Characterization of the active sites on the surface of alumina ethanol dehydration catalysts by EPR using spin probes, страница 3

The catalytic activity in ethanol dehydration was determined in the kinetic region at 330-350°C. The experiments were carried out in a catalytic installation with a differential flow reactor at atmospheric pressure (The dehydration of ethanol was carried out in a conventional flow-type reactor under atmospheric pressure.). The volume of the loaded catalyst was equal to 1 cm3 (0.5-1.0 mm fraction). Contact time 1.0 s. The products were analyzed by means of a ЦВЕТ-500 gas chromatograph equipped with a flame ionization detector and Porapak S column. The ethylene formation rate at low conversion was used as the measure of the catalytic activity.

was packed into a quartz reactor.

Results and discussion

Method for characterization of weak acceptor sites

Until now in our group we paid most attention to the investigation of the strongest acceptor sites capable of ionizing aromatic molecules with high ionization potentials, such as benzene, chlorobenzene and toluene [18, 19, 21-25]. Such sites are present in considerable concentrations only on the strongest acid catalysts – high-silica zeolites and sulfated metal oxides. In particular, it was shown such very strong acceptor sites might be responsible to skeletal isomerization and cracking of light alkanes over sulfated zirconia [19, 22, 26]. However, the concentration of such sites on conventional acid catalysts, such as Al2O3, is very low [17]. As the activity of pure and modified aluminum oxides in alcohol dehydration reactions is relatively high, there is no doubt that the strongest acceptor sites can hardly account for their activity. Meanwhile, it is well known that weaker acceptor sites present in considerable concentrations on the surface of many oxides can be characterized using aromatic molecules with lower ionization potentials, such as anthracene or perylene as spin probes. So, the development of a simple method for routine characterization of the weak acceptor sites using such probes appeared to be important for obtaining quantitative information on the concentrations of such sites and evaluation of the their role in the alcohol dehydration reactions.

To find a test donor molecule suitable for characterization of weak acceptor sites of the Al2O3 surface, first we carried out adsorption of different aromatic compounds on commercial Al2O3 sample (Condea). The liquid aromatic compounds were adsorbed in pure form, whereas solid substances were adsorbed from their solutions in p-xylene. The used compositions together with the ionization potentials of the donors and the concentrations of the radical species obtained after activation at 500°C and 300°C are reported in Table 1. The obtained data show that the highest concentration of the radical cations was obtained using anthracene that has the lowest ionization potential among the studied aromatic compounds. Since our goal was to test relatively weak acceptor sites, it seemed to be the most suitable donor for this study.

Figure 1 presents the EPR spectra obtained after adsorption of toluene, 0.04 M anthracene solution in toluene, which were used for characterization of the strong and weak acceptor sites, respectively, and 0.02 M 1,3,5-trinitrobenzene (TNB) solution in toluene used for characterization of donor sites. The spectrum obtained after TNB adsorption can be attributed to its radical anions formed on the electron acceptor sites. The predominant feature of this spectrum is an anisotropic hyperfine splitting on a single nitrogen atom with Azz = 31 G.

The spectra obtained after adsorption of the donor molecules are singlet lines with g=2.003. They can be attributed to organic radical-cations formed by polycondensation of the initial radical cations with neutral molecules on the acceptor sites. The mechanism of this process is discussed in more detail below.

Anthracene is a solid compound poorly solvable in organic solvents. So, our second step was to optimize the solvent, anthracene concentration and adsorption procedure to obtain reproducible results. Anthracene adsorption through the vapor phase used in some previous studies was unsuitable for us as it is too complicated and slow procedure that can hardly be used for routine comparison of many samples. Table 2 summarizes experimental data on the concentrations of the radical species generated after anthracene adsorption on the same commercial Al2O3 sample (Condea) activated at 300°C from solutions of different concentrations in two solvents (toluene and p-xylene) at room temperature and at 80°C. The latter temperature was previously found to be the most suitable for characterization of the electron donor sites on alumina using 1,3,5-trinitrobenzene as the spin probe. The obtained results show that the concentrations of generated radical cations substantially depended on the anthracene concentration in the solution whereas the used solvent had almost no effect on it. So, we used toluene as the solvent in the subsequent experiments.