Bimetallic Aluminum‐ and Niobium‐Doped MCM‐41 for Efficient Conversion of Biomass‐Derived 2‐Methyltetrahydrofuran to Pentadienes

Abstract The production of conjugated C4–C5 dienes from biomass can enable the sustainable synthesis of many important polymers and liquid fuels. Here, we report the first example of bimetallic (Nb, Al)‐atomically doped mesoporous silica, denoted as AlNb‐MCM‐41, which affords quantitative conversion of 2‐methyltetrahydrofuran (2‐MTHF) to pentadienes with a high selectivity of 91 %. The incorporation of AlIII and NbV sites into the framework of AlNb‐MCM‐41 has effectively tuned the nature and distribution of Lewis and Brønsted acid sites within the structure. Operando X‐ray absorption, diffuse reflectance infrared and solid‐state NMR spectroscopy collectively reveal the molecular mechanism of the conversion of adsorbed 2‐MTHF over AlNb‐MCM‐41. Specifically, the atomically‐dispersed NbV sites play an important role in binding 2‐MTHF to drive the conversion. Overall, this study highlights the potential of hetero‐atomic mesoporous solids for the manufacture of renewable materials.

Small-angle X-ray scattering patterns were collecting on a Rigaku FR-X dual wavelength rotating anode diffractometer with an AFC-11 4-circle goniometer, VariMAX TM microfocus optics and a Hypix 6000-HE area detector, using CuKα radiation (λ = 1.54184Å), beam divergence of 1.5mR and detector distance of 150mm. Data were collected at detector theta angles of 5°, 0°, -22.8° and -45.7° with 300° 300s phi scans at each detector position. Data were collected and integrated using the powder extraction tool in Crysalispro v171.41. Powder X-ray diffraction (PXRD) patterns were recorded on a Philips X'pert X-ray diffractometer The acidity concentration was measured by temperature-programmed desorption of ammonia (NH 3 -TPD) with a Quantachrome Autosorb-1 equipped with a thermal conductivity detector (TCD). Typically, 100 mg of sample was pre-treated in helium stream (30 mL min −1 ) at 600 °C for 2 h. The adsorption of NH 3 was carried out at 50 °C for 1 h. The catalyst was flushed with helium at 100 °C for 2 h to remove physisorbed NH 3 from the catalyst surface. The TPD profile was recorded at a heating rate of 10 °C min -1 from 100 to 600 °C. The Brønsted and Lewis acid sites of the samples were investigated by in situ DRIFTS of adsorbed acetonitrile-d 3 .
Wafers with a weight of 30 mg were active for 2 h under N 2 flow at 550 °C. Then acetonitrile-d 3 was admitted by bubbler system, and after equilibration, the samples were purged by N 2 flow for 0.5 h upon heating at 35, 200, and 250°C. The oxygen affinity of catalysts was verified by acetone adsorption-desorption DRIFTS experiment. Wafers with a weight of 30 mg were active for 2 h under N 2 flow at 550 °C. Then acetonitrile-d 3 was admitted by bubbler system, and after equilibration, the samples were purged by N 2 flow for 0.5 h upon heating at 35, 120, and 150 °C. X-ray photoelectron spectroscopy (XPS) was performed on an Axis Ultra Hybrid spectrometer (Kratos Analytical, Manchester, United Kingdom) using monochromated Al Kα radiation (1486.6 eV, 10 mA emission at 150 W, spot size 300 x 700 μm) with a base vacuum pressure of ~5 × 10 −9 mbar. Charge neutralisation was achieved using a filament. Binding energy scale calibration was performed using C-C in the C 1s photoelectron peak at 285 eV. Analysis and curve fitting was performed using Voigtapproximation peaks using CasaXPS.

Catalyst preparation.
AlNb-MCM-41, Al-MCM-41, Nb-MCM-41 and MCM-41 materials were synthesized using a modified method based upon a previous report 1 , and denoted as AlNb-MCM-41 (Si/Al/Nb mole ratio), Al-MCM-41 (Si/Al mole ratio), Nb-MCM-41 (Si/Nb mole ratio) and MCM-41(Si), respectively. In a typical synthesis, hexadecyltrimethyl-ammonium bromide (CTABr, 99+%, Sigma Aldrich) was used as the template reagent and firstly dissolved in a solution of deionized water, and tetrapropylammonium hydroxide (TPAOH, 1.0 M in H 2 O, Sigma Aldrich) which is an agent for directing Al 3+ ion in the tetrahedral coordination environment. Then the mixture was stirred for 2 h, which was followed closely by the addition of aluminium isopropoxide (99.99+%, Sigma Aldrich) with stirring for 2 h at room temperature. Niobium ethoxide (99.95%, Sigma Aldrich) was added and the mixture was stirred for another 2 h. Then tetraethyl orthosilicate (98%, Sigma Aldrich) was added dropwise and the mixture stirred for another 2 h, resulting in a gel with a chemical composition of 35Si: xAl: yNb: 4.2CTABr: 5.6TPAOH: 595H 2 O (x and y were determined by the target Si/Al and Si/Nb mole ratios, respectively). The gel was transferred into a 50-mL Teflon-lined stainless-steel autoclave, which was sealed and heated at 100 °C for 3 days. The solid products were centrifuged, washed with deionised water, dried overnight at 80°C, and finally calcined at 550°C under an air flow for 14 h. Al-MCM-41 samples were synthesised by the same procedure but without addition of niobium ethoxide. Nb-MCM-41 samples were synthesised by the same procedure but without addition of aluminium isopropoxide.
The selectivity is calculated by: Selectivity i (carbon basis)= Total carbon present in the product i /total carbon from the converted reactant.
Batch reactions were conducted in a Teflon-lined stainless-steel autoclave (10 mL) equipped with a temperature-controlled heating block and magnetic stirring. In a typical procedure, 100 mg of NbAl-MCM-41(35/1/0.9) and 2 mL of 2-MTHF were charged into the autoclave, which was then sealed and heated to a target temperature under stirring at 600 rpm for a given period of time. After the reaction, the system was For the EPR spectroscopy, the sample was placed in a 4.0 mm o.d. quartz tube and connected to a vacuum line.
Prior to irradiation, catalyst materials were activated by pumping at 10 -5 Torr for 12 h at 150 ˚C and then flame sealed under vacuum. The samples were exposed to γ-irradiation from a 60 Co source at 77 K to a total dose of 4.42 MRad at a dose rate of 0.34 MRad h -1 . CW EPR measurements were carried out at X-band (~9.4 GHz) using a commercial spectrometer Bruker EMX equipped with Oxford Instruments temperature control system at 40 K. EPR spectra were detected with modulation amplitudes of 0.1 and 1 mT, and microwave powers varied in the range ~0.7-70 mW. For the data presented here, 7 mW was chosen (if it was not discussed additionally in the text) to provide optimum signal intensity without saturation of spectral lines. Theoretical modelling of all EPR data was performed using EasySpin toolbox (Version 5.2.33) for Matlab 3 . Processing the time resolved EDE data was made using the DAWN software package 8 in order to crop, calibrate and normalise the XAS spectra. The processing of the Extended X-ray Absorption Fine Structure (EXAFS) data was performed using IFEFFIT with the Horae package (Athena and Artemis) 9 . Athena was used to calibrate, align and normalise the spectra with respect to the Nb foil. Figure S1. Picture of the set-up of I20-EDE beamline at Diamond Light Source.