Catalytic Cracking Of Alkanes Research Articles

High Density Alkyl Diamondoid Fuels Synthesized by Catalytic Cracking of Alkanes in the Presence of Adamantane

Alkyl diamondoid fuel mixtures have been prepared under moderate conditions by AlBr3 catalyzed cracking of nonane and heptane in the presence of adamantane. The fuel mixture prepared with heptane as the alkyl source (HA) contains primarily 1-ethyl-3-methyl adamantane and 1-propyladamantane, while the mixture prepared from nonane (NA) contains primarily C13–C15 alkyl diamondoids. Both fuel mixtures exhibit densities greater than 0.9 g/mL and volumetric net heats of combustion approximately 10 and 6% higher than conventional jet and diesel fuels, respectively. The structural diversity of the fuel blends and presence of multiple branch sites lead to lower viscosities compared to pure alkyl diamondoid fuels. The lower molecular weight blend, HA, exhibits a 40 °C kinematic viscosity of 3.22 mm2 s–1, well within the specification for diesel fuel, and both blends have derived cetane numbers of >42, suggesting that they can be used directly in conventional diesel engines.

Atomistic Investigations of the Effects of Si/Al Ratio and Al Distribution on the Adsorption Selectivity of n-Alkanes in Brønsted-Acid Zeolites

The adsorption of n-alkanes onto Brønsted-acid sites is a key step in the catalytic cracking of alkanes. Employing configurational-bias Monte Carlo simulations, we have investigated how the ratio of equilibrium adsorption constants for central C–C bonds relative to terminal bonds of n-alkanes (i.e., the adsorption selectivity ratio) in Brønsted-acid zeolites is influenced by the Si/Al ratio and the Al distribution. A new computational approach was implemented, and the developed force field was validated by a comprehensive comparison between simulation results and experimental data for a number of Brønsted-acid zeolites. While the adsorption selectivity seems to be relatively insensitive to the Si/Al ratio, our results reveal that the Al distribution plays a crucial role in determining the adsorption selectivity. Changes in the Al distribution result in a change of as much as 2-fold in the adsorption selectivity ratio for n-hexane. The selectivity generally shows larger variations with respect to Al distribution in zeolites with a larger Si/Al ratio. The two factors identified by this work that substantially influence the selectivity ratio are the siting of Al atoms among T-sites and their spatial proximity, and an atomic-level understanding of each of these effects was achieved. The siting of Al atoms at more or less selective T-sites significantly influences the overall selectivity ratio, and Al atoms in close proximity can synergistically enhance the adsorption of central C–C bonds, leading to a higher selectivity ratio relative to isolated Al atoms. We anticipate that these results will have important implications for future large-scale computational screenings and the development of advanced synthesis approaches to target certain Al distributions in zeolites.

Impact of the local environment of Brønsted acid sites in ZSM-5 on the catalytic activity in n-pentane cracking

The impact of the zeolite Brønsted and Lewis acid site concentration on the catalytic cracking of alkanes was explored using n-pentane and H-ZSM-5 as examples. Rates normalized to strong Brønsted acid sites (i.e., the turnover frequencies, TOF) showed that the two samples with the highest Al content had much higher TOF than all other samples. This difference has been unequivocally linked to the presence of extra-lattice alumina. Post-treatment of the zeolites with ammonium hexafluorosilicate and static calcination was used to vary the concentration of extra-lattice alumina. After extraction of extra-lattice alumina from the samples with high TOF, all TOFs were identical. IR spectra of adsorbed pyridine and NH3, coupled with 27Al MAS NMR, showed that the overall enhanced activity is associated with tetrahedrally coordinated extra-lattice alumina in close proximity to strong Brønsted acid sites. The TOF of these sites is approximately 40-times higher than the TOF on normal Brønsted acid sites.

Catalytic cracking of alkanes on FAU: Single‐event microkinetic modeling including acidity descriptors

AbstractSingle‐event microkinetic (SEMK) modeling is applied to catalytic cracking of 2,2,4‐trimethylpentane on a series of faujasites with Si/Al ratio ranging from 2.6 to 30. Standard activation entropies of the various elementary reaction families are calculated a priori from transition state theory and statistical thermodynamics, while activation energies are estimated on a reference faujasite by regression to experimental kinetic data. The SEMK model is then extended with two acidity descriptors. The concentration of active sites is available from independent NH3‐TPD measurements, while the change in standard protonation enthalpy, relative to the reference faujasite, is obtained by regression to experimental kinetic data. The latter parameter accounts for the effect of the zeolite average acid strength both on the stability of the intermediates and on the activation energies of the protonation and protolytic scission reactions. For these five commercially available faujasites, a variation in standard protonation enthalpy of 29 kJ mol−1 was found. © 2012 American Institute of Chemical Engineers AIChE J, 2012

Characterization and catalytic performance of mesoporous molecular sieves Al-MCM-41 materials

Mesoporous Al-MCM-41 materials of different Si/Al ratios have been synthesized and characterized by X-ray powder diffraction, 27Al and 29Si MAS NMR, differential thermogravimetric analysis, N2 adsorption measurements, FT-IR and catalytic cracking of alkanes. The experimental results show that the incorporation of aluminium into the framework of MCM-41 has a great effect on the degree of long-distance order, the surface acidities and the mesoporous structures of the materials. With increase of the aluminium content, the amounts of tetrahedral framework aluminium and the acid sites on the samples increase, but the acid strength decreases. Al-MCM-41 materials exhibit high activity for n-C160 cracking and good selectivity for producing low carbon alkylenes, particularly for i-C4=.

Catalytic cracking of alkanes on MCM-22 zeolite. Comparison with ZSM-5 and beta zeolite and its possibility as an FCC cracking additive

N-heptane cracking has been carried out on MCM-22, and its kinetic and decay behaviour is compared with that of ZSM-5 and Beta zeolites. In the case of MCM-22 cracking occurs in the 10-member ring channel system, as well as in the large cavities formed by 12-member rings. Product selectivities show that MCM-22, while presenting features which indicate structural similarities with ZSM-5, it also presents characteristics corresponding to pores or cavities larger than ZSM-5. The selectivities to the individual products together with C 1 + C 2/iC 4, C 3/C 4, C 5/C 2, iC 4/total C 4, and alkane/alkene ratios indicate that MCM-22 has good properties to be used as an FCC octane booster additive specially valuable for the production of reformulated gasolines.

ChemInform Abstract: Primary Products in Catalytic Cracking of Alkanes: Quantitative Analysis.

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Primary products in catalytic cracking of alkanes: Quantitative analysis

Catalytic cracking of several C 8 and C 10 isoalkanes over a zeolite-based catalyst at 250 and 350°C was studied and the distributions of primary cracked products (isomerized saturated hydrocarbons with the same carbon atom numbers as in the feed alkane) were analyzed. Comparison of relative yields of the primary products showed that not only the same products were formed in cracking of different isoalkanes but that their distributions did not depend on the structure of the feed molecules. The relative yields of different primary products were found to be generally proportional to the thermodynamic stabilities of corresponding isoalkanes. All isoalkane products with a methyl group in the second positions of alkyl chains represent an exception: their relative yields are uniformly 2–2.5 times higher than those expected from thermodynamics due to the additional stabilizing effect of the second methyl group on the concentration of respective carbocations.

Catalytic cracking of alkanes on large pore, high SiO2/Al2O3 zeolites in the presence of basic nitrogen compounds. Influence of catalyst structure and composition in the activity and selectivity

Catalytic cracking of alkanes on large pore, high SiO2/Al2O3 zeolites in the presence of basic nitrogen compounds. Influence of catalyst structure and composition in the activity and selectivity