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Synthesis of titanated chabazite with enhanced thermal stability by hydrothermal conversion of titanated faujasite
Microporous and Mesoporous Materials 215 (2015) 58e66
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Synthesis of titanated chabazite with enhanced thermal stability by hydrothermal conversion of titanated faujasite Yusuke Kunitake a, Tomoka Takata a, Yoshitaka Yamasaki a, Naoki Yamanaka a, Nao Tsunoji a, Yasuyuki Takamitsu b, Masahiro Sadakane a, Tsuneji Sano a, * a b
Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan Inorganic Materials Research Laboratory, Tosoh Corporation, Shunan, Yamaguchi 746-8501, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 January 2015 Received in revised form 31 March 2015 Accepted 11 May 2015 Available online 19 May 2015
Titanated chabazites (TieCHAs) with various Si/Al and Si/Ti ratios were prepared from titanated faujasite (TieFAU) by optimizing the NaOH/SiO2 and H2O/SiO2 ratios in the starting reaction mixtures. The peak assigned to isolated tetrahedrally coordinated Ti species was clearly observed at ~220 nm in the UVevis spectra and at ~960 cm1 in the FT-IR spectra of the TieCHA, confirming that Ti was incorporated in the zeolite framework. The thermal stability of the TieCHAs was higher than that of non-modified CHAs synthesized from FAU or amorphous aluminosilicate hydrogels. The framework structure of TieCHAs with Si/Ti ratios of 33 and 64 was unchanged even after calcination at 1000 C for 1 h, indicating the enhanced thermal stability by Ti modification. We also prepared Cu-loaded zeolite catalysts using Ti eCHA and non-modified CHA and investigated their catalytic performance for the selective catalytic reduction of NOx by ammonia. Although there was no difference between the NO conversion on the two fresh catalysts, the NO conversion on the Cu-loaded TieCHA catalyst after hydrothermal treatment at 900 C for 4 h was considerably higher than that on the Cu-loaded CHA catalyst, indicating the higher hydrothermal stability of Cu-loaded TieCHA catalyst. © 2015 Elsevier Inc. All rights reserved.
Keywords: Hydrothermal conversion Titanated faujasite Titanated chabazite Thermal/hydrothermal stability Selective catalytic reduction of NOx by ammonia
1. Introduction Chabazite (CHA), an aluminosilicate mineral in the zeolite family, has a three-dimensional pore system with ellipsoidal large cages (6.7 10 Å) that are accessible via eight-ring windows (free diameter: 3.8 3.8 Å). The high-silica chabazite, SSZ-13, with a Si/ Al ratio above 5, has attracted significant interest owing to its thermal and acid stabilities. There have been extensive investigations on applications of this material in catalysts for selective conversion of methanol [1e3] or bioethanol [4,5] into light olefins such as ethylene and propylene, for selective catalytic reduction (SCR) of NOx by ammonia (NH3-SCR of NOx) [6e11], and as zeolite membranes for dehydration of aqueous organic solutions [12e15]. In order to improve the physicochemical properties of CHAs, researchers are attempting to develop improved methods for the synthesis of high-silica CHAs.
* Corresponding author. Tel.: þ81 82 424 7607; fax: þ81 82 424 5494. E-mail address: [email protected] (T. Sano). http://dx.doi.org/10.1016/j.micromeso.2015.05.023 1387-1811/© 2015 Elsevier Inc. All rights reserved.
We investigated the potential of hydrothermal conversion from one zeolite into another, interzeolite conversion, or interzeolite transformation [16e18], and found that the conversion or transformation approach is an attractive strategy for zeolite synthesis [19]. Very recently, we successfully prepared high-silica CHAs with Si/Al ratios of 5e21 from faujasite (FAU) in the presence of benzyltrimethylammonium hydroxide (BTMAOH) or N,N,N-trimethyl1-adamantammonium hydroxide (TMAdaOH) as an organic structure-directing agent (OSDA). The number of structural defects in CHAs prepared by the hydrothermal conversion method was smaller compared to CHAs synthesized from amorphous aluminosilicate hydrogel (the conventional synthesis route used as a reference), resulting in the higher acid stability [15,20]. However, the thermal and hydrothermal stabilities of CHAs must be improved before they are ready for further applications. In general, the physicochemical properties of zeolites depend on the number of tetrahedrally coordinated Al atoms in the zeolite framework. In order to improve the physicochemical properties of zeolites, isomorphous substitution of various metals such as Ti, V, Ga, Fe, B, and Zn for Al in the framework has been widely studied [21e23]. Isomorphous substitution is achieved by direct
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hydrothermal synthesis or by post-synthesis treatment. Since the first report by Tarausso et al. of direct synthesis of a Ti-incorporated zeolite (titanosilicate), TS-1 with an MFI structure, there have been many investigations into the synthesis, characterization, and catalytic applications of titanosilicates [24e26]. We also reported that Ti-incorporated FAU (TieFAU) can be prepared by post-synthesis treatment of dealuminated FAU with an aqueous solution of (NH4)2TiF6 [27]. In this study, we investigated the effect of Ti incorporation into the zeolite framework on the thermal stability of CHA prepared by hydrothermal conversion of FAU in the presence of TMAda cations as an OSDA. We also prepared a Cu-loaded TieCHA catalyst and investigated its catalytic performance for the NH3-SCR of NOx. 2. Experimental 2.1. Dealumination and titanation of FAU The FAU used in this work was prepared from the zeolite NH4eY (Si/Al ¼ 2.8, Tosoh Co., Japan) through dealumination that involved a combination of steaming at 700 C and H2SO4 treatment at room temperature for 24 h. The XRD pattern of the dealuminated zeolite showed no peaks other than those corresponding to the FAU structure. The particle size of the dealuminated FAU was 0.2e0.5 mm. Incorporation of Ti into the dealuminated FAU was carried out by a post-synthesis method under acidic conditions [27]. The dealuminated FAU (1 g) was added to (NH4)2TiF6 (Aldrich, USA) solutions (100 mL) with different concentrations. The molecular size of a TiF6 anion was estimated to be ~6.2 Å by density functional theory (DFT) quantum chemical calculation (DMol3 Ver. 5.0 provided by Accelrys, Inc., USA), which allows the diffusion of TiF6 anions into the supercages (~7.4 Å) of FAU. To maintain the pH value of the suspension (pH ¼ 1) during titanation, we added a certain amount of a 30 wt% sulfuric acid solution to the suspension. The mixtures were stirred at room temperature for 24 h and the samples were filtered, washed thoroughly with hot deionized water (60 C), and dried at 120 C. The samples were then calcined at 500 C for 10 h, yielding TieFAU samples. Table 1 lists the postsynthesis treatment conditions of FAUs with various Si/Al ratios. 2.2. Hydrothermal conversion of TieFAU into TieCHA The hydrothermal conversion of TieFAU into TieCHA was performed as follows. First, TieFAU was thoroughly mixed with an aqueous solution containing TMAdaOH (25 wt%, Sachem Asia Co., Ltd., Japan) as an OSDA, seed crystals (3 wt%, Si/Al ¼ 20), and NaOH (>99%, Merck Chemicals Inc., Japan). The mixture was placed into a 30-cm3 Teflon-lined stainless steel autoclave. The hydrothermal conversion was then conducted at 125 C for 2 days in a convection oven. The solid product was collected by centrifugation and washed thoroughly with deionized water until it was nearly neutral. It was
Table 1 Preparation of TieFAUs from FAUs with various Si/Al ratios.a Sample
Treatment conditions b
1 2 3 4 5 a b
TieFAU
Si/Al of FAU
(NH4)2TiF6/mmol
30 wt% H2SO4/g
Si/Tib
Si/Alb
2.8 21 46 46 46
1.97 1.97 1.97 1.97 0.90
7.2 0.3 1.0 1.0 1.0
20 22 38 40 63
49 39 107 119 146
FAU: 3 g; H2O: 950 mL; Temperature ¼ room temperature, Time ¼ 24 h. Measured by ICP.
59
then dried overnight at 70 C. Table 2 lists the chemical compositions of starting gels and the hydrothermal-conversion conditions. 2.3. Preparation of Cu-loaded TieCHA catalyst Before the metal loading, Na cations in TieCHA (sample 7) were removed by ion exchange with an aqueous solution of NH4Cl. The obtained NH4-form was then calcined at 550 C for 1 h, yielding the H-form. The Cu-loaded zeolite catalyst was prepared by the impregnation method as follows: an aqueous solution containing a certain amount of Cu(NO3)2$3H2O (Kishida Chemical Co. Ltd., Japan) was added to the H-form zeolite and mixed thoroughly in a ceramic mortar. The resultant wet powder was dried at 110 C and then calcinated at 550 C for 1 h. For a reference, the Cu-loaded CHA catalyst was also prepared from the non-modified CHA zeolite synthesized by hydrothermal conversion of FAU. The Cu loading was maintained at 1.5 wt% for all samples. 2.4. Characterization Powder X-ray diffraction (XRD) patterns of the solid products were collected on a diffractometer (Mini Flex, Rigaku, Japan) with a curved-graphite monochromator, using Cu Ka radiation, and operated at 30 kV and 15 mA. The Si/Ti, Si/Al, Na/Al, and Cu/Al ratios were determined using inductively coupled plasma optical emission spectroscopy (ICP-OES; SPS7000, Seiko, Japan). The crystal morphology was observed using a scanning electron microscope (SEM; S-4800, Hitachi, Japan) coupled with an energy-dispersive Xray (EDX) analyzer. Magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectra of 27Al and 29Si were recorded at 104.2 and 79.5 MHz, respectively, on a solid NMR spectrometer (600 PS, Varian, USA), using a 3.2-mm-diameter zirconia rotor at a rotational speed of 15 kHz for 27Al MAS NMR and a 6-mm-diameter zirconia rotor at 6 kHz for 29Si MAS NMR. The spectra were acquired using 2.3-ms pulses, a 1- s recycle delay, and 1000 scans for 27Al, and 5-ms pulses, a 100-s recycle delay, and 1000 scans for 29Si. As chemical shift references for 27Al and 29Si MAS NMR, Al(NO3)3$9H2O and Si(CH3)4 were used, respectively. Prior to the 27Al MAS NMR measurements, the samples were moisture-equilibrated over a saturated solution of NH4Cl for 24 h. Cross-polarized (CP) MAS NMR spectra of 1He13C were measured with a spinning frequency of 20 kHz, a 90 pulse length of 2.2 ms, and a cycle delay time of 50 s. Hexamethylbenzene was used as a chemical-shift reference. Thermal analysis was performed using a thermogravimetry and differential thermal analysis (TGeDTA) apparatus (SSC/5200, Seiko Instruments, Japan). The sample (~7 mg) was heated in flowing air (50 mL min1) at 5 C min1 from room temperature to 800 C. Nitrogen adsorption isotherms were obtained at 196 C using a conventional volumetric apparatus (BELSORP-mini, Bel Japan, Inc., Japan). Prior to the adsorption measurements, the samples (~0.1 g) were evacuated at 400 C for 10 h. Ultravioletevisible light (UVevis) diffuse reflectance spectra of the zeolites were recorded on a UVevis spectrometer (V-570, JASCO, Japan) with a bandwidth of 10 mm and a scan speed of 400 nm min1. Fourier-transform infrared (FT-IR) spectra were also measured by the KBr technique at room temperature on an FT-IR spectrometer (NICOLET 6700, Thermo Scientific, USA) with a resolution of 4 cm1. 2.5. Thermal stability The thermal stability of TieCHAs was evaluated by comparing the change in the intensities of the XRD peaks at 2q ¼ 18.2, 21.0, 25.4, 26.4, and 31.1 of samples before and after calcination at 6001150 C for 1 h in air. The relative crystallinity (RC) was determined as follows:
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RC ð%Þ ¼
Sum of intensities of the peaks at 2q ¼ 18:2; 21:0; 25:4; 26:4 and 31:1 after calcination at various temperatures for 1 h Sum of intensities of the peaks at 2q ¼ 18:2; 21:0; 25:4; 26:4 and 31:1 before calcination 100 (1)
2.6. NH3-SCR of NOx activity test
3. Results and discussion
The reactions for NH3-SCR of NOx were carried out in a fixed-bed flow reactor under atmospheric pressure. The Cu-loaded TieCHA catalyst was pressed and sieved until it reached approximately 1 mm in diameter. The required amount of catalyst (1.5 mL, ~0.8 g) was retained by placing quartz wool at the center of a quartz reactor. A thermocouple inserted at the center of the catalyst bed was used to measure the temperature during the reaction. The reactant gas was composed of 200 ppm NO, 200 ppm NH3, 10 vol% O2, 3 vol% H2O, and balance N2 (NO þ NH3 þ O2 þ H2O þ N2 ¼ 100 vol%), where the gas composition was based on emissions from diesel engines. The total flow rate was fixed at 1.5 L min1 and the GHSV (gas hourly space velocity) value was 60,000 h1. During the experiments, the temperature was varied from 500 to 150 C in steps of approximately 50 C. The NO conversion was defined as follows:
3.1. Preparation and characterization of TieFAU
NOin NOout NO2out 100 NOin
NO conversionð%Þ ¼
Titanation of the dealuminated FAUs with various Si/Al ratios was carried out by varying the amounts of (NH4)2TiF6 and 30 wt% H2SO4 in the starting reaction mixtures. As shown in Table 1, TieFAUs with various Si/Ti and Si/Al ratios were obtained. Fig. 1(a) and (b) show the XRD patterns of the starting dealuminated FAU with a Si/Al ratio of 46 and the resulting TieFAU (sample 5). There were no peaks other than those corresponding to the FAU structure. The yield of TieFAU was ~80% based on the weight of the starting FAU. As seen in SEM images of the starting FAU and the prepared TieFAU (sample 5) in Fig. 2(a) and (b), the morphology of the TieFAU crystals was almost the same as that of the starting FAU crystals. The BrunauereEmmetteTeller (BET) surface area and the micropore volume of TieFAU calculated from the N2 adsorption isotherm were 835 m2 g1 and 0.32 cm3 g1, respectively, which are similar to those of commercial zeolite Y (829 m2 g1 and 0.34 cm3 g1). Fig. 3 displays the UVevis spectra of the starting FAU and the TieFAUs with different Si/Ti ratios (samples 2 and 5). Although no peaks were observed in the spectrum of the starting FAU, the peak from isolated tetrahedrally coordinated Ti species is clearly visible at ~220 nm in the spectrum of TieFAU with a Si/Ti ratio of 63 (sample 5), indicating incorporation of Ti into the zeolite framework [27]. In the spectrum of TieFAU with a larger amount of Ti (sample 2, Si/Ti ratio of 22), however, a shoulder peak appears in the 280e330 nm range together with the 220-nm peak, indicating the presence of octahedrally coordinated Ti species or TiO2, or both octahedrally coordinated Ti species and TiO2.
(2)
where NOin represents the NO inlet concentration (200 ppm), and NOout and NO2out represent the NO and NO2 outlet concentrations, respectively. To evaluate the steady state catalytic activity, the concentrations of NH3, NO, and NO2 in the outlet gas after 10 min of the time-on-stream at each reaction temperature were analyzed by an FT-IR spectrometer (FT/IR-6100, JASCO, Japan) equipped with a gas cell (LPC-12MS, 12m) and a mercury cadmium telluride detector cooled by liquid nitrogen. The concentrations were determined by the intensities of the peaks at 1033, 1875, and 2917 cm1 for NH3, NO, and NO2, respectively (see Supporting Information; Fig. 1S and Fig. S2). Thirty scans were averaged for each normalized spectrum.
3.2. Hydrothermal conversion of TieFAU into TieCHA If the hydrothermal conversion of the TieFAU was carried out in the absence of seed crystals by varying the H2O/SiO2, TMAdaOH/
Table 2 Synthesis of TieCHAs from TieFAUs with various Si/Ti and Si/Al ratios.a Sample
Synthesis conditions
6 7 8 9 10 11e 12f
TieCHA
Si/Ti of TieFAU
Si/Al of TieFAU
NaOH/SiO2
H2O/SiO2
Yield/%
Si/Tib
Si/Alb
Si/(Al þ Ti)
BET surface areac/m2 g1
Micropore volumed/cm3 g1
22 20 38 40 63
39 49 107 119 146 48(FAU) 35(Am.)
0.35 0.35 0.2 0.25 0.25 0.1 0.2
25 25 27 27 17 5 5
55 60 80 75 80 88 99
14 19 30 33 64
18 27 76 71 89 35 35
7.9 11.2 21.5 22.5 37.2
673 649 643 679 837 887 800
0.25 0.22 0.22 0.19 0.32 0.32 0.31
b
b
TMAdaOH/SiO2 ¼ 0.2; Seed (Si/Al ¼ 20) ¼ 3 wt%; Temperature ¼ 125 C; Time ¼ 2 days. Measured by ICP. c Determined by the BET method. d Determined by the t-plot. e FAU without Ti was used as Si and Al sources instead of TieFAU. f Amorphous aluminosilicate hydrogel prepared from fumed silica (Cab-O-Sil M5, Cabot, USA) and Al(OH)3 (Wako Pure Chemical Ind. Ltd., Japan) was used as Si and Al sources instead of TieFAU. a
b
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Fig. 1. XRD patterns of (a) starting dealuminated FAU (Si/Al ¼ 46), (b) TieFAU (sample 5), (c) TieCHA (sample 6), and (d) TieCHA (sample 10).
SiO2, and NaOH/SiO2 ratios in the starting reaction mixtures, highly crystalline TieCHA could not be obtained (results not shown here). Therefore, the following experiments were carried out using seed crystals. Table 2 lists the chemical compositions of starting reaction mixtures and the characteristics of TieCHAs obtained. TieCHAs with various Si/Ti and Si/Al ratios were prepared from Ti-FAUs by optimizing the NaOH/SiO2 and H2O/SiO2 ratios in the starting reaction mixtures. In Fig. 1(c) and (d), XRD patterns of the TieCHA with different Si/Ti and Si/Al ratios show the typical diffraction patterns of the CHA structure, which contained no impurities from the unconverted starting TieFAU or from a co-crystallized phase. As can be seen in Fig. 2(c) and (d), the CHA crystals exhibited a pseudo cubic morphology, which is the same as that of conventional aluminosilicate CHA. Fig. 4 shows the UVevis spectra of these TieCHAs with different Si/Ti ratios. For reference, the spectrum of the starting TieFAU is also displayed. In the UVevis spectrum of TieCHA with a Si/Ti ratio of 64 (sample 10), the peak from isolated
Fig. 2. SEM images of (a) starting dealuminated FAU (Si/Al ¼ 46), (b) Ti-FAU (sample 5), (c) Ti-CHA (sample 6), and (d) Ti-CHA (sample 10).
61
Fig. 3. UVevis diffuse reflectance spectra of (a) starting dealuminated FAU (Si/Al ¼ 46), (b) TieFAU (sample 5), and (c) TieFAU (sample 2).
tetrahedrally coordinated Ti species appears at ~220 nm, indicating that Ti was present in the zeolite framework. However, a weak shoulder was observed at ~280 nm, indicating the presence of a trace amount of octahedrally coordinated Ti species or TiO2, or both octahedrally coordinated Ti species and TiO2. On the other hand, in the case of TieCHA with a Si/Ti ratio of 14 (sample 6), the more clear shoulder peak is in the 280e330 nm range together with the 220nm peak. A reduction in the BET surface area and micropore volume also suggest the possibility of pore blocking by the formation of Ti species outside the framework. In the FT-IR spectrum of Ti-CHA (Fig. S3), a weak peak assigned to the stretching vibration of
Fig. 4. UVevis diffuse reflectance spectra of (a) TieFAU (sample 5), (b) TieCHA (sample 10), and (c) TieCHA (sample 6).
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Fig. 5. SEM image of TieCHA (sample 10) and corresponding elemental mappings.
tetrahedrally coordinated Ti species was observed at ca. 960 cm1 [28,29], which also indicates the presence of tetrahedrally coordinated Ti species. Fig. 5 shows an SEM image of TieCHA with a Si/Ti ratio of 64 and the corresponding elemental mappings. It can be seen that titanium atoms were evenly distributed in the crystals. In the 13C CP/MAS NMR spectrum of an as-synthesized TieCHA, sample 10 (Fig. 6), there are four peaks detected at approximately 32, 36, 48, and 73 ppm. The resonances at 32 and 36 ppm are assigned to H CC3 and C CH2 C, respectively, of TMAda cations, while the resonance at 48 ppm is assigned to NðCH3 Þ3 . The resonance at 73 ppm corresponds to N C C3 . Therefore, the TMAda cations were the only organic species existing in the TieCHA pores. The weight loss between 250 and 650 C in the thermogravimetric curve (Fig. 7), which corresponds to the total decomposition of organic moieties, was about 23.7 wt%, indicating the presence of approximately 3.6 molecules of TMAda in the unit cell. The 27Al MAS NMR spectrum of the as-synthesized TieCHA
Fig. 6.
13
C CP/MAS NMR spectrum of TieCHA (sample 10).
Fig. 7. TGeDTA curve of TieCHA (sample 10).
(Fig. 8) shows only one peak located at approximately 57 ppm, and it corresponds to the tetrahedrally coordinated aluminum. This indicates that all the aluminum species present in the TieCHA existed within the zeolite framework. The BET surface area and the micropore volume of the TieCHA, calculated from the N2 adsorption isotherms (Fig. 9), were 837 m2 g1 and 0.32 cm3 g1, respectively, which are similar to those of high-silica CHA SSZ-13 (800 m2 g1 and 0.31 cm3 g1). Based on the above results, we conclude that highly crystalline TieCHA was synthesized by hydrothermal conversion of titanated FAU. Although the synthesis of TieCHA with and without aluminum has already been reported [30], to our best of knowledge, this is the first report concerning synthesis of TieCHA by hydrothermal conversion of TieFAU.
Fig. 8.
27
Al MAS NMR spectrum of TieCHA (sample 10).
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3.3. Thermal stability
Fig. 9. N2 adsorption isotherms of TieCHA (sample 10) and CHA (sample 11).
Next, we investigated the thermal stability of TieCHAs with different Si/Ti ratios. Fig. 10 shows the XRD patterns of TieCHAs (samples 6, 7, 9, and 10) after calcination at various temperatures. For reference, the thermal stabilities of CHAs synthesized from FAU and amorphous aluminosilicate hydrogel are also shown (samples 11 and 12(SSZ-13), respectively). The XRD peak intensities of samples 11 and 12 gradually decreased with increasing treatment temperature, and the framework structure nearly collapsed when it was calcined at 1050 C for 1 h. Although no reduction in the peak intensities was observed for TieCHA samples after thermal treatment in the 600e900 C range, there was a large reduction in the peak intensity for samples calcined at 1000 C. However, the magnitude of the reduction depended considerably on the Si/Ti ratio. Diffraction peaks for samples with Si/Ti ratios of 33 and 64 (samples 9 and 10) were still observed even after calcination at 1100 C. Fig. 11 shows the relationship between the calcination temperature and the relative crystallinity of TieCHAs after calcination. Therefore, it became clear that the thermal stability of the TieCHAs was strongly affected by the Si/Ti ratio. Of course, we cannot rule out completely the possibility that the samples with a particular low Al content possess the higher stability. From the fact that the thermal stability of Ti-CHA with a Si/Ti ratio of 19 and a Si/Al ratio of 27 (sample 7) was considerably
Fig. 10. XRD patterns of various CHAs after calcination for 1 h at (a) 500 C (H-form), (b) 600 C, (c) 700 C, (d) 800 C, (e) 900 C, (f) 1000 C, (g) 1050 C, (h) 1100 C, and (i) 1150 C. (A) CHA (SSZ-13, sample 12); (B) CHA (sample 11); (C) TieCHA (sample 10); (D) TieCHA (sample 9); (E) TieCHA (sample 7); (F) Ti-CHA (sample 6).
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Fig. 11. Relative crystallinity of TieCHA and CHA after calcination at various temperatures for 1 h C TieCHA (sample 10); B TieCHA (sample 9); >TieCHA (sample 7); A TieCHA (sample 6); : CHA (sample 11); - CHA (SSZ-13, sample 12).
higher than those of CHAs with a Si/Al ratio of 35 (samples 11 and 12), however, we have considered that the possibility is low. Also, taking into account the fact that there is no difference between the 29Si CP/MAS NMR spectra of TieCHA and non-modified CHA, as shown in Fig. 12, there was likely no difference in the number of structural defects [31]. Therefore, we speculate that the incorporation of a small amount of Ti into the framework may have decreased the stress of the zeolite framework, although the exact reason for the high thermal stability remains unclear at the present time.
3.4. NH3-SCR of NOx over Cu-loaded TieCHA As described above, the TieCHA synthesized by hydrothermal conversion of Ti-FAU in the presence of TMAda cations exhibited higher thermal stability than non-modified CHA. Therefore, we applied the TieCHA to the NH3-SCR of NOx. This reaction has been investigated on a variety of transition-metal ion-exchanged zeolite catalysts, especially Cu- and Fe-loaded zeolite catalysts [9e11]. In this study, we selected Cu ions as a catalytic active component and
Fig. 12. (A)
29
Si MAS NMR and (B)
29
investigated the performance of a Cu-loaded TieCHA catalyst [32,33]. The Cu loading was maintained at 1.5 wt% for all samples. Although the TieCHA with a Si/Al ratio of >70 (low ion-exchange capacity) exhibited high hydrothermal stability, from an industrial viewpoint regarding the preparation of highly active catalysts, the TieCHA with a Si/Al ratio of ~20 was employed, which can give a larger number of Cu ions as an active component. The analytical data of the fresh Cu-loaded zeolite catalysts are listed in Table 3. Only a slight reduction in the BET surface area and micropore volume was observed after Cu-loading, indicating that the pore blocking by Cu-loading hardly took place. As up to 10 vol% H2O is present in emissions from diesel engines, a catalyst's resistance against steaming is important for automotive applications. In particular, we have to consider the catalyst deactivation caused by significant damage of the crystal structure or migration of Cu ions by dealumination from the zeolite framework [9e11]. Therefore, to assess the long-term hydrothermal durability of the catalysts, we also investigated the change in the TieCHAs' catalytic performance after hydrothermal treatment at 900 C in a flowing gas containing 10 vol% H2O and 90 vol% air. Fig. 13 shows the NO conversion as a function of temperature over the Cu-loaded TieCHA and the Cu-loaded CHA catalysts (Table 3) before and after hydrothermal treatment at 900 C. The quantitative results of the outlet concentrations of NO, NO2, and NH3 were listed in Table S1. Above-95% NO conversion was attained by both fresh catalysts at 300 C. After hydrothermal treatment at 900 C for 1 h, the NO conversions over these catalysts were almost the same as before. However, the marked reduction in the NO conversion was observed for the Cu-loaded Ti-free CHA catalyst after hydrothermal treatment at 900 C for 4 h, while only a slight reduction in the NO conversion was observed for the Cu-loaded TieCHA catalyst. Fig. 14 shows the XRD patterns of the Cu-loaded TieCHA and the Cu-loaded CHA catalysts after NH3-SCR of NOx. Only a few small peaks were observed for the Cu-loaded CHA catalyst, indicating that the zeolite framework nearly collapsed, while such a large reduction in the peak intensities was not observed for the Cu-loaded TieCHA catalyst. The differences in the catalytic stability of Cu-loaded CHA before and after hydrothermal treatment seem to be similar to those reported by Chen et al. [32,33]. These results suggest that the Cu-loaded CHA catalyst's deactivation resulted from the structural collapse of the zeolite framework.
Si CP/MAS NMR spectra of (a) TieCHA (sample 10) and (b) CHA (sample 11).
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Table 3 Characteristics of Cu-loaded zeolite catalysts. Catalyst
Cu-loaded Ti-CHA Cu-loaded CHA a b c
Chemical compositionsa Si/Ti
Si/Al
Na/Al
Cu/Al
19.0
25.5 17.9
0.005 0.002
0.34 0.24
BET surface areab/m2 g1
Micropore volumec/cm3 g1
667 722
0.28 0.28
Measured by ICP. Determined by the BET method. Determined by the t-plot.
For a reference, the NH3-SCR of NOx was also carried out over Cu-loaded Ti-FAU catalyst (Si/Ti ¼ 23.6, Si/Al ¼ 36.9, Cu/Al ¼ 0.52, Na/Al ¼ 0.006) (Fig. S4). As compared to the Cu-loaded Ti-CHA (Fig. 13(A)), the NO conversions in low reaction temperatures were
considerably lower and the marked reduction in the NO conversion was observed after hydrothermal treatment at 900 C. From the fact that no reduction in the XRD peak intensities was observed for the catalyst after the reaction over the hydrothermally treated Cu-
Fig. 13. NO conversion over (A) Cu-loaded TieCHA and (B) Cu-loaded CHA catalysts. fresh catalyst; C catalyst after hydrothermal treatment at 900 C for 1 h; A catalyst after hydrothermal treatment at 900 C for 4 h.
Fig. 14. XRD patterns of (a) fresh catalyst and (b) catalyst after NH3-SCR of NOx over hydrothermally treated catalyst at 900 C for 4 h. (A) Cu-loaded TieCHA catalyst; (B) Cu-loaded CHA catalyst.
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loaded Ti-FAU at 900 C for 4 h (Fig. S5), it is suggested that the Culoaded Ti-FAU catalyst's deactivation is due to the migration/aggregation of Cu ions. The above results provide strong indication that the Cu-loaded TieCHA catalyst had high hydrothermal stability. Although it has been reported that phosphorus-modification is very effective in improving the thermal and hydrothermal stability of a zeolite catalyst [34,35], to our knowledge, this is the first report of high hydrothermal stability of a Cu-loaded CHA modified with Ti. 4. Conclusions We investigated the effect of Ti incorporation into the zeolite framework on the thermal stability of CHA, prepared by hydrothermal conversion of FAU in the presence of TMAda cation as an OSDA. TieCHAs with various Si/Al and Si/Ti ratios were synthesized from titanated FAU (TieFAU) by optimizing the NaOH/SiO2 and H2O/SiO2 ratios in the starting reaction mixtures. The thermal stability of the TieCHAs was higher than that of non-modified CHAs synthesized from FAU or amorphous aluminosilicate hydrogels; the thermal stability of the TieCHAs was also strongly affected by the Si/Ti ratio. The framework structure of TieCHAs with Si/Ti ratios of 33 and 64 was unchanged even after calcination at 1000 C for 1 h, indicating the enhanced thermal stability by Ti modification. We also prepared Cu-loaded zeolite catalysts using TieCHA and non-modified CHA and investigated their catalytic performance for the NH3-SCR of NOx. Although there was no difference between the NO conversion on the two fresh catalysts, the NO conversion on the Cu-loaded TieCHA catalyst after hydrothermal treatment 900 C for 4 h was considerably higher than that on the Cu-loaded CHA catalyst, indicating the higher hydrothermal stability of the Cu-loaded TieCHA catalyst. We believe that the findings reported in this study are very useful to improve the thermal/hydrothermal stability of zeolite. Acknowledgment A part of this paper is the result of the research project sponsored by the Research Association of Automotive Internal Combustion Engines (AICE) including the subsidy from the Ministry of Economy, Trade and Industry (METI) through the project expense of R&D for the advancement of the clean diesel engine technology granted for fiscal year 2014. The authors gratefully acknowledge the concerned personnel. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.micromeso.2015.05.023.
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Synthesis of titanated chabazite with enhanced thermal stability by hydrothermal conversion of titanated faujasite Yusuke Kunitake a, Tomoka Takata a, Yoshitaka Yamasaki a, Naoki Yamanaka a, Nao Tsunoji a, Yasuyuki Takamitsu b, Masahiro Sadakane a, Tsuneji Sano a, * a b
Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan Inorganic Materials Research Laboratory, Tosoh Corporation, Shunan, Yamaguchi 746-8501, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 January 2015 Received in revised form 31 March 2015 Accepted 11 May 2015 Available online 19 May 2015
Titanated chabazites (TieCHAs) with various Si/Al and Si/Ti ratios were prepared from titanated faujasite (TieFAU) by optimizing the NaOH/SiO2 and H2O/SiO2 ratios in the starting reaction mixtures. The peak assigned to isolated tetrahedrally coordinated Ti species was clearly observed at ~220 nm in the UVevis spectra and at ~960 cm1 in the FT-IR spectra of the TieCHA, confirming that Ti was incorporated in the zeolite framework. The thermal stability of the TieCHAs was higher than that of non-modified CHAs synthesized from FAU or amorphous aluminosilicate hydrogels. The framework structure of TieCHAs with Si/Ti ratios of 33 and 64 was unchanged even after calcination at 1000 C for 1 h, indicating the enhanced thermal stability by Ti modification. We also prepared Cu-loaded zeolite catalysts using Ti eCHA and non-modified CHA and investigated their catalytic performance for the selective catalytic reduction of NOx by ammonia. Although there was no difference between the NO conversion on the two fresh catalysts, the NO conversion on the Cu-loaded TieCHA catalyst after hydrothermal treatment at 900 C for 4 h was considerably higher than that on the Cu-loaded CHA catalyst, indicating the higher hydrothermal stability of Cu-loaded TieCHA catalyst. © 2015 Elsevier Inc. All rights reserved.
Keywords: Hydrothermal conversion Titanated faujasite Titanated chabazite Thermal/hydrothermal stability Selective catalytic reduction of NOx by ammonia
1. Introduction Chabazite (CHA), an aluminosilicate mineral in the zeolite family, has a three-dimensional pore system with ellipsoidal large cages (6.7 10 Å) that are accessible via eight-ring windows (free diameter: 3.8 3.8 Å). The high-silica chabazite, SSZ-13, with a Si/ Al ratio above 5, has attracted significant interest owing to its thermal and acid stabilities. There have been extensive investigations on applications of this material in catalysts for selective conversion of methanol [1e3] or bioethanol [4,5] into light olefins such as ethylene and propylene, for selective catalytic reduction (SCR) of NOx by ammonia (NH3-SCR of NOx) [6e11], and as zeolite membranes for dehydration of aqueous organic solutions [12e15]. In order to improve the physicochemical properties of CHAs, researchers are attempting to develop improved methods for the synthesis of high-silica CHAs.
* Corresponding author. Tel.: þ81 82 424 7607; fax: þ81 82 424 5494. E-mail address: [email protected] (T. Sano). http://dx.doi.org/10.1016/j.micromeso.2015.05.023 1387-1811/© 2015 Elsevier Inc. All rights reserved.
We investigated the potential of hydrothermal conversion from one zeolite into another, interzeolite conversion, or interzeolite transformation [16e18], and found that the conversion or transformation approach is an attractive strategy for zeolite synthesis [19]. Very recently, we successfully prepared high-silica CHAs with Si/Al ratios of 5e21 from faujasite (FAU) in the presence of benzyltrimethylammonium hydroxide (BTMAOH) or N,N,N-trimethyl1-adamantammonium hydroxide (TMAdaOH) as an organic structure-directing agent (OSDA). The number of structural defects in CHAs prepared by the hydrothermal conversion method was smaller compared to CHAs synthesized from amorphous aluminosilicate hydrogel (the conventional synthesis route used as a reference), resulting in the higher acid stability [15,20]. However, the thermal and hydrothermal stabilities of CHAs must be improved before they are ready for further applications. In general, the physicochemical properties of zeolites depend on the number of tetrahedrally coordinated Al atoms in the zeolite framework. In order to improve the physicochemical properties of zeolites, isomorphous substitution of various metals such as Ti, V, Ga, Fe, B, and Zn for Al in the framework has been widely studied [21e23]. Isomorphous substitution is achieved by direct
Y. Kunitake et al. / Microporous and Mesoporous Materials 215 (2015) 58e66
hydrothermal synthesis or by post-synthesis treatment. Since the first report by Tarausso et al. of direct synthesis of a Ti-incorporated zeolite (titanosilicate), TS-1 with an MFI structure, there have been many investigations into the synthesis, characterization, and catalytic applications of titanosilicates [24e26]. We also reported that Ti-incorporated FAU (TieFAU) can be prepared by post-synthesis treatment of dealuminated FAU with an aqueous solution of (NH4)2TiF6 [27]. In this study, we investigated the effect of Ti incorporation into the zeolite framework on the thermal stability of CHA prepared by hydrothermal conversion of FAU in the presence of TMAda cations as an OSDA. We also prepared a Cu-loaded TieCHA catalyst and investigated its catalytic performance for the NH3-SCR of NOx. 2. Experimental 2.1. Dealumination and titanation of FAU The FAU used in this work was prepared from the zeolite NH4eY (Si/Al ¼ 2.8, Tosoh Co., Japan) through dealumination that involved a combination of steaming at 700 C and H2SO4 treatment at room temperature for 24 h. The XRD pattern of the dealuminated zeolite showed no peaks other than those corresponding to the FAU structure. The particle size of the dealuminated FAU was 0.2e0.5 mm. Incorporation of Ti into the dealuminated FAU was carried out by a post-synthesis method under acidic conditions [27]. The dealuminated FAU (1 g) was added to (NH4)2TiF6 (Aldrich, USA) solutions (100 mL) with different concentrations. The molecular size of a TiF6 anion was estimated to be ~6.2 Å by density functional theory (DFT) quantum chemical calculation (DMol3 Ver. 5.0 provided by Accelrys, Inc., USA), which allows the diffusion of TiF6 anions into the supercages (~7.4 Å) of FAU. To maintain the pH value of the suspension (pH ¼ 1) during titanation, we added a certain amount of a 30 wt% sulfuric acid solution to the suspension. The mixtures were stirred at room temperature for 24 h and the samples were filtered, washed thoroughly with hot deionized water (60 C), and dried at 120 C. The samples were then calcined at 500 C for 10 h, yielding TieFAU samples. Table 1 lists the postsynthesis treatment conditions of FAUs with various Si/Al ratios. 2.2. Hydrothermal conversion of TieFAU into TieCHA The hydrothermal conversion of TieFAU into TieCHA was performed as follows. First, TieFAU was thoroughly mixed with an aqueous solution containing TMAdaOH (25 wt%, Sachem Asia Co., Ltd., Japan) as an OSDA, seed crystals (3 wt%, Si/Al ¼ 20), and NaOH (>99%, Merck Chemicals Inc., Japan). The mixture was placed into a 30-cm3 Teflon-lined stainless steel autoclave. The hydrothermal conversion was then conducted at 125 C for 2 days in a convection oven. The solid product was collected by centrifugation and washed thoroughly with deionized water until it was nearly neutral. It was
Table 1 Preparation of TieFAUs from FAUs with various Si/Al ratios.a Sample
Treatment conditions b
1 2 3 4 5 a b
TieFAU
Si/Al of FAU
(NH4)2TiF6/mmol
30 wt% H2SO4/g
Si/Tib
Si/Alb
2.8 21 46 46 46
1.97 1.97 1.97 1.97 0.90
7.2 0.3 1.0 1.0 1.0
20 22 38 40 63
49 39 107 119 146
FAU: 3 g; H2O: 950 mL; Temperature ¼ room temperature, Time ¼ 24 h. Measured by ICP.
59
then dried overnight at 70 C. Table 2 lists the chemical compositions of starting gels and the hydrothermal-conversion conditions. 2.3. Preparation of Cu-loaded TieCHA catalyst Before the metal loading, Na cations in TieCHA (sample 7) were removed by ion exchange with an aqueous solution of NH4Cl. The obtained NH4-form was then calcined at 550 C for 1 h, yielding the H-form. The Cu-loaded zeolite catalyst was prepared by the impregnation method as follows: an aqueous solution containing a certain amount of Cu(NO3)2$3H2O (Kishida Chemical Co. Ltd., Japan) was added to the H-form zeolite and mixed thoroughly in a ceramic mortar. The resultant wet powder was dried at 110 C and then calcinated at 550 C for 1 h. For a reference, the Cu-loaded CHA catalyst was also prepared from the non-modified CHA zeolite synthesized by hydrothermal conversion of FAU. The Cu loading was maintained at 1.5 wt% for all samples. 2.4. Characterization Powder X-ray diffraction (XRD) patterns of the solid products were collected on a diffractometer (Mini Flex, Rigaku, Japan) with a curved-graphite monochromator, using Cu Ka radiation, and operated at 30 kV and 15 mA. The Si/Ti, Si/Al, Na/Al, and Cu/Al ratios were determined using inductively coupled plasma optical emission spectroscopy (ICP-OES; SPS7000, Seiko, Japan). The crystal morphology was observed using a scanning electron microscope (SEM; S-4800, Hitachi, Japan) coupled with an energy-dispersive Xray (EDX) analyzer. Magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectra of 27Al and 29Si were recorded at 104.2 and 79.5 MHz, respectively, on a solid NMR spectrometer (600 PS, Varian, USA), using a 3.2-mm-diameter zirconia rotor at a rotational speed of 15 kHz for 27Al MAS NMR and a 6-mm-diameter zirconia rotor at 6 kHz for 29Si MAS NMR. The spectra were acquired using 2.3-ms pulses, a 1- s recycle delay, and 1000 scans for 27Al, and 5-ms pulses, a 100-s recycle delay, and 1000 scans for 29Si. As chemical shift references for 27Al and 29Si MAS NMR, Al(NO3)3$9H2O and Si(CH3)4 were used, respectively. Prior to the 27Al MAS NMR measurements, the samples were moisture-equilibrated over a saturated solution of NH4Cl for 24 h. Cross-polarized (CP) MAS NMR spectra of 1He13C were measured with a spinning frequency of 20 kHz, a 90 pulse length of 2.2 ms, and a cycle delay time of 50 s. Hexamethylbenzene was used as a chemical-shift reference. Thermal analysis was performed using a thermogravimetry and differential thermal analysis (TGeDTA) apparatus (SSC/5200, Seiko Instruments, Japan). The sample (~7 mg) was heated in flowing air (50 mL min1) at 5 C min1 from room temperature to 800 C. Nitrogen adsorption isotherms were obtained at 196 C using a conventional volumetric apparatus (BELSORP-mini, Bel Japan, Inc., Japan). Prior to the adsorption measurements, the samples (~0.1 g) were evacuated at 400 C for 10 h. Ultravioletevisible light (UVevis) diffuse reflectance spectra of the zeolites were recorded on a UVevis spectrometer (V-570, JASCO, Japan) with a bandwidth of 10 mm and a scan speed of 400 nm min1. Fourier-transform infrared (FT-IR) spectra were also measured by the KBr technique at room temperature on an FT-IR spectrometer (NICOLET 6700, Thermo Scientific, USA) with a resolution of 4 cm1. 2.5. Thermal stability The thermal stability of TieCHAs was evaluated by comparing the change in the intensities of the XRD peaks at 2q ¼ 18.2, 21.0, 25.4, 26.4, and 31.1 of samples before and after calcination at 6001150 C for 1 h in air. The relative crystallinity (RC) was determined as follows:
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RC ð%Þ ¼
Sum of intensities of the peaks at 2q ¼ 18:2; 21:0; 25:4; 26:4 and 31:1 after calcination at various temperatures for 1 h Sum of intensities of the peaks at 2q ¼ 18:2; 21:0; 25:4; 26:4 and 31:1 before calcination 100 (1)
2.6. NH3-SCR of NOx activity test
3. Results and discussion
The reactions for NH3-SCR of NOx were carried out in a fixed-bed flow reactor under atmospheric pressure. The Cu-loaded TieCHA catalyst was pressed and sieved until it reached approximately 1 mm in diameter. The required amount of catalyst (1.5 mL, ~0.8 g) was retained by placing quartz wool at the center of a quartz reactor. A thermocouple inserted at the center of the catalyst bed was used to measure the temperature during the reaction. The reactant gas was composed of 200 ppm NO, 200 ppm NH3, 10 vol% O2, 3 vol% H2O, and balance N2 (NO þ NH3 þ O2 þ H2O þ N2 ¼ 100 vol%), where the gas composition was based on emissions from diesel engines. The total flow rate was fixed at 1.5 L min1 and the GHSV (gas hourly space velocity) value was 60,000 h1. During the experiments, the temperature was varied from 500 to 150 C in steps of approximately 50 C. The NO conversion was defined as follows:
3.1. Preparation and characterization of TieFAU
NOin NOout NO2out 100 NOin
NO conversionð%Þ ¼
Titanation of the dealuminated FAUs with various Si/Al ratios was carried out by varying the amounts of (NH4)2TiF6 and 30 wt% H2SO4 in the starting reaction mixtures. As shown in Table 1, TieFAUs with various Si/Ti and Si/Al ratios were obtained. Fig. 1(a) and (b) show the XRD patterns of the starting dealuminated FAU with a Si/Al ratio of 46 and the resulting TieFAU (sample 5). There were no peaks other than those corresponding to the FAU structure. The yield of TieFAU was ~80% based on the weight of the starting FAU. As seen in SEM images of the starting FAU and the prepared TieFAU (sample 5) in Fig. 2(a) and (b), the morphology of the TieFAU crystals was almost the same as that of the starting FAU crystals. The BrunauereEmmetteTeller (BET) surface area and the micropore volume of TieFAU calculated from the N2 adsorption isotherm were 835 m2 g1 and 0.32 cm3 g1, respectively, which are similar to those of commercial zeolite Y (829 m2 g1 and 0.34 cm3 g1). Fig. 3 displays the UVevis spectra of the starting FAU and the TieFAUs with different Si/Ti ratios (samples 2 and 5). Although no peaks were observed in the spectrum of the starting FAU, the peak from isolated tetrahedrally coordinated Ti species is clearly visible at ~220 nm in the spectrum of TieFAU with a Si/Ti ratio of 63 (sample 5), indicating incorporation of Ti into the zeolite framework [27]. In the spectrum of TieFAU with a larger amount of Ti (sample 2, Si/Ti ratio of 22), however, a shoulder peak appears in the 280e330 nm range together with the 220-nm peak, indicating the presence of octahedrally coordinated Ti species or TiO2, or both octahedrally coordinated Ti species and TiO2.
(2)
where NOin represents the NO inlet concentration (200 ppm), and NOout and NO2out represent the NO and NO2 outlet concentrations, respectively. To evaluate the steady state catalytic activity, the concentrations of NH3, NO, and NO2 in the outlet gas after 10 min of the time-on-stream at each reaction temperature were analyzed by an FT-IR spectrometer (FT/IR-6100, JASCO, Japan) equipped with a gas cell (LPC-12MS, 12m) and a mercury cadmium telluride detector cooled by liquid nitrogen. The concentrations were determined by the intensities of the peaks at 1033, 1875, and 2917 cm1 for NH3, NO, and NO2, respectively (see Supporting Information; Fig. 1S and Fig. S2). Thirty scans were averaged for each normalized spectrum.
3.2. Hydrothermal conversion of TieFAU into TieCHA If the hydrothermal conversion of the TieFAU was carried out in the absence of seed crystals by varying the H2O/SiO2, TMAdaOH/
Table 2 Synthesis of TieCHAs from TieFAUs with various Si/Ti and Si/Al ratios.a Sample
Synthesis conditions
6 7 8 9 10 11e 12f
TieCHA
Si/Ti of TieFAU
Si/Al of TieFAU
NaOH/SiO2
H2O/SiO2
Yield/%
Si/Tib
Si/Alb
Si/(Al þ Ti)
BET surface areac/m2 g1
Micropore volumed/cm3 g1
22 20 38 40 63
39 49 107 119 146 48(FAU) 35(Am.)
0.35 0.35 0.2 0.25 0.25 0.1 0.2
25 25 27 27 17 5 5
55 60 80 75 80 88 99
14 19 30 33 64
18 27 76 71 89 35 35
7.9 11.2 21.5 22.5 37.2
673 649 643 679 837 887 800
0.25 0.22 0.22 0.19 0.32 0.32 0.31
b
b
TMAdaOH/SiO2 ¼ 0.2; Seed (Si/Al ¼ 20) ¼ 3 wt%; Temperature ¼ 125 C; Time ¼ 2 days. Measured by ICP. c Determined by the BET method. d Determined by the t-plot. e FAU without Ti was used as Si and Al sources instead of TieFAU. f Amorphous aluminosilicate hydrogel prepared from fumed silica (Cab-O-Sil M5, Cabot, USA) and Al(OH)3 (Wako Pure Chemical Ind. Ltd., Japan) was used as Si and Al sources instead of TieFAU. a
b
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Fig. 1. XRD patterns of (a) starting dealuminated FAU (Si/Al ¼ 46), (b) TieFAU (sample 5), (c) TieCHA (sample 6), and (d) TieCHA (sample 10).
SiO2, and NaOH/SiO2 ratios in the starting reaction mixtures, highly crystalline TieCHA could not be obtained (results not shown here). Therefore, the following experiments were carried out using seed crystals. Table 2 lists the chemical compositions of starting reaction mixtures and the characteristics of TieCHAs obtained. TieCHAs with various Si/Ti and Si/Al ratios were prepared from Ti-FAUs by optimizing the NaOH/SiO2 and H2O/SiO2 ratios in the starting reaction mixtures. In Fig. 1(c) and (d), XRD patterns of the TieCHA with different Si/Ti and Si/Al ratios show the typical diffraction patterns of the CHA structure, which contained no impurities from the unconverted starting TieFAU or from a co-crystallized phase. As can be seen in Fig. 2(c) and (d), the CHA crystals exhibited a pseudo cubic morphology, which is the same as that of conventional aluminosilicate CHA. Fig. 4 shows the UVevis spectra of these TieCHAs with different Si/Ti ratios. For reference, the spectrum of the starting TieFAU is also displayed. In the UVevis spectrum of TieCHA with a Si/Ti ratio of 64 (sample 10), the peak from isolated
Fig. 2. SEM images of (a) starting dealuminated FAU (Si/Al ¼ 46), (b) Ti-FAU (sample 5), (c) Ti-CHA (sample 6), and (d) Ti-CHA (sample 10).
61
Fig. 3. UVevis diffuse reflectance spectra of (a) starting dealuminated FAU (Si/Al ¼ 46), (b) TieFAU (sample 5), and (c) TieFAU (sample 2).
tetrahedrally coordinated Ti species appears at ~220 nm, indicating that Ti was present in the zeolite framework. However, a weak shoulder was observed at ~280 nm, indicating the presence of a trace amount of octahedrally coordinated Ti species or TiO2, or both octahedrally coordinated Ti species and TiO2. On the other hand, in the case of TieCHA with a Si/Ti ratio of 14 (sample 6), the more clear shoulder peak is in the 280e330 nm range together with the 220nm peak. A reduction in the BET surface area and micropore volume also suggest the possibility of pore blocking by the formation of Ti species outside the framework. In the FT-IR spectrum of Ti-CHA (Fig. S3), a weak peak assigned to the stretching vibration of
Fig. 4. UVevis diffuse reflectance spectra of (a) TieFAU (sample 5), (b) TieCHA (sample 10), and (c) TieCHA (sample 6).
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Y. Kunitake et al. / Microporous and Mesoporous Materials 215 (2015) 58e66
Fig. 5. SEM image of TieCHA (sample 10) and corresponding elemental mappings.
tetrahedrally coordinated Ti species was observed at ca. 960 cm1 [28,29], which also indicates the presence of tetrahedrally coordinated Ti species. Fig. 5 shows an SEM image of TieCHA with a Si/Ti ratio of 64 and the corresponding elemental mappings. It can be seen that titanium atoms were evenly distributed in the crystals. In the 13C CP/MAS NMR spectrum of an as-synthesized TieCHA, sample 10 (Fig. 6), there are four peaks detected at approximately 32, 36, 48, and 73 ppm. The resonances at 32 and 36 ppm are assigned to H CC3 and C CH2 C, respectively, of TMAda cations, while the resonance at 48 ppm is assigned to NðCH3 Þ3 . The resonance at 73 ppm corresponds to N C C3 . Therefore, the TMAda cations were the only organic species existing in the TieCHA pores. The weight loss between 250 and 650 C in the thermogravimetric curve (Fig. 7), which corresponds to the total decomposition of organic moieties, was about 23.7 wt%, indicating the presence of approximately 3.6 molecules of TMAda in the unit cell. The 27Al MAS NMR spectrum of the as-synthesized TieCHA
Fig. 6.
13
C CP/MAS NMR spectrum of TieCHA (sample 10).
Fig. 7. TGeDTA curve of TieCHA (sample 10).
(Fig. 8) shows only one peak located at approximately 57 ppm, and it corresponds to the tetrahedrally coordinated aluminum. This indicates that all the aluminum species present in the TieCHA existed within the zeolite framework. The BET surface area and the micropore volume of the TieCHA, calculated from the N2 adsorption isotherms (Fig. 9), were 837 m2 g1 and 0.32 cm3 g1, respectively, which are similar to those of high-silica CHA SSZ-13 (800 m2 g1 and 0.31 cm3 g1). Based on the above results, we conclude that highly crystalline TieCHA was synthesized by hydrothermal conversion of titanated FAU. Although the synthesis of TieCHA with and without aluminum has already been reported [30], to our best of knowledge, this is the first report concerning synthesis of TieCHA by hydrothermal conversion of TieFAU.
Fig. 8.
27
Al MAS NMR spectrum of TieCHA (sample 10).
Y. Kunitake et al. / Microporous and Mesoporous Materials 215 (2015) 58e66
63
3.3. Thermal stability
Fig. 9. N2 adsorption isotherms of TieCHA (sample 10) and CHA (sample 11).
Next, we investigated the thermal stability of TieCHAs with different Si/Ti ratios. Fig. 10 shows the XRD patterns of TieCHAs (samples 6, 7, 9, and 10) after calcination at various temperatures. For reference, the thermal stabilities of CHAs synthesized from FAU and amorphous aluminosilicate hydrogel are also shown (samples 11 and 12(SSZ-13), respectively). The XRD peak intensities of samples 11 and 12 gradually decreased with increasing treatment temperature, and the framework structure nearly collapsed when it was calcined at 1050 C for 1 h. Although no reduction in the peak intensities was observed for TieCHA samples after thermal treatment in the 600e900 C range, there was a large reduction in the peak intensity for samples calcined at 1000 C. However, the magnitude of the reduction depended considerably on the Si/Ti ratio. Diffraction peaks for samples with Si/Ti ratios of 33 and 64 (samples 9 and 10) were still observed even after calcination at 1100 C. Fig. 11 shows the relationship between the calcination temperature and the relative crystallinity of TieCHAs after calcination. Therefore, it became clear that the thermal stability of the TieCHAs was strongly affected by the Si/Ti ratio. Of course, we cannot rule out completely the possibility that the samples with a particular low Al content possess the higher stability. From the fact that the thermal stability of Ti-CHA with a Si/Ti ratio of 19 and a Si/Al ratio of 27 (sample 7) was considerably
Fig. 10. XRD patterns of various CHAs after calcination for 1 h at (a) 500 C (H-form), (b) 600 C, (c) 700 C, (d) 800 C, (e) 900 C, (f) 1000 C, (g) 1050 C, (h) 1100 C, and (i) 1150 C. (A) CHA (SSZ-13, sample 12); (B) CHA (sample 11); (C) TieCHA (sample 10); (D) TieCHA (sample 9); (E) TieCHA (sample 7); (F) Ti-CHA (sample 6).
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Fig. 11. Relative crystallinity of TieCHA and CHA after calcination at various temperatures for 1 h C TieCHA (sample 10); B TieCHA (sample 9); >TieCHA (sample 7); A TieCHA (sample 6); : CHA (sample 11); - CHA (SSZ-13, sample 12).
higher than those of CHAs with a Si/Al ratio of 35 (samples 11 and 12), however, we have considered that the possibility is low. Also, taking into account the fact that there is no difference between the 29Si CP/MAS NMR spectra of TieCHA and non-modified CHA, as shown in Fig. 12, there was likely no difference in the number of structural defects [31]. Therefore, we speculate that the incorporation of a small amount of Ti into the framework may have decreased the stress of the zeolite framework, although the exact reason for the high thermal stability remains unclear at the present time.
3.4. NH3-SCR of NOx over Cu-loaded TieCHA As described above, the TieCHA synthesized by hydrothermal conversion of Ti-FAU in the presence of TMAda cations exhibited higher thermal stability than non-modified CHA. Therefore, we applied the TieCHA to the NH3-SCR of NOx. This reaction has been investigated on a variety of transition-metal ion-exchanged zeolite catalysts, especially Cu- and Fe-loaded zeolite catalysts [9e11]. In this study, we selected Cu ions as a catalytic active component and
Fig. 12. (A)
29
Si MAS NMR and (B)
29
investigated the performance of a Cu-loaded TieCHA catalyst [32,33]. The Cu loading was maintained at 1.5 wt% for all samples. Although the TieCHA with a Si/Al ratio of >70 (low ion-exchange capacity) exhibited high hydrothermal stability, from an industrial viewpoint regarding the preparation of highly active catalysts, the TieCHA with a Si/Al ratio of ~20 was employed, which can give a larger number of Cu ions as an active component. The analytical data of the fresh Cu-loaded zeolite catalysts are listed in Table 3. Only a slight reduction in the BET surface area and micropore volume was observed after Cu-loading, indicating that the pore blocking by Cu-loading hardly took place. As up to 10 vol% H2O is present in emissions from diesel engines, a catalyst's resistance against steaming is important for automotive applications. In particular, we have to consider the catalyst deactivation caused by significant damage of the crystal structure or migration of Cu ions by dealumination from the zeolite framework [9e11]. Therefore, to assess the long-term hydrothermal durability of the catalysts, we also investigated the change in the TieCHAs' catalytic performance after hydrothermal treatment at 900 C in a flowing gas containing 10 vol% H2O and 90 vol% air. Fig. 13 shows the NO conversion as a function of temperature over the Cu-loaded TieCHA and the Cu-loaded CHA catalysts (Table 3) before and after hydrothermal treatment at 900 C. The quantitative results of the outlet concentrations of NO, NO2, and NH3 were listed in Table S1. Above-95% NO conversion was attained by both fresh catalysts at 300 C. After hydrothermal treatment at 900 C for 1 h, the NO conversions over these catalysts were almost the same as before. However, the marked reduction in the NO conversion was observed for the Cu-loaded Ti-free CHA catalyst after hydrothermal treatment at 900 C for 4 h, while only a slight reduction in the NO conversion was observed for the Cu-loaded TieCHA catalyst. Fig. 14 shows the XRD patterns of the Cu-loaded TieCHA and the Cu-loaded CHA catalysts after NH3-SCR of NOx. Only a few small peaks were observed for the Cu-loaded CHA catalyst, indicating that the zeolite framework nearly collapsed, while such a large reduction in the peak intensities was not observed for the Cu-loaded TieCHA catalyst. The differences in the catalytic stability of Cu-loaded CHA before and after hydrothermal treatment seem to be similar to those reported by Chen et al. [32,33]. These results suggest that the Cu-loaded CHA catalyst's deactivation resulted from the structural collapse of the zeolite framework.
Si CP/MAS NMR spectra of (a) TieCHA (sample 10) and (b) CHA (sample 11).
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Table 3 Characteristics of Cu-loaded zeolite catalysts. Catalyst
Cu-loaded Ti-CHA Cu-loaded CHA a b c
Chemical compositionsa Si/Ti
Si/Al
Na/Al
Cu/Al
19.0
25.5 17.9
0.005 0.002
0.34 0.24
BET surface areab/m2 g1
Micropore volumec/cm3 g1
667 722
0.28 0.28
Measured by ICP. Determined by the BET method. Determined by the t-plot.
For a reference, the NH3-SCR of NOx was also carried out over Cu-loaded Ti-FAU catalyst (Si/Ti ¼ 23.6, Si/Al ¼ 36.9, Cu/Al ¼ 0.52, Na/Al ¼ 0.006) (Fig. S4). As compared to the Cu-loaded Ti-CHA (Fig. 13(A)), the NO conversions in low reaction temperatures were
considerably lower and the marked reduction in the NO conversion was observed after hydrothermal treatment at 900 C. From the fact that no reduction in the XRD peak intensities was observed for the catalyst after the reaction over the hydrothermally treated Cu-
Fig. 13. NO conversion over (A) Cu-loaded TieCHA and (B) Cu-loaded CHA catalysts. fresh catalyst; C catalyst after hydrothermal treatment at 900 C for 1 h; A catalyst after hydrothermal treatment at 900 C for 4 h.
Fig. 14. XRD patterns of (a) fresh catalyst and (b) catalyst after NH3-SCR of NOx over hydrothermally treated catalyst at 900 C for 4 h. (A) Cu-loaded TieCHA catalyst; (B) Cu-loaded CHA catalyst.
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loaded Ti-FAU at 900 C for 4 h (Fig. S5), it is suggested that the Culoaded Ti-FAU catalyst's deactivation is due to the migration/aggregation of Cu ions. The above results provide strong indication that the Cu-loaded TieCHA catalyst had high hydrothermal stability. Although it has been reported that phosphorus-modification is very effective in improving the thermal and hydrothermal stability of a zeolite catalyst [34,35], to our knowledge, this is the first report of high hydrothermal stability of a Cu-loaded CHA modified with Ti. 4. Conclusions We investigated the effect of Ti incorporation into the zeolite framework on the thermal stability of CHA, prepared by hydrothermal conversion of FAU in the presence of TMAda cation as an OSDA. TieCHAs with various Si/Al and Si/Ti ratios were synthesized from titanated FAU (TieFAU) by optimizing the NaOH/SiO2 and H2O/SiO2 ratios in the starting reaction mixtures. The thermal stability of the TieCHAs was higher than that of non-modified CHAs synthesized from FAU or amorphous aluminosilicate hydrogels; the thermal stability of the TieCHAs was also strongly affected by the Si/Ti ratio. The framework structure of TieCHAs with Si/Ti ratios of 33 and 64 was unchanged even after calcination at 1000 C for 1 h, indicating the enhanced thermal stability by Ti modification. We also prepared Cu-loaded zeolite catalysts using TieCHA and non-modified CHA and investigated their catalytic performance for the NH3-SCR of NOx. Although there was no difference between the NO conversion on the two fresh catalysts, the NO conversion on the Cu-loaded TieCHA catalyst after hydrothermal treatment 900 C for 4 h was considerably higher than that on the Cu-loaded CHA catalyst, indicating the higher hydrothermal stability of the Cu-loaded TieCHA catalyst. We believe that the findings reported in this study are very useful to improve the thermal/hydrothermal stability of zeolite. Acknowledgment A part of this paper is the result of the research project sponsored by the Research Association of Automotive Internal Combustion Engines (AICE) including the subsidy from the Ministry of Economy, Trade and Industry (METI) through the project expense of R&D for the advancement of the clean diesel engine technology granted for fiscal year 2014. The authors gratefully acknowledge the concerned personnel. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.micromeso.2015.05.023.
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