Evaluation of the Synthesis of Zeolite SSZ-13 using Choline Chloride and Different Crystallization Seeds and its Application in the Thermocatalytic Degradation of Ultra-High Molecular Weight Polyethylene

Abstract

The synthesis of SSZ-13 was studied by evaluating the influence of two types of crystallization seeds with CHA structure (S₁ seeds prepared in fluoride medium and S₂ seeds by the conversion of zeolite Y), SiO₂/Al₂O₃ (28, 40, 60 and 100) and OH/SiO₂ (0.6 and 0.8) ratios. Choline chloride was used as the structure-directing agent and the crystallizations occurred at 140 ºC from 24 to 96 h. S₁ seeds led to the coexistence of zeolites SSZ-13 and P, while S₂ seeds promoted in all SiO₂/Al₂O₃ ratios and OH/SiO₂ = 0.8, the formation of pure SSZ-13 after 24 h. The SSZ-13 samples are formed by clusters of crystals with cuboid-like morphology and exhibited textural properties typical of CHA framework. SSZ-13 zeolite led to a decrease in the temperature and the activation energy required for the degradation of ultra-high molecular weight polyethylene polymer, demonstrating its efficiency as catalyst in this process.

Keywords:
Zeolite SSZ-13; Crystallization seeds; Choline chloride; Ultra-high molecular weight polyethylene; Thermocatalytic degradation

1. Introduction

Zeolite SSZ-13 (framework type code CHA, with SiO₂/Al₂O₃ ratio > 6) has a three-dimensional structure formed by 4-, 6- and 8-membered ring (MR), containing large cages (0.73 nm) and intersecting channels with a diameter of 0.38 nm^1,2. Due to some properties presented by SSZ-13, such as large specific surface area, high ion-exchange capacity, strong surface acidity, remarkable molecular-shape selectivity and thermal/hydrothermal stability, several studies report the application of this material in adsorption processes and CO₂ separation^3-5, conversion of methanol to olefins (MTO), in which it can increase the yield of low-carbon olefins^6-9, in the selective catalytic reduction (SCR) of NO_x^10-13 and pyrolysis of low-density polyethylene (LDPE)¹⁴.

Crystallization of SSZ-13 has been carried out using different methodologies, the main ones being the direct synthesis (from silica and alumina precursors) and the conversion of zeolite Y¹. The first synthesis of SSZ-13 was reported by Zones in 1985¹⁵. However, the application of this material initially became limited due to the use of expensive organic structure-directing agents (SDAs)¹⁶, such as N,N,N-trimethyl-1-adamantammonium hydroxide (TMAdaOH) and N,N,N-trimethyl-1-adamantammonium iodide, making its synthesis costly and with low solid yields^17-20. In view of this, studies were focused on optimizing the synthesis of SSZ-13, in order to make its production economically attractive. Different organic molecules, like tetraethyl phosphonium hydroxide (TEPOH)²¹, copper-tetraethylenepentamine (Cu-TEPA)^22-24, tetraethylammonium hydroxide (TEAOH)^25,26, benzyltrimethylammonium hydroxide (BTMAOH)²⁷ and choline chloride^28,29 were reported as structure-directing agents to synthesize zeolite SSZ-13.

Different procedures have been described to obtain the chabazite structure (CHA) from the conversion of zeolite Y (as a source of Si and Al), which decomposes/dissolves and generates ordered aluminosilicate species, that are subsequently rearranged to form another zeolitic structure²⁷. Martín et al.²⁵ described the combination of zeolite Y with TEAOH, resulting in chabazite being obtained after 72 h of crystallization. However, these routes present as a major disadvantage the long crystallization period, around 3 to 5 days.

The addition of seed crystals of the desired zeolite phase in the synthesis gel generally leads to a faster consumption of reagents and/or secondary nucleation mechanism³⁰, resulting in higher crystallization rates^31-34 as well as allowing control of the crystal size distribution³⁵. The use of crystallization seeds combined with a less expensive structure-directing agent is one of the alternatives to produce the SSZ-13 with shorter times, which makes the synthesis more economical and with greater potential for industrial application.

Therefore, it is desirable to develop more efficient routes to synthesize SSZ-13, in order to meet the growing interest in high quality catalysts and enable new applications for this material. In the present work, the synthesis of zeolite SSZ-13 was investigated from reaction mixtures with different SiO₂/Al₂O₃ and OH/SiO₂ ratios, using choline chloride as structure-directing agent and two types of crystallization seeds (with CHA structure) obtained by different methodologies. The synthesized samples were then evaluated in the thermocatalytic degradation of ultra-high molecular weight polyethylene (UHMWPE).

2. Experimental

2.1. Reagents

The following reagents were used in the experiments: ammonium fluoride (Sigma-Aldrich, 98%), potassium hydroxide (Sigma-Aldrich, 85%), aluminum hydroxide (Merck, 63%), colloidal silica (Ludox AS-40, Sigma-Aldrich), sodium aluminate (Sigma-Aldrich, 98%), choline chloride (Sigma-Aldrich, 98%) and ammonium nitrate (Merck, 99%).

2.2. Synthesis of the crystallization seeds

S₁ seeds synthesis: The S₁ seeds were prepared based on the methodology described by Liu et al.³⁶, with modifications. The molar composition of the synthesis mixture was: 1.0SiO₂:0.2Al₂O₃:0.39K₂O:0.3NH₄F:35.0H₂O The preparation procedure of the synthesis mixture was as follows: (i) dissolution of ammonium fluoride in 50% of the distilled water required for the synthesis; (ii) dissolution of potassium hydroxide in the remaining distilled water, followed by the addition of aluminum hydroxide; (iii) the solutions obtained in steps (i) and (ii) were mixed together; (iv) then, the colloidal silica was added to the mixture obtained in step (iii). During steps (i) to (iv) mechanical stirring (400 rpm) was used for 15 min. The resulting synthesis mixture remained under mechanical stirring at 400 rpm for 4 h and was subsequently transferred to polytetrafluoroethene (PTFE) vessels and sealed in stainless steel autoclaves. The crystallization was carried out in a convection oven at 160 ºC for 72 h, under static conditions. The solids were recovered by vacuum filtration, washed with distilled water until the filtrate reached neutral pH and then dried in an oven at 120 °C for 12 h.

S₂ seeds synthesis: The S₂ seeds were synthesized based on the study by Bourgogne et al.³⁷. The following procedure was performed: (i) dissolution of potassium hydroxide in the distilled water required for the synthesis; (ii) adding zeolite faujasite (Zeolyst International, CBV 500) to the solution obtained in step (i). The resulting mixture was stirred for 5 min to form a synthesis mixture with molar composition 1.0SiO₂:0.2Al₂O₃:0.4K₂O:44.8H₂O, which was then divided and transferred to PTFE-lined stainless steel autoclaves. The synthesis was carried out in a convection oven at 95 ºC for 72 h, under static conditions. The steps of recovery, washing and drying of the solids were conducted as described for the S₁ seeds.

2.3. Synthesis of zeolite SSZ-13

Zeolite SSZ-13 was synthesized according to the methodology described by Xu et al.²⁸, with some modifications. Choline chloride (CC) was used as structure-directing agent and the mass of seed crystals was fixed at 10% of the SiO₂ mass. The molar composition of the synthesis mixture was: 0.125CC:1.0SiO₂:xAl₂O₃:yNa₂O:12H₂O, with x = 0.0357, 0.025, 0.0167 or 0.0128 and y = 0.3 or 0.4. The reaction mixture was prepared through the following steps: (i) dissolution of sodium hydroxide and sodium aluminate in 80% of the distilled water required for the synthesis (ii) addition of choline chloride in the solution obtained in step (i); (iii) addition of colloidal silica in the mixture resulting from step (ii); iv) The seeds (S₁ or S₂) were dispersed in the remaining water and then were added to the mixture obtained in step (iii). The mixtures obtained in each step were submitted to mechanical stirring (400 rpm) for 15 min, except for (iii), in which the stirring time was 40 min. Then, the resulting gel was divided and transferred to PTFE-lined stainless steel autoclaves, which were heated in a convection oven at 140 ºC for 24 to 96 h, under static conditions.

After crystallization, the solids were separated from the supernatant liquid by vacuum filtration, washed with distilled water until the filtrate reached neutral pH, and then dried in an oven at 120 °C for 12 h. Then, the samples were calcined at 550 °C for 8 h at a heating rate of 2 °C min^-1, under a synthetic air flow of 100 mL min^-1. Subsequently, the materials were subjected to ion exchange using a 0.1 mol·L^-1 ammonium nitrate solution at 80 °C for 12 h (1 g of solids per 100 mL of solution). Finally, the samples were again dried and calcined, under the same conditions previously described.

The solid yield was determined by weighing the synthesis mixture and the final dry solids. The yield was calculated on a dry basis, i.e., discounting the mass of water added in the reaction mixture, using Equation 1.

$\text{Yield}\left(\text{\%}\right)\text{=}\frac{\text{Mass of the final solid}}{\text{Mass of synthesis mixture on a dry basis}}\text{X 100}$ (1)

2.4. Characterization

X-ray diffractometry (XRD) was performed using a Shimadzu XRD-6000 diffractometer, with CuKα radiation (λ = 0.15418 nm), Ni filter, 40 kV voltage and 30 mA current intensity. Data were collected in the 2θ range between 3 and 40º, with goniometer speed of 2º·min^-1 and step of 0.02º. The area of the diffraction peaks located in the 2θ regions of 8.26° - 10.16°, 19.90° - 21.20° and 29.88° - 31.78° were used to calculate the crystallinity of the materials. Energy dispersive X-ray spectroscopy (EDX) was conducted on a Shimadzu EDX-7000/8000 spectrometer to determine the SiO₂/Al₂O₃ ratio of the samples. Thermal analysis (TG/DTG) was carried out in a Shimadzu DTG-60H thermobalance, using alumina pans and sample masses of approximately 10 mg. The samples were heated from 30 to 800 ºC at a rate of 10 ºC·min^-1, in a dynamic atmosphere of synthetic air (flow rate of 50 mL·min^-1). Nitrogen adsorption-desorption measurements were performed using a Micromeritics ASAP 2020 equipment at -196 ºC, in the relative pressure range between (P/P₀) between 0.01 and 0.99. The solids were previously degassed at 350 ºC for 12 h, under vacuum of 2 μmHg. The specific surface area (S_BET) was calculated using the BET method. The microporous area (S_Micro), external surface area (S_Ex) and microporous volume (V_Micro) were determined by the t-plot method. The total pore volume (V_T) was measured using the single-point BET method at P/P₀ = 0.975. Scanning electron microscopy (SEM) was carried out using a Shimadzu SSX-550 electron microscope. Temperature-programmed desorption of ammonia (NH₃-TPD) was conducted in a multipurpose reaction system (SAMP3 model equipment, Termolab - Brazil) equipped with thermal conductivity detector. In these measurements, a mass of approximately 100 mg of sample was submitted to a pretreatment at 400 °C for 1 h, under helium atmosphere with a flow rate of 30 mL·min^-1 Then, the temperature was reduced to 100 °C and the sample was kept in contact with the ammonia stream at a flow rate of 30 mL·min^-1 for 40 min. The next step of the process was the removal of the physisorbed NH₃ molecules at 100 °C for 1 h, under a helium flow rate of 30 mL min^-1. Finally, the NH₃ desorption curves were obtained in the temperature range between 100 and 800 °C, with a heating rate of 10 °C min^-1 and under helium flow rate of 30 mL min^-1.

2.5. Thermocatalytic degradation of UHMWPE

Ultra-high molecular weight polyethylene (UHMWPE, Sigma-Aldrich) was used in the thermocatalytic degradation tests. The catalysts were previously dried in an oven at 100 °C for 12 h and then mixed with UHMWPE using an agate mortar, in the proportion by weight of 30% catalyst/70% polymer, based on the study described by Silva et al.³⁸. The mixtures were dried in an oven at 100 °C for 4 h before each analysis. The polymer thermocatalytic degradation experiments were carried out in a Shimadzu DTG-60H thermobalance, from room temperatures to 1000 °C, under nitrogen flow of 50 mL·min^-1 and heating rates of 10, 20 and 40 °C min^-1. Approximately 10 mg of sample were used in each test. From the obtained data, the activation energy of the degradation reaction of UHMWPE with and without catalyst was determined, using the Flynn-Wall-Ozawa non-isothermal kinetic model^39-41.

3. Results and Discussion

3.1. Physicochemical Characterization

The synthesis conditions and the results of XRD and EDX characterizations are shown in Table 1.

Table 1
Synthesis and characterization results (XRD and EDX).

Sample	Synthesis conditions						Products
	SiO2/Al2O3 molar ratio	OH/SiO2 molar ratio	Seeds	Time (h)		aSolid yield (%)	bSiO2/Al2O3 final molar ratio	dPhase	cRelative crystallinity (%)
S1 Seeds	5	1.38	---	72		98	5	Chabazite	79
Z1	28	0.6	S1	96		63	---	SSZ-13 + Amorphous	8
Z2	40	0.6	S1	96		48	---	SSZ-13 + Amorphous	16
Z3	60	0.6	S1	96		20	---	SSZ-13 + Amorphous	17
Z4	100	0.6	S1	96		9	---	SSZ-13 + Amorphous	67
Z5	28	0.8	S1	24		35	9,2	SSZ-13 + Zeolite P	79
Z6	40	0.8	S1	24		20	9,0	SSZ-13 + Zeolite P	50
Z7	60	0.8	S1	24		14	8,4	SSZ-13 + Zeolite P	48
Z8	100	0.8	S1	24		13	8,2	SSZ-13 + Zeolite P	56
S2 Seeds	5	0.813	---	72		80	5	Chabazite	80
Z9	28	0.6	S2	96		44	---	SSZ-13 + Amorphous	31
Z10	40	0.6	S2	96		49	---	SSZ-13 + Amorphous	18
Z11	60	0.6	S2	96		16	---	SSZ-13 + Amorphous	65
Z12	100	0.6	S2	96		17	---	SSZ-13 + Amorphous	81
Z13	28	0.8	S2	24		41	8,3	SSZ-13	100
Z14	40	0.8	S2	24		30	8,2	SSZ-13	94
Z15	60	0.8	S2	24		25	8,7	SSZ-13	99
Z16	100	0.8	S2	24		15	8,4	SSZ-13	98

• ^a
  Obtained by equation 1.
• ^b
  Determined by EDX – The estimated values may have errors of up to ±10%
• ^c
  Determined by XRD.

The EDX results revealed that the samples composed only by SSZ-13 phase (Z13 to Z16) had similar SiO₂/Al₂O₃ ratios (between 8.2 and 8.7), suggesting an optimal range for the formation of well-crystallized zeolite SSZ-13. It can be observed that the increase in the OH/SiO₂ ratio had a strong influence on SSZ-13 crystallization, but on the other hand it promoted significant reductions in solid yield, probably due to the increase in the concentration of silicon in the liquid phase had inhibited the solid formation. During the formation of the aluminosilicate framework, some of the lattice Si⁴⁺ ions are replaced by Al³⁺, depending on the SDA content of the mixture. Therefore, the incorporation of aluminum in the lattice generates negative charges that are compensated by inorganic cations (such as sodium, introduced from the mineralizing agent NaOH) and organic cations (such as choline, which is the SDA used in the synthesis). Thus, depending on these factors, the crystallization of the SSZ-13 zeolite can generate materials with a particular range of Si/Al molar ratio⁴².

The XRD profiles of the crystallization seeds and of the samples containing the SSZ-13 phase with high relative crystallinity are illustrated in Figure 1. The diffractograms of the S₁ and S₂ seeds demonstrate the formation of well-crystallized materials with the main peaks coinciding with those of the CHA structure (JCPDS card 34-137)^27,28, indicating that both methods of preparation employed were effective in the synthesis of such material.

Figure 1
XRD profiles of selected samples synthesized with: (a) S₁ seeds and (b) S₂ seeds. P = Zeolite P phase.

The samples synthesized from both types of seeds and OH/SiO₂ ratio = 0.6 consisted of amorphous materials with small diffraction peaks related to the crystallization seeds themselves. With the increase of the SiO₂/Al₂O₃ ratio of the reaction mixture, samples with partially crystallized SSZ-13 phase were obtained. The increase of the OH/SiO₂ ratio to 0.8, in the presence of S₁ seeds, led to the formation of mixtures of zeolites SSZ-13 and P (JCPDS card 40-1464)⁴³.

On the other hand, Z13 to Z16 samples synthesized from S₂ seeds exhibited all the diffraction peaks characteristic of zeolite SSZ-13^25,27,44 and higher relative crystallinity after 24 h of crystallization (Table 1). Samples Z13 to Z15 presented a small shoulder referring to the zeolite P at the diffraction peak corresponding to the crystallographic plane (1 0 -1), which is indicative of low concentration contamination (traces of zeolite P). However, this additional peak disappeared after calcination, indicating the low thermal stability of the impurity phase. Thus, under the studied conditions, the type of seed used affected the crystallization time necessary to reach maximum crystallinity and inhibited contamination by other phases.

According to Thompson⁴⁵, the theory of nucleation and crystal growth from seed crystals establishes that the rate of crystal growth is proportional to the external surface area available in the seeds. The results obtained for the samples synthesized with the S₂ seeds can be explained by the fact that these seeds have smaller crystals (see SEM results), which in turn exhibit a larger external surface area.

Figure 2 shows the results of the thermal analysis carried out on the most crystalline and contaminant-free SSZ-13 samples (Z13 to Z16). In all cases, thermogravimetric (TG) curves and their respective derivative (DTG) exhibited three distinct mass loss events, which can be attributed to: (i) elimination of surface water and removal of strongly adsorbed water, from ambient temperature to 310 ºC; (ii) oxidative decomposition of choline molecules, between 310 and 510 ºC; (iii) degradation of carbonaceous material (coke) deposited on the sample during the decomposition of the SDA, in the temperature range of 510 to 800 ºC²⁸. The percentages of mass loss related to each event are listed in Table 2. The samples showed very similar mass losses, which is in agreement with the analogous results obtained for the relative crystallinity and SAR of the materials presented in Table 1.

Figure 2
Mass loss profiles from thermal analysis (TG/DTG) of the SSZ-13 samples.

Table 2
Mass loss of the zeolite SSZ-13 samples.

Sample		Mass loss (%) / Temperature range (ºC)
		(I)	(II)	(III)	Total loss
		30-310 °C	310-510 °C	510-800 °C	(%)
Z13		14.19	5.56	1.85	21.60
Z14		13.66	5.64	1.63	20.93
Z15		11.70	6.44	2.90	21.04
Z16		13.55	6.53	2.88	22.96

The nitrogen adsorption-desorption isotherms of the crystallization seeds and samples Z13 to Z16 are shown in Figure 3 and 4, respectively. All materials presented type I isotherm profiles according to the IUPAC classification, with large amounts of N₂ adsorbed in relative pressures (P/P₀) < 0.1, in contrast to a low nitrogen uptake in the range of medium to high relative pressures. This indicates that the solids have predominantly narrow micropores (< 1 nm)^46,47. The low amounts adsorbed by the S₁ and S₂ seeds (Table 3) can possibly be attributed to the existence of K⁺ ions in the small 8-ring channels, which strongly hinders nitrogen adsorption⁴⁸. The values of the textural properties of samples Z13 to Z16 determined from the N₂ adsorption-desorption data are listed in Table 3. Samples Z14 and Z16 showed similar values of specific area and microporous volume, while the higher values of these parameters for samples Z13 and Z15 are probably related to their greater crystallinity. The results are in agreement with the relative crystallinity of the materials (Table 1) and with those reported in the literature^1,49.

Figure 3
N₂ adsorption-desorption isotherms of the S₁ (a) and (b) S₂ crystallization seeds.

Figure 4
N₂ adsorption-desorption isotherms of the SSZ-13 samples. For clarity, the isotherms were offset along the y axis for Z14 and Z15 (10 cm³·g^-1) and Z16 (15 cm³·g^-1) samples.

Table 3
Textural and acidic properties of the seeds and zeolite SSZ-13 samples.

Sample	Textural properties				NH3–TPD
	aSBET	bSEx	cSMicro	dVMicro	Weak acid sites	Strong acid sites	Total acid sites
	(m2.g-1)	(m2.g-1)	(m2.g-1)	(cm3.g-1)	(µmol.g-1)	(µmol.g-1)	(µmol.g-1)
S1 Seeds	93	4	89	0.03	---	---	---
S2 Seeds	117	14	103	0.04	---	---	---
Z13	501	29	472	0.186	812	658	1470
Z14	421	15	406	0.160	671	608	1279
Z15	502	16	486	0.195	945	660	1605
Z16	420	14	406	0.159	662	446	1108

• ^a
  BET surface area.
• ^b
  External surface area.
• ^c
  Micropore surface area.
• ^d
  Micropore volume.

The TPD-NH₃ profiles of samples Z13 to Z16 in their acid form are illustrated in Figure 5. All catalysts exhibited desorption curves with two main peaks, typical of zeolitic materials⁵⁰. The sharp peak located in the temperature range between 100 and 300 °C can be attributed to the desorption of ammonia weakly bound to the surface that was not removed during the purging step^51-53 and/or from weak Lewis acid sites. On the other hand, the less intense peak observed between 300 and 600 °C is mainly associated with strong Brønsted acid sites^54,55.

Figure 5
NH₃-TPD profiles of the SSZ-13 samples.

The acid sites density values obtained from the NH₃-TPD curves (Table 3) leading to the following acidity order: Z15 > Z13 > Z14 > Z16. The acidity values of the samples were lower than that predicted from the aluminum content in the materials with SAR = 8.5, which is 3171 μmol/g. This fact can be attributed to two simultaneous phenomena: i) the degree of ion exchange of the compensation cations in the synthesized zeolites (Na⁺) for ammonium ions (NH₄⁺), that even being made in triplicate may not have reached a high level of exchange due to factors such as high Al content in the structure and positions of the exchange sites; ii) presence of amorphous impurities in the samples, as well as the existence of structural defects (Al atoms in octahedral coordination, narrowing and blockage of pores, among other anomalies) that limit the diffusion and adsorption of the NH₃ molecules used in the acidity measurement.

The factors that led to variations in the acidity of the samples are the differences in textural properties, such as internal surface area, total pore volume and micropore volume. These parameters can be indicative of zeolite purity, since lower microporous volume values occur due to crystalline defects such as line defects (edge and screw dislocation), pore blockage, intergrowth of impurity phases, etc^56-58. According to Table 3, samples Z13 and Z15 showed the highest micropore volume values compared to the other materials and, as these catalysts have similar amounts of aluminum, slightly higher acidity values were also observed.

Figure 6 and 7 shows the micrographs of the crystallization seeds and samples Z13 to Z16, respectively. The seeds clearly exhibited different morphologies. The S₁ material, synthesized in fluoride medium, presents a walnut-like shape (particle size around 20 µm), while the S₂ seed consists of clusters of irregular-shaped particles (< 2 µm). The different morphologies of the two types of seeds are likely due to the different crystallization temperatures (S₁ at 160 °C and S₂ at 95 °C), as well as the use of fluoride to obtain the S₁ seeds, which led to changes in the growth rate of the crystals and consequently a variation in particle sizes^36,59. The materials synthesized with the aid of S₂ seeds consist of clusters of intergrown particles (< 20 µm), which is a characteristic of seed-assisted syntheses⁴⁸. The Z15 sample is formed by cuboid-like crystals, while the Z13, Z14 and Z16 samples have crystals with an undefined morphology.

Figure 6
Scanning electron micrographs of the crystallization seeds: (a) and (b) S₁ seeds; (c) and (d) S₂ seeds.

Figure 7
Scanning electron micrographs of the SSZ-13 samples: (a) Z13, (b) Z14, (c) Z15 and (d) Z16.

3.2. Thermocatalytic degradation of UHMWPE

The mass loss curves of UHMWPE degradation under the heating rate of 10 °C min^-1 and the activation energy as a function of the conversion in the polymer decomposition process are illustrated in Figure 8. The mass loss curves reveal that the polymer degradation occurred in a single stage, associated with the evaporation or volatilization of light products, and that the addition of the catalysts reduced the decomposition temperature when compared to the purely thermal process. The polymeric macromolecules are broken down on the external surface of the catalyst, where the polymer melts and spreads among the zeolite particles during the degradation process, with the products diffusing through the polymer film or reacting inside the pores^60,61. The reactions continue through carbocation as transition state, which is governed both by the nature of the carbocation and by the type and strength of the acid sites. Thermal degradation occurs through a single reaction mechanism, whereas the presence of acid sites on the catalyst surface leads to the occurrence of different mechanisms⁶².

Figure 8
(a) Mass loss curves under the heating rate of 10 °C·min^-1 and (b) activation energy as a function of the conversion in the UHMWPE degradation process.

The use of zeolite SSZ-13 significantly reduced the energy level required for polymeric degradation. The Z15 sample showed the lowest activation energy value in the thermocatalytic degradation of UHMWPE compared to the other catalysts, which led to the following average activation energy order (Eam): Z15 (Eam = 104 kJ·mol^-1) < Z13 (Eam = 111 kJ·mol^-1) < Z14(Eam = 116 kJ·mol^-1) < Z16(Eam = 131 kJ·mol^-1) < UHMWPE pure (Eam = 241 kJ·mol^-1). Fernandes et al.⁶³ studied the degradation of high-density polyethylene (HDPE) in the presence of natural chabazite and reported average activation energy values of 277.8 kJ·mol^-1 for the thermal degradation of the polymer and 197.3 kJ·mol^-1 when using the catalyst. This indicates that zeolites with CHA framework, such as SSZ-13, can act as a catalyst for polymers degradation, enhancing the generation of light products of industrial interest.

4. Conclusions

XRD results showed that the CHA structure was obtained by hydrothermal treatment in fluoride medium (S₁ seeds) and the conversion of zeolite Y (S₂ seeds). The syntheses using choline chloride and S₁ seeds (OH/SiO₂ = 0.8) led to samples with the coexistence of zeolites SSZ-13 and P. On the other hand, pure zeolite SSZ-13 was synthesized after 24 h of crystallization with the aid of the S₂ seeds, indicating a clear effect of the type of seed on the material produced. The EDX results revealed that the samples containing only the zeolite SSZ-13 had SiO₂/Al₂O₃ ratios between 8.2 and 8.7, showing that this material crystallizes with a restricted chemical composition. All materials exhibited type I N₂ adsorption isotherms, typical of microporous materials. NH₃-TPD results showed the following acidity order for the pure SSZ-13 samples: Z15 > Z13 > Z14 > Z16. Slightly higher surface acidity was observed for samples Z13 and Z15 because, in addition to having a similar aluminum content, these materials had the highest micropore volume values. The materials synthesized with the aid of S₂ seeds consist of clusters of intergrown particles (< 20 µm), which is a characteristic of seed-assisted syntheses. The Z15 sample is formed by cuboid-like crystals, while the Z13, Z14 and Z16 samples have crystals with an undefined morphology. The use of SSZ-13 led to a decrease in the temperature and the activation energy required for the degradation of UHMWPE, demonstrating its efficiency as catalyst in the polymer decomposition process.

5. Acknowledgements

The authors are grateful for the financial support provided by Fundação de Amparo à Pesquisa do Estado de Alagoas (FAPEAL), Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) and Petrobras.

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Publication Dates

• Publication in this collection
  28 July 2021
• Date of issue
  2021

History

• Received
  25 Jan 2021
• Reviewed
  24 May 2021
• Accepted
  30 June 2021