Catalytic Transformation of Ethanol over Microporous Vanadium Silicate Molecular Sieves with MEL Structure (VS-2)

Article abstract of DOI:10.1006/jcat.1997.1755

The transformation of ethanol was carried out over vanadium silicate molecular sieves with MEL topology (VS-2) with different Si/V atomic ratios in the temperature range 523-623 K. The reaction was performed in a fixed-bed down-flow reactor at atmospheric pressure. Acetaldehyde, diethyl ether, and ethylene were the major products along with small amounts of acetone, acetic acid, ethyl acetate, and carbon oxides. The conversion increased while the selectivity toward acetaldehyde decreased with increase in reaction temperature. The kinetics of the reaction (at 5% conversion) indicated a nearly first-order dependence of the rate of formation of the major products on ethanol. The formation of acetaldehyde is suggested to be mainly through the involvement of the vanadyl species (V=O) while diethyl ether production is controlled by the simultaneous involvement of V=O and V-O-Si associated with vanadium in the lattice. The intrinsic activity of vanadium incorporated into the zeolite framework is nearly 10 times that of the vanadium present in the impregnated sample. The nature of the sites involved in the formation of the different products, as elucidated from spectroscopic techniques (NMR and ESR), and the possible reaction mechanisms are proposed.

Full text of DOI:10.1006/jcat.1997.1755

JOURNAL OF CATALYSIS 170, 304–310 (1997)
ARTICLE NO. CA971755
Catalytic Transformation of Ethanol over Microporous Vanadium Silicate
Molecular Sieves with MEL Structure (VS-2)
1
S. Kannan, T. Sen, and S. Sivasanker
National Chemical Laboratory, Pune 411 008, India
Received October 29, 1996; revised April 3, 1997; accepted May 23, 1997
hyde) was observed for the V–Ti–Si catalyst when com-
The transformation of ethanol was carried out over vanadium pared to both V–Si and V–Ti catalysts. The enhanced ac-
silicate molecular sieves with MEL topology (VS-2) with different tivity was attributed to the interactive nature of the TiO2
Si/V atomic ratios in the temperature range 523–623 K. The reac-
tion was performed in a fixed-bed down-flow reactor at atmospheric
pressure. Acetaldehyde, diethyl ether, and ethylene were the major
products along with small amounts of acetone, acetic acid, ethyl
acetate, and carbon oxides. The conversion increased while the se-
of greater heat dissipation and catalyst bed isothermicity.
lectivity toward acetaldehyde decreased with increase in reaction
temperature. The kinetics of the reaction (at 5% conversion) indi-
cated a nearly first-order dependence of the rate of formation of the
major products on ethanol. The formation of acetaldehyde is sug-
gested to be mainly through the involvement of the vanadyl species dation reactions (7–12). Such incorporation of vanadium
support and good molecular dispersion. Very recently, se-
lective dehydrogenation of ethanol was carried out over
V2O5 supported on �-Al2O3 (6). An improved selectivity
for acetaldehyde was observed, accounted for on the basis
Isomorphous substitution of vanadium in the framework
of zeolite and molecular sieves results in new materials with
remarkable catalytic properties particularly in partial oxi-
(
V == O) while diethyl ether production is controlled by the simul- into zeolite lattices creates isolated active centers in an or-
taneous involvement of V == O and V–O–Si associated with vana- dered matrix, responsible for such unusual reactivity. This
dium in the lattice. The intrinsic activity of vanadium incorporated
into the zeolite framework is nearly 10 times that of the vanadium
present in the impregnated sample. The nature of the sites involved
has been well documented in the literature by projecting
the differences in the activity of incorporated and impreg-
nated (supported) vanadium in zeolites (8a, 13, 14). Wan
in the formation of the different products, as elucidated from spec-
et al. (15) have studied the oxidation of ethanol over the
troscopic techniques (NMR and ESR), and the possible reaction
transition metal incorporated microporous aluminophos-
mechanisms are proposed.
�c 1997 Academic Press
phate molecular sieve AlPO4-5. VAPO-5 exhibited more
acidic behavior yielding mainly diethyl ether and ethylene
with small amount of acetaldehyde. Bellusi et al. (15) have
studied the oxidation of methanol over V-containing MFI
INTRODUCTION
Selective oxidation of ethanol to value-added chemicals silicalites. V silicalites showed a higher activity than V2O5
has deserved recently an increasing technological interest supported on silica with similar selectivityfor formaldehyde
in connection with the utilization of biomass as a chemi- and the difference in the activity was explained based on
cal source (1). Although silver-based catalysts are commer- the different sorption strengths.
cially used, a number of metal oxides have been tested for
The present investigation is concerned with the oxidation
this reaction in order to find a substitute for this commer- of ethanol over vanadium-containing silicalite with MEL
cial catalyst (2). Among the oxides screened, molybdenum- structure (VS-2) and its postmodified forms with different
and vanadium-based catalysts are the promising candidates Si/V atomic ratios. This study is aimed at delineating the
for the above reaction (3). Oyama and co-workers (4) have intrinsic activity of vanadium in the zeolite lattice with that
studied the oxidation of ethanol over silica-supported vana- present in the extra framework positions and to identify
dium oxide catalysts. No remarkable change in the activity the sites involved in the formation of the various reaction
was observed with vanadium loading and particle size of products.
the catalyst indicating that the reaction is structure insensi-
tive. Quaranta et al. (5) have extended the earlier study by
supporting V2O5 on a TiO2-coated SiO2 support. A good
improvement in both activity and selectivity (to acetalde-
EXPERIMENTAL
Catalyst Preparation and Characterization
1
The VS-2 samples were synthesized by a hydrother-
mal method using tetrabutyl ammonium hydroxide as the
Present address: Catalysis Division, Central Salt and Marine Chemi-
cal Research Institute, Bhavnagar–364 002, India.
304
0
021-9517/97 $25.00
Copyright �c 1997 by Academic Press
All rights of reproduction in any form reserved.

ETHANOL TRANSFORMATION OVER VANADIUM SILICATE MOLECULAR SIEVES
305
structure directing agent and VOSO4 as the source of vana- reaction temperature. Reproducible results were obtained
dium (12b). The as-synthesized samples were calcined in air after each set of experiment indicating the establishment
at 773 K for 10 h (VS2-Cal). The calcined materials were of the steady state without noticeable deactivation of the
treated with an aqueous ammonium acetate (IN) solution catalyst. A stable activity without much deactivation was
at 333 K for 12 h to remove extralattice vanadium (VS2-E). observed for a long time period (� 10 h). Carbon and oxy-
The extracted samples were exchanged with NaNO3 (IN) gen mass balances of 100 � 5% were obtained for most of
solution at 333 K for 12 h to remove the Brønsted acidic sites the experiments. Rate, when expressed asturnover rate, was
(
VS2-ENa). After extraction and exchange, the samples calculated as the number of molecules of ethanol converted
were again calcined at 773 K for 8 h. V-Sil-2 was prepared by per vanadium atom per second.
impregnating V2O5 on pure silicalite-2 at 333 K for 6 h and
the resulting sample was oven dried (383 K) and calcined
RESULTS AND DISCUSSION
at 773 K for 8 h. Two vanadosilicate samples, designated as
VS2-A and VS2-B were used in these studies. The prepara-
tion of these samples has already been described in detail
in our earlier publication (12b; samples B and C, therein).
VS2-A and VS-B samples were prepared under identical
synthesis conditions, except that different V concentrations
were used. The calcined, extracted (with NH4OAc), and
extracted and exchanged (with NaNO3) samples of VS2-A
and only the extracted sample of VS2-B were employed
for the catalytic studies. In addition silicalite-2 and V2O5-
impregnated silicalite-2 (V–Sil-2) were also used.
Table 1 summarizes the elemental composition of the
samples synthesized. It is clear from Table 1 that nearly
9
0% loss of vanadium was observed for VS2-A-Cal upon
extraction with NH4OAc indicating that vanadium is pre-
dominantly present in extralattice positions (12b). The in-
fluence of temperature on the conversion of ethanol over
various vanadium silicalite samples is presented in Fig. 1.
For comparison purposes, the reactivity of silicalite and
V-impregnated silicalite are also included in Fig. 1. Table 2
summarizes the conversion and selectivities obtained for
all the catalysts. During the reaction, on all the catalysts,
acetaldehyde, diethyl ether, and ethylene were observed as
Catalytic Studies
The reactor system employed for the present study minor products. A comparison of the activity of VS2-A-Cal
was a bench top reaction system (BTRS, Autoclave En- and its postmodified forms indicates that VS2-A-Cal and
gineers U.S.A), which is a fixed-bed continuous down-flow VS2-A-ENa samples possess similar activities, while VS2-
stainless-steel reactor (5 cc) operating under steady-state A-E possesses a lower activity. Figures 2a–2d show the plot
conditions. Ethanol was fed through a HPLC liquid injec- of selectivity to the major products at constant conversion
tion pump (Alcott, Model 760) and air was passed using levels (10 and 50% ). VS2-A-Cal and VS2-A-ENa samples
a mass flow controller (Brooks, Model 5896). The reac- exhibit a similar selectivity pattern and favor the dehydro-
tants were mixed and vaporized in a preheated oven main- genation reaction (acetaldehyde formation), whereas the
tained at 393 K and passed through the reactor (6 mm i.d.). extracted sample (VS2-A-E) favors the primary dehydra-
The temperature around the reactor was maintained by a tion product, namely diethyl ether (Figs. 2a and 2b). Fur-
temperature-controlled furnace. The outlet of the reactor thermore, the selectivity to acetaldehyde decreases with an
was directly connected to a gas chromatograph (HP-5890 increase in reaction temperature for the former samples
Series II) through a heated transfer line which was main- while it is not affected much for the later sample (Table 2).
tained at 393 K. An eight-port sampling valve was used to At higher temperatures, however, the selectivity for the sec-
inject a known amount of the product mixture or reactant ondary dehydration product, ethylene, is enhanced with a
(
250 �l) to the GC using nitrogen as the carrier gas. The loss in the selectivity of acetaldehyde and diethyl ether.
products were analyzed using a capillary column (cross-
linked methyl silicone gum, 50 m � 0.2 mm) and a FID.
The carbon oxides in the samples were estimated using an-
other GC (Shimadzu, 15-A) using a POROPAK-Q column
TABLE 1
Chemical Composition of the Samples Used in the Studies
(
2 m � 0.3 mm) and a TCD.
V/Si + V
�
3
Air after drying through a trap-containing molecular
Catalyst
Pretreatment
(� 10
)
Si/V
sieve (4A) and doubly distilled ethanol (>99% ) were used
for the reaction. A reactant gas mixture of 3 vol% C2H5OH
in air [5.3 cc/170 cc; WHSV of ethanol (mass of ethanol
VS2-A-Cal
VS2-A-E
Calcined at 773 K for 10 h
NH4OAc extraction at 333 K for 12 h
7.46
0.7
0.7
133
1427
1427
VS2-A-ENa Extraction + exchange with 1 N NaNO3
�
1
passed in 1 h/mass of the catalyst) = 2.6 h ]. The concen-
at 333 K for 12 h
tration of ethanol in air was well below the lower explosion VS2-B-Cal
Calcined at 773 K for 10 h
NH4OAc extraction of VS2-B-Cal
Impregnation with V2O5
4.1
2.4
9.8
0.0
243
415
101
∝
VS2-B-E
V-Sil-2
Sil-2
limit. Two-hundred fifty milligrams of the catalyst (mesh
size 22–30) was employed for the reaction. The catalyst was
initially activated at 673 K for 3 h in air and cooled to the
Calcined at 773 K for 10 h

306
KANNAN, SEN, AND SIVASANKER
if one were to normalize the conversion on the basis of the
V content of the samples, the activity of VS2-A-E is larger
than that of VS2-A-Cal indicating a higher intrinsic activity
of the V species in the latter catalyst (kTorr, Table 3). The
finding here of an enhanced activity with V concentration
does not have a linear dependence on the overall rate be-
cause of the presence of different types of V centers with
different intrinsic activities.
The selectivity pattern indicates (Fig. 2) that the calcined
sample favors acetaldehyde while the extracted sample fa-
vors diethyl ether. Correlating with the species present in
these samples, it is quite probable that V == O (associated
with both extralattice and lattice vanadium) is responsi-
ble for the aldehyde formation and both the V == O and
the V–O–Si are responsible for ether formation. This is
FIG. 1. Variation of total conversion with temperature over various
catalysts.
in accordance with the results reported by Trifiro et al.
(
21) on methanol oxidation, who claimed V == O bond is the
active site whose labilisation (reducibility) plays a crucial
role in controlling the selectivity of the oxidation products.
The selective oxidation of ethanol over supported metal
oxide surfaces has been studied extensively (17–19). The
important steps involved in the reaction are: (i) adsorption
and activation of ethanol on the active metal site and (ii)
decomposition of the ethoxide species (rate-determining
step) to form different products. However, the selective for-
mation of any specific product is essentially controlled by
the nature of the active sites present on the surface (M = 0,
TABLE 2
Oxidation of Ethanol over Various Catalysts
Selectivity (% )
Temp. % Conv-
Catalyst
(K)
ersion
C2H4 CH3CHO (C2H5)2O Othersa
0
M–O–M and M–O–M , where M is the active metal ion and
VS2-A-Cal
523
548
573
2.9
8.8
28.1
54.2
82.1
21
18
16
30
33
73
82
77
67
60
3
—
7
3
7
2
—
—
—
—
0
M is the support) (20). In the case of vanadium-containing
silicalite (VS-2) and its postmodified forms, it is evident
from Fig. 1 that the addition of vanadium into the silicalite
matrix enhances the activity indicating the active participa-
tion of vanadium. Although vanadium is the active site on
all these catalysts, a different selectivity pattern is observed
suggesting that the products are produced through the de-
composition of the ethoxide intermediate adsorbed on en-
5
6
98
23
VS2-A-E
523
3.1
7.3
18.1
35.3
54.7
10
14
29
52
52
35
37
31
29
34
55
49
40
19
14
—
—
—
—
—
5
5
5
48
73
98
623
ergctically different active sites. It was confirmed through VS2-A-ENa 523
5.1
11.1
32.6
52.6
67.4
16
14
20
21
39
80
77
73
61
61
4
9
6
7
—
—
—
3
11
—
spectroscopic techniques (12b) that VS2-E contains vana-
dium in a distorted tetrahedral environment (in +5 state)
incorporated into the zeolite lattice (1), while VS2-A-Cal
contains vanadium in +5 oxidation state present mainly in
extra framework positions in a symmetric tetrahedral envi-
ronment (II).
548
5
5
6
73
98
23
VS2-B-E
V-Sil-2
Sil2
523
13.3
36.3
69.4
100
4
7
21
50
74
49
43
43
33
18
49
50
35
5
—
1
12
3
5
5
5
6
48
73
98
23
100
5
—
523
3.8
15.6
27.8
68.4
92.4
8
11
21
21
28
92
88
76
77
69
—
1
3
2
3
—
—
—
—
—
5
5
5
6
48
73
98
23
523
0.8
2.2
4.8
13.1
22.7
4
5
8
17
30
51
45
33
32
30
45
50
59
51
40
—
—
—
—
—
5
5
5
6
48
73
98
23
The lower activity (conversion of ethanol) of VS2-A-E in
comparison with VS2-A-Cal is due to a lower concentra-
tion of vanadium in the former sample (Fig. 1). However,
a
Mostly acetone, acetic acid, ethyl acetate, and methanol.

ETHANOL TRANSFORMATION OVER VANADIUM SILICATE MOLECULAR SIEVES
307
TABLE 3
Kinetic Parameters for the Oxidation of Ethanol
over Various Catalysts
� 1
Catalyst
Temp. (K)
523
ln ka
Eapp (kJ/mol) kTorr (� 10� 3) (s )b
VS2-A-Cal
� 3.511
� 2.587
� 2.338
� 1.301
� 0.939
� 3.442
� 2.883
� 2.541
� 1.913
� 1.509
� 2.923
� 2.080
� 0.726
� 1.875
� 1.254
� 0.562
0.385
122.5c
(116.4)
3.7
8.9
d
5
5
5
5
33
48
63
73
11.2
27.2
35.7
VS2-A-E
523
94.1
(133.5)
42.3
72.3
5
5
5
5
33
48
63
73
99.6
174.6
247.0
VS2-A-ENa
VS2-B-E
523
109.0
(115.2)
69.6
151.4
444.8
5
5
48
73
523
135.3
(132.5)
52.7
88.1
5
5
5
5
33
48
63
73
144.0
236.0
275.3
FIG. 2. Variation of selectivity of major products at constant conver-
sion over various catalysts.
V-Sil-2
523
� 3.231
� 1.685
� 0.954
113.8
(109.4)
3.7
15.1
26.9
5
5
48
73
According to them, the activity of the catalyst is directly
connected with the metal oxygen double bond which opens
a
Calculated employing first order rate expression.
Turnover rate calculated as number of molecules of ethanol con-
b
with the addition of an H atom from the alcohol, which in verted per vanadium atom per second.
c
Eapp calculated based on the conversion of ethanol.
Eapp calculated based on the acetaldehyde formation.
turn, leaves a free valence on the metal used for bonding
with the dehydrogenated molecule (oxyreductive mecha-
nism). A similar perception was realized on MoO3-based
catalysts for ethanol oxidation in which it was suggested
that Mo == O sites were responsible for acetaldehyde forma-
d
nearly equal to unity is observed for all the three prod-
ucts. Extending the analysis, Arrhenius plots were con-
structed and the apparent activation energy values, Eapp,
were calculated for both total conversion and acetaldehyde
formation.
tion (22–25). The catalytic activity of the VS2-A-ENa sam-
ple is similar to that of the calcined sample. The objective
+
of the exchange of VS2-A-E with Na ion was to remove
the weakly Brønsted acidic sites associated with the defect
centers created with V incorporation (12c), thereby increas-
ing the selectivity toward acetaldehyde. In fact, a threefold
increase in the selectivity was observed especially at low
conversions, upon exchange (Fig. 2; Table 2).
In order to understand the kinetics of the reaction over
these samples, it is essential to know the order depen-
dence of ethanol on the rate of formation of the major
products (C2H4, CH3CHO, and (C2H5)2O). Figure 3 shows
the logarithmic plot of partial pressure of ethanol with
turnover rate. Here the turn over rates were calculated
assuming all V atoms (present both at internal and exter-
nal surface) are accessible for the reactant molecule. The
above assumption is valid in the case of zeolite molecu-
lar sieves (such as the MEL) in which all the framework
V ions are accessible through the pore system. Besides,
the small amount of the extraframework V species are
expected to be well distributed (dispersed) on the large
surface area (� 400 m /g). A linear dependence with slope various products over VS2-A-Cal.
FIG. 3. Effect of partial pressure of ethanol on the reaction rate for
2

308
KANNAN, SEN, AND SIVASANKER
Comparison of the catalysts with different Si/V atomic largely governed by the activation of the methanol on the
ratios indicate that an increase in the concentration of incor- acidic sites rather than on the oxidation function. However,
porated vanadium in the zeolite lattice increases the cata- on V-containing silicalites for ethanol oxidation, the dual
lytic activity (Table 2). Although the impregnated catalyst functionality, namely the nature of the vanadium center
contains nearly four times the concentration of the vana- (redox property) and the acidity around the active site con-
dium present in the VS2-B-E sample, a lower activity is trols this overall selectivity. In the case of V-impregnated
observed indicating the higher intrinsic activity of incor- silicalite, a selectivity pattern similar to VS2-A-Cal was ob-
porated vanadium. Figures 2c and 2d are a plot of the se- served, indicating that the sites involved in the product
lectivity of the major products for the catalysts with dif- formation on these samples are similar. Furthermore, the
ferent Si/V atomic ratios at constant conversion. Among turnover rates calculated based on number of vanadium
the samples, the impregnated catalyst shows a maximum atoms are also similar for VS2-A-Cal and V-Sil-2 (Table 3).
selectivity for acetaldehyde and a minimum selectivity for In essence, the nature of VS2-A-Cal and V-Sil-2 samples are
diethyl ether at both lower and higher conversions. The similar which is quite understandable as more than 90%
V-containing molecular sieves possess a higher selectivity of the vanadium resides outside the lattice in the former
for diethyl ether and nearly similar selectivity for acetalde- sample.
hyde. As in the case of the earlier set of samples (postmod-
ified forms), a higher selectivity for ethylene was observed formation over VS2-A-Cal at 523 K shows (Fig. 3) a rate
at the higher conversion. dependence corresponding to first order rate expression.
The partial pressure effect of ethanol for acetaldehyde
In contrast to the earlier set of samples (Figs. 2a and 2b) The Eapp values are calculated based on Arhhenius equa-
the increase in the activity is directly proportional to vana- tion [a linear plot of ln(x/1 � x) vs 1/T or ln(x1/1 � x) vs 1/T,
dium concentration. Similar turnover rates are observed where x is the conversion of ethanol and x1 is the forma-
(
Table 3) for VS2-A-E and VS2-B-E which indicates that tion of acetaldehyde] are given in Table 3. The Eapp (alde-
the nature of vanadium present in both the samples are hyde) values of samples containing framework V species
�
1
similar, even though the V concentrations are different. (VS2-A-E and VS2-B-E) are similar (� 130 kJ mol ) while
Furthermore, with an increase in vanadium concentration, the values for the other samples (VS2-A-Cal, VS2-A-ENa,
though a substantial increase in the apparent activation en- and V-Sil-2) containing extraframework species are also
�
1
ergy for total conversion is observed, the Eapp values for similar though of a lower magnitude (110–120 kJ mol ;
acetaldehyde formation are nearly the same (Table 3). A Table 3). This suggests that the formation of acetaldehyde
comparison of the activity between the impregnated sam- is less favorable over samples containing only framework
ple V-Sil-2 and VS2-B-E indicates that, despite a higher V atoms. Though the similarity of Eapp (aldehyde) values
concentration of vanadium in the former sample, the ac- for VS2-A-E and VS2-B-E are understandable, the rea-
tivity is lower by an order of magnitude (Tables 2 and 3). sons for the difference in the Eapp (conversion) are not
This clearly demonstrates the higher intrinsic activity of clear. The Eapp (conversion) values are similar for the other
the vanadium incorporated in the zeolite lattice when com- three samples containing mainly nonframework V atoms
pared to the extralattice vanadium. The selectivity pattern (Table 3).
indicates, in general, a higher selectivity for diethyl ether
A recent molecular orbital study by Weber (29) showed
at low conversions and at higher temperature and higher that during the oxidative dehydrogenation of methanol
conversions, the secondary dehydration reaction leading to over metal oxide surfaces (V and Mo), the initial destina-
ethylene is favored more over all the catalysts (Table 2). The tion of the hydrogen would seem to be the metal and not
involvement of acidity apart from redox property in con- an oxygen ligand (in opposition to proton transfer) because
trolling the selectivity of the products has been observed in the hydrogen and the metal develop a bonding interaction,
various catalytic transformations over V-containing com- while the hydrogen and any of the adjacent oxygens de-
pounds (26, 27). In all these cases, the possible influence of velop an antibonding interaction and this transfer is the
vanadium, which by itself can act as a Lewis acid site, cannot rate-limiting step of the reaction. Extending the argument
also be ignored. However, the selectivity to acetaldehyde for the oxidation of ethanol, the C–H bond breaking should
is not affected much at both conversions (10 and 50% ), be accompanied by a partial hydride transfer to the ac-
indicating that the sites responsible for aldehyde forma- tive center, vanadium. In the present series of catalysts, the
tion are not perturbed by an increase in the temperature higher selectivity of acetaldehyde observed over VS2-A-
(
Table 2). In general, the selectivity pattern appears to Cal and V-Sil-2 indicates that the labilisation of V == O bond
be considerably influenced by the acidity of the catalyst is more facile in species II compared to species I (shown
around the active site). Ai(28) has observed a linear cor- earlier). It is likely that both the V == O and Si–O–V link-
(
relation during the oxidation of methanol over V2O5-based ages are involved in the ether formation and hence higher
catalysts between formaldehyde formation and the acid- selectivity of ether is observed for VS2-A-E and VS2-B-E
ity of the catalyst, suggesting that the oxidation activity is samples. Based on these observations, a mechanism can be

ETHANOL TRANSFORMATION OVER VANADIUM SILICATE MOLECULAR SIEVES
309
SCHEME 1. Plausible reaction mechanism for the formation of products during the transformation of ethanol.
proposed for the formation of products resulting from the
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