The adsorption of methanol and water on SAPO-34: in situ and ex situ X-ray diffraction studies

The adsorption of methanol on SAPO-34 has been studied using a combination of in situ synchrotron powder X-ray diffraction to follow the process and ex situ high resolution powder diffraction to determine the structure. The unit cell volume of SAPO-34 is found to expand by 0.5% during methanol adsorption displaying greater flexibility than silicalite under similar conditions. When water is adsorbed onto the framework it contracts, while the non-polar polymethylbenzene intermediates in the methanol to olefin process cause a significant expansion. We therefore suggest that the expansion (water < methanol < polymethylbenzenes) is linked to the strength of interaction with the framework and size of the adsorbent. The position of the methanol molecule has been determined from high resolution powder diffraction data. The oxygen to framework contacts are slightly longer than reported elsewhere for adsorbed water molecules on the same framework, supporting the observation from the in situ data that water is more strongly bound than methanol. The results are consistent with previous results from thermodynamic and tapered element oscillating microbalance measurements.