Ismail Alhassan Auwal

Florent Dubray

Ka-Lun Wong

Fitri Khoerunnisa

Tau Chuan Ling

Svetlana Mintova

Eng-Poh Ng
email: epng@usm.my

Effects of Synthesis Parameters on the Crystallization and Morphological Profiles of SAPO-5 Templated by 1-Benzyl-2,3dimethyl-1H-imidazol-3-ium Hydroxide

Keywords: SAPO-5, Zeolite, Hydrothermal condition, 1-Benzyl-2,3-dimethyl-1H-imidazol-3ium hydroxide, Crystallization process

Introduction

Aluminophosphate (AlPO-n) and silicoaluminophosphate (SAPO-n) microporous materials are a class of zeolites that are commonly utilized in industries involving adsorption, catalysis and separation processes [START_REF] Alam | A new seeding method, dry rolling applied to synthesize SAPO-34 zeolite membrane for nitrogen/methane separation[END_REF][START_REF] Ng | Microwave-green synthesis of AlPO-n and SAPO-n (n= 5 and 18) nanosized crystals and their assembly in layers[END_REF]. Typically, SAPO-5 is among the most important zeotype materials due to its mild acid strength, unidimensional pore channel system, and large pore mouth opening (0.73  0.73 nm 2 ) [3]. SAPO-5 can be readily crystallized under hydrothermal conditions at 180-200 °C for 1-3 days with an aid of alkylamine organotemplates, e.g. triethylamine, tripropylamine, tetraethylammonium hydroxide, tetrabutylammonium hydroxide, and Nmethyldicyclohexylamine [START_REF] Li | Hydrothermal synthesis of SAPO-5 with novel morphologies from hydrogels containing acetic acid and high concentration of triethylamine under neutral or alkaline conditions[END_REF][START_REF] Gómez-Hortigüela | Structuredirecting role of molecules containing benzyl rings in the synthesis of a large-pore aluminophosphate molecular sieve: an experimental and computational study[END_REF][START_REF] Jiang | Synthesis of AlPO4-5 crystals using TBAOH as template[END_REF][START_REF] Ng | Discrete MnAlPO-5 nanocrystals synthesized by an ionothermal approach[END_REF][START_REF] Roldán | Influence of pH and Si content on Si incorporation in SAPO-5 and their catalytic activity for isomerisation of n-heptane over Pt loaded catalysts[END_REF]. The use of so-called structure-directing agents (SDAs) with different molecular shapes, sizes, and polarities have found to form SAPO-5 crystals with different morphological and physicochemical properties [START_REF] Geng | Efficient Manipulation of Continuous AFI-Type Aluminophosphate Membranes with Distinctive Microstructures on Macroporous α-Al 2 O 3 Substrates[END_REF][START_REF] Auwal | Crystallization profile and morphological study of SAPO-5 templated by imidazolium cations of different functional groups[END_REF].

Recently, imidazolium compounds have been explored as novel SDAs in the synthesis of AFI-type microporous solids (AlPO-5 and SAPO-5) [START_REF] Khoo | Ionothermal approach for synthesizing AlPO-5 with hexagonal thin-plate morphology influenced by various parameters at ambient pressure[END_REF][START_REF] Feng | Effect of Ionothermal Synthesis on the Acidity and Catalytic Performance of a SAPO-5 Molecular Sieve[END_REF]. Unlike aliphatic amines, the delocalized π electrons in their cyclic ring together with their bulkier molecules create a versatile platform for adding functional properties and chemical modifications, thus altering the overall crystallization process of SAPO-5 materials [START_REF] Ng | Micro-to Macroscopic Observations of MnAlPO-5 Nanocrystal Growth in Ionic-Liquid Media[END_REF][START_REF] Ng | AlPO-5 nanocrystals templated by 1-ethyl-2, 3-dimethylimidazolium hydroxide and their textural and water sorption properties[END_REF]. Besides SDAs, the crystallization process of SAPO-5 might also be affected by the synthesis parameters, particularly crystallization time, heating temperature, synthesis technique, amount and source of reactants, solvent, and aging [START_REF] Khoo | Synthesis of AlPO-5 with diol-substituted imidazolium-based organic template[END_REF]. Nevertheless, the knowledge about the impacts of those synthesis parameters on the crystallization and morphology of SAPO-5 templated by imidazolium SDA are still not well understood, and thus it is worth of further investigation.

Herein, the crystallization and morphological profiles of SAPO-5 templated by 1-benzyl-2,3-dimethyl-1H-imidazol-3-ium hydroxide as novel SDA is studied by carefully controlling the synthesis parameters. The formation mechanism of SAPO-5 is then proposed based on the X-ray diffraction and microscopy data obtained.

Experimental

Preparation of 1-benzyl-2,3-dimethyl-1H-imidazol-3-ium chloride ionic salt, [Bzm]Cl

Typically, a mixture containing 1). According to XRD analysis, the SH1 and SH2 samples were amorphous with an amorphous hump observed at 2θ = 25°-35° by the second sample indicating the phase reorganization between amorphous particles, forming secondary and more reactive amorphous entity (Fig. 1) [START_REF] Ghrear | Synthesis of Cs-ABW nanozeolite in organotemplate-free system[END_REF]. When the duration was prolonged to 8 h, the XRD peaks due to SAPO-5

started to appear at 2θ = 7. SAPO-5 nuclei started to form in the amorphous bulk solid [START_REF] Chen | Fabricating selfassembled SAPO-5 with tailored mesoporosity and acidity using a single template[END_REF]. After 10 h of heating, the amorphous solid was fully converted into crystalline SAPO-5 solid and further crystal growth of SAPO-5 crystals was observed after 14 h as indicated by the narrower and more intense AFI XRD peaks. However, a phase transformation of SAPO-5 (AFI framework) into SAPO-41 (AFO framework) was also observed with prolonged crystallization time. The XRD analysis thus demonstrated that the formation evolution of SAPO-5 underwent four distinctive stages, namely induction, nucleation, crystallization and Ostwald ripening which will be described in detail in Section 3.7.

The FESEM images of SH1, SH3, SH4 and SH5 were also captured to study the crystallization process of SAPO-5. SH1 was formed by the agglomerated and irregular-shaped particles confirming the amorphous character of this solid (Fig. 2A(a)). The particle size distribution of SH1 was plotted and a monomodal curve centered at 120 nm was obtained (Fig.

2B(a)

). This peak was shifted to larger particle sizes after 8 h indicating amorphous phase reorganization as proven by the XRD analysis (Fig. 2B(b)). In addition, another PSD centered at 500 nm was also observed, thus showing the presence of two different phases in SH3 sample. As seen, SH3 sample was a semi-crystalline solid with the SAPO-5 crystals of hexagonal prism shape were growing on the amorphous entities (Fig. 2A(b)). At 10 h, the small amorphous solids disappeared, and the SAPO-5 crystals grew further (mean size: 650 nm) as confirmed by the (SH4), the alkalinity of the reaction mixture reduced significantly to nearly neutral (pH 7.5) and a solid product was successfully precipitated. The solid was found to be SAPO-5 crystalline solid based on the XRD observation (Fig. 3). In addition, the SAPO-5 solids exhibited hexagonal prism shape with an average crystal size of 650 nm (Fig. 4A(a) and 4B(a)).

Further increasing the P 2 O 5 /Al 2 O 3 ratio to w = 3 induced crystal growth where it could be confirmed by the narrower and more intense XRD peaks in HS7 solid. The XRD observation was also in agreement with the FESEM analysis whereby large spherical particles formed by intergrown SAPO-5 crystals were captured (Fig. 4B(b)). Meanwhile, partial phase transformation of SAPO-5 into dense cristobalite phase was observed with further increasing the P 2 O 5 /Al 2 O 3 ratio (w = 3.5, pH 4.8), where an evolution change of morphology to intergrown grain secondary particles was also detected (Fig. 3 and4). Obviously, an increase in P 2 O 5 content had a profound effect on the crystallization profile of SAPO-5 [START_REF] Wragg | The role of added water in the ionothermal synthesis of microporous aluminium phosphates[END_REF].

Effect of [Bzm] 2 O/P 2 O 5 molar ratio

An equimolar amount of [Bzm] 2 O and P 2 O 5 (x) was used to control the pH of precursor hydrogel since the crystallization of SAPO-5 is more favorable at nearly neutral pH condition as observed in Section 3.2. The pH values of the hydrogels of SH9 (x = 1.5), SH10 (x = 2.0), SH4 (x = 2.5) and SH11 (x = 3.0) were measured to be 6.42, 6.15, 6.07 and 6.14, respectively. As seen, all samples successfully produced SAPO-5 crystalline phase with no impurity but with different XRD peak widths, suggesting different crystal sizes (Fig. 5). This could also be confirmed by the PSD analysis that the mean crystals size was reduced from 1.5 µm to 1.1 µm, 650 nm and 380 nm when the x value increased from 1.5 to 2.0, 2.5 and 3.0, respectively (Fig. 6B). Furthermore, the morphology of SAPO-5 crystals was also changed from hexagonal plate shape to hexagonal prism form when increasing the x values (Fig. 6A).

It is known that the crystal size of zeolites can be predetermined from the extent of nuclei present in the precusor hydrogel. On account of this, an increase in the alkalinity of the hydrogel during the addition of a large amount of organic template, followed by adding H 3 PO 4 to reach nearly neutral pH, promotes low polymerization of aluminate, phosphate and silicate species. As a result, large amount of nuclei is formed under supersaturation condition [START_REF] Ng | AlPO-5 nanocrystals templated by 1-ethyl-2, 3-dimethylimidazolium hydroxide and their textural and water sorption properties[END_REF][START_REF] Hu | Microwave-assisted hydrothermal synthesis of nanozeolites with controllable size[END_REF][START_REF] Wong | Crystal growth study of KF nanozeolite and its catalytic behavior in Aldol condensation of benzaldehyde and heptanal enhanced by microwave heating[END_REF]. For this reason, small-sized SAPO-5 crystals are produced when the x value increases.

Effect of H 2 O content

The concentration of reactants in the precursor is governed by the amount of water added which affects the properties of the final solid product (e.g. phase purity, crystallinity, size, morphology) [START_REF] Askari | Different techniques and their effective parameters in nano SAPO-34 synthesis: A review[END_REF]. In respect to this, four samples of different water contents (SH12, SH4, SH13 and SH14) were prepared by heating the precursor hydrogels having a molar ratio of It is known that the supersaturation condition favors the formation of a large number of nuclei when the water content is reduced. Concurrently, the enhanced alkalinity of the precursor hydrogel induces low polymerization of aluminate, phosphate and silicate species, thus resulting in the formation of smaller SAPO-5 particles [START_REF] Persson | The synthesis of discrete colloidal particles of TPA-silicalite-1[END_REF][START_REF] Brar | Control of crystal size and distribution of zeolite A[END_REF]. However, a less concentrated hydrogel with more acidic environment is prepared when increasing the water content, and it tends to retard the nucleation process and this leads to the formation of larger-sized SAPO-5 crystals [START_REF] An | Photocatalytic degradation kinetics and mechanism of antivirus drug-lamivudine in TiO2 dispersion[END_REF]. Furthermore, tridymite is also co-crystallized when increasing the water content because dense crystalline phases preferably crystallize under an acidic environment [START_REF] Garofalini | Kinetics of tridymite formation[END_REF].

Effect of SiO 2 /Al 2 O 3 molar ratio

In this study, the effects of silica content (expressed in SiO 2 /Al 2 O 3 molar ratio) were studied considering the significance and high tendency of silicon atoms insertion into the parent AlPO-5 during the crystallization process [START_REF] Wang | Silicon species in a SAPO-5 molecular sieve[END_REF][START_REF] Erichsen | H-SAPO-5 as methanol-to-olefins (MTO) model catalyst: Towards elucidating the effects of acid strength[END_REF]. Hence, the hydrogels with a composition of showed an XRD pattern that perfectly matched with the AFI structure (Fig. 9a). The diffraction bands were broad suggesting small crystal size and it could be confirmed from the FESEM image that showed secondary micron-sized particles with layered morphology formed by agglomerated nanocrystals (mean size of 170 nm) (Fig. 10A(a) and 10B(a)). The phase purity remains the same when the Si content was increased to z = 0.23, 0.47, and 0.7, but a significant change in crystal size and morphology was observed (Fig. 10A(b-d)). As seen, the SAPO-5 crystals tended to grow in c-direction when more Si atoms were incorporated forming hexagonal prism crystals with an elongated shape and intergrown structure. As a result, the mean particle size of SAPO-5 increased from 260 nm (z = 0.23) to 650 nm (z = 0.47) before reaching to 4.2 µm (z = 0.7) (10B(b-d)).

Effect of heating temperature

The synthesis temperature affects the crystallization process of zeolite materials since it provides the energy needed for nucleation and crystal growth [START_REF] Askari | Different techniques and their effective parameters in nano SAPO-34 synthesis: A review[END_REF][START_REF] Xu | Chemistry of zeolites and related porous materials: synthesis and structure[END_REF]. Hence, the hydrogel of a molar ratio of 1Al 2 O 3 :2.5P 2 O 5 :2.5[Bzm] 2 O:180H 2 O:0.47SiO 2 was hydrothermally heated at 100, 120, 150 and 200 °C for 10 h (Table 5.1). The SH18 sample heated at 100 °C yielded an amorphous solid (mean size of 120 nm) with a non-distinctive shape (Fig. 11a, 12A(a) and 12B(a)). Thus, it revealed that the rate of crystallization of SAPO-5 is slow at 100 °C due to insufficient energy supplied to the system [START_REF] Hu | Microwave-assisted hydrothermal synthesis of nanozeolites with controllable size[END_REF]. Increasing the temperature to 120 °C slightly accelerated the crystallization rate but the solid was partially converted to SAPO-5 as an amorphous hump was still observed at 2θ = 20 o -30 o (Fig. 11b). On the other hand, the morphology of the solids changed to a relatively hexagonal shape having a mean size of 374 nm (Fig. 12A(b) and 12B(b)). At 150 °C, fully crystalline SAPO-5 was obtained (Fig. 11c, Fig. 12A(c)). Thus, this indicated that the crystallization of SAPO-5 is a thermally activated process that requires energy to convert amorphous precursors into crystalline solids. Nevertheless, supplying excessive energy by increasing the temperature to 200 °C led to the crystallization of a tridymite dense phase. This is because SAPO-5 is a metastable crystalline phase and it tends to transform into a denser phase at very high temperatures [START_REF] Briend | Modifications of structure and Si environment upon heating of SAPO-5[END_REF].

Formation pathway of SAPO-5 crystals

In general, the formation of SAPO-5 crystals templated by [Bzm]OH organic template involves four successive stages, namely induction, nucleation, crystal growth, and Ostwald ripening (Fig. 13). At the induction stage. Al 2 O 3 , P 2 O 5 , and SiO 2 precursors are hydrolyzed in the presence of H 2 O forming low molecular-weight monomers and dimers as primary building units (Step 1) [START_REF] Grand | Mechanism of zeolites crystal growth: new findings and open questions[END_REF]. Phase reorganization also occurs when longer chain oligomers are formed via polycondensation as proven by the XRD analysis (Fig. 1a,b). These oligomers tend to align and enfold around the [Bzm] + molecules via restricted electrostatic interaction between the [Bzm] + cation and the anionic silicoaluminophosphate species forming an organic-inorganic intermediate species.

An equilibrium is established between the hydrolyzed intermediate species and the phase re-organized species, which later resulted into the nucleation stage (Step 2). During this phase, the formation of AFI framework by higher molecular-weight oligomers takes place [START_REF] Grand | Mechanism of zeolites crystal growth: new findings and open questions[END_REF]. The reaction equilibrium begins to shift to the direction of small nuclei with three-dimensional ordering so that they propagate into the eventual SAPO-5 framework structure. These nuclei serve as seeds or starting points for crystal growth in the secondary amorphous particles [START_REF] Cundy | The hydrothermal synthesis of zeolites: history and development from the earliest days to the present time[END_REF]. At this stage, the XRD and SEM measurements hardly detect the presence of SAPO-5 crystalline phase.

Further prolonging the heating time results in the complete conversion of amorphous particles into SAPO-5 crystalline solids (Step 3). This phenomenon happens due to the fact that the amorphous particles are less thermodynamically stable in the mother liquor and they tend to re-dissolve or being consumed as nutrients for further crystal growth [START_REF] Wong | KF zeolite nanocrystals synthesized from organic-template-free precursor mixture[END_REF].

The last stage involves Ostwald ripening where it is always experienced by large-pore zeolites due to their more opened structures and low metastability (Step 4) [START_REF] Jorge | Modeling spontaneous formation of precursor nanoparticles in clear-solution zeolite synthesis[END_REF][START_REF] Auerbach | New insights into zeolite formation from molecular modeling[END_REF]. Thus, a successive crystal phase transformation from SAPO-5 to SAPO-41 with less opened structure occurs.

The induction, nucleation, crystal growth, and Ostwald ripening are also affected by other synthesis parameters. In particular, heating temperature has the most profound effect on those stages because increasing the reaction temperature will supply more energy to the system to ease breaking and new formation of P-O-T bonds (T = Al, P, Si), and directly increasing the crystallization rate [START_REF] Askari | Different techniques and their effective parameters in nano SAPO-34 synthesis: A review[END_REF]. Furthermore, SAPO-5 may undergo different kinds of structural change via reconstructive phase transformation to a more metastable phase at high temperature. This is achieved by unit cell volume contraction through the removal of the water and/or [Bzm] + cations from the pores [START_REF] Hu | Microwave-assisted hydrothermal synthesis of nanozeolites with controllable size[END_REF][START_REF] Trong | Zeolite/Mesoporous Molecular Sieve Composite Materials[END_REF]. Furthermore, the addition of SiO 2 as one of the primary building units also affects the entire crystallization process of SAPO-5. During hydrothermal treatment, the silicates compete with aluminates and phosphates in the construction of AFI framework where the former species affects the crystal growth orientation forming SAPO-5 crystals with different shapes and sizes [START_REF] Jhung | Effects of silica on the synthesis of AFI molecular sieve in acid and base conditions under microwave irradiation[END_REF].

Furthermore, the pH of precursor hydrogel has been suggested to control the nucleation rate of SAPO-5, affecting the crystal size and purity. Large and intergrown SAPO-5 crystals with co-crystallized dense phase are obtained when the crystallization occurs under acidic environments (high P 2 O 5 /Al 2 O 3 ratio or high H 2 O content) because of rapid hydrolysis and polycondensation reactions at low pH [START_REF] Chen | Effect of synthesis pH and H 2 O molar ratio on the structure and morphology of aluminum phosphate (AlPO-5) molecular sieves[END_REF]. In contrast, when the crystallization takes place at nearly neutral environment (~6.0) in the excessive amount of [Bzm] + template, a supersaturation condition is formed. It thus facilitates rapid and homogeneous nucleation rather than crystal growth, forming SAPO-5 crystals with uniform and smaller sizes [START_REF] Ng | Micro-and macroscopic observations of the nucleation process and crystal growth of nanosized Cs-pollucite in an organotemplate-free hydrosol[END_REF]. The samples were heated at 100, 120, 150 and 200 °C for 10 h, respectively.

Conclusion

  3 o [100], 14.8° [200], 19.6° [210], and 25.8° [202], revealing that

  FESEM and PSD studies (Fig.2A(c), 2B(c)). Successive phase transformation of SAPO-5 into SAPO-41 was also observed in SH5 after 14 h of heating where larger secondary particles made by agglomerated primary crystals of SAPO-5 and SAPO-41 (both in hexagonal prism shape[START_REF] Auwal | Metal Chlorides Grafted on SAPO-5 (MClx/SAPO-5) as Reusable and Superior Catalysts for Acylation of 2-Methylfuran Under Non-Microwave Instant Heating Condition[END_REF][START_REF] Agliullin | Selective Crystallization of AlPO 4-41 Molecular Sieve in the Presence of Diethylamine[END_REF]) were detected (Fig.2A(d), 2B(d)).Effect of P 2 O 5 /Al 2 O 3 molar ratioP 2 O 5 and Al 2 O 3 are the basic elements for building the AFI framework. Hence, the effects of P 2 O 5 /Al 2 O 3 ratio on the crystallization of SAPO-5 were studied using a precursor hydrogel of 1Al 2 O 3 :wP 2 O 5 :2.5[Bzm] 2 O:180H 2 O:0.47SiO 2, where w varied from 2.0 to 3.5. When w = 2.0 (SH6), no solid was precipitated after hydrothermal treatment at 150 o C for 10 h which could be explained by the high alkalinity of the hydrogel (pH 10.2) as the amount of H 3 PO 4 added was much lower than that of alkaline [Bzm]OH introduced. Hence, the solid product formed tended to re-dissolve back into the mother liquor [20]. When the P 2 O 5 /Al 2 O 3 ratio increased to w = 2.5

1Al 2 O

 2 3 :2.5P 2 O 5 :2.5[Bzm] 2 O:yH 2 O:0.47SiO 2 (y = 135, 180, 225 and 270) at 150 °C for 10 h, respectively. It was observed that both the crystallinity, crystal size, and purity of SAPO-5 could be controlled by simply tuning the water content. The SH12, which contained the lowest amount of water (y = 135), showed very weak XRD peaks of AFI phase at 2θ = 12.89 o [110], 20.83 o [002], 25.90 o [220], and 29.00 o [311] (Fig.7a). Thus, the sample was a semi-crystalline nanosolid (mean size of 141 nm) with a non-distinctive shape (Fig.8A(a) and 8B(a)). The crystallinity of SAPO-5 increased when the water content was increased to y = 180 (SH4, Fig.7b). No amorphous solid was detected under microscopy investigation. Instead, it contained crystals with hexagonal prism shape (mean size of 650 nm), viz. a typical morphological feature for an AFI-type material (Fig.8A(b) and 8B(b))[START_REF] Geng | Efficient Manipulation of Continuous AFI-Type Aluminophosphate Membranes with Distinctive Microstructures on Macroporous α-Al 2 O 3 Substrates[END_REF][START_REF] Chen | Understanding the Brønsted acidity of SAPO-5, SAPO-17, SAPO-18 and SAPO-34 and their catalytic performance for methanol conversion to hydrocarbons[END_REF]. The crystallinity and crystal size further increased when the water contents were increased to y = 225 and 270 where co-crystallization of tridymite as minor dense phase was observed in the latter sample (Fig.7 and 8A). Furthermore, the SAPO-5 crystals with distinguished bimodal size distribution were also shown indicating the important role of water in the crystallization of SAPO-5 (Fig.8B(d)).

1Al 2 O

 2 3 :2.5P 2 O 5 :2.5[Bzm] 2 O:180H 2 O:zSiO 2 , where z = 0 (SH15), 0.23 (SH16), 0.47 (SH4) and 0.70 (SH17), were prepared. Without adding any SiO 2 , the so-called AlPO-5 sample (SH15)

SAPO- 5

 5 microporous solids with interesting morphologies and sizes have successfully been prepared with an aid of a novel imidazolium structure-directing agent, [Bzm]OH. In addition, the effects of the synthesis variables on the formation of SAPO-5 have also been systematically investigated. As revealed, SAPO-5 undergoes four evolutionary processes, namely induction, nucleation/propagation, crystal growth, and Ostwald ripening. Small SAPO-5 particles can be prepared under low temperature (150 °C), low water content, and concentrated template conditions where such conditions favor nucleation over crystal growth. In contrast, the high molar ratio of P 2 O 5 /Al 2 O 3 (low pH), high SiO 2 content, long crystallization time, and high heating temperature favor the formation of big SAPO-5 crystals and in most cases, tridymite and cristobalite dense phases are also formed. Similar to aliphatic amine templates, the SAPO-5 solids crystallized using [Bzm]OH also exhibit hexagonal prism morphology. Thus, this nanoporous solid is suitable to be employed in catalysis, membrane separation, sensor, heat pump, drug carrier and adsorption technologies. Comprehensive study on studying SAPO-5 in those aspects is in progress.

Figure captions Fig. 1 .

 captions1 Figure captions

Fig. 2 .

 2 Fig. 2. (A) FESEM images and (B) particle size distibutions of (a) SH1, (b) SH3, (c) SH4 and (d) SH5 samples after heating at 150 o C for 2 h, 8 h, 10 h, and 14 h, respectively.

Fig. 3 .

 3 Fig. 3. XRD patterns of (a) SH4 (w = 2.5), (b) SH7 (w = 3.0) and (c) SH8 (w = 3.5) solids where

Fig. 4 .

 4 Fig. 4. (A) FESEM images and (B) particle size distributions of (a) SH4 (w = 2.5), (b) SH7 (w =

Fig. 5 .

 5 Fig. 5. XRD patterns of (a) SH9 (x = 1.5), (b) SH10 (x = 2.0), (c) SH4 (x = 2.5) and (d) SH11 (x

Fig. 6 .

 6 Fig. 6. (A) FESEM images and (B) particle size distributions of (a) SH9 (x = 1.5), (b) SH10 (x =

Fig. 7 .

 7 Fig. 7. XRD patterns of (a) SH12 (y = 135), (b) SH4 (y = 180), (c) SH13 (y = 225) and (d) SH14

Fig. 8 .

 8 Fig. 8. (A) FESEM images and (B) particle size distributions of (a) SH12 (y = 135), (b) SH4 (y =

Fig. 9 . 2 .

 92 Fig. 9. XRD patterns of (a) SH15 (z = 0), (b) SH16 (z = 0.23), (c) SH4 (z = 0.47) and (d) SH17 (y

Fig. 10 .

 10 Fig. 10. (A) FESEM images and (B) particle size distributions of (a) SH15 (z = 0), (b) SH16 (z =

Fig. 11 .

 11 Fig. 11. XRD patterns of (a) SH18, (b) SH19, (c) SH4 and (d) SH20 prepared with hydrogels of

Fig. 12 .

 12 Fig. 12. (A) FESEM images and (B) particle size distributions of (a) SH18, (b) SH19, (c) SH4

Fig. 13 .FiguresFig. 1

 131 Fig. 13. The hydrothermal crystallization pathway of SAPO-5 templating using [Bzm]OH.

Fig. 2

 2 Fig. 2

Fig. 3 Fig. 4

 34 Fig. 3

Fig. 5

 5 Fig. 5

Fig. 6

 6 Fig. 6

Fig. 7

 7 Fig. 7

Fig. 8

 8 Fig. 8

Fig. 9

 9 Fig. 9

Fig. 10

 10 Fig. 10

Fig. 11

 11 Fig. 11

Fig. 12

 12 Fig. 12

Fig. 13

 13 Fig. 13

Table 1 .

 1

	1,2-dimethylimidazole (29.98 g, 98%, Merck), benzyl

o C under stirring. The shiny white crystalline solid formed was then purified with acetone several times before it was dried at 100 o C. The resulting product was [Bzm]Cl with a solid yield of 87.09% (Scheme 1). 1 H NMR (400 MHz, D 2 O): δ = 2.47 ppm (3H, singlet, N-C(CH 3 )-N), 3.71 (3H, singlet, N-CH 3 ), 5.27 (2H, singlet, N-CH 2 -C 6 H 5 ), 7.21 (1H, doublet, J = 3 Hz, N-CH=CH-N), 7.25 (1H, doublet, J = 3 Hz, N-CH=CH-N), 7.27-7.42 (5H, aromatic Ph-H). FTIR (cm -1 , KBr): 3074 (=C-H stretching), 2948 (C-H stretching), 1598 and 1452 (C=C), 1020 (C-N), 1536 (C=N). Anal. cacld for C 12 ClH 15 N 2 : C, 64.65%; H, 6.74%; N, 12.57%; Cl, 15.94%; experimental: C, 65.02%; H, 6.67%; N, 11.54%; Cl, 16.78%.

Preparation of 1-benzyl-2,3-dimethyl-1H-imidazol-3-ium hydroxide ([Bzm]OH)

[Bzm]OH template solution was obtained via ion exchange of [Bzm]Cl with hydroxide resin (Amberlite ® IRN-78, Sigma-Aldrich). The resultant solution was recovered by filtration before titrating with HCl solution (0.10 M) where 90% of OH -exchange was attained. The hydroxide solution was then concentrated until 33.0% by mass at 50 °C before it was further used for SAPO-5 synthesis. Scheme 1. Synthesis pathway of [Bzm]Cl before converting to [Bzm]OH via ion-exchange. Synthesis of SAPO-5 microporous solids SAPO-5 was synthesized as below: Firstly, aluminum isopropoxide (3.2260g, 98%, Acros) and [Bzm]OH solution (26.2753 g) were added into distilled water (5.8279 g) and stirred (500 rpm) for 35 min. Then, H 3 PO 4 (4.4615 g, 85%, Merck) was slowly added dropwise over about 10 min. HS-40 colloidal silicate (0.5464 g, Sigma-Aldrich) was successively introduced forming a precursor having a molar ratio of 1Al 2 O 3 :2.5P 2 O 5 :2.5[Bzm] 2 O:180H 2 O:0.47SiO 2 . The resulting precursor was loaded into a Teflon lined autoclave (50 mL) and hydrothermally treated at 150 °C for 10 h. After cooling, the solid product, denoted as SH-4, was centrifuged (

TableTable 1 .

 1 Chemical composition and crystallization conditions of SH1-SH20 hydrogels together with the products obtained after altering the individual synthesis parameter.

	Synthesis	Gel molar composition	T	t
	Samples			Products*
	parameters	Al 2 O	(℃)	(h)

3 P 2 O 5 [Bzm] 2 O SiO 2 H 2 O

		SH15				0			AFI (AlPO-5)
		SH16				0.23			AFI
	SiO 2		1	2.5	2.5	180 150 10
		SH4				0.47			AFI
		SH17				0.7			AFI
		SH18					100		Amor.
		SH19					120		AFI
	Temperature	SH1	1	2.5	2.5	0.47 180		2 10	Amor.
		SH4					150		AFI
		SH2						4	Amor.
		SH20					200		Tridymite
	Time	SH3	1	2.5	2.5	0.47 180 150	8	Amor. + AFI
	*Amor. = Amorphous; AFI = SAPO-5; AFO = SAPO-41
		SH4						10	AFI
		SH5						14	AFI + AFO
		SH6		2.0					No solid
		SH4		2.5					AFI
	P 2 O 5		1		2.5	0.47 180 150 10
		SH7		3.0					AFI
		SH8		3.5					AFI + cristobalite
		SH9		1.5	1.5				AFI
	P 2 O 5 /	SH10		2.0	2.0				AFI
			1			0.47 180 150 10
	[Bzm] 2 O	SH4		2.5	2.5				AFI
		SH11		3.0	3.0				AFI
		SH12				135			Amor. + AFI
		SH4				180			AFI
	H 2 O		1	2.5	2.5	0.47	150 10
		SH13				225			AFI
		SH14				270			Tridymite

Acknowledgement

This work was financially supported by RUI (1001/PKIMIA/8011128) and TETFund.


Zhengxing Qin

Shu Zeng

Georgian Melinte

Tomáš Bučko

Michael Badawi

Yanfeng Shen

Jean-Pierre Gilson

Ovidiu Ersen

Yingxu Wei

Zhongmin Liu

Xinmei Liu

Zifeng Yan

Shutao Xu
email: xushutao@dicp.ac.cn

Valentin Valtchev

Svetlana Mintova
email: svetlana.mintova@ensicaen.fr

Prof Y Z Qin

X Shen

Prof. S Yan

Prof. S Mintova

Prof S Y Zeng

Prof Z Wei

Prof S Liu

Xu

Prof S Z Zeng

Prof J Gilson

Prof V Valtchev

Understanding the fundamentals of microporosity upgrading in zeolites: increasing diffusion and catalytic performances

Keywords: Zeolites, Microporosity upgrading, Molecular diffusion, HP 129 Xe NMR, DFT calculation

Hierarchical zeolites are regarded as promising catalysts due to their well-developed porosity, increased accessible surface area and minimal diffusion constraints in catalytic

reactions. Thus far, the main focus has been on the creation of mesopores in zeolites, however, little is known about a microporosity upgrading and its effect on the diffusion and catalytic performance. Here we demonstrated that the "birth" of mesopore formation in FAU type zeolite, a key component of FCC and hydrocracking catalysts, starts by the removal of framework T atoms from the sodalite (SOD) cages followed by propagation throughout the crystals. This is evidenced by following the diffusion of xenon (Xe) in the mesoporous FAU zeolite prepared by post-synthesis unbiased leaching with NH 4 F in comparison to the pristine FAU zeolite. A new diffusion pathway for the Xe in the mesoporous FAU zeolite is proposed.

Xenon first penetrated through the opened SOD cages and then diffused to the supercages of the mesoporous FAU zeolite. DFT calculations indicated that the Xe diffusion between the SOD cage and the supercage occurs only in hierarchical FAU structure with defect-contained 6MR separating these two types of cages. The energy barrier decreases from 402 kJ/mol for the pure FAU to 83 kJ/mol and 34 kJ/mol for mesoporous FAU containing 3 and 4 silanol nests, respectively. The catalytic performance of the mesoporous FAU zeolite further indicated that the upgraded microporosity facilitates the intracrystalline molecular traffic and increases the catalytic performance of zeolites.

Introduction

Zeolites are crystalline aluminosilicates with unique framework topologies and adjustable acid properties widely used in catalysis, adsorption and separation. The postsynthesis removal of framework atoms and the associated formation of mesopores, i.e., the creation of "highways" is already widely used to improve the intra-crystalline molecular traffic of reactants and products. [1] Recently, significant improvements in the design, synthesis, characterization and use of these hierarchical zeolites were made. [2][3][4][5][6] Despite such efforts, many important issues still need to be addressed. For instance, the mechanism of mesopore formation is not yet fully understood on a micro-scale. [7] Although various microscopy techniques enable to visualize mesoporosity at different length scales, [8,9] the initial step of their formation, i.e., the removal of framework atoms and its consequences, is not yet fully understood. [10] In addition, the enhanced diffusion and catalytic performances of hierarchical zeolites are currently associated with improved mass transfer due to the presence of mesopores, that increase the accessibility of reactants to active sites located in the microporosity. [11] However, a quantitative description of the connectivity between the native micropores and the added mesopores required further attention. [12] The size and shape of the zeolite micropores determine the shape-selectivity of zeolite catalysts and is dramatically affected by subtle changes of such microporosity. [13,14] Considering the significant number of framework-atomdetachment events during the formation of hierarchical zeolites and the associated subtle changes in window size and shape of their micropores, referred to as microporosity upgrading, a noticeable impact on the diffusion and reaction path and product selectivity is expected.

In the present work, the connectivity between the micro-and meso-pores in FAU zeolites is studied by hyperpolarized (HP) 129 Xe NMR spectroscopy and High Resolution Electron Microscopy (HRTEM). To that end, a series of hierarchical Y zeolites (FAU-type framework structure, hereinafter referred as FY) are prepared and compared to the parent zeolite (hereinafter referred as PY). The hierarchical zeolites are prepared by the recently disclosed unbiased leaching of the parent zeolite with NH 4 F, producing derivatives with an identical Si/Al ratio. [15] Further the diffusion of Xe in the hierarchical zeolites is studied by DFT. In particular the effect of point defects in the six-membered rings (6MR), separating the sodalite cages from the supercages, on the diffusion energy barrier is computed.

Results and Discussion

The crystallinity of all zeolite samples, parent and hierarchical, remain unchanged as shown by X-ray diffraction, Figure S1. The chemical composition of the samples (Si/Al ratio) is identical as determined by 29 Si MAS NMR (framework) and ICP (bulk), Table S1.

Nitrogen physisorption results highlight a gradual formation of mesopores in the zeolite samples subjected to NH 4 F etching (FY series), Figure S2-3 and Table S1.

Microporosity upgrading in zeolites revealed by electron tomography

The dynamic evolution of the micro-and meso-porosity of FY zeolites can be revealed by electron tomography (3D-TEM), but 3D imaging of pores smaller than 2 nm is a difficult challenge for micron-sized zeolite crystals. Therefore, thin specimens of about 40 nm (Figure S4a) are prepared by ultramicrotomy of hierarchical FY-20 zeolite (Table S1). The tomographic reconstruction highlights evenly distributed small mesopores with a diameter of 2 -5 nm located in the FY-2 zeolite (Figure S4b-c). The high-resolution 3D reconstruction shows some of the zeolite cages and their merging resulting in the formation of larger mesopores. The formation of such mesopores is may be due to the etching of the walls of the cages that extends around point defects in the zeolite framework (Figure 1a). The enlargement of the mesopores seems to appear when two or multiple mesopores merge due to the close proximity (Figure 1b). The segmentation of the 3D reconstruction reveals interconnected cages (0.7 -1.5 nm, see the green frame of Figure 1c), homogenously punctured by larger mesopores (RGB colored, Figure 1c). Such high-resolution reconstruction highlights the boundary between two zeolite nanograins with different crystallographic orientations (Figure S4d). Along the 3D interface between the two zeolite grains, the concentration of mesopores increases substantially. This agrees with our previous findings highlighting the preferential formation of mesopores along defective sites. [15] To quantify the porous networks, three cylindrical volumes with no contact with the zeolite's upper and lower surfaces are evaluated (Figure 1d). This approach avoids the introduction of errors arising from the sensitive step of surface segmentation. The quantification results based on the evaluation of the three volumes shows that the mesopores alone occupy a fraction of about 6% of the total zeolite space, while the primary micropores reach a fraction of around 25%. Such a micro-to mesopore volume ratio of 4 is in good agreement with the N 2 sorption data (Table S1). The mesopore size distribution determined from the selected volumes (Figure 1e) shows that more than 55% of the secondary porosity consists of pores with a diameter of 2 -3 nm. The pores with a diameter of 3 -4 nm and 4 -5 nm account for about 24% and 11 -14%, respectively. Several pores larger than 5 nm are also present and formed by merging adjacent mesopores depicted in Figure 1b. This quantification is in line with the pore size distribution derived from nitrogen physisorption measurements (Figure S3, Table S1).

Figure 1. Electron tomography (3D-TEM) of hierarchical zeolite FY20. a) High-resolution view of a mesopore created via the merging of few microporous cages (left) and 3D representation of a mesopore and the surrounding in the zeolite crystal (right). b) Highresolution view of a pair of three mesopores merged (left) and 3D representation of mesopores and their surrounding in a zeolite crystal (right). c) 3D reconstruction image of the FY-20 zeolite visualizing the micro-cages with a size of 0.7 -1.5 nm (green frame) and larger mesopores (RGB colored). d) 3D reconstruction image containing three cylindrical volumes used for quantification of the porous network. e) Pore size distribution calculated from three distinct regions of the zeolite crystal presented in Figure 1d.

The 3D-TEM image of the FY60 hierarchical zeolite, with a lower micropore volume than of the parent PY (Table S1) is pictured in Figure 2. The 3D reconstruction shows a dense network of isolated and interconnected mesopores (Figure 2a-b). A well-connected network of mesopores extends from one external surface through the center of the zeolite crystal to the opposite surface (Figure 2c-d). The 3D-TEM quantitative analysis indicates that around 90% of the mesopores are fully connected. The isolated mesopores, or those with double and triple connections, represent about 10% of the total amount of mesopores. The quantification of the pore size distribution using three selected internal regions (Figure 2e)

shows that most of the mesopores are in the 2 -12 nm range (Figure 2f). The 2 -4 nm mesopores account for about 17% of the total mesoporosity. Such quantification, however, does not differentiate between connected and isolated mesopores. A modified segmentation of the pores, to take into account the individual pores even in the event of merging, shows that the mesopores with a size of 2 -3 nm represent around 80% of the total mesoporous network. This quantification is confirmed by the pore size distribution analysis based on N 2 physisorption measurements (Figure S3). andc) 3D distribution of the mesoporous network of the FY60 zeolite crystal (Color label based on the spatial distribution and not the pore size as the porous network is highly connected. For a color coded distribution see Figure S5). d) 3D-TEM reconstruction of the FY60 zeolite crystal showing the interconnectivity of the mesopores from one to the other surfaces. e) 3D reconstruction image containing three isolated internal volumes. f) Pore size distribution calculated from three distinct regions of the zeolite crystal presented in Figure 2e.

The important finding is that the highest density of mesopores in the 2 -3 nm range, i.e., more than 55% of the total amount of mesopores is found in the FY20 zeolite sample. This suggests a possible merging of two FAU supercages, [15] and the formation of mesopores by a continuous removal of framework atoms starting from the sodalite cage between neighboring supercages. The "birth" of mesopores in the FAU zeolite starting with the possible removal of framework T atoms from the sodalite cages was further investigated using advanced hyperpolarized (HP) 129 Xe NMR spectroscopy and supported by DFT calculations. The spatial distribution of the mesopores indicates that cages are opened in the entire volume of the zeolite crystals, and the process progresses during the etching process (sample FY60, Figure 2). As a result, while the existing mesopores increase in size and interconnectivity, new SOD cages are continuously opened, leading to the formation of new mesopores. The cage opening process leads to intermediate products with enhanced pore volume and preserved microporous characteristics. [15] Additionally, the defects may favor mesopores formation (Figure S4d) as reported before. [15] Defects may control the dissolution preference of zeolite crystals. The FAU zeolite crystals investigated here behave differently from the MFI and MOR zeolites that followed a "reverse layer-by-layer" dissolution mechanism starting from outside micropores. [16,17] Based on the above results, we suggest that the enlarged supercages and the newlydeveloped mesopores are formed as a unique interface between the mesoporous voids and the remaining micropores of the zeolite phase. Such a new porous network is likely to significantly affect the diffusion and the catalytic performance of these hierarchical zeolites.

Microporosity upgrading in zeolites tracked by hyperpolarized 129 Xe NMR spectroscopy

The diffusion pathway in the hierarchical FAU zeolites was investigated by HP 129 Xe NMR spectroscopy. 129 Xe NMR is a powerful technique to probe the porosity at the molecular level. [18][19][20] For instance, we showed earlier that Xe cannot enter SOD cages in a parent zeolite Y due to its large kinetic diameter. [15] In the current study, the HP 129 Xe NMR spectra of all hierarchical zeolites were collected at 253 K (Figure 3). All spectra display a peak at ca. 90 ppm attributed to xenon located in opened SOD cages. [15] The difference in chemical shift of this peak and the one attributed to Xe in the supercages (67.5 -81.8 ppm) narrows from the FY5 to the FY60 samples by 23 ppm to 12 ppm, respectively (Figure 3a,b). HP 129 Xe NMR provides strong evidence that the exchange of Xe is faster between the opened SOD cages and supercages as a result of the continuous opening of the SOD cages and formation of mesopores. The variable temperature HP 129 Xe NMR experiments on the hierarchical zeolites were also performed with the attempt to distinguish the mesopores from the micropores (Figure S6). Due to the quick exchange of Xe between the mesopores and micropores under the experimental conditions applied no a distinct conclusion was made. Therefore experiments at lower temperature (153 K) should be considered in the future but yet this is beyond the temperature limit allowed by our equipment set-up. inter-connectivity in different adsorption regions. [21,22] The hyperpolarized xenon quickly loses its polarization after interacting with the zeolite pore walls resulting in weakening of the signal (depolarization). The correlation peak observed in the 2D HP 129 Xe EXSY NMR spectra primarily occurs due to the adjacent micropores. On sample FY5, a cross-peak occurs at (90, 0) (F2, F1) ppm at a mixing time of 4 ms (Figure 4a). This evidences that the gaseous xenon enters the SOD cages. Two other exchange peaks occur at (90, 67) ppm and (67, 90) ppm corresponding to xenon exchange between the SOD cages and supercages almost at the same time. At mixing time of 5 ms, a cross peak occurs at (67, 0) ppm which is assigned to the diffusion of gaseous xenon in the supercages of the zeolite sample (Figure 4b). The diffusion via supercages is the only energetically feasible path for perfect FAU defect-free zeolite crystals. With increasing the mixing time to 8 ms, a cross peak occurs at (0, 67) ppm, which is attributed to the desorption of xenon confined in supercages to the gas phase (Figure 4c). A further increase of the mixing time to 50 ms provides a new cross peak at (0, 90) ppm attributed to xenon's desorption from the SOD cages to the gas phase (Figure 4d). These 2D

HP 129 Xe EXSY NMR experiments provide further proof that some SOD cages are opened and accessible for the xenon, enabling the fast exchange of xenon between opened SOD cages and supercages (Figure 4a). Moreover, by using different mixing times, the diffusion path of xenon in the microporous volume of the FY5 hierarchical zeolite is monitored. Interestingly, gaseous xenon first adsorbs in the opened SOD cages, and then diffuses to the supercages (Figure 4). For the hierarchical FY45 and FY60 zeolite samples with decreased micropore volume, increased mesopore volume (Table S1), and improved mesopore accessibility (Figure 2), the diffusion of xenon inside zeolite cages is even faster (Figure 4e-h, Figure S7).

In both hierarchical FY45 (Figure S7) and FY60 (Figure 4e-h) zeolite samples, the crosspeaks corresponding to xenon adsorption in the SOD cages from gas phase and exchange of xenon between SOD and supercages, respectively occur at a mixing time of 3 ms (Figure 4e, Figure S7a). In comparison, the xenon exchange between supercages and the gas phase for sample FY60 occurs at 5 ms mixing time (Figure 4f, Figure S7b). All signals in the spectra at 20 ms mixing time are reported in Figure 4h and Figure S7c. In the hierarchical FY60 zeolite, the desorption of xenon from the SOD cages to the gas phase occurs at the mixing time as short as 10 ms (Figure 4h).

The results above demonstrate that an increased intraparticle diffusion is provided by opening of the SOD cages and enhancing the mesoporosity of the hierarchical FAU zeolite.

The 2D HP 129 Xe EXSY NMR spectra show that opening SOD cages not only free a hitherto unreachable space, [15] but also substantially change the diffusion paths in the hierarchical FAU zeolite (Figure 4i). The current methodology applied to evaluate porosity of zeolites could oversimplify the contribution of diffusion limitations especially of concerns in catalysis and gas separation applications. The results show that even in the case of decreased micropore volume of zeolites, the micropores accessibility may change substantially via the mesopores surrounding and improved inter-connectivity and small changes of the pore (cage) openings.

Microporosity upgrading in zeolites revealed by DFT modelling

Xe diffusion in the hierarchical FAU zeolites is modelled by DFT calculations. The FAU three-dimensional structure consisting of sodalite cages (diameter 6.6 Å) and supercages (diameter of 12.4 Å) are interconnected by hexagonal prisms of 2.3 Å (D6R); the supercages have a diameter of 7.4 Å. The pure silica FAU framework is represented by the face-centered cubic cell (space group Fd3m), [23] and a lattice parameter of 25.028 Å for a unit cell consisting of 576 atoms (Si 192 O 384 ). [24,25] In the current calculations, a primitive rhombohedral cell containing 144 atoms is used (Figure S8). The lattice parameters, obtained after the relaxation of the atoms and the cell at the PBE+D2 level are a= 17. Figure 5 shows the potential energy barriers computed for the Xe diffusing from the sodalite cages to the supercages. In the case of the pure siliceous FAU structure (model A), the diffusion barrier is as high as 402 kJ/mol. As evident from Figure S9, the passage through the 6MR separating the two voids is linked with a significant structural deformation of the ring caused by the repulsive interaction between the Xe and the framework atoms. One can therefore expect that increasing the size of the 6MR via the substitution of Si for Al (the Al-O bond is 0.1 -0.3 Å longer than the Si-O bond, Figure S9) can lead to stabilization of the transition state (TS) and hence decrease the diffusion barrier. This is indeed the case, one Al substitution decreases the diffusion barrier to 376 kJ/mol, while three substitutions lead to an even more significant reduction to 339 kJ/mol (Figure 5). Since the diffusion barrier remains very high even in the latter case and the 6MR cannot accept more Al sites without violating the Löwenstein rule, [24] we conclude that the Xe diffusion between the sodalite cage and the supercage of parent FAU with a pure microporous structure is not kinetically feasible. Further, the FAU structure containing silanol nest defects created by replacing one (model D), two (model E), three (model F), and four (model G) neighboring tetrahedral sites from the 6MR separating the sodalite cage from the supercage and terminating the dangling Si-O bonds by H atoms were considered (Figure S10). In this way, the size and flexibility of the 6MR was increased since the strong covalent bonds between the tetrahedral and oxygen atoms were replaced by much weaker H-bonds. As shown in Figure 5, the introduction of silanol nests leads to a significant reduction of the diffusion barrier from 402 kJ/mol in the pure defect-free FAU zeolite to 83 kJ/mol in the structure containing three silanol nests. More importantly, the FAU zeolite structure containing four silanol defects further reduced the diffusion barrier to 34 kJ/mol. The DFT results support the experimentally observed Xe diffusion that takes place in hierarchical FAU structure with 6MR containing defects separating the sodalite cages from the supercages. Regardless of the particular structural model used in our simulations, the Xe adsorbed in the sodalite cage was ~20 kJ/mol lower in energy than in the supercage. This result is a direct consequence of a more general phenomenon of increasing van der Waals stabilization with decreasing volume of confined voids of zeolite. [26] Thus one can expect that the sodalite cages will be occupied before the supercages if both are available and accessible for the Xe adsorption.

Microporosity upgrading in zeolites changed the catalyst performance

The catalytic performance of the hierarchical FAU zeolites are assessed in two reactions:

1,3,5-triisopropylbenzene (TiPBz) dealkylation and n-octane (n-C 8 ) hydroconversion. In the dealkylation of TiPBz, the parent zeolite PY and its hierarchical derivative FY5 show identical conversions (Figure S11a). [15] The FY5 hierarchical zeolite sample with some SOD cages opened, show initially a lower 1,3-diisopropylbenzene yield (Figure S11b) but higher 1,4-diisopropylbenzene (Figure S11c) and 1,2-diisopropylbenzene (Figure S11d) yield for the bulkier isomer. During the dealkylation of TiPBz, 1,3-diisopropylbenzene is the primary product, 1,4-diisopropylbenzene and 1,2-diisopropylbenzene are the secondary isomerized products derived from 1,3-diisopropylbenzene. Apparently, the microporous FY5 zeolite favors the conversion of 1,3-diisopropylbenzene with a kinetic diameter of 0.84 nm, which is still larger than 0.74 nm (the intrinsic micropore opening of zeolite Y). Meanwhile, it favors the formation of the secondary products including 1,2-diisopropylbenzene, a bulkier secondary product than 1,3-diisopropylbenzene. These evidently demonstrate that a subtle change in microporous properties of zeolites such as opened sodalite cages, enlarged supercages and micropore deformation may cause a substantial change of their catalytic performance. This observation is in line with their thermodynamic distribution. [27] The hierarchical FY60 zeolite, with the lower micropore volume (Table S1) and lower amount of acid sites (Figure S12) shows a higher conversion in n-C 8 hydroconversion (Figure S11e-f).

These results further demonstrate that microporosity upgrading benefits the intracrystalline molecular traffic and increases the catalytic performance of zeolites.

Conclusion

The evolution of micropores and mesopores in hierarchical zeolites with the FAU-type The superior catalytic performances are a direct result of the availability of more active sites and lower transport limitations in the hierarchical FAU zeolite crystals due to newly available diffusion pathways. As several industrially relevant zeolites contain cages (FAU, CHA, LTA, AEI…), often difficult to access even for small molecules (H 2 , NH 3 , CH 4 …), the crystal engineering described in this paper could unlock further potential in catalysis, separation by selective adsorption, capture and storage of molecules. 213 K. The π/2-t1-π/2-tmix-π/2-t2 pulse sequence [28] was used in 2D EXchange SpectroscopY (EXSY) NMR experiment with the mixing time range of 1 to 100 ms under 253 K (Scheme S1). The chemical shifts were reference to gas phase xenon signal at 0 ppm. All the 27 were conducted using the dehydrated sample. 29 Si MAS NMR spectra were accumulated using high-power proton decoupling sequence with spinning rate of 10 kHz, and chemical shifts were referenced to kaolinite at -91.5 ppm.

Methods

Materials and Characterization

Transmission electron microscopy measurements: Transmission electron microscopy (TEM)

images were recorded using a JEOL 2100F microscope equipped with a high-resolution objective lens pole piece at 200 kV. The tomography series were acquired between tilting angles of ±70° with a 2° Saxton scheme, and the subsequent series alignments were performed with the IMOD software using Au nanoparticles (5-7 nm) as fiducial markers. To resolve details at maximum resolution we used the SIRT algorithm implemented in fast software running on multicore computers, Tomo3D. The size of the TEM projections used for the reconstruction was 2k x 2k pixels.

Density functional theory calculations:

The periodic density functional theory (DFT) calculations were performed using the Vienna Ab initio Simulation Package (VASP). [29,30] The Perdew Burke Ernzerhof (PBE) exchange-correlation functional in the generalized gradient approximation (GGA) proposed by Perdew et al. [31] was employed. The electron-ion interactions were described using the projector augmented plane wave (PAW) method of Blöchl [32] as adapted by Kresse and Joubert. [33] A planewave cutoff energy of 400 eV was used in all calculations. The Kohn-Sham equations were solved self-consistently until the energy difference between cycles becomes lower than 10 -6 eV. A primitive rhombohedral cell of FAU with two supercages containing total of 144 atoms was used for the calculations (Figure S8). The lattice parameters, obtained after relaxation of the atomic positions and the cell geometry at the PBE+D2 level are a= 17.34 Å and α= 60°. Considering the large size of the unit cell, the Brillouin zone sampling was realized using a single (Γ) k-point. In order to describe the adsorption process in the zeolite with a good precision, van der Waals (vdW) interactions were taken into account. An atom-pairwise D2 correction of Grimme [34,35] was used as applied previously to calculate a reasonable structure and energetics for zeolites. [36][37][38] The atomic positions were optimized until all forces were smaller than 0.005 eV/Å per atom.

The transition state (TS) optimization was performed using the optimization engine GADGET. [39,40] In order to identify the stable configurations linked with TS via a common transformation path, the intrinsic reaction coordinate [41,42] (IRC) for the forward and backward reaction steps was identified using the damped velocity Verlet algorithm. [43] The vibrational eigenspectra of all relaxed structures were checked to ensure that the given state correspond to expected stationary point on the potential energy hypersurface.

Catalytic tests:

The zeolite samples were tested in two catalytic reactions, the 1,3,5 triisopropylbenzene dealkylation and n-octane hydroconversion. The catalytic tests were carried out following the conditions reported before. [15] While the focus currently is on the creation of mesopores in zeolites, the microporosity upgrading is rarely considered. We report the fundamentals of such microporosity upgrading in zeolites and its impact on the molecular diffusion and catalyst performance using hyperpolarized 129 Xe NMR spectroscopy supported by electron tomography and DFT modelling. Understanding the fundamentals of microporosity upgrading in zeolites: increasing diffusion and catalytic performance

Figure 2 .

 2 Figure 2. 3D-TEM pictures of hierarchical zeolite FY60. a) 3D-TEM reconstruction of the FY60 zeolite crystal. b) and c) 3D distribution of the mesoporous network of the FY60 zeolite crystal (Color label based on the spatial distribution and not the pore size as the porous

Figure 3 .

 3 Figure 3. 1D HP 129 Xe spectra of hierarchical zeolites. a) HP 129 Xe spectra of hierarchical zeolites (FY5 -FY60) at 253 K; the green background highlights xenon located in opened SOD cages, the yellow background indicates xenon in supercages, the chemical shift at 0 ppm represents free xenon in gas phase. b) Difference in chemical shifts between the peaks associated with the xenon in the SOD cages and supercages in the hierarchical zeolites.

Figure 4 .

 4 Figure 4. Diffusion of xenon in hierarchical zeolites. HP 129 Xe EXSY NMR spectra xenon on the FY5 zeolite at different mixing times: a) 4 ms. b) 5 ms. c) 8 ms. d) 50 ms. HP 129 Xe EXSY NMR spectra of xenon on the FY60 zeolite at different mixing times: e) 3 ms. f) 5 ms. g) 7 ms. h) 10 ms. All spectra are recorded at 253 K. i) Schematic representation of the Xe diffusion paths in hierarchical zeolites: (1) adsorption of Xe from the gas phase in opened SOD cages, (2) simultaneous diffusion of Xe in SOD cages and exchange with supercages, (3) diffusion of Xe from the gas phase to the supercages, (4) subsequent diffusion of Xe out of supercages, and (5) desorption of Xe from SOD cages.

  34 Å and α= 60°. The primitive cell of the FAU contains two supercages and eight hexagonal windows connecting the sodalite with the supercage. In order to examine a possible reduction of diffusion barrier due to the presence of the Brønsted Acid (BA) sites or defects created upon removal of T sites from the 6MR between the sodalite cage and the supercage, seven structural models were considered: the pure siliceous structure (model A), a structure with one BA site (model B), with three BA sites (model C), with one defect (model D), two defects (model E), three defects (model F) and four defects (model G). Models A -C are displayed in Figure S9 and models D -G in Figure S10.

Figure 5 .

 5 Figure 5. Energy profiles for diffusion of Xe from SOD cages to supercages in hierarchical zeolites. Seven models are considered: pure siliceous FAU type framework structure (model A), FAU type framework structure with one (models B) and three (model C) Brønsted acid sites; pure siliceous FAU type structures with one (model D), two (model E), three (model F) and four (model G) silanol nest defects.

  framework structure are monitored by 3D-TEM and HP129 Xe NMR. The formation of mesopores in the FAU zeolite is found to be due to the unbiased removal of framework T atoms (T = Si or Al) by NH 4 F etching. The removal of the T atoms starts from the supercages, opening some SOD cages, and further allow the connection of neighboring cages. Based on the Xe adsorption and diffusion study in the hierarchical FAU zeolite samples monitored by HP129 Xe NMR spectroscopy we revealed the increased intraparticle diffusion and a new diffusion pathway was proposed. Both the experimental and theoretical results concur that Xe first diffuses into the opened SOD cages before diffusing to the supercages of FAU zeolite.

  : A commercial Y zeolite (Y-54 from UOP, Si/Al = 2.6) in its NH4 -form was used in this study (parent sample, PY). The fluoride medium etching was carried out in a 25 wt.% NH 4 F (98.0%, Sigma-Aldrich) aqueous solution with a liquid-tosolid ratio of 6. The treatment was performed under mechanical stirring and ultrasonic radiation (USC 600 TH, 45 kHz, VWR) in an ice bath (277 K) for different times (5 -60 min).The resulting samples were denoted as FYX, with the suffix X represents the etching time in minutes, X = 5, 10, 20, 45 and 60 min. For example, FY5 represents the zeolite Y sample treated by NH 4 F for 5 min. The solid products after treatments were thoroughly washed and then dried. X-ray diffraction (XRD) patterns of the parent and the treated samples were collected on a PANalytical X'Pert Pro diffractometer using Cu Kα radiation (λ = 1.5418 Å, 45 kV, 40 mA) with a scanning step of 0.0167° s -1 . The chemical composition of zeolite samples was analyzed by an inductively coupled plasma optical emission spectrometry (ICP, OPTIMA 4300 DV). Nitrogen physisorption analysis was carried out on a Micromeritics ASAP 2020 gas adsorption analyzer. Prior to analysis, the samples were degassed at 373 K for 1 h and 573 K for 10 h. The BET equation was used for the determination of the specific surface areas of the samples. The t-plot method was used to determine the micropores volume and external surface areas. The BJH model was applied to the adsorption branch of the isotherms for the mesopore size distributions.Solid-state NMR measurements: Hyperpolarized (HP)129 Xe NMR experiments were conducted on a Bruker Avance III 400 MHz spectrometer using a home built continuous-flow HP xenon delivery apparatus. Before the NMR measurements, the samples were dehydrated under vacuum at 693 K for 6 h, then transformed into a 5 mm quartz tube in an argon glove box. HP129 Xe was obtained with an optical pumping cell in the 50 Gauss field and the 60 W diode laser array (Coherent FAP-System). The mixture gas consisting of 1% Xe, 1% N 2 and 98% He was delivered to the quartz NMR tube (5 mm) at the rate of 150 mL/min. The resonance frequency of 129 Xe was 110.7 MHz. 1D HP 129 Xe NMR spectra were accumulated by a single pulse sequence with π/2 pulse width of 6 μs and 2 s recycle delay. Variable temperature (VT) experiments were conducted with the temperature varying from 298 K to

Z

  . Qin †, S. Zeng †, G. Melinte, T. Bučko, M. Badawi, Y. Shen, J. Gilson, O. Ersen, Y. Wei, Z. Liu, X. Liu, Z. Yan, S. Xu*, V. Valtchev and S. Mintova*.

  Al MAS NMR and29 Si MAS NMR experiments were performed on a Bruker Avance III 600 spectrometer equipped with a 4 mm H-X double resonance WVT probe. The resonance frequencies for27 Al,29 Si were 156.4 MHz and 119.2 MHz, respectively. The 27 Al spectra were recorded using one pulse sequence with spinning rate of 12 kHz and chemical shifts were referenced to (NH 4 )Al(SO 4 ) 2 •12H 2 O at -0.4 ppm. The 27 Al MAS NMR experiments

Acknowledgements

The authors acknowledge funding from the Sino-French joint LIA laboratory "Zeolites". This work was supported by the National Natural Science Foundation of China (21706285, 21972142, 21991090, 21991091, 21991092, 21975285, 22022202), LiaoNing Revitalization Talents Program (XLYC1807227), International Partnership Program of Chinese Academy of Sciences (121421KYSB20180007), Qingdao Applied Basic Research Project (19-6-2-70-cg), the Fundamental Research Funds for the Central Universities (18CX02013A); Thousand Talents Program for Foreign Experts (WQ20152100316). †Zhengxing Qin and Shu Zeng contributed equally to this work.

Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.