R Bhar

A Jardy

P Chapelle

V Descotes

3D NUMERICAL SIMULATION OF THE VAR CONSUMABLE ELECTRODE MELTING PROCESS

Keywords: VAR process, consumable electrode, dripping, liquid metal film, melt rate

A 3D numerical model was set-up to simulate the formation and dynamics of the liquid metal film under the consumable electrode during VAR process. In the present paper the implementation of this model is described. It was developed using the open source computational fluid dynamics (CFD) software, OpenFOAM. The model solves coupled momentum and energy equations combined with a volume-of-fluid (VOF) method to track the liquid metal free surface. The melting of the electrode material is modelled with an enthalpy-porosity approach. The electric power supplied by the arc is supposed to be uniform and distributed between the electrode as well as the liquid bath. For a given electric arc power, the model quantitatively predicts the dripping rate, hence the overall melt rate. Simulations are validated through comparisons with experimental data.

Introduction

Vacuum arc remelting (VAR) is a secondary remelting process used to improve cleanliness as well as chemical and mechanical homogeneity of metal ingots. VAR was the first remelting process to be used commercially for superalloys processing. It is also typically the final stage in the melting cycle of reactive metals such as titanium and zirconium alloys. [START_REF] Yu | Modeling for casting and solidification processing[END_REF] The process consists of melting a consumable electrode under vacuum (see Fig. 1). The heat source is a DC electric arc of low voltage and high current. The arc is created between the electrode (cathode) and the base plate of a water-cooled copper crucible at the beginning of the melt, then between the electrode and the secondary ingot (anode) forming in the crucible. The melting of the tip of the electrode generates a liquid metal film under the electrode, from which metal drops are produced that fall under the action of gravity into the crucible and progressively solidify to form the secondary ingot. At any instant, the ingot is composed of three zones: the fully solidified metal, the liquid pool fed by metal drops and an intermediate mushy zone. The quality of the produced ingots strongly depends on the operating conditions of remelting. Among them, the melt rate and the interelectrode gap play a key role, since they have significant effects on heat transfer conditions at the free surface of the liquid pool, which have important implications on the ingot structure [START_REF] Yu | control of solidification structure in VAR and ESR processed alloy 718 ingots[END_REF].

The VAR process has been investigated previously with both experimental and numerical approaches. On the experimental side, some studies were devoted to establish various correlations between the operating parameters [START_REF] Zanner | Observations of melt rate as a function of arc power, CO pressure, and electrode gap during vacuum consumable arc remelting of Inconel 718[END_REF][START_REF] Williamson | Arc voltage distribution properties as a function of melting current, electrode gap, and CO pressure during vacuum arc remelting[END_REF], whereas some research work focused on the electric arc behavior and metal transfer mechanisms in the interelectrode region, which were observed using high speed video cameras in specifically instrumented VAR furnaces [START_REF] Zanner | Metal transfer during vacuum consumable arc remelting[END_REF][START_REF] Chapelle | Characterization of the behaviour of the electric arc during VAR of a Ti alloy[END_REF]. Modelling of the VAR process is a difficult task, because the process involves a wide range of coupled physical and chemical phenomena, such as fluid flow, heat and mass transfer, solidification (macro and microsegregation), electromagnetic forces… In the literature, most modelling works deal with the development of Computational Fluid Dynamics (CFD) models of the ingot growth and solidification. The majority of authors considers a 2D axisymmetric geometry of the ingot and solves the conservation equations of mass, momentum and energy, accounting for turbulence phenomena and electromagnetic forces in the liquid pool as well as the solidification of the metal. Examples of such models are the SOLAR code [START_REF] Jardy | Solar: numerical software to optimize vacuum arc remelting operations[END_REF] and the MeltFlow-VAR code [START_REF] Kelkar | Computational modeling of the Vacuum Arc Remelting (VAR) process used for the production of ingots of Titanium alloys[END_REF]. A multiscale 3D numerical model of VAR was developed by Pericleous et al. [START_REF] Pericleous | A multiscale 3D model of the vacuum arc remelting process[END_REF], which deals also with the ingot behavior. However, studies of the liquid film formation under the electrode are non-existent in literature.

The present work focuses on the interelectrode gap of the VAR process. The aim is to numerically study the liquid film behavior under the electrode and to predict the melt rate of the electrode for a given electric arc power. A 3D model describing the melting of the consumable electrode and the dynamics of the liquid film formed at the electrode tip was developed using the CFD open source software OpenFOAM. The model considers fluid flow, heat transfer with phase change and the deformation of the free surface of the liquid film. The electromagnetic effects related to the flow of current in the electrode are not yet taken into account. The model is concerned with both large interelectrode gaps, for which the metal transfer results from the formation of molten metal drops from the electrode and their detachment before contacting the ingot, and short interelectrode gaps, for which the metal transfer involves the formation of molten metal bridges (drip-shorts) between the electrode and the ingot. In this paper, we only report results regarding large interelectrode gaps. In section 2, the model is described, including physical and mathematical issues, constitutive equations, boundary conditions and the numerical procedure. In section 3, examples of model results detailing the computed liquid film dynamics and thermal behavior of the electrode during the VAR melt of a small-scale electrode are presented. Finally, conclusions of the present study are drawn in section 4.

Numerical model

The formation and dynamics of the liquid film under the consumable electrode during the VAR process is simulated with a multiphase CFD approach. The metal phase change is accounted for using the enthalpy-porosity method [START_REF] Voller | A fixed grid numerical modelling methodology for convection-diffusion mushy region phase-change problems[END_REF] and the shape and position of the free surface of the liquid film are calculated using the volume of fluid (VOF) interface capturing method [START_REF] Hirt | Volume of fluid (VOF) method for the dynamics of free boundaries[END_REF]. Note that in a VAR furnace the liquid metal film is exposed to a low pressure arc plasma. The present model does not deal with the description of this complex latter phase, which is represented here as a neutral gas phase.

Fluid flow and behavior of the free surface

The computed behavior of the free surface of the liquid metal film is based on the VOF approach. In order to identify the metal-gas interface, the model solves an advection equation of the volume fraction of the metal αm (either solid or liquid) present in each computational cell:

+ . + . - = (1)
αm = 0 if no metal is present in the cell, αm =1 for a cell completely filled with metal, and 0 < αm < 1 if the interface is present in the cell. In the VOF formulation implemented in OpenFOAM, the volume fraction transport equation contains an additional convective term (third term in Eq. 1), referred to as "compression term". This term compresses the interface by minimizing the numerical diffusion of the volume fraction while ensuring its boundedness. It can be noticed that this additional compression term acts only at the vicinity of the interface region where αm 1αm ≠0. Therefore, it does not affect the fluid flow outside this region [START_REF] Rusche | Computational fluid dynamics of dispersed two-phase flows at high phase fractions[END_REF].

The gas-liquid metal flow is governed by a single set of continuity and Navier-Stokes equations • Continuity equation

+ . = (2)
• Momentum conservation equation

+ . = -+ . + + ! 1 -" # -# $%& ' - ( ) (3)
The thermophysical properties are calculated as volume fraction weighted averages of the properties of the phases present in the cell:

ξ = + ( ξ ( + 1 -+ ( ξ ,-. (4)
All thermophysical properties are assumed to be independent of the temperature except for the density in the gravitational term (Eq. 3), which is calculated using the Boussinesq approximation. Surface tension effects are taken into account through a continuous equivalent volumetric force fσ (Continuum Surface Force model) as proposed by Brackbill [13]. This force, acting only in the vicinity of the interface and directly linked to the interface curvature k c , is defined as:

= / 2 ( + ,-. 1 2 + ( (5)
A problem encountered in this formulation is the presence of numerical parasitic velocity vortices known as "spurious currents" [START_REF] Ubbink | Numerical prediction of two fluid systems with sharp interfaces 15[END_REF]. These "spurious velocities" appear in the gas phase near the interface despite the absence of any external forces and can be very significant when the capillary effects are predominant. These high velocities force to use very small time steps to maintain the stability of the simulation (CFL criterion), which increases the computational time. In addition, their magnitude does not decrease with neither mesh refinement nor smaller computational time steps [15]. In the current study, a filtration method consisting in setting to zero the velocities of the gaseous phase was implemented to eliminate those spurious velocities.

The last source term on the right hand side of Eq. 3 is derived from the Darcy law. The dendritic mushy zone is treated as a porous medium whose permeability is calculated using the well-known Kozeny-Carman model.

) = 3 4 4 1 -5 6 180 5² (6)
where γ is the solid fraction in the cell γ = αm (1-g l ). In practice, the effect of this term is as follows. In the fully liquid cells (γ = 0), the source term is zero and the "classical" Navier-Stokes equation is solved. In the cells where a phase change occurs, the source term dominates over the transient, convective and diffusive terms, thereby forcing to imitate the Kozeny-Carman model [START_REF] Voller | A fixed grid numerical modelling methodology for convection-diffusion mushy region phase-change problems[END_REF]. In the fully solid cells (γ = 1), the source term swamps out all other terms in the momentum equation, which forces velocities to zero [START_REF] Brent | Enthalpy-porosity technique for modeling convection-diffusion phase change: application to the melting of a pure metal[END_REF].

Heat transfer with phase change

The enthalpy-porosity method is based on the enthalpy formulation of the energy conservation equation:

ℎ + . ℎ = . 1 # (7)
The enthalpy may be computed as the sum of sensible enthalpy and latent heat absorbed when the metal changes from solid to liquid. The total enthalpy is given by: ℎ = :

; < =# + ( + ( > $ ? @ @ ABC (8)
The energy conservation may thus be expressed as follows:

; < # + . ; < # = . 1 # -( ? D + ( > $ + . + ( > $ E + F -2 (9)
The second term on the right hand side of Eq. 9 accounts for the evolution of the latent heat during phase change. The liquid-solid interface is not tracked explicitly. Instead, the melting zone is treated as a porous zone as described previously, in which the liquid fraction lies between 0 and 1. The liquid fraction is a function of temperature and can be, for example, calculated as follows:

> $ = 0 GH # < # .J$ (10) > $ = 1 GH # > # $%& > $ = # -# .J$ # $%& -# .J$ GH # .J$ < # < # $%&
The last term on the right hand side of Eq. 9 is the thermal power provided by the electric arc to the metal. In fact, this power corresponds to a boundary condition of the problem, which is treated here as a volumetric source term applied only in the cells crossed by the metal/gas interface (defined as the cells where αm = 0.5). The total arc power P tot is assumed to be uniformly distributed at both the electrode base and the ingot top surface, which leads to the following expression of P arc :

F -2 = F LJL ∑ N % N % O 2% (11)
where V ci is the volume of the cell (crossed by the interface) i, S i is the area of the portion of the interface contained in cell i and ∑Si is the total area of the interface.

Boundary conditions

The main boundary conditions of the model are outlined here based on the dependent variables under consideration, i.e. velocity, pressure and temperature.

• Velocity: A no-slip boundary condition is used at walls.

• Pressure: A pressure outlet boundary condition is used at outlets.

• Temperature: The walls are adiabatic, which results in a zero temperature gradient condition. The temperature is fixed (ambient) at the electrode top.

Numerical aspects

The pressure-velocity coupling is handled by the PIMPLE algorithm (combination of the PISO [START_REF] Issa | Solution of the implicitly discretised fluid flow equations by operatorsplitting[END_REF] and SIMPLE [START_REF] Patankar | Numerical heat transfer and fluid flow[END_REF], algorithms). The convective terms in the conservation equations are discretized with a second order VanLeer scheme while the diffusion terms are central differenced. The model is based on the existent multiphase flow solver InterFoam using the MULES correction.

Example of simulation results

The numerical model described above was applied to simulate the melt of a small diameter Ti electrode with a large interelectrode gap performed experimentally by Chapelle et al. [START_REF] Chapelle | Characterization of the behaviour of the electric arc during VAR of a Ti alloy[END_REF].

Fig. 2 shows a vertical cut of the computational domain with the initial metal represented in yellow. The electrode diameter is 160 mm, its height is 150 mm and the interelectrode gap is 60 mm. The total electric arc power is 76.8 kW, which is uniformly distributed between the electrode and liquid pool as schematized in the Fig. 2 by red lines. Initially, the metal is in solid state with a temperature of 300 K. As stated above, all the walls are supposed thermally insulated except the top boundary on which the temperature is fixed to 300 K. The computational grid is a uniform structured mesh consisting of around 2.5 million cells. The thermophysical parameters used for the simulation are reported in Table 1.

Table 1. Thermophysical parameters used for the simulation.

Liquid film behavior

At the initial state, the system is isothermal. The thermal power provided by the electric arc heats the electrode. At this stage, the heat transfer is purely conductive. As soon as the temperature of the tip reaches the solidus temperature, the electrode starts to melt and a distorted liquid film, whose thickness is about few millimeters, is formed. Owing to the Rayleigh-Taylor instability which occurs when a heavy fluid is placed above a lighter one in a gravitational field [19], small protuberances appear and are spatially distributed in lattices as shown in Fig. 3, which gives the depth reached by the protuberances in the liquid film. Continuous feeding of the film due to the metal melting eventually destabilizes these structures. The protuberances are subject to horizontal movements. Once they get close to each other, they coalesce, grow and form drops. Such a behavior is qualitatively consistent with the theory of Limat et al. The successive growth, elongation and detachment stages of a drop are compared in Fig. 4 with the experimental observations reported by Chapelle et al [START_REF] Chapelle | Characterization of the behaviour of the electric arc during VAR of a Ti alloy[END_REF]. On the left image, the protuberance which grows by drainage of the liquid metal has a cylindrical shape. Then it stretches resulting in the formation of a drop. The drop remains attached to the liquid film by a filament which gradually becomes thinner. This latter process eventually triggers the break-up of the filament and subsequent detachment of the drop. The dimension of the simulated drop is of the same order of magnitude (around 1 cm) as that of the actual drop. It is also interesting to note that the characteristic times for each stage are of the same order of magnitude, even if the "numerical" drop takes less time to detach than in reality.

Parameters

Although this comparison can only be qualitative since the simulation does not take into account the electromagnetic effects related to the presence of the arc, the simulated behavior remains close to the experimental visualizations.

Fluid flow inside a protuberance

As example of the typical fluid flow present in the protuberance, Fig. 5 shows the computed streamlines and velocity vectors in a vertical plane crossing a protuberance. The two white lines represent the free surface of the liquid film and the liquid/solid interface respectively. The fluid flow is governed by the drainage of the liquid film as well as buoyancy forces. It is characterized by a large recirculation loop that occupies the whole volume of the protuberance. The velocity magnitude reaches a maximum value of around 20 cm.s -1 located on the central axis of the protuberance (Fig. 5.c).

Heat transfer in the electrode

The computed temperature field in a vertical cutting plane of the electrode at t = 707 s is presented in Fig. 6.a, with the initial state of the electrode represented by white dashed lines. The temperature profile along the electrode axis at the same time is plotted in Fig. 6.b. Due to the consumption of the electrode, the power supplied by the electric arc affects mainly a small region at the electrode tip as shown in the Fig. 6.a. This region, usually referred to as the "heat affected zone", is about 10 centimeters high. This behavior is in a good agreement with the experimental observations of El Mir et al. [START_REF] Mir | Thermal behaviour of the consumable electrode in the vacuum arc remelting process[END_REF]. In the absence of consideration of thermal losses by radiation at the electrode lateral wall, the temperature remains essentially uniform in the horizontal direction. Note that the temperature of the liquid film exceeds the liquidus temperature with a superheat reaching around 100 °C.

Predicted melt rate

The time evolution of the electrode melt rate, calculated from the evolution of the electrode mass, is plotted in Fig. 7. The electrode begins effectively to melt after a pre-heating stage of around 10 min. The variation of the melt rate is characterized by a sharp increase until reaching a quasi-stationary regime. The average calculated melt rate is 0.96 kg.min -1 with fluctuations between 0.75 and 1.3 kg.min -1 . In the current model, it should be noted that the process starts at full power, contrary to the real process in which the electric current is gradually increased.

Conclusion

A 3D numerical model was developed in order to simulate the melting of a VAR consumable electrode. This work couples the enthalpy-porosity approach to simulate the melting process and the volume of fluid method to model the deformation of the liquid film formed under the electrode. Simulation results applied to the melting of a small diameter electrode with a large interelectrode gap have been presented, illustrating the formation and dripping of the liquid film and the thermal behavior of the electrode. The results of the simulation were found to be in good qualitative agreement with experimental observations reported in the literature. The model enables also to predict the electrode overall melt rate.

In the future, the present model will be further improved by adding a description of the thermal losses by radiation at the electrode lateral wall. In addition, simulations will be performed for the case of an industrial steel electrode. This differs from the case presented in this paper by the much larger dimension of the electrode as well as the much smaller interelectrode gap (typically around 10 mm). The latter is responsible for a radically different mode of transfer of the liquid metal (involving drip-shorts instead of fully detached drops).

Figure 1 .

 1 Figure 1. Schematic representation of the vacuum arc remelting process.

Figure 2 .

 2 Figure 2. Dimensions and initial state of the simulated system.

  [20].

Figure 3 .

 3 Figure 3. Depth of protuberances formed from the liquid film.

Figure 4 .

 4 Figure 4. Formation and detachment of a drop observed experimentally (top row) and predicted by the numerical model (bottom row).

Figure 5 .

 5 Figure 5. Streamlines and velocity vectors inside a protuberance.

Figure 6 .

 6 Figure 6. (a) Temperature field in the electrode at t = 707 s, (b) Temperature profile along the electrode revolution axis.

Figure 7 .

 7 Figure 7. Time variation of the predicted melt rate (b) from the electrode weight variation (a)

Acknowledgments

The authors would like to thank Y. Millet and J. Jourdan from TIMET Savoie, France, who have supported the acquisition of the experimental data shown in fig. 4 of the present work.


Yuchen Cui

Zhi Liu

Hailing Guo

Yongming Chai

Chenguang Liu

Svetlana Mintova

Prof Hailing Guo

T R Hellstern

J Kibsgaard

C Tsai

D W Palm

L A King

F Abild-Pedersen

T F Jaramillo

Ni 1-x Co x O y , Ni 1-x Co x S y and Ni 1-x Co x P y Catalysts Prepared from Ni 1-x Co x -ZIF-67 for Hydrogen Production by Electrolysis in Alkaline Media

Keywords: ZIF-67, Ni-Co-doping, organized bimetallic atomic arrangement, Hydrogen evolution reaction [1]

come

Introduction

Transition metals have been recently suggested as appropriate catalysts for hydrogen evolution reaction (HER) [1] . The transition metal catalysts are used in alkaline media, and the alkaline water electrolysis is a competitively priced process in comparison to proton exchange membrane water electrolysis [2] . Due to the low cost, environmental benign, and high theoretical catalytic activity, Cobased materials are particularly attractive among other transition metal-based electrocatalysts for water splitting [3] . Multiple methods were used to prepare Co-based materials. For instance, Co4N was synthesized by simple nitridation reaction and high temperature treatment with Co(OH)F nanowire arrays [3c] . CoO nanorods (NRs) were fabricated by surface strain engineering [4] .

Metal-organic frameworks (MOFs) materials have been recently proven to be promising sacrificial template or self-template to produce electrocatalysts due to their sufficient exposed active sites, abundant organic ligands, and mesoporous structure [5] . For example, Li et al. [3a] synthesized Co3O4 nanocrystals embedded in N-doped mesoporous graphitic carbon layer/multiwalled carbon nanotube (MWCNT) hybrids by a facile carbonization and subsequent oxidation of MWCNT-based MOFs, which leaded to high electrochemical performance. In 2017, Cao et al. [3d] reported the preparation of hollow CoS2 tubular arrays by a sulfidation under thermal treatment of wire-like MOFs nanoarrays for water splitting. The self-agglomeration of Co-MOFs materials usually leaded to a high overpotential and decreased reaction rate.

In order to improve the efficiency and avoid self-agglomeration of Co-based materials, it is important to understand the structure-activity relationship between their electrocatalytic activities and the dispersion and valence state of the Co-based materials [6] . Different Co-based materials including sulfides, oxides, phosphides [7] , etc. show diverse electrocatalytic activity for HER [8] [9] .In addition, reducing the size of particles of Co-based materials, especially going to ultra-small nanoclusters or even single atoms [10] , is considered as an effective way to improve their catalytic performance. Furthermore, the introduction of a second metal component in the Co-based materials improved the resistance to corrosion in the alkaline media, and prevented the Co nanoparticles from agglomeration [11] . Several investigations have focused on MOF-derived bimetallic or multi-metallic derivatives, leading to advanced electrocatalytic performance compared with single Co-based materials [12] . For examples, He and co-workers fabricated carbon containing Ni-Co mixed metal phosphide nanoboxes (NiCoP/C) by ZIF-67 [12a] . Liang and et al. reported the Ni2P-CoP bimetallic phosphides derived from the NiCo-MOFs, which showed efficient performance for both HER and the oxygen evolution reaction (OER) [12c] . Mostly Co-MOFs under thermal decomposition conditions in the presence of a second metal were applied to fabricate new multi-metallic materials [13] . Firstly, the Co-MOFs was prepared, then the second metal (Ni) under acidic etching or thermal decomposition was introduced resulting in mesoporous structures collapse with selfagglomerated metals. The preparation of nanosized homogenous bimetallic MOFs derivatives for HER is still a challenge [12c] . and Ni1-xCoxPy materials by temperature-programmed reactions with oxygen, sublimed sulfur and sodium hypophosphite, respectively. Then, the homeotypic bi-metallic materials were deposited on glassy carbon electrode (GCE) and applied in alkaline (pH=14) electrolysis for hydrogen production. The effect of the state of Ni and Co species on the electrocatalytic performance of the catalysts for hydrogen evolution in alkaline media was studied in detail.

Results and Discussion

Characterization of NixCo1-x-ZIF-67, NixCo1-xOy, NixCo1-xPy, and NixCo1-xSy catalysts

The crystal structure of Co-ZIF-67 and Ni0.1Co0.9-ZIF-67 was characterized by XRD. The XRD diffraction pattern of Ni0.1Co0.9-ZIF-67 is similar to the mono-metallic Co-ZIF-67 sample (Fig. 1a).

Cobalt and nickel with consecutive atomic numbers easily formed homeotypic crystals. As shown in Fig. S1a, the XRD patterns of samples NixCo1-x-ZIF-67 with different Ni/Co ratios contain broad peaks, but their positions are identical with the simulated pattern of Co-ZIF-67 [14] . No impurity was found in samples NixCo1-x-ZIF-67, implying that Ni 2+ ions are effectively incorporated into the Co-ZIF-67 [15] . The intensity of the peaks for NixCo1-x-ZIF-67 (Fig. S1a) decreased with an increase of nickel content, which is related to the changes of charge distribution and electrostatic fields as a result of the existence of both nickel and cobalt within the framework [16] . While the XRD patterns of samples Co3O4 and Ni0.2Co0.8Oy (Fig. 1b) contain peaks at 31.3°and 36.8°2Theta corresponding to (100) and ( 101) planes of Co3O4 [17] , and a peak at 43.0° corresponding to (200) plane of NiO [11c, 18] . This result indicates that the NixCo1-x-ZIF-67 is fully transformed to Ni0.2Co0.8Oy. The XRD patters of samples CoP and Ni0.1Co0.9Py are shown in Fig. 1c. The Bragg peaks at 31. 6, 36.6, 46.3 and 48.7° 2Theta correspond to the (011), ( 111), ( 112) and ( 211) planes of CoP [3b, 12a, 12c] , and the peak at 41.0° 2Theta corresponds to (321) plane of Ni3P [12b, 19] , which confirm the transformation from NixCo1-x-ZIF-67 to CoP and Ni0.1Co0.9Py. The nanosized bimetallic atomic arrangement in the NixCo1-x-ZIF-67 is demonstrated as well (Fig. S1c). The XRD patterns of Co9S8 and Ni0.5Co0.5Sy samples are depicted in Fig. 1d. The peaks at 15.3, 29.8, 31.2 and 47.5° 2Theta correspond to (111), ( 311), ( 222) and ( 511) planes of Co9S8 [20] , and the peak at 44.4° is related to the (102) plane of NiS [11a] , indicating the formation of Co9S8 and Ni0.5Co0.5Sy. The SEM images for all samples (Co-ZIF-67 and Ni0.1Co0.9-ZIF-6, Co3O4 and Ni0.2Co0.8Oy, CoP, Ni0.1Co0.9Py, Co9S8 and Ni0.5Co0.5Sy) are presented in Fig. 2. The Co-ZIF-67 crystals (Fig. 2a) have a diameter of 86 nm and nearly spheroidal shape. As shown in Fig. 2b, the Ni0.1Co0.9-ZIF-67 crystals have similar morphology and a size of 135 nm. After oxidation, the derived Co3O4 crystals (Fig. 2c) have dodecahedral shape with an average size of 300 nm, while the Ni0.2Co0.8Oy crystals have an average size of 206 nm (Fig. 2d). The crystal size of the samples increases about twice after oxidation of the NixCo1-

x-ZIF-67, indicating that two or three NixCo1-x-ZIF-67 nanoparticles coalesce into one NixCo1-xOy particle.

In the case of phosphides, the derived CoP showed an average size of 34 nm (Fig. 2e), and the Ni0.1Co0.9Py crystals have an average size of 23 nm (Fig. 2f), which are smaller than the NixCo1-x-ZIF-67. The derived Co9S8 crystals have an average size of 15 nm (Fig. 2g), while the Ni0.5Co0.5Sy has a size of 12 nm (Fig. 2h), which are considerably smaller than the NixCo1-x-ZIF-67 crystals. The energy dispersive X-ray (EDS) elemental mapping and the elements proportion presented in Fig. S3 S1). The content of metals (Co & Ni) in the oxide, sulfide and phosphide is much higher than in the Co-ZIF-67 and Ni0.1Co0.9-ZIF-67 samples, indicating that most of the C, N and H elements were pyrolyzed after the high-temperature treatment and the target products were prepared successfully.The transmission electron microscopy (TEM) images for all samples (Co-ZIF-67 and Ni0.1Co0.9-ZIF-6, Co3O4 and Ni0.2Co0.8Oy, CoP, Ni0.1Co0.9Py, Co9S8 and Ni0.5Co0.5Sy) are shown in Fig. S2. The TEM results are consistent with SEM. The Ni0.1Co0.9-ZIF sample contains the same Co lattice with a Co-ZIF-67 (111) interlayer spacing of 0.20 nm (Fig. 2a,b). The lattice fringes of Co3O4 and Ni0.2Co0.8Oy display interplanar spacings of 0.24 nm, which match well with the (311) planes of the Co3O4 (Fig. 3c,d). TEM images reveal that the Ni0.1Co0.9Py retained small size with rough surface (Fig. S2 e,f). Moreover, the resolved interplanar distances of 0.09 and 0.12 nm from the HRTEM image can be ascribed to the ( 211) and ( 111) planes of CoP, respectively (Fig. S2 e,f insert image). Ni0.1Co0.9Py and Ni0.5Co0.5Sy crystals with the similar sizes are revealed by both TEM and SEM.The carbon cladding outside the metal particles (Fig. S2g), the carbon layer of the entire nanoparticle are evenly formed and thinner (Fig. S2h); the carboncoated metal particles can be clearly seen. The interplanar distances of 0.14 and 0.15 nm resolved from the HRTEM image are ascribed to the (311) and ( 222 The N2 adsorption-desorption study was applied to reveal the porous nature of the samples (Figure S4). The specific surface area of samples Co-ZIF-67 and Ni0.1Co0.9-ZIF-67 is 1402.4 m 2 g -1 and 1242.8 m 2 g -1 , respectively. The isotherm of Co-ZIF-67 and Ni0.1Co0.9-ZIF-67 are identified as type I, which is characteristic of microporous materials. The corresponding Density Functional Theory (DFT) results and pore size distribution curve further indicates that Co-ZIF-67 contains the micropores. The BET specific surface area for sample Co3O4, is 2.2 m 2 g -1 , while for Ni0.2Co0.8Oy is 3.2 m 2 g -1 .The isotherms of CoP and Ni0.1Co0.9Py are identified as type IV characteristic of mesoporous materials. CoP had a BET surface area of 19.4 m 2 g -1 , while for Ni0.1Co0.9Py is 12.3 m 2 g -1 (Fig. S5 a,b). The specific surface area of Co9S8 is 105.1 m 2 g -1 and for Ni0.5Co0.5Sy is 107.5 m 2 g -1 (Figure S5 c,d and Table S2.)The above results suggest that for the oxide and sulfide, the bimetallic catalysts with larger specific surface area would provide abundant catalytic active sites and facilitates the transport of electron and active species during the catalysis process. While only the phosphide are characteristic of mesoporous materials, we assume that the mesoporous structure is beneficial for charge and mass transport for electrocatalysis.

The FT-IR spectra of Co-ZIF-67 and Ni0.1Co0.9-ZIF-67 are shown in Fig. 3a.The IR bands are primarily attributed to the ligand 2-Methylimidazole (2-MIM). The peaks in the range of 600-1500 cm -1 correspond to the stretching and bending modes of the imidazole rings, whereas the band at 1580 cm -1 is attributed to the C-N stretching mode of 2-MIM. Additionally, the bands at 2929 and 3135 cm -1 correspond to the C-H stretching mode of the aromatic ring and the aliphatic chain of the 2-MIM, respectively [21] . This result reveals that the bi-metallic Ni0.1Co0.9-ZIF-67 crystals have been successfully prepared. While several peaks in the range of 600~1500 cm -1 are not present in the spectra of Co3O4 and Ni0.2Co0.8Oy, and the stretching mode of C-N bonding and the aliphatic chain of the 2-MIM at 1580 cm - 1 and 3135 cm -1 , respectively become obscured (Fig. 4b). The C-H stretching mode of the aromatic ring at 2929 cm -1 is present. The peaks at 661 cm -1 and 570 cm -1 corresponding to the Co-O vibration [22] confirm that the ZIF-67 structure collapsed and Co is oxidized into Co3O4.

The FT-IR analysis verifies the presence of P-O bands in the spectra of CoP and Ni0.1Co0.9Py as shown in Figure 3c. Several bands in the region of 1150 -1000 cm -1 are assigned to the P-O vibrations of the PO4 group [23] . While the signals associated with the C-N stretching and the aliphatic chain of 2-MIM (1580 cm -1 and 3135 cm -1 ) are present in the spectra of Co9S8 and Ni0.5Co0.5Sy (Fig. 3d) indicating that ZIF-67 structure almost collapsed after sulfuration. The new band at 617 cm -1 belongs to the Co-S bending vibration of the Co9S8 [20b] . The peak at 1125 cm -1 is assigned to the S-O bending vibration [24] , indicating that Co is sulfured into Co9S8. The IR study confirmed that NixCo1-xOy, NixCo1-xPy and NixCo1-xSy catalysts have been successfully synthesized.

The XPS spectra of all catalysts are shown in Fig. 4. The standard binding energy of the main peak of cobalt species (Co 2p3/2) at 777.9 eV is present in all as-synthesized catalysts. While the Co 2p3/2 core level region contains a main peak at 780.97 eV (Ni0.1Co0.9-ZIF-67), 779.15eV (Ni0.2Co0.8Oy), 778.6 eV (Ni0.1Co0.9Py) and 778.56eV (Ni0.5Co0.5Sy), indicating the differences of the energy shift in the Co 2p spectra (Fig. 4b). The as-prepared materials show higher binding energy in the Ni 2p3/2 region compared with nickel species (852.3 eV), for instance, 855 eV for Ni0.1Co0.9-ZIF-67, 853.88eV for Ni0.2Co0.8Oy, 853.4eV for Ni0.1Co0.9Py and 853eV for Ni0.5Co0.5Sy (Fig. 4c). The positive metallic (Ni and Co) and negative P centers can play a role of hydride-acceptor and proton-acceptor sites, respectively, during the electrocatalytic process [19,25] .Therefore, the positively charged Co and Ni and the negatively charged O, P and S centers could act as hydride-acceptor and proton-acceptor, respectively. The acceptor could promote the formation of cobalt hydride, which could facilitate the following H2 evolution by electrochemical desorption [3b, 9a, 19, 25b, 26] .

Electrocatalytic performance of NixCo1-x-ZIF-67, NixCo1-xOy, NixCo1-xPy, and NixCo1-xSy for hydrogen evolution reaction (HER)

The electrocatalytic property of the nickel-cobalt based catalysts towards HER is evaluated. Fig. S5 shows LSV curves in alkaline media (pH =14±0.5) for NixCo1-x-ZIF (a), NixCo1-xOy (b) ,NixCo1-xPy (c) , NixCo1-xSy (d) at a scan rate of 10 mV s -1 , respectively. It was found that at a current density of 10 mA cm -2 , the catalysis showed overpotential in the following order, phosphide < oxide< sulfide < ZIF-67 in both mono-metallic and bi-metallic derivatives with specific metal ratio: CoP < Co3O4< Co9S8< ZIF-67 and Ni0.1Co0.9Py< Ni0.2Co0.8Oy< Ni0.5Co0.5Sy< Ni0.1Co0.9-ZIF-67. The comparison of alkaline HER activities of as-prepared catalysts with the same ratio in the precursor are listed in the supporting information (Table S3). Obviously, the phosphide wuith the metal ratio Co: Ni = 10:0, 9:1, and 7:3 showed the lowest overpotential at current density of 10 mA cm -2 , while the others (samples Ni0.2Co0.8Oy, Ni0.4Co0.6Oy and Ni0.5Co0.5Sy) show the lowest overpotential. The phosphide performed superior in the high Co: Ni ratio, while the oxide and sulfide showed good performance in the middle ratio, indicating the optimum synergistic effect of Ni and Co in samples NixCo1-xOy, NixCo1-xPy and NixCo1-xSy. The LSV, Tafel plot and Electrochemical Impedance Spectroscopy (EIS) results are shown in the Figure . 5. The Fig. 5a shows linear sweep voltammetry (LSV) curves for Ni0.1Co0.9-ZIF-67, Ni0.2Co0.8Oy, Ni0.5Co0.5Sy, and Ni0.1Co0.9Py catalysts in alkaline media (pH =14±0.5) in comparison with bare glassy carbon electrode (GCE) and commercial Pt/C catalyst. The Ni0.1Co0.9Py is the most active for alkaline HER with a low overpotential of 160 mV to deliver 10 mA cm -2 . As shown in Table S4, (supporting information), compared with other as-prepared catalysts and previously reported Co-based catalysts, clearly the NixCo1-xPy catalysts have relatively lower overpotentials at the current densities of both 10 and 20 mA cm -2 . Moreover, the Ni0.1Co0.9Py has the lowest Tafel slope of 100 mV dec -1 , being smaller than that of Ni0.1Co0.9-ZIF-67 (109.11 mV dec -1 ), Ni0.2Co0.8Oy (129.28 mV dec -1 ) and Ni0.5Co0.5Sy (279.43 mV dec -1 ) catalysts (Fig. 5b). To evaluate the catalytic activities, the electrochemical impedance spectroscopy analysis for Ni0.1Co0.9Py catalyst under the same operating conditions was conducted. For Ni0.1Co0.9-ZIF-67, Ni0.2Co0.8Oy, Ni0.5Co0.5Sy, and Ni0.1Co0.9Py catalysts, low charge transfer resistances of 24.8, 21.0, 49.8, and 10.2 Ω, respectively are measured (Fig. 5c). In the equivalent circuit, Rs is related to the electrical transport property of a catalyst. The charge transfer resistance (Rct) is associated with the catalytic kinetics at the catalyst/electrolyte interface, small Rct values contribute to faster charge transfer.

The sharply decreased diameter of Ni0.1Co0.9Py in the Nyquist plots of Fig. 5c suggests a much faster electron-transfer kinetics. The Ni0.5Co0.5Sy showed the largest Rct which demonstrates the lowest electron-transfer kinetics. The following trend for the catalysts at the same current density of 10 or 20 mA cm -2 is observed: Ni0.1Co0.9Py>Ni0.2Co0.8Oy>Ni0.5Co0.5Sy>Ni0.1Co0.9-ZIF-67. To evaluate the stability of the catalysts, longterm electrolysis in 1.0 M KOH is carried out. The four catalysts show almost the same LSV curve with negligible cathode current drop after 2000 cycles, which confirms their high stability for alkaline HER (Fig. S9). To get further insight into the structure-activity correlation between HER performance and different catalysts, a series of DFT calculations were performed (SI). For an ideal electrocatalyst, a moderate H adsorption free energy is close to zero (ΔGH* ≈ 0), which is in favor of the adsorption and desorption steps, thus facilitating the proton-electron-transfer process. Therefore, proton adsorption free energy was adopted to correlate the HER performance. The results are illustrated in Fig. 6 The electrochemical activity of the catalysts for HER are comparable and slightly improved than those reported in the open literature so far [11a, 18, 27] . The HER activity of Ni0.1Co0.9Py in comparison with the materials already published are summarized in Table S4. The phosphides have high affinities for H2 and they could be suitable for HER electrocatalysis. The best performance of the Ni0.1Co0.9Py catalyst is attributed to the following reasons: (i) the nanosized bimetallic structure provide abundant active sites for the electrochemical reaction, (ii) Ni and Co atoms are atomically arranged in the Ni0.1Co0.9Py, (iii) the electron transfer from Ni and Co to P improve the electron-donating ability of the catalyst, leading to the high HER activity, (iv) DFT results further indicate that the synergistic effect between Co and Ni with metal ratio of 9:1 in phosphide can increase the binding strength of H, thus improving the electrocatalytic performance. Overall, the Ni0.1Co0.9Py has been considered as a highly efficient and stable electrocatalyst for HER. The high HER activity of the Ni0.1Co0.9Py modified electrode is related to the appropriate nickelcobalt metal ratio, high crystallinity, and nickel-cobalt synergistic accelerating electron transfer from Ni and Co to P. This correlation between the composition and properties of nickel-cobalt nanoparticles and their electrocatalytic performance for hydrogen evolution in alkaline media are the base for further design of better transition metals catalyst toward the HER.

Conclusions

Experimental Section

Chemical and reagents

Cobalt acetate (Co(CH3CO2)2•4H2O), nickel acetate (Ni(CH3CO2)2•4H2O), 2-methylimidazole (C4H6N2), triethylamine ((C2H5)3N), potassium hydroxide (KOH, pellets), thiourea (CH4N2S), nafion (5 wt%), ethanol (C2H6O) were purchased from Sinopharm Chemical Reagent Co., Ltd. All reagents and solvents were used as received without further treatment.

Preparation of Co-ZIF-67: Co-ZIF-67 nanocrystals were synthesized following the procedure described by Wang et.al. [14a] with slight modification. In a typical synthesis, solution A was prepared by cobalt acetate (0.4152 g, 0.00167 mol) dissolved in 15 ml distilled water. Solution B was prepared by

Scheme 1 .

 1 Scheme 1. Schematic diagram representing the preparation of Ni1-xCox-ZIFs-67, Ni1-xCoxOy, Ni1-xCoxSy

Figure 1 .

 1 Figure 1. XRD patterns of Co-ZIF-67 and Ni0.1Co0.9-ZIF-67 (a), Co3O4 and Ni0.2Co0.8Oy (b), CoP and

Figure 2 .

 2 Figure 2. SEM images of Co-ZIF-67 (a), Ni0.1Co0.9-ZIF-67 (b), Co3O4 (c), Ni0.2Co0.8Oy (d), CoP (e),

  also confirm the uniform distribution of all elements in the Co-ZIF-67 (a) (Co ,C ,N ,O), Ni0.1Co0.9-ZIF-67 (b) (Co, Ni), Co3O4 (c) (Co, O), Ni0.2Co0.8Oy (d) (Ni, Co, O), CoP (e) (Co, P), Ni0.1Co0.9Py (f) (Ni, Co, P), Co9S8 (g) (Co, S) and Ni0.5Co0.5Sy (h) (Ni, Co, S). Additionally, the chemical composition of the materials was characterized by ICP-AES (Table

  ) planes of Co9S8, respectively (Fig.S2g,h insert image).

Figure 3 .

 3 Figure 3. FT-IR spectra of Co-ZIF-67 and Ni0.1Co0.9-ZIF-67 (a), Co3O4 and Ni0.2Co0.8Oy (b), CoP

Figure 4 .

 4 Figure 4. XPS spectra of Co-ZIF-67, Ni0.1Co0.9-ZIF-67, Co3O4, Ni0.2Co0.8Oy, CoP, Ni0.1Co0.9Py,

  The XPS spectra of NixCo1-xOy, NixCo1-xPy, and NixCo1-xSy catalysts are shown in the SI (Figs. S5-S7). The Ni0.1Co0.9Py (Fig. S6), shows low binding energy (Co 2p1/2 and Ni 2p1/2) compared to metallic Ni, Co, and P. The binding energies of Ni 2p and Co 2p exhibit positive shifts while P 2p is negatively shifted which is mainly due to the partial electron transfer from Ni and Co to P.

Figure 5 .

 5 Figure 5. Polarization curves of Ni0.1Co0.9-ZIF-67, Ni0.1Co0.9Py, Ni0.2Co0.8Oy, Ni0.5Co0.5Sy, commercial

  and the optimized adsorption structures are presented in Fig. S11. We firstly compared the H adsorption free energy of Co9S8, Co3O4 and CoP (Fig. 6a), and the H adsorption free energy of Co9S8 (1.01 eV), Co3O4 (-0.74 eV) and CoP (0.09 eV) are similar with the experimental results. Due to the lowest H adsorption free energy, the CoP shows excellent HER performance. We further built the Ni doped CoP model to simulate the Ni1-xCoxPy (Fig. 6b), of which the ΔGH* are: Co0.7Ni0.1P (0.08197 eV), Co0.6Ni0.2P (0.09327 eV), and Co0.5Ni0.3P (0.116582 eV). As the Ni decreases, the Ni1-xCoxPy showed lower ΔGH* in the order Co0.5Ni0.3P > Co0.6Ni0.2P > Co0.7Ni0.1P, which is in accordance with the experimental result demonstrating that the phosphide performance is superior in the sample with high Co: Ni ratio. In conclusion, the CoP shows superior HER performance in contrast to the Co9S8 and Co3O4. The Co0.7Ni0.1P shows the highest electrochemical performances according to the DFT calculations, while sample Co0.9Ni0.1Py with best HER performance was obtained experimentally.

Figure 6 .

 6 Figure 6. Calculated free energy diagram of the HER on the Co3O4 (110), Co9S8 (111), CoP (101)

  In summary, nanosized homeotypic bimetallic Ni1-xCoxOy, Ni1-xCoxSy and Ni1-xCoxPy catalysts were prepared from Ni1-xCox-ZIF-67 by temperature-programmed reactions with oxygen, sublimed sulfur and sodium hypophosphite, respectively. Compared to the monometallic Co-based materials, the homologous bimetallic Ni1-xCox-ZIF-67, Ni1-xCoxOy, Ni1-xCoxSy and Ni1-xCoxPy catalysts exhibit better electrocatalytic activity for hydrogen production in alkaline water electrolysis.

Acknowledgements

The authors are grateful to the National Natural Science Foundation of China [Grant No. U1862118], funding from Thousand Talents Program, China [WQ20152100316], the Fundamental Research Funds for the Central Universities, China [No. 18CX05018A], the State Key Laboratory of Advanced Technology for Materials Synthesis and Processing (Wuhan University of Technology), China [No. 2018-

dissolving 2-methylimidazole (1.3686 g, 0.0167 mol) and TEA (1.6868 g, 0.167 mol) in 15 ml distilled water. Both solutions were stirred with 800 r/min for 30 min. Then the two solutions were mixed in icewater bath for 10 min. Purple crystals were collected by centrifugation and washed several times with distilled water and dried at 60 °C oven for 12 h. The obtained samples were denoted as Co-ZIF-67.

Preparation of NixCo1-x-ZIF-67: the cobalt acetate in the synthesis precursor was replaced with a mixture of cobalt acetate and nickel acetate, the Co/Ni molar ratios were adjusted from 9:1 to 1:1. The preparation of the NixCo1-x-ZIF-67 was performed in ice-water bath for 10 min. The washing and drying procedure were same as before. The obtained samples were denoted as NixCo1-x-ZIF-67.

Synthesis of mono-and bi-metallic NixCo1-xOy, NixCo1-xSy, NixCo1-xPy catalysts

Preparation of NixCo1-xOy: the NixCo1-x-ZIF-67 samples were put into a muffle furnace and annealed at 800 °C for 4 h under air (ramping of 2 °C min -1 ). The final black powder was collected at room temperature (the samples are donated as NixCo1-xOy).

Preparation of NixCo1-xSy [20a, 20b] : NixCo1-x-ZIF-67 (0.2g) and sublimed sulfur (0.1g) were placed at two separate positions in a porcelain boat; the sublimed sulfur was placed at the upstream side of the furnace. The obtained samples were denoted as NixCo1-xSy.

Preparation of NixCo1-xPy [12c, 26, 28] : NixCo1-x-ZIF-67 (0.2g) and NaH2PO2•H2O (1g) were placed at two separate positions in a porcelain boat; NaH2PO2•H2O was placed at the upstream side of the furnace.

The molar ratio of Co/P was 1:20. The obtained samples were denoted as NixCo1-xPy.

Characterization

The structure of the samples was characterized by X-ray powder diffraction (Siemens D5005 diffractometer) with Cu Kα radiation (λ=1.5418 Å) in Bragg-Brentano (θ~2θ) configuration. The structure and morphology of the as-prepared samples were characterized by a field emission scanning electron microscope (JEOL JSM-7900F). Transmission electron microscopy (TEM) images were obtained using a JEM 2100 transmission electron microscope operating at 200 kV. The distribution of the elements of the samples was measured by EDS-SEM using JEOL-7900F SEM and Oxford Instruments X-MaxN Energy-Dispersive Spectroscopy. The hydrogen and nitrogen adsorption-desorption experiments were performed at 77K using Quantachrome autosorb iQ3 gas sorption analyzer. The external surface area was determined by the Brunauer-Emmett-Teller Method (BET).The infrared (IR) spectra of samples were collected by NEXUS FT-IR using the KBr technique. Chemical composition of the as-synthesized samples were characterized by X-ray photoelectron spectroscope with PH 5000

Versaprobe system using monochromatic Al Kα radiation (hν=1486.6 eV). Inductively Coupled Plasma (ICP) was performed by Agilent ICPOES 720.

Electrochemical Measurements

All electrochemical measurements were carried out in nitrogen-purged 1 M KOH solution (pH=14±0.5) at room temperature on a CHI660E electrochemical workstation using a standard threeelectrode system: a saturated calomel electrode (SCE) used as a reference electrode, a carbon rod used as a counter electrode, and a glassy carbon electrode (GCE, 0.1256 cm 2 , geometric area) used as a working electrode. The working electrode was prepared as follows. Typically, the catalytic powder (3.6 mg) and activated carbon (0.09 mg) are added into a mixture of 1 mL ethanol and 80 μL Nafion (5 wt%) and sonicated for 40 min to obtain a homogeneous mixture further called ink. Then (10 μL) of the catalyst ink was deposited onto the GCE surface (loading 0.358 mg cm -2 ).

The polarization curves were obtained by linear voltammetry sweeping (LSV) from 0.27 V to -0.53 V (vs RHE) with a scan rate of 10 mV s -1 in 1 M KOH. Alternating current (AC) impedance measurements were carried out at -0.3 and -0.41 V from 100 kHz to 0.1 Hz. The stability of the catalysts was characterized under continuous cyclic voltammetry (CV) measurements from -0.05 to 0.2 V (vs RHE) at a sweep rate of 50 mV s -1 . During the experiments, a flow of nitrogen was maintained over the electrolyte to eliminate dissolved oxygen. All results were calibrated with respect to the reversible hydrogen electrode (RHE) by E (RHE) = E (V vs SCE) + 0.245 V + 0.059 pH [14a] .

Theoretical calculations were performed using Vienna ab initio simulation packages (VASP) based on density functional theory (DFT). [29] The core and valence electrons were described by the projector augmented wave (PAW) pseudopotentials. The generalized gradient approximation (GGA) in the scheme of proposed by Perdew, Burke, and Ernzerhof (PBE) was adopted to express the exchange correlation interactions with a cut off energy of 400 eV. [30] A 12 A ̊ vacuum region was used to simulate the adsorption and all the top two layers of atoms in the structure are allowed to relax. The convergence of energy and forces were set to 1 × 10-5 eV and 0.03 eV, respectively. The free energy of hydrogen adsorption (∆GH*) can be calculated as follows:

∆G (H*) = ∆E + 0.24 eV And ∆E is the adsorption energy, with defined as follows: ∆E =E (substrate + adsorbate)-Esubstrate-Eadsorbate


Peng Peng

Dusan Stosic

Xin-Mei Liu

Zi-Feng Yan

Svetlana Mintova
email: svetlana.mintova@ensicaen.fr

Strategy towards enhanced

Keywords: catalyst effectiveness, effective diffusion factor, diffusion length, zeolites

come

Introduction

Zeolites are a class of microporous crystalline aluminosilicate materials with pores of molecular dimensions (0.3-1.5 nm) [START_REF] Cejka | Introduction to zeolite molecular sieves[END_REF]. Zeolites are widely used in oil refinery, petrochemical manufacture, fine chemical producing and environmental catalysis [START_REF] Martí Nez | Inorganic molecular sieves: Preparation, modification and industrial application in catalytic processes[END_REF]. Success of zeolite in many industrially important processes is based on the strong and tunable Brønsted acidity, small pore size, high internal surface area, flexible frameworks, adjustable and controlled chemistry and high thermal stability. In particular zeolites are known for exceptional catalytic performance due to selectivity control by pore confinement effects [START_REF] Weisz | Intracrystalline and molecular-shape-selective catalysis by zeolite salts[END_REF][START_REF] Frilette | Catalysis by Crystalline Aluminosilicates I. Cracking of Hydrocarbon Types over Sodium and Calcium "X" Zeolites[END_REF][START_REF] Weisz | Catalysis by Crystalline Aluminosilicates II. Molecular-Shape Selective Reactions[END_REF]. This concept is known as shape-selectivity and is crucial in the synthesis of refinery products and organic transformations [START_REF] Meyers | Handbook of Pertoleum Refining Processes[END_REF][START_REF] Gilson | Impact of Zeolites on the Petroleum and Petrochemical Industry[END_REF].

However, the size of pores and cages of zeolites also impose important mass transfer limitations that have negative influence on activity, but can also affect selectivity and lifetime of the catalyst [START_REF] Hoff | Decoupling the Role of External Mass Transfer and Intracrystalline Pore Diffusion on the Selectivity of HZSM-5 for the Catalytic Fast Pyrolysis of Biomass[END_REF]. In the chemical engineering, a catalyst effectiveness (η) is used to describe the degree of catalysts' utilization that is related to mass transport limitations [START_REF] Levenspiel | Chemical Reaction Engineering[END_REF]. Catalyst effectiveness can be generally improved by the utilization of the materials with wide pores and/or with short diffusion length.

A typical case is the fluid catalytic cracking (FCC), which is one of the most important and profitable processes in oil refining industry [START_REF] Gilson | Impact of Zeolites on the Petroleum and Petrochemical Industry[END_REF][START_REF] Primo | Zeolites as catalysts in oil refining[END_REF]. As an important component in FCC catalyst, ZSM-5 zeolite serves for raising propylene and high octane number of gasoline components due to its unique framework structure and surface properties.

Unfortunately, catalytic cracking reaction is always accompanied with coking reactions, i.e. formation of highly condensed poly-aromatic carbonaceous residues (coke species) [START_REF] Collett | Things go better with coke: the beneficial role of carbonaceous deposits in heterogeneous catalysis[END_REF]. Because the kinetic diameters of these coke species are usually much larger than the aperture of ZSM-5 (0.55 nm) [START_REF] Li | Desorption Kinetics of Naphthalene and Acenaphthene over Two Activated Carbons via Thermogravimetric Analysis[END_REF], they have to withstand strong diffusion resistance in the micropores [START_REF] Hoff | Decoupling the Role of External Mass Transfer and Intracrystalline Pore Diffusion on the Selectivity of HZSM-5 for the Catalytic Fast Pyrolysis of Biomass[END_REF][START_REF] Ngoye | Mitigating coking during methylcyclohexane transformation on HZSM-5 zeolites with additional porosity[END_REF]. Thus coke species can easily be accumulated within the microporous channels, causing the rapid deactivation of the zeolite [START_REF] Kim | Effect of mesoporosity against the deactivation of MFI zeolite catalyst during the methanol-to-hydrocarbon conversion process[END_REF].

Within the past decades, due to the quality deterioration of oil throughout the world, deactivation caused by coking reaction becomes even more series [START_REF] Vogt | Fluid catalytic cracking : recent developments on the grand old lady of zeolite catalysis[END_REF].

Integrating both meso-and micro-pores into one hierarchical zeolitic structure is an attainable way to alleviate diffusion limitations and extend the catalysis lifespan of the zeolite frameworks. For example, previous research had shown that in terms of FCC and related reactions, the ZSM-5 based hierarchical materials exhibited higher heavy oil conversion, higher light oil product yield, and lower coke yield than their conventional micronsized counterparts [START_REF] Tarach | Hydrothermal stability and catalytic performance of desilicated highly siliceous zeolites ZSM-5[END_REF][START_REF] Peng | Effects of dissolution alkalinity and self-assembly on ZSM-5-based micro-/mesoporous composites: a study of the relationship between porosity, acidity, and catalytic performance[END_REF][START_REF] Dong | Fine Tuning the Diffusion Length in Hierarchical ZSM-5 to Maximize the Yield of Propylene in Catalytic Cracking of Hydrocarbons[END_REF][START_REF] Awayssa | Modified HZSM-5 as FCC additive for enhancing light olefins yield from catalytic cracking of VGO[END_REF]. Nowadays fabricating of hierarchical structures has become an important topic of zeolites and porous material chemistry [START_REF] Masoumifard | Synthesis of Engineered Zeolitic Materials: From Classical Zeolites to Hierarchical Core-Shell Materials[END_REF][START_REF] Möller | Mesoporosity-a new dimension for zeolites[END_REF]. Various strategies for synthesizing hierarchical zeolitic structures such as dealumination [START_REF] Chang | Screening of optimum condition for combined modification of ultra-stable Y zeolites using multi-hydroxyl carboxylic acid and phosphate[END_REF], desilication [START_REF] Masoumifard | Synthesis of Engineered Zeolitic Materials: From Classical Zeolites to Hierarchical Core-Shell Materials[END_REF][START_REF] Möller | Mesoporosity-a new dimension for zeolites[END_REF], nanocrystallization [START_REF] Ng | Capturing ultrasmall EMT zeolite from template-free systems[END_REF][START_REF] Awala | Template-free nanosized faujasite-type zeolites[END_REF], zeolitization [START_REF] On | Ultrastable and Highly Acidic, Zeolite-Coated[END_REF][START_REF] Liu | Steam-Stable MSU-S Aluminosilicate Mesostructures Assembled from Zeolite ZSM-5 and Zeolite Beta Seeds[END_REF], delamination [START_REF] Corma | Preparation, characterisation and catalytic activity of ITQ-2, a delaminated zeolite[END_REF], hard templating [START_REF] Janssen | Exploratory study of mesopore templating with carbon during zeolite synthesis[END_REF], soft templating [START_REF] Choi | Amphiphilic organosilane-directed synthesis of crystalline zeolite with tunable mesoporosity[END_REF][START_REF] Choi | Stable single-unit-cell nanosheets of zeolite MFI as active and long-lived catalysts[END_REF], composites [START_REF] Wang | Single-template synthesis of zeolite ZSM-5 composites with tunable mesoporosity[END_REF], recrystallization [START_REF] Peng | ZSM-5-based mesostructures by combined alkali dissolution and re-assembly: Process controlling and scale-up[END_REF][START_REF] Ivanova | Micro-mesoporous materials obtained by zeolite recrystallization: synthesis, characterization and catalytic applications[END_REF][START_REF] Garcí A-Martí Nez | A mesostructured Y zeolite as a superior FCC catalystfrom lab to refinery[END_REF], etc. have been developed.

Although diverse strategies for hierarchical zeolites have been advanced, there is little research devoted to the improvement of catalyst efficiency of materials for diffusion-controlled catalytic reaction. In chemical reaction engineering, the descriptor for measuring effects of diffusion resistance on catalytic reactions is the catalyst effectiveness factor (η) [START_REF] Levenspiel | Chemical Reaction Engineering[END_REF][START_REF] Pé Rez-Ramí Rez | Hierarchical zeolites: enhanced utilisation of microporous crystals in catalysis by advances in materials design[END_REF], which can be quantitatively analyzed by adopting Thiele modulus (ϕ). For a reaction that is in regime of diffusion control (η<0.25), ϕ is the reciprocal of η and it is calculated by the following equation:

v eff 1 == k L D  
where kv (intrinsic kinetic constant) is fixed for a given reaction, so the Thiele modulus can only be diminished by raising the Deff (effective diffusion coefficient) or by reducing the L (diffusion length). Based on this criteria, all the strategies for preparing hierarchical zeolitic structure can be divided into two groups, depending on improvement of the catalyst' efficiency: (1) raising the effective diffusion coefficient and (2) reducing the diffusion length of zeolites (Scheme 1). In this work, these two groups are exampled by preparing two samples: (i) ordered mesoporous material with MFI secondary building units (SBU) and (ii) self-assembled nanosized ZSM-5. Their physicochemical properties, catalytic activity, selectivity and deactivation in cumene cracking reaction were evaluated and compared with a conventional micronsized ZSM-5 zeolite.

Experimental Section

Preparation of zeolite catalysts

The conventional micronsized ZSM-5 (Sample-I, SiO2:Al2O3=25) was purchased from Nankai Catalyst Company, and denoted as Sample-I. Ordered mesoporous material with MFI secondary building units (Sample-II) was prepared based on a synthesis method described elsewhere [START_REF] Zhang | Highly hydrothermally stable Al-MCM-41 with accessible void defects[END_REF][START_REF] Song | Synthesis and characterization of MCM-41-type composite materials prepared from ZSM-5 zeolite[END_REF]. Typically, parent ZSM-5 was mixed with NaOH aqueous solution (150 mL of 1.0 mol/L) at 353 K for 60 minutes. Afterwards, 4.5 g cetyl trimethyl ammonium bromide (CTAB) and 60 mL water transparent filtrate were mixed and stirred at 353 K for another 30 minutes. Then the pH of the mixture was adjusted to be 10.5 by dilution with H2SO4 (2.0 mol/L). The mixture was transferred into a Teflon-lined stainless steel autoclave and treated in an oven at 383 K for 24 hours.

The sample was filtrated, dried, and calcinated in a muffle furnace at 823 K for 6 hours.

Self-assembled nanosized ZSM-5 (Sample-III) was prepared from a mixture with a molar composition: 25SiO2: 0.227Al2O3: 3.4(TPA)2O: 441H2O. In a typical synthesis process: tetrapropylammonium hydroxide (TPAOH, 7.5 mL, 1.0 mol/L), water (2.5 mL), and Al2(SO4)3• 18H2O (0.162 g) were mixed and stirred for 10 minutes.

Afterwards, tetraethyl orthosilicate (TEOS, 6 mL, 98%) was added and stirred at room temperature for 6 hours. Then the system was transferred into a stainless steel autoclave with a Teflon-lined container and heated in an oven at 453 K for 12 hours. After crystallization, the product was purified and dried. Finally, the as-synthesized sample was calcinated at 823 K for 6 hours.

Characterization

Powder X-ray diffraction (XRD) patterns were recorded by a PANalytical X'Pert PRO MPD diffractometer equipped with Cu Kα radiation (40 kV, 40 mA, λ = 0.1541 nm) in the 2θ ranges of 2-10.0° (small-angle XRD patterns) and 5-60° (wide-angle XRD patterns).

Nitrogen adsorption-desorption isotherms were collected on a Micromeritics ASAP2020 at 77 K. Prior to analysis, samples were pretreated at 673 K under vacuum.

The Brunauer-Emmett-Teller (BET) equation and the adsorption data collected at p/p0 = 0.96 were used for calculating the specific surface area (SBET) and the total pore volume (Vtotal), respectively. Besides, micropore volume (Vmicro) was calculated by t-plot method using the Harkins-Jura equation.

Transmission electron microscopy (TEM) images were obtained on a JEOL JEM2100UHR instrument.

NH3-temperature programmed desorption (NH3-TPD) measurements were carried out to test the amount and strength of acid sites of the samples by a Micromeritics Auto 2920 chemical adsorption analyzer. Typically, 0.10 g of samples were heated in a quartz tube from room temperature to 973 K under helium flow with temperature ramp of 10 K/min. The samples were then cooled to 373 K, and ammonia was adsorbed for 30 minutes from an ammonia-helium mixture with a molar ratio of 1:9. The samples were flushed under helium for another 30 minutes to remove the gas phase and/or physically adsorbed NH3. TPD profile of ammonia was obtained from 373 K to 873 K at a ramping rate of 10 K /min equipped with thermal couple detector (TCD).

In-situ pyridine adsorption test was used to discriminate the distribution of Brønsted and Lewis acid sites using a Thermo Nicolet 6700 Fourier transform infrared spectrometer. The samples were pressed into self-supporting wafers (diameter: 1.6 cm, 18 mg). Prior to the adsorption, the samples were pre-treated at 723 K for 4 hours under vacuum (10 -6 Torr). Then 1 Torr of pyridine was adsorbed at room temperature to reach the equilibrium. Afterwards, the samples were evacuated at 423, 573 and 723 K and the spectra were collected. The molar extinction coefficients for Brønsted (B) and Lewis (L) acid sites were taken from Ref. [START_REF] Emeis | Determination of integrated molar extinction coefficients for infrared absorption bands of pyridine adsorbed on solid acid catalysts[END_REF].

Catalytic test

The three samples (granules with diameter of 20-40 mesh) were tested in cumene cracking reaction conducted at atmospheric pressure in a downstream stainless steel fixed bed reactor (10 mm internal diameter). The flow scheme of the reactor used for the cumene cracking is shown in Scheme 2. Prior to the reaction, the samples were purged under a flow of nitrogen (60 mL min -1 ) at 623 K for 2 h. The reaction temperature and weight hourly space velocity were kept at 623 K and 2.5 h -1 , respectively. The reactant and products were on-line analyzed by an Agilent 7890A gas chromatograph equipped with a 50 m × 0.25 mm × 0.5 μm PC-PONA capillary column and a flame ionization detector. Cumene conversion was calculated as the amount of converted cumene divided by the amount of cumene in the feedstock. The coke amount on the catalysts after different reaction time was analyzed by an Elementar Vario EL III CHNS/O element analyzer.

Results and Discussion

Phase analysis and textural properties of zeolite catalysts

Wide and small angle XRD patterns, nitrogen adsorption-desorption isotherms, and representative TEM images of the three samples are presented in Figure 1. In the case of Sample-I, the XRD pattern characteristic of a pure MFI phase is measured. It should be noted that there is no diffraction peaks in the low range of 2-6° 2Theta indicating the lack of ordered array of a mesostructure. In accordance with the XRD result, the nitrogen adsorption-desorption isotherm of Sample-I is a typical type I, indicating the exclusive presence of microporous structure. The Sample-I has BET surface area of any Bragg peaks in the range 5-60 ° 2Theta. However, a distinct peak at 2.2° 2Theta indexed as (100) corresponding to ordered hexagonal mesostructured material is present. In addition, two peaks at 4.1 and 4.6° 2Theta with hkl values of ( 110) and (200), respectively confirm the high order arrangement of the hexagonal mesopores. Nitrogen adsorption-desorption isotherm of Sample-II includes an H1 (p/p0=0.35) and an H4 (p/p0>0.45) hysteresis loops typical for Type IV isotherm. These two hysteresis loops are attributed to the ordered Al-MCM-41 mesostructures. The paintbrush-like morphology of the MCM-41 material is shown in Figure 1. Besides, the FTIR spectrum of Sample-II contains an intense band at 534 cm -1 (Figure 2) which is an indication for the presence double five (5-1) ring structure [START_REF] Jansen | Identification of ZSM-type and other 5-ring containing zeolites by i.r. spectroscopy[END_REF][START_REF] Shukla | Estimation of crystalline phase in ZSM-5 zeolites by infrared spectroscopy[END_REF]. This is associated with the presence of secondary building units forming pentasil chains in the MFI framework.

The band at 534 cm -1 is present in the FTIR spectra of both Sample-I and Sample-III (Figure 2). This result confirms that even though the ZSM-5 crystals collapsed after harsh alkali dissolution, some SBUs containing are preserved in the Sample-II [START_REF] Wang | Highly Acidic Mesostructured Aluminosilicates Assembled from Surfactant-Mediated Zeolite Hydrolysis Products[END_REF].

Thus Sample-II can also be regarded as a suitable model of ordered mesoporous material consisting of MFI secondary building units.

The XRD patterns of Sample-III and Sample-I are similar indicating the highly crystalline MFI zeolites. However, Sample-I and III have significantly different textural properties. The nitrogen adsorption-desorption isotherm of Sample-III is a mixture of type I and type IV, typical for hierarchical zeolite, represented by the rapid uptake at low relative pressures followed by nearly horizontal adsorption and desorption branches. The hysteresis loop at high relative pressure appears due to the formation of textural mesopores. Formation of such mesopores can be explained by the self-assembly of nanosized zeolite particles. However, no peaks in the small-angle XRD pattern of Sample-III appeared proving that no long range mesopores are formed.

TEM image further reveals that Sample-III consists of ZSM-5 nanocrystals with size around 20 nm. These nanocrystals are self-assembled in aggregates with a particle size of about 1 μm.

Acidity analysis of zeolite catalysts

Figure 3 shows NH3-TPD profiles of the three catalysts. All samples present a low temperature desorption peak at 473 K (L-peak) and a high temperature peak at 673 K (H-peak). L-peak is assigned to the weak interactions between the molecular sieves and ammonia molecules via hydrogen bonds [START_REF] Niwa | New Method for the Temperature-Programmed Desorption (TPD) of Ammonia Experiment for Characterization of Zeolite Acidity: A Review[END_REF][START_REF] Katada | Determination of the acidic properties of zeolite by theoretical analysis of temperature-programmed desorption of ammonia based on adsorption equilibrium[END_REF]. On the other hand, the high temperature peak corresponds to strong acid sites present on the catalyst surface. The positions of the H-peaks at around 673 K are almost the same. Furthermore, it can be seen that in the case of Sample-I and Sample-III, clear maximum can be observed, while the peak in Sample-II is weak. Moreover, the peak area for the three samples is different, and the area decreases in the following order: Sample-I > Sample-III > Sample-II. As the peak area is proportional to the amount of acid sites, the decrease in the acidic properties of the samples follows the same order. Furthermore, FT-IR spectroscopy study using pyridine as a probe molecule was performed to identify the differences in Brønsted and Lewis acid sites over the three catalysts (Figure 4). Bands at 1545 cm -1 and 1455 cm -1 are assigned to pyridine adsorbed on Brønsted acid sites (BAS) and Lewis acid sites (LAS), respectively.

Quantitative analysis of coordinated pyridine species on the acidic sites are shown in Table S1. The results indicate that the amount of BAS decreases in the following order: Sample-I > Sample-III > Sample-II, whereas the amount of LAS follows the following order: Sample-II > Sample-I > Sample-III. Pyridine desorption at increasing temperatures leads to a decrease of the intensity of both bands and shows preferential desorption from Brønsted acid sites for all samples. Additionally, the BAS on Samples I and III are stable up to 573 K, while the BAS on Sample-II is reduced significantly with the temperature increase. On the other hand Lewis acid sites are more stable upon temperature increase for all three samples. The ratio between BAS and LAS (B/L ratio) (Table S1) at 423 K shows that Sample l and III are primarily Brønsted acid catalysts whereas Sample-II is Lewis acid catalyst.

Distinctively different acidic properties are highly related to the different structural and textural features of the materials. Sample-I and III show strong IR band at c.a. 3610 cm -1 (Figure 5), assigned to the vibration of the bridged hydroxyl groups (Si-O(H)-Al).

In the crystalline zeolitic materials these bridged OH groups are associated with the framework aluminum and the presence of Brønsted acid sites. On the contrary, this feature is not observed for Sample-II (Figure 5), while the pyridine adsorption spectrum clearly shows the presence of Brønsted acid sites (Figure 4). This ambiguity can be explained by the presence of extra framework aluminum in the vicinity of silanols that creates Brønsted acidity. Pseudo Brønsted acid sites generated by this mode are usually weaker in comparison to the BAS formed on the surface of highly crystalline zeolites [START_REF] Hensen | Acidity Characterization of Amorphous Silica-Alumina[END_REF][START_REF] Larmier | Isopropanol Dehydration on Amorphous Silica-Alumina: Synergy of Brønsted and Lewis Acidities at Pseudo-Bridging Silanols[END_REF], as evidenced by the NH3-TPD and pyridine FTIR experiments.

Cumene cracking on zeolite catalysts

Cumene cracking is a standard test reaction used to assess the catalytic behavior of BAS [START_REF] Corma | The catalytic cracking of cumene[END_REF]. During the reaction, cumene is first adsorbed and protonated on strong BAS, then converts into benzene and propylene, which are the main products of the cracking reaction. Afterwards, the formed propylene can further undergo hydrogen transfer reaction and form propane. Figure 6 presents the main results obtained for cumene cracking reaction performed at atmospheric pressure at 623 K. During the initial stage of the reaction, Sample-I reaches the conversion of 98 %, and the ratio of propylene to propane is 1.9. As the reaction proceeds (after 197 min) the conversion decreases greatly to 56 % with the propylene/propane ratio of 0.4 (Sample-I). After 463 min reaction time, the conversion is only 27 %, and the propylene becomes undetectable among the reaction products. The coke amount of Sample-I follows the trend opposite to cumene conversion as it increases gradually from 2.45 % to 6.56 % at 120 min and 463 min, respectively.

In contrast to Sample-I, the cumene conversion of both Sample-II and Sample-III are much more stable within the tested time on stream. The cracking conversion of Sample-II is only 32 % after 63 min. Sample-III exhibits conversion of 97 % at 63 min comparable to that of Sample-I, but still maintains high conversion even after 463 min (92 %). In agreement with the conversion change, the propylene to propane ratios of Sample-II and Sample-III generally are also more stable. Between 63 min and 463 min, the propylene can hardly be detected among the products for Sample-II, whereas the propylene to propane ratio of Sample-III changes negligibly from 2.6 to 2.1.

At the initial stage of the reaction high cumene conversion of Samples I and III reflect their high amount of strong BAS. Ample amount of strong BAS also guarantees that the rate of cumene cracking conversion is much higher than that of hydrogen transfer. As a consequence the relative amount of propylene, which is presented as propylene to propane ratio (Figure 6, B), can maintain rather high level. On the other hand, due to the amorphous nature and weak acidity of Sample-II, the cumene conversion is quite low.

Additionally, the rate of hydrogen transfer is comparable to the rate of cumene cracking, making the propylene content in the product extremely low.

Beside the high initial cracking conversion, the strong BAS are also responsible for the fast formation of coke precursors. In the cracking of cumene, carbenium ions are believed to be formed on the Brønsted acid sites. The stronger BAS are, it is more difficult for the carbenium ions to desorb and they have high possibility to condense further and form large coke precursors. The pores of the MFI zeolite are narrow in comparison to the kinetic diameters of the aromatic coke species [START_REF] Li | Desorption Kinetics of Naphthalene and Acenaphthene over Two Activated Carbons via Thermogravimetric Analysis[END_REF][START_REF] Laprune | Environmental Effect of polyaromatic tars on the activity for methane steam reforming of nickel particles embedded in silicalite-1[END_REF]. These chemical species experience severe diffusion limitations and can easily block the zeolite pores. When the coking reaction reaches a point where whole parts of the active sites are completely blocked by the coke, these active sites are considered to be de facto "dead" [START_REF] Cai | Establishing a discrete Ising model for zeolite deactivation: inspiration from the game of Go[END_REF]. With the further accumulation of the coke, cumene cracking conversion gradually decreases to the point comparable or even lower to hydrogen transfer reaction, making persistent decline of the propylene/propane ratio as seen in the case of Sample-1 (Figure 6). However, in the case of Sample-III with a shortened pathlength, aromatics may pass through the connected micro-and meso-pores, thus the BAS are easily restored and high cracking activity is maintained.

Discussion on the distinctive functions of two types of hierarchical zeolite structures

Although the lifetime of both Sample-II and Sample-III are extended via soothing diffusion limitations of coke species within the MFI framework, cracking reaction performances of these two samples are quite different. Sample-II, as a representative for raising the effective diffusion coefficient, is consisting of ordered mesoporous material with zeotype SBUs (short-range micropores ordering). All active sites are easily accessible for cumene molecules. However, this no strong acid sites are formed in the pore walls of the mesoporous materials that lead to poor cracking activity. Even though the strength and amount of the BAS can be improved by some methods such as partial crystallization [START_REF] On | Ultrastable and Highly Acidic, Zeolite-Coated[END_REF] or nanocrystals assembly [START_REF] Liu | Steam-Stable MSU-S Aluminosilicate Mesostructures Assembled from Zeolite ZSM-5 and Zeolite Beta Seeds[END_REF], the acid site properties still are not comparable with the zeolitic BAS. Sample-III, as a representative of reduced diffusion path, is consisting of self-assembled nanosized ZSM-5 zeolite crystals. This sample has high crystallinity and the active sites have the same nature and strength as the classical micronsized zeolite. While the diffusion path of the reactants and products become much shorter, thus leading to a substantial increase of the catalyst effectiveness factor. Since the bulky coke precursors leave the active sites easier, thus high cumene cracking conversion for a long time on stream is maintained.

Therefore in terms of improving the lifespan of zeolite catalysts via increasing catalyst effectiveness factor, the synthesis methods for reducing diffusion length are superior to the methods for raising the effective diffusion coefficient. Nevertheless, it is should also be noticed that the poor catalytic cracking with Sample-II is due to the mesopore walls amorphous nature. Methods for raising the effective diffusion coefficient of ZSM-5 with single-unit-cell nanosheets structure by Ryoo et al. [START_REF] Choi | Stable single-unit-cell nanosheets of zeolite MFI as active and long-lived catalysts[END_REF] and self-pillared ZSM-5 nanosheets by Tsapatsis et al. [START_REF] Zhang | Synthesis of Self-Pillared Zeolite Nanosheets by Repetitive Branching[END_REF] have been reported. The samples should also contain sufficient BAS to maintain high conversion of cumene cracking reactions for long time.

Conclusions

Two strategies for improving the catalyst effectiveness by raising the effective diffusivity or reducing the diffusion path length are presented. Removing diffusion limitations by utilizing the materials with much larger pores, such as Al-MCM-41, leads to the creation of weak pseudo-Brønsted acid sites that have inferior catalytic activities. On the contrary, utilizing self-assembled nanosized ZSM-5 leads to a high cracking activity accompanied by slower deactivation. It can be concluded that constructing hierarchical zeolitic structures with a short pathways for reactants and products is a good prospective to improve the catalyst effectiveness in diffusion controlled reactions. The mesopores offer improved accessibility of larger molecules, whereas active sites located inside the micropores control the catalytic performance of the materials.

Scheme 1 .Scheme 2 .Figure 1 .

 121 Scheme 1. Strategies toward preparation of model samples with enhanced catalyst effectiveness.

Figure 2 .Figure 3 .

 23 Figure 2. FTIR spectra of Sample-I: micronsized ZSM-5 (A), Sample-II: ordered mesoporous material with zeotype secondary building units (B) and Sample-III: self-assembled nanosized ZSM-5 (C)

Figure 4

 4 Figure 4 FTIR spectra of Sample-I: micronsized ZSM-5 (A), Sample-II: ordered

Figure 5

 5 Figure 5 FTIR spectra of Sample-I: micronsized ZSM-5 (A), Sample-II: ordered mesoporous material with zeotype secondary building units (B) and Sample-III: self-assembled nanosized ZSM-5 (C) in the hydroxyl group region of 3500-3800 cm -1 .

Figure 6

 6 Figure 6 Conversion (A), propylene to propane ratio (B), and coke content (C), as a function of time on stream of Sample-I: micronsized ZSM-5 (a), Sample-II: ordered mesoporous material with zeotype secondary building units (b) and Sample-III: self-assembled nanosized ZSM-5 (c) in cumene cracking reaction.

Table 1

 1 Textural properties of Sample-I: micronsized ZSM-5, Sample-II: ordered mesoporous material with zeotype secondary building units, and Sample-III: self-assembled nanosized ZSM-5.

		SBET/	Vtotal/	Vmicro/
		(m 2 /g)	(cm 3 /g)	(cm 3 /g)
	Sample-I	455	0.21	0.17
	Sample-II	763	0.53	0.01
	Sample-III	516	0.33	0.17

Acknowledgement

PP gratefully acknowledged the China Scholarship Council (CSC201706450035) and K. C. Wong Education Foundation for their financial support and the authors gratefully acknowledged funding from Thousand Talents Program for Foreign Experts (WQ20152100284).