CC BY
36J. Ogonowski, E. Skrzynska
REVERSE WATER GAS SHIFT REACTION - THE THERMODYNAMIC ANALYSIS AND EXPERIMENTAL DATA FOR VMGOX CATALYSTS
(Institute of Organic Chemistry and Technology, Cracow University of Technology)
The results in thermodynamic analysis of the reverse water gas shift reaction were compared with the experimental data. Changes of the carbon dioxide conversion during the reaction time were monitored chromatographically% while an alterations of physicochemicai properties of vanadium-magnesium oxide catalysts were investigated using titrimetric and temperature-programmed reduction analysis.
Processes of catalytic dehydrogenation of hydrocarbons in the presence of carbon dioxide have been intensively studied recently [1]. It is known, that introduction of an oxidizing agent into the reaction zone allows to omit the thermodynamical limitations of the equilibrium process in the inert gas atmosphere (eq.l). Moreover, deep oxidation of hydrocarbons, typical for the dehydrogenation reaction with dioxy-gen, does not proceed [1-3].
From the literature [1, 3] there are two possible routes to alkene formation in the presence of CO2. In the one-step pathway (eq. 2) carbon dioxide acts as a reoxidant of the catalyst surface, reduced by a hydrocarbon. In the two-step pathway carbon dioxide reacts with hydrogen (eq. 3) and shifts the alkane equilibrium dehydrogenation (eq. 1).
CnH:
CnH
CnH2n + H2
nH2n+2
nH2n+2 + CO2 ^ CnH2n +CO + H2O
nH2n
H2 + CO2 ^ CO + H2O
(1) (2)
12O (3)
In our previous works we presented thermodynamic analysis of isobutane dehydrogenation reaction in the presence or absence of carbon dioxide [3]. The complex model, in which the main reactions (eq. 1-3) were considered with various possible side reactions, also was investigated [4]. In both cases, the equilibrium conversion of isobutane was highest for two-step reaction model. Therefore, the catalyst active in the reverse water gas shift reaction (eq. 3) should improve the conversion of isobutane.
This paper continues our previous study and consists of thermodynamic analysis of RWGS reaction for different initial conditions. The theoretical calculations are compared with the experimental data. The reverse water gas shift reaction was carried out over several vanadium-magnesium oxide catalysts active in the isobutane dehydrogenation reaction [5]. The additional analysis (i.e., titration and temperature-programmed reduction) allowed to observe the changes in the physicochemical properties of VMgOx catalysts after RWGS and to clarify which forms of vanadium are active in the process.
EXPERIMENTAL
Vanadium-magnesium oxide catalysts were synthesized in such a way as to give V/Mg molar ratio of 0.11 (20 wt% of V2O5 after calcination in static air for 6 h). The catalyst precursors were prepared in accordance with [5] using four different techniques: the citrate method (C), wet impregnation of magnesium oxide (I), co-crystallization from aqueous solution (W), and sintering the mixture of vanadium and magnesium oxides (S).
The classic thermodynamic analysis of the reverse water gas shift reaction was investigated under atmospheric pressure at the temperatures 100-1000°C and the molar ratio of H2:CО2 from 1:1 to 1:10. The thermodynamic data of reagents were taken from ChemCAD II DOS 2.601 database.
The catalytic tests of the reverse water gas shift reaction were carried out at 600°C under atmospheric pressure in a flow-type quartz reactor connected online with a TCD gas chromatograph. The mixture of hydrogen (12.5%) and carbon dioxide (87.5%) was introduced into the reaction zone at the total flow rate of 34 cmVmin. Before each of the catalytic tests," VMgOx sample (400 mg) was degassed in helium stream at 600°C for 0.5 h.
The temperature-programmed reduction analyses (TPR-H2) were carried out with a fresh VMgOx samples and the catalysts used in RWGS. The samples of 200 mg were reduced with the mixture of hydrogen (10% H2) and argon (total flow rate 34cm3/min) by increasing the temperature from room temperature to 680°C at the heating rate of 10°/min.
In the case of titration analysis of VMgOx (fresh and that used in RWGS), the samples were digested in hot mixture of phosphoric and sulfuric acids. Then the solution was titrated with potassium permanganate and ferrous ammonium sulfate, what allowed to calculate the percentage content of V+3, V+4 and V+5 ions in the catalyst, and an average oxidation number of vanadium [6,7]. Overall titration
<—
48
XHMHtf H XHMH^ECKAfl TEXHO-HOratf 2007 TOM 50 Bbm. 5
analysis proceeded according to the following scheme:
^ KMn04 titration _^ Fe(NH4)2(S04)2 titration
v3+ + V4+-> 3e"+ V5+ V5++ e"-> V4+
VMgOx cfissolving ii
catalyst H2S04/H3P04
^Fe(NH4)2(S04)2 station
V5++ e-*- V4+
RESULTS AND DISCUSSION
Figure 1 illustrates the changes of equilibrium CO2 conversion in RWGS reaction in the temperature range from 100 to 1000°C. As the standard enthalpy of the reverse water gas shift reaction (Ah°298) is +41.13 kJ/mol, the equilibrium conversion of carbon dioxide is increasing with the temperature and strongly depends on the initial molar ratio of reagents.
Fig. 1. Changes of the equilibrium conversion of CO2 in the reverse water gas shift reaction as a function of the process temperature for various molar ratios of H2:CO2
The thermodynamic analysis showed that the equilibrium conversion of CO2 at 600°C under atmospheric pressure was 10.6% for the H2:CO2 molar ratio of 1:7. For analogous process conditions, the highest conversion of carbon dioxide over VMgOx catalyst was 9.1% (fig. 2). Moreover, in the first minutes of RWGS reaction the conversion was rapidly increasing to reach constant value. This effect can be connected with the high oxidation state of vanadium ions in fresh catalysts (n°av).
The titration analysis proved that in the fresh VMgOx samples, vanadium ions were present almost
exclusively as V5+ (table Ah^g). During the first stadium of the reverse water gas shift reaction, vanadium on the catalyst surface can be reduced by hydrogen, and probably for this reason the conversion of carbon dioxide increases. This assumption was confirmed by second titration, which was done after 90 minutes of RWGS reaction, where the catalyst activity was high and stable. It was found that the catalyst samples contained mainly V3+ and V4+ ions. Therefore, an average oxidation number of vanadium (n"av) for the most active catalysts was below 3.5. Moreover, the least active sample (W) had the highest content of V4+ (81%).
Table
Comparing the results of titration and TPR-H2 analyses of VMgOx catalysts
Catalyst tpr-h2 TPR-H2 after RWGS
n°av [mmol H2/ /gcatl №/V)mol n'av nav [mmol H2/ /gcat] (H/V)mol
C 4.95 1.52 0.69 3.28 3.49 0.19 0.09
I 4.97 169 0.77 3.08 3.30 0.27 0.12
S 4.98 1.82 0.83 3.03 3.13 0.28 0.13
W 4.96 0.90 0.41 3.36 3.81 0.32 0.15
The catalysts reducibility seems to be an important parameter deciding about the catalyst activity in the reverse water gas shift reaction. In this place it has to be pointed out that the reduction of V5+ to V3+ usually occurs directly through V4+ (V2O4). Nevertheless, the reduction of vanadium can also proceed by multiple stages, and other intermediate species, such
as V6O13 (V4.3+) and V6O13 (V3 7+), can also appear [8].
A figure 3A illustrates the temperature-programmed reduction profiles of fresh VMgOx samples. Two of the catalysts (C and I), which were reduced at lower temperatures, stabilized their activity in the process after first 30 min (fig. 1). The other two samples (S and W) were less reducible, as the maximum of hydrogen consumption in TPR-H2 appeared at higher temperatures. Probably for this reason, these catalysts allowed to obtain the highest carbon dioxide conversions after adequately longer time. After the temperature-programmed reduction analysis, the catalysts contained mostly V3+ cations, what gave an average oxidation number of vanadium (n'av) close to 3 (table ).
Additional temperature-programmed reduction tests were performed on the catalysts after RWGS reaction (fig. 3B). These tests proved that vanadium cations at the oxidation number below 4+ were reduced at lower temperatures in comparison to
Fig. 2. Changes of CO2 conversion for the reverse water gas shift reaction over VMgOx catalysts as a function of the reaction time
V5+ and V4+ [8, 9]. This can mean, that during the re-
t4+
verse water gas shift reaction, vanadium at high oxidation state (i.e., that present in fresh VMgOx catalysts) is reduced first to Vn+, where n < 4+. Then, the vanadium species at lower oxidation state can work stable in the redox cycle:
Vn+Ox + H2 Vm+Ox_i + H20 Vm+0(x.i) + C02 Vn+Ox + CO where 3+ < m < n < 4+
Previously it was found that VMgOx catalysts were active in the process of isobutane dehydrogena-tion in CO2 atmosphere [5]. Nevertheless, those results did not allow to specify, which of the dehydro-genation mechanisms (i.e., one or two-step route) was more likely. The performed tests indicate that VMgOx catalysts show relatively high activity in RWGS reaction. This in turn indicates that the isobutane dehy-drogenation in the presence of carbon dioxide proceeds mostly via the two-step pathway (eq. 1 and 3), although the one-step pathway (eq. 2) cannot be completely excluded. Similar conclusions and reaction mechanisms were proposed for the process of catalytic dehydrogenation of both propane and ethyl-benzene in the carbon dioxide atmosphere [1].
CONCLUSIONS
VMgOx catalysts are active in the reverse water gas shift reaction. The highest conversion of carbon dioxide observed during the catalytic tests is close to the value calculated from thermodynamic analysis.
Catalytic tests have proven that vanadium at its higher oxidation state (close to 5+) is less active in RWGS process than the reduced vanadium species are. The initial conversion of carbon dioxide is relatively low, however it increases to a constant value with the reaction progress.
The high activity of VMgOx catalysts in RWGS reaction should improve their performance in the dehydrogenation of hydrocarbons in carbon dioxide atmosphere. However, from our previous investigations it is known [5, 10, 11], that a catalyst performance in the dehydrogenation process is also related to many other factors, such as: phase composition of the catalyst, its specific surface area and acid-base properties of the catalysts.
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Fig. 3. Graphic illustration of TPR-H2 analysis of VMgOx catalysts: A) fresh, B) used in the reverse water gas shift reaction
