R. Tesser

Lactose hydrolysis by immobilized β-galactosidase: the effect of the supports and the kinetics

Abstract The kinetic behaviour of β-galactosidase from Kluyveromices marxianus (Saccharomyces) lactis, immobilized on different oxides supports, such as alumina, silica, and silicated alumina has been studied. We observed a strong dependence of the immobilized enzyme activities on the chemical nature and physical structure of the supports. In particular, when the particle sizes of the supports are increased, the enzymatic activity strongly decreases. The hydrolysis of lactose, promoted by the mentioned enzyme immobilized on small commercial silica spheres, has been studied in different operative conditions, by changing: feed rate, reagent and products concentration and temperature, while pH has been kept constant (7.0). A depressing effect originated by both the reaction products, has been observed. Therefore, a kinetic model based on the Michaelis–Menten mechanism, in which the depressing effects of both the hydrolysis products (galactose and glucose) are also considered, have been developed and the related parameters determined.

Double bond oxidative cleavage of monoenic fatty chains

Abstract A two steps process for the production of azelaic acid and pelargonic acid or alternatively of ω-hydroxynonanoic and pelargonic acid, starting from respectively oleic acid and oleyl alcohol has been studied. In the first step, the monoenic reagent reacts with hydrogen peroxide, in the presence of pertungstic acid, as catalyst, to give the corresponding diol (hydroxylation of the double bond). In the second step, the reaction mixture obtained in the first step containing the formed diol and the exhausted catalyst, was additioned of cobalt acetate and reacted with molecular oxygen (oxidative cleavage of vicinal diols). As the first step has largely been studied in the literature, the study of the nature of the catalytic site and of the catalytic mechanism of the second step of the process is the main subject of this work with the objective of identifying, for this step, an independent and reusable catalyst. In particular, we focused our attention on the diol deriving from oleyl alcohol, because reagents and products are more easily separated and analyzed. The study of the mentioned reactions is complicated by the presence of respectively two or three phases in the reactor. The “in-situ” formed catalyst, active in the second reaction step, seems to be a lacunary poly-oxometalate in which cobalt, sequestered by the tungstate anion groups and accessible to the reagent, is the active component. This has been shown by polarographic analyses of the catalyst solution before and after the reaction, and is also confirmed by the observation that cobalt, sequestered by EDTA, in the absence of tungstic acid is active, too.

Oxidative dehydrogenation of ethanol to acetaldehyde on V2O5/TiO2-SiO2 catalysts obtained by grafting vanadium and titanium alkoxides on silica

Abstract Oxidative dehydrogenation of ethanol to acetaldehyde has been performed on vanadium based catalysts prepared by grafting on titania–silica supports with different procedures. A comparison of the performances of the prepared catalysts in terms of activity and selectivity has been made. Grafting technique gives place to well dispersed catalysts that resulted more selective than catalysts prepared by impregnation. In particular, very selective catalysts have been obtained by grafting vanadium–titanium bimetallic alkoxides directly on silica support. The effect of both the preparation methods and the used supports on the catalytic performances have been studied and an attempt to correlate the observed properties with the obtained results has been made.

Oxidative dehydrogenation of propane using V2O5/TiO2/SiO2 catalysts prepared by grafting titanium and vanadium alkoxides on silica

Abstract The oxidative dehydrogenation (ODH) of propane have been studied on three different vanadium oxide catalysts, containing comparable amounts of vanadium. All the proven catalysts have been prepared by grafting but following different procedures. One has been prepared by grafting vanadyl tri-isopropoxide, dissolved in n -hexane on a support of silica coated with a multi-layer of TiO 2 . The support has been prepared by grafting in three different steps titanium alkoxide on silica. Another catalyst has been prepared by partially hydrolysing vanadyl tri-isopropoxide, dissolved in isopropanol, before grafting the obtained product on the same support. The third catalyst has been prepared by reacting partially hydrolysed vanadyl tri-isopropoxide with titanium alkoxide in isopropanol and anchoring then the reaction product, a vanadium–titanium bimetallic alkoxide, directly on silica. The first and second catalysts have similar activities and selectivities, while the third catalyst is less active but more selective than the other two ones. A kinetic approach has been made and a pseudo-first order kinetic law has been used to interpret the results. All the observed catalytic phenomena have been interpreted also with the aid of the several used characterisation techniques.

A kinetic and mass transfer model to simulate the growth of baker’s yeast in industrial bioreactors

Abstract A structured unsegregated cybernetic model, able to simulate the growth of baker’s yeast in any possible condition in multistage industrial production has been developed. The model has first been proven in the simulation of the behavior of a laboratory batch bioreactor, describing the evolution with time of the biomass growth rate, the glucose and oxygen consumption as well as the production of ethanol and carbon dioxide. The same model with the same parameters has then been used to simulate both laboratory and industrial size fed-batch bioreactors achieving satisfactory results. The effect of the oxygen mass transfer limitation in fed-batch bioreactors has also been described and discussed. The model developed allows to find and keep the optimal conditions of biomass growth in industrial fed-batch bioreactors.

Oxidative dehydrogenation of isobutane over V2O5-based catalysts prepared by grafting vanadyl alkoxides on TiO2SiO2 supports

Following four different procedures many vanadium-based catalysts have been prepared by using the grafting technique and have been tested on the oxidative dehydrogenation of isobutane. The best results of selectivity have been obtained with catalysts prepared by grafting bimetallic vanadium–titanium alkoxides directly on silica. The alkoxide precursors have been obtained by partially hydrolysing titanium alkoxide, dissolved in isopropanol, with a stoichiometric amount of water and reacting then with vanadyl tri-isopropoxide, or alternatively by mixing the two mentioned alkoxides in isopropanol and submitting both to controlled partial hydrolysis. The bimetallic alkoxide grafted on silica show a prevalence of isolated VOTi bonds with respect to polyvanadylic VOV bonds that are prevalent, on the contrary, when vanadyl tri-isopropoxide dissolved in n-hexane is grafted on a TiO2SiO2 support. Catalysts characterised by the prevalence of VOTi bonds are slightly less active but two times more selective than catalysts in which VOV bonds prevail. The preparation of vanadium-based catalysts with a favourable TiO2 environment has been largely simplified by avoiding the use of a TiO2SiO2 support obtaining, in the meantime, a remarkable improvement in the selectivity.

DESCRIPTION OF THE VAPOR-LIQUID EQUILIBRIUM IN BINARY REFRIGERANT/LUBRICATING OIL SYSTEMS BY MEANS OF AN EXTENDED FLORY-HUGGINS MODEL

Abstract In the present work, the extended Flory–Huggins equation is applied to describe the vapor–liquid equilibrium in binary mixtures usually used in refrigerating cycle machines and constituted by a fluorinated refrigerant (CFC) and a lubricating oil. With the purpose of testing the model, some isothermal measurements of equilibrium pressure have been carried out for mixtures with different CFC compositions in oil related to the refrigerants R134a, R143a and R236fa mixed with a commercial lubricating oil (Icematic SW32). Furthermore, the model has been applied to experimental data retrieved from the literature, and the adjustable correlation parameters have been determined both for these, and for experimental isotherms.

A dynamic intraparticle model for fluid–solid adsorption kinetics

Abstract The fluid–solid adsorption batch kinetics is surely one of the most popular topic in the chemical engineering science. The water purification from pollutant components, such as metals and organic compounds, can be considered one of the main application of this field. Even if the topic is of a great scientific and industrial interest, the modeling of the mentioned systems is by now far to be reliable. As a matter of fact, most of the models reported in the literature are based on semi empirical approaches that describe the adsorption experimental data on the basis of equilibria and the kinetic terms. In this paper, a new modeling approach is proposed for adsorption kinetics investigation performed in batch reactors with a fluid–solid system. In particular, the mass balances have been developed by taking into account for both the external and internal mass transfer diffusion limitations, solving the dynamic partial differential equations (PDEs) system along the radius of the sorbent particles, considering both the fluid and solid phases that constitute the sorbent particle. From a numerical point of view, the solution of this type of problem is very challenging because it involves the simultaneous solution of many PDEs, ODEs and AEs. Here, physical parameters have to be evaluated either from existing correlations or by direct measurements. This fact makes the model predictable. In order to test the model, some Cu(II) and Pb(II) adsorption tests, taken from literature, using different kind of silica have been interpreted. The presented model can be considered of great interest, as it is the starting point for designing continuous adsorption columns for water purification.

Catalytic oxidation of methanol to formaldehyde: an example of kinetics with transport phenomena in a packed-bed reactor

In the present work the partial oxidation of methanol to formaldehyde has been studied as an example of strongly exothermic reaction affected by internal diffusion in order to deep the topic of mass and heat transfer in packed-bed catalytic reactors both at particle level, introducing the calculation of the effectiveness factor for complex reactions network, and at reactor level, for what concerns long range gradients of composition and temperature. The aim of the work is to stress the impact of the use of rigorous numerical methods, today possible for the high performances reached by the computers, in the solution of a simultaneous set of many differential equations that are necessary to describe completely the mentioned system. A complete mathematical model of the particle and the reactor is presented and a solution strategy is reported for the chosen reaction by considering a reliable kinetic law and evaluating related parameters from experimental data reported by the literature. Calculation results are reported for both particle internal profiles and reactor simulation. The described approach can easily be extended to many other devices and reactors geometry such as, e.g., the ones used in the field of environmental catalysis.

Kinetic and catalytic aspects in melt transesterification of dimethyl terephthalate with ethylene glycol in the presence of different catalytic systems

The kinetics of transesterification of dimethyl terephthalate with ethylene glycol performed in the presence of catalysts such as: Pb, Zn, Mg, Co, and Mn acetates and a mixture of Mg, Mn, and Zn acetates has been studied in semibatch conditions. The catalytic behavior of Sb2O3 has been proven, too. The performance of the different catalytic systems has been investigated following both the amount of methanol released during time and the evolution with time of the concentration of any kind of oligomer formed as consequence of the reaction. The oligomers obtained were separated, identified, and quantitatively determined by HPLC analysis. In this way, information have been achieved on both the activities and selectivities of the different catalysts. The experimental data have been interpreted through a classic kinetic model based on a complex reaction scheme. Despite the complexity of the model, only two kinetic parameters and two equilibrium constants are necessary to simulate the kinetic behavior of all the oligomers. A kinetic constant (K1) is related to the reaction of a methyl group with a hydroxyl of ethylene glycol, while the other (K2) corresponds to the reaction of a methyl group with a terminal hydroxyl of a growing chain. The Mn, Pb, and Zn acetates have shown comparable high catalytic activities; however, the Mn selectivity to give oligomers with hydroxyl-hydroxyl terminal groups is better and similar to that shown by Co and Mg acetate, at a lower activity. Sb2O3 has a very low activity in transes-terification but this activity could be important to eliminate the residual terminal methyl group during the polycondensation step. The catalytic activity of the mixture of Mg, Mn, and Zn acetate was greater than that shown by each component the mixture, while its selectivity was comparable with that of Mn and Mg acetate.