Preferred Al Locations in the Framework Structure of the Zeolite SSZ-82

Abstract

Zeolite SSZ-82 has a 2-dimensional pore system with 12-ring channels intersecting via 10-ring windows, and it is a potent catalyst in the isomerization of hydrocarbons. The location of Al in the framework structures of two differently prepared samples of Al-SSZ-82 with the same Al content has been investigated. Al-SSZ-82_IZC, was synthesized hydrothermally by interzeolite conversion of zeolite Y, using 1,6-bis(N-cyclo-hexylpyrrolidinium)hexane dications as organic structure directing agents. Then, the borosilicate molecular sieve, B-SSZ-82, was synthesized with the same organics, but its boron content (1.7 per unit cell) was lower than the Al content (2.6 per u.c.) in Al-SSZ-82_IZC. After calcination, Al-SSZ-82_exchanged was prepared by postsynthesis exchange of B by Al. The location of these heteroatoms was investigated using high-resolution synchrotron powder diffraction and ultrahigh field ²⁷Al solid-state NMR spectroscopy techniques The parent borosilicate shows preferred locations for boron and defect site vacancies in its tetrahedral framework structure. Al was also placed into defect site vacancies by the B/Al exchange reaction to yield an Al content of 2.6 per u.c. and leading to relative Al populations that differ from the B occupancies in the parent borosilicate. For both materials, Al-SSZ-82_IZC and Al-SSZ-82_exchanged, Al was found in the same T sites, T5, T10, and T11 with some minor contributions at T3/T4. However, the site occupancies were found to depend on the synthesis route. The different affinities of B, Al, and defects during the hydrothermal synthesis are discussed. Quantum-chemical methods were employed to study the charged defect site vacancies of as-made B-SSZ-82 at the T5 position, yielding predicted ¹H NMR chemical shifts in excellent agreement with the experimental value. This study addresses three questions with implications for future efforts toward a rational insertion of heteroatoms in zeolite catalysts: the location of the organic cations, the siting of heteroatoms, and defects in specific framework positions.