Enhanced Catalyst Performance of Zeolite SSZ-13 in the Methanol to Olefin Reaction after Neutron Irradiation

Abstract

Postsynthesis treatments are frequently used as a means of improving the performance of zeolitic catalysts. This study is concerned with the postsynthesis modification of zeolite H-SSZ-13 by neutron irradiation. An enhanced catalytic activity in the methanol to olefin (MTO) reaction is observed after neutron irradiation of the zeolite H-SSZ-13 catalyst. The magnitude of the neutron irradiation effect depends on the irradiation time and the Si/Al ratio of the parent material, as the highest increase in catalytic activity was observed for the irradiated high silica H-SSZ-13. FT-IR measurements with CO as a probe molecule revealed no significant change in the nature of the acid sites, and the acid strength of the catalytically important Brønsted acid sites was maintained after irradiation. Nitrogen physisorption measurements indicated a decrease in specific surface area and pore volume after neutron irradiation, whereas XRD confirmed that the crystallinity was maintained. The water content was found to be lower in the irradiated material, indicating an altered environment within the zeolite voids after irradiation. EPR measurements clearly showed the formation of radicals in consequence of the irradiation procedure. These radicals have been found to be stable in air, in daylight, and after heating to the MTO reaction temperature, and they were still detectable two years after irradiation. We suggest that the radical species consist of peroxy radicals (Si−O−O*) and nonbridging oxygen hole centers (NBOHC: ≡Si−O*) that are formed due to irradiation-induced bond rupture, and we further suggest that radiation-induced structural defects are responsible for the altered catalytic properties of zeolite H-SSZ-13.

Plasma Generation in a Gas Discharge System With Irradiated Porous Zeolite

“…It can be assumed that some hydroxyl radicals formed by radiolysis of water can disrupt the Si-O-Al or Si-O-Si bonds in the clinoptilolite framework and radiolytic production of hydrated protons [H + (H 2 O − ) n ] can generate cleavage of the bonds. In this case, irradiation leads to the formation of charged defect B-centers in ZC ir and they are placed where an electron was captured in an Al-O unit [17].…”

Radiation-modified natural zeolites for cleaning liquid nuclear waste (irradiation against radioactivity)

“…It is likely that part of the mechanism by which the cation-exchange capacity of clinoptilolite toward Cs + is so markedly improved by prior electron-irradiation involves the radiolysis and displacement of water molecules that are entrapped in the zeolite micropores, along with the creation of other point-defects and (charged) radiation damage centres where Cs + (along with other radionuclide cations) can be trapped, although there is much to be understood about the process as yet. It was proposed previously that hydroxyl radicals, formed by the radiolysis of water, might be able to break Si-O-Si or Si-O-Al bonds in the zeolite framework, and that the radiolytic production of hydrated protons [H + (H 2 O) n ] can also induce the cleavage of these bonds15. A variety of charged defect centres have also been characterized in zeolites and related materials formed by exposure to radiation of various kinds, e.g.…”

“…Positively charged (hole) sites of the type, Si-O ○ -Si and Si-O ○ -Al have been identified by E.P.R. spectroscopy in irradiated zeolite samples15, and a wide range of radical cations formed by charge-transfer from adsorbed organic molecules to the zeolite, have also been characterized16. However, the nature of the corresponding electron capture site is less clear16.…”

“…However, the nature of the corresponding electron capture site is less clear16. In a recent study, the catalytic performance of the zeolite SSZ-13 was shown to be enhanced by neutron irradiation, in which Si-O-O •  peroxyl radicals and nonbridging oxygen hole centres (NBOHC: Si-O ○ ) were identified and are proposed to be the active defects responsible for the catalyst modification15. Most noteworthy is the fact that in previous studies of zeolites that were exposed to high-energy electrons, a partial or complete amorphization of the initially crystalline structure occurred, accompanied by a dramatic reduction (by up to 95%)1314 in the cation-exchange and absorption/desorption capacities of the zeolite for radionuclides, such as Cs + and Sr 2+ .…”

Synthesis of nano-SSZ-13 and its application in the reaction of methanol to olefins

“…These reaction conditions have been used recently by Wu et al 43,46 for testing mesoporous SSZ-13 samples obtained by adding mesoporogen agents to the synthesis gel and hence, the results obtained with nano-SSZ-13 can be compared with those mesoporous SSZ-13 samples previously reported. As it can be seen, all samples show initial 100% conversion.…”