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ИЗВЕСТИЯ ВУЗОВ. ПРИКЛАДНАЯ ХИМИЯ И БИОТЕХНОЛОГИЯ, 2013, № 2 (5) УДК 544-227+544-016
DETERMINATION OF ACID-BASE SURFACE PROPERTIES OF MERCARBIDE S.A. Kiba, E.V. Kapustina, R.A. Kozlovskiy, A.E. Kapustin
Azov Sea State Technical University
7, Universitetska St., Mariupol, 87500 Ukraine, [email protected]
Mercarbide, [CHg4O2](OH)2, is a mercury derivative of methane in which all of the methane hydrogen atoms are substituted by mercury atoms. Mercarbide exhibits basic and anion exchange properties in addition to showing unique stability towards mineral acids as well as oxidizing and reducing agents.The selectivity of mercarbide towards organic anions shows selectivity dependence on the size and configuration of the pendant hydrocarbon group.
In this study characterization and surface basicity of mercarbide were investigated with Hammett basicity functions, the benzoic acid titration method, with direct acid-base titration with mineral acid and with ionexchange with alcoxides. Mercarbide has a basic strength of H0 > 15,0, Bronsted sites predominate on the surface of mercarbide. 1 figure. 3 tables. 11 sources.
Key words: mercarbide; basicity functions; ion-exchange. ОПРЕДЕЛЕНИЕ СВОЙСТВ ПОВЕРХНОСТИ МЕРКАРБИДА С.А. Киба, Е.В. Капустина, Р.А. Козловский, А.Е. Капустин
Приазовский государственный технический университет,
87500, Украина, г. Мариуполь, ул. Университетская, 7, [email protected]
Меркарбид, [CHg4OJ(OH)2, представляет собой ртуть-производное метана, в котором все атомы водорода метана замещены атомами ртути. Меркарбид обладает основными свойствами и анио-нообменными свойствами, а кроме того, показывает уникальную устойчивость по отношению к минеральным кислотам, а также к окислителям и восстановителям. Селективность меркарбида по отношению к органическим анионам зависит от размера и конфигурации углеводородного радикала.
В данной работе проведено исследование поверхностной основности меркарбида с помощью Гам-метовской функции кислотности, титрованием бензойной кислотой, прямым кислотно-основным титрованием минеральной кислоты и методом ионного обмена с алкоксидами. Меркарбид обладает основностью с силой H0 > 15,0, Бренстедовские центры преобладают на поверхности меркар-бида.
Ил. 1. Табл. 3. Библиогр. 11 назв.
Ключевые слова: меркарбид; основность; ионный обмен.
INTRODUCTION
Mercarbide, [CHg4O2](OH)2, is a mercury derivative of methane in which all of the ethane hydrogen atoms are substituted by mercury atoms [1]. Mercarbide exhibits basic and anion exchange properties in addition to showing unique stability towards mineral acids as well as oxidizing and reducing agents. The selectivity of mercarbide towards organic anions shows selectivity dependence on the size and configuration of the pendant hydrocarbon group [1].
The scientific interest in mercarbide arises from its unique properties and amazing stability.
Thus mercarbide does not undergo changes in the presence of acids and bases and is stable towards oxidizing and reducing agents. Even long heating in HNO3/HCl does not result in visible changes in its structure. However, the mercury present in mercarbide limits its development for industrial applications, especially with the present-day ecological limits.
The acidic of basic properties of solid surface are interesting aspects of surface structure. The understanding of basic nature of a solid surface is of great interest in the fields of ion-exchange and
adsorption. For these reason the acid-basic properties of solid surface have been the object of many investigations [3]. Different techniques have been used to study the surface basicity of solids. Particularly, the adsorption of water and acid probe molecules followed gravimetrically, volumetrically and microcalorimetrically and with IR, Raman and NMR spectroscopies have been applied to investigate surface basicity. Contrarily, the desorption of such probe molecules followed by temperature-programmed techniques have also been applied. Titration methods and activity measurements in catalytic test reactions have also been among the thoroughly applied methods [4-8].
The term basicity mostly discriminates between Bronsted character and Lewis character. Here we should give a reminder of the Bronsted and Lewis definitions of basicity.
The adsorption of acid probe molecules was investigated by IR of FTIR spectroscopy techniques in order to determine the Bronsted and Lewis basicity on solid surface [9-10]. The IR spectroscopy detection of surface basic sites is based on observation of the vibrational perturbation undergone when they adsorb on solid surfaces. Certain absorption bands observed in the spectra are used to distinguish between Lewis and Bronsted basicity.
The application of these techniques has added greatly to our knowledge about the surface basicity of solid surfaces however it has revealed the complexity of predicting of base strength and number of basic sites on solid surfaces. In order to be able to judge the basicity of solid surfaces, all of these methods have to be kept in mind with their advantages and disadvantages.
In this study, characterization and surface basicity of mercarbide were investigated with Ham-mett indicator titration and ion-exchange method.
EXPERIMENTAL
Materials:
The synthesis of mercarbide was carried out by the precipitation method using mercury oxide and alcohol [2]. A typical synthesis was the following. 300 ml of ethanol, 150 g of yellow mercury oxide and 30 g of potassium hydroxide were put into 500 ml glass reactor. This reactor was painted in black for the removal of influence of sun light on the reaction undergoing (in the unscreened part of the reactor the formation of metallic mercury was observed). The reaction mixture was heated at 363 K for 36 h vigorously stirring, filtered and then washed with distillated water, alkali solution and nitric acid. The obtained materials was washed with distillated water and then was drying in vacuo at 298 K.
Other chemicals, i.e. the Hammett indicators were Aldrich and Fluka reagent grades. They were used directly without further purification.
Base Strength Measurements:
The indicator method can express the strength of basic sites in a definite scale of H0, but this has disadvantages too. Although the color change is assumed to be the result of an acid-base reaction, an indicator may change its color by reaction different from an acid-base reaction. In addition, it requires a long time for benzoic acid to reach an adsorption equilibrium when titration is carried out in a solution. In some case the surface of solid may dissolved into a titration solution. If this happens, the number of basic sites should be overestimated. There for, special care should be taken with the indicator method.
Mercarbide at the OH-form has the yellow color, so it is very difficult to determine the basicity by the direct method, therefore the method of titration in the presence of white comparison standard was used [11]. Benzene was used as a solvent, calcined barium oxide was used as a comparison standard. The followed indicators were used -Bromothymol blue (pKa = 7,2), Dinitroaniline (pKa = 15,0), 4-Chloro-2-nitroailine (pKa = 17,2), 4-Chloroaniline (pKa = 26,5).
Mercarbide (1 g) was placed at the glass filled with benzene and 0,01M benzoic acid solution was used for titration; the concentration of solution of indicators in benzene was 0,1 g/l. From the results of such tests it was easy to decide whether the solid under study was acid to all indicators, basic to all indicators, or had an H0 lying between two adjacent indicators pKa's.
The amount of basic sites on the mercarbide can be measured by benzoic acid titration immediately after determination of base strength by the above method. The method consists of titration a solid acid suspended in benzene with benzoic acid, using an indicator. This method gives the sum of the amounts of both Bronsted and Lewis acid.
RESULTS AND DISCUSSION
The composition of mercarbide is given in table 1.
Mercarbide in its hydroxide form is a solid base which represents polymeric matrix, caring positive charge, localized on Hg atoms, and negative charged hydroxyl ions. While titration of solid bases it is necessary to consider, if the basic sites located in inside cavities of solid body can be reached by molecules of acid. If acid's anion has a bulky structure then impossibility of it's penetration to inside space will cause understated results. Also while titration of solid bases it is to be sure that
Table 1
Composition of mercarbide_
Hg C H
Calculated, % Analysis, % 93,22 93,05-93,10 1,85 1,80-1,83 0,30 0,33-0,34
Table 2
The concentration of mercarbide basic sites, determinated by the HI solutions
CH|, mol/l 0,01 0,01 0,05 0,1 0,2
Е, meq/g 1,35 1,45 1,56 1,59 1,68
Table 3
The concentration of mercarbide basic sites according to ionic exchange with alcoholates
Alcoholate СН3О" С2Н5О- С3Н7О" С7Н15О- С12Н25О
Е, meq/g 1,35 1,38 1,22 0,95 0,11
age. It is necessary to mention, that in despite of the fact that inside sites is absolutely inaccessible for bromthymol blue for example, it is not tells on a titration results, since that sites is accessible for acid molecules.
The titration by Hammett showed, that in presence of Bromthymol blue (pKa = 7,2), the concentration of basic sites is E = 1,5 meq/g, as well as in 2,4-Dinitroaniline presence (pKa = 15,0). The results, received by the titration in presence of 4-Clorine-2-nitroaniline (pKa = 17,2) gave following values of basic sites concentration: E = 0,8 meq/g). The sites with Нo = 26,5 intensity is not exists; indicator - 4-Clorineaniline) [11].
The base of mercarbide structure is a link with oxonic bridge - dimercurated oxonic cation C— Hg—(OH)+—Hg—C. The structure of [HgCHg3O]2+ with OH- ions and molecules of crystallizational water in internal space is present too. The polymeric trimercurated oxonic ion [OHg3CCHO]+ forms two structures in a trimercurated aldehyde, one has a pillared structure, the other has polymeric planes. It can suppose the existence of two polymeric structures, with monomercurated oxonic ion -+H2OHg—So the most reliable structure is the polymer, in which present both trimercurated and monomercurated oxonic ions, with a formula [CHg4O(OH2)] (OH)2 (figure) [1]. Such structure of mercarbide very well explains the presence only Bronsted basic sites in mercarbide, and also circumstance that basic sites have difference strength.
CONCLUSIONS
Mercarbide, the mercury derivative of methane exhibits basic and anion exchange properties in addition to showing unique stability towards mineral acids as well as oxidizing and reducing agents. Mercarbide has a basic strength of H0 > 15.0. The concentration of basic sites in mercar-
equilibrium of ionic exchange is moved to a salty form:
= - H2O =
=-OH + HAn =-An
The ionic exchange can be observed even for C18-23 alcohols fraction of alcoholates. The long hydrocarbon radicals can penetrate to the internal space of mercarbide. On account of impossibility of penetration to the basic sites in internal space of mercarbide for divaricate radical, the ionic exchange is impossible for such alcoholates with divaricate hydrocarbon radicals as 3-propyoneheptanol or 2-octadecanol. That fact gives an opportunity to estimate the portion of basic sites, situated on external surface of mercarbide which is not more, then 5% from all the quantity.
During the titration of hydroximercarbide by the solutions of sulphuric and hydrochloric acids, the similar value of exchange capacity was calculated (E = 1,35-1,37 meq/g). The same result was reached during the study of ionic exchange It is necessary to notice, that in case of complete replacement of the hydroxide ions to the iodine anions, the theoretical value of capacity for tetramer-curmethane is E = 2,2 meq/g.
The results of basic sites determination through the alcoholates of different achohols. Is shown in table 3.
The results, shown in table 3, describes the prohibition of basic sites in mercarbide by hydrocarbon radicals. At that, for the prohibition of each site the increase of hydrocarbon radical in 3 -CH2-groups required.
The definition of mercarbide basic sites intensity was carried out by the Hammett indicator us-
H2Ö+ H2O+ H2O+
HO
hg h2o
Hg ho
The polymeric structure of mercarbide
bide sites is about 1,5 meq/g. Bronsted sites predominate on the surface of mercarbide. All these
properties make very perspective the using of mercarbide as catalyst or sorbent.
LITERATURE
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Поступило в редакцию 28 октября 2013 г.
