Alkanes on metal

Luntz A C and Harris J 1992 The role of tunneling in precursor mediated dissociation Alkanes on metal surfaces J. 

Recently (67,161), evidence has been put forward for a carbene mechanism in the homologation of n-alkanes on metal films. 

This reaction, like dicarbene recombination, also has its analog in coordination chemistry, that is, reductive elimination of tetramethylene and pentamethylene ligands from platinum complexes yields cyclobutane and cyclopentane, respectively (777). According to this direct ring closure mechanism, the observed selectivity for dehydrocyclization of n-alkanes on metals (nonformation of quaternary-secondary and tertiary-secondary C-C bonds in reactions of type A and B) should be interpreted in terms of simple steric effects. However, although, in the case of platinum, the concepts of steric hindrance could account for the change of selectivity that occurs with decreasing metal particle size (i.e., cyclization of n-hexane takes place on... 

Isomerization of alkanes on metal surfaces occurs via the intermediate formation of a,y-adsorbed, metallacyclobutane, carbene and it-bounded species (Scheme III.2). 

Some reference has already been made to the chemisorption of alkanes on metal surfaces (Sections 4.8 and 6.1) it is a difficult process requiring a significant activation energy, and probably two sites , although as we have seen they may not... 

The extensive attention that has been paid to the reactions of the lower alkanes on metal catalysts reflects the wide range of phenomena encountered with structure-sensitive reactions, as opposed to those reactions met with earlier, the insensitivity of which limited the importance of variables such as particle size, crystal face and composition of bimetallic systems. Far more attention has also been paid to the careful measurement of reaction kinetics, and their interpretation by various models. This, one hopes, explains even if it does not exeuse the length of this chapter. 

Saltier JJHB, Ruiz-Martinez J, Santillan-Jimenez E, Weckhuysen BM (2014) Catalytic dehydrogenation of light alkanes on metals and metal oxides. Chem Rev 114 (20) 10613-10653... 

Figure 2.1. Selection of assumed primary reaction steps in the oxidative dehydrogenation of alkanes on metal oxides as catalyst components.

Sheppard N and De La Cruz C 1998 Vibrational spectra of hydrocarbons adsorbed on metals. Part II. Adsorbed acyclic alkynes and alkanes, cyclic hydrocarbons including aromatics and surface hydrocarbon groups derived from the decomposition of alkyl halides, etc Adv. Catal. 42 181-313... 

To illustrate how a bifunctional catalyst operates, we discuss the kinetic scheme of the isomerization of pentane [R.A. van Santen and J.W. Niemantsverdriet, Chemical Kinetics and Catalysis (1995), Plenum, New York]. The first step is the dehydrogenation of the alkane on the metal ... 

The large amounts of natural gas (mainly methane) found worldwide have led to extentive research programs in the area of the direct conversion of methane [1-3]. Ihe oxidative transformation of methane (OTM) is an important route for the effective utilization of the abundant natural gas resources. How to increase catalyst activity is a common problem on the activation of methane. The oxidation of methane over transition m al oxides is always high active, but its main product is CO2, namely the product of deep oxidation. It is because transition metal oxides have high oxidative activity. So, they were usually used as the main corrqtonent of catalysts for the conqilete oxidation of alkane[4]. The strong oxidative activity of CH4 over tran on metal oxides such as NiO indicates that the activation of C-H bond over transition metal oxides is much easier than that over alkaline earth metal oxides and rare earth metal oxides. Furthermore, the activation of C-H bond is the key step of OTM reaction. It is the reason that we use transition metal oxides as the mam conq>onent of the OTM catalysts. However, we have to reahze that the selectivity of OTM over transition metal oxides is poor. 

In some catalytic processes, it is necessary to avoid carbon-carbon bond cleavage. For example, isobutane is mainly transformed into its lower alkane homologues (hydrogenolysis products) on metal surfaces, while it can be converted more and more selectively into isobutene when the Pt catalysts contain an increasing amount of Sn (selective dehydrogenation process) [131]. 

Hydrogenolysis reactions of hydrocarbons on metal catalysts have been investigated in some detail. Extensive studies have been conducted on both alkanes and cycloalkanes. While a number of questions still remain with regard to mechanistic and kinetic details of the reactions, the general features seem reasonably clear. 

In the hydrogenolysis of the higher alkanes on the nonnoble group VIII metals (i.e., iron, cobalt, and nickel), the mode of cracking is very different from that observed on the noble metals of group VIII (49, 50). On nickel,... 

In summary a few "generalizations" have been found. First, size selective chemistry is strongly associated with chemisorption that requi res bond-breaking. Second, metal clusters react rapidly with ligands that molecularly chemisorb even when the eventual products involve dissociation of the ligand. Dehydrogenation of Cg-alkanes on small platinum clusters take exception to this. 

In 1981, we first reported on the sonochemistry of discrete organometallic complexes and demonstrated the effects of ultrasound on metal carbonyls in alkane solutions (J 8). The transition metal carbonyls were chosen for these initial studies because their thermal and photochemical reactivities have been well-characterized. The comparison among the thermal, photochemical, and sonochemical reactions of Fe(C0)5 provides an excellent example of the unique chemistry which acoustic cavitation can induce, and (because of space limitations in this review) we will focus upon it as an archetype. 

The preparation of similar precursors suitable for the deposition of metal nitrides is analogous to the preparations of phosphorus and arsenic compounds. The initial reaction of metal trialkyls MR3 (M = A1, Ga, In) with amines (NHR 2) results in the formation of oligomeric amido compounds [R2MNR 2] (n = 2 or 3) which eliminate alkanes on thermolysis. The incorporation of a proton as a substituent on the pnictide bridging ligand has been examined, and many compounds of the type [R2MNHR ]2 have been synthesized. The presence of this proton may facilitate /3-elimination, allowing lower deposition temperatures to be used. 

Sigma-bond metathesis at hypovalent metal centers Thermodynamically, reaction of H2 with a metal-carbon bond to produce new C—H and M—H bonds is a favorable process. If the metal has a lone pair available, a viable reaction pathway is initial oxidative addition of H2 to form a metal alkyl dihydride, followed by stepwise reductive elimination (the microscopic reverse of oxidative addition) of alkane. On the other hand, hypovalent complexes lack the... 

The titanosilicate version of UTD-1 has been shown to be an effective catalyst for the oxidation of alkanes, alkenes, and alcohols (77-79) by using peroxides as the oxidant. The large pores of Ti-UTD-1 readily accommodate large molecules such as 2,6-di-ferf-butylphenol (2,6-DTBP). The bulky 2,6-DTBP substrate can be converted to the corresponding quinone with activity and selectivity comparable to the mesoporous catalysts Ti-MCM-41 and Ti-HMS (80), where HMS = hexagonal mesoporous silica. Both Ti-UTD-1 and UTD-1 have also been prepared as oriented thin films via a laser ablation technique (81-85). Continuous UTD-1 membranes with the channels oriented normal to the substrate surface have been employed in a catalytic oxidation-separation process (82). At room temperature, a cyclohexene-ferf-butylhydroperoxide was passed through the membrane and epoxidation products were trapped on the down stream side. The UTD-1 membranes supported on metal frits have also been evaluated for the separation of linear paraffins and aromatics (83). In a model separation of n-hexane and toluene, enhanced permeation of the linear alkane was observed. Oriented UTD-1 films have also been evenly coated on small 3D objects such as glass and metal beads (84, 85). 

The homogeneous nucleation phenomenon was first studied by droplet crystallization experiments performed on metals [57-60], alkanes [61] and polymers [62-66] when dispersed in inert low molecular weight media. The idea was that when the polymer in the bulk is subdivided into a number of droplets that is larger than the number of active heterogeneities present in the polymer, there should be a certain number of droplets without any active heterogeneity. 

The activation of alkanes on transition metal surfaces is an important step in many catalytic reactions. Hydrogenolysis, steam reforming and isomerization of alkanes all involve alkane dissodation. Thus, much interest exists in the mechanistic and kinetic aspects of alkane dissociation. 

The possibilities afforded by SAM-controlled electrochemical metal deposition were already demonstrated some time ago by Sondag-Huethorst et al. [36] who used patterned SAMs as templates to deposit metal structures with line widths below 100 nm. While this initial work illustrated the potential of SAM-controlled deposition on the nanometer scale further activities towards technological exploitation have been surprisingly moderate and mostly concerned with basic studies on metal deposition on uniform, alkane thiol-based SAMs [37-40] that have been extended in more recent years to aromatic thiols [41-43]. A major reason for the slow development of this area is that electrochemical metal deposition with, in principle, the advantage of better control via the electrochemical potential compared to none-lectrochemical methods such as electroless metal deposition or evaporation, is quite critical in conjunction with SAMs. Relying on their ability to act as barriers for charge transfer and particle diffusion, the minimization of defects in and control of the structural quality of SAMs are key to their performance and set the limits for their nanotechnological applications. 

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