Catalytic simulation

Abstract. This paper presents results from quantum molecular dynamics Simula tions applied to catalytic reactions, focusing on ethylene polymerization by metallocene catalysts. The entire reaction path could be monitored, showing the full molecular dynamics of the reaction. Detailed information on, e.g., the importance of the so-called agostic interaction could be obtained. Also presented are results of static simulations of the Car-Parrinello type, applied to orthorhombic crystalline polyethylene. These simulations for the first time led to a first principles value for the ultimate Young s modulus of a synthetic polymer with demonstrated basis set convergence, taking into account the full three-dimensional structure of the crystal. 

Although there are examples of enzymes which maintain their catalytic activity even when ciystallized, they normally work in their natural (i.e., aqueous) environment. This is the reason why the majority of the simulations are carried out applying a technique that accounts for solvent effects. But what is the effect of a solvent ... 

Work in the area of simultaneous heat and mass transfer has centered on the solution of equations such as 1—18 for cases where the stmcture and properties of a soHd phase must also be considered, as in drying (qv) or adsorption (qv), or where a chemical reaction takes place. Drying simulation (45—47) and drying of foods (48,49) have been particularly active subjects. In the adsorption area the separation of multicomponent fluid mixtures is influenced by comparative rates of diffusion and by interface temperatures (50,51). In the area of reactor studies there has been much interest in monolithic and honeycomb catalytic reactions (52,53) (see Exhaust control, industrial). Eor these kinds of appHcations psychrometric charts for systems other than air—water would be useful. The constmction of such has been considered (54). 

A final important area is the calculation of free energies with quantum mechanical models [72] or hybrid quanmm mechanics/molecular mechanics models (QM/MM) [9]. Such models are being used to simulate enzymatic reactions and calculate activation free energies, providing unique insights into the catalytic efficiency of enzymes. They are reviewed elsewhere in this volume (see Chapter 11). 

U Ryde. Molecular dynamics simulations of alcohol dehydrogenase with a four- or five-coordinate catalytic zinc ion. Proteins 21 40-56, 1995. 

Catalytic crackings operations have been simulated by mathematical models, with the aid of computers. The computer programs are the end result of a very extensive research effort in pilot and bench scale units. Many sets of calculations are carried out to optimize design of new units, operation of existing plants, choice of feedstocks, and other variables subject to control. A background knowledge of the correlations used in the "black box" helps to make such studies more effective. 

Figure 2. Catalytic reforming flowsheet. (Used with permission of Simulation Sciences Inc.)...

The proposed mechanism for the DD process is not intended to represent that of any actual catalytic reaction, but to simulate a generic bimolecular reaction. Monte Carlo simulations of the reaction mechanism described by Eqs. (21)-(25) have shown the existence of IPTs exhibiting a rich variety of critical behavior. 

Very recently, considerable effort has been devoted to the simulation of the oscillatory behavior which has been observed experimentally in various surface reactions. So far, the most studied reaction is the catalytic oxidation of carbon monoxide, where it is well known that oscillations are coupled to reversible reconstructions of the surface via structure-sensitive sticking coefficients of the reactants. A careful evaluation of the simulation results is necessary in order to ensure that oscillations remain in the thermodynamic limit. The roles of surface diffusion of the reactants versus direct adsorption from the gas phase, at the onset of selforganization and synchronized behavior, is a topic which merits further investigation. 

H. P. Kaukonen, R. M. Nieminen. Computer simulations studies of the catalytic oxidation of carbon monoxide on platinum metals. J Chem Phys 97 4380- 386, 1989. 

J. Mai, A. Cashes, W. von Niessen. A Monte Carlo simulation of the catalytic oxidation of CO on DLA clusters. Chem Phys Lett 277 197-202, 1993. 

P. Moller, K. Wetzl, M. Eiswirth, G. Ertl. Kinetic oscillations in the catalytic CO oxidation Computer simulations. J Chem Phys 55 5328-5334, 1986. 

V. N. Kusovkov, O. Kortluke, W. von Niessen. Kinetic oscillations in the catalytic CO oxidation on Pt single crystal surfaces Theory and simulation. J Chem Phys 705 5571-5580, 1998. 

K. Fichthom, E. Gulari, R. Ziff". Self-sustained oscillations in a heterogeneous catalytic reaction A Monte Carlo simulation. Chem Eng Sci 44 1403-1411, 1989. 

In the Monsanto/Lummus Crest process (Figure 10-3), fresh ethylbenzene with recycled unconverted ethylbenzene are mixed with superheated steam. The steam acts as a heating medium and as a diluent. The endothermic reaction is carried out in multiple radial bed reactors filled with proprietary catalysts. Radial beds minimize pressure drops across the reactor. A simulation and optimization of styrene plant based on the Lummus Monsanto process has been done by Sundaram et al. Yields could be predicted, and with the help of an optimizer, the best operating conditions can be found. Figure 10-4 shows the effect of steam-to-EB ratio, temperature, and pressure on the equilibrium conversion of ethylbenzene. Alternative routes for producing styrene have been sought. One approach is to dimerize butadiene to 4-vinyl-1-cyclohexene, followed by catalytic dehydrogenation to styrene ... 

The simultaneous determination of a great number of constants is a serious disadvantage of this procedure, since it considerably reduces the reliability of the solution. Experimental results can in some, not too complex cases be described well by means of several different sets of equations or of constants. An example would be the study of Wajc et al. (14) who worked up the data of Germain and Blanchard (15) on the isomerization of cyclohexene to methylcyclopentenes under the assumption of a very simple mechanism, or the simulation of the course of the simplest consecutive catalytic reaction A — B —> C, performed by Thomas et al. (16) (Fig. 1). If one studies the kinetics of the coupled system as a whole, one cannot, as a rule, follow and express quantitatively mutually influencing single reactions. Furthermore, a reaction path which at first sight is less probable and has not been therefore considered in the original reaction network can be easily overlooked. 

The previous chapters taught us how to ask questions about specific enzymatic reactions. In this chapter we will attempt to look for general trends in enzyme catalysis. In doing so we will examine various working hypotheses that attribute the catalytic power of enzymes to different factors. We will try to demonstrate that computer simulation approaches are extremely useful in such examinations, as they offer a way to dissect the total catalytic effect into its individual contributions. 

This comprehensive article supplies details of a new catalytic process for the degradation of municipal waste plastics in a glass reactor. The degradation of plastics was carried out at atmospheric pressure and 410 degrees C in batch and continuous feed operation. The waste plastics and simulated mixed plastics are composed of polyethylene, polypropylene, polystyrene, polyvinyl chloride, acrylonitrile butadiene styrene, and polyethylene terephthalate. In the study, the degradation rate and yield of fuel oil recovery promoted by the use of silica alumina catalysts are compared with the non-catalytic thermal degradation. 9 refs. lAPAN... 

The thermal and catalytic cracking of PP, PS, and SBR waste, dissolved in light cycle oil, was studied in a riser simulator. 19 refs. 

This suggests that the primary function of the small-volume electric heater in the EHC system is to transfer the supplied electrical energy downstream for rapid lightoff of the main converter rather than to provide additional catalytic conversion. In fact, consistent with this argument, computer simulations for the EHC system with a 0.4-cm-long heater predicted very similar tailpipe HC emissions regardless of whether or not the electric heater is catalyzed (see Fig. 4). 

The authors developed a multi-layered microreactor system with a methanol reforma- to supply hydrogen for a small proton exchange membrane fiiel cell (PEMFC) to be used as a power source for portable electronic devices [6]. The microreactor consists of four units (a methanol reformer with catalytic combustor, a carbon monoxide remover, and two vaporizers), and was designed using thermal simulations to establish the rppropriate temperature distribution for each reaction, as shown in Fig. 3. 

The ratio of the observed reaction rate to the rate in the absence of intraparticle mass and heat transfer resistance is defined as the elFectiveness factor. When the effectiveness factor is ignored, simulation results for catalytic reactors can be inaccurate. Since it is used extensively for simulation of large reaction systems, its fast computation is required to accelerate the simulation time and enhance the simulation accuracy. This problem is to solve the dimensionless equation describing the mass transport of the key component in a porous catalyst[l,2]... 

Evans found that molecular hydrogen was efficiently generated by the reaction of a simple diiron complex [CpFe(CO)2]2 (Fp2) with acetic acid (pA a = 22.3) in acetonitrile [202]. Electrochemical simulations revealed that Ep2, [CpEe(CO)2] (Fp ), and [CpFe(CO)2H] (FpH) were key intermediates in this catalytic mechanism (Scheme 61). Reduction of Fp2 produces both an Fp anion and an Fp radical, which is further reduced to give an Fp anion. The oxidation of the Fp anion by proton affords FpH. This protonation was found to be the rate-limiting step. The dimerization of the FpH generates Fp2 and H2. Alternatively, the FpH is reduced to afford the FpH anion, which is subsequently protonated... 

ABSTRACT Zeolite Y modified with chiral sulfoxides has been foimd catal rtically to dehydrate racemic butan-2-ol enantioselectively depending on the chiral modifier used. Zeolite Y modified with R-l,3-dithiane-1-oxide shows a higher selectivity towards conversion of S-butan-2-ol and the zeolite modified with S-2-phenyl-3-dithiane-1-oxide reacts preferentially with R-butan-2-ol. Zeolite Y modified with dithiane oxide demonstrates a significantly higher catalsdic activity when compared to the unmodified zeolite. Computational simulations are described and a model for the catalytic site is discussed. 

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