Search

We study the effects of loading and nature of the zeolitic framework in the trans ⇌ gauche equilibrium of 1,1,2-trichloroethane (TCE) in zeolites with Faujasite (FAU) structure including sodium Y (Na-Y) and siliceous Y (Si-Y). TCE is used as a prototype molecule for the study of zeolite–chlorinated hydrocarbon interactions that determine conformational equilibrium. Conformational changes of TCE were sampled using empirical force fields and quantum mechanics/molecular mechanics (QM/MM) molecular dynamics (MD) in combination with replica exchange molecular dynamics (REMD). The population ratio of the conformers at high loading was compared with that found by Fourier transform (FT)-Raman spectroscopy and the results are in very good agreement. Comparison between the infinitely dilute limit of loading, and higher loading shows that the population ratio of the conformers at equilibrium depends strongly on the Si/Al ratio and consequently, the presence of charge compensating cations as well as the loading. At very low loading, in both Na-Y and Si-Y the population of the trans conformer is the largest with the exception of the empirical modeling that predicts the gauche conformer to be dominant in Na-Y. At the higher loading, in Na-Y the population the gauche conformer constitutes the great majority. In Si-Y the population of the trans conformer continues to dominate but the percentage of the gauche conformer increases substantially relative to the loading at the infinitely dilute limit. Such different conformational behaviors provide insight into the factors responsible for diffusion mechanisms and adsorption energies in the FAU-type zeolitic frameworks.

Molecular sieves have been scientifically and industrially important materials since their discovery. These porous, crystalline networks of tetrahedrally coordinated atoms bridged by oxygen are known as excellent catalysts. The framework of the aluminophosphate family of molecular sieves is itself non-catalytic, but through heteroatomic substitution these materials can diversify into, for example, the silicoaluminophosphate (SAPO) family of heterogeneous acid catalysts. Research into the methods by which these substitutions occur, as discoveries can lead to refined control over material property and function. SAPO-5 was chosen as model system of study. 27 Al, 29 Si and 31 P solid state NMR and PXRD were used as the primary methods of investigation. The SAPO-5 study determined that the dry gel conversion (DGC) methods of steam assisted conversion (SAC) and vapor phase transport (VPT) follow similar reaction pathways. After an initial difference caused by the lack of organic structure directing agent in the gel for the VPT method, crystallization is similar between methods, and silicon incorporation results in two populations of silicon sites. One represents isolated Si(OAl) 4 species well incorporated into the framework, while the other represents silica islands within the framework. The initial core of the nuclei is AlPO 4 in nature and the Si is gradually incorporated into the lattice as the crystals grow. The vapor phase silicon uptake (VPSU) method provides a contrast to this data, inducing nucleation that occurs concurrently with Si incorporation and producing only isolated Si(OAl) 4 species.

The crystallization of microporous material SAPO-37 was studied under hydrothermal synthesis (HTS) and dry gel conversion (DGC) conditions. The evolution of the intermediate phases as a function of crystallization time was followed by several techniques including PXRD, SEM, AFM, and solid-state NMR. For DGC, SAPO-37 crystallizes from a semi-crystalline layered precursor containing large pores and sodalite cages, whose structure is held by weak nonbonding interactions. Si atoms are only incorporated in the later stage of the crystallization via the formation of aluminosilicate species. Under HTS conditions, the crystallization pathway of SAPO-37 is somewhat similar to that for DGC. The AFM data suggest that the crystal growth mechanism is “birth and spread”, and the final SAPO-37 crystals have rough (1 1 1) surfaces terminated by different structural building units. The findings of the present work provide new physical insights into the formation of SAPO-37.

The pressure-induced anomalously high AC conductivity of zeolite NaA mixed with water was measured by impedance spectroscopy at various water/zeolite ratios. The conductivity reaches a maximum value at 0.9–1.4 GPa with increasing water content of up to 20 wt.% in NaA + water. The high ionic conductivity of NaA compressed in water is interpreted to result from cation diffusion in the quasi-liquid Na+–H2O intra-channel system enhanced by the filling of additional H2O sites. The pressure range of anomalously high AC conductivity in NaA enlarges towards higher pressures with decrease of the water/zeolite ratio. An abrupt drop in conductivity observed at P > 1.5 GPa appears to be associated with freezing of intra-channel water. After pressure release, the overhydrated state of the zeolite kept in a closed container can be preserved for a long time, providing abnormally high conductivity which is comparable with that of Na+ β-alumina.

The formation of microporous molecular sieve AlPO4-18 under the dry gel conversion (DGC) conditions was examined. The evolution of the dry gel was monitored by powder XRD, SEM and, in particular, solid-state NMR techniques. The results show that the aluminophosphate (AlPO) amorphous dry gel is first converted to a layered AlPO phase. The layered intermediate has some resemblances to AlPO4-18 and is held together by very weak non-bonding interactions. Continuous heating results in the transformation of this layered phase into AlPO4-18 via reorganizations and condensations of P and Al sites. Three non-equivalent P sites of AlPO4-18 develop at different rates during the evolution.

Several Si and Co substituted AlPO4-based molecular sieves with AFI, AEL and CHA structures including SAPO-n and CoAPO-n (where n = 5, 11 and 34) have been synthesized under dry gel conversion (DGC) conditions by using the pre-prepared AlPO-based precursors which have been demonstrated previously to be structurally related to the target molecular sieves. We show that highly crystalline SAPO-n and CoAPO-n can be prepared by this approach. Unlike conventional DGC method where the Si source is mixed with P and Al sources at the very beginning to form an initial amorphous dry gel, in this work the Si is slowly introduced to the surface of the dry powder of the semi-crystalline or crystalline AlPO-based precursor via vapor phase, and subsequently incorporated into the framework gradually. The Si environments in the SAPOs prepared by this approach appear to be more diversified compared to those in the corresponding sieves made by the normal hydrothermal synthesis (HTS) method. One feature of this synthesis is that the Si substitution in the lattice may only occur at some specific positions. This is due to the fact that since certain P sites in the precursors are directly carried into the framework of the final sieves, they are not available for Si incorporation. This finding may have implication to the locations of the catalytic sites in the SAPO-based catalysts. Similarly, for CoAPO-n, the Co source was mixed with the preformed AlPO-based precursor before DGC treatment. CoAPO-5 has a higher degree of the Co incorporation than that of the CoAPO-5 made under normal HTS conditions.

The adsorption of toluene inside zeolite Na–Y was investigated by solid-state NMR spectroscopy. The environment of Na + ions at different sites in Na–Y before and after adsorption was characterized by 23 Na magic-angle spinning (MAS) NMR. The information on the dynamic behavior of guest molecules inside the supercage of Na–Y was obtained by analyzing wideline 2 H NMR spectra. The effect of loading level and temperature on molecular dynamics was also examined. The cation–sorbate interactions were directly probed by 23 Na{ 1 H} rotational-echo double-resonance (REDOR) experiments at different temperatures. Molecular Monte Carlo simulations were also performed to assist in the interpretation of the NMR data. 23 Na MAS and 23 Na{ 1 H} REDOR results show that each toluene molecule is facially coordinated to a Na + ion at the SII site in the supercage, forming a π-complex. The adsorption also causes the Na + ions initially located at the SI′ site to slightly shift to a new position within the sodalite cage, but has little effect on the Na + at the SI site. The 2 H NMR results indicate that the toluene molecules undergo a 2-site flip around the molecular long axis in addition to the methyl group rotation about its C 3 axis. 23 Na MAS spectra suggest that the adsorptive behavior of benzene and p -xylene in Na–Y is similar to that of toluene/Na–Y. 23 Na{ 1 H} REDOR results further indicate that inside the supercage, the degree of molecular motion follows the order of benzene > toluene > p -xylene.

We have used several solid-state NMR techniques in conjunction with powder X-ray diffraction (XRD) and Raman spectroscopy to characterize the structure of the intermediate amorphous phases of AlPO 4 -5 molecular sieve synthesis. The evolution of the gel phases as a function of crystallization time was followed by 31 P and 27 Al magic-angle spinning (MAS) NMR to obtain information on the local environments of Al and P atoms and by powder XRD to detect the long-range ordering of the gel samples. We have utilized 27 Al/ 31 P double-resonance techniques such as heteronuclear correlation spectroscopy (HETCOR) to select the 31 P–O– 27 Al bonding connectivities and transfer of populations by double-resonance (TRAPDOR) to obtain information regarding the degree of condensation for P sites in the amorphous phases. The 1 H → 31 P cross-polarization (CP) was used to identify the P sites with hydroxyl groups attached. Raman spectroscopy was also employed to follow the evaluation of the pore system. The 13 C CP MAS spectra provide information regarding the chemical environment of template molecule in the gel. The work shows that a combination of the above-mentioned techniques can provide more detailed information on the structure of amorphous phases formed during hydrothermal synthesis.

We have examined the adsorption of the phosphine stabilized cluster Cu 6 (TePh) 6 (PPh 2 Et) 5 as well as the free ligands ethyldiphenylphosphine (EDPP) and ethyldiphenylphosphine oxide (EDPPO) in several MCM-41 types of mesoporous materials by 31 P solid-state NMR spectroscopy. The results indicate that upon loading into siliceous MCM-41 (Si-MCM-41), the phosphine shell of the copper cluster dissociates from the metal core. After dissociation, some EDPP molecules exist as physisorbed species and a significant amount of phosphine is oxidized to the corresponding phosphine oxide, occluded in the channels of Si-MCM-41. The reaction seems to be catalyzed by the Cu(I) species since direct loading of EDPP under the same conditions does not yield any phosphine oxide in the framework of Si-MCM-41. The 31 P NMR spectra of EDPP and EDPPO adsorbed in Si-MCM-41 suggest that neither Bronsted nor Lewis acid sites exist on the channel surface of Si-MCM-41. The adsorption of EDPP and EDPPO in the acidic form of aluminium-containing MCM-41 (Al-MCM-41), however, does indicate the interactions between the guest species and three Bronsted acid sites with different strengths. No Lewis acid site was observed in Al-MCM-41.

The adsorption of three ( p -, m -, o -) xylene isomers inside zeolite Y with different charge balancing cations including Na + , K + , Ba 2+ was studied by FT-Raman spectroscopy. Our Raman data indicate the formation of a π-complex between the xylene and cation as evidenced by observing the weakening of the C–C bonds in the aromatic ring. The strong interaction between a sorbate and a cation results in that the methyl groups of xylene are forced to interact with the local zeolite framework, namely, the six-membered ring that plays host to the cation, leading to an increase in vibrational frequency of methyl groups, upon adsorption. For a given cation, no significant differences were observed for the strength of interaction with any of the three isomers at a loading of 1 molecule per supercage.

Most of the applications of zeolites involve incorporating guest species into the framework. It is important to obtain structural knowledge of these sorbate/sorbent complexes. In this work, we outline the applications of FT-Raman spectroscopy in several areas of host–guest interactions in zeolitic systems such as detecting sorbate-induced phase transitions, investigating the conformational properties of guest species, probing the location of the guest molecules inside the zeolite and examining the cation–sorbate interactions.

The present communication demonstrates, for the first time, that 31 P/ 27 Al double resonance solid-state NMR experiments such as cross-polarization (CP) can be used to map out the connectivity in the intermediate gel phases of molecular sieve synthesis. Our work clearly highlights the advantages of using Al/P CP techniques for characterizing the gel phases.

The host–guest interactions of two related sorbates, p-dibromobenzene (pdbb) and bromobenzene, adsorbed on completely siliceous ZSM-5 have been examined by FT-Raman spectroscopy. For the pdbb/ZSM-5 system, a phase transition was identified at a loading of 4 molecules per unit cell (u.c.). The results suggest that the adsorption and phase transition behavior of pdbb/ZSM-5 are similar to those of p-dichlorobenzene/ZSM-5 and p-xylene/ZSM-5 except that the maximum loading level is 7 molecules per u.c. rather than 8 molecules per u.c. For bromobenzene, a sorbate-induced phase transition also occurred at a loading of 4 molecules per u.c. This is different from chlorobenzene which induces two successive transitions in the ZSM-5 framework upon adsorption.

In this paper, the vibrational spectra of kanemite are measured. The fact that infrared and Raman spectral bands are not coincident indicates that kanemite belongs to a space group with a center of symmetry. The observation of an IR band at 1167 cm −1 in kanemite suggests the existence of Si–O–Si linkages with a bond angle of approximately 180°. The strong Raman band at 465 cm −1 in the kanemite spectrum is associated with the six-membered rings of [SiO 4 ] tetrahedra. The strong Raman band at 1060 cm −1 is consistent with the stretching vibrations of Q 3 species (νSiO − ). The broad band centered at 1015 cm −1 is assigned to the Q 3 units involved in strong hydrogen bonding. Hydrogen bonding occurring in kanemite is also evident from a broad OH stretching band (3463 cm −1 ) observed in the IR spectrum. However, not all hydroxyl groups are involved in hydrogen bonding since a sharp OH stretching signal is observed at 3582 cm −1 in IR spectrum.