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

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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To be cited as: ChemCatChem 10.1002/cctc.201700108

Link to VoR: http://dx.doi.org/10.1002/cctc.201700108

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10.1002/cctc.201700108

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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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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.

S_BET

,

V_total

,

V_micro

,

Total

m²/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

-

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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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 Pd_3dlevel 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 (E_b=

335.2 eV) and an oxide phase (E_b= 336.8 eV) was most likely

due to the interaction of particles with the aluminosilicate

supports.^[58]

Element

State

[a]

E_b

,

C^[b], %

88.9

E_b

,

C^[b], %

eV

3d_5/2

3d_3/2

3d_5/2

3d_3/2

335.5

340.7

337.9

342.5

335.2

340.3

337.0

324.6

82.9

Pd

Pd_3d

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

X_Gly

%

,

S_mono

%

,

S_di,

%

Y_eth

%

,

Y_DIPE,

%

Y_Pr,

%

[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

3

88

92

72

79

76

79

18

22

19

87

91

71

14

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

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

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

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

X_Sol, %

Y_HP, %

Y_Gly, %

Y_others, %

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

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

[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

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

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 (X_Sol) and the yields of hydrogenolysis products (Y_HP), glycerol (Y_Gly), and other products (Y_others),

respectively.

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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 S_HPand, in some cases, X_Solsomewhat increased;

however, a maximum value of S_HPwas no higher than 50%

because of the formation of acetone condensation products and

their derivatives. Among the zeolites, the best result (X_Sol= 43%,

Y_HP= 22 %, Y_Gly= 18%, and Y_others= 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 X_Sol

mol. %,and Y_Gly, 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 (S_HP

)

60

50

40

30

20

10

0

decreased, whereas S_Glybecame greater. The reasons for the

observed S_HPdrop 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 S_Glywas observed. The addition of 5 mol % water led

to a very low effect: the value of S_HPdecreased 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 X_Sol, Y_HP, and Y_Gly

,

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 ZrO₂rotor 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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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 (X_Sol), 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

S_BET, m²/g – BET specific surface area

V_total, cm³/g – total pore volume of a catalyst

V_micro, cm³/g – total micropore volume of a catalyst

TPD – temperature-programmed desorption

XPS – X-ray photoelectron spectroscopy

TEM – transmission electron microscopy

X_i, % – conversion of the compound i

S_i, 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.

Article Doi

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

DOI: 10.1002/cctc.201700108

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Article abstract of DOI:10.1002/cctc.201700108

Full text of DOI:10.1002/cctc.201700108

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