Jean-Michel Chézeau

Influence of synthesis conditions and postsynthesis treatments on the nature and quantity of structural defects in highly siliceous MFI zeolites: A high-resolution solid-state 29Si n.m.r. study

The formation and annealing of various kinds of defects in the framework of highly siliceous MFI-type zeolites, as evidenced by high-resolution solid-state 29 Si n.m.r., can be explained by considering the systematic influence of synthesis parameters and postsynthesis treatments. High pH values and high concentrations of alkali metal and fluoride ions prevent a complete polycondensation of the silicate species and generate materials that contain significant levels of nonbonding point defects. By contrast, a high crystallization temperature results in the ready hydrolysis of the terminal Si-O − groups, leading to structures containing fewer defects. A large number of T vacancies (hydroxyl nests) are created in zeolites crystallized at low temperatures in the presence of high Pr 4 N + concentations and in the absence of F − or Na + ions. Such conditions favor the formation of double-5-ring silicate anions. The hypothesis, that such species may further condense, conducting to frameworks with an important number of empty T-sites, is proposed. On the basis of these findings, ideal synthesis conditions can be proposed to produce (Si) MFI zeolites that contain a minimum number of defects. Various postsynthesis treatments, namely, different calcination conditions followed, or preceded, by selected ionic (akali cation) exchanges, result in a partial or a nearly total healing of these defects in the final materials.

27Al MAS NMR characterization of AlPO4-14 enhanced resolution and information by MQMAS Dr. Hellmut G. Karge on the occasion of his 65th birthday

Abstract The crystalline microporous aluminophosphate AlPO4-14 was studied using 27Al NMR spectroscopy, with both conventional MAS and two-dimensional multiple-quantum MAS (MQMAS) techniques. Data obtained for the four non-equivalent framework aluminum atoms detected in AlPO4-14 are reported. They are found to occur with the ratio 1:1:1:1 according to the proposed crystal structure. The aim of this paper is also to compare the NMR methods used in this work with those found in literature. It is concluded that the new MQMAS method is by now the best approach to distinguish easily the non-equivalent Al atoms in the structure of such materials. It is also very efficient to obtain rapidly the necessary information about distribution of chemical shifts and quadrupolar interactions.

N.m.r. and i.r. study of B and BAl substitution in zeolites of the MFI-structure type obtained in non-alkaline fluoride medium

Pure boron and boron—aluminium MFI-type zeolites have been obtained by heating non-alkaline reaction mixtures containing F− instead of OH− anions and tetrapropylammonium bromide as a template. High resolution solid state magic angle spinning (MAS) 29Si-, 27Al- and 11B-n.m.r., and i.r. spectroscopy have indicated the incorporation of the trivalent elements in the zeolite framework. BO4 and AlO4 groups were shown to be present in the as-synthesized samples. For the dehydrated calcined samples, it is suggested that boron is in a trigonal environment still in the framework.

High-resolution solid-state 29Si and 13C n.m.r. on highly siliceous MFI-type zeolites synthesized in nonalkaline fluoride medium

Abstract Highly resolved 29 Si and 13 MAS -n.m.r. spectra of pure silica zeolites of the MFI-type synthesized according to a new route reveal new features, which are indicative of the very low defect content and the very high crystallinity of these materials.

1D and 2D solid state NMR investigations of the framework structure of As-synthesized AlPO4-14

The framework structure of As-synthesized AlPO4-14 has been investigated with a combination of different one-dimensional 27Al and 31P solid state NMR techniques and 27Al/31P double resonance methods. The results are found to be fully consistent with the assumed structural model. 27Al MAS and DOR experiments at three different magnetic field strengths together with simulations show the presence of two tetrahedral sites, one pentacoordinated and one octahedral aluminum site. The 27Al quadrupolar coupling constants and the 31P isotropic chemical shifts of the tetrahedral sites correlate well with tetrahedral shear-strain parameters and mean P-O-Al bond angles, respectively. These correlations allow one to assign all of the NMR resonances to specific T-sites in the proposed framework structure. The assignments are then further confirmed by the application of three different two-dimensional heteronuclear correlation methods (i.e., 27Al-->31P TEDOR, CP, and INEPT) which reveal the connectivities between AlOx and PO4 polyhedra. The two-dimensional INEPT experiment is applied here for the first time in the solid state.

FTIR and 19F MAS NMR investigation of the interactions between p-fluoroacetophenone and MFI structure type zeolites

Interactions between PFA molecules and MFI structure type zeolites were studied by FTIR and 19F MAS NMR spectroscopies. In the purely siliceous ZSM-5 sample, PFA molecules were physisorbed. In the acid samples, the interactions implicated the carbonyl group of the PFA molecules and the Bronsted acid sites of HZSM-5. This interaction could be a hydrogen bond or a protonation of the carbonyl group. The complementarity of the IR and 19F NMR spectroscopies was shown: IR specified the type of interaction whereas NMR allowed quantification of the strength of the interaction and the amount of active acid sites of the zeolite.

Low temperature orthorhombic-monoclinic transition in as-synthesized MFI siliceous zeolites

On cooling, an as-synthesized MFI siliceous zeolite presents a transition from orthorhombic to monoclinic form, at ~ 175 K. This phase transition is evidenced by several techniques and is similar to that already known in calcined MFI zeolites

1H solid-state nuclear magnetic resonance study of the mobility of the tetrapropylammonium template in a purely siliceous MFI-type zeolite

Dynamics of tetrapropylammonium (TPA) cation occluded during the synthesis in a siliceous MFI zeolite is investigated by 1H broad-line nuclear magnetic resonance (NMR) methods. Second moments M2, spin-lattice relaxation times T1 and T1 rho are measured in a large temperature domain. To al comparison, similar measurements are also reported in bulk tetrapropylammonium bromide (TPABr). Whereas methyl reorientation at low temperature and tumbling of the cation in the plastic phase are observed in crystalline TPABr in accordance with published studies, a new slower motion which could not be identified is observed below the phase transition. Such a motion is much more clearly shown by the existence of a minimum of T1 rho in a quenched sample. Our measurements in the zeolite demonstrate that the TPA template exhibits a larger and more complex mobility below 378 K. Beyond the fast methyl reorientation, the results disclose a motion which probably involves the entire propyl arms inside the channels. So the zeolite framework seems to make such a kind of motion easier. On the contrary, even at 450 K, the highest temperature investigated, the tumbling which would necessitate exchange of the propyl arms between the channels is not observed. A slow motion, responsible for a decrease of T1 rho above 350 K, could not be identified. While a simple correlation time is sufficient to describe the relaxation time dependences in TPABr, a distribution (such as Williams-Watts) is required to account for those in the zeolite.