METHANOL CONVERSION

FIELD OF THE INVENTION

The invention relates generally to direct conversion of methane to methanol. In particular, the invention relates to low temperature methods and systems for the direct selective oxidation of methane to methanol.

BACKGROUND OF THE INVENTION

Methane is a major component of natural gas and its relative abundance makes it an important source of energy. However, at ambient temperature and pressure, methane is a gas and therefore difficult to transport. One solution to this problem is the oxidation of methane to methanol. At present, this oxidation process involves an indirect method by reacting methane with steam at high temperatures (e.g. 850 degrees Celsius) and high pressures (e.g. 10-20 atm) to produce syngas, a mixture of H ₂ and CO. Methanol is subsequently formed by heating syngas in a high-pressure environment (e.g. 50-100 atm).

It would be desired to avoid the formation of the intermediate syngas allowing for a direct oxidation of methane to methanol. However, such a direct oxidation process has been a challenge, for example due to the strong C-H bond of methane compared to other hydrocarbons. Accordingly, there is a need in the art to develop new techniques whereby direct oxidation of methane to methanol can take place to avoid the high temperatures and pressures currently required for this conversion. The present invention addresses this need.

SUMMARY OF THE INVENTION

The invention provides a method of directly converting methane to methanol. An oxygen- activated catalyst is created by heating a catalyst at a first temperature in an oxidizing environment. The oxidizing environment contains less than 10 ppm water or less than 3 ppm water. A methane-containing gas stream is passed at a second temperature over the oxygen-activated catalyst to directly form methanol. Both the heating and passing step are carried out at ambient pressure. The first temperature and the second temperature are less than 300 degrees Celsius. In other examples, the first temperature or the second temperature could be in a temperature range of about 175 degrees Celsius to about 250 degrees Celsius, of about 150 degrees Celsius to about 250 degrees Celsius, of about 100 degrees Celsius to about 250 degrees Celsius, or both in the same temperature range.

Prior to the creation of the oxygen-activated catalyst, the catalyst is pre-treated. This pre- treatment involves heating of the catalyst in a gaseous environment with continuous gas flow, at a pre-treatment temperature range of about 370 degrees Celsius to about 750 degrees Celsius and at ambient pressure (preferably about 400 degrees Celsius to about 500 degrees Celsius in a continuous gas flow). The gaseous environment contains less than 10 ppm water or less than 3 ppm water. In one example, the catalyst is a Cu-based zeolite (e.g. Cu-ZSM-5, Cu-ZSM-5 with a Si/Al ratio of about 12, Cu-MOR, Cu-ZSM-11 , Cu-ZSM-12 or equivalent Cu-zeolites with different Si/Al ratios). The invention also provides a method of creating an oxygen-activated catalyst suitable for direct conversion of methane to methanol at ambient pressure. In this method a catalyst is pre-treated by heating the catalyst in a gaseous environment with continuous gas flow and at a pre-treatment temperature range of about 370 degrees Celsius to about 750 degrees Celsius. The gaseous environment contains less than 10 ppm water or less than 3 ppm water.

The pre-treated catalyst is then heated at a temperature less than 300 degrees Celsius in an oxidizing environment (with less than 10 ppm water or less than 3 ppm water) to form the oxygen-activated catalyst. In other examples, the temperature could be in a temperature range of about 175 degrees Celsius to about 250 degrees Celsius, of about 150 degrees Celsius to about 250 degrees Celsius, of about 100 degrees Celsius to about 250 degrees Celsius, or the temperature could be in the same range as a temperature range that is used for the process of direct methane to methanol conversion. The oxygen-activated catalyst could be stored in an atmosphere containing less than 10 ppm water or less than 3 ppm water conditions.

The invention further provides a chemical processing plant for direct conversion of methane to methanol. The plant includes a storage unit for methane gas, a storage unit for oxygen- activated catalyst (stored in an atmosphere containing less than 10 ppm water or less than 3 ppm water), and a unit for passing the methane gas over the oxygen-activated catalyst from the respective storage units at a temperature of less than 300 degrees Celsius and an ambient pressure for the direct conversion of methane gas into methanol. The plant could further include: (i) a methanol removing unit for removing the methanol from the passing unit, (ii) a unit for regenerating at least one active site in the oxygen-activated catalyst in an oxidizing environment containing less than 10 ppm water or less than 3 ppm water and at temperatures in a range of 100-250 degrees Celsius, or a combination of (i) and (ii). In this invention, the oxygen-activated catalyst (i.e. composition of matter) includes at least one active site characterized by having an absorption band in the region of 18000 cm ^-1 to 26500 cm ^-1 . The absorption band is further characterized by having an associated resonance- enhanced Raman spectrum that includes oxygen-isotope dependent vibrational features in a first range of 425 cm ^-1 to 475 cm ^-1 and a second range of 845 cm ^-1 to 895 cm ^-1 . The vibrational features in the second range are less intense compared to the vibrational features in the first range by a factor of at least two. The oxygen-activated catalyst has at least one active site identified as a mono-(μ-oxo)dicupric core.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the process steps involved in the direct selective conversion methane to methanol according to an embodiment of the invention.

FIG. 2 shows an example of a chemical processing plant for the direct selective conversion of methane to methanol according to an embodiment of the invention.

FIG. 3 shows according to an example of the invention resonance-enhanced Raman

(rR) spectra of oxygen-activated Cu-ZSM-5 at a wavelength of 457.9nm.

INSERT A in FIG. 3 shows according to an example of the invention absorption spectrum of oxygen-activated Cu-ZSM-5. INSERT B in FIG. 3 shows according to an example of the invention rR spectra of oxygen activated Cu-ZSM-5 formed with mixed oxygen isotope (" ¹⁶ · ¹⁸ 0 ₂ " (110)), and 1 : 1 normalized sum of ¹⁶ 0 ₂ and ¹⁸ 0 ₂ (120).

FIG. 4A shows according to an example of the invention of the rR spectra of 0 ₂ -activated Cu-ZSM-5 (Cu/Al=0.54) at various wavelengths from 351 nm to 568 nra with corresponding absorption spectrum in FIG. 4B shows according to an example of the invention rR spectra at 457.9 nm with varying Cu/Al ratio in Cu-ZSM-5. Corresponding absorption spectra are shown in FIG 4C shows 0 ₂ -activated Cu-ZSM-5 recorded before and after heating in He at 450 degrees Celsius and after reaction with methane at 200 degrees Celsius.

shows according to an example of the invention the rR spectra of Cu-ZSM-5 activated in 0 ₂ (top) and N ₂ 0 (bottom). INSERT A in FIG. 5 shows according to an example of the invention diffuse reflectance UV-vis spectra recorded at room temperature after treatment of Cu-ZSM-5 with 0 ₂ (top) and N ₂ 0 (bottom). The arrows represent the increasing temperature of the 0 ₂ and N ₂ 0 treatment (temperature difference between two spectra is 25 degrees Celsius).

shows according to an example of the invention structural models of ZSM-5 and the Cu ₂ 0 intermediate used for DFT calculations. A represents a small model, T-Cu ₂ 0. B is a 10-membered ring of ZSM-5 containing two Al-sites separated by two Si- sites. C is a large model constructed from part of a 10- membered ring (boxed atoms, B.), L-Cu ₂ 0. FIG. 7 shows according to an example of the invention the absorption band of an

0 ₂ -activated Cu-ZSM-5 (Cu/Al=0.54) during reaction at 175 degrees Celsius C (A) with CH ₄ and (B) with CD ₄ indicating decay of the 22,700 cm ^-1 absorption feature. The arrow on the figures shows the evolution in time. Time interval between any two spectra is 15 seconds. (C) Arrhenius' plots for CH ₄ (□) and CD ₄ (Δ ).

FIG. 8 shows according to an example of the invention reaction coordinate of H- atom abstraction from CH4 by singlet L-Cu ₂ ^{1 1} O. The relevant O-H and C-H distances are given.

DETAILED DESCRIPTION

The invention provides a process for the direct selective oxidation of methane to methanol at low temperatures (FIG. 1). The process takes place in two steps. In step 1, a pre-treated catalyst is heated in an oxidizing atmosphere to form an oxygen-activated catalyst. This step is crucial as it leads to the formation of an active site in the oxygen-activated catalyst, which facilitates the direct conversion of methane to methanol. Next, in step 2, methane gas is passed over the oxygen-activated catalyst to directly form methanol. The entire reaction (i.e creation of the active site (step 1) and passing methane gas (step 2)) is carried out at temperatures below 300 degrees Celsius and at ambient pressure.

In one embodiment, the catalyst is an oxide catalyst containing Cu. The catalyst can include aluminosilicates, such as zeolites. In the invention, pre-treatment of the catalyst is accomplished by heating the catalyst in a gaseous environment with continuous gas flow, at an ambient pressure and at a temperature range of about 370 degrees to 750 degrees Celsius. Preferably, the temperature is in a range of about 400 degree Celsius to about 500 degree Celsius. The gaseous environment could be helium, nitrogen, oxygen or nitrous oxide or a mixture of any of these with less than 10 ppm water or preferably less than 3 ppm water. The oxidizing atmosphere could be one of oxygen or nitrous oxide. For heat treatment in oxygen, the temperature is preferably between 175 to 250 degrees Celsius. For heat treatment in nitrous oxide, the temperature is between 100 to 250 degrees Celsius. In one aspect of the invention, the oxidizing atmosphere has less than 10 ppm water vapor. Preferably, the oxidizing atmosphere has less than 3 ppm water.

According to the invention, the oxygen-activated catalyst could be stored in an environment comprising less than 10 ppm water or less than 3 ppm water and could be used in a chemical processing plant for the production of methanol from methane. FIG. 2 shows a chemical processing plant of the invention. The plant has a storage unit for methane gas, passing unit for reaction of methane gas and the oxygen-activated catalyst. The unit also includes a methanol-removing unit for extracting the methanol gas produced in the reaction. In one example, a regeneration unit is used for regeneration of the active site. Thus methanol production could be enhanced by continuously recycling the process.

In the invention, the oxygen-activated catalyst was characterized by Absorption and resonance-Raman (rR) Spectroscopy (FIG. 3). INSERT A in FIG. 3 shows the resonance- enhancement of Raman (rR) vibrations obtained by tuning a laser (λ _ex = 457.9 nm) to the characteristic absorption feature associated with oxygen-activated Cu-ZSM-5. The oxygen activation in this case was done in an oxidizing atmosphere containing oxygen. INSERT A in FIG. 3 shows absorption spectrum of oxygen-activated Cu-ZSM-5 at an absorption band in the region between 18,000 cm ^-1 to 26,500 cm ^-1 indicating at least one active site in this region. Some of these resonance-enhanced vibrations are sensitive to isotope perturbation when the active site is generated with the oxygen isotope, ¹⁸ 0 ₂ . The most intense isotope- sensitive vibration is at 456 cm ^-1 (Δ( ¹⁸ 0 ₂ )= 8 cm ^-1 ) which supports its assignment as the symmetric stretch of an oxo group bridging two Cu centers. The weak 870 cm ^-1 vibration is then assigned as an antisymmetric metal-oxo stretch (v _as ) which should not be enhanced in the rR spectrum. The presence of both strong symmetric and weak antisymmetric stretches leads to the assignment of the Cu-ZSM-5 active site as a bent Cu-O-Cu core. All presently known copper-oxygen active site structures for the oxygen intermediate of Cu-ZSM-5 are excluded by the rR data.

FIG. 4A shows rR spectra of 0 ₂ -activated Cu-ZSM-5 (Cu/Al=0.54) at various wavelengths from 351 nm to 568 nm with corresponding absorption spectrum shown in FIG. 4B shows rR spectra at 457.9 nm with varying Cu/Al ratio in Cu-ZSM-5. These vibrations gain intensity with increasing Cu/Al ratio (as does the absorption feature shown in INSET) and are not observed after the site reacts with methane (at 200 degrees Celsius) or is heated in He at 450 degrees Celsius (FIG 4C) both of which lead to loss of the absorption feature at 22,700 cm ^-1 confirming that the vibrations observed are from the active site.

Activation of Cu-ZSM-5 by N ₂ 0 instead of 0 ₂ also results in the formation of an absorption feature at 22,700 cm ^-1 (FIG. 5, INSERT A). rR spectra were collected on the N ₂ 0-activated core of Cu-ZSM-5 and compared to the rR vibrations observed for the 0 ₂ -activated core. FIG. 5 shows the rR spectra of Cu-ZSM-5 activated in 0 ₂ (top) and N ₂ 0 (bottom). INSERT A of FIG. 5 shows diffuse reflectance UV-vis spectra recorded at room temperature after treatment of Cu-ZSM-5 with 0 ₂ (top) and N ₂ 0 (bottom) in the temperature range 100 to 200 degrees Celsius and 50 to 125 degrees Celsius respectively. The arrows on INSERT A of FIG. 5 represent the increasing temperature of the 0 ₂ and N ₂ 0 treatment (temperature difference between two spectra is 25 degrees Celsius). As can be seen, activation sets in at a lower temperature for N ₂ 0-treated samples (100 degrees Celsius) as compared to 0 ₂ -treated samples (175 degrees Celsius). Further, identical rR features are observed for both 0 ₂ and N ₂ 0 treatments indicating that the active site can be generated by either 0 ₂ or N ₂ 0. The N ₂ 0- generated active core is also capable of low-temperature selective oxidation of methane to methanol to an extent comparable to the 0 ₂ activated Cu-ZSM-5 site.

Normal coordinate analysis (NCA) that was used to correlate the observed rR symmetric (v _s ) and antisymmetric (v _as ) vibrations and their isotope shifts to the bending angle for a Cu ₂ 0 core, ^CuOCu. The symmetric (v _s ) and antisymmetric (v _as ) stretch energies correlate with ^CuOCu, and by fitting the four observables (v _s and v _as with ¹⁶ 0 ₂ and ¹⁸ 0 ₂ ) to NCA calculations, the observed stretches are consistent with a ^CuOCu of 140°. TABLE 1 shows NCA predicted Vs and Vas as a function of ^Cu-O-Cu. These NCA calculations were used to evaluate the dependence of v _s and v _as of a Cu ₂ 0 site on ^CuOCu. As this angle decreases from 140 degrees to 100 degrees, the predicted v _s increases to 600cm ^-1 . However, the observed v _s and v _as for the active site of Cu-ZSM-5 require that this site has a wide ^CuOCu (~140 degrees), thereby excluding the possibility of a second oxygen atom bridge. NCA calc'd NCA calc'd

Cu-O-Cu

v _e (A ⁸ O _j ) /cm ¹ v^ ^OJ /cm- ¹

160° 357 (-4) 909 (-46)

140° 446 (-12) 872 (-44)

120° 554 (-21 ) 813 (-40)

100° 609 (-25) 775 (-37)

TABLE 1. NCA predicted Vs and Vas as a function of ^Cu-O-Cu.

Using the NCA-calculated angle of 140° as a starting point, density functional theory (DFT) calculations were done to evaluate the capacity of the lattice to host a bent Cu-O-Cu core. Specifically, the placement of the active site in the crystal structure of Cu-ZSM-5 has been studied using two structural models.

FIG 6A shows a model with a tetrahedral Al(OH) ^4- ligand (referred to as an Al T-site) bound bidentate to each Cu atom of the Cu ₂ 0 core. The effect of the ^CuOCu of the T-Cu2 ^{1 1} O model on the DFT-calculated v _s and v _as has been evaluated and found that, in agreement with the NCA calculations, an ^CuOCu of 138 degrees results in the best correlation with experimental data. FIG. 6B shows a 10-membered ring of ZSM-5 containing two Al-sites separated by two Si-sites. Within the 10-membered ring of ZSM-5, the experimentally calibrated NCA and DFT calculated structural parameters most closely fit an active site bound to two Al T-sites connected by two intermediate Si-T-sites. Thus a larger model, as shown in FIG. 6C ("L-Cu ₂ 0") was formed with each Al T-site ligand extended with covalently bonded Si T-sites. The L-Cu2 ^ln O also electronically relaxes to a Cu ₂ ^{1 1} O core upon optimization, confirming that the ZSM-5 lattice can only stabilize a Cu ₂ ^{1 1} O core. The calculated ^CuOCu is 139 degrees with no constraints imposed on this core.

Thus, based on the proposed structural models, the calculated values for ^CuOCu are found to be in agreement with the data obtained from NCA calculations.

TABLE 2 shows the experimentally observed vibrational values for the rR features and the DFT vibrational predictions based on the proposed structural models. The calculated v _s (456cm ^-1 ) and v _as (852cm ^-1 ) and ¹⁸ 0-isotopic shifts (5 and 37 cm ^-1 respectively) for L-Cu ₂ ^{1 1} O agree well with the experimentally observed values for these vibrations (v _s =456cm ^{-1 -1} (Δ( ¹⁸ 0 ₂ ) = 40 cm ^-1 )). Further, L-Cu ₂ ⁿ O has calculated Al- T-site vibrations between 556-568 cm- , consistent with the observed O-isotope insensitive vibrations at 514cm ^-1 and 540cm ^-1 . The Cu-O-Cu bending mode of L-Cu ₂ ^{1 1} O is calculated at 253cm ^-1 (Δ( ^ι8 θ2) = 2cm ^{- 1} ), in agreement with the observed weak vibration at 237cm ^-1 indicates that the lattice restricts the coordination environment of the bound Cu atoms and their spatial orientation. The energies, intensities and isotope shifts of the observed vibrations lead to the assignment of this core as a mono-oxygen bridge binuclear Cu-sites and identified as a mono ^-oxo)dicupric core.

TABLE 2. Experimentally observed rR features and DFT vibrational predictions ( ¹⁸ 0- isotopic shifts in parentheses) and optimized geometric parameters for singlet L-Cm'O. Thus, spectroscopic measurements as well as calculations based on the proposed structural data clearly indicate the presence of an active site identified as a mono ^-oxo)dicupric core. Calculation of the activation energy (E _a ) of the oxygen-activated catalyst with methane further confirms that this active site indeed facilitates or favors the direct conversion of methane to methanol as discussed below.

FIGs 7A-B. show the absorption band of an oxygen-activated Cu-ZSM-5 during reaction at 175 degrees Celsius with CH ₄ (FIG. 7A) and CD ₄ (FIG. 7B) indicating decay of the 22,700 cm ^-1 . The disappearance of the absorption band at 22,700 cm ^-1 as a function of temperature was used to evaluate the activation energy (E _a ) for the reaction of oxygen-activated Cu-ZSM- 5 with C¾. FIG. 7C shows the Arrhenius plots of the reaction at temperatures between 1 10- 200 degrees Celsius, from which the activation energy is found to be 15.7 ± 0.5 kcal/mol. This reaction has a kinetic isotope effect ( IE) of 3.1 at 175 degrees Celsius resulting in an increase activation energy of the reaction by 3.1+0.5 kcal/mol, obtained from Arrhenius plot of the reaction with CD ₄ (FIGs. 7B-C). This indicates that C-H bond breaking is involved in the rate-limiting step of the oxidation of CH ₄ . The experimentally-determined activation energy (E _a ) for the reaction of the oxygen-activated catalyst with CH ₄ is found to be in agreement with activation energy based on DFT calculations.

DFT calculations were done to obtain a transition state structure and evaluate the reactivity of the above-discussed L-Cu ₂ ^{1 1} O model with methane (FIG. 8). The H-atom abstraction reaction is calculated to have a zero-point corrected activation energy of 18.5 kcal/mol in reasonable agreement with the experimentally observed E _a of 15.7 kcal/mol. The calculated increase in E _a upon reaction with CD ₄ is 1.3kcal/mol compared to the experimental value of 3.1kcal/mol. The calculated H-atom abstraction is endothermic by only 13.8kcal/mol (ΔΕ), reflecting the difference in bond dissociation energy of H-CH ₃ compared to that of the [Cu- OH-Cu] ²⁺ intermediate that would be generated. The strong O-H bond of the [Cu-OH-Cu] ²⁺ species (calculated bond dissociation energy of 90 kcal/mol) helps drive the reaction. The [Cu-OH-Cu] ²⁺ intermediate is best described as a delocalized-radical species, with Mulliken atomic spin densities of 0.26 and 0.44 on the Cu atoms and 0.17 on the bridging O. In the subsequent step, rebound of the hydroxyl radical (leaving 2 Cul) to couple with the methyl radical completes the reaction. Thus, the L-Cu ₂ ^{1 1} O model of oxygen-activated Cu-ZSM-5 can abstract an H-atom from CH ₄ through a low activation barrier consistent with experiment.

As one of ordinary skill in the art will appreciate, various changes, substitutions and alterations could be made or otherwise implemented without departing from the principles of the invention. Accordingly, the scope of the invention should be determined by the following claims and their legal equivalents.