H Geerts

Framework and Non-Framework Al Species in Dealuminated Zeolite Y

Abstract The hydrothermal dealumination of zeolite Y was followed by 29Si and 27Al MAS NMR. From deconvoluted MAS NMR spectra the amount of the different silicon and aluminium species, formed during steaming, could be determined. Next to well documented 27Al signals at 60 ppm of tetrahedral framework Al and at 0 ppm of octahedral Al, lines at 50 ppm, 30 ppm and in the region of 4–0 ppm are observed, the chemical nature of which is discussed. Zeolite Y with framework Si/Al ratio from 3.5 to 17 contains non-negligible amounts of extra-framework silicon and aluminium species, which complicate the determination of the framework Si/Al and non-framework:framework Al ratios. A considerable amount of extra-framework aluminium is missing in the 27Al MAS spectra. This ‘NMR-invisible’ aluminium, assigned to oligomeric alumina species, can be made ‘NMR-visible’ by complexation with acetylacetone under specific conditions.

Generation of acid catalytic activity in Si-VPI-5 by partial Si for P substitution during hydrothermal synthesis

Si-VPI-5 crystals containing Si atoms in Si(4Al) environments arc synthesized. The crystals contain occluded amine. It is protonated and neutralizes the residual negative lattice charges associated with the Si(4A1) sites. Calcined Si-VPI-5 is active in the isomerisation of decane and exhibits extra-large pore characteristics.

Carbonaceous Deposits Formed on Zeolites HY, HZSM-20 and H-Beta During the Conversion of Polynuclear Aromatics

Abstract 1-Methylnaphthalene was reacted over three large pore zeolite catalysts, viz. HY, HZSM-20 and H-Beta at 180, 250 and 300°C. All catalysts underwent deactivation due to the formation of carbonaceous deposits. The deposits were characterized by elemental analysis and CP/MAS-13C-NMR spectroscopy. HY and HZSM-20 showed very similar catalytic behaviours: At 250 and 300°C, considerable cracking and transalkylation occurred with a simultaneous formation of coke. On H-Beta, deactivation due to coking is much less severe, and there is hardly any cracking. All these observations can be accounted for by somewhat narrower pores as measured independently by the Spaciousness Index. MAS-NMR indicates that at low temperatures, the deposits are best described by unconverted methyl-naphthalene whereas at higher temperatures, condensed ring aromatics are gradually built up.

31P Solid-State Mas NMR Spectra: Fingerprints of Microporous Aluminophosphates and Silicoaluminophosphates

The structures of the silicoaluminophosphates MCM-1 and MCM-9 were characterized by 27 Al and 31 P MAS NMR. The structural identity of MCM-1 and its silicon-free homologue AlPO 4 -H 3 is demonstrated. The presence of a structural mixture in MCM-9 is confirmed. 31 P MAS NMR spectra of MCM-9 could interpreted as a superposition of spectra of VPI-5, AlPO 4 -H 3 and SAPO-11 phases.

Correct determination of aluminium in Y zeolites by magic-angle spinning n.m.r. spectroscopy

When Y zeolites are impregnated with ethanolic acetylacetone, the 29Si and 27Al magic-angle spinning n.m.r. spectra can be used to determine the correct Si/Al framework ratios and to quantify the amount of extra-framework aluminiums; evidence suggests that this impregnation does not cause further dealumination of the zeolitic material.

The Influence of Quadrupolar Effect on High-resolution 27Al Double Rotation (DOR) NMR of Aluminophosphate Molecular Sieves

ABSTRACT 27 AI DOR NMR at magnetic fields of 4.7, 8.5, 9.4 and 11.7 T is performed on a sample of calcined and rehydrated AIPO 4 -11. The application of double-rotation (DOR) NMR in magnetic fields of medium strength proves to be particularly advantageous to enhance the dispersion of the observed shifts of 27 AI nuclei, located at different crystallographic sites. The spectra allow to determine umambiguously the number of crystallographic Al sites and their occupancy, and to evaluate the symmetry of the aluminium environments and the tendency to aluminium dihydrate formation. True isotropic chemical shifts for the aluminium sites in AIPO 4 -11 are obtained by extrapolation to infinite field.

Dealumination of zeolite Y with SiCl4: a two-step reaction

Evidence is presented that the dealumination of zeolite Y with SiCl4 occurs in two distinguishable reaction steps; in the first step, lattice-attached ‘SiCl3’ species and the chloride salt of the charge compensating cation are formed, followed secondly by the isomorphic substitution of Al for Si and the subsequent formation of chloro–aluminium complexes.