Catalytic dehydration of 2,3-butanediol over P/HZSM-5: Effect of catalyst, reaction temperature and reactant configuration on rearrangement products

Article abstract of DOI:10.1039/c5ra23251a

As a type of important bio-based vicinal diol, 2,3-butanediol could be transformed into methyl ethyl ketone and 2-methyl propanal through a pinacol rearrangement mechanism under acid catalysis conditions. In this paper, a series of P/HZSM-5 (Si/Al = 360) samples with various phosphate contents were prepared and tested via the catalytic transformation of 2,3-butanediol, with particular focus on the effect of phosphate content on the ratio of methyl ethyl ketone to 2-methyl propanal. The catalyst structures were studied using several physico-chemical methods such as XRD, N₂ sorption, NH₃-TPD and FT-IR. At 180 °C, the ratio of methyl ethyl ketone to 2-methyl propanal increased from 5.1 to 37.5 when the content of phosphate increased from 0.5 to 8.0. When the reaction temperature increased from 180 °C to 300 °C over 4% P₂O₅/HZSM-5, the ratio of methyl ethyl ketone to 2-methyl propanal decreased from 15.6 to 2.5. The configuration of 2,3-butanediol would affect the conversion but not the selectivity. The characterization results demonstrated that the phosphate modification of HZSM-5 could not only reduce the strong and medium acid sites but also produce new weak acid sites. Strong acid sites and high reaction temperatures could promote the formation of 2-methyl propanal through methyl migration via carboniums. Based on these results, a possible surface reaction model was proposed.

Full text of DOI:10.1039/c5ra23251a

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PAPER
Catalytic dehydration of 2,3-butanediol over P/
HZSM-5: effect of catalyst, reaction temperature
and reactant configuration on rearrangement
products
Cite this: RSC Adv., 2016, 6, 16988
a
ab
Wengui Zhang,a Yi Hu,ab Ting Jiang,a Jie Fuc
*
Jinbo Zhao, Dinghua Yu,
ab
*
and He Huang
As a type of important bio-based vicinal diol, 2,3-butanediol could be transformed into methyl ethyl ketone
and 2-methyl propanal through a pinacol rearrangement mechanism under acid catalysis conditions. In this
paper, a series of P/HZSM-5 (Si/Al ¼ 360) samples with various phosphate contents were prepared and
tested via the catalytic transformation of 2,3-butanediol, with particular focus on the effect of phosphate
content on the ratio of methyl ethyl ketone to 2-methyl propanal. The catalyst structures were studied
using several physico-chemical methods such as XRD, N2 sorption, NH3-TPD and FT-IR. At 180 ꢀC, the
ratio of methyl ethyl ketone to 2-methyl propanal increased from 5.1 to 37.5 when the content of
phosphate increased from 0.5 to 8.0. When the reaction temperature increased from 180 ꢀC to 300 ꢀC
over 4% P2O5/HZSM-5, the ratio of methyl ethyl ketone to 2-methyl propanal decreased from 15.6 to
2.5. The configuration of 2,3-butanediol would affect the conversion but not the selectivity. The
characterization results demonstrated that the phosphate modification of HZSM-5 could not only reduce
the strong and medium acid sites but also produce new weak acid sites. Strong acid sites and high
reaction temperatures could promote the formation of 2-methyl propanal through methyl migration via
carboniums. Based on these results, a possible surface reaction model was proposed.
Received 4th November 2015
Accepted 22nd January 2016
DOI: 10.1039/c5ra23251a
www.rsc.org/advances
as lactic acid,7 sorbitol8 and ketoses (fructose, sorbose, taga-
tose).9 Excessive hydroxyl groups in polyols could particularly
1. Introduction
induce the complex reaction chemistry, which should be
studied for the development of efficient biorenewable catalysis
processes.
For the sustainable chemical industry, biomass resources could
be used as renewable feedstock to produce chemicals and fuels
and thus reduce the reliance on fossil resources.1–3 Consider-
able efforts are currently being invested in biotechnology4 and
green chemistry to develop a sustainable chemical industry
using bio-renewable sources. Although the biological fermen-
tation process could transform bio-based carbohydrates into
bulky platform chemicals,5 such as polyol, organic acids and
amino acids, the substrate specicity would limit the biological
synthesis path diversity to the acquisition of various chemicals.
Compared with the biological process, chemical catalysis could
provide a versatile protocol for bio-based chemical derivatiza-
tion.6 Many studies have been devoted to developing chemical
catalysis routes for the conversion of platform chemicals such
2,3-Butanediol, a typical platform chemical, was produced
from
a
carbohydrate through biological fermentation.10
Through chemical catalytic transformation, as shown in
Scheme 1, 2,3-butanediol could be transformed into diversied
products that could nd important applications in many elds
such as solvents, cosmetics, plasticizers, foods, moistening and
soening agents, and pharmaceuticals. For example, 2,3-buta-
nediol could be dehydrated11–13 into methyl ethyl ketone, 2-
methyl propanal, 3-buten-2-ol and 1,3-butadiene, oxidized into
3-hydroxyl-butanone and diacetyl,14 or polymerized with diacid
or isocyanate to produce polymers such as polyester15 or poly-
urea.16 These derivatives could be acquired through hydroxyl
reaction chemistry.
Recently, many studies have been devoted to the develop-
ment of heterogeneous catalysts for 2,3-butanediol. According
to the results reported by Toeroek et al.,17 diol could undergo
pinacol rearrangement to produce the corresponding carbonyl
compounds, and its selectivity is dependent on its acid
strength and the form of the catalysts. Different zeolites18–20
aCollege of Biotechnology and Pharmaceutical Engineering, Nanjing Tech University,
30 South Puzhu Road, Nanjing 211816, China. E-mail: yudh@njtech.edu.cn;
biotech@njtech.edu.cn
bState Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing Tech
University, 5 Xinmofan Road, Nanjing 210009, China
cKey Laboratory of Biomass Chemical Engineering of Ministry of Education, College of
Chemical and Biological Engineering, Zhejiang University, 38 Zheda Road, Hangzhou
310027, China
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2. Experimental section
2.1 Catalyst preparation
HZSM-5 zeolites (Si/Al ¼ 360) were obtained from the Catalyst
Factory of Nankai University. Phosphate-modied zeolites were
prepared by incipient wetness impregnation, as follows: the
mixture of 10 g HZSM-5(360) in 40 mL of deionized water was
stirred for 30 min. The specied amount of NH4H2PO4 was
added into the mixture of zeolites to yield the desired content of
P2O5 (0.5 wt%, 1.0 wt%, 2.0 wt%, 4.0 wt%, 6.0 wt% and
8.0 wt%). The impregnation process was conducted for 24 h at
room temperature.ꢀ The mixtures were then dried by rꢀotary
ꢀ
evaporators at 120 C for 4 h, heated from 120 C to 550 C at
a heating rate of 10 ꢀC minꢁ1 and calcined at 550 ꢀC for 4 h. The
samples were cooled to room temperature under ambient
conditions and then tableted and crushed into 30–40 meshes,
named “content%P/HZSM-5(360)”.
Scheme 1 Hydroxyl reaction chemistry of 2,3-butanediol and the
corresponding catalytic mechanism and product distribution.
2.2 2,3-Butanediol feedstock and distillation
Unless otherwise specied, the reactant feedstock was the 2,3-
butanediol extract from fermentation broth, and the mixture
constitution was determined as follows: 78.8% meso-2,3-buta-
nediol and 21.2% racemic-2,3-butanediol. To study the effect of
chiral conguration, the chiral ratio of reactant was changed via
reduced pressure distillation. The separation condition was as
follows: the stirring velocity was 20 rpm, the temperature was
110 ꢀC, the vacuum degree was 29.3 mmHg, and the distillation
temperature was 73 ꢀC. Aer the distillation, the distillation
residue was collected and used as a reactant, which comprises
80.5% meso-2,3-butanediol and 19.5% 2,3-butanediol of racemic
isomers.
including X, Y, modernite, b and ZSM-5 have been used for 2,3-
butanediol dehydration, and the product of methyl ethyl
ketone formation was favored. These catalysts are focused on
zeolites with a low Si/Al ratio, and the reaction was proceeded
at a high temperature (230–350 ꢀC). Under acid catalysis
conditions, 2,3-butanediol could be transformed into methyl
ethyl ketone (MEK), 2-methyl propanal (MPA), or 1,3-buta-
diene. Therefore, the reaction network adjustment would be
expected through the selection of a suitable catalyst. In our
previous report, MEK and MPA could be produced from 2,3-
BDL with HZSM-5 zeolites with different Si/Al ratios. Therefore,
acid catalytic sites of HZSM-5 could not only promote dehy-
dration to MEK but also catalyze the rearrangement of car-
bonium to MPA. Few studies have investigated the reaction
2.3 Catalytic reaction and product determination
balance of dehydration and the rearrangement process and the The catalytic reaction operation was performed similarly to the
reaction network adjustment through catalyst modication method reported in our group's previous study.11 Simply, catalytic
and reaction parameters. In total, even less attention has been dehydration was performed in a xed-bed quartz reactor with an
paid to the effect of the typical molecular structure, vicinal 8 mm inner diameter. The catalysts (1.5 g) with 30–40 meshes
hydroxyls, and their conguration on catalytic performance, were charged in the middle section of the reactor, with quartz
which is
transformation.
a
common feature of bio-based chemical wool packed in both ends. Before catalytic evaluation, the catalyst
was pretreated at the required reaction temperature for 0.5 h
The aim of the present contribution is to investigate the under highly pure N2 (0.1 MPa, 12.5 mL minꢁ1). The feedstock,
effect of catalyst structure and reaction temperature on product an aqueous solution of 2,3-butanediol (60 wt%), was then pum-
distribution in rearrangement via carbonium during 2,3-buta- ped into the preheating zone (input speed ¼ 6 mL hꢁ1), driven
nediol catalytic dehydration. Phosphate-modied HZSM-5 through the catalyst bed by nitrogen (0.1 MPa, 12.5 mL minꢁ1).
zeolites could improve the hydrothermal stabilization during Unless otherwise specied, the reactant contains 78.8% meso-2,3-
the alkylene crack process,21 and the surface structure model of butanediol and 21.2% racemic-2,3-butanediol.
phosphate-modied HZSM-5 has been proposed.21–23 In this
Product analysis was carried out offline by a Shimadzu GC-
paper, a series of HZSM-5 (Si/Al ¼ 360) catalysts with different 2010 gas chromatograph using a TCD detector equipped with
phosphate content were prepared and used to study the inu- a Satbilwax-DA capillary column (30 m, ID 0.32 mm, lm
ence of catalyst structure on the pinacol rearrangement process. thickness 0.25 mm). The injector and detector temperatures
The pore structures and surface acidity were elucidated by were 280 and 300 ꢀC, respectively. The oven was kept at 70 ꢀC for
several physico-chemical methods such as XRD, N2 sorption, 2 min then raised to 170 C at a rate of 10 C minꢁ1, where it
FT-IR and NH3-TPD. Based on this structural information, remained for 2 min before nally being raised to 215 ꢀC at 15 ꢀC
a possible surface reaction mechanism was proposed. These minꢁ1 for 2 min. For quantitative analysis, ethanol was selected
results would be useful in catalyst design and in the reaction as the internal standard. The corresponding relative correction
ꢀ
ꢀ
chemistry of bio-based chemical conversion.
factors of 2,3-butanediol, water, methyl ethyl ketone, 2-methyl
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propanal, and butadiene were 1.2249, 0.7024, 1.1934, 1.1585
and 1.0488, respectively.
2.4 Catalyst structure characterization
X-ray diffraction was performed on a Thermo ARL X'TRA X-ray
diffractometer from Thermo Electron Corporation using Cu
Ka radiation. The wavelength was 0.15406 nm. With a scanning
speed of 5ꢀ minꢁ1, the diffraction data were collected at 25 ꢀC in
a 2q range from 5ꢀ to 75ꢀ.
N2 sorption measurements were carried out on an ASAP2020
instrument from Micromeritics. Before the measurements, the
catalysts (0.1000 g) were degassed under vacuum for 10 h at
ꢀ
300 C. The specic surface area and micropore volume were
Fig. 1 Catalytic results of 2,3-butanediol dehydration at 180 ꢀC over P/
calculated by the Brunauer–Emmett–Teller (BET) method and t-
plot method, respectively. The micropore width distribution
was acquired according to the Horvath–Kawazoe method.
The temperature-programmed desorption of ammonia was
performed with a BEL-CAT-B-82 instrument connected to
a thermal conductivity detector. The catalyst surfaces were
HZSM-5(360) zeolites with various P2O5 contents.
respectively.25 As shown in Fig. 1, the ratio of methyl ethyl ketone
to 2-methyl propanal (MEK/MPA) increased from 5 to 37.5 when
the HZSM-5 was modied by phosphate via the P2O5 content
increase from 0.5% to 8.0%. These product distribution change
indicated that the catalyst modication could produce different
surface structures and could further affect carbonium rear-
rangement kinetics and nally lead to different MEK/MPA ratios.
The surface structure differences between modied catalysts
should be elucidated via physico-chemical methods.
Undoubtedly, the reaction temperature would not only
inuence conversion but also affect product distribution
because of the thermal and kinetic dynamics.26 Over HZSM-5
zeolites with different P2O5 contents, the effects of reaction
temperature on catalytic performance have demonstrated
similar regularity. The temperature effects of 2% P/HZSM-5 and
4% P/HZSM-5 on the catalytic dehydration performance of 2,3-
butanediol are shown in Fig. 2. For 4% P/HZSM-5 (Fig. 2b),
when the reaction temperature was increased from 180 ꢀC to
200 ꢀC or even higher, the conversion of 2,3-butanediol was
improved from 35.5% to 100%, which indicated that a higher
ꢀ
cleaned by a ow of helium at 550 C for 60 min. The samples
were then cooled to 120 ꢀC, and the ammonia was adsorbed at
120 ꢀC for 60 min. The adsorbed samples were ushed with the
ow of helium at 120 ꢀC for 40 min. The TPD of ammonia was
performed atꢁa1helium ow of 40 mL minꢁ1 and a heating rate
ꢀ
ꢀ
of 10 C min from 120–750 C. The acid amount was deter-
mined by peak area acquired by integrating the TPD desorption
peaks in the temperature range.
FT-IR (Fourier transform infrared spectroscopy) was carried
out on an Avatar 360 FT-IR spectrometer from Nicolet using DTGS
KBr as the detector and KBr as the beam splitter. The catalyst was
mixed together with KBr powder and tableted into a thin round
piece. The piece was then scanned in the range of 400–4000 cmꢁ1
.
3. Results and discussion
3.1 Catalytic dehydration performance of 2,3-butanediol
The modied HZSM-5 catalysts with different amounts of phos- temperature would be favorable to 2,3-butanediol conversion,
phates were tested by the catalytic dehydration of 2,3-butanediol in agreement with the thermal dynamics principle. When the
at 180 ꢀC, and the corresponding results are shown in Fig. 1. reaction temperature was increased, the selectivity to methyl
Compared with the parent HZSM-5 (Si/Al ¼ 360), the conversion ethyl ketone was improved from 66.6% to 81.3% and then
ꢀ
of 2,3-BDL over the modied zeolites (P2O5 content # 2 wt%) decreased to 74.7% at 300 C. Meanwhile, the selectivity to 2-
improved from 32.7% to more than 90%, which indicates that methyl propanal was gradually improved from 4.3% to 24.3%.
these modied zeolites could be more efficient catalysts for 2,3- Although the selectivity to both methyl ethyl ketone and 2-
butanediol conversion than the parent zeolites. When the loaded methyl propanal was improved with increasing reaction
amount of P2O5 was above 2 wt%, the modied zeolites exhibited temperature, the ratio of MEK/MPA could further reect the
very low conversions of 2,3-BDL even less than 15% over 6.0P/HZ. effect of reaction temperature on product distribution. As
At meanwhile, the 2,3-BDL conversions over the modied zeolites shown in Fig. 2b, the ratio of MEK/MPA decreased from 15.6 to
((P2O5 content # 2 wt%) were all above 90%. This decrease might 2.5 when the reaction temperature increased from 180 ꢀC to 300
be attributed to the excessive P2O5 species over the zeolite ꢀC, which indicated that over HZSM-5 zeolites catalysts, the
surface, which led to the change of surface acidic properties. high reaction temperature would promote the methyl migration
With the acid catalyst, 2,3-butanediol could undergo a dehydra- process to produce MPA. For 2% P/HZSM-5 (Fig. 2a), the inu-
tion reaction via a pinacol rearrangement mechanism.24 Over the ences of temperature on the 2,3-butanediol conversion and
HZSM-5 zeolite surface, methyl ethyl ketone is the main product methyl ethyl ketone yield were not signicant. With the rise of
of catalytic dehydration of 2,3-butanediol. During pinacol rear- temperature, the selectivity to 2-methyl propanal increased, and
rangement, the products of methyl ethyl ketone and 2-methyl the ratio of MEK/MPA decreased, similar with the reaction
propanal come from hydride migration and methyl migration, behaviors over 4% P/HZSM-5. Research of the temperature
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Fig. 3 Catalytic performance over 4.0 P/HZ at 180 ꢀC with different
feed: (A) 72.7% meso (M) +27.3% racemic (R), (B) 80.5% meso (M)
+19.5% racemic (R).
3.2 Structure and acidity characterization
Zeolites are inorganic crystalline microporous materials, and
the crystallization degree change could inuence the catalytic
performance. The effect of phosphate modication on the
crystalline structure of HZSM-5 (Si/Al ¼ 360) was studied, and
the corresponding XRD patterns are shown in Fig. 4. All modi-
ed HZSM-5 samples with different P2O5 contents showed the
characteristic diffraction peaks of ZSM-5 zeolites at 7.97ꢀ, 8.92ꢀ,
23.12ꢀ, 23.34ꢀ, 24ꢀ, which could be indexed into the typical ZSM-
5 (JCPDS 44-0003) crystal planes (011), (200), (501), (051) and
(033), respectively.27 With the increasing P2O5 content from
0.5% to 8.0%, the diffraction peak intensity gradually
decreased. Moreover, compared with parent HZSM-5 (Si/Al ¼
360) zeolites, modied zeolite samples with phosphates show
no new diffraction peaks, which suggest that no new crystalline
phases or phosphate species could exist in the form of ions or
highly dispersed clusters.
Fig. 2 The effect of reaction temperature on product distribution with
phosphate-modified zeolites. (a) 2% P2O5/HZSM-5; (b) 4% P2O5/
HZSM-5.
effect on conversion and selectivity would be benecial to
witness the complex interaction of protonation and carbonium
rearrangement during the 2,3-butanediol dehydration process.
To study the effect of 2,3-butanediol conguration on cata-
lytic results, two types of reactant feedstock with different meso/
racemic ratios (feed A: 72.7% meso (M) +27.3% racemic (R); feed
B: 80.5% meso (M) +19.5% racemic (R)) have been used to test
To study the effect of phosphate modication on ZSM-5 pore
structure, the parent and modied zeolite samples were tested
ꢀ
the catalytic performance over 4.0% P/HZSM-5 at 180 C. The
catalytic results are shown in Fig. 3. When feed B with high
content of meso-type (80.5% meso +19.5% racemic) was used as
a reactant, the conversion of 2,3-butanediol decreased from
35.5% to 22.1%, and the unreacted meso-type isomers increased
from 49.2% to 64.9%, which indicated that the meso-type 2,3-
butanediol reaction could be unfavorable over phosphate-
modied HZSM-5 catalysts compared with racemic isomers.
Meanwhile, the selectivity to methyl ethyl ketone and 2-methyl
propanal showed no obvious change when the reactants
changed from feed A to feed B. This is because 2,3-butanediol
dehydration is the typical pinacol rearrangement process, and
the product distribution would depend on the reaction condi-
tions. To elucidate the difference in catalytic results, it is
necessary to thoroughly comprehend the surface structure of
modied HZSM-5 zeolites and the mechanism of pinacol
rearrangement.
Fig. 4 XRD patterns of P/HZSM-5(360) zeolites with various P2O5
contents.
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using the N2 adsorption–desorption method, and the calculated
results are shown in Table 1. As shown in Table 1, when the
HZSM-5 sample was modied by 0.5% P2O5, the values of the
specic surface area and micropore volume remained the same
as those of the parent HZSM-5, which indicates that small
amount phosphate modication has little inuence on the pore
structure of the catalysts. When the P2O5 content was increased
from 1.0% to 8.0%, the specic surface area decreased from 325
m2 gꢁ1 to 268 m2 gꢁ1, and the micropore volume gradually
decreased from 0.21 cm3 gꢁ1 to 0.17 cm3 gꢁ1. These surface and
pore structure changes reect the small destruction effect of
phosphate modication on zeolite crystalline structures, which
could be consistent with the results of XRD determination.
On the surfaces of catalysts, the 2,3-butanediol molecules
would undergo several steps, including the protonation of the
hydroxyl group, water molecule elimination and carbonium
formation and rearrangement. Therefore, the acidity of solid
catalysts would inuence the total reaction, including activity
and selectivity with different products. To clarify the surface
acidity properties, temperature-programmed desorption
methods with NH3 probes (NH3-TPD) were used to study the
surface acidity of parent and modied ZSM-5 catalysts. The
desorption curves of ammonia are shown in Fig. 5. For the
parent ZSM-5 (Si/Al ¼ 360), the desorption curve of ammonia
Fig. 5 NH3-TPD profiles of P/HZSM-5(360) zeolites with various P2O5
contents.
peaks at low temperatures. These changes in acidity properties
could contribute to the different catalytic dehydration results of
2,3-butanediol.
From the qualitative results of acid intensity and amounts
(Table 2), phosphate modication reduced the amount of
strong acid sites from 1167 mmol gꢁ1 over the parent HZSM-5 to
359 mmol gꢁ1 over 8.0% P/HZSM-5 and increased the weak acid
amount from 176 mmol gꢁ1 over the parent HZSM-5 to 1093
mmol gꢁ1 over 8.0% P/HZSM-5. However, the total acid amount
showed no obvious change. Moreover, the desorption peak
temperature corresponding to weak acid sites shied from
138.5 ꢀC over the parent HZSM-5 to 156.5 ꢀC over 8.0% P/HZSM-
5, which indicates that further phosphate addition would
improve the weak acid intensity. These results demonstrate that
phosphate modication adjusted the acid intensity distribution
and maintained the total acid amount balance, and these
inherent acidity properties directly inuence the catalytic
dehydration results of 2,3-butanediol, including the conversion
of 2,3-butanediol and selectivity for MEK and MPA. Parent and
modied HZSM-5 zeolites could be used as Brønsted type solid
acids, whose active sites would mainly be produced by hydroxyl
groups. FT-IR spectra could disclose the hydroxyl difference
between the parent HZSM-5 and modied zeolites. Therefore,
a series of modied HZSM-5 zeolites were studied by FT-IR, and
the corresponding results are shown in Fig. 6.
ꢀ
ꢀ
shows three desorption peaks, centered at 138 C, 297 C and
580 ꢀC. The desorption temperature is higher; the acid intensity
of active sites is stronger. Moreover, the ammonia desorption
curve of parent ZSM-5 (Si/Al ¼ 360) indicates that the parent
catalyst has abundant strong acid sites. When the ZSM-5 (Si/Al
¼ 360) zeolites were modied by different amounts of phos-
phates, the ammonia desorption curves showed the regular
changes. With increasing P2O5 content, the desorption peaks at
high temperatures became gradually weaker. Meanwhile, the
desorption peaks at low and medium temperatures became
gradually stronger. Moreover, the desorption peaks at low
temperatures shied to higher temperatures, as shown in Fig. 5.
Notably, the samples with a P2O5 content higher than 2%
showed signicantly different ammonia desorption curves
compared to those with a P2O5 content lower than 2%. The
samples with a P2O5 content higher than 2% had obvious weak
desorption peaks at high temperatures and strong desorption
From the transmission spectra show in Fig. 6a, the absorp-
tion peaks at 545, 802, 1064 and 1228 cmꢁ1 could be attributed
to the typical vibration of SiO4 tetrahedron units.28 According to
the results reported by Armaroli, the absorption near 802 cmꢁ1
could be induced by the symmetric stretching of the external
linkages, and the absorption at 545 cmꢁ1 is attributed to the
double ve-ring lattice vibration of the external linkages. The
strong absorption between 1000 cmꢁ1 and 1300 cmꢁ1 could be
assigned to the internal vibration of SiO4 or the AlO4 tetrahedra
of ZSM-5. The absorption peak at 1064 cmꢁ1 could be attributed
to the asymmetric stretching vibration of the Si–O–T linkage.
Compared with the parent HZSM-5, the absorption peak at 1064
cmꢁ1 of modied HZSM-5 samples by phosphate showed
a redshi, which indicates that the phosphate modication
could affect the vibration of asymmetric stretching of Si–O–T
linkage. Meanwhile, the absorption near 960 cmꢁ1, the typical
Table 1 The texture characteristics and phosphorus content of parent
and modified HZSM-5 zeolites with phosphates
BET surface areaa,
Volume,
(cm3 gꢁ1
)
P2O5 contentsb,
(wt%)
Catalysts
(m2 gꢁ1
)
HZSM-5(360)
0.5P/HZ
1.0P/HZ
2.0P/HZ
4.0P/HZ
6.0P/HZ
8.0P/HZ
325
327
316
308
288
275
268
0.21
0.21
0.20
0.19
0.18
0.18
0.17
—
0.42
0.85
1.81
3.75
5.22
7.35
a
b
The external surface area by t-plot method. The actual phosphorus
contents have been determined by XRF technique.
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Table 2 Acid amount of modified HZSM-5 with different amounts of phosphate
Desorption peak temperature/ꢀC
Acid amount/mmol gꢁ1
Catalysts
Weak
Medium
Strong
Weak
Medium
Strong
Total acid amount/mmol gꢁ1
HZSM-5(360)
0.5P/HZ
1.0P/HZ
2.0P/HZ
4.0P/HZ
6.0P/HZ
8.0P/HZ
138.5
138.4
150.1
151.9
161.0
154.5
156.5
297.0
289.5
306.2
276.4
343.7
299.9
288.2
580.9
578.9
587.9
586.7
576.6
573.1
579.3
176
208
306
468
893
960
1093
315
329
328
261
201
212
332
1167
1028
878
678
360
379
359
1658
1565
1512
1407
1454
1551
1784
acidity, the IR absorption at 3450 cmꢁ1, shown in Fig. 6(B),
could be attributed to the strong acid sites. Meanwhile, the IR
absorption peaks at 3650 cmꢁ1 and 3744 cmꢁ1 could be
attributed to the medium and weak acidic sites. From the IR
spectra of Fig. 6(B), the parent HZSM-5 had a strong absorption
peak at 3450 cmꢁ1, which could be attributed to strong acidic
sites and should be produced by Al–OH. With the introduction
of phosphate, the absorption peak at 3450 cmꢁ1 became weak,
and the absorption peak at 3650 cmꢁ1 and 3744 cmꢁ1 became
strong, which is in agreement with the results of NH3-TPD. The
absorption peaks at 3650 cmꢁ1 and 3744 cmꢁ1 could be
attributed to the P–OH and Si–OH. In total, the absorption
between 4000–3000 cmꢁ1 is very weak, which demonstrates that
the hydrophobic surface nature of HZSM-5(360) and small
number of hydroxyl groups were produced by phosphate
modication. This small amount of hydroxyl groups was also
conrmed by the NH3-TPD results.
3.3 Surface reaction model of 2,3-butanediol over HZSM-5
2,3-Butanediol dehydration is a typical pinacol rearrangement,
which can be observed when vicinal diols interact with acid.
During the reaction process, the hydroxyl group is rst
protonated. When the water molecule is eliminated, the inter-
mediate carbonium can form. Carbonium rearrangement
through hydride shi or methyl shi can proceed selectively,
depending on catalysts and reaction conditions. According to
the classic organic chemistry principle, carbonium formation is
Fig. 6 FT-IR spectra of P/HZSM-5(360) zeolites with various P2O5
contents.
Si–O stretching vibration of the surface silanol groups,29 became the rate-determining step during the catalytic process of vicinal
gradually stronger when the zeolites were modied by increased diols. When the carbonium formed, an alkyl group or hydrogen
phosphate amount. Moreover, the absorption near 960 cmꢁ1 shis over by one carbon towards a positive charge.
showed a redshi to a low wavenumber, which demonstrated
that the phosphate modication could form a P–O–Si bond.
To further clarify the 2,3-butanediol reaction process over
modied HZSM-5, a possible surface acid sites structure and
According to the results of Derouane et al.,30 the strong acidic catalytic reaction model would be necessary. Regarding to the
sites on ZSM-5 zeolites could be produced by Al–OH located at chemical states of phosphorus in ZSM-5 modied by phos-
the channel intersections, and the corresponding IR band and phates, ve possible models have been proposed.24b The cata-
desorption peak are characterized by the peaks at 3600 cmꢁ1 lysts preparation process would inuence the possible chemical
and 773 K, respectively. Weaker acidic sites are characterized by states and acid sites. In our research, the acid amount change
an IR band at 3720–3740 cmꢁ1 and a desorption peak at and ammonia desorption curves showed that the number of OH
approximately 500 K. They probably correspond to terminal groups had decreased aer phosphate modication. Similar to
silanol groups on the external surface of the zeolite or possibly the results reported by Ding et al.,24b these results have been
to nonzeolitic impurities. Therefore, the acidity is stronger and ascribed to the condensation of two zeolitic hydroxyls and one
the IR absorption wavenumber of the hydroxyl group is lower. phosphate molecule when the catalysts were calcined in dry air.
Because ZSM-5 zeolites with high Si/Al ratio have stronger Therefore, based on the FT-IR results and the literatures, the
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MPA), the modied zeolites with higher phosphate content
could acquire higher MEK/MPA ratios than those with lower
phosphate content. Moreover, the increase in reaction temper-
ature would reduce the MEK/MPA ratio. These results demon-
strated that stronger acid sites and higher reaction
temperatures would accelerate the methyl shi process. More-
over, the chiral conguration of 2,3-butanediol would have only
the inuence on the reactant-adsorbed state and carbonium
formation. When the carbonium formed from any 2,3-butane-
diol isomers, the pinacol rearrangement product distribution
would depend not on the reactant conguration but only on the
catalysts and reaction temperature.
4. Conclusions
A series of phosphate-modied HZSM-5(360) catalysts have
been prepared and used to catalyze 2,3-butanediol dehydration.
The surface and porous structures have been disclosed by
several physico-chemical methods including XRD, FT-IR, NH3-
TPD and N2 sorption. The results showed that phosphate
modication would reduce the surface strong acid sites while
increasing weak and medium acid sites. The conguration of
2,3-butanediol would affect only the adsorption activation and
carbonium formation process. Once the carbonium formed, the
product distribution would be independent of the reactant
conguration. Owing to the methyl shi being more difficult
than the hydride shi during carbonium rearrangement, the
strong acid and high reaction temperature would accelerate the
methyl shi during the carbonium rearrangement process.
These results would be useful for new catalysts and catalytic
process design in bio-based chemical transformation, especially
bio-based polyol with various congurations.
Scheme 2 Surface acid sites and reaction process model.
possible chemical states of phosphorus have been shown in
Scheme 2.
Based on FT-IR and NH3-TPD results, the acid sites of
modied HZSM-5 with phosphate could be produced by Al–OH
and P–OH. 2,3-Butanediol protonation could be undertaken
through adsorption at acidic sites such as Al–OH and P–OH.
Meso-type 2,3-butanediol with two hydroxyl groups located on
two sides of the C–C bond would prefer to interact with the acid
sites through terminal adsorption (as shown in Scheme 2 state
(I) and state (III)). Racemic type 2,3-butanediol with two hydroxyl
groups on the same side of the C–C bond would produce
terminal adsorption (state (IV)) and synergetic adsorption
between two hydroxyls and phosphate active centers (state (III)).
Because the acid intensity of Al–OH is stronger than that of P–
OH, the terminal adsorption by Al–OH could efficiently activate
the hydroxyl of 2,3-butanediol. In comparison, the acid inten-
sity of P–OH is too weak to efficiently activate hydroxyl of 2,3-
butanediol through terminal adsorption, but the bridge
adsorption between phosphate and racemic isomers could
compensate for the activation efficiency. Therefore, the
protonation capability of acid sites could contribute to the
conversion. These adsorption models could interpret the
conversion differences induced by catalysts, reaction tempera-
ture and chiral conguration of the reactants.
Aer the H2O molecule was eliminated from the protonation
state, the corresponding carbonium would form. The products
of methyl ethyl ketone and 2-methyl propanal could come from
hydride shi and methyl shi, respectively. According to the
results reported by Alexander et al.,24a when treated with phos-
phoric acid, DL-2,3-butanediol could be converted into methyl
ethyl ketone, whereas meso-2,3-butanediol could give a mixture
of methyl ethyl ketone and 2-methyl propanal. From the
product ratio of methyl ethyl ketone to 2-methyl propanal (MEK/
Acknowledgements
The authors are grateful for the nancial support by the
National Natural Science Foundation of China (No. 21376120),
the National Key Technology R&D Program (No.
2012BAD32B08), the National Basic Research Program (No.
2011CB710806), and the PhD Programs Foundation of Ministry
of Education of China (No. 20113221110010).
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