(544eo) Effect of Different Metal Oxide Supported Cu Catalysts for 1,2-Propanediol Production Via Glycerol Hydrogenolysis Route

Effect
of different metal oxide supported Cu catalysts for 1,2-propanediol production via
glycerol hydrogenolysis route

Smita
Mondal* and Prakash Biswas

Department of Chemical Engineering, Indian
Institute of Technology Roorkee, Roorkee-

247667,
Uttarakhand, India.

                                    *Corresponding
Author: Tel.: (+91)-1332-28-6657

E-mail
address:
s08mondal@gmail.com

1.Introduction:

Biofuels,
being a potential renewable alternative of petroleum derived fuel has received significant
attention from the past two decades. In light of this, biodiesel production via
transesterification of vegetable oil and animal fat has been commercialized. Since,
glycerol is a major byproduct in transesterification process (~10 wt%), over
growing production of biodiesel has ended up with vast amount of glycerol production.
Among various glycerol value-addition methods, hydrogenolysis of glycerol to
1,2-propanediol is one of the attractive routes. 1,2-propanediol has a great
market demand as functional fluids, unsaturated polyester resin, cosmetics,
paints etc. Several noble and non-noble metal catalysts were developed and
their performances were studied for the selective conversion of glycerol
hydrogenolysis to 1,2-PDO production. High cost and poor selectivity to 1,2-PDO
over noble metal catalysts encouraged the development of non-noble metal
catalyst for this reaction Among non-noble metal based catalysts, Cu-based
catalysts have shown higher selectivity to 1,2-PDO production due to its
tendency to break C-O bond without attacking C-C bond of glycerol. Previous
literature suggested that this reaction requires bi-functional acidic and/or
basic sites in the catalysts to enhance the selectivity and yield of 1,2-PDO.
Therefore, development of bi-functional catalyst
having dehydration as well as hydrogenation sites with an appropriate surface
orientation is highly desirable for higher selectivity and yield of 1,2-PDO.
In this study, effects of various supports (MgO, La₂O₃)on monometallic Cu catalyst and Cu-Zn bimetallic catalyst for liquid
phase glycerol hydrogenolysis to 1,2-propanediol production has been evaluated.
This study aims at developing highly active, selective and stable catalyst for
1,2-PDO production from renewable glycerol. All the synthesized catalysts
showed > 94% conversion of glycerol with > 88% yield of 1,2-PDO at mild
reaction condition.

2.Experimental:

The
catalysts (Cu/La₂O₃, Cu/MgO, Cu-Zn(4:1)/La₂O₃,
Cu-Zn(4:1)/MgO) were
prepared by deposition precipitation method and were characterized by various techniques:
BET surface area, NH₃-Temperature programmed desorption, CO₂-Temperature
programmed desorption, Temperature programmed reduction, X-ray diffraction and
Field emission scanning electron microscope and X-ray photoelectron
spectroscopy. Glycerol hydrogenolysis reaction was carried out in 250 ml
stainless steel autoclave reactor at 210°C temperature, 4.5MPa pressure hydrogen
and 20 wt% glycerol concentration. Reactants and products were analyzed by gas
chromatography (GC) equipped with flame ionization detector (FID) and
Chromosorb-101 packed column (1.52 m × 3.1
mm OD × 2
mm ID).

3.Results
and discussion:

3.1
Characterization results:

BET
surface area of all the catalysts followed following order: Cu-Zn/MgO > Cu/MgO
> Cu-Zn/La₂O₃ > Cu/La₂O_3. Temperature
programmed reduction (TPR) was carried out to understand the reduction behavior
of the catalysts. As shown in figure 1, Cu/MgO showed a major peak at around
294°C temperature which indicated direct reduction of CuO to Cu^o.
After addition of Zn    the reduction temperature was shifted towards the lower
temperature and a broad major peak was identified at 260°C temperature. Cu/La₂O₃
catalyst showed reduction peak at higher temperature 360°C. Cu-Zn/La₂O₃
showed almost same reduction behavior. TPR results revealed that MgO supported catalysts
were reduced easily compared to La₂O₃ supported
catalysts. For Cu-Zn/MgO catalyst, strong interaction of Cu and Zn metal and H₂
–spillover effect of ZnO were responsible for shifting of reduction peak
at lower temperature. XRD-pattern of calcined and reduced catalysts were
observed. Crystallite sizes calculated by scherrer equation, were in the range
of 32-37 nm. NH₃-TPD analysis was carried out to determine the
acidic sites of the catalysts. Acidic sites were varied in the range of
1.67-2.13 mmol.gcat^-1. Basic sites were analyzed by CO₂-TPD
pattern of the catalysts. Basic sites were in the range of 1.21-1.81 mmol.gcat^-1.
Total basic sites of the catalysts were in the order of: Cu-Zn/MgO > Cu-Zn/La₂O₃
> Cu/MgO > Cu/La₂O₃. FESEM images showed that
morphology of the catalysts were different with different supports, which were
also responsible for different catalytic activity.

3.2
Catalytic activity:

Catalytic
activities of the supported catalysts were as follows: Cu-Zn/MgO > Cu-Zn/La₂O₃
> Cu/MgO > Cu/La₂O_3.For
all the catalysts, 1,2-PDO was the primary reaction product whereas trace
amounts of other products such as acetol, ethylene glycol, propanol were also
detected (Table 1). As shown in Table 1, It was observed that catalytic
activity of bimetallic catalysts was higher than the monometallic catalysts.
Higher catalytic activity over zinc doped catalyst were
mainly because of two reason: 1) hydrogen spillover effect from ZnO to Cu which
increased the reducibility of CuO to Cu^o  and
2) basic sites of the catalysts. Maximum 98.4% conversion with 94% selectivity
to 1,2-PDO was achieved over MgO supported Cu-Zn bimetallic catalyst at 210°C
temperature, 4.5 MPa pressure, 20 wt% aqueous solution of glycerol, 8 wt%
catalyst loading for 12 h reaction time. Higher activity of Cu:Zn(4:1)/MgO catalyst
can be correlated with larger surface area, higher acidity, basicity and higher
reducibility (88%).

4.
Conclusions:

 

           MgO and La₂O₃ supported
monometallic Cu based catalysts showed ~88% yield to 1,2-PDO at mild reaction
condition. After addition of Zn to Cu based monometallic catalysts, catalytic
activity was increased significantly due to hydrogen spillover effect of ZnO
resulting higher reducibility of Cu. Among all the catalysts, Cu-Zn(4:1)/MgO
catalyst showed very high (98.7%) glycerol conversion and selectivity (93.3%)
to 1,2-PDO. Higher catalytic activity was ascribed to
higher surface area, higher surface acidity (2.13 mmol.gcat^-1),
basicity (1.81 mmol.gcat^-1), and synergistic effect of Cu and Zn
which favored selective conversion of glycerol to 1,2-PDO via
dehydration-hydrogenation pathway. Reusability study confirmed the stability
and consistent performance of the catalysts even after successive reuse. 

Table
1: Screening of catalysts

Catalyst	Conversion (%)	Selectivity (%)		Yield (%)
		1,2-PDO	Others*
Cu/MgO	95.4	92.6	7.4	88.3
Cu/La2O3	94.6	92.7	7.3	87.7
Cu-Zn(4:1)/La2O3	96.3	93.3	6.7	89.8
Cu:Zn(4:1)/MgO	98.7	93.6	5.7	93.0

 

Reaction
condition: 210^oC, 4.5MPa, 20wt.% glycerol as feed, 8wt.% catalyst of
glycerol feed, 12h

Others*: acetol, ethylene glycol, 1-propanol, 2-propanol, methanol
and ethanol

 

Keywords:
Glycerol, 1,2-PDO, metal oxide, bi-metallic

              
Figure 1 : Temperature programmed reduction