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

carbonWashed 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)

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

Ethane-1,2-diol oxidation

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.

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 g-

1) 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.

Selection of Carbon support

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

Particle size distribution of the gold particles after deposition in the large scale preparation on (A) X40S and (B) XC72R

HRTEM image of the colloidal gold dispersion (X40S)

Physico-chemical propriety of the large scale Au/X40S and Au/XC72R catalysts compared with 2 g preparations

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

Glucose oxidation

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 NCCR-

IITM

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 choosingtwo 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.