Mn-promoted Sulfated Zirconia Research Articles

In situ diffuse-reflectance infrared spectroscopic investigation of promoted sulfated zirconia catalysts during n-butane isomerization

Sulfated zirconia, unpromoted or promoted with either 2 wt% iron or manganese, was investigated by in situ diffuse reflectance IR spectroscopy during activation, n-butane isomerization (323–378 K, 1–5 kPa n-butane), and regeneration. During activation at 773 K in N 2 or O 2, more than 95% of the adsorbed water was removed, and comparison of the resulting pattern of sulfur–oxygen vibrations with DFT calculations (A. Hofmann and J. Sauer, J. Phys. Chem. B 108 (2004) 14652) suggests S 2 O 7 2 − as a major surface species. All catalysts underwent an induction period before maximum rates were measured, which at 1 kPa n-butane were 350 and 180 μmol g −1 h −1 for the Fe- or Mn-promoted catalyst (323 K) and 5 μmol g −1 h −1 for sulfated zirconia (358 K). During isomerization several bands increased in the range of 1700–1600 cm −1; the growth of a band at 1600 cm −1 was restricted to the period of increasing catalytic activity. The rate of isomerization was linearly correlated with the total area increase of the bands in this region, indicating that the absorbing species are either active surface intermediates or side products (possibly water) formed in an amount proportional to the active surface species. The rate per such species was much higher in the presence of Mn; that is, promoters facilitate reaction steps after the initial alkane activation. The promoted materials deactivated within hours under the applied conditions, but complete recovery of the spectral and catalytic properties of Mn-promoted sulfated zirconia was possible through regeneration in O 2 at 773 K. Attempts to regenerate in N 2 produced unsaturated surface species absorbing at 1532 and 1465 cm −1.

In situ XANES study of Mn in promoted sulfated zirconia catalysts

Mn-promoted sulfated zirconia catalysts (2 wt% Mn) were investigated in situ, during the catalyst activation, isomerization of n-butane, and subsequent re-activation, using X-ray absorption spectroscopy of the Mn K-edge. The average valence of Mn in the catalysts, as determined from the edge position, was found to change from either 2.65 or 2.77 in the calcined samples to about 2.5 during activation in He (703 K for 30 min). During the isomerization of n-butane (1% in He, 80 ml min-1, 0.5 g catalyst at 333 K), the average Mn valence did not change further. When the catalyst was activated in 50% O2 the average valence only decreased from about 2.78 to 2.72. In this case, the average valence during the isomerization reaction decreased at a nearly constant rate both during the induction of activity and deactivation of the catalyst. The data do not support a stoichiometric redox reaction involving the promoter as initiator of the isomerization. However, a higher Mn valence after activation was indicative of a higher maximum conversion. It is concluded that the promoter cations function through modification of the structure of the zirconia.

Mechanical stress induced activity and phase composition changes in sulfated zirconia catalysts

Sulfated zirconia and Mn-promoted sulfated zirconia (0.5 and 2.0 wt% Mn) catalysts were subjected to mechanical stress. Pressing (10 min 540 MPa), milling (10 min vibrating mill), and grinding (manually, 10 min agate mortar) effected a partial phase transformation from the tetragonal to the monoclinic phase of ZrO 2. The mechanical stress also reduced the n-butane isomerization rate (1 kPa n-butane, 323–378 K, atmospheric pressure) to 30% and less of that measured for untreated catalyst. Standard sample preparation techniques for analytical methods thus alter the structural and catalytic properties of sulfated zirconia. Attempts to correlate data from different methods are futile unless the integrity of the sulfated zirconia is ensured.

Sulfated zirconia catalysts promoted with Fe and Mn: Mn effect in the Fe dispersion

n-Butane conversion was studied over Fe- and Mn-promoted sulfated zirconia by pulse reaction over the temperature range of 423–503 K to explore the role of Mn in the catalytic activity of Fe-promoted sulfated zirconia. On a sample containing 2 wt.% Fe, the n-butane isomerization activity was found to decrease with decreasing Fe/Mn molar ratio beyond 0.65 wt.% Mn. Concomitantly, XPS showed that the surface Fe/Zr intensity ratio increases with the Mn content until it reaches a maximum at 0.65 wt.% Mn, and then decreases. This behavior reflects a higher Fe dispersion at relatively low Mn content. Thus, low concentrations of Mn disperse the iron on the surface. Temperature programmed reduction data indicate that more sulfate species are accessible to reduction when metal promoters are present over sulfated zirconia. However, increasing Mn amount decreases the fraction of sulfate being reducible.

Influence of hydrogen on n -butane isomerization over sulfated zirconia catalysts

This work investigates the influence of hydrogen on the catalytic activity of Fe- and Mn-promoted sulfated zirconia catalysts. It was found that the effect of hydrogen on the activity of Fe-promoted SZ in n-butane isomerization significantly depended on the Fe content of the catalyst. It was also discovered that the negative effect of hydrogen is more significant at lower temperatures. The reason for the decreased activity in hydrogen is thought to be due to the interaction of hydrogen with reaction intermediates.

Probing the Low Temperature Initiation Sites in Fe-, Mn-Promoted Sulfated Zirconia via CO and H2 Adsorption

Exposure of freshly activated Fe-, Mn-promoted sulfated zirconia (SFMZ) to carbon monoxide at temperatures up to 50°C induced permanent loss of activity, while the addition of CO after the beginning of the reaction had a reversible effect, regardless of whether the butane flow has been interrupted or not. Similar experiments using dissociated hydrogen instead of CO led to irreversible poisoning in all cases. These finding were interpreted based on (i) the occurrence of initiation sites that are consumed stoichiometrically and very rapidly upon exposure to butane, (ii) such initiation sites which are also consumed by CO or dissociated hydrogen, (iii) CO which competes effectively for adsorption sites without affecting accumulated reaction intermediates, and (iv) such intermediates that are removed in the presence of dissociated hydrogen.

XAFS study of Fe- and Mn-promoted sulfated zirconia.

The local structure around Fe and Mn in Fe. Mn-promoted sulfated zirconia was investigated by means of in-situ XAFS. The Mn atoms are found to be present as MnSO 4 on the surface. The first post-edge peak of the Mn K-edge XANES spectrum was sharpened in a working state, and restored by evacuation. Fe K-edge XANES spectra showed that Fe is in a trivalent state and located at a center of a distorted octahedron. Fe atoms were suggested to be interstitially located into ZrO 2 to form a solid solution. An)' procedure did not change the Fe K-edge EXAFS and the shape and edgepositions of the XANES spectra. The reaction gases are in contact with Mn atoms, but not with the Mn atoms of the catalyst.

N-Hexane Conversion Catalyzed by Sulfated Zirconia and by Iron- and Manganese-Promoted Sulfated Zirconia: Catalytic Activities and Reaction Network

The conversion of n-hexane catalyzed by unpromoted sulfated zirconia and Fe- and Mn-promoted sulfated zirconia was investigated with a flow reactor at atmospheric pressure, temperatures of 0−25 °C, and an n-hexane partial pressure of 0.001 atm. The n-hexane conversion initially increased as a function of time on stream, followed by a rapid decline. The major primary reaction was isomerization, giving monobranched isomers (predominantly 2-methylpentane). The secondary isomerization reactions gave dibranched products (predominantly 2,3-dimethylbutane). Disproportionation reactions were apparently secondary, giving isobutane, isopentane, heptanes, and octanes from the isomerization products. Small amounts of cracking products (methane, ethane, and propane) were also observed at the shortest times on stream, before the catalyst deactivated significantly. On the basis of the different induction times observed for the various reaction products, it is inferred that the disproportionation is bimolecular and the isomerization (predominantly) monomolecular.

N-pentane isomerization and disproportionation catalyzed by promoted and unpromoted sulfated zirconia

Isomerization and disproportionation of n-pentane were catalyzed by sulfated zirconia, Fe- and Mn-promoted sulfated zirconia, and Pt-, Fe-, and Mn-promoted sulfated zirconia in a flow reactor at temperatures of −25 to 50°C and n-pentane partial pressures of 0.005–0.01 atm. Incorporation of the Fe and Mn promoters increased the activity of the sulfated zirconia by two orders of magnitude at 50°C; addition of Pt to the latter catalyst increased the activity only slightly. The primary reactions, disproportionation (to give butanes and hexanes) and isomerization, occurred in parallel; secondary disproportionation reactions gave heptanes, propane, butanes, and pentanes. The data are consistent with acid-base catalysis and carbenium ion intermediates, and the isomerization is inferred to proceed both by unimolecular and bimolecular mechanisms. H2 in the feed stream and Pt in the catalyst both led to reductions in the rate of catalyst deactivation.

The effect of pretreatment procedures on the activities of Fe- and Mn-promoted sulfated zirconia catalysts

The effect of activation and calcination conditions on the activities of Fe- and Mn-promoted sulfated zirconia (MFSZ) catalysts for the isomerization of n-butane at low temperatures (≤90°C) was investigated. Activation in air resulted in a higher activity of MFSZ than did activation in helium; this is in contrast to the situation for unpromoted sulfated zirconia (SZ) catalysts where the activation atmosphere did not affect the activity. The maximum activity of MFSZ was found to occur when the activation temperature was 450°C, while for unpromoted SZ, the optimum activation temperature was 250°C. Comparison of the n-butane conversions at these two temperatures showed that MFSZ was only about 1.6 times as active as SZ. This activity difference is much less than that reported before in the literature where comparison was made at the same activation temperature. This activity difference is attributed to the different degree of synergism between redox and acid sites on the two catalysts.

Induction of activity and deactivation of Fe, Mn-promoted sulfated zirconia catalysts

The isomerization of n-butane on sulfated zirconia catalysts promoted by the addition of Fe and Mn ions has been studied. The evolution of the catalytic activity of these materials as a function of time on stream exhibits a typical shape containing an induction period during which the activity increases. The induction period is followed by a rapid deactivation and then a slower deactivation. The shape of the conversion—time curves contains important information about the way these catalyst operate. Slight changes in the preparation and pretreatment of the catalysts result in changes in the overall activity and shape of the conversion—time curves. The results of this study suggest that the induction period is due to the formation and accumulation of reaction intermediates on the surface. These intermediates participate in the reaction as part of an inter-molecular mechanism. The n-butane molar fraction, the presence of olefins or hydrogen in the feed, and the reaction temperature strongly affect the induction period. A simple mathematical model that successfully describes the behavior of the conversion—time curves has been developed. This model is consistent with the existence of two types of sites with different reactivity and stability. One type is responsible for most of the activity observed during the first few minutes on stream, but it is rapidly deactivated. This type of sites is easily deactivated by simple exposure to hydrogen, so it is consistent with an oxidized species recently proposed. The second type of sites is more resistant to deactivation.

On the atomic structure of [formula omitted] promoted sulfated zirconia

The aim of the present work is to shed some light on the surface atomic structure of an Fe Mn-promoted sulfated zirconia catalyst using high-resolution transmission electron microscopy (HRTEM) imaging. The presence of Fe and Mn species, which were found to be dispersed at a molecular level, were apparent on the {{001}} and {{110}} surfaces of zirconia particles. A combination of the present results with additional detailed studies using other physicochemical characterization techniques is needed to elucidate the atomic configuration of the active sites responsible for the enhanced catalytic activity of the Fe Mn-promoted sulfated zirconia catalyst.

Pentane isomerization and disproportionation catalyzed by sulfated zirconia promoted with iron and manganese

Fe- and Mn-promoted sulfated zirconia was used to catalyze the conversion ofn-pentane in a flow reactor at temperatures in the range of −25 to 40‡C andn-pentane partial pressures in the range of 0.005 to 0.01 atm. The rates of reaction increased with time on stream during an induction period and then decreased rapidly. The predominant reaction at −25‡C and short times on stream was isomerization to give isopentane; no dibranched products were observed. The selectivity for isomerization decreased and that for disproportionation increased with increasing temperature, with disproportionation becoming predominant at 40‡C; the principal product was then isobutane. The product distribution data are consistent with acid-base catalysis and carbocation intermediates. However, there appears to be more to the reaction mechanism than acid-base chemistry, and the roles of the iron and manganese promoters are not yet explained.

Cracking of n-Butane Catalyzed by Iron- and Manganese-Promoted Sulfated Zirconia

Fe- and Mn-promoted sulfated zirconia was used to catalyze the conversion of n-butane at atmospheric pressure and n-butane partial pressures in the range of 0.0025-0.01 atm. At temperatures <225°C, the significant reactions were isomerization and disproportionation; in the range of 225-300°C, these reactions were accompanied by cracking, and at temperatures >350°C, cracking and isomerization occurred. Catalyst deactivation, resulting at least in part from coke formation, was rapid. The primary cracking products were methane, ethane, ethylene, and propylene. The observation of these products along with an ethane/ethylene molar ratio of nearly 1 at 450°C is consistent with cracking occurring, at least in part, by the Haag-Dessau mechanism, whereby the strongly acidic catalyst protonates n-butane to give carbonium ions. The rate of methane formation from n-butane cracking catalyzed by Fe- and Mn-promoted sulfated zirconia at 450°C was about 3 × 10−8 mol/(g of catalyst · s); for comparison, the rate of cracking of n-butane catalyzed by HZSM-5 under these conditions was estimated to be 4 × 10−9) mol/(g of catalyst · s) [as determined by extrapolation of the data of H. Krannila, W. O. Haag, and B. C. Gates (J. Catal. 135, 115, 1992)]. This comparison suggests that the catalytic activity of the promoted sulfated zirconia at 450°C is about the same as that of the zeolite, although its activity for n-butane isomerization and disproportionation at temperatures <100°C is orders of magnitude greater than those of zeolites. Thus the indication of superacidity of the promoted sulfated zirconia does not extend to high temperatures. The results raise questions about the nature of the presumed superacidity: perhaps the low-temperature reactions may involve catalyst functions other than the acidic function responsible for high-temperature cracking reactions or perhaps superacidic sites may be very rapidly poisoned at cracking temperatures.

Structure of Fe, Mn-promoted sulfated zirconia catalyst by X-ray and IR absorption spectroscopies

The rate of butane isomerization at low temperature is more than two orders of magnitude greater over sulfated zirconia with iron and manganese than over unpromoted sulfated zirconia. Therefore, the bulk and surface structures of the unpromoted and promoted catalysts have been investigated using X-ray absorption spectroscopy at the Zr and Fe K-edges and FTIR spectroscopy of adsorbed pyridine and CO. The Zr—O and Zr—Zr interatomic distances derived from EXAFS indicated that both zirconia catalysts were exclusively tetragonal regardless of composition. In addition, EXAFS revealed the Fe ions did not substitute in the tetragonal structure, but were likely to be present in small clusters or rafts supported on zirconia. The IR spectra, associated with surface sulfate groups, adsorbed pyridine and adsorbed CO, were minimally affected by the promoters indicating that Fe and Mn do not alter the surface properties of the oxides in an obvious manner. Thus, the dramatic effect of promoters on the reaction of butane is likely to be due to extremely low active site densities on the catalysts or the need for two surface sites in the reaction mechanism or both.