Deactivation Mode Research Articles

Evolution of the deactivation mode and nature of coke of HZSM-5 and USY zeolites in the catalytic cracking of low-density polyethylene during successive cracking runs

The coke deposited in the HZSM-5 zeolite at higher coke levels causes a more important pore blockage than the coke deposited in the HZSM-5 zeolite at low coke content. On the other hand, it would appear that there is an important deactivation mode change in the USY coked zeolites during the cracking runs and when the coke level in the catalyst increases an important pore blockage is observed. In the HZSM-5 zeolite, it seems that the formation of insoluble coke during the several polymer cracking runs, as well as the possible location in the external surface could be responsible for the important pore blockage observed in this zeolite. In the USY coked zeolites, the study of the nature of the soluble coke suggests that the pyrene compounds and their evolution to insoluble coke could probably be responsible for the important pore blockage observed after the second cracking run.

Importance of Counterion Reactivity on the Deactivation of Co−Salen Catalysts in the Hydrolytic Kinetic Resolution of Epichlorohydrin

Possible modes of deactivation of Jacobsen's Co-salen catalyst during the hydrolytic kinetic resolution (HKR) of epichlorohydrin were explored by UV-vis spectroscopy, X-ray absorption spectroscopy, and electrospray ionization mass spectrometry, combined with recycling studies. Although an active Co(III)-salen catalyst deactivated substantially after multiple cycles without regeneration, the catalyst maintained its +3 oxidation state throughout the runs. Thus, deactivation of Co-salen during HKR was not the result of Co reduction. The mass spectrum of a deactivated material showed that catalyst dimerization does not account for the loss of activity. Results from various catalyst pretreatment tests, as well as from catalysts containing various counterions (acetate, tosylate, chloride, iodide) indicated that the rate of addition of the Co-salen counterions to epoxide forming Co-OH during the reaction correlated with deactivation. The extent of counterion addition to epoxide was influenced by the exposure time and the nucleophilicity of the counterion. An oligo(cyclooctene)-supported Co-OAc salen catalyst, which was 25 times more active than the standard Co-salen catalyst, was recycled multiple times with negligible deactivation.

Abnormal default-mode network activation in cirrhotic patients: a functional magnetic resonance imaging study

Recently, increasing numbers of studies have demonstrated that, in humans, a default-mode functional network exists in the resting state. Abnormal default-mode network in various diseases has been reported; however, no report concerning hepatic cirrhosis has been published to date. To prospectively explore whether the resting-state network in patients with hepatic cirrhosis is abnormal or not, using functional magnetic resonance imaging (fMRI). 14 patients with hepatic cirrhosis (12 male, two female; 45+/-9 years) and 14 age- and gender-matched healthy volunteers (12 male, two female; 42+/-10 years) participated in a blocked-design fMRI study. A modified Stroop task with Chinese characters was used as the target stimulus. Statistical Parametric Mapping 99 software was employed to process the functional data. Individual maps and group data were generated for patients with hepatic cirrhosis and for healthy controls, respectively. Intergroup analysis between patients and healthy controls was also generated using the two-sample t-test model. Cluster analyses were done based on the group data, and an identical P value < or =0.01 with continuously connected voxels of no less than 10 was defined as significant deactivation. After fMRI scanning was complete, behavioral Stroop interference tests were performed on all subjects; reaction time and error number were recorded. Functionally, deactivation of the posterior cingulate cortex (PCC) and precuneus was absent when subjects performed the incongruous word-reading task; deactivation of the PCC, precuneus, and ventral medial prefrontal cortex was increased when they performed the incongruous color-naming task. The functional as well as behavioral data suggest that cirrhotic patients may have an abnormal deactivation mode. The absence of deactivation in the PCC and precuneus may be a sensitive rather than specific marker in patients with hepatic cirrhosis.

Coke deposition and characterization on titanium silicalite-1 catalyst in cyclohexanone ammoximation

In this work, the changes of catalytic activity, as well as the main deactivation mode were investigated on a commercial titanium silicalite-1 (TS-1) catalyst under simulated industrial conditions in continuous tank slurry reactor. The conclusions about deactivation behavior have been achieved based on the changes of activity, conversion and selectivity. XRD and FTIR analyses indicate the MFI framework structure of zeolite cannot be changed evidently during the reaction. It is found by comparative experiments that the main reason for catalyst deactivation is coke formation on the surface of TS-1. Results from TGA-DTA, N 2 adsorption and pyridine TPD analyses of used catalysts demonstrate there are two types of coke deposited mainly on micropores. Soft coke is located near Ti sites and can be removed by oxidation under 350 °C, while refractory coke can be oxidized by air until 700 °C. Analysis results of GC–MS for soluble coke and NMR, FTIR spectra for insoluble coke indicate that N-containing polycyclic aromatic and/or polyalkene species are the main components of coke deposits.

Spectral and Photophysical Properties of Thioxanthone in Protic and Aprotic Solvents: The Role of Hydrogen Bonds in S1‐Thioxanthone Deactivation

Spectral and photophysical properties of thioxanthone (9H-thioxanthen-9-one, TX) were determined in a few protic solvents (H2O, D2O, hexafluoro-2-propanol) and compared with those in aprotic solvents. On the basis of the time-resolved and steady-state emission measurements and available literature data, it has been shown that the dominant S1-TX deactivation process in protic solvents is the formation of the S1-complex. The important modes of deactivation of the S1-complex are fluorescence (phiF approximately 0.4-0.5) and intersystem crossing to the T1 state. The S1-complex-->S0 internal conversion plays, at most, an insignificant role in S1-complex deactivation, which is evidenced by the absence of an isotope effect of protic solvents on the lifetime and quantum yield of fluorescence.

Metal Catalyst Design and Preparation in Control of Deactivation

AbstractFor Abstract see ChemInform Abstract in Full Text.

METAL CATALYST DESIGN AND PREPARATION IN CONTROL OF DEACTIVATION

▪ Abstract Catalyst life (with activity and selectivity) is one of the three essential properties of a practical catalyst, both in laboratory and industrial usage. For most catalysts in which the active phase is a metal, high activity requires a high metal surface area. In all cases, for long life, the metal surface area has to be maintained. The various modes of catalyst deactivation leading to loss of metal surface area are described (sintering, poisoning, coking, etc.). The choice of catalyst components for both active metal phases and for support phases is shown to be subject to thermodynamic and kinetic limits. Routes for the preparation of catalysts need to be and can be designed to give not only active, but also stable catalysts. Examples described include copper/zinc catalysts for hydrogenation/dehydrogenation reactions, iron catalysts for ammonia synthesis, nickel catalysts for hydrogenation reactions, automobile exhaust catalysts, and Raney catalysts.

Vagal nerve stimulation technique: enhancing its efficacy and acceptability by augmentation with auto activation and deactivation mode of operation

The purpose of this article is to search for an additional modus operandi to improve the functioning of currently deployed vagal nerve stimulation (VNS) technique that is being used as an adjunctive therapy for intractable epilepsy, mainly complex partial seizures (partial onset with secondary generalization). The efficacy and success of current VNS technique is variable and limited, which can be attributed (to a considerable extent) to its present modi operandi. The mechanism of anti-epileptic action of VNS that has been hypothesized in the article is found to conform to observations and results in a large number of studies including those on VNS itself. Based on this mechanism in controlling seizures, the author proposes an additional mode of operation of the VNS device, (an auto activation and deactivation mode), designed to work on a feedback mechanism, which would deliver VNS as and when the brain requires it to abort/arrest the impending focal attack and/or its generalization, thus eliminating the limitations associated with the current VNS device. This mode should enhance its acceptability, efficacy and success.

Analyzing enhancement of CO2, reforming of CH4, in Pd membrane reactors

AbstractIt is demonstrated that H2, removal through Pd foil membranes can enhance the CO2, reforming of CH4. At 550°C and low flows over Pt/γ ‐Al2O3, pellets, conversions were twice that of a conventional reactor, while H2O formation was suppressed, resulting in H2/CO effluent ratios of 1.0. Larger sweep flows enhanced the reaction somewhat, and the replacement of Pt/γ ‐Al2O3, with Pt/ZrO2 mitigated catalyst deactivation in H2‐poor operating environments. A quantitative correlation was established between reaction enhancement and the overall H2 removal to production ratio. Based on this correlation, a simple membrane reactor equilibrium model was developed for comparison against experimental data. Subsequently, a unique and useful “on‐the‐fly” method of gauging membrane limitations, catalyst limitations, and modes of deactivation within a membrane reactor was demonstrated.

Final remarks

The impact of the textural properties of mordenite zeolites on the growth and toxicity of coke formed at 350 °C during propene transformation is investigated. The addition of intracrystal mesopores by alkaline treatment to a unidimensional zeolite is a simple tool for changing its classical deactivation mode, pore blocking, to a mode more usual on zeolites with a three-dimensional framework, poisoning. This change requires that the zeolite possess a high hierarchy factor, as a result of severe alkaline conditions generating a lot of intracrystal mesopores. Desilication treatment removes part of the Brønsted acid sites and generates extra-framework aluminum species that can block access to the protonic sites and/or enhance their strength, which increases the coking rate. A mild acid leaching is enough to dissolve them. The presence of a few mesopores is sufficient to enhance the access of oxygen to coke, which favor its combustion.

Use of deactivated fused-silica capillary precolumns in pesticide analysis by gas chromatography with electron-capture detection

The advantages and disadvantages of coupling a retention gap of fused-silica between the injection port and the chromatographic column are discussed. The influence on the peak width and height of several factors such as the solvent ( n-hexane, acetone, ethyl acetate and methanol), the gap (length, inner diameter, deactivation mode), the injection volume and the pesticide concentration has been examined. Those factors have very different incidences so, it is not possible to extract a general recommendation about the use of gaps. For this reason, checking its viability in each particular case is more advisable.

M-Xylene Transformation over a USHY Zeolite at 523 and 723 K: Influence of Coke Deposits on Activity, Acidity, and Porosity

The modes of coking and deactivation of a USHY zeolite during m-xylene transformation at 523 and 723 K were established. At both reactions temperatures, coke molecules result mainly from transformation of the diarylmethane intermediates of transmethylation reactions, olefinic products formed only at 723 K also participating in coke formation at this temperature. From the effect of the number of coke molecules on the accessible micropore volume, on the hydroxyl bands, and on the number of protonic sites able to retain adsorbed pyridine, it can be concluded that at both temperatures, deactivation is initially due to poisoning of the protonic sites, pore blockage by coke deposits appearing at longer time-on-stream. The effect of this latter mode of deactivation is more significant at 723 than at 523 K. No interaction of Lewis acid sites with coke molecules is observed, which suggests that these sites play no role in m-xylene transformation and coke formation.

Transmission Electron Microscopy Studies of Pd Encapsulation by Ceria-Zirconia Oxides

Abstract High-temperature catalysts containing Pd supported on high-surface area ceria-zirconia are optimum materials for fuel economy when automotive engines operate under high speed or load conditions. A prerequisite for developing such thermally stable catalysts is to gain a good understanding of the thermal deactivation modes contributing to the aging and degradation of catalysts in harsh thermal environments. It was discovered by X-ray diffraction that upon hightemperature aging, Pd may sinter into large (about 10 nm diameter) particles and become encapsulated in the ceria-zirconia. To confirm this conclusion, a prototype high-temperature catalyst containing Pd supported on high-surface area ceria-zirconia, aged at temperature above 1100 °C is studied by transmission electron microscopy (TEM). The ceria-zirconia supported Pd (0.25 wt%) catalyst was aged at 1105 °C and calcined at 700 °C for 2 h. For TEM sample preparations, first of all, a large-size aggregate with diameter about 1 mm was selected and sandwiched between by two pieces of silicon crystal.

Catalytic silane alcoholysis based on the C 5H 5(CO)(PPh 3)Fe + moiety. NMR spectroscopic identification of key intermediates

The Cp(CO)(PPh 3)Fe + fragment has been shown to catalyze silane alcoholysis in the presence of the non-coordinating counterion, B Ar′ 4 − (Ar′=3,5-(CF 3) 2C 6H 3). When ethanol is used, catalyst deactivation rapidly occurs. However, when phenol is utilized, catalytic activity continues until all of the phenol has been consumed. In each case, key intermediates in the catalytic cycle, including resting states, have been observed using low temperature NMR spectroscopy. The modes of catalyst deactivation when ethanol is used have also been determined.

Effect of coke deposition on transport and adsorption in zeolites studied by a new microbalance reactor

Coke formation and the effect of coke on transport and adsorption behaviour of a commercial HZSM-5 zeolite has been studied by means of an oscillating microbalance. This transient technique provides a powerful tool for in situ studies of adsorption and diffusion in coked zeolites. Adsorption experiments at 30°C involving ethane, toluene and n-hexane were carried out before and after coke formation during ethene oligomerization at 475°C using a partial pressure of ethene of 0.8 bar. Both adsorption rates and equilibrium adsorption were studied. Information about the location of coke was obtained from changes in characteristic diffusion time and adsorption capacity with coke content. Distinct deactivation modes such as site coverage, pore blocking and external coking were identified at different coke levels. Changes in characteristic diffusion time were found to depend on both coking level and coke location, which influenced either the diffusivity or the diffusion path.

Activation volume (.DELTA. V.dbldag.q) for energy- and electron-transfer quenching of [Cu(dpp)2+]* (dpp = 2,9-diphenyl-1,10-phenanthroline)

Author(s): Crane, DR; Ford, PC | Abstract: The quenching of emission from the metal to ligand charge-transfer (MLCT) state of the copper(I) complex Cu-(dpp)2+ (dpp = 2,9-diphenyl-l, 10-phenanthroline) by the uncharged chromium(III) complexes CrL3 (L =acetylacetonate, 1,1,1,5,5,5-hexafluoro-2,4-pentanedionate, l,l,l-trifluoro-2,4-pentanedionate) and several nitrobenzene derivatives has been investigated in dichloromethane, chloroform, and tetrahydrofuran solutions as a function of hydrostatic pressure (0.1−250 MPa). The patterns in all three solvents were similar. For those quenchers having reduction potentials E1/2(Q) l −1.6 V (vs the ferrocenium/ferrocene, Fc+/Fc, couple in dichloromethane), the principal bimolecular deactivation mode is energy transfer. In such cases, there is little effect of pressure on the quenching rate constant kq (Δq = ∼0 cm3 mol−1). In contrast, substantially more positive E1/2(Q) values lead to kq's approaching diffusion limits and significantly positive ΔV‡q values (up to +8 cm3 mol−1), reflecting the pressure-induced increases in solution viscosity. For E1/2(Q) ∼ −1.6 V, the dominant pressure-dependent term is electron transfer within the outer-sphere precursor complex, and the resulting charge creation in the transition state leads to substantially negative ΔV‡q values approaching −20 cm3 mol−1. © 1993, American Chemical Society. All rights reserved.

Evaluation of Modes of Catalyst Deactivation by Coking for Cumene Cracking over Zeolites

The deposition of coke deactivates a catalyst by the suppression of active sites and/or by choking of pores. The so-called Constant-Coke Arrhenius Plot technique can assess the magnitudes of these two modes of deactivation using reaction experiments under process conditions. For cumene cracking over REY zeolite, a TGA has been used as a continuous-flow microreactor to monitor coke levels, with activity measured by an on-line GC. Using modifications described previously for small-pore catalysts, we show that both temperature and coke level influence the relative importance of site suppression and pore choking in the deactivation of the zeolite. At relatively high temperatures, site suppression is predominant at all coke levels studied here. At lower temperatures, pore choking is more important at low levels of coke, with site suppression increasing in importance at the higher coke levels. The results are consistent with a physical model developed.

Effect of Cyanide on the S2- and S3-State in the Filamentous Cyanobacterium Oscillatoria chalybea

Abstract Low concentrations of cyanide affect the flash-induced oxygen evolution pattern in thylakoid preparations of the filamentous cyanobacterium Oscillatoria chalybea. At a cyanide concentration of 2 x 10-6 ᴍ the effect lies exclusively on the S2- and S3-state whereas the subsequent flash yields and the steady state are barely affected, showing that the reaction center is not touched by the reagent. The influence of the chemical is characterized by the fact that at basic pH (7.5 -8.5) the deactivation mode of S2 and S3 is changed, showing that the cyanide-ion is the reactant. Whereas in the control the deactivation of S2 yields as usual S1 and S-1, the deactivation pattern in the presence of cyanide shows that S2 deactivates - without giving S1 - , to a more reduced state, probably S0. In the flash pattern the two signal amplitudes of the first two flashes are lowered to zero in the presence of 2 x 10-6 ᴍ cyanide and become uptake signals at higher cyanide concentrations. It is seen that in the presence of cyanide S2 and to a lesser extent S3 in the Oscillatoria system apparently react with oxygen. In tobacco thylakoids no effect of cyanide on the flash pattern itself is observed, even at higher concentrations (e.g. 10-4 ᴍ), although the signal amplitudes of the entire pattern are affected at such concentrations. Photosystem II of cyanobacteria is characterized by the absence of the two extrinsic 16 kDa and 23 kDa peptides. As shown by mass spectrometry inside-out vesicles from tobacco thylakoids, in which these two extrinsic peptides have been removed by NaCl-washing, exhibit simultaneously flash-induced O2-evolution and an O2-uptake which both require the presence of CL- and Ca2+ ions. Both the evolution signal and the uptake signal are DCMU sensitive and are inhibited by KCN. An amperometrically measured flash pattern of inside-out tobacco vesicles shows, if the assay is supplemented with CL- ions, the usual flash pattern known from the literature. The experiments seem to indicate that even in the washed inside-out condition the S-state system of higher plants is much less sensitive to ambient oxygen than in the phylogenetically older system of the filamentous cyanobacterium Oscillatoria chalybea.

Technical Considerations for Catalysts for the European Market

The durability of automotive catalysts for European applications were investigated as a function of higher temperatures encountered in autobahn driving over extended periods of time, potentially higher lead levels anticipated in the beginning in European unleaded fuels and occasional misfuelling with leaded fuels. Increasing residual lead levels from 3 mg Pb/L to 10 mg Pb/L in laboratory experiments at 1000°C peak temperature substantially decreased three-way catalyst (TWC) performance. Higher temperature ageing conditions from 730 to 1000°C resulted in 50 per cent loss of BET area and increased stoichiometric light-off temperatures. The paper also reviews briefly the role of noble metals, rare earth oxides and base metal oxides in the formulation of practical TWCs. The operation of a TWC and the effects of A/F modulation frequency and amplitude on its performance are also discussed. Important thermal and chemical modes of TWC deactivation are discussed with specific examples. The importance of matching the active metal component to the support material to achieve the desired metal-support interactions and how this understanding has led to the development of practical TWC with proper utilization of noble metals is emphasized. Finally, the lean-burn versus stoichiometric combustion approach as it relates to catalyst requirements is also discussed.

Coking, aging, and regeneration of zeolites: III. Comparison of the deactivation modes of H-mordenite, HZSM-5, and HY during n-Heptane cracking

The rates of deactivation by carbonaceous residues (coke) of HY, H-mordenite, and HZSM-5, which have similar initial activities in n-heptane cracking are quite different: HZSM-5 forms very little coke and is very stable, while HM deactivates much faster than HY. The deactivation mechanisms of these zeolites could be specified by comparing as a function of the coke content: (i) the cracking activity of these zeolites and their capacity for adsorption of n-hexane, (ii) the volume apparently and really occupied by coke, and (iii) the number of coke molecules and of sites on which NH 3 can no longer be adsorbed. On HY, polyaromatic molecules are very rapidly formed on the strongest acid sites, these molecules obstructing partially or completely the access of the reactant to the acid sites. On HZSM-5 deactivation is initially due to the “coverage” of the acid sites, located at channel intersections, by alkylaromatics with 1 or 2 rings; later on (above 3% coke content) polyaromatic molecules, formed on the external surface, block access to a part of the pore volume. On HM deactivation is due to pore blockage: even at very low coke contents (1% coke), coke can block the access of n-heptane to a pore volume 10 times greater than the volume really occupied by the coke.