Investigation of Effective Parameters Ce and Zr in the Synthesis of H-ZSM-5 and SAPO-34 on the Production of Light Olefins from Naphtha

In this paper, Ce and Zr modiﬁed commercial SAPO-34 and H-ZSM-5 catalysts were synthesized via a wet impregnation method and used as catalysts for the production of light oleﬁns from naphtha. The synthesized catalysts were characterized using SEM, TGA, XRD, BET, and NH 3 -TPD. Thermal catalytic cracking of parent catalysts (SAPO-34 and H-ZSM-5) and modiﬁed catalysts with Ce and Zr on the production of light oleﬁns from naphtha has been studied. The eﬀects of diﬀerent loading of Ce (2–8wt.%), Zr (2–5wt.%), and diﬀerent temperatures on the yield of ethylene and propylene were also investigated. The yield of ethylene and propylene improved by 21.78wt% and 23.8wt%, respectively, over 2% Ce and 2% Zr on SAPO-34 catalyst. This is due to the higher acid sites on the surface of modiﬁed catalysts. It was found that H-ZSM-5 with 2% Zr loading has the highest yield of light oleﬁns (40.4%) at 650 ° C in comparison with unmodiﬁed parent catalysts, while Ce loading has less eﬀect on the oleﬁn yield compared to Zr loading. Finally, simultaneous loading of Ce and Zr showed no eﬀect on the light oleﬁn yield owing to the signiﬁcant decline of acid sites. on of


Introduction
Light olefins, such as ethylene and propylene, are important raw materials for synthesizing many organic compounds in the industry in a huge volume including plastics, synthetic fibers, and rubber. Common technologies for these materials production are thermal catalytic cracking and fluidized bed catalyst (FCC) using ZSM-5 zeolite catalysts [1][2][3]. Ethylene is widely used in the plastics industry to produce polyethylene, polyvinyl chloride (PVC), polyester, etc. Ethylene and propylene are among the major petrochemical base products that have benefited over other petrochemical raw materials such as gas and naphtha. In recent years, manufacturers have an increasing attention towards the development of ethylene production versus propylene production, especially methods of producing propylene [4][5][6][7][8]. e thermal catalytic cracking process of naphtha hydrocarbons has been a major source of olefin production for more than half a century [9,10] and the production of olefins varies between 24 and 55 wt. % for ethylene depending on feed type and operating conditions and between 1.5 and 18 wt. % for propylene [11]. Despite the good progress made on this process, it still has the highest energy consumption in the petrochemical industry [12,13].
is process requires a high temperature between 800 and 880°C, which comprises approximately 40% of the petrochemical fuel consumed, resulting in a large amount of CO 2 emissions [14,15]. Many studies have been carried out to reduce (1) the fuel consumption and (2) CO 2 emissions during this steam cracking process, and some improvements have contributed to the literature; however, there is still lack of control over the propylene to ethylene ratio. is ratio changes with the feed type, but the demand has grown more for propylene than ethylene, so a catalytic cracking process has been proposed [16][17][18][19][20]. ermal catalytic cracking process produces a large amount of ethylene along with other smaller molecules. During this process, the feed is preheated in an oven to 510-600°C, and the feed temperature should be shortened to prevent unnecessary chemical reaction during the preheating process. e heated feed is then transferred to a reactor, and the large molecules at high pressure break down into a small molecule. e catalytic cracking has been implemented to increase the production of light olefins, and it has been extensively investigated for various hydrocarbons on different types of catalysts. e process is carried out at temperatures of 550 to 650°C to produce ethylene and propylene [21][22][23][24][25][26]. e selectivity of light olefins depends on types of feed as well as catalysts, acid strength, and acid types such as Lewis and Brønsted acid sites. Catalytic cracking is a flexible process used also to break down less high molecular weight oils to produce more valuable light products. A wide range of solid acid catalysts have been used in this process. Among them, aluminum silicate is a good example of catalyst used in catalytic fluidized bed cracking (FCC), and it has a threedimensional crystalline structure with regular cavities of molecular size [27][28][29][30][31][32][33]. Zeolite catalysts such as ZSM-5 and SAPO-34 have been widely used in the oil refining, petrochemical, and pollution control industries [29,[34][35][36][37][38][39][40][41][42][43].
is is because these materials have a specific porous structure and high specific surface area. Aluminum phosphate molecular sieves (ALPO) were first reported in 1982. ese materials have similar structure to zeolites, but their main structure is made of Al-O-P bonds instead of Si-O-Al or Si-O-Si. Since the molecular structures of ALPO are neutral, they are not selective in catalytic process [44][45][46][47][48][49][50][51][52][53][54].
Cerium and lanthanum catalysts supported on SAPO-34 were employed in thermal catalytic cracking of naphtha to produce light olefins. SAPO-34 modified with Mg and Ni showed higher selectivity to light olefins and extended lifetime in the conversion of methanol to olefin. Stability of HZSM-5 modified with P and Fe was improved in naphtha catalytic cracking.
Zirconium with high mechanical strength and appropriate thermal resistance could improve the thermal stability of catalysts and prevent the sintering of the active particles. It has been reported that the addition of zirconium to cerium oxide can significantly increase reducibility, activity, and thermal stability of cerium oxide, resulting in increased resistance to coke formation. However, the low surface area of cerium and zirconium oxides is their major limitation. erefore, the use of cerium and zirconium oxides supported on SAPO-34 could improve the performance of SAPO-34 [68][69][70][71][72][73][74][75][76][77].
In this study, we investigate the impact of Ce and Zr modified H-ZSM-5 and SAPO-34 catalysts on the production of light olefins from naphtha. e chemical structure, morphology, surface area, surface acidity, and structure stability for the catalyst are studied using X-ray diffraction (XRD), scanning electron microscopy (SEM), N 2 adsorption-desorption isotherms, ammonia probe-temperature programmed desorption (NH 3 -TPD), and thermogravimetric analysis. e catalyst activities during the cracking of naphtha are tested using a pilot thermal catalytic cracking.

Catalyst Characterizations.
Wide-angle powder X-ray diffraction (XRD) was carried out on a Bruker D8 diffractometer or Philips with Cu Kα (λ �1.5418Å) radiation and a LynxEye detector between 5 and 80°with steps of 0.035°at 10 s per step. e particle size, morphology, and surface structure of catalysts were determined using a Hitachi operated at 15 kV. Texture properties were measured by the multipoint nitrogen adsorption-desorption method at 77 K on a Micromeritics TriStar 3000 porosimeter or NOVA-2000 Quantachrome porosimeter. Catalyst samples were degassed in vacuo overnight at xx°C prior to analysis. e temperature programmed desorption (TPD) ammonia (NH 3 ) was performed on a Micromeritics Auto Chem II analyzer coupled with a thermal conductivity detector (TCD). e catalyst sample was heated from room temperature to 800°C under a constant flow rate of helium purge gas at 30 ml/min before carrying out the measurement. e catalyst sample was firstly cooled down to 100°C and then followed by conducting ammonia adsorption for 1/2 h using an ammonia flow rate of 30 ml/min. After saturation with ammonia, the catalyst was purged with helium at 30 ml/min for 1.5 h to eliminate any unreacted ammonia before the measurement. e temperature ramped from 100 to 800°C at 10°C/min under helium purge gas of 30 ml/min and then held at 800°C for 1/2 h. e desorbed species from catalyst sample monitored by TCD and the density of acid sites was quantified.
ermogravimetric analysis (TGA) was performed on a Mettler Toledo (TGA/DSC-1) by programming the system to heat up from room temperature (25°C) to 900°C at 20°C/min under a constant flow rate of nitrogen gas at 50 ml/min.

ermal Catalytic Cracking Reactor Setup.
e thermal catalytic cracking pilot plant unit is illustrated in Figure 1. e raw material feed and water were injected into a stirred stainless steel reactor for mixing, and the resulting solution was then mixed with the steam before feeding into the preheating unit at xx. e mixture was heated to 500°C in a furnace. e obtained gases from the thermal catalytic cracking were quenched by passing through a series of condensers. e released gases were analyzed by online GC-MS (Agilent 6890 N chromatograph-560S mass spectrometer) according to the reported methods [35][36][37][71][72][73][74][75]. e physical properties and chemical composition of naphtha feed are presented in Table 2.

Operational Conditions for ermal Catalytic Cracking.
In the case of SAPO-34 catalyst, all experiments were considered at 750°C, 1.2 g/min of naphtha inlet mass flow rate and SR � 0.5 and performed over 1 g of SAPO-34 catalyst with 4 g silicon carbide.
e summary of operating conditions is given in Table 3. Sic was used as an inert to increase the amount of catalyst substrate inside the reactor. e main reason for choosing SiC was because it has low density, low thermal expansion, high strength, high thermal conductivity, and high heat shock resistance and can, therefore, be an excellent neutralizer for the thermal catalytic cracking reaction [82][83][84][85][86].
However, in the case of H-ZSM-5 catalyst, all experiments were considered at 750°C, 2 g/min of naphtha inlet mass flow rate and SR � 0.5 and performed over 1 g of H-ZSM-5 catalyst with 4 g SiC. e summary of operating conditions is presented in

Results and Discussion
3.1. XRD Analysis of Catalyst SAPO-34. Figure 2 shows the XRD results of Ce-modified SAPO-34 catalysts. Also, shown in Figure 3 are the results of XRD analysis for Zr -modified SAPO-34 catalysts, and the XRD catalysts modified by the synergy of Ce and Zr are also shown in Figure 4. Generally, in all catalysts, the peak intensities decrease with the addition of unmodified catalysts due to the presence of amorphous catalysts, which can be attributed to the presence of amorphous materials such as Ce and Zr. According to , the position of the peaks has not changed at different loading rates. According to the XRD images, all the catalysts have a chabazite structure, and their structure has not changed significantly after correction. According to Figure 2 with increasing cerium, the intensity of SAPO-34 peaks decreased, and cerium peaks appeared. According to Figure 2, with increasing cerium, new cerium peaks appeared at 2θ � 28.54, 33.08, 47.49, which is consistent with the results of Jiangnan et al. As shown in Figure 3, the crystallinity of SAPO-34 decreased with increasing Zr, but no Zr peak appeared.
is is due to the diffusion of Zr particles onto the surface of the catalyst at the nanoscale [87][88][89].

XRD Analysis of Catalyst HZSM-5.
e effect of adding different amounts of Ce and Zr on HZSM-5 is investigated in Figures 5 and 6. As can be seen by adding different amounts of Ce and Zr, the position of the zeolite peaks has not changed, indicating that the basic catalyst crystal structure is maintained after salt addition. New peaks have been added to the base catalyst, indicating the presence of metals. In Figure 5, the addition of Ce metal on the base catalyst is investigated. e apparent peak intensities for Ce are short, indicating that the metal is well dispersed on the base. e intensity of Ce metal peaks increased with increasing loading percentage. e effect of Zr metal loading on the base catalyst is investigated in Figure 6. e pigment intensity of Zr is very low, indicating that the metal is well dispersed on the surface, and the dispersed Zr dimensions are less than 3 nm. e addition of Ce metal to the catalyst with 2% Zr in Figure 7 and the addition of Zr metal to the catalyst with 2% Ce in Figure 8 are shown. Maintaining the basic structure, increasing the intensity of the peaks with increasing loading rates, and decreasing the peak intensities of these two metals are the results obtained from these shapes. e increase in peak intensity can be observed in the XRD analysis of the modified HZSM-5 catalyst with 8% Zr. is increase in crystallinity can be attributed to the removal of aluminum out of the xenolith structure due to the reaction with Zr. is results in a more regular base catalyst structure [90,91].

SEM Analysis of Catalyst HZSM-5.
As can be seen in Figure 15, the zeolite catalyst has a spherical structure, and its particle size varies. After loading the xenolith by Ce and   Advances in Materials Science and Engineering     Advances in Materials Science and Engineering Zr salts, the spherical structure is completely preserved. By loading the zeolite catalyst with the help of Ce metal, the particles accumulate, which is visible in part B. After loading with the help of Zr metal, the dispersion of the particles increased compared to the unmodified catalyst. is scattering is well observed in part C. Simultaneous inclusion of Ce vesicles reduced the particle size. e white aggregates in part D can be attributed to Ce metal, and the smaller particles can be attributed to the presence of Ce metal [95][96][97][98][99][100][101][102].

BET Analysis of Catalysts HZSM-5 and SAPO-34.
e specific surface area and volume of the catalyst cavities were measured using BET analysis, and the results are presented in Table 5. As shown in Table 5, after the modification of the SAPO-34 catalyst by Ce and Zr, the specific surface area and volume of the cavities decreased, possibly due to the penetration of CeO 2 and ZrO 2 oxides into the catalyst cavities. e BET analysis results for HZSM-5 show lower specific surface area and cavity volume for HZSM-5 than for SAPO-34, while the micro-cavity volume in HZSM-5 is significantly higher than that for SAPO-34. us, the results prove that the cavities in the SAPO-34 structure are predominantly micro, but in HZSM-5, the cavities are in both micro-and microforms with approximately equal volumes. In addition, the BET results for 2%Ce-2%Zr/HZSM-5 show that the specific surface area and volume of the cavities decreased to a lesser extent than those of the modified SAPO-34, due to the larger pores of HZSM-5 compared to SAPO-34 [103][104][105].   e TPD diagram of the basic SAPO-34 catalysts and the modified SAPO-34 catalysts with Ce and Zr metals is shown in Figure 16. e values of NH 3 absorbed per unit weight of catalyst are also given in Table 6. As shown in Figure 16, the NH 3 -TPD diagrams of all samples show two peaks at temperatures of about 150°C and 450°C. e first peak is related to the hydroxyl groups P-OH, Si-OH, and Al-OH, and these hydroxyl groups are usually attributed to weak acids. e peak at 450°C belongs to the Si-OH-Al hydroxyl groups, which attribute the hydroxyl groups to the strong acids. As can be seen in Figure 16 and Table 6, the amount of total acidity increased with the addition of Ce to the catalyst, and the total acidity decreased with the addition of Zr. But generally after loading Ce and Zr metals, in all modified catalysts, the number of strong acid centers increased compared to the basal state, and this increase was greater in the catalysts modified by the synergy of Ce and Zr. In general, the presence of empty f orbitals in Ce creates new Lewis acid sites and increases the total number of acid centers. Improvement of the catalyst with the Zr element causes the Ce oxide to sit on the outer surface of the catalyst and to destroy some of the surface acid sites on the outer surface of the catalyst. erefore, the weak acidity density decreases after the catalyst modification with Zr. It should be noted, however, that there is an optimum value for this increase in acidity as the density of the acid centers increases, and their adverse hydrogen transfer reactions increase. Figure 17 and Table 7, the effect of Ce metal loading on acidity has been investigated by TPD analysis. As expected, Ce metal  increased the acidity of zeolite. Increased acidity has occurred for both strong and weak acid centers. e reason for this can be attributed to the polarization of the Si-OH and AL-OH bonds by the quantitation and induction of Ce, which leads to an increase in electron density and an increase in acidic strength. e number of Lewis acids also increases due to the empty orbital f of Ce. Generally, this is expected for rare earth metals except for Ga [106][107][108][109][110]. Moving the location of the cycads to lower temperatures after Ce loading for weak acids indicates a decrease in the strength of these acids, and the transfer of the pics after Ce loading for the strong acids indicates an increase in the strength of these acids. e TPD analysis to investigate the loading of Zr metal is shown in Figure 18 and Table 7. Zr metal loading is particularly important in strong acidic surfaces. e increase in the number of weak and strong acid sites as well as the transfer of these peaks to higher temperatures indicating increased acidic strength can be well illustrated in these graphs. e simultaneous increase of Ce vesicles over a cone on the base catalyst has reduced the acidity. is decrease is evident from the results of the TPD analysis in Figure 19, Table 7.

TGA Analysis of Catalysts HZSM-5 and SAPO-34.
To check the amount of coke sitting on the catalysts, TGA analysis was performed on the samples at temperatures up to 900°C.
e results of the TGA analysis are presented in Figure 20. Weight loss of samples up to 250°C can be attributed to moisture removal from the catalyst structure, and weight loss at higher temperatures is due to the burning of coke sitting on the catalyst structure. According to TGA results, weight loss at temperatures above 250°C for SAPO-34, 2% Ce-2% Zi/SAPO-34, HZSM-5, and 2% Ce-2% Zi-HZSM-5 catalysts, respectively, is equal to 8.3%, 6.42%, 5.66%, and 4.26%, respectively, which are in harmony with the reduction of ethylene yields and especially propylene on the catalysts.  e results of the thermal catalytic cracking of Naphtha reactions on the SAPO-34 catalytic base with Ce, Ce and Zr and Zr elements are visible in Tables 8 and 9, respectively. In addition, to compare the yields of ethylene and propylene on the catalysts, the results can be seen in Figures 21 and 22 through Figure 23. Reactor results show an increase in the efficiency of light olefins in the catalytic thermal catalytic cracking of Naphtha process on the SAPO-34 catalyst compared to the catalytic thermal catalytic cracking of Naphtha process, so that the yields of Light olefins were 2.94 and 6.92% by SAPO -34 increase. Adding 2% Ce and 2% Zr significantly increased the yield of ethylene and propylene. is increase in the yield of ethylene and propylene can be attributed to the increased acidity of the catalyst. According to the NH3-TPD analysis results, the strong acidity of the catalysts increased by 2% Ce-2% Zr/ SAPO-34 > 2% Ce/SAPO-34 > 2% Zr/SAPO-34> SAPO-34, respectively, and the reactor results also show the same ranking in ethylene and propylene yields, with SAPO-34 having the lowest yield and 2% Ce-2% Zr/SAPO-34 having the highest yield of light olefins. As the results show, further     increase in Ce and Zr loading reduces the yield of ethylene and propylene. is can be attributed to the increase in the amount of Ce and Zr oxides on the catalyst surface and the blocking of the catalyst cavities, preventing the penetration of naphtha molecules into the catalyst cavities and preventing thermal catalytic cracking of Naphtha reactions [111][112][113][114][115].

Results of ermal Cracking Catalytic of Naphtha Catalyst HZSM-5.
To investigate the effect of loading of Ce and Zr on the zeolite catalyst in thermal catalytic cracking of naphtha, experiments were performed according to Tables 9 and 10.
In both experiments, the catalyst was added to the reactor twice, and the experiments were carried out at the operating conditions of Table 4 and the space velocity 60 h −1 . e results of the experiment are visible at 650°C in Figure 24 and for 750°C in Figure 25. In molecular sieving catalysts, acid sites are active sites [116][117][118][119][120]. It can be seen from the data in Table 7 that, with increasing base catalyst loading with Ce and Zr metals, the acidic site increased. As a result, increased production of propylene can be attributed to the sites created. According to Figures Tables 10 and 11, for propylene with increasing temper nature, some catalysts show Advances in Materials Science and Engineering increased efficiency, and some decrease efficiency. For catalysts with high acidity, the reduction in specific yields is significant. As can be seen, the formation of carbon ions on acidic sites increased the yield of propylene at a rate of bending [37,[121][122][123][124].

e Effect of Loading Ce.
According to the results presented for catalysts modified with 2% and 8% Ce, the yield of light olefins increases with the addition of this element on zeolite.
e results for the efficiency of these catalysts in Figure 26 are shown for both temperatures. According to the results presented in Figure 26, with Ce metal loading the ethylene yield, increased compared to the propylene yield and with the Ce loading, the propylene yield increased significantly. By comparing the propylene yields at the two reaction temperatures, we find that, by increasing the 700°C, the propylene yields decrease. Since propylene is an intermediate material, an increase in temperature increases secondary reactions. As the acid centers are close to each other, and the acidity increases, at high temperatures, the hydrogen transfer reactions accelerate, reducing the saturation of propylene and reducing its efficiency. In Table 12, the rate of change in yields of light olefins after the addition of Ce metal to the base catalyst is visible. Figure 27, the ethylene yield did not change much with the addition of Zr to the zeolite. e highest yield was related to 2% Zr loading, which increased the total production of olefins by 40%. is increase can be attributed to the catalytic acidity change.   According to the results of Table 7, the loading of Zr catalyst increases with increasing acidity, and on the other hand, the strength of weak acids also increases, making it easier to break down and increase efficiency. But on the other hand, increasing acidity, especially at the surface, speeds up secondary reactions and reduces the efficiency of olefins. erefore, catalysts with moderate acid sites have higher efficiency in this process. In Table 13, the rate of change in yields of light olefins after the addition of Zr metal to the base catalyst is observable.

Catalytic Stability Is Investigated.
In the previous sections, the effects of SAPO-34 and HZSM-5 with different modifications on the efficiency of light olefins were investigated. However, it should be noted that the catalyst is Advances in Materials Science and Engineering   suitable for the thermal catalytic cracking of naphtha of hydrocarbons, which, in addition to increasing the yield of light olefins, has good stability. One of the problems for catalysts for industrial applications is their inactivation due to the formation of coke on active sites. Zeolite catalysts are usually inactive due to the formation of coke and the removal of aluminum from the fast zeolite structure and need to be revived. erefore, many studies have focused on the modification of these catalysts to maintain good stability. In this section, to investigate the      stability of catalysts, in the experiment, SAPO-34, 2% Ce-2% Zr/SAPO-34, HZSM-5, and 2% Ce2-2% Zr-HZSM-5 were administered for 8 hours. e result of this experiment is shown for the ethylene yield in Figure 28 and for the propylene yield in Figure 29. Figure 29 shows that the yields of ethylene catalysts of HZSM-5 and 2% Ce-2% Zr/HZSM-5 did not change significantly over time and remained at about 14 wt%. e results show that, after 2 h of ethylene yield reaction on SAPO-34, the catalyst is inactivated to its minimum level. e 2% Ce-2% Zr/SAPO-34 catalyst showed significantly better performance and increased catalyst inactivation time. As shown in Figure 29, the propylene yields decreased by 6.36% and 8.66%, respectively, while propylene yields decreased on the SAPO-34 and 2% Ce-2% Zr-SAPO-34 catalysts, respectively, while decreasing the propylene efficiency, respectively. e HZSM-5 catalyst, and especially the 2% Ce-2% Zr/HZSM-5 catalyst, was much lower than SAPO-34 within 2 hours. It is also apparent that SAPO-34 is almost deactivated during the first 2 hours if the performance of the HZSM-5 catalyst is significantly better than that of SAPO-34.

Comparison of Results with Other Similar Studies.
Further research has been conducted on the thermal catalytic cracking of Naphtha at various universities. is article was conducted in a thermal catalytic cracking of Naphtha unit located in the Iranian Olefin Research Laboratory, and most of them, the reactor conditions, vapor to hydrocarbon ratio, residence time, catalyst weight, feed flow type, and intensity, were the same. e operating conditions, the type of catalyst, and the results of the studies are presented in Tables 13 and 14.

Conclusion
In this paper, SAPO-34 zeolites were used as catalysts for the production of Ethylene, propylene in the thermal catalytic cracking of Naphtha process. e results of the thermal catalytic cracking of Naphtha using the above catalyst showed an increase in the efficiency of the lightweight primers compared to the thermal catalytic cracking of naphtha at the same temperature. To further enhance the efficiency of light olefins, SAPO-34 was modified using Ce and Zi elements. SEM and XRD analyses were performed on some modified samples, and the results showed that the catalyst structure was not damaged after the addition of Ce and Zi. e results of the NH3-TPD analysis showed that the strong acidity of the catalyst increased after modification by Ce and Zr. Reactor results showed that the yield of light olefins increased after modification by Ce and Zr, and the highest olefin yields were obtained on catalysts with low  synergies of Ce and Zr. Comparing the performance of SAPO-34, experiments were performed under similar conditions based on HZSM-5, which is known as a reference catalyst in the thermal catalytic cracking of naphtha process. e results showed that although the yield of ethylene on SAPO-34 catalysts was higher than that of HZSM-5, propylene and total olefin production were higher than the on HZSM-5 catalyst. e stability test results showed better performance of HZSM-5 catalyst against SAPO-34. Also the stability test results showed that the performance of both SAPO-34 and HZSM-5 catalysts improved after modification with Ce and Zi.
Based on these experiments, the main results are summarized as follows: (1) Conservation of both Catalysts structures after modification with Ce and Zi elements (2) Increased strong acidity of SAPO-34 after modification with Ce and Zi elements (3) Increased ethylene and propylene yields on SAPO-34 compared with thermal cracking (4) Increased ethylene and propylene yields after modification of SAPO-34 with cerium and zirconium elements (5) Higher ethylene yields on SAPO-34 and higher propylene yields on HZSM-5 (6) ZSM-5 has better stableness than SAPO-34 (7) Increased stability of both catalysts after modification with Ce and Zi elements

Data Availability
No data were used to support this study.

Consent
Additional informed consent was obtained from all individual participants for whom identifying information is included in this paper.
Disclosure e funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results. e study submitted is part of the authors' doctoral thesis.

Conflicts of Interest
e authors declare that they have no conflicts of interest.