Preparation of pyridine and 3-picoline from acrolein and ammonia with HF/MgZSM-5 catalyst

Article abstract of DOI:10.1016/j.catcom.2016.02.011

Pyridine and 3-picoline were prepared from acrolein and ammonia using HF/MgZSM-5 as catalyst. The HF/MgZSM-5 catalyst was produced from modification of HZSM-5 by HF and Mg(NO₃)₂. It was found that the micropore structure of the HZSM-5 carrier would be destroyed when Mg and HF were loaded. This corrosion process could be promoted by employing HF onto MgZSM-5 carrier comparing with HZSM-5 one, because of the damage of MgO to ZSM-5 stability. A micro-mesoporous HZSM-5 zeolite with fewer and weaker acid sites was prepared after HF modification. Under the optimized conditions, a total yield of 68% can be reached, with 36% being 3-picoline and 30% being pyridine.

Full text of DOI:10.1016/j.catcom.2016.02.011

Preparation of pyridine and 3-picoline from acrolein and ammonia with
HF/MgZSM-5 catalyst
Xian Zhang, Zhen Wu, Wei Liu, Zi-sheng Chao
PII:
DOI:
Reference:
S1566-7367(16)30058-9
doi: 10.1016/j.catcom.2016.02.011
CATCOM 4592
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
3 July 2015
4 February 2016
9 February 2016
Please cite this article as: Xian Zhang, Zhen Wu, Wei Liu, Zi-sheng Chao, Preparation of
pyridine and 3-picoline from acrolein and ammonia with HF/MgZSM-5 catalyst, Catalysis
Communications (2016), doi: 10.1016/j.catcom.2016.02.011
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ACCEPTED MANUSCRIPT
Preparation of pyridine and 3-picoline from acrolein
and ammonia with HF/MgZSM-5 catalyst
Xian Zhang1,2, Zhen Wu1,2, Wei Liu3, Zi-sheng Chao4*
1 Department of Chemical Engineering, Ordos Institute of
Technology, Erdos, 017000, China
2 Redbud Innovation Institute of Erdos, Erdos, 017000, China
3School of Chemical＆Biomolecular Engineering and RBI
at Georgia Tech, Georgia Institute of Technology, 500 10th
Street NW, Atlanta, Georgia 30332-0620, USA
4State Key Laboratory of Chemo/Biosensing and
Chemometrics, College of Chemistry and Chemical
Engineering, Hunan University, Changsha, 410082, China
*Corresponding author: Prof. Zi-Sheng Chao, Tel.＆Fax:
+86-731-88713257; E-mail: zschao@hnu.edu.cn.
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Abstract
Pyridine and 3-picoline were prepared from acrolein and ammonia using
HF/MgZSM-5 as catalyst. The HF/MgZSM-5 catalyst was produced from
modification of HZSM-5 by HF and Mg(NO3)2. It was found that the micropore
structure of the HZSM-5 carrier would be destroyed when Mg and HF were
loaded. This corrosion process could be promoted by employing HF onto
MgZSM-5 carrier comparing with HZSM-5 one, because of the damage of
MgO to ZSM-5 stability. A micro-mesoporous HZSM-5 zeolite with fewer and
weaker acid sites was prepared after HF modification. Under the optimized
conditions, a total yield of 68% can be reached, with 36% being 3-picoline and
30% being pyridine.
Keywords：acrolein；pyridine；3-picoline；ZSM-5；mesoporous
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1. Introduction
Pyridine and 3-picoline are important intermediates in the production of
pesticides, pharmaceuticals and cosmetics [1-3]. Currently, the industrial
production of pyridine and 3-picoline is mainly performed through the reaction
of formaldehyde, acetaldehyde and ammonia over a gaseous fluidized bed
reactor. However, there are many difficulties, such as low yield of 3-picoline,
high operation cost and product separation of 3-picoline [4]. An effective way
of producing high yield of pyridine and 3-picoline is to condensate acrolein
(AN) and ammonia. This is mainly conducted over a fluidized bed. By this
method, the yield of pyridine bases can reach as high as 70%, and the yield of
3-picoline can reach 60% after the addition of acetaldehyde or propanal [5].
Although，a high selectivity to 3-picoline could be got through condensation
between acrolein and ammonia. However, there are short-commings such as the
short catalyst lifetime, complicated operation and high equipment cost.
Experimentally, oxygen, water, ketones, aldehyde, alcohol and propylene oxide
were pumped in to reduce catalyst deactivation and pipe plug for a fixed bed
method [5-8]. However, it brought series of problems, such as acrolein
oxidation, product complexity and separation difficulties.
Solid acid catalysts with high surface area, such as Al2O3-SiO2, TiO2 and
F-M-Al2O3, show good catalytic performance with a yield of pyridine bases
reaching about 70 % , while that of 3-picoline is 44-57 % [5, 9]. Espacially, The
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alkali- modified HZSM-5 [10-12] and HF modified Al2O3 [13] catalysts with
particular pore structure and surface properties are effective catalysts in the
synthesis of pyridine and 3-picoline from formaldehyde, acetaldehyde and
ammonia. However, HZSM-5 zeolite modified by HF has never been used in
the synthesis of 3-pocline.
In this paper, the pyridine and 3-picoline were prepared from acrolein and
ammonia through a fixed bed reactor with a HF and Mg(NO3)2 modified
HZSM-5 catalyst. Various factors for catalyst preparation and reaction
conditions on the yield of pyridine and 3-picoline were investigated.
2. Experimental
2.1. Chemicals
HZSM-5 zeolites were supplied by Nankai Molecular Sieve Catalyst Plant.
Acrolein, hydrofluoric acid, magnesium nitrate and ammonia were all supplied
as analytical reagents.
2.2. Catalyst preparation
The MgZSM-5 catalyst were modified by impregnation of 130 grams
HZSM-5 with 100 mL 0.62 M magnesium nitrate solution at a temperature 100
o
oC for 8 h and then activated by calcination at 700 C for 4 h. The
HF/MgZSM-5 catalyst was further modified from impregnation of the above
MgZSM-5 catalyst with 100 mL 0.62 M NH4HF2 solution at the same condition
above and then activation. The HF/HZSM-5 was prepared when only NH4HF2
solution was impregnated over HZSM-5 zeolite as the same catalyst preparation
process.
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The regeneration of HF/MgZSM-5 catalyst was conducted for three times at
o
600 C in air and steam atmosphere for 4 hours, regarded as HF/MgZSM-5
-reactivated-n. n is the number of regeneration time.
2.3. Characterization of catalyst
X-ray diffraction spectroscopy (XRD) was performed with a Bruker
D8-Advance X-Ray diffraction spectrometer. Fourier transform infrared (FT-IR)
spectroscopy was recorded on a Varian 3100 spectrometer, equipped with a
DTGS detector. N2-physisorption was carried out on a Quantachrome
Autosorb-1C/TCD Automatic Chemisorption with a TCD detector at liquid-N2
temperature. Temperature programmed desorption of NH3 (NH3-TPD) was
measured by a Micromeritics AutoChemⅡ2920 chemisorption analyzer with a
TCD detector.
2.4. Products preparation
o
The acrolein and ammonia were firstly preheated at 250 C before mixing
with each other above the catalyst bed in the fixed bed reactor. At the same time,
the preheated water vapor and nitrogen were added to reduce the deactivation of
catalyst and polymerization of acrolein. The specific reaction apparatus was
shown in Fig. 1.
3 Results and Discussion
3.1 Catalytic activities
Table 1 shows the yield of pyridine and 3-picoline over various catalysts. No
pyridine based reaction products have been collected over HZSM-5. It is
probably because that the reaction products were deposited in the micropores of
the HZSM-5 catalyst due to the small pore diameter and high diffusion
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resistance. The fast inactivation of HZSM-5 catalyst would accelerate
polymerization, leading to channel blockage. We have also found that the white
catalyst changed the color to black after reaction. The yields of pyridine bases
using HF/HZSM-5 and HF/MgZSM-5 as catalysts are obviously higher than
that using MgZSM-5 and HZSM-5 as catalysts. It shows the fact that HF offers
a great help in yield for both HF/HZSM-5 and HF/MgZSM-5 catalysts.
The yield of pyridine and that of 3-picoline are similar, which indicates their
similar synthesis mechanism. However, the selectivity of 3-picoline is higher
than that of pyridine when the catalyst shows a good selectivity to total pyridine
bases. The low selectivity to 3-picoline is probably due to demethylation
reaction of 3-picoline to pyridine [14] as shown in Eqn. 1.
The HF/MgZSM-5-reactivated-1 catalyst shows a little higher yield than the
yield before the regeneration, but it is deactivated quickly. This might be due to
the loss of F on the catalyst surface as shown in Eqn. 2.
It is apparent that the regeneration of the catalyst should be improved.
Comparing
the
catalytic
results
of
HF/MgZSM-5-reactivated-1,
HF/MgZSM-5-reactivated-2 and HF/MgZSM-5-reactivated-3, the catalyst
surface properties could not be restored completely after the regeneration. The
decrease of Vtotal, Stotal, Smic, Smic and Vmic shown in Table 2 shows the carbon
deposition on reaction pores. The diameter of micropores only decreased from
6.54 to 6.40 Å. Because pyridine bases are mainly produced on weak Lewis
acid sites (w-LAS) and hydroxyl groups of ZMS-5 catalyst, the
HF/MgZSM-5-reactivated-1 catalyst can keep high catalytic activity even
through carbon deposition occurs on the strong Lewis acid sites (s-LAS) [15].
With the continuous carbon deposition on HF/MgZSM-5-reactivated-2 and
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HF/MgZSM-5-reactivated-3, large part of pores and acid surface are covered,
which leads to decrease of pyridine bases production.
Fig.2 shows the yields of pyridine and 3-picoline versus reaction time. It is
observed that the yields of pyridine, 3-picoline and other pyridine bases during
every two hours increase firstly and decrease dramatically after the 6 hours. The
highest 62% yield of pyridine bases (about 32% of 3-picoline, 28% of pyridine
and 2% other pyridine bases) is obtained between 2 to 4 hours, with a sharp
decrease for the 3-picoline production after 4 hours. The yield of 3-picoline is
higher than that of pyridine because the catalyst has higher activity during the
first four hours. It is consistent with the results of various catalysts as shown in
Table 1.
Acrolein or propylene imine intermediate may favor cycloaddition and
Diels-Alder reaction to produce pyridine and 3-picoline products [15-17] as
shown in Eqn.3.
The total yield of pyridine bases has dropped to about 5% between 10 to 12
hours because the acrolein or propylene imine intermediate are so active that
the polymer byproducts are produced, which leads to catalyst coking. At the
same time, the total yield of pyridine bases decreases sharp at the 6th time.
Some polymerizations of acrolein and/or propylene imine were shown in Eqn.4.
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A white plastic polymer could be seen after the tars solid byproducts were
washed by method. It is possibly caused by s-LAS and often leads to channel
blockage. Other unsaturated intermediate might also take part in the above four
polymerizations [17]. However, the main polymerization byproduct is not clear.
3.2 Catalytic properties
The XRD patterns of various catalysts are shown in Fig. S1, it shows similar
ZSM-5 framework with lower peak intensity after HF modification [18, 19].
The hydroxyl stretching region (A) and ZSM-5 framework (B) for the HZSM-5,
MgZSM-5, HF/HZSM-5 and HF/MgZSM-5 are depicted in Fig. 8. The band of
SiOH(SiO)3 at 1749 cm-1 is weak and there are the strong bands at 1674-1648
cm-1, which are attributed to Si(OH)Al [18]. The broad bands at 3450-3200 cm-1
are attributed to Si(OH)Al associated with hydrogen bond [20]. The bands at
3419 and 3250 cm-1 over MgZSM-5 are attributed to the Mg(OH) and
Si(OH)Mg. The intensity of hydroxyl stretching bands decrease after HF etched
over the zeolite surface because the OH- group is partly occupied by F- ion,
which leads to BAS converting to LAS [18]. Nevertheless, F- ion could also
occupy O2- atom in the Al-O framework to from Al(OH)xFy (x + y < 3) and
make the hydroxyl group bands (3419 and 3648 cm-1) shift to higher
wavenumbers (3448 and 3674 cm-1) [13, 18].
The band at 1399 cm-1 in Fig. 3B is attributed to Al-H group since Al species
are extracted from the framework by HF [21]. While the band of 1627 cm-1 is
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attributed to the water in framework. All of the ZSM-5 framework bands at
1229, 1113, 794, 625, 584, 553 and 455 cm-1 could be seen in the spectra of the
four catalysts [22]. However, the new bands at 1160 and 1076 cm-1 shifted from
1113 cm-1 are attributed to the external asymmetric stretching vibrations. The
related peak intensities decrease because of zeolite framework containing Mg or
F. At the same time, Mg not only stays on the zeolite surface, but also enters
into pores as the bands (625, 553, 584 and 455 cm-1) intensities decrease. The
new bands at 728 and 686 cm-1 of HF/HZSM-5 and HF/MgZSM-5 catalysts are
attributed to the symmetric stretching vibration assigned to zeolite framework
containing F. The weaker interaction of HF with ZSM-5 in HF/MgZSM-5 than
HF/HZSM-5 suggests that the reaction of MgO with HF, which leads to the
formation of MgF2. This would depress the damage to ZSM-5 framework. The
band at 924 cm-1 is attributed to extra atoms for the HF/MgZSM-5 and HZSM-5
c atal ysts , which hav e the si mila r f ra me work I R spe ctra .
The N2 adsorption desorption isotherms (A) and SF pore diameter
distribution (B) of different catalyst samples are depicted in Fig. 4. The all
isotherms exhibit a type Ⅳ curve due to the micro-mesoporous structure. The
curves of SF pore diameter distribution show that there are two kinds of
micropores, one of which is smaller and the other is bigger than that of
HZSM-5. It approved the formation of MgO in a micropore leading to the
micropore expansion and the neighboring one shrinkage. The mesopores,
caused by HF modification in HF/MgZSM-5 and HF/HZSM-5, are of 27.2 and
29.2 Å respectively. These mesopores come from collapse of neighboring
micropores and may be helpful for their high catalytic activities. Fewer
mesopores could be seen in the HF/MgZSM-5 than the HF/HZSM-5 as the
neutralization of MgO with HF.
Table 2 shows the textural properties of different catalysts. The decrease of
Dmic, Vmic and Smic for MgZSM-5 catalyst suggests the blockage of micropores
because of the formation of MgO in micropores. And also, the Vtotal and Stotal
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decrease. The decrease of Sext and increase of Dmes indicates the aggregation of
HZSM-5 grains. The increase of Dmic, Dmeso, Vmic and Vtotal and decrease of Stotal,
Smic and Sext for HF/HZSM-5 show that HF dissolve amorphous species
deposited in the micropores and some micropore walls to make mesopores. The
fresh mesopore diameters are four times than that of micropore ones [21]. The
Sext and Stotal of HF/MgZSM-5 increase more obviously than that of MgZSM-5.
It shows that catalyst grains become smaller for HF/HZSM-5 than for
MgZSM-5 and 5. The deactivated HF/MgZSM-5 catalyst has fewer pores than
the fresh one is due to the deposited carbon. After it was regenerated, same
deposited carbon was burned, but not all. The regeneration method has a great
effect for the catalyst activation.
The catalyst acidities of various catalysts are summarized at Table 3 from the
NH3-TPD curves shown in Fig. 5. One can see that there are only three NH3
desorption temperature (T1, T2 and T3) for all catalysts. The peaks of
o
o
o
T1=130-250 C, T2=190-350 C and T3=450-550 C can be assigned to the
terminal silanol groups, the Brönsted acid sites (BAS) and proton acid sites, the
strong BAS from internal pore and LAS, respectively [23, 24]. The NH3
desorption temperatures of HZSM-5 and MgZSM-5 catalysts at three kinds acid
sites shift to lower temperatures, such as T1 from 150-159 oC to 134-146 oC, T2
from 345-347oC to 192-250 oC and T3 from 490-541 oC to 460-462oC. It is said
that the strong BAS and LAS are changed to weak ones. At the same time, the
HZSM-5 and MgZSM-5 catalysts contains a large quantity of acid sites (1099.6
and 2037.4 μmol/g) and have low yields of pyridine bases (0 and 8.81%), while
the HF/HZSM-5 and HF/MgZSM-5 catalysts contains a small quantity of acid
sites (56.6 and 245.5 μmol/g) and high yields of pyridine bases (52.73 and
58.86%). It can be concluded that the yield of pyridine bases is determined by
both concentration and strength of acid sites. The samples modified by HF
containing weaker and fewer acid sites show good catalytic activities. However,
a certain amount of BAS and w-LAS are highly helpful for the acrolein
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activation and pyridine bases formation because of the higher selectivity for
HF/MgZSM-5 than HF/HZSM-5 catalyst [13, 15].
From the above analysis, we conclude that the reactions between HF and
HZSM-5 zeolite shown in Eqn. 5 and Eqn. 6. Amounts of BAS and LAS have
decreased and weakened by HF modification.
It was reported that the production of pyridine bases involves both hydroxyl
groups and LAS. However, s-LAS might favor the polymerization products,
which included coke precursors [15]. It is argued that the presence of two types
of LAS exhibit high activity and selectivity in the 3-picoline synthesis by
condensation of acrolein with ammonia [9]. And also, desorption of NH3 from
strong acid sites is difficult and leads to decrease of propylene imine formation.
The proton sites and hydroxyl groups could catalyze the formation of propylene
imine from acrolein as shown in Eqn. 7.
A certain concentration of BAS and w-LAS might be favorable to
flat-adsorption of propylene imine and synthesis of 3-picoline. On the contrary,
a high concentration of strong acid sites might be favorable to point-adsorption
of propylene imine and synthesis of pyridine and polymers. The synthesis
reactions of pyridine and 3-picoline are shown in Eqn. 8 and Eqn. 9. The good
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diffusion of products in big micropores could decrease coking and increase
catalyst lifetime.
X
s-LAS
X
w-LAS
s-LAS
X=O or NH
- CX
- H2
(8)
w-LAS
N
H
N
N
Polymers
w-LAS
H+
w-LAS
H
X=O or NH
-XH2
w-LAS
(9)
N
N
w-LAS
N
H
H+
X
XH
H+
H+
4 Conclusion
A 68 % yield of pyridine and 3-picoline from the condensation of acrolein
and ammonia was observed by using HF/MgZSM-5 as catalyst. The improved
catalytic activity is seen with MgZSM-5 modified by HF. It is because that
the fresh acid sites for HF/MgZSM-5 are less and weaker than that before
modification. In addition, it is concluded that the formed bigger micropores and
mesopores can prolong catalyst lifetime and reduce diffusion resistance.
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Acknowledgments
We are grateful to the financial supports from the Project #21376068 for
National Natural Science Foundation (NSFC), the Colleges and Universities’
Scientific Research Project of Inner Mongolia Autonomous Region
(NJZZ13017) and the Innovation training program of Inner Mongolia
University.
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Figures:
Fig.1 Fixed-bed reactive apparatus for condensation of acrolein and ammonia.
Fig.2 Yield of pyridine and 3-picoline versus reaction time.
Fig.3 FT-IR spectra (A) 4000～2500 cm-1 and (B) 1800～400 cm-1 of (a)
HZSM-5, (b) MgZSM-5, (c) HF/HZSM-5 and (d) HF/MgZSM-5.
Fig.4 The N2-adsorption and desorption isotherms (A) and pore diameter
distribution (B) of (a) HZSM-5, (b) MgZSM-5, (c) HF/HZSM-5 and (d)
HF/MgZSM-5.
Fig.5 NH3-TPD curves of (a) HZSM-5, (b) MgZSM-5, (c) HF/HZSM-5 and (d)
HF/MgZSM-5
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Fig.1 Fixed-bed reactive apparatus for condensation of acrolein and ammonia.
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70
60
50
40
30
20
10
0
Pyridines
Pyridine
3-Picoline
2
4
6
8
10
12
Time / h
Fig.2 Yield of pyridine and 3-picoline versus reaction time.
Conditions: HF/MgZSM-5 Catalyst, nAN / nH2O / nNH3=1 / 2 / 2, 500 h-1, 425 oC.
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(d)
(c)
(b)
3749
(A)
3674
3648
3203
3448
(a)
3250
3419
4000
3800
3600
3400
3200
3000
2800
2600
Wavenumber / cm-1
1627
1399
(B)
626
686
924
794
728
584
553
1229
1113
455
(d)
(c)
(b)
1160
1076
(a)
1800 1600 1400 1200 1000
Wavenumber / cm-1
800
600
400
Fig.3 FT-IR spectra (A) 4000～2500 cm-1 and (B) 1800～400 cm-1 of (a)
HZSM-5, (b) MgZSM-5, (c) HF/HZSM-5 and (d) HF/MgZSM-5.
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160
(A)
140
120
100
80
HZSM-5
MgZSM-5
60
HF/HZSM-5
HF/MgZSM-5
40
20
0.0
0.2
0.4
0.6
0.8
1.0
P / P0
6.09
(B)
4.57
6.50
6.35
4.57
6.28
(a)
(b)
27.2
29.2
(c)
(d)
5
10
15
20
25
30
35
40
Diameter / Å
Fig.4 The N2 adsorption and desorption isotherms (A) and SF pore diameter
distribution (B) of (a) HZSM-5, (b) MgZSM-5, (c) HF/HZSM-5 and (d)
HF/MgZSM-5.
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T1
T3
T2
(d)
(c)
T2
T3
(b)
(a)
100
200
300
400
500
600
700
Temperature / oC
Fig.5 NH3-TPD curves of (a) HZSM-5, (b) MgZSM-5, (c) HF/HZSM-5 and (d)
HF/MgZSM-5.
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Tables:
Table 1 Yield of pyridine and 3-picoline over various catalysts
Table 2 The textural properties of different catalysts
Table 3 Acidity of different catalysts from the NH3-TPD data
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Table 1 Yield of pyridine and 3-picoline over various catalysts
Conditions: nAN: nH2O: nNH3 =1: 2: 2, 500 h-1, 4 h, 425 oC.
Catalysts
HZSM-5
MgZSM-5
Pyridine /% 3-Picoline /% Others /% Pyridines /%
—
—
—
0
—
4.55
4.26
8.81
HF/HZSM-5
HF/MgZSM-5
HF/MgZSM-5-reactivated-1
HF/MgZSM-5-reactivated-2
HF/MgZSM-5-reactivated-3
25.82
26.59
28.3
17.24
11.02
25.91
30.38
32.1
18.15
11.88
1
52.73
58.86
62.07
35.92
23.31
1.9
1.68
0.53
0.41
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Table 2 The textural properties of different catalysts
Vtotal Stotal Vmic
Smic Sext
Dmic Dmes
Catalysts
cm3/g m2/g cm3/g m2/g m 2/g
0.240 389 0.132 316 73.4
0.226 308 0.103 256 52.4
0.251 356 0.140 304 52.1
0.219 330 0.107 249 80.7
Å
Å
HZSM-5
MgZSM-5
HF/HZSM-5
HF/MgZSM-5
6.20 24.6
4.58 29.3
6.26 28.1
6.54 26.6
6.60
HF/MgZSM-5-deactivated 0.068 61.5 0.017 35.4 26.1
HF/MgZSM-5-reactivated 0.152 233 0.072 184 49.5
6.40
24

ACCEPTED MANUSCRIPT
Table 3 Acidity of different catalysts from the NH3-TPD data
Desorption temperature（Tm, oC）and acid sites（ATm, μmol/g）
Catalysts
AT
T1
AT1
667.1
1008.7
17.7
T2
AT2
288.3
207.1
17.4
T3
AT3
144.2
822.6
21.5
HZSM-5
MgZSM-5
1099.6
2037.4
56.6
159
150
138
146
345
347
192
250
492
541
462
460
HF/HZSM-5
HF/MgZSM-5
245.5
111.6
78.3
45.6
25

ACCEPTED MANUSCRIPT
Graphical abstract：
X
s-LAS
X
w-LAS
s-LAS
X=O or NH
- CX
- H2
w-LAS
N
H
N
N
Polymers
w-LAS
H+
w-LAS
H
X=O or NH
-XH2
w-LAS
N
N
w
N
H
H+
X
XH
H+
H+
The HF/HZSM-5 and HF/MgZSM-5 catalysts, containing weaker and fewer
Brösted and Lewis acid sites than HZSM-5 and MgZSM-5, have great help for
the synthesis of pyridine and 3-picoline. The flat-adsorption of propylene imine
on weak Lewis acid sites favors the production of 3-picoline. A strong
adsorption leads to pyridine or polymer products. The bigger micropores and
mesopores formed under further corrosion of HF solution have prolonged the
catalyst lifetime.
26

ACCEPTED MANUSCRIPT
Highlights
 As high as 68% yield of pyridine and 3-picoline was achieved from
concentration of acrolein and ammonia.
 A HF/MgZSM-5 catalyst containing certain weaker and fewer acid sites
after HF modification.
 The catalytic active centers might be fresh weak Lewis acid sites.
 The bigger micropores and mesopores formed by HF modification
prolonged catalyst lifetime.
27