Using reduced catalysts for oxidation reactions: Mechanistic studies of the "periana-catalytica" system for CH4 oxidation

Article abstract of DOI:10.1021/ja404895z

Designing oxidation catalysts based on CH activation with reduced, low oxidation state species is a seeming dilemma given the proclivity for catalyst deactivation by overoxidation. This dilemma has been recognized in the Shilov system where reduced Pt^II is used to catalyze methane functionalization. Thus, it is generally accepted that key to replacing Pt ^IV in that system with more practical oxidants is ensuring that the oxidant does not over-oxidize the reduced Pt^II species. The "Periana-Catalytica" system, which utilizes (bpym)Pt ^IICl₂ in concentrated sulfuric acid solvent at 200 C, is a highly stable catalyst for the selective, high yield oxy-functionalization of methane. In lieu of the over-oxidation dilemma, the high stability and observed rapid oxidation of (bpym)Pt^IICl₂ to Pt^IV in the absence of methane would seem to contradict the originally proposed mechanism involving CH activation by a reduced Pt^II species. Mechanistic studies show that the originally proposed mechanism is incomplete and that while CH activation does proceed with Pt^II there is a solution to the over-oxidation dilemma. Importantly, contrary to the accepted view to minimize Pt^II overoxidation, these studies also show that increasing that rate could increase the rate of catalysis and catalyst stability. The mechanistic basis for this counterintuitive prediction could help to guide the design of new catalysts for alkane oxidation that operate by CH activation.

Full text of DOI:10.1021/ja404895z

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Article
Using Reduced Catalysts for Oxidation Reactions: Mechanistic
Studies of the “Periana-Catalytica” System for CH4 Oxidation
Oleg A. Mironov, Steven Michael Bischof, Michael M Konnick, Brian Glenn
Hashiguchi, William A. Goddard, Mårten Ahlquist, and Roy Anthony Periana
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja404895z • Publication Date (Web): 08 Aug 2013
Downloaded from http://pubs.acs.org on August 14, 2013
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Using Reduced Catalysts for Oxidation Reactions:
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Mechanistic Studies of the “PerianaꢀCatalytica” System
for CH4 Oxidation.
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Oleg A. Mironov,a Steven M. Bischof,b Michael M. Konnick,b Brian G. Hashiguchi,b William A.
Goddard, III,*,c Mårten Ahlquist,c and Roy A. Periana*,b
.
aLoker Hydrocarbon Research Institute, University of Southern California, University Park, Los
b
Angeles, California 90089, USA. The Scripps Energy & Materials Center, The Scripps Research
c
Institute, 130 Scripps Way #3A1, Jupiter, Florida 33458, USA. Materials and Process Simulation
Center, California Institute of Technology, Pasadena, California 91125, USA.
Email: rperiana@scripps.edu
RECEIVED DATE (to be automatically inserted after your manuscript is accepted if required
according to the journal that you are submitting your paper to)
Abstract The Shilov system for methane functionalization using stoichiometric amounts of PtIV has
been shown to operate via CH activation with PtII rather than the more oxidized PtIV. It is generally
accepted that replacing PtIV with high concentrations of more practical oxidants could lead to overꢀ
oxidation of the reduced PtII and rapid deactivation by generation of PtIV. The “PerianaꢀCatalytica”
system, which utilizes (bpym)PtIICl2 in concentrated sulfuric acid solvent at 200°C, is a highly stable
catalysts for the selective, highꢀyield oxyꢀfunctionalization of methane. The high stability and observed
rapid oxidation of (bpym)PtIICl2 in the absence of methane to PtIV would seem to contradict the
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originally proposed mechanism involving CH activation by a reduced PtII species. Mechanistic studies
now show that while CH activation does proceed with PtII, the originally proposed mechanism is
incomplete. Importantly, contrary to the accepted view that the oxidation of PtII should be minimized in
systems with practical oxidants, these studies show that increasing the rate of oxidation of PtII to PtIV in
that system could increase the rate of catalysis by 20ꢀfold. The mechanistic basis for this counterꢀ
intuitive result could help to guide the design of new catalysts for alkane oxidation that operate by CH
activation.
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Introduction
The direct conversion of primary feedstocks such as alkanes to fuels and chemicals at lower
temperatures with high selectivity is an important on going challenge in catalysis.1 Homogeneous
catalysts based on the CH activation reaction are among the most effective for this transformation. The
seminal homogeneous system developed by Shilov (Eq 1) has shown this as a possible reality for the
selective functionalization of CH4 to CH3OH and CH3Cl.2 The Shilov system operated in aqueous HCl
o
at <100 C with low CH4 conversions using [PtIVCl6]2ꢀ as the overall oxidant. Extensive studies by
Shilov,3 Bercaw,4 and others5 have provided strong support for a reduced, homogeneous PtII species that
reacts with CH4 via the CH activation reaction to generate PtIIꢀCH3 intermediates.
(1)
It is notable that the more labile, reduced PtII, and not oxidized PtIV species is proposed to be
active for CH activation of CH4. Generally, coordination of the CH bond of an alkane is a key
prerequisite to generate the MꢀC bond via subsequent CH cleavage. Consequently, as alkanes are
extremely poor ligands, the reduced complex, LMn, can be much more effective at CH activation than
the more oxidized states, LM>n. Indeed, most of the wellꢀestablished CH activation systems are with
lower oxidation state metal complexes.1
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Scheme 1. General catalytic cycle for RH oxidation where only the reduced form of the catalyst is
active for CH activation.
However, it would seem paradoxical to design catalysts for hydrocarbon oxidation reactions that
are based on CH activation with reduced oxidation states, LMnX, of the catalyst. Thus, as shown in
Scheme 1, if the higher oxidation state species, LM>nꢀX, is both more stable and inactive for the CH
activation, the irreversible oxidation of the reduced species, LMnꢀX, should lead to deactivation when
the catalyst pools as the higher oxidized species, LM>nꢀX. This issue did not exist in the Shilov system
as the overall PtIV oxidant could not consume [PtII], as this oxidant itself becomes PtII upon oxidation.
However, in practical systems, the stoichiometric oxidant cannot be the higher oxidation state of the
catalyst as these are typically expensive metals.6 Furthermore, carrying out the oxidation of CH4 to
CH3OH (Eo ~ ꢀ0.6V, Eq 2),7 requires that the overall oxidant in Scheme 1, Ox, must be both relatively
strong (Eo > 0.6V) and present in large excess relative to the catalyst (for TON to be >1). Under these
conditions, it would seem unlikely to prevent deactivation from oxidation of the reduced state of the
catalyst that is. This potential issue has been recognized by several researchers. In particular, Bercaw,
noted that replacement of PtIV in the Shilovꢀsystem with more practical overall oxidants require, “strict
constraints on suitable alternative oxidants; although they must be fast enough to oxidize [PtIIꢀR]
competitively with protonolysis, they must not8 rapidly oxidize inorganic PtII, i.e., [PtIICln(H2O)4ꢀn]2ꢀn,
since that would deplete the [reduced oxidation state]9 species responsible for alkane activation.”10
Indeed, the identification of catalysts and practical oxidants that meet this requirement for slow
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oxidation of the reduced catalytic species that is active for CH activation is an important consideration
of onꢀgoing research.11
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(2)
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Herein, we report on studies of the highly efficient PerianaꢀCatalytica catalyst system that
oxidizes CH4 to CH3OH in high yield (Eq 3). This system is well suited for study as CH activation of
methane is proposed with a reduced PtII species despite the high stability of the system (TONs of 500
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were reported without deactivation12) and reaction conditions of 220 C in strongly oxidizing 101%
H2SO4 (Eq 4) as the solvent. It has been generally considered that the stability of the system resulted
from meeting the accepted requirement of oxidizing the PtIIꢀCH3 intermediate faster than the noneꢀ
methylated, reduced, PtII catalyst. Significantly, our studies now show that this is not the basis for the
high stability and that catalysis proceeds by a more complex mechanism. We believe that these new
findings could provide important guidance in the design of new catalysts for direct, selective alkane
oxidation. It should be noted that the mechanism for catalyst stability could be different from that for
product formation and that both operate at steady state. It is also possible that a single pathway is
responsible for both catalyst stability and product formation. These possibilities are examined and
discussed herein.
(bpym)PtCl2
H2SO4 220oC
CH4 + H2SO4
CH3OH + SO2 + H2O
(3)
(4)
RESULTS AND DISCUSSION
The “PerianaꢀCatalytica” system utilizes a PtII complex, (bpym)PtIICl2 (bpym = 2,2’ꢀ
bipyrimidinyl) as the catalyst for the selective oxidation of CH4 in H2SO4 to generate a mixture of
+
CH3OH, CH3OH2 , and CH3OSO3H (this mixture will be referred to herein as CH3X) and SO2. In
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addition to the high stability, the system is highly efficient for carrying out the oxidation reaction. After
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3 h, at 220 C, with 500 psig of CH4, in concentrated H2SO4 (101%, ~19 M), a TOF of ~10ꢀ3 sꢀ1 with
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~70% yield of CH3X based on added CH4 at >90% selectivity and a volumetric productivity of ~10ꢀ7
mol ccꢀ1 sꢀ1 was observed.13 A basis for reaction by CH activation was the observation that catalysis in
D2SO4 led to H/D exchange with methane. This observation combined with reported studies of the
Shilov system6 provided the basis for the proposed mechanism based on CH activation with a PtII rather
than PtIV species. While several theoretical studies of this system have been performed, no detailed
experimental studies of the reaction mechanism have been reported to date. Consistent with the original
reports, we found that the system is very efficient and remained homogeneous with no observable Pt
black or insoluble species after reaction. The thermodynamic values of a ꢀH‡ of 34 ± 2 kcal/mol and a
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ꢀS‡ of ꢀ3.8 ± 0.8 eu (ꢀG‡ 36 ± 2.5 kcal/mol at 220 C) were experimentally obtained for the overall
reaction in Eq 3.
To more fully understand the basis for the high stability of the system, we examined the original,
as well as several other plausible, independent mechanisms. These mechanisms are shown in Scheme 2,
as Pathways A, (the original pathway), B, C, and D involving only the reaction steps shown in the inset
for each pathway. Each pathway is intended to be considered as a separate mechanism that is
independent of the others. This composite diagram of the various steps is used to emphasize the
common and variable steps between the four pathways. As can be seen, on the basis of reported
theoretical studies, the (bpym)PtCl2 in liquid H2SO4 is proposed to generate various protonated forms,
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where Pt is used to designate the (Hbpym)PtX motif.
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Scheme 2. Plausible pathways for the PerianaꢀCatalytica system that may account for the high
stability.
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Pathway A
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Pathway A is the mechanism proposed by the original investigators involving steps k1, k2, and
k3 with the added consideration of the irreversible oxidation of PtII-X to X2PtIV-X, step k4. Key
characteristics of this pathway are that CH activation of CH4 involves reaction with PtII-X to generate
PtII-CH3 in step k1, oxidation of PtII-CH3 to X2PtIV-CH3 by concentrated H2SO4 solvent in step k2
followed by reductive functionalization of X2PtIV-CH3 to CH3X and PtII-X, step k3. Consideration of
step k4 in Pathway A is the fundamental basis for this study.
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Our studies show that the reaction is first order in methane and catalyst (Figures 1 and 2). The
reactions with methane were carried out with 5 mM PtII in order to operate outside of the mass transfer
limited regime. With the reactor system utilized in these studies, control studies show that the reaction
becomes mass transfer limited above ~10 mM PtIIꢀX, and we believe that this the basis for the deviation
from linearity from the reaction rate versus [PtII] (Figure 2). The rate of H/D exchange between D2SO4
solvent and CH4 was also shown to be highly dependent on the proton activity of the H2SO4 solvent (ꢀ
Ho) (Figure 3). The reaction can be followed above –Ho of ~8 (85 % H2SO4), however, due to
inaccuracies in the detection limits of our analytical methods we could not follow the reaction below
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this level of acidity. Hꢀ and CꢀNMR spectroscopic analysis of the crude liquid phase after catalysis
with 100% labeled 13CH4 in concentrated H2SO4 or D2SO4 showed no resonances that could be assigned
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to Ptꢀ13CH3 species, and all the CH3 resonances observed were from dissolved CH4 and the various
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forms of CH3X. The resonances in the region for the bpym ligand could not be analyzed due to the
various multi protonated species present in solution and proton exchange.
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1.2 104
1 104
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CH4 Pressure (psi)
Figure 1. Turnover numbers versus CH4 pressure. Conditions: 5mM Pt(bpym)Cl2, 10ꢀ145 psi CH4, 10
ml 98% D2SO4, 40 min, 165 °C.
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[(bypm)PtCl2] (mM)
Figure 2. kobs of H/D exchange (●) and MeOH production (○) versus [Pt(bpym)Cl2]. Conditions:
[Pt(bpym)Cl2
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= 5ꢀ21 mM, 500 psi CH4, 10 ml 98% D2SO4, 60 min, 165 °C.
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D2SO4 Acidity (-H0)
Figure 3. kobs H/D exchange vs –H0. Conditions: 5 mM Pt(bpym)Cl2, 500psi CH4, 10 ml of 85ꢀ98%
D2SO4, 60 min, 165 °C.
To examine whether the PtII-X species proposed for CH activation could be oxidized to X2PtIVX
by step k4 in Scheme 2, a 10 mM solution of (bpym)PtIICl2 in 98% H2SO4 was heated to 200 C under
o
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N2 (in the absence of CH4) for 1 h. The color of the reaction mixture changed from orange to yellow
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(see SI for N2 vs CH4 pictures) and ~110% ± 20% of SO2 (based on Eq 5) was observed in the gas
phase. The SO2 was identified by gas chromatography and quantified by titrimetric methods based on
SO2 trapping in H2O2(aq) solution (to form H2SO4) and back titrated with KOH(aq). Control
experiments demonstrated that SO2 is not generated without the PtIIꢀcomplex or from the bypm ligand
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alone. Attempts at analysis by in situ H and CꢀNMR studies were complicated by poorly resolved,
overlapping resonances, vide supra. Similarly, after numerous attempts at examining PtII-X in H2SO4,
195PtꢀNMR no resonances could be clearly observed. This last was not unexpected as this system was
designed to be highly labile and changing of “X” groups on Ptꢀcenters drastically change the chemical
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shift (i.e. cisꢀPt(NH3)2Cl2: δ ꢀ2168 ppm vs cisꢀPb(NH3)2(OH)2: δ −1590 ppm), exacerbating the
broadening of signals. Attempts at characterizing the proposed (bpym)PtCl2(HSO4)2 species by
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isolation or derivativization were unsuccessful due to the high solubility of the reaction product, the low
volatility and reactive properties of concentrated sulfuric acid. Although there are other explanations
for the stoichiometric formation of SO2 from PtII-X, these results would suggest that the oxidation of
PtII to PtIV is quantitative and effectively irreversible (K4 >> 1) in the absence of CH4.
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(5)
If one assumes that catalysis proceeds via Pathway A, the high observed stability of the system
would require that the rate of irreversible oxidation of PtIIꢀX required for CH activation to presumed
inactive X2-PtIV-X, ( ie, k4) be substantially slower than the overall rate of catalysis for generation of
CH3X. In this case, the system could exhibit apparent high stability as any catalyst deactivation could be
sufficiently slow and undetectable over the time required to observe the reported 500 TONs.13
Assuming that PtII-X is the resting state, this would require that the activation barrier for the oxidation
to X2-PtIV-X be substantially higher than the ~36 kcal/mol activation barrier for overall catalysis.
To obtain rate data on the oxidation of PtII-X, we attempted to follow the reaction by the rate of
SO2 formation; however, this was hampered by lack of reproducibility that resulted from the high
solubility of SO2 in H2SO4 coupled with technical challenges with mass transfer between the gas and
liquid phases. The observation that SO2 formation after 1 h is negligible at <150 °C but stoichiometric at
220 oC, vide supra, would suggest a barrier of >30 but < 36 kcal/mol for the reaction. To obtain a better
approximation of the activation barrier we examined the reaction by DFT calculations. Ziegler18
reported that the oxidation of PtII-X with SO3 is endoergic by 8.3 kcal/mol with a barrier of 35.1
kcal/mol. Attesting to the difficulty in modelling reactions under the catalytic conditions, our
calculations of the Ziegler mechanism showed an inaccessible barrier of 44 kcal/mol. However,
consistent with the interpretation of the experimental data on the oxidation of PtII-X, we found an
alternative pathway for the oxidation of PtII-X to X2PtIVX with a barrier of ~36 kcal/mol and a ꢀGrxn of
ꢀ1 to ꢀ6 kcal/mol depending on whether the oxidant was presumed to be H2SO4 or H2S2O7, respectively
(see SI).
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The results show that the rate of oxidation of PtII to PtIV is comparable to that for overall
catalysis. Under these circumstances, if the catalytic mechanism for product formation operated by
Pathway A involving only steps k1, k2, and k3, deactivation by step k4 should occur in minutes as
effectively all of the reduced, PtII-X catalytic species that is active for CH activation would pool as
presumed inactive, more stable X2PtIV-X. Given the high stability of the system, this would suggest that
either Pathway A does not operate or is not a complete description of the system. It should be
emphasized that these observations do not rule out the possibility that Pathway A is solely or partially
the basis for product formation while other reactions account for the high stability.
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Pathway B
As seen in Scheme 2, Pathway B involves only steps k4, k5, and k3. The key distinction from
Pathways A, C, or D is that CH activation is with a PtIV species, X2PtIV-X, (rather than PtII-X) to
directly generate X2PtIV-CH3. This is followed by k3 to generate PtII-X and the product CH3X.
Reoxidation of PtII-X by H2SO4, step k4, regenerates the X2PtIV-X. The key basis for considering this
pathway is that deactivation of the catalytic system is less likely if CH activation was carried out by the
more stable, high oxidation state species, X2PtIV-X. We were particularly interested in this pathway as it
seems the simplest basis for the high stability. Indeed, most efficient homogeneous catalysts systems
identified to date for the efficient oxidation of CH4 to CH3OH operate by CH activation with high
oxidation states of the catalyst, e.g., PdII,19 AuI/III,20 HgII,21 etc. Additionally, it is known that PtIV species
are competent for CH activation of aromatics in CF3CO2H.22 While alkanes are more poorly
coordinating than arenes, it is plausible that alkanes could coordinate with PtIV centers by a dissociative
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mechanism in poorly coordinating concentrated H2SO4 at 220 C. Reported calculations showing that
IV
‡
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the barrier for CH activation with Pt species is ꢀG = 41 ± 2 kcal/mol suggests that such a pathway is
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not viable. However, discrepancies in reported calculations, vide supra, are likely a result of the highly
polar, poorly coordinating, strongly oxidizing, strongly hydrogen bonding characteristics of
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Journal of the American Chemical Society
o
concentrated H2SO4 at 220 C. This increases the likelihood of highly charged species being involved
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and suggests that CH activation by PtIV should not be ruled out.
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To examine the possibility of CH activation by the higher oxidations state species, X2-PtIV-X,
the catalytic reaction was carried starting with a PtIV model complex. The expectation was that if the
original proposal, Pathway A, of CH activation by the lower oxidation state species, PtII-X, was
correct, an induction period may be observed that would rule against Pathway B. However, the lack of
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an induction period could not be interpreted as it is wellꢀknown that that PtIV can readily decomposes to
II
or contaminated with Pt . In situ synthesis of (bpym)PtIVX4 (X = Cl or HSO4) by mixing
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PtIV(OH)4(H2O)2, bpym, and HCl in a 1:1:2 molar ration were used as one model of X2PtIV-X (see SI
for details). Interestingly, these reactions gave the same results as starting with the PtII catalyst,
(bpym)PtCl2. Moreover, no induction period was observed. As noted above, while this is consistent with
PtIV as a catalyst, this result is inconclusive. Therefore additional evidence is required to provide
stronger support for this pathway.
To more definitively investigate the possibility for CH activation by X2PtIVX, Pathway B, we
examined the stoichiometric reaction of a model complex of the predicted intermediate, X2PtIV-CH3,
with concentrated D2SO4 at 180 oC, Scheme 3. The model complex of X2PtIV-CH3 utilized in this study
was the fully characterized (bpym)PtIV(CH3)(TFA)Cl2, (TFA)(X)-PtIV-CH3. This complex has the same
bpym ligand as the actual catalyst, (bpym)PtCl2, and differs from X2PtIV-CH3 only by the presence of a
TFA anion. Given the expected rapid loss of TFA in hot H2SO4, it is reasonable that (TFA)(X)-PtIV-
CH3 is a good model of the proposed intermediate in Pathway B, X2PtIV-CH3. Consistent with X2PtIV-
CH3 as an intermediate in catalysis, carrying out the catalysis starting with (TFA)(X)-PtIV-CH3 showed
identical results to starting with (bpym)PtCl2.
As noted in the original work, carrying out the catalysis in D2SO4 led to significantly more Dꢀ
incorporation into methane than generation of methanol, Eq 6. If the reaction proceeded by Pathway B,
this would suggest that the CH activation is reversible, i.e., k5 and kꢀ5 is fast compared to the conversion
of X2PtIV-CH3 to the functionalized product (k3). As can be seen in Scheme 3, kinetic analysis of
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Pathway B would predict that the molar ratio of all the isotopologues of methane, to methanol, [(mol
CHnD4ꢀn])/(mol CHmD3ꢀmX)]CAT, generated from catalytic reaction of CH4 in D2SO4 would give kꢀ5/k3,
where CAT is used to designate data obtained from the actual catalytic system. This analysis assumes
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that steps k5 and k3 are first order in X2PtIV-CH3 and (TFA)(X)-PtIV-CH3 and carried out at low
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conversions of CH4 and D2SO4 to ensure pseudoꢀfirstꢀorder kinetics and irreversible formation of the
reaction products. This analysis assumes firstꢀorder dependence on D2SO4. However, this is not
required for comparison of [(mol CHnD4ꢀn])/(mol CHmD3ꢀmX)] obtained from the catalytic reaction and
the reaction of the model complex (TFA)(X)-PtIV-CH3 (vide infra) since the comparison is carried out
under identical conditions.
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(6)
Scheme 3. Kinetic analysis assuming Pathway B
The key hypothesis is that if Pathway B was the mechanism of catalysis, then the stoichiometric
reaction of independently synthesized (TFA)(X)-PtIV-CH3 under the same reaction conditions would
give [(mol CHnD4-n])/(mol CHmD3-mX)]Pt(IV)-CH3 (where the Pt(IV)ꢀCH3 is used to designate the ratio of
products obtained from the stoichiometric reaction of TFA)(X)-PtIV-CH3) that is the same or
comparable to [(mol CHnD4-n])/(mol CHmD3-mX)]CAT from the catalytic reaction of CH4 carried out in
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Journal of the American Chemical Society
D2SO4. Conversely, any large mismatch between these ratios, while not definitive, would rule against
Pathway B as the catalytic mechanism and the basis for the stability of the system. Significantly, the
comparison shown in Scheme 3 is only possible, if the reactions are irreversible and the ratio of
products is independent of the concentration of X2PtIV-CH3 and reaction time. This is essential since the
concentration and reactions times with the model complex are almost certainly different for the
presumed intermediate, X2PtIV-CH3 in the actual catalytic system. It should be noted that there are some
differences between the catalytic and stoichiometric studies as SO2 (albeit this was shown to have no
effect on the catalytic reaction, vide infra) and other Ptꢀspecies could be present in the actual catalytic
reaction.
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To obtain [(mol CHnD4ꢀn])/(mol CHmD3ꢀmX)]CAT, the catalytic system with CH4 was examined as
previously described, but using concentrated D2SO4, Eq 6. The reactions were examined between 160ꢀ
o
190 C with 5 mM of (bpmy)PtIICl2 at <10% conversion of added CH4 to ensure pseudoꢀfirstꢀorder
conditions with respect to both CH4 and D2SO4. Consistent with the original work, analysis of the gas
phase from the reaction in D2SO4 showed H/D exchange between D2SO4 and CH4 to generate the
various isotopologues of CHnD4ꢀn (n = 0 – 4). Temperature dependent studies show that ꢀH‡ = 28 ± 2
o
kcal/mol, ꢀS‡ = ꢀ11 ± 3 eu, and ꢀG‡ = 33 ± 2 kcal/mol at 180 C was obtained for this H/D exchange
reaction. The CHmDmꢀ3X (m = 0 and 3) and was quantified by GC/MS analysis. As can be seen in Table
1, Entry 1 the molar ratio, (mol CHnD4ꢀn])/(mol CHmD3ꢀmX)]CAT, was ~ 20.
The stoichiometric reactions of the model complex were carried out by injecting 0.1 ml of a 1.04
M solution of (TFA)(X)-PtIV-CH3, in DMSO all at once into a glass vial under argon containing 5 mL
o
27
of a stirred solution of concentrated D2SO4 at 180 C. Immediately after injection, the vials were
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removed, cooled to RT, and the gaseous and liquid phases were analyzed by GCꢀMS and HꢀNMR.
Remarkably, these studies showed that even in concentrated D2SO4 at 180 oC, no CHnD4ꢀn was observed
(by GS/MS) from the stoichiometric reaction of (TFA)XPtIV-CH3 and the predominant product was
CHmD3ꢀmX in essentially quantitative yield, [(mol CHnD4ꢀn])/([mol CHmD3ꢀmX)](TFA)Pt(IV)ꢀCH3 < 0.01
(Table 1, Entry 2). Significantly, this result is dramatically different from the molar ratio of products
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(~20) obtained from the catalytic reaction of CH4 in D2SO4 with(bpym)PtCl2 Table 1, Entry 1. The poor
correlation provides strong evidence that CH activation does not proceed via the PtIV complex (k5) in
Pathway B (Scheme 2). This is consistent with the difference between the ~31 kcal/mol barrier for H/D
exchange from experiment and the ~41 kcal/mol barrier for CH activation by X2PtIV-X obtained from
DFT studies.23 There results would suggest that it is unlikely that Pathway B involving CH activation
by X2PtIV-X accounts for the high stability of the system. Importantly, while these results rule against
Pathway B, the facile, functionalization reaction of TFA-PtIV-CH3 to CH3X provides strong support
for the feasibility of the reductive functionalization step, k3. Similar functionalization reactions were
observed in model studies of the expected PtIVꢀCH3 species in the Shilov system. The model complexes
utilized in those studies contained ligands not present in this system, were carried out with different
solvents and under much milder conditions.6
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Table 1. Ratio of isotopologues:of methane and methyl bisulfate from reactions in D2SO4.a
(mol [CHnD4ꢀn])
(mol [CHmD3ꢀmX])b
/
Entry
1
Reactants
Catalytic Conditions
~20
(CH4 + D2SO4 + (bpym)PtIICl2)
TFA-PtIV-CH3
2
<0.01
~20
TFA-PtII-CH3
3
4c
<0.01
PtIV + TFA-PtII-CH3
CH4+D2SO4 + PtIV
5d
~0.03
+ (bpym)PtIICl2
aAll reactions were carried out at 180 °C in 19 M D2SO4. With each
Pt complexes at 10 mM for all experiments, except entry 5. b sum of
all the isotopologues, n = 1 – 3 and m = 0 – 3. c1 equiv of PtIV relative
to (bpym)PtCl2 as a mixture of Pt(H2O)2(OH)4, bpym and ClSO3H. d5
eq of PtIV
.
Pathway C
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Pathway C involves only k4, kꢀ4, k1, k2, and k3 in Scheme 2. Pathway C operates by CH
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activation with PtII-X. The key distinction is that, consistent with the experimental studies that shows
complete oxidation of PtII-X upon treatment with hot D2SO4, X2PtIV-X is an offꢀcycle species and
assumed to be the resting state of the catalyst. Under reaction conditions, the resulting stability of the
catalytic system could arise from equilibrium between X2PtIV-X and a very low concentration of PtII
(Eq 7, k4/kꢀ4 >> 1) that is an extremely active catalyst. This proposal could be consistent with our
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calculations that show this reaction is favourable but with a low ꢀGrxn of ꢀ1 to ꢀ6 kcal/mol, vide supra.
As shown in Scheme 4, if the reaction proceeded by Pathway C involving only steps k4, kꢀ4, k1, k2, and
k3, it is logical that an increase in [SO2] would increase the equilibrium concentration of PtII–X and
increase the rate of catalysis for formation of CHnD4ꢀn or CHmD3ꢀmX (assuming that SO2 has no impact
on the other reaction steps in Pathway C).28 To investigate this possibility, we examined the impact of
added SO2 on the catalytic rate. The reactions were carried out by comparing reactions as described in
the original reports with the same [D2SO4] (or H2SO4), [(bpym)PtCl2], temperature, and PCH4 with and
without 200 psig of SO2 at the start of reaction. The results showed no difference in initial rates (<10%
conversion) for formation of CHnD4ꢀn or CHmD3ꢀmX. These observations rule against Pathway C,
Scheme 2, as the basis for the stability of the system. While there could be many explanations for the
lack of change with increased [SO2], a plausible explanation is that PtII-X, rather than offꢀcycle X2PtIV-
X, is the resting state and active catalyst. In this case, changes in [SO2] would result in very small
changes of [PtII-X] and increases in reaction rate that could not be observed.
(7)
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Scheme 4. Kinetic Analysis of Pathway C Showing the Expected Dependence of rate of formation of
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3
products (CHnD4ꢀn or CHmD3ꢀmX) on [SO2]
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Pathway D
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Pathway D involves only steps, k4, k1, k6, and k3 in Scheme 2. In this mechanism, the CH
activation step, k1, involves reversible reaction of PtII-X with CH4 to generate PtII-CH3. X2PtIV-X is
assumed to be inactive for CH activation. A key aspect of Pathway D is that step k6 provides a basis for
the stability of the system. In this “selfꢀrepair” step, X2PtIV-X that is continuously generated from
oxidation of PtII-X (Eq. 7) with concentrated H2SO4 is brought back into the catalytic cycle by reaction
with PtII-CH3 (Eq. 8). This reaction is similar to the Shilov system where at ~60oC, PtIV is used as the
stoichiometric oxidant that upon reduction generates the active PtII catalyst. The key difference is that
o
the concentrated H2SO4 solvent (rather than XPtIV-X) at 220 C is the overall oxidant and PtIV is
continuously generated during catalysis. To be the basis for the stability of the system, it is critical that
step k6 operates at comparable or faster rates to step k4. If the resting state of the catalyst is PtII-X and
step k6 is much slower than step k4, all the catalyst would pool as X2PtIV-X in minutes and catalysis will
effectively stop.
(8)
For step k6 to be comparable to step k4, it would require that k6 be exceedingly large. Based on
calculations that show the CH activation step, k1, is endoergic with ꢀG = ~16 kcal/mol, the
concentration of the proposed PtII-CH3 intermediate from step k1 should be vanishingly small. To be
competitive with the oxidation of PtII-X (maximum concentration of 10 mM based on added
(bpym)PtCl2) by the H2SO4 solvent (k4) and the protonolysis PtII-CH3, (kꢀ1) the bimolecular reaction
between X2PtIV-X and PtII-CH3 in step k6 would likely have to proceed with a barrier of <10 kcal/mol.
Stoichiometric reactions between PtIV and PtIIꢀCH3 models of the Shilov system have been shown to
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rapidly generate CH3X at rates that are competitive with protonolysis.29 However, these studies may not
be relevant to the PerianaꢀCatalytica system as reactions were carried out at lower temperatures, in
relatively weak nonꢀoxidizing acids, and with model ligated Ptꢀcomplexes that did not show CH
activation of CH4.
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As shown in Scheme 5, if catalysis proceeds only by Pathway D, analogous to the treatment for
Pathway B, kinetic analysis would predict the molar ratio of the Dꢀisotopologues of methane to
methanol, [(mol CHnD4ꢀn])/(mol CHmD3ꢀmX)]CAT and would give kꢀ1/(kꢀ6[X2PtIVꢀX] + k2[D2SO4])
assuming that both steps k2 and k4 proceeds at similar rates. Direct PtIIꢀMe oxidation (k2) is included in
this analysis since no data has been obtained to rule out the involvement of this step for formation of
product. Our analysis was focused on determining if Pathway A was the only pathway operating and
since Pathway C only includes the additional step k2, product formation by both pathways cannot be
ruled out. However, the focus of those analyses were on the basis for catalyst stability not product
formation.
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d[CHnD4-n
]
dt
k-1[D2SO4]
(mol CHnD4-n
)
=
=
k6[PtIV] + k2[D2SO4]
(mol CHmD3-mX)
d[CHmD3-mX]
dt
CAT
CAT
Scheme 5. Kinetic analysis of the Pathway D.
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To examine whether Pathway D is the basis for catalysis, stoichiometric reactions of the
independently synthesized complex, (TFA)PtII-CH3, were examined as a model of PtII-CH3 and the
molar ratio of (mol CHnD4ꢀn/mol CHmD3ꢀmX) obtained and compared to (mol CHnD4ꢀn/mol CHmD3ꢀ
mX)CAT. The stoichiometric reactions examined are summarized in Scheme 5. To model Pathway D
and the possibility for steps k2 and k6 the reactions were carried out in the absence and presence of a
model of X2PtIV-X (Table 1, Entries 3 and 4 respectively). Attempts at using (bpym)PtCl4 as a model of
X2PtIV-X were complicated by the instability of this complex in concentrated H2SO4. Consequently, a
mixture of PtIV(OH)4(H2O)2, bpym, and HCl (PtIV in Scheme 5) was used as a model of X2PtIV-X.30 As
noted above, that mixture was found to be just as effective as PtII-X for catalyzing methane conversion.
Assuming that (TFA)PtII-CH3 reacts as expected for PtII-CH3 in Pathway D, the expected
stoichiometric reactions are summarized in Scheme 5. As can be seen, if the reactions of (TFA)PtII-
CH3 are carried out in the absence of the X2PtIV-X model (PtIV), then the molar ratio of products, [(mol
CHnD4ꢀn])/(mol CHmD3ꢀmX)](TFA)Pt(II)ꢀCH3, should give kꢀ1/k2. Step k6 with X2PtIVX should not take place
assuming that under these conditions step k4 is not competitive with these steps. However, it is possible
that rapid protonolysis of (TFA)PtII-CH3 to generate PtII, followed by fast oxidation could generate
X2PtIVX that could complicate analysis. In the case of reaction of (TFA)PtII-CH3 with X2PtIVX, the
molar ratio of products, [(mol CHnD4ꢀn])/(mol CHmD3ꢀmX)]Pt(II)ꢀCH3 +Pt(IV) should be kꢀ1/(k6[X2PtIVꢀX] +
k2(D2SO4). As for studies of Pathway B, the key hypothesis is that similarities between [(mol CHnD4-
n])/(mol CHmD3-mX)]CAT, [(mol CHnD4-n])/(mol CHmD3-mX)](TFA)Pt(II)-CH3 and [(mol CHnD4-n])/(mol
CHmD3-mX)](TFA)Pt(II)-CH3+Pt(IV) obtained from the stoichiometric reactions of the (TFA)PtII-CH3 model
complex with and without X2PtIVX would support the proposed mechanism while a poor match would
rule against.
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The stoichiometric reactions were carried out with by injecting a 0.52 M solution of TFA-PtII-CH3 in
o
DMSO into a glass vial containing a stirred solution of concentrated D2SO4 at 180 C under argon.
DMSO was used as the solvent as in situ studies by NMR showed TFA-PtII-CH3 was stable at RT for
>30 min in this solvent. Immediately, upon injection, the reaction vial was removed and allowed to cool
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to RT and the gas and liquid phases analysed by GCꢀMS and H NMR. The results (Entry 3, Table 1)
show that stoichiometric reaction of TFA-PtII-CH3 yielded CH4 as essentially the only product and only
traces of CH3X were detected with [(mol CHnD4ꢀn)/(mol CHmD3ꢀmX)](TFA)Pt(II)ꢀCH3 ≈ 50 (within detection
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limits). As shown in Scheme 5, this suggests that the kꢀ1 is faster than the k2 step (kꢀ1 ≈ 50k2).
Importantly, while this ratio of products does not exactly match [(mol CHnD4-n])/(mol CHmD3-mX)]CAT
obtained from catalysis, ~20, Table 1, Entry 2, this ratio is a much better match than that from the
stoichiometric reaction of (TFA)XPtIV-CH3, (<0.01), Table 1, Entry 2. This could provide support for
step k1 in Pathway D, CH activation by PtII–X to give PtII-CH3. This is also supported by theoretical
studies that show the calculated activation barrier for CH activation from PtII-X of ~31 kcal/mol is
lower than that obtained from experiment, ~33 kcal/mol. This contrasts with the substantially higher
calculated barrier for CH activation with PtIV of ~41 kcal/mol.
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As noted above, given the relatively fast rate of oxidation of PtII-X to X2PtIVX in the absence of
methane and the low expected concentration of PtII-CH3, the k6 step must be extraordinarily fast to be
the basis for the stability of the system. To examine the feasibility of the k6 step, the stoichiometric
reaction of TFA-PtII-CH3 in concentrated D2SO4 at 180 oC was examined under the same conditions as
before but with 1 equiv. of X2PtIV-X (added as a 1:1:1 mixture of PtIV(OH)4(H2O)2, bpym, and HCl)
added to the D2SO4 solvent prior to reaction. Remarkably, as shown in Table 1, Entry 4, this reaction
gave almost no CH4 and only CH3X with ([CHnD4ꢀn])/([CHmD3ꢀmX])(TFA)Pt(II)ꢀCH3 + Pt(IV) = <0.01 (within
detection limits). This is remarkable and shows that 10 mM PtII-CH3 (final concentration) is oxidized
by one equiv. (TFA)XPtIV-X (10 mM), step k6, at a rate significantly faster than both protonolysis, k-1,
and oxidation, k2, by the reaction solvent, concentrated D2SO4 at 180 oC! As shown in Scheme 5, using
a value of 19 M for the concentration of D2SO4, 10 mM for the concentration of X2PtIVX, we estimate
that k6 is 105 times larger than kꢀ1 and 107 larger than k2.
These results make an important, testable prediction. The high rate of step k6, relative to k2 or kꢀ1
would suggest that carrying out the actual catalytic reaction at low TON with added X2PtIV-X prior to
reaction would result in a ratio of products, ([CHnD4ꢀn])/([CHmD3ꢀmD])cat+Pt(IV), that should be the same
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or very similar to the ratio of products from the stoichiometric reaction of X2PtIV-CH3, ([CHnD4ꢀ
n])/([CHmD3ꢀmD])(TFA)Pt(IV)ꢀCH3 (<0.01) used to examine Pathway B. In effect, under catalytic conditions
added X2PtIVX should trap almost all the PtII-CH3 generated from CH activation with PtII-X to
generate primarily CHmD3ꢀmX and little CHnD4ꢀn. In this reaction, it is critical that short reaction times,
lower temperatures and pressures of CH4 are utilized to ensure that the amount of CH3X that would be
generated by the catalytic oxidation of CH4 with H2SO4 in the absence of X2PtIV-X is small relative to
the amount of added X2PtIV-X prior to the catalytic reaction.
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To test this prediction, the catalytic reaction was carried out at 150 oC and 25 psig of CH4. Under
these conditions, in the absence of an added model of X2PtIV-X, the amount of CH3X generated by
catalytic oxidation of CH4 by H2SO4 after 30 min is low (TON <0.1, less than limits of detection) and
the extent of H/D exchange with CH4 is greater (> 250%, TON = 2.5). Remarkably, repeating the
reaction with 3 eq of the 1:1:1 mixture of PtIV(OH)4(H2O)2, bpym and HCl as a model of X2PtIV-X, led
to stoichiometric amounts of CH3X relative to the added X2PtIV-X model and very little H/D exchange.
Table 1, entry 5, ( [CHnD4ꢀn])/([CHmD3ꢀmX])cat+Pt(IV) < 0.01 (within experimental error). Taken
together, these results provide strong support for Pathway D as the basis for the high stability of the
system. This type of reaction mechanism where the higher oxidation state of the catalyst is offꢀcycle
and is brought back into the catalytic cycle by reaction with another intermediate can be considered to
be a catalytic “selfꢀrepair” mechanism.
Additional evidence for CH activation by PtII-X may also be obtained by comparison of the ratio
of isotopologues CHnD4ꢀn (n = 1 to 4) generated from the rapid, stoichiometric reaction of TFA-PtII-
CH3 and the actual catalytic system. Figure 4 shows the mol % of the various CH4 isotopologues
generated from the catalytic reaction as a function of time. Remarkably, this data shows that even at
short reaction times and low conversion of CH4 (<5 %) the various isotopologues are generated
simultaneously (i.e., CH4 ꢀ CHnD4ꢀn, with n = 1 – 3 all observed) rather than sequentially (CH4 ꢀ
CH3D ꢀ CH2D2, etc.). In a sequential reaction, at low conversion of CH4, assuming no large, inverse
kinetic isotope effect, the primary product would be CH3D without any of the more substituted
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isotopologues. Significantly, GCꢀMS analysis of the gas phase from the stoichiometric reaction of PtII-
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CH3 with D2SO4 at 180 C also shows that all the isotopologues of CHnD4ꢀn (n = 0, 1, 2 and 3), are
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formed (see SI). This distribution is remarkable since the stoichiometric reaction is stopped immediately
upon mixing and is irreversible (the resulting concentration of CH4 is very low and the reaction times
very short). The similarity of simultaneous formation of the isotopologues, CHnD4ꢀn, n = 1 – 3) from the
catalytic system and stoichiometric reaction further supports step k1 in Pathway D, the CH activation by
PtII–X to give PtII-CH3. Consistent with these result DFT studies show that the formation of the various
isotopologues is due to reversible protonolysis of PtII-CH3 to yield the corresponding CH4 complex
[PtII-(CH4)]+ before loss of CH4 and generation of PtII-X.31
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Figure 4. Time dependent plot of Mol % of the CH4 Isotopologues, CHnD4ꢀn result from reaction
between D2SO4 and CH4 catalyzed by (bpym)PtCl2
DFT Calculations
These experiments show that the activation barrier for the k6 step in Pathway D must be lower
than the protonolysis of PtII-CH3, step kꢀ1, as well as for oxidation of PtII-CH3 to generate X2PtIV-CH3,
step k2, by the reaction solvent H2SO4 at 180oC! The high concentration of H2SO4 (19M) and a
maximum concentration of 10 mM X2PtIVX this would suggest that the barrier for step k6 could be
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Page 24 of 43
lower than 10 kcal/mol. To examine this reaction more closely, we examined these pathways via DFT
calculations. This type of biꢀmolecular reaction between PtII-CH3 and X2PtIV-X to generate X2PtIV-
CH3 and PtII-X could proceed via electron or methyl transfer. However, as a result of the studies of the
Shilov system showing that reactions with model complexes proceed via an electron transfer,6c we did
not examine the transition states for methyl transfer. On the basis of the classical selfꢀexchange studies
involving innerꢀsphere electronꢀtransfer (ISET) between fourꢀcoordinate, square planar [Cl4PtII]2ꢀ and
octahedral [Cl6PtIV]2ꢀ salts,6 the reaction of PtII-CH3 with X2PtIV-X could proceed as shown in Scheme
6 via nucleophilic activation of PtII-CH3. In this case, the PtII-CH3 is activated by addition of Clꢀ, the
strongest potential nucleophile in the catalytic system, to generate a 5ꢀcoordinate, [ClꢀPtIIꢀCH3]ꢀ
intermediate with a more nucleophilic Ptꢀcentered lone pair. This activated species can attack the Cl of
Cl(X)PtIV-X to generate Cl2PtIV-CH3 by “displacing” [XꢀPtII-X]- that subsequently losses Clꢀ to
generate PtII-X, Scheme 6. This ISET could also be seen as a formal Cl+ transfer.
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Since it would seem that such a nucleophilic activation of PtII-CH3 may not be plausible in a
strongly acidic media, we also considered the complimentary possibility that the reaction could be
facilitated by electrophilic activation of the X2PtIV-X species by the concentrated H2SO4 solvent. In this
case, reaction of X2PtIV-X with H+ leads to loss HX and generate a more electrophilic 5ꢀcoordinate,
cationic PtIV species, [ClꢀPtIVꢀX]+. In this species, the ꢀCl attached to the 5ꢀcoordinate Pt center would
be electrophilically activated to react nucleophilically with the lone pair of the unactivated PtII-CH3 to
generate PtII-X and [ClꢀPtIVꢀCH3]+ that can subsequently react with HX to generate X2PtIV-CH3 and
regenerate the H+.
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Scheme 6. Postulated Mechanism for Innerꢀsphere Electron Transfer between PtII-CH3 and X2PtIV-X
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by Nucleophilic or Electrophilic Activation
Figure 5. Calculated Transition States for the innerꢀsphere electronꢀtransfer transfer between PtII-
CH3 to the X2PtIV-X by Nucleophilic (Left) and Electrophilic (right) activation.
As can be seen in Figure 5, DFT calculations show that the pathway involving nucleophilic
activation of PtII-CH3 with Clꢀ TS1 proceeds with a ~20 kcal/mol barrier. The previously reported31
barrier for protonolysis of PtII-CH3 (transition state not shown, see SI) by concentrated H2SO4 to
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Page 26 of 43
generate CH4 was calculated to be ~10 kcal/mol, TS2. Consequently, since the experimental data, Table
1, entry 4, shows that the k6 step is faster than the protonolysis step, kꢀ1, it is unlikely the step k6
between PtII-CH3 with X2PtIV-X proceeds via TS1 involving nucleophilic activation of PtII-CH3.
Remarkably, consistent with the experimental observations, the reaction of PtII-CH3 with the 5ꢀ
ccordinate species resulting from electrophilic activation of X2PtIV-X with H2SO4 shows a transition of
state, TS2, of ~6 kcal/mol. This result provides strong additional support for the k6 step and Pathway D
as the basis for the catalysts stability.
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Mechanism for Product Formation:
As noted earlier, the pathways that account for product stability and product formation need not be the
same. Thus, while Pathway D can account for the high stability of the system, it is possible that
Pathway D and Pathway A (involving direct oxidation of PtII-CH3 with H2SO4 to generate X2PtIV-
CH3, the k2 step) are competitive and that both contribute to product formation. The observation that the
reaction of PtII-CH3 with 1 equiv. of X2PtIV-X generates almost exclusively CH3X, Entry 2, Table 1,
could suggest that Pathway D may be the pathway that accounts for both the high stability of the
system and the product generation. However, if the steady state concentration of X2PtIV-X is very low
then step k2 could be competitive with step k6 and Pathway A and D could be competitive. Since the k2
step in Pathway A could not be examined due to rapid protonolysis of the (TFA)-PtII-CH3 model
complex, the feasibility and energetics for the direct oxidation of PtII-CH3 with concentrated H2SO4
(containing SO3) to generate X2PtIV-CH3 was investigated using DFT.
A composite energy diagram of the two pathways, Pathway D and Pathway A are summarized in
Figures 6 and 7 respectively. These pathways were based on the lowest energy species identified.
However, there could be other lower energy pathways. It should be noted that the lowest energy
pathway identified for the direct oxidation of PtII-CH3 involves the reaction of PtII-X with H2SO4/SO3
to generate a PtIIꢀSO3H adduct with a PtꢀS bond. This is similar to the species identified by Ziegler in
studies of this reaction.18 Pathways involving electronꢀtransfer via O or Clꢀ bridges between Pt and S of
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H2SO4 were significantly higher in energy. Assuming the presence or a small energetic cost for
formation of SO3 under the reaction conditions, an important conclusion from the DFT studies is that
the rates of Pathway A and D could be comparable. This would suggest that while Pathway D is
essential to the catalyst stability that both Pathway A and D could contribute to product generation.
However, since the catalysis can be carried out between 90 – 98%, where the concentration of free SO3
is low, it is possible that Pathway A is not accessible and that Pathway D accounts for both the stability
and product formation.
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Figure 6. Energy Diagram from DFT of Pathway D.
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Perhaps the most important conclusion from this study is that the rate determining step in Pathway D
is k4, the oxidation of PtII-X to X2PtIV-X. This would suggest that increasing the rate of this step could
increase the rate of catalysis in the PerianaꢀCatalytica system. Since k1, k6, and k3 have much lower
barriers than k4, it is possible that significant increases in rate could be possible without causing the
system to deactivate. This is important to recognize since it opposite to the accepted view that oxidation
of the reduced state of a catalyst to the more stable higher oxidation state that is active for CH
activation must be suppressed to prevent catalyst deactivation. We have begun to study methods of
increasing, rather than suppressing the rate of this k4 oxidation reaction in our laboratory with the
objective of increasing the rate of catalysis. We would like to emphasize that other parallel mechanisms
for oxidation of PtII-X are also possible. However, no other mechanism with a low barrier was located.
The mechanism proposed by Ziegler via SꢀO oxidative addition was calculated to have a barrier of 44.9
kcal molꢀ1, in agreement with the previous study.18
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Figure 7. Energy Diagram from DFT of Pathway A.31
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CONCLUSION
Studies of the highly stable, PerianaꢀCatalytica (bpym)PtCl2 catalyst for the oxidation of methane with
concentrated sulfuric acid at 200oC to methanol has been shown that operate by a more complex
mechanism than previously considered. Mechanistic studies show that, consistent with the original
proposals, PtIV is not active for reaction with methane and that the reaction proceeds by CH activation
with a PtII species to generate PtIIꢀCH3. Significantly, in the absence of methane the (bpym)PtIICl2
catalyst has been found to rapidly oxidize in concentrated H2S04 at 200°C to PtIV. Unexpectedly,
contrary to the general teaching that the oxidation of the PtII to PtIV should be minimized in order to
prevent catalyst deactivation, studies show that increasing the rate of this over oxidation of PtII can
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