Martin Bülow

Metallo-organic molecular sieve for gas separation and purification

Abstract An improved synthesis process was developed for large-scale production of a highly porous material, [Cu 3 (BTC) 2 (H 2 O) x ] n , i.e., polymeric copper(II) benzene-1,3,5-tricarboxylate with molecular-sieve character, denoted as Cu-BTC, where BTC denotes benzene-1,3,5-tricarboxylate. A series of sorption properties for nitrogen, oxygen, carbon monoxide, carbon dioxide, nitrous oxide, methane, ethylene, ethane, n -dodecane, and water were studied. A detailed investigation of sorption thermodynamics was performed for carbon dioxide by a sorption-isosteric method. The experimental results show that Cu-BTC can be used for separation of gas mixtures such as carbon dioxide–carbon monoxide, carbon dioxide–methane and ethylene–ethane mixtures. In addition, air can be purified effectively from carbon dioxide, nitrous oxide, high-molecular weight hydrocarbons and moisture.

Microdynamics of methane, ethane and propane in ZSM-5 type zeolites

The n.m.r. pulsed field-gradient technique has been used to study systematically the intracrystalline self-diffusion of methane, ethane and propane in ZSM-5. In conjunction with the information obtained from nuclear magnetic relaxation studies the elementary steps of diffusion are found to be activated jumps between the channel intersections. Only for sorbate concentrations > 2.5 molecules per intersection is a decrease in the jump lengths observed. The results are compared with alkane self-diffusion measurements in zeolites A and X, as well as with the self-diffusion of water in ZSM-5 and zeolites A and X.

Theoretical prediction of uptake behaviour in adsorption kinetics of binary gas mixtures using irreversible thermodynamics

The relations proposed by Ash and Barrer [1] on the basis of the thermodynamics of irreversible processes for the description of surface flow are transferred to the adsorption kinetics of binary gas mixtures. With the use of several simplifying assumptions (highly different adsorption behaviour of the two components, negligible “cross coefficients”, applicability of the Langmuir model for the equilibria), an analytical expression for the description of sorption kinetics of binary mixtures is derived, which provides a satisfactory description of experimental data obtained by us for the mixture adsorption system benzene-n-heptane/NaX-zeolite.

Adsorptive separation of nitrogen from other gases

Type X zeolites whose charge-compensating cations are composed of 95 to 50% lithium ions, 4 to 50% of one or more of aluminum, cerium, lanthanum and mixed lanthanides and 0 to 15% of other ions. The zeolites preferentially adsorb nitrogen from gas mixtures.

On synthesis conditions for tailoring AIPO4-5 crystal dimensions

The influence of the gel composition on the dimensions of AIPO 4 -5 crystals formed in the system Al 2 O 3 :P 2 O 5 :(C 2 H 5 ) 3 N:H 2 O:H 2 SO 4 has been studied systematically. A linear correlation between the water content of the reacting gel and both the maximum length and the aspect ratio of the crystals has been found, whereas amine as well as P 2 O 5 contents are controlling the nucleation process. Crystals with sizes ranging from 20 μm to about 750 μm along the hexagonal c -axis can be synthesized with good yields when these correlations are used.

Fast adsorption-desorption kinetics of hydrocarbons in silicalite-1 by the single-step frequency response method☆

Abstract Fast adsorption-desorption kinetics with dead times of 25–35 ms have been followed using a new technique. At sorption equilibrium, a zeolite sample is subjected to small (less than ±1%) pressure changes brought about by very rapid, square-wave volume perturbations. By recording the gas pressure response on compression/expansion of the system, sorption uptake/desorption processes, which are governed by intracrystalline diffusion, can be followed. By fitting appropriate solutions of Ficks diffusion law to the adsorption/desorption curves, and correcting the as-measured diffusivities by the Darken factor, intracrystalline diffusion coefficients of short-chain hydrocarbons, n -hexane and benzene, have been determined. These corrected diffusion coefficients for methane, ethane, and propane, are ⩽ 5 smaller than the self-diffusivities measured directly by the n.m.r. pulsed-field-gradient technique. The corresponding coefficients for n -hexane and benzene agree well with those determined by piezometric measurements.

NMR and IR studies of zeolite H-ZSM-5 modified with orthophosphoric acid

By combined use of solid-state NMR, NMR self-diffusion techniques, and diffuse reflectance IR spectroscopy, important information on the nature of the interaction of orthophosphoric acid with zeolite H-ZSM-5 could be obtained. By aH NMR self-diffusion experiments it was found that the phosphoric acid enters the ZSM-5 channel system. However, with increasing amounts of H3PO 4 deposited, there is an enrichment of phosphorus species near the external surface of the zeolite crystals, forming a transport barrier for diffusing molecules. 27A1 MAS NMR shows that the decreased concentration of BrCnsted acid sites in H-ZSM-5 after impregnation with orthophosphoric acid and a subsequent thermal treatment at 500°C is a direct result of a framework dealumination. Combined 27A1, 29Si, 3ip, and 1H MAS NMR studies reveal that this dealumination process is promoted by the presence of increasing amounts of H3PO4 and results in the formation of occluded aluminum phosphate species. Furthermore, IH MAS NMR shows that H3PO 4 impregnation causes a drastic reduction in the concentration of nonacidic silanol groups. Both findings of 1H MAS NMR have been verified by diffuse reflectance IR spectroscopy. Consequently, treatment of ZSM-5 with orthophosphoric acid modifies acidity and diffusivity in the sample series under study. These effects can combine to give a useful catalyst of enhanced paraselectivity.

Investigation of internal silanol groups as structural defects in ZSM-5-type zeolites

A 1H Magic-angle spinning nuclear magnetic resonance investigation of large and uniform crystals of HZSM-5 type zeolites, synthesized either by means of TPA or without organic templates, shows that using TPA leads to a high concentration of defect centres within the ZSM-5 structure. Owing to non-intact Si—O—Si bonds, up to 8% of the lattice Si were found to be present as Si—OH groups. A comparison of the 1H—29Si cross-polarization spectra with the variation of the 1H n.m.r. chemical shift of the Si—OH groups as well as with the results of ion-exchange experiments suggests a vicinal arrangement of the silanol groups. High concentrations of such structural defects within the crystals lead to a slight decrease of crystal sorption capacity for n-hexane and significantly reduce crystal hydrophobicity. Both the intracrystalline self-diffusion and the molecular reorientation of sorbed molecules as measured by means of the 1H n.m.r. pulsed field gradient technique and 13C n.m.r. lineshape analysis, respectively, show an enhanced molecular mobility of guest molecules within the defective ZSM-5 structure

Molecular self-diffusion of methane in zeolite ZSM-5 by quasi-elastic neutron scattering and nuclear magnetic resonance pulsed field gradient technique

The translational mobility of methane sorbed on an NaZSM-5 sample of proven crystallinity has been studied by quasi-elastic neutron scattering (QENS) and an n.m.r. pulsed field gradient technique (n.m.r. PFGT) at different loadings and temperatures. In both methods, long-range self-diffusion is detected. However, owing to the different timescales of the two experimental methods applied, the mean diffusion paths followed are of different magnitude: in QENS, molecular translation is measured up to 6nm; in n.m.r. PFGT, the mean molecular displacements amount to some µm. Nevertheless, since the r.m.s. molecular displacements followed by both methods considerably exceed the distances between the pore intersections of the ZSM-5 channel network, in both techniques the translational self-diffusion coefficient of guest molecules inside the zeolite pores is detected. For the intracrystalline self-diffusion coefficient, D, as well as for the activation energy of self-diffusion, Ea, the values determined by the two independent methods agree very well. In the temperature region 200–250 K, the intracrystalline self-diffusion coefficients of methane in ZSM-5 are found to be of the order of 10–5–10–4 cm2 s–1, with only slight concentration and temperature dependence. The activation energy determined by both methods amounts to 4–5 kJ mol–1. Further agreement between QENS and n.m.r. PFGT is obtained by comparing the mean molecular jump lengths, which by both techniques are found to be ca. 1 nm for light hydrocarbons in ZSM-5, slightly decreasing with increasing loading.

A phenomenological study of surface barriers in zeolites

Abstract Over a temperature interval from 200 to 470 K the intracrystalline self-diffusion coefficients D intra and the tracer desorption times τ intra of ethane in hydrothermally treated zeolite NaCaA have been measured. While for the untreated samples the tracer desorption times coincide with the values estimated on the basis of the intracrystalline mobility, for the hydrothermally treated sample additional transport resistances at the surface of the crystallites (‘surface barriers’) become significant. Since the activation energy for tracer desorption is found to be larger by a factor of 2 than that for intracrystalline diffusion, it is concluded that the surface barrier cannot be effected by a mere deposition of impenetrable material on the outer surface of the crystallites. This conclusion is in agreement with high resolution electron micrographs which show well-preserved crystallinity and no additional amorphous material at the surface after hydrothermal treatment. Furthermore, the latter treatment is found to lead to the formation of voids in the zeolite crystals.