Catalyst modification

Reduced Emissions and Waste Minimization. Reducing harmful emissions and minimizing wastes within a process by inclusion of additional reaction and separation steps and catalyst modification may be substantially better than end-of-pipe cleanup or even simply improving maintenance, housekeeping, and process control practices. SO2 and NO reduction to their elemental products in fluid catalytic cracking units exemplifies the use of such a strategy (11). 

For most solid catalysts more detailed information concerning composition, preparation, activation, and regeneration procedures, poisons and catalyst modifications is given by Bailey 42) and Banks 43). 

The purpose of this work was to increase the A3 selectivity at low conversion through a catalyst modification. Previous studies of phenol alkylation with methanol (the analogue reaction) over oxides and zeolites showed that the reaction is sensitive to acidic and basic properties of the catalysts [3-5]. It is the aim of this study to understand the dependence of catalyst structure and acidity on activity and selectivity in gas phase methylation of catechol. Different cations such as Li, K, Mg, Ca, B, incorporated into y-Al203 can markedly modify the polarisation of the lattice and consequently influence the acidic and basic properties of the surface [5-8] which control the mechanism of this reaction. 

Enantioselectivity in acid hydrogenation was not sensitive to the reduction temperature of the catalyst. Modification by N-benzylcinchonidinium chloride substantially deactivated the catalyst and eliminated enantioselectivity. 

The hydrogenation of methyl pyruvate proceeded over 4% Pd/Fe20 at 293 K and 10 bar when the catalyst was prepared by reduction at room temperature Racemic product was obtained over utunodified catalyst, modification of the catalyst with a cinchona alkaloid reduced reaction rate and rendered the reaction enantioselective. S-lactate was formed in excess when the modifier was cinchonidine, and R-lactate when the modifier was cinchonine... 

No discussion is offered, concerning the sense of the observed enantioselectivity in this reaction because of a need to be cautious We have observed that the sense of the enantioselectivity can be reversed simply by a variation of the procedure used for catalyst modification [15], and this has been confirmed by others [16] Thus it appears that the state of the Pd surface, as well as the nature of the species adsorbed upon it and their spatial relationship, contributes to chiral direction in this reaction. 

Catalyst Modification Data Pynitrile Selectivitv Data ... 

The need for higher product specificity and milder reaction conditions (see also Section IX) has led to extensive research in hydroformylation technology. This research, as reported in technical journals, patent literature, and commercial practice has been primarily concerned with catalysis by rhodium, in addition to the traditional cobalt, and with catalyst modification by trialkyl or triaryl phosphines. These catalyst systems form the basis for the major portion of the discussion in this chapter some other catalyst systems are discussed in Section VIII. 

Photobleach mechanism, 19 203 Photobleach reversal grains, 19 201 Photocatalysis, 19 73-106. See also Photocatalysts Photoreactors aqueous pollutants eliminated and mineralized by, 19 89t catalyst modifications in, 19 94-95 catalysts in, 19 75-76 challenges in, 19 101-102 fate of photo-holes in titania, 19 82-85 in fine chemistry applications, 19 102 influence of oxygen pressure in, 19 82 ion doping in, 19 94-95 mass of catalyst in, 19 77-78 noble metal deposit in, 19 94 parameters governing kinetics in, 19 77-82... 

As a result of steric constraints imposed by the channel structure of ZSM-5, new or improved aromatics conversion processes have emerged. They show greater product selectivities and reaction paths that are shifted significantly from those obtained with constraint-free catalysts. In xylene isomerization, a high selectivity for isomerization versus disproportionation is shown to be related to zeolite structure rather than composition. The disproportionation of toluene to benzene and xylene can be directed to produce para-xylene in high selectivity by proper catalyst modification. The para-xylene selectivity can be quantitatively described in terms of three key catalyst properties, i.e., activity, crystal size, and diffusivity, supporting the diffusion model of para-selectivity. 

The phenomenon of catalyst modification by impurities (promoting by poisons, poisoning by promoters) was discovered in 1940 in the laboratory of S. Z. Roginsky. A summary of the experimental data is given in (74, 5). A theoretical interpretation of the phenomenon was given in the first papers on the electron theory of catalysis (1, 37, 66, 47)- The effect of impurities on the activity of a catalyst may be regarded as the fourth consequence of the theory. 

The above list of modifiers unambiguously shows their large diversity. There are examples indicating that the best results can be achieved by combination of several modifiers [22]. Consequently, catalyst modification is an excellent field for combinatorial research provided a library optimization method is available. 

In this section the use of amperometric techniques for the in-situ study of catalysts using solid state electrochemical cells is discussed. This requires that the potential of the cell is disturbed from its equilibrium value and a current passed. However, there is evidence that for a number of solid electrolyte cell systems the change in electrode potential results in a change in the electrode-catalyst work function.5 This effect is known as the non-faradaic electrochemical modification of catalytic activity (NEMCA). In a similar way it appears that the electrode potential can be used as a monitor of the catalyst work function. Much of the work on the closed-circuit behaviour of solid electrolyte electrochemical cells has been concerned with modifying the behaviour of the catalyst (reference 5 is an excellent review of this area). However, it is not the intention of this review to cover catalyst modification, rather the intention is to address information derived from closed-circuit work relevant to an unmodified catalyst surface. 

As optimization of chiral catalysts has progressed, some catalyst modifications have introduced structural elements that could seriously impede biodegradation. New catalytic scaffolds such as List and co-workers [60] and Jacobsen and... 

This approach requires no additional synthetic steps for catalyst modification and overcomes proline s main shortcomings of moderate yield and enantioselectivity. In some cases, an even better combination of yield and enantioselectivity could be achieved using just 0.5 mol% of (R)-BINOL, but in general both isomers of BINOL were almost equally effective. However BINOL is unlikely to be readily biodegradable, biorenewable/biodegradable alternatives could further improve the environmental profile of this co-catalytic process. Although the cocatalyst loading of just... 

Having established reliable values for all of the important rate constants as a function of alkyl substitution on dibenzothiophenes, it is now possible to examine critically how these rate constants (and associated changes in product selectivity) are affected by other components of commercial gas oils and by the H2S that is produced during the HDS process. It is also possible to evaluate how these various rate constants are affected by changes in catalyst composition and by process conditions. Knowledge of the details of these effects can lead to novel catalyst modifications and process configurations that may be able to reach the new stricter standards of 0.05% S. These topics are discussed in later sections. However, for perspective, we will first summarize what is known about present-day catalyst compositions and catalytic mechanisms that bring about the transformations observed in HDS processes. 

Another novel catalyst modification has been suggested in which the active Co-Mo-S catalyst is used in combination with an acidic catalyst such as a zeolite. This combination has the potential of opening another reaction pathway by isomerization of the alkyl groups on molecules such as 4,6-DMDBT to positions that do not sterically interfere with adsorption or oxidative addition. This is illustrated in Fig. 33. Gates and co-workers reported many years ago that the 2,8- and 3,7-dimethyldibenzothiophenes are much more easily desulfurized than 4,6-DMDBT (see Table XII) (26). Therefore, a combination of an isomerization catalyst and a desulfurization catalyst could be synergistic for removing dialkylbenzothiophenes. 

More recent research efforts have focused on the development of other possible catalysts such as promoted Raney copper,371,403 catalysts prepared from intermetal-lic precursors,362,371 386 404-406 and catalysts that tolerate high C02 content.407 Catalyst modifications allowed to shift the selectivity to the formation of higher alcohols.208,408 110 For example, in a process developed by IFP, a multicomponent oxide catalyst is applied with copper and chromium as the main components 410 By this method, 70-75% total alcohol selectivities and 30-50% of C2 and higher alcohol selectivities can be achieved at 12-18% conversion levels (260-320°C, 60-100 atm). 

This diversity of sites explains why the molecular weight distribution (MWD) of polymers produced by Cr/silica is broad (71). Model calculations which assume a single type of active site usually predict Mw/Mn 2,4 but in reality Mw/Afn = 6-15 is common, and 20-30 can be achieved with catalyst modifications. The distribution is also broader than that generally obtained from Ziegler catalysts, for which Mw/Afn = 3-6 under similar conditions. Experience with organometallic compounds suggests that a broad MWD may be a general feature of catalysts which terminate by -elimination. 

Saccharose hydrogenolysis was performed in a slurry type reactor in presence of a 5% Ru/carbon catalyst. Modification of pH during the reaction can increase the yield of 1,2-propane diol and glycerol noticeably. An adsorbed complex is proposed to account for the difference in selectivity for various Cg and Cg sugars. 

Another example of catalyst modification was achieved by synthesizing a Co(salen)-based catalyst with pendent ammonium salts (Figure 8.27) [54], This complex was able to catalyze the copolymerization of C02 and PO at TOF-values of up to 26000h 1, with high molecular weights and low polydispersities. Upon completion of the reaction the polymer/catalyst mixture was filtered through a pad of silica which yielded the purified polymer product. When the Co(salen) complex had been recovered from the silica it was recycled several times, with little to no loss of activity. 

Another approach to polymer/catalyst separation is to replace the use of costly wasteful solvents with a procedure that produces little to no waste, and requires no catalyst modification. These switchable polarity solvents (SPS), as developed by Jessop and coworkers, are able to utilize the reaction of secondary amines with C02 to reversibly form carbamate salt ionic liquids (Scheme 8.10) [66]. Upon the completion of the polymerization reaction, EtBuNH (Et = ethyl, Bu = n-butyl) is added to dissolve the polymer/catalyst mixture. The subsequent bubbling of C02 through this solution results in a precipitation of the polymer product, such that the purified polymer can be isolated by simple filtration. The catalyst can then be recovered by distilling the amine/carhamate mixture, and the entire system can (in theory) be recycled (Scheme 8.11). 

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