Jack H. Lunsford

Catalytic conversion of methane to more useful chemicals and fuels: a challenge for the 21st century

Abstract The very large reserves of methane, which often are found in remote regions, could serve as a feedstock for the production of chemicals and as a source of energy well into the 21st century. Although methane currently is being used in such important applications as the heating of homes and the generation of hydrogen for ammonia synthesis, its potential for the production of ethylene or liquid hydrocarbon fuels has not been fully realized. A number of strategies are being explored at levels that range from fundamental science to engineering technology. These include: (a) stream and carbon dioxide reforming or partial oxidation of methane to form carbon monoxide and hydrogen, followed by Fischer–Tropsch chemistry, (b) the direct oxidation of methane to methanol and formaldehyde, (c) oxidative coupling of methane to ethylene, and (d) direct conversion to aromatics and hydrogen in the absence of oxygen. Each alternative has its own set of limitations; however, economical separation is common to all with the most important issues being the separation of oxygen from air and the separation of hydrogen or hydrocarbons from dilute product streams. Extensive utilization of methane for the production of fuels and chemicals appears to be near, but current economic uncertainties limit the amount of research activity and the implementation of emerging technologies.

Partial oxidation of methane to carbon monoxide and hydrogen over a Ni/Al2O3 catalyst

Abstract Partial oxidation of methane occurs in the temperature range 450–900°C by reaction of an oxygen-deficient CH 4 /O 2 mixture over a 25 wt% Ni/Al 2 O 3 catalyst. Carbon monoxide selectivities approaching 95% and virtually complete conversion of the methane feed can be achieved at temperatures >700°C. The oxidation state and phase composition of the catalyst were characterized using X-ray photoelectron spectroscopy and X-ray powder diffractometry. This study revealed that, under operating conditions, the previously calcined catalyst bed consists of three different regions. The first of these, contacting the initial CH 4 /O 2 /He feed mixture, is NiAl 2 O 4 , which has only moderate activity for complete oxidation of methane to CO 2 and H 2 O. The second region is NiO + Al 2 O 3 , over which complete oxidation of methane to CO 2 occurs, resulting in an exotherm in this section of the bed. As a result of complete consumption of O 2 in the second region, the third portion of the catalyst bed consists of a reduced Ni/Al 2 O 3 phase. Formation of the CO and H 2 products, corresponding to thermodynamic equilibrium at the catalyst bed temperature, occurs in this final region, via reforming reactions of CH 4 with the CO 2 and H 2 O produced during the complete oxidation reaction over the NiO/Al 2 O 3 phase.

Synthesis of dimethyl ether (DME) from methanol over solid-acid catalysts

The catalytic conversion of methanol to dimethyl ether (DME) has been studied over a series of solid-acid catalysts, such as γ-Al2O3, H-ZSM-5, amorphous silica-alumina, as well as titania modified zirconia. All the catalysts are active and selective for DME formation. The apparent activation energy for DME formation over γ-Al2O3 is ca. 25 kcal/mol, a value that increases to ca. 37 kcal/mol upon the addition of 23 Torr of H2O to the reagent. The rate of methanol dehydration decreases with increasing acidity (silica content) over the amorphous silica-alumina catalysts. Although H-ZSM-5 with Si/Al = 25 is the most active among the catalysts tested, the DME selectivity is only 20% at 280°C, a typical temperature used in the syngas-to-methanol process. An amorphous silica-alumina catalyst with 20 wt.-% silica content (SIRAL20) exhibits the best catalytic performance of those tested at 280°C.

The catalytic conversion of methane to higher hydrocarbons

A process for making ethylene, ethane and other higher hydrocarbons from methane is disclosed. In this process methane is contacted with oxygen in the presence of at least one metal of Group IB of the Periodic Table of the Elements, with the proviso that if the metal is copper an additional Group IB must be present, a metal-containing chloride or a metal-containing compound capable of being formed into a metal-containing chloride in situ, where the metal is manganese, an alkali metal, an alkaline earth metal or a rare earth metal of the lanthanide series, and a volatile halide.

ESR of Adsorbed Oxygen Species

Abstract Although the catalytic oxidation of inorganic molecules, as well as the oxidation and oxidative dehydrogenation of hydrocarbons, has been the subject of extensive research, the role of oxygen in surface reactions remains uncertain. Various forms of adsorbed oxygen have been proposed on the basis of kinetic and adsorption data or electrical conductivity measurements; yet, very little direct spectroscopic evidence is available apart from that provided by electron spin resonance (ESR) spectroscopy.

The effect of sodium poisoning on dealuminated Y-type zeolites

Dealuminated zeolites prepared both hydrothermally and using silicon tetrachloride were investigated to determine the effect of sodium addition on their catalytic activities. These materials were found to have drastically reduced activities upon sodium addition with each sodium atom effectively neutralizing the catalytic activity previously thought to be due to five framework Broensted acid sites. These results are interpreted to mean that isolated framework Al atoms are a necessary but insufficient condition for strong acidity, and only about one-fifth of the framework Al atoms are associated with this strong acidity. Extraframework Al is believed to impart, through inductive effects, strong acidity to these framework Broensted sites. An infrared study of the hydroxyl region revealed bands at 3,675 and 3,595-3,610 cm{sup {minus}1} ({plus minus}2 cm{sup {minus}1}) that are attributed to protons associated with different types of extraframework aluminium oxide or oxyhydroxide. A band at 3,695 cm{sup {minus}1} is attributed to Na{sup +} interacting with water which is hydrogen-bonded to a framework oxygen atom. A narrow band at 3,602 cm{sup {minus}1} is attributed to a highly acidic Broensted site in the zeolite lattice.

Determination of framework aluminium content in dealuminated Y-type zeolites: a comparison based on unit cell size and wavenumber of i.r. bands

Abstract The framework aluminium content of normal and dealuminated zeolite-Y may be obtained from 29Si n.m.r. data, unit cell constants or positions of infrared bands due to TO stretching modes. Using n.m.r. data as the primary means for determining framework aluminium the following equations were obtained: N AI =107.1(a o −24.238) N AI =0.766(1086.7− v 1 ) N AI =1.007(838.8− v 2 ) where NAl is the numbers of aluminium ions per unit cell, ao is the unit cell constant in A, ν1 is the wavenumber for the asymmetric OTO mode and ν2 is the wavenumber for the symmetric mode.

Methane and methanol synthesis over supported palladium catalysts

Abstract Hydrogenation of carbon monoxide has been studied between 260 and 340 °C and 5 and 50 atm of pressure over palladium supported on three different silicas and on HY and NaY zeolites. Fresh and used catalysts were characterized by chemisorption, temperature-programmed desorption, X-ray diffraction, electron microscopy, and photoelectron spectroscopy studies. The selectivity and the activity of the catalysts are strongly dependent on the nature of the support and on the state of the metal on its surface. Methanol is produced on the catalysts exhibiting small size crystallites on which CO is weakly adsorbed, whereas the formation of methane is directly related to the density of acidic sites at the surface of the support. Under the experimental conditions the palladium undergoes structural and electronic modifications due to transformations into hydride phases. These transformations lead to a cracking of the metal crystallites and to changes in the reaction rate expressions.

Catalytic conversion of methane to benzene over Mo/ZSM-5

Dehydroaromatization of methane to benzene occurs over a 2 wt% Mo/ZSM-5 catalyst at 700‡C under non-oxidizing conditions. Following an initial induction period, during which CH4 reactant reduces the original Mo6+ ions in the zeolite to Mo2C and deposition of coke occurs, a benzene selectivity of ∼ 70% at a CH4 conversion of 8–10% could be sustained for more than 16 h. X-ray photoelectron spectroscopy and X-ray powder diffraction measurements indicate that the reduced Mo is highly dispersed in the channels of the zeolite. Initial activation of CH4 reactant occurs on Mo2C sites, leading to the formation of C2H4 as the primary product. The latter then undergoes subsequent oligomerization reactions on acidic sites of the zeolite to form aromatic products.

Oxidative dimerization of methane over sodium-promoted calcium oxide

Abstract Sodium-promoted calcium oxides are active and selective catalysts for the partial oxidation of methane to ethane and ethylene using molecular oxygen as an oxidant. In a conventional fixed-bed flow reactor, operating at atmospheric pressure, a 45% C 2 (sum of ethane and ethylene) selectivity was achieved at a 33% methane conversion over 2.0 g of 15 wt% Na CaO catalyst at 725 °C with a gas mixture of CH 4 O 2 = 2 . The other products were CO, CO 2 , and H 2 . EPR results indicated that [Na + O − ] centers in Na CaO are responsible for the catalytic production of CH 3 · from methane via hydrogen atom abstraction. These CH 3 · radicals dimerize, primarily in the gas phase, to form C 2 H 6 , which further oxidizes to C 2 H 4 . Increasing temperatures reverse the gas-phase equilibrium CH 3 · + O 2 ⇄ CH 3 O 2 · to produce more CH 3 · and increase the C 2 selectivity. The CH 3 O 2 · eventually is converted to carbon oxides under the reaction conditions employed; therefore, increasing O 2 pressures decrease the C 2 selectivity. There is evidence that CH 3 O 2 · in the presence of C 2 H 6 initiates a chain reaction that enhances the methane conversion. The addition of Na + to CaO also reduces the surface area of the catalysts, thus minimizing a nonselective oxidation pathway via surface methoxide intermediates.