Mechanism studies of the conversion of 13C-labeled n-butane on zeolite H-ZSM-5 by using 13C magic angle spinning NMR spectroscopy and GC-MS analysis

By using 13C MAS NMR spectroscopy (MAS = magic angle spinning), the conversion of selectively 13C-labeled n-butane on zeolite H-ZSM-5 at 430-470 K has been demonstrated to proceed through two pathways: 1) scrambling of the selective 13C-label in the n-butane molecule, and 2) oligomerization-cracking and conjunct polymerization. The latter processes (2) produce isobutane and propane simultaneously with alkyl-substituted cyclopentenyl cations and condensed aromatic compounds. In situ 13C MAS NMR and complementary ex situ GC-MS data provided evidence for a monomolecular mechanism of the 13C-label scrambling, whereas both isobutane and propane are formed through intermolecular pathways. According to 13C MAS NMR kinetic measurements, both pathways proceed with nearly the same activation energies (E(a) = 75 kJ mol(-1) for the scrambling and 71 kJ mol(-1) for isobutane and propane formation). This can be rationalized by considering the intermolecular hydride transfer between a primarily initiated carbenium ion and n-butane as being the rate-determining stage of the n-butane conversion on zeolite H-ZSM-5.     

Kinetic analysis of temperature-programmed surface reactions on porous catalysts

By means of model calculations it could be shown for an irreversible surface reaction of 1st order that the determination of the activation energy of the desorption of the reactant or, respectively, of the surface reaction is possible by application of the method of variation of the heating rate to the desorption curve of the reactant, according to circumstances whether the ratio of the activation energy of the surface reaction and of the desorption of the reactant is greater or smaller than one.The possibilities of the kinetic evaluation are applied to the isomerization of cyclopropane on a NaX-zeolite catalyst. The resulting heat of adsorption of cyclopropane and the activation energy of the reaction agree well with the values of literature obtained by isothermal measurements in a pulse reactor.

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Zusammenfassung Mit Modellrechnungen für eine irreversible Oberflächenreaktion 1. Ordnung konnte gezeigt werden, daß durch Anwendung der Methode der Variation der Heizgeschwindigkeit auf die Desorptionskurve des Ausgangsstoffes die näherungsweise Bestimmung der Aktivierungsenergie der Desorption des Ausgangsstoffes bzw. der Oberflächenreaktion möglich ist, je nachdem ob das Verhältnis der Aktivierungsenergien für die Oberflächenreaktion und der Desorption des Ausgangsstoffes größer oder kleiner als eins ist.Die Möglichkeiten der kinetischen Auswertung werden auf die isomerisierung von Cyclopropan an einem NaX-Zeoliten angewendet. Die erhaltene Adsorptionswärme für Cyclopropan und die Aktivierungsenergie für die Reaktion stimmen gut mit Literatur-werten von isothermen Messungen im Impulsreaktor überein.

     

Synthesis and immobilization of quaternary ammonium cations in acidic zeolites

A general method for the synthesis of quaternary ammonium cations in acidic zeolites by a direct reaction of tertiary amines and alcohols is described.     

Characterization of framework and extra-framework aluminum species in non-hydrated zeolites Y by 27Al spin-echo, high-speed MAS, and MQMAS NMR spectroscopy at B0 = 9.4 to 17.6 T

27Al spin-echo, high-speed MAS (nu(rot) = 30 kHz), and MQMAS NMR spectroscopy in magnetic fields of B0 = 9.4, 14.1, and 17.6 T were applied for the study of aluminum species at framework and extra-framework positions in non-hydrated zeolites Y. Non-hydrated gamma-Al2O3 and non-hydrated aluminum-exchanged zeolite Y (Al,Na-Y) and zeolite H,Na-Y were utilized as reference materials. The solid-state 27Al NMR spectra of steamed zeolite deH,Na-Y/81.5 were found to consist of four signals. The broad low-field signal is caused by a superposition of the signals of framework aluminum atoms in the vicinity of bridging hydroxyl protons and framework aluminum atoms compensated in their negative charge by aluminum cations (delta(iso) = 70 +/- 10 ppm, C(QCC) = 15.0 +/- 1.0 MHz). The second signal is due to a superposition of the signals of framework aluminum atoms compensated by sodium cations and tetrahedrally coordinated aluminum atoms in neutral extra-framework aluminum oxide clusters (delta(iso) = 65 +/- 5 ppm, C(QCC) = 8.0 +/- 0.5 MHz). The residual two signals were attributed to aluminum cations (delta(iso) = 35 +/- 5 ppm, C(QCC) = 7.5 +/- 0.5 MHz) and octahedrally coordinated aluminum atoms in neutral extra-framework aluminum oxide clusters (delta(iso) = 10 +/- 5 ppm, C(QCC) = 5.0 +/- 0.5 MHz). By chemical analysis and evaluating the relative solid-state 27Al NMR intensities of the different signals of aluminum species occurring in zeolite deH,Na-Y/81.5 in the non-hydrated state, the aluminum distribution in this material was determined.     

CaNa[Cr(OH)6] – A Layered Hydroxochromate(III) with Ordered Brucite Structure and its Thermal Decomposition

Green single-crystals of the hydroxochromate(III) CaNa[Cr(OH)₆] were grown under highly alkaline hydrothermal conditions at about 200 °C. The starting materials Ca(NO₃)₂ · 6H₂O and Cr(NO₃)₃ · 9H₂O were reacted in a mixture of water and sodium hydroxide with the molar ratio of 2.8:1. CaNa[Cr(OH)₆] crystallizes in the non-centrosymmetric trigonal space group R3 with the lattice parameters a = 583.86(2) pm and c = 1428.73(6) pm [T = 100(1) K]. Characteristically, the crystals are reverse-obverse as well as inversion twins. The crystal structure is a stack of uncharged metal hydroxide layers, which can be regarded as a cation-ordered rhombohedral variant of the Mg(OH)₂ (brucite) structure type. The oxidation state of chromium(III) and its coordination by hydroxide groups was confirmed by UV/Vis and IR spectroscopy, respectively. Temperature-dependent magnetic measurements revealed paramagnetic behavior with an effective moment of 3.82 μ_B per chromium atom. The thermal decomposition of CaNa[Cr(OH)₆] takes place at about 225 °C, where the fast elimination of 1.5 equivalents of water is followed by the oxidation of chromium(III) to chromium(VI). Upon further heating to 1000 °C and 1200 °C, the intermediate decomposition products CaCrO₄ and Na₂CrO₄ transform into the oxochromates(V) Ca₅(CrO₄)₃O_0.5 and Ca₃(CrO₄)₂, respectively.     

Enhancement of Ion Pairing of Sr(II) and Ba(II) Salts by a Tritopic Ion-Pair Receptor in Solution

Tritopic ion-pair receptors can bind bivalent salts in solution; yet, these salts have a tendency to form ion-pairs even in the absence of receptors. The extent to which such receptors can enhance ion pairing has however remained elusive. Here, we study ion pairing of M²⁺ (Ba²⁺, Sr²⁺) and X⁻ (I⁻, ClO₄⁻) in acetonitrile with and without a dichlorooxacalix[2]arene[2]triazine-related receptor containing a pentaethylene-glycol moiety. We find marked ion association already in receptor-free solutions. When present, most of the MX⁺ ion-pairs are bound to the receptor and the overall degree of ion association is enhanced due to coordinative, hydrogen-bonding, and anion-π interactions. The receptor shows higher selectivity for iodides but also stabilizes perchlorates, despite the latter are often considered as weakly coordinating anions. Our results show that ion-pair binding is strongly correlated to ion pairing in these solutions, thereby highlighting the importance of taking ion association in organic solvents into account.     

Synthesis and crystal structure of the isotypic rare earth thioborates Ce[BS3], Pr[BS3], and Nd[BS3]

The orthothioborates Ce[BS₃], Pr[BS₃] and Nd[BS₃] were prepared from mixtures of the rare earth (RE) metals together with amorphous boron and sulfur summing up to the compositions CeB₃S₆, PrB₅S₉ and NdB₃S₆. The following preparation routes were used: solid state reactions with maximum temperatures of 1323 K and high-pressure high-temperature syntheses at 1173 K and 3 GPa. Pr[BS₃] and Nd[BS₃] were also obtained from rare earth chlorides RECl₃ and sodium thioborate Na₂B₂S₅ by metathesis type reactions at maximum temperatures of 1073 K. The crystal structure of the title compounds was determined from X-ray powder diffraction data. The thioborates are isotypic and crystallize in the orthorhombic spacegroup Pna2₁ (No. 33; Z=4; Ce: , , ; Pr: , , ; Nd: , , ) . The crystal structures contain isolated [BS₃]^3‐ groups with boron in trigonal-planar coordination. The sulfur atoms form the vertices of undulated kagome nets, which are stacked along [100] according to the sequence ABAB. Within these nets every second triangle is occupied by boron and the large hexagons are centered by rare earth ions, which are surrounded by overall nine sulfur species.     

Bose-Einstein condensate coupled to a nanomechanical resonator on an atom chip

We theoretically study the coupling of Bose-Einstein condensed atoms to the mechanical oscillations of a nanoscale cantilever with a magnetic tip. This is an experimentally viable hybrid quantum system which allows one to explore the interface of quantum optics and condensed matter physics. We propose an experiment where easily detectable atomic spin flips are induced by the cantilever motion. This can be used to probe thermal oscillations of the cantilever with the atoms. At low cantilever temperatures, as realized in recent experiments, the backaction of the atoms onto the cantilever is significant and the system represents a mechanical analog of cavity quantum electrodynamics. With high but realistic cantilever quality factors, the strong coupling regime can be reached, either with single atoms or collectively with Bose-Einstein condensates. We discuss an implementation on an atom chip.