A Systematic Study of Isomorphically Substituted H-MAlPO-5 Materials for the Methanol-to-Hydrocarbons Reaction

Substituting metals for either aluminum or phosphorous in crystalline, microporous aluminophosphates creates Brønsted acid sites, well known to catalyze several key reactions, including the methanol to hydrocarbons reaction (MTH). In this work we have synthesized a series of metal-substituted aluminophosphates with AFI topology that differ primarily in acid strength and span a predicted range from high Brønsted acidity (H-MgAlPO-5, H-CoAlPO-5 and H-ZnAlPO-5), medium acidity (H-SAPO-5) and low acidity (H-TiAlPO-5, H-ZrAlPO-5). The synthesis was aimed to produce materials with homogenous properties (morphology, crystallite size, acid site density and surface area) in order to isolate the influence of metal substitution. This was verified by extensive characterization. The materials were tested in the MTH reaction at 450 °C using dimethyl ether (DME) as feed. A clear activity difference was found, where the predicted stronger acids converted DME significantly faster than the medium and weak Brønsted acidic materials. Furthermore, the stronger Brønsted acids (Mg, Co and Zn) produced more light alkenes than the weaker acids. The weaker acids, especially H-SAPO-5 produced more aromatics and alkanes, indicating that the relative rates of competing reactions change when decreasing acid strength