Bulky Isomers Research Articles

Zeolite Pore Entrance Effect on Shape Selectivity in Naphthalene Isopropylation

Naphthalene alkylation with propylene was studied over various large-pore zeolites and also over amorphous aluminosilicate catalysts. Isomeric distribution of isopropylnaphthalenes and diisopropylnaphthalenes were compared at different temperatures. Surprisingly, higher α-selectivity in monoalkylation and higher concentration of bulky isomers in diisopropylation was observed in the products obtained over zeolites than over amorphous aluminosilicate catalysts. The explanation of such results was proposed to be a specific shape-selectivity effect of alkylation reaction occurring in the entrances to the pores of zeolite. This type of shape selectivity can be very significant and conceal other shape-selectivity effects such as β-selectivity of reactions occurring inside the pores. Results available from the literature, indicating this type of shape selectivity, were also discussed.

Synthesis of high-silica [Al]-SSZ-31 by a steam-assisted conversion method and its catalytic performance in the isopropylation of biphenyl

Synthesis of high-silica [Al]-SSZ-31 zeolite by a dry-gel conversion method was performed and the catalyst was tested for the isopropylation of biphenyl with propylene. In the case of the small-scale synthesis, the product phase was found to be dependent on the alkali concentration of the initial gel, and a phase change from BEA to SSZ-31 to MFI was observed with time at the optimum alkali concentration. In the case of the large-scale synthesis, temperature had a profound effect on the crystallization of SSZ-31. A phase transformation from BEA to SSZ-31 was observed when the crystallization temperature was changed from 150°C to 175°C. [Al]-SSZ-31 was active in the isopropylation of biphenyl and selectively catalyzed the production of the least bulky isomers. Biphenyl conversion decreased with increasing silica to alumina ratio while selectivity remained nearly constant. Triisopropyl isomer yields were high in the case of lower silica to alumina ratios.

Zeolite Catalyzed Alkylation of Biphenyl. Where Does Shape-Selective Catalysis Occur?

Liquid-phase alkylation of biphenyl (BP) was studied over large-pore zeolites. Selective formation of the least bulky products, 4,4′-diisopropylbiphenyl (4,4′-DIPB) occurred only in the isopropylation of BP over one- or two-dimensional zeolites, H-mordenite (HM), ZSM-12, SSZ-24, SAPO-5, SSZ-31, and CIT-5. These shape-selective catalyses are ascribed to steric restriction of transition state and to easiness of the substrates to enter into the pores. HM gave the highest selectivity among them. The dealumination of HM enhanced catalytic activity and the selectivity for 4,4′-DIPB because of the decrease of coke deposition. Non-regioselective catalysis occurs on external acid sites over HM with the low SiO2/Al2O3 ratio because severe coke deposition deactivates the acid sites inside the pores by blocking pore openings. The selectivity of DIPB isomers was changed with propylene pressure and/or with reaction temperature. Selective formation of 4,4′-DIPB was observed at moderate temperatures such as 250 °C, whereas the decrease of the selectivity of 4,4′-DIPB occurred at higher temperatures as 300 °C. 4,4′-DIPB yielded selectively under high propylene pressure (<0.3 MPa) at 250 °C, while the selectivity of 4,4′-DIPB decreased under low propylene pressure as 0.2 MPa. However, 4,4′-DIPB was almost exclusive isomer in the encapsulated DIPB isomers inside the pores under every temperature and pressure. The decrease of the selectivity of 4,4′-DIPB is due to the isomerization of 4,4′-DIPB on the external acid sites. The deactivation of external acid sites of HM was examined by the modification with cerium and other rare earth metal oxide on HM. Selectivities of 4,4′-DIPB were improved over modified HM even at high temperatures because of the suppression of non-regioselective alkylation and isomerization at the external acid sites. The ethylation of BP to ethylbiphenyls (EBPs) and diethylbiphenyls (DEBPs) was non-regioselective. The ethylation of BP to EBPs was controlled kinetically. However, there was difference in reactivities of EBPs and DEBPs for their further ethylation. 4-EBP was ethylated preferentially among the isomers, although the formation of 4,4′-DEBP was less selective. 4-EBP and 4,4′-DEBP have the highest reactivities among EBPs and DEBPs for the ethylation to polyethylbiphenyls (PEBPs). These results show that the environments of HM pores are too loose for shape-selective formation of the least bulky isomers, 4-EBP and 4,4′-DEBP, in the ethylation of BP, and that HM pores have enough space for the further ethylation of 4,4′-DEBP.

Methylation of Aniline Over SAPO‐11

AbstractMethylation of aniline over a mildly acidic siliconaluminium phosphate catalyst (SAPO‐11) with medium pores is reported. The selectivity of N‐methylates products is about 90%. By comparison with results obtained from SAPO‐5 with large pores and with amorphous aluminium phosphate, molecular sieves with medium pores prevent the formation of bulky isomers and better resist formation of coke. The ortho isomer is most abundant among toluidine isomers. Some ortho isomers may result from the rearrangement of N‐methylaniline.

Diels–Alder reactions catalysed by cation-exchanged clay minerals

Catalysis of Diels–Alder reactions by various clay minerals exchanged with a variety of transition metal cations was shown to occur readily at ambient temperatures in a variety of solvents. Nontransition and lanthanide cation-exchanged clays were found to be ineffective. α,β-Unsaturated carbonyl compounds (such as methyl vinyl ketone, methyl acrylate and methyl methacrylate) were used as dienophile. Cyclopentadiene, furan, pyrrole, isoprene and cyclohexa-1,3-diene were successfully employed as the diene, whilst thiophene was unreactive. Such reactions (except that with isoprene which produces a mixture of 1,3- and 1,4-isomers) produced a mixture of endo- and exo-isomers, the ratios of which could be altered by manipulation of the inter-layer reaction space of the clay catalyst. Choosing a clay with a higher layer charge, and hence lower basal spacing, Δd, gave increased selection for the kinetically less favoured, but less bulky isomer.

ChemInform Abstract: Catalytic and Acidic Properties of Boron Pentasil Zeolites

Pentasil-type zeolite with boron isoraorphously substituted for silicon in the pentasil zeolite framework and aluminum pentasil (ZSM-5 type) zeolite impregnated with a boron compound have been prepared and studied. Lattice boron is shown to present trigonal or tetrahedral environment depending upon ligand adsorbates and to induce a weak acid strength and subsequently negligible catalytic activity for acidic-type reactions as methanol conversion to hydrocarbons, toluene alkylation with methanol and toluene disproportionation. Some activity was observed only if lattice aluminum was present or if aluminum based binder was used. Lattice boron was shown not to modify appreciably shape selectivity. Boron compound impregnated on aluminum pentasil sample was shown to decrease the acidity and subsequently the catalytic activity for acid type reactions but to sharply enhance shape selectivity, i.e. the yield in the less bulky para isomers of aromatics with respect to the other isomers. Boron of the impregnated compound (H3BO3) was shown to partly isomorphously subsitute for Si or Al upon calcination at 773K while calcination at 1073K results in the partial formation of a glassy borosilicate compound at the surface of the zeolite particles. These findings are discussed in light of binder-zeolite interactions.

Thermodynamics of the catalytic polymerization of alkenes in the gas phase

The polymerization of alkenes is catalyzed by zeolite catalysts in the temperature range 450–800 K. As expected from thermodynamics the degree of polymerization is higher at lower temperatures and higher pressures. Equilibrium distributions of alkene isomer groups in the ideal gas phase have been calculated over a range of temperatures and pressures. The use of isomer groups greatly reduces the number of species that have to be considered in an equilibrium calculation and provides a means for extrapolating to higher carbon numbers. Since ZSM-5 is selective, equilibrium distributions have also been calculated for three restricted isomer groups from which certain bulky isomers have been excluded. Comparison of theory and experiment provides a means for learning about the nature of isomers excluded by the catalyst.