Manganese-promoted Sulfated Zirconia Research Articles

Redox Properties of Manganese-Containing Zirconia Solid Solution Catalysts Analyzed by In Situ UV–Vis Spectroscopy and Crystal Field Theory

The optical absorption spectra of manganese-promoted sulfated zirconia, a highly active alkane isomerization catalyst, were found to be characterized by oxygen-to-manganese charge-transfer transitions at 300-320 nm and d-d transitions of manganese ions at 580 and 680 nm. The latter were attributed to Mn(4+) and Mn(3+) ions, which are known to be incorporated in the zirconia lattice. The oxygen surroundings of these ions were modeled assuming a substitutional solid solution. The crystal field splittings, vibronic coupling constants, and oscillator strengths of the manganese ions were calculated on the basis of a cluster model that considers the manganese center as a complex with the adjacent ions of the lattice as ligands. The ratio of Mn(3+) to Mn(4+) ions was determined using the spectra and the model, and the relative concentrations of Mn(2+), Mn(3+), and Mn(4+) ions were determined with the help of the average valence known from X-ray absorption data in the literature. The redox behavior of manganese-promoted sulfated zirconia in oxidizing and inert atmosphere was elucidated at temperatures ranging from 323 to 773 K.

H-D exchange between CD4 and solid acids: AlCl3/sulfonic acid resin, promoted and unpromoted sulfated zirconia, and zeolite HZSM-5

The H–D exchange reaction between CD4 and each of a family of solid acids (the zeolite HZSM-5, sulfated zirconia, iron- and manganese-promoted sulfated zirconia, and AlCl3/sulfonic acid resin) was investigated with a batch recirculation flow reactor; the data determine initial rates of the exchange reaction giving CD3H at temperatures ranging from 440 K for AlCl3/sulfonic acid resin to 688 and 703 K for the zeolite and promoted sulfated zirconia, respectively. Extrapolated results show that the reaction is three orders of magnitude faster with the AlCl3/sulfonated resin (an analogue of the very strongly acidic combination of AlCl3 and H2SO4) than with HZSM-5 or promoted sulfated zirconia and two orders of magnitude faster with the latter than with sulfated zirconia.

N-Pentane Isomerization Catalyzed by Fe- and Mn-Containing Tungstated Zirconia Characterized by Raman Spectroscopy

Although Fe and Mn promote sulfated zirconia for n-pentane isomerization, they do not promote tungstated zirconia. Raman spectra demonstrate the formation of manganese tungstate (and possibly iron tungstate), which likely hinder(s) the formation of highly dispersed oxidic promoters such as those in iron- and manganese-promoted sulfated zirconia.

Structural Analysis of Iron and Manganese Species in Iron- and Manganese-Promoted Sulfated Zirconia

The structural analyses of Fe, Mn−SO42-/ZrO2 (FMSZ), Fe−SO42-/ZrO2 (FSZ), Mn−SO42-/ZrO2 (MSZ), and Fe−Mn/ZrO2 (FMZ) were carried out by means of XAFS, XRD, and Raman spectroscopy. Local structure around Fe and Mn of each catalyst, which were at a working state for n-butane isomerization and in vacuo, were studied by in situ XAFS. The valence of the Fe atom of all the catalysts was invariantly trivalent. Coordination environment around Fe of all the catalysts was quite similar to each other. Fe atoms are present inside the bulk phase of ZrO2, the system of which forms interstitial-type solid solution and located at the center of distorted oxygen octahedra. Reaction gas does not directly make contact with Fe atoms because the introduction of reaction gas did not cause any change of coordination environment and the valence. The Mn species on FMSZ and MSZ were present as MnSO4, and their coordination environment was reversibly affected by exposure to n-butane. Without sulfate ions, Mn species are present on t...

Spectroscopic and Microscopic Characterization of Iron- and/or Manganese-Promoted Sulfated Zirconia

The structures of iron-promoted sulfated zirconia (FSZ), manganese-promoted sulfated zirconia (MSZ) and iron- and manganese-promoted sulfated zirconia (FMSZ) were investigated by ultra-violet-visible (UV-vis) diffuse reflectance, ESR, laser Raman, and X-ray photoelectron (XP) spectroscopies and by transmission electron microscopy with energy dispersive X-ray microanalysis. The bulk structure of tetragonal zirconia and the surface sulfate structure of SZ remained virtually unchanged when the promoters were added to SZ, as shown by laser Raman spectroscopy. The UV-vis spectra show that iron was present in aggregated structures and not as atomically isolated species. The Raman spectra identify these dispersed structures as Fe2O3, and the electron micrographs confirm the presence of particulate structures on the surfaces of FSZ and FMSZ, but not SZ and MSZ. The XP spectra provide confirming evidence of Fe3+, consistent with the presence of Fe2O3, and the ESR spectra show the presence of Fe3+ions in a broad distribution of coordination environments. The evidence of particulate Fe2O3is in agreement with the results of J. E. Tabora and R. J. Davis (J. Chem. Soc. Faraday Trans.91, 1825 (1995)), whose Fe-edge extended X-ray absorption fine structure spectra showed the lack of Fe–Zr and Fe–Fe contributions, consistent with particulate iron oxide clusters and inconsistent with iron in the bulk zirconia. Manganese in MSZ and FMSZ is identified by the ESR spectra as Mn2+, with the resolved hyperfine structure indicating that part of the Mn2+ions were highly dispersed in still unknown locations in FMSZ. The results are not sufficient to establish the nature of the promoter effects or to resolve the nature of the interactions between Mn and Fe indicated by catalyst performance data (F. C. Langeet al., Catal. Lett.41, 95 (1996)), showing that FMSZ is much more active than either FSZ or MSZ for isomerization ofn-butane.

Isobutane alkylation over solid acid catalysts under supercritical conditions

Solid catalyzed isobutane alkylation has been investigated for decades, but it has not yet been applied in any commercial uses because of the rapid deactivation of the catalyst. Here, the alkylation reaction has been studied under supercritical conditions using metal-promoted and unpromoted sulfated zirconia as catalysts. The catalytic activity at the supercritical condition of 5.0 MPa, 423K was significantly higher than at lower reaction pressure conditions and the deactivation rate was clearly reduced, independent on the catalyst. Iron- and manganese-promoted sulfated zirconia (SFMZ) showed higher activities under all conditions than unpromoted sulfated zirconia (SZ).

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.

Sulfated Zirconia and Iron- and Manganese-Promoted Sulfated Zirconia: Do They Protonate Alkanes?

Because of their high activities for alkane conversion, sulfated zirconia and iron- and manganese-promoted sulfated zirconia have been the objects of much recent attention as a possible next generation of solid acid catalysts for alkane conversion. These catalysts have been suggested to be superacidic on the basis of measurements with adsorbed Hammett indicator bases, but published data determined with other adsorbed bases indicate only moderately strong acid sites. The indicator methods are limited by the opaqueness of the materials and by the inability of the methods to probe a possible set of minority sites that might be responsible for the reactivity and catalytic activity for alkane conversions. Another approach to the challenge of estimating the acid strengths of the reactive and catalytic sites is to investigate the reactivities and catalytic activities of the materials for reactions which, for initiation, require donation of protons from a solid acid to a very weakly basic reactant such as an alkane. Such a test reaction is the acid-catalyzed dehydrogenation of alkanes proceeding by the Haag–Dessau mechanism (Olah type chemistry). This review includes a summary of results for conversion of ethane, propane, and n-butane that are consistent with the postulate that iron- and manganese-promoted sulfated zirconia and sulfated zirconia are capable of protonating light alkanes to give carbonium-ion transition states at temperatures as low as 200°C. The data support the postulate that these proton-donation reactions are important at low alkane conversions and in initiating alkane conversions, although conventional carbenium ion reactions predominate at high conversions.

Protolytic cracking of low-molecular-weight alkanes in the presence of iron- and manganese-promoted sulfated zirconia: evidence of a compensation effect

Low conversions of propane, n-butane, and 2,2-dimethylpropane were measured with each reactant in the presence of iron- and manganese-promoted sulfated zirconia in a once-through plug-flow reactor at 101 kPa, 473–723 K, and reactant partial pressures of 25, 250, and 1000 Pa. Rates of the reactions (overall conversion of propane, of n-butane, and of 2,2-dimethylpropane; formation of methane from each of the three reactants; formation of ethene from propane and from n-butane; and formation of ethane from n-butane) were determined by extrapolating the declining conversions to zero time on stream. The apparent activation energies and apparent pre-exponential factors characterizing these reactions, which are inferred to proceed via carbonium-ion transition states, fall near a straight line on a compensation effect plot. The plot is suggested to represent the family of reactions proceeding via protolytic cracking and dehydrogenation and carbonium-ion transition states.

Activation of Ethane in the Presence of Solid Acids: Sulfated Zirconia, Iron- and Manganese-Promoted Sulfated Zirconia, and Zeolites

Ethane was activated in the presence of solid acids [sulfated zirconia (SZ), iron- and manganese-promoted sulfated zirconia (FMSZ), HZSM-5, and USY zeolite] at 1 atm, 200–450°C, and ethane partial pressures in the range 0.01–0.2 atm. The data were measured with a flow reactor at low conversions (<0.005) such that reaction of ethane took place in the near absence of alkenes. Catalysis was demonstrated for ethane conversion in the presence of FMSZ at 450°C and 0.2 atm ethane partial pressure, but the reactions were not shown to be catalytic for the other solid acids and other conditions. FMSZ was active for converting ethane into methane, ethene, and butane at an ethane partial pressure of 0.2 atm and at temperatures of 200–300°C; the other solid acids had no detectable activities under these conditions. At higher temperatures, each of the solid acids was active for conversion of ethane into ethene; butane and methane were also formed in the presence of FMSZ, HZSM-5, and USY zeolite, whereas methane was the only other hydrocarbon observed in the presence of SZ. The initial (5 min on stream) selectivities to ethene at approximately 0.1% conversion, ethane partial pressure of 0.2 atm, and 450°C were approximately 98, 94, 97, and 99%, for SZ, FMSZ, HZSM-5, and USY zeolite, respectively. Under the same reaction conditions, the initial rates of ethane conversion were 0.15×10−8, 3.5×10−8, 3.9×10−8, and 0.56×10−8mol/(s·g) for SZ, FMSZ, HZSM-5, and USY zeolite, respectively. The reactivities are consistent with chemistry analogous to that occurring in superacidic solutions and with the suggestion that FMSZ is a stronger acid than the others investigated here.

Hydrogen Effect onn-Butane Isomerization over Sulfated Zirconia-Based Catalysts

Iron- and manganese-promoted sulfated zirconia (SFMZ) has been tested as ann-butane isomerization catalyst in the temperature range of 35 to 180°C. The catalytic activity exhibits an induction period whose length is dependent on the reaction conditions. The presence of H2in the feed stream strongly suppressesn-butane conversion over unpromoted sulfated zirconia (SZ) and over Pt-containing SFMZ (PtSFMZ). However, hydrogen had no effect onn-butane isomerization over SFMZ. These findings were interpreted on the basis of a bimolecular mechanism where unsaturated intermediates (carbenium ions and/or butene) are formed during the break-in period. The role of promoters (Fe and Mn) is not only facilitating the formation of hydrogen-deficient intermediates and their accumulation on the catalyst surface, but also enhancing their stability. The negative effect of hydrogen over PtSFMZ is attributed to the occurrence of atomic hydrogen via the dissociative adsorption of H2on Pt.

Propane Conversion Catalyzed by Sulfated Zirconia, Iron- andManganese-Promoted Sulfated Zirconia, and USY Zeolite

Solid acid catalysts, namely sulfated zirconia, iron- andmanganese-promoted sulfated zirconia, and USY zeolite, weretested for conversion of propane at 1 atm, 200–450°C, andpropane partial pressures in the range 0.01–0.05 atm. Bothpromoted and unpromoted sulfated zirconia were found to beactive for conversion of propane into butanes, pentanes,methane, ethane, ethylene, and propylene in the temperaturerange 200–350°C, but catalyst deactivation was rapid. Atthe higher temperatures, only cracking and dehydrogenation productswere observed. In contrast to the zirconia-supported catalysts,USY zeolite was observed to convert propane (into propylene,methane, and ethylene) only at temperatures ≥400°C. The initial (5 min on stream) rates of propane conversion in the presence ofiron- and manganese-promoted sulfated zirconia, sulfatedzirconia, and USY zeolite at 450°C and 0.01 atm propanepartial pressure were 3.3×10−8, 0.3×10−8, and 0.03×10−8mol/(s·g), respectively. Theproduct distributions in the temperature range 200–450°Care those of acid–base catalysis, being similar to what has beenobserved in superacid solution chemistry at temperatures <0°C. Ifpropane conversion at 450°C can be considered as a probeof acid strength of the catalyst, then the activity comparisonsuggests that the promoted sulfated zirconia is a stronger acidthan sulfated zirconia, which is a stronger acid than USY zeolite.

Studies on the Catalytic Activity of Zirconia Promoted with Sulfate, Iron, and Manganese

The catalytic properties of iron- and manganese-promoted sulfated zirconia (SFMZ) for the isomerization ofn-butane to isobutane are investigated using various catalyst pretreatments and reaction conditions. Then-butane isomerization reactivity at 30°C is effected by calcination of the catalyst at 650°C in helium and vacuum treatment at room temperature indicating that superacidity is not likely to be responsible for activity. In addition, SFMZ samples exposed to dry air at over 450°C are more active than those calcined in helium at a reaction temperature of 30°C (n-butane conversions of 18.7% vs 0.4%) suggesting the presence of an active site involving a metal “oxy” species. The oxy species is capable of reacting CO to CO2at room temperature and is present at a number density of 10–15 μmol/g. At a reaction temperature of 100°C, SFMZ catalysts calcined in air then activated in helium show similar reactivities to those activated in air up to a preheating temperature of 450°C; above 450°C the metal oxy species is formed and provides additional activity (n-butane conversions of 37.1% in air vs 15.4% in He for calcinations at 650°C). The nature of the active sites on SFMZ are investigated using temperature-programmed desorption of substituted benzenes. The liberation of CO2and SO2in the benzene TPD profile of SFMZ is attributed to the oxidation of benzene at the redox-active metal sites, resulting in the subsequent decomposition of the reduced iron (II) sulfate. Data from the TPD studies do not suggest the presence of superacidity on SFMZ that could contribute to the low-temperaturen-butane isomerization activity. Instead, a bifunctional mechanism that involves a combination of a redox-active metal site and an acid site in close proximity is proposed.

Neopentane cracking catalyzed by iron- and manganese-promoted sulfated zirconia

Cracking of neopentane was catalyzed by a sulfated oxide of zirconium promoted with iron and manganese. Reaction at 300–450°C, atmospheric pressure, and neopentane partial pressures of 0.00025–0.005 bar gave methane as the principal product, along with C2 and C3 hydrocarbons, butenes, and coke. The order of reaction in neopentane was determined to be 1, consistent with a monomolecular reaction mechanism and with the formation of methane andt-butyl cations; the latter was presumably converted into several products, including only little isobutylene. At 450°C and a neopentane partial pressure of 0.005 bar, the rate of cracking at 5 min onstream was 5×10−8 mol/(g of catalyst s). Under the same conditions, the rates observed for unpromoted sulfated zirconia and USY zeolite were 3×10−8 and 6×10−9 mol/ (g of catalyst s), respectively. The observation that the promoted sulfated zirconia is not much more active than the other catalysts is contrasted to published results showing that the former catalyst is more than two orders of magnitude more active than the others forn-butane isomerization at temperatures <100°C. The results raise a question about whether the superacidity attributed to sulfated zirconia as a low-temperature butane isomerization catalyst pertains at the high temperatures of cracking.

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.

Low-Temperature Superacid Catalysis: Reactions of n-Butane Catalyzed by Iron- and Manganese-Promoted Sulfated Zirconia

Environmental concerns are leading to the replacement of aromatic hydrocarbons in gasoline with high-octane-number branched paraffins and oxygenated compounds such as methyl t-butyl ether, which is produced from methanol and isobutylene. The latter can be formed from n-butane by isomerization followed by dehydrogenation. To meet the need for improved catalysts for isomerization of n-butane and other paraffins, researchers identified solid acids that are noncorrosive and active at low temperatures. Sulfated zirconia catalyzes the isomerization of n-butane even at 25{degrees}C, and the addition of Fe and Mn promoters increases its activity by three orders of magnitude. Little is known about this new catalyst. Here the authors provide evidence of its performance for n-butane conversion, demonstrating that isomerization is accompanied by disproportionation and other, less well understood, acid-catalyzed reactions and undergoes rapid deactivation associated with deposition of carbonaceous material. 10 refs., 3 figs.

Propane conversion in the presence of iron- and manganese-promoted sulfated zirconia: evidence of Olah carbocation chemistry

Iron- and manganese-promoted sulfated zirconia was tested for conversion of propane at 1 bar, 200, 250, and 300°C, and propane partial pressures of 0.01, 0.025, and 0.05 bar. Propane was converted to pentanes, butanes, propene, ethane, ethene, and methane at 250°C, but the conversions were low, for example, being only a fraction of a percent at a space velocity of 9.1 × 10−7 mol/(g s) and a propane partial pressure of 0.05 bar. Carbonaceous deposits formed rapidly. At 250°C, the number of propane molecules converted to gas-phase products was only about 1 per sulfate group after 16 days of operation in a continuous flow reactor. The observation of butanes and pentanes as products is consistent with Olah superacid solution chemistry, whereby propane is first protonated by a very strong acid to form a carbonium ion. The carbonium ion then decomposes into methane and an ethyl cation, which undergoes secondary reactions with propane to form higher-molecular-weight alkanes.