Wang Jia; Jin Qibing; Zhao Tianbo

(1. College of Information Science and Technology, Beijing University of Chemical Technology, Beijing 100029;2. Key Laboratory of Cluster Science of Ministry of Education, Beijing Institute of Technology, Beijing102488)

Abstract: Hierarchically porous FAU monoliths were synthesized via the gel pre-aging route using seed gel as directing agent and α-Al2O3 as monolithic carrier. The as-synthesized samples were characterized by means of the Fourier transform infrared spectroscopy (FT-IR), X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and N2 adsorption techniques. The effects of seed gel, gel pre-treatment, and gel pre-aging step were determined, while the possible mechanism for formation of alumina composites via different synthesis processes were discussed. The results showed that the crystal size, the shape, and the loading of the supported FAU could be readily tuned by varying the composition of the crystallization gel without notably changing the structure of α-Al2O3. The proposed seed gel pre-treating and gel pre-aging route are simple, reproducible, and practically easy to integrate triple porous structures into large-dimension monoliths,which are proved to be very effective in depositing pure FAU crystals on the α-Al2O3 skeleton surface and strengthening the interfacial interaction between them. Moreover, it may provide inspiration to the synthesis of other hierarchical zeolites.

Key words: FAU/α-Al2O3 monoliths; hierarchical porosity; seed gel; pre-aging; gel pre-treatment

1 Introduction

Zeolites with the feature of regularly microporous crystallite are widely used in the fields of catalysis,adsorption, separation, and ion-exchange due to their intrinsic chemical activity, high hydrothermal stability, large specific surface area, and unique shape selectivity[1-2]. However, the sole presence of micropores often imposes diffusion and transfer limitations, thus limiting their applications involving bulky molecules[3].Hierarchically porous zeolites have shown to be able to overcome some diffusion drawbacks[4]. Many methods have been proposed to fabricate hierarchically porous zeolites, such as the destructive synthesis through dealumination or desilication, the constructive method using hard-template, soft-template or template-free approaches, etc[2-4]. Unfortunately, the zeolite crystals prepared by these methods are usually in the powder form, resulting in handling and separation problems.Furthermore, the high cost also limits the large-scale industrial application of these zeolites. To overcome these limitations, a new type of shaped zeolitic materials with hierarchical porosity is expected.

The shape control of zeolite can be realized by extruding a zeolite mixture or growing zeolite on a certain shaped substrate. Yue, et al.[5]prepared the ZSM-5 monoliths with bimodal-porous hierarchical structure by transforming the aluminosilicate extrudates using ZSM-5 seed gel as the binder and the crystal nuclei provider.The aforementioned seed gel was actually the ZSM-5 mixture after crystallization. In contrast, the seed gel containing seeding nuclei with strong directing effect is usually generated via the synthesis of FAU zeolite. FAU membranes can be fabricated on the outside surface of porous substrates via in-situ growth without pre-seeding or secondary growth with seeding. It was reported that the coverage, size and purity of the supported FAU strongly depended on the nuclei in the precursor gel[6-8].Nonetheless, the development of a reliable seeding technique is required for large-scale synthesis[9].Furthermore, additional modification steps, such as calcination[6,10], acid[6,11], alkali[11]or organic solvent[12]treatment are usually required to clean and condition the support surface before the pre-seeding procedure.

In order to simplify the synthesis procedure and integrate triple porous structures into large-dimension monoliths, a directing agent induced method was proposed to fabricate FAU/γ-Al2O3monoliths using the seed gel of FAU synthesis as directing agent[13]. However, the generation of impurity phases was accompanied by the absence of seed gel in the crystallization gel. In the present work,the seed gel was used as directing agent once again, and the crystallization gel pre-aging route was developed to fabricate the pure phase FAU/α-Al2O3monoliths with varied micro-morphologies. The influences of the seed gel, gel pre-treatment and pre-aging process with different gel compositions were investigated by means of FT-IR,XRD, SEM, and N2adsorption techniques. The study of the seed gel applied as directing agent involving different crystalline alumina may provide inspiration to the synthesis of hierarchical zeolites on other kinds of supports.

2 Experimental

2.1 Fabrication of zeolite FAU supported on α-Al2O3 monoliths

Monolithic α-Al2O3used as the substrate was prepared by the large-scale synthesis method described elsewhere[13],followed by calcination at 1373 K for 5 h. The assynthesized support was denoted as M for simpli fication purpose.

In the gel pre-aging route, M was immersed into the crystallization gel and was further subject to aging altogether for 24 h at room temperature, which was the socalled gel pre-aging step, and subsequently hydrothermal crystallization was conducted at 373 K for 5 h in the Te flon-lined stainless-steel autoclaves. The impregnation process was carried out by soaking M in the synthesis gel under vacuum with sonication for 30 min. After the crystallization, the solid product was separated and rinsed with deionized water until the pH value reached below 9 and was then dried at 383 K for 1 h.

The synthesis gels contained water glass (cosisting of 26.0% of SiO2and 8.2% of Na2O, Beijing Hongxing Natron Factory), sodium aluminate (NaAlO2, with Al2O3≥41.0%, Sinopharm Chemical Reagent Co., Ltd.),sodium hydroxide pellets (containing 96.0% of NaOH,Xilong Chemical Co., Ltd), and deionized water. All reagents were used as received.

In order to understand the role of the seed gel in the structured zeolite composites, two types of synthesis gels were used as the crystallization gel, including the overall gel and the precursor gel, which were prepared as follows:

The preparation of the overall gel involved the synthesis of the seed gel and the feedstock gel, which were used to synthesize FAU according to the procedure reported by Ginter[14]. Firstly, the seed gel with a molar ratio of 10.8 Na2O: 1.0 Al2O3: 10 SiO2: 200 H2O was prepared and dynamically aged for 24 h at ambient temperature.Subsequently, the feedstock gel with a molar ratio of 4.63 Na2O: 1.0 Al2O3: 10 SiO2: 200 H2O was prepared.Finally, an appropriate amount of seed gel was added into the feedstock gel to form the overall gel with a molar ratio of 5.4 Na2O: 1.0 Al2O3: 10 SiO2: 200 H2O. The assynthesized synthesis gels were thereafter labeled as “S”,“F”, and “O”, respectively.

The precursor gel (with a molar ratio of xNa2O: 1.0Al2O3:ySiO2: 200H2O) was prepared using the same protocol as the overall gel, albeit without addition of the FAU seed gel. The as-synthesized precursor gels were thereafter named as “P”, “P1”, “P2”, and “P3”, with their molar compositions listed in Table 1.

In the gel pre-aging route without the gel pre-treatment,the overall gel and the precursor gel of P with the same molar ratio were used as the crystallization gel,respectively. The obtained samples were denoted as MO and MP, respectively.

In the gel pre-treating and pre-aging route, the synthesis gels of seed gel and feedstock gel were used as the pre-treating gel, respectively. With the aid of gel pretreatment, M was firstly soaked into the pre-treating gel,and was then dried at 383 K for 1 h before the subsequent gel pre-aging route. The seed gel pre-treated M was labeled as MS0. Similar to MS-P3, MS0 was subject to pre-aging and further crystallization with the precursor gel of P3. After the gel pre-aging step, the sample of MS-P30 was obtained after being dried at 383 K for 1 h.The sample identi fication codes under varying synthesis conditions are listed in Table 1.

Table 1 Variation of molar compositions and synthesis conditions in the transformation of α-Al2O3 monoliths into monolithic porous alumina composites(The ratio denotes the molar ratio of Na2O: Al2O3: SiO2: H2O)

2.2 Characterization

The IR spectrum was recorded on a Thermo Scientific FT-IR spectrometer with sample pressed in KBr pellets.The XRD patterns were obtained with a Shimadzu 7000 diffractometer operated at 40 kV and 30 mA with Cu Kα radiation. The scanning angle (2θ) ranged from 5° to 80° at a rate of 10(°)/min. The SEM images were captured with a field-emission scanning microscope (HITACHI S-4800).N2physisorption measurements were carried out at 77 K on a Micrometrics QUADRASORB SI surface area and pore size analyzer. Before the measurements, all samples were degassed at 573 K for 3 h. Textural properties including the total surface area, mesopore volume and mesopore surface area, micropore volume and micropore surface area, and pore size distribution were calculated using the Brunauer-Emmett-Teller (BET) equation, the Barrett-Joyner-Halenda(BJH) analysis, thet-plot, and the non-local density functional theory (NLDFT) methods, respectively. The total pore volume was evaluated atp/p0=0.99.

3 Results and Discussion

3.1 Effect of the presence of seed gel on the zeolite coating

Figure 1 SEM image, XRD pattern and IR spectrum of alumina support(Inset in (a) shows the appearance of α-Al2O3 monolith after being heat treated at 1373 K for 5 h and the plastic tube used for gelation)

Figure 1 shows the morphology and structural characteristics of alumina support. The monolithic form of the parent M maintained the appearance of the tube mold except for a reduction in volume of approximately 83% (inset of Figure 1(a)). A well-de fined pore structure of M can be observed in Figure 1(a). The support exhibited two scales of macropores with the diameter in range of ca.0.5—2 μm and less than 0.1 μm, which were derived from the phase separation and interstices between the crystals growth during the heat treatment[15]. The corresponding XRD pro file of M (Figure 1(b)) showed a typical pattern of α-Al2O3(PDF#11-0661). As for the IR spectrum, the band at 3 450 cm-1can be assigned to O-H stretching vibrations of hydroxyl groups of α-Al2O3substrate (Figure 1(c)).Without the gel pre-treatment, the XRD spectra of MP and MO (Figure 2(b), 2(d)) prepared with the same molar ratio of the crystallization gel exhibited the peaks of θ-Al2O3(PDF#35-0121) on the basis of α-Al2O3phase.The morphology of MP in the macropore range was retained even after the hydrothermal treatment as shown in Figure 2(a). Besides, the isolated particles with a size range of 0.3—1 μm were dispersed loosely on the inner support surface, and the variation might be caused by the formation of θ-Al2O3phase, which was consistent with the XRD result shown in Figure 2(b). The XRD patterns of MO prepared with the addition of seed gel exhibited strong zeolite peaks appearing at 2θ=6.2°, 15.5°, 23.5°,and 31.2°, corresponding to (111), (331), (533), and(751) reflections from the crystal facets of faujasite-Na(PDF#12-0228). The competing crystalline phases such as zeolite LTA and GIS were not observed, signifying the formation of FAU/alumina composites with a high purity of zeolite phase. The SEM image of the same sample(Figure 2(c)) displayed faintly discernible macropores,blocked with abundance of FAU crystals or aggregates of 0.6—1 μm in size with lumps of randomly oriented crystals. The blocked crystals probably nucleated in the bulk of the liquid phase and then precipitated into the macro-channels with the assistance of nutrients provided by the crystallization mixture[16], which showed weak interaction to the skeleton surface of the carrier. The improvement could be explained by the presence of seed gel in the synthesis system to promote the nucleation,and meanwhile, a certain degree of supersaturation could further increase the number of nuclei and ensure the formation of FAU phase without contaminating the foreign phases during the pre-aging step prior to the hydrothermal crystallization[10,17-18].

3.2 Effect of the addition of seed gel in the gel pretreating and pre-aging route on the zeolite coating

Figure 2 SEM images and corresponding XRD patterns of α-Al2O3 composites synthesized via gel pre-aging route without gel pre-treatment*—FAU; ◆ —θ-Al2O3

Using the same molar ratio of precursor gel of P and overall gel as the crystallization gel, M was firstly pretreated with seed gel and feedstock gel respectively to ensure the presence of seed gel in the synthesis system.The XRD patterns of the resultant samples are shown in Figure 3(b)—3(d). Besides the peaks originated from the support, reflections ascribed to FAU-type zeolite were observed, indicating that pure FAU phase could be formed after crystallizing the gel pre-treated supports through the gel pre-aging route.

Figure 3 XRD patterns of α-Al2O3 support (a) and composites obtained via gel pre-treating and pre-aging route with different gel compositions (b—d)a—M; b—MS-P; c—MS-O; d—MF-O; ◆ —θ-Al2O3

The corresponding SEM images shown in Figure 4 displayed the well preservation of the original macroporous network. Furthermore, the supported FAU phase covered over the skeleton of the substrate and exhibited a well adhesion to the skeleton surface. Apart from the irregularshaped particles with a size of ca. 700 nm (Figure 4(a)and the figure inset), the image of MS-P exhibited a layer composed of particles with a size of ca. 100 nm. Using overall gel as the crystallization gel, MS-O exhibited a slightly enhanced intensity of FAU peaks as compared to that of MS-P. Moreover, the zeolites with a particle size in the range of 100—200 nm were homogeneously crystallized onto the inner support surface as observed in Figure 4(b).It demonstrated that the size, morphology and loading of the supported FAU zeolites strongly depended on the number of nuclei that were present in the crystallization gel[19]. The abundance of nuclei showed higher reactivity to facilitate the zeolite nucleation, which was provided with the seed gel in the overall gel and the pre-treated seed gel during the synthesis process. Consequently,the crystallinity of FAU became higher and the size of precipitated crystals became smaller. The XRD patterns depicted in Figure 3(d) revealed that the peaks of θ-Al2O3phase appeared in addition to the peaks assigned to α-Al2O3and FAU phases, which was similar to that of the samples synthesized without the gel pre-treatment. The similar phase transition behavior of the alumina support might be caused by the absence of the pre-treatment with seed gel,which had been subject to pre-aging at room temperature for 24 h. The corresponding SEM image shown in Figure 4c displayed that FAU crystals precipitated on the alumina skeleton surface had a uniform distribution and a size range of 100—400 nm with an octahedral shape. Moreover, the existence of zeolites covered over the upper surface of the macropores confirmed that the feedstock gel pre-treatment could also improve the adhesion between the zeolite and the alumina skeleton surface like the seed gel pre-treatment.

Figure 4 SEM images of FAU/α-Al2O3 composites obtained via gel pre-treating and pre-aging route with different gel compositions(Inset image in (a) shows the magni fied view of macroporous structure for MS-P)

3.3 Effect of the alkalinity of precursor gel in the seed gel pre-treating and gel pre-aging route on the zeolite coating

As the seed gel pre-treatment coupled with the gel preaging route could effectively improve the zeolite adhesion to the support skeleton surface and produce a pure phase FAU/α-Al2O3composite, the in fluence of the precursor gel with different alkalinity used as the crystallization gel was investigated. The XRD patterns of the resultant samples shown in Figure 5(b)—5(d) matched well with the patterns of standard FAU zeolite and α-Al2O3, indicating that the pure FAU phase was successfully supported on the α-Al2O3substrate. Both the peak intensity and the fullwidth at half maximum (FWHM) of the FAU re flections increased slightly by increasing Na2O/Al2O3ratio from 4.7 to 6.8, implying that the crystal size decreased with an increasing alkalinity. The result seems to be in accordance with the SEM images showing the scattered irregularly-shaped FAU particles with a size range of ca.200—500 nm (Figure 6(a)), and the bi-pyramidal-shaped FAU crystals with the size in the range of 100—500 nm(Figure 6(b)) and 100—300 nm (Figure 6(c)) were coated on the skeleton surface of α-Al2O3, respectively. The decreased size and increased number of deposited crystallites could be attributed to the improved nucleation rate of zeolitic crystallization process by increasing alkalinity of the hydrogel[18,20]. It was noteworthy that the size, morphology and loading of the supported FAU crystals could be readily tuned by varying the alkalinity of precursor gel.

Figure 5 XRD patterns of α-Al2O3 support (a) and composites obtained with the seed gel pre-treating and gel pre-aging route with different alkalinity of precursor gel (b-d)a—M; b—MS-P1; c—MS-P2; d—MS-P3; e—MS0; f—MS-P30

Figure 6 SEM images of FAU/α-Al2O3 composites obtained at different alkalinity of precursor gel

Figure 7 Schematic of typical seed gel pre-treating and precursor gel pre-aging synthesis procedure of FAU/α-Al2O3 composites before crystallization(SEM images are related with α-Al2O3 composites obtained after being pre-treated with seed gel (a), thereafter precursor gel preaging step (b) before crystallization)

To study the effects of the gel pre-treating step and the gel pre-aging step on the zeolite coating, separated steps after being dried at 383 K for 1 h before crystallization were illustrated, with the SEM images shown in Figure 7 by evolution of sample MS-P3. Compared with M,the morphology change of MS0 after the seed gel pretreatment seemed to be not obvious (Figure 7(a)). The structural form displayed that the characteristic peaks of α-Al2O3weakened while new peaks appeared at 44°and 64°, which could be attributed to the formation of gibbsite phase (Figure 5(e)). The absence of FAU feature peaks on XRD pattern of MS0 suggested that the seed gel was comprised of crystal nucleus aggregates of FAU zeolite belonging to the half-baked crystals[8]. The SEM image captured on MS-P30, a dehydrated sample after being subject to pre-aging in the precursor gel of P3 is shown in Figure 7(b). The nanoparticles with a dimension of about tens of nanometers were entrapped in the gel matrix. The XRD spectrum collected on the same sample showed that the peaks at 44° and 64° almost disappeared (Figure 5(f)). The amorphous nanoparticles were derived from the embryonic zeolite species, which were probably too small in quantity to be identi fied by XRD[10,18]. After the crystallization, the submicron-sized FAU crystals were deposited on the inner support surface and exhibited a well adhesion to it.The result is contrary to the previous work[21], in which few micron-sized FAU crystals synthesized by the secondary growth method using FAU crystals as seeds were loosely precipitated onto the inner α-Al2O3surface.The reason might be attributed to the growth of smaller FAU crystals from the gel layer initially deposited on the skeleton surface during the pre-treating step[11,22]. The binding between the pre-treated gel and the substrate skeleton was attributed to the hydroxyl groups on the α-Al2O3surface[6,12], which favored the adhesion of supported zeolite to the skeleton surface. Furthermore,the successful fabrication of FAU/α-Al2O3composite via the presented route signi fied that the nuclei provided by the pre-treated seed gel appeared with the proceeding of the gel pre-aging and the succeeding hydrothermal process[13], thus favoring the formation of FAU zeolite on the wall of the macropores with the assistance of nutrients provided by the crystallization mixture[16].Meanwhile, the phase transition of alumina support could also be inhibited, which might be quite promising for practical application in large-scale preparation process as compared to the sole pre-seeding with micron-sized or nano-sized zeolite crystals.

Figure 8 N2 physisorption isotherms and NLDFT pore size distribution curves of α-Al2O3 support, the seed gel pretreated α-Al2O3 and FAU/α-Al2O3 composite synthesized via seed gel pre-treating and precursor gel pre-aging route■—M; ●—MS-P3; ▲—MS0

Table 2 Textural parameters of α-Al2O3 support and composites

The N2sorption isotherms of the parent M and the composites obtained during the seed gel pre-treating and gel pre-aging route, i.e. MS0 and MS-P3 (shown in Figure 8), were of IUPAC type IV, typical for mesoporous materials. The uptakes at p/p0＞ 0.9 indicated to the presence of macropores[23-24], which were in agreement with the SEM images shown in Figure 1(a), Figure 7(a),and Figure 6(c). Table 2 shows the physical properties of all the investigated monolith samples. The skeletons of MS0 developed the declined volumes and speci fic surface areas in comparison with the original M. The isotherm hysteresis loop changed from type H3 of M to H1 of MS0 and MS-P3, denoting that the slit-like mesopores with non-uniform shapes and sizes had been transferred to the uniform cylinder-like shape. The mesopores of MS-P3 might be associated with the inter-particulate voids of nanoparticles comprising the skeletons after crystallization under basic condition. As regards the sample MS0, the mesopore size distribution was narrowed as compared to that of M ranging from 2—30 nm to 3—17 nm (inset of Figure 8), which might be ascribed to the amorphous aluminosilicate particles deposited inside the struts of M. The result indicated that the mesopore properties changed after pre-treating M with seed gel in an alkaline environment. As the starting M only showed a small BET surface area and negligible micropore volume,the high BET surface area, micropore and mesopore volumes of MS-P3 should be mainly contributed to the FAU coatings (Table 2), which was in accordance with the XRD result (Figure 5(d)). Besides, the steep nitrogen uptake at a relative pressure of p/p0＜0.02 denoted the existence of micropores. The above analysis signified that the FAU composite fabricated via the gel pre-aging route with the aid of seed gel pre-treatment exhibited hierarchically macro-/ meso-/ microporous structure.

3.4 Formation mechanism of α-alumina composites via different synthesis routes

The α-alumina composites obtained under various conditions had different morphologies (Figure 9). The presence of the seed gel containing nuclei combined with the gel pre-aging step facilitated the nucleation process[7-8]and was the pre-requisite for the formation of pure FAU phase. Without gel pre-treatment, the nuclei provided by the seed gel increased under a certain degree of supersaturation during the following synthesis procedures, favoring the growth of pure FAU phase in the bulk liquid phase with the assistance of nutrients provided by the crystallization mixture[16]. Thus, the morphology of the sample with macropores blocked by FAU aggregates was obtained, where the zeolite showed weak interaction with the alumina support (Figure 9(b)).Indeed, when hydrothermal treatment was performed in the absence of seed gel, the formation of zeolite did not occur (Figure 9(c)). With the help of surface hydroxyl groups of M, synthesis gels involving seed gel or feedstock gel could be pre-deposited on the alumina skeleton and further used as starting materials for the zeolite coating[11,22]. The pre-coated gel layer containing aluminosilicate precursor species homogeneously covering the substrate surface could facilitate the heterogeneous nucleation on the support through the gel pre-aging route using seed gel as the directing agent[22].The succeeding crystallization via condensation reactions transferred the soluble aluminosilicate nutrients into the FAU framework at the substrate/gel interface, resulting in the formation of dispersed zeolite crystals on the wall of the macropores (Figure 9(d)). In addition, the phase transition of alumina occurred along with the formation of FAU phase when the gel preaging route with the aid of seed gel was performed in the presence of bare support or with feedstock gel pretreated support. The proposed strategy of gel pre-aging routes using seed gel as the directing agent revealed the mechanism of solution-mediated transport[22,25]. The seed gel pre-treatment not only provided the nuclei via dissolving the seed gel layer during the succeeding synthesis procedures, but also inhibited the alumina phase transition from α-Al2O3to θ-Al2O3. Additionally,the nucleation and growth rate of FAU zeolite could be tuned by adding seed gel to the crystallization mixture or varying the alkalinity of the crystallization gel via seed gel pre-treating and gel pre-aging route[10,26-27], resulting in FAU crystals with diverse dimensions, shapes and loadings adhering to the skeleton surface of α-Al2O3.The successful synthesis of FAU/Al2O3composites under different conditions demonstrated the repeatability of the proposed synthesis routes.

Figure 9 Schematic representation of α-Al2O3 composites prepared via gel pre-aging routes

4 Conclusions

By exploiting monolithic α-Al2O3featuring macro- and mesopores as substrate, pure FAU phase was shaped as centimetric monolith with hierarchical porosity through a facile gel pre-aging route using seed gel as the directing agent, free from organic template, pore generating agents or crystal seeds. The deposition of FAU strongly depended on the gel pre-aging route with the presence of seed gel, which not only could accelerate the nucleation rate but also could inhibit the transformation of FAU phase.The FAU coatings in the size range of several-hundred nanometers were distributed on the wall of the macropores or were precipitated as aggregates in the macro-channels of the substrates by conducting pre-aging step with or without the gel pre-treatment step. Moreover, the phase transition of alumina could be inhibited by pre-treating α-Al2O3substrate with seed gel. It was found that FAU crystals with diverse dimensions, shapes and loadings could be deposited on the surface of α-Al2O3skeleton by adding seed gel to the crystallization mixture or varying the alkalinity of crystallization gel involving the seed gel pre-treating and gel pre-aging route. The processes for formation of FAU/α-Al2O3composites with varied micromorphologies were supposed as a solution-mediated mechanism. With the characteristics of easy handling and low cost, the proposed method can be extended to the preseeding of FAU on the other monoliths and may provide a good prospect in large-scale production of macroporous alumina supported FAU zeolite composites.

Acknowledgments: This work was supported by the National Natural Science Foundation of China (No. 61673004 and No.11472048) and the Fundamental Research Funds for the Central Universities of China (XK1802-4).