A. Synthesis and characterization

Depolymerization of the presynthesized zeolite crystals in basic medium is a key aspect to attain the aimed zeolite–zeolite composites.^{Reference Zheng, Zeng, Yi, Wang, Ma, Qin, Zhang, Sun and Li10,Reference Zheng, Zeng, Zhang, Wang, Ma, Zhang, Sun and Li11} The metastable ZSM-5 zeolite framework in alkaline solutions can provide partial nutrients for the growth of Y zeolite during the second-step synthesis. Figure 1 shows the XRD patterns of the as-synthesized MFI–FAU zeolite–zeolite composite with the different second-step crystallization time. The peaks at 6.16°, 10.08°, and 20.24° are the characteristic diffraction peaks of FAU zeolite phase,^{Reference Yee and Yaacob28} and the peaks at 7.90°, 8.75°, and 23.00° are the characteristic peaks of MFI zeolite phase.^{Reference Jin, Cui, Rui and Li29} The above-mentioned results indicate the co-existence of Y and ZSM-5 zeolite phases in the as-synthesized samples. It can be seen from Fig. 1 that prolonging hydrothermal treatment time resulted in the decreased characteristic diffraction peaks of ZSM-5 zeolite while the enhanced characteristic peaks of Y zeolite, suggesting that the preparation procedure of MFI–FAU is a process of MFI structure transferred into FAU structure.

FIG. 1.

XRD patterns of zeolite–zeolite composites MFI–FAU(t) prepared with the different crystallization time: (a) MFI–FAU(12); (b) MFI–FAU(15); (c) MFI–FAU(18); (d) MFI–FAU(21).

When the crystallization time was shorter than 12 h, the mass fraction of Y zeolite phase in the as-synthesized zeolite–zeolite composite is negligible because the time is not enough to provide solubilization of silicate to promote crystallization of Y zeolite due to the silicon species can only be obtained from the depolymerization of ZSM-5 zeolite crystals. The result also indicates that the growth of Y zeolite crystals may be lagged after the depolymerization of ZSM-5 zeolite. When the crystallization time was 12–21 h, zeolite–zeolite composites with different ratios of Y to ZSM-5 were obtained. Perhaps the degradation of ZSM-5 crystals by the synthesis solution is such as to provide enough solubilized silicon species^{Reference Zheng, Zeng, Yi, Wang, Ma, Qin, Zhang, Sun and Li10,Reference Zheng, Zeng, Zhang, Wang, Ma, Zhang, Sun and Li11} to accelerate the growth of Y zeolite. However, when the crystallization time was further prolonged, such as over 30 h, a product abundant in Y zeolite commingled with P-type zeolite was obtained, and the characteristic peaks of ZSM-5 zeolite phase may completely disappear (not shown here). Extensive dissolution of ZSM-5 zeolite crystals makes the synthesis system to deviate from the crystallization field of the given zeolite Y.^{Reference Zheng, Zeng, Yi, Wang, Ma, Qin, Zhang, Sun and Li10,Reference Zheng, Zeng, Zhang, Wang, Ma, Zhang, Sun and Li11} As a consequence, zeolite–zeolite composites MFI–FAU are difficult to be obtained.

Crystal morphology of the zeolite–zeolite composite MFI–FAU(t) differs from those of Y and ZSM-5 zeolite crystals. As shown in Fig. 2, ZSM-5 and Y zeolite crystals show the block-like crystal particles^{Reference Teng, Wang, Chen, Liu and Wang14} and octahedral^{Reference Zheng, Zeng, Yi, Wang, Ma, Qin, Zhang, Sun and Li10,Reference Zheng, Zeng, Zhang, Wang, Ma, Zhang, Sun and Li11,Reference Ren, Bai, Sun, Zhang and Fan13,Reference Jia, Sun, Ye, Zou, Gu, Huang, Niu and Zhao17} morphology, respectively. Either ZSM-5 or Y zeolite crystal displays a smooth external surface, and their sizes are about 10 μm × 10 μm × 30 μm and 1 μm, respectively. The seeds of Y zeolite in the dry powder display highly scattered nanosized particles with the size range from 10–70 nm [Fig. 2(c)]. It can be seen from Figs. 2(d)–2(k) that the composite MFI–FAU(t) samples have rougher surface because of the inlaid Y zeolite crystals on the external surface of ZSM-5 zeolite crystals. MFI–FAU(12) displays a rather coarse surface as shown in Figs. 2(d) and 2(e), the inlaid particles on MFI–FAU(12) still seem like the seeds of Y zeolite as shown in Fig. 2(c). Moreover, some faujasite crystals with a size of about 300–500 nm are clearly visible as shown in Fig. 2(e). It can be seen from Figs. 2(f)–2(i) that the external surfaces of MFI–FAU(15) and MFI–FAU(18) are decorated by polycrystalline aggregations, which are composed of about 1–2 μm flower-like crystals, and the flower-like crystals further consist of 100–300 nm faujasite crystals. Figure 2 also suggests that a longer hydrothermal treatment time leads to a thicker Y zeolites shells layer. Figures 2(j) and 2(k) exhibit that the ZSM-5 zeolite crystal in MFI–FAU(21) has been entirely covered by a polycrystalline shell composed of nanosized faujasite crystals with a size about 150–200 nm. The nanosized faujasite crystals in the zeolite–zeolite composite can be caused by the steric hindrance provoked by the concurrently growing crystals in the polycrystalline zeolite shells layer.^{Reference Zheng, Zeng, Zhang, Wang, Ma, Zhang, Sun and Li11,Reference Bouizi, Rouleau and Valtchev25,Reference Bouizi, Diaz, Rouleau and Valtchev26}

FIG. 2.

SEM images: (a) pure Y zeolite; (b) pure ZSM-5 zeolite; (c) the as-synthesized seeds of Y zeolite; (d) and (e) MFI–FAU(12); (f) and (g) MFI–FAU(15); (h) and (i) MFI–FAU(18); (j) and (k) MFI–FAU(21); (l) and (m) MFI–FAU(18), which was crashed by 15 MPa pressure before tested by SEM.

Chemical composition of a given zeolite is an important characteristic, which defines its properties.^{Reference Zheng, Zeng, Zhang, Wang, Ma, Zhang, Sun and Li11} EDS analysis of zeolite–zeolite composites MFI–FAU(t) indicates that the SiO₂/Al₂O₃ ratios of the flower-like crystals and the block-like crystal particles are 4.8–6.6 and 30–33, respectively, close to the 3.2 of Y zeolite and the 40 of ZSM-5 zeolites used in the experiments, respectively. The results reveal that the as-synthesized zeolite–zeolite composites are composed of ZSM-5 cores and Y zeolites shells. The EDS analysis also exhibits that the ZSM-5 zeolite phase in the composite has a decreased Si/Al ratio with the increased hydrothermal treatment time during the second-step synthesis. As shown in Fig. 3, the ratios of SiO₂/Al₂O₃ of the ZSM-5 phase in the zeolite–zeolite composites MFI–FAU(12), MFI–FAU(15), MFI-FAU(18), and MFI–FAU(21) are 32, 33, 33, and 30, respectively, all lower than the 40 of the starting materials ZSM-5 zeolite. Moreover, the EDS analysis also suggests that the Y zeolite crystal in the zeolite–zeolite composites displays a relatively higher Si/Al ratio of about 4.8–6.6. The ratios of SiO₂/Al₂O₃ of Y zeolites phase in MFI–FAU(12), MFI–FAU(15), MFI–FAU(18), and MFI–FAU(21) are 6.4, 6.2, 4.8, and 6.6, respectively. The aforementioned result strongly suggests that the silicon species have been transferred from the ZSM-5 zeolite cores to the Y zeolite shells.^{Reference Zheng, Zeng, Zhang, Wang, Ma, Zhang, Sun and Li11} The transforming process of silicon species from the ZSM-5 zeolite crystals to the Y zeolites crystals guarantees the formation of the target zeolite–zeolite composite samples. The extraction and transmission of silicon species during the second-step synthesis play an important role for the formation of MFI–FAU zeolite–zeolite composite.^{Reference Zheng, Zeng, Yi, Wang, Ma, Qin, Zhang, Sun and Li10} It is short of silicon species for the growth of Y zeolite crystals during the second-step hydrothermal crystallization, the silicon species needed for the growth of Y zeolite can only be obtained from the depolymerization of ZSM-5 zeolite.

FIG. 3.

EDS analysis of the as-synthesized zeolite–zeolite composites: (a) MFI–FAU(12); (b) MFI–FAU(15); (c) MFI–FAU(18); (d) MFI–FAU(21). The as-synthesized samples were crashed by 15 MPa pressure so as to discover its nature structure before the EDS analysis.

As shown in Figs. 2(l) and 2(m), a reverse growth^{Reference Greer, Wheatley, Ashbrook, Morris and Zhou30,Reference Zeng, Bai, Zheng, Chen and Li31} of Y in ZSM-5 cores is also found in MFI–FAU(18) sample, which was crashed by 15 MPa pressure before tested by SEM, suggesting that the Y zeolite phase in MFI–FAU is also partially pregnant in ZSM-5 crystals. It is known that Al gradient affects and controls the Si extraction throughout the zeolite particles.^{Reference Groen, Zhu, Brouwer, Huynink, Kapteijn, Moulijn and Pérez-Ramírez32,Reference Groen, Bach, Ziese, van Donk, de Jong, Moulijn and Pérez-Ramírez33} In an alkaline solution, Si extraction from ZSM-5 zeolite crystals favorably occurs in the aluminum-poor bulk rather than the aluminum-rich external surface,^{Reference Groen, Zhu, Brouwer, Huynink, Kapteijn, Moulijn and Pérez-Ramírez32,Reference Groen, Bach, Ziese, van Donk, de Jong, Moulijn and Pérez-Ramírez33} due to a high framework, Al concentration prevents Si extraction. However, a low-Al concentration may lead to excessive extraction and formation of hollow zeolite crystals. During the second-step synthesis, a preferential dissolution of the aluminum-poor center results in the formation of hollow ZSM-5 crystals with a relatively preserved outer surface.^{Reference Zeng, Bai, Zheng, Chen and Li31–Reference Zheng, Zeng, Ma, Zhang, Sun and Li34} Under the direction of seeds of Y which are rather tinny as shown in Fig. 2(c), the extracted silicon species and the supplemented aluminum species, which diffuse from the synthesis solution through the ZSM-5 zeolite outer rim, promote the growth of Y zeolite crystals within the hollow ZSM-5 zeolite.^{Reference Zeng, Bai, Zheng, Chen and Li31} This allows us to propose a new formation mechanism of the zeolite–zeolite composite MFI–FAU and a new phase transformation mechanism from MFI structure to FAU structure. The formation mechanism of the zeolite–zeolite composite MFI–FAU(t) is different from that of overgrowth^{Reference Okamoto and Osafune21,Reference Bouizi, Rouleau and Valtchev25,Reference Bouizi, Diaz, Rouleau and Valtchev26} or intergrowth,^{Reference Ohsuna, Terasaki, Nakagawa, Zones and Hiraga35} and is suggested in Scheme 1. Y zeolite nanoseeds are first adsorbed on the external surface of ZSM-5 zeolite crystal due to its higher surface area,^{Reference Zheng, Zeng, Ma, Zhang, Sun and Li34,Reference Dutta and Bronic36} which is a very important step for the formation of the zeolite–zeolite composites [Scheme 1. (a) → (b)]. Meanwhile, Si species are extracted from the framework of ZSM-5 zeolite by alkaline etching action of the synthesis solution^{Reference Groen, Moulijna and Pérez-Ramírez1,Reference Pérez-Ramírez, Christensen, Egeblad, Christensend and Groen2,Reference Yang, Tian, Chen, Li, Liu, Deng, Van Tendeloo and Su6,Reference Milina, Mitchell, Crivelli, Cooke and Pérez-Ramírez24} [Scheme 1. (b) → (c)]. Secondly, the nanoseeds gradually grow up because of the interaction between aluminum species from the synthesis solution and silicon species extracted from ZSM-5 zeolite crystals,^{Reference Zheng, Zeng, Zhang, Wang, Ma, Zhang, Sun and Li11} resulting in the formation of polycrystalline Y zeolite around ZSM-5 zeolite crystals, and ZSM-5 zeolite crystals are therefore embedded into the polycrystalline Y zeolite layers^{Reference Okamoto and Osafune21} [Scheme 1. (c) → (d)]. Subsequently, the polycrystalline Y zeolite shells layer composed of small Y zeolite crystals becomes thicker and thicker along with the extracted process of silicon species from the ZSM-5 zeolite [Scheme 1. (d) → (e)]. With the extraction process of Si species, a hierarchical pore system^{Reference Groen, Moulijna and Pérez-Ramírez1,Reference Pérez-Ramírez, Christensen, Egeblad, Christensend and Groen2,Reference Yang, Tian, Chen, Li, Liu, Deng, Van Tendeloo and Su6,Reference Zheng, Zeng, Yi, Wang, Ma, Qin, Zhang, Sun and Li10,Reference Zheng, Zeng, Zhang, Wang, Ma, Zhang, Sun and Li11,Reference Milina, Mitchell, Crivelli, Cooke and Pérez-Ramírez24} is also introduced into the ZSM-5 zeolite crystal [Scheme 1. (b) → (e)]. Moreover, in an alkaline solution, Si-species extracted from ZSM-5 favorably occurs in the aluminum poor bulk rather than the aluminum-rich external surface,^{Reference Zeng, Bai, Zheng, Chen and Li31–Reference Groen, Bach, Ziese, van Donk, de Jong, Moulijn and Pérez-Ramírez33} which results in the formation of partial ZSM-5 hollow crystals. The supplemented aluminum species and highly scattered nanoseeds of Y zeolite diffuse from the synthesis solution through the ZSM-5 zeolite outer rim and promote the formation of Y zeolite crystals within the hollow ZSM-5 zeolite^{Reference Zeng, Bai, Zheng, Chen and Li31} [Scheme 1. (d) → (f)].

SCHEME 1.

Schematic representation of the process for the formation of zeolite–zeolite composite MFI–FAU(t).

Figure 4 is the FT-IR spectra of the different samples. With a prolonged crystallization time, the band at 547 cm⁻¹, which is attributed to the five member ring vibrations of ZSM-5 zeolite, is gradually weakened, while the band at 574 cm⁻¹ attributed to the hexatomic ring vibrations^{Reference Kosanović, Havancsák, Subotić, Svetličić, Radić, Cziráki, Huhn, Buljan and Smrečki37} of Y zeolite crystals is gradually enhanced. The result implies that the framework of ZSM-5 zeolite is subjected to damaging along with formation of Y zeolite phase. That also suggests that the formation of the zeolite–zeolite composite is a gradual change process of MFI structure transformed into FAU structure.^{Reference Wang, Liu, Wang, Xie and Tang38} It can be seen from Fig. 1 that the mass fraction of Y zeolite phase in MFI–FAU(12) is negligible although the characteristic diffraction peaks of ZSM-5 zeolite have been dramatically decreased. It can be observed from Fig. 4 that the bands around 574 and 1031 cm⁻¹, which respectively ascribed to the hexatomic ring vibrations and T-O-T antisymmetric stretching vibrations of Y zeolite crystals, are obviously observed in FT-IR spectra of MFI–FAU(12). The result suggests that the shorter crystallization time such as lower than 12 h, Y zeolite framework has practically been formed, and the negligible characteristic diffraction peaks of Y zeolite as shown in Fig. 1 can be therefore caused by the too small crystals to be detected by XRD. The vibration bands between 1000 and 1100 cm⁻¹ are often used to identify the Si/Al ratio of the framework.^{Reference Morsli, Driole, Cacciaguerra, Arletti, Chiche, Hamidi, Bengueddach, Quignard and Di Renzo39} It can be seen from Fig. 4 that the band ascribed to the T-O-T antisymmetric stretching vibrations band of Y zeolite phase in the as-synthesized composite samples shifts from 1031 cm⁻¹ of MFI–FAU(12), to 1027 cm⁻¹ of MFI–FAU(15), 1023 cm⁻¹ of MFI–FAU(18), and to the 1019 cm⁻¹ of MFI–FAU(21), implying that the Si/Al ratio of the Y zeolite phase in the composite exhibits a decreased trend with the prolonged hydrothermal treatment time during the second-step synthesis. This is largely in agreement with the results observed by EDS as shown in Fig. 3.

FIG. 4.

FT-IR spectra of the different samples: (a) MFI–FAU(12); (b) MFI–FAU(15); (c) MFI–FAU(18); (d) MFI–FAU(21).

As shown in Fig. 5, the adsorption–desorption of nitrogen on Y or ZSM-5 zeolite is a type-I isotherm, indicating the presence of micropores only.^{Reference Zheng, Zeng, Yi, Wang, Ma, Qin, Zhang, Sun and Li10,Reference Zheng, Zeng, Zhang, Wang, Ma, Zhang, Sun and Li11} However, curves combining type-I and type-IV isotherms are observed for MFI–FAU(t) samples. Larger hysteresis loops occur after p/p ₀ ≈ 0.45 in the adsorption–desorption isotherms of the zeolite–zeolite composite samples, indicating not only the presence of mesopores but also a broad pore size distribution.^{Reference Jin, Cui, Rui and Li29,Reference Chen, Lee, Zhang and Lu40} The mesopores created in the zeolite–zeolite composite can be ascribed to the silicon extraction^{Reference Groen, Moulijna and Pérez-Ramírez1,Reference Pérez-Ramírez, Christensen, Egeblad, Christensend and Groen2,Reference Yang, Tian, Chen, Li, Liu, Deng, Van Tendeloo and Su6,Reference Zheng, Zeng, Yi, Wang, Ma, Qin, Zhang, Sun and Li10,Reference Milina, Mitchell, Crivelli, Cooke and Pérez-Ramírez24,Reference Jin, Cui, Rui and Li29} from ZSM-5 zeolite crystals by the alkaline synthesis solution and the polycrystalline accumulations of nanosized Y zeolite crystals in the shells. As shown in Fig. 5(inset), the pore size distribution of the MFI–FAU composites obtained from the adsorption branch of the isotherm illustrates the existence of a mesopore structure with pore sizes centering at about 10 nm. Moreover, significant increase in N₂ adsorption is also observed from p/p ₀ ≈ 0.8 for the as-synthesized samples. According to the SEM results mentioned above, zeolite–zeolite composite MFI–FAU(t) displays agglomerates of nanosized Y crystals. The sharp condensation in the range of p/p ₀ ≈ 0.8 in N₂ adsorption of MFI–FAU(18) and MFI–FAU(21) can therefore be mainly attributed to capillary condensation^{Reference Bjøgen, Joensen, Holm, Olsbye, Lillerud and Svelle41} in open mesopores obtained by filling the interparticle spaces.^{Reference Simon-Masseron, Marques, Lopes, Ramô Ribeiro, Gener and Guisnet42} However, the BJH pore size distribution derived from the adsorption branch does not show a similar distribution in the corresponding pure Y or ZSM-5 as shown in Fig. 5 (inset).

FIG. 5.

N₂ adsorption–desorption isotherms and the corresponding BJH pore size distribution curves (inset) of the as-synthesized zeolite–zeolite composites MFI–FAU(t) and the references pure Y and ZSM-5 zeolites.

Some N₂ adsorption–desorption data are presented in Table I. MFI–FAU(18) has a BET surface area of 491 m²/g, lower than the 545 m²/g of the corresponding physical mixture with the similar Y/ZSM-5 ratio and similar chemical composition. Decreased Brunauer-Emmett-Teller (BET) surface area in the composites can be caused by the dissolution and partial amorphization of the starting ZSM-5 zeolite framework.^{Reference Zheng, Zeng, Yi, Wang, Ma, Qin, Zhang, Sun and Li10,Reference Zheng, Zeng, Zhang, Wang, Ma, Zhang, Sun and Li11} It can be inferred from Table I that a prolonged hydrothermal treatment time leads to an increased BET surface area in the composites. Obviously, the BET surface areas, microporous surface areas, and volume of the as-synthesized samples are decided by both of the dissolution of ZSM-5 and the growth of Y. The growth of Y zeolite crystals can offset the effect of the dissolution of ZSM-5 zeolite crystals on the microporous surface areas and volume as well as the BET surface areas in the composites. Table I also shows that the mesoporous areas and volumes in the composites largely depend on the crystallization time, the longer treatment time, the larger mesoporous areas or volumes.

TABLE I.

Physical structural properties of samples.

^a MFI + FAU is the corresponding physical mixture of sample MFI–FAU(18).

The zeolite–zeolite composite samples as well as the corresponding physical mixture, Y and ZSM-5 zeolites were investigated by NH₃-TPD of ammonia experiments. As shown in Fig. 6, the TPD profile of pure Y zeolite displays two desorption peaks with maxima in the temperature regions at about 397 and 614 K; while ZSM-5 exhibits two desorption peaks at about 400 and 697 K. The TPD profile of zeolite–zeolite composite shows three desorption peaks with a distribution at 641–661, 442–473, and 380–385 K, which can be assigned to the desorption of NH₃ from medium–strong and weak acid sites, respectively. It can be seen from Fig. 6 that the peak corresponding to the medium–strong acid sites in the curves of the TPD profiles shifts from 670 K of the corresponding physical mixture to the 641–661 K of the zeolite–zeolite composites, while the low-temperature desorption peak corresponding the weak acid sites shifts from 400 K of the physical mixture to the 380–385 K of the composite samples. The strength of the weak and the medium–strong acid sites in the composite samples is obviously weakened, which can be ascribed to the partial depolymerization of the framework of MFI zeolite phase in the zeolite–zeolite composites during the second-step synthesis. Desorption peak at about 442–473 K should be contributed to the formation of a new acid site^{Reference Zhang, Fan, Liu and Li20} resulted from the depolymerization of the ZSM-5 zeolite framework.^{Reference Zheng, Zeng, Yi, Wang, Ma, Qin, Zhang, Sun and Li10,Reference Zheng, Zeng, Zhang, Wang, Ma, Zhang, Sun and Li11} With an increased content of Y zeolite phase, the peak corresponding to ether the medium–strong acid or the weak acid shifts to the lower temperature. The peak corresponding to medium–strong acid sites shifts from 661 K of MFI–FAU(12) to 641 K of MFI–FAU(18) and MFI–FAU(21); while the peak related with weak acid sites shifts from 385 K of MFI–FAU(12) to the 380 K of MFI–FAU(21).

FIG. 6.

NH₃-TPD curves of the as-synthesized zeolite–zeolite composites MFI–FAU(t), the pure Y, and ZSM-5 zeolite samples.

B. Catalytic tests

In the present study, catalytic cracking of isopropylbenzene was selected to investigate the catalytic performances of MFI–FAU(t) and compared with Y, ZSM-5, and the corresponding physical mixture MFI + FAU with the similar Si/Al and Y/ZSM-5 ratios to those of MFI–FAU(18). It can be seen from Fig. 7 that Y zeolite has a higher conversion of isopropylbenzene than ZSM-5 because of its higher medium–strong acid sites as shown in Fig. 6. The larger micropores channel in Y zeolite may attribute to a faster diffusion for the cracking products, which also contributes to enhancing the conversion of isopropylbenzene. Figure 7 also shows that the physical mixture MFI + FAU has a better catalytic performance than the single Y or ZSM-5 during the isopropylbenzene catalytic cracking, which can be ascribed to the synergistic effect^{Reference Guo, Xiong, Huang and Li22,Reference Huang, Guo, Deng, Xue and Li23,Reference Dzikh, Lopes, Lemos and Ribeiro43,Reference Dzikh, Lopes, Lemos and Ribeiro44} of the different zeolite catalysts.

FIG. 7.

Catalytic cracking of isopropylbenzene over the zeolite–zeolite composite MFI–FAU(t), pure Y, pure ZSM-5, and the physical mixture MFI + FAU catalysts at 573 K.

It is well known that acid property of zeolite plays an important role in catalytic cracking performance. Obviously, MFI–FAU(t) zeolite–zeolite composites have different acid properties, catalytic activities, and stabilities. However, the catalytic activities and stabilities of the catalysts do not vary monotonously with the acid amounts and distributions. As shown in Fig. 7, it is difficult to observe the remarkable differences about the initial conversion of isopropylbenzene over the catalyst MFI–FAU(18) and the corresponding physical mixture MFI + FAU. However, as compared to MFI–FAU(18), the physical mixture MFI + FAU shows relatively lower stability with the prolonged reaction time. The conversion of isopropylbenzene over the physical mixture decreases from 98% at 0.1 h to about 50% at 4.5 h. While the conversion of isopropylbenzene over the zeolite–zeolite composite MFI–FAU(18) catalysts remains relatively stable. After 7 h, a 70% conversion of isopropylbenzene can still be obtained over the catalyst MFI–FAU(18). Severe diffusion limitation^{Reference van Donk, Janssen, Bitter and de Jong4,Reference Su, Sanchez and Yang7,Reference Zheng, Zeng, Yi, Wang, Ma, Qin, Zhang, Sun and Li10,Reference Zheng, Zeng, Zhang, Wang, Ma, Zhang, Sun and Li11,Reference Groen, Zhu, Brouwer, Huynink, Kapteijn, Moulijn and Pérez-Ramírez32,Reference Groen, Bach, Ziese, van Donk, de Jong, Moulijn and Pérez-Ramírez33} in the physical mixture catalyst composed of purely microporous zeolites causes a quick deactivation due to carbon deposition; Moreover, MFI–FAU(21) with more mesopores gives the catalyst a higher catalytic activity and a higher stability than MFI–FAU(18) with relatively less mesopores, which confirms that the mesopores play an important role in isopropylbenzene catalytic cracking. Similar results were also observed by several groups.^{Reference Zhang, Qu, Wang, Zhang and Liu45–Reference Diao, Wang, Zhang and Liu47}

The amount of the carbonaceous deposits formed on MFI–FAU(t) and the corresponding physical catalysts MFI + FAU after approximately 7 h on stream were studied by TG characterizations. The TG curves as shown in Fig. 8 reveal for both catalysts, a substantial weight loss in the temperature range of 400–630 °C, that correspond to the burning-off of the coke deposits.^{Reference Martínez, Peris, Derewinski and Burkat-Dulak48} The weight loss in that range is 1.1% for MFI–FAU(18) and 1.7% for the corresponding physical mixture catalysts MFI + FAU reflecting a higher amount of carbonaceous deposits in the later. Coke formed much more rapidly in the purely microporous MFI + FAU reference catalyst than in the hierarchical zeolite–zeolite composite MFI–FAU(18) (12 mg/g in the composite catalyst and approximately 17 mg/g in the corresponding physical mixture catalyst) during the initial reaction period up to 7 h.

FIG. 8.

TG analyses of coked catalysts after being used for catalytic cracking of isopropylbenzene during approximately 7 h.

It is reasonable to assume that, therefore, the introduced hierarchical pore system^{Reference Groen, Moulijna and Pérez-Ramírez1,Reference Pérez-Ramírez, Christensen, Egeblad, Christensend and Groen2,Reference Yang, Tian, Chen, Li, Liu, Deng, Van Tendeloo and Su6,Reference Su, Sanchez and Yang7,Reference Zheng, Zeng, Yi, Wang, Ma, Qin, Zhang, Sun and Li10,Reference Zheng, Zeng, Zhang, Wang, Ma, Zhang, Sun and Li11,Reference Jin, Cui, Rui and Li29} may play a crucial role in the catalytic performances. The enhanced stabilities observed in the zeolite–zeolite composite catalysts can be ascribed to the introduced mesopore or macropore structure, which avails the faster diffusion^{Reference Zheng, Zeng, Zhang, Wang, Ma, Zhang, Sun and Li11,Reference Groen, Zhu, Brouwer, Huynink, Kapteijn, Moulijn and Pérez-Ramírez32,Reference Groen, Bach, Ziese, van Donk, de Jong, Moulijn and Pérez-Ramírez33} of products from the active centers in its channel and depresses the carbon deposition as observed in TG experiment. The hierarchical pores system created in the zeolite–zeolite composites also gives the reactants easy access to the acid sites, especially the Brönsted-acid sites.^{Reference Diao, Wang, Zhang and Liu47,Reference Zheng, Zhang, Zhang, Ma and Li49} That also contributes to giving the catalyst a relatively high activity and stability.

The excellent catalytic performance of MFI–FAU during the catalytic cracking of isopropylbenzene can also be partially attributed to the special structure of the zeolites–zeolite composite. The transmission distance of the reactant between the two active components of Y and ZSM-5 is dramatically shortened as compared with the physical mixture of Y and ZSM-5 zeolite crystals. The special structure of ZSM-5 overgrown with Y (in most cases) guarantees a positive transmission way of reactant from the relatively larger micropore channels of Y zeolites (0.74 nm) to the relatively smaller micropore channels of ZSM-5 zeolite (0.54 nm) that attributes to affording a hierarchically catalytic^{Reference Yang, Tian, Chen, Li, Liu, Deng, Van Tendeloo and Su6,Reference Na and Somorjai50–Reference Zhao, Qin, Zheng, Du, Sun, Ling, Zhang and Li54} cracking process for the relatively larger reactant molecular. Reactant can be first cracked over Y zeolite crystals and the precracked products can be recracked over ZSM-5 zeolite crystals with the relatively small pore channels. That contributes to enhancing the activity and stability of the zeolite–zeolite composite catalysts. However, the transmission distance and transmission way of the reactant in the purely microporous physical mixture MFI + FAU are random and uncertain. Composites MFI–FAU may have a potential application in the heavy oil cracking procedure because of the adjustable ratios of Y to ZSM-5 and the created hierarchical pores system, which may improve the properties of the cracking products. For example, the oil/olefin ratio in the products of heavy oil cracking can be tunable by using the zeolite–zeolite composites with the adjustable ratio of Y to ZSM-5 as a cracking catalyst.