulated butyl stearate when

398

compared with the pure form (3.5°C vs 1.3°C), was attributed to the effect the foam might

399

produce in perturbing crystallization when freezing. Finally, the estimation of the heat of fusion

400

of encapsulated butyl stearate gives a value of ∆Hm = 79 J.g-1. The specific heat value, taking

401

into account the PCM fraction into the foam (61% by mass fraction), correlates rather well with

402

that of the pure PCM (∆Hm = 129.5 J.g-1). The thermal efficiency calculation of the retention of

403

the PCM (e), can be estimated using the eq 2:

404

e = ∆HPCM/ ∆Hcomposite · PCM weight fraction

405

where ∆Hcomposite = 79 J.g-1, ∆HPCM = 129.5 J.g-1 and the PCM weight fraction in composite is

406

0.61. The calculus gives e = 1. Therefore, it can be concluded that encapsulated PCM is fully

407

thermally efficient.

408

Interestingly, from an end use-like, proof-of-concept experiment, it was found that the materials

409

can withstand 10 day-night cycles without suffering appreciable leakage (2% by mass fraction),

410

thus promising adequate performances for long-term applications.

(2),

411 412

Mechanical analysis of encapsulated PCM

413 414

An uniaxial compression test of the encapsulated butyl stearate material (foam formulation from

415

Table 1, entry 2), was performed. The specimen exhibited a mechanical behavior of a cellular

416

material39, with a first slope until 10% of nominal strain approximately, following by a stress

417

plateau of about 4 MPa until 55% of nominal strain and a densification stage, finally (Figure 6).

418

Differences between samples of PCM-containing matrix are lower than between samples of

419

empty carbon matrix. Thus, the PCM seems to stabilize the collapsing process of the carbon

420

matrix porosities. Without the presence of the PCM, dispersion on the first slope and the stress

421

plateau can be observed. However, results are in the same order of magnitude and differences

422

are mainly due to influence of carbon matrix microstructure on the mechanical behavior. Some

423

unloading-loading steps were also performed during compressive tests to obtain the Young

424

modulus. An average value of E = 315±70 MPa was obtained. However, an important scattering

425

of results is observed. During the compressive test, the presence of stress irregularities can be

426

noticed, which are due to the fragile characteristic of the material. Indeed, failure occurs

427

throughout the compressive test from a nominal stress of 1.2 MPa. 14 ACS Paragon Plus Environment

Page 15 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


428

The evolution of the PCM sample encapsulated in the carbon matrix during the uniaxial

429

compression is reported in Figure 7. The PCM begins to exfilter from the matrix from 35% of

430

the nominal strain and to appear over its area. At 42% of the nominal strain, liquid PCM

431

channels are observed on the sample surface. However, this experiment shows the quality of the

432

process to maintain the PCM inside the matrix until these large deformations.

433 434

Conclusions

435 436

In this work, we have shown that the preparation of macroporous carbon foams by pyrolyzing a

437

lignosulfonate precursor is not only straightforward but tunable as well, so as to enable the

438

production of these materials on demand by modulating the process variables in a practical

439

operative range. Furthermore, the as-prepared CaLS-based foams were proven to have a TES

440

application due to they served as matrices for the microencapsulation of the PCMs. They

441

fulfilled physical and chemical criteria to a great extent so as to be considered efficient for this

442

matter, since no leakage was observed at temperature well above the fusion point of the PCM.

443

Indeed, the microencapsulation seemed to be only driven by the porosity of the carbon foam.

444

When encapsulated, the PCMs exhibited excellent thermal and mechanical performances.

445

Since the production of customizable carbon foams with high-end TES application can be

446

achieved by simply selecting a raw, inexpensive and widely-abundant lignosulfonate as the only

447

precursor through a simple process like pyrolysis as shown herein, it is thus expected that these

448

materials serve as starting point for future prototype-like developments in the field of material

449

chemistry.

450 451

Acknowledgements

452

Authors thank CONICET and Erasmus Mundus Action 2 PUEDES from which M.A.P. is a

453

postdoctoral fellow.

454 455

Abbreviations

456

CaLS, calcium lignosulfonate; Cd, average Cell size; DSC, Differential Scanning Calorimetry;

457

DTA, Differential Thermal Analysis; FTIR, Fourier Transform Infrared Spectroscopy; Max,

458

maximum; MIP, Mercury Intrusion Porosimetry; PCM, phase change material; SEM, Scanning 15 ACS Paragon Plus Environment


Page 16 of 31

459

Electron Microscopy; T, temperature; T50%, temperature value corresponding to 50% sample

460

decomposition; Te, endset temperature; To, onset temperature; Tp, peak temperature; TES,

461

Thermal Energy Storage; TGA, Thermogravimetric Analysis; TGA-MS, Thermogravimetric

462

Analysis-Mass Spectrometry; Wd, average Window size.

463 464

References

465

(1) Ragauskas, A. J.; Beckham, G. T.; Biddy, M. J.; Chandra, R.; Chen, F.; Davis, M. F.;

466

Davison, B. H.; Dixon, R. A.; Gilna, P.; Keller, M. Science 2014, 344, 1246843.

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Weckhuysen, B. M. Angewandte Chemie International Edition 2016, 55, 8164-8215.

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(3) Galkin, M. V.; Samec, J. S. ChemSusChem 2016, 9, 1544-1558.

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259-276.

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(7) Holladay, J. E.; White, J. F.; Bozell, J. J.; Johnson, D., Top Value Added Chemicals from

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Biomass-Volume II, Results of Screening for Potential Candidates from Biorefinery Lignin;

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Pacific Northwest National Lab.(PNNL), Richland, WA (United States); National Renewable

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Energy Laboratory (NREL), Golden, CO (United States)2007.

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(8) Laurichesse, S.; Avérous, L. Progress in Polymer Science 2014, 39, 1266-1290.

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(9) De Wild, P. J.; Huijgen, W. J.; Gosselink, R. J. Biofuels, Bioproducts and Biorefining 2014,

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8, 645-657.

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(13) Calvo, M.; Garcia, R.; Arenillas, A.; Suárez, I.; Moinelo, S. R. Fuel 2005, 84, 2184-2189.

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(14) Spradling, D. M.; Guth, R. A. Advanced materials & processes 2003, 161, 29-31.


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(15) Chen, C.; Kennel, E. B.; Stiller, A. H.; Stansberry, P. G.; Zondlo, J. W. Carbon 2006, 44,

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1535-1543.

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(16) Feng, S.; Cheng, S.; Yuan, Z.; Leitch, M.; Xu, C. C. Renewable and Sustainable Energy

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Reviews 2013, 26, 560-578.

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(17) Inagaki, M.; Qiu, J.; Guo, Q. Carbon 2015, 87, 128-152.

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(18) Li, S.; Tian, Y.; Zhong, Y.; Yan, X.; Song, Y.; Guo, Q.; Shi, J.; Liu, L. Carbon 2011, 49,

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618-624.

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(19) Wang, R.; Li, W.; Liu, S. Journal of Materials Science 2012, 47, 1977-1984.

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(20) Törmälä, P.; Romppanen, M. Journal of Materials Science 1981, 16, 272-274.

498

(21) Chen, X. Y.; Zhou, Q. Q. Electrochimica Acta 2012, 71, 92-99.

499

(22) Spradling, D. M.; Amie, D. R.; Google Patents, 2011.

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(23) Despaux, M.; Creunier, P.; Birot, M.; Deleuze, H. Green Materials 2013, 1, 225-230.

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(24) Sharma, A.; Tyagi, V. V.; Chen, C.; Buddhi, D. Renewable and Sustainable energy reviews

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2009, 13, 318-345.

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(25) Osterman, E.; Tyagi, V.; Butala, V.; Rahim, N.; Stritih, U. Energy and Buildings 2012, 49,

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37-49.

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(26) Raj, V. A. A.; Velraj, R. Renewable and Sustainable Energy Reviews 2010, 14, 2819-2829.

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(27) Forgacz, C.; Birot, M.; Deleuze, H. Journal of Applied Polymer Science 2013, 129, 2606-

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2613.

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(28) Foulet, A.; Birot, M.; Backov, R.; Sonnemann, G.; Deleuze, H. Materials Today

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Communications 2016, 7, 108-116.

510

(29) Vasile, C.; Gosselink, R.; QUINTUS, P.; Koukios, E.; Koullas, D.; Avgerinos, E.;

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Abacherli, D. Cellulose chemistry and technology 2006, 40, 421-429.

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(30) Hatakeyama, H.; Hatakeyama, T. In Biopolymers; Springer, 2009, pp 1-63.

513

(31) Hatakeyama, H.; Hatakeyama, T. Thermochimica acta 1998, 308, 3-22.

514

(32) Spendley, W.; Hext, G. R.; Himsworth, F. R. Technometrics 1962, 4, 441-461.

515

(33) Faix, O. In Methods in lignin chemistry; Springer, 1992, pp 83-109.

516

(34) Sazanov, Y. N.; Gribanov, A. Russian journal of applied chemistry 2010, 83, 175-194.

517

(35) Chandel, S.; Agarwal, T. Renewable and Sustainable Energy Reviews 2017, 67, 581-596.

518

(36) Vélez, C.; Khayet, M.; de Zárate, J. O. Applied Energy 2015, 143, 383-394.

519

(37) Sarı, A.; Biçer, A.; Karaipekli, A. Materials Letters 2009, 63, 1213-1216. 17 ACS Paragon Plus Environment


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520

(38) Cellat, K.; Beyhan, B.; Kazanci, B.; Konuklu, Y.; Paksoy, H. Journal of Clean Energy

521

Technologies 2017, 5.

522

(39) Gibson, L. J.; Ashby, M. F. Cellular Solids: Structure and Properties; Cambridge

523

University Press, 1999.

524


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525


Figure 1

526 527

A

B CH4 H2O C2O H2

SO2 CH3S

+



528

Page 20 of 31

Figure 2

529 530

A

B CH4 H2O C2O H2

SO2 CH3S

+


Page 21 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

531


Figure 3

532 533



534

Figure 4 80 78 76 Transmittance (/%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 31

74 72 70 68 66 64 62 60 4000 3750 3500 3250 3000 2750 2500 2250 2000 1750 1500 1250 1000 750 Wavenumber (cm-1)

500

535 536


Page 23 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

537


Figure 5

538 539



540

Page 24 of 31

Figure 6

541 542


Page 25 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

543


Figure 7

544 545 546 547 A

B

C



548

Page 26 of 31

Table 1 Experimental conditions

Entry

a

CaLS-based foam features

Max Heating Total Mass Max Open T Cd N2 Wd rate time loss T a b porosity hold (µm) (µm) (bar) (°C/min) (h) (%) (°C) (%)b (h)

1

2.8

30

350

3

5

63±1 111±8 23±1

68±2

2

1.8

30

350

3

6

63±0

16±1

76±0

3

3.6

30

350

3

4.5

62±1 102±3 20±0

74±0

4

2.8

15

350

3

5

62±0 149±6 44±2

86±7

5

2.8

52

350

3

5

62±0 103±1 20±0

63±2

6

2.8

90

350

3

5

64±1

70±3

17±1

59±2

7

2.8

30

300

3

4.5

67±1

72±1

19±1

75±4

8

2.8

30

450

3

5.5

56±1

72±2

17±1

77±4

9

2.8

30

500

3

6

52±1

91±9

19±1

75±3

10

2.8

30

350

1

3

63±1

94±2

24±2

70±3

11

2.8

30

350

6

8

62±1

90±1

17±1

72±2

69±5

Measured by SEM; b Determined by MIP.

549 550


Page 27 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

551


Table 2 Physicochemical properties of PCM Entry

PCM

1

n-Hexadecane

Features of the carbon foam

PCM microencapsulation

Fusion Density Viscosity Heat of Open Wd Cd point (g/mL) (mPa.s) fusion c d porosity (µm) (µm) a,b a,b (°C) a (%) d (J/g) a 18

0.8

3

236

2

a

PCM mass fraction (%)

74

17

76

26

65

99

18

72

26

61

48

62

3 4

T (°C)

115

24

82

26

72

5

Polyethylene glycol 600

17-21

1.2

150-190

127

93

19

74

48

72

6

Dimethyl sebacate

25-28

1.0

NA

120

99

18

72

48

67

7

Decanoic acid

27-32

0.9

4

163

94

18

75

48

68

8

Rubitherm RT 24

21-25

0.9

4

160

99

18

72

48

60

9

Butyl stearate

25-27

0.9

10

121

99

18

72

48

61

10

1-Dodecanol

24-27

0.8

19

200

81

17

77

48

66

11

n-Octadecane

26-29

0.8

3

244

99

18

72

48

60

As reported elsewhere; b At 20°C and 750 mm Hg; c Measured by SEM; d Determined by MIP; NA, not available.

552 553



Page 28 of 31

554

Figures Captions

555

Figure 1: Analysis of an as-received, wet CaLS sample by TGA (solid line) and DTA (dashed

556

line) (A) and detection by MS of its emanations (B).

557 558

Figure 2: Analysis of an oven- and vacuum-dried CaLS sample by TGA (solid line) and DTA

559

(dashed line) (A) and detection by MS of its gaseous losses (B).

560 561

Figure 3. SEM micrographs recorded for the CaLS-based foam prepared according to Table 1,

562

entry 2. Plots A-D display pictures obtained with 50X-400X augmentations.

563 564

Figure 4: FTIR spectra from CaLS (solid line) and the standard foam prepared out of it (dashed

565

line).

566 567

Figure 5. DSC curves obtained for the pure carbon matrix (yellow curve), the butyl stearate-

568

containing carbon matrix (red curve) and pure butyl stearate (blue curve).

569 570

Figure 6: Uniaxial compression curves for pure carbon matrix (red curves) and butyl stearate-

571

encapsulated carbon matrix (blue curves)

572 573

Figure 7: Evolution of the PCM-encapsulated carbon matrix sample during uniaxial

574

compression for a nominal strain of 0% (A), 35% (B) and 42% (C).

575


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576

Tables Captions

577

Table 1. CaLS-based foams prepared by varying the pyrolysis conditions

578

Table 2. Microencapsulation tests with different foams and PCMs.

579



580

Page 30 of 31

TOC/Abstract graphic “For Table of Contents Use Only”

581 582

Together we stand: high-end, on-demand thermal energy storage materials prepared from

583

pulping waste lignosulfonate-based foams and ecofriendly PCMs.


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