Methylpentane, adsorption

Figure 13.11 CBMC simulations of adsorption isotherm (a) and adsorption selectivity (b) for a 50 50 mixture of n-hexane and 3-methylpentane over MFI at 362°K [6].

Only nonpolar solvents, e.g., cyclohexane (most commonly employed with silica gel and silicic acid), methylcyclohexane, methylpentane, and carbon tetrachloride, can be utilized in these slurries since solvents of greater polarity will compete with the intended adsorbate for available binding sites and will result in incomplete sustrate adsorption. 

June et al. investigated the sorption and spatial distribution of butane and three hexane isomers within the pores of silicalite, using a Metropolis MC method (87) and MD simulations (85). Perturbations of conformation as a result of confinement within the pore were also reported. Heats of adsorption and Henry s law coefficients were found to be in good agreement with experimental values for butane (48-51 kJ/mol) (142,148,150,163-165) and n-hexane (70-71 kJ/mol) (163, 166, 167). The heats of sorption of the other two hexane isomers, 2- and 3-methylpentane, were predicted to be 5 kJ/mol lower than that of n-hexane. 

From their work on methylcyclopentane and methylpentanes Garin et al.3S0 conclude that BS is at a maximum over a Pt(557) surface. On the basis of this they propose that more than 1 metal atom must operate in the BS isomerization. They present a variety of models in which at least 2 metal atoms at a surface step are involved in the initial adsorption process. Davis et al.351 conclude also that BS is probably structure sensitive. However, they... 

Janot et al. [90] described the increased hydrogen adsorption of ball-milled Mg2Ni alloys and interpreted their findings with the removal of oxide layers. A Pd-Mg catalyst precursor used for the synthesis of methyl isobutyl ketone was prepared by milling PdO with Mg [91]. Dehydration and dehydrogenation reactions of 4-methylpentan-2-ol over ball-mined catalysts such as CuM (M = Ti, Zr, Hf) have also been investigated [92]. 

The species (2) or (3), and those from (5) to (7) (all in Fig. 4.53) are supported by both chemical and spectroscopic arguments. It is important to note [91,92] that there are important chemical arguments (exchange reactions) for the presence of multiply-bound species in the presence of hydrogen (or D2), since the presence of H2 suppresses the formation of the multiply bound species so much that they are no longer detected at the temperatures at which vibration spectra are monitored [93]. Species (4) and (5) can be considered as alternatives, both originating from the adsorption of ethene on transition metals. Species (4) is preferred on Pd, (5) on Pt [94], Labelled (C ) isohexanes have been used [95] to show that two mechanisms are operating when, for example, 2-methylpentane is converted into 3-methylpentane [94] (transition state structures are in brackets). 

Another study examined the NH3 and CO2 adsorption heats on several zirconia catalysts, differing in their preparation procedure and/or in the addition of dopants [46]. The differential heats of NH3 and CO2 adsorption show a wide range of variability, displaying either a plateau of constant heat or a continuous decrease indicative of adsorption heterogeneity [12]. The ratio between the number of the basic and acidic sites, Ub/ua, was calculated for each catalyst from the microcalorimetry results, by dividing the amount of adsorbed CO2 by the amount of adsorbed NH3. These catalysts were used to produce alk-l-ene from 4-methylpentan-2-ol. Alk-1-ene selectivity was found to first increase with the b/Wa ratio, reach a maximum and then decrease, whereas ketone formation continuously increased, being negligible for low Ub/ua values. 

The spectroscopic studies [78] of the pre-flame reactions occurring during the oxidation of 2-methylpentane have shown that the strong adsorption at 2600 A is also partly due to the jS-dicarbonyl compounds, which are subsequently consumed during the passage of the cool flame. 

It is agreed with literary data of absence of steric hindrance for adsorption of normal hydrocarbons on zeolites of a pentasile type [5-8]. Adsorption isotherms of 3-methylpentane lay below ones of n-hexane and starting US-69 sample at identical p/ps, the decrease of adsorption volume capacity made about 10%. In the investigated p/ps range adsorption isotherm of benzene on US-69 sample lays below, than for 2,3-dimethylbutane, and in initial area - even is lower than for cyclohexane contrary to a ratio of there kinetic diameters. At the same time, benzene isotherm increases faster, so that level of saturation for benzene can lay above, than for 2,3-dimethylbutane. Apparently, such character of benzene isotherm is connected to a feature of packing of molecules in pentasile channels, and also with stronger interaction adsorbate-adsorbate in comparison with interaction adsorbate-adsorbent. 

Adsorption isotherms of 3-methylpentane and benzene on the modified samples are close, though for BP-US-69 (2) they lay a little above. As to hydrocarbons with the greatest kinetic diameter - cyclohexane on the modified samples did not keep. With a sufficient reliability it was possible to receive only on one point of isotherm, for which the adsorption value has made about 0.45-0.50 from value for US-69. The greatest distinctions between the modified samples are found under adsorption of 2,3-dimethylbutane, which was kept enough well on BP-US-69 (2) and was not adsorbed on BP-US-69 (1). 

Figure 3. Adsorption isotherms of C5 hydrocarbons at 180 C on zeolite a - initial US-69 b -modified water solution boron and phosphoric acids BP-US-69(2) c - modified ethers boron and phosphoric acids BP-US-69(1) 1 - n-hexane (0,43 nm), 2 - 3-methylpentane (0,55 nm), 3 - benzene (0,58 nm), 4 - 2,3- dimethylbutan (0,61 nm), 5 - cyclohexane (0,63 nm).

The set of test molecules used here for port gauging of zeolites all enter freely at room temperature into the AlP04-5, SAPO-5, and SSZ-24 sieves. These molecules include six C-6 hydrocarbons - n-hexane (nC6), benzene, 3-methylpentane (3MP), methyl cyclopentane, cyclohexane and 2, 2-dimethylbutane (22DMB) - and iso-octane. The total adsorption of this mixture of hydrocarbons is about 0.12 ml/g for these sieves. The admission of the larger molecules such as 22DMB is compatible with the 12-ring pore openings of the sieves. 

Different solid acid catalysts like zeolite Y [2-6], beta [7-9], MCM-22 [10], solid superacids [11-13], sulphonic acid resins [14], etc. have been proposed as potential alkylation catalysts and some of them are being tested at a pilot plant scale. Zeolites and solid superacids of sulfated zirconia type were found to be the most active but they suffer rapid deactivation after an initial period. Among different zeolites studied large-pore zeolites are prefered over medium-pore type because the former favors the formation and diffusion of bulkier tri-methylpentane isomers. Beside pore size and zeolite structure, the fiamework composition (Si/Al ratio) and acid strength distribution also play an important role on the activity, selectivity and deactivation of the catalysts. It is known that the adsorption behavior of the zeolite and the extent of hydrogen transfer capacity (a crucial factor of alkylation activity) both depend on the aluminium concentration in the framework [15-16]. 

For 2-methylpentane cracking the assumed reaction mechanism was based on that proposed by Zhao et al [8], Reactions are initiated by adsorption of 2-methylpentane feed on to Bronsted acid sites. Subsequent protolysis produces a carbenium ion on the surface of the catalyst (an adsorbed olefin) and a smaller gas phase paraffin molecule. Propagation reactions can then occur by hydride transfer between carbenium ions on the surface of the catalyst and gas phase feed molecules. Zhao experimentally determined certain bimolecular reactions to be more significant than others, the most significant were implemented in our model. Reactions are terminated by desorption of carbenium ions to yield a gas phase olefin molecule... 

Many other adsorbates have been used to test pore accessibility and adsorption capacity changes upon coke deposition. Some examples are trimethylamine and ethyldiisopropylamine on ZSM-5 , m-xylene on USHY and HZSM-5, 3-methylpentane in H-Offretite xenon on ZSM-5 and USY zeolites. ... 

In the present work the behaviour of zirconia samples doped with oxides of alkali metals and alkaline-earth metals was investigated, in order to better understand the role of both the nature and the amount of the doping cation. Li-, K-, Ca-, and Ba-doped zirconia samples were prepared. Their surface acid-base properties were assessed by means of adsorption microcalorimetry, using ammonia and carbon dioxide as probe molecules. Their catalytic activity for the 4-methylpentan-2-ol dehydration was tested in a flow microreactor. 

With the TEX-PEP technique experiments on the diffusion and adsorption of mixture of n-hexane/2-methylpentane in large silicalite-1 crystals have been performed. By modeling the experimental tracer exchange curves values of intracrystalline diffusion coefficient and adsorption constant were obtained. Slight preference for the adsorption of /t-hexane was found. Diffusivity of -hexane sharply decreases with increasing fraction of its isomer, since the last one occupies channel intersections thus blocking zeolite network. 

Theoretically calculated values of the heat of adsorption for n-hexane and 2-methylpentane are 70 kj mol and 65 kj mol, respectively [46,47], which is in agreement with the average values determined by Zhu et al. [48]. As the heats of adsorption of these alkanes are very close, the difference in adsorption is caused by an entropic effect. Indeed, the conformations of the bulkier branched alkanes are much more restricted in the narrow pores of the medium-pore MEI zeoUte. Eor the branched isomer in siUcaUte-1 there is a large difference in the adsorption entropy between the molecular locations in the intersections and in the channels as shown by Zhu et al. [48]. Therefore, the adsorption of 2-methylpentane from the gas phase leads to a higher reduction in entropy compared to adsorption of n-hexane. This makes it en-tropically less favorable to adsorb the branched isomer [44]. 

Figure 9 shows the binary adsorption data of n-hexane and 2-methylpentane at 433 K as a function of the gas-phase ratio of the hydrocarbons. Obviously, the n-hexane loading monotonically decreases upon an increase of the partial pressure and loading of the 2-methylpentane. The total hydrocarbon loading only sUghtly decreases at high 2-methylpentane fraction in the gas phase. The preference for adsorption of n-hexane over the monobranched isomer is in line with the above-mentioned entropic considerations. 

The loading of n-hexane in mixtures is somewhat higher than it is expected to be if it were proportional to its partial pressure (Fig. 12). On the contrary, the 2-methylpentane loading is somewhat lower. This points to preferential adsorption of -hexane over isohexane in their mixtures in H-ZSM-5 than in silicalite-1. In earlier experimental [50] and CBMC simulation studies [44] of n-hexane/isohexane mixtures in silicalite-1, a slight preferential adsorption of the linear alkane over the branched one has been found. The most prominent explanation for this preference is the molecular siting of these two hydrocarbon molecules. Whereas -hexane exhibits no clear preference for a position in the micropore system of MFI zeolite, the branched isomer is preferentially located at the channel intersections due to entropic reasons [44]. Consequently, 2-methylpentane will be pushed out from silicalite-1 by -hexane. These effects are even stronger for H-ZSM-5, most likely due to the stronger... 

Table 2 shows the adsorbed concentrations of the pure components. At a partial pressure of 6.6 kPa the amount of n-hexane is just slightly higher than that of isohexane in silicalite-1, while the linear alkane is obviously adsorbed more strongly than 2-methylpentane in H-ZSM-5 due to the stronger interaction with the acid sites. The maximum loading of each component has been measured by a separate adsorption study. The sorption capacity of n-hexane (7 molecules per unit cell), in agreement with earlier studies [48,59-61] exceeds that of 2-methylpentane (4 molecules per unit cell). The latter value equals the number of channel intersections in the MFI pore system per unit cell. Indeed, the sorption of isohexane molecules at... 

Summarizing, we conclude that for binary mixtures of a linear and branched hexane in H-ZSM-5 and silicalite-1 two factors influence the respective diffusivities (i) the strong interaction with acid sites preferentially decreases n-hexane diffusivity and (ii) the blocking of intersection adsorption sites by 2-methylpentane decreases n-hexane diffusivity. At high loadings of the branched isomer the latter effect is dominating, and Anally the diffusivity of the linear hexane is totally determined by its branched isomer. 

The current work indicates the strong effect of acid sites on the interaction and diffusivity of hydrocarbons. To further study this effect, we determined the single-component diffusion coefficients and specifically the activation energy for diffusion. Activated diffusion is described by the Arrhenius-type Eq. 8. The pre-exponential factor Djnf is related to the jump frequency between adsorption sites in the zeolite lattice, while the exponential expresses the chance that the molecules are able to overcome the free energy barrier - act between these sites. The loadings of n-hexane and 2-methylpentane in H-ZSM-5 and silicalite-1 have been measured at temperatures between 373 and 533 K at intervals of 20 K. The hydrocarbon pressure was taken identical... 

We have discussed the adsorption and diffusion of binary mixtures of hnear (n-hexane) and branched (2-methylpentane) alkanes in silicahte-1. It turned out that not only the size but also the siting of the molecules in the particular zeohte plays an important role in the behavior of the mixture components. A shght preference for the adsorption of n-hexane over 2-methyl-pentane was observed because of the higher packing efficiency of the hnear alkane. This is due to the preferential location of the branched alkane in the zeohte intersections. A consequence of this is that the diffusivity of n-hexane... 

A comparison between sihcalite-1 and H-ZSM-5 teaches that acid sites have a profound influence on the self-diffusivity of alkanes. The self-diffusivities of both components decrease strongly, and we observe a significant preferential adsorption of the linear over the branched hexane. This is caused by the relatively stronger interaction of the linear hexane with the acid sites. On the contrary, 2-methylpentane loadings in mixtures in sihcahte-1 and H-ZSM-5 are very close. In H-ZSM-5, the diffusivity of the hnear alkane in mixtures with the branched alkane is influenced by two factors... 

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