Synergetic effects of nano-boehmite and Y nano-zeolite on catalytic cracking of residue oil

doi:10.21203/rs.3.rs-4964124/v1

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Synergetic effects of nano-boehmite and Y nano-zeolite on catalytic cracking of residue oil

https://doi.org/10.21203/rs.3.rs-4964124/v1

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Boehmite nanoparticles and NaY nanozeolite were synthesized by co-precipitation and hydrothermal methods respectively and characterized by XRD, FT-IR, TG-DTA, BET and SEM techniques. XRD and BET analyses demonstrated the formation of boehmite nanoparticles with a surface area of 350 m²/g and NaY nanozeolite with high crystallinity and surface area of 957 m²/g. In order to evaluate the effect of the content of the mesoporous boehmite nanoparticles on the catalytic performance of the Residue Fluid Catalytic Cracking (RFCC) catalyst, alumina active matrix-based and silica inactive matrix-based catalysts were prepared. Results actually demonstrated that the acidity of zeolite composition improved with the addition of boehmite nanoparticles. On the other hand, in equal zeolite content, the alumina active matrix-based catalyst possessed higher acidity (NC₃₀B₂₀, 3.44 mmol NH₃/g catalyst) than the silica inactive matrix-based catalyst (NC₃₀B₀, 2.31 mmol NH₃/g catalyst). Microactivity tests (MAT) demonstrated that in equal zeolite content, active matrix-based catalysts exhibited higher catalytic performance than the inactive matrix-based catalysts. Moreover, the active matrix-based catalyst (NC₃₀B₂₀) with a surface area of 370 m²/g showed the optimum catalytic performance in RFCC process. The synthesized NC₃₀B₂₀ catalyst with 20 wt. % mesoporous boehmite nanoparticles as an active matrix and 30 wt. % zeolite nanoparticles balanced with silica had the highest gasoline yield (42 wt. %) and gasoline selectivity (65.1 wt. %). The catalytic performance test results showed that in equal MAT conversion (almost 64 wt. %), the synthesized NC₃₀B₂₀ catalyst had higher catalytic performance than the commercial catalyst.

Physical sciences/Chemistry

Physical sciences/Energy science and technology

Physical sciences/Engineering

Physical sciences/Materials science

Physical sciences/Nanoscience and technology

Mesoporous

Nanoboehmite

Nanozeolite

RFCC catalyst

Catalytic cracking

The fluid catalytic cracking (FCC) is one of the most important units in oil refineries for conversion of atmospheric and vacuum residues into valuable products such as gasoline [1–6]. The catalyst is more effective aspect of the FCC process, since the catalyst performance as well as how the unit is operated, determine both quantity and quality of the products [2, 4]. It’s well-known that the catalyst acid sites are responsible for cracking of heavy hydrocarbon molecules via carbonium ion mechanism [2, 7–8]. Typically, RFCC catalyst is composed of four major components such as zeolite, matrix, filler and binder, among them HY zeolite with faujasite framework is the key component that provides much of the catalytic activity and product selectivity [2, 9–12]. In fact, the RFCC catalyst performance largely depends on the quality and quantity of the zeolite particles. Highly ordered crystalline structure and acidity are the unique properties of zeolites that are responsible for cracking of the heavy hydrocarbon molecules to lighter ones [13–16]. The commercial RFCC catalysts are composed of HY zeolite particles with about 1–2 µm diameter that dispersed in an amorphous matrix [2, 10].

The HY zeolite with faujasite framework is constituted of a uniform microporous structure, in which oxygen rings restrict the 7.4 Ǻ, opening connecting supercages with a 12 Ǻ diameter [17–20]. These uniform pore structures provide acidic active sites in the internal as well as the external surfaces of the zeolite crystallites with the major ones being placed within the zeolite pores [21, 22]. So, as a result of limitations related to pore size, the catalytic properties of zeolite are limited to reactant molecules with diameters lower than zeolite pore size [23]. The critical diameter of residue or heavy oil hydrocarbon molecules with the boiling point of > 400°C is from 1.2 to 15 nm [24]. These large molecules cannot penetrate to zeolite micropores and therefore must be cracked on the external surface of zeolite and mesoporous acidic sites of the active matrix [25–32]. Hence, in order to fulfill the above-mentioned requirement, mesoporous alumina nanoparticle as active matrix and nanozeolite particle can be effectively utilized. With the reduction of zeolite particle size from micrometer to nanometer the ratio of external to internal surface area strongly increases [27, 33–34]. Therefore, zeolite nanoparticles have larger external surface area and higher surface activity than the zeolite microparticles [35–37].

Unlike inactive matrix that does not have enough acidity to influence the catalyst performance, the active matrix can influence the catalyst performance but it has a lower impact on the catalytic performance compared to zeolite component [38–40]. Among the different alumina components, boehmite is one of the most important components that are used in the catalyst formulations [41]. The mesoporous structure of boehmite, which can be preserved after calcination as well as its sufficient acidity for pre-cracking of heavy hydrocarbon molecules, are the most important reasons for its use as RFCC catalyst matrix [42–43]. In fact, the large hydrocarbon molecules pre-crack on the alumina active matrix into intermediate molecules, and then these intermediate molecules penetrate into the zeolite micropores and crack further into the valuable products [44].

The use of heavier feedstock’s by RFCC units for their conversion to valuable products is growing year by year, while cracking of these heavy feedstock’s will be diffusion limited [25–26, 45–46]. Therefore, in order to decrease the diffusion resistance of large hydrocarbon molecules in catalyst pores, one has to increase the external surface area of zeolite by reducing the particle size as well as providing a mesoporous active site in the catalyst matrix structure [47–49]. In fact, using mesoporous boehmite nanoparticles as an active matrix and Y-nanozeolite in the RFCC catalyst formulation can fulfill this limitation [27, 32–40]. Therefore, studies the synergetic effect of mesoporous alumina as active matrix and HY zeolite nanoparticles in the structure of RFCC catalysts and also examination of their catalytic performance are highly favorable.

The effect of HY zeolite particle size and content on the catalytic performance of RFCC catalyst, was evaluated in our previous work [50]. So, in this work, a series of the RFCC catalysts based on the mesoporous boehmite nanoparticles and HY zeolite nanoparticles were prepared and synergetic effects of mesoporous alumina matrix and HY zeolite nanoparticle on RFCC catalyst performance were studied.

2.1. Synthesis of mesoporous boehmite nanoparticles

750 ml of deionized water was added to a precipitation reactor and heated to 57°C. Then, 306 ml sodium aluminate solution (1 mole/L) was fed to the precipitation reactor with the feed flow rate of 18 ml/min. Also, the appropriate amount of aluminum sulfate solution (0.5 mole/L) was introduced into the precipitation reactor. The flow rate of the aluminum sulfate solution was controlled in order to keep the pH of the mixed solution at 9.0. The temperature of both sodium aluminate and aluminum sulfate solutions was adjusted to 57°C and after precipitation process, the suspension was aged at 57°C for 1 hour under the vigorous stirring. After that, the precipitate was separated by centrifuging and washed three times with deionized water and finally dried at 120°C for 24 h [51].

2.2. Synthesis of nanozeolite

The NaY nanozeolite was synthesized via a hydrothermal rout in our previous work [50]. In a typical synthesis, 5.5 g pseudoboehmite (AlOOH) was initially added into the solution of 52.6 g tetramethylammonium hydroxide (TMAOH: 25 wt. %) and an appropriate amount of deionized water in 200 ml propylene bottle. The mixture was stirred overnight and then 8.3 g of tetramethylammonium bromide (TMABr, ≥ 98%) was added, and stirred vigorously for 5 h. Then 23 g sodium silicate solution (Merck, extra pure, 7.5–8.5 wt. % Na₂O, 25.5–28.5 wt. % SiO₂) was added under stirring. The resulting mixture was aged for 3 days under vigorous stirring at room temperature and then transferred into an autoclave, which was sealed tightly and kept for 6 days at 100°C without stirring. Afterward, the precipitate was recovered by centrifuging, washed three times with deionized water and finally dried at 80°C for 24 h.

2.3. Synthesis of RFCC catalysts

The RFCC catalysts with different contents of boehmite nanoparticles were synthesized, with HY nanozeolite as an active component. In a typical procedure, firstly, the ion exchange process using ammonium sulfate was conducted four times in order to transform the synthesized NaY nanozeolite to HY zeolite, which are applicable to the RFCC catalysts. After that, a certain amount of boehmite nanoparticles and 30 wt. % HY nanozeolite were added to an appropriate amount of colloidal silica at room temperature. The mixture was stirred and the gel was formed under stirring. Afterward, the resulting gel was aged at room temperature for 1 h followed by drying at 80°C for 24 h, and finally, the dried gel was calcined at 600°C with a ramp rate of 3°C/min for 5 h in air atmosphere. The nominal content of boehmite nanoparticles in the synthesized catalysts was 10, 20 and 30 wt. % and the corresponding catalysts were denoted as NC₃₀B₁₀, NC₃₀B₂₀ and NC₃₀B₃₀, respectively. Moreover, the prepared catalyst without boehmite was denoted as NC₃₀B₀.

2.4. Characterization

The X-ray diffraction (XRD) analysis was performed on a PANalytical X’Pert-Pro X-ray diffractometer with a Cu-K_α monochromatized radiation source and a Ni filter in a scanning range of 2θ = 5–80°. The infrared spectra were recorded on a SPECTRUM 2 (PerkinElmer) Fourier transform infrared (FTIR) spectrophotometer using KBr pellets containing 1 wt. % sample in KBr. Thermogravimetric (TG) and differential thermal analysis (DTA) of the prepared samples were carried out on a Netzsch STA 409 model system at a heating rate of 10°C/min in a static air atmosphere.

The nitrogen adsorption/desorption analysis (BET) was carried out at − 196°C using an automated gas adsorption analyzer (ASAP 2020, Micromeritics). The scanning electron microscope (SEM) images were obtained on a VEGA@TESCAN instrument. The acidity of the catalysts was studied using a Micromeritrics chemisorb 2900 apparatus for NH₃-TPD analysis. In a typical analysis, 0.1–0.3 g catalyst was loaded into a u-tube quartz reactor. Before the analysis, the catalyst was degassed under the He atmosphere for 2 h at 400°C. Then the catalyst was exposed to a mixture of NH₃ and He stream for 1 h at 50°C and then purged by the He stream for 30 min. Samples were than heated to 750°C under the He stream with a ramp rate of 10°C/min, and the amount of desorbed NH₃ was detected by a thermal conductivity detector (TCD).

2.5. Catalytic performance evaluation

The catalytic activity and selectivity tests for Residue Fluid Catalytic Cracking (RFCC) were performed in a fixed-bed microactivity test (MAT) unit according to ASTM D 5154. The apparatus of the test method is shown in Scheme. 1. The unit was equipped with a collection system for liquid and gas products. The boiling point (bp) range of liquid products was recorded by a simulated distillation gas chromatograph (Clarus 580 GC, PerkinElmer) equipped with a flame ionization detector.

The catalytic activity tests were performed in MAT unit at 482°C with a weight hourly space velocity (WHSV) of 16 h^− 1. All catalysts were steamed with 80% water vapor in N₂ at 800°C for 15 h before the catalytic tests. The conversion, yield of gas (H₂, H₂S, dry gas C₁-C₂ and LPG C₃-C₄ etc.), gasoline (> C₅, bp up to 216°C), light cycle oil (LCO, bp 216–343°C), heavy cycle oil (HCO, bp above 343°C) and coke were determined as MAT results. The RFCC unit feedstock of Imam Khomeini Oil Refinery Company was used for all MAT tests. The specifications of Imam Khomeini Oil Refinery Company RFCC feedstock are given in Table 1.

Table 1

Specification of Imam Khomeini Oil Refinery Company RFCC feedstock.
Item	Measured values	Test Method
Specific gravity (15.6°C/15.6°C)	0.9401	ASTM D792
Total Sulfur (wt. %)	1.01	ASTM D4294
Total Nitrogen (ppm)	1976	ASTM D4629
Conradson Carbon (wt. %)	4.45	ASTM D189
Ni (ppm)	5.9	UOP391
V (ppm)	8.7	UOP391
Distillation (°C)		ASTM D1160
IBP	225
10 vol. %	365
30 vol. %	477
50 vol. %	489
70 vol. %	507
90 vol. %	521
FBP	534

The MAT conversion and gasoline selectivity were calculated using the following equations.

Conversion, wt. %: 100% – (HCO Yield, wt. % + LCO Yield, wt. %) Eq. (1)

Gasoline Selectivity: (Gasoline Yield, wt. % / Conversion, wt. %) × 100 Eq. (2)

3.1. Synthesis of mesoporous boehmite nanoparticles

Figure 1 shows the X-ray diffraction pattern of the synthesized boehmite nanoparticles. As compared with the reference pattern (Boehmite, 00-005-0190), the synthesized boehmite powders present the diffraction characteristic of the boehmite structure with the orthorhombic crystalline system. As can be seen, the broad peaks of XRD pattern indicate the formation of nanoboehmite powder. The characteristic diffraction peaks for boehmite appear at 14.38^°, 28.11^°, 38.41^°, 49.05^° and 55.29^° (2θ) which are related to (020), (021), (130), (150) and (151) planes in the orthorhombic crystalline system, respectively.

The average crystallite size of the synthesized boehmite nanoparticles was calculated from the half-width of the main diffraction peaks using Scherer formula and the obtained results are presented in Table 2.

Table 2

Structural properties of the synthesized boehmite and Y zeolite nanoparticles.
Sample	Average crystallite size (nm)	Surface area (m²/g)	Pore volume (cm³/g)	Average Pore size (nm)
Boehmite nanoparticles	6	350	0.61	7.1
Y zeolite nanoparticles	33	957	0.35	1.13

Figure 2 shows the FTIR spectrum of the synthesized boehmite nanoparticles. Two broad bonds in the FTIR spectrum of the synthesized boehmite can be seen at 3050–3100 cm^− 1 and 3200–3300 cm^− 1, which are related to the stretching vibrations of hydroxyl groups. Moreover, the peak at 1500–1600 cm^− 1 is attributed to the adsorbed water. Two peaks at 1050–1100 cm^− 1 and 1100–1200 cm^− 1 are associated with bending vibrations of Al‒OH. Three peaks at the region of 450–750 cm^− 1 are attributed to the metal-oxygen's (Al-O) bending and stretching vibration [52].

Figure 3 shows the TG- DTA curves of the synthesized boehmite nanoparticles. The TG curve shows the weight loss (approximately 15 wt. %) in the region of ~ 50‒150°C, corresponding to a broad endothermic peak at the same region in the DTA curve. This weight loss and the endothermic peak is related to evaporation of the surface adsorbed water and the water trapped in pores. With increasing temperature, a weight loss of about 15 wt. % in the region of ~ 300‒500°C is found, which corresponds to a broad endothermic peak at the same region in the DTA curve, which is attributed to crystallization of metal hydroxide into metal oxide and removal of residual hydroxide groups. In fact, this was attributed to the phase transformation from AlOOH to γ -Al₂O₃ [52].

In order to study the specific surface area and pore structure of the synthesized boehmite nanoparticles, the BET/BJH analysis was performed. According to the results in Table 2, the synthesized sample shows a BET surface area of 350 m²/g with a pore volume of 0.61 cm³/g. Moreover, the average pore size of the synthesized sample is 7.1 nm. Figure 4 shows the N₂ adsorption/desorption isotherm and pore size distribution curve of the prepared sample. According to the IUPAC classification, the N₂ isotherm is type IV with H2 type hysteresis loop. This type of hysteresis is usually found for the solids consisting of particles crossed by partially cylindrical channels or made by agglomerates or aggregates of spheroidal particles with a non-uniform size and/or shape pores [56]. The pore size distribution curve of the prepared sample is shown in Fig. 4b. Based on this figure, the synthesized boehmite shows a pore size in the mesopore region, which is suitable for the preparation of the RFCC catalyst.

The SEM images of the synthesized boehmite nanoparticles with different magnifications are shown in Fig. 5. As can be seen, the SEM images clearly demonstrate that the structure with spherical shape is formed. The particle size of the synthesized boehmite nanoparticles was uniform, ranging from 20 to 40 nm. However, these particles were physically attached together and formed uniform spheres.

3.2. Synthesis of nanozeolite

The X-ray diffraction pattern of the synthesized NaY nanozeolite powder is shown in Fig. 6. As compared with the reference pattern (Faujasite, 00-039-1380), the synthesized nanozeolite powder presents the diffraction pattern, characteristic of a faujasite structure with the cubic crystalline system. The sharp peaks in the XRD pattern indicate the formation of zeolite powder with high crystallinity.

The average crystallite size of the synthesized NaY nanozeolite was also calculated from the half-width of the main diffraction peaks using Scherer formula, and the results are presented in Table 2.

The FT-IR spectrum of the synthesized NaY nanozeolite is displayed in Fig. 7. A wide band at 3460 cm^− 1 was due to stretching vibration of structural hydroxyl groups and water molecules. Also, the peak at 1640 cm^− 1 was due to the bending vibration of water molecules. The bands in the region of 1200 and 900 cm^− 1 were ascribed to the internal tetrahedral and external linkage asymmetrical stretching vibrations, respectively. Furthermore, the bands at 725 and 790 cm^− 1 were attributed to internal tetrahedral and external linkage symmetrical stretching vibrations, respectively [53]. The band at around 577 cm^− 1 was related to the structural vibration of the double ring (D6R) units, which is a special characteristic of Y zeolite structure [54, 55]. In fact, this band demonstrated the formation of a zeolitic structure. Likewise, the band at 465 cm^− 1 was related to the metal-oxygen bonding vibration at tetragonal holes TO₄ (T = Si or Al) in the zeolite structure [29].

The N₂ adsorption/desorption isotherm and pore size distribution of the synthesized NaY nanozeolite are shown in Fig. 8. According to IUPAC classification, the N₂ isotherm is type I, which is related to microporous materials [45]. As can be seen, the isotherm shows an intensive rise in N₂ adsorption at very low relative pressure (P/P⁰) and then a flat curve at higher relative pressure, which is a special characteristic of microporous materials. The structural properties of the prepared sample are reported in Table 2. According to the results in Table 2, the specific surface area is 957 m²/g and the external surface area based on t-plot calculation is 102 m²/g for the synthesized NaY nanozeolite. According to the N₂ isotherm and BET result, the pore size of the synthesized zeolite is in the micropore range (< 2 nm), which is in agreement with the pore size distribution, Fig. 8b.

Figure 9 shows the SEM images of the synthesized NaY nanozeolite. According to the SEM images, particles with cubic shapes are clearly formed. The particle size distribution of the synthesized NaY nanozeolite was uniform, ranging from 20 to 30 nm.

3.3. Synthesis of RFCC catalysts

The X-ray diffraction patterns of the prepared catalysts with different boehmite contents are shown in Fig. 10. It clearly demonstrates the presence of a Y zeolite structure in the prepared catalysts. The broad diffraction peak appeared at 2θ = 15–30° is related to the amorphous silica matrix.

The acidity of the catalysts was studied by NH₃ temperature programmed desorption (NH₃-TPD) analysis. The acidic sites distribution and the total acidity of the catalysts can be obtained from the peak position, shape and the area of the desorption peaks, respectively. Figure 11 illustrates the NH₃-TPD curves of the prepared catalysts without boehmite (NC₃₀B₀) and with 20 wt. % boehmite (NC₃₀B₂₀). As can be seen, the curves show two main desorption peaks. The low-temperature peak can be related to desorption of NH₃ from weak acid sites or non-acidic sites, weak Bronsted acid site, Lewis acid site, silanol groups and formation of NH₄⁺ (NH₃)n (n ≥ 1) groups [57, 58]. The high-temperature peak can be attributed to desorption of NH₃ from strong Bronsted and Lewis acid sites [59–61]. As can be seen, the acidity of the prepared catalysts increased with the addition of boehmite nanoparticles as active matrix to catalyst composition. On the other hand, in equal zeolite content (30 wt. %), the catalyst with the active matrix (NC₃₀B₂₀) has higher acidity than the catalyst with the inactive matrix (NC₃₀B₀). In fact, this higher acidity of the active matrix-based catalyst is related to the acidic sites of mesoporous alumina matrix that have a key role in precracking of heavy hydrocarbon molecules that cannot enter into zeolite micropore.

Figure 12a shows the N₂ adsorption/desorption isotherms of the catalysts prepared with different boehmite contents. According to the IUPAC classification, the N₂ isotherms are type IV with H2 type hysteresis loop. This type of hysteresis is usually found on solids consisting of particles crossed by partially cylindrical channels or made by agglomerates or aggregates of spheroidal particles with a non-uniform size and/or shape pores [55]. As can be seen, with the addition of boehmite nanoparticles to the catalyst composition, the adsorption of N₂ molecules in the saturated pressure (P/P⁰ = 1) decreased, which is attributed to the decrease of the pore volume of the catalyst. The pore size distribution curves of the prepared catalysts are shown in Fig. 12b. Based on this figure, all catalysts have a pore size in the mesopore and micropore regions. The structural properties of the prepared catalysts are presented in Table 3. Here, the BET analysis demonstrated that the addition of boehmite nanoparticles to the catalyst composition increased the surface area of all prepared samples. In fact, with the addition of boehmite nanoparticles to the catalyst composition, the pore volume of the prepared catalysts decreased, but due to the decrease of the pore size, the surface area of the catalysts increased.

Table 3

Structural properties of the (a) NC₃₀B₀, (b) NC₃₀B₁₀, (c) NC₃₀B₂₀ and NC30B30 catalysts.
Sample	Surface area (m²/g)	Pore volume (cm³/g)	Average Pore size (nm)
NC₃₀B₀	346	0.47	5.4
NC₃₀B₁₀	370	0.44	4.8
NC₃₀B₂₀	370	0.43	4.6
NC₃₀B₃₀	359	0.42	4.7

The SEM images of the NC₃₀B₀ and NC₃₀B₂₀ catalysts are presented in Fig. 13. It can be observed that the zeolite and boehmite nanoparticles are well dispersed and incorporated into a silica matrix. However, these particles were physically attached together and formed uniform spheres.

3.4 Catalytic performance

The modern RFCC catalyst is a composite that consists of different components such as zeolite, matrix, binder and different additives as promoter, among them the active matrix has a key role in precracking of heavy hydrocarbon molecules that cannot enter into zeolite micropore. The cracking of single hydrocarbon over synthesized RFCC catalyst has been reported by several authors [61–63]. This type of process can provide a general suggestion on the potential of the catalysts. However, it is necessary to evaluate the performance of the catalysts in real-life application; hence, in order to evaluate the synergetic effect of mesoporous boehmite nanoparticles as active matrix and zeolite nanoparticles on the catalytic performance of RFCC catalyst, cracking reaction was carried out using the RFCC unit feedstock of Imam Khomeini Oil Refinery Company over the prepared catalysts. In a commercial RFCC catalyst, matrix components are active and crack large hydrocarbon molecules to lighter ones. Therefore, in this work, we prepared catalysts using mesoporous boehmite nanoparticles as active matrix and zeolite nanoparticles in order to evaluate the synergetic effect of matrix activity and zeolite nanoparticles on the catalytic performance. Figure 14 (a) shows the correlation of MAT conversion and gas yield with the boehmite content of different prepared catalysts. As can be seen, the MAT conversion increased with the boehmite content. In general, increasing the boehmite content as active matrix increased the acidity and cracking of hydrocarbon molecules, so the conversion increased. However, boehmite nanoparticles as active matrix with the mesoporous active sites cause precracking of heavy hydrocarbon molecules to lighter ones. These light hydrocarbons can later enter into zeolite micropores and crack to valuable products [38–40]. On the other hand, in equal nanozeolite content, the catalysts with the alumina matrix have higher cracking activity than the catalysts with inactive matrix. In fact, this higher activity of active matrix-based catalysts is related to the mesoporous active site of alumina matrix that causes lower molecular diffusion resistance and higher heavy hydrocarbon molecular accessibility.

The gaseous products include dry gas (C₁-C₂ molecules and other molecules containing H₂, H₂S, CO, CO₂, etc.) and LPG (C₃-C₄). Dry gas is an undesirable product, while LPG is a valuable product. The obtained results revealed that the gas yield increased with the boehmite content. As it is well-known, the gaseous products are caused by thermal cracking and overcracking of hydrocarbon molecules on the catalysts acid sites. In fact, the rising of the catalyst boehmite content increased the available catalyst acid sites and catalyst conversion, hence gas yield increased.

Gasoline is one of the most important products of the RFCC unit. Figure 14 (b) illustrates the relation between the gasoline yield and gasoline selectivity with the boehmite content of different prepared catalysts. It can be observed that increasing the content of boehmite up to 20 wt.% increased the gasoline yield, and a further increase in its content had a negative influence and decreased the gasoline yield. In general, the mesoporous active site in the matrix structure of catalyst for precracking of heavy hydrocarbon that cannot enter into zeolite micropores increased with an increase in the content of boehmite in the catalyst. After that, this precracked hydrocarbon entered into zeolite micropores and selectively cracked to the lighter product, hence gasoline yield increased. In fact, mesoporous acidic sites in alumina structure precrack heavy hydrocarbon molecules and increase the accessibility of these cracked molecules to the active site of zeolite and cause lower molecular diffusion resistance, but do not result in any blockage of zeolite micropores with heavy hydrocarbon molecules. So the gasoline yield with the selective and controlled cracking increases. As the boehmite content of the catalyst reaches a critical amount, the gasoline yield reaches a steady state and eventually decreases with the further increasing of the boehmite content. In fact, further increasing of boehmite content intensifies the cracking reactions and causes overcracking of gasoline, thus gasoline yield decreases. The results clearly demonstrate the superiority of active matrix-based catalyst. In equal zeolite content, the active matrix-based catalysts have higher gasoline yield than inactive matrix-based catalyst. The highest gasoline yield was observed on the catalyst with the 20 wt. % boehmite nanoparticles (NC₃₀B₂₀), which is about 42%.

The gasoline selectivity is defined as the gasoline yield to conversion ratio, which is one of the most important parameters to evaluate the performance of RFCC catalyst. The increasing catalyst boehmite content increases the gasoline selectivity, which then reaches a constant value and eventually decreases. In fact, the addition of mesoporous boehmite nanoparticles as active matrix to catalyst composition causes selective and controlled precracking of heavy hydrocarbon molecules, and by directing the cracked heavy molecules toward zeolite micropore, gasoline selectivity increases. So, the active matrix-based catalysts have higher gasoline selectivity than the inactive matrix-based catalyst and the catalyst with the 20 wt. % boehmite nanoparticle (NC₃₀B₂₀) has the highest gasoline selectivity, which is about 65.1%. However, further increasing of boehmite content increases the catalyst acidity and causes overcracking of hydrocarbon molecules and consequently decreases the gasoline selectivity.

LCO fraction is the favorable product of the RFCC unit. The relation between LCO and HCO yield with the catalyst boehmite content are presented in Fig. 14 (c). The LCO yield increases with the catalyst boehmite content and decreases when it reaches a maximum value. In fact, the large hydrocarbon molecules cannot enter into the small pores of the zeolite, so the cracking of large hydrocarbon molecules on the mesoporous acidic site of alumina active matrix and external surface of the zeolite nanoparticle results in LCO. Hence, the active matrix-based catalyst that has higher mesoporous active site than inactive matrix-based catalyst shows higher cracking of large molecules and LCO yield. Also, due to the high frequency of large hydrocarbon molecules in low conversion, the LCO yield increases by increasing the boehmite content and conversion. However, further increase of boehmite content and conversion decreases the amount of these large molecules: thus, LCO yield drops after a maximum value. As can be seen, the catalyst with 20 wt. % boehmite nanoparticles (NC₃₀B₂₀) has the highest LCO yield, which is about 20%.

HCO fraction is the unfavorable product of RFCC process, which includes sulfur and aromatic components that should be reduced to the lowest content. As can be seen, HCO yield decreases with rising of boehmite content, due to the increase of the acidity and the number of catalytic cracking active sites of mesoporous alumina active matrix. However, the active matrix-based catalysts show lower HCO yield than inactive matrix-based catalyst.

Figure 15 displays the influence of the catalyst boehmite content on the coke formation. Coke is the undesired product of the RFCC unit, the formation of which should be kept as low as possible. The obtained results illustrate that the active matrix-based catalysts have the lowest coke yield as compared to inactive matrix-based catalyst. In fact, the large hydrocarbon molecules that cannot enter into the micropores of the zeolite can cause the blocking of these pores and lead to coke formation. The addition of alumina active matrix with the mesoporous structure to the catalyst composition, in which heavy hydrocarbon can enter and crack to lighter molecules does not cause the blockage of the micropores in zeolite and consequently, the coke yield decreases with the synergetic effect of boehmite and zeolite nanoparticles.

As it can be seen, the synthesized NC₃₀B₂₀ catalyst with 20 wt. % mesoporous boehmite nanoparticles as active matrix and 30 wt. % zeolite nanoparticles balanced with silica showed the best gasoline yield, gasoline selectivity and LCO yield in RFCC process. Hence, the catalytic performance of the synthesized NC₃₀B₂₀ catalyst was compared with the HGY series commercial catalyst of Sinopec Catalyst Company and the results are presented in Table 4.

Table 4

Structural properties and catalytic performance results of the synthesized NC₃₀B₂₀ and commercial RFCC catalysts.
Item	Synthesized NC₃₀B₂₀ Catalyst	Commercial Catalyst
Surface area (m²/g)	370	312
Pore volume (cm³/g)	0.43	0.18
Average pore size (nm)	4.6	2.3
NH₃ desorbed (mmol /g catalyst)	3.4	3.5
MAT Conversion (wt. %)	64.5	64.2
Gas Yield (wt. %)	18.5	19.0
Gasoline Yield (wt. %)	42.0	38.7
Gasoline Selectivity	65.1	60.3
LCO Yield (wt. %)	19.8	18.1
HCO Yield (wt. %)	13.2	17.7
Coke Yield (wt. %)	6.5	6.5

According to the results in Table 4, the synthesized NC₃₀B₂₀ catalyst shows higher values for surface area, total pore volume and average pore size than the commercial catalyst. In fact, the mesoporous boehmite nanoparticles produce mesopore structure and increase pore volume and pore size of the synthesized catalyst. Also, the use of zeolite nanoparticles increases the surface area of the synthesized catalyst compared to the commercial catalyst. The NH₃-TPD analysis showed approximately equal ammonia desorbed from the synthesized and commercial catalysts, which represents the same acidic strength. The catalysts also show approximately equal MAT conversion. As can be seen, the microactivity tests demonstrate that the synthesized NC₃₀B₂₀ catalyst has higher desirable product yield such as gasoline and LCO, a higher gasoline selectivity and lower undesirable product yield such as HCO and gas than the commercial catalyst in equal MAT conversion. In fact, the optimum catalytic performance of the synthesized NC₃₀B₂₀ catalyst is related to the synergetic effect of the mesoporous alumina active matrix and zeolite nanoparticle that leads to lager pore size, higher pore volume and higher surface area of the catalyst. Therefore, enlarging the pore size of the active matrix and improving the pore distribution of catalysts are the main reasons of lower molecular diffusion resistance and higher heavy hydrocarbon molecular accessibility of the synthesized catalyst compared to the commercial catalyst that result in better catalytic performance.

In this work, the boehmite nanoparticles (20–40 nm) and the NaY nanozeolite (20–30 nm) were synthesized and characterized. The XRD analyses clearly demonstrated the formation of boehmite and NaY zeolites with high crystallinity. The BET analysis showed that the prepared boehmite has a mesoporous structure with a surface area of 350 m²/g, which is an important parameter in the performance of active matrix in RFCC catalyst. Afterwards, some catalysts were prepared using the mesoporous boehmite nanoparticle as active matrix and the Y zeolite nanoparticle as the main active component in order to evaluate the synergetic effect of the mesoporous active matrix and Y zeolite nanoparticles on the catalytic performance. The NH₃-TPD analysis demonstrated that the acidity improves with the addition of boehmite nanoparticles to the catalyst composition. On the other hand, in equal zeolite content, the active matrix-based catalyst has higher acidity (NC₃₀B₂₀, 3.44 mmol NH₃/g catalyst) than the inactive matrix-based catalyst (NC₃₀B₀, 2.31 mmol NH₃/g catalyst).

However, in order to evaluate the effect of mesoporous boehmite content on the catalytic performance of RFCC catalyst, cracking reaction was carried out using the RFCC unit feedstock of Imam Khomeini Oil Refinery Company over the prepared catalysts. The microactivity tests (MAT) demonstrated that in equal zeolite content, the active matrix-based catalysts have a higher conversion, higher desirable product yield such as gasoline and LCO, a higher gasoline selectivity and lower undesirable product yield such as HCO and coke than the inactive matrix-based catalyst. In fact, the optimum catalytic performance of the active matrix-based catalysts is related to the mesoporous active site and higher acidity of alumina active matrix, which lead to lower molecular diffusion resistance and higher heavy hydrocarbon molecular accessibility. Moreover, the active matrix-based catalyst (NC₃₀B₂₀) with a BET area of 370 m²/g showed the best catalytic performance in RFCC process. The synthesized NC₃₀B₂₀ catalyst with 20 wt. % mesoporous boehmite nanoparticles as active matrix and 30 wt. % zeolite nanoparticles balanced with silica had the highest gasoline yield (42 wt. %) and gasoline selectivity (65.1 wt. %). The catalytic performance test results showed that in equal MAT conversion (almost 64 wt. %) the, synthesized NC₃₀B₂₀ catalyst has higher desirable product yield such as gasoline (42.0 wt. % synthesized catalyst versus 38.7 wt. % commercial catalyst) and LCO (19.8 wt. % synthesized catalyst versus 18.1 wt. % commercial catalyst), and higher gasoline selectivity (65.1 wt. % synthesized catalyst versus 60.3 wt. % commercial catalyst) and lower undesirable product yield such as HCO (13.2 wt. % synthesized catalyst versus 17.7 wt. % commercial catalyst) and gas (18.5 wt. % synthesized catalyst versus 19.0 wt. % commercial catalyst) than the commercial catalyst. In fact, the better catalytic performance of the synthesized NC₃₀B₂₀ catalyst is attributed to the synergetic effect of the mesoporous alumina active matrix and zeolite nanoparticle, which leads to lager pore size, higher pore volume and higher surface area of the catalyst. Therefore, lower molecular diffusion resistance and higher heavy hydrocarbon molecular accessibility are the main reasons for the superiority of the synthesized catalyst in comparison to the commercial catalyst.

Author Contribution

All authors contributed equally in conducting this research and writing and reviewing the manuscript.

Acknowledgement

The authors gratefully acknowledge the supports from Material and Energy Research Center by Grant No. 481394052.

Data Availability

All data generated or analyzed during this study are included in this published article.

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Synergetic effects of nano-boehmite and Y nano-zeolite on catalytic cracking of residue oil

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Abstract

Figures

1. Introduction

2. Experimental

2.1. Synthesis of mesoporous boehmite nanoparticles

2.2. Synthesis of nanozeolite

2.3. Synthesis of RFCC catalysts

2.4. Characterization

2.5. Catalytic performance evaluation

3. Results and discussion

3.1. Synthesis of mesoporous boehmite nanoparticles

3.2. Synthesis of nanozeolite

3.3. Synthesis of RFCC catalysts

3.4 Catalytic performance

4. Conclusions

Declarations

Author Contribution

Acknowledgement

Data Availability

References

Additional Declarations

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