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Oxidation of alcohols and sugars using Au/C catalysts
Ramana Murthy.P
M.Comotti,C.DellaPina,R.Matarrese,M.Rossi ,A.Siani, Appl.Catal.A:Gen.291(2005)204-209.
Introduction
 Catalytic oxidation based on the use of gaseous oxygen, or air, in water
solution is of great interest for economic and ecological aspects.
 Gold on carbon catalysts, designed for liquid phase oxidation, have been
prepared on 80–500 g scale and evaluated for alcohols and carbohydrates
oxidation using glucose and ethane-1,2-diol as model molecules.
 Physico-chemical properties of 0.8% gold on carbon X40S and 1% gold on
carbon XC72R catalysts are discussed along with scaling up effect.
 Gluconic acid and gluconates are important industrial products,they are used
as water-soluble cleansing agents or as additives in food and beverages.
Immobilisation method : Preparations of 1wt % Au/C
HAuCl43H2O
PVA (protecting agent )
0.1 M NaBH4
Au(0) sol
carbon
Washed with distilled water,
Dried at 120 0C, 5 h
1wt % Au/C
Chemical parameters used for 1 kg preparations of the standard catalysts
Glucose oxidation
 Reaction conditions: glucose = 50% wt.; S/M ratio = 20,000; O2 flow = 1000 N cm3
min-1; stirring rate = 1700 rpm. The pH was maintained at 9.5 by automatic NaOH
addition (Titrino)
Ethane-1,2-diol oxidation
 Reaction conditions : PO2 = 303.9 kPa; substrate/catalyst ratio = 1000; [substrate] =
0.3 mol l-1;substrate/NaOH ratio = 2; T = 343.2 K. The reaction has been carried out
in uncontrolled pH conditions
Analysis of products
 HPLC analysis
 NMR analysis -13C NMR
 Unsupported gold particles in form of aqueous sol resulted very active in
converting glucose to gluconate, showing an initial rate comparable with the
enzymatic catalysis.
 The activity of naked particles lasts only a few minutes being the colloid
unstable owing to the formation of a deactivated aggregate.
 Carbon supported gold particles show the same initial rate as the unsupported
ones having the same dimension; owing to the anchoring effect of the support,
gold on carbon maintains its activity for long time.
 Gold exhibits a catalytic activity in inverse proportion to their diameter.
 The range of 2–10 nm, meaning that only the exposed fraction of atoms in the
nanometric particles is catalytically active. Moreover, crystallites larger than 10
nm are almost inactive.
Selection of Carbon support
 The choice of the support was restricted to two different activated carbons
(a) X40S (specific area of 1100 m2 g-1, pore volume of 0.37 ml g-1) derived
from coconut shell, having low ash and sulphur content (<0.1%).
(b) Vulcan XC72R (specific area of 254 m2 g-1, pore volume of 0.19 ml g1) derived from naphtha pyrolysis, having higher sulphur content (0.5%).
 During the evaluation of metal adsorption properties, a different behaviour
was observed. In fact, XC72R allowed quick and high gold load, up to 20%
wt. in a few minutes.
 X40S allowed only a much lower and slower adsorption, with load limited
to 1 wt.%.
 This different behaviour, not related to surface area and pore volume, could
be connected to different functional groups present at the carbon surface,
where, in the case of X40S carbon, carboxylates and phenols predominate.
Chemical parameters used for 1 kg preparations of the standard catalysts
Characterization of the (a) 2 x 500 g preparations of the Au/XC72R catalyst and (b) 12 x 80 g
preparations of the Au/X40S catalyst
HRTEM image of the colloidal gold dispersion (X40S)
Particle size distribution of the gold particles after deposition in the large scale preparation
on (A) X40S and (B) XC72R
Physico-chemical propriety of the large scale Au/X40S and Au/XC72R catalysts compared
with 2 g preparations
Glucose oxidation
Comparison among the catalytic activity of the Au/C preparations and the 2 g
preparations in glucose oxidation.
Reaction conditions: glucose 50% wt.; S/M ratio 20,000; O2 flow 1000 N cm3 min-1;
stirring rate 1700 rpm. The pH was maintained at 9.5 by automatic NaOH addition
Catalytic activity and selectivity of the
(a) 12 x 80 g Au/X40S and 2 x 500 g Au/XC72R preparation in the ethylene glycol
oxidation after 30 min of reaction.
(b) mixed catalysts in comparison with 2 g preparations in the ethylene glycol oxidation
after 30 min of reaction
Experimental conditions: PO2 = 303.9 kPa; substrate/catalyst ratio = 1000; [substrate] = 0.3
mol l-1; substrate/NaOH ratio = 2; T = 343.2 K. The reaction has been carried out in
uncontrolled pH conditions. a Sample.
Conclusions
 This study shows that the immobilisation of preformed gold sol on carbon could
represent a valid technique for 80– 500 g catalyst preparations
 Peculiar proprieties, connected to the nature of carbon support, can be outlined
as the different adsorption capacity and metal superficiality.
 In the case of ethane 1,2-diol, the higher Au/C ratio derived by XPS technique
seems to be responsible of the higher activity for gold supported on X40S
carbon
.
 In the case of glucose oxidation gold particle size seems to be the major factor
influencing the reaction rate, being the smaller particles, deposited on XC72R
carbon more active than the larger particles deposited on X40S.
 A small, but definite scaling up effect has been observed which is due to the
difficulty to preserve gold particles dimension in the larger scale preparations
The following are some new
remarks on this paper – the
statements made here are not
due to the authors of this paper
and hence they are not
responsible for these remarks
the remarks are from NCCRIITM
The main observations of this
paper that will be taken up for
discussion are:
1. What is the motivation for
studying the oxidation of
glucose and ethane 1,2-diol
oxidation ( one is bio-source
and another is naphtha source)
What is the purpose of choosing
two carbons one X40S and XC
72 R once again one plant
source and another naphtha
source?
What is the essential difference
between two sources of carbon
are they representing
hydrophilic and hydrobhobic
varieties?
Why the Au loaded on these
supports have uniform particle
size on XC 72R while that on
X40S the particle sizes varies
widely?
Why on XC72R supported
system appears to be better
catalyst for glucose oxidation?
Is there any parallelism between
the activities of these two
catalyst systems and enzymatic
reactions if so what are they?
What and how does the
presence of sulphur on these
two supports affect the
behaviour in the case of XC 72
R the sulphur content is higher
that on X40S still it is better
support why?
These are some of the points on
which we have analyzed this
paper and bring out some
aspects of this paper from these
view points.