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Promoting active site renewal in heterogeneous olefin metathesis catalysts

Nature volume 617, pages 524–528 (2023)Cite this article

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Abstract

As an atom-efficient strategy for the large-scale interconversion of olefins, heterogeneously catalysed olefin metathesis sees commercial applications in the petrochemical, polymer and speciality chemical industries1. Notably, the thermoneutral and highly selective cross-metathesis of ethylene and 2-butenes1 offers an appealing route for the on-purpose production of propylene to address the C3 shortfall caused by using shale gas as a feedstock in steam crackers2,3. However, key mechanistic details have remained ambiguous for decades, hindering process development and adversely affecting economic viability4 relative to other propylene production technologies2,5. Here, from rigorous kinetic measurements and spectroscopic studies of propylene metathesis over model and industrial WOx/SiO2 catalysts, we identify a hitherto unknown dynamic site renewal and decay cycle, mediated by proton transfers involving proximal Brønsted acidic OH groups, which operates concurrently with the classical Chauvin cycle. We show how this cycle can be manipulated using small quantities of promoter olefins to drastically increase steady-state propylene metathesis rates by up to 30-fold at 250 °C with negligible promoter consumption. The increase in activity and considerable reduction of operating temperature requirements were also observed on MoOx/SiO2 catalysts, showing that this strategy is possibly applicable to other reactions and can address major roadblocks associated with industrial metathesis processes.

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Fig. 1: Promotion of heterogeneous olefin metathesis by manipulating site renewal and decay.
Fig. 2: Effect of co-fed promoters on propylene metathesis activity and kinetics.
Fig. 3: Infrared and solid-state NMR studies of olefin chemisorption on spent catalyst.
Fig. 4: Proposed mechanisms for site renewal, site decay and promotion.

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Data availability

Source data are provided with this paper. All other data supporting the findings of this study are available from the corresponding authors upon reasonable request. 

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Acknowledgements

The computations were performed on the Euler cluster at ETH Zurich. We acknowledge H. Adamji and J. Zhu for help in the synthesis and characterization of the WI MoOx/SiO2 catalyst, T. S. Wesley for a reading of the manuscript and R. R. Schrock for discussions. The work at MIT was financially supported by Saudi Aramco through MIT Energy Initiative (grant no. 6930839). We are grateful to the Swiss National Science Foundation (SNF) for financial support of this work (grant no. 200021L_157146). Z.J.B. also gratefully acknowledges financial support from the SNF (Spark award no. CRSK-2_190322).

Author information

Authors and Affiliations

  1. Department of Chemical Engineering, Massachusetts Institute of Technology (MIT), Cambridge, MA, USA

    Terry Z. H. Gani, Ran Zhu, Jong Hun Kang, John R. Di Iorio, Daniel F. Consoli & Yuriy Román-Leshkov

  2. Department of Chemistry and Applied Biosciences, ETH Zurich, Zurich, Switzerland

    Terry Z. H. Gani, Zachariah J. Berkson, Ka Wing Chan & Christophe Copéret

  3. Research & Development Center, Saudi Aramco, Dhahran, Saudi Arabia

    Sohel K. Shaikh

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  1. Terry Z. H. Gani
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Contributions

D.F.C., S.K.S., C.C. and Y.R.-L. conceptualized the project. Y.R.-L. and C.C. supervised the project. Z.J.B. and K.W.C. performed the synthesis and characterization of the SOMC catalysts. Z.J.B. conducted the solid-state NMR analyses. T.Z.H.G., R.Z., J.H.K. and J.R.D.I. performed all other experiments and data analyses. T.Z.H.G. designed and performed the computational studies. T.Z.H.G. wrote the manuscript with input from all other authors.

Corresponding authors

Correspondence to Christophe Copéret or Yuriy Román-Leshkov.

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Competing interests

D.F.C., S.K.S. and Y.R.-L. are inventors on a patent application that covers the use of olefin promoters for heterogeneous metathesis. The other authors declare no competing interests.

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Extended data figures and tables

Extended Data Fig. 1 Catalyst characterization.

a, Transmission FTIR spectra of 3%SOMC (red), grafted precursor complex (pink), and SiO2–700 (blue). b, Tauc plot of 3%SOMC. The edge energy of 4.25 eV demonstrates that the catalyst is dominated by isolated WOx sites33,59. c, Raman spectra of 3%SOMC and SiO2–700 under different environments. The in-situ dehydrated sample was heated to 450 °C (1 °C/min) for 2 h and cooled to room temperature under flowing 50 mL/min purified air. The Raman spectrum for in-situ dehydrated 3%SOMC (yellow) was dominated by strong fluorescence, and no clear Raman features could be identified. The absence of peaks at 715 and 805 cm−1 in the Raman spectrum for ambient 3%SOMC (red) indicates a lack of WO3 crystallites on the catalyst33, and the peak at 960 cm−1 is attributed to WOx under ambient conditions33. The peak at 1100 cm−1 in the SiO2–700 spectrum (blue) is due to Si-O vibration60. Vertical offsets were applied for clarity. d, Tauc plot of 3% WOx/SiO2 (IWI). The edge energy of 4.11 eV demonstrates that the catalyst is dominated by isolated WOx sites33,59. e, Raman spectra of 3% WOx/SiO2 IWI (3% W IWI) catalyst (red) and SiO2 support (blue) under ambient conditions. As before, the absence of peaks at 715 and 805 cm−1 in the Raman spectrum indicates a lack of WO3 crystallites on the catalyst33. The region from 900 to 1200 cm−1 is dominated by strong Raman features attributable to Si-OH (ca. 970 cm−1) and Si-O vibrations (ca. 1100 cm−1)60. The subtle differences between the Raman spectra of the catalyst and support (close-up in inset) could be due to the isolated surface WOx species on the 3% WOx/SiO2 IWI catalyst33. Vertical offsets were applied for clarity.

Source data

Extended Data Fig. 2 Kinetic studies of catalysts prepared by wet impregnation.

a, Reaction order plot at T = 350 °C with y(C3H6) = 0.3 to 0.6. b, Arrhenius plot at y(C3H6) = 0.5 with T = 320 °C to 350 °C. c, Arrhenius plots of propylene metathesis without promoter (blue) and with promoter (1.28 mol% 4ME + 0.22 mol% i4ME, yellow) at y(C3H6) = 0.5 with T = 230 °C to 260 °C. d, Apparent reaction order plots of propylene metathesis without promoter (blue) and with promoter (1.28 mol% 4ME + 0.22 mol% i4ME, yellow) at T = 250 °C with y(C3H6) = 0.2 to 0.5. e, Apparent reaction order plot of promoter at T = 250 °C, y(C3H6) = 0.5 with y(promoter) = 0.7 to 1.5 mol%. Reaction conditions: 10 mg 15% WOx/SiO2 (industrial) or 20 mg 3% WOx/SiO2 IWI, pretreated at 550 °C under 100 mL/min He for 1 h, 50 mL/min total gas flow rate (propylene + balance He or He saturated with promoter, WHSV = 0.0019 mol C3H6/gcat.s.

Source data

Extended Data Fig. 3 Transient kinetic studies.

a, induction period associated with 3%SOMC. Reaction conditions: 10 mg catalyst, 25 mL/min C3H6 + 25 mL/min He, WHSV = 0.0019 mol C3H6/gcat.s. The temperature was initially set to 330 °C and increased to 350 °C after ca. 15 h on stream. b, response of steady-state reaction to inert gas purge. TOS = 0 is defined as the time at which steady-state was originally achieved at 350 °C. Reaction conditions: 10 mg 3%SOMC, pretreated at 550 °C under 100 mL/min He for 1 h, T = 350 °C, 50 mL/min C3H6 + 50 mL/min He, WHSV = 0.0037 mol C3H6/gcat.s. c, response of steady-state reaction to step change in temperature. Reaction conditions: 10 mg catalyst, pretreated at 550 °C under 100 mL/min He for 1 h, 25 mL/min C3H6 + 25 mL/min, WHSV = 0.0019 mol C4H8/gcat.s, The temperature was reduced from T = 330 °C to T = 300 °C between the 7th and 8th data points (the new temperature equilibrium was achieved in ca. 10 min and the 8th data point was taken after the temperature stabilized at 300 °C). TOS = 0 is defined as the point at which steady-state at 330 °C was achieved.

Source data

Extended Data Fig. 4 Simplified reaction network for Chauvin metathesis including unpromoted and promoted versions of the proposed site renewal/decay cycle.

The derivation of a simplified steady-state kinetic model connecting our experimental observations to our mechanistic hypotheses is presented in Methods.

Extended Data Fig. 5 Unusual kinetic behavior exhibited by 3%SOMC.

a, Reaction orders for ethylene/2-butene cross-metathesis. Red: ethylene reaction order plot with y(C2H4) = 0.2 to 0.5 and y(C4H8) = 0.5. Blue: 2-butene reaction order plot with y(C4H8) = 0.2 to 0.5 and y(C2H4) = 0.5. Reaction conditions: 10 mg catalyst, pretreated at 550 °C under 100 mL/min He for 1 h, 100 mL/min total gas flow rate (balance He), WHSV = 0.0038 mol C4H8/gcat.s, T = 330 °C. b, Effect of adding 2-butene to a propylene self-metathesis reaction at steady-state. The increase in ethylene yield is modest (ca. 1.5x with 20% 2-butene co-feed) but significant, stable and fully reversible, consistent with observations for better promoters such as i4ME as discussed in the text. There are two possible alternative sources of additional ethylene production upon 2-butene addition that can both be eliminated as they are too small to account for the magnitude of the increase in ethylene yield. Firstly, traces of ethylene impurities (ca. 30 ppm determined by GC-FID on a bypass run) present in the 2-butene source are 2-3 orders of magnitude too small. Secondly, 1-butene, whether as a feed impurity or generated in situ by isomerization, can undergo cross-metathesis with propylene to yield a 1:1 ratio of pentenes and ethylene. However, the mole fraction of pentenes in the product stream is 0.0002, an order of magnitude too small. Reaction conditions: 10 mg catalyst, pretreated at 550 °C under 100 mL/min He for 1 h, 25 mL/min C3H6 + 25 mL/min He or (15 mL/min He + 10 mL/min C4H8), WHSV = 0.0019 mol C4H8/gcat.s, T = 330 °C.

Source data

Extended Data Fig. 6 Mechanistic studies and control experiments.

a, Transient behavior of purged catalyst surface pre-activated with 4ME. The initial spike in activity suggests a promotional effect of residual, adsorbed 4ME in the absence of gas-phase 4ME. A steady-state catalyst surface was first purged under helium at 350 °C for at least 3 h to destroy all active sites, then contacted with 1.5% 4ME (bal. He) for 1 h before resuming propylene flow. Reaction conditions: 10 mg catalyst, pretreated at 550 °C under 100 mL/min He for 1 h, T = 350 °C, 50 mL/min C3H6 + 50 mL/min He/4ME (corresponding to y4ME = 0.015), WHSV = 0.0037 mol C3H6/gcat.s. b, Reaction order of promoter in propylene self-metathesis. The reaction conditions are the same as in Figs. 1b and c. The promoter concentration was varied by adjusting the flow rate of promoter-saturated He (10 to 25 mL/min) and co-feeding additional He to keep the total flow rate constant. c, Solid-state 1D 15N{1H} CP-MAS NMR spectra of (A) 3%SOMC, (B) 1.5%MoSOMC, and (C) a 3 wt% WO3/SiO2 catalyst prepared by incipient wetness impregnation (IWI) after exposure to 15N-labelled pyridine and subsequent evacuation (see Methods for details). All materials show 15N NMR signals at 290 ppm and 205 ppm, which are assigned on the basis of previous computational work61 to H-bonded 15N-pyridine and 15N-pyridinium, respectively, clearly establishing the presence of strong Brønsted acid sites of sufficient acidity to protonate pyridine. Additionally, both tungsten catalysts exhibit a weaker secondary signal at 261 ppm likely arising from 15N-pyridine coordinated to irreducible mono-oxo W sites12,22. The weaker intensity of the 205 ppm peak for the IWI material (C) as compared to that of the SOMC material (A) points to a lower concentration of Brønsted acid sites that is consistent with the lower observed promotional factor. The spectra were acquired at 14.1 T, 100 K, 10 kHz MAS, with 15N-1H contact times of 3 ms. d, Thermal stability of surface intermediates from the low temperature chemisorption of propylene on 3%SOMC. The temperature was ramped from 50 °C to 200 °C (5 °C/min) under flowing 50 mL/min He and held at each labeled temperature until steady state was attained. Most of the surface intermediates were desorbed below 200 °C. Vertical offsets were applied for clarity. e–f, Propylene chemisorption on pure silica (SiO2–700, e) and pristine 3%SOMC (f). We performed two control experiments to support our claim that propylene chemisorption occurs on silanols proximal to tungsten sites. Firstly, SiO2–700 was exposed to 50 mL/min of 50% propylene in N2 for 4 h at 50 °C, and no visible propylene absorbance was observed after 4 h of N2 purging (e). This result confirms that free surface silanols are not sufficiently acidic for reaction with propylene in the absence of tungsten sites. Next, the same chemisorption experiment was performed on pristine 3%SOMC, omitting the high-temperature pretreatment step. As characterized in prior work12, the isolated, coordinatively saturated tungsten sites on pristine 3%SOMC should not undergo inner-sphere reactions with propylene at 50 °C. Observation of a spectrum virtually identical to that from propylene chemisorption on spent catalyst (f) thus confirms that propylene chemisorption does not directly involve the tungsten sites. g–h, DRIFTS spectra of surface intermediates arising from high temperature propylene exposure. Spent 3%SOMC was heated to 350 °C and exposed to 50% propylene for 4 h, followed by a gas switch and rapid cooling to 50 °C under the coolant gas (g). Cooling under propylene (h, blue) resulted in the most intense absorptions derived in large proportion from low temperature chemisorption during the cooling step. Ethylene (h, red) does not chemisorb but appears to preserve surface intermediates, resulting in greatly attenuated peaks. The similarity of the ethylene-cooled spectrum (red) to the propylene-cooled (blue) and propylene-chemisorbed (Fig. 3a) spectra suggests that the chemisorbed species remain the dominant surface intermediates during 350 °C reaction. N2 (h, black) completely destroyed all surface intermediates during the cooling step. The spectra were obtained using the clean activated 3%SOMC background. Vertical offsets were applied for clarity.

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Extended Data Fig. 7 Additional DRIFTS and solid-state NMR analysis.

a, Low temperature chemisorption of various olefins on spent 3%SOMC: (i) ethylene, (ii) propylene (presented in the main text but reproduced here for ease of comparison), (iii) 2-butene, (iv) propylene + 4ME, (v) propylene + i4ME. The spectrum of an activated blank catalyst (black) was obtained with respect to the KBr background. Vertical offsets were applied for clarity. Isobutene was omitted from the series because it oligomerized into a non-volatile oily phase. The fact that the peaks in (iv) and (v) (4ME and i4ME) differ only in intensity but not position is consistent with a common chemisorbed species but different thermodynamics of protonation. b, Solid-state 2D 1H{13C} D-HMQC NMR correlation spectra of 3%SOMC reacted with propylene without i4ME. The spectra were acquired at 9.4 T, 298 K, 40 kHz MAS, and with dipolar recoupling periods of 18 rotor periods (0.45 ms, red) or 60 rotor periods (1.5 ms, black). The 1D 13C projections of the two 2D spectra are shown along the ordinate, and a 1D 1H echo spectrum acquired under the same conditions is shown for comparison. The short (red) and long (black) recoupling times appear to select for -CH3 and -CH2- signals respectively, and the overall signals are consistent with predominantly alkyl species. Notably, the alkoxide signals of Fig. 3b are absent, likely below the detection limit of solid-state NMR. c, Comparison of the solid-state 1D 1H MAS NMR spectra of 3%SOMC after reaction with (top) propylene without i4ME or (bottom) propylene with i4ME. The signals are generally similar but with very different relative intensities indicating different distributions of the corresponding surface species. Specifically, reaction with propylene alone appears to result in larger relative quantities of surface-bound aromatic and/or olefinic species (1H signals from 4.5–8 ppm), while reaction with i4ME appears to favor greater relative proportions of surface alkoxide and alkyl moieties. The spectra were acquired at 9.4 T, 298 K, 40 kHz MAS, and with rotor synchronized echo delays rotor periods of 2 rotor periods (0.05 ms).

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Extended Data Fig. 8 Putative alkylidene formation pathways and preliminary computational studies.

a, Proton-assisted and non-proton-assisted pathways investigated. Each structure is labeled with its 350 °C enthalpy in kcal/mol relative to infinitely separated 1 and propylene. The ball-and-stick model is an illustration of the minimal cluster model used in the computations. Full computational details are provided in Methods. b, Comparison of the silanol-assisted and non-silanol-assisted pathways suggests that the presence of proximal silanol groups may help to facilitate restoration of the catalytically active sites.

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Extended Data Fig. 9 Promotion of industrial catalyst.

a, The extent of promotion of the industrial 15% WOx/SiO2 catalyst is lower than for 3%SOMC (ca. 30 at 1.5% 4ME). b, The promotion is stable over at least several hundred turnovers (calculated based on the nominal tungsten loading, which likely overestimates the actual active site count by 1-2 orders of magnitude). Reaction conditions: 50 mg catalyst, pretreated at 550 °C under 100 mL/min He for 1 h, T = 250 °C, 25 mL/min C3H6 + 25 mL/min He/4ME, WHSV = 0.0019 mol C3H6/gcat.s. Prior to promoter introduction, the catalyst was allowed to reach steady-state under 50 mL/min of 50% C3H6 (bal. He) at 350 °C (not shown), then cooled down to 250 °C and allowed to reach steady-state again. The zero value of time on stream (TOS) is defined to be the time at which steady-state at 250 °C was attained. c, i4ME promotion of industrial 15% WOx/SiO2 catalyst for cross-metathesis showing a lower promotion factor for cross-metathesis (ca. 5 at 5% i4ME) than for self-metathesis (ca. 20 at 5% i4ME) due to the intrinsic promotional ability of 2-butene as one of the reactants. d, Cross-metathesis promotion is also stable over at least several hundred turnovers. As in b, the turnover number is calculated based on the nominal tungsten loading. Reaction conditions: 50 mg catalyst, pretreated at 650 °C under 100 mL/min He for 1 h, T = 250 °C, 40 mL/min C2H4/i4ME + 10 mL/min C4H8. The same catalyst bed was used across panels a–d.

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Extended Data Fig. 10 Promotion of molybdenum-based catalysts.

a, The promotion factor for propylene self-metathesis (ca. 26 at 1.5% i4ME) at 200 °C over 1.5%MoSOMC catalyst. b, The promotion factor for propylene self-metathesis (ca. 68 at 1.5% i4ME) at 200 °C over 1.4 wt% MoO3/SiO2 WI catalyst. Reaction conditions: 20 mg catalyst, calcined at 400 °C under 40 mL/min air for 3 h and pretreated at 500 °C under 100 mL/min He for 3 h, T = 200 °C, 50% propylene with indicated percent of i4ME in helium balance, 50 mL/min total flow rate.

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Extended Data Table 1 Gas-chromatography characterization of reaction products
Full size table
Extended Data Table 2 Physical parameters used to evaluate heat and mass transfer limitations
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Gani, T.Z.H., Berkson, Z.J., Zhu, R. et al. Promoting active site renewal in heterogeneous olefin metathesis catalysts. Nature 617, 524–528 (2023). https://doi.org/10.1038/s41586-023-05897-w

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