Preparation of a core–shell structured Y@ASA composite material and its catalytic performance for hydrocracking of n-decane

Abstract

A series of core–shell structure Y@ASA composites with different contents of Y-type zeolite (abbreviated as Y@A) was synthesized in the small-crystal Y zeolite suspensoid by adding CTAB surfactant in the synthesis process of the amorphous silica alumina (ASA). Y@A composites and their corresponding catalysts were characterized by XRD, SEM, TEM, N₂ adsorption–desorption and infrared spectroscopy of pyridine adsorption (Py-IR). The results showed that the surface of the small-crystal Y zeolite was covered uniformly by ASA even if ASA content was only 10 wt%, and more importantly, ASA could provide 3–20 nm mesopores to bring about the formation of the micro- and mesoporous core–shell structured composites with internal microporous Y zeolite and external mesoporous ASA. Furthermore, due to the interaction between ASA and Y zeolite, the hydrothermal stability of the small-crystal Y zeolite was enhanced, and at the same time the acid quantity of the composite was increased. The NiW/USY@A catalysts were synthesized by loading Ni and W on the Y@A. Their hydrocracking and isomerization performance were evaluated by using n-decane as a model compound. The experimental results indicated that the selectivities of the intermediate product and isoparaffin on NiW/USY@A-90 (with 90 wt% Y-type zeolite in the Y@A) are 30.81% and 23.70% higher than NiW/USY-In in which industrial Y was used, respectively. Thus, besides its high hydrocracking performance, NiW/USY@A showed an excellent isomerization capacity.

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1. Introduction

With the continuous deepening of research and the related technology development, more and more composite materials, due to their eminent porous structure, have begun to gradually replace catalysts with a single porous structure. Most composites were generally dual structured materials. One material in the composite provided new structural characteristics to the other, while the second had a better catalytic activity. In this way, these two materials complemented each other to show their respective advantages.

Recently, Kollar et al.¹ showed that micro/mesoporous composites were prepared by applying a two-step hydrothermal synthesis procedure. They synthesized the mesoporous MCM-41 in the slurry of the MCM-22 type precursor from an additional silicate source by using hexadecyltrimethylammonium bromide (CTMABr) as template. The results suggested that the MCM-22/MCM-41 composite materials were built from nano-sized, randomly orientated stacks of thin layers of MCM-22 type zeolite and mesoporous MCM-41 material. The composite's catalytic activity was compared with the MCM-22 zeolite in the hydro-conversion of heptane. Its isomerization selectivity was up to 82.2%. Shortly after that, Zhang et al.² synthesized Y/MCM-48 composite zeolite by one-step hydrothermal method, using CTAB template. This composite zeolite had thicker pore wall, higher hydrothermal stability and highly acid quantity than the pure zeolite. Through SEM, its surface particles were observed to be dispersed uniformly, other than the heterogenous MCM-48 surface particles. Additionally, it had obvious crystal interface. Most recently, Jermy et al.³ reported that ZSM-5/MCM-41 composite material was synthesized by using alkali treated HZSM-5 and CTAB as aluminium source and template, respectively. Compared to the pure ZSM-5 and pure MCM-41, it contained secondary building units. Utilizing the hydrothermal treated ZSM-5/MCM-41 composite material for catalytic cracking, the yield of propylene was higher than MCM-41 catalyst. And owing to the ZSM-5 geometric structure, the content of arene increased obviously in products. The octane number of gasoline reached 6–12 units. Tang et al.⁴ also synthesized micro- and mesoporous composite zeolites, but by using aluminosilicate in alkali and mesoporous wall from self-assembling. Many microporous structures of the alkali treated ZSM-5 was proved to grow into mesoporous structures. According to the BET results, the pore size, pore volume and specific surface of ZSM-5 increased as well.

Hydrocracking catalyst is a typical dual-function catalyst. And Y zeolite is the main catalytic cracking component of industrial hydrocracking catalysts. But the particle diameter of industrial Y zeolite is in micron scale. Small crystal Y zeolite has thus received much concern for the special properties such as numerous active sites⁵ at the external surface, and short and regular pores.

However, with the dramatic reduction of crystal size from micro-size to nano-size, hydrothermal stability of the small crystal zeolite was restricted by its high surface energy, large number of exposed unit cells. To improve the hydrothermal stability of the small crystal zeolite, many micro- and mesoporous composites with new structures were synthesized by worldwide research groups.^6–12

Silica–alumina mixed oxide^13–17 is the most extensive solid acid catalyst and catalyst carrier material. It was mostly used for catalytic cracking, alkylation, isomerization and disproportionation. And the amorphous silica–alumina (ASA) holds the hydrocarbon pyrolysis activity. It can increase the medium oil selectivity and ease the polycyclic aromatic hydrocarbons aggregation. Hosseinpour et al.¹⁸ presented that SA/Y was synthesized by putting equal ASA on the bed of Y zeolite. SA–Y was synthesized by mechanically mixing equal Y zeolite and ASA. The cracking activity of SA–Y and SA/Y catalysts was higher than the pure Y zeolite and ASA monomer catalyst in the 1,3,5-triisopropylbenzene catalytic cracking. Meng et al.¹⁹ synthesized Y/ASA composite by adding small-crystal NaY zeolite in amorphous silica–alumina gel. The results demonstrated that the hydrothermal stability of the small-crystal Y zeolite was obviously improved by ASA. When the mass ratio of the small-crystal Y zeolite to ASA was more than 70%, the Y zeolite crystallites could not be wrapped completely by ASA layer, and that would induce the oxide particles to distribute sparsely around the Y zeolite crystallites. It is well known that ASA had dispersed pore of 2–20 nm and its pore size was less 4 nm, therefore the mesoporous distribution of ASA was relative scatter. But the research of Y@A composite in the hydrocracking and isomerization was reported rarely.

Based on the study of Meng et al.,¹⁹ the core–shell structure Y@A composites were synthesized with using CTAB as the template for mesoporous ASA. Hydrocracking catalyst was prepared by loading active metal Ni and W. Using n-decane as a model compound, hydrocracking and isomerization performance of the catalysts were studied.

2. Experiments

2.1. Preparation of the composite and catalyst

2.1.1. Preparation of Y@A composite. The small-crystal NaY zeolite was synthesized according to the Meng et al.¹⁹

1–8 g of hexadecyl trimethyl ammonium bromide (CTAB) (Sigma-Aldrich Co.), 20 ml of deionized water were mixed and stirred for 2 h at room temperature to get a homogeneous solution. This was denoted as solution A. To form solution B, 100.0 g NaAlO₂ (95 wt%, Beijing GuoHua Chemical Material Co.), 44.7 g NaOH (96 wt%, Tianjin Guangfu Technology Development Co.), 502.6 g of deionized water were mixed and stirred at room temperature to obtain a homogeneous solution. A certain amount of the small-crystal Y zeolite and deionized water were mixed into uniform suspensoids. The ASA was synthesized in the seriflux. And the silica–alumina ratio of the ASA was 1.5 : 1.

The subsequent synthesis process of Y@A composite was as follows: first, a certain amount of the small-crystal Y-type zeolite and deionized water was mixed into uniform slurry. Then a certain amount of solution A and water glass (35 wt% SiO₂, Lanzhou PetroChemical Co., PetroChina Company Ltd.) was added to the slurry under stirring and subsequent stir went on for 30 min. After that, solution B was added to the mixture and afterwards stirring continued for 1 h. Last, the pH of the ultimate mixture was adjusted to 7–8 with 2 M H₂SO₄ solution and let rest for 40 min. The Y@A composites were obtained by filtering, washing, drying at 110 °C for 18 h and calcinating at 550 °C for 3 h. The Y@A composites was denoted as Y@A-X (X = 50, 70 and 90, the mass percent of Y-type zeolite in the Y@A composite). The silica alumina ratio of ASA in the composites was 1.5. The contrast sample was the purchased industrial NaY type zeolite and denoted as NaY-In. The synthesis route was shown as Fig. 1.

Fig. 1 The synthesis route of the Y@A composites.

2.1.2. The improvement of ultrastability of Y@A and NaY-In. A certain amount of Y@A-X and NaY-In were added to 1 M (NH₄)₂SO₄ solution and stirred at 80 °C for 1 h. Then filtering and washing were done for 5 times, the wet filter cake was treated by hydrothermal ultrastabilization at 600 °C for 2 h with a water stream flux of 1 mL g⁻¹ zeolite per h in the pipe furnace and final ultrastabilized products were denoted as USY@A-X and USY-In, respectively.

2.1.3. Preparation of catalysts. A certain amount of USY@A (or USY-In), industrial ASA, the alumina sol, sesbania powder and water were mixed and kneaded to form dough, and extruded into strips. Then the strips were dried at 100 °C overnight and calcined at 550 °C for 3 h. The supporters were obtained in this way. The crystalline portion of the small crystal Y zeolite was 25 wt% in the support.

A certain amount of Ni(NO₃)₂·6H₂O and (NH₄)₆H₂W₁₂O₄₀·4H₂O were mixed with deionized water and stirred to constitute an impregnation liquid. The aforementioned catalyst supports were treated by equivalent-volume impregnation. After impregnation, they were put for 8–12 h and then dried at 120 °C for 4 h, calcined at 450 °C for 3 h. Finally, catalysts NiW/USY@A and NiW/USY-In were prepared (NiO 6 wt%, WO₃ 24 wt% and the support 70 wt%).

2.2. Physical and chemical characterization

XRD analysis was carried out on the Panalytical Empyrean X-ray diffractometer using ceramic X light pipe, the max power 2.2 kW (Cu target), the goniometer reproducibility 0.0001°, controlled min stepping 0.0001°, and the max count rate >109 cps. The 2θ range was scanned from 15° to 35°. The textural properties of the composites were measured by using a Builder KuboX1000 system at 77 K liquid nitrogen temperature. The samples were pretreated at 300 °C for 3 h. The specific area, micropore area and micropore volume were calculated using the BET equation. The mesopore volume was calculated by the t-plot method. SEM measurement was performed on Quanta 200 (FEI Co.) apparatus with an acceleration voltage of 20 kV. The samples were overgilded before being measured.

The micro-structure of the composite materials was tested by JEM-2100 LaB₆ high resolution transmission electron microscopy (HRTEM). The highest acceleration voltage is 200 kV. Magnification factor is 50× to 1500k× and the maximal slant angle is ±35°.

The pyridine IR (Py-IR) was obtained on the Gangdong FTIR-850 spectrophotometer (Gangdong Sci. & Tech. Development Co., Ltd., China). Wafers (diameter 12 mm, from 0.015 g sample) were placed in in situ quartz pool. After pyridine adsorption at 100 °C, the temperature was raised to 200 °C and held for 30 min to desorb pyridine. After that, temperature was reduced to 100 °C and IR spectra in the range of 1700 cm⁻¹ to 1400 cm⁻¹ was recorded. Then, the temperature was raised to 350 °C to desorb pyridine for 30 min and returned to 100 °C. Finally, we recorded IR spectra in the range of 1700 cm⁻¹ to 1400 cm⁻¹ again.

2.3. Catalytic performance testing

Hydrocracking reaction of n-decane was carried out on the fixed-bed microreactor. The 10 ml catalyst (40–60 mesh) was loaded in the stainless steel reaction tube (10 mm inner diameter). First presulfurization was implemented with presulfiding condition: the sulfuration fluid was cyclohexane solution with 3 wt% CS₂. The process was carried out at 4 MPa H₂ pressure, 2.0 h⁻¹ volume velocity, 600 H₂ to oil volume ratio, 290 °C for 8 h. Then n-decane was added by micropump. The reaction condition was followings: 4 MPa H₂ pressure, 360 °C reaction temperature, 400 H₂ to oil volume ratio, 3.0 h⁻¹ volume velocity. The reaction product was collected after 6 h and analyzed by SP-3420 gas chromatograph (50 m SE-30 capillary columns, FID, Beijing North Point Rayleigh Analytical Instrument CO., Ltd.).

The reaction performance of the catalysts was evaluated by the selectivity of the middle distillates (C₅–C₉) and isoparaffin. The calculation formulas were as follows:

where w_r represents unit time feed quality of n-decane, while w_p denotes the content of unreacted n-decane in the products. w_C5–C9 stands for the content of C₅–C₉ in the products.

3. Results and discussion

3.1. XRD measurements

XRD analyses of the composites are presented in Fig. 2 (Y@A-X & NaY-In) and Fig. 3 (USY@A-X & USY-In). Fig. 2 shows that the several characteristic peaks of the Y-type zeolite are in accordance with that of industrial NaY-type zeolite completely. This indicates that the skeleton structure of Y-type zeolite in Y@A composites isn't changed after ASA covered the small-crystal Y-type zeolite. In Fig. 3, the diffraction peaks shape of the ultra-stabilized Y-type zeolite is the same as before, that is to say, no peak splitting is visible indicating that the skeleton structure of Y-type zeolite is not destroyed in ultra stable process. From Fig. 2 and 3, the diffraction peaks intensity of the ultra-stabilized Y-type zeolite in USY@A composite is a little more than it in Y@A composite. But diffraction peaks intensity of USY-In is significantly lower than NaY-In. It suggests that the influence of ASA shell in USY@A to the X-ray diffraction is lower than the ASA shell in Y@A. ASA shell can improve the stability of Y-type zeolite observably.

Fig. 2 XRD patterns of the Y@A and NaY-In.

Fig. 3 XRD patterns of the USY@A and USY-In.

According to the diffraction peak area of Y-type zeolite,²⁰ the relative crystallinity of Y-type zeolite is calculated. The result is showed in Table 1. It is obviously shown that the relative crystallinity of Y-type zeolite in Y@A composites is lower than its theoretical value. This is because that ASA, dispersing on the surface of Y-type zeolite after cladding Y-type zeolite, affects the diffraction of X-ray and results in reducing the intensity of the diffraction peak. And the more the content of ASA, the lower the relative crystallinity of Y-type zeolite is, and the clad level is higher.

Table 1 The relative crystallinity and SiO₂/Al₂O₃ of the Y@A composites and NaY-In

The mass percent of Y zeolite in composite	50%	70%	90%	NaY-In
a The relative crystallinity.b The crystalline retention rate.
Xc^a of Y in Y@A	25	41	61	97
SiO2/Al2O3 of Y in Y@A	4.6	4.6	4.6	5.0
Xc of Y in USYA	34	47	66	68
SiO2/Al2O3 of Y in USY@A	15.95	13.39	12.69	12.38
XR/u,^b %	136	115	108	70

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After ultra-stabilized, the relative crystallinity of Y-type zeolite in USY@A composites is universally higher than that of Y-type zeolite in Y@A composites. It shows that when ASA repairs the Y-type zeolite skeleton in the ultra stable process, its capacity of providing silicon source is better than the capacity of providing aluminium source.²¹ Therefore, the more the content of ASA in Y@A composites, the higher the silica–alumina ratio of Y-type zeolite is after ultrastabilization. Besides, the variation of relative crystallinity for Y-type zeolite indicates that the relative crystallinity retention rate of Y-type zeolite in USY@A composites increases with the rise of ASA content and is always higher than that in Y@A composites. Conversely, the relative crystallinity of Y-type zeolite in USY-In is lower than that in NaY-In. It demonstrates that ASA plays an important role in improving the stability of Y-type zeolite.

3.2. Specific surface and pore structure analysis

Fig. 4 displays the nitrogen adsorption–desorption isotherms of USY@A-X and USY-In. The USY@A-X and USY-In exhibit type IV isotherms and H-IV hysteresis loops. Hysteresis loops of USYA-X (appear at P/P₀ = 0.3–0.4) are prior to USY-In (appears relatively backward, P/P₀ ≥ 0.5). It indicates that USY@A-X exists much smaller pore. In comparison with USY-In, the area of hysteresis loops increases significantly with the rise of ASA content in USY@A composites. It shows that the volume of mesopores in USY@A becomes bigger and bigger. As seen from Fig. 5, the pore distribution of USY-In is relatively centralized (in the range of 2–4 nm, mean pore size 3.74 nm). The pore of USY@A-X mainly distributes in the range of 2–20 nm, and its mean pore size is 7–8 nm (see Table 2). This pore distribution in USY@A-X is beneficial to the macromolecules' diffusion.

Fig. 4 Nitrogen adsorption–desorption isotherms of the samples.

Fig. 5 Pore size distribution of the samples.

Table 2 Textural properties of the samples

Samples	Surface area (m^2 g^−1)			Pore volume (cm^3 g^−1)			dmean^e (nm)
	Stotal	Smicro^a	Smeso^b	Vtotal	Vmicro^c	Vmeso^d
a t-Plot micropore surface area.b t-Plot mesopore surface area.c BJH micropore volume.d BJH mesopore volume.e Pore size range 1–100 nm.
USY@A-50	251	119	132	0.75	0.12	0.63	7.84
USY@A-70	282	133	149	0.58	0.13	0.45	7.50
USY@A-90	292	168	124	0.48	0.12	0.36	7.84
USY-In	198	112	86	0.46	0.09	0.37	3.74

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Texture properties of USY@A-X and USY-In are listed in Table 2. It can be found an interesting phenomenon from Table 2 that the S_total, S_micro and S_meso of USY@A-X are bigger than USY-In. The S_total and S_micro decrease with the rise of ASA content in USY@A. Since the micropore surface area is mainly provided by Y-type zeolite, so S_micro grows with the rise of Y-type zeolite content. Even when the content of Y-type zeolite is 50%, the S_micro of USY@A-50 is higher than USY-In. It shows that the skeleton structure stability of Y-type zeolite is improved after being covered by ASA, and not destroyed in the ultra stable process. Hence Y-type zeolite provides more micropore surface area for USY@A. The mesopore surface area increases firstly and then decreases with the reduction of ASA content. The mesopores in USY@A-X consists of two origins: one is the mesopores, which are from the framework dealumination of Y-type in the ultra stable process. And their distribution is more concentrated, such as USY-In (only 3–5 nm). The other mesopores are provided by ASA. USY provides more mesoporous and less specific area with the decrease of ASA in USY@A-X. At this time, ASA provides less mesoporous and higher specific area. When the content of Y-type zeolite is 70%, the total specific area is maximum. This means that the mesoporous in USY@A-X is mainly provided by ASA (see Table 2).

3.3. Morphology analysis

SEM analysis of USY@A-X and the small-crystal NaY-type zeolite is showed in Fig. 6. It is revealed that the surface of USY@A-X is very different from the surface of NaY-type zeolite. In contrast with the small-crystal NaY-type zeolite, the surface of USY@A-X is wrapped by ASA. When the content of Y-type zeolite is up to 90%, its surface is still wrapped by ASA.

Fig. 6 SEM image of the USY@A composites and the small-crystal Y zeolite.

The ASA coated status on the surface of USY@A-X and NiW/USY@A-X is examined by TEM. As seen from Fig. 7, whether 50% ASA content of USY@A-50 or 10% ASA content of USY@A-90, the surface of Y-type zeolite is covered with ASA. NiW/USY@A-X is also the case. It's obviously different from the work of Meng et al.¹⁹ Y@A composites, which is prepared by this research, has typical “core–shell” structure, which are microporous of regular shape Y-type zeolite as core and mesoporous amorphous silicon aluminium as shell.

Fig. 7 TEM image of the samples.

3.4. Texture properties of catalyst

Texture properties of NiW/USY@A-X and NiW/USY-In catalysts are listed in Table 3. The total specific area of NiW/USY@A-X catalysts is higher than that of NiW/USY-In catalyst. The total pore volume of NiW/USY@A-X catalysts is a little higher than that of NiW/USY-In catalysts, while its mean pore size is but much higher than NiW/USY-In catalyst. It suggests that NiW/USY@A-X catalysts inherit large mesoporous properties of USY@A-X composites. The wider channel of NiW/USY@A-X catalysts will be beneficial for the diffusion of reagent and product.

Table 3 Surface area and pore size of the NiW/USY@A and NiW/USY-In catalysts

Samples	Surface area (m^2 g^−1)			Pore volume (cm^3 g^−1)			dmean^e (nm)
	Stotal	Smicro^a	Smeso^b	Vtotal	Vmicro^c	Vmeso^d
a t-Plot micropore surface area.b t-Plot mesopore surface area.c BJH micropore volume.d BJH mesopore volume.e Pore size range 1–100 nm.
NiW/USY@A-50	187	32	156	0.36	0.09	0.27	8.08
NiW/USY@A-70	178	43	135	0.36	0.08	0.28	8.02
NiW/USY@A-90	171	30	141	0.35	0.08	0.27	8.26
NiW/USY-In	134	76	58	0.31	0.06	0.25	3.92

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3.5. Acidity characterization

Acidity property of catalysts is measured by in situ adsorption infrared spectroscopy using pyridine as adsorbent, as shown in Fig. 8. Here 1450 cm⁻¹ peak represents Lewis acid and 1540 cm⁻¹ peak represents Brønsted acid. Acid below 200 °C and 350 °C represent total acid and medium and strong acid, respectively.²² Both peak intensities at 1450 cm⁻¹ and 1540 cm⁻¹ for NiW/USY@A-X catalysts at 200 °C and 350 °C are all higher than NiW/USY-In catalyst. It suggests that the acid amounts of NiW/USY@A-X catalysts are all higher than NiW/USY-In catalyst.

Fig. 8 IR spectra of pyridine desorbed from the catalysts at (A) 200 °C, (B) 350 °C.

Based on the IR peak intensity, Brønsted acid, Lewis acid and total acid are calculated,²³ as listed in Table 4. Total Brønsted acid, total Lewis acid and total acid of NiW/USY@A-X catalysts are much higher than NiW/USY-In catalyst. The strong Brønsted acid, strong Lewis acid and total strong acid of NiW/USY@A-X catalysts also follow this trend. Acid amount of NiW/USY@A-70 catalyst is a little higher than NiW/USY@A-50 catalyst and NiW/USY@A-90 catalyst, showing a maximum. Because of the equivalent Y-type zeolite content in catalysts, this different acid amount is due to the new acid sites in the interface of Y–ASA, which are from the Y–ASA interactions.¹⁹ When the content of Y-type zeolite in composites is less, USY provides less acid amount. New acid sites in the interface of Y–ASA are also less. ASA can provide more acid amount, but its acidic strength is weaker. It leads to less total acid amount. By contrast, USY can provide more acid amount when content of Y-type zeolite is high, but ASA only provides less acid amount. It makes total acid decrease. When the content of Y-type zeolite is 70%, USY and ASA can provide maximum acid. This causes that the acid amount of NiW/USY@A-70 catalyst has a maximum.

Table 4 Amount and distribution of the acid sites of the catalysts

Catalysts	Amount (μmol g^−1) and distribution of acid sites
	Total acid (200 °C)				Medium and strong acid (350 °C)
	Brönsted	Lewis	B + L	B/L	Brönsted	Lewis	B + L	B/L
NiW/USY@A-50	71	210	281	0.34	32	169	201	0.19
NiW/USY@A-70	90	224	314	0.40	56	203	259	0.27
NiW/USY@A-90	84	214	298	0.39	47	194	241	0.24
NiW/USY-In	63	180	243	0.35	30	123	153	0.25

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3.6. Catalyst testing

n-Decane is often used as the model compound of hydrocracking reaction.^24–26 All previous literatures deal with only catalyst activity and intermediate products selectivity. But the more isomerization products in the hydrocracking products, the better the properties of obtained oil (such as gasoline and diesel) are. This research mainly focuses on the selectivity and isomerization performance of catalysts and controls the conversion rate of n-decane above 90% to make comparative study, as shown in Table 5.

Table 5 The hydrocracking of n-decane over NiW/USY@A-X and NiW/USY-In catalyst^a

Catalyst	NiW/USY@A-50	NiW/USY@A-70	NiW/USY@A-90	NiW/USY-In
a Reaction condition: 4 MPa H2 pressure, 360 °C reaction temperature, 400 H2 to oil volume ratio, 3.0 h^−1 volume velocity.
n-Decane conversion (%)	94.00	97.50	96.90	97.00
Product distribution (%)
C2	0.11	0.11	0.09	—
C3	7.20	6.49	5.33	13.27
C4	23.40	11.56	17.32	36.64
C5	10.33	9.50	8.80	29.44
C6	22.23	39.86	36.55	13.88
C7	25.73	26.36	25.19	3.15
C8	1.23	2.22	2.15	—
C9	0.13	0.06	0.06	—
C10	2.35	1.33	1.38	1.05
n-Alkanes selectivity (%)	20.83	24.91	16.39	22.75
Iso-paraffin selectivity (%)	72.96	72.59	80.46	56.76
C5–C9 selectivity (%)	60.72	78.00	77.28	46.47
Iso-decane selectivity (%)	2.49	1.36	1.42	1.08

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The content of Y-type zeolite in USY@A-X composites lies in the range of 50–90%, and the selectivity of C₅–C₉ products and isoparaffin are obviously higher than NiW/USY-In catalyst. The selectivity of C₅–C₉ products is determined mainly by catalyst texture properties. Combining with the Table 4, it suggests that NiW/USY@A catalysts provide the wide channel for n-decane and hydrocracking products. It prevents the secondary cracking and makes the selectivity of C₅–C₉ products on NiW/USY@A-X catalysts be over 15.84% higher than NiW/USY-In catalyst. The isoparaffin selectivity is dependent on catalyst acidic properties. Acid amount of NiW/USY@A-X catalyst is higher than NiW/USY-In catalyst (seen from Table 4). Especially, more Brönsted acid sites make the isoparaffin selectivity be improved over 14.25%. Furthermore, hydrocracking reaction and isomerization reaction belong to carbanium ion mechanism.^27,28 The more Brönsted acid and the stronger Brönsted acidic property, the more easy hydrocracking reaction and isomerization reaction is. It results in that NiW/USY@A-X catalyst has much higher isoparaffin selectivity.

4. Conclusions

The “core–shell” structure Y@A composites are synthesized by ASA cladding Y-type zeolite with the small-crystal NaY-type zeolite as material and CTAB as surfactant. Mesoporous ASA distributes uniformly on the surface of Y-type zeolite. Due to the interaction of Y-type zeolite and ASA, new acid sites are generated on the “core–shell” interface. At the same time, ASA provides wide channel for the diffusion of reactants and products, which makes catalysts have excellent selectivity of intermediate products and isomerization products. The research of Y@A composites presents a possibility to provide a new carrier material for hydrocracking catalyst.

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