Hussein Awala

Shrikant M Kunjir

Aurélie Vicente

Jean-Pierre Gilson

Valentin Valtchev

Hussein Seblani

Richard Retoux

Louwanda Lakiss

Christian Fernandez

Robert Bedard

Suheil Abdo

Jeffery Bricker

Svetlana Mintova

Crystallization pathway from highly viscous colloidal suspension to ultra-small FAU zeolite nanocrystals

Introduction

Zeolites are crystalline aluminosilicates with unique properties easily tuneable by engineering their chemical composition, topologies, morphologies, acidity, thermal stability, extra framework cations and porosity. The commercial relevance of zeolites has been demonstrated by a wide range of applications in oil refining, petrochemistry, fine chemicals, gas and liquid separations and purification, photonics, and drug delivery. [1][2][3][4][5][6][7] Up to now, more than 253 different framework types have been synthesized, [8] mostly in the presence of organic structure directing agents (OSDA) which enhance kinetic rates and orient the selectivity of zeolite crystallization by stabilizing metastable structures in a wide range of chemical composition. [9][START_REF] Na | [END_REF][11][12][13] It is noteworthy that only a few nanosized zeolites have been synthesized without free zeolites, many mechanistic issues remain to be addressed to optimize the syntheses and prepare novel zeolites with nanosized dimensions. When conducting synthesis work to address these issues, key parameters to be considered begin with the selection of the initial sources for silica, alumina and alkali cations (their hydrated form being the templates), followed by establishing the best conditions for the preparation of the initial precursors. For example, silicate species formed during the dissolution of the silica source play an important role in the zeolite formation and its particle size distribution and morphology. [27] In addition, the formation of the initial gel with particle sizes less than 10 nm from monomeric and low molecular weight aluminosilicate species ensuring a fast exchange between the solid and liquid phases is an important step not yet fully understood. Another important parameter is a low temperature (<60 C) hydrothermal synthesis, minimizing Ostwald ripening, during crystallization preserving the small size of the parent amorphous particles. [28] Enormous effort has been dedicated to understand the zeolite growth process in hydrogels under hydrothermal conditions to harvest zeolite crystals with desired properties. [29,30] The main challenge in zeolite synthesis is the incomplete understanding of the molecular-level interactions, the kinetic and thermodynamic driving forces governing the interactions and binding of templates (organic or inorganic) to precursors leading to specific nucleation and crystallization events. Still the open questions is what happens at the earliest stages of nucleation -where & when are the first organized « crystals » are produced. The formation of a particular zeolite could occur by more than one crystallization pathway as described. [28,31] An additional motivation for the current work is that the preparation of nanosized zeolites from OSDA-free clear precursor suspensions is highly desired from a cost and environmental perspective, especially when the synthesis takes place at moderate and low temperatures (30-130 °C). [32] For this reason, many attempts to reduce temperature and OSDA consumption have already been published [28] as well as the crystallization kinetics of several OSDA-free nanozeolites. [33] However, to improve our understanding ability to control these synthesis processes, the formation of the very first secondary building units in the nanosized amorphous particles needs further attention to better understand the selective formation of specific zeolites.

In this work, we focus on the synthesis of ultra-small FAU nanosized zeolite from highly reactive and costeffective Si and Al sources with a high colloidal stability which might be usable in larger scale production of nanosized FAU type zeolites. We then facilitate this synthesis by first preparing and stabilizing clear precursor suspensions containing well-defined, discrete amorphous particles. The size, phase purity and chemical composition of the derived FAU crystals are then controlled by adjusting the composition of this precursor suspension, with a particular focus on the water amount in the aging and de-hydration steps.

Results of our effort to track the crystallization pathway from highly concentrated clear and viscous suspension to ultra-small FAU nanosized crystals by advanced microscopic and spectroscopic techniques and the insight gained into the crystallization pathways and the specific characteristics of the synthesized nanosized zeolite will be presented.

Experimental Section

Materials and Zeolite Synthesis.

The following starting chemicals are used as such: Ludox HS-30 (Aldrich, 30% SiO 2, particle size of 3-4 nm, stabilized with sodium hydroxide, pH of 9.3), sodium aluminate (Sigma-Aldrich, anhydrous, 50-56% Al 2 O 3 ; 40-45% Na 2 O; ≤0.05% Fe) and sodium hydroxide (NaOH, Prolabo, 98.7%). This four-step process is illustrated in Figure 1: -Step 1: preparation of two solutions: Solution A (sodium aluminate) by dissolving sodium aluminate (1.02 g) in 3 g of double distilled (dd) water and Solution B (sodium silicates) by mixing 10 g of colloidal silica (Ludox HS-30) with 3.25 g of sodium hydroxide (NaOH, Prolabo, 98.7%) and 0.98 g of dd H 2 O in a closed PP bottle (process is highly exothermic). A turbid suspension first appears and evolves as a water clear suspension within two minutes. The formation of amorphous particles from both solutions A and B, providing monomeric or low molecular weight species upon mixing with sodium hydroxide is monitored by DLS (Figure S1). Solution A is then added dropwise to solution B under vigorously stirring maintained at low temperature in an ice-bath. The resulting precursor suspension has the chemical composition: 8.5 Na The precursor suspensions from steps 1, 2, and 3 are labeled P-1, P-2, and P-3, respectively. The solids harvested after the hydrothermal treatments in step 4 for 45 min, 3 h and 24 h are referred as FAU-45 min, FAU-3 h and FAU-24 h, respectively. The solid products were purified by double distilled water till pH=7.5, separated using high-speed centrifuge and freeze dried to prevent the agglomeration of the particles prior further characterizations. in PP bottles with a volume of 80 ml. The initial precursor suspensions before (step 1) and after partial evaporation of water (step 3) have weight of 18 g and 11 g, respectively.

Characterization

Powder X-ray diffraction (XRD): The crystallinity of the powders sampled during the four-step syntheses are measured with a PANanalytical X'Pert Pro diffractometer using the CuK monochromatic radiation ( = 1.5418 Å), in the 4-50° 2θ range with a steps of 0.02 o .

Transmission electron microscopy (TEM):

The size, morphology and crystallinity of nanoparticles are determined by transmission electron microscopy (TEM) on a JEOL 2010 FEG operating at 200 kV.

Dynamic light scattering (DLS):

The hydrodynamic diameters of amorphous and zeolite nanoparticles in water suspensions are determined with a Malvern Zetasizer Nano. The analyses take place after purification; the solid concentration of all suspensions is 2.5 wt. % at pH of 8. The surface charge of nanocrystals is determined by measuring the zeta potential value on the same suspensions.

Fourier-transform infrared (FTIR) spectroscopy:

The IR spectra are recorded using a Nicolet 6700 spectrometer fitted with a DTGS detector (resolution 4, 128 scans; KBr: Sample= 100:1).

Raman spectroscopy:

The spectra are recorded on a Jobin Yvon Labram 300 spectrometer fitted with a confocal microscope (Laser: 532 nm, acquisition time: 240 s).

Liquid state 29 Si NMR: 29 Si NMR spectra are acquired on a Bruker AVIII HD 500MHz (11,7 Tesla) spectrometer operating at a 29 Si frequency of 99.3 MHz equipped with a Bruker BBFO 5 mm NMR probe.

All spectra were acquired using a (π/6) single-pulse sequence with a pulse delay of 20 s, an acquisition time of 3.17 s and a minimum of 512 scans. Prior to any measurement, 250 mg of the zeolite suspensions are mixed with 1 mL Deuterium oxide (D 2 O) (mass ratio zeolite: D 2 O = 1:4) in 5 mm Teflon tubes. The in situ 29 Si NMR study was performed on a Bruker AVIII HD 400 MHz (9.4 Tesla) spectrometer equipped with a Bruker BBO 10 mm NMR probe, using 10 mm NMR FEP Teflon tubes. 256 scans were accumulated for each spectrum using a single 90 • excitation with a pulse length of 19 µs and a recycle delay of 1 s. 29 Si chemical shifts were referenced to tetramethylsilane (TMS) used as an internal standard.

HP 129 Xe NMR spectroscopy: 129 Xe NMR spectra are recorded on a Bruker Avance III-HD 400 (9.4 T) spectrometer at 110.6 MHz; 128 scans are accumulated for each spectrum using a single 90 • excitation with a pulse length of 12.5 µs and a recycle delay of 1 s. The HP 129 Xe gas is produced on a home-built xenon polarizer based on the spin exchange optical pumping (SEOP) technique, [START_REF] El Siblani | Applications de la RMN du 129Xe hyperpolarisé à l'analyse de matériaux poreux[END_REF] capable to deliver a continuous flow of hyperpolarized 129 Xe directly to the sample. [START_REF] Happer | [END_REF][36] Prior to analysis, all samples are pressed under 100 MPa, then crushed and sieved to 200-500 µm. The samples (0.5 g) are introduced in a home-designed 10 mm O.D. NMR tube and dehydrated under high vacuum (10 -6 mbar) at 275 °C overnight. HP 129 Xe NMR spectra are acquired under a continuously recirculating flow of 50 mL min -1 (90% He, 6% N 2 , and 4% of natural abundance xenon) at a pressure of 0.15 MPa. For all variable temperature (VT) measurements and after each temperature step (ramp rate of 5 C.min -1 ), 20 min elapse to ensure a homogeneous temperature all over the sample.

Results and Discussion

From Highly Viscous Colloidal Suspension to Ultra-Small Zeolite Nanocrystals

The synthesis of ultra-small FAU-type zeolites from OSDA-free precursor suspensions was fully described in the experimental section. Freshly prepared sodium aluminate supplies mainly monomeric aluminium species [37] while colloidal silica dissolved in a sodium hydroxide solution provides monomeric and low molecular weight silica species. [24,29] The two solutions A and B (see experimental section) are mixed at low temperature (4 °C) to decrease the rate of silica and alumina polymerization and thus providing highly homogeneous precursor suspensions with monomodal amorphous particles. During the preparation of the silicate solution (solution B), all sodium hydroxide is added to the silica source to achieve its complete dissolution. Sodium aluminate is highly soluble in water (Solution A) and our concentration (1.83 M; 0.56 g of Al 2 O 3 in 3 ml H 2 O) is lower than the solubility limit of alumina (5 M). The size of the particles in solutions A and B, measured prior to mixing by DLS (Figure S1) is in the 0.8-1 nm range. The high alkalinity of the precursor suspension after mixing of the two solutions guarantees the formation of monodispersed amorphous particles. [24] The resulting suspension is then aged for 24 hr at RT and becomes water clear. The water content is further decreased in step 3, stabilizing highly concentrated amorphous particles in the supersaturated viscous media. The appearance of the amorphous precursor suspension (P-1), intermediates (P-2, P-3) and final crystalline suspension (FAU-24 h) in steps 1, 2, 3 and 4 is shown in Figure 2. After steps 1 and 2, both suspensions are fully transparent, while after partial evaporation of water (step 3), viscosity increases while transparency is preserved (Figure 2). Fully transparent suspensions with a high viscosity at step 3 and after step 4 (HT treatment) are obtained (Figure 2). The precursor suspension in step 3, containing alkali cations coordinated with water and/or silicate and aluminate species, has a high viscosity (Figure S2); this feature warrants to follow the mobility of sodium cations in the suspensions at each step by conductivity measurements. Conductivity increases from step 1 to 2 (Figure 3A), as the silica remaining in suspension dissolves upon mixing and then decreases drastically from 61.3 to 3.5 mS/cm after partial evaporation of water indicating a lower sodium cations mobility (Figure 3). The amount of sodium trapped in the zeolite at the different steps is indirectly evaluated by measuring the Al content. As the Si/Al ratio decreases during crystallization as the conductivity (Figure 3B), it is related to Na + trapping in proto-zeolitic materials (X-ray amorphous) already formed at the onset of crystallization. The evolution of the crystallization of the zeolite nanoparticles is highlighted by XRD (Figure S3). The diffraction pattern of the precursor P-3 features a broad peak centred centered at 27° 2θ, typical of amorphous aluminosilicates. In the FAU-45 min pattern, peaks at 6.1, 11.7, 15.4, 23.5, 26.6 and 30.8° 2θ indicate the emergence of X-ray visible FAU type zeolite. After a prolonged HT treatment (3 and 24 hrs), a fully crystalline FAU zeolite is obtained (Figure S3d). The crystalline yield of this sample is 65 %.

The evolution of particle size, morphology and crystallinity is monitored by HRTEM (Figure 4). The TEM pictures of P-2 and P-3 both show similar regular amorphous particles, devoid of any lattice fringes, with a narrow size distribution around 15-20 nm (Figure 4a,b). FAU-45 min, harvested in step 4 after a 45 min hydrothermal treatment contains particles with fringes (Figure 4d) with a similar narrow size distribution around 10-20 nm. This implies that the P-2 and P-3 amorphous particles are converted in octahedrally shaped FAU crystals with similar dimensions (Figure 4e,f). The good agreement between TEM and DLS particle sizes (Figure 5) reinforces the occurrence of such a transformation where the size of the amorphous particles predetermines to a great extent the size of the ultimate zeolite crystals. As IR, NMR and Raman spectroscopies have demonstrated their potential to shed deeper insight on the short-range evolution of amorphous aluminosilicates to crystalline structures, they will be put to task in the next part, vide infra.

Short-Range Order Evolution during Zeolite Crystallization -Spectroscopic Characterization

NMR Spectroscopy

Clear suspensions from the four crystallization steps are characterized by liquid state 29 Si NMR to identify the nature and evolution of the aluminosilicate species present (Figure S4). The main focus here is on the formation of the first secondary building units (SBU) of the FAU structure. [38] A previous ex-situ 29 Si MAS NMR study of the time evolution of hydrogels showed that only Q 0 (4Al) aluminosilicates are formed while colloidal silica dissolves in the rate-determining step. [27] Upon mixing colloidal silica with sodium hydroxide at high pH and high temperature, monomeric silicate anions are formed. The evolution of the 29 Si NMR spectra of suspensions, from the pure silica solution B to the FAU-45 min is summarized in Figure S4. All 29 Si chemical shifts fall, as expected, in the -70 to -120 ppm range and Solution B (colloidal silica dissolved in aqueous NaOH) displays the most complex spectra as all possible Q n (n=1-4) environments are present (Figure S4a). For all samples, the spectra can be divided in 5 clusters, each one with overlapping Q n units and a distinct decreased intensity of the peaks associated with heavier units as crystallization progresses:

Q 0 :
The lowest chemical shift (-71.2 ppm) corresponds to monomeric silica Q 0 and is usually taken as the reference peak as its position is independent of the presence of other Si species; Q 1 Q 2 : two sub-clusters are located between -76 and -83 ppm: (i) the one at -79.3 ppm, includes Q 1 units in dimeric silicates and the slightly shifted peak of Q 1 end groups, or Q 1 connected with Q 3 or Q 4 units, and (ii) the other at -80.4 ppm is assigned to Q 2 in trimeric cyclosilicates; Q 2 Q 3 : this cluster located between -86 and -91 ppm highlights the presence of a wide variety of Q 2 units in P1 to FAU-45min: cyclotetrameric anions, the middle group of trimeric and tetrameric chain silicates, Q 2 units in substituted cyclosilicates and di-and tri-cyclic structures. [39] Some Q 3 units are also present, but only in solution B; Q 3 : these peaks catered around -96 ppm are only observed in solution B; Q 4 : this broad line around -110 ppm belongs to bulky Q 4 units difficult to detect in liquid-state NMR as their molecular weight put them more in the domain of solid-state NMR. Some representative silicate species of P-1 suspension are listed and attributed in Table S1. [40] The 29 Si NMR spectra of the pristine colloidal suspensions and solution B (silica solution) are recorded following the experimental protocol outlined in Figure S5. P-1 spectra are recorded every 5 min during the 24 h aging at 25°C (Figure 6) as well those of P-2 and P-3. P3 is the heated to 50°C and spectra collected every 5 min during the 24 h crystallization. Three important features of these in-situ 29 Si NMR spectra arise:

1) the appearance of new peaks at -76 and -83 ppm in P1, P2 and P3 due the formation of solid particles and/or exchange between the solid and liquid phase. They disappear upon heating at 50 °C (Figure 6 e, f, g) due to fast exchange between the silica from the solids (-76 ppm) and the monomer silica species (-72 ppm). The dynamics of the solid-liquid and/or liquid-liquid exchange is highlighted in a NOESY (Nuclear Overhauser Effect Spectroscopy) 2D spectra (Figure S6).

2) The -72 ppm peak width at half-height () increases from P-1 (=1.85 ppm), P-2 (=1.9 ppm) and P-3 (=2.02 ppm) to level from FAU-45 min onwards (=2.15 ppm). The formation of proto-zeolitic materials in P-2, with a chemical composition similar to the final crystalline material rationalizes such a behaviour.

3) The -94.8 ppm peak, assigned to the formation of double-six rings (D6R), shifts to lower values as Al is incorporated in the zeolite (Figure 6).

FT-IR Spectroscopy

Additional evidence for the formation of D6R upon aging (P-2) and further evaporation of water (P-3) of the precursor suspension P1 comes from FT-IR spectroscopy study. The P-3, FAU-45 min, FAU-3 h, and FAU-24 h spectra are gathered in Figure 7. The main changes appear in the 1250-920 cm -1 (asymmetrical stretching T-O-T vibration), 720-650 cm -1 (symmetrical stretching T-O-T mode), and 500-420 cm -1 (T-O bending mode of the TO 4 tetrahedra) regions. The D6R ring breathing mode band at 575 cm -1 in X-ray amorphous P-3 shifts to lower wave numbers on FAU-45min (566 cm -1 ), becomes more resolved and slightly shifts to 562 cm -1 on FAU-3h and further increases in intensity on FAU-24h (Figure 7). That this D6R band appears even in P-3, i.e. before any mild hydrothermal treatment indicates that ultra-small zeolite nanoparticles (proto zeolitic crystals) are already present so early in the crystallization process. Both 29 Si NMR, vide supra, and FT-IR reveal presence of double six-ring (D6R) secondary building unit (SBU) of the FAU structure in P-3, i.e., as early as step 3 of the zeolite crystallization.

Raman and Solid-State NMR Spectroscopy

Raman spectroscopy can complement such a study of the early stages of zeolite formation (Figure 8). The peaks at 501, 855 and 1020 cm -1 characteristic of amorphous aluminosilicates are present in the spectrum of P-1 (Figure 8a). The broad band centred around 501 cm -1 is assigned to out of plane bending Si-O-Si mode typical for amorphous polymeric silicates. After aging and partial evaporation of water (P-3), the bands at 296 cm -1 (six-membered rings) and 504 cm -1 (four-membered rings) narrow relative to P-1 (Figure 8c). The 865 cm -1 band, assigned to the symmetric T-O stretching mode, does not change significantly while the 970-1064 cm -1 bands, attributed to the asymmetric T-O-T stretching mode evolve. This suggests that species containing both four-and six-membered rings are present in P-3. For FAU-45min, the broad band at 504 cm -1 narrows and shifts to 506 cm -1 (Figure 8d). At the same time, bands appear at 287 cm -1 and 381 cm -1 , assigned to D6R, while the 1020 cm -1 band splits in two bands at 997 cm -1 and 1064 cm -1 both characteristic of zeolite X. [41][42][43] The appearance of a broad band at 1020 cm -1 in P-2 and P-3 is significant (Figure 8 b,c) as this can be due to the formation of the β (SOD) cages expected for zeolite X. Sodalite cages possess both 4MR and 6MR, but the framework of zeolite X has not yet been formed at this stage and the different oxygen positions may be resolved only for the fully crystalline zeolite, FAU-24h. All

Raman bands become more intense and narrower after hydrothermal treatments between 45 min and 6 h (Figure 8e) and indicate that crystallization is complete after 24 h (FAU-24h). The bands corresponding to the formation of SOD cages connected via the double six-membered rings (290 and 380 cm -1 ) already appear in FAU-45 min (Figure 8d). 29 Si solid-state NMR spectroscopy on the same series of materials (Figure 9) will determine Si/Al ratio changes during the crystallization process. As expected, the silica species exist as monomeric and oligomeric anions formed from SiO 4 tetrahedra in P-1. A broad asymmetric signal at -88.0 ppm is observed in the spectrum and can be divided in two entities corresponding to mono-and oligomeric species, respectively. The dmfit method [27] is applied to fit the spectra based on the Engelhardt and Michel equation.

The 29 Si MAS NMR spectra of fully crystalline samples contain five peaks at -84.4, -89.0, -94.1, -99.0, and -104.3 ppm attributed to Q 4 (4Al), Q 4 (3Al), Q 4 (2Al), Q 4 (1Al), and Q 4 (0Al), respectively. The degree of crystallinity of all samples calculated by 29 Si NMR are summarized in Table S2. The formation of the FAU structure in the P-2 sample is confirmed by the appearance of Q 4 (4Al) to Q 4 (0Al) peaks superimposed to the broad signal coming for the amorphous material necessary to perfectly fit spectrum. The Si/Al ratio of P-1 cannot be measured since the spectrum is dominated by silicon species in an amorphous phase. In P-3 (suspension after partial evaporation of water), the Si/Al ratio is estimated to be 2.8. After hydrothermal treatment the Si/Al ratio of FAU-45 min, FAU-3 h and FAU-24 h decreases from 1.7 to 1.41 and 1.06, respectively (Figure 9). The 29 Si NMR signals in the fully crystalline FAU-24 h are symmetric and sharp, implying high homogeneity. We can therefore suggest that the crystallization of FAU zeolite starts during the partial evaporation of water step of the precursor suspension (P-3), in agreement with the FTIR results, where the formation of the D6R is observed.

Long-Range Crystalline Order and Porosity

HP 129 Xe NMR Spectroscopy

The development of the porosity of the samples at steps 1-4 was studied by HP 129 Xe NMR (Figure 10).

The HP 129 Xe NMR spectrum of P-1 (Figure 10a) contains a broad peak at 50 ppm corresponding to Xenon in a confined space with a non-defined porosity. Another narrow peak appears at 78 ppm, 80 ppm and 82 ppm for P-3, FAU-45 min and FAU-3 h, respectively (Figure 10). The evolution of this peak is consistent with the partial crystallization of these samples as a higher chemical shift is associated with xenon located in a confined environment. The spectrum of the fully crystalline FAU-24 h contains two peaks at 77.5 and 82.5 ppm, assigned to 129 Xe located in the sodalite cages and supercages, respectively (Figure 10e). To verify the assignment of these two chemical shifts, the HP 129 Xe spectrum of the as-synthesized FAU-24 h is compared with that of ion-exchanged derivative with Li (sample Li-FAU-24 h). FAU-24 h is partially exchanged (70%) with the smaller cation (ionic radii: Li + =0.6 Å, Na + =0.95 Å). The 82.5 ppm peak is present in both the FAU-24 h and Li-FAU-24 h spectra, while the 77.5 ppm peak in the FAU-24 h spectrum shifts to 74.5 ppm (Li-FAU-24 h) after Li ion-exchange (Figure 11b); the smaller Li cation located in the supercages increasing the available volume in the latter. hexagonal prisms may facilitate the penetration of Xe into sodalite cages and its subsequent diffusion in the microporosity as was recently reported by our group . [44] Nitrogen Adsorption

The development of the porosity in the samples is monitored by N 2 physisorption. The formation of a proto zeolitic material in P-3, i.e., X-ray amorphous but with well-defined particles containing micropores is confirmed. P-3 has the highest external surface area (S ext =300 m 2 /g) and its isotherm is a mix of types I (micropore) and IV (mesopore) with a large H1 hysteresis (Figure 12). Such features are associated with textural pores formed by the close packing of monodispersed nanosized particles. The presence of microporosity at this stage indicates the formation of some unit cells corresponding to FAU zeolite. The degree of crystallinity in the samples increases with hydrothermal treatment and correlates well with the gradual increase of the micropore volume from 0.10 to 0.31 cm 3 /g (Table 1). The FAU-24 h micropore volume is comparable to that of micron-sized FAU. All samples have well developed textural mesoporosity confirming that the particle sizes in the amorphous, intermediates and final crystalline samples are very similar (Table S2). )

Partial pressure (P/P o )

Conclusion

The crystallization of OSDA-free FAU nanocrystals was followed using several complimentary characterization methods well suited for tracking zeolite synthesis. The transformation pathway from a viscous amorphous precursor suspension to fully crystalline zeolite suspensions is elucidated by measuring the mobility of the Na cations and their subsequent trapping in the zeolite structure alongside the incorporation of Al in the zeolite framework. The formation of the very first SBUs (D6R) was demonstrated by FTIR and in-situ 29 Si NMR. We demonstrated that the clear precursor suspension formed during the mixing time undergoes substantial chemical and morphological changes during the aging and partial evaporation of water steps, where the stoichiometric composition is achieved. In addition, the size of the amorphous nanoparticles in the initial precursor suspension and their transformation in crystalline nanoparticles is set by the preparation parameters (temperature, speed of mixing, aging). A high alkalinity of the suspensions produces precursors with high surface area and ultra-small sizes. In-situ 29 Si NMR spectroscopy indicates the formation and the nature of Q 3 and Q 4 silicate species surrounded by Q 1 and substituted Q 2 silicate species. Liquid state 29 Si NMR spectroscopy also indicates the existence of D6R prior to hydrothermal treatment (P-3) while solid state 29 Si NMR spectroscopy tracks the evolution of the FAU crystallization. A key finding, provided by 2D HP 129 Xe EXSY, highlights the structural flexibility of nanosized FAU zeolites leading to a faster molecular traffic between the two different cages in this structure is worth considering in many applications (catalysis, separations by adsorption) for nanosized zeolites.

The methodology outlined is applicable to better design many other zeolites where nanosized crystals prove useful.

Samples

  2 O: 1.1 Al 2 O 3 : 10 SiO 2 : 122 H 2 O. -Step 2: The step 1 suspension is kept for 24 h at room temperature for aging. -Step 3: Partial evaporation of water under vacuum. The resulting composition of the precursor suspension is: 8.5 Na 2 O: 1.1 Al 2 O 3 : 10 SiO 2 : 50 H 2 O. -Step 4: Hydrothermal treatment of the viscous step 3 suspension at 50° C from 45 min to 24 h.

Figure 1 .

 1 Figure 1. Schematic representation of the preparation of ultra-small FAU-X nanosized zeolite: Steps 1 -4

  Nitrogen physisorption: Nitrogen (N 2 ) adsorption/desorption isotherms are measured on a Micromiretrics ASAP 2020 volumetric adsorption analyser. Samples are degassed overnight at 275 C under vacuum prior to measurements. The external surface area and micropore volume are estimated by the alpha-plot method using Silica-1000 (22.1 m 2 g -1 assumed) as a reference. The micropore and mesopore size distributions are estimated by the Nonlocal Density Functional Theory (NLDFT) and Barret-Joyner-Halenda (BJH) on the desorption branch, respectively.

Figure 2 .

 2 Figure 2. Precursor suspensions after mixing solutions A and B (step 1: P-1), after aging for 24 hr at RT (step 2: P-2), after partial evaporation of water (step 3: P-3) and after crystallization for 24 h at 50 ºC (step 4: FAU-24 h).

Figure 3 .

 3 Figure 3. (A) Conductivity of precursor suspensions at different preparation steps (P-1, P-2, and P-3) and suspensions during crystallization at 50 °C (FAU-45 min, FAU-3 h, and FAU-24 h); (B) conductivity

Figure 4 .

 4 Figure 4. HRTEM images of (a) P-2, (b) P-3, (c, d) FAU-45min, and (e, f) FAU-24h.

Figure 5 .

 5 Figure 5. DLS particle size distribution of precursors (P-1, P-2, P-3) and crystalline zeolite suspensions (FAU-45 min and FAU-24 h).

Figure 6 .

 6 Figure 6. In-situ liquid state 29 Si NMR spectra of (a) solution B, (b) P-1, (c) P-2, (d) P-3, (e) FAU-45min, (f) FAU-3h and (g) FAU-24h.

Figure 7 .

 7 Figure 7. IR spectra of (a) P-3, (b) FAU-45 min, (c) FAU-3 h, (d) FAU-24 h in the 400-1500 cm -1 window.

Figure 8 .

 8 Figure 8. Raman spectra of (a) P-1, (b) P-2, (c) P-3, (d) FAU-45 min, and (e) FAU-24 h.

Figure 9 .

 9 Figure9.29 Si NMR spectra of (a) P-1, (b) P-2, (c) P-3, (d) FAU-45 min, (e) FAU-3 h and (f) FAU-24 h.

Figure 10 .

 10 Figure 10. HP 129 Xe NMR spectra of (a) P-1, (b) P-2, (c) FAU-45 min, (d) FAU-3 h and (e) FAU-24 h.

Figure 11 .

 11 Figure 11. HP 129 Xe NMR spectra of (a) FAU-24 h and (b) Li-FAU-24 h.

Figure 12 .

 12 Figure 12. N 2 sorption isotherms of samples P-3, FAU-3 h, FAU-6 h and FAU-24 h.

Table 1 .

 1 N 2 sorption data of samples obtained at different crystallization steps.

Acknowledgements

This research was supported by UOP-Honeywell.


Bowen Liu

Peng Bai
email: baipeng@upc.edu.cn

Yang Wang

Zhun Dong

Pingping Wu

Svetlana Mintova

Zifeng Yan

Highly stable Ni/ZnO-Al 2 O 3 adsorbent promoted by TiO 2 for reactive adsorption desulfurization

Keywords: Ni/ZnO-Al 2 O 3 adsorbents, Reactive adsorption desulfurization, Stability, ZnAl 2 O 4 spinel, TiO 2

The reactive adsorption desulfurization is an important and efficient technology for the ultradeep desulfurization of gasoline. The performance of an adsorbent is vital for the efficiency of the reactive adsorption desulfurization process. In the case of Ni/ZnO-Al 2 O 3 adsorbent, the formation of inert ZnAl 2 O 4 spinels is commonly recognized for the deactivation of adsorbents.

To solve this problem, in this work, TiO 2 was incorporated in the Ni/ZnO-Al 2 O 3 adsorbent by a wet impregnation method to inhibit the formation of spinel structures during the reaction and regeneration processes. The TiO 2 formed a stable protection layer on the surface of Al 2 O 3 preventing the interaction between Al 2 O 3 and ZnO, which inhibited the formation of ZnAl 2 O 4 spinel phase. The reactive adsorption desulfurization performance of the adsorbents was evaluated in a fixed bed reactor using thiophene in n-octane as a model fuel. Evaluation results indicated that the introduction of TiO 2 could significantly improve the stability of the adsorbents during cyclic hydrothermal treatments, while the sulfur capacity slightly dropped.

Characterization results revealed that TiO 2 was homogeneously dispersed on the surface of Al 2 O 3 support and the incorporation of TiO 2 could maintain a certain amount of Lewis acid sites in the adsorbents, which are favorable for the adsorption of organo-sulfur compounds.

Introduction

The air pollution problems which could bring environmental disasters and damages to human health have aroused global concern. [START_REF] Zhang | Review and challenges of policies of environmental protection and sustainable development in China[END_REF][START_REF] Hunter | Impacts of anthropogenic SO x , NO x and NH 3 on acidification of coastal waters and shipping lanes[END_REF][START_REF] Munawer | Human health and environmental impacts of coal combustion and postcombustion wastes[END_REF] Different types of techniques have been used to eliminate or control the air pollutants (NO x , SO 2 , hydrocarbon, etc.) from vehicle emissions. [START_REF] Heck | Catalytic air pollution control: commercial technology[END_REF] The ultra-deep desulfurization technology is effectively used to remove the sulfur compounds from fuel oils. [START_REF] Breysse | Deep desulfurization: reactions, catalysts and technological challenges[END_REF][START_REF] Shen | Selective adsorption for removing sulfur: a potential ultra-deep desulfurization approach of jet fuels[END_REF][START_REF] Kim | Ultra-deep desulfurization and denitrogenation of diesel fuel by selective adsorption over three different adsorbents: a study on adsorptive selectivity and mechanism[END_REF][START_REF] Song | An overview of new approaches to deep desulfurization for ultra-clean gasoline, diesel fuel and jet fuel[END_REF][START_REF] Ma | A new approach to deep desulfurization of gasoline, diesel fuel and jet fuel by selective adsorption for ultra-clean fuels and for fuel cell applications[END_REF] As fuel oils will remain as the main energy source for vehicles in recent decades, [START_REF] Bielecki | Energy security: is the wolf at the door? The quarterly review of economics and finance[END_REF] the development of efficient desulfurization technologies is still important due to the more stringent environmental regulations for clean fuel oils. [START_REF] Song | New approaches to deep desulfurization for ultra-clean gasoline and diesel fuels: an overview[END_REF][START_REF] Skea | What drives clean fuel technology[END_REF] Due to its advantages of low octane loss, low hydrogen consumption and mild operation conditions, the reactive adsorption desulfurization (RADS) technique has become a commercial process named as S-Zorb to produce clean gasoline of Stage Ⅴ (less than 10 μg/g Sulfur) in China. [START_REF] Phillips | Sulfur Removal Technology[END_REF][START_REF] Sharma | A review on reactive adsorption for potential environmental applications[END_REF][START_REF] Ma | Deep desulfurization of gasoline by selective adsorption over nickel-based adsorbent for fuel cell applications[END_REF][START_REF] Gupta | Nanowire catalysts for ultra-deep hydro-desulfurization and aromatic hydrogenation[END_REF][START_REF] Wang | Reactive adsorption desulfurization coupling aromatization on Ni/ZnO-Zn 6 Al 2 O 9 prepared by Zn x Al y (OH) 2 (CO 3 ) z •xH 2 O precursor for FCC gasoline[END_REF] A RADS mechanism has been widely accepted for the Ni/ZnO catalyst system where the S atom in the organic sulfur compounds firstly reacts with Ni (active site for the catalytic hydrogenolysis of C-S bond) and generates Ni 3 S 2 . In the presence of H 2 , Ni 3 S 2 will be converted to H 2 S and the Ni sites are restored to Ni 0 . The formed H 2 S reacts with ZnO which is the sulfur acceptor to produce ZnS. [START_REF] Huang | A sulfur K-edge XANES study on the transfer of sulfur species in the reactive adsorption desulfurization of diesel oil over Ni/ZnO[END_REF] After combustion (regeneration process) in the air, ZnO is regenerated to complete a RADS-regeneration cycle. [START_REF] Babich | Science and technology of novel processes for deep desulfurization of oil refinery streams: a review[END_REF] The study on the RADS reaction kinetics of Ni/ZnO based adsorbents revealed that the sulfur diffusion in ZnO was identified as the rate-determining step after partial sulfidation of ZnO, and the high dispersion of ZnO and NiO particles was favorable for the reaction. [START_REF] Bezverkhyy | Interaction of Ni/SiO 2 with thiophene[END_REF][START_REF] Bezverkhyy | Kinetics of thiophene reactive adsorption on Ni/SiO 2 and Ni/ZnO[END_REF][START_REF] Ryzhikov | Reactive adsorption of thiophene on Ni/ZnO: Role of hydrogen pretreatment and nature of the rate determining step[END_REF] To enhance the dispersion of Ni and ZnO, different supports have been introduced into the Ni/ZnO-based catalysts, such as Al 2 O 3 , ZrO 2 , SiO 2 , zeolites. [START_REF] Meng | Ni/ZnO-based adsorbents supported on Al 2 O 3 , SiO 2 , TiO 2 , ZrO 2 : a comparison for desulfurization of model gasoline by reactive adsorption[END_REF][START_REF] Dehghan | Zeolites for adsorptive desulfurization from fuels: A review[END_REF][START_REF] Sasaoka | Modification of ZnO-TiO 2 hightemperature desulfurization sorbent by ZrO 2 addition[END_REF] Among the supports being used, Al 2 O 3 is the most common one because of its intrinsic advantages, such as high surface area, low cost and high chemical/mechanical stabilities, triggering extensive interests about the Ni/ZnO-Al 2 O 3 adsorbents. Gao et al. [START_REF] Wen | Regeneration characteristics and kinetics of Ni/ZnO-SiO 2 -Al 2 O 3 adsorbent for reactive adsorption desulfurization[END_REF][START_REF] Wang | Reactive characteristics and adsorption heat of Ni/ZnO-SiO 2 -Al 2 O 3 adsorbent by reactive adsorption desulfurization[END_REF][START_REF] Fan | Research on reactive adsorption desulfurization over Ni/ZnO-SiO 2 -Al 2 O 3 adsorbent in a fixed-fluidized bed reactor[END_REF] investigated the reactivity and regeneration characteristics of the Ni/ZnO-SiO 2 -Al 2 O 3 . It was found that the inactive ZnAl 2 O 4 spinel formed in the reduction and reaction processes significantly decreased the desulfurization activity of adsorbents.

Moreover, surface Lewis acid sites increased the activity of the adsorbents because the sulfurcontaining compounds bearing lone pair electrons are adsorbed on the adsorbent via the Lewis acid-base interaction.

In our previous studies, a series of Ni/ZnO-Al 2 O 3 adsorbents were prepared by a homogeneous precipitation method and a cation-anion double hydrolysis (CADH) method. 29- 31 In all these adsorbents, the formation of the inert gahnite phase (such as zinc aluminate) during the reaction was commonly observed, leading to the decrease of activity and fragmentation of adsorbents, which could not be overcome by simply changing the preparation methods. [START_REF] Xu | Cause analysis of sorbent deactivation in S Zorb unit for gasoline desulfurization[END_REF] The current adsorbents regeneration process can remove the carbon deposits and convert the ZnS into ZnO via burning in the air. But the formation of the spinel phase is irreversible and the burning process would also promote the formation and growth of spinels, especially under steaming conditions of the regeneration process. [START_REF] Meng | Reactive mechanism and regeneration performance of NiZnO/Al 2 O 3 -diatomite adsorbent by reactive adsorption desulfurization[END_REF] Besides, for the preparation of industrial adsorbents, to achieve a stable adsorption performance, the adsorbents are usually prepared by the kneading method followed by an aging process to reduce the initial activity, which requires a hydrothermal treatment under the hightemperature steaming conditions. [START_REF] Gil | Passivation and reactivation of nickel catalysts[END_REF] The high-temperature steaming conditions in the aging process would also promote the formation of spinel phase. [START_REF] Zou | Study on Zn 2 SiO 4 formation kinetics and activity stability of desulfurization sorbent[END_REF] Meng et al. [START_REF] Meng | Ni/ZnO-based adsorbents supported on Al 2 O 3 , SiO 2 , TiO 2 , ZrO 2 : a comparison for desulfurization of model gasoline by reactive adsorption[END_REF] found that the presence of ZnAl 2 O 4 could reduce the reducibility and activity of the adsorbents, and the high stability of spinel renders it difficult to be removed once it formed. Moreover, the spinel formation also deteriorated the mechanical strength of the adsorbents. [START_REF] Xu | Cause analysis of sorbent deactivation in S-Zorb unit for gasoline desulfurization[END_REF] After deactivation, the difficulties and high cost for recycling result in the fact that most of the spent adsorbents are buried directly, which would cause soil and water pollution. [START_REF] Lyu | Reactivation of spent S-Zorb adsorbents for gasoline desulfurization[END_REF] Hence, from the sustainable development point of view, it is highly desirable to develop stable RADS adsorbents via costeffective approaches.

In order to inhibit the formation of the ZnAl 2 O 4 phase, it is a wise way to weaken the interaction between ZnO and Al 2 O 3 by introducing a promoter into the adsorbent. [START_REF] Reddy | Design of stable and reactive vanadium oxide catalysts supported on binary oxides[END_REF][START_REF] Hu | Partial hydrogenation of benzene to cyclohexene on ruthenium catalysts supported on La 2 O 3 -ZnO binary oxides[END_REF] Titania has been a common chemical material used as a promoter for the alumina supported catalysts, the synergetic effect of TiO 2 -Al 2 O 3 could promote the dispersion of active species and restrain the interaction between active species and Al 2 O 3 . [START_REF] Kaneko | Sol-gel synthesis of titania-alumina catalyst supports[END_REF][START_REF] Kainthla | Development of stable MoO 3 /TiO 2 -Al 2 O 3 catalyst for oxidative dehydrogenation of ethylbenzene to styrene using CO 2 as soft oxidant[END_REF] Ni/TiO 2 -Al 2 O 3 catalysts were synthesized for methane decomposition using a co-precipitation method, the incorporation of TiO 2 effectively reduced the formation of nickel aluminate (NiAl 2 O 4 ) and the carbon deposition was greatly inhibited. [START_REF] Awadallah | Hydrogen production via methane decomposition over Al 2 O 3 -TiO 2 binary oxides supported Ni catalysts: Effect of Ti content on the catalytic efficiency[END_REF] In hydrodesulfurization reactions, TiO 2 -Al 2 O 3 based catalysts also show unique advantages, TiO 2 promoted the formation of empty orbitals in multilayered MoS 2 , resulting in the strong sulfur adsorption capacity of the catalyst. The Ti 3+ species formed by the partial reduction or sulfidation of TiO 2 can act as a promoter for the MoS 2 phase. [START_REF] Coulier | TiO 2 -supported Mo model catalysts: Ti as promoter for thiophene HDS?[END_REF][START_REF] Okamoto | Active sites of molybdenum sulfide catalysts supported on Al 2 O 3 and TiO 2 for hydrodesulfurization and hydrogenation[END_REF] A series of Mo/TiO 2 -Al 2 O 3 catalysts were prepared by Segawa et al. via the chemical vapor deposition of TiCl 4 , and Mo/TiO 2 -Al 2 O 3 exhibited higher activity than the Mo/Al 2 O 3 or Mo/TiO 2 catalysts in the ultra-deep hydrodesulfurization reaction. [START_REF] Yoshinaka | Hydrodesulfurization of dibenzothiophenes over molybdenum catalyst supported on TiO 2 -Al 2 O 3[END_REF][START_REF] Saih | Tailoring of alumina surfaces as supports for NiMo sulfide catalysts in the ultra deep hydrodesulfurization of gas oil: case study of TiO 2 -coated alumina prepared by chemical vapor deposition technique[END_REF][START_REF] Saih | Ultra deep hydrodesulfurization of dibenzothiophene derivatives over NiMo/TiO 2 -Al 2 O 3 catalysts[END_REF] This was attributed to the coverage of Al 2 O 3 by TiO 2 layer, decreasing the interaction between Mo and the support and promoting the sulfidation of Mo species. In another report, by using TiO 2 -Al 2 O 3 binary supports for the Ni/Mo-based catalysts, the hydrodesulfurization activity of the catalysts was improved due to the enhanced hydrogenation activity by the existence of TiO 2 . [START_REF] Olguin | The use of TiO 2 -Al 2 O 3 binary oxides as supports for Mo-based catalysts in hydrodesulfurization of thiophene and dibenzothiophene[END_REF] In recent studies, highly ordered NiMo/TiO 2 -Al 2 O 3 hydrodesulfurization catalysts were prepared via evaporation-induced self-assembly method. [START_REF] Zhang | Synthesis of highly ordered TiO 2 -Al 2 O 3 and catalytic performance of its supported NiMo for HDS of 4, 6-dimethyldibenzothiophene[END_REF][START_REF] Zhou | Synthesis of novel NiMo catalysts supported on highly ordered TiO 2 -Al 2 O 3 composites and their superior catalytic performance for 4, 6dimethyldibenzothiophene hydrodesulfurization[END_REF] Their results demonstrated that the introduction of TiO 2 into the supports prevented the formation of strong Mo-O-Al linkages which promoted the formation of "Type II" Ni-Mo-S phase and enhanced the activity of the catalysts. Inspired by the promotion effect of TiO 2 , it can be envisaged that the adsorbents via a heterogeneous-grinding method. The performance of the adsorbents was evaluated in the RADS reaction using a model fuel and compared with the adsorbents prepared without TiO 2 .

Results

Reaction Adsorption Desulfurization (RADS) Performance of Adsorbents

In order to eliminate the influence brought by reaction products (ZnS, NiS x , etc.), we chose the fresh TiO 2 modified and unmodified adsorbents (supports) with a Zn/Al molar ratio of 1:1 to undergo the cyclic hydrothermal treatments, which can clearly reveal the promotion effect of TiO 2 addition. The adsorbents were donated as 'Ti-a-b-c' and 'C-a-b-c', where 'Ti' represents TiO 2 modified, 'C' represents unmodified, 'a-b' indicates the molar ratio of Zn to Al, and 'c' indicates the cycles of the hydrothermal treatment. The 'S' was attached behind the name of adsorbents to present the corresponded supports. Figure 1 shows the sulfur adsorption breakthrough curves and the breakthrough sulfur capacities of the fresh adsorbents and the adsorbents after 3 cycles of hydrothermal treatment, respectively. As can be seen, the fresh unmodified adsorbent exhibited a better RADS activity and a higher sulfur capacity than the fresh TiO 2 modified adsorbent. After three cycles of hydrothermal treatment, the TiO 2 modified adsorbent exhibited a higher sulfur capacity than the unmodified one, indicating the higher stability of the former. The breakthrough sulfur capacity (at 10 ppm Sulfur) of the TiO 2 modified adsorbent decreased from 10.5 mg•g -1 to 7.8 mg•g -1 which is above 2-fold that of the unmodified adsorbent (3.8 mg•g -1 ) after 3 cycles of hydrothermal treatment. The ZnO conversion revealing the ZnO transformation extent is another important indicator of the catalyst activity and stability. The ZnO conversion at the breakthrough point for the fresh TiO 2 modified adsorbent (7.04%) is slightly lower than that of the unmodified one (7.62%).

Although both adsorbents exhibit a decrease in their ZnO conversion which was caused by the hydrothermal treatments, the TiO 2 modified one retained 74.1% of its ZnO conversion in the fresh state, while the unmodified one only maintained 31.4% of its fresh ZnO conversion.

These results clearly indicated that the stability of the adsorbent could be significantly enhanced by modifying the Ni/ZnO-Al 2 O 3 adsorbent with TiO 2 . S2 show that the bulk atomic molar ratio of the TiO 2 modified adsorbents (Zn: Ti: Al = 1.22: 0.18: 1.00) is close to the set ratio (Zn: Ti: Al=1.00: 0.15: 1.00), thus indicating that most of the introduced TiO 2 was kept in the bulk structure without significant loss during the preparation process. The PXRD patterns of the TiO 2 modified and unmodified adsorbents are shown in UV-vis spectra of TiO 2 modified and unmodified supports are shown in Figure 3. The peaks at 280 nm and 314 nm could be attributed to the existence of ZnAl 2 O 4 . [START_REF] Sampath | Optical properties of zinc aluminate, zinc gallate, and zinc aluminogallate spinels[END_REF] The peak at 320 nm is due to the O 2-→Ti 4+ charge transfer transition corresponding to the excitation of electrons from the valence band (having the O 2p character) to the conduction band (having the Ti 3d character), which is the characteristic of anatase phase. [START_REF] Damyanova | Effect of mixed titania-alumina supports on the phase composition of NiMo/TiO 2 -Al 2 O 3 catalysts[END_REF] For the fresh supports, there are no discernable peaks assignable to ZnAl 2 O 4 . While after the hydrothermal treatments, the peak intensities corresponding to ZnAl 2 O 4 in the unmodified supports increased with the hydrothermal treatment cycles. However, there are no discernable ZnAl 2 O 4 peaks in the TiO 2 modified supports, evidencing the high stability of the TiO 2 modified samples which is consistent with the PXRD results.

Crystalline Structure

X-ray fluorescence (XRF) spectra results shown in Table

Morphology and Textural Properties

Figure 4 and 5 show the SEM and EDS-mapping images of both TiO 2 modified and unmodified supports. In both supports, the ZnO particles were homogeneously dispersed on the Al 2 O 3 . For the TiO 2 modified support, the Ti species are also homogeneously dispersed on the Al 2 O 3 . Considering there are no obvious peaks of titania in the PXRD patterns of TiO 2 modified adsorbents (shown in Figure 2), the introduced TiO 2 may be present as highly dispersed TiO 2 species and covered the alumina surfaces after calcination. [START_REF] Ramírez | Analysis of the hydrotreatment of Maya heavy crude with NiMo catalysts supported on TiO 2 -Al 2 O 3 binary oxides: Effect of the incorporation method of Ti[END_REF] Figure 6 shows N 2 sorption isotherms of the unmodified and TiO 2 modified adsorbents with a Zn/Al molar ratio of 1: 1. All samples display typical type IV adsorption isotherms with an H-3 hysteresis loop, indicating the existence of a mesoporous structure. [START_REF] Sotomayor | Characterization of micro/mesoporous materials by physisorption: Concepts and case studies[END_REF] The textural properties of the adsorbents are summarized in Table S3. For the fresh adsorbents, compared with the unmodified adsorbents, the TiO 2 modified one has a lower specific surface area which may be due to the partial blocking of the pores by TiO 2 , and this may also be a reason for the lower activity of fresh TiO 2 modified adsorbent in the RADS reaction (shown in Figure 1). [START_REF] Ramírez | Analysis of the hydrotreatment of Maya heavy crude with NiMo catalysts supported on TiO 2 -Al 2 O 3 binary oxides: Effect of the incorporation method of Ti[END_REF] With increasing the cycles of hydrothermal treatment, the specific surface areas of both unmodified and TiO 2 modified adsorbents decreased, which may be due to the formation of spinel, sintering and collapse of pore structure. However, the addition of TiO 2 could inhibit the formation of ZnAl 2 O 4 spinel and enhance the pore structure stability during hydrothermal treatments. The TiO 2 modified samples exhibit less reduction of the surface area. In comparison, the unmodified adsorbent lost 21% of its specific surface area after three cycles of hydrothermal treatment, while the specific surface area of the TiO 2 modified sample reduced by 17%. Increasing the Al 2 O 3 amount in the adsorbents could increase the specific surface area and enhance the dispersion of ZnO and NiO (Figure S8 and Table S3). However, this will also promote the formation of spinel under the reaction/regeneration conditions, which imposed a deleterious effect on the stability and activity of adsorbents. Moreover, the stability enhancement brought by the introduced TiO 2 is more pronounced.

State of TiO 2 and Surface Acidity

The unmodified (C-1-1-0-S and C-1-1-3-S) and TiO 2 modified (Ti-1-1-0-S and Ti-1-1-3-S) supports were characterized with Raman spectroscopy to detect the state of Ti in the TiO 2 modified supports. Both the TiO 2 modified and unmodified supports have the characteristic peaks of ZnO located at 99, 333 and 437 cm -1 (Figure 7). [START_REF] Montenegro | Non-radiative recombination centres in catalyst-free ZnO nanorods grown by atmospheric-metal organic chemical vapour deposition[END_REF] Besides, there is a peak at 146 cm -1 that appears in the Raman spectrum of the TiO 2 modified support, which reveals the presence of anatase phase. [START_REF] Dirnstorfer | Atomic layer deposition of anatase TiO 2 on porous electrodes for dye-sensitized solar cells[END_REF] As shown by SEM and EDS-mapping (shown in Figure 5), the TiO 2 was highly dispersed on the Al 2 O 3 surface; the TiO 2 as anatase formed protection layer, which could inhibit the formation of ZnAl 2 O 4 spinel phase by preventing the interaction between ZnO and Al 2 O 3 during RADS reaction and regeneration processes.

FT-IR spectra of unmodified and TiO 2 modified adsorbents are shown in Figure 8a and8b.

The peaks at 940 and 1380 cm -1 are attributed to the Zn-O bond vibration. [START_REF] Sharma | ZnO anchored graphene hydrophobic nanocomposite-based bulk heterojunction solar cells showing enhanced shortcircuit current[END_REF][START_REF] Devi | Synthesis and characterization of Mn 2+ -doped ZnS nanoparticles[END_REF] The weak shoulder at 1100 cm -1 corresponds to Al-O vibration typical for γ-alumina, which is confirmed by the presence of the peak at 1630 cm -1 . [START_REF] Toledo | Effect of aluminum precursor on physicochemical properties of Al 2 O 3 by hydrolysis/precipitation method[END_REF][START_REF] Zhang | Influence of pH, humic acid, ionic strength, foreign ions, and temperature on 60 Co (II) sorption onto γ-Al 2 O 3[END_REF] The broad band at 3200-3700 cm -1 is the stretching vibration band of -OH bonded to Al 3+ . No band assigned to Ti-O-Al bond at 1128 cm -1 is found in the spectra of TiO 2 modified adsorbents, suggesting no strong interactions between TiO 2 and γ-Al 2 O 3 . [START_REF] Bing | Enhanced mineralization of pharmaceuticals by surface oxidation over mesoporous γ-Ti-Al 2 O 3 suspension with ozone[END_REF] The FT-IR results combined with PXRD (Figure 2) and XRF (Table S2) suggest that the TiO 2 is evenly spread over the γ-Al 2 O 3 and inhibited the interactions between ZnO and γ-Al 2 O 3 .

The Py-IR spectra of unmodified and TiO 2 modified adsorbents are depicted in Figure 8c and 8d. The Lewis acid sites are identified by the presence of the bands at 1450 (υ 19b ), 1490 (υ 19a ) and 1610 (υ 18a ) cm -1 , while the peak at 1578 cm -1 is attributed to the interaction between αpyridine and -OH on the adsorbent surface. [START_REF] Andonova | Nature of the Ti-Ba interactions on the BaO/TiO 2 /Al 2 O 3 NO x storage system[END_REF] The amount of Lewis acid sites for the unmodified and TiO 2 modified adsorbents after cyclic hydrothermal treatments are summarized in Table S4. For the fresh adsorbents, the Lewis acid amount of unmodified adsorbent was higher than that of the TiO 2 modified one. This may be due to the formation of the anatase layer which partly blocked the pores and covered the defects on the surfaces. With increasing the cycles of hydrothermal treatment, the Lewis acid amounts of both unmodified and TiO 2 modified adsorbents changed. For the unmodified adsorbent, the amount of Lewis acid sites decreased by 35.48% after the first cycle of the hydrothermal treatment and almost remained the same in the following cycles. This may be caused by the pore structure destruction or sintering and the formation of ZnAl 2 O 4 . However, the TiO 2 modified adsorbent exhibited better hydrothermal stability; the amount of Lewis acid sites decreased by 23.81% after the first cycle of hydrothermal treatment. It is quite interesting to note that, the amount of Lewis acid sites started to raise with the increase of the hydrothermal treatment cycles. The Lewis acid amount recovered 14.29% in cycle 2, and dramatically increased to 3.1 mmol•g -1 which is 32.26% higher than that of the fresh TiO 2 modified adsorbent in cycle 3. This may be attributed to the formation of new acid sites resulting from the structural rearrangement during the hydrothermal treatments. Since the Lewis acid sites are beneficial for the adsorption of thiophene bearing lone pair electrons, [START_REF] Rrez-Ale Andre Ad | A FT-IR study of the adsorption of indole, carbazole, benzothiophene, dibenzothiophene and 4, 6-dibenzothiophene over solid adsorbents and catalysts[END_REF] the higher concentration of Lewis acid sites in TiO 2 modified sample after 3 cycles of hydrothermal treatment may be partially responsible for the improved RADS performance as compared to the unmodified one. [START_REF] Meng | Ni/ZnO-based adsorbents supported on Al 2 O 3 , SiO 2 , TiO 2 , ZrO 2 : a comparison for desulfurization of model gasoline by reactive adsorption[END_REF]

Discussion

The highly dispersed TiO 2 on the γ-Al 2 O 3 was verified based on the results shown above (see Figure 2 and 5, Table S2) which is in line with the study of Zhang et al. [START_REF] Bing | Enhanced mineralization of pharmaceuticals by surface oxidation over mesoporous γ-Ti-Al 2 O 3 suspension with ozone[END_REF] In addition, the Raman spectra (Figure 7) confirmed the existence of anatase TiO 2 on the support surface with good stability; no formation of new compounds after three cycles of hydrothermal treatment was observed. Based on the above results, a schematic illustration explaining the role of TiO 2 in inhibiting the ZnAl 2 O 4 formation is presented in Figure 9. As both the γ-Al 2 O 3 (the pseudo-boehmite was transformed to γ-Al 2 O 3 after calcination) and ZnAl 2 O 4 have a similar space group (Fd-3m), it is easy for the Zn 2+ (in adjacent ZnO) to diffuse into the tetrahedral sites of γ-Al 2 O 3 during the RADS reaction and regeneration process. A thin layer of TiO 2 covered on the γ-Al 2 O 3 surface would isolate ZnO from the direct contact with γ-Al 2 O 3 , and then weaken the interaction between Zn 2+ and Al 3+ to prevent the formation of ZnAl 2 O 4 structure. Along with hindering the formation of the ZnAl 2 O 4 , the TiO 2 protection layer could also be beneficial for the maintenance of the pore structure of the γ-Al 2 O 3 . The destruction of pore structures during high-temperature reaction-regeneration cycles caused by the agglomeration or Al-O-Al breakage and sintering of ZnO was a common problem, which resulted in the decrease of the specific surface area of adsorbents. [START_REF] Nair | Pore structure evolution of lanthana-alumina systems prepared through coprecipitation[END_REF][START_REF] Choi | Synthesis of mesoporous TiO 2 /γ-Al 2 O 3 composite granules with different sol composition and calcination temperature[END_REF] This problem was also observed in the unmodified adsorbents in this work (the specific surface area decreased by 20.96% after 3 cycles of hydrothermal treatment). The TiO 2 modified adsorbent exhibited higher pore structure stability with the protection from TiO 2 on the surface even though the pore structure was partially blocked by the TiO 2 . The difference in pore structure stability was more evident with the increase of the Al content. As shown in Table S3, the surface area of sample C-1-4-3 decreased by 46.93%, while for sample Ti-1-4-3 decreased only by 14.02% after 3 cycles of hydrothermal treatment. This may be due to the TiO 2 coverage on the γ-Al 2 O 3 surface, hindering the collapse of pore structures and retarding the growth or reconstruction of γ-Al 2 O 3 crystals that can result in the formation of new Al-O-Al bonds. Moreover, all spectroscopy results indicated no formation of new bonds or phases during the cyclic hydrothermal treatments, implying that the TiO 2 protection layer could not only effectively prevent the formation of ZnAl 2 O 4 but also maintain stable during the reaction and regeneration processes. Thus, it will not cause the formation of Ti-O-Zn bonds and adverse effect on the activity of ZnO. Besides, although there is no evidence of Ti-O-Al formation during the reaction and hydrothermal treatments, the possibility for its formation can't be completely excluded. [START_REF] Saih | Ultra deep hydrodesulfurization of dibenzothiophene derivatives over NiMo/TiO 2 -Al 2 O 3 catalysts[END_REF][START_REF] Segawa | Development of new catalysts for deep hydrodesulfurization of gas oil[END_REF] There are a certain amount of defects on the surface of γ-Al 2 O 3 , which are associated with the exposed O vacancies and unsaturated coordinated Al 3+ sites. As has been shown in Figure 7, the peak intensity of the anatase phase became more intense with the increase of hydrothermal treatment cycles, indicating the growth of anatase crystals. As a result, the possibility of Ti-O-Al formation is increasing during the growth or reconstruction of the TiO 2 layer as the coordination unsaturated (CUS) Al 3+ sites on the γ-Al 2 O 3 surface are close to the TiO 2 layer (see Figure 9). The formation of Ti-O-Al could be in competition to the formation of Zn-O-Al (or Al-O-Al) by occupying the unsaturated coordinated Al 3+ sites, which could enhance the stability of the adsorbent. [START_REF] Ahmed | Structural and nano-composite features of TiO 2 -Al 2 O 3 powders prepared by sol-gel method[END_REF] The high structural stability of the TiO 2 modified adsorbent ensured that the ZnO can react with sulfur by preventing the formation of ZnAl 2 O 4 , thus achieving the superior activity of ZnO after the hydrothermal treatment cycles as compared with the unmodified one.

The existence of Lewis acid sites could provide empty orbitals on the adsorbent surfaces which could promote the adsorption of sulfur (bearing lone pair electrons) in the organic sulfur molecules (such as thiophene) and then facilitate the reaction. The Lewis acid sites on the γ-Al 2 O 3 surface originate mainly from the CUS Al 3+ sites caused by the -OH removal and phase transformations during the calcination process. [START_REF] Toledo | Effect of aluminum precursor on physicochemical properties of Al 2 O 3 by hydrolysis/precipitation method[END_REF][START_REF] Morterra | IR spectroscopic and microcalorimetric characterization of Lewis acid sites on (transition phase) Al 2 O 3 using adsorbed CO[END_REF][START_REF] Zaki | In situ FTIR spectra of pyridine adsorbed on SiO 2 -Al 2 O 3 , TiO 2 , ZrO 2 and CeO 2 : general considerations for the identification of acid sites on surfaces of finely divided metal oxides[END_REF] In the unmodified adsorbent, the Lewis acid amount decreased by 35.48% after the first cycle of hydrothermal treatment. Two main reasons for this result are suggested: (1) the interaction between Zn 2+ and surface CUS Al 3+ sites generating the ZnAl 2 O 4 spinel phase (see results presented in Figure 2 and3); as a result, the empty orbital of the CUS Al 3+ was filled, causing the loss of Lewis acidity; (2) the agglomeration (see in Table S3) and reconstruction of the support surface led to the formation of new Al-O-Al bonds and the loss of CUS Al 3+ sites from the surface. However, the Lewis acid amount recovered slightly (3.22%) after three cycles of hydrothermal treatment, which may be due to the dissociation of the γ-Al 2 O 3 surface structure under the high-temperature steaming environment. During the hydrothermal treatments, a dissolution-recrystallization process would occur on the surface of γ-Al 2 O 3 . The H 2 O would dissociate and interact with the surface Al 3+ by inserting H + into the Al-O-Al to form Al-OH that can result in the formation of defects and CUS Al 3+ after dehydration. [START_REF] Li | Tailoring the surface characteristic of alumina for preparation of highly active NiMo/Al 2 O 3 hydrodesulfurization catalyst[END_REF] This process could partially recover the acidity of the adsorbent, but could not reverse the decreasing trend of the Lewis acid amount. For TiO 2 modified adsorbent, the initial Lewis acid amount (2.1 mmol•g -1 ) was lower than that of the fresh unmodified one (3.1 mmol•g -1 ), but the TiO 2 modified adsorbent showed a lower decrease of Lewis acid amount (-23.81%) than the unmodified adsorbent (-35.48%) after the first hydrothermal treatment cycle. The TiO 2 protection layer impeded the formation of Zn-O-Al bonds which could maintain the Lewis acid sites on the surface. However, the coverage of the TiO 2 layer also blocked parts of the pores and restrained the exposure of the Lewis acid sites. Moreover, the underlying formation of Ti-O-Al would further decrease the total amount of Lewis acid sites on the surface. With increasing the hydrothermal treatment cycles, the new acid sites generated by TiO 2 offset the loss of Lewis acidity caused by TiO 2 coverage and Ti-O-Al formation which was not detected. As a result, the Lewis acid amount in the sample after three cycles of hydrothermal treatment was even 32.26% higher than that in the freshly TiO 2 modified adsorbent. This is because the hydroxyl ions and H + formed during hydrothermal treatments would not only affect the γ-Al 2 O 3 but also the TiO 2 . As been recognized previously, the hydrothermal treatment would also promote the intergrowth (crystallization) or reconstruction of the TiO 2 (anatase) layers, [START_REF] Hsiang | Effects of aging on the phase transformation and sintering properties of TiO 2 gels[END_REF] similar to what would happen to γ-Al 2 O 3 , the H + may also break the Ti-O-Ti(Al) and the hydroxyl ions may coordinate with the unsaturated Ti cations. As a consequence, the growth process will inevitably produce new or break Ti-O-Ti (Al) bonds and expose coordinatively unsaturated Ti cations (5 or 4 coordinated) with the dissociation of hydroxyl ions or Ti-O-Ti (Al) cleavage, [START_REF] Ahmed | Structural and nano-composite features of TiO 2 -Al 2 O 3 powders prepared by sol-gel method[END_REF] the unsaturated Ti cations would be the new Lewis acid sites. [START_REF] Ramírez | Analysis of the hydrotreatment of Maya heavy crude with NiMo catalysts supported on TiO 2 -Al 2 O 3 binary oxides: Effect of the incorporation method of Ti[END_REF][START_REF] Lee | Anatase TiO 2 nanosheets with surface acid sites for Friedel-Crafts alkylation[END_REF] Besides, the flexible chemical valence state of Ti would also be helpful for producing more Lewis acid sites on the surface.

The H 2 treatments in reduction and reaction processes would reduce the surface of TiO 2 and produce Ti (4-δ) . Afterward, the regeneration process (calcination) would oxidize the Ti 4+ to a higher state (Ti (4+α) ); both Ti cations exhibit Lewis acidity which would be helpful for the adsorption of organic sulfur compound. [START_REF] Ramírez | Analysis of the hydrotreatment of Maya heavy crude with NiMo catalysts supported on TiO 2 -Al 2 O 3 binary oxides: Effect of the incorporation method of Ti[END_REF][START_REF] Bhattacharyya | Role of the surface lewis acid and base sites in the adsorption of CO 2 on titania nanotubes and platinized titania nanotubes: An in situ FT-IR study[END_REF]

Conclusions

In this work, a series of TiO 2 modified Al 2 O 3 supports were synthesized by the wet impregnation method and used in the preparation of Ni/ZnO-Al 2 O 3 adsorbents via a heterogeneous-grinding method. Both the TiO 2 modified and unmodified adsorbents were evaluated in the RADS reaction using a model fuel with a high sulfur concentration (500 ppm).

By incorporating TiO 2 in the ZnO-Al 2 O 3 support to form a thin anatase protection layer, the formation of the ZnAl 2 O 4 spinel phase was effectively inhibited which significantly improved the stability of the final adsorbents. By generating new Lewis acid sites, the TiO 2 modified adsorbent maintained a good adsorption ability for organic sulfur compounds in feeds, which enhanced the catalytic activity of the adsorbent. After 3 cycles of hydrothermal treatments, the breakthrough sulfur capacity (at 10 ppm Sulfur) of the TiO 2 modified adsorbent kept at 7.8 mg•g -1 which is two times higher than that for the unmodified adsorbent (3.8 mg•g -1 ), even though the initial sulfur capacity of the fresh TiO 2 modified one is slightly lower than that in the unmodified one.

Hence, to modify the ZnO-Al 2 O 3 supports by the TiO 2 protection layer is a viable strategy to inhibit the formation of inactive spinel structures and improve the stability of the RADS adsorbent.

  NiO    ZnAl 2 O 4  ZnO          Internsity (a.u) 2 Theta ( o ) Ti-1-1-0 Ti-1-1-1 Ti-1-1-2 Ti-1-1-3  (a)
    2 Theta ( o ) Internsity (a.u) C-1-1-0 C-1-1-1 C-1-1-2 C-1-1-3  ZnO  ZnAl 2 O 4             NiO  (b)

  formation of spinel structures in adsorbents may be effectively inhibited by introducing TiO 2 into the Ni/ZnO-Al 2 O 3 adsorbents. The incorporation of TiO 2 into Al 2 O 3 will construct a TiO 2 protection layer between ZnO and Al 2 O 3 , which would isolate the ZnO from Al 2 O 3 . The inserted TiO 2 would competitively react with Al 2 O 3 to form Ti-O-Al bonds, inhibiting the formation of Zn-O-Al bonds. This work aims to propose a new method to inhibit the spinel formation in Ni/ZnO-Al 2 O 3 based RADS adsorbents via TiO 2 addition. The TiO 2 modified Al 2 O 3 support was synthesized by the wet impregnation method and used for the preparation of Ni/ZnO-Al 2 O 3 based

Figure 2 .

 2 All fresh adsorbents contain hexagonal wurtzite (ZnO, JCPDS card No. 01-079-2205) and cubic NiO (JCPDS card No. 01-078-0429) phases, and there are no obvious characteristic reflections of TiO 2 phase in the TiO 2 modified adsorbents, indicating that the TiO 2 is highly dispersed. For the unmodified adsorbents, the intensities of characteristic reflections assignable to cubic gahnite (ZnAl 2 O 4 , JCPDS card No.00-001-1146, 2θ= 44.83 o , 59.2 o and 65.20 o ) raised with the increasing cycles of hydrothermal treatment. In contrast, the peak intensities of ZnAl 2 O 4 in the TiO 2 modified adsorbents are much lower than those in the unmodified ones. To support the results revealing the interactions between ZnO and Al 2 O 3 leading to the stability enhancement by TiO 2 addition, the TiO 2 modified and unmodified adsorbents with Zn/Al molar ratios of 1:2 and 1:4 were tested by cyclic hydrothermal treatments (shown in Figure S2 and S3). The peak intensities of ZnAl 2 O 4 raised with the increase of the Al 2 O 3 amount which may be due to the higher dispersion of ZnO caused by the increase in Al 2 O 3 content, promoting the interactions between ZnO and Al 2 O 3 . While the TiO 2 modified adsorbents still exhibited lower peak intensities for the ZnAl 2 O 4 phase than the unmodified ones. These results directly prove that the addition of TiO 2 in the ZnO-Al 2 O 3 based adsorbents could effectively impede the interaction between the sulfur acceptor (ZnO) and the support (Al 2 O 3 ) to form the inactive ZnAl 2 O 4 spinel, which partially explains the high activity and large sulfur capacity of the modified adsorbents after hydrothermal treatments.

Figure 1 .

 1 Figure 1. RADS breakthrough (10 ppm) (a) curves and (b) sulfur capacities of TiO 2 modified

Figure 2 .

 2 Figure 2. Powder X-ray diffraction (PXRD) patterns of (a) TiO 2 modified and (b) unmodified

Figure 3 .

 3 Figure 3. UV-visible spectra of TiO 2 modified and unmodified supports with a Zn/Al molar

Figure 4 .

 4 Figure 4. SEM and EDS-mapping images of unmodified supports.

Figure 5 .

 5 Figure 5. SEM and EDS-mapping images of TiO 2 modified supports.

Figure 6 . 3 Figure 7 .

 637 Figure 6. N 2 adsorption-desorption isotherms and PSD curves of (a) and (b) unmodified

Figure 8 . 3 Figure 9 .

 839 Figure 8. FT-IR spectra of (a) unmodified adsorbents, (b) TiO 2 modified adsorbents and Py-

Table 1 .

 1 RADS performance of TiO 2 modified and unmodified adsorbents with a Zn/Al molar ratio of 1:1 at the breakthrough point (10 ppm Sulfur).An anatase protection layer is introduced into the Ni/ZnO-Al 2 O 3 adsorbents via the wet impregnation method to inhibit the formation of inert ZnAl 2 O 4 . The modified adsorbents show high stability and good desulfurization activity compared with the unmodified adsorbents. The TiO 2 protection layer also provides a certain amount of Lewis acid sites to enhance the adsorption ability of organic sulfur compounds.Keywords: Ni/ZnO-Al 2 O 3 adsorbents; Reactive adsorption desulfurization; Stability; ZnAl 2 O 4 spinel; TiO 2 . Bowen Liu, Peng Bai * Yang Wang, Zhun Dong, Pingping Wu, Svetlana Mintova and Zifeng Yan

	Samples Breakthrough time/ h	Sulfur capacity/ mg•g -1 ZnO conversion (%)
	Ti-1-1-0	7.00	10.5	7.04
	Ti-1-1-3	5.18	7.8	5.22
	C-1-1-0	8.10	12.1	7.62
	C-1-1-3	1.95	3.8	2.39

Highly stable Ni/ZnO-Al 2 O 3 adsorbent promoted by TiO 2 for reactive adsorption desulfurization

Acknowledgments

This work was financially supported by the National Key Research and Development Program of China (2017YFB0306600), the National Key Technologies R & D Program of China (2018YFE0118200), Natural Science Foundation of China (51601223, 21206195, U1510109), the Fundamental Research Funds for the Central Universities (17CX05018, 17CX02056, 14CX02050A, 14CX02123A), Shandong Provincial Natural Science Foundation (ZR2017MB003) and State Key Laboratory of Heavy Oil Processing Fund (SKLZZ-2017008).

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Supporting Information

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


Eng-Poh Ng
email: epng@usm.my

Hidayahni Nur

Ahmad

Fitri Khoerunnisa

Svetlana Mintova

Tau-Chuan Ling

T Jean Daou

Offretite zeolite single crystals crystallized using amphiphile-templating approach

Keywords: Hierarchical zeolites, Crystal growth, Offretite zeolite, Amphiphile-templating approach, Acylation of 2-methylfuran

Offretite zeolite synthesis in the presence of cetyltrimethylammonium bromide (CTABr) is reported for the first time. The offretite crystals exhibited novel hexagonal prismatic shape with high crystallinity after only 72 h of hydrothermal treatment at 180 o C which is considerably faster than the already known synthesis procedure used for the preparation of offretite. The study showed that CTABr has dual-functions during crystallization of offretite, viz. as structure-directing agent and as mesoporogen. The resulting offretite crystals (Si/Al ratio = 4.1) possess more acid sites than the conventional offretite thanks to its high crystallinity and hierarchical structure. The synthesized zeolite is also more reactive than its conventional counterpart in the acylation of 2-methylfuran for biofuel production under nonmicrowave instant heating condition, giving 83.5% conversion with 100% selectivity to the desired product 2-acetyl-5-methylfuran. Hence, this amphiphile synthesis approach offers another cost-effective and alternative route for crystallizing zeolite materials that require expensive organic templates.

Introduction

Zeolites are crystalline microporous materials made up of tetrahedral TO 4 (T = Si or Al) repeating units [1]. These porous solids are widely used in catalysis, separation and ion exchange processes due to their unique structure with defined size and shape [2][3][4]. 253 types of zeolite structures have been recognized by the International Zeolite Association (IZA) so far [START_REF]International Zeolite Association Web site[END_REF]. The synthesis of zeolites can be achieved by using various techniques such as organotemplating [START_REF] Catizzone | [END_REF]7], organotemplate-free [8,9], ionothermal [10,11], interconversion [12,13], ADOR (Assembly, Disassembly, Organization and Reassembly) [14,15] and fluoride routes [16,17].

Offretite zeolite (OFF topology) is a 12-membered ring zeolite (diameter 6.7  6.8

Å 2 ) with 3-dimensional channel system. It is considered as a very important zeolite due to its very large pore channels that allows ease molecular diffusion and accessibility [18]. To our Knowledge, offretite can only crystallize in the presence of tetramethylammonium (TMA + ) as structure-directing agent (SDA) and both NaOH and KOH as mineralizers [19]. However, this zeolite seldom crystallizes completely free from erionite (ERI) and analcime (ANA) intergrowth, making the synthesis of pure OFF phase a challenge [20].

Recently, a new strategy for synthesizing offretite has been reported using new SDAs (pyrocatechol [21], p-dioxane [22]). The resulting crystals possess different morphologies (oval, hexagonal, broccoli-like, prismatic) with merely microporosity thanks to the unique properties (e.g. size, shape, polarity, etc.) of the SDAs. Furthermore, offretite can also be readily crystallized via interzeolite conversion method using benzyltrimethylammonium hydroxide as organic template but the method is tedious and produces small amount of erionite as competing phase [23]. Thus, any initiative to crystallize offretite zeolite with high purity using facile synthesis approach is highly appreciated.

In the present work, we applied a combined rational strategy to synthesize large offretite single crystals with hierarchical porosity using a cationic amphiphile template (cetyltrimethylammonium bromide, CTABr) where the hydrophilic head of the amphiphile serves as a SDA for the growth of the zeolite while its hydrophobic tail generates mesoporosity allowing the formation of hierarchical zeolite (micro/mesoporosity). It should therefore have superior adsorption and catalytic properties than the conventional offretite. For comparison, classical TMA + -offretite zeolite was also synthesized using tetramethylammonium salt (TMACl) as a SDA [START_REF] Mintova | Verified Syntheses of Zeolitic Materials[END_REF] and the protonated form zeolite was prepared using the same protocol.

Characterization

The XRD patterns were recorded with a PANalytical X'Pert PRO diffractometer (Cu Kα radiation, λ = 0.15418 nm, 40 kV, 10 mA, step size = 0.02°, scan speed = 0.2 °/min, scanning range 2θ = 3°-30°). The morphological properties of zeolite samples were studied using a JEOL JSM-6701F FESEM microscope operating at 20 kV. The chemical composition (Si/Al ratio) of zeolite was estimated using IR spectroscopy technique [START_REF] Ma | [END_REF]. The porous properties of solids were studied using a Micromeritics ASAP 2010 instrument. Prior to measurement, the powder sample (ca. 0.07 g) was degassed at 300 °C for 12 h before N 2 adsorption was carried out at -196 °C. The total surface area was computed using the Brunauer-Emmett-Teller (BET) equation whereas the external surface area and micropore volume were calculated using the t-plot model. The total pore volume of the samples was determined by using the amount of N 2 uptake at the relative pressure (p/p o ) of 0.996. The acidity of samples was investigated with a BELCAT-B temperature programmed desorption (TPD) instrument. First, the powder sample (ca. 0.07 g) was outgassed at 400 °C overnight before the cell was saturated with NH 3 gas at 25 °C for 30 min. The non-adsorbed NH 3 was then evacuated before the sample was heated and desorb slowly from 25 C to 600 C with a heating rate 10 C/min.

Catalytic study

Offretite zeolite (0.300 g, 400 °C, 4 h), 2-methylfuran (4.7 mmol, Merck) and acetic anhydride (14.1 mmol, Merck) were first added into a 10-mL glass vial. The vial was inserted into a non-microwave instant heating reactor (Monowave 50) and instantly heated to 170 °C for 40 min; less than 2 min was required to reach up the desired temperature. After cooling, the reaction solution was isolated using microfiltration and analyzed with an Agilent's GCMS-5975 Turbo System and 7890A GC-FID.

Results and discussion

Initially, the crystallization was carried out at 180 °C for 40 h. The XRD results revealed that the solid product remained amorphous after 40 h of heating, indicating that the crystallization of offretite zeolite is an activated process which requires induction stage. At 72 h, the amorphous solid completely transformed into offretite zeolite and no other crystalline phase, such as erionite or merlinoite, was co-crystallized (Fig. 1b). When the XRD pattern of offretite zeolite synthesized using CTA + (CTA + -offretite) is compared with those of simulated and conventional synthesized ones, the XRD peaks intensity of CTA + -offretite are different due to the preferred crystal growth orientations (Fig. 1a,c) [26]. The crystallization process was also performed in the absence of CTABr. However, pure LTL-type zeolite was obtained showing the significant role of CTABr in directing the formation of offretite zeolite. The SEM images of offretite zeolites prepared using amphiphile templating route are shown in Fig. 2a,b. As seen, this method successfully produces offretite single crystals of hexagonal prismatic shape (inset of Fig. 2a). The surface of CTA + -offretite crystals at a-and b-directions are smooth indicating that the crystals are grown along the c-direction [27],

forming crystals of 10.8  1.4 µm 2 length size with an aspect ratio of 7.7. Interestingly, the morphology of the zeolite crystals is different from that of classical TMA + -offretite zeolite which exhibits rice grain-shape (ca. 4.6  3.4 µm 2 ) which composes of intergrown acicular primary nanocrystals (ca. 880  79 nm 2 ) (Fig. 2c,d). The smaller crystallite size of TMA +offretite zeolite agrees with the XRD peak broadening effect [28] (Fig. 1c). Offretite zeolite is very useful in the catalytic reactions due to its large pore size and considerably high Si/Al ratio (> 3). According to the IR spectroscopy analysis, the Si/Al ratio of CTA + -offretite zeolite is 4.1. Its molecular formula is calculated to be K 1) [START_REF]International Zeolite Association Web site[END_REF]. The textural properties of CTA + and TMA + synthesized offretite zeolites were studied using nitrogen adsorption-desorption analysis. The calcined CTA + -offretite solid exhibits a combination of type I and IV adsorption isotherms indicating the presence of micro-and mesoporosities in the solid [29] (Fig. 3a). As a result, multimodal pore size distribution is shown thanks to the role of CTABr as a mesoporogen besides acting as structure-directing agent for the formation of offretite zeolite (inset of Fig. 3a). The zeolite also has higher specific surface area (S BET = 523 m 2 /g), micropore surface area (S Mic = 503 m 2 /g) and total pore volume (V tot = 0.26 cm 3 /g) as compared to the conventional TMA + -offretite zeolite (S BET = 504 m 2 /g, S Mic = 491 m 2 /g, V Tot = 0.19 cm 3 /g), which strongly supports the high crystallinity of CTA + -offretite zeolite (Fig. 3b). [START_REF]The molecular size of the compounds were estimated using yperChem™-Release 8.0 for Windows Molecular Modeling System[END_REF]), directs the formation of OFF framework structure via restricted electrostatic interaction with the silicoaluminate anionic oligomers (Fig. 4). The inorganic intermediate species then enfold the CTA + hydrophilic head, thus inducing nucleation points for long-range propagation and crystallization of offretite zeolite. Secondly, the CTA + cation also serves as a porogen by forming irregular micelles, whereby the hydrophobic tails point to each other leading to the formation of mesopores of wide distribution [START_REF] Ali | [END_REF]. The calcined CTA + -offretite has surface acidity after converting into protonated form (Fig. 5). Hence, NH 3 -TPD analysis was used to study the acidity of the zeolite samples. As shown, both the CTA + -and TMA + -offretites have almost similar amount of weak (T des,150 °C = 0.18 mmol/g) and mild (T des,200 °C = 0.25 mmol/g) acid sites. Nevertheless, the CTA + -offretite contains much larger amount of strong acid sites (T des,350 °C = 0.32 mmol/g) than the TMA +offretite (0.68 mmol/g) which could be due to its higher crystallinity and larger accessible (due to hierarchical structure) surface area [32].

CTA + -offretite is suitable to be used as a solid acid catalyst. Hence, its acid properties are tested in the acylation of 2-methylfuran under novel non-microwave instant heating condition (170 o C, 40 min) where the desired product 2-acetyl-5-methylfuran is a useful biofuel additive. The conversion is very low (12%) when no catalyst is added indicating that acylation of 2-methylfuran is an activated reaction and hence requires a catalyst to overcome the activation energy [33] (Table 2, Entry 1). When hierarchical CTA + -offretite is added, the conversion increases tremendously to 83.5% with 100% selectivity towards 2-acetyl-5methylfuran (molecular size 6.6 × 4.1 Å 2 [START_REF]The molecular size of the compounds were estimated using yperChem™-Release 8.0 for Windows Molecular Modeling System[END_REF]) (Entry 2). The catalytic performance of CTA +offretite is also compared with that of classical TMA + -offretite using the same reaction conditions. As expected, when using CTA + -offretite, the reaction conversion (83.5%) is higher than that of TMA + -counterpart (78.3%) thanks to its larger surface acidity and the higher accessibility of the acid sites (Entry 4). Thus, the data shows the importance of hierarchical porosity for enhanced bulk molecule diffusivity and accessibility.

The catalytic activity of CTA + -offretite is also compared with those of conventional homogeneous catalysts (e.g. HCl, HNO 3 , CH 3 COOH). The results show that CTA + -offretite has comparable catalytic activity to HNO 3 (83.6% conversion) and it exhibits even better catalytic activity than HCl (72.3%) and CH 3 COOH (47.1%) after 40 min of reaction (Entries 5-7). Most importantly, the CTA + -offretite zeolite shows high catalyst reusability after the catalytic reaction where no significant loss in catalytic reactivity is observed even after 5 consecutive runs (Entry 3). Thus, the strategy of synthesizing offretite zeolite with hierarchical structure by amphiphile-templating synthesis route is successful offering safer, cheaper, facile and scale-up feasibility comparing to the previous works [34].

Conclusion

In conclusion, offretite zeolite with high crystallinity has been successfully synthesized via amphiphile-templating approach. By using cetyltrimethylammonium bromide (CTABr) as a promising structure-directing agent (SDA), where its hydrophilic head has nearly the same dimensions as tetramethylammonium (TMA + ), offretite zeolite crystals with hexagonal prism shape (10.8  1.4 µm 2 ) are crystallized at 180 °C after 72 h of hydrothermal treatment. The resulting zeolite has larger surface areas, pore volume and pore sizes thanks to CTABr that also serves as a mesoporogen. Furthermore, CTA + -offretite (0.74 mmol/g) has higher acidity than the conventional TMA + -offretite (0.68 mmol/g) and its acidity and accessible porosity are highly appreciated in the acylation of 2-methylfuran with 83.5% conversion and 100% selectivity to the desired product after 40 min reaction at 170 °C under non-microwave instant heating conditions. Most importantly, the CTA + -offretite catalyst is reusable and more active than conventional TMA + -offretite and homogeneous acid catalysts.

Hence, the current study is beneficial for zeolite community as this amphiphile-templating strategy using other amphiphilic compounds can be applied in the preparation of many important zeolites that are normally synthesized using harmful and expensive organic templates.

Fig. 1

 1 Fig. 1 XRD patterns of (a) theoretical offretite, (b) CTA + -offretite and (c) TMA + -offretite

Fig. 2

 2 Fig. 2 SEM images of (a, b) CTA + -offretite and (c,d) TMA + -offretite zeolites under different

Fig. 3

 3 Fig. 3 Nitrogen adsorption-desorption isotherms and (inset) pore size distributions of (a) CTA + -offretite and (b) TMA + -offretite zeolites.

Fig. 4

 4 Fig. 4 Formation mechanism of offretite zeolite templated by CTA + cation. K + cations are not

Fig. 5

 5 Fig. 5 NH 3 -TPD profiles of (a) CTA + -offretite and (b) TMA + -offretite zeolites.

  3.4 Al 3.4 Si 14 O 36 which is very close to those of TMA + -offretite synthesized in the presence of KOH and NaOH mineralizers (Na 1.7 K 1.7 H 0.6 Al 4 Si 14 O 36 , Si/Al = 3.5) and theoretical one (Na 0.2 K 0.9 Al 4 Si 14 O 36 , Si/Al = 3.50) (Table

Table 1

 1 Properties of offretite zeolites.

								NH 3 -TPD acidity (mmol/g)
	Samples	Si/Al	S BET (m 2 /g)	S Mic (m 2 /g)	S Ext (m 2 /g) V Tot (cm 3 /g)
							T des,150 °C T des,200 °C T des,350 °C	Total
	CTA + -offretite	4.1	523	503	20	0.26	0.18	0.24	0.32	0.74
	TMA + -offretite	3.5	504	491	13	0.19	0.17	0.26	0.25	0.68

Table 2

 2 Catalytic performance of catalysts in acylation of 2-methylfuran at 170 °C under non-microwave instant heating conditions.

	Entry	Catalysts	10 min	Conversion (%) 20 min	40 min
	1	No catalyst	3.2	6.4	12.0
	2	CTA + -offretite	52.6	72.3	83.5
	3	CTA + -offretite a	50.2	68.0	79.1
	4	TMA + -offretite	44.7	61.7	78.3
	5	HCl	35.7	55.3	72.3
	6	HNO 3	51.1	71.9	83.6
	7	CH 3 COOH	16.0	31.9	47.1

a After 5 th run.

Acknowledgement

The financial support from RUI (1001/PKIMIA/8011128) and Institut Universitaire de France (IUF) is acknowledged.