Reactivity of a Carbon-Supported Single-Site Molybdenum Dioxo Catalyst for Biodiesel Synthesis

Abstract

A single-site molybdenum dioxo catalyst, (O c ) 2 Mo­(O) 2 @C, was prepared via direct grafting of MoO2Cl2(dme) (dme = 1,2-dimethoxyethane) on high-surface-area activated carbon. The physicochemical and chemical properties of this catalyst were fully characterized by N2 physisorption, ICP-AES/OES, PXRD, STEM, XPS, XAS, temperature-programmed reduction with H2 (TPR-H2), and temperature-programmed NH3 desorption (TPD-NH3). The single-site nature of the Mo species is corroborated by XPS and TPR-H2 data, and it exhibits the lowest reported MoO x T max of reduction reported to date, suggesting a highly reactive MoVI center. (O c ) 2 Mo­(O) 2 @C catalyzes the transesterification of a variety of esters and triglycerides with ethanol, exhibiting high activity at moderate temperatures (60–90 °C) and with negligible deactivation. (O c ) 2 Mo­(O) 2 @C is resistant to water and can be recycled at least three times with no loss of activity. The transesterification reaction is determined experimentally to be first order in [ethanol] and first order in [Mo] with ΔH ⧧ = 10.5(8) kcal mol–1 and ΔS ⧧ = −32(2) eu. The low energy of activation is consistent with the moderate conditions needed to achieve rapid turnover. This highly active carbon-supported single-site molybdenum dioxo species is thus an efficient, robust, and low-cost catalyst with significant potential for transesterification processes.

Efficient Chemoselective Reduction of N‐Oxides and Sulfoxides Using a Carbon‐Supported Molybdenum‐Dioxo Catalyst and Alcohol

“…The Mo 3d5/2 peak is found to be 232.6 eV with a spin orbit splitting of 3.1 eV. These results are in good agreement with previously reported XPS data [26] and support our hypothesis of the existence of a Mo(VI) dioxo species.…”

“…All chemicals were used as received and without further purification unless otherwise noted. Mo@C was prepared and fully characterized previously by this laboratory from (dme)Mo(=O) 2 Cl 2 (dme=1,2‐dimethoxyethane) and activated carbon …”

“…Mo@C was prepared and fully characterized previously by this laboratory from (dme)Mo(=O) 2 Cl 2 (dme = 1,2-dimethoxyethane) and activated carbon. [26] Analytical Measurements 1 H and 13 C NMR spectra were recorded at Integrated Molecular Structure Education and Research Center (IMSERC) at Northwestern University with a 500 MHz Bruker Avance III HD system equipped with a TXO Prodigy probe, a 400 MHz Bruker Avance III HD Nanobay system equipped with SampleXpress autosampler, or a 500 MHz Varian Inova-500 spectrometer. Chemical shifts (δ) for 1 H and 13 C are referenced to TMS.…”

“…Compared to homogeneous catalytic processes, heterogeneous catalysts offer several advantages including higher thermal stability and recyclability. [24] Recent work from the Marks group has shown that single-site dioxo-molybdenum species grafted on high surface area activated carbon, Mo@C, is a highly active catalyst for alcohol oxidation, [25] transesterification, [26] as well as reductive carbonyl coupling. [27] Therefore, we reasoned that in the presence of alcohols Mo@C might also be competent for catalyzing reductions.…”

Homogeneity of Surface Sites in Supported Single-Site Metal Catalysts: Assessment with Band Widths of Metal Carbonyl Infrared Spectra

“…10-12 ,. 4,5,6 Further, the ν CO values IR spectra similarly provideinge essential details of the local environments of metal sites on solid surfaces. 713 The typical mononuclear metal carbonyl complex on a metal oxide support is heterogeneous, characterized by a smear of species and broad ν CO bands, but Miessner et al 8 14 showed that mononuclear rhodium carbonyls bonded to dealuminated HY zeolite are characterized by narrow ν CO bands, indicating that the zeolite bonding sites are almost all the same.…”