Ahmad Rifqi Muchtar

Christopher L Hassam

Bhuvanesh Srinivasan

David Berthebaud
email: david.berthebaud@cnrs.fr

Takao Mori
email: mori.takao@nims.go.jp

Nugroho Soelami

Brian Yuliarto

Shape-Stabilized Phase Change Materials: Performance of Simple Physical Blending Synthesis and the Potential of Coconut Based Materials

Keywords: Phase change materials, Coconut based-materials, Latent heat storage, Activated carbon, Buildings & thermal comfort

This research reports, for the first time, the combination of food-grade coconut oil and coconut shell-based activated carbon as precursors for the synthesis of bio-based shape-stabilized phase change materials (bioSSPCM). Despite its low melting enthalpy, simple physical blending by heating and mixing is found to be a very reliable preparation method for the completely coconut-based materials, producing a thermally stable bioSSPCM with anti-leakage. No difference between food grade and analytical grade coconut oil, in terms of its application for SSPCMs, was evident. Further, comparing the performance of coconut oil against octadecane, a conventional phase change material, it was found that with the same synthesis conditions, the coconut oil exhibited improved stability, with less leakage after phase change cycling.

Introduction

In 2019 it was estimated that buildings consume 30-40% of global energy production and emit 38% of global energy-related CO2 emission [START_REF] Kumar | Punching above its weight: life cycle energy accounting and environmental assessment of vanadium microalloying in reinforcement bar steel[END_REF][START_REF] Mostafavi | Energy efficiency and carbon emission in high-rise buildings: A review (2005-2020[END_REF]. However, though energy consumption is common to all industrial, commercial, residential, and governmental buildings, the energy demands of each type vary differently, both throughout the day and throughout the year. Maintaining buildings for the thermal comfort of the occupants is necessary, and a great deal of energy is used in heating cold buildings at night and in cooling warm ones during the day. Storing thermal energy for later use is one method of reducing the energy demand for buildings, for example, by allowing heat to be stored during the day, enabling a house to be heated during the night.

In buildings, such thermal storage can be realized by the incorporation of latent heat storage materials into the building structure that could store and release heat while phase changing. Materials suited to this are referred to as phase change materials (PCMs) [START_REF] Wang | A thermal energy storage composite by incorporating microencapsulated phase change material into wood[END_REF][START_REF] Cui | A review on phase change material application in building[END_REF][START_REF] Kuznik | A review on phase change materials integrated in building walls[END_REF]. For PCMs to be of use for thermal energy storage in buildings, the phase change must occur within the correct temperature range, usually between 22 to 28 o C [START_REF] Cabeza | Materials used as PCM in thermal energy storage in buildings: A review[END_REF]. Many such PCMs in use are available whether euectic mixtures, inorganic materials including many low-melting temperature salts, and organic materials, such as fatty acids and fossil fuel-derived waxes [START_REF] Thambidurai | Review on phase change material based free cooling of buildings-The way toward sustainability[END_REF]. With the rapid depletion of fossil fuel sources throughout the world, relying on such waxes to attempt latent heat storage poses a problem with the ongoing viability of the method. Instead, the development of renewable and environmentally friendly PCMs should be emphasized. By using environmentally friendly materials with a melting temperature within the range of building comfort, electricity consumption could be reduced and waste heat repurposed; such applications have shown promise, especially in temperature peak shaving and shifting [START_REF] Frigione | Phase Change Materials for Energy Efficiency in Buildings and Their Use in Mortars[END_REF][START_REF]Eco-Efficient Materials for Mitigating Building Cooling Needs[END_REF].

One obstacle for PCM applications in buildings is the leakage of the materials while in its liquid phase. There are multiple strategies to ensure that PCMs remain stabilized in a building structure; in particular, encapsulation (microencapsulation or macroencapsulation) [START_REF] Wang | A thermal energy storage composite by incorporating microencapsulated phase change material into wood[END_REF][START_REF] He | New approach for sol-gel synthesis of microencapsulated noctadecane phase change material with silica wall using sodium silicate precursor[END_REF][START_REF] Marani | Integrating phase change materials in construction materials: Critical review[END_REF][START_REF] Royon | Optimization of PCM embedded in a floor panel developed for thermal management of the lightweight envelope of buildings[END_REF] and shape stabilization [START_REF] Atinafu | Synthesis of porous carbon from cotton using an Mg(OH) 2 template for form-stabilized phase change materials with high encapsulation capacity, transition enthalpy and reliability[END_REF][START_REF] Li | Synthesis of novel shape-stabilized phase change materials with high latent heat and low supercooling degree for thermal energy storage[END_REF][START_REF] Min | Enhanced thermal properties of novel shape-stabilized PEG composite phase change materials with radial mesoporous silica sphere for thermal energy storage[END_REF] are two well-known methods. The encapsulation techniques are well developed [START_REF] Sivanathan | Phase change materials for building construction: An overview of nano-/micro-encapsulation[END_REF], and now various commercial products in the form of micro or macro encapsulated PCMs are available [START_REF] Kalnaes | Phase change materials and products for building applications: A state-of-the-art review and future research opportunities[END_REF]. Shape stabilization also shows considerable progress for potential applications [START_REF] Frigione | Phase Change Materials for Energy Efficiency in Buildings and Their Use in Mortars[END_REF]. Comparatively, though microcapsule PCMs are possible to be used for latent energy storage in buildings, the practical application of them becomes prohibitively expensive, due to the relatively large quantity of PCMs required for a typical building. Due to this, less expensive materials and synthesis methods must be further developed.

Shape-stabilized phase change materials (SSPCMs) use the porous structure of a material to support the PCM and prevent leakage whilst in its liquid form. Three categories of nanoscale pores are defined by IUPAC [START_REF] Thommes | Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution (IUPAC Technical Report)[END_REF], and of these, it was reported that the mesopores and micropores are more preferable for SSPCM due to optimal surface tension and capillary forces [START_REF] Li | A n-octadecane/hierarchically porous TiO2 form-stable PCM for thermal energy storage[END_REF]. Activated carbon is an organic-based material that has been studied as the supporting structure for PCMs because of its high specific surface area and porosity [START_REF] Tong | Organic phase change materials confined in carbonbased materials for thermal properties enhancement: Recent advancement and challenges[END_REF][START_REF] Nicholas | Activated Carbon for Shape-Stabilized Phase Change Material[END_REF]. The use of activated carbon as the supporting structure could increase the thermal conductivity of the SSPCM [START_REF] Wan | A promising form-stable phase change material prepared using cost effective pinecone biochar as the matrix of palmitic acid for thermal energy storage[END_REF][START_REF] Li | 3D structure fungi-derived carbon stabilized stearic acid as a composite phase change material for thermal energy storage[END_REF][START_REF] Xie | Evaluation of stearic acid/coconut shell charcoal composite phase change thermal energy storage materials for tankless solar water heater[END_REF].

Biomass activated carbon is an environmentally friendly material that can be used as the SSPCM supporting structure, and critically, can be produced in low cost, and environmentally friendly ways. Kim et al. reported that the production of one kilogram of activated carbon from wood waste is capable of reducing global warming potential (GWP) by about 10 kg CO2-eq compare to coal based activated carbon, and the use of one ton of wood waste for activated carbon can reduce the GWP by 163 kg CO2-eq compared to wood waste landfill [START_REF] Kim | Analysis of environmental impact of activated carbon production from wood waste[END_REF]. The shell of coconut which is a by-product of coconut water,milk and oil production has becomes a biomass waste problem, especially in tropical countries [START_REF] Ahmad | The effects of temperature, residence time and particle size on a charcoal produced from coconut shell[END_REF]. This agricultural waste product will be present for as long as the demand for coconuts and coconut products exist. In 2020 alone it was estimated that the global coconut production was 62.5 million tons, with demand set to increase in the coming years [START_REF] Azeta | A review on the sustainable energy generation from the pyrolysis of coconut biomass[END_REF][START_REF] Ignacio | Research opportunities on the coconut (Cocos nucifera L.) using new technologies[END_REF]. The use of biomas waste product for activated carbon sources will not just lower the cost of material, but simultaneously will reduce the cost of dealing with an existing waste product. Food grade coconut oil and activated carbon are already common market products, which means an established production/synthesis procedure and inexpensive price. Adding to that, the use of a simple synthesic method such as avoiding the use of solvents and vacuum for SSPCM fabrication will lower the cost even more.

Many reports investigate the use of activated carbon as a supporting material for organic PCMs; some examples are listed in Table 1. Octadecane as a paraffin compound with high latent heat has been widely researched for thermal comfort applications. Octadecane has also been mixed with biomass-based activated carbon such as palm kernel [START_REF] Nicholas | Palm Kernel Shell Activated Carbon as an Inorganic Framework for Shape-Stabilized Phase Change Material[END_REF][START_REF] Nicholas | The effect of surface area on the properties of shape-stabilized phase change material prepared using palm kernel shell activated carbon[END_REF], and peat soil [START_REF] Hussein | Properties of N-Octadecane-Encapsulated Activated Carbon Nanocomposite for Energy Storage Medium: The Effect of Surface Area and Pore Structure[END_REF]. Despite its high latent heat, octadecane is mostly produced from the refinery process of natural gas (which is non-renewable) and hence relatively expensive. On top of the use of biomassactivated carbon, using organic PCMs, especially bio-based PCMs will increase its environmentally friendly nature. Fatty acid SSPCMs, using palmitic acid [START_REF] Wan | A promising form-stable phase change material prepared using cost effective pinecone biochar as the matrix of palmitic acid for thermal energy storage[END_REF][START_REF] Mehrali | From rice husk to high performance shape stabilized phase change materials for thermal energy storage[END_REF], stearic acid [START_REF] Li | 3D structure fungi-derived carbon stabilized stearic acid as a composite phase change material for thermal energy storage[END_REF][START_REF] Xie | Evaluation of stearic acid/coconut shell charcoal composite phase change thermal energy storage materials for tankless solar water heater[END_REF], and lauric acid [START_REF] Chen | Synthesis and thermal properties of shape-stabilized lauric acid/activated carbon composites as phase change materials for thermal energy storage[END_REF] have been successfully prepared with activated carbon acting as the supporting structure. Fewer reports are found about the combination of biomass-based activated carbon with renewably sourced PCMs [START_REF] Jeon | Latent heat storage biocomposites of phase change material-biochar as feasible eco-friendly building materials[END_REF][START_REF] Jeon | Characterization of biocomposite using coconut oil impregnated biochar as latent heat storage insulation[END_REF], but it is undoubtedly a worthwhile effort to achieve more environmentally sustainable materials with an additional economic incentive in the form of simultaneous elimination of an existing waste product.

For preparation, commonly bioSSPCM are made by physical blending using solvents or by vacuum impregnation. These two methods have shown their potential to produce SSPCMs with a PCM loading capacity of over 90% by weight [START_REF] Mehrali | From rice husk to high performance shape stabilized phase change materials for thermal energy storage[END_REF][START_REF] Jeon | Latent heat storage biocomposites of phase change material-biochar as feasible eco-friendly building materials[END_REF][START_REF] Feng | Polyethylene glycol phase change material embedded in a hierarchical porous carbon with superior thermal storage capacity and excellent stability[END_REF] and retain a melting enthalpy of over 95% compared to its pure PCM counterpart [START_REF] Li | 3D structure fungi-derived carbon stabilized stearic acid as a composite phase change material for thermal energy storage[END_REF][START_REF] Xie | Evaluation of stearic acid/coconut shell charcoal composite phase change thermal energy storage materials for tankless solar water heater[END_REF][START_REF] Khadiran | Shape-stabilised n-octadecane/activated carbon nanocomposite phase change material for thermal energy storage[END_REF]. Nevertheless, the need for solvents, filtering, and vacuum processes translate to higher production costs and environmental impact. To assess the SSPCM, besides the typical material and thermal characterization, a leakage test is unavoidable to confirm the ability of the supporting structure to retain the PCM in its structure. Heating on paper is the most favoured method, as can be seen in Table 1.

In this study coconut-based PCMs were examined, using a simple physical blending method to produce an inexpensive, eco-friendly, and high-performing bioSSPCM, using abundant and easily obtained materials. Coconut oil is a commonly used product that has a melting point near room temperature, hence the potential for maintaining thermal comfort. To stabilize the liquid form, the coconut oil will be supported by activated carbon synthesized from coconut shells. This combination of coconut oil and coconut shell is a novel bioSSPCM, that has the potential as a passive thermoregulator in buildings, especially in tropical countries with abundant production of coconut palms, and as a side note also a way to solve the problem of large coconut shell waste. Two different types of coconut oil (market and analytical grade) were used to investigate if any effect of processing impacts upon the performance for SSPCM applications. To further understand the effect of the simple physical blending synthesis for different types of organic PCM and activated carbon, octadecane as a PCM and two other types of activated carbon as supporting structures were investigated. As illustrated in Fig. 1, despite the low latent heat compared to octadecane, this work investigates the advantages of using coconut-based materials for SSPCMs by simple physical blending synthesis. To our knowledge, this research is the first that reports on the combination of coconut oil and coconut shell-based activated carbon as a bioSSPCM.

Materials and methods

Materials

Steam-activated carbon made from coconut shells, Y-180C (ACsteam) was obtained from Ajinomoto Fine-Techno Inc., food-grade organic virgin coconut oil (COfood) was purchased and used without additional purification. Basic and neutral analytical grade charcoal-based activated carbon (ACbasic and ACneutral) and analytical grade (1 st grade) coconut oil were obtained from Wako Chemical Inc. (COan), and analytical grade octadecane (99%) was purchased from Sigma Aldrich (Oct). All the materials used in this experiment were used directly without prior treatment. The photographs of all the precursors used in this experiment are shown in SI Fig. S1 and Fig. S2.

SSPCM Synthesis

The SSPCM samples were prepared by simple physical blending synthesis, as illustrated in Fig. 2. In a typical preparation, specific amounts of PCM and activated carbon (for the range of values studied, please refer to SI Loading capacity check) were heated up to 80 o C and held for 10 minutes, then the activated carbon was poured into the liquid PCM and stirred for 10 minutes. The resultant material from this process is the shape-stabilized phase change material (SSPCM), used for further analysis. Approximately 3 g of the resulting material was then shaped into a cylinder using a 10 mm diameter metal die-piston set (SI Fig. S3). Before fabricating the SSPCM samples, the PCM optimal ratios were determined by varying the quantity of activated carbon to PCM for samples (SI Loading capacity check). Unless otherwise described, measurements in this work use the determined optimal ratios, 1:0.8 ACsteam:PCM, 1:1.8 ACbasic:PCM, and 1:1.2 ACneutral:PCM.

Characterization

X-Ray Diffraction (XRD) patterns were taken with a Rigaku Smart Lab 3 diffractometer (Cu radiation, λKα1 = 1.5418 Å, and λKα2= 1.5444 Å), across a range of 10 o -90 o with a step width of 0.02 o at a scan speed of 8 o . Fourier Transform Infra-Red (FTIR) spectra were obtained using a Shimadzu IRAffinity-1S in the range of 500-4000 cm -1 . Scanning Electron Microscopy (SEM) images were taken with a Hitachi S-4800 with an accelerating voltage of 10 kV. Using data from the nitrogen adsorption isotherm (BELSORP-mini II Sorption System), surface areas were calculated by the Brunauer-Emmett-Teller (BET) method, and pore diameter and pore volume were calculated by Barrett-Joyner-Halenda (BJH) method. Thermogravimetric analysis (TGA) was conducted using a Hitachi TG/DTA6200 with a 20 o C/min heating rate under nitrogen flow 20 ml/min. Differential Scanning Calorimetry (DSC) was conducted using a Hitachi X-DSC7000 at a 5 o C/min heating rate. From DSC measurement, melting temperatures (Tm) were determined and melting enthalpies (ΔHm) were calculated. Theoretical melting enthalpies (ΔHm-T) were calculated according to equation (ΔHm-T = ΔHm x loading capacity). Thermal diffusivities (𝛼) were measured by a Laser Flash Technique with a Netzsch LFA 467-HT at room temperature using a specialized sample holder. The thermal conductivities (𝜅) were calculated according to equation (𝜅 = 𝛼. 𝐶 𝑝 . 𝜌), where 𝐶 𝑝 is specific heat (obtained from DSC measurement) and 𝜌 is the density at room temperature. Leakage testing was done by heating the samples above a filter paper, from room temperature up to ~75 o C and maintaining the elevated temperature for 30 minutes. Sample masses were taken before and after the heating process. A Testo 868 instrument was used to take the infra-red and real images of the SSPCM samples at four temperature points while conducting the leakage test.

Result and discussion

The preliminary experiment concluded that the optimal loading capacities of the PCM into activated carbon (SI Fig. S3b and3d) are 80%, 180%, and 120% of mass for ACsteam, ACbasic, and ACneutral, respectively. It was found that the loading capacity is the same regardless of PCM type. From the three types of activated carbon and three types of PCM, nine SSPCM samples were prepared, as shown in Table 2.

XRD and FTIR analysis

The XRD patterns of the three activated carbon precursors (Fig. 3a) show broad peaks at 24 o and 43 o , which belong to the (002) and (100) planes, respectively [START_REF] Atinafu | Synthesis of porous carbon from cotton using an Mg(OH) 2 template for form-stabilized phase change materials with high encapsulation capacity, transition enthalpy and reliability[END_REF][START_REF] Li | 3D structure fungi-derived carbon stabilized stearic acid as a composite phase change material for thermal energy storage[END_REF]. The graphite crystal phase at 26.5 o (002 plane) was also observed on the ACneutral compared to the other two activated carbon [START_REF] Saafie | Enhancement of the activated carbon over methylene blue removal efficiency via alkali-acid treatment[END_REF]. This indicates that the three activated carbon are largely amorphous whilst ACneutral contains some graphite crystals. The coconut oil samples show a broad peak at 20 o, indicating the amorphous phase of polymer, similar to the reported pattern by other [START_REF] Xiao | Physical, structural, and water barrier properties of emulsified blend film based on konjac glucomannan/agar/gum Arabic incorporating virgin coconut oil[END_REF], and no significant difference between COfood and COan pattern was observed. The Oct pattern shows the β-form crystal phase of n-octadecane at 19.2 o (010), 19.7 o (011), 24.5 o (-101), and 24.6 o (110) and the α-form crystal phase of n-octadecane at 11.5 o (003), 15.3 o (004), 39.5 o (022), and 44.4 o (207) [START_REF] He | New approach for sol-gel synthesis of microencapsulated noctadecane phase change material with silica wall using sodium silicate precursor[END_REF]. With this, the difference between the amorphous phase of coconut oil and the crystalline phase of octadecane was confirmed.

In the FTIR spectra Fig. 3b, the COfood and COan display absorbencies at 2960 cm -1 , 2921 cm - 1 , and 2854 cm -1, which indicates the sp 3 -CH stretching vibration, at 1742 cm -1 indicating the C=O stretching vibration, at 1466 cm -1 indicating the sp 3 -CH bending vibration, 1160-1109 cm -1 assigned to the acyl ester C-O vibration, and at around 720 cm -1 due to the -CH2 rocking vibration [START_REF] Jeon | Latent heat storage biocomposites of phase change material-biochar as feasible eco-friendly building materials[END_REF][START_REF] Jeon | Characterization of biocomposite using coconut oil impregnated biochar as latent heat storage insulation[END_REF]. The Oct spectrum shows peaks at 2913 cm -1 and 2848 cm -1 corresponding to the stretching vibrations of -CH2 and -CH3 groups respectively, at 1466 cm -1 and 1369 cm -1, which belong to the bending vibrations of the -CH2 and -CH3 groups, and 716 cm -1 which indicates in-plane rocking vibration of the -CH2 group [START_REF] Li | Synthesis and characteristics of form-stable noctadecane/expanded graphite composite phase change materials[END_REF][START_REF] Zhu | Preparation and properties of nanoencapsulated n-octadecane phase change material with organosilica shell for thermal energy storage[END_REF]. The spectrum of activated carbon shows an increasing sloping trend towards lower wavenumbers, with recognizable absorbency peaks at 2360 cm -1 (-CH stretching vibration), 1565-1725 cm -1 (C=O stretching vibration), and 1000-1200 cm -1 (C-O stretching vibration) [START_REF] Khadiran | Shape-stabilised n-octadecane/activated carbon nanocomposite phase change material for thermal energy storage[END_REF].

In the XRD patterns of the SSPCMs (Fig. 4a), peaks that belong to each precursor were observed. The peak of coconut oil at 20 o is observed instead of the peak of activated carbon at 24 o which is likely covered by the peak valley. The activated carbon peaks at 26.5 o and 43 o are observed in the SSPCM samples as well. An interesting fact is, in the XRD pattern of AC-Oct samples, the octadecane crystal peaks are not present. The possible explanation is the X-Ray caused the scattered particles of octadecane to melt [START_REF] Warren | Direct measurement of X-ray-induced heating of microcrystals[END_REF], on top of heat conduction by the activated carbon. Coincidentally, the amorphous liquid octadecane has a broad peak at 20 o [START_REF] Liang | Nanoencapsulation of noctadecane phase change material with silica shell through interfacial hydrolysis and polycondensation in miniemulsion[END_REF]. From the FTIR spectra of SSPCM samples (Fig. 4b), all the observable peaks belong to the PCMs instead of activated carbon. The existence of activated carbon is indicated by the sloping trend of the spectra. From the XRD pattern and FTIR spectra of SSPCM samples, there is no new peak present which indicates that the main interaction between the PCM and the activated carbon is physical interaction in the form of PCM absorption into the porous carbon.

Thermal stability and composition of SSPCM

Plots of the precursor TGA curves are displayed in Fig. 5a. One degradation step (evaporation) occurs at 115-250 o C for octadecane and 280-430 o C for both COfood and COan. The desorption of water particles in all activated carbon precursors mainly happened below 75 o C, as indicated in Fig. 5a. At 470 o C the activated carbon loses a total of 9%, 8%, and 12% of weight for ACsteam, ACbasic, and ACneutral, respectively. It is also interesting to note that from around 60 o C, ACneutral already shows 2-3% difference of weight loss compared to the two other activated carbon.

The thermogravimetry result of the SSPCM samples are shown in Fig. 5b. The heating of activated carbon in the synthesis process successfully expelled the water content in the activated carbon, as there is no degradation below 100 o C observed for all SSPCM samples. All the SSPCM samples show one degradation step (evaporation), just like its PCM precursor. From this TGA result, it was confirmed that for thermal comfort application, all these SSPCM samples are thermally stable.

Besides checking thermal stability, the heating in TGA measurement is also needed to confirm the composition of the SSPCM. Table 3 summarize the composition of PCMs in the samples, comparing synthesis to TGA composition. The PCM components of each sample are almost the same between synthesis and TGA results with differences of 1-2 %. A 5% difference is observed on ACneutral-Oct sample, which may be related to the weight loss of ACneutral that happened after 80 o C. From this TGA composition result, it is confirmed that the simple physical blending synthesis method used in this experiment is highly reliable.

Textural and microstructure analysis

The nitrogen adsorption measurement was performed on the three activated carbon precursors and three representatives of the samples, as shown in Fig. 6a, and the corresponding BJH pore size distribution is shown in Fig. 6b. The ACsteam precursor was identified as a type I adsorption isotherm [START_REF] Thommes | Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution (IUPAC Technical Report)[END_REF] in which a steep increase of N2 adsorption happened at very low pressure followed by negligible increment afterward. This steep increase of N2 corresponds to a very high degree of microporosity (ø < 2 nm) as detailed in the Fig. 6d, and the negligible increment afterward corresponds to a very low degree of mesoporosity (2 nm < ø < 50 nm) as detailed in Fig. 6c. The ACbasic and ACneutral precursors show a steep increase of N2 adsorption followed by a sharp knee and then a slight gradual increase which resembles the combination of type I and type II adsorption isotherm. This indicates that the pore size distribution is dominated by micropores with a small quantity of mesopores and macropores (50 nm < ø < 1000 nm) as detailed in Fig. 6c. From the difference of ACsteam with ACbasic and ACneutral curve, it was determined that ACsteam has a higher degree of microporosity compared to ACbasic and ACneutral.

The ACsteam-COfood, ACbasic-COan, and ACneutral-Oct curves are identified as type III adsorption isotherm which suggests that the material exhibits nonporous or macropores texture. The ACsteam-COfood and ACbasic-COan samples show almost no N2 adsorption in all pressure range, which indicate fully occupied pores by COfood and COan. The ACneutral-Oct shows a small increase of N2 adsorption in high relative pressure, which is indicative of interstices between the ACneutral-Oct particles. Table 4 summarizes the BET and BJH calculation results for the activated carbon precursor. The higher calculated SBET value of ACsteam compared to ACbasic and ACneutral affirms the notion of the highest degree of micropores in ACsteam that result in higher surface area. Further, based on the cumulative volume of pores, that ACbasic has the greatest, followed by ACneutral and then ACsteam, which is in line with the loading capacity result (ACbasic > ACneutral > ACsteam). The most common pore diameter of the activated carbons is below 2 nm, or microporous. Fig. 7 provides SEM images of the activated carbon precursor and SSPCM samples. The ACsteam, ACbasic, and ACneutral show similar irregular forms of particles which is common for activated carbon, with small particles size below 1 μm while the larger particle observed were in the range of 5-10 μm. This variation particle size might affect the size of interstitial area but not related to the size of the pores. The size of The six COfood and COan-based samples also show similarity in which we can observe the interconnected white glow from electron charging caused by the insulating nature of coconut oil. This interconnected white glow indicates interconnection between particles of SSPCM by the melted part of coconut oils, and also supports the nitrogen adsorption result on ACsteam-COfood and ACbasic-COan samples that no interstices exist between the particles, hence the fully occupied pores. On the other hand, the three octadecane-based samples show similarity with the activated carbon precursor in which the particles are separated, which is highly likely caused by octadecane's crystalline nature. This also explains the N2 adsorption measurement on ACneutral-Oct in which some macropores are detected which actually are the interstices between particles. This difference between the coconut oil and octadecane-based SSPCM samples further explains the difference between the amorphous and crystalline structure PCM and its effect when composed with amorphous carbon.

Comparing the N2 adsorption measurement of ACsteam-COfood and ACbasic-COan to the SEM results, it was determined that all COfood and COan-based samples fully occupy the pores of the activated carbon. Despite the SEM images of octadecane-based samples showing similarity with its activated carbon precursors, however considering the N2 adsorption measurement result of the ACneutral-Oct sample and TGA results of all octadecane-based samples, it suggests that the octadecane also fully occupy activated carbon pores. This also accentuates the importance of loading capacity determination prior to actual sample synthesis.

Thermal storage and transport properties

The melting temperature (Tm) with its onset and offset melting temperature (Tm-on, and Tm-off respectively) of the SSPCM samples are displayed in Fig. 8. These values were obtained from DSC measurement of the samples as shown in SI Fig. S5. From DSC measurement in SI Fig. S5 it was observed that COan showed multiple melting events, with a primary Tm at 22.0 o C and a secondary at 29.4 o C, caused by the inhomogeneity of the COan components. Comparatively, the COan-based SSPCMs showed a single melting event similar to that of COfood-based samples, suggesting the homogenization of the PCM during the physical blending. In Fig. 8 it can be seen that Tm and Tm-off of all coconut oil-based samples show similarity to the COfood. However, it was observed that there is a slight increase of Tm-on of the samples compared to COfood, which means the addition of activated carbon caused the melting process to start at a slightly higher temperature. This slight increase of Tm-on is most likely caused by the increase of thermal conductivity, which promotes heat conduction and results in a more homogenous melting process. Conversely, when there is no activated carbon in coconut oil, the heat conduction will be slower and cause an inhomogeneous melting process, i.e., the outer area will melt faster compared to the inner area.

The Tm, Tm-on, and Tm-off values of octadecane that were measured are in agreement with the reported values [START_REF] Lee | Development of thermal enhanced noctadecane/porous nano carbon-based materials using 3-step filtered vacuum impregnation method[END_REF][START_REF] He | New approach for sol-gel synthesis of microencapsulated noctadecane phase change material with silica wall using sodium silicate precursor[END_REF]. The mixing of octadecane with activated carbon widen the Tm-on-Tmoff range, with a significant increase of the Tm-off, and a slight decrease in Tm-on. The Tm of all octadecane-based samples also increases significantly when made into a composite. The slight decrease in Tm-on phenomena is quite the opposite of the coconut oil-based samples. Unlike the inhomogeneous melting process in coconut oil, the separate crystals of solid octadecane assure a higher surface area for heat exposure. The addition of activated carbon increases the thermal conductivity of the solid octadecane-based samples resulting in a faster start of the melting process, i.e., lower Tm-on. Once the octadecane in the samples starts melting, the addition of activated carbons increases both Tm and Tm-off values. The reason for this temperature increment is likely the same as the reason for the increment of Tm-on in coconut oil-based samples.

The ΔHm of the COfood and COan measured are consistent with the melting enthalpy of coconut oil summarized by Kahwaji and White from various reports as 105+11 J/g [START_REF] Kahwaji | Edible Oils as Practical Phase Change Materials for Thermal Energy Storage[END_REF]. The ΔHm of the Oct that was measured is also in agreement with other reported values [START_REF] He | New approach for sol-gel synthesis of microencapsulated noctadecane phase change material with silica wall using sodium silicate precursor[END_REF][START_REF] Liu | Fabrication and characterization of novel meso-porous carbon/n-octadecane as formstable phase change materials for enhancement of phase-change behavior[END_REF]. From Table 5, it is evident that the ΔHm of all SSPCM samples decreases compared to that of its PCM precursor. This decrease in ΔHm is expected because any given SSPCM consists of PCM and activated carbon. Unlike the PCM, activated carbon does not act as latent heat storage, and hence has the direct effect of reducing the heat storage capacity. However, the addition of activated carbon is important to improve the thermal conductivity and stability of PCM.

The measured melting enthalpy value (ΔHm) was compared with the theoretical melting enthalpy (ΔHm-T) for a given SSPCM sample and the percentage of theoretical (ΔH%) was calculated as a preliminary indicator of quality, resulting in values between 60-86% of ΔHm-T, as shown in Table 5. The sub-100% ΔH% performance of the composite materials was attributed to the effect of the pore size of activated carbon that causes i) limitation of the volume of expansion while melting, hence limiting the absorbed latent heat, and ii) the unreachable pores under ambient pressure synthesis, leads to air pockets that were also limiting volume expansion of PCM. Nevertheless, this synthesis method resulting in > 60% of ΔH% is satisfying -considering the simple and less expensive synthesis method.

To confirm the effect of activated carbon addition into the PCMs, four representative samples were selected, and the results compared to the thermal conductivity of the PCMs reported in the literature as shown in Fig. 9. The value of thermal conductivity of the coconut oil and octadecane were taken from the literature [START_REF] Jeon | Latent heat storage biocomposites of phase change material-biochar as feasible eco-friendly building materials[END_REF][START_REF] Khadiran | Activated carbon derived from peat soil as a framework for the preparation of shape-stabilized phase change material[END_REF]. As expected, the addition of activated carbon increases the thermal conductivity of PCM, as has been reported for some organic-based SSPCMs such as fatty acids [START_REF] Wan | A promising form-stable phase change material prepared using cost effective pinecone biochar as the matrix of palmitic acid for thermal energy storage[END_REF][START_REF] Li | 3D structure fungi-derived carbon stabilized stearic acid as a composite phase change material for thermal energy storage[END_REF][START_REF] Xie | Evaluation of stearic acid/coconut shell charcoal composite phase change thermal energy storage materials for tankless solar water heater[END_REF][START_REF] Chen | Synthesis and thermal properties of shape-stabilized lauric acid/activated carbon composites as phase change materials for thermal energy storage[END_REF], PEG [START_REF] Feng | Polyethylene glycol phase change material embedded in a hierarchical porous carbon with superior thermal storage capacity and excellent stability[END_REF][START_REF] Feng | Preparation and characterization of polyethylene glycol/active carbon composites as shape-stabilized phase change materials[END_REF], and octadecane [START_REF] Khadiran | Shape-stabilised n-octadecane/activated carbon nanocomposite phase change material for thermal energy storage[END_REF][START_REF] Khadiran | Activated carbon derived from peat soil as a framework for the preparation of shape-stabilized phase change material[END_REF] as summarized in Table 1. In this experiment, the addition of activated carbon into coconut oil increased the thermal conductivity 31-44%, while the addition of activated carbon into the octadecane increased the thermal conductivity 32-52%.

Shape-Stabilize and Leakage Performance

The leakage phenomenon in SSPCMs is explainable by two physical parameters: i) microporosity which affects the capillary force of the PCM and ii) viscosity which affects the surface tension of the PCM. Both capillary force and surface tension will affect the flowability of PCM inside the carbon pores. The viscosity of coconut oil was reported to be 36 mPas and 7.6 mPas [START_REF] Tipvarakarnkoon | Rheological properties and phase change behaviors of coconut fats and oils[END_REF] and octadecane was reported to be 3.9 mPas and 1.6 mPas [START_REF] Faden | Review of Thermophysical Property Data of Octadecane for Phase-Change Studies[END_REF] at room temperature and 80 o C, respectively. Higher temperatures will increase the possibility of leakage; however, the capillary force caused by the micropore channels can prevent this leakage. Fig. 10 shows the infra-red and actual image of the samples while heating, taken at some temperature points. After heating up to 75 o C and holding for 30 minutes, the cylindrical shape of all samples was retained and there was no leakage observed until 46 o C, which is already above the melting temperature of the samples. Observable in the last image in Fig. 10, after holding the samples at 75 o C for 30 minutes, the majority of SSPCM samples leave traces of seepage on the paper. The sample mass was measured before and after the heating and the mass loss was calculated, as shown in Table 6. Two samples (ACsteam-COfood and ACsteam-COan) showed a good result with no leakage trace on the paper. All octadecane-based samples show leakage, especially ACsteam-Oct and ACneutral-Oct with losses above 3% of the weight after the test. The other four coconut oil-based samples also showed leakage between 0.5%-1.5% of the mass.

The minimum leakage measured in ACsteam-COfood and ACsteam-COan samples is caused by the high degree of microporosity of the ACsteam precursor, which causes high capillary force, and the relatively high viscosity of coconut oil, which prevents its outflow even above the melting point. On the other hand, despite the high degree of microporosity of ACsteam, leakage is observed in the ACsteam-Oct sample, which is caused by the low viscosity of octadecane. This low viscosity of octadecane also explains the leakage observed in ACbasic-Oct and ACneutral-Oct samples, on top of the small number of macropores observed in ACbasic and ACsteam that gives lower capillary force.

Besides the leakage test, a simple three-cycles DSC measurement on COfood and the ACsteam-COfood samples were conducted to see the stability of the samples over time, as shown in SI Fig S6a and6b. The 1 st cycle of COfood shows two melting peaks and two freezing peaks, and in the second and third cycle the melting peaks converge into one while the two freezing peaks stayed the same. The ACsteam-COfood sample shows almost overlapped curve of the three cycles with one melting and one freezing peak. From this measurement it could be concluded that activated carbon addition homogenize all the coconut oil component and converged the melting/freezing peak.

Conclusion

The abundant availability of coconut materials, especially in tropical countries, coupled with the simple fabrication method is economically promising and environmentally friendly. The coconut-based bioSSPCM was successfully synthesized by a simple physical blending method. The loading capacity of coconut shell-based activated carbon (ACsteam) is 44%, and after the intrusion of coconut oil into its pores, resulting in bioSSPCM with an enthalpy of 42.6 J/s which is 60% of its theoretical value. Despite its lower loading capacity and melting enthalpy, the coconut bioSSPCM has the advantage of anti-leakage performance and thermal stability on top of its abundant-inexpensive material and synthesis process. Besides, the addition of activated carbon increased the thermal conductivity, which promoted heat conduction for faster and homogenous thermal distribution. For the purpose of building comfort, the need for large amounts of SSPCM that translate to high cost could potentially be solved with this coconutbased bioSSPCM, and the anti-leakage property also means even lower cost (low maintenance). Further research regarding integration methods, mechanical, and safety will be needed. It was confirmed that there is no distinct difference between food grade and analytical grade coconut oil in terms of its application for SSPCM materials. Further, it was shown that the simple physical blending method is not suitable for octadecane-based SSPCMs, because of its crystallinity and low viscosity, preventing stability of the SSPCM.

Fig. 1

 1 Fig. 1 Illustration of how coconut oil and coconut shell can be synthesized into shape stabilized phase change materials. The triangle diagram illustrating advantages and disadvantages of using coconut vs. octadecane for SSPCM material.

Fig. 2

 2 Fig. 2 Illustration of the simple physical blending synthesis of shape stabilized phase change materials.

Fig. 3 (

 3 Fig. 3 (a) XRD patterns and (b) FTIR spectra of activated carbon and PCM prior to physical blending.

Fig. 4 (Fig 5 .

 45 Fig. 4 (a) XRD patterns and (b) FTIR spectra of SSPCM samples after physical blending.

Fig. 6 (

 6 Fig. 6 (a) Nitrogen adsorption isotherm, (b) BJH pore size distribution for activated carbon and three SSPCM samples, and (c) 50 nm scale BJH pore size distribution, with inset in 10 nm scale

Fig. 9

 9 Fig. 9 Thermal conductivity of ACsteam-COfood, ACbasic-COan, ACneutral-Oct, and ACsteam-Oct. Error bar shows maximum possible standard deviation (6%).

Fig. 10

 10 Fig. 10 Infra-red thermal imaging (left) and its actual photo (right). The last photo was taken directly after removing the samples, and then edited to clarify the absorbed PCM trace. (Y-MCO= ACsteam-COfood;Y-WCO= ACsteam-COan;Y-SAO= ACsteam-Oct;B-MCO= ACbasic-COfood;B-WCO= ACbasic-COan;B-SAO= ACbasic-Oct;N-MCO= ACneutral-COfood;N-WCO= ACneutral-COan;N-SAO= ACneutral-Oct)

Table 1

 1 Summary of some organic based shape stabilized phase change materials

Table 2

 2 The nine SSPCM samples obtained from mixing three PCMs by three activated carbons

	Activated carbon source	Organic PCM	Preparation method	PCM loading capacity	ΔHm%	Leakage test method	Thermal conductivity (W/m.K) SSPCM PCM	Ref
	Pinecone	Palmitic acid	Vacuum impregnation	60.0%	64.3%	Heating on paper	0.39	0.27	22
	Rice husk	Palmitic acid	Vacuum impregnation	90.0%	96.1%	Heating without paper	0.28	0.28	31
	Fungus (Esculenta)	Stearic acid	Vacuum impregnation	77.5%	97.5%	Heating on paper	0.57	0.27	23
	Coconut shell	Stearic acid	Vacuum impregnation	35.3%	97.9%	Heating on paper	0.75	0.26	24
	Commercial	Lauric acid	Physical blending	48.4%	74.8%	None	0.15	0.12	32
	MOF-5	PEG2000	Physical blending with solvent	92.5%	90.6%	Heating on paper	0.31	0.33	23
	Coconut shell	PEG6000	Physical blending with solvent	70.0%	58.6%	Not specified	-		36
	Peat soil	Octadecane	Physical blending with solvent	47.4%	84.1%	Heating on paper	0.17	0.11	37
	Commercial	Octadecane	Physical blending with solvent	45.0%	94.5%	Heating on paper	0.17	0.11	38
	Rice husk	Coconut oil	Vacuum impregnation	80.0%	79.4%	Exudation stability	0.20	0.32	33
		Activated			PCM
		carbon	COfood	COan	Oct
			ACsteam	ACsteam-COfood	ACsteam-COan ACsteam-Oct
			ACbasic	ACbasic-COfood	ACbasic-COan	ACbasic-Oct
		ACneutral	ACneutral-COfood ACneutral-COan ACneutral-Oct

Table 3 .

 3 %weight content of the precursor, comparison of the actual synthesis and TGA result

Table 4 .

 4 Specific surface area calculated with BET method and pore properties calculated using BJH method

	SSPCM sample	PCM content Synthesis TGA
	ACsteam-COfood	44%	result 44%
	ACsteam-COan	44%		44%
	ACsteam-Oct	44%		46%
	ACbasic-COfood	64%		64%
	ACbasic-COan	64%		64%
	ACbasic-Oct	64%		64%
	ACneutral-COfood	55%		56%
	ACneutral-COan	55%		53%
	ACneutral-Oct	55%		60%
		SBET (Specific surface area) (m 2 /g))	BJH adsorption cumulative volume of pores (cm 3 /g)	BJH adsorption frequent pore diameter (nm)
	ACsteam	735.48	0.17		0.16
	ACbasic	656.43	0.58		0.17
	ACneutral	430.13	0.37		0.17

Table 5 .

 5 Melting enthalpy of PCM and SSPCM samples

Table 6 .

 6 Sample mass decrease (in percent) after leakage testing

	SSPCM Samples	ΔHm measure d (J/g)	ΔHm theoretic al (J/g)	ΔH% (measure d/theoreti cal) (J/g)
	COfood	96.8
	ACsteam-COfood	25.5	42.6		60%
	ACbasic-COfood	41.3	62.9		66%
	ACneutral-COfood	38.6	55.2		70%
		96.2
	ACsteam-COan	25.5	42.3		60%
	ACbasic-COan	43.8	61.6		71%
	ACneutral-COan	34.4	51.0		67%
	Oct	198.8
	ACsteam-Oct	62.0	91.4		68%
	ACbasic-Oct	109.0	127.2		86%
	ACneutral-Oct	97.5	121.3		80%
		COfood COan	Oct
	ACsteam 0.17% 0.19% 3.84%
	ACbasic 1.09% 1.52% 1.06%
	ACneutral 0.83% 0.54% 5.16%

Acknowledgements

A.R.M. acknowledges that his doctoral study in ITB has been financially supported by Lembaga Pengelola Dana Pendidikan (LPDP), Ministry of Finance of Indonesia. A.R.M. acknowledges the NIMS-ICGP program for the Indonesia-Japan mobility grant. A.R.M thanks Dr. Muhammad Iqbal from ITB for analysis N2-sorption. C.L.H and B.S. thank JSPS for the postdoctoral fellowships P21723 and P19720 respectively. T.M. acknowledges the funding from JSPS KAKENHI 19F19720 and JST-Mirai JPMJMI19A1.

Conflicts of interest

There are no conflicts to declare.

Supporting Information (SI) available: Photos of precursors, die-set for sample cylinder-shaping, loading capacity check, and DSC measurement.


Masako Uematsu
email: uematsu.masako@nims.go.jp

Kento Ishii

Sadaki Samitsu

Izumi Ichinose

Naoki Ohashi

David Berthebaud

Jean-François Halet

Takamasa Ishigaki

Tetsuo Uchikoshi
email: uchikoshi.tetsuo@nims.go.jp

Fabrication and characterization of zeolite bulk body containing mesopores and macropores using starch as pore-forming agent

Keywords: ZSM-5, starch, mesoporous material, zeolite bulk, three-dimensional observation

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Zeolites are microporous crystalline aluminosilicates known as molecular sieves that have been widely used in industry as adsorbents, catalysts, and ion exchangers. The hydrothermal method is commonly used for the synthesis of zeolites, which are generally obtained as powders consisting of nano-to micron-sized crystals [START_REF] Cundy | The hydrothermal synthesis of zeolites: History and development from the earliest days to the present time[END_REF][START_REF] Cundy | The hydrothermal synthesis of zeolites: Precursors, intermediates, and reaction mechanism[END_REF][START_REF] Corma | From Microporous to Mesoporous Molecular Sieve Materials and Their Use in Catalysis[END_REF]. The molecular sieving ability of zeolites depends on their crystalline structure containing pores and cavities of molecular dimension. Efforts have long been made to increase the pore size of zeolite crystal units due to the potential capacity of chemical processes that require larger pore spaces as reaction fields.

Consequently, the synthesis of extra-large pore zeolite has been attempted [START_REF] Jiang | Extra-Large-Pore Zeolites: Bridging the Gap between Micro and Mesoporous Structures[END_REF][START_REF] Jiang | ITQ-54: a multi-dimensional extra-large pore zeolite with 20 x 14 x 12-ring channels[END_REF][START_REF] Sun | The ITQ-37 mesoporous chiral zeolite[END_REF][START_REF] Bieseki | Synthesis and structure determination via ultra-fast electron diffraction of the new microporous zeolitic germanosilicate ITQ-62[END_REF][START_REF] Kapaca | Synthesis and Structure of a 22 x 12 x 12 Extra-Large Pore Zeolite ITQ-56 Determined by 3D Electron Diffraction[END_REF][START_REF] Sala | ITQ-69: A Germanium-Containing Zeolite and its Synthesis, Structure Determination, and Adsorption Properties[END_REF], leading to the synthesis of ITQ-37 having a 30-membered ring. However, the diameter of largest-free-sphere of zeolite units listed in the IZA database [START_REF] Baerlocher | Database of Zeolite Structures[END_REF] is still less than 10 Å. For now, the reactants or supported materials on the zeolites are limited in terms of cavity size. Mesoporous materials, such as MCM-41, are also known as zeolite-like materials with nanoscale pore size structures [START_REF] Corma | From Microporous to Mesoporous Molecular Sieve Materials and Their Use in Catalysis[END_REF][START_REF] Beck | A new family of mesoporous molecular-sieves prepared with liquidcrystal templates[END_REF][START_REF] Kresge | Ordered mesoporous molecular-sieves synthesized by a liquid-crystal template mechanism[END_REF][START_REF] Ciesla | Ordered mesoporous materials[END_REF]. However, due to their low crystallinity, their ability as acid catalysts is lower than that of the zeolites.

Zeolite bulks containing mesopores and macropores have agr eat potential to improve the capabilities of larger molecular reactors. S. Zou et al. reported the synthesis of a hierarchically porous zeolite using a polyurethan form template [START_REF] Zou | Catalytic Performance of Toluene Combustion over Pt Nanoparticles Supported on Pore-Modified Macro-Meso-Microporous Zeolite Foam[END_REF], and M. Anderson et al. proposed a biomineralization method using diatoms [START_REF] Anderson | Catalytic Hierarchical pore structures through diatom zeolitization[END_REF]. However, they did not mention the mechanical strength. The handling availability with a sufficient mechanical strength is an important factor for practical use as a bulk body. E. Igi et al. synthesized a porous zeolite bulk body by one-pot hydrothermal method and reported that it had sufficient mechanical strength for machining. [START_REF] Igi | Fabrication of Large Porous ZSM-5 Bulk Bodies by a One-pot Hydrothermal Method[END_REF][START_REF] Igi | Separation of alcohol/water mixtures by ZSM-5 bulk bodies prepared with a one-pot hydrothermal method[END_REF]. Here, we propose a new processing method for producing a porous zeolite bulk body that contains micropores, mesopores, and macropores and that has sufficiently high mechanical strength for handling. Hybrid materials consisting of zeolites and functional particles are expected to be used as novel catalysts or purification filters. Mesopore will be effective in supporting functional nanoparticles. Continuous macropores will be essential for the smooth flow of gas or liquid that needs to be treated.

Starch has been used as a pore-forming agent, and a hierarchical porous structure was introduced into the zeolite bulks [START_REF] Hizukuri | Temperature Dependence of Retrogradation of Starch Pastes[END_REF][START_REF] Buléon | Starch granules: structure and biosynthesis[END_REF][START_REF] Ishii | Fabrication of porous (Ba,Sr)(Co,Fe)O3-delta (BSCF) ceramics using gelatinization and retrogradation phenomena of starch as pore-forming agent[END_REF]. Starch forms a gel with a α-starch network when heated with water. The α-starch gel turns into solidified β-starch after a cooling process, thus keeping the network structure. This starch property is appropriate for a pore-forming agent to make a networked porous structure [START_REF] Ishii | Fabrication of porous (Ba,Sr)(Co,Fe)O3-delta (BSCF) ceramics using gelatinization and retrogradation phenomena of starch as pore-forming agent[END_REF]. In this study, rice starch was used as a pore-forming agent, and a hierarchical porous structure was introduced into the zeolite bulks.

We investigated the morphological changes of the pore structure in the zeolite bulk body involved with the gelation treatment of the pore-forming agent. In order to reveal the hierarchically pore structure, it is necessary to investigate the existence of micropores with a diameter of less than 2 nm, mesopores with a diameter of 2-50 nm, and macropores with a diameter of more than 50 nm. Micropores and mesopores can be generally characterized from N2 gas adsorption/desorption isotherms [START_REF] Barrett | The Determination of Pore Volume and Area Distributions in Porous Substances. I. Computations from Nitrogen Isotherms[END_REF][START_REF] Thommes | Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution (IUPAC Technical Report)[END_REF]. For macropores, we attempted the direct observation of the pore structure using a confocal laser fluorescent microscope (CLFM) employing the liquid immersion method. It is a high-resolution three-dimensional observation method which was developed to characterize the defects in a ceramic green-body and granules [START_REF] Uematsu | Immersion microscopy for detailed characterization of defects in ceramic powders and green bodies[END_REF][START_REF] Uematsu | Processing defects in ceramic powders and powder compacts[END_REF]. The liquid immersion method is also an internal structure observation technique that can be applied to a system in which the material is transparent to the observation light and has a little optical anisotropy. A ceramic compact or a bulk body becomes transparent when an immersion liquid having the close refractive index to the raw material of the sample is permeated into the open pores, thus reducing light scattering on the surface of the raw material. Observation inside the samples becomes available using a fluorescent microscope by dissolving a small amount of fluorescent agent in the immersion liquid. In our investigations, we used this technique to characterize the porous structure in the bulk zeolite. Finally, the mechanical strength and gas permeation rate of the bulk body were evaluated and associated with the pore structure.

Experimental procedures

Commercially-available ZSM-5 zeolite powder (HSZ-840HOA, Tosoh Corp., Japan) was used as the starting material. The particle size distribution of the ZSM-5 powder measured by the DLS method (Nanotrac UPA-UT, Nikkiso Co., Ltd.) is shown in Fig. S1. Aqueous colloidal silica (Snowtex ST-S, Nissan Chemical Corp., Japan) with the average particle size of 9 nm, containing 30 wt% SiO2, was used as a binder. The ZSM-5 powder was added to the aqueous colloidal silica so that the weight ratio of zeolite to silica was 2:1. The slurry was ball-milled at 30 rpm for 24 hours using a zirconia ball medium (2-10 mm diameter). Rice starch powder (Sigma-Aldrich Co. LLC) with the particle size range of 2.0-8.0 µm was added to the slurry as a poreforming agent in the amount of 20 wt% or 0 wt% vs. the solid contents in the slurry. The starch was mixed and dispersed in the slurry using a planetary centrifugal mixer (ARE-310, THINKY Corp., Japan) at 2000 rpm for 5 minutes. The slurry was cast into acrylic molds with a 16 mm diameter and steamed in water vapor at 80 ℃ for 10 minutes to gelatinize the starch or just dried with no heating. The green bodies were treated in an autoclave for 24 hours in steam at 180 °C without direct contact with water as shown in Fig. 1 to bind the zeolite particles by a surface reaction between the colloidal silica and the zeolite. The consolidated bodies were then thermally treated in air at 450 ℃ for 24 hours to remove the starch. Finally, three porous zeolite bulk bodies formed by different processes were obtained. In other words, the following three types of samples were prepared as sample (1); sample without starch, sample (2); sample with starch and no heat-treatment, and sample (3); sample with starch and heat-treatment. The schematic illustrations of the expected internal structure of each sample are shown in Fig. S2.

The crystal structures of the raw zeolite powder and the bulk samples were identified by an X-ray powder diffraction measurement (Miniflex600, Rigaku) with CuKα radiation. The pore size distribution in the mesoporous region was characterized by a nitrogen-gas adsorption/desorption measurement using a surface area and pore distribution size analyzer (BELSORP-Ⅱ, MicrotracBEL). The Barrett-Joyner-Halenda (BJH) method was applied to the desorption isotherm to calculate the pore size distribution. The BJH method is based on the Kelvin equation, which assumes a system of open-ended, cylindrical mesopores. The capillary radius can be calculated when capillary condensation occurs from the Kelvin equation [START_REF] Barrett | The Determination of Pore Volume and Area Distributions in Porous Substances. I. Computations from Nitrogen Isotherms[END_REF]. The specific surface area was calculated from the αs-plot since the Brunauer-Emmet-Teller (BET) theory is not applicable to microporous materials [START_REF] Thommes | Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution (IUPAC Technical Report)[END_REF]. The fracture surface of the bulk bodies was observed by scanning electron microscopy (JSM-6500F, JEOL). The internal structure of the bulk bodies was observed by a confocal scanning laser fluorescence microscope (TCS-SP5, Leica Microsystem) using the liquid immersion method [START_REF] Uematsu | Immersion microscopy for detailed characterization of defects in ceramic powders and green bodies[END_REF][START_REF] Uematsu | Processing defects in ceramic powders and powder compacts[END_REF] as schematically illustrated in Fig. 2. The sample became transparent to visible light when the immersion liquid, which has the refractive index n close to that of the base material, permeated into the samples due to reducing of the reflection of light at the interface of the powder particles and the surrounding medium. The refractive index of the synthesized ZSM-5 is 1.48 [START_REF] Olson | Chemical and physical-properties of the ZSM-5 substitutional series[END_REF], therefore, 1-methyl-2-francalboxyrate (n =1.48-1.49) [START_REF]The Chemical Society of Japan[END_REF] was selected as the immersion liquid. The fluorescent agent, Rhodamine B, was dissolved in the immersion liquid before liquid permeation to distinguish between the open pores and zeolite bulk. The samples were fractured, then ground with an abrasive paper to a thickness of 200-500 μm before the liquid immersion. The internal microstructures of the samples were observed by confocal scanning laser fluorescence microscopy. Crosssectional images of the sample from the surface to a depth of 65 μm were continuously obtained at intervals of 0.26 μm. The sliced images were binarized into the pore and base material phases, then these images were stacked to create a three-dimensional image using an image processing software (AVIZO, Thermo Fisher Scientific, Inc.). The density of the bulk bodies was measured by Archimedes' method using kerosene as the solvent. The mechanical strength of the bulk bodies was evaluated by compressive strength tests using an autograph (AG-I, SHIMADZU, Corp.) based on Japanese industrial standards [START_REF]Testing methods for compressive strength of fine ceramics[END_REF]. The samples were polished to a diameter of 12 mm and a height of 10 mm for the tests. Seven test pieces were used to measure the strength of each sample. The gas permeabilities of the samples were evaluated by a permeation membrane analyzer (PMA-601, SepraTek). A circular hole with a diameter of 6 mm was cut in the center of a round PET sheet with a diameter of 55 mm and a thickness of 110 μm. A sample with a diameter of 12 mm and a thickness of 2 mm was then fixed in the hole of the round PET sheet with epoxy resin. The sample fixed to the PET sheet was placed in the chamber of the analyzer. The amount of gas permeation per unit of time was measured by flowing 0.2 MPa of N2 gas from one side of the sample as shown in Fig. 3.

Results & Discussion

Hard, crack-free bulk samples were successfully fabricated. A zeolite bulk body with starch heated is shown in Fig. 4. The other samples are also white lumps, suggesting that the starch has been completely removed. X-ray diffraction patterns of the raw ZSM-5 powder and the bulks are shown in Fig. 5. All peaks of the bulk samples were identified as peaks of the ZSM-5 phase. The results indicated that the bulk body maintained the crystal structure of the ZSM-5 zeolite through all the processes. The colloidal silica phase did not appear in the XRD probably because it remained as an amorphous phase.

The N2 adsorption/desorption isotherms measured at 77 K are shown in Fig. 6. According to the IUPAC, the physisorption isotherm is grouped into six types [START_REF] Thommes | Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution (IUPAC Technical Report)[END_REF]. Furthermore, Type Ⅰ and Type Ⅳ have the subcategories (a) and (b), respectively. The isotherm of the raw ZSM-5 powder is classified as typeⅠ(a), indicating that it was a typical microporous material. On the other hand, the isotherms of the bulk bodies showed hysteresis loops which were provided by mesoporous adsorbents classified as Type IV(a). However, the isotherms of the bulk samples also indicated a steep uptake at a very low relative pressure the same as the raw material due to the micropore filling. In addition, the successive uptake of bulk's isotherms in the high relative pressure region revealed the presence of macropores. It suggests that bulk samples contained pores of various sizes not found in the raw zeolite powder, such as mesopores and macropores. There was no significant difference in the shape of the adsorption isotherm between the sample (1) and the sample (2). The sample [START_REF] Corma | From Microporous to Mesoporous Molecular Sieve Materials and Their Use in Catalysis[END_REF] clearly increased the amount of adsorption at the same relative pressure. For example, the amount of adsorption at a relative pressure of around 1.0 was approximately 200 cm 3 (STP)/g for the sample (1) and the sample (2), while it was approximately 240 cm 3 (STP)/g for the sample (3).

The pore size distributions obtained from the BJH plots are shown in Fig. 7. Since the capillary condensation theory cannot be applied to pores below 2 nm, the BJH plot cannot provide micropore comparisons. However, it is effective for discussion about the mesopore region. All the bulk samples had a peak distribution at 7-9 nm, corresponding to mesopores, regardless of the addition of starch. It was also found that the sample (3) contained the largest volume of mesopores. The amount of mesopores did not significantly change between the sample (1) and sample [START_REF] Cundy | The hydrothermal synthesis of zeolites: Precursors, intermediates, and reaction mechanism[END_REF]. The results mean that mesopores would be formed between the zeolite particles in the bulk, and that they were increased by the starch network structure. The zeolite bulks with mesopores would be suitable for supporting functional nanoparticles. The specific surface area of the samples calculated by the αs -plots was as follows: sample (1): 378 m 2 g -1 ; sample (2): 375 m 2 g -1 ; sample (3): 387 m 2 g -1 ; ZSM-5 raw powder: 573 m 2 g -1 . The three bulk samples have very similar specific surface areas to each other, and they were lower than that of the ZSM-5 raw powder. For the bulk sample, the contact between the particles leads to a lower surface area. The presence of the pore-forming agent did not contribute to the specific surface area because the amount of micropores was the dominant factor for the specific surface area of the bulk samples.

Macroporous structures were characterized by observation using a scanning electron microscope (SEM) and a confocal laser fluorescence microscope (CLFM). SEM images of the fracture surface at two different magnifications and CLFM images of the corresponding samples are shown in Fig. 8. The CLFM images were taken at a depth of 20 m from the sample surface. Black spots on the CLFM images correspond to pores. The structural images obtained by the two observation methods are similar demonstrating the validity of this internal observation method. Cross-sectional images by CLFM revealed that the structure of the pores clearly changes with the addition of starch and its gelation process. No large pores were observed in the sample (1).

On the other hand, large pores with a size of more than 5 μm were observed in the sample (2) and sample (3), and the two pore structures were different depending on the presence or absence of the heating process. The higher magnified SEM images revealed the existence of pores that have a size of less than 1 μm. The threedimensional images of the macropores created from 173 sliced CLFM images per each sample are shown in Fig. 9. Conventional micro-X-ray CT is generally known as a three-dimensional observation method, but it is difficult to observe a pore structure of several microns in terms of resolution, except when using large synchrotron radiation facilities [START_REF] Promentilla | Characterizing the 3D Pore Structure of Hardened Cement Paste with Synchrotron Microtomography[END_REF]. It was demonstrated that the CLFM method is excellent for facilitating the 3D high resolution observations. Results of the density measurement by Archimedes' method is shown in Table 1. As already mentioned, the sample (2) and the sample (3) had different macropore structures, but the porosity measured by Archimedes' method was similar because the same amount of pore-forming agent was added.

The compressive strength of the zeolite bulk bodies is shown in Fig. 10. All the samples had a compressive strength above 25 MPa. The compressive strength of the bulks was more than that of general concrete for construction which has a strength of 24 MPa [START_REF] Shimizu | Outline of Japanese Architectural Standard Specification for Reinforced Concrete Works (JASS 5) -2003[END_REF]. Igi et al. have reported that the machinable ZSM-5 bulk body has a bending strength of approximately 5 MPa [START_REF] Igi | Separation of alcohol/water mixtures by ZSM-5 bulk bodies prepared with a one-pot hydrothermal method[END_REF]. The bending strength is said to be about one-fifth to one-seventh of the compressive strength in the case of concrete [START_REF] Noguchi | Relationship between compressive strength and various mechanical properties of high strength concrete[END_REF]. Assuming a similar relationship applies to zeolite bulks, the strength of the ZSM-5 porous bulk body obtained in this study should be comparable to the strength of the ZSM-5 bulk body prepared by Igi et al. When comparing the samples with different processes, the strength of the sample (1) was the highest. According to the Weibull's weakest link theory [START_REF] Zok | On weakest link theory and Weibull statistics[END_REF], the destruction of ceramics starts from the largest defect. Samples that used starch as a pore-forming agent had a lower strength because they contained macropores which acted as a starting point for the fracture. Since the macropores in the sample (2) are independent, the maximum defect size is small, and as a result, stress concentration during fracture is less likely to occur. This is probably the reason why the compressive strength of the sample (2) was higher than that of the sample (3). The N2 gas permeation rate of the bulks is shown in Fig. 11. The sample (3) showed the highest gas permeability among the three, although the sample (2) and the sample (3) indicated almost the same porosity by Archimedes' method. The differences occurred due to the pore structure, the CLFM observation results showed that isolated macropores as large as the starch grains were formed in the sample with unheated starch.

The isolated macropores did not contribute to the gas permeability. Spherical pores derived from the starch irregularly expanded in the heated sample to form a network structure. The increase in the connected structure pores resulted in an increased gas permeation in the sample containing the heated starch. ZSM-5 granules are commonly used as fillers in gas chromatography columns for the purpose of separating inorganic gases. The gas flow rate through the intergranular spaces for that application is approximately 20-30 CCM [32]. On the other hand, the gas flow rate through the continuous macropores in the ZSM-5 bulk body was 2-5 CCM, which was lower than that of gas chromatography. However, the gas flow through the ZSM-5 bulk body can be controlled by optimizing the pore structure according to the amount of starch added and the subsequent treatment conditions. The zeolite bulk body proposed in this study would be applicable for gas treatment purposes.

Conclusions

Porous zeolite bulks were fabricated using starch as a pore-forming agent. An X-ray diffraction pattern indicated that the crystal structure of the starting ZSM-5 zeolite material was maintained in the bulk samples through all the processes, implying that the micropores in the zeolite crystal unit were maintained in the bulk bodies. The presence of micropores, mesopores, and macropores in the bulks was suggested by the adsorption / desorption isotherms of N2 gas. The pore size distribution of the bulk samples had a peak at 7-9 nm, corresponding to mesopores which would be suitable for supporting functional nanoparticles. Changes in the macropore structure due to the starch heating process were confirmed from the cross-sectional images inside the samples observed by confocal laser fluorescence microscopy using the liquid immersion method. The three-dimensional images of macropores created from the cross-sectional images had a higher resolution than from industrial micro-X-ray computed tomography. This observation method can be applied to other porous materials by properly selecting the refractive index of the immersion liquid. The compressive strength of the zeolite bulk bodies was above 25 MPa, which was higher than the that of general concrete used for construction. This result showed that the bulk bodies had a sufficient mechanical strength for handling. The sample [START_REF] Corma | From Microporous to Mesoporous Molecular Sieve Materials and Their Use in Catalysis[END_REF] showed the highest gas permeability among the three indicating that the starch networks as pore-forming agents are available to create continuous pore structures for a smooth gas flow.

Fig. 1 :Fig. 3 : 22 Fig. 7 :Fig. 8 :Fig. 9 :

 1322789 Fig. 1: Schematic illustration of the sample heating process in an autoclave

Fig. 10 :

 10 Fig. 10: The compressive strength of the zeolite bulk bodies

Table 1 :

 1 Results of density measurement by Archimedes' method

	Sample	sample (1)	sample (2)	sample (3)
	Apparent density(g/cm 3 )	2.07	2.06	2.08
	Bulk density(g/cm 3 )	1.36	1.12	1.13
	Porosity (%)	34.2	45.6	45.5

Acknowledgements

This study was carried out as a part of the France-Japan International Collaboration Framework (IRL3629 LINK). The authors wish to thank Mr. D. Lechevalier of Saint-Gobain KK (Tokyo, Japan) for his significant support involved in LINK and related activities. We also wish to thank Dr. H. Okuyama at NIMS for his help with the experimental setup.