Olefin system dehydrogenation

The chemical reactivity of resin acids is determined hy the presence of hoth the double- bond system and the COOH group [5], The carboxylic group is mainly involved in esterification, salt formation, decarboxylation, nitrile and anhydrides formation, etc. These reactions are obviously relevant to both abietic- and pimaric-type acids (Rgs 4.1 and 4.3, respectively). The olefinic system can be involved in oxidation, reduction, hydrogenation and dehydrogenation reactions. Given the conjugated character of this system in the abietic-type acids, and the enhanced reactivity associated with it, much more attention has been devoted to these stractures. In terms of industrial applications, salt formation, esterification, and Diels-Alder additions are the most relevant reactions of resin acids. 

HP Alkylation Process. The most widely used technology today is based on the HE catalyst system. AH industrial units built in the free world since 1970 employ this process (78). During the mid-1960s, commercial processes were developed to selectively dehydrogenate linear paraffins to linear internal olefins (79—81). Although these linear internal olefins are of lower purity than are a olefins, they are more cost-effective because they cost less to produce. Furthermore, with improvement over the years in dehydrogenation catalysts and processes, such as selective hydrogenation of diolefins to monoolefins (82,83), the quaUty of linear internal olefins has improved. 

Theoretical studies of catalytic alkane-dehydrogenation reactions by [(PCP )IrH2], PCP rf-C6H3(CH2P112)2-l, 3 and [cpIr(PH3)(H)]+, suggest that they proceed through similar steps in both cases namely (i) alkane oxidation, (ii) dihydride reductive elimination, (iii) /3-II transfer from alkyl ligand to metal, (iv) elimination of olefin.402 The calculated barriers to steps (i), (ii), and (iv) are more balanced for the PCP system than for cp(PH3). 

More recently, the same principle was applied by the same authors to cyclic alkanes for catalytic ring expansion, contraction and metathesis-polymerization (Scheme 13.24) [44]. By using the tandem dehydrogenation-olefin metathesis system shown in Scheme 13.23, it was possible to achieve a metathesis-cyclooligomerization of COA and cyclodecane (CDA). This afforded cycloalkanes with different carbon numbers, predominantly multiples of the substrate carbon number the major products were dimers, with successively smaller proportions of higher cyclo-oligomers and polymers. 

Substrates which can undergo partial oxidation are characterized by a 7T-electron system or unshared electrons olefins and aromatics contain the first, methanol, ammonia and sulphur dioxide the second. Alkanes do not contain such electrons. Their selective oxidation appears to demand (thermal or catalytic) dehydrogenation to alkenes as the initial process. 

The dehydrogenation of paraffins to olefins, while it does not take place to a large extent at typical reforming conditions (equilibrium conversion of n-hexane to 1-hexene is about 0.3% at 510°C. and 17 atm. hydrogen partial pressure), is nevertheless of considerable importance, since olefins appear to be intermediates in some of the reactions. This matter will be discussed in more detail in a subsequent section. The formation of olefins from paraffins, similar to the formation of aromatics, is favored by the combination of high temperature and low hydrogen partial pressure. The thermodynamics of olefin formation can play an important role in determining the rates of those reactions which proceed via olefin intermediates, since thermodynamics sets an upper limit on the attainable concentration of olefin in the system. 

The most interesting compounds described in this paragraph are meso-ionic naphtho[reactions with olefinic compounds. For instance, the reaction with acetylenedicarboxylates gives rise to adducts 458, which are dehydrogenated spontaneously to the new heteroaromatic systems 459 with the aromatic 147r-electron contour [75JCS(PI)556]. 

The multifunctionality is achieved through either the combination of two different compounds (phase-cooperation) or the presence of different elements inside a single crystalline structure. In antimonates-based systems, cooperation between the metal antimonate (having a rutile crystalline structure), employed for propane oxidative dehydrogenation and propene activation, and the dispersed antimony oxide, active in allylic ammoxidation, is made more efficient through the dispersion of the latter compound over the former. In metal molybdates, one single crystalline structure contains both the element active in the oxidative dehydrogenation of the hydrocarbon (vanadium) and those active in the transformation of the olefin and in the allylic insertion of the N H2 species (tellurium and molybdenum). 

The C-H/olefin coupling of aryloxazolines proceeds with unusual product selectivity. In this case, alkylation products, i.e., formally dehydrogenation products, are obtained as a major product (Eq. 22) [11]. These types of dehydrogenation compounds are believed to be formed via a carbometalation pathway. The first example of this type of alkenylation of arenes with olefins using palladium(II) complexes via C-H bond cleavage was reported in 1967 [32]. Later, several efforts were made to perform this reaction in a catalytic manner [33]. In 2001, Milstein et al. [34] reported the oxidative alkenylation of arenes with olefins using a Ru/02/C0 catalyst system (Eq. 23). Details of the reaction mechanism have not been elucidated. 

We have shown that [Ni2+]-OMS-2 and [N 2+]-OMS-1 catalyze the selective conversion of hexane to 1-hexene. Stainless steel flow reactors of 1/4 diameter containing 0.5 g catalyst, charges of 7 g n-hexane in 2 h, 1 atm pressure and temperatures of 500°C are used in these experiments. Both gas chromatography (GC) and mass spectrometry (MS) analyses are done to monitor product distributions. Conversions as high as 60% and selectivities of 90% (to the terminal olefin) have been observed for the OMS-2 system. This may be a consequence of the better shape selectivity of [Ni2+]-OMS-2 (4.6 A tunnel) versus (Ni2+]-OMS-1 (6.9 A). The latter material is not as selective or active. Systems that do not contain N 2+ are totally inactive.91 There is precedence for dehydrogenation activity of these systems since manganese nodules have been reported to be excellent catalysts for dehydrogenation of cyclohexane.63... 

At 435°C the olefin concentration is found to be only about 0.02% at 30 atm. partial pressure of hydrogen. Thus, if we were to carry out paraffin isomerization by successive and separate steps of dehydrogenation of ri-paraffin to n-olefin, followed separately by skeletal isomerization of the n-olefin produced to iso-olefin (and subsequent rehydrogenation), the over-all conversion of such a scheme could be, at best, 0.02%. Thus, the paraffin isomerization, if accomplished in a bifunctional reaction system with a high conversion as might be described by formula (2), is an example of a nontrivial case as defined by (3) above. 

Recently, Pt/Nb20s catalysts have been investigated on dehydrogenation of alkanes. These systems have presented advantages on selectivity towards olefins when compared to Pt/ALOs or even Pt-Sn/A]20 , catalysts [4-6]. The promoting mechanism is related to both the SMSI (Strong Metal Support Interaction) effect and the low acidity of the support, which produce a sharp decrease in hydrogenolysis and aromatization, respectively [5]. 

Reggel, Friedman, and Wender discovered that this reagent has remarkable power for isomerization of olefins and for dehydrogenation of dihydrobenzenoid systems. The anhydrous ethylenediamine required is prepared either by heating commercial material with sodium for a day or two and distilling or by azeotropic distillation with thiophene-free benzene, followed by distillation from potassium hydroxide pellets and then from sodium. For preparation of the reagent, purified ethylenediamine is stirred under nitrogen at 90-110° and lithium is added in portions an initial deep blue color disappears at the end of the addition, and a pale yellow solution results. ... 

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