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Micro-explosion under microwave irradiation: A facile approach to create mesopores in zeolites Bin Zhang, Yahong Zhang, Yuanyuan Hu, Zhangping Shi, Arepati Azhati, Songhai Xie, Heyong He, and Yi Tang Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b00503 • Publication Date (Web): 04 Apr 2016 Downloaded from http://pubs.acs.org on April 9, 2016

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Chemistry of Materials

Micro-explosion under microwave irradiation: A facile approach to create mesopores in zeolites Bin Zhang, Yahong Zhang,* Yuanyuan Hu, Zhangping Shi, Arepati Azhati, Songhai Xie, Heyong He and Yi Tang* Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Laboratory of Advanced Materials, Collaborative Innovation Centre of Chemistry for Energy Materials Fudan University, 220 Handan Road, Shanghai 200433, China KEYWORDS: mesopore, micro-explosion, zeolite, hierarchical, hydrogen peroxide, microwave ABSTRACT: A facile micro-explosion approach has been successfully developed to produce an interwoven mesopore network in zeolite crystals via the rushing-out of gases generated by decomposition of H2O2 under microwave irradiation. This “gas imprint” method creates the mesopores from the interior crystal towards the exterior, in line with the direction of the pristine microporous channels and is different from the previous methods in which the reagent starts attack from crystal surface and perforate inward. The created mesopores extend throughout the whole crystal and highly blend into the intrinsic micropores around. The acidity of zeolite is also well preserved due to this unique mechanism of pore creation. The continuous high quality hierarchical architecture with intact acidity leads to a notable increase both in the conversion of 2-methoxynaphthalene acylation and in the selectivity to the target molecule of 2-acetyl-6methoxynapthalene. This micro-explosion approach offers an efficient synthesis protocol of zeolitic hierarchy integrating intersected mesoporosity and zeolitic microporosity and opens the way to the rational organization of meso- and microporosity for maximal advantage in applications.

1.

INTRODUCTION

Hierarchical zeolites arise a rapidly growing interest recently in both research and industry covering a wide variety of fields from petrochemistry, biomass conversion, environmental protection to the production of fine and specialty chemicals. They integrate the advantages of zeolitic microporosity, such as large surface area, strong acidity, high (hydro)thermal stabilities, moleculardimensioned shape-selectivity from topology,1-2 and those of mesoporosity with improved accessibility and mass transport.3-7 The art of hierarchical material is where to put the pores. Apart from the amount of mesopore, the mesopore quality including the size, distribution, connectivity and topology tend to play a pronounced role.8-11 Another prominent factor determining the performance of mesoporous zeolite is the preservation of the crystalline microporosity. Therefore, an interconnected hierarchical network in which the mesopores highly “blend into” structurally integral micropores, is expected to achieve a maximized hierarchical effect. In such a nanoporous system, the mesopores alleviate the diffusion limitation whereas the micropores nearby act as sub-nanoreactor providing the intrinsic active sites and shape selectivity. Intensive efforts have been dedicated into the synthesis and fabrication of hierarchical material, which can be divided into two categories: (1) constructive route including zeolitization of mesoporous material,12-13 double/multiple templating synthesis,14-17 assembly of nanozeolites or recrystallization,18-20 and (2) destructive

approach involving selective extraction of framework atoms. The latter method boasts the advantages of easy manipulability, low cost and wide applicability to various already-existed frameworks although some scientists regarded it having the inferior capabilities for predestination and controllability of pore structure.5 Hitherto, diverse methods has been developed in this category, e.g., desilication,21-23 dealumination,23-26 detitanation27 and deboronation.28 Desilication, a widely-applied one, is achieved by extracting framework silicon atoms in alkaline solution. The attack of alkaline on the zeolite framework normally starts from crystal surface and then perforate inward. It always shows a confined range of framework Si/Al ratios, e.g., 25-50 for MFI framework, unless some extra-processes are used, such as the pretreatment of the zeolites with low Si/Al ratios by acidwashing and the addition of tetraalkylammoniumcations or trivalent cations for those with high Si/Al ratios.29 Besides, for some ZSM-5 crystals employing the typical TPAOH as template with an Al-zoned rim, the preferential dissolution of Si-rich center always produce a hollow structure.22 This isolation of larger pores from the crystalline micropores may not be an effective hierarchy as expected.9 The recently reported degermanation in ITQ22 zeolite permits a relatively uniform distribution of inter-connected mesopores through the selective extraction of germanium-containing 4-rings units in the framework.30 But this method requires the prepreparation of the Ge-containing framework and might be limited to a certain group of zeolite frameworks.

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Scheme 1. Illustration of microwave-assisted H2O2-decomposition micro-explosion process and creation of interwoven mesopore network in the zeolite.

2.2 Sample preparation. Zeolite beta was synthesized with a gel composition of SiO2 : 0.025 Al2O3 : 0.45 TEAOH : 0.40 NH4F : 5.8 H2O, according to the literature methods31 but with an improved 2-step process including a relatively low temperature at 100 oC for 3 days and then 140 oC for 10 days. The prepared zeolite beta is denoted as beta-P. Silicalite-1 was synthesized according to procedures in reports.32 The synthesized samples were calcined in air at 550 oC for 4 hours. Mordenite was synthesized on the basis of the literature33 and 5 wt% seed was added. The modernite was calcined in air at 550 oC for 4 hours and then ammonium exchanged with 10 wt% NH4NO3 aqueous solution followed by calcination in air at 550 oC for 4 hours to get H-form before microexplosion treatment. H-form ZSM-5 and Y are commercially obtained from Nankai Catalyst Company. H-form SAPO-34 is from Shanghai Novel Chemical Technology Company. MCM-22-P was obtained from Shanghai Sinopec Group. They are referred as parent samples.

In our work, a novel H2O2-decomposition microexplosion method under microwave irradiation is developed, as illustrated in Scheme 1. The gases generated from the H2O2 decomposition, which are confined within the channels of microporous crystal, would rush against the framework and create secondary mesopores from the inside out to the edge. This intersected mesopore network radially spreading across the whole zeolite crystal displays a significant accessibility of internal active sites to guest molecules. Notably, the resulted crystal after this treatment features almost no penalty of the intrinsic microporosity and minor change in the elemental composition, inheriting the acidity of the parent crystal. Compared with many other post-treatments, this approach exhibits a clean and less time-consuming process since H2O2 aqueous solution is applied as a sole reagent and water and oxygen are produced in just a few minutes. No extra metal cations are introduced and thus no further ammonium exchange or calcination processes are required. The resulted mesoporous H-form zeolites can be directly used in catalytic reaction without further treatment. Moreover, this method is proved to be effective in various framework types with a wide Si/Al ratio range from 2 to ∝, such as BEA (zeolite beta), MFI (silicalite-1, ZSM-5), MOR (mordenite), FAU (Y), MCM-22 (MWW) and CHA (SAPO-34).

2.3 Micro-explosion Process. Micro-explosion by decomposition of H2O2 was carried out in a Teflon reactor inside a microwave oven (Preekem WX-8000, Shanghai). 100-200 mg of zeolite powder was added into 8 mL solution of H2O2 with concentrations of 4-30 wt%. The microwave procedure was divided into two successive steps. In Step I, the system was heated to a relative low temperature at 120 oC with microwave power of 200 W for 1 min to make the solution evenly heated and also to avoid the possibility of high system pressure caused by the fast decomposition of H2O2. In Step II, the system was heated to 180 oC with microwave power of 600 W and kept at this temperature for 9 min to ensure the complete decomposition of H2O2. After Teflon reactor was cooled to room temperature at which the pressure in reactor is less than 5 atmospheres, the air outlet valve was twisted to let the gases off. Then the treated samples were taken out, washed with deionized water for three times and dried at 80 oC. The samples treated as above for 1, 2 and 3 times are denoted as beta-MT1, beta-MT2 and beta-MT3, respectively. The yields of the samples are higher than 80% by this approach (Table 1). In comparison experiments, the parent zeolite beta was treated in a procedure similar to that of beta-MT1 but substituting H2O2 solution by pure water or HCl solution with a similar pH value to H2O2 solution (26 wt%) to obtain the samples of betaH2O and beta-MWAc, respectively.

2. EXPERIMENTAL 2.1 Reagents. Fumed silica (Aerosil 400, Shanghai Chlorine Alkali Industry), Tetraethylorthosilicate (TEOS, ≧99%, Shanghai Lingfeng Chemical Reagent Co. Ltd.), tetraethylammonium hydroxide (TEAOH, 40 wt%, Aldrich Chemical Reagent Co. Ltd.), tetrapropylammonium hydroxide (TPAOH, 25 wt%, Yixing Dahua Chemical Co. Ltd.), aluminum foil (≧99.5%, Sinopharm Chemical Reagent Co. Ltd.), hydrogen peroxide (H2O2, 30 wt%, Shanghai Reagent Factory), ammonium fluoride (NH4F, ≧ 96%, Sinopharm Chemical Reagent Co. Ltd.), sodium hydroxide (NaOH, ≧ 96%, Shanghai Reagent Factory), sodium aluminate (NaAlO2 , Al2O3 ≧41%, Sinopharm Chemical Reagent Co. Ltd.), and distilled water were used. All chemicals were used as received without any further purification.

2.4 Alkaline and acid treatment. Alkaline treatments of zeolite were conducted in a plastic tube with magnetic stirring. 150 mg of H-form zeolite beta was added into 10 mL of NaOH or Na2CO3 aqueous solution at 40 oC for 520 min. The samples after NaOH and Na2CO3 treatment are named as beta-ATx or beta-WAx, respectively, in which x represents various treatment conditions. The alkaline treatment was also conducted in Na2CO3 aqueous solution under microwave irradiation. All other conditions remained the same as those for preparation of beta-MT1. The obtained samples are named as betaMWWAx. In acid treatment, 150 mg of H-form zeolite beta was added into a 10 mL of HCl aqueous solution (0.1 M) and kept at 40 oC for 20 min or at 80 oC for 120 min. The obtained samples are named as beta-Ac1 and beta-Ac2, respectively.


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2.5 Catalytic Reaction. Catalytic alkylation of 2methoxynaphthalene (2-MN) was conducted in a 25 mL three-necked round bottom flask, connected to a reflux cooler system with magnetic stirring. The catalyst was activated at 350 oC for 2 hours before they were used. Typically, 170 mg of catalyst was added into 5 mL of chlorobenzene. Then, 4.0 mmol of 2-MN and 2.0 mmol of acid anhydride were added in turn. The reactions were carried out for at 120 oC for 48 hours under magnetic stirring. Small amounts of sample were taken periodically during 48 hours. The products were analyzed by a gas chromatographer (Shimadzu GC 2010 Plus) with a flame ionization detector and a 30 m capillary column of crosslinked 5% phenylmethylsilicone (HP-5), using nitrobenzene as internal standard.

2.6 Characterizations. The morphology information was obtained by field emission scanning electron microscope (FESEM, Hitachi S-4800) and field emission transmission electron microscope (FETEM, JEOL JEM2100F and Tecnai G2 F20 S-Twin). The energydispersive X-ray spectroscopy (EDX) attached to SEM was used to determine the elemental compositions. The crystalline structures were characterized by X-ray diffraction (XRD) on a RigakuD/Max-RB diffractometer with Cu Kɑ radiation at 40 kV and 40 mA. The N2sorption and Ar-sorption isotherms were measured by a Quantachrome Autosorb iQ2 instrument at 77 and 87 K, respectively. Based on N2-adsorption branch, the surface area was calculated by Brunauer-Emmett-Teller (BET) method in P/P0 range of 0.05-0.20, and the external surface area and micropore volume were calculated by t-plot method in P/P0 range of 0.25-0.40. The pore size distribution was estimated with non-localized density function theory (NLDFT) method on Ar adsorption branch, adopting the cylindar pore model. The mercury intrusion of porosimetry was performed on Micromeritics Autopore IV 9500 to characterize the open mesoporous structure. 29Si Magic Angle Spinning Nuclear Magnetic Resonance (MAS NMR), {1H}-29Si cross polarization (CP) MAS NMR, 27Al MAS NMR, and 1H MAS NMR were recorded on a Bruker AVANCE III 400 WB spectrometer. The chemical shifts of 29Si, 27Al and 1H were referenced to TMS, AlCl3 (aqueous solution, 1M), TMS at 0 ppm, respectively. 29Si MAS NMR and {1H}-29Si CP MAS NMR were performed with a spinning rate of 4 kHz. 27Al and 1H MAS NMR were performed with a spinning rate of 12 kHz. The spectra of 29Si MAS NMR were recorded at 79.6 MHz with an excitation pulse length of 1.67 µs and a recycle time of 20 s. 160 scans were accumulated. {1H}-29Si CP MAS NMR was performed with a 3 s recycle time, 1200 scans and an optimized contact time of 5 ms. 1H MAS spectra were recorded by a spin echo pulse sequence (π/2-τ-π-τ-acquire), where τ equals one rotor period (rotor synchronized). The excitation pulse length was 3.3 µs (π/2), and typically 400 scans were accumulated with a 6 s delay. For 27Al MAS NMR, the samples were fully hydrated in a desiccator for 1 day and the spectra were recorded at a resonance frequency of 104.3 MHz, using a pulse of 0.5 µs, a recycle delay of 0.4 s and 8000 scans. Except for pulse changing experiment, the pulse angle was kept at 10 degrees in all experiments.

The Fourier Transform infrared (FTIR) spectra were performed on a Nicolet FT-IR 360 spectrometer. The framework vibration bands of zeolite were obtained with KBr tablet at the room temperature. For studying sample acidity, the zeolite samples were pressed into thin selfsupporting wafers and in situ activated at 400 oC for about 2 hours in a vacuum IR sample cell. Then the zeolite wafer was exposed to pyridine vapors at a pressure of 2.2 mbar for approximately 10 min at 150 oC. Spectra were recorded after evacuation for 1 hour at the required desorption temperatures of 150-350 oC. The background spectrum, recorded under identical operating conditions without sample, was automatically subtracted for all measurements.

3.

RESULTS AND DISCUSSION

3.1 Morphology and Porosity. Zeolite beta, an important industry catalyst with three-dimensional intersecting microporous system of 12-membered rings, is firstly adopted as the typical sample to study the effects of H2O2 micro-explosion method. Figures 1A and 1B show the SEM images of the parent zeolite beta (beta-P) prepared by the improved 2-step hydrothermal crystallization. The crystals possess a well-shaped truncated square bipyramide with a uniform size of 500-700 nm. The XRD patterns (Figure 1C-a) confirm its BEA framework. After micro-explosion treatment (Figure 1D, betaMT3), the obtained zeolite crystals preserve their original morphology although the rougher crystal surface could be observed. Moreover, It is worthy to note that the crystallinity of zeolite beta is even improved after the first and second times of micro-explosion (Figure 1C-b for beta-MT1 and Figure 1C-c for beta-MT2) but decreased after the third time (Figure 1C-d, beta-MT3) from the changing of the intensity of characteristic diffraction peak of BEA zeolite at 22.5o.

Figure 1. SEM images of beta-P (A and B). XRD patterns (C) of beta-P (a), beta-MT1 (b), beta-MT2 (c) and beta-MT3 (d). SEM image of beta-MT3 (D).

N2-sorption isotherm of beta-P (Figures 2A-a) displays a type-I isotherm with very small hysteresis loop. The adsorption amount at low relative pressures in the range of P/P0 < 0.1 confirms the good microporosity of this sample. After the micro-explosion treatment, the adsorp-



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tion branch shows an enhanced uptake at medium and high relative pressures, accompanied by a prominent hysteresis loop, exhibiting combined type I and IV isotherms (Figures 2A, b-d), which indicates the creation of mesopores during this process. One time treatment generates a hysteresis loop with the desorption branch which is much steeper than the adsorption branch. The characteristic step on the desorption branch down at P/P0 = 0.42 displays an ink-bottle type mesopore due to the tensile strength or cavitation effect.34,35 The three times treated sample shows a type-H1 like hysteresis loop with paralleled adsorption and desorption branches, indicating the existence of cylinder mesopores open to the outside. The sample after two times of treatment (beta-MT2) exhibits a hysteresis loop with an intermediatestate shape between the above two (beta-MT1 and betaMT3). The sequential changes in the hysteresis loop and the isotherm of the samples after the treatment of 1, 2 and 3 times can be understood by assuming the creation of mesopores from the inner crystal to the outer surface, in line with the diffusion direction of the gases generated from the decomposition of H2O2. The textural data summarized in Table 1 reveal that the mesopore volume increases from 0.08 to 0.27 cm3/g after 3 times of treatment while the microporous volume shows a little sacrifice from 0.22 to 0.18 cm3/g. The NLDFT pore size distributions (Figure 2B) are derived from Ar adsorption branches (see Figure S1 in Supporting Information). The result shows that the increasing treatment times leads to increased amounts of the mesopores in the range of 2-14 nm. The single maximum of micropore distribution at about 0.64 nm (Figure 2C) in all the samples can be attributed to the average of two kinds of 12-membered rings in zeolite beta (0.66*0.67 nm in X and Y directions and 0.56*0.56 nm in Z direction). One time treatment makes this peak higher, probably implying the clear-up effect of microexplosion process, i.e. the washing out of structure defects, imperfect crystalline parts or the debris from the pores. However, further harsher treatments entail a loss of microporosity. These are consistent with the XRD data (Figure 1C).

Figure 2. N2-sorption isotherms (A) of beta-P (a), beta-MT1 (b), beta-MT2 (c) and beta-MT3 (d). Hg intrusion curves (A, inset) from 20 to 418 MPa of beta-P (a) and beta-MT2 (c). The NLDFT pore size distributions (B and C) derived from the high-resolution low-pressure Ar adsorption branches of beta-P (a), beta-MT1 (b), beta-MT2 (c) and beta-MT3 (d).

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In order to characterize the connectivity of intracrystalline mesopores with the external crystal surface, we have adopted mercury intrusion to detect the accessibility of mesopores (Figure 2A, inset and Figure S2 in Supporting Information). It turns out that in the microporous parent sample, almost no mercury molecules can penetrate although some small-sized mesopores exist (0.08 cm3/g, Table 1 and Figure 2B-a), leading to a platform in the Hg intrusion curve. In contrast, a jump can be obviously seen in the treated beta-MT2. The accessible mesoporosity to mercury in the range of 4-100 nm is 0.10 cm3/g. This datum accounts for more than half of the total mesopore amount of 0.18 cm3/g determined by t-plot method through nitrogen adsorption. Considering the capacity of mercury penetration into the mesopores down to around 4 nm,22,36 the amount of the mesopores open to the surface might be underestimated referred to Figure 2B-c. Table 1. Physicochemical properties of zeolite beta before and after micro-explosion. SBETb Sexc VMicroc VMesod Yielde 2 2 (m /g) (m /g) (cm3/g) (cm3/g) (%)

Sample

Si/Ala

beta-P

24

557

75

0.22

0.08

/

beta-MT1

32

588

155

0.20

0.15

83

beta-MT2

29

570

150

0.19

0.18

82

beta-MT3

30

555

162

0.18

0.27

83

[a] Determined by SEM-EDX. [b] Specific surface area given by BET method. [c] t-plot method. [d] VMeso = Vp - VMicro. Vp is the total pore volume at P/P0 = 0.97. [e] Yield is calculated by the mass ratio of the resulted beta-MT after micro-explosion treatment to the sample added to the H2O2 solution.

The TEM image in Figure 3a and Figure S3 of the supporting information shows that the parent zeolite (beta-P) possesses a homogeneous distinguishable lattice fringes without apparent structure defects. However, after once micro-explosion, the fissured secondary mesopores emerge within the crystals (Figure 3b and Figure S4). These mesopores show an increase in amount and size after the second treatment (Figure 3c and Figure S5). After the micro-explosion treatment of three times, the obtained beta-MT3 exhibits abundant mesopores from center to edge throughout the crystal (Figures 3d, 3e, 3f and Figure S6), in consistence with the escape direction of the generated gases from the decomposition of H2O2. Considering the difficulty to get a clear observation of the internal structure of samples due to thickness of crystal, TEM images of ultrathin section were taken on beta-MT1 and beta-MT3. In line with the results above, beta-MT1 presents thin cracks in the rim (Figures S7a and S7b) while more intersected mesopores can be seen in the deeper internal crystal (Figures S7c and S7d) compared with the conventional TEM image (Figure 3b). In the images of beta-MT3, a well-developed interwoven mesopore network covers the entire zeolite crystal and divides the crystal into isolated microporous domains with clear lattice fringes (Figures S6 and S8 in Supporting Information).


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Figure 3. TEM images of beta-P (a), beta-MT1 (b), beta-MT2 (c) and beta-MT3 (d, e, f).

Such unique hierarchical porous network looks just like a metropolitan city with a functional traffic system of motorways, streets and alleys extending in all directions.37 These will greatly improve the availability of native topology and mass transfer rate within the particle, offering numerous opportunities for targeted applications. In addition to the treatment times, the effects of concentration of H2O2, micro-explosion temperature on the mesopore formation are also studied on zeolite beta. Detailed micro-explosion conditions (Table S1 in Supporting Information) and the N2-sorption isotherms of the resulted samples (Figures S9 and S10) are described in the supporting information. Just like the number of treatments, the concentration of H2O2 solution and the reaction temperature display a positive correlation with the amount of mesopores, which can be demonstrated by their N2-sorption isotherms, that is, the increased size and the improved uptake of adsorption branch at medium and high relative pressures with the intensified treatment (Figures S9 and S10 in Supporting Information). Considering the hydrothermal condition of microexplosion process and the weak acidity of H2O2 solution, we apply the system with pure water or HCl solution of similar pH value to prove the positive effects of H2O2 solution, and the samples obtained are called beta-H2O and beta-MWAc, respectively. From the textural information in Table S1, beta-H2O possesses much less mesopores than beta-MT1, reflecting the weak role of “steaming effect” on the formation of mesopore. Moreover, beta-MWAc has very close data to beta-H2O, meaning that this weak acidity of dilute HCl does minor changes on the porous structure. Additionally, the pressures inside the reactor after reaction and cooling down to room temperature are about 4 atmospheres for beta-MT1 and in contrast 0 atmosphere for both beta-H2O and betaMWAc. The effect of hydrogen peroxide on the generation of the mesoporosity could be further confirmed by the sequential changes in the N2-sorption isotherms (Figure

2A) as well as porous structures in the TEM images (Figure 3 and Figures S3-S6, as well as their ultra-thin sections in Figures S7-S8) from beta-P, beta-MT1 to betaMT3. Obviously, the mesopores extend radially in the crystal and their amount and width increase with the increase of the severity of treatment, implying that the gases generated by the decomposition of H2O2 under microwave irradiation are rushing out against the framework in the crystal to form mesopores. Furthermore, we also add a controlled experiment to let out the gases at 13 atmospheres. It is found that the letting-out gases at a high pressure can lead to an increased uptake and a bigger hysteresis loop at higher pressures as well as a decreased adsorption at low pressure in the N2sorption isotherm (Figure S11), demonstrating the influence of gases attack on the pore creation.

3.2 Comparison with alkaline and acid leaching treatments. Alkaline treatment is a versatile postapproach in the introduction of mesoporosity in zeolites and has found inspiring success in the synthesis of mesoporous ZSM-5 zeolite. Pérez-Ramírez38 has studied the mesoporous beta zeolite obtained by desilication and found that zeolite beta is more proactive than ZSM-5, as the BEA framework constructed of fused subunits is very susceptible towards mesopore formation by desilication in alkaline media. The proper conditions for ZSM-5 can lead to the drastic damage of crystallinity and the serious loss of microporosity in zeolite beta. Our work also confirms the fact that zeolite beta almost collapses in a moderate treatment (beta-AT1, 0.1 M, 313 K and 20 min), and shows strong deagglomeration and substantial degradation of crystallinity even decreasing the treatment period to 5 min (beta-AT2, 0.1 M, 313 K and 5 min), as shown in Figures S12 and S13 of Supporting Information. The smaller crystal size of our beta-P may be blamed for the greater tendency to collapse, compared with zeolite beta in previous reports regarding alkaline treatment. A of mesopores have been introduced as deduced from the increased external surface area and mesoporous volume (Table S2 in Supporting Information) rooted in N2-



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sorption isotherms (Figure S14 in Supporting Information). By comparing beta-AT2 and beta-AT1 with different treatment time (Figure S13 in Supporting Information), tracing the change in the porous morphology and the imaging contrast in the outer and the central region of the crystal, we can clearly observe that the creation of mesopores starts from the surface with the treatment time, in agreement with the reported results.21,39,40 As is revealed in a more detailed observation by TEM (Figure 4 and Figure S15 in the supporting information), considerable amount of mesopores can be introduced in the outer crystal, but present a different pore morphology from the interwoven mesopore network in the microexplosion treated samples (Figures 3, 4 and Figures S4S6 in Supporting Information). We also try to employ weaker alkaline Na2CO3 (Figures S16 and S17 in Supporting Information) to better control the desilication process. The samples after Na2CO3 treatment in the concentrations of 0.2 and 0.5 M are denoted as beta-WA1, beta-WA2, respectively (Table S2 in Supporting Information). It was found that Na2CO3 treatment can preserve the crystallinity to a greater degree compared with NaOH treatment but the mesoporous volume and crystallinity by Na2CO3 treatment are not comparable with those of micro-explosion treated counterparts. For example, the micropore volume of both beta-WA2 and beta-MT3 reaches 0.18 cm3/g but the crystallinity (Figures 1C and S16 of Supporting Information) and mesopore volume (Table S2 in Supporting Information) of beta-WA2 (0.11 cm3/g) is much less than beta-MT3 (0.27 cm3/g, Table 1). We have made further attempts in combining the weak alkaline Na2CO3 with microwave irradiation in microexplosion processes, and found that the Na2CO3 solution with low concentration of 0.2 M can lead to an effective birth of 0.21 cm3/g mesopores with decreased micropores to 0.14 cm3/g (beta-MWWA1, Table S2 and Figure S18, Supporting Information). When the concentration of Na2CO3 solution is lowered to 0.025 M, the resulted sample (beta-MWWA2) can preserve the microporosity of 0.20 cm3/g and good crystallinity with increased mesoporosity to 0.14 cm3/g (Table S2, Figures S18 and S19). The application of weak alkaline under microwave irradiation can better control the desilication process, which may be owed to a more evenly distributed OH- attack. As with the acid leaching of zeolite beta, the crystallinity decreases (Figure S20 in Supporting Information) and the dealumination occurs. The Si/Al ratios of betaAc1 and beta-Ac2 are 30 and 115, respectively. However, no obvious changes in the N2-sorption isotherms can be detected (Figure S21 in Supporting Information). The microporous volume increases from 0.22 to 0.23 cm3/g (beta-Ac1, Table S2 in Supporting Information), owing to the washing out of extra-framework aluminum species in the porous channels. Further increasing the leaching time and temperature does not generate mesoporosity (beta-Ac2, Figure S21 and Table S2 in Supporting Information). However, the decrease of Al will cause a direct loss of acidic sites. The mapping images of zeolite beta treated by different approaches are summarized in Figure S22 of the supporting information. The uniform elemental distribu-

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tions can be found in all the samples. A visible removal of silicon atoms can be observed in the alkaline treated beta-AT2 (Figure S22-c in Supporting Information), leading to its decreased Si/Al ratio. Although beta-MT2 and beta-Ac1 possess the same Si/Al ratio of 30 (Figure S22b and S22d, Supporting Information), no mesoporosity is introduced in the beta-Ac1 (Figure S22 and Table S2 in Supporting Information).

Figure 4. STEM-HAADF and TEM images of beta-P (a1, a2), beta-MT2 (b1, b2), beta-AT2 (c1, c2), and beta-Ac1 (d1, d2).

Combining these results, micro-explosion method has shown a superior performance in the porous hierarchy development for zeolite beta with 3-dimensional porous channel system and agglomeration growth habit. It is effective to bring about interconnected mesopores from the interior of crystal, exposing the internal active sites to a large extent. Importantly, the introduced mesoporosity is interwoven throughout the crystal and the zeolitic microporosity around remains almost intact. Furthermore, the slight change in Si/Al ratios ensures the acidic sites in catalytic reactions.

3.3 Versatility of the micro-explosion process. This micro-explosion treatment is extended to other zeolites


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and zeolite-like materials with different framework structures and compositions. As is expected, the mesopores can be introduced in all of these microporous crystals, as demonstrated by the uptake of adsorptive curves in medium and high relative pressures and the presence of hysteresis loop (Figure S23 in the supporting information), comprising silicalite-1, ZSM-5, mordenite, Y, lamellar MCM-22-P zeolites and even zeolite-like silicoalumino-phosphate SAPO-34. The parent samples before micro-explosion are denoted as Silicalite-1-P, ZSM-5-P, Mordenite-P, Y-P, MCM-22-P and SAPO-34-P. All these samples are H-form except for MCM-22-P as a template-containing sample to preserve the lamellar structure. The Si/Al ratios of these zeolites are ∞, 18, 9, 2 and 45 for Silicalite-1-P, ZSM-5-P, Mordenite-P, Y-P, MCM-22-P, respectively, covering a wide range from 2 to ∞. The (Al+P)/Si ratio is 5 for SAPO-34-P. Their microexplosion treated products are accordingly named as Silicalite-1-MT, ZSM-5-MT, Mordenite-MT, Y-MT, MCM-22-MT, with the Si/Al ratios of ∞, 19, 11, 3, and 48, respectively, as well as SAPO-34-MT with the (Al+P)/Si ratio of 7. The slight changes in the elemental composition and well-preserved crystallinities (Figure S24 in Supporting Information) prove the wide availability of this approach. In further detailed studies, we have found the shape and direction of secondary meospores after microexplosion treatment display a direct relation with those of the pristine microporous channels of the parent samples. For example, the Y zeolite with a three-dimensional 12-membered ring channel system (Figure 5a1) and the SAPO-34 with a three dimensional 8-membered ring channel system (Figure S25 in Supporting Information), exhibits a radial-oriented mesoporous system in all directions after the micro-explosion treatment (Figures 5a2 and S25 in Supporting Information), which represent the similar mesoporous direction in zeolite beta being discussed above. Silicalite-1 features an interconnected two dimensional microporous system built by the straight 10-membered ring channels in the [010] direction and the sinusoidal 10-membered ring channels in the [100] direction. After the micro-explosion treatment, the mesopores can be clearly seen in the (010) face in line with the main channels of [010] (Figures 5b1 and 5b2). The zeolite mordenite with one dimensional 12membered ring microporous channels in the [001] direction obtains secondary mesopores in [001] direction after the micro-explosion treatment (Figures 5c1 and 5c2 and Figures S26 and S27 in Supporting Information). The sample MCM-22-P, the precursor of MCM-22, is a typical lamellar zeolite made up of silicate layers. The calcination to remove the organic template will lead to the condensation of the layers of MCM-22 precursor, creating a three dimensional surpercage system. The micro-explosion treatments exert a delamination effect on MCM-22-P, as indicated by the decrease in the thickness of the layers after treatment from SEM and TEM images (Figures 5d1 and 5d2). This can be further confirmed by the disappearance of XRD peak at 6.7o/2θ in relation to (002) crystal plane (Figure S24 in Supporting Information). Additionally, the detemplation has also taken place in the meantime, as confirmed in the FTIR spectra in Figure S28 in Supporting Information. The bands at 2950, 2865 and 1470 cm–1 are ascribed to –

CH2– groups, while the band at 1460 cm–1 corresponds to –NH– groups of the template in the parent MCM-22P. These vibrations disappear in the treated MCM-22MT, implying the complete removal of the template.

Figure 5. TEM images of Y-P (a1), Silicalite-1-P (b1), Mordenite-P (c1), Y-MT (a2), Silicalite-1-MT (b2), MordeniteMT (c2). SEM images of MCM-22-P (d1) and MCM-22-MT (d2).

From the comprehensive investigation on the hierarchical porous morphology, the tight link in the shape and direction between secondary mesopores and the parent microporous channel system is clearly displayed. Considering the fact that the gases generated by the decomposition of H2O2 in the crystal will release or find their way out along the microporous channels, this phenomenon can further demonstrates the interior-to-exterior micro-explosion mechanism, i.e., the imprint of rushingout gases generated by the decomposition of H2O2. Moreover, the minor change in the elemental compositions after treatments elucidates the well preservation of acidic sites of these samples, laying a solid foundation for their favorable catalytic functions.

3.4 NMR and IR study of the micro-explosion process. The effects of micro-explosion process on the properties of the samples are further investigated by focusing on zeolite beta. The SEM-EDX data reveal that Si/Al ratios of the samples after micro-explosion treatments slightly increase to around 30 from 24 of the parent sample (Table 1).



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Figure 6. 29Si MAS NMR (A), {1H}-29Si CP MAS NMR (B),1H spin-echo MAS NMR (C) and 27Al MAS NMR (D) spectra of betaP (a), beta-MT1 (b), beta-MT2 (c) and beta-MT3 (d).27Al MAS NMR spectra (E) of beta-MT2 (c), as well as HCl-washed samples beta-MT2-Ac1 (c-Ac1) and beta-MT2-Ac2 (c-Ac2).27Al MAS NMR spectra (F) of beta-MT2 with pulse angles from 10, 30 to 50 degrees.

The 29Si MAS NMR spectra of samples are shown in Figure 6A. The peaks at -114 and -110 ppm are assigned to Q4 (Si(OSi)4) and those at -104 and -101 ppm to Q3 (Si(OSi)3(OAl) and Si(OSi)3(OH)). The multi-peaks of Q4 are regarded to originate from either the two different stacking orders of polymorphs A and B of zeolite BEA or the two T sites with different chemical environments.41,42 Q4/Q3 ratio uplifts after once treatment and slightly fluctuates after further treatments. Figure 6B is their {1H}29Si CP MAS NMR spectra. The increasing relative intensities of the peak at -104 and -101 ppm indicate the large amounts of OH groups nearby. Considering the Si/Al ratios of the samples (Table 1), we can infer a possible demetallation priority of aluminum and then silicon. In addition, the peak at -96 ppm assigned to Q2 position in parent sample disappears in the treated samples. This confirms the clear-up effect of micro-explosion treatment, through which the structure defects or imperfect crystalline part in zeolite can be removed. 1H spin-echo MAS NMR spectrum of beta-P (Figure 6C) show two main peaks at 3.8 and 1.3 ppm, assigned to the Brönsted acidic bridged hydroxyl group and terminal SiOH, respectively. After micro-explosion treatment, a broad signal of H-bonded hydroxyls around 4.9 ppm appears, indicating the formation of hydrogen bond during micro-explosion (Figure 6C).43 In the treated samples, the hydroxyls related to extra-framework aluminum can be traced by the small peaks at 3.6 and 0.2 ppm. The former is the hydroxyls connected to perturbed aluminum which is partially hydrolyzed and still connects to framework, while the latter is linked to unperturbed

aluminum species out of framework.44-46 The decreased intensity of the peak at 3.6 ppm and in parallel the increased intensity of the peak at 0.2 ppm provide a vivid picture of how aluminum moves off the framework gradually in the treatment process. In the FTIR spectra (Figure S29 in Supporting Information), no obvious change can be detected in the region of framework vibrations after micro-explosion, indicating the well preservation of BEA framework during micro-explosion. Pyridine adsorption FTIR is adopted to characterize the acidic properties of the samples (Figures S30 and S31 in Supporting Information). By comparing the pyridine adsorption curves at 150 oC for acidic sites detection, no notable decrease in the total density of acidic sites is found although slight dealumination occurs. The vibration peaks at 1542 and 1637 cm-1 are assigned to Brönsted acid sites. The minor decrease of the two after treatment can be explained by the small loss of aluminum atoms in framework. Those at 1454, 1620 cm-1 and 1444, 1595 cm-1 correspond to two types Lewis acid sites of L1 and L2, respectively.47 Although the intensity of L2 is much stronger than that of L1 at low temperature, it decreases much faster than the latter as the temperature increases, showing the weaker acidity of L2. The less acidic Lewis site L2 at lower wavenumber is generally believed to come from the interaction through H-bond with Brönsted acid sites, which causes a shift of the corresponding bands to lower frequency.48 After treatment, the relative intensity of L2 increases to an extent, in line with 1H spin-echo MAS NMR result (Figure 6C) that the micro-explosion treatment gives rise to an amount of hydroxyls while the debris and imperfect crystalline


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parts in the parent zeolite are cleared up as shown in XRD, pore size distribution and 29Si MAS NMR experiments. A sharp peak at 54 ppm with a shoulder at 58 ppm can be observed in the 27Al MAS NMR spectra of all samples (Figure 6D), which can be assigned to tetrahedral framework aluminum at two distinct lattice positions of T1-T2 and T3-T9 in zeolite beta, respectively. With the increased number of treatments, the decrease in the relative area of the peaks at 58 to 54 ppm implies that the dealumination prefers to take place at T3-T9 sites. These results are in accordance with the literatures concerning steaming or acid treatment of zeolite beta.49,50 A new signal at 41 ppm appears after micro-explosion treatment in 27Al MAS NMR and its intensity increases with increasing treatment times (Figure 6D). In literatures, this chemical shift is assigned to distorted 4coordinate tetrahedral framework aluminum or 5coordinate extra-framework aluminum.50-54 In most studies at present, this chemical shift exists as a broad peak overlapped by tetrahedral peak around 55 ppm and octahedral peak around 0 ppm. While in our case, the peak at 41 ppm appears as a distinct discrete sharp peak that has not been reported according to our best knowledge. As is known, acid washing is able to remove extra-framework aluminum species. The signal at 41 ppm can be observed even if beta-MT2 is washed by 0.05 M and 0.1 M HCl solutions, which are denoted as betaMT2-Ac1 and beta-MT2-Ac2 (Figure 6E), respectively. The possible assignment of 41 ppm to 5-coordinate extra-framework aluminum can be ruled out. Besides, taking the relative strength changes of the three peaks at 58, 54 and 41 ppm into consideration (Figure 6E), we conclude that the peak at 41 ppm is a signal of framework aluminum due to its relative higher stability towards acid treatment compared with that at 58 ppm of framework aluminum species. Furthermore, we carried out singlepulse experiments with different pulse angle of 10, 30 and 50 degrees to clarify the quadrupolar interaction of the corresponding Al species (Figure 6F). For 27Al nuclei, I = 2/5, the signals of sites with strong quadrupolar interaction are expected to decrease sharply when increasing pulse length.55 The increase of pulse angle from 10 to 50 degrees does not cause an obvious change in the relative intensity ratio of the three peaks. Thus, we confirm that the new peak at 41 ppm observed in this work has a very similar nature with those at 54 and 58 ppm. By further including that this kind of Al species accounts for about 2/5 of the total Al species in the 27Al MAS NMR spectra (Figure 6D) and the crystallinity (Figure 1C) is still well preserved, we ascribe the Al species at 41 ppm to distorted 4-coordinate framework species rather than 5-coordinate extra-framework species. Combining the results of EDX, 29Si and 27Al MAS NMR as well as FTIR above, aluminum and silicon are extracted from framework in the micro-explosion process. Aluminum at less stable sites of T3-T9 is removed at first and then other aluminium and silicon atoms. A recent work has been done in DFT calculation for the possible reaction paths in steaming process,56 demonstrating that both dealumination and desilication occur but dealumination is easier due to lower effective barriers, more stable intermediate configurations, and stronger adsorption

of the water molecules involved in hydration reaction. The similar situation may occur during the microexplosion treatment in the weak acidic H2O2 solution and hydrothermal atmosphere. Accompanying with the demetallation, a special sharp and distinct peak at 41 ppm is very clearly observed in 27Al MAS NMR spectra. We assign it to a signal of distorted 4-coordinate framework aluminum according to its relative intensity change in acid treatment and pulse angle changing experiment. The further study is still under our investigation.

3.5 Catalytic reaction. Friedel-Crafts acylation of 2methoxynaphthalene (2-MN) attracts much attention since 2-acetyl-6-methoxynapthalene (2,6-AMN) is a precursor of the anti-inflammatory drug Naproxen. The acylation of 2-MN generally produces two main isomers, i.e., 2,6-AMN and 1-acetyl-2-methoxynapthalene (1,2AMN). As regards the reaction results, 3 major factors must be taken into account: (1) The high activity of 1-position in 2-MN leads to the kinetically controlled production of 1,2-AMN, which easily takes place in the early period of the reaction and can follow by the deacylation or the isomerisation to gain 2,6-AMN; (2) 2,6-AMN is thermodynamically stable product, which will hardly undergo deacylation afterwards and will accumulate as the reaction proceeds; And (3) the difference in the molecular size of two isomers can cause a shape selectivity effect in a confined space that catalyst may play an important role. Solid acid zeolite beta is considered as a most suitable catalyst for the shape selective production of 2,6-AMN, a linear isomer 2,6-AMN molecule which exactly matches the pore diameter of zeolite beta and can be formed both within the pores and on the outer surface of zeolite beta. While the other isomer branched 1,2-AMN with a bulky molecular size can only be formed on the external surface.57,58 Herein the catalytic acylation of 2-MN may serve as a good model reaction to explore the effects of meso- and microporosity.

Figure 7. Conversion of 2-MN versus reaction time for betaP (■) and beta-MT2 (□) and selectivity to 2,6-AMN versus reaction time for beta-P (●) and beta-MT2 (○).

As is shown in Figure 7, compared with the parent counterpart (beta-P), the hierarchical beta-MT2 obtained by micro-explosion treatment shows a considerable increase in conversion from 41 to 96 %, which is attributed to its interwoven mesopore network and im-



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proved crystallinity (Schemes 2a and 2b). The crystalspread and radial-oriented mesopores provides more accessible active sites and the improved microporous crystallinity contributes to an increased acidity. As for the selectivity, 2,6-AMN is predominantly formed in the presence of both beta-P and beta-MT2 in our experiments. Remarkably, it is worth mentioning that the selectivity to 2,6-AMN in the acylation of 2-MN catalyzed by beta-MT2 (64 %) turns out to be higher than beta-P (57 %), which is different from previous reports.59,60 It is generally known that short microporous channel and large external surface area will weaken the shape selectivity of zeolite catalysts, and so as to lead to the decrease of product 2,6-AMN. This means that other factor, such as the structure of beta-MT as well as thermodynamical stability of 1,2-AMN and 2,6-AMN, has to be taken into account. It has been observed that 1,2AMN could isomerize into 2,6-AMN with the prolonging reaction time and the increasing reaction temperature in the presence of zeolite beta. This isomerization is realized by the protodeacylation to form 2-MN or by the transacylation through an intermolecular reaction with a molecule of 2-MN to obtain 2,6-AMN.61 The special porous hierarchy arrangement in beta-MT2 combines the intersected radial-oriented mesoporosity which exposes abundant micropore mouths and the retained microporsity which provides qualified acidic sites. These could facilitate the approach of produced 1,2-AMN towards the microporous channels of zeolite beta and promote the isomerization of 1,2-AMN into 2,6-AMN (Scheme 2c). Therefore, beta-MT2 display high selectivity to 2,6-AMN in 2-MN acylation reaction. We have also tried this catalytic reaction on the microwave-assisted weak alkaline Na2CO3 treated samples of beta-MWWA1 and beta-MWWA2 for comparison. It turns out that the sample of beta-MWWA1 which has the most mesoporous volume of 0.21 cm3/g suffers the lowest conversion rate and selectivity to the target product molecule of 2,6-AMN among all the treated mesoporous zeolite samples (Figures S32 and S33 in Supporting Information). The sample of beta-MWWA2, though with a higher microporous volume than beta-MT2 and wellpreserved crystallinity, shows poorer catalytic performance in terms of both conversion and selectivity. This firmly supports that the integrity or the quality of the microporous structure is of dominant importance in this acid-catalyzed shape selective reaction. The conventional alkaline treatment in which the silicon atoms are extracted from framework to form mesopores always causes a partial removal of framework chains or areas in the crystals and also interrupts the micropores left, especially in a framework like zeolite beta. In the micro-explosion process, the mesopores are created as the imprint of the rushing-out gases along the microporous directions and in this way the pristine micropores can be well preserved and the debris inside the framework channels or imperfect-crystalline parts are cleared up at the same time. Consequently, a welldeveloped hierarchical network integrating the interwoven mesoporous channels into the qualified crystalline microporous domains around is established, contributing to an inspiring performance in catalytic applications.

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Scheme 2. Illustration of conversion of 2-MN in beta-P (a) and beta-MT (b) and the promoted transacylation of 1,2-AMN to form 2,6AMN in the meso- and micro-porous intersection in beta-MT (c).

4.

CONCLUSIONS

In summary, for the first time, a micro-explosion method by microwave assisted decomposition of H2O2 has been used to create mesopores from the interior zeolite crystal towards the exterior as a facile and efficient method. The mesoporosity extends from center to edge throughout the crystal and build up a developed interwoven hierarchical network associated with microporosity, endowing the zeolitic material with shorter diffusion path length and more accessible active sites. Interestingly, this approach creates mesopores in line with the direction of the pristine crystal channels, thus preserving the microporous structure and crystallinity of zeolite to a great extent. In this process both dealumination and desilication occur, resulting in a slight change of Si/Al ratio in zeolites. Thanks to the high quality and quantity of the acidic sites as well as the rational integration of created mesoporosity and well-preserved microporosity, an enhanced catalytic performance in both the conversion of 2-MN and the selectivity to 2,6-AMN is observed, confirming the impressive advantage brought by this hierarchical structure. Moreover, this micro-explosion approach shows great potential in various framework types of zeolite family, adding to the post-synthesis category of hierarchical material and is expected to bring about desirable performances in a multitude of applications.


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Chemistry of Materials (2) Corma, A. From Microporous to Mesoporous Molecular

ASSOCIATED CONTENT

Sieve Materials and Their Use in Catalysis. Chem. Rev.

Supporting Information. 1997, 97, 2373–2420.

Ar- and N2-sorption isotherms, TEM, SEM, XRD,FTIR and

(3) Davis, M. E. Ordered Porous Materials for Emerging Ap-

catalytic results are included. This material is available free of charge via the Internet at http://pubs.acs.org.

plications. Nature 2002, 417, 813–821. (4) Hartmann, M. Hierarchical Zeolites: A Proven Strategy to Combine Shape Selectivity with Efficient Mass Transport.

AUTHOR INFORMATION

Angew. Chem. Int. Ed. 2004, 43, 5880–5882.

Corresponding Authors

(5) Chal, R.; Gérardin, C.; Bulut, M.; van Donk, S. Overview and Industrial Assessment of Synthesis Strategies towards

* (Y.T.) E-mail: [email protected].

Zeolites with Mesopores. ChemCatChem. 2011, 3, 67–81.

* (Y.H.Z.) E-mail: [email protected]. (6)

Valtchev, V.; Majano, G.; Mintovaa, S.; Pérez-Ramírez, J. Tailored Crystalline Microporous Materials by Post-

Present Addresses

Synthesis Modification. Chem. Soc. Rev. 2013, 42, 263–

† (Y.T.) Fudan University, 220 Handan Road, Shanghai

290.

200433, China. (7)

Serrano, D. P.; Escolac, J. M.; Pizarroa, P. Synthesis Strat-

† (Y.H.Z.) Fudan University, 220 Handan Road, Shanghai egies in the Search for Hierarchical Zeolites. Chem. Soc.

200433, China. Rev. 2013, 42, 4004–4035.

Author Contributions

(8) Milina, M.; Mitchell, S.; Crivelli, P.; Cooke, D.; Pérez-

The manuscript was written through contributions of all authors. All authors have given approval to the final version

Ramírez, J. Mesopore Quality Determines the Lifetime of Hierarchically Structured Zeolite Catalysts. Nat. Commun. 2014, DOI: 10.1038/ncomms4922.

of the manuscript.

(9) Groen, J. C.; Zhu, W.; Brouwer, S.; Huynink, S. J.;

ACKNOWLEDGEMENTS

Kapteijn, F.; Moulijn, J. A.; Pérez-Ramírez, J. Direct

This work is supported by 973 program (2013CB934101),

Demonstration of Enhanced Diffusion in Mesoporous ZSM-5 Zeolite Obtained via Controlled Desilication. J. Am.

NSFC (21433002, 21473037, 21171041 and U1463206), Chem. Soc. 2007, 129, 355–360.

STCMS (09DZ2271500), and National Plan for Science and (10) Pérez-Ramírez, J.; Christensen, C. H.; Egeblad, K.; Chris-

Technology (14-PET827-02).

tensend, C. H.; Groen, J. C. Hierarchical zeolites: Enhanced Utilisation of Microporous Crystals in Catalysis by

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Isomerisation

over

a

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