Post-Synthesis Conversion of Borosilicate Zeolite Beta to an Aluminosilicate with Isolated Acid Sites: A Quantitative Distance Analysis by Solid-State NMR

Abstract

Isolated acid sites were selectively generated by postsynthesis modification of a borosilicate zeolite beta. To this end, samples were prepared with pairs of adjacent boron sites balanced by Ca2+ ions, whereas isolated boron in the zeolite framework was balanced by NH4 + ions. To avoid undesired boron leaching, these ion exchange reactions were carried out in methanol solution rather than in water. Trigonal boron forms near the NH4 + ions by decomposing the latter into NH3 (and a proton), and selective extraction of the trigonal boron is achieved by water treatment, whereas the tetrahedral boron near Ca2+ ions remains in the zeolite framework. The vacancies were refilled with aluminum by treatment with an aqueous Al­(NO3)3 solution. Two Brønsted acid sites with 1H chemical shifts of 4.0 and 5.0 ppm exist in the dehydrated samples. 1H–27Al REAPDOR solid-state NMR measurements yield quantitative information on the local H–Al distances of isolated H–Al two-spin and H–Al–Al three-spin systems. The nearest H–Al distance is determined at 2.50 Å with an accuracy of 2% (or better) by fitting the oscillatory part of the REAPDOR curves, which was not observed before for zeolite acid sites. The second nearrest Al neighbors show a much larger distance of about 5 Å for the acid protons with a chemical shift of δ = 4 ppm. A second acid site at δ = 5 ppm has an approximately 50% occupation of a second Al neighbor at 3.73 Å, possibly within the same six-ring. This high resolution of dipolar interaction is not observed in a standard zeolite Al-beta prepared by direct synthesis. The method is suitable to identify well-defined local ordering in Al distributions of zeolites.

Dehydra-decyclization of 2-methyltetrahydrofuran to pentadienes on boron-containing zeolites

“…MCM-36 was prepared by swelling and pillaring of MCM-22 as reported by Maheshwari et al [43] (detailed procedure is included in Section S1.1 in the SI). Boroncontaining zeolites in three different frameworks MWW, [26] BEA, [44,45] and MFI [24] were synthesized modifying existing hydrothermal synthesis procedures, and detailed steps are included in Section S1.1 in the SI. All catalysts were calcined in a boat placed within a 1" quartz tube under air flow at 823 K using a ramp rate of 2 K min -1 for 10 hours prior to any catalytic testing.…”

Tuning the Aluminum Distribution in Zeolites to Increase their Performance in Acid‐Catalyzed Reactions

“…1 H– 1 H homonuclear correlation experiments are significantly more sensitive ( 1 H is not a quadrupolar nucleus) and simpler than 27 Al– 27 Al measurements . Moreover, this type of experiment gives information on the parameter that is directly connected to catalysis—the distance of the Brønsted acid sites of the bridging AlOHSi groups .…”

“…1 H– 1 H homonuclear correlation experiments are significantly more sensitive ( 1 H is not a quadrupolar nucleus) and simpler than 27 Al– 27 Al measurements . Moreover, this type of experiment gives information on the parameter that is directly connected to catalysis—the distance of the Brønsted acid sites of the bridging AlOHSi groups . Although this experiment does not require specific instrumentation (spectrometers with a 1 H Larmor frequency of 400 MHz seems to be fully sufficient), this type of experiment must be performed on fully dehydrated zeolites.…”

Dehydra-Decyclization of 2-Methyltetrahydrofuran to Pentadienes on Boron-Containing Zeolites

“…MCM-36 was prepared by swelling and pillaring of MCM-22 as reported by Maheshwari et al [43] (detailed procedure is included in Section S1.1 in the SI). Boroncontaining zeolites in three different frameworks MWW, [26] BEA, [44,45] and MFI [24] were synthesized modifying existing hydrothermal synthesis procedures, and detailed steps are included in Section S1.1 in the SI. All catalysts were calcined in a boat placed within a 1" quartz tube under air flow at 823 K using a ramp rate of 2 K min -1 for 10 hours prior to any catalytic testing.…”