Crystal engineering of hierarchical zeolite in dynamically maintained Pickering emulsion

Chemical Engineering Research and Design 1 5 3 ( 2 0 2 0 ) 49–62

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Crystal engineering of hierarchical zeolite in dynamically maintained Pickering emulsion Xiaoling Zhao a , Hongchang Duan b , Shanbin Gao c , Zheru Shi a , Kake Zhu a,∗ , Xinggui Zhou a a

UNILAB, State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, PR China b Lanzhou Petrochemical Research Center, Lanzhou 730060, Gansu, PR China c Daqing Petrochemical Research Center, PRI, CNPC, Daqing 163318, Heilongjiang, PR China

a r t i c l e

i n f o

a b s t r a c t

Article history:

Crystal engineering of hierarchical zeolites is regarded as a promising way to enhance

Received 14 May 2019

diffusion-dependent catalytic properties of zeolitic materials. Crystallization process control

Received in revised form 4 October

on crystal size and pore-structure is desirable over porogen based protocols and post-

2019

synthetic methods for the low cost, high yield and potential scalability. Herein, a tumbling

Accepted 8 October 2019

crystallization of hierarchical ZSM-5 zeolite in immiscible water/toluene Pickering emulsion inspired by energy dissipating structure occurring in nature is presented. The structure and acid properties of the obtained hierarchical material have been revealed using a

Keywords:

panoply of characterization techniques such as powder X-ray diffraction, N2 physisorption

Zeolite

isotherms, SEM, TEM, mercury protrusion measurements, NH3 -TPD and pyridine IR spec-

Hierarchical structure

troscopy, showing that the material contains high crystallinity, and penetrating macropores.

Energy dissipating structure

Crystallization is found to proceed through a Pickering emulsion structure maintained by

Pickering emulsion

emulsifying effect of constant tumbling. Such an emulsion structure has hindered attach-

Dimethylether to olefin

ment growth of primary nanocrystals formed at the nucleation stage to further grow into larger size via coalescense. The hierarchical zeolite exhibits architecture-dependent prolonged catalyst lifetime and light olefin yield in dimethylether-to-olefin conversion. This process control to generate hierarchical zeolites opens up new ways toward inexpensive, high level control and efficient engineering of zeolite morphology. © 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

1.

Introduction

application. In particular, artificial synthesis of zeolites is a historical breakthrough in the generation of catalytic materials that has enabled efficient use of fossil fuels including petroleum and coal (i.e., such as

The properties of crystalline materials depend greatly on their crystal structure, size, shape and textural features. It is thus important

Y zeolites used for fluid catalytic cracking (FCC) process, and others

to identify new ways to synthesize crystals with tailored architectures so that a particular performance can be enhanced for a given

in isomerization, alkylation, methanol-to-gasoline (MTG), methanolto-olefin (MTO) conversions, etc.), which is regarded as a corner stone of modern civilization. Barrer and co-workers (Barrer, 1948) pioneered

Abbreviation: FCC, fluid catalytic cracking; MTG, methanol-to-gasoline; MTO, methanol-to-olefin; DTO, dimethylether-to-olefin; SDAs, structure directing agents; TEOS, Tetraethyl orthosilicate; TPAOH, tetrapropylammonium hydroxide; TPPS, Trimethoxy[3(phenylamino)propyl]silane; DME, dimethyl ether; HC, hydrocarbons; XRD, X-ray diffraction; FE-SEM, Field-Emission Scanning Electron Microscopy; TEM, Transmission Electron Microscope; BET, Brunauer–Emmett–Teller; NLDFT, Non-Local Density Functional Theory; Py-IR, Pyridine desorption Infrared Spectroscopy; NH3 -TPD, NH3 -Temperature Programmed Desorption; TCD, thermal conductivity detector; ICP-AES, Inductively Coupled Plasma Atomic Emission Spectrometry; MAS NMR, Magic-Angle Spinning Nuclear Magnetic Resonance; WHSV, weight hourly space velocity; GC, gas chromatograph; FID, flame ionization detector; SAED, Selected Area Electron Diffraction; ESI, Electronic supporting information; TOS, time-on-stream; E/P, ethylene to propylene. ∗ Corresponding author. E-mail address: [email protected] (K. Zhu). https://doi.org/10.1016/j.cherd.2019.10.019 0263-8762/© 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

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Chemical Engineering Research and Design 1 5 3 ( 2 0 2 0 ) 49–62

artificial hydrothermal synthesis of zeolites by mimicking geological crystallization conditions necessary for the formation of the natural counterparts. The nature inspired synthesis was lately expanded to the crystallization of a family of microporous crystalline solids, including these without natural analogues, for instance, ZSM-5. Nowadays, mass production of zeolitic materials are still conducted under hydrothermal conditions, fabricating micron sized crystals. On the other hand, certain ordered structures or patterns are often observed in energy dissipating processes in nature, for instances, such as Rayleigh–Bénard convection pattern (Bénard, 1901) that possesses ordered hexagonal structure or vibrating waves formed when a stone drops into water.

Specifically, syntheses of hierarchical ZSM-5 by crystallization process control have been reported recently. Zhang et al. (Zhang et al., 2012) reported repetitive branching growth of MFI/MEL lamellae intergrown in a house-of-cards architecture when the tetrabutylphosphonium or tetrabutylammonium were used as structure directing agents (SDAs). Ding et al. (Ding et al., 2015) recently showed that it is possible to crystallize hierarchical ZSM-5 comprising of non-mesoporous core and an epitaxially grown hyper-branched shell through a stepwise regu-

These ordered structures can only form under non-equilibrium energy

lation of crystallization conditions by taking advantage of Al zoning at the crystal external surface and further epitaxially growth over Al sites. Okubo et al. (Chaikittisilp et al., 2013) have used di-quaternary ammonium cations to fabricate hierarchically organized plate-like MFI

dissipating conditions, and often entail the input of energy to maintain the otherwise unstable structure. Likewise, macro-emulsion structures

zeolites by sequential intergrowth. More recently, Zhang et al. (Zhang et al., 2019) created hierarchically organized nanozeolites of ZSM-5 by

or Pickering emulsion structures are thermodynamically unstable, and

virtue of a hindered oriented attachment crystallization in the presence of amino acids. Despite these successful preparation approaches

have a strong propensity to separate into two immiscible liquids. The maintenance or recovering of such structures often requires additional emulsifying process, i.e., an external energy input process to maintain a dynamically lived, albeit, transient structure. This gives rise to an interesting question, is it possible to tailor the morphology of crystals, such

relying on the use of organic additives, there is still a lacking of ease and high level control of structures with respect to the desired properties. As acid properties play decisive role for intrinsic catalytic

as zeolites, in a dynamically maintained emulsion system? Bearing in mind that once a certain crystalline structure has been formed in such

performance, desirable methods are those that can retain the intrinsic stability/acidity of parent zeolites when auxiliary porosities are introduced. From mass transport standpoint, crystallites size and their

a manner, the morphological features can be recorded. Henceforth, a crystal engineering through process manipulation could be inspired

distribution (Thiele theorem), and pore structure, size and connectivity (Kortunov et al., 2005), particularly, traversing transport pores across

by naturally occurring energy dissipating processes. Indeed, some of us have recently developed a novel phase-transfer synthesis route to prepare nanosized SAPO-31 (Yue et al., 2018) and hierarchically organized nanocrystalline SSZ-13 (Liu et al., 2019), and a Pickering-emulsion

crystal, are crucial parameters that influence mass transfer properties for hierarchical zeolites (Milina et al., 2014). Besides, practical industrial considerations demand methods that use low-cost starting materials, via simple procedures, yet in a scalable manner. A synthetic target

mediated crystallization pathway has been unveiled to be responsible

should consider all these aspects. Despite that many recipes for hierar-

for the formation of such structures. The exploration of the full potential of this unique synthesis route will construct a general method to create hierarchical zeolites, and the revelation of hidden crystallization

chical zeolites exist, achieving full control over the final products can be challenging, even for well-established systems. Herein, we demonstrate a toluene/water biphasic crystallization of

mechanism will not only better our understanding of the influence of Pickering emulsion structure on morphology of the crystal grown from

hierarchical ZSM-5 made up of assembled nanozeolites. As shall be shown, such a synthesis not only enables us to retain the intrinsic acid-

it, but also shed light on controlling factors that tailor such structures.

ity pertaining to zeolitic crystallinity, but also offers high level control

Zeolites are a family of microporous crystalline solids made up of

over primary crystallites size and pore-connectivity. Another motivation in this investigation is to understand the crystallization of zeolites

TO4 units (T = Si, Al, etc.) by corner-sharing O vertex, which are practically used as ion-exchangers, adsorbents, acid catalysts in numerous

in Pickering emulsion under constant tumbling and find facile ways to

industrial processes. As solid acid catalysts, zeolites possess unique shape selectivity owing to their crystallographically defined pore size, geometry and inter-connectivity. On the other hand, the solely presence of microporosity in commercial micron sized crystals also

engineer crystal morphology through process control. The investigation is structured into three parts. In the first part, the synthesis and structural features of the obtained material will be disclosed. Next, the crystallization process shall be monitored by tracking structural evo-

imparts zeolitic materials with diffusion or accessibility limitations that severely hinder their extended applications. To circumvent such

lution, with the revelation that such a design has profoundly changed the crystallization pathway. A predominant role of non-classic oriented

limitations, nanozeolites and hierarchical zeolites have been advanced since 2000 (Jacobsen et al., 2000). Hierarchically porous zeolites, i.e., zeolitic crystals that contain both auxiliary macro-/meso-pores in addi-

attachment growth in Pickering emulsion system responsible for the formation is to be uncovered. Furthermore, the catalytic consequence of hierarchical structure in dimethylether-to-olefin (DTO) will be inves-

tion to the inherent micropores, have attracted numerous attention

tigated.

from both academia and industry, as have been well documented in several excellent review articles (Li et al., 2019; Mariya et al., 2019; Na et al., 2013; Schwieger et al., 2016). Hierarchical zeolites are advantageous to nanozeolites, as the latter are technically difficult in handling or processing during synthesis, shaping and upscaling. Moreover, nanozeolites often give rise to significant pressure drop for fixed bed catalytic systems and are not as catalytically or hydrothermally stable as their conventional counterparts. Importantly, commercial applications of hierarchical ZSM-5 derived from desilication route (Pérez-Ramírez et al., 2011) and mesoporous Y zeolites generated from surfactant-templating postsynthetic recrystallization process (GarcíaMartínez et al., 2012) have been recently reported. Methods to generate hierarchical zeolites can be categorized as soft-templating using various porogen molecules (Choi et al., 2006; Inayat et al., 2012; Serrano et al., 2014; Wang and Pinnavaia, 2006; Zhu et al., 2014), hard-templating (Jacobsen et al., 2000; Wang et al., 2014; Zhu et al., 2008), dual functional structure directing agents (Na et al., 2011), post-synthetic treatments (García-Martínez et al., 2012; Milina et al., 2015), as well as crystallization process manipulations. Among them, crystallization process control such as microwave synthesis (Hao et al., 2018), dry gel conversion (Chen et al., 2014; Möller et al., 2011), etc. has the advantages of inexpensiveness, simplicity, high yields, as well as potential scalability.

2.

Experimental section

2.1.

Materials

Tetraethyl orthosilicate (TEOS, 28.0 wt.% SiO2 , Shanghai Lingfeng Chemical Reagent Co. Ltd.), sodium aluminate (NaAlO2 , AR, Shanghai Aladdin Biochemical Technology Co. Ltd.) and tetrapropylammonium hydroxide (TPAOH, 40.0 wt.% in water, Shanghai Titan Reagents Co. Ltd.) were used as silicon source, aluminum source and SDAs, respectively. Sodium hydroxide (NaOH, 98.0 wt. %, Shanghai Titan Chemical Co., Ltd.) was used as sodium source and mineralizer. Trimethoxy[3-(phenylamino)propyl]silane (TPPS, 98.0 wt.%, TCI) was used as porogenic agent. Ammonium chloride (NH4 Cl, AR, Shanghai Aladdin Biochemical Technology Co. Ltd.) was employed to exchange Na-type zeolites to H-type zeolites. Toluene (99.8 wt.%, Shanghai Titan Chemical Reagent

Chemical Engineering Research and Design 1 5 3 ( 2 0 2 0 ) 49–62

Co. Ltd.) and deionized water were used as the synthetic solvents. All materials were used without further purifications.

2.2. Synthesis of hierarchical ZSM-5 in Pickering emulsion The molar ratio of initial gel was 100 SiO2 :2 Al2 O3 :20 TPAOH:4 Na2 O:2500 H2 O:500 toluene:5 TPPS. In a typical synthesis, NaOH was dissolved in deionized water and stirred magnetically for 15 min at 300 rpm. Next, NaAlO2 was added to the above mixture under stirring, followed by addition of TPAOH solution, which was allowed to stir for 1 h. Finally, TEOS was introduced into the mixture under continuous stirring for 6 h at 298 K until hydrolyzed completely. The homogenized mixture was transferred into a Teflon-lined stainless steel autoclave, together with another mixture containing a certain amount of organosilane dissolved in toluene. Hydrothermal crystallization was conducted at 443 K for 48 h under dynamic tumbling conditions with a rotating rate of 60 rpm. The obtained product was separated via centrifugation under 10,000 rpm rotation speed for 5 min, after washing with deionized water 3 times until the pH was neutral, then dried at 353 K for 6 h in a convection oven, and calcined at 823 K for 6 h in a muffle oven in air by a heating ramp of 2 K min−1 . The synthetic sample was named ZSM-5-P, with the suffix indicating Pickering emulsion that was the crystallization media.

2.3.

Synthesis of control sample

For comparison, a control sample was synthesized under hydrothermal static conditions according to a literature (Shen et al., 2018). The molar ratio of the gel was: 100 SiO2 :2.0 Al2 O3 :20 TPAOH:4 Na2 O:2500 H2 O. In a typical synthesis, NaOH was dissolved in deionized water before adding NaAlO2 and TPAOH to the solution. The solution was further homogenized for 1 h. In the end, TEOS was introduced into the above solution and continuously stirred for 6 h at 298 K to achieve complete hydrolysis. The obtained suspension was hydrothermally crystallized at 443 K for 48 h under static conditions. Finally, after the same recovery treatment as described above, the collected sample was named ZSM-5-C, with the suffix indicating control sample. In order to prepare proton-type zeolites, both zeolites were ion-exchanged three times in 1.0 mol L−1 NH4 Cl solution at 353 K under vigorous stirring and refluxing for 8 h. Finally, the powder was dried and calcined at 823 K for 6 h in a muffle oven in air. To track the crystallization process, the crystallization was quenched with tap water at varied timespans. The corresponding samples were treated by the same process as described above. Especially, for ZSM-5-P crystallization, the Pickering emulsion mother liquor after tumbling crystallization was collected and analyzed.

2.4.

Characterizations

The phase structure of the ZSM-5 was characterized by powder X-ray diffraction (XRD) patterns recorded on a Rigaku D/Max 2550 VB/PC diffractometer, operating at 40 kV and 100 mA with Cu K␣ ( = 1.5418 Å) as X-ray source. The patterns were collected over a 2 range from 3 to 50◦ , with a scanning speed of 10◦ min−1 .

51

The size and morphological features of the samples were determined by means of micrographs images, recorded with Field-Emission Scanning Electron Microscopy (FE-SEM) NOVA Nano SEM450 microscope equipment. Transmission Electron Microscope (TEM) micrographs were obtained using a JEM2011 (JEOL) electron microscope setup operating at 200 kV. A drop of the examined solution was placed on a TEM grid covered by a perforated carbon film. N2 adsorption–desorption isotherms were collected on an ASAP 2020 (Micromeritics, USA) apparatus at 77 K. All samples were outgassed at 623 K under vacuum for 12 h prior to measurements to remove contaminates. The surface areas and pore volumes were determined by the Brunauer–Emmett–Teller (BET) method, and Non-Local Density Functional Theory (NLDFT) method, respectively. Micropore volumes were derived from a t-plot method, and the total pore volume values were estimated from the adsorbed quantity at a relative pressure P/P0 = 0.99. Mercury intrusion tests were performed by using an AutoPore IV 9500 instrument. The intrusion volumes were measured with a stepwise increase of pressures, equilibrated at each pressure step. The intrusion measurements started from a vacuum condition, and the extrusion measurements terminated at normal pressure. The pore size distribution was calculated according to the intrusion curves (Ritter and Drake, 1945). The acidity of ZSM-5 samples was probed by alkaline molecule Pyridine desorption Infrared Spectroscopy (Py-IR) on a Spectrum 100 Fourier-Transform Infrared Spectroscopy (FTIR) spectrometer (Nicolet Co., USA). All samples were pressed into self-supporting wafers (diameter: 1.4 cm, weight: 40 mg) and were pre-heated at 623 K for 2 h under vacuum (1.3 × 10−2 Pa). The background baselines of the samples were collected and subtracted at room temperature. The samples were purged at 423 K with helium for 0.5 h and cooled to room temperature, and pyridine-adsorbed IR spectrum was recorded. The amount of adsorbed probe molecules was calculated from the integrated area of given bands with their distinct molar extinction coefficients (εBrønsted (1545 cm−1 ) = 1.67 cm ␮mol−1 and εLewis (1455 cm−1 ) = 2.22 cm ␮mol−1 ) (Emeis, 1993). NH3 Temperature Programmed Desorption (NH3 -TPD) patterns were recorded on a Chemisorb 2720 analyzer (Micromeritics Co., USA). The samples were activated at 873 K for 1 h under He atmosphere to remove surface contaminates before measurements, then cooled down to 373 K to adsorb ammonia to reach saturation, purged with He for 1 h to remove weakly physisorbed ammonia, and finally heated up to 873 K with a constant rate of 10 K min−1 in helium (40 mL min−1 ). TPD profiles were obtained in the temperature range from 373 K to 873 K with a thermal conductivity detector (TCD). Elemental analyses were measured by Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES) on an IRIS 1000 instrument (Agilent Technologies, USA). Solid-state Magic-Angle Spinning Nuclear Magnetic Resonance (MAS NMR) measurements were performed using an Agilent DD2-500 MHz spectrometer operating at a magnetic field strength of 11.7 T. Before collecting NMR spectra, each zeolite sample was hydrated in a close vessel filled with vapor of NH4 Cl saturated solution overnight. 27 Al MAS NMR spectra were recorded at 130.2 MHz with a spinning rate of 13 kHz, 200 scans, and 2 s recycle delay. The chemical shifts were referenced to 1.0 wt.% aluminum nitrate (Al(NO3 )3 ) aqueous solution.

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Chemical Engineering Research and Design 1 5 3 ( 2 0 2 0 ) 49–62

Fig. 1 – XRD patterns of ZSM-5-P derived from phase-transfer synthesis and ZSM-5-C synthesized from a standard hydrothermal synthesis.

2.5.

Catalytic assessments

The DTO reaction was performed on a fixed-bed reactor at 623 K with a dimethyl ether (DME) weight hourly space velocity (WHSV) of 10 h−1 g-DME g-cat −1 . The catalyst loading was 50 mg (60–80 mesh). Prior to test, the catalyst was pretreated in air flow (30 mL min−1 ) at 823 K for 3 h. Afterwards, the catalyst was cooled down to the reaction temperature under N2 flow (15 mL min−1 ). The effluent products were analyzed using an on-line gas chromatograph (GC) equipped with a flame ionization detector (FID) and Plot-Q column (Agilent J&W GC Columns, HP-PLOT/Q 19091P-Q04, 30 m × 320 ␮m × 20 ␮m). Methanol and DME were considered as reactants, and all effluent non-oxygen hydrocarbons (HC) were perceived as products. The conversion and product selectivity were defined as follows: DME conversion =

C atoms of HC product × 100% C atoms of DME feed

Product selectivity =

C atoms of HC product × 100% Total C atoms HC products

3.

Results and discussion

3.1.

Structure and morphology of hierarchical ZSM-5-P

Hierarchical ZSM-5 was synthesized through a phase-transfer route in toluene/water mixture under tumbling crystallization. In phase-transfer synthesis, the amount of organosilane has been found to be an important factor for the tailoring of morphology and textural properties of SSZ-13 (Liu et al., 2019). In order to find a proper recipe for a successful synthesis of hierarchical ZSM-5, a series of samples with varied TPPS (100 SiO2 :2 Al2 O3 :20 TPAOH:4 Na2 O:2500 H2 O:500 toluene:x TPPS, x varied from 0 to 10) amounts were synthesized, while keeping the Si/Al ratio to fixed at 25. Tentative XRD and FE-SEM characterizations (Fig. S1 and S2, Electronic supporting information, ESI) demonstrated that although all samples were pure phase ZSM-5, only hierarchically structured ZSM-5 with small crystallites size and hierarchical architecture could be

Fig. 2 – FE-SEM micrographs for ZSM-5-C (a, b), ZSM-5-P (c, d), and TEM micrographs for ZSM-5-C (e, f), ZSM-5-H (g, h). Insets of (e) and (g) show the corresponding SAED patterns. obtained by varying x from 2.5 to 7.5. Less amount was unable to tailor the crystal size and create auxiliary porosity, whereas excessive amount of TPPS tended to generate impurities. The intensive structural characterizations were therefore focused on an optimized sample, i.e. ZSM-5-P, derived from a synthetic gel with the composition of 100 SiO2 :2 Al2 O3 :20 TPAOH:4 Na2 O:2500 H2 O:500 toluene:5 TPPS. The structural characteristics of ZSM-5-P were investigated by powder XRD, SEM, TEM, N2 physisorption measurements and Hg protrusion tests, and comparisons were made with that of ZSM-5-C synthesized from standard hydrothermal synthesis. XRD patterns of both samples demonstrated typical MFI structure (JCPDS No. 42-0023), as shown in Fig. 1. The sharp and intense characteristic peaks implied high crystallinity, pure phase of ZSM-5-P. Relative crystallinity of ZSM-5-P calculated by integrating the peak area in the 2 range of 22.5◦ –25◦ was determined to be 105%, referring to presetting the crystallinity of ZSM-5-C to be 100% (ASTM International, 2011), thus suggesting a good preservation of crystallinity by the

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Table 1 – Composition date and textural properties of calcined ZSM-5-C and ZSM-5-P derived from N2 physisorption isotherms. Sample

Si/Ala

SBET (m2 g−1 )b

Smicro (m2 g−1 )c

Sext (m2 g−1 )

Vmicro (cm3 g−1 )d

Vtotal (cm3 g−1 )d

ZSM-5-C ZSM-5-P

26 29

406 489

312 315

94 174

0.13 0.13

0.29 0.43

a b c d

Determined by ICP-AES. Calculated by the BET method. Deduced by the t-plot method. Inferred from volume absorbed at P/P0 = 0.99.

Table 2 – Py-IR Measurements of Acidity for ZSM-5-C and ZSM-5-P. CB (␮mol g−1 )a

Sample

ZSM-5-C ZSM-5-P a

CL (␮mol g−1 )a

CB /CL

423 K

623 K

723 K

423 K

623 K

723 K

423 K

623 K

723 K

155 138

126 105

110 87

63 104

22 46

17 45

2.48 1.33

5.74 2.29

6.29 1.95

The number of Brønsted acid sites and Lewis acid sites were calculated from the Py-IR band area located at 1545 cm−1 and 1455 cm−1 , respectively.

phase-transfer synthesis. FE-SEM and TEM micrographs were collected to disclose the microscopic structural features of ZSM-5-P, which were juxtaposed with these recorded for ZSM5-C in Fig. 2. FE-SEM micrographs of ZSM-5-P (Fig. 2c, d) revealed uniform ellipsoid particles with typical long axis of ca. 550 nm and short axis of ca. 250 nm, respectively, made up of fused small clusters ranging from 50 to 150 nm. Alignment and partial fusion of some primary clusters constructed a rod-like morphology in the corrugated surfaces, and porous regions could be visualized between hillocks of rods and clusters. The corresponding TEM micrographs (Fig. 2g, h) confirmed that the particles consisted of fused clusters and the black–white contrast indicated the presence of macro- or meso-porosity. The Selected Area Electron Diffraction (SAED) pattern (inset of Fig. 2g) showed spot-like pattern typical of single crystal materials, lending a further evidence for the alignment of adjacent clusters. The oriented alignment and growth is often an indicator of oriented attachment growth history (Sturm and Cölfen, 2016), as have been found in analogous synthetic systems (Qin et al., 2019; Zhang et al., 2019). In contrast, ZSM-5-C showed a disk-like to coffin-like morphology (Fig. 2a, b) that were often found for ZSM-5 with Si/Al ratio ca. 25 (Shen et al., 2018). The particle sizes were detected to be ca. 250 nm, and smooth surfaces, edges and corners of crystals were clearly visible, suggesting dense crystalline structure and absence of mesoporosity. TEM micrographs (Fig. 2e, f) and SAED (inset of Fig. 2e) pattern corroborated SEM observations, and no obvious contrast within crystal was found. N2 adsorption–desorption isotherms were measured to generate the textural properties of ZSM-5-P, and to compare with that of ZSM-5-C, as depicted in Fig. 3a. The isotherm for ZSM-5-P possesses two jumps of N2 uptakes, a low pressure one at relative pressure P/P0 < 10−3 taking its root from micropore filling process, and a high pressure uptake of N2 at P/P0 > 0.85 stemming from capillary condensation within meso- or macro-pores. The hybrid isotherm containing features of both type I and type IV implies co-existence of micropores and meso- or macro-pores in this material. The hysteresis loop represents a type III characteristic of a slitlike mesopore, and the forced closure of desorption branch at relative pressure of ca. 0.45 caused by tensile-strength-effect recommends that some of the mesopores are constricted ones

(Groen et al., 2003). In contrast, isotherm of ZSM-5-C shows a typical type I isotherm owing to the presence of solely micropores, and a condensation effect is only observed at close to saturation pressure, which is often ascribed to the presence of voids as a result of micron-sized particle packing (Sing, 2001). The inferred data for surface area, pore volumes and external surface area were compiled in Table 1. ZSM-5-P (SBET = 489 m2 g−1 ) was found to have a much larger surface area than that of ZSM-5-C (SBET = 406 m2 g−1 ), mainly because of the increase of external surface areas (94 versus 174 m2 g−1 ), as a result of reduced crystal size. Micropore volumes, which are often regarded as an indicator of crystallinity and preservation of microporosity, were detected to be identical for the two samples, hence, suggesting well preserved microporosity by the phase-transfer synthesis. As N2 physisorption often fails to determine mesopores larger than 10 nm and the condensation effect indicates rather large meso- or macro-pores exist in ZSM-5-P, complementary mercury intrusion measurements were carried out to determine the presence of larger pores. Noteworthy, mercury intrusion probes mesopores larger than 6 nm that are accessible from the external surface of crystals, i.e., the penetrating macro- or meso-pores that have been found to be important for mass transfer through diffusion process (Milina et al., 2014). The results shown in Fig. 3b corroborate the presence of two types of macropores (∼100 and ∼350 nm) in ZSM-5-P, whereas only large macropores (∼550 nm) originating from packing of bulky crystals was identified in ZSM-5-C. The chemical nature of Al sites is a decisive factor that determines the type, amount and density of catalytically relevant acidic properties of zeolitic materials. 27 Al MAS NMR technique is a proven technique to inspect the chemical environment of Al incorporation in zeolitic framework. The recorded spectra for both ZSM-5-P and ZSM-5-C were displayed in Fig. 4. Both samples showed one major resonance peak at ∼51 ppm, which was assignable to tetrahedrally incorporation of Al into the MFI framework (Sazama et al., 2011). The 0 ppm resonance peak that is associated with extraframework Al has not been detected on the samples, thus proving that most Al exists as framework AlO4 moieties in ZSM-5-P.

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Fig. 3 – N2 adsorption–desorption isotherms for ZSM-5-C and ZSM-5-P (a). Pore volumes gained from mercury intrusion measurements for ZSM-5-P and ZSM-5-C (b).

Fig. 4 – 27 Al MAS-NMR spectra of calcined ZSM-5-C and ZSM-5-P. Acidic characteristics of the proton type samples obtained after ion-exchange were also probed using NH3 -TPD and PyIR measurements. Form the profiles of NH3 -TPD patterns (Fig. 5a), two maxima of NH3 desorption temperatures were observed on the two samples, a low temperature peak (∼445 K) corresponding to NH3 desorption from physisorbed surface Si−OH groups and a high temperature (650 K–750 K) shoulder peak associated with desorption from acidic sites (Topsøe et al., 1981). A relatively weak acidity was observed on ZSM5-P with respect to ZSM-5-C. Previous studies also revealed that addition of silanes would lead to the decrease of acidity in molecular sieve materials (Serrano et al., 2010). NH3 may not be able to discriminate the types of acidic sites on these materials, therefore, the acidic properties of these materials have also been investigated by Py-IR. The quantities of Brønsted and Lewis acid sites on the two zeolite samples were evaluated by Py-IR, and the collected data were depicted in Fig. 5b and c. Vibration bands at 1454 cm−1 and 1455 cm−1 belong to pyridine adsorbed at Brønsted acid sites and Lewis acid sites, respectively, and the band at 1490 cm−1 is attributed to a combined impact from both sites (Parry, 1963; Pieterse et al., 1999). The data recorded after desorption at 423 K, 573 K and 723 K were used to calculate the corresponding numbers for total, medium and strong acid sites, respectively, and these results were tabulated in Table 2. The measured amounts of total number of Brønsted acid sites (138 ␮mol g−1 ) and acid sites of medium

(105 ␮mol g−1 ) and strong strength (87 ␮mol g−1 ) were found to slightly lower for ZSM-5-P than these of ZSM-5-C (155, 126 and 110 ␮mol g−1 for total, medium and strong Brønsted sites, respectively). Meanwhile, an increase in Lewis acid sites density were identified (Table 2), corresponding to reduced Brønsted sites to Lewis sites ratios in all measured temperature regions (column 3 of Table 2). In accordance to previous reports (Serrano et al., 2011), the formation of less numbers of Brønsted sites could be the effect of presence of TPPS, as organosilane modification of surface often gives rise to the formation of more Lewis acid sites. Attenuation of Brønsted sites and concurrent increase, albeit slight, in Lewis acid sites density were observed for ZSM-5-P with respect to ZSM-5-C. Collectively, hierarchical ZSM-5 with exceptional properties including narrow particle size distribution, high crystalline, high yields, large surface area and mesopore volumes has been achieved using this facile phase-transfer synthesis. Despite that there is no obvious increase in extraframework Al sites, the acidity of the obtained material has been altered, and less Brønsted sites were probed on hierarchical ZSM-5-P.

3.2. Structure evolution during crystallization process and proposed formation mechanism To throw light on the formation mechanism and understand factors that influence the structure of obtained materials, the crystallization processes were monitored using timedependent XRD technique coupled with FE-SEM micrographs for samples collected ex situ after quenching of hydrothermal synthesis. As crystallization was conducted in biphasic media and the final products were collected in toluene phase, the phase transfer process in the course of crystallization was also recorded by photographs (Fig. 8). At the beginning of the crystallization, the amphiphilic organosilane served as a surfactant that had modified the surface of precursor particulates to be hydrophobic, allowing the formation of an emulsion-like structure without perceivable interfaces, as shown by the photographic images in Fig. 8a. In the hydrothermal synthesis, the mother liquor collected right after discharging from tumbling crystallization of 12 h were found to be turbid without separating interfaces (Fig. 8c). However, after standing statically for ca. 2 h at ambient conditions, there was a clear separation of the two immiscible liquid phases, and milky particulate suspensions were found in the toluene phase owing to their hydrophobic nature (Fig. 8c). It was speculated that a Pickering emulsion structure had formed in this process. To prove

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Fig. 5 – NH3 -TPD patterns (a), Py-IR after desorption at 423 K, 623 K and 723 K on ZSM-5-C (b) and ZSM-5-P (c), respectively. this, a microphotograph was taken using optical microscope for a 20-fold diluted suspension of the sample crystallized for 12 h, as shown in Fig. 8d, which clearly illustrated the formation of a water-in-oil Pickering emulsion. The particulates separating layer between the interfaces could be visualized, and appeared as a dark shell encapsulating emulsion droplet. Hence, tumbling functioned as a constant emulsifying process that exerted the mechanistic energy required to maintain such Pickering emulsion structure. In the phase-transfer synthesis,

time-dependent XRD (Fig. 6a) and FE-SEM (Fig. 7) measurements disclosed that the sample consisted mainly of loosely packed amorphous nanoparticle aggregates (∼25 nm) before 1 h of hydrothermal treatments. Weak characteristic diffraction lines belonging to MFI phase became perceivable after 1.5 h, when the sample was still aggregate-like in morphology. In the 2 h crystallized sample, both precursor aggregates and MFI crystals (∼250 nm) were identified to co-exist, with coarsening of crystal surfaces indicating the presence of aux-

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Fig. 6 – Time-dependent XRD patterns for ZSM-5-P (a) and ZSM-5-C (b) during the course of crystallization obtained at varied crystallization periods. The corresponding crystallization curves (c) and particle size variation are show in (d). iliary porosity. After 5 h, the precursor had been completely transformed into zeolitic crystals possessing typical coffinlike MFI crystal shape and corrugated surfaces. Meanwhile, an abrupt increase in relative crystallinity was accompanied, as evidenced by the corresponding crystallization curve shown in Fig. 6c. From 5 to 24 h, the morphology MFI crystals remained intact with crystallization period, and only slight increase in crystallinity was inferred. In prolonged crystallization period from 24 to 48 h, a substantial change in crystal shape from coffin-like to ellipsoid-like in ripening process was detected by FE-SEM micrographs (Fig. 7), and insignificant variation in crystallinity was seen. The formation process was schematically shown in Scheme 1a. In comparison, crystallization of ZSM-5-C in standard hydrothermal synthesis was also tracked in the same way. Fig. 8b exhibited the photographs of the mother liquor, and aqueous milky suspensions containing solid particulates were observed throughout the course of crystallization. Fig. 6b showed the time-dependent XRD patterns, suggesting similar crystallization process for the evolution of order, i.e., a rapid nucleation process with quick increase in crystallinity in the initial 2 h followed by a slow but continuous crystal growth in crystallinity from 5 to 24 h (Fig. 6c). The ripening process after 24 h contributed trivial crystallinity increase. The corresponding FE-SEM micrographs (Fig. S3) demonstrated that after the formation of loosely packed amorphous worm-like particles (∼25 nm) in the first 1 h, numerous small semi-crystalline particulates with evenly distributed sizes were created in a fast nucleation process between 1 and 1.5 h, followed by an oriented attachment growth of crystallites (derived from

semi-crystalline particulates) into bulky, disc-like agglomerated crystals of ca. 350 nm after 2 h. These crystallites were uniform and constructed roughed surfaces and porous regions in between. Thereafter, from 5 to 48 h, the rough surface gradually vanished as a result of crystallites fusion, which should be energetically favorable as elimination of grain boundaries between crystallites often lead to a lowered total energy of a system (Zhang et al., 2009). These observations strongly suggest a non-classic oriented attachment growth mechanism, in line with previous reports of crystallization of MFI crystals in TPAOH-TEOS-NaOH system, regardless of presence of Al (Davis et al., 2006; Zhang et al., 2019). Structure evolution of ZSM-5-C was sketched in Scheme 1b. Comparisons between the two crystallization processes pointed to the following mechanisms for the formation of the final crystal morphology. Crystallization of ZSM-5-C could be roughly divided into three stages (Scheme 1b). First, hydrolysis of TEOS in aqueous alkali solutions containing TPAOH lead to the formation of tiny amorphous particulates that aggregated together. As the particulates already contained all the ingredients necessary for nucleation, they rapidly transformed into semi-crystalline particulates. Second, these semi-crystalline particulates developed into crystallites that grow into bulky crystals through a non-classic oriented attachment mechanism, similar to these unveiled for other crystals or zeolites (Kumar et al., 2015; Niederberger and Cölfen, 2006). Since it was an attachment growth pathway that dominated the process, the crystals formed comprised of plenty of mesopores resulting from the presence of grain boundaries, and their surfaces therefore appeared coarse. Third, a sequential ripen-

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Fig. 7 – Time-dependent FE-SEM micrographs for ZSM-5-P in the course of crystallization. ing process of crystallites fusion healed the grain boundaries between crystallites, leading to the observed bulky crystal with smooth surface and distinct crystal shape. In contrast, for ZSM-5-P crystallization in biphasic media (Scheme 1a), the crystallization could also be considered as three continuous steps, nucleation, oriented attachment growth and a ripening process. In nucleation stage, the hydrophobic modification of precursor particulates surfaces by the presence of TPPS did not change the nucleation process, but extracted the formed crystallites to the toluene/water interface. Simultaneous tumbling assisted the formation of Pickering emulsion structure that maximized the area of interface whereby crystal nucleation took place. In crystal growth step, the surface modification of organosilane hindered the growth of tiny crystals into bulky ones. Simultaneously, the formation of Pickering emulsion structure also prevented the tight agglomeration of crystallites. As surface of crystallites became progressively hydrophobic with prolonged crystallization time, these zeolitic crystals were extracted into the toluene phase and were separated from the available nutrients for their further growth. In the final ripening stage, recrystallization proceeded in the toluene phase, and the ␲–␲ stacking between toluene and benzyl ring of TPPS could have expanded the mesopore volumes between crystallites. Putting together, the surface modification of crystallites and formation of Pickering emulsion structure in biphasic media under an oriented

attachment growth mechanism is responsible for the formation of hierarchical ZSM-5-P.

3.3.

Catalytic performance of ZSM-5

The catalytic performance of hierarchical ZSM-5-P has been assessed and compared with that of ZSM-5-C in DTO conversion. Fig. 9 presented the catalytic activity as a function of time-on-stream (TOS) in DTO at 673 K with a space velocity of 10 h−1 g-DME g-cat −1 . The initial activities for both catalysts were 100% under identical testing conditions, and rapid deactivation was detected for ZSM-5-C after 4 h TOS, whereas ZSM-5-P preserved its total conversion for up to 12 h followed by a deactivation process with a slowed rate. Given the large number of acid sites in ZSM-5-C, as probed through Py-IR and NH3 -TPD measurements, a higher catalytic activity would be expected for ZSM-5-C than that of ZSM-5-P without considering diffusion properties. The significantly prolonged catalyst lifetime (by a factor of 3 times) has been attributed to an improved diffusion property associated with implementation of hierarchical structure or reduced diffusion pathway length in DTO (or MTO) processes (Xi et al., 2014). The corresponding product distributions of the two catalysts were found to differ with TOS for ZSM-5-C and ZSM-5-P, as displayed in Fig. 9b and c, respectively. At full conversions regime in TOS tests, light olefins (including ethylene, propylene and butene) con-

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Fig. 8 – Digital photographs of reaction mixture after quenching of hydrothermal synthesis for controlled crystallization time in the course of ZSM-5-P (a) and ZSM-5-C (b) synthesis. Photograph for reaction mixture of ZSM-5-P right after discharging from autoclave of 12 h hydrothermal synthesis is compared with that of the same mixture after standing statically for 2 h (c). Optical microphotograph of Pickering emulsion structure taken from 2 h crystallized mixture after discharging from autoclave and diluted by a factor of 20 times (d). 0#h in (a) represents the biphasic precursor for ZSM-5-P synthesis before mixing, while 0 h shows the same system after shaking that affords an emulsion like structure.

stituted the major product for both catalysts, with an overall selectivity of ca. 78.4% and ca. 72.6% for ZSM-5-C and ZSM-5-P, respectively. Meanwhile, paraffins (like propane and butane, 7.4%) were produced, together with C5+ compounds (mainly aromatics including benzene, toluene, xylene and ethylbenzene, <15.1%) were detected with insignificant selectivity. The selectivity for ethane was negligible in the whole testing TOS, and methane selectivity was found to be rather low (< 0.9%) in the full conversion regime. In the deactivation regime, a rapid increase in methane selectivity was observed, together with selectivity drop for ethylene and propane. In addition, simultaneous increases in selectivities of C5+ was observed with deactivation of catalysts. The corresponding integral product slates were shown in Fig. 9d in histogram, suggesting more light olefins were produced by ZSM-5-P (11.4%, 31.3% and 30.7%, for C2= , C3= and C4= , respectively) with respect to ZSM-5-C (7.0%, 34.9% and 24.9%). Moreover, the selectivities to undesirable products such as light paraffins such as methane (2.0% versus 3.5%), propane (4.0% versus and 3.2%) and butane (2.4%, versus 2.2%), as well as C5+ (18.2% versus 24.2%) were significantly reduced by replacing ZSM-5-C with ZSM-5-P. To understand the influence of diffusion over catalytic properties, it is useful to take into consideration the relevant reaction mechanism. DTO proceeds through a hydrocarbon pool mechanism including two inter-dependent cycles, i.e., an olefin cycle involving the consecutive methylation and growth of initially formed olefins to C6= to C8= and their cracking into light olefins, and an aromatic cycle associated with methy-

lation of methylbenzene species and their dealkylation into olefins (Svelle et al., 2006; Bjørgen et al., 2007; Ilias and Bhan, 2013). A high Brønsted acid site density or presence of more Lewis acid sites favors aromatic cycle over olefin cycle, which tends to produce more ethylene (formed mainly through aromatic cycle) (Almutairi et al., 2013). Ethylene to propylene (E/P) ratio can be regarded as an indicator of the respect proportion for aromatic and propylene cycles, as long-chain olefins are mainly formed through olefin cycle (Svelle et al., 2006). A high E/P ratio for ZSM-5-P suggests more propagation through aromatic cycle and ease of ethylene diffusion from micropores in the material (Fig. 9d). Diffusion enhancement by the using hierarchical zeolites tends to promote ethylene selectivity as a result of shortened residence time within micropores that avoids its further transformation (Groen et al., 2007; Kim et al., 2010). A high propane to propene ratio in ZSM-5-P recommends that there are more Lewis acid sites in the sample, as the ratio is considered as an indicator of degree of hydride transfer that is favored by the presence of more Lewis acid sites (Müller et al., 2016; Wichterlová et al., 1999; Sazama et al., 2011). This interpretation is in line with the measured acidity properties for the two catalysts, as ZSM-5-P possesses more Lewis acid sites and less Brønsted acid sites. Catalyst lifetime is decided by severe coke formation from aromatic precursors, as aromatic compounds could cover active sites or block access to these sites (Hereijgers et al., 2009). Collectively, in accordance to characterizations of acid properties, the catalytic performance of ZSM-5-P reflects presence of more Lewis

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Fig. 9 – DME conversion (a), selectivities to various products, ratio of ethylene to propylene and propane to propylene with TOS on ZSM-5-C (b) and ZSM-5-P (c), overall product selectivity (d). Reaction temperature: 673 K, WHSV = 10 h−1 g-DME g-cat −1 .

Scheme 1 – Proposed crystallization mechanism of ZSM-5-P under tumbling condition (a) and ZSM-5-C under static condition (b).

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acid sites, attenuated Brønsted site density and enhanced diffusion property.

4.

Conclusions

Phase-transfer synthesis of hierarchical ZSM-5 zeolite has been achieved in a single step toluene/water system under tumbling with the assistance of organosilane. The synthesis uses readily available starting materials and consists of simple hydrothermal conditions. The synthesis enables high level control over crystallites size, pore-network, which have been proven to be key parameters that decide mass transfer properties. An underlying three steps mechanism is disclosed for their formation: fast nucleation, crystal growth through oriented attachment in Pickering emulsion, ripening and selforganization in toluene phase. This insightful knowledge shows that oriented attachment growth as a non-classic crystallization pathway, can be utilized to tune the architecture of zeolitic materials in Pickering emulsion system. The hierarchical zeolite exhibits prolonged catalyst lifetime and high light olefin selectivity in catalytic dimethylether to olefin conversion. Extrapolation of this strategy to other zeolitic materials, fine tuning of architecture and composition for specified catalytic use is currently underway, and the new findings shall be reported in future publications.

Declaration of interest The authors declare no competing financial interest.

Acknowledgements Special thanks are sent to Prof. Marc-Olivier Coppens, as this discovery was inspired by his seminar on the dynamically formed patterns of energy dissipating processes. KZ is grateful for the financial support from National Natural Science Foundation of China (21576082).

Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.cherd. 2019.10.019.

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