Nitrided ultrastable zeolite Y: Identification and quantification of incorporated nitrogen species and their influence on the basic catalytic activity

Optimizing the Synthesis of Nitrogen-Substituted Zeolites

“…Vibrational spectroscopy is commonly used to study nitrided silica, zeolites, and aluminophosphates. ,− As such, we performed vibrational spectroscopy simulations and experiments in an effort to determine the utility of these methods for characterizing nitrided zeolites. The experiments focus on treatment G from Table , which we chose primarily because it has the least-changed X-ray pattern and adsorption isotherm but still shows substitution.…”

“…Much of this can be attributed to the vast array of synthesis conditions employed in different laboratories throughout the world and the lack of thorough understanding of the sensitivity of the substitution reaction to those conditions. For example, temperature treatment conditions used in the literature vary from as low as 200 °C using alkylamines − to as high as 1100 °C using ammonia. , Flow rates are similarly varied: from static conditions (for alkylamines − ) to flow rates of tens, − hundreds, , or even thousands of cubic centimeters per minute. A table summarizing the synthesis conditions we have extracted from the literature for crystalline and amorphous silicates and aluminophosphates is available in the Supporting Information.…”

“…Like the synthesis conditions, the variability in characterization methods so far employed is high. Most characterization relevant to nitrogen-substituted silicates and related materials has been carried out via X-ray diffraction, − which measures crystallinity; infrared ,− and Raman spectroscopy, which reveal vibrational characteristics; physical adsorption, , which gives an estimate of surface area (for non-microporous materials), pore volume, and pore size (especially for mesoporous materials); and nuclear magnetic resonance (NMR) spectroscopy, − ,− particularly 29 Si magic angle spinning (MAS) NMR for silicates and 27 Al and/or 31 P MAS NMR for aluminophosphates. We have shown in previous publications , that the 29 Si single-pulse MAS NMR spectrum can be used as an estimate of the extent of reaction (i.e., fractional substitution of oxygen sites) in the zeolite framework.…”

DFT Study of Nitrogen-Substituted FAU: Effects of Ion Exchange and Aluminum Content on Base Strength

“…Nitrided zeolites have been under study for over 40 years since the initial report by Kerr and Shipman in 1968 . Although recent work on nitrided zeolites has revealed interesting base catalytic activities, , the structures of nitrided zeolites and their active sites have remained only ambiguously identified. ,,,− By combining 29 Si solid-state NMR and quantum calculations of chemical shifts, we have shown that nitridation in high-silica H−Y (Si:Al = 15) is consistent with an intact zeolite framework and occurs first at Brønsted sites (Si−OH−Al) and subsequently at siliceous sites (Si−O−Si). ,, The story is qualitatively similar for nitridation of Na−Y (Si:Al = 2.4), although the diversity of nitrided sites in Na−Y is much higher because of the higher Al content. ,, Furthermore, through a combination of X-ray diffraction, 29 Si solid-state NMR, and high-resolution adsorption studies, we have recently reported an optimal synthesis approach for nitrided zeolites emphasizing the importance of high ammonia flow rates during heating and reacting steps. ,, Finally, density functional theory (DFT) calculations of the nitridation mechanisms in H−Y and silicalite zeolites have been used to predict that nitrided zeolites remain stable at high temperatures even at saturation water loadings . The question still remains how the Si:Al ratio and the nature of the charge-compensating cation impact strengths of basic sites in nitrided zeolites.…”

“…6 Although recent work on nitrided zeolites has revealed interesting base catalytic activities, 9,10 the structures of nitrided zeolites and their active sites have remained only ambiguously identified. 6,9,10,[12][13][14][15][16][17][18][19] By combining 29 Si solid-state NMR and quantum calculations of chemical shifts, we have shown that nitridation in high-silica H-Y (Si:Al ) 15) is consistent with an intact zeolite framework and occurs first at Brønsted sites (Si-OH-Al) and subsequently at siliceous sites (Si-O-Si). 12, 15,16 The story is qualitatively similar for nitridation of Na-Y (Si:Al ) 2.4), although the diversity of nitrided sites in Na-Y is much higher because of the higher Al content.…”