Deactivation of Zeolite Catalysts

Investigating the Flexibility of H-ZSM-5 Zeolite Upon Adsorption of Coke Precursors: A Theoretical and Experimental Approach

The flexibility of the H-ZSM-5 zeolite upon adsorption of selected coke precursors was investigated using both theoretical and experimental approaches. Four structural models with varying active site locations were analyzed through density functional theory (DFT) simulations to determine their responses to different types and quantities of aromatic molecules. Complementary experimental analysis was performed, allowing for a direct comparison with the theoretical findings, using thermogravimetric analysis (TGA), nitrogen adsorption (N2 adsorption), solid-state NMR, and X-ray diffraction (XRD). By employing proposed flexibility descriptors, significant structural changes in the MFI-type zeolite framework were identified, particularly in the unit cell parameters and the morphology of the straight channels. These changes were driven by electrostatic repulsion between adsorbates and by electrostatic attraction between adsorbates and the zeolite framework. The observed structural changes depended on both the active site location and the size and number of coke precursors. Consistent trends in structural flexibility were observed in both experimental and theoretical studies, primarily driven by variations in organic species loading. Our findings show the critical importance of active site location in influencing the magnitude of framework flexibility, which, in turn, affects the stabilization and accommodation of different coke precursors within the zeolite structure.

Pore Blocking by Phenolates as Deactivation Path during the Cracking of 4-Propylphenol over ZSM-5

Cracking of propyl side chains from 4-propylphenol, a model compound for lignin monomers, is studied for a commercial ZSM-5 zeolite catalyst. The decline of 4-propylphenol conversion with time on stream can be delayed by co-feeding water. FTIR spectroscopy shows the formation of chemisorbed phenolates during reactions and significant amounts of phenolics are detected by GC-MS of the extract from the spent catalysts. Thus, chemisorbed phenolates are identified as the main reason for deactivation in the absence of water. Regardless of the amount of co-fed water, substituted monoaromatics and polyaromatic species are formed. Comprehensive characterization of the spent catalysts including Raman and solid-state 27Al NMR spectroscopy, and thermogravimetric analysis points to a combination of deactivation processes. First, phenolates bind to Lewis acid sites within the zeolite framework and hinder diffusion unless they are hydrolyzed by water. In addition, light olefins created during the cracking process react to form a polyaromatic coke that deactivates the catalyst more permanently.

Comparative investigation of the deactivation behaviors over HZSM-5 and HSAPO-34 catalysts during low-temperature methanol conversion

Deactivation over HZSM-5 originated from overloaded HCP species, while that over HSAPO-34 was caused by methyladamantanes generated from the external surface.

The influence of straight pore blockage on the selectivity of methanol to aromatics in nanosized Zn/ZSM-5: an atomic Cs-corrected STEM analysis study

Here we report the direct atomic scale observation of ZSM-5 pores and the influence of extra-framework Al on methanol to aromatic (MTA).

Highly Dispersed X/SiO(2) and C/X/SiO(2) (X=Alumina, Titania, Alumina/Titania) in the Gas and Liquid Media

Fumed oxides, such as silica, alumina, titania, and mixed X/silicas (X=Al(2)O(3) (AS), TiO(2) (TS), CVD-TiO(2), Al(2)O(3)/TiO(2) (AST)), pristine or covered by carbon deposits formed due to pyrolysis of cyclohexene, were studied using nitrogen adsorption-desorption, photon correlation spectroscopy particle sizing, and electrophoresis. A significant influence of the nature of surface-active sites and structural features of oxides (individual silica, mixed fumed, or prepared using chemical vapor deposition (CVD)) on the pyrolysis of cyclohexene is observed with respect to the pore size distributions due to differences between primary particles in aggregates and on their outer surfaces in the filling of channels by pyrocarbon, resulting also in a decrease in fractal dimension. Structural characteristics and dependences of the particle size distribution and electrokinetic potential of X/SiO(2) and C/X/SiO(2) on the pH of aqueous suspensions suggest that the carbon deposit covers mainly acidic sites at the X/SiO(2) interfaces and X phase patches possessing catalytic activity in pyrolysis, as the negative charge of particles is reduced by pyrocarbon grafting. Copyright 2000 Academic Press.