versión On-line ISSN 0717-9707
J. Chil. Chem. Soc. v.49 n.1 Concepción mar. 2004
DRIFTS STUDIES OF ACRYLONITRILE ADSORPTION
ON Pd/SdO2 AND PdSd/SdO2.
GALO CÁRDENAS T* AND RICARDO OLIVA C.
Departamento de Polímeros
Facultad de Ciencias Químicas, Universidad de Concepción
Casilla 160-C, Concepción, Chile E-mail: email@example.com
(Received: May 26, 2003 - Accepted: July 2, 2003)
The acrylonitrile adsorption over Pd/SiO2 and PdSn/SiO2 catalysts wasprepared by "Solvated Metal Atom Dispersion" (SMAD) and analyzed by DRIFTS FTIR are studied.
The catalysts were used in pellets and treated with dry N2(g) flow inside a flask in contact with the acrylonitrile vapours for 6 h.
A change in the adsorption capacity of acrylonitrile with the increase in Sn content in the catalysts by following the Cº N IR band was studied.
The decrease in the acrylonitrile adsorption produced a decrease in the hydrogenation rates of acrylonitrile and an increase in the hydrogenation selectivity to obtain allylamine, mainly due to a greater interaction of Cº N with Sn.
From the heterogeneous catalyst point of view the interaction of metals from group VIII with unsaturated organic molecules has been widely studied, with special interest is the olefin molecular adsorption that represents the first stage in the reaction mechanism of several reactions catalyzed by this metal 5-7.
The Pd one of the transition metals, is widely used for the selective hydrogenation of dienes to mono olefins and the Sn addition allows to oxidize CO8-10, hydrocraking and hydroisomerization of n-heptane and other reactions.
The adsorption of the acrylonitrile molecule has been widely studied due to the interest that exists as a monomer in electropolymerization to form polymeric films that protect the surface of the metal 11-13. However, the adsorption over supported metallic catalyst is more unusual, even though selective hydrogenation of acrylonitrile produces two interesting molecules.
CH2 = CH-CN + H2 ¾® CH2 = CHCH2NH2 + CH3CH2CH2NH2
Propionitrile, is used as an intermediate in the herbicide industry 14 and allylamine, an easy molecule difficult to synthesize by traditional methods and well used in the polymerization via plasma and as chemical intermediator15-16. In a study carried out with such catalyst for acrylonitrile hydrogenation it can be observed that a Sn atomic fraction lower than 0.5 the TOF reaction increases and then tends to decrease, and the selectivity increases when the tin fraction increases up to an atomic fraction of Sn = 0.817.
In this work are represented the results obtained in the acrylonitrile adsorption over the Pd/SiO2 and PdSn/SiO2 catalyst prepared by "Solvated Metal Atom Dispersion" and analyzed by DRIFTS. The acrylonitrile is a monomer with two functional groups. The conjugation of the p electrons of double bond with the cyano group allows to decrease the electron density of the double bond. This is a molecule where the presence of two functional groups changes the adsorption of vinyl group over the metallic surface.
In this study the adsorption behaviour of the nitrile group will be correlated with these observations.
EXPERIMENTAL1. Catalyst preparation. The catalyst was prepared by the technique "Solvated metal Atom Dispersion" (SMAD) which Klabunde and Cárdenas 18,19 have previously described.
The catalyst formation was carried out in a bimetal atom reactor. Two tungsten crucibles (W-SiO2 from Sylvania Emissive) were charged with around 0.0872 g of Pd and 0.0137 g Sn metal in lumps. Distilled and dried solvents (e.g. 100 mL acetone) were placed in a ligand inlet tube and freeze-pump-thaw degassed for five cycles. The reactor is kept under vacuum until reaching 5-10 mm of Hg, previously the active SiO2 (5.0 g) has been introduced with a magnetic bar. A liquid nitrogen dewar was placed around the vessel and Pd, Sn and 100 mL acetone were codeposited over a 1.5 h period. The matrix was of a black colour at the end of the co-deposition. The matrix formed was allowed to warm slowly for 1.0 h at room temperature under vacuum by removal of the liquid nitrogen dewar. Upon meltdown, the black dispersion was allowed to warm for another 0.5 h at room temperature under N2(g) flow. Finally, the metal dispersed in solvent is stirred with SiO2 for 24 h in the reactor at room temperature under N2(g).
Catalysts with atomic fraction of Sn = 0.0, 0.3, 0.8 and 1.0 were prepared.2. Adsorption system
A small glass chamber was designed for this experiment, in which the acrylonitrile vapours are in contact with the pellet of the catalysts (see fig.1)
3. DRIFTS Spectrum
The catalyst samples were prepared in a pellet similar to the IR pellets of 10 mm diameter and 3 mm thickness, then treated with dry N2(g) flow over two hours in a small glass apparatus. All the samples were treated at the same time including a pure SiO2 pellet.
After this time the equipment was placed inside a dry box (LABCONCO) and purged with dry nitrogen.
The acrylonitrile substrate was distilled under vacuum and placed inside a dry box. A glass apparatus was designed as shown in figure 1. The samples were placed inside the flask in contact with the acrylonitrile vapours for 6 hrs.
|Fig. 1. Adsorption system design for acrylonitrile over the catalysts|
The DRIFTS spectra using a Pike 1393 accessory were taken with 128 scan in a FTIR Nicolet Magna-IR spectrometer 550. The background was built with the same catalyst pellets to avoid the support adsorption and the possible interaction metal - support.
RESULTS AND DISCUSSION
The spectrum under studies shows results only for the spectral region from 3000-1200 cm-1, since the higher wavenumbers are not relevant.
The FTIR spectra of acrylonitrile adsorbed on Pd/SiO2, PdSn/SiO2, Sn/SiO2 and over SiO2, recorded at zero time, showed that Cº N interaction was produced through a physisorption process. The band around 2234 cm-1 in all the catalysts spectra appears with a displacement of 5 cm-1 (fig.2-5), with respect to acrylonitrile in liquid phase (around 2227.1 cm-1), see Table 1. This fact reveals that interaction is produced by the cyano group through the free electron pair over the nitrogen in a linear coordination with a s bond 20.
|Fig. 2. FTIR of Acrylonitrile Adsorbed over Pd/SiO2 (time= 0)||Fig. 3. FTIR of Acrylonitrile Adsorbed over Pd/SiO2 (after 15min)|
|Fig. 4. FTIR of Acrylonitrile Adsorbed over PdSn/SiO2 (XSn=0.8; time=0)||Fig. 5. FTIR of Acrylonitrile Adsorbed over PdSn/SiO2 (XSn=0.8; after 15min)|
Sn Atomic Fraction
| 0.0 |
This bond is not affected in a great extent by the addition of Sn to the Pd catalyst, but there is a change in the absorption capacity of acrylonitrile with the increase of Sn content, which was found from the intensity in Cº N stretching. In this table it is also included the adsorption over the support, however, the interaction is very weak and easily eliminated. The adsorption capacity decreases almost linearly (see Table 2).
n 6 + n 7 **
p -bonded species
n 6 +n 7
p -bonded species
Mº C-CH2-Cº N ethylidine species
n 6 + n 11
n 6 +n 7
Stret. Cº N Physically adsorbed
n 9 + n 10
v(-C=C-)(n 6 + n 15)
p -bonded species
Bend asymmetric of -CH2-
Mº C-CH2-Cº N ethylidine species or d CH2 adosrbed on theSiO2
n 6 n 7
p -bonded species weak adsorbed
|n 6 + n 7 |
n 5 + n 9
n 4 + n 15
n 6 + n 11
Stret. Cº N Physically adsorbed
v(-C=C-)(n 6 + n 15)
d CH2 (n 3)
|*The combination bands are coincident with data previously reported in similar systems. 22-24|
This decrease is attributed to the low capacity of Sn to adsorb ethylene21,22. Preliminary studies by TEM, demonstrated that Pd particle sizes are not affected by the Sn addition and remain between 6-10 nm and the measurements of metallic area decrease with an increase in the Sn content, which is a consequence that Sn involve, Pd particles and eventually forms Pd-Sn alloy which decrease the adsorption capacity of Pd 17. This demonstrate, that acrylonitrile adsorption over Sn/SiO2 is produced mainly through the cyano group. This decrease in the absorption produces a decrease in the hydrogenation rates of acrylonitrile and an increase in the hydrogenation selectivity to obtain allylamine, most probably the decrease in the capacity of interaction of C=C with Sn and for the stronger affinity of Cº N with the Sn 17.
The desorption of acrylonitrile by time (15 min) produces a greater shift of the stretching Cº N at 2240 cm-1, which demonstrates that the interaction of the first layers of acrylonitrile with metals is a physisorption. In figure 2 the peak at 2234 decreases from 38 %T to 10 % T in figure 3 and appears at 2240 cm-1.The bands associated to the combined stretching of C=C and Cº N , bonds deformation CH2 (n 6 )+ rocking C-H (n 7) at 2695 cm-1, C=C stretching (n 5)+ wagging HRC=C (n 9) at 2574,8 cm-1, Cº N stretching (n 4)+ bend C-Cº N (n 15) at 2466.9 cm-1 23, are not active in all the catalysts prepared. This is an evidence that physisorption occurs through both functional groups.
The chemisorption of cyano function is produced through the Cº N bond formation with a band around 1669 cm-1 assigned to the stretching C=N previously reported.24
Also, bands at 1696,5 cm-1, 1681,9 cm-1, 1671,5 cm-1 were detected in Pd/SiO2, PdSn/SiO2 (XSn=0.3) and PdSn/SiO2 (XSn= 0.8), respectively (See fig. 4,5). The last band is not shown in Sn/SiO2 and SiO2. In all the cases the cyano stretching is very weak, meaning that a small amount of acrylonitrile is chemisorbed.
In the IR spectrum corresponding to C=CR vibrations, several bands appears between 1670-1400 cm-1. Table Nº2 summarizes the bands and can be observed that stretching bands for H2C=CR correspond to physisorption of p -adsorption complexes (1550 and 1514 cm-1, approximately) and species bonded as s (around 1420 cm-1) also are formed. The bands can be observed better after the elimination of acrylonitrile adsorbed over the support, leaving only the metals interaction, except for Sn/SiO2 and SiO2, where bands are not present. The strong band at 963.5cm-1 corresponds to a wagging of group reported for HRC=C 23 in liquid phase not found in none of the spectrum here studied, this is attributed to the C=C interaction with the Pd, PdSn or Sn particles.
In all the spectrum under study it can be observed a strong band around 1279-1323 cm-1(see table Nº2). The studies reported by several authors upon the ethylene adsorption over different metals, demonstrate that at room temperature is produced the formation of ethylidyne exhibiting a strong band similar to that reported at 1340 cm-1 6,25,26, this band can be also attributed to d CH2 adsorbed over SiO2 of the ethylene27. However, this band disappears rapidly in the Sn/SiO2 and SiO2 supported catalysts, but not with Pd and PdSn, indicating that ethylidyne species is produced by the Pd presence.
The studies carried out under these conditions reveal the acrylonitrile adsorption over Pd, PdSn and Sn supported over SiO2 is produced by a physisorption through a group C=C ( p -adsorbed species) and Cº N interaction with the lone electron pair over the nitrogen most probably related with a planar structure.
The Sn presence decreases the adsorption capacity of acrylonitrile by Pd and Sn only chemisorbe acrylonitrile by the Cº N group.
Also, the presence of ethylidyne species adsorbed over the catalysts was detected.
The authors would like to acknowledge the financial support from an operating grant Scientific Millennium Initiative (ICM 99-092) and R. Oliva the postdoctoral fellowship. We also thank Dirección de Investigación from Universidad de Concepción and laboratories from Facultad de Ciencias Químicas .
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