RICERCA ITALIANA     

INNOVATIVE CATALYTIC PROCESSES FOR THE SELECTIVE OXIDATION AND REDUCTION OF GLYCEROL IN WATER: STUDIES OF REACTION MECHANISMS AND KINETICS FOR THE PROCESS OPTIMISATION

Università degli Studi di Palermo

Abstract

The aim of this Research Project is the development of innovative processes for the selective production of organic species of great added value by following the principles of Green Chemistry and Green Engineering.
The selective transformation of alcohols is an important preparative method in organic chemistry and the resulting products (as aldehydes, ketones, etc.) are extensively used in pharmaceutical, fine chemical and fibers industries. In the field of transformation processes an important area of Green Chemistry concerns the selection of the medium in which to carry out chemical reactions. In fact, many of the solvents commonly used are dangerous and/or volatile so that they may cause contamination and smog when released to atmosphere. Then, the general trend is the utilization of environmentally benign solvents.
On the basis of previous considerations, the present Research Project is focused on the selective transformation of glycerol in water for the production of chemical intermediates of industrial interest such as 1,3-propanediol, 1,3-dihydroxyacetone, glyceraldehyde and glyceric acid. The study will be carried out by using processes which use innovative methods as heterogeneous photocatalysis, electrocatalysis and photoelectrocatalysis coupled with an innovative separation method as pervaporation. The investigation will concern with the preparation and characterisation of nanostructured semiconductor photocatalysts able to determine high quantum yields under near-UV radiation; these catalysts will adequately doped in order to exploit solar radiation. A detailed investigation on reactivity (both in batch and in continuous systems) and on surface characteristics of catalysts will be carried out for the determination of the reaction mechanisms, of the kinetic parameters and of the best operative conditions. For the continuous reactors the more efficient catalysts will be immobilised as thin films on suitable supports.
The proposed Research Program will develop a process which shows several aspects of technological innovation in a field of industrial interest; in fact it will use technologically advanced processes of transformation and separation (photocatalysis, electrocatalysis and photoelectrocatalysis together with pervaporation) coupled for producing a substantial improvement of glycerol transformation process. <<<

Principal Investigator

Vincenzo Augugliaro Università degli Studi di PALERMO

Research Objectives

The main objective of this research program is the development of a novel process able to perform the selective oxidation or reduction of glycerol in water under mild reaction conditions. This process is of great interest since some of the reaction products, such as dihydroxyacetone, glyceric acid, 1,3-propanediol and glyceraldehyde, are potentially useful as chemical intermediates. The novel process will be developed by following the principles of Green Chemistry (environmentally benign chemistry) and Green Engineering areas, i.e. the process will not use or generate hazardous substances but it will minimize the associated environmental impacts trying to reduce the amount of energy used in the process or to use cheap and clean solar energy. The novel process will be a catalytic one so that its development will concern: a) the catalyst; b) the reactor; and c) the operative conditions of the process.
By employing the following methods:
(i) heterogeneous photocatalysis,
(ii) electrocatalysis and
(iii) electrophotocatalysis
an optimal process able to maximize the production of the desired intermediate product and also to minimize the energy requirement will be identified.
As to concern the catalyst, objective of this research is that of obtaining photocatalysts able to perform the glycerol transformation with high quantum yield or under near-UV irradiation or under visible light irradiation. Moreover, this optimal catalysts will be immobilized on inert supports for allowing their utilization in continuous photoreactors or on titanium electrodes for their utilization in electrochemical and photoelectrochemical cells. Different immobilization techniques will be investigated with the objective of finding that able to maintain unaffected the catalyst features of reactivity and stability.
As to concern the process optimization, objective of this research is that of finding the best operative conditions by testing at laboratory scale various continuous photoreactors, electrocatalytic reactors and photoelectrocatalytic reactors. In order to improve the process performances, a pervaporation unit will be coupled with the reactor in order to continuously separate the desired intermediate product from the reacting mixture and so improve the process selectivity.
For optimising the process it is necessary to get information on the reaction kinetics and mechanism, and in the case of a photoprocess, also information on the radiation intensity profile inside the reacting system. Once an efficient process will be found, objective of this research will be to model it from kinetic, mechanistic and radiation absorption points of view. <<<

First Results

The most important result expected from this research program is the successful application of Green Chemistry and Green Engineering principles for performing an important preparative method in organic chemistry such as the selective transformation of alcohols to products to be used as precursors and intermediates for the pharmaceutical, fine-chemical and fiber industries. The novel process, get in working order at the accomplishment of the research program, allows to eliminate or drastically reduce severe environmental problems of traditional methods mainly related to waste generation from the use of heavy metals or stoichiometric amounts of reagents and to the production of undesirable coproducts.
Specifically the process, whose realization and characterization will be the result of this research program, concerns the selective oxidation or reduction of glycerol in water under mild reaction conditions. The oxidation of glycerol is a process of industrial interest since some of the partial oxidation products, dihydroxyacetone, glyceric acid and glyceraldehyde, are used as chemical intermediates. The reduction process of glycerol is also of great industrial importance as it allows the production of 1,3-propanediol, a starting material in the fiber industry.
For the realization of the process of selective transformation of glycerol in water the following methods will be employed: (i) heterogeneous photocatalysis ; (ii) electrocatalysis; (iii) electrophotocatalysis; and (iv) the previous methods coupled with pervaporation. It is therefore expected to obtain the following “hard” and “soft” results from the present investigation:
(a) a stable and efficient photocatalyst able to reach high quantum yields under near-UV irradiation;
(b) a stable and efficient photocatalyst able to absorb radiation in the visible region and so to be active under solar irradiation;
(c) a stable and efficient photocatalyst immobilized on inert support in order to be used in a continuous photoreactor;
(d) a stable and efficient catalyst immobilized on a conducting electrode to be used in an electrochemical cell;
(e) a stable and efficient photocatalyst immobilized on a conducting electrode to be used in a photoelectrochemical cell;
(f) an optimal photocatalytic reactor;
(g) an optimal electrocatalytical reactor;
(h) an optimal photoelectrocatalytical reactor;
(i) optimal preparation methods of pure photocatalysts and doped photocatalysts;
(j) optimal immobilization methods of bare and doped photocatalysts;
(k) informations on crystal structures and surfaces properties of (photo)catalysts to deepen the basic knowledge on these systems;
(l) informations on reaction mechanisms of glycerol transformation in the tested (photo)processes;
(m) informations on the chemical kinetics of the tested (photo)processes;
(n) informations on optimal operative conditions of tested (photo)processes;
(o) informations on optimal irradiation conditions;
(p) informations on the optimal operative conditions in the coupling of the tested (photo)processes with pervaporation operation.
(q) tuning of the modelling system for the dimensioning of the apparatuses, for the prediction of the performances and the scale up of the process.
(r) informations on the economical evaluation with rough estimations of the process and the final product costs.
Owing to the fact that the glycerol transformation is a process of great industrial importance, the results expected by this research program, both the “hard” and “soft” ones, will confirm the feasibility of the proposed green processes by constituting the basis for developing their technical application at large scale. <<<

Timescale

24 months

National and international background

With an increase in environmental consciousness throughout the world, there is a challenge for chemists and chemical engineers to develop new products, processes and services that achieve necessary social, economical and environmental objectives. The Green Chemistry (environmentally benign chemistry) and Green Engineering areas, developed in these last years, are the answers of the scientific community to that challenge. Green Chemistry is the utilization of set of principles that reduces or eliminates the use or generation of hazardous substances in the design, manufacture and application of chemical products and Green Engineering is the design, commercialisation and use of processes that are feasible and economic, reduce and prevent the generation of pollution at the source and minimise the risk to human health and the environment. As well as using and producing better chemicals with less waste, the challenge also involves reducing other associated environmental impacts including reduction in the amount of energy used in chemical processes [1-3].
An important area of green chemistry investigations concerns the selection of a medium in which to carry out a synthetic transformation. Many of the solvents commonly used are dangerous and volatile organic compounds known to cause contamination and smog when released in air so that there is an increased emphasis, both in the chemical industry and academic research, on utilization of environmentally benign solvents.
The selective oxidation of alcohols is an important preparative method in organic chemistry. The resulting products, aldehydes and ketones, are extensively used as precursors and intermediates for the pharmaceutical and fine-chemical industries [4-6]. They are particularly useful for the production of flavours, fragrances, and biologically active compounds. Traditionally, these carbonyl derivatives are produced through the selective oxidation of alcohols with a stoichiometric amount of strong oxidants
The application of these procedures, however, causes severe environmental problems due to waste from heavy metals or stoichiometric amounts of reagents and the generation of undesirable coproducts [7-11].
From the standpoint of green and sustainable chemistry, there is still a need to develop cleaner catalytic oxidation systems for this reaction. Obviously, the choice of the “green” oxidant determines the environmental impact of an oxidation process to a significant extent. In general, molecular oxygen is the ideal oxidant, and many processes have been developed with oxygen or air as oxidant in the presence of transition-metal complexes as catalysts under relatively mild reaction conditions [1-3].
The investigation here proposed deals with the selective transformation of glycerol in water under mild reaction conditions.
Today, glycerol plants are closing and others are opening that use glycerol as a raw material as a result of the large surplus of glycerol that is formed as a byproduct (10% in weight)in manufacturing biodiesel fuel. To illustrate the trend the global glycerol market was 800000 tons in 2005 with 400000 from biodiesel in comparison to 60000 tons only in 2001. Then the development of innovative processes able to use glycerol as raw material is of great industrial interest.
The products of glycerol transformation are of great interest since some of them, e.g. 1,3-propanediol, 1,3-dihydroxyacetone, glyceric acid and glyceraldehydes [12], are potentially useful as chemical intermediates.
The case of 1,3-propanediol is very interesting; in fact the recent development of a new polyester called poly(propylene terephtalate), with unique properties for the fiber industry, has determined the need of a drastic increase in the production of 1,3-propanediol [13,14]. Currently, 1,3-propanediol is made by the hydration of acrolein. The low conversion efficiency of the acrolein process, as well as the hazardous nature of acrolein, has spurred a great deal of interest in producing 1,3-propanediol from other chemical sources, especially glycerol. Recently, was developed a catalytic process coupled with a reactive distillation based on the hydrogenolisis of glycerol over a coper chromite catalyst (CuCr2O4) at 200°C and less than 10 bar (versus about 260°C and more than 150 bar for other systems) [15].
The glycerol dehydroxylation process enjoying more success and attracting the attention of more investigators is the fermentation process. Essentially, this methos is used for the oxidation of the secondary hydroxyl group by Gluconobacter oxydans to produce dihydroxyacetone; this compound is the main active ingredient in all sunless tanning skincare preparations (with a global market of about 2000 ton per year). Nevertheless, this process also has some drawbacks. One of the main drawbacks is its low theoretical yield, owing to the byproducts, such as ethanol, acetate, lactate, butyrate, H2, and CO2, formed in this process. Another main drawback is that the process is substrate-inhibited [16]. The bacteria used in the fermentation are generally not able to stand a glycerol concentration above 17%. As a result, both the product concentration and the productivity are low.
The catalytic oxidation processes of the glycerol primary hydroxy group are generally based on the aerobic oxidation in water over conventional precious metal based catalysts such as Au/C and Pt/C and yield glyceric acid. Carbon supported Au Catalyzes the oxidation of glycerol to sodium glycerate with 92% selectivity at full conversion, whereas Pt/C at 50°C yields glyceric acid with a maximum 70% yield at alkaline pH’s, ca. 11 [17,18]. For the oxidation of both primary hydroxyl groups to produce tartronic acid catalysts of Pt supported on CeO2 are used with a selectivity of 40% [19]. The drawbacks of the oxidative dehydrogenation are essentially due to a low stability of the supported metals in the oxidative environment and requires a thorough control of the reaction conditions to minimize the formation of undesired by-products.

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