3,428 research outputs found
Effects of reaction conditions on the selective oxidation of propane to acrylic acid on Mo-V-Te-Nb oxides
No full textAn effective Mo-1 V(0.3)Te(0.23)Nb(0.12)Ox catalysts for the selective oxidation of propane to acrylic acid was successfully prepared by using rotavap method. The catalyst was characterized by XRD and shown to contain (V0.07Mo0.93)(5)O-14, (Nb0.09Mo0.91)O-2.8,3MoO(2)(.)Nb(2)O(5), Mo5TeO16 and/or TeMo4O13, Te4Nb2O13 and a new TeMO (TeVMoO or TeVNbMoO; M = Mo, V and Nb) crystalline phase as the major phase. Regardless of the intrinsic catalytic characteristics of the catalyst, the external reaction conditions would have strong effects on the catalytic performance for propane oxidation. So in this paper, the effects of reaction conditions were investigated and discussed, including temperature, space velocity, V(air)/V(C3H8) ratio and V(steam)/V(C3H8) ratio. A stability test was also carried out on Mo1V0.3Te0.23Nb0.12Ox catalyst. The experimental run was performed during 100 h under the optimized reaction conditions. During the 100 h of operation, propane conversion and acrylic acid selectivity remained at about 59 and 64%, respectively. (C) 2004 Elsevier B.V. All rights reserved.An effective Mo-1 V(0.3)Te(0.23)Nb(0.12)Ox catalysts for the selective oxidation of propane to acrylic acid was successfully prepared by using rotavap method. The catalyst was characterized by XRD and shown to contain (V0.07Mo0.93)(5)O-14, (Nb0.09Mo0.91)O-2.8,3MoO(2)(.)Nb(2)O(5), Mo5TeO16 and/or TeMo4O13, Te4Nb2O13 and a new TeMO (TeVMoO or TeVNbMoO; M = Mo, V and Nb) crystalline phase as the major phase. Regardless of the intrinsic catalytic characteristics of the catalyst, the external reaction conditions would have strong effects on the catalytic performance for propane oxidation. So in this paper, the effects of reaction conditions were investigated and discussed, including temperature, space velocity, V(air)/V(C3H8) ratio and V(steam)/V(C3H8) ratio. A stability test was also carried out on Mo1V0.3Te0.23Nb0.12Ox catalyst. The experimental run was performed during 100 h under the optimized reaction conditions. During the 100 h of operation, propane conversion and acrylic acid selectivity remained at about 59 and 64%, respectively. (C) 2004 Elsevier B.V. All rights reserved
Effect of Mg addition on catalytic performance of Mo/ZSM-5 catalyst for methane aromatization
No full textThe effect of Mg addition no the Mo/ZSM-5 catalyst Was investigated by XRD, nitrogen adsorption and pyridine FTAR methods. The FTAR spectra of pyridine adsorbed on Mo-Mg/ZSM-5 indicated that the ratio of Lewis acid sites to Bronsted acid sites on the catalyst samples increased with the increase of Mg loading. When Mo-Mg/ZSM-5 was used in methane dehydro-aromatization, the carbon deposition on the catalyst decreased with the increase of Mg loading, and only the stability of 6% Mo-0.75% Mg/ZSM-5 was higher than that of 6% Mo/ZSM-5. This illuminated that the catalyst stability can be enhanced by optimizing the ratio of Lewis acid sites to Bronsted acid sites with medium/strong strength in the presence of enough Bronsted acid sites
Aromatization of methane over different Mo-supported catalysts in the absence of oxygen
No full textBoth acidity and structure of the support are important factors in converting methane to aromatics. Lower SiO2/Al2O3 ratio seems to favor the aromatization of methane over the Mo/HZSM-5 catalyst. When Pt is added as a modifier the activity of Mo/HZSM-5 catalyst will decrease slightly, but coke formation will be enhanced
MOLYBDENUM DOPED CARBON AEROGELS WITH CATALYTIC POTENTIAL
No full textMo-doped carbon aerogels were obtained in the polycondensation reaction of aqueous
resorcinol and formaldehyde by adding Mo-salt at two different stages of the synthesis: i) to
the initial sol; ii) by incipient wetting impregnation of the supercritically dried polymer gel.
Molybdenum added during the polymerization yielded a more compact gel structure with
practically no mesoporosity. With post-impregnation, by contrast, mesopores of diameter 3-15
nm were generated. Carbonization appreciably enhanced the microporous character of both
samples, but in the mesopore range their pore size distribution was conserved. The Mocontent
of the samples was also different: Mo was lost during the solvent exchange before the
supercritical drying (i.e., the Mo failed to bind chemically to the polymer matrix). The
residual Mo congregated into 25-60 nm bulk clusters of α-Mo2C. In the other carbon aerogel,
finely dispersed α-Mo2C and η-Mo3C2 crystals formed, of size 8-20 nm. On the surface of
both carbons the Mo formed oxides. In the model test reaction (acetic acid hydroconversion)
the catalytic activity of both carbon aerogels was enhanced by molybdenum. The more open
pore structure, higher concentration and finer Mo distribution, as well as its chemical form,
may all be responsible for the greater conversion and higher value products obtained with the
post-impregnated sample
Hydrothermal post-synthesis of HZSM-5 zeolite to enhance the coke-resistance of Mo/HZSM-5 catalyst for methane dehydroaromatization
No full textHydrothermal post-synthesis was used to modify the micropores and acidity of commercially available HZSM-5 zeolites. The recrystallization and the dynamic incorporation and extraction of the framework Al not only stabilized the framework with high crystallinity, but also inhibited the creation of extra-pores during the post-synthesis in NaOH aqueous solution. The resulted Mo/HZSM-5 catalyst showed rather high catalytic stability and greatly enhanced selectivity towards aromatics for methane dehydroaromatization reaction by effectively inhibiting the coke formation
Sprachen mit Zahlklassifikatoren: Analyse und Vergleich
No full textDas Ziel dieser Arbeit besteht zunächst darin, in einem 1. Teil das Phänomen 'Zahlklassifikator' zu explizieren und an Beispielen – vornehmlich aus den Sprachen Südostasiens – zu erläutern. Im 2. Teil werden dann Sprachen Ozeaniens und Mayasprachen Mittelamerikas, die nicht als 'typische' Klassifikatorsprachen gelten, dargestellt und anhand der im 10 Teil entwickelten Begriffe, diskutiert. Den Ausgangspunkt der Überlegungen bildet die Arbeit von Greenberg (1972) über Zahlklassifikatoren, deren Thesen im 1. Teil dargestellt und kommentiert werden. Die theoretische Grundlage dieser Arbeit ist die Universalienkonzeption, wie sie dem Kölner Forschungsprojekt für Universalien und Typologie (UNITYF) zugrunde liegt. Der besondere Rahmen, in dem die Bearbeitung dieses Themas steht, ist die Dimension der 'Individuation', bei der es um Prozesse zur 'Erfassung von Gegenständen' geht. (Zu den theoretischen Grundlagen von UNITYF und dem Begriff der Dimension siehe Seiler (1977 (a) und (b).
The function of Cu(II) ions in the Mo/CuH-ZSM-5 catalyst for methane conversion under non-oxidative condition
No full textNon-oxidative aromatization of methane was carried out over Mo/CuH-ZSM-5 and compared with that over Mo/H-ZSM-5. Cu(II) ions act as promoter in this reaction and improve the activity of the Mo/H-ZSM-5 catalyst notably. The effect of Cu species on the chemical state of Mo species and the changes of Cu species itself after reaction were studied by ESR and XPS methods. The introduction of Cu ions by ion-exchange method altered the reduction of Mo species, suppressed the dealumination of ZSM-5 framework and decreased coke formation over the catalyst. The results showed improved catalytic performance of Mo/H-ZSM-5 catalyst. The amount and nature of the coke which were influenced by the introduction of Cu ions were also investigated. (C) 1999 Elsevier Science B.V. All rights reserved
The effect of Cu species on the chemical state of Mo in the Mo/CuH-ZSM-5 catalyst for methane non-oxidative aromatization
No full textThe catalytic performance of Mo/CuH-ZSM-5 was compared with that of Mo/H-ZSM-5 for methane aromatization in the absence of oxygen. The Cu ions acted as promoters, and the activity and stability of catalysts for methane aromatization were enhanced to some extent. The fresh and reacted catalysts was studied by XRD, ESR and XPS methods, and it is found that by the addition of Cu species, the reduction of MoO3 was suppressed and the Cu species itself was reduced. It is supposed that the Mo species with a higher oxidative state is responsible for the activation and conversion of methane
Study of Mo-based catalysts for dehydro-aromatization of methane
No full textMethane dehydro-aromatization over Mo/HZSM-5 catalyst has been widely studied in recent years. One of the main problems for the industrial application is the short lifetime of the catalyst owing to the serious carbon deposit on the catalyst surface. The addition of CO or CO2 in feed as well as the steaming-dealumination and the silanization of the support was used to enhance the stability of the catalyst. In order to effectively suppress the formation of carbon deposit, we designed an easy and special route to prepare the Mo/HZSM-5(t) catalyst, in which HZSM-5 was precisely pretreated by calcination in air at 400 degreesC for 4 h, then re-calcination in N-2 at 500 degreesC for 4 h (precise pretreatment of support) before impregnation with ammonium molybdate. The precise pretreatment was found to be able to significantly improve the activity and the stability of the catalyst. On Mo/HZSM-5 (t), the methane conversion and benzene yield even remained at 8.8% and 5.3% respectively after 30 h. Addition of promoter Fe leads to a significant increase in methane conversion and the selectivity for benzene, and methane conversion and benzene yield increase by 18% and 52% respectively, compared with Mo/ HZSM-5(t). Through XRD and TPO characterization, it is concluded that Mo and Fe species are highly dispersed on the surface of the support or migrate into the channels of the support. The precise pretreatment of support and the addition of Fe restrain carbon deposit, hence the catalysts with high stability can be obtained
Contributions of large wood to the initial establishment and diversity of riparian vegetation in a bar-braided temperate river
No full textThe purpose of this study was to examine the effects of large wood (LW) on the physical environment and the initial establishment of vascular plant species in the Rekifune River, a large bar-braided monsoonal river in Japan. The physical environment and the diversity and composition of plant species were compared in relation to the orientation of LW pieces. We found that shading effects were more prevalent in the immediate vicinity of LW pieces than in quadrats distant from LW. The effect was especially strong at the center of LW jams (the "jam center"). Fine sand and silt were concentrated in the quadrats downstream from the LW pieces. In contrast, cobbles dominated the upstream quadrats. The highest diversity was found in the jam center, while intermediate values were observed in the quadrats surrounding LW. Indicator species analysis detected 21 indicator species only in the jam center. The LW jams favored the deposition of plant fragments and sediment and created shaded areas within and around the structures. Buried seeds may be transported with LW during a flood, and seeds dispersed by wind and stream flows may be trapped by the complex structure of LW jams. The specific environmental conditions and the trapping of seeds and plant fragments result in the early establishment of mid-successional tree species at LW jams. In conclusion, the LW pieces deposited on gravel bars altered the light and substrate conditions and thereby provided specific safe sites for various riparian plant species
- …
