Temperature butenes isomerization

Figure 24 shows the cfs-butene isomerization over zinc oxide as a function of time at room temperature (7/). On a per unit area basis the initial rate at room temperature is 4 X 1010 molecules/sec cm2, a rate roughly one third that reported for alumina (69). Since the activation energy for alumina is less than that found for zinc oxide, this means that zinc oxide is comparable (on a per unit area basis) to alumina as an isomerization catalyst at slightly higher temperatures. 

Patents assigned to Mobil (217) describe the use of boron trifluoride supported on several porous carriers. BF3 supported on silica was found to exhibit a slightly higher performance with added water in the alkylation of a mixed alkene feed at 273 K. It was also shown that self-alkylation activity was considerably lower than that with HF as catalyst. Another patent (218) describes the use of a pillared layered silicate, MCM-25, promoted with BF3 to give a high-quality alkylate at temperatures of about 273 K. BF3 was also supported on zeolite BEA, with adsorbed water still present (219). This composite catalyst exhibited low butene isomerization activity, which was evident from the inferior results obtained with 1-butene. At low reaction temperatures, the product quality was superior to that of HF alkylate. 

This assignment is supported, indirectly, by the measured activities of these experiments. For example, the activities were measured to be (in M/g-catalyst hour) 0.98 (MS-Hz) 0.61 (MS-Dz) 180 (US-Hz) and 190 (US-Dz). Hence, the 250-fold greater activities of the ultrasound systems is consistent with the expected, more rapid, statistical C-H/D dissociation process as compared to the conventional (e.g., stirred/silent) mediated systems. Additional support for this model arises from a study of gas phase cA-2-butene isomerization to fra/rs-2-butene [15] at 291 K. Here the c O extrapolated trans deuterium number of -0.27 is supportive of C3-H/D elimination predicted by tra/jsition-state theory in this system at thermal equilibrium (e.g., vibrational temperature equal to tra/jslational temperature). 

Recent studies of the kinetics and mechanism of n-butene isomerization over lanthanum oxide by Rosynek et al. (28) indicate that for this catalyst interconversion of the two 2-butene isomers (s4 in Example 8) is very slow and in that case the system could be described by mechanism m3. Studies by Goldwasser and Hall (29) indicate that as temperature is increased, there is appreciable direct conversion via s4 so that one or both of the other two direct mechanisms may be involved. These authors suggest that further studies with all three isomers, at several temperatures and with tracers, would be desirable. 

The 1-butene isomerization and the ethylene oligomerization were investigated as test reactions for solid acid sites of the Ni2+-substituted smectite catalysts. Both reactions were carried out in a closed circulating system. After pre-evacuation of a catalyst in the reactor at various temperatures for 1 h, the reactions were conducted by circulating the reactants. Products were analyzed by a gas chromatograph. 

Figure 3. Initial rates of 1-butene isomerization on the Ni2+-substituted smectite catalysts Ni-481 ( ) and Ni-359 ( ). The initial pressure of 1-butene was 13 kPa. The reaction temperature was 323 K.

Ghorbel and co-workers (329) have shown that the dependence of the rate of 1-butene isomerization at 260°C on the activation temperature of an amorphous alumina shows two maxima near 470° and 650°C. This behavior nicely parallels the surface concentration of the cation radicals formed on PhTh adsorption (see Fig. 5) as well as that of the anion radicals formed on TCNE adsorption (see... 

In view of the difference of a factor of 10 or more in peak delay between butene and thiophene at similar temperatures, butene adsorption was checked to see that chemisorption was in fact occurring below 200° C. Using the 50-foot propylene carbonate column, it was found that some butane was formed in spite of the H2S present down to 150° C. (without H2S butene was almost completely hydrogenated at this temperature) and both cis-trans and double bond isomerization of the butenes went to completion at temperatures below 100° C., indicating that chemisorption of butene must have occurred. It is therefore felt that extrapolation of the butene sorption results obtained to the temperature range of the desulfurization reaction (above 200° C.) should be valid. 

The following facts are the basis for butene isomerization (I) There is a basic similarity in the composition of alkylates produced from all four butene isomers. (2) Alkylate molecules, once formed, are relatively stable under alkylation conditions and do not isomerize to any appreciable extent alkylate fractions having the same carbon number ore not equilibrated (see Table I). (3) Thermodynamic equilibrium between the butene olefins highly favors isobutene formation at alkylation temperatures. (4) Normal butenes p>roduce only small and variable amounts of normal butane, thus indicating only a small and variable amount of chain initiation from normal butenes. Yet the alkylate composition shows a high concentration of trimethylpentanes and a low concentration of dimethylhexanes. (5) A few of the octane isomers can be explai.ned only by isomerization of the eight-carbon skeletal structure this isomerization occurs while isobutene dimer is in ionic form. For example, 2,3,3- and 2,3,4-trimethylpentanes... 

The parent NaX has been shown to be active for 1-butene isomerization in the temperature range 200°-300°C. LiX has a similar activity, but with all the other zeolites listed in Table I, an enhancement of catalytic activity resulted from the replacement of sodium by other cations. 

The highly exchanged cerium catalysts exhibited an initial deceler-atory period in the 1-butene isomerization. This may be caused by some strong sorption of the butene molecules which would be expected at the low reaction temperatures used for CeX-I and CeX-II and with the high fields associated with Ce " ions in surface sites. Such sorption might lead to a reduction in the number of sites for catalysis. 

A mechanistic study of butene isomerization was carried out using i C labelled butene, >3CH2=CH-CH2-CH3 and a GC-MS set up. The GC injector was equipped with a narrow glass tube containing 20 mg of powdered FER catalyst. In a typical experiment a butene sample (1 1, 0.9 bar) is injected, the products are separated by GC and then individually analyzed by mass spectrometry (MS) (Kratos Concept). Fresh and spent (but still active for butene isomerization) HS-FER was used for these experiments. Experiments were carried out at a catalyst temperature of 350°C. 

From the adsorption/desorption experiments (Fig. 3) it appears that butene adsorbs irreversibly on FER in the relevant temperature range for butene isomerization. Since the heat of adsorption of butenes in silicon-rich FER is estimated to be 10 kcal/mol, desorption should be facilitated at about 350°C. The low rate of desorption at 350°C proves, therefore, that butenes have reacted to higher molecular weight material, presumably oligomers, inside the pores of FER. 

Since zeolites are typical acid-base catalysts, their acid-base properties are of great importance in investigating the catalytic decomposition of hydrocarbons. Three methods-titration, temperature-programmed desorption (TPD), and characterization by test reactions — are now employed for the measurement of the acid-base properties of catalysts. We previously proposed a method to estimate the acid-base properties of catalysts by means of the reaction profiles of n-butenes isomerization [1]. 

The apparatus used for the n-butene, isomerization was a closed, circulating system with a U-shaped reactor, a conventional vacuum line and a gas-chromatograph. With the gas chromatograph, we analyzed the reaction mixtures at suitable times following the reaction. The volume of the closed circulating system corresponds to about 288 cm at a reaction temperature of 373K. 

Fio. 10. Temperature dependence of relative rates of butene isomerization over palladium-alumina (31). Q 2-butene from 1-butene at 32 4% conversion. 3= tran -2-butene from cw-2-butene at 34 5% conversion =s SO mm, Ph,= 155 mm in each case). 

In the first instance a catalyst running in l-butene/H /H O at 360°C for 4 hrs and giving <30% branched product was exposed to l-pentene/Hg/H O at 360°C. The resultant branching of 1-pentene was excellent 63%. Other runs in which the reaction temperature was lowered to 300°C prior to admission of 1-pentene agreed with the above result. It is concluded that a catalyst inactive for 1-butene isomerization may still have activity for 1-pentene isomerization. 

Butene Isomerization. Initial Deactivation. Conversion of 1-bu-tene to the 2-butenes was much more rapid initially than in the steady state for all catalysts at all temperatures studied. Initial activity of the fully-leached material was much greater than that of the original H-mor-... 

Segawa et al. (78,791 have examined the catalytic activity of several zirconium phosphates for 1-butene isomerization. They concluded that the e phase remains active even after evacuation at 773 K, while the zirconium phosphate gel and the a form show significant decreases in activity. The pyrophosphate has the highest activity. All three forms dehydrate upon evacuation at temperatures above 1000 K and eventually become the pyrophosphate ZrP207 above 1300 K as the P-OH groups condense to form P-O-P linkages simultaneous with the total loss of catalytic activity for butene isomerization. P MAS-NMR (79] of zirconium phosphate gel, the a form, and the e form of ZrP (Fig. 12) show that the a form has its phosphorus resonance at — 16.6 ppm, while in the e form it occurs at - 21.7 ppm (with reference to H3FK>4 at 0 ppm). The gel shows peaks at - 11.8,... 

For the organic syntheses mentioned above, the KF/AI2O3 was used without thermal pretreatment. We found that KF/Al20a becomes more active as it is pretreated at proper temperatures. The activity of KF/AlaOa for 1-butene isomerization showed a maximun at the pretreatment temperature of 623 K[44] as shown in Fig. 1. 

Fig. 1 Variation of the activity of Fluka KF/A OafF 5,5mmol/g) for 1-butene isomerization at 273 K as a function of pretreatment temperature...

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