Author Manuscript. Title: Aerobic Self-Esterification of Alcohols Assisted by Mesoporous Manganese and Cobalt Oxide

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Author Manuscript

Title: Aerobic Self-Esterification of Alcohols Assisted by Mesoporous Manganese

and Cobalt Oxide

Authors: Ehsan Moharreri; Sourav Biswas; Bahareh Deljoo; David Kriz; Seyoung

Lim; Sarah Elliott; Shanka Dissanayake; Marina Dabaghian; Mark Aindow; Ste-ve Suib

This is the author manuscript accepted for publication and has undergone full peer review but has not been through the copyediting, typesetting, pagination and proofrea-ding process, which may lead to differences between this version and the Version of Record.

To be cited as: 10.1002/cctc.201900704

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Aerobic Self-Esterification of Alcohols Assisted by Mesoporous

Manganese and Cobalt Oxide

Dr. Ehsan Moharreri§, Dr. Sourav Biswas†, Bahareh Deljoo‡§, David Kriz†, Seyoung Lim†, Sarah Elliott†, Shanka Dissanayake†, Marina Dabaghian+, Prof. Dr. Mark Aindow‡§, and Prof. Dr. Steven L. Suib†,§,*

§

Institute of Materials Science, University of Connecticut, Storrs, Connecticut 06269-3060.

Department of Chemistry, University of Connecticut, Storrs, Connecticut 06269-3060.

Dept. of Materials Science and Engineering, University of Connecticut, Storrs, CT 06269, USA

+

Department of Chemical and Biomolecular Engineering, University of Connecticut, Storrs, Connecticut 06269-3060.

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Abstract

Aerobic self-esterification of primary alcohols catalyzed by mesoporous metal oxides (manganese and cobalt oxides) is reported under base and solvent free conditions. For a range of aliphatic alcohols, up to 90% conversions to esters was achieved. The catalytic reaction is likewise applicable to neat aldehydes as substrates with yields of up to 86%. High pressure batch reaction for ethanol to ethyl acetate led to 22% yield. Isotope labeling studies indicated decarboxylation on the catalyst surface. Mechanistic and kinetic experiments implicate oxygen rebound and -carbon removal as intermediate steps. Mesoporous cobalt oxide showed about 20% higher catalytic activity compared to mesoporous manganese oxide.

Introduction

Ester moieties constitute major backbones, as well as functional groups of chemical significance, in numerous natural products and synthetic compounds. They are essential building blocks for various polymers, pharmaceuticals, cosmetics, agrochemicals, flavoring agents, fragrances, food emulsifiers, and soaps. Ethyl acetates are used extensively as solvent in paint, adhesives, inks, and fragrances. For thermally stable polymers and optical materials, acrylic esters are used. Transesterification of oils and fats is used in food emulsifiers and soaps. Transesterification of triglycerides with methanol is used in biodiesel fuels.

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of unwanted waste chemicals. [2–4] Direct self-esterification reaction is an attractive pathway due to simplicity and elimination of wasteful chemicals and acids.[1,5–7] In the historical context, direct catalytic conversion of alcohols to esters has been around at least since 1987 when Murahashi et al. used homogenous ruthenium di-hydride complexes to catalyze direct alcohol to ester occurring along with hydrogen formation.[8] Oxidative addition of ruthenium into the OH bonds and subsequent -elimination of (RuH) species giving corresponding carbonyl occurs as the principle pathway of oxidative condensation of alcohols to aldehydes and esters. Zhang et al. (2005) worked further on dehydrogenative oxidation of alcohols by a ruthenium complex without a sacrificial hydrogen acceptor.[9] They developed a Ru complex treated with base to get RuII with electron rich ligands which would catalyze esterification. Homogeneous catalytic complexes,[9,10] noble metal catalysts and photocatalytic systems,[5,11,12] palladium assisted carbonylation,[13] oxidative esterification,[6] chemical oxidants,[14]Au NPs,[15] ozone,[16] basic ionic liquids,[17] and embedded graphite supported nanoparticles[18] have demonstrated catalytic systems of direct esterification.

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a group of mesoporous materials synthesized by a surfactant assisted sol-gel method of making mesoporous materials [21–24] and has been shown to have broad applicability for catalysis.[25–28] Here, we report the self-esterification reaction enabled by aerobic heterogeneous UCT metal oxide catalysts mainly mesoporous manganese oxides (meso-MnOx) and mesoporous cobalt oxides (meso-CoOx).

Experimental Methods

Catalyst Synthesis

Synthesis of Meso MnOx: In a typical synthesis, 0.02 mol of Mn(NO3)2.6H2O and (0.134 mol)

1-Butanol were added into a 120 mL beaker. To this solution (0.0034 mol) P123 (PEO20PPO70PEO20, molar mass 5750 g.mol

-1

) and (0.032 mol) concentrated HNO3 was added

and stirred at room temperature until the solution became clear (light pink). The solution was then kept in an oven at 120 °C for 2 h under air. The resulting black material was washed with excess ethanol, centrifuged, and dried in a vacuum oven overnight. The dried black powder was then heated to 150 °C for 12 h, and 250 °C for 3 h, and 350 °C for 2 h under air.

Synthesis of mesoporous -MnO2 and mesoporous K2-xMn8O16 : For a typical synthesis of meso -MnO2, 0.35 g of 350 °C calcined meso MnOx was mixed with 50 mL of 0.5 M solution of

sulfuric acid, and subsequently sonicated for at least 10 minutes. The mixture was then heated in a plastic bottle at 70 °C for 2 h. The resulting material was filtered and washed with excess solution of 50% of water and ethanol. The synthesis of mesoporous K2-xMn8O16 started with 0.35

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Synthesis of meso CoOx: In a typical synthesis 0.017 mol of Co(NO3)2.6H2O and (0.163 mol)

1-Butanol were added into a 120 mL beaker. To this solution (0.00425 mol) P123 and (0.038 mol) concentrated HNO3 were added and stirred at room temperature until the solution became

clear (purple). The solution was then kept in an oven at 120 °C for 3 h under air. The resulting dark purple material was washed with excess ethanol, centrifuged, and dried in a vacuum oven overnight. The dried black powder was then heated to 150 °C for 12 h, and 250 °C for 4 h, and 350 °C for 2 h under air.

Synthesis of cesium-promoted meso CoOx, meso MnOx, mesoporous -MnO2, and

mesoporous K2-xMn8O16: In a typical synthesis of cesium-promoted meso MnOx,

[21,25]

before the addition of the surfactant Pluoronic P123, 200 μL of 1.0 M CsNO3 was added dropwise

maintaining the Mn/Cs ratio at 100/1 (mol/mol). The synthesis then followed the procedure for meso MnOx synthesis as described above. The same process was used for synthesis of cesium-promoted meso CoOx, so that before the addition of Pluoronic P123, 200 L of 1.0 M CsNO3 was added dropwise to the solution. For cesium promotion of mesoporous -MnO2 and

mesoporous K2-xMn8O16 the typical process involves starting with meso-CsMnOx-350 °C, and

the rest of the procedure involves acid treatment as described above for the synthesis of mesoporous -MnO2 and mesoporous K2-xMn8O16.

The meso MnOx, meso CsMnOx, and meso CoOx were subjected to heating cycles under air in order to obtain the desired crystal structures and mesopore sizes. The samples are designated meso [Cs]MOx(X), where X is the final heating step (calcination) temperature. In this study, samples with heat treatment temperatures of 150, 250, 350, 450, and 550 °C were prepared. For comparison, commercial lab grade manganese (III) oxide (-Mn2O3)

polycrystalline powder of 99.9%, Mn3O4 commercial sample, MnO commercial sample, and

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Esterification Reaction: In a typical reaction, 100 mg of catalyst was added to 0.03 mol of

alcohol in a two-neck round bottom flask. The reaction medium was stirred vigorously at 150 °C and 700 rpm with airflow of 100 sccm (standard cubic centimeters per minute). After the desired time (typically 48 h), a 1 mL sample of the mixture was filtered and analyzed (Figure S7) using gas chromatography–mass spectrometry (GC-MS). The conversion was determined based on the concentration of alcohols.

High Pressure Reaction: The high-pressure experiments for esterification of light alcohols

were carried out in a 50 mL high pressure reactor with volume of 50 mL model A3240HC6EB (Parr Instruments, Moline, IL) utilized with reactor controller 4848 and a mechanical stirrer. All gas sources including ultra-pure nitrogen, helium, and oxygen were purchased from Airgas, Inc (North Franklin, CT). Reagents were purchased from Sigma-Aldrich. O2 and He initial partial

pressures were typically 100 and 400 psi, respectively at room temperature. After heating to 150 °C, the reactor was mechanically stirred at 60 rpm throughout the reaction.

Catalyst Characterization

Nitrogen adsorption

To analyze the physical surface characteristics of the samples, nitrogen sorption measurements were carried out with a Quantachrome Autosorb iQ2 automated sorption system at liquid nitrogen temperature. The degassing was carried out at a temperature of 150 °C for 6 h. Using the adsorption volume at a relative pressure of 0.9918 the total pore volume was determined. Surface area through Brunauer-Emmett-Teller (BET) measurement was estimated from the adsorption data in the relative pressure range p/p0=0.0609 to 0.2681. The pore size

distribution was calculated using the desorption isotherm via the Barrett-Joyner-Halenda (BJH) method.

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The overall morphologies of the samples were analyzed using a Teneo LoVac field emission scanning electron microscope (FEG-SEM) operated at an accelerating voltage of 5 kV. A few drops of the sample dispersed in ethanol was placed on carbon tape and allowed to dry under vacuum to remove the solvent. Transmission electron microscopy (TEM) samples were prepared by first grinding the powders in a pestle and mortar set, and then dispersing them in ethanol for a uniform suspension. Drops of this suspension were then placed on a 200 Quantifoil mesh TEM copper grid coated with a holey support film, allowed to dry in air, and then kept in a vacuum desiccator overnight to remove any residual moisture. The samples were studied using an FEI Talos F200X operating at 200 kV, which is equipped with a Super-X silicon drift detector (SDD) system. Scanning transmission electron microscopy (STEM) spectrum imaging experiments were performed by acquiring EDXS spectra at each point, and then using the intensities in the respective K peaks to construct X-ray maps of Mn and O for Meso-MnOx sample, and Co and O for Meso-CoOx. The quantified chemical maps were obtained using Bruker Quantax EDS software.

X-ray Diffraction

X-ray diffraction (XRD) patterns were obtained using a Rigaku Ultima IV diffractometer with Cu K1 radiation ( = 0.15406 nm). The data were collected in the range of 2 = 5° to 75 ° with scanning rates of 2 °/min, an operating voltage of 40 kV, and a current of 44 mA.

Results

Characterization of mesoporous manganese oxides and cobalt oxides

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characterization (XRD, N2 adsorption, SEM, and TEM) results are provided to confirm the

properties of the catalysts. The wide-angle XRD patterns exhibit broad peaks suggesting that the materials exhibit some combination of both amorphous and nano-crystalline structures, but the crystallinity cannot be determined based upon these data alone. Examples of such data from meso Cs-MnOx before and after the reaction are shown in Figure S1. Further study of crystalline structures was performed by electron diffraction pattern analysis. The N2 adsorption

and desorption isotherms (Figure S2), show type IV adsorption isotherms, followed by a type I hysteresis loop, indicating regular mesoporous structures. The BET surface areas are summarized in Table 1. The BJH desorption pore size distributions of meso MnOx-350, and Cs-MnOx-350 materials peak at 4.2 nm and 4.0 nm (pore diameter) (Figure S2, Table 1). The secondary electron SEM (SE-SEM) images of meso MnOx and meso CoOx materials are shown in Figure 1.

Table 1 Textural properties of mesoporous manganese oxide and cobalt oxide samples Mesoporous material Specific surface area

(BET) m2/g

Average pore size (nm)

Total pore volume (cc/g)

Meso MnOx-350 78 4.2 0.16

Meso Cs-MnOx-350 198 4.0 0.20

Meso CoOx-350 91 14.0 0.32

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Figure 1 Representative SE SEM images of meso MnOx-350 (a), CoOx-350(b), Cs-promoted MnOx-350 (c), and CoOx-350 (d).

The HR-TEM images [Figures S3(c) and S4(c)] show the morphologies of the MnOx and CoOx samples. Based on the electron diffraction pattern [Figure S3(a)], the unused meso MnOx sample was mostly amorphous. After the esterification process [Figure S3 (d-f)], the meso MnOx sample had crystallized into Mn2O3 and/or Mn3O4, but it was difficult to distinguish

between these phases based on the TEM data because Mn2O3 and Mn3O4 have very similar

lattice spacings. The diffraction patterns for the cobalt oxide samples are nanocrystalline ring patterns with d-spacings corresponding to the Co3O4 phase for samples before and after the

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Self-Esterification of Alcohols

This section describes step by step selection of reaction conditions and catalyst synthesis optimization. Ester yield is the most significant criterion, nevertheless the catalytic reaction is very selective towards esterification, and minimal amounts of aldehyde by-products are formed in most reactions. Table 2 summarizes the time and temperature effect on conversion of phenethyl alcohol in self-esterification reaction(Scheme 1). Here, meso Cs-MnOx serves as the model catalyst.

Scheme 1 Direct esterification of phenethyl alcohol by meso Cs-MnOx and aerobic oxygen.

Table 2 Time and temperature dependent studya

Entry Time (h) Temperature (°C) Conversionb% TOF (h-1)

1 8 130 3 0.18 2 24 130 5 0.10 3 24 150 8 0.16 4 30 150 15 0.23 5 24 180 40 0.78 6 48 180 98 0.96 a

Direct esterification of phenethyl alcohol (0.03 mol) by 100 mg of meso Cs-MnOx-250. Airflow of 100 sccm. bConversions were determined by GC/MS on the basis of the concentration of alcohols.

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studies particularly compared to transition metal oxide catalysts where full conversion is achieved on the order of about 96 h. [18] At 130 °C we barely observed activity by the catalyst. Even though the temperature of 180 °C gives the highest conversion in 48 h, we choose 150 °C for all other self-esterification reactions, since for some alcohols used here as starting substrate the boiling point is below 180 °C, and the reactions were performed under reflux conditions.

Table 3 Oxidant study

Entry Oxidant Conversion % Selectivity (2) %

1 N2 0 0 2 O2 a 99 77 3 O2 51 20 4 H2O2b 0 0 5 TBHPb 93 90

Use of various oxidants (H2O2, TBHP, N2, O2) for octanol esterification in 48h. Selectivity is

reported for the dominant ester, N/A means no esterification occurred. For all entries 100 mg of meso Cs-MnOx was used as the catalyst except entry 2 where 100 mg of meso Cs-CoOxa was used. 0.03 mol of oxidant used where liquid oxidant are used as a reagent. (TBHPb: tert-Butyl hydroperoxide)

While comparing different oxidants, hydrogen peroxide solution was shown to be ineffective as an oxidant, likely due to large amounts of accompanying water and fast autooxidation of peroxide (Table 3). The tert-Butyl hydroperoxide (TBHP) is very active when used with meso Cs-MnOx. We prefer the mild reaction conditions thereby avoiding chemical

2

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oxidants and high temperatures. Therefore, molecular oxygen provided by air flow is used as the oxidant of choice in the subsequent reaction experiments.

To optimize catalyst synthesis conditions and screen over a variety of MnOx classes, we first studied heat treatment and ion promotion levels of cesium. Table 4 demonstrates the effect of heat treatment and Cs initial amount on the catalyst activity.

Table 4 Catalyst optimization: catalyst calcination temperature and Cs content

Entry Catalyst Final heat

treatmenta %Cs ion promotionb Conversion % 1 Meso Cs-MnOx 250 °C 3 h 1 40 (+/-) 4 c 2 Meso Cs-MnOx 250 °C 3 h 0.5 41 3 Meso Cs-MnOx 250 °C 3 h 0.1 47

4 Meso MnOx 250 °C 3 h none 36

5 Meso Cs-MnOx 350 °C 2 h 1 70

6 Meso Cs-MnOx 450 °C 1 h 1 70

7 Meso Cs-MnOx 550 °C 1h 1 35

8 Meso Cs-MnOx 250 °C 3 h 1.5 64

Conversions are calculated based on total alcohol (1-octanol)

a

Final heat treatment is tabulated. For example, 550 °C 1 h implies preceding 150 °C 12 h, 250 °C 3 h, 350 °C 2 h, and 450 °C 1 h treatments according to procedure. b Initial molar percentage with respect to manganese during synthesis of catalyst. c Representative error values were calculated for entry 1 based on replicating the runs 4 times and taking standard deviation. All reactions were performed with 1-octanol as the starting alcohol and 150 °C for 48 h with air flow. The catalyst was added in 100 mg and 1-octanol in 0.03 mol amount.

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were examined against commercial benchmarks demonstrated in Table S1 and all the Cs promoted samples showed higher activity compared to the non-promoted samples

Table 5 Comparison of meso MnOx and meso CoOx catalyst.

Entry Catalyst Conversion %

1 Meso MnOx 350 60

2 Meso Cs-MnOx 350 70

3 Meso CoOx 350 92

4 Meso Cs-CoOx 350 97

Conversions are calculated based on octanol. All reactions were performed with 100 mg catalyst, and 0.03 mol 1-octanol in 48 h under airflow. Cesium promotion was based on 100:1 initial ratio during synthesis and heat treatment was performed in the following sequence: 150 °C for 12 h, 250 °C for 3h and 350 °C for 2 h.

To check the role of Cs promotion, we introduced meso cobalt oxide along with meso MnOx for the esterification reaction. Supported cobalt oxide nanoparticles have previously been used for direct esterification.[18] Mesoporous cobalt oxide has shown to be effective in oxidation reactions [26] due to ease of reducibility of Co3+ (E°red= +1.81), proton abstraction by Co0 and

Co2+ sites, and high lattice oxygen content.[27,28] We examined catalytic activity of mesoporous cobalt oxide material and compared this with manganese oxide. Results provided in Table 5 show superior activity of cobalt oxide compared to manganese oxide. Activity enhancement by Cs ion promotion is general for both the materials.

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Table 6 Substrate scope: self-esterification of aliphatic and aromatic alcohols

Entry Structure Products Selectivity

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Table 6 continued.

Entry Structure Products Selectivity

% Conversion % 5 50 5 45 51 6 N/A N/A 0 7 N/A N/A 45 8 3 88 93 9 7 30

Self-Esterification of various alcohols by 100 mg of meso Cs-MnOx-350 as catalyst, 0.03 mol of substrates, in 48 h reaction time under airflow * Selectivity of the top 3 highest molecular weight esters are provided in this table. N/A: not applicable for runs where no significant esters or aldehydes are formed.

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There is significant interest in self-esterification of low molecular weight alcohols. Therefore, we performed high pressure experiments to convert ethanol to ethyl acetate as a representative example of self-esterification of lighter weight alcohols. The results of ethanol to ethyl acetate are presented in Table 7.

Table 7 Conversion of ethanol to ethyl acetate in high pressure reaction condition

Entry substrate amount (ml) Oxidant Catalyst Yield % acetaldehyde Yield % ethyl acetate TOF %O2 conversion 1a 10 O2 meso MnOx 1 11 1.2 24

2 10 Air meso MnOx 0.7 0 0.00 0

3 5 *O2 meso MnOx 7 13 0.73 40

4 5 *O2 meso Cs-MnOx 8 22 1.2 67

5 5 *O2 meso Cs-CoOx 11 21 1.2 64

Ethanol to ethyl acetate conversion under high oxygen pressure. All runs were performed with 100 psi of initial O2 pressure and 400 psi He pressure, except run 1 which was performed with

400 psi of O2 and 100 psi of He, all runs at internally controlled temperature of 150 °C. Duration

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Proposed Mechanism

Scheme 2 Proposed mechanism for esterification of alcohols by meso MnOx catalyst.

A proposed reaction mechanism is shown in Scheme 2. The mixed phase nature of the catalyst was previously elucidated with two different readily interchangeable Mn oxidation states

[22]

. Decarboxylation steps and removal of CO2 are demonstrated by TG-MS analysis of used

catalysts after reaction with isotope labeled carboxylic acid (Figure S5).

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progressed. The transformation is suggested to go through a radical formation mechanism. We performed an experiment with a radical inhibitor (2,6-Di-tert-butyl-4-methylphenol) which resulted in no ester formation in the product stream.

We propose a reaction mechanism showcased in Scheme 2 in which alcohol oxidation leads to formation of aldehyde, while aldehyde goes through decarboxylation and the co-presence of aldehyde and alcohol in the co-presence of catalyst leads to esterification. The fact that starting with aldehyde we get transformations to esters further reinforces the mechanism. Table 8 exhibits the results of aldehyde self-esterification.

Table 8 Substrate scope experimentation on aldehydes transformation to esters and carboxylic acid

Entry Structure Product Selectivity%

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Reactions were carried out with 100 mg of meso Cs-MnOx-350 as catalyst, 0.03 mol of substrates, in 48 h reaction time under airflow.

We see higher selectivity for esters in the case of the aliphatic aldehyde compared to the aromatic aldehydes. Conversion of benzaldehyde to phenyl benzoate was particularly low while the overall conversion to carboxylic acid was significant. Stability of aromatic rings precludes further decarboxylation and therefore formation of the ester. Catalyst stability and regeneration is examined by repetitive reactions and filtration steps. To highlight the importance of using a mesoporous structure, we performed regeneration on non-porous crystalline manganese oxide species (Mn3O4) as a benchmark. The results of regeneration studies are provided in Table S2.

The mesoporous structure of both cobalt oxide and manganese oxide stays active after 2 complete 48 h runs, while commercial nonporous manganese oxide ceases to be active after a single run. The regeneration of mesoporous manganese oxide despite the lack of regeneration in nonporous structure is a sign of the importance of oxygen mobility during the reaction.

Discussion and Conclusions

Previously reported self-esterification systems do not exhibit alpha carbon cleavage. Some of the esters produced here lack carbon on the aliphatic chain on one side, and therefore mesoporous metal oxide introduces the -carbon cleavage pathway. Deeper mechanistic insight into catalytic oxidation of alcohol helps with rational design of catalysts. Here we show that the oxygen rebound step is mechanistically feasible. TG-MS study indicates decarboxylation after hydrogen abstraction. Water evolution confirms that hydrogen abstraction occurs along with oxygen transfer, and radical inhibitor tests demonstrated further the possibility of oxygen rebound.

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also showed excellent activity. Since multiple esters are formed, the reaction is not selective towards a single ester, but overall no significant products other than esters are formed. The selectivity is favored towards the esters with one less carbon demonstrated in the substrate scope table. The meso CoOx catalyst however showed favorable full-length aliphatic chain esters as their main ester product. Cobalt oxide catalyst is more active, and therefore the higher rate of full-length esters could be contributed to faster formation of ester before the decarboxylation is given the chance to occur. The mesoporous manganese oxides show significantly higher activity compared to other non-porous manganese oxide material. Cesium promotion helps both CoOx and MnOx materials in achieving higher activity which was not previously shown for meso CoOx material. [25]

The active phase and catalyst stability are studied via crystallographic phase analysis before and after the reaction adding insight to understanding the reaction mechanism. Both meso MnOx and meso CoOx show a hierarchical nanostructure in TEM images (Figures S3 and S4). For meso MnOx the active phase is believed to be Mn2O3 due to the presence of Mn

3+

. Mechanistic studies on meso MnOx suggest a redox reaction between Mn3+ and Mn2+ is essential for catalysis [30,31]. Meso MnOx calcined at 350 °C is shown to be mainly amorphous while at 450 °C shows a clear presence of Mn3O4 and Mn2O3

[22]

. An amorphous appearance is evident in the unused catalyst with some local Mn3O4 crystallinity. During the reaction,

insufficient aeration of catalyst prevents regeneration of active oxygen in the surface and lattice leading to a more pronounced Mn3O4 crystal structure. For meso CoOx, more stability is

observed. The catalyst is Co3O4 before and after the reaction indicating stable catalysis

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reactor seems to be the limiting factor. We suggest slow addition of oxygen for ethanol to ethyl acetate transformation to improve ester yields. The protocol can be extended to self-esterification of aliphatic aldehydes. In summary, the transition metal oxide assisted aerobic esterification is a general protocol that provides synthetic routes to esters starting from a variety of substrates.

Acknowledgement

We would like to thank Maryam Pardakhti for assistance with manuscript editing, Panteha Toloueinia, Dustin Murray-Simmons and Ben Johnson for assistance with experimentations. Authors received funding from the Chemical, Geochemical and Biosciences Division of the Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy for supporting this work under grant DE-FG02-86ER13622-A000. The electron microscopy studies were performed using the facilities in the UConn/Thermo Fisher Scientific Center for Advanced Microscopy and Materials Analysis (CAMMA).

Keywords:

Heterogeneous Catalysis, Esterification, Transition Metal Oxides, Mesoporous Materials, Aerobic Oxidation

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