In-situ formation of [AlO_4]^0 site for confined catalytic cracking to ethylene with low methane selectivity

Domain: physics
Published: 2025-12-11
Editorial outcome: major_revision

Abstract
Ethylene, one of the major basic chemicals in the modern chemical industry, has been industrially produced mainly through intensive energy-consuming steam cracking processes from light oil fractions or shale gas operated above 800 °C following the radical mechanism and seriously suffering from high methane selectivity. Herein, we report an efficient catalytic cracking reaction of 1-pentene over a tailored ZSM-5 catalyst at 700 °C with a C2H4/C3H6 ratio of 1.31 and a CH4 selectivity below 5%, which provides an energy-saving and economically viable route for ethylene production from light oil fractions, seamlessly integrating with the existing industrial catalytic cracking processes. Mechanistic studies unveil an interesting type of zeolite catalysis different from the typical acid catalysis following the carbenium ion mechanism, termed as the confined catalytic radical (CCR) mechanism in which 1-pentene dehydrogenates to gaseous pentenyl radicals (C5H9•) at non-acidic [AlO4]0 sites of ZSM-5 in situ formed by dehydrogenation of bridging hydroxyls, whose subsequent reactions confined in ZSM-5 produce more C2H4 than C3H6. These findings significantly broaden the concept and application of zeolite catalysis.

Introduction
Ethylene (C2H4), one of the major basic chemicals in the modern chemical industry, has been industrially produced mainly through steam cracking processes from light oil fractions or shale gas operated above 800 °C for more than one hundred years. In addition to the very intensive energy consumption, steam cracking processes following the radical mechanism seriously suffer from high methane (CH4) selectivity (10~30%) due to the difficulty in directional regulation of free radical generations and transformation pathways, leading to low utilization efficiencies of C and H atoms. It is thus of significant importance to develop energy-saving processes to produce ethylene with low CH4 selectivity capable of competing with steam cracking processes in terms of scale and cost. Catalytic cracking of olefins using zeolite catalysts with Brønsted acid sites (BAS) at lower temperatures presents significant advantages in energy consumption and reaction controllability over steam cracking processes, but they follow the carbenium ion mechanism of acid catalysis to mainly produce propylene (C3H6) and butylene (C4H8) with limited C2H4 productions. A solution to enhance the C2H4 production by catalytic cracking reactions is to strengthen the monomolecular-cracking pathway of carbenium ions by increasing the reaction temperature, but such an approach yields C2H4/C3H6 ratios theoretically no higher than 1.0 and enhanced CH4 selectivity due to the structure of pentene molecular and the inherent bond-breaking mechanisms. It was also reported that propylene and butene underwent the BAS-catalyzed dimerization cracking reactions to produce ethylene under olefin cracking reaction conditions. By adopting a high reaction temperature of 550 °C to enhance monomolecular catalytic cracking pathway and a very low weight hourly space velocity (WHSV) of 7.68 h−1 and consequently extended residence time to facilitate secondary dimerization cracking reactions of propylene and butene, primary products of 1-pentene cracking reaction, to ethylene, Lin et al. achieved a C2H4/C3H6 ratios of 1.13 by BAS-catalyzed cracking of 1-pentene. However, it is worth noting that very low WHSV and correspondingly low hydrocarbon partial pressures are practically infeasible for industrial implementation. Herein, we report an efficient catalytic cracking reaction of 1-pentene (C5H10) over a tailored ZSM-5 catalyst at 700 °C with a WHSV of 75 h−1, achieving a C2H4/C3H6 ratio of 1.31 and a CH4 selectivity below 5%. Mechanistic studies unveil a novel type of zeolite catalysis following a confined catalytic radical (CCR) mechanism, in which 1-pentene dehydrogenates to gaseous pentenyl radicals (C5H9•) at non-acidic [AlO4]0 sites of ZSM-5 in situ formed by dehydrogenation of bridging hydroxyls (Si-(OH)-Al), whose subsequent reactions confined in ZSM-5 produce more C2H4 than C3H6.

Results
Catalytic performance
Figure 1a compares catalytic performance of ZSM-5 zeolite with a Si/Al ratio of 13.8 (denoted as Z-1), Z-1 catalyst aged in steam at 800 °C (denoted as Z-2) and ZSM-5 zeolite with a Si/Al ratio of 107.6 (denoted as Z-3) in 1-pentene cracking at 700 °C with a WHSV of 75 h−1. The results of a blank test using the same amount of quartz sand (SiO2) and of the steam cracking of naphtha at 860 °C are also included. The blank experiment gave a pentene conversion of 22% and CH4, C2H4, C3H6 and C4H8 selectivity of 15.3%, 22.5%, 18.7% and 8.9%, respectively. Similar results were also observed when using BAS-free silicate-1 zeolite as the catalyst. With Z-1, the pentene conversion increased largely to 97.0% and the CH4, C2H4, C3H6 and C4H8 selectivity were 4.8%, 38.9%, 30.0% and 9.1%, respectively. The pentene conversion remained unchanged while the CH4, C2H4, C3H6 and C4H8 selectivity were 3.6%, 38.4%, 34.8% and 11.1%, respectively, when Z-2 zeolite was used as the catalyst. Using Z-3 as the catalyst, the pentene conversion was 56.6% and the CH4, C2H4, C3H6 and C4H8 selectivity were 5.8%, 34.7%, 37.4% and 10.3%, respectively.

Figure 1b, c further compares the C2H4/C3H6 ratio and C2H4/CH4 ratio of various 1-pentene cracking reactions at 700 °C, respectively. The blank reaction using SiO2 as the catalyst exhibited a C2H4/C3H6 ratio of 1.20, a high CH4 selectivity of 15.3% and a C2H4/CH4 ratio of 1.47, which, similar to the steam cracking of naphtha at 860 °C, are characteristic of cracking reactions following the thermal radical mechanism. The reaction using Z-3 catalyst presented a characteristic of cracking reactions following the acid-catalyzed carbenium ion mechanism, including a C2H4/C3H6 ratio of 0.93, a low CH4 selectivity of 5.8% and a C2H4/CH4 ratio of 5.97. Its relatively high C2H4/C3H6 ratio arises from the confinement of ZSM-5 catalyst and adopted high reaction temperature that favors the monomolecular-cracking pathway of carbenium ions, beneficial for C2H4 production. Unexpectedly but excitingly, the reactions catalyzed by Z-1 and Z-2 exhibited C2H4/C3H6 ratios respectively of 1.30 and 1.11, low CH4 selectivity respectively of 4.8% and 3.6% and C2H4/CH4 ratios respectively of 8.21 and 10.72. The effect of WHSV on the 1-pentene cracking reaction over Z-1 zeolite was further studied at 550 and 700 °C. At 550 °C where the 1-pentene cracking reaction followed the BAS-catalyzed mechanism, lowering the WHSV from 75 h−1 to 7.68 h−1 increased the C2H4/C3H6 ratio from 0.83 to 1.15, closely matching the reported value, which could be attributed to a low WHSV and consequently extended residence time facilitated secondary dimerization cracking reactions of propylene and butene, primary products of 1-pentene cracking reaction, to ethylene. However, at 700 °C, lowing the WHSV from 75 h−1 to 7.68 h−1 only resulted in a marginal increase in the C2H4/C3H6 ratio, suggesting that the observed C2H4/C3H6 ratios well above 1.0 at 650 and 700 °C could not be attributed to the BAS-catalyzed secondary dimerization cracking reactions of primary products (propylene and butene). Moreover, the results of 1-pentene cracking reaction catalyzed by Z-1 and Z-2 catalysts at 450–525 °C strictly following the BAS-catalyzed carbenium ion mechanism showed a gradual increase of the C2H4/C3H6 ratio with reaction temperature but theoretically well below 1.0 at 700 °C, consistent with the previous results. Therefore, the observed characteristic of 1-pentene cracking reaction over Z-1 and Z-2 zeolites at 650 and 700 °C is neither that of thermal cracking reactions nor that of acid-catalyzed cracking reactions, but combines the advantages of thermal cracking reactions and acid-catalyzed cracking reactions regarding the C2H4 production by olefin cracking reactions.

1-Pentene cracking reaction over Z-1, Z-2 and Z-3 catalysts at 550 °C all exhibited the characteristic of acid-catalyzed cracking reactions, including C2H4/C3H6 ratios below 1.0 and low CH4 selectivity. With the reaction temperature elevating up to 700 °C, such a characteristic remained for the Z-3 catalyzed pentene cracking reaction, but not for the Z-1 or Z-2 catalyzed pentene cracking reaction, in which the C2H4/C3H6 ratio was 1.14 and 1.30 at 650 and 700 °C for Z-1 catalyzed pentene cracking reaction, respectively, and 1.11 at 700 °C for the Z-2 catalyzed pentene cracking reaction. These reaction data strongly suggest that 1-pentene catalyzed by Z-1 or Z-2 catalysts at 700 °C should involve a novel reaction mechanism (confined catalytic radical mechanism) with a characteristic of C2H4/C3H6 ratios above 1.0 and low CH4 selectivity (<5%), different from the well-established thermal radical mechanism and acid-catalyzed carbenium ion mechanism. The 1-pentene catalytic cracking reaction to ethylene over Z-1 or Z-2 catalyst at 700 °C reported herein outperformed largely the traditional naphtha thermal cracking to ethylene at 860 °C. The coke content was quantified below 1.0 wt% for all catalysts under the adopted experimental conditions. The regeneration test of Z-1 catalyst in the 1-pentene cracking reaction at 700 °C demonstrated its good stability.

Reaction mechanism
Synchrotron radiation vacuum ultraviolet photoionization mass spectroscopy (SVUV-PIMS), a technique capable of sensitively characterizing gaseous radical intermediates in combustion reactions and heterogeneous catalytic reactions, was used to probe gaseous components of the 1-pentene cracking reaction catalyzed by SiO2 and Z-1 with photons of different energies. At a photon energy of 9.2 eV, in addition to the stable molecules, gaseous ethyl (C2H5•, m/z = 29) and allyl (C3H5•, m/z = 41) radicals were observed in the reaction with SiO2 at 750 °C resulting from the C–C bond breaking of 1-pentene molecule which is the weakest bond in pentene, consistent with the thermal radical mechanism in which the weakest chemical bond of a reactant is preferentially broken, whereas gaseous radicals were not observed for the reaction catalyzed by Z-1 at 550 °C, agreeing with the acid-catalyzed carbenium ion mechanism. However, a weak but clear m/z = 69 signal and tiny gaseous C2H5• and C3H5• radical signals appeared for the reaction catalyzed by Z-1 at 700 °C. Photoionization Efficiency (PIE) profiles of m/z = 69 and m/z = 70 (C5H10) signals within the energy range between 8.3 and 11.5 eV showed that the profile of m/z = 69 signal differed significantly from that of m/z = 70 (C5H10) signal at photon energies below 11.0 eV while the m/z = 69 signal grew greatly at a photon energy of 11.5 eV, probably arising from the fragmentation process of C5H10 previously reported to occur above at photon energies above 11.1 eV. Thus, the m/z = 69 signal did not likely result from the fragmentation process of C5H10 in the SVUV-PIMS spectra acquired with photon energies below 11.0 eV, evidencing that the m/z = 69 signal should come from the gaseous pentenyl radicals (C5H9•). As shown in Fig. 2b, the C2H5•, C3H5•, and C5H9• radicals emerged at 650 °C in the 1-pentene cracking reaction over Z-1, and the C2H5• and C3H5• radicals kept increasing with the reaction temperature, whereas the C5H9• radicals increased up to 700 °C and then weakened at 725 and 750 °C. An near-linear correlation existed between the C2H5• and C3H5• intensities, but not between the C5H9• intensity and the C2H5• or C3H5• intensity. These observations demonstrate that the C2H5• and C3H5• radicals are generated simultaneously by the thermal cracking of 1-pentene, but the C5H9• radicals are not, directly proving the presence of a novel reaction mechanism with C5H9• radicals as the key intermediate in Z-1 catalyzed 1-pentene cracking reaction at around 700 °C.

The gaseous C2H4 intensity correlated linearly with the C5H9• intensity but did not show any obvious correlations to the C2H5• or C3H5• intensity, suggesting that C2H4 is mainly produced via the C5H9• intermediate. Meanwhile, the gaseous C3H6 intensity did not correlate to either of the C2H5•, C3H5• and C5H9• intensity, consistent with that C3H6 in Z-1 catalyzed 1-pentene cracking reaction should be largely contributed by the acid catalysis with the carbenium ion intermediates. Normalized to the C2H4/C3H6 ratio at 550 °C strictly following the acid-catalyzed carbenium ion mechanism, the C2H4/C3H6 ratio was 1.23, 1.26, 1.39, 1.40 and 1.41 at 650, 675, 700, 725 and 750 °C, respectively. Its increase between 550 and 700 °C is in accordance with the growth of C5H9• signal, indicating that the confined catalytic radical mechanism with the C5H9• key intermediate of 1-pentene cracking is beneficial for the C2H4 production. The C2H4/C3H6 ratio remained nearly unchanged with the temperature increasing from 700 to 750 °C despite the large decrease of the C5H9• signal, which could be attributed to the enhanced probability of the 1-pentene thermal cracking pathway, favoring the C2H4 production. The C2H4/CH4 ratio at 650, 675, 700, 725 and 750 °C, normalized to the C2H4/CH4 ratio at 550 °C, was 0.36, 0.35, 0.26, 0.20 and 0.15, respectively. The 1-pentene cracking reaction over SiO2 at 750 °C strictly following the thermal radical mechanism gave normalized C2H4/C3H6 and C2H4/CH4 ratios of 2.62 and 0.14, respectively. It is worth noting that the gaseous C5H9• radicals formed within the pores of zeolites are not likely to escape into the bulk gas phase; thus, the detected gaseous C5H9• radicals in the SVUV-PIMS spectra are probably formed at the outer surface and pore windows of zeolites.

Our SVUV-PIMS results clearly demonstrate that the acid-catalyzed carbenium ion mechanism, thermal radical mechanism, and confined catalytic radical mechanism of 1-pentene cracking reaction exhibit characteristics of low C2H4/C3H6 ratio and high C2H4/CH4 ratio, high C2H4/C3H6 ratio and low C2H4/CH4 ratio, and high C2H4/C3H6 ratio and high C2H4/CH4 ratio, respectively. The acid-catalyzed carbenium ion mechanism dominates in the 1-pentene cracking reaction catalyzed by Z-1 catalyst at 550 °C, featuring a low C2H4/C3H6 ratio of 0.82 and a low CH4 selectivity of 1.54%. The contribution of confined catalytic radical mechanism initially increases with the temperature and reaches the maximum at 700 °C, leading to a high C2H4/C3H6 ratio of 1.30 and a low CH4 selectivity of 4.8%. With the further increase of the temperature, the contribution of confined catalytic radical mechanism decreases while the contribution of thermal radical mechanism keeps growing and eventually dominates, leading to a high C2H4/C3H6 ratio and high CH4 selectivity.

The confined catalytic radical mechanism contributed more to the 1-pentene cracking reaction at 700 °C catalyzed by Z-1 than by Z-2 and barely contributed in the reaction catalyzed by Z-3. The acidity of Z-1, Z-2 and Z-3 was characterized by NH3 differential adsorption heats measured by flow-pulse adsorption microcalorimetry. As shown in Fig. 3a, the Z-1 zeolite with a Si/Al ratio of 13.8 exhibited three types of BAS with NH3 differential adsorption heats of 128, 119 and 102 kJ/mol and corresponding site densities of 133.1, 135.3 and 39.7 μmol/g, respectively. After high-temperature steam aging at 800 °C, the density of BAS with NH3 differential adsorption heats of 128 and 119 kJ/mol decreased to 18.5 μmol/g, while the density of BAS with NH3 differential adsorption heat of 102 kJ/mol remained almost unchanged at 35.6 μmol/g. The Z-3 zeolite with a high Si/Al ratio of 108 only showed two types of BAS with NH3 differential adsorption heat of 115 and 102 kJ/mol and corresponding densities of 17.9 and 23.8 μmol/g, respectively. NH3 adsorption heat on BAS within zeolite micropores was shown to be significantly influenced by the pore confinement effect, where smaller pore sizes typically result in higher adsorption heat due to the multiple interactions between protonated ammonia and the surrounding framework oxygen atoms. The MFI zeolite topology comprises two types of 10-membered ring channels: sinusoidal channels and straight channels, both with pore sizes of approximately 0.5 nm, as well as intersection channels with a pore size of around 0.7 nm. The strength of the confinement effect in these channels was reported to follow an order of sinusoidal channels > straight channels > intersection channels. NH3 adsorption heat in the intersection channels was demonstrated to be approximately 20 kJ/mol lower than in the straight channels. Thus, we assigned the BAS with NH3 differential adsorption heats of 128, 119 and 102 kJ/mol to the BAS in the sinusoidal channels, straight channels and intersection channels of ZSM-5 catalysts. The confined catalytic radical reaction pathway of 1-pentene cracking reaction at 700 °C strengthens with the BAS acidity of the used ZSM-5 catalyst, indicating that the BAS not only exerts the well-known acid catalysis for 1-pentene cracking reaction at 550 °C but also is responsible for the confined catalytic radical reaction pathway of 1-pentene cracking reaction at 700 °C.

BAS in zeolites is known to convert to Lewis acid sites at high temperatures by dehydration reaction, meanwhile, they were also found to generate non-acidic [AlO4]0 sites by decomposition reaction at higher temperatures. As shown in the H2 temperature-programmed desorption (H2-TPD) profiles of dehydrated Z-1, Z-2 and Z-3 catalysts (Fig. 3b), an obvious H2 peak initiating at approximately 550 °C was observed for Z-1, weakened for Z-2 and could barely be observed for Z-3. In the corresponding in situ diffuse reflectance infrared Fourier transform (DRIFT) spectra and ex situ 1H MAS NMR spectra of dehydrated catalysts, the features of BAS in Z-1 and Z-2 catalysts decreased. Meanwhile, an electron paramagnetic resonance (EPR) signal (g ≈ 2.0032) assigned to the lattice oxygen ion of zeolites with an electron hole emerged upon high-temperature treatments of Z-1 and Z-2 zeolites but barely of Z-3 zeolite and could be associated with the formation of non-acidic [AlO4]0 sites. This EPR signal grew with the treatment temperature increasing and was much stronger for Z-1 zeolite than for Z-2 zeolite, consistent with the H2-TPD results. All these observations demonstrate the occurrence of the dehydrogenation reaction of BAS to generate [AlO4]0 sites which is more in Z-1 than in Z-2 but seldom in Z-3, consistent with the contribution of confined catalytic radical reaction pathway to 1-pentene cracking reaction at 700 °C. The BAS of dehydrogenated Z-1 and Z-2 catalysts were partially recovered after a subsequent treatment in 1-pentene at 700 °C. This demonstrates the activation of 1-pentene at the [AlO4]0 site, forming a BAS and, deduced by the charge balance, a gaseous C5H9• radical, while the resulting BAS undergoes a dehydrogenation reaction to produce H2 and re-generates the [AlO4]0 active site. Such a surface reaction cycle features a novel type of zeolite catalysis in the 1-pentene cracking reaction in addition to the acid catalysis, following the confined catalytic radical mechanism in which the 1-pentene activates at the [AlO4]0 active site in situ generated by the dehydrogenation reaction of BAS to the gaseous C5H9• radical. The confined catalytic radical mechanism is responsible for the characteristic of 1-pentene catalyzed by Z-1 or Z-2 catalyst at 700 °C, with a characteristic of C2H4/C3H6 ratios above 1.0 and low CH4 selectivity (<5%). The H2-TPD profile of the used Z-1 catalyst demonstrated the stability of BAS sites that underwent the dehydrogenation reaction to form the [AlO4]0 sites.

Theoretical simulations
Density functional theory (DFT) calculations were carried out to explore the reaction mechanisms of the 1-pentene cracking reaction over the ZSM-5 catalyst. The dehydrogenation reaction of different types of BAS in ZSM-5 zeolite to [AlO4]0 site was firstly calculated and it was found that the barrier followed an order of BAS in the intersection channels > BAS in the straight channels > BAS in the sinusoidal channels. This is consistent with our experimental results that the Z-1 catalyst with the highest density of BAS in the sinusoidal channels and straight channels showed the best catalysis performance following the confined catalytic radical mechanism. Then, the activation of 1-pentene in ZSM-5 following the acid catalysis mechanism and confined catalytic radical mechanism was computed. At the BAS active site, 1-C5H10 adsorbs with an adsorption energy of −1.01 eV and transforms to C5H11+ carbenium ion with reaction energy and barrier of 0.94 and 1.33 eV, respectively. At the [AlO4]0 active site, 1-C5H10 adsorbs with an adsorption energy of −1.28 eV and transforms to an adsorbed C5H9 species with reaction energy and barrier of 0.34 and 0.97 eV, respectively, and the adsorbed C5H9 species facilely desorbs from the surface to the gaseous C5H9• radical with a reaction energy of 0.02 eV. These calculation results suggest that the confined catalytic radical mechanism of 1-pentene cracking reaction over ZSM-5 does exist to produce the gaseous C5H9• key intermediate, agreeing with our experimental observation of gaseous C5H9• radicals. Meanwhile, the energetics of the confined catalytic radical mechanism barrier at the [AlO4]0 active site is thermodynamically and kinetically slightly more favorable than the acid catalysis mechanism at the BAS site.

Reactive molecular dynamics (ReaxFF MD) capable of reproducing reaction networks in hydrocarbon cracking systems and heterogeneous catalytic reactions without predefining reactive sites was further used to explore the complex reaction network of 1-pentene cracking reaction catalyzed by ZSM-5 at 1200–1800 K and at the gas phase of 2000 K. Consistent with the DFT calculation results, the lower activation energy of 1-C5H10 at the [AlO4]0 site than at the BAS site was also in the ReaxFF MD simulations. Figure 4c–e shows simulated reaction networks with branching ratios of gaseous C5H9• radicals generated in the confined catalytic radical mechanism at 1600 K, of 1-C5H10 cracking following the carbonium ion mechanism at the BAS site at 1200 K, and of 1-C5H10 cracking following the thermal radical mechanism at 2000 K. Table S4–S6 summarize corresponding simulated reactions with reaction potential energies of 1-pentene activation, C2H4, C3H6 and CH4 formations by calculating the differences of potential energies between the products and reactant that can, to some extent, indicate how easily the reaction occurs, where a positive value corresponds to an endothermic reaction and a negative value indicates an exothermic reaction. Figure 4f compares the acquired C2H4/C3H6 and C2H4/CH4 ratios. In the confined catalytic cracking mechanism, five C5H9• radical isomers are generated by 1-pentene activation at the [AlO4]0 site, and the CH2•CH2CH2CH=CH2, CH3CH2CH•C(•)=CH2 and CH3CH•CH2CH=CH2 radicals decompose to produce C2H4 and C3H6 with reaction potential energies around 50.0 kcal/mol whereas the CH3CH2CH=CHCH2• and CH3CH2CH•CH=CH2 radicals decompose to produce CH3• radicals, the precursor for CH4 formation, with reaction potential energies around 110.0 kcal/mol. In the carbonium ion mechanism at the BAS site, the monomolecular-cracking pathway of carbenium ions produces C2H4 and C3H6 with the reaction potential energies around 10–20 kcal/mol, whereas the bimolecular-cracking pathway produces C2H4 and C3H6 with reaction potential energies around 80 kcal/mol. Such a reaction network favors a monomolecular-cracking pathway and produces the simulated C2H4/C3H6 ratio of 0.9. In the thermal radical mechanism, except that the potential energy required for the formation of C2H4 from C2H5• is around 50 kcal/mol (which is significantly lower than that of the other reactions), the potential energies for the formation of C2H4, C3H6, and CH4 are all around 100 kcal/mol. These reactions compete fiercely at high temperatures, resulting in the 28.95% C2H4, 8.33% C3H6 and 10.75% CH4, giving the simulated C2H4/C3H6 ratio and C2H4/CH4 ratio being 3.5 and 2.7, respectively.

The simulated C2H4/C3H6 and C2H4/CH4 ratios are 2.7 and 7.3 for gaseous C5H9• radical reactions, and 3.5 and 2.7 for 1-C5H10 cracking following the thermal radical mechanism, consistent with their corresponding experimentally observed characteristics. The C2H4/C3H6 ratio of 0.9 in 1-C5H10 cracking following the carbonium ion mechanism, which approaches the theoretical limit (1.0) of the acid-catalyzed carbonium ion mechanism, was found due to the strengthened monomolecular-cracking pathway of carbenium ions in ZSM-5 catalyst with a branched ratio of 71.4%. The contributions of each reaction pathway in 1-C5H10 cracking over ZSM-5 at different temperatures were further simulated, from which the apparent C2H4/C3H6 and C2H4/CH4 ratios were calculated. As shown in Fig. 4g, Table S7, only the acid catalysis mechanism occurred at 1200 K and its contributions decreased to 83.9%, 72.9% and 63.2% at 1400, 1600 and 1800 K, respectively; the confined catalytic radical mechanism emerged at 1400 K and its contribution was 16.1%, increased to 22.5% at 1600 K and did not change much at 1800 K (23.6%); and the thermal radical mechanism emerged at 1600 K and its contribution was 4.6% at 1600 K and increased to 13.2% at 1800 K. The simulated apparent C2H4/C3H6 ratio was 0.9, 1.5, 2.1 and 2.3 at 1200, 1400, 1600 and 1800 K, respectively, while the simulated apparent C2H4/CH4 ratio was 7.0, 4.2 and 3.4 at 1400, 1600 and 1800 K, respectively, whose trends are consistent with those experimentally observed as a function of reaction temperature. The simulation results at 1400~1600 K gave apparent C2H4/C3H6 and C2H4/CH4 ratios consistent with the characteristics of 1-pentene cracking over ZSM-5 at 700 °C. Both DFT and ReaxFF MD simulation results support our experimental results that the confined catalytic radical mechanism at the [AlO4]0 active site with a characteristic of high C2H4/C3H6 ratio and low CH4 selectivity occurs in the 1-pentene cracking over ZSM-5 catalyst at the temperatures between the typical temperature following the acid catalysis mechanism and the typical temperature following the thermal radical mechanism, leading to the 1-pentene cracking reaction with C2H4/C3H6 ratios well above 1.0 and low CH4 selectivity below 5%.

Methods
Catalyst synthesis
Z-1 is ZSM-5 zeolite acquired from SINOPEC Catalyst CO, LTD (Qilu Division) with a Si/Al ratio of 13.8. Z-2 zeolite was procured following the high-temperature pretreatment of Z-1: The powdered Z-1 was pressed, crushed, and sorted into grains of 20~40 mesh, and then the granular Z-1 was treated at 800 °C for 4 h under steam atmosphere (the flow rate of water is 0.66 mL min−1), yielding Z-2 sample. A sample of Na-ZSM-5 acquired from SINOPEC Catalyst CO., LTD (Qilu Division) with a SAR of 30 was subjected to post synthesis treatment. As a typical run for the ion exchange, 50 g Na-ZSM-5 powder was dispersed in 400 mL water containing 20 g ammonium chloride (NH4Cl), stirred at 70 °C for 1 h, and after drying at 120 °C, the NH4-ZSM-5 was obtained. Subsequently, the NH4-ZSM-5 zeolite was treated with fluosilicic acid (H2SiF6) solution. Typically, 50 g NH4-ZSM-5 powder was mixed with 500 mL deionized water, and 47.4 g H2SiF6 solution (30 wt%) was added into the mixture drop by drop at 80 °C, stirring for 1.5 h. After cooling to ambient temperature, Z-3 zeolite was finally obtained by washing with deionized water, drying at 120 °C, and calcining at 550 °C for 4 h.

Catalytic reactions
Various reactions were conducted using ZSM-5 (Z-1, Z-2, Z-3) or the SiO2 particles (20~40 mesh). Reactions were carried out in a fixed-bed reactor (6 mm I.D.), as shown in Fig. S29. The temperature was detected with a thermocouple in the inner center of the catalyst bed. In general, 1.00 g zeolite was packed in the reactor (For the reaction over SiO2, the bed height should be maintained consistent with that when packing zeolite particles), and the 1-pentene (TCI, >98%,) with nitrogen (N2/1-C5H10 = 0.175, molar ratio) were passed through the reactor when the catalyst bed was heated to the desired reaction temperature, where the flow rate of 1-pentene was adjusted by the injection pump. The output products were condensed at −19 °C and separated into vapor and liquid phases. The gaseous products were monitored by a gas chromatography (GC) (Agilent 7890A), which is equipped with a flame ionization detector (FID) and a thermal conductivity detector (TCD). The composition of gaseous products analyzed hereby GC is mole results (according to the calibration gas used to standardize GC), and nitrogen was used as a reference. The quality of liquid products was measured by an analytical balance, and the composition was determined with another gas chromatograph (Agilent 7890A) equipped with an FID detector. For simplicity, we grouped all types of pentenes as the overall feedstock. Following reaction completion, the catalyst was regenerated in situ by flowing air (90 mL min−1) at 670 °C. The evolved CO2 was continuously monitored with a QGS-08B gas analyzer, and the coke content was quantified from the integrated CO2 evolution profile.

Conversions of reactant were calculated as follows:
Conversion = (nC5=,in − nC5=,out) / nC5=,in × 100% (1)

Selectivities of product were calculated as follows:
Selectivity of CxHy = nCxHy,out / (Σni,in − Σni,out) × 100% (2)

During the catalyst recycling test, 1.0 g Z-1 catalyst was firstly tested in 1-pentene cracking reaction for 60 s (1-pentene flow rate, 2 mL min−1; reaction temperature, 700 °C; pressure, 0.1 MPa; N2 flow rate, 70 mL min−1; WHSV, 75 h−1) and then was regenerated in situ by flowing air (90 mL min−1) at 670 °C till no CO2 could be detected with a gas analyzer (15–30 mins) and purged in N2 (70 mL min−1) for 1 h.

Characterizations
X-ray diffraction (XRD) patterns were collected on an EMPYEEAN powder diffractometer (PANalytical Corporation, Netherlands) with CuKα radiation (λ = 1.54 Å). N2 adsorption was carried out at 77 K on a Micro-meritics ASAP 2420 instrument after outgassing the samples for 4 h under 10−2 Pa. Scanning electron microscopy (SEM) was carried on with a HITACHI S-4800. The zeolites were characterized by FT-IR spectroscopy of pyridine adsorption (Py-IR) (Thermo Fisher, NICOLET 6700). A self-supported catalyst wafer with a weight of 25 mg in an in situ cell with CaF2 windows was heated to 400 °C for 1 h under 10−6 Pa and then cooled to room temperature. A blank spectrum from 1300 to 1700 cm−1 was recorded. Pyridine was adsorbed until equilibrium. Desorption was performed at 200 and 350 °C for 30 min, then the spectra were recorded, respectively. The EPR spectra were obtained on a Bruker A300 EPR spectrometer. The instrumental parameters were as follows: scanning frequency, 9.45 GHz; central field, 3300.00 G; scanning width, 600 G; scanning power, 5.03 mW; scanning temperature, 573 K.

Flow-pulse adsorption microcalorimetric measurements. Microcalorimetric measurements of NH3 isothermal adsorption on the samples were performed on a home-designed flow-pulse adsorption-microcalorimetry system consisting of a chemisorption apparatus (Micromeritics Autochem II 2920) and a microcalorimeter (Setaram Sensys EVO 600). In a typical NH3 pulse adsorption experiment, an accurately weighted zeolite sample (20 mg) was placed in a quartz sample tube and pretreated with helium at 500 °C for 1 h. After cooling down to 150 °C, a sequence of NH3 pulses containing a mixture gas (2% NH3/He or 10% NH3/He) was then introduced through the sample, and the microcalorimeter detected the heat changes. The concentration of NH3 in the gas flow was continuously monitored by a down-stream thermal conductivity detector (TCD). For comparison, the same experiments for a blank sample tube were also performed. The reversable adsorbed NH3 was allowed to desorb between each pulse, and their reversible adsorbed amount of NH3 was determined using the TCD signal and heat-flow peaks.

For low NH3 coverage, a low dose of 0.5 ml 2% NH3/He (~0.32 μmol per pulse) was employed for the initial sequence of pulses to differentiate small concentrations of sites with strong acidity. Then, larger doses of 10% NH3/He were used to cover the rest of the large, concentrated acid sites. The molar heats of adsorption for irreversibly adsorbed NH3 were plotted against the total amount of irreversibly adsorbed NH3 per gram of the sample. Multiple experiments were performed on each zeolite under identical conditions, and satisfactory reproducibility was obtained for both heat-flow and TCD peak integrations.

Temperature programmed desorption. Temperature programmed desorption (TPD) experiments were performed on a Micromeritics Autochem II 2920 apparatus. A quartz tube was used as the reactor. The thermocouple, which is in physical contact with the zeolite sample outside the quartz tube, monitors the temperature. The downstream gas (~1 atm) was sampled by the online mass spectrometer (HIDEN QIC-20) through a thin stainless-steel capillary. A bed of quartz chips was placed in the reactor, and 0.08 g of zeolite was added on top of the bed. The sample was pretreated in an air atmosphere by heating to 500 °C for 30 min, followed by cooling to 200 °C. After switching to an Ar carrier gas (50 mL min−1), the temperature was increased from 200 to 700 °C at a rate of 10 °C min−1, then held at 700 °C for 20 min. The m/z = 2 (H2) and m/z = 18 (H2O) signals were recorded during the experiments. No water was detected to desorb during the TPD experiments.

In-situ diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. In situ temperature-programmed treatments of zeolites were performed on a Nicolet 6700 FTIR spectrometer equipped with an in situ high-temperature DRIFTS reaction cell (Harrick Scientific Products, Inc.). The DRIFTS spectra were measured with 256 scans and a resolution of 4 cm−1 using a MCT detector. Prior to the experiments, background spectra were recorded for KBr powder in a dried Ar atmosphere at various temperatures. A 100 mg zeolite sample was loaded onto the sample stage of the reaction cell. The sample was first heated in a dried Ar atmosphere to 500 °C for 30 min, and the spectra of the zeolite framework were recorded before the temperature programmed treatment. The first temperature-programmed desorption (TPD) experiment involved heating the sample from 200 to 700 °C at a rate of 10 °C min−1, followed by a 20-minute hold at 700 °C. Spectra were recorded after the sample was cooled back to 500 °C. For comparison, controlled experiments were conducted in parallel. In these experiments, 1-pentene reactant gas in Ar was introduced at 700 °C for 10 min following the TPD experiment. After cooling the sample to 500 °C, the atmosphere was switched to air, and the sample was reheated to 600 °C to remove carbon deposits, then the atmosphere was switched back to Ar while spectra were recorded.

Solid-state 1H magic-angle-spinning (MAS) NMR experiments. 1D 1H, 27Al magic-angle-spinning (MAS) NMR spectra were acquired at 14.0 T on a Bruker AVANCE II 600 spectrometer equipped with a 4.0 mm probe. The spinning speed was set at 12 kHZ. The repetition time and π/2 pulse width of the 1H MAS NMR spectra were optimized to 2.0 and 2.5 μs, respectively. The quantitative 27Al MAS NMR spectrum was recorded using a small-flip-angle technique with a repetition time of 1.0 s and a π/12 pulse width of 0.5 μs.

In-situ synchrotron vacuum ultraviolet photoionization mass spectroscopy. SVUV-PIMS was performed at the National Synchrotron Radiation Laboratory (NSRL, Hefei, China). Synchrotron radiation from an undulator beamline (BL03U) was monochromatized with a 200 lines/mm laminar grating, which covered a photon energy range of 7.5 to 22 eV with an energy resolution of 3000 (E/ΔE @ 10 eV). A quartz catalytic reactor was connected to the SVUV-PIMS spectrometer, with the catalyst bed (7 × 15 mm) positioned 10 mm from the sampling nozzle of the spectrometer. Before the reaction, the catalyst was pretreated by heating from room temperature to the desired reaction temperature in an Ar flow (100 mL min−1). The 1-pentene reactant gas (flow rate 100 mL min−1) was introduced into the reactor via an Ar carrier gas at room temperature. During the cracking reaction, the feed gas mixture was pumped through the catalyst bed at a total pressure of 2 Torr (the partial pressure of 1-pentene is about 1.4 Torr). Once the catalytic reaction reached a steady state at the desired temperature, the composition of the effluent gas was analyzed using the online SVUV-PIMS spectrometer.

Density functional theory (DFT) calculations
All calculations are performed in the framework of the spin-polarized density functional theory with the projector augmented plane-wave method, as implemented in the Vienna ab initio simulation package (VASP 6.3.2). The generalized gradient approximation (GGA) proposed by Perdew, Burke, and Ernzerhof (PBE) is selected for the exchange-correlation potential. The long-range van der Waals interaction is described by the DFT-D3 approach. The cut-off energy for the plane wave is set to 480 eV. The energy criterion is set to 10−5 eV in the iterative solution of the Kohn-Sham equation. All the structures are relaxed until the residual forces on the atoms have declined to less than 0.02 eV/Å. Data analysis and visualization are carried out with the help of the VASPKIT code and VESTA. To avoid interlaminar interactions, a vacuum spacing of 20 Å is applied perpendicular to the slab. The adsorption energy Eads is expressed as:
ΔEads = EA+B − EA − EB (3)
where EA+B is the total energy of slab A model with B adsorption, EA is the energy of a A slab, and EB is that for a B molecule.

The minimum energy pathways for elementary steps, dehydrogenation in BAS and the reaction steps of 1-pentene, were explored by using the Climbing Image Nudge Elastic Band (CI-NEB) method. The energy barriers for dehydrogenation in BAS at different adsorption sites are reported as DFT electronic energy. For the reaction energy profiles of 1-C5H10 conversion on the BAS and [AlO4]0, the relative Gibbs free energy (ΔG) of each intermediate and transition state was calculated as:
ΔG = ΔE + ΔEZPE − TΔS (4)
where ΔE is the DFT electronic energy, ΔEZPE is the zero-point energy correction, and TΔS is the entropy difference in between adsorbed states of reaction intermediates and gap phase, respectively. T is the room temperature (298.15 K). The Gibbs free energy of gas phase molecules was corrected to a standard pressure of 1 bar.

Reactive force field molecular dynamics (ReaxFF MD) simulation methods
Model construction. A reasonable molecular model is necessary for ReaxFF MD simulation to explore 1-pentene cracking reactions. It is well known that ZSM-5 zeolite is an active catalyst and/or supports for a variety of reactions like cracking, alkylation, aromatization, isomerization of gasoline, etc., owing to their shape selectivity, ion-exchange properties and tri-dimensional micropore topology etc. Fig. S30 shows the 3D configurations of the ZSM-5 unit cell containing 96 T.

The initial simulated box for ZSM-5 has 96 tetrahedral (96 T) SiO2. The initially constructed ZSM-5 model was expanded along with the corresponding direction of 4 × 4 × 1 of the unit cell to construct large clusters of ZSM-5 catalysts by using Materials Studio (MS). It is well recognized that the acidity of the zeolites is largely dependent on their Si/Al molar ratios, and the SiO2/Al2O3 ratio determines the amount of cation required in the structure and the level of acidity. Thus, a ZSM-5 cluster model was built by replacing 32 superficial silicon atoms randomly with 32 Al atoms to have a Si/Al ratio in T12 sites to obtain 32 Bronsted acid sites. Moreover, 128 hydroxyl groups were introduced on the surface of ZSM-5 catalyst to represent 128 weak acid sites. In order to allow major reactions to be completed in the acceptable simulation time, the surface defects containing dangling bonds of Si and O atoms were introduced into the ZSM-5 surface. These exposed –Si and –O on the ZSM-5 surface would absorb 1-pentene molecules at the very initial stage to create an active environment, which can accelerate the reaction process. Subsequently, the modified ZSM-5 zeolite was located centrally in a periodic boundary box, where 160 optimized 1-pentene molecules were put around the zeolites. In addition, a model containing only 200 optimized 1-pentene molecules was built to study a typical simulation temperature for major thermal cracking reactions occurring, while the model containing only ZSM-5 zeolite was constructed to confirm the structure transformation of zeolites at the specific simulated temperature.

ReaxFF MD simulation details. Temperature is an important factor to influences zeolite stability, catalytic performance, cracking rate and reactions taking place on zeolite catalyst. Because of significant difference in reaction time between simulation and experiments, the manually increased temperature strategy was used usually to allow reactions taking place and completed at the acceptable computational time. Higher temperatures would cause unexpected results such as lots of thermal decomposition of hydrocarbons, the collapse of zeolites or a change of catalyzed reaction mechanisms. In order to select the appropriate simulation temperature for competitive reactions for catalytic cracking and thermal cracking reactions, a heat-up ReaxFF MD simulation for the model containing only 1-pentene molecules was performed at 100–3000 K with a heating rate of 1 K/ps. The thermal radical decomposition reactions appear from the temperature of 1400–1600 K and dominate from 2000 K. Thus, the simulated temperatures of 1400–1600 K are speculated most likely to correspond to those in the experiments of 650–750 °C, where thermal radical mechanisms start to appear. Consequently, the temperature ranges of 1200–1800 K were selected in this work to investigate the temperature effects on the competitive reactions of acid-catalyzed carbenium ion mechanism, confined catalytic radical mechanism and thermal radical mechanism during 1-pentene cracking at ZSM-5.

All the ReaxFF MD simulations were performed by using Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS-29 Oct 20). During the ReaxFF MD simulation, the ZSM-5 catalyzed 1-pentene cracking model first went through energy minimization and then equilibrated at 300 K for 50 ps. Subsequently, the temperatures increased from 300 K to a specific temperature within 50 ps by using the NVT ensemble. The isothermal ReaxFF MD simulations were the last steps for 5000 ps. During all NVT ReaxFF MD simulations, the Berendsen thermostat with a damping constant of 0.1 ps was employed to maintain temperature equilibration and a time-step of 0.1 fs was used to integrate Newton’s equation of motion by the velocity-Verlet algorithm. The zeolite parameters developed by Pitman et al., were used in ReaxFF MD simulation, which is demonstrated the close accuracy to DFT for zeolites and applied in catalytic decomposition of hexadecane with the presence of ZSM-5 catalysts at high temperature. It should be noted that three parallel simulations were performed for each condition to obtain results more statistically. And all the reactions occurring during the simulation period were identified automatically by using the reaction analysis tool of VARxMD (Fig. S31).

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The authors' main claim is that their catalysts present unusual active sites, non-acidic Al-occupied oxygen tetrahedra, which\nwould be involved in unconventional cracking of 1-pentene, leading to large amounts of ethylene. This would be useful in\ncertain oil-to-chemicals settings.\nI suggest to reject this work as their complicated explanation is not supported by strong experimental mechanistic evidence.\nIt is easy to see that making ethylene directly from 1-pentene presents a high barrier (secondary to primary carbenium ion\ntransition states). Increasing the temperature will lead to an increasing contribution of ethylene in the direct cracking of 1-\npentene as for instance can already be seen in the data in the following paper: ACS Catal. 2015, 5, 7, 4048–4059. Indeed, at\na temperature of 550C, this paper also reports ethylene/propylene ratios in excess of 1.\nObviously, a further increase of the temperature will increase said ratio, explaining the results. Their analysis of radicals in\nthe gas phase cannot be used as a direct evidence for a mechanism, although we should not directly discard these results.\nBut the work lacks experimental evidence for their mechanism.\nNothing new under the sun, I would think.", "reviewer_2": "This paper described the conversion of 1-peneten over ZSM-5 at high temperature. High ethylene selectivity was obtained\nover a tailored ZSM-5 catalyst at 700 ℃ with low methane selectivity. Detailed study showed that the high ethylene\nselectivity was related with the non-acidic [AlO4]0 sites of ZSM-5 in situ formed by dehydrogenation of bridging hydroxyls.\nThe authors proposed the formation of gaseous pentenyl radicals (C5H9•) by dehydrogenation over these new catalyst\ncenters and further cracking through confined catalytic radical (CCR) mechanism, which is responsible for high ethylene\nselectivity and is different from the carbenium ion mechanism of acid catalysis. These findings are important for zeolite\ncatalysis and are recommended for publication after minor revision. The suggestions and questions are listed as follows.\n1.The ethylene selectivity and C2H4/C3H6 ratio are closely related to thermodynamic equilibrium, and high temperature is\nresponsible for high ethylene selectivity. The control experiments are necessary to clarify the influence of thermodynamics\non product distribution at different temperature by only acid catalysis.\n2.As shown in Figure 3b, hydrogen was produced during the temperature-programmed desorption in an Ar atmosphere from\n200 to 700 oC over Z-1, Z-2, and Z-3 zeolites. The question is what is the structure changes of zeolite according to the\ndehydrogenation reaction? How to balance the electric charge of framework after H was removed?\n3.If the reactor is operated at 700 oC, the regenerator will be operated at even higher temperature. The dealumination of\nZSM-5 and the catalyst stability will be very important for the proposed process for ethylene production. How does the\nproperty of these non-acidic [AlO4]0 sites change under harsh regeneration conditions?\n4.High methane selectivity is mainly related to formation of hydrogen-deficient substances. Except diminishing the coke\nformation, the intrinsic reason of the methane yield reduction in confined catalytic radical (CCR) mechanism compared with\nthermal cracking is still not clear.\n5.The catalytic cracking of alkanes over acid zeolite also suffers from the lower ethylene selectivity. Activation of alkanes\nover non-acidic [AlO4]0 sites might also give higher ethylene selectivity. What is the difference between alkane and alkene\nactivation over these new type catalytic sites?", "reviewer_3": "In their most interesting manuscript titled “In situ formation of non-acidic [AlO4]0 active site of zeolite for confined catalytic cracking of 1-peneten to ethylene with low methane selectivity”, the authors claim the discovery of a novel “confined” radical pathway for 1-pentane cracking at moderate temperatures, which results in superior C2H4 selectivity, lower CH4 production, and potentially higher profits. From the experimental perspective, these findings are based on comparisons of product selectivities over several ZSM-5 (MFI) samples (low Si/Al, steam-treated low Si/Al, and high Si/Al) and quartz, with literature data on gas-phase steam cracking as a benchmark. Furthermore, the results are substantiated with photoionization-based mass-spectrometry of the catalytic reactor effluent using SUV radiation and several complementary catalyst characterization techniques. The mechanistic interpretation is supported by DFT and MD simulations. In general, this work is of great interest to the catalysis community, as it put forth an alternative mechanism for an extremely important industrial reaction using state-of-the-art methods. The article is well-written and clearly illustrated. However, I am reluctant to accept this manuscript without major revisions, primarily due to the following concerns:\n1) Silicalite-1 would appear to be a more convincing choice of a reference, BAS-free material for the reaction studies, as it features the same high surface-area MFI pore network as ZSM-5.\n2) Not a single error bar appears in the entire manuscript when it comes to conversion and selectivites, which for catalytic test data is a major shortcoming in quality of work. How uncertain are the derived quantities, such as C2/C3 and C2/C1 ratios for different materials upon which major conclusions are drawn, given that selectivities are not replicated?\n3) In some ways, the entire deduction of the “confined” radical mechanism rests on a single SUV-PIMS data point whereby the ion current for the hypothetical C5H9 radical at 700C drastically exceeds that of the C3H5 radical (Figure 2b). Yet, the authors did not bother to record additional data points at intermediate temperatures slightly above and below 700C to confirm the systematic nature of their observations. I find this sub-standard to the level of the journal. At the very least, the authors should discuss and present data convincing that it is only the PIY of C5H9 radical at that energy that peaked at that temperature with respect to some other PIY at that energy that remained unchanged.\n4) Incomplete data are presented for various methods and the experimental section is not sufficiently detailed for other researchers to reproduce their work. For example, DRIFTS and NMR data (Fig 3c,d) are presented only for Z1, but not Z2 and Z3 samples. Signal at m/z 69 is not compared between blank and Z1 at 9.2 and 10.6 eV, similarly to that presented for m/z 70 in Fig S5. The catalytic setup is very poorly described (no diagram) and no information is given regarding the experimental treatment sequence. Another inconsistency raises concerns – in the legend to Figure 1, the authors claim 100% pentene feed, in which case it is not clear who the GC data were quantified without any internal standard.\n5) The manuscript completely lacks standard characterization data for zeolite materials that any researcher in the field would expect – nitrogen adsorption, SEM/EDX, particle size and morphology, DRIFTS in the proton range and probe molecules to distinguish LAS and BAS, etc.\n6) In the absence of information regarding external sites present on these materials, I find it hard to believe that any radicals originating in the inner pores of the catalyst would be able to escape into the bulk phase and survive for detection with SUV-PIMS. The authors should discuss this in more details and at least clearly state that the detection of C5H9 in their work is rather a hypothesis than confirmed observation, which will need to be more directly substantiated in future works using, e.g. velocity imaging detectors that can distinguish radical species more clearly. It would be very helpful to see the absorption spectra for PIY 69 m/z and PIY 70 m/z covering the entire photoionization step and compare those to the spectra for C5H9 and C5H10 from the experimental and/or theoretical literature.\n7) Looking at the NH3 pulse chemisorption data in Fig S12, I cannot see the NH3 breakthrough in the TCD signal, which looks like a noise background line all throughout the experiment. Can the authors elaborate on this?\n8) The theoretical calculations are documented very poorly! While I lack competence in DFT and MD to scrutinize their results properly, but it appears that no info is given regarding the estimation of entropic contributions, and all the energy barriers are given in electronic terms, rather than Gibbs free energy. At the same time, Gibbs energy barriers are mentioned for diffusion barriers in the methods section, but diffusion results never even mentioned in the text or the supplementary, which makes me wonder to what degree were the authors focused on self-consistency of the manuscript. The reaction network in Figure 4 is very extensive, but it is highly speculative and not documented at all! How was that network and the branching ratios of reaction obtained?\n9) No discussion of catalytic stability was provided. While coke content quantification is mentioned in the experimental section, it is never actually discussed in the manuscript.\n10) Last, but not least, I suggest that the authors provide at least a surface-level discussion of their work in view of urgent sustainability concerns associated with petroleum-based production of chemicals and ethical implications of maximizing profits of the plastics industry."}, "author_response": "Author reply to reviewer 1’ comments\n[... full author response text ...]\nAuthor reply to reviewer 2’ comments\n[... full author response text ...]\nAuthor reply to reviewer 3’ comments\n[... full author response text ...]"}, {"round": 2, "reviewer_comments": {"reviewer_2": "All my concerns were addressed in the revised manuscript and publication is recommended.", "reviewer_3": "After carefully considering all the additional experiments provided by the authors in response to the previous round of\nreviews and the changes they introduced into the manuscript, I can recommend accepting this publication."}, "author_response": ""}]

Decision letter