Effect of stabilizer type on the physicochemical properties of rigid PVC. III

The crystallinity of rigid PVC specimens, stabilized by a tin mercaptide or a lead stabilizer, has been studied. It is found that the crystallinity of rigid PVC is greatly influenced by the type of stabilizer added, and that the response of the PVC toward subsequent UV exposure was also different. The effects of unstable structures and crystallinity, after UV irradiation, on the mechanical properties of rigid PVC have been investigated.     

Effect of alumina-deficiency and promotion with platinum and rhenium on the catalytic and diffusional behaviour of mordenite catalysts

The hydroconversion of n-heptane on catalysts containing platinum hydrogen-mordenite, platinum/alumina-deficient hydrogen mordenite and platinum-rhenium/alumina-deficient hydrogen mordenite was carried out in a continuous high-pressure plug-flow reactor system and product analysis was carried out by gas chromatography (GC). The effectiveness factor for each catalyst was calculated by iteration on estimated rate constant at various reaction temperatures and the effect of diffusion on the catalytic behaviour was correlated. Catalysts exhibiting larger values of effectiveness factor were found to enhance production of 2,3-dimethylpentane on account of 3-methylhexane, whereas the reverse is true with respect to catalysts exhibiting lower values of effectiveness factor.     

Dual growth of graphene nanoplatelets and carbon nanotubes hybrid structure via chemical vapor deposition of methane over Fe–MgO catalysts

Abstract

In this study, Fe–MgO catalyst substrates with various Fe and MgO combinations were evaluated for the growth of different types of carbon nanostructure materials (CNMs), particularly graphene nanoplatelets (GNPs) and carbon nanotubes (CNTs) via chemical vapor deposition using methane as a carbon source. The hydrogen yield was also determined as a valuable by-product in this process. Therefore, a set of Fe–MgO catalysts with different iron loadings (30, 80, 85, 90 and 100?wt %) were prepared by the combustion method to realize this target. The physicochemical properties of freshly calcined Fe–MgO catalysts were investigated by XRD, TPR and BET, while the as-grown CNMs were studied by HR-TEM, XRD and Raman spectroscopy. The results verified that the morphology of as-grown CNMs as well as the H₂ yield was directly correlated to the iron content in the catalyst composition. The XRD and TPR results showed that various FeMgO_x species with deferent levels of interactions were produced with the gradual incorporation of MgO content. TEM images indicated that GNPs were individually grown on the surface of high loaded iron-containing catalysts (90–100?wt %) due to the presence of highly aggregated iron particles. While multi-walled carbon nanotubes (MWCNTs) with uniform diameters were grown on the low iron-loaded catalyst (30%Fe/MgO) due to the formation of highly dispersed FeMgO_x particles. On the other hand, GNPs/MWCNTs hybrid materials were grown on the surface of 80%Fe and 85%Fe/MgO catalysts. This behavior can be interpreted by the co-existence of highly aggregated and highly dispersed Fe₂O₃ particles in the catalyst matrix. The results demonstrated that the catalyst composition has a notable effect on the nature of CNMs products and H₂ yield.     

Non-oxidative Decomposition of CH4 Over CeO2 and CeO2–SiO2 Supported Bimetallic Ni–Mo Catalysts

Catalysis Letters - The development of highly active and durable catalysts for H2 production through CH4 decomposition process is still a great challenge. In this study, CeO2 and CeO2–SiO2...     

Synthesis of carbon nanotubes by CCVD of natural gas using hydrotreating catalysts

Carbon nanotubes have been successfully synthesized using the catalytic chemical vapor deposition (CCVD) technique over typical refining hydrotreating catalysts (hydrodesulfurization and hydrodenitrogenation) containing Ni–Mo and Co–Mo supported on Al₂O₃ catalysts at 700°C in a fixed bed horizontal reactor using natural gas as a carbon source. The catalysts and the as-grown CNTs were characterized by transmission electron microscopy, HRTEM, X-ray diffraction patterns, EDX and TGA–DTG. The obtained data clarified that the Ni–Mo catalyst gives higher yield, higher purity and selectivity for CNTs compared to Co–Mo catalyst. XRD, TEM and TGA reveal also that the Ni–Mo catalyst produces mostly CNTs with different diameters whereas the Co–Mo catalyst produces largely amorphous carbon.     

Nanosized Platinum-Loaded H-ZSM-5 Zeolite Catalysts for n-Hexane Hydroconversion

Abstract

A series of nanosized platinum-containing catalysts was successfully loaded on/in zeolite H-ZSM-5 via exchanging the zeolitic proton with platinum from Pt tetramine dichloride complex. Another series of Pt/H-ZSM-5 catalysts was prepared via wet impregnation of H₂PtCl₆ solution for comparison. The latter series was found to produce lower Pt dispersion. Pt dispersion was determined by H₂ chemisorption. Catalyst characterization via ammonia temperature-programmed desorption (TPD), temperature-programmed reduction (TPR), and transmission electron microscopy (TEM) was examined for all catalysts and showed large differences in particle sizes. The data on n-hexane reactions of the catalysts of both series confirmed the formation of Pt nanoparticles in the exchanged catalysts. The relatively lower density and strength of acid sites acquired by Pt-exchanged catalysts contributed to this difference; stronger acid sites in the impregnated catalysts are in favor of hydrocracking reactions, which inhibit isomerization selectivity.     

Direct Conversion of Natural Gas to Petrochemicals Using Monofunctional Mo/SiO2 and H-ZSM-5 Zeolite Catalysts and Bifunctional Mo/H-ZSM-5 Zeolite Catalyst

Abstract

The components of the standard catalyst globally used for natural gas direct conversion (6% Mo/H-ZSM-5) have been separately prepared and examined to represent: (a) monofunctional metallic component (6% Mo/SiO₂) and (b) acidic component (H-ZSM-5 zeolite) to numerically investigate the extent of activity of each catalytic component in comparison to the activity of the standard catalyst in a fixed-bed flow-type reactor. The temperature and gas hourly space velocity are 700°C and 1500 mLg^?1 h ^?1, respectively, which are very close to those used in the industrial gas conversion reactions. Time on stream up to 240 min was examined. The gaseous products were ethylene, propylene, and hydrogen, whereas the liquid products were benzene, toluene, and naphthalene. Carbon was also produced as deposited particles on the catalyst.     

Direct conversion of natural gas into COx-free hydrogen and MWCNTs over commercial Ni–Mo/Al2O3 catalyst: Effect of reaction parameters

A commercial hydrotreating nickel molybdate/alumina catalyst was used for the direct conversion of natural gas (NG) into CO_x-free hydrogen and a co-valuable product of multi-walled carbon nanotubes (MWCNTs). The catalytic runs were carried out atmospherically in a fixed-bed flow reactor. The effect of reaction temperature between 600 and 800 °C, and dilution of the NG feed with nitrogen as well as pretreatment of the catalyst with hydrogen were investigated. At a reaction temperature of 700 °C and dilution ratio of NG/N₂ = 20/30, the optimum yield of H₂ (～80%) was obtained with higher longevity. However, using the feed ratio of NG/N₂ = 30/20, the optimum yield of MWCNTs was obtained (669%). X-ray diffraction pattern for the catalyst after the reaction showed that the MWCNTs were grown on the catalyst at all reaction temperatures under study. TEM pictures revealed that the as-grown MWCNTs at 600, 650 and 800 °C are short and long with a low graphitization degree. At 700 °C a forest of condensed CNTs is formed, whereas both carbon nanofibers and CNTs were formed at 750 °C.     

Performance of ultrasonic – Treated nano-zeolites employed in the preparation of dimethyl ether

Catalytic dehydration of methanol to dimethyl ether was carried out over nano-zeolites and their modified samples via ultrasonic technique. Parent H-Beta and Parent H-Mordenite zeolites are used as synthesized and after sonication. H-Mordenite sonicated for 20 and 120 min whereas H-Beta sonicated for 20 min only. The reaction temperature was varied between 100 and 225 °C at three different contact times. The different catalyst samples were characterized using: XRD, FTIR, TEM, SEM and NH₃-TPD techniques. The results revealed that sonication of parent zeolite samples affects on the unit cell dimensions and their crystal size. FTIR-spectroscopic analysis indicated that sonication may decrease the pore opening and cause framework structure defects. TEM and SEM micrographs showed that sonication broke-up and re-ordered zeolite crystals with longer time resulted in a different morphology relative to parents and also change the particle size. Sonicated samples have a good performance in methanol dehydration with complete conversion and complete selectivity to dimethyl ether at lower temperature relative to the corresponding parent zeolites. TPD results indicated that the concentration of strong acid sites decreased in sonicated H-Mordenite samples leading to an increase of their catalytic activity and the selectivity to DME. On the other hand, ultrasonic treatment of H-Beta sample decreased its catalytic activity.     

Effect of hydrochlorination and hydrofluorination of Pt/H-ZSM-5 and Pt–Ir/H-ZSM-5 catalysts for n-hexane hydroconversion

Pt/H-ZSM-5 and Pt–Ir/H-ZSM-5 catalysts were hydrochlorinated or hydrofluorinated with 3.0 wt%HCl or HF, respectively. These mono- and bimetallic catalysts were tested for n-hexane hydroconversion in a pulsed microcatalytic reactor in a flow of H₂ gas. The catalysts were characterized via XRD, metal dispersion via H₂ chemisorption and acid site strength distribution using temperature programmed desorption of ammonia (TPD). Although metallic promoters frequently cause inhibition of the catalytic activities of Pt and since iridium is less active than platinum, fortunately, Ir was found to enhance the hydroconversion activities of the current catalysts, particularly after hydrochlorination.