1-Butene adsorption

Fig. 17. FTIR spectra of 1-butene adsorption and oligmerization on H( zeolite in the 77-300 K temperature range. (Reproduced with permission from Bjorgen et al. (163) ...

These results obtained at room temperature favor the specific interactions of a "ir-complex" type. Similar conclusions were drawn from 1-and trans 2-butene adsorption studies on NaY zeolites as well as on HY zeolite activated at 750 K (29). ... 

Fig. 1. The temperature programmed desorption profiles for a-Fe203 (a) Blank desorption without adsorbates (b) ris-2-butene adsorption (c) butadiene adsorption (d) cis-2-butene adsorption from a catalyst depleted of selective oxidation sites. From ref. 5, reprinted with permission, copyright 1979 by the American Chemical Society.

The temperature programmed desorption profile for the adsorption of butadiene in place of cis-2-butene is shown in Fig. 1, curve c. Two sets of products are observed. The product below 210°C is unreacted butadiene, and the products above 210°C are carbon dioxide and water. The similarity in the evolution of the combustion products of butene and butadiene is an indication that their combustion proceeds via similar reaction mechanisms. The similarity in the desorption of butadiene suggests that in butene adsorption, butadiene desorption is desorption limited. Indeed, that both butene and butadiene adsorb on the same type of sites has been confirmed by sequential adsorption experiments. The results are shown in Table III. It was found that if the C4 hydrocarbons are adsorbed sequentially without thermal desorption between adsorptions, the amounts of the final desorption products are the same as those in experiments where only the first hydrocarbon... 

The separation of the two sets of desorption products may indicate that they are from different sites. That is, branching of the selective and nonselec-tive oxidation takes place on adsorption of butene. This can be confirmed if the two sets of products can be varied independently. This is shown by two experiments. The first experiment makes use of the fact that butene and butadiene adsorb on the same sites. Butadiene is first adsorbed onto the catalyst (5). The catalyst is then heated to 210°C, desorbing all of the unreacted butadiene, but leaving on the surface the precursors of the combustion products. Since desorption of the unreacted butadiene does not involve a net chemical reaction, the adsorpton sites involved are not affected. The catalyst is then cooled to 22°C, and cis-2-butene is adsorbed. If selective oxidation and combustion take place on the same site, the adsorbed butene would undergo both reactions. If they take place on separate sites, and butene adsorbs only on the selective oxidation site (because the combustion site is covered by species from butadiene adsorption), the adsorbed butene would form only butadiene. Subsequent desorption yields a profile similar to that for a single adsorption of ds-2-butene (Fig.l, curve b). More importantly, within experimental errors, the amount of butadiene evolved is the same as in a ds-2-butene adsorption experiment, and the amount of C02 evolved is the same as in a butadiene adsorption experiment. Thus, the adsorbed butene forms only butadiene. These results show that under these experimental conditions (i.e., in the absence of gas-phase oxygen), the production of butadiene and carbon dioxide takes place on separate sites. 

Fig. 3. The amounts of thermally desorbed products from cis-2-butene adsorption on a-Fe203 versus temperature of adsorption and purging. Data for total products include 1-butene, ris-2-butene, butadiene, and C02/4 data for total desorption include c/s-2-butene as well. From ref. 6, reprinted with permission from Academic Press.

Experiments were then performed to preadsorb the various forms of oxygen on an iron oxide surface. These were then followed by the butene adsorption-desorption experiments to determine the amounts of the two types of oxidation sites. Typical results are shown in Table V (6). The amounts of oxidation products are independent of the presence of preadsorbed oxygen. In fact, even the thermally desorbed isomers are not affected by the preadsorbed oxygen. This absence of effect of the preadsorbed oxygen was observed also in pulse experiments. It was found that the amounts of butadiene, C02, and butene isomers formed from a butene pulse passing over the catalyst at 100,200, or 300°C are independent of preadsorbed oxygen (6). 

Fig. 7. The fraction of butadiene and C02 precursosr left adsorbed on a-Fe203 and y-Fe203 as a function of desorption temperature in a butene adsorption experiment. Filled circles are for adsorption-desorption experiments without oxygen, open circles are for those in which the desorption process was interrupted for passage of an oxygen pulse over the catalyst at the indicated temperatures. From Ref. 18, reprinted with permission from Academic Press.

The only heat of adsorption we have found in the literature is a figure of 17 kcal. per mole (27) for thiophene on a supported chromia catalyst. This figure was obtained by analysis of reactions carried out in a static system, assuming a Langmuir mechanism for thiophene and hydrogen adsorption and neglecting the effects of H2S and butene adsorption. 

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 adsorption of 3,3-dimethyl-1-butene at 303 K on the heat-treated sample is shown in Fig. 3. The 3,3-dimethyl-l-butene adsorption yielded a Type 1 isotherm. The BET sur ce area value of 161 m g obtained, fi om 3,3-dimethyl-l-butene is lower than those derived fi om nitrogen and 1-hexene (Table 1). The pore volume determined fi-om this sorptive was also lower (0.13 cm g ) than that for 1-hexene on the heat-treated sample. This suggests that adsorption of 3,3-dimethyl-l-butene, which approximates to a spherical shape, is sterically hindered, whereas in 1-hexene, which is a straight chain molecule, is less sterically hindered to interact with the surface. 

The poisoning of the strongest acid sites was also shown indirectly by ammonia adsorption and TPD or butene adsorption and TPD. The coked sample adsorbed a smaller amount of each base than did the fresh catalyst. 

Chemical methods of determining the nature and surface concentrations of active components have also been attempted Mo oxide by butene adsorption and, in reduced catalysts, by oxygen chemisorptionM0S2 and/or active site concentrations by pyridine adsorption and thiophen adsorption and temperature-programmed desorption of thiopen cf. p. 200), surface acidity by NH3 adsorption cf. p. 199). Sulphides in a Ni-Mo/ AI2O3 catalyst have been characterized by differential thermal analysis of the catalyst in oxygen. ... 

Uematsu et al. have investigated the interrelation between the electronic properties of a ZnO surface and its activity for butene isomerization. They conclude that as butene adsorption instantaneously increases the bulk electronic conductance of ZnO that the molecule donates electron(s) partly to the surface. Thus, isomerization rate should depend on the surface electronic state, a prediction seemingly confirmed as this rate was modified by aliavalent dopant ions, e.g. Ga +, AP +, and Li". Although this explanation of their observations may be correct, such studies a priori suffer from the disadvantage that foreign cations... 

Fig. 3. FT-IR spectra of (a) surface species arising from cis-2-butene adsorption on MgFe204 at r.t. (b) liquid cis-2-butene (c) surface species arising from 1,3-butadiene adsorption on MgFe204 (d) liquid 1,3-butadiene. The bars denote the species arising from reactive adsorption of cis-2-butene, i.e. but-3-en-2-oxides, and but-2-en-l-oxides.

On alkaline earth metal oxides butene is adsorbed as methylallyl anions (CH3—CH—CH—CH2). This carbanion is an intermediate in the double bond isomerization of butene. Adsorption was shown to be stronger on CaO (higher basicity) than on MgO. Activation of CaO at 700-900 °C results in maximum Lewis basicity and optimum activity for the isomerization to 2-butene. 

In the physical separation process, a molecular sieve adsorbent is used as in the Union Carbide Olefins Siv process (88—90). Linear butenes are selectively adsorbed, and the isobutylene effluent is distilled to obtain a polymer-grade product. The adsorbent is a synthetic 2eohte, Type 5A in the calcium cation exchanged form (91). UOP also offers an adsorption process, the Sorbutene process (92). The UOP process utilizes ahquid B—B stream, and uses a proprietary rotary valve containing multiple ports, which direct the flow of Hquid to various sections of the adsorber (93,94). The cis- and trans-isomers are alkylated and used in the gasoline blending pool. 

This is the same case with which in Eqs. (2)-(4) we demonstrated the elimination of the time variable, and it may occur in practice when all the reactions of the system are taking place on the same number of identical active centers. Wei and Prater and their co-workers applied this method with success to the treatment of experimental data on the reversible isomerization reactions of n-butenes and xylenes on alumina or on silica-alumina, proceeding according to a triangular network (28, 31). The problems of more complicated catalytic kinetics were treated by Smith and Prater (32) who demonstrated the difficulties arising in an attempt at a complete solution of the kinetics of the cyclohexane-cyclohexene-benzene interconversion on Pt/Al203 catalyst, including adsorption-desorption steps. 

From the results of other authors should be mentioned the observation of a similar effect, e.g. in the oxidation of olefins on nickel oxide (118), where the retardation of the reaction of 1-butene by cis-2-butene was greater than the effect of 1-butene on the reaction of m-2-butene the ratio of the adsorption coefficients Kcia h/Kwas 1.45. In a study on hydrogenation over C03O4 it was reported (109) that the reactivities of ethylene and propylene were nearly the same (1.17 in favor of propylene), when measured separately, whereas the ratio of adsorption coefficients was 8.4 in favor of ethylene. This led in the competitive arrangement to preferential hydrogenation of ethylene. A similar phenomenon occurs in the catalytic reduction of nitric oxide and sulfur dioxide by carbon monoxide (120a). 

These equations represent the adsorption-desorption reactions and the surface reaction E, P, and B are, respectively, ethene, propene, and 2-butene, and s represents an active site. If the surface reaction is rate deter-... 

The effect of electronegative additives on the adsorption of ethylene on transition metal surfaces is similar to the effect of S or C adatoms on the adsorption of other unsaturated hydrocarbons.6 For example the addition of C or S atoms on Mo(100) inhibits the complete decomposition (dehydrogenation) of butadiene and butene, which are almost completely decomposed on the clean surface.108 Steric hindrance plays the main role in certain cases, i.e the addition of the electronegative adatoms results in blocking of the sites available for hydrocarbon adsorption. The same effect has been observed for saturated hydrocarbons.108,109 Overall, however, and at least for low coverages where geometric hindrance plays a limited role, electronegative promoters stabilize the adsorption of ethylene and other unsaturated and saturated hydrocarbons on metal surfaces. 

To probe the formation of coke we conducted TPO measurements on samples previously used in the butene TPD experiments. The TPO profiles corresponding to catalyst INiSZ(s) are shown in Fig. 6. Significant evolution of CO2 was detected, indicating the formation of coke during the adsorption/desorption of 1-butene. The INiSZ(s) catalyst exhibited almost twice as much CO2 as the unpromoted SZ sample. 

Figure 6 TPO of carbonaceous deposits left on the surface of INiSZ(s) after the adsorption/desorption of 1-butene. Evolution of CO2 and SO2 (curves A and B, respectively). Evolution of SO2 during the butene TPD (curve C, added for comparison).

GP 3] ]R 3h] [R 4a] Safe operation in the explosive regime was demonstrated [103]. Catalytic runs with 1-butene concentrations up to 10 times higher than the explosion limit were performed (5-15% 1-butene in air 0.1 MPa 400 °C). A slight catalyst deactivation, possibly due to catalyst active center blockage by adsorption, was observed under these conditions and not found for lower 1-butene concentrations. Regeneration of the catalyst is possible by oxidation. 

FIGURE 2.19 Adsorption of cis-2-butene at the B4 site on Pt(755) metal atoms not shown. 

Thiophene is the typical model compound, which has been extensively studied for typifying gasoline HDS. Although, some results are not completely understood, a reaction network has been proposed by Van Parijs and Froment, to explain their own results, which were obtained in a comprehensive set of conditions. In this network, thiophene is hydrodesulfurized to give a mixture of -butenes, followed by further hydrogenation to butane. On the considered reaction conditions, tetrahydrothiophene and butadiene were not observed [43], The consistency between the functional forms of the rate equations for the HDS of benzothiophene and thiophene, based on the dissociative adsorption of hydrogen, were identical [43,44], suggesting equivalent mechanisms. 

These results are similar to those with propylene insofar as they indicate dissociative adsorption of the olefin. The hydrogen that yields the hydroxyl has not been identified but it seems reasonable to suppose that, once again, the allylic hydrogen is lost. Results with butene, however, do differ from those with propylene in two respects first, the dissociation (as evidenced by the OH band) is rapid but not instantaneous as found for propylene second, dissociatively adsorbed butene is more easily removed by room temperature evacuation than dissociatively adsorbed propylene. These facts suggests that steric effects are present hence, the kinetic behavior of these two species may be quite different. 

Cutlip and Kenney (44) have observed isothermal limit cycles in the oxidation of CO over 0.5% Pt/Al203 in a gradientless reactor only in the presence of added 1-butene. Without butene there were no oscillations although regions of multiple steady states exist. Dwyer (22) has followed the surface CO infrared adsorption band and found that it was in phase with the gas-phase concentration. Kurtanjek et al. (45) have studied hydrogen oxidation over Ni and have also taken the logical step of following the surface concentration. Contact potential difference was used to follow the oxidation state of the nickel surface. Under some conditions, oscillations were observed on the surface when none were detected in the gas phase. Recently, Sheintuch (46) has made additional studies of CO oxidation over Pt foil. 

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