Pulse Microreactor Research Articles

The modification of CO adlayers on [formula omitted] catalysts by preadsorbed oxygen: An infrared and pulse microreactor study

Changes in the structure of CO adlayers as the result of co-adsorbed oxygen on Pt SiO 2 catalysts have been studied using transmission infrared spectroscopy and pulsed microreactor studies. Preadsorbed oxygen was observed to cause a blue shift in the absorption frequency of adsorbed CO. The extent of the blue shift was observed to increase with the severity of the O 2 treatment. The origin of this blue shift is explained by invoking an increase in the extent of COCO dipole coupling. Both the infrared and microreactor studies strongly suggest that the increase in dipolar coupling occurs as the result of a compressed CO adlayer in which CO is segregated into isolated patches.

Catalytic activity of HY-type zeolite for the disproportionation of nitrogen monoxide

The disproportionation of NO on HY-type zeolite was carried out at 15°C by a pulsed microreactor. The catalytic activity began to appear at the pretreatment temperature 500°C and showed its maximum in the region of 600 to 700°C. The Lewis acid sites of HY-type zeolite seem to act as the active sites. TPD profiles after the reaction were composed of three peaks. The amount of peak β also showed its maximum at the pretreatment temperature ca. 600°C.

Isomerization of m-xylene on H-ZSM-5: Part I: Influence on catalytic activity and selectivity of si/al ratio, degree of cation exchange, deammoniation conditions and poisoning of stronger acid sites

Isomerization of m-xylene on H-ZSM-5 zeolites with different Si/Al ratios and degrees of cation exchange, and on the zeolites obtained at different deammoniation conditions from NH 4-ZSM-5 has been investigated in a pulse microreactor. The results are correlated to the number of strong acid sites as measured by the irreversible adsorption of pyridine at 673 K. The reaction on the poisoned zeolite has been studied by selectively blocking the stronger acid sites in the order of their strength with ammonia, quinoline and pyridine at different temperatures in the range 673–773 K. The catalytic activity and selectivity of the zeolite are seen to be strongly affected by the changes in its Si/Al ratio, degree of cation exchange and deammoniation conditions and also by the poisoning of the stronger acid sites of the zeolite by pyridine or quinoline. The selectivity in the Isomerization increases with the decrease in the number of strong acid sites on the zeolite and with the increase in the extent of poisoning of the stronger acid sites. The strength of the acid sites required for catalysing the various parallel reactions occurring in the overall conversion of m-xylene is in the following order:- isomerization < dealkylation < disproportionation.

Transformation of cyclic hydrocarbons on H, Ca, and La forms of zeolites

1. The transformation of cyclohexane, cyclohexene, and 1,4-cyclohexadiene was studied on H, Ca, and La forms of type Y zeolites, erionite, and mordenite as well as on amorphous aluminosilicate, γ-aluminum oxide, and silicon dioxide in a pulse microreactor. These hydrocarbons undergo cracking, isomerization, dispraportionation, and dehydrogenation at 200–500°C. 2. The catalysts studied fall into two groups relative to the methylcyclopentane/cyclohexane ratio in the cyclohexene transformation products: the catalysts without acid sites (Na forms of zeolites and SiO2) have ratios less than unity, while the catalysts with acid sites (multiply charged cationic and H forms of zeolites and amorphous aluminosilicate) have ratios above unity. 3. The dehydrogenating activity of the catalysts studied increases in going from non-acidic to acidic catalysts.

Hydrodechlorination and oligomerization of carbon tetrachloride over nickel Y zeolites

A pulsed microreactor was used to study the reaction of carbon tetrachloride with hydrogen at 370 °C over NiNaY, NiCuNaY, NiCrNaY, NiCoNaY, and CuNaY. Nickel was present at about 30 wt%. Activity was in the above order when catalysts were incompletely reduced at 370 °C. Ni 0 in NiCoY was measured to be 40% of the total Ni by X-ray photoelectron spectroscopy. Upon complete reduction at 530 °C, NiCoNaY became the most active and most selective catalyst for production of 1,1,1,2-tetrachloroethane (I). The highly specific conversion of CCl 4—0.40 mole Cl 3CCH 2Cl/mole CCl 4 at 80–100% conversion—is postulated to proceed by a free-radical mechanism initiated by H 2 dissociation on the metal H 2 ⇆ 2H ·, followed by hydrodechlorination propagation reactions such as CCl 4 + · H → · CCl 3 + HCl, CH 2Cl 2 + · H → · CH 2Cl + HCl and oligomerization termination reactions such as · CCl 3 + · CH 2Cl → Cl 3CCH 2Cl. Catalyst activity and selectivity to I correlated with the order of Ni diffusion out of the supercages. Outside, on the crystallite, hydrogen from the gas stream is readily available, and hydrodechlorination is favored by excess hydrogen. Inside the supercage there should exist a relative deficiency of hydrogen, and oligomerization could be favored. That the latter process proceeds inside the supercage is evidenced by the abrupt deactivation of the catalysts after a highly active initial period. The deactivation would be related both to HCl generation and to formation of higher-molecular-weight oligomers inside the zeolite. In the absence of metal, NaY is inactive. It is probable that the Ni 0 inside and outside the supercage is not of a different nature, but the proportions in effect tailor the catalyst selectivity. Such a remarkable selectivity to Cl 3CCH 2Cl was not anticipated.

A combined infrared and gas chromatographic reaction system for in situ catalytic studies

A novel flow system including an infrared cell with the capability of operating either as a pulse microreactor or a single-pass differential flow reactor has been constructed. With this new design, extensive in situ surface characterisation studies of working catalysts can be made. The following applications are considered: the measurement of extinction coefficients; the measurement of turnover numbers and activation energies; the measurement of product distributions; temperature programmed reaction and temperature programmed desorption studies; and the measurement of the surface coverage of adsorbed species under reaction condition. As an example, the reaction CO(g)+H2(g) on silica-supported Pt-Ru bimetallic clusters is considered. Results obtained using the IR cell as a single-pass reactor are in excellent agreement with those obtained using a powdered catalyst placed in a conventional microreactor.

Redox kinetics of bismuth molybdate ammoxidation catalysts

The response of several bismuth molybdate-based catalysts to reduction under propylene ammoxidation conditions in the absence of gaseous oxygen, and to reoxidation by gaseous oxygen, was studied using a pulse microreactor method. The catalysts investigated were Bi 2Mo 3O 12, Bi 2Mo 2O 9, Bi 2MoO 6, Bi 3FeMo 2O 12, and a multicomponent system (M a 2+M b 3+Bi x Mo y O z ). The unit area rates of lattice oxygen participation at 430 °C decrease in the order: multicomponent system > Bi 2Mo 2O 9 > Bi 2Mo 3O 12 > Bi 3FeMo 2O 12 ≳ Bi 2MoO 6. Maximum selective utilization of reactive lattice oxygen occurs after partial reduction for the multicomponent system, Bi 2Mo 3O 12 and Bi 2Mo 2O 9. These results are consistent with a mechanism requiring coordinately unsaturated metal ions in complex shear domains for selective ammoxidation. Conversely, Bi 3FeMo 2O 12 and Bi 2MoO 6 show maximum lattice oxygen activity at their highest oxidation states. The overall reoxidation rates of partially reduced catalysts at 430 °C decrease in the order: Bi 2MoO 6 > Bi 2Mo 2O 9 > Bi 2Mo 3O 12 > Bi 3FeMo 2O 12 ≳ multicomponent system. These reoxidation rates of partially reduced catalysts are first order in oxygen vacancy concentration and half order in gaseous oxygen. A general mechanism for catalyst reoxidation is proposed based on these kinetics. This series of catalysts exhibits two reoxidation regimes. One regime is characterized by a low activation energy at low degrees of initial reduction and involves the reoxidation of surface vacancies. A second regime is observed for deeper degrees of reduction which is characterized by a higher activation energy and involves the reoxidation of anion vacancies in the bulk of the catalyst. The observed activation energies for the reoxidation of the catalyst bulk are strongly dependent upon the structure and composition of the catalyst.

Pulse microreactor examination of the vapor-phase aldol condensation of acetone

Pulse microreactor techniques and a small flow reactor have been employed to examine the nature of the MgOAl 2O 3-catalyzed aldol condensation of acetone to principally mesityl oxide and isophorone. It has been shown that a large number of transient intermediates are involved in a series of very rapid, reversible equilibria. The last step in this sequence is the irreversible formation of isophorone which appears to be the net driving force of this condensation reaction. The nature of the catalyst, the effect of various additives, and the reaction intermediates and by-products have been detailed in order to arrive at an overall description of the base-catalyzed oligomerization of acetone.

Kinetics of thermal cracking of a hydrocarbon mixture by an empty pulsed microreactor

The kinetics of thermal cracking of a mixture of hydrocarbons (ethane, propane, isobutane and n-butane) was investigated in an empty pulsed microreactor in the temperature range 600 – 710 °C. The results were compared with those obtained in earlier work with a quartz packed pulsed microreactor. While the kinetic parameters agreed well, better yields of ethylene and propylene were found with the empty pulsed microreactor

Kinetic Analysis of 1-Butene Oxidation to Maleic Anhydride with a Polyfunctional Catalyst

Kinetic analysis of 1-butene oxidation to maleic anhydride (MA) with a polyfunctional catalyst, a phosphorus/vanadium oxide (PVO) with P/V ratios of 1.3:1 or 1.4:1, carried out in a pulse microreactor or a continuous-flow integral microreactor, suggested the presence of two different types of active sites on the catalyst surface responsible for 1-butene oxidation to butadiene (the main intermediate) and oxidation of the latter to MA. Carbon oxides are formed at both types of active sites. Butadiene is probably formed via a redox mechanism involving lattice oxygen, whereas the second stage, which is considerably more selective, involves adsorbed oxygen species. The selectivity of the first stage was improved by adding a bismuth molybdate (2:1 Bi/Mo) at 1:2 by wt to the PVO. This modification also increased considerably the over-all reaction rate and MA yield, which approached that obtained in oxidation of pure butadiene.

Reaction Mechanism of Ethylbenzene Isomerization

The reaction mechanism of gas-phase ethylbenzene isomerization over bifunctional alumina- or zeolite-supported 0.5% by wt platinum catalysts was studied in a fixed-bed pulse microreactor by determining the product distributions in the conversion of ethylbenzene and of eight probable C/sub 8/ intermediates at 300/sup 0/-400/sup 0/C and 1.8 atm. Over the Pt/Al/sub 2/O/sub 3/ catalyst, xylenes are formed from ethylbenzene mainly via ethylcyclohexene to 1,2-methylethylcyclopentene to 1,2-dimethylcyclohexene to o-xylene, which corresponds to predictions from a bifunctional mechanism, in which only skeletal rearrangements of tertiary carbocations are considered to be rate-determining. Over Pt/zeolite catalysts with higher support acidity, skeletal isomerizations via both tertiary and secondary carbocations contribute to the measured product distributions. In ethylbenzene isomerization, the yield of C/sub 8/ naphthenes decreased from 60 mole % at 300/sup 0/C to zero at 400/sup 0/C and those of C/sub 8/ aromatics and xylenes passed through maxima of approx. 90% at 380/sup 0/C and 20% at 340/sup 0/C, respectively.

Kinetics of thermal cracking of light hydrocarbon mixture by pulsed microreactor

The kinetics of thermal cracking of a mixture of hydrocarbons (ethane, propane, isobutane and n-butane) was investigated in a packed-bed pulsed microreactor in the temperature range 600 – 725 °C. The reactor was made up of stainless steel (304) packed with quartz, and nitrogen was used as diluent. From experimental data it was found that all the saturated hydrocarbon, taken together as reactant in terms of total carbon, yielded first-order rates of disappearance to give products which were mainly methane, ethylene and propylene. The values of the Arrhenius factor and activation energy were found to be 4.779 × 10 11 s −1 and 214.24 kJ/g mol, respectively. The kinetics of disappearance of individual hydrocarbons and formation of individual products were also investigated. Optimum temperature and residence time conditions for the formation of ethylene and propylene are reported. Surface effects of the packing were not detected when different sizes of packing were used.

The Influence of Chloriding on Isomerization and Hydrogenolysis of n-Pentane on Pt-AI2O3 Reforming Catalysts

The influence of the catalysts chloriding on isomerization and hydrogenolysis of n-pentane on Pt/Al/sub 2/O/sub 3/ catalysts was studied at 300/sup 0/-436/sup 0/C in a pulse microreactor and a continuous-flow, bench-scale tubular reactor with 0.75 and 0.59Vertical Bar3< by wt Pt/Al/sub 2/O/sub 3/ commercial reforming catalysts that contained 0.03, 0.066, or 0.087Vertical Bar3< sulfur. Fully sulfided catalysts (Vertical Bar3: 0.072Vertical Bar3< sulfur) showed high and stable isomerization activity but low-sulfur samples were more active in hydrogenolysis. Chlorination of the catalysts with CCl/sub 4/ in hydrogen flow at 400/sup 0/C decreased n-pentane conversion, effectively suppressed hydrogenolysis, and increased isomerization selectivity to 100Vertical Bar3<. By contrast, catalyst treatment with gaseous hydrogen chloride to the same chlorine content (5-7Vertical Bar3< by wt), had only a minor effect on the catalyst activity and selectivity. Apparently, the deactivation of the Pt sites specifically responsible for hydrogenolysis is mainly due to sulfiding or the deposition of coke formed from CCl/sub 4/ and to a much lesser extent to their chlorination. CCl/sub 4/ treatment also enhances the acidity of the alumina support and thus accelerates olefin isomerization. Hydrogen chemisorption and desorption data were used to characterize the Pt surface of the catalysts and their selectivities.

Active centers in fluorinated montmorillonites

The catalytic activity of fluorinated montmorillonite in cumene cracking has been studied by the pulse microreactor technique. The existence of acid-base and radical centers on the catalyst surface were detected, on which cracking and dehydrogenation occur in parallel but α-methylstyrene is then cracked consecutively.

Kinetics of thermal cracking of light hydrocarbon mixture by pulsed microreactor

The kinetics of thermal cracking of a mixture of hydrocarbons (ethane, propane, isobutane and n-butane) was investigated in a packed-bed pulsed microreactor in the temperature range 600 – 725 °C. The reactor was made up of stainless steel (304) packed with quartz, and nitrogen was used as diluent. From experimental data it was found that all the saturated hydrocarbon, taken together as reactant in terms of total carbon, yielded first-order rates of disappearance to give products which were mainly methane, ethylene and propylene. The values of the Arrhenius factor and activation energy were found to be 4.779 × 10 11 s −1 and 214.24 kJ/g mol, respectively. The kinetics of disappearance of individual hydrocarbons and formation of individual products were also investigated. Optimum temperature and residence time conditions for the formation of ethylene and propylene are reported. Surface effects of the packing were not detected when different sizes of packing were used.

Hydrodesulfurization of thiophene, benzothiophene, dibenzothiophene, and related compounds catalyzed by sulfided CoO$z.sbnd;MoO3/$gamma;-Al2O3: Low-pressure reactivity studies

Hydrodesulfurization experiments were carried out with a sulfided CoOMoO3γ-Al2O3 catalyst in a pulse microreactor operated at atmospheric pressure and temperatures of 350 to 450 °C. The reactants were hydrogen and pure sulfur-containing compounds (or pairs of compounds), including thiophene, benzothiophene, dibenzothiophene, several of their hydrogenated derivatives, and various methyl-substituted benzothiophenes and dibenzothiophenes. The aromatic compounds appeared to react with hydrogen by simple sulfur extrusion; for example, dibenzothiophene gave H2S + biphenyl in the absence of side products. The reactivities of thiophene, benzothiophene, and dibenzothiophene were roughly the same. Each hydrogenated compound (e.g., tetrahydrothiophene) was more reactive than the corresponding aromatic compound (e.g., thiophene). Methyl substituents on benzothiophene had almost no effect on reactivity, whereas methyl substituents on dibenzothiophene located at a distance from the S atom slightly increased the reactivity, and those in the 4-position or in the 4- and 6-positions significantly decreased the reactivity. In contrast to the observation of a near lack of dependence of low-pressure reactivity on the number of rings in the reactant, the literature shows that at high pressures the reactivity decreases with an increased number of rings. The pressure dependence of the structure-reactivity pattern is suggested to be an indication of relatively less surface coverage by the intrinsically more reactive compounds (e.g., thiophene) at low pressures but not at high pressures. The relative reactivities are also suggested to be influenced by differences in the structures of the catalyst at low and high hydrogen partial pressures, which may be related to the concentrations of surface anion vacancies and the nature of the adsorbed intermediates.

Simple pulse micro-reactor combined with a gas chromatograph

Simple pulse micro-reactor combined with a gas chromatograph

Hydrodechlorination of polychlorinated biphenyl

The catalytic hydrodechlorination of a complex polychlorinated biphenyl (PCB) mixture was studied in a batch reactor using a 61% Ni on kieselguhr catalyst from 25 to 100 °C at 50 atm of H 2. Ethanol was used as a solvent and NaOH as an acid acceptor. Aromatic hydrodechlorination of chlorinated biphenyls is a consecutive single-step process; ortho-substituted chlorine is the least labile due to steric effects. In the reaction sequence below, φφ i indicates a biphenyl molecule substituted with i chlorine atoms distributed over the two rings. First-order pseudo-homogeneous relative rate constants for the liquid phase study are noted: ▪ Similar behavior was observed in a gas-phase pulse microreactor over a series of 0.00, 0.05, 0.10, and 0.35 wt% Pd on α-Al 2O 3 catalysts at 220 ± 5 °C, 2.3 atm of H 2, and space time of ~10 −2 sec (based on H 2 carrier gas flow rate). The corresponding values of the relative rate constants were 1.0, 0.85, 0.50, 0.70, and 0.95. These values of relative rate constants pass through a minimum as a result of the combined influence of statistical and steric factors. Highly ortho-substituted materials cannot adsorb in a planar configuration. The relative first-order rate constant for ortho to ( meta + para) hydrodechlorination k o k mp = 0.52 for the nickel catalyst. Reaction behavior is consistent with the mechanism requiring adsorption of the benzene ring and charge delocalization at the aromatic chlorine: ▪ For hydrodechlorination, the ▪ carbon-chlorine double bond is regarded as the reactive species.

The catalytic isomerisation of cyclopropane over NaCoX zeolites using a pulsed Microreactor

The catalytic isomerisation of cyloprane to propane was investigated over a series of NaCoX zeolites using a pulsed microreactor. First order kinetic parameters were calculated for the surface reaction from measured enthalpies of adsorption and apparent activation energies. Surface activation energies were not very dependent on the extent of ion exchange of Co(II) for Na(I) ions. Rates of reaction did not increase significantly with Co(II) concentration until after about 30% exchange. At levels of ion exchange greater than about 30%, rates of reaction with cobalt concentration due to an increase in the number of active sites. In agreement with previous reports the active sites were presumed to be hydroxyl groups and the increase in rate at higher concentrations of cobalt was attributed to dissociation of coordinated water molecules, during dehydration, to produce protons. Location of these protons on oxygens in the large cages provides the active sites.

Chromatographic pulsed micro-reactor system for studying the kinetics of hydrocarbon conversions

Chromatographic pulsed micro-reactor system for studying the kinetics of hydrocarbon conversions