Catalytic reaction steps

Hydrogen production from carbonaceous feedstocks requires multiple catalytic reaction steps For the production of high-purity hydrogen, the reforming of fuels is followed by two water-gas shift reaction steps, a final carbon monoxide purification and carbon dioxide removal. Steam reforming, partial oxidation and autothermal reforming of methane are well-developed processes for the production of hydro-... 

Finally, it is worth mentioning that a successful integration of catalytic reaction steps with product separation and catalyst recovery operations will also be dependent on innovative chemical reaction engineering. This will require the widespread application of sustainable engineering principles [48].In this context process intensification , which involves the design of novel reactors of increased volumetric productivity and selectivity with the aim of integrating different unit operations to reactor design, and miniaturization will play pivotal roles [49, 50]. 

Overview In many industrial reactions, the overall rate of reaction is limited by the rate of mass transfer of reactants between the bulk fluid and the catalytic surface. By mas,s transfer, we mean any proces.s in which diffusion plays a role. In the rate laws and catalytic reaction steps described in Chapter 10 (diffusion, adsorption, surface reaction, desorption, and diffusion), we neglected the diffusion steps by saying we were operating under conditions where these steps are fast when compared to the other steps and thus could be neglected. We now examine the assumption that diffusion can be neglected. In this chapter we consider the external resistance to diffusion, and in the next chapter we consider internal resistance to diffusion. 

The radiolysis/EPR study of isobutene was supported by experiments on Cj-Cg olefins on both HZSM5 and NaZSMS (Table 1). This eillowed screening for possible radical cation reactions (minimal for these compounds), and the survey of related compounds aided spectroscopic assignment and tested the catalytic reaction steps from different starting points. For example, 2 was also formed from C and Cg feed molecules, corroborating the conclusion that 2 can be formed from isobutene by cracking the dimer as opposed to addition of C4 and Cj units. 

Moreover, Fig. 2.2 points out further statistics data on palladium membranes applied in the field of membrane reactors (MRs), devices combining the separation properties of the membranes with the typical characteristics of catalytic reaction steps in only one unit. In particular, this figure reports the number of publications on palladium-based membranes reactors with respect to the total number of publications in the membrane reactors area. 

As our understanding of the elementary catalytic reaction steps and our analytical capabilities improved, the basic kinetic models were expanded in order to reflect this finer level of detail. 

Reforming. Reforming of the natural gas feedstock takes place in two distinct catalytic reaction steps. The first step is the primary reforming and is carried out in a furnace in the presence of steam to produce a partially reformed gas. The second step constitutes the secondary reforming, in which the reaction is carried out in a refractory-lined pressure vessel to produce a low-methane product. 

Various Pd(0) or Pd(II) species are used in the catalytic reactions. Step 1 involves oxidative addition of one of the organic species to the palladium. A transmetallation in Step 2 results in the palladium having two carbon-based ligands. A reductive elimination in Step 3 couples the two carbon fragments together. The relative simplicity and variability of each component involved has made this catalytic cycle a very powerful one. 

As already outlined above, the OCM reaction consists of surface-catalyzed and non-catalytic reaction steps occurring in the gas phase the latter type of reactions was dealt with in the above section 2. Within the present section (1) kinetic results obtained for the primary catalytic reaction steps which lead to the formation of ethane plus ethylene and carbon oxides from methane are presented, and (2) suggestions are put forward of how to account for both the catalytic and non catalytic homogeneous gas-phase reactions occurring simultaneously. 

Basis of Simulation. It is basically assumed that the primary catalytic reaction step is the formation of methyl radicals which further react either on the catalytic surface or in the gas phase. The rate of catalytic methyl-radical formation was set equal to the rate of methane consumption. A power-law rate equation, valid for temperatures around 1020 K as reported earlier [27], was applied. 

Since the iridium(III) complex [(Cp )IrCl2]2 (Cp = pentamethylcyclo-pentadienyl) is an active catalyst for the p-alkylation of secondary alcohols with primary alcohols, a series of iridium(III) complexes 26-28 bearing a Cp unit tethered to an imidazolyhdene was synthesized (Equation (8.15)). These complexes displayed similar activities in the p-alkylation of secondary alcohols with primary alcohols as electrophiles (Equation (8.15)), and surpassed the performance of their parent compound [(Cp )IrQ2]2. Control of the reaction time was found to be crucial to avoid the undesirable dehydrogenation of the product (see Section 8.4.2 for further details). The sequence of catalytic reaction steps was thought to involve the oxidation of both alcohols and the formation of an iridium hydride species. Base-promoted cross-aldolization and elimination to form the ot-enone and hydrogenation of the C=C and C=0 bonds to regenerate an iridium-alkoxide species would complete the cycle. 

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