Hydrogenolysis of sorbitol over Ni and Pt loaded on NaY

Article abstract of DOI:10.1016/j.catcom.2010.12.026

The hydrogenolysis of sorbitol (15% aqueous solution) was carried out at 60 bar pressure at 220 °C in a stirred batch reactor using Ni-NaY (2, 4, 6 wt.% Ni) with and without added Pt(1 wt.%). 1,2-Propanediol was the major product over Ni-NaY and glycerol was the main product over Pt(1 wt.%)-NaY. The addition of 1 wt.% Pt to the Ni-catalysts had only a marginal effect on conversion and selectivity of the catalysts. Addition of Ca(OH)₂ as the promoter to both Ni and Pt catalysts increased the conversion significantly without any significant effect on selectivity.

Full text of DOI:10.1016/j.catcom.2010.12.026

Catalysis Communications 12 (2011) 673–677
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Catalysis Communications
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Short Communication
Hydrogenolysis of sorbitol over Ni and Pt loaded on NaY
b
c
M. Banu a, S. Sivasanker a, , T.M. Sankaranarayanan , P. Venuvanalingam
⁎
a
National Centre for Catalysis Research, Indian Institute of Technology, Madras, Chennai, 600 036, India
b
Department of Chemistry, Anna University, Chennai, 600 025, India
School of Chemistry, Bharathidasan University, Tiruchirapalli, 620 024, India
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 2 August 2010
Received in revised form 9 December 2010
Accepted 17 December 2010
Available online 28 December 2010
The hydrogenolysis of sorbitol (15% aqueous solution) was carried out at 60 bar pressure at 220 °C in a stirred
batch reactor using Ni–NaY (2, 4, 6 wt.% Ni) with and without added Pt(1 wt.%). 1,2-Propanediol was the
major product over Ni–NaY and glycerol was the main product over Pt(1 wt.%)–NaY. The addition of 1 wt.% Pt
to the Ni-catalysts had only a marginal effect on conversion and selectivity of the catalysts. Addition of Ca
(OH)2 as the promoter to both Ni and Pt catalysts increased the conversion significantly without any
significant effect on selectivity.
Keywords:
Sorbitol
© 2010 Elsevier B.V. All rights reserved.
Hydrogenolysis
Glycerol
Propanediol
Glycols
Ni–NaY
Pt–NaY
1. Introduction
a fixed bed reactor over Ni/SiO2 and obtained 30% conversion to glycerol
[6]. Sohounloue et al. conducted a study on sorbitol hydrogenolysis over
Cellulosic biomass is an abundant organic material on earth, and is
therefore an ideal feedstock for the production of bio-based chemicals
[1]. In a simple processing scheme, cellulose is hydrolyzed/hydro-
genolyzed into simple sugars/sugar alcohols [2] that are then
transformed into valuable chemicals through subsequent processing
[3–5]. For example, hydrogenolysis of sorbitol (obtained from
glucose) can be used to produce valuable di- and polyhydric alcohols
like ethylene glycol, propanediols and glycerol, which are generally
produced by multi-step transformations from non-renewable petro-
leum based feed-stocks. The discovery and easy availability of
petroleum and the ease of transformation of petroleum derived
olefins into the various alcohols and chemicals, decreased the
importance of the carbohydrate route for the manufacture of organic
chemicals. Presently, the transformation of carbohydrates obtained
from renewable sources into value added chemicals has assumed
significance owing to the decreasing petroleum reserves and
increasing carbon accumulation in the atmosphere. Hence, for
environmental and economic reasons, conversion of biomass derived
sorbitol into commodity chemicals like propanediol and ethylene
glycol has received increasing attention in recent years.
Ru/SiO2 and discussed the dependence of bond cleavage on temperature
with respect to a retro-aldol condensation mechanism in a basic
medium [7]. Montassier et al. found a substantial decrease in the activity
of Ru/carbon on S-poisoning and proposed that the main reaction
causing C–C bond cleavage in sorbitol hydrogenolysis was a retro-
Michael reaction under the action of adsorbed nucleophilic species [8].
Zhao et al. carried out sorbitol hydrogenolysis over Ru supported on
carbon nanofiber and graphite felt composite catalysts and reported 68%
conversion and 79% propylene glycol selectivity [9,10].
Ru catalysts are more expensive than Ni catalysts in spite of using
lower Ru loadings. In this communication, we report our studies on
the use of Ni and Pt loaded on NaY (zeolite) as catalysts for the
hydrogenolysis of sorbitol. We have investigated the influence of the
mode of preparation of the Ni catalyst, the effect of Pt addition and the
role of Ca(OH)2.
2. Experimental
2.1. Catalyst preparation
Clark et al. carried out sorbitol hydrogenolysis using Ni on Kieselguhr
and reported 40% glycerol yield [4]. Chao et al. carried out the reaction in
NaY zeolite (CBV100, Zeolyst International) with a silica/alumina
ratio of 5.1 was used in the preparation of the catalysts. Ni was loaded
on the zeolite by an incipient wetness method using different
amounts of NiNO3·6H2O in the required solution volume to obtain
catalysts with the desired loading of Ni (2, 4 and 6 wt.%). The Ni
loaded samples were dried at 120 °C for 12 h and calcined at 450 °C
⁎
Corresponding author. Tel.: +91 44 2257 4238; fax: +91 44 2257 5245.
E-mail address: ssivasanker@iitm.ac.in (S. Sivasanker).
1566-7367/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2010.12.026

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M. Banu et al. / Catalysis Communications 12 (2011) 673–677
for 4 h in air. Pt was loaded by exchanging the NaY with an aqueous
solution containing 0.025 M of [Pt(NH3)4]2+ at 80 °C for 6 h. After
exchange, the solution was filtered and the solid was dried at 120 °C
for 12 h and calcined in air at 450 °C for 4 h. Different Ni–Pt–NaY
catalysts were prepared by impregnation of Ni (using a NiNO3
solution) on calcined Pt(1 wt.%)–NaY by the incipient wetness
method. These samples were also dried at 120 °C for 12 h and
calcined at 450 °C for 6 h in air. An 8% Ni–NaY (IE) catalyst was
prepared by exchanging the NaY with an aqueous solution containing
0.05 M NiNO3 at 80 °C for 6 h. After exchange, the solution was filtered
and dried at 120 °C for 12 h and calcined at 450 °C for 4 h in air. All the
catalysts prepared by both incipient wetness and ion-exchange
methods were reduced in H2 at 400 °C for 4 h before carrying out
the hydrogenolysis experiments.
3. Results and discussion
The physicochemical characteristics of the catalysts are presented in
Table 1. The total surface areas (SL) were calculated using the Langmuir
equation as the BET equation is not suitable in the case of microporous
materials like zeolites. The surface areas are 721 44 m2/g for the
samples (Ni–NaY and Ni–Pt–NaY; Table 1). The external surface areas
(SExt) of the samples calculated by the t-plot method are 33 2 m2/g
(Table 1). The external surface area presumably arises mainly from the
surface of the crystallites with contributions from mesopores present (if
any) in the crystallites and amorphous material (if any). The parent
zeolite (NaY) itself has SL and SExt areas of 765 and 34 m2/g. The
similarity in the surface area values for the samples and the parent
zeolite suggest that the NaY did not undergo any significant damage
during metal impregnation and calcination procedures. XRD patterns of
the samples and the parent NaY were also similar confirming the
absence of structural damage during catalyst preparation. The XRD
patterns of two catalyst samples are compared with that of NaY in Fig. 1.
Metal dispersion values were obtained for many of the samples by a
pulse H2 chemisorption method. The dispersion values (%) for the
samples (Table 1) are: Pt(1%)–NaY, ~100; Ni(2%)–NaY, 24; Ni(6%)–NaY,
12; Ni(2%)–Pt(1%)–NaY, 20; Ni(6%)–Pt(1%)–NaY, 22%, Ni(8%)–NaY
(Exchanged), 6%. The dispersion values are in the range of 20–24% for
the low Ni and Ni–Pt samples. Earlier workers have reported increased
dispersion values for Pt–Ni than for Ni in the case of supported metal
catalysts [12]. The low dispersion value (6%) of the exchanged Ni sample
is due to the smaller amount of reduced Ni present in it. Temperature
programmed reduction (TPR) studies carried out up to 600 °C reveal a
H2 consumption of 1.018 mmol/g by the impregnated catalyst and
0.254 mmol/g by the ion-exchanged sample corresponding, respective-
ly, to about 100 and 20% reduction of the Ni in the samples. Further, the
reduction temperature maximum was lower for the impregnated
sample (439 °C) than for the ion-exchanged sample (510 °C) suggesting
a greater ease of reduction of the Ni ions in the impregnated sample. The
large dispersion value (~100%) obtained for the Pt (1%) catalyst is to be
expected as the Pt loading was done by ion-exchanging and the Pt
content is also low. TPR studies of this sample revealed near quantitative
reduction of the Pt ions.
2.2. Catalyst characterization
XRD patterns of the calcined catalysts were obtained using a Rigaku
Miniflex II instrument with Cu Kα as the radiation source. Surface areas
and pore volumes of the samples were obtained by N2 adsorption at
liquid N2 temperature (Micromeritics, ASAP 2020). TPR profiles
were obtained on a commercial apparatus (Micromeritics TPD/TPR
2900) interfaced with a computer. Prior to the measurements, the
catalyst (ca. 50 mg) was dried in a TPR cell at 600 °C for 2 h in a stream
of He to remove water and adsorbed impurities. The TPR profiles
were then obtained by passing a 10% H2/Ar flow (60 mL min−1
)
through the sample at temperatures from 30 to 600 °C. The temperature
was increased at the rate of 15 °C min−1 and the amount of H2
consumed was determined with a Thermal Conductivity Detector
(TCD). A cooling trap was placed between the sample and the TCD to
retain the water produced during the reduction process. Metal
dispersion measurements were carried out on the reduced sample
(0.2 g) by pulsed chemisorption of H2 at 25 °C using the Micromeritics
instrument mentioned above [11]. The sample was reduced at450 °C for
2 h and flushed with Ar at the same temperature for 2 h prior to H2
chemisorption.
Catalytic reactions were carried out with Ni, Pt and Ni–Pt loaded on
NaY zeolite. The choice of NaY was based on earlier reports that the
sorbitol hydrogenolysis reaction proceeds well when alkaline catalysts are
used [9]. As the cage opening in NaY is only slightly larger (~0.74 nm)
than the molecular dimensions of sorbitol [0.53 nm×0.67 nm×2.9 nm],
one would expect the presence of substantial diffusion constraints, and,
therefore, it is likely that much of the reaction will take place on the
external surface of the zeolite crystallites. The hydrogenolysis of sorbitol
produces a number of products [5]. The major ones that are discussed in
this paper are presented in Scheme 1. A cleavage of the central C–C bond
will lead exclusively to glycerol (II). The other products (III to VI) are
obtained directly from sorbitol or from (I) or (II) depending on the
cleavage/reaction of different bonds. Experiments using glycerol as the
reactant revealed that it did not undergo much hydrogenolysis (b2%
conversion) under the experimental conditions over 1% Pt–NaY, though
2.3. Hydrogenolysis experiments
Sorbitol hydrogenolysis was carried out in a 300 mL stainless steel
(SS) autoclave (Parr 4843) using 100 g of an aqueous solution of
15 wt.% sorbitol. After the aqueous mixture of sorbitol was introduced
into the autoclave, the H2-reduced catalyst (powder) was added. The
sealed autoclave was purged by flowing H2 at room temperature. The
contents were then stirred (300 rpm) and the autoclave pressurized
with H2 to 50 bars and heated to the reaction temperature. Once the
desired temperature was reached, the pressure was adjusted to
60 bars and timing was started. During the reaction, the H2 pressure in
the autoclave was kept constant by periodic addition of small
amounts of hydrogen. After 6 h, the autoclave was cooled rapidly
with cold water. The product mixture was then collected, weighed,
filtered and analyzed.
Table 1
Physicochemical properties of the catalysts.
2.4. Product analysis
Sample
SL (m2/g)
SExt (m2/g)
Dispersion, D (%)
Polyols were analyzed in a Gas Chromatorgraph (GC) with a capillary/
column (FFAP) after derivation by silylation using hexamethyldisilazane
(HMDS) and trifluoroacetic acid in dimethylformamide [5]. The liquid
product mixture was also analyzed by HPLC (LC20AT, Shimadzu)
equipped with an RI detector (RID-10A, Shimadzu). The separation of
the product mixture was achieved using a NH2 column (Phenomenex) at
25 °C. Redistilled water was used as the mobile phase at a flow rate of
0.5 mL min−1 and a typical analysis run lasted 20 min.
Ni (2%)–NaY
Ni (4%)–NaY
Ni (6%)–NaY
Ni (8%)–NaY(exchanged)
Ni (2%)–Pt (1%)–NaY
Ni (4%)–Pt (1%)–NaY
Ni (6%)–Pt (1%)–NaY
Pt (1%)–NaY
725
739
754
740
677
691
705
760
765
33.6
33.9
34.2
32.1
31.0
31.0
31.1
33.0
33.6
24
–
12
6
20
–
22
100
–
NaY

M. Banu et al. / Catalysis Communications 12 (2011) 673–677
675
Fig. 1. Powder XRD patterns of (a) NaY, (b) reduced 8% Ni–NaY (ion-exchanged) and
(c) reduced 6% Ni–NaY (impregnated).
significant conversion (32%) of glycerol was noticed over 6% Ni–NaY, the
major products over both the catalysts being methanol and ethanol.
Earlier workers have reported the occurrence of steam reforming of
polyols in the presence of Pt/alumina catalysts [13,14] and the formation
of alkanes over Pt/acidic supports [15]. We did not observe any reforming
reaction or any alkane formation in our experiments over NaY based
catalysts under our experimental conditions. Small amounts (b0.1 wt.%)
of CH4 was, however, formed over 6% Ni–NaY. The overall mass balances
were always better that 98% suggesting negligible conversion of sorbitol
or the intermediates into gaseous products.
The results of the catalytic hydrogenolysis experiments carried out
over NaY and Ni(2–6%)–NaY are presented in Fig. 2. The temperature
of the reaction was 220 °C, the total pressure (with H2) was 60 bar and
the duration of the reaction was 6 h. NaY alone (without any metal) is
found to be slightly active (15% conversion). Conversion increases
rapidly with Ni-content, the selectivity for the desired propanediol
increasing with Ni loading. Conversion over the 2% (Ni) catalyst
(D, 24%) is found to be 39%, while it is 66% over the 6% sample (D,
12%). The increase in conversion with Ni-loading (from 2 to 6 wt.%)
appears to be nearly proportionate to the amount of available surface
Ni-atoms (D×metal content) in the catalysts. Selectivities for glycerol
and ethylene glycol (EG) go through a small maximum at about 2% Ni.
The reason for small maximum observed for the two products is
because in the absence of the metal NaY produces mainly the polyols
(Table 1) and the selectivity for glycerol and EG are low. So, when the
data is plotted (Fig. 2), a small maximum is noticed for the two
Fig. 2. Sorbitol conversion and product selectivity over Ni–NaY (conditions: 15% sorbitol in
water; temperature: 220 °C; pressure: 60 bar; time: 6 h; stirring speed: 300 rpm).
products in the case of the 2% Ni-catalyst. The influence of
temperature (in the range 200 to 230 °C) on the reaction over Ni
(6%)–NaY is presented in Fig. 3. Though, initially there is a significant
increase in conversion on going from 200 to 220 °C, the increase is not
significant between 220 and 230 °C. Selectivity for 1,2-propanediol
(1,2-PD) is found to increase slightly with temperature.
Results of experiments carried out on the support NaY and its
acidic form (HY) are presented in Table 2. Though the basic NaY is
only marginally active (15% conversion, Table 2), the acidic HY is even
less (~7% conversion) active. The selectivity over the two catalysts is
also different; HY produces more of the cracked lighter alcohols and
the polyols than NaY. This suggests that the mechanisms for the
transformation of sorbitol are different over acidic and basic catalysts,
basic catalysts being more active than the acid catalysts. The influence
of catalyst usage on the reaction is also presented in Table 2. An
increase in the amount of the catalyst used from 0.2 g to 0.5 g
increases the conversion of sorbitol significantly though the increase
is only marginal beyond 0.5 g (Table 2, compare #3, 4 and 5). The
large increase in conversion noticed on increasing the loading from
0.2 to 0.5 g is attributed to the increase in the number of active sites.
The smaller increase in conversion on going from 0.5 to 1.0 g is
probably due to the fairly large conversion values (66–68%) recorded
at the higher loadings and possible poisoning of the catalysts by one or
more of the products. Though the product selectivity at the two higher
Fig. 3. Influence of temperature on the hydrogenolysis of sorbitol over Ni(6%)–NaY
catalyst (conditions: 15% sorbitol in water; catalyst amount: 0.5 g; pressure: 60 bar;
time: 6 h; stirring speed: 300 rpm).
Scheme 1. Products of hydrogenolysis of sorbitol.

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M. Banu et al. / Catalysis Communications 12 (2011) 673–677
Table 2
Hydrogenolysis of sorbitol over Ni–NaY and other catalysts.a
#
Catalyst [g]
Sorbitol
Product selectivity (%)
conversion (%)
Glycerol
Dihydric alcoholsb
Monohydric
alcoholsc
Polyolsd
Otherse
1,2-PD
EG
1
2
3
4
5
6
NaY [0.5]
15
7
24
66
68
54
17
10
12
14
15
8
8
–
1
–
4
59
5
7
7
41
27
16
10
11
33
29
4
4
HY [0.5]f
Ni (6%)–NaY [0.2]
Ni (6%)–NaY [0.5]
Ni (6%)–NaY [1.0]
Ni (8%)–NaY (IE) [0.5]
53
62
60
20
10
7
7
–
–
9
14
16
a
b
c
d
e
f
15% sorbitol in water; temperature: 220 °C; pressure: 60 bar; time: 6 h; stirring speed: 300 rpm.
1,2-PD=1,2-propanediol; EG=ethylene glycol.
Methanol and ethanol.
Trihydric (except glycerol) and higher alcohols.
Light products (some unidentified).
NH4-Y (Zeolyst, CBV300) calcined at 450 °C for 6 h.
loadings is similar, it is somewhat different at the lowest loading
(0.2 g). The reason for this is the slightly larger production of the
polyols (xylitol and erithyritol) and others (total selectivity 20%) at
0.2 g loading when compared to the lower (10–11%) selectivity for
these compounds observed at 0.5–1.0 g (Table 2, compare #3, 4 and
5). The greater selectivity for the polyols translates to a lower
selectivity for the desired 1,2-PD.
The performance of Pt(1%)–NaY and Ni(2–6%)Pt(1%)–NaY are
presented in Table 3 and Fig. 4. The Pt(1%)-only catalyst, though it has
a higher activity (43%) than the Ni(2%)Pt(1%) catalyst (31%), produces
mainly glycerol and not the desired propanediol (Table 3, #3). Also,
the Ni(2%)Pt(1%) catalyst is found to have a lower activity than the Ni
(2%) catalyst (Fig. 2). It appears that the introduction of Pt in the Ni
catalyst decreases its activity slightly. However, the addition of Pt to
the Ni catalysts does not appear to alter significantly the overall
selectivity of the Ni-only catalysts (compare Figs. 2 and 4) in spite of
the slight decrease in conversion (especially for 2 and 6% Ni catalysts).
The absence of significant influence by Pt may be due its low
concentration in the catalysts compared to Ni (Ni/Pt atomic ratios are,
respectively, 7, 14 and 20 for 2, 4 and 6% Ni–Pt(1%) catalysts). The
reason for the small decrease in conversion in the case of the 6% Ni
catalyst (in spite of the increase in dispersion from 12% to 22% on
adding Pt) is not clear. As the present reaction system is a complex
one and the effects of alloying could also be complex, more studies
will be needed to understand the phenomena better. The nature of the
interaction between Pt and Ni in Pt–Ni supported catalysts, the
amount of alloy formation, and the enrichment of Pt on the Ni surface
(or vice-versa) have been a matter of debate for a long time [12,16].
The overall conclusion in our studies is that though both Pt and Ni
cleave primarily the C3–C4 bond, the adsorbed species are different
over the two metals leading to different products (glycerol over Pt and
1,2-PD over Ni).
The influence of loading Ni by ion-exchange on the performance
of the catalyst is presented in the last row (# 6) in Table 2.
Ion-exchanging decreases conversion besides leading to a lower
yield of 1,2-PD (Table 2, compare # 6 with 4). The catalyst also
produces more polyols and monohydric alcohols (methanol and
ethanol). In the calcined impregnated catalysts, one would expect
most of the Ni to be present as Ni-species on the external surface of
the zeolite crystallites and only a small amount is expected to be
present at exchange positions inside the cages. On reduction, the Ni
present on the external surface is likely to get reduced more easily
into small Ni crystallites. In the case of the catalyst prepared by ion-
exchanging, one would expect nearly all of the Ni ions to be present in
ion-exchange positions with only a small amount getting reduced in
H2 during the reduction step. These expectations are confirmed by
XRD and TPR studies (discussed earlier) of the two samples. A weak
line (at 2θ=44.5o) corresponding to the [111] plane of cubic Ni-metal
is seen in the XRD pattern (Fig. 1) of the impregnated sample and not
in the pattern of the ion-exchanged sample.
The lower activity of the exchanged catalyst (in spite of its higher
metal content) is, therefore, due to the lower amount of reduced Ni
present in it. This is confirmed by the fact that substantial amounts of
monohydric and polyhydric products are obtained as in the case of
NaY alone (Table 2, compare # 1 and 6) revealing that the support
(NaY) contributed significantly to the reaction in the case of the
ion-exchanged catalyst.
Earlier workers have reported that Ca(OH)2 added to the reaction
mixture increases conversion [9]. This is found to be true in our case
also both for the Ni (Table 3, compare # 1 and 2) and Pt (Table 3,
compare # 3 and 4) catalysts; it is seen that the addition of Ca(OH)2 to
the reaction mixture enhances conversion without significantly
affecting the inherent selectivity of the Ni and Pt catalysts. However,
Ca(OH)2 alone (without any metal catalyst) possesses a moderate
Table 3
Hydrogenolysis of sorbitol over Ni and Pt-catalysts: influence of Ca(OH)2 addition.a
#
Catalyst [g]
Sorbitol
Product selectivity (%)
conversion (%)
Gly-cerol
Dihydric alcoholsb
Monohydric
alcoholsc
Polyolsd
Otherse
1,2-PD
EG
1
2
3
4
5
Ni (6%)–Pt (1%)–NaY [0.5]
Ni(6%)–NaY [0.5]+Ca(OH)2 [0.5]
Pt (1%)–NaY [0.5]
Pt (1%)–NaY [0.5]+Ca(OH)2 [0.5]
Ca(OH)2 [0.5]
59
75
43
60
24
7
4
62
70
3
72
69
6
6
9
10
7
3
3
2
1
5
24
11
23
10
8
1
2
45
–
7
4
8
18
a
b
c
.15% sorbitol in water; temperature: 220 °C; pressure: 60 bar; time: 6 h; stirring speed: 300 rpm.
1,2-PD=1,2-propanediol; EG=ethylene glycol.
Methanol and ethanol.
Trihydric (except glycerol) and higher alcohols.
Light products (some unidentified).
d
e

M. Banu et al. / Catalysis Communications 12 (2011) 673–677
677
Therefore, it appears that in the case of Pt, the direct hydrogenolysis
of the C3–C4 bond is the major reaction leading preferentially to
glycerol, while over Ni, the hydrogenation of the C3–O/C4–O bonds
also occurs along with the hydrogenolysis of the C3–C4 bond to
produce 1,2-PD as the major product. It is likely that, over Pt,
adsorption occurs mainly at the C3 and C4 carbons, while over Ni,
adsorption occurs at the C3 and C4 carbons and the associated O-atoms
(multipoint adsorption).
4. Conclusions
The hydrogenolysis of an aqueous solution of sorbitol takes place
in the presence of H2 over both Pt and Ni loaded on NaY at 220 °C. 1,2-
PD is the major product over Ni–NaY, while glycerol is the major
product over the Pt–NaY. The addition of Pt to the Ni catalysts does
not influence their activity or selectivity significantly. Ca(OH)2
promotes the performance of both Ni and Pt catalysts without
affecting product selectivity. The difference in the product selectivity
observed over Pt and Ni catalysts is probably due to the different
modes of adsorption of sorbitol over the two metals.
Fig. 4. Sorbitol conversion and product selectivity over of Pt(1%)–NaY and Ni(2–6%)–Pt
(1%)–NaY catalysts (conditions: 15% sorbitol in water; temperature: 220 °C; pressure:
60 bar; time: 6 h; stirring speed: 300 rpm).
(24%) activity with a different product selectivity producing mainly
mono and polyhydric alcohols (Table 3, # 5). These observations
suggest that, over the metal containing catalysts, the reaction
proceeds mainly through the metal catalyzed hydrogenolysis of the
C–C bonds, and over Ca(OH)2 or NaY (Table 2; # 1), the cleavage of the
C–C bonds occurs through a base catalyzed retro-aldol type of
mechanism as proposed by earlier workers [9]. The lack of a major
change in selectivity on addition of Ca(OH)2 to the metal catalysts
indicates that the base probably acts more to promote the metal
catalyzed reaction and to a lesser extent as a separate catalytic agent.
Our experiment (Table 3; # 5) reveals that no solid Ca(OH)2 is present
in the product mixture at the end of the reaction (carried out at 200 °C
and 50 bar pressure). One would, therefore, expect much of the Ca
(OH)2 to be in solution with a small amount getting into the exchange
positions of the zeolite. Analysis of the used catalyst samples did
reveal the presence of substantial amounts of Ca in them (59%
exchange of Na in NaY in the experiment carried out with Pt(1%)–
NaY; Table 3, # 4). The presence of substantial amount of Ca-ions in
the exchange positions in the zeolite in close proximity to the metal
specie and the change in the basicity of the support may be
responsible for the observed influence of Ca(OH)2. This effect is still
not clearly understood, and is under investigation.
Acknowledgement
We thank Prof. B. Viswanathan, Head, NCCR and DST for support.
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