Energy-efficient templating method for the industrial production of

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Materials and Interfaces

Energy-efficient templating method for the industrial production of porous carbon particles by a spray pyrolysis process using poly(methyl methacrylate) Annie Mufyda Rahmatika, Yuan Weilin, Aditya Farhan Arif, Ratna Balgis, Keita Miyajima, Gopinathan M. Anilkumar, Kikuo Okuyama, and Takashi Ogi Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b02564 • Publication Date (Web): 27 Jul 2018 Downloaded from http://pubs.acs.org on July 30, 2018

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Industrial & Engineering Chemistry Research

Energy-efficient templating method for the industrial production of porous carbon particles by a spray pyrolysis process using poly(methyl methacrylate) Annie M. Rahmatika1,2, Yuan Weilin1, Aditya F. Arif1, Ratna Balgis1, Keita Miyajima3, Gopinathan M. Anilkumar3, Kikuo Okuyama1, Takashi Ogi1*

1

Department of Chemical Engineering, Graduate School of Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashihiroshima 739-8527, Japan 2

Department of Biotechnology and Veterinary, Vocational School, Gadjah Mada University, Sekip Unit 1 Catur Tunggal, Depok Sleman, D.I. Yogyakarta 55281, Indonesia.

3

Research and Development Center, Noritake Co., Limited, 300 Higashiyama, Miyoshi-cho, Miyoshi, Aichi 470-0293, Japan

ACS Paragon Plus Environment

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KEYWORDS: Nanostructured Particle; PMMA; Phenolic Resin; Colloidal Template, Selfassembly.

ABSTRACT Template selection is a critical step to determine an effective and efficient method for the synthesis of porous materials. We introduce the use of the low-cost poly(methyl methacrylate) (PMMA) as a template in a spray pyrolysis process for the synthesis of a porous carbon particle . Monodisperse, negatively charged, with three sizes of PMMA particles, are used to evaluate the porous structure formation. The highest surface area of the prepared porous carbon particle is 172 m2/g, which is double that of carbon particles produced using a polystyrene latex (PSL) template. The high surface area is attributed to the low PMMA decomposition residue and the formation of a micropore and a mesopore. The use of PMMA reduces the energy consumption for template decomposition by 32% compared with PSL. This benefit suggests an excellent potential for the use of PMMA in the scaled-up production of porous materials.


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1. Introduction The precise control of porous structures has increasingly attracted the attention of researchers in the past decades1–4. Porous carbon materials have unique properties that are anticipated to be useful for many applications such as adsorption and separation, the electrode of Li-ion battery, drug delivery, and catalysis4–12. The templating method has been widely used for the production of porous carbon13. This method enables easy control of the pore size and pore structure depending on the characteristics of the used template. Template selection is a critical factor in the production of porous carbon. The chosen template should not only enable the formation of the desired pore structure without contaminating the carbon but also be economical and support a green production process. Our group has reported investigations on the synthesis of porous carbon particles using polystyrene latex (PSL) as the template through a spray pyrolysis process14–17. PSL is versatile and has been proven to be effective for the synthesis of various nanostructured particles, especially macroporous and hollow structured particles17–20. Although PSL decomposes at elevated temperatures, the removal of PSL by thermal decomposition leaves a residue of approximately 22.4% of its initial weight21. This residue is unfavorable for the production of nanostructured carbon, as it potentially blocks the formation of micropores and mesopores. The disappearance of micropores and mesopores decreases the surface area, which constricts the performance of carbon and its application. In addition, this residue leads to the drawback of a decrease of the material purity. Therefore, it is highly desirable to synthesize a porous carbon particle that is template residue-free and has a high surface area with an appropriate template selection.


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A promising alternative template is poly(methyl methacrylate) (PMMA). PMMA is as versatile as PSL, with at least two advantages that distinguish it from PSL. First, the decomposition temperature of PSL is higher than that of PMMA which allows for a complete decomposition of PMMA in a process involving a rapid reaction, such as spray pyrolysis. Second, the low residual amount after a complete decomposition. PMMA can maintain a high surface area because the low amount of residue will not block the micropore and mesopore formation. In addition to these advantages, which are attributed to its chemical characteristics, PMMA offers a significant economic advantage owing to its economical price. PMMA is cost effective which will potentially be more profitable on the industrial scale22. Although PMMA has been used for films, monoliths and foam materials 2,6,7,12,21,23,24, there have been no reports on the synthesis of porous particles, especially carbon particles, using PMMA as the template in a spray pyrolysis process. The current work demonstrates, for the first time, the synthesis of porous carbon particles with exceptionally high surface area using PMMA as the template through a one-pot facile spray pyrolysis process. The resulting surface area of the porous carbon particle obtained using a PMMA template is twice as high as those produced using a PSL template with similar morphology from the same carbon precursor, that is, a phenolic resin.

2. Experimental Section 2.1. Synthesis of Porous Carbon Porous carbon particles were produced by an aqueous precursor solution containing phenolic resin (PR, 50 wt.%, Sumitomo Bakelite Co., Ltd., Tokyo, Japan) as the carbon source, polymethyl methacrylate (PMMA, Sekisui Plastics Co., Ltd., Tokyo, Japan) as the template and


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ultrapure water as the dispersant media. The concentration of PR in the precursor was set as high as 0.25 wt.%. Three sizes of PMMA particles (PMMA (A): 108 nm, PMMA (B): 306 nm, and PMMA (C): 503 nm) were used in this study. PMMA was used in various weight ratios of 6, 7 and 8 to PR. The aqueous precursor was then sonicated by an ultrasonic device (IKA, T 10 basic ULTRA-TURRAX S004) with a speed rotation of 25600 rpm for 1 hour to ensure dispersion of PMMA. Then, the precursor was put in an ultrasonic nebulizer (0.8 MHz, NE- U17, Omron Healthcare Co., Ltd., Kyoto, Japan) with a circulation system. The precursor was sprayed through a tubular furnace with four stacked temperature zones set to 150, 350, 1000, and 1000°C. The produced particles were collected by filter paper that was placed on the top of the furnace using N2 gas (0.8 L/min). The filter was maintained at 150 °C to avoid water condensation. Details of the spray pyrolysis experiment setup have been reported in our previous work16.

2.2. Characterization Scanning electron microscopy (SEM) (S-5000, Hitachi Ltd., Tokyo, Japan) and transmission electron microscopy (TEM; JEM-2010, 200 kV, JEOL, Tokyo, Japan) were used to investigate the morphology of the products. Dynamic light scattering (DLS) with a ZS nano analyzer (Malvern Instrument Inc., London, U.K.) was used to examine the size distribution and zeta potential of the PMMA particles. Thermogravimetric analysis (TGA-50/50H, Shimadzu Corp, Kyoto, Japan) was used to determine the temperature for the PMMA removal. The surface areas were determined quantitatively by N2 adsorption–desorption (BELSORP-max, BEL Japan, Osaka, Japan) using the Brunauer–Emmett–Teller (BET) method, the pore distribution was


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determined by the Barrett–Joyner–Halenda (BJH) method, and the size of micropore by Horvath–Kawazoe method.

3. Results and discussions Figure 1 shows the SEM images of the three sizes of PMMA particles used in this study. All PMMA particles exhibited a spherical shape. The size and zeta potential of the PMMA particles were shown in Table 1. The average sizes of the PMMA particles were 108, 305 and 503 nm for PMMA (A), (B) and (C), respectively. As shown in Table 1, the deviation percentage (σ) of the particle size was below 5%, which meant a uniform size. Considering that the pore size of the final carbon product is highly dependent on the template size, the uniformity of PMMA becomes important. The uniform size of PMMA particles also indicated a good dispersion in the precursor system. PMMA (A), (B) and (C) had a negative charge, that is, −34 mV, −24 mV, and −39mV, respectively. As reported previously14,15, a negatively charged template will produce porous carbon particles in a spray pyrolysis system with PR as the carbon source. The negative charge of the precursors causes both sets of particles to repel each other and leads them to be independently distributed in the droplet16.

Figure 1. SEM image of a) PMMA (A); b) PMMA (B); and c) PMMA (C)


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Table 1. Size and zeta potential of the PMMA particles and PR Precursor Name PMMA (A) PMMA (B) PMMA (C) PR

Diameter (nm) ± σ (%) 108 ± 4.2 306 ± 3.4 503 ± 1.9 -

Zeta Potential (mV) -34 -24 -39 -40

To analyze the characteristics of PMMA and to determine the reaction temperature of the furnace in the pyrolysis process, thermogravimetric analysis was conducted on the samples of PR, PMMA and the PR-PMMA blend with a mass ratio of 1:6 (Figure 2). The starting material for the TG analysis, of which result is provided in Figure 2, was a prepared via spray drying in 180 oC to promote self-assembly. Therefore, the results of TG analysis could closely represent the actual condition in the spray pyrolysis process. In the PR curve, the PR lost its weight owing to the decomposition which released various gases (CO, CO2, CH4, H2 and H2O)25 as shown in the steep slope in the temperature range of 20–500 °C. After that, the weight of PR became relatively stable, as shown by the gentle slope, because carbonization had already started10.

Figure 2. TGA thermogram of PR; PMMA; and PR-PMMA blend in N2 atmosphere with heating rate 10oC/min


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Previously, Malhotra et al. explained that longer PMMA chains depolymerize first at low temperatures, followed by the shorter chains at higher temperature26. Recent research has divided the decomposition of PMMA into steps. According to the TGA curve of PMMA samples (Figure 2, red curve), there are three steps in the weight loss. The weight loss trend of pure PMMA was similar to that reported previously27. The first step was a gradual weight loss below 220 °C (number 1 in Figure 2) arising from the initiation of radical transfer to the unsaturated chain ends. The second step was a weight loss of PMMA in the temperature range from 220 to 340 °C, marked as number 2 in Figure 2. In this temperature range, the PMMA became thermally unstable, which increased the decomposition rate. The weight loss in this step was the result of main chain scission arising from H-H bonds and radical transfer to unsaturated ends. Decomposition of the main chain scission reaction is generally expected as a first-order reaction. The last step was hemolytic scission of a methoxycarbonyl side group at temperatures of 340°C and above, which resulted in a small weight loss indicated by number 3 in Figure 2. This was a zero-order reaction to form low molecular weight compounds28. The decomposition process was completed at 450 °C, which was indicated by the constant weight. The remaining weight was less than 2%, or a half of the residue from the PSL decomposition at the same heating rate (Figure S1 in the Supporting information). In the case of PSL, the decomposition occurred in one step, which was a first-order reaction29,30. Meanwhile, the PMMA decomposition was a combination of first- and zero-order reactions. The combination with a zero-order reaction makes the overall activation energy of PMMA lower than that of PSL28,31. Furthermore, a PMMA particle has a lower decomposition temperature. The lower activation energy and lower decomposition temperature increased the opportunity for complete decomposition in a process where the temperature is elevated within a short residence time. These characteristics indicated


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that PMMA could be a good template for particle synthesis using spray pyrolysis. Comparison of the thermograms between the PR-PMMA blend and pure PMMA revealed that the starting temperature of the PR-PMMA blend decomposition was significantly shifted from that of pure PMMA. This shift indicated a self-organization between PMMA and the PR network.


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Figure 3. SEM and TEM images of porous carbon particles prepared using a) PMMA (A); b) PMMA (B); c) PMMA (C) at ratio of PMMA/PR is 6 with same mass concentration of PR and PMMA. d) Average pore size of porous carbon particles by different size of PMMA; e) Porous particle formation mechanism using PMMA template


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Figure 3 (a-c) show the morphology of the porous carbon particles produced using differently sized PMMA particles. Overall, the carbon particle was spherical in shape with a porous morphology. The TEM and SEM images revealed the porous structure on the surface and inside of the carbon particle. This morphology was similar to the results from our previous works which used PSL particles as the template10,14–16. Increasing the size of the template increased the pore size but decreased the number of pores. Even so, as indicated in Figure 3 (d), the average pore sizes were different from the sizes of the PMMA particles. The average pore sizes using PMMA (A), (B) and (C) were 55, 194 and 370 nm, which implied a shrinkage from the initial size of PMMA by 45, 35 and 26%, respectively. An earlier completion of the PMMA decomposition process than the curing process makes the shrinkage ratio inversely proportional to the PMMA size, as will be described later. Many factors affect the resultant particle morphology in spray pyrolysis. They include thermodynamic stabilities, hydrodynamic stabilities, and the self-assembly of the precursors in the droplet26,27. Based on the similar morphology of the produced porous carbon particle10, we predicted the particle formation mechanism using the PMMA and PSL templates to approximately be the same. As described in Figure 3(e), porous carbon particles were formed through four steps, that is, i) water evaporation, ii) PR cross-linking and self-assembly, iii) PMMA template decomposition and PR curing, and iv) carbonization. The self-assembly step involved water evaporation simultaneously with the colloidal assembly of PR-PMMA in the droplet. The droplet shrank owing to the evaporation of water. An attractive force among the solid PMMA particles was then initiated because of the water evaporation. However, they remained independently distributed because of the stabilization by the repulsive force from negatively charged PR, which filled the space between PMMA particles, and from other PMMA


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particles. Then, phenol oligomers crosslinked as the water was continuously evaporated. Along with a temperature increment, PMMA decomposition took place simultaneously with the curing process. However, unlike PSL decomposition, which is complete after the curing process, the PMMA decomposition was completed earlier than the curing process. Completion of the curing process and PMMA decomposition transformed the droplet into a porous PR particle. As the curing process continued, the pores left by PMMA particles shrank along with the particle shrinkage. This implied that the presence of the pore in the curing process increased the possibility of pore size shrinkage in the final product. Therefore, when the small size of PMMA was used as the template, the parts of the skeleton that contained many pores experienced a higher shrinkage in the curing process, which resulted in greater pore shrinkage as shown in Figure 3(d). The pore size further shrank after the carbonization process by elevating the temperature under a nitrogen atmosphere.

Table 2. Skeleton thickness of porous carbon and energy to stabilize PMMA

PMMA type (A) (B) (C)

Skeleton Thickness (nm) Experiment 8 16 35*

a)

Calculation 7.72 12.00 21.20

b)

Total interaction energy c) 37.6 105.2 295.5

*) outer wall thickness a) Measured from SEM images b) Calculated via DLVO theory c) Total colloidal interaction energy as the resultant of attractive and repulsive forces

Interestingly, in this study, we found that increasing the template size tended to increase the thickness of the skeleton wall formed between the pores, as observed from the measurement results of hundreds of particles shown in Table 2. The term “skeleton thickness” might not be applicable for the porous carbon synthesized using type (C) because most of the particle


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contained only one pore. However, we measured the thickness of the outer wall to represent the shell thickness. The repulsion and attraction forces between the colloidal PMMA particles arising from the colloidal interaction, droplets evaporation and capillary in the self-assembly process determined the inter-particle distance and, in the end, the skeleton thickness. The colloidal interaction, explained by DLVO theory, was used as a representing force to illustrate the assembly (Figure S2 in the Supporting information). The calculation results were consistent with the measurement that the bigger size of PMMA produced a thicker skeleton, as shown in Table 2. As mentioned before, the bigger size of PMMA leads to a higher repulsive force (Figure S2), which in turn, requires more total interaction energy to be stabilized, as shown in Table 2. The higher total interaction energy produced a longer distance between the PMMA particles where the gap was filled by PR then was transformed into a thicker skeleton in the porous carbon particle after carbonization. Deviations of the calculation from the experimental results imply that the skeleton thickness was not only determined by the colloidal force but also the other forces mentioned earlier.


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Figure 4. SEM analysis of porous carbon particles with various ratios of PMMA/PR: a, d) 6; b, e) 7; c, f) 8; j)16; TEM images of carbon particles prepared using various ratios of PMMA/PR: g) 6; h) 7; i) 8; k) distribution of skeleton thickness; l) average size of porous carbon particle in different ratio of PMMA/PR The ratio of PMMA/PR was varied to control the porous structure (Figure 4). The morphology results of the PMMA/PR ratio of 6–8 are shown in Figure 4 (a–f). By increasing the PMMA/PR


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ratio, the structures of carbon particles gradually changed, as shown in detail in Figure 4 (d–f). At a low PMMA/PR ratio, the pores formed only on the surface of the particle (Figure 4 (d)). Increasing the PMMA concentration increased the number of template in the droplet which later transformed into pores. The pores were not only present on the surface, but also inside the particle (Figure 4 (e–f)). This phenomenon was caused by the self-assembly of the precursor in the droplet. When the droplets shrank isotropically, the temperature gradient between the droplet center and surface during droplet evaporation triggered the PMMA particles to move towards the droplet surface and be stabilized thereon33. Therefore, PMMA particles tended to assemble only on the surface at a low PMMA concentration. When the PMMA concentration was increased, the number of PMMA particles in the droplets increased. Owing to the space constraint on the surface, some of the PMMA particles filled the inner layer towards the droplet core. The TEM images in Figure 4 (g–i) confirmed that the addition of PMMA led to the formation of highly ordered porous structures inside the porous carbon particle. In addition to increasing the number of pores, the skeleton size decreased. The distribution of the skeleton thickness gradually became narrow for a small value of skeleton thickness, as shown in Figure 4 (k). As previously explained, increasing the number of templates decreased the space between the template particles. When the number of templates in the droplet was above some critical value, the space between template particles became frustrated which led to a collapse. In this research, the porous carbon particle started to collapse when the mass ratio of PMMA/PR reached 8 and was entirely broken when the ratio was further increased to 16, as shown in Figure 4 (j). However, increasing the template concentration hardly changed the average size of the carbon particles (Figure 4 (l)).


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Figure 5. a) N2 -adsorption-desorption and b) pore size distribution from Barrett-Joyner-Halenda (BJH) desorption analysis of carbon particles The specific surface area values calculated using the BET equation, for the dense, porous carbon particles using PMMA (A) and PMMA (B) are 361 m2/g, 172 m2/g, and 142 m2/g respectively. The high surface area of the dense carbon is contributed by the micropores present in the carbon structure, as evidenced by the type II isotherm and form no hysteresis (Figure 5 a), showing a high adsorption capacity at a relative pressure lower than 0.1 P/Po. At low relative pressures (< 0.1), dense carbon particles adsorbed a higher quantity of gas (N2) compared to that of porous carbons of PMMA (A) and PMMA (B). The results implied that increasing the size of PMMA decreased the formation of micropore in the skeleton. A steep slope was observed in the region of high relative pressure (>0.9 P/Po) in the adsorbed nitrogen volume of porous carbon particle, which indicated the existence of a high volume of macropores. Porous carbon particle shows type IV isotherm with N2 loop hysteresis above P/Po ~ 0.9 arose from the presence of mesopores. Utilization PMMA (A) shows type of


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H2 hysteresis which means disordered and wide size distribution of pore, and type of H3 hysteresis for PMMA (B) which indicates slit-shape pores or a lot of space between parallel plates were detected. These results indicated that the porous carbon particle using PMMA (A) and PMMA (B) contained a micropore, mesopore and macropore. The distribution of pore size from 2–100 nm was calculated using the Barrett–Joyner–Halenda method, as shown in Figure 5 (b). The average size of the macropores are 55 nm for PMMA (A) and 194 nm for PMMA (B) derived porous carbon. The mesopore size was approximately 2.4 nm for all type of carbon particles. The number of mesopores increased along with the increase in size of PMMA. The formation of mesopores are attributed to the release of gas during PMMA decomposition and phenolic resin carbonization. Meanwhile, the average size of the micropore was 0.6 nm and 0.8 nm for dense and porous carbon particles respectively, as calculated by the Horvath–Kawazoe method. However, the specific surface area of porous carbon particle in this research was higher than that from our previous research in the production of a porous carbon particle using PSL15, in which the specific surface area (SBET) was 70 m2/g with the dominating mesopore size of 2.8 nm. The high surface area arose from the presence of a lower size of the micropore and mesopore, and had a higher intensity than that using PSL. In the pyrolysis process using PMMA as the template, the decomposition process of PMMA occurred earlier than that with PSL. Because the decomposition of PMMA coincided with the curing of PR, the curing stabilized the small pores formed by the released volatile gas. We determined that the micropore surface area was 94 (m2/g) from the t-plot method, which was 57.3% of the total BET surface area. This indicated that the micropore donated a significant portion of the surface area.


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Previous studies reported the synthesis of porous carbon particles with templating method (Table S1, supporting information). These studies mainly used a template with small size (< 50 nm) and therefore produced mesopore-dominated carbon with high surface area. However, the reported methods required additional steps such as chemical etching to remove the template and activation to promote the micro- and mesopore formation. Therefore, one step process and the utilization of large size template (> 50 nm) while keeping the surface area high become the unique points of this research.

Table 3. Energy consumption in decomposition process

PMMA PSL

Sensible Heat (kJ/kg) 277 390

Activation Energy (kJ/mol) 129 212

Total (kJ/mol) 406 602

A PMMA template can produce porous carbon particles with similar morphology, but with a higher surface area compared with a PSL template. From the commercial perspective, there was a considerable impetus to develop the porous carbon particle with an economical production process. Considering that the energy consumption determines the economy of production, Table 3 compares the energy consumption of PSL and PMMA decomposition. The energy consumption was calculated based on TGA data (Figure S1). In the decomposition process, the total energy consumption was approximated by two parameters, that is, sensible heat and activation energy of the decomposition process. The sensible heat was calculated by Newton's law of heating, within the temperature ranges of 25–220 °C and 25–345 °C for PMMA and PSL, respectively. The activation energy for the decomposition was calculated using Arrhenius’s law by assuming the decomposition as a first-order reaction. As shown in Table 3, the results showed that the sensible heat of PMMA was lower than PSL because of the lower decomposition


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temperature. The activation energy of PMMA was also lower than that of PSL. This result is in good agreement with previous work by Cheng etal.30, which implied that the decomposition of PMMA occurred earlier than PSL. Overall, the energy required for PMMA decomposition was 32% lower than that for PSL.

4. Conclusions The porous carbon particle has been synthesized from PR by spray pyrolysis using PMMA as the template. The low decomposition temperature (300 °C) and low residue left by PMMA in the decomposition process renders them effective as a template for particle synthesis using spray pyrolysis. The size of the PMMA particle affected the thickness of the porous carbon particle skeleton because of the different PR-PMMA interaction force during self-assembly. Increasing the mass ratio of PMMA to PR had little effect on the size of porous carbon particle. The morphologies of the resultant carbon particles were the same as those obtained using the PSL template. Despite having the same morphology, the PMMA template produced a higher number of micropores and mesopores with a size of 0.8 and 2.4 nm, respectively. These small pores contributed to a specific surface area of 172 m2/g, which was double that of the carbon particles produced from the same carbon precursor using a PSL template. The low decomposition energy of PMMA, which was 32% lower than that for PSL, suggests a large potential for the use of PMMA in the scaled-up production of a porous material.


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ASSOCIATED CONTENT Supporting Information. Resume specific surface area of carbon particles synthesized using various methods ,TGA thermogram of PMMA and PSL, and calculation of skeleton thickness using DLVO theory. The following files are available free of charge (PDF).

AUTHOR INFORMATION Corresponding Author E-mail: [email protected]. Phone: +81-82-424-7850. Fax: +81-82-424-7850) Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources JSPS KAKENHI Grant Number 26709061 and 16K13642. ACKNOWLEDGMENT This work was supported by JSPS KAKENHI Grant Number 26709061 and 16K13642. This work is partly supported by the Center for Functional Nano Oxide at Hiroshima University. The authors would like to thank the Japanese Ministry of Education, Culture, Sports, Science, and Technology (MEXT) for providing a doctoral scholarship (A. M. R.). We thank Edanz Group (www.edanzediting.com/ac) for editing a draft of this manuscript.


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