The Synthesis of YNU-5 Zeolite and Its Application to the Catalysis in the Dimethyl Ether-to-Olefin Reaction

During prior investigations of the synthesis of the novel zeolite YNU-5 (YFI), it was found that a very slight amount of an impurity phase contaminated the desired zeolitic phase. This impurity was very often ZSM-5 (MFI). The phase composition was determined to be sensitive to the water in the synthesis mixture, and it was possible to obtain a pure phase and also to intentionally generate a specific impurity phase. In the present work, trials based on the dimethyl ether-to-olefin (DTO) reaction using a fixed-bed downflow reactor were performed to assess the effect of the purity of YNU-5 on its catalytic performance. Dealuminated pure YNU-5 exhibited rapid deactivation due to coking at time on stream (TOS) values exceeding 5 min. Surprisingly, this deactivation was greatly suppressed when the material contained a trace amount of ZSM-5 consisting of nano-sized particles. The formation of ZSM-5 nanoparticles evidently improved the performance of the catalytic system during the DTO reaction. The product distributions obtained from this reaction using highly dealuminated and very pure YNU-5 resembled those generated by 12-ring rather than 8-ring zeolite catalysts. The high selectivity for desirable C3 and C4 olefins during the DTO reaction over YNU-5 is beneficial.

Effects of the Selenium Precursor on the Growth of ZnSe by Metalorganic Chemical Vapor Deposition

ABSTRACTThe growth of ZnSe on GaAs substrates by metalorganic chemical vapor deposition was investigated in a specially designed vertical downflow reactor. Dimethylzinc was used as the Zn source while different Se source compounds (hydrogen selenide (H2Se), diethylselenide and methylallylselenide) were employed to determine the effect of different source combinations on morphology, thickness uniformity, growth rate, electrical properties and photoluminescence (PL) characteristics of the grown ZnSe films. The H2Se was produced in situ by reaction of H2 and Se followed by distillation to control the amount of H2Se entering the reaction zone. H2Se produced very high mobility films with good PL spectra but poor surface morphology. Diethylselenide led to layers of good morphology and PL characteristics but the films were highly resistive. Unusual surface features were observed for methylallylselenide.