Adsorption calorimetric measurements and aluminum-27 DOR NMR studies on the molecular sieve aluminum phosphate-18

Abstract

Highly crystalline samples of the chabazite-related aluminophosphate AlPO4-18 (AEI) have been prepared and characterized by adsorption measurements as well as by *7Al double rotation (DOR) N M R studies. Whereas the heat curves and isotherms of n-paraffins on AEI show the common feature for adsorption on nonpolar molecular sieves with a given pore dimension, methanol gives an unusual heat curve with a deep minimum for a loading >4 molecules per cavity. This corresponds to the extended low-pressure hysteresis loop of the isotherm which is absent in SAPO-34. In accordance with the structure determination and Rietveld refinement of the as-synthesized AEI, three crystallographically inequivalent A1 positions including one pentacoordinated A1 could be detected and assigned by 27Al DOR NMR. Calcination as well as the adsorption of polar molecules results in a structure change. While water and ammonia generate octahedrally coordinated Al, methanol gives only AIV as has been found for the template-containing sample. The methanol adsorption has been studied in more detail. It has been found that the formation of the A P requires more than one methanol molecule per All site. This process occurs for a loading between one and four molecules per cavity; it is isobaric and compares to a hydration process. Two additional molecules can be accommodated in the AEI cavity which seems to be related to the second step of the methanol isotherm and the extended low-pressure hysteresis loop which is absent in the isotherm of water. This is accompanied by another reversible structure change resulting in four N M R lines for the six different A1 positions in the double 6-ring, the secondary building unit of the AEI. IntroductionAluminophosphate molecular sieves represent a new family of microporous adsorbents of broad structural diversity1 which includes now 23 accepted structure types with 14 novel structures.2 One of these new structures is the small-pore molecular sieve AlP04-18 (AEI), which recently has been characterized by synchrotron-based X-ray powder diffraction and Rietveld refinement as a chabazite-related three-dimensional 8-ring channel system. After calcination structural changes occur.3 In the assynthesized form, it contains pentacoordinated A1 caused by the presence of the tetraethylammonium hydroxide in its cavities. Therefore, some unusual adsorption properties, especially regarding polar molecules, can be expected as has been shown for other structures of this family such as VFI, AFI, AEL, ERI, and others using NMR or adsorption techniques.&lS Very little, however, is known about the adsorption properties of AEI as only a few related studies have been carried out so far.I6In the present contribution the adsorption behavior of the pure aluminophosphate AEI has been investigated using adsorption calorimetry. To explain the unusual adsorption properties of this molecular sieve with regard to polar molecules, the27Al DOR (double rotation) NMR technique has been applied to samples with different loadings. For the first time ...

Characterization of Zeolites — Infrared and Nuclear Magnetic Resonance Spectroscopy and X-Ray Diffraction

“…The framework of aluminophosphate-type zeolites is built of AI0 4 and P04 tetrahedra in an alternating arrangement. The 27 Al DOR NMR spectra of these materials consist of narrow lines due to aluminum atoms on crystallographically inequivalent T -sites [259 -263] or caused by framework aluminum atoms interacting with probe molecules [264][265][266][267] . At the top of Fig.…”

Separation of isotropic chemical and second-order quadrupolar shifts by multiple-quantum double rotation NMR

“…This process satisfies the conditions necessary to time-average both first-and second-order perturbations to the Zeeman interaction which have spatial dependences described by the second-and fourth-order Legendre polynomials, P 2 (cos b) and P 4 (cos b), respectively [1,2]. A number of studies have been reported utilizing DOR NMR for the differentiation of sodium, boron, aluminium and oxygen sites in minerals [9][10][11], materials [12][13][14][15][16] and molecular sieves, including, zeolites [17][18][19][20], sodalites [21][22][23] and aluminophosphates [24][25][26][27][28]. Recently, focus has turned to exploiting DOR for the study of oxygen sites in organic solids [29][30][31][32][33][34][35].…”

Phosphate-Based Zeolites and Molecular Sieves

“…With water and other polar adsorbates, AIP04s exhibit hystereses extending to very low pressures, as illustrated with AlP04-5 in Fig. 2.5 [88,[92][93][94][95]. This sorption behavior of polar molecules in AlP04s is explained by coordinative adsorption of the polar molecule on specific framework aluminum sites.…”

“…Janchen et al suggested that the sorbate structure effectively interacting with the framework aluminum atoms and causing the phase transition is a methanol dimer [95]. Based on 27 Al DOR NMR, it could be shown that the Al atoms that adopt the Al v coordination in the as-synthesized material and after methanol adsorption are the same atoms [95]. Their position in the narrow part of the AEI cage is shown in Fig.…”