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Section 15. Chemistry
Section 15. Chemistry
Aliyeva Nushaba Musa,
Institute of petrochemical processes, Azerbaijan National Academy of Sciences, senior researcher, Department of physical, physical-chemical studies
Email: [email protected]
The nature of catalytic active centers of ethanol to hydrocarbons conversion reaction over alumina based catalysts
Abstract: The conversion of ethanol to hydrocarbons as a function of the content of electron-acceptor (EA) and surface -OH centers is studied. It was shown that the conversion of ethanol into hydrocarbons over the alumina based catalyst proceeds in two stages and Bronsted and EA centers is assumed as the catalytic active centers of this reaction on the first and next stages, respectively.
Keywords: ethanol, hydrocarbons, alumina based catalysts, electron-acceptor centers, surface OH groups.
Introduction
The conversion of ethanol to hydrocarbons over solid catalysts is one of the most studied reactions in heterogeneous catalysis. The analysis of the available literature shows that the aluminum oxide and its modified with various elements forms can be used as active catalyst for conversion of alcohols to hydrocarbons [1-6]. Despite of intensive research in this direction in the last 20-25 years, mechanism of the nature of the catalytically active sites and mechanism of this reaction are still the subject to debate [1, 4-6].
The aim of this work is the study of the role of EA centers and the surface -OH groups as catalytic active centers for the conversion reaction of ethanol to hydrocarbons over the y-Al2O3 modified with different content (1-5 wt %) of iron and zirconium, by FTIR, UV/VIS and EPR methods in combination with GC/MS analysis of gas phase products of this reaction.
Experimental/methodology
The catalysts were prepared by wet impregnation technique of y-alumina support with the ZrOCl2 - 8H2O and FeCl3 - 2H2O solutions. The prepared samples of catalysts were dried at 393 K and then calcined in the presence of flow of purified air at 673 K for six hours. These catalysts are tested in the conversion of ethanol to hydrocarbons at 473-578 K
and atmospheric pressure. The contents of EA centers in the samples is determined based on the EPR spectra of adsorbed over catalysts diphenylamine molecules usingJES-PE-3X, Jeol spectrometer. FTIR spectra were recorded using a Bruker IR spectrometer ALPHA FTIR. Concentration of the -OH group is determined using UV/VIS and brilliant -green as an indicator using UV/VIS 6850, Jenway spectrometer. The gas-phase products of ethanol conversion has been studied by GC/MS method using Thermo Scientific chromato-mass-spectrometer GC/MS Focus. Element composition on the surface and phase composition of the samples before and after conversion of ethanol are determined by X-ray fluorescence microscopy and X-ray diffraction methods using XGT 7000, Horiba, Japan, microscope and XRD TD 3500, China diffractometer, respectively.
Results and discussion
The reaction products of catalytic conversion of ethanol to hydrocarbons at reaction temperature range from 473 to 578 K are given in the table 1. The hydrocarbons in the gas phase were detected starting from 473 K. It was shown that at the initial stage of the reaction during the first 30-40 min. the yield of the ethylene is increased and then during the next 3 hours the yield of the ethylene decreases and the formation of aliphatic (saturated, unsaturated) and aromatic hydrocarbons are observed.
Table 1. - Gas-phase products of Ethanol Conversion over Al2O3 based catalysts at 578 K and atmospheric pressure
Catalyst Conversion, % Yield, %:
Ethy- lene Aliphatic hydrocarbons Aromatic h/c-s
Saturated Unsaturated
1.0 %Zr/Al2O3 100 62 15 13 10
3.0 %Zr/Al2O3 100 72 5 16 7
0.3 %Fe-1.0 %Zr/Al2O3 100 65 18 12 5
1.0 %Fe-1.0 %Zr/Al2O3 100 58 21 10 11
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The nature of catalytic active centers of ethanol to hydrocarbons conversion reaction over alumina based catalysts
In the EPR/FMR spectra for these catalysts two different by nature signals are observed: belong to ferro-/super-paramagnetic FeOx for Fe/y- Al2O3 (or their modified by Zr forms) catalysts with effective g-factor g = 2.14-3.65 and line width ДЫ = 110-320 mT and paramagnetic carbon deposits with g = 2.003 and ДЫ = 0.5-0.7 mT. The formation of hydrocarbons is accompanied with the appearance of signals at g = 2.14-2.15 and ДЫ = 125-131 mT in the EMR spectra. It was established that the active catalyst are characterized with the symmetrical EMR signal, due to super-paramagnetic particles of the size 15-20 nm. At the same time grain size of identified from diffractogram phases of active elements for this catalyst based on Scherrer formula was (35-45 nm). The scanning
the surface of the catalyst using X-ray fluorescent microscope shows the noticeable differences in distribution of the active elements (Fe, Zr) in the samples before and after the reaction. Regeneration of catalyst with calcination in air at 673 K during the 2h leads to the initial state of catalyst. In the fig. 2 the X-ray fluorescent spectrum of modified y-Al203 samples is given.
In the table 2 the concentration of EA centres as a function of the concentration of Cl- ions in the samples and influence of the concentration of Cl- ions in the samples on the rate of the conversion of ethanol to ethylene are given. As can be seen from this table the concentration of the EA centres and the yield of the ethylene are increased with increasing of the concentration Cl- ions in the samples.
Fig. 1. X-ray fluorescence spectrum of calcined in air at 873 K sample 3%Fe-3%Zr/Al2O3
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Fig. 2. XRD patterns of calcined in air at 1073 K sample 3%Zr/Al2O3
Table 2. - The number (N) of EA and centres and the yield of the formation of ethylene (in mas.%) as a function of the concentration of Cl- ions in the catalyst
1.0 %Zr/Al2O3
Cl- ions in the samples, mas. %
- 1 2 4
Yield of ethylene, mas. % 62 67 78 86
Number of EA centres, N x 10 17 spin/g 4 6,5 5.3 5.5
Number of -OH centers, N x 10-3 mmol/g 55 62 78 86
1.0 %Fe-1,0 %Zr/Al2O3
Cl- ions in the samples, mas. %
- 1 2 4
Yield of ethylene, mas. % 54 57 58 641
Number of EA centres, N x 10 17 spin/g 2.8 2.3 2.0 2.0
Number of -OH centers, N x 10-3 mmol/g 52 68 72 76
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Section 15. Chemistry
The number of EA and -OH centers is determined on the base of adsorbed diphenylamin EPR spectra and it was shown that the concentration of EA centers increase with increasing
of temperature (473-1073 K) for the treated in air samples. The EPR spectra of adsorbed at room temperature diphenyl-amine are given in the fig. 3.
Fig. 3. EPR spectra of the calcined at 873 K Zr/Al oxide catalyst with adsorbed at room temperature diphenylamine after: a) 0.5; b) 4 hours and c) 2 days
Fig. 4. FTIR spectrum at room temperature of the of Fe, Zr/Y- Al2O3 - sample treated at 673 K in air during the 2 hours
Conclusion
Two type — surface -OH groups and EA centers are discussed as active centers for the conversion of ethanol over y-Al2O modified with Fe and Zr. It was shown that -OH groups is responsible for the dehydration of ethanol at first and EA centers are responsible for the conversion of ethylene and diethyl-ether into ethane, propane, butane, pentane, hexane — aliphatic hydrocarbons and benzene, toluene and xylene — aromatic hydrocarbons at the next
stages of the reaction. t was established that conversion of ethanol to aromatics increases with increasing the content of Zr in the sample and that the active sites necessary for formation of aromatics are the Zr ion based structures, phases (ZrFeO3 , ZrO2 ) in vicinity with y-alumina acid sites that facilitate the formation of aromatic compounds. The obtained data show that Zr active sites catalyze the formation of aromatic compounds and FeOx species the cracking reaction.
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