Catalytic Oxidation of Light Alkanes Mediated at Room Temperature

Publication Date (Web): March 13, 2018 .... Immobilization of the Tricopper Complex in the MSN-TP .... indicate that these MSN-TP are formed with a hi...
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Research Article Cite This: ACS Sustainable Chem. Eng. 2018, 6, 5431−5440

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Catalytic Oxidation of Light Alkanes Mediated at Room Temperature by a Tricopper Cluster Complex Immobilized in Mesoporous Silica Nanoparticles∥ Chih-Cheng Liu,† Damodar Janmanchi,† Da-Ren Wen,‡ Jung-Nan Oung,‡ Chung-Yuan Mou,§ Steve S.-F. Yu,† and Sunney I. Chan*,†,§ †

Institute of Chemistry, Academia Sinica, No. 128, Sec. 2, Academia Road, Nankang District, Taipei 11529, Taiwan Exploration and Development Research Institute, Chinese Petroleum Corporation, No. 3, Songren Road, Sinyi District, Taipei 11010, Taiwan § Department of Chemistry, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Daan District, Taipei 10617, Taiwan ‡

S Supporting Information *

ABSTRACT: The controlled oxidation of light alkanes is challenging chemistry. Here, we describe the further development of a catalytic system consisting of a tricopper cluster complex immobilized in mesoporous silica nanoparticles, which we have recently shown to be capable of efficient conversion of methane into methanol without overoxidation under ambient conditions, to follow the oxidation of (i) ethane to ethanol, (ii) propane to isopropanol and acetone, and (iii) a 1:1:1 mixture of methane, ethane and propane. The efficacy of the system to mediate the catalytic conversion of these light alkanes in natural gas into liquid oxidized products is also assessed. KEYWORDS: alkane oxidation, heterogeneous catalysis, mesoporous silica nanoparticles, methane oxidation, natural gas conversion, tricopper cluster complex

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INTRODUCTION Methane (CH4) accounts for about 20% of the global warming problem.1−3 Although there is over 200 times more carbon dioxide (CO2) than CH4 in the atmosphere, CH4 is a more potent greenhouse gas. Molecule for molecule, CH4 warms the planet roughly 30 times more than CO2.1−4 The principal sources of CH4 emissions on planet earth are the ocean floor;5 seeping of natural gas near volcanic areas and oilfields;6 permafrost;7 production of CH4 by methanogens and related microbes;8 soil and decaying vegetation;9 lakes, ponds and landfills;10,11 rice paddies;12 livestock husbandry;13,14 not to mention incomplete burning of natural gas during energy production and domestic utilization.15,16 The amounts of CH4 in the atmosphere have more than doubled in the past 250 years, largely due to human activities.17,18 However, with the increasing exploration for natural gas and fracking of shale gas in the U.S. and elsewhere,19 not to mention the increasing consumption of natural gas for purpose of energy production, there is concern for acceleration of the rise in CH4 emissions. For this reason, there is increasing impetus to develop methods to monitor and control CH4 emissions into the atmosphere. One way to control CH4 emission is to convert the gas into methanol (MeOH) and other oxidized liquid products before it has the opportunity to escape into the atmosphere. However, © 2018 American Chemical Society

the controlled oxidation of CH4 is extremely difficult chemistry requiring extreme conditions of temperature and pressure as well as the deployment of expensive catalysts.20,21 Although oxidative C−H functionalization has been an active field for many years and significant progress has been made recently toward the development of inorganic complexes capable of catalytic transformation of many liquid alkanes by H2O2 and/or O2, into their corresponding alcohols, aldehydes, and ketones, the selective oxidation of CH4 as well as other light alkane (C2, C3, and C4) gases remains challenging.22−28 Recently, we have developed a simple catalyst based on the tricopper cluster complex CuICuICuI(7-N-Etppz)1+, where 7-N-Etppz denotes the organic ligand 3,3′-(1,4-diazepane-1,4-diyl)bis(1-(4-ethylpiperazin-1-yl) propan-2-ol), which is capable of mediating the efficient conversion of CH4 into MeOH under ambient conditions.29,30 This catalyst, which we will henceforth refer to as Cu3(7-N-Etppz), is also capable of oxidizing other light alkane gases efficiently under the same conditions.30 Unfortunately, as a homogeneous catalyst in acetonitrile (MeCN), the process suffers from low overall catalytic efficiencies due to the Received: January 17, 2018 Revised: March 5, 2018 Published: March 13, 2018 5431

DOI: 10.1021/acssuschemeng.8b00270 ACS Sustainable Chem. Eng. 2018, 6, 5431−5440

Research Article

ACS Sustainable Chemistry & Engineering low solubility of the light alkane gases in the solvent system.30 In an attempt to remedy this limitation, we have immobilized the tricopper complex in the nanochannels of mesoporous silica nanoparticles (MSN) to take advantage of the “over-solubility” of small nonpolar gaseous molecules in the liquid trapped within the nanopores of the nanoparticles.31−35 Indeed, dramatic enhancement in the catalytic efficiency of the conversion of CH4 into MeOH is observed.36 In this study, we extend this technology platform to examine the selective oxidation of ethane (C2H6) and propane (C3H8), the two principal remaining alkane components of natural gas. The goal of this exercise is to assess the efficacy of this technology platform for the control of natural gas emissions in general. We assume that methods for the recovery of the product liquid oxygenated compounds are in place or could be developed so that these liquid condensates could be used as feedstocks for conversion into value-added chemicals including fuels.

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Etppz) complex in situ and give the CuEtp@MSN-TP catalyst. The mixture was degassed, followed by injecting O2 (10 mL at 1 atm, 25 °C, 0.41 mmol) and substrate gas (C2H6 or C3H8; 100 mL at 1 atm, 25 °C, 4.1 mmol) using gas syringes to fill the total volume of the sample bottle with these two gases. The total gas pressure was 2.44 atm in the 50 mL glass bottle. After stirring for 10 min, an aliquot of different amounts of H2O2 solution (35%) was added. The reaction mixture was then stirred continuously for 3 h at room temperature. The reaction solution was analyzed periodically by using GC-MS. The products of the oxidation reaction of the pure alkane gas (C2H6 or C3H8), were analyzed and confirmed by comparing with the commercially available standards (EtOH, isopropanol, acetone, and benzotrifluoride as an internal standard) and the built-in MS database software. Product yields and the turnover numbers are based on the loading of the tricopper complexes in the MSN-TP. The same procedures were employed to follow the catalytic oxidation of the light alkane gases in a (C1+C2+C3) sample consisting of a 1:1:1 mixture of CH4, C2H6 and C3H8, as well as the light alkane gases in samples of natural gas. In the experiment on the (C1+C2+C3) mixture, a total 500 equiv. of the (C1+C2+C3) gas (27.9 mL at 1 atm) and 3 mL O2 at 1 atm were injected into a 20 mL glass bottle and the total pressure was 2.06 atm In the experiments on the natural gas samples, a smaller amount of catalyst was used to compensate for the low partial pressures of C2H6 and C3H8 in natural gas. Here, 10 mg of the CuEtp@MSN-TP catalyst was well suspended in anhydrous MeCN (5 mL) in a 50 mL glass sample bottle sealed tightly with a rubber cap. 120 mL of natural gas (CHK: Chu-Huang Keng, Maoli County and CS-71: Chun-Shui, Maoli County), two local sources of natural gas in Taiwan) was used and injected into the sample bottle using gas syringes. The total alkane (C1+C2+C3) content was 2,540.7 equiv. in the experiment on the CHK sample; and 3,659.7 equiv. for the CS-71 sample. The total pressure (including CO2 and N2) was 2.67 atm in the 50 mL glass bottle. Otherwise, the reaction procedure was the same as in the oxidation reaction of the pure alkane gases (C2H6 or C3H8) described above. In the natural gas experiments designed to exhaust the less abundant alkanes (C2H6 and C3H8) in the samples by catalytic oxidation, we used a total alkane (C1+C2+C3) content of 500 equiv. 23.6 mL of the natural gas CHK and 7 mL of O2 at 1 atm were added to a 20 mL glass bottle containing 10 mg of the CuEtp@MSN-TP catalyst, and the total pressure was 2.04 atm. In the case of CS-71 natural gas experiment, 16.4 mL of the natural gas was used and 14 mL of O2 at 1 atm was added to bring the total pressure in the glass bottle to 2.03 atm.

EXPERIMENTAL SECTION

Synthesis of the Ligand 7-N-Etppz and Preparation of the Tricopper Cluster Complex CuIICuIICuII(7-N-Etppz)4+. The synthesis and characterization of the 7-N-Etppz ligand, as well as the preparation of the tricopper cluster complex CuIICuIICuII(7-NEtppz)4+, have been described earlier.29,36,37 We have repeated the chemical syntheses of these compounds and verified their structures according to the methods established earlier. The details are presented in the ESI. Preparation of the Negatively Charged Nanoparticles (MSNTP). The MSN were synthesized using tetraethylorthosilicate (TEOS) as a precursor under basic conditions (ammonia solution). In order to facilitate anchoring of the positively charged tricopper complexes, the MSN were functionalized by 5% anionic 3-(trihydroxysilyl)propylmethyl-phosphonate (TP) to generate a negatively charged surface within the MSN channels. As described previously,28,36,37 0.58 g of cetyltrimethylammonium bromide (C16TAB) and 5 mL of 0.21 M dilute ethanol (EtOH) solution of TEOS were first dissolved in 0.51 M ammonia solution (NH4OH(aq)). After stirring at 40 °C for 5 h, 250 mL of TP dissolved in 1 mL of deionized H2O and 5 mL of 0.88 M EtOH solution of TEOS were then added under vigorous stirring for another 1 h. After aging the solution at 40 °C for 24 h, the assynthesized materials were collected by centrifugation at 12 000 rpm for 25 min, washed and redispersed with EtOH several times. The surfactant templates were then removed by extraction twice in ammonium nitrate/EtOH solution (1 g of ammonium nitrate/50 mL of EtOH at 60 °C for 1 h). Finally, the modified-MSN were collected by centrifugation, washed with EtOH several times, and dried under vacuum for 10 h to obtain a silica powder [(MSN-TP−) (NH4+)]. Immobilization of the Tricopper Complex in the MSN-TP. The CuEtp@MSN-TP was prepared by the ion-exchange method via electrostatic attraction according to the procedure that we established earlier.36 To begin, 150 mg of the as-synthesized MSN-TP was well dispersed in 25 mL of EtOH, and then 25 mL of 10 mM exchange acetonitrile (MeCN) solution of the CuIICuIICuII(7-N-Etppz)4+ was added slowly and stirred at 25 °C for 24 h. The solid was collected by centrifugation, washed with EtOH, and dried under vacuum. The loading of the CuIICuIICuII(7-N-Etppz)4+ in the MSN-TP was determined by measuring the copper content by ICP-MS and the ligand content by C/N elemental analysis. Catalytic Oxidation of Light Alkanes Mediated by the CuEtp@MSN-TP Catalyst Initiated with H2O2. For studies of the catalytic oxidation of the pure alkane gases, the following procedure was devised. First, 20 mg of the CuEtp@MSN-TP was well suspended in anhydrous MeCN (5 mL) in a 50 mL glass sample bottle with a rubber cap sealed tightly by a metal cap. Then sodium ascorbate was added as a reducing agent (4 equiv. based on the amounts of the tricopper complex in the MSN, freshly prepared 1 M solution in deionized water) under a N2 atmosphere. The heterogeneous mixtures were stirred vigorously at room temperature to form the Cu3(7-N-

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RESULTS AND DISCUSSION Preparation and Characterization of the CuEtp@MSNTP Catalyst. To provide the negative charges on the MSN to immobilize the positively charged CuIICuIICuII(7-N-Etppz)4+ complexes by electrostatic Coulombic interactions, we first functionalize the surface of the nanochannels of the MSN by anionic TP groups to obtain the MSN-TP. As reported in our earlier work,36,37 29Si solid-state NMR spectra (Supporting Information (SI) Figure S1) indicate that these MSN-TP are formed with a high degree of silica poly condensation with mainly Q3 and Q4 substructures. Transmission electron microscopy (TEM) images of the MSN-TP (SI Figure S2) demonstrate that the nanoparticles are well dispersed, with uniform sizes and clear nanochannels. The average particle diameter of the MSN-TP nanoparticles is 71.4 ± 7.8 nm (calculated by the statistical analysis of at least 100 particles), the silica pore diameter is 2.8 ± 0.14 nm, which is large enough to accommodate the tricopper complexes within the nanochannels of the MSN-TP that provide the confined space for the selective catalytic oxidation of the light alkane gases considered in this study. Finally, the N2 adsorption−desorption isotherm of the bare MSN-TP is shown in SI Figure S3. The Brunauer−Emmett−Teller (BET) surface area and pore 5432

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Scheme 1. Catalytic Cycle: Comparison of a Productive Cycle That Mediates the Oxidation of Alkane (RH) to Alkanol (ROH) versus an Abortive Cycle That Merely Leads to the Oxidation of H2O2 to O2 and H2O

volume of the MSN-TP are 913 m2 g−1 and 0.71 cm3 g−1, respectively. To assemble the CuEtp@MSN-TP catalyst, we use the ionexchange method to immobilize the CuIICuIICuII(7-NEtppz)4+ complexes in the nanochannels of the MSN-TP.36,37 The loading amount of the CuII Cu II Cu II (7-N-Etppz) 4+ complex in the nanoparticles is 114 μmol g−1, as determined by measuring the copper content by ICP-MS and the quantity of the ligand from C, N elemental analysis. The catalyst is robust, but typically there is ∼10% loss of the tricopper complexes from the MSN at the end of a 7 h catalytic run.36,37 Electron paramagnetic resonance (EPR) measurements (SI Figure S4) on the MSN samples reveal that the CuIICuIICuII(7N-Etppz)4+ complexes have remained intact during the immobilization into the functionalized MSN. From comparison of the N2 adsorption−desorption isotherms of the MSN-TP before and after incorporation of the CuIICuIICuII(7-NEtppz)4+ complexes (SI Figure S3), we deduce that the immobilization of the tricopper complexes into the nanochannels of the MSN-TP has reduced the BET surface area from 913 to 702 m2 g−1 and the BET pore volume from 0.71 to 0.39 cm3 g−1. Moreover, the silica pore diameter has decreased from 2.8 to 1.7 nm. From the decreased surface area and pore volume, and the reduction of the average pore size, we infer that the CuIICuIICuII(7-N-Etppz)4+ complexes are immobilized within the nanochannels of the MSN-TP and partially occupy the pore space. Catalytic Oxidation of Pure C2H6 and C3H8 Mediated by the Cu3(7-N-Etppz) Immobilized in the MSN-TP. We have undertaken the catalytic oxidation of both pure C2H6 and C3H8 mediated by the Cu3(7-N-Etppz) complex immobilized in MSN-TP. For comparison with the oxidation of CH4 by the same catalytic system, we reproduce in SI Figure S5 some of the data reported in our earlier publication on CH4 oxidation.36

In these experiments, the Cu3(7-N-Etppz) complex is activated by O2 and the catalyst is regenerated by H2O2 reduction of the “spent” catalyst after O atom transfer from the activated tricopper cluster complex to the alkane substrate at room temperature.22,31,32 As shown in Scheme 1, substrate oxidation is linked to the consumption of one H2O2 molecule during a productive catalytic cycle (Scheme 1, left panel) without the net consumption of O2, so the net effect of the catalytic turnover is “indirect oxidation” of the alkane by H2O2. Nonetheless, the H2O2 is functioning as a reductant and not an oxidant in the catalytic turnover mechanistically. Moreover, at sufficiently high concentrations of H2O2 or under sufficiently low alkane concentrations, the H2O2 added to the system can also be oxidized by the activated tricopper cluster complex (Scheme 1, right panel). This competition between the alkane substrate and H2O2 for the activated tricopper cluster limits the catalytic efficiency of the H2O2 used to drive the catalytic turnover.30 From Scheme 1, it is evident that abortive cycling begins to set in when kabortive [H2O2] ≥ kCH [alkane], where kCH is the second-order rate constant for the oxidation of the C−H bond by the activated tricopper cluster complex in the productive cycle; and kabortive is the second-order rate constant associated with the deactivation of the activated tricopper cluster complex by reduction with a molecule of H2O2 in the abortive cycle. Given the low solubility of the light alkane gases in a typical solvent, we can expect to encounter this difficulty even at the rather low H2O2 concentrations (≤150 mM) employed in the present study. In fact, we presented evidence for this competition between productive and abortive cycling of the catalyst in our earlier work on CH4 oxidation, where the Cu3(7-N-Etppz) complex was used as a homogeneous catalyst in MeCN.29 We subsequently learn to alleviate this problem, at least in part, by immobilizing the tricopper complex in MSN, 5433

DOI: 10.1021/acssuschemeng.8b00270 ACS Sustainable Chem. Eng. 2018, 6, 5431−5440

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ACS Sustainable Chemistry & Engineering exploiting the “over-solubility” of CH4 in the mesopores of the nanoparticles.36 We are adopting the same strategy here. To activate the Cu3(7-N-Etppz) complex immobilized in the MSN-TP, the CuIICuIICuII complex is first reduced to form the CuICuICuI complex. This reduction is performed by adding a slight excess of sodium ascorbate solution (ca. 4 equiv. based on the amount of the catalyst) into a glass sample bottle with a rubber cap sealed tightly by a metal cap containing 20 mg of the CuEtp@MSN-TP well dispersed in 5 mL of MeCN. After stirring for 10 min and vacuum drying, 10 mL of O2 (ca. 180 equiv., based on the amount of catalyst) and 100 mL (at STP) of C2H6 or C3H8 (ca. 1800 equiv., based on the amount of catalyst) are injected to fill the total volume of the catalytic system. An aliquot of different amounts of H2O2 solution (50, 100, 200, and 500 equiv., based on the amount of catalyst) is then added to drive the catalytic turnover mediated by the Cu3(7-N-Etppz) complex. (Note that a small amount of the H2O2 is consumed reducing the excess ascorbate (ca. 1 equiv.) left over from the amounts required to generate the catalyst at the outset.) The reaction mixture is stirred continuously for 3 h at room temperature and analyzed periodically at various intervals over this 3 h period by using GC−MS to monitor the formation of products. Limiting Turnover Numbers (TONs). The results of the C2H6 and C3H8 oxidation mediated by the Cu3(7-N-Etppz) complex immobilized in the nanochannels of the MSN-TP at room temperature when the catalytic turnover is initiated with different starting amounts of H2O2 ([H2O2]0) are shown in Figure 1 and summarized in Table 1.

Table 1. Summary of the TONs, Selectivity, and OCE Obtained for the Oxidation of C2H6 and C3H8 Mediated by the Cu3(7-N-Etppz) Complex Immobilized in MSN-TP Using Different [H2O2]0 to Drive the Catalytic Turnover.a entry

[H2O2]0 (eq )

TONolb

1 2 3 4 5 6 7 8

50 100 200 500 50 100 200 500

39.4 78.6 155.2 382.8 22.3 43.9 86.7 209.5

TONoneb

selectivityc (-ol, %)

d conversion (%)

OCEe (%)

8.6 17.3 34.6 86.8

100 100 100 100 72.2 71.7 71.5 70.7

2.2 4.4 8.7 21.3 1.7 3.4 6.8 16.5

78.8 78.6 77.6 76.6 79.0 78.5 78.0 76.6

a The starting amount of alkane gas (C2H6 or C3H8) in each of the experiments is 4.1 mmol; and the quantity of the tricopper complex immobilized in the MSN-TP is 2.28 μmole. bTON-ol: TON of alcohol [EtOH (entry 1−4) and isopropanol (entry 5−8)]; TON-one: TON of acetone. cSelectivity of alcohol. dConversion = (moles of alcohol + moles of ketone)/(mole of alkane gas) × 100%. eOCE = (TON-ol +2 TON-one)/[H2O2]0.

Only EtOH is produced in the catalytic oxidation of C2H6. No other product is detected by GC/MS. When the catalytic turnover is driven by 100 equiv. of H2O2, the limiting TON of EtOH produced by the immobilized Cu3(7-N-Etppz) complex can approach to 78.6 after reaction for 3 h. With the present quasi-heterogeneous catalytic system, the observed TON is significantly higher compared to when the same tricopper complex is employed for C2H6 oxidation in MeCN as a homogeneous catalyst without the silica support, where the limiting total TON is only 11 under otherwise similar conditions.30 On the other hand, the time course of C3H8 oxidation mediated by the CuEtp@MSN-TP catalyst at room temperature follows a consecutive kinetic conversion of C3H8 into isopropanol and then to acetone. When the catalytic turnover is driven by 100 equiv. of H2O2, we obtain high product TONs (limiting total TON of 61.2 for isopropanol and acetone) with good product selectivity toward isopropanol (72%) after the 3h reaction at room temperature. As expected, dramatically higher TONs are obtained here, where the Cu3(7-N-Etppz) complex is reformulated as a heterogeneous catalyst compared to when it was used earlier as a homogeneous catalyst for C3H8 oxidation in MeCN without the silica support, where the limiting total TON is only 18.2.30 To evaluate the catalytic performance of the heterogeneous CuEtp@MSN-TP toward C2H6 and C3H8 oxidation, we use different [H2O2]0 (50, 100, 200, and 500 equiv., based on the amounts of the catalyst) to drive the catalytic turnover. In the case of C2H6 oxidation, the limiting product TONs are directly proportional to the [H2O2]0. When the catalytic turnover is initiated with 500 equiv. of H2O2, the limiting TON of EtOH can approach to 382.8, corresponding to ∼21% conversion of the C2H6 substrate (ca. 1800 equiv. initially) to EtOH with 100% selectivity. With C3H8 oxidation, both of the limiting individual product TONs (TON−ol and TON−one) (SI Figure S6) as well as the total product TON (TON−ol + TON−one) SI Figure S7) and the productive turnovers of the catalyst (TON −ol + 2 TON−one, Figure 1b) are also roughly proportional to the initial amounts of H2O2 used to drive the turnover. When the catalytic turnover is initiated with 500 equiv. of H2O2, the limiting total TON is 296.3 and the limiting

Figure 1. Time course of (a) the product TON of EtOH in the C2H6 oxidation, and (b) the product TON of isopropanol and acetone (TON-ol +2 TON-one) obtained in the C3H8 oxidation, catalyzed by the CuEtp@MSN-TP catalyst at room temperature using different [H2O2]0 (equiv.) to drive the catalytic turnover. The starting amount of alkane gas (C2H6 or C3H8) in each of the experiments is 4.1 mmol; and the quantity of the tricopper complex immobilized in the MSNTP is 2.28 μmole. 5434

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ACS Sustainable Chemistry & Engineering

However, there is clearly some abortive cycling of the catalyst during turnover as the OCE decreases gradually with increasing [H2O2]0 for both C2H6 and C3H8 oxidation (Table 1). Turnover Frequency (TOF). In Scheme 1, the regeneration of the Cu3(7-N-Etppz) complex from the “spent” catalyst, namely, the CuICuII(μ-O)CuII(7-N-Etppz)1+ species formed after the facile O atom transfer from the activated tricopper cluster complex to the substrate, is rate-limiting in the catalytic cycle.36,37 Accordingly, the rate of product formation is controlled by the reduction of the “spent” catalyst by the H2O2 in the medium. As a bimolecular process, the rate law is d[product]/dt = k2[catalyst][H2O2] = k[H2O2], where k2 denotes the second-order rate constant, and k denotes the pseudo first-order rate constant (k2[catalyst]).37 This rate equation predicts the following turnover frequency (TOF) for the tricopper catalyst: TOF = [catalyst]−1(d[product]/dt) = d(TON)/dt = k[H2O2] = k([H2O2]0 − TON), if the small degree of abortive cycling of the catalyst may be ignored (the H2O2 concentrations are now expressed in equiv of the catalyst). Upon integration, we obtain Ln[1 − TON/[H2O2]0] = −kt. Application of this result to the data summarized in Figure 1 on the productive catalytic turnovers of C2H6 oxidation (TON-ol) and C3H8 oxidation (TON-ol +2 TONone) catalyzed by the CuEtp@MSN-TP catalyst at various reaction times is shown in Figure 3. All the data ([H2O2]0 = 50,

number of productive turnovers is 383.1, corresponding to ∼17% conversion of the C3H8 substrate (ca. 1800 equiv. initially); and the limiting TON of isopropanol (TON−ol) and acetone (TON−one) are 209.5 and 86.8, respectively, with ∼71% selectivity toward isopropanol as the major product (Table 1 and Figure 2).

Figure 2. Time course of the product TONs for the C3H8 oxidation reaction catalyzed by the Cu3(7-N-Etppz) complex immobilized in MSN-TP at room temperature initiated using 500 equiv. of H2O2 to drive the catalytic turnovers. The TONs of isopropanol and acetone denote the moles of the two products formed per mole of the tricopper complex mediating the C3H8 oxidation. The starting amount of C3H8 in the experiment is 4.1 mmol; and the quantity of the tricopper complex immobilized in the MSN-TP is 2.28 μmole.

The Overall Catalytic Efficiency (OCE). The performance of the heterogeneous CuEtp@MSN-TP catalyst toward the alkane oxidation can be assessed by the catalytic efficiency (CE). From the kinetic scheme highlighted for the catalytic turnover in Scheme 1, the CE is given by the ratio of the catalytic turnovers to the amounts of H2O2 consumed to accomplish the formation of the products. This quantity takes into consideration the quantities of H2O2 expended for productive catalytic turnovers (one molecule per turnover) as well abortive cycling of the catalyst (two molecules per cycle); the latter process is wasteful of the H2O2 used to drive the turnover of the catalyst. In our experiments, we estimate an overall catalytic efficiency (OCE) from the total number of catalytic turnovers at the end of the 3h catalytic run to the amounts of H2O2 initially added to the system to carry out the catalysis; namely, OCE = [limiting TON−ol]/[H2O2]0 for C2H6 oxidation, and [limiting (TON-ol +2 TON-one)]/[H2O2]0 in the case of C3H8 oxidation. Note that the OCE is equal to the CE if all the H2O2 introduced to accomplish the catalysis is totally consumed at the end of the experimental run. Thus, the OCE offers a measure of the effectiveness of the tricopper complex as a catalyst for alkane oxidation. The OCEs for C2H6 and C3H8 oxidation mediated by the CuEtp@MSN-TP catalyst are summarized in Table 1. Typically, the OCE is 75−80% for the various amounts of H2O2 used to drive the catalytic turnover, indicating that the bulk of the H2O2 is consumed in turning over the tricopper catalyst for products formation in C2H6 or C3H8 oxidation reaction. These catalytic efficiencies are similar to the results previously reported for methane oxidation, 36 and are remarkable, given the limited solubility of these light alkanes at the normal pressures used in our experiments and that the catalysis is performed at room temperature. The conclusion is that the activated tricopper cluster is enormously efficient in transferring the harnessed “oxene” to the alkane substrate.

Figure 3. Best fit of the kinetic data, obtained for (a) C2H6 and (b) C3H8 oxidation reaction catalyzed by the CuEtp@MSN-TP catalyst at room temperature using different [H2O2]0 to initiate the catalytic turnover.

100, 200, and 500 equiv.) can be seen to fall on the same straight line with a negative slope of k = 0.0083 min−1 for C2H6 oxidation and k = 0.0085 min−1 for C3H8 oxidation. These results yield similar TOF of 4.15 min−1 (or 249 h−1) for C2H6 oxidation and 4.25 min−1 (or 255 h−1) for C3H8 oxidation at the H2O2 concentration of 500 equiv. Thus, the TOF of the catalyst is essentially independent of the substrate, as expected. The reader is reminded that these values are upper limits to the TOF as the H2O2 consumption has not been corrected for the 5435

DOI: 10.1021/acssuschemeng.8b00270 ACS Sustainable Chem. Eng. 2018, 6, 5431−5440

Research Article

ACS Sustainable Chemistry & Engineering small degree of abortive cycling in the analysis. A TOF of 210 h−1 has been previously reported for cyclohexane oxidation by the same catalytic system under the same condition, where abortive cycling of the catalyst is not apparent.37 Control Experiments: Is the Real Catalyst the Tricopper Cluster Complex Immobilized in the MSN-TP? Based on our previous reports on CH4 and cyclohexane oxidations,36,37 we have assumed here that the conversion of C2H6 and C3H8 into their corresponding oxidized products catalyzed by the immobilized Cu3(7-N-Etppz) complex in the MSN-TP is a quasi-heterogeneous process. To confirm this assumption, we have tested for catalytic C2H6 and C3H8 oxidations by the bare MSN-TP under the same conditions. No catalytic activities are detected by GC-MS (SI Figure S8). In addition, we have looked for catalytic activity in the solution phase after removing the nanoparticles from the solution by centrifugation following a 40 min catalytic reaction, until the end of the 3-h experiment. As expected, no catalytic activities are detected for the solutions without the nanoparticles (SI Figure S9), indicating that the catalysis is heterogeneous in nature. We have also confirmed that there are no differences in the TONs during the time course of both the C2H6 and C3H8 oxidation mediated by the CuEtp@MSN-TP catalyst at room temperature in the presence of the radical trapping agent (2,6-ditert-butyl-p-cresol) (SI Figure S10), indicating that the catalytic oxidations are not promoted by auto-oxidation triggered by the immobilized tricopper complexes. Thus, the catalytic activity is indeed intrinsic to the active Cu3(7-N-Etppz) complex immobilized in MSN-TP, ruling out the production of diffusing radicals by reaction(s) of the tricopper complexes with O2 or H2O2 as the source of the catalysis. Finally, to confirm that the conversion of isopropanol into acetone is indeed catalyzed by the Cu3(7-N-Etppz) complex immobilized in the MSN-TP in the presence of H2O2, and not by the MSN-TP support, we use isopropanol as the starting substrate for oxidation with the bare MSN-TP in the presence of 200 equiv. H2O2. As shown in SI Figure S11, no acetone is produced. Catalytic Oxidation of a Mixture of Light Alkanes (C1+C2+C3) by H2O2 Mediated by the Cu3(7-N-Etppz) Complex Immobilized in the MSN-TP. Since the thrust of the present study is to assess the efficacy of the heterogeneous CuEtp@MSN-TP catalyst for the conversion of natural gas into liquid oxidation products, we proceed next to examine the performance of the catalyst toward a mixture of CH4, C2H6 and C3H8, the principal components of natural gas. To begin, we have carried out a catalytic oxidation of a mixture comprising equal molar fractions of these light alkanes gases (total alkane (C1+C2+C3) = 500 equiv.) by 200 equiv. of H2O2 in the presence of the CuEtp@MSN-TP catalyst. These results are depicted in Figure 4. The outcome is dramatic. While we find that all the catalytic results, including the productive catalytic turnovers and OCEs, are similar to the oxidation of the pure alkane gases initiated by 100 or 200 equiv. of H2O2, the conversion in the case of the gaseous mixture is strongly selective toward the oxidation of C3H8 and C2H6. We observe the following product selectivity: MeOH (1.9%), EtOH (41.0%), isopropanol (40.8%), and acetone (16.3%). However, the OCE (62.5%) is rather similar, albeit lower by ca. 20%, compared with the pure alkane gases. To account for the observed product throughputs, there must be strong kinetic competition among the three light alkane gases for the harnessed “oxene” in the activated tricopper cluster complex

Figure 4. Time course of the individual product TONs for oxidation of the light alkanes in a 1:l:1 mixture of CH4, C2H6, and C3H8 (total (C1+C2+C3) = 500 equiv. (1.14 mmol)) catalyzed by the CuEtp@ MSN-TP catalyst (20 mg containing 2.28 μmole of the tricopper complex) at room temperature initiated by 200 equiv. of H2O2 to drive the catalytic turnovers.

according to the following order of reactivity: C3H8 > C2H6 ≫ CH4. The factors that can influence the reactivity of the activated tricopper cluster complex toward the three different alkane gases are quite complex. While there might be subtle differences in the C−H bond energies of the different alkanes that can influence the intrinsic first-order rate constants for transfer of the harnessed “oxene” (kOT) to the various alkane substrates, the dominating effects are more likely to come from different affinities (K) of the alkane substrates toward the binding pocket at the base of the tricopper complex30,36 as well as different partitioning of the gas molecules into the liquid trapped within the nanopores of the MSN (Henry’s law constant, kH). In simplistic terms, the product throughput for each alkane substrate is expected to be directly proportional to the product (K)(kOT)(kH)(xgas), where xgas is the mole fraction of the individual alkane component in the gas in equilibrium with the liquid trapped in the nanoconfined space of the MSN. Since K can vary dramatically with the molecular size of the alkane substrate36 and the partitioning of the gas molecules can also be dependent on the molecular size, it is evident that the competitive oxidation of CH4, C2H6 and C3H8 can lead to dramatic kinetic fractionation of the oxidized products. Thus, these results offer insights into the performance of the tricopper cluster complex at the level of the O atom transfer step of the catalytic turnover cycle, unlike the TOF, which give information only on the rate-limiting step associated with the regeneration of the catalyst during turnover. From the productive turnovers of the catalyst toward C3H8, C2H6 and CH4 oxidation to their corresponding alcohols in this experiment, we deduce the following selectivity for the alkane oxidation: C1:C2:C3 = 1:22:30. Catalytic Oxidation of Natural Gas Mediated by the Cu3(7-N-Etppz) Complex Immobilized in the MSN-TP. To examine the efficacy of our CuEtp@MSN-TP catalyst for catalytic oxidation of natural gas, two samples of natural gas were collected by Chinese Petroleum Corporation (CPC) from different locations in Miaoli County in Taiwan. CPC also performed GC analyses of the compositions of the natural gas (SI Figure S12). The contents of the major gas components are summarized in Table 2. The natural gas samples are complex mixtures of CH4, C2H6, C3H8, N2, and CO2, as well as trace amounts of other minor components. While the major fractions are CH4 and CO2, the 5436

DOI: 10.1021/acssuschemeng.8b00270 ACS Sustainable Chem. Eng. 2018, 6, 5431−5440

Research Article

ACS Sustainable Chemistry & Engineering

Similar results are obtained with the CS-71 natural gas sample (Table 3b). Here the oxidized product TONs and selectivity are substantially higher for C2H6 and C3H8 oxidation than in the case of the CHK sample (Table 3a); the C2H6/ C3H8 content is double for these gases in the CS-71 sample. The productive catalytic turnovers for CH4, C2H6, and C3H8 oxidation are 23.9%, 44.7%, and 31.4%, respectively. From the product selectivity in the natural gas oxidation: MeOH (25.4%), EtOH (47.5%), isopropanol (20.8%), and acetone (6.3%), we deduce that C2H6 oxidation is 35 times more selective than CH4 oxidation (molecule for molecule), and C3H8 oxidation to produce isopropanol is in turn 2.0 times more selective than C2H6 oxidation in the competitive oxidation. Thus, the selectivity toward oxidation of the three light alkane gases is essentially the same between the two natural gas samples. For comparison, the corresponding alkane substrate selectivity in the oxidation of the 1:1:1 (C1+C2+C3) mixture of pure alkane gases is C2H6:CH4 = 22 and C3H8:C2H6 = 1.4. Compared with the catalytic oxidation of pure C2H6 and pure C3H8 gas, as well as the 1:1:1 (C1+C2+C3) mixture of the pure alkane gases described earlier, the OCEs of the catalytic oxidation observed for the natural gas samples are substantially lower (∼40%), although still reasonable. Substantial amounts of C2H6 and C3H8 are remaining in the catalytic system after the 3-h reaction in these experiments. We have carried out the catalytic oxidation with reduced amounts of natural gas. The results with natural gas samples each consisting of a total alkane (C1+C2+C3) content of 500 equiv. in the catalytic oxidation initiated with 200 equiv of H2O2 are shown in Table 4. We find that the selectivity of MeOH (59.6%) is better than that of EtOH (32.4%), isopropanol (1.0%), and acetone (7.0%) for the CHK natural gas sample. The similar observation is seen in the case of the CS-71 natural gas: MeOH (46.9%), EtOH (41.4%), isopropanol (1.1%), and acetone (10.6%). However, in these experiments, only small amounts of C2H6 and C3H8 are remaining in the reaction bottle after the 3-h catalytic run. Evidently, the levels of these gases are diminishing and becoming less available to the tricopper catalyst so that their oxidation becomes less competitive relative to CH4, which is present in significantly greater abundance. Finally, we show that the catalytic activity of natural gas oxidation is sustainable by simply providing additional H2O2 to drive the catalytic turnovers. If an additional 200 equiv. of H2O2 is injected into the catalytic system after a 3-h catalytic reaction, essentially all the C2H6 and C3H8 are consumed and converted into EtOH and acetone at the end of second 3-h catalytic run. The catalyst works then only on oxidizing the CH4. These results are summarized on Table 5.

Table 2. Contents and the Compositions of the Principal Gases in the Natural Gas Samples sample

CH4 (%)

C2H6 (%)

C3H8 (%)

N2 (%)

CO2 (%)

CHK CS-71

56.4 79.5

2.1 4.3

0.51 1.2

2.57 1.01

37.7 12.8

two samples differ substantially in the contents of these two gases: Sample CS-71 is more abundant in CH4 (79.5%) relative to sample CHK (56.4%), but the CHK sample contains substantially greater amounts of CO2 (37.7% versus 12.8%). Thus, the alkane content is substantially lower in the CHK sample. However, relative to CH4, the fraction of C2H6 and C3H8 is also ∼50% higher in the CS-71 sample (6.9% versus 4.6%). These differences in the composition of the alkanes between the two samples should be reflected in the catalytic performance of the CuEtp@MSN-TP catalyst between the two natural gas samples, given that we find C2H6 and C3H8 oxidation to be much more efficient relative to CH4 in the experiments described above on the mixture of the three pure gases (C1+C2+C3). We observe the same kinetic competition among these alkane gases in the catalytic oxidation of the natural gas samples: C3H8 > C2H6 ≫ CH4. The catalytic results, including the TONs, selectivity, and OCE, obtained upon incubation of the two natural gas samples with the Cu3(7-N-Etppz) complex immobilized in the MSNTP are summarized in Table 3. In these experiments, the catalytic turnover is initiated by 200 equiv. of H2O2. However, given the low levels of C2H6 and C3H8 in the natural gas samples, the experiments are performed using much higher amounts of the natural gas in the catalytic oxidation (total alkane (C1+C2+C3) = 2540.7 equiv. in the experiment on the CHK sample; and 3659.7 equiv. for the CS-71 sample), to ensure that the C2H6 and C3H8 are not depleted in the reaction bottle during the time course of the 3 h catalytic run. For the CHK natural gas sample, we observe the following product selectivity: MeOH (36.3%), EtOH (42.6%), isopropanol (15.7%), and acetone (5.4%). (Table 3a) The turnover of the catalyst to produce EtOH (40.5%) is greater than that of MeOH (34.4%) and isopropanol plus acetone (25.1%). However, before comparing these results with those described earlier for the (C1+C2+C3) pure gas mixture, where the three gases are present in essentially equal proportions, we normalize the production yields of the oxidation products according to their molar ratios in the natural gas mixture. When this is done, we find that C2H6 oxidation is 32 times more selective than CH4 oxidation (molecule for molecule), and C3H8 oxidation to produce isopropanol is in turn 1.6 times more selective than C2H6 oxidation in the competitive oxidation.

Table 3. Summary of the TONs, Selectivity, and OCE Obtained for the Oxidation of Natural Gas Mediated by the Cu3(7-NEtppz) Complex Immobilized in the MSN-TPa When the Catalytic Turnover is Initiated by 200 equiv. of H2O2 (a) CHKb

a

(b) CS-71c

products

TON

selectivity (%)

alkane gas remainingd (%)

TON

selectivity (%)

alkane gas remainingd (%)

methanol ethanol isopropanol acetone OCE

29.1 34.2 12.6 4.3

36.3 42.6 15.7 5.4

98.8 62.2 23.0

21.4 40.0 17.5 5.3

25.4 47.5 20.8 6.3

99.4 78.4 55.9

42.3%

44.8%

10 mg of CuEtp@MSN-TP containing 1.14 μmole of the tricopper catalyst. b(C1+C2+C3) content 2.90 mmol. c(C1+C2+C3) content 4.17 mmol. Alkane gas remaining after the 3-h catalytic run.

d

5437

DOI: 10.1021/acssuschemeng.8b00270 ACS Sustainable Chem. Eng. 2018, 6, 5431−5440

Research Article

ACS Sustainable Chemistry & Engineering

Table 4. Summary of the TONs, Selectivity, and OCE Obtained for the Oxidation of Reduced Amounts of Natural Gas (Total Alkane (C1+C2+C3) = 500 equiv.) Mediated by Cu3(7-N-Etppz) Complex Immobilized in the MSN-TPa When 200 equiv. of H2O2 is Used to Initiate the Catalytic Turnover (c) CHKb

(d) CS-71b

products

TON

selectivity (%)

alkane gas remainingc (%)

TON

selectivity (%)

alkane gas remainingc (%)

methanol ethanol isopropanol acetone OCE

31.2 17.0 0.5 3.7

59.6 32.4 1.0 7.0

93.5 4.6 3.5

27.1 23.9 0.6 6.1

46.9 41.4 1.1 10.6

94.2 5.5 4.6

28.0%

31.9%

10 mg of CuEtp@MSN-TP containing 1.14 μmole of the tricopper catalyst. b(C1+C2+C3) content 570 μmole. cAlkane gas remaining after the 3 h catalytic run. a

Table 5. Summary of the TONs, Selectivity, and OCE Obtained for the Oxidation of Natural Gas (Total Alkane (C1+C2+C3) = 500 equiv.) Mediated by Cu3(7-N-Etppz) Complex Immobilized in the MSN-TPa After 200 equiv. of H2O2 are Used to Initiate the Catalytic Turnover for 3 h, and an Additional 200 equiv. of H2O2 are Introduced to Sustain the Catalytic Oxidation up to 6 h (e) CHKb

(f) CS-71b c

products

TON

selectivity (%)

alkane gas remaining (%)

TON

selectivity (%)

alkane gas remainingc (%)

methanol ethanol isopropanol acetone OCE

50.2 17.7