Hydrogenolysis of sorbitol into valuable C3-C2 alcohols at low H2 pressure promoted by the heterogeneous Pd/Fe3O4 catalyst

Article abstract of DOI:10.1016/j.mcat.2017.12.038

The hydrogenolysis of sorbitol and various C5-C3 polyols (xylitol; erythritol; 1,2- 1,4- and 2,3-butandiol; 1,2-propandiol; glycerol) have been investigated at low molecular hydrogen pressure (5 bar) by using Pd/Fe₃O₄, as heterogeneous catalyst and water as the reaction medium. Catalytic experiments show that the carbon chain of polyols is initially shortened through dehydrogenation/decarbonylation and dehydrogenation/retro-aldol mechanisms followed by a series of cascade reactions that include dehydrogenation/decarbonylation and dehydration/hydrogenation processes. At 240 °C, sorbitol is fully converted into lower alcohols with ethanol being the main reaction product in liquid phase.

Full text of DOI:10.1016/j.mcat.2017.12.038

Molecular Catalysis 446 (2018) 152–160
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Hydrogenolysis of sorbitol into valuable C3-C2 alcohols at low H2
pressure promoted by the heterogeneous Pd/Fe O catalyst
3
4
a
b,∗
b
a
Bianca Gumina , Francesco Mauriello , Rosario Pietropaolo , Signorino Galvagno ,
Claudia Espro
a
a,∗
Dipartimento di Ingegneria, Università di Messina, Contrada di Dio–Vill. S. Agata, I-98166 Messina, Italy
Dipartimento DICEAM, Università Mediterranea di Reggio Calabria, Loc. Feo di Vito, I-89122 Reggio Calabria, Italy
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 3 October 2017
Received in revised form
The hydrogenolysis of sorbitol and various C5-C3 polyols (xylitol; erythritol; 1,2- 1,4- and 2,3-butandiol;
1
,2-propandiol; glycerol) have been investigated at low molecular hydrogen pressure (5 bar) by using
Pd/Fe3O4, as heterogeneous catalyst and water as the reaction medium. Catalytic experiments show
that the carbon chain of polyols is initially shortened through dehydrogenation/decarbonylation and
dehydrogenation/retro-aldol mechanisms followed by a series of cascade reactions that include dehydro-
2
1 December 2017
Accepted 28 December 2017
◦
genation/decarbonylation and dehydration/hydrogenation processes. At 240 C, sorbitol is fully converted
Keywords:
Sorbitol
Xylitol
into lower alcohols with ethanol being the main reaction product in liquid phase.
©
2017 Elsevier B.V. All rights reserved.
Polyol
Erythritol
Glycerol
Ethanol
Hydrogenolysis
Heterogeneous catalysis
Palladium
Bimetallic Pd-Fe system
1
. Introduction
of biofuels, chemicals [17–19] and renewable hydrogen (through
aqueous-phase reforming reactions–APR) [17,20–24]. Xylitol (C5
polyol) is commonly produced by catalytic hydrogenation of xylose
as well as from the hydrogenation of hemicellulose and it is
widely applied as sweetener and inhibitor for the development of
bacteria in foods [16,25]. Erythritol (C4 polyol) is the main start-
ing substrate for the synthesis of butanediols (BDOs), that are
widely employed as precursors of polyester resins, polyurethanes
and polymers such as polybutylene-terephthalate (PBT) [16,25].
Glycerol (C3 polyol) is cheaply available in continuously growing
quantities, being the main by-product in bio-diesel production. The
selective hydrogenolysis of glycerol can lead to 1,2-propanediol
(1,2-PDO), that is a useful compound in several industrial fields [4].
The shortest polyol is ethylene glycol (EG, C2 polyol), which is typi-
cally used as antifreeze agent and as a precursor of polyesters, such
as polyethylene terephthalate (PET) and polyethylene-naphthalate
In the last decade, a growing attention has been devoted to
the production and the utilization of bio-sugars and bio-alcohols
obtainable from renewable cellulose, hemicellulose and lignin (the
main fractions of lignocellulosic biomasses) [1,2]. To this regard,
cellulose and hemicellulose derived sorbitol (C6 polyol), xylitol (C5
polyol) and glycerol (C3 polyol) have been inserted in the list of top
1
2 biomass-based building blocks [3–5] representing key renew-
able resources for modern biorefineries.
With a global market expected to overcome 2.3 Mt/year, sor-
bitol is attracting intense industrial and research interests [6,7].
At present, it is mainly produced by hydrogenation of glucose
[5,6,8] (easily obtained by the hydrolysis of cellulose [9–15]) and
largely used in pharmaceutical, food and cosmetic industries [16].
Moreover, it has been proposed as building block for production
(PEN). Catalytic epoxidation of ethylene is presently the main
technology applied in the production of EG, although industrial
processes to obtain it directly from cellulose are in continuous
development [1]. Among others, a valuable desirable product from
∗ Corresponding authors.
E-mail addresses: francesco.mauriello@unirc.it (F. Mauriello),
claudia.espro@unime.it (C. Espro).
https://doi.org/10.1016/j.mcat.2017.12.038
2
468-8231/© 2017 Elsevier B.V. All rights reserved.

B. Gumina et al. / Molecular Catalysis 446 (2018) 152–160
153
C6-C2 polyol is surely ethanol, that is currently extensively pro-
duced through fermentation of glucose (generally derived from
corn or sugar cane [4]) and can be used as bio-fuel as well as feed-
stock to produce building block chemicals.
(III) nitrate nonahydrate was added dropwise into a 1 M aqueous
solution of Na CO . The obtained catalyst was filtered, washed and
2
3
◦
dried for 1 day under vacuum at 120 C. Before use, the catalyst
◦
was reduced at 200 C for 2 h under a hydrogen flow.
Biomass-derived polyols are characterized by a higher O/C ratio
with respect to fossil-derived feedstocks, because, in their struc-
ture, every carbon atom is linked to a hydroxyl group. Therefore,
deoxygenative technologies could face the need of the effective
reduction of the oxygen content allowing the production of new
high added value chemicals ready to be integrated into the modern
market chain [26,27].
The Pd/C catalyst was acquired from a commercial source (Alfa
Aesar) and used after reduction, under H2 flow, at 200 C for 2 h.
The main characteristics of the investigated Pd-based catalysts
are reported in Table S1.
◦
2
.2. Catalysts characterization
In this context, the catalytic hydrogenolysis is an easily avail-
able technology, which is gaining increasing attention since it
allows possible breaking of C O, C C, C H and O H bonds
XRD data were acquired at room temperature on a Philips X-
Pert diffractometer by using the Ni ␤-filtered Cu K␣ radiation
◦
(
␭ = 0.15418 nm) in the 2␪ range of 20–80 at a scan speed of
with the simultaneous addition of H . The process is typically
◦
−1
2
0.5 min .
ruled by dehydration, decarbonylation, retro-aldol condensation
and/or hydrogenation reactions [28,29]. Polyols hydrogenolysis is
a long time studied reaction, often carried out in severe oper-
ating conditions and in presence of hydroxides as base, to drive
the selectivity toward the desired products [6,28,30]. In particular,
sorbitol hydrogenolysis was deeply investigated by using cata-
lysts based on Ni, Ru, Cu, Pt metals [5,28]. Since the pioneering
work of I.T. Clark [31], nickel based catalysts have been widely
investigated for their high intrinsic activity [28,30,32–35]. Progres-
sively, the investigation was shifted toward ruthenium systems
that were found to show high performance both in conversion
and selectivity [28,36–43]. Heterogeneous copper catalysts repre-
sent another interesting alternative to promote the conversion of
polyols [44–46], being less active than Ni and Ru in reducing the
The particle size and the relative morphology of investigated
catalysts were analysed by performing Transmission Electron Mis-
croscopy (TEM) measurements using a JEM-2100F (JEOL, Japan)
operating at an acceleration voltage of 200 kV and directly
interfaced with a computer controlled-CCD for real-time image
processing. Particle size distributions were obtained by counting
several hundred particles visible on the micrographs on each sam-
ple. From the size distribution, the average diameter was calculated
by using the expression: dn = ꢀn d /n where n is the number of
i
i
i
i
particles of diameter di.
H -TPR measurements were performed using a conventional
2
TPR apparatus. The dried samples (50 mg) were heated at a linear
◦
−1
◦
rate of 10 C min from 0 to 1000 C in a 5vol% of H /Ar mixture at
2
3
−1
a flow rate of 20 cm min . H2 consumption was monitored with
a thermal conductivity detector (TCD).
C
C cleavage, so favouring C4-C6 products [47,48]. Besides, also
platinum was found very active in hydrogenolysis and hydrodeoxy-
genation of sorbitol [28,42,49–51].
XPS measurements were performed on a JPS-9010MC photo-
electron spectrometer using an Al K␣ (1486.6 eV) radiation source.
After the reduction treatment, samples were introduced into the
XPS chamber, avoiding exposure to air. All spectra were recorded
at room temperature, and the binding energies (BE) were set taking
the C 1 s peak at 284.6 eV as reference.
Bimetallic catalytic systems play a crucial role since they address
the selectivity towards a desired product, also starting from lig-
nocellulosic biomasses (cellulose, hemicellulose, lignin) [52–65].
Generally, the introduction of a second metal has been found
helpful to increase the performance [66–75]. Palladium based cat-
alysts, for example, were found commonly less reactive towards
hydrogenolysis of polyols [5,28]. However, addition of iron to pal-
ladium, through the co-precipitation technique, creates intimate
interactions that generate a synergistic effect [76,77] enabling a
remarkable reactivity in several important reactions for chemical
industry [78–94]. In recent years, some of the authors investigated
the hydrogenolysis of glycerol both in presence and in absence
2.3. Catalytic tests
Hydrogenolysis reactions were carried out in a 300 ml stain-
less steel autoclave at a stirring speed of 500 rpm. The reactor was
purged three times with He (99.99%) and subsequently pressurized
at the desired H2 pressure and heated at the reaction tempera-
ture, monitored using a thermocouple fixed into the autoclave and
connected to the reactor controller.
of added hydrogen by using the co-precipated Pd/Fe O4 cata-
3
lyst under mild operating conditions. With the aim to extend the
substrate scope, in this paper, we evaluate the performance and
The range of operative conditions used for hydrogenolysis reac-
◦
tions was: 150–240 C, 5–20 bar initial H2 pressure, 80 ml of 4 wt%
the reaction pattern of the co-precipitated Pd/Fe O4 catalyst in
3
C6–C3 polyol aqueous solutions and 500 mg of reduced catalyst.
After 6–24 h of reaction, the system was cooled and, when at room
temperature, the pressure was released carefully and the liquid
phase analysed.
The reactant and products, in the liquid phase, were analysed
using an off-line Shimadzu HPLC equipped with an Aminex HPX-
the hydrogenolysis of sorbitol and lower polyols (such as xylitol,
erythritol, butanediols, glycerol, 1,2-propanediol). Under the con-
ditions adopted, C2-C3 alcohols were obtained in high selectivity
with ethanol becoming the main product in liquid phase at higher
reaction temperatures demonstrating also that the co-precipitated
Pd/Fe O system can be a suitable catalyst for the chemical valori-
3
4
8
7-H column and a TOC analyzer Shimadzu, in order to confirm
sation of bio-derived polyols.
the carbon balance in the liquid phase. The gas phase products
were analysed with a GC (Agilent 7890A) equipped with a molec-
ular sieve column (Supelco, Porapak Q column 80/100 mesh) and a
capillary column (HP-PLOT/Q, internal diameter: 0,53 mm; length:
2
. Experimental section
3
0 m; film thickness: 40 ␮) linked with the TCD detector and a capil-
2
.1. Catalysts preparation
lary column (HP-Al, internal diameter: 0.53 mm; length: 50 m; film
thickness: 15 ␮m) connected to a FID detector.
The conversion and product selectivity in the liquid phase were
calculated on the basis of the following equations:
All chemicals were purchased and used without further purifi-
cation.
The Pd/Fe O
catalyst was prepared through the co-
precipitation technique, designed with nominal palladium
loading of 5 wt%. An aqueous solution of palladium nitrate and iron
3
4
a
mol of reacted substrate
Conversion [%] =
× 100
mol of substrate feed

1
54
B. Gumina et al. / Molecular Catalysis 446 (2018) 152–160
mol of specific product in liquid phase
sum of mol of all product in liquid phase
Liquid phase selectivity [%] =
× 100
The products yield was calculated on carbon basis and defined
as:
mol of specific product × C atoms in specific product
Product Yield [%C] =
× 100
mol of total C atoms in substrate
The carbon balance at the end of each reaction was confirmed
by using a liquid Total Organic Carbon analyzer (Shimadzu TOC-
VCSH). Through the comparison of the carbon balance evaluated
by HPLC analysis and by TOC measurements it has been estimated
a percentage of carbon loss in liquid phase lower than 5%. The total
organic carbon of the gas phase, when calculated, was estimated
as the difference between the TOC measurements before and after
hydrogenolysis reactions. In all experiments, the analytical results
of GC analysis showed a carbon balance in gas phase higher than
9
5%.
3
. Results
3.1. Catalyst preparation and characterization
The Pd/Fe O
catalyst was prepared through the co-
3
4
precipitation of palladium nitrate and iron (III) nitrate nonahydrate,
simultaneously added dropwise into a 1 M aqueous solution of
Na CO . The precipitated solid was filtered, washed with distilled
2
3
water, calcinated overnight and reduced under a flow of molecular
◦
hydrogen at 200 C [81,84].
In the last decade, the co-precipitated Pd/Fe O has been deeply
3
4
analysed by several research groups, including that of authors,
and a detailed description of its physico-chemical and structural
properties was discussed in detail in different reports [76–95]. For
clarity, the main peculiarities of the co-precipitated Pd/Fe O cata-
3
4
lyst are here concisely summarized. The catalyst is characterized
by palladium nanoparticles ranging mainly between a diameter
of 1–2 nm (as revealed by the high-resolution transmission elec-
Fig. 1. Size distribution of the palladium nanoparticles and HR-TEM images of the
Pd/Fe3O4 catalyst.
tron microscopy, HR-TEM) well dispersed over the Fe O4 surface
3
(
as shown by X-ray diffraction (XRD), Fig. S1). H -TPR analysis of
2
the Pd/Fe O catalyst shows a profile having only an intense peak
3
4
◦
centred at about 80 C, in which reductions of either palladium
Pd(II) → Pd(0)] and iron [Fe(III) → Fe O ] are involved, indicating
(Xyl) and glycerol (Gly), followed by lesser amounts of ery-
[
3
4
thritol (Ery) and ethylene glycol (EG). When the temperature
increases, xylitol and glycerol are gradually converted into
smaller polyols such as butanediols (BDOs = 1,2-butandiol + 1,4-
butandiol + 2,3-butandiol), 1,2-propanediol (1,2-PDO), propanols
that Pd nanoparticles are effective in promoting the reduction of
hematite (Fe O ) into magnetite (Fe O ) (Fig. S2). Extended x-ray
2
3
3
4
absorption fine structure (EXAFS) characterization confirms for-
mation of Pd–Fe bimetallic ensembles, having the Pd-Fe distance
shorter (0.259 nm) than that of Pd–Pd atoms (0.266 nm) [78,81,83].
X-ray photoelectron spectroscopy (XPS) evidences that the Pd 3d5/2
binding energy is 0.5 eV higher than the binding energy of metal-
(
(
POs = 1-propanol + 2-propanol), ethylene glycol (EG) and ethanol
EtOH). It is worth noting that, as the reaction proceeds toward
products having lower molecular weight, the quantity of ethanol
progressively increases, so that it becomes the main product, in
lic Pd, denoting the presence of partial positively charged metal
◦
liquid phase, at 240 C (63.5% mol-based selectivity, 25.4% carbon-
␦
+
species (Pd ) (Table S1) [81,95,96]. All these findings indicate the
presence of Pd-Fe ensembles/alloy and that strong interactions
between palladium nanoparticles and the iron oxide support are
based yield).
On the contrary, the commercial Pd/C exhibits a significant
◦
lower activity. Indeed, at 210 C the sorbitol conversion is less than
present on the Pd/Fe O catalyst Fig. 1.
◦
3
4
30% and the maximum is 60% at 240 C, the highest temperature
investigated. The selectivity pattern, on increasing the temper-
3.2. Catalytic tests on the hydrogenolysis of sorbitol
ature, follows a similar trend as that reported for the Pd/Fe O4
3
catalyst; furthermore, a much lower ethanol yield is always reg-
◦
The catalytic performance of the bimetallic Pd/Fe O4 catalyst
istered (12.0% carbon-based yield at 240 C–Table S2.2).
3
was compared with that of the commercial Pd/C and results are
reported in Table 1 and Table S2.2.
Furthermore, pure Fe O4 was tested within the same tempera-
3
ture range and no appreciable sorbitol conversion was found.
The lack of significant reactivity using either Pd/C or pure Fe O ,
The bimetallic Pd/Fe O4 catalyst shows excellent perfor-
3
3
4
◦
mances affording an almost complete conversion at 180 C (91%)
and a total conversion (100%) at 210 C. The distribution pat-
confirms that the marked activity shown by the bimetallic Pd/Fe O4
3
◦
catalyst has to be attributed to the strong interaction between
palladium and iron, as consequence of the preparation method (co-
precipitation) [76,77] in analogy with other reports attaining to C3
and C2 polyols.
tern of products, in liquid phase, was found to change within
the temperature range investigated. Indeed, at lower tempera-
◦
◦
tures (150 and 180 C) the main reaction products are xylitol

B. Gumina et al. / Molecular Catalysis 446 (2018) 152–160
155
Table 1
Conversion of sorbitol promoted by Pd/Fe3O4 and Pd/C catalysts in presence of hydrogen. Conditions: 0.5 g catalyst; 80 ml sorbitol solution 4% wt; 5 bar of initial H2 pressure;
2
4 h.
◦
Catalyst
Temp [ C]
Conv. [%]
Liq.Ph. [%]
mol-based selectivity [%] of liquid phase products
C5
C4
C3
C2
EG
Xyl
Eryth
1,2-BDO
1,4-BDO
2,3-BDO
Gly
1,2-PDO
1-PO
2-PO
EtOH
OP
Pd/Fe3O4
Pd/C
Pd/Fe3O4
Pd/C
Pd/Fe3O4
Pd/C
Pd/Fe3O4
Pd/C
240
240
210
210
180
180
150
150
100.0
60.0
100.0
28.0
91.0
13.6
17.0
<1
40.0
44.0
60.0
100.0
80.0
100.0
100.0
100.0
0.0
0.0
0.0
0.0
5.5
0.0
33.0
–
0.0
0.0
0.0
0.0
1.8
0.0
7.0
–
0.0
3.2
8.4
11.4
7.2
12.0
0.5
–
0.3
0.0
0.0
0.0
0.0
0.0
0.0
–
2.8
2.0
7.4
2.1
5.0
0.0
0.0
–
0.0
0.0
1.2
4.3
19.5
18.0
35.0
–
5.9
13.0
13.4
10.0
4.2
2.0
0.0
0.0
–
1.0
0.0
0.0
0.0
0.0
0.0
0.0
–
0.0
12.4
8.1
26.1
13.4
22.0
8.0
63.5
45.4
41.3
16.9
8.5
6.0
0.5
–
13.5
3.2
1.6
0.0
9.2
0.0
6.0
–
20.4
22.0
35.0
27.9
42.0
10.0
–
–
Xyl: xylitol; Eryth: erythritol; 1,2-BDO: 1,2-butandiol; 1,4-BDO: 1,4-butandiol; 2,3-BDO: 2,3-butandiol; Gly: glycerol; 1,2-PDO: 1,2-propanediol; 1-PO: 1-propanol; 2-PO:
2
-propanol; EG: ethylene glycol; EtOH: ethanol; OP: other products.
◦
◦
◦
Fig. 2. Schematization of gas phase composition at 180 C (20%), 210 C (40%) and 240 C (60%) after 24 h of reaction using the Pd/Fe3O4 as heterogeneous catalyst.
Gaseous products were also quantitatively analysed, pulling
out the gas phase from the reaction vessel, cooled down at room
temperature, after every experiment without accounting for the
plete; however, the decrease of ethanol selectivity in liquid phase
was ascertained (Fig. 3, bottom). A similar effect was also observed
at 180 and 210 C (Fig. S4). The result suggests an inhibitory effect,
◦
amount of molecular hydrogen, added as pure reactant (5 bar at
on the hydrogenolysis reaction, of the hydrogen pressure, probably
ascribed to a competitive adsorption of substrate and hydrogen on
the catalyst surface [84–97].
◦
2
5 C) at the start of each reaction. The amount of gas products rises
◦
on increasing the reaction temperature (180–240 C), although its
composition was found quite constant, being composed by mainly
of carbon dioxide and only small amounts of alkanes (methane,
ethane and propane). Gaseous compositions, detected at different
temperatures, are depicted in Fig. 2 (the detailed distribution of liq-
uid and gas products–including carbon-based yield–is available in
Table S2.1). It is worth to highlight that carbon monoxide was never
3.3. Catalytic tests on the hydrogenolysis of C5-C3 polyols
In order to get a complete reactions pattern, the hydrogenolysis
of C5-C3 polyols, over the Pd/Fe O4 catalyst, was also investigated
3
under the same operative conditions. The results obtained at 180
detected in the gas phase, indicating that the Pd/Fe O catalyst is
◦
3
4
and 210 C are reported in S.I. (Sections 2.7 and 2.8), whereas the
also effective in promoting the water gas shift (WGS) reaction, as
previously observed in the hydrogenolysis of glycerol and ethylene
glycol, carried out under APR conditions [84].
◦
results relative to 240 C are detailed in Table 2 and Table S2.3.
◦
The conversion of all C5-C3 polyols at 240 C is very high rang-
ing from 78.7 to 100% except for 2,3-butanediol (2,3-BDO) that, as
expected, is a stable molecule and does not give any appreciable
reactivity [98]. From the products distribution in liquid phase, it is
quite clear that ethanol is mainly formed from xylitol, 1,4-BDO, 1,2-
PDO and glycerol hydrogenolysis, demonstrating that, under the
reaction conditions adopted, both C O and C C cleavages simulta-
neously occur.
Xylitol is mostly converted into C2 and C3 alcohols, respectively
ethanol (64.1% mol-based selectivity, 25.6% carbon-based yield)
and POs (16% mol-based selectivity, 8.6% carbon-based yield) and,
in a lesser amount, di-alcohols, such as butanediols and 1,2-PDO.
◦
Considering the 240 C value as a significant reaction tempera-
ture to maximize the ethanol production in liquid phase, sorbitol
hydrogenolysis was also investigated at different reaction times
(
3, 6, 12 and 24 h). The sorbitol conversion was complete already
after 3 h of reaction, although the ethanol selectivity in liquid phase
increases as the reaction proceeds from 3 to 24 h (Fig. 3, top). Results
obtained from investigations at different reaction times at 180 and
◦
2
10 C are reported in S.I (Sections 2.4 and 2.5)
The effect of the hydrogen pressure increase from 5 to 20 bar was
◦
also investigated. At 240 C the conversion of sorbitol remains com-

1
56
B. Gumina et al. / Molecular Catalysis 446 (2018) 152–160
Fig. 3. Reaction time (top) and initial H2 pressure (bottom) effects on the product distribution obtained in the sorbitol conversion (selectivity%) and carbon liquid phase (%)
◦
in the reaction carried out in presence of H2 at 240 C.
Table 2
◦
Conversion of C5-C3 polyols promoted by the Pd/Fe3O4catalystunder hydrogen pressure. Conditions: 240 C, 0.5 g catalyst; 80 ml polyols solution 4% wt; 5 bar of initial H2
pressure; 24 h.
Entry
Polyol type
Conv.
Liq. Ph.
mol-based selectivity [%] of liquid phase products
C4
C3
C2
EG
[%]
[%]
1,2-BDO
1,4-BDO
2,3-BDO
1,2-PDO
1-PO
2-PO
EtOH
OP
Xyl
Eryth
C5
C4
100.0
100.0
98.0
100.0
0.0
40.0
36.0
63.0
73.0
100.0
57.2
67.7
2.4
2.1
0.0
0.0
0.0
–
0.0
0.0
0.0
0.0
0.0
–
2.9
15.1
0.0
0.0
0.0
–
5.4
9.1
18.0
0.0
0.0
9.3
0.0
16.0
31.0
80.0
6.5
0.0
8.0
0.0
1.0
0.0
0.0
0.0
2.0
1.0
1.2
0.8
0.0
0.0
0.0
0.5
0.0
64.1
34.3
1.0
92.5
0.0
8.0
6.6
1.0
1.0
0.0
9.7
1.4
1
1
2
,2-BDO
,4-BDO
,3-BDO
Gly
,2-PDO
C3
100.0
78.7
70.5
90.0
1
–
–
–
7.6
Xyl: xylitol; Eryth: erythritol; 1,2-BDO: 1,2-butandiol; 1,4-BDO: 1,4-butandiol; 2,3-BDO: 2,3-butandiol; Gly: glycerol; 1,2-PDO: 1,2-propanediol; 1-PO: 1-propanol; 2-PO:
2
-propanol; EG: ethylene glycol; EtOH: ethanol; OP: other products.
Erythritol follows a similar pattern affording C2 and C3 alcohols
C3 polyols reactivity, in analogy with our previous report [84],
follows the order: glycerol (100%) > 1,2-PDO (78.7%) being both
very selective in ethanol production.
in almost equal amount. However, di-alcohols are also obtained,
being BDOs the most abundant products, confirming the efficiency
of erythritol in BDOs manufacture [98].
Investigated BDOs show a different pattern in the hydrogenol-
ysis reaction. 1,2-BDO produces preferentially 1-propanol (80%
mol-based selectivity, 49.4% carbon-based yield) followed by a
smaller amount of 1,2-PDO (18% mol-based selectivity, 11.1%
carbon-based yield). On the contrary, 1,4-BDO breaks preferentially
the C2-C3 bond leading to an ethanol mol-based selectivity, that
slightly overcomes 90% (67,5% carbon-based yield). As previously
mentioned, the 2,3-BDO does not give any reactivity.
4
. Discussion
4.1. Understanding the C O and C C bond breaking: the reaction
mechanism
Consistent with the catalytic results obtained from the
hydrogenolysis of sorbitol and C5-C3 polyols, a global reaction
pathway, promoted by the bimetallic co-precipitated Pd/Fe O cat-
3
4
alyst, is proposed in Fig. 4.

B. Gumina et al. / Molecular Catalysis 446 (2018) 152–160
157
Fig. 4. Schematic representation of cascade reactions involved in the sorbitol hydrogenolysis carried out over Pd/Fe3O4 catalyst.
It is well accepted that the initial reaction step in the
hydrogenolysis of polyols is the dehydrogenation of primary
tions such as dehydrogenation/decarbonylation and dehydra-
tion/hydrogenation occur, leading, to final products including
butanediols and C2-C3 alcohols, particularly ethanol.
or secondary alcoholic group, leading to an aldehyde or
a
ketone [99–101]. In the light of observed reaction products, the
hydrogenolysis of sorbitol follows, at the beginning, two main
alternative ways: (i) the dehydrogenation/decarbonylation of ter-
minal C OH groups and (ii) the cleavage of internal C C bonds via
dehydration/retro-aldol reaction.
The first steps in sorbitol hydrogenolysis is sketched in
Scheme 1. Three possible reaction routes have to be considered. In
the case of an adsorption of a primary hydroxyl group on the cata-
lyst surface, the dehydrogenation process allows at first formation
of glucose (open chain). In the following reaction pathway leading
to short the polyols chain, the terminal C CO breaking enables xyl-
The contribution of other reaction mechanisms, such as the
dehydration of sorbitol into cyclic compounds (e.g. isosorbide and
1,4-sorbitan) was not found to be operating in the reaction network
over the Pd/Fe O4 catalyst.
3
Therefore, sorbitol appears to be simultaneously and consec-
◦
utively subjected to two alternative patterns, as data at 150 C,
where the conversion degree is low but useful to appreciate
the initial steps, demonstrate (Table 1). The dehydrogena-
tion/decarbonylation of terminal C OH groups produces xylitol
(33% mol-based selectivity), while the retro-aldol condensation
operates on the internal chain either breaking the C3-C4 bond,
that forms two molecules of glycerol (35% mol-based selectivity
itol production (Route A). On the other hand, if the internal C
C
◦
bond of glucose is cleaved by a retro-aldol mechanism erythritol
and ethylene glycol are formed as reaction products (Route B).
at 150 C) or, to a lesser extent, breaks the C2-C3 bond produc-
ing erythritol (7% mol-based selectivity) and ethylene glycol (8%
mol-based selectivity).
Conversely, if
a secondary hydroxyl group is dehydro-
genated, the retro-aldol cleavage of C3-C4 bond affords two
molecules of glycerol (Route C). Subsequently, consecutive reac-
Xylitol is, in turn, converted into erythritol through dehydro-
genation/decarbonylation, or through retro-aldol condensation,
Scheme 1. First steps in sorbitol conversion: the dehydrogenation is followed by retro-aldol condensation or decarbonylation reactions.

1
58
B. Gumina et al. / Molecular Catalysis 446 (2018) 152–160
affording glycerol and ethylene glycol. Production of butanediols
probably stems from erythritol rather than directly from xyli-
tol. Indeed, erythritol, through the dehydration/hydrogenation,
leads to BDOs in considerable amount, while, through retro-aldol
condensation, ethylene glycol is obtained. Glycerol undergoes
successive hydrogenolysis reactions leading, in liquid phase, preva-
lently to POs and ethanol.
Butanediols (BDOs) react through different hydrogenolysis
patterns depending on the isomer considered. The 2,3-BDO
does not give any appreciable conversion in the reactive
conditions investigated. The 1,2-BDO, probably through dehydro-
genation/decarbonylation, affords mainly 1-propanol whereas the
sorbitol in analogy with a recent report on the use of simple pri-
mary and secondary alcohols as H-donor molecules for the transfer
hydrogenolysis of benzyl phenyl ether [88].
5. Conclusions
The catalytic conversion of biomass-deriving sorbitol and C5-
C3 polyols was investigated under mild hydrogenolysis conditions
over the bimetallic Pd/Fe O4 catalyst, exhibiting a higher perfor-
3
mance compared to that of the Pd/C catalyst. The hydrogenolysis
reactions of C5-C3 polyols with the Pd/Fe O4 catalyst were also
3
carried out in order to completely understand the main pathways
occurring in the reaction.
The starting reactions were found to be the dehydrogena-
tion/decarbonylation of the terminal C COH group and the
1
,4-BDO through retro-aldol condensation breaks the C2-C3 bond
producing ethanol, and leads, marginally, to POs through dehydro-
genation/decarbonylation.
The glycerol detected during the hydrogenolysis of sorbitol,
almost entirely, derives from the initial retro-aldol condensation of
sorbitol and only in slight percentages, from xylitol and erythritol
hydrogenolysis. Furthermore, glycerol dehydration/hydrogenation
or dehydrogenation/decarbonylation affords 1,2-PDO and EG,
respectively. 1,2-PDO then leads to POs or ethanol, after dehydra-
tion/hydrogenation or dehydration/decarbonylation, respectively.
Finally, the hydrogenolysis of ethylene glycol could lead to ethanol
or methanol.
dehydrogenation followed by the retro-aldol reaction (internal C
C
bond cleavage). At the beginning, the sorbitol carbon chain is pref-
erentially shortened leading to xylitol and C3 or C4-C2 polyols. After
the initial cleavage of sorbitol, a series of cascade reactions (such as
H /-CO, H O/H , retro-aldol condensation and hydrogenation)
2
2
2
occur, leading to production of shorter diols, alcohols and gaseous
products. A similar cascade pattern of reactions leads to the prefer-
ential formation of ethanol, at the highest temperature investigated
◦
(
240 C).
It is worth noting that, at the beginning of the reaction, hav-
With the exception of 2,3-butanediol, C2-C3 alcohols are
the main reaction products obtained by the hydrogenolysis of
the investigated substrates suggesting that the co-precipitated
ing 5 bar of H2 pressure, the initial molar ratio of H /sorbitol is
2
about 2,5. However, in order to perform the complex pattern of
reactions observed (Fig. 4) the amount of H2 should be higher than
that available on the vessel volume.
Pd/Fe O4 catalyst can be a suitable catalyst for the production of
3
ethanol from bio-derived C6-C3 polyols.
Nevertheless, it is useful to remember that H can be also formed
2
Finally, the gas phase analysis, in all experiments, shows the
total absence of CO revealing the excellent performance of the
Pd/Fe O catalyst in promoting the water-gas-shift (WGS) reaction.
by APR of sorbitol itself (as well as from the other polyols formed
through the hydrogenolysis process):
3
4
CxOxH2
+ xH O → xCO + (2x + 1)H
(x+1)
2
2
2
Appendix A. Supplementary data
where the stoichiometry involves the sum of the two reactions:
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.mcat.2017.12.
(
1) Reforming of polyol into hydrogen and carbon monoxide
(
CxOxH(2x+1) → x CO + (x + 1) H )
2
0
38.
(
2) Water Gas Shift (WGS) reaction of carbon monoxide into carbon
dioxide and hydrogen (xCO + x H O → x CO + x H )
2
2
2
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