Effect of Phosphate Modification on the Brønsted Acidity and Methanol‐to‐Olefin Conversion Activity of Zeolite ZSM‐5

Abstract

Phosphate-modified ZSM-5 zeolites were studied by standard characterization techniques and solid-state nuclear magnetic resonance spectroscopy, and in the methanol-to-olefin (MTO) conversion. Considering the physicochemical properties of the ZSM-5 zeolites, the most important effects of the phosphate modification are a deposition of polyphosphates in the pore system and a decrease of the acid site density, but not of the acid site strength. The significant increase in the selectivity to C 2 -C 4 alkenes and the decrease of C 5+ formation in the MTO reaction for a phosphate coverage of about 5 wt % are explained by extra-framework phosphate species near the crossing intersections of the ZSM-5 pore system, which hinder the formation of large intermediates and reaction products.

Selective production of propylene from methanol over high-silica mesoporous ZSM-5 zeolites treated with NaOH and NaOH/tetrapropylammonium hydroxide

“…Beside the acidic properties of zeolite catalysts, structural properties such as pore size distribution and channel length also play a key role on the stability of the alkali-treated ZSM-5 catalysts [13,53]. Since the majority of the active sites of the parent ZSM-5 catalysts are located in its micropore channels, it thus has a particular shape selectivity.…”

Active intermediates formed on phosphorous-modified MTW zeolites in methanol to olefin conversion: An ESR study

“…S5. Aluminum atoms of P(x)-MTW exhibited two 27 Al MAS NMR peaks at 54 and −2 ppm; the former peak was attributed to the aluminum atoms with tetrahedral coordination and the latter peak to those with octahedral coordination [25,27]. The negligibly small latter peak indicated that most aluminum atoms of P(0.0)-MTW were located in its framework with a tetrahedral coordination.…”

“…The increase in the amount of phosphorous induced the increase in the relative intensity of the peaks at 1 and −5 ppm, while that of the peak at −11 and −18 ppm decreased considerably. The peaks at −11 and −18 ppm is attributed to the phosphorous atoms of Al O P bonds, while those at 1 and −5 ppm are from the phosphorous atoms of phosphorous oxides located in the zeolite pores [25,27]. When the amount of phosphorous was small, most phosphorous atoms were combined to aluminum atoms through oxygen atoms.…”

“…The 27 Al chemical shifts were referenced to Al(H 2 O) 6 3+ solution. The 31 P MAS NMR spectra, at a spinning rate of 11.0 kHz, were recorded on the same spectrometer at a 31 P frequency of 202.450 MHz.…”

“…The 27 Al MAS NMR spectra were obtained on a Bruker AVANCE 500 spectrometer at a 27 Al frequency 130.325 MHz with spinning a 4 mm rotor at 12.0 kHz. The 27 Al chemical shifts were referenced to Al(H 2 O) 6 3+ solution.…”