富山大学研究推進機構水素同位体科学研究センター研究報告 39：17－24，2019.

ノート

低 H2O/SiO2 比における純シリカ CHA 型ゼオライトの合成

田口

富山大学

明*，中森

研究推進機構拓実

水素同位体科学研究センター〒930-8555 富山市五福 3190

Pure-silica CHA-type zeolites synthesized using low H2O/SiO2 ratios

Akira Taguchi* and Takumi Nakamori

Hydrogen Isotope Research Center
Organization for Promotion of Research, University of Toyama
Gofuku 3190, Toyama 930-8555(Received March 30, 2020; accepted August 7, 2020)

Abstract
Highly crystalline pure-SiO2 zeolites with CHA-type framework were prepared from threedifferent silica sources of an alkoxide (tetraethyl orthosilicate), colloidal silica (AS-40) andfumed silica (Cab-O-Sil M5) in fluoride media. Single-phase CHA-type zeolites weresuccessfully obtained with H2O/SiO2 molar ratios in the range of 1.2 – 1.3 in the final gel.

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Research note
Introduction
Aluminosilicate zeolites with chabazite (CHA)-type framework topology are known tohave three-dimensional pore systems with ellipsoidal cages that are accessible via eightmembered windows with dimensions of 0.38×0.38 nm [1]. Both a high-silica CHA-type
aluminosilicate (SSZ-13) and a silicoaluminophospate (SAPO-34) have attracted significant
attention as potential catalysts for the selective catalytic reduction of NOx [2] and the methanolto-olefin conversion [3], respectively. We have also reported the possible hydrogen isotope (H2and D2) separation of Na-CHA prepared by the interzeolite conversion method [4]. In addition,pure-silica zeolites are highly important because their adsorption and diffusion properties arecomplementary to those of the aluminosilicate zeolites [5]. As an example, while the hydrogenstorage capacity of the pure-SiO2 CHA zeolite is insufficient for mobile applications (2.86mass%) [6], this material shows an intrinsic property in propylene/propane separation [7, 8].
The pure-silica CHA zeolite is typically synthesized using the fluoride anion as amineralizer, along with tetraethyl orthosilicate [9] or colloidal silica as silica sources [10, 11].
A single crystalline CHA phase has been found to appear over a very narrow range of watercontent (that is, a narrow range of H2O/SiO2 ratios) between 3.0 and 5.3 [9, 10]. Either loweror higher H2O content produce a mixture of CHA and STT-type zeolites [10, 12], and so careful
control over the synthesis conditions is required to obtain the desired product.
The present paper describes the preparation of pure-SiO2 CHA zeolites from an alkoxide,
colloidal silica and fumed silica. Although the preparation conditions were not fully optimized,
highly crystalline pure-SiO2 CHAs were obtained regardless of the silica source. Pure-SiO2
CHA zeolites could be synthesized under the preparation conditions with low H2O/SiO2 molar
ratios of approximately 1.2 – 1.3.

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低 H2O/SiO2 比における純シリカ CHA 型ゼオライトの合成

In this work, pure-SiO2 CHAs were synthesized according to the procedure described byDíaz-Cabañas et al., with slight modifications [9]. Specifically, three different silica sourceswere used: tetraethyl orthosilicate (TEOS, Fujifilm Wako Pure Chemical Co.), colloidal silica(AS-40, Sigma-Aldrich) and fumed silica (Cab-O-Sil M5, Cabot Corporation, USA). N,N,Ntrimethyladamantan-1-ammonium hydroxide (TMAdaOH, Sachem Japan GK) was employed
as the structure directing agent (SDA), dissolved in Millipore water in a Teflon® beaker. Thesilica source was then added slowly with vigorous stirring, and was continued stirring for
approximately 12 h at room temperature while the mixture was covered with a polyethylene
film. The molar composition in the resulting solution was 1.00 SiO2 : 0.41 TMAdaOH : 31.3
H2O. The solution was subsequently heated to 70 °C in a water bath with continuous stirring
for approximately 7 h to evaporate the solvent. The resulting sticky gel was further dried in anoven at 70 °C for 20 h. The light yellow-to-white solid obtained was finely ground in an agate
mortar. This material is referred to herein as the green precursor. Following, 1 g of the green
precursor was loaded into a Teflon® vessel together with 0.148 mL (0.136 g based on a densityof 0.922 g/cm3) of HF solution (46.0 – 48.0 %, assumed to be 47.0 %; Fujifilm Wako Pure
Chemical Co.), and mixed manually with a stir rod to produce a homogeneous dispersion. The
molar composition of SiO2, HF and H2O in this final gel mixture are provided in Table 1, wherethe amount of H2O containing in green-precursors will be shown below have been taking intoaccount. The Teflon® vessel was set in a stainless steel autoclave reactor and heated at 160 °C
for 48 h in an oven under static. After cooling to room temperature, Millipore water was added
Table 1 The final gel compositions used for the synthesis of the pure-SiO2 CHA
zeolites and the SiO2 and H2O content in the respective green precursors.

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to the paste-like product, which was then removed
by filtration, washed and dried at 70 °C. Finally, thematerial was calcined at 600 °C for 1 h (at a ramprate of 0.5 °C/min) under an air flow ofapproximately 100 cm3/min to remove the SDA.
The resulting pure-SiO2 CHA zeolites preparedfrom TEOS, colloidal silica and fumed silica are
denoted herein as psCHA(alk), psCHA(cs) andpsCHA(fs), respectively. The yields of thesematerials ranged from 75 to 80 % based on the SiO2amounts.
The X-ray diffraction (XRD; X’Part PRO MPD

Fig. 1 The XRD patterns of pure-SiO2 CHAzeolites. The triangles mark the main peaks thatcan be assigned to a CHA-type structure.

PW3040, PANalytical, The Netherlands) patterns of the prepared CHAs are presented in Figure
1. A single crystalline CHA phase was observed in all cases, with no impurity phases (expected
STT-type zeolite with the most intense 020 peak at 2θ = 8.11°) [11-13]. The diffraction signals
could be assigned to a trigonal crystal system (space group: R3ത m (#166)) and the optimized cellparameters were determined to be a = 1.354(2), c = 1.480(3) nm for the psCHA(alk), a =1.354(2), c = 1.480(3) nm for the psCHA(cs), and a = 1.352(1), c = 1.489(3) nm for thepsCHA(fs). These parameters were almost equivalent to one another.
Scanning electron microscopy (SEM; JSM-6701F, JEOL) images of the psCHA(alk),psCHA(cs) and psCHA(fs) are shown in Figure 2. The prepared CHAs exhibited theintergrowth of crystals, as seen in Figures 2(A) to (C). These crystals were evidently quasicubic and likely comprised parallelepiped single particles, as shown in Figures 2(D) to (F). It
is worth noting that the plate-like particles found in previous research were not observed in thisstudy [11]. The energy-dispersive spectroscopic (EDS; JED-2300, JEOL) analyses20低 H2O/SiO2 比における純シリカ CHA 型ゼオライトの合成

Fig. 2 SEM images of the pure-SiO2 CHA zeolites. (A and D) psCHA(alk), (B and E) psCHA(cs) and (C andF) puCHA(fs).

demonstrated the absence of Na in these specimens (less than 0.1 wt%), even in the case of thepsCHA(cs), although the colloidal silica (AS-40) contained a small amount of Na (SiO2/Na2O= 160 – 270 in catalog).
Figure 3 shows the N2 sorption isotherms of the pure-SiO2 CHA zeolites measured at 77 K(Autosorb-iQ, Quantachrome Instruments). Type I isotherm with steep increase (approximately
200 cm3/g-STP) of N2 adsorption in the low relative pressure region was clearly seen. The
apparent surface areas (SBET) calculated by the Brunauer-Emmett-Teller (BET) method in theP/P0 region of 0.005 – 0.05 were 783, 778 and 803
m2/gforthepsCHA(alk),

psCHA(cs)andpsCHA(fs), respectively. The pore volumes (Vp)less than approximately 1.0 nm of pore diameter,
which arisen from about 0.0122 (P/P0) according tothe Kelvin equation, were determined to be 0.283,
0.283 and 0.290 cm3/g for the psCHA(alk),
psCHA(cs) and psCHA(fs), respectively. The Fig. 3 N2 sorption isotherm of the pure-SiO2CHA zeolites.

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psCHA(fs) exhibited the largest SBET and Vp. It is important to note, however, these values donot reflect a realistic accessible pore structure in ultramicropores [14]. These are probably
valuable as an identification information concerning crystallinity. For comparison, SBET and Vpvalues for pure-SiO2 CHA zeolites have been reported in the range of 679 – 850 m2/g and on
the order of 0.30 cm3/g, respectively [13, 15]. Our results therefore indicate that the psCHA(alk),psCHA(cs) and psCHA(fs) compared favorably with other pure-SiO2 CHA zeolites reported.
The results described above thus confirm the successful synthesis of pure-SiO2 CHA-typezeolites. Significant differences were not found between these materials, indicating that theproduct was unaffected by the silica source employed during synthesis. However, prior work
has shown that the hydrophobicity of the product was changed when switching between TEOSand AS-40 [10]. The clear type I isotherms in Figure 3 suggests that these zeolites could havesignificant H2 and D2 sorption abilities [4], and the psCHA(fs) was found to adsorb 6.05 and6.77 mmol/g H2 and D2 at 95 kPa and 77 K, respectively. Such large H2 and D2 sorption capacity
suggests that pure-SiO2 CHAs are not well suited for effective hydrogen isotope separation [4],however, we intend to further investigate D2/H2 separation in the future.
Pure-SiO2 CHA zeolites have been commonly prepared from TEOS or colloidal silica as
the silica sources [9-11]. In this study, we also performed the synthesis using fumed silica. The
thermogravimetricanalysis(TG;Rigaku,ThermoPlus2) curves of the green precursor for thepsCHA(fs) in air (up to 800 °C) and N2 (up to600 °C) are shown in Figure 4. Both curves are
similar and three distinctive weight losses areapparent at about below 110, about 110 – 300 andabove 300 °C. The weight loss below 110 °C
Fig. 4 TG curves for the green precursor for(approximately 7.1 wt%) was attributed to the psCHA(fs) acquired under air and N2 (100cm3/min) with a heating rate of 2 °C/min.

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低 H2O/SiO2 比における純シリカ CHA 型ゼオライトの合成

desorption of H2O, while the steep change between 110 and 300 °C was ascribed to thecombustion (under air) or desorption (under N2) of the SDA. Above 300 °C, the condensationof silanol groups in addition to the combustion or desorption of residual SDA-related materials
gave gradual weight loss. The final weight (38.4 wt%) at 800 °C under air corresponded to themass of SiO2 in the specimen. The weight loss between about 110 and 800 °C was 54.5 wt%,
which was in good agreement with the initial content (55.4 wt%) of SDA based on the molar
ratio of 1.00 SiO2 : 0.41 TMAdaOH. Taking the amounts of H2O and SiO2 in the green precursor
into consideration, the composition of SiO2, HF and H2O in the final gel after adding the HF
solution could be calculated to be 1.00 : 0.50 : 1.24, respectively. Similarly, the final gel
compositions for the psCHA(alk) and psCHA(cs) were calculated based on the TG data and are
given in Table 1. The H2O/SiO2 molar ratios in the final gels were determined to be 1.26, 1.33
and 1.24 for the psCHA(alk), psCHA(cs) and psCHA(fs), respectively.
The water content in the final gel is a crucial for the formation of pure-SiO2 CHA zeolites.
Specifically, higher H2O concentrations (H2O/SiO2 =5.8) [13] lead to the formation of STTtype zeolite (SSZ-23) as a minor impurity. Furthermore, Miyamoto et al. reported that not only
a higher water content (6.0 < H2O/SiO2) but also a lower water content (H2O/SiO2 <3.0) resulted
in the formation of the STT-type zeolite [10]. Although a direct comparison with prior work isdifficult, our results demonstrate the potential formation of pure-SiO2 CHA zeolites at lower
water content below 3.0 (H2O/SiO2).
In conclusion, pure-SiO2 CHA-type zeolites were successfully synthesized from analkoxide, colloidal silica and fumed silica. No significant differences were identified in theresulting materials when varying the silica source, based on analyses by XRD, SEM and N2sorption. In all cases, highly crystalline and single phase zeolites were obtained. Based on TG
analyses of the green precursors, the H2O/SiO2 molar ratios in the final gel were found to be in
the range of 1.2 – 1.3, which are lower than previously reported values.

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Acknowledgements
This work was partly supported by KAKENHI(16K06935) and (19H01872).

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