MeAPO acidity

Aluminosilicates (zeolites) are widely used as acidic and bifunctional catalysts. The formation of carbocationic intermediates is generally ascribed to the protons present in the open zeolite structure. A newer class molecular-sieve catalyst is the aluminophosphates. These may contain sihcon (SAPO) or metal (MeAPO) in their AiP04 frameworks. These framework substitutions in several cases generate protonic acidity that makes SAPO and MeAPO acid catalysts. There is already an extensive literature of this subject (20, 21). 

The spectrum of adsorption pore sizes and pore volumes and the hydrophilic surface selectivity of the MeAPOs are similar to those described for the SAPOs. The observed catalytic properties vary from weakly to strongly acidic and are both metal- and structure-dependent. The thermal and hydrothermal stability of the MeAPO materials is somewhat less than that of the AIPO4 and SAPO molecular sieves. 

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 ... 

Further variation of the stmctural and catalytic properties of four-coimected tetrahedral frameworks is obtained by the substitution of silicon or metal cations,giving materials known as SAPO s and MeAPO s, respectively. More than twenty metal aluminophosphate frameworks have been identified with Mg, Mn, Fe, Co, or Zn substituents. These give the possibility of framework redox activity (e.g. Fe +/Fe +) in catalysis as well as the usual Bronsted acidity. For further information about zeolitic and microporous phosphate frameworks see Porous Inorganic Materials and Zeolites) and recent reviews. ... 

Similar behavior was observed for ZSM-23 (38), MeAPO-11 (14), and ZSM-22 (35). For these unidimensional pore systems (MTT, AEL, and TON), the coking rate was less than that for FER catalysts furthermore, the magnitude of the differences in activity and selectivity between fresh and coked samples was less than that for catalysts with bidimcnsional pore systems. For example, when ZSM-23 was used at 693 K and a W.HSV of 171 h , the micropore volume decreased from 58.4 to 11 ptl/g after 20 h on stream. Also, the number of acidic sites estimated by butene TPD decreased from 0.45 to 0.06 mol per unit cell, whereas the butene conversion decreased only from 41 to 31% the isobutylene selectivity increased from 72 to 92% (38). Evidently, the catalyst pore geometry significantly affects the coke deposition and thus the selectivity. The relevant literature is discussed in the following section. 

The changes of the acid - base properties of metal substituted aluminophosphate based molecular sieves (MeAPO) as function of the chemical composition and the crystal structure are proposed to be complicated and to be substantially different compared to zeolites (1,2). 

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). 

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. 

However, the incorporation of metal cations whose valence is different from that of A1 or P leads to the formation of electronically unsaturated sites, as schematically shown in Figure 3. This addition of aliovalent metal cations into the lattice of AlPO-n generates solid acidity and ion-exchange sites. There are numerous reports on the incorporation of many different metal cations into the lattice of AlPO-n. Table 2 summarizes the reported isomorphous substituted AlPO-n. The family of AlPO-n substituted with metal cations is generally called metal aluminophosphates (MeAPO-n). The typical metal cations substituted into AlPO-n are Li, B, Be, Mg, Ti, Mn, Fe, Co, Zn, Ga, Ge, Si, and As. The Si-substituted AlPO-n is called a silicoaluminophosphate and denoted as SAPO-n, where n also means the framework structure, and it is distinct from the MeAPO-n materials.SAPO-n exhibits both structural diversity and compositional variation. In particular, the crystal structure of SAPO-n is of greatest interest, because the distribution of the Si atom in the framework is quite complicated. Some crystal structures, such as SAPO-40, are only found in SAPO-n and not in AlPO-n or zeolite. The mole... 

Solid Acidity of SAPO-n and MeAPO-n. - The largest difference between AlPO-n and AlPO-n substituted with metal cations (SAPO-n or MeAPO-n) is its solid acidity and, consequently, its ion-exchange properties. Solid acidity is caused by substitution of a part of A1 or P in framework with metal cations. However, the number of acid sites caused by metal substitution does not increase linearly as the amount of substituted metal cations increases. This is very much in contrast with aluminosilicate zeolites. It is well known that, in the case of zeolites, the number of acid sites increases linearly as the number of A1 increases in the framework. Consequently, the acidity is often expressed simply by the Si/Al ratio in the zeolite. However, the number of acid sites as well as their strength depends on the amount of substituted metal cations in a complicated way. This is because two sites for substitution, A1 and P, exist in the framework of AlPO-n, and the substituted cations are not always substituted at the same site. The acidity of SAPO-n or MeAPO-n has been studied in connection with the state of substituted metal. In this section, acidity of SAPO-n and MeAPO-n will be briefly reviewed. 

Catalysis of AlPO-n, MeAPO-n, or SAPO-n as a Solid Acid... 

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