Environmentally Friendly Mesoporous Material Derived From Thin-Film Transistor Liquid Crystal Display and Sandblasting and Its Application of Environmental Humidity Control

The preparation of an aluminum-mesoporous humidity controlling material (Al-MHCM) by hydrothermally synthesizing a mixture of thin-lm transistor liquid crystal display (TFT-LCD) waste glass and sandblasting (SB) waste was studied; additionally, the remaining residue was completely reused. The synthesized Al-MHCM was analyzed, and its moisture controlling performance along with its pore and surface structure characteristics were determined. In addition, the inuence of the hydrothermal synthesis temperature and silicon-to-aluminum molar ratio on the product was also studied. The characteristic analysis results of the product conrm that the product has a typical mesoporous structure, with a specic surface area of up to 1013 m 2 /g, and the pore size distribution calculated by the BJH method is 3-4 nm with a pore volume of 0.97 cm 3 /g. The 27 Al NMR analysis veries that all the aluminum atoms of the product are in the form of tetrahedral aluminum (Td-Al) in the mesoporous Al-MHCM framework, which conrms the successful synthesis of Al-containing mesoporous MCM-41 materials. In addition, as the aluminum content increased, the water adsorption/desorption capacity of Al-MHCM also increased. Results show that when the hydrothermal synthesis temperature is 105°C, the product synthesized from a mixture with a silicon-to-aluminum molar ratio of 41.8 exhibits excellent performance (91.45 m 3 / m 3 ) in terms of the equilibrium moisture content and moisture adsorption capacity. Al-MHCM, which can be synthesized at a low cost, can be used as a humidity controlling material for adjusting the relative humidity of indoor environmental spaces. Thus, this study provides a novel method for producing Al-MHCM to use as a future humidity-controlling building material. also analyzed, and each material was subjected to a heavy metal leaching test. The concentrations of the leached heavy metals from each material was measured by ame atomic absorption spectrometer (FL-AAS). The analysis results of the total amount of heavy metals show that the TFT-LCD waste glass is dominated by Zn, with a content of 160 mg/kg, and the main source are the wires in the panel etching tank. The predominant heavy metal in the SB waste is mainly Cr, which is caused by the SB waste from the coating treatment of a solar panel, and its content is 90 mg/kg, followed by the contents of Zn and Cu, which are 65 and 25 mg/kg, respectively. The experimental results show that the leaching concentrations of TFT-LCD waste glass and SB waste all met Taiwan’s EPA regulatory limits.


Introduction
In the early 1990s, scientists at Mobil Petroleum Corporation discovered an ordered mesoporous silicate/aluminosilicate material, which they called the M41S series [1][2][3]. Since then, a series of studies have been conducted, in which the in uence of the concentration of the surface active agent on the prepared mesoscopic particles was studied, and three structural arrangements were determined: a (MCM-50). Among them, MCM-41 is the most studied mesoporous material because of its good thermal, hydrothermal and hydrolytic stability along with its high surface area and pore volume [4][5][6]. In addition, MCM-41 can prevent the agglomeration of a supported metal oxide by forming a bond between the surface silicon hydroxyl bridge and the deposited oxide [7]. It has been reported that the incorporation of Al into the MCM-41 framework can provide the resulting Al-MCM-41 material with Bronsted acid sites while exhibiting good adsorption a nity, thereby broadening its potential application [8][9]. However, the cost of silicon dioxide precursors commonly used in the synthesis process (such as sodium silicate or silicon alkoxide) represents a major problem in the use of such materials in regard to economic and environmental considerations [10]. Therefore, some research has been conducted to develop mesoporous silica using low-cost silica resources, including y ash [11], wheat stem ash [12] and packaging resin waste [13]. Despite this plethora, reports on MCM-41mesoporous material synthesis from thin-lm transistor liquid crystal display waste glass and sandblasting waste are still scarce in literature. In the optoelectronic industry, a large amount of thin lm transistor liquid crystal display waste glass (TFT-LCD waste glass) is generated during panel manufacturing, assembly and cutting. In addition, sandblasting waste (SB waste) is the surface etching process after the silicon wafer is cut, and the surface of the silicon material is optimized through the use of abrasive sand, thereby mass production of SB waste in the upstream factory of the solar cell module. In the past, most of these wastes were treated by incineration or land ll, thus wasting valuable materials. Therefore, from an environmental and economic point of view, a cleaner process to add value and provide resource reuse and safe disposal research for solid waste is of great signi cance. In addition, a mesoporous material of MCM-41 can be prepared by a variety of synthetic methods, such as a hydrothermal treatment [14], a sol-gel method [15], cocondensation method [16], and an impregnation method [17].The hydrothermal treatment method is the preferred method because it is simple and the operational complexity is low; furthermore, it has been reported that the hydrothermal treatment can increase the structural stability, acidity, activity and surface area of the mesoporous material [18].
Taiwan has a subtropical island climate, with warm and humid weather conditions year round; thus, the hot climate and high humidity of Taiwan provide ideal conditions for mold. According to the literature, the most comfortable range of relative humidity for the human body is 40-70% [19]; additionally, excessively dry or humid environments are not conducive to human health and life [20]. Therefore, to achieve effective humidity control while also avoiding the use of energy-consuming methods, such as mechanical air conditioning, the development of new and energy-free indoor humidity control technologies has been explored to save energy and provide healthy living environments [21][22]. If low-cost silicon dioxide resources (thin-lm transistor liquid crystal display waste glass and sandblasting waste) can be used to produce mesoporous MCM-41 materials for the preparation of Al-MHCM with humidity controlling performance, then the humidity of the environment can be adjusted while decreasing the amount of pollution in the environment. With no need to consume energy or use other equipment, as long as the original capillary phenomenon of the mesoporous material is used, the designed Al-MHCM exhibits moisture absorption and desorption characteristics, thereby achieving moisture control. Therefore, the purpose of this research is to con rm the application of MCM-41 as a humidity controlling material. This study is the rst attempt to measure the in uence of various temperatures and Si/Al molar ratios on the pore structure and surface characteristics of Al-MHCM produced from waste materials and its performance toward environmental humidity control. In addition, the feasibility of producing zeolite, from the remaining residue of the primary synthesis, was studied to achieve the goal of zero environmental resource waste.

Materials (TFT-LCD waste glass and SB waste)
The TFT-LCD waste glass and SB waste were provided by the Central Taiwan Resource Recycling Plant and Solar Energy Technology Company, respectively. These wastes were dried, ground and sieved to achieve a particle size ≤ 74 µm. As shown in Table 1, its chemical composition was measured and analyzed by X-ray uorescence (XRF) spectroscopy. The main components of the TFT-LCD waste glass were SiO 2 , Al 2 O 3 and CaO, which accounted for 69.70, 15.30 and 8.45%, respectively, and the molar ratio of SiO 2 to Al 2 O 3 was approximately 4.5. The main components of the SB waste were SiO 2 and Al 2 O 3 , which accounted for 70.10 and 13.90%, respectively. The molar ratio of SiO 2 to Al 2 O 3 was calculated to be approximately 5.0. Among them, TFT-LCD waste glass mainly came from material cut from glass substrates, and SiO 2 and Al 2 O 3 were the main bodies of the glass network structure, thereby providing an abundance of SiO 2 and Al 2 O 3 . The mineralogical phase analysis of TFT-LCD waste glass and SB waste using XRD is shown in Fig. 1. TFT-LCD waste glass had an amorphous structure inside, so there was no obvious diffraction peak. SB waste produced diffraction peaks of silicon oxide, aluminum oxide and Cu3Ge. ltered, and then dried in an oven at 105°C overnight. Finally, the obtained product was calcined up to 550°C at a heating rate of 5°C/min in air to burn off the surfactant from the matrix. The obtained mesoporous material was called Al-MHCM.

Test and methods
The used TFT-LCD waste glass and SB waste were analyzed by an XRF uorescence analyzer (RIX 2000) to obtain the chemical element composition of the raw materials. The pore structure of Al-MHCM was determined by X-ray powder diffraction (D8AXRD) to identify the crystalline phases in the material.
Diffraction data were collected between 2θ = 1-8°. Scanning electron microscopy (SEM; American FEI company (Nova Nanosem 230)) was used to characterize the surface morphology and structural changes of the sample. A solid-state 27 Al NMR spectrum of Al-MHCM was obtained. A nitrogen adsorption and desorption instrument (Tristar 3000) was used to obtain isotherms, and the surface area was measured by the Brunauer-Emmett-Teller (BET) method; nally, the total pore volume and pore diameter were calculated by the BJH method.

Moisture adsorption/desorption test
The moisture adsorption/desorption test was performed in accordance with the method for measuring the equilibrium moisture content in Japanese Industrial Standards JIS A 1475. First, the test body was dried in an oven at 105°C for 24 h, and its constant weight was recorded. Next, the sample was placed in a constant temperature and humidity controller, and the relative humidity was changed to 10, 33, 55, 75, 85, and 95% at a xed temperature of 23°C. The constant weight of adsorption (from low to high) and desorption (from high to low) of the sample was recorded at this relative humidity. Finally, the moisture absorption per unit area of mesoporous Al-MHCM was obtained at different times. The standard of the Japanese Industrial Regulations JIS A 1470 humidity controlling building materials regulations was veri ed, and the standard value, based on the average equilibrium moisture content (> 5 kg/m 3 ), was evaluated.

Heavy metal leaching concentration of the raw materials
The leaching concentration of heavy metals that passed the toxicity characteristic leaching procedure (TCLP) test is shown in Table 2. In addition, the total leaching concentration of heavy metals was also analyzed, and each material was subjected to a heavy metal leaching test. The concentrations of the leached heavy metals from each material was measured by ame atomic absorption spectrometer (FL-AAS). The analysis results of the total amount of heavy metals show that the TFT-LCD waste glass is dominated by Zn, with a content of 160 mg/kg, and the main source are the wires in the panel etching tank. The predominant heavy metal in the SB waste is mainly Cr, which is caused by the SB waste from the coating treatment of a solar panel, and its content is 90 mg/kg, followed by the contents of Zn and Cu, which are 65 and 25 mg/kg, respectively. The experimental results show that the leaching concentrations of TFT-LCD waste glass and SB waste all met Taiwan's EPA regulatory limits.  peaks of MCM-41, respectively [23]. With the increase in the Si/Al molar ratio, the two higher-order hexagonal structure peaks (d (110) and d (200) ) tend to increase. At lower Si/Al ratios, the main characteristic peaks are slightly shifted, which is mainly due to the 2θ angle shift caused by the high aluminum content in the source material; this result means that Al atom doping in the crystal lattice changes the crystallinity of the product and causes poor sorting. Therefore, with increasing aluminum content, the intensity of the diffraction peak gradually decreases, indicating that the order of the structure decreases [24][25]. In addition, it can be found from the change in the hydrothermal temperature that when the temperature increased from 90 to 120°C, the main characteristic peak d (100) has relatively stable crystallinity. The characteristic peaks (d 110 and d 200 ) representing the hexagonal structure also have a tendency to gradually form as the hydrothermal temperature increased because when the hydrothermal temperature increased, the balance between the solid and liquid phases of the initial gel is destroyed; this imbalance accelerates the aggregation of silicate species on the micelle surfaces. Therefore, a high hydrothermal temperature is conducive to the formation of a well-ordered MCM-41 structure because the high temperature will accelerate the condensation rate of silicate on the silicon dioxide wall [26].

Solid-state 27 NMR analysis of Al-MHCM
The 27 Al NMR spectrum was used to track the change in the position of Al added to Al-MHCM. All the spectra of the current Al-MHCM material are dominated by the signal of the tetrahedral-coordinated aluminum (Al IV ) species with 27 Al = 53 ppm (Fig. 3), and the weak signal at 27 Al = 0 ppm is due to the octahedral-coordinated aluminum (Al VI ) species (Fig. 3(c)) [27]. The appearance of Al VI species is usually related to the Lewis acid on the surface, and the strength will increase with an increasing content of Al IV species [28]. However, when the Si/Al molar ratio is lower (Fig. 3 (a)-(b)), more Al atoms (Al VI ) are introduced into the silicon dioxide framework, thereby generating and enhancing the Al IV signal. It is well known that Al IV can enhance the acid strength of adjacent SiOH groups, thereby forming Brønsted acid sites (BAS) with high catalytic activity [29]. In addition, from the area content of the tetrahedralcoordinated aluminum (Al IV ) species, it can be seen that when the hydrothermal temperature is low (90°C), the occupied area is between 38.50-52.02%, and when the hydrothermal temperature is increased to 105°C, the area occupied by Al IV species is between 52.67-59.61%. In addition, when the hydrothermal temperature is 120°C, the area occupied by species at 53 ppm increased to 54.98-67.17%. These results show that as the hydrothermal temperature increased, Al VI can be more effectively converted to a tetrahedron (Td-Al), which is the form that enters the skeleton.

SEM analysis of Al-MHCM
In this study, the microstructure changes of Al-MHCM were observed through FE-SEM. The morphology of the Al-MHCM material at a 50,000x magni cation is shown in Fig. 4. During the hydrothermal treatment under alkaline conditions (pH = 10), silicate and aluminosilicate are extracted from the TFT-LCD waste glass and SB waste. Then, the extracted silicate and aluminosilicate are mixed with the CTAB surfactant to form a mesoporous silica material similar to MCM-41 [30]. In addition, it can be seen that even after the calcination process (550°C), the macrostructure of Al-MHCM remains intact, thus con rming the high thermal stability of Al-MHCM [31]. The morphology of Al-MHCM is clearly observed as round nanoparticles, and the appearance of the particles is mainly spherical. From the observations in Fig. 4 increase in crystal size was mainly due to the increased synthesis temperature and rapid hydrolysis reaction that affected the uniformity of the generated particles [32]. In addition, Ekloff et al. stated that the difference between products obtained by the same synthesis method was due to the formation of delayed structures due to changes in the reaction medium pH or the occurrence of uneven precipitation [33].

N 2 isothermal adsorption-desorption of Al-MHCM
The N 2 adsorption-desorption phenomenon provides a technique for determining the surface area, pore volume and pore size distribution. The N 2 isotherm adsorption-desorption curve is shown in Fig. 5. All the synthesized Al-MHCM junctions exhibit a type-IV nitrogen adsorption-desorption isotherm. According to the IUPAC nomenclature, this is a typical feature of homogeneous mesoporous materials [34]. The isotherm shows the following three stages: when the low relative pressure is 0.2-0.3, the monolayer adsorption of nitrogen on the mesoporous wall is observed; and when the relative pressure is 0.3-0.4, a sharp increase occurs under the same parameter conditions with different Si/Al ratios. This is characteristic of capillary condensation in mesopores, which shows a narrow hysteresis loop. Finally, when the relative pressure is 0.5-0.9, the synthesized Al-MHCM shows slightly tilted plateau, which is caused by multilayer adsorption on the outer surface of particles [35]. In addition, when the Si/Al ratio is 26, 41.8 and 56.3, the speci c surface area ranges from 506-587, 733-795 and 781-1013 m 2 /g, respectively. The adsorption-desorption characteristics for all samples are typical of mesoporous materials, indicating that mesoporous materials have better adsorption performance. Figure 6 shows the total pore volume and pore diameter calculated by the N 2 adsorption-desorption measurements and the BJH method and are listed in Table 3. The pore size distribution curves of all Al-MHCM shown in Fig. 6 are unimodal and belong to a narrow range of mesopores that are approximately 3-4 nm, indicating the existence of mesopores in the particles. Thus, this result con rms the successful synthesis of a uniform mesoporous material, and is consistent with the BJH method for calculating the pore size (Table 3). In addition, Fig. 6 (a) shows that when the Si/Al ratio is 26, the pore volume of the synthesized Al-MHCM is 0.4-0.5 cm 3 /g. When the Si/Al ratio is increased to 56.3, the pore volume is increased to 0.9-1.4 cm 3 /g. Based on the above knowledge, the increase in the Si/Al ratio and hydrothermal temperature can form an ordered mesoporous structure. The pore volume and speci c surface area of the synthesized Al-MHCM increase accordingly, with the highest pore volume being 0.97 cm 3 /g, and the highest speci c surface area being 1013 m 2 /g (Al-MHCM 9 ). The results of Sohrabnezhad and Mooshangaie show that adding metal (such as Ag/AgBr) to an MCM-41 material will increase the pore size and pore volume, which indicated that some of the metal was dispersed on the inner pore surfaces of the MCM-41 material [36]. 3.6. The 24 h equilibrium moisture content curve of Al-MHCM Figure 7 shows the equilibrium moisture content of Al-MHCM at different relative humidities (RHs). The solid line in Fig. 7 shows the process of water vapor adsorption, and the dotted line is the process of water vapor desorption. The results show that the equilibrium water contents of all samples increase with increasing RH. However, there is a signi cant difference in the equilibrium moisture content in the material, especially when the relative humidity is high (75 to 95%). Figure 7 shows that when the Si/Al ratio is 26, the RH of the synthesized Al-MHCM is 95%, the equilibrium moisture content is in the range of 65.20-76.60 m 3 /m 3 ( Fig. 7 (a)). When the Si/Al ratio is 41.8, the highest equilibrium moisture content is shown to be in the range of 81.34-91.45 m 3 /m 3 ( Fig. 7(b)). In addition, when the Si/Al ratio is 56.3 and the RH of the synthesized Al-MHCM is 95%, the equilibrium moisture content is 69.63-75.15 m 3 /m 3 (Fig. 7 (c)). These results show that the adsorption characteristics are closely related to the speci c surface area and pore size. In the process of water vapor absorption and desorption, the pore size of Al-MHCM is signi cantly related to capillary condensation [37]. In addition, Jansen et al. observed that water saturation increased with relative humidity in a relatively nonlinear manner; furthermore, they found that the diffusion coe cient did not depend on the water concentration itself because there was no difference √ 3 in the diffusion rate between absorption and desorption [38]. In addition, when the low hydrothermal synthesis temperature is 90°C, the equilibrium moisture content of Al-MHCM at 95% RH is 73.19 m 3 /m 3 , and when the hydrothermal temperature is increased to 120°C, the equilibrium moisture content increased signi cantly to 76.60 m 3 /m 3 ( Fig. 7(a)). Moreover, the adsorption-desorption curve shows a hysteresis loop. Since the inner wall of the pore is in a wet state after the loss of water during the dehumidi cation process, the contact angle of the water molecules on the inner wall of the pore is small, thereby delaying the desorption effect. In contrast, in the process of moisture absorption, the inner wall of the dry pore has a better moisture absorption effect with its large water contact angle. The above results show that the change in the Si/Al ratio exhibits no obvious difference in regard to the change in equilibrium moisture content, while an increase in the hydrothermal temperature can destroy the balance between the solid and liquid phases of the initial gel; thus, the concentration of silicate and aluminosilicate in the liquid phase results in the aggregation of silicate species on the microcell surface. Aggregation promotes the crystallization process and increased the pore structure and speci c surface area of Al-MHCM; therefore, the equilibrium moisture content gradually increased.

Feasibility of synthesizing microporous zeolite
In this study, after the alkaline fusion process and extraction of SiO 2 and Al 2 O 3 , we tried to use the remaining residue as a starting material for a direct hydrothermal synthesis of zeolite with a one-pot hydrothermal method. Through the use of a 3 M NaOH solution as the alkali treatment, the formation and crystal phase in uence of the synthesized microporous zeolite at different hydrothermal temperatures (100, 150, 200°C) were discussed; furthermore, this process is expected to achieve the purpose of completely reusing the remaining residue. Figure 8 shows the XRD pattern. It can be seen from Fig. 8 that when the silicon-to-aluminum ratio is xed, comparing different temperatures will produce different types and crystal phases of zeolite. The results show that it is mainly zeolites such as analcime, sodalite, and cancrinite [39]. TCLP dissolution test results show that the heavy metal leaching amount of each material meets the legal standards, so TFT-LCD waste glass and SB waste have the potential to be recycled. The XRD analysis shows that a high hydrothermal temperature is conducive to the formation of a well-ordered structure. The 27 Al-NMR analysis shows that Al VI can be converted more effectively to a tetrahedron (Td-Al), which is the form that enters the skeleton. These results also show that the increase in hydrothermal temperature makes the crystal size signi cantly smaller. However, the crystal size increased as the Si/Al ratio increased, and the pore volume and speci c surface area of the synthesized Al-MHCM increase accordingly; the highest pore volume is 0.97 cm 3 /g, and the highest speci c surface area is 1013 m 2 /g (Al-MHCM 9 ). These results show that the addition of Al metal to Al-MHCM will increase the pore size and pore volume; therefore, some of the metal is dispersed on the inner pore surface of the material. From the equilibrium moisture content test, it is found that during the absorption and desorption of water vapor, the pore size of Al-MHCM is signi cantly related to the capillary condensation, and it is expected that the adsorption characteristics are closely related to the speci c surface area and pore size. Finally, when the Si/Al ratio is 41.8, the highest equilibrium moisture content is shown to be 91.45 m 3 /m 3 .

Declarations
Availability of data and materials All data generated or analyzed during this study are available from the corresponding author upon request.

Competing interests
The authors declare they have no competing interests.