Hierarchically structured porous materials have triggered extensive research because of their fascinating features such as high surface areas, interface-facilitated transport and advanced mass transport kinetics [1-3]. Recently, there have been successful attempts to synthesize hierarchically porous zeolites. For example, post-synthesis treatments can be used to generate hierarchically porous zeolites through selective leaching of aluminum (dealumination) or silicon (desilication) from the zeolite framework [4, 5]. The major disadvantage of such a process is the partial degradation of the crystalline nature of the zeolite, and consequently, a reduction in the catalytic activity. More recently, other processes to create zeolites with a hierarchical porosity using a hard sacrificial templates have also been examined; carbon materials [6-8], mesoporous silica spheres [9, 10] and aerogels [11, 12] have been investigated. However, these processes were somewhat expensive and time consuming. Cationic polymers have also been successfully employed to produce hierarchical zeolites [13, 14], but the synthetic zeolites were not suitable for industrial catalysis.

In the case of industrial catalysis, a certain amount of binders, such as alumina, silica or clay, is generally used for cementing zeolite crystals into large sticks or granules that are mechanically stable. However, the inorganic binders may dilute the active zeolite and partially block the pore system, which results in diffusion limitations and inaccessibility of the active species [15]. To overcome these problems, continuous progress has been made on the development of mechanically stable zeolite monoliths with uniform shapes and hierarchical structures. Such monolithic zeolite materials facilitate the diffusion of the reactants and products as well as the reaction efficiency and can be used in practical application. For example, Shi et al. [16, 17] have reported the preparation of HSZs by using ordinary mesoporogens such as triethanolamine (TEA), P123, F127 and even sucrose as in situ carbonaceous templates. Ryoo et al. [18] designed several bifunctional templates, combing the features of both structural guide agent (SDA) and mesoporogens within one molecule, and successfully synthesized zeolite nanosheets with hierarchical structures. However, the reported synthesis of the hierarchically structured zeolites by massive mesoporogens was neither cost efficient nor friendly because most of the template agents were difficult to make and nocuous. Novel synthesis routes using few mesoporogens are highly desirable and have become a promising research field.

In this study, we prepared high-quality zeolite monoliths with hierarchical porosity through a steam-assisted crystallization (SAC) method using sponges as rigid scaffolds. X-ray diffraction (XRD) and nuclear magnetic resonance (NMR) spectroscopy analysis showed the ZSM-5 monoliths were highly crystalline. Because of their hierarchical porous structure, the zeolite monoliths had a higher surface area, stronger acidity, and showed better catalytic activity than that of traditional ZSM-5 in the liquid phase esterification reaction.

Tetraethylorthosilicate (TEOS, 98 wt%), tetrapropylammoniumhydroxide (TPAOH, 25 wt%), hexanoic acid (98.8%), benzyl alcohol (98.8%), ammonium nitrate (98.5%), toluene (99.5%), 4-methylbenzyl alcohol (98%), 4-nitrobenzyl alcohol (98%), 4-methoxybenzyl alcohol (98%) and 3-phenylpropan-1-ol were purchased from J & K Corporation. Aluminum isopropoxide (AIP, 97%) was obtained from Beijing Chemical Reagent Corporation (Beijing, China). Aqueous ammonia solution (28% in H₂O), sodium hydroxide and ethanol (99.9%) were purchased from Beijing Chemical Company. All the reagents were used without further purification. Nanosponges were purchased from Henan.

The sponges were cut into small cubes that were suitable for the size of the autoclave. To synthesize M-ZSM-60 (M indicates monolith, ZSM represents ZSM-5 zeolite, 60 represents the synthesized materials with Si/Al molar ratio of 60), 0.275 g of AIP was dissolved in 16 mL of ethanol at room temperature for 2 h and then added to 14 g of TEOS. The solution was stirred for 5 h and then the sponge was immersed in the solution. The impregnated sponge was then placed into a Teflon-lined autoclave containing 2 mL of 30% aqueous ammonia solution. The autoclave was then closed and treated at 80 ℃ for 12 h in a hot air oven. The autoclave was then quenched to room temperature and the sponge was once again impregnated with the ethanol solution containing AIP and TEOS. This procedure was repeated three times to ensure efficient loading of the silica source and aluminum source into the sponge. The finally loaded sponge was adsorbed with appropriate quantity of TPAOH. The TPAOH/Si molar ratio was maintained as 0.27. The dried sponge containing silica, aluminum and TPAOH was placed inside a Teflon-lined autoclave containing 2 mL of distilled water for steam-assisted crystallization. The autoclave was closed and placed in a hot air oven at 130 ℃ for 36 h. After the crystallization period, the autoclave was taken out and quenched to room temperature. The product was collected, dried at 80 ℃ for 8 h and then calcined at 550 ℃ for 8 h in air to remove the TPAOH and the sponge. The synthesis of M-ZSM-40 and M-ZSM-90 with different Si/Al ratios were similarly conducted by accordingly changing the AIP content in the precursor solution while keeping the addition of TEOS constant. For comparison, traditional ZSM-5 (denoted as ZSM-5(60)) was synthesis with a molar ratio of 28Na₂O:1Al₂O₃:120SiO₂:4000H₂O.

Prior to the catalytic tests, all the materials were exchanged to the H-form with an aqueous solution of NH₄NO₃ (1.0 mol/L) at 90 ℃ for 4 h and then converted to the H⁺ form through calcination at 550 ℃ for 5 h. Esterification of the alcohol with an acid was carried out in a double-necked round bottom flask fitted with a reflux condenser. The alcohol (15 mmol), acid (18 mmol) and toluene (15 mL) were added to the round bottom flask, as well as n-nonane, which is an internal standard. The solution was heated to 110 ℃ using a silicone oil bath and then 250 mg of the catalyst was added. The reaction mixture was stirred using a magnetic stirrer and the stirring speed was maintained at 1200 r/min to avoid external mass transfer limitations. The reaction products were analyzed by GC (Agilent 6890 N) with a capillary column (DB-5, 30.0 m × 320 mm × 0.25 mm) and FID, and the products were further identified by GC-MS (Shimadzu, GCMS-QP 2010S) with a capillary column (DB-5 ms, 30.0 m × 320 mm × 0.25 mm).

The morphology and size of the resultant powders were characterized with a field emission scanning electron microscope (FESEM JEOL-6701F). Transmission electron microscopy (TEM) images were taken using a JEM-2100F (JEOL) operated at 100 kV. Power XRD patterns were collected on a Rigaku-2500 in the 2 θ range of 5°-55°. The relative crystallinity (RC) of the synthesized M-ZSMs was calculated from the ratio of the sum of the intensities of the four most intense reflections in the 2 θ range of 22.5°-24° and the corresponding sum of the parent ZSM-5, which was purchased from the Catalyst Plant of Nankai University. N₂ adsorption isotherms were measured at -196 ℃ on a Quantachrome Autosorb AS-1 instrument; the samples were outgassed at 120 ℃ for 12 h prior to testing. Pore size distributions were evaluated from the adsorption isotherms using the Barret-Joyner-Halenda (BJH) formula and the microporous volume was evaluated by nonlocal density functional theory. ²⁷Al and ²⁹Si solid-state NMR spectroscopy experiments were performed on an AVANCE Ⅲ 400WB spectrometer. The spectra were collected at a frequency of 104.1 MHz (0.4 s recycle delay time) for ²⁷Al-NMR and at a frequency of 79.3 MHz (2 s recycle delay time) for ²⁹Si-NMR. Compressive strength tests were conducted in an Instron5567. SiO₂/Al₂O₃ molar ratios were analyzed by XRF. Thermogravimetric measurements were performed on a Netzsch STA 494C Jupiter TG/DSC instrument. Ammonia temperature-programmed desorption (NH₃-TPD) was conducted using a Micromeritics Autochem 2920 instrument. The catalyst (0.15 g) was charged in a U-shaped quartz cell and pretreated in Ar (20 mL/min) at 500 ℃ for 1 h (ramp rate 10 ℃/min) then cooled to 100 ℃. The gas flow was then changed to a mixture of 10% NH₃-90% Ar (40 mL/min) for 2 h. The sample was then purged with Ar (20 mL/min) at 100 ℃ for 2 h to remove free and weakly adsorbed ammonia. The NH₃-TPD profile was measured by rising the temperature up to 600 ℃ (ramp rate 10 ℃/min) using a TCD detector.

Scheme 1 illustrates the synthesis of M-ZSM monolith through the SAC method with a sponge as a rigid scaffold. First, TEOS (silicon source) and AIP (aluminum source) were dispersed in ethanol to form aluminosilicate seeds, which were then adsorbed by the PUF sponge. The sponge was then treated under hydrothermal conditions in an autoclave using the SAC method with ammonium hydroxide as steam. The same process was repeated three times to load more seeds, ensuring the mechanical stability of the final monolithic product. Second, the sponge obtained in the first step was impregnated in TPAOH solution to adsorb an appropriate quantity of the template and then crystallized under hydrothermal conditions in an autoclave through the SAC method with H₂O as steam. Finally, the M-ZSM monolith was obtained after calcination at 550 ℃ to remove the sponge and the TPAOH template. As shown in the photos (Scheme 1), the zeolite monolith maintained the shape of the original sponge template after the sequential treatments. Depending on the exact conditions, 0.1 g of the nanosponge template produced approximately 2 g of zeolite, indicating a high yield based on the proposed synthetic strategy. Because the sponge scaffolds are easy to handle, M-ZSMs monoliths with different shapes and sizes can be produced, which would facilitate their potential applications.

The strong Brönsted acidity of ZSM-5 zeolite originates from the heterogeneous substitution of Al³⁺ with Si⁴⁺ in the framework. However, because of the difference in the chemical valence and ionic radius between Al and Si, such a substitution is limited [19]. Here, to extend the range of the hierarchical zeolites with variable Si/Al ratios and surface acidity, and consequently, to satisfy the requirements for diverse catalytic applications, M-ZSMs with varied Si/Al ratios from 40 to 90 were prepared under identical conditions at a fixed TPAOH/Si ratio of 0.25 (Table 1, entries 1, 2 and 3). The syntheses of M-ZSMs with different Si/Al ratios were conducted by changing the AIP content in the precursor solution and keeping the amount of TEOS constant.

The microstructure of the obtained M-ZSM-60 monolith was characterized by SEM and TEM. As shown in Fig. 1(a), the sponge scaffold had an interconnecting macroporous structure. The obtained M-ZSM-60 monolith was composed of many dense blocks, among which macropores were formed (Fig. 1(b)). The high-magnification SEM image showed that the zeolite consisted of many close-packed particles (Fig. 1(c)). The TEM image further confirmed that the monolithic zeolite consisted of intergrown particles with a globular morphology (Fig. 1(d)). The corresponding SAED pattern demonstrated the good crystallinity of the zeolite (inset in Fig. 1(d)). The elemental mapping images of M-ZSM-60 showed that the Si and Al were uniformly dispersed in the M-ZSM-60.

Fig. 2(a) shows the corresponding XRD patterns of the M-ZSMs with different Si/Al ratios and traditional ZSM-5 for comparison. All diffraction peaks could be indexed to the standard ZSM-5. No other peaks were observed, indicating the purity of the obtained M-ZSMs. Moreover, depending on the Al doping content, the calculated RC increased from 88% for sample M-ZSM-40 to 98% for sample M-ZSM-90 (Table 1). This result suggested that the increase of Al content in M-ZSMs is unfavorable for the crystallization transformation during SAC treatment.

The obtained M-ZSMs were further characterized by NMR spectroscopy to confirm the structure of the zeolite framework. As shown in Fig. 2(b), the ²⁹Si MAS-NMR spectra clearly showed one major band with a chemical shift at -112 ppm corresponding to Si (OSi)₄(Q₄), which suggested that most of the silicon atoms were located in Q₄ sites, as expected for a silicate framework. The shoulder peak at -100 ppm was ascribed to (HO) Si (OSi)₃(Q₃), also implying that the MFI zeolite was highly crystalline. Furthermore, another weak shoulder peak at -105 ppm was ascribed to (AlO)-Si (OSi)₃, suggesting the existence of some amorphous silicate species in the monolithic zeolites. The ²⁷Al MAS-NMR spectra (Fig. 2(c)) exhibited two peaks at ~55 ppm and 0 ppm. The peak at ~55 ppm was ascribed to aluminum species with tetrahedral coordination. The peak with a lower intensity at ~0 ppm was assigned to the octahedral Al in the form of polymeric oxo-hydroxo-Al-cations and extra-framework Al species or the framework Al species located at the defect sites.

Nitrogen adsorption-desorption experiments were carried out to determine the textural porosity of the obtained M-ZSMs monoliths (Fig. 3). The isotherm of ZSM-5(60) exhibited a typical type-I profile with a high uptake at low relative pressures (p / p ₀ < 0.1) and a plateau at high relative pressures (0.4 < p / p ₀ < 1.0), indicating that the resultant materials had a purely microporous phase with negligible mesoporosity. For M-ZSM-90, a similar type-I isotherm was observed. However, for M-ZSM-40 and M-ZSM-60, typical type-IV isotherms with obvious hysteresis loops and apparent uptakes at high relative pressures were observed, demonstrating the existence of both microporosity and mesoporosity. The corresponding surface areas and pore volumes are listed in Table 1.

Fig. 4(a) presents the NH₃-TPD profiles for ZSM-5(60) and the M-ZSMs. The density of the surface acid sites increased in the order ZSM-5(60) < M-ZSM-90 < M-ZSM-40 < M-ZSM-60. There were two NH₃ desorption peaks for ZSM-5(60) and the M-ZSMs, which were attributed to weak and strong acidic sites. Generally, the peak at a higher temperature is related to a strong acid site, which was attributed to the Al-OH-Si bridge in the 10-member ring (10 MR) of ZSM-5. The peak at a lower temperature was attributed to a weak or medium strength acid site, which was associated with the nonframework Al atoms and zeolite defects. The peaks for the M-ZSMs were stronger than those for ZSM-5(60), indicating that the acidity of the M-ZSMs was higher than that of ZSM-5(60). It was unusual that M-ZSM-60 had a higher acid density than M-ZSM-40. It is likely that the method used in this work does not allow high aluminum content into the MFI framework. As shown in Fig. 2(c), there was a significant amount of extra-framework Al species on M-ZSM-40, indicating that less Al atoms were incorporated into the framework.

The liquid-phase esterification reaction between benzyl alcohol and hexanoic acid was then employed as a probe reaction to examine the catalytic performance of the M-ZSMs and ZSM-5(60). The M-ZSMs exhibited higher conversions of benzyl alcohol than ZSM-5(60) (Fig. 4(b)). The esterification reaction involves large molecular species, which is mainly catalyzed by Brönsted acids located at the surface of the mesoporous walls [20]. Thus, the low conversion of ZSM-5(60) could be attributed to diffusion limitations owing to its small pore diameter. The high catalytic activities of M-ZSM-60 and M-ZSM-40 indicated that hierarchical frameworks can lead to easier access to the active sites for reactants and faster diffusion of products. These results were consistent with the NH₃-TPD results.

Hierarchically structured zeolites can be used in many other catalytic reactions involving large molecules, in which diffusion constraints and/or the adsorption of the reactant molecules onto the acid sites are the main concern. Various esterification reactions of alcohols and acids could also be catalyzed with high yields (Table 2), indicating the superior efficiency of the M-ZSMs catalyst. The reactants containing an electron-donating group (MeO^-) showed a higher reactivity relative to those containing electron withdrawing groups (NO₂^-; Table 2, entries 3 and 4). Furthermore, the M-ZSM-60 catalyst showed good recyclability (Fig. 4(c)).

ZSM-5 monoliths with hierarchical porous structures were prepared through a steam-assisted crystallization method using sponges as rigid scaffolds. The obtained ZSM-5 monoliths were characterized by XRD and NMR. The ZSM-5 monoliths were highly crystalline. Owing to their hierarchical porous structure, they had strong acidity and exhibited superior catalytic performance in the liquid-phase esterification reaction.