B. Tijsebaert et al. / Journal of Catalysis 278 (2011) 246–252
247
and thus product distribution are controlled by the ratio of ammo-
Table 1
Overview of Al-RUB-41 samples employed.
nia versus methanol as well as by the reaction temperature. For
thermodynamic reasons, TMA is predominantly formed, but the
demand for monomethylamine and dimethylamine is much high-
er. Therefore, an ammonia excess and a recycle of undesired TMA
are often applied in the process. However, this approach requires
expensive purification steps, also because of the azeotropic mix-
tures intermediately formed. To overcome these limitations,
shape-selective catalysts have been introduced [10].
2
À1
Al source
Si/Al
BET (m /g)
NH3 mmol g adsorbed
A1
A2
A3
A4
B
Al isopropoxide
Al isopropoxide
Al isopropoxide
Al isopropoxide
Sodiumaluminate
19
25
51
163
20
69
412
350
323
270
0.413
0.285
0.156
0.062
0.218
The first commercially available shape-selective catalysts were
applied by Nitto and consisted of a mordenite treated with tetra-
ethoxysilane to enhance the selectivity to the mono- and dialkylat-
ed amines [11]. A large body of further work focused on the
shape-selective properties of small pore zeolites containing
typically cages that can be accessed via 8-membered ring pore
windows [12–19]. Active and selective amination catalysts are
for instance H-Rho, H-chabazite, and H-levyne zeolites. The
windows of the cages in these materials are sufficiently small to
prevent the egress of trimethylamine.
tios. Samples A1–A4 were synthesized using aluminum isopropox-
ide; sample B was synthesized using sodium aluminate.
ZSM-5 CBV 2314 was obtained from ZEOLYST and has a Si/Al ra-
tio of 20. A chabazite sample (Si/Al = 16) was obtained from BASF
Ludwigshafen. XRD showed the pattern of pure and fully crystal-
line chabazite (checked as-synthesized and after ion exchange).
FE-SEM analysis showed the typical cube morphology. Al-MCM-
4
1 (Si/Al = 10) was synthesized according to a modified Stoeber
method [21].
In this work, we report on the activity and shape-selective prop-
erties of (H)Al-RUB-41 in methanol amination. As is apparent from
the literature background, this reaction is a sensitive one to detect
even minor amounts of non-selective sites. Important questions
are whether the RUB-41 pore system of 8-rings and distorted 10-
rings may allow shape-selective methanol amination and whether
the Al-siting can be controlled so as to mostly generate active sites
only in a shape-selective environment. The effect of aluminum
content on selectivity and conversion and the influence of post-
2
.2. Characterization
27Al MAS NMR spectra of the samples were acquired on a Varian
Infinity Plus-400 spectrometer at 104.2 MHz using a 4 mm MAS
NMR probe head with a spinning rate of 10 kHz. Chemical shifts
were referenced to (NH
ary reference. All spectra were accumulated for 12,000 scans with a
/4 flip angle and a 2-s pulse delay. The X-ray powder diffraction
analysis was carried out with a Siemens 5000D diffractometer
using CuK radiation (k = 0.15401 nm). Nitrogen adsorption iso-
4
)Al(SO
4
) Á12H
2
2
O at À0.4 ppm as a second-
p
synthesis modification by
investigated.
a silylating agent will also be
a
therms were determined by physisorption of nitrogen at 77 K on
a Coulter Omnisorp 100 CX. Prior to measurements, the samples
were outgassed under vacuum at 473 K overnight. SEM
micrographs were recorded using a Philips XL30 FEG. The temper-
2
. Experimental
3
ature-programmed desorption of ammonia (NH -TPD) experi-
2.1. Catalyst preparation
ments were performed using a Micromeritics AutoChem II 2920
automated chemisorption analysis unit with a thermal conductiv-
ity detector (TCD) under helium flow. The sample was heated with
a temperature ramp of 20 °C/min–500 °C under He flow. After stay-
ing at that temperature for 10 min, it was cooled down to 100 °C in
He atmosphere. Ammonia saturation was carried out at 100 °C
using a 10% NH –He gas mixture. After saturation, excess ammonia
3
was purged from the chamber under flowing He at 100 °C for 1 h.
The desorption step was performed with a temperature ramp of
Different Al-RUB-41 materials with varying Al content were
synthesized. Typically, the molar composition of the synthesis gel
in the first step was 1 SiO : 0.5 SDA: 0–0.08 NaOH: 2–10 H O. Dim-
ethyldipropylammoniumhydroxide was used as structure-direct-
ing agent (SDA). After aging of the gel for 1 h at room
temperature, it was kept in Teflon-lined stainless steel autoclaves
and stirred at a temperature of 140 or 150 °C at 15 rpm for 2–
2
2
4
days. Seeding crystals of RUB-39 were added to shorten crystal-
lization times. After this first stage, the synthesis mixture was
cooled, and a minute amount of Al isopropoxide or NaAlO was
added to the gel. This was carried out using synthesis mixtures
with SiO /Al = 30–200 and SiO /SDA = 2.
1
0 °C/min up to 500 °C under He flow. Desorbed species were ob-
served with the on-line mass spectroscopy unit, which confirmed
that the TCD signal indeed corresponded to ammonia desorption.
2
2
2
O
3
2
Crystallization was continued for another 2–4 days. The as-syn-
thesized, layered Al-RUB-39 material was converted to Al-RUB-41
by slow heating at 1 °C/min until 520 °C in a furnace under static
air. After 12 h at this temperature, heating was continued until
2.3. Catalytic experiments
Catalytic experiments were performed in a continuous flow
fixed-bed reactor. Prior to reaction, 200 mg of calcined zeolite sam-
+
5
60 °C for 4 h.
To eliminate any Na that could be present as a residue from the
synthesis, samples were three times ion-exchanged in a solution of
.5 M NH NO at 80 °C for 24 h and washed with distilled water.
ple in the H -form was pelletized. Pelletizing was done by pressing
+
the catalyst between 2 iron bolts at 200 bar followed by sieving to
obtain the 250–500 lm fraction. Then, the catalyst was pretreated
0
4
3
in the reactor at 400 °C under a helium flow of 5.6 mL/min. There-
after, the reactor was cooled to reaction temperature (300 or
340 °C). The reaction was carried out by feeding the reactor with
a 1:1 or a 2:1 mixture (on mole basis) of ammonia and methanol
diluted in helium. Methanol was fed to the reactor by passing a he-
lium flow through a methanol-filled saturator. Mass flow control-
lers enabled to adjust the ammonia to methanol ratio. Typically,
Next, these samples were dried at 70 °C and calcined for 6 h in a
furnace under static air with a constant temperature increase of
1
°C/min from 20 °C to 450 °C. Silylation was performed with a
hexamethyldisilazane (HMDS) treatment. One gram of the
calcined(H)Al-RUB-41 catalyst was dried at 200 °C and added to
a solution of 0.38 g HMDS in 10 g toluene. The resulting mixture
was refluxed under N
2
atmosphere at 120 °C for 2 h. The silylated
the gas flow of a 2:1 mixture contained 1.6 mL/min NH
3
and
sample was filtered, abundantly washed with toluene, and subse-
quently dried at 70 °C [20]. A sample designation list can be found
in Table 1. Al-RUB-41 could be synthesized with different Si/Al ra-
5.6 mL/min MeOH+He, or reactant partial pressures of 22.5 kPa
NH
3
and 11.25 kPa MeOH, which resulted in a WHSV of 0.66 gfeed
. For a 1:1 mixture, gas flows of 0.8 mL/min NH and
À1
g
3
catalyst