Ni promoters

Around 500 K, the catalyst consumes H2, as shown by the sharp peak, while simultaneously H2S and some additional H2O are produced, which indicates that the catalyst has taken up too much sulfur at lower temperatures, which is now released in the form of H2S. At higher temperatures, the catalyst continues to exchange oxygen for sulfur until all the molybdenum is present as M0S2. TPS has proven very useful in studying the sulfidation of M0O3 as well as Co and Ni promoted catalysts. 

In spite of much theoretical work, we still do not have a complete picture of why the Co and Ni-promoted M0S2 catalyst is so successful. Interpretations range from the promoter-induced weakening of the metal-to-sulfur bond strength to the presence of unique sulfur species bound between molybdenum and the promoter. 

Previous reports on FMSZ catalysts have indicated that, in the absence of added H2, the isomerization activity exhibited a typical pattern when measured as a function of time on stream [8, 9], In all cases, the initial activity was very low, but as the reaction proceeded, the conversion slowly increased, reached a maximum, and then started to decrease. In a recent paper [7], we described the time evolution in terms of a simple mathematical model that includes induction and deactivation periods This model predicts the existence of two types of sites with different reactivity and stability. One type of site was responsible for most of the activity observed during the first few minutes on stream, but it rapidly deactivated. For the second type of site, both, the induction and deactivation processes, were significantly slower We proposed that the observed induction periods were due to the formation and accumulation of reaction intermediates that participate in the inter-molecular step described above. Here, we present new evidence to support this hypothesis for the particular case of Ni-promoted catalysts. 

Figure 2 Conversion - time curves on several Ni - promoted catalysts under n-C4Hio/He mixture. n-Butane conversion to isobutane as a function of time on stream over 0 4 g cat., at 150°C, n-C4Hio flow rate = 5.46 cmVmin, He flow rate = 10.4 cm7min. 3NiSZ(s) (squares) 2NiSZ(s) (triangles) INiSZ(s) (circles).

The promoting effect on the n-C4Hio isomerization increases with the amount of Ni, and further increases with the addition of Mn. We have presented evidence that on the Ni-promoted sulfated zirconia catalysts, the nickel is, at least partially, in the form of sulfate. 

Rieke Ni-promoted (Scheme 11) and cat. NiCk/Mg/MesSiCl-mediated pinacol coupling have been investigated [38]. The similar nickel-catalyzed reaction was also reported later [39]. 

Rh, Ru, Pd) and oxides (<4wt% Fe jO4/Cr2O3, La2O3, SnO2, K2O) was recently performed by Lodeng et al. [134]. A comparison with Ni- and Fe-based catalysts was also addressed. It was found that addition of metal promoters, particularly Rh and Pt, enhanced the catalyst activity at low temperatures (which resulted in delayed extinction of the reaction during ramping at —1 Tmin ). However, addition of Ni promoted carbon formation. Addition of surface oxides typically promoted instability, deactivation and combustion (hence the formation of a stable Co metallic phase was hindered). It was found that Ni performed better than Co-based catalysts at all temperatures. However, Fe-based catalysts showed high combustion activity. 

Figure 4 in Scheme 2.3-2 reveals that with comparable and Pt-intermediary complexes, differences in local symmetries may result. In addition, Ni promotes interactions with a primary carbon atom of the a-allyl group whereas Pt interacts with the substituted carbon atom of this group. 

McDonald et al. applied their dihydropyran formation to the oligosaccharide synthesis featuring reiterative alkynol cycloisomerization [19c-f]. Dihydropyran 49 was prepared by the method described above and then NIS-promoted glycosyla-tion with the acyclic alkynol 50 followed by Ph3SnH-promoted dehalogenation and... 

As has been discussed above, knowledge of the dispersion of the catalyst is extremely difficult to obtain, especially in the promoted systems. This is the main obstacle to clear differentiation between the promoter models proposed and remains so today. Chemisorption techniques to count the active sites present on the sulfided surfaces have had limited success. 02 chemisorption has been associated with edge vacancies (active sites) on pure M0S2 (100). However, the same authors showed that it was impossible to correlate activity and amount of 02 chemisorbed on Co- or Ni-promoted molybdenum sulfide (101). The use of other test molecules was disputable, particularly NO, which can strongly modify the structure of the surface during the measurement (102). 

Catalytic hydrodesulfurization (HDS) is a very important industrial process that involves removal of sulfur from crude oils by high-temperature ( 400°C) treatment with hydrogen over Co- or Ni-promoted Mo or W catalysts supported on alumina. In an attempt to determine the mechanism of this process, many transition metal complexes of thiophene, a sulfur-containing heterocycle that is particularly difficult to desulfurize, have been prepared and their reactivities studied in order to compare their behavior with those of the free thiophenes that give H2S and C4 hydrocarbons under HDS conditions (88ACR387). Thiophene can conceivably bind to the catalyst surface by either cr-donation via a sulfur electron pair or through a variety of -coordination modes involving the aromatic system... 

In a study of NIS-promoted additions to glycals it was noted that when the nu-cleophilicity of the alcohol was low, a competing reaction occurred whereby the succinimide anion itself acted as a nucleophile, attacking the anomeric carbon.76,77... 

Prins summarizes advances in understanding of the reactions in catalytic hydrodenitrogenation (HDN), which is important in hydroprocessing of fossil fuels. Hydroprocessing is the largest application in industrial catalysis based on the amount of material processed. The chapter addresses the structures of the oxide precursors and the active sulfided forms of catalysts such as Ni-promoted Mo or W on alumina as well as the catalytically active sites. Reaction networks, kinetics, and mechanisms (particularly of C-N bond rupture) in HDN of aliphatic, aromatic, and polycyclic compounds are considered, with an evaluation of the effects of competitive adsorption in mixtures. Phosphate and fluorine promotion enhance the HDN activity of catalysts explanations for the effect of phosphate are summarized, but the function of fluorine remains to be understood. An account of HDN on various metal sulfides and on metals, metal carbides, and metal nitrides concludes this chapter. 

Stratz compared Raney Ni without inhibitor with the Raney Ni poisoned by dicy-andiamide in the hydrogenation of o-chloronitrobenzene in methanol below 120°C and 1 MPa H2. Proportion of dechlorination decreased from 44.3% without inhibitor to 0.3% in the presence of the inhibitor, and the yield of o-chloroaniline increased from 54.5% without inhibitor to 98.8% with the inhibitor. The dechlorination decreased further to 0.2%, and the yield of o-chloroaniline increased to 99.3% over a Raney Ni promoted with chromium and iron (eq. 9.50).114 Similarly, dehalogenation over the chromium- and iron-promoted Raney Ni was only 0.6% with m-chloroni-trobenzene, 0.2% with 3,4-dichloronitrobenzene, and 0.5% with p-bromonitroben-zene. 

Alumina-supported Co- and Ni-promoted molybdenum sulphide hydrotreating catalysts are the main workhorses in many refineries and have, therefore, attracted a lot of attention from catalytic chemists. They are usually prepared via co-impregnation, i.e. pore-volume impregnation with both Mo and the promoter atom present in solution. After drying and calcining, the catalyst manufacture is complete, but it has to be sulphided before use. Traditionally, this is done in situ... 

Oxidized V (Oxidation state +5) has a greater mobility due to its higher vapor pressure, and the vanadic acid produced is very destructive towards the zeolite. This is a fundamental difference between Ni and V deactivation while Ni promotes undesirable side reactions (H2 production and coke), V is also lethal for the zeolite. In the case of the FAU zeolite, the following irreversible reaction takes place and destroys the zeolite ... 

Fig. 39. Model representing the interaction between MoSj and alumina through phosphorus oxo-species. (a) Unpromoted MoP/Al (b) Ni-promoted MoP/Al.

Poleski and co-workers (232) measured differential heats of Hj adsorption on Co- and Ni-promoted Mo catalysts at 483 and 593 K. Increasing the ratio Co/Mo from 0 to 0.02 increased the heat of Hj adsorption from 15 to 100 kJ mol. The heat remained high to a Co/Mo ratio of about 0.5, whereas it decreased at higher ratios. The Ni-containing Mo catalysts showed qualitatively similar behavior. 

Recently, an additional version of intramolecular aglycone delivery starting from 2-O-allyl derivatives and using iodonium ion-promoted tethering was described [83]. As illustrated in Scheme 5.28, the 2-(9-allyl ether (154) was first isomerized to the propenyl ether (155). NIS-promoted coupling with the acceptor (146) then afforded the mixed acetal (156), which was transformed into the (3-mannoside (148) by using NIS and AgOTf for activation. 

In searching for the origin of the regioseiectivity observed in the formation of trisaccharides 206 and 212 (O Scheme 28 and Scheme 29) several factors were considered. The reactions in O Scheme 28 and Scheme 29c were carried out with excess NIS promoter, conditions under which the intermolecular halonium ion transfer (responsible for the armed-disarmed effect) is not operative. A study of the three types of -pentenyl donors indicated that their relative reactivities were in the order NPOE > armed > disarmed (e. g. 202a > 64 > 200) [111]. Therefore, the most and the least reactive donors have chosen their preferred —OH in the final trisac-... 

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