In the Laboratory
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Experiments on Heterogeneous Catalysis Using a Simple Gas Chromatograph Flávia Cristina Camilo Moura, Frederico Garcia Pinto, Eduardo Nicolau dos Santos, Luis Otávio Fagundes do Amaral, and Rochel Montero Lago* Departamento de Química, ICEx, Universidade Federal de Minas Gerais, Belo Horizonte MG, Brazil 31270-901; *
[email protected] Catalysis is an important area in chemical education owing to its multidisciplinary nature and its importance to industry and to environmental protection. However, only a few catalysis experiments with pedagogical purposes have been suggested in the literature (1–13). This is probably due to the complexity of a catalytic reaction setup. For example, a gas– solid heterogeneous catalysis experiment requires gas cylinders, gas flow controllers and meters, tube fittings, a reactor, usually a glass or quartz tube placed in a temperature-controlled oven, injection valves, and an analytical instrument, such as a gas chromatograph (GC), to identify and quantify the reaction products. In this work, it is shown how a GC can be used to carry out heterogeneous catalysis experiments with no other equipment. In this setup the catalyst is placed inside the GC injector liner and the reactants are injected with a micro syringe to produce a pulsed catalytic experiment. This system offers several advantages such as: • It is applicable to any split–splitless GC equipment since all models have glass inserts that can be removed by a simple procedure
The removal of the liner, the introduction of the catalyst, and the insertion of the liner in the GC injector is a simple procedure. Some instructions to replace the liner in the GC are presented in the Supplementary Material.W The hydrogenation of 1,5-cyclooctadiene (1,5-COD) was studied over two different palladium catalysts: a Pd metal supported on Al2O3 at 5 wt % and a Pd metal dispersed on the surface of quartz wool prepared by injecting 1 mL of the precursor Pd2(π-allyl)2Cl2 solution at 10᎑1 mol L᎑1, which decomposes on the silica wool inside the liner at 300 ⬚C to form Pd0. The preparation of these catalysts and a suggestion of other commercially available supported catalysts are given in the Supplementary Material.W Volumes of 0.2–3.0 µL (1.6–24 µmol) of 1,5-COD were injected into the GC with a micro syringe. The isomerization reactions were carried out using the same procedure as described above only replacing H2 by N2 as carrier gas. For the methanol reaction a commercial catalyst Cu兾ZnO (BASF) was used. The preparation of the Cu兾ZnO catalysts is described in detail in the Supplementary Material.W
• It simplifies the catalytic experiment • It allows a simple control of the reaction temperature by modifying the injector temperature • A great variety of reactions can be studied by modifying the catalyst, the carrier gas, and the reactants injected • It allows the study of contact time by varying the carrier gas flow rate • It allows the study of the reactant concentration effect by simply varying the quantity injected • The reaction products are analyzed online taking only a few minutes
These different parameters allow the study of several aspects in heterogeneous catalysis such as conversion, selectivity, activation energy, and the study of the reaction mechanism. Experimental The experiments were carried out in a Shimadzu GC17A gas chromatograph equipped with a flame ionization detector (FID) and a Carbowax 20M capillary column (30 m). In this GC the injector temperature can be varied typically from room temperature up to 350 ⬚C. Different carrier gases can be used, such as N2, He, H2, and so forth. The carrier gas flow rate can be varied by controlling the inlet carrier pressure. In a typical experiment the solid catalyst (ca. 2–5 mg) was placed between two silica wool plugs inside the injector liner, as shown schematically in Figure 1. www.JCE.DivCHED.org
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Figure 1. Schematic representation of the GC injector with a catalyst placed inside the glass liner.
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Hazards Special care to prevent H2 leakage should be taken when introducing the catalyst into the GC injector. Hydrogen is a highly flammable gas with explosion risk. The organic compounds 1,5-COD, 1,3-COD, ethanol, propanol, butanol, 1hexene have pungent irritant odors and are flammable liquids. Carbon tetrachloride, formaldehyde, and chloroform are highly toxic and cancer suspect agents. Methanol is a toxic flammable liquid and poisoning may occur by ingestion, inhalation, or percutaneous absorption. These liquids should be handled in a hood. The solid catalysts, Al2O3 and silica wool, should be handled with gloves and mask to avoid contact and inhalation of the fine powders or fibers. The copper, zinc, and manganese salts are oxidizers and corrosives.
+
COE
H2
COA +
1,5-COD
+
1,4-COD
1,3-COD
Scheme I. Reaction products for the hydrogenation of 1,5-COD over Pd catalyst.
Results and Discussion GC equipment typically operates with carrier gases such as N2, argon, or He. These inert gases can be used to study several reactions, especially decompositions and isomerizations. Hydrogenation reactions can also be studied by using H2 as a carrier gas. The use of H2 as carrier in GC is fairly common, especially in equipment with an FID detector. However, as H2 is a flammable gas several precautions must be taken to avoid leakages and accumulation in the GC room. Oxidation reactions are limited in this GC reactor system owing to the use of oxygen as a carrier gas, which might damage the GC column. In a GC reactor system the reaction temperature is limited by the GC injector, which typically can be raised up to 350 ⬚C. Hereon, we describe some reactions studied in the GC catalytic reactor system.
Catalytic Hydrogenation of 1,5-Cyclooctadiene over Pd Catalysts The hydrogenation of 1,5-cyclooctadiene (1,5-COD) was selected owing the variety of products possible in this reaction: cyclooctene (COE), cyclooctane (COA), 1,4cyclooctadiene (1,4-COD), and 1,3-cyclooctadiene (1,3COD) (Scheme I) (14). The product selectivity is strongly
dependent on the catalyst used and the reaction conditions employed. This allows an interesting discussion on the effect of different parameters on the reaction selectivity. Several commercial hydrogenation catalysts can be used for this purpose. In this work, the reaction was carried out in the presence of two different Pd metal catalysts: Pd (5 wt %)兾Al2O3 and Pd supported on quartz wool. Completely different selectivities were observed for the reactions carried out in the presence of the different Pd catalysts. The Pd兾Al2O3 resulted in the total hydrogenation of 1,5-COD to COA. The 1,5COD conversion at different temperatures in the presence of Pd兾Al2O3 is shown in Figure 2. These results can be used to investigate the effect of the temperature on the reaction. It can be observed that the conversion increases with temperature reaching 90% at 350 ⬚C. Using these data and the Arrhenius equation (lnk = lnA − Ea兾RT ), the activation energy can be calculated by plotting ln(conversion) versus 1兾T (Figure 3). Considering the constant R equal to 8.314 J K᎑1 mol᎑1, an Ea of approximately 52 kJ mol᎑1 was obtained. On the other hand, the Pd0兾quartz wool catalyst showed much higher activity and produced COE as a major product
4.7
ln(conversion as percent)
100
Conversion (%)
80
60
40
20
0 200
225
250
275
300
325
350
375
4.5
4.3
4.1
3.9
3.7 1.5
1.6
T
Temperature / °C Figure 2. 1,5-COD conversion over Pd/Al2O3 at different temperatures.
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1.7
•
ⴚ1
/ (10
1.8
−3
1.9
2.0
ⴚ1
K
)
Figure 3. Arrhenius plot for the hydrogenation of 1,5-COD in the presence of Pd/Al2O3 catalyst.
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In the Laboratory
Table 1. Isomerization of 1,5-COD to 1,3- and 1,4-COD
150
05
40/60
250
17
60/40
NOTE: Reaction in the presence of Pd/quartz wool catalyst, carrier N2, and Vinj 0.2 mL.
(Figure 4). The effect of the temperature on the reaction selectivity is also shown in Figure 4. As the reaction temperature increases the selectivity for the total hydrogenation product COA increases concomitantly with the decrease of the selectivity for COE. These results clearly show that at higher temperature COE is hydrogenated to COA. Very low selectivities for the isomers 1,3- and 1,4-COD were obtained in this reaction. The effect of 1,5-COD concentration on the product selectivity can also be demonstrated by injecting different volumes of the reactant. The effect of the 1,5-COD injected volume, Vinj, on the COE兾COA ratio is shown in Figure 5. As the injected volume increases the selectivity for COE increases up to four times. This result can be discussed in terms of a reaction pathway of the type: 1,5-COD
H2 step 1
COE
H2 step 2
Catalytic Isomerization of 1,5-Cyclooctadiene over Pd Catalyst It is also possible to study the isomerization of 1,5-COD to 1,3-COD and 1,4-COD (Scheme I) using the Pd0兾quartz wool catalyst and employing N2 as carrier gas in place of hydrogen. Conversions of approximately 5 and 17% are obtained at 150 and 250 ⬚C, respectively. The major product observed at 150 ⬚C is the isomer 1,4-COD. As the temperature increases the conversion and the selectivity for the 1,3COD also increase (Table 1). Methanol Decomposition over Cu/ZnO Catalysts A simple reaction that can be studied by the GC catalytic reactor is the methanol decomposition using N2 as carrier gas. The decomposition produces mainly two products: hydrogen and carbon monoxide. 2H2 + CO
A typical catalyst for this reaction is Cu兾ZnO, which is the industrial catalyst for methanol synthesis from syngas (a mixture of CO and H2) (15). The reaction also takes place on supported noble metal catalysts, such as Pd, Pt, and Rh (16). The methanol conversion with the reaction temperature is shown in Figure 6. From these data an activation energy of 58 kJ mol᎑1 for the reaction can be calculated.
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1,5-COD conversion 80
COE selectivity
60
COA selectivity
40
20
1,3 and 1,4 selectivity 0 80
100
120
140
160
200
220
Temperature / °C Figure 4. 1,5-COD conversion and product selectivity over Pd0/quartz wool catalyst at different temperatures.
4.5 4.0 3.5
COA
Steps 1 and 2 can be viewed as competitive processes. As the 1,5-COD concentration increases step 1 is favored, producing more COE compared to COA. If necessary the 1,5-COD can be diluted in an inert solvent, such as hexane, and injected as a solution. The presence of the inert solvent should not interfere significantly in the reaction.
CH3OH
100
Selectivity and Conversion (%)
1,3-COD/1,4-COD Ratio
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COE Ratio COA
1,5-COD Conversion (%)
3.0 2.5 2.0 1.5 1.0 0.5 0.0 0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Vinj / µL Figure 5. Effect of 1,5-COD concentration on the COE/COA ratio.
50
CH3OH Decomposition (%)
Reaction Temp/⬚C
40
30
20
10
0 0
50
100
150
200
250
300
Temperature / °C Figure 6. Methanol decomposition over Cu/ZnO catalyst (catalyst 5 mg, Vinj 0.2 mL, carrier N2) as a function of temperature.
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Table 2. Examples of Catalytic Reactions That Can Be Studied in the GC Reactor System Reaction
Catalyst
Substrate
GC gas
Hydrogenation
Supported noble metal (e.g., Pd, Pt, Rh)
Olefins (e.g.,1-hexene, cyclohexene, cyclooctadiene)
H2
Hydrogen–dechlorinationa
Supported noble metal (e.g., Pd, Pt, Rh)
Organochloro compounds (e.g., CCl4, CHCl3, CH2Cl2)
H2
Hydroisomerization
Pt/zeolite
Heptane
H2
Dehydration of alcohols
Acid catalysts: zeolites
Ethanol, propanol, butanol
N2 or He
Olefin isomerization
Supported noble metal catalysts (e.g., Pd, Pt, Rh)
Olefins (such as 1-hexene) or diolefins (1,5-COD)
N2 or He
Decomposition
Cu/ZnO
Methanol, formaldehyde
N2 or He
Decomposition
Metal oxides (e.g., MnO2, Fe3O4)
Alkylhydroperoxide
N2 or He
a
Small quantities of HCl are formed in the reaction that might be corrosive to some parts of the GC.
Several catalytic reactions can be studied in this gas chromatography reactor system by varying the carrier gas, the catalyst, and the substrate. Some examples, such as dehydration, hydrogenation, isomerization, decomposition, are given in Table 2. Conclusion The GC reactor system described in this article is simple and versatile. Laboratories equipped with only a simple gas chromatograph can run complex experiments allowing the study of different gas–solid heterogeneous catalytic reactions. Several topics in heterogeneous catalysis such as kinetics, catalyst deactivation, product selectivity, and activation energy can be explored by the students using this experimental setup. These experiments are suitable for undergraduate and graduate courses that give a basic knowledge in catalysis and need a short and illustrative experimental work. The students will also profit by learning several aspects of GC equipment. W
Supplemental Material
Instructions for the students and notes for the instructor are available in this issue of JCE Online.
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