Glycerol Isopropyl Ethers: Direct Synthesis from Alcohols and Synthesis by the Reduction of Solketal

Article abstract of DOI:10.1002/cctc.201700108

The catalytic reduction of solketal ((2,2-dimethyl-1,3-dioxolan-4-yl)methanol) over bifunctional heterogeneous palladium catalysts is proposed as an alternative to the synthesis of glycerol isopropyl ethers by the etherification of glycerol. The direct synthesis of glycerol isopropyl ethers from isopropanol and glycerol requires severe conditions (T=130–150 °C, p(H₂)=20–35 bar) and a large excess of isopropanol to reach a considerable yield. The main reaction products in the catalytic reduction of solketal are glycerol mono- and di-isopropyl ethers and solketal isopropyl ether. Solketal conversion over Al-HMS-supported palladium catalysts (T=120 °C and p(H₂)=20 bar) affords a mixture of ethers with a high degree of conversion (87 %), 78 % selectivity, and excellent regioselectivity between isomeric ethers. Zeolite-BEA-supported palladium catalysts also exhibit high activity but much lower selectivity because of intense acetone aldol condensation. The effects of Si/Al ratios in BEA zeolites and Al-HMS aluminosilicates and the amounts of supported palladium (1 and 2 wt %) on the properties of the catalysts at different reaction temperatures and hydrogen pressures are considered.

Full text of DOI:10.1002/cctc.201700108

Accepted Article
Title: Glycerol isopropyl ethers: direct synthesis from alcohols and
synthesis by the reduction of solketal
Authors: Vadim Samoilov, Maria Onishchenko, Dzhamalutdin
Ramazanov, and Anton Maximov
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Glycerol isopropyl ethers: direct synthesis from alcohols and
synthesis by the reduction of solketal
Vadim Samoilov,[a] Dr. Maria Onishchenko,[a] Dr. Dzhamalutdin Ramazanov,[a] and Prof. Dr. Anton
Maximov*[a,b]
Abstract: The catalytic reduction of solketal over bifunctional
heterogeneous palladium catalysts is proposed as an alternative to
the synthesis of glycerol isopropyl ethers by the etherification of
glycerol. The direct synthesis of glycerol isopropyl ethers from
isopropanol and glycerol requires severe conditions (T = 130-150°С,
p(H2) = 20-35 bar) and a large excess of isopropanol to reach a
considerable yield. The main reaction products in the catalytic
reduction of solketal are glycerol mono- and di-isopropyl ethers and
solketal isopropyl ether. Solketal conversion over Al-HMS-supported
palladium catalysts (T = 120°C and p(H2) = 20 bar) affords a mixture
of ethers with a high degree of conversion (87%), 78% selectivity,
and excellent regioselectivity between isomeric ethers. Zeolite-BEA-
supported palladium catalysts also exhibit high activity but much
lower selectivity because of intense acetone aldol condensation. The
effects of Si/Al ratios in BEA zeolites and Al-HMS aluminosilicates
and the amounts of supported palladium (1 and 2 wt %) on the
properties of the catalysts at different reaction temperatures and
hydrogen pressures are considered.
described. A sharp decrease in the rate of conversion on going
to higher alcohols caused by the mutual immiscibility of
reactants is a disadvantage of this method. Moreover, the
conversion of glycerol with primary alcohols requires relatively
high temperatures (120°C or higher) and an excess of alcohol,
which leads to the formation of by-products. The catalytic
telomerization of glycerol with dienes[28–31] with the formation of
unsaturated ethers (which can be subsequently reduced) is a
special alternative method.
The production of glycerol ethers from acetals, ketals, and
glycerol esters (reductive alkylation) is of considerable interest.
On the one hand, the acetalization (ketalization) of glycerol can
be performed under mild conditions (usually, at 20-80°C and
atmospheric pressure using standard acid catalysts) with high
yields of products. For example, the ketalization of glycerol with
acetone on an Amberlyst 36 sulfonic cation exchanger occurred
with a fourfold molar excess of acetone at 25°C with the yield of
solketal at a level of 92-96%[32]; the condensation of glycerol with
other aldehydes and ketones, both aliphatic[33–36] and
aromatic[37,38], resulted in high yields and selectivity. On the other
hand, the reduction of acetals and ketals to ethers (in essence,
this is the reduction of a bound carbonyl compound to a
corresponding bound carbinol) is a familiar and well-studied
reaction; it is convenient to use lithium aluminum hydride for
preparative purposes in the synthesis of ethers.[39,40] Similarly to
the reduction of carbonyl compounds, the reaction requires
bifunctional, both acid and redox, catalysis. The ketals of lower
alcohols with acetone are reduced to ethers in high yields at a
minimum excess pressure of hydrogen and at temperatures to
100°C with the use of the Pd(Rh)/Al2O3 – HCl bifunctional
catalytic system.[41] The conversion of a mixture of 1-octanal and
1-butanol into butyl octyl ether occurs under analogous
conditions. [42]
Introduction
Glycerol ethers are important glycerol derivatives, which are
promising for use as motor fuel components,[1–4] solvents,[5,6]
surfactants,[7] and personal care product components.[8] They
can be potentially competitive with their nearest analogs such as
ethylene glycol and propylene glycol ethers. Although glycerol
ethers are also toxic[9] like some of their glycol analogs, they are
less expensive than propylene glycol and its derivatives;
furthermore, both of the glycols are currently petroleum-derived
products, whereas the main sources of glycerol are renewable
raw materials.
In addition to the classical Williamson synthesis, glycerol ethers
can be obtained from glycerol by a number of methods,
including direct synthesis with the participation of an alcohol or
an olefin. The direct synthesis is especially effective in the case
of lower alcohols, which are miscible with glycerol; the process
can be performed on a heterogeneous catalyst. Processes for
the heterogeneous catalytic conversion of glycerol with
ethanol[2,10–12], tert-butanol[13–16], n-butanol [17–19], and higher
Shi and coworkers[43] studied the reduction of in situ generated
glycerol acetals over Pd/C with camphorsulfonic acid as a co-
catalyst. At T = 140°C and p(H2) = 10 bar, the yields of glycerol
ethers varied from 70 to 94% with high regioselectivity between
1-O- and 2-O-ethers. A starting mixture of an aldehyde and
glycerol in a molar ratio of 1 : 10 was used to ensure a maximum
degree of acetalization. Analogously, the high yields of ethers
from acetals were obtained with the use of tetramethyldisiloxane
as a donor of hydrogen.[44] In a comprehensive survey by Sutter
et al.,[8] only one description of the heterogeneous catalytic
reduction of glycerol acetals to ethers with the use of an
aluminosilicate-supported palladium catalyst at T = 190°C and
p(H2) = 7 bar was cited. As is evident, the reaction does not
require high pressures and occurs at moderate temperatures.
Furthermore, the possibility of increasing the regioselectivity of
conversion is a definite advantage of the production of ethers by
the reduction of acetals and ketals.
alcohols[7,20] were proposed; the use of olefins (isobutylene[21–23]
isoamylenes[24,25], and 1-octene[26,27]) for glycerol alkylation was
,
[a]
Russian Academy of Sciences Full Professor, Dr. A.L. Maximov
Petroleum Chemistry and Organic Catalysis Laboratory
Faculty of Chemistry of Moscow State University
Leninskiye Gory 1, Moscow, Russia
E-mail: max@ips.ac.ru
[b]
V.O, Samoilov, Dr. M.O. Onishchenko, Dr. D.N. Ramazanov, Prof.
Dr. A.L. Maximov
This study was devoted to the synthesis of glycerol
isopropyl ethers (GIPEs) by the catalytic reduction of solketal
using bifunctional heterogeneous palladium catalysts. The
preparation of GIPEs by the reductive conversion of solketal was
Hydrocarbons Chemistry Laboratory
A.V. Topchiev Institute of Petrochemical Synthesis RAS
Leninsky prospect 29, Moscow, Russia
Supporting information for this article is given via a link at the end of
the document
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10.1002/cctc.201700108
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compared with experimental data on the direct synthesis of
GIPEs from glycerol and isopropanol. Because there are no
reliable data on glycerol etherification with isopropanol
(particularly the reaction regioselectivity) in the scientific
literature, we performed the direct alkylation of glycerol with
isopropanol over an Amberlyst 36 ion-exchange resin, which is a
suitable catalyst for glycerol etherification.[2,13,17,20,45]
We studied the reductive conversion of solketal (the best
known glycerol derivative)[1,46–49] into a mixture of GIPEs.
Solketal is a convenient intermediate in the chemistry of
glycerol: it can be easily prepared by the ketalization of glycerol;
it has a free hydroxyl group, which makes it possible to perform
selective alkylation (acylation); and the protective ketal group
can be readily removed by acid hydrolysis. Solketal is soluble in
hydrocarbons, and it possesses a high octane number;[50,51] its
viscosity is lower and its volatility is considerably higher than
those of glycerol; therefore, it is a potential fuel component.
Glycerol can be further hydrophobized by the production of
glycerol monoether ketals and acetals. For example, the
ketalization of glycerol mono-tert-butyl ether with acetone leads
to the formation of solketal tert-butyl ether (STBE) as a potential
additive to motor fuels.[52,53] Under the conditions of catalytic
indicate the presence of pores with a diameter of 4 nm. The
specific surface areas and pore volumes of the aluminosilicates
(Table 1) are indicative of the high quality of the materials. The
isotherm of a BEA-38 sample corresponds to type I according to
the IUPAC classification.[55] This type of isotherms is
characteristic of microporous systems. The isotherm of a BEA-
25 zeolite sample corresponds to type IV isotherms. Such
isotherms are characteristic of samples having a platelet shape
or slit-like pores.
Table 1. Characteristics of the catalyst supports
Support
SiO2/Al2O3,
mol.
SBET
,
Vtotal
,
Vmicro
,
Total
m2/g
cm3/g
cm3/g
concentration
of acid sites,
µmol/g
BEA-25
BEA-38
25.0
38.0
9.8
543
572
680
0.51
0.34
0.70
0.17
0.21
-
858
630
256
Al-
HMS(10)
reduction, solketal isopropyl ether (SIPE),
a completely
Al-
HMS(20)
19.7
39.8
850
890
1.00
1.00
-
-
170
118
hydrophobized glycerol derivative, can be formed on the
reduction of solketal as a result of a redistribution of acetone. On
this basis, we also studied its synthesis from glycerol
monoisopropyl ether (m-GIPE) by ketalization with acetone.
The reductive conversion of solketal was performed on
heterogeneous bifunctional catalysts based on acidic
aluminosilicates: BEA zeolites and mesoporous amorphous
structured aluminosilicates of the HMS type. Beta zeolites (the
most active catalysts for the ketalization of glycerol with
acetone)[54] with Si/Al = 25 and 38 and HMS aluminosilicates
with Si/Al = 10, 20, and 40 were chosen as noble metal supports,
and palladium (1 and 2 wt %), which is an accessible and easy-
Al-
HMS(40)
700
600
500
400
300
200
100
0
to-reduce
metal,
was
a
hydrogenating
component.
Relationships between the catalytic activity and the textural and
acidic properties of the catalysts were studied. The activity of
aluminosilicate catalysts in ketalization depends, among other
things, on catalyst surface hydrophobicity, which is determined
by a silicate module.[54]
Results and Discussion
The atomic absorption analysis of the aluminosilicates showed
that the Si/Al atomic ratio in the materials was close to a molar
ratio between the precursors of silicon and aluminum in the
reaction mixture; this fact is indicative of the quantitative
precipitation of aluminosilicate components (Table 1). The
porous structure characteristics of the synthesized mesoporosus
aluminosilicates and zeolites were calculated based on the low-
temperature adsorption isotherms of nitrogen (Fig. 1, Table 1).
The shape of the isotherms (Fig. 1) of the Al-HMS materials (an
Al-HMS(20) sample) allowed us to ascribe them to type IV
according to the IUPAC classification, which corresponds to
mesoporosus materials. Jumps in the isotherms at p/p0 ~ 0.5
0
0,2
0,4
0,6
0,8
1
p/p0
Figure 1. Isotherms of the low-temperature adsorption of nitrogen on Al-
HMS(20) aluminosilicate and zeolite supports. (■) – Al-HMS(20), (▲) – BEA-
25, and (●) – BEA(38).
The acidic properties of the supports were studied using the
temperature-programmed desorption of ammonia. Table
1
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10.1002/cctc.201700108
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summarizes the concentrations of acid sites a0(NH3), which were
evaluated from the areas under the TPD curves (Fig. 2).
0,5
0,4
0,3
0,2
0,1
0
Figure 3. Solid-state 27Al NMR spectra of the synthesized Al-HMS
aluminosilicates and the Pd-containing catalysts on their basis. From top to
bottom: 2%Pd/Al-HMS(40), 2%Pd/Al-HMS(20), 2%Pd/Al-HMS(10), Al-
HMS(40), Al-HMS(20), Al-HMS(10).
0
100
200
300
400
T, °C
500
600
700
Figure 2. Curves of the TPD of ammonia from the catalyst supports. Black –
BEA-25, blue – BEA-38, red – Al-HMS(10), green – Al-HMS(20), and orange –
Al-HMS(40).
Table 2. Results of the processing of XPS data for the 2% Pd/Al-HMS(20) and
2% Pd/BEA(25) catalysts isolated after conducting the reaction
Sample
Electron
level
2%Pd/Al-HMS(20)
2%Pd/BEA(25)
The TPD spectra had identical asymmetric profiles regardless of
the aluminum contents of the mesoporous aluminosilicates (Fig.
2). The materials were characterized by the presence of weak
acid sites (150-260 °C) and medium strength sites (300-
500 °C).[56] The profile of the TPD curves of BEA-25 and BEA-38
supports is typical of zeolite materials. Maximums at 210 and
370 °C indicate the presence of the weak and medium strength
acid sites, respectively. The high-temperature region (above
500 °C) corresponds to strong acid sites. Note that maximums in
the low-temperature region of the TPD curves can correspond to
physically adsorbed ammonia.
27Al NMR-spectroscopic analysis confirmed the coordination of
aluminum atoms in the structures of the aluminosilicates, namely,
the presence of atoms in octahedral and tetrahedral
environments, as evidenced by characteristic signals in the
spectra at 1 and 55 ppm, respectively (Fig. 3).[56] A signal at 30
ppm corresponds to the pentacoordinated atoms of aluminum. It
is well known that such atoms can be a peculiar kind of binding
(stabilization) sites for supported metal particles.[57]
The 2% Pd/Al-HMS(20) and 2% Pd/BEA(25) catalysts isolated
after conducting reactions were analyzed by XPS (Table 2). The
deconvolution of the Pd3d level allowed us to establish that
palladium occurred in two (metallic and oxide) forms. In this
case, palladium predominantly occurred in a metal form on the
surfaces of both of the catalysts. An increase in the binding
energies of the states of palladium in a bulk metal phase (Eb =
335.2 eV) and an oxide phase (Eb = 336.8 eV) was most likely
due to the interaction of particles with the aluminosilicate
supports.[58]
Element
State
[a]
[a]
Eb
,
C[b], %
88.9
Eb
,
C[b], %
eV
eV
3d5/2
3d3/2
3d5/2
3d3/2
335.5
340.7
337.9
342.5
335.2
340.3
337.0
324.6
82.9
Pd
Pd3d
11.1
17.1
PdO
[a] Binding energy, eV; [b] concentration of the corresponding state, mol. %
The Pd-containing catalysts isolated after the reaction were
studied by TEM. An analysis of the TEM images showed that the
metal particles were localized both in the support pores as
particles smaller than 1 nm and on the external surfaces of
aluminosilicates (Figs. 4a-d). Based on an analysis of TEM
micrographs and the statistical processing of data for particles
whose size was greater than 1 nm, we plotted size-distribution
curves (Fig. 5).
We found that the catalysts prepared based on an Al-HMS
mesoporous aluminosilicate were characterized by a narrower
%
particle size distribution (2% Pd/Al-HMS(20) and 1% Pd/Al-
HMS-(20) samples). With the use of BEA zeolites as supports,
maximums in the diagrams were shifted toward larger particles
and the resulting metal particles were characterized by a wider
distribution (from 2-3 to 15-16 nm). The most highly dispersed
palladium occurred in the 2% Pd/Al-HMS(20) sample; the
particle sizes varied from 1-2 to 6-7 nm with a distribution
Particle size, nm
maximum
at
about
3-4
nm.
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(а)
(b)
Condensation of glycerol with isopropanol and ketalization
with acetone
We studied the direct alkylation of glycerol with isopropanol over
Amberlyst 36, which is a well-known catalyst for glycerol
etherification[2,13,17,20,45]. In this reaction, an excess of the alcohol
is required to increase the conversion of glycerol and to inhibit
side reactions like glycerol self-etherification. Usually,
etherification is conducted with a 2- to 12-fold excess of an
alcohol;[10,11,19] we conducted the reaction with the use of a
sixfold molar excess of isopropanol. Table 3 summarizes the
results of the experiments.
(d)
(c)
At 130-140°C, the conversion of glycerol was higher than 80%;
in this case, glycerol conversion by-products were not detected
in amounts higher than trace concentrations.
Table 3. Conversion of glycerol with isopropanol over Amberlyst 36 in a flow
reactor. Raw material: glycerol–isopropanol (6 : 1, by volume)
[b]
T[a]
°C
,
p,
bar
VHSV,
h-1
XGly
%
,
Smono
%
,
Sdi,
%
Yeth
%
,
YDIPE,
%
YPr,
%
[b]
Figure 4. TEM micrographs of the catalysts obtained from the reaction
mixtures: (a) 2%Pd/Al-HMS(20); (b) 1%Pd/Al-HMS(20);(c) 2%Pd/BEA(38);
and (d) 2%Pd/BEA(25)
130
140
150
18
35
50
1
1
3
88
92
72
79
76
79
18
22
19
87
91
71
14
17
17
32
26
16
30
25
20
[a] The maximum reaction temperature was limited by the thermal stability of the
Amberlyst cation-exchange resin; [b] Yields of DIPE and propylene on an
isopropanol basis.
Isopropanol, as a secondary alcohol, occupies an intermediate
place between primary and tertiary alcohols. A reaction with tert-
butanol catalyzed by sulfo cation exchangers occurs with high
yields even at 50-110°C.[13,16,45] The conversion of glycerol with
primary alcohols into ethers requires substantially more severe
conditions. Thus, temperatures of 140-150°C are necessary for
reaching a nearly quantitative glycerol conversion on sulfo
cation-exchanger catalysts even in the presence of a 12-fold
molar excess of n-butanol.[17,18] Analogous conditions are
required for the conversion of glycerol with other primary
alcohols.[2,7,10–12] High reaction temperatures lead to the
formation of oligoglycerols and glycerol degradation products in
considerable amounts.[20]
15
10
5
%
0
0
5
10
15
20
In the synthesis of glycerol alkyl ethers, regioselectivity for
isomeric ethers becomes an important issue. The
regioselectivity of the formation of monoethers is quite low: the
1-m-GIPE/2-m-GIPE and 1,3-di-GIPE/1,2-di-GIPE (di-GIPE
refers to glycerol di-isopropyl ether) ratios were about 85 : 15
and 50 : 50, respectively, regardless of the conversion of
glycerol.
Particle size, nm
Figure 5. Curves of the particle size distributions of the active constituent of
the catalysts. Black – 2%Pd/Al-HMS(20), grey – 1%Pd/Al-HMS(20), blue –
2%Pd/BEA(38), and green – 2%Pd/BEA(25).
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catalytic activity. The textural properties and acid strength
distributions in all of the Al-HMS aluminosilicates were almost
the same; therefore, the observed regularity is a consequence of
an increase in the surface hydrophobicity with the Si/Al ratio.
Ketalization of m-GIPE with acetone
The isolated m-GIPE was used for the preparation of SIPE by
ketalization with acetone (Fig. 6). Because the ketalization of m-
GIPE can occur under the conditions of the catalytic reduction of
solketal in the presence of acetone, we studied this reaction
separately. The following aluminosilicates, which were
subsequently used as the supports of palladium-containing
catalysts for the hydrogenolysis of solketal, were chosen as
ketalization catalysts: BEA-25 and BEA-38 zeolites and Al-HMS
mesoporous aluminosilicates with Si/Al = 10, 20, and 40. Figure
7 shows the experimental results.
The high activity of BEA zeolites in the ketalization of polyols is
well known:[59] they possess acceptable textural properties and
acidity. The silicate module of zeolite also plays a significant role
because it is responsible for surface hydrophobicity. Zeolites
with a relatively high module, which possess higher surface
hydrophobicity,[54,60] exhibited higher efficiency in the alkylation
and ketalization of polyols. At the same time, the ketalization of
glycerol and its derivatives on Al-HMS mesoporous
aluminosilicates was studied insufficiently because their catalytic
activity was lower than that of zeolites and sulfo cation
exchangers due to a high fraction of weakly acidic active sites.
70
60
50
40
30
20
10
0
0
50
100
150
200
Reaction time, min
Figure 7. Ketalization of 1-m-GIPE with acetone (T = 50°C, a fourfold molar
excess of acetone):. (◆) – BEA-25, (◇) – BEA-38, (○) – Al-HMS(40), ( ) –
Al-HMS(20), and (●) - Al-HMS(10).
◍
Solketal hydrogenolysis
OH
HO
O
Primary catalyst screening. Determination of reaction
conditions
1-m-GIPE
H+, - H2O
O
O
+
O
O
SIPE
For the primary evaluation of catalytic activity, the catalysts
containing 1 wt % palladium supported on beta zeolite and Al-
HMS(20) aluminosilicate were used (Table 4). At T = 80-120°C,
a noticeable change in the composition of reaction products was
observed. The 1% Pd-BEA-38 catalyst was found the most
active. The main hydrogenolysis products were m-GIPEs and
SIPE. The formation of di-GIPE was not observed. Note that by-
products were formed in significant amounts upon catalysis by
zeolites, whereas the formation of glycerol was the only side
reaction on Al-HMS. Partial hydrolysis also occurred on the
zeolite catalysts. The formation of MIBK (yield to 3 mol % on 1%
Pd-BEA-38 at 120°C) and glycerol ketals with MIBK (to 3%) was
the most important difference observed on catalysis by zeolites
under the same conditions.
The yield of solketal reduction products about 36% with 74%
selectivity was obtained by increasing a catalyst-to-feed ratio by
a factor of 4 (Table 1S). At this conversion, small amounts of di-
GIPEs were formed. The reaction on an aluminosilicate catalyst
occurred selectively, and condensation products were not
detected; isopropanol was found in small amounts (yield, to 1%)
among by-products in addition to glycerol.
acetone
Figure 6. Ketalization of 1-m-GIPE with acetone.
The catalysts were compared at equal ratios between the
numbers of moles of a substrate and acid sites determined from
the TPD of ammonia. On the ketalization of glycerol 1-mono-tert-
butyl ether under analogous conditions, the hydrolysis of an
alkoxyl moiety was observed,[61] which was not detected in the
case of 1-m-GIPE because the isopropyl ether of glycerol is
more stable than the tert-butyl ether. As expected, the BEA-38
zeolite was found the most active catalyst; this is consistent with
a previously described effect of the surface hydrophobicity of
zeolite on its catalytic activity. Moreover, the increased activity of
BEA-38 can be due to the presence of stronger acid sites than
those in BEA-25. The equilibrium concentrations of products
upon the catalysis with BEA-25 and BEA-38 were almost equal.
Mesoporous Al-HMS aluminosilicates exhibited somewhat lower
catalytic activity. A comparison of mesoporous aluminosilicates
with each other showed that Al-HMS(40) possesses the highest
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by evacuation before the reaction did not lead to a significant
decrease in the resulting amount of glycerol. Glycerol can be
formed by the hydrolysis of solketal or the irreversible binding of
acetone because of ketal reduction to an ether. A few sources of
water in the test system include the reduction of palladium oxide
with the release of a negligible quantity of water even in the
case of a maximum palladium/substrate ratio (about 0.2 mol %).
Another source of water is the occurrence of side reactions
(mainly, acetone aldol condensation) in the system with the
release of water. The concentration of acetone in a reaction
mixture for catalysis on zeolites was as high as 14 mol %,
whereas it was no higher than 3-4 mol % with HMS-based
catalysts. The equilibrium concentration of glycerol was always
much higher (by a factor of 3-6), and the yield of glycerol was
proportional to the yield of diethers. Consequently, glycerol was
accumulated in the system mainly due to the irreversible
consumption of acetone in the course of reduction.
Table 4. Hydrogenolysis of solketal. p(H2) = 20 bar; reaction time, 6 h; catalyst
amount, 2 wt % on a feed basis (Pd/feed = 0.016 mol %).
Catalyst
T, °C
80
XSol, %
YHP, %
YGly, %
Yothers, %
5
0
4
1
1%Pd/
BEA-25
100
120
80
9
1
6
2
14
8
2
9
3
1
5
1
1%Pd/
BEA-38
100
120
80
17
39
7
7
7
3
11
4
16
3
12
0
The formation of free acetone initiated side reactions in which it
was a starting compound. First, acetone was reduced to
isopropanol (no more than 3 mol % in the resulting reaction
mixtures). Second, MIBK was formed due to the aldol
condensation of acetone to mesityl oxide followed by the
hydrogenation of the latter. The resulting MIBK competed with
acetone in the ketalization of polyols; because of this, glycerol-
MIBK ketals were detected in the reaction mixtures. Note that
their hydrogenation products (glycerol – methyl isobutyl carbinol
ethers) were not detected in the reaction mixtures. These side
reactions were characteristic of only zeolites with strong acid
sites.
The reaction scheme of ketal conversions was analogous in
many respects to the reactions observed on the reduction of in
situ generated glycerol acetals and ketals.[43,44,62] Analogously,
the reduction occurred with high regioselectivity. The absence of
side reactions from the direct synthesis of ethers from glycerol
and aldehydes in the Pd/C – camphorsulfonic acid system[43] can
be explained by a large excess of glycerol, which ensured the
rapid and quantitative aldehyde binding into acetal.
1%Pd/
Al-HMS(20)
100
120
5
1
4
0
12
4
8
0
Reaction network
Figure
8 shows the chemical reactions that occur on
aluminosilicate-supported palladium catalysts during the
conversion of solketal (T = 80-120°C, p(H2) = 5-30 bar).
The reduction of solketal with the formation of a mixture of m-
GIPEs is the main reaction that occurs in the system. At
relatively low solketal conversions, solketal isopropyl ether
(SIPE) is the main reduction product formed from m-GIPE
because of acetone redistribution between two polyols: glycerol
and m-GIPE. This acetone redistributuion can be catalyzed by
traces of water. SIPE can also be reduced with the formation of
di-GIPE. The regioselectivity of reduction is high: in the
overwhelming majority of cases, in a mixture of 1-O-m-GIPE and
2-O-m-GIPE upon the reduction of solketal, the concentration of
the latter was about 2-3% (a maximum of 5%); this ratio was
always observed upon the heterogeneous catalytic reduction of
solketal. The ratio between the isomers of solketal remained
constant in all of the experiments: the dioxolane isomer content
was about 2.0-2.5%. It is believed that the regioselectivity of
reaction depends on a ratio between the isomers of solketal in a
substrate and the reduction of isomeric ketals occurs with 100%
regioselectivity. Upon the reduction of SIPE into a mixture of
diethers, the 1,2-di-GIPE : 1,3-di-GIPE ratio was always about
15 : 85, which differs significantly from the ratio (about 1 : 1)
upon direct acid-catalyzed glycerol etherification. Therefore, the
reduction approach to the production of symmetrical 1,3-di-GIPE
is advantageous.
Comparison of catalysts: Effects of support properties and
palladium contents
The performance characteristics of the prepared catalysts were
compared under identical conditions at T = 100 and 120°C,
p(H2) = 20 bar, a Pd/feed ratio of 0.064 mol %, and a reaction
time of 6 h. Figure 9 summarizes the results of the comparative
tests. The influence of external-diffusion factors was minimized
by determining the dependence of substrate conversion on the
rate of reaction mass stirring (Fig. 3S).
An optimum ratio between the acid and hydrogenating functions
of a catalyst is required for reaching maximum selectivity and a
high rate of conversion; this ratio can be found by varying the
amount of supported palladium and the type of the support.
The most important difference between the selectivity of
catalysts based on strongly and weakly acidic supports (zeolites
and HMS aluminosilicates, respectively) consists in the
occurrence of side reactions (namely, the crotonic condensation
The formation of glycerol was the most important side reaction in
the test system. The drying of the initial solketal with molecular
sieves followed by vacuum rectification, the use of freshly
calcined catalysts, and the removal of oxygen from the reactor
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H+, - H2O
isopropanol
max - 4%
OH
O
DIPE
max - 1%
[H]
OH
H+
O
+ acetone
- H2O
H+, - H2O
OH
O
O
O
O
[H]
OH
98 : 2
O
HO
+
O
OH
glycerol
min - 3%
OH
Solketal
O
SIPE
acetone
max - 14%
HO
max - 27%
OH
m-GIPEs
[H]
H+, [H]
H+
+ glycerol
- H2O
OH
O
O
O
O
O
O
O
O
MIBK
max - 3%
85 : 15
OH
HO
O
MIBK ketals
max - 3%
OH
di-GIPEs
Figure 8. Solketal hydrogenolysis reaction network (the maximum and minimum yields of by-products in mol
%
are indicated).
50
70
60
50
40
40
30
20
10
0
%
%
30
20
10
0
a
b
Figure 9. Comparison of catalysts for solketal hydrogenolysis: p(H2) = 20 bar; Pd/feed ratio, 0.064 mol %; reaction time, 6 h; T = (a) 100 and (b) =120°C. Dark
blue, light blue, green, and red bars refer to solketal conversion (XSol) and the yields of hydrogenolysis products (YHP), glycerol (YGly), and other products (Yothers),
respectively.
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10.1002/cctc.201700108
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of acetone on the zeolite catalysts). In accordance with the
results of the TPD of ammonia, zeolite catalysts have a
considerable number of strong acid sites, whereas the main
fraction of the acid sites of Al-HMS catalysts consists of weak
and medium-strength acid sites. The strong acid sites of zeolites
are responsible for the condensation of free acetone.
At the same time, it is well known that the ketalization of glycerol
with acetone does not require strong acidity, and it successfully
occurs at medium-strength and weak acid sites[59] (or without a
catalyst in the case of aldehydes),[36] as evidenced by the
formation of SIPE on the Al-HMS catalysts.
hydrogenating rather than acid function of the catalyst in this
case. Furthermore, the activity of aluminosilicates in the
ketalization of m-GIPEs increased with the silicate module; this
was most likely due to an increase in the surface hydrophobicity.
A decrease in the activity on going to Al-HMS(40) was related to
an extreme decrease in the acid function, which began to limit
catalytic activity.
In all of the experiments (Fig. 9), the catalyst amount was
chosen to ensure the same Pd/feed ratio. In spite of
considerably lower acidity, the Al-HMS-based catalysts exhibited
conversions comparable with those obtained over zeolites. For
example, conversions over 2% Pd/BEA-25 and 2% Pd/Al-
HMS(20) at 120°C were 64 and 60%, respectively, although the
acidity of Al-HMS is lower by a factor of 5.5 (858 mol/g in BEA-
25 against 157 mol/g in Al-HMS(20)). At the same time, the
yield of reduction products on the HMS-based catalyst was
higher by a factor of almost 2 (38% on 2%Pd/Al-HMS(20)
against 22% on 2%Pd/BEA-25).
An increase in the amount of supported palladium in the zeolite
catalysts led to a slight increase in the catalyst selectivity. The
values of SHP and, in some cases, XSol somewhat increased;
however, a maximum value of SHP was no higher than 50%
because of the formation of acetone condensation products and
their derivatives. Among the zeolites, the best result (XSol = 43%,
YHP = 22 %, YGly = 18%, and Yothers = 3%) was obtained on the
2%Pd/BEA-38 catalyst under the following conditions: Т =
100°С; p(H2) = 20 bar; and reaction time, 6 h. A wide particle-
size distribution and a considerable amount of coarse palladium
particles on the surfaces of zeolite catalysts can be responsible
for the relatively low hydrogenating activity of palladium, as
compared with that in the HMS-based catalysts.
70
60
50
40
30
20
10
0
2% Pd/Al-HMS (10)
2% Pd/Al-HMS (20)
2% Pd/Al-HMS (40)
0,064
0,128
Pd/feed ratio, mol. %
0,193
Figure 10. Effect of a catalyst-to-feed ratio on the hydrogenolysis of solketal: T
= 120°C; p(H2) = 20 bar; reaction time, 6 h.
70
60
50
40
30
20
10
0
At the same Pd/feed ratio, the conversion of solketal on 2%
Pd/Al-HMS(20) was higher than that on 1% Pd/Al-HMS(20).
Upon catalysis performed under the same conditions at the
same catalyst-to-feed weight ratio, the conversion on 2% Pd
was higher by a factor of 2.3 or 2.2, respectively (Table 2S). The
doubled concentration of supported palladium did not lead to an
imbalance between the acidic and hydrogenation functions of Al-
HMS(20). Moreover, hydrogenation was a rate-determining step
of the reaction.
0
5
10
15
Water amount, mol. %
Figure 11. Effect of a water-to-feed ratio on the hydrogenolysis of solketal: T =
120°C; p(H2) = 20 bar; reaction time, 6 h. Circles and squares refer to XSol
mol. %,and YGly, mol. %, respectively; filled and empty markers refer to 2%
The catalyst activity (Fig. 10) increased in the order Al-HMS(20)
,
>
Al-HMS(10)
>
Al-HMS(40). Because the textural
Pd/Al-HMS (20) and 2% Pd/Al-HMS (40), respectively.
characteristics of aluminosilicates with different modules are
very close, it is believed that internal-diffusion processes
similarly occur on these catalysts. The average sizes of
palladium particles and the states of the metal in the catalysts
were similar (according to TEM and XPS data, respectively).
Therefore, the catalysts differed from each other in terms of a
ratio between acid and hydrogenating functions, that is, a ratio
between the numbers of moles of palladium and acid sites. An
increase in the acidity relative to the palladium content (of Al-
HMS(10) and Al-HMS(20)) did not lead to an increase in the
activity because the reaction rate was limited by the
Water in the test system can exert versatile effects on the
course of reaction. First, in the contact of aluminosilicates with
water, active sites were solvated to cause the conversion of
Lewis acid sites into Brønsted acid sites and a decrease in the
acid strength.[63,64] Second, the presence of water led to the
hydrolysis of solketal and shifted an equilibrium toward glycerol.
We experimentally evaluated the influence of water on the
catalytic performance. Catalysts with different surface
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hydrophobicity demonstrated completely different behaviors in
contact with water (Fig. 11). Upon increasing the concentration
of water in feed, selectivity for the hydrogenolysis products (SHP
)
60
50
40
30
20
10
0
decreased, whereas SGly became greater. The reasons for the
observed SHP drop were different in the supports with different
hydrophobicity. The conversion of solketal on the 2%Pd/Al-
HMS(20) catalyst decreased with the retention of the yield of
glycerol. On the contrary, the conversion on the 2%Pd/Al-
HMS(40) catalyst remained almost unchanged, whereas a clear
increase in SGly was observed. The addition of 5 mol % water led
to a very low effect: the value of SHP decreased by no more than
3-5 mol %. At a 15 mol % water content of feed, YHP decreased
by a factor of 2.2 or 1.4 for 2% Pd/Al-HMS(20) or 2% Pd/Al-
HMS(40), respectively. Thus, the catalyst that is more
hydrophobic exhibited a more stable catalytic effect in the
presence of water. Both of the catalysts could be reused with a
slight decrease in activity.
%
5
10
20
30
p(H2), bar
Figure 13. Effect of hydrogen pressure on solketal hydrogenolysis over 2%
Pd/Al-HMS(20): reaction time, 6 h; catalyst-to-feed ratio, 4 wt % (Pd/feed,
0.064 mol %). Dark grey, shaded and light grey bars refer to XSol, YHP, and YGly
,
respectively.
Figure 12 shows the reaction kinetics of solketal reduction on
2% Pd/Al-HMS(20). The curves clearly demonstrate changes in
the composition of hydrogenation products with the degree of
conversion: the concentration of SIPE in the reaction products
decreased with conversion. This was due to the irreversible
consumption of acetone (in the ketal form) for the formation of
ethers with shifting the SIPE – m-GIPE equilibrium towards the
ether.
Conclusions
We prepared glycerol ethers (SIPE, m-GIPEs, and di-GIPEs) by
the reduction of solketal and compared the results with those
obtained by the traditional direct acid-catalyzed synthesis. The
activity of zeolites and HMS aluminosilicates in the ketalization
of m-GIPEs increased with the Si/Al ratio. Among catalysts for
the reduction of solketal based on palladium-doped BEA zeolites
and aluminosilicates, 2% Pd/Al-HMS(20), which possessed an
optimum ratio between the hydrogenating and acid functions,
exhibited the greatest activity. Because of the presence of a
great number of strong acid sites, conversion on the zeolite
100
80
60
40
20
0
catalysts occurred nonselectively with the formation of
a
noticeable amount of by-products; in the case of the
aluminosilicate catalysts, the formation of glycerol was the most
important side reaction. The increased surface hydrophobicity of
aluminosilicates led to higher activity in ketalization and
increased stability in contact with water in the catalytic reduction
of solketal.
0
2
4
6
8
Reaction time, hours
At T = 120°C and p(H2) = 20 bar, the conversion of solketal was
as high as 87% at 68% selectivity for the hydrogenation
products. The reduction of solketal to a mixture of m-GIPEs
occurred with high regioselectivity: 98% for 1-m-GIPE and 85%
for 1,3-di-GIPE. At the same time, a conversion of 92% with
76% selectivity was observed in the direct synthesis (T = 140°C,
p = 35 bar, VHSV = 1 h-1, and glycerol : isopropanol = 6 : 1, by
volume); the regioselectivity was much lower (85% for 1-m-GIPE
and about 50-55% for 1,3-di-GIPE). The yields were
comparable, but the direct synthesis required more severe
conditions and was less selective.
Figure 12. Solketal hydrogenolysis over 2% Pd/Al-HMS(20). T = 120°C; p(H2)
= 20 bar; and catalyst-to-feed ratio, 4 wt %.: the reaction profile. (●) – solketal,
(○) – m-GIPE, (▲) – SIPE, and (■) – di-GIPE.
As a rule, the rate of hydrogenation reactions increases with the
pressure of hydrogen to reach a limiting value. Figure 13 shows
the dependence of the yield of hydrogenation products on the
pressure of hydrogen. A decrease in the pressure of hydrogen
from 20 to 10 bar and an increase to 30 bar did not lead to a
dramatic change in the yield of hydrogenation products. Upon a
decrease in the pressure to 5 bar, the reaction rate decreased
considerably. Note that, in addition to the effect of pressure on
the concentration of hydrogen as a reactant in solution, the
reduction of palladium on the catalyst surface is also a pressure-
sensitive process. Finally, because the reaction mixture was a
suspension, the pressure of hydrogen had a strong effect on its
transfer from a gas phase through a liquid to the catalyst surface.
We consider the reduction of glycerol acetals and ketals over
bifunctional heterogeneous catalysts as an interesting approach
to the selective synthesis of glycerol ethers.
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and then dried in a flow of argon. The heterogeneous catalyst thus
prepared was used for TEM and XPS studies.
Experimental Section
Materials
Ketalization of m-GIPE with acetone
Solketal, glycerol, glycerol formal, isopropanol, and acetone (≥97.0%,
Sigma Aldrich) were used as the initial reagents.
In control experiments, a glass batch reactor (internal volume, 15 cm3)
was charged with a catalyst sample (50-300 mg) and a magnetic stirrer.
When the temperature of water in a heating jacket reached a specified
level, a feed mixture (5.0 mL of acetone and 2.2 g of m-GIPE) was
loaded and the reaction time was measured. The stirrer was stopped at
regular intervals and 0.1-mL samples were taken for GLC analysis.
Physicochemical characterization of catalysts and supports
The textural characteristics of the synthesized aluminosilicates and
commercial supports were determined on an ASAP 2020 instrument from
Micromeritics with the use of the low-temperature adsorption of nitrogen.
The parameters were calculated by the BET method using the instrument
software.
Synthesis of pure glycerol derivatives
The direct synthesis of GIPEs was performed with the use of a laboratory
flow system of (1) a feed flask, (2) a Gilson 305 HPLC pump, (3) an
The localization of aluminum atoms in the structure of the
aluminosilicates was determined based on solid-state 27Al NMR-
spectroscopic data obtained on a Varian Unity Inova AS 500 instrument.
The sample was placed in a ZrO2 rotor and rotated at a rate of 15 kHz.
Chemical shifts were measured with reference to a solution of AlCl3.
electrically heated catalytic fixed-bed reactor (290*18 mm), (4)
a
condenser, and (5) separator. The temperature conditions were
a
controlled with two thermocouples (at the center of the catalyst bed and
at the reactor wall) and a thermoregulator. The reactor pressure was
tuned by a backpressure regulator and controlled by two manometers at
the reactor inlet and outlet. The gaseous reaction products from the
separator were vented to a flare through a bubble gage.
The structure and morphology of the supported catalyst samples were
studied by high-resolution transmission electron microscopy (TEM) on a
JEM 2100 electron microscope from JEOL at an accelerating voltage of
200 kV.
To convert glycerol into GIPEs, the reactor was loaded with commercial
Amberlyst 36 sulfonic ion-exchange resin in the H+ form. A mixture of
isopropanol and glycerol (16.67 vol %) was pumped through the reactor,
cooled down, and collected in the separator. After completion of the
reaction, diisopropyl ether, unreacted isopropanol, and water were
distilled off at atmospheric pressure. Pure m-GIPEs (the structures were
confirmed by GC-MS analysis and 1H NMR spectroscopy) were
recovered from the mixture by vacuum rectification (residual pressure, 3
Torr) as a fraction with a boiling point of 110°C.
The catalysts were studied by X-ray photoelectron spectroscopy (XPS)
on a PHI 5500 ESCA X-ray photoelectron spectrometer from Physical
Electronics. Nonmonochromatic AlKα radiation (hν = 1486.6 eV) with a
power of 300 W was used for the excitation of photoemission. Powders
were pressed into an indium plate. The diameter of an analysis zone was
1.1 mm. The photoelectron peaks were calibrated based on the C 1s line
of carbon with a binding energy of 284.9 eV. The deconvolution of
spectra was carried out by a nonlinear least squares method with the use
of Gaussian and Lorentzian functions.
To obtain SIPE, 1-m-GIPE was mixed with acetone (molar ratio, 10 : 1)
and a small amount of p-toluenesulfoniс acid (p-TSA) was added. The
mixture was stirred for 1 h; then, the catalyst was neutralized by the
addition of an excess of NaCO3, and acetone and water were distilled off.
The vacuum rectification of the residue at 92°C (10-12 Torr) yielded pure
SIPE.
The acid properties of the supports were studied with the use of the
temperature-programmed desorption of ammonia (TPD of NH3). The
experiments were performed on a USGA-101 sorption analyzer from
Unisit. Before the adsorption of ammonia, the samples were in situ
calcined in a flow of dry air at 500°C for 1 h and then cooled to 60°C in a
flow of nitrogen. Saturation was carried out in a flow of ammonia diluted
with helium (1 : 1) at 60°C for 30 min. Physically adsorbed ammonia was
removed at 100°C in a flow of helium for 1 h. The thermal desorption
curves of ammonia were measured in a temperature range of 100-800°C
in a flow of helium at a heating rate of 8 K/min.
To obtain pure glycerol and MIBK ketals, solketal was mixed with freshly
distilled MIBK (molar ratio, 20 : 1) and a scintilla of p-TSA was added.
The mixture was stirred for 1 h; then, the catalyst was neutralized with an
excess of NaCO3, and acetone and water were distilled off. As a result, a
mixture of glycerol-MIBK ketals (≥97% GC purity) was obtained.
Catalytic experiments
Analysis of products
Solketal reduction
Reaction mixtures were analyzed by GLC on a Kristallyuks-4000M
chromatograph with a flame-ionization detector using a Supelcowax 10
(30 m*0.25 mm) column. The temperature program included an
isothermal exposure at 60°C (5 min), a temperature rise from 60 to
230°C at a heating rate of 5 K/min, and an exposure at 230°C for 10 min.
For quantitative analysis, the response factors of components were
determined. GC calibration was carried out with the solutions of pure
compounds in solketal, whose response factor was taken as 1.
Solketal (2.5 mL), a catalyst sample in the oxide form after calcination
(50-300 mg; 2–12 wt % on a raw material basis), and a magnetic stirrer
were placed in a stainless steel autoclave (internal volume, 50 mL)
equipped with a manometer for pressure control and a thermocouple.
The closed autoclave was evacuated to a residual pressure of 3 Torr,
filled with hydrogen (5-30 bar), and placed in a furnace; stirring was
turned on, and this point in time was taken as the onset of reaction. At
the end of the experiment time, the autoclave was cooled with cold water,
and the products of catalysis were decanted, centrifuged, and analyzed
by GLC and GC–MS. The catalyst was washed five times with acetone
(the suspension was centrifuged and acetone was decanted each time)
The GC-MS analysis was performed on
a Finnigan MAT 95 XL
instrument (Varian VF-5 ms capillary column, 30 m*0.25 mm) with helium
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10.1002/cctc.201700108
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as a carrier gas. The mass spectrometer operating conditions were the
following: ionization energy, 70 eV; source temperature, 230°C; scanning
over a range of 20–800 Da at a scan rate of 1 s per decade and 1000
resolution. The components were identified based on reference mass
spectra available from the NIST/EPA/NIH 11 database.
The high-resolution 1H NMR spectra of solutions in deuterochloroform
were measured on an MSL-300 spectrometer (Bruker): operating
frequency, 300.13 MHz; number of accumulations, 20 at
a scan
frequency of 8928 Hz (29.8 ppm); the PAPS.PC pulse program with the
subsequent Fourier transform.
The conversion of solketal (XSol), selectivity, and the yields of hydrolysis
and hydrogenation products were calculated only for molecules
containing a glycerol moiety. Thus, the presence of acetone, isopropanol,
and MIBK in the reaction mixtures was ignored in these calculations. The
conversion of solketal was calculated by difference between the initial
and final molar concentrations in the reaction mixture.
Glossary
Solketal - (2,2-dimethyl-1,3-dioxolan-4-yl)methanol
GIPE – glycerol isopropyl ether
mono-GIPE - glycerol mono-isopropyl ether
di-GIPE - glycerol di-isopropyl ether
SIPE – solketal isopropyl ether, (2,2-dimethyl-1,3-dioxolan-4-
yl)isopropoxymethane
DIPE – diisopropyl ether
MIBK – methyl isobutyl ketone
MIBC – methyl isobutyl carbinol
SBET, m2/g – BET specific surface area
Vtotal, cm3/g – total pore volume of a catalyst
Vmicro, cm3/g – total micropore volume of a catalyst
TPD – temperature-programmed desorption
XPS – X-ray photoelectron spectroscopy
TEM – transmission electron microscopy
Xi, % – conversion of the compound i
Si, mol. % - reaction selectivity for the compound i
Yi, mol. % - yield of the compound i
T, °C – reaction temperature
p, p(H2) – pressure in the reaction apparatus
VHSV, h-1 – volume hourly space velocity
Pd/feed ratio – a ratio between the molar amounts of palladium
and a substrate introduced into the reaction
GLC – gas–liquid chromatography
GC–MS – gas chromatography–mass spectrometry
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Acknowledgements
This work was supported in part by the Russian Academy of Sciences (program no. 9 Catalytic and Thermal Conversion of Fossil,
Alternative, and Renewable Hydrocarbon Raw Materials and Industrial Wastes). We are grateful to Dr. R.S. Borisov for his
assistance in performing GC–MS analysis, to Dr. M.P. Filatova for performing NMR-spectroscopic analysis, and to Dr. V.A. Demidov
(Dow Europe GmbH, Moscow) for providing us with Amberlyst 36 ion-exchange resin.
Keywords: hydrogenation • mesoporous materials • glycerol • solketal • green chemistry
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Entry for the Table of Contents
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Vadim Samoilov, Maria Onishchenko,
Dzhamalutdin
Maximov*
Ramazanov,
Anton
Page No. – Page No.
Glycerol isopropyl ethers: direct
synthesis from alcohols and
synthesis by the reduction of solketal
A
roundabout route for bioglycerol hydrophobization: Solketal was
hydrogenated in mild conditions over Pd-doped mesoporous aluminosilicas to yield
a mixture of ethers and the isopropyl ether of solketal with high selectivity and yield.
For high activity and selectivity, the balance between reductive and acid catalytic
functions is essential. The significance of the approach for glycerol
hydrophobization was discussed in comparison with the traditional acid-catalyzed
route.
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