Catalytic activity of MEL zeolites modified with metallic couples for the conversion of ethane

Article abstract of DOI:10.3390/50300560

The transformation of ethane into aromatic hydrocarbons over Zn-metal- containing zeolites at W/F=10 gh/mol and 550°C was studied in a flow reactor at atmospheric pressure. Zn-metal-zeolite modified the activation mode of the alkane, generating highly reactive intermediates and enhancing the aromatic selectivity.

Full text of DOI:10.3390/50300560

560
Molecules 2000, 5
Catalytic Activity of MEL Zeolites Modified with Metallic
Couples for the Conversion of Ethane
Griselda Eimer, Pedro Girola, Lorena Tomas, Liliana B. Pierella and Oscar A. Anunziata
CITeQ(Centro de Investigacion y Tecnologia Quimica), Facultad Cordoba, Universidad Tecnologica
Nacional, CC36 -Suc16(5016)- Cordoba, Argentina
Tel/Fax: 54-351-4690585, E-email: anunziat@sa.frc.utn.edu.ar
Abstract: The transformation of ethane into aromatic hydrocarbons over Zn-metal- con-
taining zeolites at W/F=10 gh/mol and 550°C was studied in a flow reactor at atmospheric
pressure. Zn-metal-zeolite modified the activation mode of the alkane, generating highly re-
active intermediates and enhancing the aromatic selectivity.
Introduction
The transformation of light alkanes into more valuable compounds, such as aromatic hydrocarbons
(AH), is of great importance from both the industrial and academic points of view. MEL and MFI zeo-
lites loaded with metal cations, such as zinc and molybdenum have been successfully used for this
purpose showing very pronounced selectivity for aromatic hydrocarbons [1-6]. At the moment, the
utilization of zeolites modified with metalic couples for the paraffin conversion has not been reported.
Here we discuss the catalytic behavior of MEL zeolites modified with Zn-metal couples in the activa-
tion of ethane and suggest a relation between structure - activity.
Experimental
Zeolites with a Si/Al molar ratio of 17 were synthesized by hydrothermal crystallization in Na2O-
Al2O3-SiO2 systems, in the presence of tetrabutylammonium hydroxide as template agent. The cations
were incorporated by ion exchange of NH4-zeolite except the molybdenum, which was incorporated by
incipient impregnation. Catalytic reactions were carried out in a continuous flow quartz reactor at at-
mospheric pressure. Products were withdrawn periodically from the outlet of the reactor and analyzed
by an on-line gas chromatograph equipped with a FID detector. Conversion and products distribution
were expressed on a carbon-atom basis.
Results and Discusion
Table 1 shows the results of catalytic activity of acidic and Zn-metal- containing zeolites for ethane

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Molecules 2000, 5
transformation. The ethane conversion is very low over the protonic form of ZSM-11. The metal spe-
cies loading improved the conversion of ethane as well as the selectivity to aromatics.
Table 1. Ethane conversion and reaction products selectivity over various catalysts at 550°C, W/F=
10 gh/mol, total pressure of 1 atm and TOS= 20min.
Catalyst
Conversion
mol % (C)
1.5
Selectivity, mol % (C)
C1
C2=
46.7
C3-C5
7.3
AH
H-ZSM-11
Zn-ZSM-11
29.3
2.44
2.82
5.47
1.68
16.7
11.26
54.74
43.05
40.08
59.03
9.10
8.59
8.38
7.69
33.72
45.54
46.05
31.60
ZnPb-ZSM-11
ZnMo-ZSM-11
ZnCu-ZSM-11
8.24
12.34
5.72
FT-IR data for chemisorbed pyridine after adsorption at room temperature and after further out-
gassing the samples at 250, 350 and 400°C indicate that the number and strength of Lewis sites in-
creases by incorporation of metal cations into the zeolite. Zn-metal-ZSM-11 could act as a hydride ab-
stractor in the ethane activation and. the function of metal species would be the dehydrogenation of
ethane into ethylene and of the naphthenic intermediates into aromatics [3,5].
Conclusions
MEL zeolites modified with metallic couples are effective for the activation and aromatization of
ethane. The first step would be the direct abstraction of a hydride from ethane, producing a ethyl-
carbenium surface ion through the electron-donor acceptor adduct (EDA) formation, and then by
deprotonation ethylene. Ethylene would undergo secondary transformations toward the aromatization.
References and Notes
1. Inui, T.; Okasumi, F. J. Catal. 1984, 90, 366.
2. Scurrel, M.S. Appl. Catal. 1987, 32, 1.
3. Pierella, L.B.; Eimer, G.A.; Anunziata, O.A. React. Kinet.Catal.Lett. 1998, 63(2), 271.
4. Pierella, L.B.; Eimer, G.A.; Anunziata, O.A. Stud.Surf.Scie.Catal. 1998, 119, 235.
5. Anunziata, O.A.; Eimer, G.A.; Pierella, L.B. Appl.Catal. 1999, 192, 267.
6. Anunziata, O.A.; Eimer, G.A.; Pierella, L.B. Catal.Lett. 1999, 58, 235.