Bart H. Wouters

Methylalumoxane MCM‐41 as Support in the Co‐Oligomerization of Ethene and Propene with [{C2H4(1‐indenyl)2}Zr(CH3)2]

Superior to the homogeneous catalyst or physisorbed catalyst system is a system with the ansa-metallocene catalyst [{C2 H4 (1-ind)2 }Zr(CH3 )] on a support formed by covalently anchoring methylalumoxane (MAO) on the internal pore walls of MCM-41. This system is a highly active and shape-selective mesoporous host in the co-oligomerization [shown schematically in Equation (a)] of ethene and propene with ansa-metallocenes. TMA=trimethylaluminum, ind=indenyl.

Aluminium Coordinations in Zeolite Mordenite By 27Al Multiple Quantum MAS NMR Spectroscopy

27Al 3Q MAS NMR spectroscopy has been applied to study the coordination state of the species giving the 30 ppm Al NMR signal in the 27Al MAS NMR spectrum of activated mordenite materials. From the 27Al 3Q MAS NMR measurements it is evident that the broad peak at 30 ppm in the 27Al NMR spectrum of the mordenite calcined at temperatures up to 600 °C comes mainly from the distorted four-coordinated Al. By simulation a quadrupolar coupling constant of 5.8 MHz was estimated for the distorted tetrahedral Al. For samples calcined at 650 and 700 °C, a small amount of pentacoordinated Al emerges. The majority of the signal, however, arises from distorted tetrahedral Al. A two-step calcination results in a significant contribution of the pentacoordinated Al to the signal at 30 ppm. From the simulated line-shape, a quadrupole coupling constant of 6.2 MHz is obtained for the latter signal. These data show that during the calcination of the mordenite, the coordination environment at the Al centre gradually becomes distorted to give rise to the shoulder at 30 ppm. With the increase of the calcination temperature, pentacoordinated Al species form

Crystallization of MAZ-type zeolites using tetramethylammonium, sodium and n-hexane derivatives as structure- and composition-directing agents

Abstract An attempt was made to synthesize MAZ-type zeolites with enhanced Si/Al ratio by adding organic molecules to conventional aluminosilicate hydrogels for MAZ zeolites containing sodium and tetramethylammonium cations. Molecular models of candidate templates were built and optimized inside the 12-membered ring (12-MR) pore of the MAZ structure. The hexamethonium cation, 1,6-diaminohexane and 1,6-hexanediol show the most favorable calculated Lennard-Jones potential. The crystallization in presence of these organic molecules led to the formation of MAZ-type zeolite. The samples were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive analysis of X-rays (EDAX), chemical analysis, nitrogen adsorption, thermogravimetric analysis/differential thermal analysis (TGA/DTA), 29 Si and 27 Al magic-angle spinning (MAS) nuclear magnetic resonance spectroscopy (NMR) and 13 C cross-polarization (CP)/MAS NMR. The hexamethonium cation and 1,6-diaminohexane are incorporated into the MAZ crystals, whereas 1,6-hexanediol is not. Incorporation of 1,6-diaminohexane caused a higher incorporation of silicon in the framework, especially in the T1 sites delineating the 12-MR channels. The upper limit of silicon incorporation was investigated by systematically reducing the aluminum content of the gel. The highest Si/Al ratio achieved by this approach was 5.3. A molecular crystallization mechanism for mazzite is proposed, explaining this upper limit as well as the crystal morphology changes depending on the aluminum content of the synthesis gel.

Enhanced resolution of Al sites in the molecular sieve SAPO-37 by 27Al MQ MAS NMR

Abstract The Al coordination state of molecular sieve SAPO-37 has been studied by 27 Al multiple quantum (MQ) MAS NMR spectroscopy. Although the 1D 27 Al MAS NMR spectrum of the aluminum sites show a complex pattern, the 3Q MAS NMR spectra of SAPO-37, submitted to different treatments, lead to the detection of four different Al species in the as-synthesized sample. The comparison of the MQ spectra of the as-synthesized sample, as well as of the dehydrated, partial template extracted, and full template calcined samples, made it possible to distinguish: (i) tetrahedral Al sites with 4P; (ii) tetrahedral Al sites with one (or more) Si as nearest neighbors interacting with TPA, and framework Al coordinated with; (iii) TMA; or (iv) water.

Alumoxo-heteropoly compounds as weakly coordinating anions for metallocenes in the oligomerization of alkenes

Abstract The in situ synthesis of methylalumoxane (MAO) on the crystalline structure of heteropoly acids generates a highly active and weakly coordinating compound for the metallocene catalyst. The MAO-phase formed by the controlled hydrolysis of trimethylaluminum covers the Keggin unit of the heteropoly compound, as is verified with several spectroscopic techniques. The primary Keggin structure of the heteropoly compound is preserved and MAO-formation induces an intensive charge transfer in the VIS-region. 27 Al-NMR of alumoxo-phosphotungstate illustrates the reversible interaction of the Lewis acid Al of the anchored alumoxane with the bridging oxygen atoms of the phosphotungstate Keggin structure. The interaction of the alumoxo-heteropoly compound with an ansa-metallocene produces a highly active catalyst for the co-oligomerization of ethene and propene. The softness of the heteropoly compound combined with the pseudo-liquid phase formation of the alumoxo-heteropoly compounds exceeds the activity of the homogeneous system and the stability of the heterogeneous MAO-anchored materials. The composition of the Keggin structure determines the catalytic potential of the metallocene which is transformed in a temperature dependent oligomerization and a fluctuating alternation of monomer and comonomer. The molecular weight and the methyl branching of the oligomer molecules are designing tools for the physicochemical features of a synthetic lubricant. A comparable charge delocalization over each heteropoly compound results in a single-sited catalytic system inducing a small dispersity of oligomer products.

13C MAS NMR of organic templates in zeolites

Abstract Cross-polarization (CP) combined with magic-angle spinning (MAS) is the most popular technique used for the high-resolution solid 13 C study of occluded templates in zeolitic materials. Depending on the mobility of the incorporated templates the high-power decoupling (DEC) technique is more suitable to detect and to quantify templates in zeolites. By this technique, high-resolution MAS NMR spectra, displaying quantitative reliable 13 C signal intensities of the occluded templates, are obtained in ZSM-5, SAPO-37 and the mazzite structure.

Immobilization of triazacyclononane-type metal complexes on inorganic supports via covalent linking: spectroscopy and catalytic activity in olefin oxidation

Publisher Summary As an alternative to porphyrin and phthalocyanine catalysts, the study of complexes of Mn and the cyclic triamine 1,4,7-trimethyl-1,4,7-triazacyclononane (tmtacn) is important. Here an immobilization of Mn-tmtacn based on zeolite Y has been proposed. However, a major problem is that hydrophilic matrix attracts H 2 O 2 . This is a drawback from the peroxide efficiency viewpoint. Therefore, a purely siliceous matrix seems more attractive. As pure SiO 2 lacks ion exchange capacity, reverting to other immobilization strategies is necessary, such as the covalent route. The chapter discusses various routes for covalent attachment of the tacn macrocycle to a pre-formed support matrix. This work is the first example of a covalently anchored, catalytically active nonheme Mn compound. Two different spacers are used to link the surface and the tacn: propyl (P) and glycidoxypropyl (GP). The affinity of the modified surface for metals is probed with the test ions Cu 2+ and Mn 2+ and styrene is the test substrate for the selective hydrocarbon oxidation. To improve the catalyst performance, the effect of reacting the anchored tacn with two molecules of propylene oxide (PO) was tested.

13-P-10-Evidence of partially broken bridging hydroxyls in molecular sieves from 1H MAS spin echo NMR spectroscopy

Publisher Summary This chapter presents evidence of partially broken bridging hydroxyls in molecular sieves from 1 H magic angle spinning (MAS) spin echo nuclear magnetic resonance (NMR) spectroscopy. By spin echo editing NMR method, combined with the l H ( 27 Al) spin echo double resonance, a new 1 H signal is found in the thermally treated molecular sieves, reflecting the complexity of hydroxyls as well as the aluminum state in the dehydrated state. It represents an initial stage of the dehydroxylation or may be related with the initial stage of dealumination.

13-P-18-Aluminium species in activated zeolites: solid-state NMR spectroscopy of the active sites

Publisher Summary This chapter discusses aluminium (Al) species in activated zeolites. Framework Al-OH defect species are at the origin of reversible tetrahedral–octahedral transformation in mild calcined zeolites. Depending on the calcination degree, the 30 ppm line in the 27 Al magic angle spinning nuclear magnetic resonance (MAS NMR) spectra of zeolites is a superposition of deformed tetrahedrally coordinated and penta-coordinated Al species.

13-P-11-Structure change of molecular sieve SAPO-37 at high temperature studied by 27Al MQ MAS NMR

Publisher Summary This chapter describes the structure change of molecular sieve SAPO-37 at high temperature studied by aluminium ( 27 Al) multiple-quantum magic angle spinning nuclear magnetic resonance (MQ MAS NMR). When SAPO-37 is calcined at 1173 K, silicon (Si) atoms become mobile and detached from the framework to aggregate to form polymorph silica while Al-phosphorus (P) dense phase is formed, corresponding to a new Al signal, which can be distinguished by the 27 Al MQMAS spectrum. The move of atoms in SAPO-37 is the start of the collapse of the framework.