The IR transmission windows of hydrogen bonded complexes in zeolites: A new interpretation of IR data of acetonitrile and water adsorption on zeolitic Broensted sites

The 2800, 2400 and 1700 cm-1 infrared OH bands recently found upon adsorption of many basic molecules on Bronsted sites of zeolites, including acetonitrile and water, are suggested to be analogous to the (A,B,C) triplet of medium-strong molecular X-OHY complexes in vapours, liquids and solids. IR studies of CD3CN and CCI3CN adsorptions on H-ZSM5, H-FeSil, H/D-ZSM5 and H/D-FeSil zeolites show these bands to be in a good agreement with the resonance theory of the (A,B,C) triplet, developed for molecular H-complexes. A new interpretation of IR data of water adsorption on zeolites, using this theory and results of ab initio calculations, suggests the water complex with the bridging OH group of zeolites to be "neutral", the one water OH group interacting by hydrogen bonds both with the bridging OH group and with the nearest AlOSi oxygen and the other OH group being free

(A,B,C) triplet of infrared OH bands of zeolitic H-complexes

The phenomenon of the (A,B,C) triplet of infrared OH bands at similar to 2800, similar to 2400 and similar to 1700 cm(-1), well-known for strong X-OH ... Y molecular H-complexes in solutions, liquids, and solids, is studied for the first time for surface H-complexes using CD3CN and CCl3CN adsorption on deuterated H-ZSM5 and H-FeSil zeolites. A direct experimental proof is given that the minimum between the A and B bands of D- complexes occurs at nearly exactly the 2 delta(OD) in-plane bending overtone frequency of the perturbed OD group. This verifies the resonance theory of the (A,B) doublet by Claydon and Sheppard. In reference to zeolites this means that the similar to 2800 and similar to 2400 cm(-1) OH bands recently found in adsorption of many basic molecules on zeolitic OH groups are actually pseudobands, caused by the subdivision of the very broad v(OH) +/- kv(OH ... Y) superposition band of the perturbed OH groups by Evans transmission window at the 2 delta(OH) similar to 2600 cm(-1) frequenc

Acidic properties of metal substituted aluminophosphates studied by adsorption calorimetry and IR spectroscopy

The heats of adsorption of MeCN on metal substituted AlPO4-5 and AlPO4-11 compared to the pure AlPO's indicate the formation of strong acidic sites. Besides these strong sites some weaker acidic centers, probably P-OH groups were found. Accordingly, IR measurements on the loaded MeAPO's show some weakly Broensted bonded MeCN in combination with a strong Lewis complex. The Lewis bonded MeCN in the MeAPO's originate from coordinative interaction of the lone pair with the bivalent framework metal ions such as Mg2+, Mn2+, Co2+ or Zn2+. [on SciFinder (R)

Adsorption calorimetric measurements and 27Al DOR NMR studies on the molecular sieve AlPO4-18

Highly cryst. samples of the chabazite-related aluminophosphate AlPO4 18 (AEI) were prepd. and characterized by adsorption measurements as well as by 27Al double rotation (DOR) NMR studies. While the heat curves and isotherms of n-paraffins on AEI show the common feature for adsorption on nonpolar mol. sieves with a given pore dimension, MeOH gives an unusual heat curve with a deep min. for a loading > 4 mols. per cavity. This corresponds to the extended low-pressure hysteresis loop of the isotherm which is absent in SAPO 34. In accord with the structure detn. and Rietveld refinement of the as-synthesized AEI, 3 crystallog. inequivalent Al positions including one pentacoordinated Al can be detected and assigned by 27Al DOR NMR. Calcination as well as the adsorption of polar mols. results in a structure change. While H2O and NH3 generate octahedrally coordinated Al, MeOH gives only AlV as was found for the template-contg. sample. The MeOH adsorption was studied in more detail. The formation of the AlV requires > 1 MeOH mol. per Al1 site. This process occurs for a loading between 1 and 4 mols. per cavity; it is isobaric and compares to a hydration process. Two addnl. mols. can be accommodated in the AEI cavity which seems to be related to the 2nd step of the MeOH isotherm and the extended low-pressure hysteresis loop which is absent in the isotherm of H2O. This is accompanied by another reversible structure change resulting in 4 NMR lines for the 6 different Al positions in the double 6-ring, the secondary binding unit of the AEI. [on SciFinder (R)

Acid sites in sulfated and metal-promoted zirconium dioxide catalysts

Unpromoted sulfated zirconia (SZ) and sulfated zirconia that was promoted with iron and manganese ions (FMSZ) have been tested as catalysts for the isomerization of n-butane to isobutane. FMSZ is the superior catalyst; its activity at 60°C is similar to that of SZ at 180°C. Both catalysts deactivate rapidly. FTIR analysis of adsorbed CO and acetonitrile reveals the presence of Lewis and Brønsted sites in both catalysts. Acetonitrile is strongly adsorbed on FMSZ. Subsequent desorption is destructive; CO2, SO2, and O2 are formed, while temperature-programmed oxidation shows that no carbon-containing products are left on the surface. CO-FTIR reveals equal acidity, within experimental error, for the Lewis sites on SZ and FMSZ. Changes in proton NMR and FTIR parameters caused by adsorption of acetonitrile show that the acid strength of the Brønsted sites of SZ and FMSZ is similar to that of the lower OH-frequency protons in HY, but weaker than that of the protons in HZSM-5. The results indicate that the remarkable activity of SZ and FMSZ is not caused by exceptionally strong acid sites, but by stabilization of the transition state complex at the surface

Acidic properties of metal substituted aluminophosphates studied by adsorption calorimetry and IR spectroscopy

The heats of adsorption of MeCN on metal substituted AlPO4-5 and AlPO4-11 compared to the pure AlPO's indicate the formation of strong acidic sites. Besides these strong sites some weaker acidic centers, probably P-OH groups were found. Accordingly, IR measurements on the loaded MeAPO's show some weakly Broensted bonded MeCN in combination with a strong Lewis complex. The Lewis bonded MeCN in the MeAPO's originate from coordinative interaction of the lone pair with the bivalent framework metal ions such as Mg2+, Mn2+, Co2+ or Zn2+. [on SciFinder (R)