Platinum/phosphotungstic acid/(Zr)MCM-41 catalysts in glycerol dehydration

Full text of DOI:10.1016/j.mencom.2014.04.015

Mendeleev
Communications
Mendeleev Commun., 2014, 24, 167–169
Platinum/phosphotungstic acid/(Zr)MCM-41
catalysts in glycerol dehydration
Wimonrat Trakarnpruk
Department of Chemistry, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand.
E-mail: wimonrat.t@chula.ac.th
DOI: 10.1016/j.mencom.2014.04.015
MCM-41 and ZrMCM-41 impregnated with 2% Pt and 30–40% phosphotungstic acid exhibit catalytic activity in the dehydration of
glycerol to acrolein, and Pt doping increases the stability.
Glycerol dehydration to acrolein is an attractive sustainable
alternative to the conventional acrolein process (oxidation of
propene with Bi/Mo-mixed oxide catalyst). Solid acid catalysts
Table 1 Chemical analysis, textural parameters and acidity of catalysts.
HPW
ana-
Acidityc
1
Pore
Pore Total
S
/
BET
b
Catalyst
volume/ size/ acidity /
B/L
–
including sulfates, phosphates and zeolites have been tested for
lyzeda m g
2
–1
3
–1
–1
cm g
nm mmol g
I(B) I(L)
2
–4
the dehydration of glycerol in either gaseous or liquid phases.
(%)
Heteropoly acids, which have strong Brønsted acidity, showed
decreasing activity in the liquid phase dehydration of glycerol to
MCM-41
ZrMCM-41
0
0
993
792
0.80
0.71
0.64
3.8
0
–
–
3.6 0.26
3.1 0.63
0.19 0.23 0.8
0.83 0.72 1.1
5
acrolein in the order H SiW O > H PW O > H PMo O .
4
12 40
3
12 40
3
12 40
3
5%HPW/
34.6 722
Due to their very low surface area and high solubility in polar
solvents, the synthesis of supported catalysts was focused on
activated carbon-supported H SiW O (10% loading, yield
MCM-41
Pt/30%HPW/ 29.4 765
MCM-41
Pt/35%HPW/ 34.4 753
MCM-41
Pt/40%HPW/ 38.8 736
MCM-41
Pt/30%HPW/ 29.0 712
ZrMCM-41
Pt/35%HPW/ 32.8 686
ZrMCM-41
Pt/40%HPW/ 36.6 680
ZrMCM-41
0.70
0.67
0.65
0.69
0.67
0.65
3.4 0.55
3.5 0.67
3.5 0.71
3.5 0.51
3.5 0.68
3.3 0.73
0.70 0.66 1.0
0.83 0.72 1.1
1.09 0.80 1.4
0.74 0.68 1.1
0.85 0.71 1.2
1.12 0.80 1.4
4
12 40
–
1
–1 6
of 68.5 mmol h g ), ZrO -supported H PW O (10–40%
loading, 42–54% yield, 65–71% selectivity), alumina- and
aluminosilicate-supported H SiW O (20% loading, 75% selec-
2
3
12 40
7
4
12 40
8
tivity) and SiO -supported H SiW O (30% loading, >85%
2
4
12 40
9
selectivity). The major drawback to the gas-phase dehydration
of glycerol on acid catalysts is catalyst deactivation due to coke
deposition. Addition of alkaline and alkaline earth cations
improved the catalytic performance, but they still deactivated
over a short reaction time.10 The decrease in glycerol conver-
3
sion was reported: >60% after 8 h (zeolite), >20% after 10 h
a
Deduced from the chemical analysis of W by ICP AES. b Measured by
(
ZrO -supported H PW O ) and 40% after 5 h (SiO -supported
NH -TPD techniques. cMeasured from pyridine adsorption FTIR.
2
3
12 40
2
3
7
H PW O ). The catalyst deactivation is overcome by co-
feeding O2, or doping with noble metals and co-feeding H ,
with the order of activity Ru, Pt < Pd. Zr doped mesoporous
silica catalysts are more thermally stable and have high selectivity
for acrolein. H SiW O supported on SBA-15 grafted with
3
12 40
1
1
Table 1 summarizes the specific surface area, pore volume, pore
2
12
‡
size and acidity of the support and the catalysts. The decreases
of specific surface area, pore volume and pore size with Pt and
HPW loadings indicate that the HPW was either introduced in the
porosity, filling the pores and/or being deposited on the external
surface of the mesoporous material.
1
3
4
12 40
1
4
ZrO was reported to have a long-time performance. Mixed
2
Zr–Nb oxide exhibited stability (82% glycerol conversion after
1
5
1
77 h). Previously, we used MCM-41-supported Cu and Ce
16
phosphotungstates for the dehydration of ethanol and MCM-41-
supported H PW O for the esterification of palm fatty acids
distillate.
material was filtered off and dried at 80°C for 12 h. The sample was
calcined at 500°C for 6 h. The determined Si/Zr molar ratio was 8.9.
MCM-41 or ZrMCM-41 was first impregnated with an aqueous solution
of H PtCl ·6H O (2 wt% Pt). The sample was dried overnight at 110°C and
3
12 40
1
7
The aim of this work was to improve the long-time stability of
MCM-41-supported catalysts for selective dehydration of glycerol
2
6
2
calcined in air at 300°C for 3 h. Then, it was impregnated with H PW O
3
12 40
solutions (30, 35 and 40 wt%), dried overnight at 110°C and calcined
†
into acrolein. The supports and catalysts were synthesized and
at 350°C for 2h. 35%HPW/MCM-41 without Pt was also prepared for
comparison.
‡
characterized.
‡
Specific surface areas were measured using the BET method on
†
MCM-41 was synthesized as follows: 1.988 g of cetyltrimethylammo-
BELSORP-mini. XRD measurements were performed on a Rigaku DMAX
nium bromide (CTAB) was dissolved in 120 g of water; 8 ml of aqueous
2002/Ultima Plus powder X-ray diffractometer. The acidity of the catalysts
NH (conc.) and 10 ml of tetraethyl orthosilicate (TEOS) were added
was studied by the temperature-programmed desorption of ammonia
3
with vigorous stirring. The solution became milky and slurry formed.
After 1 h, the material was filtered off and dried at 80°C for 12 h. The
sample was calcined at 500°C for 6 h. ZrMCM-41 with the molar ratio
Si/Zr = 10 was synthesized as follows: 12.2 g of CTAB was dissolved in
(NH -TPD). To evaluate the types of acid sites, pyridine adsorption on
3
the samples was performed on a 170-SX FTIR spectrometer. The FTIR
spectra were obtained on a Nicolet FT-IR Impact 410 spectrophotometer
with a pressed KBr pellet. The amounts of Zr and W were determined by
inductively coupled plasma atomic emission spectrometry (ICP, Perkin
Elmer model PLASMA-1000). The carbon content of spent catalysts was
determined from combustion analysis.
1
10 ml of water and 110 ml of aqueous NH (conc.). A solution prepared
3
from 5.5 ml of zirconium n-propoxide (70% in propanol) and 22.8 ml
of TEOS was then dropwise added with vigorous stirring. After 2 h, the
©
2014 Mendeleev Communications. All rights reserved.
– 167 –

Mendeleev Commun., 2014, 24, 167–169
5
1
2
4
3
2
1
1
400
1450 1500
1550 1600
n/cm–1
1650 1700
Figure 1 Pyridine-adsorption FTIR spectra of (1) Pt/35%HPW/ZrMCM-41
and (2) Pt/35%HPW/MCM-41.
8
60
86
9
8
11
1
200
1000
n/cm–1
800
600
5
3
2
1
00
75
50
1
2
Figure 4 FTIR spectra of (1) HPW, (2)–(4) Pt/HPW/MCM-41 (30, 35 and
45% loading, respectively) and (5) MCM-41.
peak intensities decreased with the loading amount of HPW. The
XRD patterns of the MCM-41-supported catalysts (Figure 3) show
diffraction peaks corresponding to HPW at 2q angles of 25.3,
2
9.3, 34.6 and 37.7° (PDF#38-0178). The XRD patterns of the
25
0
ZrMCM-41-supported catalysts are similar but with a slight shift
of the (100) peak position towards smaller angle resulting from an
increase in the lattice cell parameter from the substitution of Zr.
The FTIR spectra of the MCM-41 and Pt/HPW/MCM-41
catalysts are depicted in Figure 4. The IR spectrum of HPW in
0
.0
0.2
0.4
0.6
0.8
1.0
P/P
0
–
1
the region of 600–1300 cm contains characteristic bands at 960
(W=O), 886 and 811 cm (W–O–W). A tiny band at 886 cm
Figure 2 N adsorption–desorption isotherms of (1) Pt/35%HPW/MCM-41
and (2) Pt/35%HPW/ZrMCM-41.
2
–
1
–1
still appears in the spectra of the supported catalyst, which
confirmed the retention of the HPW structure after impregnation.
The FTIR spectra of pyridine adsorption on the supported
catalysts (Figure 1) show Brønsted (B) and Lewis (L) acidities,
the integrated areas of the absorbance bands at 1444 (B) and
–
1
However, the bands at 960 and 811 cm are not clearly evident,
–
1
as well as the bands at 1065–1100 cm attributed to n (P–O)
as
−
1
18
1
542 (L) cm are quantified. The total acidity and B/L ratios
vibration were obscured by an intense and broad band of MCM-41.
The FTIR spectra of the Pt/HPW/ZrMCM-41 catalysts are similar
are given in Table 1. The incorporation of Zr into the MCM-41
framework weakens the SiO–H bond, generates Brønsted acid
–
1
except that the tiny band at 886 cm was observed only in case
of the 40% HPW loaded catalyst.
19
sites and increases the total acidity. The supported catalysts had
a larger amount of acid sites than pure supports. With increasing
acid loading, not only the total acidity but also both Lewis and
Brønsted acidities increase. Compared with Pt/HPW/MCM-41
catalysts (30–35% HPW loadings), the B/L ratio of Pt/HPW/
ZrMCM-41 catalysts is a little higher. It was reported that
Pt/HPW/ZrMCM-41 showed a stronger interaction of HPW
with the ZrMCM-41 support than MCM-41.20
§
The catalytic activity is given in Table 2. The main product is
acrolein with minor amounts of acetaldehyde, hydroxyacetone,
propanal and trace amounts of others. In Scheme 1, when protona-
tion occurs at the secondary hydroxy group of glycerol 1, a water
molecule and a proton are eliminated from the protonated glycerol,
and then 3-hydroxypropanal 2 is produced by tautomerism. Since
this 3-hydroxypropanal intermediate is unstable, readily dehydrated
into acrolein 3, it was not detected, similarly to that reported
The N adsorption–desorption isotherms of the MCM-41-
2
1
8
and ZrMCM-41-supported catalysts are similar (Figure 2). They
exhibit a reversible type IV isotherm with a hysteresis loop typical
of mesoporous materials.
for TiO -supported HPW catalyst. When protonation proceeds
2
O
OH
‡
O
The powder XRD patterns of MCM-41 and ZrMCM-41
H+
exhibit a low-angle diffraction peak due to the (100), (110), (200)
and (210) planes characteristic of the MCM-41 structure. The
–
H2O
HO
OH
OH
OH
H+
2
3
–
H2O
OH
HO
OH
O
OH
1
2
1
H2C
Me
4
3
4
5
Scheme 1
§
Dehydration of glycerol was carried out in a vertical tubular fixed-bed
quartz reactor (20 cm in length and 10 mm in internal diameter). The
catalyst (20–40 mesh, 0.4 g) was put into the center of the reactor between
–
1
quartz wool. The catalyst was first reduced under H flow (15 ml min )
2
0
2
4
6
8
20
q/°
30
40
50
60
for 1 h at 250 °C. Then, a 10% aqueous solution of glycerol was fed into
–1
an evaporator at 250 °C with a flow rate of 0.14 ml min (or WHSV of
2
–1
–1
2
.1 h ). Then, the gas stream was transported into the reactor in 15 ml min
Figure 3 XRD patterns of (1) HPW, (2) MCM-41 and (3)–(5) Pt/HPW/
MCM-41 (30, 35 and 40% HPW, respectively).
N2 flow under atmospheric pressure and reacted at different reaction
temperatures (300–400°C).
–
168 –

Mendeleev Commun., 2014, 24, 167–169
1
00
90
80
Table 2 Glycerol dehydration (5 h).
Acrolein
selectivity Ref.
×
×
×
×
Conver-
sion (%)
×
Entry Catalyst
T/°C
(
%)
×
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
MCM-41
300 10
30
28
79
74
78
77
80
69
73
77
76
78
76
81
69
77
77
71
95
94
This worka
This worka
This worka
This worka
This worka
This worka
This worka
This worka
This worka
This worka
This worka
This worka
This worka
This worka
This worka
This worka
This worka
8
7
0
ZrMCM-41
300 66
350 70
35%HPW/MCM-41
60
50
Pt/30%HPW/MCM-41 300 68
350 76
5
10
20
30
40
50
Pt/35%HPW/MCM-41 300 76
350 82
Time/h
400 80
Figure 5 Time dependence of glycerol dehydration at 350°C over (×) Pt/
35%HPW/ZrMCM-41, ( ) Pt/35%HPW/MCM-41 and ( ) 35%HPW/
MCM-41.
Pt/40%HPW/MCM-41 300 79
350 83
1
1
1
1
1
1
1
1
1
1
2
Pt/30%HPW/ZrMCM-41 300 70
350 80
The long term stability of the catalysts was tested with
Pt/35%HPW/ZrMCM-41 and Pt/35%HPW/MCM-41 compared
to the Pt-free catalyst at 350°C (Figure 5). Both Pt loaded
catalysts retain conversion until 40 h; at 50 h, the glycerol conver-
sion decreased to 78–80%, while the Pt-free catalyst demonstrated
a drastic decrease in conversion (60% in 20 h). This might be due
to coke deposition (7.5%). A lower coke deposition was found
for the Pt loaded catalysts (£1.0–1.5%). The Pt reduces catalyst
coking by hydrogenating coke precursors.
Pt/35%HPW/ZrMCM-41 300 79
350 87
400 76
Pt/40%HPW/ZrMCM-41 300 80
350 86
20%HPW/SiO2b
300 67
275 60
275 72
c
20%HPW/SiO (in N )
12
2
2
2%Pd/20%HPW/SiO
12
2
(
in H2)c
ZrMCM-41 (Si/Zr = 10)d 325 73
24
13
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4
5
2
at a terminal hydroxy group of glycerol, hydroxyacetone 4 is
produced through dehydration and deprotonation accompanied by
tautomerism. Other by-products include acetaldehyde, propanal
and trace unidentified compounds (each less than 4%).
MCM-41 and ZrMCM-41 produce significantly low glycerol
conversion and selectivity for acrolein (Table 2, entries 1, 2). The
supported catalysts provide much higher glycerol conversion and
selectivity for acrolein. The 35%HPW/MCM-41 shows 70%
conversion with 79% selectivity. When loaded with Pt, under
the same reaction condition, a higher conversion (82%) was
achieved (entry 3 vs. 7). This reveals the higher catalyst stability
attributed to the role of Pt in reducing coke formation.
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acrolein is related to Brønsted acid sites, while Lewis acid sites
show higher selectivity for hydroxyacetone.14 Increasing HPW
loading results in a higher B/L ratio (Table 1); therefore, the
selectivity for acrolein increased. Among the test catalysts,
Pt/35%HPW/ZrMCM-41 shows the best performance (82%
conversion with 80% selectivity, entry 7). As for other by-products
over the Pt-loaded catalysts, the selectivity for hydroxyacetone is
1
1
1
2
2
1
1–18%, along with acetaldehyde (<8%) and propanal (<5%).
There is a difference in the selectivity for hydroxyacetone between
Pt/35%HPW/MCM-41 and 35%HPW/MCM-41 (16% vs. 7%),
similarly to a result reported for Pt/SBA-15 and SBA-15.
2
2
Selectivities for hydroxyacetone and acetaldehyde from the bare
support are almost equal.
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The glycerol conversion increased with the reaction tempera-
ture (300–400°C). However, the selectivity for acrolein decreased
at 400°C (entries 8 and 15) due to the formation of acrolein
oligomers or the cracking of acrolein. The experimental results
showed that both the reaction temperature and the acid loading
affected the conversion of glycerol and the selectivity for acrolein.
Received: 13th May 2013; Com. 13/4119
–
169 –