Borivoj Adnadjevic

Effect of different physicochemical properties of hydrophobic zeolites on the pervaporation properties of PDMS-membranes

The paper describes the investigation of the effect of three different types of hydrophobic zeolites (ultrastable zeolite type Y, pentasyl type zeolite (ZSM-5), and ALPO-5 type zeolite) on the pervaporation properties of zeolite-filled polydimethylsiloxane membranes. The procedure of obtaining membranes of various thicknesses varying in zeolite content was mastered. The physicochemical properties of the utilized zeolites were investigated and determined. The effect of zeolite type and concentration, as well as the membrane thickness and pervaporation temperature on the membrane permeability and selectivity were investigated. The physicochemical properties of the zeolite used, primarily the degree of hydrophobicity, as well as the sorption capacity for ethanol, the specific pore volume, specific area and mean crystallite size of the zeolite, significantly influence the membranes pervaporation properties. An increase in the zeolite content results in an increase of both membrane permeability and selectivity, while an increase in the pervaporation temperature results in an increase of the permeability and a decrease of selectivity, as opposed to the effect of membrane thickness.

Novel Microwave-Assisted Synthesis of Poly(D,L-lactide): The Influence of Monomer/Initiator Molar Ratio on the Product Properties

Poly(D,L-lactide) synthesis using tin(II) 2-ethylhexanoate initiated ring-opening polymerization (ROP) takes over 30 hours in bulk at 120 °C. The use of microwave makes the same bulk polymerization process with the same initiator much faster and energy saving, with a reaction time of about 30 minutes at 100 °C. Here, the poly(lactide) synthesis was done in a microwave reactor, using frequency of 2.45 GHz and maximal power of 150 W. The reaction temperature was controlled via infra-red system for in-bulk-measuring, and was maintained at 100 °C. Different molar ratios of monomer and initiator, [M]/[I], of 1,000, 5,000 and 10,000 were used. The achieved average molar masses for the obtained polymers (determined by gel permeation chromatography) were in the interval from 26,700 to 112,500 g/mol. The polydispersion index was from 2.436 to 3.425. For applicative purposes, the obtained material was purified during the procedure of microsphere preparation. Microspheres were obtained by spraying a fine fog of polymer (D,L-lactide) solution in tetrahydrofuran into the water solution of poly(vinyl alcohol) with intensive stirring.

The Effects of Microwave Heating on the Isothermal Kinetics of Chemicals Reactions and Physicochemical Processes

Microwave heating is a widely accepted, non-conventional energy source which is now applied in numerous fields. In many of the examples published in the literature, microwave heating has been shown to lead to dramatically reduce reaction times, increased product yields and enhanced product purities by reducing unwanted side reactions compared to the conventional method of heating. Several reviews have been published on the application of microwaves in organic synthesis (Kingston & Haswall, 1997; Loupy, 2002), polymer synthesis (Bogdal & Prociak, 2007), material science (Perelaer et al., 2006), nanotechnology (Tsuji et. al., 2005), homogeneous and heterogeneous catalyses (Larhed et. al., 2002; Will, et. al., 2004), medicinal chemistry (Shipe et. al., 2005), drug delivery (Kappe & Dallinger, 2006), biochemical processes (Collins & Leadbeather, 2007), synthesis of radioisotopes (Elander et. al., 2000), combination chemistry (Kimini et. al., 2000), green chemistry (Nücther et. al., 2004 ), sintering (Saberi et. al., 2009), ceramics (Vanderach, 2002), extraction (Luchesi, et. al., 2004), nucleation and crystallization (Sung et. al., 2007; Cundy 1998), annealing (Bhaskar, et. al., 2007), solid-state reactions (Li et. al., 2007), adsorption (Turner, 2000) and combustion (Y-Pee, et. al., 2005). The region of microwave radiation in the electromagnetic spectrum is located between infrared radiation and radio waves. Microwaves have wavelengths of 1 mm to 1 m, corresponding to frequencies between 0.3 and 300 GHz. In industrial and household microwave equipment, a frequency of 2.45 GHz with a wave number 12.2 cm is used. The process by which matter absorbs microwave energy is known as dielectric heating. Three mechanism of dielectric heating of materials under the influence of microwave irradiation are recognized: the dipolar polarization mechanism, the conduction mechanism and the interphase polarization mechanism (Gabriel, et. al., 1998). Microwave heating of materials which occurs due to the dipolar polarization mechanism is provoked by the molecular friction caused by the re-orientation of the dipole moments of the material under the influence of a microwave field, i.e., by the breaking and reforming of intermolecular bonds, due to which irreversible dissipation of the kinetic energy and thermal energy of the sample occurs. In the case when a substance contains mobile ions, the solvated ion due to attending the change in the oscillation of electrical component of the microwave field, increase the energy of collision and provokes the breakage and re-establishment of

Investigation of the effects of NaA-type zeolite on PDMS composites

The aim of this article was to investigate the effect of a NaA-type zeolite on a polydimethylsiloxane (PDMS)–zeolite composite. The obtained results show that the introduction of a NaA-type zeolite into polysiloxane matrices results in a change of the equilibrium degree of swelling (DSe) of composites in toluene, their hardness, and their thermal stability. With increase in the zeolite, the time required to achieve the DSe is reduced, DSe decreases, and hardness increases. A double effect of the zeolite concentration on the change of the composite characteristics was noted. At zeolite concentrations of up to 20%, there is a drastic change of the DSe and hardness with zeolite concentration, while this effect is considerably less pronounced at higher concentrations. The results obtained show that a NaA-type zeolite plays the role of a reinforcing filler. A new mechanism of bonding the zeolite into the composite was suggested. According to this model, there is a possibility that bonds are established between the polymer and the zeolite via interaction of the π-bond of terminal vinyl groups of PDMS with the Na cation of the zeolite. With increase in the zeolite concentration in the composite, the total number of established polymer–zeolite bonds increases, but the density of the interaction decreases. This explains the change in the characteristics of the composite with the change of zeolite concentration.

An Investigation of the Dehydration of Superabsorbing Polyacrylic Hydrogels

Samples of super-absorbing hydrogels based on cross-linked poly(acrylic acid), having different cross-linking degrees, were synthesized using a procedure of simultaneous polymerization and cross-linking. The equilibrium swelling degrees of the synthesized hydrogel samples in water were determined at 25C. A correlation was established between the equilibrium swelling degree and the cross-link density of the investigated hydrogels. The dehydration kinetics of the swollen hydrogels was investigated by non-isothermal kinetics (TG and DSC methods). The basic kinetic parameters of dehydration of the swollen hydrogels: reaction order, pre-exponential factor, and activation energy were determined. It was concluded that an increase in the cross-linking degree led to the decrease in the activation energy and to the increase in the pre-exponential factor. A possible mechanism of dehydration of super-absorbing polyacrylic hydrogels was discussed.

Hydrothermal Transformation of Sawdust into Synthetic Coke-Mechanism and Influence of Experimental Parameters

Abstract The influence of temperature, pressure, and time of interaction on the yield, caloric power, and elemental composition of solid product of hydrothermal transformation of sawdust into synthetic coke has been studied. With other parameters fixed, transformation of sawdust occurs between 350°C and 400°C. Further increase of temperature up to 580°C as well as pressure above 80 atm, result in the increase of the yield of synthetic coke. Rate of formation of coke is governed by precipitation rate of asphaltene liquid associates, while the activation energy of the process is close to the activation energy of diffusion of asphaltene molecules in reaction environment.

Hydrogel Synthesis Directed Toward Tissue Engineering: Impact of Reaction Condition on Structural Parameters and Macroscopic Properties of Xerogels

The existence of correlation and functional relationships between reaction conditions (concentrations of crosslinker, monomer and initiator, and neutralization degree of monomer), primary structural parameters (crosslinking density of network, average molar mass between crosslinks, and distance between macromolecular chains), and macroscopic properties (equilibrium swelling degree and xerogel density) of the synthesized xerogels which are important for application in tissue engineering is investigated. The structurally different xerogels samples of poly(acrylic acid), poly(methacrylic acid), and poly(acrylic acid-g-gelatin) were synthesized by applying different methods of polymerization: crosslinking polymerization, crosslinking polymerization in high concentrated aqueous solution, and crosslinking graft polymerization. The values of primary structural parameters and macroscopic properties were determined for the synthesized xerogels samples. For all of the investigated methods of polymerization, an existence of empirical power function of the dependence of primary structural parameters and macroscopic properties on the reaction conditions was established. The scaling laws between primary structural parameters and macroscopic properties on average molar mass between crosslinks were established. It is shown that scaling exponent is independent from the type of monomer and other reaction conditions within the same polymerization method. The physicochemical model that could be used for xerogel synthesis with predetermined macroscopic properties was suggested.

Comparison of adsorbent materials for herbicide diuron removal from water

AbstractDiuron herbicide is a persistent and frequently detected compound in surface and groundwater. In this work, a comparative study of the performance of four types of adsorbent (two precipitated silica, zeolite Y and carbon molecular sieves (CMS)) for removing diuron from water was carried out. Precipitated silica samples were obtained by controlled precipitation of SiO2 with sulphuric acid from water glass; zeolite Y was synthesized by a microwave-assisted hydrothermal method; CMS were synthesized by the method of controlled pyrolysis of wheat straw and chemical vapour deposition of organic matter. It was found that adsorption isotherms for precipitated silica and zeolite type Y are of linear shape, whereas the one for CMS exhibits at low concentrations a concave shape followed by an inflection point which suggests cooperative adsorption and the formation of adsorbate multilayers. CMS adsorbent showed the highest adsorption capacity and this was likely due to its high hydrophobicity. Temperature has...

A novel approach to the explanation the effect of microwave heating on isothermal kinetic of crosslinking polymerization of acrylic acid

The effect of microwave heating (MWH) on the isothermal kinetic of crosslinking-polymerization of acrylic acid (CPAA) was investigated. The kinetic curves of CPAA were determined in the temperature range from 303 to 328 K. By applying the model-fitting method it was revealed that the isothermal kinetics of CPAA was described by the first order chemical reaction kinetics model under the MWH and by the second order chemical reaction rate model for the conventionally heated (CH) process. The values of the reaction rate constants of CPAA are about 40 times higher for the microwave heated system than for the conventional heating. The kinetic parameters (activation energy (Ea) and pre-exponential factor (lnA)) of the CPAA are significantly lower than the corresponding values for CH process. It was found that the increase in the reaction rate of CPAA for MWH was not a consequence of overheating neither the hot spots in the reaction system. Based on model of selective energy transfer between the reacting molecules and the heating bath a novel explanation of the established effects of MWH on the kinetics of CPAA is given. A quantized nature and value of activation energy was confirmed. The decrease in the value of activation energy of CPAA under microwave heating is explained by the increased value of energy of ground vibration level of resonant oscillator in the AA molecule (v = 1417 cm−1) caused by the absorption of non-thermal energy of MW field.

Prevention of trace and major element leaching from coal combustion products by hydrothermally-treated coal ash

Abstract The most important structural components of coal ash obtained by coal combustion in “Nikola Tesla A” power plant located near Belgrade (Serbia) are amorphous alumosilicate, α-quartz, and mullite. The phase composition of coal ash can be altered to obtain zeolite type NaA that crystallizes in a narrow crystallization field (SiO2/Al2O3; Na2O/SiO2; H2O/Na2O ratios). Basic properties (crystallization degree, chemical composition, the energy of activation) of obtained zeolites were established. Coal ash extracts treated with obtained ion-exchange material showed that zeolites obtained from coal ash were able to reduce the amounts of iron, chromium, nickel, zinc, copper, lead, and manganese in ash extracts, thus proving its potential in preventing pollution from dump effluent waters.