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Hierarchical porous MCM-68 zeolites: Synthesis, characterization and catalytic performance in m-xylene isomerization
Microporous and Mesoporous Materials 263 (2018) 135–141
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Hierarchical porous MCM-68 zeolites: Synthesis, characterization and catalytic performance in m-xylene isomerization
T
He Haoa, Yue Changa, Wenjun Yua, Lan-Lan Loua,∗, Shuangxi Liua,b,∗∗ a
Institute of New Catalytic Materials Science and MOE Key Laboratory of Advanced Energy Materials Chemistry, National Institute of Advanced Materials, School of Materials Science and Engineering, Nankai University, Tianjin 300350, People's Republic of China b Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, People's Republic of China
A R T I C L E I N F O
A B S T R A C T
Keywords: Hierarchical zeolite MCM-68 H2O2 microexplosion NH3(aq) treatment m-Xylene isomerization
Novel hierarchical MCM-68 zeolite materials were successfully prepared by two simple, effective and environmentally friendly methods: H2O2 microexplosion under microwave and NH3(aq) treatment. The characterization of the materials with powder XRD, SEM, TEM, N2-sorption, and NH3-TPD proved that abundant mesopores across zeolite crystals were created while the microporous MSE framework was well preserved. The mesopore volume of hierarchical MCM-68 zeolites is up to 0.19 cm3 g−1. The crystallinities of hierarchical ones and microporous MCM-68 are nearly the same according to the XRD result. The acidities of hierarchical MCM-68 are generally lower than their raw materials. Gas-phase isomerization reaction of m-xylene was carried out over these hierarchical MCM-68 zeolites as well as HMCM-68 zeolite under different conditions to characterize the catalytic property of newly prepared hierarchical zeolites. It was found that the m-xylene conversion and disproportionation/isomerization rate ratio were closely related to the created mesopores in hierarchical MCM-68 zeolites under high reaction temperature and low weight hourly space velocity (WHSV). Moreover, these hierarchical MCM-68 zeolites exhibited improved resistance to coke formation compared with HMCM-68.
1. Introduction Zeolites are a class of crystalline microporous aluminosilicate materials with highly defined porous structures. Their frameworks consist of corner-sharing TO4 tetrahedral units (T is generally Si or Al). Zeolites have been used in lots of industrial fields because of their unique framework structures, high cation-exchange capacities, good adsorptivities, and moderate acidities [1]. The application of zeolites as catalysts for cracking, alkylation, isomerization and other petrochemical reactions is of great importance [2]. Furthermore, there have been increasing attention on zeolites used as catalysts for methanol to olefins process [3], nitrogen oxides selective catalytic reduction [4], Fischer–Tropsch synthesis [5], and biomass conversion [6] in recent years. The well-defined microporous framework contributes to the high surface area, excellent thermal stability, and “shape-selective” feature of zeolites. However, on the other hand, narrow pores (usually below 1 nm) always cause diffusion limitations in zeolite particles and the acid sites inside zeolite crystals are inaccessible to bulk molecules, both of which lead to a low utilization of the catalysts [1,7]. Furthermore,
micropores and high acid amount may cause coke deposition and then the catalytic deactivation during organic reactions [8]. Therefore, in the last decade, the catalytic exploitation of hierarchical zeolites, which combine the advantages of improved diffusion ability and the welldefined microporous structure, has attracted widespread research and industry interest. Mesopores and/or macropores can be fabricated by post-synthesis treatments of zeolite crystals, such as steaming [7], acid leaching [9–11], and NaOH solution treatment [12,13]. These postsynthesis treatments are simple and widely usable to generate hierarchical zeolites, but they often suffer from some problems such as environmental pollution and the demand of additional ion-exchange processes. Besides these traditional methods, two novel approaches towards hierarchical zeolites were reported very recently. Aelst et al. adopted weak alkaline NH3(aq) as modification agent instead of traditional NaOH solution to generate mesopores in commercial dealuminated USY zeolite [14,15]. NH4+ form zeolite product was obtained that can be used directly after calcination without additional ion-exchange. Zhang et al. reported the fabrication of hierarchical zeolites, including beta, ZSM-5, mordenite, Y and so on, by using H2O2 decomposition under microwave treatment [16]. H2O2 molecules diffused
∗
Corresponding author. Corresponding author. Institute of New Catalytic Materials Science and MOE Key Laboratory of Advanced Energy Materials Chemistry, National Institute of Advanced Materials, School of Materials Science and Engineering, Nankai University, Tianjin 300350, People's Republic of China. E-mail addresses: [email protected] (L.-L. Lou), [email protected] (S. Liu). ∗∗
https://doi.org/10.1016/j.micromeso.2017.12.009 Received 21 June 2017; Received in revised form 19 November 2017; Accepted 10 December 2017 Available online 11 December 2017 1387-1811/ © 2017 Published by Elsevier Inc.
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into zeolite crystals can release big amount of gases under microwave irradiation and generate mesopores from inside crystals to the outside. MCM-68 (framework type code: MSE) is a type of aluminosilicate zeolite invented by Mobil researchers in 1997 and first reported as patent in 2000 [17]. It is a multipore zeolite with three-dimensional 12 × 10 × 10-ring channel system, including a 12-ring (6.4 × 6.8 Å) straight channel and two tortuous 10-ring (5.2 × 5.8 Å and 5.2 × 5.2 Å) channels that intersect with each other [18–20]. There is a cylindrical 24-hedral supercage circumscribed by 12- and 18-rings in the framework, accessible only through 10-ring pores, which can accommodate relative bulk transition-state species. Because of its unique three-dimensional pore structure, MCM-68 zeolite has received increasing attention. Various post-synthesis treatments, such as steam treatment [8], metal elements insertion [21–23], phosphate impregnation [24], and acid treatment [25,26], have been reported to modify the catalytic properties of MCM-68. And MCM-68 zeolites have been used as catalysts in different reactions, such as phenol oxidation [21–23], alkylation of aromatics [25,26], disproportionation of ethylbenzene [27] or toluene [28], isomerization of n-alkanes [27], hexane cracking [29–33], and dimethyl ether-to-olefin reaction [24]. However, up to now, there was no report focused on the fabrication of hierarchical porous MCM-68 zeolite through alkaline treatment. Here we reported the novel hierarchical MCM-68 zeolite materials with mesopores generated by NH3(aq) treatment of dealuminated MCM-68 and by exploding of H2O2 molecules inside HMCM-68 zeolite under microwave irradiation. In addition, their catalytic performance for isomerization of m-xylene, an important industrial reaction to produce p-xylene, was studied. For comparison, HMCM-68 and dealuminated MCM-68 were also used. To the best of our knowledge, this is the first report focused on the utilization of MSE-type zeolite in m-xylene isomerization.
Table 1 Physicochemical properties of various MCM-68 zeolites. Sample
SBET (m2·g−1)a
SEx (m2·g−1)b
VMicro (cm3·g−1)b
VMeso (cm3·g−1)c
Si/Ald
HMCM-68 DA-5h DS-10h DS-24h HMCM68MW
504 504 514 503 567
73 80 152 147 102
0.20 0.20 0.17 0.17 0.21
0.08 0.08 0.19 0.19 0.12
11.0 38.7 26.1 24.4 11.9
a b c d
BET method. t-Plot method. Total pore volume minus micropore volume. Determined by ICP-AES.
calcined zeolite was ion-exchanged by 0.5 mol L−1 NH4NO3 aqueous solution at 80 °C in order to turn the zeolite to NH4+-form. This ionexchange procedure was repeated twice. The NH4+-form MCM-68 was calcined at 550 °C for 10 h to afford HMCM-68, which had a Si/Al ratio of 11.0 (Table 1).
2.2.2. Preparation of hierarchical MCM-68 zeolites The preparation procedure of hierarchical MCM-68 zeolites is shown in Scheme 1. HMCM-68 was treated by microexplosion under microwave irradiation [16] to fabricate hierarchical MCM-68 zeolite, and the detailed preparing procedure was as follows. HMCM-68 zeolite (0.2 g) and H2O2 aqueous solution (15 wt%, 8 mL) were charged in a high pressure microwave reactor (HP-100, SINEO Microwave Chemistry Technology Co. Ltd.), which was placed in a microwave oven (MDS-10, SINEO). The reaction system was heated to 120 °C for 5 min, then to 140 °C for 10 min, then to 180 °C for 20 min, and finally cooled down to room temperature. The solid product was separated by centrifugation and dried overnight. This process was repeated for another two times, and the obtained material was denoted as HMCM-68-MW. Considering that low Si/Al ratio zeolites show good resistance to alkaline treatment [34], which is inappropriate for preparing hierarchical zeolites, dealumination of HMCM-68 was performed with 1 mol L−1 HCl solution (50 mL g−1 sample) in 80 °C water bath for 5 h. The solid was filtrated, dried overnight, and calcined at 500 °C for 4 h to afford dealuminated MCM-68 sample, which was denoted as DA-5h. DA-5h was applied as the raw material for the preparation of hierarchical MCM-68 zeolites by NH3(aq) treatment [15]. In a typical preparation, DA-5h was treated with 0.1 mol L−1 ammonia solution at room temperature under drastically stirring for different time (10 h and 24 h), followed by filtration, thoroughly washing with distilled water and calcination at 550 °C for 10 h. The resulted materials were denoted as DS-n (n = 10h and 24h, according to the ammonia treatment time).
2. Experimental 2.1. Chemicals and reagents Bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic dianhydride (> 95 .0%, Tokyo Chemical Industry Co. Ltd.), aluminum hydroxide (Al(OH)3, 77%, Tianjin Heowns Biochem LLC), colloidal silica (40 wt% suspension in H2O, LUDOX HS-40, Aldrich), hydrogen peroxide (H2O2, 30%, GR, Tianjin Damao Chemical Reagent Factory), ammonia solution (ACS, 28.0–30.0% NH3 basis, Aladdin Industrial Corporation), m-xylene (> 99.0%, GC, Aladdin Industrial Corporation), potassium hydroxide (KOH, GR, Tianjin Chemical Reagent Company), and ammonium nitrate (NH4NO3, AR, Tianjin Chemical Reagent Company) were used as received. HZSM-5 (Si/Al = 14.0) and Hβ (Si/Al = 12.5) were obtained from Nankai University Catalyst Co. Ltd. 2.2. Sample preparation 2.2.1. Synthesis of HMCM-68 MCM-68 zeolite was synthesized according to previous literature [8,25]. The organic structure-directing agent (OSDA) of N,N,N′,N′-tetraethylbicyclo[2.2.2]oct-7-ene-2,3,5,6-dipyrrolidinium diiodide (TEBOP2+(I−)2) was synthesized from bicyclo[2.2.2]oct-7-ene-2,3,5,6tetracarboxylic dianhydride through three steps. The typical gel composition is 1.0SiO2:0.1TEBOP2+(I−)2:0.375KOH:0.1Al(OH)3:30H2O. In a typical synthesis, colloidal silica (3.0 g), distilled water (9.0 g), and Al (OH)3 (0.20 g) were mixed and stirred at room temperature for 10 min, then KOH (0.42 g) was added to the solution and stirred for another 15 min. After that, TEBOP2+(I−)2 (1.12 g) was added and the mixture was stirred for 3 h. Then the gel was transferred to a 25 mL Teflon-lined autoclave and placed in a 160 °C oven for 14 days. The solid product obtained was separated by filtration, washed several times with distilled water, and dried overnight. The as-synthesized MCM-68 was calcined in a muffle furnace at 650 °C for 6 h to remove the OSDA. The
Scheme 1. The preparation procedure of various materials.
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2.3. Characterization The X-ray powder diffraction (XRD) patterns were obtained on a Bruker D8 FOCUS diffractometer, using Cu Kα radiation at 40 kV and 40 mA operating from 5° to 50°. The morphologies and sizes of materials were observed by filed-emission scanning electron microscopy (SEM, JEOL JSM-7500F) and high-resolution transmission electron microscopy (HRTEM, Philips Tecnai G2 F30). N2 sorption isotherms were measured by a Micromeritics ASAP2020 physisorption instrument at 77 K. The specific surface areas were calculated by the BrunauerEmmett-Teller (BET) method, and the micropore volumes and external surface areas were calculated by t-plot method. The pore size distribution was estimated by DFT method. The ammonia temperature programmed desorption (NH3-TPD) was measured on a Micromeritics ChemiSorb 2750 instrument. The sample (0.1 g) was preheated at 480 °C for 1 h under He flow. After cooled down to 100 °C, the sample was saturated with 5% NH3/Ar, followed by He flow sweeping off physisorbed NH3. Desorption was performed by heating from 100 °C to 800 °C at a rate of 10 °C min−1. The inductively coupled plasma-atomic emission spectrometry (ICP-AES) was applied to determine the Si/Al ratios of zeolites by using a Perkin-Elmer Optima 8300 instrument. Thermogravimetric analysis (TGA) was performed by using a TA SDT Q600 instrument under air atmosphere.
Fig. 1. Powder XRD patterns of (a) as-synthesized MCM-68, (b) HMCM-68, (c) HMCM-68MW, (d) DA-5h, (e) DS-10h, and (f) DS-24h.
68 zeolite from 11.0 (HMCM-68) to 38.7 (DA-5h), and the successive mild NH3(aq) treatment decreased the Si/Al ratio to 24.4–26.1 (DS-n). Compared with typical desilication methods like NaOH treatment, NH3(aq) treatment is a relatively mild way because NH3(aq) acts as a weak base and the treatment is carried out at room temperature. More importantly, this method is simple (without the need of additional NH4+ exchange), energy-saving and high yield. The yields of DS-n materials were about 70 wt% without big differences between 10 h and 24 h. For comparison, similar desilication experiments towards DA-5h material using NaOH solution with the same concentration were also carried out at room temperature. It turned out that the crystallinity of the zeolite decreased very sharply and nearly no solid product could be collected after several hours. As for HMCM-68-MW, the Si/Al ratio was about 11.9, changing slightly from that of HMCM-68, which was also consistent with the paper reported before [16]. N2 adsorption-desorption isotherms of various zeolites are shown in Fig. 3. Clearly, HMCM-68 exhibited type I isotherm with a very steep rise in N2 amount adsorbed in the low relative pressure (p/p0) range and a small hysteresis loop of type H3 in the high p/p0 range of 0.9–1.0, indicating its typical microporous feature as well as the existence of some mesopores and/or macropores derived mainly from the pile-up of zeolite crystals [37]. In addition, no obvious increase in N2 amount adsorbed was observed in the medium p/p0 range of 0.05–0.9. The sample of DA-5h showed very similar isotherm as that of HMCM-68, suggesting the present dealumination process by HCl treatment had little influence on the pore structure of HMCM-68. As for the zeolites of DS-n and HMCM-68-MW, a gradual increase in N2 uptake in the medium p/p0 range of 0.05–0.9 could be observed in their N2 adsorption-desorption isotherms, along with a small hysteresis loop of type H2 in the range of 0.4–0.8, demonstrating the appearance of some mesopores in the zeolites after microexplosion or NH3(aq) treatment process. Fig. 4 shows the DFT pore size distribution curves of different samples. It could be found that, compared with HMCM-68 and DA-5h, abundant mesopores in the range of 2–15 nm appeared for HMCM-68-MW and DS-n zeolites. In the microporous region, all the samples had two main peaks centered at around 0.53 nm and 0.70 nm, which could be attributed to 10-ring and 12-ring channels in MCM-68 framework, respectively. The textural parameters of various MCM-68 zeolites are listed in Table 1. The BET surface area, micropore volume and mesopore volume of DA-5h were 504 m2 g−1, 0.20 cm3 g−1 and 0.08 cm3 g−1, respectively, which were as same as those of HMCM-68. And the following desilication treatment by ammonia affected the BET surface areas of zeolites to a small degree, ranging from 503 m2 g−1 to 514 m2 g−1. While for the pore volume, DS-10h and DS-24h samples showed an obviously increased mesopore volume of 0.19 cm3 g−1 and a slightly decreased micropore volume of 0.17 cm3 g−1 compared with DA-5h. The sample of HMCM-68-MW owned the highest BET surface
2.4. Catalytic reaction Catalytic m-xylene isomerization was chosen as the probe reaction for various MCM-68 catalysts. The m-xylene isomerization was performed in a tubular, down-flow stainless steel fixed bed reactor (inner diameter 8 mm, length 50 cm, Tianjin Pengxiang Technology Co. Ltd.). The catalyst (20–40 mesh) was activated at 500 °C for 2 h under nitrogen flow prior to the catalytic run. m-Xylene was injected from the top through a pulsation-free pump (2PB-00C, Beijing Xingda Science and Technology Development Co. Ltd.) and was vaporized before flowing into the reactor. The isomerization reaction was carried out at atmospheric pressure under nitrogen flow (5 mL min−1), and the reaction temperature was 300–400 °C. The liquid products were collected by condenser and quantified by gas chromatography (GC, FL9790II) equipped with a PEG-20M column. The products obtained from 3 h to 10 h were adopted to study the activity and selectivity of the catalyst. Product distributions were reported in mol% [35,36]. 3. Results and discussion 3.1. Properties of the materials The XRD patterns of different MCM-68 samples are presented in Fig. 1. The zeolites of as-synthesized MCM-68 and HMCM-68 showed a typical MSE topology with good crystallinity. Other samples obtained from post-synthesis modification maintained MSE topology with no obvious decrease in crystallinity compared with HMCM-68, suggesting that MCM-68 zeolite structure was very well preserved upon dealumination, desilication and microexplosion. Fig. 2 shows the SEM images of these MSE type zeolites. It could be found that as-synthesized MCM-68 and HMCM-68 exhibited cuboid shaped crystallites with sizes of about 100–200 nm, which was similar to the previous report [8]. The shapes and particle sizes of HMCM-68-MW and DA-5h showed no obvious change compared with HMCM-68. As for DS-n, with the post treatment going severer, the shape of particles became more irregular and the sizes varied in a wider range, like 100–400 nm. This may be caused by the dissolution and recrystallization processes during NH3(aq) treatment. The Si/Al molar ratios of the bulk zeolites were determined by ICPAES, and the results are listed in Table 1. Generally, dealumination treatment increases Si/Al ratio, while desilication treatment is just the contrary. In this case, HCl treatment increased the Si/Al ratio of MCM137
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Fig. 2. FE-SEM images of (a) as-synthesized MCM-68, (b) HMCM-68, (c) HMCM-68-MW, (d) DA-5h, (e) DS-10h, and (f) DS-24h.
Fig. 4. DFT pore distribution curves of HMCM-68 (black), DA-5h (red), DS-10h (orange), DS-24h (purple), and HMCM-68-MW (green). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
outside to inside and microexplosion treatment creates mesopores from inside to outside. The results of TEM characterization further confirmed the hierarchical pore structure of HMCM-68-MW and DS-n zeolites. NH3-TPD curves of the zeolites are shown in Fig. 6, which suggested that HMCM-68 possessed higher acid sites concentration and stronger acid sites than other samples. Two distinct ammonia desorption peaks centered at about 200 °C and 450 °C, attributed to the weak and strong acid sites, respectively, could be observed for HMCM-68. The order of the amount of both weak and strong acid sites is HMCM-68 > HMCM68-MW > DA-5h > DS-n. Besides the decreased amount of acid sites, HMCM-68-MW showed a shift of the peak of strong acid sites from 450 °C to 400 °C compared with HMCM-68, indicating the decline in the strength of the strong acid sites. The dealuminated sample DA-5h exhibited much lower acid sites concentration than HMCM-68, due to the removal of Al atoms from the framework, especially the Al atoms on the external surface [38]. As for the zeolites obtained after NH3(aq) treatment, the acid amount were even lower than that of raw DA-5h sample. It could be seen from Fig. 6 that the amount of acid sites in DS-n zeolites was lower than that of DA-5h and that the peak of strong acid sites moved towards low temperature direction from 450 °C to 380 °C. In other words, NH3(aq) treatment lessened the acid amount of the raw DA-5h zeolite and weakened the strength of strong acid sites at the same time, though Al atom concentration in the sample was increased. This abnormal relationship between acid amount and Si/Al ratio
Fig. 3. Nitrogen adsorption-desorption isotherms of (a) HMCM-68, (b) DA-5h, (c) HMCM68-MW (d) DS-10h, and (e) DS-24h. (Isotherms are shifted by + 50.)
area of 567 m2 g−1 among all these zeolites as well as an enhanced mesopore volume of 0.12 cm3 g−1 compared with HMCM-68. The results of N2 sorption evidenced that some mesopores were successfully created in MCM-68 zeolites by NH3(aq) treatment or H2O2 microexplosion, both of which kept the zeolite frameworks very well. In addition, the method of NH3(aq) treatment seems more effective to create mesopores. Whereas microexplosion method has advantages of fast synthesis, no wastewater and no need of post-treatment. Fig. 5 presents the TEM images of different MCM-68 zeolites. No obvious mesopores could be found in the crystals of DA-5h sample obtained from dealumination treatment. While for DS-n samples, abundant mesopores throughout zeolite crystals could be clearly observed from TEM images. HMCM-68-MW sample also had abundant mesopores through crystals, which were caused by microexplosion from H2O2 decomposition under microwave irradiation. One of the differences between these two ways of creating mesopores, proved by TEM characterization, is that desilication treatment creates mesopores from 138
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Fig. 5. TEM images of (a) DA-5h, (b) HMCM-68-MW, (c) DS-10h, and (d) DS-24h.
Fig. 7. Conversion of m-xylene (solid) and p-xylene yield (open) over HMCM-68 zeolite: 400 °C, 1.73 h−1 (square); 300 °C, 1.73 h−1 (circle); 300 °C, 5.18 h−1 (triangle-up); 300 °C, 10.37 h−1 (triangle-down).
Fig. 6. NH3-TPD curves of (a) HMCM-68, (b) HMCM-68-MW, (c) DA-5h, (d) DS-10h, and (e) DS-24h.
Table 2 Catalytic data of m-xylene isomerization over different catalysts.a
sometimes happened on alkaline treated zeolites according to previous researches, and may be connected with Si-OH-Al hydrolysis or extra framework Al. However, the accurate reason still needs to be further studied [39].
3.2. Catalytic performance 3.2.1. m-Xylene isomerization over HMCM-68 zeolite As m-xylene isomerization reaction had never been studied over MCM-68-type zeolites, we investigated the catalytic performance of parent HMCM-68 firstly, to lay a foundation for further study. The mxylene conversion and p-xylene yield over HMCM-68 under different conditions are shown in Fig. 7. To investigate the effect of reaction temperature, the isomerization reaction was carried out under 400 °C and 300 °C with the same weight hourly space velocity (WHSV) of 1.73 h−1. It was found that the average conversion under 300 °C was less than that under 400 °C by about 2.2% (Table 2). Inversely, an increase in p-xylene yield from 13.3% to 14.1% was observed with decreasing reaction temperature from 400 °C to 300 °C. The effect of the WHSV on the m-xylene conversion and p-xylene yield was also investigated by carrying out the isomerization reaction under the same temperature of 300 °C and different WHSVs of 1.73 h−1, 5.18 h−1 and 10.37 h−1, and the results are given in Fig. 7 and
Catalyst
T (°C)
WHSV (h−1)
Conv. (%)
PX Sel. (mol%)
PX yield (%)b
PX/ΣX
D/I
HMCM-68 HMCM-68 HMCM-68 HMCM-68 HMCM-68MW DA-5h DS-10h DS-24h HMCM-68MW DA-5h DS-10h DS-24h HZSM-5 Hβ
400 300 300 300 400
1.73 1.73 5.18 10.37 1.73
69.3 67.1 61.2 53.6 71.8
19.3 21.1 21.8 28.0 15.3
13.3 14.1 13.3 14.9 11.0
0.23 0.23 0.21 0.20 0.21
1.50 1.33 1.27 0.85 2.01
400 400 400 300
1.73 1.73 1.73 10.37
69.7 72.5 73.1 45.8
19.0 16.2 16.1 30.9
13.2 11.7 11.8 14.1
0.23 0.23 0.23 0.18
1.52 1.83 1.93 0.71
300 300 300 400 400
10.37 10.37 10.37 1.73 1.73
42.8 38.4 40.6 54.0 64.2
29.1 32.6 31.2 35.3 23.7
12.4 12.5 12.6 19.1 15.1
0.15 0.15 0.15 0.23 0.23
0.71 0.59 0.66 0.42 1.10
a b
139
The catalytic results were the average values of 5–10 h. PX yield is the mole fraction of PX in liquid products.
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70
Table 2. It was obvious that after 5 h of reaction, the conversion decreased while the selectivity of p-xylene increased. The increase in pxylene selectivity could be mainly attributed to that less side reactions, such as disproportionation, took place at relatively low WHSV, which could be proved by the disproportionation/isomerization ratio (D/I) in Table 2 [40]. As an interaction of both conversion and selectivity, it turned out that the highest p-xylene yield of 14.9% was obtained under 300 °C and 10.37 h−1. Contrast experiments were carried out to thoroughly understand the catalytic behavior of HMCM-68 in the m-xylene isomerization reaction by using HZSM-5 (Si/Al = 14.0) and Hβ (Si/Al = 12.5) as catalysts under the same reaction conditions (400 °C, 1.73 h−1), and the results are listed in Table 2. The order of conversion over these three catalysts was HMCM-68 (69.3%) > Hβ (64.2%) > HZSM-5 (54.0%); however, the order of the p-xylene yield was on the contrary: HZSM-5 (19.1%) > Hβ (15.1%) > HMCM-68 (13.3%). In previous studies, the high p-xylene selectivity over ZSM-5-type zeolites was usually attributed to the faster diffusion of p-xylene in the 10-ring channels of ZSM-5 compared with o- and m-xylene. However, under the present conditions, p-xylene produced accounted for 23% of all xylenes in effluent regardless of the kind of catalysts used, suggesting the diffusion limit of products was not the main factor. The difference in m-xylene conversions and p-xylene selectivities among the three catalysts may be connected with the void space in zeolite frameworks that could accommodate bulk transition-state complexes for disproportionation [41]. The 10-ring channel system of HZSM-5 cannot afford as many as bulk transition-state complexes compared with the 12-ring channel systems of Hβ and HMCM-68. In addition, there is a 24-hedral supercage in HMCM-68 framework, which is also in favor of the formation of bulk transition-state molecules, as can be proved by the relatively high D/I ratio over HMCM-68 (Table 2).
25
Conversion ( % )
50 20 40 30 15 20
p-Xylene Yield ( % )
60
10 0
10 3
4
5
6
7
8
9
10
Reaction Time ( h ) Fig. 9. Conversion of m-xylene (solid) and p-xylene yield (open) at 300 °C and 10.37 h−1 over HMCM-68 (square), DA-5h (triangle-up), HMCM-68-MW (circle), DS-10h (triangleright), and DS-24h (diamond).
that of HMCM-68 zeolite, which indicated that the acidity was not the main factor that effected the conversion. The zeolite frameworks of these catalysts were the same, however the hierarchical catalysts of HMCM-68-MW, DS-10h and DS-24h owned abundant intracrystal mesopores compared with HMCM-68 and DA-5h. According to the foregoing experiments, lager void space was in favor of disproportionation reactions, which in this case was the mesopore structure. It could be clearly observed that the molar ratios of p-xylene to all xylene in effluents (PX/∑X) were very similar among different catalysts and were close to the value under thermodynamic equilibrium state. Under this situation, more disproportionation reactions would lead to higher conversion, which corresponds to the great D/I ratios of reactions over hierarchical MCM-68 zeolites in Table 2 [40,42]. On the contrary, the pxylene yields over hierarchical MCM-68 zeolites were lower than that of HMCM-68 zeolite due to the relatively low p-xylene selectivities, which was also in accord with the foregoing results over HMCM-68. Under the condition of 300 °C and 10.37 h−1, the conversion order of these catalysts was: HMCM-68 > HMCM-68-MW > DA-5h > DS-n, which was basically accordant with the order of acidity. This indicated that under high WHSV and low reaction temperature, the conversion depended mainly on the zeolite acidity. When the WHSV is high, there are too many m-xylene molecules participating in the reaction, which means the acid sites on the zeolite may not be enough to turn the substrate into the product adequately. It also matches the PX/∑X ratio. As listed in Table 2, the p-xylene fraction among all xylene effused was lower over treated MCM-68 zeolites compared with HMCM-68 zeolite. In this case, the porosity of zeolite seems no longer the most important factor determining the conversion. The D/I ratios also indicated that the conversions decreased as disproportionation reactions were suppressed, along with improved p-xylene selectivity. Affected by both conversion and selectivity, the final p-xylene yields of DA-5h and DS-n were around 12.5%, lower than those of HMCM-68-MW (14.1%) and HMCM-68 (14.9%). Fig. 10 shows the TG curves of the samples of HMCM-68, DA-5h, HMCM-68-MW, DS-10h, and DS-24h used in isomerization reaction under 400 °C and 1.73 h−1 for 24 h. The weight losses in the range of 350–800 °C due to coking removal for used HMCM-68 and DA-5h were 5.8 wt% and 5.0 wt%, respectively. The decrease in coking over DA-5h could be attributed to the loss in acidity by dealumination treatment compared with HMCM-68. For the samples of used HMCM-68-MW, DS10h and DS-24h, the coking weight losses were 4.7 wt%, 4.4 wt% and 4.5 wt%, respectively, which were lower than those of HMCM-68 and DA-5h. This indicated that hierarchical MCM-68 zeolites had a higher resistance to coke formation, owing to the broad mesopores across zeolite crystals.
3.2.2. m-Xylene isomerization over hierarchical MCM-68 zeolites Hierarchical MCM-68 zeolites prepared by H2O2 microexplosion of HMCM-68 and by NH3(aq) treatment of dealuminated MCM-68 were used as catalysts for m-xylene isomerization reaction to evaluate their catalytic properties. Based on the catalytic results over HMCM-68, two reaction conditions of 400 °C, 1.73 h−1 and 300 °C, 10.37 h−1, under which the highest conversion and p-xylene yield were obtained, respectively, were chosen. The results are listed in Table 2, Fig. 8 and Fig. 9. It could be found that, under 400 °C and 1.73 h−1, hierarchical MCM-68 zeolites HMCM-68-MW, DS-10h and DS-24h exhibited higher m-xylene conversion compared with HMCM-68 zeolite. The highest conversion in this paper achieved over DS-24h catalyst, which was 73.1% on an average of 5–10 h (Table 2). Based on NH3-TPD results in Fig. 6, the acidities of post-treated MCM-68 zeolites were lower than
Fig. 8. Conversion of m-xylene (solid) and p-xylene yield (open) at 400 °C and 1.73 h−1 over HMCM-68 (square), DA-5h (triangle-up), HMCM-68-MW (circle), DS-10h (triangleright), and DS-24h (diamond).
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Fig. 10. TG curves of HMCM-68 (black), DA-5h (red), HMCM-68-MW (blue), DS-10h (green), and DS-24h (purple) after 24 h of reaction. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
4. Conclusion In summary, novel hierarchical MCM-68 zeolites have been successfully prepared by H2O2 microexplosion and NH3(aq) treatment for the first time. These hierarchical MCM-68 zeolites were characterized by multiple methods, and the presence of abundant mesopores across zeolite crystals was well demonstrated with the preservation of MCM68 zeolite framework. Especially, the H2O2 microexplosion method only changed the Si/Al ratio slightly from its raw material, while acid or alkaline treatment changed the Si/Al ratio much more. m-Xylene isomerization reaction was firstly tested over hierarchical MCM-68 zeolites and HMCM-68 as well. It turned out that high temperature and low WHSV were in favor of m-xylene conversion as well as disproportionation side reactions. Meanwhile, the created mesopores in hierarchical zeolite crystals would lead to higher conversion and D/I ratio, but lower p-xylene selectivity. A high conversion of over 72.5% was achieved over DS-n, which owned relatively large mesopore volumes. Moreover, the hierarchical MCM-68 zeolite structure was found to be beneficial to the enhancement of resistance to coke formation. Acknowledgements This work was financially supported by the Tianjin Municipal Natural Science Foundation (Grant Nos. 14JCQNJC06000 and 14JCZDJC32000), China Scholarship Council (Grant No. 201606200087), MOE (IRT13R30) and 111 Project (B12015). References [1] R. Bai, Q. Sun, N. Wang, Y. Zou, G. Guo, S. Iborra, A. Corma, J. Yu, Chem. Mater. 28 (2016) 6455–6458. [2] E. Koohsaryan, M. Anbia, Chin. J. Catal. 37 (2016) 447–467. [3] P. Tian, Y. Wei, M. Ye, Z. Liu, ACS Catal. 5 (2015) 1922–1938. [4] W. Shan, H. Song, Catal. Sci. Technol 5 (2015) 4280–4288. [5] F. Jiao, J. Li, X. Pan, J. Xiao, H. Li, H. Ma, M. Wei, Y. Pan, Z. Zhou, M. Li, S. Miao,
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Hierarchical porous MCM-68 zeolites: Synthesis, characterization and catalytic performance in m-xylene isomerization
T
He Haoa, Yue Changa, Wenjun Yua, Lan-Lan Loua,∗, Shuangxi Liua,b,∗∗ a
Institute of New Catalytic Materials Science and MOE Key Laboratory of Advanced Energy Materials Chemistry, National Institute of Advanced Materials, School of Materials Science and Engineering, Nankai University, Tianjin 300350, People's Republic of China b Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, People's Republic of China
A R T I C L E I N F O
A B S T R A C T
Keywords: Hierarchical zeolite MCM-68 H2O2 microexplosion NH3(aq) treatment m-Xylene isomerization
Novel hierarchical MCM-68 zeolite materials were successfully prepared by two simple, effective and environmentally friendly methods: H2O2 microexplosion under microwave and NH3(aq) treatment. The characterization of the materials with powder XRD, SEM, TEM, N2-sorption, and NH3-TPD proved that abundant mesopores across zeolite crystals were created while the microporous MSE framework was well preserved. The mesopore volume of hierarchical MCM-68 zeolites is up to 0.19 cm3 g−1. The crystallinities of hierarchical ones and microporous MCM-68 are nearly the same according to the XRD result. The acidities of hierarchical MCM-68 are generally lower than their raw materials. Gas-phase isomerization reaction of m-xylene was carried out over these hierarchical MCM-68 zeolites as well as HMCM-68 zeolite under different conditions to characterize the catalytic property of newly prepared hierarchical zeolites. It was found that the m-xylene conversion and disproportionation/isomerization rate ratio were closely related to the created mesopores in hierarchical MCM-68 zeolites under high reaction temperature and low weight hourly space velocity (WHSV). Moreover, these hierarchical MCM-68 zeolites exhibited improved resistance to coke formation compared with HMCM-68.
1. Introduction Zeolites are a class of crystalline microporous aluminosilicate materials with highly defined porous structures. Their frameworks consist of corner-sharing TO4 tetrahedral units (T is generally Si or Al). Zeolites have been used in lots of industrial fields because of their unique framework structures, high cation-exchange capacities, good adsorptivities, and moderate acidities [1]. The application of zeolites as catalysts for cracking, alkylation, isomerization and other petrochemical reactions is of great importance [2]. Furthermore, there have been increasing attention on zeolites used as catalysts for methanol to olefins process [3], nitrogen oxides selective catalytic reduction [4], Fischer–Tropsch synthesis [5], and biomass conversion [6] in recent years. The well-defined microporous framework contributes to the high surface area, excellent thermal stability, and “shape-selective” feature of zeolites. However, on the other hand, narrow pores (usually below 1 nm) always cause diffusion limitations in zeolite particles and the acid sites inside zeolite crystals are inaccessible to bulk molecules, both of which lead to a low utilization of the catalysts [1,7]. Furthermore,
micropores and high acid amount may cause coke deposition and then the catalytic deactivation during organic reactions [8]. Therefore, in the last decade, the catalytic exploitation of hierarchical zeolites, which combine the advantages of improved diffusion ability and the welldefined microporous structure, has attracted widespread research and industry interest. Mesopores and/or macropores can be fabricated by post-synthesis treatments of zeolite crystals, such as steaming [7], acid leaching [9–11], and NaOH solution treatment [12,13]. These postsynthesis treatments are simple and widely usable to generate hierarchical zeolites, but they often suffer from some problems such as environmental pollution and the demand of additional ion-exchange processes. Besides these traditional methods, two novel approaches towards hierarchical zeolites were reported very recently. Aelst et al. adopted weak alkaline NH3(aq) as modification agent instead of traditional NaOH solution to generate mesopores in commercial dealuminated USY zeolite [14,15]. NH4+ form zeolite product was obtained that can be used directly after calcination without additional ion-exchange. Zhang et al. reported the fabrication of hierarchical zeolites, including beta, ZSM-5, mordenite, Y and so on, by using H2O2 decomposition under microwave treatment [16]. H2O2 molecules diffused
∗
Corresponding author. Corresponding author. Institute of New Catalytic Materials Science and MOE Key Laboratory of Advanced Energy Materials Chemistry, National Institute of Advanced Materials, School of Materials Science and Engineering, Nankai University, Tianjin 300350, People's Republic of China. E-mail addresses: [email protected] (L.-L. Lou), [email protected] (S. Liu). ∗∗
https://doi.org/10.1016/j.micromeso.2017.12.009 Received 21 June 2017; Received in revised form 19 November 2017; Accepted 10 December 2017 Available online 11 December 2017 1387-1811/ © 2017 Published by Elsevier Inc.
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into zeolite crystals can release big amount of gases under microwave irradiation and generate mesopores from inside crystals to the outside. MCM-68 (framework type code: MSE) is a type of aluminosilicate zeolite invented by Mobil researchers in 1997 and first reported as patent in 2000 [17]. It is a multipore zeolite with three-dimensional 12 × 10 × 10-ring channel system, including a 12-ring (6.4 × 6.8 Å) straight channel and two tortuous 10-ring (5.2 × 5.8 Å and 5.2 × 5.2 Å) channels that intersect with each other [18–20]. There is a cylindrical 24-hedral supercage circumscribed by 12- and 18-rings in the framework, accessible only through 10-ring pores, which can accommodate relative bulk transition-state species. Because of its unique three-dimensional pore structure, MCM-68 zeolite has received increasing attention. Various post-synthesis treatments, such as steam treatment [8], metal elements insertion [21–23], phosphate impregnation [24], and acid treatment [25,26], have been reported to modify the catalytic properties of MCM-68. And MCM-68 zeolites have been used as catalysts in different reactions, such as phenol oxidation [21–23], alkylation of aromatics [25,26], disproportionation of ethylbenzene [27] or toluene [28], isomerization of n-alkanes [27], hexane cracking [29–33], and dimethyl ether-to-olefin reaction [24]. However, up to now, there was no report focused on the fabrication of hierarchical porous MCM-68 zeolite through alkaline treatment. Here we reported the novel hierarchical MCM-68 zeolite materials with mesopores generated by NH3(aq) treatment of dealuminated MCM-68 and by exploding of H2O2 molecules inside HMCM-68 zeolite under microwave irradiation. In addition, their catalytic performance for isomerization of m-xylene, an important industrial reaction to produce p-xylene, was studied. For comparison, HMCM-68 and dealuminated MCM-68 were also used. To the best of our knowledge, this is the first report focused on the utilization of MSE-type zeolite in m-xylene isomerization.
Table 1 Physicochemical properties of various MCM-68 zeolites. Sample
SBET (m2·g−1)a
SEx (m2·g−1)b
VMicro (cm3·g−1)b
VMeso (cm3·g−1)c
Si/Ald
HMCM-68 DA-5h DS-10h DS-24h HMCM68MW
504 504 514 503 567
73 80 152 147 102
0.20 0.20 0.17 0.17 0.21
0.08 0.08 0.19 0.19 0.12
11.0 38.7 26.1 24.4 11.9
a b c d
BET method. t-Plot method. Total pore volume minus micropore volume. Determined by ICP-AES.
calcined zeolite was ion-exchanged by 0.5 mol L−1 NH4NO3 aqueous solution at 80 °C in order to turn the zeolite to NH4+-form. This ionexchange procedure was repeated twice. The NH4+-form MCM-68 was calcined at 550 °C for 10 h to afford HMCM-68, which had a Si/Al ratio of 11.0 (Table 1).
2.2.2. Preparation of hierarchical MCM-68 zeolites The preparation procedure of hierarchical MCM-68 zeolites is shown in Scheme 1. HMCM-68 was treated by microexplosion under microwave irradiation [16] to fabricate hierarchical MCM-68 zeolite, and the detailed preparing procedure was as follows. HMCM-68 zeolite (0.2 g) and H2O2 aqueous solution (15 wt%, 8 mL) were charged in a high pressure microwave reactor (HP-100, SINEO Microwave Chemistry Technology Co. Ltd.), which was placed in a microwave oven (MDS-10, SINEO). The reaction system was heated to 120 °C for 5 min, then to 140 °C for 10 min, then to 180 °C for 20 min, and finally cooled down to room temperature. The solid product was separated by centrifugation and dried overnight. This process was repeated for another two times, and the obtained material was denoted as HMCM-68-MW. Considering that low Si/Al ratio zeolites show good resistance to alkaline treatment [34], which is inappropriate for preparing hierarchical zeolites, dealumination of HMCM-68 was performed with 1 mol L−1 HCl solution (50 mL g−1 sample) in 80 °C water bath for 5 h. The solid was filtrated, dried overnight, and calcined at 500 °C for 4 h to afford dealuminated MCM-68 sample, which was denoted as DA-5h. DA-5h was applied as the raw material for the preparation of hierarchical MCM-68 zeolites by NH3(aq) treatment [15]. In a typical preparation, DA-5h was treated with 0.1 mol L−1 ammonia solution at room temperature under drastically stirring for different time (10 h and 24 h), followed by filtration, thoroughly washing with distilled water and calcination at 550 °C for 10 h. The resulted materials were denoted as DS-n (n = 10h and 24h, according to the ammonia treatment time).
2. Experimental 2.1. Chemicals and reagents Bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic dianhydride (> 95 .0%, Tokyo Chemical Industry Co. Ltd.), aluminum hydroxide (Al(OH)3, 77%, Tianjin Heowns Biochem LLC), colloidal silica (40 wt% suspension in H2O, LUDOX HS-40, Aldrich), hydrogen peroxide (H2O2, 30%, GR, Tianjin Damao Chemical Reagent Factory), ammonia solution (ACS, 28.0–30.0% NH3 basis, Aladdin Industrial Corporation), m-xylene (> 99.0%, GC, Aladdin Industrial Corporation), potassium hydroxide (KOH, GR, Tianjin Chemical Reagent Company), and ammonium nitrate (NH4NO3, AR, Tianjin Chemical Reagent Company) were used as received. HZSM-5 (Si/Al = 14.0) and Hβ (Si/Al = 12.5) were obtained from Nankai University Catalyst Co. Ltd. 2.2. Sample preparation 2.2.1. Synthesis of HMCM-68 MCM-68 zeolite was synthesized according to previous literature [8,25]. The organic structure-directing agent (OSDA) of N,N,N′,N′-tetraethylbicyclo[2.2.2]oct-7-ene-2,3,5,6-dipyrrolidinium diiodide (TEBOP2+(I−)2) was synthesized from bicyclo[2.2.2]oct-7-ene-2,3,5,6tetracarboxylic dianhydride through three steps. The typical gel composition is 1.0SiO2:0.1TEBOP2+(I−)2:0.375KOH:0.1Al(OH)3:30H2O. In a typical synthesis, colloidal silica (3.0 g), distilled water (9.0 g), and Al (OH)3 (0.20 g) were mixed and stirred at room temperature for 10 min, then KOH (0.42 g) was added to the solution and stirred for another 15 min. After that, TEBOP2+(I−)2 (1.12 g) was added and the mixture was stirred for 3 h. Then the gel was transferred to a 25 mL Teflon-lined autoclave and placed in a 160 °C oven for 14 days. The solid product obtained was separated by filtration, washed several times with distilled water, and dried overnight. The as-synthesized MCM-68 was calcined in a muffle furnace at 650 °C for 6 h to remove the OSDA. The
Scheme 1. The preparation procedure of various materials.
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2.3. Characterization The X-ray powder diffraction (XRD) patterns were obtained on a Bruker D8 FOCUS diffractometer, using Cu Kα radiation at 40 kV and 40 mA operating from 5° to 50°. The morphologies and sizes of materials were observed by filed-emission scanning electron microscopy (SEM, JEOL JSM-7500F) and high-resolution transmission electron microscopy (HRTEM, Philips Tecnai G2 F30). N2 sorption isotherms were measured by a Micromeritics ASAP2020 physisorption instrument at 77 K. The specific surface areas were calculated by the BrunauerEmmett-Teller (BET) method, and the micropore volumes and external surface areas were calculated by t-plot method. The pore size distribution was estimated by DFT method. The ammonia temperature programmed desorption (NH3-TPD) was measured on a Micromeritics ChemiSorb 2750 instrument. The sample (0.1 g) was preheated at 480 °C for 1 h under He flow. After cooled down to 100 °C, the sample was saturated with 5% NH3/Ar, followed by He flow sweeping off physisorbed NH3. Desorption was performed by heating from 100 °C to 800 °C at a rate of 10 °C min−1. The inductively coupled plasma-atomic emission spectrometry (ICP-AES) was applied to determine the Si/Al ratios of zeolites by using a Perkin-Elmer Optima 8300 instrument. Thermogravimetric analysis (TGA) was performed by using a TA SDT Q600 instrument under air atmosphere.
Fig. 1. Powder XRD patterns of (a) as-synthesized MCM-68, (b) HMCM-68, (c) HMCM-68MW, (d) DA-5h, (e) DS-10h, and (f) DS-24h.
68 zeolite from 11.0 (HMCM-68) to 38.7 (DA-5h), and the successive mild NH3(aq) treatment decreased the Si/Al ratio to 24.4–26.1 (DS-n). Compared with typical desilication methods like NaOH treatment, NH3(aq) treatment is a relatively mild way because NH3(aq) acts as a weak base and the treatment is carried out at room temperature. More importantly, this method is simple (without the need of additional NH4+ exchange), energy-saving and high yield. The yields of DS-n materials were about 70 wt% without big differences between 10 h and 24 h. For comparison, similar desilication experiments towards DA-5h material using NaOH solution with the same concentration were also carried out at room temperature. It turned out that the crystallinity of the zeolite decreased very sharply and nearly no solid product could be collected after several hours. As for HMCM-68-MW, the Si/Al ratio was about 11.9, changing slightly from that of HMCM-68, which was also consistent with the paper reported before [16]. N2 adsorption-desorption isotherms of various zeolites are shown in Fig. 3. Clearly, HMCM-68 exhibited type I isotherm with a very steep rise in N2 amount adsorbed in the low relative pressure (p/p0) range and a small hysteresis loop of type H3 in the high p/p0 range of 0.9–1.0, indicating its typical microporous feature as well as the existence of some mesopores and/or macropores derived mainly from the pile-up of zeolite crystals [37]. In addition, no obvious increase in N2 amount adsorbed was observed in the medium p/p0 range of 0.05–0.9. The sample of DA-5h showed very similar isotherm as that of HMCM-68, suggesting the present dealumination process by HCl treatment had little influence on the pore structure of HMCM-68. As for the zeolites of DS-n and HMCM-68-MW, a gradual increase in N2 uptake in the medium p/p0 range of 0.05–0.9 could be observed in their N2 adsorption-desorption isotherms, along with a small hysteresis loop of type H2 in the range of 0.4–0.8, demonstrating the appearance of some mesopores in the zeolites after microexplosion or NH3(aq) treatment process. Fig. 4 shows the DFT pore size distribution curves of different samples. It could be found that, compared with HMCM-68 and DA-5h, abundant mesopores in the range of 2–15 nm appeared for HMCM-68-MW and DS-n zeolites. In the microporous region, all the samples had two main peaks centered at around 0.53 nm and 0.70 nm, which could be attributed to 10-ring and 12-ring channels in MCM-68 framework, respectively. The textural parameters of various MCM-68 zeolites are listed in Table 1. The BET surface area, micropore volume and mesopore volume of DA-5h were 504 m2 g−1, 0.20 cm3 g−1 and 0.08 cm3 g−1, respectively, which were as same as those of HMCM-68. And the following desilication treatment by ammonia affected the BET surface areas of zeolites to a small degree, ranging from 503 m2 g−1 to 514 m2 g−1. While for the pore volume, DS-10h and DS-24h samples showed an obviously increased mesopore volume of 0.19 cm3 g−1 and a slightly decreased micropore volume of 0.17 cm3 g−1 compared with DA-5h. The sample of HMCM-68-MW owned the highest BET surface
2.4. Catalytic reaction Catalytic m-xylene isomerization was chosen as the probe reaction for various MCM-68 catalysts. The m-xylene isomerization was performed in a tubular, down-flow stainless steel fixed bed reactor (inner diameter 8 mm, length 50 cm, Tianjin Pengxiang Technology Co. Ltd.). The catalyst (20–40 mesh) was activated at 500 °C for 2 h under nitrogen flow prior to the catalytic run. m-Xylene was injected from the top through a pulsation-free pump (2PB-00C, Beijing Xingda Science and Technology Development Co. Ltd.) and was vaporized before flowing into the reactor. The isomerization reaction was carried out at atmospheric pressure under nitrogen flow (5 mL min−1), and the reaction temperature was 300–400 °C. The liquid products were collected by condenser and quantified by gas chromatography (GC, FL9790II) equipped with a PEG-20M column. The products obtained from 3 h to 10 h were adopted to study the activity and selectivity of the catalyst. Product distributions were reported in mol% [35,36]. 3. Results and discussion 3.1. Properties of the materials The XRD patterns of different MCM-68 samples are presented in Fig. 1. The zeolites of as-synthesized MCM-68 and HMCM-68 showed a typical MSE topology with good crystallinity. Other samples obtained from post-synthesis modification maintained MSE topology with no obvious decrease in crystallinity compared with HMCM-68, suggesting that MCM-68 zeolite structure was very well preserved upon dealumination, desilication and microexplosion. Fig. 2 shows the SEM images of these MSE type zeolites. It could be found that as-synthesized MCM-68 and HMCM-68 exhibited cuboid shaped crystallites with sizes of about 100–200 nm, which was similar to the previous report [8]. The shapes and particle sizes of HMCM-68-MW and DA-5h showed no obvious change compared with HMCM-68. As for DS-n, with the post treatment going severer, the shape of particles became more irregular and the sizes varied in a wider range, like 100–400 nm. This may be caused by the dissolution and recrystallization processes during NH3(aq) treatment. The Si/Al molar ratios of the bulk zeolites were determined by ICPAES, and the results are listed in Table 1. Generally, dealumination treatment increases Si/Al ratio, while desilication treatment is just the contrary. In this case, HCl treatment increased the Si/Al ratio of MCM137
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Fig. 2. FE-SEM images of (a) as-synthesized MCM-68, (b) HMCM-68, (c) HMCM-68-MW, (d) DA-5h, (e) DS-10h, and (f) DS-24h.
Fig. 4. DFT pore distribution curves of HMCM-68 (black), DA-5h (red), DS-10h (orange), DS-24h (purple), and HMCM-68-MW (green). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
outside to inside and microexplosion treatment creates mesopores from inside to outside. The results of TEM characterization further confirmed the hierarchical pore structure of HMCM-68-MW and DS-n zeolites. NH3-TPD curves of the zeolites are shown in Fig. 6, which suggested that HMCM-68 possessed higher acid sites concentration and stronger acid sites than other samples. Two distinct ammonia desorption peaks centered at about 200 °C and 450 °C, attributed to the weak and strong acid sites, respectively, could be observed for HMCM-68. The order of the amount of both weak and strong acid sites is HMCM-68 > HMCM68-MW > DA-5h > DS-n. Besides the decreased amount of acid sites, HMCM-68-MW showed a shift of the peak of strong acid sites from 450 °C to 400 °C compared with HMCM-68, indicating the decline in the strength of the strong acid sites. The dealuminated sample DA-5h exhibited much lower acid sites concentration than HMCM-68, due to the removal of Al atoms from the framework, especially the Al atoms on the external surface [38]. As for the zeolites obtained after NH3(aq) treatment, the acid amount were even lower than that of raw DA-5h sample. It could be seen from Fig. 6 that the amount of acid sites in DS-n zeolites was lower than that of DA-5h and that the peak of strong acid sites moved towards low temperature direction from 450 °C to 380 °C. In other words, NH3(aq) treatment lessened the acid amount of the raw DA-5h zeolite and weakened the strength of strong acid sites at the same time, though Al atom concentration in the sample was increased. This abnormal relationship between acid amount and Si/Al ratio
Fig. 3. Nitrogen adsorption-desorption isotherms of (a) HMCM-68, (b) DA-5h, (c) HMCM68-MW (d) DS-10h, and (e) DS-24h. (Isotherms are shifted by + 50.)
area of 567 m2 g−1 among all these zeolites as well as an enhanced mesopore volume of 0.12 cm3 g−1 compared with HMCM-68. The results of N2 sorption evidenced that some mesopores were successfully created in MCM-68 zeolites by NH3(aq) treatment or H2O2 microexplosion, both of which kept the zeolite frameworks very well. In addition, the method of NH3(aq) treatment seems more effective to create mesopores. Whereas microexplosion method has advantages of fast synthesis, no wastewater and no need of post-treatment. Fig. 5 presents the TEM images of different MCM-68 zeolites. No obvious mesopores could be found in the crystals of DA-5h sample obtained from dealumination treatment. While for DS-n samples, abundant mesopores throughout zeolite crystals could be clearly observed from TEM images. HMCM-68-MW sample also had abundant mesopores through crystals, which were caused by microexplosion from H2O2 decomposition under microwave irradiation. One of the differences between these two ways of creating mesopores, proved by TEM characterization, is that desilication treatment creates mesopores from 138
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Fig. 5. TEM images of (a) DA-5h, (b) HMCM-68-MW, (c) DS-10h, and (d) DS-24h.
Fig. 7. Conversion of m-xylene (solid) and p-xylene yield (open) over HMCM-68 zeolite: 400 °C, 1.73 h−1 (square); 300 °C, 1.73 h−1 (circle); 300 °C, 5.18 h−1 (triangle-up); 300 °C, 10.37 h−1 (triangle-down).
Fig. 6. NH3-TPD curves of (a) HMCM-68, (b) HMCM-68-MW, (c) DA-5h, (d) DS-10h, and (e) DS-24h.
Table 2 Catalytic data of m-xylene isomerization over different catalysts.a
sometimes happened on alkaline treated zeolites according to previous researches, and may be connected with Si-OH-Al hydrolysis or extra framework Al. However, the accurate reason still needs to be further studied [39].
3.2. Catalytic performance 3.2.1. m-Xylene isomerization over HMCM-68 zeolite As m-xylene isomerization reaction had never been studied over MCM-68-type zeolites, we investigated the catalytic performance of parent HMCM-68 firstly, to lay a foundation for further study. The mxylene conversion and p-xylene yield over HMCM-68 under different conditions are shown in Fig. 7. To investigate the effect of reaction temperature, the isomerization reaction was carried out under 400 °C and 300 °C with the same weight hourly space velocity (WHSV) of 1.73 h−1. It was found that the average conversion under 300 °C was less than that under 400 °C by about 2.2% (Table 2). Inversely, an increase in p-xylene yield from 13.3% to 14.1% was observed with decreasing reaction temperature from 400 °C to 300 °C. The effect of the WHSV on the m-xylene conversion and p-xylene yield was also investigated by carrying out the isomerization reaction under the same temperature of 300 °C and different WHSVs of 1.73 h−1, 5.18 h−1 and 10.37 h−1, and the results are given in Fig. 7 and
Catalyst
T (°C)
WHSV (h−1)
Conv. (%)
PX Sel. (mol%)
PX yield (%)b
PX/ΣX
D/I
HMCM-68 HMCM-68 HMCM-68 HMCM-68 HMCM-68MW DA-5h DS-10h DS-24h HMCM-68MW DA-5h DS-10h DS-24h HZSM-5 Hβ
400 300 300 300 400
1.73 1.73 5.18 10.37 1.73
69.3 67.1 61.2 53.6 71.8
19.3 21.1 21.8 28.0 15.3
13.3 14.1 13.3 14.9 11.0
0.23 0.23 0.21 0.20 0.21
1.50 1.33 1.27 0.85 2.01
400 400 400 300
1.73 1.73 1.73 10.37
69.7 72.5 73.1 45.8
19.0 16.2 16.1 30.9
13.2 11.7 11.8 14.1
0.23 0.23 0.23 0.18
1.52 1.83 1.93 0.71
300 300 300 400 400
10.37 10.37 10.37 1.73 1.73
42.8 38.4 40.6 54.0 64.2
29.1 32.6 31.2 35.3 23.7
12.4 12.5 12.6 19.1 15.1
0.15 0.15 0.15 0.23 0.23
0.71 0.59 0.66 0.42 1.10
a b
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The catalytic results were the average values of 5–10 h. PX yield is the mole fraction of PX in liquid products.
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Table 2. It was obvious that after 5 h of reaction, the conversion decreased while the selectivity of p-xylene increased. The increase in pxylene selectivity could be mainly attributed to that less side reactions, such as disproportionation, took place at relatively low WHSV, which could be proved by the disproportionation/isomerization ratio (D/I) in Table 2 [40]. As an interaction of both conversion and selectivity, it turned out that the highest p-xylene yield of 14.9% was obtained under 300 °C and 10.37 h−1. Contrast experiments were carried out to thoroughly understand the catalytic behavior of HMCM-68 in the m-xylene isomerization reaction by using HZSM-5 (Si/Al = 14.0) and Hβ (Si/Al = 12.5) as catalysts under the same reaction conditions (400 °C, 1.73 h−1), and the results are listed in Table 2. The order of conversion over these three catalysts was HMCM-68 (69.3%) > Hβ (64.2%) > HZSM-5 (54.0%); however, the order of the p-xylene yield was on the contrary: HZSM-5 (19.1%) > Hβ (15.1%) > HMCM-68 (13.3%). In previous studies, the high p-xylene selectivity over ZSM-5-type zeolites was usually attributed to the faster diffusion of p-xylene in the 10-ring channels of ZSM-5 compared with o- and m-xylene. However, under the present conditions, p-xylene produced accounted for 23% of all xylenes in effluent regardless of the kind of catalysts used, suggesting the diffusion limit of products was not the main factor. The difference in m-xylene conversions and p-xylene selectivities among the three catalysts may be connected with the void space in zeolite frameworks that could accommodate bulk transition-state complexes for disproportionation [41]. The 10-ring channel system of HZSM-5 cannot afford as many as bulk transition-state complexes compared with the 12-ring channel systems of Hβ and HMCM-68. In addition, there is a 24-hedral supercage in HMCM-68 framework, which is also in favor of the formation of bulk transition-state molecules, as can be proved by the relatively high D/I ratio over HMCM-68 (Table 2).
25
Conversion ( % )
50 20 40 30 15 20
p-Xylene Yield ( % )
60
10 0
10 3
4
5
6
7
8
9
10
Reaction Time ( h ) Fig. 9. Conversion of m-xylene (solid) and p-xylene yield (open) at 300 °C and 10.37 h−1 over HMCM-68 (square), DA-5h (triangle-up), HMCM-68-MW (circle), DS-10h (triangleright), and DS-24h (diamond).
that of HMCM-68 zeolite, which indicated that the acidity was not the main factor that effected the conversion. The zeolite frameworks of these catalysts were the same, however the hierarchical catalysts of HMCM-68-MW, DS-10h and DS-24h owned abundant intracrystal mesopores compared with HMCM-68 and DA-5h. According to the foregoing experiments, lager void space was in favor of disproportionation reactions, which in this case was the mesopore structure. It could be clearly observed that the molar ratios of p-xylene to all xylene in effluents (PX/∑X) were very similar among different catalysts and were close to the value under thermodynamic equilibrium state. Under this situation, more disproportionation reactions would lead to higher conversion, which corresponds to the great D/I ratios of reactions over hierarchical MCM-68 zeolites in Table 2 [40,42]. On the contrary, the pxylene yields over hierarchical MCM-68 zeolites were lower than that of HMCM-68 zeolite due to the relatively low p-xylene selectivities, which was also in accord with the foregoing results over HMCM-68. Under the condition of 300 °C and 10.37 h−1, the conversion order of these catalysts was: HMCM-68 > HMCM-68-MW > DA-5h > DS-n, which was basically accordant with the order of acidity. This indicated that under high WHSV and low reaction temperature, the conversion depended mainly on the zeolite acidity. When the WHSV is high, there are too many m-xylene molecules participating in the reaction, which means the acid sites on the zeolite may not be enough to turn the substrate into the product adequately. It also matches the PX/∑X ratio. As listed in Table 2, the p-xylene fraction among all xylene effused was lower over treated MCM-68 zeolites compared with HMCM-68 zeolite. In this case, the porosity of zeolite seems no longer the most important factor determining the conversion. The D/I ratios also indicated that the conversions decreased as disproportionation reactions were suppressed, along with improved p-xylene selectivity. Affected by both conversion and selectivity, the final p-xylene yields of DA-5h and DS-n were around 12.5%, lower than those of HMCM-68-MW (14.1%) and HMCM-68 (14.9%). Fig. 10 shows the TG curves of the samples of HMCM-68, DA-5h, HMCM-68-MW, DS-10h, and DS-24h used in isomerization reaction under 400 °C and 1.73 h−1 for 24 h. The weight losses in the range of 350–800 °C due to coking removal for used HMCM-68 and DA-5h were 5.8 wt% and 5.0 wt%, respectively. The decrease in coking over DA-5h could be attributed to the loss in acidity by dealumination treatment compared with HMCM-68. For the samples of used HMCM-68-MW, DS10h and DS-24h, the coking weight losses were 4.7 wt%, 4.4 wt% and 4.5 wt%, respectively, which were lower than those of HMCM-68 and DA-5h. This indicated that hierarchical MCM-68 zeolites had a higher resistance to coke formation, owing to the broad mesopores across zeolite crystals.
3.2.2. m-Xylene isomerization over hierarchical MCM-68 zeolites Hierarchical MCM-68 zeolites prepared by H2O2 microexplosion of HMCM-68 and by NH3(aq) treatment of dealuminated MCM-68 were used as catalysts for m-xylene isomerization reaction to evaluate their catalytic properties. Based on the catalytic results over HMCM-68, two reaction conditions of 400 °C, 1.73 h−1 and 300 °C, 10.37 h−1, under which the highest conversion and p-xylene yield were obtained, respectively, were chosen. The results are listed in Table 2, Fig. 8 and Fig. 9. It could be found that, under 400 °C and 1.73 h−1, hierarchical MCM-68 zeolites HMCM-68-MW, DS-10h and DS-24h exhibited higher m-xylene conversion compared with HMCM-68 zeolite. The highest conversion in this paper achieved over DS-24h catalyst, which was 73.1% on an average of 5–10 h (Table 2). Based on NH3-TPD results in Fig. 6, the acidities of post-treated MCM-68 zeolites were lower than
Fig. 8. Conversion of m-xylene (solid) and p-xylene yield (open) at 400 °C and 1.73 h−1 over HMCM-68 (square), DA-5h (triangle-up), HMCM-68-MW (circle), DS-10h (triangleright), and DS-24h (diamond).
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Fig. 10. TG curves of HMCM-68 (black), DA-5h (red), HMCM-68-MW (blue), DS-10h (green), and DS-24h (purple) after 24 h of reaction. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
4. Conclusion In summary, novel hierarchical MCM-68 zeolites have been successfully prepared by H2O2 microexplosion and NH3(aq) treatment for the first time. These hierarchical MCM-68 zeolites were characterized by multiple methods, and the presence of abundant mesopores across zeolite crystals was well demonstrated with the preservation of MCM68 zeolite framework. Especially, the H2O2 microexplosion method only changed the Si/Al ratio slightly from its raw material, while acid or alkaline treatment changed the Si/Al ratio much more. m-Xylene isomerization reaction was firstly tested over hierarchical MCM-68 zeolites and HMCM-68 as well. It turned out that high temperature and low WHSV were in favor of m-xylene conversion as well as disproportionation side reactions. Meanwhile, the created mesopores in hierarchical zeolite crystals would lead to higher conversion and D/I ratio, but lower p-xylene selectivity. A high conversion of over 72.5% was achieved over DS-n, which owned relatively large mesopore volumes. Moreover, the hierarchical MCM-68 zeolite structure was found to be beneficial to the enhancement of resistance to coke formation. Acknowledgements This work was financially supported by the Tianjin Municipal Natural Science Foundation (Grant Nos. 14JCQNJC06000 and 14JCZDJC32000), China Scholarship Council (Grant No. 201606200087), MOE (IRT13R30) and 111 Project (B12015). References [1] R. Bai, Q. Sun, N. Wang, Y. Zou, G. Guo, S. Iborra, A. Corma, J. Yu, Chem. Mater. 28 (2016) 6455–6458. [2] E. Koohsaryan, M. Anbia, Chin. J. Catal. 37 (2016) 447–467. [3] P. Tian, Y. Wei, M. Ye, Z. Liu, ACS Catal. 5 (2015) 1922–1938. [4] W. Shan, H. Song, Catal. Sci. Technol 5 (2015) 4280–4288. [5] F. Jiao, J. Li, X. Pan, J. Xiao, H. Li, H. Ma, M. Wei, Y. Pan, Z. Zhou, M. Li, S. Miao,
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