1,720,962 research outputs found
Probing the Catalytic Function of External Acid Sites Located on the MFI Nanosheet for Conversion of Methanol to Hydrocarbons
No full textThe surfactant-directed MFI zeolite nanosheet of 2.5-nm thickness has been re-investigated to clarify
whether the catalytic function in the methanol-to-hydrocarbon(MTH) reaction originates from acid sites located solely in the internal micropores or also on the external surfaces. Two catalytic reactions were measured after all acid sites on the external surfaces were poisoned with triphenylphospine
oxide. One reaction was the catalytic cracking of triisopropylbenzene, which can occur only on
external surfaces. This reaction was completely killed by poisoning. The second reaction was MTH conversion. Unlike the catalytic cracking of triisopropylbenzene, the MTH reaction proceeded at the same rate, even when the external acid sites were poisoned. The result indicated that the MTH reaction occurred inside the internal pores, not at the external surfaces. This information will be useful for
understanding MTH catalysis of other hierarchical zeolites.122221sciescopu
High utilization of methanol in toluene methylation using MFI zeolite nanosponge catalyst
No full textA highly mesoporous MFI zeolite nanosponge, which was composed of a disordered assembly of 2.5-nm thick zeolite frameworks, was synthesized using a meso- and micropore dual structure-directing surfactant. The zeolite nanosponge was investigated as a catalyst for toluene methylation by methanol. The results showed remarkably high toluene conversion and xylene yield, in comparison with the bulk zeolite counterparts. The high catalytic performance was attributed to the suppression of side reactions that could convert methanol to linear hydrocarbons. This suppression can be attributed to the fact that the dealkylation of polymethylbenzenes, which must take place before linear hydrocarbon formation, did not occur significantly in the zeolite nanosponge as compared to the case of bulk zeolites. In addition to the aforementioned high performance, the MFI zeolite nanosponge was also superior to the bulk MFI in terms of catalytic longevity. © 2017 Elsevier B.V
Upgrading of bio-oil derived from biomass constituents over hierarchical unilamellar mesoporous MFI nanosheets
No full texttUnilamellar mesoporous MFI nanosheets (UMNs) were, for the first time, applied to the catalytic pyrol-ysis of the constituents of lignocellulosic biomass: cellulose, hemicellulose and lignin. A representativemesoporous catalyst, Al-SBA-15, was also applied to the same process for the purpose of comparison. Thecatalysts were characterized by N2adsorption–desorption, X-ray diffraction, temperature programmeddesorption of ammonia, and transmission electron microscopy. The synthesized UMN catalyst was com-posed of a random assembly of zeolite nanosheets with a thickness of 2.5 nm. UMN was shown to havestrong Brønsted acid sites, whereas Al-SBA-15 had weak acid sites. Catalytic pyrolysis experiments werecarried out using Py-GC/MS. The product distribution was analyzed in order to evaluate catalytic activity.Acidity had a considerable influence on catalytic activity; UMN, with a higher acidity than Al-SBA-15,exhibited higher activities for cracking and deoxygenation, producing bio-oil with a lower oxygen con-tent and with a better overall quality. Levoglucosan, which was the main product of the non-catalyticpyrolysis of cellulose, was converted to aromatics and furans on the acid sites of the catalysts. Ketones,alcohols and cyclo-compounds, the main products of the non-catalytic pyrolysis of xylan, were also con-verted to aromatics and furans by catalytic upgrading. The main products of the non-catalytic pyrolysisof lignin were phenolics, which were converted to aromatics and lighter phenolics (alkyl phenolics andmethoxy phenolics) in the presence of catalysts. Increasing the catalyst dose improved the quality of theproduct oil even further.136361sciescopu
Conversion of kraft lignin over hierarchical MFI Zeolite
No full textCatalytic pyrolysis of kraft lignin was carried out using pyrolysis gas chromatography/mass spectrometry. Hierarchical mesoporous MFI was used as the catalyst and another mesoporous material Al-SBA-15 was also used for comparison. The characteristics of mesoporous MFI were analyzed by X-ray diffraction patterns, N2 adsorption-desorption isotherms, and temperature programmed desorption of NH3. Two catalyst/lignin mass ratios were tested: 5/1 and 10/1. Aromatics and alkyl phenolics were the main products of the catalytic pyrolysis of lignin over mesoporous MFI. In particular, the yields of mono-aromatics such as benzene, toluene, ethylbenzene, and xylene were increased substantially by catalytic upgrading. Increase in the catalyst dose enhanced the production of aromatics further, which is attributed to decarboxylation, decarbonlyation, and aromatization reactions occurring over the acid sites of mesoporous MFI. Copyright © 2014 American Scientific Publishers All rights reserved.116161sciescopu
External Surface Catalytic Sites of Surfactant-Tailored Nanomorphic Zeolites for Benzene Isopropylation to Cumene
No full textNanomorphic *BEA, MTW, and *MRE zeolites
were investigated as catalysts for isopropylation of benzene. From
the deactivation pattern of the zeolites, we evaluated the
contribution of external and internal active sites. These nanomorphic
zeolites exhibited a high activity and long catalytic lifetime.
Such catalytic properties can be explained by a large contribution of
external sites, which have the advantage of slow deactivation.148411sciescopu
n-Heptane hydroisomerization over Pt/MFI zeolite nanosheets: Effects of zeolite crystal thickness and platinum location
No full textPlatinum was supported on MFI-type (ZSM-5) zeolites with various crystal thicknesses ranging from 300
to 2 nm (i.e., from bulk crystal to nanosheet). Two series of Pt/MFI were prepared by the ion exchange of PtðNH3Þ2þ4 and the impregnation of colloidal Pt nanoparticles. The ion exchange yielded Pt nanoparticles
that were supported inside zeolite pores, whereas the impregnation yielded Pt nanoparticles supported
exclusively on the external crystal surfaces. The two series of Pt/MFI were used as catalysts for the hydroisomerization
of n-heptane to investigate the effects of zeolite crystal thickness and Pt location. This
research showed that the product selectivity to branched isomers could be significantly improved by
decreasing zeolite crystal thickness to nanosheets. The selectivity improvement was attributed to short
diffusion path lengths for branched products to escape before cracking.158621sciescopu
Mesopore-selective incorporation of strong Brønsted acid catalytic sites via aluminium grafting on hierarchically porous siliceous MFI zeolite
No full text© 2020 Elsevier Inc. Post-synthetic Al grafting onto the surface silanols in mesoporous MFI zeolite has been investigated in an attempt to obtain a model catalytic material that possesses strong Brønsted acid sites at mesopore surfaces of the zeolite, but not inside the microporous framework. To this end, a siliceous MFI zeolite with a mesopore/micropore hierarchy was synthesized using a meso-micro dual structure-directing diammonium surfactant. The hierarchically porous zeolite was treated with anhydrous AlCl3 for the formation of Si–O–Al bonding with silanols on the mesopore surfaces. According to solid-state 31P NMR spectroscopy of the surface-adsorbed phosphine oxides and catalytic activity for decalin cracking, the Al-grafting treatment performed in the conventional manner resulted in the formation of weak Brønsted acid sites, similar to Al grafting on SBA-15 mesoporous silica. To obtain strong Brønsted acid sites, we have developed a method of heating the AlCl3-grafted zeolite in a controlled-humidity chamber before calcination. The resultant zeolite was catalytically active for cracking of hydrocarbons requiring strong acid sites, but not for methanol-to-hydrocarbon conversion. The results support that the moisture treatment caused a local reconstruction of the initially grafted Al–O–Si framework portion to a strong Brønsted acid site. In addition, our model catalytic zeolite material unveiled the catalytic activity of Brønsted acid sites at the mesopore surfaces in hierarchically porous zeolite in n-octane cracking. Furthermore, we confirmed that this method was effective for other mesoporous MFI zeolites obtained through a post-synthetic desilication route11sciescopu
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
- …
