(112g) Tpr Analysis of Fe/Zsm-5 for the Reaction of Syngas to Gasoline

Introduction

Zeolites loaded with iron have been used as catalysts for the reaction of synthesis gas, syngas, to gasoline.[1] The metal promotes the conversion of carbon monoxide, CO, into low molecular weight olefins and the zeolite promotes the oligomerization of these olefins into longer carbon chains and aromatics consisting of C₅-C₁₀ which is part of the blend called gasoline. Temperature-Programmed reduction (TPR) is a useful tool to characterize the reducible metal ion(s) in these catalysts and such a characterization can be related to the catalytically active sites which are believed to be the lower oxidations states of iron.  The reduction of the iron ions by H₂ in a TPR device is attended by the production of gaseous water according to the follow stoichiometry for the case of Fe₂O₃ reducing to Fe in three steps.

3Fe₂O₃ + H₂à 2Fe₃O₄ + H₂O

2Fe₃O₄ + 2H₂à 6FeO + 2H₂O

6FeO + 3H₂à 6Fe + 3H₂O

Thus, one may interrogate for the reduction of Fe ions by analyzing for the production of water in the effluent stream using a quadrapole mass spectrometer.  In addition, one may also examine the consumption of H₂ in the same stream also using the mass spectrometer.  For the present case, we wish to characterize a Fe catalyst supported on H⁺ZSM-5 zeolite using TPR to identify the possible Fe oxide phases that may be present in the sample.  This characterization will be used to interpret data for the syngas to gasoline reaction so as to identify the active phases for the production of gasoline from syngas.

Experimental

Three samples of pure iron oxides (Fe₂O₃, Fe₃O₄, and FeO) were obtained as standard reference materials from Aldrich and were used without further purification.  One other reference material, FeAl₂O₄, was prepared by heating an equimolar mixture of FeO and Al₂O₃ in air to 800^oC overnight.  The samples were introduced into the TPR device and purged with hydrogen at room temperature before starting the TPR run.  This reduction protocol was to program the temperature at a constant rate of 10ºC/min from room temperature to  800ºC by passing hydrogen over the samples. The Fe/H⁺ZSM-5 samples were prepared by aqueous phase impregnation of the Fe(III) nitrate salt into the proton form of the ZSM-5 having a silica/alumina ratio of 23/1 as determined by elemental analysis.  The nominal iron loading of this sample was 10 wt%.

Results

           Figure 1-a

             Figure 1-b

                Figure 1-c

            Figure 1-d

             Figure 2

         Figure 3

Fe oxide standards.  The pure iron oxide standards showed reductions occurring from 300ºC to 700ºC. Consider the TPR spectrum of Fe₂O₃ (Figure 1-a) which shows two water production peaks at 300^o and 550^oC, whereas, the Fe₃O₄ (magnetite) standard, Figure 1-b, showed TPR reduction peaks at 323^o, 500^o, and 550ºC.  The FeO sample, Figure 1-c, showed TPR peaks at 500^o and 660^oC.  The FeAl₂O₄ sample, Figure 1-d, showed two higher temperature peaks at 580 and 670^oC.  These iron oxide standards will be used to interpret the zeolite supported Fe oxide catalyst used for syngas to gasoline.  Two of these samples will be tested:  1) a fresh catalyst before it is used, and 2) a used catalyst that has been heated in air to regenerate the oxide(s).

Fe/H⁺ZSM-5 samples.  TPR studies were performed on fresh and used Fe supported ZSM-5 catalysts (Figure 2). The fresh catalyst showed peaks at 200^o, 350 ^o, 500^o, 580 ^o and 660^oC; whereas, the used and regenerated catalyst showed TPR peaks at 180^o, 375 ^o, 530^o and 650^oC.  The used catalyst showed peaks at 530 ^o and 650^oC which were of different intensities from the fresh sample, suggesting that the heating and regeneration produced an Fe-environment(s) that was more difficult to reduce.  Upon comparison these TPR spectra with the reference iron oxides, we conclude that very little Fe₂O₃ is present in the Fe/H+ZSM-5 catalysts but significant amounts of Fe₃O₄, FeO, and FeAl₂O₄ may be present in the supported Fe catalyst.  Moreover, the higher temperature peak (650^oC) which grew upon regeneration of the used catalyst is similar to the peak which appears in the FeAl₂O₄ standard.  The formation of this aluminate is possible as a result of the reaction of extra-framework alumina which is present in the zeolite sample.  For example, this zeolite was examined by Dufreche to show that the framework silica/alumina ratio was 35/1[2] which means that roughly 1/3 of the alumina present in the sample is outside of the framework and is available for reaction with the iron oxide to form the iron aluminate (FeAl₂O₄).   We prepared an additional sample by combining a physical mixture of Fe₂O₃ and H⁺ZSM-5 in the proportions that would be present in the catalyst sample prepared by incipient wetness as a means to understand better the lower temperature peak at 180-200^oC which did not appear in any of the iron oxide reference standards. The TPR spectra of these samples (Figure 3) shows the appearance of the lower temperature peak which suggests that water in the zeolite is probably responsible for this peak.

Discussion and Conclusions

            Temperature programmed reduction of zeolite-supported, iron oxide catalysts showed the existence of several phases of iron oxide which might be attributed to Fe₃O₄, FeO, and FeAl₂O₄.  Of these three phases, only magnetite, Fe₃O₄, is reducible at reaction conditions which are 350^oC.  We conjecture that this phase is responsible for the reaction of CO and H₂ to form olefins.  Additional studies will be required to confirm these conclusions.  We suggest that powder, X-ray diffraction studies be completed on these standards and unknowns to determine the crystalline phase(s) present in the supported Fe catalyst.  In addition, one might perform X-ray photon electron spectroscopy on the same samples to determine the surface oxidation state(s).  Finally, the particle sizes of the Fe particles should be determined by transmission electron microscopy.

Acknowledgements

Partial support for this work is acknowledged from the Earnest W. Deavenport, Jr. Endowed chair funds.  The bulk of this material is based upon work performed through the Sustainable Energy Research Center at Mississippi State University and is supported by the Department of Energy under Award Number DE-FG3606GO86025.

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[2] DuFreche, S. A., Ph. D. thesis,
Mississippi State University, 2007.