Continuous Flow Alkane Dehydrogenation by Supported Pincer

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Continuous Flow Alkane Dehydrogenation by Supported Pincer-Ligated Iridium Catalysts at Elevated Temperatures Boris Sheludko, Molly T Cunningham, Alan S. Goldman, and Fuat E. Celik ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b01497 • Publication Date (Web): 16 Jul 2018 Downloaded from http://pubs.acs.org on July 16, 2018

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ACS Catalysis

Continuous Flow Alkane Dehydrogenation by Supported Pincer-Ligated Iridium Catalysts at Elevated Temperatures

Boris Sheludko,1,2 Molly T. Cunningham,1 Alan S. Goldman,1 Fuat E. Celik2*

1

2

Department of Chemistry and Chemical Biology

Department of Chemical and Biochemical Engineering

Rutgers, The State University of New Jersey, Piscataway, NJ 98 Brett Road, Piscataway, New Jersey 08854, United States

* Tel.: +1 848 445 5558. E-mail: [email protected]

1 ACS Paragon Plus Environment

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Abstract Pincer-ligated iridium complexes of the form [Ir(R4PCP)L] (R4PCP = κ3-C6H3-2,6(XPR2)2; X = CH2, O; R = tBu, iPr) are efficient homogeneous alkane dehydrogenation catalysts that have been reported to be highly active at temperatures of 240 °C or below. In this work, silica-supported [Ir(C2H4)(p-tBu2PO-tBu4POCOP)] (1/SiO2) was used to study a model continuous-flow gas-phase acceptorless alkane dehydrogenation system. This particular supported framework is thermally stable at temperatures up to 340 °C, 100 °C above the highest temperature at which analogous homogeneous complexes have been reported to show stable activity, with observed butane dehydrogenation rates of ca. 80 molbutenes molcat-1 h-1. Solid-state 31

P MAS-NMR and ATR IR are used to demonstrate that the backbone pincer ligand remains

intact and coordinated at 340 °C. The complex is fully converted to [Ir(CO)(p-tBu2POtBu4

POCOP)] (3/SiO2) above 300 °C. 3/SiO2 is observed to be catalytically active at the higher

temperatures tested, and reaction rates are comparable to those of 1/SiO2. 3/SiO2 and 1/SiO2 act as resting states for the active 14-electron fragment, through dissociation of the CO or olefin ligand respectively. Given that 3/SiO2 is air resistant at ambient temperature and is structurally stable and catalytically active at elevated temperatures, it is a suitable candidate as a catalyst for the highly endothermic acceptorless dehydrogenation of alkanes.

Key Words alkane dehydrogenation, pincer ligand, silica, supported molecular catalyst, thermal stability


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1. Introduction Olefins are highly versatile chemical building blocks, useful as precursors for polymers, fuels, and a wide range of chemicals. They are commercially obtained from dehydrogenation of alkanes in high-energy processes such as STAR, Catofin and Oleflex.1 However, such systems have significant shortcomings, including the fact that catalysts used industrially deactivate over time by coking due to the high temperatures used and need to be regenerated, leading to nontrivial downtime and energy expenditure. In addition, products need to be separated due to the numerous olefins produced from even short-chain alkane cracking. With the advent of the "shale gas revolution,"2-4 a new feedstock for such dehydrogenation processes has emerged. On-purpose dehydrogenation plants for light alkanes such as propane are being opened5 or planned at an increased rate. these processes also suffer from the same issues as more traditional petroleum cracking including a need for high-energy input, a lack of selectivity and catalyst deactivation over time.1 Selectivity generally presents a major challenge for high-temperature alkane conversion processes. Thermal cracking is likely at high temperatures, leading to n-alkyl radicals which can then perform hydrogen atom abstraction, or radical addition to olefins to form longer chain alkanes which can crack further. Shorter contact times can typically mitigate this, leading to a higher selectivity for the desired dehydrogenation products with minimal side products formed. Pincer-ligated iridium catalysts6 of the form [Ir(R4PCP)L] (R4PCP = κ3-C6H3-2,6-(XPR2)2; X = CH2, O; R = tBu, iPr) have been studied extensively in the context of homogeneous (solution-phase) alkane dehydrogenation both for their high activity as well as their regioselectivity for formation of terminal olefin. Dehydrogenation by such catalysts can occur at or below 240 °C, allowing for reaction conditions much milder than current industry standards.


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The lower equilibrium conversion at these milder conditions do not allow these catalysts to reach the higher olefin yields achieved in industrial processes at much higher temperatures, but they may be incorporated into tandem processes that upgrade and consume the olefins as they form, producing value-added products in a single reactor.7 Since their discovery, various frameworks including [Ir(POCOP)], [Ir(PCOP)], [Ir(PSCOP)], [Ir(CCC)], and [Ir(NCN)], as well as complexes of metals other than iridium have been developed and investigated for various reactions including dehydrogenation.8 Recently, [Ir(iPr4PCP)] complexes analogous to the parent [Ir(tBu4PCP)] complex have been used in the dehydrogenation of gas-phase alkane,9 functioning as discreet molecular catalysts in a solventless system, suggesting their potential for dehydrogenation of shorter-chain olefins such as ethane and propane.

Acronym [Ir(PCP)] [Ir(POCOP)] [Ir(PCOP)] [Ir(PSCOP)] [Ir(CCC)] [Ir(NCN)]

Substituents Z = P, E1 = E2 = CH2 Z = P, E1 = E2 = CH2 Z = P, E1 = CH2, E2 = O Z = P, E1 = S, E2 = O Z = C, E1 = E2 = CH2 Z = N, E1 = E2 = CH2

In order to develop industrially relevant processes based on alkane dehydrogenation by pincer-iridium complexes, it would be desirable to implement such selective catalysts in a continuous-flow system. Supporting catalyst (precursors) of the form [Ir(COD)L2] on metal oxide surfaces has afforded dehydrogenation activity which remained stable with the initial addition of excess phosphine.10-12 Pincer-ligated catalysts have also previously been supported to


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ACS Catalysis

the same end: supported complexes of the form [Ir(X-R4PCP)] and [Ir(X-R4POCOP)] (X = functional group) on alumina also result in active dehydrogenation catalysts.13 Iridium complexes of triptycene-based pincer ligands have also been supported on alumina,14 presumably via a Lewis acid-Lewis base interaction anchoring the catalyst in place. More recently, MOF-supported pincer-iridium complexes have also been observed to be catalytically active in hydrogenation which, while being the reverse of acceptorless dehydrogenation, suggests promise in the development of systems incorporating MOF-supported dehydrogenation catalysts.15 In all reports of supported pincer-ligated catalysts, a backbone substituent, most often in the para- position of a central aryl group, is required to maintain activity. In the case of alumina as the supporting material, Lewis basic substituents such as -OK, -OMe, and -NMe2 have been incorporated into active, supported catalysts.13 In the absence of such a para-substituent, oxidative addition of support hydroxyl groups to the iridium center occurs, resulting in Ir(III) species.15-17 Such catalysts are effective in olefin hydrogenation, with reported turnover frequencies of up to 0.95 s-1 and stable activity for several days in the case of ethylene.16,17 However, these Ir(III) catalysts are not expected to be good alkane dehydrogenation catalysts compared to pincer-ligated Ir(I) complexes as they would not have access to the established Ir(I)Ir(III) couple responsible for dehydrogenation with molecular catalysts. While more active for dehydrogenation than their unfunctionalized analogues, γ-aluminasupported [Ir(p-KO-tBu4POCOP)] and [Ir(p-tBu2PO-tBu4POCOP)] catalysts still show a 60-80% drop in activity upon recycling under transfer dehydrogenation conditions, while γ-aluminasupported [Ir(p-Me2N-tBu4PCP)] showed no added recyclability as compared with solution-phase systems. Deactivation can occur by a variety of mechanisms including inhibition by products,


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thermal decomposition, or undesirable interaction with the support such as the generation of a resting-state Ir(III) species outside of the catalytic cycle of interest. In this work, we report development and kinetic studies of a continuous-flow gas-phase system for alkane dehydrogenation by pincer-iridium complexes. Such a system allows access to reaction conditions unattainable in a batch system – fine control over pressure and temperature, reagent partial pressures, and residence time. Unlike in batch or semi-batch experiments where they may accumulate, products are continuously removed from the reactor, limiting the extent of secondary reaction or product inhibition. In this system, we use [Ir(C2H4)(p-tBu2PO-tBu4POCOP)] on various supports as a model catalyst precursor, and we demonstrate catalyst stability under acceptorless alkane dehydrogenation conditions. Through MAS-NMR, XPS and ATR FTIR, we describe catalyst speciation as a function of reaction conditions, providing insight into the catalyst species involved in the reaction.

2. Experimental Methods 2.1 Catalyst Preparation All chemical syntheses and material preparations were performed under an air-free atmosphere unless otherwise noted. The ethylene complexes [Ir(C2H4)(p-tBu2PO-tBu4POCOP)] (1) and [Ir(C2H4)(tBu4POCOP)Ir] (2) were synthesized according to previously established procedures.13 The carbonyl complex [Ir(CO)(p-tBu2PO-tBu4POCOP)] (3) and dihydride complex [IrH2(p-tBu2PO-tBu4POCOP)] (4) were obtained from 1 using procedures used for similar complexes.18


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(p-tBu2PO-tBu4POCOP): To a Schlenk flask charged with a stir bar were added 100 mg (0.79 mmol) phloroglucinol and 76 mg (3.17 mmol) sodium hydride and dissolved in 15 mL THF with the evolution of hydrogen. This mixture was stirred at reflux for two hours, at which point 570 mg (3.17 mmol) chloro-di-tert-butylphosphine was added and let stir an additional two hours. Volatiles were removed in vacuo and the solid residue was extracted several times with pentane and filtered through a kimwipe plug. The washes were combined and volatiles were pulled off in vacuo at 55 °C to yield the product as 391 mg (0.70 mmol, 88.6%) of a white solid. Purity was confirmed by comparison to literature.13 [IrClH(p-tBu2PO-tBu4POCOP)]: To a Schlenk flask charged with a stir bar were added 229 mg (0.409 mmol) (p-tBu2PO-tBu4POCOP) and 137 mg (0.204 mmol) [Ir(COD)Cl]2 and dissolved in 10 mL toluene. This solution was stirred at reflux for 16 hours, by which point the solution was deep red in color. All volatiles were removed in vacuo and the residue was extracted several times with benzene and filtered through a kimwipe plug. Solvent was removed in vacuo once again to leave the product as a bright red microcrystalline powder (256 mg, 80% yield). 31P and 1

H NMR spectra matched well with those in the literature.13

[Ir(C2H4)(p-tBu2PO-tBu4POCOP)] (1): To a Schlenk flask charged with a stir bar were added 100 mg (0.127 mol) [IrClH(p-tBu2PO-tBu4POCOP)] and 14.3 mg (0.127 mmol) potassium tertbutoxide and dissolved in 5 mL benzene. This solution was stirred under ethylene at room temperature for 16 hours, with a slight change in color to red-brown. All volatiles were removed in vacuo and the residue was extracted several times with benzene and filtered through a kimwipe plug. Solvent was removed in vacuo once again to leave the product as a brown microcrystalline powder (90 mg, 91% yield). 31P and 1H NMR spectra matched well with those in the literature.13


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(tBu4POCOP): To a Schlenk flask charged with a stir bar were added 202 mg (1.83 mol) resorcinol and 92 mg (3.83 mmol) sodium hydride and dissolved in 10 mL THF (caution: evolution of hydrogen). This mixture was stirred at reflux for one hour, at which point 0.73 mL (690 mg, 3.8 mmol) chloro-di-tert-butylphosphine was added and let stir an additional 16 hours. Volatiles were removed in vacuo and the solid residue was extracted several times with pentane and filtered through a Kimwipe plug. The washes were combined and volatiles were pulled off in vacuo at 55 °C to yield the product as a translucent colorless oil (637 mg, 89% yield). 31P and 1H NMR spectra matched well with those in the literature.19 [IrClH(tBu4POCOP)]: To a Schlenk flask charged with a stir bar were added 195 mg (0.489 mmol) (tBu4POCOP) and 150 mg (0.223 mol) [Ir(COD)Cl]2 and dissolved in 5 mL toluene. This solution was stirred at reflux for 24 hours, by which point the solution was deep red in color. All volatiles were removed in vacuo and the residue was extracted several times with toluene and filtered through a kimwipe plug. Solvent was removed in vacuo once again to leave the product as a brown microcrystalline powder (246 mg, 88% yield). 31P and 1H NMR spectra matched well with those in the literature.19 [Ir(C2H4)(tBu4POCOP)] (2): To a Schlenk flask charged with a stir bar were added 50.7 mg (0.0810 mmol) [IrClH(tBu4POCOP)] and 9.5 mg (0.085 mmol) potassium tert-butoxide and dissolved in 10 mL toluene. This solution was stirred under ethylene at room temperature for 16 hours, with a slight darkening in color. All volatiles were removed in vacuo and the residue was extracted several times with pentane and filtered through a kimwipe plug. Solvent was removed in vacuo once again to leave the product as a brown microcrystalline powder (43 mg, 86% yield). 31

P and 1H NMR spectra matched well with those in the literature.13


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ACS Catalysis

[Ir(CO)(p-tBu2PO-tBu4POCOP)] (3): To a J. Young NMR tube was added 30 mg (0.039 mmol) [Ir(C2H4)(p-tBu2PO-tBu4POCOP)] (1) dissolved in 400 µL C6D6. This solution was then degassed and charged with 1 atm of carbon monoxide. The NMR tube was spun overnight, by which point the color of the solution was a dark orange and quantitative conversion to the carbonyl complex was observed by NMR along with generation of free ethylene. Volatiles were removed in vacuo and the isolated yellow solid was used without further purification. Elemental analysis calculated for C31H56O4P3Ir (777.91 g/mol): C 47.86, H 7.26; found: C 42.35, H 8.79. 1H NMR (400 MHz, C6D6): δ = 1.09 (d, 3JP,H = 11.6 Hz, 18 H, 2 x P(tBu)2), 1.31 (virtual t, 3JP,H = 7.2 Hz, 36 H, 4 x P(tBu)2), 7.00 (s, 2 H, Ar-H). 31P{1H} NMR (162 MHz, C6D6): δ = 154.3 (s, 1 x uncoordinated P(tBu)2), 202.6 (s, 2 x coordinated P(tBu)2). [IrH4(p-tBu2PO-tBu4POCOP)] (4): To a J. Young NMR tube was added 10 mg (0.013 mmol) [Ir(C2H4)(p-tBu2PO-tBu4POCOP)] (1) dissolved in 400 µL D8-toluene. This solution was then degassed and charged with 1 atm of H2. The NMR tube was spun overnight and >99% conversion to the H2 and H4 complexes was observed by NMR along with generation of ethane. Volatiles were removed in vacuo and the isolated brown solid was used without further purification. Elemental analysis calculated for C30H58O3P3Ir (751.92 g/mol): C 47.92, H 7.77; found: C 44.05, H 9.80. 1H NMR (400 MHz, D8-toluene): dihydride: δ = -8.42 (bs, 2H, Ir-H), 1.10 (d, 3JP,H = 11.6 Hz, 18 H, 2 x P(tBu)2), 1.30 (virtual t, 3JP,H = 7.13 Hz, 36 H, 4 x P(tBu)2), 7.09 (s, 2 H, Ar-H). tetrahydride: δ = -17.6 (bs, 4 H, Ir-H), 0.95 (d, 3JP,H = 14.7 Hz, 18 H, 2 x P(tBu)2), 7.00 (s, 2 H, Ar-H). Note: Signals from chelating tert-butyl protons overlapped with signals from dihydride and thus were not observed.

31

P{1H} NMR (162 MHz, D8-toluene):

dihydride: δ = 156.4 (s, 1 x uncoordinated P(tBu)2), 211.5 (s, 2 x coordinated P(tBu)2). tetrahydride: δ = 189.3 (s, 2 x coordinated P(tBu)2). Note: Signal from chelating phosphorus


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atoms overlapped with signals from the same in the dihydride complex and thus were not observed. Mg(Al)O was prepared by post-synthesis modification of hydrotalcite. Aluminum nitrate nonahydrate, magnesium nitrate hexahydrate, sodium carbonate and sodium hydroxide were obtained from Acros and used as received. Hydrotalcite was synthesized by the coprecipitation method.20,21 A solution prepared from Al(NO3)3·9H2O and Mg(NO3)2·6H2O in deionized water was added dropwise into a stirred solution of Na2CO3 and NaOH in deionized water at a constant temperature of 60 °C, and stirring continued for an hour after mixing. The rinsed and filtered hydrotalcite precipitate was dried at 110 °C, then ground. The powder was calcined at 700 °C for three hours in a tube furnace to yield the hydrotalcite-derived Mg(Al)O. Mesoporous silica (Sigma-Aldrich) and γ-alumina (Strem) were obtained commercially. SiO2, Al2O3, and Mg(Al)O were calcined in a gas-tight reactor equipped with valves to seal the samples in dry air following calcination. Calcination was carried out under 100 mL min-1 using a 2 °C min-1 ramp to 550 °C, held for 3 hours, and cooled to room temperature at 2 °C min-1. The reactor was then sealed using the valves and transported to a high-vacuum line, evacuated, and brought into a glovebox for use as catalyst support. In a typical experiment, 5 mg of catalyst (1, 2, or 3) was supported onto 150 mg of support (SiO2, Al2O3, or Mg(Al)O) using incipient wetness impregnation. Approximately 3.2 wt% catalyst/support was prepared by dissolving the catalyst in a volume of benzene or toluene equivalent to the pore volume of the support material. The nominal catalyst coverage calculated from the amount of metal catalyst impregnated in the support is approximately 0.8 metal wt% based on iridium. The actual amount of iridium deposited was determined by ICP-MS analysis (Robertson Microlit Laboratories) to be 0.71 wt% and by XPS to be 0.77 wt%, in good


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ACS Catalysis

agreement with the expected value. After impregnation, the solid was dried for one hour in the glovebox prior to packing in the gas-phase flow reactor. The appearance of 1/SiO2 is of a redpurple powder, 2/SiO2 is a brown powder, and 3/SiO2 is a yellow powder. 2.2 Steady-state Gas-phase Continuous-flow Catalytic Data Reactions were carried out in a 6.4 mm outer diameter (OD) quartz tube reactor with an expanded section of 12.5 mm OD packed with quartz wool to hold the catalyst powder in place. The reactor was packed with supported catalyst under argon atmosphere and sealed with valves prior to connection to the gas-flow manifold, and connected under inert gas flow. The reactor was placed inside a resistively heated ceramic furnace with external temperature control, and the catalyst bed temperature was measured with a K-type thermocouple placed in direct contact with the catalyst bed. Butane (Airgas, 99.99%) was used as received. Helium (Praxair, 99.999%) was passed through an on-stream oxygen and moisture trap. Reaction products were analyzed using an Agilent 7890B GC equipped with a GS-GASPRO capillary column (0.32 mm x 60 m) fitted with a splitter plate connected to a flame ionization detector and a 5977A mass spectrometer. Unless stated otherwise, all experiments were carried out at 1.27 atm total pressure by throttling a needle valve located downstream from the reactor. Gas flow rates reported were measured at room temperature and pressure. For experiments at elevated pressure, the gas flow rates were increased to maintain constant volumetric flowrate at the reactor pressure. 2.3 Batch Catalytic Data In a typical batch experiment, 1.5 mL glass ampules were sealed with either 100 µL solution of 1 or 2 (1 mM) in dodecane or the equivalent amount of catalyst supported on 6.7 mg


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SiO2 (to match catalyst loading used in continuous-flow runs) in the presence of 100 µL dodecane. These ampules were heated from room temperature to 340 °C and held for 30 minutes or 24 hours, at which point the oven was cooled down and the vials were removed. The ampules were opened in a glovebox and the liquid contents sampled via GC. In the case of recycling experiments, all volatiles were removed by mild heating under vacuum. Fresh dodecane was added and the ampules were resealed. All experiments were performed in duplicate. 2.4 Catalyst Characterization Solution phase NMR spectra were recorded on a Varian-400 or Varian-500 spectrometer. The samples for solid-state 31P MAS NMR spectra were packed into 3.2 mm zirconia solid-state NMR rotors under an argon atmosphere and sealed with tight-fitting rotor caps. Solid-state

31

P

MAS NMR spectra were recorded on a Bruker Avance III spectrometer operating at 162 MHz with magic angle spinning (MAS) of 10 KHz. The rotors were spun with air and no change to sample spectra was observed on the time scale of spectrum collection. All spectra were recorded at room temperature. Spinning side bands were observed at 10 KHz intervals upfield and downfield of immobile species’ signals, corresponding to the frequency of the rotor spin. Attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectra were measured on a Bruker ALPHA spectrometer equipped with a germanium crystal plate. X-Ray Photoelectron Spectroscopy (XPS) was measured under UHV using a Thermo Fisher K-Alpha XPS instrument equipped with a monochromatic Al Kα line with photon energy of 1486.7 eV. The estimated depth sensitivity in this geometry is about 10 nm.22 Spectra were deconvoluted using the method described by Nesbitt and Banerjee.23 All reported binding energies are in reference to the C (1s) peak at 284.8 eV.


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Multi-point BET surface area and pore volume distribution were measured at -196 °C by nitrogen physisorption using an Autosorb1 (Quantachrome). Pore diameter and volume were measured via NLDFT equilibrium physisorption at -196 °C with nitrogen as the adsorbent and silica as the oxygen model. Samples were degassed under nitrogen at 100 °C for six hours prior to surface area measurement in a 6 mm tube.

3. Results and Discussion

O P Ir O P

[Ir(C2H4)(p-tBu2POtBu4 POCOP)] 1

[Ir(C2H4)(tBu4POCOP)] [Ir(CO)(p-tBu2PO-tBu4POCOP)] [IrH2(p-tBu2PO-tBu4POCOP)] 2

3

4

O P O P H

Ir

O O

1/SiO2

2/SiO2

3/SiO2

Si

O

O P

O

4/SiO2

Figure 1. Molecular catalyst species (1, 2, 3, and 4) prepared and supported on SiO2 in the present study. Expected structures for covalently anchored supported molecular catalysts from chemisorption (1/SiO2, 2/SiO2, 3/SiO2 and 4/SiO2). Silica surface represented by one SiO4 tetrahedron.

3.1 As-prepared Supported Catalyst


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Upon supporting 1 on SiO2, chemisorption is expected to lead to a condensation reaction between the para-substituent phosphinite group and a surface hydroxyl group to form a covalent Si-O-C bond between the molecular catalyst and the surface of the silica.24 The resulting supported complex, 1/SiO2, is anchored to a single site on the surface of the support, as illustrated in Figure 1. The red-purple color (Figure S1) of the supported complex is consistent with the color of the complex in solution prior to supporting on SiO2. To confirm that supported catalyst structure was consistent with 1/SiO2 and with previous reports, solid-state

31

P MAS-

NMR was conducted to examine the environment of the chelating phosphorus atoms of the pincer ligands. Figure 2a shows the solution-phase

31

P NMR spectrum of 1, with two signals for

phosphorus corresponding to the para-substituent phosphinite group at 150.6 ppm13 and the chelating phosphorus atoms at 181.0 ppm13 with an approximate peak area ratio of 1:2, consistent with the structure of 1. Upon supporting 1 on SiO2, the majority of the complex undergoes condensation with surface silanol groups at room temperature to generate 1/SiO2. The spectrum of the supported complex (Figure 2b) shows the formation of two major new peaks at 174.7 ppm and 72.5 ppm. A small amount of complex 1 remained in physisorbed or molecular form, evidenced by the minor peaks at 150.6 and 181.0 ppm. The new peak at 174.7 ppm13 is assigned to the chelating phosphorus atoms of the supported complex 1/SiO2, shifted upfield from 181.0 ppm for the physisorbed species. The chemisorbed and covalently anchored species is anchored strongly to the surface and is relatively immobile, as indicated by the presence of spinning side bands which would be absent in the case of more mobile species. The new peak at 72.5 ppm is assigned to di-tert-butylphosphine oxide24, the leaving group from the condensation reaction of the para substituent phosphinite group of 1. The peak area ratio between the 72.5


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ACS Catalysis

ppm and 174.7 ppm peaks was approximately 1:2, consistent with the stoichiometry of the condensation reaction between 1 and a silanol group to form 1/SiO2 and di-tert-butylphosphine oxide. This binding interaction preserves the iridium in the Ir(I) oxidation state. The interaction of pincer complexes lacking para-substituents, including 2, with SiO2 surfaces has been studied extensively and leads to interaction through the iridium center as shown for 2/SiO2 in Figure 1.16,17,25,26 These chemisorbed Ir(III) complexes have been shown to be active for hydrogenation reactions in batch and continuous flow systems. The impact of supporting molecular iridium complexes on surface area was investigated using the carbonyl complex 3 instead of the olefin complex 1. Pincer-type iridium carbonyl complexes have been anecdotally reported to be more stable with respect to air exposure relative to olefin complexes. For example, the time scale over which the color of the complex changes upon air exposure is significantly longer for carbonyl complexes, which we have also observed. This higher air-tolerance permitted the collection of accurate BET surface area measurements with low risk of the catalyst changing oxidation state or structure over the time-scale of the BET experiment. The surface area of the calcined SiO2 was 578 m2 g-1. Upon supporting 3, the surface area of 3/SiO2 decreased to 510 m2 g-1 and the pore volume decreased from 0.77 cm3 g-1 to 0.62 cm3 g-1, indicating the presence of catalyst within the mesopores of the support. Also observed were a small drop in larger diameter pores (6 – 10 nm pores) and a decrease in the modal pore width from 5.4 to 5.1 nm, both consistent with a submonolayer surface coverage of iridium complexes. Assuming 1.67 silanol groups per square nanometer for SiO2 calcined at 550 °C,27 the expected ratio of SiOH groups to Ir atoms is approximately 3,000. Comparison of the solution-phase 31P NMR of 3 (Figure 2h) with the solid-state NMR of 3/SiO2 (Figure 2g) confirmed that the carbonyl complex was supported in a manner analogous to


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that of the olefin complex 1/SiO2. As observed with 1, the chemical shift of the para-phosphinite group in 3 at 151.9 ppm13 disappeared upon supporting on SiO2, yielding a signal at 73.4 ppm attributable to di-tert-butylphosphine oxide. The substitution of the ethylene ligand by CO has a strong effect on the chemical shift of the chelating phosphorus atoms, resulting in a downfield shift from 181.0 ppm in 1 to 200.1 ppm in 3; the corresponding chemical shifts for the supported species are 174.7 ppm and 202.8 ppm for 1/SiO2 and 3/SiO2, respectively. The dihydride analogue of 1, species 4, could be considered as a viable catalyst precursor for supporting on SiO2, and yet was found to interact with the surface in a very different way than 1/SiO2. 4 was prepared and supported on SiO2 using the same procedure as for 1 and 3. The 31

P NMR showed a significant shift in peak corresponding to the chelating phosphorus atoms

from 206.8 ppm in the solution phase spectrum of 4 to 175.3 ppm in the MAS spectrum of the supported 4/SiO2 (Figure S2). The new peak is similar in chemical shift to the chelating phosphorus atoms in 2/SiO2 at 170 ppm,16 where a surface hydroxyl group is oxidatively added to the iridium center, with simultaneous loss of H2. A para-phosphinite peak appears in the spectrum of 4/SiO2 at 153.4 ppm, similar to 1/SiO2. However, the spectrum of 4/SiO2 shows spinning side bands for this peak indicating an immobilized species, unlike in 1/SiO2, where spinning side bands for this peak are totally absent, indicating a mobile and physisorbed species. The preservation of the para-phosphinite peak shows that the complex has not anchored via condensation reaction with a silanol group. As this binding mode leads to an Ir(III) complex, activity towards dehydrogenation is not expected. These results serve to shed light on future rational design of such supportable catalysts. Even with an established supporting strategy, a highly reactive and unhindered iridium center will preferentially react with the surface and form an undesired product. While the olefin and


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ACS Catalysis

carbonyl ligands in 1 and 3 were sufficient to prevent interaction of the Ir center with surface silanol groups, the analogous iridium dihydride complex behaves much more similarly to 2 when supported on silica. It is important to note, however, that since the oxidative addition of surface silanol groups is reversible while phosphinite generation is irreversible, given sufficient time under reaction conditions (200 °C under alkane flow), this species may ultimately form the parasupported surface species, presumably with bound olefin in the fourth coordination site.

a) b) c)≠ d)≠

*

*

*

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*

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*

*

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Figure 2. Solution-phase 31P NMR of a) 1 and h) 3 in C6D6; solid-state 31P MAS-NMR of freshly supported b) 1/SiO2 and g) 3/SiO2, and c-f) 1/SiO2 following continuous butane dehydrogenation for 1.5 hours at 200 °C, 250 °C, 300 °C, and 340 °C respectively. Spinning sidebands denoted with *. ≠ Catalyst = 1/SiO2, V̇total = 20 mL min-1, Pbutane = 0.32 atm, PHe = 0.95 atm, Ptotal = 1.27 atm.

3.2 Steady-State Continuous Flow Butane Dehydrogenation The primary advantage of a properly-supported heterogeneous catalyst is the trivial separation of catalyst from reactants and products, and this is best exemplified in continuous 17 ACS Paragon Plus Environment

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flow reaction conditions. To explore the effectiveness of supported pincer-type iridium catalysts, the acceptorless dehydrogenation of butane over catalyst 1/SiO2 was selected. Prior to the present study, 240 °C was the highest temperature for which data was available for catalytic alkane dehydrogenation by pincer-ligated iridium complexes.9 Above this temperature, rate data were found to be unreliable as the catalyst decomposed and yielded a black solid that precipitated from solution or collected on reactor vessel walls.13 To test the temperature stability of supported 1/SiO2, steady-state butane dehydrogenation was measured at temperatures between 200 °C and 340 °C, shown in Figure 3a. These experiments revealed an unprecedented stable reactivity for pincer-ligated iridium catalysts at 340 °C. Previously, 200240 °C was considered high temperature for molecular dehydrogenation catalysts.28 Due to the unfavorable thermodynamics for alkane dehydrogenation at the temperatures studied here, it is important to note that reaction can potentially reach equilibrium conversion of the starting alkane within the catalyst bed of the reactor. Reaction rates reported in this study primarily denote the net rate of forward reaction under these conditions, although reaction rates far from equilibrium can be measured by manipulating the residence time in the reactor (see e.g. Figure 6 discussed below). At 200 °C the net forward rate of reaction was low, on the order of 1 h-1, but the activity was stable and did not decline with time on stream. The 31P MAS-NMR spectrum of the catalyst after 1.5 hours of exposure to reaction conditions at 200 °C (Figure 2c) was essentially indistinguishable from freshly supported 1/SiO2, except that no signal could be found for physisorbed catalyst species, suggesting that the heating resulted in physisorbed species covalently attaching to the surface. The similarities in the NMR spectra between 1/SiO2 freshlysupported and after 1.5 hours of reaction at 200 °C indicate the similar chemical environments of


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ACS Catalysis

the chelating phosphorus atoms, which did not change significantly with the identity of the coordinating olefin (ethylene prior to reaction, and presumably butene(s) after reaction). The catalyst remained unchanged in color, indicative of very little change in electronic properties of the complex upon binding a different, albeit very similar, ligand (Figure S1). Upon heating the supported catalyst to 250 °C, the

31

P MAS-NMR spectrum showed a

new small peak at 201.2 ppm (Figure 2d). A reduction in the di-tert-butylphosphine oxide peak was also observed relative to the peak corresponding to the chelating phosphorus atoms, possibly due to desorption of the molecule from the surface at higher temperature. At this temperature, the reaction rate was still stable and so higher reaction temperatures were attempted. At 300 °C, a steady butene formation rate of 35 h-1 was observed over 1.5 hours of time on stream. At this temperature (Figure 2e), the peak in the

31

P MAS-NMR spectrum corresponding to the olefin

complex at 174.7 ppm had disappeared, and the peak at 201 ppm had grown. Thus, the resting state of the catalyst was now a different species than the starting olefin complex, though it showed similar reactivity. The di-tert-butylphosphine oxide peak also disappeared, indicating that desorption of the volatile molecule was complete. Upon continued heating of the catalyst, a reaction rate of 84 h-1 was reached at a temperature of 340 °C, and was stable for 1.5 hours. By this temperature, the color had changed to brown-yellow, though the 31P MAS-NMR (Figure 2f) showed no significant changes relative to the spectrum at 300 °C. The peak at 201 ppm coincided well with the freshly supported carbonyl complex 3/SiO2 (this is further discussed in Section 3.4). It is worth noting that there is no evidence for deactivation of the catalyst via oxidation at the phosphine positions or dimerization up to at least 340 °C, both of which would result in significant changes in the chemical shift of the corresponding phosphorus signals.


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ACS Catalysis

320 1750

Time (min)

Figure 3. Catalyst activity as a function of temperature. a) 200 - 340 °C. b) 360 - 440°C. Catalyst = 1/SiO2, V̇total = 20 mL min-1, Pbutane = 0.32 atm, PHe = 0.95 atm, Ptotal = 1.27 atm. Heating the catalyst above 340 °C under butane dehydrogenation conditions led to a color change to gray and then black (Figure S1), the appearance of a red-orange band of color on the quartz tube wall downstream of the reactor heated zone, and declining reaction rates as a function of time on stream (Figure 3b). The color change is consistent with that observed with pincer-iridium catalysts in solution when heated above reaction temperatures of ca. 200-240 °C. The band of color downstream of the reactor likely arises from the sublimation29 of a small amount of pincer-iridium complex at high temperature, and the leachate then condensing in a colder spot downstream of the heated zone. This condensate was analyzed by

31

P and 1H

solution-phase NMR and showed several peaks consistent with symmetric, chelated pincer species. While the activity of the catalyst was not stable above 340 °C, it did continue to increase as a function of temperature despite eight hours of exposure to temperatures where the activity was unstable. The highest initial reaction rate measured at any temperature was 315 h-1 at 440 °C, which declined to 186 h-1 after four hours. This suggests that some of the supported molecular catalyst survived at 440 °C and was responsible for the reaction rates observed, while the concentration of such species decreased as a function of time. Continued exposure to reaction 20 ACS Paragon Plus Environment

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ACS Catalysis

conditions at 440 °C for over three days led to a further decline in reaction rate to 149 h-1, at which it stabilized. This new steady state activity can therefore be attributed to the thermal decomposition product of the molecular catalyst, which retains some lower activity. The reaction temperature was subsequently reduced to compare the steady state activity of the decomposed catalyst to the anchored molecular catalyst. At 340°C, the reaction rate had dropped to 21 h-1, down from 77 h-1 in the fresh catalyst, and at 200 °C, the reaction rate was 0.27 h-1, down from 1.1 h-1. 31

P MAS-NMR of the catalyst following reaction at 440 °C showed no strong signals,

indicating a complete loss of chelating pincer ligands. This means that both the oxidation of the chelating phosphines and dimerization of the anchored complexes are unlikely deactivation mechanisms. The P (2p) region of the XPS spectrum (Figure 4) also showed a complete loss of phosphorus signal after heating to 440 °C, compared to the strong phosphorus signal in the asprepared sample. Thus, relative to the liquid phase, covalently supporting the catalyst on SiO2 greatly increases the thermal stability of the pincer-Ir catalysts and delays the onset temperature for catalyst deactivation by ca. 100 °C. Analysis of the Ir (4f) region of the as-prepared sample XPS spectrum (Figure 4) shows a spin-orbit doublet consistent with Ir(I) species, while lacking evidence of metallic iridium or Ir(0).30 Following reaction conditions for over three days at 440 °C, the peaks broadened slightly and shifted to higher binding energy. The shift in the spectrum to higher binding energy indicates an increase in the oxidation state upon decomposition of the catalyst, rather than a reduction to iridium metal. The similar Ir (4f) peak intensity between the as-prepared and spent catalyst indicates that the catalyst loss due to sublimation discussed above was small compared to the total catalyst loading, and not a major source of deactivation. The loss of chelating phosphorus


ACS Catalysis

from the pincer ligand and the oxidation of the iridium are responsible for the reduction in activity following high temperature treatment.

Intensity (a.u.)

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440 °C

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Binding Energy (eV) Figure 4. P (2p) and Ir (4f) regions of XPS spectra of as-prepared 1/SiO2 compared with those of a sample after exposure to reaction conditions at 440 °C for three days. V̇total = 20 mL min-1, Pbutane = 0.32 atm, PHe = 0.95 atm, Ptotal = 1.27 atm. While the reactivity of 1/SiO2 observed at 340 °C appeared to be stable over 1.5 hours of reaction time, the fact that 360 °C was sufficient to cause deactivation necessitated a longer timeon-stream experiment to determine if any deactivation occurs at 340 °C. The results, shown in Figure 5, show that even after three days of continuous operation, there was no appreciable decline in activity for butene formation, with a calculated turnover number over 3,500. Covalently anchoring pincer-Ir species on high surface area SiO2 support creates a true heterogeneous catalyst with enhanced thermal stability relative to the molecular homogeneous catalyst. This stability seems likely to arise from the reduced mobility of the Ir species, limiting their ability to engage in intermolecular reactions.


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Figure 5. Catalyst activity as a function of time on stream. Catalyst = 1/SiO2, V̇total = 5 mL min-1, Pbutane = Ptotal = 1.27 atm. T = 340 °C. These results compare favorably with prior reports of homogeneous catalysis by solutionphase [Ir(R4PCP)]-type pincer complexes. To the best of our knowledge, there are no prior reports on batch-phase acceptorless linear alkane dehydrogenation making use of [Ir(R4POCOP)]-type pincer complexes. [Ir(tBu4PCP)] complexes afforded no discernible products at reflux in the acceptorless dehydrogenation of n-decane (at the boiling point of n-decane, 174 °C) in a sealed system.31 In the dehydrogenation of n-undecane (bp 196 °C) at reflux in an open system that allows the escape of hydrogen, an initial rate of 46 h-1 has been observed with [IrH2(tBu4PCP)], which decreased by about a factor of two within the first two hours.32 In contrast, the continuous-flow gas-phase system described here displays negligible activity loss over several days. The approach to chemical equilibrium between butane and the reaction products was explored by manipulation of the space time in the reactor. At long space times, with sufficient contact time between reactant (butane) and product gases (1-butene, cis-2-butene, trans-2butene, hydrogen) and the catalyst, the gases should be fully equilibrated with each other. This


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equilibrium conversion is the maximum achievable at a fixed set of reaction conditions. The measured reaction rate at this point is the net forward reaction rate at equilibrium, and will not be affected by further increasing the space time. Reducing the space time will lead to a monotonic decrease in conversion, accompanied by an increase in the net forward reaction rate as the concentration of product gases is reduced. This expected behavior was observed during a space time (τ) test at 340 °C. Increasing the space time led to an increase in conversion, with a plateau at 3.7% conversion (Figure 6). Conversely, the net reaction rate reduced with increasing space time as reactants and products approached equilibrium concentration in the gas phase. However, at the shortest space times, the reaction was sufficiently far from equilibrium, and much larger net reaction rates were measured. The net reaction rate was only 13.3 h-1 at sufficiently long space times, but increased to 200 h-1 at very short contact times. The continuous reaction conditions used in the present study thus have another advantage in the study of molecular catalysts relative to batch operation, in that the true reactivity of the catalyst at sufficiently small contact times can be studied. Extrapolating the measured data to zero space time and zero conversion gives the intrinsic forward reaction rate, as the reverse reaction rate is zero at zero conversion. The value obtained from Figure 6 is approximately 235 h-1 at 340 °C. Assuming that the reaction rate is first order in butane when far from equilibrium, this gives a first order rate constant of 3.6 × 103 atm-1 h-1.


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catalyst butenes

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ACS Catalysis

-1 -1 rate (mol butenes mol TOF (mol molcatalyst -1hr hr-1))

Page 25 of 43

0.0005

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Figure 6. Effect of space time on catalyst activity and conversion. Catalyst = 1/SiO2, T = 340 °C, V̇total = 4 - 100 mL min-1, Pbutane = 0.32 atm, PHe = 0.95 atm, Ptotal = 1.27 atm. While the above calculation assumed a first order dependence between the butane partial pressure and the reaction rate when far from equilibrium, the expected relationship when close to equilibrium is quite different. If the partial pressures of olefin (Pbutenes) and hydrogen (PH2) from the dehydrogenation of butane are assumed to be equal to one another, then the equilibrium constant (Keq) can be rearranged to give: Pbutenes·PH2 = Pbutenes2 = Keq·Pbutane. The partial pressure of butenes, and therefore the formation rate of butenes, is therefore expected to have ½-order dependence on Pbutane regardless of temperature. The measured dependence in rate of formation of butenes at 200 °C and 340 °C on butane partial pressure at long space time was found to follow a ½-order power law (Figure 7), confirming that at long space time the reaction reached equilibrium regardless of temperature.


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

rate (mol molcatalyst-1-1hr-1)-1 TOF (molbutenes butenes molcatalyst hr )

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160 140

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Figure 7. Effect of butane partial pressure at varying temperature and space time. Catalyst = 1/SiO2, at T = 200 °C ( ) (values ×100) and 340 °C ( ),V̇total = 20 - 32 mL min-1, Pbutane = 0.32 2 atm, PHe = 0.95 - 0 atm, PTot = 1.27 - 2 atm. Curves fit to ½ power law functions. The selectivity towards each butene isomer as a function of temperature between 200 °C and 340 °C is given in Figure 8. Selectivity was constant as a function of time on stream, a further indication of the stability of the active site. Selectivity also did not vary with residence time (Figure S3), regardless of the distance from equilibrium conversion. The product distribution did vary with temperature up to 340 °C, but only slightly, and closely matched changes expected from equilibrium calculations. As predicted by equilibrium, the selectivity between butenes declined as trans-2-butene>cis-2-butene>1-butene. Equilibrium calculations predict a higher yield of trans-2-butene and a lower yield of 1-butene than that observed experimentally. In contrast to the stable selectivity with time on stream at 340 °C over three days, the selectivity changed continually at 440 °C over a similar time period (Figure S4). This highlights the stable nature of the active site at a temperature insufficient to cause catalyst decomposition, compared to significant changes in the nature of the catalyst active site at the higher temperature.


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Selectivity (Pproduct/Pbutenes)

Page 27 of 43

0.8 0.6 0.4 0.2 0.0 200

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350

Temperature (°C) Figure 8. Effect of temperature on selectivity between 200 – 340 °C. Dashed lines indicate expected product distribution from equilibrium calculations. 1-butene ( ), trans-2-butene ( ), and cis-2-butene ( ). Catalyst = 1/SiO2, V̇total = 20 mL min-1, Pbutane = 0.32 atm, PHe = 0.95 atm, Ptotal = 1.27 atm, T = 200 – 340 °C.

3.3 Effect of Catalyst Support The influence of support acidity on the catalytic properties of supported 1 was investigated by comparing SiO2 to γ-Al2O3, a more acidic support, and hydrotalcite-derived Mg(Al)O, a less acidic support. Mg(Al)O is a slightly basic support that is inert towards reaction of olefins at elevated temperatures.21 Experimental results show that dehydrogenation activity of 1 supported on all three oxide supports was comparable and stable over several hours (Figure 9), with nearly identical steady state activities between. The choice of support and its acidity did not appear to influence the activity of the catalyst. The selectivity of the reaction was similarly unaffected.


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rate (mol1-butene molcatalyst-1 hr-1)

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Page 28 of 43

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time (min) Figure 9. Comparison of 1-butene formation rates when supported on SiO2 ( ), Mg(Al)O ( ) and Al2O3 ( ). Catalyst = 1/support, V̇total = 20 mL min-1, Pbutane = Ptotal = 1.27 atm, T = 200 °C.

3.4 Supported Carbonyl Complex as an Active Catalyst In section 3.2, butane dehydrogenation conditions at temperatures above 300 °C lead to the formation of a

31

P MAS-NMR signal consistent with a carbonyl complex 3/SiO2. This

assignment was further examined by ATR FTIR. The spectra of as-prepared samples of 1, 1/SiO2, 3, and 3/SiO2 were examined for evidence of carbonyl stretching frequencies (Table 1). As expected, the ethylene complexes 1 and 1/SiO2 showed no carbonyl stretches. The molecular complex 3 showed a single intense peak at 1932 cm-1. Supported 3/SiO2 showed two signals; a weaker signal also at 1932 cm-1 possibly from physisorbed 3, and a stronger signal at 1945 cm-1 in good agreement with the reported C=O stretch of an authentic 3/SiO2 structure.13 Table 1. C-O stretching frequencies of complexes in solution and adsorbed on SiO2. Condition ṽCO (cm-1) intensity Sample as-prepared 1 as-prepared 1/SiO2 as-prepared 1932 strong 3 1932 weak as-prepared 3/SiO2 1945 strong post-reaction at 200 °C a 1/SiO2 a post-reaction at 340 °C 1945 strong 1/SiO2 a

V̇Tot = 20 mL/min, PButane = PTot = 1.27 atm, Duration = 1.5 hours.


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ACS Catalysis

As the 201.2 ppm peak in the 31P MAS-NMR was not observed after reaction at 200 °C, no carbonyl peak was expected in ATR FTIR either, and none was observed. Following reaction at 340 °C, a strong band at 1945 cm-1 corresponded well with the as-prepared and reference spectra,13 providing further evidence that the catalyst was converted from ethylene/olefin complex at lower temperatures to carbonyl complex at higher temperatures under reaction conditions, while maintaining the chelated pincer-Ir structure. If the CO complex 3/SiO2 were formed in situ from olefin complex 1/SiO2 under reaction conditions, at temperatures where the conversion of 1/SiO2 → 3/SiO2 was complete, the activity of either precursor would be expected to be the same. Indeed, this can be seen in Figure 10. At 340 °C, where the 31P MAS-NMR spectrum showed only carbonyl complex present, the activity of both precursors were the same. Interestingly, under identical temperature profiles, the decomposition profiles at temperatures above 340 °C, where activity was unstable, also fully coincided. Again, this points to identical molecular species present, even at 440 °C, which rapidly lose activity by the same deactivation mechanism with the same deactivation kinetics. However, at temperatures below 340 °C, the activity of the carbonyl complex was significantly lower than that of the olefin complex. The much higher expected binding affinity of carbon monoxide for the iridium center relative to ethylene (or other light olefin) presumably explains this. The active catalyst species in either case is the 14-electron complex with a coordination site opened by ligand dissociation.9,33 Both 1/SiO2 and 3/SiO2 can lose their respective small ligands to generate this species (Figure 11), but ethylene is much more labile than carbon monoxide. Importantly, the composition of the catalyst appears to be uniform even at the highest temperatures at which stable activity is observed. This is in contrast to results reported with


ACS Catalysis

2/SiO2, which forms two major species upon exposure to carbon monoxide – the four-coordinate iridium(I) carbonyl species as well as the six-coordinate iridium(III) species [IrH(OSi)(CO)(POCOP)] which possesses a phosphorus resonance at 159 ppm.26 This latter species is presumably formed first, followed by reductive elimination over the course of 15 hours to attain 60-70% conversion to the four-coordinate carbonyl complex in what is a kinetically slow process. Since no signals for six-coordinate grafted complex are observed in the IR or NMR spectra of the spent catalysts collected in work, even when collected less than an hour after reaction, this serves as further evidence that both 1 and 3 are quantitatively converted to, and remain as, their para-supported analogues 1/SiO2 and 3/SiO2 with no oxidation of the iridium center. 350 440 300 400 250 360 200 320 150 280 100 240 50

Temperature (°C)


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Figure 10. Comparison of dehydrogenation activities of 1/SiO2 ( ) with 3/SiO2 ( ) from 200 440 °C. V̇total = 20 mL min-1, Pbutane = 0.32 atm, PHe = 0.95 atm, Ptotal = 1.27 atm. At high temperature, the conversion to carbonyl complex is complete, and at such temperatures the dissociation of CO is facile enough to allow catalysis. At each relevant temperature, the species present when starting from ethylene complex are active enough to reach equilibrium. Starting with CO complex, optimal activity is not seen until 340 °C. However, as carbonyl complexes deactivate in the presence of O2, N2, and H2O much more slowly than the


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ACS Catalysis

corresponding ethylene complexes, they offer other practical benefits, such as easier handling of supported pincer complexes without concern for air contamination. This property was exploited in the present study during BET analysis for example. 3/SiO2 was found to be air stable on the order of days to weeks, as evidence by color change from oxidation. By contrast, oxidation took only minutes for 1/SiO2. Accidental exposure of 1/SiO2 to even minute quantities of air led to visible color change and a loss of activity in catalytic tests. Thus, 3/SiO2 is a significant step to a practical immobilized molecular catalyst. Airresistant and stable at elevated temperatures, it may also be used for reactions in which the thermodynamics favor a higher temperature, including dehydrogenation. These applications would be impossible for solution-phase catalysts, which decompose rapidly at such temperatures.

Figure 11. 1/SiO2 is converted to 3/SiO2 at 300 °C under butane, both 16 electron Ir(I) complexes. Both 1/SiO2 and 3/SiO2 serve as reservoirs for the 14 electron active species.


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The mechanism for conversion of the ethylene complex to the carbonyl complex is at present uncertain. It appears likely however that surface hydroxyl groups, resembling chemisorbed water species, may hydrate olefins to alcohols. C4H8 + 2 Si-OH → C4H9OH + Si-O-Si These alcohols would be readily dehydrogenated by the iridium catalyst,34 yielding aldehydes and ketones containing a carbonyl functional group. Preliminary experiments showed that cofeeding propene and small quantities of water over supported pincer- iridium complexes formed acetone. As there are approximately 3,000 SiOH groups per iridium atom, it is possible that a small number of these sites provide the oxygen needed to form CO via this mechanism.

3.5 Comparison of Supported Pincer-Complexes at Elevated Temperatures Previously, unsupported pincer-iridium complexes were found to be effective catalysts for gas-solid reaction in batch-type alkane transfer dehydrogenation reactions.9 The complexes used (including 2) lacked functional groups that would lend themselves to covalent interaction with the glass reactor walls, and so were presumably physisorbed on glass reactor walls. However, they remained catalytically active at 240 °C. Our attempts to effect continuous gas-phase alkane dehydrogenation with physisorbed catalysts including 1 at similar temperatures were unsuccessful. Without a strong interaction with the support, weakly bound catalyst leached from the reactor, and could be found condensing at cold spots downstream of the reactor outlet (approximately 110 °C). This limits the effectiveness of physisorbed catalyst to batch-type operation, where it is contained in the reactor. To compare the activity and stability of supported and unsupported (physisorbed) catalyst


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at 340 °C, a batch-type acceptorless dehydrogenation reaction was studied. Sealed glass ampules of dodecane with silica-supported catalyst (1/SiO2, 2/SiO2) and with unsupported catalyst (1 and 2) were heated to 340 °C for 30 minutes or 20 hours. At this high temperature, all of the dodecane evaporated, yielding conditions similar to those reported for alkane transfer dehydrogenation in reference 9, albeit 100 °C higher in temperature.

Samples

of

silica-

supported and solution-phase 1 and 2 were heated to 340 °C in sealed glass ampules for 30 minutes or 20 hours. Following reaction, the reaction mixtures were condensed, the ampules were opened inside a glove box, and concentrations of products quantified by GC. The remaining solution was evaporated under vacuum, and fresh dodecane was added. The ampules were resealed and heated to 340 °C for 30 additional minutes. The initial loading of catalyst was 1 mM in dodecane for the unsupported catalysts, and an equivalent molar loading of iridium for supported catalysts. In addition to dodecenes (the primary product from dodecane dehydrogenation), secondary reaction products were present in all reaction samples (Figure 12). The general reaction scheme to produce these products is summarized in Figure 13. The initial dehydrogenation of dodecane produces dodecenes and molecular hydrogen. Cracking products of dodecenes, including pentenes and heptenes, were found in all samples. Hydrogen from the dehydrogenation step would hydrogenate the lighter olefins, yielding their corresponding alkanes. Figure 12 illustrates the concentrations of dodecene and the C2-C11 alkanes and alkenes formed from cracking dodecene.


ACS Catalysis

175

175

a)

b) 150

Concentration (mM)

Concentration (mM)

150 125 100 75 50

125 100 75 50 25

25

0

0

1/SiO2

2/SiO2

1

2

1/SiO2

none

175

2/SiO2

1

2

none

175

c)

d) 150

Concentration (mM)

150

Concentration (mM)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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125 100 75 50

125 100 75 50 25

25

0

0

1/SiO2 2/SiO2

1

2

SiO2

none

1/SiO2 2/SiO2

1

2

SiO2

none

Figure 12. C2-C11 alkanes( ), C2-C11 olefins( ) and dodecenes( ) from dehydrogenation of dodecane at 340 °C. Runs with 1/SiO2; 2/SiO2; 1 in the absence of silica; 2 in the absence of silica, and control (no pincer-iridium catalyst or silica). a) 30-minute reaction time. b) 30-minute recycle with same catalyst sample used in (a). c) 20-hour reaction time. d) 30-minute recycle with same catalyst sample used in (c). 1 mM iridium catalyst in solution equivalent, 6.7 mg SiO2 where present. [dodecane]0 = 4.4 M.


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Figure 13. Proposed dehydrogenation-cracking-hydrogenation scheme explaining distribution of products in dodecane dehydrogenation.

Control experiments without any catalyst or silica showed very little product formation after 30 minutes. After 20 hours, a small amount of light olefin and alkane had formed, possibly from thermal cracking of dodecane at 340 °C. The same yield of these lighter hydrocarbons was obtained in the 20-hour experiments where silica was added to the reactor, suggesting that surface area did not affect the rate of alkane cracking. The dodecane cracking mechanism must therefore be homogeneous in the gas phase. The small concentrations of cracking products in neat dodecane and in the presence of silica reveal that the homogeneous dodecane cracking rate is small and that silica is essentially inert. After 30 minutes of reaction (Figure 12a), both supported catalysts 1/SiO2 and 2/SiO2 had reached the same yield of 100 mM of dodecene, with very small quantities of lower olefins and alkanes. The near-identical concentrations reached by these dissimilar species likely indicates that equilibrium was reached between dodecane and dodecene/hydrogen in 30 minutes. The unsupported catalysts 1 and 2 were unable to reach this same concentration within 30 minutes of reaction, either due to lower reaction rates or loss of activity at 340 °C. The low concentration of C2-C11 olefins and alkanes were comparable to the control experiment, and the rate of dodecene cracking was small. A recycle experiment carried out by removing the reaction


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mixture and replacing it with fresh dodecane revealed that the unsupported catalyst had indeed completely deactivated at 340 °C, with activity upon recycle no better than the control experiment without any catalyst (Figure 12b). These samples contained black, insoluble material on the walls of the ampules, consistent with catalyst decomposition observed in other reaction systems. Both supported catalysts had reached the same concentration of dodecene, indicating that they both retained activity. The amount of dodecene formed was less after the recycle compared to fresh catalyst by approximately a factor of three. As an Ir(III) complex, 2/SiO2 is not expected to be highly active for alkane dehydrogenation. However, it is known that the Ir(III) species 2/SiO2 can reductively eliminate the surface hydroxyl group to which it is bound, regenerating the Ir(I) species 2,17,25 which is present as a reaction product and accumulates in the sealed reactor. This small amount of free 2 is likely responsible for the catalytic activity, which over 30 minutes of reaction after recycle appears as effective as in 1/SiO2. Both supported catalysts were less effective upon recycle compared to fresh reaction. Since 1/SiO2 showed activity at 340 °C over three days (Figure 5), the loss of activity is unlikely to arise from catalyst decomposition during the course of the reaction. Instead, the likely source of the activity loss is an artifact of batch-type operation, where the products accumulate in the presence of the catalyst, compared to continuous operation, where the products are removed in the gas flow. The accumulation of olefin product could possibly lead to a loss of activity by either leading to coke formation or through overbinding in the case of product olefins binding to iridium more strongly than ethylene. In general, dienes would be expected to bind to iridium centers more strongly than monoenes, and small quantities of dienes were formed during the reactions, enough to poison the low catalyst loading. To determine if dienes were responsible for


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inhibiting reaction, reactions were carried out with piperylene added to the reactant mixture with fresh 1/SiO2. No inhibition was observed and the results were identical to Figure 12a. The likely candidate for the observed loss of activity upon recycling is therefore coke formation. As this contrasts with the continuous flow gas phase butane dehydrogenation (Figure 5), preventing the accumulation of coke precursors via continuous flow is an important consideration for stable activity. The product distributions after 20 hours of reaction (Figure 12c) were significantly different from those obtained after 30 min. Dodecene cracking was clearly a contributor at this longer reaction time, as hydrocarbons smaller than C12 were prevalent in the product mixtures. Cracking of dodecane, the parent alkane, was slow by contrast, as evidenced by the control reactions. Both reaction without any catalyst and with silica only showed the same product distribution, and lacked dodecene, as was observed after 30 minutes of reaction. The yield of lighter alkanes and olefins therefore must have originated from the slow alkane cracking reaction instead. In the presence of any iridium-containing catalyst, both dodecane and dodecene were present even after 30 minutes, so the much higher yield of lighter hydrocarbons suggests dodecene cracking as the source. Further, given the identical product distributions between silica and no catalyst, the silica was completely inert towards any reaction with dodecane and its cracking products, so any observed reaction occurred in the gas phase. The equilibrium concentration of dodecenes reached after 30 minutes of reaction (Figure 12a) was surpassed at longer reaction times by cracking and subsequent hydrogenation (Figure 13). Thermal cracking of dodecene at 340 °C yields two equivalents of lighter olefins. These lighter olefins may react with either H2 generated in the first reaction, or with dodecane via transfer dehydrogenation. In either case, the alkenes are hydrogenated and the dehydrogenation


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of dodecane is promoted, giving rise to the higher dodecene concentration in Figure 12c compared to Figure 12a. The overall reaction is then the transformation of two equivalents of dodecane to one equivalent of dodecene and two equivalents of lighter alkane. However, the rate of cracking is slow relative to dehydrogenation, and independent of catalyst, leading to similar results regardless of which catalyst is employed. The high dodecene yield after 20 hours on unsupported catalyst is unexpected, considering the total lack of activity observed in the recycle reaction following just 30 minutes of reaction at 340 °C. Most likely, the decomposition product from the unsupported catalysts retains some alkane dehydrogenation activity. Over 20 hours, even this residual activity could be sufficient to reach the same or similar product distributions as the supported catalysts. This is especially plausible if the black particulate formed contained Ir metal clusters, which would be expected to be active for dodecane dehydrogenation, albeit slower than with molecular Ir(I) complexes. The net rate of formation of dodecene, beyond the initial dodecane/dodecene equilibrium, is limited by the rate of cracking, which as discussed above, occurs independently of the catalyst. A 30-minute recycle experiment was carried out on all samples following 20 hours of reaction (Figure 12d), confirming that the activity of the active material following decomposition of the unsupported catalysts is insufficient to form any appreciable product concentration in 30 minutes. This recycle experiment also revealed that 2/SiO2 was much less recyclable than 1/SiO2. The reductive elimination of SiO-H is required to convert 2/SiO2 to a catalytically active Ir(I) species 2, but this process lead to the loss of ⅔ of the activity following two hours reaction relative to the recycle following only 30 minutes reaction. By contrast, 1/SiO2 showed identical reaction rates in both Figures 12b and 12d. The loss of activity, most likely from coke formation,


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in the first 30 minutes of reaction was not exacerbated in the following 19.5 hours, and dodecene yield and product distribution were unchanged. The activity of 1/SiO2 and 2/SiO2 was also compared for the continuous-flow gas-phase dehydrogenation of butane at 200 °C (Figure S5). Over the short duration of the experiment (ca. 75 minutes), the activity of 2/SiO2 appeared to be stable with time on stream, However, the rate of butene formation was lower than that over 1/SiO2 by a factor of 10. This is consistent with the different inherent activity expected for alkane dehydrogenation by Ir(I) and Ir(III) complexes.

4. Conclusions In this work, a stable, continuous-flow gas-phase alkane dehydrogenation system making use of supported, pincer-iridium catalyst was developed. Anchoring via a covalent linkage at a backbone position avoided catalyst leaching over the course of the reaction while also eliminating decomposition pathways available in homogeneous catalytic systems. At 340 °C, rates as high as 200 h-1 were measured at very short contact times, and deactivation was negligible over time scales of several days. By comparison to batch studies, flow conditions allowed the catalyst to retain its activity better by avoiding coking arising from accumulation of the reaction products. Even at this high temperature, the pincer-iridium moiety remained intact as determined by

31

P MAS-NMR spectroscopy. The starting supported olefin complex converted fully to the

corresponding iridium carbonyl complex. Starting with either complex gave identical reactivity at high temperature, consistent with either species acting as a reservoir of the 14-electron, threecoordinate catalytically active fragment. This allowed the use of the carbonyl complex, a more air-stable catalyst than either the olefin species, for reactions at higher temperatures.


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Supporting Information 31

P NMR of 3 in C6D6; 1H NMR of 3 in C6D6; 31P NMR of dihydride (4) and tetrahydride mixture in d8-toluene; 1H NMR of 4 in d8-toluene; color and appearance of catalyst; 31P MASNMR of 4/SiO2; selectivity between butene isomers as a function of residence time and reaction temperature; comparison of continuous-flow alkane dehydrogenation over 1/SiO2 and 2/SiO2

Acknowledgements This work was supported by NSF under the CCI Center for Enabling New Technologies through Catalysis (CENTC) Phase II Renewal, CHE-1205189. The authors would like to thank Dr. Ashley M. Pennington for assistance in performing BET measurements, and Rachel A. Yang for assistance in analyzing kinetic and spectroscopic data. The authors would also like to thank Dr. Nagarajan Murali for assistance in collecting, processing and interpreting solid-state MAS NMR spectra and Shinjae Hwang and Dr. Eric Garfunkel for collection and interpretation of XPS spectra.

References

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Kumar, A.; Zhou, T.; Emge, T. J.; Mironov, O.; Saxton, R. J.; Krogh-Jespersen, K.; Goldman, A. S., Dehydrogenation of n-Alkanes by Solid-Phase Molecular Pincer-Iridium Catalysts. High Yields of alpha-Olefin Product. J. Am. Chem. Soc. 2015, 137 , 9894-911. 10 Alt, H. G.; Böhmer, I. K. Catalytic Dehydrogenation of Isopentane with Iridium Catalysts. Angew. Chem. Int. Ed. 2008, 47, 2619-2621. 11 Taubmann, S.; Alt, H. G. Heterogeneous Catalysts for the Dehydrogenation of Saturated Hydrocarbons. J. Mol. Catal. A: Chem. 2008, 287, 102-109. 12 Alt, H. G.; Böhmer, I. K. Influence of Triphenylphosphine on the Activity of Heterogeneous Iridium, Rhodium and Platinum Catalysts for the Dehydrogenation of Saturated Hydrocarbons. J. Organomet. Chem. 2009, 694, 1001-1010. 13 Huang, Z.; Brookhart, M.; Goldman, A. S.; Kundu, S.; Ray, A.; Scott, S. L.; Vicente, B. C. Highly Active and Recyclable Heterogeneous Iridium Pincer Catalysts for Transfer Dehydrogenation of Alkanes. Adv. Synth. Catal. 2009, 351, 188 – 206. 14 Bézier, D.; Brookhart, M. Applications of PC(sp3)P Iridium Complexes in Transfer Dehydrogenation of Alkanes. ACS Catal. 2014, 4, 3411-3420. 15 Rimoldi, M.; Nakamura, A.; Vermeulen, N. A.; Henkelis, J. J.; Blackburn, A. K.; Hupp, J. T.; Stoddart, J. F.; Farha, O. K. A Metal-Organic Framework Immobilised Iridium Pincer Complex. Chem. Sci. 2016, 7, 49804984. 16 Rimoldi, M.; Fodor, D.; van Bokhoven, J. A.; Mezzetti, A. Catalytic Hydrogenation of Liquid Alkenes with a Silica-Grafted Hydride Pincer Iridium(III) Complex: Support for a Heterogeneous Mechanism. Catal. Sci. Technol. 2015, 5, 4575-4586. 17 Rimoldi, M.; Mezzetti, A. Batch and Continuous Flow Hydrogenation of Liquid and Gaseous Alkenes Catalyzed by a Silica-grafted Iridium(III) Hydride. Helv. Chim. Acta 2016, 99, 908-915. 18 Gottker-Schnetmann, I.; White, P. S.; Brookhart, M. Synthesis and Properties of Iridium Bis(phosphinite) Pincer Complexes (p-XPCP)IrH2, (p-XPCP)Ir(CO), (p-XPCP)Ir(H)(aryl), and {(p-XPCP)Ir}2{µ-N2} and Their Relevance in Alkane Transfer Dehydrogenation. Organometallics 2004, 23, 1766-1776. 19 Gottker-Schnetmann, I.; White, P. S.; Brookhart, M. Iridium Bis(phosphinite) p-XPCP Pincer Compexes: Highly Active Catalysts for the Transfer Dehydrogenation of Alkanes. J. Am. Chem. Soc. 2004, 126, 1804-1811. 20 Akporiaye, D.; Jensen, S.F.; Olsbye, U.; Rohr, F.; Rytter, E.; Ronnekleiv, M.; Spjelkavik, A.I. A Novel, Highly Efficient Catalyst for Propane Dehydrogenation. Ind. Eng. Chem. Res. 2001, 40, 4741–4748. 21 Galvita, V.; Siddiqi, G.; Sun, P.; Bell, A. T. Ethane Dehydrogenation on Pt/Mg(Al)O and PtSn/Mg(Al)O Catalysts. J. Catal. 2010, 271, 209-219. 22 Biedron A. B.; Garfunkel E. L; Castner Jr, E. W.; Rangan S. Ionic Liquid Ultrathin Films at the Surface of Cu(100) and Au(111). J. Chem. Phys. 2017, 146, 054704. 23 Nesbitt H. W.; Banerjee D. Interpretation of XPS Mn(2p) Spectra of Mn Oxyhydroxides and Constraints on the Mechanism of MnO2 Precipitation. Am. Mineral. 1998, 83, 305-315. 24 Vicente, B. C.; Huang, Z.; Brookhart, M.; Goldman, A. S.; Scott, S. L. Reactions of Phosphinites with Oxide Surfaces: A New Method for Anchoring Organic and Organometallic Complexes. Dalton Trans. 2011, 40, 4268-4274. 25 Rimoldi, M.; Fodor, D.; van Bokhoven, J. A.; Mezzetti, A. A Stable 16-Electron Iridium(III) Hydride Complex Grafted on SBA-15: a Single-Site Catalyst for Alkene Hydrogenation. Chem. Commun. 2013, 49, 11314-11316. 26 Rimoldi, M.; Mezzetti, A. Silica-Grafted 16-Electron Hydride Pincer Complexes of Iridium(III) and Their Soluble Analogues: Synthesis and Reactivity with CO. Inorg. Chem. 2014, 53, 11974-11984. 27 Zhuravlev, L. T.; Potapov, V. V. Density of Silanol Groups on the Surface of Silica Precipitated From a Hydrothermal Solution. Russ. J. Phys. Chem. A. 2006, 80, 1119-1128. 28 Dobereiner, G.E.; Crabtree, R.H. Dehydrogenation as a Substrate-Activating Strategy in Homogeneous Transition-Metal Catalysis. Chem. Rev. 2010, 681-703. 29 Moulton, C. J.; Shaw, B. L. Transition Metal-Carbon Bonds. Part XLII. Complexes of Nickel, Palladium, Platinum, Rhodium and Iridium with the Tridentate Ligand 2,6-Bis[(di-t-butylphosphino)methyl]phenyl. J. Chem. Soc. Dalton. 1976, 0, 1020-1024. 30 Katrib, A.; Stanslaus, A.; Yousef, R. M. XPS Investigations of Metal-Support Interactions in Pt/SiO2, Ir/SiO2 and Ir/Al2O3 Systems. J. Mol. Struct. 1985, 129, 151-163.


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Liu, F.; Goldman, A. S. Efficient Thermochemical Alkane Dehydrogenation and Isomerization Catalyzed by an Iridium Pincer Complex. Chem. Commun. 1999, 655-656. 32 Zhu, K.; Achord, P. D.; Zhang, X.; Krogh-Jespersen, K.; Goldman, A. S. Highly Effective Pincer-Ligated Iridium Catalysts for Alkane Dehydrogenation. DFT Calculations of Relevant Thermodynamic, Kinetic, and Spectroscopic Properties. J. Am. Chem. Soc. 2004, 126, 13044-13053. 33 Renkema, K.B.; Kissin, Y.V.; Goldman, A.S. Mechanism of Alkane Transfer-Dehydrogenation Catalyzed by a Pincer-Ligated Iridium Complex. J. Am. Chem. Soc. 2003, 125, 7770-7771. 34 Morales-Morales, D.; Redon, R.; Wang, Z.; Lee, D.W.; Yung, C.; Magnuson, K.; Jensen, C.M. Selective Dehydrogenation of Alcohols and Diols Catalyzed by a Dihydrido Iridium PCP Pincer Complex. Can. J. Chem. 2001, 79, 823-829.


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TOC and Abstract Graphic 350 440 300 400 250 360 200 320 150 280 100

+ catalyst/SiO2

50

continuous flow steady state

+ H2

0 0

250

500

750

240

+

Temperature (°C)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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200

1000 1250 1500 1750

Time (min)


Continuous Flow Alkane Dehydrogenation by Supported Pincer

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