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Preparation of mesoporous mordenite for the hydroisomerization of n-hexane
Catalysis Communications 125 (2019) 21–25
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Short communication
Preparation of mesoporous mordenite for the hydroisomerization of nhexane
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Xinqing Lu, Yuping Guo, Chunhui Xu, Rui Ma , Xue Wang, Ningwei Wang, Yanghe Fu, ⁎ Weidong Zhu Key Laboratory of the Ministry of Education for Advanced Catalysis Materials, Institute of Physical Chemistry, Zhejiang Normal University, 321004 Jinhua, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Mesoporous zeolite Bifunctional catalyst Mordenite Desilication Hydroisomerization
A mesoporous mordenite (MOR) zeolite was prepared from a commercially available MOR sample by a sequential acid-alkaline-acid post-treatment. The latter was used as support of Pt to prepare a bifunctional Pt/HMOR catalyst for the hydroisomerization of n-hexane. The results showed that the available mesopores in the catalyst shorten the diffusion length of reactants and of the produced branched hexanes inside the micropores. In addition, the dispersion of supported Pt in the zeolite was increased and an enhancement in the conversion of nhexane and at the same time the reduction of the non-selective cracking of hexane isomers and of the formation of coke (catalyst deactivation) was observed.
1. Introduction
sites in zeolites could be enhanced by introducing mesopores in the zeolite pore structure by either direct syntheses with soft and hard templates [9,10] or post-treatments with acid [11], alkaline [12] and NH4F [13]. Alkaline-derived desilication has been used as an efficient strategy for the generation of mesopores in zeolite secondary crystals [14–21]. However, the Si/Al molar ratios in zeolites should be in an optimal range with the purpose to preserve the zeolite's crystallinity and acidity [22–24]. If these ratios become too low, no mesoporosity is formed, while if these ratios become too high, then excessive silicon dissolution results in the collapse of the zeolite structure. Acid treatment was used to increase the Si/Al ratio of MOR into its optimal range via dealumination, making mesopores successfully formed by an alkaline treatment [23]. van Laak et al. [24] found an improved reactivity in the liquid-phase alkylation of benzene with propylene to cumene over the mesoporous MOR catalyst prepared by a sequential acid-alkaline-acid post-treatment. A similar strategy was also applied for preparing a mesoporous MOR-type titanosilicate that showed remarkably improved catalytic properties in the hydroxylation of toluene and the ammoximation of cyclohexanone [25]. Most recently, Pastvova et al. [26] applied a sequential acid-alkaline-acid post-treatment to generate the mesoporosity in MOR and then used this mesoporous zeolite as a support to prepare a Pt/H-MOR catalyst for the hydroisomerization of n-hexane, showing a significantly improved catalytic activity due to the simultaneous enhancement of the accessibility of the acid sites and of molecular transport. However, less work dealt with the effects of the introduced mesopores on the dispersion of Pt in the zeolite
The hydroisomerization of n-paraffins into isoparaffins plays an important role in improving the quality of gasoline by increasing the octane number. The commercial process consists of (de)hydrogenation on metallic Pt and skeletal isomerization on acid sites typically catalyzed by bifunctional Pt/H-MOR catalysts, which is generally accompanied with cracking reactions [1]. The balance between (de)hydrogenation and skeletal isomerization plays a determinant role on the activity and selectivity for branched products in this isomerization process [2,3]. Additionally, the product selectivity is significantly affected by the proximity between metallic Pt and acid sites [4,5]. MOR has two types of channels, 12-membered-ring channels with an opening size of 0.65 nm × 0.70 nm, and 8-membered-ring channels with an opening size of 0.26 nm × 0.57 nm. Therefore, in general, MOR is regarded as one-dimensional pore system, because the 8-membered-ring channels are too small for most molecules to enter, inducing one-dimensional diffusion, i.e., molecules cannot overtake each other in the channels so a molecule located at the pore mouth blocks the entrance or escape of other molecules [6,7]. Hence, the molecular transport in the micropores is strongly hindered by this one-dimensional diffusion in comparison with the 2-D or 3-D pore systems [6–8]. Additionally, the lodgment of extra-framework species in the MOR channels can significantly restrict the accessibility to adsorbate and/or reactant molecules [7]. It has been repeatedly demonstrated that the accessibility of the acid
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Corresponding authors. E-mail addresses: [email protected] (R. Ma), [email protected] (W. Zhu).
https://doi.org/10.1016/j.catcom.2019.03.017 Received 16 January 2019; Received in revised form 18 March 2019; Accepted 19 March 2019 Available online 20 March 2019 1566-7367/ © 2019 Published by Elsevier B.V.
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2.2. Catalyst characterization
and its derived catalytic selectivity and stability. In the present work, a rich mesoporosity was introduced into MOR secondary crystals via a sequential acid-alkaline-acid post-treatment. A comparative study was conducted on the catalytic properties of Pt/HMOR catalysts prepared using commercially available, acid posttreated, and sequential acid-alkaline-acid post-treated MOR samples as supports for the hydroisomerization of n-hexane in order to investigate the effects of the introduced mesopores in MOR secondary crystals on the derived catalytic activity, selectivity and stability.
The powder X-ray diffraction (XRD) patterns of the solid catalysts prepared were recorded by a Philips PW3040/60 powder diffractometer using Ni-filtered Cu Kα radiation (λ = 0.1541 nm) in the range of 5–50° with a scanning rate of 2° min−1. The zeolite crystal morphology was examined by a Hitachi S-4800 scanning electron microscope (SEM). Transmission electron microscopy (TEM) for Pt particle size and mesoporosity characterization was carried out on a JEOL JEM-2100 microscope after the sample was deposited onto a holey carbon foil supported on a copper grid. The textural properties of the samples were determined by N2 adsorption-desorption at 77 K using a Micromeritics ASAP 2020 instrument after the samples were degassed in vacuum at 573 K for 6 h. The specific surface area of the investigated samples was calculated using the Brunauer-Emmett-Teller (BET) method in the relative pressure range of p/p0 = 0.05–0.35. The micropore volume and the external specific surface area were determined by the t-plot method, and the total pore volume was determined by converting the amount adsorbed at a relative pressure of 0.995 to the volume of liquid nitrogen. The mesopore volume was computed by subtracting the volume of micropores from the total pore volume. The bulk Si/Al ratio and the amount of Pt loading in the catalyst were determined by inductively coupled plasma-atomic emission spectrometry (ICP-AES) on a Thermo IRIS Intrepid II XSP after the samples were dissolved in liquated NaOH. The 27Al MAS NMR spectra were collected on a Varian VNMRS-400 MB NMR spectrometer. The spectra were collected at a frequency of 104.18 MHz, a spinning rate of 9.0 kHz and a recycling delay of 4 s. KAl(SO4)2·12H2O was used as reference for chemical shift. The amount of the acid sites was determined by temperature-programmed desorption of NH3 (NH3-TPD) on a Micromeritics AutoChem II 2920 instrument equipped with a thermal conductivity detector (TCD). Typically, 80 mg of the sample was pretreated in flowing helium (25 mL min−1) at 823 K for 1 h. The adsorption of NH3 was carried out at 393 K for 15 min, and then weakly chemisorbed NH3 was purged by flowing helium (25 mL min−1) at the same temperature for 30 min. To ensure the complete removal of weakly chemisorbed NH3, the purging process was repeated three times. The NH3-TPD profile was then recorded from 393 to 973 K with a heating rate of 15 K min−1 using He as carrier gas. The signal of desorbed NH3 was measured by TCD detector.
2. Experimental 2.1. Catalyst preparation 2.1.1. Acid and alkaline post-treatments The MOR zeolite with a Si/Al molar ratio of 10 in an ammonium form was purchased from Zeolyst (CBV 21A), denoted as NH4-MOR-p. The mesoporous MOR was obtained by a sequential acid-alkaline-acid treatment, similar to the procedure described elsewhere [23,24]. The as-received Zeolyst MOR was calcined at 773 K for 3 h with a heating rate of 1 K min−1 to get a proton-form MOR, denoted as H-MOR-p. Afterwards, H-MOR-p was treated with 3 M HNO3 aqueous solution with a solid-to-liquid weight ratio of 1:10 (w/w) at 373 K for 1 h under stirring. The acid-treated product was filtered, washed with hot-deionized water until the pH of the filtrate became ~ 7. Then, the obtained filter cake was dried at 383 K overnight and calcined at 773 K for 3 h with a heating rate of 1 K min−1 to get a partially dealuminated MOR (denoted as H-MOR-p-a). The alkaline treatment was carried out over H-MOR-p-a in 0.2 M NaOH aqueous solution with a solid-to-liquid weight ratio of 1:30 (w/w) under stirring at 343 K for 5 min. The solidliquid mixture was subsequently centrifuged and decanted of the liquid. The alkaline-treated product was washed with hot deionized water until the pH of the filtrate became ~ 7, and then dried at 383 K overnight and calcined at 773 K for 3 h with a heating rate of 1 K min−1. The calcined sample was ion-exchanged with 1 M NH4NO3 aqueous solution with a solid-liquid weight ratio of 1:12 (w/w) at 353 K for 12 h. To ensure the complete exchange of Na+, this procedure was repeated three times, and the obtained solid sample was dried at 383 K overnight and calcined at 773 K for 3 h with a heating rate of 1 K min−1 to get a protonform MOR (denoted as H-MOR-p-a-a). Finally, H-MOR-p-a-a was washed with 0.1 M HNO3 aqueous solution with a solid-liquid weight ratio of 1:20 (w/w) at 323 K for 15 min under stirring, and the obtained solid sample was then dried at 383 K overnight and calcined at 773 K for 3 h with a heating rate of 1 K min−1 to get an acid-alkaline-acid treated MOR (denoted as H-MOR-p-a-a-a). The flow diagram for the separate synthesis steps, including sample designations for the as-received Zeolyst MOR to the sequential acid-alkaline-acid post-treated MOR, is presented in Scheme 1.
2.3. Hydroisomerization of n-hexane The hydroisomerization of n-hexane was carried out in a fixed-bed reactor (6 mm inner diameter) at 1 atm pressure. 0.2 g of Pt/H-MOR catalyst was loaded into the reactor and in-situ activated in 80 vol% H2 /20 vol% N2 gas mixture at 673 K for 2 h (total flow rate of 10 mL min−1). After the reactor temperature was reduced to the reaction temperature of 513 K, n-hexane was introduced into the reactor by a high-pressure constant flow pump. The weight hourly space velocity (WHSV) of n-hexane and the H2/n-hexane molar ratio for all the experiments conducted were maintained at 2 h−1 and 6, respectively. The reaction products were analyzed by a gas chromatograph (Agilent 6820) equipped with a 50 m OV-101 capillary column and a flame ionization detector (FID).
2.1.2. Pt/H-MOR preparation Pt/H-MOR catalysts were prepared by the impregnation method as follows: a proton-form MOR sample (1 g) was impregnated with a Pt (NH3)4Cl2 aqueous solution (0.5 mg g−1) of 2.5 mL initially to become a slurry, followed by dropwise addition of the remaining Pt(NH3)4Cl2 aqueous solution of 7.5 mL into the slurry under vigorous stirring at 308 K for 24 h. Then the final slurry was filtered, washed with hotdeionized water until the complete removal of chloride ions and dried at 383 K overnight. Subsequently, the dried sample (1 g) was loaded in a tubular reactor (5 cm inner diameter) and heated in flowing oxygen at the flow rate of 500 mL min−1 at 673 K for 2 h with a heating rate of 0.5 K min−1. The resultant catalysts with nominal loading of 0.5 wt% Pt and the various zeolite treated samples (see Section 2.1.1) are named as Pt/H-MOR-p, Pt/H-MOR-p-a, and Pt/H-MOR-p-a-a-a.
3. Results and discussion 3.1. Catalyst characterization As shown in Fig. S1 (ESI), all the investigated catalysts showed the powder XRD characteristic patterns of MOR, indicating that the crystal structures are well preserved after the different post-treatments of the zeolite applied. Both Pt/H-MOR-p and Pt/H-MOR-p-a show a type-I N2 adsorption-desorption isotherm (Fig. S2A), which is a characteristic of microporosity. The acid-derived dealumination makes the micropore 22
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Scheme 1. Flow diagram of the different post-treatment steps, starting from the commercial zeolite. Table 1 Textural properties and chemical composition of the catalysts investigated. Catalysts
SBET (m2 g−1)
Vmicroa (cm3 g−1)
Sexta (m2 g−1)
Vtotal (p/p0 = 0.995) (cm3 g−1)
Vmesob (cm3 g−1)
Si/Alc
Pt loadedc (wt%)
Pt/H-MOR-p Pt/H-MOR-p-a Pt/H-MOR-p-a-a-a
419 452 492
0.17 0.18 0.17
39 61 126
0.25 0.27 0.46
0.08 0.09 0.29
10 28 15
0.45 0.43 0.46
a b c
Determined from the measured N2 adsorption-desorption isotherms at 77 K using the t-plot method. Vmeso = Vtotal–Vmicro. Determined by the ICP-AES analysis.
volume and the external specific surface area sharply from 0.08 cm3 g−1 and 39 m2 g−1 for Pt/H-MOR-p to 0.29 cm3 g−1 and 126 m2 g−1 for Pt/H-MOR-p-a-a-a, respectively, while the micropore volume almost remains unchanged. Hence, these results confirm the validity of the sequential acid-alkaline-acid post-treatment as an effective method to create mesoporosity in MOR crystals, while the microporosity is preserved. Furthermore, the pore size distribution profiles determined by the Barrett-Joyner-Halenda (BJH) method from the adsorption branch, shown in Fig. S2B, indicate that Pt/H-MOR-p-a-a-a possesses mesoporosity centered at ca. 10 nm, while Pt/H-MOR-p and Pt/H-MOR-p-a are purely microporous. Additionally, the Si/Al molar ratio is decreased from 28 for Pt/H-MOR-p-a to 15 for Pt/H-MOR-p-a-aa, respectively, due to the desilication (Table 1). To elucidate the effects of dealumination and desilication of zeolites on their acidity, 27Al MAS NMR spectra and NH3-TPD profiles were measured. In the 27Al MAS NMR spectra (Fig. S3), Pt/H-MOR-p shows only a resonance at 58 ppm ascribed to tetrahedral Al, the so-called framework Al species. The dealumination can develop extra-framework Al species showing a new resonance at 0 ppm in the spectrum of Pt/HMOR-p-a, while these extra-framework Al species can be effectively removed by the alkaline-acid post-treatment, as the resonance at 0 ppm in the spectrum of Pt/H-MOR-p-a-a-a is almost absent. Fig. 1 shows the NH3-TPD profiles recorded after removing the weakly chemisorbed NH3 species on the investigated catalysts. The lowtemperature (400–525 K) and high-temperature (525–973 K) desorption peaks are ascribed to the desorption of NH3 from the weak and strong acid sites of MOR, respectively [26,27]. As shown in Fig. 1, the strong acid sites, which are mainly considered as the catalytic sites in the hydroisomerization, have been well preserved during the dealumination and desilication post-treatments. Compared to the hightemperature desorption peak, the low-temperature peak was more obviously affected by the variation of the Si/Al ratio (Table 1 and Fig. 1).
Fig. 1. NH3-TPD profiles of Pt/H-MOR-p (a), Pt/H-MOR-p-a (b), and Pt/HMOR-p-a-a-a (c).
volume and the specific surface area slightly increased from 0.17 cm3 g−1 and 419 m2 g−1 for Pt/H-MOR-p to 0.18 cm3 g−1 and 452 m2 g−1 for Pt/H-MOR-p-a, respectively (Table 1) due to the enlargement in the micropore size after the removal of Al in the framework [24,25]. Moreover, the Si/Al molar ratio is increased from 10 for Pt/HMOR-p to 28 for Pt/H-MOR-p-a, where the latter is in the optimum range of Si/Al molar ratio in the zeolite for the subsequent alkaline post-treatment [24]. A further alkaline-acid post-treatment changes the isotherm to the combined one of type-I and type-IV with a well pronounced hysteresis loop. The latter indicates the generation of mesopores in the zeolite crystals (Fig. S2A). As shown in Table 1, the sequential acid-alkaline-acid post-treatment increases the mesopore 23
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The concentration of weak acid sites is in the order: Pt/H-MOR-p > Pt/ H-MOR-p-a-a-a > Pt/H-MOR-p-a, which is in accordance with the Al amount in the zeolite. Additionally, the presence of a shoulder peak at higher temperatures in the Pt/H-MOR-p-a indicates that the acid strength is increased after dealumination due to the formation of extraframework Al species acting as Lewis acid sites [28]. As shown in Table 1, the Pt amounts in all the catalysts, determined by the ICP-AES analysis, are within their nominal values of 0.5 wt%, indicating the efficiency of the impregnation method applied for preparing the catalysts. The SEM image of H-MOR-p shows that MOR particles are aggregates of nanocrystals (Fig. S4a), while TEM images reveal the mesoporosity in Pt/H-MOR-p-a-a-a and the distribution of platinum oxide particle size in all catalysts (Figs. S4b-d). Pt/H-MOR-p and Pt/H-MOR-p-a both show opaque crystals with some dark spots, corresponding to the presence of platinum oxide particles located on the surface of the MOR crystals (Figs. S3b and c). In contrast, the TEM image of Pt/H-MOR-p-a-a-a shows many white spots with a diameter of ca. 10 nm (Fig. S4d), indicating the presence of mesopores in the MOR crystals, which is in accordance with the N2 adsorption-desorption characterization results (Fig. S2). Moreover, the created mesopores are indeed embedded in the crystals as shown in the TEM images of Fig. S5. The mean diameter of platinum oxide particles decreases from ca. 3.2 nm in Pt/H-MOR-p and ca. 3.0 nm in Pt/H-MOR-p-a to ca. 1.8 nm in Pt/H-MOR-p-a-a-a, implying that the presence of mesopores in the zeolite enhanced the dispersion of Pt.
Fig. 3. Distribution of products and the yield of total branched hexanes in the hydroisomerization of n-hexane over Pt/H-MOR-p (a), Pt/H-MOR-p-a (b) and Pt/H-MOR-p-a-a-a (c) in the first 60 min of reaction. The reaction conditions are the same as those presented in Fig. 2. Others stands for C1-C5 products from the cracking reaction.
Lewis acid sites (Fig. 2), and the cracking products are pronounced as shown in Fig. 3. It is thus illustrated that Pt/H-MOR-p-a-a-a shows the highest conversion of n-hexane and yield of hexane isomers, with a stable activity and at the same time negligible cracking products (Figs. 2 and 3). The enhanced catalytic activity must be related to the introduction of mesoporosity with the preservation of the zeolite framework crystallinity, microporosity and acidity. The introduced mesopores in the MOR by the sequential acid-alkaline-acid post-treatment can shorten the diffusion length of the reactants and of the produced branched hexanes inside the micropores [26], affect the proximity between metallic Pt and acid sites [4,5], and enhance the dispersion of Pt in the zeolite. All these factors eventually lead to the increase of conversion of n-hexane and reduction of the non-selective cracking of hexane isomers and the formation of coke. For all the investigated catalysts, the product distributions almost remain unchanged with time on stream (Fig. S6). Additionally, the selectivity for the branched isomers for all the investigated catalysts follows the order: 2, 2-dimethylbutane < 2, 3-dimethylbutane < 3-methylpentane < 2-methylpentane, similar to the order of the molecular diameters of these hexane isomers. Moreover, Pt/H-MOR-p-a-a-a provides almost the same selectivity for the branched isomers as Pt/H-MOR-p, indicating that the introduced mesopores do not alter the shape selectivity of Pt/H-MOR catalysts. This observation agrees well with that most recently reported [27].
3.2. Hydroisomerization of n-hexane The catalytic hydroisomerization of n-hexane consists of the dehydrogenation of n-hexane to n-hexene on metallic Pt, the skeletal isomerization of n-hexene to monobranched methylpentenes and dibranched dimethylbutenes on acid sites, and the hydrogenation of the branched alkene intermediates to the corresponding branched alkanes on metallic Pt [26]. In addition, the hydroisomerization of n-hexane to isohexane is usually accompanied with the cracking reaction and coke formation [1]. As shown in Figs. 2 and 3, Pt/H-MOR-p-a possesses a higher conversion of n-hexane and a higher yield of hexane branched isomers than Pt/H-MOR-p in the initial reaction stage. The available Lewis acid sites due to the formation of extra-framework Al species in Pt/H-MOR-p-a have a promoting effect on the rates of the hydroisomerization, cracking and coking that leads to catalyst deactivation [28]. Therefore, although the initial conversion of n-hexane is enhanced over Pt/H-MOR-p-a in comparison with that over Pt/H-MOR-p, the catalytic activity will be rapidly dropped due to coke formation on the
4. Conclusions The simple acid post-treatment applied over a commercial MOR zeolite does not obviously introduce mesoporosity in the zeolite crystals but it can significantly increase the Si/Al ratio due to partial dealumination that results in the formation of extra-framework Al species. The prepared corresponding bifunctional Pt/H-MOR catalyst does not show an improved catalytic activity in the hydroisomerization reaction of n-hexane since the extra-framework Al species can enhance the formation of cracking products and coke. These results may depend on the specific MOR under investigation because other synthesis/post-treatment procedures or commercial MOR may lead to different results regarding the acid leaching treatment. On the other hand, the sequential acid-alkaline-acid post-treatment can introduce significant mesoporosity in the MOR crystal structure. The prepared corresponding bifunctional zeolite-supported Pt catalyst shows then an enhanced catalytic activity with long-term stability, due to the facts that the available mesopores in the zeolite catalyst can shorten the diffusion length of the
Fig. 2. Comparative results on the hydroisomerization of n-hexane over Pt/HMOR-p (a), Pt/H-MOR-p-a (b) and Pt/H-MOR-p-a-a-a (c). Reaction conditions: WHSV (n-hexane) = 2 h−1, H2/n-hexane molar ratio = 6:1, and T = 513 K. 24
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reactants and produced branched hexanes inside the micropores, and enhance the dispersion of Pt in the zeolite. The latter contributes to the enhancement of proximity of Pt and acid sites in the mesopores of zeolite which is a very important aspect in the present catalytic reaction.
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Acknowledgements The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (20971109, 21036006, 21476214, and 21706239) and the Programme of Introducing Talents of Discipline to Universities (D17008). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.catcom.2019.03.017. References [1] R.A. Asuquo, G. Eder-Mirth, K. Seshan, J.A.Z. Pieterse, J.A. Lercher, J. Catal. 168 (1997) 292–300. [2] Y. Hong, J.J. Fripiat, Microporous Mater. 4 (1995) 323–334. [3] Y.S. Tao, H. Kanoh, L. Abrams, K. Kaneko, Chem. Rev. 106 (2006) 896–910. [4] N. Batalha, L. Pinard, C. Bouchy, E. Guillon, M. Guisnet, J. Catal. 307 (2013) 122–131. [5] J. Zečević, G. Vanbutsele, K.P. de Jong, J.A. Martens, Nature 528 (2015) 245–254. [6] G.D. Lei, B.T. Carvill, W.M.H. Sachtler, Appl. Catal. A Gen. 142 (1996) 347–359. [7] D. Lozano-Castello, W.D. Zhu, A. Linares-Solano, F. Kapteijn, J.A. Moulijn, Microporous Mesoporous Mater. 92 (2006) 145–153. [8] M. Tromp, J.A. van Bokhoven, M.T. Garriga Oostenbrink, J.H. Bitter, K.P. de Jong, D.C. Koningsberger, J. Catal. 190 (2000) 209–214.
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Short communication
Preparation of mesoporous mordenite for the hydroisomerization of nhexane
T
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Xinqing Lu, Yuping Guo, Chunhui Xu, Rui Ma , Xue Wang, Ningwei Wang, Yanghe Fu, ⁎ Weidong Zhu Key Laboratory of the Ministry of Education for Advanced Catalysis Materials, Institute of Physical Chemistry, Zhejiang Normal University, 321004 Jinhua, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Mesoporous zeolite Bifunctional catalyst Mordenite Desilication Hydroisomerization
A mesoporous mordenite (MOR) zeolite was prepared from a commercially available MOR sample by a sequential acid-alkaline-acid post-treatment. The latter was used as support of Pt to prepare a bifunctional Pt/HMOR catalyst for the hydroisomerization of n-hexane. The results showed that the available mesopores in the catalyst shorten the diffusion length of reactants and of the produced branched hexanes inside the micropores. In addition, the dispersion of supported Pt in the zeolite was increased and an enhancement in the conversion of nhexane and at the same time the reduction of the non-selective cracking of hexane isomers and of the formation of coke (catalyst deactivation) was observed.
1. Introduction
sites in zeolites could be enhanced by introducing mesopores in the zeolite pore structure by either direct syntheses with soft and hard templates [9,10] or post-treatments with acid [11], alkaline [12] and NH4F [13]. Alkaline-derived desilication has been used as an efficient strategy for the generation of mesopores in zeolite secondary crystals [14–21]. However, the Si/Al molar ratios in zeolites should be in an optimal range with the purpose to preserve the zeolite's crystallinity and acidity [22–24]. If these ratios become too low, no mesoporosity is formed, while if these ratios become too high, then excessive silicon dissolution results in the collapse of the zeolite structure. Acid treatment was used to increase the Si/Al ratio of MOR into its optimal range via dealumination, making mesopores successfully formed by an alkaline treatment [23]. van Laak et al. [24] found an improved reactivity in the liquid-phase alkylation of benzene with propylene to cumene over the mesoporous MOR catalyst prepared by a sequential acid-alkaline-acid post-treatment. A similar strategy was also applied for preparing a mesoporous MOR-type titanosilicate that showed remarkably improved catalytic properties in the hydroxylation of toluene and the ammoximation of cyclohexanone [25]. Most recently, Pastvova et al. [26] applied a sequential acid-alkaline-acid post-treatment to generate the mesoporosity in MOR and then used this mesoporous zeolite as a support to prepare a Pt/H-MOR catalyst for the hydroisomerization of n-hexane, showing a significantly improved catalytic activity due to the simultaneous enhancement of the accessibility of the acid sites and of molecular transport. However, less work dealt with the effects of the introduced mesopores on the dispersion of Pt in the zeolite
The hydroisomerization of n-paraffins into isoparaffins plays an important role in improving the quality of gasoline by increasing the octane number. The commercial process consists of (de)hydrogenation on metallic Pt and skeletal isomerization on acid sites typically catalyzed by bifunctional Pt/H-MOR catalysts, which is generally accompanied with cracking reactions [1]. The balance between (de)hydrogenation and skeletal isomerization plays a determinant role on the activity and selectivity for branched products in this isomerization process [2,3]. Additionally, the product selectivity is significantly affected by the proximity between metallic Pt and acid sites [4,5]. MOR has two types of channels, 12-membered-ring channels with an opening size of 0.65 nm × 0.70 nm, and 8-membered-ring channels with an opening size of 0.26 nm × 0.57 nm. Therefore, in general, MOR is regarded as one-dimensional pore system, because the 8-membered-ring channels are too small for most molecules to enter, inducing one-dimensional diffusion, i.e., molecules cannot overtake each other in the channels so a molecule located at the pore mouth blocks the entrance or escape of other molecules [6,7]. Hence, the molecular transport in the micropores is strongly hindered by this one-dimensional diffusion in comparison with the 2-D or 3-D pore systems [6–8]. Additionally, the lodgment of extra-framework species in the MOR channels can significantly restrict the accessibility to adsorbate and/or reactant molecules [7]. It has been repeatedly demonstrated that the accessibility of the acid
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Corresponding authors. E-mail addresses: [email protected] (R. Ma), [email protected] (W. Zhu).
https://doi.org/10.1016/j.catcom.2019.03.017 Received 16 January 2019; Received in revised form 18 March 2019; Accepted 19 March 2019 Available online 20 March 2019 1566-7367/ © 2019 Published by Elsevier B.V.
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2.2. Catalyst characterization
and its derived catalytic selectivity and stability. In the present work, a rich mesoporosity was introduced into MOR secondary crystals via a sequential acid-alkaline-acid post-treatment. A comparative study was conducted on the catalytic properties of Pt/HMOR catalysts prepared using commercially available, acid posttreated, and sequential acid-alkaline-acid post-treated MOR samples as supports for the hydroisomerization of n-hexane in order to investigate the effects of the introduced mesopores in MOR secondary crystals on the derived catalytic activity, selectivity and stability.
The powder X-ray diffraction (XRD) patterns of the solid catalysts prepared were recorded by a Philips PW3040/60 powder diffractometer using Ni-filtered Cu Kα radiation (λ = 0.1541 nm) in the range of 5–50° with a scanning rate of 2° min−1. The zeolite crystal morphology was examined by a Hitachi S-4800 scanning electron microscope (SEM). Transmission electron microscopy (TEM) for Pt particle size and mesoporosity characterization was carried out on a JEOL JEM-2100 microscope after the sample was deposited onto a holey carbon foil supported on a copper grid. The textural properties of the samples were determined by N2 adsorption-desorption at 77 K using a Micromeritics ASAP 2020 instrument after the samples were degassed in vacuum at 573 K for 6 h. The specific surface area of the investigated samples was calculated using the Brunauer-Emmett-Teller (BET) method in the relative pressure range of p/p0 = 0.05–0.35. The micropore volume and the external specific surface area were determined by the t-plot method, and the total pore volume was determined by converting the amount adsorbed at a relative pressure of 0.995 to the volume of liquid nitrogen. The mesopore volume was computed by subtracting the volume of micropores from the total pore volume. The bulk Si/Al ratio and the amount of Pt loading in the catalyst were determined by inductively coupled plasma-atomic emission spectrometry (ICP-AES) on a Thermo IRIS Intrepid II XSP after the samples were dissolved in liquated NaOH. The 27Al MAS NMR spectra were collected on a Varian VNMRS-400 MB NMR spectrometer. The spectra were collected at a frequency of 104.18 MHz, a spinning rate of 9.0 kHz and a recycling delay of 4 s. KAl(SO4)2·12H2O was used as reference for chemical shift. The amount of the acid sites was determined by temperature-programmed desorption of NH3 (NH3-TPD) on a Micromeritics AutoChem II 2920 instrument equipped with a thermal conductivity detector (TCD). Typically, 80 mg of the sample was pretreated in flowing helium (25 mL min−1) at 823 K for 1 h. The adsorption of NH3 was carried out at 393 K for 15 min, and then weakly chemisorbed NH3 was purged by flowing helium (25 mL min−1) at the same temperature for 30 min. To ensure the complete removal of weakly chemisorbed NH3, the purging process was repeated three times. The NH3-TPD profile was then recorded from 393 to 973 K with a heating rate of 15 K min−1 using He as carrier gas. The signal of desorbed NH3 was measured by TCD detector.
2. Experimental 2.1. Catalyst preparation 2.1.1. Acid and alkaline post-treatments The MOR zeolite with a Si/Al molar ratio of 10 in an ammonium form was purchased from Zeolyst (CBV 21A), denoted as NH4-MOR-p. The mesoporous MOR was obtained by a sequential acid-alkaline-acid treatment, similar to the procedure described elsewhere [23,24]. The as-received Zeolyst MOR was calcined at 773 K for 3 h with a heating rate of 1 K min−1 to get a proton-form MOR, denoted as H-MOR-p. Afterwards, H-MOR-p was treated with 3 M HNO3 aqueous solution with a solid-to-liquid weight ratio of 1:10 (w/w) at 373 K for 1 h under stirring. The acid-treated product was filtered, washed with hot-deionized water until the pH of the filtrate became ~ 7. Then, the obtained filter cake was dried at 383 K overnight and calcined at 773 K for 3 h with a heating rate of 1 K min−1 to get a partially dealuminated MOR (denoted as H-MOR-p-a). The alkaline treatment was carried out over H-MOR-p-a in 0.2 M NaOH aqueous solution with a solid-to-liquid weight ratio of 1:30 (w/w) under stirring at 343 K for 5 min. The solidliquid mixture was subsequently centrifuged and decanted of the liquid. The alkaline-treated product was washed with hot deionized water until the pH of the filtrate became ~ 7, and then dried at 383 K overnight and calcined at 773 K for 3 h with a heating rate of 1 K min−1. The calcined sample was ion-exchanged with 1 M NH4NO3 aqueous solution with a solid-liquid weight ratio of 1:12 (w/w) at 353 K for 12 h. To ensure the complete exchange of Na+, this procedure was repeated three times, and the obtained solid sample was dried at 383 K overnight and calcined at 773 K for 3 h with a heating rate of 1 K min−1 to get a protonform MOR (denoted as H-MOR-p-a-a). Finally, H-MOR-p-a-a was washed with 0.1 M HNO3 aqueous solution with a solid-liquid weight ratio of 1:20 (w/w) at 323 K for 15 min under stirring, and the obtained solid sample was then dried at 383 K overnight and calcined at 773 K for 3 h with a heating rate of 1 K min−1 to get an acid-alkaline-acid treated MOR (denoted as H-MOR-p-a-a-a). The flow diagram for the separate synthesis steps, including sample designations for the as-received Zeolyst MOR to the sequential acid-alkaline-acid post-treated MOR, is presented in Scheme 1.
2.3. Hydroisomerization of n-hexane The hydroisomerization of n-hexane was carried out in a fixed-bed reactor (6 mm inner diameter) at 1 atm pressure. 0.2 g of Pt/H-MOR catalyst was loaded into the reactor and in-situ activated in 80 vol% H2 /20 vol% N2 gas mixture at 673 K for 2 h (total flow rate of 10 mL min−1). After the reactor temperature was reduced to the reaction temperature of 513 K, n-hexane was introduced into the reactor by a high-pressure constant flow pump. The weight hourly space velocity (WHSV) of n-hexane and the H2/n-hexane molar ratio for all the experiments conducted were maintained at 2 h−1 and 6, respectively. The reaction products were analyzed by a gas chromatograph (Agilent 6820) equipped with a 50 m OV-101 capillary column and a flame ionization detector (FID).
2.1.2. Pt/H-MOR preparation Pt/H-MOR catalysts were prepared by the impregnation method as follows: a proton-form MOR sample (1 g) was impregnated with a Pt (NH3)4Cl2 aqueous solution (0.5 mg g−1) of 2.5 mL initially to become a slurry, followed by dropwise addition of the remaining Pt(NH3)4Cl2 aqueous solution of 7.5 mL into the slurry under vigorous stirring at 308 K for 24 h. Then the final slurry was filtered, washed with hotdeionized water until the complete removal of chloride ions and dried at 383 K overnight. Subsequently, the dried sample (1 g) was loaded in a tubular reactor (5 cm inner diameter) and heated in flowing oxygen at the flow rate of 500 mL min−1 at 673 K for 2 h with a heating rate of 0.5 K min−1. The resultant catalysts with nominal loading of 0.5 wt% Pt and the various zeolite treated samples (see Section 2.1.1) are named as Pt/H-MOR-p, Pt/H-MOR-p-a, and Pt/H-MOR-p-a-a-a.
3. Results and discussion 3.1. Catalyst characterization As shown in Fig. S1 (ESI), all the investigated catalysts showed the powder XRD characteristic patterns of MOR, indicating that the crystal structures are well preserved after the different post-treatments of the zeolite applied. Both Pt/H-MOR-p and Pt/H-MOR-p-a show a type-I N2 adsorption-desorption isotherm (Fig. S2A), which is a characteristic of microporosity. The acid-derived dealumination makes the micropore 22
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Scheme 1. Flow diagram of the different post-treatment steps, starting from the commercial zeolite. Table 1 Textural properties and chemical composition of the catalysts investigated. Catalysts
SBET (m2 g−1)
Vmicroa (cm3 g−1)
Sexta (m2 g−1)
Vtotal (p/p0 = 0.995) (cm3 g−1)
Vmesob (cm3 g−1)
Si/Alc
Pt loadedc (wt%)
Pt/H-MOR-p Pt/H-MOR-p-a Pt/H-MOR-p-a-a-a
419 452 492
0.17 0.18 0.17
39 61 126
0.25 0.27 0.46
0.08 0.09 0.29
10 28 15
0.45 0.43 0.46
a b c
Determined from the measured N2 adsorption-desorption isotherms at 77 K using the t-plot method. Vmeso = Vtotal–Vmicro. Determined by the ICP-AES analysis.
volume and the external specific surface area sharply from 0.08 cm3 g−1 and 39 m2 g−1 for Pt/H-MOR-p to 0.29 cm3 g−1 and 126 m2 g−1 for Pt/H-MOR-p-a-a-a, respectively, while the micropore volume almost remains unchanged. Hence, these results confirm the validity of the sequential acid-alkaline-acid post-treatment as an effective method to create mesoporosity in MOR crystals, while the microporosity is preserved. Furthermore, the pore size distribution profiles determined by the Barrett-Joyner-Halenda (BJH) method from the adsorption branch, shown in Fig. S2B, indicate that Pt/H-MOR-p-a-a-a possesses mesoporosity centered at ca. 10 nm, while Pt/H-MOR-p and Pt/H-MOR-p-a are purely microporous. Additionally, the Si/Al molar ratio is decreased from 28 for Pt/H-MOR-p-a to 15 for Pt/H-MOR-p-a-aa, respectively, due to the desilication (Table 1). To elucidate the effects of dealumination and desilication of zeolites on their acidity, 27Al MAS NMR spectra and NH3-TPD profiles were measured. In the 27Al MAS NMR spectra (Fig. S3), Pt/H-MOR-p shows only a resonance at 58 ppm ascribed to tetrahedral Al, the so-called framework Al species. The dealumination can develop extra-framework Al species showing a new resonance at 0 ppm in the spectrum of Pt/HMOR-p-a, while these extra-framework Al species can be effectively removed by the alkaline-acid post-treatment, as the resonance at 0 ppm in the spectrum of Pt/H-MOR-p-a-a-a is almost absent. Fig. 1 shows the NH3-TPD profiles recorded after removing the weakly chemisorbed NH3 species on the investigated catalysts. The lowtemperature (400–525 K) and high-temperature (525–973 K) desorption peaks are ascribed to the desorption of NH3 from the weak and strong acid sites of MOR, respectively [26,27]. As shown in Fig. 1, the strong acid sites, which are mainly considered as the catalytic sites in the hydroisomerization, have been well preserved during the dealumination and desilication post-treatments. Compared to the hightemperature desorption peak, the low-temperature peak was more obviously affected by the variation of the Si/Al ratio (Table 1 and Fig. 1).
Fig. 1. NH3-TPD profiles of Pt/H-MOR-p (a), Pt/H-MOR-p-a (b), and Pt/HMOR-p-a-a-a (c).
volume and the specific surface area slightly increased from 0.17 cm3 g−1 and 419 m2 g−1 for Pt/H-MOR-p to 0.18 cm3 g−1 and 452 m2 g−1 for Pt/H-MOR-p-a, respectively (Table 1) due to the enlargement in the micropore size after the removal of Al in the framework [24,25]. Moreover, the Si/Al molar ratio is increased from 10 for Pt/HMOR-p to 28 for Pt/H-MOR-p-a, where the latter is in the optimum range of Si/Al molar ratio in the zeolite for the subsequent alkaline post-treatment [24]. A further alkaline-acid post-treatment changes the isotherm to the combined one of type-I and type-IV with a well pronounced hysteresis loop. The latter indicates the generation of mesopores in the zeolite crystals (Fig. S2A). As shown in Table 1, the sequential acid-alkaline-acid post-treatment increases the mesopore 23
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The concentration of weak acid sites is in the order: Pt/H-MOR-p > Pt/ H-MOR-p-a-a-a > Pt/H-MOR-p-a, which is in accordance with the Al amount in the zeolite. Additionally, the presence of a shoulder peak at higher temperatures in the Pt/H-MOR-p-a indicates that the acid strength is increased after dealumination due to the formation of extraframework Al species acting as Lewis acid sites [28]. As shown in Table 1, the Pt amounts in all the catalysts, determined by the ICP-AES analysis, are within their nominal values of 0.5 wt%, indicating the efficiency of the impregnation method applied for preparing the catalysts. The SEM image of H-MOR-p shows that MOR particles are aggregates of nanocrystals (Fig. S4a), while TEM images reveal the mesoporosity in Pt/H-MOR-p-a-a-a and the distribution of platinum oxide particle size in all catalysts (Figs. S4b-d). Pt/H-MOR-p and Pt/H-MOR-p-a both show opaque crystals with some dark spots, corresponding to the presence of platinum oxide particles located on the surface of the MOR crystals (Figs. S3b and c). In contrast, the TEM image of Pt/H-MOR-p-a-a-a shows many white spots with a diameter of ca. 10 nm (Fig. S4d), indicating the presence of mesopores in the MOR crystals, which is in accordance with the N2 adsorption-desorption characterization results (Fig. S2). Moreover, the created mesopores are indeed embedded in the crystals as shown in the TEM images of Fig. S5. The mean diameter of platinum oxide particles decreases from ca. 3.2 nm in Pt/H-MOR-p and ca. 3.0 nm in Pt/H-MOR-p-a to ca. 1.8 nm in Pt/H-MOR-p-a-a-a, implying that the presence of mesopores in the zeolite enhanced the dispersion of Pt.
Fig. 3. Distribution of products and the yield of total branched hexanes in the hydroisomerization of n-hexane over Pt/H-MOR-p (a), Pt/H-MOR-p-a (b) and Pt/H-MOR-p-a-a-a (c) in the first 60 min of reaction. The reaction conditions are the same as those presented in Fig. 2. Others stands for C1-C5 products from the cracking reaction.
Lewis acid sites (Fig. 2), and the cracking products are pronounced as shown in Fig. 3. It is thus illustrated that Pt/H-MOR-p-a-a-a shows the highest conversion of n-hexane and yield of hexane isomers, with a stable activity and at the same time negligible cracking products (Figs. 2 and 3). The enhanced catalytic activity must be related to the introduction of mesoporosity with the preservation of the zeolite framework crystallinity, microporosity and acidity. The introduced mesopores in the MOR by the sequential acid-alkaline-acid post-treatment can shorten the diffusion length of the reactants and of the produced branched hexanes inside the micropores [26], affect the proximity between metallic Pt and acid sites [4,5], and enhance the dispersion of Pt in the zeolite. All these factors eventually lead to the increase of conversion of n-hexane and reduction of the non-selective cracking of hexane isomers and the formation of coke. For all the investigated catalysts, the product distributions almost remain unchanged with time on stream (Fig. S6). Additionally, the selectivity for the branched isomers for all the investigated catalysts follows the order: 2, 2-dimethylbutane < 2, 3-dimethylbutane < 3-methylpentane < 2-methylpentane, similar to the order of the molecular diameters of these hexane isomers. Moreover, Pt/H-MOR-p-a-a-a provides almost the same selectivity for the branched isomers as Pt/H-MOR-p, indicating that the introduced mesopores do not alter the shape selectivity of Pt/H-MOR catalysts. This observation agrees well with that most recently reported [27].
3.2. Hydroisomerization of n-hexane The catalytic hydroisomerization of n-hexane consists of the dehydrogenation of n-hexane to n-hexene on metallic Pt, the skeletal isomerization of n-hexene to monobranched methylpentenes and dibranched dimethylbutenes on acid sites, and the hydrogenation of the branched alkene intermediates to the corresponding branched alkanes on metallic Pt [26]. In addition, the hydroisomerization of n-hexane to isohexane is usually accompanied with the cracking reaction and coke formation [1]. As shown in Figs. 2 and 3, Pt/H-MOR-p-a possesses a higher conversion of n-hexane and a higher yield of hexane branched isomers than Pt/H-MOR-p in the initial reaction stage. The available Lewis acid sites due to the formation of extra-framework Al species in Pt/H-MOR-p-a have a promoting effect on the rates of the hydroisomerization, cracking and coking that leads to catalyst deactivation [28]. Therefore, although the initial conversion of n-hexane is enhanced over Pt/H-MOR-p-a in comparison with that over Pt/H-MOR-p, the catalytic activity will be rapidly dropped due to coke formation on the
4. Conclusions The simple acid post-treatment applied over a commercial MOR zeolite does not obviously introduce mesoporosity in the zeolite crystals but it can significantly increase the Si/Al ratio due to partial dealumination that results in the formation of extra-framework Al species. The prepared corresponding bifunctional Pt/H-MOR catalyst does not show an improved catalytic activity in the hydroisomerization reaction of n-hexane since the extra-framework Al species can enhance the formation of cracking products and coke. These results may depend on the specific MOR under investigation because other synthesis/post-treatment procedures or commercial MOR may lead to different results regarding the acid leaching treatment. On the other hand, the sequential acid-alkaline-acid post-treatment can introduce significant mesoporosity in the MOR crystal structure. The prepared corresponding bifunctional zeolite-supported Pt catalyst shows then an enhanced catalytic activity with long-term stability, due to the facts that the available mesopores in the zeolite catalyst can shorten the diffusion length of the
Fig. 2. Comparative results on the hydroisomerization of n-hexane over Pt/HMOR-p (a), Pt/H-MOR-p-a (b) and Pt/H-MOR-p-a-a-a (c). Reaction conditions: WHSV (n-hexane) = 2 h−1, H2/n-hexane molar ratio = 6:1, and T = 513 K. 24
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reactants and produced branched hexanes inside the micropores, and enhance the dispersion of Pt in the zeolite. The latter contributes to the enhancement of proximity of Pt and acid sites in the mesopores of zeolite which is a very important aspect in the present catalytic reaction.
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