Synthesis and catalytic properties of large-pore Sn-β and Al-free Sn-β molecular sieves

Article abstract of DOI:10.1039/a607038h

Sn-β and Al-free Sn-β (large pore, 12-membered ring channels) molecular sieves prepared by hydrothermal synthesis and characterised by XRD, FTIR and sorption techniques are distinguished by their acidic and oxidation properties, in the acetylation of 1,3,5-trimethylbenzene (1,3,5-TMB) with acetyl chloride and in the oxidation of m-cresol and 1,3,5-TMB with aqueous H₂O₂, respectively.

Full text of DOI:10.1039/a607038h

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Synthesis and catalytic properties of large-pore Sn-b and Al-free Sn-b
molecular sieves
N. K. Mal and A. V. Ramaswamy*
National Chemical Laboratory, Pune-411 008, India
Sn-b and Al-free Sn-b (large pore, 12-membered ring
channels) molecular sieves prepared by hydrothermal
synthesis and characterised by XRD, FTIR and sorption
techniques are distinguished by their acidic and oxidation
properties, in the acetylation of 1,3,5-trimethylbenzene
(1,3,5-TMB) with acetyl chloride and in the oxidation of
m-cresol and 1,3,5-TMB with aqueous H2O2, respectively.
The XRD profiles of all as-synthesized and calcined samples
match well with the BEA topology. No peaks due to SnO2 or
crystalline impurity phases are seen. The crystallinity of as-
synthesized b is maximum, and this sample was considered to
be the reference sample. Sn-b (A), Al-free Sn-b (C), de-
aluminated Sn-b (B) and dealuminated b (silicalite) are
relatively less crystalline (approximately 96, 87, 67 and 55%,
respectively) with respect to the reference sample. The SEM
micrographs show that the particle size of all the samples is
< 0.3 mm. The framework IR spectra of Sn samples (in Nujol
mull) show a peak at ca. 957 cm21 (Fig. 1). This band which is
intense in the case of Al-free Sn-b (C), is not observed in Sn-
free b-zeolites. This band has been assigned to Si–O–M
vibrations or Si–O(H) defect groups promoted by framework M
from the possible substitution of heteroatom (M) in the Si–O–Si
units.9,10 N2 adsorption isotherms at 77 K of Sn-b samples are
characteristic of microporous materials. The surface areas are
typically in the region of 650 m2 g21 for all the samples (Table
1). The mesopore (t-area) contribution is rather small in the case
Following the synthesis of medium-pore (10-membered ring)
TS-11 and its use in shape-selective oxidation reactions on a
commercial scale in the hydroxylation of phenol,2 many
attempts have been made to synthesize large pore (12 MR),
three-dimensional Al-free Ti-b, without much success, unless
trivalent metal ions of Al3+ were present in the lattice.3–6
Recently, Camblor et al.7 reported the synthesis of Al-free Ti-b
using seed crystals, and this material has been used effectively
for the epoxidation of alkenes and for oxidation of relatively
bulkier organic substrates.8 We earlier reported the synthesis of
medium-pore Sn-sil-1 (MFI structure), which was found to be
less active than TS-1 in the hydroxylation of phenol9 but more
active than TS-1 in the oxidation of ethylbenzene10 under
similar conditions. In this report, for the first time, we describe
the synthesis of large-pore, Sn-b and Al-free Sn-b molecular
sieves, their characterization by XRD, FTIR and sorption
techniques and catalytic activity in the hydroxylation of phenol,
oxidation of m-cresol and 1,3,5-TMB in presence of aqueous
H2O2 and acetylation of 1,3,5-TMB with acetyl chloride
(AcCl).
The hydrothermal synthesis of Sn-b was carried out using
gels with the following molar composition: 1.0 SiO2 :0.0–
0.0125 SnO2 :0.0–0.03 Al2O3 :0.52–0.70 TEAOH:20 H2O. In
a typical preparation, 0.71 g of SnCl4·5H2O (98%, Loba Chem)
in 10 g of H2O was added to 41.7 g of tetraethylorthosilicate
(TEOS) (99.9%, Aldrich). After 15 min, 58.9 g of tetra-
ethylammonium hydroxide (TEAOH) (35 mass% aqueous
solution, Aldrich) was added under vigorous stirring for 1 h. To
this reaction mixture 0.63 g of Al2(SO4)3·16H2O in 15 g H2O
was added and stirred for another 1 h. 8.7 g of H2O was then
added to the clear solution and stirred for 2 h. The resultant clear
solution was charged into a stainless-steel autoclave and heated
at 415 K under autogenous pressure and static conditions for 10
days to complete the crystallization. The resultant product was
filtered, washed with deionized water, dried at 383 K and
calcined at 823 K for 16 h. This sample is referred to as A. One
more such Sn-b sample was prepared with different Al and Sn
contents (Si/Al = 30.0 in gel and 14.7 in crystalline product and
Si/Sn = 200 in gel and 150 in crystalline product). Dealu-
minated Sn-b (sample B) was prepared by refluxing 4 g of
sample A with 60 ml of 5 m HCl for 3 h at 383 K. This treatment
was repeated once more after filtration. This sample was filtered
and washed thoroughly with distilled water until free of all acid
and finally dried and calcined at 723 K for 12 h. Sample C was
prepared following a similar procedure that was used for the
synthesis of sample A, except that 0.5 g of B was added in place
of 0.63 g of Al2(SO4)3·16H2O. Two b zeolite samples (without
Sn) were also prepared with different SiO2/Al2O3 ratios (30.0
and 250). Al-free b (or silicalite) was prepared by the
dealumination of zeolite-b of SiO2/Al2O3 molar ratio of 250.
of Sn-b and Al-free Sn-b samples (35.6 and 57.0 m2 g21
,
respectively) but significant in the dealuminated b samples
(120–160 m2 g21). The micropore volumes are in the range
0.20–0.25 ml g21 for all samples. The amounts of H2O,
n-hexane, cyclohexane and m-xylene adsorbed on these samples
at 298 K and at p/po of 0.5 (gravimetric, Cahn electrobalance)
are included in Table 1. The comparable sorption capacity of
Sn-containing and Sn-free b samples indicates that the micro-
pore volumes are well maintained and that occluded SnO2-type
species may not be present in the Sn-containing samples.
The results on the catalytic activity of all the b samples are
summarised in Table 2. The catalytic activity of the b, Sn-b and
*
8 %
*
*
(a)
(b)
(c)
1300
1150
1000
850
ν / cm
700
550
400
–1
Fig. 1 FTIR spectra of Al-free Sn-b sample (a), Sn-b sample with Si/Al
= 28.5 and Si/Sn = 78.8 (b) and b sample with Si/Al = 50.3 (c). The
absorption at 957 cm21 is similar to that observed for Ti-, V- and Sn-MFI
and -MEL structures. * Nujol peak.
Chem. Commun., 1997
425

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Table 1 Physicochemical characteristics of Sn-b zeolitesa
In gel, molar
ratio
Products, molar
ratioa
Sorption capacityb (mass%)
Surface
areac/m2 g21
Micropore
volume/ml g21
Sample
Si/Sn
Si/Al
Si/Sn
Si/Al
H2O
n-hexane
cyclohexane
m-xylene
Sn-b (A)
100
—
100
—
100
—
—
125
—
78.8
85.4
54.5
H
28.5
26.7
23.3
25.6
20.5
18.4
19.3
18.6
19.4
18.8
18.2
21.2
20.4
21.3
20.7
19.7
25.0
23.9
25.4
24.2
22.3
670
665
677
652
645
0.25
0.22
0.25
0.23
0.20
deAl-Sn-b (B)
Al-free Sn-b (C)
b
> 3000
> 4000
50.3
deAl-b (silicalite)
—
—
> 3000
a Chemical analysis using EDX and ICP. b Gravimetric adsorption at p/po = 0.5 and at 298 K (Cahn electrobalance). c N2 adsorption isotherms (Coulter 100
CX analyser) at 77 K.
Table 2 Reactions over Sn-containing b zeolitesa
Products,
molar ratio
Phenol
Oxidation of
Acetylation of
1,3,5-TMBe
hydroxylationb
m-cresolc
Oxidation of 1,3,5-TMBd
H2O2
H2O2
efficiency
H2O2
Sample
Si/Sn
Si/Al
28.5
TON
30.3
> 4000 83.6
> 3000 80.1
TON
efficiency
TON
efficiency
Selectivityf
AcCl conv.g
Selectivityh
Sn-b (A)
78.8
85.4
54.5
H
150
H
33.0
44.7
65.0
8.1
—
—
6.6
32.3
30.6
1.3
20.3
52.3
75.4
4.6
1.0
7.7
6.6
0.0
—
4.2
18.9
24.5
0.0
—
—
50
80
77
0.0
—
—
32.5
—
6.4
34.8
99.9
99.9
96.5
—
99.1
83.6
94.6
82.0
deAl-Sn-b (B)
Al-free Sn-b (C)
b
Sn-b
b
50.3
14.7
13.9
8.8
—
—
—
—
—
—
—
a
Catalyst/substrate = 10, 20, 20 mass% in the case of phenol, m-cresol and 1,3,5-TMB, respectively; substrate/H2O2 (mole) = 3.0; solvent/substrate
b
(mole) = 20; reaction time = 15 h; TON is defined as the mole of substrate converted per mole of metal ion (Al + Sn). Batch reactor; T = 348 K;
solvent = H2O, H2O2 efficiency = mol% H2O2 consumed in the formation of parabenzoquinone, catechol and hydroquinone. c Parr autoclave; T = 373 K;
solvent
= H2O:acetonitrile (2:1), H2O2 efficiency = mol% H2O2 consumed in the formation of 2,5-dihydroxytoluene, 3,4-dihydroxytoluene,
3-hydroxybenzyl alcohol and 3-hydroxybenzaldehyde. d Batch reactor; T = 348 K; solvent = acetonitrile, H2O2 efficiency = mol% H2O2 consumed in the
formation of 3,5-dimethylbenzyl alcohol, 3,5-dimethylbenzaldehyde and 2-hydroxy-1,3,5-TMB. e Down-flow, fixed-bed reactor; T = 428 K; 1,3,5-TMB/
acetyl chloride (AcCl) (mol) = 3.0, WHSV = 1.0 h21, analysis after 1 h run. Product selectivity (mass%) for 3,5-dimethylbenzaldehyde. Conversion
(mass%) of acetyl chloride (AcCl). h Product selectivity for 2,4,6-trimethylacetophenone.
f
g
Al-free Sn-b samples can be rationalised in terms of activity due
to acidic properties (depending on the Si/Al ratio) and oxidation
properties (isolated Sn4+ sites) or a combination of the two. All
the Sn-containing samples are active in the hydroxylation of
phenol, and oxidation of m-cresol and 1,3,5-TMB with aqueous
H2O2 to different degrees. Both deAl-Sn-b (B) and Al-free Sn-b
(C) samples show consistantly higher H2O2 efficiency in all the
three test reactions, indicating the absence of acid sites (Si/Al
> 3000). Al-free b (silicalite) shows negligible activity in the
oxidation of m-cresol and 1,3,5-TMB (no active Sn4+ ions) and
in the acetylation of 1,3,5-TMB (no acidity). In the hydroxyla-
tion of phenol and oxidation of m-cresol and 1,3,5-TMB, the
H2O2 efficiency over deAl-Sn-b (B) was lower than over Al-
free Sn-b (C), while the TON is in the same order. The H2O2
efficiency in hydroxylation of phenol over Sn-containing b
samples was less than that obtained with Sn-sil-1 (MFI
structure).9 This may be due to the more selective nature of the
medium-pore Sn-sil-1 than the large pore Sn-containing b
samples for relatively small molecules like phenol. In the
oxidation of m-cresol, 75.4 mol% H2O2 efficiency could be
achieved over Al-free Sn-b and 2-methylhydroquinone was
detected as the major product. In the oxidation of m-cresol, both
deAl-Sn-b (B) and Al-free Sn-b (C) show better H2O2
efficiency (18.9 and 24.5%, respectively) and TON (7.7 and 6.6,
respectively) than Sn-b (A) (H2O2 efficiency = 4.2% and
TON = 1.0). Sn-b (A) is not a good catalyst for the oxidation
reaction. 3,5-Dimethylbenzaldehyde was detected as the major
product in the oxidation of 1,3,5-TMB.
selectivity over Sn-containing b zeolite is not clear at the
moment. When the acetylation of 1,3,5-TMB was carried out in
the absence of any catalyst (only porcelain beads were loaded in
the reactor), the conversion of acetyl chloride was only 5.2%,
which is slightly less than over Al-free Sn-b (C) sample (6.4%).
This indicates that very weak acidity is associated with Sn4+
ions in the zeolite structure.
In conclusion, a large-pore zeolite (BEA) with Sn in the
framework has been synthesized for the first time, which is able
to catalyse oxidation of bulkier organic substrates with aqueous
H2O2 in the absence of acidity (Al-free state) and acetylation
with acetyl chloride when acidic (low Si/Al ratio), with some
selectivity advantages due to the presence of Sn4+ ions.
Nawal Kishore is thankful to CSIR, New Delhi for a senior
research fellowship.
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Received, 15th October 1996; Com. 6/07038H
426
Chem. Commun., 1997