Zeolite cage size and its effect on conversion of methanol to light olefins – ethylene and propylene
The methanol-to-olefins (MTO) reaction can be accomplished using solid acid catalysts, such as aluminosilicates (zeolites) and silicoaluminophosphates (SAPOs), and is an industrially viable method for the conversion of methanol to light olefins. This reaction provides a route for the production of light olefins from non-petroleum sources such as natural gas, coal and biomass by using methanol as an intermediate.
In the MTO reaction mechanism (shown below), methanol initially equilibrates with its dehydrated product – dimethyl ether. This equilibration can occur over weak acid sites, such as the Si-OH defect sites present in zeolite crystals, or at the crystal termini. Over stronger acid sites, such as the acid sites in zeolites or SAPOs, the equilibrium mixture is then further converted to light olefins, such as ethylene and propylene, with the evolution of water as a byproduct.
These light olefins can then either be left unconverted and evolved as reaction products or be further converted into a variety of carbon-chain lengths. The olefins may also react among themselves to yield aromatic compounds. The hydrogen evolved in the formation of these aromatic species is then transferred to the olefins, yielding paraffins.
In this reaction mechanism, different zeolites or SAPOs influence the reaction products based on their internal structures. Two basic types of structures exist: frameworks containing simple channels through which molecules can flow, and frameworks containing channels that open up into wider cavities within the structure (cages). Structures containing only channels will allow the aromatic products to escape the zeolite or SAPO as organic products; however, the structures with channels narrower than the cavities they enclose will trap the aromatic species and prevent them from escaping. Consequently, the product mix for channel-only structures contains a high fraction of aromatic and paraffinic species. These species are absent from the product mix for structures with cavities, since the cavity structures tend to trap aromatic species in the methylation/cracking cycle.
Over time, this accumulation causes a carbonaceous buildup within the cavities of the catalyst. When the rate of alkylation exceeds cracking such that polyaromatic species (such as the methylated napthalenes shown in the figure above) are formed, the cavities become blocked with these aromatic species, and the cavity is “deactivated” (unable to perform further reactions). Channel-type frameworks suffer from minimum carbonaceous buildup (and thus exhibit longer catalyst lifetimes) since the aromatic species formed are continuously “purged”, keeping the channels clear for reactions to occur.
Thus, the structure and size of a channel or cavity in a material can affect the conversion of methanol and the selectivity towards ethylene and propylene production in the MTO process. SAPO-34, a silicoaluminophosphate with the chabazite (CHA) topology (three-dimensional cage structure with 8-membered ring (8MR) pores) is one of the most studied catalysts for this reaction. We investigated additional zeolites with the LEV, CHA and AFX frameworks. These 8MR zeolites with different cavity geometries are synthesized at similar Si/Al ratios and crystal sizes and then tested as catalysts for the selective conversion of methanol to light olefins. Previous work has revealed that variations in crystal parameters such as Al content in the zeolite and crystallite size can alter the reactivity. Thus, we investigated the effects of cage size over a range of Si/Al ratios with small crystallite sizes.
We have found that the ethylene selectivity decreases as the cage size increases. Variations in the Si/Al ratio of the LEV and CHA also show maximum selectivities by optimizing the Si/Al ratio. Because lower Si/Al ratios tend to produce faster deactivation rates and poorer selectivities, reactivity comparisons between frameworks are performed with solids having an optimum Si/Al ratio. At similar Si/Al and primary crystallite size, the propylene selectivity for the material with the CHA structure exceeds those from either the LEV or AFX structure. The AFX material gives the shortest reaction lifetime, but has the lowest amount of carbonaceous residue after reaction. Thus, there appears to be an intermediate cage size for maximizing the production of light olefins and propylene selectivities equivalent to or exceeding ethylene selectivities.