Shape selectivity extending to ordered supermicroporous aluminosilicates

Article abstract of DOI:10.1039/c4cc08784d

In the synthesis of polyoxymethylene dimethyl ethers (PODE_n) catalyzed by ordered supermicroporous aluminosilicates, shape selectivity was observed and the high selectivity for target products (PODE_3-8) was attributed to the particul

Full text of DOI:10.1039/c4cc08784d

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Shape selectivity extending to ordered
supermicroporous aluminosilicates†
Cite this:DOI: 10.1039/c4cc08784d
Wen Hua Fu,a Xiao Min Liang,a Haidong Zhang,*b Yi Meng Wang*a and
Ming Yuan Hea
Received 4th November 2014,
Accepted 2nd December 2014
DOI: 10.1039/c4cc08784d
www.rsc.org/chemcomm
In the synthesis of polyoxymethylene dimethyl ethers (PODEn) However, hierarchical zeolites always have a negative effect on
catalyzed by ordered supermicroporous aluminosilicates, shape shape selectivity, since activity and selectivity often have contra-
selectivity was observed and the high selectivity for target products dictory requirements.8 To develop the so-called supermicropor-
(PODE3–8) was attributed to the particular pore diameter.
ous materials, with the pore size between those of microporous
zeolites and OMMs, is of significant importance, due to their
The selectivity of a heterogeneous catalyst is a significant issue, medium large pores to facilitate the mass transfer and to fit
importance of which is even greater than its activity.1 Shape-selective molecules with medium size.9
catalysis was coined by Weisz and Frilette to describe the unique
In this work, we prepared ordered supermicroporous alumino-
catalytic properties of microporous molecular sieves/zeolites.2 Shape silicates with Pm3n symmetry and applied them to the preparation
selectivity can be divided into reactant selectivity, product selectivity of polyoxymethylene dimethyl ethers (CH3O(CH2O)nCH3, PODEn).
and transition state selectivity.3 Each kind of shape selectivity has Shape selectivity was observed in the pores of ordered supermicro-
been disclosed in the micropores of zeolites and used in industrial porous aluminosilicates. To our best knowledge, this is the first
applications, such as alcohol dehydration, xylene isomerization, and report on the shape selectivity of supermicroporous materials.
the methanol to hydrocarbon process.4
The preparation of PODEn was carried out by using trioxane and
Scientists developed ordered mesoporous materials (OMMs) and methylal as reactants and the details of reaction tests can be found in
mesoporous zeolites (so-called hierarchical zeolites) to overcome the electronic supplementary information. The N2 physisorption
motion restriction that is inevitable in the restricted micropores.5 isotherms recorded at 77 K and the corresponding pore size distribu-
However, the pore sizes of OMMs are of the order of several tion (PSD) curves are shown in Fig. 1. The isotherm of C10-AS-50 is a
nanometers, while molecules on this scale will have a high number typical one corresponding to supermicroporous materials,10 and the
of degrees of freedom, probably leading to many different confor- pore size of C10-AS-50 is in the supermicroporous range. Although the
mers with similar energies but different molecular dimensions. This distribution may extend to the borderline between zeolitic micropore
variability in size hinders a suitable match to nanopores, which is and mesopore ranges, the most probable distribution is still in the
characteristic of zeolites.6 Although shape selectivity can be expected supermicroporous range.11 In other words, given that the pore
in exceptional cases where extremely bulky molecules together with
matched pore sizes are involved,7 diminished or even eliminated
shape selectivity related to enhanced mass transfer will be observed
in mesopores under most circumstances. Creation of mesoporo-
sity in zeolite crystals has the opportunity of increasing the
catalytic activity because of the fast diffusion and mass transfer.
a Shanghai Key Lab of Green Chemistry and Chemical Processes, Department of
Chemistry, East China Normal University, Shanghai 200062, P.R. China.
E-mail: ymwang@chem.ecnu.edu.cn; Fax: +86-21-62232251; Tel: +86-21-62232251
b Engineering research centre of waste oil recovery technology and equipment of
Chinese ministry of education, Chongqing Key laboratory of catalysis science and
technology, Chongqing Technology and Business University, Chongqing 400067,
Fig. 1 (A) N2 adsorption–desorption isotherms and (B) pore size distribution
curves of aluminosilicate catalysts calculated from the NLDFT algorithm:
(a) USY-1, (b) USY-3 (60), (c) C10-AS-50, and (d) C16-Al-SBA-1.
P.R. China. E-mail: haidongzhang@ctbu.edu.cn; Fax: +86-23-62768317
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4cc08784d
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diameter of C10-AS-50 is between the conventional zeolitic micropore in zeolite Y.13 The increased 6% yield of gasoline has given rise to
and the ordered mesopore, C10-AS-50 is termed ordered super- tremendous economic benefits. In the current work, the apparent
microporous aluminosilicate for simplicity as defined in the title. increase of around 5% in PODE3–8 selectivity is also of great value to
C16-Al-SBA-1 possesses the same cubic Pm3n symmetry (Fig. S2 the industrial economy likewise, because the worldwide demand for
and S4, ESI†) and a similar specific area as C10-AS-50 does, yet the diesel fuel, especially green diesel fuel, is extensively increasing.
pore size of the former is larger and in the mesoporous range. It is Furthermore, C10-AS-50 can be reused several times without remark-
no doubt that the pore diameter increases in the order of zeolite able loss of activity or product selectivity, which makes it a promising
Y o C10-AS-50 o C16-Al-SBA-1.
heterogeneous catalyst (Fig. S6, ESI†).
The characterization and reaction results of different catalysts
As stated above, the supermicroporous aluminosilicate catalyst
used in this work are summarized in Table 1. PODE3–8 (PODEn with C10-AS-50 yielded the highest selectivity for the target products and
the value of n = 3–8) possess comparable properties, including high could be considered as the most suitable catalyst for the synthesis of
density, moderate melting point, boiling point and viscosity, to those PODEn in the diesel pool. On account of the effect of catalyst acidity
of conventional diesel fuel, but a higher cetane number than that of on the product selectivity, the acidic properties of the catalysts
conventional diesel fuel, making them appropriate for blending should be demonstrated.14 From the temperature programmed
in diesel pool.12 Then in the preparation of PODEn from trioxane desorption of ammonia (NH3-TPD) profiles (Fig. S3, ESI†), we can
and methylal, PODE3–8 was the target product. The conversion of conclude that neither the acid amount of the supermicroporous
trioxane over these four catalysts was all higher than 90%. However, aluminosilicate catalyst C10-AS-50 was larger than that of the other
the distributions of the PODEn product of these four catalysts were catalysts nor the acid strength was stronger. The FTIR spectra of
significantly different. When microporous zeolites USY-1 and USY-3 adsorbed pyridine were measured in order to estimate the strength
(60) were used, nearly half of the product was PODE2. For C10-AS-50, and nature of acid sites in these four aluminosilicate catalysts
whose pore size increased to supermicropores, the production (Fig. 2). For the non-crystalline aluminosilicate catalysts, the inten-
of PODE2 obviously decreased and much more PODE3–8 can be sity of the band at 1455 cmÀ1 ascribed to Lewis acid sites was always
obtained. When C16-Al-SBA-1, whose pore size further increased to higher than the intensity of the band at 1545 cmÀ1 assigned to
mesopores, was used, the selectivity for PODE2 increased and the Bronsted acid sites and the latter band decayed when the desorption
selectivity for PODE3–8 decreased.
temperature reached 350 1C.15 It occurred to us that the Bronsted
We must notice that the absolute value of the PODE3–8 acid strength of the non-crystalline aluminosilicates with Pm3n
selectivity for C10-AS-50 is about 5% higher than that for other symmetry was weak and it seemed that Lewis acid was dominant
catalysts. This absolute value (5%) is not very sensational at the in these catalysts, although the possibility that the transformation of
first look but can reflect a big difference among those four Bronsted acid sites to Lewis acid sites under vacuum and elevated
catalysts. All the product distribution of PODEn curves versus the temperature conditions during the catalyst activation procedure
n value showed an exponential distribution (Fig. S5, ESI†). When could not be excluded.16 For USY-1 and USY-3 (60), the bands
n was in the range of 3–8, the selectivity for every single PODEn assigned to both Bronsted acid sites and Lewis acid sites diminished
product over C10-AS-50 was higher than that over other three as the pyridine desorption temperature increased but they still
catalysts. In addition, the reaction was thermodynamically con- remained after evacuation at 450 1C, indicating that the acid sites
trolled and there existed more than ten kinds of products. The in zeolite Y were much stronger than those in non-crystalline alumino-
above-mentioned reasons disclosed that the discrepancy in silicates. In other words, judged by both the NH3-TPD curves and
PODE3–8 selectivity over different catalysts was not from experi- pyridine-FTIR spectra, we can conclude that the C10-AS-50 catalyst
mental error. For a complex reaction with dozens of possible showed no advantage in either the acid amount or acid strength
products, a variation in selectivity is a result of the cumulative over other catalysts. Therefore, the acidic properties should not be
effect and the value is not as ‘‘huge’’ as in a simple reaction with responsible for its superior PODE3–8 selectivity. Some other possible
only two or three possible products. A similar example is that the factors should be considered to explain the different product
utilization of zeolite Y over amorphous silica alumina gel distribution for these four catalysts.
catalysts in the FCC (fluid catalytic cracking) process results in
Inspired by the mechanism of shape selectivity in zeolite
an increase from 54 to 60% in gasoline yield, where the pores, the most probable factor that determines the product
increased selectivity is ascribed to the presence of supercages selectivity is the matching between the pore dimensions and
Table 1 Characterization and reaction results of different catalysts
PODEn product distributiond (%)
SiO2/
Structure Al2O3
Acidityb
SBET
Vtotal
dBJH
(nm) (nm)
dNLDFT Trioxane
conv.c (%) n = 2
(mmol NH3 gÀ1
)
(m2 gÀ1
)
(cm3 gÀ1
)
n = 3–8
n Z 8
a
Catalysts
USY-1
USY-3 (60)
C10-AS-50
FAU
FAU
Pm3n
5.6
60
48
54
2.75
0.27
0.31
0.28
790
768
1045
1090
0.40
0.47
0.53
0.60
—
—
1.3
2.0
1.1
1.2
2.0
3.0
92.0
93.4
92.1
90.6
49.9
50.6
45.1
47.7
49.0
48.9
53.5
50.6
1.1
0.5
1.4
1.7
C16-Al-SBA-1 Pm3n
a
b
c
Measured by ICP-AES. Measured by NH3-TPD. Reaction conditions: methylal 5 g, trioxane 2 g, catalyst 0.525 g, N2 pressure 1.3 MPa, temperature
d
105 1C, time 2 h, and mesitylene as internal standard. Area normalization method.
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of PODE8. The molecules of PODE3–8 in the supermicropore
would be partially restricted and the fine matching between
the pore diameter and the molecule size leads to a superior
selectivity for PODE3–8. While for mesoporous C16-Al-SBA-1, its
pore diameter is too large to fit the PODE3–8 molecules and the
product molecules can diffuse out facilely. It should be noted
here that the product molecules could vary their structures or
conformations to adapt to the catalyst pores, and thus each of
these products could be generated. The confinement effect of
the pore size lies in altering the distribution instead of the kind
of products. For clarification, the conformity of the pore
diameter of the catalysts to the selectivity of PODEn products
is facilely illustrated in Scheme 1.
Fig. 2 FTIR spectra for pyridine desorption on different catalysts (A) USY-1,
(B) USY-3 (60), (C) C10-AS-50, and (D) C16-Al-SBA-1 at different tempera-
tures (a) 100 1C, (b) 150 1C, (c) 250 1C, (d) 350 1C and (e) 450 1C.
the sizes of product molecules. For USY-1 and USY-3 (60) catalysts,
The meaning behind the discovery of shape selectivity in
diffusion in the pores would be highly restricted due to smaller supermicropores is not limited to the application of the probe
opening of their micropores in the size of 0.74 nm despite the reaction we discussed herein. The potential application of ordered
presence of supercages with the diameter of 1.3 nm. It was difficult supermicroporous aluminosilicates in industrial catalysis, parti-
for their micropores to accommodate and diffuse PODEn products cularly the petroleum and petrochemical industry, needs to be
with large molecular size. As a result, shape selectivity of PODEn43 further elucidated. It has been stated that the usage of zeolite Y
in the micropores of USY-1 and USY-3 (60) was the lowest among all (FAU type) in the FCC (fluid catalytic cracking) process results in a
the catalysts (Table S2, ESI†). On the other hand, when C16-Al-SBA-1 much higher gasoline yield.13 The fine matching of pore dimen-
was used, a large pore diameter makes the diffusion of PODEn in sions of the supercages in zeolite Y with the size of gasoline
the mesopores much more unrestricted. However, the rapid diffu- hydrocarbons contributes to the increased production in gasoline
sion of the products might result in a low polymerization degree pool.18 Nevertheless, to fit larger molecules, it is essential to
between methylal and trioxane. Once the newly generated light increase the pore size in catalysts. The fit between the super-
products (for example, PODE2) desorbed from the acid sites and microporous pore size and the dimensions of diesel molecules
diffused out fast, further polymerization and chain propagation can result in an increased selectivity for products in diesel pool.
would be inhibited. As we can observe in Table 1, enlarging the Taking into account that the dimensions of the micropores
pore diameter of the catalyst from supermicropores to mesopores hamper reactions between bulky molecules and shape selectivity
leads to an increased selectivity for PODE2 but a reduced selectivity is not expected in mesopores, ordered supermicroporous alumino-
for PODE3–8. For a similar case, Christensen et al. have observed silicates possessing shape selectivity can be a potential valid
that the zeolite catalyst with the hierarchical pore architecture extension of zeolites for industrial processes.19
possesses higher product selectivity for ethylbenzene than the
conventional zeolite catalyst in the ethylation of benzene reaction,
the reason for which lies in the fact that the shorter diffusion path
in mesoporous zeolite suppresses the further alkylation of ethyl-
benzene.17 In the current work, when supermicroporous alumino-
silicate catalyst C10-AS-50 was used, the diffusion rate in the pores
was moderate so that chain propagation could proceed successfully
while pores with medium diameter would prevent excessive
polymerization and the generation of a heavy product component.
As a result, high selectivity for PODEn with moderate molecular
size/weight (PODE3–8) can be expected.
The conformity of the pore diameter to the molecular size of
PODE3–8 can be further substantiated with the help of PODE3–8
models optimized by Gaussian 09. The sizes of the optimized
PODE3 and PODE8 models under the reaction conditions
Scheme 1 Illustration of PODEn product selectivity (PODE8 as an exam-
ple) in the pores with increased diameters of different aluminosilicate
are 0.70 nm  0.52 nm and 1.49 nm  0.28 nm, respectively
catalysts. In the supercages of zeolite Y, the PODE8 molecule can only exit
(see Table S3, ESI†). For catalysts USY-1 and USY-3 (60), the
in the form of some particular conformer and the diffusion through the
dimensions of their micropores are narrower than the length of
PODEn43 molecules. Although the possibility of oriented motion
of the molecules of PODEn43 parallel to the pores could not
be ruled out, it is still difficult for the pores of zeolite Y to
accommodate the ‘‘bulky’’ molecules. As a result, the selectivity
for PODEnZ3 in the products was low. For supermicroporous
opening is highly restricted, resulting in a lower selectivity for PODE3–8
.
The diameter of the PODE8 molecule is commensurate with the pore size
of C10-AS-50 (Pm3n symmetry) while the diffusion of PODE8 is partially
restricted, leading to higher selectivity for PODE3–8. In the case of
mesoporous C16-Al-SBA-1 (Pm3n symmetry), the diameter of the PODE8
molecule is smaller than the pore size. The molecule can be accommo-
dated in the form of a random conformer and diffused out facilely,
resulting in a lower PODE3–8 selectivity.
C
10-AS-50, the pore diameter is commensurate with the length
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