104809-90-7Relevant articles and documents
Facile Hydrodehalogenation with H2 and Pd/C Catalyst under Multiphase Conditions. 2. Selectivity and Kinetics
Marques, Carlos Alberto,Selva, Maurizio,Tundo, Pietro
, p. 3830 - 3837 (1994)
Hydrodehalogenation of polyhalogenated aromatics with Pd/C catalyst carried out in the presence of a quaternary onium salt follows zero-order kinetics in the substrate and first-order kinetics in the Pd/C catalyst; the related rate constants were determined for o-, m- and p-bromotoluenes, o-, m- and p-chloroalkylbenzenes (methyl, ethyl, and propyl derivatives), and other aryl halides.Reaction rates, depending on the aromatic to be reduced, may be strongly enhanced by the presence of quaternary onium salts: the isomeric chloroethylbenzenes were reduced 50 times faster when operating in the presence of Aliquat 336 (1).Also the hindered 2-chloro-m-xylene easily yielded m-xylene.The cocatalyst onium salts operate by being adsorbed on the Pd/C surface, as shown when kinetic constants are reported by varying the onium salt amount: classical Langmuir adsorption isotherms are observed.The presence of the onium salt may also influence selectivity in the reduction of isomeric aryl halides: when 1 is present, p-dichlorobenzene reacts in diethyl ether at 20 deg C, 5-fold slower than the ortho isomer; whereas the reduction rates of the two compounds are almost the same in its absence.
Generation of Acidic Sites in Metal Salts of Heteropolyacids
Baba, T.,Watanabe, H.,Ono, Y.
, p. 2406 - 2411 (1983)
The effects of water and hydrogen on the generation of acid sites in the metal salts of dodecatungstophosphoric acid (H3PW12O40, HTP) are studied by means of catalysis amd infrared spectroscopy.The silver salt has no activity for o-xylene isomerization and pyridinium ion was not formed by adsorption of pyridine.By the reaction of the salt with hydrogen at 300 deg C, the activity for xylene isomerization appeared and the development of acid sites is confirmed by infrared spectroscopy.The scheme of the generation of acid sites is expressed as Ag+ + 1/2 H2 --> Ago H+.The features of the copper(II) salt are similar to those of the silver salts.The high acidity of the copper(II) salt is demonstrated by the inhibition of the catalytic activity for dehydration of 1-butanol by the sorption of pyridine.For the aluminium salt, hydrogen has no effect on the isomerization activity.But the activity and the capacity for pyridinium ion formation are enhanced by treatment with water.A plausible scheme for acid-site formation in aluminium salt is expressed as Al3+ + H2O --> 2+ + H+.
Fine Control of the Pore-Opening Size of Zeolite ZSM-5 by Chemical Vapor Deposition of Silicon Methoxide
Niwa, Miki,Kato, Masaaki,Hattori, Tadashi,Murakami, Yuichi
, p. 6233 - 6237 (1986)
The pore-opening size of ZSM-5 was finely controlled by the chemical vapor deposition (CVD) of silicon methoxide.The silicon compound vapor was deposited, and the deposited material was calcined by oxygen, finally forming a silica-coated zeolite (SiHZSM-5).This was characterized by temperature-programmed desorption of ammonia and adsorption experiments using various molecules.It was found that the silica was deposited only on the external surface and narrowed the pore-opening size effectively with the internal structure unaltered.The fine control of the pore-opening size was tested, exemplified by the conversion of methanol.Hydrocarbon product distribution shifted to smaller molecules over the SiHZSM-5: the formation of smaller olefins such as ethylene and propylene increased in place of the complementary decrease of large aromatics.Enhancement of the preoduct shape selectivity was remarkable in xylene isomers and larger aromatics, which indicated the fine controls of the pore-opening size according to the deposition extent.Structures of the CVD zeolite and the enhanced shape selectivity in the methanol conversion were discussed.
CO2 atmosphere-enhanced methanol aromatization over the NiO-HZSM-5 catalyst
Li, Junhui,Hu, Chao,Tong, Kai,Xiang, Hao,Zhu, Zhirong,Hu, Zhonghua
, p. 44377 - 44385 (2014)
The aromatization of methanol over parent HZSM-5 and the modified NiO-HZSM-5 in a fixed-bed reactor was investigated under CO2 and N2 flow. The as-prepared catalysts were characterized by H2-TPR, XRD, BET, NH3-TPD, CO2-TPD and probe-molecule reaction. Compared with parent HZSM-5 in CO2 or N2 flow and with NiO-HZSM-5 in N2 flow, NiO-HZSM-5 showed greatly improved aromatization activity and BTX (benzene, toluene and xylene) yield in CO2 atmosphere. This is attributed to the cooperation of acid sites with the activated CO2 (over NiO species), which not only can effectively promote dehydrogenation of alkanes to form olefin intermediates, but also can accelerate dehydrogenation in the conversion of olefin intermediates to aromatics. In particular, an optimized NiO-HZSM-5 with 2.0 wt% NiO loading showed 50.1% total aromatic yield and 35.5% BTX yield in CO2 atmosphere. It also showed high catalytic stability resulting from the restriction of coking due to its acidity, carbonaceous elimination by the activated CO2-reacting deposited coke, and the suppressed reduction of highly active NiO species by activated CO2.
Enhanced para-xylene selectivity in the toluene alkylation reaction at ultralow contact time
Breen, John,Burch, Robbie,Kulkarni, Manisha,Collier, Paul,Golunski, Stan
, p. 5020 - 5021 (2005)
Dramatic improvements in the para-xylene selectivity of the toluene alkylation reaction can be effected by operating the catalytic reaction at ultralow contact time. Unexpectedly, the rate of alkylation is sustained, while unwanted side reactions are suppressed. By demonstrating that contact time directly influences the fate of para-xylene, which is known to form and diffuse preferentially within the zeolite catalyst, we conclude that external mass transfer is a key parameter in controlling selectivity. Even non-optimized catalysts can be made to achieve near-perfect selectivity, without sacrificing conversion. Copyright
Transient puffs of trace organic emissions from a batch-fed waste propellant incinerator
Hart, John R.
, p. 559 - 569 (2001)
Emissions data have been obtained from a waste propellant incinerator. The incinerator is a dual fixed hearth, controlled air incinerator equipped with acid gas and particulate scrubbing. Puffing has been evident in this waste propellant incinerator by spikes in the CO concentration. Transient puffs of organics may travel down the combustion chambers and lead to stack emissions. The major conclusions from this study are that (1) transient puffs are formed due to the semi-batch feed nature of the combustion process (causing a local oxygen deficiency) and high water content of the desensitized propellant; (2) in batch-fed combustors, puffs can contribute to most of the organic emissions (which are relatively low) measured with US EPA sampling and analytical methods; (3) it is estimated that batch-fed combustion contributes up to 7-18 times more emissions than steady-state combustion will generate; (4) by applying dispersion analyses to determine the amount of oxygen deficiency in the flame zone, the combustion zone concentration of CO during batch-fed operation could be as high as 160,000 ppm, compared to a measured peak stack concentration of 1200 ppm CO; and (5) an organic sample is collected and averaged over at least a 2-h period that smooths out the transient peaks of organics emissions during batch-fed operation. For emissions that are associated with long-term potential health impacts, this is an appropriate sampling method. However, if a compound has a short-term potential health impact, it may be important to measure the time-resolved emissions of the compound.
Structural and catalytic characterization of solid acids based on zirconia modified by tungsten oxide
Barton, David G.,Soled, Stuart L.,Meitzner, George D.,Fuentes, Gustavo A.,Iglesia, Enrique
, p. 57 - 72 (1999)
Tungsten oxide species form strong acid sites on ZrO2 supports and inhibit ZrO2 crystallite sintering and tetragonal to monoclinic structural transformations. W-LI X-ray absorption near-edge spectra suggest that the W centers are in a distorted octahedral oxygen environment, even after dehydration at 673 K, in all WOx-ZrO2 samples (2-21 wt.% W) oxidized at 1073 K. Maximum o-xylene isomerization turnover rates (per W atom) on WOx-ZrO2 solids occur at WOx surface densities (10 W nm-2) that exceed the theoretical monolayer capacity of ZrO2. Similar turnover rates are obtained on WOx-ZrO2 samples with similar WOx surface densities (W nm-2) over a large range of oxidation temperatures (773-1223 K) and WOx concentrations (5-21 wt.% W). UV-visible spectra suggest an increase in WOx domain size with increasing surface density. High isomerization turnover rates appear to require the presence of WOx domains of intermediate size on ZrO2 surfaces. WOx domains of intermediate size appear to provide a compromise between reducibility and accessibility of WOx centers. These domains are necessary to delocalize a temporary charge imbalance that forms Bronsted acid sites in the presence of H2 and stabilizes carbocation intermediates. The presence of H2 during o-xylene isomerization increases turnover rates and prevents rapid deactivation. Slow D2/o-xylene exchange reactions indicate that H atoms from H2 are not frequently involved in the activation or desorption of xylenes. H2 is required, however, in order to reverse the occasional desorption of H atoms during o-xylene isomerization reactions. These desorption processes lead to the destruction of Bronsted acid sites by the formation of strongly adsorbed unsaturated species in the absence of H2. After promotion with Pt (0.3 wt.%), WOx-ZrO2 solids catalyze n-heptane isomerization in the presence of H2 at 400-500 K with much higher selectivity than sulfated oxides or zeolitic acids at similar turnover rates. On Pt/WOx-ZrO2, efficient hydrogen transfer steps prevent extensive cracking of adsorbed carbocations by limiting their surface lifetimes.
Alkylation Reactions over Ion-exchanged Molecular Sieve Zeolite Catalysts. Part 1. Alkylation of Toluene with Methanol: Consideration of the Effects of Reaction Parameters on Catalyst Deactivation and the Extent of Polysubstitution
Coughlan, Brendan,Carroll, William M.,Nunan, John
, p. 281 - 296 (1983)
The alkylation of toluene with methanol under non-isomerizing conditions has been studied over a range of ion-exchanged zeolite catalysts.Rapid deactivation with increasing time on stream was observed in the case of LaY and NH4Y catalysts, whereas a NiY and a range of Ni,SnY samples manifested a high steady-state activity; the catalytic activity at the steady state could be related to the chemical composition of these latter samples.Catalyst deactivation was always associated with the formation of polysubstituted products.From a detailed study of the systems at the steady state it was concluded that (i) methylation of toluene is always a ssociated with the formation of tri- or tetra-methylbenzenes to a greater or lesser extent over the partially deactived catlysts; (ii) the origin of the polymethylated products is a weakly adsorbed o-xylene species: (iii) the selectivity towards polysubstitution varies with time on stream.This results from a change in the strength of interaction between the xylenes and the catalyst surface as shown by studies of the heats of immersion of the catalysts.
Simple two-step ipso substitution of aromatic carboxylic acids by alkyl halides
Vorndran, Katja,Linker, Torsten
, p. 2489 - 2491 (2003)
Methyl-substituted arenes can be synthesized with high regioselectivity in only two steps through formal exchange of an aromatic carboxylic acid function with an alkyl substituent. The results obtained with toluic acid illustrate that good to very good yields can be obtained from inexpensive reagents (see scheme).
Surface-Passivated Hierarchically Structured ZSM5 Zeolites: High-Performance Shape-Selective Catalysts for para-Xylene Production
Lv, Jian,Hua, Zile,Zhou, Jian,Liu, Zhicheng,Guo, Hangle,Shi, Jianlin
, p. 2278 - 2284 (2018)
Hierarchically structured ZSM-5 zeolites (HSZ) were synthesized and used as high-performance catalysts for para-xylene (p-X) production by tuning their pore structures and external surface acidity, which was achieved by two independent passivation approaches: dealumination in oxalic acid solution and chemical liquid deposition of tetra-ethoxysilane (CLD of TEOS). The mesoporous structures of HSZ were well-preserved after dealumination or CLD of TEOS, and the external surfaces of HSZ were passivated significantly. Compared to dealumination, CLD is more efficient because not only the external surface could be passivated, but also the pore-openings of HSZ had been effectively narrowed, which both favored the enhancement of product p-X selectivity in ortho-xylene (o-X) isomerization. The constraint index (CI) of as-synthesized catalysts derived from the competitive reactions of ethylbenzene (EB) dealkylation and meta-xylene (m-X) isomerization and gravimetric adsorption measurements were introduced to investigate the extent of pore-narrowing by CLD of TEOS. Benefitting from the auxiliary mesopores and surface-passivation treatments, the optimized catalyst HSZ(Si0.5) showed remarkably enhanced catalytic activity over the microporous ZSM-5 counterpart in the model reaction of o-X isomerization.
