Infrared spectroscopic investigation of hydroxy-group siting in H faujasites

Abstract

The total hydroxy-group and the separate high-frequency (h.f.) and low-frequency (1.f.) hydroxy-group concentrations in H faujasites, observed by i.r. spectroscopy and corroborated by other methods, show a dependence of the maximum occupation of h.f. OH groups on the Si/AI ratio of between 15 and 32 OH groups per unit cell for zeolites HY and HX, respectively. In the case of HX zeolites above 70% exchange, decomposition is unavoiable. As a result of band separation the separate amounts of seven kinds of h.f. OH subspecies have been obtained from measurements of their i.r. absorbance and the integrated extinction coefficients of the h.f. OH stretching band for HX and HY zeolites. These subgroups show the same i.r. sub-bands in both HX and HY zeolites, but of different intensities. This may be explained on the basis of different Si/AI ordering schemes by 0 , H groups bonded to silicon atoms, surrounded by I.R. STUDY OF H-FAUJASITES group12 and more than one h.f. OH group, as has been shown by i.r. spectroscopic splitting of the h.f. OH stretching vibration into l 4 o r more sub-bands.', In deep-bed activated DY zeolites (apparently in the ultrastable form, as indicated by large bands at 2760, 2725 and 2670 cm-l) a splitting of the OD stretching vibration into two sub-bands was also 0bser~ed.l~Although the concentrations of individual portions of h.f. and 1.f. OH groups were determined by other authors for some H forms of f a u j a ~i t e s , ~~~ 2o the individual concentrations of OH subspecies (calculated from the split h.f. OH stretching bands) are still unknown. Furthermore, the individual h.f. and 1.f. OH concentrations of the H faujasites were determined by different methods of pyridine adsorptionlg? 2o or using different assumptions concerning overlapped h.f. and 1.f. OH stretching bands, and there are still unanswered questions concerning the dependence of the h.f. OH concentrations on the Si/AI ratios in H faujasites. In order to estimate the location and quantitative distribution of the h.f. OH subgroups we have investigated various HXand HY-type zeolites by means of i.r. spectroscopy.

Heterogeneity of Brønsted Acidic Sites in Faujasite Type Zeolites due to Aluminum Content and Framework Structure

“…Some authors believe that the HF band can be resolved into as many as seven submaxima, each belonging to an O 1 H group with different Al environments. 9 The 1 H NMR spectrum of the HY zeolite with Si/Al ) 2.6 shows a line at 3.9 ppm and a shoulder at 4.6 ppm which are assigned to O 1 H and O 3 H groups, respectively. 10 The O 1 H peak shifts with increasing Si/Al ratio from 3.8 ppm (Na-HX zeolite, Si/Al ) 1.2) to 4.4 ppm (HY zeolite, Si/Al ) 10) but remains constant for Si/Al > 10.…”

“…These bands shift to lower frequencies with increasing Si/Al ratio. , It has been suggested that on adsorption of aromatic hydrocarbons the HF band (O 1 H) of Na−HY with Si/Al = 2.56 splits into three submaxima, indicating the presence of O 1 H sites with different properties. Some authors believe that the HF band can be resolved into as many as seven submaxima, each belonging to an O 1 H group with different Al environments . The 1 H NMR spectrum of the HY zeolite with Si/Al = 2.6 shows a line at 3.9 ppm and a shoulder at 4.6 ppm which are assigned to O 1 H and O 3 H groups, respectively .…”

Structure of the Hydroxyl Groups and Adsorbed D2O Sites in the DX Zeolite: DFT and Experimental NMR Data

“…Moreover, Si-O1H-Al groups in zeolites of faujasite type were found to be heterogeneous, and hydroxyls of various acid strength exist. [9][10][11] The acid strength of (AlO) n (SiO) 3−n Si-O1H-Al(OSi) 3 was found to decrease with number n i.e. the number of Al atoms in close vicinity of OH group (chemical factor).…”

“…This last effect was explained by the higher contribution of Si atoms with electronegativity greater than that of Al. Moreover, Si–O1H–Al groups in faujasite-type zeolites were found to be heterogeneous, and hydroxyls of various acid strength exist. − The acid strength of (AlO) n (SiO) 3– n Si–O1H–Al­(OSi) 3 was found to decrease with number n , i.e., the number of Al atoms in close proximity to the OH group (chemical factor). As 29 Si MAS NMR evidenced the presence of Si­(0Al), Si­(1Al), Si­(2Al), and Si­(3Al), three kinds of hydroxyls [(SiO) 3 Si–O1H–Al­(OSi) 3 , (AlO) (SiO) 2 Si–O1H–Al­(OSi) 3 , and (AlO) 2 (SiO)­Si–O1H–Al­(OSi) 3 ] of decreasing acid strength exist in NaHY.…”