Regeneration of Cu/SAPO-34(MO) with H₂O only: too good to be true?

Abstract

Illustration of the proposed mechanism for deactivation and regeneration of Cu/SAPO-34(MO) with H2O.

Destructive and Protective Effects of NH3 on the Low-Temperature Hydrothermal Stability of SAPO-34 and Cu-SAPO-34

“…When checking the formation of terminal Si–OH by 1 H– 29 Si CP MAS NMR (Figure S4), it is discovered that even the most severe aging condition, i.e., aging with gaseous NH 3 , does not lead to a remarkable increase in the intensity of Si–OH signal. It may be explained by that the condensation of Si–OH to Si islands consumes Si–OH. , So the significantly increased amount of terminal hydroxyls revealed by NH 3 -TPD and DRIFTS (Figures d and a) should be attributed to Al–OH and P–OH. Moreover, the SAPO-34 framework has the ability of self-healing that vacancies caused by desilication can be refilled by extra-framework Al and P, by which the structure integrity of SAPO-34 can be maintained .…”

“…Although NH 3 and H 2 O compete for the strong adsorption sites (i.e., Si–O­(H)–Al), the weakly adsorbed ones promote each other (Figure ). Besides NH 4 + acting as a H 2 O adsorption site, the ammoniation and/or NH 3 -catalyzed hydrolysis of P–O–Al bonds (Figure ) largely enhances the hydrophilicity of the SAPO-34 framework. ,,, Water molecules condense as liquid-like clusters inside zeolite pores, ,,, in which NH 3 can be captured by hydrogen bonds, or in other words, “dissolved”, so the amount of weakly adsorbed NH 3 largely increases. Indeed, such a type of H-bonded NH 3 is confirmed by 1 H MAS NMR (Figure ), so the molecularly adsorbed NH 3 detected by DRIFTS (Figure ) is not merely on Lewis acid sites of SAPO-34 (e.g., framework and extra-framework Al) but mostly combined with adsorbed H 2 O (acting as a Lewis acid).…”

“…The degradation mechanisms of Cu-SAPO-34 upon exposure to moisture are understood as the hydrolysis of the SAPO-34 framework, aggregation of substituted Si to “Si islands” without acidity and transformation of active Cu­(II) ions (i.e., Z 2 Cu and ZCuOH, Z representing framework negatively charged [Si–O–Al] − site) to other inactive Cu species. ,,− Focusing on the fatal hydrolysis of the SAPO-34 framework, several attempts have been reported to minimize the density of vulnerable Si–O­(H)-Al bonds and protect them from hydrolysis, including tuning the distribution of substituted Si atoms ,− and employing cocations (e.g., Nd 3+ , Ce 3+ , Ag + , and NH 4 + ) to neutralize Si–O­(H)–Al acid sites. − Among these cocations, NH 4 + may be the most practicably feasible one since it can be automatically produced via adsorbing gaseous NH 3 during SCR operation and participate in SCR after a cold start. Indeed, Cao et al have reported that NH 4 + loaded by 200–300 °C preadsorption shows a notable protection effect upon Cu-SAPO-34 against low-temperature hydrothermal aging, attributed to inhibited framework hydrolysis and hereby preserved isolated Cu­(II) ions, while decreased NH 4 + coverage with increased preadsorption temperature leads to a decline in the protection effect.…”

One‐pot facile synthesis of Cu‐SAPO‐34 via addition of tetraethylenepentamine and its enhanced catalytic activity in selective catalytic reduction of NOx with NH3

“…5(A), the Si signals at −91, −95, −100, −105 and −110 to −120 ppm are assigned to Si4Al, Si3Al1Si, Si2Al2Si, Si1Al3Si and Si4Si species, respectively. And the signals at −80 and −88 ppm are assigned to the structure defects for Cu‐SAPO‐34 16,32,33 . The percentage composition of silicon species was calculated by fitting deconvolution of 29 Si NMR spectra and the results are listed in the supporting information (Table S4).…”

“…And the signals at −80 and −88 ppm are assigned to the structure defects for Cu-SAPO-34. 16,32,33 The percentage composition of silicon species was calculated by fitting deconvolution of 29 Si NMR spectra and the results are listed in the supporting information (Table S4). These catalysts show similar Si species, but fewer Si islands and more defects.…”