Sustainable production of acrylic acid: potassium-ion
exchanged zeolites for gas-phase dehydration of lactic acid
Bo Yan,1,2,* Bo-Qing Xu1,*
1
Innovative Catalysis Program, Department of Chemistry, Tsinghua University, Beijing, 100084, CHINA
Fluocarbon Chemical R&D and Technology Center, Zhejiang Research Institute of Chemical Industry Ltd., Hangzhou, 310023,
CHINA
*Corresponding author: [email protected] (Bo Yan), [email protected] (Bo-Qing Xu)
2
Keywords: ion-exchanged zeolites, acid-base catalysis, catalytic synthesis, acrylic acid, lactic acid
1. Introduction
Development of high-performance solid acid-base
catalysts for chemicals and materials production
from bio-resourced feedstock has become an
important research topic in heterogeneous catalysis
for renewable energy and green chemistry. As a bioderived platform molecule, LA can be dehydrated to
acrylic acid (AA), which is an important
intermediate for a number of key chemicals and
materials [1]. Research efforts to develop highly
efficient and selective solid acid-base catalysts for
this LA-to-AA (LTA) reaction in the gas-phase
would entitle a green and sustainable alternative to
current commercial AA production technology
based on petroleum.
Previous investigations on the LTA reaction over
various solid acids and bases have shown that the
most effective catalysts would be those having both
weakly acidic and weakly basic sites with suitably
balanced acidity and basicity [2-6]. Our recent study
of this reaction over various alkali-ion (Li+, Na+, K+,
Rb+ and Cs+) exchanged Beta zeolite catalysts
(SiO2/Al2O3 = ca. 40) for this reaction discovered
that KxNa1-xβ catalysts with high fractional exchange
degree of K+ [molar K/(Na + K) ratio, x = 0.92~0.98]
were highly selective for AA production [3,4]. We
provide herein a comprehensive study on the
catalytic performance of various K+-exchanged
zeolites (KxNa1-xZ_y) with similar K+ exchange
degree and molar K/Al ratios for the LTA reaction
and discuss the effects of zeolite type (Z = ZSM-22,
ZSM-35, MCM-22, ZSM-11, ZSM-5, ZSM-5/ZSM11 and β) and SiO2/Al2O3 ratio (y).
2. Experimental
The KxNa1-xZ_y catalysts were prepared by ion
exchange of their corresponding zeolites in sodium
form (NaZ) with an aqueous solution of 0.5 M KBr,
followed by filtration, drying and calcination. The
catalyst samples were characterized by nitrogen
adsorption, XRF, XRD, NH3-TPD, CO2-TPD, etc.
The gas-phase LTA reaction was conducted at 360
o
C under atmospheric pressure in a vertical fixed bed
quartz reactor, using an aqueous LA solution
containing 35.7 wt% LA (10 mol%) as the reaction
feed and N2 as the reaction carrier gas. The weight
hourly space velocity of LA (WHSVLA) was 2.1 h-1.
3. Results and discussion
Seven zeolites were selected as the representatives
with one-dimensional (1D: ZSM-22), twodimensional (2D: MCM-22 and ZSM-35) or threedimensional (3D: ZSM-5, ZSM-11, co-crystalline
ZSM-5/ZSM-11, and β) pore structures with 10- or
12-ring openings. The catalytic data obtained at
TOS = 9~10 h are given in Table 1 to show the
stabilized product distribution over the different
KxNa1-xZ_y catalysts with a similar K+ exchange
degree (x = 0.90~0.97) and K/Al ratio (ca. 1.9). The
AA selectivity was higher than 50 mol% over all the
catalysts except K0.96Na0.04ZSM-22_50 (ca. 35
mol%) and K0.96Na0.04ZSM-11_46 (ca. 45 mol%),
and could reach as high as 58~65 mol% over the
K0.94Na0.06ZSM-5_43 and K0.96Na0.04β_42 catalysts.
AD (9-35 mol%) and/or 2,3-PD (1~29 mol%)
appeared as the most abundant byproduct, followed
by PA (1~7 mol%) and 1-hydroxyacetone (1-HA:
0~4 mol%). Except the products clearly specified in
Table 1, any other product including all the very
minor liquid byproducts, gases and surface
carbonaceous deposits are arbitrarily added up and
shown as “Others” in the table. Thus, K+-exchanged
ZSM-5 and β zeolites are found far superior to the
others in the LTA reaction in terms of AA selectivity
and yield.
Besides, the Si/Al ratios (y) for these KxNa1-xβ_y and
KxNa1-xZSM-5_y (x = 0.90~0.98) catalysts are
shown to greatly impact on not only the AA
selectivity and yield but also the catalyst stability;
lower y number leads to better catalytic performance.
For example, Figure 1 presents the effects of y on
the catalytic performance of KxNa1xZSM-5_y by the
time courses of LA conversion and AA selectivity,
indicating that the catalyst with a lower y often
deactivated more slowly and produced higher AA
selectivity. A K0.97Na0.03ZSM-5_27 is identified as
the best-performing catalyst, which offers AA
selectivity higher than 78 mol% and yield higher
than 72 mol% for longer than 10 h. This catalyst also
shows a remarkable stability and regeneration
property in long-term reaction up to 80 h (Figure 2),
during which the AA selectivity and yield are kept
higher than 70 mol% and 55 mol%, respectively.
Furthermore, an in situ calcination of the used
catalyst at 450 C in an air flow entitles complete
catalyst regeneration.
Table 1. Catalytic performance of K+-exchanged zeolite catalysts for gas-phase dehydration of aqueous lactic acida
Catalyst
Product selectivityb (mol%)
1.6
2.1
2.2
2.1
2.2
LA
conv.
(%)
92
71
81
100
75
2.1
2.0
90
83
K/Al
(molar)
K0.96Na0.04ZSM-22_50
K0.90Na0.10ZSM-35_28
K0.97Na0.03MCM-22_26
K0.96Na0.04ZSM-11_46
K0.96Na0.04ZSM5/ZSM-11_50
K0.94Na0.06β_42
K0.94Na0.06ZSM-5_43
AA
AD
2,3-PD
PA
1-HA
Others
33
49
53
44
50
33
9
17
25
25
1
25
19
10
6
1
3
3
5
6
0
3
3
1
1
32
11
5
15
12
AA
yield
(mol%)
29
34
43
44
37
58
60
18
23
12
3
7
2
0
0
5
12
52
50
c
Carbon
depositsd
(mgcarbon⋅mcat-2)
0.23
0.45
0.52
0.12
0.11
0.32
0.13
a
Catalyst loading: 500 mg; WHSVLA: 2.1 h-1; rxn temp.: 360 oC; TOS = 9~10. b AA: acrylic acid; AD: acetaldehyde; PA: propionic
acid; 2,3-PD: 2,3-pentanedione; 1-HA: 1-hydroxyacetone. c The “Others” refer to all unlisted products including gaseous products
and surface carbonaceous deposits, whose selectivity (mol%) = 100 - ∑(selectivity for each listed product). d Surface carbonaceous
deposits accumulated after reaction for 10 h.
100
LA conv. (%)
LA conv. (%)
95
90
85
100
80
AA
60
40
80
AD
20
75
80
60
40
20
0
70
0
2
4
6
8
0
0
10
20
40
60
80
100
TOS (h)
TOS (h)
Figure 1. Effect of SiO2/Al2O3 (y) on the catalytic performance by
the time courses of (A) LA conversion and (B) AA selectivity for
KxNa1-xZSM-5_y (0.92 ≤ x ≤ 0.98): y = 27 (), y = 36 (), y = 43
(), y = 68 (), and y = 75 ().
4. Conclusions
KxNa1-xβ_y and KxNa1-xZSM-5_y of x = 0.90~0.97
and y < 50 are identified as the highly selective ones
for AA production (sel.: 60~81 mol%) among the
various zeolite catalysts. In particular, the aluminarich ZSM-5 catalyst (K0.97Na0.03ZSM-5_27) shows
the best catalytic performance in terms of high AA
selectivity (70-81 mol%) and yield (55-78 mol%),
catalytic durability (> 80 h) and easy regeneration.
The correlations between the product selectivity and
acidity/basicity ratio reveal that suitably balanced
acidity and basicity at the catalyst surface are critical
in offering the high selectivity for AA production. It
is also shown that the acidity/basicity ratio for
enabling the possible highest AA selectivity is
dependent of the zeolite type and SiO2/Al2O3 ratio in
the framework.
AA or AD sel. (mol%)
B
Regeneration with flowing air at 450 oC
A 100
Figure 2. Stability and regeneration performance
by the time courses of LA conversion (, ),
AA selectivity (, ) and AD selectivity (,
) of the K0.97Na0.03ZSM-5_27 catalyst.
Acknowledgments
This work is partly supported by NSF of China (grant:
21533004, 21033004), the National Basic Research of China
(grant: 2013CB933103) and Tsinghua University (grant:
20131089311).
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