Chemical, physical and catalytic properties of ZSM-5 and ZSM-11 zeolites : a study by electron microscopy, EDX-STEM and XPS

Abstract

In order to ascertain the relative importance of shape-selectivity due to channel size limitations and of zeolitic grain size in the conversion of methanol to higher hydrocarbons on Pentasil-type zeolites, several catalysts were prepared using different synthesis and acidification procedures. Acidity of the samples was determined by microcalorimetry of the differential heat of NH 3 adsorption. Crystallite size and morphology were determined by high resolution electron microscopy while the Al distribution across the crystallites was monitored by high resolution EDX-STEM and XPS. The ZSM-11 samples were composed of spherical aggregates having diameter of 0.6 ± 0.2 μ m for one batch and 6 ± 2 μm for another batch. The former sample was composed of aggregates of crystals of 5 to 10 nm in diameter and gave an XRD pattern with broad peaks. However, infrared vibrational spectra and microdiffraction were consistent with ZSM-5 or -11 material. The latter sample was composed of aggregate cores with crystalline needles of 200 to 400 nm in size emerging from the surface. EDX-STEM technique shows that within aggregate crystals the Al distribution is rather homogeneous while a great heterogeneity exists between grains. XPS data show that the outermost layers are in average richer in Al than the inner layers. In the sample with needles the AI content within the needles is homogeneous but varies from one grain to another one. However this Al content is 1.5 to 4 times lower than in aggregate core of each grain. XPS technique also shows that the outer layers of the needles are very poor in Al (Si:Al = 70 ± 10). ZSM-5 samples were shown to be composed of parallelipipeds about 0.5 to 1 μm in size with a narrow size distribution. The Al content was shown to be approximately homogeneous within the grain but very different from one grain to another (factor 2 to 3). Acidification by HCl was also shown by XPS to eliminate lattice Al from the outerlayers. It appears from the selectivities in aromatic hydrocarbons in the reaction of methanol conversion to a mixture of hydrocarbons that H-ZSM-5 yields preferentially A 8 aromatics whereas H-ZSM-11 gives more A 9 aromatics. This holds true for both acidification procedures and for different crystallite sizes. This was attributed to different physical constraints resulting from the difference in the free void space at the channel intersections between ZSM-5 and ZSM-11 zeolites. Large crystallites of ZSM-11 (second sample) gave more light aliphatics (C 1 + C 2) and less C 6 + aliphatics and A 9 + aromatics than the small crystallites (first sample) and resemble the ZSM-5 zeolite of about the same crystallite size for this particular aspect. The acid strength of active sites, as characterized by microcalorimetry of NH 3 adsorption, and the acid site distribution were observed not to play an important role in methanol conversion reaction, as long as sufficient acid sites are present. Particular emphasis is placed upon misleading conclusions on the particle size effect which may stem from inaccurate SEM characterization and on the oversimplification of steric limitations due to the size of the channels.