Creation of Al‐Enriched Mesoporous ZSM‐5 Nanoboxes with High Catalytic Activity: Converting Tetrahedral Extra‐Framework Al into Framework Sites by Post Treatment

Abstract ZSM‐5 zeolite nanoboxes with accessible meso‐micro‐pore architecture and strong acid sites are important in relevant heterogeneous catalysis suffering from mass transfer limitations and weak acidities. Rational design of parent zeolites with concentrated and non‐protective coordination of Al species can facilitate post‐synthetic treatment to produce mesoporous ZSM‐5 nanoboxes. In this work, a simple and effective method was developed to convert parent MFI zeolites with tetrahedral extra‐framework Al into Al‐enriched mesoporous ZSM‐5 nanoboxes with low silicon‐to‐aluminium ratios of ≈16. The parent MFI zeolite was prepared by rapid ageing of the zeolite sol gel synthesis mixture. The accessibility to the meso‐micro‐porous intra‐crystalline network was probed systematically by comparative pulsed field gradient nuclear magnetic resonance diffusion measurements, which, together with the strong acidity of nanoboxes, provided superb catalytic activity and longevity in hydrocarbon cracking for propylene production.


Synthesis of mesoporous ZSM-5 nanoboxes
All chemicals were obtained from Sinopharm Chemical Reagent Co., Ltd and used as received. The procedure to prepare the parent zeolite is described as: aluminium chloride (AlCl3) was added to a mixture of tetrapropylammonium hydroxide (TPAOH) and deionised (DI) water and continually stirred for 1 h at room temperature (RT), then the mixture was heated to 40 C. Subsequently, tetraethylorthosilicate (TEOS) was added rapidly into the mixture under vigorous stirring and continuously stirred for 30 min until the synthesis gel become clear. The molar composition of the sol gel mixture is SiO2:0.04Al2O3:0.3TPAOH:19H2O. After rapid aging of the sol gel synthesis mixture, it was transferred into a Teflon-lined stainless-steel (SS) autoclave (500 ml capacity) for crystallisation at 160 °C (for different h). The resulting product was recovered by centrifugation and washed with DI water (100 mL for each wash, 3 times), then dried at 110 °C (overnight) and calcined in static air at 550 °C with for 6 h (heating rate = 1 °C min −1 ). The as-synthesised parent zeolite is denoted as AS-MFI. For comparison, conventional ZSM-5 (denoted as C-ZSM-5) was also prepared using the conventional aging method (i.e. precursor aging at RT for 24 h), then followed by the same hydrothermal synthesis and work-up.
ZSM-5 nanoboxes are synthesised using a post-synthetic treatment method. Post-synthetic hydrothermal treatment of AS-MFI using TPAOH aqueous solution was carried by mixing the sample (4 g) with TPAOH solution (0.1 M, 40 mL, pH ≈ 13) at 400 rpm (for 20 min to form a slurry). Then the slurry was transferred to a Teflon-lined SS autoclave (100 ml) and left at autogenous pressure and 160 °C under hydrostatric condition for various h (specifically, 6, 12, 24, 48 and 96 h). Finally, the mixture was filtered, washed with DI water, dried at 120 °C for 12 h, and calcined at 550 °C for 6 h. The post-treated sample was denoted as ZSM-5-P-x-y, in which P represents via post-synthetic treatment, x represents the TPAOH concentration using in the post-treatment (in M) and y represents post-treatment time (in h), respectively.

Post-synthetic treatment of AS-MFI using concentrated TPAOH solutions (0.3 and 0.5 M)
Post-synthetic treatment of AS-MFI using 0.3 and 0.5 M TPAOH aqueous solutions was carried by mixing AS-MFI (4 g) with TPAOH solution (0.3 M and 0.5 M, 40 mL) at 400 rpm (for 20 min to form a slurry). Then the slurry was transferred to a Teflon-lined SS autoclave (100 ml) and left at autogenous pressure and 160 °C for 24 h. Finally, the mixture was filtered, washed with DI water, dried at 120 °C for 12 h, and calcined at 550 °C for 6 h. The treated sample was denoted as ZSM-5-P-0.3-24 and ZSM-5-P-0.5-24. Relevant pH values of the systems are ~14.

Synthesis and post-synthetic treatment of conventional ZSM-5
Standard procedure was used to synthesise the conventional ZSM-5 zeolite (denoted as C-ZSM-5), and it is described as: AlCl3 was added to a mixture of TPAOH and DI water and continually stirred for 1 hour at RT. Subsequently, TEOS was added dropwise into the mixture under vigorous stirring at ice bath temperature (~12 h) and continuously stirred for 12 h at the same temperature. The molar composition of the sol gel mixture is SiO2:0.04Al2O3:0.3TPAOH:19H2O. After the 24 h aging of the sol gel synthesis mixture at RT, it was transferred into a Teflon-lined SS autoclave (500 ml capacity) for crystallisation at 160 °C for 48 h. The resulting product was recovered by centrifugation and washed with DI water (100 mL for each wash, 3 times, then dried at 110 °C (overnight) and calcined in static air at 550 °C (with a ramp rate of 1 °C min −1 for 6 h) to obtain C-ZSM-5.
The post-synthetic treatment of C-ZSM-5 using 0.1 M TPAOH aqueous solution was carried by mixing the C-ZSM-5 (4 g) with TPAOH solution (0.1 M, 40 mL) at 400 rpm (for 20 min to form a slurry). Then the slurry was transferred to a Teflon-lined SS autoclave (100 ml) and left at autogenous pressure and 160 °C for 6 h. Finally, the mixture was filtered, washed with DI water, dried at 120 °C for 12 h, and calcined at 550 °C for 6 h. The resulting sample of the post-treated C-ZSM-5 was denoted as C-ZSM-5-P-0.1-6 (6).

Synthesis of conventional hollow ZSM-5 with a SAR of ~45 (C-HO-ZSM-5)
The procedure to prepare the relevant parent ZSM-5 is described as: AlCl3 was added to a mixture of TPAOH and DI water and continually stirred for 1 h at RT. Subsequently, TEOS was added into the mixture under vigorous stirring and continuously stirred for 24 h. The molar composition of the precursor sol is SiO2:0.0125Al2O3:0.3TPAOH:19H2O. After the 24 h aging of the precursor sol at RT, it was transferred into a Teflon-lined SS autoclave (500 ml capacity) for crystallisation at 160 °C for 48 h. The resulting product was recovered by centrifugation and washed with DI water (100 mL for each wash, 3 times, then dried at 110 °C (overnight) and calcined in static air at 550 °C with a ramp rate of 1 °C min −1 for 6 h.
The post-synthetic treatment of the parent zeolite using 0.7 M TPAOH aqueous solution was carried by mixing the sample (4 g) with TPAOH aqueous solution (0.7 M, 40 mL) at 400 rpm (for 20 min to form a slurry). Then the slurry was transferred to a Teflon-lined SS autoclave (100 ml) and left at autogenous pressure and 160 °C for 24 h. Finally, the mixture was filtered, washed with DI water, dried at 120 °C for 12 h, and calcined at 550 °C for 6 h. The treated sample was denoted as C-HO-ZSM-5.

Characterisation of Materials
Powder X-ray diffraction (XRD) XRD patterns of materials were recorded using a PANalytical X'Pert Pro system fitted with CuKα1 X-ray source (λ = 0.15406 nm) operated at 40 kV and 40 mA. The measurement was performed over a 2θ range of 5°-35° in 0.02 step size at a scanning rate of 1° min −1 . The relative crystallinity (RC) of zeolites was determined by using a standard Integrated Peak Area Method, which involves a comparison of the integrated peak areas in the range of 22.5 to 25.0° 2θ. Details of the method has been described elsewhere. [1] The sample with the strongest peak at 23.1° 2θ was selected as the reference, which was ZSM-5-P-0.1-12 in this study. It is worth mentioning that, in this work, the parent zeolite, i.e. AS-MFI, was not perfectly crystallised with the non-coordinated tetrahedral Al and Si species, and hence the crystallinity of AS-MFI is comparatively poor. The subsequent post-treatment with the aqueous TPAOH solution (at 0.1 M) under hydrothermal conditions can facilitate the re-crystallisation of the parent AS-MFI zeolite during the post-treatment, and hence producing the relevant ZSM-5-P zeolites with the improved crystallinity.

Nitrogen (N2) adsorption-desorption
N2 physisorption measurements at the liquid nitrogen temperature (−196.15 C) using a Micromeritics 3Flex surface area and pore size analyser. Prior to the measurement, the sample (~100 mg) was degassed at 350 °C under vacuum overnight. The micropore size distribution was S4 calculated by the Horvath-Kawazoe (HK) method, and mesopore size distribution was determined from the adsorption branch of the isotherms by the Barrett-Joyner-Halenda (BJH) method.

Solid-state nuclear magnetic resonance (NMR)
Solid-state NMR spectra were recorded on an Agilent 600 DD2 spectrometer operating at a Larmor frequency of 600 MHz for 1H. 29 Si magic angle spinning (MAS) NMR spectra were recorded at 119.15 MHz (with the proton decoupling (TPPM) during acquisition) using a 3.6 μs pulse with a 30 s recycle delay and 1024 scans (tetramethylsilane as the reference). 27 Al MAS NMR spectra were recorded at 156.25 MHz (with the proton decoupling (TPPM) during acquisition) using a 3.6 μs pulse with a 5 s recycle delay and 512 scans (aluminium chloride as the reference).

Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX)
SEM-EDX analysis was performed on a JEOL 7401 high-resolution field emission scanning electron microscope with an Oxford INCA 350 EDX system.

High resolution transmission electron microscopy (HRTEM)
HRTEM analysis of the microstructure of zeolites was performed using a Fei Tecnai F20 field emission gun transmission electron microscope at 200 kV. Before TEM analysis, the sample was dispersed in ethanol (which was sonicated for 5 min in an ultrasonic bath), and a droplet of the solution was casted on a carbon-coated mesh grid.

Inductively coupled plasma-atomic emission spectroscopy (ICP-AES)
ICP-AES analysis of various zeolites for the elemental analysis of their Al and Si contents was performed on a Varian Vista AX ICP-AES spectrometer. To prepare the solutions for ICP-AES analysis, the zeolite sample (0.1 g) was dissolved using hydrofluoric acid (HF) solution (10 wt.% HF in water, 5 ml) at RT, then diluted using DI water to 100 ml.

X-ray photoelectron spectroscopy (XPS)
XPS analysis was carried out using ThermoScientific ESCALAB 250 spectrometer equipped with monochromated AlKα X-ray source (1486.6 eV, 150W, spot size = 500 μm), a charge neutraliser and a hemispherical electron energy analyser. During data acquisition, the chamber pressure was kept below 10 −9 mbar. The spectra were analysed using the CasaXPS software pack and corrected for charging using C1s binding energy (BE) as the reference at 284.8 eV.

Ammonia temperature-programmed desorption (NH3-TPD)
NH3-TPD measurements were performed using a Micromeritics AutoChem II 2920 chemisorption analyser (Micromeritics, USA) to determine the acidic property of the zeolites. ~100 mg zeolite was pre-treated at 823 K for 1 h and then cooled down to 323 K under Helium (He). A gas mixture of NH3 in He (10%:90%, 30 cm 3 min −1 ) was then introduced to saturate the catalyst followed by the purge of pure He (60 cm 3 min −1 ) at 373 K for 2 h to remove the physically adsorbed NH3. Finally, NH3-TPD was performed by heating the catalyst from 373 K to 873 K with a heating rate of 10 K min −1 under He flow (30 cm 3 min −1 ) and the desorbed NH3 was monitored by a gas chromatography (GC) equipped with a thermal conductivity detector (TCD).

Pyridine Fourier transform infrared (py-FTIR)
py-FTIR analysis was performed using a nexus Model Infrared Spectrophotometer (Termo Nicolet Co, USA) operating at 2 cm −1 full width at half maximum (FWHM) equipped with an in situ cell containing CaF2 windows. Adsorption of pyridine was performed at RT and then evacuated at 200 C measuring pyridine adsorbed at all acid sites. After that, the sample was evacuated in situ at 350 C corresponding to the pyridine adsorption at the strong acid sites.

Thermogravimetric analysis (TGA)
TGA of the used zeolites from the cracking reaction was performed using a TG analyser (Q600 TGA-DSC, TA Instruments, Germany) under air (flow rate = 100 ml min −1 ). The temperature ramp was from RT to 800 °C with the heating rate of 10 °C min −1 .

Pulsed-Field Gradient Nuclear Magnetic Resonance (PFG-NMR) Diffusion Experiments
Zeolites were pelletised with particle sizes of 1.6-1.8 mm for PFG-NMR measurements. The zeolite particles were dried at 60 °C in a vacuum oven overnight and calcined at 550 °C for 12 h. To prepare samples for PFG-NMR experiments, the zeolite particles were soaked in n-octane, cumene or 1,3,5-triisopropylbenzene (TIPB) for 2 days to ensure full saturation of the porous matrix of the zeolite samples with the respective probe molecule. The saturated samples were then dried on a filter paper to remove any excess liquid from the external surface of the particles and transferred to 5 mm NMR tubes. To minimise relevant errors due to evaporation of the volatile liquid, a small amount of pure liquid was dropped onto a filter paper, which was placed under the cap of the NMR tube. The tube was then placed into the magnet and left for approximately 15 min to achieve thermal equilibrium before measurements started. NMR experiments were performed on a Magritek SpinSolve benchtop NMR spectrometer operating at a 1 H frequency of 43 MHz. PFG-NMR experiments were carried out using a diffusion probe capable of producing magnetic field gradient pulses up to 163 mT m −1 . Diffusion measurements were performed using the pulsed-field gradient stimulated echo sequence (PGSTE sequence). [2] The sequence is made by combining a series of radiofrequency pulses (RF) with magnetic field gradients (g, Figure S1). The NMR signal attenuation of PFG-NMR experiments as a function of the gradient strength, E(g), is related to the experimental variables and the diffusion coefficient (D) by: where E0 is the NMR signal in the absence of gradient, γH is the gyromagnetic ratio of the nuclei being studied (i.e. 1 H in this case), g is the strength of the gradient pulse of duration δ, and Δ is the observation time (i.e. the time interval between the leading edges of the gradient pulses). In Eq. S1, it is often convenient to define the product 2 The measurements using cumene and octane were performed by fixing Δ = 200 ms and δ = 3 ms, and measurements with TIPB were performed by fixing Δ = 500 ms and δ = 5 ms. The magnitude of g was varied linearly with 16 spaced increments. To achieve full signal attenuation, maximum values of g of up to 163 mT m −1 were necessary. All the measurements were performed at atmospheric pressure and 25 °C. The diffusion coefficients D were calculated by fitting Eq. S1 to the experimental data. Root-mean-square displacement (RMSD) values of the probing molecules are determined using Eq. S2.
To understand the diffusion process occurring within the pore structures under investigation, the PFG interaction parameter is calculated, [3] which in the case of using hydrocarbons as probe species, can be approximated to the tortuosity of the pore network, τ, defined as in Eq. S3. [4] b u lk where D is the diffusivity of probing molecules within the zeolite samples, and Dbulk is the diffusivity of the bulk liquid.

Cracking reactions
Cracking of n-octane and cumene over the catalyst was carried out in a fixed bed reactor (I.D. = 10mm). The packed bed consists of 1 g pelletised zeolite catalysts (particle size = 1. temperature for 2 h, then n-octane (0.1 ml min −1 with N2 as the carrier gas at 240 ml min −1 ) or cumene (0.1 ml min −1 with N2 as the carrier gas at 150 ml min −1 ) was introduced to start the reaction at the same temperature (i.e. 540 °C for n-octane and 320 °C for cumene). The gaseous products of the reactions were analysed using an in-line gas chromatograph (GC, Agilent 7890B) equipped with a flame ionisation detector (FID) and Agilent PoraPLOT Q column. The carbon balance is above 95%.

Hydrothermal steam aging of zeolite
Hydrothermal aging of ZSM-5 nanoboxes (~100 mg) using steam was carried out in a tube furnace (Carbolite Gero Ltd., UK) at 500 °C. N2 (at 200 mL min −1 ) was used as the carrier gas which was passed through a bubbler containing DI water (at 50 °C) to generate steam. The sample were treated by steaming for 10 h prior to XRD and N2 physisoprtion analysis.