Article pubs.acs.org/EF
Catalytic Cracking of Endothermic Hydrocarbon Fuels over Ordered Meso-HZSM‑5 Zeolites with Al-MCM-41 Shells Li Wang,†,‡ Zhenheng Diao,† Yajie Tian,† Zhongqiang Xiong,§ and Guozhu Liu*,†,‡ †
Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology and Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin University, Tianjin 300072, People’s Republic of China § Tianjin Entry-Exit Inspection and Quarantine Bureau, Tianjin 300457, People’s Republic of China ‡
S Supporting Information *
ABSTRACT: Ordered meso-HZSM-5 zeolites with Al-MCM-41 shells were synthesized by a surfacant-directed encapsulating process. It was found that ordered meso-HZSM-5, which was used as a core, could be synthesized using a designed amphiphilic organosilane as a surfactant and that the mesopore size of the MCM-41 shell can be adjusted from 2.5−3 to 4.5−10 nm by changing the structures of alkyltrimethylammonium bromide and the trimethylbenzene amount. Catalytic cracking performances of supercritical n-dodecane (500 °C and 4 MPa) show that ordered meso-HZSM-5 zeolites with Al-MCM-41 shells of 4.5−10 nm exhibit a 28% higher catalytic activity and a 25% lower deactivation rate compared to the conventional HZSM-5 zeolites. With an increasing mesopore size of the MCM-41 shell, the acid sites accessibility increased gradually and, thus, led to enhanced catalytic activity and decreased secondary reaction ability. In addition, ordered hierarchical zeolites show a well connection of ordered mesopores between the core and shell, which could also lead to the enhanced acid site accessibility.
1. INTRODUCTION Over the last few decades using liquid hydrocarbon fuels as both propellants and coolants is usually considered as one promising technique for the thermal management of advanced aircrafts.1−3 Cooling capacity of hydrocarbon fuel was attributed to both the physical and chemical heat sink of endothermic reactions. Typically, catalytic cracking of hydrocarbon fuels using wall-coated catalyst (for example, HZSM-5 zeolite) is considered to be one of the most important candidates for its low-pressure drop and thermal resistances.4,5 The pressure of hydrocarbon fuel (>3.0 MPa) in the cooling system is usually above its critical pressure, and thus, the hydrocarbon fuel will become the supercritical fluid when its temperature rises above its critical temperature (385−420 °C).6 Previously, attempts were made to obtain more insight into the catalytic cracking of supercritical hydrocarbon in the presence of various zeolite catalysts, such as coatings. Fan et al.4 obtained a total heat sink of ∼3.4 MJ/kg of a Chinese commercial aviation kerosene using HZSM-5 coatings as catalysts under the same working conditions of scram jet applications. Wu et al.7 observed the significant chemical heat sink of n-heptane over HZSM-5 coatings directly grown on the cooling channel wall. Sicard et al.8 found that conversion of supercritical n-dodecane was enhanced approximately 100% over HZSM-5 coating compared to thermal cracking. The influence of the coating thickness of HZSM-5 zeolites on the catalytic cracking performances was also studied with n-dodecane at 550 °C and 4 MPa.9 Therefore, wall-coated HZSM-5 is an excellent candidate for the applications of catalytic cracking of supercritical hydrocarbon fuels in the micro cooling channels.10 Recently, Süer et al.11 reported that supercritical hydrocarbons have lower diffusion rates in the channels of a zeolite catalyst as a result of the liquid-like and extraordinary dense behavior © XXXX American Chemical Society
within micropores. In this view, an important concern for the further improvement of the catalytic performances of the catalysts (activity and stability) is to enhance the pore diffusion rate of reactants. With high pore diffusion and acid accessibility, core−shellstructured zeolites consisting of a HZSM-5 crystal core and a mesoporous shell were recognized as the most important catalyst. Up to now, various approaches have been adopted to synthesize core−shell-structured zeolites. Ordomsky et al.12 prepared BEA@MCM-41 composites by recrystallization of BEA zeolite in the solution containing cetyltrimethylammonium bromide (CTAB). Qian et al.13 prepared ZSM-5@mesoSiO2 by means of a CTAB-directed process using tetraethylorthosilicate (TEOS) as the silica precursor and found that the core−shell zeolites showed better catalytic cracking performance of n-dodecane compared to conventional HZSM-5 zeolite. Na et al.14 also synthesized ZSM-5@MCM-41 materials using desilication in alkaline solution and recrystallization directed by CTAB, which was used as a catalyst in n-dodecane cracking. Very recently, our group developed a method to synthesize meso-HZSM-5@Al-MCM-41 composites by sequential desilication and recrystallization of HZSM-5. The introduction of intracrystal mesopores to the HSZM-5 crystals could improve the diffusion rates of both reactant and products in a zeolite crystal. At the same time, the precracking of hydrocarbon in the mesoporous shell helps the diffusion of primary products inside micropore channels, and thus, it is benifical for the improvement of catalytic activity and lifetime in the catalytic cracking of supercritical n-dodecane.15 However, for the core−shell Received: May 16, 2016 Revised: August 26, 2016
A
DOI: 10.1021/acs.energyfuels.6b01160 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
2.3. Characterization. X-ray diffraction (XRD) patterns of the samples were taken on a diffractometer (Philips X’Pert MPD) with Cu Kα radiation. The morphologies of the samples were observed using transmission electron microscopy (TEM, JEM-2100F). The pores of the samples were measured on a volumetric adsorption analyzer (Micromeritics ASAP 2020) using N2 adsorption−desorption isotherms. The Brunauer−Emmett−Teller (BET) method was used to calculate the specific surface area (SBET),18 and the t-plot method was used to calculate the volume of micropores (Vmicro).19,20 For the mesopore, the Barrett−Joyner−Halenda (BJH) model was applied using the adsorption isotherm.21 Contents of aluminum and silicon were measured using an inductively coupled plasma (ICP) spectrophotometer (Thermo Jarrell-Ash Corp. ICP-9000). Ammonia temperature-programmed desorption (NH3-TPD) was performed on a Quantachrome CHEMBET 3000. For a typical run, 50 mg of sample was treated in the He flow to remove physisorbed NH3 and then the temperature was raised to 600 °C with a rate of 10 °C/min to obtain chemically adsorbed NH3. The Brønsted and Lewis acid sites of the samples were identified on a Fourier transform infrared (FTIR) spectrometer (Bruker Equinox Vertex 70) with pyridine as a probe molecule. The pyridine adsorption on a pressed self-supported wafer of the sample was performed in an in situ infrared (IR) cell at 50 °C under vacuum of 10−3 Pa. IR spectra were recorded at 150 and 350 °C after evacuation desorption assigned to the total Brønsted and total Lewis acid sites. The adsorption of another probing molecule, 2,6-ditert-butylpyridine (DTBPy), was used to obtain the accessibility index (ACI). 2.4. Catalytic Performances. To study the catalytic performance of the prepared catalysts, catalytic cracking of n-dodecane under supercritical conditions was selected as a model reaction.5,22 The prepared samples were coated on the inner surface of a stainless-steel tube (l = 300 mm, and d = 2 mm), which was used on the reactor for the performance evaluations.23 The coatings were prepared by the following steps: mixing zeolites (HZs), binder (colloidal silica), and ethanol together, and then an additional ball milling of the mixture was performed for 3 h to obtain the coating slurry. After then, the slurry was used to flow through the tube and a uniform zeolite coating was obtained after a complete drying at 110 °C overnight and calcinating at 600 °C. The same compositions and preparation method were used to obtain a similar thickness of coatings and zeolite loading amounts. An equal quality of zeolite and SiO2 was weighted by an analytical balance (Mettler Toledo) to control the loading amount on the reactor. As shown in the experimental apparatus (Figure S1 of the Supporting Information), n-dodecane was added with a flow rate of 10 mL/min by a plunger pump. K-type thermocouples were inserted into union crossjunctions to measure the outlet temperature, and direct current power was used to heat the reactor. A Tescom back-pressure valve was employed to keep the reactor pressure at 4 MPa. A condenser was used to cool the products, which were allowed to flow into a gas− liquid separator. To guarantee the sufficient mass for the material balance, gas and liquid products were gathered each for 5 min. The heat sinks of n-dodecane catalytic cracking are measured by the total power input (I2R) minus the calibrated heat lost.24 An Agilent 7890A gas chromatograph with a flame ionization detector was used to analyze the composition of the liquid products by a capillary column (PONA, 50 m × 0.2 mm × 0.5 μm). The gaseous products were analyzed with a multichannel gas chromatograph (Agilent Micro GC 3000A) equipped with three thermal conductivity detectors and three analytical columns, i.e., Plot U (10 m × 30 μm), molecular sieve (10 m × 12 μm), and alumina (10 m × 8 μm). The ndodecane conversion (X), product mass selectivity j (Sj), and deactivation rate (rd) are defined as follow:
structure catalyst, there is still some room to improve its catalytic performance by further enhancing the acid amount of the desilicated zeolite core and the accessibility of reactants and their primary products in it.16 Ryoo et al.17 demonstrated that crystalline MFI zeolites synthesized using a designed amphiphilic organosilane as a surfactant with tunable mesoporosity might have better technological implications than mesoporous materials with amorphous frameworks based on its uniform intracrystal mesoporous distribution and strong acidity. Therefore, a core−shell zeolite composite with both an ordered porous core and a shell might be a promising candidate for the catalytic cracking of supercritical hydrocarbons in view of its enough acid amount, better acid accessibility, and suitable meso-/micropore junctions. In this work, ordered meso-HZSM-5 zeolites with an AlMCM-41 shell were prepared and their catalytic cracking performance was investigated. The ordered meso-HZSM-5 zeolite was synthesized using amphiphilic organosilane as a mesoporogen, and then the Al-MCM-41 shells were synthesized by CTAB-directed recrystallization. The mesopore sizes of the MCM-41 shell were controlled by adjusting the chain length of CTAB and the amount of trimethylbenzene (TMB). Finally, n-dodecane was selected as endothermic hydrocarbon fuel, and the catalytic cracking of supercritical n-dodecane was performed to study the catalytic activity of the ordered mesoHZSM-5@Al-MCM-41 zeolites.
2. EXPERIMENTAL SECTION 2.1. Materials. [3-(Trimethoxysilyl)propyl]octadecyldimethylammonium chloride (TPOAC, 60 wt % methanol solution), tetrapropylammonium hydroxide (TPAOH, 25 wt % in water), tetraethylammonium hydroxide (TEAOH, 25 wt % in water), decyltrimethylammonium bromide (C10TAB), and cetyltrimethylammonium bromide (C16TAB) were purchased from J&K Chemical, Ltd. (China). Colloidal silica (40 wt % in water) from Sigma-Aldrich (St. Louis, MO) was used as a binder. n-Dodecane (99.5% purity) from Sinopharm Chemical Reagent (China), TEOS, acetone, n-pentane, Al(NO3)3·9H2O, and TMB from Guangfu Chemical Company (China), and hydrochloric acid (37 wt %) from Real & Lead Chemical Company (China) were used without further purification. 2.2. Preparation of Samples. Meso-HZSM-5 zeolite was prepared using TPOAC missle as a mesoporogen. For a typical procedure, a certain amount of Al(NO3)3·9H2O was dissolved in water and then added to a given TPAOH. After that, a TEOS and TPOAC mixture were dropped into them under vigorous stirring for 24 h to give a solution with a molar ratio of 0.32TPAOH/1TEOS/165H2O/ 0.01Al(NO3)3/0.09TPOAC. The gel was then moved and kept in a Teflon-lined autoclave at 130 °C. After 48 h, the temperature was raised to 170 °C for 24 h. The products were thoroughly washed by water, then dried overnight at 100 °C, and calcined at 550 °C in flowing air for 6 h. The product is denoted as HZ-TPO. Conventional HZSM-5 zeolite (HZ-P) was prepared in the absence of TPOAC by a similar procedure. The meso-HZSM-5 zeolites with Al-MCM-41 shells were prepared using CnTAB with different chain lengths (n = 10 and 16) as surfactants and TMB as the swelling agent. In a typical run, a measured amount of TEOS was slowly dropped into the mixture containing CnTAB, water, TEAOH, and Al(NO3)3 under vigorous stirring for 24 h to obtain a solution with a molar composition of 0.8TEAOH/ 1TEOS/200H2O/0.02Al(NO3)3/0.8CnTAB/xTMB (where x = 0 and 1). The solution pH was then adjusted to ca. 8.5 by 2.0 M HCl. Then, a required amount of zeolite powder (HZ-TPO) and TMB was added and stirred for 5 h. The mixture was then moved into a Teflon-lined autoclave at 110 °C for 24 h. Then, the products were washed with distilled water, dried at 100 °C for 12 h, and then calcined at 550 °C in flowing air for 6 h. The prepared catalysts with C10TAB, C16TAB, and C16TAB and TMB are denoted as HZ-C10, HZ-C16, and HZ-C16&T.
X=
Sj = B
W × 100% W0
mj w
× 100% DOI: 10.1021/acs.energyfuels.6b01160 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels Table 1. Textural and Acid Properties of HZs acid densitya (μmol of pyridine g−1) sample
Si/Al
HZ-P HZ-TPO HZ-C10 HZ-C16 HZ-C16&T
67 81 76 77 79
b
−1
−1
−1
SBET (m g )
Sexter (m g )
Vt (cm g )
Vmicro (cm g )
BW
BS
LW
LS
TB
TL
ACIe
100 88 84 84 83
420 500 575 596 617
78 225 431 454 472
0.250 0.514 0.496 0.529 0.679
0.159 0.126 0.067 0.065 0.065
85 33 42 39 39
90 65 48 50 47
45 91 98 104 103
41 49 41 41 44
175 98 90 89 86
86 140 139 145 147
0.11 0.32 0.42 0.44 0.49
2
d
−1
Rcc
2
3
3
a
Acid density calculated by combining the NH3-TPD and pyridine-adsorbed FTIR methods. B, L, and T represent Brønsted, Lewis, and total acid sites, respectively. Subscripts S and W represent strong and weak acid sites, respectively. bSi/Al ratio measured by ICP. cXRD relative crystallinity based on the sum of the peak intensities in the 22° < 2θ < 25° region compared to that of the commercial HZSM-5 zeolite. dSexter calculated by the tplot method. eAccessibility index determined as the ratio of acid sites accessible to DTBPy (the band at 1617 cm−1) to those of pyridine (the band at 1543 cm−1). rd =
X t = 5 − X t = 30 × 100% Xt =5
where w0 and Δw are the mass of n-dodecane fed and consumed, respectively, mj is the mass of component j, and Xt = 5 and Xt = 30 are the n-dodecane conversions at 5 and 30 min, respectively.
3. RESULTS AND DISCUSSION 3.1. Texture Properties of Conventional and Hierarchical Zeolites. 3.1.1. Chemical Compositions. The Si/Al ratio of HZ-P was 67, which increased to 81 for HZ-TPO (see Table 1), indicating that the introduction of TPOAC into the synthesis recipe might make a negative impact on incorporating Al species into the zeolite framework.25 The Si/Al ratios of core−shell zeolites (ca. 77) were close to that of ordered mesoHZSM-5, suggesting that the Si/Al ratios of the MCM-41 shell were similar to that of the meso-HZSM-5 core. 3.1.2. Crystalline Structures. As shown in the XRD patterns (Figure 1a), the peaks in the range of 2θ from 7° to 10° and from 22° to 25° are attributed to the (101), (020), (501), (151), and (303) reflections of the MFI phase, implying the successful crystallization of the MFI phase after introducing TPOAC to the synthesis gel, and the framework was wellretained during the encapsulating process. By consideration of the relative crystallinity (Rc) of HZ-P as a reference, that of HZTPO was calculated to be 88% (Table 1), implying that the TPOAC addition had a negative effect on HZSM-5 crystallization. The further drop of Rc to ca. 84% after the encapsulating process might be ascribed to the shield effect of amorphous shells to X-rays.16 The small-angle X-ray scattering pattern (see Figure 1b) of HZ-TPO displays a diffraction peak at a 2θ of ca. 2.3°, corresponding to the (100) reflection crystal faces of the hexagonal phase, indicating that mesopores with a long-range order were fabricated by the TPOAC micelles. The patterns of HZ-C10, HZ-C16, and HZ-C16&T show one resolved peak at 2.3°, and two weak scattering peaks at 3.9° and 4.5° were also observed on the patterns of HZ-C10, HZ-C16, and HZC16&T, indicating that the MCM-41 phase appeared during the encapsulating process, while the peak of HZ-C16&T is distinctly weaker and broader than that of HZ-C10 and HZC16, indicating that less ordered mesopores were formed when using TMB as the swelling agent. 3.1.3. Crystal Morphologies. TEM images may show the intracrystal mesopores and MCM-41 phases (see Figure 2). As indicated in the TEM image (Figure 2a), hexagonal pseudoprism morphology with a particle size of approximately 320 nm and no intracrystal mesopore was observed for HZ-P, while worm-like mesopores in HZ-TPO crystals were distinctly
Figure 1. (a) Wide-angle and (b) small-angle XRD patterns of HZs.
observed, in agreement with the decrease of Rc, indicating that worm-like mesopores with a long-range order can be fabricated in HZSM-5 crystals using the TPOAC micelle as a mesoporogen.17 After the encapsulating process, each HZTPO crystal was uniformly coated by a MCM-41 shell with a thickness of 50−60 nm, in agreement with XRD results. It is worth noting that the MCM-41 shell was less ordered and the pore size of HZ-C16&T was less uniform in comparison to HZC10 and HZ-C16, corresponding to its weaker intensity in lowangle range of XRD patterns.26 A good coherence of the mesopore structure between the core and shell could also be observed in HZ-C10 and HZ-C16 from the TEM image (panels c and d of Figure 2), which might imply well connections of ordered mesopores between the meso-HZSM-5 core and the Al-MCM-41 shells. 3.1.4. Textural Structures. Nitrogen adsorption−desorption isotherms (Figure 3) of HZ-P show a type-I curve, C
DOI: 10.1021/acs.energyfuels.6b01160 Energy Fuels XXXX, XXX, XXX−XXX
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intracrystal mesopores. For HZ-C10 and HZ-C16&T, relative weak peaks in the range of 2.5−3 and 4.5−10 nm are observed, respectively, indicating that the pore in the MCM-41 shell may be enlarged by long-chain CTAB and TMB. Furthermore, the curves of HZ-C10 and HZ-C16&T also displayed similar peaks (mesopore size = 3−4 nm) with HZ-TPO and HZ-C16, corresponding to intracrystal mesopores, implying that the core−shell-structured zeolites contained dual-model pores, i.e., intracrystal mesopores and mesopores from the MCM-41 shell. After the encapsulating process, both SBET and Vt show a higher degree compared to those of HZ-P and show a slight increase with the increasing CTAB chain length and TMB amount. Notably, a great enlargement of Vt and Sexter was observed for core−shell-structured zeolites, which should be the contribution of the increased mesopore size in the MCM41 shell. 3.2. Catalytic Performances. All zeolite coatings used in performance evaluation had similar thickness around 40.0 μm (see Table 2). The carbon balance between reactants and products was examined for each case, and the agreement, 98.8 ± 0.3%, was excellent for each measurement. For HZ-P zeolite, the initial n-dodecane conversion is only 45.37%, which decreases to 27.22% at time on stream (TOS) = 30 min with a deactivation rate (rd) of 40.0% (Figure 4 and Table 2). For HZ-TPO, the initial conversion of n-dodecane reaches up to 55.07% and rd reduced to 19.63%. After the encapsulating process, HZ-C16 and HZ-C16&T showed higher catalytic activity and stability than HZ-TPO. Although the initial conversion over HZ-C10 was slightly lower than that of HZTPO, the deactivation rate of HZ-C10 decreased by 30% compared to that of HZ-TPO. Therefore, catalytic activity and stability improvement change as the mesopore size of the MCM-41 shell enlarged, i.e., HZ-C10 < HZ-C16 < HZC16&T. A similar trend was also observed from the heat sink of n-dodecane with the increasing catalytic activity, as shown in Table S1 of the Supporting Information. Specifically, the size of mesopores enlarged from 2.5−3 nm (HZ-C10) to 4.5−10 nm (HZ-C16&T), and rd significantly decreased from 14.54 to 7.29%, together with ca. 8% improvement in the initial catalytic activity, indicating that the larger mesopores in the shell are in favor of better catalyst activity and stability. In comparison to HZ-P, HZ-C16&T shows a better catalytic performance up to 28% improvement and a lower deactivation (only one-fifth). As a contrast, the conversion obtained with a bare tube falls from 12.5 to 11.3% which illustrates that there exists thermal pyrolysis of n-dodecane under such experimental conditions. In comparison to our previous research,15 the catalyst with longrange ordered intracrystal mesopores shows a higher conversion of n-dodecane and a lower deactivation rate. The presence of long-range ordered mesopores in the core may improve the n-dodecane diffusion in the micropores and, thus, help with the removal of coke precursors. The reaction products were also listed in Table 3 (detailed mass selectivity shown in Table S2 of the Supporting Information). It is well-known that primary paraffin and olefin formed during n-dodecane cracking, and then these molecules would be cracked into light hydrocarbons.27 The highest selectivity to gaseous products was observed (see Table 3) over HZ-P, which indicated the highest degree of overcracking on it. Among hierarchical zeolites, the selectivities to gaseous products decrease in the order HZ-TPO > HZ-C10 > HZC16 > HZ-C16&T, which was in good agreement with catalytic stability of zeolites. This implies that deep cracking is limited
Figure 2. TEM images of (a) HZ-P, (b) HZ-TPO, (c) HZ-C10, (d) HZ-C16, and (e) HZ-C16&T.
Figure 3. Porous properties of HZs: (a) N2 adsorption−desorption isotherms and (b) BJH adsorption pore size distribution.
demonstrating the microporous framework. HZ-TPO and core−shell-structured zeolites show type-I and type-IV isotherms, implying the presence of both micro- and mesoporous phases. The BJH method was used to calculate the mesopore size. The pore size distribution (Figure 3b) of HZ-P reflected that there is no 2−18 nm pores, while the 3−4 nm pore of HZ-TPO is observed. Similarly, that of HZ-C16 also showed an intensive peak centered at 3−4 nm, implying that the mesopore size of the MCM-41 shell is similar to that of D
DOI: 10.1021/acs.energyfuels.6b01160 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels Table 2. Properties and Catalytic Performances of Zeolite Coatings loading amount (mg cm−2)
zeolite coating
slurry mass composition
HZ-P HZ-TPO HZ-C10 HZ-C16 HZ-C16&T
1HZ-P/2.5binder/4.5ethanol 1HZ-MT/2.5binder/4.5ethanol 1HZ-ET/2.5binder/4.5ethanol 1HZ-MR/2.5binder/4.5ethanol 1HZ-ER/2.5binder/4.5ethanol
5.48 5.54 5.49 5.53 5.52
± ± ± ± ±
0.18 0.23 0.17 0.20 0.15
mean thicknessa (μm)
rib (mol g−1 min−1)
rdc (%)
± ± ± ± ±
0.167 0.198 0.202 0.209 0.213
40.00 19.63 14.54 10.66 7.29
37.8 40.4 38.7 40.1 38.9
1.4 2.5 2.2 2.5 2.2
a Mean thickness determined by scanning electron microscopy (SEM). bri is defined as the ratio of the n-dodecane mole number per gram of catalyst per minute reacted to n-dodecane entering. crd is defined as rd = (Xt = 5 − Xt = 30)/Xt = 5 × 100%, where in Xt = 30 and Xt = 5 are n-dodecane conversions at TOS of 5 and 30 min, respectively.
As we know, formation of small alkanes (such as ethane, propane, etc.) and dehydrogenation−cyclization to form aromatics are the typical secondary reactions.27,28 The selectivity to aromatics and olefin/paraffin (o/p) ratios are used to evaluate the secondary reactions. It can be seen in Table 3 that core−shell zeolites give lower selectivity to aromatics and higher o/p ratios in comparison to HZ-P and HZ-TPO, indicating that the secondary reactions over core− shell zeolites were limited; that is to say, the secondary reactions were significantly impressed after introducing the MCM-41 shell. 3.3. Acid Properties. To explore the reason for the above results, NH3-TPD and FTIR were used to characterize the acid nature and amount of the prepared samples. There are generally two NH3-adsorbed peaks around 190 and 385 °C on the NH3-TPD (Figure S2 of the Supporting Information).29 However, the amounts and strength of hierarchical zeolites are less than those of HZ-P, which may be due to the presence of the mesoporous structure. Pyridine-adsorbed FTIR spectra were taken at evacuation desorption temperatures of 150 and 350 °C (Figure 5) assigned to the total and strong acid sites, respectively. The band at 1547 cm−1 is assigned to pyridine adsorbed on the Brønsted acid sites, whereas that at 1452 cm−1 is assigned to the Lewis acid sites. The acid distributions were calculated and listed in Table 1. The amount of Brønsted acid sites decreases from HZ-P to HZ-TPO and further to core− shell zeolites. Among three core−shell-structured zeolites (HZC10, HZ-C16, and HZ-C16&T), there was no discernible difference in acid properties being observed, indicating that the change of the MCM-41 mesopore size has negligible influence on zeolite acid properties.
Figure 4. Catalytic cracking activities of HZCs for supercritical ndodecane (reaction conditions: 500 °C, 4 MPa, and 10 mL/min).
Table 3. Conversion and Mass Selectivity of n-Dodecane Catalytic Cracking over Zeolite Coatingsa
a
product
HZ-P
HZ-TPO
HZ-C10
HZ-C16
HZ-C16&T
aromatics gasous products liquid products C32−/C3 i-C42−/i-C4 conversion (%)
1.10 62.62 37.38 1.68 3.33 45.37
0.91 60.18 39.82 2.07 4.65 55.07
0.85 57.28 42.72 2.06 5.11 53.81
0.81 55.74 44.26 2.15 6.86 56.65
0.78 55.09 44.91 2.19 8.73 57.97
Reaction conditions: 4 MPa and 500 °C, with TOS = 5 min.
over core−shell-structured zeolites, especially over HZ-C16&T with the larger mesopore size of the MCM-41 shell.
Figure 5. Probe molecule-adsorbed FTIR spectra of HZs: (a and b) pyridine at 150 and 350 °C and (c) DTBPy at 150 °C, with (I) HZ-P, (II) HZTPO, (III) HZ-C10, (IV) HZ-C16, and (V) HZ-C16&T. E
DOI: 10.1021/acs.energyfuels.6b01160 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels The adsorption of DTBPy is used commonly to study the accessibility of the Brønsted acid in zeolites because its kinetic diameter (1.05 nm) exceeds the size of HZSM-5 micropores (0.51 × 0.55 nm and 0.53 × 0.56 nm).30,31 New bands at 1617 and 1530 cm−1 were observed after adsorption of DTBPy (Figure 5c), which would be attributed to DTBPy bonded to Brønsted sites.30 The absence of the band at 1543 cm−1 provided evidence that dealkylation of DTBPy did not occur. Moreover, no band was observed at 1452 cm−1 because there is no DTBPy bonded to Lewis acid as a result of the possible steric effect.30 Indeed, an evident difference in the peak areas at 1617 cm−1 among zeolites could be found, revealing the difference in acid site accessibility. In this work, the ACI was determined as the ratio of the amount of sites accessible for DTBPy to those for pyridine,30 which was presented in Table 1. The ACI gradually increased from 0.11 for HZ-P to 0.32 for HZ-TPO as a result of the formation of intracrystal mesopores and then sharply exceeded 0.42 for core−shell zeolites, indicating that the MCM-41 shell can act as precracking location for n-dodecane. Furthermore, the ACI increased with increasing mesopore sizes of the shell, indicating that the relatively larger mesopores in the MCM-41 shell are beneficial for improving the accessibility of active sites. The ACI of HZ-P was the lowest, which lead to the lowest catalytic activity and stability because of the rapid pore-mouth plugging.32 The introduction of ordered mesopores in HZTPO enhanced the ACI, resulting in the catalytic performance. After the encapsulating process, the precracking of the MCM41 shell may be helpful for the further improvement of diffusion and, thus, the ACI, which leads to the enhanced catalytic activity and decreased secondary reaction ability.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (21522605).
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REFERENCES
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4. CONCLUSION Ordered meso-HZSM-5 with MCM-41 shells was synthesized using CnTAB as surfactants and TMB as the swelling agent. It was found that ordered meso-HZSM-5, which was used as the core, could be synthesized using a designed amphiphilic organosilane as the surfactant and that the mesopore size of the MCM-41 shell can be adjusted from 2.5−3 to 4.5−10 nm by changing the structures of surfactants and the amount of TMB. The precracking of hydrocarbon in the mesoporous shell helps the diffusion of primary products inside micropore channels, and thus, it is benifical for the improvement of the catalytic activity and lifetime in the catalytic cracking of supercritical n-dodecane. In addition, the acid site accessibility increased gradually with increasing the mesopore size of the MCM-41 shell and, thus, led to enhanced catalytic activity and decreased secondary reaction ability. Ordered hierarchical zeolites also show a well connection of ordered mesopores between the core and shell, which could enhance acid site accessibility.
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catalytic cracking over zeolite coatings (Table S1), and conversion and detailed mass selectivity of n-dodecane catalytic cracking over zeolite coatings (Table S2) (PDF)
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.6b01160. Schematic diagrams of the experimental supercritical apparatus for catalytic cracking of endothermic hydrocarbon (Figure S1), NH3-TPD profiles of HZs: (I) HZP, (II) HZ-TPO, (III) HZ-C10, (IV) HZ-C16, and (V) HZ-C16&T (Figure S2), heat sink of n-dodecane F
DOI: 10.1021/acs.energyfuels.6b01160 Energy Fuels XXXX, XXX, XXX−XXX
Article
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DOI: 10.1021/acs.energyfuels.6b01160 Energy Fuels XXXX, XXX, XXX−XXX
