Investigations of methanol-to-hydrocarbons catalysis in zeolites by synchrotron infrared microspectroscopy

Abstract

The catalytic conversion of methanol to hydrocarbons (MTH reaction) has been investigated by a synchrotron-based spectroscopic method that was developed at the Diamond Light Source. The operando synchrotron infrared microspectroscopy (OIMS) method presented here measures time-resolved spectra from large zeolite crystals with simultaneous on-line analysis of gas phase products by mass spectrometry. The results showed that OIMS can follow reactions on crystals down to 30 microns in size, with a time resolution of 2 s routinely, and down to 0.25 s. Heterogeneities in the acid site density of steamed samples can be visualised.

Large crystals of two commercially-relevant materials, HZSM-5 and HSAPO-34, were prepared to gain mechanistic insights into the MTH, methanol-to-olefins (MTO) and dimethyl ether-to-olefins (DMTO) reactions. Initial studies focused on the elementary steps of olefins formation in the early stages of the MTH reaction over HZSM-5 during pulses of methanol and continuous flow of dimethyl ether. Surface bound methoxy groups (SMS) were identified as the key reactive species for the direct formation of olefins. The studies of both HZSM-5 and HSAPO-34 suggest that SMS (produced from methanol or dimethyl ether) couple directly via a carbene-like transition state to make the first olefins. This was confirmed by studies with deuterium-labelled reactants. The reactivity of SMS was found to be dependent on zeolite topology and acid site strength with lower reactivity seen for HSAPO-34. Identification of methylated cyclopentenyl carbenium ions, oligomers and cracking reactions were compared for HZSM-5 and HSAPO-34 and for three different crystal sizes of HZSM-5. Varying the crystal size reduced the diffusion path lengths and shortened the induction period, and reduced induction periods were also seen at higher reaction temperatures in the 473–673 K range studied.

The last chapter explored the effect of steaming on the structure and catalytic properties of HSAPO-34. A steaming protocol was developed to investigate the effect of varying the severity of the treatment (up to 1023 K) on the structural and textural properties of the materials and their catalytic performance in the MTO reaction. Ex-situ characterisation, MTO catalytic performance and operando FTIR studies were analysed, suggesting a three-stage transformation is occurring during steaming of HSAPO-34. Under mild to moderate steaming there is redistribution of silica, followed by the development of an optically dark ‘core’ that contains meso- and macropores, but under harsh steaming conditions the samples rapidly recrystallise to a denser phase losing its micropore volume.