The formation and growth of tetrapropylammonium-templated crystal nuclei of silicalite from a clear homogeneous solution was recorded in situ as a function of time at temperatures between 90 and 115 °C using small-angle X-ray scattering (SAXS). The kinetics of the nucleation process was studied and give an apparent activation energy of 70 kJ mol-1. A small-angle neutron scattering (SANS) contrast variation study confirms that the nuclei contain the tetrapropylammonium template in the expected stoichiometry of the completed crystal. Infrared spectroscopy provides evidence that the silicalite framework structure is present in the nuclei. Our SAXS and SANS data both show that a cylindrical form factor gave the best fit to the measured scattering functions for the particles that developed from nucleation to the end of the induction period. Dynamic light scattering measurements were used to follow the crystal growth in the 100−6000 Å size domain, beyond the range of the SAXS measurements. Scanning electron microscopy was also used to determine crystal morphology and particle sizes of sedimented crystals and freeze-dried solutions. We have proposed a detailed model for the nucleation and crystallization processes wherein cylindrical primary nuclei, 2 × 2 unit cells in cross section, form very quickly upon heating and then assemble end-to-end along the crystal c axis into 330 Å long primary crystallites during an extended induction period, followed by the aggregation of the primary crystallites into polycrystalline ellipsoids of length 6000 Å.

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Protolytic cracking of ethane in zeolites has been investigated using quantum-chemical techniques and a cluster model of the zeolite acid site. An aluminosilicate cluster model containing five tetrahedral (Si, Al) atoms (5T) was used to locate all of the stationary points along a reaction path for ethane cracking at the HF/6-31G(d), B3LYP/6-31G(d), and MP2(FC)/6-31G(d) levels of theory. The cracking reaction occurs via a protonated structure that is a carbonium-like ion and is a transition state on the potential energy surface. The activation barrier for cracking calculated at each level of theory was refined by including (i) vibrational energies at the experimental reaction temperature of 773 K, (ii) electron correlation and/or an extended basis set at the B3LYP/6-311+G(3df,2p) or MP2(FC)/6-311+G(3df,2p) levels, and (iii) the influence of the surrounding zeolite lattice from a 58T cluster model of the zeolite H-ZSM-5. The barrier is especially sensitive to the long-range electrostatic effect of the lattice, which reduces it by 14.5 kcal/mol from the value obtained with the 5T cluster. The final calculated barrier of 54.1 kcal/mol at the MP2(FC)/6-311+G(3df,2p)//MP2(FC)/6-31G(d) level, including corrections, is significantly smaller than values obtained by previous theoretical studies and is in reasonable agreement with typical experimental values for short alkanes. The other levels of theory give similar values for the barrier.

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A critical analysis is given of the EPR spectrum exhibited by the rare-earth S-state ions, Gd3+ and Eu2+, in glassy and disordered polycrystalline materials. The analysis of this spectrum and of its previous interpretations is based on (a) a set of criteria derived from a wide range of experimental EPR and optical data, and (b) a first principles computer simulation method which explicitly incorporates broad distributions in the crystal field interaction parameters. It is found that all four previous interpretations of the glassy spectrum are unsatisfactory, each failing to satisfy two or more of the criteria imposed by the full range of data. The correct general solution to the spectrum is unequivocally established and shown to be a convolution of (a) a broad and essentially unimodal distribution of second-order crystal field parameters, b02, with a maximum in the approximate range 0.051≲b02 ≲0.056 cm−1, and (b) a broad distribution of asymmetry parameters, λ′=b22/b02, with appreciable probability over the whole range 0.0≤ λ′≤1.0. The prominent features in the X-band spectrum at g∼6.0 and 2.8 are found to be the result of specific EPR transitions that are stationary with respect to b02, λ′, and the orientation angles of the applied field H over a wide range. The quantitive results indicate that the site symmetries of the RE ions are essentially very low and disordered, and are best characterized by a single low-symmetry ‘‘glassy type’’ site.

Ab initio molecular orbital calculations using Hartree−Fock theory and Møller−Plesset perturbation theory have been used to study the interaction of H2O with the Brønsted acid site in the zeolite H-ZSM-5. Aluminosilicate clusters with up to 28 T atoms (T = Si, Al) were used as models for the zeolite framework. Full optimization of a 3 T atom cluster at the MP2/6-31G(d) level indicates that the “ion-pair” structure, Z-···HOH2 +, formed by proton transfer from the acid site of the zeolite (ZH) to the adsorbed H2O molecule, is a transition state, while the “neutral” adsorption structure, ZH···OH2, is a local energy minimum. Partial optimization of a larger 8 T cluster at the HF/6-31G(d) level also gave results suggesting that the ion-pair structure is a transition state. Calculations were carried out to obtain corrections for high levels of theory, zero-point energies, and larger cluster size. The resulting energy difference between the neutral and ion-pair structure is small (less than 5 kcal/mol and possibly close to zero). The interaction energy of ZH···OH2 is 13−14 kcal/mol, in agreement with experiment. We find that addition of a second H2O molecule to Z-···HOH2 + in the 3 T atom cluster stabilizes the ion-pair structure, Z-···H(OH2)2 +, making it a local energy minimum. Finally, calculated vibrational frequencies for a 3 T atom cluster are used to help interpret experimental IR absorption spectra.