Understanding the Role of Gallium as a Promoter of Magnesium Silicate Catalysts for the Conversion of Ethanol into Butadiene

Article abstract of DOI:10.1002/cctc.201601630

In this study we explore the use of Ga as a new component in MgO-SiO₂ catalysts for the Lebedev reaction (the one-pot conversion of ethanol to butadiene). Several characterisation techniques (XRD, temperature-programmed desorption of NH₃, BET measurements, IR spectroscopy) and in situ spectroscopic studies (DRIFTS-MS) were performed with the aim to correlate the properties of the modified materials with the catalytic results. We concluded that the wet impregnation of Ga³⁺ on the MgO-SiO₂ catalyst creates new Ga?O(H)?Si sites. These sites interact strongly with alcohol and not only facilitate its dehydrogenation to acetaldehyde and its transformation into the intermediate crotyl alcohol but also enhance the dehydration of the latter because of an improved acidity. However, an appropriate amount of gallium oxide is needed to avoid excessive acidity, which is conducive to an increased selectivity to ethylene.

Full text of DOI:10.1002/cctc.201601630

Accepted Article
Title: Understanding the role of Gallium as a new promoter of MgO-
SiO2 catalysts for the conversion of Ethanol into Butadiene
Authors: Juliana Velasquez Ochoa, Andrea Malmusi, Carlo Recchi,
and Fabrizio Cavani
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To be cited as: ChemCatChem 10.1002/cctc.201601630
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Understanding the role of Gallium as a new promoter of MgO-
SiO2 catalysts for the conversion of Ethanol into Butadiene
Juliana Velasquez Ochoa,[a] Andrea Malmusi, [a] Carlo Recchi,[a] and Fabrizio Cavani*[a,b]
acetylene via hydroformylation to 1,4-butynediol (Reppe
process) or hydration to acetaldehyde and further
condensation.[6] Instead, in Russia it was implemented the first
successful one-pot process to obtain BDE directly either from
grain ethanol or from potatoes.[7] The details of this process,
known as Lebedev reaction (after the scientist Sergei Lebedev
who developed it) were kept secret during several years since
rubber was a very precious good especially during the war
period. This type of approach was later on abandoned when the
boom of petroleum made it economically less competitive until
the beginning of the XXI century, when the trend started to go
back to the use of renewable raw materials due to the economic
and environmental problems related to the use of fossil-derived
resources.[8–10] What it was revealed afterwards about the
Lebedev process was not very detailed, it was only an indication
on the composition of the catalysts used and of the reaction
conditions. For this reason, during the last years several studies
have been devoted to the optimization of the parameters for this
process, but until now there is no general agreement on how it
really works, that is, what is the real mechanism of this catalyzed
reaction, and on reasons why the MgO-SiO2 catalyst shows the
best performance.[11,12] Nevertheless, what is sure is that the
catalyst properties must be carefully tuned in terms of Mg/Si
ratio, acid/base properties and incorporation of promoters.[13–15]
Among the metals that have been studied to increase the
activity of MgO-SiO2 catalysts, Cr[16–18], Ni[19], Zn[20], Cu[21,22]
Ag[23,24] and Au[25] (or combinations such as Zn-Zr[26,27] or Cu-
Ag[28]) have shown to give an acceptable performance (from 5 to
56% BDE yield). In this work, it is proposed the use of Ga due to
its well-known dehydrogenating properties that have made it
relevant in different catalytic applications, i.e., in olefins
Abstract: This study explores the use of Gallium as a new
component in MgO-SiO2 catalysts for the Lebedev reaction (one-pot
conversion of ethanol to butadiene). Several characterization
techniques (XRD, NH3-TPD, BET, ATR) and in-situ spectroscopic
studies (DRIFTS-MS) were performed with the aim of correlating the
properties of the modified materials with the catalytic results. It was
concluded that the wet-impregnation of Ga3+ ion on the MgO-SiO2
catalyst creates new Ga-O(H)-Si sites, that interact strongly with the
alcohol and not only facilitate its dehydrogenation to acetaldehyde
and transformation into the intermediate crotyl alcohol, but also
enhance the dehydration of the latter compound because of an
improved acidity. On the other hand, a proper amount of Ga oxide
content is needed, in order to avoid excessive acidity which is
conducive to an increased selectivity to ethylene.
Introduction
Because of the forecasted decrease in 1,3-butadiene (BDE)
production by means of the conventional extraction from the C4
fraction produced by naphtha cracking, as well as of the possible
increase of demand for bio-sourced rubber, several alternative
routes are currently under investigation for the synthesis of bio-
BDE, starting from various renewable sources.[1] Several options
are being investigated, such as (a) the direct fermentation route,
(b) the one-step or two-step dehydration of butanediols – which
appears to be one of the most promising routes [2,3] - and (c) the
direct transformation of bio-ethanol.
Indeed, the interest in the development of better catalysts for
the transformation of ethanol into BDE, has observed an
exponential increase in the last decade due to the higher
availability and lower cost of this 2nd generation bio-alcohol that
constitutes an important renewable building block for a more
sustainable chemical industry.[4,5] However, the reaction itself is
not new, it is known since the early XX century when many
countries were trying to overcome their dependence on natural
rubber by developing a synthetic version, and the most
successful formulations all contained BDE as monomer.
production by propane or ethane dehydrogenation[29–31]
methane activation and aromatization[32]
methanol to
,
,
hydrocarbon conversion[33]. Ga oxide also shows acid properties,
that make it active, amongst others, in Friedel-Crafts benzylation
and acylation reactions.[34,35] The present study is an attempt to
understand the behaviour of Ga oxide-modified MgO-SiO2
catalysts.
Synthetic BDE was produced at that time starting from
Results and Discussion
[a]
[b]
J. Velasquez Ochoa, A. Malmusi, C. Recchi. and F. Cavani
Dipartimento di Chimica Industriale “Toso Montanari”
Università di Bologna
Viale Risorgimento 4, 40136, Bologna, Italy.
E-mail: fabrizio.cavani@unibo.it
A MgO-SiO2 catalyst was prepared by means of wet kneading
since it has been stated (and we verified as well, see Table S1)
that this method produces more active catalysts for this
reaction.[15,22] Figure 1 shows the X-ray diffraction patterns of the
unmodified MgO-SiO2 catalyst (MS-1, where 1 indicates the
Mg/Si atomic ratio) and the samples containing different
amounts of Ga, after calcination at 500°C for 3h. The MS-1
catalyst presents reflections attributable to MgO and a broad
band typical of amorphous SiO2. On the other hand, the
Consorzio INSTM, Firenze, Research Unit of Bologna
Supporting information for this article is given via a link at the end of
the document.
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impregnated samples were mainly amorphous with some broad
reflections that seem attributable, with some uncertainty, to
MgSiO3 (JCPDS 00-047-1750), and 2MgO·3SiO2·2H2O (JCPDS
00-002-1009). However due to their broadness and low intensity,
an unambiguous assignment could not be done. There was no
evidence of any crystalline phase containing Ga. From the figure,
it is clear that the impregnation method used caused changes in
the original structure and that probably during the stirring of the
catalyst with the Ga(NO3)3 solution, there was some hydration
and re-dissolution of the Mg and Si phases creating new Mg-
silicate compound with the Ga species dispersed in this matrix.
Table 1. Surface area and results of catalytic tests for samples impregnated
with different Ga content.
Sample
Surface
area
Ethanol
conv. [a]
(mol %)
BDE
BDE
Ethylene
Selectivity[a]
(mol %)
Yield[a]
(mol %)
Selectivity[a]
(mol %)
(m2/g)
MS-1
82
262
149
168
48
65,9
99,7
99,9
99,9
98,8
99,7
32,5
37,5
34,6
48,2
52,4
42,1
49,3
37,6
34,6
48,2
53,1
42,2
26,7
49,4
28,8
30,4
15,7
13,3
0,5% Ga
1,0% Ga
3,0% Ga
5,0% Ga
7,0% Ga
51
[a] Conditions: Feed 2% Ethanol in N2, W/F 0,65 g s/mL, WHSV=0,075
gEtOH/(gcat h), T 400°C. Average values of the last 3h of time-on-stream.
Other products obtained with higher yield for catalysts with Ga
include butenes (the sum of the isomers), propylene and
acetaldehyde (See Table S2). This indicates that Ga introduces
dehydrogenating properties but also some acidity, and this was
confirmed by the NH3-TPD and pyridine adsorption tests (vide
infra).
In similar studies by Larina et al.[36] for a Zn-modified MgO-
SiO2 catalyst, it was found that a 4%ZnO on a MgO:SiO2 1:1 was
the best compromise between the acid/base characteristics of
the support and the dehydrogenating capacity of Zn.
The effect of temperature for the best performing catalyst (MS-
1+5%Ga) was studied. Results presented in Table 2 confirm that
400°C is the more suitable temperature to maximize the yield to
BDE. Below this temperature the conversion was lower whereas
at higher temperatures the production of heavy compounds was
greater (as seen from the C loss).
Figure 1. X-ray diffraction pattern of the MS-1 sample before impregnation
(bottom) and after impregnation with different Ga content (wt% Ga) after
calcination at 500°C. Symbols: *MgO, SiO2, 2MgO·3SiO2·2H2O, +MgSiO3.
Table 1 presents the surface area for the unmodified and the
impregnated samples. The structural change caused by the
impregnation procedure resulted in higher area materials. The
trend indicates that a low Ga content originated compounds with a
higher surface area; conversely, at higher load of the metal, the
area sensibly decreased probably due to the formation of Ga
oxide aggregates.
The samples were tested in the direct conversion of ethanol to
BDE (Lebedev process). Results are presented in Table 1. An
increase of conversion was shown in the presence of Ga. Already
at 0,5 wt% Ga, conversion was complete. BDE yield also
increased in all cases with respect to the sample without Ga.
However, the amount of by-products also increased (mainly
ethylene and butenes). Indeed, the selectivity to BDE first declined,
because of the remarkable increase of selectivity to ethylene,
shown by sample with 0,5% Ga. However, a further increase of
the Ga content led to a lower selectivity to ethylene (16 and 13%
for samples with 5 and 7% Ga, respectively); catalyst MS-1+5%Ga
was that one showing both the best yield and selectivity to BDE,
higher than 50%. It is also important to notice that even though the
higher activity of samples containing 0,5-3% Ga compared to MS-
1 can be attributed to their higher surface area, this is not the case
for catalysts containing > 3% Ga.
Table 2. Temperature effect for ethanol transformation with MS-1+ 5%Ga
catalyst.
Temp.
(°C)
Ethanol
conversion[a]
(mol %)
BDE
BDE
Ethylene
Selectivity[a]
(mol %)
C loss
(mol %)
Yield[a]
(mol %)
Selectivity[a]
(mol %)
255
300
350
400
450
6,7
4,5
8,0
73,8
65,0
48,4
53,1
40,2
0,0
5,3
6,4
6,2
13,1
37,8
98,8
99,1
18,4
52,4
39,9
9,1
4,8
15,7
11,3
9,0
19,9
[a] Conditions: Feed 2% Ethanol in N2, W/F 0,65 g s/mL, WHSV=0,075
gEtOH/(gcat h). Average values of the last 3h of time-on-stream.
The introduction of 5% Ga was then carried out using different
methods other than wet impregnation (WI). Specifically, the Ga
source was directly added to the slurry during the synthesis of
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the catalyst (in-situ method), or deposited by means of incipient
wetness impregnation (IWI). Figure 2 compares the XRD
patterns for samples prepared with the three different methods
used. It is observed that only the WI method produced changes
in the structure of the sample; in fact, the other two samples
presented the same diffraction pattern as the original MS-1,
albeit with different intensity.
heavy compounds formed (measured as C-loss), that was even
greater than BDE yield. For catalyst MS-2 the measured BDE
yield was ca 32% (Entry 7) and in presence of 5% Ga the yield
was 47% (Entry 8). These results are in line with recent findings
(including our previous research work) that in this reaction the
preparation method of MgO-SiO2 and the Mg/Si ratio are critical
parameters.[22,36,39]
In recent studies it has been reported that the nature and
amount of Mg-silicate phases formed during the synthesis
procedure has a direct influence on the catalytic performance for
this reaction.[37] Moreover, when this type of catalysts are
modified with a dehydrogenating promoter, the formation of a
solid solution with either SiO2 or MgO might be responsible for
the increased activity of these materials, as explained by
Angelici et al.[38] They studied this reaction with a catalysts
prepared supporting Cu on either SiO2 or MgO prior to the
addition of the other component during the wet-kneading, and
found that materials where Cu was supported on MgO prior to
wet-kneading with SiO2 showed better overall performances and
Table 3. Results for direct ethanol transformation to BDE with Ga/MgO-SiO2
catalysts prepared using different methods.
C
Ethanol
Conv.[a]
(mol %)
BDE
BDE
Sel.[a]
(mol %)
Ethylene
Sel.[a]
(mol %)
loss
(mol
%)
Entry
Sample
MS-1
Yield[a]
(mol %)
1
2
3
4
5
6
7
8
65,9
98,8
96,5
98,6
80,0
99,9
79,1
98,8
32,5
52,4
36,2
28,4
18,6
28,0
31,9
46,5
49,3
53,1
34,4
28,8
23,3
28,0
40,4
47,1
26,7
15,7
13,8
5,3
3,8
9,0
MS-1+5%
Ga
MS-1+5%
Ga in-situ
MS-1+5%
Ga IWI
23,1
41,9
9,0
a large portion of Cu was found to be present in a CuxMg1−x
O
solid solution and CuO subnanometric clusters.
FS
(forsterite)
47,7
8,0
FS+5%Ga
MS-2
37,0
1,2
39,7
9,6
MS-2+5%
Ga
18,5
[a] Conditions: Feed 2% Ethanol in N2, W/F 0,65 g s/mL, WHSV=0,075
EtOH/(gcat h) T 400°C. Average values of the last 3h of time-on-stream.
g
,
Spectroscopic studies: In order to correlate the properties of
the catalysts and their performances, catalyst MS-1+5%Ga was
further characterized and compared to MS-1. Figure 3 shows the
DRIFT spectra of samples, recorded at both 85°C and 400°C
(using KBr as background). For the sample containing Ga, a
sharp band at 3672 cm-1 is present. This band has been
ascribed to the vibration of surface OH multi-coordinated to Ga3+
in an octahedral environment.[40,41] The band at 3730 cm-1 may
be assigned to mono-coordinated OH groups.[41]
For the same samples, attenuated total reflection IR spectra
(ATR) were taken in order to observe the spectral range at lower
wavenumbers that is difficult to analyze by means of DRIFTS.
Spectra are shown in Figure 4; it is possible to observe that the
presence of Ga caused the appearance of a Si-O-Si stretching
band (at ca 1014 cm-1), which is attributable to a newly
developed interaction, i.e., to a Ga-O-Si bond. The fitting of the
spectra in the region of this band (Figure S1) confirms that the
component at lower wavenumber is not present in the sample
without Ga.
Figure 2. XRD pattern for sample MS-1 (A) and samples with 5% Ga
introduced by the in-situ method (B; MS-1+5%Ga in-situ), by incipient wetness
impregnation (C; MS-1+5%Ga IWI) and by wet impregnation (D; MS-1+5%
Ga) (all patterns are on the same intensity scale).
The catalytic tests showed that the in-situ sample (Table 3,
Entry 3) afforded a slightly lower conversion (96,5%) and it also
formed more heavy-products (C loss). The BDE yield was higher
for the in-situ sample compared to the IWI catalyst (36% vs 28%,
Entries 3 and 4 of Table 3). Nevertheless, both samples
presented a lower yield to BDE and the C balance was worse
than in the case of the WI sample (entry 2 in Table 3).
Two more MgO-SiO2 samples were used for the impregnation
with 5% Ga, one synthesized by sol-gel and calcined at high
temperature (forsterite, FS), and one synthesized by wet-
kneading but with a higher Mg/Si ratio (MS-2). Catalytic results
for FS without Ga showed an average of 19% yield to BDE
(Entry 5, Table 3); Ga addition (Entry 6, Table 3) increased the
conversion. However, there was a considerable amount of
In-situ DRIFT spectra of both samples, during ethanol
adsorption at room temperature, are shown in Figure 5. The
results (after subtraction of the catalyst spectrum) showed that
the alcohol adsorbs molecularly as well as in its dissociated form
(ethoxy group, C2H5O-). The negative peak, clearly shown in
spectra of both samples (at high wavenumbers), is due to the
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interaction of the free OH groups on the catalyst surface that
become unavailable due to the interaction with the alcohol.
sample with Ga (48 vs 82 m2g-1 for MS-1+5%Ga and MS-1,
respectively). Therefore, the Ga-modified sites are responsible
for an additional direct interaction with the alcohol; in fact, the
negative peak is more intense in the case of the Ga-containing
sample. On the other hand, the peaks associated to the
intermediate species (800-1500 cm-1) seem to be less intense
for the Ga-containing sample (compared to the negative peak,
for instance). This could be due to the fact that in the case of
MS-1+5%Ga, the adsorbed intermediates were more easily
transformed and desorbed. This hypothesis was verified
afterwards in the experiments carried out by increasing the
temperature (vide infra).
Figure 3. DRIFT spectra of samples MS-1 (Blue, dotted) and MS-1+5%Ga
(Red, continuous) at 85 and 400°C under He flow.
Figure 5. DRIFT spectra of ethanol adsorption at room temperature for
samples MS-1 (Blue, bottom) and MS-1+5%Ga (Red, top).
After the saturation with ethanol at room temperature, the cell
temperature was increased until 400°C at a rate of 10°C min-1
(with continuous flow of ethanol in He). Figure 6 shows the
results of ethanol reaction at different temperatures for MS-1
and MS-1+5%Ga. The spectra were obtained after subtraction of
the background (bare catalyst mixed with KBr), registered at
each temperature.
At high wavenumbers, there was a negative band that
corresponds, in both catalysts, to an interaction of the alcohol
with the free OH groups. This interaction is stronger in the case
of the catalyst with Ga, due to the presence of additional Ga-OH
groups that can react with ethanol, and even at high temperature
the intensity of this negative band continued to be strong,
whereas in the case of MS-1 (top spectra), it became less
intense with the increasing temperature. One important
difference is that the sample with Ga showed a band at around
Figure 4. ATR spectra of samples MS-1 (Blue, dotted) and MS-1+5%Ga (Red,
continuous).
1610 cm-1 that has been attributed to the νas of an adsorbed
[11,39]
crotyl alcohol species, precursor of BDE formation.
This
band can be seen at lower temperature (starting at 300°C) in
comparison with the sample without Ga, which presents this
band only at 400°C. Moreover, the formation of this band is
concomitant with the disappearance of the bands attributable to
the ethoxy species (1039-1076 cm-1), which according to our
previous proposed mechanism of reaction [11] is the precursor of
acetaldehyde. Therefore, this demonstrates that the sample
modified with Ga is more active for two reasons: it has a higher
dehydrogenating power (which explains the higher conversion)
Table 4 presents the assignment of the bands highlighting the
most characteristic ones for each species, whereas some of
them are in common to both adsorbed ethanol and ethoxy
species.[42–44] The sample with Ga adsorbed more ethanol (more
or less twice total intensity). This is also true if we consider that
the quantity of sample used in the experiment was the same in
the two cases, but the specific surface area was smaller for the
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and it also readily transforms the intermediates into crotyl
alcohol which is then dehydrated to BDE, due to the newly
developed acidic character. Figure S2 shows a comparison for
the spectra of the two samples recorded at 350°C.
Table 4. Assignment of the bands after ethanol adsorption on samples.
Vibration
Wavenumber (cm-1)
Metal-O
878
νas(C-O)bident/νas(C-C) ethoxy
νas(C-O)monodent ethoxy
δ(OH) in ethanol
δs(CH3) in ethanol
δas(CH3)
1043
1083
1276
1403
1450
δas(CH2)
1479
δs(H2O) (negative)
νas(α-C-H) ethoxy
νs(CH2)
1630
2743
2885
νs(CH3)
2900
νas(CH2)
2933
νas(CH3) in ethoxy
ν(OH) in ethanol
ν(free OH) (negative)
2975
3234
3714
Figure 6. DRIFT spectra of samples MS-1 (Top) and MS-1+5%Ga (Bottom)
during ethanol adsorption at increasing temperatures.
Another set of experiments was performed; in this case,
ethanol was sent at low temperature as a pulse (0,6 l min-1 for
15 min). Afterwards, the samples were left under He flow until
weakly adsorbed ethanol was removed (ca 30 min) and then a
temperature program was performed (until 400°C, ramp 10°C
During the ethanol-TPD experiments, the desorbed products
were followed with an on-line quadrupole (mass spectrometer).
The main products detected for the two samples are shown in
Figure 8. It can be inferred that for both catalysts, desorption of
unreacted ethanol and acetaldehyde formation and desorption
occurred at low temperature (slightly lower for the sample MS-
1+5%Ga). Ethylene formed as well with both samples; the profile
of the MS signal for this molecule was particular since it formed
at least in two stages; the first one was probably due to the acid-
catalysed ethanol dehydration, whereas the second one can be
explained by a particular mechanism of ethanol adsorption and
activation to generate a carbanion species, which then may
either decompose to ethylene (Scheme 1) or react with
acetaldehyde.[11]
On the other hand, it can be observed that ethylene and BDE
(the two main products of the reaction) showed similar trends for
the two samples, but both compounds were released earlier for
the sample with Ga (see Figure S3 for a comparison), and this is
in agreement with the spectroscopic observations regarding the
easier formation of BDE precursor (i.e., crotyl alcohol) in the
case of MS-1+5%Ga. However, this observation can also
suggest a higher acidity of this sample, since acidity is an
essential property for both the formation of ethylene and the
dehydration of crotyl alcohol to BDE.
min-1). Figure
7 shows the result of this ethanol-TPD
experiments for MS-1 and MS-1+5%Ga. In this case, samples
were not diluted in KBr. For both samples, at low temperature
the bands corresponding to adsorbed ethanol and ethoxy
species were present, even if some of the characteristic bands
below 1000 cm-1 were not seen probably due to the high
absorbance of the undiluted samples in this region.
There were two main differences between the samples; the
first is the way the OH groups interact with ethanol (negative
band at around 3730 cm-1): a sharper band was observed for the
sample with Ga, which might indicate that ethanol interacts
preferentially with only one type of OH group, the one mono-
coordinated with Ga (observed in the spectra of catalysts before
ethanol adsorption (Figure 3)). Instead, in the sample without Ga
this band was broader and less strong, indicating an interaction
of ethanol with different types of surface OH (Mg-OH or Si-OH).
The second difference was that the sample with Ga showed a
band at around 1617 cm-1 that has been ascribed to the
formation of an adsorbed crotyl alcohol, precursor for BDE
formation; this band was observed already at 300°C.
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Figure 8. Main products (signal from the mass spectrometer) during the
ethanol-TPD experiment for samples MS-1 (Left) and MS-1+5%Ga (Right).
CH
2
(-)
CH
CH
OH
H (+)
2
OH
OH
2
H C
-H2O
2
(-)H C
H C
2
(+)
H
O
2
H
Mg
Mg
O
Mg
O
Scheme 1. Formation of ethylene from ethanol via carbanion.[11]
As regards pyridine adsorption (Figure S4), it was observed
that MS-1+5%Ga contains a small amount of Brnsted sites
which are absent in MgO-SiO2,[30] and might correspond to Ga-
OH groups; besides, the Lewis acid sites are slightly stronger
(blue shifted). An interesting observation is a band usually
assigned to pyridone species [45] which indicates the presence of
very reactive OH species or acid/base pairs in the case of the
sample with Ga.
Figure 7. DRIFTS spectra of samples MS-1 (Top) and MS-1+5%Ga
(Bottom).during ethanol TPD.
The samples without Ga and with 0,5 and 5% Ga were
analyzed by means of NH3-TPD in order to evaluate their acidic
properties. From Figure 9 and Table 5 it is observed that the
sample containing the lower amount of Ga (0.5%) holds a
greater number of acid sites, which are of moderate-high
strength (i.e., with higher temperature of desorption); this can
explain its higher selectivity to ethylene. On the other hand, the
sample with 5% Ga has a lower number of acid sites but a
higher acid site density. However, it is important to notice that
the acid strength of these sites was relatively weak (Tmax= 265-
270°C vs 305-310°C for the other two samples). This might
explain its better performance in the Lebedev reaction (i.e., with
lower selectivity to ethylene and better selectivity to BDE), which
agrees with our previous research work in the sense that
selectivity to BDE is favoured by a higher acid site density but of
low/moderate strength.[39] These results are also in line with
recent findings concluding that a delicate balance between the
amount and strength of the acid sites in MgO-SiO2 catalysts is
needed in order to be selective in the Lebedev reaction.[14,24,36]
Figure 9. Ammonia-TPD for samples MS-1, MS-1+0.5%Ga and MS-1+5% Ga.
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Bragg law, using the d value: 2d sen = n. The specific surface area
was measured applying the single point BET method. The instrument
used for this analysis was a Carlo Erba Sorpty 1700. In the analysis
around 0.5g of the sample was placed inside the sample holder and then
heated at 150°C under vacuum (4 Pa) in order for it release the water, air
or other molecules adsorbed. Afterwards the sample was put in liquid
nitrogen and the adsorption of the gaseous N2 was carried out. NH3-
temperature-programmed desorption (TPD) measurements were
obtained with a TPD/TPR/TPO Micromeritics instrument. 100–300 mg of
catalysts were pre-treated at the calcination temperature for 45 min
under He flow. After cooling down to 100 °C, NH3 was adsorbed by
flowing of a 10% NH3 in He gas mixture for 20 min (30 mL min−1, NTP),
with subsequent He treatment for 60 min to remove physisorbed
Table 5. Results of ammonia-TPD tests for the samples impregnated with
different Ga content.
Sample
Surface area
(m2/g)
Acid sites
number
Acid sites density
(10-5 mol/m2)
(10-5 mol/g)
MS-1
82
262
48
7,6
11,5
8,9
0,093
0,044
0,185
MS-1+0,5%Ga
MS-1+5%Ga
molecules. Catalysts were then heated under He flow (50 mL min−1
,
NTP) at a heating rate of 10 °C min−1 up to 600°C. Attenuated total
reflectance spectra of the materials were recorded at room temperature
with an ALPHA-FTIR instrument at a resolution of 2 cm−1. First a
background was taken to eliminate the contribution of atmospheric water
and carbon dioxide. Later on, the powder was put in intimate contact with
the crystal to perform the measurement. DRIFTS-MS: In a typical
experiment, the samples were pre-treated at 450°C in a He flow (10 mL
min-1) for 45 min, in order to remove any molecules adsorbed on the
material. Then the sample was cooled down to room temperature and
ethanol was fed at 0.6 mL min-1 and vaporized. Subsequently, He was
left to flow until weakly adsorbed ethanol was evacuated. The
temperature was raised to 400°C at 10°C min-1 while registering the
spectra (DRIFT and on-line MS). The following selected mass
spectroscopy signals (m/z) were monitored continuously with time (and
temperature): 2, 16, 25, 28, 29, 30, 31, 40, 41, 43, 44, 45, 56, 58, 59, 60,
and 61. By combining the information obtained from several different m/z
signals, it was possible to obtain unambiguous information on the various
products formed. The IR apparatus used was a Bruker Vertex 70 with a
Pike DiffusIR cell attachment. Spectra were recorded using a MCT
detector after 128 scans and 2 cm−1 resolution. The mass spectrometer
was an EcoSys-P from European Spectrometry Systems.
Conclusions
When Ga is deposited on MgO-SiO2 catalysts by means of the
wet impregnation method, it affects the structure and creates
new surface Ga-OH groups (and Ga-O(H)-Si bonds) that
strongly interact with ethanol. This interaction seems to be more
effective on the OH groups coordinated to only one Ga cation.
This modification of the sample affects positively the
performance for BDE production. In general, it enhances BDE
yield by increasing the adsorption of ethanol on catalysts surface,
its dehydrogenation to acetaldehyde, and further transformation
to the adsorbed intermediate, crotyl alcohol. Moreover, Ga
modifies the acidic properties of the catalyst. These properties
can be tuned by changing the amount of Ga in the sample. In
particular, 5% of Ga was found to give an optimal balance
between acid sites density and strength, thus increasing not only
the rate of ethanol dehydrogenation, but also the selectivity to
BDE and decreasing ethylene formation.
The main limits of the direct Lebedev technology still remain (a)
the moderate selectivity to BDE, (b) the relevant number of by-
products formed (ranging from ethylene to heavier hydrocarbons,
such as aromatics), and (c) the low productivity, mainly due to
the low inlet concentration of ethanol used.
Catalytic tests: Reactivity experiments were carried out using
a
continuous flow reactor, operating under atmospheric pressure. Ethanol
percent was fixed to a 2 mol.% in N2 . The catalyst amount and flux of the
carrier were varied in order to achieve the desired residence time. Most
of the experiments were performed at W/F 0,65 g s/mL, WHSV=0,075
gEtOH/(gcat h), at 400°C. Downstream products were continuously
monitored by online gas chromatography (GC) using an Agilent-6890
instrument equipped with two columns (HP-5 50 m, 0.20 mm and HP-plot
Al2O3-KCl 30 m, 0.50 mm) and two detectors (FID and TCD).
Compounds were identified by means of GC-MS and then quantified by
external standard calibration curve. The values reported were taken as
the average of the last 3h of time on stream in a 4h run (stationary state).
Figure S5 shows the catalytic performance in function of time-on-stream
for samples MS-1 and MS-1+5%Ga.
Experimental Section
Synthesis: The MgO-SiO2 sample with a Mg/Si molar ratio of 1 (labelled
MS-1) was prepared as follows: The Mg source was Mg(OH)2 prepared
by precipitation of Mg(NO3)2·6H2O with NH3. The Si source was obtained
by precipitation of TEOS with NH3. Afterwards, the Mg and Si sources, in
molar ratio 1/1, were mixed under stirring in water for 4h and then dried
at 120°C during the night to be finally calcined at 500°C for 3h in static air.
The MS-1 catalyst was impregnated with different loading of Ga. The
method used was the wet impregnation (WI) with aqueous solutions of
Ga(NO3)3 in different weight percentage (of Ga), from 0.5 to 7%. Further
impregnation methods were as well explored: i) Incipient wetness
impregnation (IWI): dissolving the Ga source in the minimum amount of
water needed to wet the catalyst, then dropping this solution onto the
catalyst. ii) In situ: the Ga source was added directly during the synthesis
of the Mg-Si catalyst.
Keywords: Lebedev • Butadiene • Ethanol • DRIFTS
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Gallium oxide is an activity
and selectivity promoter for
MgO-SiO2 catalysts for the
direct transformation of
ethanol into butadiene
(Lebedev process).
Juliana Velasquez Ochoa,
Andrea Malmusi, Carlo Recchi,
Fabrizio Cavani*
Page No. – Page No.
Understanding the role of
Gallium as a new promoter
for the catalytic conversion
of Ethanol into Butadiene
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