Supported Embryonic Zeolites and their Use to Process Bulky

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Synthesis of Embryonic Zeolites and Their Use to Process Bulky Molecules Kok-Giap HAW, Jean-Pierre Gilson, Nikolai Nesterenko, Mariame Akouche, Hussein El Siblani, Jean-Michel Goupil, Baptiste Rigaud, Delphine Minoux, Jean-Pierre Dath, and Valentin Valtchev ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b01936 • Publication Date (Web): 25 Jul 2018 Downloaded from http://pubs.acs.org on July 25, 2018

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ACS Catalysis

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Synthesis of Embryonic Zeolites and Their Use to

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Process Bulky Molecules

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Kok-Giap Haw, †, § Jean-Pierre Gilson, † Nikolai Nesterenko, ‡ Mariame Akouche, † Hussein El

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Siblani, † Jean-Michel Goupil, † Baptiste Rigaud, † Delphine Minoux, ‡ Jean-Pierre Dath, ‡

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Valentin Valtchev †,*

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†

Normandie Univ, ENSICAEN, UNICAEN, CNRS, Laboratoire Catalyse et Spectrochimie,

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14000 Caen, France ‡

Total Research and Technology Feluy (TRTF), Zone Industrielle C, 7181 Feluy, Belgium

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Abstract

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X-ray

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tetrapropylammonium (TPA+) hydroxide as a structure directing agent. Their physicochemical

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properties are compared to those of a highly crystalline ZSM-5. Embryonic zeolites contain

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fewer acid sites, but their micropore volume and SBET area are higher than crystalline MFI-type

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material synthesized with TPA+. They can be introduced in the mesopores of a shaped silica-

amorphous

zeolite

precursors,

embryonic

zeolites,

ACS Paragon Plus Environment

are

prepared

using

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doped alumina matrix by two procedures: i) impregnation of externally bred embryos, ii) in-situ

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growth of embryos to prepare composite catalysts. Their catalytic performances in the

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dealkylation of 1,3,5-triisopropylbenzene (TiPBz), a bulky molecule hardly penetrating the

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micropores of most zeolites, are superior to their highly crystalline ZSM-5 counterpart and the

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silica doped alumina support. This is attributed to the highly accessible active sites of embryonic

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zeolites, located in an open microporosity leading to shorter diffusion pathlengths. They offer

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interesting prospects to process bulky molecules in fields such as oil refining, petrochemistry,

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biomass upgrading.

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Keywords: embryonic zeolites, ZSM-5, composite, catalysis, bulky molecules processing,

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dealkylation

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1. INTRODUCTION

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Molecular sieve zeolites have proved exceptionally useful as heterogeneous catalysts in a

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number of industrial processes.1,2 They typically offer high specific surface area located in

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ordered micropores of molecular dimensions (ca. 0.25 – 1 nm) and active sites promoting

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superior shape selectivity in catalytic transformations.3-5 However, the downside for catalysis in

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such a confined environment is some often severeoften-severe transport limitations of reactants

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and products. Due to their limited pore sizes (< 1.0 nm), current zeolites cannot be applied to

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significantly process molecules with kinetic diameters much larger than 1.0 nm.5 Thus tTheir

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only active sites accessibility accessibile to bulky molecules is limited toare those located on the

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surface of the zeolite crystals and their pore mouths of zeolites. This handicap is particularly

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pronounced in the conversion of heavy fractions of crude and bio-sourced oils.


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Zeolites based catalysts are for instancehowever very efficient to transform convert heavy

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gasoil fractions or residues to olefins and gasoline (FCC) or middle distillates (Hydrocracking),

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but because they are engineered as composite catalysts where other components (e.g. amorphous

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silica alumina) first reduce somewhat the molecular weight of the feeds. The ongoing

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“whitening” of the oil barrel (ever heavier fractions to be converted in transportation fuels and

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petrochemicals) and biomass conversion will require extra-large pore zeolites.6 A number of

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zeolite structures possess extra-large pores, but none of them reached the commercial stage due

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to their limited (hydro)thermal stability.7 The quest for alternative materials combining zeolite-

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type of activity with better accessibility to their active sites is one of the top priorities of in

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zeolite chemistscatalysis. Since their first commercial use, much work has been expandedwas

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devoted to prepare hierarchical or mesoporous zeolites to improve the catalytic performance of

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purely microporous zeolites.8-10 A mesoporous network connected to the native micropores in

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zeolite crystals decreases the impact of mass transport limitations, but it does not change affect

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the intrinsic limited ability of hierarchical zeolites to process bulky molecules. Decreasing the

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zeolite crystal size leads to a largermore (active) external surface accessible to bulky molecules

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and is also an very effective way to reduce diffusion pathlengths.

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Advances in understanding zeolite nucleation and growth lead to more rational zeolite

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synthesis, especially to control their crystal size.9-17 This Such newly acquired knowledge

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allowed to control better the crystallization of zeolites and tune their physicochemical

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properties.18 For instance, ultra-small non-aggregated FAU- and EMT-type zeolites were

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produced with narrow particle size distribution.19,20 Such A dramatic decrease of crystal size to

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10 – 20 nm resulted in materials with external surface areas up to 250 m2 g-1. These crystals

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contain between 5 and 10 unit cells. Such drastic reduction ina crystal size is also beneficial for


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on the mass transport of small molecules. However, with these nanozeolites, and any other type

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of hierarchical zeolites, the major portion of the active sites remains inaccessible for molecules

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with size larger than the pore openings.

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A further reduction of thein crystal size is a promising alternative that wouldto combine zeolite

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activity with and more accessible surfaces. However, the preparation of zeolitic units with a size

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below 8 – 10 nm results in X-ray amorphous materials and brings difficulties in their

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identification. Some earlier works reported the preparation of X-ray amorphous materials under

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condition typical for zeolite formation. Manton and Davitz used tetraalkylammonium cations,

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largely usedubiquitous in the synthesis of high silica zeolites, to prepare amorphous

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aluminosilicates (ASA).21 They found reported a relationship between the size of the cation and

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the median size of the pores. The A solid synthesized with tetrapropylammonium (TPA+)

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showed the highest catalytic activity in the cracking of n-heptane. Shortly after this

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studythereafter, Jacobs et al. reported that precursors of ZSM-5 exhibit catalytic activities similar

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to their crystalline counterpart.22 This was attributed to zeolite units with a size below their

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detection limit by X-ray diffraction (XRD); they resorted to Infrared (IR) spectroscopy to

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evaluate the crystallinity of XRD amorphous materials. Corma and Pérez-Pariente also studied

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the physicochemical properties of amorphous silica-alumina prepared from tetraalkylammonium

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solutions in the presence of sodium and potassium.23 Their goal was to obtain amorphous

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matrices with a controlled porosity and acid site density. They paid particular attention to the

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(hydro)thermal stability of the resulting materials: upon severe steaming, their zeolite precursors

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dealuminated and their micropores collapsed. A ZSM-5 precursor prepared in the presence of

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TPA·OH was used for the synthesis of amorphous mesoporous silica-alumina (MSA) by Bellussi

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et al.24 They highlighted the occurrence of some order in the zeolite precursor species, by a


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combined combining IR and

Al MAS NMR spectroscopy characterization. Such MSA

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materials showed outstanding performance in propene oligomerization, attributed to their a

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peculiar pore system combined with remarkable superior acidity. They same group further

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performed a more detailed study on the impact of the ratio between the silica source, TPA·OH

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content and the type of solvent (methanol, ethanol, 1-propanol) on the pore structure of their

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MSA.25 Pinnavaia and co-workers studied the preparation of mesopousres materials from zeolite

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precursor systems. Their materials were substantially more (hydro)thermally stable and

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displayed a high catalytic activity.26,27 Corma and co-workers prepared amorphous zeolite

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precursors of ZSM-12, Beta, NU-87, ITQ-7 and -21 and showed that they are stable for under

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steaming calcination up to 923 K.28 The same group reported the room temperature synthesis of

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micro-mesoporous material by dual templating, i.e., combining TPA with simple molecules as

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tromethamine, cysteamine and ethanolamine.29 Recently, FAU-type zeolite precursors were used

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as basic catalysts for in the Knoevenagel condensation of benzaldehyde with ethyl

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cyanoacetate.30 The product, ethyl α-cyanoacetate, can be formed in the super cage of the FAU-

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type structure, but it is too big to be released through the 12-member ring (12MR) window. The

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crystal growth kinetics of the zeolite was used to control improve their catalystic performances.

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A sample taken at the end of the induction period, i.e. just before the appearance of a X-Ray

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crystalline phase showed displayed the highest catalytic activity. A combined Raman-HEXRD-

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solid-state NMR analysis revealed that substantial changes in the ring structure of the

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aluminosilicate precursor occurred. Thus, their high catalytic activity was attributed to a

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combination of zeolite type basic sites combined with a more open structure of semi- crystalline

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zeolite units.


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Recently we developed an advanced catalyst based on ultra-small zeolite crystals, the so-called

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embryonic zeolites, supported on a shaped (microspheres and extrudates) silica-doped alumina

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matrix providing the necessary mechanical resistance for such a composite catalyst.31 The goal of

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the present study is to shed more light on the structure, properties and catalytic potential of

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embryonic zeolites in bulky hydrocarbons conversion. Two catalyst preparation procedures are

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used in this study: i) the deposition, by nano-casting, of externally bred embryonic zeolites on a

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silica-doped alumina matrix (Siralox 30), ii) the in-situ growth of embryonic zeolites on a silica-

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doped alumina matrix (Siralox 30. Their catalysts catalystic performances are evaluated in the

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dealkylation of a model bulky molecule, 1,3,5-triisopropylbenzene (1,3,5-TiPBz, kinetic

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diameter, 0.95 nm) hardly penetrating the micropores of any industrially relevant zeolite.

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2. EXPERIMENTAL SECTION

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2.1 Preparation of ZSM-5 precursor suspension

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ZSM-5 precursors were prepared using the following reagents: tetrapropylammonium

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hydroxide (TPA·OH 20% in water, Alfa Aesar) solution, distilled water, aluminum sulfate (98%,

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Aldrich), and tetraethylorthosilicate (TEOS 98%, Aldrich). After mixing the reactants, the

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solution was allowed to hydrolyze at room temperature for 4 h in a closed bottle. A complete

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hydrolysis was confirmed by the appearance of a monophasic clear solution. The composition of

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the initial clear solution was: 4.5(TPA)2O:0.25Al2O3:25SiO2:430H2O:100EtOH, where EtOH is

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the side product of TEOS hydrolysis. The following series of zeolite precursor (ZP) solutions

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was prepared:

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ZP-1: SiO2/Al2O3=100, hydrolyzed at room temperature for 4 h followed by hydrothermal treatment at 100 °C for 12 h.


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ZP-2: SiO2/Al2O3=100, hydrolyzed at room temperature for 4 h.

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As prepared zZeolite precursor suspensions, without any any further purification, were further

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used for theto prepareation of the catalysts (see below). In order tTo study the physicochemical

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properties of the ZP-1 and ZP-2 solids, their suspensions were freeze dried. The calcination was

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performed at 550 °C (at 2 °C·min-1 ramp rate) in air flow. As mentioned, the obtained the parent

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zeolite precursor suspensions were not subjected to purificationpurified, thus the SiO2/Al2O3 was

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similar to the initial gel and the solid yield was close to 100 %.

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Reference samples: A commercial ZSM-5 sample (MFI90) with Si/Al ratio of 45 was provided

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by Sud-ChimieChemie (now Clariant). A highly crystalline nano-sized ZSM-5 prepared from the

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same initial suspension described above at 100 °C for 29 h was used as a reference sample. A

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highly crystalline micron-sized ZSM-5 prepared from an initial suspension with the molar

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composition 4.5(TPA)2O:0.07Al2O3:25SiO2:1500H2O:100EtOH at 160 °C for 24 h was also

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used for comparative purposes.

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2.2 Catalyst carrier

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A commercial silica-doped alumina microsphere, Siralox 30 (γ-Al2O3 doped with 30 % SiO2,

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Sasol - Germany) was used as a catalyst carrier. The initial (green) matrixIt was calcined under

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air at 600 °C for 10 h. A cationic polymer, PDDA (poly(diallyldimethylammonium) chloride, 20

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wt. % in water, Aldrich) diluted to 0.5 wt. % with distilled water was used to reverse the negative

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surface charge of Siralox 30. The pH of the 0.5 wt. % PDDA solution was adjusted to 8 – 9 by

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using a 0.1 M ammonia solution. A weight ratio Siralox 30/PDDA ratio of 0.3 follows by drying

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at 60 °C overnight.

23


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2.3 Preparation of Embryonic ZSM-5/Siralox 30 composites by impregnation

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In a typical experiment, the Siralox 30 was impregnated with thea pre-ageda clear suspension

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containing externally bred embryonic zeolites. The weight ratio of aged precursor/Siralox 30 was

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2 for the 1st first impregnation. The sample was then dried at 60 °C for 24 h and subsequently

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dried at 100 °C for 12 h. The impregnation step was repeated twice with a suspension/carrier

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ratio of 1.5 and 1 for the 2nd second and 3rd third impregnations, respectively. After a drying step,

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both organic template and the cationic polymer were decomposed by air calcination from room

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temperature toat 550 °C (2 °C·min-1 ramp rate) and heldfor 4 h under airat this final temperature.

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The calcination temperature was reached with a 2 °C·min-1 ramp. The calcined composite was

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ion-exchanged with 0.3 M NH4NO3 solution (solid:liquid=1:100, wt./wt.). The H-form of the

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ZSM-5/Siralox 30 composites was obtained by calcination at 550 °C (at 2 °C·min-1 ramp) in air

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flow. Samples prepared by Impregnation (WI) with ZP-1 and ZP-2 are denoted WI-1 and WI-2,

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respectively.

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2.4 In-situ preparation of Embryonic ZSM-5/Siralox 30 composites by hydrothermal treatment

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The Siralox 30 matrix was mixed with ZP-1 precursor suspension at solid/liquid ratio of 0.5/25

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and homogenized on a shaker at a speed of( 125 rpm) for 2 h. The crystallization was then

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performedproceeded under hydrothermal conditions at 100 °C and samples were taken every 12

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h up to 192 h (8 days). The samples prepared by hydrothermal treatment are denoted HT-x,

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where x is the duration (days) of the hydrothermal treatment. The composite was separated from

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its mother liquor and treated in distilled water under ultrasonic radiation for 5 min to disperse

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any loosely attached particles, followed by washing under vacuum filtration until pH 8 – 9 and


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dried at 100 °C. The composites were calcined at 550 °C (at 100 °C·h-1 ramp rate) in air flow for

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4 h, subjected to ion- exchanged with 0.3 M NH4NO3 solution (solid:liquid=1:100, wt./wt.),

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washed, dried and calcined at 550 °C (at 2 °C·min-1 ramp rate) in air flow.

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2.5 Characterization

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All materials were characterized by X-ray diffraction (XRD) using a PANalytical X’Pert Pro

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diffractometer with Cu Kα radiation (λ = 1.5418 Å). The samples diffractograms were scanned

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recorded in the 2θ range of 3-50° with a step size of 0.02°. Electron micrographs were taken on a

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TESCAN Mira scanning electron microscope (SEM) operated at 30 kV. To improve the

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electrical conductivity of the sample, they were sputtered with gold or platinum. Nitrogen

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adsorption measurements were carried out on a Micromeritics ASAP 2020 surface area analyzer.

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The calcined samples were analyzed after degassing at 300 °C. The isotherms were recorded

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using ASAP 2020 analysis program. The microporous volume (Vmic / cm3 g-1) and external

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surface area (Sext / m2 g-1) were obtained from t-plot based on the Harkins-Jura equation. The

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macro-mesopores size distribution was obtained from the desorption branch using the Barrett

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Joyner Halenda algorithm assuming cylindrical pores. For refined analysis, micro-mesopores

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size distribution was extracted from Density Functional Theory modelling of the adsorption

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branch. Particles size analysis was performed by dynamic light scattering (DLS) with a Malvern

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ZetaSizer Nano Series instrument. Before transferring samples into the test cells, the zeolite

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precursor particles were stabilized in water at 25 °C. TG-DSC measurements of different

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samples were carried out on a SETSYS evolution instrument (SETARAM). About 10 mg of each

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sample was introduced in an alumina crucible and that is loaded in the analyzer chamber. The


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samples was were heated from 30 °C to 800 °C with a heating ramp of 5 °C·min-1 under air (flow

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rate: 40 mL·min-1). Prior to the analysis the physisorbed TPA+ was eliminated by ion exchange.

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Each sample was exchanged two times with 1.0 M sodium sulfate solution (composite/liquid =

4

1/50 wt./wt.) at ambient temperature under agitation on a shaker at a speed of 175 rpm for 3 h.

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After that, tThe composite was recovered from the solution, washed with distilled water and

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dried. A Siralox 30 that has been impregnated by with a TPA·OH solution (4.5TPA2O:430H2O)

7

was used as a reference sample. It was also exchanged two times with Na+ before subjected toits

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TG-DSC analysis.

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All NMR measurements were done with 4-OD mm zirconia rotors and with a spinning (speed

10

rate of 12 kHz) at the magic angle. The solid state {1H}-13C CP-MAS NMR spectra were

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recorded on a Bruker Avance 400 spectrometer operating at (100.6 MHz for

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pulse program, a π/2 pulse of 3.7 µs on 1H, a contact time of 2 ms and a recycle delay of 2 s were

13

used.

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MHz for. 27Al) and MAS NMR spectra was obtained with a π/12 pulse and a recycle delay of 1 s.

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The amount of non-tetrahedral Al was estimated by the integration of peak area using the Dm-fit

16

program.32,33 The results were verified employing with the Top Spin program. The sSolid state

17

29

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T) operating at 99.3 MHZ, using zirconia rotors of 4 mm outer diameter spun at 12 KHZ. A

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single pulse excitation (30° flip angle) is was used with a recycle delay of 30s. TMS was used

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asthe reference for 29Si and 13C, while a 1M Al(NO3)3 solution was employed as athe reference

21

for 27Al.

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27

13

C). During the

Al MAS NMR was recorded on a Bruker Avance 500 spectrometer operating at( 130.3

Si NMR (1pulse) was were recorded also on a Brucker Avance III-HD 500 spectrometer (11.7

129

Xe NMR experiments were performed on a Bruker AV III 400 wide bore spectrometer

equipped with a 10 mm BBO probe, operating at a frequency of 110.64 MHz for 129Xe. Prior to


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each experiment, the sample was introduced into a home-designed (10 mm O.D.) NMR tube and

2

dehydrated under high-vacuum overnight at 573K. For thermally polarized (TP)

3

experiments, xenon gas (99% enriched

4

by varying the pressure from the lower to the higher value. For hyperpolarized (HP) 129Xe NMR

5

experiments, the hyperpolarized xenon gas was obtained using a home-built xenon polarizer34

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based on the spin-exchange optical pumping (SEOP) technique.35,36 HP 129Xe NMR spectra were

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acquired under a continuous re-circulating flow at 150 mL·min-1 of a gas mixture containing 96

8

% He, 2 % N2 and 2 % of natural abundance xenon, with a 129Xe partial pressure of ~20 torr. For

9

all variable temperature (VT) measurements and after each temperature change performed at a

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speed of 5 K·min-1, a waiting delay of about 20 min was generally necessary to ensure a

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homogeneous temperature all over the sample, and this stability was checked by directly

12

monitoring the xenon chemical shifts. All spectra are were referenced to free xenon gas at 0 ppm.

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The IR spectra of ZP-1, ZP-2 and highly crystalline ZSM-5 reference samples were recorded

14

on a SHIMADZU IRAffinity-1 Fourier Transform Infrared Spectrophotometer using the KBr

15

pellet technique. Pyridine adsorption was used to evaluate their acidity of the samples. The study

16

was performed using a standard in-situ FTIR set-up. These Infrared IR spectra were recorded on

17

a Nicolet Magna 550 FTIR spectrometer equipped with a DTGS detector at 4 cm-1 optical

18

resolution, with one level of zero-filling for the Fourier transform. Prior to the measurement,

19

sample was grinded and pressed into a self-supporting disc (diameter: 2 cm, approx. 5 mg cm-2)

20

and activated in vacuum (ca. 10-6 hPa) at 460 °C for 2 h at 2 °C·min-1. After cooling to room

21

temperature, the spectrum of the sample was recorded for further use as a reference. Then, a

22

pressure of 1.33 hPa of pyridine was established in the cell at ambient temperature to reach

23

saturation. The wafer was heated at 100 °C for 15 min to facilitate diffusion of pyridine into

129

129

Xe NMR

Xe isotope) was directly adsorbed into on the sample


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throughout the sample. Successive evacuations were performed at 25 °C, 50 °C, 100 °C, 150 °C,

2

200 °C, 250 °C, 300 °C, and 350 °C at 15 min intervals. All spectra were normalized to 2.5 mg

3

cm-2 wafer. Quantification of the acid sites was performed on the difference spectra (spectra of

4

pyridine - coveredadsorbed on a sample - reference spectra). The amounts of acidic Lewis site

5

(L) and Brønsted site (B) were determined using the band area from the coordinated pyridine at

6

1450 cm-1 and that of the adsorbed pyridinium at 1545 cm-1 respectively. The molar extinction

7

coefficients (Ɛ) used for quantification were taken from Khabtou et al.37: Ɛ1545 (B-pyridine) = 1.8

8

cm µmol-1 and Ɛ1455 (L-pyridine) = 1.5 cm µmol-1. The OMNIC version 7.3 SP1 program was

9

used for data processing.

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Inductively coupled plasma–atomic emission spectroscopy (ICP-AES) on an OPTIMA 4300

11

DV (Perkin–Elmer) was used for theto determineination of theall chemical composition by using

12

an OPTIMA 4300 DV (Perkin–Elmer) instrument.compositions

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2.6 Catalytic testing

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The performances of the cCatalystsic performances were evaluated in the dealkylation of a

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model bulky molecule, 1,3,5-triisopropylbenzene (TiPBz) in a down flow atmospheric reactor.

17

Its kinetic diameter (0.95 nm), is well above the pore openings of the MFI-type zeolite (0.56 nm)

18

and TiPBz is therefore commonly used to study the external surface properties of zeolites,

19

including the large pores (12 MR) ones. In a typical catalytic test, 20 mg of catalyst were loaded

20

at the center of a stainless-steel tubular reactor (internal diameter of 1/2'') and activated in-situ at

21

460 °C (ramping from room temperature at 5 °C·min-1) under air flow (50 ml·min-1) for 1 h and

22

then under a dry nitrogen flow (50 ml·min-1) for 0.5 h. The temperature was subsequently

23

lowered to 300 °C, the nitrogen flow diverted to a saturator (maintained at operated at 70 °C)


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filled with TiPBz (Alfa Aesar, 97%), generating aand the resulting TiPBz partial pressure of (170

2

Pa) . sentfed to the reactor. The WHSV (Weight Hourly Space Velocity) was held constant at 8

3

h-1 for all tests. The conversions were measured at 300 °C over a period of 180 min. The online

4

analysis of the products and unconverted reactant, transferred via a heated line heated at (150 °C)

5

to a gas sampling valve, were monitored by a Varian CP-3800 Gas Chromatograph equipped

6

with a flame ionization detector (FID) detector and a HP-PONA cross-linked methyl siloxane

7

column, 50 m (L) x 0.2 mm (ID) x 0.5 µm (film thickness). The temperature of the column

8

temperature was kept constant at 160 °C,

9

temperature was maintained at 200 °C while and the detection temperature was maintained at

10

250 °C.

11

3.0 RESULTS

throughout the measurement. Tthe injection

12

We reported elsewhere the synthesis of pure embryonic zeolites.38 The impact of Si/Al ratio,

13

temperature and time on their formation of embryonic zeolitic units were screened. In the present

14

study we employed use an optimized system (SiO2/Al2O3=100) to prepare embryonic zeolites at

15

100 °C (ZP-1) and room temperature (ZP-2) and proceed to their loading on a shaped silica-

16

doped alumina matrix, Siralox 30.

17 18

3.1 Physicochemical characterization

19

All synthesized materials are X-ray amorphous (Figure S1). However, the IR spectra (KBr

20

pellet) of ZP-1 and ZP-2 exhibit absorption bands characteristic of pentasil type zeolites.39 More

21

precisely, the 1225 cm-1 (external asymmetric stretching) and 550 cm-1 (double five-membered

22

rings) bands from the MFI structure are clearly present (Figure S2). These bands are larger and

23

less intense than those of the highly crystalline reference sample. Although not intense, they


13

ACS Catalysis

1

illustrate that some elements of a zeolite structure are already present in the X-ray amorphous

2

precursors. According to DLS, the particle size of both samples, ZP-1 and ZP-2, is 3 – 4 nm

3

(Figure S3). The material synthesized at 100 °C (ZP-1) contains small amounts of a second

4

population of larger (ca. 18 nm) particles. TEM inspection of zeolite embryos shows highlights a

5

uniform mass of apparently amorphous particles (Figures S4a). Upon drying the solid product

6

agglomerates and form large particles with straight edges (Figure S4b). The TEM inspection of

7

the large particles showed shows identical features identical with to the freeze dried ZP-1

8

suspension (Figure S4c). No morphological differences are observed between ZP-1 and ZP-2. N2

9

adsorption analysis of calcined ZP-1 and ZP-2 indicates that they are essentially microporous

10

(Figure 1). They display a type Ib isotherm,40 characteristic of extra-large microporous materials

11

with pores ranging between 1 and 2.5 nm. Such characteristics are substantially different from

12

highly crystalline MFI-type zeolites. The ZP-1 and ZP-2 BET specific surface area and

13

micropore volume are respectively 627 and 640 cm2 g-1 and 0.31 and 0.30 cm3 g-1. This suggests

14

a microporous structure more open than the crystalline material obtained with the same template.

15

For the sake of comparison, the isotherm of a highly crystalline nano- and micron-sized ZSM-5

16

zeolite synthesized with TPA+ is included in Figure 1.

17 18 19 20 21 22 23

500

ZP-1 ZP-2

Vads / mL (STP) g-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 49

400

Nano-sized ZSM-5 Micron-sized ZSM-5

300 200 100 0 0.0

0.2

0.4

0.6

0.8

1.0

P / P°


14

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ACS Catalysis

1

Figure 1. N2 adsorption/desorption isotherms of ZP-1, ZP-2 and highly crystalline micron- and

2

nano-sized ZSM-5 samples.

3 4

The state of aluminum in as-synthesized and calcined zeolite embryos is studied by 27Al MAS

5

NMR (Figure S5). The spectra of as-synthesized embryonic zeolites exhibit a single peak

6

centered at 53 ppm. This peak is characteristic of tetrahedral aluminum in low aluminum silicon

7

rich zeolites frameworks. After calcination at 550 °C a decrease in intensity suggests that part of

8

the Al leaves this tetrahedral coordination. Indeed, a second peak at 0 ppm, typical of

9

octahedrally coordinated Al, is observed in calcined samples. A large peak below 0 ppm reveals

10

the presence of other aluminum species, their nature is difficult to be specified at this stage. The

11

amount of nnon tetrahedralnon-tetrahedral Al is estimated to be about 30 % in ZP-1 and ZP-2

12

samples. and The their 53 ppm peak broadens indicating some distorted tetrahedral units. The

13

amount of Al leaving the tetrahedral coordination is substantially higher than in the ZSM-5

14

zeolite, where the extra framework Al formed upon calcination is usually below 10 %. However,

15

the amount of Al remaining in tetrahedral coordination is substantially higher than what is

16

observed for many commercial amorphous silica-aluminas, indicating an excellent retention of

17

potential active sites.41

18

The

29

Si NMR spectra of Asas-synthesized and calcined ZP-1 and ZP-2 samples were 29

19

subjected to

20

and Q4, were observed. Their positions of the peaks match well with theose already literature

21

date reported for the ZSM-5 intermediates.42-44 Based on tThe deconvolution of the spectra by

22

using the dm-fit program (SI), ) yields the relative fraction of each of the observed species was

23

determined (Table S1). As can be expected, the number of Q2 and Q3 environments was is larger

Si NMR analysis (Figure S6). shows The different silicon environments, Q2, Q3


15

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Page 16 of 49

1

in ZP-1 and ZP-2 samples in respectthan in to fully crystalline ZSM-5. These two chemical shifts

2

retained their positionare unchanged after calcination, while their peak areas decreaseed. This

3

The latter decreased wais coupled with an increase of in Q4 species. and indicates Later result is

4

related with thethe dealumination of ZP-1 and ZP-2 samples (cf., revealed by

5

spectra), and condensation of silica species upon during calcination.

27

Al NMR

6 7 8

9 10 11

Table 1. Physical properties of the ZP-1, ZP-2 and highly crystalline ZSM-5. Samples

SBET/ m2g-1

a

ZP-1

627

9

0.31

0.33

ZP-2

640

9

0.30

0.31

Nano-sized ZSM-5

525

108

0.20

0.40

Micron-sized ZSM-5

369

6

0.18

0.19

a

Sext/ m2g-1

a

Vmicro/ cm3g-1

Vt/ cm3g-1

Sext and Vmic were determined by t-plot. Thermogravimetry (TG/DSC) and

13

C NMR spectroscopy provide valuable insights in the

12

interaction of organic structure directing agents with zeolites.50 TG analysis of highly crystalline

13

ZSM-5 nanocrystals shows a weight loss of about 2 wt. % (water), below 130 °C (Figure S7) and

14

its associated endotherm in DSC. A second peak (weight loss of about 9 wt. %) in the

15

temperature range 250 – 450 °C associated with a DSC exotherm is due to the decomposition of

16

TPA+ cations. The TPA+ release exhibits a low temperature shoulder at about 280 °C,

17

characteristic of loosely attached cations probably located on the surface of zeolitic units. The

18

peak centered at 340 °C is typical of TPA+ cations at the intersection of straight and zig-zag

19

channels. No weight loss or other changes are observed at temperatures higher that 500 °C

20

(Figure S8). ZP-1 and ZP-2 show higher weight losses and somewhat different TG profile than

21

the reference ZSM-5. ZP-2 contains a small amount (ca. 7 wt. %) of surface water released at


16

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ACS Catalysis

1

about 100 °C. A much larger release (ca. 13 wt. %) takes place between 180 °C and 280 °C and

2

the associated endotherm suggests it is due to water release, located in a more confined space or

3

strongly bound to the aluminosilicate structure. It cannot be excluded, however, that some

4

loosely attached TPA+ is also released within this temperature range. An exothermic weight loss

5

(6 wt. %) is recorded between 280 and 380 °C and signals the thermal degradation of TPA+. A

6

similar peak appears inis observed on ZP-1, synthesized at 100 °C. The larger release (ca. 40 wt.

7

%) in the temperature range 50 to 280 °C is attributed to loosely adsorbed water and TPA+

8

species. Again, a peak typical of the combustion of TPA+ interacting with the aluminosilicate

9

matrix is observed at about 340 °C.

10

In the 13C CP MAS NMR spectrum of TPA+ occluded in zeolite ZSM-5, Figure 2, the C1, C2

11

and C3 signals are characteristic of the TPA+ carbons in an MFI-type structure. The splitting of

12

the C3 peak, visible solely in the spectrum of ZSM-5, was previously attributed to methyl groups

13

in two different environments.45-47 The central nitrogen atom is positioned at the channel

14

intersections and two propyl groups are located in the linear channels (all-trans conformation)

15

while the two remaining propyls chains occupy the zig-zag elliptical channels (V-shape

16

conformation). In addition, the methyl groups of adjacent TPA+ ions are in close contact in the

17

linear channels, leading to a higher field chemical shift. Such a phenomenon splitting is not

18

observed in the spectra of the zeolite precursors (ZP-1 and ZP-2), indicating a higher mobility

19

and a looser fit of the TPA+ cations.

20

129

Xe NMR is very useful to characterize the pore structure of calcined embryonic zeolite

21

units.48 ZP-1 exhibits a sharp peak at 113 ppm with a small shoulder at about 123 ppm (Figure

22

3a). A peak at 153 ppm is observed on the (reference) nanosized ZSM-5 (MFI90) (Figure 3b).

23

The higher chemical shift of the latter points to Xe located in a more confined environment. This


17

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Page 18 of 49

1

again indicates, again, tthat the embryonic zeolite units possess pores larger than a typical MFI-

2

type material. The relatively narrow linewidth of the embryonic zeolites peak reveals is

3

indicative of a more open structure allowing a faster exchange than in the case of fully

4

crystallized zeolites.

5 6 7 8 9 10 11 13

C CP MAS NMR spectra of TPA+ in nanosized ZSM-5, ZP-1and ZP-2. Inset:

12

Figure 2.

13

tetrapropylammonium cation with the positions of carbon atoms.

14 15 16 17 18 19 20


18

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ACS Catalysis

1

Figure 3. 129Xe NMR spectrum of ZP-1 (a) and ZSM-5 (b) acquired under a pressure of 150 kPa

2

at ambient temperature.

3

3.2 Composite catalyst preparation

4

The very small size and easy sintering of zeolite embryos make their handling and shaping in a

5

form appropriate for catalytic testing difficult. We used aA mesoporous silica-doped (30 wt. %)

6

alumina matrix (Siralox 30) is used as a carrier to accommodate zeolite embryos by either

7

impregnation or in-situ growth, vide-infra. Prior to any treatment the matrix is heated at high

8

temperature to control its reactivity. We determined Screening experiments indicated that 600 °C

9

is to be the most appropriate temperature, as it provides a good balance between matrix reactivity

10

and retention of its specific surface area; higher temperatures decreasing substantially the

11

specific surface area (Table S2). The incorporation of zeolite embryos in the matrix is takes place

12

either by impregnation of pre-formed units or by in-situ synthesis under hydrothermal

13

conditions. However, both preparations routes take place in alkaline conditions where matrix and

14

embryos are negatively charged. To promote electrostatic interactions with the negatively

15

charged zeolitic embryos, the charge of the matrix is inverted inverted by adsorbing a cationic

16

polymer, PDDA-Cl (Poly(diallyldimethylammonium chloride)), as described elsewhere.49 The

17

embryos can then be nano-casted on such a modified matrix.

18

3.3 Nano-casted composites

19

ZP-1 and ZP-2 are casted on the modified Siralox 30 matrix by impregnation (WI). XRD

20

analysis indicates their amorphous nature composites are amorphous (Figure S9). The intensity

21

of the broad peak in the 2θ range 25° - 30° 2θ range increases with the number of impregnations

22

thus revealingindicating the increase amountpresence of more zeolitic embryos in the matrix, in


19

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Page 20 of 49

1

line increases and is visualized with as athe thick layer coating on the matrix observed by SEM

2

(Figure S10).

3

The Nitrogen physisorption sorption isotherm ofon the bare pristine matrix is a combination of

4

type II and IV isotherms together with including a mesoporous contribution (Figure 4a). The

5

composites isotherms of the composites show an increased microporosity at the expense of the

6

total pore volume (Vt) (Table 1) indicating . This clearly shows that zeolite embryos occupy

7

some of the Siralox 30 mesoporosity. BJH pore size distributions show that WI-1 has smaller

8

pores than WI-2 (Figure 4b) and a lower mesopore volume. WI-1 and WI-2 are prepared in a

9

similar way from ZP-1 and ZP-2 embryonic suspensions, respectively. The difference in

10

mesopore volume is attributed to the bigger particles obtained during hydrothermal treatment

11

(ZP-1) compared to a room temperature preparation of zeolite embryos (ZP-2).

12 13

Table 1. Textural properties of the parent matrix and composites obtained after 1 and 3

14

impregnations with ZP-1 and ZP-2. Samples

Number impregnations

of SBET

Vmic

Vt

/ m2 g-1 / cm3 g-1 / cm3 g-1

Siralox 30

-

317

0.00

1.09

WI-1 (ZP-1)

1

450

0.04

0.74

3

598

0.09

0.53

1

445

0.06

0.85

3

566

0.09

0.66

WI-2 (ZP-2)

15 16

The amount of embryonic zeolite in the composite can be estimated by the oxidative

17

decomposition of the TPA+ cations occluded using thermal analysis. Prior to each measurement,


20

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ACS Catalysis

1

the physisorbed TPA+ is removed by ion exchange with a 1M NaNO3 solution as TPA+

2

molecules trapped in cages or channel intersections cannot be exchanged. The TPA+ content is

3

evaluated by the weight loss between 320 °C and 550 °C where the oxidative decomposition of

4

the TPA+ is known to take place.50 The average loss, in a similar temperature range, for the pure

5

embryonic ZSM-5 (ZP-1) is about 8 wt. %.1,8 In the Siralox composites, the amount of zeolite

6

embryos was evaluated tois 18, 20 and 22 % after 1, 2 and 3 nano-castings, respectively.

7 8 9 10 11 12 13 14 15 16

Figure 4. (a) Nitrogen adsorption isotherms and (b) BJH pore size distribution of Siralox 30

17

matrix and the composites obtained after 3 impregnations with ZP-1 and ZP-2 suspensions.

18

The composite containing only strongly bound TPA+ cations is further studied by 13C CP MAS

19

NMR (Figure 5). In addition to the TPA+ peaks (Cx), four broad peaks are attributed to the

20

adsorbed PDDA (c-x); their positions are highlighted in the inset of Figure 5 and a single methyl

21

group is present on the composite, as in their parent embryos, vide supra.

22


21

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Page 22 of 49

1 2 3 4 5 6 7 8

Figure 5. 13C CP MAS NMR spectra of WI-1 composites obtained after 1 and 3 impregnations.

9

Inset: PDDA monomer with the positions of carbon atoms.

10 11

The

27

Al MAS NMR spectrum of the pristine matrix shows signals at 6.6 and 63.4 ppm

12

attributed to octahedrally (AlVI) and tetrahedrally (AlIV) coordinated Al atoms as observed in γ-

13

Al2O3 (Figure 6). The composites spectra, WI-1 and WI-2, exhibit an additional signal at 53.4

14

ppm, indicative of aluminum atoms in tetrahedral coordination integrated in a silicate

15

framework. Tetrahedrally coordinated AlIV is the origin of Brønsted acidity in zeolites and

16

related materials. The acidity of the composites is studied by in-situ IR spectroscopy using

17

pyridine as a probe molecule. Their IR spectra after activation and before pyridine adsorption are

18

shown in Figure S11. ; The they spectrum exhibits a distinct adsorption band at 3740 cm-1 and a

19

shoulder centered around 3600 cm-1. Owing to the intense silanol band of the Siralox matrix, the

20

bridged hydroxyls (3610 cm-1) appear as a shoulder in the IR spectra. The difference spectra (the

21

spectra of the bare pristine matrix is subtracted from those after pyridine adsorption and


22

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ACS Catalysis

1

evacuation), are reported in Figure S12. The higher acidity of the composite, compared to the

2

pristine matrix, is evidenced with by an increased intensity on of the protonated pyridine (B-Py)

3

peak at 1545 cm-1. The number and strength of these acid sites is further evaluated by desorbing

4

pyridine at different temperatures, Figure 7; . in particular, tThe number of Brønsted acid sites is

5

significantly higher in the composites than in their parent pristine matrix. Moreover, strong acid

6

sites remain on the WI-1 and WI-2 composites, in contrast to the Siralox matrix and illustrate

7

that their nature is probably more zeolite-like than ASA-like as discussed by Chizallet and

8

Raybaud.51

9 10 11 12 13 14 15 27

16

Figure 6.

17

reference highly crystalline nano-sized ZSM-5 sample.

Al MAS NMR spectra of parent the Siralox 30, WI-1 and WI-2 composites and the

18 19 20 21 22


23

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Page 24 of 49

1 2 3 4 5 6 7 8 9

Figure 7. TPD of Pyridine adsorbed on embryonic units monitored by IR Analysis. The Si/Al

10

ratio of reference nanosized ZSM-5 sample (MFI90) is 45.

11

3.4 Hydrothermally prepared composites

12

In addition to the above mentioned nano-casting of embryos on a Siralox matrix, in-situ

13

growth of embryonic zeolites on Siralox 30 carrier is also considered. Preliminary experiments

14

showed a poor interaction between the matrix and the zeolite precursor suspension as no

15

substantial changes were observed in the matrix even though the precursor was fully converted to

16

zeolite in the bulk solution. Therefore, prior to the hydrothermal step, the matrix is, as above,

17

pre-treated with a cationic polymer to promote electrostatic interactions with the negatively

18

charged zeolite precursor species. A series of scouting experiments with a ZP-1 precursor (100

19

°C, ½ to 8 days) allowed to optimize the composite formation under hydrothermal conditions.

20

XRD patterns show that all samples are amorphous, including the one hydrothermally treated

21

for 192 h (Figure S13). Although no crystalline phase is detected, some changes in the XRD

22

patterns of the composites are observed. The peaks characteristic of gamma-alumina (37 – 48°

23

2θ) gradually decrease with treatment time while a broad peak, centered at 23° 2θ, appears


24

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ACS Catalysis

1

emerges and its intensity increases over prolonged hydrothermal treatment. The crystallization in

2

the bulk solutions of a similar composition is much faster and nanosized ZSM-5 are harvested

3

after only 24 h.

4

The nature of the materials formed on the surface of Siralox 30 is, as above, studied by

5

combining TG analysis and N2 adsorption. The amount of embryonic zeolite is calculated, vide

6

supra, on the basis of the weight loss of TPA+ (Figure S14). As for crystalline MFI-type

7

materials, the main weight loss takes place in the temperature range 300 – 550 °C and the

8

amount of zeolite embryos increases with crystallization time (Tables S2). It is worth mentioning

9

that even though the amount of TPA-containing phase in the composites is higher than the

10

detection limit of XRD, i.e. 3 – 5 wt. %, no X-ray crystalline material is found highlighting . This

11

unambiguously proves the absence lack of long range order in these composites. The fact that

12

ZSM-5 crystals are formed in the bulk solution after 24 h, while the content of embryonic units

13

in the matrix increases for up to 7 – 8 days, suggests that the formation of embryonic zeolites in

14

Siralox 30 is due to left over TPA+ cations reacting with the matrix.

15

N2 adsorption shows that external surface areas and micropore volumes gradually increase

16

with treatment time and to reach a maximum after 8 days of treatment (Table 2). Further

17

extension of theLonger crystallization times leads to a decrease of the SBET area (data not

18

presented). The gradual increase in micropore volume indicates the formation of microporous

19

materials in the matrix. In contrast to the impregnation, zeolitization on the matrix does not

20

decrease the total pore volume (Vt); in fact, a slight increase in the total pore volume (Vt) is

21

observed. The most plausible explanation is that the growth of zeolite embryos comes at the

22

expense of a partial dissolution of the matrix; this explains also the slow formation of the

23

microporosity, vide supra.


25

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Page 26 of 49

1

Table 2. Textural properties of the parent matrix and the X-ray amorphous composites obtained

2

after different treatment time. Samples

SBET

Vmic

Vt

/m2 g-1

/cm3 g-1

/cm3 g-1

Siralox 30

322

0.00

1.04

HT-0.5

317

0.03

1.20

HT-1.5

328

0.03

1.10

HT-3

342

0.03

0.98

HT-6

397

0.04

1.12

HT-7

414

0.04

1.17

HT-8

419

0.05

1.12

3

To better understand the phenomenon taking place during these hydrothermal treatments, we

4

studied the chemical compositions of the solid and liquid phases harvested are measured. The

5

molar SiO2/Al2O3 ratio in the solid phase (composites) and mother liquor are shown in Figure 8.

6

The temporal changes of silica to alumina molar ratio of the solid are inversely proportional to

7

those of the mother liquor. They This are related toindicates a partial leaching of alumina from

8

the Siralox 30 and the incorporation of some silica species from the ZP-1 precursor. The

9

abundant presence of reactive aluminum species may also cause a slow crystallization of zeolite

10

units. The lowest known SiO2/Al2O3 ratio for ZSM-5 synthesized with TPA+ is 24. Thus, after

11

the rapid bulk crystallization of pre-aged ZP-1 precursor solution, the process slows down,

12

particularly in the pores of the matrix where the mismatch between zeolite stoichiometry and

13

available silica and alumina species hinders the further crystallization process.

14 15


26

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ACS Catalysis

1 2 3 4 5 6 7 8 9 10

Figure 8. Changes in SiO2/Al2O3 mole ratio in the solid (a) and liquid (b) phase during the hydrothermal synthesis of composite catalysts.

11 12

Although the formation of the zeolite phase is partially inhibited, a constant increase in the

13

amount of TPA-occluded in the solid phase is observed. N2 adsorption also indicates changes in

14

the pore structure of the material. In the absence of clear XRD evidence for ZSM-5 formation,

15

the existence of embryonic ZSM-5 units is again studied by a combination of

16

NMR39 and

17

in ZSM-5 typically shows peaks at 62.6, 16.3, and a doublet at 10.1/11.3 ppm for C1, C2, and C3

18

carbons, respectively. As mentioned above, a splitting of the C3 is due to different confinements

19

of the terminal methyl carbon in the straight and zig-zag channels. As in the TG-DSC study, the

20

TPA+ impregnated Siralox 30 is used as a reference sample and any physisorbed TPA+ is

21

removed by sodium exchange; the observed NMR spectra represents only TPA+ confined in an

22

aluminosilicate environment.

129

13

C CP MAS

Xe NMR.52 The peak assignments of TPA+ is shown in Figure 9. TPA+ occluded


27

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Page 28 of 49

1 13

C CP MAS NMR spectra on of the C2 and C3 of TPA+ in the

2

Figure 9. Close-up of on the

3

composites (solid line) and nano-sized ZSM-5 (dashed line). From bottom to top the treatment

4

time is increased in the following sequence: 24 (HT-1), 60 (HT-2.5), 72 (HT-3), 96 (HT-4), 144

5

(HT-6), and 192 (HT-8) h.

6

The intensity of the C2 and C3 of TPA+ cations in the embryonic composites increases with

7

time indicating that more TPA+ is occluded; at the same time, their positions also gradually shift

8

to lower fields. This indicates that the structure of embryonic units is increasingly organized

9

upon prolonged hydrothermal treatment but never reaches the configuration of a fully crystalline

10 11

MFI type zeolite (ZSM-5). 129

Xe NMR provides additional information since xenon is a probe very sensitive to confined

12

spaces.53 Low chemical shift peaks, close to 0 ppm indicate large (meso-)pores, while high

13

chemical shifts indicate more confined space (micro-)pores. Figure S15 compares the

14

NMR spectra of xenon adsorbed on a 1:1 wt. ratio mechanical mixture of nanosized ZSM-5 and

15

Siralox 30, a nanosized ZSM-5, a pristine Siralox 30 matrix and an X-ray amorphous composite


129

Xe

28

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ACS Catalysis

1

obtained after 6 days hydrothermal treatment (HT-6). The last sample is selected because of its

2

high catalytic activity (vide infra). All spectra are acquired under a pressure of 150 kPa at

3

ambient temperature with enough scans to enhance signal/noise ratio. The 129Xe NMR spectrum

4

of the composite shows a single resonance at 108 ppm, between typical micropore (ZSM-5, 158

5

ppm) and mesopore (pristine matrix, 57 ppm) peaks. The most plausible explanation is that the

6

micro- and mesopores in the composites are in close proximity promoting a fast exchange and

7

thus a single resonance. Such an observation on the intermediate exchange regime is further

8

confirmed by NMR spectra of thermal polarized 129Xe (≈ 99% enriched isotope) adsorbed on the

9

mechanical mixture at different pressure and at room temperature (Figure S16). Each spectrum

10

consists of 3072 scans, using a single 90◦ excitation pulse with a pulse length of 15 µs and a

11

recycle delay of 5 s. The pressure is variatedvaries from the lower lowest to higher highest value.

12

At a pressure of 20 kPa, the 129Xe NMR spectrum shows two resonances: the first, centered at

13

82 ppm, is assigned to xenon adsorbed in the mesopores of the matrix while the second, centered

14

at 127 ppm, corresponds to xenon atoms in the micropores of ZSM-5. The difference between

15

these two resonances increases with the pressure, as the chemical shifts of the first peak

16

decreases (red line) while the second one increases (green line) when pressure increases. The

17

coalescence of both peaks at lower pressure indicates that xenon atoms exchange rapidly

18

between micro and mesopores. When the pressure increases, the exchange is rate slower slows

19

and the single peak evolves in two distinct resonances. It is worth nothing that the intensity of the

20

first resonance increases strongly with the pressure while the increase of intensity is less

21

significant in the case of the second resonance. This is a strong proof that the xenon atoms

22

preferentially adsorb on in the ZSM-5 micropores.


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Page 30 of 49

1

To shed more light on the intimate structure of the composite, both the pristine Siralox 30

2

matrix and the X-ray amorphous HT-6 composite are further investigated with the highly

3

sensitive hyperpolarized (HP)

4

between these two different porous environments. The spectrum of Siralox 30 recorded at 245 K

5

displays a wide

6

reflects a wide distribution of mesopores in the matrix. The chemical shift and intensity of the

7

peaks increase with decreasing temperatures. Below 200 K, a new peak appears at a much higher

8

chemical shift and persists even after the xenon polarization is turned off, possibly due to

9

adsorption of

129

129

129

Xe NMR, at different temperatures to vary the exchange rate

Xe resonance between 50 and 100 ppm (Figure 10a); such a wide resonance

Xe on Al2O3.54 Figure 10b shows the HP

129

Xe NMR spectrum of the X-ray

10

amorphous HT-6 composite. The spectrum acquired at 290 K shows a single peak at 88.2 ppm.

11

As the temperature decreases, the peak shifts to higher values, a behavior typical of

12

adsorbed in porous materials due to more Xe – surface and Xe – Xe interactions. A second peak

13

at a much lower chemical shift (~110 ppm chemical shift) is detectedappears at 170 K and its

14

intensity increases in intensity at 150 K. This indicate two types of pores in the HT-6 composite

15

at 150 K: i) the 110 ppm resonance is already attributed to the mesopores of the Siralox 30

16

matrix, ii) the 180 ppm resonance is therefore assigned to the micropores in the embryonic

17

zeolite units.

129

Xe gas

18 19 20 21 22


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129

1

Figure 10. Hyperpolarized

Xe NMR spectra of: (a) Siralox 30 matrix recorded at variable

2

temperature condition and (b) composite obtained after 6 days hydrothermal treatment (HT-6).

3 4

3.5 Acidity and catalytic performances of the embryonic composites

5

Figure 11a compares the TiPBz conversion, under identical T and WHSV, on Siralox 30 and

6

WI-1 and WI-2 composites obtained after 3 impregnations of ZP-1 and ZP-2 precursors,

7

respectively. The two composites are more active than the pristine matrix, as expected from the

8

presence of tetrahedrally coordinated aluminum in the embryonic zeolites. The initial

9

conversions on WI-1 and WI-2 composites are comparable. However, differences appear as a

10

function of time on stream; the deactivation rate of WI-1 is significantly lower than WI-2.

11

although they The two composites have similar micropore volume and small differences in SBET

12

and Vmic (Table 1). These Such small textural differences do notcannot explain the substantial

13

difference in their catalytic performances. One plausible cause could be the stability of the active

14

sites obtained at 100 °C (ZP-1) and 25 °C (ZP-2); namely, zeolite embryos harvested at elevated

15

temperature are more resilient due to a more locally organized network compared to their room

16

temperature counterparts.

17 18 19 20 21


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Page 32 of 49

1

Figure 11. TiPBz conversion on composites prepared by (a) three impregnation procedures and

2

(b) hydrothermal treatment after 2 and 180 min on stream. Conditions: Treaction= 300 °C, WHSV

3

= 8 h-1. Note: errors bars are provided in (b).

4

Figure 11b compares the TiPBz conversion on Siralox 30 and HT-x composites containing

5

different levels of embryonic zeolites (x = Duration of hydrothermal treatment in day(s)). While

6

the catalytic performance of composites obtained by hydrothermal treatment is always higher

7

than the pristine matrix, those obtained during the first 48 h display relatively low conversions.

8

As discussed above, this treatment leads first to a partial dissolution of the matrix followed by a

9

slower growth of zeolite embryos in the pores of the matrix. This explains a variation in

10

performance and a maximum in activity (6 days) as a function of hydrothermal treatment time.

11

The number of Brønsted acid sites increases with crystallization time and reaches a maximum

12

after six days of synthesis (Figure 12). The catalytic impact of in-situ synthesized zeolite

13

embryos is compared with the reference nanosized ZSM-5 (MFI90) and the pristine matrix. A

14

(modest) increase in the number of Brønsted acid sites (up to 16 µmol g-1) leads to a substantial

15

increase in TiPBz conversion with no further increase in conversion up to 24 µmol g-1. With

16

nano-sized ZSM-5, MFI90, the number of Brønsted acid sites dramatically increases fivefold (up

17

to 135 µmol g-1) but most of them are now confined in a microporous network denying access to

18

bulky molecules like TiPBz: this dramatically limits the conversion to the external surface sites

19

of the zeolite crystals. This is an indirect proof that the composite containing embryonic zeolites

20

possesses a more open structure.


32

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ACS Catalysis

1 2 3 4 5 6 7 8 9 10

Figure 12. Acidity–activity relationship of (a) Siralox 30, (b) HT-2, (c) HT-5, (d) HT-6, (e) HT-7

11

composites and (f) nano-sized ZSM-5 (MFI90).

12 13

4. DISCUSSION

14

The physicochemical characterization of zeolite precursors, the so-called embryonic zeolites,

15

highlights their amorphous nature. However, they display some structural organization (Figure

16

S2) and a well-defined pore structure (Figure 1). N2 adsorption reveals the presence of

17

micropores, templated by the TPA+ cations incorporated in their aluminosilicate matrix.

18

Combined TG – 13C MAS NMR analysis shows that the interaction between the organic template

19

and aluminosilicate species are similar with those observed in crystalline MFI-type material.

20

Indeed, only a part of TPA+ cations (ca. 8 wt. %) are strongly bound to the inorganic matrix and

21

begin their thermal degradation at temperatures similar to those of species occluded in the pores

22

of crystalline ZSM-5. On the other hand, 13C MAS NMR does not show a splitting of the (end)


33

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Page 34 of 49

1

methyl group of TPA+. Such splitting is characteristic of the different environments found in the

2

straight and zig-zag channels of the MFI-type structure. This illustrates that the TPA+ is in a

3

local environment not possessing such a well-defined structure. The size of these pores is

4

between and 1 and 2 nm, as shown by N2 adsorption analysis. This conclusion is confirmed by a

5

129

6

aluminosilicate matrix with a very open structure. The rapid exchange is also due to the small

7

size (3 – 4 nm) of zeolite embryos. A material obtained after 4 h mixing at room temperature

8

(ZP-2) with size 3 – 4 nm exhibits an impressive micropore volume and properties similar with

9

the hydrothermally treated counterpart (ZP-1). Actually, ZP-2 possesses slightly higher

10

micropore volume than ZP-1 (Figure 1). We attribute this result to the beginning of the

11

crystallization process in ZP-1, which results in a decrease of micropore volume. The

12

hydrothermal treatment leads to an aggregation of zeolite embryos and the formation of textural

13

mesopores as revealed by the hysteresis of ZP-1 isotherm. The aggregation of primary particles

14

yields a second population of particles with a size of about 16 – 20 nm (Figure S3). In summary,

15

one can state that:

16

i) at room temperature, during hydrolysis of tetraethylortosilicate, (alumino)silicate species

17

condense around the TPA+. This is a result of electrostatic interactions between negatively

18

charged inorganic and positively charged organic components of the system. Thus, a sharp

19

distribution of 3 – 4 nm particles, incorporating TPA+ cations are formed.

Xe NMR analysis (Figure 3). The Xe atoms exchange very fast and this points to an

20

ii) upon calcination, the TPA+ is removed, leaving behind pores of 1 – 2 nm; the inorganic

21

material produced exhibits substantially higher micropore volume and specific surface area

22

compared to fully crystalline ZSM-5 obtained with the TPA+ structure directing agent.


34

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1

This indicates that tThe TPA+ trapped in the embryonic zeolite will does not fit as tightly as in

2

its corresponding zeolite, ZSM-5. This conclusion is further supported by the presence of much

3

water in the zeolite embryos and the larger micropore volume left after combustion of the

4

template.

5

For practical applications, zeolite crystals need to be shaped (beads, microspheres, extrudates)

6

as technical bodies, most of the time in the presence of a binder (amorphous silica, alumina,

7

clay) to provide thermal and mechanical resistance of the catalyst and avoid substantial pressure

8

drops. This approach, however, is difficult to apply to ultra-small embryonic particles. In

9

addition, the complex shaping process (mixing, mulling, extrusion, drying, calcination) could

10

change substantially the properties of zeolite embryos. To avoid such potential problems, we

11

used a pre-formed mesoporous matrix (Siralox 30, available as mm-sized extrudates and µm-

12

sized spheres), as a carrier for the zeolite embryos. We developed two preparation procedures,

13

impregnation and in-situ hydrothermal synthesis, to prepare composite catalysts. Both

14

procedures lead to Siralox 30 – zeolite embryos composites. In the case of impregnation, the

15

amount of zeolite embryos in the composite depends on the number of impregnation steps and

16

the content of active phase in the composite can be adjusted precisely. Besides, the properties of

17

the zeolite embryos can be set during their preparation and will remain almost unchanged after

18

deposition on the carrier. The in-situ preparation is more complex due to the interaction between

19

the zeolite synthesis solution and the Siralox 30 matrix. This process includes a partial

20

dissolution of the matrix changing the stoichiometry of the zeolite precursor. Consequently, the

21

embryos formation in the pores is slow and their composition difficult to control. We attribute

22

the slow formation of such zeolite embryos in the matrix to the presence of large quantities of

23

dissolve alumina species and therefore a stoichiometry not favorable to ZSM-5 crystallization.


35

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Page 36 of 49

1

Longer treatment times increase the activity of the composites significantly until it reaches a

2

maximum after 6 days hydrothermal treatment (HT-6). All experimental results fully support the

3

conclusions based on the physical characterization of composites, namely that an active phase

4

comprising strong acid sites (embryonic zeolite units) is present in the composites and enhances

5

the catalytic activity of the pristine Siralox 30 matrix. The conversion of 1,3,5-TiPBz on in-situ

6

prepared composite catalysts is higher than its counterpart prepared by impregnation. We

7

underline, however, that each procedure offers certain advantages. The impregnation is rapid and

8

the number of zeolite units can be controlled by the number of impregnation steps. In addition,

9

the properties of zeolite embryos are well defined. In-situ synthesis takes time, but the amount of

10 11

active phase is higher and the mesopore structure of the carrier remains almost unchanged. 27

Al MAS NMR characterization of pre-formed embryonic zeolites shows only tetrahedrally

12

coordinated aluminum. Its position is characteristic of Al in zeolite frameworks (Figure S5).

13

Upon calcination at 550 °C a portion (ca. 30 %) of this aluminum change its coordination from

14

tetrahedral to octahedral. This is substantially higher than for ZSM-5 subjected to the same high

15

temperature treatment, but still lower than for most amorphous silica-aluminas. The lower

16

thermal stability of Al in zeolite embryos is not surprising owing to their amorphous nature and

17

very small size. Nevertheless, enough Al retains its tetrahedral coordination and provides

18

Brønsted acidity evidenced by FTIR monitoring of adsorbed pyridine (Figure 7). The composite

19

materials containing ZP-1 and ZP-2 particles showed substantially higher Brønsted acidity than

20

the pristine matrix but much fewer than a reference nanosized ZSM-5 zeolite (MFI90, Si/Al=45).

21

However, although the number of Brønsted acid in the zeolite embryos is relatively low, the

22

derived composite catalysts show high catalytic activity compared to both the reference zeolite

23

(nanosized, so with a high external surface) and the pristine matrix (Figures 11 and 12). We


36

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ACS Catalysis

1

attribute the remarkable catalytic performance of embryonic zeolites in the dealkylation of TiPBz

2

(ca. 0.95 nm kinetic diameter) to their open and high accessibility structure. These results and the

3

physisorption analysis indicate that their pores are larger than 1 nm.

4

Although, the elucidation of zeolite nucleation/growth mechanism is not the goal of this study,

5

it should be addressed in order to shed more light on the formation of zeolite precursor species.

6

Our data show that TPA+ reacts with the hydrolyzed silica species and form some “cages” where

7

the interaction between the organic structure directing agent and mineral species are fairly strong

8

and close, but not identical to those in highly crystalline zeolites. We attribute this interaction to

9

the charge balancing function of TPA+ compensating the negative charge of an Al tetrahedra

10

surrounded by four silica tetrahedra. The cages contain relatively large amount of water, which is

11

not typical of MFI-type materials synthesized with TPA+. This shows, first that the cages are

12

large enough to accommodate a TPA+ cation and water molecules and second that the

13

aluminosilicate matrix does not fit tightly to the morphology of the structure directing agent. The

14

Ar analysis of ZP-1 and ZP-1 confirmed that the embryonic zeolites exhibit type Ib isotherms

15

characteristic of materials with extra-large micro- or small mesopores (Figure S17). The pore

16

size distribution is presented in Figure 13. As can be seen both sample contain pore larger than

17

0.83 nm, including a fraction of pore centered at 1.42 and 2.25 nm. This finding is in

18

disagreement with nanoslab hypothesis, where the structure of zeolite precursor units is

19

described as fully formed MFI-type framework.55,56 Our results unambiguously show that TPA+

20

cation is situated in a cage, where the four propyl chains are in similar environment. Therefore,

21

the channel system of MFI-type materials is not yet formed, although structural elements as 5-

22

member rings were detected by infrared spectroscopy (Figure S2). Thus, the most adequate

23

description of zeolite embryos as highly hydrated (alumino)silicate cages, formed around


37

ACS Catalysis

1

hydrated TPA+ cation and does not contain long range order. This study indicates that first cage-

2

type units forms around the TPA and later evolve to a MFI-type structure. These units are

3

isometric as the Si-O-Si shell is not following tightly the geometry of TPA cation. The curvature

4

of the embryonic zeolites is thus different from the crystalline MFI-type zeolite and their T-O-T

5

angles are most probably larger than in the crystalline MFI-type zeolite channels leading to the

6

relatively low acid strength observed in the zeolite embryos. A graphical presentation (Scheme

7

S1) highlights this conclusion.

9 10 11 12 13 14

(a)

0.83 nm

1.42 nm 2.25 nm

0

2

4

6

8 10 12 14 16 18 20 Pore width / nm

(b)

0.83 nm

Incrementral pore volume / cm3 g-1

8

Incrementral pore volume / cm3 g-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 49

1.42 nm 2.25 nm

0

2

4

6

8 10 12 14 16 18 20 Pore width / nm

15

Figure 13. DFT pore size distribution of (a) ZP-1 and (b) ZP-2, obtained from Argon sorption at

16

87K.

17 18

5. CONCLUSIONS

19

X-ray amorphous zeolite precursors, embryonic zeolites, are formed with TPA+ cation as an

20

organic structure directing agent. Their micropore volume and specific surface area are higher

21

than that of the crystalline material (MFI-type) prepared with the same structure directing agent.

22

This is due to the very open structure of cage-like units formed in the early stage of reaction

23

between the TPA+ and (alumino)silicate species. Upon calcination, part of the Al atoms leaves


38

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ACS Catalysis

1

the tetrahedral coordination, but the majority of the aluminum remains however tetrahedral to

2

provide Brønsted acidity.

3

We used such embryonic zeolite units as the active phase of composite catalysts where a

4

mesoporous silica-doped alumina matrix is used as carrier. Two preparation procedures, nano-

5

casting of pre-formed zeolite embryos and their in-situ formation under hydrothermal conditions,

6

were developed. Both procedures provide catalysts with high activity in the dealkylation of

7

1,3,5-TiPBz. Thus, depending on the particular demand or application, a preparation procedure

8

meeting the desired catalytic properties could be selected. Substantially improved catalytic

9

activity with respect to the pristine matrix and highly crystalline ZSM-5 is due embryonic

10

zeolites units with active sites in a relatively open structure able to process bulky molecules.

11

The preparation procedure could be extended to other systems/structure directing agent thus

12

providing a wide array of embryonic zeolites with different pore openings and surface properties.

13

The methodology developed in this study offers highly accessible composite material for bulky

14

molecules processing. In petroleum refining and petrochemistry, one could cite for instance i)

15

the catalytic hydrocracking function of slurry-based residue upgrading processes (eg. EST from

16

ENI, UNIFLEX from Honeywell UOP)57 where the cracking function is still thermal, ii)

17

hydrocracking of very long linear molecules (from the Fisher-Tropsch process), iii)

18

hydroisomerization of waxes (synthetic or mineral) to optimize their viscosity (viscosity index)

19

and cold flow properties. Finally, as most bio sourced feedstocks can be quite bulky and very

20

polar due to the presence of oxygen, they would probably benefit from the advantages of

21

embryonic zeolites.

22 23

ASSOCIATED CONTENT


39

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1

AUTHOR INFORMATION

2

Corresponding Author

3

*[email protected]

4

ORCID

5

Valentin Valtchev: 0000-0002-2341-6397

6

Kok-Giap Haw: 0000-0002-9791-0611

7

Present Addresses

8

Kok-Giap Haw

9

§

10

Page 40 of 49

State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University,

Changchun 130012, P. R. China.

11 12 13 14 15

SUPPORTING INFORMATION The Supporting Information is available free of charge on the ACS Publications website at DOI: Characterization data: XRD, FTIR, DLS, SEM, TGA-DSC, N2 sorption,

129

Xe NMR,

27

Al

16

MAS NMR (PDF).

17

ACKNOWLEDGMENT

18

The financial supports from Total Research & Technology Feluy TOTAL Raffinage Marketing

19

is and ANR-17-CHIN-005-01 are highly appreciated.

20


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ACS Catalysis

1

Notes

2

The authors declare no competing financial interest.

3 4

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Supported Embryonic Zeolites and their Use to Process Bulky

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