Research Article www.acsami.org
Mesoporogen-Free Synthesis of Hierarchically Structured Zeolites with Variable Si/Al Ratios via a Steam-Assisted Crystallization Process Xiaoyun He,† Tongguang Ge,† Zile Hua,*,† Jian Zhou,‡ Jian Lv,† Jinling Zhou,† Zhicheng Liu,‡ and Jianlin Shi† †
State Key Lab of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, P.R. China ‡ Sinopec Shanghai Research Institute of Petrochemical Technology, 1658 Pudong North Road, Shanghai 201208, P.R. China S Supporting Information *
ABSTRACT: In the absence of additional mesoporous template, hierarchically structured zeolites (HSZs) with variable Si/Al ratios (30−150) have been successfully synthesized via a newly developed steam-assisted crystallization process. The synthesized materials were characterized with powder X-ray diffraction, nitrogen sorption measurement, scanning electron microscopy, transmission electron microscopy, inductively coupled plasma optical emission spectrometry, solid-state nuclear magnetic resonance, and ammonia temperature-programmed desorption. All these results prove that the synthesized materials feature high crystallinity (microporous framework) and auxiliary mesoporous structure. In the model reactions of isopropylbenzene and 1,3,5-triisopropylbenzene cracking, compared to purely microporous ZSM-5 counterparts, here synthesized HSZs exhibited markedly enhanced catalytic performances resulting from their enlarged external surface area and shortened diffusion length in the microporous system. KEYWORDS: hierarchically structured zeolites, mesoporogen-free, steam-assisted crystallization, Si/Al ratios, cracking
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INTRODUCTION ZSM-5 zeolites are crystalline aluminosilicates with well-defined microporous structures. Because of their intrinsic acidity, large specific surface area, high stability, and molecular shape selectivity, they have been an important class of solid acid catalysts in industry.1 However, the sole microporous system of pure zeolites not only leads to catalyst deactivation and consequently unfavorable catalyst regeneration but also restricts their applications in bulky-molecule-involved process. 2 Although ordered mesoporous materials, such as M41S and SBA-15, possess adjustable large pore sizes (2.0−30 nm in diameter), their practical applications are still far from extensive success, owing to their amorphous framework and the resultant low stability and weak acidity.3 Therefore, in recent years, hierarchically structured zeolites (HSZs), which integrate microporous crystalline framework with auxiliary mesoporous structure, have attracted great interest.4 About HSZs synthesis strategies, the “bottom-up” (or constructive) method has been developed, which strongly relies on the rational design/selection of porous agents and/or synthesis procedure.4 Generally, the “bottom-up” method requires two types of porogens, namely, a small molecular structure-directing agent (SDA) responsible for microporosity formation and a large template aimed at creating desired mesoporosity. Depending on the physicochemical nature and interactive effect with inorganic sources, mesoporogens can be © XXXX American Chemical Society
further divided into hard templates and soft templates. Hard templates are rigid and could only physically mix with inorganic sources while serving as a sacrificial scaffold for mesoporosity. In 1999, Holland et al. first used arrays of monodispersed polystyrene spheres as hard template and tetrapropylammonium hydroxide (TPAOH) as SDA and successfully synthesized silicates with bimodal pore structures of macropores (250 nm in diameter) that are surrounded by microporous silicalite walls.5 Besides, carbon materials such as carbon pearls,6 carbon nanotubes,7 and carbon nanofibers8 are also adopted. Soft templates, on the other hand, could not only act as physical scaffolds but also interact chemically with inorganic sources. The group of soft templates consists of silylated surfactants,9,10 polymers,11,12 hydrophilic carbonhydrates,13 and so on. More importantly, Ryoo et al. designed a number of bifunctional templates, combing features of both SDA and mesoporogen within one molecule, and successfully synthesized zeolite nanosheets with hierarchical structure.14,15 On the basis of the steam-assisted crystallization (SAC) process, Shi and coworkers have also reported the preparation of HSZs by using ordinary mesoporogens, for example, triethanolamine (TEA), P123, F127, and even sucrose as an in situ carbonaceous Received: January 7, 2016 Accepted: March 1, 2016
A
DOI: 10.1021/acsami.6b00141 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces template.16−18 However, reported templating synthesis of HSZs accompanied by massive mesoporogen usage is neither costefficient nor environmentally friendly because most template agents are expensive, irritative, and nocuous. Moreover, it would emit a large amount of greenhouse gases, such as NOx and CO2, in the followed calcination step to remove the organic or inorganic templates. Novel synthesis routes using fewer mesoporogens or those that are even free of mesoporogens are highly desirable and have become a promising research field. Using tetrabutylphosphonium hydroxide or tetrabutylammonium hydroxide as the SDA, Tsapatsis et al. first synthesized hierarchical pentasil zeolite made of orthogonally connected microporous nanosheets by a novel repetitive branching method.19,20 The house-of-cards arrangement of the nanosheets created a permanent network of 2−7 nm mesopores. However, the yield of HSZs with respect to silica source is relatively low, approximately 40%. Shortly afterward, Okubo and co-workers devised a sequential intergrowth approach for the construction of hierarchically organized silicalite-1 zeolite with the use of self-synthesized SDA.21 Unfortunately, this synthesis process requires a long hydrothermal duration of about 264 h and extraframework Al species exist in the synthesized aluminosilicate HSZs. On the other hand, by the addition of a nucleationpromoting agent22 or the new process design,23−26 without any mesoporogens, hierarchically porous structured aggregates of ZSM-5,22 ZSM-12,22 beta,23−25 and TS-126 nanocrystals had also been synthesized. Very recently, only with ordinary small-molecular SDA, TPAOH, we have reported the successful synthesis of singlecrystalline ZSM-5 HSZs via a SAC process.27 Since the zeolite surface acidity is closely related to the Al-substitution content in the framework, herein we extend the ZSM-5 HSZs synthesis to a wide range of Si/Al ratios from 30 to 150. When the TPAOH SDA concentration in the synthetic system is tuned, ZSM-5 HSZs with favorable hierarchical micro-/mesoporous structure are obtained under different Si/Al ratios. Finally, their catalytic performances are characterized with model reactions of alkylbenzene cracking.
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addition of TEOS constant. Table 1 summarizes the composition and reaction conditions of synthesized materials.
Table 1. Composition and Reaction Conditions of Synthesized Materials entry
sample
1 2
HSZ30-0.1 HSZ300.15* HSZ50-0.1 HSZ50-0.14 HSZ80-0.1 HSZ80-0.06 HSZ150-0.1 HSZ1500.04 ZSM-5 (50)
3 4 5 6 7 8 9
TPAOH/ SiO2
reaction temperature (°C)
reaction time (h)
0.10 0.15
150 180
10 24
0.10 0.14 0.10 0.06 0.10 0.04
150 150 150 150 150 150
10 10 10 10 10 10
0.10
150
24
For comparison, purely microporous ZSM-5 zeolite with Si/Al molar ratio of 50 was synthesized according to the previous literature.28 First, 0.3066 g of Al(iPrO)3 and 15.62 g of TEOS were mixed, followed by the addition of 25.92 g of deionized water. Then 10.98 g of TPAOH solution was added dropwise into the above mixture and stirred at 40 °C for 2 h to form a clear solution. Meanwhile, 0.54 g of NaOH was dissolved into 120.00 g of deionized water. The dissolved NaOH solution was then added into the above clear solution, which then underwent hydrothermal treatment at 150 °C for 24 h. The product was filtered, dried, and calcined at 550 °C for 6 h. The final product was designated as ZSM-5(50). The microporous ZSM-5 with Si/Al ratio of 80 was brought from the catalyst plant of Nankai University. All the materials were ion-exchanged with 1 M NH4NO3 solution three times at 80 °C for 4 h and then calcined at 550 °C for 4 h to convert the materials into H-form. Characterization. X-ray diffraction (XRD) patterns were recorded on a Rigaku Ultima IV X-ray diffractometer with Cu Kα radiation (40 kV and 40 mA) at a scanning rate of 6°·min−1 and an angular step size of 0.02°. The relative crystallinity (RC) of the synthesized HSZs was calculated by comparing their integrated peak areas in the 2θ range of 22.5−25° with that of purely microporous zeolite ZSM-5(50).29 Nitrogen sorption measurement was conducted on a Micromeritics ASAP 3000 porosimeter at 77 K. The surface area and pore size distribution were calculated by the Brunauer−Emmett−Teller (BET) and the Barrett−Joyner−Halenda methods, respectively, using the desorption branch of the isotherms. The micropore volume were calculated by the t-plot method. The scanning electron microscopy (SEM) images were taken on an Hitachi SU8220 under the voltage of 1 kV. Transmission electron microscopy (TEM) images were performed on a JEOL-2010F at 200 kV. Si/Al ratios were determined by the Agilent Technologies 725 inductively coupled plasma optical emission spectrometer. The solid-state 27Al spectra were obtained on a Bruker Avance III spectrometer. The ammonia temperatureprogrammed desorption (NH3-TPD) was performed on an apparatus built by Tianjin Golden Eagle Tech. Co., Ltd. The materials (0.1500 g) was pretreated at 600 °C for 1 h and then cooled down to 50 °C in He flow. Mixture of 10% NH3/90% N2 was injected until adsorption saturation, followed by the flow of N2 for 30 min. The temperature was raised to 150 °C afterward. When the baseline became stable, the temperature was further raised from 150 to 550 °C with a heating rate of 10 °C·min−1 and the amount of ammonia desorbed was detected by a TCD detector. Thermogravimetric measurements were performed on Netzsch STA 494 C Jupiter TG/DSC instrument. The sample weight loss between 573 and 973 K was taken as the total coke content. Catalytic Tests. The catalytic cracking of isopropylbenzene (IPB) and 1,3,5-triisopropylbenzene (1,3,5-TIPB) was carried out using a continuous flow microreactor in a quartz fixed bed under atmospheric
EXPERIMENTAL METHODS
Materials Preparation. HSZs with variable Si/Al ratios were synthesized according to a reported procedure.27 In a typical process (Si/Al = 80), 10.42 g of tetraethyl orthosilicate (TEOS, Shanghai Lingfeng Chemical Co., Shanghai, China), 0.1276 g of aluminum isopropoxide (Al(iPrO)3, Adamas Reagent Co. Ltd., Shanghai, China), and 18.00 g of deionized water were mixed at 25 °C for 1 h. Then 4.10 g of tetrapropylammonium hydroxide solution (TPAOH, 25 wt % in water, Yixing Dahua Chemical Co. Yixing, Jiangsu, China) was added dropwise into above mixture. The molar ratio of the resultant sol was 1:0.00625:0.1:20 SiO2:Al2O3:TPAOH:H2O. The mixture was further stirred at 40 °C until a clear precursor gel was formed. After the mixture was dried at 40 °C for 48 h, the obtained gel precursor was crushed and transferred into a crucible container and then put into an 80 mL Teflon liner. Then 0.7 g of deionized water was added into the bottom of the Teflon liner without contacting the gel precursor. The Teflon liner was sealed in a stainless steel autoclave and heated in a preheated oven at 150 °C for 10 h. Afterward, the converted gel was filtrated and washed repeatedly with distilled water, dried at 100 °C for 12 h, and then calcined at 600 °C for 6 h in air. The resultant was named HSZx-y, in which x and y refer to the molar ratio of Si/Al and TPAOH/Si in the materials composition, respectively. For example, HSZ80-0.10 represents the synthesized materials with Si/Al molar ratio of 80 and TPAOH/Si molar ratio of 0.10. The synthesis of HSZs with different Si/Al ratios was similarly conducted by accordingly changing the Al(iPrO)3 content in the precursor gel while keeping the B
DOI: 10.1021/acsami.6b00141 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces pressure. The catalysts were first pelleted to obtain the particles with diameter in the range of 40−60 mesh. Before reaction, catalysts (0.2000 g) were activated at 500 °C in nitrogen flow for 1.5 h. The cracking of IPB was performed at 500 °C in nitrogen flow (30 mL· min−1) for 12 h time on stream (TOS). The reactant was fed into the microreactor at the rate of 0.02 mL·min−1. The cracking of 1,3,5-TIPB was performed at 500 °C in nitrogen flow (80 mL·min−1) for 12 h TOS, with the reactant feed rate of 0.06 mL·min−1. The products were analyzed using online gas chromatography of GC/2010 plus (Shimadzu, Rtx-Wax capillary column) with a flame ionization detector.
sample HSZ30-0.1, all of them show the characteristic patterns of MFI-type zeolite and strong diffraction peaks, reflecting the successful production of highly crystallized ZSM-5 particles. Moreover, depending on the Al-doping content in the synthesized gels, their calculated RCs increase from 74% of sample HSZ50-0.1 to 91% of sample HSZ80-0.1 and then to 98% of sample HSZ150-0.1, as listed in Table 2. This confirms that, under a fixed TPAOH concentration condition, the increase of Al content in HSZs is unfavorable for the crystallization transformation during SAC treatment and, as a result, leads to the decrease of product crystallinity. With the highest Al-doping content, sample HSZ30-0.1 only shows a few subtle diffraction peaks in its XRD pattern (Figure 1) and in an enlarged profile (Figure S1 of the Supporting Information), a broad halo peak at about 23° is apparent, both of which demonstrate that sample HSZ30-0.1 retains most of the amorphous nature of gel precursors. N2 sorption isotherms of synthesized HSZx-0.1 and ZSM5(50) are shown in Figure 2 and their textural properties are
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RESULTS AND DISCUSSION HSZx-0.1 Synthesized under a Fixed TPAOH Concentration. The strong Brönsted acidity of ZSM-5 zeolite originates from the heterogeneous substitution of Al3+ for Si4+ in the framework. However, because of the difference of chemical valence and ionic radius between Al and Si, such a substitution is limited in concentration.30 In our previous report, we discussed our successful synthesis of HSZs with Si/ Al ratio 50 under a fixed TPAOH/Si ratio of 0.10.27 Here, in an attempt to extend the hierarchically micro-/mesoporous zeolite synthesis range with variable Si/Al ratios and surface acidity and consequently satisfy the requirement of diverse catalytic applications, first, HSZx-0.1 with varied Si/Al ratios from 30 to 150 were prepared under identical conditions at a fixed TPAOH/Si ratio of 0.10 (entries 1, 3, 5, and 7, Table 1). Figure 1 gives the XRD results of synthesized HSZx-0.1 and microporous ZSM-5(50) in the 2θ range of 5−50°. Except for
Figure 2. N2 sorption isotherms of HSZ30-0.1, HSZ50-0.1, HSZ800.1, HSZ150-0.1, and ZSM-5(50).
summarized in Table 2. The isotherm of ZSM-5(50) exhibits a typical type-I profile with a high uptake at low relative pressures (P/P0 < 0.1) and a plateau at high relative pressures (0.4 < P/P0 < 1.0), indicating that the resultant materials is a purely microporous phase with negligible mesoporosity. The corresponding BET surface area (SBET) and total pore volume (Vpore) are 367 m2·g−1 and 0.22 cm3·g−1, respectively. For sample HSZ150-0.1, similar type-I isotherm is present and the calculated surface area and total pore volume (entry 7, Table 2) are almost equal to those of sample ZSM-5(50). When considering its high crystallinity shown by the above XRD
Figure 1. XRD patterns of HSZ30-0.1, HSZ50-0.1, HSZ80-0.1, HSZ150-0.1, and ZSM-5(50).
Table 2. Textural Parameters, Composition, and RC of All Synthesized Materials entry
sample
SBET (m2·g−1)
Smicro (m2·g−1)
Sext (m2·g−1)
Vpore (cm3·g−1)
Vmicro (cm3·g−1)
Dmeso (nm)
Si/AlICP
RC (%)
1 2 3 4 5 6 7 8 9 10
HSZ30-0.1 HSZ30-0.15* HSZ50-0.1 HSZ50-0.14 HSZ80-0.1 HSZ80-0.06 HSZ150-0.1 HSZ150-0.04 ZSM-5(50) ZSM-5(80)
632 494 472 466 428 460 406 431 367 259
204 259 252 256 302 287 288 244 245 206
428 236 221 210 126 173 118 187 122 53
1.12 0.79 0.56 0.46 0.29 0.44 0.22 0.32 0.22 0.19
0.09 0.12 0.12 0.12 0.14 0.13 0.13 0.11 0.11 0.1
11.0 18.8 15.3 9.4 18.8 17.9 3.5 33.2
26.0 25.7 50.8 50.8 76.6 71.6 148.8 111.4 38.9 75.3
64 74 81 91 89 98 94 100 85
C
DOI: 10.1021/acsami.6b00141 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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sample HSZ30 with the highest Al-doping amount, the increase of TPAOH usage has a limited effect on accelerating the crystallization transformation of amorphous gels (Figure S1). In fact, to get the significant improvement of materials crystallinity, SAC treatment of HZS30-0.15* was performed at a higher temperature of 180 °C for a longer time of 24 h, though its RC is only 64%, lower than those of the others. Given the presence of auxiliary mesoporous structures in the synthesized HSZs, it is inferred that the near-complete crystallization conversion of amorphous gel precursors has been realized for sample HSZs with Si/Al molar ratio from 50 to 150, though further work is needed to improve the materials crystallinity of HSZ30. Figure 4 gives the N2 sorption isotherms of synthesized HSZx-y with optimized TPAOH concentration and their
results, it proves that the synthesized HSZ150-0.1 is comparable to purely microporous zeolites rather than the expected hierarchically micro-/mesoporous materials. With increased Al-doping content, sample HSZ80-0.1, though exhibiting a slightly higher Vpore in comparison with sample HSZ150-0.1, retains poor mesoporosity as indicated by its low external surface area (Sext) and type-I isotherm profile. When the Al-doping content in HSZs further increases, the isotherm of HSZ50-0.1, in addition to a high uptake at low relative pressures, shows the typical type-IV profile characterized by a hysteresis loop and an apparent uptake at high relative pressures, demonstrating the copresence of microporosity and mesoporosity in them. Accordingly, it gets larger BET surface area, external surface area, and total pore volume (entry 3, Table 2) as compared to ZSM-5(50). Further increase of Al content in HSZs, i.e., HSZ30-0.1, because of its lower crystallinity and amorphous nature, the highest BET surface area and largest total pore volume (entry 1, Table 2) are obtained among the synthesized HSZx-0.1. HSZx-y Synthesized with Optimized TPAOH Concentration. As mentioned above, except for HSZ50-0.1, the synthesized HSZx-0.1 are with either low crystallinity (HSZ300.1) or poor auxiliary mesostructures (HSZ80-0.1 and HSZ150-0.1). Since the variation in synthesis parameters, for example, materials composition, gel processing conditions, and SAC reaction temperature and time, would alter the morphology, crystallinity, and textural properties of the final products, aimed at the preparation of highly crystallized HSZs with abundant mesostructures, the following work would be focused on the optimization of TPAOH concentration according to the variation of Si/Al ratios in the compositions. Figure 3 shows the XRD results of synthesized HSZx-y with optimized TPAOH concentration (entries 2, 4, 6, and 8, Table
Figure 4. N2 sorption isotherms of HSZ30-0.15*, HSZ50-0.14, HSZ80-0.06, and HSZ150-0.04.
textural properties are summarized in Table 2. Interestingly, HSZ150-0.04 demonstrates the typical type-IV isotherm with a hysteresis loop and a high uptake at high relative pressures, but not the type-I profile of HSZ150-0.1. The corresponding BET surface area and total pore volume are 431 m2·g−1 and 0.32 cm3·g−1, respectively (entry 8, Table 2), which are significantly larger than those of HSZ150-0.1. Combined with the above XRD results (Figure 3), N2 sorption results prove the successful production of highly crystallized HSZs with well-defined hierarchical micro-/mesoporous structure at a relatively low Al-doping content. Similar results could be found in HSZ800.06 sample. Although it shows similar zeolite crystallinity with HSZ80-0.1, the increased BET surface area and total pore volume implies that a lower TPAOH concentration favors the development of mesostructures in the lower Al-doping ZSM-5 HSZs while keeping its high crystallinity, as shown in Figure 3, Figure 4, and Table 2. For sample HSZ50-0.14 (entry 4, Table 2), the slightly decreased mesoporosity is also reasonable due to the increase of TPAOH concentration in the synthesized gels. The above results further verify the effectiveness of tuning TPAOH concentration on the production of HSZs with varied Al-doping levels and textural properties. SEM and HRTEM images of synthesized HSZ80-0.06 and HSZ30-0.15* are shown in Figure 5. Figure 5a,b of HSZ800.06 exhibits clean and micrometer-sized globular particles with rough and porous surfaces and no irregularly shaped amorphous phase is present, which is consistent with its high crystallinity and above N2 sorption results. Its HRTEM image reveals the clear lattice fringes throughout the entire particle,
Figure 3. XRD patterns of HSZ30-0.15*, HSZ50-0.14, HSZ80-0.06, and HSZ150-0.04.
1) and Table 2 (entries 2, 4, 6, and 8) lists their corresponding RCs. All the patterns show the typical diffraction peaks of ZSM5 phase and no other impurity is present. For the materials with smaller amounts of Al doping, compared to sample HSZx-0.1, although the adopted TPAOH concentration was reduced by 60% for HSZ150-0.04 and 40% for HSZ80-0.06, its negative influence on gel crystallization transformation is negligible as both of them still maintain 94% and 89% RCs, respectively. On the other hand, for sample HSZ50-0.14, the increase of TPAOH concentration leads to an elevation of material crystallinity from 74% to 81%. It should be noted that, for D
DOI: 10.1021/acsami.6b00141 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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TPAOH concentration slowed down the dissolution rate of precursors during SAC treatment. Consequently, the crystallization transformation rate was decreased and thus favored the production of expected hierarchically porous structures. Moreover, lower Al-doping content accompanied with a lower amount of TPAOH is preferred for the corresponding HSZ synthesis combined with high crystallinity and abundant mesoporous structures, as shown by the optimized TPAOH/Si ratio of 0.04 with sample HSZ150 and of 0.06 with sample HSZ80. In addition to TPAOH concentration, SAC treatment temperature and time could also kinetically control the materials nucleation/growth process.33,35 Higher reaction temperature would accelerate nucleation/growth rate and the prolonged reaction time would promote the crystallization conversion. Thus, although the improvement of TPAOH concentration is not enough for the preparation of HSZ30-0.15 with expected hierarchically porous structure, combined with further adjustment of SAC reaction temperature and time, HSZ30-0.15* exhibits obviously increased crystallinity and abundant mesostructures. Surface Acidity and Catalytic Performance. Entries 6, 8, and 10 in Table 2 list the Si/Al molar ratios of synthesized HSZ80-0.06, HSZ150-0.04, and ZSM-5(80), which are close to the designed composition. The coordination state of Al species in the selected samples were investigated by 27Al MAS NMR spectroscopy (Figure S5). All the samples exhibit a strong peak centered at about 54 ppm, which can be ascribed to tetrahedrally coordinated Al (framework Al, FAl) with Brönsted acidity, and a small peak centered at around 0 ppm corresponding to octahedrally coordinated Al (extra-framework Al, EFAl).36 These results confirm that, in all the samples, most of the Al atoms are well-incorporated into the microporous frameworks. The NH3-TPD analysis was conducted to evaluate the surface acidity of selected samples. As depicted in Figure S6, all these profiles can be divided into two separated desorption peaks centered at ca. 200 °C and ca. 380 °C, which correspond to the weak and strong acid sites, respectively.37 The smaller peak intensity with that of HSZ150-0.04, indicative of a lower amount of acidic sites, is attributed to its lower Al content compared to those of HSZ80-0.06 and ZSM-5(80). To evaluate the effect of Al content and porous structures on the catalytic performance, the cracking reactions of IPB and 1,3,5-TIPB were conducted. IPB, with a molecular cross section of 0.50 nm, can freely diffuse in the micropore system (∼0.56 nm) of ZSM-5 zeolites and its cracking reaction is often used to assess the surface acidic strength and density of the materials.38 TIPB, with a molecular cross section of 0.74 nm, are not able to enter the 10-membered ring channels of ZSM-5 zeolites and its cracking process would mainly occur at the external surface and near the micropore entrance of the catalysts.38 Thus, it is a suitable probe for the assessment of the accessibility of surface acid sites. Figure 6 shows the IPB conversions as a function of time on stream over selected catalysts. HSZ80-0.06 and ZSM-5(80) depict similar catalytic activity during the whole testing period. Moreover, resulting from its larger external surface area and shortened diffusion length in micropore channels, a slightly higher activity is obtained for sample HSZ80-0.06 in comparison to ZSM-5(80).38 In contrast, although sample HSZ150-0.04 possesses comparable specific surface area and pore volume to HSZ80-0.06, only a lower initial IPB conversion (99% vs 93%) is observed, mostly because of its
Figure 5. SEM and TEM images of HSZ80-0.06 (a, b, c) and HSZ300.15* (d, e, f).
which further identifies the single-crystalline nature of the synthesized HSZ80-0.06. In contrast, Figure 5d,e of HSZ300.15* are the aggregates of globular particles and some unclear little particles which is believed to be the residues of incomplete conversion of amorphous gel precursors. More obvious evidence could be found in its HRTEM image of Figure 5f. Beside the lattice fringes in the internals, the particle rims are surrounded by a few little amorphous particles. All these results are in accordance with its above XRD results. Discussion on Formation Mechanism. As suggested in our previous report, here adopted SAC process for the synthesis of HSZs is a kinetics-controlled nucleation/growth process.27 Compared to the explosive crystal growth after a long-term induction period in the conventional hydrothermal synthesis, its key point lies in the moderate crystallization transformation of amorphous precursors in the discrete and localized water pools during this SAC treatment. By the adjustment of TPAOH concentration, the successful preparation of ZSM-5 HSZs with various Si/Al ratios (30−150) provides additional supports for the formation mechanism with this new mesoporogen-free method. In the case of ZSM-5 zeolite synthesis, aluminum-doping content in compositions affects the crystallization behavior. Higher Al-doping content means increased crystallization barrier and, consequently, decreased crystallization rate, and vice versa. Therefore, although the TPAOH/Si ratio of 0.10 was suitable for the synthesis of HSZ50-0.1 with the expected hierarchically micro-/mesoporous structures, under identical conditions, low Al-doping materials (HSZ80-0.1 and HSZ1500.1) were close to microporous zeolites with negligible mesostructures resulting from their rapid crystallization transformation and high Al-doping sample (HSZ30-0.1) was the amorphous mesoporous phase suffering from the retarded crystallization kinetics. On the other hand, TPAOH plays a dual role in the synthesis of ZSM-5 zeolites. First, TPA+ cations could electrostatically adsorb the negatively charged silicate or aluminosilicate species and work as the effective SDA for MFI structure.31,32 Second, the variation of TPAOH concentration would change the solution alkalinity in the meantime, which would alter the dissolution rate of amorphous species and/or nanocrystals and the solubility and, as a result, the crystal nucleation/growth rate during zeolite synthesis.33,34 Therefore, for the relatively low Al-doping HSZ80 and HSZ150 samples, the decreased E
DOI: 10.1021/acsami.6b00141 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
ACS Applied Materials & Interfaces
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CONCLUSIONS By a newly developed mesoporogen-free process, ZSM-5 HSZs with varied Si/Al ratios from 30 to 150 have been successfully prepared based on the adjustment of TPAOH concentration in the composition. Higher Al-doping amount in the synthesized gels means the additional requirement of TPAOH agent for the production of HSZs with high zeolite crystallinity and abundant mesostructures, and vice versa. These results also support our previous suggestion that this SAC process follows a kineticscontrolled nucleation/growth mechanism. In the model reactions of alkylbenzene cracking, the catalytic performance of synthesized materials are simultaneously determined by their Al-doping content and textural properties. As an example, HSZ80-0.06 demonstrates the comparable catalytic activity to microporous counterparts ZSM-5(80) when the small-molecule IPB substrate was adopted, but much enhanced performance for bulky molecule-involved reactions.
Figure 6. Conversions of IPB over HSZ80-0.06, HSZ150-0.04, and ZSM-5(80) during the cracking reactions.
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smaller Al-doping content in the synthesized gels and consequently a lower amount of surface acidic sites. For the same reason, the relatively fast catalyst deactivation was found on it. After 12 h reaction, the IPB conversion decreased by ∼20%. However, the catalyst activity of sample HSZ80-0.06 and ZSM-5(80) decreased by only 5%. Figure 7 shows the TIPB conversions as a function of time on stream over selected catalysts. As expected, due to the lack
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b00141. XRD patterns of HSZ30-0.1 and HSZ30-0.15; pore size distributions of HSZ30-0.1, HSZ50-0.1, HSZ80-0.1, HSZ150-0.1, and ZSM-5(50); pore size distributions of HSZ30-0.15*, HSZ50-0.14, HSZ80-0.06, and HSZ1500.04; SEM and TEM images of HSZ150-0.04 (a, b, c) and HSZ50-0.14 (d, e, f); 27Al MAS NMR spectra of HSZ80-0.06 and HSZ150-0.04; NH3-TPD profiles of HSZ80-0.06, HSZ150-0.04, and ZSM-5(80); TG curves of HSZ80-0.06, HSZ150-0.04, and ZSM-5(80) after 12 h IPB cracking reactions; TG curves of HSZ80-0.06, HSZ150-0.04, and ZSM-5(80) after 12 h TIPB cracking reactions (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (Z.H.). Notes
The authors declare no competing financial interest.
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Figure 7. Conversions of TIPB over HSZ80-0.06, HSZ150-0.04, and ZSM-5(80) during the cracking reactions.
ACKNOWLEDGMENTS This work was sponsored by the National Basic Research Program of China (2013CB933200), Natural Science Foundation of China (21403303, U1510107), and the Opening Projects of State Key Laboratory of Heavy Oil Processing (SKLOP201402003).
of apparent mesoporosity, microporous ZSM-5(80) shows the lowest initial TIPB conversion of ∼26% among the tested materials. Furthermore, because the accessible amount of acidic sites on the limited external surface of ZSM-5(80) is very low, the formation of a small amount of carbonaceous deposits (2.04%, Figure S8) would lead to fast catalyst deactivation and its activity to decrease by ∼50% in the first 2 h and continuously decline to ∼5% after 12 h reaction. As a comparison, HSZ80-0.06 and HSZ150-0.04 demonstrate greatly higher initial catalytic activity up to ∼100% and keep the conversions at 74% and 60%, respectively, in the end of tests, benefiting from their auxiliary mesoporous structures. In addition, the relatively larger weight losses for the used HSZs (5.05% and 2.62%, Figure S8) support the conclusion that hierarchically porous materials possess higher coke-tolerance capacity compared to microporous zeolites.
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NOTE ADDED AFTER ASAP PUBLICATION This paper was published on the Web on March 9 with errors in the first three rows in Table 1, column 1. The corrected version was reposted on March 12, 2016.
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DOI: 10.1021/acsami.6b00141 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
