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Insights into the Mechanism of Zeolite Detemplation by Positron Annihilation Lifetime Spectroscopy Robbie Warringham, Lars Gerchow, Asier Zubiaga, David Cooke, Paolo Crivelli, Sharon Mitchell, and Javier Perez-Ramirez J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b08931 • Publication Date (Web): 17 Oct 2016 Downloaded from http://pubs.acs.org on October 20, 2016

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The Journal of Physical Chemistry

Insights into the Mechanism of Zeolite Detemplation by Positron Annihilation Lifetime Spectroscopy Robbie Warringham,†,‡ Lars Gerchow,§,‡ Asier Zubiaga,† David Cooke,§ Paolo Crivelli,§ Sharon Mitchell,† and Javier Pérez-Ramírez*,† †

Institute for Chemical and Bioengineering, Department of Chemistry and Applied Biosciences,

ETH Zurich, Vladimir-Prelog-Weg 1, CH-8093 Zurich, Switzerland §

ETH Zurich, Department of Physics, Institute for Particle Physics, Otto-Stern-Weg 5, 8093

Zurich, Switzerland.

ACS Paragon Plus Environment

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ABSTRACT Templating approaches play a key role in expanding the structural diversity of many important classes of porous materials. Understanding the interaction of the templating agent with the resulting porous framework and the mechanism of detemplation is critical to ensure their effective application. This study explores the scope of positron annihilation lifetime spectroscopy (PALS) to gain insight into the location of tetrapropylammonium (TPA+) industrially employed as a structure directing agent (SDA) in the synthesis of MFI-type zeolites. Ortho-positronium (o-Ps) formed upon positron implantation are initially confined within the zeolite crystals. Simulations using a modified Tao potential confirm that o-Ps atoms localize between the propyl chains of adjacent TPA+ molecules within the sinusoidal channels of the micropore network. Controlled removal of the SDA leads to a linear increase in the micropore volume determined by nitrogen sorption. In contrast, the out-diffusion of o-Ps is found to be crystal size dependent. Analysis of the expected percolation threshold shows that the experimental data is best represented by an isotropic detemplation mechanism favoring the formation of small isolated SDA-free volumes. The restricted accessibility of the zeolite micropores due to the presence of the SDA is supported by confocal fluorescence microscopy. These findings emphasize the unique sensitivity of PALS to the presence of guest species within the micropore network opening new doors to study their impact on the textural properties of other porous materials.


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INTRODUCTION Templating strategies continue to play an important role in zeolite synthesis, where the application of structure directing agents (SDAs) such as amines and alkyl ammonium cations has enabled the crystallization of a large proportion of the over 200 different framework types and related structures discovered to date.1-5 Although the use of SDAs introduces additional costs and synthetic complexity (efforts have been directed to develop SDAfree synthesis routes)6,7 SDA-directed syntheses have prevailed due to the structural flexibility they afford, particularly for the preparation of highsilica zeolites.2,3 Recently templating strategies have also been widely applied in the development of zeolites with hierarchical pore structures.8-12 A zeolite synthesized with the aid of SDAs is the wellstudied MFItype ZSM-5, which has applications in a broad range of catalytic processes.13,14 The MFI framework consists of intersecting straight (5.4 x 5.6 Å) and sinusoidal channels (5.1 x 5.5 Å).5 These channels cross to form spherical-like intersection volumes of ca. 8 Å diameter. MFI is commonly synthesized using tetrapropylammonium cations (TPA+) as SDA, with the ammonium ion occupying the intersection volume and the propyl arms oriented along the straight and sinusoidal channels (Figure 1a).15 As is the case for most templating strategies, TPA+ needs to be removed to vacate the micropore network for other functions, which is typically achieved by calcining the zeolite at high temperatures. However, such procedures can profoundly impact the structural integrity of the microporous framework.16 Milder alternatives such as the removal by chemical means17,18 and ozonication19 have been explored in attempts to address this issue. The location and distribution of TPA+ in MFI has also been correlated to the presence of silanol defects.20-22 Therefore a thorough understanding of the detemplation process is important to gauge the possible effects on the resulting pore network.


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The detemplation in MFI-type zeolites has been studied by NMR,20,21,23 XRD,24 thermoanalytical

methods,25,26

and

fluorescence

microspectroscopy.27,28

However,

the

development of the micropore network upon SDA removal has not been spatially resolved. One technique that can aid in this regard is positron annihilation lifetime spectroscopy (PALS). The method exploits orthopositronium, a bound state between a positron and an electron in the solid (herein denoted as oPs) as a diffusional probe that actively localizes in pores within a material before annihilating.29–31 Unique facets of PALS are that the formation of o-Ps occurs within the material and that the lifetime of o-Ps depends on the pore size.32,33 This enables the study of isolated pore volumes that would be inaccessible to standard diffusion-based techniques as gas sorption, while the dynamic response directly reflects the pore architecture. PALS has been utilized to study different zeolite frameworks,34–36 porous silica powders37-39 and films,37,40,41 and the crystallization of zeolites from aluminosilicate gels.42 Recently, it also yielded key insights into the connectivity of hierarchical pore networks in MFI43,44 and FAU45 zeolites. This manuscript studies the porosity evolution upon SDA removal in MFItype zeolites by PALS coupling measurements with simulation and modeling studies on the behavior of oPs within the micropores. A series of partially detemplated samples are prepared from a well characterized

micronsized

ZSM5.

Simulations

of

oPs

distributions

within

the

SDAcontaining framework are developed employing a modified Tao potential to describe the localization behavior. Based on the observed PALS response, several detemplation mechanisms are modeled using adapted percolation theory to describe the evolution of SDAfree regions. The experimental results were best simulated by a highly random detemplation mechanism. This


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analysis is extended to a more industrially relevant series of nanosized ZSM5, confirming the similar mechanism of detemplation. METHODS Zeolite Synthesis. Two MFI-type ZSM-5 zeolites with micron- (coded ZM) or nanosized (coded ZN) crystals were prepared via hydrothermal synthesis using tetrapropylammonium bromide (TPABr, 98%, ABCR) as a structure-directing agent (SDA). ZM was synthesized in fluoride media. Briefly, sodium aluminate (NaAlO2, Sigma Aldrich) and TPABr were dissolved in distilled water until a clear solution was attained. Ammonium fluoride (NH4F, 98%, Acros organics) was added and finally fumed silica (Aerosil 130, Evonik) was slowly introduced in the mixture under agitation. The pH was adjusted to 7 through the addition of hydrofluoric acid (40%, Sigma Aldrich) and left to age for 2 h at room temperature, leading to a final gel composition of 100SiO2:1NaAlO2:56NH4F:4.3TPA2Br:7990H2O. The hydrothermal synthesis was conducted in a stainless steel autoclave at 448 K for 48 h, and the template-containing zeolite was recovered by filtration, thoroughly washed with distilled water, and dried at 338 K overnight. ZN was synthesized in alkaline media. An autoclave was charged with a mixture of NaAlO2, TPABr, colloidal silica (30 wt% in water, Sigma Aldrich), sodium hydroxide (NaOH, Sigma

Aldrich,

99.9%)

and

water,

yielding

a

final

gel

composition

of

30SiO2:1Al2O3:5.6Na2O:2.4TPA2Br:1374H2O. The autoclave was subsequently heated to 423 K for 24 h before cooling, filtering and washing the SDA-containing zeolite. The resulting sample was dried overnight at 333 K. Partial detemplation was achieved by isothermal calcination at 693 K for varying durations (0.25-16 h) with complete template removal reached after 16 h for both samples. The samples are coded ZM-x or ZN-x, where x refers to the fraction of detemplation (0.00 to 1.00).


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Zeolite Characterization. Inductively coupled plasma-optical emission spectroscopy (ICPOES) was conducted using a Horiba Ultra 2 instrument. Samples were dissolved in hydrofluoric acid (HF, Sigma Aldrich, >99.99%) and neutralized with boric acid (Sigma Aldrich, >99.8%) before analysis. Scanning electron microscopy (SEM) was undertaken by using a Zeiss Leo 1530 microscope operated at 5 keV. Transmission electron microscopy (TEM) was performed using an FEI Tecnai F30 FEG microscope (300 kV). X-ray diffraction (XRD) patterns were acquired using a PANalytical X’Pert PRO-MPD diffractometer operated in Bragg-Brentano geometry using Ni-filtered Cu-Kα radiation (λ = 0.1540 nm). Data were recorded in the 2θ range of 5-70° with an angular increment of 0.1° and a counting time of 2 s per step. Nitrogen sorption at 77 K was performed using a Micromeritics TriStar II instrument after sample evacuation at 573 K for 3 h. Thermogravimetric analysis in air (20 cm3 min-1) was conducted using a Mettler Toledo TGA/DSC 1 system with a heating ramp of 10 K min-1. Elemental analysis of carbon, hydrogen and nitrogen was performed using a LECO TruSpec Micro elemental analyzer. Sample were dried at 373 K under vacuum overnight prior to analysis. Confocal fluorescence microscopy (CFM) was undertaken using a Zeiss LSM 780-FCS laser scanning microscope with a PlanApochromat ×100/1.4 oil objective lens. Sample preparation included heating the zeolite powder at 373 K for 15 min followed by the addition of thiophene (1 µl, Merck Chemicals), and subsequently quenching the reaction after 10 s. Samples were transferred to an open cell and excited using a monochromatic laser (488 nm) to study the oligomeric products resulting from the acid-catalyzed reaction of thiophene. Detection range of 508-562 nm was used to record the stimulated. Slices through the z-axis of individual crystallites were obtained at a similar focal depth using an identical laser power for each sample, with an average pixel size of 0.30 µm.


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Positron Annihilation Lifetime Spectroscopy (PALS). PALS measurements were performed using the ETHZ slow positron beam.44 Powdered samples (ca. 0.1 mg) were degassed in situ under vacuum (

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