# 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

Authors: Omer Dhia Aldeen Salah Aldeen; Mustafa Z. Mahmoud; Hasan Sh. Majdi; Dhameer A. Mutlak; Khusniddin Fakhriddinovich Uktamov; Ehsan kianfar
Journal: Advances in Materials Science and Engineering
 (2022)

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## Abstract

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 NH3-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 (2–8 wt.%), Zr (2–5 wt.%), and different temperatures on the yield of ethylene and propylene were also investigated. The yield of ethylene and propylene improved by 21.78 wt% and 23.8 wt%, 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.


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## Body

## 1. 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 [1–3]. 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 [4–8]. The 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]. This 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 CO2 emissions [14, 15]. Many studies have been carried out to reduce (1) the fuel consumption and (2) CO2 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. This ratio changes with the feed type, but the demand has grown more for propylene than ethylene, so a catalytic cracking process has been proposed [16–20]. Thermal 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. The heated feed is then transferred to a reactor, and the large molecules at high pressure break down into a small molecule. The 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. The process is carried out at temperatures of 550 to 650°C to produce ethylene and propylene [21–26]. The 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 three-dimensional crystalline structure with regular cavities of molecular size [27–33]. Zeolite catalysts such as ZSM-5 and SAPO-34 have been widely used in the oil refining, petrochemical, and pollution control industries [29, 34–43]. This is because these materials have a specific porous structure and high specific surface area. Aluminum phosphate molecular sieves (ALPO) were first reported in 1982. These 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 [44–54]. Therefore, the addition of silicon to the ALPO structure resulted in the formation of silicoaluminophosphate (SAPO) molecules. The addition of Si+4 with P+5 results in the formation of Bronsted acid sites, which makes these molecules more selective as catalyst for specific reaction [55–67]. Among the various SAPOs, SAPO-34 has a good selectivity to ethylene and propylene.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. Therefore, the use of cerium and zirconium oxides supported on SAPO-34 could improve the performance of SAPO-34 [68–77].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. The chemical structure, morphology, surface area, surface acidity, and structure stability for the catalyst are studied using X-ray diffraction (XRD), scanning electron microscopy (SEM), N2 adsorption-desorption isotherms, ammonia probe-temperature programmed desorption (NH3-TPD), and thermogravimetric analysis. The catalyst activities during the cracking of naphtha are tested using a pilot thermal catalytic cracking.


## 2. Experimental

### 2.1. Materials

SAPO-34 and H-ZSM-5 catalysts were purchased from ZEOCHEM, Switzerland, and the specifications of these catalysts are given in Table1. Cerium (III) nitrate hexahydrate [Ce(NO3)3.6H2O, 99.99% purity] and zirconium (IV) oxynitrate hydrate [ZrO(NO3)3. H2O, 99.99% purity] were obtained from Sigma-Aldrich, while the naphtha fraction was provided from Iran and used without further purifications.Table 1
Texture properties and compositions of SAPO-34 and H-ZSM-5 catalysts.
CatalystsBET surface area (m2/gr)Pore volume (cm3/g)Crystallite size (µm)Si/AlSAPO-345730.2870.24HZSM-54430.26450


### 2.2. Synthesis of Catalysts

Cerium and zirconium with specific weight ratios were impregnated into SAPO-34 and H-ZSM-5 catalysts using wet impregnation method. A certain amount of [Ce(NO3)3.6H2O and ZrO(NO3)3. H2O was added into a round-bottomed flask containing dispersed catalyst in 25 ml of distilled water and gently stirred for 2 h at room temperature. The resulting suspension was refluxed at 75°C in a water bath for 1 h. The obtained solution was then oven-dried at 120°C for 2 h, prior to calcination in furnace under static air at 750°C for 1 h in order to get rid of any remaining impurities. The obtained catalysts were sieved in the range of 10–20 mesh size [78–81].


### 2.3. Catalyst Characterizations

Wide-angle powder X-ray diffraction (XRD) was carried out on a Bruker D8 diffractometer or Philips with Cu Kα (λ = 1.5418 Å) radiation and a LynxEye detector between 5 and 80° with steps of 0.035° at 10 s per step. The 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. The temperature programmed desorption (TPD) ammonia (NH3) was performed on a Micromeritics Auto Chem II analyzer coupled with a thermal conductivity detector (TCD). The 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. The 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. The 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. The desorbed species from catalyst sample monitored by TCD and the density of acid sites was quantified. Thermogravimetric 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.


### 2.4. Thermal Catalytic Cracking Reactor Setup

The thermal catalytic cracking pilot plant unit is illustrated in Figure1. The 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. The mixture was heated to 500°C in a furnace. The obtained gases from the thermal catalytic cracking were quenched by passing through a series of condensers. The released gases were analyzed by online GC-MS (Agilent 6890 N chromatograph-560S mass spectrometer) according to the reported methods [35–37, 71–75].Figure 1
A pilot scale of thermal catalytic cracking setup.The physical properties and chemical composition of naphtha feed are presented in Table2.Table 2
Physical properties and composition of naphtha feed.
Physical properties of naphthaDensity (g/ml)0.655Initial boiling point (°C)60.7Final boiling point (°C)120.3Chemical composition (wt. %)AromaticNaphthaIso-paraffinn-paraffinCarbon number000.122.16C403.5821.3827.34C51.583.8412.2910.19C61.574.343.823.29C70.550.921.21.04C80.0300.50.26C93.7312.6839.3144.28Total


### 2.5. Operational Conditions for Thermal 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. The summary of operating conditions is given in Table3. Sic was used as an inert to increase the amount of catalyst substrate inside the reactor. The 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 [82–86].Table 3
Operational conditions for thermal catalytic cracking over SAPO-34.
ParametersAmountsFeed flow rate (g/min)1.2Ratio of hydrocarbon vapor (g/g)0.5Catalyst weight (g)1Weight hourly space velocity (WHSV, h−1)12.4However, 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. The summary of operating conditions is presented in Table4 [85–87].Table 4
Operational conditions for thermal catalytic cracking of naphtha.
ParametersAmountsFeed flow rate (g/min)2Ratio of hydrocarbon vapor (g/g)0.5Catalyst weight (g)1WHSV (h−1)60


## 2.1. Materials

SAPO-34 and H-ZSM-5 catalysts were purchased from ZEOCHEM, Switzerland, and the specifications of these catalysts are given in Table1. Cerium (III) nitrate hexahydrate [Ce(NO3)3.6H2O, 99.99% purity] and zirconium (IV) oxynitrate hydrate [ZrO(NO3)3. H2O, 99.99% purity] were obtained from Sigma-Aldrich, while the naphtha fraction was provided from Iran and used without further purifications.Table 1
Texture properties and compositions of SAPO-34 and H-ZSM-5 catalysts.
CatalystsBET surface area (m2/gr)Pore volume (cm3/g)Crystallite size (µm)Si/AlSAPO-345730.2870.24HZSM-54430.26450


## 2.2. Synthesis of Catalysts

Cerium and zirconium with specific weight ratios were impregnated into SAPO-34 and H-ZSM-5 catalysts using wet impregnation method. A certain amount of [Ce(NO3)3.6H2O and ZrO(NO3)3. H2O was added into a round-bottomed flask containing dispersed catalyst in 25 ml of distilled water and gently stirred for 2 h at room temperature. The resulting suspension was refluxed at 75°C in a water bath for 1 h. The obtained solution was then oven-dried at 120°C for 2 h, prior to calcination in furnace under static air at 750°C for 1 h in order to get rid of any remaining impurities. The obtained catalysts were sieved in the range of 10–20 mesh size [78–81].


## 2.3. Catalyst Characterizations

Wide-angle powder X-ray diffraction (XRD) was carried out on a Bruker D8 diffractometer or Philips with Cu Kα (λ = 1.5418 Å) radiation and a LynxEye detector between 5 and 80° with steps of 0.035° at 10 s per step. The 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. The temperature programmed desorption (TPD) ammonia (NH3) was performed on a Micromeritics Auto Chem II analyzer coupled with a thermal conductivity detector (TCD). The 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. The 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. The 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. The desorbed species from catalyst sample monitored by TCD and the density of acid sites was quantified. Thermogravimetric 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.


## 2.4. Thermal Catalytic Cracking Reactor Setup

The thermal catalytic cracking pilot plant unit is illustrated in Figure1. The 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. The mixture was heated to 500°C in a furnace. The obtained gases from the thermal catalytic cracking were quenched by passing through a series of condensers. The released gases were analyzed by online GC-MS (Agilent 6890 N chromatograph-560S mass spectrometer) according to the reported methods [35–37, 71–75].Figure 1
A pilot scale of thermal catalytic cracking setup.The physical properties and chemical composition of naphtha feed are presented in Table2.Table 2
Physical properties and composition of naphtha feed.
Physical properties of naphthaDensity (g/ml)0.655Initial boiling point (°C)60.7Final boiling point (°C)120.3Chemical composition (wt. %)AromaticNaphthaIso-paraffinn-paraffinCarbon number000.122.16C403.5821.3827.34C51.583.8412.2910.19C61.574.343.823.29C70.550.921.21.04C80.0300.50.26C93.7312.6839.3144.28Total


## 2.5. Operational Conditions for Thermal 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. The summary of operating conditions is given in Table3. Sic was used as an inert to increase the amount of catalyst substrate inside the reactor. The 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 [82–86].Table 3
Operational conditions for thermal catalytic cracking over SAPO-34.
ParametersAmountsFeed flow rate (g/min)1.2Ratio of hydrocarbon vapor (g/g)0.5Catalyst weight (g)1Weight hourly space velocity (WHSV, h−1)12.4However, 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. The summary of operating conditions is presented in Table4 [85–87].Table 4
Operational conditions for thermal catalytic cracking of naphtha.
ParametersAmountsFeed flow rate (g/min)2Ratio of hydrocarbon vapor (g/g)0.5Catalyst weight (g)1WHSV (h−1)60


## 3. Results and Discussion

### 3.1. XRD Analysis of Catalyst SAPO-34

Figure2 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 Figures 2–4, 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. This is due to the diffusion of Zr particles onto the surface of the catalyst at the nanoscale [87–89].Figure 2
XRD images of SAPO-34 and Ce/SAPO-34 catalysts.Figure 3
XRD images of SAPO-34 and Zr/SAPO-34 catalysts.Figure 4
XRD images of SAPO-34 and Ce-Zr/SAPO-34 catalysts.


### 3.2. XRD Analysis of Catalyst HZSM-5

The effect of adding different amounts of Ce and Zr on HZSM-5 is investigated in Figures5 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. The apparent peak intensities for Ce are short, indicating that the metal is well dispersed on the base. The intensity of Ce metal peaks increased with increasing loading percentage. The effect of Zr metal loading on the base catalyst is investigated in Figure 6. The 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. The 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. The increase in peak intensity can be observed in the XRD analysis of the modified HZSM-5 catalyst with 8% Zr. This increase in crystallinity can be attributed to the removal of aluminum out of the xenolith structure due to the reaction with Zr. This results in a more regular base catalyst structure [90, 91].Figure 5
XRD images of HZSM-5 and Ce/HZSM-5 catalysts.Figure 6
XRD images of HZSM-5 and Zr/HZSM-5 catalysts.Figure 7
XRD images of HZSM-5 and Ce-Zr/HZSM-5 catalysts.Figure 8
XRD images of HZSM-5 and Ce-Zr/HZSM-5 catalysts.


### 3.3. SEM Analysis of Catalyst SAPO-34

Figure9 shows the morphology of SAPO-34, and Figures 10–12 show the morphology of 2% Ce-SAPO-34, 8% Ce-SAPO-34, and 15% Ce-SAPO-34, respectively. From Figures 10–12, it is clear that the morphology of the catalysts is preserved after modification by Ce [92–94]. In addition, Figures 13 and 14 are SEM catalysts of 2% Zr/SAPO-34 and 2% Ce-2% Zr/SAPO-34, respectively. The images also show that the morphology of the catalysts has not changed. The white particles on the catalyst surface at 500 nm can be attributed to Ce and Zr particles.Figure 9
SEM analysis image of unmodified SAPO-34 zeolite.Figure 10
SEM image of 2% zeolite Ce-SAPO-34.Figure 11
SEM image of 8% zeolite Ce-SAPO-34.Figure 12
SEM image of 15% zeolite Ce-SAPO-34.Figure 13
SEM image of 2% zeolite Zr-SAPO-34.Figure 14
SEM image analysis of 2% Ce-2% Zr-SAPO-34 zeolite.


### 3.4. SEM Analysis of Catalyst HZSM-5

As can be seen in Figure15, the zeolite catalyst has a spherical structure, and its particle size varies. After loading the xenolith by Ce and 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. This scattering is well observed in part C. Simultaneous inclusion of Ce vesicles reduced the particle size. The white aggregates in part D can be attributed to Ce metal, and the smaller particles can be attributed to the presence of Ce metal [95–102].Figure 15
SEM image of (a) HZSM-5. (b) 8% Ce- HZSM-5. (c) 8% Zr- HZSM-5. (d) 8% Ce-8% Zr- HZSM-5.


### 3.5. BET Analysis of Catalysts HZSM-5 and SAPO-34

The specific surface area and volume of the catalyst cavities were measured using BET analysis, and the results are presented in Table5. 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 CeO2 and ZrO2 oxides into the catalyst cavities. The 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. Thus, 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 [103–105].Table 5
HZSM-5/SAPO-34 catalyst structural information prepared based on BET analysis.
CatalystBET surface area (m2/g)Total pore volume (cm3/g)Vmicro (cm3/g)Vmeso (cm3/g)SAPO-345730.2800.2460.0342%Ce/SAPO-344070.2040.1800.0248%Ce/SAPO-343900.1940.1750.0192%Zr/SAPO-343920.1850.1640.0215%Zr/SAPO-343850.1790.1620.0172%Ce-2%Zr/SAPO-344170.2370.2060.031HZSM-5442.720.2580.1320.1262%Ce/HZSM-5366.650.2200.0690.1518%Ce/HZSM-5361.210.2100.0850.1252%Zr/HZSM-5375.420.2300.0890.1418%Zr/HZSM-5370.310.2100.0980.1122%Ce-2%Zr/HZSM-5386.410.2370.0620.1752%Ce-8%Zr/HZSM-5368.640.2230.0840.1398%Ce-2%Zr/HZSM-5351.890.1950.0920.1038%Ce-8%Zr/HZSM-5360.350.2100.0960.116


### 3.6. NH3-TPD Analysis of Catalyst SAPO-34

The TPD diagram of the basic SAPO-34 catalysts and the modified SAPO-34 catalysts with Ce and Zr metals is shown in Figure16. The values of NH3 absorbed per unit weight of catalyst are also given in Table 6. As shown in Figure 16, the NH3-TPD diagrams of all samples show two peaks at temperatures of about 150°C and 450°C. The 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. The 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. Therefore, 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 16
NH3-TPD analysis of the effect of increasing Ce and Zr metals on SAPO-34 catalyst.Table 6
Acid properties of catalysts NH3-TPD analysis.
CatalystsAcidity (mmol NH3/g-catalyst)Weak acidMiddle and strong acidTotal acidSAPO-340.6250.7021.3272%Ce-SAPO-340.6770.9421.6192%Zr-SAPO-340.3580.8431.2012%Ce-2%Zr-SAPO-340.7250.9731.698


### 3.7. NH3-TPD Analysis of Catalyst HZSM-5

In Figure17 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. The 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. The 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 [106–110]. 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. The 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. The 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. The simultaneous increase of Ce vesicles over a cone on the base catalyst has reduced the acidity. This decrease is evident from the results of the TPD analysis in Figure 19, Table 7.Figure 17
NH3-TPD analysis of the effect of Ce loading on acidic properties of HZSM-5.Table 7
Acid properties of catalysts NH3-TPD analysis.
CatalystsAcidity (mmol NH3/g-catalyst)Weak acidMiddle and strong acidTotal acidHZSM-50.400.821.182%Ce- HZSM-50.720.851.578% Ce - HZSM-51.051.492.542%Zr- HZSM-50.531.031.558%Zr- HZSM-50.711.332.042%Ce-8%Zr- HZSM-50.270.140.418%Ce-2%Zr- HZSM-50.180.000.18Figure 18
NH3-TPD analysis of the effect of Zr loading on acidic properties of HZSM-5.Figure 19
NH3-TPD analysis of the effect of Ce and Zr loading on acidic properties of HZSM-5.


### 3.8. 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. The results of the TGA analysis are presented in Figure20. 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.Figure 20
TGA analysis of the catalysts after the reaction.


## 3.1. XRD Analysis of Catalyst SAPO-34

Figure2 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 Figures 2–4, 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. This is due to the diffusion of Zr particles onto the surface of the catalyst at the nanoscale [87–89].Figure 2
XRD images of SAPO-34 and Ce/SAPO-34 catalysts.Figure 3
XRD images of SAPO-34 and Zr/SAPO-34 catalysts.Figure 4
XRD images of SAPO-34 and Ce-Zr/SAPO-34 catalysts.


## 3.2. XRD Analysis of Catalyst HZSM-5

The effect of adding different amounts of Ce and Zr on HZSM-5 is investigated in Figures5 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. The apparent peak intensities for Ce are short, indicating that the metal is well dispersed on the base. The intensity of Ce metal peaks increased with increasing loading percentage. The effect of Zr metal loading on the base catalyst is investigated in Figure 6. The 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. The 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. The increase in peak intensity can be observed in the XRD analysis of the modified HZSM-5 catalyst with 8% Zr. This increase in crystallinity can be attributed to the removal of aluminum out of the xenolith structure due to the reaction with Zr. This results in a more regular base catalyst structure [90, 91].Figure 5
XRD images of HZSM-5 and Ce/HZSM-5 catalysts.Figure 6
XRD images of HZSM-5 and Zr/HZSM-5 catalysts.Figure 7
XRD images of HZSM-5 and Ce-Zr/HZSM-5 catalysts.Figure 8
XRD images of HZSM-5 and Ce-Zr/HZSM-5 catalysts.


## 3.3. SEM Analysis of Catalyst SAPO-34

Figure9 shows the morphology of SAPO-34, and Figures 10–12 show the morphology of 2% Ce-SAPO-34, 8% Ce-SAPO-34, and 15% Ce-SAPO-34, respectively. From Figures 10–12, it is clear that the morphology of the catalysts is preserved after modification by Ce [92–94]. In addition, Figures 13 and 14 are SEM catalysts of 2% Zr/SAPO-34 and 2% Ce-2% Zr/SAPO-34, respectively. The images also show that the morphology of the catalysts has not changed. The white particles on the catalyst surface at 500 nm can be attributed to Ce and Zr particles.Figure 9
SEM analysis image of unmodified SAPO-34 zeolite.Figure 10
SEM image of 2% zeolite Ce-SAPO-34.Figure 11
SEM image of 8% zeolite Ce-SAPO-34.Figure 12
SEM image of 15% zeolite Ce-SAPO-34.Figure 13
SEM image of 2% zeolite Zr-SAPO-34.Figure 14
SEM image analysis of 2% Ce-2% Zr-SAPO-34 zeolite.


## 3.4. SEM Analysis of Catalyst HZSM-5

As can be seen in Figure15, the zeolite catalyst has a spherical structure, and its particle size varies. After loading the xenolith by Ce and 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. This scattering is well observed in part C. Simultaneous inclusion of Ce vesicles reduced the particle size. The white aggregates in part D can be attributed to Ce metal, and the smaller particles can be attributed to the presence of Ce metal [95–102].Figure 15
SEM image of (a) HZSM-5. (b) 8% Ce- HZSM-5. (c) 8% Zr- HZSM-5. (d) 8% Ce-8% Zr- HZSM-5.


## 3.5. BET Analysis of Catalysts HZSM-5 and SAPO-34

The specific surface area and volume of the catalyst cavities were measured using BET analysis, and the results are presented in Table5. 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 CeO2 and ZrO2 oxides into the catalyst cavities. The 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. Thus, 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 [103–105].Table 5
HZSM-5/SAPO-34 catalyst structural information prepared based on BET analysis.
CatalystBET surface area (m2/g)Total pore volume (cm3/g)Vmicro (cm3/g)Vmeso (cm3/g)SAPO-345730.2800.2460.0342%Ce/SAPO-344070.2040.1800.0248%Ce/SAPO-343900.1940.1750.0192%Zr/SAPO-343920.1850.1640.0215%Zr/SAPO-343850.1790.1620.0172%Ce-2%Zr/SAPO-344170.2370.2060.031HZSM-5442.720.2580.1320.1262%Ce/HZSM-5366.650.2200.0690.1518%Ce/HZSM-5361.210.2100.0850.1252%Zr/HZSM-5375.420.2300.0890.1418%Zr/HZSM-5370.310.2100.0980.1122%Ce-2%Zr/HZSM-5386.410.2370.0620.1752%Ce-8%Zr/HZSM-5368.640.2230.0840.1398%Ce-2%Zr/HZSM-5351.890.1950.0920.1038%Ce-8%Zr/HZSM-5360.350.2100.0960.116


## 3.6. NH3-TPD Analysis of Catalyst SAPO-34

The TPD diagram of the basic SAPO-34 catalysts and the modified SAPO-34 catalysts with Ce and Zr metals is shown in Figure16. The values of NH3 absorbed per unit weight of catalyst are also given in Table 6. As shown in Figure 16, the NH3-TPD diagrams of all samples show two peaks at temperatures of about 150°C and 450°C. The 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. The 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. Therefore, 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 16
NH3-TPD analysis of the effect of increasing Ce and Zr metals on SAPO-34 catalyst.Table 6
Acid properties of catalysts NH3-TPD analysis.
CatalystsAcidity (mmol NH3/g-catalyst)Weak acidMiddle and strong acidTotal acidSAPO-340.6250.7021.3272%Ce-SAPO-340.6770.9421.6192%Zr-SAPO-340.3580.8431.2012%Ce-2%Zr-SAPO-340.7250.9731.698


## 3.7. NH3-TPD Analysis of Catalyst HZSM-5

In Figure17 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. The 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. The 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 [106–110]. 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. The 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. The 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. The simultaneous increase of Ce vesicles over a cone on the base catalyst has reduced the acidity. This decrease is evident from the results of the TPD analysis in Figure 19, Table 7.Figure 17
NH3-TPD analysis of the effect of Ce loading on acidic properties of HZSM-5.Table 7
Acid properties of catalysts NH3-TPD analysis.
CatalystsAcidity (mmol NH3/g-catalyst)Weak acidMiddle and strong acidTotal acidHZSM-50.400.821.182%Ce- HZSM-50.720.851.578% Ce - HZSM-51.051.492.542%Zr- HZSM-50.531.031.558%Zr- HZSM-50.711.332.042%Ce-8%Zr- HZSM-50.270.140.418%Ce-2%Zr- HZSM-50.180.000.18Figure 18
NH3-TPD analysis of the effect of Zr loading on acidic properties of HZSM-5.Figure 19
NH3-TPD analysis of the effect of Ce and Zr loading on acidic properties of HZSM-5.


## 3.8. 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. The results of the TGA analysis are presented in Figure20. 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.Figure 20
TGA analysis of the catalysts after the reaction.


## 4. Catalyst Performance

### 4.1. Results of Thermal Catalytic Cracking of Naphtha Catalyst SAPO-34

The 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 Tables8 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. This 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. This 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 [111–115].Table 8
Results of thermal catalytic cracking of naphtha on Ce-SAPO-34 at 750°C.
CatalyticYield (wt. %)MethaneEthanEthylenePropyleneButyleneC5+SAPO-346.113.9417.0519.822.3740.172%Ce/SAPO-346.94.6119.6921.262.4235.558%Ce/SAPO-346.674.3919.2420.463.0936.2415%Ce/SAPO-346.534.1818.2119.072.7438.71Table 9
Results of thermal catalytic cracking of naphtha on Zr -SAPO-34 at 750°C.
CatalyticYield (wt. %)MethaneEthanEthylenePropyleneButyleneC5+SAPO-346.113.9417.0519.822.3740.172%Zr/SAPO-346.534.0718.8520.182.3937.515%Zr/SAPO-346.583.5619.9118.372.6438.268%Zr/SAPO-345.613.2917.8417.232.4446.67Figure 21
Comparison of ethylene yields produced by thermal catalytic cracking of naphtha.Figure 22
Comparison of propylene yields produced by thermal catalytic cracking of naphtha.Figure 23
Comparison of ethylene + propylene yields produced by thermal catalytic cracking of naphtha.


### 4.2. Results of Thermal 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 Tables9 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. The 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 [116–120]. 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 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, 121–124].Table 10
The results of thermal catalytic cracking of naphtha at 650°C.
YieldProduct name8%Zr/HZSM-52%Zr/HZSM-58%Ce/HZSM-52%Ce/HZSM-5HZSM-5650°C650°C650°C650°C650°C6.606.807.219.348.47Methane3.533.143.854.043.01Ethane17.3118.9317.8516.9915.87Ethylene2.572.402.842.201.10Propane30.022.6429.1124.8220.85Propylene0.330.481.351.080.10i-butane0.100.171.321.030.04n-butane0.200.751.671.540.42Propadien2.942.984.223.832.74Acetylene0.693.721.911.570.56T2-butene3.223.414.104.943.811-butene3.210.901.270.940.0Iso-butene0.210.191.401.180.10C2-butene0.060.071.321.000.03n-pentane0.831.792.412.181.17MA0.580.882.152.671.93Hydrogen25.7833.0325.6719.8938.00C+Figure 24
Comparison of modified olefin catalyst yields at 650°C.Figure 25
Comparison of modified olefin catalyst yields at 700°C.Table 11
The results of thermal catalytic cracking of naphtha at 700°C.
YieldProduct name8%Zr/HZSM-52%Zr/HZSM-58%Ce/HZSM-52%Ce/HZSM-5HZSM-5700°C700°C700°C700°C700°C6.666.949.929.8110.41Methane3.563.214.304.253.71Ethane19.9219.1019.0618.8419.29Ethylene2.602.452.342.311.35Propane27.6021.9626.4726.5922.64Propylene0.330.491.141.130.13i-Butane0.200.171.091.080.05n-Butane0.210.761.641.620.52Propadien2.973.044.074.020.37Acetylene3.701.741.661.640.69T2-Butene3.253.485.265.194.691-Butene3.250.921.000.990Iso-butene0.210.191.251.240.12C2-Butene0.060.081.071.050.04n-Pentane0.841.832.312.281.43MA0.590.902.832.802.38Hydrogen25.6135.6017.9418.9331.97C+


### 4.3. The 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. The results for the efficiency of these catalysts in Figure26 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 26
Ce loading effect on the yield of light olefins.Table 12
Percentage of olefin yield changes after Ce loading.
No.Cerium loading percentageTemperature (°C)YieldEthylenePropyleneEthylene + propylene126507.0623.8316.582865012.4639.6227.88327002.38.613.57487001.1916.938.89


### 4.4. The Effect of Loading Zr

As shown in Figure27, the ethylene yield did not change much with the addition of Zr to the zeolite. The highest yield was related to 2% Zr loading, which increased the total production of olefins by 40%. This 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. Therefore, 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.Figure 27
Zr loading effect on the yield of light olefins.Table 13
Percentage of olefin yield changes after Zr loading.
No.Cerium loading percentageTemperature (°C)YieldEthylenePropyleneEthylene + propylene1265019.2856.5340.43286509.0843.8828.84327004.1823.4814.6487001.1721.912.61


### 4.5. 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 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. Therefore, 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. The result of this experiment is shown for the ethylene yield in Figure28 and for the propylene yield in Figure 29.Figure 28
Results of ethylene production efficiency with increasing reaction time.Figure 29
Results of propylene production efficiency with increasing reaction time.Figure29 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%. The results show that, after 2 h of ethylene yield reaction on SAPO-34, the catalyst is inactivated to its minimum level. The 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. The 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.


### 4.6. Comparison of Results with Other Similar Studies

Further research has been conducted on the thermal catalytic cracking of Naphtha at various universities. This 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. The operating conditions, the type of catalyst, and the results of the studies are presented in Tables13 and 14.Table 14
Research on thermal catalytic cracking of naphtha unit.
Yield wt. %Temperature operating (°C)Size meshCatalyst typeYearsReferencesEthylene + propylenePropyleneEthyleneActive elementBasic31.8412.7819.0662810–25FeCNT2012[125]42.2922.8619.4368010–25Ce-La-MoZSM-52010[83]52.53616.568010–25CeZSM-52010[84]49.3622.5626.875010–25Ce-LaSAPO-342012[90]


## 4.1. Results of Thermal Catalytic Cracking of Naphtha Catalyst SAPO-34

The 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 Tables8 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. This 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. This 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 [111–115].Table 8
Results of thermal catalytic cracking of naphtha on Ce-SAPO-34 at 750°C.
CatalyticYield (wt. %)MethaneEthanEthylenePropyleneButyleneC5+SAPO-346.113.9417.0519.822.3740.172%Ce/SAPO-346.94.6119.6921.262.4235.558%Ce/SAPO-346.674.3919.2420.463.0936.2415%Ce/SAPO-346.534.1818.2119.072.7438.71Table 9
Results of thermal catalytic cracking of naphtha on Zr -SAPO-34 at 750°C.
CatalyticYield (wt. %)MethaneEthanEthylenePropyleneButyleneC5+SAPO-346.113.9417.0519.822.3740.172%Zr/SAPO-346.534.0718.8520.182.3937.515%Zr/SAPO-346.583.5619.9118.372.6438.268%Zr/SAPO-345.613.2917.8417.232.4446.67Figure 21
Comparison of ethylene yields produced by thermal catalytic cracking of naphtha.Figure 22
Comparison of propylene yields produced by thermal catalytic cracking of naphtha.Figure 23
Comparison of ethylene + propylene yields produced by thermal catalytic cracking of naphtha.


## 4.2. Results of Thermal 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 Tables9 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. The 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 [116–120]. 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 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, 121–124].Table 10
The results of thermal catalytic cracking of naphtha at 650°C.
YieldProduct name8%Zr/HZSM-52%Zr/HZSM-58%Ce/HZSM-52%Ce/HZSM-5HZSM-5650°C650°C650°C650°C650°C6.606.807.219.348.47Methane3.533.143.854.043.01Ethane17.3118.9317.8516.9915.87Ethylene2.572.402.842.201.10Propane30.022.6429.1124.8220.85Propylene0.330.481.351.080.10i-butane0.100.171.321.030.04n-butane0.200.751.671.540.42Propadien2.942.984.223.832.74Acetylene0.693.721.911.570.56T2-butene3.223.414.104.943.811-butene3.210.901.270.940.0Iso-butene0.210.191.401.180.10C2-butene0.060.071.321.000.03n-pentane0.831.792.412.181.17MA0.580.882.152.671.93Hydrogen25.7833.0325.6719.8938.00C+Figure 24
Comparison of modified olefin catalyst yields at 650°C.Figure 25
Comparison of modified olefin catalyst yields at 700°C.Table 11
The results of thermal catalytic cracking of naphtha at 700°C.
YieldProduct name8%Zr/HZSM-52%Zr/HZSM-58%Ce/HZSM-52%Ce/HZSM-5HZSM-5700°C700°C700°C700°C700°C6.666.949.929.8110.41Methane3.563.214.304.253.71Ethane19.9219.1019.0618.8419.29Ethylene2.602.452.342.311.35Propane27.6021.9626.4726.5922.64Propylene0.330.491.141.130.13i-Butane0.200.171.091.080.05n-Butane0.210.761.641.620.52Propadien2.973.044.074.020.37Acetylene3.701.741.661.640.69T2-Butene3.253.485.265.194.691-Butene3.250.921.000.990Iso-butene0.210.191.251.240.12C2-Butene0.060.081.071.050.04n-Pentane0.841.832.312.281.43MA0.590.902.832.802.38Hydrogen25.6135.6017.9418.9331.97C+


## 4.3. The 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. The results for the efficiency of these catalysts in Figure26 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 26
Ce loading effect on the yield of light olefins.Table 12
Percentage of olefin yield changes after Ce loading.
No.Cerium loading percentageTemperature (°C)YieldEthylenePropyleneEthylene + propylene126507.0623.8316.582865012.4639.6227.88327002.38.613.57487001.1916.938.89


## 4.4. The Effect of Loading Zr

As shown in Figure27, the ethylene yield did not change much with the addition of Zr to the zeolite. The highest yield was related to 2% Zr loading, which increased the total production of olefins by 40%. This 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. Therefore, 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.Figure 27
Zr loading effect on the yield of light olefins.Table 13
Percentage of olefin yield changes after Zr loading.
No.Cerium loading percentageTemperature (°C)YieldEthylenePropyleneEthylene + propylene1265019.2856.5340.43286509.0843.8828.84327004.1823.4814.6487001.1721.912.61


## 4.5. 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 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. Therefore, 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. The result of this experiment is shown for the ethylene yield in Figure28 and for the propylene yield in Figure 29.Figure 28
Results of ethylene production efficiency with increasing reaction time.Figure 29
Results of propylene production efficiency with increasing reaction time.Figure29 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%. The results show that, after 2 h of ethylene yield reaction on SAPO-34, the catalyst is inactivated to its minimum level. The 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. The 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.


## 4.6. Comparison of Results with Other Similar Studies

Further research has been conducted on the thermal catalytic cracking of Naphtha at various universities. This 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. The operating conditions, the type of catalyst, and the results of the studies are presented in Tables13 and 14.Table 14
Research on thermal catalytic cracking of naphtha unit.
Yield wt. %Temperature operating (°C)Size meshCatalyst typeYearsReferencesEthylene + propylenePropyleneEthyleneActive elementBasic31.8412.7819.0662810–25FeCNT2012[125]42.2922.8619.4368010–25Ce-La-MoZSM-52010[83]52.53616.568010–25CeZSM-52010[84]49.3622.5626.875010–25Ce-LaSAPO-342012[90]


## 5. 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. The 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. The 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. The 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. The 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



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