Chinese Journal of Chemical Physics  2018, Vol. 31 Issue (4): 393-403

#### The article information

Xin-hua Gao, Qing-xiang Ma, Tian-sheng Zhao, Jun Bao, Noritatsu Tsubaki

Recent Advances in Multifunctional Capsule Catalysts in Heterogeneous Catalysis

Chinese Journal of Chemical Physics, 2018, 31(4): 393-403

http://dx.doi.org/10.1063/1674-0068/31/cjcp1805129

### Article history

Accepted on: June 14, 2018
Recent Advances in Multifunctional Capsule Catalysts in Heterogeneous Catalysis
Xin-hua Gaoa, Qing-xiang Maa, Tian-sheng Zhaoa, Jun Baob, Noritatsu Tsubakic
Dated: Received on May 31, 2018; Accepted on June 14, 2018
a. State Key Laboratory of High-Efficiency Utilization of Coal and Green Chemical Engineering, College of Chemistry & Chemical Engineering, Ningxia University, Yinchuan 750021, China;
b. National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230029, China;
c. Department of Applied Chemistry, School of Engineering, University of Toyama, Gofuku 3190, Toyama 930-8555, Japan
*Author to whom correspondence should be addressed. Noritatsu Tsubaki, E-mail:tsubaki@eng.u-toyama.ac.jp
Abstract: Capsule catalysts composed of pre-shaped core catalysts and layer zeolites have been widely used in the tandem reactions where multiple continuous reactions are combined into one process. They show excellent catalytic performance in heterogeneous catalysis, including the direct synthesis of middle isoparaffins or dimethyl ether from syngas, as compared to the conventional hybrid catalysts. The present review highlights the recent development in the design of capsule catalysts and their catalytic applications in heterogeneous catalysis. The capsule catalyst preparation methods are introduced in detail, such as hydrothermal synthesis method, dual-layer method, physically adhesive method and single crystal crystallization method. Furthermore, several new applications of capsule catalysts in heterogeneous catalytic processes are presented such as in the direct synthesis of liquefied petroleum gas from syngas, the direct synthesis of para-xylene from syngas and methane dehydroaromatization. In addition, the development in the design of multifunctional capsule catalysts is discussed, which makes the capsule catalyst not just a simple combination of two different catalysts, but has some special functions such as changing the surface hydrophobic or acid properties of the core catalysts. Finally, the future perspectives of the design and applications of capsule catalysts in heterogeneous catalysis are provided.
Key words: Capsule catalyst    Tandem reaction    Zeolite    Heterogeneous catalysis
Ⅰ. INTRODUCTION

Many kinds of capsule catalysts with pre-shaped core catalysts enwrapped by zeolite layers have been designed and used as heterogeneous catalysts for the tandem catalytic reactions in the past few years [1-10]. Generally, a capsule catalyst consists of one core catalyst and one zeolite layer with a core-shell structure (see FIG. 1). The core catalyst could be a pre-shaped pellet such as amorphous SiO$_2$ (diameter 0.85-1.7 mm) or a tailor-made millimeter-sized metal oxide catalyst [11-15]. The shell catalyst is a kind of zeolite such as H-ZSM-5, H-Beta, Silicalite-1, H-MOR, SAPO-11, and so on [16-22].

 FIG. 1 Illustration of a capsule catalyst

At the very beginning, the capsule catalyst is applied to the direct synthesis of middle isoparaffins from syngas based on Fischer-Tropsch synthesis reaction (FTS) [1, 11, 23, 24]. It also shows high selectivity in the direct synthesis of dimethyl ether (DME) from syngas [9, 25]. These two reactions are typical tandem reactions, in which two or multiple reactions (as shown in FIG. 1) are combined into one synthetic operation [26-30]. The tandem reaction is an effective approach to reducing the use of chemicals, minimizing chemical waste, as well as saving reaction time [28-31]. In order to satisfy the requirement of the tandem reaction, different catalyst components need to be assembled together to obtain a hybrid catalyst with multifunctional sites. Usually, a hybrid catalyst is prepared by a simply physical mixing of different catalysts. However, the selectivity of the final reaction product is not high enough owing to the unrestricted, open reaction environment where the intermediate products tend to leave the catalyst surface directly without further reaction [8]. The capsule catalyst with a special core-shell structure is an effective way to enhance the selectivity of desired products. Before leaving the catalyst surface, the intermediates have to enter the shell layer, where the subsequent reactions happen.

So far, numerous capsule catalysts have been designed and synthesized. Several chemical and physical methods have been developed for the synthesis of these materials. For example, a capsule catalyst with a Co/Al$_2$O$_3$ core and an H-Beta zeolite shell was prepared by a hydrothermal synthesis method [1]. Another CuZnAl/SAPO-11 capsule catalyst was synthesized by a physically adhesive method [19]. These capsule catalysts exhibit excellent catalytic performance in the heterogeneous catalysis. Heterogeneous catalysis is very important in many areas of the chemical and energy industries. The great majority of heterogeneous catalysts are solids and the reactants are gases or liquids. With the development of catalyst preparation techniques, many new capsule catalysts are expected to be designed for the new catalytic processes. In this review paper, we present a brief overview of the recent development and applications of capsule catalysts in heterogeneous catalysis.

Ⅱ. METHODS FOR CAPSULE CATALYSTS PREPARATION A. Hydrothermal synthesis method

Traditionally, a hydrothermal method is used to synthesize capsule catalyst. For example, an H-Beta zeolite coated Co/Al$_2$O$_3$ capsule catalyst was prepared as follows [1]. To begin with, a Co/Al$_2$O$_3$ core catalyst for FTS was prepared by an incipient wetness impregnation method. An aqueous solution of Co(NO$_3$)$_2\cdot$6H$_2$O was used to obtain 10 wt% cobalt loading in the pre-shaped commercial Al$_2$O$_3$ pellets. Then, the H-Beta zeolite layer was coated onto the Co/Al$_2$O$_3$ core catalyst surface by a liquid-phase hydrothermal method. The molar ratio of the reactants was 48.24 SiO$_2$/17.40 TEAOH/1.00 Al$_2$O$_3$/519.30 H$_2$O. The hydrothermal treatment was carried out at 428 K and a rotation speed of 2 r/min for 72 h. The H-Beta coated Co/Al$_2$O$_3$ catalyst was collected and washed until reaching pH value of 7. The calcination was performed at 823 K for 8 h after drying at 393 K for 12 h. This capsule catalyst shows excellent performance for the direct synthesis of isoparaffins from syngas. Another H-ZSM-5 coated Co/Pd/SiO$_2$ capsule catalyst prepared by the hydrothermal synthesis method also exhibits high selectivity for the C5-C11 isoparaffins production based on FTS [13]. However, it is not easy to prepare the defect-free H-type zeolite layer on the core catalyst such as coprecipitated catalysts, which are easy to be decomposed or dissolved during the hydrothermal treatment process. To solve this problem, we developed a new zeolite layer preparation method, which is called the aluminum migration method.

B. Aluminum migration method

We reported a well-designed aluminum migration method used for the preparation of H-ZSM-5/Cu-ZnO-Al$_2$O$_3$ capsule catalyst for the first time in 2010 [9]. The acidic H-ZSM-5 zeolite layer was directly prepared over the coprecipitated Cu-ZnO-Al$_2$O$_3$ core catalyst surface by employing a close-to-neutral Silicalite-1 zeolite synthesis recipe of 0.48 TPAOH/2 TEOS/8 EtOH/120 H$_2$O/0.24 HNO$_3$. The mixed H-ZSM-5 zeolite synthesis precursor together with the Cu-ZnO-Al$_2$O$_3$ core catalysts were added into a Teflon container and then treated at 453 K and a rotation speed of 2 r/min for 72 h. After the hydrothermal treatment, the samples were collected and dried at 393 K for 12 h. The final capsule catalysts were obtained after calcination at 773 K for 5 h.

It is noteworthy that no aluminum resource was added for H-ZSM-5 zeolite layer preparation. The H-type H-ZSM-5 coated Cu-ZnO-Al$_2$O$_3$ catalyst was successfully prepared in one step. As shown in FIG. 2, the Cu-ZnO-Al$_2$O$_3$ catalyst was well enwrapped by a zeolite shell with a thickness of 5 ${\rm{ \mathsf{ μ} }}$m. However, all these liquid-phase hydrothermal synthesis methods are not cost-effective because the zeolite layer yield is low. For this reason, an economical, green strategy capsule catalyst preparation method is desired.

 FIG. 2 Cross-section SEM image and EDS line analysis of the H-ZSM-5/Cu-ZnO-Al$_2$O$_3$ capsule catalyst [9]
C. Direct liquid membrane crystallization method

In order to increase the yield of zeolite layer, a modified vapor phase transport method for capsule catalyst preparation was designed in our team [32]. A Co/SiO$_2$ pellet prepared by a conventional incipient wetness impregnation method used as core catalyst was coated by an H-Beta zeolite layer. For the preparation of zeolite layer, a zeolite precursor was pre-coated onto the Co/SiO$_2$ pellet. Typically, the fumed SiO$_2$, TEAOH, and Al(NO$_3$)$_3\cdot$9H$_2$O were mixed with a molar ratio of 30.0 SiO$_2$/13.3 TEAOH/1.0 Al$_2$O$_3$/326 H$_2$O. Then, the zeolite precursor was transformed into an H-Beta zeolite layer by a hydrothermal treatment at 428 K for 96 h with a proper template solution (1.75 g H$_2$O and 0.25 g TEAOH). In this case, more than 60% of the total Si and Al species were transformed into zeolite and coated onto the core catalyst. Furthermore, during the crystallization process, those core catalysts do not contact the liquid template solution, the separation and washing is unnecessary in this method. As a result, this direct liquid membrane crystallization method is a promising, high efficient, and green capsule catalyst synthesis method.

D. Dual-layer method

The hydrothermal process is the most common method to synthesis zeolite shell, however, the structure of the core catalyst is easily damaged by the strong alkaline condition, which is necessary for the synthesis of acidic zeolite (such as H-ZSM-5, H-Beta, and H-Y). Therefore, a dual-layer method has been developed to solve this problem [18, 25, 33]. As shown in FIG. 3, one Slicalite-1 zeolite layer was firstly coated onto the alkali-sensitive silica-based core catalyst under a close-to-neutral synthesis condition [7]. The Silicalite-1 zeolite layer was synthesized by a hydrothermal method with a synthesis recipe of 0.48 TPAOH/2 TEOS/8 EtOH/120 H$_2$O/0.24 HNO$_3$. Then, the pre-coated sample was used as a new core catalyst for the acidic H-ZSM-5 zeolite layer preparation. After a hydrothermal treatment, a dual-layer coated catalyst was obtained.

 FIG. 3 Illustration of the H-ZSM-5 zeolite shell prepared on a silica-based catalyst by the dual-layer method [7]

The pre-coated Silicalite-1 zeolite layer onto the core catalyst can protect the silica-based core catalyst from corrosion and breakage by the alkaline condition during the H-ZSM-5 zeolite synthesis. This dual-layer method is a very simple method and easy to synthesize other zeolite layers. However, the dual-layer structure will reduce the diffusion rate of reactants and products, which may cause the relative low activity and short life time of the capsule catalyst.

Although the direct liquid membrane crystallization method and the dual-layer method can effectively avoid the corrosion damage of the core catalyst during the hydrothermal process of constructing zeolite layers, the hydrothermal synthesis process still faces many problems such as the vulnerable property of core catalyst, and difficulty of different zeolite layer synthesis on varied supports.

In order to overcome the drawbacks of the hydrothermal process, a new capsule catalyst preparation way has been proposed [8, 34]. The target zeolite layer was prepared by a physically adhesive (PA) method. That is, the zeolite powders are pasted onto the core catalyst by using an adhesive. Typically, a Co/SiO$_2$ pellet is used as core catalyst. The core catalyst was first impregnated by silica sol (Ludox), then moved into a flask and mixed well with the prepared H-ZSM-5 zeolite powders. After dried at 393 K and calcined at 773 K, a Co/SiO$_2$ catalyst enwrapped with an H-ZSM-5 zeolite layer was obtained.

Another CuZnAl/SAPO-11 capsule catalyst was also prepared by the PA method and used as a catalyst for DME synthesis from syngas [19]. To start with, the Cu/ZnO/Al$_2$O$_3$ (CuZnAl) core catalyst was prepared by an oxalate co-precipitation method. In brief, an ethanol solution containing metal nitrates of Cu, Zn, and Al was mixed with another ethanol solution containing oxalic acid at ambient temperature under vigorous stirring. After aging for 24 h, the precipitate was separated, dried and calcined. The calcined samples were compressed to form granules and sized to 0.85-1.70 mm fraction for capsule catalyst synthesis. Then the Silicoaluminophosphate-11 (SAPO-11) zeolite powders used for capsule catalyst synthesis were independently prepared by the conventional hydrothermal method. Next, the CuZnAl core catalyst was immersed in a silica sol and mixed with the as-prepared SAPO-11 zeolite powders as shown in FIG. 4. In the end, the CuZnAl/SAPO11 capsule catalyst was obtained after calcined at 773 K for 2 h. In summary, the PA method is very cost-saving, reliable and scalable for capsule catalyst synthesis, which also brings new opportunities to develop various zeolite capsule catalysts.

 FIG. 4 Illustration of the CuZnAl/SAPO-11 capsule catalyst prepared by the physically adhesive method [19]
F. Single crystal crystallization method

In previous reports, capsule catalysts usually have large sizes in the millimeter range. Recently, single crystal capsule catalysts with diameters in the micrometer range have been prepared and applied in heterogeneous catalysis [22, 35-37]. For example, a micro-capsule catalyst with interior Fe/Silica core and exterior H-ZSM-5 shell was designed and used for the synthesis of middle isoparaffin from syngas. The size of the prepared Fe/Silica@H-ZSM-5 capsule catalyst is about 1-2 ${\rm{ \mathsf{ μ} }}$m [35]. Recently, another Zn/Z5@S1 single crystal capsule catalyst was synthesized and applied in the direct synthesis of para-xylene from syngas [22]. The catalyst was prepared as follows: an H-ZSM-5 zeolite was firstly synthesized through a simple hydrothermal synthesis method. Then it was further modified by zinc through ion-exchange method. The obtained Zn/H-ZSM-5 (Zn/Z5) sample was used as core catalyst for the preparation of single crystal capsule catalyst. Finally, a Silicalite-1 zeolite layer was coated onto the Zn/Z5 surface by the hydrothermal synthesis method as aforementioned. As shown in FIG. 5, the Zn/Z5@S1 single crystal capsule catalyst has a hexagonal exterior shape with a particle length of 2.5 ${\rm{ \mathsf{ μ} }}$m.

 FIG. 5 FE-SEM images of (a) Zn/Z5 and (b) Zn/Z5@S1 single crystal catalysts. (c) STEM image of the Zn/Z5@S1 zeolite component and the corresponding STEM EDS mapping of (d) Si, (e) Al, (f) O, (g) Zn, and (h) the combined Si, Al, O, and Zn [22]

On the basis of our work, a lot of capsule catalyst synthesis methods were developed besides the conventional hydrothermal synthesis and the PA method. For example, Ma et al. synthesized a micron-sized FeMnK/Al$_2$O$_3$@Silicalite-2 core-shell catalyst by the combination of Ströber method and hydrothermal crystallization method [21]. Typically, FeMnK/Al$_2$O$_3$ core catalyst was mixed with polyvinyl pyrrolidone and ethanol in a flask. Then tetraethyl orthosilicate and ammonium hydroxide were added into the mixed solution. After stirring for 2 h at room temperature, the mixed solution was heated at 333 K to remove the ethanol and then dried at 373 K. Next, the obtained FeMnK/Al$_2$O$_3$@SiO$_2$ and tetrabuthyl ammonium hydroxide were grounded for 20 min and heated at 343 K for 10 min. Subsequently, the mixture was moved into an autoclave and hydrothermal crystalized at 463 K for 72 h. Finally, the micron-sized FeMnK/Al$_2$O$_3$@Silicalite-2 catalyst was obtained after calcination at 823 K for 8 h.

G. Chemical vapor deposition method

Different from the micron-sized capsule catalyst, a SiC monolith (2 cm$\times$2 cm) coated by one CNFs (carbon nanofibers) layer with a thickness of 4 ${\rm{ \mathsf{ μ} }}$m was prepared by a catalytically chemical vapor deposition method (see FIG. 6) [38]. In brief, a SiC foam impregnated with 1 wt% of nickel was placed into a tubular quartz reactor. After the reduction treatment under hydrogen flow at 673 K for 2 h, the gas flow was switched to a mixture of ethane and hydrogen (60/40 mL/min) to grow CNFs. After that, the obtained sample was treated in an HNO$_3$ solution to remove the pre-loaded Ni. Finally, the CNFs-SiC composite with a core-shell structure was obtained and used as catalyst support for the methane dry reforming Ni-based catalyst.

 FIG. 6 Schematic image of methane dry reforming over Ni/CNFs-SiC catalyst [38]

During the past few years, many capsule catalyst synthesis methods have been designed and developed. The H-ZSM-5, H-Beta, SAPO11, Silicalite-1, Silicalite-2, and other zeolite catalysts could be successfully coated onto the surface of SiO$_2$, Al$_2$O$_3$ pellets or other metal oxide as core catalysts. The hydrothermal synthesis method is the most common method for the capsule catalyst synthesis. However, it is not suitable to synthesis some zeolite shells which need alkali condition. In contrast, the physically adhesive method is suitable to enwrap various defect-free zeolite layers onto different core catalysts. Moreover, new function of capsule catalyst was discovered recently. For example, a Silicalite-1 was designed as a non-acidic Silicalite-1 layer, which aimed to cover the exterior acid sites on the external surface of the H-ZSM-5 in the Zn/Z5@S1 single crystal capsule catalyst [22]. Another CNFs layer coated onto the SiC surface was used as Ni-based catalyst support [38]. Furthermore, the modification of the capsule catalyst could enhance the catalytic performance. A palladium modified Co/SiO$_2$-Z-Sp-Pd capsule catalyst by sputtering method exhibits high product selectivity for isoparaffin direct synthesis by Fischer-Tropsch synthesis reaction from syngas [12]. With the development of these new concepts, the capsule catalysts have been designed to catalyze new catalytic reactions.

Ⅲ. APPLICATIONS IN HETEROGENEOUS CATALYSIS A. Fischer-Tropsch synthesis

FTS is an effective process for the conversion of syngas into clean liquid fuels (gasoline, diesel fuel, and jet fuel) and valuable chemicals (light olefins and aromatics) [39-49]. It is a stepwise polymerization reaction and the products generally follow an Anderson-Schulz-Flory (ASF) distribution, which is inherently wide and unselective [50-55]. In the past years, we have designed many kinds of capsule catalysts for the isoparaffin synthesis based on FTS [11-18]. At the very beginning, a capsule catalyst with a H-ZSM-5 zeolite shell and Co/SiO$_2$ core was designed for the high selective production of isoparaffin based on FTS in 2005 [11, 23]. An anti-ASF distribution was obtained on this H-ZSM-5/Co/SiO$_2$ catalyst. The formation of C11$^+$ hydrocarbons was suppressed completely in this system by the H-ZSM-5 zeolite layer, where the formed long chain hydrocarbons over the Co/SiO$_2$ catalyst undergo further hydrocracking and isomerization [11, 23]. On the basis of these studies, a micro-capsule catalyst with Fe/Silica core and H-ZSM-5 shell achieved an isoparaffin selectivity of 46.5% [28], another Co/SiO$_2$-Z-PA capsule catalyst showed an isoparaffin selectivity of 49.3% and C11$^+$ selectivity of 0.4% for FTS [8].

Recently, we designed a new miniaturized Co/SiO$_2$-M-Z6ET capsule catalyst (see FIG. 7) for liquid fuel synthesis from syngas [56]. Different from conventional capsule catalyst, this new catalyst possesses a strong crystallization intensity and Brønsted acidity. It has a small core size of 7$\times$10$^{-4}$ mm$^3$ in volume, which is in favor of zeolite layer growth. It is about 1500 times smaller than that of the conventional Co/SiO$_2$ core catalyst (around 1 mm). As a result, a striking gasoline selectivity of 72% was obtained over this capsule catalyst from FTS.

 FIG. 7 Illustration of the liquid fuel synthesis over Co/SiO$_2$-M-Z6ET capsule catalyst [56]

Light olefins (C2-C4) are key building blocks in the current chemical industry [57-67]. Recently, a methyl modified Fe$_2$O$_3$@SiO$_2$-(CH$_3$)$_3$ capsule catalyst was prepared for light olefins synthesis from syngas [57]. The production of CO$_2$ was suppressed by the hydrophobic surface of the Fe$_2$O$_3$@SiO$_2$-(CH$_3$)$_3$ capsule catalyst, which prevented the readsorption of water and then inhibited the water gas shift (WGS) reaction. For the synthesis of Fe$_2$O$_3$@SiO$_2$-(CH$_3$)$_3$ capsule catalyst, a Fe$_2$O$_3$ core catalyst was first prepared using a solvent thermal method. Then a Stöber method was used to prepare Fe$_2$O$_3$@SiO$_2$ sample. In brief, the obtained Fe$_2$O$_3$ powders were dissolved in ethanol solution. Then the tetraethoxysilane (TEOS), ammonia, and deionized water were added in order under stirring with a volume ration of 1 TEOS/5 NH$_3\cdot$H$_2$O/20 H$_2$O. The obtained samples were then washed, dried and calcined. Finally, the modified Fe$_2$O$_3$@SiO$_2$ catalyst was prepared by a silylation reaction, which provides a hydrophobic environment for iron active centers in the FTS reaction. The synthesized catalysts showed high activity and stability for FTS with a CO$_2$ selectivity lower than 5%, which is much lower than that of the traditional FTS catalyst with a CO$_2$ selectivity higher than 40%. In the conventional capsule catalyst system, the core and shell are two independent catalysts for two reactions. In this Fe$_2$O$_3$@SiO$_2$-(CH$_3$)$_3$ capsule catalyst, however, a new function of shell was developed which just changes the hydrophobic properties of the Fe$_2$O$_3$ core catalyst.

B. Syngas to DME

DME is a useful chemical intermediate and a promising clean fuel, which can be produced from syngas [68-72]. The direct synthesis of DME from syngas (STD) could be accomplished with a bifunctional catalyst consisting of methanol synthesis catalyst and methanol dehydration catalyst. A capsule catalyst with Cu/ZnO/Al$_2$O$_3$ (CZA) as core and H-ZSM-5 zeolite as shell was first designed and used as catalyst for DME direct synthesis in 2010 [9]. As shown in FIG. 8, the syngas first entered the CZA core catalyst forming methanol, and then the formed methanol was converted to DME on the H-ZSM-5 zeolite layer. Two capsule catalysts synthesized by the hydrothermal synthesis method were tested in STD reaction. Their catalytic activities are listed in Table Ⅰ. Both CZA-Z and CZA-S capsule catalysts exhibit much higher DME selectivities than the physical mixing of CZA with H-ZSM-5 zeolite powder. Since then, Cu-ZnO@H-ZSM-5, Cu/ZnO/Al$_2$O$_3$@SiO$_2$, Cu/ZnO/Al$_2$O$_3$@SiO$_2$-Al$_2$O$_3$, Cr/ZnO-S-Z et al. have also been prepared as STD catalysts [4, 5, 19, 25, 71]. The selective synthesis of DME from syngas with these capsule catalysts are due to the special confinement effect and synergistic function.

 FIG. 8 Illustration of the DME direct synthesis from syngas on a single capsule catalyst [9]
Table 1 Catalyst properties and catalytic performance in STD reaction$^{\rm{a}}$ [9]
C. Syngas to liquefied petroleum gas

Liquefied petroleum gas (LPG) is a mixture of propane and butanes. It has been used as clean fuel, chemical feed, propellant for aerosols, and so on [33, 73]. LPG can be produced from either natural gas or crude oil. The direct synthesis of LPG from syngas is more potential and advantageous to large-scale application, as compared to the conventional indirect or semi-indirect processes [40]. Recently, we have developed an H-ZSM-5 zeolite enwrapped Pd-based capsule catalyst for the direct synthesis of LPG from syngas [33]. As shown in FIG. 9, the capsule catalyst was prepared by a dual-layer method consisting of Silicalite-1 and H-ZSM-5 zeolite double layers. A Pd/SiO$_2$ catalyst prepared by incipient wetness impregnation method was used as core catalyst. The Silicalite-1 and H-ZSM-5 zeolite layers were coated onto the Pd/SiO$_2$ catalyst by a dual-layer method. Syngas was first converted to methanol over the Pd/SiO$_2$ catalyst, and then the LPG fraction hydrocarbons were obtained by methanol dehydration in the H-ZSM-5 zeolite layer. This well-designed Pd/SiO$_2$-SZ capsule catalyst realized a LPG selectivity of 34.4% and maintained over 100 h on the stream. It is much better than the physical mixing of Pd/SiO$_2$ catalyst and SZ zeolite, which exhibits a LPG selectivity of 13.3%.

 FIG. 9 Schematic image of the LPG direct synthesis from syngas on the Pd/SiO$_2$-SZ capsule catalyst [33]

Another capsule catalyst CZA@H-Beta comprising Cu/ZnO/Al$_2$O$_3$ (CZA) as core and H-Beta zeolite as shell was prepared for the direct synthesis of LPG from syngas [73]. It involves methanol synthesis from syngas on CZA core catalyst, methanol dehydration to DME on H-Beta zeolite, and DME dehydration to hydrocarbons on H-Beta zeolite. As shown in FIG. 10, a multifunctional interface between CuZnAl core and H-Beta zeolite shell is able to catalyze the dehydration of methanol to olefins on acid sites and olefin hydrogenation to LPG over Cu sites on the interface. This capsule catalyst realized a LPG selectivity of 77% in hydrocarbons with a CO conversion of 50.2%.

 FIG. 10 Schematic image of the LPG direct synthesis from syngas on the CZA@H-Beta capsule catalyst [73]
D. Syngas to para-xylene

Aromatic hydrocarbons, especially para-xylene (PX) is an important basic chemical, which is generally produced from petroleum resource. At present, the PX is mainly produced by the catalytic reforming of naphtha followed by the separation procedure from the BTX aromatics (benzene, toluene, and the xylene isomers) [22]. The one-pass selective synthesis of PX from syngas was accomplished by our group recently over a hybrid catalyst [22]. As shown in FIG. 11, it consists of a Cr/Zn catalyst and a Zn/Z5@S1 capsule catalyst. The Cr/Zn catalyst was prepared by a coprecipitation method. The Zn/Z5@S1 capsule catalyst was synthesized by the single crystal crystallization method as we mentioned above. Generally, the Silicalite-1 layer is used as a protection layer and seed-layer-like intermediate. It is noteworthy that, a new function of Silicalite-1 layer in the Zn/Z5@S1 capsule catalyst was discovered. The H-ZSM-5 zeolite is always used for the methanol to PX reaction owing to its suitable channel architecture and alterable acidity. However, the acid sites on the external surface of the H-ZSM-5 are excessive, which leads to the side reactions. Here, a non-acidic Silicalite-1 layer successfully covered the exterior acid sites on the external surface of the H-ZSM-5, and avoided the undesirable xylene isomerization. As a result, this Cr/Zn-Zn/Z5@S1 catalyst exhibits a CO conversion of 55.0% and a PX selectivity of 27.6% in the total products and 77.3% in xylene. The physical granule mixture of Cr/Zn and Zn/Z5@S1 catalyst only exhibits a PX selectivity of 5.2%. This Cr/Zn-Zn/Z5@S1 hybrid catalyst is extremely promising as an industrial catalyst.

 FIG. 11 Illustration of the one-pass selective conversion of syngas to para-xylene over the Cr/Zn-Zn/Z5@S1 catalysts [22]
E. Methane dehydroaromatization

Benzene is an important organic chemical intermediate to make other chemicals, such as ethylbenzene, cumene, cyclohexane, nitrobenzene, and alkylbenzene. It is manufactured from catalytic reforming petroleum-based naphtha [74]. Direct synthesis of benzene from methane, named as methane dehydroaromatization (MDA), is a promising strategy to get valuable chemical products from shale gas or natural gas resources [74, 75]. Unfortunately, it is limited by thermodynamics, and the catalyst deactivates because of carbon buildup. Therefore, a hollow structure Silicalite-1-H-ZSM-5 zeolite capsule catalyst with pores was designed to facilitate the mass transfer rate of the formed benzene, which leads to the formation of coke [75]. The hollow capsule H-S-Z zeolite was prepared by a modified dual-layer method, which used activated-carbon as a hard template for the hollow structure (see FIG. 12). After the removal of activated-carbon by calcination in air flow, the obtained hollow capsule H-S-Z zeolite was further modified by molybdenum. The Mo/H-S-Z catalyst prepared by incipient wetness impregnation method was used to catalyze methane dehydroaromatization. Compared with the conventional solid catalyst, the Mo/H-S-Z capsule catalyst with a hollow structure accelerated the mass transfer of the benzene product, and realized an enhanced catalytic activity and stability.

 FIG. 12 Schematic representation of the MDA reaction on the hollow capsule catalyst [75]
F. Methane reforming

Methane reforming is an efficient route for the conversion of CH$_4$ and CO$_2$ into syngas [76-80]. As the reforming reaction proceeds at high temperatures, a catalyst with high thermal conductivity, high mechanical strength and high mass transfer is of great interest. The Silicalite-1, Al$_2$O$_3$, and carbon nanofibers (CNFs) layers coated silicon carbide (SiC) foam used as supports of Ni catalysts were designed and applied in methane reforming reactions [38, 79, 80]. For example, one Silicalite-1 zeolite layer was coated over the SiC via a hydrothermal synthesis method. The Silicalite-1 coated SiC foam was then used to prepare the Ni catalyst by an impregnation method. The obtained Ni/S-1/SiC catalyst exhibited excellent catalytic activity and reliable stability in the combined methane dry reforming (CO$_2$+CH$_4$$\rightarrow2CO+2H_2) and methane partial oxidation reaction (CH_4+1/2O_2$$\rightarrow$CO+2H$_2$). Another SiC monolith coated by one CNFs layer was used as the support of Ni-based catalyst for methane dry reforming [38]. The CNFs layer provided more anchorage sites for improving the Ni dispersion and enhanced the interaction between Ni particles and the support. The prepared Ni/CNFs-SiC catalyst exhibited high catalytic activity and stability during a 100 h time on stream methane dry reforming reaction. The Silicalite-1 or CNFs layers coated SiC foam was just used as Ni catalyst support. This is different from the conventional capsule catalyst, which has a core catalyst and shell catalyst, and catalyzes two or more different reactions independently. This kind of development in the capsule catalyst design could broaden the application of traditional capsule catalyst in heterogeneous catalysis.

Recently, encapsulation of metal nanoparticles within zeolite crystals has been developed by Xiao et al. [81-86]. For example, a nano-sized manganese oxide catalyst fixed inside the silicalite-1 zeolite crystal (MnO$_x$@S-1), exhibits high selectivity for producing nitriles by efficiently facilitating the oxidative cyanation reaction and hindering the side hydration reaction [82]. Another Pd@Beta capsule catalyst with Pd nanoparticle inside Beta zeolite crystal exhibits superior selectivity for hydrogenation of the nitro group, outperforming both conventional Pd nanoparticles supported on zeolite crystals and a commercial Pd/C catalyst [83].

Since the capsule catalyst has been successfully prepared by a hydrothermal synthesis process, the aluminum migration method, direct liquid membrane crystallization method, dual-layer method, physically adhesive method, single crystal crystallization method, and chemical vapor deposition method were further developed and used to prepare capsule catalysts. These catalysts have shown excellent performance compared to the corresponding hybrid catalysts in heterogeneous catalysis. The high conversion and high selectivity of desired product can be obtained over these capsule catalysts in the tandem reaction, such as the direct synthesis of isoparaffin, LPG, DME, or PX from syngas. Besides, it can also be used as a catalyst for MDA or as a catalyst support for methane reforming reaction.

Ⅳ. CONCLUSIONS AND OUTLOOK

In summary, the preparation methods and applications of capsule catalysts in heterogeneous catalysis have been extensively developed in recent years. This review summarizes our recent works on the preparation and application of capsule catalyst. On the basis of hydrothermal synthesis method, a single crystal crystallization method was developed for Zn/Z5@S1 capsule catalyst synthesis. It exhibits high selectivity for the synthesis of PX from syngas. In order to accelerate the mass transfer of products, a hollow structure Mo/H-S-Z capsule catalyst was designed, which realized high catalytic activity and stability in methane dehydroaromatization. We also reported two capsule catalysts of Pd/SiO$_2$–SZ and CZA@H-Beta, which are selective for the direct LPG synthesis from syngas.

In addition, the concept of capsule catalyst design was further expanded. Usually, a capsule catalyst composed of one core catalyst and one shell catalyst is designed to accomplish the tandem catalytic reactions. Some new concepts of the capsule catalyst design were developed recently. For example, the Silicalite-1 zeolite was used as a non-acidic Silicalite-1 layer, which covered the exterior acid sites on the external surface of the H-ZSM-5, and avoided the side reactions in PX synthesis from syngas. Moreover, the shell of Fe$_2$O$_3$@SiO$_2$-(CH$_3$)$_3$ capsule catalyst is just designed to change the hydrophobic properties of the Fe$_2$O$_3$ core catalyst. Both the core and the shell of Ni/S-1/SiC and Ni/CNFs-SiC catalysts are applied as catalyst supports, which provide high thermal conductivity, high mechanical strength, and high mass transfer.

It is clear that the development of new functions of the capsule catalyst is strongly desired. In previous reports, the concept of capsule catalyst was designed for the tandem reactions, which achieves the one-step synthesis of the desired products from multiple continuous reactions. Recently, more and more new functions of the core or shell materials were developed for capsule catalyst preparation and application. The capsule catalyst will be applied in a growing number of heterogeneous catalytic reactions via the development of multifunctional capsule catalysts. Although some important advances have been reported on the preparation and application of capsule catalyst, the mechanism studies of zeolite growth over the core catalyst surface are rarely reported. More work is necessary to understand the growth mechanism of the zeolite layer onto the core catalyst, which is helpful to obtain more multifunctional capsule catalysts in heterogeneous catalysis.

Ⅴ. ACKNOWLEDGEMENTS

The work was supported by the Key R & D plan (East-West Cooperation) of Ningxia and the First-rate Discipline Construction Project of Ningxia (NXYLXK2017A04).

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a. 省部共建煤炭高效利用与绿色化工国家重点实验室, 化学化工学院, 宁夏大学, 银川 750021;
b. 中国科学技术大学国家同步辐射实验室, 合肥 230029;
c. 日本富山大学工学部应用化学科, 富山 930-8555