E. G. Khelkovskaya-Sergeeva

Cyclohexane ring opening on metal-oxide catalysts

Ring opening of cyclohexane on various metal-containing oxide catalysts is studied in detail. Rhodium supported on neutral alumina is shown to exhibit the highest selectivity toward n-hexane, whereas the formation of side products of cracking, isomerization and dehydrogenation (C 1 –C 4 hydrocarbons, isohexanes + methylcyclopentane, and benzene, respectively) is essentially suppressed.

Physicochemical characterization and catalytic properties of solid superacids based on sulfated zirconia modified with supported platinum

The acidic properties and electronic state of platinum in superacidic catalysts based on sulfated zirconia are studied by diffuse-reflectance IR spectroscopy. The spectroscopic data indicate a positive charging of platinum microparticles due to the interaction with strong Broensted acid sites, which presumably represent delocalized protons and are characterized by a broad absorption band at ~3100 - 2950 cm-1. For the ZrO2 sample, however, it is likely that a partial negative charge is induced on the metal particle due to the influence of the ZrO2 support which reveals basic properties. The sulfated systems were shown to exhibit high activity and selectivity in n-butane isomenzation. Plausible reaction pathways and intermediates are discussed.

Hydroisomerization of C6–C14 n-Alkanes over Hybrid Catalysts

Various hybrid catalysts prepared by mechanically mixing Pt(Pd) promoted oxides (Al2O3, SiO2) and an acidic component (RE-FAU, H-Beta zeolites and SO4/ZrO2 solid Superacid) were studied in C6–C14 n-alkane skeletal isomerization. The hybrid catalysts exhibited high activity and selectivity in paraffin isomerization. The enhancement in the activity of hybrid catalysts as compared to individual components was rationalized in terms of spillover of reactive species between metal promoted oxide systems and acidic sites.

Cyclohexane transformations over metal oxide catalysts. 1. Effect of the nature of metal and support on the catalytic activity in cyclohexane ring opening

The activities of monometallic Pt-, Ru-, and Rh-containing catalysts supported on Al2O3, Al2O3—F, SiO2, WO3/ZrO2, and La2Î3/ZrO2, in cyclohexane ring opening to form n-hexane were studied. The most active catalyst is Rh/Al2O3. Cyclohexane hydrogenolysis to n-hexane also occurs over the Pt/Al;>2O3 and Pt/La2Î3/ZrO2 catalysts. Ring opening over the Ru catalysts proceeds at significantly lower temperatures (210—230 °C) than over the Pt and Rh catalysts (350—400 °C), but the ruthenium systems are less selective for n-hexane formation than Rh/Al2O3 catalysts. The effects of acid-basic properties of the support and the reaction conditions on the activities of the catalytic systems in cyclohexane ring opening was studied.

The transformations of toluene on alumina and bifunctional catalysts

The activity of Pt, Rh, and Ni catalysts deposited on Al2O3 and tungsten-containing catalysts 20% H4SiW12O40/ZrO2 and 15% WOx/ZrO2 in the hydrogenation of toluene and toluene ring opening and isomerization in the presence of hydrogen was studied. Under experimental conditions (160–360°C, 2.2 MPa), the main reactions on Rh/Al2O3 were the hydrogenation of toluene into methylcyclohexane, hydrogenolysis into isoheptanes, and hydrocracking into alkanes C1–C6. On Pt, Rh, and Ni catalysts on carriers with strong acid properties, the isomerization of the six-membered into five-membered ring followed by hydrogenolysis (hydrocracking) of alkylcyclopentanes occurred. The yield of heptane isomers, however, did not exceed 13%. The activity of Pt and Rh catalysts on a high-acidity carrier (WOx/ZrO2) in hydrocracking was much higher than that of catalysts based on deposited heteropoly acid. The yields of hydrogenolysis (hydrocracking) products on Ni/WOx/ZrO2 were much lower than on Pt(Rh)/WOx/ZrO2. The highest yield of ring opening products (isoheptanes and n-heptane) was obtained with layered loading of two catalysts; it reached 58 wt % at 300°C and a 2.2 MPa pressure, which was 4.5 and 2 times higher than the yield obtained on Ni-Pt/WOx/ZrO2 and 2% Rh/Al2O3 catalysts. Hydrodemethylation was not the main direction of toluene transformations on any of the catalysts studied.

Cyclohexane transformations over metal oxide catalysts: 2. Selective cyclohexane ring opening to form n-hexane over mono- and bimetallic rhodium catalysts

The activity of monometallic Rh and Pt catalysts and bimetallic Pt—Rh catalysts on oxide supports in cyclohexane ring opening to form n-hexane was studied. The Rh-containing catalysts are highly active and selective in this reaction. Cyclohexane dehydrogenation predominates in the case of the Pt catalysts. The use of the bimetallic alumina-supported Pt—Rh catalysts allows one to minimize the contribution of cyclohexane cracking and to enhance the selectivity for n-hexane with the yield of the latter slightly depending on the metal ratio in the bimetallic system under the experimental conditions.

Isomerization ofn-pentane andn-hexane on modified zeolites

The activity and selectivity of catalysts based on TsVM (an analog of ZSM-5), Beta, and La-H-Beta zoelites modified by Pt, Pt−Fe, and Pt−Ga were studied in the isomerization of C5 and C6 linear alkanes. The Pt/HTsVM, Pt/H-Beta, and Pt/La-H-Beta catalysts are efficient inn-pentane isomerization, whereas the Pt/H-Beta and Pt/La-H-Beta are most active inn-hexane isomerization. Nearly equilibirum isoparaffin yield at a selectivity of at least 95–96% is reached on these catalysts unlike other zeolite systems. The overall yield of 2,2-and 2,3-dimethylbutanes is 22 wt.%. The hexane isomers are not formed over the Pt/HTsVM catalyst due to the molecular-sieve properties of this type of zcolites.

n-Butane isomerization in the gas phase and under supercritical conditions

The aromatization of n-butane under supercritical conditions on gallium-, zinc-, and platinum-modified high-silica zeolites with a modulus of 30–70 was first studied, and the experimental data were compared to the results of a study of this process in the gas phase. It was found that the operational efficiency of catalysts for n-butane conversion under supercritical conditions was much higher than that for the gas-phase reaction in terms of activity, productivity, and resistance to poisoning by condensation products. The aromatization of gaseous n-butane at 530°C and 1 atm was characterized by rapid catalyst deactivation. The selectivity for the benzene-toluene-xylene (BTX) fraction was higher than 50%. Under supercritical conditions at 430–560°C and 100–200 atm, the selectivity of formation of aromatic compounds decreased by a factor of 2, whereas the yield of C1-C3 cracking products increased by the above factor. On the other hand, it was found that an increase in the productivity of catalysts by a factor of 20–50 with the retention of almost 100% activity for several days of operation is an advantage of the process performed under supercritical conditions. The almost complete conversion of butane under supercritical conditions was found on promoted HZSM-5 zeolite samples. The thermogravimetric analysis of spent samples suggested a higher degree of catalyst carbonization under supercritical conditions, as compared with that in the reaction performed in the gas phase. However, the deposition of 20–30 wt % condensation products on the catalysts had no detectable effect on the high activity of the catalysts in the reaction performed under supercritical conditions.

26-O-03-Cyclohexane ring opening on metal-zeolite catalysts

Publisher Summary This chapter discusses cyclohexane ring opening on metal–zeolite catalysts. Various zeolite catalysts, such as H-ZSM-5, H-MOR, H-Beta, and La-FAU modified with noble metals, are tested in the ring opening of cyclohexane. Methylcyclopentane, gas products (C 1 -C 4 ), n-hexane, and isohexanes are shown to be the main reaction products. The highest ring opening selectivity is found for Pt/H-ZSM-5, Rh/H-ZSM-5, and Pt/H-Beta catalysts. Acidity is found to be important for achieving reasonable activity in the ring opening of cyclohexane and simultaneous isomerization into methylcyclopentane and isohexanes.

n-Alkane isomerization on heteropolyacids

Acid-base properties of bulk 12-tungstophosphoric heteropolyacid (HPWA) and HPWA supported on alumina and fluorinated alumina were studied by DRIFT spectroscopy using adsorption of probe molecules. Catalytic properties of Pt-containing catalysts carried by these supports were studied inn-pentane isomerization. It was shown that the Pt/HPWA/Al2O3 system is almost inactive in isomerization as a result of the interaction of HPWA with basic sites of the alumina support. On the contrary, the presence of acidic sites in fluorinated alumina prevents HPWA destruction and favors the uniform distribution of HPWA on the support surface. As a result, enhancement of the activity and selectivity of the Pt/HPWA/Al2O3-F catalyst inn-pentane isomerization is observed.