Propane Aromatization over HZSM‐5 and Ga/HZSM‐5 Catalysts

The selective transformation of light alkanes to aromatics that are more valuable and versatile feedstocks for the chemical industry is one of the major challenges of catalytic chemistry. The complexity of the aromatization chemistry makes it difficult to unravel reaction mechanisms and, mechanistic information is largely developed from observed product distributions. This article reviews the current mechanistic understanding for the conversion of propane to aromatic compounds over HZSM‐5 and Ga/HZSM‐5 catalysts based on experimental as well as theoretical studies.

Following a general discussion of acidity and confinement effects in these systems, this review focuses on understanding specific reactions occurring on Brønsted acid sites in HZSM‐5. Mechanistic details available from Density Functional Theory (DFT) calculations, as well as kinetic modeling efforts for various complex hydrocarbon systems are critically reviewed. A detailed, tabulated review of the literature compares the catalytic performance of gallium modified ZSM‐5 catalysts and subsequently the promotional effect of gallium as an additive is critically discussed in terms of the nature of the active sites, as well as the new reaction pathways introduced by gallium addition.     

Cobalt‐catalyzed amination of 1,3‐cyclohexanediol and 2,4‐pentanediol in supercritical ammonia

The onestep procedure of amination of bifunctional secondary alcohols to diamines has been investigated in a continuous fixedbed reactor. Application of supercritical NH₃ as a solvent and reactant suppressed catalyst deactivation and improved selectivities to amino alcohol intermediates, whereas selectivities to diamines remained poor (8–10%). The main reason for the low diamine selectivity of 1,3dihydroxy compounds is water elimination leading to undesired monofunctional products via ,unsaturated alcohol, ketone or amine intermediates. This side reaction does not occur with 1,4dihydroxy compounds which afford high aminol and diamine selectivities under similar conditions. Amination of secondary diols with ammonia was found to be faster, but less selective than that of the corresponding primary 1,3propanediol.     

Stereoselective Formation of 1,4‐Anhydro‐2‐C‐carboxytetritols in the Degradation of 1,5‐Anhydroribitol and 1,5‐Anhydroxylitol with Oxygen in Aqueous Alkali

Abstract

In addition to other acid products, degradation of 1,5‐anhydroribitol (5) and 1,5‐anhydroxylitol (6) with oxygen in 1.25 M NaOH produced diastereomeric 1,4‐anhydro‐2‐C‐carboxy‐D‐erythritol (7) and 1,4‐anhydro‐2‐C‐carboxy‐D‐threitol (8) and their enantiomers as major products. However, the ratio of the diastereomers differed for the two reactants. Thus, their formation could not proceed solely by benzilic acid‐type rearrangements through α‐dicarbonyl intermediates as typically proposed for formation of alkyl C‐carboxyfuranosides from alkyl glycopyranosides in similar reactions. The α‐dicarbonyl species that can form from 5 and 6 are identical. Potential mechanisms to account for stereoselective formation of 7 and 8 are presented.     

Simultaneous dehydrogenation and isomerization of η‐butane to isobutene over ZSM‐22 and zinc‐modified ZSM‐5 zeolites

Direct formation of isobutene from η‐butane was investigated over a zinc‐impregnated potassium‐ion‐exchanged ZSM‐5 dehydrogenation catalyst and an acidic shape‐selective ZSM‐22 skeletal isomerization catalyst. The experiments were performed in a fixed‐bed microreactor system operating at near‐atmospheric pressure. High selectivity to η‐butene was obtained over the zinc‐impregnated potassium‐ion‐exchanged ZSM‐5 dehydrogenation catalysts. The yield of isobutene increased after adding the acidic ZSM‐22 skeletal isomerization catalyst, although the selectivity to butene isomers slightly decreased because the skeletal isomerization of η‐butene was competing with other acid‐catalyzed reactions such as cracking and aromatization. This revised version was published online in July 2006 with corrections to the Cover Date.     

Quasi‐Homogeneous and Pseudospin Modes of Zirconium‐Wire Combustion in Air

A novel experimental technique is proposed for examining the transition mechanism from quasihomogeneous to heterogeneous combustion — burning of a variablepitch spring. Depending on the pitch of aircombustible zirconium springs, two combustion modes are possible. Quasihomogeneous (layerbylayer) combustion is observed in the case of smallpitch springs; as the spring pitch increases, quasihomogeneous combustion transforms into heterogeneous (pseudospin) combustion. Conditions for the occurrence of various combustion modes, depending on the spring diameter and pitch, are studied.     

Synergistic Solvent Extraction and Separation of Lanthanide(III) Ions with 4‐Benzoyl‐3‐Phenyl‐5‐Isoxazolone and the Quaternary Ammonium Salt

The solvent extraction of the lanthanide(III) ions (without Pm) with a 4‐benzoyl‐3‐phenyl‐5‐isoxazolone(HPBI) alone and in the presence of the quaternary ammonium salt Aliquat 336 in perchlorate form (QClO₄) in C₆H₆ was investigated by the slope analysis method. The composition of the extracted species was determined as Ln(PBI)₃ and Q[Ln(PBI)₄] (Q⁺ is the quaternary ammonium salt cation). The values of the equilibrium constant were calculated. Synergistic effects were found for all lanthanide metals when they were extracted with a binary mixture of HPBI and QClO₄. The influence of the synergistic agent on the extraction process has been discussed. The parameters of the extraction process were determined. The separation factors between adjacent metals were evaluated.     

Variable‐temperature IR spectroscopic studies of CO adsorbed on Na‐ZSM‐5 and Na‐Y zeolites

CO interacts with extraframework alkali metal cations (M⁺=) of zeolites to form both M⁺CO and M⁺OC species. By using variabletemperature FTIR spectroscopy, these Cbonded and Obonded species were found to be in a temperaturedependent equilibrium. For the same cation, the difference in interaction energy depends upon the zeolite framework. Thus, for the equilibrium process ZNa⁺=CO ZNa⁺OC, where Z represents the zeolite framework, H ⁰ was found to take the values 3.8 and 2.4 kJ mol^– for CO/NaZSM5 and CO/NaY, respectively. The Cbonded species show always the highest cation–CO interaction energy.     

Hydrothermal deactivation of Ce‐ZSM‐5, Ce‐beta, Ce‐mordenite and Ce‐Y zeolite deNOx catalysts

The hydrothermal stability of Ce³⁺ zeolite catalysts used for selective catalytic reduction of NO _x was investigated. Aging of Ce‐ZSM‐5, Ce‐beta, Ce‐mordenite and Ce‐Y catalysts consisted of steaming in 10 or 12 vol% water at 600°C for 3–99 h. Ce‐ZSM‐5 (Si/Al ratios: Si/Al = 17.1, 22.6 and 146.6) and Ce‐mordenite (Si/Al = 6.4, IE = 77.2%) showed fast deactivation. Ce‐beta (Si/Al = 12, IE = 68.4%) and Ce‐Y (Si/Al = 2.8, IE = 122%) are significantly more stable zeolite catalysts, Ce‐beta being the most active of these two. Ce‐beta and Ce‐ZSM‐5 catalysts – both having high initial activities – were characterized with ²⁹Si‐NMR and ²⁷Al‐NMR. Especially Ce‐ZSM‐5 showed an increase of non‐framework Al, meaning that the zeolite suffered from dealumination. This revised version was published online in July 2006 with corrections to the Cover Date.     

Nonoxidative dehydrogenation and aromatization of methane over W/HZSM‐5‐based catalysts

Highly active and heat‐resisting W/HZSM‐5‐based catalysts for nonoxidative dehydro‐aromatization of methane (DHAM) have been developed and studied. It was found from the experiments that the W−H₂SO₄/HZSM−5 catalyst prepared from a H₂SO₄‐acidified solution of ammonium tungstate (with a pH value at 2–3) displayed rather high DHAM activity at 973–1023 K, whereas the W/HZSM‐5 catalyst prepared from an alkaline or neutral solution of (NH₄)₂WO₄ showed very little DHAM activity at the same temperatures. Laser Raman spectra provided evidence for existence of (WO₆)^n- groups constructing polytungstate ions in the acidified solution of ammonium tungstate. The H₂‐TPR results showed that the reduction of precursor of the 3% W–H₂SO₄/HZSM‐5 catalyst may occur at temperatures below 900 K, producing W species with mixed valence states, W⁵⁺ and W⁴⁺, whereas the reduction of the 3% W/HZSM‐5 occurred mainly at temperatures above 1023 K, producing only one type of dominant W species, W⁵⁺. The results seem to imply that the observed high DHAM activity on the W–H₂SO₄/HZSM‐5 catalyst was closely correlated with (WO₆)^n- groups with octahedral coordination as the precursor of catalytically active species. Incorporation of Zn (or La) into the W–H₂SO₄/HZSM‐5 catalyst has been found to pronouncedly improve the activity and stability of the catalyst for DHAM reaction. Over a 2.5% W–1.5% Zn–H₂SO₄/HZSM‐5 catalyst and under reaction conditions of 1123 K, 0.1 MPa, and GHSV=1500 ml/(h g−cat.), methane conversion (XCH₄) reached 23% with the selectivity to benzene at ∼96% and an amount of coke for 3 h of operation at 0.02% of the catalyst weight used. This revised version was published online in July 2006 with corrections to the Cover Date.     

Preparation and characterization of alumina‐supported metal‐carbonyl‐derived model catalytic systems in UHV conditions

Active catalysts for metathesis of alkenes, hydrodesulfurization, and hydrogenation can be prepared by exposing a high‐surface‐area alumina support to molybdenum hexacarbonyl at room temperature. This strategy is mimicked in ultrahigh vacuum by adsorbing molybdenum hexacarbonyl onto an ultrathin hydroxylated alumina film grown onto a Mo(100) substrate. In contrast to results found on high surface area, no Mo(CO)₆ is found to adsorb on alumina at 300 K, and significant molybdenum deposition is only found by heating the sample to above 670 K. Alternatively, molybdenum hexacarbonyl adsorbs on alumina when cooled to 80 K. In this case the majority of the carbonyl desorbs intact and temperature‐programmed desorption and X‐ray photoelectron spectroscopy indicate that ∼2% of a monolayer of the carbonyls undergoes decarbonylation. Auger and X‐ray photoelectron spectroscopy measurements reveal that molybdenum carbide (MoC) is deposited onto the alumina surface heated to 700 K forming a monolayer after an exposure of ∼50 L. This layer is reduced to the metal by heating to ∼1500 K by reaction with the alumina substrate to evolve CO and form metallic molybdenum. The carbide can be reformed by heating the metal‐covered alumina sample in ethylene at 900 K, and the carbide can once again be reduced to the metal by heating to 1500 K. This process can be repeated so that the carbide can be regrown by reaction with ethylene and reduced by annealing in vacuo to 1500 K. Subcarbonyl species are detected after adsorbing Mo(CO)₆ on hydroxylated alumina at 80 K as the sample is heated to ∼200 K. At higher temperatures, the molybdenum is oxidized to an approximately 4+ oxidation state and deposits primarily oxalate species on the surface. The adsorbed oxalates thermally decompose at ∼300 K to evolve CO to form surface formates. These are stable to ∼560 K and react to evolve CO at this temperature. It is also found that the extent of decarbonylation depends on the degree of alumina hydroxylation so that heating hydroxylated alumina to 900 K, which removes ∼50% of the surface hydroxyls, decreases the both CO desorption yield and the oxalate coverage by 50%. This revised version was published online in June 2006 with corrections to the Cover Date.     

Characterization and catalytic properties of Ti‐ZSM‐5 prepared by chemical vapor deposition

Ti‐ZSM‐5 is prepared via a chemical vapor deposition method by reacting HZSM‐5 with TiCl₄ at temperatures of 200–400°C. Ti‐ZSM‐5 is characterized by skeletal‐ and surface hydroxy‐FT‐IR, XPS, and XANES spectroscopy. It seems that Ti is incorporated in the zeolite surface with tetrahedral coordination. Contents of incorporated Ti atoms in Ti‐ZSM‐5 zeolite increase with increasing SiO₂/Al₂O₃ ratio and a CVD reaction temperature of 400°C is optimal. Cyclohexanone ammoximation was used as test reaction and Ti‐ZSM‐5 catalysts show similar catalytic activities compared to TS‐1. This revised version was published online in July 2006 with corrections to the Cover Date.     

On the reactivity of K2O‐, CaO‐ and P2O5‐doped nickel molybdate catalysts in a periodic‐flow reactor

The propane oxydehydrogenation with monolayer lattice oxygen of undoped and K₂O–, CaO– and P₂O₅–NiMoO₄ was investigated by using a periodic‐flow reactor (PFR). The influence of the nature and the extent of the promoter has been emphasized relative to the doped catalysts with respect to pure NiMoO₄ phases. It was observed that calcium and potassium promoters satisfactorily enhance propylene selectivity, and phosphorus promoter specifically increases the total activity while maintaining the propylene selectivity. Evidence found by thermogravimetric (TG) analyses (oxygen depletion rate) has shown a dependence on lattice oxygen mobility due to the presence of promoters. This dependence has been correlated to the propane conversion while the propylene selectivity was attributed to the acid–base properties. This revised version was published online in July 2006 with corrections to the Cover Date.     

Comparison of the catalytic properties of Al‐ZSM‐22 and Fe‐ZSM‐22 in the skeletal isomerization of 1‐butene

The catalytic activities of Al‐ZSM‐22 (Al in the framework) and Fe‐ZSM‐22 (Fe in the framework) were compared in the skeletal isomerization of 1‐butene to isobutene. The catalysts synthesized in the laboratory were characterized by means of XRD, FTIR spectroscopy, SEM and surface area measurements. The activity of the zeolites was investigated using a fixed‐bed microreactor system. Al‐ZSM‐22 demonstrated higher activity in 1‐butene transformation compared to Fe‐ZSM‐22, while the selectivity to isobutene, on the other hand, was higher over Fe‐ZSM‐22. Coke formation was monitored using a microbalance and the results showed that the weight gain of Fe‐ZSM‐22 was slightly higher compared to Al‐ZSM‐22. This revised version was published online in July 2006 with corrections to the Cover Date.     

Pulsed Disc‐and‐Doughnut Column Performance

Abstract: A study of the hydrodynamic variables, drop size, continuous phase axial dispersion, and mass‐transfer coefficients of a pulsed annular disc‐and‐doughnut liquid extraction column are presented for three different systems. The results indicate that the characteristic velocity plot of Gayler et al. (1953) can be used to describe the variation of holdup with flow rate for a range of pulsation velocities. The existence of several different operating regimes, namely streamline, mixer‐settler, and emulsion regimes, was observed when the input energy was altered. Mass‐transfer data from 72.5 mm i.d. and 2.5 m i.d. columns were interpreted in terms of the differential axial‐dispersion model; the number of transfer units in a unit length of column is proposed as the basis for scale‐up of the mass‐transfer performance. By considering the free areas in the column, a method is proposed for the geometric scale‐up of pulsed disc‐and‐doughnut columns.     

Effect of Channel Geometry and Mixture Temperature on Detonation‐to‐Deflagration Transition in Gases

Results from theoretical and experimental studies of deflagrationtodetonation transition in hydrocarbon–air mixtures are reported. The effects of internal geometry, turbulence transition of the flow, and the temperature and fuel concentration in the unburned mixture on detonation initiation are considered.     

Vapor‐phase synthesis of acetophenone in benzene acylation over CeHZSM‐5(30) zeolite

The benzene acylation was carried out with acetic anhydride as an acylating agent in vapor phase over HZSM5, Si/Al =30 and 280, HY and Ce, Vmodified HZSM5(30) zeolite catalysts at atmospheric pressure. The yield of acylated product acetophenone was found to be 82.1 wt% with 95.0% selectivity at 86.4 wt% conversion of acetic anhydride over Cemodified HZSM5(30) where Brønsted acidic sites are active for this reaction.     

C,N‐Pyridylpyrazole‐Based Ligands: Synthesis and Preliminary Use in Metal Ion Extraction

Abstract

The synthesis of new N‐donor pyridylpyrazole ligands with a functionalized arm is described. The complexation capabilities of these compounds towards bivalent metal ions (Hg²⁺, Cd²⁺, Pb²⁺, Cu²⁺, and Zn²⁺) and alkali metal ions (K⁺, Na⁺, and Li⁺) were investigated using the liquid‐liquid extraction process. The percentage limits of extraction were determined by atomic absorption measurements.     

Zeolite effect in the enantioselective transhydrogenation over a Co‐salen “ship‐in‐the‐bottle” complex

Our investigations were focused on the transhydrogenation of carbonyl groups using Co(II)‐salen complexes entrapped in zeolites. The Co‐salen 1 complex was occluded in the zeolite as a “ship‐in‐the‐bottle” (SIB) catalyst, as could be demonstrated by means of DRIFT‐IR spectra, DTG, nitrogen adsorption isothermes and soxhlet extraction. The results of the test reactions show that an immobilization of the Co‐salen 1 complex in the zeolite as a “SIB” complex results in an enantiomeric excess as high as in the homogeneous case. The Co‐salen 1 USY SIB catalyst shows its best ee‐value at higher temperature (25°C) than the homogeneous one (-10°C). The reason could be the steric constraints of the Co‐salen complex in the zeolite framework. This revised version was published online in July 2006 with corrections to the Cover Date.