Yanming Jia*a,
Qinghua Shia,
Junwen Wang
b,
Chuanmin Ding
b and
Kan Zhangc
aDepartment of Chemistry, Taiyuan Normal University, PO Box 030619, Jinzhong, China. E-mail: jiayanmingtynu@163.com
bCollege of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan, 030024, China
cState Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, 030001, China
First published on 10th August 2020
Hierarchical nano-ZSM-5 zeolites (Z5-X) with different grain sizes were synthesized by varying amounts of 3-glycidoxypropyltrimethoxysilane (KH-560) in the hydrothermal synthesis strategy. Moreover, the conventional ZSM-5zeolite(Z5), which was prepared without KH-560, was used as the reference sample. The crystalline phases, morphologies, porous characteristics, Si/Al molar ratios and acidic properties of all fresh catalysts were characterized using the X-ray diffraction (XRD), Fourier transform infrared spectra (FT-IR), scanning electron microscopy (SEM), N2 adsorption–desorption, inductively coupled plasma atomic emission spectroscopy (ICP) and temperature programmed desorption of ammonia (NH3-TPD) techniques. Results show that the grain size and strong acid amount of zeolite decreased with the increasing amount of KH-560. The micropore surface areas and the corresponding volume of Z5-X changed less compared with Z5. Consequently, the high shape-selectivity of zeolite was preserved well under the addition of KH-560. However, the mesopore surface areas and the corresponding volume increased significantly with the increasing amount of KH-560. Benefiting from the abundant hierarchical structure, the Z5-X catalysts exhibited a larger coke capacity than the Z5 catalyst. The coke depositions of all the deactivated catalysts were characterized by the thermogravimetric technique (TG), and the results are indicative of the decreased average rate of coke deposition with an increasing amount of KH-560, which could result from the gradually reduced strong acid amount and the nano-sized crystallites. The catalytic performance of methanol-to-aromatics (MTA) indicates that the Z5-0.12 catalyst exhibited higher catalytic activity and selectivity of BTX as the reaction was prolonged, which could result from the synergistic effect among the proper strong acid amount, the smaller zeolite grain size, and the abundant hierarchical structure.
Conventional ZSM-5 zeolites have a large grain size of the micrometer or sub-micrometer level, which inevitably increases the residence time of macromolecular intermediates and products in the inter channel.9–11 Thus, the narrow pores of such zeolites are prone to be blocked quickly because of the coke deposition, leading to the sharp deactivation of zeolite catalysts.
Thus, many methods, like the preparation of nano-sized ZSM-512–15 as well as hierarchical ZSM-5,16–18 have been applied for the promotion of the diffusion property to enhance the catalyst stability of ZSM-5 zeolites. As the hierarchical ZSM-5 zeolites are concerned, the micropores of such zeolites involve remarkable shape-selectivity for the target products. Furthermore, the mesopores are beneficial for the transfer of macromolecules, and the coke capability of such zeolites are improved significantly. Accordingly, the catalytic performance of hierarchical ZSM-5 is obviously enhanced due to the advantageous properties of microporous zeolites and mesoporous materials. For the nano-sized ZSM-5, a small grain size implies a shorter inner diffusion path, which could transfer the heavy macromolecular products within a short period. Thus, the pore blockage derived deactivation of the catalyst will decelerate, and the catalyst lifetime of such zeolites increases significantly.
In our previous communication,19 organosilane named 3-glycidoxypropyltrimethoxysilane (KH-560) was applied to prepare a hierarchical nano-ZSM-5 zeolites in a hydrothermal synthesis system. The use of organosilane could effectively inhibit zeolite crystal growth by the bond-blocking function, thereby producing the nano-sized zeolite crystals. Meanwhile, the intercrystalline mesopores among these nano-sized crystals could greatly enhance the coke capability of such zeolites. Thus, the hierarchical nano-ZSM-5 zeolites combine the advantages of nano-sized zeolites and hierarchical zeolites, and are a potential alternative for practical utilization.
In this research, the effects of the contents of KH-560 on the physical and chemical characteristics of Z5 zeolites are studied and discussed. Moreover, all the as-prepared Z5 zeolites are used for methanol to aromatics (MTA) reaction, respectively. The relationship between the MTA catalytic performance and the catalyst characteristics is built.
(1) 0.528 g of NaOH and 1.45 g of NaCl were dissolved in 24 ml of H2O. Afterward, 1.2384 g of Al(NO3)3·9H2O was gradually added into the above aqueous solution. The resultant mixture was electromagnetically stirred continuously for 1 h, and the resultant clear solution was marked A.
(2) Quantitative (0–4.5 ml) KH-560 was added dropwise to 10 ml of TPAOH solution. The resultant solution was electromagnetically stirred for 1 h, the resultant clear solution was marked B.
(3) B was added to A slowly, and the resultant mixture was first stirred for 0.5 h. Then, 22.5 ml of SD-Si was dropwise added and, the resultant mixture was continuously stirred for another 2 h to achieve a homogeneous Si–Al sol. In particular, the molar compositions of the Si–Al sol was 38SiO2:
1Al2O3
:
4Na2O
:
7.5TPAOH
:
(0–12)KH-560
:
800H2O
:
15Cl−.
In the end, the homogeneous sol was diverted to an autoclave, which was preheated for 12 h at 100 °C and then heated for another 72 h at 170 °C. The generated white solid was filtered and washed with distilled H2O, then dried for 12 h at 100 °C and calcined for 5 h at 550 °C.
In order to obtain the acidic zeolites, the calcined solid was ion-exchange twice with 1 mol L−1 of NH4NO3 solution at 80 °C for 4 h each time. Subsequently, the resultant solid was dried for 12 h at 100 °C and calcined for 5 h at 550 °C. The resulting solid powder was denoted as Z5-X, where X refers to the molar ratio of KH-560/Si, and X = 0.06, 0.12, 0.18, 0.24 as is shown in Table 1.
KH-560 aq (ml) | 0.91 | 1.94 | 3.13 | 4.50 |
---|---|---|---|---|
KH-560/Si | 0.06 | 0.12 | 0.18 | 0.24 |
The conventional ZSM-5 zeolites were similarly prepared without KH-560, and denoted as Z5. For obtain the extruded catalyst, four prepared zeolites powder were mixed respectively with pseudo boehmite, and the mass ratio of which was 4:
1. Then, quantitative Sesbania powder was slowly added under constant stirring. After mixing completely, quantitative dilute HNO3 (3%) was dropwise added, the mixture was squeezed to obtain a blocky catalyst. Finally, the blocky catalyst was extruded to strips, which were further dried for 12 h at 100 °C and calcined for 5 hat 550 °C.
Fig. 2 displays the FT-IR spectra of the five prepared ZSM-5 catalysts. As shown, the Z5, Z5-0.06, Z5-0.12 and Z5-0.18 catalysts show the FT-IR bands near 450, 547, 796, 1099 and 1222 cm−1, which are characteristics of ZSM-5 framework.23,24 Specially, the vibrational modes near 450, 796 and 1099 cm−1 are assigned to internal vibrations of SiO4 or AlO4 tetrahedra, which can also be observed in silica, quartz and cristobalite. However, the vibrational modes near 547 and 1222 cm−1 are assigned to the double-rings tetrahedra vibration and the asymmetric stretching of SiO4 and AlO4 tetrahedra in the zeolite framework, respectively.25 For Z5-0.24 catalyst, no absorbance band at 547 and 1222 cm−1 could be observed, and the wavenumber near 450 cm−1 is ∼20 cm−1 higher (blue shift) than that of Z5, Z5-0.06, Z5-0.12 and Z5-0.18 catalysts, these results further prove the amorphous nature of Z5-0.24 catalyst.
The morphologies of all the investigative ZSM-5 catalysts were observed by SEM techniques. As is shown in Fig. 3, all of the investigative ZSM-5 catalysts exhibit a spherical morphology, and the zeolite particle sizes of them are around 1.5 μm. However, their zeolite grain size is clearly different. As shown in Fig. 3A, the zeolite particles of Z5 catalyst are built by the accretion of abundant small crystallites. Moreover, the crystal size of Z5 catalyst are centered at 105–306 nm, and their average crystal diameter is around 193.9 nm as shown by the crystal size distribution. The average grain size of the Z5-0.06, Z5-0.12 and Z5-0.18 catalysts decreased with the addition of KH-560. Moreover, the average grain size gradually decreased with the increasing amount of KH-560. In particular, the zeolite particles of Z5-0.06 (Fig. 3B), Z5-0.12 (Fig. 3C) and Z5-0.18 (Fig. 3D) catalysts are built by the accretion of abundant nano-crystallites, and their average crystal diameters are approximately 61.5 nm, 53.7 nm and 24.5 nm, respectively. This result is well consistent with the XRD result. The accumulation of these nano-crystallites contributes to the formation of extensive intercrystal mesopores within these zeolite particles, which effectively increases the coke capacity of such zeolites.19
![]() | ||
Fig. 3 SEM images (left) and corresponding crystal size distribution (right) of Z5 (A), Z5-0.06 (B), Z5-0.12 (C) and Z5-0.18 (D) catalysts. |
Fig. 4 displays the N2 adsorption–desorption isotherms of the Z5, Z5-0.06, Z5-0.12 and Z5-0.18 catalysts. Similarly, all of the ZSM-5 catalysts show a high N2 uptake at low relative pressure of P/P0 < 0.2. Moreover, a hysteresis loop is observed at high relative pressure of P/P0 > 0.4, indicating that the intrinsic micropores and mesopores coexist in the four ZSM-5 catalysts. In particular, all of the ZSM-5 catalysts exhibit the IV-type isotherm with an H4-type hysteresis loop, which is generally observed in slit-like porous materials.26 According to the SEM images (Fig. 3), we can reasonably come to the conclusion that the mesopores in the four ZSM-5 catalysts were built by the accretion of small crystallites or nanocrystals. It should be noted that a forced closure of hysteresis at a relative pressure of P/P0 = 0.42 is observed in Z5-0.06, Z5-0.12 and Z5-0.18 catalysts, which could be due to the fact that the N2 molecules in the mesopores are surrounded by the “micropore wall”. Only when the specific release pressure of the molecule is reached, the N2 molecules blocked in the mesoporous could be released rapidly. Specially, for further comparative investigation, the specific surface area and pore volume of the four ZSM-5 catalysts are calculated in Table 2.
Sample | Ala (wt%) | Sia (wt%) | Si/Ala | SBETb (m2 g−1) | Smicroc (m2 g−1) | Smesod (m2 g−1) | Vtotale (cm3 g−1) | Vmicrof (cm3 g−1) | Vmesog (cm3 g−1) |
---|---|---|---|---|---|---|---|---|---|
a Determined by the ICP analysis.b Calculated by the BET method.c Calculated by the t-plot method.d Smeso = SBET − Smicro.e Calculated from the adsorbed data at P/P0 = 0.995.f Calculated by the t-plot method.g Vmeso = Vtotal − Vmicro. | |||||||||
Z5 | 2.25 | 43.17 | 18.50 | 353.6 | 261.3 | 92.3 | 0.214 | 0.102 | 0.112 |
Z5-0.06 | 2.09 | 43.35 | 20.00 | 390.4 | 263.9 | 126.5 | 0.236 | 0.102 | 0.134 |
Z5-0.12 | 2.03 | 43.60 | 20.71 | 418.0 | 276.1 | 141.9 | 0.249 | 0.105 | 0.144 |
Z5-0.18 | 1.95 | 44.18 | 21.85 | 429.0 | 268.5 | 160.5 | 0.256 | 0.103 | 0.153 |
As is shown in Table 2, the Z5-0.06, Z5-0.12 and Z5-0.18 catalysts exhibited higher BET surface areas (SBET) with the addition of KH-560. Moreover, the BET surface areas increased with an increasing amount of KH-560. However, no trend in micropore surface areas (Smicro) could be determined, so no changes could be observed in relation to SBET where an upward trend is indicated. Therefore, the increase in BET surface areas was caused by the increase in mesopore surface areas (Smeso). This analysis is well matched with the data shown in Table 2. Meanwhile, the trends of change of the total pore volume (Vtotal), the micropore volume (Vmicro) and the mesopore volume (Vmeso) is similar to those of SBET, Smicro, and Smeso, respectively. All in all, the micropores intrinsically possessed by the ZSM-5 crystals changed lightly with the increasing amount of KH-560, thereby preserving the shape-selectivity of the ZSM-5 zeolite, while the mesopores generated by the accumulation of nanocrystals increased, which effectively increases the coke capacity of the ZSM-5 zeolites.
The Si/Al molar ratios of the four investigated ZSM-5 catalysts were determined by ICP analysis, and the results were also presented in Table 2. It is clear that the Si/Al ratio increases with the increasing amount of KH-560. We tentatively ascribe this to the increased Si content in synthetic gel.27 Specifically, the KH-560 in synthetic gel could hydrolyze to produce the Si–OH, which were grafted on the MFI skeleton and acted as the Si source in the hydrothermal synthesis process, as a result, the ratio of Si/Al in the zeolite product increases.
The acidic properties of the Z5, Z5-0.06, Z5-0.12 and Z5-0.18 catalysts are measured by NH3-TPD technique, and the corresponding NH3 desorption profiles are shown in Fig. 5. According to the literature, the high temperature desorption peak above 350 °C is ascribed to the desorption of NH3 from strongly acidic sites.28,29 The low temperature desorption peak below 300 °C is due to the desorption of weakly bound NH3. In particular, the weakly bound NH3 is not the NH3 adsorbed on weakly acidic sites, but possibly the hydrogen-bonded ammonia species.30–32 The area of the NH3 desorption peak can be used to measure the acid amount.33,34 In order to achieve the strong acid amounts of the Z5, Z5-0.06, Z5-0.12 and Z5-0.18 catalysts, every NH3-TPD profile was deconvoluted into three Gaussian peaks (green dashed lines), and the corresponding strong acid amounts were calculated in Table 3. Apparently, compared with the Z5 catalyst, the strong acid amount of the Z5-0.06, Z5-0.12 and Z5-0.18 catalysts decreased. Moreover, the strong acid amounts gradually decreased with the increasing amount of KH-560, which we tentatively ascribe to the increased Si/Al molar ratio as shown in Table 2.
![]() | ||
Fig. 5 NH3-TPD profiles of Z5 (A), Z5-0.06 (B), Z5-0.12 (C) and Z5-0.18 (D). Green dashed lines show the Gaussian deconvolution peaks, red dashed lines show the cumulative fit peak. |
Catalyst | Strong acid center (°C) | Strong acid amount (μmol g−1) |
---|---|---|
Z5 | 409 | 685.6 |
Z5-0.06 | 374 | 550.6 |
Z5-0.12 | 370 | 496.2 |
Z5-0.18 | 371 | 422.3 |
![]() | ||
Fig. 6 Relationships between methanol conversion and time on-stream in the Z5, Z5-0.06, Z5-0.12 and Z5-0.18 catalysts. |
In particular, the Z5 catalyst displayed a high catalytic activity in a short time on-stream of about 102 h. Subsequently, the methanol conversion quickly decreased to 40% at 138 h. Compared with Z5 catalyst, the Z5-0.06 catalyst has a high catalytic activity in a longer time on-stream, and the methanol conversion remained as high as >98% after 138 h, which is 58% higher than that of the Z5 catalyst. Moreover, the methanol conversion still remained more than 90% after 150 h. Similarly, the Z5-0.12 and Z5-0.18 catalysts showed a high catalytic activity within 180 h and 186 h of the reaction (>98%), respectively; subsequently, the methanol conversion dramatically decreased to less than 50% at 210 h and 216 h, respectively. According to the reports in the literature,35 in the MTH reaction, the zeolite catalysts can be considered completely deactivated when the conversion of methanol is lower than 50%. Thus, the catalyst lifetimes of the Z5, Z5-0.06, Z5-0.12 and Z5-0.18 catalysts are around 135 h, 170 h, 208 hand 214 h, respectively.
The relationships between the BTX selectivity and the time on-stream in the Z5, Z5-0.06, Z5-0.12 and Z5-0.18 catalysts are shown in Fig. 7. Apparently, the selectivity of BTX in the four investigated ZSM-5 catalysts decreased as the reaction prolonged. This phenomenon may be largely attributed to the formation of coke species as the reaction prolonged. Specifically, the MTA reaction is a typical acid catalytic reaction,36–38 in which numerous strongly acidic sites are conducive for generating BTX hydrocarbons. Nevertheless, the strongly acidic sites of catalysts are covered gradually by the coke species with the reaction process, and thus the BTX selectivity decreased gradually. Particularly, the Z5 catalyst displayed the highest BTX selectivity in the first 24 h of reaction, moreover, the initial BTX selectivity could reach as high as 36.95%, which we tentatively ascribe to its abundant strongly acidic sites as shown in the NH3-TPD profiles (Fig. 5). After that, however, the Z5 catalysts exhibited lower selectivity of BTX than the Z5-0.06 and Z5-0.12 catalysts, and the selectivity of BTX falls to 14.58% after 102 h of reaction. This may be largely attributed to its higher average rate of coke deposition as shown in the TG profiles (Fig. 8). Therefore, a large number of coke species were deposited on the strongly acidic sites of the Z5 catalyst in a short time, and the BTX selectivity of such catalyst decreased significantly. Compared with Z5 catalyst, the Z5-0.06 and Z5-0.12 catalysts showed a lower BTX selectivity in the first 24 h of reaction. In particular, the initial selectivity of BTX of the two catalysts could respectively reach 35.97% and 35.25%, which is closely related to the less strongly acidic sites of the two catalysts. After that, however, the Z5-0.06 and Z5-0.12 catalysts exhibited a higher BTX selectivity, which respectively decreased to 16.24% and 17.68% after 102 h of reaction. This phenomenon is closely related to their lower average rate of coke deposition. The Z5-0.18 catalyst exhibited the lowest BTX selectivity during the entire reaction, and the initial BTX selectivity could reach 33.72%, which decreased to 12.12% after 102 h of reaction. This phenomenon may be largely attributed to its much fewer strongly acidic sites than Z5 catalyst.
![]() | ||
Fig. 7 Relationships between BTX selectivity and time on-stream in the Z5, Z5-0.06, Z5-0.12 and Z5-0.18 catalysts. |
The above results strongly indicate that the selectivity of BTX during the MTA reaction shows significant associations with the strong acid amount and the average rate of coke deposition. Moreover, the average rate of coke deposition shows significant associations with the strong acid amount and the diffusion characteristics of the catalyst. Therefore, the selectivity of BTX is closely related to the strong acid amount, grain size and hierarchical structure of the catalyst. A high selectivity of BTX could be obtained under the synergistic effect of a proper strong acid amount, smaller zeolite grain size, and abundant hierarchical structure.
In order to discuss the coke deposition on the deactivated ZSM-5 catalysts, we perform a thermogravimetric analysis. Moreover, the TG curves of the four deactivated catalysts after MTA reaction are shown in Fig. 8. As the figure shows, the TG curves of the four deactivated catalysts are quite similar. Nevertheless, the weight losses of these catalysts are clearly different. Specifically, the Z5 catalyst exhibited a low weight loss of around 19.32%, which is lower than that of the Z5-0.06 (21.26%), Z5-0.12 (23.27%), and Z5-0.18 (21.51%) catalysts. That is, the coke capacity of the ZSM-5 zeolite increased after KH-560 was added, which we tentatively ascribe to their increased mesopores formed by the buildup of the ZSM-5 nano-crystallites. Additionally, the weight loss per hour can be defined as the average coke deposition rate of the catalyst.39 As shown in Fig. 6, the catalytic lifetimes of the Z5, Z5-0.06, Z5-0.12, and Z5-0.18 catalysts are approximately 135 h, 170 h, 208 h, and 214 h, respectively. Therefore, the average rates of coke deposition in the Z5, Z5-0.06, Z5-0.12, and Z5-0.18 catalysts are approximately 0.143%, 0.125%, 0.112%, and 0.100% per hour, respectively. Clearly, the average coke deposition rates decreased with an increasing amount of KH-560, which could be reasonably ascribed to the synergistic effect between the reduced strong acid amount and the nano-sized crystallites.
This journal is © The Royal Society of Chemistry 2020 |