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PubMed2001

Phosphodiester hydrolysis by lanthanide complexes of bis-tris propane.

Potentiometric titrations of the mixtures of lanthanide(III) perchlorates and bis-Tris propane (BTP) reveal formation of dinuclear hydroxo complexes M2(BTP)2(OH)n(6-n), where M = La(III), Pr(III), Nd(III), Eu(III), Gd(III), and Dy(III) and n = 2, 4, 5, or 6, in the pH range 7-9. ESI-MS data confirm the presence of dinuclear species. Kinetics of the hydrolysis of bis(4-nitrophenyl) phosphate (BNPP), mono-4-nitrophenyl phosphate (NPP), and 4-nitrophenyl acetate (NPA) in the lanthanide(III)-BTP systems has been studied at 25 degrees C in the pH range 7-9. The second-order rate constants for the hydrolysis of BNPP by individual lanthanide hydroxo complexes have been estimated by using the multiple regression on observed rate constants obtained at variable pH. For a given metal, the rate constants increase with increasing in the number n of coordinated hydroxide ions. In a series of complexes with a given n, the second-order rate constants decrease in the order La > Pr > Nd > Eu > Gd > Dy. Hydrolysis of NPP follows Michaelis-Menten-type "saturation" kinetics. This difference in kinetic behavior can be attributed to stronger binding of NPP dianion than BNPP monoanion to the lanthanide(III) species. Activities of lanthanide complexes in the hydrolysis of NPA, which is 10(6) times more reactive than BNPP in alkaline or aqueous hydrolysis, are similar to those in BNPP hydrolysis indicating unique capability of lanthanide(III) cations to stabilize the transition state of phosphate diester hydrolysis. Results of this study are analyzed together with literature data for other metal cations in terms of the Brønsted correlation and transition state-catalyst complexation strength.

Catalysis↗
PubMed2004

Synthesis, crystal structure, and spectroscopic characterization of trans-bis[(mu-1,3-bis(4-pyridyl)propane)(mu-(3-thiopheneacetate-O))(3-thiopheneacetate-O)]dicopper(II), [[Cu2(O2CCH2C4H3S)4mu-(BPP)2]]n: from a dinuclear paddle-wheel copper(II) unit to a 2-D coordination polymer involving monatomic carboxylate bridges.

From the reaction between a dinuclear paddle-wheel carboxylate, namely [Cu2mu-(O2CCH2C4H3S)4] (1), and the flexible ligand 1,3-bis(4-pyridyl)propane (BPP) a neutral 2-D coordination polymer [[Cu2(O2CCH2C4H3S)4mu-(BPP)2]]n (2) was obtained. Compounds 1 and 2 were characterized by means of elemental analysis, thermal analysis (TG/DSC), vibrational spectroscopy, and electron paramagnetic resonance (EPR). The crystal structure of 2 reveals that each Cu(II) is coordinated by two nitrogen atoms from different BPP ligands and two 3-thiopheneacetate groups within a distorted square planar geometry in a trans-[N, N, O, O] arrangement. The BPP ligand adopts a TG conformation bridging two copper centers giving rise to a 1-D sinusoidal polymeric chain along the crystallographic c axis. Adjacent 1-D chains are extended into a 2-D coordination network through pairs of monatomic carboxylate bridges in direction of the b axis. This bridging mode affords centrosymmetric dimeric units Cu2O2, and therefore, the copper ions are involved in a CuN2O2O' chromophore displaying a (4 + 1) square pyramidal coordination in the resultant 2-D polymeric network. The polycrystalline X-band EPR spectrum of 2 at room temperature is characteristic of a triplet state with nonnegligible zero-field splitting in agreement with the crystal structure. Crystal data for 2: monoclinic, space group P2(1)/c, a = 9.4253(10) A, b = 10.9373(10) A, c = 23.6378(10) A, beta = 98.733(4) degrees, Z = 2.

Journal Article↗
PubMed2005

Dynamic formation of coordination polymers versus tetragonal prisms and unexpected magnetic superexchange coupling mediated by encapsulated anions in the cobalt(II) 1,3-bis(pyrid-4-ylthio)propan-2-one series.

The reactions of cobalt(II) halides and flexible ligand L [L=1,3-bis(pyrid-4-ylthio)propan-2-one] under different conditions generated a series of complexes with various structural motifs ranging from tetragonal-prismatic cages to 1-3D coordination polymers. The layer diffusion of cobalt(II) chloride and L in methanol/acetone at 25 degrees C gave rise to a 3D polymer, [Co(L)2Cl2].Me2CO. At 30 degrees C, the slow diffusion of diethyl ether into the blue dimethylformamide (DMF) solution of complex 1 afforded a 1D polymer, Co(L)Cl2(DMF)2. However, at 10 degrees C, the diffusion of diethyl ether into the DMF solution of complex 1 produced a tetragonal-prismatic cage, [Co2(L)4Cl2]Cl2.Et2O.DMF.2MeOH.4H2O. The reaction of cobalt(II) bromide and L in DMF at 10 degrees C yielded a dimer, [Co2(L)4Br2]Br2.6DMF.2H2O, with a cage structure similar to. The preparation of the series of compounds indicates the subtle relationship between structures and tunable reaction conditions. It is also found that the structural motifs vary according to the ligand conformations and that the formation of tetragonal-prismatic cages and may be templated by anionic guests. Magnetic studies on complexes in a temperature range 4-300 K disclose that L is unfavorable for a long-range magnetic interaction; however, intramolecular spin-coupling constants of -19.6 and -21.5 cm-1 for and indicate rather strong magnetic superexchanges arising from the overlap of the dz2 orbitals of the cobalt(II) and pz orbitals of the encapsulated halide anions. Electron paramagnetic resonance (EPR) spectra of complexes 3 and 4 in solution and solid give information that both complexes are high-spin cobalt(II) compounds with a rhombic distortion of the axial zero-field splitting. Interestingly, the intramolecular magnetic-exchange coupling in 3 and 4 mediated by the encapsulated anion Cl- or Br- is also reflected by the EPR spectra.

Journal Article↗
PubMed2005

Coordination networks with flexible ligands based on silver(I) salts: complexes of 1,3-bis(phenylthio)propane with silver(I) salts of PF6-, CF3COO-, CF3CF2COO-, CF3CF2)CF2COO-, p-TsO-, and CF3SO3-.

The synthesis and characterization of nine coordination networks based on 1,3-bis(phenylthio)propane, L(3), and silver(I) salts of PF(6)(-) (1), CF(3)COO(-) (2), CF(3)CF(2)COO(-) (3), CF(3)CF(2)CF(2)COO(-) (4), p-TsO(-) (5, 6), and CF(3)SO(3)(-) (7-9) are reported. Only 1 and other "isostructural" complexes with weakly coordinating anions such as ClO(4)(-) and SbF(6)(-) are of the host-guest type. In all the other complexes, the anions and the acetone molecules, when present, are coordinated to the metal. Most of the complexes studied here form a 2D-coordination network. Only 4 and 5 adopt a polymer-like chain structure. The packing of the chains of 4 is pseudohexagonal compact, while that of 5 is of the centered type. In complex 1, the silver atom is tetrahedrally coordinated to the sulfur atoms of four different ligands. The PF(6)(-) anions and acetone molecules, sandwiched between silver-ligand cationic sheets, are held through van der Waals interactions. In each of the three perfluorocarboxylates (2-4), two silver atoms are joined by the anions in a diatomic bridging mode. The Ag...Ag distances are sufficiently short to indicate weak metal...metal interactions. The dimeric units in 2 and 3 are interconnected through the ligands, thereby generating a 2D-network of neutral sheets, while, in 4, the dimeric units are bound to four ligands and a 1D-coordination polymer is generated. In the case of the sulfonate anions (p-TsO(-) and CF(3)SO(3)(-)), the crystallization solvent influences the structure adopted. Thus, in 5, 7, and 9 obtained from petroleum ether, or other nonpolar solvents, two silver atoms are bound in a double-bridge fashion, while a monobridge mode is noted for 6 and 8, both recrystallized from diethyl ether. In 8, both bridging types are observed. The thermogravimetric investigation, in the room temperature-450 degrees C interval, of complexes 1, 3, and 7, which incorporate acetone molecules in their crystal structures, reveals a two-step weight loss for 1 (the acetone molecules are lost first followed by the ligands, leaving behind the silver salt), while complexes 3 and 7 decompose in a single step to metallic silver.

Journal Article↗
PubMed2000

Synthesis, crystal structure, EXAFS, and magnetic properties of catena [mu-tris(1,2-bis(tetrazol-1-yl)propane-N1,N1')iron(II)] bis(perchlorate). First crystal structure of an iron(II) spin-crossover chain compound.

[Fe(btzp)3](ClO4)2 (btzp = 1,2-bis(tetrazol-1-yl)propane) represents the first structurally characterized Fe(II) linear chain compound exhibiting thermal spin crossover. It shows a very gradual spin transition (T1/2 = 130 K) which has been followed by magnetic susceptibility measurements and 57Fe Mössbauer spectroscopy. The structure has been solved at 200 and 100 K by single-crystal X-ray analysis. It crystallizes in the trigonal space group P3c1 with Z = 2 Fe(II) units at both temperatures. The molecular structure consists of chains running along the c axis in which the Fe(II) ions are linked by three N4,N4' coordinating bis(tetrazole) ligands. The main difference between the two forms appears to be in the Fe-N bond lengths, which are 2.164(4) A at 200 K and 2.038(4) A at 100 K. The Fe-Fe separations are 7.422(1) A at 200 K and 7.273(1) A at 100 K. The EXAFS results are consistent with the crystal structure. In both spin states, the FeN6 octahedron is almost regular within the EXAFS resolution. The Fe-N distance is found as 2.16(2) A at 300 K and 2.00(2) A at 40 K. The absence of the "7 A peak" in the EXAFS spectra of [Fe(btzp)3](ClO4)2, in contrast with what has been observed for the [Fe(4-R-1,2,4-triazole)3]-(anion)2 chain compounds, confirms that this peak can be used as the signature of a metal alignment only when it involves a strongly enhanced multiple scattering M-M-M path, with M-M spacing less than 4 A. Irradiation with green light at 5 K has led to the population of the metastable high-spin state for the iron(II) ion. The nature of the spin-crossover behavior has been discussed on the basis of the structural features.

Journal Article↗
PubMed2005

Enantioselective 1,3-dipolar cycloaddition of nitrones to methacrolein catalyzed by (eta5-C5Me5)M{(R)-prophos} containing complexes (M = Rh, Ir; (R)-prophos = 1,2-bis(diphenylphosphino)propane): on the origin of the enantioselectivity.

The rhodium and iridium Lewis-acid cations [(eta(5)-C(5)Me(5))M{(R)-Prophos}(H(2)O)](2+) ((R)-Prophos = 1,2-bis(diphenylphosphino)propane) efficiently catalyze the enantioselective 1,3-dipolar cycloaddition of nitrones to methacrolein. Reactions occur with perfect endo selectivity and with enantiomeric excesses up to 96%. Intermediates [(eta(5)-C(5)Me(5))M{(R)-Prophos}(methacrolein)](SbF(6))(2) (M = Rh (3), Ir (4)) have been spectroscopically and crystallographically characterized. The nitrone complexes [(eta(5)-C(5)Me(5))M{(R)-Prophos}(nitrone)](SbF(6))(2) (M = Rh, nitrone = 1-pyrrolidine N-oxide (5), 2,3,4,5,-tetrahydropyridine N-oxide (6), 3,4-dihydroisoquinoline N-oxide (7); M = Ir, nitrone = 1-pyrrolidine N-oxide (8)) have been isolated and characterized including the X-ray crystal structure of compounds 6 and 8. The equilibrium between methacrolein and nitrone complexes is also studied. [Ir]-adduct complexes are detected by (31)P NMR spectroscopy. A catalytic cycle involving [M]-methacrolein, [M]-nitrone, as well as [M]-adduct species is proposed, the first complex being the true catalyst. The absolute configuration of the adduct 4-methyl-2-N,3-diphenyl-isoxazolidine-4-carbaldehyde (9) was determined through its (S)-(-)-alpha-methylbenzylamine derivative diastereomer. Structural parameters strongly suggest that the disposition of the methacrolein in 3 and 4 is fixed by CH/pi attractive interactions between the pro-S phenyl ring of the Ph(2)PCH(CH(3)) moiety of the (R)-Prophos ligand and the CHO aldehyde proton. Proton NMR data indicate that this conformation is maintained in solution. From the structural data and the results of catalysis the origin of the enantioselectivity is discussed.

Acrolein↗
PubMed2001

New 1-aryl-3-(4-arylpiperazin-1-yl)propane derivatives, with dual action at 5-HT1A serotonin receptors and serotonin transporter, as a new class of antidepressants.

In a search toward new and efficient antidepressants, 1-aryl-3-(4-arylpiperazin-1-yl)propane derivatives were designed, synthesized, and evaluated for 5-HT reuptake inhibition and 5-HT1A receptor antagonism. This dual pharmacological profile should lead, in principle, to a rapid and pronounced enhancement in serotoninergic neurotransmission and consequently to a more efficacious treatment of depression. The design was based on coupling structural moieties related to inhibition of serotonin reuptake, such as gamma-phenoxypropylamines, to arylpiperazines, typical 5-HT1A ligands. In binding studies, several compounds showed affinity at the 5-HT transporter and 5-HT1A receptors. Antidepressant-like activity was initially assayed in the forced swimming test with those compounds with Ki < 200 nM in both binding studies. Functional characterization was performed by measuring the intrinsic effect on rectal temperature in mice and also the antagonism to 8-OH-DPAT-induced hypothermia. The most efficacious compounds (12f, 23gE, 28a, and 28b) were further explored for their ability to antagonize 8-OH-DPAT-induced inhibition of forskolin-stimulated cAMP formation in a cell line expressing the 5-HT1A receptor. Furthermore, the antidepressant-like properties of 12f, 28a, and 28b, which exhibited 5-HT1A receptor antagonistic property in the latter study, were also evaluated in the learned helplessness test in rats. Among these three compounds, 28b (1-benzo[b]thiophene-3-yl)-3-[4-(2-methoxyphenyl)-1-ylpropan-1-ol) showed the higher affinity at both the 5-HT transporter and 5-HT1A receptors (Ki = 20 nM in both cases) and was also active in the other pharmacological tests. Such a pharmacological profile could lead to a new class of antidepressants with a dual mechanism of action and a faster onset of action.

Animals↗
PubMed1986

Novel 1,3-bis(aryloxy)propanes as leukotriene D4 antagonists.

The synthesis and structure-activity relationships of a number of 1,3-bis(aryloxy)propanes, which are in vivo antagonists of LTD4 in the guinea pig, are described. One of these compounds, 4 (Wy-44,329), was not only approximately equipotent with the standard 1 (FPL 55712) in the LTC4 (ID50 = 0.17 and 0.23 mg/kg iv, respectively) and LTD4 (ID50 = 0.11 and 0.15 mg/kg iv, respectively) challenge models but also possessed greater potency in the ovalbumin challenge model (ID50 = 0.47 mg/kg and 4.1 mg/kg iv, respectively) and a longer duration of action. This compound was a competitive LTD4 antagonist on guinea pig ileum (pA2 = 9.4) and possessed mediator release (rat PCA, ID50 = 0.26 mg/kg iv) and 5-lipoxygenase (IC50 = 32 microM vs. 5-HETE) inhibitory activities.

Animals↗
PubMed1986

Synthesis and beta-adrenergic receptor blocking potency of 1-(substituted amino)-3-(4-indolyloxy)propan-2-ols.

Although (-)-[125I]iodopindolol (IPIN) can be used to label beta-adrenergic receptors in the central nervous system in vivo, use of this ligand for receptor imaging studies in humans may be limited due to its relatively poor penetration into the brain. As a first step toward the development of radioligands for imaging studies, we report the synthesis and measurement of in vitro binding affinity to beta-receptors of a series of 1-(substituted amino)-3-(4-indolyloxy)-propan-2-ol derivatives. The synthesized compounds vary widely in their lipophilicity as measured by their distribution coefficients between phosphate buffer and octanol at pH 7.4. The affinity of these compounds for beta-receptors, as measured by their inhibition of binding of IPIN to rat cortical and cerebellar membranes in vitro, ranges from 2- to 100-fold less potent than pindolol; the most potent compounds have Ki values of 2-5 nM. The radiolabeled analogues of some of these compounds may prove useful for receptor imaging studies.

Adrenergic beta-Antagonists↗
PubMed1980

Beta-adrenergic blocking agents. 20. (3-Hydroxyprop-1-enyl)-substituted 1-(aryloxy)-3-(alkylamino)propan-2-ols.

The synthesis of a series of (3-hydroxyprop-1-enyl)-substituted 1-(aryloxy)-3-(alkylamino)propan-2-ols is described. These compounds were investigated for their beta-adrenoreceptor blocking properties and their selective of action. Among the o-(hydroxypropenyl)-substituted derivatives we have found some potent noncardioselective beta-adrenoreceptor blocking agents which have a greater blocking action on the beta 2 receptor, thus resembling propranolol. The p-(hydroxypropenyl)-substituted analogues were generally less potent and tended to be cardioselective. The structure-activity relationships are discussed in the light of the hypothesis that the cardioselective of p-amido-substituted (aryloxy)propanolamines is attributable, in part, to binding of the amide group to some additional site on the beta receptor; our findings argue against a similar interaction for the allylic hydroxyl group.

Adrenergic beta-Antagonists↗
PubMed1982

beta-adrenergic blocking agents. 24. Heterocyclic substituted 1-(aryloxy)-3-[[(amido)alkyl]amino]propan-2-ols.

The synthesis of a series of 1-(aryloxy)-3-[[(amido)alkyl]amino] propan-2-ols where either the aryl moiety is heterocyclic or the amidic group is substituted by a heterocyclic moiety is described. Several of the compounds were more potent than propranolol when given intravenously to anesthetized rats. In contrast to previous findings with beta-blockers based on heterocyclic moieties and with either an isopropylamino or tert-butylamino substituent on the side chain, several compounds proved to be cardioselective when further examined in anesthetized cats. The detailed structure-activity relationships shown by this series of compounds are discussed.

Adrenergic beta-Antagonists↗
PubMed1983

beta-Adrenergic blocking agents. 23. 1-[Substituted-amido)phenoxy]-3-[[(substituted-amido)alkyl]amino] propan-2-ols.

The synthesis of a series of 1-phenoxy-3-[(amidoalkyl)amino]propan-2-ols, in which the phenoxy ring is variously substituted with ortho and para amidic moieties, is described. Several of the compounds have beta-blocking potency comparable to that of propranolol and cardioselectivity similar to that of practolol, when given intravenously to anesthetized cats. In contrast to previous findings with cardioselective beta blockers, both ortho and para substitution give variable degrees of cardioselectivity. Potency, however, is favored by ortho substitution.

Adrenergic beta-Antagonists↗
PubMed2000

Toward an understanding of the selectivity in domino reactions. A DFT study of the reaction between acetylenedicarboxylic acid and 1, 3-Bis(2-furyl)propane

The mechanism of the domino reaction between acetylenedicarboxylic acid and 1,3-bis(2-furyl)propane has been theoretically studied in the framework of density functional theory. This domino process comprises two consecutive cycloaddition reactions: the first one is initialized by the nucleophilic attack of the C5 position of the furan ring to a conjugate position of acetylenedicarboxylic acid to give a zwitterionic intermediate, which by a subsequent ring-closure process affords an oxanorbornadiene intermediate. The second reaction is an intramolecular concerted cycloaddition of this intermediate to give the final dioxapentacyclic adduct. For the second cycloaddition, which corresponds to the step controlling the selectivity, eight alternative reaction pathways are found. Chemoselectivity, facial selectivity, and stereoselectivity of this domino reaction are related with the different approach modes of the tethered furan to the oxanorbornadiene system of the intermediate. The most favorable pathway takes place along an endo/syn approach of the furan ring relative to the bridged oxygen atom of the oxanorbornadiene system, with participation of the substituted double bond. An analysis of energetic contributions to the potential energy barriers for the intramolecular cycloadditions identifies the different factors controlling the reactive channels. Selectivity outcome is reproduced by these calculations.

Journal Article↗
PubMed2000

Polymeric reagents with propane-1,3-dithiol functions and their precursors for supported organic syntheses

Reliable completely odorless syntheses of soluble copolymeric reagents of styrene type containing propane-1,3-dithiol functions able to convert carbonyl compounds into 1,3-dithiane derivatives and to support other useful transformations are reported together with their progenitor copolymers containing benzenesulfonate or thioacetate groups perfectly stable in open air and suitable for unlimited storage. The effectiveness of the prepared reagents as tools for polymer-supported syntheses to produce ketones by aldehyde umpolung and alkylation is tested in the conversion of benzaldehyde to phenyl n-hexyl ketone starting from copolymers with different contents of active units and molecular weights. To facilitate the adaptation of the prepared soluble copolymeric reagents to other possible applications, a table of solvents and nonsolvents is presented.

Journal Article↗
PubMed1996

Novel Rearrangements of 2,3-Dimercapto-1-propanol Derivatives: Formation of Dialkyl Sulfide, Dialkyl Disulfide, and 1,2,3-Tris(alkylthio)propane.

The base alkylation of 2,3-dimercapto-1-propanol with alkyl halides leads in good yield to 2,3-bis(alkylthio)-1-propanol and dialkyl sulfide. The reaction of 2,3-bis(alkylthio)-1-propanol with catalytic concentrated sulfuric acid proceeds with rearrangement to give 1,2,3-tris(alkylthio)propane and dialkyl disulfide. The rearrangement reaction takes place through a common intermediate thiaranium intermediate 8.

Journal Article↗
PubMed2005

Oxidative dehydrogenation of propane over V2O5/MoO3/Al2O3 and V2O5/Cr2O3/Al2O3: structural characterization and catalytic function.

The structure and catalytic properties of binary dispersed oxide structures prepared by sequential deposition of VO(x) and MoO(x) or VO(x) and CrO(x) on Al(2)O(3) were examined using Raman and UV-visible spectroscopies, the dynamics of stoichiometric reduction in H(2), and the oxidative dehydrogenation of propane. VO(x) domains on Al(2)O(3) modified by an equivalent MoO(x) monolayer led to dispersed binary structures at all surface densities. MoO(x) layers led to higher reactivity for VO(x) domains present at low VO(x) surface densities by replacing V-O-Al structures with more reactive V-O-Mo species. At higher surface densities, V-O-V structures in prevalent polyvanadates were replaced with less reactive V-O-Mo, leading to lower reducibility and oxidative dehydrogenation rates. Raman, reduction, and UV-visible data indicate that polyvanadates predominant on Al(2)O(3) convert to dispersed binary oxide structures when MoO(x) is deposited before or after VO(x) deposition; these structures are less reducible and show higher UV-visible absorption energies than polyvanadate structures on Al(2)O(3). The deposition sequence in binary Mo-V catalysts did not lead to significant differences in structure or catalytic rates, suggesting that the two active oxide components become intimately mixed. The deposition of CrO(x) on Al(2)O(3) led to more reactive VO(x) domains than those deposited on pure Al(2)O(3) at similar VO(x) surface densities. At all surface densities, the replacement of V-O-Al or V-O-V structures with V-O-Cr increased the reducibility and catalytic reactivity of VO(x) domains; it also led to higher propene selectivities via the selective inhibition of secondary C(3)H(6) combustion pathways, prevalent in VO(x)-Al(2)O(3), and of C(3)H(8) combustion routes that lead to low alkene selectivities on CrO(x)-Al(2)O(3). VO(x) and CrO(x) mix significantly during synthesis or thermal treatment to form CrVO(4) domains. The deposition sequence, however, influences catalytic selectivities and reduction rates, suggesting the retention of some of the component deposited last as unmixed domains exposed at catalyst surfaces. These findings suggest that the reduction and catalytic properties of active VO(x) domains can be modified significantly by the formation of binary dispersed structures. VO(x)-CrO(x) structures, in particular, lead to higher oxidative dehydrogenation rates and selectivities than do VO(x) domains present at similar surface densities on pure Al(2)O(3) supports.

Journal Article↗
PubMed2005

Framework composition effects on the performance of steam-activated FeMFI zeolites in the N2O-mediated propane oxidative dehydrogenation to propylene.

The N(2)O-mediated oxidative dehydrogenation of propane (ODHP) to propylene has been investigated in the temperature range of 673-773 K over steam-activated FeMFI zeolites with different framework compositions (Si-Al, Si-Ga, Si-Ge, and pure Si). The catalysts, which were characterized by ICP-OES, XRD, SEM, N(2) adsorption, NH(3)-TPD, and FTIR of pyridine adsorbed, HRTEM, and UV/vis, have a very similar iron content (0.6-0.7 wt %). A tapered element oscillating microbalance (TEOM) coupled to on-line analysis of products has been applied to simultaneously monitor activity and mass changes during the ODHP reaction. FeAlMFI and FeGaMFI zeolites display higher propylene yields (up to 25%) and a much slower deactivation due to coking than do FeGeMFI and FeMFI zeolites. The higher density and strength of acid sites in the former samples did not induce a faster catalyst deactivation. In general, catalyst deactivation was accelerated upon decreasing the reaction temperature. The total amount of coke formed in the samples during 400 min on stream ranged from 6 to 23 wt %, increasing linearly with the amount of propylene produced. The distinct performance of both groups of zeolites is likely related to the different iron speciation, which is influenced by the composition of the framework. The smaller primary crystallites of FeAlMFI and FeGaMFI, as well as the presence of mesopores in these samples, which are created by dislodgement of aluminum and gallium to extraframework positions, appear to be also beneficial for higher catalyst effectiveness and a reduced deactivation.

Journal Article↗
PubMed2005

Reactivity of alkanes on zeolites: a computational study of propane conversion reactions.

In this work, quantum chemical methods were used to study propane conversion reactions on zeolites; these reactions included protolytic cracking, primary hydrogen exchange, secondary hydrogen exchange, and dehydrogenation reactions. The reactants, products, and transition-state structures were optimized at the B3LYP/6-31G level and the energies were calculated with CBS-QB3, a complete basis set composite energy method. The computed activation barriers were 62.1 and 62.6 kcal/mol for protolytic cracking through two different transition states, 30.4 kcal/mol for primary hydrogen exchange, 29.8 kcal/mol for secondary hydrogen exchange, and 76.7 kcal/mol for dehydrogenation reactions. The effects of basis set for the geometry optimization and zeolite acidity on the reaction barriers were also investigated. Adding extra polarization and diffuse functions for the geometry optimization did not affect the activation barriers obtained with the composite energy method. The largest difference in calculated activation barriers is within 1 kcal/mol. Reaction activation barriers do change as zeolite acidity changes, however. Linear relationships were found between activation barriers and zeolite deprotonation energies. Analytical expressions for each reaction were proposed so that accurate activation barriers can be obtained when using different zeolites as catalysts, as long as the deprotonation energies are first acquired.

Journal Article↗