The oxidative dehydrogenation of С1-С2 alcohols on copper-contained zeolites and HTSC

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The oxidative dehydrogenation of С1-С2 alcohols on copper-contained zeolites and HTSC

L. Akhalbedashvilia,

G. Maisuradzeb,

A. Mskhiladzec,

Sh. Sidamonidzeb

Summary

The oxidative dehydrogenation of CH3OH and C2H5OH was studied on different matrix, containing copper, and compared with each other. It was found that Y123 and Bi2212 are less active to compare with Cu-containing zeolites. It should be mentioned that many uncertainties associated with the mechanism of the catalytic action of copper associates in high-temperature superconductors, as well as the localization, nature and mechanism of the catalytic action of Cu-ion sites in the copper-containing zeolites.

Keywords: alcohols, zeolite, copper, oxidation

Introduction

In the oxidation reactions with molecular oxygen the oxides of transition metals™ are active catalysts due to the low activation energy of change the oxidation state that facilitates transfer the electrons between the substrate and catalyst. The specifity of zeolite catalysts in oxidation reactions is caused, on the one hand by the fact that the activity of the transition metals™ cations fixed in the matrix depends on the coordination of skeleton with atoms, on the other hand, the matrix itself contributes to the formation and conversion of intermediate products as well as in the diffusion of reagents and reaction products. It should be noted that the catalytic transformation of alcohols on heterogeneous, including zeolite contacts studied quite extensively, but the conversion of methanol and ethanol in an oxidizing environment on zeolite catalysts today are still little studied.

It was previously shown [1−5] that the low activity of initial synthetic and natural zeolites in the oxidative conversion of methanol and ethanol due to the presence of molecular oxygen near the active alcohol adsorbed on the alkali cations, but the introduction of transition metals changes the nature of the catalytic action.

In this study it was carried out the comparison the activity of copper-containing catalysts on various zeolite and high-temperature superconductor (HTSC) carriers in the oxidative conversion of C1-C2 alcohols.

oxidative dehydrogenation alcohol zeolit

Experimental part

As a matrix the synthetic zeolites Х, Y, mordenite, ZSM-5 and natural zeolite clinoptilolite (CL) were applied; as the oxidic form high-temperature superconductors (HTSC) Y1Ba2Cu3Ox and Bi2Sr2CaCu3Ox were used. Zeolite catalysts were prepared by an ion-exchange method from solutions of copper-ammonia complexes and copper nitrate; HTSC-samples were synthesized by the traditional ceramic technique from the corresponding metal oxides. The catalytic experiments were carried out by the microflow method with varying of temperature, the size of catalyst grain and gas flow rate. The composition of the initial mixture and yields were analyzed chromatographically using porapac Q, porapac R and molecular sieve 13X as phases.

Results and discussion

Conversion of the ethanol in an oxidizing atmosphere at zeolite catalysts, as shown previously [2, 5] proceeds in four directions: the intermolecular and intramolecular dehydration to ether and ethylene, respectively, as well as complete to CO2 and partial oxidation to acetaldehyde (Scheme 1). Thus on initial NaX and NaY zeolites the intramolecular and intermolecular dehydration of ethanol pass through a maximum and in the range of lower temperature (550 — 650 K), in contrast to the oxidative reactions, i.e., in deep and partial oxidation (Figure 1).

Scheme 1

Fig. 1. The relation of ethanol conversion products on NaY zeolite:

1 — (C2H5)2O; 2 — C2H4; 3 — CH3CHO; 4 — CO2.

Unlike ethanol, methanol conversion on NaY proceeds a little differently — intermolecular dehydration, oxidative dehydrogenation to CH2O and full and partial oxidation to CO and CO2 (scheme 2).

Scheme 2

The temperature of reaction shifted to a higher temperature region (see Fig. 2). According to data the processes of oxidative dehydrogenation, dehydration and deep oxidation of alcohol on faujasite proceed simultaneously. Introduction the copper cations to the structure of Y zeolite sharply alters the nature of the conversion of alcohols (see Fig. 3).

Fig. 2. The relation of methanol conversion products on NaY zeolite:

1 — CH2O; 2 — (CH3)2O; 3 — CO2; 4 — unreacted CH3ОН

Fig. 3. The relation of methanol conversion products on CuNaY:

1 — CH2O; 2 — (CH3)2O; 3 — CO2; 4 — unreacted CH3ОН

The catalytic activity of initial mordenite and especially of ZSM-5 is extremely low up to 773 K and introduction of Cu2+ in their structure and formation of coordinative-unsaturated isolated Cu (II) ions with symmetry of plane square result in appearance of high activity and selectivity in deep oxidation of alcohols.

On initial CL ethanol and methanol and formed C2H4, CH3CHO and CH2O undergo to full oxidation at temperature high than 623K (fig. 4). The reactions as complete and partial oxidation, so intermolecular dehydration of methanol and ethanol up to dimethyl ether and С2Н4 accordingly begins on CuCL from 493 K. The ethoxy ethane in yields of a reaction missed. The dehydration of ethanol to C2H4 prevails up to 553 К, and the degree of conversion up to СН3СНО does not exceed 10% in all temperature range (fig. 5).

Fig.4. Dependence of ethanol conversion products on temperature on initial CL: 1 — unreacted C2H5OH; 2 — C2H4; 3 — CH3CHO; 4 — CO2

Fig.4. Dependence of ethanol conversion products on temperature on CuCL: 1 — unreacted C2H5OH; 2 — C2H4; 3 — CH3CHO; 4 — CO2

The conversion of ethanol on zeolites occur under two parallel — consecutive paths:

Scheme 3

Scheme 4

Scheme 4 prevails.

Mechanism of methanol conversion to СН2О occurs by formation of methylene groups with following oxidation according scheme 5:

Scheme 5

The major reaction products on HTSC in conversion of alcohols were aldehydes, CO2, CO (fig. 6, 7). Neither ethers nor C2H4 and H2 were detected. The catalytic activity of Bi2Sr2CaCu3Ox in oxidative dehydrogenation and deep oxidation was much lower than of Y1Ba2Cu3Ox [6]. It may be suggested that the observed difference in activity and in the mechanism of alcohol conversion can be caused by the structural differences between Y1Ba2Cu3Ox and Bi2Sr2CaCu3Ox — the difference in the number of CuO-CuO2 layers per unit cell, or in the number of active centers; as well as different coordination environments of these centers, and their different accessibilities to reagent molecules.

Fig.6. The relation of methanol conversion products on Y1Ba2Cu3Ox: 1 — СН2О;2 — СО; 3 — СО2; 4 — unreacred СН3ОН

Fig.7. The relation of methanol conversion products on Bi2Sr2CaCu3Ox: 1 — СН2О; 2 — СО; 3 — СО2; 4 — unreacted СН3ОН

Review of many studies of catalytic properties of copper-based systems — oxides Cu (II) and Cu (I), various kinds of mixed cuprates, copper-contained zeolites — in heterogeneous catalytic reactions lead to the conclusion that the high catalytic activity of these compounds is associated with the active centers formed by copper atoms in a particular charge state and coordination environment [1, 7, 8, 9], and in high-temperature superconductors is due to the presence of non-stoichiometric labile oxygen.

From the obtained data follows that copper-exchanged Y zeolites are characterized with the greatest total activity. They differ by the lowest temperatures of conversion of alcohols and high oxidative ability. On an example of CuNaY samples is clearly that not only nature and amount of a substituting cation, but also its state in a zeolite matrix, which, in turn, depends on conditions of heat treatment and ion exchange, substantially define a direction and depth of catalytic process.

It is known that on zeolites, not containing transition metals, the conversion of methanol is carried out according the acid-base mechanism with the formation of DME [10]. Moreover, it is now established that the interaction of zeolite with the OH- of methanol promotes to release CH3+ ions, which are intermediates in reactions catalyzed by protons and can exist freely in zeolites [11]. On the other hand, the monovalent cations are not active centers of the redox processes [12].

The influence of conditions of modifying on a state of a cation is proved both with our catalytic data. It is known that at рН=5 the formation of cluster structures as a result of hydrolysis of using salt is carried out with participation of OH-groups. The heat treatment causes formation of clusters of copper ions, which exchange interaction causes weakening of EPR-signal strength [10]. The identical clusters, which include non-lattice oxygen, determine high catalytic activity in deep oxidation of alcohols. The mobility and reactivity of copper clusters is incremented also by additional coordination of copper ions with reagent molecules. The sample prepared at рН=10 contains the copper ions, which are coordinated with ammonia molecules at the expense of the greater coordination ability of NH3 in comparison with OH- groups. Such complexes interfere with formation of cluster structures. The heat treatment of this sample causes the transference of isolated copper cations on SII and SI ' sites in synthetic X and Y zeolites. Cu2+ ions localized in sodalite cavities in SII', under influence of molecules of alcohol at reaction temperatures migrate in large cavities — sites SIII and complete coordination up to octahedral at the expense of molecules of alcohol.

Conclusions

So, the catalytic activity of the zeolites and HTSC-materials in the reaction of the oxidative dehydrogenation of C1-C2 alcohols is determined by presence of the identical active sites — the associates of copper ions with oxygen; for zeolites they are migrate in to the big cavities — copper ions with extra coordination by oxygen, and for superconductive cuprates they are the fixed — O — Cu — O — chains or CuO2 planes, which are capable to change their coordination at loss of oxygen.

References

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