Effects of alkaline additives on the formation of lactic acid in sorbitol hydrogenolysis over Ni/C catalyst

Article abstract of DOI:10.1016/S1872-2067(15)60976-7

Lactic acid is produced as a major byproduct during sorbitol hydrogenolysis under alkaline conditions. We investigated the effects of two different alkaline additives, Ca(OH)₂ and La(OH)₃, on lactic acid formation during sorbitol hydrogenolysis over Ni/C catalyst. In the case of Ca(OH)₂, the selectivity of lactic acid was 8.9%. In contrast, the inclusion of La(OH)₃ resulted in a sorbitol conversion of 99% with only trace quantities of lactic acid being detected. In addition, the total selectivity towards the C2 and C4 products increased from 20.0% to 24.5% going from Ca(OH)₂ to La(OH)₃. These results therefore indicated that La(OH)₃ could be used as an efficient alkaline additive to enhance the conversion of sorbitol. Pyruvic aldehyde, which is formed as an intermediate during sorbitol hydrogenolysis, can be converted to both 1,2-propylene glycol and lactic acid by hydrogenation and rearrangement reactions, respectively. Notably, these two reactions are competitive. When Ca(OH)₂ was used as an additive for sorbitol hydrogenolysis, both the hydrogenation and rearrangement reactions occurred. In contrast, the use of La(OH)₃ favored the hydrogenation reaction, with only trace quantities of lactic acid being formed.

Full text of DOI:10.1016/S1872-2067(15)60976-7

Chinese Journal of Catalysis 37 (2016) 177–183
a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c h n j c
Article
Effects of alkaline additives on the formation of lactic acid in
sorbitol hydrogenolysis over Ni/C catalyst
Junjie Zhanga,b, Fang Lua,#, Weiqiang Yu a, Rui Lu a,b, Jie Xua,*
a State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian National Laboratory for Clean Energy,
Dalian 116023, Liaoning, China
b University of Chinese Academy of Sciences, Beijing 100049, China
A R T I C L E I N F O
A B S T R A C T
Article history:
Lactic acid is produced as a major byproduct during sorbitol hydrogenolysis under alkaline condi‐
tions. We investigated the effects of two different alkaline additives, Ca(OH)2 and La(OH)3, on lactic
acid formation during sorbitol hydrogenolysis over Ni/C catalyst. In the case of Ca(OH)2, the selec‐
tivity of lactic acid was 8.9%. In contrast, the inclusion of La(OH)3 resulted in a sorbitol conversion
of 99% with only trace quantities of lactic acid being detected. In addition, the total selectivity to‐
wards the C2 and C4 products increased from 20.0% to 24.5% going from Ca(OH)2 to La(OH)3.
These results therefore indicated that La(OH)3 could be used as an efficient alkaline additive to
enhance the conversion of sorbitol. Pyruvic aldehyde, which is formed as an intermediate during
sorbitol hydrogenolysis, can be converted to both 1,2‐propylene glycol and lactic acid by hydro‐
genation and rearrangement reactions, respectively. Notably, these two reactions are competitive.
When Ca(OH)2 was used as an additive for sorbitol hydrogenolysis, both the hydrogenation and
rearrangement reactions occurred. In contrast, the use of La(OH)3 favored the hydrogenation reac‐
tion, with only trace quantities of lactic acid being formed.
Received 28 August 2015
Accepted 24 September 2015
Published 5 January 2016
Keywords:
Lactic acid
Alkali
Sorbitol
Catalytic hydrogenolysis
Nickel catalyst
© 2016, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Published by Elsevier B.V. All rights reserved.
1. Introduction
propylene feedstocks. The development of a direct method for
the conversion of sugar alcohols to EG and PG is therefore
Sugar alcohols, such as sorbitol and xylitol, are attractive
platform chemicals because they can be readily produced by
the hydrogenation of sugars, which are available in large quan‐
tities from natural sources [1–3]. Furthermore, sugar alcohols
can be converted to ethylene glycol (EG) and 1,2‐propylene
glycol (1,2‐PG) [4–6]. EG and 1,2‐PG are important commodity
chemicals that are widely used as starting materials for the
preparation of polyesters. The main manufacturing route cur‐
rently used for the industrial production of EG and PG is based
on the two‐step conversion of petroleum‐derived ethylene and
highly desired as an energy‐efficient and environmental
friendly alternative to the existing procedures in terms of uti‐
lizing renewable resources [7–9].
The mechanism for sugar alcohols hydrogenolysis is gener‐
ally believed to involve two key steps [10,11]. According to the
first step of this mechanism, the polyol substrates would be
dehydrogenated over a metal catalyst under alkaline conditions
to give the corresponding carbonyl intermediates. The subse‐
quent cleavage of the C–C bonds in these intermediates under
basic conditions would give the EG and PG products, most like‐
* Corresponding author. Tel/Fax: +86‐411‐84379245; E‐mail: xujie@dicp.ac.cn
# Corresponding author. Tel/Fax: +86‐411‐84379245; E‐mail: lufang@dicp.ac.cn
This work was supported by the National Natural Science Foundation of China (21203183, 21233008, 21473188).
DOI: 10.1016/S1872‐2067(15)60976‐7 | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 37, No. 1, January 2016

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Junjie Zhang et al. / Chinese Journal of Catalysis 37 (2016) 177–183
ly via a retro‐aldol condensation. Sugar alcohols hydrogenolysis
proceeds much more efficiently under basic conditions, which
appear to facilitate the dehydrogenation and C–C bond cleavage
reactions. Several inorganic bases, including Ca(OH)2 [12–14],
KOH [15], NaOH [16], CaO [17–19], and Ba(OH)2 [20], have
been investigated as additives for the hydrogenolysis of sorbi‐
tol in terms of their effects on the conversion of sorbitol and the
selectivity of the products. In general, the results of these stud‐
ies showed that the addition of an inorganic base favored the
formation of glycols compared with the use of neutral condi‐
tions. However, the addition of an inorganic base to these hy‐
drogenolysis reactions typically results in the formation of lac‐
tic acid as a major byproduct. Liu et al. [11] evaluated the ef‐
fects of several different inorganic bases, including CaCO3,
Mg(OH)2, and Ca(OH)2, on xylitol hydrogenolysis and found
that the nature of the base had a significant effect on the selec‐
tivity towards lactic acid, as well as the activity of the reaction.
The selectivity towards lactic acid increased as the pH of the
reaction medium increased. In a later study, Xia et al. [21] re‐
ported that the conversion of xylitol and selectivity towards
lactic acid increased as the amount of Ca(OH)2 added to the
reaction increased. Unfortunately, lactic acid formation could
be problematic for sugar alcohols hydrogenolysis. For example,
the presence of lactic acid would result in the consumption of
the alkaline material present in the reaction mixture, which
would suppress the conversion of the sugar alcohols. Further‐
more, the presence of lactic acid could complicate the analysis
of the product mixture. To address these limitations, we want‐
ed to develop a deeper understanding of the hydrogenolysis
process. For example, we intended to understand why the ad‐
dition of an inorganic base led to lactic acid formation. We also
wanted to identify the pathway responsible for lactic acid for‐
mation. Based on this improved understanding, we then want‐
ed to develop a new process capable of achieving high levels of
sugar alcohols conversion with only trace quantities of lactic
acid.
acid were obtained from Tianjin Kemiou Chemical Reagent Co.,
Ltd. Glycerol was purchased from Sinopharm Chemical Reagent
Co., Ltd. La(OH)3 (99.9%) was obtained from Aladdin Industrial
Inc. Sorbitol (98%) and pyruvic aldehyde (35%–45%) were
obtained from Alfa Aesar (Tianjin, China). Activated carbon
(80–100 mesh, BET surface area 1318 m2/g) was purchased
from Beijing Guanghua Timber Mill.
2.2. Catalyst preparation
The 10 wt% Ni/C catalyst was prepared according to a
modified incipient wetness impregnation method. Briefly, acti‐
vated carbon (1.0 g) was added to a aqueous solution of
Ni(NO3)2 (1.2 mol/L, 1.6 mL) under stirring, and the resulting
solution was sonicated for 0.5 h. The mixture was then aged for
24 h at ambient temperature before being dried at 383 K for 12
h. The resulting catalysts were then reduced with H2 at 723 K
for 3 h prior to being used in the hydrogenolysis reactions.
2.3. Catalytic reactions and product analysis
The catalytic reactions were carried out in a 50‐mL stainless
steel autoclave. For a typical run, the reactor was charged with
a 10 wt% solution of sorbitol in water (20 mL), the pre‐reduced
catalyst (0.3 g), and the required amount of the additive (i.e.,
0.18 g of Ca(OH)2 or 0.3 g of La(OH)3). The reactor was then
purged several times with H2 to remove any air and pressur‐
ized up to 5.0 MPa with H2. The reactor was then heated to 493
K and stirred at 1000 r/min for 1 h.
The products of the reaction were analyzed by gas chroma‐
tography using an Agilent 7890A gas chromatograph equipped
with a 19091j‐323 capillary column and a flame ionization de‐
tector. The starting materials and products were analyzed by
HPLC on a Waters e2695 separations module equipped with a
refractive index detector (Waters 2414) and an UV/visible de‐
tector (Waters 2489). All of the samples were acidified (pH = 2)
using a aqueous H2SO4 solution (2 mol/L) prior to being ana‐
lyzed by HPLC. The HPLC system was equipped with an
OA‐1000 column, which was eluted with an 0.2 wt% aqueous
H2SO4 solution as the mobile phase at a flow rate of 0.5
mL/min.
Supported Ni systems have been reported as effective cata‐
lysts for sorbitol hydrogenolysis [22]. Furthermore, the intro‐
duction of a La species into these catalysts led to an increase in
the conversion of xylitol, as well as an increase in the selectivity
towards glycols [23]. La(OH)3 has been used as a heterogene‐
ous alkaline catalyst for the Knoevenagel reaction [24] and has
also been used as an additive for the conversion of cellulose
[25]. In this work, we evaluated the effects of two different al‐
kaline additives, Ca(OH)2 and La(OH)3, on the formation of lac‐
tic acid during sorbitol hydrogenolysis over Ni/C catalyst. We
also studied the reactions of pyruvic aldehyde over Ni/C cata‐
lyst in the presence of Ca(OH)2 and La(OH)3, with the aim of
exploring the pathway as plausible mechanism for lactic acid
formation.
3. Results and discussion
3.1. Effects of types and amount of alkaline additives on
catalytic hydrogenolysis of sorbitol
Sorbitol hydrogenolysis was examined over Ni/C catalyst in
the absence and presence of three different inorganic bases to
determine the impact of these additives on the conversion and
selectivity of the reaction (Table 1). NaOH, Ca(OH)2, and
La(OH)3 were used as the additives in this study based on their
different water solubility properties. When Ni/C (Table 1, entry
1) was used as the catalyst in the absence of any additive, the
sorbitol conversion was only 19.2%. The addition of NaOH
(Table 1, entry 2) led to a significant increase in the sorbitol
conversion to 73.8%. However, much greater increases were
2. Experimental
2.1. Materials
All of the chemicals were purchased at analytic grade and
used as received. Ni(NO3)2·6H2O, NaOH, Ca(OH)2, and lactic

Junjie Zhang et al. / Chinese Journal of Catalysis 37 (2016) 177–183
179
Table 1
Effects of the different alkaline additives on the conversion and selectivity for sorbitol hydrogenolysis over Ni/C catalyst.
Amount of OH–
(mmol)
Conversion
(%)
Selectivity a (%)
BG b Lactic acid Glycolic acid Acetic acid Others c
Entry
Additive
EG
1,2‐PG Glycerol
1
2
3
4
5
6
7
8
—
NaOH
—
19.2
73.8
68.8
81.3
100
2.9
1.4
21.6
14.1
20.0
30.0
33.1
30.3
5.7
14.3
15.3
17.8
19.2
21.5
23.6
19.6
—
6.0
6.7
20.0
14.7
7.1
1.1
0.5
16.8
9.2
8.6
13.2
16.8
13.7
9.6
4.3
5.7
2.2
3.2
3.0
4.0
4.9
4.4
4.0
4.6
5.2
5.7
7.3
7.8
8.0
9.0
7.3
—
—
15.1
2.0
3.7
8.9
10.3
11.7
—
trace
trace
trace
trace
0.1
—
0.3
0.1
0.2
0.2
0.5
0.7
—
—
—
—
—
—
2.6
0.3
0.5
0.7
1.2
1.2
—
—
—
—
—
87.5
34.4
49.5
44.6
31.5
33.8
38.9
66.0
56.8
56.2
46.1
40.5
40.8
42.1
54.6
66.9
4.74
0.79
1.58
4.74
7.90
11.06
0.02
0.08
0.16
0.79
1.58
4.74
7.90
11.06
1.58 d
16.1
10.9
12.4
16.6
15.6
12.8
6.9
14.5
14.2
15.6
15.7
15.9
15.5
13.9
—
Ca(OH)2
Ca(OH)2
Ca(OH)2
Ca(OH)2
Ca(OH)2
La(OH)3
La(OH)3
La(OH)3
La(OH)3
La(OH)3
La(OH)3
La(OH)3
La(OH)3
La(NO3)3
100
100
29.1
99.0
99.0
99.0
99.0
99.0
100
9
10
11
12
13
14
15
16
—
—
—
10.1
—
—
—
0.8
0.2
0.3
16.5
100
15.3
Reaction conditions: 10% Ni/C 0.3 g, 10 wt% aqueous sorbitol solution 20 mL, 493 K, H2 pressure (RT) 5 MPa, 1 h.
EG = ethylene glycol, 1,2‐PG = 1,2‐propylene glycol.
a C‐based selectivity.
b BG includes 2,3‐butanediol, 1,2‐butanediol, 1,4‐butanediol, and 1,2,4‐butanetriol.
c Others include methane, methanol, ethanol, and some unidentified.
d 1.58 mmol La(NO3)3.
observed following the addition of Ca(OH)2 (Table 1, entry 5)
or La(OH)3 (Table 1, entry 13), which gave sorbitol conversions
of 100% and 99%, respectively. With regard to the selectivities
of these reactions, the total selectivity towards EG (C2 product)
and the BG products (C4 products) was only 5.1% in the ab‐
sence of any additive (Table 1, entry 1). In contrast, the total
selectivity values for EG and BG were 20.0% and 24.5% fol‐
lowing the addition of 7.90 mmol OH– for Ca(OH)2 (Table 1,
entry 6) and La(OH)3 (Table 1, entry 14) to the hydrogenolysis
reaction, respectively. These results therefore indicated that
addition of an alkaline additive led to a significant improve‐
ment in the sorbitol conversion and selectivity of the hydro‐
genolysis reaction. Furthermore, these results were consistent
with those from several previous reports [11,20]. Notably, the
La(OH)3 additive favored the cleavage of the C2–C3 bond in
sorbitol.
Consideration of the selectivity of the reaction towards lac‐
tic acid revealed that there were significant differences be‐
tween the different additives. For example, no lactic acid was
detected when the reaction was conducted in the presence of
the Ni/C catalyst without additive (Table 1, entry 1). When
NaOH (Table 1, entry 2) or Ca(OH)2 (Table 1, entry 5) was used
as the additive, the selectivities of the reaction towards lactic
acid were 15.1% and 8.9%, respectively. However, when
La(OH)3 (Table 1, entry 13) was used as an additive, the selec‐
tivity towards lactic acid was only 0.1%. For Ca(OH)2 and
La(OH)3, the selectivities of the reaction towards C3 products
(i.e., 1,2‐PG, glycerol, and lactic acid) were 46.0% and 35.3%,
respectively. These results therefore indicated that the addition
of Ca(OH)2 to the hydrogenolysis reaction favored the cleavage
of the C3–C4 bond of sorbitol. However, this increase in the
C3–C4 bond cleavage products led to higher levels of lactic acid
instead of the C3 polyols.
We subsequently proceeded to investigate the effects of dif‐
ferent amounts of alkali on lactic acid formation using Ca(OH)2
and La(OH)3 as the additives; the results are shown in Table 1.
When Ca(OH)2 was used as an additive (Table 1, entries 3–7),
the conversion of sorbitol increased as the amount of Ca(OH)2
added to the reaction increased. For example, the use of 0.79
mmol OH– led to a sorbitol conversion of 68.8% (Table 1, entry
3), whilst the use of 4.74 mmol OH– led to a much higher sorbi‐
tol conversion of 100% (Table 1, entry 5). With regards to the
production of lactic acid, the selectivity increased as the
amount of Ca(OH)2 added to the reaction increased, reaching
11.7% with 11.06 mmol OH– (Table 1, entry 7). Increasing the
amount of Ca(OH)2 initially led to a gradual increase in the se‐
lectivity of the reaction towards 1,2‐PG, with further increase
leading to a gradual decrease in 1,2‐PG. Furthermore, the selec‐
tivity of glycerol decreased as the amount of Ca(OH)2 increased.
When La(OH)3 was used as the additive, the sorbitol conversion
reached 99.0% with only 0.08 mmol OH– (Table 1, entry 9).
Notably, these conditions afforded only 0.3% lactic acid up to
11.06 mmol OH– (Table 1, entry 15). The selectivity of the reac‐
tion towards 1,2‐PG and glycerol initially increased and then
decreased with further increase in the amount of La(OH)3.
Based on these above results, it was clear that the nature and
the amount of the alkaline additive had a significant impact on
lactic acid formation. In the case of Ca(OH)2, the selectivity to‐
wards lactic acid was dependent on the amount of the alkaline
additive. However, the use of La(OH)3 led to only trace quanti‐
ties of lactic acid.
We wanted to develop a deeper understanding of the dif‐
ferences between these additives for the conversion of sorbitol.
We noticed that the pH of the hydrogenolysis reaction mixture
changed before and after the reaction with the Ca(OH)2 addi‐
tive. The pH value was 12 before the reaction and decreased to

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Junjie Zhang et al. / Chinese Journal of Catalysis 37 (2016) 177–183
7 after the reaction. However, when La(OH)3 was used as an
additive, the pH of the reaction mixture was around 7 both
before and after the reaction. It is well known that Ca(OH)2 is
only partially water‐soluble and that its solubility in water at
room temperature is 0.165 g/100 mL. In contrast, La(OH)3 is
completely insoluble in water at room temperature. The basic
environment provided by Ca(OH)2 would therefore promote
lactic acid formation, which would subsequently neutralize and
consume the remaining Ca(OH)2.
develop a better understanding of the role of La3+ in this reac‐
tion, we evaluated the use of La(NO3)3 (Table 1, entry 16) as an
additive, which gave a sorbitol conversion of only 15.3%. This
result indicated that La3+ was not the main factor responsible
for the high sorbitol conversion observed with La(OH)3. The
pronounced promoting effect of La(OH)3 was therefore at‐
tributed to the OH– anions of La(OH)3. These results therefore
show that La(OH)3 is an effective alkaline additive for enhanc‐
ing the conversion of sorbitol with negligible lactic acid for‐
mation.
To further explore the nature of the interactions between
lactic acid and the alkaline additives, we investigated the hy‐
drogenation of lactic acid in the presence of Ca(OH)2 and
La(OH)3. The lactic acid remained unchanged in the presence of
Ca(OH)2, whilst it was hydrogenated to give 1,2‐PG with a con‐
version of 67.9% in the presence of La(OH)3. It has been re‐
ported that lactic acid can be hydrogenated to 1,2‐PG under
acidic and neutral conditions [26]. In the presence of Ca(OH)2,
lactic acid would exist as the corresponding calcium lactate salt,
which would be very stable in the solution [27]. These results
also indicated that lactic acid could neutralize Ca(OH)2 but not
La(OH)3. Ca(OH)2 could therefore be consumed by lactic acid. It
has also been reported that the first step in the dehydrogena‐
tion of sorbitol could be promoted by an alkaline additive. The
continuous consumption of the alkaline additive would conse‐
quently suppress the conversion of sorbitol. A large amount of
Ca(OH)2 would therefore be required to achieve the complete
conversion of sorbitol. Furthermore, the selectivity towards
lactic acid increased as the amount of Ca(OH)2 added to the
reaction increased. In contrast, the La(OH)3 additive could not
be consumed in the same way, meaning that a much smaller
amount of La(OH)3 could be used to achieve the complete con‐
version of sorbitol. Given that lactic acid can be hydrogenated
to give 1,2‐PG in the presence of La(OH)3, it seems unlikely that
the trace quantities of lactic acid formed during this reaction
could be attributed to the consumption of lactic acid. Based on
our result for sorbitol hydrogenolysis in the presence of
La(OH)3 (Table 1, entry 13) and the conversion of lactic acid,
we calculated the total selectivity towards lactic acid, which
was much lower than that of lactic acid (8.9%) in the presence
of Ca(OH)2 (Table 1, entry 5) under the same conditions. To
3.2. Pathway for lactic acid formation in sorbitol
hydrogenolysis on Ni/C in the presence of the alkali
The effect of reaction time on sorbitol hydrogenolysis over
Ni/C catalyst was investigated to develop a better understand‐
ing of the reaction process (Fig. 1). When Ca(OH)2 was used as
the additive, the selectivity towards lactic acid increased from
6.8% to 8.4% over 30 min. Furthermore, the selectivity to‐
wards glycerol gradually decreased during the first 30 min,
whilst the selectivity towards 1,2‐PG increased. Further in‐
crease in the reaction time (> 30 min) did not lead to any dis‐
cernible changes in the selectivity of lactic acid, glycerol, or
1,2‐PG. In the case of La(OH)3, the selectivity towards lactic acid
was about 0.1% and remained almost unchanged throughout
the entire reaction. As for glycerol, its selectivity initially in‐
creased up to a reaction of 15 min and then gradually de‐
creased with further increase in the reaction time. The selectiv‐
ity towards 1,2‐PG in the presence of La(OH)3 increased gradu‐
ally with time, even after the sorbitol had been completely
consumed.
These results showed that the selectivity profiles of the re‐
action towards glycerol and lactic acid were different for the
Ca(OH)2 and La(OH)3 additives. To develop a deeper under‐
standing of the underlying reasons for these differences, we
investigated the reactions of glycerol under the same condi‐
tions to those used for sorbitol hydrogenolysis, and the results
are shown in Table 2. In the presence of Ca(OH)2 (Table 2, entry
1), 1,2‐PG and lactic acid were formed as the major products.
The glycerol substrate was mainly converted to 1,2‐PG when
40
50
(a)
Conversion
(b)
100
80
60
40
20
100
95
90
85
80
Conversion
35
30
25
20
15
10
5
40
30
20
10
0
1,2-Propylene glycol
1,2-Propylene glycol
Lactic acid
Glycerol
Glycerol
Lactic acid
0
0
20
40
60
80
100
120
0
20
40
60
80
100
120
Time (min)
Time (min)
Fig. 1. Effect of the reaction time on sorbitol hydrogenolysis over Ni/C catalyst in the presence of the different additives. (a) Ca(OH)2; (b) La(OH)3.
Reaction conditions: 10% Ni/C 0.3 g, Ca(OH)2 0.18 g or La(OH)3 0.3 g, 10 wt% aqueous sorbitol solution 20 mL, 493 K, H2 (RT) 5 MPa, 1 h.

Junjie Zhang et al. / Chinese Journal of Catalysis 37 (2016) 177–183
181
Table 2
Conversion of glycerol and pyruvic aldehyde over Ni/C in the presence of the additive.
Selectivity a (%)
Entry
Substrate
Additive
Conversion (%)
EG
9.9
10.7
—
—
—
1,2‐PG
Lactic acid
26.4
1.0
Others b
23.1
34.1
15.0
64.3
68.2
14.9
56.0
1
2
3
4
5c
6d
7d
Glycerol
Glycerol
Ca(OH)2
La(OH)3
Ca(OH)2
La(OH)3
—
54.6
62.5
100
100
100
100
100
40.6
54.2
0.8
35.7
31.8
—
Pyruvic aldehyde
Pyruvic aldehyde
Pyruvic aldehyde
Pyruvic aldehyde
Pyruvic aldehyde
84.2
—
—
85.1
44.0
Ca(OH)2
La(OH)3
—
—
—
Reaction conditions: 10% Ni/C 0.3 g, Ca(OH)2 0.18 g or La(OH)3 0.3 g, 10 wt% aqueous glycerol solution or 0.05 wt% aqueous pyruvic aldehyde solu‐
tion 20 mL, 493 K, H2 (RT) 5 MPa, 1 h.
a C‐based selectivity.
b Others include glycolic acid, acetic acid, methane, and some unidentified.
c Reaction time 5 min.
d Reaction time 5 min, without 10% Ni/C.
La(OH)3 was used as an additive (Table 2, entry 2), which was
consistent with the results for sorbitol hydrogenolysis. Glycerol
hydrogenolysis was also conducted without the Ni/C catalysts
both in the absence and the presence of an additive (Ca(OH)2 or
La(OH)3), but no reaction occurred.
It has been suggested that the lactic acid produced during
glycerol hydrogenolysis is formed via a pyruvic aldehyde in‐
termediate [28]. To explore the pathway responsible for lactic
acid formation in more detail, we investigated pyruvic alde‐
hyde conversion under the same conditions as those used for
sorbitol. As shown in Table 2, pyruvic aldehyde conversion in
the presence of Ca(OH)2 (Table 2, entry 3) resulted in the for‐
mation of lactic acid as the main product, with 0.8% selectivity
towards 1,2‐PG. In contrast, pyruvic aldehyde conversion in the
presence of La(OH)3 (Table 2, entry 4) resulted in the for‐
mation of 1,2‐PG as the main product, with no lactic acid being
detected.
To explain these differences in the selectivities of the addi‐
tives towards 1,2‐PG and lactic acid, we conducted three addi‐
tional reactions using pyruvic aldehyde as a substrate, and the
results of these reactions are shown in Table 2 (entries 5–7).
When Ni/C was used as the catalyst, pyruvic aldehyde was hy‐
drogenated to give 1,2‐PG as the main product. Furthermore,
the reaction of pyruvic aldehyde only resulted in lactic acid
formation when Ca(OH)2 or La(OH)3 was added to the reaction.
It is noteworthy that the selectivity towards lactic acid was
0.9‐fold greater for Ca(OH)2 than it was for La(OH)3. Based on
these results, we proposed a pathway for lactic acid formation
starting from glycerol. According to this pathway, the hydro‐
genation reaction would occur on the active sites of the metal
catalyst, and the rearrangement reaction would be catalyzed by
the alkali. The reactions responsible for the formation of 1,2‐PG
and lactic acid would require different active sites, which indi‐
cates that the formation of these products would be competi‐
tive. The use of a metal catalyst in conjunction with an alkaline
additive would therefore lead to different selectivities towards
1,2‐PG and lactic acid, which would be dependent on the dif‐
ferences in the additives. For Ca(OH)2, which is only partially
soluble in water, both the hydrogenation and rearrangement
reactions would occur, resulting in the formation of 1,2‐PG and
lactic acid. However, for La(OH)3, which is largely insoluble in
water, the pyruvic aldehyde substrate would be mainly con‐
verted to 1,2‐PG through a hydrogenation reaction.
Fig. 2 shows the relationship between the selectivity ratio of
S(lactic acid) to S(1,2‐PG) and the mass ratio of Ca(OH)2 to sor‐
bitol for the hydrogenolysis reactions of sorbitol, which were
calculated using the data in Table 1 (entries 3–7). The selectiv‐
ity ratio of S(lactic acid) to S(1,2‐PG) increased as the amount of
Ca(OH)2 added to the reaction increased. These results also
indicated that the reactions responsible for the formation of
1,2‐PG and lactic acid were competitive. More alkali was favor‐
able for lactic acid formation.
Based on the above results, we proposed a pathway for lac‐
tic acid formation during sorbitol hydrogenolysis over Ni/C in
the presence of an alkaline additive, which is shown in Scheme
1. Given that a pathway for sorbitol hydrogenolysis has already
been reported in the literature, we have focused primarily on
the pathway responsible for lactic acid formation during this
reaction. Pyruvic aldehyde would be formed as an intermediate
during the course of this reaction and subsequently converted
to lactic acid via a rearrangement reaction. Pyruvic aldehyde
could also be converted to 1,2‐PG through a hydrogenation
reaction. The addition of different additives would lead to dif‐
ferent selectivities towards lactic acid. For example, the addi‐
tion of Ca(OH)2 would lead to the hydrogenation and rear‐
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.00
0.05
0.10
0.15
0.20
m(Ca(OH)2)/m(sorbitol)
Fig. 2. Effect of the mass ratio of Ca(OH)2 to sorbitol on the ratio of
S(lactic acid) to S(1,2‐PG) during sorbitol hydrogenolysis over Ni/C
catalyst. These results were calculated using the data in Table 1 (entries
3–7).

182
Junjie Zhang et al. / Chinese Journal of Catalysis 37 (2016) 177–183
Scheme 1. Proposed pathway for lactic acid formation via sorbitol hydrogenolysis over Ni/C catalyst in the presence of an alkaline additive.
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rangement reactions, whilst the addition of La(OH)3 would
favor the hydrogenation reaction as the major pathway, leading
to only trace quantities of lactic acid.
4. Conclusions
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The total selectivity of the hydrogenolysis reaction of sorbi‐
tol towards C2 and C4 products was enhanced by the presence
of La(OH)3. Furthermore, the addition of La(OH)3 resulted in a
sorbitol conversion of 99% with only trace quantities of lactic
acid being detected. In contrast, the use of Ca(OH)2 as an addi‐
tive led to an increase in the selectivity towards C3 products,
which was most likely be related to the formation of higher
levels of lactic acid. Pyruvic aldehyde, which is formed as an
intermediate during sorbitol hydrogenolysis, was converted to
1,2‐propylene glycol and lactic acid via hydrogenation and re‐
arrangement reactions, respectively. We found that these two
reactions were competitive and that the hydrogenation reac‐
tion became the dominant transformation in the presence of
La(OH)3. These results are therefore of fundamental im‐
portance in terms of enhancing our understanding of sugar
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Graphical Abstract
Chin. J. Catal., 2016, 37: 177–183 doi: 10.1016/S1872‐2067(15)60976‐7
Effects of alkaline additives on the formation of lactic acid in sorbitol hydrogenolysis over Ni/C catalyst
Junjie Zhang, Fang Lu*, Weiqiang Yu, Rui Lu, Jie Xu *
Dalian Institute of Chemical Physics, Chinese Academy of Sciences; University of Chinese Academy of Sciences
La(OH)3 has been identified as an efficient additive for sorbitol hydrogenolysis over Ni/C with very little lactic acid formation. The path‐
ways for 1,2‐propylene glycol and lactic acid formation were competitive, with the hydrogenation to 1,2‐propylene glycol dominating for
La(OH)3.

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