Control of Selectivity

Selectivity is a key topic for the design of reactors in recycles. From the standalone viewpoint the means to influence selectivity are reactor type, conversion level and mixing method. In the standalone view low conversion and PFRs are recommended for achieving good selectivity. By contrast, following the results of RSR analysis, the recycle policy and the plantwide control of reactant feeds can play the determinant role the reactor type or the conversion level is less important 

---

Tannor D J, Kosloff R and Rice S A 1986 Coherent pulse sequence induced control of selectivity of reactions exact quantum mechanical calculations J. Chem. Rhys. 85 5805-20, equations (1)-(6)... 

Tannor D J and Rice S A 1985 Control of selectivity of chemical reaction via control of wave packet evolution J. 

The number of theoretical investigations of hetero-Diels-Alder reaction is very limited. The few papers dealing with this class of reactions have shown that the influence of the Lewis acid on the reaction course can to a high extent be compared to those found the carbo-Diels-Alder reactions. At the present stage of investigations, however, more work is needed if we are to understand the influence and control of selectivity in Lewis acid-catalyzed hetero-Diels-Alder reaction - we are probably at the beginning of a new era in this field. 

Methanol remains the most widely used modifier because it produces highly efficient separations, but it does not always produce the highest selectivity [8]. Recent studies have provided insight into the role of the modifier in enantioselectivity in SFC [69]. Blackwell and Stringham examined a series of phenylalanine analogues on a brush-type CSP and developed a model that allowed prediction of selectivity based on the bulk solvation parameters of various modifiers [70]. Careful choice of modifiers can be used to mask or enhance particular molecular interactions and ultimately provide control of selectivity [71]. 

Direct Current (DC). This current is transmitted for industrial uses only in exceptional situations. The most common sources of direct current are storage batteries and industrial devices called rectifiers, in which alternating current is changed (rectified) to direct current, as is used in electrolytic cells for the manufacture of chlorine gas, magnesium, aluminum, and a few other chemicals. The direct current is flowing from the source through the user application and back to the source, in one direction. The motor is primarily used for speed control of selected equipment. 

The catalyst composition has a role in the control of selectivity. The rutile-type V/Sb/(Nb) mixed oxide activates the hydrocarbon and ammonia. However, most of the ammonia is burnt to N2, rather than being inserted on the hydrocarbons this likely occurs because the catalyst is not veiy efficient in the generation of the selective Me=NH species when reaction temperatures lower than 400°C are used (11). In fact, with all catalysts the selectivity to A -containing compounds increased when the reaction temperature was increased, and the selectivity to N2 correspondingly decreased (Figure 40.6). The dilution of the active phase with tin... 

In the present chapter an attempt is made to summarize the available material concerning the control of selectivity in the nickel-catalyzed homogeneous oligomerization of olefins and related reactions [for a previous review of this topic, see Bogdanovic et al. (4)]. No attempt has, however, been made to give a complete literature survey. 

The cross-metathesis reaction has evolved extensively during the past few years, but there is still a considerable amount of work to be done before the full potential of this reaction is realised. The development of new metathesis catalysts, greater understanding and control of selectivity, and more extensive applications in synthesis that will surely follow in the near future, make this a particularly exciting time in the evolution of the alkene cross-metathesis reaction. 

A certain control of selectivity for simple ligands without altering their fluorescence features is possible when integrating them into more developed, often macrocyclic receptor units [45,46]. 

In the following decades, researchers in catalysis turned their efforts to controlhng molecular structure as well as size. The catalyst zeolite paved the way. In the late 1960s, researchers at Mobil Oil Co. were able to s)uithesize zeolite by deliberately designing and preparing the structure of catalysts at the atomic and molecular levels. The resulting nanostructured crystalline material (ZSM-5)—with a 10-atom ring and pore size of 0.45-0.6 nm—enabled the control of selectivity in petrochemical processes at the... 

Yube, K. and Furuta, M. and Aoki, N. and Mae, K. (2007). Control of selectivity in phenol hydroxylation using microstructured catalytic wall reactors. Applied Catalysis A General 327, 278-286... 

Maintenance dose Most patients will be controlled with 5 or 10 mg taken once daily. However, some patients may require up to the maximum recommended daily dose of 20 mg. While the glycemic control of selected patients may improve with doses that exceed 10 mg, clinical studies... 

Furthermore, a catalyst can stabilize a carbanion not only through a stronger or weaker basicity of the site but by means of its coordination with the conjugated surface Lewis acid. It is apparent that the interplay between the acid and base pair can markedly help in the control of selectivity. 

K. N. Houk, N. G. Randan, and J. Mareda, Theoretical Studies of Halocarbene Cycloaddition Selectivities. A New Interpretation of Negative Activation Energies and Entropy Control of Selectivity, Tetrahedron, 1985, 41, 1555. 

Key to these applications has been the control of selectivity-chemoselectivity, regioselectivity, diastereoselectivity, and, especially, enantioselectivity. Here the design of chiral catalysts, begun already in the 1960s, has allowed access to one product when multiple products would, in the past, have been expected. Both electronic and steric control are important, and different metal ions with their associated chiral ligands can have unexpected effects. 

M. Shapiro, Z. Chen, and P. Brumer. Simultaneous control of selectivity and yield of molecular dissociation pulsed incoherent interference control. Chem. Phys., 217(2) 325—340(1997). 

Electrochemical and Electrocatalytic Reduction. The one-electron reduction of C02 yields the radical anion C02-, which reacts with an H source to give the formate ion. The reaction, however, is not selective because various other reactions may take place. An alternative and more promising approach is the two-electron reduction of C02 in the presence of a proton source to afford formic acid. The latter process requires a considerably lower potential (—0.61 V) than does the one-electron reduction (—1.9 V) consequently, the electrolysis in the presence of catalysts may be performed at lower voltages. The control of selectivity, however, is still a problem, since other two-electron reductions, most importantly reduction to form CO and H2, may also occur.101127 The reduction of C02 to CO, in fact, is the subject of numerous studies. Electrochemical and electrocatalytic reductions of C02 in aqueous solutions have been studied and reviewed.11,128-130... 

Behr A, Urschey M (2003) Palladium-catalyzed telomerization of butadiene with ethylene glycol in liquid single phase and biphasic systems control of selectivity and catalyst... 

Control of Selectivity Using Carbene-Derived Ylides... 

---