Pulse Microreactor Research Articles

Isoprene by Prins Condensation over Acidic Molecular Sieves

The Prins condensation of isobutylene with formaldehyde overdifferent catalysts (HY, USY, H-ZSM-5, H-Boralite, H-B-MCM-41)having various pore sizes and acidities has been investigatedin a pulse microreactor. The results show unambiguously that highselectivities to isoprene are obtained only in the presence ofweakly acidic Brønsted acid sites.

Mechanistic Studies of Methane Partial Oxidation to Syngas over SiO2-Supported Rhodium Catalysts

The reaction behaviors of CH4and CH4/O2(2/1) with reduced (Rh/SiO2) and oxidized (Rh(O)/SiO2) SiO2-supported rhodium catalysts were investigated at temperatures ranging from 600 to 800°C over a pulse microreactor. The interaction of methane with Rh/SiO2led to CO, H2and surface carbon formation. The conversion of methane increased with the increase in rhodium loading. During the interaction of methane with Rh(O)/SiO2catalysts having rhodium loading ≥0.5%, besides CO, H2, and surface carbon, CO2and H2O were formed. Total oxidation of methane occurred over rhodium oxide together with the simultaneous reduction of Rh3+to Rh0. The partial oxidation of methane as well as the decomposition of methane took place over the resulting metallic rhodium. Within the first pulse of CH4/O2(2/1), rhodium oxide in the oxidized catalysts was reduced to metallic rhodium. From the second pulse onward, both methane conversion and CO selectivity were similar to those observed over the reduced catalysts. The results indicate that metallic rhodium is the active site for methane partial oxidation and chemisorbed oxygen species have participated in the methane oxidation reaction. We suggest that OH groups and/or certain low coordinated oxygens of SiO2were also involved in the oxidation of methane, especially at low rhodium loadings. Deuterium isotope effects in the methane partial oxidation reaction were investigated at 700°C by performing CH4+O2and CD4+O2reactions alternately over the Rh/SiO2catalysts. Normal deuterium isotope effects of similar magnitude were observed on the overall reaction of methane oxidation as well as on the CO and CO2formation reactions with insignificant change in product selectivities. The results indicate that methane dissociation is a key step and that CO and CO2are formed via some common intermediates, viz. surface CHx(x=0–3) species. The activation energies for methane dissociation with or without the involvement of chemisorbed oxygen on a Rh(111) surface were calculated by means of the bond-order conservation Morse-potential approach.

Influence of O2 and H2 pretreatments on acidity/acid strength distribution and acid functions of Ga/H-ZSM-5, H-GaMFI and H-GaAl MFI zeolites

H-gallosilicate (H-GaMFI), H-galloaluminosilicate (H-GaAlMFI) and Ga, impregnated H-ZSM-5 (Ga/H-ZSM-5) zeolites pretreated with O2 and H2 (at 600°C for 10 h) have been characterized for their acidity/acid strength distribution (by chemisorption and stepwise thermal desorption of pyridine from 100°–400°C) and also for their acid functions by acid catalysed reactions [viz. isooctane cracking (at 400°C) (for characterizing external acid sites) and toluene disproportionation (at 500°C) and methanol-to-aromatics conversion (400°C)] using a pulse microreactor. The catalysts were also characterized by XPS,29Si,27Al and71Ga MAS NMR. The acidity/acid strength distribution, activity in the acid catalyzed reactions, frame-work Si/Ga ratio and surface Ga/Si ratio of the zeolites are significantly affected by their pretreatment by O2 or H2.

Gas-generating agent for automobile airbags

The relation between the surface acidity of H-mordenite synthesized in nonaqueous solvents (HNAM) and H-mordenite synthesized in aqueous solvents (HAM) and isomerization ofortho-xylene was investigated with techniques of pulsed microreactor (carrier H2), temperature programmed desorption of NH3 (t.p.d.), and pulsed poisoning experiment of 2,6-dimethylpyridine. The selectivity of catalyst is dependent on peak 1 of t.d.p. chromatogram. The activity of isomerization is affected by the total acid sites. The activity of dealkylation is increased with an increase of the strength of acid sites. F− added increases the intensity of the (350) reflection and decreases that of (202) Na2CO3 added in synthesis promotes the growth rate of crystal. F− and Na2CO2 added enable mordenite to have different crystal morphology. The activity and selectivity ofo-xylene isomerization on HNAM are higher than that on HAM.

Transalkylation of the alkylaromatic hydrocarbons in the presence of ultrastable Y zeolites Transalkylation of toluene with trimethylbenzenes

The catalytic properties of Y zeolites, dealuminated by steam stabilization, for the transalkylation of toluene with trimethylbenzene were investigated in a pulse microreactor. Some factors influencing this reaction were taken into account: the reaction temperature, the degree of dealumination, the nature of acid sites and the deposited coke. The transalkylation reaction proceeds with good yields on USY. At bulk silica to alumina ratio's around 5, a higher activity is observed for the transalkylation of toluene than for the disproportionation of trimethylbenzenes. Both HNaY and USY samples were characterized using FTIR and XPS of chemisorbed pyridine. These techniques allowed to detect the presence of the three kinds of acidic sites: the Brønsted acid sites, the Na + cations and the extra framework AlO + species. The effect of these three sites on catalytic results are discussed as well as the reactivity of alkylaromatic hydrocarbons in the reaction network and the reaction mechanism.

CH4/CD4 isotope effects in the carbon dioxide reforming of methane to syngas over SiO2-supported nickel catalysts

By performing the CH4 + CO2 and CD4 + CO2 reactions alternately over SiO2-supported nickel catalysts in a pulse micro-reactor, normal deuterium isotope effects on both the methane conversion reaction and on the CO formation reaction have been observed in the process of CO2 reforming of methane. Based on the observed CH4/CD4 isotope effects, the pathways for the formation of CO are discussed.

Pulse study of methane partial oxidation to syngas over SiO2-supported nickel catalysts

The reaction behaviors of CH4/O2 (2/1) with unreduced and reduced SiO2-supported nickel (10 wt% metal) catalysts were investigated in a pulse micro-reactor at temperatures ranging from 600 to 800° C. In the interaction of CH4/O2 with unreduced NiO/SiO2, the products are exclusively CO2 and H2O at or below 700°C. At 800° C, besides CO2 and H2O, CO and H2 are formed. The reactivity at 800°C can be ascribed to the net generation of metallic nickel site at this temperature. In the reaction of CH4/O2 over reduced Ni/SiO2, the main products are CO and H2 with CO2 and H2O being the minor ones. The results indicate metallic nickel is the active site for methane partial oxidation to syngas. Normal deuterium isotope effects of similar magnitude were observed on the overall, as well as on the CO and CO2 formation reactions with insignificant change in the product selectivities when CD4 was used instead of CH4, indicating that methane dissociation is a key step and that CO and CO2 are formed via some common intermediates which are subject to kinetic deuterium isotope effect. The results of deuterium isotope effects can be reasonably explained based on the methane pyrolysis mechanism.

Mechanistic Studies of CH4/O2Conversion over SiO2-Supported Nickel and Copper Catalysts

Methane utilization as received much attention in the past few years. Besides the methane oxidative coupling process, the partial oxidation of methane to syngas is another route being considered. In this note, the authors report the reaction behaviors of CH{sub 4} and CH{sub 4}/O{sub 2} with reduced and unreduced SiO{sub 2}-supported copper and nickel (10 wt% metal) catalysts in a pulse microreactor at 600, 650, and 700{degrees}C. 18 refs., 5 figs.

Methanol-to-aromatics conversion over H-gallosilicate (MFI): Influence of [formula omitted] ratio, degree of H + exchange, pretreatment conditions, and poisoning of strong acid sites

The conversion of methanol to aromatics over H-gallosilicate (H-GaMFI) zeolite with different Si Ga ratios and degrees of H + exchange, calcined at different temperatures (600–1,100°C), and pretreated hydrothermally at different temperatures and partial pressures of steam, has been investigated in a pulse microreactor at 400°C. The aromatization activity and product distribution in the methanol-to-aromatics conversion are found to be influenced strongly by the above zeolite factors and calcination/pretreatment parameters. The aromatization activity of the zeolite shows a close relationship with its strong acidity (measured in terms of pyridine chemisorbed at 400°C). H-GaMFI and H-ZSM-5 zeolites (having almost the same Si (Ga or Al) ratio and degree of H + exchange and pretreated under similar conditions) have been compared for their catalytic activity/selectivity in the methanol-to-aromatics conversion and also for the poisoning of their strong acid sites by pyridine chemisorption at 400°C. H-GaMFI shows much higher aromatization activity. Unlike H-ZSM-5, H-GaMFI shows appreciable aromatization activity even after the poisoning of its strong acid sites or in the presence of only weak acid sites because of its higher dehydrogenation activity.

The role of carbonaceous deposits and support impurities in the selective hydrogenation of ethyne

In order to elucidate the role of carbonaceous deposits, the hydrogenation of ethyne over silica supported Pt and Pd catalysts has been studied in a micro-pulse reactor. When a pure silica support (Aerosil) was used, the conversion decreased with an increasing number of ethyne pulses led over the reactor. Simultaneously, the selectivity towards ethene increased. When less pure silica (Kieselgel) was used as a support, the opposite effect was observed. These results are explained by assuming that the carbonaceous deposits are not active, but influence the selectivity of the uncovered metal surface by diminishing the average ensemble size available for the reaction. Impurities in the support can alter the behaviour of the catalysts.

Initial activity/selectivity of H-gallosilicate (MFI) in propane aromatization: influence of H+ exchange and thermal/hydrothermal pretreatments

Initial activity/selectivity of H-gallosilicate (MFI) zeolite with different degrees of H+ exchange and pretreated under different thermal and hydrothermal conditions in propane aromatization (at 500‡C) has been determined using a pulse microreactor connected to GC. It is found to be strongly influenced by the degree of H+ exchange, calcination temperature and hydrothermal treatment at different temperatures and concentrations of steam. There exists a close relationship between the acidity (measured in terms of pyridine chemisorbed at 400‡ C) of the gallosilicate and its initial propane conversion and aromatization activity. Presence of strong acidic sites (attributed to FW Ga) at high concentration is essential for the well dispersed non-FW Ga oxide species to be active for dehydrogenation in the propane aromatization over the zeolite.

Structural, surface, and catalytic properties of bismuth molybdovanadates containing foreign atoms: V. Oxidative Dehydrogenation of 1-Butene on Iron-Containing Bismuth Molybdovanadate Catalysts

The oxidative dehydrogenation of 1-butene has been studied over catalysts with the scheelite structure and general formula Bi 1- x/3[ x/3-yMe y [V 1-x Mo x-y Fe y ]O 4 with x = 0.45; y = 0, 0.10, or 0.15; Me  Fe or Bi. Experiments were carried out in a pulse microreactor in the temperature range 473–673 K. The trend of total conversion with catalyst composition is similar to that found previously for the same catalysts in propene oxidation. Moreover, the total conversion was identical in the presence and in the absence of gaseous oxygen in the pulse (first pulse with 1-butene alone). On the other hand, the production of butadiene was markedly different from that of acrolein in the propene reaction for the catalyst in which the cationic vacancies were completely filled with bismuth ions ( y = 0.15, Me  Bi). This finding points to a differing importance of the cationic vacancies depending on whether the selective reaction occurs with oxygen insertion. Also the oxidation to isomerization ratios and the reduction-oxidation properties changed with catalyst composition. The results are discussed in terms of participation of cationic vacancies and iron ions in the steps of the allylic oxidation.

Development and screening of selective catalysts for the synthesis of clean liquid fuels

This article is a compilation of the research carried out under EEC contract EN3V-0400-D at the Institut fur Energieverfahrenstechnik in Julich and at the Faculty of Chemical Technology and Materials Science, Delft, concerning the development and screening of copper/cobalt-based catalysts for the synthesis of alcohol mixtures from syngas. Analogous work, based on copper/zinc oxide/alumina catalysts, has been performed at the Faculty of Chemical Technology in Twente University at Enschede. This work is described in a companion paper. Comparative tests of several catalysts in a pressure micropulse reactor and in a plug flow tubular reactor, carried out at the Institut fur Technische Chemie, TU Braunschweig, are presented in a second companion paper. In the discussion section of the present article the results obtained by the joint groups are compared with the initial objectives of the programme.

Calcination of Metal Oxide/Activated Carbon Catalysts Under Electromagnetic Fields

ABSTRACTEffects of microwave heating were investigated for calcining metal-oxide precursors that were supported on an activated carbon. To obtain the heterogeneous metal oxide/activated carbon catalysts, calcination of the precursors impregnated onto the activated carbon was executed for 10 minutes at 300°C within a cavity of 2.45 GHz microwave. Such an electromagnetic treatment was compared with the calcination in a conventional muffle furnace. Activities for the oxidation of the benzene or oil adsorbed on the catalysts prepared by the two methods were examined with a micro-pulse reactor as well as with thermogravimetric analysis. Experiments revealed that the time required for the calcination in the electromagnetic field is much shorter than that in the muffle furnace. The catalytic activities in the low-temperature oxidation were also enhanced by the application of microwave in the calcination. In addition, the oxidative reaction of the adsorbates could be obtained without the activated carbon being burned. The microwave effects were outstanding for Co3O4/activated carbon catalysts.

Hydroconversion of meta-xylene on catalysts containing Pt, Re and PtRe on H-mordenite

Hydroconversion of m-xylene on catalysts containing Pt, Re and PtRe on H-mordenite (HM) was investigated in a pulse micro-reactor in flowing hydrogen at atmospheric pressure and temperatures between 300 and 400°C. The extent of m-xylene hydroconversion, the selectivity for p-xylene production and the hydrodealkylation of m-xylene for the catalysts was of the order PtReHM ≫ PtHM > ReHM. Disproportionation of m-xylene did not occur on PtReHM but took place on the two other catalysts. Some ring saturation took place at lower temperatures on PtHM and PtReHM. Ethylbenzene was not detected in any reaction product.

A study of the properties of active sites on PtY/Al2O3

The catalytic performance and the properties of active sites on PtY/Al2O3 have been studied using a pulse microreactor, TEM and poisoning technique. The study shows that PtY/Al2O3 shows higher activity and selectivity for n-hexane conversion and the dehydrogenation of cyclohexane, as compared to Pt/Al2O3. The changes in PtY/Al2O3 geometric patterns and in both the total numbers of active sites (TNAS) and the active site distribution (ASD) caused by Y are likely to be responsible for the catalytic improvements.

Selectivity and Yield of Components of a Composite Cracking Catalyst: Effect of Interactions between Composite Components

Coking, activity, and product distribution of a composite catalyst and its components, an REY zeolite and a silica-alumina matrix, are studied in the cracking of n-hexadecane at 500°C in a pulse microreactor equipped with a thermogravimetric analysis unit (TGA), an integral sample collector, and a GC. The matrix component alone does not show significant activity beyond thermal cracking. The zeolite component has high activity and high coking rate. The composite catalyst shows much higher conversion, much higher yield to C 5-C 10, and much lower coking rate than the values predicted from its components. The activity and the product distribution are affected by coke deposition, and this effect depends on coke level of the catalysts. Product yields for the three catalysts as functions of coke levels allow us to determine yield enhancement factors and selectivity enhancement factors (referring to the zeolite component in the composite catalyst relative to the pure zeolite catalyst at the same zeolite coke level) for four product groups. Examination of the selectivity enhancement factors indicates how the heaviest products (C 17+) from the matrix component of the composite catalyst convert to kerosene, gasoline, and lightest (C 4−) product groups over the zeolite component of the composite catalyst, and how these interconversions change with the coke level on the zeolite.

Carbon dioxide hydrogenation on FeMn oxide catalyst doped with Rh and La

The hydrogenation of CO 2 was investigated on a pure FeMn oxide catalyst and on catalysts doped with Rh and La in a pulse microreactor. Temperature-programmed reduction of the catalysts showed that Rh addition led to a much faster and more complete reduction, by a probably catalytic effect of Rh, whereas the addition of La led to a slower and less complete reduction than in the case of the pure FeMn catalyst. This was also shown by XPS investigation of the surfaces of the reduced catalysts. Additionally, XPS showed an enrichment of the surfaces with Mn on all three catalysts and a strong increase in La surface concentration in comparison to the bulk of the La-containing catalyst. The formation of a higher amount of OH species was observed on the La-containing catalyst than on the other ones after reduction. Three subsequent steps were distinguished during hydrogenation of CO 2 pulses: a fast methanation connected with CO evolution, and two slower steps related to hydrogenation of carbonaceous surface species. The proportion of fast methanation was highest on FeMnRh and absent on the FeMnLa catalyst. A mechanism similar to the hydrogenation of CO combined with hydrogenolysis was postulated for the FeMn and FeMnRh catalysts. A different mechanism had to be assumed for the CO 2 hydrogenation on the FeMnLa catalyst.

Pulse microreactor studies on conversion of methane, ethane, and ethylene over rare earth oxides in the absence and presence of free oxygen

Rare earth oxides and rare earth promoted alkaline earth oxides show high activity in the oxidative coupling of methane (OCM) to C[sub 2] hydrocarbons. Oxidative coupling of methane has been covered in a number of recent reviews. In the authors' recent studies, rare earth oxides have been compared for their acid/base strength distribution and catalytic activity of the OCM process, and a mechanism for the formation of methyl radicals involving acid-base pairs on the catalyst surface has been suggested. According to this mechanism, the formation of the methyl radical involves the transfer of an electron from CH[sub 3][sup [minus]] to an oxygen molecule. Hence, for the rare earth oxide catalyst to be active in the OCM process, the presence of free oxygen is essential. Earlier, Lin et al. have observed the requirement of free O[sub 2] for the formation of methyl radicals on La[sub 2]O[sub 3] (at 773 K) in substantial concentrations. It is therefore very interesting to compare the catalytic activity/selectivity of the rare earth oxides in the conversion of methane and also of ethane and ethylene (which are the desirable products of the OCM) in the absence and presence of free oxygen for understanding the catalytic process. Thesemore » studies could be carried out very conveniently using a pulse microreactor. The present investigation was undertaken for this purpose. The rare earth metal oxide catalysts studied were La[sub 2]O[sub 3], CeO[sub 2], Sm[sub 2]O[sub 3], Eu[sub 2]O[sub 3], and Yb[sub 2]O[sub 3].« less

Activity, selectivity, and deactivation of high-sodium HY zeolite

The effect of deactivation on the activity and selectivity of (very high) Na + content HY zeolite catalysts was studied in a pulse microreactor equipped with a thermogravimetric analysis unit (TGA). Cumene was used as the reactant. The Na + level studied was from 3.2 to 11.8% of Na 2O. As the Na + content decreases, the acidity of the zeolite increases and deactivation decreases. The effect of coke level on selectivity to benzene, toluene, ethylbenzene, dimethylbenzene, ethyltoluene, cymene, and diisopropylbenzene depends on the Na + content. For example, the selectivity to benzene, the major aromatic formed by the fresh catalyst, decreases with increasing coke level and Na + level, while the selectivity to ethyltoluene increases under the same conditions. A maximum in selectivity with coke level is obtained for most other products. At intermediate Na + levels (around 7% Na 2O), ethylbenzene shows a maximum in selectivity while cymene passes through a minimum. The maximum coke deposition and the coke level for the maximum incremental coke deposition are two important parameters that distinguish conversion and selectivity patterns.