An efficient, selective process for the conversion of glycerol to propylene glycol using fixed bed raney copper catalysts

Article abstract of DOI:10.1021/op400123f

Propylene glycol is formed in yields of up to 95% at 100% conversion using a Raney Cu catalyst in a fixed bed reactor. The reaction uses an 80% aqueous glycerol solution and a hydrogen pressure of 600 psi. The primary byproduct is ethylene glycol formed in 1-3% yield. In a reaction run continuously for 24 d using a sample of a commercial preparation of Raney Cu, the selectivity to propylene glycol at 100% glycerol conversion was 94.6% with a space-time yield (STY) of 0.49 g of 1,2-propylene glycol/mL Raney Cu/h. Ethylene glycol was formed in 2.5% yield, while methanol, ethanol, n-propanol 1,3-propylene glycol, and acetol were present in less than 1% yield.

Full text of DOI:10.1021/op400123f

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Full Paper
An Efficient, Selective Process for the Conversion of Glycerol
to Propylene Glycol Using Fixed Bed Raney® Copper Catalysts
Setrak K Tanielyan, Norman Marin, Gabriela Alvez, Ramesh Bhagat,
Balaraju Miryala, Robert L. Augustine, and Stephen Schmidt
Org. Process Res. Dev., Just Accepted Manuscript • DOI: 10.1021/op400123f • Publication Date (Web): 06 Aug 2013
Downloaded from http://pubs.acs.org on August 18, 2013
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An Efficient, Selective Process for the Conversion
of Glycerol to Propylene Glycol Using Fixed Bed
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
Raney Copper Catalysts
a
a
a
a
a
Setrak K. Tanielyan, Norman Marin, Gabriela Alvez, Ramesh Bhagat, Balaraju Miryala,
a
b
Robert L. Augustine * and Stephen R. Schmidt
a
Center for Applied Catalysis, Department of Chemistry and Biochemistry, Seton Hall
b
University, South Orange, NJ 07079, USA. W.R. Grace and Co.-Conn. 7500 Grace Drive,
Columbia MD 21044, USA
Raney  is a trademark, registered in the United States and/or other countries, of W.R. Grace &
Co.-Conn.
Keywords: Glycerol, Propylene Glycol, 1,2-Propanediol, Raney Copper, Hydrogenolysis,
Fixed Bed Reaction
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ABSTRACT: Propylene glycol is formed in yields of up to 95% at 100% conversion using a
Raney  Cu catalyst in a fixed bed reactor. The reaction uses an 80% aqueous glycerol solution
and a hydrogen pressure of 600 psi. The primary by-product is ethylene glycol formed in 1-3%
yield. In a reaction run continuously for 24 d using a sample of a commercial preparation of
Raney Cu, the selectivity to propylene glycol at 100% glycerol conversion was 94.6% with a
Space Time Yield (STY) of 0.49 g of 1,2-propylene glycol/cc Raney Cu/h. Ethylene glycol
was formed in 2.5% yield while methanol, ethanol, n-propanol 1,3-propylene glycol and acetol
were present in less than 1% yield.
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1. INTRODUCTION
The increasing world-wide production of biodiesel by the transesterification of fats and oils has
resulted in a corresponding increase in the production of glycerol. One gallon of glycerol is
produced for every nine gallons of biodiesel. Even though there are over 2000 different
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applications for which glycerol is used, new high volume commercial processes are needed to
improve the economics of biodiesel production. Some early work in this area concentrated on
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the selective oxidation of glycerol to compounds such as dihydroxyacetone, hydroxypyruvic
acid and glyceric acid. For a variety of reasons this approach was not sufficiently effective to
utilize much of the glycerol which was being produced. Other research was directed toward
finding commercially applicable procedures for the conversion of glycerol into commodity
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chemicals. Processes have been developed which use a glycerol feedstock to replace the
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petrochemical feedstock for the preparation of acrolein and epichlorohydrin. More recently
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procedures have been reported for converting glycerol to glycerol carbonate, a practical,
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ecofriendly solvent, and glycerol alkyl ethers which are safe and effective fuel additives.
The most recent research efforts, though, have been directed to developing practical methods for
the conversion of glycerol (GLY) to 1,2-propylene glycol (PG) which is used primarily in
polymer synthesis but is also an ingredient in antifreeze and de-icing solutions. Since PG is
considered as Generally Recognized As Safe (GRAS) by the USFDA, it has also found use in
pharmaceuticals, personal care products and as a preservative in food.
The commonly proposed mechanistic pathways for the conversion of glycerol to glycols are
shown in Scheme 1. The formation of PG apparently takes place through an initial dehydration
of one of the primary hydroxyl groups on GLY to give the enol, 1, which is in equilibrium with
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Scheme 1. Glycerol hydrogenolysis reactions
OH
O
OH
H2
C
C
CH
H C
CH2 OH
H C
CH2 OH
H C
CH2 OH
2
1
3
3
ACT
PG
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-
H O
2
OH
CH
-H O
CH
CH2
2
HO
CH2
CH2 OH
HO
CH
CH2 OH
O
CH
CH2 OH
3
2
GLY
-
H O
H
2
2
-
H2
H2
OH
CH
CH2
CH2 OH
1,3PG
O
CH
CH2
HO
CH2
H
CH
ACR
O
CH
CH
CH2 O
4
-CO
OH
CH
H
B
O
CH2
O
4
OH
OH
H2
H C
O
+
CH2
2
CH
H C
OH
+
HO
CH2
HO
CH
3
5
EG
-
H O
2
O
CH2
H2
HOCH CH
C
CH3
HO
CH
2
3
H
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the hydroxy ketone, acetol, (ACT). Hydrogenation of ACT gives PG. Dehydration of the
secondary hydroxyl group in GLY gives the enol, 2, which is in equilibrium with the hydroxyl
aldehyde, 3. Hydrogenation of 3 produces 1,3-propylene glycol (1,3PG) while dehydration
gives acrolein (ACR). One of the most common by-products in this reaction is ethylene glycol
(EG). One possible route to its formation is that initiated by the dehydrogenation of GLY to
glyceraldehde (4). A retro-Aldol reaction leads to the formation of formaldehyde and the enol, 5,
which on hydrogenation give EG and MeOH. Other potential routes to EG are the direct
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hydrogenolysis of a C-C bond of GLY and the decarbonylation of 4 which would form EG
directly. A dehydration-hydrogenation sequence on EG leads to the formation of EtOH.
The catalysts involved in the conversion of GLY to PG generally contain Ru, Ni or Cu.
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Supported Ru was used in conjunction with an acidic ion exchange resin to facilitate the initial
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dehydration step.
Other reports cited the use of supported Ru catalysts which also contained
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Pt Au, or Fe. Supported Ni catalysts included Ni/C, Ni-Ce/C, and Ni/NaX. Co/MgO
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was also used as a catalyst for this reaction. The most commonly used catalysts, however,
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contained copper, usually either supported on ZnO,
modified with Cr
or as a component
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in a mixed oxide system
such as copper-chromite (Adkins Catalyst) (Cu-CrO).
Most of
these reports described the use of batch processes for this reaction but some, primarily patents,
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also discussed the use of continuous reactions.
It is difficult to compare these procedures since they were run using a wide range of reaction
conditions: glycerol concentrations ranging from 10% aqueous GLY to pure GLY and technical
grade GLY; reaction temperatures from 140 °C to 270 °C; hydrogen pressures from 150 psi to
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500 psi with one patent describing the use of 3700-4400 psi of H . The batch reactions were
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run for 5 to 40 hours giving widely different results, reporting GLY conversions ranging from
3% to 100% and selectivities to PG from 20% to 95% as well as giving varying amounts of by-
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products, primarily EG, ACT, n-propyl alcohol, ethanol and methanol. The continuous
processes utilized primarily copper containing catalysts, again with differing reaction conditions
making any comparisons difficult.
Probably the most extensive study of batch and continuous reactions for this transformation was
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that reported by Suppes
which used Cu-CrO as the catalyst. His approach was unique in that
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it involved a two-stage reaction sequence in which ACT was formed first and then hydrogenated
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in a separate step. Both batch and continuous reactions were used in this approach.
Little has been published on the use of Raney catalysts for the conversion of GLY to PG. One
paper described using Raney Ni as the catalyst for batch reactions comparing the extent of PG
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formation between pure GLY and the GLY obtained from a biodiesel reaction. Another
reported using Raney Ni in a batch process to yield selectivities of 77% PG, 15% ethanol and
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% CO at 63% conversion after 20 h. In a survey of catalysts screened for the GLY to PG
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conversion it was reported that with 80% aqueous glycerol, 200 °C and 200 psi of H , a Raney
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Ni catalyst gave a selectivity of 53% for PG at 50% conversion after 24 h. Under the same
conditions a Raney Cu catalyst yielded 69% selectivity to PG at 49% conversion. Some ACT
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was also produced. Interestingly, no EG formation was reported in these articles even though
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EG was the primary by-product observed in reactions run over supported Ni catalysts.
An
early paper describing GLY conversion over Raney Cu at 240 °C and 750 psi of H reported
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an 80% yield of PG and 20% EG at 85% conversion.
Most of what has been published in the open literature described the use of batch reactions for
the GLY to PG reaction. However, continuous fixed bed processes are preferred over batch
procedures because the former provide a higher catalyst/feed ratio which offsets the low specific
activity of catalysts, such as those containing Cu, as well as having the ability to remove by-
products such as water and CO and, thus, minimizing any equilibrium constraints which may
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have led to the low yields observed in most batch reactions. We report here an examination of
the effect which reaction variables have on the activity and selectivity of these Raney catalysts
in the continuous conversion of GLY to PG. A preliminary report on the use of Raney Cu for
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the valorization of glycerol has been published. The data cited in this previous report have been
modified in the present description by the use of an improved analytical procedure involving five
point curves for the internal standard calibrations.
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2
. RESULTS AND DISCUSSION
2.1. Raney Ni Catalysts. Relatively little has been published on the use of either Raney
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Ni
or Raney Cu
for the conversion of GLY to PG. This seeming discrepancy
indicated that a re-investigation of the use of Raney Ni and Raney Cu catalysts in this
reaction was warranted but, given the reported relative inactivity of these catalysts in batch
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processes
as well as the advantages of fixed bed reactions cited above, the Raney Ni and
80
Conversion
PG Selectivity
EG Selectivity
EtOH Selectivity
MeOH Selectivity
60
40
20
1
80
185
190
195
200
205
210
Temperature, °C
Figure 1. Effect of reaction temperature on the percent conversion and product
composition for the GLY to PG reaction run over Raney Ni with an 80% aqueous
GLY solution flow rate of 0.075 cc/min and a H pressure of 300 psig and a flow
2
rate of 150 cc/min.
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Conversion
PG Selectivity
EG Selectivity
EtOH Selectivity
MeOH Selectivity
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Pressure, psi
Figure 2. Effect of H pressure on the percent conversion and product composition
for the GLY to PG reaction run over Raney Ni at 190 °C, with an 80% aqueous
GLY solution flow rate of 0.075 cc/min and a H flow rate of 150 cc/min.
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2
Raney Cu promoted reactions were run in a continuous mode using a fixed bed reactor.
Research was initiated by an examination of the effect which the individual reaction parameters,
temperature, H pressure, GLY feed flow rate and H flow rate, had on the outcome of the
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Raney Ni catalyzed reaction. Figure 1 and Figure 2 depict the effect of temperature and
hydrogen pressure, respectively, on the GLY conversion and reaction selectivity while the effect
of hydrogen and GLY feed flow rates are shown in Table 1. As Scheme 1 illustrates, the overall
reaction sequence for GLY conversion is complex with the potential of having several reactions
taking place simultaneously. In the equation at the top of Scheme 1, the dehydration of GLY
followed by hydrogenation of ACT, leads to the production of PG. The pathways at the bottom
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of Scheme 1 are responsible for the degradation of GLY to EG, EtOH and MeOH. The totality
of these reactions is indicated by the percent conversion of GLY with the most significant effect
on this aspect of the reaction being brought about by changes in the reaction temperature since
this factor influences virtually all the reactions in Scheme 1. Increasing the hydrogen pressure,
however, had a more modest effect on the percent conversion since not all of the reactions used
hydrogen. A faster GLY flow rate can cause a decrease in conversion because the reactants have
insufficient time in contact with the catalyst to react completely.
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The effect of these variables on the product composition, though, is more complex. A low GLY
feed flow rate resulted in the reaction mixture being in contact with the catalyst longer, thus,
giving more degradation products. Increasing the hydrogen flow rate also increased the extent of
degradation since there was more hydrogen available for hydrogenolysis. An increase in
temperature apparently favored the hydrogenation steps over the C-C hydrogenolysis while
increasing the hydrogen pressure appeared to have the opposite effect. The n-PrOH and ACT
selectivities were less than 1% and the 1,3PG selectivity was between 1% and 1.5% in all of
2
Table 1. Effect of H and GLY feed flow rates on percent conversion
and selectivity of PG, EG, EtOH and MeOH in a Raney Ni
a
catalyzed conversion of GLY.
H Flow,
2
100
150
200
100
150
200
cc/min
GLY Feed
Flow, cc/min
0.025 0.025 0.025 0.075 0.075 0.075
%
%
Conversion
Selectivity
PG
EG
EtOH
77
75.4
73
46
39.2
38
17.0
20.0
14.9
7.6
17.8
24
13.5
7.4
20.7
32.3
11.1
6.1
18.8
25.1
7.1
20.8
34.6
7.0
22.7
39.5
7.7
MeOH
2.8
3.2
3.6
a
Temp. 190 °C, H
2
pressure, 600 psi, 80% aqueous GLY.
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these reactions.
2.2. Raney Cu Catalysts. The large amount of EG, EtOH and MeOH formed in the Raney
Ni promoted reactions showed that it was not a viable catalyst for the selective conversion of
GLY to PG. It seemed, therefore, that since the use of other Raney type catalysts had not been
explored to any great extent primarily because of their commercial rarity and obscurity to date, a
non-Ni catalyst would be a prime candidate for further study especially when used in continuous
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reactions. Given the interest in the use of Cu containing catalysts for the GLY to PG
19-38
conversion,
it was decided to investigate the use of Raney Cu and modified Raney Cu
catalysts for this reaction.
The ranges of the reaction parameters investigated in the Raney Cu promoted reactions were
based on the data obtained with Raney Ni. The effect of temperature in the range of 190 °C to
205 °C on the conversion and product composition in the Raney Cu catalyzed reactions are
shown in Table 2. As expected, increasing the
temperature increased the conversion and the
yield of PG. The amount of EG observed
was significantly less than that formed during
the Raney Ni catalyzed reactions. Table 3
lists the percent conversion and the PG and
EG selectivities observed from reactions run
Table 2. Temperature effect on Raney Cu
a
catalyzed conversion of GLY to PG.
Temp.
°C
%
Conv.
% PG % PG % EG
b
c
c
Yield
73.2
80.5
81.4
Sel.
Sel.
190
200
205
86.3
99.9
100
84.9
80.6
81.4
2.6
2.3
1.7
a
Reactions run at a H flow rate of 150 cc/min
2
and pressure of 200 psi and 80% aqueous GLY
at a flow rate of 0.05 cc/min.
with varying GLY feed and H flow rates as
2
b
well as H pressures. In all reactions the
Conversion
2
c
Selectivity
1
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Table 3. Effect of GLY feed flow rate and H
Raney Cu catalyzed conversion of GLY to PG.
2
flow rate and pressure on the
a
GLY Feed
Flow Rate
cc/min
H
2
Flow
Rate
H
2
Pressure
psi
%
% PG
% EG
Conversion Selectivity Selectivity
cc/min
375
375
375
375
375
375
450
450
450
515
515
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32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
0
.05
.06
200
200
200
200
175
150
175
150
150
150
125
100
100
96.3
95.3
93.9
93.7
90.1
89.3
90.2
88.9
85.8
85.3
83.5
1.5
2.3
1.9
2.2
2.1
1.9
1.9
1.9
1.9
2.0
1.7
0
0.075
0.10
100
99.1
98.6
97.0
98.7
98.5
94.9
95.5
95.1
0.10
0.10
0.10
0.10
0.125
0.125
0
.125
a
Reactions run using an 80% aqueous GLY at 205 °C.
amounts of MeOH, EtOH and ACT were less than 1% with no n-PrOH or1,3PG observed.
Increasing the GLY feed flow rate, with the concomitant decrease in residence time in the
catalyst bed, resulted in a decrease in PG selectivity regardless of the H flow rate. Increasing
2
the GLY feed flow rates above 0.1 cc/min decreased the percent conversion as well. As
anticipated from the reactions shown in Scheme 1, decreasing the hydrogen pressure resulted in
lower GLY conversions and PG formations. A higher H flow rate somewhat compensated for a
2
lower pressure. The data in Table 4 show that increasing the GLY concentration to 90% resulted
in a decrease in PG selectivity. Since virtually all of the previous research cited in the
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1
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2
2
2
2
2
2
2
2
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3
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3
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Introduction used aqueous solutions of GLY it
would appear that the presence of some water
may be needed for this reaction. One possible
scenario is that sufficient water in the GLY
solution is needed to facilitate the passage of
the substrate and hydrogen through the liquid-
solid barrier at the catalyst surface. If this is
the case then it would appear that the 90%
GLY does not contain sufficient water for this
to happen.
Table 4. Effect of glycerol concentration on
the Raney Cu catalyzed conversion of
a
GLY to PG.
GLY
Conc.
%
Conv.
% PG
Sel.
% EG
Sel.
b
c
d
d
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
8
0%
0%
100
100
96.7
91.4
1.1
1.5
9
a
Reactions run at 205 °C with a H flow rate
2
of 375 cc/min and pressure of 200 psi and a
GLY feed flow rate of 0.05 cc/min.
b
Concentration.
c
Conversion.
d
Selectivity.
Since the addition of Cr has been shown to increase the activity and selectivity of supported Cu
25
catalysts, some Cr modified Raney Cu catalysts were also used to promote the GLY to PG
reaction. They were Raney Cu containing 2% Cr and Raney Cu containing 0.75% Cr and
2
.8% Ni. These catalysts along with a commercial CuCrO catalyst and the unmodified Raney
Cu were used in long term GLY to PG reactions with the results shown in Table 5. While all of
the reactions went to complete conversion, the use of the unmodified Raney Cu catalyst
resulted in the highest PG selectivity. The drop in PG selectivity using Raney Cu-Cr suggests
that the final hydrogenation step shown in Scheme 1 takes place primarily on the Cu surface.
With the Raney Cu-Cr catalyst, the Cu is probably partially covered by CrO moieties. This
x
decrease in hydrogenation ability allows time for the competing degradation reactions to take
place. The Cu-CrO catalyst gave a differing product distribution, less PG and more EtOH. In
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the first days of operation with this catalyst the product mixture was strongly colored suggesting
Table 5. Percent conversions and product compositions from GLY hydrogenolysis over copper
a
catalysts.
%
Selectivities
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
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59
60
b
TOS
(
%
n-
PrOH
Catalyst
PG
EG
MeOH EtOH
ACT 1,3PG
Days) Conv.
Raney
Cu
Raney
Cu-Cr
Raney
Cu-Cr-Ni
Cu-CrO
6.2
4.9
3.9
5.0
100
100
100
100
95.0
87.4
82.9
82.5
1.9
0.6
1.8
0.7
0.9
1.0
0.2
0.9
0
0.2
0
0.4
0.5
0.4
0.5
0
3.9
0
1.0
0
0.2
0
2.2
0.5
a
Reactions run at 205 °C with a H flow rate of 375 cc/min and pressure of 200 psi and an 80%
GLY feed flow rate of 0.05 cc/min.
2
b
Time on Stream.
either some leaching or the presence of colored unanalyzed oligomers. This later is more
consistent with the lower material balance observed with Cu-CrO (~92%) as opposed with
9
8.5% with Raney Cu. Since there was no interest in pursuing the use of Cu-CrO or the other
modified catalysts, this matter was not investigated further.
The unmodified Raney Cu was stable under these same reaction conditions in runs of up to
2
4 d with the conversion remaining at 100% and a stable PG selectivity at 95%. However, under
these reaction conditions the STY was only 0.17 grams of PG/cc of catalyst/h. Data in the
literature on the use of supported Cu catalysts show that higher temperatures could be used with
Cu catalysts but this was usually associated with higher H pressures because the increase in
2
1
9-21,25
temperature would decrease the H coverage of the catalyst.
With this in mind, the effect
2
of increasing the reaction temperature was examined using a 50% aqueous GLY solution. The
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2
2
2
2
2
2
2
2
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3
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data in Table 6 show that increasing the
temperature to 240 °C had no effect on the
GLY conversion. The PG selectivity was
stable between 210 °C and 225 °C but
decreased at the higher temperature. It was
decided that further reactions would be run
Table 6. Effect of higher temperatures on
the Raney Cu catalyzed conversion
a
of GLY to PG.
Temperature
%
% PG
°C
Conversion
Selectivity
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
210
100
100
100
96.2
2
2
25
40
96.3
83.8
a
Reactions run with a H
2
flow rate of 300 cc/min
at 225 °C with the H pressure increased to
2
and pressure of 600 psi and a GLY feed flow rate
of 0.10 cc/min.
600 psi and a H flow of 300 cc/min.
2
With these changes the effects of GLY feed concentration and flow rate would have to be
reexamined. In comparing the effect of different GLY feed concentrations, though, the flow rate
is not a useful parameter since the use of different concentrations of GLY feed at the same flow
rate would result in differing amounts of GLY feed passing through the catalyst over a given
time. The factor which should be used is the Weight Hourly Space Velocity (WHSV) which is
defined as the grams of substrate/gram of catalyst/h. The results from reactions run using 50%,
6
5% and 80% aqueous GLY feeds at different WHSV’s are shown in Figures 3 and 4. These
data indicate that there is a combination of factors involved in the GLY to PG reaction. Among
these is the effect which increasing the water concentration has on the competitive adsorption on
the catalyst surface between water and GLY. Another factor is the changing of the contact time
(flow rate or WHSV) of the reactants with the catalyst and the effect it has on the kinetics of the
reaction. A third factor is the effect which increasing the water concentration had on the
reactions shown in Scheme 1. There are two separate aspects of this reaction which are
summarized in Figures 3 and 4, the percent conversion of GLY and the selectivity in the
formation of PG and EG from the GLY.
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100
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9
9
8
5
0
5
5
6
8
0% Glycerol
5% Glycerol
0% Glycerol
0.2
0.3
0.4
0.5
0.6
0.7
WHSV
Figure 3. WHSV effect on the percent conversion observed using different aqueous GLY
concentrations for the GLY to PG reaction run over Raney Cu at 225 °C
with a H pressure of 600 psig and a flow rate of 300 cc/min.
2
Increasing the flow rate will decrease the contact time of the water and GLY on the catalyst
surface. This fact, combined with the competitive adsorption of water on the Cu surface, would
indicate that at a particular flow rate GLY adsorption could be diminished to the extent that the
conversion of GLY decreased. As shown in Figure 3, this point was reached with the 50%
aqueous GLY at a flow rate of about 0.15 cc/min (WHSV = 0.35) while with the 65% GLY it
was at a flow rate of about 0.19 cc/min (WHSV = 0.57) and with 80% GLY about 0.15 cc/min
(
WHSV = 0.55). This was not an indication of catalyst deactivation since when the flow rate
was decreased the conversion returned to 100%. This type of behavior was observed at other
times when changes in the GLY feed flow rate was being examined. When a decrease in GLY
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1
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1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
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conversion was noted with an increase in the GLY feed flow rate the conversion could be
recovered by returning to the original flow rate.
100
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
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0
PG
5
6
8
0% Glycerol
5% Glycerol
0% Glycerol
80
8
EG
6
4
2
0.2
0.3
0.4
0.5
0.6
0.7
WHSV
Figure 4. WHSV effect on the PG and EG selectivities observed using different
aqueous GLY concentrations for the GLY to PG reaction run over Raney Cu at
2
25°C with a H pressure of 600 psig and a flow rate of 300 cc/min.
2
The product compositions shown in Figure 4, though, appear to depend on the effect which water
had on the different reaction pathways depicted in Scheme 1. It is possible that an increase in the
water content of the feed could inhibit the initial dehydration of GLY to form ACT and this
would decrease the formation of PG while shifting the catalytic activity to those paths which
lead to the formation of EG, pathways in which water does not play a role. As the water content
of the feed decreased, the amount of PG increased, though slightly, while the EG selectivity
decreased significantly.
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Table 7. Product composition from 24 d (580 h) continuous GLY hydrogenolysis
a
catalyzed by Raney Cu.
%
Selectivities
% Conv.
100
PG
EG
MeOH
0.4
EtOH
0.7
n-PrOH
0.6
ACT
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
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29
30
31
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59
60
94.6
2.5
1.0
a
Reaction run at 225 °C with a H flow rate of 300 cc/min and pressure of 600 psi and
2
an 80% GLY flow rate of 0.12 cc/min.
In Table 7 are shown the average product compositions for a reaction run continuously for 24 d
using 80% aqueous GLY over a Raney Cu catalyst at 225 °C, a H flow rate of 300 cc/min, a
2
H pressure of 600 psi and a GLY feed flow rate of 0.12 cc/min (WHSV = 0.440). Product
2
compositions varied by less than 1% throughout the reaction. The STY, under these conditions,
was 0.49 g PG/cc catalyst/h corresponding to 3.98 kg of PG being produced over the course of
this reaction. The catalyst used for the reaction was a sample obtained from a commercial scale
production of Raney Cu.
3. CONCLUSION
A commercially prepared Raney Cu catalyst has been shown to be an efficient and selective
catalyst for the conversion of glycerol to propylene glycol under moderate conditions; 80%
aqueous glycerol, 225 °C, H pressure of 600 psig in a liquid phase fixed bed reactor. The
2
unmodified Raney Cu catalyst was shown to be superior to Raney Νi, Cu-CrO and Raney
Cu catalysts containing Cr and Ni modifiers.
4
. EXPERIMENTAL SECTION
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4
.1. Materials. The Raney catalysts used in this study were made using proprietary
techniques involving the partial leaching of Al from 8 to 12 mesh sized particles of Ni-Al, Cu-
Al, Cu-Al-Cr and Cu-Al-Ni-Cr alloys using dilute NaOH solutions. The copper chromite (Cu-
CrO) catalyst was a 1/8”extrudate obtained from BASF, Elyria, OH. The sample used was
obtained by first crushing the extrudates and sieving the resulting material to 10-20 mesh before
packing the catalyst in the reactor. The Cu-CrO catalyst was reduced, in situ, in a 20 mL/min
flow of hydrogen at 205 °C for 2 h, after raising the temperature at 20°/h from ambient to 205
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
°
C. The Raney catalysts did not require any in situ activation.
4
.2. Description of the fixed bed reactor system. The continuous hydrogenation of glycerin
over particulate Raney catalysts was carried out in the fixed bed reactor depicted in Figure 5. The
gas control section, A, consisted of a channel for low pressure inert gas, a low pressure hydrogen
line, a by-pass channel for pressurizing the system and a high pressure hydrogen delivery
channel. All four channels were fitted with Brooks mass flow controllers (6) and high pressure
two-way (7) and three-way (8) solenoid valves (Parker Instruments). All controllers and the
solenoid valves were interfaced to an in-house developed data acquisition and control system.
Section B was the GLY feed unit. The GLY solution, in a glass reservoir (9), was kept under 30
psi nitrogen pressure and placed on a balance for continuous monitoring and recording of the
amount of feed added over time. The solution was pumped into the reactor using a high precision
Shimadzu LC-10AT pump (10) while the liquid flow rate was also monitored by a Brooks liquid
mass flow meter (11).
Section C was a 0.5 inch id tube reactor fitted with three band-heaters and quarter inch inlet and
outlet ports. The reactor was partitioned into three zones with the temperatures in each zone
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controlled by two Omega temperature controllers; one for the internal reactor temperature and
the second for controlling the temperature of the band heater. This combination of two
PC
6-1
A
B
10
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12
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60
Aqueous Glycerol
HP
Argon
LP
7
-1 PC
LP
9
6
-2
LP
PC
PC
LP
Pump 10
MFM 11
HP
PC
7-2
6
-3
PC
PC
8
-1
Hydrogen
8
-2
HP
Inlet
Pressure
8-3
PC
PC
PT
PC
13-1
TC-1
C
TC-3
TC-4
1
2
Outlet
Pressure
PC
PC
PC
PT
Gaseous product
D
BPR
Condenser
13-2
12
TC-5
Sample
Collector
1
5
14
TC-2
PC
16
Liquid
Product
Figure 5. Fixed bed reactor
controllers connected in series enabled maintaining the temperature of each zone within ±0.2 °C.
The top, pre-heating, zone was fitted with 2 ꢀm stainless steel filters, 12, and used for pre-
heating and mixing the aqueous glycerin and hydrogen. The second zone was the “hot catalyst”
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temperature zone with a thermocouple inserted 0.5 inch into the catalyst bed. The third zone was
the catalyst bed temperature zone with a second thermocouple placed 0.5 inch into the lower
section of the catalyst bed. The reactor inlet and outlet pressure were monitored by Omega
pressure transducers (13).
0
1
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The fourth section,D, consisted of a condenser (14), a domed back pressure regulator (15) and a
o
gas–liquid separator (16) operated at a temperature of 6 C. The excess hydrogen flow was
directed to a trap kept in dry ice.
All two and three way valves, the mass flow controllers and meters, the reactor temperatures, the
inlet and outlet pressures and the balance reading were constantly under computer control and
monitoring. Special attention was paid to the safety of the flow system so when any one of the
process parameters (temperatures in the reactor, pressure, gas and liquid flows) varied by more
than 10% from the pre-set values, the system was programmed to enter an “abort regime” which
terminated the power to the heaters and the pump and vented the reactor to a by-pass line.
4.3 Hydrogenolysis of GLY. Three filtering elements, 12, were inserted over the bottom
thermocouple to serve as a catalyst support (Figure 5). The catalyst, 14 cc (16.5 g), was
suspended in water and carefully transferred into the reactor under a constant flow of water for
uniform packing and protection from direct contact with air. A GC vibrator was used to avoid
gaps between the particles. The reactor was sealed on the top with three additional filtering
elements, 12, to contain the catalyst bed. The reactor was attached to the feed and the exit lines.
The GLY solutions were prepared using de-ionized water at pH 7. At each set of reaction
conditions the catalyst performance was stabilized by running the reaction for 6-7 h after which
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time 3 collections were made; 2 day-time collections for an average TOS of 8 h each and one
overnight collection with an approximate 15 h TOS. All reactions were run in a down flow
mode with both the aqueous GLY and the hydrogen introduced at the top of the reactor. The
product flow exited the reactor, passed through a condenser, 14, which was kept at 25 °C, and
then to the BPR, 15, which lowered the pressure to ambient. The exit flow was switched to a
waste flask during the stabilization steps or to a collection flask during the reaction steps and
analyzed offline. The gas phase was passed through a liquid trap held at -36 °C to condense low
boiling products which may have been in the hydrogen flow.
10
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4.4 GC analysis was done using an HP 5890 with FI detector and an RTX1701 capillary
column (60 m, 0,53 mm ID and film thickness of 1 m). 1,4-Butanediol was used as the internal
ꢀ
standard with the response factor (RF) calibrations for the reaction products obtained using 5
point curves.
FORMULAS
%
%
Conversion of GLY = [mmol converted GLY] * 100 / [mmol GLY]o
Selectivity to PRODUCT = [mmol PRODUCT formed] * 100 / [mmol converted GLY
]
Space-Time-Yield (STY) = g PRODUCT/cc catalyst/h
Weight Hourly Space Velocity (WHSV) = g GLY/g catalyst/h
Liquid Hourly Space Velocity (LHSV) = cc GLY/cc catalyst/h
REFERENCES
(1) Stelmachowski, M. Ecological Chemistry and Engineering 2011, 18, 9-30.
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