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DME than that could be obtained from sequential processing. The process is based on dualcatalytic
synthesis in a single reactor stage, and also based on a combination of an equilibrium
limited reaction (methanol synthesis) and an equilibrium unlimited reaction (methanol
dehydration).
2.3.2.3 Catalyst systems for the single-stage DME process
Typically, there are two types of catalyst systems for the single-stage DME process[51].
The first type, called the dual catalyst system, consists of a physical mixture of a methanol
synthesis catalyst and a methanol dehydration catalyst. The methanol synthesis catalyst is
generally a copper and/or zinc and/or aluminum and/or chromium based commercial catalyst
while the methanol dehydration catalyst is generally selected from solid acid materials, including
γ-alumina, silica alumina, other metal oxides and mixed oxides, crystalline aluminosilicates,
crystalline zeolites, clays, phosphates, sulfates, metal halides, acidic resins, supported phosphoric
acid, and heteropoly acids.
Among them, γ-Al2O3 has been mostly employed due to its low price, easy availability
and high stability. In gas phase applications using a fixed or fluidized bed reactor, the powders of
the two catalysts can be mixed followed by being formed into pellets or beads; or, separate
pellets or beads can be prepared of the two catalysts. The pellets can be placed in a fixed bed
reactor either in well mixed form or in a layer-by-layer arrangement. In liquid phase applications
using a slurry bed reactor containing an inert liquid medium, a powder mixture of the two
catalysts can be directly used.
In the second type of catalyst system for the single-stage DME process, the two
functionalities are built into a single catalyst. This has been achieved either by coprecipitating
methanol synthesis and dehydration components together to form one catalyst, or by
precipitating methanol synthesis components onto an existing, high surface area solid acid
material.
Regardless of which type of catalyst system is used and regardless of whether the process
is conducted in the gas or liquid phase, maintenance of the catalyst activity is a major challenge.
This is especially true when a dual catalyst system is used. Which mainly contributed to the truth
that acidic component and the Cu/ZnO/Al2O3 component are totally different; the lateral
interactions between the two components must be considered for the direct synthesis of DME.
The currently used industrial Cu/ZnO/Al2O3 catalysts are usually operated at 220–280 o C. The
reaction at lower temperature leads to the low reaction activity, while higher temperature results
in the sintering of the catalysts. Thus, an ideal dehydration component must be operated at the
temperature range for the Cu/ZnO/Al2O3 catalyst if it is used with Cu/ZnO/Al2O3 for the direct
synthesis of DME. It must be highly active and stable in the temperature range from 220 to 280
o C.
Among the solid acids used for methanol dehydration, H-ZSM-5 and γ-Al2O3 are the two
catalysts that have been studied intensively both for academic and commercial purposes[52].
They can be used for the direct dehydration of methanol to DME or as the dehydration
components in the STD process. H-ZSM-5 was reported to be a good dehydration catalyst by
several groups. For example, Ge et al. prepared some bi-functional STD catalysts using H-ZSM-
5 as a dehydration component[53]. Kim et al. reported that both Na-ZSM and H-ZSM-5 zeolites
could be used as effective dehydration components in STD process[54]. They pointed out that
the optimized ZSM-5 composition in the admixed catalysts was determined by the acid strength
of the acid component. On the other hand, some researchers reported that hydrocarbons were
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