MeAPO-5 molecular sieves

The chemical analysis of the synthesized samples expresses in the framework composition, that is, (MeIAlyPz)02 provides the as-synthesized sample framework composition, which is shown in Table 3.7, indicating the presence of about 1% of Me in the synthesized aluminophosphate [29], From the x-ray powder diffractograms (Figure 3.14), it is noted that the crystallized products exhibit all the characteristic reflections of the MeAPO-5 molecular sieves [140] and a high crystallinity and degree of purity [29],... 

Cracking of light alkanes over MeAPO-5 molecular sieves... 

SAPO-5, MAPO-5, and MeAPO-5 molecular sieves are also active catalysts for methanol conversion into hydrocarbons. However, high concentrations of aromatics can also be obtained on these molecular sieves. In SAPO-5, the selectivity toward olefins can be improved by decreasing the Si/Al ratio, therefore, the concentration of strong acid sites. Incorporating bivalent elements to the aluminophosphate freunework also modifies the acid properties. Cations like and Co " " lead to active catalysts for methanol conversion, but the production of aromatics is high so that the olefin selectivity is lower. 

In-situ DRS spectroscopy has been used to investigate the crystallization process of MeAPO-5 molecular sieves (with Me = Co, Cr, Ni and V) as a function of the synthesis time and conditions. A specially developed synthesis cell was used, which allows to probe the coordination environment of the transition metal ions in-situ during hydrothermal conditions and to obtain high-quality molecular sieves starting from a homogeneous synthesis gel. 

The MeAPSO family further extends the structural diversity and compositional variation found in the SAPO and MeAPO molecular sieves. These quaternary frameworks have Me, Al, P and Si as framework species [27]. The MeAPSO structure types include framework topologies observed in the binary AIPO4 and ternary (SAPO, MeAPO) compositional systems and the novel structure -46 with a 0.7 nm pore. The structure of -46 has been determined [34]. 

The synthesis of a MeAPO molecular sieve typically uses an aqueous reaction mixture formed by combining a dissolved form of the divalent metal, orthophosphor1c acid, a reactive alumina, and an amine or quaternary ammonium templatlng agent (R>. The metal Is typically Introduced as the acetate or sulfate salt, or as the metal oxide dissolved In dilute phosphoric acid. A synthesis mixture Is prepared In one of two ways ... 

Table I. Typical Templates and Structure-Types for MeAPO Molecular Sieves...

Chemical Analysis. The chemical analyses of these and other MeAPO molecular sieves are consistent with tetrahedral frameworks in which Me and A1 occupy 50% of the T sites and P, the other 50%. All of the available structural evidence on A1P0< and MeAPO also supports frameworks based on alternating (Al.Me) and P in the lattice T sites. The negative framework charge produced by such an incorporation pattern is balanced primarily by the positively charged template and, secondarily, by other occluded species, such as H or Me5. ... 

Thermal Stability. Most MeAPO molecular sieves exhibit good thermal stability, retaining crystallinity after a 400-600°C air calcination to remove the organic template. The ultimate thermal stability depends on structure-type, metal type, and metal concentration. 

Along with A1P0 and SAPO, the MeAPO molecular sieves have extended the structural and compositional variety found among the growing numbers of AlPO,-based molecular sieves. Both the metal and the organic template exert a primary influence on... 

Novel metal aluminophosphate molecular sieves (MeAPO-n) have been synthesized, where Me can be the divalent form of Co, Fe, Mg, Mn, or Zn, or trivalent Fe. These MeAPO molecular sieves have tetrahedral frameworks containing metal, aluminum, and phosphorus, and exhibit a wide range of compositions within the general formula 0 - 0.3 R (MexAlyP2)02 where... 

New crystalline microporous molecular sieves have been synthesized by incorporating other elements into the AIPO4 freunework. Some of these elements are Co, Be, Mn, and Fe. ° They carry the generic naunes MAPO and MeAPO molecular sieves. The acidity of MAPO and MeAPO molecular sieves can vary widely (see Table 2). 

Tapp et al. ° used SAPO-5 and CoAPO-5 to convert methanol to hydrocarbons. The catalytic experiments were carried out at 360 C, 1 bar total pressure, and LHSV=2. The typical hydrocarbon fraction contained 45 wt% of light olefins, 15 wt% of higher aliphatics, 40 wt% of aromatics, and less than 1% of methane. The main aromatics were penta- and hexa-methylbenzene. SAPO-5 remained active for approximately 300 g methanol/g catalyst, while CoAPO-5 remained active for only 30 g methanol/g catalyst. The lower stability of CoAPO-5 with respect to SAPO-5 is in agreement with that observed in small-pore SAPO and MeAPO molecular sieves. The methanol transformation on AIPO4-5 mainly yielded dimethylether and water. 

The principles governing the formation of a porous crystalline material are not yet understood because of the complexity of hydrothermal crystallization. Namely, the hydrothermal crystallization occurs in a closed vessel, in which many chemical processes, interactions, and equilibriums take place and change with time (105). In this sense, some novel in situ technique such as synchrotron X-ray powder diffraction or in situ solid-state NMR may contribute to a better understanding of the crystallization process (106,107). In situ DRS-ESR studies also allow for a better understanding of the hydrothermal crystallization of MeAPO molecular sieves by means of following the coordination chemistry of transition metal ions during the crystallization of the reaction mixture (105). 

JDF-20 is not stable to calcination and converts to a mixture of AlP04-5 and tridymite at 500°C. The JDF-20 synthesis gel also produces the 12-MR AlP04-5 when heated at 200°C rather than at 180°C. This will limit the utility of JDF-20, however, this molecular sieve can be prepared in MeAPO compositions (where Me = Li, Mg, Mn, Ga, B, Si, Ti, and Sn) that may provide interesting redox properties. 

There is no systematic nomenclature developed for molecular sieve materials. The discoverer of a synthehc species based on a characteristic X-ray powder diffraction pattern and chemical composihon typicaUy assigns trivial symbols. The early syn-thehc materials discovered by Milton, Breck and coworkers at Uruon Carbide used the modem Lahn alphabet, for example, zeoHtes A, B, X, Y, L. The use of the Greek alphabet was inihated by Mobil and Union Carbide with the zeoHtes alpha, beta, omega. Many of the synthetic zeoHtes which have the structural topology of mineral zeoHte species were assigned the name of the mineral, for example, syn-thehc mordenite, chabazite, erionite and offretite.The molecular sieve Hterature is replete with acronyms ZSM-5, -11, ZK-4 (Mobil), EU-1, FU-1, NU-1 (ICI), LZ-210, AlPO, SAPO, MeAPO, etc. (Union Carbide, UOP) and ECR-1 (Exxon). The one pubHcaHon on nomenclature by lUPAC in 1979 is Hmited to the then-known zeoHte-type materials [3]. 

---