Lifetime, catalysts

Propionic acid is accessible through the Hquid-phase carbonylation of ethylene over a nickel carbonyl catalyst (104), or via ethylene and formic acid over an iridium catalyst (105). Condensation of propionic acid with formaldehyde over a supported cesium catalyst gives MAA directiy with conversions of 30—40% and selectivities of 80—90% (106,107). Catalyst lifetime can be extended by adding low levels (several ppm) of cesium to the feed stream (108). 

A process based on a nickel catalyst, either supported or Raney type, is described ia Olin Mathieson patents (26,27). The reduction is carried out ia a continuous stirred tank reactor with a concentric filter element built iato the reactor so that the catalyst remains ia the reaction 2one. Methanol is used as a solvent. Reaction conditions are 2.4—3.5 MPa (350—500 psi), 120—140°C. Keeping the catalyst iaside the reactor iacreases catalyst lifetime by maintaining a hydrogen atmosphere on its surface at all times and minimises handling losses. Periodic cleaning of the filter element is required. 

In this appHcation, ZSM-5 acts as a strong, soHd acid, and may be viewed as supported on the surfaces of the crystalline zeoHte stmcture. The older, Friedel-Crafts aluminum chloride catalyzed process for ethylbenzene produces considerably more by-products and suffers from the corrosivity of the catalyst system. Because of the intermediate pore size of ZSM-5, those reactions that produce coke from larger molecules that cannot enter the ZSM-5 pore stmcture are significantly reduced, which greatly extends catalyst lifetime. 

Catalyst lifetime for contemporary ethylene oxide catalysts is 1—2 years, depending on the severity of service, ie, ethylene oxide production rate and absence of feed poisons, primarily sulfur compounds. A large percentage (>95%) of the silver in spent catalysts can be recovered and recycled the other components are usually discarded because of thek low values. 

Catalyst lifetimes are long in the absence of misoperation and are limited primarily by losses to fines, which are removed by periodic sieving. Excessive operating temperatures can cause degradation of the support and loss of surface area. Accumulation of refractory dusts and chemical poisons, such as compounds of lead and mercury, can result in catalyst deactivation. Usually, much of such contaminants are removed during sieving. The vanadium in these catalysts may be extracted and recycled when economic conditions permit. 

Ruthenium- and cobalt-catalyzed hydroformylation of internal and terminal alkenes in molten [PBuJBr was reported by Knifton as early as in 1987 [2]. The author described a stabilization of the active ruthenium-carbonyl complex by the ionic medium. An increased catalyst lifetime at low synthesis gas pressures and higher temperatures was observed. 

Obviously, the ionic liquid s ability to dissolve the ionic catalyst complex, in combination with low solvent nucleophilicity, opens up the possibility for biphasic processing. Furthermore it was found that the biphasic reaction mode in this specific reaction resulted in improved catalytic activity and selectivity and in enhanced catalyst lifetime. 

No differences in operability and catalyst behavior (activity and deactivation) in the two plants were discernible. The expected catalyst lifetime in a commercial plant, calculated from the movement of the temperature profile down the catalyst bed with time, in both cases will be more than 16,000 hrs under the design conditions. 

Longer catalyst lifetime Higher linear oligomers (>99%)... 

Olefin disproportionation order to have an increased ration of P-olefins. This caused higher catalyst lifetime. Mo or W03 and alumosilicate as a carrier. 47... 

An interesting way to retard catalyst deactivation is to expose the reaction mixture to ultrasound. Ultrasound treatment of the mixture creates local hot spots, which lead to the formation of cavitation bubbles. These cavitation bubbles bombard the solid, dirty surface leading to the removal of carbonaceous deposits [38]. The ultrasound source can be inside the reactor vessel (ultrasound stick) or ultrasound generators can be placed in contact with the wall of the reactor. Both designs work in practice, and the catalyst lifetime can be essentially prolonged, leading to process intensification. The effects of ultrasound are discussed in detail in a review article [39]. 

Turnover number - the amount of product per mole of catalyst (this is related to catalyst lifetime and hence to cost and waste). 

Again, standard snbstrates 1, 3 and 5 were ntilised to monitor the new catalysts behavionr in RCM catalysis. It was found that increased backbone substitution led to increased catalyst lifetimes but decreased rates of reaction. 

Many studies have been devoted to the hydroamination of isobutene with NH, since BASF started the production of FBuNHj in Antwerp in 1986 (6000 t/yr) [60, 61]. These studies were aimed mainly at improving conversion, selectivity, catalyst lifetime and space time yields, using less expensive catalysts than zeolites, decreasing the NHj/isobutene ratio to nearly 1/1, and recycHng of the NHj/isobutene mixtures. 

Finally the catalyst lifetime was tested, as together with activity it influences the costs of catalyst. The catalyst after completing the hydrogenation was filtered. 

We evaluated a number of potential catalysts and conditions using xylitol as a model compound in a batch reactor. A catalyst was selected from this initial screening and examined in a continuous trickle-bed reactor to develop operating conditions. Finally, as resources allowed, the catalyst was evaluated in a trickle bed reactor to gain a concept of potential catalyst lifetime. 

In 2003, Efskind and Undheim reported dienyne and triyne domino RCMs of appropriately functionalized substrates with Grubbs type II or I catalysts (Scheme 6.71, reactions a and b, respectively) [151]. While the thermal processes (toluene, 85 °C) required multiple addition of fresh catalyst (3 x 10 mol%) over a period of 9 h to furnish a 92% yield of product, microwave irradiation for 10 min at 160 °C (5 mol% catalyst, toluene) led to full conversion. The authors ascribe the dramatic rate enhancement to rapid and uniform heating of the reaction mixture and increased catalyst lifetime through the elimination of wall effects. In some instances, use of the Grubbs I catalyst was more efficient than use of the more common Grubbs II equivalent. 

Addition of pvp significantly extends catalyst lifetime (Scheme 2). This is a well-known protocol whereby the Pd nanoparticles are wrapped up in the polymer chains, which prevents their aggregation into catalytically inactive palladium black. 

The lifetime of a zeolitic alkylation catalyst depends on the concentration of Brpnsted acid sites. This has been shown by Nivarthy et al. (78), who used a series of zeolites H-BEA with varied concentrations of back-exchanged sodium ions. The sodium decreased the concentration of Brpnsted acid centers, which led to a concomitant decrease in the measured catalyst lifetime during alkylation. 

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