Joe W. Hightower

Oxidative dehydrogenation of butenes over magnesium ferrite kinetic and mechanistic studies

Abstract Deuterium and 14C-labeled isotopic tracers were used in kinetic experiments to study the oxidative dehydrogenation (OXD) of n-butenes to 1,3-butadiene over a MgFe2O4 catalyst in the temperature range 300–400 °C. The OXD reaction is approximately zero order in O2 when O2 is in excess, and it is near first order in butene when the partial pressure of the hydrocarbon is relatively low. OXD occurs even in the absence of gaseous O2 over an oxidized catalyst, but the reaction becomes quite slow when the surface is reduced. Use of [14C]butadiene confirmed that both butene and butadiene are directly oxidized to CO2 and water. Large kinetic isotope effects ( k h k d ~ 2 ) indicate that both OXD and n-butene isomerization involve CH cleavage in the rate-limiting step, but there is no H D scrambling in the product molecules when the butene reactant is an equimolar mixture of C4H8 and C4D8. Most of the observations can be accounted for by a modified Rennard-Massoth mechanism involving an oxidation-reduction cycle between Fe2+ and Fe3+ with C4H7, C4H6−, and OH species on the surface.

The catalytic decomposition of petroleum into natural gas

Abstract Petroleum is believed to be unstable in the earth, decomposing to lighter hydrocarbons at temperatures > 150°C. Oil and gas deposits support this view: gas/oil ratios and methane concentrations tend to increase with depth above 150°C. Although oil cracking is suggested and receives wide support, laboratory pyrolysis does not give products resembling natural gas. Moreover, it is doubtful that the light hydrocarbons in wet gas (C 2 C 4 ) could decompose over geologic time to dry gas (> 95% methane) without catalytic assistance. We now report the catalytic decomposition of crude oil to a gas indistinguishable from natural gas. Like natural gas in deep basins, it becomes progressively enriched in methane: initially 80% (wet gas) to a final composition of 100% methane (dry gas). To our knowledge, the reaction is unprecedented and unexpectedly robust (conversion of oil to gas is 100% in days, 175°C) with significant implications regarding the stability of petroleum in sedimentary basins. The existence or nonexistence of oil in the deep subsurface may not depend on the thermal stability of hydrocarbons as currently thought. The critical factor could be the presence of transition metal catalysts which destabilize hydrocarbons and promote their decomposition to natural gas.

1,3-Butadiene selective hydrogenation over Pd/alumina and CuPd/alumina catalysts

Abstract The selective hydrogenation of 1, 3-butadiene was studied in the presence of a 10:1 excess of 1-butene over supported bimetallic catalysts containing palladium in a vapour phase atmospheric plug flow reactor in the temperature range 288–313 K. Addition of copper to alumina-supported Pd increases its selectivity for converting butadiene to n-butenes without saturating or isomerizing the n-butenes. For example, with atomic ratio Cu/Pd = 2, the bimetallic catalyst selectively converts > 99% of the diene to n-butenes while isomerizing or saturating less than 1% of the starting 1-butene. Under similar conditions, Pd alone maintains high n-butene selectivity to only about 50% conversion of the diene. Over both catalysts the kinetics of 1, 3-butadiene hydrogenation are first order in hydrogen and zero order in butadiene. Hydrogen sorption experiments showed that Cu addition decreases both the surface adsorption and the bulk absorption, the former suggesting surface decoration of Pd by Cu and the latter indicating significant bulk Cu-Pd interaction in the supported bimetallic samples.

Sorption and catalysis on sodium-montmorillonite interlayered with aluminum oxide clusters

Abstract Exchanging aluminum hydroxyl cations in Na-bentonite pioduces an expanded clay mineral characterized by a pore volume of 0.16–0.20 cm3g-1 and The pillared bentonite has high initial activity for methanol conversion to olefins, but is substantially deactivated within an hour by coke deposition

Tracer studies of acid-catalyzed reactions. Part 10.—Deuterium exchange and isomerization of cyclic olefins over alumina

The mechanisms of double-bond migration and the exchange of D2 with the hydrogen atoms of small cyclic olefins (methylenecyclobutane, methylenecyclopentane, cyclopentene, 3-methylcyclopentene, cyclohexene, and bicyclo-(2,2,1)-hepta-2, 5-diene) were investigated over a pure alumina catalyst. At temperatures below 100°, only those hydrogen atoms which were initially vinyl, or which could become vinyl by isomerization of the olefin, underwent exchange. A primary kinetic isotope effect of about 2.8 was found, indicating that cleavage of the C—H bond was probably the slow step in the exchange reaction. Rapid intermolecular scrambling of all vinyl hydrogen atoms was observed, and the presence of the olefins greatly reduced the usually fast rate of H2+ D2 equilibration. Double-bond migration below 100° was sensitive to the geometry of the olefin. Only molecules having a three carbon chain, including the double bond, which could appear concave when viewed from outside the molecule underwent isomerization; those which did not fulfill this requirement did not isomerize. Nearly pure 1,2-cyclopentene-d2 could be prepared, suggesting a dissociative mechanism for the exchange reaction. Hence, exchange and double-bond migration are independent processes, although both reactions may have involved the same sites and may have had a common intermediate. Poisoning experiments using radioactive CO2 indicated an active site density of about 1.4 × 1013 cm–2 for the exchange reaction after 530° pretreatment.

Ferrite spinels as catalysts in the oxidative dehydrogenation of butenes

Both CoFe2O4 and CuFe2O4 spinels are active catalysts for oxidative dehydrogenation of n-butenes to butadiene. However, both suffer from their tendency to catalyze total oxidation as well. Neither is an effective n-butene isomerization catalyst, even at high temperatures. Surface and lattice oxygen atoms participate in the reactions, and the steady state in the presence of gaseous oxygen, at least for CoFe2O4, is near the totally oxidized state. Selectivity for the dehydrogenation reaction is strongly dependent on the presence of gaseous oxygen in the case of the Co catalyst, although the selectivity over the Cu catalyst is not a strong function of gaseous oxygen. The kinetics are not simple; the pressure dependencies are less than first order in both butene and oxygen pressures and the reactions are inhibited by the presence of butadiene. CH bond cleavage is probably involved in the rate-determining step of all reactions, although the reactions showed some characteristics of both interand intramolecular mechanisms.

Oxidative dehydrogenation of butenes over magnesium ferrite: Catalyst deactivation studies

Abstract Magnesium ferrite (MgFe 2 O 4 ) is a moderately selective catalyst for butadiene formation from n -butenes via oxidative dehydrogenation (OXD). However, both the activity and selectivity decrease irreversibly as the catalyst ages, and these changes have been correlated with bulk phase solid-state changes that occur in reducing atmospheres at temperatures above 300 °C. The fresh catalyst contained a 5% α-Fe 2 O 3 impurity phase whose disappearance paralleled the activity/selectivity decline. These conclusions were confirmed by X-ray, Mossbauer, and magnetic susceptibility measurements. In addition, TGA (thermogravimetric analysis) experiments suggested that oxygen equivalent to about 6.4 layers could be slowly removed by evacuation at elevated temperatures in the range of 530 °C. This labile oxygen is probably related to the OXD activity. A proposed deactivation mechanism attributes the loss of selectivity to an increased population of Fe 2+ sites at the expense of Fe 3+ sites. A process that would stabilize the Fe 3+ population in the MgFe 2 O 4 matrix would presumably decrease the deactivation and result in a more stable catalyst.

The dual-site nature of gamma alumina catalysts: Isomerization vs D2 exchange reactions of 1-butene

Additional evidence has been obtained to support the contention that there are at least two types of active sites on alumina which catalyze different types of reactions and which are quite independent of each other. Two test reactions were employed: 1-butene isomerization, and the exchange of olefinic H atoms in the butene molecules with D atoms from D2 gas. Small amounts of adsorbed CO2 (corresponding to coverages of less than 1.6 × 1013 molecules/cm2) completely eliminated the exchange reaction but had very little effect on either the absolute rate or the selectivity of the isomerization reaction. When the catalysts were exposed to comparable concentrations of SO2, HCl, NH3, or pyridine, at reaction temperature (~25 °C), there was no signficant effect on the rates of either isomerization or exchange. Speculation about the two types of sites for these reactions are discussed.

Participation of support sites in hydrogenation of 1,3-butadiene over Pt/Al2O3 catalysts

Abstract The effect of acidic sites on activity and product selectivity has been investigated in 1,3-butadiene hydrogenation over platinum catalysts prepared with high purity Al2O3 supports (Condea SB, Woelm N, Degussa Alumina C, ICN-N). Degassing the reduced samples at 723 K has increased the selectivity of n-butane formation on catalysts of high platinum dispersion (DCO=68–93%). Over Woelm and ICN aluminas isomerization of 1-butene formed on platinum sites to cis- and trans-2-butene can also be observed. Poisoning the catalysts with water, ammonia and ethylenediamine (EDA) suppresses isomerization of 1-butene, but only EDA hinders markedly formation of n-butane. The results have been interpreted by participation of Lewis acid sites and by contribution of electron deficient platinum sites in the vicinity of the metal-support interface.

Nitric oxide reduction by methane over Rh/Al2O3 catalysts

Abstract The mechanism of nitric oxide reduction with methane has been investigated over an alumina-supported rhodium catalyst. A series of kinetic studies were performed using initial rate data obtained in a recirculation reactor. Between 300 and 400 °C NO elimination is initially fast over a reduced catalyst, but the reaction rate rapidly decreases due to oxidation of the catalyst surface. The decomposition is apparently a noncatalytic stoichiometric reaction between nitric oxide and surface rhodium atoms. The initial rate of disappearance of NO is adequately described by a dualsite Langmuir-Hinshelwood expression. In presence of reducing agents such as CO or CH 4 , oxygen is effectively removed as CO 2 (plus H 2 O). In the reduction of NO with CH 4 , the initial rate of NO disappearance fits the following empirical rate expression Rate = Ae − E RT (P No ) −0.63 (P CH 4 ) where A = 3.57 × 10 3 NO Rh s · sec · ( N m 2 ) 0.37 and E = 77 kJ/mole. A deuterium isotope effect of 1.9 is observed in the reduction of NO with mixtures of CH 4 and CD 4 . This, along with the linear rate dependence on CH 4 partial pressure, indicates that the dissociative adsorption of CH 4 is the rate limiting step of the reaction. An experiment run with a 15 NO, N 2 O, and CH 4 mixture indicated that N 2 O is not an exclusive gas phase intermediate in the pathway to N 2 formation from NO. However, all these results are consistent with N 2 O being a true surface intermediate. A reaction mechanism is proposed for NO reduction by methane. It is based on the assumption that two adsorbed NO molecules disproportionate to (N 2 O) a + (O) a . Adsorbed (N 2 O) a either desorbs as N 2 O or decomposes to N 2 and (O) a . The role of the reductant is to remove the strongly adsorbed (O) a and to keep the catalyst in an active reduced state for NO reaction.