Disproportionation activity

Fig. 5. The disproportionation activity of Al-mont( O) and Al-sapo( A) as a function of the concentration of acid sites adsorbing ammonia at 250 C.

Various methods have been developed to maximize p-xylene formation. A mild hydrothermal treatment of H-ZSM-5 and the incorporation of Pt greatly reduce disproportionation activities and, consequently, increase para selectivity.355 Suppressing the activity of the external surface can also lead to enhanced para selectivity. By judicious choice of reaction conditions and organometallic reagents... 

The ruthenium complexes were tested for disproportionation activity using (CH3)3A12C13 as cocatalyst, PhCl as solvent, and 2-pentene as substrate. The results, along with that obtained for MoCl2(NO)2(PPh3)2 under the same conditions, are summarized in Table V. 

The disproportionation activity in the supported species is parallel to the increased activity of ethylene polymerization on supported catalysts. Many of the steps in the reaction may be identical for example, the initial coordination of olefin to the metal center will be common to both systems. Indeed, some of these catalysts are also ethylene polymerization catalysts (see Table IV) although their activities are much less than the corresponding zirconium derivatives. A possible intermediate common to both disproportionation and polymerization could be the hydrocarbyl-olefin species (Structure I). Olefin disproportionation would result if the metal favored /3-hydrogen elimination to give the diolefin intermediate (Structure II) which is thought to be necessary for olefin disproportionation. Thus, the similarity between the mechanism and activation of olefin disproportionation and polymerization is suggested. 

These results strongly pointed toward the involvement of the acidic hydroxyl groups in the catalytic reaction as suggested by Benesi (157), since the maximum activity was obtained when the zeolite was completely deammoniated. In addition, catalysts which had been dehydroxylated by high-temperature calcination demonstrated low activity. Thus, Benesi proposed that the Brtfnsted acid sites rather than the Lewis acids were the seat of activity for toluene disproportionation. This conclusion was supported by the enhancement in toluene disproportionation activity observed when the dehydroxylated (Lewis acid) Y zeolite was exposed to small quantities of water. As discussed previously, Ward s IR studies (156) indicated a substantial increase in Brdnsted acidity upon rehydration of dehydroxylated Y sieve. 

As indicated in Chapter 1, principally, the reactions may be conjugated, when one of them slows another one down, and somewhat inhibits it. The mechanism of slowing down the secondary reaction may have different origins (e.g. if the target reaction is of the catalytic type, and intermediate products (IP) poisoning the catalyst are formed in the primary reaction). Another possible case is realized when IP of both reactions recombine or disproportionate (active sites are eliminated). Such negative effects of chemical reactions allow us to... 

Increased disproportionation activity of tungsten oxide-silica following treatment of the catalyst with hydrogen chloride, or hydrocarbon chlorides that decomposed to hydrochlorides at the temperature of the treatment, was disclosed by Pennella40). Table 4 shows propylene conversions before and after 20—80 minute treatments of the catalysts with various chloride compounds. 

A decrease in 1-butene yield indicated that double-bond isomerization activity of the catalyst decreased more rapidly than disproportionation activity. Patents to Wilson and Larson 90-92) relate to the start-up and operation of olefin disproportionation processes. 

Figure 22.1 Dextrin represents the initial starch hydrolyzate used for the reaction. It is a hydrolyzate resulting from alpha-amylase and the disproportionation activity of the CDTGase, or a combination of these activities. This dextrin and cydodextrins are both substrates and products of the CDTGase.

Disproportionation is a special form of double bond migration in which the double bond is transferred from one molecule to another. Reactions of this type are especially liable to occur over palladium, and for this reason palladium sometimes is best avoided in olefin hydrogenation when the double bond is contained in an incipient aromatic system. Disproportionation activity in the hydrogenation of cyclohexene (and presumably other incipient aromatic systems will follow the same order) decreases with the metal in the order palladium >> platinum > rhodium (16). An example of the complication that can be caused by disproportionation during hydrogenation is found in the attempted reduction of... 

According to a recent study, the highest values of isomerization selectivity can be obtained with bifunctional Hmordenite catalysts with high values of the ratio between the concentrations of Pt and protonic sites accessible by reactant molecules, i.e. with catalysts on which the acid step is the limiting step of ethylbenzene isomerization. Furthermore, because of the reduction of the disproportionation activity, Na exchange of HMOR has a positive effect on the selectivity. 

Disproportionation of 1,2,4- trimethylbenzene Al-B, A1-M Al-S NH3 treatment reduced pillar density wdiich can be increased by re-intercalation. Catalytic activity increased with the number of pillars, so was the selectivity for 1,2,4,5-tetramethylbenzene. Al-M have less acidity than Al-S hence greater disproportionation activity. 49 65 66... 

The production of para-xylene is of interest to the petrochemical industry because of its use as monomer in polyester production. In addition to Cg aromatic isomerization, there are a number of important routes to para-xylene including the alkylation of toluene with methanol and the disproportionation of toluene. The catalytic properties of the SAPO molecular sieves for toluene methylation reactions have been described(11). While both large and medium pore SAPO s were active for the alkylation reaction, the medium pore materials were distinguished by remarkably high selectivity for methylation reactions, with disproportionation of the toluene feed representing less than 2% of the total conversion. By comparison, large pore SAPO-5 had nearly 60% disproportionation selectivity and the zeolite reference LZ-105 had nearly 80% disproportionation selectivity. The very low disproportionation activity of the medium pore SAPO s, attributed to their mild acid character, resulted in reduced losses of toluene to benzene and increased xylene yields relative to LZ-105 and SAPO-5. 

Under these conditions all molecular sieves evaluated give essentially complete isomerization of m-xylene feed to a thermodynamic equilibrium mixture of xylene isomers, while the disproportionation activity to toluene and trimethylbenzenes varies significantly. The results of this study are summarized in Table III and in Figure 3, where xylene disproportionation activity is plotted as a function of molecular sieve pore size. In general, a rough trend can be seen in which the molecular sieves with larger pore sizes are more active for this undesirable side reaction. 

Cg Aromatic Reactions with Hydrogen. The mild acid nature of the family of aluminophosphate based sieves renders them selective for a number of rearrangements as observed in the reactions of olefins and paraffins described above. This property as well as their apparent low disproportionation activity observed in the alkylation of toluene suggests that they be evaluated as the acid function in bifunctional Cg aromatic isomerization. As described above, cyclo-olefins are most likely involved in the conversion of ethylbenzene to xylenes. Strong acid functions, such as in mordenite, actively isomerize cyclo-olefinic intermediates but also catalyze ring-opening reactions which lead to loss of aromatics. A more selective acid function must still effectively interconvert ethyl cyclohexene to dimethylcyclohexenes but must leave the cyclohexene rings intact. 

Throughout these tests no substantial occurrence of metathesis was observed. Only minor disproportionation activity is evident upon initial contact of hydrocarbon with the catalyst and this behaviour is short lived. 

PdZr-c showed significantly lower carbon incorporation under the same experimental conditions, whereas no CO disproportionation and carbon incorporation could be observed with the palladium powder below 400°C. Thus the intimate interaction of palladium and zirconia seems to be crucial for the high CO disproportionation activity observed with the catalysts derived from the glassy alloy. 

The numbers of acid sites on the outer surfaces of partially coked ZSM-5 samples, which had low activities for psuedocumene disproportionation, were almost the same as the number on an untreated sample that had a high disproportionation activity. In contrast, external acidities of chemically modified ZSM-5 samples with low disproportionation activities were an order of magnitude lower than those of untreated ZSM-5. 

In order to examine the disproportionation activity separately we incubated the p-nitrophenyl-a-D-malto-pentaoside with the enzymes under the same conditions as in synthesis but in the absence of glucose-1-phosphate. The results of HPLC analysis are shown in Fig. 

There are different modes of termination combination and disproportionation. Active chains terminated by disproportionation will have the same molecular weight, where one of the chains will have an unsaturation and the other will be fully saturated. When chains are terminated by combination, because two propagating chains combine, the number of chains decreases by one, and the resultant molecular weight is the sum of the two macroradicals, thereby increasing the final molecular weight distribution. 

In systems where Cu and Cu can coexist due to disproportionation, activation of alkyl halides by copper in both oxidation states can take place. From a thermodynamic point of view, the relative activity of Cu and Cu as ATRP activators can be correlated to the ability of the Cu complex to disproportionate. The disproportionation of a Cu complex yields the corresponding Cu complex and Cu . Even if one assumes that the formed Cu is so finely... 

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