Effect of the pore structure of an active alumina catalyst on isobutene production by dehydration of isobutanol

Article abstract of DOI:10.1039/d1ra00136a

An alumina catalyst was prepared by mixing and pinching with pseudo-boehmite, and the catalyst was reamed with polyethylene glycol. The catalysts prepared were characterized by means of XRD, mercury injection and NH₃-TPD, and the dehydration properties of the catalysts prepared with different amounts of reamer were evaluated in a 10 mL fixed bed reactor with 5% water as a raw material. The results showed that the addition of reamer did not affect the crystal structure and the amount of acid of the catalyst. With the increase of the amount of reamer, the pore volume of the catalyst increased continuously, the number of large pores increased, the conversion rate of isobutanol increased, and the selectivity of isobutene remained basically unchanged. When the amount of reamer is 30%, the isobutanol conversion rate is the best. The isobutanol conversion rate and the isobutene selectivity were 97% and 93% respectively under the conditions of 330 °C, 0.1 MPa and 12 h^?1air velocity of the body liquid.

Full text of DOI:10.1039/d1ra00136a

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Effect of the pore structure of an active alumina
catalyst on isobutene production by dehydration of
isobutanol
Cite this: RSC Adv., 2021, 11, 11952
Kaige Tian,
and Guanglin Zhou
Qin Li, Weili Jiang, Xiaosheng Wang,
Shicheng Liu, Yapeng Zhao
*
An alumina catalyst was prepared by mixing and pinching with pseudo-boehmite, and the catalyst was
reamed with polyethylene glycol. The catalysts prepared were characterized by means of XRD, mercury
injection and NH3-TPD, and the dehydration properties of the catalysts prepared with different amounts
of reamer were evaluated in a 10 mL fixed bed reactor with 5% water as a raw material. The results
showed that the addition of reamer did not affect the crystal structure and the amount of acid of the
catalyst. With the increase of the amount of reamer, the pore volume of the catalyst increased
continuously, the number of large pores increased, the conversion rate of isobutanol increased, and the
selectivity of isobutene remained basically unchanged. When the amount of reamer is 30%, the
isobutanol conversion rate is the best. The isobutanol conversion rate and the isobutene selectivity were
97% and 93% respectively under the conditions of 330 ꢀC, 0.1 MPa and 12 hꢁ1 air velocity of the body liquid.
Received 7th January 2021
Accepted 12th March 2021
DOI: 10.1039/d1ra00136a
rsc.li/rsc-advances
increase.23–25 The preparation method is complicated and
expensive and not conducive to industrial use.
1. Introduction
Both HZSM-5 molecular sieve and alumina catalysts are used
in the reaction of ethanol dehydration to ethylene on an
industrial scale. Molecular sieves are well known for their
adjustable acidity and shape-selective catalysis,26 but they are
rapidly deactivated due to coke deposition. Therefore, the
preparation of thermally stable catalysts with anti-coking
properties has important meaning and high value. This can
be achieved through a exible catalyst design.27 The catalyst
carrier is not only a support, but also a carrier to provide
a complex microenvironment—this is a key factor in deter-
mining the catalytic function.28 The catalyst carrier should have
sufficient thermal stability to maintain the shape of the catalyst
and provide an open structure for the catalyst. This can make it
easy to contact the reactants and prevent the coke from clogging
the pores.29,30
Porous alumina is one of the most widely used materials in
industry. It has good structural properties and thermal stability.
Unfortunately, the pore size of alumina is small, and the reac-
tants tend to stay in the small pores leading to coking and side
reactions that inevitably deactivate the catalyst.30,31 According to
reports, glucose, hydroxy acids, and urea can be used as pore
formers in catalysts to change the morphology/texture of
different oxide catalysts.32–34
Isobutylene is an important part of the petrochemical
industry.1–3 As an important organic raw material in the global
chemical industry, a large amount of isobutylene is needed
every year to produce other chemicals, including downstream
chemicals such as MTBE,4 polyisobutylene,5 butyl rubber, tert-
butanol, and ABS resin.6–9 The current industrial processes for
the production of isobutylene are mainly based on steam
cracking and uid catalytic cracking, which involve vast
amounts of energy and signicant CO2 emissions.10–14 The
dehydration of biomass alcohol to obtain isobutylene is a more
economical and environmentally friendly approach than
petroleum-based systems.15,16 The process of dehydrating
biomass alcohol to olens is relatively well-established.17 For
example, a joint venture established by Dow Chemical and
Brazil's CRYSTALSEV to produce polyethylene from sugar cane
has been in production for many years. Here, the main catalysts
for alcohol dehydration to olens are alumina,18–20 hetero-
polyacids, transition-metal oxides,21 and HZSM-5 molecular
sieve catalysts.22 The stability of heteropolyacid and transition
metal oxide catalysts is poor, and the catalyst is easily deacti-
vated when the temperature rises. Moreover, the selectivity of
heteropolyacid catalysts becomes poor as the temperature
increases. Olens tend to aggregate, and by-products
Here, we studied the relationship between polyethylene
glycol as a low-cost pore-forming agent and the structure and
catalytic performance of alumina. We adjusted the pore struc-
ture of alumina by changing the amount of polyethylene glycol
added. A series of rod-shaped porous alumina materials were
College of New Energy and Materials, China University of Petroleum-Beijing, 18, Fuxue
Road, Changping District, Beijing, 102249, China. E-mail: zhouguanglin2@cup.edu.
cn; Tel: +86-10-89731160
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2. Results
2.1. X-ray diffraction (XRD)
Fig. 1 shows XRD patterns of PEG-modied Al2O3 at different
contents of PEG. A diffraction peak of g-Al2O3 at 2q ¼ 67.00ꢀ,
45.84ꢀ, 39.47ꢀ, and 37.59ꢀ is observed at PEG loadings of 10 wt%
or higher. This indicates that the crystal structure of the catalyst
is g-Al2O3 (COD: 01-079-1158).35 The crystalline modications of
g-Al2O3 were not observed; that is, the crystal structure of the
catalyst with different additive contents is the same. The PEG
additive does not affect the crystal structure of g-Al2O3.
2.2. Automatic mercury porosimeter and the adsorption–
desorption isotherms of nitrogen
The pore size distribution and physical structure data of the
catalysts prepared with different amount of reamer during the
preparation process are shown in Fig. 2 and Table 1, respec-
tively. Table 1 shows the total pore volume (Vp) and the mean
pore size (MPS). This changed to a higher extent for the
supports obtained with different PEG contents. Table 1 shows
that, upon adding the expanding agent to the catalyst, the
average pore diameter is sufficiently greater than that without
the expanding agent. Moreover, with increasing proportion of
the added expanding agent, a signicant increase in the pore
volume and average pore diameter of the catalyst occurs. The
pore size distribution of the expanding agent has a more
signicant inuence on the entrance and pore diameter. This
may be caused by the fact that the polymer PEG 400 has short
helices and, when added during sample preparation, can aid in
obtaining materials with a somewhat tight structure.36
Fig. 1 XRD patterns of PEG-modified Al2O3 at different PEG loadings.
As can be seen from Fig. 2, macropores were produced by the
addition of reamer. With increasing proportion of reamer, the
proportion of pore diameters of 2–5 nm in the prepared catalyst
gradually decreased, from 92.61% to 21.02%, while the
proportion of 1000–7000 nm macropores increased signi-
cantly, from 1.05% to 50.81%. The macroporous aperture ratio
increased to improve the mass transfer performance of the
catalyst. Upon injecting isobutanol into the hole and the living
heart center of the contact reaction, the reaction of isobutylene
molecules easily spreads out at the same time, reducing the
secondary reaction of isobutylene, which occurs as a result of
the coked catalyst deactivation, thus improving the catalytic
Fig. 2 Pore size distribution of PEG-modified Al2O3 at different PEG
loadings.
prepared with the equal volume impregnation method. The
modied alumina served as a catalyst for the reaction of iso-
butanol dehydration to prepare isobutene with higher selec-
tivity and activity. This greatly reduces the deactivation rate.
Table 1 Specific surface area and pore structure properties of PEG-modified Al2O3 at different PEG loadings
Pore size distributiona/%
103–7
ꢂ
7 ꢂ
Catalyst
SBETb (m2 gꢁ1
)
VPa (mL gꢁ1
)
2–5
5–10
10–103
103
103–104
S104
MPS/nm
Al2O3
308.61
239.60
223.95
130.24
0.3088
0.4974
0.5432
0.6175
92.61
46.04
36.01
21.02
0.83
4.96
4.64
3.09
0.44
13.19
13.88
16.53
1.05
0.12
0.64
0.6
4.96
6.06
6.92
7.49
4
10% PEG-Al2O3
20% PEG-Al2O3
30% PEG-Al2O3
29.11
37.95
50.81
8.3
11.8
19
1.06
a
MIP analysis. b The specic surface area of the catalyst sample was calculated with the Brunauer–Emmett–Teller (BET) method.
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subject activity stability.37 Therefore, the catalyst with reaming
agent has good catalytic de-watering activity.
2.3. Thermo gravimetry-differential thermal analysis (TG-
DTA)
The amount of coke formed on the spent catalysts was evaluated
by TG in airow. Fig. 3(a) shows the difference in the TG proles
between the Al2O3 and 30% PEG-Al2O3 catalysts. Two used
samples were prepared by being recovered aer the reaction at
330 ꢀC for 24 h. All the used catalysts had a decrease in weight at
temperatures higher than 200 ꢀC. The difference in weight loss
between the Al2O3 and 30% PEG-Al2O3 catalysts must be
attributed to the carbon deposition. The carbon content, which
was calculated from the difference in the TG curves between the
Al2O3 and 30% PEG-Al2O3 catalysts, was 16.81 and 12.34 wt%.
This result showed that the amount of coke formed over the
Fig. 4 NH3-TPD of PEG-modified Al2O3 at different PEG loadings.
Al2O3 catalyst was signicantly higher than that formed over the
30% PEG-Al2O3 catalyst.
Fig. 3(b) shows the DTG curve. The DTG curve indicates that
ꢀ
the weight losses of the spent catalysts occurred at 400 C and
560 ꢀC. The peak at 400 ꢀC in the DTG curve might be related to
the amorphous coke that was easily oxidized. The peak at 560 ꢀC
could be assigned to the coke with high crystallinity.38
Thus, it is assumed that carbon is relatively difficult to
accumulate on 30% PEG-Al2O3 because of the large pores of
30% PEG-Al2O3. In other words, PEG-modied alumina exhibits
the suppression of coking.
2.4. The temperature-programmed desorption of adsorbed
NH3 (NH3-TPD)
NH3-TPD experiments were carried out to analyze the number
and strength of the acid sites. It was observed from Fig. 4 that all
the samples presented a broad desorption peak ranging from
ꢀ
ꢀ
150 C to 500 C. The shapes of the proles were simple and
symmetric, suggesting the presence of the same acid sites. To
further investigate the distribution of acid sites, the asymmetric
proles were tted by a Gaussian function. The patterns
exhibited one desorption peak, while the peak centered between
220 ꢀC and 290 ꢀC was assigned to the deliberation of medium-
strong acid sites. The distribution of surface acidity was similar
in Al2O3, and the fraction of the type of acid sites in the former
does not show a signicant change with the loading of different
PEG contents. In the isobutanol dehydration reaction, higher
acid content and acid strength are conducive to the dehydration
reaction. However, if the acid strength is too high, side reac-
tions, superposition, and cracking reactions will also occur.39
Therefore, the dehydration reaction needs an appropriate acid
content and strength. The amount of acid and acid strength of
the four catalysts basically changed slightly. With increasing
amount of reamer added, the total amount of acid in the cata-
lyst decreased slightly, as did the amount of medium strong
acid. When the amount of reamer added was 30%, the amount
of medium strong acid in the catalyst was the lowest. This is
Fig. 3 Characterization of the spent catalysts after the isobutanol
dehydration, after 24 hours of reaction. (a) TG-air test, and (b) DTG
curves in air.
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Fig. 5 Catalytic activity of PEG-modified Al2O3 at different PEG loadings: (a) Al2O3; (b) 10% PEG-Al2O3; (c) 20% PEG-Al2O3; and (d) 30% PEG-
Al2O3.
benecial to inhibit the occurrence of side reactions and adding the pore expander is about 57% (Fig. 5a), and the cata-
improve selectivity.40,41
lyst isobutanol conversion rate when the pore expander is 10%
is 86%. The isobutanol conversion rate of the catalyst added
with the pore expander is signicantly increased and constant.
The isobutanol conversion rate of the catalyst is signicantly
increased when 30% of the pore expander is added to the
catalyst. The conversion rate of isobutanol reached 97%. The
addition of the pore expander increases the pore volume and
pore diameter of the catalyst. The addition of the pore expander
increases the contact surface between the reactant and the
active center of the catalyst. The addition of the pore expander
improves the mass transfer performance of the catalyst,
promotes the diffusion of raw materials and product molecules,
2.5. Catalytic activity of PEG-modied Al2O3 at different PEG
loadings
The pressure is 0.1 MPa when the reaction temperature is
330 ꢀC, and the volume liquid space velocity is 12 hꢁ1. The
reaction performance of isobutanol dehydration to isobutene
with different pore expander amounts of the g-Al2O3 catalyst is
shown in Fig. 5. The specic data of the main products are
shown in Table 2. Under 24 h of continuous reaction condi-
tions, the isobutanol conversion rate of the catalyst without
Table 2 Dehydration of isobutanol over PEG-modified catalystsa
Selectivityb/%
Catalyst
Conversionb/%
CH4
C2H4
C3H6
1-C4H8
cis-2-C4H8
trans-2-C4H8
iso-C4H8
tert-C4H8
Al2O3
56.9
86.0
93.9
97.0
0.4
0.3
0.3
0.1
0.8
0.6
0.9
0.5
0.9
0.5
0.8
0.6
1.6
2.3
1.9
2
0.7
0.5
0.4
0.3
0.6
0.4
0.4
0.2
93.3
93.2
93
1.2
1.3
1.5
2.1
10% PEG-Al2O3
20% PEG-Al2O3
30% PEG-Al2O3
92.9
a
b
Reaction conditions: P ¼ 0.1 MPa; T ¼ 330 ꢀC; LHSV ¼ 12 hꢁ1
.
Average activity in the initial 24 h.
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suppresses the occurrence of plugging and coking, and
improves the conversion rate of the catalyst.42 Therefore, there
is better dehydration performance of the catalyst for isobutanol
with increasing amounts of pore expander. The order of the
pore size of the four catalysts from large to small is 30% pore
expander >20% pore expander >10% pore expander >0% pore
expander. The 30% pore expander catalyst has the largest pore
size and pore volume. The 30% pore expander catalyst has the
highest conversion rate of isobutanol. However, the selectivity
of isobutylene did not change much during the experiment, and
the selectivity of isobutylene of the four catalysts was about
93%.
3.2. Catalytic reaction
The dehydration of isobutanol was performed in a xed-bed
down-ow stainless steel reactor with an inner diameter of 17
mm. Prior to the reaction, 15 mL of catalyst was placed in the
ꢀ
catalyst bed and heated at 550 C with an ambient pressure of
N2 for 2 h to activate the catalyst. Aer activation, isobutanol
containing 5% water was used as the raw material. The raw
material was fed through the top of the reactor at a liquid feed
rate of 12 hꢁ1. The temperature was maintained at 330 C and
ꢀ
the pressure was 2 MPa. The product was divided into gas phase
and liquid phase aer passing through the condensing tank
and the distributor tank. The liquid phase was collected every
2 h and analyzed by a FID-GC (GC-9790II, FL) with a 50 m ꢂ
0.53 mm capillary column of GS-ALUMINA (Agilent). The
conversion and selectivity were calculated as follows:
2.6. Compressive strength test
The compressive strength of the catalyst is an important indi-
cator to measure whether the catalyst can be used in a xed bed
reactor. According to the measurement results in Table 3, as the
amount of PEG added continues to increase, the compressive
strength of the alumina catalyst gradually decreases. This may
be because, by adding PEG, the macropores in the catalyst
gradually increase, making the catalyst easier to fragment under
external stress conditions. However, the reduction in
compressive strength is very small and can be ignored. It can
still be applied to xed-bed reactors, which means that the
catalyst is easier to industrialize for application.
sum of moles of all products
conversion ð%Þ ¼
selectivity ð%Þ ¼
ꢂ 100;
(1)
mole of the reactant
moles of carbon in specific product
moles of carbon in all products
ꢂ 100
(2)
3.3. Characterization of the catalysts
The ammonia temperature programmed desorption (NH3-TPD)
technique was conducted by an Auto Chem II 2920 instrument
(Micromeritics, USA) using a TCD detector. In the sample
pretreatment stage, around 200 mg of sample was activated
3. Materials and methods
3.1. Samples
The pseudo-boehmite powder and Sesbania powder were
purchased from Shandong Zibo Hengyi Chemical Technology
Co., Ltd. The polyethylene glycol (PEG) with a molar weight of
400, isobutanol (AR.), and HNO3 (98 wt%) were purchased from
Beijing Chemical Plant; deionized water was made in the lab.
The alumina catalyst modied by polyethylene glycol was
prepared as follows. First, pseudo-boehmite powder (100 g) was
mixed with Sesbania powder (2.3 g) with different proportions
of PEG (10 g, 20 g, and 30 g, marked as 10%, 20%, and 30%) and
even mixing. The mixture was then stirred vigorously for 30 min
at 25 ꢀC to form a homogeneous mixture. A 2 wt% dilute nitric
acid solution was added to the mixture and mixed well. Aer
mixing into a dough shape, we used a 3 mm diameter template
for extrusionꢀ. The catalyst was dried at 25 ꢀC for 12 h and then
with Ar (99.999%) gas at a rate of 30 mL minꢁ1 and 550 C for
ꢀ
1 h. Prior to the experiment, NH3 was adsorbed at 150 ꢀC by
owing NH3 (10%) at 40 mL minꢁ1. The TPD prole was ob-
tained in the temperature range of 150–550 ꢀC under Ar
(99.999%).
The X-ray diffraction (XRD) measurements of the sample
were performed with a Bruker spectrometer model D8 Advance
using Cu Ka radiation (27.5 kV, 30 mA) with a wavelength of
0.01 nm.
The compressive strength of the samples was measured
using a ZQJ-III intelligent particle strength testing machine
(Shanghai Biao Science Instrument Co., Ltd).
The total pore volumes of the samples were measured using
a Micromeritics AutoPore IV 9510 Mercury Intrusion Poros-
imeter (MIP). A mercury contact angle of 130ꢀ and a mercury
interfacial tension of 485 dyne cmꢁ1 were used to calculate the
pore size distribution.
ꢀ
dried at 120 C for 8 h, followed by calcination in air at 550 C
for 6 h to prepare the strip-shaped gamma-Al2O3 catalyst.
Table 3 Compressive strength of PEG-modified Al2O3 at different
PEG loadings
The specic surface area of the alumina solids was estimated
from the adsorption–desorption isotherms of nitrogen at
ꢁ196 ꢀC, using Micromeritics equipment, model ASAP 2020,
Compressive strength N
ꢀ
Catalyst
mmꢁ1
and previous degasication at 100 C for 1 h.
The thermo gravimetry-differential thermal analysis (TG-
DTA) was performed using a Thermoplus 8120 ꢂ 102 (Rigaku,
Japan) under the following conditions: sample weight, ca. 5 mg;
heating rate, 5 ꢀC minꢁ1; heating range, from room temperature
to 800 ꢀC.
Al2O3
11.6
10.4
9.8
10% PEG-Al2O3
20% PEG-Al2O3
30% PEG-Al2O3
9.6
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selectivity of b-H elimination, Polymer, 2013, 54(9), 2235–
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The pore structure of an activated alumina catalyst has
a signicant effect on the performance of isobutanol dehydra-
tion to isobutene. Adding polyethylene glycol during the prep-
aration process of the activated alumina catalyst does not affect
the crystal structure of the catalyst, can produce macropores
and pore volume, improves the mass transfer performance of
the catalyst, increases the conversion rate of isobutanol, and
inhibits the superposition reaction of isobutylene. The conver-
sion rate of isobutanol of the catalyst is continuously improved
with increasing pore expander addition.
The isobutanol dehydration performance of the catalyst is
better when 30% pore expander is added to the activated
alumina catalyst. The compressive strength can meet the
requirements of industrial applications. When the temperature
is 330 ꢀC, the pressure is 0.1 MPa, and the volume liquid space
velocity is 12 hꢁ1. The conversion of isobutanol is maintained at
about 97%, and the selectivity of isobutene is maintained at
about 93% for 24 hours.
Hydroxy-3-Methylbutyrate
by
Diphosphomevalonate
Decarboxylase, Appl. Environ. Microbiol., 2010, 76(24), 8004–
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This research received no external funding.
14 D. Gogerty Isobutene formation from 3-hydroxy-3-
methylbutyrate (3-HMB) by the Saccharomyces cerevisiae
diphosphomevalonate decarboxylase (ScMDD) and directed
enzyme evolution to improve enzyme function, Iowa State
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Author contributions
Data acquisition and formal analysis, Zhao Yapeng; writing ꢁ
original dra preparation, Tian Kaige and Li Qin; writing ꢁ
review and editing, Tian Kaiege; methodology, Zhou Guanglin;
supervision, Jiang Weili. All authors have read and agreed to the
published version of the manuscript.
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byRhodotorula minuta, Appl. Microbiol. Biotechnol., 1987,
25(5), 430–433.
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Kinetic modeling of polymer-grade ethylene production by
diluted ethanol dehydration over H-ZSM-5 for industrial
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Conflicts of interest
The authors declare no conict of interest.
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11958 | RSC Adv., 2021, 11, 11952–11958
© 2021 The Author(s). Published by the Royal Society of Chemistry