Efficient carbon-based acid catalysts for the propan-2-ol dehydration

Article abstract of DOI:10.1016/j.catcom.2012.06.018

Halogenated activated carbons (AC-Hal, Hal = F, Br and Cl) used as precursors were functionalized with SO₃H groups to prepare (AC-Hal-S) solid acid catalysts. ACs obtained were subjected to chemical and elemental analysis, characterized by miscellaneous physicochemical techniques and were tested in the propan-2-ol dehydration. The high catalytic activity of AC-Hal-S (Hal = F, Cl) is attributed to the presence of F and Cl affect on the catalysts performance and stability at the operation conditions.

Full text of DOI:10.1016/j.catcom.2012.06.018

Catalysis Communications 27 (2012) 33–37
Contents lists available at SciVerse ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Efficient carbon-based acid catalysts for the propan-2-ol dehydration
Vitaliy E. Diyuk ⁎, Alexander N. Zaderko, Liudmyla M. Grishchenko,
Andrii V. Yatsymyrskiy, Vladyslav V. Lisnyak
Department of physical chemistry, Chemical Faculty, Kyiv National Taras Shevchenko University, 64, Volodymyrska str., 01601 Kyiv, Ukraine
a r t i c l e i n f o
a b s t r a c t
Article history:
Halogenated activated carbons (AC–Hal, Hal=F, Br and Cl) used as precursors were functionalized with SO H
groups to prepare (AC–Hal–S) solid acid catalysts. ACs obtained were subjected to chemical and elemental
analysis, characterized by miscellaneous physicochemical techniques and were tested in the propan-2-ol de-
hydration. The high catalytic activity of AC–Hal–S (Hal=F, Cl) is attributed to the presence of F and Cl affect
on the catalysts performance and stability at the operation conditions.
3
Received 23 March 2012
Received in revised form 12 June 2012
Accepted 17 June 2012
Available online 23 June 2012
©
2012 Elsevier B.V. All rights reserved.
Keywords:
Carbon-based solid acids
Surface halogenation
Propan‐2‐ol dehydration
1
. Introduction
3
SO H groups (AC–Hal–S). Functionalized solids acids (AC–Hal–S) were
tested in a model propan-2-ol dehydration reaction and their perfor-
mances were compared.
Textural characteristics and unique surface chemistry of activated car-
bons (ACs) make them prospective candidates to be used for the prepara-
tion of efficient acid catalysts [1,2]. The oxidation treatment of ACs [3] can
result in the formation of O-containing surface groups and the strongest
acidic groups of them are carboxylic groups, whose acidity is too low rel-
ative to the most acidic sulfonic groups.
2. Experimental
2.1. Preparation of AC–Hal
The sulfonation of carbon materials with sulfuric acid and oleum are
used mainly to prepare sulfonated carbon catalysts [4–7]. The highest
SO H groups' concentration can be obtained by sulfonation of the partly
3
AC prepared from Prunus armeniaca fruit stones was used in this
study. The samples of AC with halogenated surface were obtained as
follows:
carbonized carbon materials [7]. However, solid acid catalysts obtained
have a relatively low specific surface area and poor mechanical character-
istics. A surface treatment with diazonium benzenesulfonic salt can be
considered as an alternative route to prepare solid acid catalysts [8–10].
Thus, this route is complicated and non-selective.
Activated carbons (ACs) are characterized by advanced surface area
and good mechanical properties. However, high temperature treatment
of precursors at the ACs preparation stages caused a loss of surface reac-
(
1) AC (5g) was immersed in (1a) a bromine aqueous solution
20g of KBr and 10g of Br in 60ml H O) or in (1b) a dry liquid
Br (5ml), and was kept at 298K for 1h to prepare AC–Br(1)
and AC–Br(2), respectively. The brominated samples were
treated with a 10% K aqueous solution (200ml) for 3h
, were washed with water and
(
2
2
2
2 2 4
C O
to remove physisorbed Br
dried at 393K;
2
−3
(
(
4 4
2) AC was treated in a CCl /Ar mixture (C(CCl )=2.03⋅10 mol/l,
tivity, so the treatment with H
ized SO H groups. To change the ACs surface reactivity one can use
surface chemical reactions for grafting halogen-containing groups and
strong acidic groups, such as SO H.
2 4
SO gives a minor amount of functional-
3
GSV=50cm /min) at 723K for 1.5h to prepare AC–Cl. The AC–
Cl was then flushed with Ar for another 1h to remove
3
physisorbed CCl
3) AC–Cl (5g) was immersed in a solution for fluorination
25mmol KF and 20mmol of 18-crown-6 in 50ml of CH CN)
4
and cooled down to 298K;
3
In the present communication we reported a route to prepare solid
acids catalysts, which is based on the reactivity of carbons C=C bonds.
The route includes a stage of halogenation to prepare halogenated AC sur-
faces (AC–Hal, Hal=Br, Cl, F) as precursors. The AC–Hal obtained was
functionalized with necessary moieties to form ACs with strong acidic
(
3
at 298K for 24h to prepare AC–Cl–F. The AC–Cl–F obtained
was washed with distilled water and dried at 393K.
2.2. Preparation of AC–Hal–S
⁎
Corresponding author at: Chemical Faculty, Kyiv National Taras Shevchenko
The Hal-atoms were substituted for the S-containing groups as fol-
University, Kyiv 01601, Ukraine. Tel./fax: +380 44 2581241.
E-mail address: dve@univ.kiev.ua (V.E. Diyuk).
lows. AC–Hal (1g) was immersed in 5ml of an aqueous solution of (4)
1566-7367/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2012.06.018

34
V.E. Diyuk et al. / Catalysis Communications 27 (2012) 33–37
Fig. 1. The routes used to prepare the AC–Hal and AC–Hal–S and the proposed structure of surface functionalities.
2
5% NaSCH
2
COONa or(5) 20% Na
2
S to prepare AC–Hal–(1) or AC–Hal–S(2),
The elemental analysis (E.A.) of samples was performed with Oxford
Inca 350 energy dispersive X-ray spectrometer mounted on a Jeol
JSM-6490 scanning electron microscope. Titrimetric Volgard's (Br, Cl)
and Eschke's (total S content) methods were used for chemical analysis
(C.A.) of functionalized ACs.
respectively. The samples obtained were transferred into an autoclave
glass beaker to be heated for 12h at 393K to form SX-adducts. The
ACs with SX-adducts were immersed in a 25% HCl aqueous solution
(
3
100ml) at 373K for 2h, washed with water and then treated with a
0% H solution (50ml) for 3h. The AC–Hal–S samples obtained
O
2 2
The ACs surface chemistry was analyzed by X-ray photoelectron
spectroscopy (XPS, Kratos 800 Ultra System, MgKα=1253.6eV) and
after the wet oxidation were washed with water and dried at 393K. A
sample AC–S(1), to be used for the comparison, was prepared by the
same technique omitting the halogenation stage. Fig. 1 illustrates the
routes used and schematized structures of surface functionalities.
The stages of ACs functionalization are based on strategic application
by thermal desorption analysis. The concentrations of CO, CO
2
and
SO (CCO, ССO2, СSO2) evolved due to thermal destruction of the ACs
2
surface groups were determined by temperature-programmed de-
sorption with IR spectrometric registration of gaseous desorption
products (TPD-IR). An apparatus combining TG/DTG analyzer and
Specord 400 IR-spectrometer was used for all the TPD-IR studies.
The sample (0.1g) stored in a quartz pan, which is situated in a cus-
tom quartz tubular reactor placed inside an electrical furnace, was
of organic chemistry reactions as follows: electrophilic addition of Br
11] or of CCl [12] to double C=C bonds with formation of AC–Br and
AC–Cl, respectively; nucleophilic substitution of Cl in CCl groups with F
to form AC–Cl–F; nucleophilic substitution of active surface halogen in
AC–Hal (Hal=Cl, Br) with NaSX (X=CH COONa, Na) or nucleophilic ad-
dition of NaSX to double bonds activated with CF groups (AC–Cl–F) to
form SX adducts; hydrolysis and oxidation with H of SX adducts to
form SO H groups. Nucleophilic substitution of surface Br with NaSX
X=CH COONa, Na) is accompanied with HBr elimination and C=C
2
[
4
3
−5
3
2
heated in a flow of a purge Ar gas (GSV=7⋅10 m /min). The tem-
perature was increased from 303K to 1073K with a rate of 10K/min
and the sample weight change was recorded in a form of TG/DTG pro-
files. Evolved gases, which are transferred with the Ar flow from the
furnace heated area into the flow cell of the IR-spectrometer, were
tested on-line, and TPD-IR profiles were registered. The IR intensities
3
2 2
O
3
(
2
bond formation. Earlier, a principal possibility of the bromine electrophilic
addition and its nucleophilic substitution with amines, alcohols and thiols
was shown for Norit activated carbon in Ref. [11].
2 2
related to the concentrations of CO, CO and SO were calibrated daily
with diluted gases mixtures. For the temperature-programmed de-
sorption, ultra-high-purity Ar, was used as the inert purge gas, since
besides desorption no oxidation could occur. The value of СSO2 was
2
.3. Catalysts characterization
considered as a measure of SO
spectra were recorded on an Avance 400 NMR spectrometer at
298K. The concentration of strong acid sites (C ) was determined
H groups content. The MAS 19F NMR
3
2
The N adsorption–desorption analysis was performed for degassed at
423K ACs samples on a Micromeritics ASAP 2010 volumetric adsorption
A
system at 77K. The specific surface area (SBET) was determined using
the BET method. The total pore volume (V ) was estimated from the
adsorbed at a relative pressure of 0.95.
by acid/base pH-metric titration. The quantum-chemical (QC) com-
putation (see Appendix A for procedure) was performed to mimic
qualitatively the affect of substitutes on the ACs relative acidity.
S
amount of N
2
Table 1
The textural properties (SBET, V
S
), results of E.A. and C.A., CСО and ССO2 determined by TPD-IR.
2
3
Sample
S
BET, m /g
V
S
, cm /g
E.A.
C.A.
TPD-IR
СО, mmol/g
Element content, at. %
C
ССO2, mmol/g
C
O
Br
Cl
F
CНal, mmol/g
AC
1350
1085
1050
950
0.45
0.41
0.40
0.36
0.38
95.9
87.2
88.9
86.3
86.0
4.1
11.1
9.3
7.2
7.4
–
–
–
–
0.76
1.41
1.44
1.34
1.36
0.12
0.25
0.29
0.88
0.91
AC–Br(1)
AC–Br(2)
AC–Сl
1.7
1.8
–
–
–
0.62
0.52
4.60
2.52(Cl);1.91(F)
–
–
6.5
3.7
–
AC–Cl–F
985
–
2.9

V.E. Diyuk et al. / Catalysis Communications 27 (2012) 33–37
35
Table 2
Binding energies (E
b
) of the chemical states of C, O, Cl, Br and S and the states relative content (in parenthesis, in %) for selected ACs.
Sample
Eb, eV
C 1s
O 1s
Cl 2p3/2
Br 3p3/2
S 2p3/2
SH
C―C
C―O
C=O
O―C=O
O―I
O―II
C―Cl
CCl
3
C―Br
3
SO H
(C=O+S=O)
(C―O+S―O)
AC
284.4
285.6
(14.4)
285.6
(16.6)
285.6
(17.3)
285.5
(16.6)
286.6
(12.4)
286.9
(9.6)
286.7
(7.7)
286.6
(9.0)
288.7
(2.8)
288.3
(3.7)
288.1
(2.8)
288.4
(4.2)
532.4
(43.4)
532.2
(39.7)
532.4
(36.3)
532.3
(41.0)
533.8
(56.6)
533.8
(60.3)
533.8
(63.7)
533.8
(59.0)
(
70.4)
284.4
70.1)
284.4
72.2)
284.4
70.2)
AC–Br(1)
AC–Сl
183.6
(100)
(
200.1
201.2
(
(82.1)
(17.8)
AC–Br(1) –S(1)
163.8
(9.3)
168.9
(90.7)
(
surface functionalization with Hal. The content of Hal in the AC–Сl and
AC–Cl–F samples is 8–10 times higher than that in the AC–Br ones. The
2
.4. Catalysts test
The catalytic activity of ACs obtained was examined in a flow reactor
bromination is realized by the electrophilic addition of Br
C=C bonds. In contrast to the bromination, the formation of C―Cl and
CCl groups (cf., Table 2) was realized at the high temperature chlorina-
tion by the reaction of CCl
2
to double
at atmospheric pressure (see Appendix A for details). As the propan-2-ol
catalytic dehydration over ACs functionalized with strong acidic groups
gave propene mainly and diisopropyl ether traces [13,14], the
propan-2-ol conversion to propene (α) and the reaction rate (r) were cal-
culated by means of Eqs. (1) and (2), respectively
3
•
4
with C=C bonds [12]. Alternatively, Cl and
•
3
CCl
radicals can react with the AC surface centers, e.g. OH or C―H. The
3
formation of surface CCl groups by the radical reaction causes the high
content of Hal in the AC–Сl (>4.5mmol/g).
CðC H Þ
F is able to substitute only about 40% of Cl in the AC–Cl (cf., Table 1).
Fig. 2 shows a typical MAS F NMR spectrum of AC–Cl–F sample.
3
6
α ¼
;
ð1Þ
19
С ðС H OНÞ
0
3
7
The main peak registered at −64ppm/CFCl
face AC–CF groups in accordance with data of [15,16]. Signals at −117
and −162ppm/CFCl were attributed to CF groups in the aromatic and al-
iphatic fragments of carbon matrix, respectively. The partial substitution
of Cl by F is realized due to the difference in the reactivity of CCl groups,
which depends on the nearest-neighbor groups. The reactivity of CCl
3
was assigned to the sur-
where C(C
3
H
6
) is the propene outlet concentration, and C
0
(C
3
H
7
OH) is
3
−3
the propan-2-ol inlet concentration equal to 1.07⋅10 mol/l, and
3
r ¼ CðC H Þ⋅GSV;
ð2Þ
3
3
6
3
3
groups increases if electron acceptor groups (e.g. carbonyl, activated
aryl, lactonic or carboxylic groups) are located closely to them.
where GSV is the gas velocity rate equals 45cm /min. The propan-2-ol
dehydration reaction heating-cooling cycles were carried out to evaluate
catalyst deactivation. The temperature at 100% conversion of propan-2-ol
to propene (T100%) was used as a measure of the catalytic activity. The
evolution of T100% value from I to III cycle was used to estimate the cata-
lysts deactivation.
3
. Results and discussion
3
.1. Characterization of AC–Hal
Table 1 summarizes the textural properties, result of analysis and
TPD-IR data for the AC and AC–Hal.
The initial AC is characterized by significant amount of phenolic, car-
bonyl groups (evolved CO) and carboxyl groups (evolved CO
halogenation of ACs leads to decrease of SBET (by 20–30%) and V
–20%). This observation can indicate on the preferential micropore
2
) [3]. The
S
(by
7
Fig. 3. Typical TG (1), DTG (2) and TPD-IR profiles of CO
(a); deconvoluted TPD profile of SO2 (b).
2 2
(3), CO (4) and SO (5) desorption
Fig. 2. 19F MAS NMR spectrum of AC–Cl–F–S(1) (spinning rate of 10kHz).

36
V.E. Diyuk et al. / Catalysis Communications 27 (2012) 33–37
Table 3
The textural properties (SBET, V
cycles over modified ACs.
S
), results of C.A., C(SO
2 A
) and Tmax, (1), Tmax, (2) from the TPD-IR data, the concentration of strong acid centers (C ) and T100% at I–III heating–cooling
2
3
Sample
S
BET, m /g
V
S
, cm /g
C.A.
TPD-IR
C
A
,
T100%, K
mmol/g
C(S), mmol/g
C(SO
2
), mmol/g
T
m(1), K
T
m(2), K
I
II
III
(C
1
·10−4, mol/g)
(C
2
·10−4, mol/g)
AC–S(1)
1280
1120
1075
1055
980
0.44
0.42
0.41
0.40
0.37
0.36
0.39
0.22
0.36
0.36
0.55
0.45
0.56
0.50
0.16
0.21
0.21
0.13
0.42
0.24
0.29
583 (0.8)
568 (1.3)
568 (1.2)
523 (0.4)
543 (2.1)
533 (1.2)
533 (0.4)
643 (0.8)
653 (0.8)
653 (0.9)
623 (0.9)
643 (2.1)
633 (1.2)
638 (2.5)
0.14
0.20
0.18
0.09
0.41
0.23
0.29
531
514
521
525
479
488
488
577
529
543
546
485
494
492
592
544
553
562
492
498
494
AC–Br(1)–S(1)
AC–Br(2)–S(1)
AC–Br(2)–S(2)
AC–Cl–S(1)
AC–Cl–S(2)
AC–Cl–F–S(1)
960
1050
As significant amounts of CO
2
and especially CO were registered
surface groups of the active carbon [17]. So, O(1s) core level XP spec-
trum can be deconvoluted on the two components from O―I (S=O
and C=O) and O―II (S―O and C―O) type oxygen [17]. The formation
of sulfonic acid groups of up to 0.5mmol/g gives two oxygen forms S=O
and S―O in 2:1 ratio; the latter should lead to increase of the total con-
tent of O―I form. This effect is observed experimentally as the relative
amount of the O―I form increases from 39.7% for AC–Br(1) to 41.0% for
AC–Br(1)–S(1).
by TPD-IR for the AC–Hal, consequently, significant ACs surface oxida-
tion is accompanied with the halogenation (Table 1). It was found, if
the XPS data for the initial AC and AC–Hal are compared, that the ratio
of (C―O)/(C=O) forms increases in the AC–Hal; the latter corre-
sponds to the formation of phenolic groups (cf., Table 2). Hydrolysis
of the most active surface Br or Cl can cause the formation of phenolic
groups (up to 0.7mmol/g). In summary, it is clearly seen that the ACs
can be functionalized with 0.5–4.5mmol/g of Hal-containing groups
through the halogenations routes used.
Fig. 3a represents typical TG/DTG, TPD-IR profiles registered for
the AC–Hal–S samples. The TPD profile of SO
grafted SO H groups can be deconvoluted into two Gaussian compo-
nents (Fig. 3b).
Two maximum of SO
tered by TPD-IR (Table 3) were assigned to two forms of SO
forms can be associated with SO H groups grafted in the meso-/
macropores and in the micropores, respectively.
The concentration of SO H groups in the AC–Br–S determined
2
that evolved from the
3
3
.2. Characterization of AC–Hal–S
2
thermal desorption (Tmax, (1), Tmax, (2)) regis-
. These
XPS data confirm the formation of sulfonic (SO
on the AC–Hal–S. The main sulfur XPS peak S(2p3/2) at 168.9eV (cf.,
Appendix A, Table 2) was assigned to the surface SO H groups. The
peak at 163.8eV was assigned to the rest of SH surface groups. The XPS
data on O(1s) are not informative for investigation of the concentration
3
H) groups grafted
2
3
3
3
form the TPD-IR data is 1.7–4.2 times lesser than the total amount
of S (C.A.) and 2.5–4 times lesser than the total amount of Br in the
AC–Br (Table 3).
of SO
of SO
3
H groups. The binding energies for the oxygen in S=O and S―O
H groups are close to those in C=O and C―O of oxygen functional
3
The SO
of the surface Br groups at the treatment of AC-Br with basic Na
NaSCH COONa reagents or/and by 2) partial hydrolysis of the
SX-adduct. The higher SO groups concentrations of 0.42 and
3
H groups' low concentration is caused by 1) partial hydrolysis
2
S or
2
3
H
3 3
Fig. 5. The protonated (C24H10XSO H) and deprotonated (C24H10XSO
−) forms of a
Fig. 4. Conversion of propane-2-ol to propene with the temperature (a) and T100%
against the concentration of functionalized SO H groups in the ACs (b).
model object used for the difference in energy estimation (a), correlation plot of
ΔE(X)−ΔE(OH) and T100% with substituent X.
3

V.E. Diyuk et al. / Catalysis Communications 27 (2012) 33–37
37
0
.29mmol/g were achieved if use the AC–Cl and the AC–Cl–F precursors,
respectively. This fact can be explained by the higher concentration of ac-
tive Hal and by the acceptor properties of CCl and CF groups facilitate
the stage of the Na S or NaSCH COONa addition. The solid acids prepared
contain up to 0.42mmol/g of the strong acidic group. These data are in a
good agreement with SO concentration determined from the TPD-IR
Table 3).
the performance for at the least the next 5 heating–cooling cycles.
The catalysts deactivation occurs due to the reduction of S in the
3
3
SO
3
H groups with the AC matrix; the latter is accompanied with
gas. The surface of chlorinated and fluorinated AC sam-
2
2
evolving of SO
2
ples is more chemical inert to the oxidation due to the deactivation of
reductive surface centers at the stage of the radical chlorination.
2
(
4
. Conclusions
3
.3. Catalytic properties
A novel synthetic route for the preparation of carbon-based solid acids
Fig. 4a shows that all modified ACs samples exhibit a high activity
was elaborated. The halogenation of AC with subsequent reaction of the
AC–Hal with NaSCH COONa or Na S allow obtaining ACs with up to
.5mmol/g of the strong acid SO H groups. The carbon-based solid
level and convert 100% propane-2-ol to propene. The standard indus-
trial grade strong acid catalyst amberlyst-15 WET was also tested in
this reaction. It was found that propan-2-ol converted to propene
over amberlyst-15 at 403K with a yield of 77%. At higher temperature
α value decreases from 77% at 403K to 65% at 428K (see Appendix A
for details). The catalyst deactivation is caused by the thermal degra-
2
2
0
3
acids prepared show a high activity level in the model reaction of cata-
lytic propan-2-ol dehydration to propene. The most efficient catalysts
are that obtained from the fluorinated and chlorinated ACs. The
halogen-containing surface moieties not only increase the catalytic ac-
tivity of the AC-based solid acids, but also increase the reactive groups
thermal stability in the case of AC–Cl–S(1) and of AC–Cl–F–S(1) to the
utmost.
3
dation of the surface SO H groups.
The catalyst activity and the reaction rates (r ⋅ 10 mol g min−1 at
4
−1
4
70K) decrease in a sequence AC–Cl–S(1) (4.36)>AC–Cl–S(2) (3.69),
AC–Cl–F–S(1) (3.24)>AC–Br(2)–S(1) (1.10), AC–Br(1)–S(1) (1.05), AC–
Br(2)–S(2) (0.82)>AC–S(1) (0.61).
Appendix A. Supplementary data
The dependence of catalytic activity of studied samples (in fact
T
3
100%) against the concentration of functionalized SO H groups has a
Supplementary data to this article can be found online at http://
dx.doi.org/10.1016/j.catcom.2012.06.018.
complicated character that cannot be trivially interpreted as the two
parameter dependence (Fig. 4b). In general the catalytic activity in-
creases (T100% decreases) with increase of the concentration of func-
tionalized SO
3
H groups.
References
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activity of the AC–Cl–S(1), AC–Cl–S(2) and AC–Cl–F–S(1) samples is
realized due to affect of CCl
[
[
[
[
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of SO H groups.
3
The effect of substituents on the ACs relative acidity was modeled
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3
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−
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24
H10XSO
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that the presence of the nearest CCl and CF groups decreases
ΔE(X)−ΔE(OH) and so increases the deprotonation of SO H groups.
The lowest T100% values are found for the ACs containing CCl and
CF groups; this confirms for the some extent the suggestion done.
The ΔE(X)−ΔE(OH) values decrease with in sequence
H>Br>F>COOH>CCl >CF >NO >OH. T100% values decrease in a
similar way for selected X-substituents. So, the ΔE(X)−ΔE(OH)
values could be used to estimate the influence of a substituent on
3
the activity of carbon solid acids functionalized with SO H groups.
[
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3
3
[7] B.V.S.K. Rao, K.C. Mouli, N. Rambabu, A.K. Dalai, R.B.N. Prasad, Catalysis Communi-
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3
[
[
3
3
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X
a
Lopez-Manchado, Carbon 45 (2007) 1669–1678.
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3
3
2
(
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The value of T100% for the functionalized ACs increases from I to III
heating–cooling cycle at the operation condition (Table 3). The
highest deactivation, which is caused by the partial destruction of ac-
tive centers of the catalysts, is observed for the AC–Br(2)–S(2) and
AC–S(1). The chlorinated and especially fluorinated ACs preserve
Chemistry 44 (2008) 331–336.
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[
[
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