Hydroisomerization of cis-Stilbene into trans-Stilbene on Supported Heterogeneous Metal Catalysts (Rh, Pd, Pt, Ru, Ir/Α-Al2O3)

Article abstract of DOI:10.1134/S0023158417060064

The hydroisomerization of a cis-isomer to produce a trans-isomer on Rh, Pd, Pt, Ru, and Ir/α-Al₂O₃ catalysts is studied. It is shown that Rh and Ru catalysts on which the hydroisomerization reaction mostly takes place exhibit the most favorable characteristics, whereas on the other metals, the main route is the hydrogenation reaction. Rh/α-Al₂O₃ is the optimum catalyst, since it has much higher activity than Ru/α-Al₂O₃. It is found that the increased selectivity of the trans-isomer formation is facilitated by a decrease in the hydrogen pressure and by an increase in the substrate concentration. The maximum selectivity is achieved when the reaction is carried out in nonpolar n-hexane and toluene, whereas in the case of the more polar tetrahydrofuran (THF), dimethylformamide (DMFA), and methanol both the reaction rate and the selectivity of the trans-isomer formation decline.

Full text of DOI:10.1134/S0023158417060064

ISSN 0023-1584, Kinetics and Catalysis, 2017, Vol. 58, No. 6, pp. 771–779. © Pleiades Publishing, Ltd., 2017.
Original Russian Text © P.V. Markov, I.S. Mashkovsky, G.N. Baeva, A.Yu. Stakheev, 2017, published in Kinetika i Kataliz, 2017, Vol. 58, No. 6, pp. 751–759.
Hydroisomerization of cis-Stilbene into trans-Stilbene on Supported
Heterogeneous Metal Catalysts (Rh, Pd, Pt, Ru, Ir/α-Al O )
2
3
P. V. Markov, I. S. Mashkovsky, G. N. Baeva, and A. Yu. Stakheev*
Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Moscow, 119991 Russia
*
e-mail: st@ioc.ac.ru
Received January 29, 2017
Abstract—The hydroisomerization of a cis-isomer to produce a trans-isomer on Rh, Pd, Pt, Ru, and Ir/α-Al O
2
3
catalysts is studied. It is shown that Rh and Ru catalysts on which the hydroisomerization reaction mostly takes
place exhibit the most favorable characteristics, whereas on the other metals, the main route is the hydrogena-
tion reaction. Rh/α-Al O is the optimum catalyst, since it has much higher activity than Ru/α-Al O It is
2
3
2
3.
found that the increased selectivity of the trans-isomer formation is facilitated by a decrease in the hydrogen
pressure and by an increase in the substrate concentration. The maximum selectivity is achieved when the reac-
tion is carried out in nonpolar n-hexane and toluene, whereas in the case of the more polar tetrahydrofuran
(
THF), dimethylformamide (DMFA), and methanol both the reaction rate and the selectivity of the trans-iso-
mer formation decline.
Keywords: hydroisomerization, trans-stilbene, cis-stilbene, Rh, hydrogenation
DOI: 10.1134/S0023158417060064
INTRODUCTION
and the absence of by-products. In this respect, many
research groups are trying to develop new catalytic sys-
tems that allow to obtain trans-isomers via the selec-
tive hydrogenation of disubstituted alkyne com-
pounds. In recent years, a number of publications have
appeared that show the fundamental possibility of
making a highly selective hydrogenation catalyst in the
trans-alkene isomers formation.
In recent years, specialists in the field of organic
chemistry and heterogeneous catalysis are being
increasingly attracted by the selective hydrogenation
of disubstituted alkyne compounds to produce trans-
alkenes. This is caused by the fact that many com-
pounds used in the pharmaceutical and food indus-
tries contain a double C=C bond in their structure and
that the nature of their biological activity is largely
Thus the authors of [20] proposed a heterobime-
determined by the geometry of substituent arrange- tallic complex (IMes)Ag–RuCp(CO) capable of
2
ment (cis or trans) with respect to the double bond. hydrogenating DPA to produce highly selective
Typical examples of such structures are the beta-caro- trans-stilbene (trans-ST). A number of works have
tene molecules [1], the polyene molecules of antifun- shown the possibility of making selective hydrogena-
gal agents [2, 3], polyunsaturated fatty acids, phero- tion catalysts based on Ru complexes. Lindhardt
mones [4–6], and other biologically active com- et al. [21] described the selective hydrogenation of
pounds [7, 8].
DPA and other arylacetylenes on a commercial cata-
(CO)(H)Cl that highly selectively
One of the most efficient traditional methods for lyst Ru(PPh
)
3 3
preparing cis-isomers is the selective hydrogenation of formed trans-ST at a hydrogen pressure of 3 bar. In a
disubstituted alkyne compounds on heterogeneous later paper [22], a comparative study was performed
metal catalysts. The use of modified Pd catalysts concerning the selectivity and activity of 17 metal-
allows to carry out this process with a selectivity level complex Ru catalysts in the hydrogenation of DPA
higher than 95% and an acceptable rate [9, 10].
into trans-ST using formic acid as a hydrogen source.
However, the synthesis of trans-isomers is much
At the 100% conversion level of the initial DPA, a
more complicated. Methods based on alkene metath- selectivity higher than 98–99% was achieved using the
esis reactions [11, 12], cross coupling [13] and other [RuPPh ]Cl and Ru(PPh ) (CO)(H)Cl catalysts, as
3
2
3 3
methods for the formation of a new double bond are
used to produce them [14–19].
Nevertheless, the reaction of selective hydrogena- catalyst also highly selectively formed trans-isomers
tion remains a very attractive alternative to the above- upon the hydrogenation of a number of substituted
mentioned methods due to the ease of performing it alkyne compounds having different structures. By
well as a Grubbs–Hoveda type metathesis catalyst
representing a benzylidene complex of Ru. The latter
771

7
72
MARKOV et al.
studying the mechanism of hydrogenation, the alkyne molecule is hydrogenated to form cis-alkene
authors concluded that the reaction process via a two- that further isomerizes into a more thermodynami-
storage mechanism. At the first stage, a disubstituted cally stable trans-isomer (see Scheme 1):
[Ru]
[Ru]
H2
Scheme 1
Unfortunately, there is almost no data available in gen pressure, substrate concentration, and solvent
the literature concerning a selective liquid-phase nature. Cis-ST was used as the model substrate, since
hydrogenation of the substituted alkynes to yield the most of the works devoted to liquid-phase cis–trans-
corresponding trans-isomers on heterogeneous cata- isomerization were performed using this compound.
lysts. Only in recent years, Japanese scientists have
shown the possibility of obtaining trans-isomers on
EXPERIMENTAL
Preparation of Catalysts
heterogeneous catalysts using this method. Thus,
Komatsu et al. [23] achieved a high yield of trans-ST
(
74%) via DPA hydrogenation on a composite system
To obtain a series of metal-supported catalysts,
obtained by mechanically mixing the bimetallic
2
α-Al O (Alfa Aesar, S = 8 m /g), preliminarily cal-
2
3
sp
(
Pd Bi/SiO ) and zeolite (H–USY) components. In a
3 2
cined in air flow (550°C) was used as the carrier. Met-
als were applied using an incipient wetness impregna-
tion.
later work [24], a yield of about 58% of trans-ST was
obtained on a bimetallic Rh Sb/SiO catalyst. The yield
2
2
of the trans-isomer was increased to 88% when chang-
Pd was supported from a solution of Pd(NO ) in
3
2
ing the monocomponent Rh Sb/SiO catalyst to the
2
2
1
0% nitric acid. After impregnation, the sample was
tandem [RhSb/SiO + Pd Bi/SiO ] catalytic system.
2
3
2
dried in air at room temperature, then reduced at a
The investigation of the reaction in the presence of 5 vol % H /Ar flow for 1 hour at 600°C. The metal
2
heterogeneous catalysts showed that the process pro-
ceeds according to a two-stage mechanism similar to
the mechanism proposed for a Ru metal-complex cata-
lyst: at the 1st stage, the DPA is hydrogenated to form
cis-stilbene (cis-ST) that is then isomerized at the sec-
ond stage to give trans-ST (see Scheme 1). The isomer-
ization on the [Pd Bi/SiO + H–USY] catalyst pro-
content in the catalyst was 1 mass %.
Aqueous solutions of Rh(NO ) , [Pt(NH ) ]Cl ,
3
3
3 4
2
IrCl , and RuCl were used to prepare the Rh-, Pt-,
3
3
Ir-, and Ru-based catalysts, respectively. After
impregnation, the samples were dried at room tem-
perature, then calcined in air flow: Rh/α-Al O for 4 h
2
3
3
2
at 450°C, Ir, Ru/α-Al O for 3 h at 450°C, and Pt/α-
2
3
ceeds according to the mechanism of acid catalysis on
the zeolite component. On [RhSb/SiO + Pd Bi/SiO ],
Al O for 4 h at 500°C. The catalysts were activated in
2
3
2
3
2
a flow of 5 vol % H /Ar for 1 h: Rh/α-Al O at 550°C,
2
2
3
the hydroisomerization of cis-ST occurs on the RhSb
surface, whereas the hydrogenation proceeds on
Pd Bi/SiO .
Pt/α-Al O at 500°C, and Ir, Ru/α-Al O at 450°C.
2
3
2
3
Since the activity of Rh, Pt, Ir, and Ru in the studied
reaction is significantly lower than that of Pd, catalysts
were prepared with a 3 wt % metal content.
3
2
It should be noted that it is possible to hydroisom-
erize cis-ST into trans-ST on the monocomponent
Rh Sb/SiO , although in this case the yield of the trans
2
2
product is somewhat lower (~58%) [24]. A key param-
eter determining the yield of the trans-isomer is the
activity of the catalyst in the hydroisomerization. As a
rule, hydroisomerization rate is slower than the rate of
the competing hydrogenation of the resulting olefin.
Unfortunately, in the current literature there is almost
no data on the activity of heterogeneous catalysts
based on different metals in the liquid-phase
hydroisomerization of complex molecules.
The aim of this work consisted in comparing the accelerating voltage. The average size and the particle
hydroisomerization activity of the supported catalysts size distribution were calculated based on the mea-
based on Pd, Ir, Pt, Rh, and Ru, as well as in deter- surement of 300–1000 particles on the TEM images of
mining the optimal process parameters such as hydro- different parts of the samples.
Electron Microscopy
The catalysts were examined by transmission elec-
tron microscopy (TEM) using an HT7700 electron
microscope (Hitachi, Japan). The analytical measure-
ments were optimized as described elsewhere [25].
Before the imaging, the powder samples were depos-
ited from an isopropanol suspension onto copper grids
(
d = 3 mm) covered with a layer of carbon. Images
were acquired in bright-field TEM mode at 100 kV
KINETICS AND CATALYSIS
Vol. 58
No. 6
2017

HYDROISOMERIZATION OF cis-STILBENE INTO trans-STILBENE
773
Hydroisomerization of cis-Stilbene
The following solvents were used: n-hexane (98%,
LiChrosolv), methanol (99.8%, LiChrosolv), toluene
The hydroisomerization of cis-ST (97%, Alfa
Aesar) was carried out in an autoclave-type reactor
mounted on a magnetic stirrer and equipped with a gas
supply and a pressure control system. The substrate,
catalyst, and solvent (6 mL) were mixed in a glass vial
(
99.9%, LiChrosolv, tetrahydrofuran ((THF), 99.9%,
LiChrosolv), and dimethylformamide ((DMFA),
9%, Acros).
9
(
12 mL) and placed into the autoclave. The setup was
RESULTS AND DISCUSSION
Electron Microscopy
The TEM images of the studied catalysts and the
purged with He to remove air and was filled with
hydrogen to the required pressure. The reaction
occurred at a temperature of 25°C. The kinetic mode
of the process was confirmed according to a technique histograms of the size distribution for the metal parti-
described in details earlier [26].
cles are shown in Figs. 1 and 2. All the catalysts are
characterized by a rather broad monomodal particle
The process was monitored by periodically sam-
size distribution. In the 1%Pd/α-Al O sample, metal
pling and analyzing the reaction mixture using a Crys-
2
3
tal 5000 chromatograph (Chromatec, Russia) with a particles with the average size (Dav) ~ 14 nm are
flame ionization detector. The components of the formed. The particles of Pt in the 3%Pt/α-Al O cat-
2
3
mixture were separated using a HP5–MS column (5% alyst are much larger; their average size is ~ 26 nm.
phenyl dimethylsiloxane, 30 m, 0.25 mm internal The size of metal nanoparticles in the catalysts based
diameter, 0.25 μm thick fixed film, and helium as the on Ru, Rh, and Ir is ~10, 5, and 4 nm, respectively.
carrier gas).
Only hydroisomerization products such as trans-ST
and hydrogenation products such as diphenylethane the other catalysts could be caused by the fact that a
The significantly larger size of the supported metal
particles in the 3%Pt/α-Al O sample compared to
2
3
(
DPE) have been found in the reaction mixture. The complex salt containing NH ligands was used for its
3
substrate conversion level was calculated using the fol- preparation.
lowing formula:
X DPE + X trans-ST
X cis-ST + X DPE + X trans-ST
Activity and Selectivity of Various Metals
in the Hydroisomerization of cis-Stilbene
K =
,
The experimental results presented in Table 1 allow
us to compare the hydrogenating and hydroisomeriz-
ing activity of various metals based on the formation
rates for DPE (hydrogenation) and trans-ST
where X
X
, and X
are the mole fractions
DPE, trans-ST
cis-ST
of DPE, trans-ST and cis-ST, respectively, in the reac-
tion mixture.
The specific rates of hydroisomerization (rh-isom)
and hydrogenation (r ) (mmol min–1
(hydroisomerization).
−
1
g
) were
cat
hydr
It is clearly seen from the comparison of the data
determined at the conversion level of cis-ST amount- that 1%Pd/α-Al O has the highest hydrogenation
2
3
ing to less than 15% using the following formula:
activity, whereas the hydrogenation rate for 3% Rh/ α-
−
1
–1
n
Al O is slightly lower: 0.26 and 0.23 mmol min
g
,
cat
2
3
r =
,
τmcat
respectively. It should be taken into account that the
Rh content in the sample is triple that of Pd; therefore,
the TOF value for the Rh catalyst is lower by a factor
of 3.5 than for Pd. A slightly lower hydrogenation
activity is observed for the platinum catalyst with DPE
where n is the amount of product (mmol) formed in
time τ (min) and m is the mass of the catalyst (g).
cat
The conversion rate of the substrate was calculated
using the following formula:
–1
formation rate ~0.18 mmol min .
rconv = rh-isom + rhydr
.
The systems based on Ru and Ir are much less
The catalytic activity of metals in the hydrogena- active than the Pd, Rh, and Pt ones. The rate of DPE
tion and hydroisomerization reactions was evaluated formation on Ru and Ir is lower by 7–8 factors: 0.034
–1
−1
cat
basing on the turnover number (TOF, min ), i.e., the
ratio between the number of DPE or trans-ST mole-
cules formed per minute and the total number of active
metal atoms in the catalyst. The calculation was car-
ried out according to the following formula:
and 0.048 mmol min–1
g
, respectively.
The obtained results make it possible to arrange the
investigated metals in the following series according to
their activity in the hydrogenation of cis-ST, which is
in good agreement with the published data [27]:
rMMe
Pd @ Pt > Rh @ Ru ~ Ir.
TOF =
,
[
Me]×1000
From the standpoint of using the studied catalysts
where [Me] and M are the active metal content in in the synthesis of trans-isomers, the most important
Me
the catalyst (wt %) and its molar weight (g/mol), parameters are their hydroisomerizing activity, as well
respectively.
as the ratio between the rates of hydroisomerization and
KINETICS AND CATALYSIS Vol. 58 No. 6 2017

7
74
MARKOV et al.
(а)
100 nm (b)
100 nm (c)
100 nm
(d)
100 nm (e)
100 nm
Fig. 1. TEM images of catalysts: (a) 1%Pd/α-Al O , (b) 3%Pt/α-Al O , (c) 3%Rh/α-Al O , (d) 3%Ru/α-Al O , (e) 3%Ir/α-Al O .
2
3
2
3
2
3
2
3
2 3
(
а)
(b)
Dav = 26 nm
(c)
3
3
2
2
5
0
5
0
25
25
20
15
10
5
Dav = 14 nm
Dav = 5 nm
20
15
1
5
10
1
0
5
0
5
0
0
5
15 25 35 45 55 65 75 85 95
Particle size, nm
5
25 45 65 85 105
Particle size, nm
1
5
9
13 17 21 25 27
Particle size, nm
(d)
(e)
3
2
2
0
5
0
45
4
0
5
Dav = 10 nm
Dav = 4 nm
3
30
2
5
0
15
2
10
15
1
0
5
0
5
0
2
10 18 26 34 42 50 58
Particle size, nm
1
5
9
13 17 21 25 27
Particle size, nm
Fig. 2. Histograms of size distribution for metal particles in catalysts: (a) 1%Pd/α-Al O , (b) 3%Pt/α-Al O , (c) 3%Rh/α-Al O ,
2
3
2
3
2 3
(
d) 3%Ru/α-Al O , (e) 3%Ir/α-Al O .
2 3 2 3
hydrogenation. Since these two processes are compet-
The analysis of the data on the hydroisomerization
ing, particular interest is provided to catalysts with the rates presented in Table 1 shows that 3%Ru/α-Al
O is
2
3
the most active catalyst with the rate of trans-isomer
following characteristics: (1) the cis-stilbene conversion
−
cat
1
proceeds predominantly via the hydroisomerization formation about 0.69 mmol min–
1
g
, which is triple
route and (2) the ratio of the rh-isom/rhydr rates is maximal. the hydrogenation rate (r
/rhydr = 3.0). As a result,
h–isom
KINETICS AND CATALYSIS
Vol. 58
No. 6
2017

HYDROISOMERIZATION OF cis-STILBENE INTO trans-STILBENE
Table 1. Kinetic characteristics of cis-ST hydroisomerization on different metals*
775
rconv
rhydr
rh-isom
TOFconv
TOFhydr
min–1
TOFh-isom
Catalyst
rh-isom/rhydr
mmol min–1
−1
cat
g
3
1
3
3
3
%Rh/α-Al O3
0.92
0.27
0.22
0.11
0.23
0.26
0.18
0.034
0.048
0.69
3.0
3.14
2.83
1.42
0.36
0.33
0.78
2.71
1.19
0.11
0.31
2.36
0.12
0.23
0.24
0.02
2
%Pd/α-Al O3
0.011
0.035
0.072
0.003
0.04
0.19
2.1
2
%Pt/α-Al O3
2
%Ru/α-Al O3
2
%Ir/α-Al O3
0.051
0.06
2
*
3
Substrate concentration, 0.33 mol/L; 2.5 bar H ; catalyst loading: 3%Rh/α-Al O 2.5 mg; 3%Pt/α-Al O , 1%Pd/α-Al O 15.0 mg;
2 2 3 2 3 2 3
%Ru/α-Al O , 3%Ir/α-Al O , 30.0 mg, solvent n-hexane.
2 3 2 3
the overall conversion rate of cis-ST on this catalyst is trans-ST in the reaction products obtained on
significantly higher than for all the tested samples—0.92 3%Ir/α-Al O is much lower, whereas the selectivity
2
3
−
1
of trans-product formation on 1%Pd/α-Al O is less
2 3
mmol min–1
, including the Pd catalyst exhibiting
cat
g
than 5%.
the maximal hydrogenation activity (see below).
In addition to the Rh catalyst, a high ratio of
The obtained data allow us to conclude that the
most promising system is Rh/α-Al O , since it has the
2
3
rh-isom/r
= 2.1 is observed for the reaction occur-
hydr
best r
/r ratio and a high activity in hydroisom-
h-isom hydr
ring on 3%Ru/α-Al O ; however, its activity in both
2
3
erization. Therefore, for this catalytic system a detailed
study was made of the hydroisomerization efficiency
as a function of the solvent nature, the hydrogen pres-
sure, and the substrate concentration.
hydrogenation and hydroisomerization processes is low
and the overall conversion rate of cis-ST on this catalyst
is almost an order of magnitude lower than that for
−
1
–1
3
%Rh/α-Al O : 0.11 and 0.92 mmol min
g
,
cat
2
3
respectively.
Effect of the Solvent
The 1%Pd/α-Al O catalyst exhibits a low activity
in hydroisomerization; the rate of trans-ST formation
2
3
Since the hydroisomerization reaction of complex
organic molecules such as cis-ST needs to be carried
out in the liquid phase, the nature of solvent is an
important factor that can significantly affect the over-
all process rate and the ratio between the hydroisom-
−
cat
1
on the catalyst is ~0.011 mmol min–1
sample is most active in hydrogenation, the lowest
ratio r /r among the studied catalysts (0.04) is
. Since this
g
h-isom hydr
observed, which agrees with the published data con-
cerning the high selectivity of Pd in the formation of
cis-isomers in the course of the hydrogenation of
alkyne compounds [9]. Somewhat higher ratios
between the rates of hydroisomerization and hydroge-
nation are exhibited by the Pt and Ir catalysts (0.19 and
Composition of products, %
trans-ST
DPE
100
80
60
40
20
0
0.06, respectively); however, the values of r
/r
h-isom hydr
inherent in them are substantially smaller than the val-
ues for the Rh and Ru samples.
Thus, the analysis of the activities of catalysts based
on various metals in hydrogenation and hydroisomer-
ization reactions shows that Rh and Ru exhibit the best
characteristics for trans-isomers formation. Figure 3
shows a comparison of product compositions for the
cis-ST’s interaction with hydrogen on the studied
samples when the initial substrate reaches a 35% con-
version level. The maximum selectivity in the forma-
tion of trans-ST is exhibited by 3%Rh/α-Al O and
Rh
Pd
Pt
Ru
Ir
2
3
3
%Ru/α-Al O , for which the fraction of the trans-
2 3
isomer in the reaction products is ~75 and 68%,
respectively. In the case of a catalyst containing Pt as
the active component, the product of complete hydro-
genation of DPE is predominant, and the amount of
trans-isomer does not exceed 15%. The fraction of
Fig. 3. Composition of products of cis-ST hydroisomeriza-
tion on different metals. Substrate concentration
0
2
.33 mol/L, 2.5 bar H , catalyst loading: 3%Rh/α-Al O
2 2 3
.5 mg; 3%Pt/α-Al O , 1%Pd/α-Al O 15.0 mg; 3%Ru/α-
2
3
2 3
Al O , 3%Ir/α-Al O 30.0 mg, solvent n-hexane. Substrate
2
3
2 3
conversion level 35%.
KINETICS AND CATALYSIS Vol. 58 No. 6 2017

7
76
MARKOV et al.
Table 2. Effect of solvent on kinetic characteristics of cis-ST hydroisomerization*
rconv
rhydr
rh-isom
TOFconv
TOFhydr
min–1
TOFh-isom
Solvent
n-Hexane
Toluene
THF
rh-isom/rhydr
mmol min–1
−1
cat
g
0.92
0.55
0.44
0.31
0.66
0.23
0.14
0.12
0.15
0.26
0.69
0.42
0.31
0.16
0.40
3.0
3.0
2.5
1.1
1.6
3.14
1.89
1.50
1.05
2.27
0.78
0.47
0.43
0.51
0.88
2.36
1.43
1.08
0.54
1.38
DMFA
MeOH
*
Substrate concentration⎯ 0.33 mol/L, 2.5 mg %Rh/α-Al O , 2.5 bar H .
2 3 2
erization and hydrogenation rates. In connection with and the selectivity of trans-isomer formation reaches
this, we studied the effect of the solvent on the cis-ST 75% (Fig. 4). As the polarity of the solvent increases,
hydroisomerization. Both polar and nonpolar solvents the selectivity decreases, and DMFA is the most unfa-
such as n-hexane, toluene, THF, DMFA, and methyl vorable from this standpoint. In the case of DMFA,
alcohol were used.
the r
/r ratio is 1.1, which leads to low selectivity
h-isom hydr
From the data presented in Table 2 we can see that in the formation of trans-isomer (Fig. 4). A somewhat
the maximum rate of cis-ST conversion is achieved higher selectivity is observed when the reaction is car-
when the reaction is carried out in n-hexane— ried out in the less polar THF and methanol (71 and
−
1
62%, respectively).
1
0
.92 mmol min–
g
. In the case of toluene and
cat
The observed relationship between the selectivity
of the process and the polarity of the solvent can be
explain in the following way. A significant role can be
played by the competitive adsorption of the solvent
and substrate on the catalyst surface, which leads to a
reduction in the surface concentration of the latter,
and thus negatively affects the selectivity of trans-iso-
mer formation (Fig. 4). This allows us to suggest that
the displacement of the substrate’s molecules from the
catalyst surface by the solvent’s molecules results in a
decrease in the selectivity of the entire process due to
the substantial decrease in the rate of the hydroisom-
erization reaction.
methanol, the rate is somewhat lower amounting to
.55 and 0.66 mmol min–1
lowest rate is observed for DMFA 0.31 mmol min
−
1
0
g
, respectively. The
cat
–
1
−
1
g
. The most probable cause consists in the differ-
cat
ent solubility of hydrogen in these solvents [28].
It should also be noted that the nature of the sol-
vent has a significant effect on the ratio of r
/r
h-isom hydr
and on the overall selectivity of the process. The anal-
ysis of the data presented in Table 2 shows that when
the reaction proceeds in nonpolar solvents such as n-
hexane and toluene, the r
/r
ratio is maximal
h-isom hydr
The effect of hydrogen pressure and substrate con-
centration on the kinetics and selectivity of
hydroisomerization was investigated using n-hexane
as the solvent in which a high selectivity and a high
reaction rate were achieved.
Composition of products, %
00
trans-ST
DPE
1
8
6
4
2
0
0
0
0
0
Effect of Hydrogen Pressure
The data concerning the effect of the hydrogen
pressure in the range from 1 to 10 bar on the cis-ST
hydro-conversion are presented in Table 3. It can be
seen that as РН2 decreases the rate of the total conver-
−
1
–
1
sion decreases from 1.28 to 0.75 mmol min
g
,
cat
which corresponds to the kinetic order of ~0.21 with
respect to hydrogen. It should be noted that the effect
of the hydrogen pressure on the rate of hydrogena-
tion and hydroisomerization is significantly differ-
ent: the rate of the hydrogenation decreases by a fac-
−1
Hexane Toluene THF DMFA MeOH
Fig. 4. Effect of solvent on composition of cis-ST
–1
tor of ~3.5 from 0.44 to 0.13 mmol min g , while the
cat
hydroisomerization products. Substrate concentration
rate of hydroisomerization decreases to a much lesser
0
.33 mol/L, 2.5 mg of 3%Rh/α-Al O , 2.5 bar H , sub-
2 3 2
–
1
strate conversion level 35%.
extent, by a factor of 1.3 (from 0.84 to 0.62 mmol min
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2017

HYDROISOMERIZATION OF cis-STILBENE INTO trans-STILBENE
777
Table 3. Effect of hydrogen pressure on kinetic characteristics of cis-ST hydroisomerization*
rconv
rhydr
rh-isom
TOFconv
TOFhydr
min–1
TOFh-isom
P , bar
rh–isom/rhydr
H2
mmol min–1
−1
cat
g
1
5
2
1
0.0
.0
.5
.0
1.28
0.93
0.92
0.75
0.44
0.28
0.23
0.13
0.84
0.66
0.69
0.62
1.9
2.4
3.0
5.0
4.39
3.20
3.14
2.57
1.51
0.95
0.78
0.43
2.88
2.25
2.36
2.14
*
Substrate concentration⎯ 0.33 mol/L, 2.5 mg 3% Rh/α-Al O , solvent⎯ n-hexane.
2 3
−
cat
1
lnr [mmol min−1
−1
g
). The calculation of the orders of the hydrogena-
g
]
cat
tion and isomerization reactions (Fig. 5) has shown
that the kinetic order of the hydrogenation with
0.5
nconv = 0.21
respect to H is 0.52 and corresponds to the order
2
0
observed for hydrogenation on palladium catalysts
[
26]. The dependence of the hydroisomerization rate
–0.5
on the hydrogen pressure is much less pronounced,
and the order with respect to hydrogen is 0.11. As a
result, a decrease in the hydrogen pressure from 10 to
nh-isom = 0.11
–
1.0
1.5
1
bar, leads to a significant increase in the r
/r
h-isom hydr
–
rconv
ratio, and, in turn, to an increase in the selectivity of
nhydr = 0.52
trans-isomer formation from 66 to 78% (Fig. 6).
rhydr
–2.0
rh-isom
The observed dependences are in good agreement
with the existing concepts of the differences in the
reactions of hydrogenation and hydroisomerization.
The hydrogenation involves one C–C bond breaking
with the simultaneous or sequential addition of two
hydrogen atoms and the subsequent desorption of the
formed alkane molecule. In contrast, the hydroisom-
erization proceeds through a double-bond semihydro-
genation stage followed by a rotation around the C–C
bond, after which 1–H–dehydrogenation and the
desorption of the thermodynamically more stable
trans-isomer take place. At the same time, the H atom
remains on the surface of the catalyst, which makes the
reaction significantly less sensitive to a change in the
hydrogen pressure [24, 29, 30].
–
2.5
–
0.5
0
0.5 1.0 1.5 2.0 2.5 3.0
ln P 2
bar]
[
H
Fig. 5. Effect of hydrogen pressure on hydrogenation and
hydroisomerization rates, and on rate of cis-ST conver-
sion. Substrate concentration 0.33 mol/L, 2.5 mg
3
%Rh/α-Al O , solvent n-hexane.
2 3
Composition of products, %
100
trans-ST
DPE
80
Effect of cis-Stilbene Concentration
The data concerning the effect exerted by the sub-
strate concentration on the hydro-conversion of cis-ST
are given in Table 4. A 10-fold increase in the concen-
tration from 0.083 to 0.83 mol/L leads to a significant
decrease in the total conversion rate of the substrate
60
4
0
0
0
−
1
from 1.28 to 0.23 mmol min–1
. The character of
cat
g
2
the curves for the rates of the hydrogenation and
hydroisomerization depending on the concentration
of cis-ST is fundamentally different. Thus, the hydro-
genation rate significantly decreases as the substrate
concentration increases, and the order of the hydroge-
nation reaction with respect to the concentration of
cis-ST in the range of 0.083–0.333 mol/L (Fig. 7) is
1.0 bar
2.5 bar
5.0 bar
10.0 bar
Fig. 6. Effect of hydrogen pressure on composition of cis-ST
hydroisomerization products. Substrate concentration
–
0.91. The negative order of alkene hydrogenation on
0
.33 mol/L, 2.5 mg 3%Rh/α-Al O , solvent n-hexane.
2 3
Rh could most likely be related to the competitive
Substrate conversion level 35%.
KINETICS AND CATALYSIS Vol. 58 No. 6 2017

7
78
MARKOV et al.
Table 4. Effect of substrate concentration on kinetic characteristics of cis-ST hydroisomerization*
rconv
rhydr
rh–isom
TOFconv
TOFhydr
TOFh-isom
Concentration
of cis-ST, mol/L
rh–isom/rhydr
mmol min–1
−1
cat
min–1
g
0
0
0
0
.083
.17
.33
.83
1.28
1.08
0.92
0.23
0.80
0.51
0.23
0.013
0.48
0.57
0.69
0.22
0.6
1.1
3.0
4.38
3.70
3.14
0.80
2.74
1.75
0.78
0.04
1.64
1.95
2.36
0.75
17.3
*
2.5 mg 3%Rh/α-Al O , 2.5 bar H , solvent—n-hexane.
2
3
2
−
1
−1
adsorption of olefin and hydrogen on the metal sur-
face. As the olefin concentration increases, the
amount of adsorbed hydrogen decreases, and the rate
of the hydrogenation reaction slows to a significant
extent [27] (Fig. 8).
lnr [mmol min
g
]
cat
0.5
nconv = –0.24
0
–0.5
nh-isom = 0.26
At the same time, the hydroisomerization rate
increases slightly with an increase in the substrate con-
centration from 0.083 to 0.333 mol/L, which corre-
sponds to the observed order of the hydroisomeriza-
tion rate with respect to the substrate of about ~0.21. It
can be assumed that the hydroisomerization is less
sensitive to the amount of adsorbed hydrogen, since,
in contrast to the hydrogenation, this process requires
only one H atom, which, in addition, is not consumed
in the reaction and remains on the metal surface after
the desorption of the trans-ST molecule.
–
–
1.0
1.5
nhydr = –0.91
–
–
–
–
–
–
2.0
2.5
3.0
3.5
4.0
rconv
rhydr
rh-isom
4.5
–
2.7 –2.2 –1.7 –1.2 –0.7 –0.2 0.3
–1
When the concentration of cis-ST is increased from
.33 to 0.83 mol/L, the rates of both reactions
lnCcis-ST[mol L ]
0
Fig. 7. Effect of substrate concentration on rates of hydro-
genation and hydroisomerization, as well as on conversion
rate of cis-ST. 2.5 mg 3%Rh/α-Al O , 2.5 bar H , sol-
decrease to a considerable extent, and the linear
dependence of the rate on the substrate concentration
in logarithmic coordinates is violated for both the
hydrogenation and the hydroisomerization (Fig. 7).
This is associated with exceeding the solubility limit of
the formed trans-ST in n-hexane used as a solvent.
2
3
2
vent—n-hexane.
Composition of products, %
trans-ST
DPE
CONCLUSIONS
100
Thus, the results of the study allow us to conclude
that catalysts based on Rh and Ru are most promising
for the formation of trans-isomers of complex organic
molecules through the hydroisomerization of the cor-
responding cis-isomers or the selective hydrogenation
of the disubstituted alkynes. The selectivity of their
action can be significantly increased by carrying out
the process in nonpolar solvents. A higher selectivity
in the formation of trans-isomers is also facilitated by
an increase in the concentration of the initial sub-
strate, as well as by carrying out the process under a
reduced hydrogen pressure.
8
0
0
0
0
0
6
4
2
0.08
0.17
0.33
0.83
Concentration, mol/L
ACKNOWLEDGMENTS
The work was financially supported by the Russian
Science Foundation (grant 16–13–10530). The
authors are grateful to the Department of Structural
Research of the Zelinsky Institute of Organic Chemis-
Fig. 8. Effect of substrate concentration on composition
of cis-ST hydroisomerization products: 2.5 mg of
3
%Rh/α-Al O , solvent—n-hexane. Substrate conver-
2 3
sion level 35%.
KINETICS AND CATALYSIS
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2017

HYDROISOMERIZATION OF cis-STILBENE INTO trans-STILBENE
779
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1
961, vol. 83, p. 1733.
7. Blakemore, P.R., J. Chem. Soc., Perkin Trans., 2002,
vol. 1, p. 2563.
8. Peterson, D.J., Org. Chem., 1968, vol. 33, p. 780.
samples using electron microscopy.
1
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Translated by O. Polyakov
KINETICS AND CATALYSIS Vol. 58 No. 6 2017