The role of Lewis acidic centers in stabilized zirconium dioxide

Applied Catalysis A: General 249 (2003) 313–326

The role of Lewis acidic centers in stabilized zirconium dioxide Helena Teterycz a,∗ , Roman Klimkiewicz b , Marek Łaniecki c b

a Wrocław University of Technology, Faculty of Microsystem Electronics and Photonics, ul. Z. Janiszewskiego 11/17, Wrocław, Poland Polish Academy of Sciences, W. Trzebiatowski Institute of Low Temperature and Structure Research, ul. Okólna 2, 50-422 Wrocław, Poland c Adam Mickiewicz University in Pozna´ n, Faculty of Chemistry, ul. Grunwaldzka 6, 60-780 Pozna´n, Poland

Received 25 September 2002; received in revised form 10 March 2003; accepted 11 March 2003

Abstract The physico-chemical properties of ZrO2 , MgO and admixtured (stabilized) zirconium dioxide are presented. The synthesis of stabilized ZrO2 was conducted in a low-temperature process. The catalysts obtained on the basis of zirconium dioxide were monophase materials of a regular structure. The lattice parameter of the materials did not differ much from the standard value. The character of the surface active centers was being changed as a result of the admixturing. Synthesized monophase materials of catalytic properties, Zr-Mg-O and Zr-Mg-Y-O, have many strong Lewis centers at surfaces. There appeared not only the stabilization of the regular polymorphous structure of ZrO2 , but also a modification of catalytic properties of the materials when the basic admixture (MgO and Y2 O3 ) was being introduced to basic zirconium dioxide. The authors present a mechanism of alcohol or aldehyde condensation to ketone at the surface of the materials with the participation of Lewis acidic centers. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Zirconium dioxide; Magnesium oxide; Yttrium oxide; Ketonization; Lewis acidic centers; Oxygen vacancy

1. Introduction After a difficult technology of obtaining pure zirconium without hafnium admixture for the needs of nuclear power engineering and new generation of alloys, there was an increase in the production of not only zirconium but also its compounds [1,2]. The consumption of the materials is still increasing. In last decades, most of the research is devoted to materials containing zirconium, which were used as superionic conductors in oxygen sensors [3,4], fuel links [5–8] and as catalysts of “secondary generation” [9,10]. The catalysts are applied in environmental protection, petrochemistry, ∗ Corresponding author. Tel.: +48-71-320-2982; fax: +48-71-328-3504. E-mail address: [email protected] (H. Teterycz).

electrocatalysis, polymerization, etc. Oxygen sensors with solid electrolyte, based on zirconium dioxide, are used in car and air transport to control the ratio of fuel to air suction and to achieve the most favorable combustion conditions at the lowest emission of pollutants. Zirconium dioxide is the most important component of Lambda probe (oxygen sensor) and also a significant component of catalysts used in fume purification (three-way catalysts (TWC)), for catalytic fuel combustion, etc. [11,12]. High-temperature oxygen sensors based on zirconium dioxide which are placed directly on chimneys of power and heating plants enable the optimization of fuel combustion process and reduction of the emission of harmful substances [13]. Zirconium dioxide can be applied in the catalysis as a single unit or as a component of compound catalysts [12,14,15]. ZrO2 is used in sensors as a main

0926-860X/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0926-860X(03)00231-X

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component, but not in its pure form. It is also a very good support for catalysts or an additive to supports, which improves their properties, e.g. it improves rhodium surface stability in three-way catalysts [16]. Zirconium dioxide is characterized by high chemical resistance and is stable at a wide temperature range. It possesses both acidic and basic centers [14]. Its properties enable surface processes as a mechanism of bifunctional acid–base catalysis [12]. The number of acidic and basic centers can be changed using proper preparations in ZrO2 synthesis. Zirconium dioxide can have three crystallographic forms. Up to the temperature of 1200 ◦ C a stable variety is a monoclinic form. From 1200 to 1900 ◦ C it crystallizes as a tetragonal form, and over 1900 ◦ C to the melting point at 2670 ◦ C the regular structure is stable. There is also a metastable tetragonal structure to 650 ◦ C. Its appearance is usually explained by the influence of impurities or crystallites effect [15]. The stability ranges for the structural varieties can be modified introducing various admixtures. All polymorph types of ZrO2 are used in catalysis. Obtaining the required variety of ZrO2 is a complicated process and depends on many parameters of hydroxide precipitation [17,18]. Regular structure of ZrO2 can be stabilized at the whole temperature range by admixturing with ions of lower valence than +4 [19]. In oxygen sensors and fuel cells magnesium, yttrium or calcium oxides are mostly used. Admixtured zirconium dioxide can be asserted as a solid solution described as, e.g. Zr1−x Cax O2−x . The dopands introduced to ZrO2 not only stabilize its regular crystallographic structure but also bring about a certain balance in oxygen vacancies (free nodes positions usually filled with oxygen ions). The gaps and dopand ions cause the relaxation of neighboring oxygen ions that ensure fluorite stability even at the room temperature. In favorable conditions, e.g. at high temperature, oxygen ions can go to free positions, which involves the transfer of electrical loads. Thus, in solid solutions of a regular structure ZrO2 –Mex Oy (where Me is Ca2+ , Mg2+ , Y3+ , or Sc3+ ) ions of admixture cause crystallographic structure stabilization and generate ionic load carriers. The carriers concentration (i.e. oxygen vacancies) is approximately proportional to the percentage share of the admixture. The concentration of dopands is chosen so that:

• the ionic conductance is the strongest; • the regular structure is stable at the temperature of electrolyte operation; • the superionic conductor has good thermo-mechanical properties during work as the galvanic cell or sensor. In catalysis, the oxides added to zirconium dioxide cause increase specific surface [20], stabilize specific surface at high temperature [21], have impact on the improvement in mechanical properties [22], improve activity [23] and selectivity [24], ensure possibly low thermal expansion coefficient [25], enable acidity control [26] and enable active centers control [26]. Active centers, in the case of stabilized zirconium dioxide, are mostly oxygen vacancies whose concentration can be modified by controlling the amount of the introduced stabilizing admixture. ZrO2 samples with stabilizing admixtures are obtained by mixing the pure zirconium dioxide with a selected dopands in a proper ratio. Next, the mixture of oxides is heated at the temperature over 1400 ◦ C [27]. Then, the obtained sinter is milled, sintered and milled again obtaining powder of the right grain size. Many articles have been devoted to catalytic properties of pure ZrO2 [5–12,14–18,28–30]. It is a material of weak basic and acidic centers, in some conditions showing very good catalytic properties, e.g. in the dehydration of 1-cyclohexylethanol to vinylhexane [31]. The yield of the reaction is improved by adding TiO2 , Y2 O3 , ThO2 , etc. Moreover, it is reported that acidic properties of ZrO2 are used for the synthesis of ketones from aldehydes, alcohols and carboxylic acids [29]. In our research on the reaction of obtaining ketones from alcohols, we have not observed that the acidic properties of ZrO2 decided about their high yield and selectivity. In order to explain such behavior we have performed physico-chemical analyses of the following materials: • • • •

pure ZrO2 ; pure MgO; pure Y2 O3 ; ZrO2 admixtured with magnesium, described as Zr-Mg-O; • and ZrO2 admixtured with magnesium and yttrium, described as Zr-Mg-Y-O.

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In the article, we present physico-chemical properties of the materials. Basing on the research, the authors assert that catalytic properties of stabilized ZrO2 are determined mainly by nucleophilic and electrophilic centers connected with the existence of oxygen vacancies in the materials. 2. Experimental Zirconium dioxide and stabilized ZrO2 were produced with the use of low-temperature method [32,33]. Zirconium tetrachloride, analytically pure for semiconducting purposes, was used as a substrate. ZrCl4 was hydrolyzed at high temperature in the environment of ammonia hydroxide according to the equation: ZrCl4 +4NH4 OH → ZrO(OH)2 ↓ +4NH4 Cl + H2 O (1) Precipitated zirconyl hydroxyoxide was washed with distilled water with some isopropyl alcohol until the negative result of chloride ions test. The sediment was filtered off and dried at 50 ◦ C, and next heated at 600 ◦ C in order to obtain anhydrous ZrO2 : 600 ◦ C

ZrO(OH)2 → ZrO2 + H2 O

(2)

Stabilized zirconium dioxide is usually obtained by mixing and grinding a mixture of oxides, sintering at the temperature over 1400 ◦ C. The obtained sinter is ground once more and sintered again at a high temperature [19,27]. In the present work, in order to obtain zirconium oxide stabilized with magnesium and yttrium (Zr-MgY-O: Zr:Mg:Y = 90 mol:9 mol:1 mol) the sediment of zirconyl hydroxide devoid of chloride ions was mixed with a solution containing magnesium and yttrium ions. In order to obtain the solution, hydrated magnesium chloride was diluted in water, a small amount of magnesium oxide was added and it was heated for 90 min at the temperature of 70–75 ◦ C, stirring it frequently. After clearing, the solution was filtered. A proper amount of yttrium chloride was added to the resultant solution and it was mixed with zirconyl hydroxide sediment. The obtained mixture was heated up to the temperature of 80–85 ◦ C and a cold solution of sodium carbonate was added until the solution was getting weakly basic (over phenolphthalein). Next, the

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solution was heated for 30 min, and decanted repeatedly with hot water. After the decantation process, the sediment was washed with isopropyl alcohol, dried at the temperature of 50 ◦ C and heated at the temperature of 600 ◦ C. In order to obtain MgO, hydrated magnesium chloride was diluted in water, a small amount of magnesium oxide was added and it was heated for 90 min at the temperature of 70–75 ◦ C, frequently stirring the solution. After clearing, the solution was filtered and heated up to the temperature of 80–85 ◦ C, and next a cold solution of sodium carbonate was added until weakly basic reaction (over phenolphthalein). Next, the solution was heated for another 30 min, and was decanted repeatedly with hot water. The sediment was additionally washed with isopropyl alcohol, dried at the temperature of 50 ◦ C and heated at 600 ◦ C [34]. Powders of pure ZrO2 , MgO, Y2 O3 , Zr-Mg-O (90 mol.% Zr:10 mol.% Mg), and Zr-Mg-Y-O (Zr:Mg:Y = 90 mol:9 mol:1 mol) obtained by this method, were analyzed with XRD. Their microstructure was evaluated with an electron microscope and X-ray microprobe, their specific surface was estimated with BET method. The number and kinds of acidic centers were studied with the use of pyridine. The research on catalytic activities of the above mentioned materials were performed in relation to transformations of n-butyl alcohol in the gas phase at the atmospheric pressure. The reaction was carried in a vertical quartz down flow reactor with fixed bed heated in a Thermolyne Tube Furnace type F21100. In all cases, a volume of 3 cm3 of each catalyst of 0.6–1.2 mm grain size was placed in the middle of the reactor. The space below catalyst bed was filled with quartz wool. The substrate was supplied from the top of the reactor using an infusion pump (610-2 Medipan) with a flow intensity of 3 cm3 of liquid/h (load, 1 h−1 ). No carrier gas was applied. The reactions were studied in the function of increasing temperature at the range of 280–480 ◦ C. The reaction products were analyzed with mass spectrometer HP MSP 59 and gas chromatography.

3. Research results X-ray studies on the obtained catalysts were carried with the use of Philips Materials Research Materials

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(Cu K␣ radiation). One kind of scan was applied, i.e. typical Θ/2Θ for powders. The average grain sizes and their distributions were determined on the basis of the half-width at half-maximum (FWHM) of the peaks and diffractometric profile analysis. AWPX software tool was used for this purpose. X-ray studies revealed that the obtained pure zirconium dioxide had a monoclinic structure (Fig. 1), and the average size of crystallites was about 50 nm, while ZrO2 admixtured in various ways (Fig. 1(b) and Table 1) was a monophase polycrystalline material of a regular structure. Diffractograms XRD of stabilized zirconium dioxide (Zr-Mg-O and Zr-Mg-Y-O) did not reveal the presence of peaks characteristic for magnesium and yttrium oxide (III). Lattice constant determined for the materials changed slightly depending on the applied admixtures (Table 1). The average size of crystallites indicated on the basis of half-width of the characteristic peak equaled 85 nm. The microstructure of tested materials grains was observed by means of JSM 5800 LV scanning microscope, and the content and distribution of the components in the catalyst with microprobe X-ray Oxford ISIS 300. The studies revealed that pure ZrO2 was built of needles (Fig. 2(a)), while stabilized with magnesium and yttrium oxides—of grains from several to several hundreds nm diameter—being a conglutination of many crystallites (Fig. 2(b)). The measurements with the use of X-ray microprobe Oxford ISIS 300 (Fig. 3) revealed only the presence of primary elements building the catalytic

Fig. 1. (a) The X-ray diffractogram for pure ZrO2 and (b) ZrO2 admixtured with magnesium.

Table 1 Kind of studied material, its crystallographic structure and lattice parameters Material

Crystal structure

Lattice constant (Å)

Content of the elements (%)

MgO

Regular of NaCl type

4.211

100

ZrO2

Monoclinic of baddeleyite type

a = 5.1507 b = 5.2028 c = 5.3156

100

100

Y 2 O3

Cubic body centered

a = 10.61

Zr-Mg-O

Regular of fluorite type (a = 5.107 Å standard)

5.106

10 (Mg) 90 (Zr)

Zr-Mg-Y-O

Regular of fluorite type (a = 5.107 Å standard)

5.1045

9 (Mg) 1 (Y) 90 (Zr)

Note: a = 5.107 Å standard.

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Fig. 2. Photo SEM: powder microstructure (a) pure zirconium dioxide and (b) stabilized with magnesium.

materials. The determined content of the elements in the studied materials is presented in Table 1. Specific surface as well as the number and kinds of active centers of the obtained catalysts are very important parameters. The information about the surface area and porosity was obtained by the evaluation of the nitrogen adsorption–desorption data with a Spectrometer Philips. The area of the specific surface depended on the sort of studied pure material and admixture stabilizing the structure of zirconium dioxide (Fig. 2 and Table 2).

The largest specific area, equaling ca. 85 m2 /g, had pure magnesium oxide, while the smallest: pure zirconium dioxide, 16 m2 /g (Table 2). The average size of pores clearly depended on the kind of the studied catalytic material. The material of the smallest specific area and also the smallest pore diameter was zirconium dioxide (Fig. 4(a)). The results of studies on the isotherms of nitrogen adsorption show that the addition of magnesium oxide, apart from changing other properties, clearly widens the area of mezo- and macro-pores in stabilized ZrO2 . The lack of uniformity

Fig. 3. Exemplary spectrum obtained with X-ray microprobe.

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Table 2 Specific area, pores volume and average size of pores in the studied materials Material

BET surface area (m2 /g)

Vpore for o.d. 8.5–1500 Å (desorp.) (cm3 /g)

BJH desorption average pore radius (2 V/A) (Å)

ZrO2 MgO Zr-Mg-O Zr-Mg-Y-O

16.5036 85.3378 41.4369 36.7878

0.079764 0.653299 0.278355 0.220267

94.5333 119.7091 224.3314 108.0054

Fig. 4. Distribution of pores and their volume in the studied catalysts: (a) MgO and ZrO2 ; (b) Zr-Mg-O and Zr-Mg-Y-O.

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in pores distribution is considerable in the area from 150 to 700 Å (Fig. 4). The type and number of active centers were determined with IR studies with the use of pyridine, after activating the sample at 400 ◦ C and pyridine’s adsorption at 100 ◦ C. To achieve this, nitrogen (carrier gas of 99.999% purity) was flown through a flow cell of an original construction. The activation of “self-supported wafer” (weight, 4–5 mg/cm2 ) was carried in the carrier gas stream (3 dm3 /h) at the temperature of 400 ◦ C for 2 h. After cooling down the flow cell to ca. 50 ◦ C, 3 ␮l of pyridine (Py) were injected through septum placed in front of the cell.

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Next, in order to remove physically adsorbed pyridine and excessive pairs in the flow cell, the whole sample was heated to 100 ◦ C and IR spectra measurements were taken every 0.5 h. After the stabilization of conditions, when the intensity of pyridine bands connected with Lewis acidic centers did not change, the proper intensity measurement was taken and calculations were made. FT–IR measurements were done in “Vector 22” spectrometer produced by BRUKER. The studies revealed that the number of active centers in the materials differed considerably depending on the sort of admixture (Fig. 5). Magnesium oxide

Fig. 5. Spectrum IR: dependence of transmittance on wavenumber for materials on the base of zirconium dioxide.

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did not show any pyridine adsorption, which implies the lack of Lewis acidic centers at its surface (Table 3). In the case of materials based on zirconium dioxide a high number of the centers was reported. Lewis centers were especially strong for Zr-Mg-Y-O, as even after desorption at 200 ◦ C there was a strong band for wave value 1444 cm−1 (Fig. 6). Moreover, it was observed that zirconium dioxide stabilized with magnesium and yttrium, after the activation at various temperatures to 300 ◦ C there appeared only a band of hydroxyl groups ca. 3650 cm−1 (non-brönstedian), while after the activation at 400 ◦ C

Table 3 The character of sorption centers in pure MgO and ZrO2 and stabilized zirconium dioxide Material

Type of active sites according to publications [12,15]

Lewis acid sites concentration (a.u.)

MgO ZrO2 Y2 O 3 Zr-Mg-O Zr-Mg-Y-O Zr-Mg-Y-O

Basic Acid/basic Basic Basic Basic (no acid) Basic (no acid)

– 0.381 – 0.441 1.491 (100 ◦ C) 1.070 (200 ◦ C)

Fig. 6. Spectrum IR: dependence of transmittance on wavenumber for zirconium dioxide stabilized magnesium and yttrium, after (a) thermal activation at 400 ◦ C and pyridine activation—(b) 3 ␮l at 50 ◦ C; (c) after 30 min at 100 ◦ C; (d) after next 30 min at 100 ◦ C; and (f) after next 30 min at 200 ◦ C.

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Fig. 7. Spectrum IR: dependence of transmittance on wavenumber for zirconium dioxide stabilized magnesium and yttrium after thermal activation in argon atmosphere (a) at room temperature; (b) after 30 min at 100 ◦ C; (c) after next 30 min at 200 ◦ C; (d) after next 30 min at 300 ◦ C; and (e) after next 30 min at 400 ◦ C.

there was another band at 3740 cm−1 (Fig. 7). However, this band was also of non-brönstedian type, since there was no clear band at the range of 1540– 1550 cm−1 after the adsorption of pyridine (Fig. 6). Catalytic properties of pure initial oxides and stabilized zirconium dioxide were determined in n-butyl alcohol conversion. Clear differences in the behavior of the analyzed materials were observed (Figs. 8 and 9). The level of n-butyl alcohol conversion over catalysts based on zirconium dioxide revealed a strong dependency on temperature (Fig. 8). A rapid increase

in conversion appeared over the temperature of ca. 370 ◦ C, reaching the maximum at about 450 ◦ C. Magnesium oxide and yttrium oxide behaved in a different way; the level of conversion in such a reaction slowly increased at the whole studied temperature range. Significant differences appeared in the behavior of the studied catalysts with respect to selectivity and yield (Fig. 9). Pure ZrO2 did not favor any direction—in comparable amounts were observed dehydration,

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Fig. 8. The level of conversion in n-butyl alcohol condensation over various catalysts.

dehydrogenation and bimolecular condensation products. Products of the alcohol transformations over ZrO2 contain relatively considerable amount of butenes. Different specificity, depending on the preparation, can feature this oxide, even high activity and selectivity towards alkenes [35]. In this respect, the method of preparation of Zr-Mg-Y-O visibly modifies the properties of the initial ZrO2 , especially, that dehydration can get according to different mechanisms depending on kind of centers [36] and their mutual relation [37]. MgO in such conditions revealed clearly that dehydrogenating behavior and the low level of conversion caused a low yield. Even at higher temperatures, when there were observed some condensation products (ketone and ester), the liquid product was of a

light color, which meant that the reactions of condensation and degradation were proceeding to a lower extent (Table 4). Zirconium dioxide stabilized with magnesium or magnesium and yttrium behaved in a different way. Although the level of n-butanol conversion depending on the temperature for both materials was similar to the dependency observed for pure ZrO2 , the materials showed a clear selectivity to dipropylketone. The Studies of the selectivity of the reaction of n-butyl alcohol over stabilized zirconium dioxide revealed that it proceeded with a clear selectivity towards dipropylketone. At the temperature range from 325 to about 440 ◦ C the selectivity and yield of ketone increased, reaching the maximum just at 440 ◦ C,

Table 4 The level of conversion, selectivity and yield of dipropylketone and aldehyde in the process of n-butyl alcohol condensation over various catalysts at the temperature of 450 ◦ C Catalysts

Conversion (%)

Selectivity to ketone (%)

Select. to aldehyde (%)

Yield to ketone (%)

Yield to aldehyde (%)

MgO ZrO2 Y2 O3 Zr-Mg-O Zr-Mg-Y-O

21 86 18 88 88

0 23 41 80 80

26 21 29 8 8

0 18 8 70 69

5 17 5 7 8

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Fig. 9. Dependence of yield and selectivity in obtaining butene, aldehyde, ketone and other products (non-analyzed).

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while in the chosen conditions the selectivity was 80% and yield, 72%. Over this temperature, the selectivity and yield of dipropylketone rapidly decreased.

4. Discussion The studied materials can be divided into two groups: 1. pure oxides MgO, ZrO2 and Y2 O3 ; and 2. monophase compositions of stabilized zirconium dioxide: Zr-Mg-O and Zr-Mg-Y-O. The studies on physico-chemical properties of the materials revealed that as a result of admixturing zirconium dioxide with magnesium and yttrium there appear: • stabilization of regular crystallographic structure of ZrO2 ; • modification of the materials microstructure (decay of pores); • change in the character of active centers (there appear strong Lewis acidic centers); • and in consequence also change in their catalytic properties. For studying alcohols ketonization, zirconium dioxide stabilized with magnesium (∼9 at.%) and yttrium (∼1 at.%) was used. That composition is a superionic conductor in oxygen sensor. The material in the studied reaction of n-butyl alcohol ketonization showed a high activity and selectivity. The results were similar to values obtained over iron catalysts [38], which are completely different materials in respect to their electronic properties. Polycrystalline iron oxides are p-type semiconductors [39], while zirconium dioxide stabilized with magnesium, calcium or yttrium is a typical anion conductor of fast ions. For this reason, we started studies in order to explain why two materials (stabilized ZrO2 and Fe2 O3 ), which are so different from each other as for electrical properties, show such a high and similar activity and selectivity in the reaction. The first step was to check if ZrO2 stabilized with magnesium oxide (Zr-Mg-O) would show similarly good catalytic properties in the studied reaction. The studies revealed that Zr-Mg-O was also a good catalyst of alcohol condensation to ketone. If electrical properties are considered, both forms Zr-Mg-O and

Zr-Mg-Y-O have almost identical electrical properties, they slightly differ in mechanical durability. Yttrium oxide is added during the production of stabilized zirconium ceramics in order to improve its mechanical durability [40]. At the next stage, the studies were carried out for the reaction parameters over pure zirconium and magnesium oxides. It was shown that both pure oxides did not show selectivity towards ketone or any other component (aldehyde, alkene, or ester) appearing during the reaction. Moreover, in the presence of magnesium oxide the conversion level was considerably lower than in the presence of pure ZrO2 . In order to explain a different behavior of pure zirconium and magnesium oxides and stabilized ZrO2 several physical studies were carried on the materials. The evaluation of specific area of the materials revealed that pure MgO powders had the largest specific area (Table 2). In the aspect of active centers character the obtained results differ a lot from the literature data (Table 3). In the literature [12,14], the authors found that the distance (0.2455 nm) between acidic center Zr+4 and basic center O2− of zirconium dioxide as well as the distance (0.264 nm) between basic group (C–OH) and acidic group (C–H) in 1-cyclohexylethanol (2.64 Å) was of importance. Performed calculations showed that the distances did not differ too much between the studied materials (Table 5). Moreover, according to the literature [41], pure oxides of MgO, ZrO2 , Y2 O3 reveal decisively basic properties. As a result of admixturing ZrO2 , the obtained materials changed not only their crystallographic structure (at the whole temperature range of the experiments the regular variety is stable), but also in the materials there was a large number of oxygen vacancies. Zirconium dioxide stabilized with magnesium or calcium (Me2+ ) can be asserted as a solid solution and described with the following formula: Zr1−x Mex O2−x . As a result of admixturing (e.g. Me2+ ), the ions of admixture enter node positions causing the appearance of a balancing number of defects—oxygen vacancies—in ZrO2 structure. The process can be denoted in Kröger–Vink symbols as follows: ZrO2

••

MeO → Me

Zr + OX O + VO

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Table 5 Studied materials, their crystalline structure, valence (zi ), central ion radius (rion ) and calculated distance between the central ion and oxygen ion Material

Structure

zi

rion (Å)

Distance between zi and O2− (Å)

MgO ZrO2 Y2 O3 Zr-Mg-O Zr-Mg-Y-O

Regular of NaCl type a = 4.211Å Monoclinic of baddeleyite type Cubic body centered Regular of fluorite type a = 5.106 Å Regular of fluorite type a = 5.1045 Å

2+ 4+ 3+ 4+ 4+

0.78 0.87 0.93 0.87 0.87

2.105 2.65 9.19 2.452 2.450

where: Me

Zr is bivalent cation of admixture Me2+ in the node of cation sub-lattice Zr4+ ; OX O the bivalent 2− in the node of anion sub-lattice O2 ; and anion O •• VO the anion vacancy. The authors assert that oxygen vacancies appearing as a result of admixturing are acidic centers of Lewis type. In the analyzed n-butyl alcohol condensation they interact with oxygen from carbonyl group. The carbon atom from the carbonyl group (poor in electrons) becomes a center of nucleophilic attack of nucleophilic factors (base—electron donor), which in the materials are surface-lattice oxygen ions. It is compliant with the results of Biela´nski and Haber studies [42]. Depending on the temperature on the surfaces of intermediary metal oxides, as a result of chemisorp− − tion, there can appear ions O− 2 or O . Ion O2 is clas2− sified as “electrophilic” factor, while ions O bound to the lattice on the surface as “nucleophilic” factor [15]. Alcohol particles can be easily fixed to the surface of such a catalyst by oxygen atom of its OH group. Considering such a character of centers at the surface a general n-butyl alcohol course of ketonization at a higher temperature can be described as follows. At the first stage of the reaction on active centers of the catalyst, at the right temperature, there undergoes the adsorption of the alcohol molecule and its dissociation, the hydrogen ion is adsorbed at the electrophilic center and the alcohol radical at Lewis acidic center according to the reaction: RCH2 OH

Zr 1−x Mex O2−x ;T

→

RCH2 Oads + Hads

(3)

Alcohol radical can react with oxygen vacancy (Eq. (4)) and with oxygen that is in a close lattice node (Eq. (5)): ••

RCH2 Oads + VO → RCH2 Olat

(4)

RCH2 Olat + Olat → RCOlat Olat + 2Hads

(5)

The resultant bonds RCOlat Olat undergo transformation to the proper ketone according to Eq. (6): ••

2RCOlat Olat → RCORgas ↑ +CO2 gas ↑ 2 Olat +2VO

(6) The adsorbed hydrogen can react to water and undergo desorption [43,44]. Taking into account the transient products the authors maintain that also the condensation to ketone of aldehydes, organic acids and their mixtures can proceed on the stabilized zirconium dioxide containing Lewis acidic centers. Comparing physico-chemical properties of the materials Sn-Ce-Rh-O [45], Zr-Mg-O, Zr-Mg-Y-O, iron catalysts [38] that were used in process of primary alcohols condensation to ketones, their only common feature is the existence of oxygen vacancies. Thus, the materials become either p-type semiconductors, as Fe2 O3, or n-type, as Sn-Ce-Rh-O composition, or superionic conductors, as stabilized zirconium dioxide. For this reason, the authors assert that oxygen vacancies, which bring about the formation of Lewis acidic centers at the surfaces of the materials, are responsible for their catalytic properties.

5. Summary Research on various catalytic materials, both pure and admixtured (stabilized) zirconium dioxide, was carried out. The synthesis of stabilized ZrO2 was conducted in a low-temperature process. The obtained materials were monophase products of a regular structure. Their lattice constant did not differ much from

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