Production of propylene glycol (1,2-propanediol) by the hydrogenolysis of glycerol in a fixed-bed downflow tubular reactor over a highly effective Cu-Zn bifunctional catalyst: Effect of an acidic/basic support

Article abstract of DOI:10.1039/c9nj01180c

In this study, different acidic (V₂O₅, ZrO₂, and TiO₂) and basic (CaO and MgO) oxide-supported copper-zinc bimetallic catalysts were prepared by a deposition-precipitation method, and they were evaluated for the vapor-phase hydrogenolysis of glycerol to propylene glycol at 0.1 MPa and at 220 °C. The catalysts were thoroughly characterized by different techniques such as BET, XRD, H₂-TPR, NH₃ and CO₂ TPD, N₂O adsorptive decomposition, TEM, XPS, FE-SEM and TGA. Among all the supported catalysts, the Cu-Zn/MgO catalyst was found to be the most selective to propylene glycol. High copper-metal dispersion (～5%), surface area (～23 m² g^-1), the highest basicity (0.25 mmol CO₂ g cat^-1) and the availability of partially reduced copper species (Cu₂O, CuO and Cu⁰) were the primary reasons for the higher propylene glycol selectivity. At optimum reaction conditions, i.e., at 220 °C, 0.72 MPa and a weight hourly space velocity (WHSV) of 0.073 h^-1, ～98.5% conversion of glycerol with ～89% selectivity to propylene glycol was obtained over the Cu-Zn/MgO catalyst. This catalyst was also found to be stable for a long period of time (84 h) without much deactivation and without decrease in the selectivity to propylene glycol.

Full text of DOI:10.1039/c9nj01180c

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Production of propylene glycol (1, 2-propanediol) by the hydrogenolysis of glycerol in a fixed
bed downflow tubular reactor over highly effective Cu-Zn bifunctional catalyst: Effect of
acidic/basic support
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Dinesh Kumar Pandey and Prakash Biswas*
Department of Chemical Engineering, Indian Institute of Technology Roorkee, Roorkee-247667,
Uttarakhand, India.
*Author to whom correspondence should be addressed: email: prakbfch@iitr.ac.in;
prakashbiswas@gmail.com,
Tel.: (+91)-1332-285184; Fax: +91-1332- 276535
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Abstract
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In this study, different acidic (V2O5, ZrO2, TiO2) and basic (CaO, MgO) oxides supported
copper-zinc bimetallic catalyst were prepared by deposition-precipitation method and evaluated
for vapor phase hydrogenolysis of glycerol to propylene glycol at 0.1 MPa and at 220oC. The
catalysts were thoroughly characterized by different techniques such as BET, XRD, H2-TPR,
NH3 and CO2 TPD, N2O adsorptive decomposition, TEM, XPS, FE-SEM and TGA. Among all
the supported catalysts, Cu-Zn/MgO catalyst was found to be the most selective to propylene
glycol. High copper metal dispersion (~5%), surface area (~23 m2. g-1), highest basicity (0.25
mmol CO2. g.cat-1) and the availability of partially reduced copper species (Cu2O, CuO and Cuo)
were the primary reasons for higher propylene glycol selectivity. At optimum reaction condition
i.e. at 220oC, 0.72MPa, and at the weight hourly space velocity (WHSV) of 0.073 h-1, ~98.5 %
conversion of glycerol with ~89% selectivity to propylene glycol was obtained over Cu-Zn/MgO
catalyst. The catalyst was found to be stable for longer period of time (84 h) without much
deactivation and affecting the selectivity to propylene glycol.
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Keywords: Glycerol hydrogenolysis, vapor phase, sectional packed downflow reactor, Cu-
Zn/MgO catalyst, propylene glycol.
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1. Introduction
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Due to the scarcity of petroleum reserves and environmental issues, biodiesel has been
regarded as one of the promising and acceptable alternatives to the conventional diesel.
However, the major concern of the modern biodiesel industries is the value addition of the excess
amounts (~10 wt.%) of low-grade glycerol obtained as a by-product [1,2]. The various routes for
glycerol value addition have been reviewed and documented [3,4]. Among these, hydrogenolysis
of glycerol to propylene glycol (1,2-propanediol) has received much attention in the last two
decades because of very high demand of 1,2-propanediol (1,2-PDO) [5,6]. 1,2-PDO is a valuable
chemical and widely used as a feedstock for the manufacture of unsaturated polyester resin, as a
functional fluid, paints, cosmetics, starting materials in pharmaceuticals, printing etc. [5]. The
production of 1,2-PDO from glycerol is not only a lucrative to biodiesel producer but also a
green route as compared to the current commercial hydration of propylene oxide derived from
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petroleum resources [7,8]
.
Glycerol hydrogenolysis was attempted over various supported noble and non-noble
metal catalysts, primarily in a liquid phase as summarised by various authors [5,6,9]. Therein noble
metals were reported as less selective to 1,2-PDO due to their more affinity towards the C-C
bond cleavage [10-13]. While among transition metals, copper-based catalysts were reported as
more active and selective towards 1,2-PDO [14-18]. Hydrogenolysis in the liquid phase has many
limitations including very high reaction pressure (0.1-8.0 MPa) [19, 20], structural damage to
catalyst due to hydrolytic effect, difficulties in catalyst separation, catalyst leaching etc., which
impedes its commercialization. Therefore, vapor phase hydrogenolysis of glycerol to 1,2-PDO in
a continuous flow fixed bed reactor has gained significant interest now a day. As of now, very
limited numbers of research publications are available on the production of 1, 2-PDO in vapor
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phase hydrogenolysis of glycerol in a continuous flow reactor [21-30]. Akiyama and his co-workers
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[22, 31]
reported 97% selectivity to 1, 2-PDO in presence of a commercial Cu/Al2O3 catalyst. The
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experiments were conducted in a continuous reactor at various gradient temperature. 1, 2-PDO
yield was increased to ~2% in presence of Ag as a catalyst promoter [24]. Ba doped Cu-Cr
catalyst rendered higher catalytic activity, however, 1,2-PDO selectivity was marginally
improved, the addition of Ba increased acidic sites on the catalyst surface [27]. Huang et al.
2008[32] developed Cu/ZnO/Al2O3 catalyst by precipitation method and reported 96.2 %
conversion of glycerol with 92.2 % selectivity to 1,2 PDO at 190◦C, 0.64 MPa. It was suggested
that acid catalyzed dehydration of glycerol proceed over metal oxides (ZnO and Al2O3) and
hydrogenation of intermediate was facilitated by copper metal. SiO2 supported Cu catalyst
prepared by ion-exchange technique showed 87% selectivity to 1,2-PDO at 255 oC and 1.5 MPa
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pressure, the catalytic activity was found to be dependent to the metallic surface area [23]
.
Most of these studies were carried out at higher H2 to glycerol mole ratio and they were
devoid of stability analysis. Mitaa et al. (2015) [33] reported 90% conversion of glycerol with
84% selectivity to 1,2-PDO in presence of Cu/SBA-15 catalyst. However, catalyst deactivation
was observed after 10 h of reaction. Ni-based catalysts were found to be poor selective to 1,2-
PDO, nickel metal promoted C-C bond breakage and lower alcohol, ethylene glycol, CH4 and
CO were detected as a major product [22,32,34]. Although Cu based catalyst shown excellent
activity, however, there are sufficient evidence for their poor stability. Since the long-time
stability is the prime requirement for a commercial catalyst, therefore, various techniques have
been employed to achieve higher stability of copper metal [35]. Various studies were performed
[35,
over a series of Cu-ZnO based catalysts, where ZnO was reported as an anti-sintering agent
36]. For glycerol hydrogenolysis reaction particularly it was observed that ZnO promoted
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hydrogenation capability of the catalyst [37] because ZnO adsorbed atomic hydrogen and acted as
a hydrogen reservoir [56]. ZnO also promoted CuO reduction in Cu/ZnO catalyst [37]. Previous
literature also suggested that incorporation of ZnO enhanced the acidity and basicity of the
catalyst, which promoted the catalytic activity significantly [55]. It was proposed that glycerol
hydrogenolysis proceed with glycerol dehydration to acetol via acid protonation of terminal
hydroxide group, and further acetol hydrogenated to 1,2-PDO in presence of active metal sites of
the catalyst [38,55,59]. While in basic condition glycerol first dehydrogenated to glyceraldehyde
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which subsequently dehydrated to 2-hydroxyacrolein. Further, the hydrogenation of 2-
[39,40]
hydroxyacrolein produced 1,2-PDO
Given the fact that glycerol dehydration facilitates on
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the acidic and basic sites, various mild and strong acidic oxide such as Al2O3, SiO2, boehmite[41]
,
[45]
, Cr2O3, ZnO, ZnO/ Al2O3 [31], HY, H-β, H-ZSM [42], Al2O3[43], ZnO[44] γ-Al2O3 SiO2
and
basic oxide (MgO[46], CeO2[21]), modified solid base( Mg-Al-O)[47] supported catalysts were
developed and their performance was discussed in terms of the nature of acidic/basic sites
present of the catalysts. It was shown that in presence of bronsted acid sites glycerol converted to
acrolin and lower alcohols whereas glycerol dehydration to acetol was favoured in presence of
Lewis acid sites [41, 48- 50].
In contrast to that, few studies reported that the dehydration of glycerol to acetol was
favoured in presence of active metal sites and independent to the acidic sites of the catalysts [20-22,
51]. Moreover, there is another opinion about the dehydration of glycerol to acetol. Cu species
present as Cu+ protonated the glycerol to from acetol via dehydration and copper species at the
[62]
state of Cu0, activated the hydrogen molecules [63]. Further, Vila et al
also explained that in
reduced Cu/Al2O3 catalyst, the ratio of Cuo / Cu + was the governing factor for the conversion of
glycerol to acetol and 1,2-PDO, respectively. In this context and aforesaid background,
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development of a highly stable and bi-functional catalyst, capable of catalyzing glycerol
dehydration and hydrogenation of dehydrated product to 1,2-PDO in vapour phase is highly
desirable.
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In this study, copper-zinc bimetallic catalysts supported on various acidic (TiO2, V2O5,
ZrO2) and basic (MgO, CaO,) oxide were synthesized by the deposition-precipitation method.
The effect of support on the catalytic performance was evaluated and compared in a sectional
packed, downflow tubular reactor. MgO supported copper-zinc catalysts was found to be most
active, selective and stable. Almost complete conversion (~98.5%) with very high selectivity
(~89%) to 1,2-PDO was obtained at very low reaction pressure (0.72 MPa). The presence of
acidic sites (Cu+ and Cu++) distributed over the larger surface area of MgO supported catalyst
and the availability of Cuo species, metal dispersion, and basicity were correlated to high
selectivity to 1,2-PDO. The structural stability of the MgO supported catalyst was examined by
the time-on-stream (TOS) study. The results demonstrated that the catalyst was very stable and
selective to 1,2-PDO for 84 h.
2. Catalyst preparation and characterization
For catalyst synthesis, Cu (NO3)2.3H2O (99.5%, Thomas Baker, India), Zn (NO3)2.6 H2O
(98.5%, Thomas Baker, India) were used as metal precursors and MgO light (98 %, Thomas
Baker, India), CaO (95 %, Thomas Baker, India), V2O5 (99 %, Thomas Baker, India), ZrO2 (99%,
Thomas Baker, India), TiO2 (98 %, Thomas Baker, India) were used as catalyst support.
NaHCO3 (99.9 %, Thomas Baker, India) was used as the precipitating agent. The standard
chemicals such as glycerol (99.9%, Thomas Backer Pvt. Ltd. India) 1, 2-PDO (99.99%, Merck
Specialities, India), acetol (95%, Alfa Aesar), ethylene glycol (99%), 1-propanol (99%), 2-
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propanol (99.5%), ethanol (99%) and methanol (99%) were used for calibration. Hydrogen
(99.99%) and nitrogen (99.99%) were procured from Sigma Gases New Delhi, India.
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Cu-Zn bimetallic catalyst supported on MgO lights, CaO, V2O5, ZrO2, and TiO2 were
synthesized by deposition-precipitation technique [30]. For all the catalyst, copper to zinc metal
weight ratio was 7:3, and the total metal loading was kept constant at 50 wt.% in the catalyst.
During the synthesis, the required amount of metal precursors were dissolved in distilled water
and metal hydroxides were precipitated by the dropwise addition of 1 (M), NaHCO3 solution
under continuous stirring till the solution pH reached to 8-9. Further, the required quantity of
support oxide was added and the slurry was stirred for 6 h followed by aging at ambient
condition for 12 h. After aging, the upper layer of liquid was spilled out and remaining thick
slurry was washed with distilled water and filtered in a vacuum filter. The filtered cake was dried
at 120oC for 12 h followed by calcination at 400oC for 4 h in air atmosphere. The synthesized
catalysts were designated as Cu-Zn/MgO, Cu-Zn/CaO, Cu-Zn/TiO2, Cu-Zn/V2O5, and Cu-
Zn/ZrO2.
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The physicochemical properties of the catalysts were characterized by Nitrogen
adsorption-desorption technique, X-ray diffraction (XRD), temperature-programmed reduction
(TPR), temperature-programmed desorption (TPD), N2O chemisorption and scanning
photoelectron spectroscopy (XPS). The morphology of the catalysts was obtained by field
emission scanning electron microscope (FE-SEM) and transmission electron microscope (TEM).
Nitrogen adsorption-desorption isotherms of all the catalysts were obtained at liquid nitrogen
temperature (-196oC) in a Micromeritics ASAP 2020 instrument. Prior to each analysis, catalyst
sample was vacuumed degassed at 200oC for 3 h. XRD pattern of fresh calcined and reduced
catalysts were recorded in Bruker AXS D8 Advance diffractometer with Ni filtered Cu-K훼
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monochromatized radiation source (휆 = 1.5418 Å). Diffraction patterns were obtained at the 2θ
interval of 10-90o with the steps of 0.02o/s and a time constant of 3 s. The reduction behavior of
the catalysts was determined in a Micromeritics Pulse Chemisorb 2720 instrument following the
standard procedure mentioned in our previous study [17,30, 67]. The morphology of the catalysts
was obtained by FE-SEM (Carl Zeiss Ultra Plus microscope) and by TEM (Tecnai G2 20 S-Twin
TEM, FEI model), respectively. To record Fe- SEM images, catalyst sample was spread
homogeneously over the sample holder and coated with gold using sputter coater (Edwards
S150). TEM image was recorded by dispersing sample in pure ethanol and put it on the copper
grid coated with carbon. The oxidation state of the active metals in the catalyst was determined
by XPS in a PHI 5000 Versa Probe III, XPS spectrometer.
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For XRD and XPS analysis of reduced catalyst, catalysts were reduced for 2 h under the
flow (20cc/min) of a gas mixture (10% Ar balance He) in a small quartz reactor at their
respective reduction temperature of catalysts obtained from TPR study. After reduction, catalyst
samples were flushed with He gas at same temperature for 1 h to remove trace impurity and
adsorbed H2 present in the catalyst. After completion of reduction process, catalyst sample was
kept in vacuum desiccators for XRD and XPS analysis.
TPD experiments were performed in Micromeritics Chemisorb 2750 machine. Prior to
the analysis, catalyst was degassed at 200oC under the flow of He (20 cc/min) for 1 h and
subsequently reduced in-situ at their respective reduction temperature obtained from TPR
experiments for 2h under the flow of a gas mixture (10% H2 and balance Argon). After
reduction, samples were flushed with He (20 cc/min) at the same temperature for 1 h in order to
remove trace of H2 remained adsorbed over catalyst surface and cooled to room temperature.
After cooling, catalyst samples were saturated with the respective analysis gases for 1 h. The
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analysis gas (10%NH3 balance He for NH3 TPD and pure CO2 for CO2 TPD) flow was
maintained at 20 cc/min. After the saturation, to remove physically adsorbed ammonia or CO2
He gas was flushed at a rate of 20cc/min for 1h. Thereafter, samples were heated from room
temperature to 1000oC at a temperature ramp of 10oC/min and desorption of gas at different
temperature was recorded with the help of a thermal conductivity detector.
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The percentage copper metal dispersion and copper metallic surface area in the catalysts
were measured by N2O chemisorption technique in Micromeritics Chemisorb 2750 machine
]
following the standard experimental procedure described in various literature [37, 72,
.
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3. Catalyst test
Hydrogenolysis of glycerol was conducted in a sectional packed, downflow tubular
reactor (Chemito Technologies, India), the detail description of the experimental set-up is
discussed elsewhere [29]. In a typical experiment, the catalyst was sectionally packed inside the
reactor with the help of glass wool. Prior to each experiment, the catalyst was reduced in-situ at
the reduction temperature of the individual catalyst obtained from TPR experiments for 3 h under
the flow of hydrogen (50 cc. min-1), followed by cooling the reactor at the desired reaction
temperature. The reactor was pressurized with the help of a back pressure regulator connected at
the reactor outlet. After achieving the desired reaction temperature and pressure inside the
reactor, the feed was introduced into the reactor through an evaporator with the help of a liquid
feed pump (Lab Alliance, USA, HPLC dosing type) and pre-mixed with the gas stream at the
reactor inlet. The reaction was carried out at different conditions. Reaction pressure, temperature
and weight-hourly-space-velocity (WHSV) was varied in the range of 0.1-0.8 MPa, 180-240oC,
and 0.061-0.37 h-1, respectively. The gas and liquid products were separated in a gas-liquid
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separator connected at the outlet of the reactor. The products were analyzed in an online (GC
5765 Nucon, India) and offline (GC 6800, Newchrom Technologies, India) gas chromatography
(GC), respectively. The online GC was equipped with a packed column (Porapak-Q) and a
thermal conductivity detector (TCD). The offline GC was equipped with a flame ionization
detector (FID) and a Chromosorb-101 packed column. Butanol was considered as an internal
standard for product selectivity calculation. For, all the data reported in this study, the carbon
balance was agreed 100 ± 5 %. The reported glycerol conversion, product selectivity, and
product yields were calculated following the equations [(1)-(3)].
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Glycerol conversion (%) = [(Initial moles of glycerol – Final moles of glycerol)/Initial moles of
glycerol] × 100
(1)
Product selectivity (%) = [Moles of carbon in the specific product/Total carbon in all the
products] × 100
(2)
Yield (%) = [Conversion (%) × selectivity (%)]/100
(3)
4. Results and discussion
4.1.
Catalyst characterization
Nitrogen adsorption-desorption isotherms of copper-zinc bimetallic catalysts with
different supports are shown in Figure 1. Typical type-IV isotherms were observed for
irreversible capillary evaporation of condensed N2 in the mesoporous structure of the catalysts
[50]. A pronounced hysteresis loop at a relative pressure range of 0.5<P/Po<0.90 were observed.
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This result demonstrated a typical mesoporous nature of catalysts with hysteresis loop
intermediate to type H3 and H4, indicated the presence of aggregate of plate-like particles with
narrow slit-like pores within the catalysts [51]. The obtained BET surface area (SBET), total pore
volume (Vp), Barrett–Jovner–Halenda (BJH) pore diameter (DBJH) of catalysts are summarized in
Table 1. The BET surface area was measured at the relative pressure (P/Po) ranging from 0.05-
0.3 and pore volume and pore size were calculated at P/Po of 0.987. The highest BET surface
area of 55.5 m2 g-1 and pore volume of 0.17 cm³/g were obtained for MgO supported catalyst
while the surface area (5.6 m2. g-1) and pore volume (0.05 cm³.g-1) of CaO supported catalyst
was lowest. The largest size pore (41.6 nm) obtained for CaO supported catalyst indicated the
formation of large clusters of particles while pore size of other catalysts was in the range of 12.1-
20.5 nm.
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Adsorption
Desorption
Cu-Zn/MgO
Cu-Zn/ TiO
2
Cu-Zn/ZrO
2
Cu-Zn/CaO
Cu-Zn/V O
2 5
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
Relative pressure (P/P )
o
Figure 1. Nitrogen adsorption-desorption isotherms of catalysts. (a) Cu-Zn/MgO, (b) Cu-
Zn/TiO2, (c) Cu-Zn/ZrO2, (d) Cu-Zn/CaO, (e) Cu-Zn/V2O5.
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XRD
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X-ray diffraction pattern of fresh calcined catalyst is displayed in Figure 2A. For all the
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catalysts, the peaks corresponding to CuO was detected at the 2θ value of 35.5o and 38.8o
corresponding to (111) crystal plane [JCDPS-80-1916]. The characteristics peaks of ZnO were
detected at the 2θ value of 31.7o, 34.6o, 56.5 o, 62.8o, 67.8o corresponding to (100), (002), (110),
(103) and (112) crystal planes, respectively [JCPDS-87-0075]. The diffraction peaks identified
for various supports and their corresponding 2θ values are marked in Figure 2A.
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The diffraction pattern of the reduced catalysts is shown in Figure 2B. The reduction
temperature of individual catalyst was decided based on the TPR profile of the respective
catalyst. The peaks corresponding to metallic copper were identified at the 2θ value of 43.1o
corresponding to (111) crystal plane, at 50.2o correspond to (200) crystal plane and at 74.2o
corresponding to (220) crystal planes, respectively [JCPDS-85-1326]. The diffraction peak
corresponding to the (222) plane of cubic Cu2O crystal was detected at the 2θ value of 36.4o
[JCPDS-78-2076]. However, for (111) crystal plane of the cubic CuO phase was detected at the
2θ value of 35.4o and 38.2o, respectively [JCPDS-80-1268]. The peaks corresponding to metallic
zinc was not detected in any catalysts and the peak corresponding to (100), (002), (110), 103),
(112) crystal plane of ZnO was detected at the 2θ value of 31.2o, 34.4 o, 56.5 o, 62.8 o, 67.8 o,
respectively [JCPDS-87-0075]. This results demonstrated that ZnO was not reduced to metallic
zinc under the reduction condition used [54,74]. The characteristics diffraction peaks detected for
MgO, TiO2, ZrO2 and CaO are marked in Figure 2B. The diffraction peak corresponding to V2O5
was not detected in the reduced Cu-Zn/V2O5 catalyst. For Cu-Zn/CaO catalyst, additional high-
intensity peaks were detected at the 2θ value of 29.4o, 47.2 o, 48.6 o, respectively, corresponding
to CaCO3 [JCPDS01-086-017]. This results depicted that some of CaO might be converted to
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CaCO3 in presence of trace amount of bicarbonate present in the catalyst or the CaCO3 could
have formed during calcination due to adsorption of ambient CO2 [73]. The crystallite size of Cu,
CuO and ZnO was calculated by using the Scherrer equation and the values are displayed in
Table 1. The average crystallite size of Cu, CuO and ZnO was in the range of 26.4-32.5 nm,
17.1-35.5 nm and 24.5-55.5 nm respectively.
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Table 1
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6
Structural properties of catalysts
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a
a
a
Catalyst
SBET
Vp
DBJH
Copper metal Copper metal
Metal loading#
(wt.%)
Crystallite size
of calcined
catalyst (nm)
CuO
Crystallite size
of reduced
catalyst (nm)
(m2.g-1) (cm³. g-1)
(nm)
dispersion
(% D)*
surface area*
(m2. g-1)
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48
Cu
-
Zn
-
Cu
ZnO
MgO
TiO2
94.2
23.0
14.0
6.8
0.18
0.034
0.012
0.02
0.04
0.17
0.11
0.06
0.05
0.08
81.0
16.0
6.9
-
-
-
-
-
-
-
-
-
-
-
-
ZrO2
-
-
-
-
-
-
-
CaO
14.0
13.8
12.6
20.1
20.5
41.6
12.1
-
-
-
-
-
-
-
V2O5
27.6
55.5
8.8
-
-
-
-
-
-
-
Cu-Zn/MgO
Cu-Zn/TiO2
Cu-Zn/ZrO2
Cu-Zn/CaO
Cu-Zn/V2O5
4.88
4.81
2.52
1.1
1.94
23.1
21.4
11.9
3.5
9.1
41.8
42.1
37.8
37.5
43.7
12.8
16.9
15.2
14.7
15.4
25.8
30.3
28.8
35.5
17.1
27.0
26.4
31.7
32.5
28.9
50.0
38.3
55.5
32.9
24.5
6.9
5.6
10.7
Note: aN2 adsorption-desorption data, # SEM-EDX, *N2O chemisorptions
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 CuO
 MgO
 ZrO  CaO # CaCO
 TiO
 ZnO
(A)
2
3
2
 V O

2 5





Cu-Zn/MgO




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Cu-Zn/TiO
2





Cu-Zn/ZrO
2

#
#
#



Cu-Zn/CaO

Cu-Zn/V O
2 5

10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90
2 degree

(B)
 MgO
 ZrO
 Cu  CuO
 TiO
 ZnO
2
2
 Cu O
 CaO
2








Cu-Zn/MgO




Cu-Zn/TiO

2


#
Cu-Zn/ZrO
2
#

Cu-Zn/CaO


Cu-Zn/V O
2 5
20
30
40
50
60
70
80
2 degree
Figure 2. XRD patterns of (A) fresh calcined catalyst and (B) reduced catalysts
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N2O chemisorption
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Cupper metal dispersion and copper metal surface area in the catalysts were determined
by N2O chemisorption method and the obtained results are summarized in Table 1. The metal
dispersion and metallic surface area of the catalysts followed the order as Cu-Zn/MgO> Cu-
Zn/TiO2> Cu-Zn/ZrO2> Cu-Zn/V2O5/ Cu-Zn/CaO. The maximum metal dispersion (~4.9 %) and
metallic surface area (23.1 m2. g cat.-1) were obtained for MgO supported catalyst. However, for
CaO supported catalyst, the metal dispersion (~1.1 %) and metallic surface area (3.5 m2. g-1) was
lowest. Large crystallite size and low surface area were obtained for Cu-Zn/ZrO2 catalyst as
compared to Cu-Zn/V2O5 catalyst. Still the metal dispersion was higher for Cu-Zn/ZrO2 catalyst.
This result indicated that metal support interaction also affected the distribution of active metals
on the catalyst [74]. Metal dispersion and metallic surface area of different supported catalysts
were dependent on the reduction–oxidation and reduction processes followed in N2O
chemisorptions technique. This reduction–oxidation and reduction behaviour varied in different
catalysts due to their different metal-support interactions. Metal dispersion was lowest for Cu-
Zn/CaO catalyst because of lowest surface area and due to higher degree of particle
agglomeration as evident from FE-SEM images of calcined and reduced catalyst, respectively.
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TPR
TPR patterns obtained for all the catalysts are shown in Figure 3. Since ZnO was not
reducible in the temperature range studied, the reduction peaks obtained for all the catalysts were
due to the combined reduction of CuO to Cu2O and Cu2O to Cuo, respectively [55, 56, 75]. For the
Cu-Zn/MgO catalyst, the broad reduction peak was detected at (300-490oC), whereas the broad
o
reduction peak was shifted to the slightly higher temperature (300-520 C) for Cu-Zn/V2O5
catalyst. For Cu-Zn/TiO2 and Cu-Zn/ZrO2 catalyst, the reduction peaks were shifted to lower
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o
o
temperature i.e. (250-370 C) and (200-300 C), respectively. For Cu-Zn/CaO catalyst the
reduction peaks obtained at lower temperature was due to the reduction of CuO to Cuo and higher
temperature (>650oC) peak was may be due to the decomposition of CaCO3. The lower side
shifting of H2 consumption peaks over TiO2, ZrO2 and CaO supported catalysts suggested that the
reduction of Cu was facilitated over these supports [75]. The TPR patterns obtained clearly
demonstrated the variation of metal support interactions in the different catalyst
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Cu-Zn/MgO
Cu-Zn/TiO
2
Cu-Zn/ZrO
2
Cu-Zn/CaO
Cu-Zn/V O
2 5
100 200 300 400 500 600 700 800 900
o
Temperature C
Figure 3 TPD profile of calcined catalysts.
TPD
The acidity and basicity of supports as well as catalysts was determined by NH3-TPD and
CO2-TPD analysis, respectively. The NH3 and CO2 desorption pattern obtained for supports are
shown in supplementary document [Figure 4(S)]. Total acidity and basicity of the support was
calculated based on the total amount of NH3 and CO2 desorbed at different temperature region.
Total acidity of the supports was in the range of 0.13-2.9 mmol NH3 g cat.-1. MgO was found to
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be most acidic and ZrO2 was the least acidic in nature. However, total basicity was in the range
of 0.03-1.17 mmol CO2. g cat.-1. TiO2 was the most basic and V2O5 was found to be the least
basic in nature.
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The obtained desorption pattern of catalysts (Figure 4A) suggested desorption of
ammonia from weak (80-300oC), moderate (300-550oC) and strong (>550oC) acidic sites present
on the catalyst surface, respectively [17]. The total acidity of all the catalysts was varied in the
range of 0.057-1.12 mmol NH3. g cat.-1 (Table 2). For Cu-Zn/MgO catalyst, total acidity was
found to be minimum (~ 0.057 mmol NH3. g cat.-1). However, for the Cu-Zn/CaO catalyst, the
acidic sites were absent. The basicity of the catalyst was determined by CO2-TPD analysis.
Similar to the NH3-TPD pattern obtained, three different types i.e. weak, medium and strong
basic sites were detected (Figure 4 B). The obtained basicity on the catalyst surface was in the
range of 0.1-0.25 mmol CO2. g cat.-1 (Table 2). Maximum basicity of ~0.25 mmol CO2. g cat.-1
was obtained for Cu-Zn/MgO catalyst and it was lowest (~ 0.1 mmol CO2. g cat.-1) for Cu-
Zn/V2O5 catalyst. By comparison, it was observed that the acidity/basicity values of the support
material were affected significantly after the addition of metals as shown in Table 2.
To verify the NH3/CO2 desorption from the catalyst surface during TPD analysis
primarily at higher temperature (>500oC), a blank experiment was performed in absence of NH3
and CO2. In the typical experiments, reduced catalyst was heated from room temperature to
1000oC at a temperature ramp of 10oC/min under the flow of helium (20 cc/min). Desorption was
monitored continuously with the help of a TCD. The results (Figure 3(S)) showed that, no
o
desorption peaks were detected for all the catalyst up to 1000 C, except for CaO supported
catalyst. For CaO supported catalyst, a desorption peak was detected at 650-840oC due to the
decomposition of CaCO3.
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The NH3 and CO2 desorption pattern of Cu-Zn/CaO catalyst were plotted separately
(Figure 4(C)). For this catalyst, a large desorption peak was detected at >600oC, which indicated
the decomposition of CaCO3 at high temperature (>600oC). The decomposition of CaCO3 present
in the Cu-Zn/CaO catalyst was also confirmed by the thermo-gravimetric analysis (TGA) (Figure
4(D)). Results demonstrated significant weight loss of the catalyst sample at ≥ 600oC, which is in
agreement with the XRD, NH3-TPD and CO2-TPD results reported in Figure 4(C).
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(B)
(A)
Cu-Zn/MgO
Cu-Zn/MgO
Cu-Zn/TiO
2
Cu-Zn/TiO
2
Cu-Zn/ZrO
2
Cu-Zn/ZrO
2
Cu-Zn/V O
2
5
Cu-Zn/V O
2
5
100 200 300 400
500 600 700 800
100 200 300 400 500 600 700 800 900
Temperature, (oC)
o
Temperature, ( C)
(D)
(C)
Cu-Zn/MgO
Cu-Zn/TiO
2
Cu-Zn/ZrO
2
Cu-Zn/V O
2 5
NH TPD
3
Cu-Zn/CaO
CO TPD
2
100 200 300 400 500 600 700 800 900
o
Temperature( C)
100 200 300 400 500 600 700 800
o
Temperature, ( C)
Figure 4. TPD profile of reduced catalysts (A) CO2-TPD and (B) NH3-TPD, (C) NH3 and CO2 TPD
profile of Cu-Zn/CaO catalyst, (D) Thermogravimetric analysis of calcined catalysts.
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FE-SEM
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Morphology of supports and catalysts obtained from Field Emission Scanning Electron
microscope (FE-SEM) are displayed in Figure 1(S) in the supplementary document. Rock like
structure was detected for CaO and ZrO2 support, while the morphology of MgO support was a
layered flakes type. The morphology of TiO2 has appeared as the mixture of quasi cylinders and
spheres. The V2O5 support showed a combination of slits of different size. The morphology of all
supported calcined catalysts exhibited the combination of rods and spherical shaped particles.
However, after reduction, particles morphology was transformed into quasi-spherical structure.
The SEM images obtained for different catalysts demonstrated the significant variation of
catalyst morphology with the variation of supports, after calcination and reduction. The metal
weight percentage obtained from SEM-EDX analysis is shown in Table 1. Theoretical and
experimental values of metal percentage in the catalyst were in good agreement.
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XPS
The existence of Cuo, Cu+, Cu++ and ZnO species over the surface of reduced Cu-Zn/MgO
catalyst was recorded in X-ray photoelectron spectrometer (XPS) and the XPS profile of
Cu2p3/2 and Zn2p3/2 are displayed in Figure 2(S) (A) and (B), respectively. The single intense
and wide spectrum at the binding energy in the range of 930eV-935eV was the indicator of the
presence of Cu2p3/2 species. The peaks referring to Cuo Cu+ and Cu++ at the binding energy
range of 933-934eV were also reported earlier [57, 58]
.
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4.2.
Catalytic activity
Catalytic activity was evaluated in a sectional packed bed, downflow tubular reactor.
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Initial catalyst screening was performed at 220oC, 0.1 MPa and at the WSHV of 0.18 h-1. After
starting the feed, the reaction was continued for 6 h to achieve a steady state. Further, the
reaction data were collected after a regular time interval of 2 h. It was observed that glycerol
conversion was increased up to 12 h and then it was almost unchanged for all the catalyst.
Therefore, all the data reported in this study were collected after a reaction time of 12 h.
Initially, the activity of different support materials was evaluated at 220oC, 0.1 MPa and
at the WSHV of 0.18 h-1 and compared with the performances of supported catalysts. The results
obtained are summarized in Table 2. As shown in Table 2, the activity of the supports was very
poor. The highest glycerol conversion of 16.1% was obtained over CaO followed by TiO2
(13.4%). The conversion obtained over other supports was < 7%. In presence of all the supports,
acetol was detected as the main reaction product with the selectivity range of 45-55.2%, followed
by 1-propanol (24.7-39.1 %), 2-propanol (7.2-11.1 %) and methanol (3.4-13.9%), respectively.
1,2-PDO was detected only in presence of ZrO2, support with ~9% selectivity. This results
demonstrated that, the role of the supports for selective hydrogenolysis of glycerol to 1,2-PDO
was not very insignificant.
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In presence of supported catalyst, the glycerol conversion and 1,2-PDO selectivity was
increased significantly as shown in Table 2. The catalytic activity followed the order as Cu-
Zn/CaO ≈ Cu-Zn/MgO > Cu-Zn/V2O5 > Cu-Zn/ZrO2 > Cu-Zn/TiO2. Maximum glycerol
conversion of 40-41% was obtained in presence of Cu-Zn/CaO and Cu-Zn/MgO catalyst,
respectively. In presence of the supported catalyst, the primary reaction product was acetol. Cu-
Zn/V2O5 catalyst showed the highest acetol selectivity of 89.5 % followed by Cu-Zn/CaO
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(89.2%), Cu-Zn/TiO2 (84.2%), Cu-Zn/ZrO2 (83.0%) and Cu-Zn/MgO (76.6%), respectively. The
obtained 1,2-PDO selectivity over various catalysts followed the order as Cu-Zn/V2O5 < Cu-
Zn/CaO < Cu-Zn/TiO2 < Cu-Zn/ZrO2 < Cu-Zn/MgO. Maximum 1,2 PDO selectivity of 20.2 %
was obtained over Cu-Zn/MgO catalyst. Results reported in Table 2 suggested that all catalysts
were highly selective to acetol at atmospheric pressure because of low hydrogenation rate at
ambient pressure due to the poor hydrogen adsorption [21, 32]. Trace amounts of other products
including 1-propanol (1.1-2.1%), 2- propanol (0.7-1.8 %), and methanol (0.2-1.1 %) were also
detected.
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Gas phase dehydration of glycerol followed two different routes based on the nature of
the acidic and basic sites present in the catalyst surface. Over the strong/bronsted acid sites,
glycerol dehydrated to acrolein while moderately acidic centres/lewis acidic sites catalysed
glycerol to acetol [41, 48- 50]. It has been reported that over Cu/Cr2O3 catalyst, at N2 ambience,
acetol was the primary product in the gas phase dehydration of glycerol at atmospheric pressure
and at 230oC. At this condition, further hydrogenation of acetol to 1,2-PDO was insignificant
[76]. However, in presence of bronsted acid sites incorporated by solid acids (niobium and
tungsten oxide) on the Al2O3, SiO2, and TiO2 supports, glycerol was mainly dehydrated to
acrolein at 305oC and at inert atmosphere [49]. Cu/ HZSM-5, Cu/ Hb catalysts were reported as
less selective to 1,2-PDO as compared to Cu/Al2O3 catalyst due to their higher acidity. This
results indicated that support had strong effect on the performance of the supported Cu base
catalysts [42]. Sato et al. [21] investigated the catalytic performance of various acidic (Al2O3, SiO2,
ZrO2, Fe2O3) and basic (MgO, CeO, ZnO) supported Cu catalysts for gas phase dehydration of
glycerol at 250 oC and at N2 atmosphere. Catalyst over acid oxide supports were shown as more
selective to acetol as compared to basic oxide supported catalysts. From previous literature
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discussion, major conclusion can be drawn that under strong acidic catalyst, glycerol was mainly
transformed to acrolein and dehydration of glycerol to acetol favored over moderately acidic
catalyst.
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In this study, higher glycerol conversion over CaO and TiO2 support and higher acetol
selectivity over all the support was possible due to combine effect of acid and basic nature of an
individual support. However, 1-propanol (1-PO) and 2-propanol (2-PO) were formed either
directly form glycerol or due hydrogenolysis of 1,2-PDO in presence of bi-functional metal/acid-
base catalyst [77]. In contrast to that, X. Lin et al. [78] reported 69% selectivity to 1-propanol over
sequential two-layer H-β-Ni/Al2O3 catalysts at 220 °C, 2MPa H2 pressure, due to hydrogenation
of acrolein. Manifold increased in glycerol conversion after the addition of Cu-Zn over the
supports suggested that metal species also participated in the process of dehydration as suggested
by various previous reports [20-22, 51]
.
The product distribution obtained clearly suggested that glycerol hydrogenolysis reaction
over Cu-Zn bimetallic catalyst in vapor phase followed the two simultaneous steps, initially,
acid/base sites of the catalysts catalyzed glycerol dehydration to acetol followed by metal
catalysis hydrogenation of acetol to 1,2-PDO [38, 59-61]. Higher selectivity to acetol suggested that,
at atmospheric pressure, dehydration of glycerol was favored. Results also demonstrated that the
activity trend obtained over various catalysts were not followed any regular trend with their
physicochemical properties i.e. acidity, basicity, BET surface area, metal dispersion (% D),
crystallite size obtained for all the catalysts. Although the percentage dispersion and metallic
surface area of MgO and TiO2 supported catalysts were very close, still higher 1,2-PDO
selectivity over MgO supported catalyst indicated the intrinsic characteristic of this support for
this reaction.
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1
2
3
4
Table 2
5
6
Acidity, basicity and catalytic performance
7
8
9
Catalyst
Conversion
(%)
Selectivity (%)
Total Acidity
Total Basicity
(mmol NH3. g (mmol CO2. g
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12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
cat.-1)
cat.-1)
1,2-PDO Acetol 1-PO 2-PO Methanol EG
MgO
TiO2
6.6
13.4
3.6
-
55.2
51.6
48.2
52.4
45.0
76.6
84.2
83.0
89.1
89.3
24.7
38.2
27.5
26.1
39.1
1.2
9.0
10.2
7.2
10.5
-
-
-
2.9
2.07
0.13
-
0.932
1.17
0.056
-
-
ZrO2
8.95
-
7.4
-
CaO
16.1
5.8
7.5
13.9
3.4
-
V2O5
-
11.1
0.7
-
1.61
0.057
1.1
0.029
0.25
0.24
0.11
-
Cu-Zn/MgO
Cu-Zn/TiO2
Cu-Zn/ZrO2
Cu-Zn/CaO
Cu-Zn/V2O5
40.3
27.3
32.2
40.9
37.1
20.7
11.8
14.3
7.7
6.9
1.1
0.5
--
1.2
1.8
0.82
0.46
0.69
0.18
1.4
0.79
1.19
0.73
--
0.49
-
1.07
2.13
0.19
0.67
0.2
0.1
Reaction condition: 10 wt% aqueous glycerol solution, 220oC, 0.1 MPa, 1.0 g catalyst, H2/glycerol mole ratio = 66.8, WHSV =
0.18 h-1.
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The basicity, copper metal dispersion (~5%) and the active metal surface area (~23 m2. g-1) was
highest for Cu-Zn/MgO catalyst. The uniform distribution of metal particles and the presence of
partially reduced Cu species (Cu2O, CuO and Cuo) as evidenced from TEM and XRD results
were the governing factor for higher selectivity to 1,2-PDO in presence of Cu-Zn/MgO catalyst
[52]. The presence of Cu+ favored the dehydration of glycerol, while Cuo activated the hydrogen
molecule for hydrogenation of intermediate acetol [63]. Since Cu-Zn/MgO catalyst was most
selective to 1,2-PDO, therefore, this catalyst was chosen further for detail reaction process
parameter study to increase the 1,2-PDO selectivity and yield. The time-on-stream stability of
Cu-Zn/MgO catalyst was also investigated.
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4.3.
Reaction parameter study
4.3.1. Effect of temperature
Effect of reaction temperature on glycerol conversion and product selectivity obtained
over Cu-Zn/MgO catalyst at atmospheric pressure (0.1 MPa) is shown in Figure 5 (A). As
expected, glycerol conversion was increased from 56.7% at 190oC to 92.3% at 260oC. At lower
temperature (<200oC), acetol was obtained as the primary reaction product with very high
selectivity (87.9 %) and the total selectivity to 1,2-PDO and other products was very low (~12.1
%). Acetol was formed as a primary product due to the dehydration of glycerol, whereas 1,2-
PDO was obtained by the hydrogenation of acetol [63]. Further, with increasing the reaction
temperature, it was observed that the selectivity to acetol was decreased with simultaneous
increasing the selectivity to 1,2-PDO up to a reaction temperature of 240oC. Acetol selectivity
was decreased from ~87.9 % to 67.9 % and the selectivity to 1,2-PDO was increased from 8.4%
to 28.8% with increasing the reaction temperature from 190oC to 240oC. Moreover, within this
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temperature range, the selectivity to other products was almost constant (~3.5%). This selectivity
trend suggested that, at the higher reaction temperature (>200oC), acetol was converted
selectively to 1,2-PDO by the hydrogenation process in presence of the active metal surface of
the catalyst [65]. At very high temperature (>240oC), the selectivity to 1,2-PDO was decreased
significantly with simultaneous increasing the selectivity to the other degradation products,
which indicated that very high temperature (>240 oC) facilitated 1,2-PDO degradation reaction
and produced the other products i.e. ethylene glycol, methanol, and ethanol, 1-PO and 2-PO,
respectively [65]. The effect of temperature variation at atmospheric pressure suggested that lower
reaction temperature (<240oC) was beneficial for higher selectivity to 1,2-PDO and the preferred
reaction temperature was ~ 220oC for higher selectivity to 1,2-PDO.
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4.3.2. Effect of pressure
Influence of reactor pressure was investigated in the pressure range of 0.1 MPa to 0.8
MPa at 220oC, H2/glycerol mole ratio of 66.8 and WHSV of 0.12 h-1. As shown in Figure 5 (B),
glycerol conversion was found to increase with pressure and nearly 98.3% conversion of glycerol
was achieved at 0.72 MPa pressure. This is very interesting to note that, the selectivity to acetol
was decreased drastically (~60%) with a sharp increase in selectivity to 1,2- PDO (~57.5 %)
with increasing reaction pressure from ~0.12 to ~ 0.72 MPa at the constant reaction temperature
of 220oC. Moreover, the variation in the selectivity to other products were very marginal (~2-3
%). This results demonstrated that, at lower pressure (~0.1 MPa), conversion of glycerol to
acetol via dehydration reaction was highly favorable. However, with increasing reaction pressure
from ambient to 0.72 MPa, acetol was converted to 1, 2- PDO by the hydrogenation reaction in
presence of more hydrogen atom. At higher pressure, the higher concentration of active
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hydrogen species on the metal site increased the rate of acetol hydrogenation to 1,2-PDO [32]. At
very high pressure (i.e. > 0.72 MPa), 1, 2-PDO selectivity was started to decline due to over
hydrogenolysis of 1, 2-PDO to lower alcohol [30]. The maximum 1,2-PDO selectivity 82.9 % was
achieved at 0.072 MPa pressure.
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4.3.3. Effect of catalyst amount
Reactions were conducted at 220oC and at 0.1 MPa pressure and the catalyst weight was
varied from 0.5 g to 3.0 g. In each experiment, the catalyst was divided into two equal parts and
placed sequentially over a glass wool bed inside the reactor. As shown in Figure 5(C), the
glycerol conversion was increased sharply from 14.3% to 91.3% with increasing the catalyst
amount up to 2 g and further it was gradually reached to 97.5 % in presence of 3 g of catalyst.
The selectivity to acetol, 1,2-PDO and other products was also showed a similar trend as
obtained with the variation of pressure. Selectivity to acetol gradually dropped from 88.1% to
53.6% against the slow rise of 1, 2-PDO selectivity from 10.7% to 39.1%. Increasing glycerol
conversion and 1, 2- PDO selectivity were attributed to the accessibility of reactant to a higher
[67, 56].
number of active catalyst sites at higher catalyst loading as expected
Interestingly, no
significant rise in other product selectivity was observed. Further increasing the catalyst weight
(>3 g), increased the pressure drop inside the reactor, the maximum 1,2-PDO selectivity of 39.1
% was obtained in presence of 3 g catalyst.
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90
80
70
60
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40
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10
0
100
90
(A)
(B)
80
70
Conversion
1, 2-PDO
Acetol
60
Conversion
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0
1, 2-PDO
Acetol
Others
Others
0.0
0.2
0.4
0.6
0.8
1.0
190 200 210 220 230 240 250 260
o
Pressure (MPa)
Temperture ( C)
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90
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0
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90
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50
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0
(C)
(D)
Conversion
1, 2-PDO
Acetol
Others
Conversion
1, 2-PDO
Acetol
Others
0.5
1.0
1.5
2.0
2.5
3.0
0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40
-1
Catalyst weight(g)
WHSV (h )
100
90
80
70
60
50
40
30
20
10
(E)
Conversion
Acetol
1, 2-PDO
Others
0
20
30
40 50
H2/Glycerol mole ratio
60
70
Figure 5. Variation of glycerol conversion and product selectivity over Cu-Zn/MgO catalyst.
(A) Effect of temperature (at 0.1 MPa, 1.5 g catalyst, WHSV 0.122 h-1, H2/glycerol mole ratio =
66.8). (B) Effect of pressure (at 220oC, 1.5 g catalyst, WHSV = 0.122 h-1, H2/glycerol mole
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ratio= 66.8). (C) Effect of catalyst weight (at 220oC, 0.1 MPa, H2/glycerol mole ratio = 66.8).
(D) Effect of weight hourly space velocity (at 220oC, 0.1 MPa, H2/glycerol mole ratio = 66.8).
(E) Effect of Hydrogen to glycerol ratio (at 220oC, 0.1 MPa, catalyst weight 1 g).
*Others Methanol, 1-PO, 2-PO
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4.3.4. Effect of weight hourly space velocity (WHSV)
Variation of glycerol conversion and products selectivity in terms of WHSV (0.06-0.37 h-
1) are presented in Figure 5 (D). At the WHSV of 0.06 h-1, the glycerol conversion was almost
100 % and a drastic drop in glycerol conversion and 1,2-PDO selectivity were observed with
increasing the WHSV with simultaneous increasing the selectivity to acetol. The selectivity to
other products was also decreased with WHSV. Poor access of reactants to active sites at higher
WHSV may likely be the possible reason for diminishing the glycerol conversion and 1,2-PDO
selectivity [67]. The maximum 1,2-PDO selectivity to 39.1% was obtained at the WHSV of 0.06
h-1.
4.3.5. Effect of hydrogen to glycerol mole ratio
The variation in glycerol conversion and product selectivity obtained with the variation in H2
to glycerol mole ratio is displayed in Figure 5 (E). The hydrogen to glycerol mole ratio was
varied by varying the glycerol concentration in the feed. The results indicated that higher
H2/glycerol ratio favored high glycerol conversion and high selectivity to 1,2-PDO. However, at
lower H2/glycerol mole ratio, the glycerol conversion was decreased. This results demonstrated
that lower glycerol concentration favored glycerol conversion and 1,2-PDO selectivity. Possibly
higher glycerol concentration produced adsorption hindrance for hydrogen over the limited
29