Spectroscopic Characterization of Hydroxyl Groups in SAPO-40. 1. Study of the Template-Free Samples and Their Interaction with Ammonia

Abstract

FTIR, MAS 1H-NMR, and molecular graphics data are reported concerning the hydroxyl species in a series of SAPO-40 samples with variable Si contents, as well as FTIR data concerning the interaction of ammonia with the different acid sites. Up to seven types of Si(OH)Al Brønsted sites are present, mainly differing by the O−H stretching frequency. Three high-frequency bands (3637−3624 cm-1) correspond to hydroxyls located in the 12-member channels and two low-frequency bands (3559 and 3528 cm-1) to OH species in eight-member rings and side pockets of the AFR structure. Two bands at exceptionally low frequency (3485 and 3445 cm-1 ) are due to OH species interacting with silica islands. All low-frequency bands behave with temperature as H-bonded species. All types of hydroxyls form ammonium ions, the thermal decomposition of which shows some difference in acidity among the OH species. IR evidence indicates that NH4 + species interact strongly with the basic oxygen atoms of the lattice. No Lewis sites were evidenced by ammonia adsorption.

Role of Brønsted acidity in NH3 selective catalytic reduction reaction on Cu/SAPO-34 catalysts

“…In this work, the bands at 3347, 3286, 1496, and 1465 cm À1 were detected in the spectra of all the catalysts, which are assigned as the symmetric and asymmetric N-H stretching modes and asymmetric vibration of NH 4 + adsorbed on the Brønsted acid sites (r as (NH 4 + )), respectively [27] (see Table 4). Generally, the NH 3 adsorption spectra of K/SAPO-34 showed lower intensity than that of H/SAPO-34.…”

“…A similar effect of potassium on the NH 3 adsorption at the Brøn-sted acid sites was found in the series of Cu-(K)/SAPO-34 catalysts. With increasing potassium content, the negative bands at 3600 and 3627 cm À1 assigned as the Brønsted OH groups gradually decreased in intensity and eventually disappeared, meanwhile the bands at 1496 and 1465 cm À1 (r as (NH 4 + )), as well as those at 1035 and 1002 cm À1 , decreased in intensity [29][30]. For the Cu sample without any potassium exchanged, the adsorbed ammonia on copper species showed a band at 3179 cm À1 , corresponding to Cu + -NH 3 species on the exchanged sites [12,31].…”

On the reactive adsorption of ammonia on activated carbons modified by impregnation with inorganic compounds

“…12A). For instance, N-H deformation bands from coordinated NH 3 are seen at $1600 cm À1 and $1200 cm À1 [57,71,72]. New broad bands are also observed in the range 3200-3700 cm À1 for the samples run in moist conditions (EM, EPD and EPM).…”

Potassium poisoning on Cu-SAPO-34 catalyst for selective catalytic reduction of NOx with ammonia

“…adsorbed onLewis acid sites (σ as (NH 3 )), while bands in the range 1390-1496 cm −1 are assigned to NH 4 + adsorbed on the Brønsted acid sites (σ as (band of the tetrahedral ammonium ion[24] gradually merge into one broad band with the increasing potassium content. In addition, their intensities are decreasing after the Cu-SAPO-34 catalyst was impregnated with the increasing potassium content.…”