Intergrowth Materials for Improved Methanol-To-Olefin Catalysts

Catalysts are central to industrial processing. Catalysts save energy; make process faster and cheaper; and reduce by-products. The INTERCAT project has been aiming to apply such improvements through understanding an industrial catalyst on at the atomic level and optimize it through custom synthesis. SAPO-34 is a microporous catalyst used for the conversion of methanol (made from natural gas, coal or biomass) to olefins. The latter constitutes one of the most useful chemical building blocks for the preparation of products from plastics and polymers to medicine. A micro porous catalyst has tiny holes sized to fit with small chemical molecules that will be converted by a catalytic reaction. In fact, the shape and size of the cavities selectively control what kind of chemical products formed. A key issue in INTERCAT is to understand specific defects in such catalysts, which occur when the structure blocks were placed incorrectly in the crystal structure, known as stacking faults or intergrowth (IG). The presence of such stacking fault means that the pores take on a somewhat different geometry. An extreme case is that the entire particle changes structure type to SAPO-18 instead of SAPO-34. SAPO-18 has a very different overall structure, to SAPO-34 but is built from the same structural units. A particularly important aspect of the process is that there are indications that a high percentage of IG leads to increased lifespan for industrial catalysts. Our approach has been to first develop robust synthetic methods for providing well-defined samples with different amounts of IG. Then, our main focus was to develop a methodology for determining the degree and type of stacking faults through the use of X-ray diffraction (partly by synchrotron, ESRF, Grenoble) and to understand the formation of defects by studying the surfaces of crystallites by atomic force microscopy (AFM). Over the past years we have made great progress with respect to understanding such complex IG materials. We have mastered the synthesis of materials with a wide range of stacking faults (IG) levels and have established a library of different samples. The materials are characterized by scanning electron microscopy, X-ray diffraction and AFM. Major breakthroughs have been achieved in 2012/13 thanks to the development of a new method for fitting the observed powder XRD diffraction patterns of IG materials; in which stacking faults are included as a variable parameter. In situ AFM has revealed mechanisms for crystal growth of catalytic particles. Recently, catalytic testing has been performed to identify possible correlations between stacking faults, distinctive surface features observed by AFM and catalytic activity.

The Methanol-to-Olefin process (MTO) is an important step in the route of natural gas to petrochemicals and fine chemicals, originally developed by Norsk Hydro (now INEOS Chlorvinyls). Deactivation of the catalyst is a major feature of the process and rec ent results indicate that the catalyst structure and composition may have a role in the deactivation mechanism. Systematic studies of the composition (Silicon distribution) of intergrowths of the dominant MTO catalyst, Silicoaluminophosphate (SAPO) -34, w ith a closely related polymorph, SAPO-18, are expected to reveal important aspects of the deactivation process. High resolution in situ synchrotron studies of a family of materials prepared and characterised using isotopic labelling techniques aim to link structure/composition to performance under realistic testing conditions. Studies of the crystal growth mechanisms of the intergrowth materials aim to establish the controlled synthesis of materials of desired composition and structure. Collaboration with international partners in the UK will provide complimentary modelling and in situ studies. Links to industrial users INEOS Chlorvinyls will ensure the commercialisation of interesting materials.