Synthesis of zeolite from multilayer food packing and sugar cane bagasse ash for CO₂ adsorption

Abstract

The X/A zeolite crystal mixtures were synthesized using sugar cane bagasse ash (SCBA) as a silicon source and multilayer food packing (MFP) as an aluminum source under hydrothermal conditions at 80 °C for 79–296 hours. The silicon was extracted by alkaline fusion for 40 min at 550 °C with an alkali–SCBA weight ratio of 1 : 1. The aluminum solution was obtained from MFP using NaOH 1 M (3 : 1 water–acetone) solution. The synthesized zeolites were analyzed by XRD, FTIR, SEM, and BET. In the XRD results, most of the signals were indexed to zeolite X, and some signals were indexed to zeolite A. The vibration bands in the region 1200–400 cm⁻¹ suggested the presence of the double-six-ring (D6R) zeolite X structure. The crystal morphology is characteristic of the zeolite X, and the specific area found by the BET method was 810.47 m² g⁻¹. The zeolite with the higher specific area was applied in the CO₂ adsorption process until it reached 25 bar by the gravimetric method. The experimentally adsorbed amounts were adjusted with the Langmuir, Freundlich, and Toth models.

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Introduction

Zeolites are aluminosilicates of the alkaline and alkaline-earth metals with porous framework structures of corner-sharing SiO₄ and AlO₄ tetrahedra. Many industries use these materials, and decreasing production costs are relevant. One of the promising strategies to prepare zeolites at a low cost is to replace commercial chemicals with waste products as the starting precursor.¹ The disposal of large amounts of sugarcane bagasse ash and multilayer food packing has become a serious environmental problem. However, these solid wastes can be used in the zeolite synthesis process.

Sugar cane bagasse is a hazardous solid waste generated in large amounts in sugar mills. Combustion of sugar cane bagasse in boilers, used for steam and electricity generation, produces a great amount of another solid waste, denominated sugar cane bagasse ash (SCBA).² Employing this quartz-abundant waste as a silicon source can avoid its accumulation.³

The food packaging industry needs to develop multilayer films containing different polymers. Multilayer films may be manufactured by lamination or co-extrusion. These physical processes combine polymers into a film with special chemical, physical, and mechanical properties.^4,5 Another material that is used in these packages is aluminum, which protects food from the effects of sunlight and ultraviolet radiation.⁶ A single medium-sized food packaging factory, for instance, produces about 8 tons monthly of parings of multilayer films containing aluminum. These films are gathered in the factory or turned into waste. Thus, the future of multilayer packages has become a great environmental concern.

Many researchers have used waste as a low-cost source of silicon and aluminium to produce zeolites. Different types of zeolites such as X,^7–9 ZSM-5,¹⁰ hydroxysodalite,^7,11 Na-P1,^12,13 and zeolite A (ref. 3 and 14) were synthesized through many methods. Considering this, zeolite synthesis using solid waste as aluminium and silicon sources is a promising technique to recycle these wastes. This green strategy has received extensive attention over the last decade.

On the other hand, greenhouse gases are considered the cause of the global temperature increase, and this has attracted attention to the need to develop strategies to decrease carbon dioxide (CO₂) emissions.¹⁵ The most commonly used techniques for CO₂ capture and separation from fuel gases include the ammonium absorption process,¹⁶ dual-alkali absorption,¹⁷ the membrane separation process,^18,19 and adsorption on solid adsorbents.^20–24 Intense research is currently focused on the design of new and effective CO₂ adsorbents. The main challenge for greenhouse gas adsorption is to find a way to decrease the cost of the process and to make it more attractive than other market technologies. Therefore, CO₂ capture based on cheap technology with great potential for reducing the global cost of the sorbents is a very promising alternative for the future.^25,26

One of the promising strategies to prepare low-cost sorbents is the replacement of commercial chemicals with waste products as the starting precursor.²⁷ Furthermore, the disposal of large amounts of sugar cane bagasse ash and multilayer food packing has become a serious environmental problem. Considering this, zeolite synthesis can be adopted as a promising technique for recycling these wastes and has received extensive attention over the last decade.

The purpose of this study was to synthesize zeolite using SCBA as a silicon source and MFP as an aluminum source and apply this green, low-cost zeolite in the CO₂ adsorption process by the gravimetric method at high pressure. This research demonstrates the potential of SCBA and MFP extract to be used as a reliable silica and aluminum source for preparing zeolites for CO₂ capture.

Experimental

Zeolite synthesis

The sugar cane bagasse ash (SCBA) was collected from the sugar cane industry located in the region of Maringá City, Paraná, Brazil. We placed the quartz material in a horizontal furnace and heated it in air at 20 °C min⁻¹ from room temperature to 600 °C and kept it for 4 h (SCBA600). Previous work has shown the characterization of these materials.³ The silicon solution from SCBA was obtained via alkaline fusion treatment with NaOH at 773 K and a molar ratio of 1 : 1.5 (SCBA : NaOH) for 40 minutes. The solid resultant was diluted using distilled water (solution 1). The multilayer food packing was obtained from Inovaflex Rótulos e Etiquetas (Maringá, PR – Brazil). The parings of multilayer film containing 19 ± 1.0 wt% of PET, 47 ± 1.0 wt% of PE, and 34 ± 1.0 wt% of Al were cut to 20 × 30 mm. Treating MFP with NaOH yielded the aluminum solution 1 mol L⁻¹ (3 : 1 water–acetone) after 24 hours. After silicon and aluminum extraction, both solutions were mixed in the molar ratio 1SiO₂ : 0.4Al₂O₃ : 3.3Na₂O : 173.8H₂O. We transferred the mixture (2.0 L) to 10 polypropylene reactors (0.2 L each) and kept them at 80 °C for different crystallization periods (79, 121, 149, 163, 212, 235, 247, 272, 284, and 296 hours). Then, the solid was separated by filtration, washed with distilled water, and dried overnight at 100 °C.

Characterization

The zeolites were characterized by Fourier transform infrared spectrometry (Bomem-Michelson MB-100 with a resolution of 4 cm⁻¹ using a KBr disc method). XRD analysis (Shimadzu, model XRD-6000 X-ray operated at 40 kV and 40 mA, with Cu Kα (1.54 Å) as the radiation source, diffraction angle – 2θ – in the range 4°–60°). The relative crystallinity was calculated using the area of diffraction signals localized in 2θ = 6, 10 15, 23, 26, and 31°. Scanning electron microscopy (SEM) (Shimadzu SSX-550 Superscan) characterized the morphology and the N₂ adsorption/desorption isotherm at 77 K (ASAP 2020 – Micromeritcs).

CO₂ adsorption

Adsorption equilibrium studies were performed with zeolite synthesized for 149 hours due to its higher specific surface area. The mass measurement was achieved using a magnetic suspension balance from Rubotherm (Bochum, Germany). The adsorbent was degassed in situ at 573 K until no mass variation was observed. Soon after, the measuring chamber was cooled down to the experiment temperature (298 K), and the gas pressure (CO₂) was increased stepwise (until 25 bar). The mass variation at equilibrium (m) was recorded for each pressure step. For the selected sample, a previous experiment with helium was carried out to determine the specific volume of the solid phase and the sample container volume, characteristic of the suspended parts inside the chamber. The sum of these volumes was used to account for the buoyancy effects on measurements with the adsorbed phase.

For a given gas pressure P, the adsorbed phase concentration may be calculated according to eqn (1):^28–30

m_ex(P, T) = Δm(P, T) + [(V_b + V_s)ρ(P, T)] (1)

where m_ex is the adsorption excess uptake (g per g sample), Δm is the mass difference sensed by the equipment (g per g sample), V_b is the volume of the balance-suspended components (cm³), V_s is the specific volume of the sample (cm³ per g sample), ρ is the gas density (g cm⁻³), P is the pressure (bar), and T is the temperature (K).

To clearly describe the CO₂ adsorption behavior on the synthesized zeolite, the Toth, Freundlich, and Langmuir models were used to fit the isotherm using the software Origin 7.0®.^31,32,33,34 The description of adsorption models and equations was described in ESI.†

Results and discussion

Fig. 1 displays the X-ray diffraction patterns of zeolite synthesis for each period of time. The diffraction peaks were indexed to zeolite type X and A as indicated in the figure. These zeolites exhibit Pm3̄m and Fd3̄ space groups, respectively (standard pattern number 71-0784 and 85-2064 – ICDD database and standard pattern of International Zeolite Association – IZA). After 79 h, X/A zeolite crystals mixtures were detected. Following the crystallization time, zeolitization increases until 149 h. After 149 h, crystallinity decreases, indicating an alkaline attack due to high crystallization time. This can be observed in the main signals to zeolite X (localized in 6, 10, 15, 23, 26, and 31° – 2 theta degrees). The materials with higher relative crystallinity were 149, 163, 212, and 247 h. However, the sample 247 h was ignorated to the adsorption test due to the long synthesis time. Available times verify Ostwald's rule:³⁵ the crystalline phase did not change successively, indicating that increasing time would not obtain pure-phase zeolite X.

Fig. 1 X-ray diffraction patterns of zeolite synthesis for each period of time.

Fig. 2 presents the FTIR spectra of the zeolites as a function of the hydrothermal process period. Peaks in the lattice region of 1200–400 cm⁻¹ suggest the existence of zeolite X. The spectrum of X zeolite illustrates the presence of absorptions at 458, 559, 666, 746, and 974 cm⁻¹. The 974 cm⁻¹ band is due to the Si–O–Al asymmetric stretching vibration mode of T–O bonds, (where T = Si or Al). The band at 746 cm⁻¹ is due to the S4R T–O–T symmetric stretching, while the absorption at 559 cm⁻¹ is attributed to D6R T–O–T symmetric stretching and is very close to the external vibration of double four-rings (D4R) in the zeolite A framework localized at 557 cm⁻¹. The two bands at 666 and 458 cm⁻¹ are assigned to the Si–O–Al symmetric stretching and S4R symmetric bending modes, respectively. This band is slightly shifted and sharpens as the amorphous material transforms to crystalline zeolite. These results agree well with XRD and microscopy results. The OH band, related to deformational vibrations of adsorbed water molecules in zeolite channels, also appeared at about 1655 cm⁻¹.³⁶

Fig. 2 FTIR spectra of the zeolites as a function of the hydrothermal process period.

The SEM observations in Fig. 3 surveyed the external morphology of synthesized zeolites in different periods. A well-defined octahedral morphology is typical of zeolite X (ref. 7–9) and appeared in hydrothermal synthesis at 79 hours. After 212 h, the corrosion surface set in, indicating an alkaline attack on the surface due to the high crystallization time. This evidence corroborates the XRD and FTIR results.

Fig. 3 SEM images of zeolites.

The textural properties were determined using BET, t-plot, and Dubinin–Radushkevich methods.³⁷ The specific surface area was used only for an internal comparison of the samples. The BET method obtained the total specific surface areas (we chose the linear region in the range of 0.004 < P/P₀ < 0.04), which were 810.47 m² g⁻¹, 767.38 m² g⁻¹, and 757.20 m² g⁻¹ to materials 149 h, 163 h, and 212 h, respectively, whereas the micropore areas determined by the t-plot method were 808.22 m² g⁻¹, 764.01 m² g⁻¹, and 755.41 m² g⁻¹, indicating that these materials are priority microporous. This fact is also clearly using an internal comparison between the micropore and the total pore volumes determined by the t-plot, Dubinin–Radushkevich (DR), and BET methods (t-plot: 0.3062 cm³ g⁻¹, 0.2870 cm³ g⁻¹, and 0.2840 cm³ g⁻¹ of micropores – DR: 0.3086 cm³ g⁻¹, 0.2906 cm³ g⁻¹, 0.2869 cm³ g⁻¹ of micropores and total pore volume as determined by BET method were 0.3173 cm³ g⁻¹, 0.3026 cm³ g⁻¹, 0.2915 cm³ g⁻¹ to materials 149 h, 163 h, and 212 h, respectively). The energy of nitrogen adsorption at 77 K was measured by the Dubinin–Radushkevich method presenting ca. 38 kJ mol⁻¹, indicating a physisorption process. The zeolite at 149 h exhibits the higher specific area and micropore volume, corroborating with XRD results that indicate that the 149 h material presented higher relative crystallinity. This fact points to a potential application of this green material because zeolite X has a large pore size (7.3 Å) and a high cation exchange capacity (5 meq g⁻¹), which make this zeolite an interesting molecular sieve and a high-cation exchange material (Fig. 4).¹⁴

Fig. 4 N₂ adsorption–desorption isotherms at 77 K.

Fig. 5 shows the experimental CO₂ adsorption isotherm and the adjustment with the three models on the zeolite prepared for 149 h (Z-149) at 298 K at the pressures between 0 and 25 bar. It can be seen that the maximum adsorbed amounts of CO₂ on the zeolite sample is close to 7 mmol g⁻¹. However, this amount is achieved up to 3 bar pressure, indicating a fast saturation of the porous structure. This behavior at low pressure can be explained by the strong interaction between CO₂ molecules and the Z-149 surface. In fact, the high-energy sites are first occupied by CO₂ molecules.³⁰ The literature shows similar results for the amount of CO₂ adsorbed in porous materials.^38–44 Therefore, the results indicate that zeolite obtained from the sugar cane bagasse ash is a promising low-cost sorbent and has potential as an efficient gas-adsorption process.

Fig. 5 Experimental CO₂ adsorption isotherm and the adjustment with the adsorption models on the zeolite prepared by 149 h (Z-149) at 298 K.

Table 1 shows the adjusted parameters of the Langmuir, Toth, and Freundlich models fitted on the CO₂ adsorption isotherm at 293 K. The CO₂ adsorption can be better fitted by the Toth adsorption equation. The Toth isotherm assumes that adsorption occurs on a heterogeneous surface containing sites with different energy and availability for adsorption.^28–30,32 In addition, when the affinity of the Toth constant b is larger, there is a stronger affinity of the adsorbate molecule toward the surface; in other words, the surface is covered by an organized layer of adsorbate molecules.

Table 1 Langmuir, Toth and Freundlich adjustments parameters of CO₂ isotherm at 293 K

Toth				Freundlich			Langmuir
qm (mmol g^−1)	b (bar^−1)	n	r^2	n	k	r^2	qm (mmol g^−1)	b (bar^−1)	r^2
7.01345	6.97833	0.75255	0.97622	8.64597	5.0041	0.90026	6.79188	4.16527	0.94069

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In summary, the mechanism proposed for CO₂ adsorption in zeolite from solid waste indicates that this process mostly occurs in the cavities because this 149 h is predominantly a microporous material, due to the demonstrated micropore-specific area of the 808.22 m² g⁻¹ (determined by the t-plot method), while the total area was 810.47 m² g⁻¹ (determined by the BET method). Linear OCO–X⁺ complexes are formed (X⁺ is the cation) that also involve the perturbation of Si–O–Al bonds according to Coluccia S. et al. (1999)⁴² who obtained the formation of molecules linearly co-ordinated to X⁺ cations (Lewis acidity) of the zeolitic supercages, and several different kinds of carbonate-like species form complex interactions, which was also reported by Martra, G. et al. (1999)⁴³ and Montanari, T. and Busca G. (2008).⁴⁴

Conclusions

This work showed that sugar cane bagasse ash and multilayer food packing can be successfully used as raw material for the hydrothermal synthesis of green and low-cost zeolites, which have potential applications in the CO₂ adsorption process. This research contributes to materials and environmental science, suggesting the recycling of contaminant solid wastes generated in large amounts around the world. In fact, this reveals a new green and low-cost material for CO₂ adsorption, which is possibly doubly beneficial to environmental management because decreased contamination of hazardous solid wastes and greenhouse gas capture costs.

Acknowledgements

The authors acknowledge the financial support of CNPq. Murilo Pereira Moisés, Paula Pomaro de Almeida, and Cleiser Thiago Pereira da Silva would like to thank CNPq and Capes for the scholarships.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra04513k

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