CoAPO molecular sieve acidity investigated by adsorption calorimetry and IR spectroscopy

Abstract

The acidic properties of cobalt-and silicon-substituted AIPO-5, -11 and -44 have been characterized by adsorption calorimetry and IR measurements. Adsorption calorimetric measurements indicate that the adsorption potential of the samples for acetonitrile is enhanced upon cobalt incorporation. The calorimetrically measured heats of adsorption indicate the formation of strong acid sites, due to this cobalt incorporation, as well as the presence of weaker acid sites, probably terminal P-OH groups. IR measurements of adsorbed ammonia and acetonitrile indicate that some Brsnsted-bonded ammonia and acetonitrile are present, in combination with Lewis-bonded species. The Brsnsted-bonded species presumably originate from interaction of the base with the weakly acidic P-OH groups. The Lewis-bonded species are due to the coordinative interaction of the lone pair of the base with framework cobalt ions,

X-ray Absorption Spectroscopic Study of Brønsted, Lewis, and Redox Centers in Cobalt-Substituted Aluminum Phosphate Catalysts

“…IR studies, with acetonitrile as a probe, clearly show that extraframework Co(II) (if present) does not contribute to the Lewis acidity. 40 On the basis of these observations, we see that the degree of static disorder (see Table 1) obtained from the EXAFS analysis, especially in CoAlPO-18 which has predominantly Brønsted acid sites, is attributable to a distorted environment around Co(II). Further analysis of CoAlPO-18 using constrained refinement, as discussed above, indeed showed the presence of Table 1).…”

Effect of the different modifications of A1PO-5 molecular sieve on its acidity and catalytic properties in methanol conversion to hydrocarbons

“…The desorption peak appearing in the case of AIPO-5 at Tm,x 470 K shifts now to 500 K for SAPO-5 which can be explained by the formation of protons with moderate acid strength [12,16]. In the case of Cosubstituted materials, two TPD peaks are observed : a low temperature (LTP) peak at about 500 K and a high temperature (HTP) one at 650-670 K. IR measurements of adsorbed ammonia on CoAPO-5 [6,9,10] have shown that both Bronsted and Lewis type bonded ammonia was present. Thus, the Co incorporation likely results in formation of acidic P-OH groups in the neighborhood of Lewis acid sites.…”

“…These new materials are usually denoted as SAPOn, MeAPO-n and MeAPSO-n depending on the element (silicon and/or divalent metal) incorporated in the crystal framework. The material composition determines whether Bronsted or Lewis acid sites predominate, or a certain ratio between Bronsted and Lewis type surface acidity can be attained [3][4][5][6][7][8][9][10][11][12]. In the present work we compare the catalytic behavior of CoAPO-5, CoAPSO-5 and SAPO-5 molecular sieves in the methanol conversion to hydrocarbons.…”

Lewis Acidity in Transition‐Metal‐Doped Microporous Aluminophosphates

“…As a general rule, we predict therefore that the microporous polymorphs of zeolites and AlPOs with intersecting channels, or with single walls between large cages, such as the Chabasite (AlPO-34 and its closely related AlPO-18) and Ferrierite framework topologies (Figure 4 a,b), will have more active Lewis acid sites than those framework structures with one-dimensional channels made of ™double-wall∫ building units, such as those found in the AlPO-5 and AlPO-11 structures (Figure 4 c,d). This result may explain why, experimentally, the Lewis-acidic behaviour of Co-AlPO-5 towards the adsorption of acetonitrile was found to be similar to that of Co-AlPO-11, [17] but different from that of Co-AlPO-18. [15,16] We leave open for further experimental verifications this prediction of the computational work.…”