Catalysts for the production of methanol from carbon dioxide

Abstract

Catalysts for the reduction of CO.sub.2 are described herein. More specifically, catalysts of Formula I and Formula II: ##STR00001##
wherein LB is a Lewis base; LA is a Lewis acid; R.sup.1 is selected from hydrogen, alkyl, substituted alkyl, cycloalkyl, substituted cycloalkyl, phenyl and substituted phenyl; and R.sup.9 and R.sup.10 are independently selected from alkyl, substituted alkyl, cycloalkyl, substituted cycloalkyl, phenyl and substituted phenyl; are described. A process for the production of methanol from CO.sub.2 using such catalysts is also described.

Claims

1. A catalyst for the reduction of CO.sub.2, where the catalyst is represented by Formula I: ##STR00043## wherein: LB is a Lewis base of formula PR.sup.2R.sup.3; LA is a Lewis acid of formula BR.sup.4R.sup.5; R.sup.1 is selected from hydrogen, alkyl, substituted alkyl, cycloalkyl, substituted cycloalkyl, phenyl and substituted phenyl; R.sup.2 and R.sup.3 are independently selected from alkyl, substituted alkyl, cycloalkyl, substituted cycloalkyl, phenyl and substituted phenyl; R.sup.4 and R.sup.5 are connected together by an OBO bridge to form, together with the carbon atoms to which they are attached, a 7-, 8- or 9-membered mono or bicyclic ring system or a substituted 7-, 8- or 9-membered mono or bicyclic ring system.

2. The catalyst of claim 1, wherein PR.sup.2R.sup.3 is selected from: ##STR00044##

3. The catalyst of claim 1, wherein BR.sup.4R.sup.5 is: ##STR00045##

4. The catalyst of claim 1, having the structure: ##STR00046## wherein Cat is a catechol group.

5. A process for the production of methanol from CO.sub.2, the process comprising: combining a catalyst of claim 1 and a hydrogen source to produce a mixture; exposing the mixture to CO.sub.2under conditions to convert the CO.sub.2 into methoxyboranes; and hydrolyzing the methoxyboranes to produce methanol.

6. The process of claim 5, wherein the hydrogen source is a hydroborane.

7. The process of claim 6, wherein the hydroborane is selected from the group consisting of HBCat, HBPin, 9-BBN and BH.sub.3SMe.sub.2.

8. A catalyst for the reduction of CO.sub.2, having the Formula: ##STR00047## wherein: A is PR.sup.2R.sup.3; and R.sup.1 is selected from hydrogen, alkyl, substituted alkyl, cycloalkyl, substituted cycloalkyl, phenyl and substituted phenyl; and R.sup.2 and R.sup.3 are independently selected from alkyl, substituted alkyl, cycloalkyl, substituted cycloalkyl, phenyl and substituted phenyl.

9. A molecule having the formula: ##STR00048## wherein Cat is a catechol group.

Description

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

(1) In the appended drawings/figures:

(2) FIG. 1 is an ORTEP drawing of derivative 2 with anisotropic atomic displacement ellipsoids shown at the 50% probability level in accordance with an embodiment of the present disclosure. Selected bond lengths [] and angles [] are as follows: P(1)-C(13): 1.8450; C(13)-C(18): 1.413; C(18)-B(1): 1.560; C(7)-P(1)-C(13): 101.32 (6); C(1)-P(1)-C(13): 101.76 (6); C(18)-C(13)-P(1): 119.85 (10); and C(13)-C(18)-B(1): 126.12 (13).

(3) FIG. 2 is a graph illustrating the Turn-Over Numbers (TON) for the formation of CH.sub.3OBCat in accordance with an embodiment of the present disclosure. More specifically, the CH.sub.3OBCat is obtained from a solution of 2 in benzene-d.sub.6 (9 mM) in the presence of 100 equivalents of HBCat under an atmosphere of CO.sub.2 (1 atm.): reaction carried out at 23 C. (.diamond-solid.); reaction carried out at 23 C. in presence of two equivalents of HC(O)OMe (.box-tangle-solidup.); reaction carried out at 70 C. (.square-solid.). The TON's are based on the number of hydrogen atoms transferred to CO.sub.2.

(4) FIG. 3 is an illustration of a calculated theoretical enthalpy profile (in kcal/mol) for the reduction of CO.sub.2 by 2 and catecholborane in accordance with an embodiment of the present disclosure.

(5) FIG. 4 is an illustration of a calculated theoretical enthalpy profile (in kcal/mol) for the reduction of methylformate by 2 and catecholborane in accordance with an embodiment of the present disclosure.

(6) FIG. 5 is an illustration of a theoretical mechanistic pathway for the reduction of CO.sub.2 to CH.sub.3OBCat by 2.

(7) FIG. 6 is an illustration of a theoretical mechanistic pathway for the first reduction step of CO.sub.2 to HCOOBCat by 2.

DETAILED DESCRIPTION

(8) I. Glossary

(9) In order to provide a clear and consistent understanding of the terms used in the present disclosure, a number of definitions are provided below. Moreover, unless defined otherwise, all technical and scientific terms as used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this specification pertains.

(10) The word a or an when used in conjunction with the term comprising in the claims and/or the specification may mean one, but it is also consistent with the meaning of one or more, at least one, and one or more than one unless the content clearly dictates otherwise. Similarly, the word another may mean at least a second or more unless the content clearly dictates otherwise.

(11) As used in this specification and claim(s), the words comprising (and any form of comprising, such as comprise and comprises), having (and any form of having, such as have and has), including (and any form of including, such as include and includes) or containing (and any form of containing, such as contain and contains), are inclusive or open-ended and do not exclude additional, unrecited elements or process steps.

(12) As used in this specification and claim(s), the word consisting and its derivatives, are intended to be close ended terms that specify the presence of stated features, elements, components, groups, integers, and/or steps, and also exclude the presence of other unstated features, elements, components, groups, integers and/or steps.

(13) The term consisting essentially of, as used herein, is intended to specify the presence of the stated features, elements, components, groups, integers, and/or steps as well as those that do not materially affect the basic and novel characteristic(s) of these features, elements, components, groups, integers, and/or steps.

(14) The terms about, substantially and approximately as used herein mean a reasonable amount of deviation of the modified term such that the end result is not significantly changed. These terms of degree should be construed as including a deviation of at least 1% of the modified term if this deviation would not negate the meaning of the word it modifies.

(15) As used herein, the term alkyl includes both straight-chain and branched. This also applies if they carry substituents or occur as substituents on other residues, for example in alkoxy residues, alkoxycarbonyl residues or arylalkyl residues. Substituted alkyl residues are substituted in any suitable position. Examples of alkyl residues containing from 1 to 10 carbon atoms are methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl and decyl, the n-isomers of all these residues, isopropyl, isobutyl, isopentyl, neopentyl, isohexyl, isodecyl, 3-methylpentyl, 2,3,4-trimethylhexyl, sec-butyl, tert-butyl, or tert-pentyl. A specific group of alkyl residues is formed by the residues methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl and tert-butyl.

(16) As used herein, the term cycloalkyl is understood as being a mono- or bicyclic carbon-based ring system, non-limiting examples of which include cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl and cycloheptyl.

(17) As used herein, the term aryl is understood as being an aromatic substituent which is a single ring or multiple rings fused together and which may optionally be substituted. When formed of multiple rings, at least one of the constituent rings is aromatic. In an embodiment, aryl substituents include phenyl, and naphthyl groups.

(18) The term substituted as used herein, means that a hydrogen radical of the designated moiety is replaced with the radical of a specified substituent, provided that the substitution results in a stable or chemically feasible compound. Non-limiting examples of substituents include halogen (F, Cl, Br, or I) for example F, and C.sub.1-4alkyl.

(19) The term frustrated Lewis pair is used herein to refer to a compound or mixture of compounds containing a Lewis acid and a Lewis base which, because of steric hindrance, cannot combine to form a strongly bound adduct, or may not in fact form any adduct at all.

(20) As used herein, the term Lewis acid refers to an electron pair acceptor.

(21) As used herein, the term Lewis base refers to an electron pair donor.

(22) The term catechol or CAT as used herein refers to the group:

(23) ##STR00039##
wherein the two oxygen atoms are bonded to two separate atoms (which are the same of or different) or to a single atom.

(24) The term HBPin as used herein refers to pinacolborane.

(25) The term BBN as used herein refers to 9-borabicyclo[3.3.1]nonane.

(26) The term suitable as used herein means that the selection of the particular compound or conditions would depend on the specific synthetic manipulation to be performed, and the identity of the molecule(s) to be transformed, but the selection would be well within the skill of a person trained in the art. All process/method steps described herein are to be conducted under conditions sufficient to provide the product shown. A person skilled in the art would understand that all reaction conditions, including, for example, reaction solvent, reaction time, reaction temperature, reaction pressure, reactant ratio and whether or not the reaction should be performed under an anhydrous or inert atmosphere, can be varied to optimize the yield of the desired product and it is within their skill to do so.

(27) The expression proceed to a sufficient extent as used herein with reference to the reactions or process steps disclosed herein means that the reactions or process steps proceed to an extent that conversion of the starting material or substrate to product is maximized. Conversion may be maximized when greater than about 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95 or 99% of the starting material or substrate is converted to product.

(28) II. Preparation of Ambiphilic Catalyst 2and Use Thereof in the Production of Methanol by Catalytic Reduction of CO.sub.2

(29) In a general way, the catalysts corresponding to the compounds of Formula I and II can be prepared and isolated prior to their use in the process according to the general methods described in the literature or using the methods described herein. In an embodiment of the disclosure, formation of the catalyst is performed by reacting a lithiated phosphine with a suitable borane reagent.

(30) The preparation of ambiphilic catalyst 2, in accordance with an embodiment of the present disclosure, is illustrated hereinbelow in Scheme 1.

(31) ##STR00040##

(32) Catecholborane derivative 2 was readily prepared from o-lithiated triphenylphosphine 1 in 80% yield. Multinuclear NMR characterization of derivative 2 demonstrated the species to be monomeric in solution and having no observable P-B interaction. The .sup.31P{.sup.1H} and .sup.11B{.sup.1H} NMR chemical shifts were measured to be 4.57 and 33.1 ppm respectively. Furthermore, the solid state structure did not reveal any evidence of P-B interaction which is likely due to the intramolecular distance separating the phosphorus and boron atoms. This is corroborated by the P1-C13-C18 and C13-C18-B1 bond angles, which were measured to be 119.85(10) and 126.12(13) respectively (FIG. 1). These values are in close correlation with that expected for a sp.sup.2 hybridized carbon (i.e. 120). Catecholborane derivative 2 was demonstrated to be an active catalyst for the catalytic reduction of carbon dioxide to methanol.

(33) The reaction of 2 under an atmosphere of CO.sub.2 (1 atm.) at room temperature resulted in no observable spectroscopic changes in solution (confirmed by .sup.1H, .sup.31P, and .sup.11B NMR). Although an adduct between CO.sub.2 and 2 could not be observed, the addition of 100 equivalents of HBCat to a solution of 2 in benzene-d.sub.6 (9 mM) in a J-Young tube under an atmosphere of CO.sub.2 (1 atm.) resulted in the formation of a white precipitate following a reaction period of 24 hours. The precipitate was subsequently identified as being CatBOBCat based on a comparison with the independently synthesized product (Scheme 2). Monitoring of the solution by .sup.1H NMR spectroscopy revealed the presence of a new signal at 3.37 ppm, attributed to the presence of CH.sub.3OBcat. The hydrolysis of the latter product afforded methanol as characterized by GC-MS. Carrying out the reaction under identical conditions using .sup.13CO.sub.2 resulted in the formation of .sup.13CH.sub.3OH.

(34) ##STR00041##

(35) Monitoring the reaction using both .sup.1H and .sup.31P{.sup.1H} NMR spectroscopy revealed an induction period of 30 minutes in which no spectroscopic changes could be observed. However, after the initial induction period, the reaction starts such that after 2 hours a 34% yield of CH.sub.3OBCat was obtained (FIG. 2, .diamond-solid.). The observed yield corresponds to a TON of 34 and a TOF of 17 h.sup.1. The rate of the reaction diminished as the reaction progressed, suggesting that conversion is dependent on the concentration of HBCat in solution. Indeed, 50% conversion to CH.sub.3OBcat was obtained in less than 5 hours whereas 69% conversion to CH.sub.3OBcat was observed after 24 hours.

(36) The induction period is significantly reduced by the addition of 2 equivalents of methyl formate. In fact, a TON of 40 was observed following a reaction period of 2 hours (FIG. 2, .box-tangle-solidup.). No significant impact was observed on the reaction rate. The reduction of CO.sub.2 also proceeds in the presence of BH.sub.3.SMe.sub.2 (100 eq.) as the hydrogen source to generate (CH.sub.3OBO).sub.n. However, a longer induction period is observed (>2 hours). Nonetheless, the formation of the methoxyborane species occurs rapidly once catalysis is initiated. TONs of 108 and 200 were observed following a reaction period of 2 and 5 hours respectively. A TON of 257 was obtained following a reaction period of 14 hours. The greater TON numbers suggest that all hydrogen atoms of BH.sub.3.SMe.sub.2 are available for the reduction of CO.sub.2.

(37) The temperature has an effect on the efficiency of the catalytic system. Heating a solution comprising 2 and HBCat (100 eq.) under an atmosphere of CO.sub.2 (1 atm.) at a temperature of 70 C. resulted in the immediate generation of CH.sub.3OBCat without requiring any induction period (FIG. 2, .square-solid.). A TON of 86 was observed following a reaction period of 30 minutes (TOF=172 h.sup.1). The TON increased to 92 following a reaction period of 90 minutes. Following a 24 hour reaction period at 70 C. (using a J-Young tube), additional HBCat (100 eq.) was added to the reaction mixture followed by continued heating at 70 C. The catalytic reaction resumed, however with a rate that seemed somewhat slower. This is possibly due to the presence of a large amount of precipitate (CatBOBCat) in the solution, reducing its homogeneity. After an additional 60 minutes of reaction time, following the addition of the further portion of HBCat, an overall TON of 185 was observed. Under similar reaction conditions, BH.sub.3.SMe.sub.2 proved to be an excellent hydrogen source, generating (CH.sub.3OBO).sub.n in less than an hour of reaction time (100% yield and TOF>300 h.sup.1). The reaction was similarly carried using other hydroborane sources and the results summarized in Table 1. The addition of HBPin (100 eq.) to a solution of 2 under an atmosphere of CO.sub.2 (1 atm.) at 70 C., generated the desired product in 60% yield following a reaction period of 3 hours. The borane reagent HBPin was significantly less active when compared to the HBCat reagent. This observation seems to corroborate the reduced reactivity of HBPin in hydroboration reactions when compared with HBCat. Similarly, a TON of 34 was observed for 9-BBN following a reaction period of 3 hours.

(38) Density functional theory studies were performed using catecholborane derivative 2 as the catalyst and HBCat as the hydrogen source in order to gain further insight into the reaction mechanism for the catalytic conversion of CO.sub.2 into methanol. Only potential intermediates were considered in the study and the results are summarized in FIG. 3. As observed experimentally, the coordination of CO.sub.2 with species 2, to generate the corresponding intermediate IM1, is energetically disfavored by 9.86 kcal/mol. This is likely a result of the weak coordination of CO.sub.2 with species 2. Indeed, despite the bending of the molecule (indicative of CO.sub.2 activation), the CO bonds appear to be only marginally elongated as compared to free CO.sub.2. Nevertheless, adduct IM1 undergoes the addition of an equivalent of HBCat to yield intermediate IM2, whose formation is energetically favored by 14.4 kcal/mol relative to 2. The consumption of a further equivalent of HBCat yields intermediate IM3 and CatBOBCat. The formation of IM3 is also energetically favored by 47.7 kcal/mol relative to 2. The addition of a third equivalent of HBCat regenerates catalyst 2 as well as methanol precursor species CH.sub.3OBCat. This last step is energetically favored by a further 34.0 kcal/mol relative to 2. While not wishing to be limited by theory, these results indicate that as soon as the initial coordination of CO.sub.2 with 2 has been achieved, the subsequent reduction steps are thermodynamically highly favorable. This study corroborates the initial observation of a rather long induction period for the catalytic reduction process. Although these results show a number of similarities with the formation of CH.sub.3OSiR.sub.3 in the CO.sub.2 reduction by N-heterocyclic catalysts or to the formation of CH.sub.2OBR.sub.2 with a nickel pincer complex, the ambiphilic system shows unique characteristics, notably the stepwise reduction and the three hydride transfers. Indeed, the catalytic system of the present disclosure comprises the activation of the CO.sub.2 species, whereas the prior art systems include the activation of the reducing agent. Furthermore, since no trace of HCOOBCat could be observed in solution during the catalytic process, as monitored by .sup.1H NMR, suggests that the ambiphilic catalyst remains attached to the reduced carbonyl species, as suggested by the thermodynamic data.

(39) To confirm these computational results, a further density functional theory study was performed using catecholborane derivative 2 as the catalyst and HBCat as the hydrogen source for the catalytic reduction of methylformate. As for the previous computational study, only potential intermediates were considered and the results are summarized in FIG. 4. The coordination of methylformate with 2 was predicted to result in the formation of intermediate IM1mf. The formation of this intermediate was calculated to be energetically disfavored by 3.84 kcal/mol. When carried out in the absence of HBCat, no trace of intermediate IM1mf could be observed in solution as monitored by .sup.1H NMR. However, upon the addition of HBCat to the catalytic system, the conversion to CH.sub.3OBCat could be observed (FIG. 4). This observation confirms that even though the formation of the initial adduct IM1mf is thermodynamically disfavored, the presence of a hydrogen source, such as a hydroborane, is sufficient to push the reaction forward and initiate the reduction process. Furthermore, the reaction does not display any significant induction period, which corroborates the computed thermodynamic preference for the formation of IM1mf over IM1. A similar structure to formaldehyde intermediates IM3 and IM2mf was identified as a key intermediate for both the formation of CH.sub.3OSiR.sub.3 in the CO.sub.2 reduction by N-heterocyclic catalysts and the formation of CH.sub.2OBR.sub.2 with a nickel pincer complex, but could not be observed experimentally. In order to yet further corroborate the proposed reaction mechanism, catecholborane derivative 2 was reacted with paraformaldehyde and heated at 70 C. to provide IM3, which was subsequently identified by multinuclear NMR spectroscopy. The subsequent addition of HBCat to the reaction mixture yielded CH.sub.3OBCat. While not wishing to be limited by theory, together, these results suggest the reduction pathway illustrated in Scheme 3, and further suggest that activation of carbon dioxide is the rate limiting step of the catalytic reduction.

(40) ##STR00042##

(41) Further theoretical studies have shown that although the catalyst lowers the energy gap for the reduction of CO.sub.2 to HCOOBCat, it also plays a significant role in enhancing the rates of the subsequent reduction steps. Moreover, the species in the first reduction step appears to involve the hydridoborato/boronium bifunctional system IM0D. The Lewis base center plays a role in binding carbon dioxide with an ideal geometry, allowing the hydride delivery while the boronium fragment ensures the electrophilic activation of CO.sub.2 (TS1D). Compared to the catalyst free reduction, together these factors lead to a lowering of the energy barrier by 6.7 kcal.Math.mol.sup.1. A slight increase in energy of only 2.4 kcal.Math.mol.sup.1 provides for the possibility of the Lewis acidic site of the catalyst binding CO.sub.2, while the phosphine activates the borane for delivery of a hydride to the electrophilic carbon of CO.sub.2 (TS1C) (FIG. 6). Both pathways put emphasis on the fact that the role of the catalyst appears to be the simultaneous activation of both reagents (the borane reagent and CO.sub.2) as opposed to the activation of CO.sub.2 alone.

(42) The reduction of both HCOOBCat and CH.sub.2O was shown to be possible without any implication from the catalyst and consequently, some of these reductions are expected to occur catalyst-free in the presence of a large excess of HBCat. However, activation of the HBCat moiety by the Lewis base center while the substrate is fixed and activated by the Lewis acidic boron center results in lowering the transition state energies by 11.1 and 2.3 kcal.Math.mol for the hydroboration of HCOOBCat and CH.sub.2O, respectively (TS2C and TS3C respectively). The rapid reduction of HCOOBCat by the catalyst explains why it could not be observed experimentally. On the other hand, the 14.7 kcal.Math.mol.sup.1 bonding interaction of the catalyst with formaldehyde (IM2C) rationalizes the fact that this particular adduct can be observed spectroscopically during catalysis. The entire catalytic process is summarized and illustrated in FIG. 5.

(43) These results are indicative of the importance of designing FLPs that incorporate a moderate Lewis base and Lewis acid pair. Indeed, a moderate Lewis base would not induce strong binding of CO.sub.2 which might hinder the hydride transfer or generate stable adducts with the intermediates. Similarly, a moderate Lewis acid would allow for the release of the various hydroboration products from the catalyst into the reaction medium. Furthermore, the presence of both the Lewis acid and Lewis base in a single molecule reduces the entropic cost associated with the catalyzed steps.

(44) While the present disclosure has been described with reference to what are presently considered to be the preferred examples, it is to be understood that the disclosure is not limited to the disclosed examples. To the contrary, the disclosure is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.

(45) All publications, patents and patent applications are herein incorporated by reference in their entirety to the same extent as if each individual publication, patent or patent application was specifically and individually indicated to be incorporated by reference in its entirety.

REFERENCES

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(47) TABLE-US-00001 TABLE 1 Reaction Conditions and TONs for the Catalytic Reduction of CO.sub.2 Using Various Hydroboranes..sup.a Entry Borane # eq. Time (h) Temp ( C.) TON.sup.b 1 HBCat 100 36 70 86 2 HBCat 100 98 70 92 3 HBCat 100 + 100.sup.d 30 70 136 4 HBCat 100 + 100.sup.d 60 70 185 5 HBPin 100 174 70 60 6 9-BBN.sup.e 50.sup.e 174 70 34 7 HBBz 100 1440 70 0 .sup.aReaction conditions: 2.0 mg (0.0053 mmol) of 2 in 0.6 mL of benzene-d.sub.6. .sup.bBased on mole of B-H consumed per mole of 2 (determined by .sup.1H NMR integration using hexamethylbenzene as internal standard). .sup.c2.0 equiv. (0.0106 mmol) of HCOOMe were added. .sup.dA second addition of 100 equivalents of HBCat was carried out 24 hours after the first addition. .sup.eLimited at 50 equivalents because of low solubility of 9-BBN.