Claus J. H. Jacobsen

Zeolites by confined space synthesis – characterization of the acid sites in nanosized ZSM-5 by ammonia desorption and 27Al/29Si-MAS NMR spectroscopy

ZSM-5 zeolites prepared by confined space synthesis, i.e., by crystallization inside a mesoporous matrix, are characterized by X-ray powder diffraction (XRPD), ammonia temperature-programmed desorption (TPD), 27Al and 29Si-MAS NMR spectroscopy, and transmission electron microscopy (TEM). It is shown by XRPD that confined space synthesis can be used for the preparation of pure and highly crystalline ZSM-5 with a uniform crystal size distribution governed by the pore size of the inert matrix. Ammonia desorption demonstrates that the small zeolite crystals have the same number of acid sites as large crystals when the Si/Al ratio is similar. The number of acid sites have been determined by ammonia TPD and converted into framework Si/Al ratios. These ratios are higher than the bulk Si/Al ratios but agree with those obtained from 27Al/29Si-MAS NMR. Furthermore, the 27Al MAS NMR spectra show that the synthesis conditions are important in order to prevent the formation of non-framework Al and that the best result is obtained using NaAlO2 as the aluminum source. By proper control of the synthesis conditions, it is possible to produce a ZSM-5 zeolite with a framework Si/Al=50 and without non-framework aluminum. The relative numbers of Si*(OSi)4, HOSi*(OSi)3, and AlOSi*(OSi)3 units have been determined from 29Si-MAS NMR in combination with the Si/Al ratios from 27Al-MAS NMR. These results indicate a higher number of HOSi*(OSi)3 sites for ZSM-5 zeolites of the nm size when compared to those of the μm size. Transmission electron micrographs provide independent support for the formation of small zeolite crystals.

Electronic factors in catalysis: the volcano curve and the effect of promotion in catalytic ammonia synthesis

Abstract The activity and selectivity of heterogeneous catalysts are determined by their electronic and structural properties. In many cases, the electronic properties are determined by the choice of both the catalytically active transition metal and promoter elements. Density functional theory is used to calculate how these two factors affect the energies of the intermediates and transition states in the ammonia synthesis reaction. We show that a linear relationship exists between the activation energy for N 2 dissociation and the binding energy of adsorbed nitrogen. The ammonia synthesis activity under industrial conditions can be determined as a function of the nitrogen–surface interaction energy by combining the calculated adsorption energy–activation energy relation with a micro-kinetic model. The result is a volcano curve and we illustrate such relationships for both the non-promoted and alkali metal promoted transition metals. We conclude that promotion is most effective for the best non-promoted catalysts and that promotion will always be essential for obtaining an optimal ammonia synthesis catalyst. Analysis of the micro-kinetic model show that the best catalysts are those with the lowest apparent activation energies and they exhibit reaction orders between two asymptotic behaviors.

HDS, HDN and HYD activities and temperature-programmed reduction of unsupported transition metal sulfides

Several transition metal sulfides have been shown to exhibit high catalytic activities in various hydrotreating reactions. The simultaneous catalytic activities for hydrodesulfurization of dibenzothiophene, hydrodenitrogenation of indole and hydrogenation of naphthalene have been studied using high surface area bulk sulfides: NbS2, MoS2, ReS2, RuS2 and Co9S8. The metal–sulfur bond strengths have been assessed by temperature‐programmed reduction of H2S‐pretreated sulfides, and it is demonstrated that this pretreatment is necessary to obtain reliable TPR data for the sulfides. The catalytic HDS activity is shown to follow the trend in the metal–sulfur bond strengths estimated by this improved method. Furthermore, the metal–sulfur bond strengths determined this way follow the trend estimated previously from theory. The TPR data suggest that a large number of sulfur vacancies exist in active catalysts under industrial reaction conditions. Such vacancies have recently been observed on model catalysts by use of STM.

Nanosized zeolite crystals—convenient control of crystal size distribution by confined space synthesis

Confined space synthesis is a novel method in zeolite synthesis, which allows preparation of nanosized ZSM-5 crystals with a controlled crystal size distribution.

Support Effect and Active Sites on Promoted Ruthenium Catalysts for Ammonia Synthesis

The catalytic activities of three supported, barium-promoted ruthenium catalysts for ammonia synthesis are reported. The three supports are silicon nitride (Si3N4), magnesium aluminum spinel (MgAl2O4), and graphitized carbon (C). The effect of the promoter on the activity is strongly dependent on the choice of support material in accordance with several previous observations. Here, this dependence is ascribed to a difference in the affinity of the promoter for the different supports. It is shown how it is possible to image the barium promoter present on the surface of ruthenium crystals in passivated catalysts by conventional high-resolution transmission electron microscopy (HRTEM). By comparison with in situ HRTEM images obtained lately from similar catalysts, and with reference to recent density functional theory (DFT) calculations, we suggest that active B5-type sites on the surfaces of the ruthenium crystals are promoted by nearby promoter atoms via electrostatic interactions.

Effect of the activation procedure on the performance of Mo/H-MFI catalysts for the non-oxidative conversion of methane to aromatics

Abstract The effect of pre-treatment on the performance of Mo/H-MFI zeolite catalysts applied to the non-oxidative conversion of methane to aromatics has been investigated. It is demonstrated that catalyst performance depends on activation conditions. Activation of the Mo-oxide/H-MFI precursor with an n-butane/hydrogen mixture results in higher catalyst stability and benzene selectivity which are tentatively attributed to the formation of the α-MoC1−x carbide, with f.c.c. structure as revealed by XRD.

Novel class of ammonia synthesis catalysts

Ternary nitrides Fe3Mo3N, Co3Mo3N and Ni2Mo3N, exhibit high catalytic activities in ammonia synthesis; promotion of Co3Mo3N with caesium results in higher activity than that of the commercial multi-promoted iron based catalyst.

Unusual observation of nitrogen chemical shift anisotropies in tetraalkylammonium halides by 14N MAS NMR spectroscopy.

14N Magic-angle spinning (MAS) NMR spectra for a number of polycrystalline, symmetrical tetraalkylammonium halides with short alkyl chains (C2H(5)- to n-C4H(9)-) have been recorded following a careful setup of the experimental conditions. Analysis of the spectra demonstrates the presence of 14N chemical shift anisotropies (CSAs) on the order of |delta sigma| = 10-30 ppm along with 14N quadrupole coupling constants in the range of 10-70 kHz. The magnitude and sign of the CSAs determined from 14N MAS NMR are confirmed by recording and analysis of the corresponding slow-speed spinning (500-650 Hz) 15N CP/MAS NMR spectra. Most interestingly, it is observed experimentally and demonstrated theoretically and by simulations, that these CSAs are reflected in the spinning sideband (ssb) intensities of the 14N MAS spectra at much higher spinning speeds than can be applied to retrieve the corresponding 15N CSAs from the ssb pattern in the 15N CP/MAS spectra.

NIR FT Raman spectroscopic studies of η-Al2O3 and Mo/η-Al2O3 catalysts

Abstract NIR FT Raman spectra excited at 1064 nm have been obtained from samples of Mo/η-Al2O3 catalysts after treatment at 773 K in vacuum for 12 h. The samples were heated in quartz tubes which were sealed and transferred directly to the spectrometers without exposure to the ambient atmosphere. The broad background previously observed in the NIR FT Raman spectra of hydrated Mo/η-alumina samples disappeared after the dehydration. Upon exposure to ambient air the broad background signal reappeared suggesting that the broad background feature can be attributed to surface hydroxyl groups present in the hydrated samples. Furthermore, it was observed that upon treating the η-Al2O3 with water the broad background signal disappeared and the presence of bayerite (Al(OH)3) or boehmite (AlO(OH)) was detected depending on the experimental details. The identification of the various phases is supported by XRPD analysis. The overall conclusion is that the broad NIR FT Raman background can be attributed to surface hydroxyl groups in hydrated η-Al2O3.

Catalytic epoxidation of alkenes with hydrogen peroxide over first mesoporous titanium-containing zeolite

Novel mesoporous TS-1 catalyst is shown to be active in epoxidation of oct-1-ene and significantly more active in epoxidation of cyclohexene than conventional TS-1.