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Microwave assisted in situ synthesis of USY-encapsulated heteropoly acid (HPW-USY) catalysts
Applied Catalysis A: General 352 (2009) 259–264
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Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
Microwave assisted in situ synthesis of USY-encapsulated heteropoly acid (HPW-USY) catalysts Dingfeng Jin a, Jing Gao a, Zhaoyin Hou a,*, Yan Guo a, Xiuyang Lu b, Yinghong Zhu c, Xiaoming Zheng a a
Key Lab of Applied Chemistry of Zhejiang Province, Department of Chemistry, Zhejiang University (Xixi Campus), Hangzhou, Zhejiang 310028, China Department of Chemical and Biochemical Engineering, Zhejiang University, Hangzhou, Zhejiang 310027, China c College of Chemical Engineering and Materials, Zhejiang University of Technology, Hangzhou, Zhejiang 310012, China b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 18 June 2008 Received in revised form 9 October 2008 Accepted 9 October 2008 Available online 22 October 2008
Under microwave irradiation, 12-tungstophosphoric acid (HPW) could be successfully synthesized in situ and encapsulated in the supercage of ultra stable Y (USY) in several minutes. But the framework of USY collapsed easily in traditional hydrothermal synthesis routine. 31P MAS NMR, transmission electron microscopy (HR-TEM), N2 adsorption, inductively coupled plasma (ICP) and X-ray diffraction (XRD) characterizations found that the formed HPW molecule located separately in the supercage of USY. Temperature-programmed desorption of NH3 (NH3-TPD) showed that HPW-USY exhibited stronger acidity than that of pure USY, and adsorbed pyridine infrared (Py-IR) disclosed that the concentration of Bro¨nsted acid sites was enhanced. This hybrid solid acid exhibited higher activity in the synthesis of 4,40 dimethyldiphenylmethane via toluene and formaldehyde and could be utilized as a solid acid catalyst in aqueous solutions. ß 2008 Elsevier B.V. All rights reserved.
Keywords: 12-Tungstophosphoric acid USY Encapsulation Microwave irradiation
1. Introduction Substitution of strong liquid acids (such as HCl and sulfuric acid) by solid acids has attracted much attention in past years. In published papers, heteropoly acids with Keggin structure (abbreviated as HPA) were popularly reported for their strong acidity, ease in separation and possible utilization in recycling [1–4]. However, the high solubility of HPA in polarity solvent brings about a serious loss in reactant, which hindered the application of HPA. Many efforts have been put into avoiding the loss of HPA, such as immobilization of HPA on a support material and partial substitution of proton by Cs+ ions [5]. As the Keggin heteropoly anions is larger (ca. 11–12 A˚) than the pore diameter of zeolite Y, HPA anions cannot enter its pore channels and a severe leaching of heteropoly anions into water was observed of the supported PW catalysts, which led to their poor reusability [6–7]. Sulikowski et al. proposed that this problem could be solved by encapsulating HPA in the supercage of zeolitic matrix [8]. It is well known that ultra stable Y (abbreviated as USY) zeolite consists of almost spherical 13 A˚ cavities interconnected tetrahedrally through smaller apertures of 7.4 A˚ diameter [9]. And a Keggin
* Corresponding author. Tel.: +86 571 88273283; fax: +86 571 88273283. E-mail address: [email protected] (Z. Hou). 0926-860X/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2008.10.020
structured HPA encapsulated in a supercage of USY cannot escape from its smaller apertures. Several attempts were reported in order to encage HPW and 12-molydophosphoric acid (HPMo) in supercage of USY by a ‘‘ship in the bottle’’ method via a hydrothermal routine [10–14]. However, major drawbacks of the reported hydrothermal routine are long time (3 h), high temperature (90–95 8C) and strong acidity of the aqueous medium. The zeolitic matrix collapses easily under these conditions and amorphous SiO2 derived from collapsed zeolite would enwrap the formed HPAs, which hindered the contact of acid sites with reactants. And the final product has a lower activity than that of pure USY [10]. It is of great practical interest to overcome the collapse of the zeolitic matrix during the preparation. Microwave irradiation is an alternative thermal energy source to conventional heating thanks to its rapid rise in temperature and higher energy efficiency [15–20]. In a reaction irradiated with microwaves, reaction time could be reduced greatly and the procedure would be helpful to avoid the collapse of zeolitic matrix during the in situ synthesis of HPW. In this contribution, we want to report an efficient synthesis routine of HPW encapsulated in a supercage of USY (abbreviated as HPW-USY) under microwave irradiation. The synthesized HPWUSY was characterized by 31P MAS NMR, X-ray diffraction (XRD), Fourier transform infrared (FT-IR), nitrogen adsorption-desorption isotherms and inductively coupled plasma studies (ICP). And the
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location of the formed HPW was detected by transmission electron microscopy (HR-TEM), X-ray photoelectron spectroscopy (XPS), adsorbed pyridine infrared (Py-IR) spectroscopy and temperatureprogrammed desorption of NH3 (NH3-TPD). This hybrid solid acid was utilized in the synthesis of 4,40 -dimethyldiphenylmethane (abbreviated as 4,40 - DMDPM) via toluene and formaldehyde, it exhibited higher activity than pure USY. 2. Experimental Sodium hydrogen phosphate (Na2HPO4), sodium tungstate (NaWO4H2O) and hydrochloric acid (HCl) were purchased from Shanghai Chemical Reagent Co. (China). USY zeolite was kindly supplied by Huahua Catalysis Co. (Wenzhou, China). All chemicals were used as received without further purification. Encapsulation of HPW in zeolitic matrix was carried out via in situ synthesis under microwave irradiation. USY was first calcined at 550 8C for 3 h, and then put into an aqueous solution of well mixed Na2HPO4 and NaWO4H2O with a controlled P/W atom ratio (1/6). A stoichiometric amount of HCl was added dropwise into the solution until pH 1.0. The flask containing the reaction mixture was placed into a microwave oven with a working frequency of 2.45 GHz (Nanjing Robot Co., LWMC-201, 650 W). Microwave irradiation was performed from 3 to 9 min. The formed HPW-USY (x) (where, x denoted the microwave irradiation time in min) was separated by filtration and washed 20 times with hot distilled water (80 8C) in order to remove the HPW anions formed on the external surface of USY completely. Finally, HPW-USY (x) were dried at 100 8C overnight and calcined at 250 8C for 3 h. At the same time, a reference sample was also prepared by a hydrothermal method according to reference [12]. USY zeolite was added to a solution of Na2HPO4 in deionized water and the mixture was stirred for 2 h at room temperature. Then a solution containing a known amount of NaWO4H2O was added dropwise. After 1.5 h of stirring, a stoichiometric amount of HCl was added dropwise at 90 8C. This suspension was stirred for 3 h at 90 8C. The synthesized sample (PW-USY (hydrothermal)) was separated and washed intensively with hot distilled water, dried at 100 8C overnight and finally calcined at 250 8C for 3 h. Solid-state 31P MAS-NMR spectra were obtained at room temperature with a 300 MHz spectrometer (Avance 300, BRUKER). For 31P spectra, 908 pulse (2 ms) and 60 s delay were used to acquire about 200–300 transients of 2k complex data. XPS analyses of the fresh PW-USY (7) and in situ Ar ionic sputtered PW-USY (7) were conducted on a PHI 5600 spectrometer (Physical Electronics) under 1010 Torr, Al-Ka source (1486.6 eV). The analyzed area was 8000 A˚ in diameter and the binding energies were calibrated by contaminated carbon (C1s at 285.0 eV). Ar ionic sputtering was performed with a beam energy of 4 keV and a sputtering rate of 25 A˚ min1 for 4 minutes. In situ pyridine adsorption was carried out by FT-IR spectroscopy (Nikolet 560, USA). The sample was first degassed (103 mbar, 200 8C for 4 h) in an IR cell and then the spectra of adsorbed pyridine were recorded. The textural structure of HPW-USY was measured by nitrogen adsorption at 196 8C using an OMNISORP 100CX system (COULTER Co., USA). All samples were pretreated at 250 8C in vacuum, and the pore structure was calculated from adsorption isotherms. The micropore diameter in a range of 5.5–18 A˚ was calculated by the Horvath–Kavazoe (HK) method and a mesopore size in a range of 30–1000 A˚ was calculated by Barrett–Joyner–Halenda (BJH) method. The element contents were measured using ICP in IRIS Intrepid II XSP (Thermo Fisher Scientific, USA) after leaching the metal ions with HF and diluting with distilled water (for aluminum,
phosphorus and tungsten) or very dilute aqueous NaOH solution (for silicon) to specific volumes. HR-TEM images were obtained using an accelerating voltage of 200 kV in JEM 2010HR (JEOL Ltd., Japan). Samples were first ground to powder, suspended in tetrachloromethane under supersonic shaking and finally dispersed on a copper grid. Energy dispersive Xray analysis (EDX) was performed with an INCA Energy 300 (Oxford, UK). XRD patterns were obtained on an D8 ADVANCE (BRUKER, Germany) instrument using nickel-filtered Cu Ka radiation at 40 kV and 40 mA. Diffraction data were recorded with a rate of 0.018 s1. The acidity of synthesized HPW-USY catalysts was performed via NH3-TPD. Each sample was first treated at 300 8C in Ar flow of 30 ml min1 for 1 h, and then cooled to room temperature, exposed to 20% NH3/Ar for 30 min, and purged by Ar at 100 8C for 5 h in order to eliminate the physical adsorbed ammonia. Temperature programmed desorption was conducted by ramping to 750 8C at 10 8C min1 and NH3 (m/e = 16) in effluent was detected and recorded as a function of temperature by a quadrupole mass spectrometer (OmniStarTM, GSD301, Switzerland). Condensation between toluene and formaldehyde over these HPW-USY catalysts was carried out in a custom-designed 150 cm3 stainless autoclave (Lanzhou, China). A mixture of toluene (450 mmol), paraformaldehyde (30 mmol of HCHO) and nitrobenzene (20 mmol, internal standard) was poured into the catalyst. The suspension was vigorously stirred with a magnetic stirrer at 140 8C (4 h). After the reaction, the reactor was cooled to room temperature, solid catalyst was separated by centrifuging, and organic reactant solution was diluted with ethanol and analyzed using an FID gas chromatography–mass spectrometer equipped with a 25 m capillary column of a cross-linked 5% phenylmethylsilicone (HP5988A, USA). 3. Results and discussion The detected 31P MAS NMR spectrum of PW-USY (7) is a broader (ca. 20.0 ppm) peak with a chemical shift at 15.0 ppm from H3PO4 (Fig. 1); this profile was wider than that of bulk HPW (a sharp signal at 15.3 ppm with a width of ca. 1 ppm) [8,21]. This broader spectrum could be due to the fact that the encapsulated Keggin unit located separately in the supercage of USY and its electronic environment would be disturbed by the surrounding Si, Al and O of the zeolitic matrix.
Fig. 1. Solid-state
31
P MAS NMR spectra of HPW-USY (7).
D. Jin et al. / Applied Catalysis A: General 352 (2009) 259–264
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XPS spectra of HPW-USY (7) are shown in Fig. 2. On the surface of fresh HPW-USY (7), the detected binding energies of Al2p, Si2p and O1s locate at 74.9, 103.9 and 533.5 eV. Neither phosphorous nor tungsten was detected (curve a in Fig. 2), which indicated that the HPW that formed on external surface of USY was removed completely during preparation. After Ar ionic sputtering, contaminated carbon was removed completely and the detected binding energy of Al2p, Si2p, O1s remained constant (curve b in Fig. 2). But phosphorous with a binding energy at 134.4 eV (of P2p in Fig. 3) and tungsten with binding energies at 35.4 and 38.4 eV (of W4f 72 in Fig. 4) were detected. These binding energy levels fit well with those of P and W in a Keggin anion [22]. These observations confirmed that phosphorous and tungsten in the matrix of USY combined into Keggin anions. And these formed HPW might be located in the supercage of USY (about 13 A˚ in diameter) because the size of a spherical HPW molecule is about 11–12 A˚ (diameter). HPW molecules encapsulated in the supercages of USY cannot escape from its smaller apertures of 7.4 A˚ diameters. The spectra of Py-IR of pure USY zeolite and HPW-USY (7) catalysts are illustrated in Fig. 5. A typical absorption band assigned to the vibration modes of adsorbed pyridine, forming Lewis-type adducts (Py-L, Py-M) with bands at 1610 cm1 and 1450 cm1, and to the mode of pyridine in interaction with Bro¨nsted acid sites (Py-B) as a band at 1540 cm1 were detected. And a band at 1490 cm1 is assigned to pyridine adsorbed on both Lewis and Bro¨nsted acid sites [23,24]. It can be found that HPWUSY (7) has an enhanced concentration of Bro¨nsted acid sites over that of pure USY. One can conclude that such enhanced Bro¨nsted acid sites contributed to the protonic acidity of synthesized HPW. As XPS analysis has confirmed that the HPW that formed on external surface of USY was removed completely during preparation, the enhanced Bro¨nsted acid sites are from these of HPW in supercage of USY. Nitrogen adsorption isotherms disclosed that the amount of adsorbed nitrogen of USY before P/Po = 0.3 was higher than that of HPW-USY (7), which indicated that HPW molecule encapsulated in supercage of USY would fill part of the micropores. Calculated structure data of USY and synthesized HPW-USY are summarized in Table 1. It can be found that the surface area of HPW-USY decreased from 693.6 (of pure USY) to 331.4 m2 g1 (of HPW-USY(9)) and the pore volume also decreased from 0.25 to 0.11 ml g1 with the
Fig. 2. High-resolution XPS spectra of the entire region in HPW-USY (7): (a) surface of HPW-USY and (b) down to 100 A˚ below the surface of HPW-USY.
Fig. 3. High-resolution XPS spectra of the P2p region in HPW-USY (7).
increasing microwave irradiation time because part each of supercage was occupied by a synthesized HPW molecule. The percentages of element content in pure USY and HPW-USY (7) determined by ICP are presented in Table 2. It shows that the atomic ratio between Si and Al in USY is lower than that of HPWUSY (7), which means that some aluminium could be dissolved. Phosphorus and tungsten were detected in HPW-USY (7) and the atom ratio between P and W was in accord with HPW due to the presence of HPW in the super cages of the USY zeolite. The calculated amount of formed HPW encapsulated in USY is about 5.6 wt% of the parent USY. HR-TEM of PW-USY (7) reveals a random distribution of dark regions (marked by smaller circles) in a zeolitic matrix (Fig. 6). The circled diameter of this region is 20 A˚, in which HPW molecules (about 10 A˚) could be identified clearly. EDX analyse around this area (marked by bigger circles) indicated that both P and W with atom ratio around 1/12 were detected. This result is the most promising image of the encapsulated HPW in USY. It is the first time that the location of HPW has clearly been identified. As a contrast, a sample in which the zeolitic matrix partly collapsed (confirmed by XRD analysis) and in which HPW crystals
Fig. 4. High-resolution XPS spectra of the W4f
72
region in HPW-USY (7).
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Fig. 5. The infrared spectra of pyridine adsorbed infrared over USY and HPW-USY (7) after degassing at 200 8C: (a) USY and (b) HPW-USY (7).
assembled into big clusters (PW-USY (9)) is shown in Fig. 7. It can be found that one part of bigger-sized HPW crystals are assembled in the external surface of USY and another part of the HPW crystals are enwrapped by the collapsed zeolitic matrix. Fig. 8 shows the XRD patterns of pure HPW, USY, HPW-USY (7) and HPW-USY (hydrothermal). Traces a and b are of HPW and faujasite zeolite, respectively. A perfect zeolite crystalline of USY was retained in HPW-USY (7). However, the structure of the faujasite collapsed in HPW-USY (hydrothermal), which was caused by dissolution of Al in the aqueous HCl solution during the hydrothermal process [25]. The XRD patterns of HPW-USY prepared at different microwave irradiation time also confirmed that no HPW crystal on the external surface of zeolite was detected as the formed HPW molecule dispersed highly and located separately in the supercage of the zeolitic matrix. However, longer irradiation time has a striking effect on the structure of the zeolitic matrix and amorphous silicon dioxide can be detected in HPW-USY (9). Amorphous silicon dioxide caused the formed HPW clusters to be enwrapped (also confirmed in HRTEM, Fig. 7) and so they cannot contact with reactant. We suppose that this is the major objection of traditional hydrothermal synthesis routine, which caused the final product to have a lower activity than that of pure USY [10]. The acidity of USY, HPW-USY (7) and HPW-USY (hydrothermal) detected by NH3-TPD is shown in Fig. 9. A desorption peak at about 200 8C could be assigned to weakly adsorbed ammonia on zeolite
Fig. 6. The HR-TEM image of HPW-USY (7).
matrix and the peak at about 450 8C could be ascribed to ammonia desorbed from those strong acid sites of zeolitic matrix [26–29]. When HPW was encapsulated in supercages of USY zeolite, the desorption temperature of NH3 on the strong acid site rose to 520 8C and the total amount of desorbed NH3 increased. The desorbed amount of NH3 from these acid sites was calculated and is summarized in Table 3. It can be found that the acid sites from zeolite matrix decreased slightly with the increasing irradiation time from 3 to 7 min. When the irradiation time prolonged to 9 min, this part of acid sites decreased quickly from 1.05 mmol g1 (of pure USY) to 0.19 mmol g1. The stronger acid sites of zeolitic matrix reduced from 1.59 to 1.28 mmol g1, too. However, total acidity remained almost constant because of the formation of HPW. It is found that HPW formed after 3 min irradiation with a microwave and that the amount of HPW increased with irradiation time. Because HPW that formed on external surface of zeolitic matrix was removed completely during
Table 1 The structure of all samples irradiated with microwaves for different times. Catalysts USY HPW-USY HPW-USY HPW-USY HPW-USY HPW-USY
(0 min) (3 min) (5 min) (7 min) (9 min)
SBET (m2 g1)
Pore volume (ml g1)
693.6 680.2 542.8 383.2 324.5 331.4
0.25 0.28 0.22 0.13 0.11 0.10
Table 2 Chemical compositions of USY and HPW-USY (7 min).
USY HPW-USY (7 min)
Al%
Si%
P%
W%
Si/Al
P/W
3.34 2.68
30.09 30.10
– 0.05
– 2.99
8.69 10.83
– 11.91
Fig. 7. The HR-TEM image of HPW-USY (9).
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Table 3 The acid strength distribution of the catalysts calculated by the results of NH3-TPD. Total acidity (mmol g1)
Catalysts
Acidity (mmol g1)a Weak (T1)
USY HPW-USY HPW-USY HPW-USY HPW-USY HPW-USY a b c
(0 min) (3 min) (5 min) (7 min) (9 min)
2.64 2.30 2.39 2.35 2.40 2.35
1.05 0.92 0.88 0.67 0.51 0.19
Strong (T2) T2–1b
T2–2c
1.59 1.38 1.33 1.31 1.27 1.28
0 0 0.18 0.37 0.62 0.88
Desorption temperature: T1 = 150–300 8C, T2 = 300–700 8C. Acidity supplied by USY zeolitic matrix. Acidity supplied by HPW heteropoly acid.
Table 4 The conversion of paraformaldehyde and selectivity of DMDPMs product catalyzed by USY zeolite and HPW-USY prepared with different methodsa. Fig. 8. XRD patterns of pure HPW, USY, HPW-USY (7) and HPW-USY (hydrothermal): (a) Pure HPW, (b) USY, (c) HPW-USY (7) and (d) HPW-USY (hydrothermal).
preparation, this enhance acidity comes from those HPW molecules in the matrix of USY. The catalytic properties of these samples have been examined in the synthesis of DMDPMs from toluene and formaldehyde, and the reaction results are summarized in Table 4. It was found that pure USY zeolite was less active for this reaction (with a 20.1% conversion of HCHO). On the other hand, the detected conversion of HCHO increased to 30.8% on HPW-USY (7). This increased activity contributed to the formation of more active sites as HPW became encapsulated in supercage of USY. Though the detected amount of formed HPW in HPW-USY (9) is larger than that of HPWUSY (7), its activity (with a HCHO conversion of 7.9%) is lower than that of pure USY. These results were because HPW heteropoly acid was covered by amorphous silicon derived from collapsed zeolitic matrix that hindered the contact of acid sites with reactants. And the lower activity of HPW-USY (hydrothermal) could be attributed to the same reason. According to previous experimentation, pure HPW exhibited high activity in synthesis DMDPMs from toluene and paraformaldehyde, while the selectivity of 4,40 -DMDPM was low [30–32]. And those zeolites with pore sizes large enough to allow diffusion of large molecules could increase the selectivity of 4,40 - DMDPM [33]. After heteropoly acid was packed into the supercages of the zeolitic
Catalysts
USY HPW-USY HPW-USY HPW-USY HPW-USY HPW-USY HPW-USY
Conversion (mol %)
(0 min) (3 min) (5 min) (7 min) (9 min) (hydrothermal)
20.1 13.1 23.8 25.2 30.8 7.9 11.7
Selectivity (mol %) 2,20
2,40
4,40
1.5 1.8 2.2 2.3 2.3 1.1 1.7
33.6 36.2 42.1 40.4 42.6 31.0 36.8
64.9 62.5 55.7 57.3 55.1 67.9 61.5
a Reaction conditions: toluene 450 mmol, paraformaldehyde 30 mmol, catalyst 1.0 g, reaction time 4 h, reaction temperature 140 8C.
matrix, the HPW-USY catalyst can also increase the selectivity of 4,40 - DMDPM more than pure HPW could. 4. Conclusions In conclusion, we found that HPW could be synthesized successfully in situ in supercages of USY without destruction of the zeolitic matrix. 31P MAS NMR, HR-TEM, N2 adsorption, ICP and XRD characterizations gave a promising image of the encapsulated HPW in USY and the location of HPW is clearly identified. And both the stronger acid sites and the concentration of Bro¨nsted acid sites were enhanced in HPW-USY. HPW-USY (7) exhibited higher activity for the synthesis of DMDPM via toluene and formaldehyde than pure USY. Acknowledgements This Project is supported by National Natural Science Foundation of China (90610002, 20433030), Zhejiang Provincial Natural Science Foundation (Z406142), the Ministry of Science and Technology of China through the National Key Project of Fundamental Research (Contract No. 2007CB210207) and the State Key Laboratory Breeding Base of Green Chemistry-Synthesis Technology. References [1] [2] [3] [4] [5]
Fig. 9. NH3-TPD profiles of USY, HPW-USY (7) and HPW-USY (hydrothermal): (a) USY, (b) HPW-USY (7) and (c) HPW-USY (hydrothermal).
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Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
Microwave assisted in situ synthesis of USY-encapsulated heteropoly acid (HPW-USY) catalysts Dingfeng Jin a, Jing Gao a, Zhaoyin Hou a,*, Yan Guo a, Xiuyang Lu b, Yinghong Zhu c, Xiaoming Zheng a a
Key Lab of Applied Chemistry of Zhejiang Province, Department of Chemistry, Zhejiang University (Xixi Campus), Hangzhou, Zhejiang 310028, China Department of Chemical and Biochemical Engineering, Zhejiang University, Hangzhou, Zhejiang 310027, China c College of Chemical Engineering and Materials, Zhejiang University of Technology, Hangzhou, Zhejiang 310012, China b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 18 June 2008 Received in revised form 9 October 2008 Accepted 9 October 2008 Available online 22 October 2008
Under microwave irradiation, 12-tungstophosphoric acid (HPW) could be successfully synthesized in situ and encapsulated in the supercage of ultra stable Y (USY) in several minutes. But the framework of USY collapsed easily in traditional hydrothermal synthesis routine. 31P MAS NMR, transmission electron microscopy (HR-TEM), N2 adsorption, inductively coupled plasma (ICP) and X-ray diffraction (XRD) characterizations found that the formed HPW molecule located separately in the supercage of USY. Temperature-programmed desorption of NH3 (NH3-TPD) showed that HPW-USY exhibited stronger acidity than that of pure USY, and adsorbed pyridine infrared (Py-IR) disclosed that the concentration of Bro¨nsted acid sites was enhanced. This hybrid solid acid exhibited higher activity in the synthesis of 4,40 dimethyldiphenylmethane via toluene and formaldehyde and could be utilized as a solid acid catalyst in aqueous solutions. ß 2008 Elsevier B.V. All rights reserved.
Keywords: 12-Tungstophosphoric acid USY Encapsulation Microwave irradiation
1. Introduction Substitution of strong liquid acids (such as HCl and sulfuric acid) by solid acids has attracted much attention in past years. In published papers, heteropoly acids with Keggin structure (abbreviated as HPA) were popularly reported for their strong acidity, ease in separation and possible utilization in recycling [1–4]. However, the high solubility of HPA in polarity solvent brings about a serious loss in reactant, which hindered the application of HPA. Many efforts have been put into avoiding the loss of HPA, such as immobilization of HPA on a support material and partial substitution of proton by Cs+ ions [5]. As the Keggin heteropoly anions is larger (ca. 11–12 A˚) than the pore diameter of zeolite Y, HPA anions cannot enter its pore channels and a severe leaching of heteropoly anions into water was observed of the supported PW catalysts, which led to their poor reusability [6–7]. Sulikowski et al. proposed that this problem could be solved by encapsulating HPA in the supercage of zeolitic matrix [8]. It is well known that ultra stable Y (abbreviated as USY) zeolite consists of almost spherical 13 A˚ cavities interconnected tetrahedrally through smaller apertures of 7.4 A˚ diameter [9]. And a Keggin
* Corresponding author. Tel.: +86 571 88273283; fax: +86 571 88273283. E-mail address: [email protected] (Z. Hou). 0926-860X/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2008.10.020
structured HPA encapsulated in a supercage of USY cannot escape from its smaller apertures. Several attempts were reported in order to encage HPW and 12-molydophosphoric acid (HPMo) in supercage of USY by a ‘‘ship in the bottle’’ method via a hydrothermal routine [10–14]. However, major drawbacks of the reported hydrothermal routine are long time (3 h), high temperature (90–95 8C) and strong acidity of the aqueous medium. The zeolitic matrix collapses easily under these conditions and amorphous SiO2 derived from collapsed zeolite would enwrap the formed HPAs, which hindered the contact of acid sites with reactants. And the final product has a lower activity than that of pure USY [10]. It is of great practical interest to overcome the collapse of the zeolitic matrix during the preparation. Microwave irradiation is an alternative thermal energy source to conventional heating thanks to its rapid rise in temperature and higher energy efficiency [15–20]. In a reaction irradiated with microwaves, reaction time could be reduced greatly and the procedure would be helpful to avoid the collapse of zeolitic matrix during the in situ synthesis of HPW. In this contribution, we want to report an efficient synthesis routine of HPW encapsulated in a supercage of USY (abbreviated as HPW-USY) under microwave irradiation. The synthesized HPWUSY was characterized by 31P MAS NMR, X-ray diffraction (XRD), Fourier transform infrared (FT-IR), nitrogen adsorption-desorption isotherms and inductively coupled plasma studies (ICP). And the
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location of the formed HPW was detected by transmission electron microscopy (HR-TEM), X-ray photoelectron spectroscopy (XPS), adsorbed pyridine infrared (Py-IR) spectroscopy and temperatureprogrammed desorption of NH3 (NH3-TPD). This hybrid solid acid was utilized in the synthesis of 4,40 -dimethyldiphenylmethane (abbreviated as 4,40 - DMDPM) via toluene and formaldehyde, it exhibited higher activity than pure USY. 2. Experimental Sodium hydrogen phosphate (Na2HPO4), sodium tungstate (NaWO4H2O) and hydrochloric acid (HCl) were purchased from Shanghai Chemical Reagent Co. (China). USY zeolite was kindly supplied by Huahua Catalysis Co. (Wenzhou, China). All chemicals were used as received without further purification. Encapsulation of HPW in zeolitic matrix was carried out via in situ synthesis under microwave irradiation. USY was first calcined at 550 8C for 3 h, and then put into an aqueous solution of well mixed Na2HPO4 and NaWO4H2O with a controlled P/W atom ratio (1/6). A stoichiometric amount of HCl was added dropwise into the solution until pH 1.0. The flask containing the reaction mixture was placed into a microwave oven with a working frequency of 2.45 GHz (Nanjing Robot Co., LWMC-201, 650 W). Microwave irradiation was performed from 3 to 9 min. The formed HPW-USY (x) (where, x denoted the microwave irradiation time in min) was separated by filtration and washed 20 times with hot distilled water (80 8C) in order to remove the HPW anions formed on the external surface of USY completely. Finally, HPW-USY (x) were dried at 100 8C overnight and calcined at 250 8C for 3 h. At the same time, a reference sample was also prepared by a hydrothermal method according to reference [12]. USY zeolite was added to a solution of Na2HPO4 in deionized water and the mixture was stirred for 2 h at room temperature. Then a solution containing a known amount of NaWO4H2O was added dropwise. After 1.5 h of stirring, a stoichiometric amount of HCl was added dropwise at 90 8C. This suspension was stirred for 3 h at 90 8C. The synthesized sample (PW-USY (hydrothermal)) was separated and washed intensively with hot distilled water, dried at 100 8C overnight and finally calcined at 250 8C for 3 h. Solid-state 31P MAS-NMR spectra were obtained at room temperature with a 300 MHz spectrometer (Avance 300, BRUKER). For 31P spectra, 908 pulse (2 ms) and 60 s delay were used to acquire about 200–300 transients of 2k complex data. XPS analyses of the fresh PW-USY (7) and in situ Ar ionic sputtered PW-USY (7) were conducted on a PHI 5600 spectrometer (Physical Electronics) under 1010 Torr, Al-Ka source (1486.6 eV). The analyzed area was 8000 A˚ in diameter and the binding energies were calibrated by contaminated carbon (C1s at 285.0 eV). Ar ionic sputtering was performed with a beam energy of 4 keV and a sputtering rate of 25 A˚ min1 for 4 minutes. In situ pyridine adsorption was carried out by FT-IR spectroscopy (Nikolet 560, USA). The sample was first degassed (103 mbar, 200 8C for 4 h) in an IR cell and then the spectra of adsorbed pyridine were recorded. The textural structure of HPW-USY was measured by nitrogen adsorption at 196 8C using an OMNISORP 100CX system (COULTER Co., USA). All samples were pretreated at 250 8C in vacuum, and the pore structure was calculated from adsorption isotherms. The micropore diameter in a range of 5.5–18 A˚ was calculated by the Horvath–Kavazoe (HK) method and a mesopore size in a range of 30–1000 A˚ was calculated by Barrett–Joyner–Halenda (BJH) method. The element contents were measured using ICP in IRIS Intrepid II XSP (Thermo Fisher Scientific, USA) after leaching the metal ions with HF and diluting with distilled water (for aluminum,
phosphorus and tungsten) or very dilute aqueous NaOH solution (for silicon) to specific volumes. HR-TEM images were obtained using an accelerating voltage of 200 kV in JEM 2010HR (JEOL Ltd., Japan). Samples were first ground to powder, suspended in tetrachloromethane under supersonic shaking and finally dispersed on a copper grid. Energy dispersive Xray analysis (EDX) was performed with an INCA Energy 300 (Oxford, UK). XRD patterns were obtained on an D8 ADVANCE (BRUKER, Germany) instrument using nickel-filtered Cu Ka radiation at 40 kV and 40 mA. Diffraction data were recorded with a rate of 0.018 s1. The acidity of synthesized HPW-USY catalysts was performed via NH3-TPD. Each sample was first treated at 300 8C in Ar flow of 30 ml min1 for 1 h, and then cooled to room temperature, exposed to 20% NH3/Ar for 30 min, and purged by Ar at 100 8C for 5 h in order to eliminate the physical adsorbed ammonia. Temperature programmed desorption was conducted by ramping to 750 8C at 10 8C min1 and NH3 (m/e = 16) in effluent was detected and recorded as a function of temperature by a quadrupole mass spectrometer (OmniStarTM, GSD301, Switzerland). Condensation between toluene and formaldehyde over these HPW-USY catalysts was carried out in a custom-designed 150 cm3 stainless autoclave (Lanzhou, China). A mixture of toluene (450 mmol), paraformaldehyde (30 mmol of HCHO) and nitrobenzene (20 mmol, internal standard) was poured into the catalyst. The suspension was vigorously stirred with a magnetic stirrer at 140 8C (4 h). After the reaction, the reactor was cooled to room temperature, solid catalyst was separated by centrifuging, and organic reactant solution was diluted with ethanol and analyzed using an FID gas chromatography–mass spectrometer equipped with a 25 m capillary column of a cross-linked 5% phenylmethylsilicone (HP5988A, USA). 3. Results and discussion The detected 31P MAS NMR spectrum of PW-USY (7) is a broader (ca. 20.0 ppm) peak with a chemical shift at 15.0 ppm from H3PO4 (Fig. 1); this profile was wider than that of bulk HPW (a sharp signal at 15.3 ppm with a width of ca. 1 ppm) [8,21]. This broader spectrum could be due to the fact that the encapsulated Keggin unit located separately in the supercage of USY and its electronic environment would be disturbed by the surrounding Si, Al and O of the zeolitic matrix.
Fig. 1. Solid-state
31
P MAS NMR spectra of HPW-USY (7).
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XPS spectra of HPW-USY (7) are shown in Fig. 2. On the surface of fresh HPW-USY (7), the detected binding energies of Al2p, Si2p and O1s locate at 74.9, 103.9 and 533.5 eV. Neither phosphorous nor tungsten was detected (curve a in Fig. 2), which indicated that the HPW that formed on external surface of USY was removed completely during preparation. After Ar ionic sputtering, contaminated carbon was removed completely and the detected binding energy of Al2p, Si2p, O1s remained constant (curve b in Fig. 2). But phosphorous with a binding energy at 134.4 eV (of P2p in Fig. 3) and tungsten with binding energies at 35.4 and 38.4 eV (of W4f 72 in Fig. 4) were detected. These binding energy levels fit well with those of P and W in a Keggin anion [22]. These observations confirmed that phosphorous and tungsten in the matrix of USY combined into Keggin anions. And these formed HPW might be located in the supercage of USY (about 13 A˚ in diameter) because the size of a spherical HPW molecule is about 11–12 A˚ (diameter). HPW molecules encapsulated in the supercages of USY cannot escape from its smaller apertures of 7.4 A˚ diameters. The spectra of Py-IR of pure USY zeolite and HPW-USY (7) catalysts are illustrated in Fig. 5. A typical absorption band assigned to the vibration modes of adsorbed pyridine, forming Lewis-type adducts (Py-L, Py-M) with bands at 1610 cm1 and 1450 cm1, and to the mode of pyridine in interaction with Bro¨nsted acid sites (Py-B) as a band at 1540 cm1 were detected. And a band at 1490 cm1 is assigned to pyridine adsorbed on both Lewis and Bro¨nsted acid sites [23,24]. It can be found that HPWUSY (7) has an enhanced concentration of Bro¨nsted acid sites over that of pure USY. One can conclude that such enhanced Bro¨nsted acid sites contributed to the protonic acidity of synthesized HPW. As XPS analysis has confirmed that the HPW that formed on external surface of USY was removed completely during preparation, the enhanced Bro¨nsted acid sites are from these of HPW in supercage of USY. Nitrogen adsorption isotherms disclosed that the amount of adsorbed nitrogen of USY before P/Po = 0.3 was higher than that of HPW-USY (7), which indicated that HPW molecule encapsulated in supercage of USY would fill part of the micropores. Calculated structure data of USY and synthesized HPW-USY are summarized in Table 1. It can be found that the surface area of HPW-USY decreased from 693.6 (of pure USY) to 331.4 m2 g1 (of HPW-USY(9)) and the pore volume also decreased from 0.25 to 0.11 ml g1 with the
Fig. 2. High-resolution XPS spectra of the entire region in HPW-USY (7): (a) surface of HPW-USY and (b) down to 100 A˚ below the surface of HPW-USY.
Fig. 3. High-resolution XPS spectra of the P2p region in HPW-USY (7).
increasing microwave irradiation time because part each of supercage was occupied by a synthesized HPW molecule. The percentages of element content in pure USY and HPW-USY (7) determined by ICP are presented in Table 2. It shows that the atomic ratio between Si and Al in USY is lower than that of HPWUSY (7), which means that some aluminium could be dissolved. Phosphorus and tungsten were detected in HPW-USY (7) and the atom ratio between P and W was in accord with HPW due to the presence of HPW in the super cages of the USY zeolite. The calculated amount of formed HPW encapsulated in USY is about 5.6 wt% of the parent USY. HR-TEM of PW-USY (7) reveals a random distribution of dark regions (marked by smaller circles) in a zeolitic matrix (Fig. 6). The circled diameter of this region is 20 A˚, in which HPW molecules (about 10 A˚) could be identified clearly. EDX analyse around this area (marked by bigger circles) indicated that both P and W with atom ratio around 1/12 were detected. This result is the most promising image of the encapsulated HPW in USY. It is the first time that the location of HPW has clearly been identified. As a contrast, a sample in which the zeolitic matrix partly collapsed (confirmed by XRD analysis) and in which HPW crystals
Fig. 4. High-resolution XPS spectra of the W4f
72
region in HPW-USY (7).
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Fig. 5. The infrared spectra of pyridine adsorbed infrared over USY and HPW-USY (7) after degassing at 200 8C: (a) USY and (b) HPW-USY (7).
assembled into big clusters (PW-USY (9)) is shown in Fig. 7. It can be found that one part of bigger-sized HPW crystals are assembled in the external surface of USY and another part of the HPW crystals are enwrapped by the collapsed zeolitic matrix. Fig. 8 shows the XRD patterns of pure HPW, USY, HPW-USY (7) and HPW-USY (hydrothermal). Traces a and b are of HPW and faujasite zeolite, respectively. A perfect zeolite crystalline of USY was retained in HPW-USY (7). However, the structure of the faujasite collapsed in HPW-USY (hydrothermal), which was caused by dissolution of Al in the aqueous HCl solution during the hydrothermal process [25]. The XRD patterns of HPW-USY prepared at different microwave irradiation time also confirmed that no HPW crystal on the external surface of zeolite was detected as the formed HPW molecule dispersed highly and located separately in the supercage of the zeolitic matrix. However, longer irradiation time has a striking effect on the structure of the zeolitic matrix and amorphous silicon dioxide can be detected in HPW-USY (9). Amorphous silicon dioxide caused the formed HPW clusters to be enwrapped (also confirmed in HRTEM, Fig. 7) and so they cannot contact with reactant. We suppose that this is the major objection of traditional hydrothermal synthesis routine, which caused the final product to have a lower activity than that of pure USY [10]. The acidity of USY, HPW-USY (7) and HPW-USY (hydrothermal) detected by NH3-TPD is shown in Fig. 9. A desorption peak at about 200 8C could be assigned to weakly adsorbed ammonia on zeolite
Fig. 6. The HR-TEM image of HPW-USY (7).
matrix and the peak at about 450 8C could be ascribed to ammonia desorbed from those strong acid sites of zeolitic matrix [26–29]. When HPW was encapsulated in supercages of USY zeolite, the desorption temperature of NH3 on the strong acid site rose to 520 8C and the total amount of desorbed NH3 increased. The desorbed amount of NH3 from these acid sites was calculated and is summarized in Table 3. It can be found that the acid sites from zeolite matrix decreased slightly with the increasing irradiation time from 3 to 7 min. When the irradiation time prolonged to 9 min, this part of acid sites decreased quickly from 1.05 mmol g1 (of pure USY) to 0.19 mmol g1. The stronger acid sites of zeolitic matrix reduced from 1.59 to 1.28 mmol g1, too. However, total acidity remained almost constant because of the formation of HPW. It is found that HPW formed after 3 min irradiation with a microwave and that the amount of HPW increased with irradiation time. Because HPW that formed on external surface of zeolitic matrix was removed completely during
Table 1 The structure of all samples irradiated with microwaves for different times. Catalysts USY HPW-USY HPW-USY HPW-USY HPW-USY HPW-USY
(0 min) (3 min) (5 min) (7 min) (9 min)
SBET (m2 g1)
Pore volume (ml g1)
693.6 680.2 542.8 383.2 324.5 331.4
0.25 0.28 0.22 0.13 0.11 0.10
Table 2 Chemical compositions of USY and HPW-USY (7 min).
USY HPW-USY (7 min)
Al%
Si%
P%
W%
Si/Al
P/W
3.34 2.68
30.09 30.10
– 0.05
– 2.99
8.69 10.83
– 11.91
Fig. 7. The HR-TEM image of HPW-USY (9).
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Table 3 The acid strength distribution of the catalysts calculated by the results of NH3-TPD. Total acidity (mmol g1)
Catalysts
Acidity (mmol g1)a Weak (T1)
USY HPW-USY HPW-USY HPW-USY HPW-USY HPW-USY a b c
(0 min) (3 min) (5 min) (7 min) (9 min)
2.64 2.30 2.39 2.35 2.40 2.35
1.05 0.92 0.88 0.67 0.51 0.19
Strong (T2) T2–1b
T2–2c
1.59 1.38 1.33 1.31 1.27 1.28
0 0 0.18 0.37 0.62 0.88
Desorption temperature: T1 = 150–300 8C, T2 = 300–700 8C. Acidity supplied by USY zeolitic matrix. Acidity supplied by HPW heteropoly acid.
Table 4 The conversion of paraformaldehyde and selectivity of DMDPMs product catalyzed by USY zeolite and HPW-USY prepared with different methodsa. Fig. 8. XRD patterns of pure HPW, USY, HPW-USY (7) and HPW-USY (hydrothermal): (a) Pure HPW, (b) USY, (c) HPW-USY (7) and (d) HPW-USY (hydrothermal).
preparation, this enhance acidity comes from those HPW molecules in the matrix of USY. The catalytic properties of these samples have been examined in the synthesis of DMDPMs from toluene and formaldehyde, and the reaction results are summarized in Table 4. It was found that pure USY zeolite was less active for this reaction (with a 20.1% conversion of HCHO). On the other hand, the detected conversion of HCHO increased to 30.8% on HPW-USY (7). This increased activity contributed to the formation of more active sites as HPW became encapsulated in supercage of USY. Though the detected amount of formed HPW in HPW-USY (9) is larger than that of HPWUSY (7), its activity (with a HCHO conversion of 7.9%) is lower than that of pure USY. These results were because HPW heteropoly acid was covered by amorphous silicon derived from collapsed zeolitic matrix that hindered the contact of acid sites with reactants. And the lower activity of HPW-USY (hydrothermal) could be attributed to the same reason. According to previous experimentation, pure HPW exhibited high activity in synthesis DMDPMs from toluene and paraformaldehyde, while the selectivity of 4,40 -DMDPM was low [30–32]. And those zeolites with pore sizes large enough to allow diffusion of large molecules could increase the selectivity of 4,40 - DMDPM [33]. After heteropoly acid was packed into the supercages of the zeolitic
Catalysts
USY HPW-USY HPW-USY HPW-USY HPW-USY HPW-USY HPW-USY
Conversion (mol %)
(0 min) (3 min) (5 min) (7 min) (9 min) (hydrothermal)
20.1 13.1 23.8 25.2 30.8 7.9 11.7
Selectivity (mol %) 2,20
2,40
4,40
1.5 1.8 2.2 2.3 2.3 1.1 1.7
33.6 36.2 42.1 40.4 42.6 31.0 36.8
64.9 62.5 55.7 57.3 55.1 67.9 61.5
a Reaction conditions: toluene 450 mmol, paraformaldehyde 30 mmol, catalyst 1.0 g, reaction time 4 h, reaction temperature 140 8C.
matrix, the HPW-USY catalyst can also increase the selectivity of 4,40 - DMDPM more than pure HPW could. 4. Conclusions In conclusion, we found that HPW could be synthesized successfully in situ in supercages of USY without destruction of the zeolitic matrix. 31P MAS NMR, HR-TEM, N2 adsorption, ICP and XRD characterizations gave a promising image of the encapsulated HPW in USY and the location of HPW is clearly identified. And both the stronger acid sites and the concentration of Bro¨nsted acid sites were enhanced in HPW-USY. HPW-USY (7) exhibited higher activity for the synthesis of DMDPM via toluene and formaldehyde than pure USY. Acknowledgements This Project is supported by National Natural Science Foundation of China (90610002, 20433030), Zhejiang Provincial Natural Science Foundation (Z406142), the Ministry of Science and Technology of China through the National Key Project of Fundamental Research (Contract No. 2007CB210207) and the State Key Laboratory Breeding Base of Green Chemistry-Synthesis Technology. References [1] [2] [3] [4] [5]
Fig. 9. NH3-TPD profiles of USY, HPW-USY (7) and HPW-USY (hydrothermal): (a) USY, (b) HPW-USY (7) and (c) HPW-USY (hydrothermal).
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