9 research outputs found
Studies of Aluminum Reinsertion into Borosilicate Zeolites with Intersecting Channels of 10- and 12-Ring Channel Systems
The work here describes the kinetic analyses of aluminum replacement for boron in a suite of borosilicate molecular sieves. While the method has been described before as a means of converting synthesized borosilicates (with weak inherent acidity) to aluminosilicates (with much stronger acid strength) when there are large pores in the structure, here we carry out the transformation under less than optimal replacement concentrations, in order to better follow the kinetics. We examined several zeolite structures with boundary conditions of boron MEL where there are only 10-ring (or intermediate) pore structures and no Al is taken up, to multidimensional large pore zeolites, like boron beta, where Al substitution can occur everywhere. We also studied materials with both intermediate and large pores, SSZ-56, 57, 70, and 82. In the case of 57 up to 90% of the structure is made up of boron MEL. We observe that the pH drop is proportional to the Al reinsertion and is the same for all zeolites we studied. In one case, we compared a zeolite (SSZ-24) with boron and then no boron sites and found that Al does not go into defect sites. It was again confirmed (shown in earlier work) that Al will go into nest sites created by boron hydrolysis out of the substrate before Al treatment. Along those lines we also made two new observations: (1) the profile for Al uptake, as followed by pH drop, is the same kinetically, whether the boron is there or not; and (2) NMR showed that the boron is leaving the structure faster than Al can go back in (SSZ-33 study), even when we treat a material with boron in the lattice
Studies of Aluminum Reinsertion into Borosilicate Zeolites with Intersecting Channels of 10- and 12-Ring Channel Systems
The work here describes the kinetic analyses of aluminum replacement for boron in a suite of borosilicate molecular sieves. While the method has been described before as a means of converting synthesized borosilicates (with weak inherent acidity) to aluminosilicates (with much stronger acid strength) when there are large pores in the structure, here we carry out the transformation under less than optimal replacement concentrations, in order to better follow the kinetics. We examined several zeolite structures with boundary conditions of boron MEL where there are only 10-ring (or intermediate) pore structures and no Al is taken up, to multidimensional large pore zeolites, like boron beta, where Al substitution can occur everywhere. We also studied materials with both intermediate and large pores, SSZ-56, 57, 70, and 82. In the case of 57 up to 90% of the structure is made up of boron MEL. We observe that the pH drop is proportional to the Al reinsertion and is the same for all zeolites we studied. In one case, we compared a zeolite (SSZ-24) with boron and then no boron sites and found that Al does not go into defect sites. It was again confirmed (shown in earlier work) that Al will go into nest sites created by boron hydrolysis out of the substrate before Al treatment. Along those lines we also made two new observations: (1) the profile for Al uptake, as followed by pH drop, is the same kinetically, whether the boron is there or not; and (2) NMR showed that the boron is leaving the structure faster than Al can go back in (SSZ-33 study), even when we treat a material with boron in the lattice
Guest/Host Relationships in the Synthesis of the Novel Cage-Based Zeolites SSZ-35, SSZ-36, and SSZ-39
Here, we report the synthesis and structure of three high-silica molecular sieves, SSZ-35, SSZ-36, and SSZ-39, that are prepared from a library of 37 different cyclic and polycyclic quaternized amine molecules that are used as structure-directing agents (SDAs). The size and shape of the quaternized amine molecules are purposely designed in order to obtain novel zeolite structures, and the synthesis of these molecules is presented. The selectivity for the three molecular sieve phases is found to depend on both the SDA and the degree of heteroatom lattice substitution of Al^(3+) or B^(3+) in the silicate framework. Molecular modeling is utilized to probe the effects of the nonbonded SDA/zeolite-framework interaction energy on the selectivity for the observed molecular sieve phase. The Rietveld refinement of the powder X-ray data confirms the structure of the SSZ-39 zeolite to be isomorphous with the aluminophosphate molecular sieve, SAPO-18 (AEI). The structure of SSZ-36 is found to possess a range of fault probabilities between the two-dimensional channel system, end-member polymorphs, ITQ-3 and RUB-13 (International Zeolite Association Codes ITE and RTH, respectively). The SSZ-35 structure is reported to contain a one-dimensional pore system possessing stacked cages circumscribed by alternating rings of 10 and 18 tetrahedral atoms (10- and 18-membered rings)
Locating Organic Guests in Inorganic Host Materials from X‑ray Powder Diffraction Data
Can the location
of the organic structure-directing agent (SDA)
inside the channel system of a zeolite be determined experimentally
in a systematic manner? In an attempt to answer this question, we
investigated six borosilicate zeolites of known framework structure
(SSZ-53, SSZ-55, SSZ-56, SSZ-58, SSZ-59, and SSZ-60), where the location
of the SDA had only been simulated using molecular modeling techniques
in previous studies. From synchrotron powder diffraction data, we
were able to retrieve reliable experimental positions for the SDA
by using a combination of simulated annealing (global optimization)
and Rietveld refinement. In this way, problems arising from data quality
and only partially compatible framework and SDA symmetries, which
can lead to indecipherable electron density maps, can be overcome.
Rietveld refinement using geometric restraints were then performed
to optimize the positions and conformations of the SDAs. With these
improved models, it was possible to go on to determine the location
of the B atoms in the framework structure. That is, two pieces of
information that are key to the understanding of zeolite synthesisî—¸the
location of the organic SDA in the channel system and of the positions
adopted by heteroatoms in the silicate frameworkî—¸can be extracted
from experimental data using a systematic strategy. In most cases,
the locations of the SDAs determined experimentally compare well with
those simulated with molecular modeling, but there are also some clear
differences, and the reason for these differences can be understood.
The approach is generally applicable, and has also been used to locate
organic guests, linkers, and ligands in metal–organic compounds
Computationally Guided Synthesis of SSZ-52: A Zeolite for Engine Exhaust Clean-up
Computationally Guided Synthesis of SSZ-52: A Zeolite
for Engine Exhaust Clean-u