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Optimization of Hierarchical Structures for Beta Zeolites by Post-Synthetic Base Leaching Ke Zhang, Sergio Fernandez, Sarah Kobaslija, Tatiana Pilyugina, Jeremy T O'Brien, John A. Lawrence III, and Michele L Ostraat Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b00912 • Publication Date (Web): 12 Jul 2016 Downloaded from http://pubs.acs.org on July 19, 2016
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Introduction Zeolites are a class of crystalline aluminosilicates with various framework structures and compositions extensively applied in heterogeneous catalysis, adsorption, and separation due to their high surface area, ion-exchange capability, strong acidity, shape selectivity, and excellent thermal/hydrothermal stability.1-3 Conventional zeolites are typically constructed from 8, 10, or 12 tetrahedral atoms with pore sizes less than 1 nm. For these microporous materials, the pore channels may impose significant diffusion limitations when the dimensions of penetrating molecules approach the sizes of the relatively rigid zeolite pore apertures, which is likely to reduce the accessibility of active sites within micropores and increase the possibility of coking in catalytic reactions. To address these challenges, hierarchical zeolites were conceived with the purpose of introducing additional porosity, usually in the mesopore range, by either top-down or bottom-up approaches.4-8 The top-down approach uses microporous zeolites that are post-synthetically leached to create hierarchical structures, while bottom-up approaches often involve more complicated templating routes, such as hard- or soft-templating strategies. Zeolite beta (*BEA type) is one of several zeolites important to the petrochemical and chemical industries that is able to catalyze a series of reactions, including alkylation, isomerization, and aromatic acylation.9-13 Zeolite beta is a large-pore, low-aluminum microporous material first reported by Mobil researchers that features an intergrowth of two or more polymorphs comprising a three-dimensional, 12-membered ring channel system.14,15 The bottom-up synthesis of hierarchically structured beta zeolites has been reported by various routes with or without the use of templates. Xiao et al. synthesized hierarchical zeolite beta with tunable mesoporosity by using a cationic polymer polydiallyldimethylammonium chloride (PDDA-Cl).16,17 The presence of PDDA-Cl effectively enhanced the interaction with inorganic silicate species and alleviated the phase separation issues frequently observed in conventional hexadecyltrimethylammonium bromide (CTAB) dual-templating systems.18-20 Hierarchical beta zeolites can be obtained either in the structure of nanoparticle assembly21,22 or single crystals23 via this PDDA-Cl cationic polymer route. Other cationic polymers, such as ammonium modified chitosan was also applied to the hierarchical synthesis of zeolite beta, wherein the modified chitosan exhibited good solubility in aqueous solution.24 Ryoo et al. synthesized mesostructured zeolite beta by designing multi-functional single templates of organosilanes to address phase separation issues, wherein hierarchical zeolites were obtained by 2
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conventional hydrothermal operations with simultaneous addition of surfactants in the precursor solutions.25 Alternatively, the organosilanes can be added after a separate precrystallization step for surface functionalization on zeolite beta seeds or protozeolitic nanoparticles. In this way, hierarchical beta zeolites are obtained through secondary crystallization, and the organosilanes serve as the mesoporosity generators and crystal growth inhibitors.26-28 When the zeolite precursor gel is sufficiently concentrated, mesoporous zeolite beta can be synthesized via a template-free route to achieve hierarchical structure via nanocrystal aggregation.29 Under this scenario, the drygel conversion method was very effective in constructing hierarchical structured beta zeolites.30 As compared to the previous bottom-up templating and template-free methods, the top-down approach by post-synthetic base leaching (e.g. desilication) is a highly reproducible, convenient, and cost-effective approach for the creation of hierarchical zeolites, which is favorable for commercial zeolite production.31-33 Pioneering work in this area focused primarily on MFI-type zeolites after Ogura et al. reported the formation of mesopores in ZSM-5 with preserved crystallinity after leaching in alkaline solutions.34-39 The crucial role of framework Al content was later identified and only MFI zeolites with Si/Al ratios of 25-50 could be conveniently leached with mesopore formation and micropore preservation upon a standard alkaline treatment (0.2 M NaOH at 65 oC). At lower Si/Al ratios, very limited mesoporosity was created, while higher Si/Al ratios resulted in excessive Si extraction and framework dissolution.40-43 Zeolite beta’s framework is less stable than MFI zeolites, and base leaching within the optimal Si/Al ratios (25-50) identified for MFI zeolites leads to extensive Si extraction, which negatively impacts the microporosity and acidity of the resulting zeolites.44 Unlike MFI-type zeolites, the critical effect of Al contents (Si/Al ratio) on the extent and efficiency of direct NaOH desilication for beta zeolites has not been fully established. Pé rez-Ramírez et al. developed a modified top-down approach by introducing pore-directing agents (PDAs) for the protection of less stable zeolite frameworks in alkaline solutions, and found that an effective PDA required cationic charge and alkyl moieties in the range of ca. 10-20 carbon atoms, such as TPA+.45-48 Recently, a fluorination pretreatment was suggested to facilitate the subsquent NaOH desilication of high Al MFI zeolites.49 It was proposed that the formation of F-bearing tetrahedral Al species alleviates the resistance of Al sites to alkaline medium and allows Si extraction in both Al-rich and Al-deficient zones. A sequential fluorination-desilication treatment enabled controllable framework dissolution with mesopores around 6 nm for a high Al ZSM-5 with Si/Al ratio of 14. However, the applicability of this protocol to other high Al zeolite topologies needs to be investigated. 3
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The present study reports a comprehensive optimization of hierarchical structures via different post-synthetic base leaching routes for beta zeolites with Si/Al ratios in the range of 14-250, including direct desilication in NaOH solutions and alkaline treatments with TPA+ as PDA. The sequential fluorination-desilication was applied for high Al zeolite beta (Si/Al = 14) to explore the applicability of this protocol on beta framework. According to the resulting findings, a general strategy will be summarized for fabricating mesostructured zeolite beta by post-synthetic base leaching.
Experimental The parent beta zeolites, provided by TOSOH Corporation, are coded as “Bx”, where “x” refers to the nominal Si/Al ratios. These zeolites are in their protonic form. The alkaline-treated samples in NaOH solutions are labeled as “Bx-yNa”, where y represents the molar concentration of NaOH solutions. When the sequential fluorination-desilication protocol was used, the treated samples were referred to as “FBx-yNa”, where “F” indicates a fluorination step before alkaline treatments. When TPA+ was added in the alkaline solutions as the PDA, the treated zeolites were labeled as “BxyNa+zTPA”, where “z” is the molar concentration of TPA+. NaOH, TPAOH (40 wt % in water), and NH4F were purchased from Fisher, Alfa, and Acros Organics, respectively. All base leaching treatments were carried out at 65 oC for 30 min. In a typical experiment, the alkaline solution (y M NaOH + zM TPA+) was stirred at 400 rpm and heated to 65 oC before the parent or fluorinated zeolite samples were added in the amounts of 3.3 g zeolites per 100 ml solution. The leached samples in the suspension were retrieved by quenching, centrifuging, washing with distilled water, and drying overnight at 110 oC. When an ion-exchange step was needed, the samples were transformed to the protonic form by a three-fold ammonium-exchange in 0.8M NH4NO3 (Fisher) and a subsequent calcination at 550 oC for 5 h at a heating rate of 10 oC/min. Mass yields of the obtained zeolites were estimated by weighing the samples without taking into account the operational loss during experiments. For fluorination, zeolite samples were stirred in NH4F solution (6 g zeolite; 50 mL, 0.36 M NH4F, 600 rpm) at room temperature for 8 h. The impregnated sample was dried at 110 oC overnight and then calcined at 550 oC for 5 h. Powder X-ray diffraction (XRD) measurements were taken using a Bruker D8 Discover diffractometer equipped with a Copper tube (l=0.15418nm) and a VANTEC-500 2-D detector. Data 4
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were recorded in the range of 5-60°. Scanning electron microscopy (SEM) imaging and elemental composition was determined using a JEOL JSM-7100F equipped with an Oxford Energy Dispersive X-Ray Spectroscopy (EDS) system. Images were taken using an operating voltage in the range of 210 kV. Transmission electron microscopy (TEM) was carried out in a JEOL 2100 TEM microscope operated at 200 kV and equipped with a Gatan Osiris 2k x 2k digital camera. To prepare TEM samples, a few drops of the sample suspended in ethanol were drop casted on a carbon-coated copper grid and allowed to evaporate at ambient conditions. NH3-TPD analysis was performed using a Micromeritics AutoChem II Chemisorption Analyzer with a flow-through reactor connected to a thermal conductivity detector (TCD). The samples were activated at 560 oC for 1h followed by the adsorption of NH3 at 180 oC. The NH3-adsorbed zeolites were purged in high purity helium flow gas for an extended 5 hours to minimize the extent of NH3 physisorption. Then, the TPD spectra were recorded by heating the samples from 180 oC to 550 oC at a rate of 10-20 oC/min in helium flow. The maximum temperature measured at different ramp rates was used to calculate the desorption activation energy (Ed).50 Nitrogen physisorption measurements were performed at -196 oC on a Micromeritics ASAP 2460. Prior to the measurements, all samples were degassed at 350 oC overnight under vacuum in a Micromeritics Smart VacPrep. The apparent surface areas were determined with the BrunauerEmmett-Teller (BET) method in the range between P/P0 0.02-0.12. The t-plot method was used to estimate the micropore volume (Vmic) and total pore volume (Vtot). An abacus was used to correct the underestimated Vmic by the t-plot method for hierarchical zeolites.51 The mesopore size distribution was obtained by the Barrett−Joyner−Halenda (BJH) model applied to the adsorption branch of the isotherm. An indexed hierarchy factor (IHF) is introduced to quantitatively evaluate the changes of textural properties of zeolite samples after base leaching. The IHF is defined as (Vmic/Vmic,max)×(Vmes/Vmes,max), wherein the Vmic and Vmes are the micropore volume and mesopore volume, while Vmic,max refers to the maximum micropore volume of the untreated zeolites, and Vmes,max refers to the maximum mesopore volume among the leached hierarchical zeolites. Solidstate magic-angle spinning nuclear magnetic resonance (MAS NMR) spectra were recorded on a Bruker Ascend 500 MHz spectrometer operating at a static field of 11.74 T. MAS NMR measurements were performed at a resonance frequency of 130.32 MHz. Spectra were recorded at a spinning rate of 10 kHz, a pulse length of 15 μs and a delay time of 1 s.
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Results Fig. 1 shows the X-ray diffraction patterns for various zeolite beta samples. For the B14 series, the crystalline structures were maintained after one-step leaching in 0.2 M NaOH at 338 K whether or not the pore-directing agent TPA+ was added. In the presence of TPA+, XRD results demonstrate excellent preservation of the structure integrity with high characteristic peak intensities. On the contrary, the fluorinated sample experienced severe structural amorphization after treating in 0.2 M NaOH at 65 oC with significant peak broadening and intensity decline. For beta zeolites with higher Si content (B20, B50, B250), the crystal structure is well preserved when TPA+ exists in the alkaline solutions. In the absence of TPA+, base leaching in 0.1 M NaOH at 65 oC almost resulted in a complete loss of crystallinity for the high Si zeolite beta (B250). Table 1 lists the values of 2θ and the corresponding d spacing of the two main reflections from XRD patterns of different beta samples. The reflections shifted to lower angles after base leaching, and the corresponding d spacing values increased. Fig. 2 illustrates the morphology changes for the original and treated zeolite beta (B14 series), as measured by SEM (a-d) and TEM (e-h). The crystal size of parent B14 is in the range of 0.2-0.4 µm (Fig. 2a). Upon leaching in NaOH solution, the external surface of zeolite crystals became more distinct with newly created surfaces and pores as seen in Fig. 2b. In the fluorinated sample, the damage to the crystals upon base leaching is more severe with visible detached and/or amorphous segments as shown in Fig 2c. The sample treated with TPA+ also featured some crevices and apertures on the outer surface (Fig. 2d). The created mesoporosity is also visible from TEM images (Fig.2e-2h). As compared to B14 (Fig. 2e), the NaOH treated zeolites display more open structures (Fig. 2f and 2g), while the alkaline-treated zeolites in the presence of TPA+ exhibit smaller and more uniform mesopore systems (Fig. 2h). Table 2 lists the porous properties of beta zeolites treated at different alkaline conditions. The N2 physisorption isotherms (Fig. 3) display large uptakes at low relative pressures (
