Bioinspired Ant-Nest-Like Hierarchical Porous Material Using CaCl2

Article pubs.acs.org/IECR

Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Bioinspired Ant-Nest-Like Hierarchical Porous Material Using CaCl2 as Additive for Smart Indoor Humidity Control Xiaopeng Liu, Zhang Chen, Guang Yang, and Yanfeng Gao* School of Materials Science and Engineering, Shanghai University, 99 Shangda Road, Shanghai 200444, China

Ind. Eng. Chem. Res. Downloaded from pubs.acs.org by BOSTON UNIV on 04/07/19. For personal use only.

S Supporting Information *

ABSTRACT: Inspired by the functional microstructure of the ant nest, a humidity control material was prepared by the sintering of modified low-grade sepiolite. A hierarchical porous structure accelerates the diffusion of water vapor. Meanwhile, CaCl2 was applied subtly to enhance absorption/desorption of water vapor in response to the change of air relative humidity. The water vapor adsorption− desorption content reaches 550 g·m−2 with a steady performance after 10 cycles. The flexural strength of the specimen is excessive, 10 MPa. Furthermore, two model houses were used to evaluate the performance of the material in a real environment. The result indicated that it could narrow indoor humidity fluctuation by more than 10% RH spontaneously and mainly maintained the humidity within a healthy range (RH 40−70%) without energy consumption. This invention makes it possible for large-scale fabrication of this material in terms of wall bricks for smart indoor humidity control.

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INTRODUCTION The air relative humidity (RH) plays an important role in indoor comfort. The World Health Organization (WHO) has recommended indoor RH at a range of 40−70% as optimal for healthy housing.1 In practice, there are periods of time spans during the year, such as the “plum rain season” in Japan and China, at which indoor relative humidity is uncomfortable.2,3 At present, the main products such as dehumidifiers and humidifiers are used to control indoor humidity. However, they cannot meet the current needs of energy savings and passive regulation. Humidity control material (HCM) can selectively absorb or desorb water vapor according to the change of RH and narrows the humidity fluctuation without any energy consumption. It has been reported that HCM can reduce heating and cooling energy consumption by 5−30%.4,5 Porous materials are considered the most promising HCM candidates. It was calculated by Kelvin’s equation that the RH variation could be damped and controlled within a range of 40−70% by a material with a pore diameter of 3.2−7.4 nm.6 Eddaoudi et al.7 synthesized MOF-5 with a water vapor adsorption− desorption content of 0.35−0.4 g·g−1. Silica-based porous materials with different pore sizes were synthesized by Jing and Tomita et al.8,9 The water vapor adsorption−desorption content of the material with 3.7 nm pores reached 330 g· m−2, but it was reduced quickly with pore size increasing from 4.2 to 7.1 nm. In most cases, natural porous materials, such as diatomite,10,11 volcanic ash,12 zeolite,13 and sepiolite,14,15 etc., have been used as raw materials for preparing HCM because they are cost effective and environmentally friendly. However, compared with the aforementioned synthesized material, © XXXX American Chemical Society

natural porous material has a wide pore-size distribution, ranging from micropore to macropore,16,17 and only partial pores are sized in a span for generating capillary condensation, reflecting a relatively low water vapor adsorption−desorption content.5,18 Therefore, it is meaningful to improve and take full advantage of this kind of material. A low-temperature hydrothermal method has been developed to prepare HCM using natural porous material.19,20 It can sustain the inherent porous property of these materials effectively. Nevertheless, the water vapor adsorption−desorption content is not satisfactory. CaCl2 has been applied previously as a desiccant to improve the dehumidification of the natural porous materials.15,21 The role of CaCl2 in this way is only its natural property. However, as a smart control material, the water vapor desorption performance at room temperature is equally important. In this regard, designing a HCM with excellent humidity control property and low cost is still a challenge. Ants can maintain a suitable humidity level within the nest for living and reproducing because this nest possess a very subtle microstructure of clay-made tunnel networks,22−24 in which clay acts as a moisture controller, and the network of broad tunnels support channels for quick inside−outside moisture exchange. Inspired by the functional microstructure of the ant nest, we propose a hierarchical porous HCM using low-grade sepiolite by low-temperature sintering (Figure 1a and 1b). We found that the addition of CaCl2 can expand the magnitude of available mesopores that generate capillary Received: December 9, 2018 Revised: February 23, 2019 Accepted: March 25, 2019

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DOI: 10.1021/acs.iecr.8b06092 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

Figure 1. (a) Schematic illustration of an ant nest with the moisture exchange, (b) Bio-HCM used in indoor humidity control, (c) digital images of Bio-HCM, and (d) XRD patterns of AAS and Bio-HCM.

The compositions of samples were characterized by the Xray diffraction method (18kw-D/MAX2500 V, Rigaku) using Cu Kα radiation (λ = 1.5406 Å) at a scan speed of 2°/min. Microstructure studies were performed using scanning electron microscopy (SEM, JFM-7500F, FEI) and a transmission electron microscopy (TEM, JEM2010, JEOL). Surface area, pore volume, and porosity analyses were performed using an automatic nitrogen gas sorption porosity analyzer (AutosorbIQ2, Quantachrome). The specific surface area was calculated by the Brunauer−Emmett−Teller (BET) method. The mesopore size distribution was derived from the adsorption branch of the isotherms using the Barrett−Joyner−Halenda (BJH) model. The macropore size distribution was obtained by mercury intrusion porosimetry (MIP, AutoPore Iv 9510, Micromeritics). Three-point flexural strengths of the sintering samples were measured with a strength testing machine (Instron-5566, 10 kN). Three specimens were tested for each parameter, and the experimental results presented in this study are the averaged data.

condensation. The synergy effect of hierarchical porous and CaCl2 excellently improves the water vapor adsorption− desorption content of the HCM. Real indoor humidity evaluation with two model houses (one as reference) for 31 days in Shanghai (from March 19, 2018 to April 19, 2018) indicated that it could narrow indoor humidity fluctuation by more than 10% RH spontaneously; as a result, the humidity was mainly maintained within a recommended healthy range.

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EXPERIMENTAL SECTION

The raw sepiolite was obtained from Henan Province (China). Hydrochloric acid, sodium hydroxide, calcium chloride, magnesium chloride hexahydrate, sodium chloride, and potassium nitrate were purchased from Sinopharm Chemical Reagent Co., Ltd. (China). Low-melting point waste glass was obtained from Fanghuan New Materials Co., Ltd. (Shanghai, China). All reagents were used without further purification. Preparation of Sepiolite/CaCl2 Bio-HCM. The raw sepiolite was activated by HCl (3 mol/L). The chemical compositions of acid-activated sepiolite (AAS) and raw sepiolite are shown in Table S1. Subsequently, CaCl2 was added into the AAS dispersion. The mixture was adjusted to pH 10 using a NaOH solution (5 mol/L) and separated by vacuum filtering. The filter residue was mixed with 20 wt % of low-melting point waste glass powder (compositions see in Table S3). The green specimen was obtained by pressing precursor powders uniaxially in a mold (50 × 20 mm2) at a pressure under 4, 8, 12, and 20 MPa. Large specimens were made by a mold (100 × 80 mm2) at 8 MPa and used in a simulation test. The green specimen was sintered at 700 °C for 40 min in an electric-heated furnace, and then Bio-HCM with a thickness of near 4.5 mm was obtained. Characterization. Two virtually identical model houses with 0.45 m3 of space volume for each were used for a simulation test. The variations of temperature and relative humidity inside the two model houses were recorded by hygrothermographs every 20 min.

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RESULTS AND DISCUSSION Material Characterization. Using a facile method, the Bio-HCM containing 0, 1.9, 4.8, 9.6, and 11.0 wt % CaCl2 was produced (Figure 1c). Figure 1d reveals the XRD patterns of AAS and Bio-HCM. The diffraction peaks at 2θ ≈ 7.3°, 10.5°, 11.7°, 27.1°, 27.9°, 28.5°, 30.8°, and 35.2° were assigned to the sepiolite with quartz and talc as the main impurities.7,25,26 Compared with the AAS, some diffraction peaks of sintering specimens such as 7.3° and 11.7° in 2θ disappeared. It originated from the acid treatment and sintering.27,28 With the CaCl2 increased, diffraction peaks at 10.5°, 21°, and 27.1° weakened, probably due to the generation of calcium silicate hydrate (C−S−H) after sintering, which consumed silicon dioxide in the quartz and sepiolite. It can be seen later that the C−S−H can enhance the strength of Bio-HCM.19,29 The diffraction peak of CaCl2 was not observed because of the hygroscopic nature.15,30 After drying at 150 °C, the diffraction peaks at 29.6° and 32° were assigned to the CaCl2·2H2O B


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Figure 2. TEM images of acid-activated sepiolite fibers: (a) SEM images of the Bio-HCM prepared with the CaCl2 at contents of (b) 0, (c) 1.9, (d) 4.8, (e) 9.6, and (f) 11.0 wt %.

Figure 3. (a) Nitrogen adsorption and desorption isotherms and (b) pore size distribution curve of Bio-HCM specimens.

inducing a poor absorption of nitrogen.32 Correspondingly, the specific surface areas of Bio-HCM decreased from 37.6 to 6.5 m2·g−1. Figure 3b exhibits the diameter distribution of mesopores (pore diameter, 3−40 nm). Compared to AAS, the larger mesopores in Bio-HCM with CaCl2 increased which may be caused by alkali activation in the sintering process.19,33 After sintering, the pore volume of mesopores less than 10 nm decreased, while large mesopores increased for Bio-HCM with CaCl2 below 5 wt %. With increasing CaCl2 to 9.6 or 11 wt %, large mesopores decreased as well. It is mainly because CaCl2 occupies the mesopores, evidence of increased average pore diameter (from 7.3 to 16.4 nm) but decreased pore volume (from 0.15 to 0.03 cm3·g−1) in Table S2. We can see later that the CaCl2 improves the water vapor adsorption−desorption content of Bio-HCM though it decreases the porosity. Humidity Control Property. The core part of the apparatus for measuring the water vapor adsorption− desorption content is shown in Figure 4a (details are described in the Supporting Information). For comparison, the weight and size of each fresh specimen are almost the same. The water vapor adsorption−desorption content of the Bio-HCM in an RH of 33−75% at 25 °C is revealed in Figure 4b. For the BioHCM without CaCl2, the maximal adsorption−desorption content is nearly 100 g·m−2, which is due to the mesopores of the sample. The water vapor adsorption−desorption capabilities of the samples increase significantly with the addition of CaCl2 into the Bio-HCM. It reaches 450 g·m−2 for Bio-HCM with 11.0 wt % CaCl2.

(JCPDS No. 70-0385). The amplified main peak at 32° reveals that the amount of CaCl2·2H2O increased with addition of CaCl2 in the sintering specimen. Figure 2a presents the characteristic fibrous morphology of AAS fibers with a diameter in the range of 50−100 nm. “Unit fibers” flock into groups in parallel to form the tape-like shapes.16 The morphologies of the Bio-HCM are significantly affected by the addition of CaCl2. Interfiber mesopores remain in the specimen after sintering. When CaCl2 content is less than 4.8 wt %, the sepiolite fibers in the Bio-HCM seem clear and distinct in Figure 2b and 2c. In contrast, fibers in BioHCM with high concentrations of CaCl2 exhibit fuzzy fibers morphology (see Figure 2d−f). Elemental mappings of BioHCM with 4.8 wt % CaCl2 show that Ca and Cl are homogeneously distributed in the interfiber mesopores or the surface of sepiolite fibers (Figure S2). This outcome indicates that CaCl2 is retained in the sintering specimen. The variation of morphologies may be caused by the existence of CaCl2. The nitrogen adsorption and desorption isotherms of AAS and Bio-HCM are plotted in Figure 3a. According to the IUPAC classification,31 they belong to typical Type IV isotherms and H3 hysteresis loops that are frequently associated with slit-like pores, indicating the coexistence of meso- and macropores. This can be confirmed by the morphologies in Figure 2b−f. However, the nitrogen adsorption capacities of Bio-HCM with high concentrations of CaCl2 are smaller than those with low concentrations. It may result from CaCl2 which occupies the pore volume, C


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available mesopores is enlarged, because of the higher γ and Vm of the CaCl2 solution. The capillary condensation can sequentially form in larger pores (e.g., RH 70%, 17 nm) under the same humidity. A larger pore occurring capillary condensation means a higher adsorption content in water vapor in a high RH and so as to the desorption contents in a low RH. Therefore, the synergy effect of mesopores and CaCl2 makes the water vapor adsorption−desorption content of the Bio-HCM with CaCl2 higher than that without CaCl2, which allows for full attainment of the high adsorption potential of the interfiber mesopores of sepiolite. Every regime of the interfiber mesopores and CaCl2 functions as a “minireservoir” in the Bio-HCM and can adsorb/desorb water vapor spontaneously according to the changes of outside humidity. However, more CaCl2 in a specimen means more CaCl2 solution can form in high humidity. When Bio-HCM encounters long-term high-humidity conditions, such as “plum rain season”, leakage may occur due to the adsorption/condensation property of high-concentration CaCl2 solution.37 The increasing volumetric CaCl2 solution may overflow when it exceeds the carrying capacity of the mesopores in the specimen. A diffusion−oozing test was performed to assess the applicable amount of CaCl2. As shown in Figure 6a, the Bio-HCM was put on an alkaline litmus paper, which was then placed into an airtight box with RH 92% at 25 °C for 4 days. After that the Bio-HCM was taken away, and the results are presented in Figure 6b−f. Significant leakages appeared for the Bio-HCM with 11.0 and 9.6 wt % of CaCl2, which are depicted within the white dotted−dashed line in Figure 6b and 6c. Hardly any leakage appeared for the BioHCM with CaCl2 less than 4.8 wt %. Therefore, Bio-HCM with 4.8 wt % of CaCl2 possesses potential application in areas with an annual plum rain season, while Bio-HCM with 9.6 and 11.9 wt % of CaCl2 is more suitable for arid areas. Furthermore, the CaCl2 modification may be effective for other natural porous materials, such as diatomite, volcanic ash, or zeolite.

Figure 4. (a) Core part of the test apparatus, and (b) water vapor adsorption−desorption contents of Bio-HCM with CaCl2.

At thermal equilibrium, researchers have validated the Kelvin capillary condensation in nanoporous media.34−36 Therefore, the variation of water vapor adsorption−desorption contents between Bio-HCM with or without CaCl2 can be illustrated by the classical theory (see Supporting Information). As shown in Figure 5a, the surface tension (γ) and molar volume (Vim) increase with the concentration of CaCl2 increasing, and the fitting curves are guidelines. The relations between pore diameter that generate capillary condensation and the relative water vapor pressure are shown in Figure 5b. The dashed line and the dot-fit line were calculated by purified water and a CaCl2 solution, respectively. It can be seen that the pore diameter is expanded to 8−17 nm compared with 3.2−7.4 nm of purified water. Hence, the water vapor adsorption− desorption mechanism of Bio-HCM with or without CaCl2 is proposed in Figure 5c. Without CaCl2, the available mesopores can adsorb water vapor and generate capillary condensation in a high RH. Correspondingly, it can be desorbed spontaneously when the ambient humidity is reduced. Beyond the dimension size (e.g., RH 70%, 7.4 nm), capillary condensation does not occur continuously. In contrast, once CaCl2 is added into the Bio-HCM, the CaCl2 solution is generated in interfiber mesopores when it adsorbs water vapor.15 The magnitude of

Figure 5. (a) Surface tension and molar volume of CaCl2 solution. (b) Relation between pore diameter and relative water vapor pressure.(c) Profile map of interfiber mesopores adsorption/desorption moisture mechanism of Bio-HCM (i) without or (ii) with CaCl2. D


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Figure 6. Digital images of the diffusion-oozing test: (a) sample on the litmus paper before the test and after the test for samples with (b) 11.0, (c) 9.6, (d) 4.8, (e) 1.9, and (f) 0 wt % of CaCl2. Once leakage appears, the CaCl2 solution (pH 5−6) touches the litmus paper and the color of the touch area changes from blue to red. Litmus paper: Φ70 mm.

Figure 7. Effects of compaction pressure on (a) bulk density and tensile strength. Error bars are smaller than the symbols. (b) Water vapor adsorption/desorption capabilities of Bio-HCM with 4.8 wt % CaCl2. (c) Macropores distribution and the corresponding cumulative intrusion curve. (d) Stability test of Bio-HCM with 4.8 wt % CaCl2 and 10.3 MPa in flexural strength. (e) Morphologies of hierarchical porous structure.

point waste glass can strengthen the Bio-HCM.12,18 Hence, a suitable porosity endows the Bio-HCM with an equal mechanical strength for practical application. The distribution of macropore and hierarchical porous morphology are shown in Figure 7c and e. The porosity and median pore diameter are 39.2% and 481 nm, respectively. Under 10 cycles of RH from 33% to 92% to 33%, the water vapor adsorption−desorption content reaches 550 g·m−2. During the cycles, the Bio-HCM shows a stable property with the variation within 40 g·m−2, as shown in Figure 7d. The water vapor adsorption−desorption contents at different temperatures are indicated in Figure S4. A comparison of the properties between the prepared Bio-HCM and the representative HCM is provided in Table 1. The results suggest that the Bio-HCM exhibits good comprehensive performance. Simulation Test. Furthermore, a simulation test was performed to evaluate the practical humidity control property

Mechanical Properties and Stability. In addition to a defined amount of 4.8 wt % CaCl2, which ensures large water vapor adsorption−desorption content, mechanical strength and stability are also important for the practical application of the HCM. Macropores in the Bio-HCM can accelerate the diffusion of water vapor, exactly as the broad tunnels in the ant nest play a fast-diffusion role for humidity regulation. However, it is also a negative factor for strengthening the Bio-HCM.20,38 It is shown in Figure 7a that the bulk density and flexural strength of Bio-HCM increase with the increase of pressing pressure for green specimen. On the contrary, the water vapor adsorption−desorption content decreases (see Figure 7b). The adsorption−desorption contents of the Bio-HCM with flexural strengths of 4.4 and 10.3 MPa are near 350 g·m−2. It indicates that the macropore porosity for specimen with 10.3 MPa is enough to acquire a high water vapor adsorption−desorption content. As mentioned above, the C−S−H and low-melting E


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RH without any energy consumption. Thus, the Bio-HCM is an effective material for smart indoor humidity control.

Table 1. Comparison of Properties between Bio-HCM and Other HCM Reported Previously HCM sepiolite/ CaCl2 volcanic ash mordenite loess silica-based materials

humidity control propertya (g·m−2)

flexural strength (MPa)

sintering

350

10.3

sintering hydrothermal hydrothermal hydrothermal

255 70 100 250

6 15.3 23

preparation method

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CONCLUSIONS We propose a humidity control material with a bioinspired antnest structure. In the Bio-HCM, a regime of interfiber mesopores and CaCl2 functions as a minireservoir and macropores work as accelerated channels. According to the climatic features, the Bio-HCM can be produced with different contents of CaCl2 for applications. An optimized Bio-HCM with a 4.8 wt % CaCl2 for regions with an annual plum rain season possesses a specific surface area of 19.3 m2·g−1 and pore volume of 0.06 m3·g−1. The water vapor adsorption− desorption content reaches 350 and 550 g·m−2 at RH 33− 75% and 33−92%, respectively. The flexural strength is 10.3 MPa. A simulation test indicates that the Bio-HCM is an effective material for smart indoor humidity control.

ref present work 12 39 19 8

a Water vapor adsorbed−desorbed contents within 48 h for RH 33− 75% at 25 °C.

of the Bio-HCM by using two model houses (see in Figure 8b). The model houses were placed outdoors in the Baoshan

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.8b06092.

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Test apparatus for water vapor adsorption−desorption content; SEM images and elemental mappings; saturated vapor pressure and test apparatus; water vapor adsorption/desorption at different temperatures; compositions of raw sepiolite, AAS, and low-melting point waste glass; physical properties of Bio-HCM; discussion on Kelvin’s capillary condensation equation and Arai’s correction equation (PDF)

AUTHOR INFORMATION

Corresponding Author

*Tel./fax: +86 21 6990 6213. E-mail: [email protected]. ORCID

Yanfeng Gao: 0000-0001-7751-1974 Figure 8. Simulation test of (a) the Bio-HCM and (b) the model houses. (c) Indoor humidity and temperature change with time in the two model houses.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by Innovation Program of Shanghai Municipal Education Commission (No. 2019-01-07-00-09E00020). Y.G. also acknowledges fundings from the Changjiang Scholars programs (T2015136).

Campus of Shanghai University in Shanghai. Before the test, the conformance was examined for 2 days. The left breakpoint in Figure 8c shows that the “indoor humidity” of the two model houses tended to be the same in both high and low humidity. An approximate 2% RH error existed because of a weak difference in gas tightness. Then 16 pieces of the BioHCM (Figure 8a, total area 0.128 m2) were put into the test house (0.45m3 in space volume); meanwhile, the reference house was kept empty. The test duration was from March 19, 2018 to April 19, 2018, and it included 10 rainy days and 20 sunny days. The short-dotted line in Figure 8c shows that the temperature variation of the two model houses was almost the same. Therefore, we believe that the difference of “indoor humidity” between the two model houses resulted from regulation of the Bio-HCM. The humidity in the test house was maintained mainly within a range of 40−70% (black line in Figure 8c), while the humidity in the reference house fluctuated seriously (red line in Figure 8c). It can narrow the indoor humidity fluctuation spontaneously by more than 10%

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REFERENCES

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Bioinspired Ant-Nest-Like Hierarchical Porous Material Using CaCl2

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