The Effect of Acid Strength on the Conversion of Methanol to Olefins Over Acidic Microporous Catalysts with the CHA Topology

Abstract

The present work addresses the influence of acid strength on the stability and product selectivity of microporous catalysts with CHA framework type. The two studied catalysts, H-SAPO-34 and H-SSZ-13, have the same topology, density of acid sites (approximately one acid site per cage), and crystal size (0.2-2 microns), but their acid strength differ due to the framework composition. The difference in acid strength was determined by infrared spectroscopy, using CO as probe molecule. Catalytic tests were performed in a fixed bed flow reactor at 300-425°C and WHSV = 6.0 h -1 . It was observed that the acid strength has significant influence on reaction rates, enhancing the production rate of olefins in the reactor effluent as well as aromatics retained in the catalyst pores and leading to a lower optimal temperature of operation for the more acidic H-SSZ-13 catalyst. The activation and deactivation patterns and the intermediates formed are very similar for the two materials. The ethene to propene ratio increases with temperature and time on stream for both catalysts, and is higher over the more acidic H-SSZ-13 catalyst at similar reaction conditions.

Incorporation of mixed metals into SAPO-34 frameworks by the dry-gel conversion method using mixed templates: investigating catalysts characterisation and performance

“…A similar behaviour can be observed in some other works. [23,24] The induction period in the early time on stream is causing to incomplete conversion of methanol, which can be explained by the hydrocarbon pool (HP) mechanism. Also, due to the small cage size of the MeAPSO-34 molecular sieve, the hydrocarbon materials formed in the cages, and they work as a diffusion barrier for methanol and lower olefins.…”

Methanol to olefins: activity and stability of nanosized SAPO-34 molecular sieves and control of selectivity by silicon distribution

“…In the CO stretching vibration region a band at 2171 cm -1 appears after CO adsorption (Fig. 9), corresponding to CO interacting with Brönsted acid sites 42 . The position of this band is the same for the different nano-SAPO-34 hydrated samples.…”

Local silico-aluminophosphate interfaces within phosphated H-ZSM-5 zeolites

“…[6][7][8][9][10][11][12] Even though there is still some debate on the effect of acid site strength on the MTO product distribution, the promoting effect of phosphorus on the hydrothermal stability of H-ZSM-5 is undisputed. 4,9,[13][14][15][16][17][18][19][20][21][22][23][24][25] Zeolite H-ZSM-5 catalysts that contain phosphorus are more stable under hydrothermal conditions i.e. they retain more framework aluminium species and strong acid sites than their non-phosphated counterparts.…”