Catalyst loss of activity

Dialkylaminoethyl acryhc esters are readily prepared by transesterification of the corresponding dialkylaminoethanol (102,103). Catalysts include strong acids and tetraalkyl titanates for higher alkyl esters and titanates, sodium phenoxides, magnesium alkoxides, and dialkyitin oxides, as well as titanium and zirconium chelates, for the preparation of functional esters. Because of loss of catalyst activity during the reaction, incremental or continuous additions may be required to maintain an adequate reaction rate. 

The methanation reaction is carried out over a catalyst at operating conditions of 503—723 K, 0.1—10 MPa (1—100 atm), and space velocities of 500—25,000 h . Although many catalysts are suitable for effecting the conversion of synthesis gas to methane, nickel-based catalysts are are used almost exclusively for industrial appHcations. Methanation is extremely exothermic (AT/ qq = —214.6 kJ or —51.3 kcal), and heat must be removed efficiently to minimise loss of catalyst activity from metal sintering or reactor plugging by nickel carbide formation. 

In the second phase, performed at a maximum temperature of about 370°C, the sulfur and a portion of the coke are removed by combustion. The rate and exothermicity are controlled by limiting the flow of combustion gas through the catalyst. Spent base metal catalysts may have sulfur levels of from 6 to 12 wt % in the form of metal sulfides. A high degree of sulfur removal must be achieved in these first two regeneration steps to avoid the formation of sulfate on the support during the final combustion step. Such a formation causes a loss of catalyst activity. 

There are two general temperature poHcies increasing the temperature over time to compensate for loss of catalyst activity, or operating at the maximum allowable temperature. These temperature approaches tend to maximize destmction, yet may also lead to loss of product selectivity. Selectivity typically decreases with increasing temperature faster deactivation and increased costs for reactor materials, fabrication, and temperature controls. 

Loss of catalyst activity. The higher regenerator temperature combined with the formation of steam in the regenerator reduces catalyst activity by destroying the catalyst s crystalline structure. 

The presence of increased basic nitrogen compounds, such as pyridines and quinoline in the FCC feedstock, also attack catalyst acid sites. The result is a temporary loss of catalyst activity and a subsequent increase... 

Crabtree s catalyst is an efficient catalyst precursor for the selective hydrogenation of olefin resident within nitrile butadiene rubber (NBR). Its activity is favorably comparable to those of other catalyst systems used for this process. Under the conditions studied the process is essentially first order with respect to [Ir] and hydrogen pressure, implying that the active complex is mononuclear. Nitrile reduces the catalyst activity, by coordination to the metal center. At higher reaction pressures a tendency towards zero order behavior with respect to catalyst concentration was noted. This indicated the likelihood of further complexity in the system which can lead to possible formation of a multinuclear complex that causes loss of catalyst activity. 

The more interesting situation occurs when the catalyst is partially and reversibly poisoned by impurities in the reactant gas. The degree of loss of catalyst activity then depends on the operating conditions. 

Reduction in activity of the catalyst may be caused by polymer and coke deposition due to excessive temperature, low pressure, insufficient water in the feed, or too low a space velocity (25). The presence of diolefins, oxygen, caustic, or nitrogen bases, such as ammonia or amines in the feed, also causes loss of catalyst activity. 

While catalytic HDM results in a desirable, nearly metal-free product, the catalyst in the reactor is laden with metal sulfide deposits that eventually result in deactivation. Loss of catalyst activity is attributed to both the physical obstruction of the catalyst pellets pores by deposits and to the chemical contamination of the active catalytic sites by deposits. The radial metal deposit distribution in catalyst pellets is easily observed and understood in terms of the classic theory of diffusion and reaction in porous media. Application of the theory for the design and development of HDM and HDS catalysts has proved useful. Novel concepts and approaches to upgrading metal-laden heavy residua will require more information. However, detailed examination of the chemical and physical structure of the metal deposits is not possible because of current analytical limitations for microscopically complex and heterogeneous materials. Similarly, experimental methods that reveal the complexities of the fine structure of porous materials and theoretical methods to describe them are not yet... 

Next, we employed the timed catalyst 33 containing a benzoyl group at the catechol moiety30b under high concentration (>12 M to THF), and the product was obtained in synthetically acceptable reaction time, yield, and enantioselectivity (entry 6). The reaction was performed on a 50-g scale without any difficulty (entry 7). The chiral ligand was recovered in 95% yield after silica gel column chromatography, and could be used at least several times without any loss of catalyst activity. 

Metals cause particular problems because they poison catalysts used for sulfur and nitrogen removal as well as other processes such as catalytic cracking (Chapter 5). Thus, serious attempts are being made to develop catalysts that can tolerate a high concentration of metals without serious loss of catalyst activity or catalyst life. 

Metals accumulate more slowly on the catalyst surfaces because the inlet concentrations of metals are lower than for coke precursors. The accumulation of metals can be even greater than coke, for example the vanadium concentration can reach 30-50 wt% of the catalyst on a fresh catalyst basis (Thakur and Thomas, 1985). Demetallization reactions can be considered autocatalytic in the sense that once the surface of the catalyst is covered with metal sulfides the catalyst remains quite active and continues to accumulate metal sulfides. The final loss of catalyst activity is usually associated with the filling of pore mouths in the catalyst by metal sulfide deposits. 

Pilot plant results indicated that satisfactory catalyst life could be realized by gradual temperature increase to offset the decrease in activity with time. This decrease in activity was caused mainly by the formation of more refractory aromatic recycle oils. In commercial operation the activity loss of the catalyst was more rapid. The decline in catalyst activity could be slowed down by decreasing the end point of the feed middle oil or by withdrawing small amounts of heavy ends formed. Commercial operation indicated further that the use of recycle hydrogen was a cause of the more rapid loss of catalyst activity. Ammonia and volatile ammonium salts formed by the reduction of tar bases in the feedstock might have been a factor in the accelerated-catalyst-activity loss. 

While the ethenolysis of fatty acids and esters yields a-olefins and oo-unsaturated acids and esters, the use of higher olefins or functionalized olefins as CM partners gives access to a wide spectrum of platform chemicals. The wide availability of ethylene makes ethenolysis particularly attractive however, the associated problems regarding loss of catalyst activity already explained have motivated the search for alternative low molecular weight olefins. In this way, 2-butene (butenolysis) has been used to avoid the mentioned problems. Patel et al. reported the butenolysis of different natural oils in the presence of C5 [63], TONs between... 

The loss of catalyst activity through metal deposition can be attributed to the interaction of the deposited metals with the original active sites of the catalyst ( active site poisoning ) and the loss of pore volume due to the obstruction of catalyst pores ( pore plugging ) as depicted in Figure 1. 

The rapid loss of catalyst activity in catalytic cracking has had a profound influence on the engineering and commercialization of cat cracking. In this work we propose a plausible chemical mechanism for catalyst decay and derive an expression for the kinetics of the decay process based on this mechanism. 

The loss of catalyst activity in catalytic cracking must be connected in some way with one or both of the following root causes ... 

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