Chemistry of Materials
Article
molecular modeling predictions. This study was prompted by
our recent work in which we screened a number of
2.2. Microporous Materials Synthesis. 2.2.1. Characterization.
A general synthesis procedure for the microporous materials can be
found below. In all situations the samples were spun at 43 rpm in a
rotating oven. The 27Al MAS NMR were recorded using a Bruker AM
13
imidazoliums to make pure-silica STW. We found with
many of these OSDAs, that the addition of aluminum to
fluoride-mediated systems caused RTH to form instead of
STW. These results are not trivial, as in the original work to
produce SSZ-50, modifying the OSDA by a single carbon atom
3
00 MHz spectrometer with a 4 mm rotor at a spinning rate of 8 kHz
and were conducted in a 7.0 T magnetic field corresponding to a
Larmor frequency of 78.172 MHz. The Al spectra are referenced to
27
27
1
.1 M Al(NO ) as an external standard. Prior to Al NMR analysis,
3
3
9
led to a change in product. Additionally, the hydrothermal
the samples were hydrated by placing them in a desiccator containing a
saturated aqueous salt solution. All powder X-ray diffraction (PXRD)
characterization was conducted on a Rigaku MiniFlex II with Cu Kα
radiation. Nitrogen adsorption isotherms were performed on a
Quantachrome Autosorb iQ at 77 K. Micropore volume was
determined using the t-plot method. Scanning electron microscope
stability of the RTH materials is evaluated, and the zeolites are
found to be stable under severe hydrothermal conditions and
maintain good catalytic activity (MTO).
2
. EXPERIMENTAL SECTION
(
SEM) images were acquired on a ZEISS 1550 VP FESEM, equipped
2
.1. OSDA Synthesis. Two different types of reactions were used
with in-lens SE. EDS spectra were acquired with an Oxford X-Max
to produce the imidazolium OSDAs in this work. Specific details
including reaction types and product characterizations for each OSDA
can be found in Table 1.
SDD X-ray Energy Dispersive Spectrometer system.
For all microporous material syntheses, aliquots of the material were
taken periodically (generally every 3−4 days) by first quenching the
reactor in water and then removing enough material for PXRD.
Syntheses were considered to be finished when a crystalline product
was observed via PXRD and the broad peak indicative of amorphous
material, between 20 and 30° 2θ, had disappeared. If incomplete
crystallization or no crystalline product was observed in PXRD, the
syntheses were replaced in the oven. For all syntheses that produced
RTH, representative synthesis times are included in Table 2. As some
variation in synthesis times is generally observed in zeolite synthesis,
all samples were evaluated using PXRD to determine completion.
Table 1. Characterization of Imidazolium Cations
reaction
type
organic
parent imidazole
supplier
13C NMR δ (ppm)
1
2
N-methylimidazole
Aldrich
Aldrich
1
1
36.32, 123.76, 136.86
8.52, 34.48, 121.64,
1,2-dimethylimidazole
1
44.63
3
4-methylimidazole
Aldrich
2
8.52, 33.37, 35.87,
1
1
20.37, 132.33,
35.86
2
.2.2. Fluoride-Mediated Reactions. Tetraethylorthosilicate
(
TEOS) and aluminum isopropoxide (if necessary) were added to
4
5
2-ethylimidazole
Aldrich
2
2
9.92, 16.59, 34.71,
22.07, 148.16
8.62, 9.38, 31.46,
the OSDA in its hydroxide form in a Teflon Parr Reactor. The
container was closed and stirred overnight to allow for complete
hydrolysis. The lid was then removed, and the ethanol and appropriate
amount of water were allowed to evaporate under a stream of air.
Once the appropriate mass was reached, aqueous HF was added, and
the mixture was stirred by hand until a homogeneous gel was obtained.
In the cases of H O/SiO = 4 a second evaporation step was normally
1
2,4-dimethylimidazole
Synquest
3
1
4.36, 118.64,
30.14, 143.89
6
2-ethyl-4-
methylimidazole
Aldrich
2
8.80, 10.17, 16.93,
31.52, 34.36,
1
1
18.83, 120.23,
30.23, 147.32
2
2
used. The final gel molar ratios were
7
8
2-isopropylimidazole
TCI
TCI
2
1
17.55, 24.79, 35.68,
22.65, 149.69
1
1
SiO :0.5ROH:0.5HF:xH O, x = 4, 7
2
2
1,2,4,5-
tetramethylimidazole
7.99, 9.76, 31.58,
125.42, 142.21
0
.95SiO :0.05Al:0.5ROH:0.5HF:4H O
2
2
The autoclave was sealed and placed in a rotating oven at 175 °C.
Aliquots of the material were taken periodically by first quenching the
reactor in water and then removing enough material for PXRD.
Reaction type 1: The parent imidazole was dissolved in methanol
and then cooled in a dry ice/acetone bath. Then a 3-fold molar excess
of methyl iodide was slowly added. (Caution: These reactions can be
highly exothermic, use appropriate precautions.) The mixture was then
allowed to slowly warm to room temperature and finally refluxed
overnight. After cooling, the solvent and excess methyl iodide were
removed using rotary evaporation (Caution: Highly toxic vapors
present, use appropriate precautions), and the product was recrystal-
lized from acetone and washed with ether.
2.2.3. Si/Al = 15 (NH −Y and Sodium Silicate). Following the
4
1
4
method of Wagner et al. 2 mmol of OSDA was mixed with 0.20 g of
1 M NaOH, and water was added to give a total mass of 6 g. Then 2.5
g of sodium silicate (PQ Corporation, 28.6 wt % SiO and 8.9 wt %
2
Na O) was added to the mixture, and finally 0.25 g of NH −Y
2
4
(Zeolyst CBV 500, Si/Al = 2.55) was added. The solution was heated
at 140 °C in a rotating oven.
Reaction type 2: The parent imidazole was dissolved in chloroform,
and then a 2-fold molar excess of potassium carbonate was added. The
mixture was cooled in a dry ice/acetone bath, and then a 2-fold molar
excess of methyl iodide was slowly added. (Caution: These reactions
can be highly exothermic, use appropriate precautions.) The mixture
was then allowed to slowly warm to room temperature and finally
refluxed overnight. After cooling to room temperature the potassium
carbonate was filtered, and the solids were rinsed with extra
chloroform to recover all the product. Then the solvent and excess
methyl iodide were removed using rotary evaporation (Caution:
Highly toxic vapors present, use appropriate precautions), and the
product was recrystallized from acetone and washed with ether.
In both types of reactions the structure was verified using 13C NMR
2.2.4. Si/Al = 15 (CBV 720). Three mmol of OSDA was mixed with
1 g of 1 M NaOH, and water was added to bring the total mass to 7 g.
Then 1 g of CBV 720 (Zeolyst, Si/Al = 15) was added. The solution
was heated at 160 °C in a rotating oven.
2.2.5. Si/Al = 30 (CBV 760). Three mmol of OSDA was mixed with
1 g of 1 M NaOH, and water was added to bring the total mass to 7 g.
Then 1 g of CBV 760 (Zeolyst, Si/Al = 30) was added. The solution
was heated at 175 °C in a rotating oven.
2.2.6. Si/Al = 50 (Ludox AS-40 and Sodium Aluminate). Four
mmol of OSDA was mixed with 1.56 g of 1 M NaOH, and the total
mass was brought to 9.66 g with the addition of water. Then 0.038 g of
sodium aluminate (Pfaltz & Bauer) was added and stirred until
dissolved. Finally 3 g of Ludox AS-40 was added and stirred until a
homogeneous gel was obtained. Seeds were added, and then the gel
was heated at 160 °C in a rotating oven.
2.3. Calcination and Ammonium Exchange. All products were
calcined in breathing grade air. The material was heated to 150 °C at 1
°C/min, held for 3 h, then heated to 580 °C at 1 °C/min, and held for
in D O with methanol added as an internal standard. The products
2
were then converted from iodide to hydroxide form using hydroxide
exchange resin (Dowex Marathon A, hydroxide form) in water, and
the product was titrated using a Mettler-Toledo DL22 autotitrator
using 0.01 M HCl as the titrant.
B
Chem. Mater. XXXX, XXX, XXX−XXX