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 (40 kV and 30 mA) using Cu Kα radiation (λ=1.5406Å).N 2 adsorption was carried out at 77 K on a Micromeritics 3Flex instrument after activating the samples for 12 h under dynamic vacuum at 453 K.The crystal morphology and size were measured by scanning electron microscopy (SEM) on a Quanta FEG 650 microscope.The ratios of Si/Al/Nb in the sample were quantified by EDX on multiple regions over a sample on a Bruker XTrace instrument.The morphologies of the samples were characterized by the transmission electron microscope (JEOL-1011).
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.NbOPO 4 was synthesized by a hydrothermal method according to a reported method 2 .Nb 2 O 5 was purchased from Sigma Aldrich.

Catalytic testing.
Catalytic reactions were carried out in a stainless steel continuous-flow reactor (12.7 mm i.d.).0.5 Gram of the catalyst was pressed, crushed, and sorted into grains in range of 40-60 meshes, which were then activated at 550 °C for 3 h under a N 2 flow before the reaction.2-MTHF was injected into the N 2 flow by syringe pump (Cole-Parmer) and passed through the reactor at the target temperature.The output liquid products were collected and analysed by GC (Agilent 7890B, equipped with an HP-5 column 30 m × 0.32 mm × 0.25 µm) and GC-MS (Agilent 6890A-Agilent 5973N, equipped with an HP-5MS column 30 m × 0.25 mm × 0.25 µm).
The output gas products were collected and analysed by GC (Agilent Micro GC 490 equipped with a PoraPLOT U column, length 10 m) and GC-MS (Agilent 7890A-Agilent 5975C, equipped with an HP-PLOT/Q column 30 m × 0.53 mm × 40 µm).The conversion of 2-MTHF is calculated by: 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 quenched by an ice bath and the liquid phase was collected for analysis.The liquid products were collected and analysed by GC (Agilent 7890B, equipped with an HP-5 column 30 m × 0.32 mm × 0.25 µm) and GC-MS (Agilent 6890A-Agilent 5973N, equipped with an HP-5MS column 30 m × 0.25 mm × 0.25 µm).
The yield is calculated by: Yield i = mole of the product i /mole of 2-MTHF(initial) × 100%.
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 .

Solid-state Nuclear Magnetic Resonance Spectroscopy.
NMR spectra were acquired at the UK High-Field Solid-State NMR Facility on a 20.0 T (850 MHz 1 H Larmor frequency) Bruker AVANCE NEO spectrometer equipped with a 1.3 mm HXY MAS probe in doubleresonance 1 H/ 27 Al mode, and a 4 mm HXY MAS probe in double resonance 1 H/ 93 Nb mode. 1 H and 27 Al NMR spectra were acquired at ambient temperature using a MAS frequency of 60 kHz, 1 H (/2)-pulses of 2.5 s, and 27 Al hard pulses of 1 s. 1 H NMR spectra were recorded after a Hahn-echo ( - -), with a total echo time (2) of two rotor periods.32 transients were co-added with inter-scan repetition delays of 1 s. 27Al NMR spectra were recorded with 8192 co-added transients with inter-scan repetition delays of 0.2 s. 93 Nb NMR spectra were acquired under static conditions using a solid echo ((/2) x - -( y -) with (/2)-pulses of 1.5 s and total echo time (2) of 40 s.32880 and 140512 transients were co-added with inter-scan repetition delays of 0.5 s, for the pristine and 2-MTHF-dosed AlNb-MCM-41(35/1/0.9),respectively.All samples were activated for 48 hours prior to packing into 4 mm o.d.zirconia NMR rotors.Following this, for the adsorbed samples, 2-MTHF was added and drying was repeated to remove any excess within the rotor in a vacuum oven at 60°C for at least an hour.Spectral simulations were performed in the solid lineshape analysis (SOLA) module v2.2.4 in Bruker TopSpin v4.0.9 and using Dmfit 4 for a Gaussian Isotropic Model to represent the distribution in isotropic chemical shift and quadrupolar parameters.The 1 H NMR chemical shifts were referenced to neat TMS externally, the 27 Al chemical shifts were referenced externally to 1.1 mol/kg Al(NO 3 ) 3 in D 2 O, and the 93 Nb chemical shifts were referenced externally to a saturated solution of NbCl 5 in acetonitrile.

In situ combined EDE/DRIFTS measurement for AlNb-MCM-41(35/1/0.9).
A combined energy dispersive EXAFS (EDE) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements have been carried out on the I20-EDE beamline at Diamond Light Source, Harwell Innovation Campus, Oxfordshire, UK. 5 The storage ring energy was 3 GeV and was operated at 300 mA in top-up mode (with an injection period of 20 seconds every 10 mins.I20-EDE is an energy-dispersive EXAFS beamline that was optically optimised at the Nb absorption edge (18986 eV), in transmission mode.The synchrotron radiation from an 8 pole wiggler insertion device (18.5 mm gap between magnets (1.2 T)) is conditioned with an initial aperture of size 0.77 mm x 0.84 mm (h x v) and beam defining primary slits of 0.5 mrad horizontally and 0.1 mrad vertically.In horizontal the beam is focused using a curved Si(311) polychromator (radius of curvature = 12.0 m) in Bragg geometry. 6This curvature leads to polychromatic beam with the energy range at the sample measured was 18807 to 19708 eV.The beam is vertically focused by a curved mirror ME1 at 36m from the beam source (radius of curvature = 8.6 km) with a Pt coating and 3.1 mrad pitch angle.The unwanted low energies were filtered out using a 2.8 mm thick pyrocarbon filter and two Be windows of 300 um thickness along the beamline.This gave a beam size of 90 x 200 um (FWHM h x v) at the focal spot on the sample position.To remove the background a reference sample of MCM-41Al without Nb was measured before each XAS measurement.The detector used was the FReLoN CCD (14 µm/pixel), 7 with a typical measurement of 7143 accumulations at 600 us each for a total of 10 seconds per spectrum.The detector's pixel to energy calibration was calculated by measuring a 25 μm thick Nb foil and comparing to the same foil measured on the monochromatic XAS beamline, B18, at Diamond Light Source.and rechecked daily during the experiment.The overall result is that the whole X-ray absorption spectrum to be collected simultaneously.Additionally, the size of the focussed beam at the sample position is very stable (+/-1 um horizontal and +/-10 um vertical beam position) as there is no requirement for the optical elements to be moved during the data collection.Simultaneous DRIFTS was facilitated through integration of a Bruker Invenio R spectrometer on the beamline, mounted on an in house designed trolley and stage to deliver the IR beam to the sample (FigureS1).The IR beam was focused from an external port to a Harrick Praying Mantis DRIFTS accessory mounted on an external optics board and aligned to an MCT detector.The set-up utilised a bespoke high temperature IR cell, fitted with 2 mm ZnSe IR windows, and one 0.5 mm glass carbon window and one 1 mm diamond window for transmission XAS.The solid sample was contained within a heated ceramic crucible, with glass flow configuration through the catalyst bed.
Measurements were employed as the following steps: 1).In situ activation of the sample at 550 °C in N 2 (30 mL min -1 ) flow for 120 min.(ramp rate 5 °C min -1 ).2) Cooled down the sample in N 2 (30 mL min -1 ) flow to room temperature.Then flowed 2-MTHF saturated in N 2 (10 mL min -1 ) into the cell through a bubbler system.3).Temperature programmed reaction of the sample in a constant flow of 2-MTHF saturated in N 2 (10 mL min -1 ) from 30-300°C (1 °C min -1 ), and then maintained for 120 min.Time-resolved Nb K-edge (18986 eV) XAS (6s per scan) measurements were collected in the range of 30-275 °C (ramp rate 1 °C min -1 ).4) Pure N 2 (30 mL min -1 ) was flowed into the cell for 60 min to remove all adsorbed species on Nb(V) sites.The complementary coupling of surface infrared (DRIFTS), average spectra collected every ~60 seconds, probed the chemical speciation of adsorbates, whilst MS enabled real-time activity and selectivity to be monitored.
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.Intensity (a.u.) Binding Energy (eV) The O 1s deconvoluted spectrum of MCM-41AlNb(35/1/0.9)

Figure S1 .
Figure S1.Picture of the set-up of I20-EDE beamline at Diamond Light Source.

Figure
FigureS12.Simulated IR spectrum of trans-1,3-pentadiene.The simulated IR spectrum of trans-1, 3pentadiene was obtained via DFT calculation using ORCA (version 4.2.1).Geometry optimization and consequent frequency calculation were carried out using the B3LYP functional and the def2-TZVP basis set for all atoms[13][14][15][16] .Imaginary frequency was checked and the peak frequencies corrected with a vibrational scaling factor of 0.965 for the used functional and basis set according to the Computational Chemistry Comparison and Benchmark DataBase (CCCBDB).

Figure S15 .
Figure S15.GC-MS analysis of the liquid products generated in the batch-mode reaction over NbAl-MCM-41(35/1/0.9) as a function of reaction time.(a) 0.5 h, (b) 1h, (c) 1.5 h, and (d) 2h.(e) Yields of different intermediates as a function of reaction time.Reaction condition: 2-MTHF, 2 ml; catalyst, 0.1 g; reaction temperature, 220 o C. Compared with the flow reaction at 275 o C, the lower reaction temperaure used in batch reactions is due the limit of the Telfon liner in the reactor.

Table S1 .
Specific surface areas of all zeolites used in this study (data determined from N 2 sorption isotherms at 77 K) Value of d 100 of calcined samples collected from small angle PXRD patterns.[b] S BET obtained based on Brunauer-Emmett-Teller (BET) equation.[c] Pore volume of the material obtained through Barrett-Joyner-Halenda (BJH) method according to the adsorption curve.d] D BJH represents adsorption average pore width calculated with Barrett-Joyner-Halenda (BJH) method.[e] Pore-wall thickness calculated form equation t BJH =2d 100 /1.732-D BJH .(Based on the relationship between unit cell a and d 100 for 2D hexagonal space group, d hkl =[(4/3)(h 2 +hk+k 2 )/a 2 +l 2 /c 2 ] -1/2 )

Table S3 .
Spin-Hamiltonian parameters of radiation induced Nb(IV) and electron hole Al-O֗ -Si defects, from simulation of CW X-band EPR spectra in FigureS7: lw is the homogeneous Lorentzian linewidth, g and A are the principle values of the g and hyperfine interaction tensors.

Table S4 .
Comparison of the catalytic performance between AlNb-MCM-41 and reported catalysts for the conversion of 2-MTHF to pentadienes.

Table S5 .
1 st Shell fitting of EXAFS data on the Nb sites for as-prepared and 2-MTHF-adsorbed AlNb-MCM-41.
a The amplitude factor S 0 2 was set to 1. b CN was set to 1 according to the structural mode