MTO Paper Background

I. Overview of MTO Process

MTO Process

The Methanol to Hydrocarbons process was discovered at Mobil Oil in 1977. This process is used to convert methanol to products such as olefins and gasoline. The methanol can first be obtained from coal or natural gas. In the Methanol to Olefins (MTO) process, the methanol is then converted to olefins such as ethylene and propylene. The olefins can be reacted to produce polyolefins, which are used to make many plastic materials. An MTO process flow diagram advertised by Honeywell is shown below.

process diagram

Process flow diagram for an MTO process advertised by Honeywell Corporation

Critical to the successful application of the MTO process are acidic zeolite catalysts, which are discussed in detail on the zeolites page. Without these catalysts, the chemical reactions involved in the MTO process would be too slow for the process to be economically feasible.

plasticsplastics2

Polyolefin plastic containers (left) and shrink film (right) (Source: 1, 2)

II. Overview of MTO Mechanism

The conversion of methanol to olefins on acidic zeolites takes place through a complex network of chemical reactions. The distribution of products and thus the “selectivity” depends on the temperature, among other factors. Selectivity is a measure of the amount of one product produced relative to others when the possibility to form multiple products exists. Selectivity depends on temperature through the Arrhenius law for the different rate constants.

In general, at lower temperatures methanol reacts to form dimethyl ether (DME). At higher temperatures, the desired products (olefins) are produced and the selectivity for DME decreases. Typical percent yields at steady state for methanol conversion on the zeolite H-SAPO-34 are shown below. “Steady-state” refers to a process in which the properties, such as temperature or concentrations, are not changing with time. The percent conversion of methanol is denoted by Xme and the Y terms represent percent yields. The subscript dme refers to dimethyl ether. The subscripts C2=, C3=, C4= and C4 refer to ethylene, propylene, butylenes, and butane, respectively. Based on the yield curves, the temperature at which the olefin products are first observed is near 523 K for H-SAPO-34.

% Conversion

Percent conversion and yield at steady-state for methanol conversion on H-SAPO-34

Intermediate Species

The main focus of Wang et al. is to review investigations of the intermediate species involved in the MTO process. Some of the olefins, methanol, and DME react further to yield heavier species. These species, termed the “hydrocarbon pool,” are occluded in the pores and catalyze the formation of olefins from methanol. The composition and catalytic role of the hydrocarbon pool are discussed in detail here.

In the formation of DME, the role of surface methoxy groups has been debated. Surface methoxy groups may be formed from the reaction of methanol with a zeolite acid site. The possible role of these species in DME formation and the formation of initial hydrocarbons are also investigated (see surface methoxy).

Dahl and Kolboe. J. Catalysis. 1994, 149, 458.
Wang et. al. Catalysis Today. 2006, 113, 102.