High-Quality Single-Crystalline MFI-Type Nanozeolites: A Facile

Apr 3, 2018 - International Center of Future Science, Jilin University, 2699 Qianjin ... Center for Electron Microscopy, Tianjin Key Laboratory of Adv...
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Article Cite This: Chem. Mater. 2018, 30, 2750−2758

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High-Quality Single-Crystalline MFI-Type Nanozeolites: A Facile Synthetic Strategy and MTP Catalytic Studies Qiang Zhang,† Guangrui Chen,† Yuyao Wang,† Mengyang Chen,† Guanqi Guo,† Jing Shi,§ Jun Luo,§ and Jihong Yu*,†,‡ †

State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, 2699 Qianjin Street, Changchun 130012, P. R. China ‡ International Center of Future Science, Jilin University, 2699 Qianjin Street, Changchun 130012, P. R. China § Center for Electron Microscopy, Tianjin Key Laboratory of Advanced Functional Porous Materials, Institute for New Energy Materials & Low-Carbon Technologies, School of Materials, Tianjin University of Technology, Tianjin 300384, P. R. China S Supporting Information *

ABSTRACT: A facile strategy affording high-quality singlecrystalline MFI-type nanozeolites (10−55 nm) with hexagonal prism morphology, good monodispersity, high crystallinity, and high product yield (above 97%) has been developed. This is achieved by synergistically using an L-lysine-assisted approach and a two-step crystallization process in a concentrated gel system (H2O/Si = 9). The morphological evolution of nanosized silicalite-1 is monitored by highresolution transmission electron microscopy (HRTEM). In this process, metastable irregular nanoparticles are initially obtained at 80 °C as the first step. Consequently, a rearrangement in morphology toward equilibrium crystal shape and without excessive growth for the metastable nanoparticles occurs at 170 °C as the second step. Throughout the whole process, L-lysine acts as an inhibitor to effectively limit the crystal growth of zeolites. Thanks to the high-quality nanosized crystals, the as-prepared ZSM-5 catalysts exhibit superior performance in methanol-to-propylene (MTP) reactions, which deliver a prolonged lifetime of 54 h with a total light olefin selectivity of 74% and a high propylene selectivity of 49% at 470 °C at a high methanol weight hourly space velocity (WHSV) of 7.2 h−1. This synthetic route provides a general strategy for preparing other types of zeolites with good monodispersity, nanosize, high yield, and high crystallinity.

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ogy.19 Mintova et al. studied the effect of silica source on the formation of nanosized silicalite-1. Depending on the silica source employed, the size of silicalite-1 crystals decreased in the following order: colloidal silica Ludox LS 30, fumed silica CabO-Sil, and tetraethylorthosilicate.20 Water content has been proven to strongly influence the ultimate size of zeolites. A substantial reduction of crystal size was achieved by reducing the water content in the synthesis.21 Kuechl et al. also demonstrated that concentrated systems were more efficient in improving the yield of nanosized zeolite β (BEA) crystals.22 It is widely accepted that the increase in the number of nuclei causes the decrease of the ultimate particle size. Generally, lowtemperature conditions favor nucleation over crystal growth for zeolite synthesis, giving rise to smaller zeolite crystals. Thus, variations in synthesis temperature and time allowed the synthesis of zeolite A (LTA) with size ranging from 90 to 300 nm.23 Very small zeolite A crystals (40−80 nm) with narrow particle size distributions were obtained at room temperature.24

INTRODUCTION The synthesis of nanozeolites is of great interest, since the decrease in crystal size to nanometer scale endows zeolites with large external surface area, high stability in suspension, short diffusion length, and more exposed active sites compared with micron-sized counterparts. These characteristics not only provide new possibilities for traditional applications in heterogeneous catalysis1−3 and molecule separation,4−6 but also lead to the expansion of their use to other emerging fields, such as chemical sensing,7−9 cosmetics and food,10−12 optical devices,13,14 and biomedical and biological analyses.15−18 Zeolites are conventionally synthesized using hydrosol precursors in closed reactors via conventional heating, where the initial precursor source, templates, alkalinity of medium, crystallization temperature and time, water content, and aging process are generally modulated to control the crystal sizes. Several strategies have been developed for the synthesis of nanosized zeolites by varying the synthetic conditions. For instance, Sharma et al. obtained nanosized mordenite (MOR) with size below 50 nm by changing the initial gel composition, and concluded that the alkalinity of medium played an important role in determining the zeolite size and morphol© 2018 American Chemical Society

Received: February 4, 2018 Revised: April 2, 2018 Published: April 3, 2018 2750

DOI: 10.1021/acs.chemmater.8b00527 Chem. Mater. 2018, 30, 2750−2758

Article

Chemistry of Materials Table 1. Molar Composition of the Initial Mixtures, Crystallization Conditions, and Particle Size of Silicalite-1

a

sample

SiO2

TPAOH

L-lysine

H2O

S-L0C0-80 S-LC0-80 S-L0C-80 S-LC-80 S-L0C0-170 S-LC0-170 S-L0C-170 S-LC-170 S-L0C0-80/170 S-LCo-80/170 S-L0C-80/170 S-LC-80/170 S-LC-70/170

1 1 1 1 1 1 1 1 1 1 1 1 1

0.45 0.45 0.45 0.45 0.45 0.45 0.45 0.45 0.45 0.45 0.45 0.45 0.45

0 0.1 0 0.1 0 0.1 0 0.1 0 0.1 0 0.1 0.1

50 50 9 9 50 50 9 9 50 50 9 9 9

particle sizea (nm)

temperature (time) 80 °C (2 days) 80 °C (2 days) 80 °C (2 days) 80 °C (2 days) 170 °C (3 days) 170 °C (3 days) 170 °C (3 days) 170 °C (3 days) 80 °C (2 days)/170 80 °C (2 days)/170 80 °C (2 days)/170 80 °C (2 days)/170 70 °C (2 days)/170

°C °C °C °C °C

(1 (1 (1 (1 (1

day) day) day) day) day)

142 130 94 55 275 193 226 138 137 127 273 56 37

Particle size: estimated on the basis of SEM results by counting more than 200 particles.

the assistance of L-lysine that serves as crystal growth inhibitor, the obtained irregular nanoparticles are evolved into good crystalline nanozeolite crystals without excessive growth. By using the above synthetic strategy, high-quality single-crystalline MFI nanozeolites with diameters ranging from 10 to 55 nm can be obtained with good crystallinity, excellent monodispersity, as well as high product yield. Importantly, the as-prepared nanosized ZSM-5 catalysts exhibit outstanding performance in MTP reactions compared with conventional ZSM-5 crystals. This strategy offers a general approach for the synthesis of nanozeolites, and may open more opportunities for the applications of nanozeolites in separation, adsorption, catalysis, and others.

However, lowering the synthesis temperature may result in nanozeolites at the expense of crystallinity. Recently, the ultrasmall EMT-type zeolite was obtained from an organictemplate-free and sodium-rich system.25,26 This is a great achievement that solves the problem of high synthetic cost of zeolites and further promotes their practical applications. Alternatively, some new methods have been developed for the synthesis of nanozeolites utilizing confined-space synthesis and other synthetic approaches. For instance, the confinedspace synthesis by a physical barrier overcomes the difficulties in the control of zeolite growth. Madsen et al. obtained nanosized ZSM-5 (MFI) crystals by utilizing the voids in the porous carbon to confine the zeolite synthesis.27 Afterward, some organic matrices, such as polymer hydrogel28 and starch,29 were used as soft templates to replace the hard carbon templates. Drawbacks of the confined-space synthesis are that the matrix must be stable and possess narrow mesopore size distributions to yield uniform crystal size distributions of the zeolite crystallized inside the confined spaces. On the other hand, microwave heating and sonication were utilized to facilitate abundant and rapid nucleation in the zeolite synthetic system, and consequently yielded nanosized crystals.30−32 In addition, an ionothermal approach could also be applied to reduce the size of zeolite crystals, which may contribute to a greener and more sustainable synthesis of nanozeolites.33 So far, over 20 types of nanozeolites (e.g., BEA, LTA, EMT, FAU, MOR, CHA, and MFI) have been successfully synthesized by conventional and alternative methods. It is noted that these synthetic methods greatly vary from different types of nanozeolites. Moreover, the as-prepared nanozeolites are usually heavily aggregated with irregular morphologies and low product yields, and suffer from poor crystallinity and poor hydrothermal stability. Thus, a facile and general approach to synthesize high-quality nanozeolites with good monodispersity and high product yields, crystallinity, and hydrothermal stability is highly desired to meet the great demands in the application of nanozeolites. In this work, we have developed a facile synthetic strategy for the synthesis of high-quality nanozeolites via synergetic use of the L-lysine-assisted approach and two-step crystallization in a concentrated gel system. The concentrated gel system ensures the formation of abundant nuclei and high product yield. At the first step (80 °C), the low-temperature condition favors nucleation over growth for synthesis of zeolites resulting in nanosized irregular crystals. At the second step (170 °C), with

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EXPERIMENTAL SECTION

Chemicals and Reagents. Tetraethylorthosilicate (TEOS, 28 wt %) was taken from Tianjin Fuchen Chemical Reagents Company. Tetrapropylammonium hydroxide solution (TPAOH, 25 wt %), sodium aluminate (NaAlO2), and L-lysine (C6H14N2O2, 98%) were supplied by Sinopharm Chemical Reagent Company. Sodium hydroxide (NaOH, 98 wt %) was purchased from Beijing Chemical Reagent Company. Synthesis of Silicalite-1 Zeolites with Different Sizes. The molar compositions of the mixtures were set to 1.0 SiO2/0.45 TPAOH/x L-lysine/y H2O (x = 0 and 0.1, y = 9 and 50). Typically, TEOS was first mixed with TPAOH and deionized water at room temperature with fast stirring until hydrolyzed completely, followed by the addition of L-lysine. The gel mixture was continuously stirred under infrared lamp to evaporate the excess water for a desired water ratio and then transferred into a Teflon-lined stainless steel autoclave for crystallization. The as-synthesized solid products were centrifuged, washed with water and ethanol several times, and then dried at 80 °C in the oven overnight, followed by calcination at 550 °C for 6 h. The obtained samples synthesized under varied conditions are denoted as S-LC-T, where S means sample, L represents employing the L-lysineassisted approach (L0 indicates that no L-lysine is used), C represents concentrated system (C0 indicates that no concentrated system is employed), T represents the crystallization temperature. The crystallization conditions for silicalite-1 zeolites are summarized in Table 1. For investigation of the crystallization process, samples were synthesized at different crystallization times. Synthesis of Nanosized ZSM-5 Zeolites. The molar compositions of the mixtures were set to 1.0 SiO2/0.45 TPAOH/x Al2O3/ 0.015 Na2O/0.1 L-lysine/9 H2O (x = 0.0025 and 0.003), for synthesizing nanosized ZSM-5 with different Si/Al ratios. The detailed synthetic conditions are listed in Table S1. The synthesis procedure was similar to that for silicalite-1 zeolites except that NaAlO2 was 2751

DOI: 10.1021/acs.chemmater.8b00527 Chem. Mater. 2018, 30, 2750−2758

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Chemistry of Materials

Figure 1. SEM images of silicalite-1 samples synthesized under varied conditions. Scale bar = 100 nm. automated chemisorption analysis unit with a thermal conductivity detector (TCD) under helium flow. Catalytic Test. The catalytic performance of the prepared ZSM-5 catalysts for methanol to propylene (MTP) reactions was analyzed in a quartz tubular fixed-bed reactor under atmospheric pressure. The catalyst (150 mg, 40−60 mesh) loaded in the quartz reactor (6 mm inner diameter) was activated in situ at 500 °C for 1 h under N2 flow (30 mL min−1) before starting each reaction run, and then, the temperature was adjusted to a reaction temperature of 470 °C. The methanol was fed by passing the carrier gas (15 mL min−1) through a saturator containing methanol at 48 °C, which gave a WHSV of 7.2 h−1. The reaction products were analyzed on-line by a gas chromatograph (Agilent GC 7890N), equipped with a flame ionization detector (FID) and Plot-Q column (Agilent J&W GC Columns, HPPLOT/Q 19091P-Q04, 30 m × 320 μm × 20 μm). Methanol conversion was calculated on the basis of reactor inlet and outlet concentrations. Selectivity to hydrocarbons was defined as the molar ratio of carbon in the hydrocarbon formed versus the methanol converted. Both methanol and dimethyl ether (DME) were regarded as reactants for calculation.

added to the reaction mixtures. The crystallization was conducted in a conventional oven initially at 80 °C for 2 days and subsequently at 170 °C for 1 day under static conditions. The as-synthesized solid products were centrifuged, washed with water and ethanol several times, and then dried at 80 °C in the oven overnight, followed by calcination at 550 °C for 6 h. For acquisition of the H-type ZSM-5, the samples were ion-exchanged two times in 1 M NH4NO3 solution at 80 °C for 7 h, and then calcined at 450 °C for 7 h. The obtained samples are denoted as NZ-1 and NZ-2. Synthesis of Conventional ZSM-5 Zeolites. The molar composition of the mixture was set to 1.0 SiO2/0.45 TPAOH/0.003 Al2O3/0.015 Na2O/x H2O (x = 35 and 100), for the synthesis of ZSM-5 crystals with different sizes. The detailed synthesis conditions are listed in Table S1. The crystallization was conducted in a conventional oven at 170 °C for 4 days under static conditions. The as-synthesized solid products were separated, and then ion-exchanged by NH4+ ions followed by calcination. The obtained samples are denoted as CZ-1 and CZ-2. Characterizations. The crystal size and morphology were measured by transmission electron microscopy (TEM) and highresolution TEM (HRTEM) at 200 kV using a Tecnai F20 and a Talos F200X (FEI) electron microscope. Scanning electron microscopy (SEM) observations of samples were performed on a JSM-6510 (JEOL) electron microscope. Dynamic light scattering (DLS) measurements were performed by photon correlation spectroscopy employing a Nano ZS90 laser particle analyzer (Malvern Instruments) at 25 °C. Thermogravimetric (TG) analysis was carried out on a TGA Q500 analyzer from instruments in air, with a heating rate of 10 °C min−1. Elemental analysis was performed on a PerkinElmer 2400 elemental analyzer. The crystallinity and phase purity of the samples were characterized by powder X-ray diffraction (XRD) on a Rigaku DMax 2550 diffractometer using Cu Kα radiation (λ = 1.5418 Å). The Si/Al ratios were determined with inductively coupled plasma (ICP) analyses carried out on a PerkinElmer Optima 3300 DV ICP instrument. Nitrogen adsorption/desorption measurements were carried out on a Micromeritics 2020 analyzer at 77.35 K after the samples were degassed at 350 °C under vacuum. Infrared (IR) spectra of the samples dispersed in KBr pellets were measured on a PerkinElmer spectrum 430 FT-IR spectrometer. The acidic properties of samples were tested by temperature-programmed desorption of ammonia (NH3-TPD) on a Micromeritics AutoChem II 2920

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RESULTS AND DISCUSSION Influence of Synthetic Parameters on Crystal Sizes of Silicalite-1. Silicalite-1 crystals with different sizes are synthesized under varied synthetic conditions (Table 1). Figure 1 shows the SEM images of silicalite-1 crystals, and corresponding crystal sizes and size distributions are presented in Table 1 and Figure S1. DLS measurements are performed to analyze the size of silicalite-1 samples (Figure S2), which is in good consistency with the observation from SEM. All of the samples show five distinct XRD peaks at 7.96°, 8.82°, 23.27°, 23.97°, and 24.43° ascribable to 011, 200, 051, 033, and 133 reflections, respectively, which are characteristic of MFI zeolite (Figure S3). As can be seen from Table 1, every single approach, namely, a two-step crystallization process, addition of L-lysine in the synthesis gel, and adopting a concentrated gel system can reduce the crystal size. Via synergetic use of an Llysine-assisted approach and a two-step crystallization in a concentrated gel system, we have successfully synthesized highquality nanosized silicalite-1 (see S-LC-80/170 and S-LC-70/ 2752

DOI: 10.1021/acs.chemmater.8b00527 Chem. Mater. 2018, 30, 2750−2758

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Chemistry of Materials

Figure 2. TEM and HRTEM images of (a−c) S-LC-70/170 and (d−f) S-LC-80/170.

Figure S5 reveal that no decomposition of L-lysine occurs in the synthesis gel throughout the crystallization at 170 °C. As can be seen from elemental analysis (Table S2) and TG analysis (Figure S6), no L-lysine molecules exist in the final crystallized crystals. The effect of L-lysine on crystal size is ascribed to the buffering action of L-lysine and the H-bonding between amino groups of L-lysine and external surface silanols of zeolite, which restricts the crystal growth, and thus leads to the formation of nanozeolites.36−38 Furthermore, on the basis of the utilization of a two-step crystallization and introduction of L-lysine, the smallest S-LC80/170 crystals are obtained by applying a concentrated gel into the synthetic system. Compared with a diluted system where the H2O/Si ratio is 50, the concentrated synthesis system where the H2O/Si ratio is 9 provides a favorable condition for the synthesis of the smaller zeolite (see S-L0C0-80 versus S-L0C-80; S-LC0-80 versus S-LC-80; S-L0C0-170 versus S-L0C-170; and S-LC0-80/170 versus S-LC-80/170). In addition, it can be also found that the product yields of SLC-70/170 and S-LC-80/170 are noticeably higher than that of the S-L0C0-170 (98% and 97% versus 80%). Generally, a fast nucleation rate and a slow growth rate are desirable for synthesis of nanoparticles.23,24 In a concentrated gel system, the nucleation process has proven to be fast, and the nuclei are abundant.39 As is known, in classical crystal growth theory, there are two growth mechanisms including diffusion-limited growth and reaction-limited growth.40 Within a concentrated synthesis system, the growth rate controlled by the diffusionlimited process is slow because of the poor diffusion rate of elementary units including atoms, molecules, assemblies, and particles.41 In contrast, the aggregation is dramatically facilitated in a diluted system, causing the formation of large crystals. It is noted that S-L0C-80/170 synthesized by applying a two-step crystallization in a concentrated synthesis system but without Llysine possesses a large size of 273 nm. This is because the nanocrystals may grow larger without the assistance of L-lysine serving as a growth inhibitor. The above studies demonstrate that each of the three approaches, i.e., a two-step crystallization

170). As shown in Figure 2, S-LC-80/170 (∼56 nm) and S-LC70/170 (∼37 nm) present high monodispersity, hexagonal prism morphology, and good crystallinity. The HRTEM images in Figure 2 show clear lattice fringes with consistent orientations over the entire image region, confirming the single-crystalline features of nanosized silicalite-1. It can be seen that some intracrystalline mesopores could be observed in the nanocrystals. Notably, the nanocrystals contain ultrasmall crystals with size even below 10 nm, which possesses regular hexagonal prism morphology (Figure 2f). To the best of our knowledge, this is the smallest silicalite-1 single crystal with regular morphology.34,35 First, low crystallization temperature contributes to decreasing the size of crystals because the low-temperature conditions favor nucleation over growth for synthesis of zeolite.23,24 However, the crystals obtained at 80 °C suffer from poor crystallinity and irregular morphology as shown in Figures S3 and S4 (see S-L0C0-80 with average crystal size of 142 nm). In contrast, high crystallization temperature contributes to improving crystallinity and endowing regular morphology for crystals. However, the crystals obtained at 170 °C are large in size (see S-L0C0-170 with average crystal size of 275 nm). A two-step crystallization process is thus adopted to ensure small crystals with good crystallinity. As a result, S-L0C0-80/170 exhibits an average crystal size of 137 nm, regular morphology, and good crystallinity. This demonstrates that the two-step crystallization is crucial for the synthesis of high-quality nanosized silicalite-1. Second, the introduction of L-lysine into the abovementioned two-step crystallization process could further decrease the crystal size. S-LC0-80/170 synthesized via a twostep crystallization and in the presence of L-lysine presents an average crystal size of 127 nm, which is smaller than S-L0C0-80/ 170 synthesized in the absence of L-lysine. L-Lysine acts as a growth inhibitor in the synthesis system, effectively reducing the crystal size of zeolites under varied synthetic conditions (see S-LC0-80 versus S-L0C0-80; S-LC0-170 versus S-L0C0-170; and S-LC0-80/170 versus S-L0C0-80/170). The IR spectra in 2753

DOI: 10.1021/acs.chemmater.8b00527 Chem. Mater. 2018, 30, 2750−2758

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Chemistry of Materials Table 2. Porosity and Product Yields of Selected Samples sample

SBETa (m2 g−1)

Smicrob (m2 g−1)

Sexternal‑surfaceb (m2 g−1)

Vmicrob (cm3 g−1)

Vtotalb (cm3 g−1)

yieldc (%)

S-LCT-70/170 S-LCT-80/170 S-000-170 NZ-1 NZ-2 CZ-1 CZ-2

619 505 386 418 423 353 375

425 326 255 240 243 233 272

194 179 131 178 180 120 103

0.20 0.16 0.12 0.12 0.12 0.11 0.13

1.36 1.00 0.25 0.56 0.66 0.34 0.28

98 97 72 98 97 85 78

a

SBET (total surface area) calculated by applying the BET equation using the linear part (0.05 < P/P0 < 0.30) of the adsorption isotherm. bSmicro (micropore area), Sexternal‑surface, Vmicro (micropore volume), and Vmeso (mesopore volume) calculated using the t-plot method. cYield = w1/w2, where w1 and w2 are the weight of the calcined sample and theoretical zeolite sample, respectively.

Figure 3. (a) Low-magnification and (b) high-magnification TEM images of S-LC-80 crystallized at 170 °C for different times, and (inset) corresponding FFT diffractograms of individual silicalite-1 crystals.

(Figure 3, 24 h). No excessive growth occurred during the whole process, and the particles keep the single-crystalline structure throughout the morphological evolution (Figure 3). The fast Fourier transformation (FFT) diffractograms (Figure 3b, 0 h, inset) indicates that the individual S-LC-80 particle possesses a single-crystalline feature. At elevated temperature the initial particles undergo a morphological evolution. As shown in Figure 3, some small and crystal-like blocks emerge at the corners of nanoparticles at 170 °C (1−3 h). No further growth occurs as the size of the particles remain unchanged. This is mainly due to the fact that, as discussed above, L-lysine protects the isolated metastable nanoparticles from aggregating. After 5 h at 170 °C, sharp corners and crystalline edges appear, and the nanoparticles gradually exhibit regular morphology (Figure 3, 5 h). The bright diffraction spots in FFT diffractograms (Figure 3b, inset) indicate that the samples are crystalline crystals. After 24 h, isolated crystals with hexagonal prism morphology and good crystallinity are developed, which can be verified by HRTEM images and FFT diffractograms (Figure 3, 24 h and inset). The structure evolution was also monitored by XRD measurements (Figure S8). With crystallization proceeded at 170 °C for S-LC-80, the (101) and (200) planes of S-LC-80/ 170 are well-oriented at [101] and [100] directions compared with those of S-LC-80 as indicated by the increased intensity of corresponding peaks. It is noted that the XRD patterns of the samples exhibit broadened Bragg peaks, suggesting the presence of very small crystals throughout the entire crystallization. In addition, the crystal sizes along [101] and

process, addition of L-lysine, and a concentrated gel system, or the combination of two approaches can give rise to a reduction in crystal size to some extent. However, high-quality nanozeolites with regular morphology and size below 50 nm can only be obtained by simultaneously combining the three approaches. The highly crystalline structure of as-prepared nanosized silicalite-1 zeolites is also made evident by the N2 adsorption− desorption isotherms (Table 2). As shown in Figure S7, a steep rise uptake at low P/P0 pressure is typical for microporous materials, and a hysteresis loop at P/P0 between 0.9 and 1.0 is usually associated with the existence of interparticle voids caused by the stacking of nanosized crystalline particles.42 SLC-70/170 and S-LC-80/170 exhibit a larger BET surface area (505 and 619 m2 g−1), higher micropore surface area (326 and 425 m2 g−1), and larger micropore volume (0.16 and 0.20 cm3 g−1) than S-L0C0-170 (386 m2 g−1, 255 m2 g−1, and 0.12 cm3 g−1), revealing that nanosized S-LC-70/170 and S-LC-80/170 possess higher crystallinity than their conventional counterpart. Detailed porosity data are summarized in Table 2. Morphological Evolution of Nanosized Silicalite-1 Zeolites. It is noted that S-LC-80 and S-LC-80/170 possess a similar crystal size of 55 nm but distinct morphologies: S-LC80 shows irregular morphology and S-LC-80/170 regular hexagonal prism morphology. For an understanding of the formation process from S-LC-80 at 80 °C to S-LC-80/170 at 170 °C, the morphological evolution with different crystallization times has been monitored by HRTEM. As shown in Figure 3, irregular S-LC-80 (Figure 3, 0 h) gradually evolves to S-LC-80/170 with regular morphology and good crystallinity 2754

DOI: 10.1021/acs.chemmater.8b00527 Chem. Mater. 2018, 30, 2750−2758

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Chemistry of Materials

Scheme 1. Proposed Formation Processa of Nanosized Zeolites via Synergetic Use of L-Lysine-Assisted Approach and Two-Step Crystallization in a Concentrated Gel System

In the first step, metastable irregular nanoparticles are initially obtained at 80°C. Then, the crystallization is further carried out at 170°C. Consequently, a rearrangement in morphology towards equilibrium crystal shape occurs without excessive growth for the metastable nanoparticles. In this process, L-lysine serves as inhibitor to limit the crystal growth.

a

[100] estimated by the Scherrer equation show slight changes (Table S3), which is in good consistency with the TEM observation in morphological evolution (Figure 3). Notably, the peaks corresponding to (121) and (220) planes become indiscernible because of the rearrangement in morphology toward equilibrium crystal shape. The evolution of silicalite-1 nanocrystals in morphology and structure might be understood according to the context of a Wulff construction, where the equilibrium crystal shape is determined by minimization of the surface free energy for a given volume.43−47 Throughout the entire crystallization at 170 °C, the irregular metastable S-LC80 nanoparticles with low crystallinity gradually evolve into SLC-80/170 nanozeolites with high crystallinity and regular morphology while keeping the crystal size unchanged because of the growth-inhibiting role of L-lysine. Scheme 1 illustrates the proposed process for the evolution of nanosized silicalite-1 zeolites toward regular morphology and high crystallinity. In this process, the first step at low temperature favors the formation of metastable nanocrystals with irregular morphology and poor crystallinity. At the second step, these irregular metastable nanocrystals transform into the good crystalline nanosized zeolite with regular morphology at an elevated temperature (170 °C), where confined crystallization and the ripening process occur. Throughout the whole process, the size of the nanocrystals remains unchanged via the assistance of Llysine serving as a crystal growth inhibitor. This facile synthetic route could provide a general strategy for the formation of other types of high-quality nanozeolites. Preparation of Nanosized ZSM-5 Zeolites. As is commonly known, nanosized zeolites may provide improved properties for catalytic applications due to the short diffusion length and more exposed active sites.48 High-quality nanosized ZSM-5 catalysts with a high product yield up to 98% have been successfully prepared via synergistically using the L-lysineassisted approach and two-step crystallization process in a concentrated gel system. The as-prepared NZ-1 and NZ-2 catalysts with Si/Al ratios of 157 and 193 have the average crystal size of 58 and 57 nm, respectively (Table S1). These nanosized ZSM-5 catalysts present a single-crystalline feature, high monodispersity, regular morphology, and good crystallinity (Figure 4). For comparison, conventional CZ-1 (av 238 nm) and CZ-2 (av 442 nm) ZSM-5 catalysts with Si/Al ratios of 154 and 158, respectively, are synthesized (Figure S9 and Table S1). Notably, as seen from Figure 4, intracrystalline mesopores with diameters of ca. 2−5 nm could be clearly observed in the

Figure 4. TEM and HRTEM images of (a, b) NZ-1 with Si/Al ratio of 157 and (c, d) NZ-2 with Si/Al ratio of 193.

interior crystals of the NZ-1 and NZ-2 catalysts. These intracrystalline mesopores are randomly distributed throughout the whole crystals. By contrast, CZ-1 and CZ-2 catalysts synthesized without L-lysine show the conventional morphology of the ZSM-5 zeolites without visible voids (Figure S9). The highly crystalline structure of nanosized ZSM-5 catalysts is made evident by the XRD patterns (Figure S10) and N2 adsorption−desorption isotherms (Figure S11). Nanosized ZSM-5 catalysts show improved physicochemical properties compared with conventional ZSM-5 catalysts. NZ-1 and NZ-2 crystals exhibit larger BET surface areas (418 and 423 m2 g−1), higher external surface areas (178 and 180 m2 g−1), and larger total volumes (0.56 and 0.66 cm3 g−1) than CZ-1 (353 m2 g−1, 120 m2 g−1, and 0.34 cm3 g−1) and CZ-2 (375 m2 g−1, 103 m2 g−1, and 0.28 cm3 g−1). These reveal that NZ-1 and NZ-2 catalysts possess higher crystallinity than their conventional counterparts. Detailed porosity data are summarized in Table 2. The acidity of the catalysts was evaluated by NH3-TPD measurements (Figure S12). Catalysts CZ-1, CZ-2, and NZ-1 show similar acidic strength and acidic concentration in both weak acid sites (around 170 °C) and strong acid sites (around 2755

DOI: 10.1021/acs.chemmater.8b00527 Chem. Mater. 2018, 30, 2750−2758

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Chemistry of Materials 370 °C), which is consistent with their similar Si/Al ratios. Because of the higher Si/Al ratio, NZ-2 shows lower acidic strength and acidic concentration in both weak acid sites and strong acid sites. Catalytic Performance of Nanosized ZSM-5 Catalysts in Methanol Conversion. The catalytic performance of the nanosized ZSM-5 catalysts compared with conventional ZSM catalysts for the MTP reaction is investigated under atmospheric pressure at 470 °C and WHSV of 7.2 h−1. All the catalysts show 100% methanol conversion at the initial stages, but different catalytic stability with time on stream as shown in Figure 5. Catalyst NZ-1 shows longer catalytic

propylene selectivity (48.6%), P/E ratio (4.2), and light olefin selectivity (73.6%) compared to the other three catalysts (Table 3). In general, smaller crystal size and higher Si/Al for ZSM-5 catalysts could afford a long catalytic lifetime and high propylene selectivity in the MTP reaction. It is worth noting that NZ-2 and NZ-1 catalysts perform much better in propylene selectivity than previously reported aggregates of nanosized ZSM-5 crystals (40%, WHSV of 8 h−1).54 The above work demonstrates that high-quality nanosized ZSM-5 are ideal catalysts for MTP conversion.

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CONCLUSIONS

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ASSOCIATED CONTENT

We have demonstrated a facile strategy to prepare high-quality single-crystalline MFI-type nanozeolites (10−55 nm) with regular morphology, good monodispersity, high crystallinity, and high product yield (above 97%). This strategy is achieved in a concentrated system via synergetic use of an L-lysineassisted approach and a two-step crystallization, where the twostep crystallization endows zeolite crystals with good crystallinity and regular morphology; L-lysine acts as a growth inhibitor to limit the crystal growth, and the concentrate gel system ensures high product yield. HRTEM studies reveal the morphological evolution process of nanosized silicalite-1. At the first step (80 °C), metastable irregular nanoparticles are initially obtained. At the second step (170 °C), with the assistance of Llysine which serves as crystal growth inhibitor, these metastable irregular nanoparticles evolve to high-quality nanozeolites with regular morphology and good crystallinity while keeping the crystal size unchanged. The first step determines the ultimate crystal size, while the second step determines the ultimate morphology and crystallinity. As the result of high crystallinity and nanocrystal size, high-quality nanosized ZSM-5 catalysts synthesized via this strategy exhibit excellent performance in MTP reactions compared with conventional ZSM-5 catalysts. This synthetic approach can be also applied to the synthesis of other zeolite types with nanosize, high yield, and high crystallinity, which may open more applications of nanosized zeolites.

Figure 5. Methanol conversion as a function of time on stream over CZ-1, CZ-2, NZ-1, and NZ-2. Reaction conditions: T = 470 °C, Ptotal = 1 atm, WHSV = 7.2 h−1, catalysts weight = 150 mg.

lifetime (42.7 h) and higher propylene selectivity (45.8%) than conventional catalysts CZ-1 (32.0 h, 41.8%) and CZ-2 (17.7 h, 43.4%) (Figure S13 and Table 3). Additionally, the propylene/ ethylene (P/E) ratio of catalyst NZ-1 is considerably higher than that of conventional ZSM-5 catalysts (3.2 versus 2.0 and 2.6). As an important index parameter in the MTP process, the high P/E ratio enables the high propylene yield in the recirculation process.49,50 Since these three samples possess similar acidity (Figure S12), the excellent MTP performance of NZ-1 catalyst could be attributed to its smaller particle size on the basis of the hydrocarbon pool mechanism.51,52 As is wellknown, nanosized zeolites can enormously shorten the micropore channels to improve the diffusion properties.53 Light olefin can more easily escape from the zeolite channels, and the secondary reactions are thus inhibited, which consequently leads to high propylene selectivity and good catalytic stability. Significantly, NZ-2 catalyst with higher Si/Al ratio exhibits the longest lifetime (53.7 h) and highest

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.8b00527. Size distributions, XRD patterns, TEM images, HRTEM images, FTIR spectra, N2 adsorption/desorption, NH3TPD, and product selectivity (PDF)

Table 3. Catalytic Performances of MTP Reaction over ZSM-5 Catalystsa selectivityc (%) sample

lifetimeb (h)

CH4

C2H4

C2H6

C3H6

C3H8

C4H8

C4H10

C5+

light olefins

P/Ed

NZ-1 NZ-2 CZ-1 CZ-2

42.7 53.7 32.0 17.7

0.2 0.2 0.2 0.9

9.5 7.7 14.2 11.3

0.0 0.0 0.1 0.1

45.8 48.6 41.8 43.4

0.9 0.6 2.1 1.5

17.6 17.3 17.0 17.1

11.3 11.5 9.8 10.7

14.7 14.1 14.8 15.1

72.9 73.6 72.9 71.7

3.2 4.2 2.0 2.6

Reaction conditions: T = 470 °C, Ptotal = 1 atm, WHSV = 7.2 h−1, catalyst weight = 150 mg. bMethanol conversion above 90%. cData were collected as the average of the first 4 h. dC3H6/C2H4 molar ratio.

a

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Qiang Zhang: 0000-0002-2837-8473 Jun Luo: 0000-0001-5084-2087 Jihong Yu: 0000-0003-1615-5034 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Key Research and Development Program of China (Grant 2016YFB0701100), the State Basic Research Project of China (Grant 2014CB931802), the National Natural Science Foundation of China (Grant 21320102001 and 21621001), and the 111 Project (B17020) for supporting this work.

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