Chunzheng Wang

Ningkun Xu

Ting-Ting Liu

Weisong Xu

Hailing Guo

Yanpeng Li

Peng Bai

Xin-Ping Wu
email: xpwu@ecust.edu.cn

Xue-Qing Gong

Xinmei Liu

Svetlana Mintova
email: svetlana.mintova@ensicaen.fr

Mechanical pressure-mediated Pd active sites formation in NaY zeolite catalysts for indirect oxidative carbonylation of methanol to dimethyl carbonate

Keywords: Mechanical pressure, Active site, Palladium, NaY, Carbonylation, Dimethyl carbonate

teaching and research institutions in France or abroad, or from public or private research centers.   Mechanical pressure-mediated Pd active sites formation in NaY zeolite catalysts for indirect oxidative carbonylation of methanol to dimethyl carbonate

Introduction

In commercial applications, heterogenous catalysts are seldom utilized in a powder form, and thus the catalyst shaping is indispensable to be implemented in industrial reactors [13]. Several shaping methods are used, among which the most important are extrusion, spray drying, tableting and granulation [4,5]. Among these methods, the tableting method produces the most regularly shaped bodies such as pellets and cylinders, which often have high mechanical strength [6,7]. However, the tableting process is often conducted by trial-and-error approach [8]. The tableting method is often proprietary, and the related patents provided a range of shaping conditions and rarely disclosed any details [START_REF] Ertl | Handbook of Heterogeneous Catalysis[END_REF][START_REF] Bobadilla | [END_REF]. In most cases, there is a huge difference in the performance of catalysts in powder and the shaped forms [11].

Therefore, understanding the chemical and structural features of catalysts behind tableting process is desirable for both fundamental research and commercial applications.

High-pressure inorganic chemistry is one of the "Top Ten Emerging Technologies in Chemistry 2020" reported by the International Union of Pure and Applied Chemistry (IUPAC). Previous high-pressure studies of zeolites revealed unusual structural and chemical changes [1214]. Several zeolites were amorphized under high pressure, i.e., pressure-induced full or partial amorphization reverting back to the original crystalline structure upon decompression [15,16]. The partly disordered structure resulted from the local collapse and distortion of zeolite framework, which was closely related to the properties of charge-balancing cations in the zeolites [17]. Lee et al. reported that high pressure of 1.9 GPa led to the irreversible migration and rearrangement of charge-balancing cations, i.e., pressure-induced migration, and to the formation of super-hydrated zeolite with a doubled water content described as pressure-induced hydration [18]. They also suggested that the structural changes of zeolites under high pressure were mediated by the charge-balancing cations and their coordination environment, which could drive the changes in the order-disorder/disorder-order transitions and framework distortion [19]. Jordá et al. demonstrated that ITQ-29 zeolite with LTA topology was transformed into a new ITQ-50 zeolite under a high pressure of 3.2 GPa [20]. The behavior of metal-containing zeolite catalysts under high pressure has not been systematically investigated. It is worthwhile to study whether the catalytic performance of metal-containing zeolite catalysts is dependent on the mechanical pressure, and if so, to what extent.

Green and sustainable manufacture of valuable commercial chemicals based on C1 chemistry via one-carbon feedstocks from abundant carbon resources such as shale gas and coal, is an emerging alternative to the petroleum routes [2123].

Dimethyl carbonate (DMC) is a green, versatile compound which can be used as a methylating and methoxycarbonylating agent, fuel additive, electrolyte liquid carrier for lithium-ion batteries, as well as for polycarbonate and isocyanate syntheses [24,25]. The traditional production of DMC ((CH 3 O) 2 CO) involves the utilization of a toxic feedstock, phosgene (COCl 2 ) [26]. In the past decade, extensive studies have been carried out to develop environmentally sustainable and efficient processes for DMC production including the transesterification of methanol (CH 3 OH) with ethylene/propylene carbonate, the direct synthesis from carbon dioxide (CO 2 ) and CH 3 OH, the alcoholysis of urea, and direct/indirect oxidative carbonylation of CH 3 OH [2729]. Among these methods, the indirect oxidative carbonylation of methanol to DMC has gained substantial interest due to the cheap raw materials, mild operation conditions and high atom efficiency [30]. This process involves two separate reactions presented in Scheme S1 in Supporting Information (SI): [START_REF] Mitchell | [END_REF] Pd-catalyzed oxidative carbonylation to DMC: CO + 2CH 3 ONO  (CH 3 O) 2 CO +2NO, and (2) non-catalytic methyl nitrite (CH 3 ONO) regeneration with recycled NO:

2CH 3 OH + 2NO + 1/2O 2  2CH 3 ONO + H 2 O. The overall reaction (CO + 2CH 3 OH + 1/2O 2  (CH 3 O) 2 CO + H 2 O
) is a green and efficient process converting C1 feedstocks (CO and CH 3 OH) to C2 product.

The gas-phase indirect oxidative carbonylation of methanol to DMC has been industrially implemented by Ube Industries, Ltd., Japan, using a PdCl 2 /active-carbon catalyst [31,32]. However, chlorine such as hydrogen chloride (100 L L 1 ) must be added into the reactants to prevent the Pd sintering and maintain the activity of chlorine-containing catalyst [32,33]. There is an incentive to eliminate chlorine from this process to reduce costs associated with equipment corrosion, chlorine regeneration and generation, product purification and emissions. Consequently, the development of chlorine-free, high-performance Pd catalysts is highly desirable for the DMC synthesis. Various chlorine-free Pd catalysts have been investigated, and the most appropriate catalyst support was NaY zeolite in comparison to NaX, -Al 2 O 3 , -Al 2 O 3 , silica, and MgO [31,34,35]. Guo et al. revealed that the copper promoter could improve the Pd dispersion and maintain the Pd 2+ active sites on the Pd/NaY catalyst [36]. Dong et al. reported that potassium-doped Pd/NaY catalyst enhanced the activation of CO reactant by forming a Pd-COK + structure [37]. They also revealed that Lewis acidity of the Pd/NaY catalyst favored the DMC formation, while Brønsted acidity could accelerate the undesired decomposition of CH 3 ONO reactant into methyl formate and dimethoxymethane byproducts [34]. Tan et al. found that the catalytic activity of the Pd/NaY catalyst was related to the high oxidation state of Pd species, which could be tuned by the organic ligands [38]. They also demonstrated, by performing density functional theory (DFT) calculations, that the mononuclear-isolated Pd 2+ centers are critical for high DMC selectivity while the aggregation of Pd species was responsible for the formation of dimethyl oxalate byproduct [35]. The possible mechanism of the reaction followed oxidative addition and reductive elimination, and consisted of (1) oxidization of Pd 0 species by methyl nitrite (CH 3 ONO) reactant to generate NO-Pd 2+ -OCH 3 , (2) insertion of adsorbed CO into NO-Pd 2+ -OCH 3 to form NO-Pd 2+ -COOCH 3 , and (3) the reaction between NO-Pd 2+ -COOCH 3 and NO-Pd 2+ -OCH 3 to yield DMC [33,39]. The Pd 2+ species are generally considered to be the active sites for the formation of DMC [30,33,35]. In view of the redox cycle of active sites, the function of Pd 0 species is still under discussion. Yamamoto et al. reported that Pd/NaY catalyst showed a delayed deactivation and decreased by about 50% after 300 h test [40]. Tan et al. found that Pd/NaY catalyst with mononuclear-isolated Pd 2+ centers was stable for at least 100 h [35], but the selectivity of inevitable byproducts such as methyl formate and dimethoxymethane was not disclosed. Recently, we reported on the preparation of stable Pd/EMT catalyst tested for 130 h. We found that the EMT zeolite framework could inhibit the palladium sintering and maintain the Pd 2+ active sites [41]. The chlorine-free Pd-based catalysts are still under development, and the bottleneck for their industrialization is the lack of sufficiently active and stable catalysts.

Herein, we presented the mechanical pressure-mediated preparation of Pd-containing NaY zeolite catalyst (Pd/NaY) by a tableting method. The Pd/NaY catalyst was tested in the reaction of indirect oxidative carbonylation of methanol to DMC. The structural changes that took place during the mechanical compression of the Pd/NaY catalyst and their effects on the catalytic performance in the DMC synthesis were disclosed.

Experimental

Catalyst preparation

NaY zeolite was kindly supplied by Daqing Chemical Research Center, China.

Prior to the ion exchange, the NaY zeolite was pre-activated at 550 ºC for 2 h under static air. In a typical process, 16.7 mg palladium chloride (PdCl 2 , Sinopharm Chemical Reagent Co., Ltd., China) was dissolved in 1.2 mL, 0.94 mol L 1 ammonia (NH 3 ) solution to form a clear palladium ammonia complex (Pd(NH 3 ) 4 Cl 2 ) solution.

The Pd(NH 3 ) 4 Cl 2 solution was added dropwise into the activated NaY zeolite slurry containing 1.0 g zeolite and 100.0 g deionized water. The resulting suspension was stirred at room temperature (25 ºC) for 12 h and subsequently was heated at 80 ºC to evaporate the excess ammonia until the pH value of the suspension decreased to 7.07.2 [35,36]. Then the solid was collected by vacuum filtration and washed thoroughly with deionized water. After drying at 80 ºC for 6 h, the sample was calcined at 200 ºC for 2 h under static air. The as-prepared Pd/NaY catalyst had a Si/Al molar ratio of 2.8 and a Pd loading of 1.0 wt%, corresponding to roughly 1.2 Pd atoms per unit cell, which was determined by inductively coupled plasma-atomic emission spectroscopy (ICP-AES).

As known, the shaping pressure applied for commercial catalysts is in the range of 1001000 MPa using the tableting process [START_REF] Ertl | Handbook of Heterogeneous Catalysis[END_REF]. To study the effect of the mechanical pressure, the as-prepared Pd/NaY catalyst was subjected to treatment under pressure in the range of 60600 MPa (Fig. S1). After the mechanical compression, the samples were crushed and sieved into 6080 mesh. The as-obtained catalysts in pellet form were denoted as Pd/NaY-x, where x corresponds to the mechanical pressure (an example, the catalyst shaped at 300 MPa was named as Pd/NaY-300).

Characterizations

Transmission electron microscopy (TEM) images of the catalysts were recorded using a JEOL JEM-2100 TEM instrument. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images were taken on a JEOL JEM-2100F STEM instrument. The chemical composition of the samples was determined by an energy dispersive X-ray spectrometer (EDS). Scanning electron microscopy (SEM) images were acquired on a JEOL JSM-7900F SEM instrument.

The Pd loading and Si/Al molar ratio of the samples were measured by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) on a Thermo IRIS Intrepid II XSP. The X-ray photoelectron spectroscopy (XPS) spectra were recorded on an Escalab 250xi spectrometer with an Al Kα radiation. All binding energies were referenced to the C 1s peak at 284.6 eV. The solid-state 29 Si cross-polarization magic-angle spinning nuclear magnetic resonance (CP MAS NMR), and single pulse 29 Si MAS NMR spectra were measured on a Bruker Advance 400 spectrometer. The chemical shifts were referenced to the kaolin (91 ppm).

X-ray diffraction (XRD) patterns of the catalysts were obtained using a Bruker D8

Advance diffractometer with a Cu K radiation. The relative crystallinity of the samples was determined by measuring the sum of eight Bragg peak areas, namely (331), (511), (440), (533), (642), (822), ( 555) and (664), and related to the peaks area of the reference sample [42]. The Pd/NaY powder catalyst before the mechanical compression was taken as a reference assigning 100% of crystallinity. After subtracting the background and k2 line broadening from the obtained patterns, the average crystallite sizes were estimated using the Scherrer equation [43].

The unit cell parameter () of the samples was determined from the (555) reflection, using pure silicon as an internal standard [42]. The framework Si/Al molar ratio of the samples was estimated from the unit cell parameter () by the following equation [42]: i Al ratio 25. 5 2 .

Textural features of the samples were analyzed by N 2 physisorption isotherms measured at 196 ºC using a Quantachrome Autosorb-IQ3 instrument. The specific surface area was calculated using the Brunauer-Emmett-Teller (BET) method; the pore size distribution was determined by the DFT model from the adsorption branch of the N 2 sorption isotherms.

In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS)

experiments with a carbon monoxide (CO) probe molecule were performed in a

Harrick Scientific HVC-DRP-4 cell fitted with ZnSe windows using a Bruker Vertex 70 spectrometer, equipped with a mercury-cadmium-telluride (MCT) detector. The sample temperature was monitored by a K-type thermocouple welded onto the top center of the cell. Typically, about 0.002 g sample without diluents was pre-treated in N 2 flow (80 mL min 1 ) at 200 ºC for 1 h, and then cooled down to 30 ºC for taking a background spectrum. The adsorption of CO was conducted at 30 ºC using a gas mixture of 10 vol% CO in N 2 . After that, the sample was purged thoroughly with pure N 2 and then the DRIFTS spectra were recorded at 30 ºC and 50 ºC. Each spectrum was obtained using the Kubelka-Munk mode from 4000 cm 1 to 600 cm 1 (4 cm 1 resolution and 32 scans).

Reactivity tests

The The 99.99 vol% N 2 , gas mixtures of CO/N 2 (1/9, mole) and CH 3 ONO/N 2 (1/4, mole)

were fed via three calibrated mass flow meters, respectively. The gas hourly space velocity (GHSV) and reaction temperature were varied in the range of 4000800000 mL g cat. 1 h 1 and 100120 ºC, respectively. Analysis of product stream was conducted using an online gas chromatography (Agilent 7890B).

Results and discussion

General properties of Pd/NaY catalysts prepared under treatment with different mechanical pressures

The morphologies of the Pd/NaY catalysts prepared under treatment with different mechanical pressures are presented in Fig. 1. The Pd/NaY-60 catalyst consisted of aggregated crystals with a size of 300500 nm (Fig. 1a). With increasing the mechanical pressure from 60 MPa to 150 MPa, the mechanical compression led to the fragmentation of several zeolite crystals, and consequently, sporadic small crystals were found on the Pd/NaY-150 catalyst as marked with red circles in Fig. 1b. With further increasing the mechanical pressure, more and more primary zeolite crystals were fragmented into small pieces as observed by SEM (Fig. 1ce). In line with the SEM, the TEM pictures revealed the gradual decrease of average crystal size from 370 nm to 180 nm with enhancing the mechanical pressure (Table 1). Even though the Scherrer equation is not highly accurate when the crystal size is larger than 100 nm [44], the crystal size estimated based on the XRD study showed a similar trend to the b Size calculated using the Scherrer equation.

c Size evaluated by TEM analysis.

d Unit cell parameter () determined from the (555) reflection, and the framework Si/Al molar ratio estimated from the unit cell parameter ().

e Si/Al molar ratio determined from the XPS spectra.

f Apparent activation energy (E a ) calculated from the Arrhenius plots (Fig. S2).

g TOF based on the CO conversion and surface Pd atoms calculated by decreasing the CO conversion below 10 % with GHSV of 800000 mL g cat.

1 h 1 at 110 ºC.

With increasing the mechanical pressure from 60 MPa to 300 MPa, the catalysts still had a high crystallinity (93%98%), indicating that the mechanical compression caused a negligible damage of the zeolite framework (Fig. 2). However, the relative crystallinity was only 78% with further increasing the mechanical pressure up to 600 MPa (Fig. 2). The obvious decrease in crystallinity was attributed to the pressure-induced amorphization [15,16], suggesting that more framework defects and amorphous aluminosilicate parts were formed in the Pd/NaY-450 and Pd/NaY-600 catalysts. These five catalysts showed almost the same unit cell parameter of 2.4632.464 nm calculated using the silicon internal standard method (Table 1) [42].

The Si/Al molar ratios measured by XRD and XPS were determined to be 2.72.8 on all five catalysts (Table 1), which was consistent with that of 2.8 measured by ICP-AES on the Pd/NaY catalyst before the shaping. In addition, the Bragg peaks of PdO (JCPDS 88-2434) and Pd 0 (JCPDS 01-1201) were not observed on all five catalysts (Fig. S3), which might be due to the high Pd dispersion and/or low Pd loading. The N 2 adsorption isotherms of all five catalysts exhibited the typical profile of microporous material (type Ⅰ) with a high adsorption capacity at low P/P 0 (Fig. 3a) [45]. All five catalysts showed similar pore size distribution consisting of micropores of about 0.74 nm corresponding to the FAU-type zeolite, and mesopores of 4.1 nm originated from the aggregation of zeolite crystals (Fig. 3b) [46]. This suggested that the mechanical compression had a negligible effect on the pore size distribution of the catalysts. As shown in Table 1, the Pd/NaY-60, -150 and -300 catalysts presented similar surface area of 841893 m 2 g 1 and micropore volume of 0.300.32 cm 3 g 1 .

When the mechanical pressure increases to 600 MPa, the surface area and volume of the Pd/NaY-600 catalyst decreased to 732 m 2 g 1 and 0.26 cm The 29 Si CP MAS NMR spectra of the catalysts prepared under different shaping pressures were recorded to examine the structural deformation, and the results are shown in Fig. 4. The spectra of all samples contained four resonance peaks at 88, 94, 100 and 105 ppm corresponding to Q 1 , Q 2 , Q 3 and Q 4 species, respectively [47,48]. The resonance peak at 88 ppm of the Pd/NaY-600 catalyst appeared with enhanced intensity in the 29 Si CP MAS NMR spectrum in comparison to the 29 Si MAS NMR (Fig. S4). The existence of HSi dipolar interaction due to the presence of framework defects (silanols) in the Q 1 species is indicated [4951]. As shown in Table 2, the fraction of Q 1 species was gradually increased from 7.2% to 16.7% with increasing the shaping pressure applied to the catalysts from 60 MPa to 600 MPa. This confirmed that more framework defects (Q 1 species) in the catalysts were formed under treatment at high shaping pressure, which was consistent with the XRD and N 2 sorption results. a Fraction of Si species determined based on the peak areas in the solid-state 29 Si CP MAS NMR spectra presented in Fig. 4.

Catalytic performance of Pd/NaY catalysts prepared under treatment with different mechanical pressures

The catalytic activities of the series of Pd/NaY catalysts in the indirect oxidative carbonylation of methanol to DMC are presented in Fig. 5. When the mechanical pressure was less than 300 MPa, the CO conversion of about 90.4% was preserved while the DMC selectivity increased progressively from 74.4% to 80.4%. At the 20 mechanical pressure larger than 300 MPa, the CO conversion declined from 90.4% to 78.3% while the DMC selectivity of about 80% was preserved. In addition, the DMC selectivity using the Pd/NaY-60, -150, -300, -450 and -600 catalysts gradually increased, while the selectivity of methyl formate, dimethoxymethane byproducts progressively decreased with prolonging the reaction time from 1 h to 6 h (Fig. S5).

For example, the selectivities of DMC, methyl formate and dimethoxymethane were 75.7%, 13.7% and 10.6% after 1 h catalytic test, respectively. After 6 h, the selectivities of DMC, methyl formate and dimethoxymethane using the Pd/NaY-300 catalyst were 80.4%, 10.9% and 8.6%, respectively (Fig. S5). The increase of DMC selectivity with prolonging the reaction time would be discussed in posterior XPS analysis. To study both internal and external diffusion, the kinetic experiments were conducted, and the results are shown in Fig. S6 and Table S1. The GHSV was in the range of 400024000 mL g cat. 1 h 1 for the Pd/NaY-300 catalyst with different amounts of 0.1000 g and 0.1600 g; a similar CO conversion under the same GHSV was measured (Fig. S6). This suggested that the external diffusion effect could be neglected with a GHSV larger than 4000 mL g cat. 1 h 1 [52,53]. The Thiele modulus was determined to be 0.156, 0.126, 0.113, 0.086 and 0.075 for the Pd/NaY-60, -150, -300, -450 and -600 catalysts, respectively (Table S1 and details are provided in SI).

All the effectiveness factors approached the value of 1 and thus the DMC synthesis could be regarded as surface-limited reaction (Table S1) [54,55]. In addition, a thermocouple was placed in the middle of the catalyst bed for monitoring the reaction temperature during the catalytic tests, and no temperature rise ( 1 ºC) was detected between the catalyst bed and the reactor during the kinetic experiments. Therefore, the heat and mass transfer should have a negligible effect on the reaction rate.

The apparent activation energies for the indirect oxidative carbonylation of methanol to DMC were measured in the temperature range of 100120 ºC with an interval of 5 ºC. As presented in Table 1, these five Pd/NaY catalysts showed comparable apparent activation energies of 5567 kJ mol 1 under the same reaction conditions (Fig. S2). This indicated that the dominant reaction pathway and the rate control step were identical for these catalysts [56,57]. Furthermore, the CO conversion was decreased below 10% at 110 ºC and GHSV of 800000 mL g cat. 1 h 1

(details provided in SI). The intrinsic activity, i.e. the TOF decreased from 0.25 s 1 to 0.08 s 1 , with increasing the shaping pressure from 60 MPa to 600 MPa (Table 1).

This was possibly related to the amorphization of zeolite support after the mechanical treatment.

Understanding the formation of mechanical pressure-mediated active sites in

Pd/NaY catalysts TEM study of the used Pd/NaY catalysts prepared under treatment with different mechanical pressures was conducted to evaluate the size and distribution of palladium.

As described previously, the Pd/NaY catalyst was prepared by an ion-exchange method with Pd precursor of Pd(NH 3 ) 4 2+ ; and the low-temperature calcination at 200 ºC caused the Pd(NH 3 ) 4 2+ to lose partially the NH 3 ligands and to generate the Pd(NH 3 ) n 2+ (n  2) sites on the catalyst [41]. Neither Pd clusters (below 2 nm) nor nanoparticles were observed on the fresh Pd/NaY-60 catalyst before the catalytic test (Fig. S7). While the used Pd/NaY-60 catalyst after 6 h test contained a mixture of both small Pd clusters (about 1 nm) and large Pd nanoparticles (Fig. 6a1,a2). The average size of the Pd nanoparticles was estimated to be 13.1 nm based on measuring about 200 nanoparticles (inset of Fig. 6a1). The TEM images of fresh and used Pd/NaY-60 catalysts indicated that the large Pd nanoparticles were formed because of the catalytic test. This was in line with the previous observation revealing the palladium sintering on the Pd/NaY catalyst after the catalytic test [41]. The used Pd/NaY-150 catalyst contained Pd nanoparticles of 6.3 nm that were remarkably smaller than the counterparts on the Pd/NaY-60 catalyst (Fig. 6a1,b1). Interestingly, no big Pd nanoparticles were detected on the used Pd/NaY-300 catalyst and only mono-dispersed Pd clusters of about 1 nm confined within the zeolite framework were identified (Fig. 6c1,c2). The used Pd/NaY-450 was similar to the used Pd/NaY-300 catalyst with the exception that sporadic large Pd nanoparticles of about 20 nm could be seen in the TEM pictures (Fig. 6c1,d1). Bigger Pd particles up to 110 nm were detected on the used Pd/NaY-600 catalyst (Fig. 6e1). The XRD pattern of this sample contained the Bragg peaks corresponding to the metallic Pd 0 (111) and Pd 0 (200) (JCPDS 01-1201) with a non-homogeneous dispersion (Fig. S3). As stated above, the mechanical compression resulted in the formation of framework defects, such as collapse of micropore, framework distortions, fragmentation of primary zeolite crystals, and even amorphization of zeolites (Table 1) [16,19]. With increasing the mechanical pressure from 60 MPa to 300 MPa, the average size of the Pd nanoparticles decreased from 13.1 nm to 6.3 nm, but only mono-dispersed Pd clusters (about 1 nm) were detected on the Pd/NaY-300 catalyst (Fig. 6a1c1,a2c2). It was thus reasonable to infer that the framework defects possibly played an important role in hindering the aggregation of Pd clusters. This was in line with the report disclosing that the zeolite defects served as hosting place for the Pd species, thus separating them from each other and preventing their growth [5860]. At the mechanical pressure larger than 300 MPa, more amorphous aluminosilicate areas were formed, and thus possibly led to sintering of the Pd and formation of bigger metal particles (Fig. 6e1) [35,58]. As stated above, the Pd 2+ species were generally believed to be the active sites for the formation of DMC [35,39]. Therefore, XPS study was performed to check the effect of mechanical pressure on the electronic properties of Pd species. As shown in Fig. 8A, the XPS spectra confirmed the presence of Pd, Si, Al, Na, O and C elements on all five catalysts. The Pd 3d regions of these catalysts exhibited the 3d 5/2 and 3d 3/2 peaks with a spin-orbit splitting of 5.3 eV; only the Pd 3d 5/2 peak was discussed herein (Fig. 8). Generally, Pd 0 was characterized by a Pd 3d 5/2 peak at 334.8335.6 eV, and it shifted to higher binding energy with the increase of palladium oxidation state.

The peaks detected at 336.4337.3 eV and 338.0338.9 eV corresponded to the Pd 2+ and Pd 4+ species, respectively [57,6164]. Consequently, the peaks at 336.1 eV and 338.0 eV were assigned to Pd + and Pd (2+)+ species (0    2) on all five catalysts (Fig. 8B); similar observations had been reported previously [65,66]. The XPS peak area ratio corresponding to the Pd (2+)+ /Pd + species for these five catalysts decreased remarkably from 1.16 to 0.45 with increasing the mechanical pressure from 60 MPa to 600 MPa applied to the catalysts (Table 3). Generally, the charge of Pd species increases with decreasing the particle size based on the intrinsic size effect [61,67].

Herein, the average Pd charge monotonically decreased with increasing the mechanical pressure (Table 3), while the Pd particle size distribution showed a volcano-like behavior according to the TEM observations (Fig. 6). The lack of correlation between the Pd particle size and the charge could be explained as follows:

(1) The Pd (2+)+ species possibly result from the Pd clusters and nanoparticles charged by the zeolite framework through the formation of PdO ionic bonds [35,40] and (2) Lewis acid sites of zeolite support are considered to be electron acceptors and thus make the Pd species lose the electrons [34]. The increase of the mechanical pressure caused the formation of more framework defects and partial amorphization, which inhibited the formation of the Pd (2+)+ species and thus decreased the Pd (2+)+ /Pd + ratio [68]. ). This indicated that the Pd (2+)+ /Pd + ratio played a critical role in the DMC selectivity which would be further discussed in posterior Section 3.4.

Fig. 9.

The relationship between the area ratio of Pd (2+)+ /Pd + and DMC selectivity of the used catalysts (Pd/NaY-60, -150, -300, -450 and -600) after 1 h and 6 h catalytic tests (Pd (2+)+ /Pd + ratio shown in Table 3).

Combining the TEM and XPS results, it could be concluded that the highly dispersed Pd species contributed to the high CO conversion, while the low Pd (2+)+ /Pd + ratio led to the high DMC selectivity for the indirect oxidative carbonylation of methanol to DMC.

Correlation between the Pd (2+)+ /Pd + ratio and DMC selectivity: Enhanced adsorption of reactants

Carbon monoxide is not only a common probe molecule but also the reactant for the DMC synthesis. The DRIFTS spectra of CO adsorbed on the used Pd/NaY catalysts were presented in Fig. 10. The band at 2103 cm 1 was assigned to the linear CO adsorption over Pd + species (0    2), and the band at 2140 cm 1 corresponded to the linear CO adsorption on Pd (2+)+ species [69,70]. With increasing the mechanical pressure of the catalysts from 60 MPa to 600 MPa, the band corresponding to Pd (2+)+ species declined while the band related with Pd + species gradually increased (Fig. 10a). This was consistent with the evolution of the Pd (2+)+ /Pd + ratio determined by XPS (Fig. 8 and Table 3). When the temperature of the DRIFTS reaction cell was increased from 30 ºC to 50 ºC, the band corresponding to Pd (2+)+ species decreased noticeably while the intensity of the band corresponding to Pd + species was well preserved. This suggested that CO adsorption on the Pd (2+)+ species was weaker than on the Pd + species, which was due to the weak  back-donation [57]. This finding further indicated that the change of the Pd (2+)+ /Pd + ratio could affect the adsorption of CO reactant on these catalysts. thoroughly purged with N 2 . After that, the spectra were obtained at 30 ºC and 50 ºC.

In order to further understand the effect of the Pd 1). Furthermore, 12 Na atoms were added into the cell based on the reported distribution, i.e., 17% in site Ⅰ (~2 Na atoms), 29% in site I' (~ Na atoms) and 5 % in site Ⅱ (~6 Na atoms) [71,72]. A Pd 13

cluster reported by a previous study [37] was embedded into the supercage of NaY (Pd 13 @NaY) to model the synthesized Pd/NaY catalysts and the optimized structure was shown in Fig. 11a. Since the synthesized Pd/NaY catalysts had mixed valence states of (2+)+ and +, we also constructed Pd 13 O 8 @NaY and Pd 13 O 10 @NaY models by adding eight or ten oxygen atoms onto the Pd 13 cluster of Pd 13 @NaY (Fig. 11). To locate the stable structures of Pd 13 O 8 @NaY and Pd 13 O 10 @NaY, we carried out a molecular dynamics (MD) simulation of ~ 25 ps (time step: 1 fs) on each of these two systems in the canonical (NVT) ensemble with T = 137 ºC. For each system, geometry optimizations were performed on the selected low-energy structures obtained from the MD simulation, and the structure with the lowest energy was selected for further study. Charge analyses showed that the Pd atoms of Pd 13 @NaY were partially transformed into Pd (2+)+ species after adding eight or ten oxygen atoms onto the Pd 13 cluster (Table S2). The Pd 13 O 10 @NaY had the highest Pd (2+)+ /Pd + ratio, followed by Pd 13 O 8 @NaY and then Pd 13 @NaY (Table S2). Since the DRIFTS spectra suggested linear adsorption of CO on all five Pd/NaY catalysts (Fig. 10), only the top-site adsorption was considered in our DFT calculations. The optimized structures of linear adsorption of CO on the Pd 13 @NaY, Pd 13 O 8 @NaY, and Pd 13 O 10 @NaY models are presented in Fig. S9. We found that the CO preferred to be adsorbed on the Pd + sites in comparison with the Pd (2+)+ sites (Table 4). This was mainly due to the stronger  back-donation of Pd + compared to the Pd (2+)+ sites, [57] which was consistent with the DRIFTS results (Fig. 10). The most favorable adsorption energy of CO at Pd + site increased significantly with decreasing the oxygen atoms of the models (1.32 eV for Pd 13 O 10 @NaY → 2.06 eV for Pd 13 O 8 @NaY → 2.61 eV for Pd 13 @NaY, Table 4) [73]. In addition, the dissociative adsorption of CH 3 ONO on the considered models showed a similar trend to the adsorption of the CO reactant. More specifically, the dissociative adsorption energies of CH 3 ONO on the Pd 13 O 10 @NaY, Pd 13 O 8 @NaY, and Pd 13 @NaY models were 2.06, 2.44, and 2.71 eV, respectively (Fig. S9 and Table S3). Therefore, our DFT results suggested that the lower the Pd (2+)+ /Pd + ratio of the catalyst was, the stronger the adsorption of CO and CH 3 ONO reactants were expected. a Pd-x, where x represents the number of Pd atoms in the models (Fig. S9).

b The valence states of Pd were determined by calculating their Bader charges (see Table S2 in SI for details).

c Calculated adsorption energy (E ads ) of CO.

The calculated high adsorption energies of CO and CH 3 ONO also suggested that these two reactants could be easily adsorbed on the Pd/NaY catalysts (Tables 3 andS2) [73]. This indirectly indicated that the Langmuir-Hinshelwood mechanism was preferred for the DMC synthesis over the Pd/NaY catalysts [40,74]. Since DMC synthesis and catalytic decomposition of CH 3 ONO reactant were parallel competing reactions (details provided in SI), the catalytic decomposition of CH 3 ONO was examined on all five Pd/NaY catalysts at 110 ºC using a feed of CH 3 ONO/N 2 = 1/4 (mole). With increasing the shaping pressure from 60 MPa to 600 MPa, the activity of undesired decomposition of CH 3 ONO reactant gradually decreased from 30.0% to 20.9% (GHSV = 30000 mL g cat. 1 h 1 ) and from 21.7% to 15.2% (GHSV = 90000 mL g cat. 1 h 1 ) on these five Pd/NaY catalysts, which gave a reverse trend to the DMC selectivity for the DMC synthesis (Figs. 5 andS10). This suggested that Pd/NaY catalyst with a low Pd (2+)+ /Pd + ratio could hinder the undesired decomposition of CH 3 ONO reactant.

In summary, combining the XPS, DRIFTS and DFT results, it could be concluded that the mechanical pressure tuned the Pd (2+)+ /Pd + ratio and further adjusted the

Conclusions

Palladium-containing NaY zeolite catalysts were prepared by mechanical-pressure controlled treatment and tested in the indirect oxidative carbonylation of methanol to dimethyl carbonate (DMC). The mechanical pressure applied to the Pd/NaY catalysts was found to have a pronounced effect on the Pd dispersion and the catalysts stability.

The Pd clusters with a size of 1.3 nm homogeneously distributed in the NaY zeolite were obtained under mechanical-pressure treatment of 300 MPa. This catalyst not only showed the high CO conversion of 89% and DMC selectivity of 83%, but also was very stable for at least 150 h. The DMC selectivity was closely related to the

  Scheme S2 (Supporting Information, SI). The indirect oxidative carbonylation of

  A thermal conductivity detector (TCD, 2.5-m molecular sieve 5A and 1.0-m HayeSep Q packed columns, Agilent) was used to measure the CO and N 2 . A flame ionization detector (FID, 30-m HP-Innowax capillary column, Agilent) was employed to detect the CH 3 ONO, DMC, dimethoxymethane and methyl formate. The kinetic experiments, and the calculations of the turnover frequency (TOF), Thiele modulus, effectiveness factor, CH 3 ONO conversion, CO conversion, DMC, dimethoxymethane and methyl formate selectivity were presented in SI.

Fig. 1 .

 1 Fig. 1. SEM images of the Pd/NaY catalysts prepared under treatment with different

Fig. 2 .

 2 Fig. 2. XRD patterns of the Pd/NaY catalysts with the corresponding degree of

Fig. 3 .

 3 Fig. 3. (a) N 2 sorption isotherms and (b) pore size distribution of the Pd/NaY catalysts

Fig. 4 .

 4 Fig. 4. Solid-state 29 Si CP MAS NMR spectra of the Pd/NaY catalysts prepared under

Fig. 5 . 21 1/ 6 /

 5216 Fig. 5. CO conversion (black circles) and DMC selectivity (red triangles) for the

Fig. 6 .

 6 Fig. 6. TEM images of the used catalysts after 6 h catalytic test: (a1,a2) Pd/NaY-60,

Fig. 7 .

 7 Fig. 7. (a) HAADF-STEM image and the size distribution of Pd (inset), (b)

Fig. 8 .

 8 Fig. 8. (A) XPS spectra and (B) Pd 3d spectra of used catalysts after 6 h catalytic test:

Fig. 10 .

 10 Fig. 10. DRIFTS spectra of CO adsorbed on the used Pd/NaY-60, -150, -300, -450

Fig. 11 .

 11 Fig. 11. Optimized structures of (a) Pd 13 , (b) Pd 13 O 8 , and (c) Pd 13 O 10 clusters

  adsorption of CO and CH 3 ONO reactants. The increased amount of Pd + species could enhance the adsorption of CO and CH 3 ONO reactants and inhibit the undesired decomposition of CH 3 ONO reactant into byproducts, thereby improving the DMC selectivity.

Fig. 12 .

 12 Fig. 12. Durability performance of the Pd/NaY-300 catalyst in the indirect oxidative

Table 1

 1 Physicochemical properties of Pd/NaY catalysts prepared under treatment with different mechanical pressures and their catalytic performance in the indirect oxidative carbonylation of methanol to DMC.

	Catalyst	S BET	V micro	Size,	Size,		Si/Al,	Si/Al,	E a	TOF
		(m 2	(cm 3 g 1 ) a	XRD	TEM	(nm) d	XRD d	XPS e	(kJ	(s 1 ) g
		g 1 ) a		(nm) b	(nm) c				mol 1 ) f
	Pd/NaY-60 893	0.32	89	370	2.463 2.8	2.8	67	0.25
	Pd/NaY-150 853	0.31	84	300	2.463 2.8	2.8	67	0.22
	Pd/NaY-300 841	0.30	79	270	2.464 2.7	2.7	67	0.13
	Pd/NaY-450 800	0.28	64	200	2.464 2.7	2.7	58	0.11
	Pd/NaY-600 732	0.26	54	180	2.464 2.7	2.7	55	0.08

a BET surface area, and the micropore volume determined by the t-plot method.

  3 g 1 , respectively. This indicated the formation of abundant framework defects and amorphous aluminosilicate areas on the Pd/NaY-600 catalyst, which was consistent with the SEM and XRD results (Figs. 13 and Table1).

		300	(a)
	)
	1
	g
	STP	200
	3
	(cm Volume	100					Pd/NaY-60 Pd/NaY-150 Pd/NaY-300 Pd/NaY-450
		0	0.0	0.2	0.4	0.6	0.8 Pd/NaY-600 1.0
				Relative pressure (P/P 0 )
			(b)				Pd/NaY-60
		2.0					Pd/NaY-150
	1 )						Pd/NaY-300
	dV/d(D) (cm 3 nm g 1	0.0 0.5 1.0 1.5		dV/d(D) (cm 3 g 1 1 ) nm	0.00 0.01 0.02	Pore width (nm) 5 10 15 Pd/NaY-450 20 Pd/NaY-600
			1			10
					Pore width (nm)

Table 2

 2 Fraction of Si species of the Pd/NaY catalysts prepared under treatment with different mechanical pressures determined by solid-state29 Si CP MAS NMR spectroscopic study. a

	Catalyst	Q 1 (88 ppm) Q 2 (94 ppm) Q 3 (100 ppm)	Q 4 (105 ppm)
	Pd/NaY-60	7.2%	38.9%	43.1%	10.8%
	Pd/NaY-150	11.0%	38.0%	39.7%	11.3%
	Pd/NaY-300	13.3%	39.5%	37.0%	10.2%
	Pd/NaY-600	16.7%	37.4%	34.7%	11.2%

Table 3

 3 Peak areas of Pd + (0    2), Pd (2+)+ , and Pd (2+)+ /Pd + area ratio of used catalysts after 1 h and 6 h catalytic tests determined by XPS. a

	Catalyst	Time	Area of Pd +	Area of Pd (2+)+	Pd (2+)+ /Pd + ratio
	Pd/NaY-60	1 h	5744	10037	1.75
	Pd/NaY-150	1 h	8660	11574	1.34

  (2+)+ /Pd + ratio on the DMC selectivity, we conducted DFT calculations to probe the adsorption of CO and CH 3 ONO reactants on the Pd species with different valence states. A primitive cell (Si 24 Al 24 O 96 ) of the faujasite structure with the Fd space group symmetry was used to model the NaY zeolite. Based on the Löwenstein rule, we replaced 12 Al atoms by 12 Si atoms to obtain a Si/Al molar ratio of 3.0, which was close to that of the synthesized Pd/NaY catalyst (Si/Al=2.8, Table

Table 4

 4 Calculated most favorable adsorption of CO at the Pd + and Pd (2+)+ sites in the Pd 13 @NaY, Pd 13 O 8 @NaY, and Pd 13 O 10 @NaY models.

	Model	Adsorption mode	Site a	Valence state b	E ads (eV) c
	Pd 13 O 10 @NaY Linear	Pd-13	+	1.32
		Linear	Pd-6	(2+)+	0.64
	Pd 13 O 8 @NaY	Linear	Pd-3	+	2.06
		Linear	Pd-4	(2+)+	0.46
	Pd 13 @NaY	Linear	Pd-13	+	2.61

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21908246, 21975285, 21991090, 21991091), Shanghai Sailing Program (20YF1410000), Fundamental Research Funds for the Central Universities (18CX02148A) and the Sino-French International Laboratory (LIA) "Zeolites".

mechanical pressure applied as it was related with the amount of Pd (2+)+ and Pd + (0    2) active sites formed in the Pd/NaY catalysts. The in situ DRIFTS and DFT calculations revealed clearly that the Pd + in contrast to Pd (2+)+ species enhanced the adsorption of CO and CH 3 ONO reactants and inhibited the undesired decomposition of CH 3 ONO reactant into byproducts, and thus enhanced the DMC selectivity.

 S8) and 6 h (Fig. 8) tests.

The XPS results and catalytic performance of these five catalysts after 6 h catalytic test showed that the area ratio of Pd (2+)+ /Pd + decreased gradually, while the DMC selectivity increased with raising the mechanical pressure from 60 MPa to 600 MPa (Figs. 5 and8, Table 3). As stated above, the DMC selectivity also increased on each catalyst with extending the reaction time from 1 h to 6 h (Fig. S5). The Pd (2+)+ /Pd + area ratios of catalysts Pd/NaY-60, -150, -300, -450 and -600 after 6 h test were smaller than the ones measured after 1 h (Table 3). The relationship between the Pd (2+)+ /Pd + ratio and DMC selectivity on these five catalysts after 1 h and 6 h catalytic tests was shown in Fig. 9. A linear relationship with a correlation coefficient R 2 = 0.9 between the DMC selectivity and the Pd (2+)+ /Pd + ratio was observed (Fig.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.


Lei Zhao

Ge Yang

Hailing Guo

Chunzheng Wang

Lijuan Wang

Svetlana Mintova
email: mintova@ensicaen.fr

Effect of sodium concentration on the synthesis of faujasite by two-step synthesis procedure

Keywords: synthesis, zeolite, morphology, alkali metal

The relationship between concentration of sodium cations and the properties of faujasite (FAU) zeolite using a two-step synthesis procedure including (1) formation of amorphous aluminosilicate precursors and separation of amorphous nanoparticles, and (2) transformation of these amorphous particles into zeolite crystals by treatment with alkali solutions (NaOH) were studied. Three representative samples including two nano-sized zeolites and one micron-sized zeolite were prepared using different concentration of sodium hydroxide. The crystallization process of these zeolites was studied in details by FTIR, NMR, XRD, SEM, TEM, N 2 adsorption. The results indicated that minor changes in the concertation of inorganic cations can significantly shorten the induction period and crystallization time and thus affect the morphology, size and chemical composition of the zeolite crystals.

Introduction

The synthesis of zeolites continues to receive attentions due to its great industrial value in catalysis, adsorption, separation, medicine, food, etc [1][2][3][4][5] . Organic amine template, surfactant, seeding aid, inorganic cation [6][7][8][9] are widely used in the preparation of zeolites. Among them, the inorganic structural direction agents (single or mixtures of alkali cations) are highly desired due to their green nature and low cost. Therefore, efforts have been made to develop organic template-free methods to synthesize zeolites with emphasis on their particle size and chemical composition.

Several inorganic ions (e.g. Na + , K + , Li + , Cs + ) are introduced to prepared nanosized zeolite [10][11][12][13][14][15] . Na + is commonly used in the synthesis of nanosized zeolites with EMT, LTA, FAU, SOD, MFI, MTW framework structures [16][17][18][19][20] . However, the effect of the inorganic ions on the nucleation and crystallization process of nanozeolites is still under debate. At present, the inorganic cations with higher charge density, which can compensate more AlO 4 -are used to synthesize zeolite with low Si/Al ratio. Therefore, the concentration of sodium cations in the initial precursor mixtures is of importance and is considered to predetermine partially the Si/Al ratio of final zeolites. However, the Si/Al ratio of the zeolite is controlled by the whole reaction process [21] . The classical one-step hydrothermal synthesis promotes the simultaneous nucleation and crystallization in highly diluted colloidal precursor suspensions. But defining the role of each component in the reaction system and the entire sequence of events during the whole reaction process requires further study [22][23] . Recently we have reported on the two-step synthetic strategy for preparation of aluminosilicate amorphous precursor particles and their transformation into nanosized zeolites crystals [24] . In this paper, we focus on the synthesis of various zeolites with FAU type framework structure (zeolites X and Y) by controlling precisely the formation and transformation of preformed amorphous nanosized particles into zeolites under treatment with alkali solutions. The influence of tiny variations of content of NaOH on the properties of zeolites such as morphology, size, Si/Al ratio was studied.

Materials and methods

Chemicals and reagents

Aluminum powder (Al, 325 mesh, 99.5%, Alfa Aesar), sodium hydroxide (NaOH, Sinopharm, 97%), colloidal silica (SiO 2 , Ludox-HS30, 30 wt.% SiO 2 , pH=9.8, Aldrich), 1,3,5-triisopropylbenzene (Aladdin, 98%), NH 4 Cl (Aladdin, AR) were used as received.

Preparation of amorphous precursor

The amorphous precursor was prepared from a clear precursor suspension with a molar composition: 9 Na 2 O: 0.59-0.

Synthesis of FAU zeolite

The 500 mg amorphous power was mixed with 10 ml NaOH solution, the concentration of the NaOH solution varied from 0.5 wt.% to 10 wt.%. The suspensions were placed in an oven at 50 °C for 15 h, 36 h and 11d different time.

The crystalline materials were collected by centrifugation and washed several times with distilled water till pH of 7. Samples are presented in Table 1.

Ammonium exchange

The samples FAU-1-nano, FAU-2-nano and FAU-3-micron were ion-exchanged with solution of 0.1 M NH 4 Cl at room temperature for 6 hours, washed with distilled water and calcined at 200 °C.

Characterizations

Powder samples were measured using a Bruker D8 Advance diffractometer with CuKα monochromatized radiation (λ=1.5418 Å). The crystal size, morphology and crystallinity of solids were determined by a scanning electron microscope (SEM) using a JEOL JSM-7900F and transmission electron microscope (TEM) using a FEI LaB6 TECNAI G230UT operated at 200 kV. FTIR spectra were recorded on a Bruker Vertex 70 spectrometer with an average of 64 scans. Nitrogen adsorption/desorption isotherms were measured using Quantachrome, Autosorb IQ, samples activated at 300 °C for 10 h with a heating rate 2 °C• min -1 . The chemical composition was analyzed by inductively coupled plasma atomic emission spectrometer (ICPAES, a Leeman Labs Prodigy High Dispersion ICP instrument). TG-MS was performed using a Netzsch TG-MS (STA449 F5) instrument in a nitrogen atmosphere in the range of 50-1000 °C with a ramp of 2 °C•min -1 . 29 Si NMR spectra were recorded on a Bruker Avance 400 (9.4 T) spectrometer using 4 mm-OD zirconia rotors and a spinning frequency of 12 kHz.

Catalytic test

The catalytic conversion of 1,3,5-triisopropylbenzene (TiPBz) was performed in a fixed bed reactor operating in a gas phase. For each experiment, catalyst (50 mg) was loaded in the reactor and enclosed between two layers of quart sand (20-40 mesh).

The catalyst was activated at 450 °C in N 2 flow of 50 mL/min for 1 h before reaction, and then the temperature was set to the to a reaction temperature of 350 °C. The TiPBz was delivered using a carrier gas (50 mL/min N 2 ) through a saturator containing TiPBz at 90 °C. The reaction products were analyzed using an online gas chromatograph (Scion 456-GC GC 7890N) equipped with a flame ionization detector (FID).

Results and discussion

The amorphous nature of the solid precursor was confirmed by X-ray diffraction (Fig. 1a). As shown no diffraction peaks were found in the amorphous precursor noted as sample S1 in Table 1. This amorphous precursor was treated by 7 wt.% NaOH solution at 50 °C for 15 h (sample S2, Table 1 Further the size and morphology of the amorphous and crystalline samples were evaluated using microscopy (SEM and TEM). The amorphous particles (Fig. 2a) have uniform size of 20-30 nm without sharp edge. Remarkably, spherical monodispersed nanosized crystals with sizes in the range of 30-45 nm were obtained for sample FAU-1-nano (Fig. 2b). The size of the crystals determined by SEM is consistent with the XRD results. As shown in Fig. 2c, the FAU-2-nano crystals have a diameter of 50-80 nm. While sample FAU-3-micron (Fig. 2d) contains crystals with a size of 500 nm and with pronounced octahedral-morphology. These results point out that via decreasing the concentration of sodium, bigger FAU crystals with more defined octahedral shape cane be synthesized. Further the crystalline nature, morphology and size of particles in the samples are revealed based on TEM. As shown in Fig. 3a, the amorphous particles with size of 25 nm do not show any crystalline fringes. While the lattice fringes corresponding to [111] plan in the samples FAU-1-nano (30-45 nm, FAU-2-nano (50-80 nm) and FAU-3-micron (500 nm) are clearly present (Fig. 3b,c,d).

The porosity of the amorphous and crystalline samples was evaluated by N 2 physisorption measurements. As shown in the Fig. 4, the amorphous sample exhibits a combination of type I and IV isotherm containing a H1-type hysteresis. The high mesoporosity is attributed to the interparticle voids originated from packing of amorphous particles that delineate regular mesopores with diameters of 25 nm. The amorphous precursor shows a feature of micropore filling at low P/P 0 (1×10 -7 -1×10 -3 ), indicating the micropore formation during the aging process. FAU-1-nano, FAU-2-nano and FAU-3-micron exhibit high microporosity and in addition textural mesoporosity. The textural mesoporosity is associated with the close packing of monodispersed and well-shaped nanosized crystallites. The total pore volume, micropore volume, specific surface area, and the external surface area of the samples are summarized in Table 2. The results imply that the amorphous particles were transferred to FAU zeolite with well-developed porosity.

The transformation of the amorphous nanoparticles into FAU zeolite under treatment with different concentration of NaOH is followed by FTIR (Fig. 5). The FTIR spectra of faujasite zeolite samples contain two bans, the symmetric stretching band at 744 cm -1 and double six ring vibration at 566 cm -1 thus confirming the crystalline nature of the samples. In addition, the T-O-T asymmetric vibration for samples FAU-1-nano, FAU-2-nano and FAU-3-micron appear at 983 cm -1 , 995 cm -1 and 1003 cm -1 , respectively implying that the Si/Al ratio increases.

The loss of zeolite-type water from the amorphous precursor is about 15.7% as determined by TG (Fig. 6). While the weight losses of FAU-1-nano, FAU-2-nano and FAU-3-micron are 21.8%, 22.8% and 16%, respectively. Compared with the FAU-3-micron, samples FAU-1-nano and FAU-2-nano contain more water due to the smaller crystal sizes and higher Al content as shown by ICP and IR characterizations. The crystallization process of FAU-2-nano and FAU-3-micron was studied by X-ray diffraction. As shown in Fig. S1, the weak diffraction peak located at 6.11 o belonging to FAU zeolite can be found in the pattern of sample FAU-2-nano after 22 h. While after 24 h, about 60% of the amorphous precursor is transform to faujasite zeolite. After 36 h, the amorphous phase completely transformed to zeolite crystals.

The SEM images corresponding to these samples are shown in Fig. S2. The sample treated for 22 h contains few particles with a size of 40 nm. After extension of the crystallization time to 24 h, the particles became with sharper edges. While after 36 h, the amorphous particles transform into crystals of size of 50-80 nm. The porosity of these samples was measured by N 2 physisorption (Fig. S3 and Table S1). The microporous specific surface area of the amorphous precursor is 85 m 2 /g, whereas, the microporous specific surface area for sample after 20 h treatment is 19 m 2 /g. The microporous specific surface area of samples after crystallization for 22 h and 24 h are 31 m 2 /g and 542 m 2 /g, respectively, and for the fully crystalline sample FAU-2-nano is 875 m 2 /g. Similar trends were observed for the other samples. The XRD and SEM results for sample FAU-3-micron are shown in Fig. S4-S5. Similar results were observerd except the kinetics of crystal growth. After crystallization for 8 d only 10%

crystalline product was obtained. The crystals exhibit octahedral morphology with a size in the range 100-250 nm. About 65% of the amorphous precursor is transform to FAU crystals with a size of 400 nm after 9 d. All amorphous precursor nanoparticles are transformed to FAU after 11 d. The evolution of porous structure in the FAU-3-micron series is followed by N 2 and the results are shown in Fig. S6 and Table S2. The results are in a good agreement with the XRD and SEM.

The sodium cations act as inorganic template and ensure the high degree of supersaturation of the suspensions. The high concentration of sodium in the suspensions lead to the formation of small nanosized crystals with uniform particle size distribution (samples FAU-1-nano and FAU-2-nano). While the lower sodium content leads to the formation of bigger crystals (sample FAU-3-micron). During the crystal growth of sample FAU-3-micron, the low concentration of sodium penetrates in the amorphous precursor particles slower. Then large amount nuclei are formed and grow via aggregation due to the limited amount of sodium in the system. The

Conclusion

In summary, controlling the amount of sodium cation is the key to control both the nucleation and growth processes of FAU zeolite using the two-step synthesis approach. The amorphous nuclei with high Si/Al ratio were obtained under the growth-limiting nutrient (Na + ) at room temperature followed by crystallization at 50 °C for different times. The FAU-1-nano zeolite with nanosized crystals of 30-45 nm and Si/Al ratio of 1.1 was obtained for a short crystallization time and high Na + concentration. On the contrary, larger crystals (the FAU-2-nano zeolite) with a dimension of 50-80 nm and Si/Al ratio of 1.5 are obtained at lower sodium concentration. The FAU-3-micron crystals with a diameter of 500 nm and Si/Al ratio of 1.7 were obtained for longer crystallization time and even lower sodium concentration. The FAU-2-nano crystals showed good catalytic performance due to the uniform nanosized crystals and high Si/Al ratio. The Si/Al ratio and crystal size of the FAU zeolites obtained by two-steps crystallization approach increase with a slight decrease of the concentration of sodium in the precursor suspensions.

Further the chemical composition

  and the silica arrangement in the faujasite samples are determined by29 Si NMR. As shown in the Fig.7, the amorphous precursor after aging 24 h at room temperature present a broad peak around 90 ppm typically observed for amorphous silica materials. From the 29 Si NMR of sample FAU-1-nano the Si/Al ratio of 1.1 is calculated, which is consistent with the ICP result. The spectrum of sample FAU-2-nano contains five bands at 84.8, 89.5, 94.2, 99.3 and 103 ppm (Fig.7c). The Si/Al ratio calculated for sample FAU-2-nano is 1.5, while for FAU-3-micron is 1.7 (Fig.7d); five bands at 83.7, 88.1, 93.4, 98.4 and 101.1 ppm in the29 Si NMR spectra are present for both samples with slight difference in their intensity. The NMR and ICP study indicate that a higher Na 2 O/Al 2 O 3 ratio in the amorphous mixture favors the formation of FAU-1-nano with a crystalline yield of 85% after a shorter crystallization time, while the lower Na 2 O/Al 2 O 3 ratio of the precursor mixture produces FAU-2-nano and FAU-3-micron crystals with a higher yield of 90% after prolonged crystallization time.

  significant decrease in particle size from FAU-2-nano and FAU-3-micron to FAU-1-nano indicates that short crystallization time and high concentration NaOH facilitate the formation of mono-disperse nanosized crystals, while the lower Na + amount and longer crystallization time are beneficial for the formation of larger particles. The XRD patterns of the three fully crystalline samples of FAU-1-nano, FAU-2-nano and FAU-3-micron in H-form are shown in Fig. S7. The high crystallinity of these samples indicates their stability after ion-exchange. Besides, a gradually increased of medium strong acid cites is observed. The change peaks at 201 o C, 232 o C and 250 o C in the NH 3 -TPD profiles of FAU-1-nano, FAU-2-nano and FAU-3-micron, respectively are shown in Fig. S8. Dealkylation of 1,3,5-triisopropylbenzene (TiPBz, a kinetic diameter of 0.95 nm larger than kineticdiameter of FAU of 0.74 nm) is selected as a model reaction to characterize the samples FAU-1-nano, FAU-2-nano and FAU-3-micron. As shown in Fig.8, the conversions of TiPBz on FAU-1-nano, FAU-2-nano and FAU-3-micron are 10%, 55% and 10%, respectively. The conversion of FAU-2-nano is much higher than that of FAU-1-nano and FAU-3-micron at the initial stage of the reaction; the conversion is maintained 20% higher for sample FAU-2-nano even after 160 min of reaction time. The results of TiPBz dealkylation indicates that the enhanced catalytic performance of sample FAU-2-nano is originated from the decreased crystal size and increased Si/Al ratio. The FAU-1-nano has the highest external surface areas (230 m 2 /g) but weaker acidity. On the contrary, the FAU-3-micron sample has strong acidity but low external surface area (72 m 2 /g).

Fig. 1

 1 Fig. 1 XRD patterns of amorphous precursor (a), FAU-1-nano (b), FAU-2-nano (c) and FAU-3-micron (d).

Fig. 2

 2 Fig. 2 SEM picture of amorphous precursor (a), FAU-1-nano (b), FAU-2-nano (c) and FAU-3-micron (d).

Fig. 3

 3 Fig. 3 TEM picture of amorphous precursor (a), FAU-1-nano (b), FAU-2-nano (c) and FAU-3-micron (d).

Fig. 4

 4 Fig. 4 Nitrogen sorption isotherms of amorphous precursor (a), FAU-1-nano (b), FAU-2-nano (c) and FAU-3-micron (d).

Fig. 5 FIIR

 5 Fig. 5 FIIR patterns of amorphous precursor (a), FAU-1-nano (b), FAU-2-nano (c) and FAU-3-micro (d).

Fig. 6

 6 Fig. 6 TG curves of amorphous precursor (a), FAU-1-nano (b), FAU-2-nano (c) and FAU-3-micro (d).

Fig. 7 29

 7 Fig. 7 29 Si NMR spectra of amorphous precursor (a), FAU-1-nano (b), FAU-2-nano (c) and FAU-3-micron (d).

Fig. 8

 8 Fig. 8 Conversation for dealkylation on FAU-1-nano (a), FAU-2-nano (b) and FAU-3-micron (c) in H-form.

  g of NaOH in 20 g double distilled water (dd H 2 O) followed by slow addition of 0.8-0.945 g aluminum powder. Solution B was prepared by mixing of 50 g colloidal silica with 8 g NaOH and 17 g dd H 2 O; as a result a turbid suspension was obtained. In order to obtain clear suspension, the container B was placed in an oven at

	100 °C for 6 minutes. Solution A was added drop wise under vigorously stirring to the
	solution B; during the mixing, solution B was kept in ice. The resulting clear
	suspension was kept 24 h at room temperature for aging. Then the suspension was put
	into the freezer to remove the water with 46-48 g. Finally, the aluminosilicate
	precursor as white amorphous power was collected by centrifugation and washed
	several times with distilled water.

7 Al 2 O 3 : 10 SiO 2 : 160 H 2 O. The initial reactants were mixed to prepare two initial solutions denoted A and B. Solution A was prepared by dissolving 10

Table 1

 1 Al ratio. Sample FAU-3-micron required longer crystallization time in comparison to samples FAU-1-nano and FAU-2-nano. The intensity of diffraction peaks of sample FAU-1-nano is significantly low and they are broader indicating the smaller crystallite sizes. The average size of the crystals of sample FAU-1-nano is c.a. 25 nm;

	While the addition of 2 wt.% of NaOH to the amorphous solid precursor and
	crystallization at 50 °C for extended time (36 h,) resulted in a sample with Si/Al of
	1.5 (sample FAU-2-nano). Further, decreasing of the concentration of sodium
	hydroxide to 1 wt.% leads to slower crystal growth, and after HT treatment at 50 °C
	for 11 d, a fully crystalline sample with a Si/Al of 1.7 was obtained (sample
	FAU-3-micro,

). The XRD pattern of this sample contains diffraction peaks belonging to FAU type framework structure. Further, the sample was characterized by

29

Si NMR and ICP. The results indicate that the sample is fully crystalline with Si/Al=1.1 that correspond to zeolite X (sample FAU-1-nano). ). It was found that the concentration of sodium cations is the growth-limiting nutrient in the formation of zeolite FAU. High concentration of sodium favors the crystallization rate and lead to the formation of zeolite with lower Si/

Table 1 .

 1 Transformation of amorphous precursors into crystalline FAU samples under treatment with NaOH with different concentrations and different crystallization conditions.

	Sample	Concentration of NaOH (LiOH/KOH)	Solid/ Liquid ratio	Synthesis temperature /time
	amorphous precursor	0 wt.%	--	--
	FAU-1-nano	7 wt.% NaOH	0.05	50 °C/15 h
	FAU-2-nano	2 wt.% NaOH	0.05	50 °C/36 h
	FAU-3-micron	1 wt.% NaOH	0.05

Table 2

 2 Physicochemical properties of amorphous, FAU-1-nano, FAU-2-nano, and FAU-3-micron.

	Sample	SBET	Vmic	Smic	Vmes	Sext	Vtot
		(m 2 /g)	(cm 3 /g)	(m 2 /g)	(cm 3 /g)	(m 2 /g)	(cm 3 /g)
	amorphous precursor	175	0.01	85	0.32	90	0.33
	FAU-1-nano	850	0.31	620	1.05	230	1.36
	FAU-2-nano	985	0.34	859	0.17	126	0.67
	FAU-3-micron	897	0.32	825	0.08	72	0.40

Acknowledgements

The authors gratefully acknowledged, the National Natural Science Foundation of China (Grant No. U1862118,No. 21975285, No.21991091, No. 21991090, No. 21908246), the Fundamental Research Funds for the Central Universities (NO. 21CX06024A), and Postgraduate Innovation Projects (China University of Petroleum (East China)) (Grant No. YCX2021053).

Conflicts of interest

There are no conflicts to declare.