Catalytic hydrogenolysis of glycerol into propyl acetate with ruthenium complexes

Article abstract of DOI:10.1016/j.catcom.2019.105743

Ru complexes have been utilized as catalyst precursors for glycerol hydrogenolysis under mild conditions, which gave propyl acetate (PA) as a major product. Notably, the esterification reaction of acetic acid with glycerol can prevent glycerol from polymerization. In(OTf)₃ played a critical role in facilitating esterification of glycerol and sequential dehydration, while the Ru complexes' function was hydrogenation. The promoter (FeCl₃) can suppress the reduction of Ru complex to Ru particles, improving the catalytic performance. The present catalytic system can give full glycerol conversion and 57% yield of PA. Finally, the reaction pathway was proposed accordingly.

Full text of DOI:10.1016/j.catcom.2019.105743

Accepted Manuscript
Catalytic hydrogenolysis of glycerol into propyl acetate with
ruthenium complexes
Zichen Xu, Honghui Gong, Manyu Chen, Ruihan Luo, Wei Qian,
Qingpo Peng, Zhenshan Hou
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DOI:
Article Number:
Reference:
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https://doi.org/10.1016/j.catcom.2019.105743
105743
CATCOM 105743
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Please cite this article as: Z. Xu, H. Gong, M. Chen, et al., Catalytic hydrogenolysis
of glycerol into propyl acetate with ruthenium complexes, Catalysis Communications,
https://doi.org/10.1016/j.catcom.2019.105743
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ACCEPTED MANUSCRIPT
Catalytic hydrogenolysis of glycerol into propyl acetate with
ruthenium complexes
Zichen Xu, Honghui Gong, Manyu Chen, Ruihan Luo, Wei Qian, Qingpo Peng, Zhenshan Hou*
Key Laboratory for Advanced Materials, Research Institute of Industrial Catalysis, School of
Chemistry and Molecular Engineering, East China University of Science and Technology,
Shanghai 200237, China.
Corresponding author: E-mail: houzhenshan@ecust.edu.cn; Tel.: +86-21-64251686
Abstract: Ru complexes have been utilized as catalyst precursors for glycerol
hydrogenolysis under mild conditions, which gave propyl acetate (PA) as a major
product. Notably, the esterification reaction of acetic acid with glycerol can prevent
glycerol from polymerization. In(OTf)3 played a critical role in facilitating
esterification of glycerol and sequential dehydration, while the Ru complexes’
function was hydrogenation. The promoter (FeCl3) can suppress the reduction of Ru
complex to Ru particles, improving the catalytic performance. The present catalytic
system can give full glycerol conversion and 57% yield of PA. Finally, the reaction
pathway was proposed accordingly.
Keywords: Glycerol hydrogenolysis; Propyl acetate; Ruthenium complexes;
Homogeneous catalysis.
1. Introduction
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The dependence of human well-being on fossil fuels will have to be reduced in
the future, mainly owing to geopolitical, natural resource scarcity and environmental
factors [1]. The use of renewable biomass resources as a partial replacements for
petroleum-derived chemicals and fossil fuels is essential for the sustainable
development of our society [2, 3]. The transesterification of vegetable oils with
alcohols gives bio-diesel, a fuel that can be used directly in diesel engines. However,
the transesterification process co-generates glycerol at a rate of 1 mol for every 3 mol
of alkyl-fatty esters synthesized [4, 5], decreasing the atom efficiency and economic
feasibility of the process. It is therefore essential to find effective approaches to use
this oversupply of glycerol, which would enhance the biodiesel economy [6].
Based on the current research, glycerol can be catalytically converted into
value-added chemicals by hydrogenolysis, oxidation, dehydration, etherification,
esterification and polymerization, etc [7-10]. One of the most important approach is
the hydrogenolysis of glycerol to propanediols (PDOs), which have found
considerable application in the manufacture of fine chemicals and polymers [11, 12].
Supported heterogeneous metal (Rh, Ir, Ru, Pd, Co, Ni, etc.) catalysts have been
widely applied for the hydrogenolysis of glycerol under a temperature condition of
250-350 C [13-15]. Vapor phase hydrogenolysis of glycerol to PDOs has also been
studied recently. Zeolites or metal oxide supported Ru catalysts exhibited good
catalytic activity at 230-280 C [16,17].
However, much less efforts were constructed to convert glycerol into 1-propanol
(1-PO) up to date, although 1-PO is a valuable chemical used as solvent, organic
intermediate and raw materials. 1-PO is industrially produced via hydroformylation of
ethylene and subsequent hydrogenation [18]. The synthesis of 1-PO from biomass is a
potential alternative to petroleum-derived processes, which has attracted great interest
recently. It has been reported that Pd/C catalyst gave a promising selectivity to 1-PO
(87.7%) under batch-reaction conditions, but the conversion of glycerol was poor
(4.5%) [19]. Furthermore, the sequential two-layer catalysts containing Hβ zeolite and
Ni/Al2O3 catalyst can provide good 1-PO selectivity (69%) at full glycerol conversion
[20]. In our previous studies, we found that the two sequential zirconium phosphate
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and Ru/SiO2 layer catalyst system also showed a good selectivity (77%) to 1-PO at
full of glycerol conversion [21]. Nevertheless, the reactions above were normally
conducted under fairly harsh conditions (>200 C).
Comparatively, although the easily available RuCl2(PPh3)3, RuHCl(PPh3)3 and
RuCl2(dppp)2 complexes were highly active for the hydrogenation reaction [22-24]
and these Ru complexes have rarely applied to glycerol hydrogenolysis. It was
reported that the HI/Ru(CO)4I2 catalyst system was active for the conversion of
glycerol into 1-PO (25.4% yield) [25]. In addition, the Ru complexes
-
-
{[Cp*Ru(CO)2]2(-H)}+OTf
and
[Cp*Ru(OH2)(N-N)]+OTf
(Cp*=5-pentamethylcyclopentadieny;
N-N=2,2’-ipyridine or
-
phen=1,10-phenanthroline; OTf =trifluoromethanesulfonate) also afforded moderate
yield of 1-PO (<20%) in the presence of HOTf [26, 27]. These results demonstrated
the homogeneous Ru complexes were indeed capable of catalyzing hydrogenolysis of
glycerol under mild conditions, but the catalytic efficiency of this process needs to be
substantially improved.
In this work, glycerol was hydrogenolyzed by using Ru(II) complexes in acetic
acid (HOAc) under mild conditions. After the reaction, PA was detected as a dominant
product, and 1,3-propyleneglycol diacetate (1,3-PGDA), 1,2-propyleneglycol
diacetate (1,2-PGDA), triacetin (TA) and monoacetin (MA) as minor products. It is
well known that propyl acetate (PA) can be readily hydrolyzed to 1-PO and is also
primarily intended as a good solvent and a flavor additive. PGDAs, TA and MA are all
important chemicals and applied in the manufacture of dyes, plasticizers and
pharmaceuticals etc [28, 29]. Hence, it is highly promising that glycerol can be
transformed into PA and other valuable chemicals like PGDAs, TA and MA
effectively through an alternative, biomass-derived approach.
2. Results and discussion
2.1 Glycerol hydrogenlysis with Ru(II) complexes
Initially, the Ru(II) complexes (Fig. S1-S6) have been prepared and screened for
the glycerol hydrogenlysis in the presence of the acidic additives and HOAc solvent.
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For the sake of comparison, commercially supplied RuCl33H2O and Ru(acac)3 were
also used as catalysts. As shown in Fig. 1, it indicated that although glycerol was
consumed completely after 12 h, only a trace of PA and TA were found without Ru
catalysts. All products were obtained in the form of acetates due to the presence of
excess HOAc. The yield of PA can reach up to 40% with RuCl2(PPh3)3 (Fig. 1).
However, if RuCl2(PPh3)3 was replaced by RuCl2(dppp)2, RuCl33H2O, RuHCl(PPh3)3
and Ru(acac)3, the yield of PA was lower (Fig. 1). Thus RuCl2(PPh3)3 was the most
efficient catalyst for the glycerol hydrogenolysis.
<Insert Fig. 1>
Next, the influence of dosage of Ru(II) complex on the reaction have been
examined. Fig. S7 demonstrated that PA yield increased with the concentration of
RuCl2(PPh3)3 catalyst (Ccat.) ranged from 2 mmol/L to 7 mmol/L, which can be
attributed to more active ruthenium centres accessible to substrate molecules.
However, if Ccat. raised continuously, the yield of PA leveled off. This demonstrated
that Ccat. had a great influence on yield of PA, and the optimal yield was achieved at
Ccat.=5 mmol/L.
Since the acid was shown to be crucial in the hydrogenolysis of glycerol [30], the
impact of different types of Lewis acids and Brønsted acids on PA yield was examined
thereafter. As shown in Fig. 2, PA yield was improved by adding acids. It can be seen
that the addition of Brønsted acids gave rise to an increase of PA yield following the
order: HOTs·H2O<HOTf<PW12<HNTf2 (Fig. 2). Interestingly, except for PW12, the
catalytic activity order was consistent with the acidity order of different acids, that is,
HOTs·H2O(pKa=-1.90)<HOTf(pKa=-3.91)<HNTf2(pKa=-10.42)<PW12(pKa=-13.16).
The effect of Lewis acids on the catalytic activity was also examined. The catalytic
activity towards PA formation increased in the order: Bi(OTf)3<
Zn(OTf)2<Ce(OTf)3<Y(OTf)3<Al(OTf)3<Sc(OTf)3<In(OTf)3 (Fig. 2). According to
the previous report [31], (ion charge)2/(ion radius) (e2/r) can be used as an
approximate measure for the electron-withdrawing ability, or in other word, Lewis
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acidity of metal cations. The catalytic activity order was partially consistent with e2/r
order, that is, Zn2+ (5.4 Å-1)<Bi3+ (8.3 Å-1)<Ce3+ (8.7 Å-1)<Y3+ (10.0 Å-1)<Sc3+ (11.1
Å-1)=In3+ (11.1 Å-1)<Al3+ (18.0 Å-1). Moreover, Lewis acidity was expressed by H0
(Table S1), and the H0 order of different Lewis acids was consistent with e2/r order.
These results indicated that strong Lewis acids like In(OTf)3 activate glycerol
molecule at the beginning of reaction, with the intermediate product undergoing
sequential dehydration and hydrogenation.
<Insert Fig. 2>
Because In(OTf)3 was an effective acidic additive for producing PA (Fig. 2), the
effect the concentration of In(OTf)3 was examined and the results were shown in
Table S2. It was observed that the yield of PA increased from 15% to 40% as the
nsub./ acid ratio increased from 6:1 to 8:1. However, when the nsub./nacid further
n
increased to 16:1 , the PA yield decreased continuously from 40% to 12%. This
indicated that an excess of acidic additive could result in more side-reactions due to
the occurrence of glycerol polymerization and coking [32].
2.2 The role of FeCl3 in glycerol hydrogenolysis
In the next step, promoters were added to further improve the PA yield. As
shown in Fig. S8, the use of Co(CH3COO)2·4H2O and CoCl2·6H2O had an only
marginal effect on PA yield, while the addition of Co2(CO)8, Cu(CH3COO)2·4H2O,
AlCl3, CuCl2·4H2O, Ni(CH3COO)2·4H2O and InCl3 produced a negative effect (Fig.
S8). To our delight, the use of FeCl3 led to a significant increase of PA yield. It was
worth noticing that MA and TA were main products if In(OTf)3 was replaced by FeCl3
(Fig. S8), indicating that In(OTf)3 was essential for the formation of PA.
It was visually observed that the homogeneous mixture changed from brick-red
to brown after reaction. It was hypothesized that RuCl2(PPh3)3 was reduced by H2 and
deactivated in the course of reaction. To further identify the role of FeCl3, HRTEM
was employed to discern the nature of reaction mixture. It can be seen that Ru
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nanoparticles (around 5-8 nm) were generated in the absence of FeCl3 (Fig. 3, left).
Nevertheless, the reaction mixture was transparent and no particles can be observed
when FeCl3 was added to the reaction system (Fig. 3, right). It indicated that FeCl3
31
can prevent the formation of Ru nanoparticles. P NMR spectra of RuCl2(PPh3)3 did
not change after treatment with FeCl3 (Fig. S9), indicating that the Ru complex was
actually not influenced by the FeCl3. Additionally, the images of HRTEM showed that
Ru nanoparticles indeed disappeared once FeCl3 was added. Meantime, the color of
reaction mixture became almost colorless in the presence of FeCl3 (Fig. S10). As a
result, we proposed that the oxidizing ability of Fe(III) may be involved in impeding
the tendency of forming Ru(0) cluster [33].
< Insert Fig. 3>
As shown in Fig. S11, after reaction, the 31P NMR resonance signal of the Ru
complex (around 42 ppm) was still seen although the intensity became weaker. The
signal at 24 ppm was assigned to OPPh3. New peaks around 46-48 ppm could be
attributed to the ruthenium dihydride [RuH2(PPh3)4] or its derivatives after
hydrogenation reaction. This indicated that a part of Ru(II) complexes underwent a
possible structural evolution in the course of hydrogenation reaction [34].
Based on the activity evaluation, the effect of reaction temperature on the
catalytic activity was examined. As shown in Table S3, the conversion of glycerol
increased from 83% to 100% as the reaction temperature increased from 120 to 180
oC (Table S3, entries 1-3). Actually, PA was generated starting at 150 oC, and the yield
o
increased remarkably with the temperature until 180 C. This indicated that the
production of PA was facilitated at higher reaction temperature. However, the amount
of glycerol lost to unidentified products also increased at the elevated temperature
condition (Table S3, entries 4 and 5). Also, a large amount of black precipitate was
observed at the bottom of liner after the reaction. The glycerol polymerization under
high temperature may account for the partially deficient mass balances. Another
possibility that would explain the mass balance is that the glycerol is deoxygenated to
6

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the gas phase products, which due to their low solubility in the polar reaction mixture
and are lost upon venting the reactor. GC analysis of gas samples collected from the
head space indicated that propane, methane (CH4) and CO2 were indeed formed. But
total yield of the three gases was less than 5%. Hence, the main reason for the loss of
mass balance resulted from glycerol polymerization. This demonstrated that an
appropriate reaction temperature was crucial for the conversion of glycerol to PA.
Thus the optimal yield of PA can reach 57% around 180 oC.
Subsequently, the impact of H2 pressure on the glycerol hydrogenolysis was
studied, and the results are shown in Fig. S12a. The conversion of glycerol was
normally close to 100% in the pressure range of 1-7 MPa. However, the yield of PA
was increased obviously as the H2 pressure was increased from 1 MPa to 5 MPa. A
reasonable explanation was that more H2 can be dissolved in HOAc as increasing
reaction pressure, which was favorable for the hydrogenation of intermediates into PA.
However, the PA yield did not show more obvious changes and propane was found in
a small amount under higher pressure of H2 (>5 MPa). On the other hand, the impact
of reaction time on the glycerol hydrogenolysis was also examined. As shown in Fig.
S12b, the conversion of glycerol gradually increased from 2 h to 12 h reaching close
to 100 % after 12 - 16 h. The yield of PA increased slowly within 8 h, but it raised
sharply as the reaction was further prolonged to 12 h. The obvious increase of PA
yield (57%) was obtained, accompanying with a decrease of other intermediate
products (MA, TA, PGDAs). However, when reaction time was longer than 12 h, the
yield of PA declined slightly and the yields of propane, CH4 and CO2 increased (Fig.
S12b). It indicated that PA was possibly converted into gas-phase products at the
prolonged reaction times.
Different organic solvents also produced a significant effect on the product yields.
When HOAc was used as solvent, the main product was PA. As shown in Table S4,
when HOAc was replaced by acetic anhydride, a large amount of black precipitate
was produced after reaction. We believed that the deficient mass balance could result
from the formation of the polymer under this condition. A similar tendency was
observed when sulfolane and acetonitrile were used as solvents. As a result, HOAc
7

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was advantageous for the glycerol hydrogenolysis due to esterification reaction of
glycerol by HOAc, suppressing the polymerization of glycerol to some extent.
Glycerol conversion in the presence of different components was summarized in
Table S5. It demonstrated that the absence of FeCl3, PPh3 or In(OTf)3 led to a
significant decrease of PA yield (Table S5, entries 4, 6 and 7). The results above
indicated that all components are essential for glycerol hydrogenolysis into PA (Table
S5, entries 1-10).
2.3 Reaction pathway
In order to explore the reaction pathway, the possible intermediate species in
glycerol hydrogenolysis were hydrogenated with RuCl2(PPh3)3 under the same
condition. As shown in Table 1, both 1,2-PDO and HA could be converted completely
and the main product was PA (Table 1, entries 1 and 3). Nevertheless, 1,3-PDO only
gave esterification products without further hydrogenation (Table 1, entry 2). If AE
was used as substrate, self-polymerization occurred and only a trace of PA was
detected (Table 1, entry 4). Especially, when MA and TA were used as substrates, PA
was formed in substantial amounts and even the former was converted into PA with
much better yield (Table 1, entries 5 vs 6). Notably, it was observed that when only
either acid or Ru catalyst was present, the main products were MA and TA, and no PA
was found in the products (Table S5, entries 8-10), demonstrating that both the acid
and Ru catalyst are important for the production of PA. In addition, it can be inferred
that FeCl3 played an important role in not only preventing from forming Ru
nanoparticles, but also enhancing the esterification reaction of glycerol with HOAc
(Table S5, entries 8 vs 9). With respect to the results above, it is highly believable that
MA was an important intermediate due to the excess of HOAc as solvent, although TA
was not excluded as an intermediate.
< Insert Table 1>
Overall, a conceivable reaction mechanism is proposed in Scheme 1. Glycerol
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was initially esterified with HOAc through an acid-catalyzed reaction to form
intermediate (MA) and then MA was subsequently dehydrated and hydrogenated to
generate 1,3-propyleneglycol monoacetate (1,3-PGMA) and 1,2-propyleneglycol
monoacetate (1,2-PGMA). PA was produced by further dehydration and the
sequential hydrogenation over Lewis acid and Ru(II) complex. In addition, 1,3-PGDA
and 1,2-PGDA were obtained as minor by-products via hydrogenation and
esterification (Scheme 1). The preferential dehydration reaction of PGMAs over
esterification reaction is the main cause for high selectivity towards PA. Noteworthy
is that the esterification reaction of HOAc with glycerol played a key role in inhibiting
the polymerization of glycerol to polyether, in comparison with that of the other
solvents (Table S4).
< Insert Scheme 1>
3. Conclusion
In summary, an alternative approach has been developed for the catalytic
transformation of glycerol to C3 alcohol acetate over Ru(II) complexes. It indicated
that the RuCl2(PPh3)3 as a catalyst precursor gave 57% PA yield at full of glycerol
conversion under reaction conditions. FeCl3, In(OTf)3 and RuCl2(PPh3)3 were
indispensable for the glycerol hydrogenolysis involving esterification, dehydration
and hydrogenation process. Compared with heterogeneous catalytic system, the
present homogeneous catalytic system, due to its easy preparation, high efficiency,
and mild conditions, has a great potential for the transformation of glycerol into
value-added chemicals.
Acknowledgement
The authors are grateful for support from the National Natural Science Foundation of
China (21373082, 21773061) and the innovation Program of Shanghai Municipal
Education Commission (15ZZ031), and the Fundamental Research Funds for the
Central Universities.
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Figure and Table captions
Fig. 1 The Ru complex-catalyzed hydrogenolysis of glycerol. Reaction conditions:
glycerol 5 mmol, Ru catalysts 0.025 mmol, PPh3 0.075 mmol, In(OTf)3 0.6 mmol,
o
HOAc 5 mL, 180 C, 12 h, 5 MPa H2. The glycerol conversion was normally more
than 99%.
Fig. 2 Impact of the acidic additives on product yields. Reaction conditions: glycerol
5 mmol, RuCl2(PPh3)3 0.025 mmol, PPh3 0.075 mmol, acid 0.6 mmol, HOAc 5 mL,
180 oC, 12 h, 5 MPa H2. The glycerol conversion was normally more than 99%.
Fig. 3 HRTEM images of the reaction mixture without FeCl3 (Left) and with FeCl3 as
promoter (Right). Inset (Left): the image with higher magnification.
Table 1 The conversion of the possible intermediates in glycerol hydrogenolysis.
Scheme 1 The proposed mechanism for glycerol hydrogenolysis catalyzed by Ru
catalyst in HOAc solvent. In(OTf)3 was used as acid catalyst (Acid Cat.) and
RuCl2(PPh3)3 as Ru catalyst (Ru Cat.).
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ACCEPTED MANUSCRIPT
60
TA
MA
50
40
30
20
10
0
1,3-PGDA
1,2-PGDA
1-PA
)
)
3
3
2
)
3
)
3
t
3
p
p
c
Cl
Ru
3
a
lys
c
p
a
PPh
a
PPh
(
2
t
d
(
(
(
Ru
l
2
ca
Cl
Ru
Cl
HC
Ru
No
Ru
Fig. 1 The Ru complex-catalyzed hydrogenolysis of glycerol. Reaction conditions:
glycerol 5 mmol, Ru catalysts 0.025 mmol, PPh3 0.075 mmol, In(OTf)3 0.6 mmol,
o
HOAc 5 mL, 180 C, 12 h, 5 MPa H2. The glycerol conversion was normally more
than 99%.
15

ACCEPTED MANUSCRIPT
TA
MA
1,3-PGDA
1,2-PGDA
PA
60
45
30
15
0
3
2
3
3
3
3
3
d
2
O
12
Tf
Tf)
Tf
Tf)
Tf)
Tf)
Tf)
S
2
Tf)
H
O
O
N
O
O
(
PW

(
O
(
Zn
i
H
(
H
l
o aci
Y
c(O
n(O
Ts
B
I
A
N
Ce(OTf)
O
H
Fig. 2 Impact of the acidic additives on product yields. Reaction conditions: glycerol
5 mmol, RuCl2(PPh3)3 0.025 mmol, PPh3 0.075 mmol, acid 0.6 mmol, HOAc 5 mL,
180 oC, 12 h, 5 MPa H2. The glycerol conversion was normally more than 99%.
16

ACCEPTED MANUSCRIPT
.
Fig. 3 HRTEM images of the reaction mixture without FeCl3 (Left) and with FeCl3 as
promoter (Right). Inset (Left): the image with higher magnification.
17

ACCEPTED MANUSCRIPT
Table 1 The conversion of the possible intermediates in glycerol hydrogenolysis.
Yield(%)
Con
Entry Substrates
PA 1,2-PGMA 1,2-PGDA 1,3-PGMA 1,3-PGDA
(%)
100
100
100
100
100
95
1
2
3
4
5
6
1,2-PDO
1,3-PDO
HA
41
3
2
-
1
-
-
6
-
-
28
-
30
3
2
-
-
AE
-
-
-
MA
39
15
-
1
3
-
1
-
TA
-
-
Reaction conditions: substrate 5 mmol, RuCl2(PPh3)3 0.025 mmol, PPh3 0.075 mmol,
o
In(OTf)3 0.6 mmol, FeCl3 0.1 mmol, HOAc 5 mL, 180 C, 12 h, 5 MPa H2.
(1,2-PDO=1,2-propanediol,
1,3-PDO=1,3-propanediol,
HA=hydroxyacetone,
AE=acrolein, MA=monoacetin, TA=triacetin, PA=propyl acetate, 1,2-PGMA=
1,2-propyleneglycol monoacetate, 1,3-PGMA=1,3-propyleneglycol monoacetate,
1,2-PGDA=1,2-propyleneglycol diacetate, 1,3-PGDA=1,3-propyleneglycol diacetate.)
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ACCEPTED MANUSCRIPT
Scheme 1 The proposed mechanism for glycerol hydrogenolysis catalyzed by Ru
catalyst in HOAc solvent. In(OTf)3 was used as acid catalyst (Acid Cat.) and
RuCl2(PPh3)3 as Ru catalyst (Ru Cat.).
19

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Highlight
◆ Easily available Ru complexes have been utilized for hydrogenolysis of glycerol.
◆ The reaction can be carried out under relatively mild conditions.
◆ The hydrogenolysis of glycerol gave propyl acetate (PA) as a major product.
◆ The In(CF3SO3)3 played a very critical role in the esterification and dehydration.
◆ The co-catalyst (FeCl3) can suppress the reduction of Ru complex to Ru particles.
20